1. PRECISION SATURATED ABSORPTION SPECTROSCOPY OF H3+ Yu-Chan Guan1, Yi-Chieh Liao1, Yung-Hsiang Chang1, Jin-Long Peng2, Li-Bang Wang3, Jow-Tsong Shy1,3 1. Institute of Photonics Technologies, National Tsing Hua University, Taiwan 2. Center for Measurement Standards, Industrial Technology Research Institute, Taiwan 3. Department of Physics, National Tsing Hua University, Taiwan

2. History of Experimental Observation of H3+
Above, new stars burst into being in the star-forming nebula Messier 78, imaged by NASA's Spitzer Space Telescope. (Image credit: NASA/JPL-Caltech)
Lurking in the vast, chilly regions between stars, the unassuming molecule known as a triatomic hydrogen ion, or H3+, may hold secrets of the formation of the first stars after the Big Bang.
At the University of Arizona, then doctoral candidate Michele Pavanello spent months doing painstaking calculations to find a way to spot H3+ and unveil its pivotal role in astronomy and spectroscopy, supervised by Ludwik Adamowicz, a professor in the UA's department of chemistry and biochemistry.
The groundbreaking results have been published in a recent edition of Physical Review Letters.
"Most of the universe consists of hydrogen in various forms," said Adamowicz, "but the H3+ ion is the most prevalent molecular ion in interstellar space. It's also one of the most important molecules in existence."
Believed to be critical to the formation of stars in the early days of the universe, H3+ also is the precursor to many types of chemical reactions, said Adamowicz, including those leading to compounds such as water or carbon, which are essential for life.
Early stars would have become hotter and hotter until they exploded before they ever formed, according to Pavanello, unless there was a way to release some of that pent-up energy.
"There wouldn't be any star formation if there weren't molecules that slowly cool down the forming star by emitting light," said Pavanello. Not many molecules can do that, he added, partly because very few molecules existed in the early days of the universe.
"Astronomers think that the only molecule that could cool down a forming star in that particular time is H3+."
A perfect asymmetry
Another molecule, molecular hydrogen, would have been present, but it would have had a much harder time cooling a forming star than H3+. "Hydrogen does not like to emit light, while H3+ can bend and vibrate, and in doing so it is able to emit light." said Pavanello.
H3+ is an electrically charged molecule, called an ion. It consists of three hydrogen atoms with only two, as opposed to a healthy three, electrons to share between them. Lacking a negatively charged electron, the molecule takes on a plus-one positive charge.
H3+ has a triangular shape, explained Adamowicz. "As it is excited it starts to vibrate in various ways."
"One has to involve a large amount of computations at the quantum mechanical level to predict those vibrations," said Adamowicz. "The role of theory is essentially to simulate those vibrations in the computer and then describe how the molecule is swinging or dancing."
Understanding the various vibrations of H3+could help astronomers deduce to what extent it played a role in the formation of the early stars.
"In the 1990s, H3+ was observed surrounding stars," said Adamowicz. "The stars emit radiation, which not only contributes to the production of H3+ but also excites the molecule to higher energy states. The molecule can also become excited through leftover energy from chemical reactions it was involved in or through collisions with other molecules. In the process of de-excitation the molecule emits photons that are detected by our radio telescopes."
"That can only happen with H3+ because molecular hydrogen is too symmetric," said Pavanello. "And so H3+ has a very important cooling function in the formation of the first stars after the Big Bang."
"The only way we can predict how the stars form is if we know very well what the cooling abilities of H3+ are, and we cannot know its cooling ability until we know its vibrational spectrum. We need to know what these energy levels are," said Pavanello.
"With this paper we have pinpointed the energy levels up to a certain energy threshold that is already good enough to generate accurate predictions of the cooling ability of H3+," said Pavanello.
It happened almost by chance
The group didn't set out to unlock the secrets of H3+, said Pavanello,
The transitions in the ν2 fundamental band were first detected by Professor Oka in 1980.
A summary of the high-resolution spectroscopic works of H3+ up to 2001 was given by Lindsay and McCall.
Our group first observed the Lamb dip of H3+ in 2012 using frequency modulation spectroscopy.
Afterward, McCall’s and Schlemmer’s group re-measured some fundamental H3+ lines with high accuracy (sub-MHz).
在銀河間的分子密度很高的星雲裡，或是木星電離層上都可透過H3+來觀測。
但天文上的觀測結果受限實驗室的量測數據，為了得到更好的觀測結果所以我們需要更精準的躍遷頻率

