This PDF file contains the front matter associated with SPIE Proceedings Volume 7197, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing

We describe new sources of tunable, high-power radiation in the blue and ultraviolet. Continuous-wave (cw), singlefrequency blue radiation tunable across 425-489 nm and femtosecond ultraviolet (UV) radiation tunable across 250-355 nm is generated by intracavity frequency-doubling of resonant signal radiation in cw and ultrafast optical parametric oscillators (OPOs) in singly-resonant oscillator (SRO) configuration. The cw SRO, pumped in the green, uses a 30-mm MgO:sPPLT as the nonlinear material and a 5-mm BiB₃O₆ (BIBO) crystal for internal doubling. Using this approach, we generate 1.27 W of cw blue power with a linewidth of 8.5 MHz and a TEM₀₀ profile. The device also generate a singlefrequency signal output of ~100 mW across 850-978 nm and up to 2.6 W of idler power in the 1167-1422 nm spectral range. The femtosecond SRO, based on a 400-μm BIBO crystal and pumped at 415 nm in the blue, can provide visible femtosecond signal pulses across 500-710 nm. Using a 500-μm crystal of β-BaB₂O₄ internal to the SRO cavity, efficient frequency doubling of the signal pulses into the UV is achieved, providing tunable femtosecond pulses across 250-355 nm with up to 225 mW of average power at 76 MHz. Cross-correlation measurements result in UV pulses with durations down to 132 fs for 180 fs blue pump pulses.

High-efficiency (5%-10% wall-plug efficiency) high-power continuous-wave (CW) visible lasers have been developed for large-format-display applications (e.g., planetariums, visualization centers, etc.). Using an approach pioneered by Evans & Sutherland (E&S), a fiber based master-oscillator-power-amplifier (MOPA) architecture is employed to generate high power near-infrared (NIR) tunable lasers that are then converted to visible wavelengths in external enhancement nonlinear ring cavities. Depending on the wavelength generated, either second-harmonic generation or sum-frequency mixing (or both) in lithium triborate (LBO) are utilized to convert 1064 nm and/or 1550 nm to visible wavelengths, with NIR-to-visible optical-conversion efficiencies of 65%-95% routinely obtained. The resulting visible lasers are single-axial-frequency (FWHM bandwidth < 200 kHz) spatially pure (m² < 1.05) Gaussian beams, and are used as light sources in ultrahigh-resolution projectors manufactured by E&S. The current systems reliably produce 6 W of visible laser power at 448 nm, 532 nm, and 631 nm, with short-term CW operation yielding up to 18 W visible-laser output per color. Laser-induced damage (LID) on nonlinear-crystal facets is the primary limitation to long-term operation at visible powers > 6 W, and efforts are underway to increase crystal LID thresholds to allow reliable operation at greater power levels.

A 70-Watt green laser with M²<1.4 has been demonstrated. This green laser consists of an all-fiber-based IR pump laser at 1064 nm and a frequency-conversion module in a compact and flexible configuration. The IR laser produces up to 150 Watts in a polarized diffraction-limited output beam with high spectral brightness for frequency conversion. The IR laser is operating under QCW mode, e.g. 10 MHz with 3~5 ns pulse width or 700 MHz with 50 ps pulse width, to generate sufficient peak power for frequency doubling in the converter module. The IR laser and conversion module are connected via a 5-mm stainless-steel protected delivery fiber for optical beam delivery and an electrical cable harness for electrical power delivery and system control. Both the IR laser and converter module are run through embedded software that controls laser operations such as warm up and shut down. System overview and full characterization results will be presented. Such a high power green laser with near diffraction-limited output in a compact configuration will enable various scientific as well as industrial applications.

