Frequency Comb Development for Ultra- Precise Space Based Applications Jordan Wachs Systems Engineer Ball Aerospace
Ball has a strong history of working with space based and precision laser systems Calipso Used for cloud aerosol measurement Full decade of successful measurements Grace Follow-On Cavity stabilized reference for GRACE gravity mission Optical interferometry expected to be at least an order of magnitude more precise than microwave interferometry Picometer scale metrology Lab demo showing sensitivity to changes of length on the pico-meter scale Calipso is part of the A-Train constellation of satellites ~24 cm Grace FO cavity and shroud prototype Image Courtesy: NASA ~7.5 cm Image Courtesy: NASA ESTO 2
Applications of Frequency Combs Picometer Metrology Ongoing development at Ball <10pm displacement measurements >1mm measurement range Nanometer precision long distance ranging Demonstrated technique for kilometer scale ranging Single nanometer ranging demonstrated with 60ms integration times [Coddington, 2009] Multiple km/s velocity tracking possible Femtosecond precision time transfer Demonstrated technique for highest precision time transfer Femtosecond (1e-15) scale jitter on clock synchronization [Deschênes, 2016] 3
What is a Frequency Comb? F CEO Drives rigid translation of comb Controlled with oscillator pump power Linearly proportional to F REP and the carrier phase picked up per round trip in the cavity ν CEO = Δφ CEOmod 2π 2π f REP F CEO Spacing between teeth. This is identical between all teeth over full spectrum. 4 Directly controlled by tuning repetition length (PZTs on cavity) Fourier Transform of pulse train (pulsed laser) is pulse train (frequency comb)
Ball Aerospace Frequency Comb Top: Unbroadened (soliton) and fully broadened comb spectra. Right: Resultant heterodyne between 1064nm and 2128nm light for F CEO locking. 40 db >150 THz bandwidth once broadened 44fs pulse width (autocorrelated) ~200 MHz rep rate Fully selfreferenced FPGA controlled 5
Applications 6
Picometer Metrology Unequal Arm Michelson Utilize a modulated, tunable laser to lock to a single fringe. Sensitivity related to optical path length (OPL) difference between Measurement and Reference Arms Inverse of typical fringe counting approach to interferometry I = E 1 2 (e ikz 1) 2 + E 2 2 (e ikz 2) 2 + E 1 E 2 e ikz 1e ikz 2 λ = c f = z 2 z 1 n + 1/2 7
Comb Based Picometer Metrology (1) Change in wavelength measured directly by heterodyning against comb teeth This technique has exhibited measurement resolution on order of 100s of femtometers using fabry perot cavities (Schibli et al). Currently seeing 10s of picometers with Michelson 8
Comb Based Picometer Metrology (2) Ability to measure across comb teeth greatly expands measurement range without losing resolution Original picometer metrology experiment measurement range on order of tens of microns Current iteration should have interferometer measurement range on the order of millimeters with picometer precision 9
Anticipated Comb Measurement Precision Top 1kHz noise seen in frequency comb corresponds to ~1pm displacement uncertainty Expect final comb state to see ~200Hz noise (Rb source limited) Can use alternate lock scheme to bring noise <1Hz Bottom Plots showing measurements that are possible using current tunable laser setup 10
I Linear Optical Sampling Δt A Δt B Due to offset, relative time can be measured as an intensity signal, so timing jitter in detector is irrelevant t LOS requires two separate combs with slightly different rep-rates - Cross correlation of the two signals allows measurement of interferogram - Fourier relationships can be used to measure pulse peak arrival at a chosen reference plane 11
Long Range Nanometer Metrology By cross-correlating femtosecond scale pulses, a time of flight measurement can be made with great enough precision to enable extremely precise ranging with short integration times. I Δt A t 5nm precision has been demonstrated with 60ms integration times Δt B 12
Femtosecond Scale Time Transfer Time transfer is different from frequency transfer Not only need to match frequency, but need to control oscillator phase at both locations Femtosecond pulses Basic setup requires one stabilized comb at each site, A and B. Comb A is phase locked to ultrastable source such as optical atomic clock Comb B rep-rate will be locked to Comb A rep-rate A third comb serves for information transfer. Transfer comb has slightly different rep-rate so LOS can be performed to compare directly to Comb A and Comb B. X-fer comb held at fixed offset from Comb B, and Stepwise Function 1. Local Time Scale A Drives Comb A 2. Comb B compared to comb A via X-fer comb 3. Comb B locked to Comb A 4. Local Time Scale B driven by Comb B Spacecraft A X-fer Comb LOS Comb A Lock Local Time Scale A LOS Spacecraft B Comb B Lock Local Time Scale B 13
Femtosecond Scale Time Transfer Results Laboratory based demonstrations have shown system operation with parameters approaching real-world application 50 hours of system operation 4km path length through atmosphere Link availability rarely greater than 90% System drift typically less than 6fs during dropouts on scale of milliseconds Results demonstrate clock synchronization with less than 40fs (40e-15 sec) drift for 50 hours of continuous operation 14
Conclusion Frequency combs have represented a paradigm shift in laboratory science, providing access to timescales previously indecipherable to researchers. Ball is actively developing frequency comb technology for flight systems to leverage recent scientific breakthroughs in solving future mission challenges. 15