3. Our Previous Measurements of H3+
Above, new stars burst into being in the star-forming nebula Messier 78, imaged by NASA's Spitzer Space Telescope. (Image credit: NASA/JPL-Caltech)
Lurking in the vast, chilly regions between stars, the unassuming molecule known as a triatomic hydrogen ion, or H3+, may hold secrets of the formation of the first stars after the Big Bang.
At the University of Arizona, then doctoral candidate Michele Pavanello spent months doing painstaking calculations to find a way to spot H3+ and unveil its pivotal role in astronomy and spectroscopy, supervised by Ludwik Adamowicz, a professor in the UA's department of chemistry and biochemistry.
The groundbreaking results have been published in a recent edition of Physical Review Letters.
"Most of the universe consists of hydrogen in various forms," said Adamowicz, "but the H3+ ion is the most prevalent molecular ion in interstellar space. It's also one of the most important molecules in existence."
Believed to be critical to the formation of stars in the early days of the universe, H3+ also is the precursor to many types of chemical reactions, said Adamowicz, including those leading to compounds such as water or carbon, which are essential for life.
Early stars would have become hotter and hotter until they exploded before they ever formed, according to Pavanello, unless there was a way to release some of that pent-up energy.
"There wouldn't be any star formation if there weren't molecules that slowly cool down the forming star by emitting light," said Pavanello. Not many molecules can do that, he added, partly because very few molecules existed in the early days of the universe.
"Astronomers think that the only molecule that could cool down a forming star in that particular time is H3+."
A perfect asymmetry
Another molecule, molecular hydrogen, would have been present, but it would have had a much harder time cooling a forming star than H3+. "Hydrogen does not like to emit light, while H3+ can bend and vibrate, and in doing so it is able to emit light." said Pavanello.
H3+ is an electrically charged molecule, called an ion. It consists of three hydrogen atoms with only two, as opposed to a healthy three, electrons to share between them. Lacking a negatively charged electron, the molecule takes on a plus-one positive charge.
H3+ has a triangular shape, explained Adamowicz. "As it is excited it starts to vibrate in various ways."
"One has to involve a large amount of computations at the quantum mechanical level to predict those vibrations," said Adamowicz. "The role of theory is essentially to simulate those vibrations in the computer and then describe how the molecule is swinging or dancing."
Understanding the various vibrations of H3+could help astronomers deduce to what extent it played a role in the formation of the early stars.
"In the 1990s, H3+ was observed surrounding stars," said Adamowicz. "The stars emit radiation, which not only contributes to the production of H3+ but also excites the molecule to higher energy states. The molecule can also become excited through leftover energy from chemical reactions it was involved in or through collisions with other molecules. In the process of de-excitation the molecule emits photons that are detected by our radio telescopes."
"That can only happen with H3+ because molecular hydrogen is too symmetric," said Pavanello. "And so H3+ has a very important cooling function in the formation of the first stars after the Big Bang."
"The only way we can predict how the stars form is if we know very well what the cooling abilities of H3+ are, and we cannot know its cooling ability until we know its vibrational spectrum. We need to know what these energy levels are," said Pavanello.
"With this paper we have pinpointed the energy levels up to a certain energy threshold that is already good enough to generate accurate predictions of the cooling ability of H3+," said Pavanello.
It happened almost by chance
The group didn't set out to unlock the secrets of H3+, said Pavanello,
在銀河間的分子密度很高的星雲裡，或是木星電離層上都可透過H3+來觀測。
但天文上的觀測結果受限實驗室的量測數據，為了得到更好的觀測結果所以我們需要更精準的躍遷頻率