Advances in the periodic poling process have led to longer and larger PPLN crystals. Today it is also possible to prepare PPLN samples with a thickness of about 5 mm which allows the use of pump laser beams with a larger aperture, and so with a higher energy. Moreover thicker samples give the possibility to consider quasi-phase-matching (QPM) at any angle with respect to the grating periodicity. We called this scheme angular quasi-phase-matching (AQPM). In order to illustrate the potentiality of AQPM, we compared its tunability and spectral acceptance with that of BPM in the case of second harmonic generation (SHG) and difference frequency generation (DFG) in the periodically poled negative uniaxial 5%MgO:PPLN crystal, with a grating periodicity Λ = 32.2 μm We found that AQPM exhibits complementary spectral range and acceptances compared with BPM. We experimentally performed the first validation of the theory of AQPM by cutting 5%MgO:PPLN as a polished sphere with a diameter of 3.9 mm in order to propagate beams in any direction of the crystal by keeping normal incidence. It allowed us to measure any SHG and DFG AQPM direction, with the associated efficiencies, the spectral and angular acceptances. They are reported with calculations.

The combination of high brightness laser diodes and periodically poled (PP) waveguide crystals for the generation of blue light at the technically interesting wavelength of 488 nm is promising. Although PPKTP has a lower nonlinear coefficient compared to PPLN it is of interest for the use in such devices. Because of its higher photorefractive damage threshold, it is well suited for operation at room temperature. In this work, a DFB laser as well as a tunable external cavity enhanced broad area diode laser (ECDL) are used for second harmonic generation using a waveguide PPKTP crystal. Both lasers yield several hundred Miliwatts of diffraction limited light around a center wavelength of 976 nm with excellent spectral properties. The ECDL system is further tunable over a broad range of 40 nm. The PPKTP crystal has a length of 12 mm and the 4 μm x 8 μm waveguides are manufactured by ion exchange followed by a patented submount poling technique. By using a DFB laser diode as pump source a laser to waveguide coupling efficiency of more than 55% could be achieved. A maximum output power of 66.7 mW could be generated out of 220 mW infrared light inside the waveguide channel at room temperature. This results in a conversion efficiency of more than 260%/W.

Quasi phasematched domain periods for bulk and waveguide GaAs nonlinear frequency conversion were calculated. Waveguide beam confinement resulted in a decrease in effective refractive index and the magnitude of this index change was found to increase supralinearly with wavelength. Domain periodicity required for a 2.056-μm pumped, 10x10 μm² GaAs core, Al_0.5Ga_0.5As cladding waveguide OPO at degeneracy decreased from 60.4 μm for bulk GaAs to 57.9 μm for a rib waveguide and 49.8 μm for a buried waveguide in a GaAs OPO. Careful accounting for waveguide effects is necessary to fabricate a periodic waveguide structure with the desired function.

The U.S. Air Force is developing orientation-patterned GaAs (OPGaAs) for nonlinear frequency conversion in the 2-5 μm and 8-12 μm regions. We report recent progress in OPO device performance which reflects continued improvement in material quality. Seven new OPGaAs samples, representing four distinct growth regimes, were evaluated in terms of threshold, slope efficiency, and output spectral content using a Q-switched Tm,Ho:YLF laser operating at 2 μm as the pump source. The samples were identical in base template, grating period, length, and AR coating, permitting direct comparison of results to identify favorable growth conditions. As anticipated, performance varied significantly among the sample set, with slope efficiencies from 12% to 35% and thresholds from 9 μJ to 40 μJ. Less anticipated was the low level of uniformity across each sample and between samples grown in the same growth run. Significant variations in slope efficiency and threshold were not uncommon. Still, most of the samples performed manifestly better than previously grown material, indicating an overall improvement in OPGaAs quality. Research continues on understanding growth processes, optical loss mechanisms, and how these translate into device performance.

We demonstrate concepts for compact and cost effective THz technology based on semiconductor diode lasers. In detail, we analyze diode laser based THz sources and detectors. Continuous wave THz radiation is generated by two color diode lasers either with external photomixers or direct difference frequency generation in the diode laser. For time domain THz sampling applications we present a suitable mode-locked diode laser system. Further we present a method to detect THz radiation with diode lasers at room temperature: A THz signal coupled into the active region of a diode laser results in a variation of the voltage across the p-n-junction.

