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Journal of Physical Chemistry & Biophysics

Superluminal ring laser as a Holy-Grail of precision metrology: Rotation sensing, accelerometry, fiber-optic sensing and gravitational wave detection

M. S. Shahriar

Published Oct 29, 2014• 0 citations
OpenAlex

Abstract

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.

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Techniques to control dispersion in a medium have attracted much attention due\nto potential applications to devices such as ring laser gyroscopes, interferometric\ngravitational wave detectors, data buffers, phased array radars and quantum information\nprocessors. Of particular interest is an optical resonator containing a medium with an\nanomalous dispersion corresponding to fast-light, which behaves as a White Light\nCavity (WLC). A WLC can be tailored to improve the sensitivity of sensing devices as\nwell as to realize an optical data buffering system that overcomes the delay-bandwidth\nproduct of a conventional cavity.\nThis dissertation describes techniques to tailor the dispersion for fast-light in\nintracavity media. We present first a demonstration of fast-light in a photorefractive\ncrystal. When placed inside a cavity, such a medium could be used to enhance the\nbandwidth of a gravitational wave detector. We then describe how a superluminal laser\ncan be realized by adding anomalously dispersive medium inside a ring laser. We\nidentify theoretical conditions under which the sensitivity of the resonance frequency to a change in the cavity length is enhanced by as much as seven orders of magnitude. This\npaves the way for realizing a fast-light enhanced ring laser gyroscope, for example. This\nis followed by the development of a novel data buffering system which employs two\nWLC systems in series. In this system, a data pulse can be delayed an arbitrary amount\nof time, without significant distortion. The delay time is independent of the data\nbandwidth, and is limited only by the attenuation experienced by the data pulse as it\nbounces between two high-reflectivity mirrors. Such a device would represent a\nsignificant breakthrough in overcoming the delay-time bandwidth product limitation\ninherent in conventional data buffers.\nWe then describe our experimental effort to create a fiber-based WLC by using\nstimulated Brillouin scattering (SBS). Experimental results, in agreement with our\ntheoretical model presented here, show that the WLC effect is small under the conditions\nsupported by current fiber optic technology. We conclude that future efforts to induce a\nlarge WLC effect would require fibers with high Brillouin coefficient and low\ntransmission loss, as well as optical elements with very low insertion loss and high\npower damage thresholds.

•Oct 12, 2010•0 citations
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.
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.
Superluminal lasers operate under the condition in which the average group velocity in the cavity exceeds the vacuum speed of light, or alternatively, the group index is between 0 and 1 [1]. It has been shown in previous works that the sensitivity of the lasing frequency of such a laser with respect to a change in its optical roundtrip due to an external perturbation is expected to be substantially larger than that of a conventional laser, with an enhancement factor which is inversely proportional the group index [2]. This property makes superluminal lasers highly attractive for various sensing applications, primarily for challenging tasks such as gravitational wave detection and navigation grade rotation sensing. The realization of superluminal lasers requires the presence of a narrow dip in the center of a broad gain profile. The depth and width of the dip must be tailored carefully to produce the required dispersion profile yielding the enhanced sensitivity.
We investigate the feasibility of realizing a superluminal ring laser with enhanced sensitivity by using the gain produced via double-Raman pumping in an inverted three-level system. If the laser cavity is tuned to the center between the two gain peaks produced in such a system, it is expected to experience an effective negative dispersion, which can be tuned to the condition necessary for enhancing the sensitivity of the laser frequency to a change in the cavity length. Using a model of two idealized, independent Lorentzian gains, we solve the laser equations to show the enhancement of sensitivity. We then extend our analysis to calculate the gain profile in a real lambda system. Unlike the idealized system mentioned above, where the dual gain peaks are modeled as two adjacent Lorentzians in a manner that ignores the relative coherence between the Raman pumps, we present a more accurate model where this coherence is not neglected. As such, the Hamiltonian remains time dependent after making the rotating wave transformation. Using an approximation --- valid for practical systems --- where higher order terms in the density matrix are neglected, we determine the gain experienced by a weak probe beam. Numerical techniques are needed to incorporate the resulting gain and dispersion profiles into the laser equation to determine accurately the behavior of the superluminal laser. This work will be carried out in the near future. Experimental realization of a superluminal laser based on this approach may prove to be easier than other approaches proposed previously.
Techniques to control dispersion in a medium have attracted much attention due\nto potential applications to devices such as ring laser gyroscopes, interferometric\ngravitational wave detectors, data buffers, phased array radars and quantum information\nprocessors. Of particular interest is an optical resonator containing a medium with an\nanomalous dispersion corresponding to fast-light, which behaves as a White Light\nCavity (WLC). A WLC can be tailored to improve the sensitivity of sensing devices as\nwell as to realize an optical data buffering system that overcomes the delay-bandwidth\nproduct of a conventional cavity.\nThis dissertation describes techniques to tailor the dispersion for fast-light in\nintracavity media. We present first a demonstration of fast-light in a photorefractive\ncrystal. When placed inside a cavity, such a medium could be used to enhance the\nbandwidth of a gravitational wave detector. We then describe how a superluminal laser\ncan be realized by adding anomalously dispersive medium inside a ring laser. We\nidentify theoretical conditions under which the sensitivity of the resonance frequency to a change in the cavity length is enhanced by as much as seven orders of magnitude. This\npaves the way for realizing a fast-light enhanced ring laser gyroscope, for example. This\nis followed by the development of a novel data buffering system which employs two\nWLC systems in series. In this system, a data pulse can be delayed an arbitrary amount\nof time, without significant distortion. The delay time is independent of the data\nbandwidth, and is limited only by the attenuation experienced by the data pulse as it\nbounces between two high-reflectivity mirrors. Such a device would represent a\nsignificant breakthrough in overcoming the delay-time bandwidth product limitation\ninherent in conventional data buffers.\nWe then describe our experimental effort to create a fiber-based WLC by using\nstimulated Brillouin scattering (SBS). Experimental results, in agreement with our\ntheoretical model presented here, show that the WLC effect is small under the conditions\nsupported by current fiber optic technology. We conclude that future efforts to induce a\nlarge WLC effect would require fibers with high Brillouin coefficient and low\ntransmission loss, as well as optical elements with very low insertion loss and high\npower damage thresholds.