Proceedings Volume 2700

Nonlinear Frequency Generation and Conversion

Mool C. Gupta, William J. Kozlovsky, David C. MacPherson
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Proceedings Volume 2700

Nonlinear Frequency Generation and Conversion

Mool C. Gupta, William J. Kozlovsky, David C. MacPherson
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Volume Details

Date Published: 10 May 1996
Contents: 12 Sessions, 54 Papers, 0 Presentations
Conference: Photonics West '96 1996
Volume Number: 2700

Table of Contents

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Table of Contents

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  • Bulk Crystals and Nonlinear Conversion I
  • Bulk Crystals and Nonlinear Conversion II
  • Waveguide Materials and Nonlinear Conversion I
  • Waveguide Materials and Nonlinear Conversion II
  • Bulk Crystals and Nonlinear Conversion I
  • Optical Parametric Oscillators I
  • Optical Parametric Oscillators II
  • Devices and Applications I
  • Devices and Applications II
  • Bulk Crystals and Nonlinear Conversion II
  • Waveguide Materials and Nonlinear Conversion II
  • Stimulated Raman Scattering in Liquids and Fibers
  • Quantum and Injection Seeding of Stimulated Raman Scattering
  • Infrared Generation with Stimulated Raman Scattering
  • Ultrafast and Backward Stimulated Raman Shifting
  • Waveguide Materials and Nonlinear Conversion II
Bulk Crystals and Nonlinear Conversion I
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CsLiB6O10 crystal: growth and properties
Yusuke Mori, Takatomo Sasaki
We report on the growth and various properties of CsLiB6O10 (CLBO). The fourth and fifth harmonic generations of Nd:YAG laser radiation were realized in CLBO. The values of 500 mJ and 180 mJ at 266 nm and 213 nm were obtained from 2200 mJ of fundamental energy. The Al doping was found to give rise to an enhancement of mechanical and chemical properties of CLBO.
Internal field in lithium tantalate single crystals
An in-built internal field of 5 KV/mm was measured in Z-cut LiTaO3 single crystals in the direction of the original polarization. This was reflected in the asymmetry (along the field axis) in the polarization hysteresis curve as a function of external electric field. Measurement of internal field as a function of crystal thickness revealed that the internal field is a volume effect. Using infrared absorption measurements, the spectrum of the hydroxl ions (OH-) present in these crystals was measured at room temperature. The shape of the spectrum changes with polarization reversal in these crystals indicating a strong correlation with the direction of internal field with respect to the polarization direction. It suggests that the origin of internal field lies in point defect complexes involving OH- ions.
Visible quasi-phase-matched harmonic generation by electric-field-poled lithium niobate
Gregory D. Miller, Robert G. Batchko, Martin M. Fejer, et al.
Laser-based displays and illumination systems are applications which can capitalize on the brightness and efficiency of semiconductor lasers, provided that there is a means for converting their output into the visible spectrum. Semiconductor laser manufacturers can adjust their processes to achieve desired wavelengths in several near-infrared bands; an equally agile conversion technology is needed to permit display and illumination system manufacturers to choose visible wavelengths appropriate to their products. Quasi- phasematched second harmonic generation has the potential to convert high-power semiconductor laser output to the visible with 50% optical-to-optical conversion efficiency in a single-pass bulk configuration, using electric-field-poled lithium niobate. Lithographically- defined electrode structures on the positive or negative polar faces of this crystal are used to control the formation of domains under the influence of electric fields applied using those electrode structures. The quality of the resulting domain patterns not only controls the efficiency of quasi-phasematched second harmonic generation, but also controls the degree of resistance to photorefractive damage. We present a model which is used to identify the optimum electrode duty cycle and applied poling field for domain patterning and compare the predicted domain duty cycle with experimental results. We discuss factors which contribute to inhomogeneous domain pattern quality for samples poled under otherwise ideal conditions and our progress in limiting their influence. Finally, we present optical characterization of a 2.4 mm long 500 micrometers thick sample which produced an average second harmonic power of 1.3 W of 532 nm green from a 9 W average power Q-switched 1064 nm Nd:YAG laser in a loose- focus single-pass configuration.
Bulk Crystals and Nonlinear Conversion II
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Development of nonlinear crystals and violet-blue SHG laser
Yasunori Furukawa, Satoshi Makio, Tsuyoshi Miyai, et al.
Compact solid-state blue laser have great potential for use in optical data storage, laser beam printing, particle countering, reprographics, holography, and fluorescent bioanalysis. We report recent progresses in qualities of LiB3O5 and K3Li2 (TaxNb1-x)5O15 nonlinear crystal which are essential in manufacturing bulk-type blue SHG devices. We also review newly developed violet-blue laser, 20 mW output, using intracavity frequency doubling of a diode laser pumped Cr:LiSrAlF6 laser with low loss LiB3O5 crystal as a frequency doubler.
Photorefractive damage to bulk crystals and channel waveguides in periodically poled lithium tantalate
William V. Davis, Mool C. Gupta
We present the results of an experimental study of photorefractive damage caused by laser light at a wavelength of 488 nm in z-cut lithium tantalate bulk crystals and channel waveguides. The photorefractive damage was measured using both imaging technique and interferometry. Initially, we measured the photorefractive damage in virgin-poled crystal samples of various grades purchased from several sources. As expected, the amount of photorefractive damage is least in optical-grade crystals. In addition, we compared photorefractive damage in waveguides fabricated in virgin-poled and periodically poled optical-grade z-cut crystals. Our results confirm that the amount of photorefractive damage that occurs during irradiation is less in periodically poled waveguides. Furthermore, heating the waveguide devices to a temperature of 100 degree(s)C during irradiation removes most of the visible distortions of the waveguide mode profile that result from photorefractive damage for both the virgin-poled and periodically poled waveguides.
Walk-off-compensated type-I and type-II SHG using twin-crystal AgGaSe2 and KTiOPO4 devices
Jean-Jacques Zondy, M. Abed, S. Khodja, et al.
