This PDF file contains the front matter associated with SPIE Proceedings Volume 9731, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Degenerate optical parametric oscillators (OPOs) divide optical frequencies by two with only modest pumping, making them promising mid-infrared frequency comb sources. Our measurements show that our degenerate OPO preserves the frequency comb stability of the pump to sub-Hz levels. However, degenerate OPOs are often overlooked due to their interferometric cavity stabilization requirements. We find that the stability requirements of our system are actually much simpler, because thermal feedback results in self-stabilization. When the OPO is oscillating, absorption of the intracavity field increases the crystal temperature, and subsequently the effective cavity length, which is fortunately the right direction to stabilize degenerate oscillation in our system. Our OPO is based on an orientation-patterned GaAs crystal, pumped by a stabilized 2 W, 418 MHz, optically-referenced Tm frequency comb, generating a broadband, midinfrared frequency comb centered at 4 μm. We have observed continuous OPO oscillation for almost an hour without cavity length feedback. These measurements show that a degenerate OPO can serve as a simple device to downconvert a frequency comb.

We study coherence properties of a more-than-octave-wide (2.6-7.5 μm) mid-IR frequency comb based on a 2-μm Tmfiber- laser-pumped degenerate (subharmonic) optical parametric oscillator (OPO) that uses orientation-patterned gallium arsenide (OP-GaAs) as gain element. By varying intracavity dispersion, we observed a 'phase' transition from a singlecomb state (at exactly OPO degeneracy) to a two-comb state (near-degenerate operation), characterized by two spectrally overlapping combs (signal and idler) with distinct carrier-envelope offset frequencies. We achieve this by generating a supercontinuum (SC) from the mode-locked Tm laser that spans most of the near-IR range, and observing RF beats between the SC and parasitic sum-frequency light (pump + OPO) that also falls into the near-IR. We found RF linewidth to be <15 Hz (a resolution of our spectrum analyzer), which proves that coherence of the pump laser comb is preserved to a high degree in a subharmonic OPO. Transition to a two-comb state was characterized by a symmetric splitting of the RF peak. Low pump threshold (down to 7 mW), high (73 mW) average power and high (up to 90%) pump depletion make this comb source very attractive for numerous applications including trace molecular detection and chemical sensing with massively parallel spectral data acquisition.

Generation of wideband optical frequency combs requires precise balance between nonlinear photon interaction and parasitic effects. While near-octave combs can be generated in both silica and silicon waveguides, it is not always possible to suppress the noise across the operational bandwidth. Principles and challenges of noiseinhibited, tunable frequency comb generation in cavity-free parametric mixers are described and discussed.

We demonstrate high power, deep ultraviolet (DUV) conversion to 266 nm through frequency quadrupling of a nanosecond pulse width 1064 nm fiber master oscillator power amplifier (MOPA). The MOPA system uses an Yb-doped double-clad polarization-maintaining large mode area tapered fiber as the final gain stage to generate 0.5-mJ, 10 W, 1.7- ns single mode pulses at a repetition rate of 20 kHz with measured spectral bandwidth of 10.6 GHz (40 pm), and beam qualities of M_x ²=1.07 and M_y ²=1.03, respectively. Using LBO and BBO crystals for the second-harmonic generation (SHG) and fourth-harmonic generation (FHG), we have achieved 375 μJ (7.5 W) and 92.5 μJ (1.85 W) at wavelengths of 532 nm and 266 nm, respectively. To the best of our knowledge these are the highest narrowband infrared, green and UV pulse energies obtained to date from a fully spliced fiber amplifier. We also demonstrate high efficiency SHG and FHG with walk-off compensated (WOC) crystal pairs and tightly focused pump beam. An SHG efficiency of 75%, FHG efficiency of 47%, and an overall efficiency of 35% from 1064 nm to 266 nm are obtained.

