Proceedings Volume 9342

Solid State Lasers XXIV: Technology and Devices

W. Andrew Clarkson, Ramesh K. Shori
Proceedings Volume 9342

Solid State Lasers XXIV: Technology and Devices

W. Andrew Clarkson, Ramesh K. Shori
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Volume Details

Date Published: 16 April 2015
Contents: 14 Sessions, 61 Papers, 0 Presentations
Conference: SPIE LASE 2015
Volume Number: 9342

Table of Contents


Table of Contents

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  • Front Matter: Volume 9342
  • Crystal Fiber Lasers I
  • Crystal Fiber Lasers II
  • Eye-Safe and Mid-IR Lasers
  • Airborne and Space Qualified Lasers
  • UV and Visible Lasers
  • Disk Lasers
  • Novel Concepts I
  • Novel Concepts II
  • Laser Materials and Characterization
  • Ultrafast Lasers
  • Pulsed Lasers I
  • Pulsed Lasers II
  • Poster Session
Front Matter: Volume 9342
Front Matter: Volume 9342
This PDF file contains the front matter associated with SPIE Proceedings Volume 9342, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.
Crystal Fiber Lasers I
Single crystal fiber for laser sources
Xavier Délen, Adrien Aubourg, Loïc Deyra, et al.
Single crystal fiber (SCF) is a hybrid laser architecture between conventional bulk laser crystals and active optical fibers allowing higher average powers than with conventional crystals and higher energy than with fibers in pulsed regime. The pump beam delivered by a fiber-coupled laser diode is confined by the guiding capacity of the SCF whereas the signal beam is in free propagation. In this paper, we study the pump guiding in the SCF and give an overview of the results obtained using SCF gain modules in laser oscillators and amplifiers. We report about up to 500 μJ nanosecond pulses at the output of a passively Q-switched Er:YAG SCF oscillator at 1617 nm. High power experiments with Yb:YAG allowed to demonstrate up to 250 W out of a multimode oscillator. High power 946 nm Nd:YAG SCF Q-switched oscillators followed by second and fourth harmonic generation in the blue and the UV is also presented with an average power up to 3.4 W at 473 nm and 600 mW at 236.5 nm. At 1064 nm, we obtain up to 3 mJ with a nearly fundamental mode beam in sub-nanosecond regime with a micro-chip laser amplified in a Nd:YAG SCF. Yb:YAG SCF amplifiers are used to amplify fiber based sources limited by non-linearities such as Stimulated Brillouin Scattering with a narrow linewidth laser and Self Phase Modulation with a femtosecond source. Using chirped pulse amplification, 380 fs pulses are obtained with an energy of 1 mJ and an excellent beam quality (M2<1.1).
High-power Yb:YAG single-crystal fiber amplifiers for femtosecond lasers
Fabien Lesparre, Igor Martial, Julien Didierjean, et al.
We describe a multi-stages single crystal fiber (SCF) amplifier for the amplification of femtosecond pulses with radial or azimuthal polarization in view of high speed material processing (surface structuring, drilling). We demonstrate a three stages diode-pumped Yb:YAG single crystal fiber amplifier to achieve femtosecond pulses at an average power of 85W at 20 MHz in radial and azimuthal polarization.
Erbium distribution in single crystal YAG fibers grown by laser-heated pedestal growth technique
Craig D. Nie, Subhabrata Bera, Jeffrey E. Melzer, et al.
Single crystal (SC) yttrium aluminum garnet (YAG, Y3Al5O12) as a host material has the ability to be doped with high concentrations of Er3+ ions. We utilize this ability to grow a 50% Er3+ doped YAG SC fiber, which was inserted into a SC YAG tube. This rod-in-tube was used as a preform in our laser-heated pedestal growth (LHPG) apparatus to grow a fiber with a radial distribution of Er3+ ions. The work shows that there is a distribution of Er3+ ions from their fluorescence and two different techniques were used to measure the index of refraction.
Investigation of the amplification properties of Ho:YAG single crystal fiber
Yuan Li, Zeyu Zhang, Ian Buckley, et al.
0.5% Holmium (Ho) doped YAG single crystal fiber (SCF) was fabricated using the laser heated pedestal growth (LHPG) method and amplification properties of the fabricated Ho:YAG SCF were studied. The relatively large lengthto- diameter ratio provides guiding for both the pump and signal beams propagating in the SCF. The propagation and gain of signals with different modes were studied. A numerical method based on finite difference (FD) beam propagation method (BPM) combined with the rate equations was developed for theoretical simulation. The results are encouraging to demonstrate the advantages of SCF for its fiber-like beam guiding property and solid state material gain property. The simulation tool provides details about how the fiber shape and launched mode affect the gain and output beam shape as well as predicts the amplification behavior of such unique specialty fibers.
Crystal Fiber Lasers II
Beam quality investigation in Nd:YAG crystal fiber amplifier pumped at > 110 W
Aleksej M. Rodin, Aidas Aleknavicius, Andrejus Michailovas, et al.
We present results of beam quality investigation in Nd:YAG crystal fiber amplifier seeded by ns, sub-ns and ps laser pulses counter-propagating to continuous pump of < 110 W power at 808 nm wavelength. The maximum amplified power of 44 W and energy of 3.2 mJ has been achieved with ns seed. We observed gain rise to < 75 with ps seed of 1.6 mW average power when tuned emission spectra to ~ 1064.3 nm. Beam propagation coefficient M2 approach 1.2 at the maximum pump power with high quality seed pulse emitted by 6 ps fiber laser. Amplification of sub-ns seed pulses from microchip laser led to an M2 increase from ~1.2 to ~< 1.5. New analytical solution for temperature distribution in end-pumped thin long single crystal fiber with temperature dependent thermal conductivity coefficient is found for polynomial transverse pump distribution. The analytical relation between thermal coefficients of refractive index at zero stresses and zero strains is found for YAG type cubic crystals. Using plane strain approximation the analytical expression for thermal radial and tangential changes of refractive index is found, and the relation between different expressions for so-called photoelastic constants Cr,θ is established. The methods of numerical calculation of rays and Gaussian beam propagation in a graded-index medium of active element are developed. The error in widely used formula for M2 of Gaussian beam with quartic phase aberration is corrected. It is shown that beam quality degradation can be explained by active thermal lens in power amplifier when changes of transverse beam shape or beam width during amplification are taken into account.
Laser diode pumped high efficiency Yb:YAG crystalline fiber waveguide lasers
Xiaodong Mu, Stephanie Meissner, Helmuth Meissner
Single-clad and double-clad Yb:YAG crystalline fiber waveguides (CFWs) have been prepared with Adhesive-Free Bonding (AFB®) technology. By using a fiber coupled laser diode as pump source, a single-mode laser with near diffraction limited beam quality M2=1.02 has been demonstrated in a double-clad CFW. The laser output power and efficiency are 13.2 W and 34%, respectively. In a single-clad CFW, core pumping was used. The laser output has top-hat beam profile. An output power of 28 W and a slope efficiency of 78% have been achieved respectively.
Templated growth of II-VI semiconductor optical fiber devices and steps towards infrared fiber lasers
Pier J. A. Sazio, Justin R. Sparks, Rongrui He, et al.
ZnSe and other zinc chalcogenide semiconductor materials can be doped with divalent transition metal ions to create a mid-IR laser gain medium with active function in the wavelength range 2 - 5 microns and potentially beyond using frequency conversion. As a step towards fiberized laser devices, we have manufactured ZnSe semiconductor fiber waveguides with low (less than 1dB/cm at 1550nm) optical losses, as well as more complex ternary alloys with ZnSxSe(1-x) stoichiometry to potentially allow for annular heterostructures with effective and low order mode corecladding waveguiding.