The walkoff-compensated (WOC) twin-crystal device enables to reduce the aperture effects in critically phase-matched parametric generation. In that device, instead of using a monolithic bulk crystal of length 21c, two identically-cut crystals of length lc are disposed in tandem such that the relative sign of the walkoff angle (rho) in the first and second items are opposite, while keeping unchanged the relative sign of their non-linear effective susceptibilities through a proper orientation of their optic axes. Applied to KTiOPO4 (KTP) for type-II (oeo) second-harmonic generation (SHG) of a color center laser, the 2 X lc twin- crystal device leads to a conversion efficiency 3.5 times higher than that of a 21c single bulk crystal. The same WOC technique has been applied successfully in the type-I (ooe) SHG of a CO2 laser using an AgGaSe2 (AGSE) device. The extension of the WOC method to a periodical structure composed of N twin-device stacks is considered. We report the first demonstration of the SHG efficiency enhancement of such a structure, which is made of four KTP plate optically contacted in the WOC configuration.
Electro-optic wafer beam deflector in LiTaO3
Jun Li, Hsing-Chine Cheng, Matthew J. Kawas, et al.
A novel electro-optic beam deflector is reported based on ferroelectric domain inversion extending through the thickness of a Z-cut LiTaO3 wafer. The selective domain inversion is achieved by electric-field poling assisted by proton exchange, rather than proton exchange followed by rapid thermal annealing. The deflection sensitivity of the device was measured to be 5.0 mrad/KV. This is 93% of the theoretical value for this geometry, and a significant improvement over the value of 80% of theoretical previously reported for a waveguide deflector.
Phase-matching characteristics of periodically poled KTP
William P. Risk
The phasematching characteristics of bulk periodically-poled KTP (potassium titanyl phosphate), fabricated by electric-field poling, are measured for first-order quasi- phasematched second-harmonic generation and compared to theory.
Waveguide Materials and Nonlinear Conversion I
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Periodic domain inversion in ion-exchanged LiTaO3 by electric field application
Cynthia Baron, Hsing-Chine Cheng, Mool C. Gupta
We report a process for the formation of inverted ferroelectric domain regions in LiTaO3. This process involves ion exchange followed by domain inversion by a uniform electric field. The resultant periodic domain structure is applicable to first order quasi-phase-matched second-harmonic generation. In first order periodically poled crystals of Z-cut LiTaO3, initial results have shown bulk conversion efficiencies of up to 3%/(W-cm). A second- harmonic conversion of 3 mW of blue wavelength power was observed from waveguide devices made on these crystals.
Short-time rapid thermal annealing processes for periodically domain-inverted structures with nearly straight domain walls in proton-exchanged LiTaO3
Cangsang Zhao, Reinhart Engelmann
Short-period domain inverted gratings with considerable inversion depth can be formed on the -z face of LiTaO3 by proton exchange in benzoic rather than pyrophosphoric acid followed by rapid thermal annealing at temperatures around 540 - 610 degree(s)C. This procedure allows the fabrication and characterization of first- second-, and third-order domain- inverted gratings for quasi-phase-matching blue-light generation. Annealing for extremely short time (approximately 6 - 12 s) is crucial for achieving domain-inverted structures with near-straight domain walls and least lateral expansion. Lateral domain expansion can be effectively suppressed by pre-annealing at low temperatures (
Ferroelectric channel waveguides with high SHG conversion efficiency and noncritical phase-matching characteristics
Nandakumar Ramanujam, James J. Burke, Lifeng Li
Second harmonic generation (SHG) conversion efficiency in quasi-phasematched waveguides can be enhanced significantly by optimizing the linear properties of the guiding structure. We describe here a method for fast and accurate computation of the modal properties of continuous, periodically-poled, ferroelectric channel waveguides, and a multi-parameter optimization algorithm which we have used to increase normalized internal SHG conversion efficiency. We will show, with results from our simulations, how both mode confinement at the fundamental wavelength and overlap of the transverse modes at the fundamental and second harmonic wavelengths influence the SHG conversion efficiency. Finally, we will also present the theoretical phasematching characteristics of these designs, and discuss the possibility of high SHG conversion efficiency (2900% W(DOT)cm2) near non- critically phasematched regimes of operation.
Optical characterization of lithium tantalate channel waveguides
William V. Davis, Mool C. Gupta
We present the results of a study of the optical properties of channel waveguides fabricated in z-cut lithium tantalate single crystals through proton-exchange in pyrophosphoric acid. A mask with lines 4 micrometers wide was used to fabricated these waveguides. The extraordinary effective refractive indices and near-field mode profiles of a waveguide proton-exchanged at 260 degree(s)C for 12 min and annealed at 400 degree(s)C for up to 3 h, are measured for TM polarized laser light at wavelengths 442, 543, 632, and 780 nm. The effective indices of the waveguide were measured through prism coupling. We observed that the variation of the extraordinary effective index with annealing time displays the anomalous behavior reported previously in the literature and it is maximum for an annealing time of 9 min. Using a prism coupler to individually excite each mode, we measured the near-field mode profiles by magnifying and imaging the end of the waveguide onto an optical multichannel analyzer. The variation of the FWHM in the width and depth directions as a function of wavelength and annealing time are presented. For the longest annealing time, we compare our results with the theoretical predictions obtained through numerical modeling.
Waveguide Materials and Nonlinear Conversion II
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Anomalous dispersion phase-matched second harmonic generation in poled polymer waveguides
We demonstrate anomalous dispersion phase-matched second harmonic generation in a poled polymer waveguide. Phase-matching was achieved between lowest order fundamental and harmonic modes. TMwo to TM2wo, at a fundamental wavelength of 815 nm over a propagation length of 32 micrometers . The maximum conversion efficiency was (eta) exp equals 39%/Wcm2, in good agreement with theory. Methods to further enhance the conversion efficiency using a combination of phase-matching techniques are also discussed.
