Longer baselines and how it impacts the ALMA Central LO 1 C. Jacques - NRAO October 3-4-5 2017 ALMA LBW
Quick overview of current system Getting the data back is not the problem (digital transmission), getting the LO there will be (analog signals) Assume 15 km (current), 30 km, and 60 km Assume ALMA antenna, ALMA cryostat (correct?) A look at 20 to 30 km path lengths A look at 60 km path lengths Some questions needing to be answered 2 October 3-4-5 2017 ALMA LBW
Current Central LO References AOS Central Bldg Master Frequency Standard 5 MHz 125 MHz Correlator Antenna WDM LO Photonic Receiver 1 st LO Offset 1 st LO ref 27-122 GHz 20-45 MHz 1 st LO PLL ref Central Reference Generator 10 MHz 5 MHz Central Variable Reference LO Reference Receiver Buried Fiber to Antenna 2 nd, 3 rd LO 125 MHz 2 nd LO ref 8-14 GHz comb 48 msec Key Electronic Fiber Optic 13.5 20 GHz Master Laser and Laser Synthesizer 1556 nm 27-122 GHz WDM Line Length Corrector Fiber Patch Panel 1532 nm, 2 GHz, 125 MHz, 48 msec ` 3 October 3-4-5 2017 ALMA LBW
Current specifications (guaranteed vs. temp, time, fiber length, antenna motion, ) Tunable LO for all ALMA bands, 27~ 122 GHz, including an extended Band 1 tuning range of 27~ 39 GHz 1 st LO RMS integrated phase noise < 53 fs (at highest frequency) 1 st LO phase drift < 18 fs, over 15 km (w/. active stabilization). < 1 second fast-switching time to any frequency 5 independently tunable SubArrays, expandable to 6 For all 66 antennas 1 hour reset free operation / 24 hour polarization calibration 1E-11 stability 4 October 3-4-5 2017 ALMA LBW Phase Structure Function (sec) 1E-12 1E-13 1E-14 1E-15 Measured Phase Expected Phase without Correction LO spec 10 100 1000 Tau (Seconds)
To state the obvious: For the LO, the PAD to AOS-TB distance is the metric used (15 km), not the antenna-antenna baseline distance (16 km) Pad placement could yield a bigger baseline without increasing the LO path by the same amount AOS-TB 21.2 km 5 October 3-4-5 2017 ALMA LBW
We can achieve these longer baselines We already have, in place, all the connections required for 14 additional antennas (assuming current ALMA cryostat and Analog Rack) 6 October 3-4-5 2017 ALMA LBW
Simply extending may not be so simple Example: the upcoming Band 1 was originally specified with a 27 to 33 GHz LO, which was achieved. The built-in margin + software upgrade now allows us to tune to 39 GHz, based on an upgraded requirement. The newest requirement, a 40 GHz LO (52 GHz sky frequency), is simply not possible with current hardware: f LO (max) = 2 x f CVR 125 MHz which gives us 39.875 GHz, just because of one component (CVR, the RF synthesizer) 7 October 3-4-5 2017 ALMA LBW
A clear definition of what will be required, early Right now: ALMA site expected Sky delay variations < 500 fsec 50% of the time < 150 fsec 10% of the time < 90 fsec 2.5% of the time 1 st LO rms integrated phase noise < 53 fsec (LO was designed to support highest frequency band and best observing condition all the time) Phase noise spec? Highest observing frequency? 373 (B7)/690 (B9) GHz? What will the max residual drift specification be? Will impact the technology used to deliver the LO, and if we need to develop new systems (ex: allow use of different lasers) 8 October 3-4-5 2017 ALMA LBW
20 ~ 30 km path lengths -1 (2 x baseline) In addition to potential degradations at higher bands due to elevation: Based on lab measurements performed with cvclots, the current CLOA can reliably lock at 22.5 km at Band 6 (meeting ALMA specs). This is mostly due to the Coherence Length of the Master Laser: X Coherence specification: > 50 % at 30 km 9 October 3-4-5 2017 ALMA LBW
20 ~ 30 km path lengths - I1 New Master Laser with double the coherence length, and better frequency stability, is needed (or a new correction scheme). < 2e-11 ASD Frequency Stability required (stabilizes 15km fiber to 0.3 microns) Result < 10-12 10-13 10-16 frequency stability of 10 MHz reference achieved over 100 km (van Tour, Koelemeij, NRAO internal memo) using White Rabbit + PLL + OCXO 10 October 3-4-5 2017 ALMA LBW
20 ~ 30 km path lengths - III We would also need an upgraded Line Length Correction system. Larger range: 10 mm? (currently 4.5mm) Higher resolution (target 16 bit) Higher bandwidth (limited by signal return delay) We should look at alternative technologies to stretching fiber, to achieve greater range and speed, less polarization sensitivity (because of polarization to phase conversion) Less than 0.4 radians SOP change for 540-degree antenna rotation. SOP change plotted against input polarization 11 October 3-4-5 2017 ALMA LBW
60 km path lengths -1 Overcoming the link loss is not an issue, as we demonstrated locking and correction over a 70 km equivalent span, using a very simple proof-of-concept bi-directional EDFA. Other groups report the same. LO Signal (to antenna) Er doped fiber Return Signal (from Antenna) 5/95 ASE filter ASE filter 5/95 Return Signal Output Monintor LO Signal Input Monitor pump/signal combiner Return Signal Input Monitor LO Signal Output Monintor 980 nm pump laser Fringe Counter @70 km 25000 20000 15000 10000 5000 0 0 5 10 15 20 25 30 12 October 3-4-5 2017 ALMA LBW
