What is a Single Frequency Laser?
A single-frequency laser (rarely called a single-wavelength laser) is a laser that operates on a single resonator mode so that it emits quasi-monochromatic radiation with a very small linewidth and low phase noise.
Types of Single Frequency Laser
Mode Frequencies in Lasers
Those modes which have gained more than a loss will be able to oscillate leading to multi-frequency oscillation
Laser oscillation in continuous-wave operation usually occurs with one or several frequencies which correspond fairly precisely to certain mode frequencies. However, the frequency-dependent gain can cause some frequency pulling (slightly nonresonant oscillation), and the mode frequencies themselves can be influenced e.g. by thermal lensing in the gain medium.
The single-frequency operation of a laser means that only a single resonator mode (nearly always a Gaussian one) is excited; this leads to a much lower emission bandwidth than in cases where multiple resonator modes are excited.
If different modes of a laser resonator are simultaneously excited in continuous-wave operation, there is usually the phenomenon of mode competition.
When a laser has a poor beam quality, this is usually (although not always) the result of the excitation of higher-order transverse cavity modes. When the output light is sent to a fast photodiode, one can detect beat notes involving higher-order modes, i.e., with frequencies that substantially deviate from integer multiples of the round-trip frequency.
In mode-locked operation, the optical spectrum is a frequency comb, consisting of exactly equidistant spectral lines (ignoring possible laser noise), where the line spacing is the inverse pulse repetition rate. Due to the not exactly equidistant mode frequencies, there is some amount of mismatch between emission frequencies and mode frequencies; the larger that mismatch is, the stronger needs to be the effect of the mode-locking device, for example, a saturable absorber. Based on this insight, it is easy to understand why in cases with substantial chromatic dispersion it is hard to achieve mode-locking with a broad emission bandwidth and a correspondingly short pulse duration.
Normal Fabry-Perot laser diode oscillates in several longitudinal modes
Many applications require single-frequency oscillation
Single Frequency Laser Application
Typical applications of single-frequency lasers occur in the areas of optical such as fiber-optic sensors and interferometry, optical data storage, high-resolution laser spectroscopy, e.g. LIDAR, and optical fiber communications. In some cases such as spectroscopy, the narrow spectral width of the output is directly important. In other cases, such as optical data storage, a low intensity noise is required, thus the absence of any mode beating noise.
Single-frequency sources are also attractive because they can be used for driving resonant enhancement cavities, e.g. for nonlinear frequency conversion, and for coherent beam combining. The latter technique is currently used to develop laser systems with very high output powers and good beam quality.