Large electro-optic coefficient observed in ion-exchanged potassium titanyl phosphate waveguides
Yi Chiu, Daniel D. Stancil, Tuviah E. Schlesinger
We report the observation of an anomalously large electro-optic coefficient in potassium titanyl phosphate (KTiOPO4, KTP) waveguides fabricated under various conditions. An interferometric method was used to measure the effective electro-optic coefficient r33 in the waveguides by measuring the phase retardation of an optical mode when a voltage is applied across the substrate. We observe that at low frequency the waveguide electro-optic coefficient can be higher than the bulk value by a factor up to about 100. Since KTP crystals are known to have a high concentration of mobile ions, a model relating the mobile charges, space charge fields, and observed enhancement in r33 is proposed to explain the mechanism of this effect. Possible device application and long term effects on system operation are also discussed.
Sputter-deposited c-axis-oriented LiNbO3 thin films on silicon
Songshen Tan, J. Zou, Daniel D. Stancil, et al.
We report on the deposition of c-axis oriented LiNbO3 planar waveguides on silicon substrates. We have employed a combined SiO2/Si3N4 cladding layer to provide both optical confinement in the LiNbO3 as well as acting as a barrier layer to prevent Li diffusion into the SiO2. LiNbO3 films displaying exclusively the (006) reflection (no other orientations) in x-ray diffraction spectra have been produced. Post deposition rapid thermal annealing has been employed to improve the crystallinity of these films. Optical losses of about 18 dB/cm were measured for these films. We discuss the deposition conditions, the role of the silicon nitride in determining the LiNbO3 texture, as well as the performance of these layers as planar optical waveguides.
Correlation between ordinary and extraordinary refractive indices and structural parameters of HxLi1-xNbO3 and HxLi1-xTaO3 waveguides
The Optical Communications Group of the German Aerospace Research Establishment (DLR) has investigated the feasibility of a fiberless receiver telescope for high sensitive coherent optical space communication, resulting in an elegant Pointing, Acquisition and Tracking (PAT) concept. To demonstrate the feasibility of this new concept, an optical receiver terminal that coherently obtains both the spatial error signal for tracking and the data signal with only one set of detectors has been built. The result is a very simple and compact setup with few optical surfaces. It does not require fibers for superpositionning and is capable to compensate for microaccelerations up to about one kilohertz. Keywords: free-space optical communication, coherent optical receiver terminal, satellite vibration compensation, fiberfree heterodyning, Pointing, Acquisition and Tracking
Bulk Crystals and Nonlinear Conversion I
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Applications of high-average-power nonlinear optics
Stephan P. Velsko, William Franklin Krupke
Nonlinear optical frequency converters (harmonic generators and optical parametric oscillators) are reviewed with an emphasis on high-average power performance and limitations. NLO materials issues and NLO device designs are discussed in reference to several emerging scientific, military and industrial-commercial applications requiring approximately equals 100 watt average power level in the visible and infrared spectral regions. Research efforts required to enable practical approximately equals 100 watt class NLO based laser systems are identified.
Optical Parametric Oscillators I
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Progress in quasi-phase-matched optical parametric oscillators using periodically poled LiNbO3
Lawrence E. Myers, Robert C. Eckardt, Martin M. Fejer, et al.
We review progress on quasi-phasematched optical parametric oscillators (OPOs) in bulk periodically poled LiNbO3. We have extended the electric-field poling process so that we now reliably fabricate crystals over 60-mm long in full 3-inch-diameter, 0.5-mm-thick wafers. Periodically poled material retains the low loss and bulk power handling properties of single domain LiNbO3, and QPM allows noncritical phasematching with the highest value of the nonlinear coefficient. OPOs pumped by 1.064-micrometers Nd:YAG lasers have been operated over the wavelength range 1.36 micrometers to 4.9 micrometers with tuning by temperature or QPM period. We have shown oscillation threshold as low as 0.006 mJ with a Q-switched pump laser, and pumping over 25 times threshold without damage. We have also demonstrated a doubly resonant OPO pumped directly with a commercial cw diode laser at 978 nm, and a 1.064-micrometers -pumped cw singly-resonant OPO with threshold < 3 W.
Optical and thermo-optical characterization of KTP and its isomorphs for 1.06-um-pumped OPOs
Christopher A. Ebbers, Stephan P. Velsko
We have characterized the phasematching angle, bandwidth, thermal conductivity, and d(lambda) /dT for potassium titanyl phosphate, potassium titanyl arsenate and rubidium titanyl arsenate optical parametric oscillators.
Solitonlike pulse compression in a synchronously pumped optical parametric oscillator
John D.V. Khaydarov, James H. Andrews, Tony C. Kowalczyk, et al.
We describe in detail the mechanism of 20-fold pulse compression in a synchronously pumped optical parametric oscillator. Nonlinear compression of these soliton-like pulses arises from pump depletion by the leading front of the compressed pulse and is limited by the round-trip cavity losses.
Optical Parametric Oscillators II
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OPO performance with an aberrated input pump beam
The performance of an optical parametric oscillator (OPO) with non-ideal input pump fields is investigated numerically. The analysis consists of a beam propagation calculation based on Fourier methods including walk-off in the non-linear crystal coupled with the three-wave interaction in the crystal. The code is time dependent enabling analysis of laser pulses. The pump beam aberrations are described by Zernike polynomials. The OPO investigated is a LiNbO3 crystal in a flat-flat resonator. The LiNbO3 crystal is cut to produce a 1.5 micrometers signal and 3.6 micrometers idler from a 1.06 micrometers input pump field. The results show that the type of aberration is significant when predicting the output performance of the OPO and not simply the beam quality or M2 angular divergence of the pump beam. While thresholds for input pump beams with M2 equals 2 only increase on the order of 10% over unaberrated beams, the divergence of the output fields can be much worse than the pump beam divergence. The output beam divergence is also a function of the input pump energy. Aberrated pump fields can also lead to angular displacements between the generated signal and idler fields.