60 km path lengths - II Single path EDFA cannot use isolators, forcing a reduction in gain to prevent oscillation. This approach (similar to others) chosen because circulators introduce substantial polarization effect (split optical paths). ML 10 db 2:2 EDFA FS SL PR Laser Tuning port 2:1 DRIVER FFS PM1 RF AMP PM2 25 MHz PLL 100 Mhz 20 GHz RF AMP VVM 13 October 3-4-5 2017 ALMA LBW
60 km path lengths -III Many new systems can achieve significant spans: SKA (University of Western Australia): 185 km White Rabbit GbE (CERN): 100 km Lower frequency LO: MHz, 9 GHz None report sub-picosecond residual phase drift. We can explore a direct photonic approach, using Integrated Microwave Photonics (if phase noise spec OK) ex: Teraxion Laser Synthesizer on-a-chip (Photonics West 2016) Fast switching: many active-correction techniques cannot achieve unambiguous return to phase new development needed Active correction: current and planned upgraded Correlators cannot accommodate post-processing of phase drift. 14 October 3-4-5 2017 ALMA LBW
60 km path lengths -IV Another enabling technology: J. Campbell and A. Belling at UVA, high output power at 100 GHz +) CD now comes into play. Lasers cannot have a ROF in the 4~12 GHz range of the IF band Keep in mind this is a holistically designed CLO system Options considered Cost to Build Direct Photonic Photonic Hybrid Photonic Reference Development Risk, NRE 15 October 3-4-5 2017 ALMA LBW
To realize this vision Photonic LO V 1.0 Line Length Correction V. 1.0 Current implementation Photonic LO V 2.0 Line Length Correction V. 2.0 Bidirectional Optical amplifier LO Regeneration n3 km Direct LO? Future implementation 16 October 3-4-5 2017 ALMA LBW
We can deliver this, but it may be expensive As a quick example: The current Line Length Correction active correction has a 4.5 mm range (enough for 15 km + antenna motion): it relies on a low PMD fiber cable being buried at a 1meter depth. Based on recent ALMA contracts, we can estimate that each additional 10 km span length will cost almost USD 1 million, just for the fiber cable (from purchasing to commissioning). (Ex: for 6 antennas at 50 km, the cost is USD 30 M, in 2016 dollars, based on 50/50 soft soil/rock ratio) 17 October 3-4-5 2017 ALMA LBW
Specifications to consider for ALMA 2030 What is, realistically, the maximum baseline distance target? Can you tailor it to minimize span lengths? Which Bands will be used, which will not? Ex: Band 3 requires a 121 GHz LO reference Ex: the Round-trip Phase Correction uses Band 9 Is fast-switching a requirement? What is the target reset-free integration time? What is the residual phase error spec? Currently < 18 fs @ Band 10 (3.6 microns for 15 km of fiber) Acceptable Phase noise (coherence vs resolution)? What compromises could be made? 18 October 3-4-5 2017 ALMA LBW
www.nrao.edu science.nrao.edu public.nrao.edu The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. 19 October 3-4-5 2017 ALMA LBW
Serial Port (RS-232) Trigger (TTL) Fluorescence Microcontroller Error Signal Detector Narrow linewidth laser Laser Controls Frequency Noise User Optical Output 1556.2108 nm Modulator Oven Freq. Doubler Freq. Doubler Temp. Controller 778 nm Optical isolator Oven Rb Rb Oven Temp. Controller Reflector 20 October 3-4-5 2017 October Long 3-4-5 Baseline 2017 Workshop ALMA LBW 20
1532 nm Input from Laser Synthesizers 1:6 Switch PC PBS 5% WDM PC Fiber Stretcher Polarimeter FBG l M l s PC Subarray Switch Beatnote Det 50 MHz Div by N 5 MHz ref Phase Detector Fringe Counter Stretcher Driver MicroControlller Line Length Corrector Band 1 LO RCVR WDM Compensating Fiber EDFA Frequency Shifter 1:10 Switch LO Photonic Receiver 1 st LO Offset Compensating Fiber FRM Band 10 LO 20-45 MHz 1 st LO PLL ref 1st LO Driver YIG+Active Multiplier Chain and PLL LO Reference Receiver 2 nd, 3 rd LO 125 MHz 2 nd LO ref 8-14 GHz comb 48 msec Round-trip phase correction path in red 21 October 3-4-5 2017 October Long 3-4-5 Baseline 2017 Workshop ALMA LBW 21
AOS Web data from 2012-05-19 to 2012-05-19 3.4 3.2 3 2.8 CONTROL_DV02_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV05_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV08_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV09_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV10_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV11_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV12_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV13_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV14_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV15_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV16_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV17_LLC VF_MON_VF_MON_0 (N/A) CONTROL_DV18_LLC VF_MON_VF_MON_0 (N/A) 2.6 2.4 2.2 16:00 20:00 Time (Timezone: localtime) 22 October 3-4-5 2017 October Long 3-4-5 Baseline 2017 Workshop ALMA LBW 22