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Modulation Depth

Definition: a relative modulation amplitude, or (for a saturable absorber) the maximum change in absorption

German: Modulationstiefe

Units: dimensionless or %

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When a quantity such as the transmission of an optical modulator is sinusoidally modulated, the modulation depth can be defined as the modulation amplitude (i.e. one-half of the peak-to-peak changes) divided by the mean value. A modulation depth of 100% thus corresponds to a situation where the minimum value of the modulated quantity is zero, and the maximum is twice the mean value.

modulation depth
Figure 1: A signal with a modulation depth of 50%. Here, the modulation amplitude is half the mean value.

Modulation Depth of Saturable Absorbers

In the context of saturable absorbers, as used for passive mode locking or Q switching of lasers, the modulation depth is the maximum change in absorption (or reflectance) which can be induced by incident light with a given wavelength. This is an important design parameter in passively mode-locked lasers. A large modulation depth leads to strong pulse shaping by the saturable absorber, which can lead to a short pulse duration and reliable self-starting, but also to unwanted Q-switching instabilities.

For semiconductor saturable absorber mirrors (SESAMs), the modulation depth is specified as the maximum light-induced change of reflectance ΔR, and may substantially depend on the operation wavelength. For use in mode-locked bulk lasers, it is typically of the order of 1%, while substantially larger values (order of 10%) are normally required for mode-locked fiber lasers. The achieved modulation depth is a substantial fraction of the unsaturated reflectance loss caused by the integrated absorber. It may be increased by using a thicker absorber or multiple thin absorbers, and it also depends on other design features; for example, it can be substantially reduced in anti-resonant designs, which decrease the optical intensity within the absorber.

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See also: saturable absorbers, semiconductor saturable absorber mirrors

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