Femtosecond optical parametric oscillator frequency combs ......optical parametric oscillator frequency comb 1-GHz Ti:sapphire laser OPO SHG SFM f-to-2f reference Heterodyne locking

Femtosecond optical parametric oscillator

frequency combs for high-resolution

spectroscopy in the mid-infrared

Zhaowei Zhang, Karolis Balskus, Richard A. McCracken, Derryck T. Reid

Institute of Photonics and Quantum Sciences,

School of Engineering and Physical Sciences,

Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK

ultrafast.hw.ac.uk

Introduction to OPO frequency comb technology

Concepts and operating principles

Comb-offset control in a fs OPO

METROCOMB project

Recent results

• Frequency combs tunable across one octave of bandwidth

• OPO frequency combs at 1 GHz and above

• Precision metrology and stabilisation

Outline




METROCOMB project

Recent results



• Precision metrology

Outline

femtosecond laser

An optical parametric oscillator is effectively a "photon splitter"

Every converted pump photon ( ), yields signal ( ) and idler ( ) photons

Wavelength coverage from the visible to the mid-infrared is readily available by

adjusting:

• crystal phasematching

• OPO cavity length (tens of µm adjustment)

Operating Principles

femtosecond

OPO

Femtosecond laser

f

signal (s)idler (i) pump (p)

Synchronously pumped OPOs have near-IR and mid-IR frequency combs with

spacing equal to the pump.

Offsets of pump, signal and idler combs described by:

OPO comb offsets

are easily tuned by

small (nm) changes

to the cavity lengthPower


sum-

frequency-mixing (SFM)

The carrier envelope offset can be measured by using interference between the pump super-

continuum and the pump + signal, or pump + idler sum-frequency mixing light e.g. idler CEO:

fi = nfrep +di fp =mfrep +dpfSFM = (m+n) frep

+ dp +di

fS/C = lfrep +dp

f

pump super-continuum (s/c)

idler (i) signal (s) pump (p)

The beat signal between fS/Cand fSFM contains the

frequency di, and harmonics of the laser repetition

frequency, frep.

Detecting the OPO comb offset frequencyTeresa I. Ferreiro, Jinghua Sun and Derryck T. Reid, Opt. Lett. 35, 1668 (2010)




METROCOMB project

Recent results




Outline

The METROCOMB consortium

Development of high-repetition-rate OPO frequency combs for mid-IR

spectroscopy

Bring together 5 SME partners from across the supply chain

Research carried out by 3 RTD performers with extensive comb experience


Development of high-repetition-rate OPO frequency combs for mid-IR

spectroscopy

Bring together 5 SME partners from across the supply chain

Research carried out by 3 RTD performers with extensive comb experience

xc

xc

xcxc


Project led to increased collaborations between suppliers and researchers

Strong benefits for both parties

Patent applications

IP and knowledge transfer

Sales!

Engineering support

Rewarding research

Papers!




METROCOMB project

Recent results




Outline

Quasi-phasematching: designer

wavelength conversion devices

L

l1

l2

f

l1

l2

Quasi-phasematched crystals (top) are tuned for different processes by

changing the domain pattern, unlike conventional angle-tuned crystals (bottom)

We utilise QPM PPKTP, engineering it to produce multiple colours in the visible /

IR

Balskus et al, CLEO 2015, Paper STh1N.7

PPLN PPKTP

d33 coefficient 27 pm/V 14 pm/V

Refractive index @ 1.3 µm 2.14 1.82

Dispersion @ 1.3 µm 160 fs2mm-1 56 fs2mm-1


wavelength conversion devices

L1 L2 L3 L4

(µm)

