Advances on Portable Frequency Advances on Portable Frequency References in LUMOSReferences in LUMOS

Kansas State UniversityKansas State UniversityKevin KnabeKevin Knabe

Advising Professors: Kristan Corwin & Brian WashburnAdvising Professors: Kristan Corwin & Brian WashburnColleagues: Rajesh Thapa, Andrew Jones, Aaron Pung, Asma Al-RawhiColleagues: Rajesh Thapa, Andrew Jones, Aaron Pung, Asma Al-Rawhi

OutlineOutline

GoalsGoals Overview of Frequency StandardsOverview of Frequency Standards Saturated Absorption SpectroscopySaturated Absorption Spectroscopy Fiber CellsFiber Cells Reflected Pump Setup (single Reflected Pump Setup (single

beam!)beam!) LockingLocking Update on the Cr:F LaserUpdate on the Cr:F Laser

GoalsGoals

Studying Studying νν11 + + νν33 vibrational band vibrational band of acetylene (optimize signal for of acetylene (optimize signal for locking)locking)

Create an acetylene cell using Create an acetylene cell using PPhotonic hotonic BBandandGGap (PBG) Fiber ap (PBG) Fiber and and SSingle ingle MMode ode FFiber (SMF)iber (SMF)

Lock a laser to absorption signalLock a laser to absorption signal

Acetylene as Frequency Acetylene as Frequency ReferenceReference

Moderate accuracy (± 100 MHz)Moderate accuracy (± 100 MHz) High accuracy ( ±2 kHz)High accuracy ( ±2 kHz)

>500 MHz

Complex, fragile

1- 40 MHz

W.C. Swann and S.L. Gilbert. (NIST), Opt. Soc. Am. B, 17, 1263 (2000).

100 mW

8.5 mW

Figure from: K. Nakagawa et al., JOSAB 13, 2708 (1996)

Our Goal: Combine portability with improved accuracy

Saturation spectroscopy – pump-probe techniqueSaturation spectroscopy – pump-probe technique Long interaction lengthsLong interaction lengths

Portable, robust -> Large linewidths

SMFPBG Fiber

Splice Splice

SMF

Saturated AbsorptionSaturated Absorption

Pump burns hole in velocity distribution, probe samples different velocity class, except when on resonance.

Frequency (MHz)

Fra

cti

on

al ab

sorp

tion Doppler-broadened line width

Sub-Doppler line width

Pump and probeat same frequency

l“Bennett Hole” or Saturated Absorption Feature

Theory of Saturated Theory of Saturated AbsorptionAbsorption

Pump

Probe

Pump and probe burn independent holes while scanning velocity classes

The only time they see each other is when they are resonant

Ipump >> Iprobe (Iprobe < (0.05) Ipump)

Gas Cell

Beer’s Law

Ab

so

rptio

n

Pump-Probe SetupPump-Probe Setup

ECDL EDFA2x

AOM 70%

30% PC

PBGF

PBS PBS 2

λ

PD

40%

60%

Iso .

Diagnostics

VC4

λ

2

λ

Michelson Interferometer for

Frequency Calibration

Pump

Probe

PBGF acts like a waveplate!!

Extended Cavity Diode Laser

-Tunable from 1510 nm to 1580 nm

-Can sweep using external voltage control

Sweep ~ 4 GHz at 1530 nm

Erbium Doped Fiber Amplifier - Can amplify up to 500

mW

Photonic Bandgap FibersPhotonic Bandgap Fibers

““10 10 m fiber”- 7 missing cellsm fiber”- 7 missing cells 7.5 7.5 μm mode field diameter mode field diameter

““20 20 m fiber”- 19 missing cellsm fiber”- 19 missing cells 13.5 13.5 μm mode field diameter mode field diameter

Images by Crystal Fibre A/S

Allows for long interaction lengths!