4. Our Previous Measurements of H3+
Above, new stars burst into being in the star-forming nebula Messier 78, imaged by NASA's Spitzer Space Telescope. (Image credit: NASA/JPL-Caltech)
Lurking in the vast, chilly regions between stars, the unassuming molecule known as a triatomic hydrogen ion, or H3+, may hold secrets of the formation of the first stars after the Big Bang.
At the University of Arizona, then doctoral candidate Michele Pavanello spent months doing painstaking calculations to find a way to spot H3+ and unveil its pivotal role in astronomy and spectroscopy, supervised by Ludwik Adamowicz, a professor in the UA's department of chemistry and biochemistry.
The groundbreaking results have been published in a recent edition of Physical Review Letters.
"Most of the universe consists of hydrogen in various forms," said Adamowicz, "but the H3+ ion is the most prevalent molecular ion in interstellar space. It's also one of the most important molecules in existence."
Believed to be critical to the formation of stars in the early days of the universe, H3+ also is the precursor to many types of chemical reactions, said Adamowicz, including those leading to compounds such as water or carbon, which are essential for life.
Early stars would have become hotter and hotter until they exploded before they ever formed, according to Pavanello, unless there was a way to release some of that pent-up energy.
"There wouldn't be any star formation if there weren't molecules that slowly cool down the forming star by emitting light," said Pavanello. Not many molecules can do that, he added, partly because very few molecules existed in the early days of the universe.
"Astronomers think that the only molecule that could cool down a forming star in that particular time is H3+."
A perfect asymmetry
Another molecule, molecular hydrogen, would have been present, but it would have had a much harder time cooling a forming star than H3+. "Hydrogen does not like to emit light, while H3+ can bend and vibrate, and in doing so it is able to emit light." said Pavanello.
H3+ is an electrically charged molecule, called an ion. It consists of three hydrogen atoms with only two, as opposed to a healthy three, electrons to share between them. Lacking a negatively charged electron, the molecule takes on a plus-one positive charge.
H3+ has a triangular shape, explained Adamowicz. "As it is excited it starts to vibrate in various ways."
"One has to involve a large amount of computations at the quantum mechanical level to predict those vibrations," said Adamowicz. "The role of theory is essentially to simulate those vibrations in the computer and then describe how the molecule is swinging or dancing."
Understanding the various vibrations of H3+could help astronomers deduce to what extent it played a role in the formation of the early stars.
"In the 1990s, H3+ was observed surrounding stars," said Adamowicz. "The stars emit radiation, which not only contributes to the production of H3+ but also excites the molecule to higher energy states. The molecule can also become excited through leftover energy from chemical reactions it was involved in or through collisions with other molecules. In the process of de-excitation the molecule emits photons that are detected by our radio telescopes."
"That can only happen with H3+ because molecular hydrogen is too symmetric," said Pavanello. "And so H3+ has a very important cooling function in the formation of the first stars after the Big Bang."
"The only way we can predict how the stars form is if we know very well what the cooling abilities of H3+ are, and we cannot know its cooling ability until we know its vibrational spectrum. We need to know what these energy levels are," said Pavanello.
"With this paper we have pinpointed the energy levels up to a certain energy threshold that is already good enough to generate accurate predictions of the cooling ability of H3+," said Pavanello.
It happened almost by chance
The group didn't set out to unlock the secrets of H3+, said Pavanello,
Our Previous Measurements of H3+
Transition
Our Work
Other's Work
Diff.
R(1,1)l
(250)
(165)a
9.187
(15)b
10.637
R(1,0)
(250)
(86)a
-5.74
(11)b
-5.07
R(1,1)u
(250)
(84)a
7.888
(20)b
8.508
R(2,2)l
(250)
(70)a
-8.869
(65)b
R(3,3)l
(250)
(88)a
8.076
在銀河間的分子密度很高的星雲裡，或是木星電離層上都可透過H3+來觀測。
但天文上的觀測結果受限實驗室的量測數據，為了得到更好的觀測結果所以我們需要更精準的躍遷頻率
a. J. N. Hodges et al. J. Chem. Phys. 139, (2013).
b. P. Jusko et al. J. Mol. Spectrosc. 319, 55 (2016).
The discrepancy comes from the frequency modulation on the DFB pump laser. The frequency modulation of 30 MHz induces the error in frequency counting.

