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Construction and Optimization of a Tapered Amplifier System for Applications in Ultra-Cold Plasma Research

R. A. Cole

Published Jan 1, 2015• 0 citations

Abstract

The number density of cold atoms confined in a magneto-optical trap (MOT) is critically dependent on the intensity of the lasers used to cool the sample. To generate large optical powers while retaining the practicality of homemade external cavity diode lasers (ECDLs), a tapered amplifier (TA) system was designed and constructed to amplify the output of an existing 780 nm, continuous-wave ECDL. The amplifier’s performance is discussed in terms of its gain and power output. Under standard operating conditions, optical amplification of 12 dB is achieved, with a maximum power output of 0.75 W. The completed amplifier is installed into an existing MOT at Colby College, and its effect is analyzed in terms of the number density of trapped samples of rubidium-85. MOT measurements are given demonstrating an increase in these number densities by a factor of ten. We conclude with initial results to quantify the amplifier’s effect on the dynamics of ultra-cold plasmas created from the trapped atom samples.

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The number density of cold atoms confined in a magneto-optical trap (MOT) is critically dependent on the intensity of the lasers used to cool the sample. To generate large optical powers while retaining the practicality of homemade external cavity diode lasers (ECDLs), a tapered amplifier (TA) system was designed and constructed to amplify the output of an existing 780 nm, continuous-wave ECDL. The amplifier’s performance is discussed in terms of its gain and power output. Under standard operating conditions, optical amplification of 12 dB is achieved, with a maximum power output of 0.75 W. The completed amplifier is installed into an existing MOT at Colby College, and its effect is analyzed in terms of the number density of trapped samples of rubidium-85. MOT measurements are given demonstrating an increase in these number densities by a factor of ten. We conclude with initial results to quantify the amplifier’s effect on the dynamics of ultra-cold plasmas created from the trapped atom samples.

We present plans for the construction and operation of a tapered optical amplifier (TA) system seeded by a single-mode, frequency-tunable, near-IR external-cavity diode laser. Our plans include machine drawings for the parts, electronic circuit diagrams, and information on prices and vendors. Instructions are provided on how to safely couple light into and out of the TA chip. Practical aspects of handling the chip are discussed as well. Because many cold atom experiments require light beams with Gaussian spatial profiles, measurements of the tapered amplifier light output through a single-mode optical fiber are presented as a function of seed intensity, polarization, and driving current.

This thesis describes the design, construction and characterization of an external cavity diode laser (ECDL) within the context of AMO research – specifically, ultracold rubidium experiments. The main benefit over other forms of laser light is the ECDL’s low cost and narrow linewidth. Having a more narrow linewidth allows us to affect specific electron states, such as for laser cooling, more precisely than broader alternatives. We find that building such a laser in house is feasible with scan range up to 4 GHz. We also note that attention to external noise, through mechanical vibrations but more importantly through temperature drifts, is necessary to produce an ECDL with a stable optical frequency.

Cold atoms coupled to an optical cavity provide an ideal platform for creating lasers without conventional population inversion. Such lasers can generate very high spectral purity, and the output can be precisely controlled by varying the atom-drive or atom-cavity parameters. In this letter, we report experimental observation of gain, amplification, and self-lasing within a continuously operated 85 Rb magneto-optic trap (MOT), in the collective strong coupling regime of the atom-cavity system. The presence of MOT cooling lasers significantly alters the transmission properties of the cavity, and gain is observed in one of the vacuum Rabi peaks. The system makes a transition from an amplifying medium to a self-sustained laser as the MOT parameters are tuned. The underlying lasing mechanism is shown to be Mollow gain in the driven atomic ensemble, and is confirmed through the free-space absorption spectroscopy of the MOT. The free space gain required for the lasing action is significantly smaller in our system as compared to the cases where there is no collective strong coupling.

In this paper,we report an optical power amplifier operating at 780 nm.The optical power amplifier is a semiconductor tapered amplifier(Eagleyard,EYP-TPA-0780-01000);the output power from the optical power amplifier is up to about 600 mW when a seeding laser beam with a power of 25 mW is injected into the amplifier.After beam shaping by lens,frequency shifting by an acousto-optical modulator(AOM) and spatial filtering by a single-mode fiber,the output power is up to 110 mW.The presented optical power amplifier may be applied into the MOT for achieving a larger number of trapped atoms.

This thesis will describe a cold atom ring laser that uses a cloud of magneto-optically trapped potassium atoms as a gain medium and a ring cavity for feedback. Some upgrades to the experiment were necessary, which include a new imaging axis in the vacuum system, a new atom source, alignment of some new optics and detectors to measure the powers and transverse intensity profiles of both counterpropagating cavity modes, and a new tapered amplifier for the magneto-optical trapping beams. The lasing threshold, gain mechanism, intensity correlations, and lasing into the two counterpropagating cavity modes were investigated. The results showed several transverse patterns at the lasing outputs, a threshold as a function of atom number, Mollow gain, a at second-order coherence above threshold, clearly distinguishable from that of fluorescence, nonreciprocity of the TEM\\(_{00}\\) transverse mode as a function of cavity length, and anticorrelated counterpropagating modes.