Abstract
A novel approach to monitoring the laser injection-locking (IL) state is proposed and verified using the side-mode suppression ratio (SMSR). In a photonics experiment for laser IL, an optical spectrum analyzer with the conventional criterion of a 35-dB SMSR is conventionally used to detect the locking state of a Fabry–Pérot (FP) laser with multiple longitudinal modes to an external master laser with one longitudinal mode. Since the 35-dB criterion is not always a sufficient locking condition, we propose a microwave-photonic technique to determine the stable-locking regime based on the observation of the radio-frequency (RF) components. A novel approach to monitoring the generated additional spectral components uses the well-known delayed-self-homodyne technique and the RF spectrum analyzer. For the novel generation of additional longitudinal groups on each FP laser’s resonator mode in the optical spectrum and consequently the overlapping RF components in the RF spectrum, an additional external resonator with low reflectivity was connected to the slave FP laser. The novel monitoring approach was experimentally verified by connecting a 1-m-long external cavity with 0.5% reflectivity and observing the optical IL phenomenon of a 1550-nm FP semiconductor laser.
Highlights
Accepted: 27 October 2021In the last two decades, injection locking (IL)—a nonlinear phenomenon for synchronizing outputs where two or more self-holding lasers are coupled—attracted much attention due to its numerous applications in optical communications [1,2,3,4], optical signalprocessing systems [5], oscillators [6,7,8], laser cooling [9], optical phased arrays [10], etc
A conventional observation of the injection-locked stability of a FP laser is performed with the setup shown in Figure 5, where a single-longitudinal-mode laser is used for the master laser, whose measured linewidth is shown in Figure 4, while the multilongitudinal-mode FP laser is used for the slave laser
The stability analysis focuses on the case of positive frequency detuning, since the of th instabilities are more pronounced in this region [16,24]
Summary
Accepted: 27 October 2021In the last two decades, injection locking (IL)—a nonlinear phenomenon for synchronizing outputs where two or more self-holding lasers are coupled—attracted much attention due to its numerous applications in optical communications [1,2,3,4], optical signalprocessing systems [5], oscillators [6,7,8], laser cooling [9], optical phased arrays [10], etc. In the IL regime, most of the fundamental limits of the laser, such as the mode-distribution noise, relaxation oscillation frequency, nonlinear electron-photon coupling, relative intensity noise, etc., are improved. Using the IL technique, a Fabry–Pérot (FP) laser with multiple longitudinal modes can be operated in a single longitudinal mode, which greatly improves its properties [11,12]. The performance of optical IL and direct-modulated laserdiode emitters are investigated to operate as single longitudinal mode lasers with reduced linewidth [13], frequency chirp reduction [14], improved modulation bandwidth [15], and so on. Due to the complex system dynamics of an injection-locked FP laser and its extensive dynamic behavior, it is very difficult to determine the stable-locking conditions, especially in the boundary regions [16]. The IL can be unstable, especially as a result of temperature fluctuations, vibrations, and other possible environmental effects, where an already-locked slave laser can be unlocked by external environmental effects
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