Solid State Lasers for Coherent Communication NASA Grant

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Solid State Lasers for Coherent Communication and Remote Sensing. NASA Grant NAGW 1760,Robert L Byer,Applied Physics Department. Stanford University,Stanford California 94305, Work in the stabilization of monolithic Nd YAG lasers and the. application of these lasers to nonlinear optical frequency conversion is. discussed The intrinsic stability of semiconductor diode laser pumped. solid state lasers has facilitated a number of demonstrations in external. resonant cavity harmonic generation and stable optical parametric. oscillation Relative laser frequency stabilization of 0 3 Hz has been. achieved and absolute stability of a few hundred hertz is anticipated The. challenge is now to reproduce this frequency stability in the output of. tunable nonlinear optical devices Theoretical and experimental work. toward this goal are continuing, Solid State Lasers for Coherent Communication and Remote Sensing. NASA Grant NAGW 1760,Table of Contents, ltll luo u la elQlloeg U UellgQmQI I Ig O U UUg UO ii. Table of Contents m,Personnel Associated with the Program iv.
I Intttxlt on 1,II Frequency Stable Operation of Nd YAG Lasers. to be published in The Proceedings of the Fifth International. School on Laser Applications in Atomic Molecular and. Nuclear Physics Vilnius University Press Vilnius,Lithuania USSR 2. m Optical parametric oscillator frequency tuning and control. to be published in the Journal of the Optical Society of America B. March 1991 18, Solid State Lasers for Coherent Communication and Remote Sensing. NASA Grant NAGW 1760,Personnel Associated with the Program. Robert L Byer Principal Investigator,Robert C Eckardt Senior Research Associate.
Martin M Fejer Assistant Professor of Applied Physics. Eric Gustafson Research Associate,Nick Sampas Post Doctoral Research Affiliate. Timothy Day Ph D September 1990,Steven T Yang Graduate Student. Christopher Pohalski Graduate Student,Darwin Serkland Graduate Student. Solid State Lasers for Coherent Communication and Remote Sensing. Robert L Byer,Principal Investigator,I Introduction. This progress report is presented in the form of two manuscripts that have been submitted. for publication The first manuscript describes a presentation given in Vilnius Lithuania in. August of 1990 Our work in the development and frequency stabilization of. semiconductor diode laser pumped solid state lasers was presented Emphasis was placed. on research in nonlinear optical frequency conversion made possible through the use of. these lasers to generate pump radiation for the nonlinear optical devices These lasers are. now being stabilized to better than one hertz relative to a high finesse resonant cavity. Preliminary demonstrations have shown the possibility absolute stabilization to a few. hundred hertz using sub Doppler spectroscopy techniques with molecular absorptions in. The second manuscript describes a theoretical analysis of the tuning properties of. monolithic doubly resonant optical parametric oscillators RO s This paper will be. published in the March issue of the Journal of the Optical Society of America B The. tuning properties of the DRO s are complicated and stable operation requires that an. number of critical tolerances be met Monlithic DRO s that we have operated have. displayed extremely good coherence properties low thresholds 10 mW for cw parametric. oscillation and good conversion efficiency The potential of optical parametric oscillators. for optical communication remote sensing spectroscopy and even projection displays is. extensive This potential for application proves the motivation for stabilizing the DRO. We are currently pursuing a number of projects in these areas Work on both. improved relative laser frequency stabilization and absolute stabilization is continuing The. single mode output power of our injection locked Nd YAG laser has recently been. upgraded to 18 W and cw harmonic generation has be demonstrated at 6 5 W power. levels This is adequate power to pump even a single resonant optical parametric oscillator. In addition we will be studying cascaded harmonic generation to the ultraviolet and a. number of additional DRO and SRO schemes,Frequency Stable Operation of Nd YAG Lasers.
R C Eckardt T Day E K Gustafson and R L Byer,Ginzton Laboratory. Stanford University,Stanford California,94305 4085 USA. Semiconductor diode laser pumped solid state lasers are inherently frequency. stable when compared with other lasers and this stability can be improved by. active feedback control Monolithic nonplanar ring oscillators constructed from. solid pieces of the laser material provide better than 10 kHz frequency stability. over 0 1 sec intervals Active feedback stabilization of the cavity length has been. used to demonstrate 0 3 Hz stabilization relative to a reference cavity resonance. frequency This range of frequency stability has application in nonlinear optical. frequency conversion, Resonantly enhanced nonlinear frequency conversion techniques such as external resonant. cavity harmonic generation and optical parametric oscillation are particularly sensitive to the. frequency stability of the pump radiation High stability diode laser pumped solid state lasers. permit efficient cw second harmonic generation SHG and optical parametric oscillation OPO. An interesting aspect of this work is an analysis of the conditions required to produce single. mode pair optical parametric oscillation The exceptional frequency stability of the diode pumped. NonPlanar Ring Oscillator NPRO compared to dye lasers gas lasers or semiconductor lasers. also provides some unique advantages for active frequency stabilization Sub Hertz relative. frequency stability has been achieved by locking to reference cavities and absolute stability of a. few hundred Hz by locking to molecular or atomic absorptions appears possible. Remeasurement of some nonlinear optical coefficients is a by product that grew out of this. work when it was found that some of the accepted values were predicting results that were. inconsistent with experimental observations,1 The NonPlanar Ring Oscillator. The solid state monolithic NPRO 1 2 3 generates stable single axial mode output which is. important for resonant cavity nonlinear optical frequency conversion Semiconductor diode laser. pumping of the NPRO s 4 2 provides advantages of efficiency and reduced heat loading In. addition the low amplitude fluctuations of the diode laser pump source provide improved. frequency stability, A schematic illustration of a NPRO is shown in Fig 1 Spatial hole burning in the popula.
