# Optical Injection Locking for Generation of Tunable Low-Noise Millimeter Wave and THz Signals  
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Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/pdf/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/111/11/111/11/111/11/111/111/111/111/111/111/111/111/1](https://www.mdpi.com/journals/pdf/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/111/11/111/11/111/11/111/111/111/111/111/111/111/111/1)]([https://www.mdpi.com/journals/pdf/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/111/11/111/11/111/11/111/111/111/111/111/111/111/111/1](https://www.mdpi.com/journals/pdf/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/11/111/11/111/11/111/11/111/111/111/111/111/111/111/111/1))  
pair of semiconductor lasers [9; 10]. Optoelectronic systems stand out due to their tunability, simplicity, and low propagation loss in optical fibers. The possibility and efficiency of the generation of highly tunable THz and sub-THz signals in the optical domain and transmitting them through a fiber were proven in many experiments [10; 11; 12]. At the same time, Ref. [13] demonstrated cointegration of lasers and a terahertz antenna on a single chip, which enables generation of highly tunable THz signals in the range of 0.15-3 THz. The main restriction of optoelectronics in the generation of sub-THz and THz signals is the low efficiency of the conversion of optical signal into electrical [14] and resulting low output power of the p-i-n and unitraveling carrier (UTC) photodiodes [9]. This problem can be solved with hybrid solutions based on combinations of the optoelectronic signal generators and electronic amplifiers for the THz signal [10]. In addition, simple optical THz generation circuits using two free-running lasers usually suffer from broad linewidths and a high level of phase noise, which makes them unsuitable for use in communications employing high-order modulation formats [4]. A similar situation is observed in THz spectroscopy if a high resolution is required for determining spectra of molecules with closely spaced or very narrow absorption or reflection lines [15]. Several techniques allow achieving high resolution with continuous-wave systems. One of them is based on quantum cascade lasers (QCL), which can provide high output power and low linewidth. However, QCL requires high temperature stability at cryogenic temperatures making it extremely difficult to use [16; 17]. Another way to generate low-noise THz signals is to lock two semiconductor lasers, which decreases the linewidth of the beating tone and, consequently, the linewidth of the THz signal. This can be achieved with an optical feedback loop [18; 19], but this method is limited by the bandwidth of the electronics components. The use of optical frequency combs (OFCs) for the generation of coherent optical tones is another alternative, but achieving the generation of a high-power THz signal requires additional filtering and amplification [20]. In order to overcome these limitations of OFC use while maintaining the benefits of using a wideband comb, recent work has demonstrated the implementation of optical injection locking (OIL) [21; 22]. Optical injection locking is a technique of frequency and phase synchronization based on the photon-photon interaction when external light is injected into the laser cavity [23]. This method is usually used to synchronize two lasers, but it can also be used to reduce laser linewidth or to filter out a single optical tone out of the optical frequency comb [24]. In [21; 22], the synchronization of monolithically integrated lasers employing OIL for sub-THz and THz signal generation was demonstrated. These are important steps for demonstrating of the feasibility of the generation of high-quality THz signals at room temperature with photonic integrated components.