13,14 July 2005 Feasibilty/Concept Study Mid Term Status Review Laser Metrology for Segmented Telescopes Feng Zhao, Tom Cwik Jet Propulsion Laboratory

JPL Laser Metrology for Segmented Telescopes What is laser metrology? The proposal for CCAT R&D progress at JPL

Laser metrology is an optical ruler Two concentric laser beams reflect from two retro-reflectors. The reflected beams interfere with a beam of a different frequency. The two oscillating beat signals are measured with a photodiode. The length difference is proportional to the phase difference between the two signals. Measurement beam, f o Local Oscillator (LO) beam, f o +f h Signal #2 Signal #1

One calibrated reference can support many mirror segments Optical Domain Electrical Domain Near 1xN G G G G Far Near Far Phase meter Swept Freq Lasers AOM Metrology source Fiber distribution Beam Launcher REF beam launcher with a calibrated REF cavity x1x1 xRxR RR 11 nn

Absolute distance measurement Use an optical ruler between a primary segment and the secondary mirror. Use a second optical ruler on a calibrated delay line of known length (REF). Continually change the laser frequency ( Δ f o ) Use a phasemeter to monitor the phase changes. Compare the phase shift from the delay line to that from the mirror segment (MEAS).

CCAT: An all-optical metrology truss uses the secondary mirror as a common reference for each primary mirror segment. Segmented primary mirror Metrology truss Secondary mirror (common reference)

Three corner cubes are mounted on the secondary mirror.

Compact beam launchers are mounted behind the primary mirror segments. Beam launchers (6)

Each segment has an Optical Hexapod (OHP) Six degrees of freedom per segment All measurements are relative to the secondary mirror. The secondary mirror needs only 3 corner cubes. No heat sources or wiring at the secondary mirror. Each panel can be controlled separately. Absolute alignment is ~ 1 micron. Control loops are adapted from mechanical hexapods. Mechanical hexapod

Finding the optimum cost/benefit for CCAT A design study will balance cost, performance, and complexity for CCAT. Electronics can be behind each mirror segments for low cost, or elsewhere for low heat. CCAT may only need 3 degrees of freedom Tip, Tilt, & Piston Tip, Tilt, & Piston A tripod has half as many parts as a hexapod. A tripod has half as many parts as a hexapod. Use tripods with conventional edge sensors to form “panel groups” Use current edge sensor capability for a smaller number of panels Use current edge sensor capability for a smaller number of panels Laser metrology to align these panel groups Laser metrology to align these panel groups Corner cube for laser sensors Conventional edge sensors

R&D progress at JPL SoftwareOpticsLasers Beam launcher mechanics Phasemeter electronics Interfaces

Software LabView and C interfaces are being developed The first demonstration will calibrate a delay line and use that to measure a variable distance.

Optics Concentric beams require a corner cube with a hole Vendors are identified Vendors are identified Parts are being delivered Parts are being delivered Φ5mm hole

Lasers Swept frequency diode laser External cavity control External cavity control Computer-controlled frequency sweeps are verified with an optical spectrum analyzer. Computer-controlled frequency sweeps are verified with an optical spectrum analyzer. Fiber-coupled acousto-optic modulator generates beats

Beam launcher and mixer Attaches directly to the bottom of the primary mirror Optics are precision-aligned during assembly JPL will deliver a fabrication and assembly process JPL will deliver a fabrication and assembly process Lower cost options for CCAT This design is for space missions This design is for space missions CCAT could use thumbscrews instead of flexures CCAT could use thumbscrews instead of flexures CCAT could use lower quality optics CCAT could use lower quality optics

Phasemeter electronics Integrated multi-channel zero-crossing detector Converts beats to a square wave Converts beats to a square wave Mature and reliable circuit design reduces systematic error Mature and reliable circuit design reduces systematic error VME 6U size But it only needs +5, -5, GND. But it only needs +5, -5, GND. $600 for parts and assembly Optional test switches 6 fiber optic inputs Analog and digital power isolation AGC switch Digital signals to VME bus or jumpers

