# Optical Injection Locking for Generation of Tunable Low-Noise Millimeter Wave and THz Signals
## 4 Results and Discussion
### Injection Locking with Two Lasers  
In this section, the usage of OIL as an active filter for selecting an optical tone from the OFC and amplifying is shown. As described in the previous section, an OFC is generated using a PM and is injected into the two slave lasers, which are tuned such that two of the comb lines correspond to the free-running frequencies of the slave lasers. Respective spectra of the comb (orange) and combined locked slave lasers (blue) are shown in Figure 7. To demonstrate the tunability of such a system, the same slave lasers are locked to different pairs of comb lines with different spacings (top and bottom of Figure 7). Robust tuning  
Figure 6: (**a**) Phase noise for different injection ratios. Optical spectra of free-running laser (**b**), and of locked laser with injection ratios of \(-31\) dB (**c**) and \(-21\) dB (**d**). of the slave lasers was provided by thermal tuning, which was limited by the current flow into the SOA section. Fine-tuning was provided using the phase section, but was similarly limited by the breakthrough voltage of the device. \"Selective amplification\" of two different sets of comb lines with spacings of 34.6 and 70.0 GHz was shown. In this context, \"selectivity\" represents the suppression ratio (SR) between the desired amplified lines and the remaining undesired tones of the original optical comb, which was as high as 37 dB in the combined signal from the two slave lasers. \"Amplification\" in this case refers to a power gain of 10 dB compared with the original optical tone. In this work, the measured lasers do not have tuning contacts on the DBRs, but this can be easily changed in future designs, which may significantly expand the tuning range of the slave lasers. Another useful addition to the design of the laser can be an external heater for each cavity. Further, a pair of slave DBR or DFB lasers can be designed to initially operate at different wavelengths by introducing differences in grating pitch, which can be used for selection of the different lines from a broadband OFC. Fine-tuning in such a system can be achieved by adjusting the RF frequency \(f_{rf}\) of the source used for comb generation. Figure 8 depicts a tuning of the RF tone, generated with two locked SLs, during continuous changing of the OFC spacing. In this measurement, one of the SLs was locked to the central line of the optical comb and the other SL was locked to the first harmonic (as in Figure 7 top). Both SLs were fixed and the injection ratio was \(-27\) dB. A tuning range of \(\sim\)1 GHz is observed, which is in good agreement with the locking range of the single laser itself (Figure 5b). Combined with the coarse-tuning by comb line selection, such easily realized fine adjustment provides flexible tunability thanks to the phase section within the cavity and the thermal tunability of the laser, which can be useful in both communication systems and spectroscopy applications. Analysis of the phase noise was also performed for the beating of the two locked lasers. Figure 9 illustrates the PN of the beating of each of the lasers with the remainder of the OFC, when one of the lasers was turned off (Figure 9a,b). Figure 9c depicts the beating of the two lasers, locked to the different lines of the same comb. Significant differences in performance  
Figure 7: Normalized optical spectra of frequency comb (orange) and locked slave lasers (blue) for two different spacings: (**top**) 34.6 GHz and (**bottom**) 70.0 GHz. Note: the apparent difference in noise floors results from the normalization of the spectra and is the shifted noise floor of the OSA rather than actual noise in the signal. in terms of phase noise of the two lasers are observed. For both lasers, PN has the similar profile, which copies the PN of the master signal, for frequency offsets up to 100 kHz, and the same PN profile was demonstrated for other lasers. However, for frequency offsets higher than 100 kHz, a significant divergence in PN level was recorded.