A 25.4 6.8 – 21.1

B 25.9 7.1 – 21.0

C 26.4 7.5 – 20.5

D 26.9 8.2 – 20.0

E 27.15 9.0 – 19.0

F 27.25 10.0 – 18.0

G 27.1 11.8 – 16.4

H 26.85 – – 14.5

I 26.6 18.2 18.2 12.5

J 26.5 23.0 23.0 10.5

OPO section

Signal SHG

Unpoled

Pump+idler

SFG

Quasi-phasematched crystals (top) are tuned for different processes by

changing the domain pattern, unlike conventional angle-tuned crystals (bottom)

We utilise QPM PPKTP, engineering it to produce multiple colours in the visible /

IR


Phase-matching SFM section G in the PPKTP

crystal design

L1 L2 L3 L4

(µm)

A 25.4 6.8 – 21.1

B 25.9 7.1 – 21.0

C 26.4 7.5 – 20.5

D 26.9 8.2 – 20.0

E27.1

59.0 – 19.0

F27.2

510.0 – 18.0

G 27.1 11.8 – 16.4

H26.8

5– – 14.5

I 26.6 18.2 18.2 12.5

J 26.5 23.0 23.0 10.5


wavelength conversion devicesBalskus et al, CLEO 2015, Paper STh1N.7

Pump laser (333-MHz Gigajet)

repetition rate stabilisation

achieved via PZT1 placed in the

laser cavity (@ 2-GHz 6th

harmonic)

OPO (CEO) stabilisation: via PZT2

OPO frequency-comb stabilisationBalskus et al, CLEO 2015, Paper STh1N.7

Supercontinuum

p+i SFG (@2um idler)

p+i SFG (@4um idler)

Near-continuous coverage from 1.1–4.0 µm

frep

frep-fCEOfCEO

OPO frequency-comb stabilisation

tune


10 Hz

@ -3dB

Comb offset beat (fCEO)

Stabilised to a 10-MHz reference

(synthesizer)

Sidebands present due to 30-kHz noise

on green pump laser for Ti:Sapphire

Note narrow (instrument limited)

linewidth of the CEO beat frequency


1.2 rad integrated phase noise

Strong peak at 30 kHz originating from the pump has a high contribution to

the fCEO integrated phase noise (>0.6 rad)

Stability (phase-noise) measurementBalskus et al, CLEO 2015, Paper STh1N.7

Mode filtering to 10-GHz

in a Fabry-Perot cavity

10.3 GHz filtered comb mode spacing

Pulse repetition period = 97 ps

830-MHz resolution (data window 36 cm)

1.0-GHz linewidth after apodization

Modes resolved with high contrast

Zhang et al, "Mode-resolved 10-GHz frequency comb from a femtosecond OPO," Opt. Lett. 40, 2692 (2015)

10

Multi-section PPKTP design enhanced SFG process needed for

fCEO locking over broad range of wavelengths

Demonstrated continuously tunable frequency comb operation

across >2000 nm in the mid-IR region

The fCEO locking quality is preserved across all idler tuning range –

the locking is as good at 2000 nm as it is at 4000 nm

Signal tuning range can also be stabilised as a comb 1.1–1.6 µm

Modes can be subsequently filtered in a high-finesse filter cavity

Summary




METROCOMB project

Recent results




Outline

Doubly-resonant 10-GHz optical parametric oscillator frequency comb

10-GHz

Ti:sapphire

laserOPO

SHG

SFM

Rb-ECDL

reference

Dither

locking


1-GHz

Ti:sapphire

laserOPO

SHG

SFM

f-to-2f

reference

Dither locking +

heterodyne test

Singly-resonant 1-GHz optical parametric oscillator frequency comb

1-GHz

Ti:sapphire

laserOPO

SHG

SFM

f-to-2f

reference

Heterodyne

locking

OPO comb concepts

Towards higher mode spacings:

fundamentally-pumped 1-GHz OPO comb

Operation at 1-GHz requires some compromises

The peak power of the pump laser is lower...

...so more average power is needed to generate the super-continuum used for

locking

...leaving less power to pump the OPO

...restricting its tuning range

But it can be done!

