Comb-referenced sub-Doppler resolution infrared spectrometer

Comb-referenced sub-Doppler resolution infrared spectrometer
International Symposium on Molecular Spectroscopy University of Illinois, Urbana-Champaign June, 23, 2015 Mini-Symposium: High-Precision Spectroscopy TF01 Comb-referenced sub-Doppler resolution infrared spectrometer Kana Iwakuni and I will talk about an infrared spectrometer developed in Keio University. Keio is the first university in Japan. We celebrated 150th anniversary two years ago. Hiroyuki Sasada

High-Resolution IR Spectroscopy
the ν3 band of 12CH4 2900　　 2950　　3000　　3050　　3100　 (cm-1)　 P(7) R Grating spectrometer Resolution: ~10 GHz A. H. Nielsen et. al., Phys. Rev. 48, 864 (1935). 1.5 THz 5 GHz F1(1) F2(1) A2 F2(2) E F1(2) DFG source Doppler limited Resolution: 140 MHz 波長３ミクロン帯には、CH、NH、OH結合の伸縮振動運動による吸収があります。これらは分子構造の特徴を反映した固有の吸収スペクトルを示すため、この波長域の分光は分子構造の決定や微細な相互作用の解明などへ応用されています。特にメタンは特に強い吸収スペクトルを持ち、取扱いも楽なため、この波長域の標準ガスとして利用されます。これは異なる分解能で記録したメタンの吸収スペクトルです。メタンの振動バンドでは8THzにわたり回転構造が広がっています。その中の一本を拡大すると、水素の超微細構造で10kHzで三本に分かれたスペクトルが観測されます。さらにその１本１本が反跳分裂で1kHz程度分かれています。このように、中赤外領域では広い波長域で高分解能分光ができる分光計が求められています。広い範囲で分光を行うためには、光源の同調範囲が広い必要があります。また高分解能で分光を行うには光源の中心周波数が安定で、線幅が狭いことが必要になります。しかしいままでこれらすべてを満たす光源は存在しませんでした。 3.39-μm He-Ne laser Resolution: ~1 kHz J. L. Hall, C. Bordé, K. Uehara, Phys. Rev. Lett. 37, 20 (1976).

John Hall and Kiyoji Uehara
1983 at Keio University at his lab. From Nobelprize.org

Contents Widely tunable difference-frequency-generation (DFG) source
Enhanced-cavity absorption cell (ECAC) and Optical frequency comb (OFC) Lamb-dip-referenced spectrometer Comb-referenced spectrometer Towards reduction of the linewidth of the Lamb dip Linewidth Transfer Conclusion TF02　　　by Dr. Kana Iwakuni, Hyperfine splitting of HCl TF03　　　by Dr. Masashi Abe, A1-A2 splitting of CH3D The first talk by me will introduce a 3.4 micrometer DFG spectrometer for sub-Doppler resolution spectroscopy and frequency measurements of methane lines using an optical comb. The second talk by Kana Iwakuni will discuss about comb-referenced spectroscopy.

Spectrometer at Keio Univ.
3.3 μm region for　the ν3 band of 12CH3 1) wide tunability DFG source 2) sub-Doppler resolution ECAC, Optical Frequency Comb 3)　 high sensitivity 　 ECAC 4)　 precise frequency measurement Optical frequency comb Our spectrometer covers the 3.4 micrometer region. It has wide tunable range, high sensitivity, and sub-Doppler resolution. It is also able to determine transition frequencies with an accuracy corresponding to the resolution and the signal to noise ratio.

Enhanced-cavity absorption cell (ECAC) and Optical frequency comb (OFC) Lamb-dip-referenced spectrometer Comb-referenced spectrometer Towards reduction of the linewidth of the Lamb dip Linewidth Transfer Conclusion The first talk by me will introduce a 3.4 micrometer DFG spectrometer for sub-Doppler resolution spectroscopy and frequency measurements of methane lines using an optical comb. The second talk by Kana Iwakuni will discuss about comb-referenced spectroscopy.

