• Proposal 531: Photonic Local Oscillator for Multibeam Focal Plane Arrays

  • Reviewer 1

Grade: 4.5

This is a very detailed proposal that unfortunately is relatively far outside my areas of expertise. As compared to the other proposals it appears much more detailed and much more ambitious in terms of deliverables for the funding requested. This may be due to my lack of familiarity with the area but it means I have somewhat more difficulty in grading this project with confidence.

Title: Photonic Local Oscillator for Multibeam Focal Plane Arrays

1. Alignment with NA ALMA Partnership strategic goals;

Comment.

This project aligns with the strategic goal of increased wide field mapping speed for ALMA. The goal is to investigate the photonic LO concept as an enabling technology for LO distribution in multibeam SIS arrays.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ‘ALMA 2030’ development documents.

Comment.

The ability to map larger fields of view has high scientific value across a wide range of scientific areas. A few significant studies noted in the proposal were recent programs which mapped large areas at high resolution.

3. Quality of the upgrade conceptual design; Comment:

The goals are to demonstrate and develop a photonic LO system for ALMA receivers that could replace the entire ALMA 1st LO with a simpler and less expensive assembly. The ultimate goal is technology enabling a multibeam array for expanded FoV.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development);

Comment

This project will provide the photonic LO basis needed for future multi-beam focal plane arrays on ALMA.

5. Strength of the consortium organization (if applicable);

Comment.

6. Qualifications of the key personnel of the Study;

Comment.

The key personnel for the program appear ell equipped to plan and lead this program through completion. Experience includes work with the Central LO Article and Photonic LO reference for ALMA as well as many other relevant areas.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study;

Comment.

Team members appear very well places to undertake the proposed project with significant experience and expertise in the relevant areas.

8. Assessment of the level of risk inherent in the design;

Comment.

Overall this project is well laid out and appears very well understood by the team. The risks as presented are minor with clear remediation path.

9. Strength of the Scientific Team supporting the Study;

Comment.

10. Level of support guaranteed by the Institutes;

Comment.

No indicated In-kind contribution.

  • Reviewer 2

Review of proposal "Photonic Local Oscillator for Multibeam Focal Plane Arrays"

Proposal index number: 531

Principal Investigator: Bill Shillue

Cost: $399,082

In-kind contributions: none

Review date: 8 July 2017

Overall score: 2.0 (0.1 best to 9.9 worst) [but see comments under Scoring]

Proposal Summary The proposed project is to develop and test a new photodetector that can serve as a direct LO for ALMA SIS mixers up to band 6. This is intended to facilitate the development of multi-pixel receivers to address the strategic goal of increasing wide-field mapping speed, as stated in the ALMA 2030 Roadmap. At present, the centrally-generated LO at 27-122 GHz is recovered from the optical fiber by a photodetector at each antenna and then used to lock an electronic oscillator. For higher bands, active and/or passive multipliers are used to generate the final LO signal for an SIS mixer. In contrast, a direct photonic LO connects the millimeter wavelength signal from the photodetector directly to the SIS mixer with no intervening electronics. Such a large simplification is needed to enable implementation of multi-pixel receivers, which would be too complex and voluminous if the present approach were duplicated. The concept includes transmitting the photonic LO into the receiver's vacuum chamber on a single fiber, splitting it with an optical power divider, and delivering it to separate photodetectors attached to the SIS mixers of all pixels. The proposal discusses several challenges with the direct-photonic approach, including making the centrally-generated signal sufficiently pure and stable, but the issue addressed by the proposed work is the construction of a photodetector that will produce sufficient power at millimeter wavelengths when operated at cryogenic temperatures. A plan to design the appropriate epitaxial structure and to fabricate it into photodetector chips is given, and recent work at UVa on room-temperature photodetectors of similar structure is described. The project continues by packaging the chips into fiber/waveguide mounts at NRAO and testing them with existing ALMA SIS mixers at bands 3 and 6, first with the photodetectors at room temperature and then with the photodetectors integrated with the SIS mixers at cryogenic temperature. The packaged photodetector is considered a deliverable of the project.