Widely tunable KTP optical parametric oscillator amplifier and its applications to fluorescence-lifetime measurement in Yb-doped laser materials
Hiromitsu Kiriyama, Narayanan Srinivasan, Takashi Kimura, et al.
The design and construction of an efficient near-infrared holosteric optical parametric oscillator amplifier based on KTiOPO4 (KTP) crystal are reported. The system is tunable from 840 to 1451 nm and generates up to 0.45 mJ at 920 nm with 27% efficiency in 4 - 6 ns pulses. The 532 nm pump source is a frequency doubled laser-diode pumped Nd:YAG laser producing up to 2 mJ in 8 ns, 50 Hz pulses. We have used the system to study the fluorescence lifetimes of Yb:YAG and Yb:glass as a function of wavelength. The data reveal interesting variations in the reabsorption enhanced Yb-ion lifetime. Some Yb:YAG samples exhibiting a sharp increase in their lifetime near the peak absorption wavelengths. The longest lifetime measured in a few mm thick sample of Yb:YAG was 3.3 ms, which would be highly beneficial for energy storage applications using laser diodes.
Multiwavelength injection-seeded midinfrared optical parametric oscillator for DIAL
Mark S. Webb, Kenneth A. Stanion, David J. Deane, et al.
We have constructed and fielded a multi-wavelength injection seeded mid-IR OPO source for DIAL. This OPO system was built for ground based remote sensing measurements of species with both broad (300 cm-1) and narrow absorption bandwidth (0.07 cm-1 FWHM). The OPO utilizes a single frequency tunable diode laser for the injection seeded signal wavelength in the region from 6400 to 6700 cm-1 and an angle phase-matched 5 cm LiNbO3 crystal to provide large tuning excursions on a slow time scale. The pump was a diode pumped Nd:YAG MOPA (9398 cm-1) running at 180 Hz. This pump source was repeatedly injection seeded with a different wavelength on each of three sequential shots forming a set of three pulses having wavelength separations on the order of 0.4 cm-1 at a three color set repetition rate of 60 Hz. This combination of OPO signal and pump source produced a set of three time staggered idler wavelengths separated by 0.4 cm-1 with the center wavelength tunable from 2700 to 3000 cm-1. This OPO system was used in field test experiments to detect the release of chemicals from a standoff distance of 3.3 Km. We present key OPO design criteria, performance data, and numerical simulations that agree with our observation of pump induced spectral impurities in the OPO output.
Devices and Applications I
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Progress in all-solid-state deep-ultraviolet coherent light sources
Michio Oka, L. Y. Liu, Werner Wiechmann, et al.
High power deep ultraviolet (UV) radiation has attracted much attention in areas such as photo-lithography, micro fabrication, material processing and ultra high density optical disk mastering. We report progress in improving the quality of Czochralski-grown (beta) -BaB2O4 (BBO) which is essential for generating high average output power in the deep ultraviolet regime. We obtained 1.5 W of cw 266 nm output using an external resonant doubler, 1.3 W of 213 nm output from a flashlamp pumped Q-switched Nd:YAG laser and more than 0.4 W of 213 nm radiation from a commercial 6 W diode-pumped high repetition rate Q-switched Nd:YAG laser using our melt-grown BBO crystals for the nonlinear frequency conversion. Using the cw 266 nm output speckle-free 0.25 micrometers photolithography and micro-fabrication was demonstrated.
Diode-pumped Nd:YAG/KNbO3 monolithic blue laser assembly producing greater than 10 mW at 473 nm
Gregory J. Mizell, Douglas T. Essex, Kurt Yates, et al.
An ultra compact monolithic crystal assembly (MCA) producing greater than 10 mw of coherent blue light at 473 nm is described. The MCA is comprised of Nd:YAG and off-axis cut KNbO3 crystal etalons, bonded together to form a flat and parallel optical cavity only 3 mm thick. Narrow-band HR coatings centered near 946 nm are applied to the exterior surfaces of the assembly, permitting 946 nm oscillation while suppressing competing 1064 and 1320 nm laser action. The KNbO3 crystal is cut for critically phase matched SHG (946/473) at approximately 35 degrees C. Using a commercially available 808 nm laser diode pump source, greater than 10 mw of blue light is obtained. Merits of this blue light emitting MCA are discussed.
High-power cw Nd:YAG lasers pumped with fiber-coupled diode lasers
Dirk Golla, S. Knoke, Wolfram Schoene, et al.
We report on diode laser side-pumped, cw Nd:YAG rod lasers operating at output powers of several hundred Watts. Applying fiber-coupled diode lasers as pump sources, linear pump power densities of more than 200 W/cm are attainable. A nearly free adjustment of the pump light distribution inside the laseractive medium allows a significant decrease of thermally- induced effects and minimizes aberration-related losses. In multimode operation at 1064 nm, output powers of more than 320 W cw are observed. Applying an improved resonator design, output powers of more than 80 W in TEM00 mode operation are realized with an optical- to-optical efficiency of more than 20%. Frequency conversion into the visible spectral range with optical-to-optical efficiencies (green power to pump power) of 5% is realized by intracavity second harmonic generation in KTP crystals. Thermal properties and laser performance of laser systems excited by fiber-coupled diode lasers will be discussed.
High-power Q-switched and frequency-doubled Nd: YVO4 miniature lasers
Takunori Taira, Tsuyoshi Suzudo, Takao Kobayashi
We have studied the electro-optic Q-switching on a frequency doubling KTP crystal. It is possible to achieve approximately 1 kW, 6 - 8 ns green pulse generation on diode pumped Nd:YVO4 miniature lasers.
Devices and Applications II
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cw/Q-switched 40-W green beam generation by frequency doubling with a KTiOPO4 crystal
Koji Yasui, Susumu Konno, Tetsuo Kojima
High average power of 40-W class green beam (532 nm) was generated in a wide range of repetition rates of 2 kHz - 10 kHz in addition to pure cw mode by placing a KTiOPO4 crystal in a resonator cavity of a Nd:YAG laser.