Terahertz-frequency (THz) waves have shown potential for a wide range of applications. We have developed tunable THz-wave sources using nonlinear optical crystals, which have several advantages, including frequency agility, wide tunability, high output, and high coherency. We found that the organic nonlinear crystal of 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) had particular potential for ultra-wide THz-wave generation from sub-THz to mid-infrared frequencies. Using DAST, we manufactured a coherent, tunable source (1-40 THz) with frequency agility. Moreover, we demonstrated THz-wave detection through up-conversion using DAST or MgO:LiNbO₃ nonlinear optical crystals, which provided a fast response, high sensitivity, and room-temperature operation.

Conversion efficiency is boosted by using tandem crystal optical parametric amplifier geometry. We have undertaken comprehensive numerical modeling of the optical systems, where the output beam for the first stage are calculated and then propagated to the second stage. As a concrete example periodically poled lithium niobate crystals are experimentally examined. The phase and amplitude information about the pump and signal beams were retrieved using the Fresnel phase retrieval method and used as input in the theory. This enabled real laboratory conditions to be modeled. Subsequently, the amplitude and phase of the complex pump and signal beam from the crystal was calculated and the results were validated by experiments. Non-collinear interactions in the second stage also yielded an efficient idler output over a broad range of temperatures. ZnGeP₂ crystals were used in the numerical simulations with first and second stage amplification. We predicted an idler energy increase by a more than a factor of 3.

In this paper we present a study of the single pass normalized second harmonic generation (SHG) conversion efficiency as a function of the beam propagation factor M² and the beam diameter in the lateral and vertical direction. It can be shown that an increase in M² results in dramatic changes for the optimal focusing conditions, in comparison to the SHG with a Gaussian beam. Based on the results of the measurements we developed a model to simulate the focusing conditions for partial coherent beams.

A quasi-noncritical phase-matching (QNCPM) technology has been developed employing adhesive-free bonding (AFB) for high efficiency and high beam quality frequency conversion. A 16-layer KTP composite with total length of 32 mm was fabricated for a low pulse energy pumped 2-μm optical parametric oscillator (OPO). Our calculations indicate that the KTP composite has a 16 times lower walk-off effect and 16 times higher angle acceptance compared with the OPO in the same length single KTP crystal. Even only considering the walk-off correction, the threshold pump pulse energy in such a QNCPM OPO can be expected to be reduced by 256 times. In addition, the AFB technique was demonstrated to have uniform bonding quality and immeasurably small interface loss. Therefore, it can be expected to allow engineering of other critical phase-matched nonlinear optical devices into QNCPM devices.

Here we report a high power, pulsed optical parametric oscillator (OPO) at 3.5 μm by using a MgO:PPLN crystal as the gain medium. The OPO itself was pumped by a semiconductor diode-seeded, Yb³⁺-doped fiber Master Oscillator Power Amplifier (MOPA) operating at 1062nm. An OPO output power as high as 11W at an overall slope efficiency of 67% was achieved, with nearly 2.7W and 8.2W of optical power obtained at 3.5μm and 1.5μm respectively. Due to the fast response time of the external modulator, it is possible to implement active pulse shaping on a nanosecond time-scale. Using adaptive pulse shaping of the seed laser (using an external modulator) we demonstrated a reduction in the impact of dynamic gain saturation and optical Kerr/Raman nonlinearities within the fibre MOPA obtaining shaped signal and idler pulses at the OPO output and reduced spectral bandwidths. We have also investigated the dependence of the OPO build-up time and energy transfer efficiency on pump pulse peak power and shape. The build-up time shows an exponential dependence on the pulse peak power and as expected decreases with an increase in pulse peak power. Analyzing the shift in spectral peak at 1.5μm it is possible to estimate the internal temperature of the crystal for various pump powers. Our experiments were pump-power limited and considerable scope remains for further power-scaling of the OPO output using this approach.

