Replica Gratings vs. Master Gratings

Manufacturing process. Replica gratings are made by the replication process – they are resin castings of master gratings. The master gratings themselves, though, are not castings: their grooves are created either by burnishing (in the case of ruled gratings) or by optical exposure and chemical development (in the case of holographic gratings).

Composition. Replica gratings are composed of a metallic coating on a resin layer, which itself rests on a substrate (usually glass). Master gratings also usually have glass substrates but have no resin (the grooves of a ruled master are contained entirely within a metallic layer on the substrate, and those of a holographic master are contained entirely within a layer of photoresist or similar photosensitive material).

The differences in manufacturing processes for master gratings and replica gratings naturally provide an advantage in both production time and unit cost to replica gratings, thereby explaining their popularity, but the replication process itself must be designed and carried out to ensure that the performance characteristics of the replicated grating match those of the master grating. Exhaustive experimentation has shown how to eliminate loss of resolution between master and replica – this is done by ensuring that the surface figure of the replica matches that of the master, and that the grooves are not displaced or distorted as a result of replication. The efficiency of a replica matches that of its master when the groove profile is reproduced faithfully. Other characteristics, such as scattered light, are generally matched as well, provided care is taken during the transfer coating step to ensure a dense metallic layer. [Even if the layer were not dense enough, so that its surface roughness caused increased scattered light from the replica when compared with the master, this would be diffuse scatter; scatter in the dispersion plane, due to irregularities in the groove spacing, would be faithfully replicated by the resin and does not depend significantly on the quality of the coating.] Circumstances in which a master grating is shown to be superior to a replicated grating are quite rare and can often be attributed to flaws or errors in the particular replication process used, not to the fact that the grating was replicated.

In one respect, replicated gratings can provide an advantage over master gratings: those cases where the ideal groove profile is not obtainable in a master grating, but the inverse profile is obtainable. Echelle gratings, for example, are ruled so that their grooves exhibit a sharp trough but a relatively less sharp peak. By replicating, the groove profile is inverted, leaving a first-generation replica with a sharp peak. The efficiency of the replica will be considerably higher than the efficiency of the master grating. In such cases, only odd-generation replicas are used as products, since the even-generation replicas have the same groove profile (and therefore the same efficiency characteristics) as the master itself.* The most prominent hazard to a grating during the replication process, either master or replica, is scratching, since the grating surface consists of a thin metal coating on a resin layer. Scratches involve damage to the groove profile, which generally leads to increased stray light, though in some applications this may be tolerable. Scratches faithfully replicate from master to submaster to product, and cannot be repaired, since the grating surface is not a polished surface, and an overcoating will not repair the damaged grooves.

Another hazard during replication is surface contamination from fin-gerprints; should this happen, a grating can sometimes (but not always) be cleaned or recoated to restore it to its original condition. [In use, accidentally evaporated contaminants, typical of vacuum spectrometry pumping systems, can be especially harmful when baked on the surface of the grating with ultraviolet radiation.]