Plane gratings More...
For a plane blazed grating, the groove spacing and blaze angle determine the distribution of energy. The blaze direction for most gratings is specified for first order Littrow use. In Littrow use, light is diffracted from the grating back toward the source. Gratings used in the Littrow configuration have the advantage of maximum efficiency, or blaze, at specific wavelengths.
Concave gratings More...
For the spectral region below 250 nm, concave gratings are preferable. Concave gratings are also used frequently between 120 and 400 nm, functioning as both the dispersing and focusing element for spectrographs as well as monochromators. For wavelengths below 30 nm, reflectivity factors mandate switching from a normal incident angle to a grazing incident angle and accepting the increased astigmatism that results. Concave holographic gratings make possible short radii gratings with low ƒ/# and flat-field imaging suitable for array spectrographs. These gratings are part of a system design and usually call for interaction between the designer and the manufacturer.
Echelle gratings More...
Echelle gratings are special rulings with high-blaze angles that are used mainly in high diffraction orders. Echelles provide very high dispersion and resolution, combined with high efficiency and compact design. Some type of order separation is essential, with cross-dispersion provided by a prism or another grating, producing an image-plane format compatible with CCD or CID detectors.
Large astronomical gratings More...
Large astronomical gratings have ruled areas from 128 x 254 mm (5" x 10") to 304 x 406 mm (12" x 16"). Replication of smaller replicas from these large gratings may require an additional tooling charge.
Holographic gratings More...
Holographic gratings normally have a sinusoidal groove shape, which is the result of recording interference fringe fields in photoresist material. Since the grooves are symmetrical, they do not have a preferred blaze direction and hence the gratings carry no blaze arrows. The range of useful diffraction efficiency is controlled by varying the modulation (the ratio of groove depth to groove spacing). The lower the modulation, the shorter the wavelength limit to which the grating can be used, but the peak efficiency may be lowered as well. We have found that three modulation levels are adequate for nearly all purposes.
Blazed holographic gratings More...
- Sheridon gratings
In addition to sinusoidal grooves, it is possible to make triangular groove gratings in photoresist by recording fringe fields inclined at a small angle with respect to the resist layer. This Sheridon method leads to blaze performance very similar to a ruled grating, but these gratings are restricted to blaze peaks near 250 nm. They have the same low stray light performance as gratings with sinusoidal grooves.
- Ion-etched gratings
Gratings can also be blazed by bombarding their grooves with a beam of ions. This ion etching process changes the groove profile from sinusoidal to triangular, which can in certain cases increase the peak efficiency of the grating.
Laser tuning gratings More...
- Diode and dye laser tuning grating
Dye laser wavelength tuning, in the visible region of the spectrum, is done in two different modes. The classical one uses a grating in the autocollimating (Littrow) mount where the beam requires expansion to fill the grating in order to obtain adequate resolution. Telescope or prism optics fulfill this need. The alternative approach is to use the grating in a fixed grazing incidence mode together with a rotating reflecting tuning element in the form of either a mirror or a second grating.
Littrow tuning is done either with fine pitch first-order gratings (typically 1800 or 2400 g/mm frequency, either ruled or holographic) or a coarser grating used in higher orders. For the latter, a 600 g/mm, 54° blaze angle grating is particularly useful, because it covers the visible spectrum in orders 3 to 7 with free spectral ranges that match the dyes and prevent overlap.
Grazing incidence tuning is done in first order only and 1800 g/mm, 2000 g/mm, and 2400 g/mm holographic gratings are preferred. The gratings have their ruled width filled by incidence angles of 80° to 88°.
Steep angle usage leads to special grating dimensions such as 16.5 x 58 x 10 mm.
Gratings for this application are listed in Table 8.