Eye-Safe and Mid-IR Lasers
4.5 W mid-infrared supercontinuum generation in a ZBLAN fiber pumped by a Q-switched mode-locked Tm3+- doped fiber laser
C. Kneis, B. Donelan, A. Berrou, et al.
The generation of mid-infrared (mid-IR) supercontinuum (SC) radiation, ranging from 2 - 5 μm, is subject of intense research due to its wide range of applications. A very popular host media for mid-IR SC generation are soft glass fibers owing to their low-loss transmission in the mid-IR wavelength regime, particularly fluoride fibers are very attractive for high-power operation. In this research study, a diode-pumped Q-switched mode-locked (QML) thulium (Tm3+)-doped double-clad silica fiber laser is used to pump a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber for mid-IR SC generation. The QML regime of the fiber laser is actively generated by two acousto-optic modulators. The Tm3+-fiber laser provided up to 23.5 W (26 W) of average output power in QML (continuous wave) regime with a slope efficiency of 36 % (32 %). The measured beam quality has been close to the diffraction-limit in QML regime. The system delivered mode-locked pulses with a duration of 7.5 ps, measured with a commercial autocorrelator system, at a repetition rate of 46 MHz. The Q-switched envelopes had a width between 50 and 150 ns depending on the output power level and the adjustable repetition rate. Mid-IR SC with an average output power in all spectral bands of 4.5 W have been achieved with more than 3 W/ 1.7 W/ 1 W/ 0.36 W after a long-wave-pass filter with a 3 dB-edge at 2.15 μm/ 2.65 μm/ 3.1 μm/ 3.5 μm.
Multi-wavelength resonant pumping of Er:YAG lasers for energy efficient trace gas detection systems
Haro Fritsche, Oliver Lux, Casey Schuett, et al.
The multiplicity of narrow absorption lines of erbium ions in the spectral range from 1450 to 1540 nm is exploited for the development of a highly efficient Er:YAG laser emitting at 1645 nm. Resonant pumping of the active medium with an absorption efficiency of up to 96% is achieved using a novel diode laser system consisting of two narrowband modules with a combined output power of 80 W ex fiber. The utilization of multiple pump wavelengths allows for both substantial power scaling and reduction of the laser threshold, thus providing a low power consuming laser system feasible for LIDAR applications.
Ho:YLF non-planar ring laser with fractional image rotation
A Tm:fiber-laser-pumped Ho:YLF non-planar ring laser with fractional image rotation (~77.45 degree per round trip) is presented. The ring laser cavity consists of six flat mirrors with an incident angle on all six mirrors of ∼32.7 degree with a total cavity length of 222 mm. To make the resonator stable, an intracavity lens has been used with different focal lengths of 75, 100 and 300 mm resulting in different TEM00 spot radii. With this configuration, a maximum laser power of 17.3 W was obtained at a wavelength of 2064 nm, corresponding to a slope efficiency of 28% (57%) with respect to incident (absorbed) pump power. The beam quality factor M2 was measured to be 6.4, 5.2 and 1.7 with focal length of 75, 100 and 300 mm, respectively. Near and far field images show, that the mode exhibits a strong narrow central spot with some ring structure around it, which can be understood as a superposition of Laguerre-Gaussian modes (l, p) with l = 0. Identical experiments have been carried out with a planar ring configuration without beam rotation resulting in comparable power performance but somewhat better beam quality with M2 values of 5.2, 3.5 and 1.4, respectively. The intensity distribution of near and far field in case of a planar cavity is more or less Gaussian.
Gain-switched operation of ultrafast laser inscribed waveguides in Cr:ZnSe
Sean A. McDaniel, Patrick A. Berry, Kenneth L. Schepler, et al.
We report the first demonstration of a gain-switched chromium-doped zinc selenide channel waveguide laser. The guided-wave structure was produced by ultrafast laser inscription and exhibited output pulse energies up to 12 μJ . The laser exhibited narrow spectral output with a linewidth less than 1 nm. The beam quality was measured to be M2 ≤ 7 with a highly multimode output profile. The laser had a maximum slope efficiency of 9.8% and no deleterious thermal effects were observed up to an average pump power of 3.3 W .
A continuous wave Fe:ZnSe laser pumped by efficient Er:Y2O3 laser
Fe:ZnSe lasers have been pumped by several types of diode-pumped solid state laser, including Cr:Er:YSGG (2800 nm),1 Cr:CdSe (2970 nm),2 and Er:YAG (2698 nm,3 2936 nm4). None of these sources has exceeded 1.5 W of true continuous-wave (CW) output power. In this work, we report demonstration of a CW Fe:ZnSe laser pumped by a 10 W Er:Y2O3 laser emitting at 2740 nm,5 which had not been previously attempted. The Er:Y2O3 pump laser was characterized with respect to propagation losses, beam quality, mode size, and pointing stability. It was determined that the limit of output power from the Fe:ZnSe laser was limited by the output stability of the pump laser. The Fe:ZnSe laser operated with < 22% slope efficiency and 800 mW output power was achieved at approximately 4050 nm.
Radiation-enhanced thermal diffusion of transition metal and rare earth ions into II-VI semiconductors
Alán Martinez, Lamario Williams, Ozarfar Gafarov, et al.
We report on study of gamma radiation-enhanced thermal diffusion of Transition Metal and Rare Earth ions into IIVI semiconductor crystals. ZnSe and ZnS samples with of iron thin film deposited on one facet were sealed in evacuated quartz ampoules at 10-3 Torr. The crystals were annealed for 14 days at 950°C under γ-irradiation from 60Co source. The irradiation dose rates of 43.99 R/s, 1.81 R/s were varied by distance between 60Co source and furnaces. For comparison, the samples were also annealed without irradiation at the same temperature. The spatial distributions of transition metal were measured by absorption of focused laser radiation at 5T2-5E mid-IR transitions of iron ions. In addition, samples of ZnSe were similarly sealed in evacuated quartz ampoules in the presence of Praseodymium metal and annealed at 950°C under 43.99 R/s and 0 R/s and the diffusion lengths and Pr concentrations were compared. The γ-irradiation results in better intrusion of the iron ions from the metal film and increase of the diffusion length at ~25%, while Praseodymium diffusion is dramatically enhanced by γ-irradiation during the annealing process.
Airborne and Space Qualified Lasers
Radiation tests on erbium-doped garnet crystals for spaceborne CH4-Lidar applications
Ansgar Meissner, Martin Kreitler, Miroslaw Cubera, et al.
A test campaign for assessing the radiation hardness of different Erbium-doped garnet crystals including Er:YAG and a compositionally tuned Er:YAG/Er:LuAG mixed garnet is reported. Tests with proton and gamma radiation have been performed with parameters mimicking a 3-year low-earth-orbit satellite mission like MERLIN or ADM-Aeolus. For each test sample broadband transmission spectra in the wavelength range of 500 nm – 1700 nm and characteristic laser curves from a test laser oscillator have been measured. Radiation-induced losses have been calculated from the obtained data. The results indicate that gamma radiation is the dominant loss source with about 0.5 %/cm radiation-induced losses for the nominal dose of the chosen mission scenario.
Multi-pulse detection technique to improve the timing/range resolution in a scanning LADAR system
The temporal resolution of a LADAR system is primarily decided by the pulse duration of the laser source, response time of the detector and the resolution of the processing electronics. A combination of the timing jitter associated with these components can deteriorate the system performance by reducing the range resolution of the complete system. The range information in a LADAR system is obtained by measuring the time-of-flight using an electronic time-to-amplitude converter or a multi-channel analyzer. In this paper, we discuss a multi-pulse detection scheme which can be utilized to improve the range resolution of a LADAR system, and allow us to determine the target range with higher accuracy. We present results showing improvement by a factor of three in the range resolution of the scanning LADAR system using this scheme in our laboratory.
Single frequency and wavelength stabilized near infrared laser source for water vapor DIAL remote sensing application
Ti Chuang, Brooke Walters, Tim Shuman, et al.
Fibertek has demonstrated a single frequency, wavelength stabilized near infrared laser transmitter for NASA airborne water vapor DIAL application. The application required a single-frequency laser transmitter operating at 935 nm near infrared (NIR) region of the water vapor absorption spectrum, capable of being wavelength seeded and locked to a reference laser source and being tuned at least 100 pm across the water absorption spectrum for DIAL on/off measurements. Fibertek is building a laser transmitter system based on the demonstrated results. The laser system will be deployed in a high altitude aircraft (ER-2 or UAV) to autonomously perform remote, long duration and high altitude water vapor measurements.
