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Annual Meeting Optical Society of America

High-resolution optical multiplex spectroscopy

M. P. Winters, J. L. Hall, Klaus‐Peter Dinse

Published Jan 1, 1987• 0 citations

Abstract

We have developed a frequency-multiplex laser spectrometer using the concept of stochastic excitation to provide an effective optical excitation bandwidth of >200 MHz and an inherent spectral resolution of a few kilohertz. A frequency-stabilized Ar+ laser is phase-modulated by a pseudorandom binary sequence (PRBS) running at a clock frequency of 200 MHz. The detected system response to the stochastic optical field is cross-correlated with an identical time-delayed PRBS to give the equivalent of a free induction decay signal. A time-to-frequency transformation then gives the transfer function of the optically absorbing/dispersing system. The calibration and linearity of the frequency axis are dependent only on the oscillator used to clock the PRBS generators, thus avoiding the problems of drift and nonlinearities in ordinary sequential-scan spectroscopy. The multiplexing scheme allows a spectrum to be obtained in much less time than sequential scanning or, equivalently, gives a higher SNR for the same amount of data taking time. We have demonstrated the spectrometer by observing Doppler-free transitions in molecular iodine. We also describe the use of digitally filtered binary sequences and active control of residual amplitude modulation to improve the SNR of the spectrometer and possibilities for increasing the excitation bandwidth of the system.

DOI: 10.1364/oam.1987.wn4

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We have developed a frequency-multiplex laser spectrometer using the concept of stochastic excitation to provide an effective optical excitation bandwidth of >200 MHz and an inherent spectral resolution of a few kilohertz. A frequency-stabilized Ar+ laser is phase-modulated by a pseudorandom binary sequence (PRBS) running at a clock frequency of 200 MHz. The detected system response to the stochastic optical field is cross-correlated with an identical time-delayed PRBS to give the equivalent of a free induction decay signal. A time-to-frequency transformation then gives the transfer function of the optically absorbing/dispersing system. The calibration and linearity of the frequency axis are dependent only on the oscillator used to clock the PRBS generators, thus avoiding the problems of drift and nonlinearities in ordinary sequential-scan spectroscopy. The multiplexing scheme allows a spectrum to be obtained in much less time than sequential scanning or, equivalently, gives a higher SNR for the same amount of data taking time. We have demonstrated the spectrometer by observing Doppler-free transitions in molecular iodine. We also describe the use of digitally filtered binary sequences and active control of residual amplitude modulation to improve the SNR of the spectrometer and possibilities for increasing the excitation bandwidth of the system.

By using the concept of stochastic excitation, a multichannel optical spectrometer was developed with an inherent spectral resolution in the kilohertz domain. Stochastic optical fields are obtained by phase modulating a frequency-stabilized Ar+ laser with a pseudorandom binary sequence. This sequence is characterized by an autocorrelation time of 5 nsec, with a resulting excitation bandwidth of approximately 200 MHz. A signal related to an optical free-induction decay is obtained after cross correlating the response of the system with the time-delayed stochastic excitation sequence. The performance of the spectrometer was demonstrated by observing Doppler-free transitions of molecular iodine.

A frequency-doubled diode laser spectrometer in the blue-wavelength region with a continuous-frequency tuning range of up to ∼7 GHz has been developed, comprising a powerful fundamental light source based on a master/slave laser combination coupled with a KNbO3 crystal for second-harmonic generation in a power-enhancement cavity. It has been employed for the observation of saturated absorption spectra in 52S1/2–62P1/2 hyperfine transitions of atomic rubidium with the aid of a modulation transfer spectroscopic technique, and also the frequency stabilization to one of them has been demonstrated, suggesting the possibility of the establishment of a precision optical frequency marker in the blue range. A relative stability of better than 2.8×10−10 is estimated based on the locked error signal in the preliminary stabilization. With a further study of the frequency instability and uncertainty, in addition to supplementary improvements of apparatus, one may expect a realization of a stabilized laser in the blue-spectral domain. Details of the developed blue-light source are described and the demonstrated results of the rubidium spectroscopy are also described and discussed.

Time-domain spectroscopy using dual, coherent frequency combs is used to measure free-induction decay from a molecular gas sample in the near-IR with a time-domain signal-to-noise ratio of approximately 10(6) over a approximately 6 ns window at 55 fs time resolution (corresponding to the 9 THz source bandwidth) and a frequency/timing accuracy set by the frequency combs. The free-induction decay exhibits the expected periodic pulses from the rephasing of the multiply excited rovibrational levels. This demonstration represents the first high-resolution, high-accuracy, broadband measurement of optical free-induction decay, to our knowledge.

We demonstrated a compact 780 nm rubidium optical clock, which includes an optical frequency standard and an optical frequency comb, with an optical volume of 11.6 liters. Unlike the 778 nm rubidium atomic clocks based on two-photon transition, here, the laser frequency is stabilized to the Rb D2 transition, using modulation transfer spectroscopy. This approach effectively eliminates Doppler background and provides a high signal to noise ratio and high sensitivity. A nearly 300 MHz microwave signal, whose phase exactly tracks that of the optical frequency standard, is generated via the optical frequency comb, yielding a frequency instability of 1.91 E-13 @1 s and 5.29 E-14 @1000 s in the electronic domain. To the best of our knowledge, this is the most precise frequency stabilization result for the first-excited-state transition of alkali metal atoms to date and represents the first optical clock based on this transition. These results offer a promising approach for the development of portable optical clocks.

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