Typical DataTypical Data

Uneven background in 10 μm attributed to coupling into

surface modes due to small mode field diameter

-750 -500 -250 0 250 500 750

0.0

0.5

1.0

1.5

2.21 torr 0.93 torr 0.66 torr 0.47 torr 0.22 torr

L

(u

nit

less

)

Frequency Offset (MHz)

20 m diameter, 0.80 m long, 29 mW Pump Power

-750 -500 -250 0 250 500 750-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

L

(u

nit

less

)

10 m diameter, 0.90 m long, 30 mW Pump Power

0.84 torr 0.79 torr 0.60 torr 0.48 torr 0.40 torr 0.31 torr

Frequency Offset (MHz)

Good signal quality, small widths

Problem: Splices are not commercially available!!

Pressure ResultsPressure Results

R. Thapa, K. Knabe, M. Faheem, A. Naweed, O. L. Weaver, and K. L. Corwin, "Saturated absorption spectroscopy of acetylene gas inside large-core photonic bandgap fiber," Opt. Lett. 31, 2489 (2006).

20 μm fiber shows lower fundamental width at zero pressure – larger transit time, so less broadening!

Mode field radius of

fiber

Power ResultsPower Results

Power broadening widens transition, but discrimination keeps going up

Because of availability of additional laser sources, staying under 50 mW may be a requirement (EDFA’s are expensive)

0 20 40 60 80 1000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

D (

khz-1

)

Power (mW)

GoalsGoals

Studying Studying νν11 + + νν33 vibrational band vibrational band of acetylene (optimize signal for of acetylene (optimize signal for locking)locking)

Create an acetylene cell using Create an acetylene cell using PPhotonic hotonic BBandandGGap (PBG) Fiber ap (PBG) Fiber and and SSingle ingle MMode ode FFiber (SMF)iber (SMF)

Lock a laser to absorption signalLock a laser to absorption signal

“Arc fusion splicing of hollow-core photonic bandgap fibers for gas-filled fiber cells”R. Thapa, K. L. Corwin, and B. R. Washburn, Accepted to Optics Express 2006

LUMOS Spliced Fiber (2005) - 20 μm core

Splice Loss:PBG ->SMF ~ 2.0 dB (30%)SMF ->PBG ~ 0.5 dB (10%)

The Splice is RightThe Splice is Right

20 μm core PBG supports more than 1 mode!

“Half Cell”

Making Fiber CellsMaking Fiber Cells

SMFPBGSMF

Vacuum Chamber filled to a low

pressure with C2H2

CO2 Laser

Arc Fusion Splicer

We will have a robust portable fiber cell!Aaron

Pung

GoalsGoals

Studying Studying νν11 + + νν33 vibrational band vibrational band of acetylene (optimize signal for of acetylene (optimize signal for locking)locking)

Create an acetylene cell using Create an acetylene cell using PPhotonic hotonic BBandandGGap (PBG) Fiber and ap (PBG) Fiber and SSingle ingle MMode ode FFiber (SMF)iber (SMF)

New GoalNew Goal: Check quality of half : Check quality of half cellscells

Lock a laser to absorption signalLock a laser to absorption signal

Saturated Saturated Absorption Absorption

Spectroscopy With Spectroscopy With Only 1 Beam in PBG Only 1 Beam in PBG

Fiber*Fiber*

*Patent pending

ECDL EDFA EDFA

PC

PBGF

PBS PBS 2

λ

PD

Iso .

Diagnostics

VC 4

λ

2

λ

SMF

Splice

2x AOM

Pump-Probe Setup for Pump-Probe Setup for Spliced Half-CellsSpliced Half-Cells

High Loss

Low Loss

Pump

Probe

-800 -600 -400 -200 0 200 400 600 800

0.0

0.2

0.4

0.6

0.8

Ab

sorp

tio

n (

Arb

. u

nit

s)

Frequency Offset (MHz)

P11 line at 0.9 torr, 23 mW pump power

Pump-Probe Setup for Pump-Probe Setup for Spliced Half-Cells – No Spliced Half-Cells – No

AOMAOM

ECDL EDFA EDFA

PC

PBGF

PBS PBS 2

λ

PD

Iso .