5. Outline Saturated absorption spectrometer for molecular ion in mid-IR
Above, new stars burst into being in the star-forming nebula Messier 78, imaged by NASA's Spitzer Space Telescope. (Image credit: NASA/JPL-Caltech)
Lurking in the vast, chilly regions between stars, the unassuming molecule known as a triatomic hydrogen ion, or H3+, may hold secrets of the formation of the first stars after the Big Bang.
At the University of Arizona, then doctoral candidate Michele Pavanello spent months doing painstaking calculations to find a way to spot H3+ and unveil its pivotal role in astronomy and spectroscopy, supervised by Ludwik Adamowicz, a professor in the UA's department of chemistry and biochemistry.
The groundbreaking results have been published in a recent edition of Physical Review Letters.
"Most of the universe consists of hydrogen in various forms," said Adamowicz, "but the H3+ ion is the most prevalent molecular ion in interstellar space. It's also one of the most important molecules in existence."
Believed to be critical to the formation of stars in the early days of the universe, H3+ also is the precursor to many types of chemical reactions, said Adamowicz, including those leading to compounds such as water or carbon, which are essential for life.
Early stars would have become hotter and hotter until they exploded before they ever formed, according to Pavanello, unless there was a way to release some of that pent-up energy.
"There wouldn't be any star formation if there weren't molecules that slowly cool down the forming star by emitting light," said Pavanello. Not many molecules can do that, he added, partly because very few molecules existed in the early days of the universe.
"Astronomers think that the only molecule that could cool down a forming star in that particular time is H3+."
A perfect asymmetry
Another molecule, molecular hydrogen, would have been present, but it would have had a much harder time cooling a forming star than H3+. "Hydrogen does not like to emit light, while H3+ can bend and vibrate, and in doing so it is able to emit light." said Pavanello.
H3+ is an electrically charged molecule, called an ion. It consists of three hydrogen atoms with only two, as opposed to a healthy three, electrons to share between them. Lacking a negatively charged electron, the molecule takes on a plus-one positive charge.
H3+ has a triangular shape, explained Adamowicz. "As it is excited it starts to vibrate in various ways."
"One has to involve a large amount of computations at the quantum mechanical level to predict those vibrations," said Adamowicz. "The role of theory is essentially to simulate those vibrations in the computer and then describe how the molecule is swinging or dancing."
Understanding the various vibrations of H3+could help astronomers deduce to what extent it played a role in the formation of the early stars.
"In the 1990s, H3+ was observed surrounding stars," said Adamowicz. "The stars emit radiation, which not only contributes to the production of H3+ but also excites the molecule to higher energy states. The molecule can also become excited through leftover energy from chemical reactions it was involved in or through collisions with other molecules. In the process of de-excitation the molecule emits photons that are detected by our radio telescopes."
"That can only happen with H3+ because molecular hydrogen is too symmetric," said Pavanello. "And so H3+ has a very important cooling function in the formation of the first stars after the Big Bang."
"The only way we can predict how the stars form is if we know very well what the cooling abilities of H3+ are, and we cannot know its cooling ability until we know its vibrational spectrum. We need to know what these energy levels are," said Pavanello.
"With this paper we have pinpointed the energy levels up to a certain energy threshold that is already good enough to generate accurate predictions of the cooling ability of H3+," said Pavanello.
It happened almost by chance
The group didn't set out to unlock the secrets of H3+, said Pavanello,
Saturated absorption spectrometer for molecular ion in mid-IR
Light source →
Production of H3+ →
Frequency calibration →
Results
Frequency calibration test by methane
Summary and Future Works
OPO
Extended negative glow discharge
Optical frequency comb & Iodine stabilized system
Frequency measurement results of H3+

6. Optical Parametric Oscillator (OPO)
Nd:YAG
Laser
Ytterbium-doped Fiber Amplifier
ISO
λ 𝟐
PBS
Beam Dump
Signal
使用NDYAG雷射經過ytterbium光纖放大器後做為OPO的pump wave。
經過fan out的非線性晶體產生signal 光與idler光。敘述波長
Cavity 共振 signal 光
(跟阿毛主要的差異為，阿毛的非線性晶體為通道式，需換溫度調整出光波長，造成不穩。Fanout只需換晶體周期極可調整出光波長，不用改變溫度。)
優點 很寬的出光波長範圍 、 輸出功率高達 um
1064 nm Pump
Etalon
PZT
3.2 – 4 μm idler
Fan-out type PPLN
Rough tuning idler frequency → PPLN position, etalon angle
Fine tuning idler frequency → PZT, pump frequency

7. Idler Wave of Our OPO The idler wavelength range is 3.2 to 4 μm.
使用NDYAG雷射經過ytterbium光纖放大器後做為OPO的pump wave。
經過fan out的非線性晶體產生signal 光與idler光。敘述波長
Cavity 共振 signal 光
(跟阿毛主要的差異為，阿毛的非線性晶體為通道式，需換溫度調整出光波長，造成不穩。Fanout只需換晶體周期極可調整出光波長，不用改變溫度。)
優點 很寬的出光波長範圍 、 輸出功率高達 um
The idler wavelength range is 3.2 to 4 μm.
The idler power reaches 1.7 W at 3.6 μm.

