20 April 1995 Testbed measurements of subcarrier OQPSK versus digital OOK laser intensity modulation at 266 to 1244 Mbps
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We report here on measurements made on a lasercom crosslink hardware testbed built on internal MITRE funds. Laser diodes rated at 150 mwatts were characterized to be flat to 100 MHz, rolling off 5 dB at 500 MHz. A microstrip laser driver with equalizer was implemented to provide flat, highly linear frequency response for analog modulation out to 550 MHz. Microstrip and hybrid versions of APD receivers were also fabricated with flat, linear response to 700 MHz (typically -3 dB at 850 - 1000 MHz), suitable for datarates up to 1.25 Gbps. The optical crosslink testbed with equalized driver, laser, and APD receiver exhibits +/- 0.25 dB flatness, +/- 2.5 degrees phase linearity deviation, and +/- 0.25 (eta) sec group delay variation over the full bandwidth for 650 Mbps. This testbed was evaluated with two modulation approaches: analog laser intensity modulation using an OQPSK subcarrier scheme, and baseband digital NRZ OOK laser intensity modulation, at datarates from 266 to 1244 Mbps. The QPSK subcarrier hardware characterization includes the high speed modem/demodulator and a pair of frequency converters. The digital NRZ hardware characterization includes the clock recovery and amplitude/timing decision circuit. Both the subcarrier QPSK and the digital OOK tests used the same laser and equalized laser driver, the same modulated laser power with 95% intensity modulatino depth, and the same APD receiver for these tests. As such, the comparison between these modulation schemes is an even-handed comparison of the end-to-end performance. Results and conclusions are presented.
© (1995) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Robert T. Carlson, "Testbed measurements of subcarrier OQPSK versus digital OOK laser intensity modulation at 266 to 1244 Mbps", Proc. SPIE 2381, Free-Space Laser Communication Technologies VII, (20 April 1995); doi: 10.1117/12.207414; https://doi.org/10.1117/12.207414

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