Frank Havermeyer* , Christophe Moser Lawrence Ho

11/3/2011 Ondax confidential

Discrete tunable laser for 3D imaging

Frank Havermeyer*a, Christophe Moserb, Lawrence Hoa

a Ondax Inc, Monrovia, California, U.S.A b Laboratory of Applied Photonics Devices, School of

Engineering, Swiss Federal Institute of Technology Lausanne (EPFL)

8261-7

*Presenter: Frank Havermeyer, Advanced Technology Development, Ondax, Inc.

OWNER

テキストボックス

courtesy of Dr. Havermeyer


Precision 3 D Imaging

3D Imaging / digital holography : • Imaging object interferograms with multiple wavelengths and computer aided

reconstruction of accurate depth information.

Multiple phase and wavelength Interferograms

Computer-generated

precision 3-dimensional

data set

Tunable laser

Inte

rfere

nce

CCD

Reference

Mirror

Object

Beam

splitter


Motivation

3D Imaging brings many benefits to industrial metrology: • sub m depth resolution over centimeter range • Non contact / non destructive • High throughput • Large area with sub-millimeter 2D resolution

Laser source requirements: • Wavelength stability and repeatability • Narrow line width (long coherence length) • Rapidly tunable • Robust • Compact /portable • Low power consumption


Laser Source - Basic Principle

LD

p-pol.

L1

multiplexed

VHG

/4 PBS /4

L2

s-pol.

Digital Micromirror

Device (DMD)

‘Off’ ‘On’ ‘Off’ ‘Off’

No

lasing LD L1

PBS /4

multiplexed

VHG L2

/4

‘Off’ ‘On’ ‘Off’ ‘Off’

lasing

• External Cavity Laser (ECL) • AR coated broadband laser diode (750-790nm)

• Stable optical design

• “Self-aligned” feature with folded cavity • Compact and easy to assemble

• “Digital” tuning between λ • Digital micromirror, not thermal or mechanical

• Multiplexed Volume Holographic Grating (VHG) as wavelength selector

• Multiple, discrete wavelengths at any spacing (within diode bandwidth)

• 10 wavelengths demonstrated

• Low power consumption


Volume Holographic Gratings

784.90 784.95 785.00 785.05 785.100

10

20

30

40

50

Effic

iency

[%]

Wavelength [nm]

single path

double path

34 pm

46 pm

j2

qin

qout1qout2

L2

VHG

Allows design of

discrete and

arbitrary

dispersion

relation

• Periodic refractive index modulation inside a proprietary PTR glass substrate

• “Solid State”

• Unlimited lifetime

• Very narrow bandwidth • Highly selective for small diameter beams

• Non-dispersive • Discrete diffraction

• Multiplexing • Multiple discrete λ can be recorded at

arbitrary diffraction angles


5 Channel Prototype

• Compact: 80 mm roundtrip cavity in only 10x10x10 mm^3 volume

• Optical components bonded on TEC

• Operated at ~ 10 C above RT

-> Low power, fast and precise temperature control

CH

Discrete Wavelength [nm]

1 783.53

2 781.83

3 780.13

4 778.46

5 776.80

• 5 wavelengths selected to achieve ~ 7 nm (> 4 THz) tuning range

• VHG slant angles selected according to DMD pitch (7.5 m), mirror size and other optical constraints


Wavelength Repeatability

Better than +/-3 pm wavelength repeatability over 3600 cycles (5h)

10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0

776.78

776.80

776.82

time [s]

CH1

778.44

778.46

778.48

CH2

780.12

780.14

Wa

ve

leng

ht

[nm

]

CH3

781.82

781.84

781.86

CH4

783.52

783.54

CH5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

776.78

776.80

776.82 776.799 nm ± 1.5 pm

time [h]

CH1

778.44

778.46

778.48 778.460 nm ± 2.9 pm

CH2

780.12

780.14780.131 nm ± 2.3 pm

Wavele

nght

[nm

]

CH3

781.82

781.84

781.86781.833 nm ± 2.0 pm

CH4

783.52

783.54783.529 nm ± 2.3 pm

CH5


Spectral Performance

-80 -60 -40 -20 0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

Instrument: SA200

Resolution: 7.5 MHz

CH1 CH2 CH3

CH4 Ch5

FWHM: (6±1) MHz

Frequenzy Deviation [MHz]

Fab

ry P

ero

t S

ign

al [r

.u.]

