## 2 Experiment and Results  
Experimental configuration of the MOPA system includes an external-cavity master oscillator and a double-stage power amplifier, as shown in Figure 1.  
The master oscillator is a Littrow-type external-cavity oscillator, as shown in Figure 2. It is comprised of a Fabry-Perot laser diode with GaInP/AlGaInP quantum wells and AR/HR facet coating, an aspherical collimated lens with 2.75-mm focal length, a blazed grating of 1800 lines/mm, and a reflector. The first order of the grating feeds back to the laser diode for locking the wavelength and the zero order is the output of the master oscillator. The external-cavity length is about 6 cm, which is defined from rear facet of the laser diode to the blazed grating. The optical spectrum is shown in Figure 3. A single-frequency operation at 670.8 nm, with side mode suppression ratio (SMSR) of more than 16 dB, is observed. A piezoelectric ceramic component is attached to the back of the grating. By properly designing the rotation arm length and PZT moving step, the oscillating wavelength can be finely tuned at 0.2-pm resolution. The mode-hop free tuning range of 49 pm (32.6 GHz), from 670.820 to 670.771 nm, was achieved by tuning the PZT voltage from 1 to 69.9 V, as shown in Figure 4. Wider tuning range of more than 10 nm with mode hopping was also achieved, combined with a driving current tuning of the laser diode. The maximal output power is about 30 mW under CW driving current of 83 mA. The master oscillator is temperature controlled at 25 \({}^{\circ}\)C by two, single-stage, Peltier coolers for suppressing the working wavelength perturbation. Besides that, an optical isolator (Thorlabs IO-5-670-VLP) is inserted to protect the laser diode from the cascaded optical pre-amplifier. Figure 1: Configuration of the master oscillator power amplifier (MOPA) system. The inset shows the output and focused beam spots. DG: Dammann grating. Figure 2: Configuration of the tunable master oscillator. In order to supply sufficient seed power for the amplifier array, a SOA pre-amplifier is utilized. All the SOA chips in the MOPA system are uniform, which are based on GaInP/AlGaInP multiple quantum wells embedded in 150-nm AlGaInP p-waveguide and n-waveguide layers. The chips are designed and fabricated into 2-mm long, single-pass (with double-facet antireflection coating), tapered waveguide, with a tapered angle of 6\({}^{\circ}\). Figure 5a shows the SOA spontaneous emitting spectrum without seed injection. A total gain spectral range of 17 nm (FWHM), centering at 665 nm, was obtained. A \(\lambda\)/2 plate was inserted before the SOA to adjust the polarization of the seed beam, in order to maximize the amplified efficiency, as shown in Figure 1. When 22 mW seed from the master oscillator was injected, the amplified output power reached 610 mW at driving current of 1.0 A, as shown in Figure 5b. The small-signal gain is above 14 dB. A pair of D-ZLAF52 La aspherical lenses with 2.75-mm focal length were used for coupling the input light and collimating the output light. Besides that, a fused-silica cylindrical lens with 25-mm focal length was inserted to compensate the astigmatism of the tapered SOA. Figure 4: The wavelength tuning versus PZT piezoelectric ceramic control voltage. Figure 5: Gain spectrum without seed injection: (**a**) and P-I with seed injection; (**b**) of the 670 nm semi-conductor optical amplifier (SOA). Figure 3: The optical spectrum of the external-cavity master oscillator. For coupling the beam from the pre-amplifier to the amplifier array, multiple beam splitting was needed. We designed and fabricated a Dammann grating, with a profile of (0, \(\pi\)) phase step on fused silica substrate to function. The repetition period was 8.15 \(\upmu\)m and the split angle was 4.7\({}^{\circ}\). The total diffractive efficiency was 76% with AR coating. The optical power of each effective order was measured and achieved more than 50 mW, which means the uniformity of the split beams was good enough for our application.