Decades of research and development at Newport have contributed to the process for manufacturing replicated diffraction gratings (replicas) of spectroscopic quality. This process is capable of producing thousands of duplicates of master gratings which equal the quality and performance of the master gratings themselves. The replication process has reduced the price of a typical diffraction grating by a factor of one hundred or more, compared with the cost of acquiring a master grating, as well as greatly increasing their commercial availability.

The process for making replica gratings results in a grating whose grooves are formed in a very thin layer of resin that adheres strongly to the surface of the substrate material. The optical surface of a reflection replica is usually coated with aluminum (Al), but gold (Au) or silver (Ag) is recommended for greater diffracted energy in certain spectral regions. Transmission gratings have no reflective coating.

Most commercially-available surface-relief gratings are made using a casting process, which faithfully reproduces the three-dimension nature of the grating surface. [It is for this reason that photographic replication techniques are not generally sufficient.²⁸]

The casting process for the production of a replicated diffraction grating is a series of sequential steps:

• Submaster selection. The replication process starts with the selection of a suitable submaster grating that has the desired specifications (groove frequency, blaze angle, size, etc.). [A submaster grating is a grating replicated from a master, or from another submaster, but is itself used not as a final optical product but as a mold for the replication of product gratings; for this reason, it is not strictly required that a submaster grating meet all of the performance specifications of the product grating (e.g., it need not have a suitably reflective coating).]

• Application of parting agent. A parting agent is applied to the surface of the submaster grating. The parting agent serves no optical purpose and should have no deleterious optical effects but aids in the separation of the delicate submaster and product grating surfaces. Since the replicated optical surface is intended to match that of the submaster as closely as possible, the parting agent must be very thin and conformal to the surface of the submaster.²⁹

• Application of transfer coating. After the parting agent is applied, a reflective coating (usually aluminum) is applied to the surface of the submaster. This coating will form the optical surface of the product grating upon separation. To obtain an optical quality coating, this step is performed in a vacuum deposition chamber. [Since this coating is applied to the submaster, but transfers to the product grating upon separation, it is called a transfer coating.] Typical transfer coating thicknesses are about one micron.

• Cementing. A substrate is then cemented with a layer of resin to the grooved surface of the master grating; this layer can vary in thickness, but it is usually tens of microns thick. It is the resin that holds the groove profile and replicates it from the submaster to the product; the transfer coating is much too thin for this purpose. The "sandwich" formed by the substrate and submaster cemented together is shown in Figure 5-1.

  Since the resin is in the liquid state when it is applied to the submaster, it must harden sufficiently to ensure that it can maintain the groove profile faithfully when the product grating is separated from the submaster. This hardening, or curing, is usually accomplished by a room-temperature cure period (lasting from hours to days) or by heating the resin to accelerate the curing, though gratings can also be replicated using a UV-curable resin.³⁰

Figure 5-1.   The replication "sandwich", showing the substrates, the resin layers, the metallic coatings, and the parting agent.

• Separation. After the resin is fully cured, the groove profile is faithfully replicated in the resin when the submaster and product are separated. The parting agent serves as the weak interface and allows the separation to take place between the submaster coating and the transfer metallic coating. The groove profile on the product is the inverse of the groove profile on the submaster; if this profile is not symmetric with respect to this inversion, the efficiency characteristics of the product grating will generally differ from those of the submaster grating. In such cases, an additional replication must be done to invert the inverted profile, resulting in a profile identical to that of the original submaster. However, for certain types of gratings, inversion of the groove increases efficiency significantly.

  At this stage, if a transmission grating is desired, the transfer coating is removed from the product, leaving the groove structure intact in the transparent resin.

• Inspection. After separation, both the submaster and the product gratings are inspected for surface or substrate damage. The product grating may also be tested for key performance characteristics (e.g., efficiency, wavefront flatness (or curvature), scattered light, alignment of the grooves to a substrate edge) depending on requirements.

The product grating formed by this replication process may be used as an optical component, or it may serve as a mold (replication tool) by being considered a submaster. In this way, a single master grating can make several submasters, each of which can make several more submasters, etc., to form a replication tree (see Figure 5-2).

The replication tree shown in Figure 5-2 illustrates two important features of replication: extension horizontally (within a generation) and vertically (to subsequent generations). Replication within a generation is accomplished by the successive replication of a single grating (much as a parent can have many children). Replication to additional generations is accomplished by forming a replica (child), which itself forms a replica (grandchild), etc. Thus replication can extend both within generations (X-1, X-2, X-3, X-4, …) and to subsequent generations (X-1, X-1-3, X-1-3-1, X-1-3-1-4, …) to create a large number of replicas from a single master grating.

As an example, consider a master grating X from which five first-generation replicas are made (X-1 through X-5). Each of these is used as a submaster to form five replicas: X-1 forms X-1-1 through X-1-5, X-2 forms X-2-1 through X 2-5, and so on. This forms twenty-five second generation replicas. If each of these replicas is itself replicated five times, we arrive at 125 third-generation products (X-1-1-1, X-1-1-2, …, through X-5-5-5). This example illustrates that a large number of replicas can be made from a single master grating, assuming a conservative number of replicas and a reasonable number of generations.

Figure 5-2.   A replication tree. Master X is replicated to create several first-generation replicas (X-1, X-2, …), which themselves are replicated to form second-generation replicas (X-1-1, …), etc.

