Stability of Repicated Gratings

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 110 °C and above. 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.

Temperature and Relative Humidity. Fiber optic telecommunications applications often require diffraction gratings that meet harsh environmental standards, particularly those in the Telcordia 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.

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 λ = 10.6 um was reported by Huguley and Loomis to generate damage to the surface of replicated grating at about 150 kW/cm² or above.

Gill and Newnam 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 λ = 1.06 um. 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.

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.

Table 5-1. Damage thresholds reported by Gill and Newnam. For these gratings, the difference in damage 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.

Experimental damage thresholds for continuous wave (cw) beams, reported by Loewen and Popov, 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 who studied optical components illuminated by a pulsed Nd:YAG laser operating at λ = 1.06 um.

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