05-10-2012, 04:57 PM
Performance comparison of slow-light coupled-resonatoroptical gyroscopes
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In the second of these papers, the authors
cleared up some errors in the first paper and arrived at two important conclusions. First, the apparent dependence of the group index on the rotation sensitivity is coinciden-tal, not fundamental. Second, rotation sensitivity cannot
be enhanced by replacing a single side-coupled resonator of area A with N smaller resonators (N = 10 in Fig.1a), each having an area A/N.
Two years later, another publication by Scheuer et al. [14] claimed enhanced sensitivity as a result of the use of slow light in high-finesse coupled resonator optical waveg-uides (CROW) that are now coupled to each other, and arranged in the form a Sagnac loop (Fig.1b).
A more recent publication by Steinberg et al. [16] describes a different CROW gyroscope in which the res-onators are not arranged in a Sagnac loop but in a straight line, as shown in Fig.1c.
The authors observed that this device exhibits a rotation-induced bandgap in its transmis-sion, resulting in a transmitted power that is asymptotically a decaying exponential function of rotation rate.
Another recent series of publications by Peng et al. [17,18] point out that by reversing the direction of propagation of light in adjacent rings, so that now light propagates in the same direction in all rings (see Fig.1d), the sensitivity can be further enhanced.
However, as we will show, even when taking this enhancement into account, this unidirectional CROW gyro still does not perform any better than an RFOG once loss is taken into account.
Thereisnobenefit to coupling several resonators together in a gyroscope: for equal footprint and waveguide loss, CROW gyros never ex-ceed the sensitivity of a simple resonant gyro, which is the CROW gyro with
N =1 .
This is because loss ultimately limits the maximum sensitivity of all passive resonant opti-cal gyroscopes, whether they consist of a single resonator or multiple resonators coupled together.
Since coupling multiple resonators together does not offer the potential for sensitivity enhancement, it is better to instead use a single resonator with as little loss as possible.
Gyroscope classification
(1) non-resonant optical gyroscopes, of which the only representative is the FOG;
(2) resonant optical gyroscopes (ROGs), defined as any gyroscope utilizing a single resonant loop, such as a reso-nant fiber optic gyroscope (RFOG); and
(3) gyros utilizing multiple coupled resonant optical waveguides (CROWs), such as the gyros of Fig.1.
It is useful to further divide CROW gyros into two sub-classes: bidirectional and unidirectional.
In each of the three gyroscope categories, the waveg-uide may be provided by one of several technologies, for example optical fiber [19], microspheres [12], or photonic-crystal waveguides [20].
First, because these waveguides can be made with considerably tighter bend radii than a fiber without suffering dominant bending loss (e.g., 100 μmin a microsphere versus ∼1cm in a fiber), a gyro utilizing such a waveguide can have a much smaller footprint than its fiber counterpart.
Second, these waveguides can be fabricated in materials exhibiting a much lower loss than silica, e.g., CaF2 [21].