The number N of replicas of a particular generation that can be made from a single master can be estimated using the following formula,

(5-1)

where R is the number of replications per generation and g is the number of generations. Reasonable values of R are 5 to 10 (though values well above 20 are not unheard of), and g generally ranges from 3 to 9. Conservatively, then, for R = 5 and g = 3, we have N = 125 third-generation replicas; at the other end of the ranges we have R = 10 and g = 9 so that N = 1,000,000,000 ninth-generation replicas. Of course, one billion replicas of a single grating has never been required, but even if it were, Eq. (5-1) assumes that each replica in every generation (except the last) is replicated R times, whereas in practice most gratings cannot be replicated too many times before being damaged or otherwise rendered unusable. That is, some branches of the replication tree are truncated prematurely. Consequently, Eq. (5-1) must be taken as an upper limit, which becomes unrealistically high as either R or g increase. In practice, N can be in the thousands, and can be even higher if care is taken to ensure that the submasters in the replication tree are not damaged.

There are two fundamental differences between master gratings and replica gratings: how they are made and what they are made of.

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 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 fingerprints; 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.]

Temperature. There is no evidence of deterioration or change in standard replica gratings with age or when exposed to thermal variations from the boiling point of nitrogen (77 K = –196 °C) to 50 °C. Gratings that must withstand higher temperatures can be made with a special resin whose glass transition temperature is high enough to prevent the resin from flowing at high temperatures (thereby distorting the grooves). In addition to choosing the appropriate resin, the cure cycle can be modified to result in a grating whose grooves will not distort under high temperature.

Gratings replicated onto substrates made of low thermal expansion materials behave as the substrate dictates: the resin and aluminum, which have much higher thermal expansion coefficients, are present in very thin layers compared with the substrate thickness and therefore do not expand and contract appreciably with temperature changes since they are fixed rigidly to the substrate.

Relative Humidity. Standard replicas generally do not show signs of degradation in normal use in high relative humidity environments, but some applications (e.g., fiber-optic telecommunications) require extended exposure to very high humidity environments. Coatings and epoxies that resist the effects of water vapor are necessary for these applications.

Instead of a special resin, the metallic coating on a reflection grating made with standard resin is often sufficient to protect the underlying resin from the effects of water vapor. A transmission grating that requires protection from environmental water vapor can be so protected by applying a dielectric coating (e.g., SiO) to its grooved surface.³¹

Temperature and Relative Humidity. Recent developments in fiber optic telecommunications require diffraction gratings that meet harsh environmental standards, particularly those in the Telcordia (formerly Bellcore) document GR-1221, "Generic Reliability Assurance Requirements for Passive Optical Components". Special resin materials, along with specially-designed proprietary replication techniques, have been developed to produce replicated gratings that can meet this demanding requirement with no degradation in performance.

High Vacuum. Even the highest vacuum, such as that of outer space, has no effect on replica gratings. Concerns regarding outgassing from the resin are addressed by recognizing that the resin is fully cured. However, some outgassing may occur in high vacuum, which may be a problem for gratings used in synchrotron beamlines; in certain cases ruled master gratings are used instead.

Energy Density of the Beam. For applications in which the energy density at the surface of the grating is very high (as in some pulsed laser applications), enough of the energy incident on the grating surface may be absorbed to cause damage to the surface. In these cases, it may be necessary to make the transfer coat thicker than normal, or to apply a second metallic layer (an overcoat) to increase the opacity of the metal film(s) sufficiently to protect the underlying resin from exposure to the light and to permit the thermal energy absorbed from the pulse to be dissipated without damaging the groove profile. Using a metal rather than glass substrate is also helpful in that it permits the thermal energy to be dissipated; in some cases, a water-cooled metal substrate is used for additional benefit.³²

Pulsed lasers often require optical components with high damage thresholds, due to the short pulse duration and high energy of the pulsed beam. For gratings used in the infrared, gold is generally used as the reflective coating (since it is more reflective than aluminum in the near IR).

A continuous-wave laser operating at l = 10.6 µm was reported by Huguley and Loomis³³ to generate damage to the surface of replicated grating at about 150 kW/cm² or above.

Gill and Newnam³⁴ undertook a detailed experimental study of laser-induced damage of a set of master gratings and a set of replicated gratings using 30-ps pulses at l = 1.06 µm. They reported that the damage threshold for the holographic gratings they tested was a factor of 1.5 to 5 times higher than for the ruled gratings they tested. Differences in the damage threshold for S- vs. P-polarized light were also observed: the threshold for S-polarized light was 1.5 to 6 times higher than for P-polarized light, though how this correlates to grating efficiency in these polarization states is not clear. The (holographic) master gratings tested exhibited lower damage thresholds than did the replicated gratings. Some of the experimental results reported by Gill and Newnam are reproduced in Table 5-1.

Table 5-1.   Damage thresholds reported by Gill and Newnam. For these gratings, the difference in dame threshold measurements between Au and Al coatings, between P- and S-polarization, and between the 1800 g/mm holographic gratings and 300 and 600 g/mm ruled gratings are evident.

Increasing the thickness of the reflective layer can, in certain circumstances, greatly increase the damage threshold of a replicated grating used in pulsed beams, presumably by reducing the maximum temperature which the metallic coating reaches during illumination.³⁵

Experimental damage thresholds for continuous wave (cw) beams, reported by Loewen and Popov³⁶, are given in Table 5-2.

Coating defects can play a critical role in the incidence of laser damage, as reported by Steiger and Brausse,³⁷ who studied optical components illuminated by a pulsed Nd:YAG laser operating at l = 1.06 µm.

Table 5-2.   Damage thresholds for continuous wave (cw) beams.