The 1.55 μm waveband integrated external cavity tunable diode lasers have excellent merits such as their small volume, low cost, low power consumption, wide tuning range, narrow linewidth, large side mode suppression ratio, and high output power. These merits have attracted many applications for the lasers, such as in wavelength division multiplexing, passive optical networks, mobile backhaul, and spectral sensing technology. In this paper, firstly, the basic structure and principle of integrated external cavity tunable diode lasers are introduced, and then two main integrated structures of 1.55 μm waveband external cavity tunable diode lasers are reviewed and compared in detail, namely the hybrid integrated structure and monolithic integrated structure of 1.55 μm waveband integrated external cavity tunable diode lasers. Finally, the research progress in 1.55 μm waveband integrated external cavity tunable diode lasers in the last decade are summarised, and the advantages and disadvantages of 1.55 μm waveband integrated external cavity tunable diode lasers are analysed. The results show that, with the transformation of optical communication into more complex modulation formats, it is necessary to integrate miniature 1.55 μm waveband external cavity tunable diode lasers. Low-cost integrated 1.55 μm waveband external cavity tunable diode lasers are expected to be used in the next generation of optical transceivers in small-factor modules.

Narrow-linewidth broadband tunable external cavity diode lasers (NBTECDLs), with their broadband tuning range, narrow linewidth, high side-mode suppression ratio (SMSR), and high output power, have become important laser sources in many fields such as optical communication, spectral analysis, wavelength division multiplexing systems, coherent detection, and ultra-high-speed optical interconnection. This paper briefly describes the basic theory of NBTECDLs, introduces NBTECDLs with diffraction grating type, fiber Bragg grating (FBG) type, and waveguide type, and conducts an in-depth analysis on the working principles and performance characteristics of NBTECDLs based on different NBTECDL types. Then, it reviews the latest research progress on Littrow-type, Littman-type, FBG-type, and waveguide-type NBTECDLs in detail and compares and summarizes the characteristics of Littrow-type NBTECDLs, Littman-type NBTECDLs, FBG-type NBTECDLs, and waveguide-type NBTECDLs. Finally, it looks at the structural features, key technologies, optical performance, and application fields of the most cutting-edge research in recent years and summarizes the challenges and future development directions of NBTECDLs.

The 1.55 μm TECSL has excellent characteristics such as wide tuning, narrow linewidth, high SMSR, and high output power and has a wide range of applications in optical communications, spectral sensing, gas detection, atomic physics, and biomedicine. For the TECSL, the choice of collimating lens is very significant. In order to obtain a wider tuning range, five structures are constructed in this paper to investigate the effect of lens focal length on the output characteristics of 1.55 μm TECSL. It is shown that when the lens focal length is 4.51 mm, the minimum threshold current is 52 mA, the maximum output power is 42.36 mW, the maximum SMSR is 62.15 dB, the narrowest linewidth is 0.26 nm, and 152.3 nm (1458.2~1610.5 nm) can be tuned continuously. It is shown that different lens focal lengths affect the output characteristics of the TECSL, and the performance of the TECSL can be improved by appropriately changing the lens focal length.

The TECSL has attracted much attention due to its wide tuning range, narrow linewidth, high output power, and excellent SMSR. It holds irreplaceable value in optical communication, spectroscopy analysis, and biomedical applications. The demand for a wide tuning range, high power, narrow linewidth, and a high SMSR has driven the development of high-performance TECSL structures. This paper comprehensively discusses five key TECSL structures: Littrow-type structures, Littman-type structures, filter-type structures, fiber-type structures, and waveguide-type structures, and elaborates on their structures and principles. This paper reviews the research process of different type-structure TECSLs, analyzes the advantages and disadvantages of different external cavity structures, and explores the future development trends of TECSLs. The review shows that the Littrow-type structure TECSL achieved an extremely wide tuning range using diffraction gratings, reaching up to 360 nm. The Littman-type structure TECSL demonstrated excellent spectral purity, achieving an SMSR of 71.03 dB. The filter-type structure TECSL was able to achieve flexible wavelength selection using tunable filters, achieving a linewidth of 570 Hz. The fiber-type structure TECSL has a linewidth of up to 600 Hz. The waveguide-type structure TECSL can achieve a linewidth as low as 0.252 kHz and a tuning range of up to 120.9 nm.

This article investigates fiber coupling techniques for low numerical aperture 808 nm semiconductor lasers. A coupling optical system combining fast-axis/slow-axis collimators (FAC/SAC) with a focusing lens was designed, achieving efficient coupling through high-precision optical integration packaging. First, a high-power GaAs-based 808 nm semiconductor laser chip was designed and fabricated. Its thermal performance and operational stability were enhanced by optimizing packaging materials and structures. The coupling system employs a fast-axis collimating lens, slow-axis collimating lens, and aspheric focusing lens to shape the beam and focus it into a 200 μm/0.12 NA fiber. Experimental results show that the developed coupling module achieves the threshold current of 1.2 A at 298 K, the continuous output power of 9.59 W, with the slope efficiency of 1.1 W/A, a coupling efficiency of 95%, the maximum output numerical aperture of 0.116, the wavelength temperature drift coefficient of approximately 0.2 nm/°C, and the peak brightness of 0.72 MW/cm²·sr. This study validates the feasibility and superiority of the FAC/SAC combined with focusing lens approach for low-NA fiber coupling. It provides theoretical and practical foundations for fiber coupling in high-brightness, high-power laser systems, offering promising applications in solid-state laser pumping, enhancing system integration, and enabling long-distance, high-brightness transmission.

A seven-layer broadband antireflection coating consisting of Al₂O₃, TiO₂, SiO₂, and MgF₂ was designed and deposited on both sides of K9 glass for the 400–1100 nm range. The deposition parameters for TiO₂ and SiO₂ single layers were optimized, and the measured optical constants were used in TFCalc to refine the multilayer design. Layer-specific thickness-tolerance analysis was performed to identify the layers requiring strict process control. The initially characterized double-sided coating showed an average transmittance of 98.72% over 400–1100 nm. Four samples prepared in one additional deposition batch exhibited closely matching transmittance spectra, indicating low sample-to-sample variation within that batch. Five-position profilometry indicated consistent total-thickness control. Cross-sectional scanning electron microscopy (SEM) revealed well-defined multilayer contrast without obvious cracking, and regional energy-dispersive X-ray spectroscopy (EDS) analysis detected the constituent elements of the coating. These results demonstrate a practical design-to-fabrication approach that combines process-specific optical constants with tolerance analysis, with potential applications in imaging systems, optical windows, and lidar.