Holographic Gratings

Wavelength Tuning of Lasers
Holographic gratings are often used for wavelength tuning of lasers. The grating acts as a wavelength selective end mirror inside the laser cavity. There are two basic configurations used, Littrow configuration and Grazing incidence or Littman configuration.

Littrow configuration
The grating is mounted so that light of the desired wavelength is diffracted back along the incident beam, and the wavelength is scanned by rotating the grating. Generally, an intracavity achromatic lens is used, which expands the laser beam to fill a relatively large area of the grating. The zero order diffracted beam can be used as the output laser beam; however, a disadvantage is that the beam will have different directions as the grating is rotated.

Grazing incidence Littman configuration
The grating is kept fixed at an angle of incidence near 90 degrees, and the wavelength is tuned by rotating a special tuning mirror. No beam expanding lens is needed, and therefore a smaller grating can be used. The large incidence angle implies, however, that the ruled width of the grating has to be considerably greater than the groove length.

The efficiency in grazing incidence may be very high for light polarized perpendicularly to the grating grooves (TM polarization), but is always very low for TE polarization. The dye laser beam will therefore be plane polarized.

Laser Pulse Compression
When a short laser pulse is transmitted through an optical fibre, the pulse will be stretched, or “chirped” due to nonlinear effects (selfphase modulation).The group velocity dispersion in the fibre results in that the front of the pulse will have a longer wavelength than the tall. By using a pair of gratings one can arrange so that the long wave length pulse will travel a longer path than the short wavelength pulse, with the result that, after the grating pair, they arrive at the same time. The grating pair not only compensates for the pulse broadening in the fibre, but makes the pulse even shorter than the input. Up to 90 times compression can be achieved.

Chirped pulse amplification
Very short pulses (100 femtoseconds) can be produced by some types of mode locked lasers. For many applications, these pulses have too low peak power. The technique of chirped pulse amplification (CPA) can be used for amplifying such pulses, to achieve peak powers in the order of Terawatts.

The amplifier is basically a laser crystal inside a resonator. To avoid strong nonlinear effects which would destroy the crystal, the input pulse is stretched in time, so that the peak power is decreased. This chirped pulse is then amplified, and subsequently compressed to obtain a high power pulse with a duration nearly equal to the input pulse.

Stretching and compression
Both stretching and compressing utilize grating pairs, arranged in subtractive dispersive mode; so that the angular dispersion of the first grating is cancelled by the second grating. Two parallel beams of different wavelengths, incident on the first grating, are still parallel when they leave the second grating, but they have travelled different distances.

A grating pair arranged parallel will introduce a negative group velocity dispersion, i.e. pulses of long wavelength arrive later than short wave pulses.

In order to achieve a positive dispersive delay, a more complicated arrangement is necessary. An afocal lens system (telescope) is inserted between the gratings. The telescope reverses the sign of the angles so that the beams will hit the second grating at the same angle as they leave the first one.

Both stretcher and compressor are normally used in double pass. The advantages are twofold: the dispersion is doubled, and all wavelength components of the beam emerge colinear, not linearly translated as shown in the figure for single pass.

Spectroscopic Instruments
A spectroscopic instrument consists generally of an entrance slit, a collimator, a dispersive element, focusing optics, and sometimes an exit slit. Radiation entering the entrance slit is collected by the collimator, generally a concave mirror.

The dispersive element, in this case a grating, deviates the radiation in a direction which depends on the wavelength. The dispersed radiation is focused onto the image plane, where a spectrum (a series of monochromatic images of the entrance slit) is formed.

Monochromators
In a monochromator there is an exit slit, which transmits a narrow portion of the spectrum. The entrance and exit slits are fixed, and the spectrum is scanned by rotating the grating. The grating thus works with a constant angular deviation between the incident and diffracted light. This is true for most types of monochromators, such as the Czerny-Turner, Ebert and Littrow types.

Fibre Optics
Holographic gratings are well suited for fibre optics applications. By using high frequency gratings, high efficiency can be achieved, and high angular dispersion makes it possible to design small compact instruments.

Raman Spectroscopy and Laser Scattering Experiments
In laser scattering studies, such as Raman spectroscopy and Thomson scattering for plasma diagnostics, the requirements on the grating are very high. The specimen is illuminated by laser light, and resonance scattering gives rise to weak spectral lines which are very close to the strong laser line. In Raman spectroscopy, the peaks may have an intensity of only 10-12 of the laser light, and may be separated only 10 cm-1 from the laser line.
The necessary high resolution is achieved by using large instruments with long focal lengths, where all optical surfaces are of the highest quality. When working very close to a strong spectral line, aberrations of the optical system, and Fraunhofer diffraction from aperture stops may yield considerable stray light. Spectrogon low stray light gratings are manufactured on high optical quality substrates and such a grating will have practically no influence on the optical aberrations. Double or triple spectrometers are frequently used in order to reduce the stray light. Holographic gratings are necessary, since even the best ruled gratings give rise to ghosts, which are orders of magnitude stronger than the spectral peaks to be detected.

Absorption Spectroscopy
Absorption spectroscopy is another application where the low stray light of holographic gratings is of great advantage. The stray light level is directly related to the absorbance range of the instrument, the smaller amount of stray light present, the higher absorbance values can be measured.

The light source in absorption spectroscopy is generally a broad band source, and the stray light will therefore consist of a continuum of wavelengths. Each wavelength component of the incident light gives rise to stray light, centered around the actual wavelength.

The resulting stray light is the sum of all wavelength components.