Demonstration of a vapor-density monitoring system using UV radiation generated from waveguide quasi-phase-matched SHG waveguide devices
Shirley A. Galanti, Leon Val Berzins, J. B. Brown, et al.
Many industrial application require non-intrusive diagnostics for process monitoring and control. One example is the physical vapor deposition of titanium alloys. In this paper we present a system based on laser absorption spectroscopy for monitoring titanium vapor. Appropriate transitions for monitoring high rate vaporization of titanium require extension of available IR diode technology to the UV. The heart of this vapor density monitoring system is the 390 nm radiation generated from quasi-phase matched interactions within periodically poled waveguides. In this paper, key system components of a UV laser absorption spectroscopy based system specific for titanium density monitoring are described. Analysis is presented showing the minimum power levels necessary from the ultraviolet laser source. Performance data for prototype systems using second harmonic generation waveguide technology is presented. Application of this technology to other alloy density monitoring systems is discussed.
Bulk Crystals and Nonlinear Conversion II
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Aluminophosphate-doped glasses: new materials for optoelectronics
Rodica Rogojan, Mihai Elisa, Anca I. Stanculescu
Until now, all studies were done over the semiconductor glasses doped with CdSxSe1-x, in which the microcrystallite phases was grown into a vitreous silica matrix. Our study was made as for growing microcrystallites of CdSxSe1-x into a vitreous aluminophosphate matrix. In the first step, the samples of semiconductor glass were obtained by melting the raw materials batch and then by adding sulphur selenium and cadmium into the molten glass. In the second step, the resulted glass was annealed in order to bring about the microcrystallite nucleation and growing process. Both linear and non-linear optical properties are deeply influenced by the microcrystallite size, which, on its turn is determined by the duration and temperature of the annealing treatment. By means of a spectrophotometer (SPECORD M-42), were drawn absorption, transmission and fluorescence spectra, in visible domain. From the shape and the absorption edge of these spectra, we are able to characterize the microcrystallite growing process. From these spectra it can be observed a shift of the absorption edge to red which depends on the microcrystallite size.
m-dinitrobenzene: optical nonlinear organic crystal growth for optoelectronics
Anca I. Stanculescu, Alexandru Popina
A high purity dinitro derivate of benzene for crystal growth has been obtained by chemical purification, vacuum distillation and two stages directional freezing. The crystallization process of m-dinitrobenzene has been investigated by a Bridgman method using a special heating and displacement arrangement. The quality of organic crystalline material has been analyzed by chemical and optical methods.
Improvement of BBO NLO figures of merit by postgrowth thermal treatment
Valery D. Antsygin, Vladimir A. Gusev, Alexander M. Yurkin
The effect of post-growth thermal treatment on optical figures of merit of BBO was studied. It was established that annealing in an atmosphere improved bulk transparency and homogeneity of the samples but simultaneosly accelerated the surface destruction. Annealing in organic solutions, in general, decreased a volume and surface optical loss, increased a material's homogeneity and its laser damage resitance. Key words: beta barium borate, annealing, laser damage.
Wideband tunable and multiple noncritical phase matchings in nonlinear optics
Victor I. Zadorozhnii, Nikolai E. Kornienko, Lyudmila I. Konopaltseva, et al.
A general classification of the phase-matching (PM) conditions noncritical to frequencies and angular apertures of signal and pumping () radiations is given; the sumand difference-frequency generation processes in uniaxial and biaxial crystals are covered in this investigation. The multiple noncriticality conditions are analysed, and physical concepts on group, tangential, and conditional centers are introduced. It has been found that the group centers (GC) can be tuned over the entire transparency region of the KTIOPO4 (KTP) crystal when changing , and the GC-tuning curve is finished at the distinctive critical point of PM (CPPM) where the triple noncriticality of PM is realized under conditions of collinear interaction and the greatest nonlinearity. It is shown that near the GC in KTP the spectral width of PM can exceed the value of lO3cm', and the considerable magnitude of tuning is possible at small change in in the vicinity of the CPPM. The possibility of generating narrow-band and wide-band radiations using the wide-band (narrow-band) pump radiation in optical parametric oscillator (OPO) is estabilished. A new phenomenon of division of the GC region into three regions of a double noncriticality is found. Key words: noncritical phase-matching, sum- and difference-frequency generation, optical parametric oscillator, wide-band UP-conversion.
Waveguide Materials and Nonlinear Conversion II
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Periodic domain inversion in Z-cut LiTaO3 by electron beam scanning assisted by an internal field within the crystal
The coercive field for domain reversal in a 0.5 mm Z-cut LiTaO3 was measured to be 21 KV/mm. However, if the polarization of the crystal is reversed using an electric field of 21 KV/mm, the coercive field for a second reversal is found to be 11 KV/mm. This asymmetry is due to the presence of an in-built internal field of 5 KV/mm in the virgin crystal. If the electron beam writing is done on the new C-face of the polarization reversed region of the crystal, (1) no cracking was observed, (2) the writing required less charge for domain inversion because of the reduced coercive field, thereby facilitating shorter scan times. We have obtained uniform 3.6 micrometers period domain inversion grating using this technique.
