Beat Note Signal Phase Noise Suppression Of Dual-polarization Fiber Grating Laser Through External Delayed Optical Feedback

In the field of optical fiber communication and optical fiber sensing, fiber laser is one important group of laser source, which finds a wide range of applications,including fiber-optic communications, laser radar, and some precision interferometric measurements, etc.. The dual-polarization fiber laser studied in this thesis can output two orthogonally polarized modes with different frequencies due to the intra-cavity birefringence which can generate a beat signal after photodetection. The frequency of the beat note is highly sensitive to the birefringence of the laser cavity. Moreover, the beat note normally shows advantages in narrow-linewidth, high signal-to-noise ratio(SNR), relatively stable and tuning friendly, etc.. Because the measurands can be detected based on the beat signal, dual-polarization fiber lasers are very popular in fiber optic sensing field.In order to improve the sensing performance of dual-polarization fiber laser,the noise of the beat note of a dual-polarization fiber grating laser should be decreased as low as possible. The thesis has studied the following issues regarding the beat note signal phase noise suppression of dual-polarization fiber grating laser through external delayed optical feedback:1. A summary of the operation theory and the application technologies has been presented for dual-polarization fiber laser. The recent development of frequency stabilization technology has also been studied. Based on those results, a scheme to suppress the phase noise of the beat note of dual-polarization fiber grating laser has been proposed based on external delayed optical feedback, which feedback a small fraction of the dual-polarization laser output to the laser cavity by a reflector through a single-mode fiber delay line to suppress the phase noise of the beat note. The theoretical fundamental of the proposed scheme has also been discussed.2. Experiment study: Different lengths of single- mode optical delay line have been employed as the feedback delay for external optical feedback. A Faraday rotationmirror(FRM) has been used as the reflector to reflect a small fraction of the output power of dual-polarization fiber laser back to the fiber laser cavity through the optical delay line. The stability of the beat note frequency has been observed and recorded to study the phase noise of the beat note for different lengths of the single-mode delay line.3. Based on the above results, a transmissive FRM has been inserted into the external optical delay feedback loop. A small fraction of the dual-polarization fiber laser output is reflected back to the laser cavity after polarization rotation due to the FRM.In this case the phase noise of the beat note can be suppressed significantly.4. By using a fiber Bragg grating as a reflector, the phase noise of the beat note can be reduced by feedback a small part of the laser output power to the laser cavity after delay through a fiber delay line. The results demonstrate that longer feedback time results in more phase noise reduction. However, the suppression will eventually saturate gradually. Meanwhile, the experiments also confirm that the polarization of the feedback laser shows significant impact on the operation of the fiber laser.