Monolithic solid-state lasers for spaceflight
A new solution for building high power, solid state lasers for space flight is to fabricate the whole laser resonator in a single (monolithic) structure or alternatively to build a contiguous diffusion bonded or welded structure. Monolithic lasers provide numerous advantages for space flight solid-state lasers by minimizing misalignment concerns. The closed cavity is immune to contamination. The number of components is minimized thus increasing reliability. Bragg mirrors serve as the high reflector and output coupler thus minimizing optical coatings and coating damage. The Bragg mirrors also provide spectral and spatial mode selection for high fidelity. The monolithic structure allows short cavities resulting in short pulses. Passive saturable absorber Q-switches provide a soft aperture for spatial mode filtering and improved pointing stability. We will review our recent commercial and in-house developments toward fully monolithic solid-state lasers.
ICESat-2 laser technology readiness level evolution
We report on the completion of the space qualification testing program for NASA Goddard Space Flight Center’s (GSFC) Ice, Cloud, and Land Elevation Satellite 2 (ICESat-2) program. This paper describes the final performance results of the fully integrated (laser and electronics) flight laser system with an emphasis on the system design evolution from a breadboard demonstration to a fully space-qualified laser system. The 532 nm ICESat-2 laser transmitter generates diffraction limited pulse energies of 1 mJ, pulsewidths of < 1.5 ns, and 10 kHz pulse repetition frequency and has minimum lifetime of 1 trillion pulses on-orbit. A combination of engineering design units and correlated structural thermal optical analysis was used to systematically improve reliability and performance over the operating environment. The laser system qualification and acceptance test programs included electromagnetic interference (EMI), vibration, and thermal vacuum (TVAC) testing. This paper presents key laser performance results and lessons learned on the multi-year laser development to facilitate future space-qualified laser developments, improve reliability, and increase performance.
Laser Amplifier Development for IPDA Lidar measurements of CO2 from Space
Anthony W. Yu, James B. Abshire, Mark Storm, et al.
Accurate global measurements of tropospheric CO2 mixing ratios are needed to better understand the global carbon cycle and the CO2 exchange between land, oceans and atmosphere. NASA Goddard Space Flight Center (GSFC) is developing a pulsed lidar approach for an integrated path differential absorption (IPDA) lidar as a candidate for the NASA’s planned ASCENDS mission to allow global measurements of atmospheric CO2 column densities from space. Our group has developed and demonstrated an airborne IPDA lidar for this purpose. It uses two tunable pulsed laser transmitters allowing simultaneous measurement of a single CO2 absorption line in the 1570 nm band, absorption of an O2 line pair in the oxygen A-band (765 nm), and atmospheric backscatter profiles in the same path. In the airborne lidar, both lasers are pulsed at 10 kHz, and the two absorption line regions are sampled at typically a 300 Hz rate. A space version of this lidar must have a much larger laser power-telescope area product to compensate for the signal losses in the ~40x longer range. An analysis of signal to noise ratios indicated that for a 400 km orbit, a 1.5 m diameter telescope and a 10 second integration time, that 1.5 to 2 mJ laser energy is required to attain the needed measurement precision. To meet the laser energy requirements we have pursued two parallel power-scaling approaches for the space laser. These include a single-amplifier approach consists of a multi-pass Er:Yb:Phosphate glass based planar waveguide amplifier (PWA) and a parallel amplifier approach using multiple (typically 8) large mode area (LMA) fiber amplifiers. In this paper we summarize the laser amplifier design approaches and preliminary results.
UV and Visible Lasers
1W frequency-doubled VCSEL-pumped blue laser with high pulse energy
We report on a Q-switched VCSEL side-pumped 946 nm Nd:YAG laser that produces high average power blue light with high pulse energy after frequency doubling in BBO. The gain medium was water cooled and symmetrically pumped by three 1 kW 808 nm VCSEL pump modules. More than 1 W blue output was achieved at 210 Hz with 4.9 mJ pulse energy and at 340 Hz with 3.2 mJ pulse energy, with 42% and 36% second harmonic conversion efficiency respectively. Higher pulse energy was obtained at lower repetition frequencies, up to 9.3 mJ at 70 Hz with 52% conversion efficiency.
Development of high coherence, 200mW, 193nm solid-state laser at 6 kHz
T. Nakazato, M. Tsuboi, T. Onose, et al.
The high coherent, high power 193-nm ArF lasers are useful for interference lithography and microprosessing applications. In order to achieve high coherence ArF lasers, we have been developing a high coherence 193 nm solid state laser for the seeding to a high power ArF laser. We used the sum frequency mixing of the fourth harmonic (FH) of a 904-nm Ti:sapphire laser with a Nd:YVO4 laser (1342 nm) to generate 193-nm light. The laser system consists of a single-mode Ti:sapphire oscillator seeded by a 904-nm external cavity laser diode, a Pockels cell, a 6-pass amplifier, a 4-pass amplifier, a 2-pass amplifier and a wavelength conversion stage. The required repetition rate of 6 kHz corresponding to the ArF laser, along with a low gain at 904 nm induces serious thermal lens effects; extremely short focal lengths of the order of cm and bi-foci in the vertical and horizontal directions. From the analysis of thermal lens depending on pump intensity, we successfully compensated the thermal lens by dividing a 527-nm pump power with 15, 25 and 28 W to 3-stage amplifiers with even passes, resulting in the output power above 10W with a nearly diffraction limited beam. This 904-nm output was converted to 3.8 W in the second harmonic by LBO, 0.5 W in FH by BBO sequentially. Finally the output power of 230 mW was obtained at 193 nm by mixing the FH with a 1342-nm light in CLBO.
Annealing temperature dependence of random lasing properties in a diamond nanoparticle film
Ryo Niyuki, Hideaki Takashima, Hideki Fujiwara, et al.
We demonstrated a novel ultraviolet random lasing in a diamond nanoparticle film, in which above the threshold, several sharp peaks appeared at the center of spontaneous emission around 380 nm. In order to improve the lasing properties, we measured the annealing temperature dependence to examine the influence of the sp2 layer on the surface of diamond nanoparticles, which was considered to quench the emission from a diamond body (sp3 diamond). From the results, we found that UV random lasing was able to observe in a diamond nanoparticle film when the annealing temperature was from 400 to 700 ˚C, although the lasing was hard to observe when the annealing temperature was below 300 ˚C. However, regardless of annealing temperature (above 400 ˚C), UV random lasing properties did not change, in which random lasing peaks clearly concentrated around the center of spontaneous emission and their thresholds were dispersed from several tens to several hundred MW/cm2. These results suggest that annealing at suitable temperature is important to induce UV random lasing, whereas random lasing properties (lasing wavelength, lasing threshold) do not change even if annealing temperature change from 400 to 700 ˚C.
Demonstration of miniaturized 20mW CW 280nm and 266nm solid-state UV laser sources
Nicolas Landru, Thierry Georges, Julien Beaurepaire, et al.
Visible 561 nm and 532 nm laser emissions from 14-mm long DPSS monolithic cavities are frequency converted to deep UV 280 nm and 266 nm in 16-mm long monolithic external cavities. Wavelength conversion is fully insensitive to mechanical vibrations and the whole UV laser sources fit in a miniaturized housing. More than 20 mW deep UV laser emission is demonstrated with high power stability, low noise and good beam quality. Aging tests are in progress but long lifetimes are expected thanks to the cavity design. Protein detection and deep UV resonant Raman spectroscopy are applications that could benefit from these laser sources.
Disk Lasers
Front end for high-repetition rate thin disk-pumped OPCPA beamline at ELI-beamlines
The ELI-Beamlines facility, currently under construction in Prague, Czech Republic, will house multiple high power laser systems with varying pulse energies, pulse durations, and repetition rates. Here we present the status of a high repetition rate beamline currently under construction with target parameters of 20 fs pulse duration, 100 mJ pulse energy, and 1 kHz repetition rate. Specifically we present the Yb:YAG thin disk lasers which are intended to pump picosecond OPCPA, synchronization between pump and signal pulses in the OPCPA, and the first stages of OPCPA.
