Laser Line-width Measurement And Research Of The External-cavity Performance Improvement

With the rapid development of laser technology, semiconductor laser with a narrow line-width,on kHz orders of magnitude or even less, has a very broad application prospects in the field ofcoherent communications, high-resolution spectroscopy, wavelength division multiplexing, opticalfiber sensing and so on. Thus improving the existing semiconductor laser’s performance, stabilizingthe frequency of the laser, as well as reducing the laser line-width has become a hot research. Further,accurate measurement of the laser line-width is required to characterize the system performance.The main contribution and innovation of this dissertation are as follows:1. First, this paper elaborates the semiconductor laser line-width theory and analyzes theadvantages and disadvantages of the traditional line-width measurement approaches. And this paperoriginally proposed the delayed self-heterodyne line-width measurement method based on theMach-Zehnder modulator (MZM). Both of formula derivation and experimental demonstration arepresented. The utilization of MZM’s carrier suppression modulation substitutes traditionalacousto-optic modulator’s (AOM) single-sideband modulation. The measurement data result fromexperimental comparison has fully demonstrated the feasibility of the program. Meanwhile, thephenomenon of the measured line-width increases with the delay of the optical fiber, were analyzed.2．Secondly, as the high requirements of semiconductor laser’s coherence in application fields, weanalyze the approaches of the external cavity feedback control of semiconductor lasers: opticalfeedback and electrical feedback. Following with the analysis of the common electrical feedbackfrequency stabilization methods such as Lamb dip frequency stabilization, saturated molecularabsorption frequency stabilization, fiber interferometer stabilization and FP resonator frequencystabilization, π-phase shifted fiber Bragg grating (Fiber Bragg Grating, FBG) is proposed as afrequency discriminator in the laser frequency stabilization system. Theoretical derivations of thecoupled mode theory and simulation analysis of the frequency discriminator properties have beencompleted. By optimizing the parameters, the3dB-bandwidth of the reflection spectrum is about4MHz, which satisfies the requirements of high-sensitivity frequency discriminator to provide highSNR error feedback signal for the feedback circuit.3. Finally, we design the overall block diagram of the frequency stabilization system based onπ-phase shifted fiber Bragg grating, and a detailed discussion of the various parts of the feedbackcontrol circuit is demonstrated, including a photoelectric receiver module, a local oscillator drive circuit, a phase identification module, a feedback control module.