Owing to their compact size, light weight, high reliability, and long lifespan, tunable semiconductor lasers have broad application prospects in fiber-optic communication, fiber-optic sensing, and spectral analysis. In particular, modulated grating Y-branch (MG-Y) lasers, which are a type of distributed Bragg reflection lasers, are characterized by a wide tuning range, high output power, high side-mode suppression ratio (SMSR), and high response speed. However, MG-Y lasers are adversely affected by wavelength jumping and power fluctuations when tuned based on a wavelength-current look-up table (LUT). These issues limit the practical applications of MG-Y lasers; therefore, the tuning methods for MG-Y lasers must be investigated and optimized.

The phase-region tuning characteristics of an MG-Y laser were investigated, which resulted in the proposal of a novel tuning method based on increasing and decreasing currents in the phase region. First, the currents in the left and right reflectors were scanned along an arc trajectory, where smooth wavelength-tuning paths were fitted rapidly within a tuning range of 40 nm. Next, the phase-region currents were loaded in increments of +0.2 mA and -0.2 mA, which revealed the existence of phase-jump windows. To address this issue, a new target-wavelength retrieval scheme was designed. The overlapping tuning segments of the phase region were obtained from the non-uniform feature points to splice the wavelength maxima and minima while ensuring sufficient current compensation. The remaining fine-wavelength data were filled with arithmetic progressions. Finally, a closed-loop calibration model was developed, through which the expected tuning performance of the MG-Y laser was realized.

The stability of the proposed tuning method is attributed to two key aspects. The reflector current-tuning paths are located in the center of supermodes. Besides, the retrieved target wavelength is not only distant from the boundary of the overlapping tuning segments in the phase region but also exhibits monostable characteristics. An experimental system was used to evaluate the tuning performance of the MG-Y laser. First, an arbitrary wavelength-switching test was performed to verify the effectiveness and reliability of the optimized tuning method. The laser was tuned in increments of 0.1 nm from 1549.5 nm to 1550.5 nm and then back to 1549.5 nm to monitor wavelength switching at the same feature point, where 20 switches were regarded as one cycle. Subsequently, the tuning step was increased to 4 nm and the same operation was performed over the range of 1528?1568 nm to monitor wavelength switching between different feature points. The results show that the MG-Y laser not only effectively avoids the problem of wavelength jumps when switching any wavelength but also outputs lasers with stable wavelengths (1 pm drift in 12 h) and high SMSRs (>40 dB) for a long time. Second, to verify the effectiveness and practicality of the closed-loop calibration model, the MG-Y laser was tuned from 1528 nm to 1568 nm in increments of 4 pm. Because of the nonlinear tuning of the phase-region current and the periodic variations of the carrier absorption rate, the linear scanning test based on the uncalibrated LUT shows a maximum wavelength bias of 11.4 pm, in addition to sawtooth-shaped periodic power variations with a power drift of 2.31 mW (the corresponding absolute value of power is 0.895 dBm). By contrast, the wavelength and power are well controlled after calibration. The maximum tuning wavelength bias reduced to 1.1 pm with an accuracy of 0.243, whereas the maximum power drift is limited to 0.188 mW (the corresponding absolute value of power is 0.065 dBm) with a flatness of 1.49%, which verifies the feasibility of the MG-Y laser for practical engineering applications.

In this study, the phase-region tuning characteristics of an MG-Y laser were investigated, and a novel tuning method based on increasing and decreasing currents in the phase region was proposed. After the tuning paths were determined, the target wavelengths were retrieved from the overlapping tuning segments of the phase region to avoid wavelength jumps. Subsequently, the LUT was iteratively updated using a closed-loop calibration model, which improved the linear scanning performance of the laser. A corresponding experimental system was constructed to evaluate the tuning performance of the MG-Y laser. The results show the absence of wavelength jumps during arbitrary wavelength tuning based on the LUT, with SMSRs exceeding 40 dB and a wavelength drift of approximately 1 pm within 12 h. The tuning wavelength bias of the linear scanning across the entire wavelength range was less than ±1.1 pm, with an accuracy of 0.243 pm, whereas the output power drift was only 0.188 mW (the corresponding absolute value of power is 0.065 dBm), with a flatness of 1.49%. Thus, one can conclude that the optimized tuning method enables the MG-Y laser to achieve stable tuning over the 1528?1568 nm range at a wavelength spacing of integer multiples of 4 pm, thus demonstrating its practical application value.