Thin-disk laser multi-pass amplifier
K. Schuhmann, M. A. Ahmed, A. Antognini, et al.
In the context of the Lamb shift measurement in muonic helium [1,2,3,4] we developed a thin-disk laser composed of a Q-switched oscillator and a multi-pass amplifier delivering pulses of 150 mJ at a pulse duration of 100 ns. Its peculiar requirements are stochastic trigger and short delay time (< 500 ns) between trigger and optical output [5]. The concept of the thin-disk laser allows for energy and power scaling with high efficiency. However the single pass gain is small (about 1.2). Hence a multi-pass scheme with precise mode matching for large beam waists (w = 2 mm) is required. Instead of using the standard 4f design, we have developed a multi-pass amplifier with a beam propagation insensitive to thermal lens effects and misalignments. The beam propagation is equivalent to multiple roundtrips in an optically stable resonator. To support the propagation we used an array of 2 x 8 individually adjustable plane mirrors. Astigmatism has been minimized by a compact mirror placement. Precise alignment of the kinematic array was realized using our own mirror mount design. A small signal gain of 5 for 8 passes at a pump power of 400 W was reached. The laser was running for more than 3 months without the need of realignment. Pointing stability studies is also reported here.
Ultrafast thin-disk multipass amplifier with 1.4 kW average power and 4.7 mJ pulse energy at 1030 nm converted to 820 W and 2.7 mJ at 515 nm
Jan-Philipp Negel, André Löscher, Andreas Voss, et al.
In recent years, there has been a growing interest in increasing the output power of ultrafast lasers to the kW-range. This allows higher productivity for laser material processing, e.g. for cutting of carbon-fiber reinforced plastics (CFRP) or for micro-machining. We developed an Yb:YAG thin-disk multipass amplifier delivering sub-8 ps pulses with 1.4 kW average power which is – to the best of our knowledge – the highest output power reported for a sub-100 ps ultrafast laser system so far. The amplifier is seeded by a regenerative amplifier with 6.5 ps pulses and 115 W of average power at a repetition rate of 300 kHz. Taking this repetition rate into account, the energy of the amplified pulses is as high as 4.7 mJ. This was achieved using a scheme with 40 mirrors in an array to geometrically fold the seed beam 40 times over the thin-disk. The beam quality was measured to be better than M2=1.4. This system was used in first experiments to cut CFRP with very good quality and with unprecedented efficiency. Additionally, the output beam of the amplifier was frequency-doubled in an LBO crystal to 820 W (70 % conversion efficiency) output power at the second harmonic wavelength (515 nm) and 106 W (26.5 % conversion efficiency) at the third harmonic wavelength (343 nm). Both results are record output powers for ultrafast laser systems at the respective wavelengths. In the presentation, we will show concepts on further power scaling of the system.
First demonstration of passively mode-locked Yb:CaF2 thin-disk laser
Benjamin Dannecker, Xavier Délen, Katrin S. Wentsch, et al.
The need for ultra-short (sub-ps) pulsed laser systems with high power and high energy has advanced the mode-locked Ytterbium-doped thin-disk technology in the last decade. Therefore several research groups have made efforts to explore new laser crystals e.g. Yb:SSO, Yb:CAlGO or Yb:Lu2O3 for the generation of sub-500 fs pulses in thin-disk oscillators. Another promising and known candidate for ultra-short pulsed lasers is Yb:CaF2, which has been so far only used in bulk laser architecture. In this work, we present the first demonstration of a mode-locked Yb:CaF2 laser in thin-disk configuration. The resonator cavity was designed for eight passes through the disk per roundtrip at a repetition rate of 35 MHz. A saturable absorber mirror (SESAM) was used to obtain the soliton mode-locking. We achieved close-to transform-limited pulses with a pulse duration of less than 445 fs and an emission spectral width of 2.6 nm at FWHM (i.e. time-bandwidth product of 0.323). At the average output power of 6.6 W this corresponds to a peak-power of 430 kW and pulse energy of 190 nJ. To the best of our knowledge, this is the highest average output power and pulse energy using Yb:CaF2 as gain material reported to date. Taking into account the dispersion, self-phase modulation, pulse energy, output coupling ratio and laser gain, the pulse-duration estimated from the soliton-equation and our numerical calculations of pulse-propagation is in good agreement with the pulse-duration obtained in the experiment. Higher powers and shorter pulse-durations with this material are the subject of our future investigations.
Latest advances in high brightness disk lasers
Vincent Kuhn, Tina Gottwald, Christian Stolzenburg, et al.
In the last decade diode pumped solid state lasers have become an important tool for many industrial materials processing applications. They combine ease of operation with efficiency, robustness and low cost. This paper will give insight in latest progress in disk laser technology ranging from kW-class CW-Lasers over frequency converted lasers to ultra-short pulsed lasers.

The disk laser enables high beam quality at high average power and at high peak power at the same time. The power from a single disk was scaled from 1 kW around the year 2000 up to more than 10 kW nowadays. Recently was demonstrated more than 4 kW of average power from a single disk close to fundamental mode beam quality (M²=1.38). Coupling of multiple disks in a common resonator results in even higher power. As an example we show 20 kW extracted from two disks of a common resonator.

The disk also reduces optical nonlinearities making it ideally suited for short and ultrashort pulsed lasers. In a joint project between TRUMPF and IFSW Stuttgart more than 1.3 kW of average power at ps pulse duration and exceptionally good beam quality was recently demonstrated.

The extremely low saturated gain makes the disk laser ideal for internal frequency conversion. We show >1 kW average power and >6 kW peak power in multi ms pulsed regime from an internally frequency doubled disk laser emitting at 515 nm (green). Also external frequency conversion can be done efficiently with ns pulses. >500 W of average UV power was demonstrated.
Novel Concepts I
Double-beam, mode-controlling diode side-pumped Nd:YLF laser with near 60% efficiency
The double-beam, mode-controlling technique (DBMC) is a compact side-pumped laser design, very advantageous for systems based on active media with intermediate absorption cross section, such as Nd:YLiF4 (Nd:YLF). Recently, a record optical efficiency of 53.6% and 63.5% slope efficiency has been achieved for a Nd:YLF laser emitting at 1053 nm with diffraction limited beam quality. In this work we review our results using the DBMC design and present the latest achievements exploiting new ways to push the limit of this technique to higher pump powers. By narrowing down the laser emission bandwidth of the pump diode bar using a volume Bragg gratings (VBG) we increased the effective absorption cross section in the Nd:YLF crystal, improving the spatial overlap between pump and laser beam. With this setup the laser delivers 68 W peak fundamental mode output power at 115 W of QCW absorbed peak power, resulting near 60% of optical-to-optical efficiency which is, to the best of our knowledge, the highest efficiency ever reported for a Nd:YLF laser, considering even longitudinal pump schemes. The results reported here highlight the remarkable advantages of the side pumped DBMC laser scheme versus longitudinal pumped laser set-ups, showing that the efficiency of the side-pumped DBMC laser is not suppressed by its spatially weaker overlap between pump and laser mode and preserving single mode laser operation, even at high pump powers.
LED side-pumped Nd3+:YVO4 laser at room temperature
Adrien Barbet, Hugo Grardel, Amandine Paul, et al.
The lighting market has recently improved LED performance by orders of magnitudes. In parallel, massive production decreases dramatically LED price. Those improvements triggered new interests for LED pumping of lasers which was first studied in the early 1980s on neodymium doped and ytterbium doped lasers at low temperature. Since the 2000’s, several research teams started to revisit the concept of LED pumped lasers: polymer laser, fiber laser and semiconductors have recently demonstrated laser effect under visible LED pumping. However, no experimental results were reported on LED pumped bulk crystals. In this paper, we demonstrated for the first time a LED pumped Nd:YVO4 laser operating at room temperature. We investigated two pumping wavelengths: in the amber around 600 nm and in the near infrared around 850 nm. The laser operated in quasi-cw-pumping regime to increase the LED intensity. We performed a two-mirror cavity transversely pumped by 36 LEDs. Laser operation was achieved at room temperature for the both pump wavelengths: a maximum output energy of 40μJ for an emitted energy of 7.4 mJ with infrared pumping and an energy of 11.7 μJ for an emitted energy or 2.3 mJ with amber pumping.

