Tunable filters are optical filters that are suitable for applications requiring wavelength tuning. Two of their main specifications are the optical density (OD) and the out-of-band rejection. The definitions of the characteristics of tunable filters, however, vary from manufacturer to manufacturer and their properties are not well defined. The aim of this article is to provide rigorous definitions of the optical density and out-of-band rejection.
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Figure 1: Comparison of the the out-of-band rejection (OOBR), optical density (OD) and filter peak efficiency. The red line represents an optical source and the blue curve represents the spectrum of the optical source following filtering.
As a result, it is important that end users take additional care when examining the specifications of a filter. They will also find it challenging to measure these properties on their own. To be useful in their respective applications highly-sensitive instruments must be used that provide a high power source and have a broad spectral and dynamic range.
The Challenge
Today, a wide range of technologies are available in the market which makes it difficult to select a perfect tunable filter for a specified experiment. Given that each filter has inherent benefits and drawbacks, making a comparison between their ambiguous specifications can be challenging.
Tunable filters are capable of separating a narrow band of light. This unique ability will become more apparent once the concepts of out-of-band rejection, also known as isolation, and OD are properly defined and interpreted. Ultra-low noise measurement and highly sensitivity setups need to be used to provide accurate and consistent results.
The out-of-band rejection describes the ability of a filter to attenuate signals that lie either side of a specific wavelength and is often incorrectly referred to as the isolation. The out-of-band rejection is defined relative to the filters peak efficiency (see Figure.1), it specifies a rejection value (normally in dB) centered around λc. A high out-of-band rejection value is indicative of a filter that can produce a clean output beam with low contributions from neighbouring spectral wavelengths.
The OD describes the ability of a filter to block unwanted wavelengths and is often confused with the out-of-band rejection. The OD is given by: OD=-log10(T) where T is the transmittance (T=It/I0), It the transmitted light intensity and I0 is the incident light intensity.
It is defined relative to the unfiltered light source (see Figure 1).
The Main Assets of the LLTF Contrast™
The OD is defined as the logarithmic ratio between the radiated power transmitted via the system and the radiated power of the incident beam. The out-of-band rejection of an optical bandpass filter describes the capablity of the filter to choose accurate wavelengths with low attenuation levels either side of a specific band.
Once the concepts of OD and out-of-band rejection are clearly defined, it is easy to interpret the potential of the LLTF Contrast™, as shown in Figure 2.
As a sub-nanometric, non-dispersive bandpass optical filter the LLTF Contrast has an efficiency of up to 65% (including coupling losses and polarizations). This efficiency refers to the signal throughput, which is the ratio of the filtered intensity across the input intensity at a particular wavelength.
The LLTF Contrast filter has a high optical density of more than OD 6.5 and high out-of-band rejection of less than -60 dB. It also acts as a widely tunable optical filter. Depending on volume Bragg grating (VBG) angle, a particular configuration will allow the resonant filter to choose a narrow part of a laser spectrum, which can be either broadband or monochromatic.
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Figure 1. Photon etc’s laser line tunable filter (LLTF) Contrast™
In contrast to other filters that are commercially available the LLTF Contrast filter offers a broad tunability, spanning from 1000 to 2300 nm in the SWIR range and from 400 to 1000 nm in the VIS range, with less than 5 nm and 2.5 nm bandwidths respectively. This bandwidth is the spectral full width at half maximum (FWHM) of the filtered beam. The LLTF contrast filter technology offers an exceptional out-of-band rejection of less than -60 dB.
The LLTF Contrast is the only tunable optical filter that has an optical density of greater than OD 4. To determine these specifications, precise performance as well as quality control tests must be conducted on setups that are highly sensitive. The LLTF Contrast is mainly used to produce an ultra-broadband tunable laser source - a special kind of source achieved by coupling a commercial supercontinuum source with the tunable filter.
Conclusion
OD and out-of-band rejection are the two major optical properties of a widely tunable filter, in addition to the covered bandwidth and spectral range. When users opt for this kind of filter, they must consider the definitions of these characteristics.
This article clearly defines the OD and out-of-band rejection of an optical bandpass filter. However, manufacturers do not always use the same definitions and there may be confusion when attempting to compare specifications from one manufacturer to another.
In such instances, users must contact their suppliers to clarify their meanings and specifications. In case users want to carry out the measurements to verify the specifications, then a high-sensitive and ultra-high dynamic range measurement setup must be used for the out-of-band rejection.
This information has been sourced, reviewed and adapted from materials provided by Photon etc.
For more information on this source, please visit Photon etc.