Diagnostics

VC 4

λ

2

λ

SMF

Splice

-500 0 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Frequency Offset (MHz)

High Loss

Low Loss

Pump

Probe

ECDL EDFA

PBGF

BS 2λ

PD

40%

60%

Diagnostics

VC SMF

Splice

Reflected Pump SetupReflected Pump Setup

Pump

Probe

-500 0 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

*L

(Arb

. Uni

ts)

Frequency Offset (MHz)

R-Pump Results: R-Pump Results: Signal QualitySignal Quality

-1000 -500 0 500 1000

0.0

0.2

0.4

0.6

0.8

*L

(uni

tless

)

Frequency (MHz)

0.31 t 0.39 t 0.45 t 0.50 t 0.55 t 0.66 t

Keeping saturated absorption feature centered with Doppler broadened profile has locking benefits

Wings exhibit very small interference pattern which has a free spectral range associated with the length of the PBG fiber

Reflections occurring at front end of splice, causing interference

R-Pump Results: R-Pump Results: Comparison Of WidthsComparison Of Widths

R-Pump : P11 Line : Pump Power = 30 mW

Pump-Probe : P11 Line : Pump Power = 29 mW

0.00 0.25 0.50 0.75 1.00

20

25

30

35

L (M

Hz)

Pressure (torr)

R-Pump : P11 Line : Pressure = 0.5 torr

Pump-Probe : P11 Line : Pressure = 1 torr

0 25 50 75 100

25

30

35

40

45

L (M

Hz)

Power (mW)

GoalsGoals

Studying Studying νν11 + + νν33 vibrational band of vibrational band of acetylene (optimize signal for acetylene (optimize signal for locking)locking)

Create an acetylene cell using Create an acetylene cell using PPhotonic hotonic BBandandGGap (PBG) Fiber and ap (PBG) Fiber and SSingle ingle MMode ode FFiber (SMF)iber (SMF)

Check quality of half cellsCheck quality of half cells Lock a laser to absorption signal, Lock a laser to absorption signal,

then compare with locked Cr:F then compare with locked Cr:F frequency combfrequency comb

LockingLocking

– Lock laser to sat. abs feature.Lock laser to sat. abs feature.– Measure with comb referenced to GPS.Measure with comb referenced to GPS.– Lock comb to fiber-based reference, output Lock comb to fiber-based reference, output

stable microwaves.stable microwaves.

Frequency domain

0 fn = nfr + fo

I(f)

f

fo fr

EDFA

C2H2 molecules

AOM

PBG Cell

PumpProbe

1550nmECLD Locking

Electronics

-500 0 500

0.0

0.2

0.4

0.6

0.8

*L

(u

nitl

ess

)

Frequency (MHz)

0.31 t 0.39 t 0.45 t 0.50 t 0.55 t 0.66 t

Chromium Forsterite Chromium Forsterite UpdateUpdate

Cr:F SpectraCr:F Spectra

Supercontinuum

Using HNLF to generate supercontinuum from

~1000nm to ~2250 nm

Laser out of oscillator

70 nm BW @ 1280 nm center λ

PPLN

f-2f Interferometerf-2f Interferometer

Using Periodically Poled Lithium Niobate to get 2nd harmonics

Got 47 dB beat note signal

Have also locked repetition rate

Currently Rajesh is playing with pump power modulation and other servo controls to lock f0 1200 40 80f (MHz)

ConclusionConclusion

Characterization has been done on Characterization has been done on acetylene filled PBG fibersacetylene filled PBG fibers

Advances have been made on Advances have been made on making gas filled cellsmaking gas filled cells

Discovery of the Reflected-Pump Discovery of the Reflected-Pump techniquetechnique

Cr:F is close to being locked!Cr:F is close to being locked! Next: Lock Laser to saturated Next: Lock Laser to saturated

absorption signalabsorption signal

ThanksThanks

Funding Agencies: Funding Agencies: – AFOSRAFOSR– NSF CAREERNSF CAREER– Kansas NSF EPSCoR programKansas NSF EPSCoR program– Kansas Technology Enterprise Kansas Technology Enterprise

CorporationCorporation– Kansas State UniversityKansas State University

Mike Wells and the JRM StaffMike Wells and the JRM Staff