Frequency-modulation saturation spectroscopy of molecular iodine hyperfine structure near 640 nm with a diode laser source
V. M. Khodakovskiy, V. I. Romanenko, I. V. Matsnev, R.A. Malitskiy, A. M. Negriyko, L.P. Yatsenko
aa r X i v : . [ phy s i c s . a t o m - ph ] D ec Frequency-modulation saturation spectroscopy of molecular iodine hyperfine structurenear 640 nm with a diode laser source
V. M. Khodakovskiy, V. I. Romanenko, I. V. Matsnev, R.A. Malitskiy, A. M. Negriyko, L.P. Yatsenko
Institute of Physics, Nat. Acad. of Sci. of Ukraine,46, Nauky Ave., Kyiv 03680, Ukraine ∗ In a frequency-modulation spectroscopy experiment, using the radiation from a single frequencydiode laser, the spectra of molecular iodine hyperfine structure near 640 nm were recorded on thetransition B Π + u − X Σ + g . The frequency reference given by the value of the modulation frequency(12.5 MHz in given experiment) allows determination of the frequency differences between hyperfinecomponents with accuracy better than 0.1 MHz using the fitting procedure in experiment with onlyone laser. I. INTRODUCTION
The dense spectrum of molecular iodine is widely usedfor optical wavelength reference in for laser spectroscopicapplications and laser frequency stabilization in a wideregion of the optical spectrum from the green (500 nm)to the near infrared (900 nm). High frequency stability ofHe-Ne laser at 543 nm, 612 nm, 633 nm, 640 nm, Nd:YAGlaser at 532 nm ( second harmonic), Ar + laser at 514.5nm is achieved by use of sub-Doppler techniques such assaturation spectroscopy for locking to an iodine molec-ular transitions. Seven of the 20 recommended wave-lengths for the realization of the metre are based on thehyperfine transitions wavelengths of I [1–2].The precise frequency locking onto the iodine hyperfinetransitions is used now for the determination of the nu-clear electric quadrupole interaction and the nuclear spin-rotation interaction parameters of iodine molecules. The I and I I molecules are the attracting objectsfor these investigations. The very promising set of iodinetransitions for laser frequency stabilization includes thewavelengths 502 nm ( I R(51) 68-0), 633 nm ( I R(33) 6-3), 793 nm ( I R(92) 0-15). The additionalreferences could be created with transitions of I Imolecules.The standard approach to the experiments with satu-rated absorption resonances is based on the use of twoidentical lasers stabilised to different hyperfine structurecomponents and on the measurements of the beating fre-quencies of two laser radiations. In this work, we reporton the Doppler-free saturation spectroscopy of the molec-ular iodine hyperfine lines at 640 nm using much simplerexperimental setup. We have used so- called frequency-modulation spectroscopy [3-4] for which the laser fieldis phase modulated at a frequency higher than the reso-nance linewidth. When the mean laser frequency is closeto some hyperfine component due to saturation of theabsorption and dispersion the phase modulation is trans-formed to amplitude modulation. The amplitude modu-lation is detected by lock-in amplifier and the output sig- ∗ Electronic address: [email protected] nal consists of the saturated absorption or saturated dis-persion depending on the reference oscillator phase. Theimportant feature which we use in our approach is thatthe FM-resonances are a sum of individual resonancesshifted on half of the modulation frequency what givesthe frequency reference. The frequency reference givenby the value of the modulation frequency (12.5 MHz ingiven experiment) allows determination of the frequencydifferences between hyperfine components with accuracybetter than 0.1 MHz using the fitting procedure in ex-periment with only one laser.
