High-efficiency and low-noise 1550 nm semiconductor lasers are essential for analog optical links to maximize the system spurious free dynamic range (SFDR), which is a key feature for numerous applications such as microwave photonics systems, signal distribution in broadband analog communications as cable TV (CATV), fiber-optic sensors, high-resolution spectroscopy as well as light detection and ranging devices (LiDARs). The buried heterostructure (BH) laser has proven to be effective at reducing the relative intensity noise (RIN) and the threshold current through tight confinement of charge carriers and photons within the device active region as defined by a lateral current-blocking structure. However, the BH laser requires an additional regrowth process, which greatly increases the process complexity and cost, and highly reduces the device reliability. By conducting dual-channel H⁺ ion implantation to restrict the current transverse diffusion, we design and fabricate a 1550 nm high-efficiency and low RIN fundamental transverse mode DFB laser, and study the RIN and linewidth characterizations.

We adopt the AlGalnAs material that exhibits sound temperature characteristics and high differential gain as a quantum well and waveguide layer to achieve high slope efficiency and high power. Additionally, an asymmetrical cladding is employed to reduce internal loss by lowering the optical overlap between the optical eigenmode and the p-doped layers. The dual-channel laser ridge-waveguide, 11 μm/2.5 μm/11 μm wide, is formed by inductively coupled plasma (ICP) etching (Fig. 2). Lateral current spreading is suppressed by proton implantation of 350 keV with doses of 1.0×10¹⁵ cm^-2 adjacent to the ridge (Fig. 1). In continuous-wave operation at room temperature, the RIN (Fig. 5), linewidth (Fig. 6), slope efficiency, and threshold current (Fig. 3) are analyzed.

In continuous-wave operation at room temperature, the threshold current of the designed H⁺ ion-implanted DFB laser is less than 40 mA (Fig. 3). With injection current of 200 mA, the output power is greater than 60 mW, the slope efficiency is greater than 0.35 mW/mA (Fig. 3), the RIN is less than -160 dB/Hz (Fig. 5), and the Lorentz linewidth is less than 200 kHz (Fig. 6). In comparison, the threshold current of the non-implanted DFB laser with the same epitaxial structure is above 50 mA (Fig. 4). With injection current of 200 mA, the slope efficiency is about 0.3 mW/mA (Fig. 4), the RIN is less than -145 dB/Hz (Fig. 5), and the Lorentz linewidth is about 350 kHz (Fig. 6). At the lasing threshold, the increase in series resistance from 2.0 to 2.5 Ω caused by H⁺ ion implantation decreases the slope efficiency from 0.6 to 0.48 mW/mA. With the increasing injection current, the current lateral spreading predominates to improve slope efficiency and RIN via H⁺ ion implantation.

By conducting dual-channel H⁺ ion implantation to restrict the current transverse diffusion, we design and fabricate a 1550 nm high-efficiency and low RIN fundamental transverse mode DFB laser based on AlGaInAs material. In continuous-wave operation at room temperature, the laser yields a threshold current of less than 40 mA. With an injection current of 200 mA, the output power is greater than 60 mW, the slope efficiency is greater than 0.35 mW/mA, the RIN is less than -160 dB/Hz, and the Lorentz linewidth is less than 200 kHz. The results show that H⁺ ion implantation limits the current lateral spreading, improves the slope efficiency, reduces the RIN, and narrows the linewidth. Finally, a simple and highly manufacturable method of creating a low RIN and high-efficiency DFB laser is created and demonstrated.