Transcript National Institute of Standards and Technology 1

National Institute of
Standards and Technology
Broadband Spectroscopy of CO2 Bands Near
2μm Using a Femtosecond Mode-Locked Laser
Andrew Klose1, Daniel L. Maser1, Gabriel Ycas1, Ian Coddington2,
Nathan Newbury2, and Scott A. Diddams1
National Institute of Standards and Technology
1Time and Frequency Division
2Quantum Electronics and Photonics Division
Boulder, Colorado
ISMS 2014 - Session TI.09
17 June 2014
Outline
• Why use optical frequency combs?
• 2 micron fiber laser source
• 2D Virtually-imaged phased array spectrometer
• Preliminary carbon dioxide measurements near 2 microns
• Future plans
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Optical Frequency Combs
• Optical frequency comb consists of many
(>105) lines equally-spaced in frequency
• All lines defined by two radio frequencies:
𝑓 𝑛 = 𝑓0 + 𝑛 ∙ 𝑓𝑟𝑒𝑝
• Spectroscopy with combs is advantageous
=> broadband source of many “cw” lasers
Femtosecond Optical Frequency Comb. Ye and Cundiff (Eds.). Springer, Norwell, MA (2005).
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Spectroscopy with Combs
• Femtosecond mode-locked fiber lasers are a welldeveloped technology => straightforward to
implement
• Er:fiber laser + nonlinear optical processes =>light
in IR/MIR => broadband/precision spectroscopy
• Spectroscopy measurements benefit from stable
source of laser light
250 MHz Er:fiber Laser
• Polarization maintaining optical fibers =>
increased robustness of optical source
• 2µm spectral region interesting for CO2
spectroscopy
• 2µm => supercontinuum generation from
Er:fiber
For previous work, see, for example:
S. Kumkar, et al. Opt. Lett. 37, 554 (2012)
A. Sell, et al. Opt. Exp. 17 1070 (2009)
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G. Ycas, et al. Opt. Lett. 37, 2199 (2012)
I. Hartl, et al. CLEO Technical Digest. CTh1J.2 (2012)
H. Hoogland, et al. Opt. Exp. 21, 31390 (2013)
Blue – H20
Red – CO2
L. S. Rothman, et al. J Quant. Spec. Rad.
Trans. 130, 4 (2013)
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Source Setup
• 25mW of from 250 Er:fiber oscillator (non-PM)
• EDFA Amplifier => 350 mW average output power, 70 fs pulses
• EDFA output launched into highly nonlinear fiber (HNLF)
(small mode field diameter=>nonlinear effects)
• 240 mW of average power spanning an optical octave after HNLF
Connectorized
Output
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Output from Source
• Output spectrum spans
1.1 μm - 2.2μm
Total power = 240mW
• The spectral peak near 2μm can
be tuned by altering the optical
power incident the HNLF
• Altered average EDFA
output power by varying
980 nm pumping current
• Relatively flat individual
spectra spanning 50-100 nm
3
• 90 fs pulses in 2μm region at
connectorized output (can
compress to 35 fs)
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Stability of Source
• Recorded 250 optical spectra at intervals of 15 minutes over 2.5 days
• Free Running setup - no stabilization of Er:fiber oscillator
• Analyzed variation of spectrum
• For each resolution element from the optical spectrum analyzer
• Integrated intensity variation of 2μm region
• Overall intensity drift of 2μm spectral region is a few percent over
a timescale of days
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VIPA Spectrometer
Grating
• Comb teeth are spatially
dispersed in 2D
• Virtually Imaged Phased
Array (VIPA)
=> vertical dispersion
Fiber-Coupled
Light
[1]
• Orth. Diffraction Grating
=>horizontal dispersion
[2]
• Simultaneous
measurement of ~50nm
bandwidth of spectrum
Cylindrical
Lens
Spherical
Lens
InSb
2D-camera
• Typical resolution
0.5 – 2 GHz
[1] M. Shirasaki. Opt. Lett. 21, 366 (1996)
[2] S. Diddams, et al. Nature. 293, 627 (2007)
