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Ask RP Photonics for advice on any aspect of laser noise, be it origins, simulation and modeling, optimization, measurement, or its effects. Note that Dr. Paschotta has a particularly strong expertise in this area. A Powerpoint presentation gives more details.
Definition: fluctuations of various parameters of laser light, such as the optical power and phase
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 is intensity noise (or amplitude noise) as well as phase noise. The latter is causing 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 strong mode partition noise (also called mode beating noise) of the optical intensity. This is particularly the case when the number of modes is small, and when higher-order transverse modes are involved.
- A mode-locked laser exhibits noise in the temporal position of the pulses (→ timing jitter) but also 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.
(This article is not considering acoustical noise emitted by laser devices.)
Figure 1: Intensity noise spectrum of a solid-state laser.
Origins of Laser Noise
The origins of laser noise can be divided into two groups:
- quantum noise, in particular associated with spontaneous emission in the gain medium
- technical noise, arising e.g. from excess noise of the pump source, from vibrations of the laser resonator, or from temperature fluctuations
Particularly for solid-state lasers, noise is often strong around the relaxation oscillation frequency and weak at frequencies well above this frequency (see Figure 1). Semiconductor lasers exhibit strongly damped relaxation oscillations, but with very high frequencies, which can be well above 1 GHz.
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 achievable data transmission rates of 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 as well as 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, as well as the parameters determining the laser's sensitivity to thus noise influences. Depending on the case, it can be more effective to either reduce noise influences themselves or the laser's sensitivity.
Bibliography
| [1] | A. L. Schawlow and C. H. Townes, "Infrared and optical masers", Phys. Rev. 112 (6), 1940 (1958) |
| [2] | C. C. Harb et al., "Intensity-noise dependence of Nd:YAG lasers on their diode-laser pump source", J. Opt. Soc. Am. B 14 (11), 2936 (1997) |
| [3] | B. C. Buchler et al., "Feedback control of laser intensity noise", Phys. Rev. A 57 (2), 1286 (1998) |
| [4] | T. C. Ralph et al., "Understanding and controlling laser intensity noise", Opt. Quantum Electron.. 31, 583 (1999) |
| [5] | R. Paschotta, "Noise of mode-locked lasers. Part I: Numerical model", Appl. Phys. B 79, 153 (2004); R. Paschotta, "Noise of mode-locked lasers. Part II: Timing jitter and other fluctuations", Appl. Phys. B 79, 163 (2004) |
| [6] | R. Paschotta et al., "Optical phase noise and carrier-envelope offset noise of mode-locked lasers", Appl. Phys. B 82 (2), 265 (2006) |
| [7] | R. Paschotta, H. R. Telle, and U. Keller, "Noise of Solid State Lasers", chapter 12 in "Solid-State Lasers and Applications", editor: Alphan Sennaroglu, CRC Press, Taylor and Francis Group, LLC, pp. 473–510, 2007, ISBN 0-8493-3589-2 |
See also: noise specifications, quantum noise, shot noise, intensity noise, phase noise, linewidth, Schawlow-Townes linewidth, coherence, stabilization of lasers, mode hopping, Spotlight article 2006-08-01, Spotlight article 2006-08-20
Categories: fluctuations and noise, lasers