Micro-Raman and luminescence spectroscopic techniques for the characterization and process control of Rb+-exchanged KTiOPO4 waveguides
David D. Tuschel, Suzanne D. Lau, William P. Risk
The fabrication of ion-exchanged waveguides with high-frequency doubling conversion efficiency requires high-quality crystalline substrates, an understanding of the effects of partial cation exchange on the optical properties of the waveguide, and control of the degree and effects of ion-exchange. To address these needs we have developed micro-Raman and luminescence spectroscopic techniques for the characterization and process control of Rb+ exchanged KTiOPO4 (R/KTP) waveguides. We report on the use of laser excited luminescence to screen device substrates for unacceptable levels of impurity transition metals, which contribute to photorefraction and optical losses due to absorption. Micro-Raman spectroscopy has been used to probe types of R/KTP channel waveguides for the degree and effects of Rb+ exchange. The high spatial resolution and nondestructive nature of micro-Raman spectroscopy make it suitable as a probe for in situ characterization of photonic devices. Specifically, micro-Raman spectroscopy can detect cation-exchange induced changes in the polarizability, reduction of crystal symmetry, and changes in the chemical bonding and orientation of TiO6 octahedra, the anionic groups primarily responsible for the nonlinear properties of the materia. Individual R/KTP waveguides from different devices have been studied by micro-Raman spectroscopy and structural differences have been detected. The uniformity of a channel waveguide is another quality that can be readily probed and quantified by micro-Raman spectroscopy.
Stimulated Raman Scattering in Liquids and Fibers
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Enhancing stimulated Raman scattering of weaker gain Raman modes in microdroplets by seeding and efficient pumping
Md. Mohiuddin Mazumder, Mitchell H. Fields, Justin M. Hartings, et al.
High-Q morphology-dependent resonances (MDR's) in microdroplets provide optical feedback for nonlinear processes such as stimulated Raman scattering (SRS). The SRS signal from weaker gain modes or from the minority species in a binary mixture droplet is small, because of depletion of the pump beam by the strongest-gain Raman mode. We present an effective- average Raman gain formula and discuss the factors that contribute to the enhancement of SRS intensity. Spatial overlap between pump wave and SRS wave is an important factor in determining the SRS intensity. We have observed that in a binary mixture microdroplet, the SRS of the majority species is more efficient (because of good spatial overlap) in pumping the minority species than the input laser itself. Laser-induced distortion can couple light efficiently into a droplet. We have used a variable number (3 - 6) of mode-locked 100-psec laser pulses focused on the droplet rim. The SRS emission from a minority species increases as the number of input pulses is increased, because cumulative laser-induced surface distortion couples more energy into the droplet. SRS signal at the Stokes shift of a weaker gain mode or of the mode of a minority species can be enhanced by seeding light at the SRS wavelengths. The seed signal is obtained from the fluorescence of a dye added to the microdroplet or from external coupling of light into the droplet. A lasing dye (Rhodamine 6G) is added to pure ethanol droplet. By using an excimer-pumped tunable dye laser as the excitation source, the weaker gain C-C-O mode (882 cm-1 Stokes shift) of ethanol is overlapped spectrally with the fluorescence and lasing of R6G. The SRS signal of the C-C-O mode is enhanced, because (1) the fluorescence of R6G at the SRS wavelength adds to the spontaneous SRS emission and (2) the gain from population inversion adds to the Raman gain. We have also observed enhanced SRS emission from a minority species (benzene) in dodecane by using internal dye seeding. We externally seeded ethanol SRS (2928 cm-1) in a binary mixture (12% ethanol and 88% water) microdroplet by using a frequency-doubled Nd-YAG pumped tunable dye laser. A large (factor of 45) enhancement in the ethanol SRS (integrated over 100 shots of a 10-ns laser) has been observed.
Newly observed nonlinear scattering feature occurring in optically pumped cylindrical liquid jets
Thomas E. Ruekgauer, Paul Nachman, Jing-Gang Xie, et al.
Stimulated Raman scattering (SRS) produced by a high intensity laser (Nd:YAG) pulse illuminating a micron-sized cylindrical liquid jet will be discussed. Due to reflection at the liquid-air interface, a cylindrical jet can act as a `leaky' optical resonator, providing the necessary feedback for the generation of the nonlinear SRS. The spectral character of SRS generated in a dielectric cylinder is discrete, with the emission maxima corresponding to normal mode frequencies of a cylindrical microcavity. Results from our experiments incorporating an auxiliary source of light passing through the SRS-generation region of the cylinder reveal that the SRS fields can grown to intensities high enough to drive nonlinear index of refraction changes (n2 or other processes) in the liquid. This process is conjectured to be responsible for the existence of a previously unreported, spatially localized, nonlinear scattering feature, appearing as a ring of light in the SRS-generation region. In this presentation, we describe characteristics of the ring feature and provide evidence for the importance of self-focusing in its generation.
Stimulated rotational Raman scattering in H2
Bruce G. Oldaker, Timothy Pritchett, Thomas R. Moore
The development of stimulated emission from molecular transitions between vibrational energy levels of diatomic gases exposed to high intensity laser pulses through the Raman effect has been studied for many years. The energy levels are non-degenerate, the pump pulse is usually linearly polarized, and there are amenable classical models of the scattering. We have started a research program to study the development and gain of stimulated emission from molecular transitions between rotational energy levels in molecular hydrogen when exposed to a high intensity laser pulse. The rotational energy levels are necessarily degenerate in angular momentum and therefore the scattering is sensitive to the angular momentum of the pump light, i.e. its polarization. We present initial experimental verification of simplified classical theories for the polarization dependence of this type of scattering for the Stokes radiation and the Stokes--Anti-Stokes coupling common to all Raman scattering. Extensions of the classical theory to higher rotational orders and similarities and differences to stimulated vibrational Raman scattering and plans for further experiments are discussed. An exciting prospect is the prediction of generating coherent emission at the rotational Raman frequency itself by using an electric field perpendicular to the laser's direction of propagation.
Laser cavitation and Raman pulse compression
Shao-Xian Meng, Zhongmin Gu, Jiran Zhao, et al.
The cavitation and cavity are produced near the focus when high power laser is focused into the liquid, and Raman seed source is generated in this cavity, which will give a backward and forward Raman light pulse due to amplification.
