2.1

Light source:

As source for the radiation, a COTS halogen bulb (100W, 12V, Type Osram 64628 FDT A1/261 in a GY9.5 socket) is used. This radiation is then focused on the entrance slit of the spectrometer using a spherical mirror. The life time of the source is given as 50 hours, so it needs to be exchanged regularly. The IR-source assembly (type LSH-T100) is a part of the standard monochromator setup (Horiba IHR320).

2.2

Chopper

As the intensities received at the detector are very low, a chopper is used, together with a lock-in amplifier for the detector signal. The frequency used for the copper is 146.57Hz. The Chopper assembly is a part of the standard monochromator setup (Horiba IHR320).

2.3

Order sorting filter

The order sorting filter is a standard filter wheel from Thorlabs FW120C. It is equipped with 5 low pass filters and has 6 filter positions. The filters are low-pass filters, and can be used as order sorting filters for measuring the following spectral ranges:

The order sorting filter was first located at the exit of the spectrometer before the radiation is fed into the fiber. As the filter elements have wedges, however, which are not oriented in the same way, this led to a small change in the beam path so that the radiation did not enter the fiber for some filters. For this reason, the OSFs were placed in front of the spectrometer. The source image on the input slit is rather large, so that some small deviations have no or only minimal impact onto the signal strength.

2.4

Monochromator

The monochromator used is the grating spectrometer iHR320 from Horiba. The iHR320 is a Czerny-Turner type monochromator. The system consists of a motorized input slit that is imaged onto a motorized output slit via two concave mirrors. The collimated beam is reflected off a grating and thereby spectrally dispersed. The image of an infinitesimal small input slit illuminated with white light is thus a continuous spectrum in the plane of the output slit. The position of the spectrum with respect to the slit can be set by rotating the grating. The spectral properties of the light transmitted through the output slit depend furthermore on the properties of the used grating and the widths of input and output slit. A specific wavelength is selected by commanding the spectrometer to a specific wavelength. The transformation to the correct angle of the grating is taken over by the firmware itself. The following gratings can be selected:

Table 1

Monochromator gratings

The F-Number of the spectrometer is approximately 4.

The coupling of the radiation into the fiber after the monochromator was realized with 2 off axis parabolic mirrors (OPAs) of the same type as used for the optics in the proximity of the filter (see section below); both with a focal length of 2”. As this is a 1:1 imaging, the F-Number of the spectrometer matches the F-Number of the glass fiber.

2.5

Fiber coupling and front optics

The glass fibers used are made from ZrF4, as these are capable to transmit radiation above 2400nm. Typical glass fibers have a transmission of 50%/m only at 2400nm, becoming even worse above that. The transmission for ZrF4 is shown in the following figure.

Two fibers types were used:

Figure 4

Transmission for a ZrF4 Fiber

To create the measurement spot for the filter, a setup as shown in the following figure was chosen. The mirrors were off axis parabolic (OAP) mirrors with 1’ diameter and protected gold coating. By changing the aperture diameter, the F-Number can be changed. When using higher F-Number (smaller apertures), a lot of radiation is blocked by the aperture.

For this reason, the setup can be changed with two OAPs of 2” effective focal lengths, allowing to open the aperture while still having a small NA.

Figure 5

Optical path of the transmission optics with filter functional surface in the intermediate focus. The dark sections on the filter upper surface indicate the aperture mask, which separates the individual spectral channels and covers the glue gap between the individual filter bars.

Figure 6

Front end fiber optic coupling optics and filter positioning stage (left) and Filter holder (blue) in between the optics

The following plots show the achievable spot sizes of ~50μm for F# 4 and F# 2.5:

Figure 7

Spot size for F# 4

Figure 8

Spot size for F# 2.5

2.6

Detectors

3 alternative detectors can be applied to cover the full spectral range:

The responsivity is very high up to about 2400nm, however dropping very rapidly as shown in the following figure:

Figure 9

Responsivity of the Gamma photonics InGaAs radiometer

2.7

Scanning Translation stage and filter orientation

The filter is mounted on a precision x/y stage, which is capable to position each point of the filter under the measurement spot with a high spatial resolution of less than a μm.