Since resonant absorption of light caused by a variety of different molecular bond occurs in the mid-infrared (MIR) wavelength region, many applications using tunable MIR lasers have been reported. However, the applicable fields of the MIR tunable lasers have been restricted by their large size and high cost equipments. Therefore, we are developing a compact tunable MIR laser using an optical parametric oscillator (OPO). To obtain a long term stability and a high conversion efficiency, a diode-pumped and Q-switched Tm,Ho:YAG ceramic laser with a wavelength of 2.1 μm was adopted for the pump source. A maximum output energy of 40 mJ was obtained with the Tm,Ho:YAG ceramic laser at a pulse width of 150 ns and a repetition rate of 10 Hz. An experiment was performed using a singly-resonant OPO with a ZnGeP₂ crystal pumped by another OPO with a wavelength of 2.1 μm. A threshold pump fluence of 0.2 J/cm² and a slope efficiency of 60% were obtained at a signal and idler wavelengths of 3.3 and 5.6 μm, respectively. Using these results and a theoretical model calculation, the maximum output energy of MIR-OPO pumped with the Tm,Ho:YAG ceramic laser was estimated to be about 20 mJ.

A new setup for efficient blue light generation that consists of two passively coupled optical resonators is presented. The first resonator is based on a broad area laser diode (BAL) in a Littrow external cavity with a special off-axis design. This external cavity diode laser provides more than 450 mW diffraction limited and narrow bandwidth emission at 976 nm. A compact cavity design with 40 mm length could be realized. The second resonator is a monolithic high finesse ring cavity containing a 10 mm bulk periodically poled lithium niobate (PPLN) crystal for resonant second harmonic generation. This ring resonator consists of four small mirrors with appropriate reflectivities and two GRIN lenses for stability reasons. All parts of this ring cavity are mounted monolithically on a glass substrate with a size of 19.5 mm x 8.5 mm. First experiments showed good passive matching of both cavities without any active closed-loop control. With this setup efficient SHG was achieved. A maximum optical output power of 70 mW blue light at 488 nm was obtained. The conversion efficiency was better than 15%.

High efficiency parametric fiber devices are used for frequency generation, band-invariant amplification and general signal processing. Present generation of high-confinement fibers used for mixer design posses transverse variation control measured in silica molecular diameters, a critical feature for long-scale phase matching. We introduce new energy delivery method based localized four-photon mixing in order to achieve dispersion mapping of arbitrary dispersion fibers. The technique improves the sensitivity of existing dispersion mapping methods by orders of magnitude and is applicable to arbitrary waveguide type. Implications of the new technique is illustrated on general mixer applications.

The new nonlinear crystal for the mid-IR CdSiP₂ was discovered only very recently but the interest in this chalcopyrite is enormous because it possesses most of the attractive properties of the related ZnGeP₂ but allows in addition pumping at 1064 nm without two-photon absorption and uncritical phase-matching for 6 μm generation with maximized effective nonlinearity. The last feature is due to the fact that this crystal is negative uniaxial in contrast to ZnGeP₂ which shows positive birefringence. We now measured its nonlinear coefficient using SHG of femtosecond pulses generated near 4.6 μm from a seeded KNbO₃ optical parametric amplifier. The SHG efficiency was compared for uncoated samples of CdSiP₂ and ZnGeP₂, both ≈0.5 mm thick, in the low conversion limit (<10% internal conversion efficiency) which justifies the use of the plane wave approximation. Taking into account the experimentally determined phase-matching angles for type-I SHG (oo-e type in CdSiP₂ and ee-o type in ZnGeP₂), which were in good agreement with the existing Sellmeier approximations, we arrived at d₃₆(CdSiP₂)~d₃₆(ZnGeP₂) which is rather unexpected having in mind the larger band-gap of CdSiP₂. The reliability of the measurement was tested at the same wavelength by comparing ZnGeP₂ with HgGa₂S₄ which led to the result d₃₆(ZnGeP₂)~3d₃₆(HgGa₂S₄), in very good agreement with previous estimations.

A new nonlinear optical crystal, CdSiP₂, has recently been developed and a bulk crystal cut for phase-matching of second harmonic generation from 4.8 to 2.4 μm has been tested. Numerical and theoretical results are presented. Accounting for Fresnel losses, internal conversion efficiency was over 55% at a peak incident irradiance of 56 MW/cm².

Phase-matching properties for harmonic generation via second- and third-order nonlinearities have been investigated in LiIO₃ using the Nd:YAG laser and its harmonic pumped parametric oscillator. It was found that this crystal is 90° phase-matchable for direct type-2 third-harmonic generation (THG) at 0.3568 μm at 20 °C. In addition, the temperature acceptance bandwidths have been measured for second-order processes and were used for construction of the thermo-optic dispersion formula. The Sellmeier equations that correctly reproduce almost all of the experimental data in the whole transmission range are also presented.