References [1] Hall, J. L. (2006). Defining and Measuring Optical Frequencies: The Optical Clock Opportunity And More (Nobel Lecture). ChemPhysChem,7(11), 2242-2258. doi:10.1002/cphc.200600457 [2] Newbury, N. R., Swann, W. C., Coddington, I., Lorini, L., Bergquist, J. C., & Diddams, S. A. (2007). Fiber laser-based frequency combs with high relative frequency stability. 2007 IEEE International Frequency Control Symposium Joint with the 21st European Frequency and Time Forum. doi:10.1109/freq.2007.4319226 [3] Phillips, D. F., et. al. (2016). An astro-comb calibrated solar telescope to search for the radial velocity signature of Venus. Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II. doi:10.1117/12.2232452 [4] Schibli, T. R., Minoshima, K., Bitou, Y., Hong, F., Inaba, H., Onae, A., & Matsumoto, H. (2006). Displacement metrology with sub-pm resolution in air based on a fs-comb wavelength synthesizer. Optics Express,14(13), 5984. doi:10.1364/oe.14.005984 [5] Coddington, I., Swann, W. C., Nenadovic, L., & Newbury, N. R. (2009). Rapid and precise absolute distance measurements at long range. Nature Photonics,3(6), 351-356. doi:10.1038/nphoton.2009.94 [6] Diddams, S. A., Hollberg, L., & Mbele, V. (2007). Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb. Nature,445(7128), 627-630. doi:10.1038/nature05524 [7] Exoplanets Exploration: Technology Overview. (2017, February 03). Retrieved March 20, 2017, from https://exoplanets.nasa.gov/exep/technology/technology-overview/ [8] Folkner W.M.,de Vine G, Klipstein W.M., McKenzie K, Spero R, Thompson R, Yu N, Stephens M, Leitch J, Pierce R, Lam TTY, Shaddock DA (2011) Laser frequency stabilization for GRACE-2 In: Proceedings fo the 2011 Earth Science Technology Forum [9] Sinclair, L. C., Deschênes, J., Sonderhouse, L., Swann, W. C., Khader, I. H., Baumann, E.,... Coddington, I. (2015). Invited Article: A compact optically coherent fiber frequency comb. Review of Scientific Instruments,86(8), 081301. doi:10.1063/1.4928163 [10] Wachs J, Leitch J, Knight S, Pierce, R, Adkins M (2016) Development and test of the Ball Aerospace optical frequency comb: a versatile measurement tool for aerospace applications. Proc. SPIE 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, 991206 (July 22, 2016); doi:10.1117/12.2230006. [11] Cundiff, S. T. (2002). Phase stabilization of ultrashort optical pulses. Journal of Physics D: Applied Physics,35(8). doi:10.1088/0022-3727/35/8/201 [12] Leitch, J., Kopp, G., and M.C. Noecker, Laser Metrology for Space Interferometry, SPIE Vol. 3350, Astronomical Interferometry, ed Robert Reasenberg, p. 526 (1998). doi: 10.1117/12.317126 [13] Coddington, I., Swann, W., & Newbury, N. (2008). Coherent Multiheterodyne Spectroscopy Using Stabilized Optical Frequency Combs. Physical Review Letters,100(1). doi:10.1103/physrevlett.100.013902 [14] Giorgetta, F. R., Swann, W. C., Sinclair, L. C., Baumann, E., Coddington, I., & Newbury, N. R. (2013). Optical two-way time and frequency transfer over free space. Nature Photonics,7(6), 434-438. doi:10.1038/nphoton.2013.69 [15] Deschênes, J., Sinclair, L. C., Giorgetta, F. R., Swann, W. C., Baumann, E., Bergeron, H.,... Newbury, N. R. (2016). Synchronization of Distant Optical Clocks at the Femtosecond Level. Physical Review X,6(2). doi:10.1103/physrevx.6.021016 16
Backup Slides 17
Ball Aerospace Frequency Comb (1) Note: Based on NIST design. See Sinclair et al. Right: Packaging for Ball Aerospace fiber comb containing all key optics, including PPLN and interferometer (dimensions: 23 cm x 18 cm x 3 cm) Mode locked fiber oscillator Soliton resulting from balancing of normal and anomalous dispersion (GVD) Anomalous GVD from fiber Normal GVD from Self Phase Modulation (Optical Kerr Effect) Highly nonlinear fiber (HNLF) used to achieve full octave width 18
Dispersion Compensation Dispersion Compensation stage simple, but key functionality Broad pulses created in amplification stage have increased pulse energy from gain medium, but need higher peak powers Anomalous dispersion fiber shortens pulse after broadening in gain medium. Significant effort went into tuning this length (~30cm) so as to minimize pulse width and maximize pulse power. Minimum pulse measured (autocorrelation): 44fs Peak power: >10GW Normal Dispersion from Self-Phase Modulation and standard fibers Anomalous Dispersion from certain fibers 19
Supercontinuum Generation OFS HNLF-PM fiber Birefringence induced in HNLF with an elliptical core rather than standard stress rod approach Most challenging splices were between panda and elliptical core fibers High peak powers need to be reached in HNLF (short pulses) to take advantage of the many nonlinearities achievable in fiber 4 wave mixing, 2 nd harmonic generation, sum frequency generation, difference frequency generation etc etc etc 1064 nm 2128 nm 1 octave width Width at 3dB down: ~10nm 1520 nm 1600 nm 1.0 μm 2.2 μm 20
PPLN waveguide 1064 nm 2128 nm 2128 nm 1064 nm Periodically Poled Lithium Niobate waveguide serves as efficient means to frequency double light at 2128 to 1064 Conversion efficiency of ~10% and bandwidth of ~3nm, centered at 2128 nm 21
In-line interferometer (F CEO control) 45 deg splice Narrowband 1064nm bandpass filter selects fundamental and doubled light from pulses and propagates down fiber. Temporal misalignment between 2128 nm and 1064 nm light from dispersion relations in HNLF and other fibers. Temporal overlap of pulses is necessary for heterodyne. 45 degree splice introduced to put some of each light in both fast and slow axes. Speed of light difference in each axis ensures that light will overlap after some propagation distance Fine tuning of interferometer length necessary for good SNR. Our SNR: 40dB 40 db 22