Practical space-based coherent laser radar systems envisioned for global winds measurement must be very efficient and must contend with unique problems associated with the large platform velocities that the instruments experience in orbit. To compensate for these large platform-induced Doppler shifts in space-based applications, agile-frequency offset-locking of two single-frequency Doppler reference lasers was thoroughly investigated. Such techniques involve actively locking a frequency-agile master oscillator (MO) source to a comparatively static local oscillator (LO) laser, and effectively producing an offset between MO (the lidar slave oscillator seed source, typically) and heterodyne signal receiver LO that lowers the bandwidth of the receiver data-collection system and permits use of very high-quantum-efficiency, reasonably-low-bandwidth heterodyne photoreceiver detectors and circuits. Similar techniques are being applied in atmospheric CO2 differential-absorption lidar work, where MO sources need to be actively offset-locked to CO2 reference cells for continuous absolute-calibration purposes. Active MO/LO offset-locking is also highly applicable to lidar problems involving very high target velocities with respect to a static or moving lidar platform.

Figure 1. Tm,Ho:YLF MO/LO Offset Locking System incorporates the described improvements.
Efforts to date have focused on development of Tm,Ho:YLF lasers operating near 2.05 μm, which have much potential for both efficient space-based wind lidar systems and CO2 DIAL measurements. The locking techniques are readily applicable to any number of other wavelengths and laser formats.

Figure 2. Heterodyne Beat Note Stability is depicted when Tm,Ho:YLF MO and LO lasers are actively offset-locked 7.5 GHz apart. Locking stability is ≈30 kHz over many seconds. [SA = signal analyzer; RBW = resolution bandwidth; VBW = video bandwidth; SWP = duration of frequency sweep; Att = attenuation; Ag = antenna gain.]
Recent work on MO/LO offset locking has focused on increasing the offset locking range, improving the graded-In-GaAs photoreceiver performance, and advancing the maturity of the offset locking electronics. Figure 1 provides a schematic diagram of the offset-locking system. Improvements to the design of the tunable MO laser resonator resulted in continuous, fast, SLM piezo-tuning range of 25 GHz—more than double the range of the initial prototype. Major progress was also made in the performance of very wideband, 2-μmsensitive heterodyne photoreceivers. The fiber-coupled, hybridized-preamplifier photo-receivers developed most recently exhibited heterodyne detection bandwidth of 4 GHz to the 3 dB point, and adequate bandwidth to demonstrate robust offset-locking to 10 GHz. This advanced component is now offered as a standard product. Remarkably, these very small (30-μm active area diameter), thin, fast PIN (positive/intrinsic/ negative) devices exhibit ≈70 percent quantum efficiency to 4 GHz, adequate for direct use as a heterodyne receiver in many applications. With some degradation in locking robustness, MO/LO offsets of as much as 13.2 GHz were obtained. Settling times were typically 15 ms for 1 GHz steps, and locking stability was measured at 30 kHz over 20-s intervals. The system incorporated a LabVIEW-based GUI and robust auto-locking servo, greatly enhancing its usefulness in offset locking experiments and use as a wideband photoreceiver calibration instrument. Figure 2 shows a typical locking stability result.

This work was done by Sammy W. Henderson, Charley P. Hale, and David M. E’Epagnier of Coherent Technologies, Inc. for Marshall Space Flight Center. For further information, contact Kent Blanchard at or (303) 379-3264. MFS-31434

Photonics Tech Briefs Magazine

This article first appeared in the May, 2006 issue of Photonics Tech Briefs Magazine.

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