DFG source 1.064 μm 100mW Nd:YAG Line width < 2 kHz
Tunability: 6 GHz 3.2~3.6 μm 100 μW ECLD Line width 250 kHz Tunability: 12 THz Waveguide PPLN　 Efficiency: 10 %/W Tunability: 2.6 THz The spectrometer consists of a difference-frequency-generation source. The pump source is a Nd:YAG laser. The line width is very narrow, but the tunable rage is also narrow. The signal source is an extended-cavity laser diode. It has wide tunable range, but the line width is broad. The signal wave is amplified by a fiber amplifier, and combined with the pump wave by a fiber coupler. The both waves enter a waveguide PPLN with an efficiency of 10%/W. This value is three orders of magnitude larger than the bulk PPLN. The idler wave is 100 microW when the pump and signal waves are 100 mW. F.A. 1.5~1.6 μm 100 mW

Tunable Range Nd:YAG pump + tunable 1.55 μm signal Frequency/ THz 85
90 95 80 Waveguide PPLN single element ECLD Fiber amp. This figure illustrates the tunable range of the DFG source. You can buy waveguide PPLN between 3.2 to 3.6 micrometer from NTT Electronics Corporation. You also can ask the company to provide PPLN in the neighboring wavelength regions. A single element covers about 2.6 THz. If you use different combinations of pump and signal wavelengths, the tunable range of the single element varies. The ECLD is extensively tunable. Fiber amplifiers usually cover over one band. S-band C-band L-band 2700 2800 2900 3000 3100 3200 Wavenumber/ cm–1

Enhanced-cavity absorption cell
bellows bellows PZT DFG beam PZT Mirrors are optical windows. The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily. mirror separation (FSR) reflectivity (transmittance) finesse (FWHM) 23.6 cm (636 MHz) 99.0% (0.7%) 300 (2.1 MHz) effective absorption length sensitivity optical field strength at antinodes ×198 ×139 ×17

Optical frequency comb for frequency determination
TAI Rb clock Er fiber comb Repetition rate: 65 MHz ~ 1.57 μm Highly nonlinear fiber 1.0 ~ 2.0 μm We measure the DFG frequency using a frequency comb. This picture shows an Er fiber comb made by Kana Iwakuni for this measurement. The repetition rate is 65 MHz, and the comb spectrum is spread between and 1.57 micrometer. It is broadened further using a nonlinear fiber. Then the comb spectrum covers 1 to 2 micrometer.

Lamb-dip-referenced DFG spectrometer
Enhanced-cavity ECLD Nd:YAG laser 3.4 μm ECAC PPLN frep Optical frequency comb

Enhanced-cavity ECLD Pound-Drever-Hall Nd:YAG laser 3.4 μm ECAC PPLN frep Optical frequency comb

Enhanced-cavity ECLD Pound-Drever-Hall Nd:YAG laser sweep 3.4 μm ECAC PPLN frep CH4 　ν３ P(7) F2(2) Optical frequency comb

Enhanced-cavity ECLD Pound-Drever-Hall Nd:YAG laser sweep 3.4 μm ECAC PPLN linewidth: 300 kHz transit-time: 80 kHz pressure: 12 kHz source: 25 kHz power: ~200 kHz frep CH4 　ν３ P(7) F2(2) Optical frequency comb

Enhanced-cavity ECLD Pound-Drever-Hall Nd:YAG laser 3.4 μm ECAC PPLN linewidth: 300 kHz transit-time: 80 kHz pressure: 12 kHz source: 25 kHz power: ~200 kHz frep CH4 　ν３ P(7) F2(2) Optical frequency comb

Frequency Control t T τ Stabilization of the center frequency
Reference: long-term stability Servo bandwidth ~ 1/T Narrowing of the source linewidth Reference: narrow linewidth Servo bandwidth > 1/τ ~Δν τ

Lamb-dip-referenced ECAC
Frequency Control t T Stabilization of the center frequency Reference: long-term stability Servo bandwidth ~ 1/T Narrowing of the source linewidth Reference: narrow linewidth Servo bandwidth > 1/τ ~Δν τ long-term stability linewidth response bandwidth Lamb-dip-referenced ECAC ~ 1 kHz ~30 kHz ~ 30 kHz 1.064-μm Nd:YAG Laser 50 kHz 1.5-μm ECLD ~250 kHz >250 kHz