An overview of recent advances in exact phase matching technologies of second order nonlinear optical processes in compound semiconductors is reported. The technique used utilizes dispersion engineering in Bragg reflection waveguides (BRWs) or 1-dimensoinal photonic bandgap structures to achieve phase matching between the interacting waves. One of its distinguishing features in comparison to other techniques is that it does not involve any demanding technological steps such as oxidation, nor does it rely on periodic modulation of the optical properties of the materials used in the propagation direction. This in turn provides phase matching with significantly lower optical losses in comparison to other techniques. Nonlinear conversion efficiency matching what is achievable in periodically poled lithium niobate is obtained for ridge BRWs fabricated in GaAs/AlGaAs. Most notable applications that would benefit from integrable ultrafast second order optical nonlinearities include monolithically integrated optical parametric oscillators, correlated photon pair sources and monolithic tunable frequency conversion elements.

Periodically poled ferroelectric crystals form highly efficient quasi-phase matched optical frequency conversion devices. For optimal performance of such devices, accurate period and duty-cycle are required throughout the poled region. For the quality evaluation we demonstrate a simple and a powerful technique using far-field diffraction measurement. Periodically poled lithium niobates were fabricated and etched out to reveal a surface-relief grating. The far-field diffraction pattern was analyzed to obtain statistical information for the duty-cycle. We explored the equivalence between the linear diffraction experiment and the conventional second-harmonic generation method for poling quality evaluation, through the Fourier-transform of the spatial modulation of domains.

The photorefractive effect in iron-doped lithium niobate crystals is investigated, using femtosecond pulses and cw light, both at a wavelength of 532 nm, in direct comparison. For that purpose, measurements about "light-induced" or "holographic" scattering of a single beam as well as writing of index gratings with two interfering beams are performed. We find that light-induced scattering is reduced for femtosecond pulses, and even absent for a sufficiently low Fe²⁺ concentration, in comparison and in contrast to cw light. Additional differences include a slower buildup time and a weaker angular selectivity for the scattering of pulses. Our observations can be attributed to the smaller temporal coherence of the pulses. When writing index gratings into as-grown lithium niobate crystals, the saturation value of the refractive index unexpectedly decreases with increasing pulse energy fluence. Furthermore, in oxidized crystals, writing with femtosecond pulses turns out to be much faster than with cw light. A model about the charge excitation, migration and trapping is proposed that explains these differences.

Optical pumping is the efficient way to polarize a nuclear spin of helium-3 (³He). The nuclear spin polarization of 3He has been often demonstrated in a discharge cell under the low B-field through the metastability exchange optical pumping (MEOP) technique for a short time. Since the appearance of a high power near-infrared light source, optical pumping with a circularly polarized light tuned to the resonant frequency of 2³S→2³P transition at 1083nm has been investigated extensively. We, however, are focusing on another optical transition 2³S1→2³P₀ at 389nm that has not been investigated yet for the polarization. Therefore, we developed a single-frequency 389-nm coherent light source which enables to optical-pump metastable 3He atoms via the transition. This light source is based on the second harmonic generation of a single-frequency 778-nm CW Ti:Sapphire laser light with a BiB₆O₃ (BIBO) nonlinear crystal in an external cavity for the enhancement. We have demonstrated very efficient frequency doubling with high conversion efficiency of 56%, which is obtained at the second harmonic generation of 380mW in the cavity and is corresponding to an efficiency of 81%/W, and we examined the optical pumping for the nuclear spin polarization and polarization analysis.

We report proof-of-concept experiments generating 5.5W of rapidly tuneable 589nm laser radiation through SHG of 1178nm radiation generated by optical parametric amplification in Lithium Triborate. The two stage OPA was pumped by a frequency doubled mode-locked Nd:YVO₄ laser (15ps, 1.5MHz, 50W) and seeded by a tuneable diode laser, allowing convenient tuning on and off the Na D2 line. Our findings imply that if scaled up in power and with modified pump pulse duration, this approach can lead to a versatile high-power Na guide star source which can be built from readily available commercial components.

