Performance of Resonant Modulation in the mm-Wave Frequency Range: Multi-Subcarrier Modulation

Optical transmitters capable of efficiently transporting several millimeter-wave (mm-wave) subcarriers to/from fiber-fed antenna sites in indoor/ out-door mm-wave mobile/point-to-point wireless networks are of considerable importance in mm-wave free space links [101, 102]. Future deployment of a fiber infrastructure in these systems rests primarily upon the availability of low-cost mm-wave optical transmitters. Optical transmission of a single narrowband (50Mbs⁻¹) channel at 45 GHz was demonstrated using resonant modulation of an inexpensive, conventional semiconductor laser with a baseband direct modulation bandwidth of <5 GHz [103]. It was shown that this technique provides a means of building simple, low-cost, narrowband (<1 GHz) mm-wave subcarrier optical transmitters for frequencies approaching 100 GHz. In this chapter, the multichannel analog and digital performance of these transmitters at a subcarrier frequency of ̃40 GHz are described. Two-tone dynamic range is characterized in detail as a function of bias to the laser, and a maximum dynamic range of 66 dB-Hz^−2/3 is found. Although this is modest by conventional, say, CATV standard, it is adequate for serving a typical indoor picocell with a 40 dB variation in received RF power for a per-user voice channel bandwidth of 30 kHz and a carrier-to-interference ratio of 9 dB. A multichannel system implementation of resonant modulation is also presented in which two signals centered around 41 GHz operating at 2.5 Mb s⁻¹ BPSK are transmitted over 400 m of single mode optical fiber. The required RF drive power to the laser to achieve a bit-error-rate (BER) of 10⁻⁹ for both channels transmitting simultaneously is measured to be <5 dBm per channel. Based on these transmission results and by taking advantage of conventional wireless time-division multiplexing techniques in which up to eight users can share a single channel [104], these mm-wave links are potentially adequate in remoting signals from an antenna serving up to 16 mobile users in an indoor environment.

The setup used to perform two-tone measurements and multichannel digital transmission test of the mm-wave optical transmitter is illustrated in Fig. 10.1. The laser used was a GaAs quantum-well laser with a cavity length of ̃900 μm and emitting at 850 nm. First, the small-signal modulation response at the cavity round-trip frequency is measured. The modulation signal is delivered to the laser with the aid of a single-section microstrip matching circuit having a response shown in Fig. 10.2. The matching circuit reduces the reflection coefficient S₁₁ of the laser to −15 dB at 41.15 GHz as measured. Modulation response around 41 GHz is shown in Fig. 10.3 for several bias conditions. By simply adjusting the bias to the laser, a higher modulation efficiency is achieved at the expense of passband bandwidth [103]. At a modulation efficiency of −5 dB (relative to that at dc) the passband bandwidth is ̃200MHz.