Technical Note 2
PULSE COMPRESSION AND STRETCHING WITH DIFFRACTION GRATINGS

Diffraction gratings, used in conjunction with chirped pulses, are the key to compressing or stretching a laser pulse (i.e., increasing or decreasing their peak power with little loss in energy).

PULSE COMPRESSION

When a spectrally broad laser pulse is incident on a diffraction grating, the various wavelengths that make up the pulse will diffract from the grating at angles that depend on wavelength. If this pulse has its wavelength "chirped" so that the frequency increases during the length of the pulse, then diffraction will spread out the light in proportion, with the first part of the pulse (longer wavelengths) diffracting at a larger angle. The trailing portion of the pulse (shorter wavelengths) will diffract at a smaller angle and is directed to the leading edge of the second grating (see Figure 1). When the light diffracts from the second grating, which is oriented parallel to the first grating, the different parts of the pulse (with their correspondingly different wavelengths) will diffract at angles which yield a pulse whose parts are nearly in time synchronism. Peak power greatly increases, while total energy remains nearly the same.

PULSE STRETCHING

A pulse is sometimes stretched instead, in order to extract the maximum stored energy as well as reduce possible damage to optical components from excessive powers. This can be accomplished by using a pair of gratings in an "antiparallel" arrangement (see Figure 2). This works exactly opposite to the compression scheme in that the chirped pulse is spread out more by the gratings so that the resulting laser beam is longer in duration.

DISPERSION

The grating equation,

can be differentiated to give the angular spread (dispersion) of the spectrum:

.

(1)

Here a and b are the angles of incidence and diffraction, m is the diffraction order, l is the wavelength and d is the groove spacing.

EFFICIENCY BEHAVIOR

The efficiency behavior of several typical gratings used for pulse compression and stretching are shown in the figures below.

ORDERING INFORMATION

Popular transmission grating sizes and prices are listed in the Diffraction Grating Catalog and Price Guide. Different specifications can be accommodated: please contact us for price quotations. Prices are subject to change without notice.

FOR FURTHER INFORMATION

For additional information, please contact us.

SOME TECHNICAL REFERENCES

• A. Sullivan et al., "Multiterawatt 100-fs laser", Optics Letters 16, 1406-1408 (1991).
• F. G. Patterson, R. Gonzales & M. D. Perry, "Compact 10-TW, 800-fs Nd:glass laser", Optics Letters 16, 107-1109 (1991).
• D. Strickland & G. Mourou, Opt. Commun. 56, 219 (1985).
• K. Yamakawa, C. Barty et al., "Generation of a High-Energy Picosecond Laser Pulse with a High-Contrast Ratio by CPA", IEEE JQE 27, (1991).

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