This works demonstrated that LED pumping has an interesting potential to realize ultra low cost solid state lasers operating in pulsed regime at kHz repetition rate and with energies in the mJ range.
Novel Concepts II
Fully vectorial laser resonator modeling by vector extrapolation methods
The optimization of multi-parameter resonators requires flexible simulation techniques beyond the scalar approximation. Therefore we generalize the scalar Fox and Li algorithm for the transversal eigenmode calculation to a fully vectorial model. This modified eigenvalue problem is solved by two polynomial-type vector extrapolation methods, namely the minimal polynomial extrapolation and the reduced rank extrapolation. Compared to other eigenvalue solvers these techniques can also be applied to resonators including nonlinear components. As an example we show the calculation of an azimuthally polarized eigenmode emitted by a resonator containing a discontinuous phase element and a nonlinear active medium. The simulation is verified by experiments.
High power tube solid-state laser with zigzag propagation of pump and laser beam
Michael Savich
A novel resonator and pumping design with zigzag propagation of pumping and laser beams permits to design an improved tube Solid State Laser (SSL), solving the problem of short absorption path to produce a high power laser beam (100 - 1000kW). The novel design provides an amplifier module and laser oscillator. The tube-shaped SSL includes a gain element fiber-optically coupled to a pumping source. The fiber optic coupling facilitates light entry at compound Brewster’s angle of incidence into the laser gain element and uses internal reflection to follow a “zigzag” path in a generally spiral direction along the length of the tube. Optics are arranged for zigzag propagation of the laser beam, while the cryogenic cooling system is traditional. The novel method of lasing uses advantages of cylindrical geometry to reach the high volume of gain medium with compactness and structural rigidity, attain high pump density and uniformity, and reach a low threshold without excessive increase of the temperature of the crystal. The design minimizes thermal lensing and stress effects, and provides high gain amplification, high power extraction from lasing medium, high pumping and lasing efficiency and a high beam quality.
In-phase synchronization of array laser using intra-Talbot-cavity second harmonic generation
Kenichi Hirosawa, Fumio Shohda, Takayuki Yanagisawa, et al.
Talbot cavity is passive method to synchronize the phase of array lasers. Because the Talbot cavity does not need any electrical feedback systems, we believe that Talbot cavity is the most suitable technique to combine a considerable number of laser array into a compact system. A well-known drawback of the Talbot cavity is that it can produce out-phased array and their far-field image has 2-peak profile. To solve this drawback, we developed a frequency doubled laser array based on intra-Talbot-cavity second harmonic generation. Basic concept is second harmonic generation of the out-phased array generated from the Talbot cavity. Because the second harmonic wave is generated proportionally to the square of the fundamental wave, out-phase flips to in-phase. Our Talbot cavity is composed of a pumping 808-nm laser diode array with 15 emitters, an Nd:YVO4 planar waveguide, a PPLN planar waveguide, an f =10 cylindrical lens, and an output coupler (high reflection for 1064 nm and high transition to 532 nm). The pump laser beams are directly launched into the Nd:YVO4. The fundamental wave (1064 nm) oscillates between the Nd:YVO4 and the output coupler and generates second harmonic wave (532 nm) at the PPLN placed next to the Nd:YVO4. The round-trip optical path of the cavity length is set to 1/2 Talbot length so that Talbot cavity forms for the fundamental wave. As a result, we obtained 1-peak far-field image of second harmonic wave from the intra-Talbot-cavity second harmonic generation.
Laser Materials and Characterization
Schlieren imaging of bulk scattering in transparent ceramics
Saurabh Sharma, J. Keith Miller, Ramesh K. Shori, et al.
Bulk scattering in polycrystalline laser materials (PLM), due to non-uniform refractive index across the bulk, is regarded as the primary loss mechanism leading to degradation of laser performance with higher threshold and lower output power. The need for characterization techniques towards identifying bulk scatter and assessing the quality. Assessment of optical quality and the identification of bulk scatter have been by simple visual inspection of thin samples of PLMs, thus making the measurements highly subjective and inaccurate. A modified white light Schlieren Imaging Setup utilizing variable focusing capability is demonstrated. The white light Schlieren Imaging Setup makes it possible to image the spatial variations in the refractive index in the PLMs regardless of dimensions, which are the cause of bulk scattering loss in a transparent material over the entire cross-sectional area of the sample. The high sensitivity of white light Schlieren provides the ability of directly imaging the local spatial variations in refractive index across the entire sample dimension and compare different samples.
Energy transfer upconversion measurements for popular neodymium-doped crystals
Sung Jin Yoon, RenPeng Yan, Stephen J. Beecher, et al.
We report our investigations on measuring the energy transfer upconversion (ETU) parameter in various neodymiumdoped laser crystals (YAG, YVO4, GdVO4, KGW, and YLF) via the z-scan technique. Starting with a simple two-level macro-parameter spatially dependent rate equation model we obtain a good correlation for Nd:YAG at different concentrations and crystal temperatures, however the other crystals illustrate significant deviation between simulation and measurement. Currently we attribute this difference to additional ion-ion interactions in the respective samples, for which a more detailed model is currently being considered. Of the tested materials Nd:YAG appears to have the lowest ETU macro parameter, at around 0.35 x10-16 cm3/s for a 0.6 at.% doping concentration, compared with nominally thrice this for 0.5 at% Nd:YLF and almost an order of magnitude higher for the 0.5 at.% vanadates (YVO4 and GdVO4). These values are significant for determining additional heat load in the respective gain media, especially when trying to increase the output power/energy from lasers employing these crystals, typically achieved by increasing the pump and cavity mode size.
Ultrafast Lasers
First experimental results towards a 100 W wavelength tunable femtosecond OPCPA
M. J. Prandolini, H. Höppner, A. Hage, et al.
Optical parametric chirped-pulse amplification (OPCPA) is the most promising method for providing compact, wavelength tunable, high power, femtosecond lasers. We have recently achieved a 112 W OPCPA with wavelength tunability around 800 nm and 30 fs pulse duration in burst mode (100 kHz in a 800 µs burst at 10 Hz). In this work, we discuss the various laser architectures and the critical parameters in achieving similar laser parameters but in continuous operation.
High-energy multiwatt femtosecond diode-pumped Yb:CaAlGdO4 and Yb:CaF2 regenerative amplifiers
E. Caracciolo, M. Kemnitzer, A. Guandalini, et al.
We study and compare the performance of Yb:CaAlGdO4- and Yb:CaF2-based regenerative amplifiers at low (5 to 10 kHz) and high (up to 500 kHz) repetition rates. Both materials allow for pulse energies of <1 mJ with sub-400-fs at low repetition rates and up to 9.4 W of average output power at 500 kHz.

Thanks to the good thermal properties of Yb:CaF2 and Yb:CALGO, the extracted energy has the potential to be significantly increased with further pump power scaling. Shorter pulses are also potentially achievable by optimizing the design of stretcher and compressor in order to better compensate higher-order dispersion and reduce nonlinear effects.

These laser sources are extremely interesting for industrial applications where high pulse energies at relative high repetition rates allow to considerably reduce the manufacturing throughput time.
Single grating mirror intracavity stretcher design for chirped pulse regenerative amplification
E. Caracciolo, M. Kemnitzer, M. Rumpel, et al.
We demonstrated for the first time, to the best of our knowledge a new intracavity pulse stretching design, employing a single grating-mirror based on a leaky-mode grating-waveguide design. The extremely compact and flexible layout allows for femtosecond pulses to be easily stretched up to nanosecond durations. The stretcher was implemented in a diode-pumped Yb:CALGO regenerative amplifier followed by a standard transmission grating compressor. We demonstrated sub-200 fs long pulses (stretched pulses ≈ 110 ps) with a maximum energy of 205 μJ at 20 kHz repetition rate. As a proof of the robustness and potential energy scaling of leaky-mode grating-waveguide intracavity stretcher, energies up to 700 μJ and 400 ps long pulses before compression at a lower repetition rate of 10 kHz, have been achieved. A simple model is proposed to investigate the cavity behavior in presence of induced spatial chirp.
