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DIFFRACTION GRATINGS
Technical information
A gratings consists of a series of equally spaced parallel grooves made on the surface of a suitable material such as polished glass or copper and over coated with a reflective material. Two types of gratings are available. Ruled gratings are made by ruling parallel grooves using precision ruling engine with diamond tool whiles holographic gratings are made using interferometer technique to produce interference fringes on a material coated with photo resist followed by developing. The distance between the adjacent grooves or fringes and the angle the grooves form with respect to the substrate surface help to determine the dispersion and efficiency of a grating. In order to maximise the performance of a grating, energy must be concentrated into one of the orders (except the zero order) as energy distribution depends on the groove shape. The principle is to rule the grating so that the reflecting groove is tilted with respect to the grating surface. For holographic gratings the profile of the sinusoidal shape is important.
Transmission gratings are made on polished glass surfaces deposited with index matched epoxy resin. Ruled and holographic transmission gratings are commonly used for laser beam division and multiple laser line separation in wide spectral region in the visible region. The transmitted beam is diffracted into multiple orders. By optimising grating parameters such as materials, coating and groove profile various gratings can be produced offering different dispersion, polarisation and power distributions.
The general grating equation which enables selection to separate polychromatic
radiation into its constituent wavelengths is usually written as:
nλ = d (sin α ± sin βn)
Where n = order number of diffraction, λ = diffracted wavelength, d = distance between successive groves or fringes, α = angle of incidence measured from the grating normal and βn = angle of diffraction of the nth order measured from the grating normal. θ = brazed angle.
In the Littrow configuration λ = βn and the grating equation reduces to
nλ = 2d sinα
For Transmission gratings at normal incidence, the grating equation becomes
nλ = d sinβn
The selection of a grating requires consideration of a number of variables related to the intended use of the grating including efficiency, blaze wavelength, wavelength range, resolving power and stray light.
Medway Optics offers both original and replicated gratings in ruled and holographic versions for laser and spectroscopy applications from UV-VIS to Mid-IR. Original gratings are ruled directly into an aluminium coating deposited on kanigen coated copper substrate resulting in an inherently higher damage threshold. Replicas are produced onto Pyrex substrate. Replication is a useful technique for making a number of gratings with identical properties and almost equivalent properties and quality as the original. High peak efficiencies and damage thresholds values associated with these types of grating make them highly favoured as end reflectors in laser cavity for tuning molecular lasers.
Optional overcoat of the aluminium with gold increases the reflectivity in the near infrared (from 650nm) to the far infrared and also offers better protection against oxidation.
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| General Specifications
| Clear aperture | 90% |
| Groove parallelism to edge | ± 0.5° |
| Dimensional tolerances | ± 0.5mm |
| Thickness | Variable 6-20mm |
| Sizes | Variable ( squares, rectangular, circular) |
| Shape | Flats and Concave |
| Laser Damage threshold | From 0.1 to 1.5 KW/cm2 CW depending on type |
| Grating absolute efficiency | Up to 95% depending on brazed wavelength. Values for holographic gratings at peak wavelength are gennerally lower.
| | Spectral range | 300nm to >16 microns
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Ordering
Please request for a quote specifying:
Size (blank size, clear aperture and thickness)
Shape (with radius of curvature where necessary)
Spectral range and brazed wavelength (for ruled gratings)
Number of grooves per mm
Coating preference (Al, Al + SiO2 overcoat, Al + MgF2 overcoat, or Gold )
Source characteristics
Quantity
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