How to specify a filter

The optical filter specifications required will depend on the filter application and how it is to be used. In general, typical filter optical specifications will include the wavelength ranges over which to reflect and transmit light, angle of incidence tuning range,clear aperture of the filter, the polarization state of light for non-normal angles of incidence, light coherency (laser or white light), optical beam size and cone angle of light, desired substrate material (with dimensions), incident light medium (air or glass) and so forth. In addition, physical specifications such as the operating temperature range and the ambient environment of the filter should also be specified.

AOI and CHA:

Angle of Incidence, or AOI, is the angle at which a collimated beam of light is incident on the filter’s first surface, measured relative to the surface normal.

Cone half-angle, or CHA, refers to the angular range associated with a non-collimated incident beam, and is measured from the AOI to the largest cone angle.

Changing the AOI or CHA of your incident light will change the spectral response of the filter. For more information on how the edge pass filters respond to changes in AOI, please review our technical note Edge Filters for Raman Spectroscopy. For a custom filter, you need to provide the AOI and CHA ranges in order for our filter design engineer to optimize the filter design working for these ranges.

It is also desired to provide the beam size information when you provide the filter specifications.

Clear Aperture:

While the optical coating extends to the edge of every filter, the clear aperture will be less than the transversedimensions for each filter.

For unhoused, round filters, the clear aperture is typically guaranteed to be greater than 90% of the outer diameter. For square andrectangular parts, the guaranteed clear aperture will be greater than 80% of the filter’s transverse dimensions and is elliptically shaped. The clear aperture of a housed filter is dependent on the size and thickness of the housing ring. All filter specifications are guaranteed only within the clear aperture region.

Orientation of the filters:

To maximize intended transmission and blocking and to minimize auto fluorescence, filters are recommended to be oriented in such a way that the light is incident on a specific surface of the filter.

Ring mounted filters can have an arrow on the mounting ring, whereas the un-mounted filters are indicated with either scribed or diced mark on one surface of the filters. The filter arrow indicates the first surface upon which the light should be incident.

Polarization:

When light is incident on an optical filter at a non-normal angle of incidence, the polarization of the light can be described by twoorthogonal vector components associated with the orientation of the electric field of the light wave. The polarization is referenced to the plane of incidence or the plane that is parallel to the normal to the surface of the filter and contains both the incident andthe reflected light rays. The polarization component that is perpendicular to the plane of incidence is called the “s” component, andthe component that is parallel to the plane of incidence is called the “p” component.

Substrate and filter dimensions:

Substratescan be Borofloat, Schott BK7, Fused Silica, Corning 7980 Fused silica, UV Grade Fused Silica and WMS-15 glass with high CTE value. Substrate thickness can be specified and requested. Some standard thicknesses are available depending on the substrate type. Please check with our sales team for the available thickness.

Iridian can process custom filter sizes in round shapes, rectangular or square dimensions to your required size. The dimension tolerance should be provided.

Filter mounts with standard diameter 12.5mm and 25 mm are typical and custom diameter can also be requested as well.

Finished quality

The finished surface quality requirement needs to be defined as part of the non-optical quality specifications. Iridian’s scratch-dig standard is based on the US Military Standard MIL-PRF-13830.

If you require certain surface flatness or wavefront distortion (transmitted wavefront or reflected wavefront) on the finished filters, should provide your target value.

Filter types

In general, the full wavelength range needs to be defined for a given optical filter.

There are 4 major types of filters including Band pass filter (BPF), Edge pass filter (long pass and short pass, LPF and SPF), notch filter and dichroic filter (long pass dichroic and short pass dichroic, DLP and DSP). The telecom filters include BPF, LPF and Gain flattening filter (GFF) filters and Solid etalons.

The following need to be specified in addition to the operational wavelength range, AOI, CHA, substrate type, substrate thickness and tolerance, filter dimensions (un-mounted or mounted and mount size) and tolerance, polarization requirement, filter clear aperture, and finish surface quality as well as the operating temperature range.

(1) Band pass filter
The passband wavelength range and transmittance level, the FWHM, reflection band range and blocking level need to be defined. If you have a ripple spec for the pass band, you need to request this as well.

(2) Edge pass filter (long pass and short pass)
Transmitted wavelength range and pass band transmittance level, reflection wavelength range and blocking level, and cut-off value or edge steepness need to be defined.For the definitions of cut-off and edge steepness, please refer to the Edge Filters for Raman Spectroscopy. If you have a ripple spec for the pass band, you need to request this as well.

(3) Notch filter
The passband wavelength range and transmittance level, reflection band range and blocking level need to be defined. If you have a ripple spec for the pass band, you need to request this as well.

(4) Dichroic filter (long pass dichroic and short pass dichroic)
Transmitted wavelength range and pass band transmittance level, reflection wavelength range and reflection level need to be defined. If you have a ripple spec for the pass band, you need to request this as well.

(5) Telecom Band Pass Filters
The passband width and –dB level, the -3dB width (FWHM), transmission isolation, reflection band range and blocking isolation need to be defined. If you have a ripple spec for the pass band, you need to request this as well.

(6) Gain Flattening Filters (GFF)
Need to provide the target transmittance curve data along with desired Peak-to-Peak Error Function (PPEF).

(7) Solid Etalons and Single Cavity Etalons
Need to specify the Free Spectral Range (FSR), Finesse (F) and FSR temperature variation. For the Single cavity etalon, the-3dB bandwidth (FWHM) needs to be specified.