The bandwidth, linewidth, side mode suppression ratio and spectral width of the laser are the performance parameters of the laser. These parameters are used to describe the frequency or wavelength distribution range of laser output light and are important indicators for evaluating laser performance and application performance.
Among them, bandwidth and linewidth are used to describe the distribution range and stability of the laser output signal in frequency, and are mainly used in fields such as optical communications and optical measurement;
Spectral width is used to describe the monochromaticity and narrowness of laser output light, and is mainly used in optical guidance, laser medical and other fields.
The size and requirements of these laser performance parameters will vary according to different application fields and needs, so precise control and adjustment are required in laser applications and designs.
Bandwidth refers to the frequency range contained within a certain wavelength range. In the field of communications, bandwidth usually refers to frequency bandwidth, that is, the degree to which a signal is broadened in frequency. Since there is a reciprocal relationship between wavelength and frequency, wavelength bandwidth is also defined accordingly in the field of optical communications.
In digital communications, bandwidth is often used to measure the effective data transmission rate of a communications channel. The wider the bandwidth, the faster the data transfer rate. In wireless communications, bandwidth is also a factor that needs to be considered, because bandwidth limitations will affect the transmission rate and distance of wireless signals. In fiber optic communications, the bandwidth of a wavelength usually refers to the data transmission rate that a wavelength can carry.
Linewidth refers to the range of frequency or energy distribution of the wavelength of laser output light, also called spectral line width. Use an interferometer to measure the laser wavelength bandwidth. Interferometers can be used for testing. The principle is to use the interference phenomenon of light waves to measure the properties of light. When two beams of light cross, interference fringes will be generated. By analyzing the fringe morphology, information such as the phase and frequency of the light waves can be obtained.
By introducing the laser beam into the interferometer, the laser wavelength bandwidth can be calculated based on characteristics such as the width and shape of the interference fringes. Laser wavelength bandwidth can also be measured using a spectrometer. By introducing the laser beam into the spectrometer, the spectral curve of the laser output can be obtained. Based on characteristics such as the width and shape of the spectral curve, the laser wavelength bandwidth can be calculated. In spectroscopy, spectral linewidth is the full width of a wave crest, indicating how wide the frequency differences of light are in time and space.
The narrower the linewidth of the laser wavelength, it means that the spectrum generated by the laser is very narrow and has good monochromaticity. Line width is related to the characteristics of the laser itself, such as the length of the laser's resonant cavity, reflectivity, characteristics of the laser medium, etc. In addition, the linewidth of the laser wavelength will also be affected by environmental factors such as laser current, temperature and other parameters. Linewidth is a very important indicator of laser output light and is critical to many applications, such as optical communications, optical storage, laser medicine, photonics, optical measurement and other fields. Therefore, in order to obtain a narrow linewidth laser, it is necessary to select a high-quality laser and a stable working environment, and adopt appropriate control methods, such as heaters and feedback control.
Spectral width refers to the wavelength range of the light output by the laser, and broadly refers to the range of spectral distribution on the wavelength axis. Since the light generated by lasers is usually monochromatic, its spectral distribution range is very narrow, so the spectral width is also generally very narrow, usually at the nanometer or sub-nanometer level. The spectral width of the laser wavelength is the longest wavelength corresponding to 50% of the maximum relative spectral intensity minus the longest wavelength corresponding to 50% of the maximum relative spectral intensity.
The spectral width of the laser wavelength is mainly determined by the characteristics of the laser's resonant cavity, the laser medium and other factors, and is generally a fixed value. However, in practical applications, the wavelength output by the laser will be affected by some factors, such as the temperature of the laser, pump power, current and other factors. These factors will cause changes in the length of the laser's resonant cavity or the refractive index of the medium, and then affect the The spectral width of the laser wavelength. In optical communications, the spectrum width of the laser is required to be as narrow as possible to meet the requirements of high-speed transmission. In applications such as laser spectrum analysis and optical guidance, the spectral width of the laser needs to be precisely controlled and adjusted.
The side mode suppression ratio refers to the ratio between the main mode peak and the side mode peak in the laser output wavelength, and is used to evaluate the cleanliness and monochromaticity of the laser output wavelength. The main mode peak is the strongest frequency component of the laser output, while the side mode peak refers to the frequency component secondary to the main mode peak in the laser output wavelength distribution curve.
The higher the side mode suppression ratio, the better the monochromaticity of the laser output and the smaller the output power of the side mode component. The calculation formula for the side mode suppression ratio is as follows: SMSR = 10 × log10 (Pmain / Pside), where Pmain is the optical power of the main mode peak, and Pside is the optical power of the side mode peak secondary to the main mode peak. Therefore, the larger the value of the side mode suppression ratio, the better the monochromaticity of the laser and the smaller the output power of the side mode component. If the peak power of the main mode of a laser is 1 mW and the peak power of the side mode secondary to the main mode is 10 μW, then its side mode suppression ratio is 10 ×log10 (1/0.01) = 20 dB. This means that the power of the side mode is only about 1% compared to the power of the main mode, and the output of this laser is a high-quality monochromatic laser.
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