Spectra of composite comb

formed by pumping OPO directly

at 1-GHz (Ti:sapphire pump):

Self-referenced 1-GHz OPO comb

lsignal 1160 nm 1230 nm 1280 nm 1350 nm

85 fs 87 fs 83 fs 71 fs

Mirror

on PZT silica plate as OC

1 GHz

1.3 W

30 fsp

, i+

SF

M

CEO

interferometer

McCracken et al, CLEO 2015, Paper JTh2A.75


Ti:sapphire stabilisation achieved in a standard f-2f nonlinear interferometer

The detected CEO frequency was used in a feedback loop that modulated the

diode current in the 532-nm pump laser.

For OPO CEO control, SHG signal pulses

were heterodyned against a portion of the

pump supercontinuum to detect a beat

frequency

Associated feedback loop modulated the

OPO cavity length



CEO stabilisation to 400 mrad (pump) and 1.5 rad (OPO)

Milli-radian stability of repetition rate

Sufficient stability for applications but limited tuning range


Atomically-referenced 1-GHz OPO comb

Direct comb stabilisation to an atomically-referenced cw laser demands

Needs much less average power, allowing OPO to tune further

780.2 nm lock

CEO stabilisation to 1.6 rad (pump) and 3.4

rad (OPO) i.e. noisier than self-referencing

McCracken et al, "Atomically referenced 1-GHz optical parametric oscillator frequency

comb," Opt. Express (in press)


10-GHz

Ti:sapphire

laserOPO

SHG

SFM

Rb-ECDL

reference

Dither

locking


1-GHz

Ti:sapphire

laserOPO

SHG

SFM

f-to-2f

reference

Dither locking +

heterodyne test


1-GHz

Ti:sapphire

laserOPO

SHG

SFM

f-to-2f

reference

Heterodyne

locking

OPO comb concepts

Degenerate 1-GHz OPO for astronomy

29

OPO behaves like an optical frequency divider

Maintains frequency comb structure of high quality

Ti:sapphire 1-GHz pump comb

Two outputs, with the signal typically resonant

one pump mode

w

w/2idler | signal

w/2idler | signal


30

OPO behaves like an optical frequency divider

Maintains frequency comb structure of high quality

Ti:sapphire 1-GHz pump comb

Two outputs, with the signal typically resonant

one pump mode

w


31

Degenerate OPO mirror coating is centered at

half the pump frequency

Signal and idler merge into one pulse

Intrinsically broadband and smooth spectrum

one pump mode

w

w/2idler | signal

1-GHz

Ti:sapphire

laser

OPO nonlinear l

conversion

f-to-2f

locking

Dither locking +

heterodyne test

Low oscillation threshold and high efficiency

High starting mode spacing (1 GHz) relaxes

performance needed from Fabry-Pérot filter cavity

Requires active stabilisation of OPO cavity

Max bandwidth is limited by

group delay dispersion

Edges of signal/idler pulse

must stay in sync to achieve

gain

Δτ=0

50 mm

Doubly-resonant degenerate

1-GHz OPO comb

Five-cycle optical pulse generated at 1.6 µm

Duration 27 fs and bandwidth 145 nm (full coverage from 1.45 – 1.75 µm)

Self-referenced locking of the pump laser leaves plenty power for OPO

Still requires filtering for comb modes to be resolved


10-GHz

Ti:sapphire

laserOPO

SHG

SFM

Rb-ECDL

reference

Dither

locking


1-GHz

Ti:sapphire

laserOPO

SHG

SFM

f-to-2f

reference

Dither locking +

heterodyne test


1-GHz

Ti:sapphire

laserOPO

SHG

SFM

f-to-2f

reference

Heterodyne

locking

OPO comb concepts

Summary

Degenerate OPO combs provide:

Low pump thresholds, compatible with direct 10-GHz pumping

Removes (or at least reduces) the dependence on filter cavities

Eliminates sidebands

Broadband outputs

Short pulses, compatible with implementing further coherent nonlinear

broadening

Locked OPO comb is produced when pumped by another locked comb

Limitations?

