High-energy Nd:Cr:GSGG lasers based on phase and polarization conjugated multiple-pass amplifiers

Lasers based on Nd:Cr:GSGG low-energy oscillator Imultiple-pass amplifiers produced 1.7 J pulses in a M22 divergence beam at 2.4 % electrical efficiency. Thermal lensing and birefringence correction were major factors. L Introduction High energy, high efficiency laser development is advancing. Electrical efficiency of high energy, diode pumped YAG lasers can reach 5%. Flashlamp pumped YAG has an efficiency of 1 to 2 % in Q-switched power oscillators. Higher efficiencies and higher energies may be achieved using other flashlamp pumped media. Thus Cr3 in Nd:Cr:GSGG (gadolinium scandium gallium garnet) crystals absorbs over broad bands at 450 and 640 nm, and transfers energy to Nd via nonradiative and radiative transitions.1 A twofold improvement in electrical efficiency results. GSGG has a lower crosssection than YAG, resulting in twice the energy storage capacity.2 GSGG costs three times as much as YAG. Its thermal characteristics are inferior, resulting in significant aberrations at power levels 20% that of YAG.3 Thermal limitations manifest themselves as beam distortions and reduced output energy. In this work, phase conjugate mirrors (PCMs) and Faraday rotators (FRs) were used to correct phase distortions and birefringence in a quadruple-passed GSGG rod amplifier. IL Experimental Setup The high brightness laser consisted of a ThM 00 multiple longitudinal mode (MLM) oscillator followed by a telescope and then the PCMPA (figure 1). The GSGG oscillator was of the reentrant type and contained a 3 m radius back mirror separated from the 40% reflectivity output coupler and 100% reentrant mirror by a 1.5 m cavity. 4)Tilted etalons consisting of a 0.2 cm thick 20% reflectivity etalon and a 0.02 cm thick uncoated etalon were used to obtain multipeaked spectra with bandwidths (measured with a Burleigh Wavemeter) of 40 and 10 GHz (fw'/10m) without and with etalons. (PCM fidelity was better with the narrower bandwidth). Durations varied with PFN energy, but were kept at 50 to 70 ns. The beam into the PCMPA was circular, of Gaussian intensity profile without diffraction rings, and had a 0.5 cm beam diameter. The undersized input beam, with 0.5 fill factor, is typical of high gain, heavily saturated systems where unsaturated edge gain converts a Gaussian input into a flat-top output with only modest diffraction moduIation.5 The PCMPA consisted of a laser head surrounded by optics required for beam path control and aberration correction. The Kigre FDM 104 head housed a 1 by 10 cm rod and two close coupled flashlamps (xenon fill, bore =0.5 cm, arc length = 8.4 cm, K, = 22.7) in a diffuse reflector. The diffuser substantially reduced, but did not totally eliminate, pump inhomogeneities. The power supply, from Kigre, provided 150 s pulses at energies up to 91 J for average power to 2 KW. The GSGG amplifier rod was from Litton-Airtron. Rod faces were 6° parallel and AR coated. The barrel surface was initially fine ground, but was reworked to have 0.03 cm deep circular grooves to test whether whispering mode parasitic oscillations limited gain at the highest pump energies. The Nd and Cr concentrations were 1.65 and 2.47 at. % respectively. All optics used to control the beam were dielectric coated. AR coatings were important not only to minimize losses, but also to limit parasitic oscillations and stray damage producing beams. Because double pass gain could reach 730, reflections from AR coated surfaces had to be less than 0. 14 % to prevent parasitic oscillations with the 100 % reflectivity dielectric double pass mirror. This is lower than specified AR reflectivities, so all surfaces were tilted. Stray beams ( 10 mJ) were a hazard if they originated from or impacted on concave beam focusing surfaces. Damage occurred during testing. Faraday rotators (FR) were from Electro-Optics Technology (TGG crystals in permanent magnet housings). The damage issue was critical because once initiated, a black compound formed and rapidly increased in size. Oscillator pulse width was fixed by damage threshold. Beam overlap in the heavily saturated amplifier resulted in light electric fields that were largest in the second polarizer, where three passes overlapped. This is one reason that 50-70 ns pulses were. Extra protection (1.4X) to some optics was provided by inserting the 1/42. plate after polarizer 2. One reflection from a single phase conjugate mirror was used. This arrangement was based on past experience that showed beam quality degradation (filaments) as the number of single or multiple cell reflections increased. The present arrangement produced no hot spots, but stressed the PCM in that it had to correct for two passes of phase distortion. The PCM consisted of a stainless steel cell containing 70 atm of CH4. The rear of the cell contained a conical, diffuse scattering beam dump that eliminated parasitic oscillation inducing retroreflections. The cell was sealed with a 1 cm thick