In order to be able to scan along filter stripes, the orientation of the filter around vertical axis must be known, with respect to the directions the translation stages move the filter. For this, a “Translation Stage” coordination system X_T and Y_T and a “Filter Coordination System” X_F and Y_F was defined, as shown in the following sketch:

Figure 10

Coordinate system of the filter. The translation stage coordinates (not shown) were assumed to be exactly horizontal and vertical in the image. The clocking angle α is determined with the setup can compensated for each measurement spot.

The 2 coordinate systems are defined relative to each other by two points A and B. By translating the filter slowly in Y_T direction while observing the signal on the detector (of course the correct wavelength must be set in the spectrometer) it is possible to determine the positions of A and B (located exactly in the middle of the filter stripe). The positions along the long axis of the filter (X_F) can be chosen freely within the size the filter allows.

Having done this coordinate system determination, the controlling software enables a movement of the measurement spot in the Filter coordinate system

2.8

Spectral calibration and accuracy

To calibrate the spectral axis in order to have a good confidence in the spectral accuracy, a common method is to use calibration sources. For the current setup, the Xe emission source XE2 from Ocean Insight was purchased. Compared to other gases like Hg, Ne, Kr or Ar, Xe shows the best coverage of spectral lines in the spectral region of interest (800nm to 2500nm).

Figure 11

Xe lines as given by Ocean Insight. The lines marked with * are not real lines, but second orders of the grating when omitting an order sorting filter

A second way to calibrate the spectral axis is by using gas cells containing known gas mixtures which exhibit adsorption lines at well-known positions. In the spectral range of interest there are strong spectral bands from the following gases (not complete list):

H2O: 950nm, 1360nm, 1830nm,

CH4: 1650nm, 2300nm

CO: 2320nm

As the radiation passes through lab air anyway inside the spectrometer and at the optics surrounding the filter, there are always water vapour lines in the spectra. The CH4 lines can easily be created by adding some CH4 to the inside of the spectrometer. For CO, a gas cell would be necessary due to the toxicity. For CH4 and H2O no gas cell is needed, which has the advantage of both being very easy to be performed with little or no effort

Using these two methods and carefully optimizing the setup, the accuracy of the wavelength axis for the measurements achievable with this setup is estimated to be smaller than ±0.2nm.

2.9

Intensity Calibration

Several components have impact onto the accuracy of the intensity measured by the system. The main known sources are:

The source intensity variations are in the order of about ±0.7%, which is a typical value for this type of source Due to the setup, the receiving optics below the filter must be positioned precisely relative to the illumination optics, so that the two foci of the optical elements are at the same position in space. A shift of the focus due to the refractive index of the filter must be compensated by re-positioning the receiving optics. As the reference measurement after each filter measurement was done at a glass substrate having the same thickness (1.4mm), there should be no shift of focus. To measure the transmission without any filter or glass substrate, however, the receiving optics needed to be re-positioned. This was done by maximizing the signal.

In total, the intensity accuracy is estimated to be in a range of ±1.5 to 2%.

3.1.

Filter under test

For testing and demonstrating the performance of the metrology setup, a 10 channel butcher block filter was selected. This filter was procured by OHB Digital connect in the framework of a remote sensing application and was characterized at OHB Munich in a joint activity. Supplier is Iridian, CA.

This butcher block filter features 10 individual spectral channels, each channel having a length of 20mm, and clear aperture widths varying from 550μm down to 330μm.

The overall filter substrate has a dimension of 28 x16 mm, including edges for mounting in front of the detector.

The spectral coverage of the individual channels ranges from 900nm up to 2.5 μm.

The channel bandwidth of the individual spectral channels ranges between 7nm up to 90nm.

Figure 12

Geometry of filter under test – section of production drawing.

No further details on the individual channels, their specific line positions and tolerances, the foreseen detector and the application for this multispectral filter are mentioned here, as the focus of the paper is on the metrology setup and its performance and not on the filter itself.

