Due to various influences of quantum noise and fluctuations of various technical origins, the output of a laser always contains some noise. There are different kinds of laser noise:
- In a single-frequency laser, there are intensity noise (or amplitude noise) and phase noise. The latter causes a finite laser linewidth, and is strongly related to frequency noise. It also limits the temporal coherence.
- In a laser operating on multiple resonator modes, there is usually stronger intensity noise due to mode beating noise and also mode partition noise, i.e., fluctuations of the power distribution over the resonator modes. The optical power in one of these modes may fluctuate much more than the total power.
- A mode-locked laser exhibits additional types of noise – for example, noise in the temporal position of the pulses (→ timing jitter), noise in the center frequency, pulse duration, and chirp. For harmonic mode locking, there is also so-called supermode noise.
- Any laser can exhibit beam pointing fluctuations.
Origins of Laser Noise
The origins of laser noise can be divided into two groups:
Impacts of Laser Noise
Laser noise is important for many laser applications. Some examples are:
- High precision optical measurements, e.g. in frequency metrology, precision spectroscopy or interferometry, require low intensity and phase noise.
- The data transmission rates achievable with optical fiber communications systems are usually limited by noise of lasers and amplifiers.
- For precise laser material processing, it is often necessary that beam pointing fluctuations and pulse energy variations are minimized.
Methods for Noise Reduction
Laser noise can be reduced in many ways:
- Quantum noise can be reduced e.g. by increasing the intracavity power level and by minimizing optical losses.
- Technical noise influences can be reduced, e.g. by building a stable laser resonator, by temperature stabilization of the setup, or by using a low-noise pump source.
- Laser parameters can be optimized so that the laser reacts less strongly to certain noise influences.
- Mode hopping may be suppressed, e.g. with an optical filter.
- There are various active or passive techniques for the stabilization of lasers.
A prerequisite for effective noise reduction is that the origin of the most disturbing noise is known, in addition to the parameters determining the laser's sensitivity to thus noise influences. Depending on the case, it can be more effective to reduce either noise influences themselves or the laser's sensitivity.
The RP Photonics Buyer's Guide contains 5 suppliers for low-noise lasers or equipment for the characterization of laser noise. Among them:
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See also: noise specifications, laser specifications, quantum noise, shot noise, intensity noise, phase noise, linewidth, Schawlow–Townes linewidth, amplifier noise, coherence, stabilization of lasers, mode hopping, Spotlight article 2006-08-01, Spotlight article 2006-08-20, Spotlight article 2006-10-09
and other articles in the categories fluctuations and noise, lasers
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