# A Compact Self-Injection-Locked Narrow-Linewidth Diode Laser with Narrowband Dielectric Filter
###### Abstract  
The self-injection-locked diode laser can significantly reduce the linewidth down to a sub-100 kHz level but requires optical filtering that is much narrower than the laser cavity linewidth. Such optical filtering is usually sophisticated and beyond the capability of the normal dielectric coating approach. Here we develop a 0.16 nm narrowband dielectric coating using a Fabry-Perot dielectric filter (HFFPF) for the narrow linewidth self-injection-locked laser demonstration using a cost-effective Fabry-Perot laser diode. Single-longitude-mode output is achieved, with beat linewidth down to \(\sim\)66 kHz and a high side mode suppression ratio over 60 dB. This setup of self-injection-locked laser is cost-effective for mass production and thus is suitable for various applications such as laser ranging, LIDAR, optical communication, and holographic. self-injection locking; Fabry-Perot cavity; dielectric filter; narrow-linewidth laser +
Footnote †: journal: Applied Physics Letters  
**Citation:** [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] A Compact Self-Injection-Locked Narrow-Linewidth Diode Laser with Narrowband Dielectric Filter. _Appl. Sci._**2023**, 13, 4765. [[https://doi.org/10.3390/app13084765](https://doi.org/10.3390/app13084765)]([https://doi.org/10.3390/app13084765](https://doi.org/10.3390/app13084765))  
**Cognition**: 2023 by the authors. Liconse MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))). **Cognition**: 2023 by the authors. Liconse MDPI, Basel, Switzerland. This article is an open access article distributed through the non-assisted deposition method in the HFFPFPF; hence, the high reflectivity and thin resonance cavity thickness are achieved at the same time. In a single batch of coating, we fabricated a large number of HFFPFs that can be used for \(\sim\)23,000 narrow-linewidth lasers, demonstrating its extraordinary mass production capability. Our result shows a new approach for narrow-linewidth laser fabrication with a low requirement for the laser diode (LD), thus reducing the system cost and enabling mass-production capability, which would push forward its practical application in the relevant fields. For Fabry-Perot laser diodes (FP-LD), the FSR is normally in the order of sub-nanometer, which is smaller than the semiconductor gain bandwidth. A free-running FP-LD is thus usually lasing in multi-longitudinal modes. Here, we choose to use a Fabry-Perot (FP) cavity for the self-injection locking of an FP-LD, not only to narrow the laser linewidth but also to achieve single-longitudinal-mode lasing. In an FP cavity [22], the transmission bandwidth \(\Delta v\) is calculated by \(\Delta v_{c}=\Delta v_{FSR}/F\), where the \(F\) is the finesse of the FP cavity and the FSR \(\Delta v_{FSR}\) is related to the cavity thickness \(t\), by  
\[\Delta v_{FSR}=\frac{c}{2n_{g}t} \tag{1}\]  
where \(n_{g}\) is the group refractive index of the resonance cavity layer, and \(c\) is the light speed. It is beneficial to have a large FSR and a small bandwidth for the self-injection locking laser based on the FP cavity for achieving a single-longitudinal-mode lasing and a narrow linewidth, respectively. We developed a dielectric coating method, which can realize the large FSR through a thin resonance cavity layer and the small bandwidth through a pair of high-reflection coatings on both sides of the resonance cavity layer. In the experiment, we use the ion-assisted deposition (IAD) [23] method to fabricate the FP cavity, where a thickness accuracy of \(\sim\)1 nm can be achieved with precise control of the deposition time.