- Pulse compression gratings
Gratings used for pulse compression of lasers generally require a diffracted wavefront free of aberrations as well as high diffraction efficiency and a high damage threshold. Several of our gratings, both ruled and holographic, can be used for pulse compression at wavelengths of 800 nm, 1.06 microns, 1.3 microns, 1.5 microns, etc. The groove frequencies most commonly used are 300 g/mm, 600 g/mm, 1200 g/mm and 1800 g/mm.
Newport Corporation is continually developing new gratings, so please contact us if you have a question regarding the best grating for your particular application.
- Molecular laser tuning gratings
Molecular lasers, operating both pulsed and continuous-wave (cw) in the infrared, typically have their output wavelength tuned by Littrow-mounted gratings. High efficiency is obtained by operating in the first order at diffraction angles > 20°. This corresponds to wavelength-to-groove-period ratios from 0.67 to 1.8, which ensures that only the zero and first orders can diffract. The output will be polarized in the S-plane (i.e., with the electric vector perpendicular to the grooves) because the efficiency will be several times greater than in the P-plane (for which the electric vector is parallel to the grooves).
Dispersion is a function of the tangent of the diffraction angle β and is chosen from medium (β 
20°) to very high (β > 50°) as required. Note from Table 9, which summarizes gratings most suitable for this application, that high efficiency corresponds to diffraction angles that can be significantly greater than the groove or blaze angles. This is a consequence of the electromagnetic nature of diffraction from deep groove gratings. For maximum efficiency, any of these gratings can be supplied in the form of gold replicas.
Some molecular lasers operate at high power, capable of destroying gratings. In the case of pulsed lasers, extra thick replica films may be of help, but at maximum intensity levels only original gratings survive. In the case of cw lasers, replicas on metal substrates are superior to glass because of far greater thermal conductivity; in some cases it is advisable to supply the substrates with water cooling. Otherwise, ruled originals on metal substrates are advisable. In all cases, close attention to groove geometry maximizes reflection, minimizes absorption and leads to improved grating performance.
The table below serves as a guide to the typical power levels a grating can be expected to survive.
Grating damage thresholds
Since the applications in which gratings are used vary widely, we do not certify damage threshold figures. Instead, we offer the following general thresholds, which have been determined by independent researchers and published in the open literature:
Pulsed lasers at 1.06 µm
Standard gold replica gratings can withstand 300 mJ/cm2 pulses of 100 ps duration.
Cw lasers at 10 µm
Standard gold replica on copper 100 W/cm2
Water-cooled gold replica on copper 200 W/cm2
There are a number of masters available which are used to produce replicas with high S-plane efficiency for use with CO2, CO, HF, or DF lasers (see Table 9). For this type of application, we suggest you advise us of the following specifications:
Spectral region of interest
Peak power
Pulse duration
Beam size
Transmission gratings More...
Transmission gratings (Tables 6 and 7) can be made from any plane master in the catalog. Special-quality substrates have anti-reflectance coating on the back face to reduce light loss and internal reflections. Geometrical optics considerations require relatively coarse spacings (no more than 600 g/mm). Finer grating pitches are possible, but at sharply reduced efficiencies. For transmissions gratings, the blaze angle is defined to be the angle at which a normally incident beam at the blaze wavelength is diffracted. It is not equal to the groove angle (which is given in Tables 6 and 7). In addition, transmission gratings can serve as beam dividers. When two beams are required, a coarse, low blaze angle transmission grating will direct most of the incident energy into the zero and first orders. When three beams are required, a lamellar groove transmission grating divides light between the zero order and two equal first orders in almost any intensity ratio desired.
Grisms (grating prisms) More...
Transmission gratings can also be replicated onto the face of a prism (to form a grism), which produces a straight-through spectrum, undeviated at one central wavelength. In such cases the groove angle is often chosen to be approximately equal to the prism angle. For a thorough treatment of grism equations, see W. A. Traub, Journal of the Optical Society of America A, Volume 7, September 1990, page 1779.
New gratings More...
Newport Corporation is continually developing new gratings, so please contact us if you have a question regarding the best grating for your particular application.