In this work three different concepts for micro integrated laser sources emitting light at 560 nm are investigated. The modules have different near infrared diode laser sources and different geometries of the crystals for second harmonic generation. The power emitted by the modules varies from 112mW achieved with a simple module with a ridge-waveguide laser and a ridge-waveguide crystal to 548mW coming from a module using a laser with subsequent amplifier and a planar-waveguide crystal. The article features a detailed description of the near infrared sources and the used crystals as well as the discussion of the complete modules.

Deep ultraviolet (DUV) lasers emitting below 300 nm are of great interest for many applications, for instance in medical diagnostics or for detecting biological agents. Established DUV lasers, e.g. gas lasers or frequency quadrupled solid-state lasers, are relatively bulky and have high power consumptions. A compact and reliable laser diode based system emitting in the DUV could help to address applications in environments where a portable and robust light source with low power consumption is needed. In this work, a compact DUV laser system based on single-pass frequency doubling of highpower GaN diode laser emission is presented. A commercially available high-power GaN laser diode from OSRAM Opto Semiconductors serves as a pump source. The laser diode is spectrally stabilized in an external cavity diode laser (ECDL) setup in Littrow configuration. The ECDL system reaches a maximum optical output power of 700 mW, maintaining narrowband emission below 60 pm (FWHM) at 445 nm over the entire operating range. By direct single pass frequency doubling in a BBO crystal with a length of 7.5 mm a maximum DUV output power of 16 μW at a wavelength of 222.5 nm is generated. The presented concept enables compact and efficient diode laser based light sources emitting in the DUV spectral range that are potentially suitable for in situ applications where a small footprint and low power consumption is essential.

Cr²⁺ doped ZnS and ZnSe possess a unique blend of physical, spectroscopic, and technological parameters. These laser materials feature ultra-broadband gain in 1.9 – 3.3 μm mid-IR range, low saturation intensities, and large pump absorption coefficients. The II-VI semiconductor hosts provide a low phonon cut-off, broad IR transparency, and high second and third order nonlinearity. Cr:ZnS and Cr:ZnSe are available in polycrystalline form: the material consists of a multitude of microscopic single-crystal grains with a broad distribution of grain sizes and orientations, which results in random quasi-phase-matching (RQPM). The distinctive features of RQPM are a linear dependence of the conversion yield with length of the medium and ultra-wide bandwidth of three-wave mixing. We review resent experimental results on optically pumped mid-IR ultrafast lasers based on polycrystalline Cr:ZnS and Cr:ZnSe. We demonstrate that Kerrlens mode-locking of polycrystalline Cr:ZnS and Cr:ZnSe lasers allow for generation of few-cycle mid-IR pulses with MW-level peak power. This opens several avenues for efficient nonlinear frequency conversion of short optical pulses directly in the laser gain medium via RQPM process. We implemented Kerr-lens mode-locked Cr:ZnS oscillators, which feature high power (up to 0.25 W), spectrally broad (up to 22 THz) second harmonic generation (SHG) in the laser medium. We also demonstrate simple and robust ultrafast source based on single-pass continuously pumped polycrystalline Cr:ZnS laser amplifier: mid-IR pulses with 6.8 W average power and the spectrum spanning 2.0–2.6 μm as well as SHG pulses with 0.52 W average power and 1.05 – 1.25 μm spectral span were obtained.

Gallium nitride’s (GaN) material properties of broadband transparency, high thermal conductivity, and wide-band gap make it a promising candidate for high-power frequency conversion devices. The strong internal polarization of GaN leads to large second-order nonlinearity, but conventional phase matching is prevented due to weak birefringence. To obtain efficient nonlinear optic frequency conversion, patterned inversion growth has been developed to induce quasiphase matching (QPM). We have fabricated and tested periodically oriented GaN (PO-GaN) devices to obtain QPM frequency conversion. This report discusses our recent measurements of second harmonic generation resonances for these devices.

We present measurements of the temporal and polarization dependence of the nonlinear optical (NLO) response of selected organic solvents using our beam deflection (BD) method. These measurements allow us to separately determine the bound-electronic and nuclear responses which then determines the NLO response function. With this NLO response function the outcome of other experiments such as Z-scan as a function of pulse-width can be predicted. By performing similar measurements on the gas phase of these solvents we can compare the hyper-polarizabilities in the two phases.

