Improving Ultra Narrowband Optical Filters

A new computer-controlled deposition system for multicavity filters enhances their spectral precision and contrast. Narrowband filters are an important technology for a range of applications such as light detection and ranging (LIDAR), instrumentation, chemical and gas sensing, laser cleanup, and astronomy.

The design principles are well established and fairly simple. All designs rely on stacked Fabry-Pérot resonant cavities with dielectric reflectors composed of layers a quarter of a wavelength thick, separated by cavities multiple half-wavelengths across.

A number of cavity filters are used in combination to ‘square up’ the spectral wave shape, resulting in the transmitted light with a ‘flat-topped’ spectrum when compared with that from light passed via single-cavity filters, which has a sharp, peaked spectral shape.

This type of multicavity filters also have much steeper rejection responses compared to single-cavity filters: the less-steep spectral slopes of single-cavity filters can affect signal-to-noise in narrowband detection.

Computer-Controlled Deposition System

Producing multicavity filters is difficult for deposition process control systems. This invariably introduces unwanted ripple (wavelength-dependent variation in attenuation of the signal as it enters the filter), causing signal loss in the pass band, the spectral range over which the filter exhibits high transmittance.

Alluxa use a computer-controlled variation on the turning point technique¹ of thickness control for each individual layer, where the filter is frequently measured and variations in thickness compensated for, taking into consideration thickness errors linked with a number of layers.

The system enables the company to make unparalleled reproducible filters with very low ripple, offering a signal shape with steep slopes that consistently match theory.

Ultra-Narrowband Filters

Figure 1 shows high-contrast images produced by ultra-narrowband filters. This image of the Sun has had its light passed via an H-alpha filter, which is designed to only pass the emission of the H-alpha line at 656.28 nm while blocking the rest of the light. This considerably enhances the image contrast.

When the transmission is higher and the filter is narrower, the contrast enhancement is better. A filter of about 1 nm bandwidth at this wavelength provides roughly a thousandfold increase in contrast to a silicon-based detector camera system.

It must noted that at these narrow bandwidths, commercial instruments are not able to measure the filters effectively and specialized metrology methods are required, as explained elsewhere in other articles.^2,3

Figure 1. Solar image using a narrowband H-alpha filter combined with a narrowband etalon.

Figure 2 shows the measured and theoretical response of a 532 nm three-cavity flat-top band pass filter that is completely blocked from 350 to 900 nm at OD4 levels (an optical density of 4). Displayed alongside are normal advanced narrowband pass filters from legacy deposition processes, for example ion-assisted electron beam. Filters from 330 to >2000 nm can be constructed to similar or even narrower specifications.

Figure 2. Measured results of fully blocked three-cavity flat-top band pass filter with 0.94 nm bandwidth compared with the other known deposition methods for ultra-narrowband pass filters.

Due to the resonance in the pass band, ultra-narrowband filters require very low extinction coefficients of the deposited materials which causes loss of intensity. A filter with a bandwidth of 1 nm needs extinction coefficients of below 10 ppm to restrict losses to acceptable levels.

The plasma physical vapor deposition (PVD) hard-coated filters that are routinely manufactured by Alluxa have transmission over 95% with a completely blocked configuration requiring OD4, OD5, or greater  level of blocking above 200 – 1200 nm and longer.

All thin-film filters reveal a shift in the wavelength of the pass band dependant on the angle of incidence. The net effect of greater cone angle is to round the slopes of the spectrum and decrease the wavelength. Alluxa reduced the angle shift using design methods that take into consideration refractive index.

An example is the filter illustrated in Figure 2, which can function with F-numbers as low as F5. The measured response of a variety of F-numbers on the filter response is illustrated in Figure 3. As specified by the plot, filters to be used in low F-number systems should be biased a little higher for the central peak to achieve 100% transmittance.

Figure 3. Filter function as calculated for various F-numbers. T: Transmission.

The bulk of narrowband applications use filters in a variety of diameters from 12.5 to 50 mm. By cautiously controlling uniformity of optical and physical thickness, Alluxa has manufactured filters up to 250 mm in diameter. Figure 4 displays the measured result of various radii of a 250 mm flat-top band pass filter on a 250 mm wafer that is specifically designed to transmit the 532 nm laser line

Figure 4. Measured results of 250 mm-diameter, fully blocked three-cavity flat-top band pass filter with 0.94 nm bandwidth.

In contrast to narrowband filters manufactured using other deposition technologies, Alluxa’s ultra-narrowband filters can be combined with broad and deep blocking performance at state-of-the-art levels (up to and over OD6) without compromising transmission performance.

Standard performance specifications are a 400 to 900 nm range around the pass band with OD4, OD5, or OD6 blocking performance. Alluxa’s filters are routinely subjected to both MIL and telecom standards (as used by the US Department of Defense).

The MIL standard for functioning in humid environments has a prerequisite of 10 × 24 hours temperature/humidity cycles with extremes of 20°C/95% relative humidity, 30°C/95% relative humidity, and 60°C/95% relative humidity, whereas the Telcordia GR-1221 Damp Heat UNC prerequisite is 2000 hours at 85°C/85% relative humidity.

The filters produced by Alluxa do not display measurable change to physical appearance or spectral performance after these tests. In addition to environmental testing, Alluxa filters also comply with Department of Defense durability requirements (MIL 48497A).

Alluxa hard-coated ultra-narrowband pass filters have a low wavelength reliance on temperature. It differs based on the type of substrate and the type of coating design. However, for a regular 532 nm ultra-narrowband filter on a Schott BK7 glass substrate, the center wavelength will change about 2.5 pm/°C. Lower values of temperature change can be created by enhancing the design parameters on special request.

Conclusion

Alluxa’s computer-controlled technique of thin-film deposition enables the company to develop multicavity filters with better wavelength precision and enhanced contrast. The company plans to focus on further reducing the bandwidths of its high-transmission, hard-coated flat-top filters to values of below 0.5 nm, and further enhancing the filter function squareness. Alluxa also plans to investigate the reduction of the transmitted wavefront error that considerably distorts the images.

References:

1. H. A. Macleod, D. Richmond, The effects of errors on the optical monitoring of narrow-band all-dielectric thin film optical filters, Opt. Acta 21, p. 429-443, 1974.

2. Alluxa Engineering Staff, New metrology techniques for advanced thin film optical filters, Alluxa White Paper Series, 2012. http://www.alluxa.com/learning-center

3. T. Burt, Characterizing sub-nanometer narrow bandpass filters using a Cary 400/500, Tech. Rep. SI-A-1193 Agilent Technologies Inc., 2011.

This information has been sourced, reviewed and adapted from materials provided by Alluxa

For more information on this source, please visit Alluxa