Compact, passively Q-switched Nd:YAG laser for the MESSENGER mission to Mercury Danny J. Krebs, Anne-Marie Novo-Gradac, Steven X. Li, Steven J. Lindauer, Robert S. Afzal, and Anthony W. Yu A compact, passively Q-switched Nd:YAG laser has been developed for the Mercury Laser Altimeter, an instrument on the Mercury Surface, Space Environment, Geochemistry, and Ranging mission to the planet Mercury. The laser achieves 5.4% efficiency with a near-diffraction-limited beam. It passed all space-flight environmental tests at subsystem, instrument, and satellite integration testing and success- fully completes a postlaunch aliveness check en route to Mercury. The laser design draws on a heritage of previous laser altimetry missions, specifically the Ice Cloud and Elevation Satellite and the Mars Global Surveyor, but incorporates thermal management features unique to the requirements of an orbit of the planet Mercury. © 2005 Optical Society of America OCIS codes: 140.3480, 120.2830. 1. Introduction from the rest of the satellite. In terms of laser per- The Mercury Surface, Space Environment, Geochem- formance it is necessary to achieve more than 18 mJ istry, and Ranging (MESSENGER) mission to the of output energy in a near-diffraction-limited beam planet Mercury requires a laser altimeter capable of with ϳ6᎑ns pulses at an 8᎑Hz repetition rate, while performing range measurements to the surface of the the laser bench temperature is executing a thermal planet over highly variable distances and with a con- ramp from 15 to 25 °C at a rate of approximately 0.4 stantly changing thermal environment.1–3 Specifi- °C͞min. cally the satellite will execute an orbit with a periapsis of 200 km and an apoapsis of approxi- 2. Description of the Laser mately 15,200 km, with an orbital period of 12 h. For To satisfy the requirements above, we chose an oscil- the altimeter instrument, science observations are lator͞amplifier architecture with passive Q switching taken during the 0.5 h of closest approach to the (see Fig. 1). This approach is similar to one taken by planet. The laser must support the instrument re- the Geoscience Laser Altimeter System5 (GLAS) la- quirement of achieving a range resolution of less than ser instrument, previously developed by NASA for 40 cm to the surface of the planet. The anticipated the Ice Cloud and Elevation Satellite mission. The radiation exposure over the mission life is 30 krad primary difference between this laser and the lasers (Si), total dose, assuming an effective shielding of in the GLAS instrument is that only one amplifier 0.1 cm of aluminum.4 During the close approach to slab is used in this laser, whereas the GLAS lasers the day side of the planet the satellite is heating, and have two stages of amplification. The oscillator͞am- it is not possible to fully isolate the laser subsystem plifier approach has several advantages for this ap- plication: (1) The saturated loss of the passive Q switch affects only the efficiency of the oscillator sec- All authors are affiliated with NASA Goddard Space Flight Cen- tion, thereby enabling a reasonable overall efficiency ter, Greenbelt, Maryland 20771. S. Lindauer is with Northrop with a passive Q switch. (2) The small mode diameter Grumman Laser Systems, 2787 South Orange Blossom Trail, Ap- ͑ϳ0.1 cm͒ of the oscillator permits a compact and opka, Florida 32703. R. Afzal is with Spectra Systems Corporation, stable laser resonator with reasonable alignment tol- 321 South Main Street, Providence, Rhode Island 02903. A. Yu is erance. (3) The ratio of internal to external optical with Northrop Grumman Electronic Systems, 1745 West Nursery Road, MS1155, Linthicum, Maryland 21090. fluence is lessened relative to the oscillator-only sys- Received 4 August 2004; revised manuscript received 16 Novem- tems. ber 2004; accepted 19 November 2004. The oscillator section comprises a crossed-porro op- 0003-6935/05/091715-04$15.00/0 tical resonator6–8 with polarization output coupling, © 2005 Optical Society of America a Brewster’s angle Nd:Cr:YAG slab pumped by a sin- 20 March 2005 ͞ Vol. 44, No. 9 ͞ APPLIED OPTICS 1715 Fig. 1. Optical layout of the MLA laser. Fig. 2. View of the laser bench. gle two-bar stack of GaInAsP laser diode bars (Co- herent G-2), an air gap polarizer (Synoptics), a passive Q switch (Cr4ϩ, 0.46 optical density, Scientific Materials Inc.), zero-order quartz wave plates for po- passive Q switch and lessening of the output energy. larization control, and fused-silica Risley wedges for We used beam stops in key locations to intercept optical alignment. A thermoelectric cooler is em- backreflected energy. Unlike the oscillator pump ar- ployed between the pump array and the laser