Phasemeter electronics FPGA-based fringe counter New and simplified single-chip realization New and simplified single-chip realization On-board clock or external high-quality oscillator. On-board clock or external high-quality oscillator. Data is filtered and averaged on-board, then sent via ethernet to a computer at 1kHz. Data is filtered and averaged on-board, then sent via ethernet to a computer at 1kHz. FPGA code is reprogrammable. FPGA code is reprogrammable. $100 Xilinx prototype board, with a 50MHz crystal oscillator Spartan-III FPGA chip PROM $50 Breakout card Ethernet out Reference in Measure in

Integration progress Building a beam launcher Building a white light interferometer for verification

Advantages of JPL’s laser metrology Accurately measures the distance from the secondary mirror to each primary mirror segment. Continually measures the telescope’s mechanical deformation Designed for straightforward integration with the telescope’s control systems Simple. Power, cabling, and costs scale controllably with system size 2 wires for ethernet out 2 wires for ethernet out 2 wires for a high-quality clock 2 wires for a high-quality clock 2 wires for power 2 wires for power 2 fibers with laser light 2 fibers with laser light

Extra slides Common path heterodyne interferometer Common path heterodyne interferometer Mirror segment beam launcher attachments Mirror segment beam launcher attachments Beam launcher Beam launcher Scaling laws Scaling laws How a phasemeter works How a phasemeter works Phasemeter HDL Phasemeter HDL Timing diagram Timing diagram Ethernet data format Ethernet data format Severe vibration Severe vibration Old beam launchers Old beam launchers

Common Path Heterodyne Interferometer Two signals from separated parts of one wavefront Less optical cross-talk than polarization schemes Less optical cross-talk than polarization schemes Better thermal stability than adjacent beams Better thermal stability than adjacent beams Reconfigurable for different types of measurements Reconfigurable for different types of measurements x

Panels with embedded metrology beam launchers Fiber optic cables for laser input and output

Fiducials on rigid segments

Scaling Laws Mechanical optical error scales to the 4 th power of aperture diameter (Peterson, et al.) The larger the aperture, the more (a lot more!) challenge on structures Metrology sensor error scales linearly with the dimension Save your $$ and kg from structures to build metrology? Build a very rigid back structure and a simple metrology system or a low-cost back structure and a very capable metrology system to correct for deformations? Where is the optimum?

f2 f1 (f1-f2) Fringe counter Reference Measure Clock Zero-crossing detectors To the customer’s data acquisition system Light sources Reference Measure Shanti Rao, , 2005-Apr-13 How a phasemeter works

200MHz Clicks Frequency Integer & fractional Averaging Clocked delay 1kHz 256 bit synchronous- Load RAM Ethernet out Based on the SIM phasemeter (Halverson et. al.) Phasemeter HDL

Phasemeter HDL timing diagram Count fractional and integral fringes between signals from near and far fiducials. End fractional Start fractional Measurement update Ready pulse Fringe wrap Integer counter updates with fractional counter

32-byte UDP packet every 1 ms Could be faster Could be faster Could be 3-wire, RS422, etc. Could be 3-wire, RS422, etc. IP+MAC identify each phasemeter struct phasemeter { char hello[4]; unsigned short ref_tick ; unsigned short meas_tick ; unsigned long ref_sum ; //ref_period = basetick * ref_count / ref_sum unsigned short ref_count ; unsigned long meas_sum ;//meas_period = basetick * meas_count / meas_sum unsigned short meas_count; signed long int_sum ; //number of integer fringes = int_sum/int_count unsigned short int_count ; unsigned long frac_sum ; //distance unsigned short frac_count; } Ethernet data format

Measuring severe vibration (2 lasers) Use WDM (wavelength division multiplexing) to combine 2 nd laser (not tunable) to monitor vibration Remove vibration from the ABS (swept frequency) ranging measurement

Old beam launchers 2001 Polarization-separation type Spatial separation type Hewlett-Packard JPL prototypes QP SAVV SIM Internal Launcher SIM External Launcher COPHI