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Single-frequency fiber lasers have a very narrow limit linewidth, and their spectral line shape is Lorentz type, which is significantly different from single-frequency semiconductors. The reason is that single-frequency fiber lasers have longer laser resonant cavities and longer photon lifetimes in the cavity. This means that single-frequency fiber lasers have lower phase noise and frequency noise than single-frequency semiconductor lasers.

Narrow linewidth lasers are widely used as light sources and receivers in fiber optic communication systems. In terms of light sources, narrow linewidth lasers can provide high-quality and highly stable optical signals, which can reduce signal distortion and bit error rates. In terms of receivers, narrow linewidth lasers can provide high sensitivity and high-precision light detection, which can enhance the signal detection capabilities of the receiver. In addition, narrow linewidth lasers can be used for functions such as optical filtering and frequency conversion.

The linewidth of a laser, especially a single-frequency laser, refers to the width of its spectrum (typically full width at half maximum, FWHM). More precisely, it is the width of the radiated electric field power spectral density, expressed in terms of frequency, wavenumber, or wavelength. The linewidth of a laser is closely related to temporal coherence and is characterized by coherence time and coherence length. If the phase undergoes an unbounded shift, phase noise contributes to the linewidth; this is the case with free oscillators. (Phase fluctuations confined to a very small phase interval produce zero linewidth and some noise sidebands.) Shifts in the resonant cavity length also contribute to the linewidth and make it dependent on the measurement time. This indicates that the linewidth alone, or even a desirable spectral shape (lineform), cannot provide the full information about the laser spectrum.

Optical fiber transmits signals through light guide, non-conductive, and not afraid of lightning strikes, so there is no need to use grounding protection. According to the transmission mode of light in the optical fiber, we divide it into multi-mode optical fiber and single-mode optical fiber.

Let's take a look at which scenarios the DFB Butterfly Lasers is mainly used in.