Frequency-Comb Based Approaches to Precision Ranging Laser Radar

Coherent Laser Radar Conference XVI Long Beach, CA

June 23, 2011

Nate Newbury, Tze-An Liu, Ian Coddington, Fabrizio Giorgetta, Esther Baumann, Kevin Knabe, Bill Swann

NISTBoulder, CO

Outline

•

Introduction: Combs

•

Combs for ranging

•

Dual Comb ranging–

Initial phase-locked system: nm-level precision–

Simplified free-running system: sub-m precision in < 1 ms

•

Comb-assisted coherent FMCW ranging Ladar–

Tracking a fast swept laser–

Preliminary results

•

Conclusion

Frequency Comb Coherent, stabilized femtosecond, mode-locked lasers

2005 Nobel prize to J. Hall & T. Hänsch

frep

I(f)

Frequency Comb (Spectrum)

Pulse Train

Stabilization of any two degrees of 

freedom → Entire comb stabilized→ Entire pulse train stabilized

frequency

f

Exact periodControlled phase

Exact spacingControlled offset

FemtosecondPassively modelockedLaser

Er+

1/frep

ReferenceLasers

Properties of Combs For Ranging

time

Time‐domain

Frequency‐domain

frequency

Stable pulse timing → time-of-flightStable carrier frequency→ interferometry

Comb = thousands to millions of “cw lasers”with Hz-linewidths and known frequencies

→ Multi-wavelength interferometry→ Calibration of fixed/swept lasers

cw laser

known toothfrequencies

1 Hz

Different Comb-Based Ranging Systems

lase

r 2

lase

r 3

lase

r 1

Comb

2 3 1

Multi-WavelengthInterferometer

Swept laser

Comb

FM CW LIDAR

Processortime

Comb

Comb Interferometertranslate

vary frep

X‐correlator Spectrometer

Comb

Modulated waveform

RF phaseMinoshima et al, AO 39, 5512 (2000)Yokoyama et al, OE, 17, 17324 (2009).

Schuhler et al, OL 31, 3101 (2006)Salvade et al, AO, 47, 2715 (2008)Joo et al, OE 16, 19799 (2008).

Joo et al, OE, 14, 5954 (2006)Cui et al. OE 19, 6549 (2011)Balling et al, OE 17, 9300 (2009)Lee

et al, NP 4, 716 (2010).

Barber et al, OL 36, 1152 (2011).)This work

Comb 1

Dual-Comb Interferometer

Cooddington et al, NP. 3, 351 (2009).Godbout et al, OE 18, 15981 (2010).

Comb 2

“Direct”

Comb Ranging Comb-Assisted Ranging

Dual Comb System in Time Domain: Linear Optical Sampling

Phaselock combs Coherent carrier

Slightly different repetition rate frep

LOComb

SourceComb

Interferogram

(cross‐corr.)

time

Source

LO

T T

A cross‐correlation with timing is as precise as combs

x x

x

xx x x xx x x x x x xx x x x x x x x x x xx xx

x

Measurement 

repeats every 

1/frep

time of arrival

T = comb timing difference

Dual-Comb Ranging: Lab Demo

Reference

Movable Target

6040200-20Effective Time (ps)

InterferometricRange

Reference Target

Time-of-FlightRange = (vgroup

/2)×

time-of-flight

time-of-flight

= T R ) /(4+ N

TR

LO

Signal

Lab Experiment

Combs phase‐locked 

together‐

Coherent carrier‐

Precisely offset 

repetition rate

Absolute Range

High‐precision Range

ambiguity

1 nm

10 nm

100 nm

1 µm

10 µm

Ran

ge U

ncer

tain

ty

1 ms 10 ms 100 msAveraging time

Time-of-Flight

Interferometric

Range Uncertainty versus Observation Time nm-level absolute distance measurements

< 5 nmat 60 msec

“Range Window”

or Ambiguity is 1.5 meters (but can extend to > 30 km)

/2

Range

Time-of-Flight

Inteferometric

Handover

(Also verified Time‐of‐Flight and Interferometric Agree)

1 nm

10 nm

100 nm

1 µm

10 µm

Ran

ge U

ncer

tain

ty

1 ms 10 ms 100 msAveraging time

Time-of-Flight

Interferometric

Range1 kmDelay

after 1 km delay

across 1 km spool

Time-of-Flight

Inteferometric

Range Uncertainty versus Observation Time nm-level absolute distance measurements

“Range Window”

or Ambiguity is 1.5 meters (but can extend to > 30 km)

CLRC_201110

Fully Phase-Locked Dual Comb System

Four phase locks to two cavity‐stabilized laser

Can we trade off precision for simplicity?‐

Nanometer precision excessive for typical terrestrial 

ranging of ~ 10‐7

to 10‐8 

precision

Try a free‐running system with no phase locks…

Interferometric ranging lost

Time‐of‐flight ranging still possible?

phase 

locks

cavity

source comb

LO comb clock D

igiti

zer

AOMAmp

TargetReference

Range

-100 -50 0 50 100Optical Frequency Offset (Hz)

1 Hz

Optical Linewidth

CLRC_201111

Free-running Dual Comb System Time-of-Flight Ranging Without Phase Locks

source “comb”(fs Er laser)

LO “comb”

clock Dig

itize

r

TargetReference

Range

counter

157015601550nm

Simplified 200 MHZ Er fs laser

Free‐running rep rate stable to 10‐8

@ 1 secAdjust for frep = 7 kHz between combsFree‐running carrier stable to ~ MHz

sample #

XY

Simplified Ratio Processing

Volts

ReferenceTarget

CLRC_201112

Time-of-Flight Ranging with Simpler System Sub-micron in Sub-milliseconds with 0.75 m range window

