Abstract
The Mars Laser Communication Demonstration will use a PPM communication system operating from 25-Kbps to greater than 30-Mbps at 1064-nm with a 5-W transmitter. Design, performance, and qualification of the transmitter subsystems will be described.

Introduction
The Mars Laser Communications Demonstration (MLCD) [1,2] planned for launch in 2009 will use variable-rate M-ary pulse position modulation (M-PPM) at data rates from 25 Kbps to greater than 30 Mbps. Previously, binary-PPM transmitters have demonstrated variable-rate performance consistent with near-quantum-limited communication performance at 1550 nm wavelengths [3,4]. The MLCD transmitter design presented here is capable of transmitting variable M-PPM waveforms with 5 W average power at 1064 nm. The transmitter is based on a flexible master-oscillator, power-amplifier (MOPA) design [3] and will primarily make use of commercial-off-the-shelf (COTS) fiber optic components, which greatly accelerates development and brings modularity and simplicity to the design. The performance of flight-worthy components screened for sensitivity to radiation, shock, vibration, thermal cycling, and operation in vacuum is being evaluated in a laboratory breadboard system.
The variable-rate system uses 32-ary and 64-ary PPM signals with slot widths (pulses) of 1.6, 3.2, and 4.8 ns. A Ytterbium-doped fiber amplifier (YDFA) operating in saturation boosts the average signal power to greater than 5 W with peak pulse powers exceeding 320 W.

Figure 1: MLCD transmitter optical schematic.

Design
Figure 1 shows a schematic of the MLCD transmitter optical design. The master laser is a 1064 nm fiber distributed feedback (DFB) laser manufactured by Koheras, which outputs 10 mW when pumped with about 250 mW from a single-mode 976 nm pump laser. The 1064 nm signal exits the laser through a polarization maintaining (PM) wavelength division multiplexer (WDM). A polarizing isolator protects the laser from feedback.

Figure 2: Theoretical transmitter power penalty vs. extinction ratio for 32-ary and 64-ary PPM.

The CW master laser signal is modulated with a cascade of two LiNbO3 Mach-Zehnder modulators manufactured by EOSpace. The cascade is required to achieve sufficient (>35 dB) extinction ratio (ER) necessary to avoid signal losses at the output of the average-power-limited YDFA [5]. Figure 2 shows the power penalty as a function of ER for 32-ary and 64-ary PPM. Active bias control circuits keep the modulator DC bias optimized for high ER. The modulators are driven with 1.6 ns to 4.8 ns electrical pulses to generate the required PPM waveforms.
A YDFA preamplifier is used after the modulators to boost the signal power to greater than 1 mW to saturate the power amplifier. A fixed ~1 nm filter after the preamplifier reduces amplified spontaneous emission (ASE) noise. Higher order PPM systems have greater communication efficiency but require higher amplifier gain in order to extract full power. In our system, for 64-ary PPM there is about -17 dBm input power to the preamplifier and 37 dBm average (320 W peak) at the transmitter output which implies a net gain of ~54 dB. Fiber nonlinearities presently limit the peak powers that can be achieved to about 1 kW, although higher peak power systems are expected as fiber amplifier technology matures. As low duty-cycle communication formats take advantage of these developments, the requirement for low noise preamplifiers and narrow ASE filters will increase. The latter may impact transmitter capabilities by limiting, for instance, master laser tunability. In addition, the impact of the long-term space environment on preamplifier noise figure will need to be carefully considered. The availability of modulators capable of handling high average optical power without photorefractive damage would lessen these high gain requirements.
The system uses PM YDFAs manufactured by IPG Photonics. Double-clad fiber amplifiers pumped by multimode laser diodes efficiently generate high average (and peak) powers with a net wall-plug efficiency exceeding 15%. Uncooled multimode laser diodes are power efficient and can be reliably coupled to multimode fibers that deliver the pump light to the doped fiber. Furthermore, multimode pump fibers can be efficiently combined and coupled into the double-clad fiber, allowing for the use of redundant derated pumps. This also facilitates heat management since the fiber coupled diodes can be spread over an extended radiator area.

Figure 3: Measured preamplifier extinction ratio vs. master laser pump current for 32-ary and 64-ary PPM.

Performance
The properties measured in the breadboard transmitter operation include spectral width, on-off extinction ratio, electrical efficiency, power stability, and polarization extinction ratio. Performance over the range of duty-cycles and pulse widths used in the system is consistent with requirements for near-quantum-limited communication performance.
Figure 3 shows representative measurements of modulation ER after the preamplifier as a function of master laser pump current (input power), which indicate significant drive margin over ASE limited extinction. Such measurements performed with a typical high-speed oscilloscope are limited to about 20 dB. But for the low duty-cycle PPM waveforms employed here, greater than 40 dB extinction ratio is measured by raising the signal level into the photodetector to allow for accurate measurement of background leakage during the long off period between pulses. Although the photodiode output is saturated in the vicinity of the signal pulse, the much longer off periods allow for easy measurement of the background.

Space Qualification
In preparation for full space qualification, a preliminary screening of the fiber and electro-optic components has been completed. In contrast to operation in some Earth orbits [6], the Martian radiation environment is relatively benign and does not present a limitation for any of the components. Nevertheless, radiation sensitivity has been assessed through tests with a Cobalt-60 gamma radiation source. All components have been subjected to shock and vibration testing beyond the mission requirements. In addition, many of the components have successfully passed thermal-vacuum testing.
When possible, the transmitter design has incorporated mature Telcordia qualified components. However, 1064 nm components often lack telecom heritage or extensive field use. These components pose a greater risk and need additional screening, performance testing, and life testing in a more comprehensive space-qualification process in order to ensure reliable performance.

Conclusion
We have presented a design for the transmitter to be used for the MLCD program. The performance required for the mission has been demonstrated in a laboratory breadboard with a MOPA design employing many COTS components. The presentation will include additional details on the MLCD transmitter design, performance, and qualification.

References
[1] B. L. Edwards, et al., “Overview of the Mars Laser Communications Demonstration Project,” American Institute of Aeronautics and Astronautics Space 2003, Sept. 2003.
[2] S.F. Franklin et al., “The 2009 Mars Telecom Orbiter Mission,” IEEE Aerospace Conference Proceedings, pp. 1-20, 2004.
[3] D. O. Caplan, M. L. Stevens, and D. Boroson, “A Multi-rate Optical Communications Architecture with High Sensitivity,” LEOS 1999.
[4] M. L. Stevens, D. M. Boroson, D. O. Caplan, “A novel variable-rate pulse-position modulation system with near quantum limited performance,” LEOS 1999.
[5] D. O. Caplan, “A technique for measuring and optimizing modulator extinction ratio,” CLEO, May 2000.
[6] F. Hakimi, D. O. Caplan, H. Hakimi, and A. L. Tuffli, “Radiation effects on a two-stage double-pass single-polarization erbium fiber amplifier,” CLEO, May 2002.
1 This work was sponsored by the National Aeronautics and Space Administration under Air Force Contract F19628-00-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Air Force.