Energy is absorbed in the medium, creating excited states in the atoms. Population inversion is achieved when the number of particles in an excited state exceeds the number of particles in the ground state or less excited states. In this case, a mechanism of stimulated emission can occur and the medium can be used as a laser or optical amplifier.
Some laser applications require the laser to have a very narrow linewidth, that is, a narrow spectrum. Narrow linewidth lasers refer to single-frequency lasers, that is, there is a resonant cavity mode in the laser value, and the phase noise is very low, so the spectral purity is very high. Typically such lasers have very low intensity noise.
The gain medium of an optical amplifier can only achieve a limited gain. One approach achieves higher gain by geometrically setting up light so that it passes through multiple channels as it passes through the amplifier, known as a multipass amplifier. The simplest is a two-pass amplifier, where the beam passes through the crystal twice, usually with exactly or nearly opposite directions of propagation.
Time-domain OCT is mainly composed of Michelson interferometers. The light emitted by the light source is divided into two beams after passing through the coupler, and enters the sample arm and the reference arm of the Michelson interferometer respectively.
The eye diagram is a series of digital signals accumulated and displayed on the oscilloscope. It contains a wealth of information. From the eye diagram, the influence of intersymbol crosstalk and noise can be observed, which reflects the overall characteristics of the digital signal, so as to estimate the system optimization. Therefore, eye diagram analysis is the core of signal integrity analysis for high-speed interconnect systems.
Single-frequency lasers based on Er3+-doped or Er3+/Yb3+ co-doped gain fibers mainly work in the 1.5 μm band (C-band: 1530-1565 nm) and part of the L-band (1565-1625 nm). Its wavelength is in the C window of optical fiber communication, which makes the 1.5 μm band single-frequency fiber laser with narrow linewidth and low noise characteristics very important in coherent optical communication. It is used in high-resolution sensing, optical frequency domain reflectometer , laser radar and other fields also have a wide range of applications.
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