When light gets reflected from the back surface to impede with light incident on the front surface, there should be a phase relationship between the incoming light and the reflected light, or there will be no interference. Light from any source has a distinct coherence length. If the two points along a light ray are separated by less than this length, there will be a phase relationship between them. If the points are isolated by more than this length, then there will be no phase relationship. This means that the layers must have an optical thickness (nt) less than that of the source coherence length.
White light has a coherence length of approximately 1µm, and light from monochromatic sources, such as lasers, has much longer coherence lengths and, in principle, one can construct “thick thin films”. With these sources, interference effects from use of normal optics, such as windows or lenses, lead to unnecessary variations in reflectance.
Figure 1. Single Fabry-Perot cavity
Interference filters combine Fresnel reflections and interference in thin layers for wavelength dependent transmittance. It is important to understand the filter construction in terms of Fabry-Perot cavities or etalons. Figure 1 depicts the basic Fabry-Perot cavity.
Figure 2. Transmittance of ideal Fabry-Perot cavity
Reflective layers are present at each surface of a thin, transparent dielectric slice. The dielectric could be glass, air, or one of the thin film dielectrics utilized for optical coatings. Internally, multiple reflections occur from the partially reflecting mirrored surfaces. When broadband light is incident on the Fabry-Perot cavity, destructive and constructive interference leads to transmittance of bands of light, with core wavelengths where the thickness of the optical cavity is ½ an optical wave thick:
nt = kλ/2 ........................... (1)
Where nt is the optical thickness of the cavity, λ denotes center wavelength, and k represents any integer.
Figure 2 shows the transmittance of an ideal Fabry-Perot cavity.
Multi-layer Thin Film Coatings
In order to give better control over the bandshape, multi-layer, thin film coatings are utilized. The location of the pass bands i.e. λ1, λ 2 and λ3, etc. depend on the thickness of the optical cavity. If the filter is to pass λ1, and the cavity is made just one half wave thick, for instance nt = λ1 / 2, only wavelengths shorter than λ1 can satisfy Equation 1. Wavebands centered around λ1/2 and λ1/3 also pass through. In case the thickness of the cavity is greater than a single half wave, for example 3λ1, then longer wavelengths such as 3λ1 and 6λ1 can pass through.
Since most filters are for separation of a single pass band, the unnecessary pass bands must be suppressed. Partial reflectors, and absorbance of the cavity materials, often suppress short wavelengths. For effective blocking, absorbing filter glasses can also be used.
In a Fabry-Perot of given spacing, as reflectance of the reflecting stacks increases, the bandwidth decreases. The reflectance depends on the number of layers in the stack. The outcome is that the bandwidth is about a logarithmic function of the number of layers in the reflector stacks. Using a number of Fabry-Perot cavities, for example reflector-spacer-reflector, in a single filter adds flexibility in bandshaping, and enables an improved combination of transmittance at the wavelength of interest, with enhanced rejection of other wavelengths. This type of filter is called as a multi-cavity filter.
A standard 3 cavity 10nm bandwidth filter in the mid-visible range has about 50 individual thin film layers. When compared to the simple Fabry-Perot, multi-cavity filters have distinct square tops with steeper sides, but nevertheless they require additional blocking to suppress unwanted harmonics.
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.
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