Kevin Knabe, Ahmer Naweed, Aaron Pung Saturation Spectroscopy -Inside Photonic Band Gap Fiber Rajesh Thapa Kevin Knabe, Ahmer Naweed, Aaron Pung Larry Weaver, Brian Washburn, Kristan Corwin

Outline An overview:- optical frequency references IR wavelength standard Pump probe spectroscopy Observations of saturation spectra Modeling of Light Inside Fiber Conclusion and future direction

An Overview : Optical Frequency References CH4 C2H2 C2HD Rb Ca I2 H 3390 1556 1064 778 657 532 486 Well studied optical frequency references v1+v3 combination band of acetylene Robust and stable frequency communication and navigation System - Frequency division multiplexing - Spectrum analyzer Relevance to telecommunication industry. Well separated transitions. ~ms long lifetime, ~kHz linewidth. Lack of permanent dipole moment. Relatively immune to external field and shifts. Sarah L. Gilbert, W.C.S., Acetylene 12C2H2 Absorption Reference for 1510 nm to 1540 nm Wavelength Calibration-SRM 2517a. 2001

Higher-accuracy IR wavelength standard: nonlinear spectroscopy Comité International des Poids et Measures, 2000 13C2H2 P(16) ± 100 kHz (2000) Comb-based meas. ± 2 kHz (2005) Great Britain, Japan, Canada, Japan Existing portable wavelength references for the telecom industry Line centers:±130 MHz or ±13 MHz Used to calibrate optical spectrum analyzers (OSA’s) pressure → broadening & shift laser or LED C2H2 W.C. Swann and S.L. Gilbert. (NIST), Opt. Soc. Am. B, 17, 1263 (2000).

Spectroscopy Inside Power Build up cavity Basis for Highest-accuracy measurements Cavity: Long Interaction length High Intracavity Power Cavity and laser locked to resonance independently Not Portable, Fragile Figure from: K. Nakagawa, M. de Labachelerie, Y. Awaji, and M. Kourogi, JOSAB 13, 2708 (1996)

Hollow Core Photonic Band Gap Fiber Blaze Photonics (www.blazephotonics.com) J. C. Knight et al., Science, 282, 1476, 1998 photonic band gap fiber. Advantages: Long interaction length High laser intensities More portable Proximity of fiber surfaces to the molecules Small beam size inside the fiber (~15µm)as compared to cavities (500µm)

Predicted loss from the fundamental mode in ordinary hollow core fiber and PBG fiber Knight, j.C., Photonic crystal fibers. nature, 2003. 424: p. 847.

How Popular is Acetylene in PBG Hollow Core Fiber? Gas sensors (Helsinki U. of Tech., Crystal Fiber 2004) Optical frequency standards (Bath, 2005) Sealed PBG fiber cells Saturated absorption in hollow-core photonic bandgap fibers (J. Henningsen et al., 2005) Figure from F. Benabid et al., Nature (2005).

Recent Paper Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap fibers Jes Henningsen, Jan Hald, and Jan C. Peterson Opt. Express 13, 10475-10482 (2005) See background noise, We have at least 40 times higher signal to noise ratio Fiber Diameter :- 10 µm Width :- 22.4 MHz Psat :- 23 mW The improved SNR makes overtone transitions in the near-infrared region accessible to frequency metrology.

Application and Motivation Basis of international frequency reference in near-IR region with accuracy in KHz limit . Basis of portable frequency reference for telecom industry . Splice Splice Step Index, Single Mode Fiber (SMF) Step Index, Single Mode Fiber (SMF) PBG Fiber Fiber Cell

Splicing hollow-core photonic bandgap fibers for gas-filled First Fiber Cell LUMOS Spliced Fiber (2005) 74% coupling efficiency splice loss - 1.6 dB Splice Loss ~ 1.3 dB mechanical strength of the splices [ 80 bar -1 µbar ] Evacuated up to the pressure of ~10 mT for ~14 hours “Compact, stable and efficient all-fibre gas cells using hollow-core photonic crystal fibres” Benabid et al., Nature (2005). Splicing hollow-core photonic bandgap fibers for gas-filled optical frequency references to solid core fiber using an arc fusion splicer R. Thapa, K. L. Corwin, and B. R. Washburn, Submitted in CLEO 2006