Frequency Control t T τ Stabilization of the center frequency
Reference: long-term stability Servo bandwidth ~ 1/T Narrowing of the source linewidth Reference: narrow linewidth Servo bandwidth > 1/τ ~Δν τ long-term stability linewidth (Stabilized linewidth) response bandwidth Lamb-dip-referenced ECAC ~ 1 kHz ~30 kHz ~ 30 kHz 1.064-μm Nd:YAG Laser 50 kHz 1.5-μm ECLD ~250 kHz (DFG: 25 kHz) >250 kHz

DFG frequency measurement using a comb
νn = n× frep 　+ fCEO mode number difference Δn comb mode fCEO f1.55μm fbeat frep f1.06μm fbeat ・・・・・・・・・ ≈ ≈ ν ≈ νDFG This figure illustrates the principle of the frequency measurement. The neighboring comb modes are precisely separated by the repetition rate. These are the signal and pump frequencies. The difference between the laser and the closest comb mode corresponds with the beat frequency. This equation gives the DFG frequency. The delta n is the mode number difference and approximately one million. The repetition rate is controlled with a relative uncertainty of 10–12 level. This equation is free from the carrier envelope offset frequency. ECLD 1.55 μm Nd:YAG 1.06 μm νDFG =Δn× frep 　 – f1.06μm fbeat f1.55μm Δn ~1.32 × 106 frep~ 65 MHz fCEO free

CH4 ν3 band: 204 lines from P(12) to R(8)
Result CH4 ν3 band: 204 lines from P(12) to R(8) transitio n sym. sp. measured fr. / kHz stnd. dev. / kHz previous works / kHz discrepancy / kHz P(12) F1(2) 2.2 x 103 –2.40 x 103 … P(7) E 2.4 (20) 1.3 F2(2) 2.1 (20) –0.2 R(8) A1 3.9 x 103 –3.29 x 103 We have measured frequencies of 56 transitions from 88.2 to THz. This table shows a part of results. All tetrahedral components of P(1) to P(7) and Q(1) to Q(8) are included. It also includes 2 forbidden transitions in blue and 2 13C methane transitions in green. The standard deviations are typically 2 kHz, which is comparable with that of microwave spectroscopy. The measured value of the P(7) F2(2) transition excellently agrees with the CIPM recommendation value. CIPM recommendation Relative uncertanty: 2 x 10–11 S. Okubo, H. Nakayama, K. Iwakuni, H. Inaba, and H. Sasada, Opt. Express, 19, (2011) M. Abe, K. Iwakuni, S. Okubo, and H. Sasada, JOSA B, 30, 1027 (2013)

CH4 ν3 band: 204 lines from P(12) to R(8)
Result CH4 ν3 band: 204 lines from P(12) to R(8) transitio n sym. sp. measured fr. / kHz stnd. dev. / kHz previous works / kHz discrepancy / kHz P(12) F1(2) 2.2 x 103 –2.40 x 103 … P(7) E 2.4 (20) 1.3 F2(2) 2.1 (20) –0.2 R(8) A1 3.9 x 103 –3.29 x 103 We have measured frequencies of 56 transitions from 88.2 to THz. This table shows a part of results. All tetrahedral components of P(1) to P(7) and Q(1) to Q(8) are included. It also includes 2 forbidden transitions in blue and 2 13C methane transitions in green. The standard deviations are typically 2 kHz, which is comparable with that of microwave spectroscopy. The measured value of the P(7) F2(2) transition excellently agrees with the CIPM recommendation value. CIPM recommendation Relative uncertanty: 2 x 10–11 S. Okubo, H. Nakayama, K. Iwakuni, H. Inaba, and H. Sasada, Opt. Express, 19, (2011) M. Abe, K. Iwakuni, S. Okubo, and H. Sasada, JOSA B, 30, 1027 (2013) HITRAN 2012: cm–1 = 30 kHz

Comb-referenced DFG spectrometer
For detection of weak Lamb dips

For detection of weak Lamb dips TAI Rb clock Rb clock synthesizer frep Enhanced-cavity OFC ν Nd:YAG laser 1.06 μm 3.4 μm PPLN ECAC ECLD 1.55 μm

For detection of weak Lamb dips TAI Rb clock Rb clock synthesizer frep Enhanced-cavity OFC ν Pound- Drever-Hall Nd:YAG laser 1.06 μm 3.4 μm PPLN ECAC ECLD 1.55 μm

For detection of weak Lamb dips TAI Rb clock synthesizer frep Sweep Enhanced-cavity OFC ν Pound- Drever-Hall Nd:YAG laser 1.06 μm 3.4 μm PPLN ECAC ECLD absolute frequency 1.55 μm

Result K. Iwakuni, S. Okubo, and H. Sasada, Opt. Express, 21, (2013) Demonstration; CH4, CH3I 2) M. Abe, K. Iwakuni, S. Okubo, and H. Sasada, JOSA B, 30, 1027 (2013) CH4 forbidden transition 3) K. Iwakuni, H. Sera, M. Abe, and H. Sasada, JMS, 306, 19 (2014) HCl The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily.