Strengths and Weaknesses The investigators are exceptionally well qualified to carry out the proposed work. Existing facilities at UVa are well suited to support the chip fabrication and initial testing, and facilities at the CDL are well suited to support the packaging work and integrated testing. The tasks given in the Statement of Work (section 4.10 of the proposal) can be expected to be completed within the allowed time (24 months) and within the proposed cost ($399k). However, certain critical questions are not addressed in the proposal. The emphasis is on getting enough power from the photodiode to drive an SIS mixer, but that amount of power is never mentioned. Test results are shown for a similar room-temperature photodetector that delivers +8 dBm at 110 GHz, a "breakthrough" level, but that is about the only quantitative information provided. This is said to be "sufficient" to drive a band 3 SIS mixer, and extrapolation at –6 dB per octave is said to imply that it is also sufficient at 300 GHz (band 6). More importantly, there is no mention of the photodiode's conversion efficiency; how much optical power does it take to produce "sufficient" millimeter wave power? How much power is needed to drive the 2SB mixers that ALMA uses in these bands, considering that each includes two individual SIS mixers, and taking into account their LO coupling efficiency? To support a receiver with P dual-polarization pixels, we will need drive 2P SIS mixers, or 4P elementary SIS mixers for 2SB. Even the smallest useful multi-pixel receiver at P=4 requires driving 16 elementary mixers using either 8 or 16 photodetectors. It is important to understand the total amount of optical power needed to do this, since all of it will be dissipated as heat and will need to be absorbed by the 4K stage of the cryocooler. The proposal mentions that an optical amplifier at the antenna will be necessary to get enough power but it does not say how much power that is. Unless it is known that the required optical power is small enough to be practical, the entire concept is not worth further study. While an accurate value may not yet be available, failure to give even an order-of-magnitude estimate is a serious deficiency of the proposal.

Details of Proposal Contents Sec 4.10, Statement of Work. Only the "design" of the photodiode chip is described, not fabrication of the wafer and chips. Clearly fabrication is intended, as mentioned in the previous section under "Design Tools." Sec 9, Staffing. The PI at NRAO will devote 30% of his time to this project over its entire 24 month duration. Others at NRAO will devote smaller fractions of time for 12 months. The time commitments of the two CoIs at UVa are not stated. It appears that no person will devote a majority of his time to this project, so there will be no one whose main responsibility is this project. That creates a risk that it will get insufficient attention and not be completed on schedule. Sec 10, Cost Breakdown. From Tables 4.0–6.0, it appears that 285.1k$ is intended to be spent at NRAO and 108.6k$ is intended to be spent at UVa, for a total cost to the ALMA Development Program of 399.1k$. Table 4.0 gives a breakdown of what appear to be NRAO expenditures, but the body of the proposal (especially section 4.9) makes it seem that Task 1.0 of Table 4.0, photodetector design, will be done at UVa. The UVa expenditures in Table 5.0 are not broken down. Is the 62.5k$ for Task 1 in Table 4.0 being counted twice? The actual flow of funds to the two institutions needs to be made clear. Will the 62.5k$ for Task 1 go to UVa in addition to the 108.1k$ in Table 5.0? Some breakdown of the UVa costs is needed; all of the 20k$ for Materials & Services in Table 4.0 is for Task 2 (Packaging, presumably at NRAO), but some M&S ought to be needed for the photodetector chip fabrication (presumably at UVa). Since all testing will be done at UVa and NRAO in Charlottesville, it is not clear why 10k$ is budgeted for travel under the "Testing" task. Sec 12.5, Risk Management. Three risks are identified with a total cost to recover from them estimated at 27k$. But the probability of each is estimated to be 20%, so only 5.4k$ is allocated to contingency and included in the total cost of Table 6.0. However, if any one of the risks materializes, then 5.4k$ will not be enough to recover. The budgeted contingency is less than 2% of the total cost — not nearly enough.

Scoring The proposal deserves high marks for its clear description of the work, excellent background discussion, competence of the PI and CoIs, and relevance to the ALMA 2030 Roadmap. On this basis I am inclined to assign a score of 1.0, nearly as good as possible. However, my final score is degraded to 2.0 to reflect the serious deficiency of not addressing the photodetector efficiency and the total optical power needed, and for the minor deficiencies in budgeting and risk management, as noted in this review. These deficiencies could easily be corrected, in which case the score should be adjusted toward the better value.

  • Reviewer 4

Grade: 1.0

Title: Photonic Local Oscillator for Multibeam Focal Plane Arrays

1. Alignment with NA ALMA Partnership strategic goals;

Proposal is well aligned with ALMA strategic goals.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ‘ALMA 2030’ development documents.

This proposal is for a study of ALMA multibeam arrays for expanded field of view. The study will focus on packaging of devices to form multibeam focal plane arrays. These devices are based only already existing capabilities : ALMA photonic LO system, and mmwave photodiodes developed at Univ. of Virginia. The proposal is extremely well written and the project is in fact low risk but high impact.

3. Quality of the upgrade conceptual design;

See above.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development);

This proposal is on the LO distribution in multibeam SIS arrays. Parallel development in other areas will be needed for the full implementation of wide-field mapping capability. Compatibility with ALMA is discussed in section 4.8.

5. Strength of the consortium organization (if applicable);

Extremely strong consortium since it is NRAO and University of Virginia.

6. Qualifications of the key personnel of the Study;

PI is the leader of the Photonic LO group for ALMA, and extremely well qualified, so are the two co-Is.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study;

More than adequate exertise exists at both institutes for the proposed study.