Frequency conversion in orientation-patterned quasi-phase matched materials is a leading approach for generating tunable mid- and long-wave coherent IR radiation for a wide variety of applications. A number of nonlinear optical materials are currently under intensive investigation. Due to their unique properties, chiefly wide IR transparency and high nonlinear susceptibility, GaAs and GaP are among the most promising. Compared to GaAs, GaP has the advantage of having higher thermal conductivity and significantly lower 2PA in the convenient pumping range of 1– 1.7 μm. HVPE growth of OPGaP, however, has encountered certain challenges: low quality and high price of commercially available GaP wafers; and strong parasitic nucleation during HVPE growth that reduces growth rate and aggravates layer quality, often leading to pattern overgrowth. Lessons learned from growing OPGaAs were not entirely helpful, leaving us to alternative solutions for both homoepitaxial growth and template preparation. We report repeatable one-step HVPE growth of up to 400 μm thick OPGaP with excellent domain fidelity deposited for first time on OPGaAs templates. The templates were prepared by wafer fusion bonding or MBE assisted polarity inversion technique. A close to equilibrium growth at such a large lattice mismatch (-3.6%) is itself noteworthy, especially when previously reported attempts (growth of OPZnSe on OPGaAs templates) at much smaller mismatch (+0.3%) have produced limited results. Combining the advantages of the two most promising materials, GaAs and GaP, is a solution that will accelerate the development of high power, tunable laser sources for the mid- and long-wave IR, and THz region.

Several types of infrared sensors are based on sensitive focal plane arrays. In such sensors, the intensity will typically increase by a factor ~10⁷ at the focal plane, compared to the intensity of the incoming radiation. Such arrays are thus vulnerable when illuminated with high-intensity laser pulses. One solution for protecting the array against such pulses is to use an optical limiter. We here present results where carbon disulfide (CS₂) has been tested as an optical limiting material against high-energy laser pulses at 2.05 μm wavelength.

We demonstrate a tunable and robust femtosecond supercontinuum source with a maximum output power of 550 mW and a maximum spectral width of up to 2.0 μm which can cover the mid-infrared region from 2.3 μm up to 4.9 μm by tuning the pump wavelength. As light source we use a synchronously pumped fiber-feedback OPO and a subsequent OPA which delivers femtosecond, Watt level idler pulses tunable between 2.5 μm and 4.1 μm. These pulses are launched into As₂S₃ chalcogenide step-index fibers with core diameters of 7 and 9 μm. The spectral behavior of the supercontinuum is investigated by changing the pump wavelength, core diameter, fiber length, and pump power. Self-phase modulation is identified as the main broadening mechanism in the normal dispersion regime. This source promises to be an excellent laboratory tool for infrared spectroscopy owing to its high brilliance as demonstrated for the CS₂- absorption bands around 3.5 μm.

The use of supercontinuum light sources in different optical measurement methods, like microscopy or optical coherence tomography, has increased significantly compared to classical wideband light sources. The development of various optical measurement techniques benefits from the high brightness and bandwidth, as well as the spatial coherence of these sources. For some applications, only a portion of the broad spectral range can be used. Therefore, an increase of the spectral power density in limited spectral regions would provide a clear advantage over spectral filtering. This study describes a method to increase the spectral power density of supercontinuum sources by amplifying the excitation wavelength inside a nonlinear photonic crystal fiber (PCF). An ytterbium doped photonic crystal fiber was manufactured by a sol-gel process and used in a fiber amplifier setup as the nonlinear fiber medium. In order to characterize the fiber’s optimum operational characteristics, group-velocity dispersion (GVD) measurements were performed on the fiber during the amplification process. For this purpose, a notch-pass mirror was used to launch the radiation of a stabilized laser diode at 976 nm into the fiber sample for pumping. The performance of the fiber was compared with a conventional PCF. Finally, the system as a whole was characterized in reference to common solid state-laser-based photonic supercontinuum light sources. An improvement of the power density up to 7.2 times was observed between 1100 nm to 1380 nm wavelengths.