Narrow-band single photons represent an important resource for quantum memories due to their efficient interaction with atomic resonances. In this paper, we report on the generation of photons with 3 MHz linewidth by cavity-enhanced parametric down-conversion and demonstrate direct proof of their single-photon character by detection of heralding idler photons. Compared to a Poissonian source, a suppression of higher-order photon numbers by nearly two orders of magnitude could be achieved. Moreover, the brightness of our source exceeds previous realizations by more than a factor of 100.

We investigate coherence properties of a degenerate synchronously-pumped optical parametric oscillator (SPOPO) as a divide-by-2 subharmonic generator. Type 0 (e-ee) periodically-poled MgO : LiNbO₃ was used as a nonlinear gain crystal and a femtosecond mode-locked Ti:Sapphire laser at 775 nm - as a pump source. We observed that the SPOPO longitudinal modes at degeneracy were phase-coherent with that of the pump. The self-phase-locking and self-stabilization effect can be explained in terms of mutual injection locking between the signal and the idler frequencies of the SPOPO. We confirmed the phase-locking effect by performing interference between pump and frequency-doubled output, as well as beat frequency measurements between the SPOPO output and (i) the pump laser and (ii) an independent continuous-wave (CW) laser. A frequency locking range around SPOPO degeneracy Δf was measured as a function of pump power, when the SPOPO operated in the phase-locking regime. We have found that Δf increased monotonically with the pump power and decreased with the cavity Q, in good accord with our theoretical model based on coupled nonlinear optical wave equations. When the proper regime was chosen, the SPOPO remained phase-locked to the pump without any active stabilization even in the presence of environmental noise. At degeneracy (around 1550 nm), the SPOPO produced 70-fs output pulses with the FWHM spectral width of 210 cm^-1 that is 2.6 times broader than the spectrum of the pump laser.

We present an approach to distributed fiber-optic temperature sensing utilizing a dual-Brillouin-frequency optical fiber. Traditional distributed sensor systems employ a heterodyne detection scheme to measure a temperature-dependent microwave frequency Stokes' shift. Our approach toward realizing an RF, rather than microwave, detection scheme is the development of an optical fiber engineered to have two gain-equalized Brillouin frequencies (dual-Brillouin-frequency fiber, or DBFF). The design goal is that the two acoustic modes respond differently to temperature variations, and thus the detection of their beat signal (in the RF) would provide temperature data. One approach is to structure the core to have two or more dissimilar layers that are 'quasi-independent' such that their resulting Brillouin frequencies have a dissimilar dependence on temperature. Proper tailoring of the overlap integrals with the optical mode results in gain equalization between resulting acoustic modes. A slightly different approach is presented, where two Brillouin frequencies are achieved through core-cladding Brillouin-gain equalization via the reduction of Brillouin gain in the core. Temperature sensing is then accomplished by the direct detection of the RF beat frequency between them (~175MHz). A linear temperature dependence of -1.07 MHz/C was measured for the beat frequency of a tailored fiber.

Optical spatial solitons are of interest at present due their possible application to integrated all optical circuitry where light controls light. This optical circuitry utilises the various novel properties of the optical spatial soliton, such as rewritable waveguides, and phase dependent interactions. Of all the types of optical spatial solitons, photorefractive (PR) solitons are the subject of much research due to their ease of production and stability. They are readily produced in either self-focusing PR media (photovoltaic solitons), or self-defocusing PR media with an applied external bias (screening solitons). The external bias, typically an applied DC filed, is used to manipulate the self-defocusing PR media to act like self-focusing PR media. However, solitons produced in self-focusing PR media run the risk of over focusing causing permanent damage to the PR media, while applying an external bias to the PR media requires many additional components, increasing the complexity of the system. Recently, we outlined for the first time a theoretical model of soliton-like low divergence fields in unbiased self-defocusing PR media. Numerical analysis of these soliton-like fields showed stability over distances well in excess of both the confocal distance of the beam, and the physical size of the PR media. The present research examines the existence of the low-divergence soliton-like fields and the fundamental nature of the interactions of these low-divergence soliton-like fields in unbiased PR self-defocusing media. Here we show that low-divergence soliton-like fields can be produced in unbiased self-defocusing photorefractive media, and that when the two of these soliton-like fields interact within the PR media, they are forced away from each other.

