Objective The InP-based mode-locked semiconductor lasers have attracted considerable interest for their ability to generate pulses with high repetition frequency. However, they can hardly reach 1.7 μm wavelength range due to the difficulty on growing indium-rich highly strained quantum wells, which limits their application on optical frequency combs, medical photoacoustic imaging and gas detection. For the purpose of chip-scale sensing in the mid-infrared region, this paper designs a 1.7 μm InP-based monolithic mode-locked semiconductor laser.

Methods The laser structure was grown an (100) oriented n-InP substrate by Metal-organic Chemical Vapor Deposition (MOCVD). The undoped active zone of the laser contains three compressively strained 8-nm-thick In_xGa_1-xAs quantum wells separated by 12-nm-thick In_xGa_1-xAs_yP_1-y, which is enclosed between 300-nm-thick InGaAsP layers (Fig.1(a)). The MOCVD-grown wafer is processed into ~1.9-μm-wide ridge waveguide using standard optical processing (Fig.1(b)). The 4178-μm-long colliding-pulse mode-locked laser (MLL) is achieved with an ~168-μm-long saturable absorber located at the center of the cavity (Fig.1 (c)).

Results and Discussions The threshold current of the device is 83 mA without bias voltage, and the maximum output power is 25.83 mW (Fig.3(b)). According to the relation between external quantum differential efficiency and cavity length (Fig.3(a)), the internal loss of this epitaxial structure is calculated to be 14.378 cm⁻¹. An efficient mode locking has been achieved at 1.74 μm with a repetition frequency of 19.3 GHz, the narrowest linewidth of RF spectra is 14 kHz (Fig.4(a)), and the period of pulse train is 51.88 ps (Fig.4(b)). A microwave signal appears at a gain current of 130 mA when V_SA=−1.6 V, and its RF spectrum drops down to tens kHz with increasing current (Fig.5). Decreasing the reverse bias voltage from −1.4 V to −2 V with forward current at 520 mA, the laser emission spectrum gradually broadens (Fig.6). The 9.88-nm-wide spectrum contains more than 40 longitudinal modes spaced by 0.2 nm when V_SA=−2 V.

Conclusions This paper presents a monolithic colliding-pulse mode-locked semiconductor laser based on high-strained In_xGa_1-xAs/In_xGa_1-xAs_yP_1-y multi-quantum wells structure. The laser exhibits stable mode-locking operation with a repetition frequency of 19.3 GHz at 1.74 μm. By comparing the current and the reverse bias voltage influences on RF spectrum and emission spectrum, it proves that the monolithic InP-based device can provide a stable and efficient mode-locking above 1.6 μm range.