8. Extended Negative Glow Discharge
Applying few hundred gauss magnetic field would extend the negative glow discharge.
要如何把富光輝區域延伸
軸向磁場的目的為減少電子與管壁的碰撞，電子能量不會損失，離子反應繼續進行。
低氣壓放電(mTorr)可減少碰撞，獲得較窄線寬
延伸後吸收路徑變長可以獲得不錯的訊號大小
Extended
The axial magnetic field will reduce the collisions of electrons with the wall.
Long optical absorption path → Enhance absorption signal
Low pressure → Narrow linewidth

9. Extended Negative Glow Discharge Tube
講氫氣進入放電管，軸向磁場的線圈延伸負光輝，抽出氫氣
講截面
外真空隔絕線圈產生的熱
Ethanol Cooling
99.95 % hydrogen gas
Ethanol cooling -70 ℃
Outer vacuum 10 mTorr
Solenoid coil & Water cooling

10. Experimental Setup OPO AOM _ + Idler Signal Computer Double modulation
Filter
-1st
Pump beam
Probe beam
InSb
Detector
Preamp.
Solenoid Power Supply
AOM
Function
Generator 1
Lock-in 2
Computer
Generator 2
70 kHz
Square Wave
5 Hz
Lock-in 1
Discharge Power Supply
Wavemeter
Idler
OPO
Fiber Comb
Frequency Offset Locking Loop
PZT
Signal
Nd : YAG Laser
Iodine-stabilized Nd : YAG Laser
Tunable Offset Locking Loop
RF Synthesizer
Yb : Fiber
Amplifier PD L2 L1
BS2
BS3
BS1
Sample Inlet
Pump
Beam Dump + _
將濃度調至加進來，Probe帶有兩個調製的訊號，濃度調制與pump強度調至。
Double modulation
Pump intensity modulation by AOM
Ion concentration modulation by magnetic field

11. Idler Frequency Determination
Pump frequency
fpump ＝ fIodine ± fsyn ± fbeat1
Signal frequency
fsignal ＝ n × frep ± foffset ± fbeat2
Idler frequency
fidler ＝ fpump － fsignal
分析飽和吸收的結果，發現不是單純勞羅茲曲線
放電館充滿中性H2，因為H3+與H2質量相近，在質量相近的情況下因為碰撞而重覆貢獻的速度偏移需要考慮。
Each ± sign can be predetermined by tuning the frequency of Nd:YAG laser, OPO, comb repetition rate and synthesizer and observing the beat frequency 1 and 2.
Comb number n can be determined by wavemeter.

12. Frequency Calibration Test by Methane
Pressure of CH4: 14 mTorr
Pump power : 8 mW Lock-in time constant : 100 ms
Probe power : 0.7 mW AOM modulation frequency : 70 kHz
Linewidth ~ 0.88 MHz
由doppler量測結果得知，離子在放電管內有漂移速度，所以我們必須做飽和吸收光光譜才能得到準確的頻率中心
利用AOM調製pump光的強度。probe光帶著調製訊號送進lockin做解調

13. Frequency Calibration Test by Methane
Pressure of CH4: 14 mTorr
Pump power : 8 mW Lock-in time constant : 100 ms
Probe power : 0.7 mW AOM modulation frequency : 70 kHz
由doppler量測結果得知，離子在放電管內有漂移速度，所以我們必須做飽和吸收光光譜才能得到準確的頻率中心
利用AOM調製pump光的強度。probe光帶著調製訊號送進lockin做解調
Standard (kHz)
This Work (kHz)
Difference (kHz)
P(7)
(3)
(160)
6.8

14. Lineshape Fitting Model
The strong collisions between H3+ and H2 redistribute excited H3+ uniformly over the Maxwellian velocity distribution.
Gaussian + Lorentzian
Gaussian
Smith, P. W. & Hänsch, R., Phys. Rev. Lett., 1971, 26,
分析飽和吸收的結果，發現不是單純勞羅茲曲線
放電館充滿中性H2，因為H3+與H2質量相近，在質量相近的情況下因為碰撞而重覆貢獻的速度偏移需要考慮。
Lorentz model
Strong-collision model

15. Sub-Doppler Signal of H3+ R(1,0) transition
Pressure of Hydrogen : 80 mTorr
Ethanol cooled : -70 °C Lock-in 1 time constant : 1 ms
Pump power : 1000 mW Lock-in 2 time constant : 300 ms
Probe power : 5 mW Discharge current : 17 mA
AOM modulation frequency : 70 kHz
Apply magnetic field : AC amplitude(P-P) 167 Gauss(25 A) with frequency 5 Hz and
DC offset 135 Gauss(10.3 A)
80 mTorr
SNR ~ 460

