| Summary: | 碩士 === 國立臺灣科技大學 === 電子工程系 === 100 === The direct-modulation of semiconductor lasers is the most compact and simplest choice to transmit data onto an optical fiber; however, their intrinsic limitations under direct-modulation such as wavelength chirp and inherent relaxation oscillation frequency constraints prevent their high-speed and long-distance capabilities. The injection-locking of semiconductor lasers improves the characteristics of free running laser's under direct-modulation, attracting a large degree of interest over the past decade.
In this thesis, the characteristics of semiconductor lasers under optical injection locking are presented and analyzed in various aspects. The measured frequency responses of optical injection locking show distinctive modulation characteristics, controllable by the frequency detuning between the master and the free-running lasers and injection ratios. These are confirmed both experimentally and theoretically. By increasing injection ratio and the frequency detuning, we have experimentally achieved a higher resonance frequency of 22 GHz only limited by spectrum analyzer which is more than four times that of free running resonance of 5.25 GHz. Along with increase in resonance, by optimizing the frequency detuning reduction in noise level is achieved. The noise level reduction at low frequency regime (0-10 GHz), implies that injection-locking technique can be rather robust for noise reduction.
The transmission performances of a radio-over-fiber (RoF) system were evaluated at RF frequencies of 2, 2.5 and 3 GHz 16 QAM WiMAX signal for broadband wireless access. Error vector magnitude (EVM) measurements were carried out to evaluate the performance both the free running and injection locked laser. The Injection locked laser can tolerate higher range of RF input power than that of free running with better signal to noise ratio and linearity. For 20mA bias current, the EVM value is less than 3.1% for all the range of measuring input power at RF frequency of 2 and 2.5 GHz. These entire enhancement in the performance of semiconductor lasers by optical injection locking, extend their applications to larger bandwidth, higher performance systems making directly modulated semiconductor lasers a much attractive choice as transmitter in analog optical communication.
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