ultra-narrow bandpass coatings for deep space optical communications (dsoc)€¦ · ultra-narrow...

Omega Optical, Inc. tr: 9/13/2017

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Ultra-narrow Bandpass Coatings for Deep Space Optical

Communications (DSOC)

Thomas Rahmlow, Timothy Upton, Markus Fredell, Terry Finnell, Stephen Washkevich, Kirk

Winchester, Tina Hoppock and Robert Johnson

Omega Optical, Inc.

21 Omega Drive Brattleboro, VT, 05301

[email protected]

(802) 251-7390

NASA Phase 1 SBIR: NNX17CP58P

Program Monitor: Michael Peng, PhD

mailto:[email protected]


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Problem Statement

Deep Space Optical Communication has the potential for higher data rates and information density.

Once developed, the protocol can handle a large number of channels in parallel.

Ground Terminal:

1550.1 nm, 0.175 nm FWHM ‘Flat Top’ ultra-narrow bandpass filter

3o operating angle

Tilt tunable

Well collimated (F#48) beam

> OD 12 off-band rejection

Flight Terminal:

1064 nm FWHM ‘Flat Top’ bandpass filter

Space qualified, thermally stable

Signal strength is very low: photon counting

Very high background noise: can be within 5% of the sun when Earth and Mars are in

opposition.

Sun

Mars

Earth


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Ground-Based Terminal Filter Design

Figure 1: Modeled transmission for the ground base terminal filter. The design is a multi-cavity, flat top design to maximize throughput and signal to noise

Figure 2: Modeled transmission for the same design as presented in the previous figure is plotted on a log plot to highlight off band optical density.

Parameter Goal Design Phase 1 Goal

Phase 2 Goal

Comment

Center Wavelength 1550.1 nm at 3o AOI

1550.1 nm at 3o AOI

+- 0.01 nm +- 0.01 nm Center wavelength can be angle tuned +- 0.5 degrees

Temperature Shift: 1oC < 0.01 0.02 0.02 < 0.01 Substrate selection (CTE of 0.92)

OD 350 nm to 4000 nm Average

12 >8 no add’l blocker

>10 >12 We propose an additional blocker element


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Flight Terminal Filter Design

Figure 3: Modeled transmission for the flight terminal ultra-narrow band-pass filter. The design is a multi-cavity, flat top design to maximize throughput and signal-to-noise.

Figure 4: Modeled transmission for the same design as presented in the previous figure is plotted against optical density (OD = -Log10 T). Both the filter on S1 and blocker design on S2 is modeled.

Parameter Goal Design Phase 1 Goal

Phase 2 Goal

Comment

Center Wavelength 1064 nm at 3o AOI

1064 nm at 3o AOI

+- 0.01 nm +- 0.01 nm Center wavelength can be angle tuned +- 0.5 degrees

Wavelength Shift: 1o Temperature Shift: 1oC < 0.01 0.02 0.02 < 0.01 Substrate selection (CTE of

0.92)

OD 350 nm to 4000 nm Average

12 10 >10 >12 Alternate blocker schemes are being considered


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State of the Art

Figure 5: Measured transmission for two 2.5 nm, multi-cavity bandpass filters at 0 and 5º AOI are overlaid with the target laser wavelengths. The application is for free space laser communication. The operational angle of incidence is 0 to 5º. The laser wavelengths are 1552.3 and 1548.7.

Figure 6: The same measured transmission data presented in the previous figure are plotted on a log scale to highlight filter slope and rejection of the corresponding adjacent laser line. The filters provide high in-band transmission at 0 to 5º and OD 4 rejection of the adjacent laser bands.

Figure 7: Measured transmission of three ultra-narrow notch filters fabricated at Omega Optical, Inc. is presented. These are laser wavelength scanning data for 1.0nm wide, 0.65nm wide, and 0.3nm wide bandpass filters. Ultra-narrow notch bandpass filters can be reliably fabricated, but spectral shift with angle and temperature need to be matched to system requirements. (SPIE Paper 9612-21: Sub-nanometer band pass coatings for LIDAR and astronomy)


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State of the Art Hardware and Processes

Figure 8: The Helios cleanroom provides a clean environment for final cleaning and inspection of substrates prior to coating. Tight controls limit surface contamination and particles that can lead to pinholes in the surface.

Figure 9: The Helios multi-target high volume reactive sputtering coater provides high volume capability and reliable performance for the most challenging designs. Even so – uniformity of 0.25% across a 200 mm plate and 5 to 20 ang layer thickness errors limit capability for the manufacture of sub-nm multi cavity designs.


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Significance of the Innovation: Why the State of Art Doesn’t Meet the Need

Filter Design

SEM of a Multi-cavity Interference Filter


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Single Cavity versus Multi-Cavity Design

Figure 10: The transmission of a single cavity (red) and multi-cavity (olive dashed) bandpass filter designed to the same bandwidth is overlaid. The multi-cavity design gives a flat top response and a sharper edge and deeper shirt.

Figure 11: The same filter designs presented in the previous figure are plotted on a log scale. The multi-cavity design drives down to an optical density of 6 within 1 nm of the CW. Signal to noise for the multi-cavity is estimated to be 7.4x better than the single cavity.