16. Sub-Doppler Signal of H3+ R(1,0) transition
Pressure of Hydrogen : 80 mTorr
Ethanol cooled : -70 °C Lock-in 1 time constant : 1 ms
Pump power : 1000 mW Lock-in 2 time constant : 300 ms
Probe power : 5 mW Discharge current : 17 mA
AOM modulation frequency : 70 kHz
Apply magnetic field : AC amplitude(P-P) 167 Gauss(25 A) with frequency 5 Hz and
DC offset 135 Gauss(10.3 A)
Line center = (18) MHz

17. Frequency Measurements of H3+ Transitions
Unit : MHz
Transition
This Work
Other’s Work
Diff.
Q(1,0)
(85)
(109)a
-0.23
Q(1,1)
(93)
(54)a
0.33
Q(2,2)
(158)
(139)a
1.5
Q(3,3)
(255)
(135)a
-2.29
R(1,1)l
(28)
(165)c
-1.08
(15)d
0.37
R(1,0)
(18)
(86)c
-0.88
(11)d
-0.21
R(1,1)u
(28)
(84)c
-0.49
(20)d
0.13
R(2,2)l
(33)
(70)c
-0.24
(65)d
R(2,1)l
(231)
(279)c
-0.66
R(2,2)u
(56)
(121)c
-0.19
(48)d
-0.75
R(3,3)l
(38)
(88)c
-0.54
R(3,3)u
(87)
(143)e
-4.15
R(3,2)u
(92)
(254)e
-3.79
R(5,5)l
(77)
(144)e
-4.05
R(4,4)u
(390)
(150)b
130.3
R(4,3)u
(83)
(232)e
-0.67
做了一些修正，標準差
相減後結果
a. A. J. Perry et al. ISMS, MH03 (2016).
b. C. M. Lindsay and B. J. McCall, J. Mol. Spectrosc. 210, 60 (2001).
c. J. N. Hodges et al. J. Chem. Phys. 139, (2013).
d. P. Jusko et al. J. Mol. Spectrosc. 319, 55 (2016).
e. A. J. Perry et al. J. Mol. Spectrosc. 317, 71 (2015).

18. Signal of H3+ Q(3,3) Line Q(3,3) Pressure of Hydrogen: 80 mTorr
Ethanol cooled : 10 °C Lock-in 1 time constant : 1 ms
Pump power : 680 mW Lock-in 2 time constant : 300 ms
Probe power : 5 mW Discharge current : 17 mA
AOM modulation frequency : 70 kHz
Apply magnetic field : AC amplitude(P-P) 167 Gauss(25 A) with frequency 5 Hz and
DC offset 135 Gauss(10.3 A)
Q(3,3)
SNR ~ 10

19. Summary and Future Works
We have measured 16 transitions of H3+ with ~ 1 MHz precision.
In addition to transition frequency, our system provides information of Doppler profile, sub-Doppler profile, and the linewidth.
The frequency determination of molecular ion will be more precise after we improve the saturation signal further.
More transitions of H3+ and other molecular ions will be measured.

20. Acknowledgements Advisor : Jow-Tsong Shy Li-Bang Wang Group Members:
Yu-Chan Guan
Yi-Chieh Liao
Yung-Hsiang Chang
Funding Agencies

22. Optical Frequency Comb
An optical frequency comb, based on a fiber mode-locked laser, links the optical frequency to microwave frequency standard designed by Dr. Jin-Long Peng in ITRI in Taiwan.
250 MHz repetition rate mode-locked fiber comb with octave-spanning SC
Frequency accuracy : 2 × at 1 s (depending on our lab GPS disciplined RF source)
Repetition rate monitor > 10 mW
F-2f construction for fCEO > 40 dB at 100 kHz RBW
Broadband super-continue > 200 mW from 1050 to 2200 nm
The optical frequency accuracy < 5 kHz at 1 s
Broadband SC

23. Iodine Stabilized System
The molecular iodine spectrum had been proven to be an excellent method for laser frequency stabilization.
Beat with OPO pump.
Nd : YAG Laser
λ/2
EOM
AOM
1064 nm HR
PPLN
1st
I2 cell
The optical frequency accuracy < 6 kHz at 5 ms