Result K. Iwakuni, S. Okubo, and H. Sasada, Opt. Express, 21, (2013) Demonstration; CH4, CH3I 2) M. Abe, K. Iwakuni, S. Okubo, and H. Sasada, JOSA B, 30, 1027 (2013) CH4 forbidden transition 3) K. Iwakuni, H. Sera, M. Abe, and H. Sasada, JMS, 306, 19 (2014) HCl The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily. TF02

Linewidth reduction of Lamb dips
Reduce transit-time broadening ECAC coupled with a wide beam The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily.

Reduce transit-time broadening ECAC coupled with a wide beam previous ECAC new ECAC beam radius at beam waist 0.7 mm 1.9 mm CH4 Q(12) Lamb dips width 360 kHz 80 kHz source width 25 kHz 12 kHz The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily.

Reduce transit-time broadening ECAC coupled with a wide beam Narrow Lamb dips are NOT observed in comb-reference spectroscopy. previous ECAC new ECAC beam radius at beam waist 0.7 mm 1.9 mm CH4 Q(12) Lamb dips width 360 kHz 80 kHz source width 25 kHz 12 kHz The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily.

TAI referenced Rb clock
Frequency Control Comb-referenced spectrometer long-term stability linewidth response bandwidth TAI referenced Rb clock 1 x ~ 100 THz ~ 10 THz 1.064-μm Nd:YAG Laser ~ 1 kHz 50 kHz 1.5-μm ECLD 250 kHz >200 kHz frep controled comb 25~100 kHz fceo linewidth ~10 kHz

Frequency Control Comb-referenced spectrometer long-term stability
linewidth response bandwidth TAI referenced Rb clock 1 x ~ 100 THz ~ 10 THz 1.064-μm Nd:YAG Laser ~ 1 kHz 50 kHz 1.5-μm ECLD 250 kHz >200 kHz frep controled comb 25~100 kHz fceo linewidth ~10 kHz frep and fceo controled comb with an EOM reference to Rb clock 400 kHz advanced applications of combs dual-comb spectroscopy linewidth transfer

A1-A2 splitting of CH3D Linewidth of Lamb dip: 60 ~ 90 kHz

Result M. Abe, K. Iwakuni, S. Okubo, and H. Sasada, Opt. Lett., 39, 5277 (2014) Enhanced-cavity absorption cell coupled with a wide beam 2) M. Abe, H. Sera, and H. Sasada, JMS, 312, 90 (2015) A1A2 splittings of CH3D The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily.

Result M. Abe, K. Iwakuni, S. Okubo, and H. Sasada, Opt. Lett., 39, 5277 (2014) Enhanced-cavity absorption cell coupled with a wide beam 2) M. Abe, H. Sera, and H. Sasada, JMS, 312, 90 (2015) A1A2 splittings of CH3D The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily. TF03

Frequency Control Comb-referenced spectrometer long-term stability
linewidth response bandwidth TAI referenced Rb clock 1 x ~ 100 THz ~ 10 THz 1.064-μm Nd:YAG Laser ~ 1 kHz 50 kHz 1.5-μm ECLD 250 kHz >200 kHz frep controled comb 25~100 kHz fceo linewidth ~10 kHz frep and fceo controled comb with an EOM reference to Rb clock ~1 kHz reference to Nd:YAG laser 400 kHz

Linewidth Transfer νn = n × frep + fCEO frep fCEO Frequency stabilize
H. Inaba et al. Opt. Express 21, 7891 (2013) νn = n × frep + fCEO stabilize Intensity fCEO frep Frequency 線幅の細いYAGレーザーにコムを安定化すると、コムはFREPを調整してモードの周波数を安定化します。従って、FREPはYAGレーザーに安定化されたことになります。コムのすべてのモードはこの形の式で与えられるため、コムをYAGレーザーに安定化すると、コムのすべてのモードがYAGレーザーの線幅、揺らぎを反映します。

