Laser noise has emerged as a significant area of research alongside the advancement of laser technology. In 1960, the laser was invented, and as science and technology progressed, researchers began to notice fluctuations in various laser parameters—such as optical power and phase—which came to be known as laser noise. 

In-depth research and effective control of noise have not only advanced the development of lasers themselves but have also directly given rise to a series of revolutionary scientific and engineering breakthroughs—enabling us to "listen" to the ripples of the universe (through gravitational-wave detection), create clocks of unprecedented precision (such as optical atomic clocks), and lay the foundation for tomorrow's ultra-fast information networks. 
1. Definition of laser noise 
The "noise" of a laser refers to the unavoidable random fluctuations over time in various output parameters, such as intensity, frequency and phase, as well as beam quality and pointing stability.

All kinds of random fluctuations generated by lasers during operation may originate from the lasers' internal mechanisms, such as temperature fluctuations and current instability, or be affected by the external environment, such as mechanical vibrations and electromagnetic interference.

Understanding and suppressing these noises is critical for enhancing laser performance, advancing precision measurement technologies, and driving the development of high-speed communication systems. In many cutting-edge applications, lasers are no longer just sources of energy—they’ve become incredibly precise "rulers." For instance, gravitational wave detection requires measuring spatial strains thousands of times smaller than the diameter of an atomic nucleus; even the tiniest fluctuations in the laser could drown out the faint signals. Similarly, optical atomic clocks, which serve as the ultimate timekeeping standard, rely directly on the stability of their laser light—any noise in the laser can compromise their accuracy. In coherent optical communication, noise degrades the signal-to-noise ratio, increasing error rates and limiting both communication capacity and transmission distance. Meanwhile, in precision spectroscopy, noise can obscure the fine spectral structures of materials, severely compromising measurement accuracy. As a result, laser noise has emerged as one of the key metrics for evaluating a laser’s overall performance. 
 

2. Main Types of Laser Noise 
The laser operates by generating coherent light through stimulated emission. However, during this process, factors such as the laser's operating environment, its structural design, and its operational state can cause fluctuations in the laser's parameters, leading to noise. Laser noise primarily falls into two main categories: 
 

2.1 Intensity Noise 
Intensity noise refers to the random fluctuations of laser output power (light intensity) over time. It is the most intuitive and significant form of noise.

Quantitative indicator: Relative Intensity Noise (RIN), defined as the ratio of noise power to the square of average optical power, typically expressed in dB/Hz.

Main sources:(1) Quantum noise (shot noise limit): Originates from the quantum nature of photons. Its generation and detection follow a random Poisson process. This is the fundamental noise limit in classical laser theory, an unavoidable physical limit.(2) Technical noise: Mainly includes fluctuations in the pump source (driving current), mechanical vibrations, and temperature variations. These factors cause changes in the population inversion density of the laser gain medium, thereby leading to fluctuations in the output light intensity.(3) Relaxation oscillations: In devices such as semiconductor lasers, the interaction between carrier density and photon density produces a characteristic damped oscillation (usually on the MHz scale), which manifests as high-frequency fluctuations in intensity.

2.2 Frequency and Phase Noise 
Frequency noise and phase noise are essentially different descriptions of the same phenomenon (frequency is the derivative of phase). They describe the random drift of the oscillation frequency and the random jitter of the phase of the laser electromagnetic wave.

Quantitative indicator: Linewidth. An ideal single frequency laser has an infinitely narrow linewidth, while the spectrum of a practical laser is broadened due to the presence of phase noise. Linewidth is the ultimate indicator for measuring laser monochromaticity.

Major sources: (1) Environmental disturbances: Temperature fluctuations cause thermal expansion and contraction in the laser cavity, altering its length (Δν/ν = ΔL/L); additionally, acoustic pressure and mechanical vibrations can also modulate both cavity length and refractive index. (2) (2) Spontaneous emission: Each photon generated by spontaneous emission randomly changes the phase of the laser field. This is the physical essence of laser linewidth broadening (Schawlow-Townes linewidth formula).
Impact: Frequency noise directly determines the coherence time of the laser. The greater the noise, the shorter the coherence time and the weaker the interference capability, which is critical in interferometric measurements and coherent communication.

3. Reducing Laser Noise 
Noise has a significant impact on the performance of lasers. Intensity noise causes instability in laser output, which in turn affects the accuracy and reliability of applications such as laser processing and measurement; phase noise reduces the coherence of lasers, impairing their application effectiveness in experiments like interference and diffraction. Therefore, reducing laser noise is crucial for improving laser performance.

To reduce laser noise, a range of measures can be implemented. For instance, optimizing the laser design and enhancing manufacturing precision; improving the laser's cooling system to minimize temperature fluctuations; and using a stable power supply to minimize current variations. Additionally, an external feedback control system can be employed to continuously monitor and adjust the laser's output in real time, further helping to lower noise levels. 

In laser design, when no frequency-selection measures are implemented, lasers typically operate in a multi-longitudinal-mode state.  Multi-longitudinal-mode operation causes many problems, such as broadening the laser output frequency band, degrading the laser's temporal and spatial coherence, and leading to unstable laser output power due to longitudinal mode competition. In particular, when nonlinear optical components are inserted into the cavity, different longitudinal modes will also couple with each other in the polarizable material, generating random sum-frequency, difference-frequency, and other phenomena, which further degrade the performance of the output laser.

Single-longitudinal-mode low-noise state (left: single longitudinal mode; right: photocurrent)

Multi-mode coupling generates noise (left: multi-mode; right: photocurrent) 
　　

A direct way to significantly reduce laser output noise is to force the laser to operate in a single longitudinal mode (single-frequency). Achieving single-frequency operation not only enhances the laser's stability but also produces a more monochromatic, highly coherent light source. 

Therefore, single-frequency lasers are an excellent choice for applications requiring ultra-low-noise laser output. Single-frequency or frequency-stabilized lasers are widely used in fields such as spectroscopy, coherent communications, LiDAR, gravitational wave detection, high-speed printing, image processing, optical data storage, DNA chip technology, and flat-panel display inspection, among many others. 
 

4. Noise of Single-Frequency Lasers
The method to measure the Relative Intensity Noise (RIN) of a laser involves photoelectric detection and spectrum analysis. The typical RIN of single-frequency lasers produced by CNI can reach 0.01% - 0.05%.

1064nm, 1W Single-Frequency Laser (RIN Noise <0.012%)

532nm, 10W Single-Frequency Laser (RIN Noise <0.043%)

The low-noise or single-frequency lasers produced by CNI include three types: DPSS,diode and fiber lasers. They have been supplied in bulk to fields such as interferometric measurement, holographic imaging, atomic physics, precision spectroscopy, particle counting, flow cytometry, and semiconductor inspection. For high-end applications like quantum computing, customized noise  specifications are possible.

For more details, please visit www.cnilaser.com.