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Spectral Linewidth
The spectral linewidth of the laser
The spectral linewidth of a laser, also known as the full width at half maximum (FWHM) of the emission spectrum of the laser source, refers to the width between the two frequencies corresponding to half the height of the peak. It describes the distribution range of the laser's output light in the frequency domain and is a key indicator for evaluating the monochromaticity and coherence of the laser. A narrower linewidth means better monochromaticity and coherence of the laser, which is particularly important for fields such as optical communication, coherent measurement, holography, and spectral analysis.
The spectral linewidth of a laser is determined by the quality factor of the resonant cavity — the higher the quality factor of the cavity, the narrower the laser linewidth. When considering the gain of the laser medium, the theoretical limit of the laser linewidth is determined by the spontaneous emission of the gain medium. Of course, in actual lasers, there are various linewidth broadening mechanisms such as spontaneous emission, so the laser linewidth generally cannot reach its theoretical limit.
There are usually two expressions for spectral linewidth: one in hertz (Hz) and the other in meters (m). The two can be converted into each other. The specific formulas are:linewidth
(in units such as Hz, kHz, MHz) and linewidth
(in units such as fm, pm, nm).
The spectral linewidth of ordinary light sources (such as LEDs) covers tens of nanometers, while that of lasers is generally only a few nanometers or less. For example, the spectral linewidth of single-frequency lasers can be less than 1 MHz (10⁻⁵ nm), featuring excellent monochromaticity.
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365/470/579/630 LED light source linewidth | 633nm single-frequency laser linewidth: |
Lasers are categorized by linewidth as follows:
Conventional linewidth laser
The spectral linewidth of conventional linewidth lasers is generally around 0.2 nm for DPSS lasers such as those operating at 1064 nm and 532 nm, around 2 nm for 405 nm diode lasers, and between 150 nm and 300 nm for quantum cascade diode lasers at 4.3 µm. The type of laser can deliver higher output power. It is mainly used in fields like laser medicine, fluorescence excitation, and laser printing.
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Conventional 532nm laser linewidth:0.097 nm | Conventional 405nm laser linewidth: 1.6 nm |
Narrow linewidth laser
Narrow linewidth lasers narrow the linewidth of conventional lasers by technologies like external cavity grating mode selection. For example, the 1.6 nm linewidth of 405 nm ordinary laser can be reduced to <0.03 nm, and they are mainly used in fields such as DNA sequencing, Raman spectroscopy, and confocal microscopy.
Linewith for 405nm narrow linewidth laser: 0.0133 nm
Single frequency laser (ultra-narrow linewidth laser)
The single frequency laser has only one longitudinal mode oscillating in its resonant cavity and an extremely narrow spectral linewidth—generally MHz-order or narrower (frequency) and 10⁻⁵ nm or narrower (wavelength). With excellent monochromaticity and coherence, it is mainly used in holography, Raman scattering, Brillouin scattering and other fields. For details, visit www.
Linewith for 457nm single frequency laser:754.1 kHz | Linewith for 532nm single frequency laser: |
Measurement of spectral width
Choose a suitable spectrometer based on wavelength range and resolution.CNI's spectrometers offer ranges of 200~1100nm, 185~240nm, 950~1200nm, and resolution <0.1 nm.
Aurora4000 UV pro measured wavelength: 375.33 nm, linewidth: 1.01 nm
For measuring single-frequency or narrower spectral linewidths, an expensive linewidth analyzer is required. CNI offers adjustable options along with related testing services. For more details, please visit www.
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