LightSmyth has a new concept in grating design – gratings with integral calibration features. A user of these gratings will enjoy the same high performance and low cost of low scatter reflection gratings with the additional benefit of advanced wavelength calibration offered by a thin array of embedded micro-gratings. A schematic representation of such a grating is shown in the figure to the right. The Primary Grating is used in the traditional manner, but the 12 alignment/calibration gratings, when illuminated by a HeNe laser or other reference, provide a family of calibration markers in the detection plane. These markers allow high precision and convenient calibration of the Primary Gratings dispersion line. As described in the application note “Gratings with Integral Calibration Features“, the markers also allow precise monitoring of the input signal wavefront and the proper detection-plane focus. Significantly, the primary grating wavelengths indicated by the markers are independent of the angle of incidence of the input light. Essentially, one is calibrating the primary grating’s output plane through differential comparison of the ratio of line densities of the primary and alignment/calibration gratings.

In this product, the Primary and alignment/calibration gratings are spatially coherent (i.e. positioned with interferometric accuracy relative to one another) and are part of a single monolithic structure. The product comes in two versions. In the first, the lines of all gratings are parallel and marker output signals fall along the dispersion line of the Primary Grating. In the second, lines of the alignment/calibration gratings have tilts precisely calculated to provide NeHe generated markers that fall along a line slightly displaced from but parallel to the Primary Grating dispersion line. Alternative reference light sources can be employed as well in which case calibration markers are still precisely correlated with specific primary grating output wavelengths in an input-angle-independent manner. The markers shift their primary grating reference wavelength (as given in Table 1) in proportion to the ratio of the new reference wavelength pided by 632.8 nm.

Table 1 details the properties of both the tilted and non-tilted versions of the product. One inch gratings contain the full set of 12 alignment/calibration gratings. Half-inch gratings contain a subset of 6. The alignment/calibration gratings are designed so that subsets of 6 gratings provide a useful set of markers. For Table 1, signal light is incident at 30 degrees from the normal on the same side as the output. Other input angles may be employed without changing the marker wavelengths.

Typical grating thickness: approximately 0.73 mm
Substrate height and width tolerance: 0.3 mm
Grating substrate: single crystal silicon
Grating coating: aluminum (other coating types are optional)