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.
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.
The spatial coherence of a light source can be easily controlled by varying the source size. The temporal coherence of a light source, on the other hand, has proved to be much more difficult to control. The dependence of the laser diode emission wavelength on temperature offers an opportunity for direct control of their temporal coherence. By modulating the Injection current, temperature fluctuations in the semiconductor junction produce a controlled wavelength drift In the emitted radiation. The amount of wavelength drift can be varied by regulating the depth and period of the modulation which over the integration period translates into variable spectral bandwidths. Thus, a wide range of temporal coherence lengths can be synthesized by appropriately modulating the laser diode current. Possible applications of the technique are described.
Optical Transient Experiments allow one to probe the coherent interaction of light with matter, as well as measure the time for energy transfer, T1, and loss of phase coherence, T2, in atoms, molecules and solids. Coherent optical transients are induced when the interaction of light with the sample is changed in a time which is short compared to T2. The interaction can be modified by changing the amplitude, frequency or phase of the existing laser light. This can be accomplished with an electro-optic modulator which makes it possible to electronically control a wide variety of laser experiments. The method of electro-optic laser phase and frequency switching with an extra-cavity modulator is discussed below.
We demonstrate an optical ranging technique using stationary light implemented on a semiconductor laser diode operating below threshold, with the required time-of-flight information extracted from thermal photon bunching. In contrast, conventional ranging techniques require a light source to be timing modulated in intensity, phase, or frequency, with which to correlate the reference and probe-to-target beams. This reference-probe correlation provides for time-of-flight information from which the target distance can then be measured. Presently, quantum ranging schemes derive timing correlation information from spontaneous parametric down-conversion sources which need not be timing modulated [1] , [2] . However, such light sources are complex and expensive to construct.
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.
A series of simple and low-cost devices for switching, amplifying, and chirping diode lasers based on current modulation are presented. Direct modulation of diode laser currents is rarely sufficient to establish precise amplitude and phase control over light, as its effects on these parameters are not independent. These devices overcome this limitation by exploiting amplifier saturation and dramatically outperform commonly used external modulators in key figures of merit for quantum technological applications. Semiconductor optical amplifiers operated on either rubidium D line are recast as intensity switches and shown to achieve ON:OFF ratios >106 in as little as 50 ns. Current is switched to a 795 nm wavelength (Rb D1) tapered amplifier to produce optical pulses of few nanosecond duration and peak powers of 3 W at a similar extinction ratio. Fast rf pulses are applied directly to a laser diode to shift its emission frequency by up to 300 MHz in either direction and at a maximum chirp rate of 150 MHz ns-1. Finally, the latter components are combined, yielding a system that produces watt-level optical pulses with arbitrary frequency chirps in the given range and <2% residual intensity variation, all within 65 ns upon asynchronous demand. Such systems have broad application in atomic, molecular, and optical physics and are of particular interest to fast experiments simultaneously requiring high power and low noise, for example, quantum memory experiments with atomic vapors.
The art of imparting information onto a light wave by optical signal modulation is fundamental to all forms of optical communication. Among many schemes, direct modulation of laser diodes stands out as a simple, robust, and cost-effective method. However, the simultaneous changes in intensity, frequency, and phase have prevented its application in the field of secure quantum communication. Here, we propose and experimentally demonstrate a directly phase-modulated light source which overcomes the main disadvantages associated with direct modulation and is suitable for diverse applications such as coherent communications and quantum cryptography. The source separates the tasks of phase preparation and pulse generation between a pair of semiconductor lasers leading to very pure phase states. Moreover, the cavity-enhanced electro-optic effect enables the first example of subvolt half-wave phase modulation at high signal rates. The source is compact, stable, and versatile, and we show its potential to become the standard transmitter for future quantum communication networks based on attenuated laser pulses.