Figure 13

multispectral filter in visual inspection: glue gaps are visible as thin dark stripes, covered by the aperture mask of the filter.

Figure 14

multispectral filter with aperture mask and outer edge unmasked (the individual filter bars and the glue gaps between them can be nicely seen).

3.2.

Results for center wavelength and channel width

All measurements from all filter strips with 5 positions on each stripe are shown in the following figure, together with the transmission of the uncoated glass sample.

Figure 15:

complete scan of all filters and substrate, measured with F# of 4.

It can be seen, that the maximum transmission is very high at the centre of each band. It is significantly higher than the uncoated glass sample (having reflection losses), indicating very good antireflection properties on both sides of the filter. The dip in band 5 (2200nm) clearly is no fault in the coating, but stems from the bulk absorption inside the substrate. The narrow band 10 at 2318nm expectedly has a lower value for the maximum transmission due to the F-number.

Two exemplary channels out of the 10 spectral channels of the filter are shown in the following section, to demonstrate the measurement of the centre wavelength tolerance along the spectral channels.

In the following plots, the solid lines are the five measurements along each filter stripe at positions 0, 5, 10, 15 and 19mm with the colours Red, Green, Blue, Yellow and Magenta, respectively. The black dotted lines are the measurements made by the filter supplier Iridian at wafer level (at higher F-number and, very probably, at different positions on the filter). The actual filter characteristics at the design F-Number of 2.5 will be slightly shifted and broadened compared to these measurements (up to -6nm as for the 2318nm band).

Table 2

Spectral Band 1, comparison of design values and measured results

The measurement by the supplier Iridian is shifted towards the higher wavenumber side, however still well agreeing to the OHB measurements. The shift is due to the infinite F-number measurements from Iridian compared to the OHB measurements made at F-Number of 4.

Table 3

Spectral band 7, comparison of design values and measured results

Filter band 7 is perfectly within the requirement. The Iridian measurement is shifted by about 2.5nm to higher wavelengths.

The results for the two exemplary filter bands shown in Figure 16 Figure 17 demonstrate, that the measurement along the filter stripes can resolve minor inhomogeneity’s in center wavelength and channel bandwidth.

Figure 16:

Band 1: The first 3 positions lie close together; there is a clearly visible inhomogeneity. Still the transmissions are well within the requirement.

Figure 17:

Filter band 7 with F/# of 4

Calculation of transmission measurements:

As described above, the glass substrate (both coated and uncoated) in between the measuring optics induces a shift of the focal point. Therefore, when the optics is aligned for the setup with a glass in between (e.g. the filter), it is not possible to move this filter out for measuring the intensity without filter. Due to the shift in focal position the focal point is not any more in the focal point of the receiving mirror, and therefore the light is not well coupled into the receiving fiber. For this reason the receiving optics has to be translated manually to compensate this effect. As this is not automatized, this cannot be done in between a measurement sequence. For this reason, directly after scanning a position on the filter, a reference measurement was done at a common position at the uncoated glass substrate, to avoid a shift in focal position. To cancel out the transmission characteristics of the uncoated glass substrate, this then was measured separately.

The accuracy of the transmission measurement is estimated to be ±1.5 to 2%.

Optical density:

To check the out-of-band blocking of the coatings outside of the transmission region, each filter bar was measured at 5 positions for the spectral range 630nm to 2500nm with a step size of 2nm. As different detectors, order sorting filters and gratings have to be used for specific spectral regions, and each measurement of a sample has to be followed by a measurement at a common position on the uncoated glass substrate, this measurement is rather complex and with a measurement time of about 20 hours (with the standard settings used) rather time consuming. In total, it consists of 500 single measurement sequences.

Figure 18

complete scan of all channels showing the requirement for out of band blocking as black line

It can be seen, the out-of-band blocking is well below the filter requirement (horizontal lines in the figure) except for very few positions. The metrology setup is capable to characterise this out-of-band blocking for optical densities of OD3 or even higher over the full spectral regime.