Locked linewidth remains to be fully investigated

Not readily tunable

Motivation – HIRES calibration requirements




Recent results



• Precision metrology and stabilisation

Outline

Institut fédéral de métrologie METAS

UniNE

Thomas Südmeyer

Stéphane Schilt

Valentin Wittwer

Pierre Brochard

Nayara Jornod

HWU

Karolis Balskus

Laser Quantum

Albrecht Bartels

Tobias Ploetzing

Benchmarked the stability and metrology performance of a 333-MHz OPO comb

against a 250-MHz Menlo Systems fibre comb at 1.5 µm

Characterized noise, stability and linewidth for frep, fCEO and nopt

Metrology demonstration (absolute optical frequency measurement)

Two campaigns, in March and April 2015

Frequency metrology with an OPO comb

HWU OPO frequency comb

UniNE ultra-stable

optical cavity laser

Parameter OPO measurement Menlo comb measurement

frep [Hz] 333’273’459.020 ± 0.051 250’000’413.00000 ± 0.00039

fceo [Hz] 10’000’000.000 ± 0.093 20’000’000.000 ± 2.271

fbeat [Hz] -104’685’653 ± 33’300 29’731’235 ± 28’500

N (calculated) 576’446 (576’445.999787) 768’455 (768’454.999844)

Measured frequency

[Hz]

192’114’057’672’589 ± 44’400 192’114’057’640’680 ± 28’500

Theoretical value [Hz] 192’114’057’601’610.0

Frequency offset [Hz] +70’979 +39’070

OPO and Menlo Er:fibre comb measurements in agreement within uncertainty margin

Difference between the measured frequencies arises from different settings in the

lock of the cw laser to the Rb transition and from etalon fringes occurring in the laser

setup

Short-term noise on the measured optical frequency at the kHz level, mainly limited

by the MHz linewidth of the cw laser

OPO measurement 2 conducted under conditions of better cw laser stability

Parameter OPO measurement 1 OPO measurement 2

frep [Hz] 333’261’086.66666 ± 0.00046 333’260’610.66667 ± 0.00046

fceo [Hz] 10’000’000.000 ± 0.084 10’000’000.001 ± 0.172

fbeat [Hz] 28’785’300.558 ± 37’478.230 -30’036’067.888 ± 7680

N (calculated) 576’446 (576’446.999906) 576’446 (576’467.999784)

Measured frequency

[Hz]

192’114’057’632’772 ±

37’500

192’114’057’673’724 ± 7’680

Theoretical value [Hz] 192’114’057’601’610.0

Frequency offset [Hz] +31’200 +72’100

Metrology results

Summary

Performance characterisation of 333-MHz OPO comb showed it to be very

comparable to near-IR Menlo Systems fibre comb in its stability

Equivalent accuracy observed in an absolute metrology measurement

Limitations of the measurement ultimately determined by

residual noise in synthesizers

noise on the 532-nm laser used to pump the Ti:sapphire laser

bandwidth of the OPO fCEO lock

OPO combs

provide wavelengths which are (at best) marginal from existing laser combs

can reach 1 GHz and potentially up to 10 GHz directly, without F-P filtering,

with potential advantages of lower noise and better sideband suppression

Ongoing research attempting to demonstrate multi-GHz comb operation over an

extended wavelength range

Doubly-resonant OPO architecture expected to enable 10-GHz operation

Direct stabilisation to a Rb optical transition will ensure traceability of comb line

frequencies, without f-to-2f stabilisation

Applications beyond spectrograph calibration such as dual-comb spectroscopy

Acknowledgements

Dr. Zhaowei Zhang, Dr. Karolis Balskus, Stuart Leitch.

Conclusions

Documents

Femtosecond optical parametric oscillator frequency combs ......optical parametric oscillator frequency comb 1-GHz Ti:sapphire laser OPO SHG SFM f-to-2f reference Heterodyne locking