Formation and relaxation dynamics of electron polarons in lithium niobate crystals is investigated by measuring transient absorption induced by blue femtosecond pulses. Anisotropy in absorption change distinguishes between small free polarons and small bound polarons, revealing that the dynamics is influenced by MgO-doping and stoichiometry control. In crystals doped with MgO at concentrations above threshold, small free polarons are generated within 100 fs and decay at tens of nanosecond. In the presence of antisite defects, sequential formation of polaronic states occurs: electrons initially trapped as small free polarons become trapped as small bound polarons at picosecond time scale. The results are relevant for nonlinear optical applications of pulsed or high-power lasers.

A temperature induced variable chromaticity phosphor based upon a rare-earth multi-doped solid-state frequency upconverter is presented. The phosphors are composed of ytterbium-sensitized multiple doped(Tm, Er, Ho) lead-cadmium fluorogermanate glass samples excited by a laser source around 1064 nm. The temperature induced color variation exploits the heat enhanced effective absorption cross-section of the ytterbium sensitizer under multiphonon-assisted anti-Stokes excitation. The temperature enhancement of the energy-transfer mechanism between the sensitizer and the appropriate active light emitter ion allows the selective intensity control of the RGB emission wavelengths due to different upconversion excitation routes. The suitable combination of rare-earth active ions yielded the generation of variable chromaticity light with CIE-1931 coordinates changing from CIE-X=0.283;Y=0.288 at 20°C to CIE-X= 0.349;Y=0.412 at 190 °C, and CIE-X=0.285;Y=0.361 at 25°C to CIE-X=0.367;Y=0.434 at 180°C in Yb³⁺/Tm³⁺/Ho³⁺ and Yb³⁺/Tm³⁺/Er³⁺ multidoped samples, respectively. The viability of producing a low cost solid-state changeable visible color remote distributed temperature indicator in the 25°C - 300°C range is also discussed.

We investigated numerically the transmission of a set of solitons through a nonlinear optical loop mirror (NOLM) and found that under some conditions a single soliton can be extracted. We analyzed the evolution of relatively long (20-50 ps) and no very strong (10 W) pulses. The results show that the input pulse duration and the amplification of the solitons resulting from pulse breakup play important role to extract the highest quality pulse. The transmitted pulses correspond to fundamental solitons with subpicosecond durations. We believe that the proposed method can be useful for the study of nonlinear phenomena in optical fibers.

Herein we report our experimental results on nonlinear optical properties of (H2)₂SnPc (I), Sn(OH)₂Pc (II), and Sn(Cl)₂Pc (III) studied using Z-scan technique with 800 nm, 100 fsec pulses, and 633 nm continuous wave (cw) laser excitation. Femtosecond open-aperture Z-scan data revealed these molecules exhibited strong 3PA coefficient (α₃). The estimated values of α₃ were ~4.0×10^-5, ~2.0×10^-5 cm³/GW², and ~1.5×10^-5 cm³/GW² for I, II, and III respectively obtained after deducting the solvent contribution. Closed aperture data recorded with femtosecond pulses revealed positive nonlinearity for all the molecules. We also observed large nonlinear response in the cw regime at 633 nm. Closed aperture scans performed with 633 nm indicated strong negative nonlinearity while open aperture scans depicted mixed response. The performance of these alkyl phthalocyanines in various time domains vis-à-vis recently reported phthalocyanines is discussed in detail.

We investigated the output efficiency of signal wave as a function of reflectance of output coupler in nanosecond optical parametric oscillators. The measurements are performed for a type-I critically phase matched signal-resonant optical parametric oscillator of Beta-Barium-Borate crystal. It is observed that the threshold fluence and slope efficiency increases as the reflectance of output coupler decreases, resulting that the maximum efficiency of the signal output was achieved with ~ 60% reflectance of output coupler. It is found that the experimental result of output characteristics is in good agreement with the numerical prediction by a simple numerical model.

