Characterization of Neutral Density Filters for In-Lab Optical Tests

Shreya Sutariya,

Kathleen Harrington, Thomas Alford, Carlos Sierra, Grace Chesmore, Jeff McMahon

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SPIE Astronomical Telescopes + Instrumentation

July 20th, 2022

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The Simons Observatory

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• A new ground-based CMB observatory located in the Atacama Desert in Chile

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• 3 Small Aperture Telescopes (SATs) and 1 Large Aperture Telescope (LAT), aiming to map the millimeter wave sky at large (degree) and small (arcminute) angular scales.
• Covered in earlier talks by Nicholas Galitzki and Katie Harrington

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The Simons Observatory

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• Combined ~60, 000 transition-edge sensor detectors

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• This increase in sensitivity requires an increase in understanding of telescope systematics.

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• For in-lab optical testing, we need neutral density filters (NDFs).

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LATR-tester (LATRt)

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SAT

SAT

• In-lab testing setup makes use of a Small Aperture Telescope.

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• Modified with the LATRt extension to study the Large Aperture Telescope’s Optics Tube

LATRt Extension

Instrument mounting structure with XY-Stages

Holds our testing equipment like the FTS & FLS for bandpasses, the warm load and chopper for optical efficiency measurements, etc.

LATR-tester (LATRt)

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SAT

LATRt Extension

• In-lab testing setup makes use of a Small Aperture Telescope.

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• Modified with the LATRt extension to study the Large Aperture Telescope’s Optics Tube (OT).

• First OT tested: Mid-Frequency (MF) (93/145 GHz)

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• The optics tube holds all filters, lenses, baffling, and detector arrays that we want to test.

Instrument mounting structure with XY-Stages

Holds our testing equipment like the FTS & FLS for bandpasses, the warm load and chopper for optical efficiency measurements, etc.

LATR-tester (LATRt)

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▶ See Joseph Golec’s poster #110 on AR Coatings.

Anti-reflection (AR) coated silicon lenses

LATR-tester (LATRt)

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NDF mount 1 (4K)

Detector arrays (100mK)

NDF mount 2 (100mK)

• The first NDF mount is to take calibration data at 4K.
• Location makes it easy to remove.

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• The second NDF mount is located right next to the detectors to account for systematics.

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• Measurements taken with one NDF.

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Holography

UFM (detector array)

Single Pixel Box (SPB)

▶ See Grace Chesmore’s poster #124 for more on holography.

• The UFM is the detector array that we are testing.
• This requires the neutral density filter.

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• The high-G SPB is designed to look directly at 300K and is useful, in particular, for 4K NDF calibration.

Testing Configuration of the Focal Plane:

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Neutral Density Filters for In-Lab Optical Testing

In order to not saturate the detector array, we require attenuation by at least factors of:

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0.089 (-10dB) for MF-1 (93 GHz band)

0.05 (-13 dB) for MF-2 (145 GHz band)

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The solution is to use neutral density filters. In our case, we use iron loaded rigid epoxies.

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• These are absorptive and machinable.

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• Available with varying amounts of iron loading.
• Eccosorb MF-110, 112, 114

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NDF Characterization: Things to Consider

1. Reflection:
1. Significant reflection can cause ghosting so knowing reflection inform design decisions.
1. We’ve gone with absorptive NDFs over reflective ones to mitigate this.
2. Transmission:
• Attenuation levels for our bands
3. Cryogenic Transmission:
• Our detectors and NDFs are cooled to 100 mK for optical testing of the detector array.
• How do NDF properties change with temperature?
4. Material repeatability:
• How does performance vary between different samples?
1. We are choosing rigid (machinable) instead of cured samples.

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NDF Characterization: Things to Consider

• Reflection:
• Significant reflection can cause ghosting so knowing reflection inform design decisions.
• We’ve gone with absorptive NDFs over reflective ones to mitigate this.
• Transmission:
• Attenuation levels for our bands
• Cryogenic Transmission:
• Our detectors and NDFs are cooled to 100 mK for optical testing of the detector array.
• How do NDF properties change with temperature?
• Material repeatability:
• How does performance vary between different samples?
• We are choosing rigid (machinable) instead of cured samples.

