An important type of noise in a light beam is noise of its intensity. Strictly, the noise of the optical power, rather than of the optical intensity, is usually considered, but the common term is intensity noise rather than power noise.
Specifications for Intensity Noise
Intensity noise is usually quantified as relative intensity noise (RIN), i.e. as noise of the power divided by the average power. Common specifications are based on
- an r.m.s. (root-mean-square) value for a certain measurement bandwidth
- a power spectral density S(f)
Sometimes specifications encountered such as “±0.1%” are not valid, since statistical aspects are not properly described, and it is not clear which range of noise frequencies is considered.
More details are found in the article on noise specifications.
Measurement of Intensity Noise
Intensity noise is normally measured by detecting the intensity (or power) with a fast photodetector (e.g. with a p–i–n photodiode) and evaluating the noise spectrum with an electronic spectrum analyzer. Although this appears simple in principle, there can be various technical challenges:
- The calibration of the intensity noise level often involves difficulties. Although an electronic spectrum analyzer displays noise spectra in proper units like dBc/Hz (where dBc = dB below carrier), the calibration usually has to be corrected because it is valid for sinusoidal signals, but not for random noise. It is common that 2 decibels have to be added to the noise level, but this can depend on the detailed settings of the spectrum analyzer such as the detection mode. Further difficulties arise when the DC component of the photocurrent needs to be suppressed in a preamplifier; a separate calibration measurement may then be required.
- The photodetector must of course be operated in a region with linear response, i.e., it should never be saturated. For measurements on low repetition rate pulse trains, this can imply that the recorded average power is fairly low, so that a high sensitivity is difficult to achieve.
- For pulse trains, the measured signal is influenced not only by intensity noise, but also by timing jitter, and both types of noise can even be correlated. Ignoring this can easily lead to wrong results.
For the characterization of low-frequency noise, it may be required to record the power variations in the time domain and process them numerically.
Origins of Intensity Noise
Intensity noise of a laser results partly from quantum noise (associated with laser gain and resonator losses) and partly from technical noise sources such as excess noise of the pump source, vibrations of resonator mirrors, thermal fluctuations in the gain medium, etc. The resulting intensity noise also depends on the operation conditions; in particular, it often becomes weaker at high pump powers, where relaxation oscillations are strongly damped. There are methods to reduce the noise further by using a feedback system (→ stabilization of lasers).
In most cases, the lowest possible intensity noise level for laser beams results from shot noise. At least at high noise frequencies, well above the relaxation oscillation frequency, this noise level is approached by many lasers. However, for so-called squeezed states of light, the intensity noise can be below the shot noise, at the cost of increased phase noise.
Intensity noise of a laser can be reduced in various ways:
- Influences of external noise sources can be reduced, e.g. by operating laser diodes with a well-stabilized injection current.
- The laser design can be optimized so that its susceptibility to external noise and the influence of quantum noise are minimized.
|||R. Paschotta, "Noise in Laser Technology – Part 1: Intensity and Phase Noise"|
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