We demonstrate all-normal dispersion supercontinuum generation in the 1080 nm-1600 nm range by propagating subnanosecond pulses in a high numerical aperture standard optical fiber. The extreme saturation of the Raman gain provides a flat spectrum in the considered range, making this broadband source particularly suitable for coherent Raman spectroscopy. This unusual regime of supercontinuum generation (Raman gain saturation regime) is investigated through an experimental spectrotemporal study. The possibility of operating spectrometer-free time-coded coherent Raman spectroscopy is introduced.

Sandia National Laboratories is pursuing a variation of Magneto-Inertial Fusion called Magnetized Liner Inertial Fusion, or MagLIF. The MagLIF approach requires magnetization of the deuterium fuel, which is accomplished by an initial external B-Field and laser-driven pre-heat. While magnetization is crucial to the concept, it is challenging to couple sufficient energy to the fuel, since laser-plasma instabilities exist, and a compromise between laser spot size, laser entrance window thickness, and fuel density must be found. Nonlinear processes in laser plasma interaction, or laser-plasma instabilities (LPI), complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation and beam-spray. Key LPI processes are determined, and mitigation methods are discussed. Results with and without improvement measures are presented.

We demonstrate a linearly-polarized high efficiency random Raman lasing of the 1st-order Stokes wave and cascaded generation in 0.5- and 1-km-long PM fiber, respectively, under polarized pumping. Quantum efficiency of converting input pump radiation (1.05μm) into the 1st (1.11μm), 2nd (1.17μm) and 3rd-order (1.23μm) Stokes waves is about 80% in the cascaded generation, regardless of the order, and amounts to 92% for the 1st-order Stokes wave in the 0.5-km PM fiber. Polarization extinction ratio is >22 dB for all the waves at output powers of up to 10 W. An analytical model describing adequately the generated power for all components of the cascaded random Raman fiber laser has been developed. The laser bandwidth increases with Stokes order, amounting to ~1, ~2 and ~3 nm for the consecutive orders, respectively.

An external cavity BaWO₄ Raman source pumped by a Q-switched Ho:YAG laser is demonstrated. Watt-level average output power is generated at the first Stokes wavelength of 2602 nm. Output pulse width as short as 8.5 ns was measured at a repetition rate of 5 kHz. Near-diffraction limited beam quality is observed (M²≈1.2). This simplified Raman laser configuration can harness the high average power levels offered by Thulium- and Holmium-doped solid-state and fiber lasers to generate fixed-wavelength and tunable output at 2.3-2.8 μm interval.

Broadband infrared spectroscopy employing optical parametric oscillation in bow-tie cavities, including a periodically- poled lithium niobate (PPLN) crystal, is well known. We demonstrate, however, that such spectroscopy is also possible using 2-mm-size monolithic whispering gallery resonators (WGRs). This is achieved in a radially-poled WGR by controlling wavelength tuning despite triple resonance of pump, signal, and idler light. Simulated and measured tuning characteristics of the Type-0 OPOs, pumped at about 1 μm wavelength, coincide. Tuning branches, which are crossed or curved at degeneracy, are present over a spectral range of up to 0.9 µm. As a proof-of-principle experiment, we show that all spectroscopic features of ethanol can be resolved using the idler light between 2.2 and 2.55 μm.