8. Assessment of the level of risk inherent in the design;

Risk is low since the basic two elements the design uses, are well established already: the current photonic LO system, and the mmwave photodiodes development.

9. Strength of the Scientific Team supporting the Study;

See above.

10. Level of support guaranteed by the Institutes;

The commitment section 14.0 is not signed as in other proposals- but I assume this is OK since the PI is from NRAO.

11. Budgeted cost of the Study;

Total cost is $399,082, over two years (for PI) and one year for other staff to be hired. Collaborating institute’s cost is about 1/4th of total.


  • Reviewer 6

Grade: 1

Title: Photonic Local Oscillator for Multibeam Focal Plane Arrays

1. Alignment with NA ALMA Partnership strategic goals;

This proposal is relevant to NA ALMA strategic goals.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ‘ALMA 2030’ development documents.

LO generation and distribution, subject to power and volume constraints of the ALMA cryostats and receiver areas, is entirely relevant to ALMA 2030 directions toward adding multi-beam receiver capabilities to the array.

3. Quality of the upgrade conceptual design;

Very thorough, clearly explained, and of uniformly high quality.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development);

This is a study that would contribute to long-term goals in the ALMA Development Plan. It could also provide more capable photomixers of use to ALMA in applications other than multi-beam receivers.

5. Strength of the consortium organization (if applicable);

Excellent combination of experts in the field.

6. Qualifications of the key personnel of the Study;

Excellent.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study;

Proposers have world-class technical expertise and uniquely valuable experience.

8. Assessment of the level of risk inherent in the design;

Low risk to complete the study and provide information valuable for future planning.

9. Strength of the Scientific Team supporting the Study;

N/A: scientific goals from ALMA 2030 properly acknowledged.

10. Level of support guaranteed by the Institutes;

N/A

11. Budgeted cost of the Study;

Both effort and materials costs are reasonable.

  • Reviewer 13

Grade: 5.0

Title: Photonic Local Oscillator for Multibeam Focal Plane Arrays

1. Alignment with NA ALMA Partnership strategic goals; Comment: This proposal is aligned with the long-term strategic goals in that it would help support increased wide-field mapping with ALMA.

2. Strength of the scientific case for the proposed ALMA upgrade concept; Comment on the relevance to the ‘ALMA 2030’ development documents. Comment: The scientific case for larger throughput is wellestablished, and no funds are requested to enhance or support the science case.

3. Quality of the upgrade conceptual design; Comment: This proposal principally targets the development of high power photometers and receiver tests incorporating the output of a photometer LO without the need/use of a microwave YIG clean up oscillator. It is not at all clear that current photometer power levels are insufficient to be used in multi-beam receivers, especially in the lower frequency ALMA bands: 3, 4, 5, 6. Furthermore, there is no indication as to the number of pixels that may be required in a multibeam system for ALMA, so one cannot estimate the LO power requirements. In addition, the proposal suggests that a multi-beam receiver set would be incorporated into the current ALMA cryostat simply by replacing the single receiver set in that given band. This seems highly unlikely, since any multi-beam system would necessarily require additional space for multiple beams to couple effectively to the telescope. The aim is to evaluate LO performance by comparing receiver noise measured using the proposed scheme to that of the same receiver pumped with an existing LO. Success would be declared if the proposed scheme adds < 5% additional noise - this would add 10% more time to achieve the same sensitivity as compared to the single pixel case, which becomes significant in a multi-beam array of just a few pixels.

4. Readiness for production in the context of the ALMA Development Plan (the aim is to support a range of upgrades including both those which can be implemented rapidly and those requiring longer-term research and development); Comment: It would seem that a photonics LO could be tested using an ALMA mixer without the need to develop a new photometer.

5. Strength of the consortium organization (if applicable); Comment: Close ties already exist between staff at NRAO and UVA, and the team would work together effectively.

6. Qualifications of the key personnel of the Study; Comment: The key personnel are competent to perform the study as presented.

7. Technical expertise, past experience (also in series production, if relevant) and technical facilities in the Institutes taking part in the Study; Comment: The PI and his team were responsible for the development of the current photonic LO system used at ALMA, which performs well, and was delivered as planned.

8. Assessment of the level of risk inherent in the design; Comment: The risks outlined in the proposal are connected to device design and packaging. The greater risk might be that the scheme as proposed would add more noise to the receiver than can be tolerated. This could be tested without developing a new photodiode.

9. Strength of the Scientific Team supporting the Study; Comment: Funds have not been requested for scientific support.

10. Level of support guaranteed by the Institutes; Comment: There is no cost sharing provided for in this proposal.

11. Budgeted cost of the Study; Comment: If the photodiode development part of this proposal were dropped, the cost of this proposal would be cut by ~ 50%.

-- AlWootten - 2017-07-18
Topic revision: r1 - 2017-07-18, AlWootten
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