Continuously Tunable Narrow Linewidth External-Cavity Semiconductor Lasers

Objective The advancement of continuously tunable, miniaturized, narrow-linewidth external-cavity semiconductor lasers holds a key position in applications such as continuous-wave LIDAR and quantum technologies. By coupling a distributed feedback (DFB) laser with a short Fabry-Perot (FP) cavity and using piezoelectric ceramics (PZT) for cavity length modulation, a miniaturized tunable laser system operating at a wavelength of 1550 nm is realized. Experimental validation using a Mach-Zehnder interferometer (differential fiber length of 1 m) reveals a mode-hop-free tuning range exceeding 10 GHz. The frequency tuning ranges of 25.0, 21.6, 18.0, and 10.0 GHz are demonstrated at the repetition rates of 0.1, 1.0, 10.0, and 100.0 kHz, respectively. Additionally, utilizing the self-delayed heterodyne technique, the laser exhibits a 3 dB linewidth approaching 10 kHz. Forthcoming endeavors would aim to achieve a broader tuning range with a narrower linewidth by optimizing the optical pathway and modifying the cavity mirror reflectivity. Methods This study introduces a meticulously designed butterfly encapsulated laser module with dimensions of 20.8 mmx 12.7 mmx 8.9 mm. The configuration leverages a dual-output 1550 nm DFB laser diode that is controlled in terms of both current and temperature via an in-house circuit system. A schematic of the laser architecture is shown in Fig. 1. The optical resonator is formed by a planar mirror (M1) and concave mirror (M2), with M2 affixed to the PZT. This fosters cavity length modulation. Emissions from the DFB diode undergo meticulous collimation via a silicon lens and propagate within the cavity. Subsequently, these leak out of the cavity and are fed back into the diode. This results in a self-injection-locked state. The feedback phase is controlled using a heating resistor placed on top of a silicon lens. Results and Discussions Linewidth evaluations are conducted using a 20 km fiber-based self-delayed heterodyne measurement system. The spectral characterizations reveal a 20 dB linewidth close to 200 kHz, as shown in Fig. 3. The corresponding self-injection frequency-locking signal is shown in Fig. 2. To demonstrate the continuous-tuning capabilities of the laser, we use a function generator with two output channels to apply two triangular modulation signals to the PZT and the current of the diode with the same phase. Both feedback phase and amplitudes of the two modulation signals are optimized to extend the continuous tuning to the extent feasible. Using a fiber Mach-Zehnder interferometer with a differential length of 1 m, we measure a continuous tuning range of 25 GHz at a repetition frequency of 0.1 kHz. This is shown in Fig. 4. An increase of the repetition frequency to 100 kHz still permits continuous tuning exceeding 10 GHz. Conclusions We successfully develop a wide-range continuously tunable narrow-linewidth external cavity semiconductor laser. By scanning the DFB laser current, synchronizing the phase control, and manipulating the PZT to alter the effective cavity length, the external cavity laser consistently maintains its self-injection lock during dynamic tuning. This results in a continuous tuning range of 25 GHz and linewidth of 10 kHz at a low repetition frequency of 0.1 kHz. In scenarios with higher repetition frequencies, the laser achieves continuous tuning beyond 10 GHz. Future plans would involve the design of a more compact feedback optical path and optimization of the control circuitry to achieve a wider continuous tuning range.