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Definition: two closely spaced optical pulses

Category: light pulses

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Related: light pulsesultrashort pulsesQ-switchingQ-switched lasersmode lockingmode-locked lasersphotodiodesautocorrelatorsUnderstanding Fourier Spectra

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DOI: 10.61835/j5q Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Double Pulses?

Light pulses sometimes occur in the form of double (or multiple pulses), with one following the other within a short time. This phenomenon can arise under different circumstances, as discussed in the following.

Double Pulses in Q-switched Lasers

The normally desired mode of operation of a Q-switched laser leads to the generation of a single pulse (typically with a nanosecond duration) per Q-switching cycle. Under certain circumstances, however, multiple pulses may be emitted within a single Q-switching cycle. Typical reasons for such behavior are:

• If the switching time of the modulator is too long, the first pulse may not be able to extract fully the energy stored in the gain medium. A second pulse may then be generated at a later time, where the modulator has reached the state of lowest loss.
• If the laser is continuously pumped and the opening time of the modulator is too long, sufficient energy may be accumulated in the gain medium to form a second pulse.

For predicting or analyzing such effects, it can be very helpful to simulate the laser dynamics numerically.

Double Pulses in Mode-locked Lasers

With mode locking of a laser, one usually obtains a single ultrashort pulse circulating in the laser resonator, or (with harmonic mode locking) multiple pulses with equidistant spacing. In some situations, however, a circulating pulse may break up into two or more closely spaced pulses, leading to multiple pulses in the output pulse train. This occurs particularly often in passively mode-locked lasers operating in the femtosecond domain, e.g. with soliton mode locking.

Figure 1: A pair of 120-fs pulses with a 1-ps temporal spacing.

Figure 2: Optical spectrum of a double pulse as shown above. Both pulses are assumed to have a center frequency of 300 THz.

The spectrum (solid curve) is modulated with a frequency of 1 THz, which is the inverse pulse spacing. For comparison, the dashed curve shows the spectrum of a single pulse.

Multiple pulses can sometimes form a relatively stable configuration, since there can be various physical effects which permit some kind of interaction between the pulses. These pulses can then do many resonator round trips with constant spacing and even constant phase, i.e., they remain mutually coherent. As a result, the optical spectrum is strongly modulated (see also a RP Photonics Spotlight article). In fact, such a spectrum often leads to the discovery of this phenomenon.

In other cases, however, multiple pulses can be very unstable, at least concerning their relative phase. A modulated spectrum then cannot be observed.

Detecting Double Pulses

Detection of double pulsing of mode-locked lasers is possible, e.g. with the following methods:

• It can often be seen as a modulation of the optical spectrum, recorded with an optical spectrum analyzer (see above). This method may fail, however, when the relative phase is not sufficiently stable, or when the spectral resolution is insufficient (for large pulse spacings).
• A more direct way is to use a fast photodiode and a sampling oscilloscope. Correct triggering may require special care and is challenging particularly in unstable situations. Only for pulse spacings of around 100 ps or larger is the measurement bandwidth of a fast photodiode sufficient for detecting double pulses.
• For small enough pulse spacing, double pulsing may also be detected with an autocorrelator.
• A sign of double or multiple pulsing can be that the conversion efficiency of a frequency doubler is lower than that theoretically expected or it fluctuates without apparent reason. This results from the reduced peak power, when a certain total energy (as obtained from the measured average laser power) is spread over several pulses, or from fluctuations in peak power in situations with unstable pulse numbers.

In cases where the pulse spacing varies significantly over time, it may be more difficult to detect the double pulsing.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What are laser double pulses?

Double pulses are a phenomenon where a laser emits two or more light pulses in close succession within a cycle where a single pulse is normally expected. This can occur in different types of lasers, such as Q-switched or mode-locked lasers.

What causes double pulses in Q-switched lasers?

In Q-switched lasers, double pulses can occur if the modulator's switching time is too long, preventing the first pulse from fully depleting the stored energy. They can also arise if the modulator stays open too long in a continuously pumped system, allowing the gain to recover and form a second pulse.

Why does double pulsing happen in mode-locked lasers?

In passively mode-locked lasers, a single circulating pulse can break up into two or more closely spaced pulses. This is particularly common in femtosecond lasers operating with soliton mode locking.

How does the presence of double pulses affect the optical spectrum?

If the double pulses are mutually coherent (i.e., have a stable phase relationship), their interference causes a strong periodic modulation of the optical spectrum. The frequency of this modulation is the inverse of the temporal spacing between the pulses.

How can one detect double pulses from a laser?

Double pulses can be detected by observing the characteristic modulation in the optical spectrum, by direct measurement with a fast photodiode and oscilloscope for larger separations, or with an autocorrelator for shorter separations. Reduced efficiency in nonlinear processes like frequency doubling can also be an indicator.

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