Using Ultra-Narrowband Optical Bandpass Filters for Improving Transmission and Temperature Stability

Compared to legacy soft coatings, hard coated ultra-narrowband optical filters made using sophisticated plasma processes provide improved temperature stability, transmission, and out of band blocking. These filters are used in optical systems as diverse as light detection and ranging (LIDAR), laser cleanup, Doppler shift detection of plasma velocity, cutting-edge astronomy, chemical and gas sensing, and instrumentation applications.

In the 1990s, the telecom industry manufactured ultra-narrowband filters displaying square passbands for the NIR, but until recently, these filters have not been feasible in large format sizes. Larger filters can be difficult to manufacture because of uniformity restrictions, but are in high demand for instrumentation purposes.

Narrowband filters with compulsory operation in the UV and Visible present an additional challenge because of loss and scatter; the deposition control system has to be adept to handle lower levels of light used for optical monitoring.

Novel Deposition Techniques

Alluxa have developed novel deposition methods to overcome these problems and accommodate large format demands, resulting in extremely square ultra-narrowband filters. Alluxa’s techniques incorporate an advanced computer-controlled variation of the turning point technique¹, where the filter is constantly measured at a single wavelength and errors in prior layers are dealt with in real time at the current layer.

This provides tight control of optical thickness, enabling highly reproducible square top filters with low ripple, which consistently match the theory. Practical commercial limits to ultra-narrows are in the 50 mm to 100 mm diameter range. Although larger formats are possible, the costs can be prohibitive.

Figure 1 displays the measured variation of a 75 mm format ultra-narrow filter at four separate quadrants starting 12.5 mm from the edge. Center wavelength differs < 0.06% for all positions, and bandwidth (FWHM) is controlled to better than 0.02 nm.

Figure 1. Uniformity plot for a 75 mm-diameter measured at four quadrants in from the edge.

Progress in monitoring techniques has enabled narrowband filters to be fully customizable. For instance, a filter can be fabricated to convey very discrete wavelengths of light for a plasma physics application, enabling excellent contrast imaging of the ELMs (edge localized modes) in a tokomak (Figure 2).

A custom narrowband filter with approximately 2 nm bandwidth and T > 90% has contributed to a significant increase in spatial and temporal resolution for a system measuring carbon impurities in ELMing plasmas.² Imaging contrast can be additionally improved by boosting passband transmission while decreasing the bandwidth of the filter.

Figure 2. Spherical tokomak from plasma fusion research.

Alluxa continues to expand the manufacturing limits for ultra-narrow filters, creating extremely narrow bandwidths with excellent transmission. Figure 3 shows measurement of a 0.3 nm multi-cavity filter that can be angle-tuned to maintain p-polarization performance.

Figure 3a. Ultra-narrow, 0.3 nm, bandpass filter at 0°.

Figure 3b. Ultra-narrow, 0.3 nm, bandpass filter angle-tuned by 26° for p-polarization.

It is important to note that commercially available instruments cannot measure the filters efficiently at these narrow bandwidths. Specialized metrology methods are often required to confirm performance, as described in other articles.^3,4

When Alluxa compared its ultra-narrow results to those made using alternative technologies, the difference is quite remarkable. Figure 4 displays the measured versus theoretical response of a 532 nm three-cavity flat-top band pass filter that is completely blocked from 350 nm to 900 nm at OD4 levels (an optical density of 4).

Shown alongside are regular high-tech narrowband filters from legacy deposition processes, such as ion-assisted electron beam.

Figure 4. Measured performance for a 0.94 nm three-cavity ultra-narrowband filter compared with results of other known deposition methods for narrowband filters.

Figure 5. Filter function as calculated for various F-numbers.

Filters

Real world filters are frequently used with some defined cone angle rather than a perfectly collimated source. All thin-film filters display a shift in the wavelength of the passband which is based on the angle of incidence. The net effect of using a larger cone angle is rounded slopes in conjunction with the shift.

The angle shift can be reduced by using design methods that consider the refractive index of the total design. An example is illustrated in Figure 5, where a filter with F number as low as F5 still retains a portion of its nominal shape. As revealed by the plot, filters used in low F-number systems should be biased slightly higher for the central peak to attain optimal transmittance.

In contrast to narrowband filters manufactured with other deposition technologies, Alluxa ultra-narrowband filters offer broad and deep blocking performance at levels up to and beyond OD6 without compromising transmission performance.

Standard performance specifications need blocking in the range of 400 nm to 900 nm outside of the passband at levels of OD4, OD5, or OD6; this is largely dependent on the detector response and can be customized to individual system requirements. A typical blocking curve is shown in Figure 6.

Figure 6. Blocking curve with OD6 levels out of band.

Alluxa filters are regularly exposed to both MIL and telecom standard operating conditions (as used by the US Department of Defense). The MIL standard for operation in humid environments defines a requirement of 10 × 24 hours temperature/humidity cycles with extremes of 60°C/95% relative humidity, 30°C/95% relative humidity, and 20°C/95% relative humidity.

The Telcordia GR-1221 Damp Heat UNC requirement is 2000 h at 85 °C/85% relative humidity. Alluxa filters do not display measurable change to physical appearance or spectral performance after enduring these cycle tests. In addition to environmental testing, Alluxa filters also meet Department of Defense durability requirements (as specified by MIL 48497A).

Alluxa hard-coated ultra-narrowband pass filters have an extremely low wavelength dependence on temperature. It differs according to the substrate choice and the type of coating design, however for a standard 532 nm ultra-narrowband filter on a Schott BK7 glass substrate, the center wavelength will shift approximately 2.5 pm/°C.

Figure 7 further shows the effect of shift with increasing temperature for a 659.2 nm centered ultra-narrow – this particular filter shows an average shift of < 4.2 pm/°C from room temperature (27 °C) to 105 °C. Lower values of temperature shift can be produced on request by optimizing the design parameters.

Figure 7. Measured filter performance for 1.2 nm narrowband, heated from room temperature to 105 °C.

Conclusion

Alluxa’s computer-controlled technique of thin-film deposition enables the development of multi-cavity filters with greater wavelength precision, excellent contrast, and enhanced performance in challenging environments.

Alluxa’s future operations will focus on further reducing the bandwidths of its high-transmission, hard-coated flat-top filters to values of less than 0.5 nm, and further enhancing filter functions and transmission levels. Alluxa is also planning to look at ways for reducing the transmitted wavefront error that considerably contributes to image distortion.

References

1. 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. 2. S.L. Allen, W.H. Meyer, G.D. Porter, and J. Howard, Carbon ion flow measurements in DIII-D divertors by coherence imaging, US DOE in conjunction with Lawrence Livermore National Laboratory and Australia National University, 2013. https://e-reports-ext.llnl.gov/pdf/775621.pdf
3. 3. Alluxa Engineering Staff, New metrology techniques for advanced thin film optical filters, Alluxa White Paper Series, 2012. http://www.alluxa.com/learning-center
4. 4. 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