Patent Application
20090219545
A fiber optic gyroscope (FOG) uses the interference of light to measure angular velocity. Rotation is sensed in a FOG with a large coil of optical fiber forming a Sagnac interferometer. To measure rotation, two light beams are introduced into the coil in opposite directions by an electro-optic modulating device such as an integrated optical circuit (IOC). If the coil is undergoing a rotation, then the beam traveling in the direction of rotation will experience a longer path to the other end of the fiber than the beam traveling against the rotation. This is known as the Sagnac effect. As the beams exit the fiber they are combined in the IOC, and the phase shift between the counter-rotating beams due to the Sagnac effect and modulation in the IOC causes the beams to interfere, resulting in a combined beam, the intensity and phase of which depends on the angular velocity of the coil.
When testing FOGs using a proton exchanged IOC in a vacuum environment, it has been found that a corruption of the electro-optic modulation occurred and grew with time, eventually rendering the FOG inoperable. The exact phenomenon that corrupts the modulation in FOG output is only partially understood and appears to involve ionic migration along the electric fields near the electrodes of the IOC.
Referring to
Normally, in a LiNbO3 IOC the response of the refractive index to the electric field applied to the electrodes follows the voltage very accurately. However, after soaking in a vacuum, the phenomenon called RDS manifests itself and corrupts the response.
More specifically, the voltage Vφ(t), where t is time, across the electrodes changes the phase of light in a waveguide by Δφ. During normal IOC operation in air, Δφ(t) follows the shape of the trace of Vφ(t) exactly. After the IOC has been in vacuum for a nominal time, instead of following Vφ(t), Δφ(t) is corrupted, as shown in the lower trace as Δφ(t), and overshoots the desired Δφ at both the up and down voltage steps.
In an embodiment, an integrated optical circuit includes a first waveguide portion of a first material. The first waveguide portion includes an input-port section terminating in a junction section of the first waveguide portion from which first and second branch sections of the first waveguide portion are formed. Second and third waveguide portions are respectively coupled to the first and second branch sections. The second and third waveguide portions are diffused with a second material different from the first material. First and second modulators are respectively coupled to the second and third waveguide portions. Each of the modulators provides respective modulating voltages generating respective electric fields across the second and third waveguide portions. The second and third waveguide portions are coupled to the first and second branch sections at respective locations where the modulating electric fields are substantially zero.
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
As will be described more fully hereinafter, an optical circuit of a Sagnac gyroscope according to an embodiment may be comprised of an IOC, a light source, polarizing circulator, detector, and fiber coil. An embodiment may employ crystalline LiNbO3 integrated IOCs useful in navigation grade gyros because an applied voltage changes the refractive index of the LiNbO3. This property provides superior performance in closed loop gyros by allowing fast, accurate, and sophisticated modulation of light transiting the rate-sensing coil.
Early work with LiNbO3 created IOCs by diffusing titanium (Ti) waveguides into LiNbO3 designed to create a y-junction. A fiber containing light from a fiber laser is attached to the single tail of the y, and the two ends of a fiber coil were attached to the two outputs of the y-junction. Low voltage waveforms on the electrodes, parallel and close to the waveguides, change the refractive index of the waveguide and allow precise phase modulation between the clockwise and counterclockwise light beams propagating in the fiber coil.
Using a proton exchange process on LiNbO3 to create waveguides has a distinct benefit: light of only one polarization is transmitted through the IOC. This greatly increases the precision of the fiber optic gyro rate measurement to a level necessary for the most demanding navigation requirements and, in an embodiment, eliminates the need for an external polarizer in the gyro circuit.
Titanium-diffused waveguide portions 225, 230 are respectively coupled to the first and second branch sections 215, 220. First and second modulators, such as electrodes 235, 240, are respectively coupled to the waveguide portions 225, 230. Each of the electrodes 235, 240 provide respective modulating voltages generating respective electric fields. The IOC 115 may further include second and third proton-exchanged waveguide portions 245, 250 coupled to the waveguide portions 225, 230.
An approach to solving the RDS problem for gyroscopes includes, in an embodiment, a method called “stitching.” Stitching involves creating connected segments of Ti-diffused and proton-exchanged waveguides on the same substrate 255.
Referring again to
Additionally, and preferably, the respective locations 270, 275 are approximately halfway between the electrodes 235, 240 and the bent regions 260, 265. As such, the stitching occurs a distance away from the bent regions 260, 265 sufficient to avoid modal transition effects that may occur at the bent regions.
Further advantages to the approach illustrated in
Linearly polarized light propagating along the fast or slow axis of a birefringent material such as LiNbO3 will remain in that axis, as coupling between the axes cannot occur for the reason that it is not possible to phase match the light in both beams simultaneously.
Since waveguides may be physically formed by well known processes for diffusing Ti or H+ along the crystal planes which develop the birefringence in the crystal, the angular alignment between the fast and slow axes of the stitched waveguides is virtually perfect, a property that maintains the very high extinction ratio provided by the proton exchange waveguides.
In anisotropic substances such as a birefringent crystal, electric vectors oscillate normal to the propagation vector in orthogonal planes (H and V). The azimuths and refractive indices of H and V are determined by the stoichiometric arrangement of the molecules comprising the crystal. The refractive index is proportional to the area density of atoms in the respective H and V planes (viz., atoms/mm2); the birefringence is proportional to the difference of the refractive indices along the planes.
for small ε. The extinction ratio is commonly expressed in decibels (dB) as 10·log [ε2].
In the embodiment illustrated in
Moreover, the LiNbO3 crystal planes determine the alignment of both the birefringent axes in Ti-diffused waveguides, and the pass axis of the light in proton-exchanged waveguides. This makes the angular alignment at the stitch nearly perfect, thus avoiding gyro rate errors due to angular misalignments in the IOC.
Additionally, the extinction ratio of the stitched waveguide IOC 115, which includes polarizing proton-exchanged waveguides and Ti-diffused waveguides, is substantially the same as that of a proton-exchanged IOC.
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
