# A Compact Self-Injection-Locked Narrow-Linewidth Diode Laser with Narrowband Dielectric Filter
###### Abstract
As shown in Figure 1a, high-refractive films using eight-layer pairs of Ta\({}_{2}\)O\({}_{5}\) and SiO\({}_{2}\) are first deposited on the 8-inch BK7 glass circular substrate, with an effective thickness of 1/4 \(\lambda\) for each layer. Then \(\sim\)12 \(\upmu\)m Ta\({}_{2}\)O\({}_{5}\) is deposited on the refractive film as the resonance cavity layer; hence, the theoretical FSR is \(\sim\)11.8 nm. Finally, more eight-layer pairs of Ta\({}_{2}\)O\({}_{5}\) and SiO\({}_{2}\) with the same reflectivity are deposited on the cavity layer as the second reflective layer. For the 97%-reflectivity layer, the corresponding finesse is \(\sim\)103.1; hence, the theoretical full-width half-maximum (FWHM) bandwidth is \(\Delta v=0.114\) nm. In our fabrication, 10 pieces of 8-inch HFFPF are coated in the same batch, and 3 mm \(\times\) 3 mm square blocks of HFFPF are sufficient for the self-injection locking experiment. Therefore, \(\sim\)23,000 narrow-linewidth lasers can be made from a single batch of coating, which is beneficial for mass production (In this estimation, we considered the edge effect for coating and only used the central area with a diameter of 197 mm in each wafer for HFFPF fabrication). Figure 1: (**a**) Structure of the HFFPF. The dark blue layer is the glass substrate, and the two yellow layers are high-reflection films, while the blue layer is the resonance cavity formed by 12-\(\upmu\)m Ta\({}_{2}\)O\({}_{5}\). The inset shows the structure of the high-reflection film, which is formed with eight-layer pairs of Ta\({}_{2}\)O\({}_{5}\) and SiO\({}_{2}\), with a designed reflectivity of 97%. The Ta\({}_{2}\)O\({}_{5}\) and the SiO\({}_{2}\) layers are marked in blue and pink, respectively. (**b**) The measured transmission spectrum of HFFPF. (**c**) The setup diagram for the transmission spectrum measurement. L: aspherical lens; BS: beam splitter; PD: photodetector. Then, we measure the transmission spectrum of the HFFPF using a tunable external cavity diode laser (DL PRO, TOPTICA Photonics AG, Munich, Germany) with a linewidth of 10 kHz. The measurement setup is shown in Figure 1c, where the fiber-coupled laser beam is directed to a 50:50 fiber coupler for the separation into two output ports. One port is connected to a wavelength meter (WS Ultimate, High Finesse GmbH, Tubingen, Germany) for the precise monitoring of the laser wavelength during the tuning. The laser beam in the other port is used for the HFFPF transmission measurement, which is collimated into free space by an aspherical lens and further split by a 50:50 cubic beam splitter. The transmitting beam is focused onto the HFFPF and then detected by a photodetector (PD). The reflecting beam is detected by another PD for the intensity normalization against the laser power fluctuation during the scanning process, with a wavelength sweeping from 772.5 nm to 785.5 nm. The measured HFFPF transmission is plotted in Figure 1b, and the FSR is measured to be 11.2 nm. There is a small deviation between this value and the theoretical one. It is because to achieve resonance at 785 nm, the actual resonance cavity thickness must be slightly larger than 12 \(\upmu\)m. The transmission peak is further zoomed-in in the inset of Figure 1b, where its FWHM bandwidth is fitted to be 0.164 nm. Compared to the theoretical value, the experimental bandwidth is broadened because the actual reflectivity, due to fabrication deviation, is slightly lower than 97%. For the laser source in the self-injection, we use a low-cost FP-LD with a TO-can package working at around 785 nm. With a 102.6 mA driven current supplied by a commercial constant-current laser driver module, it has an output power of 49.8 mW. Then we build the self-injection locking setup, as schematically shown in Figure 2. The LD output is first collimated by an aspherical lens, with the beam waist on a 90%-transmissivity partially reflecting mirror, which is used to generate feedback for the self-injection locking.