• Consistent MHz line width for all 5 channels

• Instrument-limited measurement (~7.5MHz)

• Estimated linewidth of < 1MHz

(> 4 m)


Tuning Performance

0 10 20 30 40 50

0.00

0.02

0.04

0.06

0.08

Inte

nsity [

r.u

.]

time [ms]

780 nm, ~ 70% duty cycle

776 nm, ~ 30% duty cycle

• 50 s exponential rise and fall times (~120 µs for 90/10 % level)

• No intermediate wavelength

• No settling required

• Highly repeatable:

• Switching dynamic

• Power levels

• Cycling speed (120 Hz) limited by DMD


Wavelength Fine Tuning

Wavelength fine tuning:

• Easily integrated

• 4 pm continuous tuning range

• Enables active locking

0 2 4 6 8 10 12 14 16-200

0

200

400

600

800

1000

1200

1400

1600

1800

-0.4

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.7

Wa

ve

len

gth

Devia

tio

n [

pm

]

Fre

qu

en

cy D

evia

tio

n [

MH

z]

Voltage at Piezoelectric Actuator [V]

CH 2

CH 4

Linear fits

Slope: 179 MHz/V

LD

PBS

VHG on piezo-

electric actuator

DMD ‘Off’ ‘On’ ‘Off’ ‘Off’


Optical Power / Efficiency

• Free space power demonstrated up to 40 mW at max current

• ~< 0.5 W overall electrical power consumption

• consistent, balanced operation > 20 mW for all 5 channels at nominal operating current (110mA)

LD

PBS

VHG

DMD ‘Off’ ‘On’ ‘Off’

Single Mode

PM fiber

coupling

‘Off’

• 50 – 55% PM fiber coupling efficiency for all 5 channels

• Independent of wavelength or power


Summary

• New laser source introduced for Digital Holography • Discretely tunable, external cavity laser design

• Multiplexed VHG, folded “self-aligning” cavity

• Compact low power consumption

• 5 channel prototype demonstrated • Fast and repeatable tuning (±3 pm, 120 s rise time, 120 Hz

• Narrow line width (~1 MHz)

• Up to 40 mW optical power (~<0.5 W consumption)

• 55% coupling efficiency into PM fiber independent of wavelength or power

• Other applications: THz Wave generation, Raman spectroscopy


Acknowledgements

National Science Foundation:

Grant #0956430


Ondax Product Line


Coherent THz Photomixing

DFB laser (EMCORE, PB1316-783)

THz

Source

THz

Detector

Source

Photomixer

(current biased)

Detector

Photomixer

THz

Collecting

optics

DFB

50/50

beam-

splitter

Fre

e

sp

ace

dis

tan

ce

Optical

fibers

10 15 20 25 30 35 40

784.0

784.5

785.0

785.5

784.65 + 0.052*(x-25)

Wavele

ngth

[nm

]

Temperature [C]

Measured Wavelength @ 110 mA

Linear Fit

Terahertz

Spectrometer

Emcore, PB2700

DFB Discrete tunable

laser

Discrete tunable laser

Backup


Coherent THz Photomixing

• Clear THz signal for 2 channels up to 1.5 THz

• Signal strength in line with estimates based on optical power

•Power-limitations and noise floor compromised broader range measurements

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

104

105

106

107

108

DFB thermal

tuning range

CH2 CH1

System Noise Limit

TH

z S

ignal [a

.u.]

Mixing Frequency [THz]

Backup


Tuning Performance Detail

• 50 us exponential rise and fall times (185 µs for 99/1 % level)

• No intermediate wavelength

• No settling required

• Speed limited by DMD

-500 0 500 1000 1500 2000

0.00

0.02

0.04

0.06

0.08

50 s exponential

rise and fall fits

Inte

nsity [

r.u

.]

time t [s]

780 nm

776 nm

Backup

Documents

Frank Havermeyer* , Christophe Moser Lawrence Ho