bench ray there is no direct control of the amplifier pump to keep the oscillator pump array at its design tem- array temperature. Therefore the output wavelength perature and output wavelength. The (0.5%) Cr3ϩ co- and pumping efficiency of the amplifier pump arrays doping of the Nd:Cr:YAG slab enhances the vary with the optical bench temperature. resistance of that element to radiation darkening.9 A(15ϫ magnification) beam expander10 is The nominal oscillator mode diameter is 1.0 mm, and mounted to the underside of, and perpendicular to, its output energy is 3.0 mJ in a 5᎑ns pulse. The phys- the laser optical bench. A quarter-wave plate be- ical length of the oscillator is 10.8 cm. The cavity Q tween the beam expander and the laser prevents was adjusted by rotating the angle of the quarter- backreflected energy from external optics from enter- wave plate to produce the Q-switched laser pulses ing the amplifier. The output polarization of the laser after 0.15 ms of pumping. subsystem is therefore circular. On the backside of The oscillator output beam undergoes a (2ϫ mag- the optical mount for the exit mirror, i.e., the mirror nification) beam expansion and is amplified 8.7 dB by that directs the output beam to the 15ϫ beam ex- a Nd:Cr:YAG amplifier slab. The slab is double- pander, we attached a diffuser plate to intercept the passed with the first pass p polarized with respect to leakage through the exit mirror. A quadrant photo- the Brewster endfaces and the second pass s polar- diode (UDT Sensors, SPOT-4D) staring at this dif- ized with respect to the endfaces. The input and out- fuser plate provides (1) a timing signal for put faces of the slab are coated to achieve low loss in terminating the laser diode pump pulse, (2) laser both p and s polarizations. The amplifier slab is energy monitoring, and (3) a start signal for the rang- pumped by two, four-bar stacks of GaInAsP laser ing electronics. To provide a greater signal level for diode bars (Coherent G-4). The pump arrays for the the critical function of triggering the ranging system, oscillator and the amplifier sections are operated in the signals from three quadrants of the detector are series, electrically, driven at a 100᎑A peak current. As summed to provide the ranging start signal. The re- described in detail below the laser electronics termi- maining quadrant of the detector provides the signal nates the laser diode drive pulse once a Q-switched for terminating the laser pumps and energy monitor- laser output pulse is detected. The isolation of the ing. The laser electronics is designed to provide a amplifier output beam from its input beam is accom- maximum laser diode drive pulse of 0.24᎑ms dura- plished by polarization. A 0.57-retardation wave tion. As configured the oscillator Q switches at plate together with the porro-prism reflector provides 0.15 ms, and the signal from the quadrant photo- a polarization change in the backreflected beam to diode terminates the laser diode drive pulse at that the orthogonal linear polarization. Size and weight time. As the laser ages the time to fire increases. The constraints preclude use of a Faraday isolator. Be- pump duration capacity of the electronics provides an cause the isolation level achieved with this approach approximate 35% specific gain margin to accommo- is only approximately Ϫ17 dB, we found it necessary date the pump array degradation and additional res- to offset the pointing of the input and output beams of onator losses at end of life. the amplifier so that the backreflected amplified The mechanical design of the laser utilizes a beryl- spontaneous emission from the amplifier does not lium optical bench ͑9.27 cm ϫ 14.1 cm ϫ 1.1 cm͒ for overlap the oscillator mode in the Q switch. reduced mass and enhanced thermal performance Overlap results in the premature bleaching of the (Fig. 2). A titanium spacer is used to optimize the 1716 APPLIED OPTICS ͞ Vol. 44, No. 9 ͞ 20 March 2005 Fig. 3. Laser output energy versus bench temperature, measured in vacuum. Fig. 4. Laser pulse temporal profile. Horizontal scale, 5 ns. related to the Fabry–Perot resonances of the oscilla- thermal isolation of the laser bench from the rest of tor. the instrument subsystem, and a small heater en- Figure 4 shows the laser pulse temporal profile. We sures that the laser bench starts each science obser- observed no mode beating in the large majority of vation period at a temperature of 15 °C. The weight pulses. allocations of 0.52 kg for the laser and 0.32 kg for the The laser maintained a stable TEM00 spatial mode laser electronics are satisfied. over the entire range of the thermal sweep (Fig. 5). The best Gaussian-fit divergence was 75 ␮rad (ϳ1.67 3.