Superior performance (due to higher repetition rate sources) 

Also verified 200 nm linearity across 1 meter

10 nm2

4

100 nm2

4

1 µm2

4

10 µmR

angi

ng U

ncer

tain

ty

1 ms 10 ms 100 msAveraging time

“old”

phase‐locked system

new free‐running system

Rang

ing Precision      

CLRC_201113

Dual-Comb Ranging

•

Advantages–

Rapid, absolute, high precision ranging (sub-micron in sub-

millisecond)

–

Immunity to spurious reflections–

No significant dead zones

•

Disadvantage–

Inefficient use of photons: Multiplexed disadvantage of √N10’s of nW to W return powers (100’s of photons per pulse)

•

Suitable for ranging to “cooperative”

target with retroreflector–

Not suitable for diffuse target–

(True of all direct comb ranging systems….)

Can we combine accuracy of combs with efficiency of FM CW LIDAR?

Combing Swept Cw Lasers & Combs

Tunable diode lasers offer • ~100 nm tuning ranges

• Rapid scan rates >1000 THz/s in MEMS or Y‐branch design

• >10 mW ‐

many Watts and single mode operation.

• Much higher SNR than direct‐comb illumination

But •Optical frequency is poorly defined during sweeps 

(orders of magnitude worse than linewidth)•Also fastest sweeps will not be linear (but quasi‐sinusoidal) 

Goal: Use frequency combs to track the phase of a swept laserWith high accuracyAt high speedsOver arbitrary waveforms

Metrology of a Swept CW laser using Frequency Combs

T.-A. Liu, et al. , Opt. Exp., 16, 10728, (2008).P. Del’Haye et.al., Nat. Photon., 3, 529, (2009).F. R. Giorgetta, et.al., Nat. Photon., 4, 853, (2010)Z. W. Barber, et. al., Opt. Lett., 36, 7, 1152, (2011).I. Coddington, et. al., JSTQE (Invited), IEEE Early Access, (2011)

Absolute optical frequency vs time Requires dual phase-locked combs >1000x faster than wavemeter Unique tool for metrology of fast

swept lasers

Single Comb Spectrometer

SweptLaser

Dual Comb Spectrometer

Relative optical frequency vs time Single, free-running comb ~20 nanosecond time resolution Much higher linearity, speed,

flexibility than etalon approaches

Basic Approach: Heterodyne laser against comb & digitize (fast)

Digitizer

Sweptlaser

Comb 1

Comb

2

CW laser

f

f

190.1234

190.1233 

Absolute frequency(THz)

n -n-1

n -n-1

Time

Heterod

yne Freq

uency

Maximum sweep rate (chirp)

fr fr = 0.1-1 THz/s (0.8-8 nm/s)

Swept CW laser + Dual Comb SpectrometerAbsolute Frequency Determination

fr

fr

Digitizer

Sweptlaser

Comb 1

CW laser

n -n-1

Time

Heterod

yne Freq

uency

Swept CW laser + Single Comb SpectrometerFast Relative Frequency Determination

Maximum sweep rate (chirp)

fr fr /2 = 5,000 THz/s (40,000 nm/s)

fLaser vs. time

Unwrap

Freq

uency un

certainty 1 MHz

100 kHz

10 kHz

1 kHz

(s)

1 10 100 1000 10000

10 ns 1 s 0.1 ms

# of sampling pulses

Example Measurement of a Fast Swept Laser MEMS-based ECL

Chirp rates > 1015/sec(10,000 nm/sec)

Accuracy ~  30 kHz/s (1.5 MHz @ 20 ns)

I. Coddington, et. al., JSTQE (Invited), IEEE Early Access, (2011)

10 ms

CLRC_201119

FM CW Ranging Lab DemonstrationPreliminary Results 

Comb Spectrometer

rough Al(diffuse target)

Digitize

r

AOM

1.0

0.8

0.6

0.4

0.2

0.0Opt

ical

Fre

quen

cy (G

Hz)

151050Time (ms)

~ 1 THz amplitude20 Hz Modulation

controlvoltage(sine)

Post‐processV(t) x e‐i[(t) - (t-

10 mW

50 nW

Sig

nal

4.9404.9354.9304.9254.920Time Delay (ns)

1 ps (1 THz)‐1

(150 m)

Processed Signal vs DelayEach up/down ramp separately processed

3 msec acq.

>10x faster should be possible

-10

-5

0

5

10D

ista

nce

- 0.7

3907

1m (µ

m)

151050 Time (ms)

Range Precision +‐

5 microns (35 fsec) in 3 msec

•

Combs used in many ways for ranging•

Direct Dual Comb Ranging system–

Phase-locked system: nanometer absolute ranging at a distance–

Free-running system: sub-micron ranging in < 1 msec

at distance–

Multiplexed penalty compared to cw

coherent laser radar•

Comb-based metrology of swept lasers can track “arbitrary”

cw

laser waveforms–

Follow waveforms with chirps of few thousand THz/sec–

Absolute or relative frequency measurments•

Preliminary comb-assisted FMCW–

Processing intensive–

Range resolution ~ 1/Bandwidth~ 1 psec

~ 150 microns in 3 msec–

Range precision of 5 microns in 3 msec–

Potential 10x improvement in speed possible….

Conclusion