The ubiquitous techniques of fluorescence and Raman imaging and spectroscopy rely heavily on spectrally precise, high-quality and high-throughput optical filter technologies. As both fluorescence and Raman-based techniques move from traditional R&D-based environments into medical and clinical (diagnostic) settings, even higher demands are placed on system performance. Therefore, it is necessary to continue to improve system components and architectures to meet the demanding challenges often encountered in biological applications.

Figure 1. Illustration of tunable bandpass (center) and edge (right) filters resulting from angle-tuning.
One simple and straightforward means to do this without a complete system redesign and overhaul is to conceive of new spectrally precise, high-quality and high-throughput optical filters that can perform in more than one mode of operation. A recent advance in this area has been the development of thin-film interference filters that are tunable over a wide range of wavelengths with little or no sacrifice in filter performance, i.e. transmission characteristics. This innovation is not only simpler than alternative approaches (e.g. liquid crystal technologies) but can lead to cost reductions as fewer filters are required to cover a broad wavelength range. The advent of this new technology serves to improve and expand the flexibility and capability of both fluorescence and Raman instrumentation/modalities across a wide range of imaging and spectroscopy applications.

Thin-film filters are the ideal solution for wavelength selection in most optical systems due to their exceptionally high transmission (close to 100%), very steep spectral edges, and blocking of optical density 6 or higher over wide spectral regions for maximum noise suppression. However, until now thin-film filters have been considered “fixed” such that changing the spectral characteristics required swapping filters. Mechanical means to perform filter swapping, like filter wheels, exist but these are generally large in size, relatively slow (minimum switching times are typically 50 to 100 ms), and permit only a limited number of filters. (Typically, filter wheels can contain anywhere from 4 to 12 filters, depending on the instrument and application.) As a result, size, speed, and filtering function flexibility are all limited. Therefore, in order to overcome these shortcomings new innovations in filter technology are required.

Angle-Tuned Thin-Film Filters

Figure 2. Example of a fluorescence bandpass thin-film filter comprised of a combination of long-wave-pass and short-wave-pass filter coatings (FWHM ~ 35 nm). Six spectra associated with light of average polarization and for angles ranging from 0° to 60° are shown. (b) Transmission spectra for a green-to-blue tunable thin-film filter shown for comparison. All spectra shown are calculated.
It is well-known that the spectrum of any thin-film filter shifts toward shorter wavelengths when the angle of incidence (AOI) is increased from 0° to larger angles *. However, in general the spectrum becomes highly distorted at larger angles, and the shift can be significantly different for both s- and p-polarized light, leading to strong polarization dependence.

altMathematically, when the AOI of light impinging upon the filter is increased beyond 0° (normal incidence) to larger angles, the resulting wavelength shift is generally described quite accurately by the equation where θ is the angle of incidence and neff is called the “effective index of refraction,” which is unique for each filter design and for the two orthogonal states of polarization. This effect can be used to tune the spectrum of an optical filter, albeit over a limited spectral range.

Multi-cavity Fabry-Perot thin-film filters are one example of tunable filter technology. However, although they can be designed to provide a narrow passband (about 2nm) at 561nm, the passband can become considerably narrower for s-polarizations and wider for p-polarization, and tune at different rates resulting in polarization splitting as the AOI is increased. Such polarization dependent features are undesirable and can severely limit filter and system performance in many applications.

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