Quantum and Injection Seeding of Stimulated Raman Scattering
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Competition between spontaneous scattering and stimulated scattering in an injection-seeded Raman amplifier
Jim G. Wessel, Kevin S. Repasky, John L. Carlsten
It has been shown that only a few photons of input signal into a H2 Raman amplifier are enough to ensure that the amplified signal is larger than the amplified spontaneous scattering. This low noise property of Raman amplifiers makes it feasible to perform injection seeding experiments using a low power continuous wave laser diode. In this paper we study, experimentally and theoretically, the amplification of a laser diode beam in an optically pumped Raman amplifier. It is found that for larger laser diode powers the signal grows nearly exponentially in the pump laser power. At low laser diode powers (<EQ 5 nW), on the other hand, the behavior is more complicated because the spontaneously scattered light which is then amplified is comparable in power to the amplified signal. We also find that due to an increased number of spatial spontaneous scattering modes, the ratio of amplified spontaneous scattering to amplified signal is larger at low pump powers than high pump laser powers. The experiments, including experiments with no laser diode input, are described by a 3D theory of a gain-guided amplifier. This theory expresses the amplified electric field as a linear combination of nonorthogonal modes and, in slightly different forms, has been used to describe a wide range of amplifiers from Raman amplifiers to x-ray lasers.
Intensity dependent index guiding in a seeded Raman amplifier
Kevin S. Repasky, Jim G. Wessel, John L. Carlsten
This paper examines the effect of index guiding in a Raman amplifier. The slowly varying Maxwell wave equation including the total Raman susceptibility for a seeded Raman amplifier is explored. Numerical calculations indicate that the real part of the Raman susceptibility cause the Raman medium to act as a lens when the Stokes seed is detuned from the Raman resonance. This focusing effect leads to a gain enhancement of up to 5% and a shift in the peak Stokes energy by up to 145 MHz to the blue side of the Raman resonance for Raman scattering in H2 with a pump laser at 532 nm and an input seed near 683 nm.
Quantum coherence and squeezing in stimulated Raman scattering
Joseph W. Haus, K. C. Yeong, A. V. Chizhov
Two models are considered to examine quantum correlations between the Stokes and anti- Stokes fields generated in stimulated Raman processes; the first is a cavity model and the second treats propagating fields. The coherence introduced through the dynamics evolution are manifest through their higher-order correlation functions where sum-squeezed states are identified.
Gain-guiding and quantum noise in a transient Raman amplifier
Gain-guiding in an optical amplifier can have a significant effect on the quantum noise properties of the amplified signal. It has been demonstrated, both theoretically and experimentally, that gain-guiding in a steady state Raman amplifier can lead to excess noise in the output signal. Experiments using a Raman amplifier operating in the extreme transient regime also indicate that gain-guiding can have a significant effect on the output Stokes field. In this paper, a theoretical description of transient stimulated Raman scattering in a focused gain geometry is developed. The theory accounts for diffraction, gain-guiding, and quantum initiation.
Stimulated scattering of Bessel beams
Stephan Herminghaus, Steffen Klewitz, Paul Leiderer, et al.
Light beams whose transverse intensity profile is given by the zero order Bessel function of the first kind, so-called Bessel beams, exhibit a sharp intensity maximum on the optic axis which, quite remarkably, does not spread even over macroscopic propagation distance. It is shown that this quasi non-diffracting behavior can lead to interesting novel pumping schemes in nonlinear optics. In experiments on stimulated Raman and Brillouin scattering of Bessel pump beams, we demonstrate that the frequency of the scattered radiation may be tuned without the use of a resonator, merely by changing the propagation constant of the Bessel beam. Due to the peculiar intensity distribution of the Bessel beam, the scattered beam is of particularly high quality and in the far field becomes intrinsically separated from the pump beam in the far field. Conversion efficiencies are comparable to those achieved with Gaussian beams.
Temporal dynamics quantum fluctuations in spectrum of stimulated Raman scattering
Victor G. Bespalov, Yuri N. Efimov, Dmitry I. Staselko
The investigation showed step by step picture of formation and development of SRS spectra fine structure including processes of: separation of the central and side modes from broadband smooth spectrum; narrowing of the components linewidth; frequency shifting of the side components to the central component; generating modes competition leading to the smooth changing in their relative intensities; the narrowing of SRS spectrum envelope; and frequency shifting of all the lines towards long wave region. These processes are general and are observed practically in all the cases however each of them has its own peculiarities and characteristic times, determined by pump threshold exceeding and Stokes emission duration.
Infrared Generation with Stimulated Raman Scattering
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Raman shifting in hydrogen with a Cr:LiSAF pump
Mark D. Eickhoff, David C. MacPherson
Double Raman shifting in molecular hydrogen is used in our laboratory as a method to generate tunable infrared radiation. The pump pulses from a Q-switched Cr:LiSAF laser at 885 nm are typically 50 mJ with a pulse width (FWHM) of about 120 ns. the spectral width of the pulses is narrowed to < 1 GHz by injection seeding using a stable, tunable AlGaAs diode laser coupled through a single-mode fiber. A Cr:LiSAF amplifier increases the pulse energy by a factor of two and these amplified pulses are focused (Rayleigh range approximately equals 7 cm) into a cell filled with 12 atm of hydrogen. A resonator which is high-Q at the first Stokes wavelength (1.40 micrometers ) improves the efficiency of the conversion process. Further enhancement of the desired second Stokes radiation is expected by phase-matching with a small admixture of carbon dioxide. 2.5 mJ of 3.33 micrometers radiation has been observed from this system without phase-matching. The tunability of the pump laser will allow the second Stokes output to span the 2 - 5 micrometers wavelength range.
Raman shifting in the absence of multiple Stokes orders with a 1.3-um Nd:YAG laser in hydrogen
A Q-switched Nd:YAG laser generating 160 mJ in 60 ns at 1.3 micrometers has been shifted to 2.9 micrometers by Raman conversion in hydrogen. The energy of the 2.9 micrometers photons is insufficient to undergo further Stokes shifts. Thus the first Stokes line is the terminal wavelength for this process. While terminal Stokes components have been reached in previous Raman shifting studies, these components have always been the result of multiple Stokes shifts. In a single pass configuration photon conversion of up to 0.37 was realized. Conversion into anti-Stokes lines of up to third order was observed, albeit at insignificant energies. With the addition of an unoptimized resonator the photon conversion rose to 0.49. The pressure dependence of Stokes conversion was also investigated. The data show a smooth increase in output followed by saturation. This is in accordance with conversion into a single terminal Stokes lines. However, in contradiction with theory, the pump linewidth was observed to matter a great deal. Amplified elastic or near elastic scattering is suggested as an explanation for this result. Overall, the success of this work bodes well for forthcoming attempts to obtain tunable mid-IR radiation by means of a terminal first Stokes shift.
