By Bruce M. Lairson
Infrared optics are often used at oblique angles of incidence and over a range of wavelengths. While the best performance is obtained when spectral performance does not change with angle of incidence, such a shift is intrinsic to interference filters because of changes in optical-path length and effective index with angle. Design can mitigate spectral shifts, however, by using materials with relatively high refractive index, and by changing the coating thickness across an optic to accommodate the change in weighted-average angle of incidence.
It is a good idea in many applications to use spectral filters in which the beam is incident at an oblique angle, yet the change in the filtered spectrum with angle can present a problem. What can make this spectral shift worse is the need to minimize the s- and p-polarization separation of the filter. The polarization separation can occur in transmission, reflection, or in the amount of absorbed or scattered light.
Designers can eliminate polarization separation at one wavelength, or mitigate it over a range of wavelengths, by applying design constraints to the design of the thin-film coating. They also can improve angular sensitivity somewhat by tailoring the stack design using the materials available to the particular spectral requirements for a filter.
The designer can improve design flexibility by using relatively high-index materials-if these materials are available. The effective index varies less with angle for high-index materials, and the dispersion between s- and p-polarizations is lower. The reduced native polarization separation allows more of the available design freedom toward accommodating angle shift, and facilitates designs with improved overall performance.
The basic angular sensitivity, and degree to which a design must accommodate s and p splitting, compares with quarter-wave stacks of various materials. The different physical thicknesses are the result of holding the optical thickness constant. Interestingly, even though the Si:H/SiO2 has a smaller thickness than the niobia/silica stack, and thus a higher average index, it exhibits greater s and p splitting.
The Si:H/silicon nitride quarter-wave stack exhibits much lower s-p splitting and angle shift, at about the same index ratio, as the niobia/silica stack. As a result, when this stack is incorporated into a design, minimizing polarization splitting imposes a smaller constraint on the stack design.
Generally, even though choosing higher-index materials does not eliminate angle shifts and polarization splitting, these design constraints are less dominant if higher-index materials are used for the coating.
Experimental filters
Coatings were deposited in a Deposition Sciences Inc. (DSI) MicroDyn reactive-sputtering chamber, which holds the substrates on a rotating drum and sequentially exposes them to sputtered materials and a microwave plasma. The magnetrons went into the balanced configuration and supported Si targets with dimensions of 5 by 15 by 0.25 inches. The filters were deposited with DC power applied to the sputter targets.
Bandpass filters that perform acceptably over a wide angular range typically require extinction coefficients of less than about 10-03 in the bandpass wavelength range. Optimizing materials processes, and the transition processes between the layers, engineers can obtain acceptable extinction coefficients with Si:H for wavelengths above 1000 nanometers.
Compatible silicon dioxide and silicon nitride process were developed for use with this material. Durability testing of model multilayers and filters showed acceptable durability against abrasion and humidity. Filter characteristics did not change for temperatures below about 200 degrees Celsius.
For a given design, it is difficult to quantify the advantage of relatively high-index materials versus design skill. The “Maximum Principle” holds that designers can achieve the best design at normal incidence with the materials with the highest and lowest refractive indices. There is no such general principle for optical designs at large oblique angles, however. Also, the optical constants of a materials set may be offset by deficiencies in durability, morphology, or film stress.
A shortwave-pass edge filter constructed from Si:H and silicon nitride for operation at 45 degrees C has a pass band with more than 95 percent transmission in the range 1450 to 1500 nanometers, and a blocking band above 1530 nanometers. The polarization splitting has been reduced from 32 to 2 nanometers via design optimization. The filter has about 30 dB rejection in the rejection band.
This filter is considerably better than the performance achieved with lower-index materials. The two filters were produced to the same design targets, using the same sputtering system. The table shows considerably better performance for the edge sharpness, the angle shift, and polarization splitting.
Not surprisingly, the filter composed of high-index materials has reduced angle shift and reduced polarization spitting. Perhaps more surprising is the reduced thickness and sharper edge of the Si:H/Si3N4 filter. This is due to the smaller design accommodation required to correct the native polarization splitting and angle shift. Generally, it appears that to achieve similar performances near 45 degrees C, less than half the film thickness is required for Si:H/Si3N4 than for niobia/silica multilayers.
High-index materials are particularly useful in fabricating filters at near-grazing incidence. A near IR bandpass at 65 degrees C and at 82 degrees C achieves greater than 50 percent transmission at the wavelength of interest over this angular range, with a bandpass width of approximately 50 nanometers.
The filter operates well over the angular range of interest, despite coating functions near the substrate Brewster angle. For this application, the use of Si:H as a high-index material dramatically improved the filter capability, such that it was not possible to find an comparable design solution based on niobia/silica or other lower-index materials.
Designers also can improve oblique performance by grading the pass band of a filter across a lens or other curved optic. In one case, the bandwidth of the coating can drop by 50 percent by optimizing the bandpass characteristics across the optic. This approach is particularly useful in devices such as combiners, where the reflection and transmission characteristics must be optimized.
Bruce M. Lairson, Ph.D., is engineering manager of Deposition Sciences Inc. (DSI) in Santa Rosa, Calif. His e-mail address is [email protected].