H. Inaba et al. Opt. Express 21, 7891 (2013) νn = n × frep + fCEO stabilize Intensity fCEO frep Frequency ECLD Nd : YAG linewidth ~ 250 kHz ～ 1 kHz 線幅の細いYAGレーザーにコムを安定化すると、コムはFREPを調整してモードの周波数を安定化します。従って、FREPはYAGレーザーに安定化されたことになります。コムのすべてのモードはこの形の式で与えられるため、コムをYAGレーザーに安定化すると、コムのすべてのモードがYAGレーザーの線幅、揺らぎを反映します。

Nd:YAG H. Inaba et al. Opt. Express 21, 7891 (2013) Intensity fCEO frep EOM Frequency ECLD Nd : YAG linewidth ~ 250 kHz ～ 1 kHz 線幅の細いYAGレーザーにコムを安定化すると、コムはFREPを調整してモードの周波数を安定化します。従って、FREPはYAGレーザーに安定化されたことになります。コムのすべてのモードはこの形の式で与えられるため、コムをYAGレーザーに安定化すると、コムのすべてのモードがYAGレーザーの線幅、揺らぎを反映します。

Nd:YAG H. Inaba et al. Opt. Express 21, 7891 (2013) Intensity fCEO frep EOM Frequency ECLD Nd : YAG linewidth ~ 1 kHz ～ 1 kHz 線幅の細いYAGレーザーにコムを安定化すると、コムはFREPを調整してモードの周波数を安定化します。従って、FREPはYAGレーザーに安定化されたことになります。コムのすべてのモードはこの形の式で与えられるため、コムをYAGレーザーに安定化すると、コムのすべてのモードがYAGレーザーの線幅、揺らぎを反映します。

Nd:YAG H. Inaba et al. Opt. Express 21, 7891 (2013) Intensity fCEO frep EOM Frequency ECLD Nd : YAG PPLN linewidth ~ 1 kHz ～ 1 kHz 線幅の細いYAGレーザーにコムを安定化すると、コムはFREPを調整してモードの周波数を安定化します。従って、FREPはYAGレーザーに安定化されたことになります。コムのすべてのモードはこの形の式で与えられるため、コムをYAGレーザーに安定化すると、コムのすべてのモードがYAGレーザーの線幅、揺らぎを反映します。

Servo System linewidth Nd:YAG 1.06 μm TAI pump center fr. PPLN Comb
signal linewidth ECLD 1.5 μm

Narrow linewidth DFG source
< 1 kHz Beat note with a 3.39 μm He-Ne laser calculated from the fluctuation of frep FREPが一定になるように、YAGレーザーにフィードバックしてドリフトを止めました。すると中赤外光は安定になり、中心周波数の揺らぎも1秒で3kHz程度に抑えられました。 7 kHz

Experimental Setup Narrow Linewidth DFG Source InSb Detector Methane
Liquid Nitrogen Corner Cube

Observed Spectrum CH4 ν3 band R(2) E (I = 0) cf. 80 kHz recorded using
the ECAC coupled with a wide beam 20.5 kHz DFG Frequency / kHz –

Observed Spectrum CH4 ν3 band R(2) E (I = 0) Source width
cf. 80 kHz recorded using the ECAC coupled with a wide beam 20.5 kHz DFG Frequency / kHz – Source width < 7 kHz Pressure width 3.5 kHz @ 0.1 Pa Transit-time width < 12 kHz @Beam radius 3.0 mm, 77 K

Conclusion A comb-referenced DFG spectrometer has realized a high resolution of 80 to 300 kHz, a precise frequency determination of a few kilohertz, and high sensitivity in the frequency range of 87 to 94 THz. ２. A linewidth transfer technique reduces the linewidth of the DFG source to less than 7 kHz, and the observed Lamb dip of CH4 gets narrowed to 20.5 kHz.

Acknowledgements PEOPLE JST CREST, SENTAN, ERATO MEXT APSA, KAKENHI
Abe Takahata Kobayashi Okubo Iwakuni Nakayama Sera Inaba JST　 CREST, SENTAN, ERATO MEXT　 APSA, KAKENHI

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