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Optics Letters

Observation of coherent transients by use of current switching of a semiconductor diode laser

R. M. Macfarlane, Miao Zhu

Published Feb 15, 1997• 16 citations
Publisher

Abstract

We have observed optical free induction decay on the (3)H(6)(1) ? (3)H(4)(1) transition of Tm(3+) ions in LaF(3), using an external-cavity diode laser. The diode-laser frequency was switched by the application of a current step, and the resulting transient was detected by heterodyne beating with the laser at its new frequency. This provides a simple new way to measure coherent transients. The beat note shows a frequency chirp that gives a real-time measure of the diode-laser frequency. This result is attributed to changes in the laser lattice temperature on the microsecond time scale.

DOI
10.1364/ol.22.000248

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We have observed optical free induction decay on the (3)H(6)(1) ? (3)H(4)(1) transition of Tm(3+) ions in LaF(3), using an external-cavity diode laser. The diode-laser frequency was switched by the application of a current step, and the resulting transient was detected by heterodyne beating with the laser at its new frequency. This provides a simple new way to measure coherent transients. The beat note shows a frequency chirp that gives a real-time measure of the diode-laser frequency. This result is attributed to changes in the laser lattice temperature on the microsecond time scale.
A simple method for observing coherent optical-transient phenomena is described. Effects such as photon echoes and free induction decay (FID) should allow studies of the dynamic properties of atoms, molecules, and solids on a time scale ranging from milliseconds to \ensuremath{\sim} 50 ps. The frequency of a cw dye laser is switched with an intracavity electro-optic modulator, producing in an external sample coherent transients which are detected in the forward beam. Measurements of long dephasing times are possible, as in FID, because the laser can oscillate indefinitely at the new frequency following a step-function switching pulse. Measurements of short decay times are also practical because the switching time is not restricted by the laser gain or cavity ringing time. In addition, the dye's dephasing time of a few picoseconds or less is too rapid to interfere with these observations. Since the pulse sequence can be preselected, the entire class of coherent optical transients is accessible. The method thus incorporates all of the advantages inherent in Stark switching, including heterodyne detection and high sensitivity, without being restricted to Stark-tunable systems. Laser frequency switching is discussed also in terms of a theoretical model which exposes the time-dependent properties of a phase-modulated cavity mode. Dephasing studies have been carried out thus far in atomic and molecular gases and low-temperature solids containing inorganic impurity ions or large organic molecules. The utility of the FID effect is emphasized where precise dephasing times are obtained from time-frequency Fourier transformation of FID signals using a digital computer. The first quantitative and detailed test of FID theory is provided by these experimental techniques.
We describe transient spectroscopic measurements with a current-switched semiconductor diode laser. Following a fast pulse modulation in the injection current of the diode laser, the diode laser frequency is switched by a few hundred MHz and, then, is scanned over 10 GHz range in a short-time evolution (<0.5 µs). Free-induction decay based on the frequency switching and transient absorption spectroscopy of rubidium D1 lines based on the fast frequency-scan are experimentally demonstrated. These fast spectroscopic measurements are accomplished in a single-shot pulse modulation and are, thus, applicable to short-lived samples such as in flowing or combustion systems.
We report observation of coherent transient amplification that is due to free-induction decay and optical nutation in inhomogeneously broadened rubidium atoms by a sudden switch of diode-laser frequency. An amplification coefficient of 2.2 x 10(-2) cm(-1) is observed when the diode-laser frequency, initially tuned to the blue side of the Doppler-broadened absorption profile, is switched to be out of resonance by application of a step-function pulsed injection current. The transition from free-induction decay to optical nutation is observed and discussed.
By extending the laser-frequency-switching technique to a 100-psec time scale, we have observed for the Na D1{D}_{1}D1​ line the first-order free-induction decay, its inhomogeneous dephasing time T2∗{{T}_{2}}^{*}T2​∗, its interference with the nonlinear free-induction decay, and a 1.8-GHz interference beat of the ground-state hfs. Detailed theoretical predictions of these new coherence effects are faithfully observed.
Semiconductor diode lasers have been widely used in atomic physics and quantum optics. Advanced semiconductor lasers have the capability of modulation in intensity or optical frequency at the rate greater than a few GHz by modulating the driving current. This characteristics makes semiconductor laser diodes ideal light sources in the study of optical coherent transient phenomena with the time scale ranging from few milliseconds to sub-nanoseconds. In this paper, we discuss the applications of semiconductor lasers in coherent transient measurements and report our experimental results.
A novel form of laser frequency switching is devised that extends coherent optical transient studies to a 100-picosecond time scale, the measurements being performed in real time. Free-induction decay (FID) on a subnanosecond time scale reveals new features, such as a first-order FID that dephases with the inhomogeneous dephasing time T2* and interferes with the well-known nonlinear FID. A complete analytical expression for optical FID is derived and supports our FID observations for the sodium D1transition.