Laser Noise

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: fluctuations in various parameters of laser light, such as the optical power and phase

More specific terms: intensity noise, phase noise, timing jitter

Categories: article belongs to category laser devices and laser physics laser devices and laser physics, article belongs to category fluctuations and noise fluctuations and noise

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DOI: 10.61835/2ty Cite the article: BibTex plain text HTML Link to this page! LinkedIn

Due to various influences of quantum noise and fluctuations from 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 in 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:

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

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 laser 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.

intensity noise spectrum — Figure 1: Intensity noise spectrum of a solid-state laser.

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, by minimizing optical losses and by increasing the resonator length.
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.

More to Learn

Schawlow–Townes linewidth

Timing jitter

Amplifier noise

Stabilization of lasers

Noise specifications

Lasers Disturbed by Vacuum?

Lower Noise from Longer Lasers

Correct Specifications for Laser Noise – Valuable but Hard to Obtain

Suppliers

The RP Photonics Buyer's Guide contains four suppliers for laser noise measurement equipment. Among them:

Thorlabs

Thorlabs manufactures a high-precision intensity noise analyzer for measuring intensity noise in optical systems. Featuring a DC to 3 MHz input frequency bandwidth, this device overlaps data from several sample rates to optimize resolution bandwidth and provide high frequency-axis resolution over its full bandwidth; the 9 Hz minimum resolution bandwidth is ideal for resolving low-frequency noise sources. With a nominal noise floor less than −140 dBV²/Hz, this low-noise instrument is designed to identify environmental noise sources in an optical experiment, such as ambient lighting or electrical line noise. Ideal applications also include light source and instrumentation development, where characterizing noise levels or detection limits is critical.

Bibliography

[1]	A. L. Schawlow and C. H. Townes, “Infrared and optical masers”, Phys. Rev. 112 (6), 1940 (1958); https://doi.org/10.1103/PhysRev.112.1940 (ground-breaking work; also contains the famous Schawlow–Townes equation)
[2]	E. I. Gordon, “Optical maser oscillations and noise”, Bell Sys. Tech. J. 43, 507 (1964); https://doi.org/10.1002/j.1538-7305.1964.tb04076.x
[3]	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); https://doi.org/10.1364/JOSAB.14.002936
[4]	B. C. Buchler et al., “Feedback control of laser intensity noise”, Phys. Rev. A 57 (2), 1286 (1998); https://doi.org/10.1103/PhysRevA.57.1286
[5]	T. C. Ralph et al., “Understanding and controlling laser intensity noise”, Opt. Quantum Electron. 31, 583 (1999); https://doi.org/10.1023/A:1006943801659
[6]	R. Paschotta, “Noise of mode-locked lasers. Part I: numerical model”, Appl. Phys. B 79, 153 (2004)“,”http://link.springer.com/article/10.1007%2Fs00340-004-1547-x; R. Paschotta, “Noise of mode-locked lasers. Part II: timing jitter and other fluctuations”, Appl. Phys. B 79, 163 (2004); https://doi.org/10.1007/s00340-004-1548-9
[7]	R. Paschotta et al., “Optical phase noise and carrier–envelope offset noise of mode-locked lasers”, Appl. Phys. B 82 (2), 265 (2006); https://doi.org/10.1007/s00340-005-2041-9
[8]	C. J. McKinstrie, “Stochastic and probabilistic equations for three- and four-level lasers: tutorial”, J. Opt. Soc. Am. B 37 (5), 1333 (2020); https://doi.org/10.1364/JOSAB.379976
[9]	J. Peng et al., “Principles, measurements and suppressions of semiconductor laser noise – a review”, IEEE J. Quantum Electron. 57 (5), 2000415 (2021); https://doi.org/10.1109/JQE.2021.3093885
[10]	C. J. McKinstrie, T. J. Stirling and A. S. Helmy, “Laser linewidths: tutorial”, J. Opt. Soc. Am. B 38 (12), 3837 (2021); https://doi.org/10.1364/JOSAB.439882
[11]	C. J. McKinstrie, T. J. Stirling and A. S. Helmy, “Stochastic systems: tutorial”, J. Opt. Soc. Am. B 38 (12), 3818 (2021); https://doi.org/10.1364/JOSAB.439879
[12]	R. Paschotta, H. R. Telle, and U. Keller, “Noise of Solid State Lasers”, in Solid-State Lasers and Applications (ed. A. Sennaroglu), CRC Press, Boca Raton, FL (2007), Chapter 12, pp. 473–510
[13]	R. Paschotta, “Noise in Laser Technology”. Part 1 – Intensity and Phase Noise; Part 2: Fluctuations in Pulsed Lasers; Part 3: Beam Pointing Fluctuations

(Suggest additional literature!)

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