modulation depth (original) (raw)

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

Categories: light detection and characterization, lightwave communications

Related: saturable absorberssemiconductor saturable absorber mirrors

Units: dimensionless or %

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DOI: 10.61835/mda Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is a Modulation Depth?

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.

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 in reflectance ($\Delta R$), and may substantially depend on the operating 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.

Frequently Asked Questions

What is modulation depth?

For a sinusoidally modulated quantity, the modulation depth is defined as the modulation amplitude (half the peak-to-peak change) divided by the mean value. A 100% modulation depth implies that the modulated quantity swings between zero and twice its mean value.

How is modulation depth defined for a saturable absorber?

In the context of saturable absorbers, the modulation depth is the maximum change in absorption or reflectance that can be induced by incident light. For a SESAM, it is the maximum light-induced change in reflectance ($\Delta R$).

Why is the modulation depth important for passive mode locking?

A large modulation depth of the saturable absorber leads to strong pulse shaping, which can result in a short pulse duration and reliable self-starting. However, if it is too large, it can cause unwanted Q-switching instabilities.

What are typical values for the modulation depth of a SESAM?

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