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Annual Meeting Optical Society of America

Sagnac spectroscopy

Jean‐Claude Diels, Scott A. Diddams

Published Jan 1, 1992• 0 citations
Publisher

Abstract

In a passively mode-locked femtosecond ring dye laser the two counterpropagating pulses have a relative frequency difference Av that can be adjusted from a few hertz to megahertz by rotation of the cavity. These opposing pairs of pulses can thus be used to establish coherence between sublevels of a A transition in a resonant vapor. Sagnac spectroscopy is the distortion of the (previously linear) Sagnac response of the laser when such an atomic vapor (Sm) is inserted in the cavity. Even though the bandwidth of this unstabilized laser is of the order of 1013 Hz, spectroscopic measurements can be performed with 10 Hz accuracy. Applications include magnetic field measurement, lasing without inversion, and giant index of refraction.

DOI
10.1364/oam.1992.fz7

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•Jan 1, 1978•0 citations
In a passively mode-locked femtosecond ring dye laser the two counterpropagating pulses have a relative frequency difference Av that can be adjusted from a few hertz to megahertz by rotation of the cavity. These opposing pairs of pulses can thus be used to establish coherence between sublevels of a A transition in a resonant vapor. Sagnac spectroscopy is the distortion of the (previously linear) Sagnac response of the laser when such an atomic vapor (Sm) is inserted in the cavity. Even though the bandwidth of this unstabilized laser is of the order of 1013 Hz, spectroscopic measurements can be performed with 10 Hz accuracy. Applications include magnetic field measurement, lasing without inversion, and giant index of refraction.
Summary form only given. The fact that the two counterpropagating pulses in a modelocked dye ring laser can be completely decoupled in frequency makes such a laser a sensitive probe of phase differences between the counterpropagating pulses. The difference in center frequency between the two counterpropagating pulses can be resolved down to the noise limit of 100 Hz. This limit corresponds to a phase difference between the counterpropagating pulses of 10/sup -6/ or an effective cavity length difference of 10/sup -12/ m. When combined with fluorescence measurements and the intensities of the counterpropagating fields, the steady-state elements of the density matrix describing the intracavity system can be inferred.
Synchronous mode locking of two dye lasers was employed to obtain pairs of independently tunable picosecond pulses that were precisely synchronized in time. Overlapped pulses in strontium vapor were used to generate tunable picosecond pulses in the vacuum uv. The tunability was employed to study resonant enhancement of a four-wave parametric process by the 4d6p autoionizing resonance. Mutually separated pulses were employed to measure the build-up and decay of coherence for a two-quantum excited state. The decay was found to be faster than theoretical predictions that assume only velocity dephasing.
Resonant interactions of a mode-locked pulse train with intracavity samples, namely rubidium-87 (Rb-87) vapor and Fabry-Perot etalon, placed inside a laser cavity are studied in the light of developing ultra-sensitive laser sensors to measure a small magnetic field and a minute change of index of refraction of a sample material, respectively. A Rb-87 vapor provides an opportunity for a compact high-sensitivity atomic magnetometer due to its accessibility by the standard laser sources and to the large ensemble magnetization. By employing the ultrasensitive interferometric technique utilizing the intracavity properties of a mode-locked laser, the performance of the Rb-87 magnetometer can be further improved. The fundamental properties of coherent interaction between a mode-locked pulse train and a Rb-87 vapor are studied in numerical calculations of 3 x 3 density matrix equations and the reduced wave equation, which are then examined in experiments. In particular, a coherent dark-state is created by the pulse train and is further enhanced by means of spectral shaping or polarization modulation of the excitation pulse train. Experiments performed inside a laser cavity show that the atomic coherence is still preserved due to the coherent nature of interaction between the Rb-87 vapor and the ultrashort pulses occurred within a short time scale compared to the atomic relaxation times, which results in nonlinear propagation of the pulses as well as an observation of the dark-line resonance inside the laser cavity. A Fabry-Perot etalon is a type of optical cavity and serves as a tuning element of the frequency of cw-lasers. By inserting a Fabry-Perot etalon inside a mode-locked laser, the cavity resonance modes are modified due to a coupling between the two cavities, which leads to unique temporal and spectral characteristics of the resultant pulse train and its frequency comb. Both the temporal and spectral properties of the pulse train are studied in detail in experiments as well as in numerical calculations. In particular, it will be shown that the coupling between low frequency modes from the laser repetition rate and high frequency modes from the Fabry-Perot pulse train can be exploited for a new interferometric technique to measure a minute change in the index of refraction of the etalon.
A technique of differential intracavity phase spectroscopy is introduced. The measured beat frequency between the two counterpropagating fields in a ring laser directly yields the differential phase shift due to the presence of an atomic vapor. In an unstabilized laser cavity, phase differences near ${10}^{\ensuremath{-}6}$ are measured. This technique is used to study the nonlinear dispersion associated with a three-level $\ensuremath{\Lambda}$ system in atomic samarium. Sub-Doppler resonances and population trapping are observed, both of which are attributed to the coherences between the two lower levels of the $\ensuremath{\Lambda}$ level structure. Solutions of the density matrix describing the system are also presented, and good qualitative agreement is found between the calculations and the measurements.
We demonstrate an experiment which utilizes a Sagnac interferometer to measure a change in optical frequency of 129 $\ifmmode\pm\else\textpm\fi{}$ 7 kHz/Hz\text{kHz}/\sqrt{\text{Hz}}kHz/Hz​ with only 2 mW of continuous-wave, single-mode input power. We describe the measurement of a weak value and show how even higher-frequency sensitivities may be obtained over a bandwidth of several nanometers. This technique has many possible applications, such as precision relative frequency measurements and laser locking without the use of atomic lines.
I. A new technique utilizing the electro-optic frequency switching of a single mode ( 6 Mhz) dye laser was used to observe free induction decay and optical nutation in iodine gas and optical nutation in an organic mixed crystal (pentacene in p-terphenyl) at low temperature. A new technique, incoherent resonance decay (IRO) was also developed. These experiments allow one to determine T_1 and T_2, the longitudinal and transverse relaxation times of the excited ensemble, in analogy with magnetic resonance. For I_2(g) at O∙C; T_l = 2.3 x 10^(-7) sec, T_2 ≃ 1 x 10^(-7) sec. These techniques are limited to a time scale > 10 nsec. II. The construction and characterization of cw, mode-locked dye laser capable of putting out a continuous train of reproducible sub-picosecond pulses is described. Experiments utilizing this laser for the measurement of picosecond coherent transients and time resolved absorption spectroscopy are proposed.