Pump Probe Spectroscopy Theoretical Approach Pump Probe Spectroscopy

Absorption Of Light Absorption ∆I I = -() ∆z Laser Fiber Molecules (Beer’s law) ∆I I = -() ∆z ‘a ‘is the absorption coefficient True basically If, ∆z This leaves most of population in Ground state Or, I Laser Fiber Molecules IO I ∆z ∆I

What if laser light is Intense It Significantly begin to Deplete the Population of Ground State N1(v) N2(v) v Doppler Broadened Profile, ~ 500 MHz Wide

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

What a mess!!! Schematic Of Experimental Set up Experimental Set-Up Diode Laser PBG Fiber Photo Diode Vacuum Chamber Probe Pump AOM BS EDFA 70% 30% 5 mW 400 mW Diode Laser Michelson Interferometer Isolator AOM PBG Fiber Glass Cell Photo Diode Vacuum Chamber Probe Pump Squeezer squeezer BS EDFA BS(30/70) (10/90) 90% 70% 30% Experimental Set-Up What a mess!!!

Saturated absorption spectra as a function of 5 different pressures P (13) line, 10µm diameter fiber More Background Noise Surface mode? Transmission of light through glass 138 mT 219 mT 317 mT 435 mT 540 mT P (11) line, 20µm diameter fiber Saturated absorption spectra as a function of 5 different pressures We need some equation to fit this profile. SEARCH!!!

Our Fitting Equation Fitting Parameters (Laser Spectroscopy, W. Demtroder) True for s0<<1 Keep in Mind !!!

See Our Fit It Seems Our Equation Works!!!

Pressure-Broadening measurement of the different lines. 10µm diameter fiber , Width varies from 35 to 45 MHz 20µm diameter fiber , Width varies from 20 to 35 MHz ~10 MHz/Torr of Pressure Broadening What determines width?? Perhaps other broadening mechanism ???

Broadening Mechanism Transit time broadenings For 20µm diameter fiber -Interaction time of the molecule with laser beam In terms of frequency, V, most probable velocity of molecule For 20µm diameter fiber For 10µm diameter fiber Power broadenings See some Power broadening effect!!!

Optimum Signal Size change in fractional transmission due to pump signal width Discrimination =

Power broadening effect on the lineshape for P (11) lines Higher the power – bigger the amplitude of narrow feature- good for freq. reference Higher the power – wider the width of narrow feature- bad for freq. reference

Power-Broadening measurements Ps ~ 20mW Ps ~ 45mW Why Psat different ??? A big question!!!

Calculation of Saturation Parameter Theoretical Approach and Numerical Analysis Calculation of Saturation Parameter Remember:- Demtroder eq. is valid only for S<<1 But Psat is small so S is large !!!

The total absorption coefficient Absorption cross-section of probe in presence of pump. Population change is due to Pump only Integration over entire velocity spectrum gives, This is true for any S0 Usual linear attenuation of the Weak probe beam Change in signal induced by the Intense field, a Lorentzian of width w When S<<1, , above eq reduces to, Calculated By Demtroder Which is exactly as Now, we have our Own Equation!!!

Numerical Calculation of Psat Using Pump Propagation into account Fiber P0 P1 P2 Pn Pump Power Pump Small Finite Segment Saturated absorption in acetylene and hydrogen cyanide in hollow-core photonic bandgap fibers. Jes Henningsen, Jan Hald, and Jan C. Peterson Opt. Express 13, 10475-10482 (2005) Recent Paper Psat~ 23 mW We Know P0 and Pn, vary Ps until we get measured pump output power. Pump Power (mW) Distance along fiber Psat ~ 20 mW

Modeling Of Light inside the fiber Pump P0 P1 P2 Pn Pump Power Small Finite Segment Probe P’1 P’2 P’n Probe Power P’0 From Beer’s Law (For Probe laser) ∆P’ P’ = -S() ∆z This Way we can find out Probe transmission in presence of pump for entire frequency spectrum

Transmission effect along the fiber Pump Power Probe Power Vary Ps , Calculate Signal height (Al), Compared to Experiment , Repeat We can find different S0 at each different segment along the fiber and thus Psat.