Optical pump energy transformation in system with Raman converter and generated wideband medium
Vitaliy M. Khulugurov, Nikolai Ivanov, Eugeniy A. Oleynikov
Processes of the optic pump energy transformation in system Raman converter-wideband laser medium with homogeneously broadening amplification band have been investigated. Raman converter and wideband medium were used in common cavity. As a Raman converter a barium nitrate and benzol were used, as a wideband media a LiF crystals with F-2 or F2 color centers and oxazyn-1 were used. Thanks to four-levels excitation scheme of the wideband media, pump output excitation threshold of them is less than for SRS excitation. In such system generation start from the widespread medium. The wideband medium amplify seeding radiation of the Raman converter at the Stokes frequencies, which get to the amplification band of the wideband medium. Then, thanks to additional Raman amplification, condensation of the wideband generation spectrum arise. Thus the threshold of the SRS excitation decreased and efficiency of the SRS increased in Stokes or anti-Stokes regions. Under certain conditions in liquid Raman media arise generation on the different types of the vibrations and generation on the combined frequencies too.
Ultrafast and Backward Stimulated Raman Shifting
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Amplification of femtosecond signals by stimulated Raman scattering in hydrogen gas
Aleksander Rebane, Vitaly N. Krylov, Daniel Erni, et al.
We report efficient amplification of weak femtosecond signals by a stimulated Raman scattering process on vibrational and rotational components of pressurized H2 gas excited with 200-fs-duration frequency-doubled pulses from a regenerative-amplified Ti:sapphire laser. The amplification factor of 108 is obtained at the wavelength of 465 nm for weak seed pulses produced by white light continuum generation in glass.
Stimulated Raman scattering by frequency-doubled 1-kHz amplified femtosecond Ti:sapphire laser in H2, D2, HD, and CH4 gases
Vitaly N. Krylov, Aleksander Rebane, Daniel Erni, et al.
We present results on stimulated Raman scattering in H2, D2, HD and CH4 excited by 200 fs-duration 1 kHz repetition rate and 0.6 mW average power Ti:sapphire laser (780 nm) and 0.2 W average power second harmonic (390 nm) pulses with linear and circular polarization. Spectral and temporal measurements of the vibrational and rotational components at Stokes- and anti-Stokes frequencies in the wavelength range 289 - 1154 nm are reported. We observed 2.5 times shortening of the vibrational Stokes component pulses in H2 and D2 exited by 350 fs-duration second harmonic pulses.
Experimental investigation of vibrational dephasing relaxation in liquid bromine
Igor V. Alexandrov, Zinaida V. Nesterova, Vuatcheslav O. Archirejev
The cascade stimulated Raman scattering by single vibration of molecular bromine in capillary optical fiber, filled by liquid Br2 was detected when the IR picosecond YAG-Nd pulses were used as the pump ones. In a frame of modern transient SRS theory and on the basis of modern models of molecular relaxation the vibrational dephasing constant T2 of molecular bromine was calculated for the first time.
Extreme shock wave compression of ultrashort light pulses
Igor V. Alexandrov, Zinaida V. Nesterova, Vassily V. Solovjov
The experiments on an envelope deformations for the picosecond light pulses in capillary fibers, filled by neat organic liquids were made. The combined effects of Raman depletion and the light energy concentration due to the shock wave regime were considered.
Calculation of nonlinear resonant Raman effect cross sections in intense laser and magnetic fields
Tamara Ya. Karagodova
The method of computer simulations on nonlinear resonant magnetooptical effects developed for the first time for real multilevel atoms in the two laser fields of arbitrary intensity and external magnetic field is applied for Raman-effect cross section calculations and investigations of the dependence on magnetic field strength, intensities, detunings of laser fields for alkaline atoms, thallium. Calculations of nonlinear resonant Raman-effect cross- sections in the intense laser fields resonant to adjacent transitions and magnetic field show the opportunity of appearing of the new effects in Raman scattering in atomic gases.
Waveguide Materials and Nonlinear Conversion II
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Growth of lithium niobate thin films by solid source MOCVD
S. Y. Lee, Roger K. Route, Robert S. Feigelson
Lithium niobate (LiNbO3) epitaxial thin films have been grown on c-plane sapphire and LiTaO3 (LT) substrates using solid-source MOCVD. The structural quality of LiNbO3 (LN) films grown on LT was found to be higher than for films grown on sapphire. Films on LT have a single in-plane orientation over a wide range of deposition temperatures (500 degree(s) to 750 degree(s)C), while growth on sapphire often leads to a small percentage of 60 degree(s) misoriented grains. The surface roughness of LN films on LT substrates is also much less than for films deposited on c-sapphire under similar conditions. While sapphire is a less suitable substrate for the deposition of optical quality films, it has provided an opportunity to study the effects of surface microstructure on film quality. Our findings are described. Prism- coupled, thin film analysis has revealed optical losses as low as 2 dB/cm (TMo mode) in 1000 angstroms thick LN films deposited on c-sapphire and 5 approximately 6 dB/cm (TEo mode) in 5000 angstroms films deposited on LT. Using LT, we have been able to deposit high crystalline quality LN films with very smooth surfaces below the Curie temperature of the substrate (600 degree(s)C), and etching techniques have revealed that the polarity of these films follows that of the substrate. This result suggests the possibility of depositing LN on periodically poled LT substrates for quasi-phase matched optical interactions.