High average power picosecond laser for selective material processing at 1342 nm wavelength
We demonstrate results of design and optimization of high average output power picosecond laser operating at 1342 nm wavelength for selective material processing. This laser is comprised of mode locked master oscillator, regenerative amplifier and output pulse control module. Passively mode locked by means of semiconductor saturable absorber and pumped with 808 nm wavelength Nd:YVO4 master oscillator emits pulses of ~ 13 ps duration at repetition rate of 55 MHz with average output power of ~ 140 mW. The four-pass confocal delay line with image relay forms a longest part of the oscillator cavity in order to suppress thermo-mechanical misalignment. Optimization of the intracavity pulse fluence ensures significant lifetime improvement for the saturable absorber. This oscillator was used as the seeder for regenerative amplifier based on composite diffusion-bonded Nd:YVO4 rod pumped with 880 nm wavelength. When operating at 300 kHz repetition rate the laser delivers high quality output beam of M2 ~ 1.1 with average power in excess of 10 W at 1342 nm wavelength.
Pulsed Lasers I
1 mJ single-rod fiber Er:glass laser for rangefinding
Andrey A. Mak, Vadim M. Polyakov, Vladimir V. Vitkin, et al.
We demonstrate a compact Er:glass single-rod fiber laser for rangefinding with 1 mJ energy Q-switched at 1.54μm. Double-pass pumping with 16 W power and 5 ms pulse duration was used. Active medium was enveloped with diffuse reflector. Thus efficient output power operation achieved. Free-running mode output pulse energy was 12 mJ with a slope efficiency of 16%. Transparent glass-ceramics containing Co2+:MgAl2O4 nanocrystals were selected as the optimal passive gate to ensure Q-switching in a temperature range and transverse mode selection. The Q-switch mode had steady operation at 1 Hz repetition rate with thermal effects playing no visible role.
A cryo-cooled high-energy DPSSL system delivering ns-pulses at 10 J and 10 Hz
Klaus Ertel, Saumyabrata Banerjee, Thomas J. Butcher, et al.
Lasers generating multi-J to kJ ns-pulses are required for many types of laser-plasma interactions. Such lasers are either used directly for compressing matter to extreme densities or they serve as pump lasers for short-pulses laser chains based on large-aperture Ti:sapphire or parametric amplifiers. The thus generated high-energy fs-pulses are most useful for laser driven secondary sources of particles (electrons, protons) or photons (from THz to gamma). While proof-of-principle experiments have been carried out with flashlamp-pumped glass lasers, lasers with much higher efficiency and repetition rate are required to make this applications practically viable. We have developed a scalable new laser concept called DiPOLE (diode pumped optical laser for experiments) based on a gas-cooled ceramic Yb:YAG multi-slab architecture operating at cryogenic temperatures. While the viability of this concept has been shown earlier [1], we have now reached our target performance of 10 J pulse energy at 10 Hz repetition rate at an optical-to-optical efficiency of 21%. To the best of our knowledge, these are record values for average power and efficiency for lasers of this type. We have also upgraded the system by adding a fibre-based front-end system with arbitrary pulse shaping capability and by installing an image-relayed multipass system enabling up to eight passes of the main amplifier. We have then used this system to demonstrate frequency doubling with 65 % conversion efficiency and a long-term shot-to-shot stability of 0.5% rms over a total of nearly 2 million shots, achieved in runs extending over 4 to 6 hours.
Pulsed Lasers II
Mode-locking in intracavity frequency doubled Nd:YVO4 laser
Anton V. Kovalev, Vadim M. Polyakov, Andrey A. Mak, et al.
We experimentally study passive mode-locking in Nd:YVO4 laser based on second harmonic generation in KTP crystal. We characterized RF spectra and optical spectra versus pump power, the KTP crystal temperature and position, the output coupler reflectivity, and the intracavity polarizer. We discuss the device performance considering cascaded χ(2) lensing in KTP, frequency doubling nonlinearity, and Kerr lens formed in Nd:YVO4. Implementing an intracavity Lyot filter and cavity length modulation via PZT does not affect mode-locking capability. These results and ultra-low noise mode beat signal open a new perspective for stable RF signal generation by transferring optical reference stability (iodine absorption lines) into RF domain.
Current status of Kumgang laser system
Hong Jin Kong, Sangwoo Park, HeeKyung Ahn, et al.
In KAIST, Kumgang laser is being developed for demonstration of the kW level coherent beam combination using stimulated Brillouin scattering phase conjugation mirrors. It will combine 4 modules of DPSSL rod amplifier which produces 1 kW output power. It is composed of the single frequency front-end, pre-amplifier module, and main amplifier. The output powers of the pre-amp and main amplifier module are 200 W (20 mJ @ 10 kHz / 10 ns) and 1.07kW (107 mJ @ 10 kHz / 10 ns), respectively.
50W CW output power and 12mJ pulses from a quasi-2-level Yb:YAG ceramic rod laser end-pumped at the 969nm zero-phonon line
Christian Fries, Marco Weitz, Christian Theobald, et al.
With the advent of high power and narrow bandwidth 969 nm pump diodes, direct pumping into the upper laser level of Yb:YAG and hence quasi-2-level lasers became possible. Pumping directly into the emitting level leads to higher quantum efficiency and reduction of non-radiative decay. Consequently, thermal load, thermal lensing and risk of fracture are reduced significantly. Moreover pump saturation and thermal population of uninvolved energy-levels in ground and excited states are benefical for a homogenous distribution of the pump beam as well as the reduction of reabsorption loss compared to 3-level systems, which allows for high-power DPSS lasers. Beside continuous-wave (cw) operation, nanosecond pulses with a repetition rate between 1 and 5 kHz are an attractive alternative to flashlamp-pumped systems (10-100 Hz) in various measurement applications that require higher data acquisition rates because of new faster detectors. Based on measurements of the absorption and a detailed numerical model for pump beam distribution, including beam propagation and saturation factors, power-scaling of a ceramic rod Yb:YAG oscillator was possible. Finally a cw output power of 50 W with 33 % pump efficiency at 1030 nm has been demonstrated (M2 < 1.2). Nanosecond pulses have been produced by cavity-dumping of this system. The cavity-dumped setup allowed for 3-10 ns pulses with a pulse energy of 12.5 mJ at 1 kHz (M2 < 1.1). In order to achieve these results a systematic experimental and numerical investigation on gain dynamics and the identification of different stable operating regimes has been carried out.
Investigation of mechanically Q-switched lasers
Brian Cole, Lew Goldberg, Nathaniel Hough, et al.
Using a resonant scanner mirror Q-switch to provide a time varying change in cavity alignment, 1535nm lasers based on Er/Yb-doped glass and 1064nm lasers based on Nd:YAG were evaluated. Using a side pumping architecture, the Er/Yb glass laser used a compact mechanical Q-switch with a mirror rotation rate of 330 Rad/s, enabling generation of <3 mJ pulses with a pulse width of 16ns. The laser output was a diffraction limited TEM00 mode. A mechanical Q-switch based on a MEMS tilting mirror was also used; its performance in a laser cavity was found to be similar to the resonant mirror. The technique of mechanical Q-switching was also extended to a side pumped Nd:YAG laser (mirror sweep rate of 1300 Rad/s), enabling generation of Q-switched pulses of <30mJ and 25ns duration. The far-field divergence showed this laser to be highly multi-moded within the pump plane, with a measured beam-product-parameter greater than 30 mm-mRad.