Coherent sources that are broadly and continuously tunable in the mid- and longwave infrared are of interest for a variety of scientific, commercial, and military applications. The advantages in an OPO of quasi-phasematched materials like orientation-patterned gallium arsenide (OPGaAs) come at the cost of the angle tuning possible in birefringent nonlinear crystals. Temperature tuning is limited by the material’s dn/dT value, and lacks speed and stability. A better alternative is to tune the OPO by tuning the pump laser. Here we report an OPGaAs OPO pumped by a gain-switched Cr:ZnSe laser which was continuously tuned by an intracavity etalon. The etalon also narrowed the output linewidth to around 4 nm. The Cr:ZnSe laser operated at a repetition rate of 500 Hz with a 45 ns pulsewidth. The pump was focused to a spot size (1/e²) of 100 μm at the center of a simple linear resonator formed by two 5-cm ROC mirrors. The OPGaAs crystal was 14 mm long, with a period of 97 μm, and was mounted with no active cooling. Tuning the pump laser over a range of 90 nm (2385-2475 nm) produced OPO output over a range of almost 4.5 μm (3500-7450 nm). OPO tuning was ultimately limited by coatings on the crystals and the resonator mirrors, as the Cr:ZnSe laser is capable of much broader tuning as a pump source. A maximum slope efficiency of 21% was obtained, with a pulse energy threshold of 84 μJ.

We report here on the performance of a narrow-line, mid-IR source based on a PPLN-crystal optical parametric generator (OPG). The crystal was pumped by a pulsed, 20-MHz-rate, 1064-nm Yb:fiber-based source operating with 20- psec pulses. The OPG produced a broad spectrum between 2027 nm and 2239 nm. By placing a band-pass filter after the OPG we were able to select a 30-nm bandwidth output, and we achieved further line reduction (0.7 nm) and 4.5 mW of average power at 2039 nm, using a reflective Volume Bragg Grating (VBG). Devices such as piezo-controlled etalons can provide rapidly tunable, narrow-linewidth power from this system.

The mid-IR wavelength range is highly relevant for a number of applications related to gas spectroscopy and spectral analysis of complex molecules such as those including CH bounds. The main obstacles for exploitation of mid-IR applications include suitable and affordable mid-IR light sources for excitation of the sample and sensitive mid-IR detectors. With the advent of mid-IR Quantum cascaded lasers and super continuum light sources new possibilities has emerged. However, low-noise, mid-IR (2-15 μm) detection is still challenging requiring cryogenic cooling to gain sensitivities needed for measurements of fluorescence or absorptions signals. Mid-IR upconversion imaging and detection using nonlinear crystals offers good promise as an alternative, sensitive mid-IR imaging and detection technology. In this paper the fundamental properties of upconversion is discussed.

Anderson localization, also known as strong localization, is the absence of diffusion in turbid media resulting from wave interference. The effect was originally predicted for electron motion, and is widely known to exist in systems of less than 3 dimensions. However, Anderson localization of optical photons in 3 dimensional systems remains an elusive and controversial topic. Random Raman lasing offers the unique combination of large gain and virtually zero absorption. The lack of absorption makes long path length, localized modes preferred. The presence of gain offsets what little absorption is present, and preferentially amplifies localized modes due to their large Q factors compared with typical low Q modes present in complex media. Random Raman lasers exhibit several experimentally measured properties that diverge from classical, particle-like, diffusion. First, the temporal width of the emission being 1 to a few nanoseconds in duration when it is pumped with a 50 ps laser is a full order of magnitude longer than is predicted by Monte Carlo simulations. Second, the random Raman laser emission is highly multi-mode, consisting of hundreds of simultaneous lasing modes. This is in contrast to early theoretical results and back of the envelope arguments that both suggest that only a few modes should be present. We will present the evidence that suggests a divergence from classical diffusion theory. One likely explanation, that is consistent with all of these anomalies, is the presence of high-Q localized modes consistent with Anderson localization.

This paper presents the tunable Stokes laser characteristics based on the stimulated polariton scattering in KTiOAsO₄ crystal. With a given pumping laser wavelength of 1064.2 nm, discontinuously tunable first-Stokes wave was obtained from 1077.9 to 1079.0 nm, from 1080.1 to 1080.8 nm, from 1082.8 to 1083.6 nm, from 1085.5 to 1085.8 nm, from 1086.8 to 1088.4 nm. With a pumping pulse energy of 125.0 mJ, the maximum first-Stokes laser pulse energy at 1078.6 nm was 24.7 mJ. The second-Stokes wave at 1093.4 nm was obtained and investigated as well.

