Bandpass Filter Tutorial - Thorlabs

Hard-Coated Bandpass Filter Structure

A bandpass filter is created by depositing layers of material on the surface of the substrate. For our hard-coated bandpass filters, the coating is comprised of dielectric stacks alternating with dielectric spacer layers. Each dielectric stack is composed of a large number of alternating layers of low-index and high-index material. The thickness of each layer in the dielectric stack is λ/4, where λ is the central wavelength of the bandpass filter (i.e. the wavelength with the highest transmittance through the filter). The spacer layers are placed in between the stacks and have a thickness of (nλ)/2, where n is an integer. A Fabry-Perot cavity is formed by each spacer layer sandwiched between dielectric stacks. The filter is mounted in an engraved metal ring for protection and ease of handling.

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Filter Operation Overview

The constructive interference conditions of a Fabry-Perot cavity allow light at the central wavelength, and a small band of wavelengths to either side, to be transmitted efficiently, while destructive interference prevents the light outside the passband from being transmitted. However, the band of blocked wavelengths on either side of the central wavelength is small. To increase the blocking range of the filter, materials with broad blocking ranges are used as the substrate or to coat the spacer layers. Although these materials effectively block out-of-band transmission of incident radiation, they also decrease the transmission through the filter in the passband.

Filter Orientation

An engraved arrow on the edge of the filter is used to indicate the recommended direction for the transmission of light through the filter. Orienting the coated side toward the source will reduce unwanted scattering and minimize reflections sent back toward the source. Using the filter in the opposite orientation will not, however, significantly affect the performance of the filter. Figure 1.2 was made by illuminating the filter with a low intensity broadband light and measuring the transmission as a function of wavelength. The plot shows that the direction of transmission through the filter has very little effect on the intensity and the spectrum of the light transmitted through the filter. The minimal variation between the forward and backward traces is most likely due to a small shift in the incident angle of the light on the filter introduced when the filter was removed, flipped over, and replaced in the jig.

The filter is intended to be used with collimated light normally incident on the surface of the filter. For uncollimated light or light striking the surface at an angle not normally incident to the surface, the central wavelength (wavelength corresponding to peak transmission) will shift toward lower wavelengths and the shape of the transmission region (passband) will change. Varying the angle of incidence (AOI) by a small amount can be used to effectively tune the passband over a narrow range. Large changes in the incident angle will cause larger shifts in the central wavelength but will also significantly distort the shape of the passband and, more importantly, cause a significant decrease in the transmittance of the passband, as seen in Figure 1.3.

Filter Temperature

The central wavelength of the filter can be tuned slightly (~1 nm over the operating range of the filter) by changing the temperature of the filter. This is primarily due to the slight thermal expansion or contraction of the layers.

Some optical industry workers may be wondering what is the difference between narrowband filters and bandpass filters? Let's discuss it today.

1. Band pass filter optics

Band pass filter optics allow optical signals to pass through in a specific wavelength band, while optical signals on both sides that deviate from this band are blocked. The passband of bandpass filters is relatively wide, generally half a The bandwidth is above 40nm!

2. Narrowband optical filter

The narrowband optical filter is divided into the bandpass filter, which belongs to a kind of bandpass filter. Its definition is the same as that of the bandpass filter, which allows the optical signal to pass through in a specific wavelength band. .

The optical signals on both sides that deviate from this band are blocked, but the narrow-band filter is relatively narrow.

The narrow-band filter mainly adopts the technology of all-dielectric hard coating and the principle of medium interference. On the basis of highlighting the characteristics of the narrow-band filter, the optical performance has nothing to do with the thickness of the substrate. The narrow-band filter is more convenient for built-in instrument imaging systems.

In order to improve its optical performance and apply it effectively, special optical material substrates are used to solve the problems of mildew and unstable optical performance of traditional absorption synthetic glass. The products are produced according to the customer's requirements.

The single-piece narrow-band filter does not use gluing or hard coating, and has long service life, small temperature drift, strict control of production process, and small error in product size and optical index.

Ultra-thin optical glass, high transmittance, good finish, small thickness tolerance, good flatness and other optical characteristics strong characteristics.

Its definition is the same as that of band-pass filtering, that is, this filter allows optical signals to pass in a specific wavelength band, and the optical signals on both sides that deviate from this band are blocked. The passband of narrow-band filters is relatively narrow. Generally, it is less than 5% of the central wavelength value.

Both of the above make the image band chromatic and seriously affect the imaging quality, even in the paraxial region. We define the wavelength of this diffraction direction, that is, the wavelength of the reflected light from the groove surface with the right diffraction direction on the grating as blaze wavelength.

This feature can be used to align the center wavelength within a certain range. The advent and application of electronic computers have greatly promoted the work of optical design.

The monitoring distance is long, up to - meters. The object distance ratio infrared thermometer to determine the target can be divided into monochromatic thermometer and dual color thermometer (radiation colorimetric thermometer) according to the principle.

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Differences Between Band Pass Filter Optics and Narrowband Filters

1. Wavelength Range:

• Bandpass Filter: A bandpass filter allows a broader range of wavelengths to pass through. It has a specified central wavelength and a bandwidth that defines the range of wavelengths transmitted. The central wavelength is typically chosen based on the application's requirements.

• Narrowband Filter: A narrowband filter, on the other hand, only allows a very specific and narrow range of wavelengths to pass through. It has a much smaller bandwidth compared to a bandpass filter.

2. Bandwidth:

• Bandpass Filter: The bandwidth of a bandpass filter is relatively wide compared to a narrowband filter. It allows transmission of a range of wavelengths around the central wavelength.

• Narrowband Filter: The bandwidth of a narrowband filter is very narrow, often just a few nanometers or less. It is designed to transmit light within a highly restricted wavelength range.

3. Applications:

• Optical Bandpass Filter: Bandpass filters are often used in applications where a specific range of wavelengths needs to be transmitted, such as fluorescence microscopy, environmental monitoring, or colorimetry.

• Narrowband Filter: Narrowband filters are utilized in applications where precise isolation of a particular wavelength is crucial. This includes tasks like laser line filters for spectroscopy, astronomy filters, or applications where interference from other wavelengths must be minimized.

4. Selectivity:

• Bandpass Filter: Bandpass filters are less selective in terms of wavelength isolation because they allow a wider range of wavelengths to pass through.

• Narrowband Filter: Narrowband filters offer higher selectivity by allowing only a very narrow range of wavelengths to pass, effectively rejecting other unwanted wavelengths.

5. Light Intensity:

• Bandpass Filter: Bandpass filters transmit a higher intensity of light because they allow a broader spectrum to pass through.

• Narrowband Filter: Narrowband filters transmit a lower intensity of light but provide better spectral purity within the selected wavelength range.

The choice between a bandpass filter and a narrowband filter depends on the specific requirements of the optical system and the application's need for wavelength selectivity and bandwidth. Bandpass filters are suitable for applications where a broader range of wavelengths is acceptable, while narrowband filters are preferred for tasks requiring highly selective wavelength transmission.

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