# Pulse Compression and Stretching with Diffraction Gratings

Pulse compression is a useful technique that takes advantage of the relationship between pulse duration and spectral width of a pulse. This enables the amplification of laser pulses above the normal damage threshold limits imposed by the optical components in the laser system. 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 mλ=d(sinα + sinβ) can be differentiated to give the angular spread (dispersion) of the spectrum:

Here α and β are the angles of incidence and diffraction, m is the diffraction order, λ is the wavelength and d is the groove spacing.

Figure 1. Pulse compression using two gratings with the same groove frequency and efficiencies peaked for the polarization and wavelength of the laser. The damage threshold is about 200 to 300 mJ/cm2 for pulses of duration under 1 nsec.
Figure 2. Pulse stretching. This arrangement of two identical gratings allows for lower peak power to be transmitted through the laser system, thereby increasing the amount of stored energy which can be extracted.

### Master Grating Options for Pulse Compression

Gratings used for pulse compression of lasers generally require a diffracted wavefront free of aberrations as well as high diffraction efficiency. Several of our gratings, both ruled and holographic, can be used for pulse compression at wavelengths of 800 nm, 1.06 μm, 1.3 μm, 1.5 μm, etc. The groove frequencies most commonly used are 300 g/mm, 600 g/mm, 1200 g/mm and 1800 g/mm. Nominal Blaze wavelengths listed are for the first-order Littrow configuration. The maximum ruled area is groove length x ruled width. Click on a Master Grating Code (last 4 digits of a grating's part number) below to view master grating efficiency curves. Use the request a quote to get a quote based on your requirements. We are continually developing new gratings, so please contact us if you have a question regarding the best grating for your particular application.

Master Grating Code Grooves per mm Nominal Blaze Wavelength Nominal Blaze Angle (R) or Modulation Depth (H) Maximum Ruled Area (mm) Request a Quote
430H 2400 300 nm high 102 x 102 Quote
059H 2000 750 nm high 102 x 128 Quote
330H 1800 500 nm high 102 x 128 Quote
233H 1760 550 nm high 102 x 102 Quote
239H 1500 750 nm high 102 x 102 Quote
230H 200 800 nm high 110 x 110 Quote
340R 1200 600 nm 21.1° 204 x 306 Quote
360R 1200 750 nm 26.7° 156 x 206 Quote
727R 672 832 nm 16.4° 64 x 64 Quote
351R 600 800 nm 13.9° 154 x 206 Quote