Coherent spectral expansion of the mid-infrared femtosecond pulses is beneficial for monitoring and controlling molecular vibrational dynamics. We investigate the spectral broadening of mid-infrared pulses due to nonlinear optical effects in semiconductor materials. The mid-infrared pulses of 100 fs duration and 180 cm^-1 bandwidth at the center wavelength of about 5 micron are focused onto the semiconductor materials. With only few-micro-joule pulse energy, the spectral broadening by a factor of more than 3 is observed for Si, Ge, and GaAs. The output spectral component extends from 1500 cm^-1 to 3000 cm^-1. The intensity and the phase profiles of the self-phase modulated pulses are characterized by the modified auto-interferometric autocorrelation method and its phase-retrieval algorithm, indicating the spectral phase to be compensated for pulse compression.

The present paper analyzes different methods for dynamic control of parameters of super-continuum generated in fibres both under pulse and CW pumping as well as peculiarities of temporal structure of different super-continua. In particular we show experimentally and numerically a sensitivity of supercontinuum spectral power density to wavelength and repetition rate of pump pulses. We find also that chirp variation of pump pulses results in change of super-continuum coherence for short-wavelength wing. A novel method for control of SC generation under CW pumping is proposed. We discuss the method for control of repetition rate and duration of pulses generated with the help of dual-wavelength pumping by means of adjusting input power and frequency difference. Developed super-continuum generators with enhanced set of controlled parameters are essential for cytometry, tomography, spectroscopy, communications and for other applications.

This paper reports studies of the performance of a phase-conjugate mirror (PCM) with its operation based on the stimulated Brillouin scattering (SBS) of the focused laser beam in ultra-pure fluorocarbon FC-75 (C₈F₁₈). A pulsed laser with 15-ns pulse width from a home-made injection seeded single frequency MOPA configuration Nd:YAG laser with variable output energy (from several mJ to 300 mJ) was used as a coherent light source in these experiments. The PCM reflectivity better than 90% has been achieved at optimized focusing conditions of an incoming beam, and the output energy of the phase conjugate pulse linearly followed the input pulse energy after reaching the threshold level of about 3.3 mJ. The slope efficiency was estimated to be approximately 95% without taking into account of losses in some optical components, including the thin film polarizer. We believe that a higher level of PCM efficiency can be achieved with more careful selection of the critical optical components, including the thin film polarizer, quarter-wave plate and focusing lens.

Experiments were carried out in order to investigate Second Harmonic Generation (SHG) in germanium-doped optical glass fibers, relying on a mode-locked Q-switched neodymium yttrium aluminum garnet (Nd:YAG) laser with high peak power intensity. The preparation or writing conditions for optical fibers were investigated. SHG was observed successfully under a synchronous pump of the fundamental infrared and seeding green lights, with a maximum of 82 μW from second harmonic generation being observed in the optical fiber from a 60 mW input, corresponding to a conversion efficiency of 0.14%. The pump laser used in the experiments was a Nd:YAG laser, operated Q-switched and mode-locked at 1064 nm. The laser delivered approximately 100 ps mode-locked pulses at a 75.6 MHz repetition rate, modulated by a 2 kHz frequency Q-switched pulse envelope of 260 nanoseconds' duration (Full Width Half Maximum or FWHM).

High pulse energy continuum generation in conventional multimode optical fibers has been studied for potential applications to a holographic optical coherence imaging system. As a new imaging modality for the biological tissue imaging, high-resolution holographic optical coherence imaging requires a broadband light source with a high brightness, a relatively low spatial coherence and a high stability. A broadband femtosecond laser can not be used as the light source of holographic imaging system since the laser creates a lot of speckle patterns. By coupling high peak power femtosecond laser pulses into a multimode optical fiber, nonlinear optical effects cause a continuum generation that can be served as a super-bright and broadband light source. In our experiment, an amplified femtosecond laser was coupled into the fiber through a microscopic objective. We measured the FWHM of the continuum generation as a function of incident pulse energy from 80 nJ to 800 μJ. The maximum FWHM is about 8 times higher than that of the input pulses. The stability was analyzed at different pump energies, integration times and fiber lengths. The spectral broadening and peak position show that more than two processes compete in the fiber.