Wafers Used in Bandpass Filters
Bandpass filters are precision optical components that transmit a selected wavelength range while blocking unwanted light. They are fabricated on high-quality optical substrates such as glass wafers, Corning Eagle Glass, fused silica, BK7 glass, and sapphire. These substrates are widely used in spectroscopy, fluorescence microscopy, laser systems, biomedical imaging, machine vision, and telecommunications, where high optical transmission, low surface roughness, and durable thin-film coatings are critical.
Glass Substrates for Bandpass Filter Fabrication
Thin glass wafers are commonly used as substrates for bandpass filters, NIR filters, AR-coated optics, and other thin-film optical components. Materials such as Corning Eagle Glass, fused silica, BK7, and specialty glass wafers are selected for their optical transmission, flatness, thickness control, and coating compatibility.
A senior microsystems engineer requested the following glass substrate for bandpass filter fabrication:
Research Request:
We need a 0.2 mm thin glass substrate similar to Corning Eagle Glass. One side will receive a bandpass filter coating, similar to the NIR filter shown below. The required substrate size is 4 × 4 inches.
NIR Bandpass Filter with Backside AR Coating
The researcher also asked about the following optical design considerations:
- Angle shift: How much the passband shifts from 0° to 25° based on the effective index of the filter design.
- Out-of-band blocking: The required optical density (OD) outside the passband, since blocking performance is an important cost driver.
- Coating stress: Whether a second-surface coating can be added to help balance stress on very thin glass.
For thin optical glass substrates, coating stress, substrate thickness, flatness, and backside anti-reflective coatings should all be considered before finalizing the bandpass filter design.
Reference #236378 for specifications and pricing.
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What is a Bandpass Filter?
A bandpass filter is an optical filter that transmits a selected wavelength range while blocking wavelengths outside that range. Bandpass filters are used when an optical system needs to isolate one part of the spectrum for imaging, spectroscopy, fluorescence microscopy, laser systems, machine vision, or telecommunications.
In many research and production applications, bandpass filters are made by depositing precision thin-film coatings onto glass or optical wafer substrates. The substrate material, coating design, thickness, and angle of incidence all affect the final filter performance.
Important Bandpass Filter Specifications
- Center wavelength: The main wavelength the filter is designed to transmit.
- Bandwidth: The wavelength range transmitted around the center wavelength, often measured as full width at half maximum (FWHM).
- Peak transmission: The percentage of desired light that passes through the filter.
- Blocking range: The wavelength region outside the passband that must be blocked or reflected.
- Optical density: The blocking strength outside the passband. Higher OD means stronger blocking.
- Angle shift: The movement of the passband when the filter is used at an angle instead of normal incidence.
Substrates Used for Bandpass Filters
Bandpass filters can be fabricated on several optical substrates depending on wavelength range, thickness requirements, coating stress, and transmission needs.
How Bandpass Filters Work
Most precision bandpass filters are interference filters. They use alternating dielectric thin-film layers deposited on a substrate. These layers create constructive interference for the desired wavelength band and destructive interference for unwanted wavelengths.
Bandpass filters may also include anti-reflective coatings on the back side to reduce unwanted reflections and improve transmission. For very thin glass substrates, coating stress must be considered because stress can bend, warp, or distort the substrate.
Common Bandpass Filter Applications
- Spectroscopy: Isolates specific spectral lines for measurement and analysis.
- Fluorescence microscopy: Separates excitation and emission wavelengths.
- NIR imaging: Selects near-infrared wavelength bands for sensors and cameras.
- Laser systems: Transmits a target laser wavelength while blocking unwanted light.
- Machine vision: Improves contrast by allowing only selected wavelengths to reach the detector.
- Telecommunications: Filters wavelength channels in optical communication systems.
Choosing a Bandpass Filter Substrate
When selecting a wafer or glass substrate for a bandpass filter, researchers should consider substrate thickness, flatness, optical transmission, coating compatibility, thermal expansion, and whether the second surface needs an anti-reflective or stress-balancing coating.
UniversityWafer supplies glass, fused silica, sapphire, BK7, Corning Eagle Glass, and other optical substrates used for bandpass filters, NIR filters, AR-coated optics, and thin-film optical coating research.
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