1. National Institute of Standards and Technology Broadband Spectroscopy of CO 2 Bands Near 2μm Using a Femtosecond Mode-Locked Laser ISMS 2014 - Session TI.09 17 June 2014 Andrew Klose 1, Daniel L. Maser 1, Gabriel Ycas 1, Ian Coddington 2, Nathan Newbury 2, and Scott A. Diddams 1 National Institute of Standards and Technology 1 Time and Frequency Division 2 Quantum Electronics and Photonics Division Boulder, Colorado

2. A. Klose2ISMS:2014 TI09 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

3. A. Klose3ISMS:2014 TI09 Optical Frequency Combs Optical frequency comb consists of many (>10 5 ) lines equally-spaced in frequency All lines defined by two radio frequencies: Spectroscopy with combs is advantageous => broadband source of many “cw” lasers Femtosecond Optical Frequency Comb. Ye and Cundiff (Eds.). Springer, Norwell, MA (2005).

4. A. Klose4ISMS:2014 TI09 Spectroscopy with Combs Femtosecond mode-locked fiber lasers are a well- developed 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 Polarization maintaining optical fibers => increased robustness of optical source For previous work, see, for example: G. Ycas, et al. Opt. Lett. 37, 2199 (2012) S. Kumkar, et al. Opt. Lett. 37, 554 (2012) I. Hartl, et al. CLEO Technical Digest. CTh1J.2 (2012) A. Sell, et al. Opt. Exp. 17 1070 (2009) H. Hoogland, et al. Opt. Exp. 21, 31390 (2013) 2µm spectral region interesting for CO 2 spectroscopy 2µm => supercontinuum generation from Er:fiber L. S. Rothman, et al. J Quant. Spec. Rad. Trans. 130, 4 (2013) Blue – H 2 0 Red – CO 2 250 MHz Er:fiber Laser

5. A. Klose5ISMS:2014 TI09 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

6. A. Klose6ISMS:2014 TI09 Output from Source 3 Output spectrum spans 1.1 μm - 2.2μm 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 90 fs pulses in 2μm region at connectorized output (can compress to 35 fs) Total power = 240mW

7. A. Klose7ISMS:2014 TI09 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

8. A. Klose8ISMS:2014 TI09 VIPA Spectrometer Fiber-Coupled Light Cylindrical Lens Grating Spherical Lens InSb 2D-camera Comb teeth are spatially dispersed in 2D Virtually Imaged Phased Array (VIPA) => vertical dispersion Orth. Diffraction Grating =>horizontal dispersion Simultaneous measurement of ~50nm bandwidth of spectrum 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) [1] [2]

9. A. Klose9ISMS:2014 TI09 Calibration of 2D Image 250 MHz Source 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 VIPA FSR ~50GHz => count dots, fit dots, generate x,y pixel to frequency map Use unfiltered source for spectroscopy measurement Spectrometer and Camera 5 GHz Fabry Perot Cavity

10. A. Klose10ISMS:2014 TI09 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

11. A. Klose11ISMS:2014 TI09 Example Spectra [1] L. S. Rothman, et al. J Quant. Spec. Rad. Trans. 130, 4 (2013) 12 cm path 200 mbar CO 2 [1] Two Grating Positions Resolution of <2 GHz achieved (~0.05cm -1 ) Simultaneous measurement of 40 nm bandwidth (~2000 spectral elements) < 1ms integration time Fluctuations at time dependent etalon Detailed analysis currently underway

12. A. Klose12ISMS:2014 TI09 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)

13. A. Klose13ISMS:2014 TI09 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

14. A. Klose14ISMS:2014 TI09 Acknowledgements Thank You! NIST Scott Diddams Gabe Ycas Dan Maser Dan Hackett Nate Newbury Ian Coddington Esther Baumann Fabrizio Giorgetta Laura Sinclair Lindsey Sonderhouse Bill Swann CU-Boulder/JILA Jun Ye CU-Boulder Greg Rieker U of Campinas Flavio Cruz St. Johns Todd Johnson Funding: NRC NIST Climate Science

15. A. Klose15ISMS:2014 TI09

16. A. Klose16ISMS:2014 TI09 Source Coherence and Noise f rep 2f rep Heterodyne beats 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 2000nm LPF 1850nm LPF Red = 1650nm LPF Green = Full Supercontinuum Black = Er:fiber Oscillator Relative intensity noise (RIN) from supercontinuum increases with decreasing spectral window Green – Full SC Red – 1650nm LPF Blue – 1850nm LPF Purple – 2000nm LPF

17. A. Klose17ISMS:2014 TI09 Pulse Width SHG-FROG Connectorized Laser Output Output of source was temporally compressed using silicon prisms Pulses were analyzed via SHG-Frequency Resolved Optical Gating (SHG-FROG) Achieved FWHM pulse width of 35 fs Transform limit of 20 fs SHG-FROG-retrieved optical spectrum in good agreement with direct spectrum measurement