Frequency Stabilized Lasers for LIDAR 6/29/2016 Mark Notcutt and SLS Team Stable Laser Systems Boulder CO Lasers stabilized to Fabry-Perot cavities: good Signal to Noise Compact Frequency stabilized lasers on a vibrating platform Cavity designs that we typically use Feedforward compensation of platform vibration 1
SLS sells frequency stable lasers ~ 1 Hz Variety of packaging, wavelengths, and applications (defense, timekeeping, spectroscopy) 2
Fiber coupled filter cavities 10 GHz and upwards free spectral range Acceleration coefficient at the 10-9 /g level < 1 db loss fiber to fiber Rugged Fiber noise cancelation electronics 3
Laboratory-style frequency stabilized laser Cavity in temperature controlled vacuum housing: cavity resting on springs Pound-Drever-Hall discriminator Frequency Stabilized light 0 V - Servo Laser PDH FP Cavity Vibration isolation platform underneath breadboard Laser and fiber components The laser is tightly servo locked to the cavity, and faithfully follows the cavity resonance So the frequency stability properties of the stabilized laser = frequency/length stability of the reference cavity 4
Evolution to more compact packaging Available In development 5
Portable FP cavities with low acceleration coefficient Till Rosenband and Jim Bergquist at NIST devised this squeeze insensitive cavity Stephen Webster extended this to a cube held tetrahedrally These designs are optimized using FEM 6
Portable FP cavities with low acceleration coefficient Mirrors cause mass asymmetry Mounting and thermal isolation take up volume Till Rosenband and Jim Bergquist at NIST devised this squeeze insensitive cavity Stephen Webster extended this to a cube held tetrahedrally Balancing the mounting forces is difficult These designs are optimized using FEM 7
SLS measurements of cavity vibration sensitivity
A Portable FP Cavity with df/f~ 10-15 Cavity is firmly held to be portable, 25 mm or 50 mm in length Low frequency drift > 1 Hz/s - improve w/ temperature feedforward Good acceleration coefficients in the df/f ~ 3 x 10-11 /g range improve with acceleration feedforward NIST has done this before, in style (Rosenband, Leibrandt, et al) + Acceleration - Outside the lab, accelerations can be large and low frequency, so passive filtering is not effective - Mass asymmetry of the off-axial mirror position on the cavity (50 um) is a present limit to the acceleration coefficient - Accelerations can easily be measured Temperature - Compact packaging lot of thermal insulation - Ambient temperature and gradients are easily measured Stable Lasers Systems - SBIR Data Rights 9
Good Laser Frequency Stabilized Laser Drift measured wrt comb/rb osc - Linear drift of ~10 khz/day - No visible correlation against lab temp - Two stages of temperature control - Operation at temperature at which expansion coefficient is zero Stable Lasers Systems - SBIR Data Rights 10
Frequency beat measurement of 2 nd laser against good laser 2 nd laser cavity is has low Tzc, though is controlled at ~ 30 C, so has significant expansion coefficient - 3 days of data here - Significant cooling cycle at 7 am daily - Daily variation of 600 Hz when drift subtracted - This laser is in a different lab to the first laser Stable Lasers Systems - SBIR Data Rights 11
Ambient Room Temperature Change Wiener Filter Feedforward Step 1 Calibration: Derive the impulse response function of the plant and derive Wiener Predictive Filter Step 2 Operation: Input the noise to the Wiener filter predictor to calculate a frequency shift, and apply this frequency shift to an AOM on the output of the frequency stabilized laser Room temp Change Cavity Stabilized Laser AOM Output Light Thermometer FPGA Use Wiener Predictor Coefficients DDS Stable Lasers Systems - SBIR Data Rights 12
Correlate Frequency and Room Temperature Change Stable Lasers Systems - SBIR Data Rights 13
Results of Room Temperature Change Wiener Filter Feedforward When good, residual is ~20 %, when not so good, residual ~ 50% The non-convergence starts at the big morning cooldown in our lab Stable Lasers Systems - SBIR Data Rights 14
Acceleration Feedforward With Acceleration feedforward, the Wiener filter predictions should be done to correct perturbations at ~ 300 Hz, so a microprocessor or FPGA is used for the calculation Reference Stabilized Laser Stabilized Laser under test Drive Acceleration Noise Beat Frequency Accelerometer x 6 ADCs Data stored for post processing Wiener filter calculation Stable Lasers Systems - SBIR Data Rights 15
Cavity on Gimbal setup for Acceleration filter coefficients measurement The gimbal will be used to drive translational and angular accelerations Gimbal driven by hand for large low frequency motions, or by a voice coil for acoustic frequency drive Six accelerometers, arranged on diagonals of faces of cube with center approx. coincident with cavity We thank Dave Leibrandt at NIST for his help with this Stable Lasers Systems - SBIR Data Rights 16
Experimental setup: Wires and Fibers Laser coupled into fiber Locking Electronics Frequency Counter Differential Amp for Accelerometers DACs FPGA DDS AOM Out of the picture on a table to the right Second Stabilized Laser Stable Lasers Systems - SBIR Data Rights 17
Vibrations applied to 25 mm cavity to simulate real-world environments
Results: Feed-forward correction of large low frequency platform motion LMS minimization routine used to determine filter coefficients from 6 accelerometers. Filter length of 5 used. Allan deviation dramatically improved when feed-forward correction applied.
Correlation between drive signal and noise of corrected signal Correlation between drive signal and noise of corrected signal
A great deal of thanks to With great thanks to: Nate Newbury and his group, and Scott Diddams, Frank Quinlan and Tara Fortier at NIST for their help Jun Ye, Wei Zhang, and Jan Hall at JILA for their help Jim Bergquist, Dave Leibrandt, for help with and discussions on the feedforward compensation. A cavity-stabilized laser with acceleration sensitivity below 10 12/g Phys. Rev. A 87, p023829 (2013) DARPA for the opportunity and funding Colleagues at SLS Charles Fabry Alfred Perot 21
Results: Feed-forward correction of 40 Hz vibration