A single-frequency laser operates with only one longitudinal mode, and thus can emit quasi-monochromatic radiation with a very narrow linewidth and low noise. Such lasers, both in continuous waveform and pulsed operations, are of interest for interferometric sensing, coherent light detection and ranging, laser spectroscopy, and laser nonlinear frequency conversions.

The distributed Bragg reflector (DBR) fiber laser is based on a short cavity combined with a narrowband fiber Bragg grating (or FBG, an optical fiber that reflects particular wavelengths of light and transmits the rest). We can fabricate the cavity by cleaving the high-reflection (HR) and partial reflection (PR) FBGs very close to the index modulation region and directly splicing to a centimeter's length a glass fiber highly doped with rare earth elements. This laser provides robust single-frequency operation with watt-level output power and at sub-kilohertz linewidth. Its most significant component is the rare-earth-doped glass fiber, which enables high gain (usually several dB/cm) per unit length. Developers have used rare-earth-doped soft glass fiber, including phosphate glass and germanate glass fibers for DBR single-frequency fiber lasers operating in the 1, 1.5, and 2μm spectral regions. However, fusion splicing between these soft glass active fibers with the passive silica fiber is challenging, and is usually undertaken only by specially trained and experienced technicians using expensive and complex fiber fusion splicers.

So It is usually cost so much for a factory during laser production. We Shinho fiber communication has Specialty fusion splciers like LGD splicer and PM splicer for PM fiber and 250um,400um,600um fiber splicing and cleaving. It cost only half of Fujikura and Sumitomo. reduce a lot of cost for factories.

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