This thesis describes fabrication and transmission experiments of the distributed feedback(DFB) laser diode(LD) module that is applied to the performance improvement of the mode-locked fiber laser gyroscope(MLFLG) using DFB-LD as gain medium.
The DFB laser module with stable single mode operation was developed for a 2.5Gbps optical transmission systems. DFB lasers with various active layer structures including Bulk, multiple quantum well(MQW), and strained-MQW in planar buried heterostructure(PBH) were fabricated by three step crystal growth and two step mesa etching. Typical characteristics of the fabricated DFB-LD shows side mode suppression ratio(SMSR) of more than 35dB and the threshold current of 11.5mA. The key parameters of DFB-LD such as linewidth enhancement factor, chirping, and differential gain were also investigated and compared for various active layers. Single mode fiber pigtailed DFB laser module was designed and fabricated. A single planar-convex GRIN lens was adopted for the simple construction of optical coupling system, and laser welding was used for assembling the submodule assemble for an accurate alignment of optical elements. The fabricated laser module shows 40% coupling efficiency and modulation bandwidth of 2.6 GHz. The optical transmission performances of the fabricated laser module were evaluated by a bit error rate(BER) measurement system. Received power of -33dBm was obtained after transmitting 47 km in a conventional optical fiber, and dispersion penalty was 0.7dB.
The second part of the thesis deals with a recently demonstrated MLFLG with a new form of an output in time domain. The output of the MLFLG is a series of mode-locked optical pulses, with two pulses per every round-trip time of light in the laser cavity. The separation of the two sets of optical pulses in time domain varies as a function of rotation rate. However, the MLFLG built with a fiber amplifier exhibits very strong gain competition between the two sets of mode-locked optical pulses, leading to detrimental errors in the measurements of rotation rate.
In this thesis, a solution to the above stated problem is presented by using a semiconductor laser amplifier(SLA) with a fast gain recovery for the gain medium of the MLFLG. The SLA used in the MLFLG was DFB-LD fabricated during this work and it proved itself as suitable gain medium and detector of optical pulses. Experimental results show an almost complete suppression of the gain competition resulting in the much improved operation of the MLFLG with stable optical pulses. An additional simplification of the optical circuit is also realized by using the SLA as a photodetector while providing the required gain for the MLFLG. With the stability of the pulses, we could analyze, for the first time, the performance of the MLFLG in rotation rate measurements. The shift of the timing of the mode-locked optical pulses as a function of rotation rate measured by a time interval analyzer (HP 5371A) agreed well with theoretical expectation. The mean time deviation of the time interval between each set of mode-locked pulses were also evaluated by a universal counter/timer (Tek DC5010) when the gyroscope is at rest. The rms time jitter was 11.08 ps around the mean value of 390 ps for integration time of 2.66 sec. The rms time jitter corresponds to about 40.6 mrad of optical phase error and 14.6 deg/h of rotation rate error. Considering the integration time of 2.66 seconds, the random walk noise is calculated to be 0.4 deg/√h. The offset of 390 ps from the half period of the applied modulation signal corresponds to 1.43 mrad of optical phase offset and about 515 deg/h of rotation rate offset.
