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Conference on Lasers and Electro-Optics

Controllable anomalous dispersion and group index nulling via bi-frequency Raman gain for ultraprecision rotation sensing

G. S. Pati, Renu Tripathi, M. Messall, Venkatesh Gopal, K. Salit, M. S. Shahriar

Published Jan 1, 2006• 1 citation
Publisher

Abstract

Using a bi-frequency pumped Raman amplifier in a Rb vapor cell, we demonstrate experimentally the condition for achieving a null group index, necessary for fast-light enhanced rotation sensing in a passive cavity Sagnac gyroscope.
DOI
10.1109/cleo.2006.4629099

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We have recently proposed [9], the use of fast-light media to obtain ultrahigh precision rotation sensing capabilities. The scheme relies on producing a critically anomalous dispersion, in a suitable dispersive medium, which is introduced in the arms of a Sagnac interferometer. We present here an experimental investigation of the anomalous dispersion properties of bi-frequency Raman gain in rubidium vapor, with the goal of using this medium for producing the critically anomalous dispersion condition. A heterodyne phase measurement technique is used to measure accurately the index variation associated with the dispersion. The slope of the negative linear dispersion (or group index) is experimentally varied by more than two orders of magnitude while changing the frequency separation between pump fields, responsible for producing gain. Using this result, we have identified the experimental parameters for achieving a null value of the group index, corresponding to the critically anomalous dispersion condition necessary for enhanced rotational sensitivity.
We describe a resonator-based optical gyroscope whose sensitivity for measuring absolute rotation is enhanced via use of the anomalous dispersion characteristic of superluminal light propagation. The enhancement is given by the inverse of the group index, saturating to a bound determined by the group velocity dispersion. We also show how the offsetting effect of the concomitant broadening of the resonator linewidth may be circumvented by using an active cavity. For realistic conditions, the enhancement factor is as high as 106{10}^{6}106. We also show how normal dispersion used for slow light can enhance relative rotation sensing in a specially designed Sagnac interferometer, with the enhancement given by the slowing factor.
We show that group velocity of light far exceeding the vacuum speed can be realized when a medium that produces a narrow band dip in the gain profile is placed inside a ring laser. The dip leads to an effective negative dispersion, which can be tuned to produce a very small group index. The rotational sensitivity of the ring laser is enhanced by a factor equaling the inverse of the group index. For a realistic system, the enhancement factor can be as high as 1.8*105. In order to realize such a device, the background gain can be produced by using, for example, an optically pumped Ti:Sapphire crystal, a semiconductor optical amplifier, or a diode pumped alkali laser. The narrow dip can be produced, for example, by a Rb cell configured for Raman depletion. Here, we present the theoretical model behind such a superluminal laser, and describe a preliminary experiment for realizing such a device.
In this paper, we present the experimental observation of simultaneous bi-directional superluminal lasing in a triangular ring cavity without gain competition and crosstalk as needed for realizing a gyroscope based on the Sagnac effect. The gain spectrum for each of the lasers is tailored to be a narrow dip on top of a broad gain using two stable isotopes of Rb. Specifically, we make use of 85Rb to produce a broad gain spectrum via the optically pumped Raman gain process and 87Rb to produce a narrow absorption spectrum via the optically pump Raman depletion process. A separate gain cell is used for the laser in each direction. Inferred from the simulation, the spectral sensitivity enhancements of the clock-wise and counter-clock-wise superluminal ring lasers are ∼362 and ∼505, respectively, with the imbalance attributed to differences in pump powers.
A superluminal ring laser (SRL) is a laser in which the group velocity of light far exceeds, by a factor as large as a million, the vacuum speed of light, without violating special relativity or causality. This behavior results from the so-called fast light effect, corresponding to critically tuned anomalous dispersion. The anomalous dispersion is produced by adding inside the ring cavity an auxiliary medium that produces a narrow dip in the broad-band gain profile produced by the main gain medium. In an SRL, the change in the laser frequency, as a function of a change in the cavity length, is enhanced by a factor of nearly a million, when compared to that of a conventional ring laser. This effect can be used for enhancing the sensitivity of many devices, including a gyroscope, and accelerometer, and a fiber optic sensor. It can also be configured to realize a novel type of gravitational wave detector. We have realized an SRL using a buffer gas loaded Rb vapor cell, pumped on the D1 line, as the main gain medium, and an additional Rb vapor cell for producing the anomalous dispersion via Raman induced pump depletion. In this talk, the author will describe the behavior of this SRL, and outline the various applications we are pursuing, including rotation sensing and accelerometry for inertial navigation under GPS-denied conditions, and gravitational wave detection. He will also describe a fiber-optic based SRL, employing dual peaked Brillouin gain, for ultra-sensitive, general purpose fiber optic sensing.