Figure 3. (a) Transmission spectra at four AOIs for both s- and p-polarization light for a green-to-blue filter. Note the almost nonexistent polarization dependence. (b) Shift of center wavelength with increasing angle of incidence (AOI) for the same filter design; the full-width-at-half-maximum (FWHM) bandwidth remains fixed at 20 nm.
Multi-cavity Fabry-Perot thin-film filters are one special case. Many fluorescence imaging and quantization applications require filters with passbands that are considerably wider — often 30 to 50nm or more (at visible wavelengths). One approach is to form such filters using a combination of long-wave and shortwave pass edge filters. Such filters exhibit qualitatively similar behavior to multi-cavity Fabry-Perot filters in that distortion of the filter spectrum increases as the AOI is increased. The useful tuning range for such filters is typically limited to 10° to 15°, resulting in a wavelength tuning range of only 0.5-1.0%.

The above examples stress the importance of the polarization state of light on tunable filter performance. However, for many applications unpolarized light is used. Presented in Figure 2 are two transmission spectra for average polarization which clearly illustrate this aspect. Figure 2(a) shows the average polarization spectra at six angles (ranging from 0° to 60°) for a fluorescence filter. The spectrum is highly distorted (i.e. loss of steep edges) even at angles of 20° to 30°, and almost unusable for larger angles (30° to 60°).

In order to overcome these shortcomings it is necessary to develop new filter technologies with advanced designs that maintain steep (bandpass) edges, high transmission, and out-of-band blocking with essentially no or little polarization dependence. The introduction of new (proprietary) filter technology has led to the development and introduction of both bandpass and edge filters that overcome all of the shortcomings discussed above. An example of the filter performance of this new filter type is displayed in Figure 2(b). In contrast to the spectrum of the bandpass filter shown in Figure 2(a), this new innovative design results in high transmission, steep edges, and excellent out-of-band blocking over the full range of angles from 0° to 60°.

In addition to maintaining the above desired filter attributes, this new filter technology also solves the problem of polarization-dependent spectral distortion. The plot shown in Figure 3(a) shows a series of transmission spectra of a green-to-blue filter and clearly demonstrates improved transmission fidelity even at high tuning angles for both sand p-polarized light. Furthermore, from the plot displayed in Fig. 3(b) it is clear that the FWHM bandwidth, rather than narrowing, remains near constant as the AOI is varied from 0° to 60°. Continual design improvements will likely lead to even narrower passbands that are tunable over a much wider wavelength range.

Conclusion

Fluorescence microscopy and other fluorescence imaging and quantitation applications, hyperspectral imaging, high-throughput spectroscopy, and fiber optic telecommunications systems should all benefit from tunable optical filters with the spectral and two-dimensional imaging performance characteristics of thin-film filters and the center wavelength tuning flexibility of a diffraction grating. There exist several technologies that combine some of these characteristics, including liquid-crystal tunable filters, acousto-optic tunable filters, and linear-variable filters, but none are ideal and all have significant additional limitations. The introduction of tunable thin-film filters overcomes the shortcomings of alternative technologies and should pave the way for their introduction into fluorescence and Raman spectroscopy systems, leading to enhancement of the size, speed and filtering function in addition to expanding current fast-imaging capabilities.

This article was written by Neil Anderson, Ph.D., Technology Development Analyst, and Turan Erdogan, Ph.D., co-founder and CTO, Semrock, Inc. (Rochester, NY). For more information, contact Dr. Anderson at nanderson@ idexcorp.com, Dr. Erdogan at terdogan@ idexcopr.com, or visit http://info.hotims.com/28055-201.

References

* Optical Waves in Layered Media, P. Yeh, Wiley, New York, 1988, Section 7.6