[3] L. Nugent-Glandorf, et al. Opt. Lett. 37, 3285 (2012)
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Calibration of 2D Image
• Resolution of VIPA is ~1GHz
• Filter 250 MHz source lines
to 5 GHz
• Use 5 GHz filtered comb to
calibrate camera pixels to
laser frequency
250 MHz
Source
5 GHz
Fabry Perot
Cavity
Spectrometer
and Camera
• VIPA FSR ~50GHz => count
dots, fit dots, generate x,y
pixel to frequency map
• Use unfiltered source for
spectroscopy measurement
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Obtaining Spectrum
• Background image
• Sample image
• Dark image
• Subtract dark image
from background
and sample
• Spectrometer is fiber
coupled=> No
alignment change
between calibration,
background, or
sample
=
Frequency →
Light passed through
~12 cm cell at 700 mbar with
~200 mbar CO2
re-coupled into fiber and sent to
spectrometer
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Example Spectra
[1]
Two Grating
Positions
12 cm path
200 mbar CO2
• Resolution of <2 GHz achieved (~0.05cm-1)
• Simultaneous measurement of 40 nm bandwidth
(~2000 spectral elements)
• < 1ms integration time
• Fluctuations at <1% level => time dependent etalon
• Detailed analysis currently underway
[1] L. S. Rothman, et al. J Quant. Spec. Rad. Trans. 130, 4 (2013)
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Future Plans
• Continued Spectroscopy
with VIPA-based system
• Multiheterodyne dual comb
spectroscopy [1,2]
• Open air atmospheric
measurements on 2km path
on NIST-Boulder Campus [3]
• Implement source with PM
oscillator [3]
[1] F. Keilmann, et al. Opt. Lett. 29, 1542 (2004)
[2] A. Schliesser, et al. Opt. Exp. 13, 9029 (2005)
[3] G. Rieker, et al. arXiv:1406.3326v1 [physics.optics]
[3] L.C. Sinclair, et al. Opt. Exp. 22, 6996 (2014)
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Summary
• 2 micron source was constructed => based on supercontinuum generation from
Er:fiber oscillator
• <100fs pulses
• ~30mW power in relevant bandwidth region
• Flat, tunable spectrum
• Robust comb source
• Spectroscopy of carbon diode using 2D VIPA-based Spectrometer
• Resolution on the order of 1 GHz
• 40 nm simultaneous bandwidth measured
• <1ms integration time
• Continued work on
• VIPA spectroscopy
• Dual Comb Spectroscopy
• Open-air measurements
• Portable source
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Acknowledgements
Thank You!
NIST
CU-Boulder/JILA
Scott Diddams
Gabe Ycas
Dan Maser
Dan Hackett
Nate Newbury
Ian Coddington
Esther Baumann
Fabrizio Giorgetta
Laura Sinclair
Lindsey Sonderhouse
Bill Swann
Jun Ye
CU-Boulder
Greg Rieker
U of Campinas
Flavio Cruz
St. Johns
Todd Johnson
Funding:
NRC
NIST Climate Science
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Source Coherence and Noise
frep
2frep
• Relative intensity noise (RIN) from
supercontinuum increases with decreasing
spectral window
Heterodyne beats
2000nm LPF
1850nm LPF
• 2µm portion of spectrum was
frequency doubled in PPLN crystal
• Free-running heterodyne beat
note with 980nm external cavity
diode laser measured
• 25 dB S/N beat note was
recorded with 100 kHz
resolution bandwidth
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Red = 1650nm LPF
Green = Full Supercontinuum
Black = Er:fiber Oscillator
Green – Full SC
Red – 1650nm LPF
Blue – 1850nm LPF
Purple – 2000nm LPF
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Pulse Width
• Output of source was temporally compressed using
silicon prisms
• Pulses were analyzed via SHG-Frequency Resolved
Optical Gating (SHG-FROG)
Connectorized
Laser Output
• Achieved FWHM pulse width of 35 fs
• Transform limit of 20 fs
• SHG-FROG-retrieved optical spectrum in good
agreement with direct spectrum measurement
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SHG-FROG
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