TITLE AND SUBTITLE Phase Stability of Injection - Locked Beam of Semiconductor Lasers

An experiment on the phase stability of an injection-locked beam was done by using AlGaAs semiconductor lasers. The coherence of two beams from master and slave lasers was measured by interference between the beams in the Twymann-Green interferometer. The phase change of the output beam of the slave laser as a function of the driving current was measured in a Mach-Zehnder interferometer consisting of the master and slave lasers, and a value of 2.5 radians/mA was obtained. Introduction The improvement of AlGaAs crystal growth technology and the new laser cavity con guration have enabled the rapid development of room-temperatureoperated, high-power semiconductor lasers. Recently, the laser diode array (LDA) has achieved 38 W continuous wave (CW) output power (ref. 1), while a single stripe semiconductor laser did not exceed an output power of more than 100 mW. Most of the LDA's developed earlier had shortcomings| such as a widely distributed spectrum and an uneven spatial distribution of amplitude|and are perhaps adequate only for pumping solid-state lasers. Accordingly, to overcome these shortcomings, continuous e orts have been poured into the development of room-temperature-operational, high-power, di raction-limited LDA's (ref. 2). The applications of the LDA are divided into two categories: (1) spectroscopy and coherent transmission require a narrow bandwidth of the output laser beam and (2) solid-state laser pumping, beam combining, manufacturing, and beacon beams generally require high-power output. The increase in beam power of a single laser diode array has a limit, so beam combining is necessary to obtain a high-power output (refs. 3 and 4). The output-power improvement in a single-chip base is achieved either by phase coupling the evanescent wave between parallel cavity stripes or by phase coupling Y-junction arrays (ref. 5). However, the power from a single-chip diode seems to be limited to only tens of watts; hence, high-power output beyond that of a single chip requires the combination of a master oscillator and power ampli er (MOPA) diode system. The beam from the master oscillator is split into several beams, and then the split beams are fed into every ampli er diode element simultaneously. Mutual coherence of the beams from the ampli er array is important and achieved either by injection-locking or by travelingwave ampli cation (refs. 6 and 7). The injectionlocking ampli er (ILA) has a narrow gain bandwidth (in a magnitude of GHz) but has the advantage of low noise and high ampli cation with a simple system con guration. On the other hand, the traveling-wave ampli er (TWA) has a wide gain bandwidth (in a magnitude of THz) but also has a slightly noticeable drop in power extraction e ciency. Manufacturing requirements are also more stringent for the TWA, as a thinlm facet coating is required to reduce the re ectivity to the order of 1 percent. In the experiments, the variability of the degree of interference and phase stability of the ILA, which are the important factors for beam combining, was studied. The variation in driving current of a semiconductor laser causes changes in electron density and temperature. These changes eventually cause the refractive index to change and give rise to an effect similar to cavity-length variation (refs. 8 and 9). Accordingly, the frequency from a self-operating diode laser may change. The slave laser which has a xed frequency by the injection-locking method is subject to phase variation. The fareld pattern from a laser diode ampli er array is therefore a ected by phase mismatching. Thus, the information on the phase variation due to the change in driving current of the laser diode array becomes very important for the quality of the fareld beam pattern. Correlation of Driving Current and Phase The beam-combining methods are determined by the requirement of beam coherence or incoherence. For instance, wavelength division multiplexing (WDM) is an incoherent beam-combining method. On the other hand, ILA and TWA are regarded as coherent beam-combining methods (refs. 10 and 11). Accordingly, for ber optic telecommunications which carry various multifrequency signals in a single ber, WDM is necessary. To yield high-output beam power, coherent beam-combining methods are required. When gain saturation is induced by the injected beam intensity (quenching e ect), an output power which carries only the mode of the injected beam is generated (ref. 12). Thus, this output beam has the same phase and frequency as the injected beam. This principle can be e ectively implemented for the generation of a single (coherent) high-power beam, as shown in gure 1. The beam from a master laser is split into several beams, and then each split beam is injected into the respectively aligned slave lasers.