Poster Session
Numerical simulations of the optical gain of crystalline fiber doped by rare earth and transition ion
A. K. Daoui, B. Boubir, A. Adouane, et al.
A fiber laser is a laser whose gain medium is a doped fiber, although lasers whose cavity is made wholly of fibers have also been called fiber lasers. The gain media in a fiber laser is usually fiber doped with rare-earth ions, such as erbium (Er), neodymium (Nd), ytterbium (Yb), thulium (Tm), or praseodymium (Pr), which is doped into the core of the optical fiber, similar to those used to transmit telecommunications signals. Fiber lasers find many applications in materials processing, including cutting, welding, drilling, and marking metal. To maximize their market penetration, it is necessary to increase their output power. In this work, we present a detailed study based on the numerical simulation using MATLAB, of one of the principal characteristics of a fiber laser doped with rare earth ions and transition ion. The gain depends on several parameters such as the length of the doped fiber, the density, the pump power, noise, etc.). The used program resolves the state equations in this context together with those governing the light propagation phenomena. The developed code can also be used to study the dynamic operating modes of a doped fiber laser.
Temperature influence on diode pumped Er:CaF2 laser
The goal of this work was an investigation of the temperature influence (in range from 80 up to 330 K) on the laser properties of Er:CaF2 ceramics, which is suitable as a gain medium for generation of radiation at 2.7 μm. The tested Er:CaF2 ceramics sample, prepared using a hot-forming technique, was doped with 5.5% of ErF3. The sample was in the form of plane-parallel face-polished 5.8mm thick plate (without AR-coatings). It was mounted in a temperature controlled cupreous holder, placed inside the vacuum chamber of the liquid nitrogen cryostat. A fiber coupled laser diode, operating in pulsed regime (3 ms pulse length, 20 Hz repetition rate) at wavelength 968 nm, was used for Er:CaF2 sample pumping. The 145mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (HR @ 2.65 − 2.95 μm, HT @ 0.97 μm) and a curved (r = 150mm) output coupler with a reflectivity of ∼ 97% @ 2.65 − 2.85 μm. From the results it follows that the temperature of the active medium has a strong influence mainly on laser threshold (more than 8 times lower threshold power corresponded to the temperature 80K in respect to 330 K). The highest slope efficiency (2.3% in respect to absorbed power), obtained for the temperature 80 K, was more than twice higher than the slope efficiency for 330 K.
Wavelength tunability of laser based on Yb-doped YGAG ceramics
The wavelength tunability of diode pumped laser based on Yb-doped mixed garnet Y3Ga2Al3O12 (Yb:YGAG) ceramics was investigated. The tested Yb:YGAG sample (10% Yb/Y) was in the form of 2mm thick plane-parallel face-polished plate (without AR coatings). A fiber (core diameter 100 μm, NA= 0.22) coupled laser diode (LIMO, LIMO35-F100-DL980-FG-E) with emission at wavelength 969 nm, was used for longitudinal Yb:YGAG pumping. The laser diode was operating in the pulsed regime (2 ms pulse length, 10 Hz repetition rate). The duty-cycle 2% ensured a low thermal load even under the maximum diode pumping power amplitude 20W (ceramics sample was only air-cooled). The 145mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (HR @ 1.01 − 1.09 μm, HT @ 0.97 μm) and curved (r = 150mm) output coupler with a reflectivity of ∼ 97% @ 1.01 − 1.09 μm. Wavelength tuning of the ytterbium laser was accomplished by using a birefringent filter (single 1.5mm thick quartz plate) placed inside the optical resonator at the Brewster angle between the output coupler and the laser active medium. The laser was continuously tunable over ∼ 58nm (from 1022nm to 1080 nm) and the tuning band was mostly limited by the free spectral range of used birefringent filter. The maximum output power amplitude 3W was obtained at wavelength 1046nm for absorbed pump power amplitude 10.6W. The laser slope efficiency was 34%.
On the efficiency of Tm-doped 2µm lasers
K. van Dalfsen, S. Aravazhi, C. Grivas, et al.
A potassium double tungstate layer with the composition KY0.40Gd0.29Lu0.23Tm0.08(WO4)2 was grown onto a pure KY(WO4)2 substrate by liquid-phase epitaxy, microstructured by standard lithography and Ar-ion etching, and overgrown by a pure KY(WO4)2 layer. The end-facets were polished. Laser experiments were performed on these buried, ridge-type channel waveguides in a resonator with one butt-coupled mirror and Fresnel reflection from the other end-facet, resulting in a high output-coupling degree of 89%, compared to intrinsic round-trip losses of only 2%. By pumping with a Ti:Sapphire laser at 794 nm, 1.6 W of output power at 1.84 μm with a maximum slope efficiency of ~80% was obtained. To the best of our knowledge, this result represents the most efficient 2-μm channel waveguide laser to date. We determined the optimum Tm3+ concentration in double tungstate channel waveguides to be at least 8at.% for efficient lasing. The theoretical limit of the slope efficiency depends on the Stokes efficiency which here is 43.2%, the outcoupling efficiency which here is 99%, and the pump quantum efficiency. The pump quantum efficiency of a 2-μm Tm3+ laser pumped around 800 nm hinges on the efficiency of its cross-relaxation process. By fitting the macroscopic cross-relaxation parameter which linearly depends on the Tm3+ concentration to concentration-dependent luminescence- decay data, calculating the overall decay rate of the pump level, and deriving the concentration-dependent pump quantum efficiency, we obtain a theoretical limit for the slope efficiency of 83% for the chosen Tm3+ concentration. The experimental slope efficiency of ~80% closely approaches this limit.
Fe:ZnSe and Fe:ZnMgSe lasers pumped by Er:YSGG radiation
H. Jelínková, M. E. Doroshenko, M. Jelínek, et al.
The aim of the presented work was to design and characterize bulk Fe:ZnSe and Fe:Zn(1-x)Mg(x)Se (Mg content x = 0.19) lasers coherently pumped by electro-optically Q-switched Er:YSGG laser. This laser generated pumping radiation at 2.79 μm with the maximum energy of 50 mJ in 80 ns long pulse with the repetition-rate of 1 Hz. The 25 mm long optical resonator of Fe:ZnSe or Fe:ZnMgSe lasers was formed by a plan dichroic pumping mirror and a concave output coupler (r = 200 mm) with reflectivity 88 % @ 4-5 μm. Both lasers were operated at room temperature. Measured maximum output energy/slope efficiency in respect to the absorbed energy was ~ 3.8 mJ/42 % for the Fe:ZnSe laser and ~ 0.48 mJ/10 % for the Fe:ZnMgSe laser. The generated output pulse duration was 150 – 200 ns in both cases and the output beam spatial profile was approximately gaussian. The Fe:ZnSe and Fe:ZnMgSe lasers output spectra line-width was ~ 200 nm (FWHM) and their maxima were centered at 4.45 μm and 4.8 μm, respectively. The results were compared to pumping the same crystals by a Q-switched Er:YAG laser in similar conditions.
Moderate high power 1 to 20µs and kHz Ho:YAG thin disk laser pulses for laser lithotripsy
An acousto-optically or self-oscillation pulsed thin disk Ho:YAG laser system at 2.1 μm with an average power in the 10 W range will be presented for laser lithotripsy. In the case of cw operation the thin disk Ho:YAG is either pumped with InP diode stacks or with a thulium fiber laser which leads to a laser output power of 20 W at an optical-to-optical efficiency of 30%. For the gain switched mode of operation a modulated Tm-fiber laser is used to produce self-oscillation pulses. A favored pulse lengths for uric acid stone ablation is known to be at a few μs pulse duration which can be delivered by the thin disk laser technology. In the state of the art laser lithotripter, stone material is typically ablated with 250 to 750 μs pulses at 5 to 10 Hz and with pulse energies up to a few Joule. The ablation mechanism is performed in this case by vaporization into stone dust and fragmentation. With the thin disk laser technology, 1 to 20 μs-laser pulses with a repetition rate of a few kHz and with pulse energies in the mJ-range are available. The ablation mechanism is in this case due to a local heating of the stone material with a decomposition of the crystalline structure into calcium carbonate powder which can be handled by the human body. As a joint process to this thermal effect, imploding water vapor bubbles between the fiber end and the stone material produce sporadic shock waves which help clear out the stone dust and biological material.