II. EXPERIMENTAL SETUP
The experimental setup is shown in Fig. 1. The single
Figure 1: Experimental setup for molecular iodine FM-spectroscopy: DL100, diode laser; DigiLock 110, electronicmodule; PD, photodiode; PMT, photomultiplier; FPI, Fabry-Perot interferometer; USB2000, spectrometer; Ne, hollowcathode lamp frequency diode laser (Toptica Model DL100) emitting640-nm radiation with 40-mW output power has beenused. Within the frequency tuning range (more than 5nm) of the laser more than 1000 strong rovibronic linesof the I , I and I I molecules can be observed.To reduce the laser-frequency drift induced by temper-ature changes the laser temperature was stabilized withaccuracy about 0.01 C with laser drift better than 10MHz per hour. A controllable frequency tuning with-out mode jumps was achieved in range about 20 GHzby means of the electronic module DigiLock with use ofa piezoelectric actuator mounted on the external lasermirror. The probe beam of 7 mW and the pump beamof 16 mW with beam diameter of 2 mm counterpropagateinside a 8-cm-long iodine cell. The iodine vapor pressurewas kept constant by keeping the cold-finger temperaturewithin 0.001 C. Phase modulation of the laser outputradiation is produced by a laser diode injection currentmodulation at the frequency 12.5 MHz. This frequencyis higher than the iodine resonance linewidth (about 1-2MHz), it is much higher than the characteristic techni-cal noise frequencies and it is comparable with with fre-quency differences between hyperfine components. Thephotomultiplier PMT detects the iodine fluorescence, thephotodiode PD detects the probe beam power. Thespectral control of the laser output provide the scanningFabry-Perot interferometer (FPI) with finesse about 250and FSR 2 GHz, the spectrometer USB2000 with resolu-tion 0.5 nm. The absolute wavelength of the laser can bedetermined by help of optogalvanic efects in the Ne filledlamp with hollow cathode. In the described experimentswe have used the optogalvanic resonance with the 0.6403nm Ne line. The signal from the output of the numericallock-in amplifier built in DigiLock was processed by thePC.
III. EXPERIMENTAL RESULTS
Here we present some preliminary results of study ofthe hyperfine structure of the I I line closed to the0.6403 nm Ne line. The figure 2 shows an example of thehyperfine structure obtained with FM spectroscopy tech-nique (resonances of saturated absorption). The I P(90) 5-3 and I P(10) 8-5 line contributes in the dis-played spectrum. The figure 3 shows an example of ap- -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.025-0.024-0.023-0.022-0.021 S i gna l , a . u frequency, a.u Figure 2: An example of the hyperfine structure obtainedwith FM spectroscopy technique proximation of the two hyperfine line by theoretical for-mula describing FM spectroscopy. The fitting are very close to the experimental data. As one can see fromFig.3c, the frequency difference of the displayed compo-nents is 30.8 MHz.
80 100 120 140-0.00020.00000.0002 A pp r ox ., ze r o b ac kg r ound n - n , MHz (c) S i gn a l a nd a ppox i m a ti on frequency, a.u (b) S i gn a l , a . u . frequency, a.u. (a) Figure 3: An example of fitting the hyperfine structure ob-tained with FM spectroscopy technique by the theoretical for-mula: a — experimental spectrum, b — fitting is shown bythe thick line is compared with the experimental spectrum,c — the spectrum with the subtracted background
IV. CONCLUSION
In a frequency-modulation spectroscopy experiment,using the radiation from a single frequency diode laser,the spectra of molecular iodine hyperfine structure near640 nm were recorded on the transition B Π + u − X Σ + g .The frequency reference given by the value of the modu-lation frequency (12.5 MHz in given experiment) allowsdetermination of the frequency differences between hy-perfine components with accuracy better than 0.1 MHzusing the fitting procedure in experiment with only onelaser. V. ACKNOWLEDGEMENTS
This study is supported by the joint Russian-Ukrainiangrant RFFR/1-09-25 [1] T. J. Quinn, “Practical realization of the definition of themetre, including recommended radiations of other opti-cal frequency standards (2001),” Metrologia , 103–133(2003).[2] R. Felder, “Practical realization of the definition of themetre, including recommended radiations of other opti-cal frequency standards (2003),” Metrologia , 323–325(2005). [3] J. L. Hall, L. Hollberg, T. Baer, and H. G. Robinson, “Op-tical heterodyne saturation spectroscopy,” Appl. Phys.Lett. , 680–682 (1981).[4] G. C. Bjorklund, M. D. Levenson, W. Lenth, and C. Or-tiz, “Frequency modulation (FM) spectroscopy-theory oflineshapes and signal to noise analysis,” Appl. Phys. B32