![]() ![]() These longitudinal diffraction-grating mirrors reflect the light back in the cavity, very much like a multi-layer mirror coating. If one or both of these end mirrors are replaced with a diffraction grating, the structure is then known as a DBR laser (distributed Bragg reflector). At the important wavelengths of 1.55 μm and 1.3 μm, the peak gain typically moves about 0.4 nm to the longer wavelengths as the temperature increases, while the longitudinal modes shift about 0.1 nm to the longer wavelengths. This causes the spectrum to be unstable and highly temperature-dependent. ![]() At the same time, the longitudinal modes of the laser also vary, as the refractive index is also a function of temperature. If the temperature of a semiconductor Fabry–Perot laser changes, the wavelengths that are amplified by the lasing medium vary rapidly. Since the mirrors are generally broad-band and reflect many wavelengths, the laser supports multiple longitudinal modes, or standing waves, simultaneously and lases multimode, or easily jumps between longitudinal modes. The light then leaks out of the front mirror and forms the output of the laser diode. The back reflector generally has high reflectivity, and the front mirror has lower reflectivity. The light bounces back and forth between these two mirrors and forms longitudinal modes, or standing waves. The simplest kind of a laser is a Fabry–Perot laser, where there are two broad-band reflectors at the two ends of the lasing optical cavity. Semiconductor DFB lasers in the lowest loss window of optical fibers at about 1.55 μm wavelength, amplified by erbium-doped fiber amplifiers (EDFAs), dominate the long-distance communication market, while DFB lasers in the lowest dispersion window at 1.3 μm are used at shorter distances. DFB lasers tend to be much more stable than Fabry–Perot or DBR lasers and are used frequently when clean single-mode operation is needed, especially in high-speed fiber-optic telecommunications. ![]() The strongest grating operates in the first order, where the periodicity is one-half wave, and the light is reflected backwards. The periodic change can be either in the real part of the refractive index or in the imaginary part (gain or absorption). This longitudinal diffraction grating has periodic changes in refractive index that cause reflection back into the cavity. The structure builds a one-dimensional interference grating ( Bragg scattering), and the grating provides optical feedback for the laser. ( January 2016) ( Learn how and when to remove this template message)Ī distributed-feedback laser ( DFB) is a type of laser diode, quantum-cascade laser or optical-fiber laser where the active region of the device contains a periodically structured element or diffraction grating. Please help improve it to make it understandable to non-experts, without removing the technical details. This article may be too technical for most readers to understand. ![]()
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