AR/AR coated window. The focusing lens was a R=50 cm planoconvex AR/AR coated lens. The lens could be tilted to add 48 SPIE Vol. 3092 • 0277-786X/97/$10.O0 astigmatism, and thus counter some of the astigmatism incurred during the first two passes. This resulted in better ray mixing within the focal region and in better high average power PCM fidelity. Uncorrected astigmatism in nonstigmatic focus PCMS has been reported.6 Methane was selected because its reflection characteristics were the least sensitive to input bandwidth of all materials tested by us. The long focal length lens insured that high energy ( 1 J) breakdown did not occur. CH4 is a slow SBS material with a characteristic response time of4O ns (Another reason to use 50-70 ns pulses). A divergence compensating telescope was placed between FR2 and the O' dielectric mirror. This telescope was added when it was found that strong thermal lensing caused the beam to focus onto the PCM input window resulting in damage. The telescope was set to produce a collimated double pass beam (It did nothing to compensate for astigmatism.). A secondaiy damage route that the telescope was to eliminate, was the residual beam that did not undergo birefringence correction after four passes and was reflected from the PCM for another two passes. This beam, too, could, focus down and cause damage. Beam collimation, to the extent possible, minimized this threat. The primary job of the telescope was damage prevention. It was the task of the PCM to correct phase distortions. Use of the telescope aided to a limited extent in birefringence correction since rays must exactly retrace to be fully effective. llI.Stored Energy Single pass small signal gain was measured to determine stored energy available for amplification. Stored energy Estored can be calculated using the relation EstoredhvA[Ln(G,,)]/c=O.49[Ln(G,,)J,7 where h is Planck's constant, v is the laser frequency, A is the beam area, G,, is the small signal gain, and c is the stimulated emission cross-section between the specific lasing sublevels. We take a=3x10'9 cm2, assume uniform gain cross section, and a beam that fills the rod aperture. Note that E,t01d represents stored energy available for pulsed lasing, i.e., without change in the Ri R2 sublevel energy distribution. The total energy stored in the 4F312 level is approximately 2.4 times as great as the calculated value. Measurements were performed by placing an Ophir 30A-P-CAL calorimeter after the first pass. A first series defined the oscillator input range over which small signal gain could be measured (< 0.5 mJ). Use of the equation for Estored together with G, data as a function of PFN energy, yielded energy storage efficiency (figure 2). Efficiency peaks at 2.5% and drops to 1.8% at the highest pump energies. The efficiency drop could be due to saturation in the Cr to Nd transfer efficiency, or due to increased losses by amplified spontaneous emission or parasitic oscillations. In order to check for whispering mode parasitic oscillation, the gain was compared to earlier data obtained before the rod was grooved. Results were identical, so parasitic oscillations appear to be an unlikely cause of efficiency decrease. In concluding this section, the stored to electrical pump energy efficiency of GSGG and YAG are compared. In previous work, YAG PCMPAs were developed and completely characterized. From this work, GSS(YAG) = 33 at a PFN energy of 88 J. For a stimulated emission cross-section of = 6x1019 cm2, the calculated stored energy efficiency is 0.0098. Thus, GSGG is measured to be 1.9 times more efficient than YAG in converting electrical into stored laser energy. lv. Birefrin2ence Correction A primary consideration in developing this PCMPA was to conjugate out static and dynamic birefringence. The second Faraday rotator (FR2) performed this task.5 Alternative schemes exist such as a "Sagnac" interferometer." To see how a polarization conjugator (POC) [FR or Sagnac] works, a simple model can be considered: A polarizer in front of the amplifier defines x,y axes. After one amplifier pass, the polarization is elliptical and the relative phase difference between components along the ellipsoid axes (x', y') is given by , where $ itself may be a function of x', y'. The beam passes through the POC and again through the amplifier. We want to calculate the E,, ES,, double pass components. The end result is: E,(double pass)=O and E(double pass)=(-)E0e. Output double pass polarization is linear, so there is no birefringence. The difference between the input and output is a 900 polarization rotation and addition of a e phase factor. Since i4 varies from point to point, the beam will suffer from a birefringence induced lensing. The POC requires exact ray retracing to fully compensate for birefringence. Thermal lensing does, however, occur whenever there is spatially dependent birefringence. A PCM used by the FR will correct for temperature dependent refractive index and rod length induced ray deflections, but not e' POC lens