In this work we show the changes of polarization at different wavelengths in the end of a photonic crystal fiber (PCF) by means bandpass filters in a supercontinuum light source. A linear and circular polarization was introduced in a piece of PCF, showing the changes of the polarization for each wavelength of each one of the filters from 450 to 700nm. We used a microchip laser as pumping source with wavelength of 532nm and short pulses of 650ps with repetition rate of 5kHz. We obtained a continuous spectrum in the visible spectral region, showing a comparison of the polarization state at the fiber input with respect to polarization state in the fiber output for different wavelengths by rotating the axes of the PCF.

Raman spectroscopy in the visible spectral range is of great interest due to resonant Raman effects. Nevertheless, fluorescence and ambient light can mask the weak Raman lines. Shifted excitation Raman difference spectroscopy is a demonstrated tool to overcome this drawback. To apply this method, a light source with two alternating wavelengths is necessary. The spectral distance between these two wavelengths has to be adapted to the width of the Raman signal. According to the sample under investigation the width of the Raman signal could be in the range of 3 cm^-1 – 12 cm^-1. In this work, a micro-integrated light source emitting at 488 nm with a continuous wavelength tuning range up to 2 nm (83 cm^-1) is presented.

The pump source, a DFB laser emitting at 976 nm, and a periodically poled lithium niobate (PPLN) ridge waveguide crystal is used for the second harmonic generation (SHG). Both components are mounted on a μ-Peltier-element for temperature control. Here, a common wavelength tuning of the pump wavelength and the acceptance bandwidth of the SHG crystal via temperature is achieved.

With the results the light source is suitable for portable Raman and SERDS experiments with a flexible spectral distance between both excitation wavelengths for SERDS with respect to the sample under investigation.

This paper reports on the phase-matching properties of GaS_0.4Se_0.6 for type-2 difference-frequency generation (DFG) between a Nd:YAG laser and a Nd:YAG laser-pumped β-BaB₂O₄ optical parametric oscillator (BBO/OPO) in the 100.4- 1030.6μm (0.291-2.988THz) range together with the refined Sellmeier equations for GaS_xSe_1-x (x=0, 0.29, 0.40, and 1.0) that provide a good reproduction of the present experimental results as well as second-harmonic generation (SHG) to sixth-harmonic generation (6HG) of a CO₂ laser at 10.5910μm and a Ti:Al₂O₃ laser-pumped THz generation in the 1~1.8THz range.

Principally new features of the non-collinear two-phonon light scattering governed by elastic waves of finite amplitude in birefringent bulk crystals are detected and observed. The main goals of our investigations are to reveal novel important details inherent in the nonlinearity of this effect and to study properties of similar parametric nonlinearity both theoretically and experimentally in wide-aperture crystals with moderate linear acoustic attenuation. An additional degree of freedom represented by the dispersive birefringence factor, which can be distinguished within this nonlinear phenomenon, is characterized. This physical degree of freedom gives us a one-of-a-kind opportunity to apply the strongly non-linear two-phonon light scattering in practice for the first time. The local unit-level maxima in the distribution of light scattered into the second order appear periodically as the acoustic power density grows. It makes possible to identify a few transfer function profiles peculiar to these maxima in the isolated planes of angular-frequency mismatches. These maxima give us an opportunity to choose the desirable profile for the transfer function at the fixed angle of incidence for the incoming light beam with a wide spectrum .The needed theoretical analysis is developed and proof-of-principle experiments, performed with a specially designed wide-aperture acousto-optical cell made of the calomel (α-Hg₂Cl₂) crystal, are presented. The obtained spectral resolution ~0.235 Å at 405 nm (i.e. the resolving power ~17,200) can be compared with the most advanced acousto-optical spectrometers for space/airborne operations. Evidently, our results with the calomel-based acousto-optical cell look like the best we can mention at the moment.