Influence of temperature on spectroscopic and lasing properties of Pr:YLF crystal
We report on influence of temperature on spectroscopic and lasing properties of Pr:YLF crystal. Fluorescence lifetime, polarization-resolved absorption and emission spectra, and laser characteristics in the orange spectral range are described. Using 1-W InGaN pump laser-diode, 350 mW of continuous-wave output power at 604 nm wavelength at 80 K crystal temperature is reported. The corresponding slope efficiency related to absorbed pump power was 46 %.
60W Ho:YLF oscillator-amplifier system
Wayne Koen, Cobus Jacobs, Lorinda Wu, et al.
We developed a compact Ho:YLF oscillator–amplifier system end-pumped by two 54 W unpolarised Tm:fibre lasers, and produced 60.2 W of output power at 2064 nm. The oscillator consisted of a flat input coupler mirror, a 50 mm long 0.5 % doped Ho:YLF crystal rod, a 45 degree folding mirror, an AOM, and a concave output coupler mirror. The oscillator operated vertically polarised on the holmium crystal’s σ–polarisation, ensuring good beam quality from the weak thermal lens. The concave output coupler had a radius of 300 mm and a reflectivity of 82 % at 2064 nm. The oscillator gave a maximum output of 24 W with an M2 of 1.06. The single-pass amplifier consisted of two 40 mm long, 0.5 % doped, Ho:YLF crystal rods and four folding mirrors. While the seed laser was pumped by a single fibre laser, the amplifier utilized the transmitted pump light from the seed laser in addition to the second fibre laser. With the first crystal amplifying on the σ–polarisation and the second crystal on the π-polarisation, the amplifier delivered 60.2 W with an M2 of 1.09, representing a gain of 2.5 while achieving an optical-to-optical efficiency of 55.5 %. When Q-switched with the AOM, the system delivered pulse lengths of between 43 and 113 ns at repetition rates from 15 to 40 kHz.
Development of a closed-loop cryogenically cooled sub-picosecond regenerative amplifier
P. Sikocinski, T. Miura, Venkatesan Jambunathan, et al.
We are developing joule level picosecond pulse duration laser system operating at repetition rate of 120 Hz. The laser system consists of a seed mode-locked fiber laser and two cryogenically cooled amplifier stages: a single slab regenerative amplifier and a single slab multi-pass amplifier. We have found the displacement of the probe beam caused by mechanical vibrations of the cryostat is less than 0.7 mrad. We have also improved our in-situ gain measurement system by decreasing the spectral line of the narrow band source down to 50 pm. We have found that the emission bandwidth of Yb:YGAG at 160 K is around 7 nm.
Continuous-wave hybrid index-antiguided and thermal-guided planar waveguide laser with large mode area
Yuanye Liu, Lee Casperson, Tsing-Hua Her
We present lasing experiments in index-antiguided (IAG) planar waveguides made of Nd:YAG (refractive index = 1.820) sandwiched by Terbium Gallium Garnet (refractive index = 1.954). For a core width of 220 μm, we observe fundamental mode oscillation with maximal 2.62 W output power and 0.109 slope efficiency. An analytical model is developed taking into account simultaneous index-antiguiding, gain guiding and thermal induced refractive focusing. For a core width of 400 μm, fundamental mode oscillation is only observed near the lasing threshold, suggesting that the maximally core width in IAG waveguides for fundamental mode operation depends on the strength of index antiguiding.
Underwater laser detection system
The conventional method used to detect an underwater target is by sending and receiving some form of acoustic energy. But the acoustic systems have limitations in the range resolution and accuracy; while, the potential benefits of a laserbased underwater target detection include high directionality, high response, and high range accuracy. Lasers operating in the blue-green region of the light spectrum(420 : 570nm)have a several applications in the area of detection and ranging of submersible targets due to minimum attenuation through water ( less than 0.1 m-1) and maximum laser reflection from estimated target (like mines or submarines) to provide a long range of detection. In this paper laser attenuation in water was measured experimentally by new simple method by using high resolution spectrometer. The laser echoes from different targets (metal, plastic, wood, and rubber) were detected using high resolution CCD camera; the position of detection camera was optimized to provide a high reflection laser from target and low backscattering noise from the water medium, digital image processing techniques were applied to detect and discriminate the echoes from the metal target and subtract the echoes from other objects. Extraction the image of target from the scattering noise is done by background subtraction and edge detection techniques. As a conclusion, we present a high response laser imaging system to detect and discriminate small size, like-mine underwater targets.
Transmitted beam profile for determining bulk scattering in transparent ceramics
Saurabh Sharma, J. Keith Miller, Ramesh K. Shori, et al.
Bulk scattering in polycrystalline laser materials (PLM), due to non-uniform refractive index distribution across the bulk, is regarded as the primary loss mechanism leading to degradation of laser performance with higher threshold and lower output power. There is a need for characterization techniques, towards identifying bulk scatter and assessing the quality. Assessment of optical quality and the identification of bulk scatter have been by simple visual inspection of thin samples of PLMs, thus making the measurements highly subjective and inaccurate. Transmitted Beam Wavefront Profiling (TBWP) allows for the direct and quick imaging of the distortions introduced by bulk scattering, which is a direct manifestation of the presence of refractive index inhomogeneities in the PLM sample. As a laser beam propagates through the PLM sample, different regions of the incident beam experience different refractive index profiles, which cause spatial distortions to the beam. TBWP is able to directly and quickly image these distortions introduced to a propagating laser beam caused by the presence of bulk scattering in the PLMs.
Angle resolved scatter measurement of bulk scattering in transparent ceramics
Saurabh Sharma, J. Keith Miller, Ramesh K. Shori, et al.
Bulk scattering in polycrystalline laser materials (PLM), due to non-uniform refractive index across the bulk, is regarded as the primary loss mechanism leading to degradation of laser performance with higher threshold and lower output power. The need for characterization techniques towards identifying bulk scatter and assessing the quality. Assessment of optical quality and the identification of bulk scatter have been by simple visual inspection of thin samples of PLMs, thus making the measurements highly subjective and inaccurate. Angle Resolved Scatter (ARS) measurement allows for the spatial mapping of scattered light at all possible angles about a sample, mapping the intensity for both forward scatter and back-scatter regions. The cumulative scattered light intensity, in the forward scatter direction, away from the specular beam is used for the comparison of bulk scattering between samples. This technique employ the detection of scattered light at all angles away from the specular beam directions and represented as a 2-D polar map. The high sensitivity of the ARS technique allows us to compare bulk scattering in different PLM samples which otherwise had similar transmitted beam wavefront distortions.
Spectroscopic characterization of Cr2+ ions in ZnSe/ZnS crystals under visible excitation
Jeremy M. Peppers, Vladimir V. Fedorov, Sergey B. Mirov
Detailed spectroscopic characterization of Cr:ZnSe (Cr:ZnS) crystals under visible excitation into the charge transfer bands of Cr2+ impurities was performed. Middle infrared photoluminescence of Cr:ZnS under this excitation exhibits shorter rise time (~150ns) than that previously observed in Cr:ZnSe (~4-10 μs). As a result the quantum yield of Cr:ZnS mid-IR photoluminescence under 10ns pulsed 416nm excitation into the charge transfer band was estimated at close to 100%, which contrasts with low (~14%) quantum yield measured in Cr:ZnSe under 532nm pulsed excitation, indicating the possibility of efficient excitation of the upper laser level of Cr:ZnS using this mechanism. The rise time can be caused by cascade relaxation from higher lying levels through the 3T1 metastable level, producing luminescence in the near-IR. Measurements of the temperature dependence of the middle- and near infrared photoluminescence signals are reported. These values indicated that more efficient pumping of Cr:ZnSe under 532nm excitation can be achieved at temperatures greater than 300 K. Results of high temperature laser experiments supporting this idea are presented.
Direct measurement of up-conversion processes in diode pumped erbium-doped YAG
Brad Chun-Ting Liu, Ramesh K. Shori, Oscar M. Stafsudd
Observation of the time dependence of the up-conversion fluorescence allows the determination of the relative importance of the excited state absorption and energy transfer up-conversion in 0.5% and 50% erbium-doped YAG. Pump radiation up to 10 kW/cm2 shows drastic changes in the effective life time of nearly all excited energy states.