# Optical Injection Locking for Generation of Tunable Low-Noise Millimeter Wave and THz Signals
## 3 Materials and Methods--Experimental Setups and Employed Lasers  
Figure 2 shows the experimental setup used to demonstrate sub-THz signal generation based on injection locking of a pair of lasers to different tones of an OFC. The optical signal from an external, off-the-shelf laser with a linewidth less than 100 kHz (CoBright-DX1) was modulated with an external phase modulator (PM, PM-DVES-40-PFA-PFA-LV) to generate OFC. The PM was driven with a sinusoidal signal \(f_{RF}\) boosted by a power amplifier (ERZ-HPA-3000-4000-32-E) with an operational bandwidth of 30-40 GHz. Modified electrodes of PM are able to dissipate high RF powers up to 2 W. This configuration allows generating an OFC with a width above 300 GHz and repetition rate \(f_{RF}\). To monitor the OFC, a 50:50 coupler was used to direct half the signal to an optical spectrum analyzer (OSA). The remainder of the signal was split again into two parts and injected into the two on-chip slave lasers (SLs) through circulators. The frequencies of the free-running lasers were tuned to match one of the comb lines of the OFC. Subsequently, the signal from both SLs was mixed and directed in equal parts to the OSA and the photodiode (PD). The frequency of the beat note, generated on PD, is equal to the frequency difference of the two selected optical tones, and the electrical signal of PD can be expressed as  
Figure 1: General schematic of using a combination of optical frequency comb and injection locking for generation of THz wave. SA—“selective amplification”; SP—signal processing; PCA— photoconductive antenna. \[E_{\text{RF}}\propto\sqrt{P_{1}P_{2}}\cos(\omega_{RF}t+\phi(t)) \tag{1}\]  
where \(P_{1}\) and \(P_{2}\) are the powers of the optical tones, and \(w_{RF}=w_{1}-w_{2}\) and \(\phi(t)\) describe phase variations conditioned by frequency instabilities of the free running lasers and differences in optical path length between two channels. The signal from the PD was amplified by a low-noise amplifier (LNA) and analyzed using an electrical spectrum analyzer (ESA). Slave lasers were represented by monolithically integrated DBR lasers (Figure 3), fabricated with standard MPW run [39]. Using MPW service significantly decreases the costs of the fabrication process due to the fact of shared costs with all participants of the run. However, on the other hand, such manufacturing imposes restrictions on the choice of components, since only building blocks represented by the platform can be used. The cavity of the laser was formed by a DBR mirror and broadband multimode interference reflector (MIR). The DBR has a uniform pitch of 236 nm, which determines a central reflection wavelength of roughly 1550 nm. The gain was provided by two semiconductor optical amplifier (SOA) sections. The wavelength was adjusted by thermally tuning cavity length by changing the feed current of the lasers and fine-tuning was carried out by reverse biasing of the phase section. The output waveguides of the lasers are bent at an angle of \(7^{\circ}\) at the chip facets to suppress reflections. A set of two chips with different types of lasers with a different length of DBR and SOA\({}_{1}\) were used in this study, as shown in Table 1. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Laser** & **DBR, (\(\mu\)m)** & **SOA\({}_{1}\), (\(\mu\)m)** & **SOA\({}_{2}\), (\(\mu\)m)** & **PS, (\(\mu\)m)** & **Chip** \\ \hline Laser 1 & 350 & 500 & 100 & 500 & 1 \\ Laser 2 & 250 & 500 & 100 & 500 & 1 \\ Laser 3 & 250 & 500 & 100 & 500 & 2 \\ Laser 4 & 250 & 400 & 100 & 500 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the InP DBR lasers employed as slave lasers. Figure 2: Experimental setup for sub-THz signal generation based on injection locking of a pair of InP DBR lasers to different lines of an optical comb. PM—phase modulator; OS—optical switch; OSA—optical spectrum analyzer; PD—photodiode; LNA—low-noise amplifier; ESA—electrical spectrum analyzer.