Psat vs. Pressure P11 line, 10 micron, 0.9 m Fiber at 31 mW P13 line, 20 micron, 0.8 m Fiber at 14 mW P13 line, 10 micron, 1.9 m fiber at 60mW P13 line, 10 micron, 1.9 m fiber at 20mW We can not say anything about dependence of Psat on Pressure Psat varies from 25 mW to 60 mW!!!

It seems there is linear dependence of Psat on Power. Psat vs. Power P-11, 20 um, 1T,0.8m P-13, 20 um, 1T,0.8m P-13, 10 um, 430 mT,1.92m P-11, 10 um, 500 mT, 0.9m It seems there is linear dependence of Psat on Power. We don’t believe it, Should not Psat be constant? An Open question!!!

Conclusions Saturated absorption is readily achievable in photonic bandgap fibers with power <10 mW. We have characterized linewidth in terms of pressure and power. Linewidth dominated by transit-time broadening. larger-core photonic bandgap fibers desirable. Counter-propagation prone to noise- careful polarization control required We have got Satuation Power to be somewhere between 25 to 50 mW. We have also spliced the fiber outside the vacuum chamber and got very good splice.

We are on the process of making splice with CO2 Future: We are on the process of making splice with CO2 Laser to splice fiber inside the vacuum chamber. We are in final stage of generating frequency comb. - Measure frequency shift and stability of those narrow feature using frequency comb. We are also in the final stage to peak-lock these narrow feature. To Narrow the line (Target ~1 MHz) larger core size, coated cell? To make fiber cell for portable frequency references.

Funding generously provided by: AFOSR NSF CAREER Kansas NSF EPSCoR program Kansas Technology Enterprise Corporation Kansas State University Thanks to: Sarah Gilbert Mohammad Faheem Dirk Müller Bill Swann Kurt Vogel Mikes Wells and JRM staff

Mode of vibration of acetylene ν1(cm-1) ν3(cm-1) 3373.7 3278 ν1+ν3=6651.7(cm-1) Wavelength=1.5 V1 and v3 are doubly degenerate (equal energy) bending vibration. υ1 mode alone is dipole-forbidden, it can be excited in combination with the dipole-allowed υ3 mode excitation

[ R. F. Cregan et al., Science 285, 1537 (1999) ] Photonic Band-gap (PBG) Fiber 10µm 5µm [ R. F. Cregan et al., Science 285, 1537 (1999) ]

Guidance of light SMF Fiber PBG Fiber Total Internal Reflection multiple Interference and scattering at Bragg’s condition

Calculation Of Saturation Power It Gives Sat. Power on resonance with out taking Propagation effect into account. Psat (from) calculation Comparing Al vs. S Psat (from) mathematica by taking pump and probe attenuation into account. S P13 line, 20 micron, 0.8 m long Fiber.

Demtroder eq. Fitting eq in origin My Calculation Larry’s Calculation Or, Our calculation Demtroder Calculation

Effect of Probe saturation upon Saturation Power When Probe saturation is taken into account, the total absorption must incorporate two different Saturation Parameter due to both pump and probe. S1=Saturation Parameter due to probe S2=Saturation Parameter due to pump There is almost no effect on Psat due to probe power Psat (mW) 20 mW of pump Input power 60 mW of Pump power Without probe saturation 33.9 40.6 Probe saturation into consideration 33.5 40

Transit time broadenings: a dominant factor in small core fiber Molecules Laser FWHM In terms of the beam diameter, D=2w, most probable velocity In terms of frequency, For acetylene, M=26.016 g/mole; Room Temp, T=2950K for 13x10-6m mode field diameter

Diode Laser EDFA Pump Probe PBG Fiber Vacuum Chamber BS Photo Diode BS Isolator AOM Squeezer 70% BS(30/70) EDFA Squeezer 30% BS Diode Laser (10/90) BS Isolator Squeezer Photo Diode Glass Cell 90% BS Michelson Interferometer Photo Diode squeezer

Mode of vibration of acetylene ν1(cm-1) ν3(cm-1) 3373.7 3278 ν1+ν3=6651.7(cm-1) Wavelength=1.5 V1 and v3 are doubly degenerate (equal energy) bending vibration. υ1 mode alone is dipole-forbidden, it can be excited in combination with the dipole-allowed υ3 mode excitation