Holographic Gratings

Plane Gratings

High efficiency diffraction gratings
When ordering a plane grating, please use the following example format:

P 1200 W x H x Thk 700-900 nm (TM+TE)/2 (-1) constant deviation angle 10°

  • P for Plano and L for Littrow gratings (optional information)
  • 1200 is the groove density (groove frequency) in grooves/mm
  • W is the blank dimension in mm parallell with the grating grooves
  • H is the blank dimension in mm perpendicular to the grating grooves
  • Thk is the blank thickness in mm
  • 700-900 nm is the desired optimisation range. A specific wavelength or range with peaked wavelength can also be specified
  • Average (TM+TE)/2 (-1) is desired polarisation state and diffraction order the grating should be optimised for. TM and TE can also be specified
  • Constant deviation angle 10° is the configuration the grating should be optimised for. A constant incidence angle α° can also be specified.

Groove densities: Spectrogon manufactures 600 grooves/mm to 3600 grooves/mm on a regular basis. For lower or higher groove densities please contact our sales team.

Wavelength range from UV to approximately 2000 nm

Standard sizes: 25 x 25 x 6 mm, 30 x 30 x 6 mm, 50 x 50 x 6 mm, , 50 x 50 x 10 mm, 58 x 58 x 10 mm, 64 x 64 x 10 mm, 90 x 90 x 16 mm, 110 x 110 x 16 mm, 100 x 140 x 20 mm, 120 x 140 x 20 mm

Standard tolerances on W, H, Dia: ± 0.2 mm Thk ± 0.5 mm. CA > 90 % of each dimension.

Other specifications available on request, contact our sales department!

A Plane type of grating is the choice for high resolution spectroscopy and applications where low stray light levels are of high importance. With these gratings the spectral lines will be sharper, accurately on wavelength, and, in the case of absorption lines, deeper than with other gratings on the market.

Extremely low stray light
The gratings are holographically recorded with two highly collimated, clean and homogeneous beams, which give straight and equispaced grooves. The diffracted light from these gratings is free from ghost spectral lines. The randomly scattered light is as low as that from a good front surface aluminium mirror.

Optimised efficiency
The groove profile is symmetric sinusoidal, with a groove depth optimised for the spectral region of use. For obtaining the highest efficiency, these gratings are preferably used in configurations where only two diffracted orders (-1 and 0) are present, i.e. high groove frequency is preferred. In such case, the efficiency is comparable or better than for ruled blazed gratings. The groove depth variation across the grating surface is very small, also for the very highest groove frequencies. This means you can make full use of all the grating surface, for obtaining maximum throughput in your instrument.

The combination of a flat grating surface, extremely straight and equally spaced grooves gives a flat diffracted wavefront making it possible to obtain maximum wavelength resolution.

Accurate groove frequency
The groove frequency of the gratings is accurate within ±0.2 grooves/mm of the nominal value. This means a reliable wavelength reading in your instrument.

A plane grating is designed to meet the specifications for size, wavelength range, angle of incidence and angle of diffraction, but not for a specific focal length for the optical system. Therefore it is possible to use the same grating for different optical arrangements as long as the four previously mentioned parameters are the same.

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.

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.

Wavelength scale
For a constant deviation mounting and with an angular deviation of, the grating equation can be written (assuming -1 order diffraction):

sin(α + δ/2) = λ/(2dcos δ/2)

We see that the wavelength transmitted by the monochromator is proportional to the sine of the rotation angle of the grating. Monochromators are often equipped with a special sine bar mechanism which facilitates the wavelength reading.

Light throughput
The throughput of a grating based spectroscopic instrument depends on a number of factors, such as the radiance of the light source, the F-number of the optical system, the width and height of the entrance slit, the spectral bandwidth of the instrument and on the sensitivity of the detector.

In a monochromator it is often more efficient to use a high frequency holographic grating, than a classically ruled grating of lower frequency, though the efficiency may be higher for the classically ruled grating. A grating with high frequency gives higher wavelength dispersion. For a given wavelength resolution, one is therefore able to use wider slits in the monochromator, which improves the light throughput.

In a spectrograph the grating is fixed and the detector simultaneously detects the different spectral components in the focal plane of the instrument. Modern instruments often utilize array detectors. Spectrographs with plane gratings are often made as modified Czerny-Turner configuration specially designed to give a flat focal plane.

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