Signal to noise for the multi-cavity is estimated to be 7.4x better than the single cavity …


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But … Making the Multi-cavity comes with some very strong challenges

Single Cavity

Figure 12: Transmission for 100 trials of a Monte Carlo simulation of a 0.5 nm single cavity design assuming a 1% error in layer thickness is overlaid.

Figure 13: Relative sensitivity of thickness layer errors by layer. The impact of error is greatest for errors in the thickness of the central cavity layer. Thickness errors in the outer reflector are small.

Random errors (1%) in layer thickness for a single cavity Fabry Perot

design do not significantly distort the shape, band width or

transmission, only the center wavelength.


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Multi-Cavity

Figure 14: Transmission for 10 trials of a Monte Carlo simulation of a 0.175 multi-cavity design for a 0.005% error in layer thickness is overlaid. Random errors in layer thickness for the multi-cavity design distort the bandpass shape and depress in band transmission.

Figure 15: Relative sensitivity of thickness layer errors by layer. The design is comprised of three cavities. Thickness errors in any of the three cavity layers drives the filter out of coherence and destroys the filter’s in band transmission.

Random errors of only 0.005% in layer thickness for the multi-cavity design

distort the bandpass shape and depress in band transmission.


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Uniformity, Layer Thickness Control and Plate to Plate Variability

We reliably meet uniformity across a 200 mm plate of +/-0.25% of the

center wavelength.

o At 1550 nm, 0.25% non-uniformity translates to a gradient error of

0.75 nm across a 25.4 mm aperture.

o This non-uniformity is 100x too high to produce the 0.175 nm target

bandwidth filters using state of the art technology.

Similarly, layer thickness accuracy using the turning point monitor on the

Helios coater is estimated from measured scans to be in the range of 8 to

20 angstroms (depending on the algorithm used)

o This error can push the multiple cavities apart by as much as 2 nm

and thereby destroy cavity-to-cavity coherence.


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Work in Progress

Filter design studies using the current material system were completed

Four test deposition runs were completed demonstrating the limits of the

Helios systems monitor algorithms for making ultra-narrow bandpass

filters

Annealing studies demonstrate an effective method for annealing filters

locally to correct for inhomogeneity across the filter sample

High confidence in measuring filter performance was gained from

measurements of test filters with and without AR coating

In-situ process monitoring development is continuing


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Filter Fabrication

We are currently doing a number of filter test runs.

These test runs provide test samples for process characterization as well

as provide an ultra-narrow band metrology test set.

Test Runs

Design Monitor Result

1 Single Cavity

2 layers – rest by rounds

Low %T (60%) and off wavelength: 1598nm

2 Single Cavity

First order monitoring

Monitor failed to correctly count turning pts coming out of cavity

3 Single Cavity

Second order monitoring

Ran well – high %T (97%) with AR and 2x750nm monitor placed CW at 1496 nm.

4 Two Cavity

Second order monitoring

Ran well – Low %T (OD4) – cavities not aligned


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Metrology – In Situ

Figure 16: A circulator and scrambler were added to the laser source path but no improvement is signal stability was noted.

Figure 17: Screen display of the optical monitor stability using the laser alone, with the laser and circulator and with the laser, circulator and scrambler. No significant differences were noted.

Metrology – Post Process


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Metrology – Filter Characterization

Figure 18: Laser measured transmission of the filter fabricated in test run 3 using 2 order monitoring. There was no AR on the second surface

Figure 19: Laser measured transmission of the filter fabricated in test run 3 using 2 order monitoring. There was no AR on the second surface

Figure 20: Laser measured transmission of the filter fabricated in test run 3 using 2 order monitoring. There is an AR on the second surface

Figure 21: Laser measured transmission of the filter fabricated in test run 3 using 2 order monitoring. There is an AR on the second surface


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Uniformity

Figure 22: Distribution of center wavelength Figure 23: Distribution of bandwidth (FWHM)

Figure 24: Distribution of peak transmission Figure 25: Filter transmission and OD


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Annealing Studies

Exposure power Percent change CWL Percent Change FWHM*

15% 0.135%-0.167% 1.17%-1.33%

20% 0.301%-0.335% 3.3%-3.38%

30% 0.646%-0.668% 7.34%

35% 0.721%-0.790% 7.43%-7.68%


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Optical Density

Optical Density is high by design – but several factors limit what can be realized

Pin holes and inclusion in the film

Material absorption

Scatter

The impact of Pin Holes on Optical Density

Density Dia: 0.001 mm Dia: 0.01 mm Dia: 0.1 mm

0 0 0 0

1 3.14E-14 3.14E-12 3.14E-10

10 3.14E-13 3.14E-11 3.14E-09

100 3.14E-12 3.14E-10 3.14E-08

1000 3.14E-11 3.14E-09 3.14E-07


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In Conclusion

We are working with state of the art tools to extend the our capability to

fabricate ultra-narrow band high performance optical filters

Current work quantifies process and metrology capability and defines

areas of measureable improvement

We are developing in-process and post process annealing techniques to

precisely tune the center wavelength of each cavity and of the completed

filter

We are developing micro-mapping tooling to precisely measure

performance across the apperature

Work is in process – stay tuned.

ultra-narrow bandpass coatings for deep space optical communications (dsoc)€¦ · ultra-narrow...

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