Multi-wavelength distributed feedback laser array based tunable semiconductor lasers have been widely used owing to their high mode stability and simple tuning scheme. Comparing with the in-parallel laser array, the in-series one can avoid a large power loss caused by the optical combiner. However, the single-mode stability of an in-series laser array is vulnerable to the reflections of other lasers. Here we experimentally demonstrated a tunable in-series laser array with high single-mode stability and high uniformity of wavelength spacing. To reduce the impact of such reflections, the Bragg gratings in different lasers were designed in phase, and the grating phase error was reduced by utilizing the reconstruction-equivalent-chirp technique. Besides, a three-section laser structure was applied in each laser to increase the priority of the dominant lasing mode. Four lasers with small wavelength-spacings of 2.5 nm were integrated in the in-series laser array. According to the measurement results of 40 in-series laser arrays (160 lasers), wavelength deviations of 90.6% lasers were within ±0.2 nm, average wavelength spacings of 97.5% of the measured laser arrays were deviated from the design within ±0.2 nm, and SMSRs of 96.3% lasers were above 45 dB. The output power was above 25 mW and the relative intensity noise was below -130 dB/Hz. Like the in-parallel laser array, the proposed laser array can be applied in either continuous wavelength tuning or fast channel switching. For the continuous wavelength tuning applications, the 10-nm tuning range was obtained with a temperature tuning range of less than 20 °C. For the fast channel switching applications, 4 channels with 2.5-nm spacing are available, and the switching time between two channels was less than 300 ns.

We propose and experimentally demonstrate a novel multi-wavelength DFB laser with high mode stability and uniform spacing for the optical I/O technology. By employing the linearly chirped sampled grating and multiple equivalent π phase shifts among the cavity, the four-wavelength and six-wavelength lasers with 100 GHz spacing are implemented and experimentally validated. The proposed multi-wavelength DFB laser works with high mode stability, high uniform wavelength spacing, and high uniform output power with varied bias currents and operating temperatures. A semiconductor optical amplifier is monolithically integrated to amplify the output and the output power exceeds 25 mW when the current injected into the SOA is 120 mA. Besides, the relative intensity noise of the filtered wavelengths is below -130 dB/Hz for each wavelength, and clear 25 Gb/s NRZ eye grams are obtained for the filtered each wavelength with the external lithium-niobate Mach-Zehnder modulator, which indicates the capability of the proposed structure to be an alternative approach for the multi-wavelength optical sources in DWDM optical I/O links.

A four-channel wavelength-selectable in-series DFB laser array with good single-longitudinal-mode (SLM) property is proposed and theoretically analyzed. To realize good SLM property, a linearly chirped Bragg grating with multiple phase shifts are implemented in the laser cavity. In addition, two reflector sections with chirped Bragg grating are specially designed for reflection compensation at both ends. In our simulation, the split-step time-domain model with coupled-wave equations and carrier rate equations are used. Based on the simulation results, all the channels work with low threshold currents and the SLM properties are considerably enhanced compared with the conventional in-series laser array. Notably a high precision control of grating phase is required in the practical fabrication due to the ultra-small variation of grating period. Therefore, the reconstruction-equivalent-chirp technique is discussed to provide the fabrication of high-precision grating. In addition, the complex heterogeneous integration or butt-joint technique is not required during the whole fabrication process, which simplifies the fabrication and reduces the cost. The proposed wavelength-selectable in-series laser array is deemed to be applied in the dense wavelength division multiplexing technology, especially when fast wavelength switching is needed, such as in the application of NGPON2.

We have proposed and experimentally demonstrated a four-channel wavelength-tunable in-series DFB laser array with 100-GHz channel spacing. Compared to the in-parallel laser array, the optical combiner is not utilized and the extra power loss induced by the combiner is avoided. A linearly chirped Bragg grating with multiple phase shifts implemented in the laser cavity is utilized to reduce the grating interference among different lasers. Besides, the reflector sections are inserted at two ends to provide grating feedback for two side channels. With the feedback, all the channels work with low threshold currents. The linearly chirped Bragg grating requires precise control of grating phase variation, which is quite difficult for traditional e-beam-lithography-based method. Here we used the reconstruction-equivalent-chirp technique to fabricate the grating, and precise control of the grating phase is achieved. From the experimental results, the laser arrays work with high output power (>10 mW), relative intensity noise near the relaxation oscillation frequency of below -130 dB/Hz, stable single-mode operation, and high-uniform channel spacing. Fast channel switchings (<600 ns) are obtained by on/off laser switching. Such laser performance enables the proposed laser array to be applied in the dense wavelength division multiplexing technology, especially when fast channel switching is needed, such as the NG-PON2 system.

We propose and experimentally demonstrate an O-band multi-wavelength high-power DFB laser array for optical I/O technology. The proposed DFB laser array operates with 400-GHz wavelength spacing, which meets the CW-WDM MSA specifications. The reconstruction equivalent chirp technology is used to precisely control the grating phase and introduce the equivalent $\pi $ phaseshift in the laser cavity. The AR and HR films are coated on the front and rear facets to increase the output power. The equivalent $\pi $ phaseshift is placed near the HR film facet to improve the single longitudinal mode yield. The optical spectra were measured at a bias current of 150 mA, with the wavelength deviations of below 1.958 GHz, and the SMSRs of above 55 dB. Statistical analysis shows 94.8% of SMSRs are above 40 dB, and 90.0% of the wavelength spacings deviate from the designed value within ±12.5%. The laser output power is over 100 mW at the bias current of above 370 mA. The far-field divergence angle of the laser was measured to be $25.5^{\circ } \times 29.9^{\circ }$ and the measured Lorentzian linewidth is 1.68 MHz at the bias current of 150 mA. The proposed O-band multi-wavelength DFB laser array shows uniform wavelength spacing, high output power, and good single longitudinal mode stability, which is promising to be used as the multiple wavelength laser source in the optical I/O technology.

We demonstrate a two-section distributed feedback (DFB) laser for direct modulation based on the reconstruction equivalent chirp (REC) technology. The two-section DFB laser exhibits a stable single-mode characteristic. Utilizing the photon-photon resonance (PPR) effect and detuned-loading effect, the 3-dB modulation bandwidth of the two-section DFB laser is improved from 8 GHz to 23 GHz. A 25 Gb/s non-return to zero (NRZ) signal is applied to the two-section DFB laser and eye diagrams with large open extent are obtained. In addition, the relative intensity noise is reduced to below −143 dB/Hz. The frequency chirp is also suppressed. The chirp parameter is reduced from 5.67 to 1.37, which is a small value in directly modulated DFB laser. We compared the eye diagrams after transmitting different distances, the results show that the two-section structure reduces the frequency chirp significantly. The modulation of the two-section DFB laser also has good linearity, with a 1-dB compression point of more than 18 dBm.