1. Reflection
2. Transmission
3. Cryogenic Transmission
4. Material Repeatability

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NDF Characterization: Things to Consider

• Reflection:
• Significant reflection can cause ghosting so knowing reflection inform design decisions.
• We’ve gone with absorptive NDFs over reflective ones to mitigate this.
• Transmission:
• Attenuation levels for our bands
• Cryogenic Transmission:
• Our detectors and NDFs are cooled to 100 mK for optical testing of the detector array.
• How do NDF properties change with temperature?
• Material repeatability:
• How does performance vary between different samples?
• We are choosing rigid (machinable) instead of cured samples.

• Reflection
• Transmission
• Cryogenic Transmission

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NDF mount

NDF to cover the test detector array

Absorber Model

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ñ = n + ik

k = a (ν/ν₀ )^b

a : absorption coefficient

ν₀ : pivot frequency

b : power-law relation for frequency ν

Extracting parameters using the ABCD matrix formalism and reflection and transmission measurements:

Reflection and Transmission Measurements

• Frequency-tunable Laser Source (FLS) to do room temperature measurements.

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• Coherent source with a wide frequency range and fine resolution

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• Transmitter and receiver photomixers, with the receiver being able to read electric field amplitude and phase information.

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▶ See Shreya Sutariya’s poster #124 on using the FLS for detector bandpass calibration.

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Reflection Measurement Setup

Receiver

Transmitter

Sample

Measurement Procedure:

We use the receiver to measure electric field amplitude.

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1. Measurement of reflective aluminum plate: E_plate
2. Measurement of sample under test: E_sample

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R = ( E_sample / E_plate )^2

• Reflection
• Transmission
• Cryogenic Transmission

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• We end up choosing MF-114.

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• Reflection of ~22% requires careful design consideration ⇒

we place the NDF mount right next to the detectors to mitigate systematics like ghosting.

• Reflection
• Transmission
• Cryogenic Transmission

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• We end up choosing MF-114.

​

• Reflection of ~22% requires careful design consideration ⇒

we place the NDF mount right next to the detectors to mitigate systematics like ghosting.

• Reflection
• Transmission
• Cryogenic Transmission

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Measurement Procedure:

We use the receiver to measure electric field amplitude.

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• Measurement with no sample: E_open
• Measurement with sample under test: E_sample

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T = ( E_sample / E_open )^2

Transmitter

Receiver

Transmission Measurement Setup

• Reflection
• Transmission
• Cryogenic Transmission

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• Fits are shown as solid lines.

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• Well understood behavior after 60 GHz that we can model using an exponential fall-off.

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• Below 60 GHz
• Seen in Zivkovic and Murk, 2011
• Model it with a varying magnetic susceptibility?
• If you have ideas about how to model this better - talk to us

• Reflection
• Transmission
• Cryogenic Transmission

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ñ = n + ik

k = a (ν/ν₀ )^b

• Reflection
• Transmission
• Cryogenic Transmission

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ñ = n + ik

k = a (ν/ν₀ )^b

• Properties do not change much from 300 K → 77 K.
• This is not what we expected
• Good news in terms of predicting behavior at colder temperatures

• Reflection
• Transmission
• Cryogenic Transmission

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• 4 K Measurements done by Thomas Alford using a Fourier Transform Spectrometer (FTS) with the high-G SPB

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• Place and remove NDF in the 4 K mount

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• We’re able to model this behavior with an exponential
• Useful when applying NDF corrections to detector array measurements

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• Behavior consistent with measurements at 300K

• Reflection
• Transmission
• Cryogenic Transmission

▶ See T. Alford’s poster #174 on FTS bandpass calibration.

Conclusion

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Using Eccosorb MF-114 as our NDF and characterizing it well allowed us to do a series of in-lab optical tests:

▶ See Carlos Sierra’s poster #123 for all in-lab optical LATRt tests.

Detector Bandpasses with the FTS

Detector Efficiencies

▶ See K. Harrington and C. Sierra for more on efficiency measurements.

• Bandpasses
• Efficiency Measurements
• Thermal Beam Maps

T. Alford

Conclusion

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Using Eccosorb MF-114 as our NDF and characterizing it well allowed us to do a series of in-lab optical tests:

▶ See Carlos Sierra’s poster #123 for all in-lab optical LATRt tests.

Detector Bandpasses with the FTS

Detector Efficiencies

▶ See K. Harrington and C. Sierra for more on efficiency measurements.

• Bandpasses
• Efficiency Measurements
• Thermal Beam Maps

T. Alford

Thank You

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• We end up choosing MF-114.

​

• Reflection of ~22% requires careful design consideration ⇒

we place the NDF mount right next to the detectors to mitigate systematics like ghosting.

• Reflection
• Transmission
• Cryogenic Transmission