Radio-over-fiber uplink transmission using semiconductor laser dynamics


Student thesis: Master's Thesis

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Award date4 Oct 2010


Combining the capabilities of photonic and microwave technologies, radio-over-fiber (RoF) has emerged as a promising solution for the next generation wireless communication systems. In RoF, an optical carrier is modulated by a subcarrier microwave signal, which is then transmitted between the central office and remote base stations through optical fibers. RoF has the attractive advantages of centralizing high-speed electronics, low signal attenuation, large bandwidth, immunity to radio frequency interference, and high cell density. In this thesis, RoF uplink transmission based on the nonlinear dynamics of an optically injected semiconductor laser is investigated. A master laser at the central office optically injects a slave laser at the base station. The slave laser exhibits nonlinear dynamical period-one (P1) oscillation such that the output intensity is modulated at a microwave frequency, which is regarded as the RoF subcarrier frequency. When the slave laser receives an uplink current modulation in tune with the P1 oscillation frequency, it is double-locked by both the optical injection and the current modulation. Uplink microwave phase-shift keying (PSK) signal is converted to optical PSK signal accordingly, which allows both electrical and optical demodulations at the central office. The approach offers large tunability of the subcarrier frequency through optical injection, bandwidth enhancement beyond the relaxation resonance frequency, and elimination of high-speed electronics. Experimentally, RoF uplink transmission is demonstrated at 16 GHz using simply a 2.5-Gbps-grade single-mode semiconductor laser. By employing an optical delay-line interferometer, differential demodulations into non-return-to-zero (NRZ) and return-to-zero (RZ) signals are achieved. The bit-error rate (BER) remains below 10-9 with a temperature tolerance of at least 10°C through compensations of optical injection. In order to investigate the feasibility of operating at the 60-GHz window, numerical simulations are conducted using the well-established rate-equation model. The characteristics of double-locking are quantified by the associated phase variance as the modulation index and the modulation frequency vary. Compared to stable locking, the double-locked P1 oscillation requires a smaller modulation index for low BER due to bandwidth enhancement. With a moderately strong injection strength of 0.25, uplink transmission with BER below 10-10 at 60 GHz is possible by optimizing the injection detuning frequency. The results illustrate the capability of using double-locked semiconductor lasers for optically controlled RoF uplink transmissions.

    Research areas

  • Wireless communication systems, Semiconductor lasers