## 3 Optical Characterization Measurement Results
### Hybrid External Cavity Laser Operation  
An external cavity laser is set up using a variation of the end-fire measurement system, see Figure 3. A commercial, packaged SOA (Kamelian SOA OPA-20-N-C-F-A) is used as the laser gain medium, lenses are used to couple the broadband SOA output to the a-Si PhC cavity which acts as a wavelength selective mirror, and a 98% retro-reflector coupled to standard single-mode fibre (Thorlabs P5-SMF28 ER-P01-1) is used to form the second mirror of the laser cavity. The 5 m length of the cavity in this instance is determined by the length of the fibre pigtails on the commercial SOA and the spacing between the lenses that couple light from the SOA to the a-Si chip. Since the PhC acts as a wavelength selective mirror only light with wavelength within the PhC resonance bandwidth gets reflected from the PhC back into the SOA. As the wavelength selected by the PhC mirror propagates in the SOA it is amplified. Further details concerning the operation of this type of laser can be found in [15]. The temperature of the SOA and the PhC cavity were separately maintained  
Figure 4: Measured optical transmission spectrum of the PhC cavity with lattice spacing 386 nm. The PhC cavity Free Spectral Range (FSR) is greater than 11.8 nm. Figure 5: Measured resonant wavelengths of the PhC cavity and their variation with PhC lattice constant spacing. For each of the modes it is observed that \(\Delta\lambda/\Delta a\,=\,-1.4\,\pm\,0.6\). at a constant temperature of \(T\,=\,25\,^{\circ}\)C. The SOA used in these measurements has a gain spectrum peak at \(\lambda\,=\,1560\) nm. The power-current characteristic for the laser with lattice spacing of 390 nm was measured using an optical power meter; see Figure 6. After correcting for the lens system insertion losses, approximately 10 dB, the laser is seen to output optical power of 4 mW for 100 mA of current across the SOA, which is more than double the output power observed in a similar system reported in [15] where 1.8 mW of output power was measured for 80 mW of SOA current. The laser threshold current is approximately 22 mA and is typical for lasers of this type; in [15] a threshold current of 23.6 mA is reported. The slope efficiency of the laser above threshold is 0.054 W/A. The optical spectrum of the laser output was measured on an Optical Spectrum Analyser (OSA) for several samples with different lattice spacings. Figure 7 shows the measured optical spectrum of three lasers with PhC cavity lattice spacings of 386 nm, 388 nm, and 390 nm operating in a single-mode regime with wavelengths of 1562.76 nm, 1569.42 nm, and 1577.95 nm respectively. Observed lasing wavelengths are consistent with the measured PhC cavity resonant wavelengths shown in Figures 4 and 5. The side-mode suppression ratio (SMSR) in each case is 26 dB, 24 dB, 33 dB. The current across the SOA in each case was 27 mA, and with the retro-reflector in place the SOA output power at this current was \(-10.9\) dBm. Single-mode operation of the lasers was not maintained as the SOA current increased. Figure 8 shows spectral broadening of the laser line as the SOA current increased from 30 mA to 90 mA. The spectrum bandwidth increased from 0.2 nm at \(I\,=\,30\) mA to 1.6 nm at \(I\,=\,90\) mA. The data shown in Figure 8 is for the PhC sample with lattice spacing of 386 nm, and similar behavior was observed with the other samples. Broadening is caused by the simultaneous lasing of multiple lines within the laser cavity. In a cavity of length 5 m the longitudinal mode spacing was approximately 0.33 pm. The bandwidth of the PhC resonances was approximately 0.25 nm which means that there were many modes available for lasing.