tion inversion is avoided by unidirectional oscillation in a ring cavity which is important for single. frequency oscillation The monolithic cavity non planar path includes three total internal. Fig 1 Semiconductor diode laser pumped Non Planar Ring. Oscillator NPRO, reflections and a fourth reflection at non normal incidence from a multilayer dielectric coated. curved output coupler The total internal reflections provide reciprocal polarization rotation and. non reciprocal rotation is produced by Faraday rotation when the laser material is placed in a. magnetic field The result is four nearly degenerate modes two for the different eigen. polarizations in each direction of oscillation One of the four modes wiLl have minimum loss. Relatively small loss differences result in single mode unidirectional laser oscillation Designs. have been produced that maximize loss difference to increase the resistance to optical feedback an. effect that can reduce frequency stability 5, The beamote linewidth of monolithic Nd YAG neodymium doped yttrium aluminum. garnet and Nd GGG neodymium doped gadolinium gallium garnet lasers has been observed to. be between 3 kHz and 10 kHz for short periods of time typically 100 ms 6 4 7 Over longer. periods of time the frequency drifts through greater excursions due to the temperature tuning rate. which is 3 1 GHz C for monolithic Nd YAG oscillators Temperature fluctuations in the. millidegree range produce a frequency wander of several MHz Short term stability of a few kHz. and MI Iz stability over periods of minutes however is adequate for a number of resonant cavity. nonlinear frequency conversion demonstrations And long term experiments can be carried out. when the laser is slaved to the nonlinear cavity by active frequency control The temperature. sensitivity can be used for tuning but the response is slow typically 1 sec Piezoelectric crystals. bonded directly to non optical surfaces of the monolithic lasers provide limited tuning range but. much faster response extending to frequencies of hundreds of kHz 8 9 A split servo with. fe back to both the temperature, andthepiezoelectric crystal can have both narrow jitter linewidth. and longtermfrequencycontroL,2 Pulsed Applications. Pulsed pumping of monolithic lasers usually results in a frequency chirp during laser output. causedby heating Continuouslypumped lasers reacha steady state. conditionwithconstant, temperature However thecontinuouswave outputcan be used toinjection.
seedhigh power, pulsed lasers or the cw laser output can be time gated and amplified Another way in which. stable frequency pulsed operation is obtained from the monolithic lasers is by applying a small. amount of modulation to the pump radiation at the relaxation oscillation frequency to produce. spiking in the laser output Each of these methods has been used to generate pulsed pump. radiation for optical parametric oscillators and the results of these investigations provide a. heuristic review of the requirement for achieving single nxxie pair oscillation of OPO s. a Injection seeded Q switched laser operation, High power Q switched lasers have been a traditional source of pump radiation for singly. remnant OPO s SRO s The short buildup times and high gain of the Q switched lasers usually. result in highly multimode output If the oscillation of one mode is injection seeded with a few. mW of cw radiation oscillation on that laser mode can buildup and deplete the population. inversion before other laser modes have a chance to grow to significant levels Several. commercial lasers are available that offer injection seeded operation with nearly time bandwidth. limited output of typically lO nsec duration, SRO s have been forced to operate resonated single mode signal even with highly. multimode pumping 10 The combined frequency selection of phase matching a dispersing. grating and an intracavity eta Ion were necessary The multi axial mode pump resulted in a. multimode output at the nonresonated idler field When this type of frequency control is used. with a pulse SRO pumped with an injection seeded Q switched laser the result is single mode. output at both the signal and idler wavelengthsJ 1 With the addition of piezoelectric control of the. cavity length and computer control of all the adjustable parameters spectrographic measurements. with 300 MHz resolution were possibleJ 2 These SRO s used angle tuned LiNbO 3 pumped at. The single mode pump alone is not sufficient to produce single mode pair oscillation in a. simple SRO with no frequency selection except that of phase matching This was observed in a. BaB204 SRO pumped with the 354 nm third harmonic of the output of an injection seeded Q. switched Nd YAG laserJ 3 The single frequency pump did reduce OPO output energy fluctuation. to 10 from 30 which was observed with multimode pumping when injection seeding of the Q. switched laser was blocked The plane parallel cavity SRO with 1 2 era long BaB204 crystal and. 3 era overall length pumped by a 6 nsec pulse would oscillate on typically 8 axial modes at the. signal wave The broad tuning range of the BaB204 OPO extending from 412 nm to 2 55 I tm. included both the 1 064 1am fundamental and 532 nm second harmonic of the laser which were. available for injection seeding the SRO With seeding the buildup time of the SRO oscillation. was substantially decreased and single mode pair oscillated was achieved Measurements of the. SRO threshold yielded values that were less than predicted from calculations based on reported. values of the nonlinear coefficient of BaB204 This lead to a reassessment of the scale for optical. nonlinear coefficients,b Nonlinear optical coefficient measurements. The injection seeded spatially filtered Q switched Nd YAG laser provided a suitable pump for. phase matched second harmonic measurements of the nonlinear optical coefficient High optical. quality in both the pump beam and the non mat al is needed for precise control of phase. matching which is also a requirement of these measurements The measmements included careful. characterization of phase matching for each observation In addition it was determined that there. were no extraneous spatial temporal or spectral components of the pump. The results of careful nonlinear optical coefficient measurements 14 are reproduced in. Table I The investigation which at first was motivated by the BaB20 SRO results grew to. include a total of six materials This was necessary because the earlier BaB20 4 measurements. were relative to KDP and a controversy exists over the value of the nonlinear coefficient of that. material The measurements described here were performed in a two beam setup which permitted. both relative measurements between two crystal and absolute measurements with harmonic power

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