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Journal of the Optical Society of America B

Doppler-free optical multiplex spectroscopy with stochastic excitation

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

Published Sep 1, 1988• 20 citations
Publisher

Abstract

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.

DOI
10.1364/josab.5.001825

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Journal of the Optical Society of America B•Mar 1, 2001•14 citations
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.
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.
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.
This paper describes a new class of techniques, called time-delayed laser saturation spectroscopy, which combine frequency- and time-domain methods of laser spectroscopy to provide a way of studying a molecular system as it evolves from an initially prepared stationary state to a second, final state. The specific example analyzed here is three-level free induction decay, in which the time-dependent gain of a Doppler-broadened molecular transition is probed after the sudden termination of an intense field resonating with a coupled transition. The theoretical calculation is based on the coupled density-matrix equations of motion in the slowly-varying envelope approximation. The time-delayed line shapes, which may be studied in either transmission or side fluorescence, exhibit linewidth asymmetries, line-shape deformations, Ramsey-type fringes, power broadening and dephasing, and dynamic Stark splittings and oscillatory decays. The technique provides a unique way of distinguishing the influence of Raman-type processes from that of population saturation and a means to separately measure the associated decay rates. The relationship of the present work to other studies is also discussed.
We propose a novel pulsed optical method based on saturated absorption spectroscopy, where the pump beam is periodically switched on and off using pulse amplitude modulation. This periodic alternation enables time-division multiplexing of the same probe beam into both probe and reference signals, achieving differential Doppler-free spectroscopy with a deterministic zero background. This method effectively mitigates Doppler broadening and common-mode optical noise by addressing disturbances such as nonuniform background absorption and environmental noise, thereby offering enhanced accuracy and robustness. Using this technique, we measured the transition frequency of Yb+{\mathrm{Yb}}^{+}Yb+ isotopes in the $^{2}S_{1/2}\ensuremath{\rightarrow}^{2}P_{1/2}$ transition. By employing an error signal derived from the dispersive signal of 174Yb+^{174}\mathrm{Yb}^{+}174Yb+, we achieved efficient stabilization of a 369.5-nm ultraviolet diode laser, demonstrating a frequency stability of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ (26 kHz) over a 1500-s averaging period and a locking point standard deviation of 850 kHz sustained over 10 days. Furthermore, we report the first in situ observation of Doppler-free Zeeman sublevel spectra in a hot ion system, highlighting the precision of this method and its potential application in measuring magnetic field gradients.
Photon-echo modulation spectroscopy is extended into a new regime to study coherences between states that are widely separated in frequency compared with the bandwidth of the exciting laser pulses. Time-delayed four-wave mixing (TDFWM) is observed on the Na DDD doublet by use of 7-nsec pulses of light spectrally peaked at each of the DDD-line transitions. As the two double-frequency, 7-nsec light pulses are delayed with respect to each other a 1.9-psec beating of the integrated TDFWM signal is observed, even for delays greater than the excitation pulse width.
We study theoretically ultrafast modulation spectroscopy in Doppler-broadened cascading three-level and four-level systems. The pump beams consist of two frequency components that drive two-pathway excitation. The four-wave mixing (FWM) signal exhibits damping oscillation as the relative time delay τ between the two pump beams is varied, from which the resonant frequencies between energy levels can be deduced. Furthermore, Doppler-free precision in the measurement of the resonant frequencies can be achieved when the pump beams have either narrow-band or broadband linewidths. We study two-photon resonant FWM induced by time-delayed correlated fluctuating fields in a Doppler-broadened cascading three-level system. It is related intrinsically to the sum-frequency trilevel echo when the pump beams have broadband linewidths. However, unlike the corresponding incoherent-light stimulated photon echo, the maximum of the FWM signal is shifted from τ≃0. If we consider the fluctuations of the pump beams, the FWM signal exhibits damping oscillation when the laser frequency is off resonant from the two-photon transition.