1. Field of the Invention
The present invention relates to path length control apparatus (PLC) for optical devices and in particular to a physical design of the apparatus that removes the likelihood of imparted stresses on a mirror resulting in a flatter surface for reflecting a laser beam of a ring laser gyroscope (RLG).
2. Description of the Related Art
A ring laser gyroscope (RLG) is commonly used to measure the angular rotation of an object, such as an aircraft. Such a gyroscope has two counter-rotating laser light beams that move within a closed loop optical path or “ring” with the aid of successive reflections from multiple mirrors. The closed path is defined by an optical cavity that is interior to a gyroscope frame or “block.” In one type of RLG, the block includes planar top and bottom surfaces that are bordered by six planar sides that form a hexagon-shaped perimeter. Three planar non-adjacent sides of the block form the mirror mounting surfaces for three mirrors at the corners of the optical path, which is triangular in shape.
Operationally, upon rotation of the RLG about its input axis (which is perpendicular to and at the center of the planar top and bottom surfaces of the block), the effective path length of each counter-rotating laser light beam changes and a frequency differential is produced between the beams that is nominally proportional to angular rotation. This differential is then optically detected and measured by signal processing electronics to determine the angular rotation of the vehicle. To maximize the signal out of the RLG, the path length of the counter-rotating laser light beams within the cavity must be adjusted. Thus, RLGs typically include a path length control apparatus (PLC), the purpose of which is to control the path length for the counter-rotating laser light beams for maximum output signal.
FIG. 1 illustrates one such known PLC 10 for a laser block assembly (LBA) 12 of an RLG, such as that described as prior art in U.S. Pat. No. 6,515,403, herein incorporated by reference. This PLC 10 includes a piezoelectric transducer (PZT) 16 which is secured to a mirror 18 via an epoxy-based adhesive 20. The epoxy adhesive 20 completely covers the interface (defined by a lower surface 22 of the PZT 16 and an upper surface 24 of the mirror 18 between the PZT 16 and the mirror 18. The mirror 18 is secured to a mirror mounting surface 26 of the optical LBA 12. The mirror 18 communicates with laser bores 32 (only partially shown) of an optical cavity 34 (only partially shown) of the LBA 12. The bores 32 form a portion of the closed loop optical path 38 defined by the optical cavity 34. As seen in FIG. 1, the mirror 18 reflects counter-rotating laser light beams 40 at its respective corner of the closed loop optical path 38.
A conventional PZT 16 (FIGS. 2 & 3A) is defined by a pair of piezoelectric elements 42 and 44. The PZT 16 takes an applied voltage delivered by a regulated voltage source (not shown), in response to a signal provided by a detector (not shown) that monitors the intensity of the light beams 40, and turns this voltage into small but precisely controlled mechanical movement in a direction perpendicular to a top surface of the PZT 16. This mechanical movement of the PZT 16 affects translational movement of the mirror 18, and thereby controls the laser light beam path length.
FIG. 3B illustrates a known multi-layered PZT, such as that described by U.S. Pat. No. 6,515,403, in which a stack of alternating negative and positive co-fired ceramic layers is provided. Co-fired ceramic layers are those that are “fired”, when they are made, together, as opposed to being made separately and then later bonded together. These may then form a resultant multilayered stack.
This structure may include a top layer 62, a bottom layer 68, and alternating negative 64 and positive 66 layers. This multi-layer PZT 16 has contacts, which are electrically connected to other layers within the multi-layer PZT 16, formed directly on the top layer of the PZT 16, and the regulated voltage source can be coupled directly to the PZT 16 at the top layer contacts. The multi-layer PZT 16 includes a plurality of ceramic layers 62, 64, 66, 68 so as to form a stack in which each ceramic layer has first and second opposing surfaces.
The plurality of ceramic layers includes a top layer 62 at a first end of the stack having a top conductive pattern formed on its first surface. The top conductive pattern includes a negative contact and a positive contact.
The plurality of ceramic layers 62, 64, 66, 68 also includes at least one poled ceramic layer 64 having a conductive pattern formed on its first surface. The plurality of ceramic layers 62, 64, 66, 68 include additional poled ceramic layers 66, 68 having alternating conductive patterns formed on the first surface thereof. In such a multi-layer configuration, the layers are more tightly coupled to the mirror since they lack extra epoxy layers. Almost all the distortion in the ceramic is directly imparted into the mirror 18.
Sometimes, with conventional PZTs 16 in which the PLC driver is bonded directly to the transducer mirror, curvature in the mirror due to stresses or other factors may cause multimoding of the laser beam. In multi-layer PZTs 16, this occurs more often, i.e., in approximately 30–50% of the LBAs. This is particularly true, e.g., because only thin layers 20 (such as 0.0005″ to 0.001″) of epoxy are typically used to attach the mirror 18 to the driver. This multimoding interferes with the laser mode that the LBA uses to get an accurate count data (and therefore navigation data).
The problem of multimoding is described in more detail below.
Multimoding occurs when a higher order transverse mode becomes resonant with the fundamental TEM00 (Transverse Electro-Magnetic) mode. The fundamental TEM00 mode is characterized by an intensity distribution, which can mathematically be described by a Gaussian function centered on the direction of propagation. Mathematically, the intensity distribution is
      I    ⁡          (              x        ,        y            )        =            I      0        ⁢          exp      ⁡              [                  -                                                    x                2                            +                              y                2                                                                    ω                ⁡                                  (                  z                  )                                            2                                      ]            
Here I0 is the intensity in W/cm2 at the center of the beam and ω is the 1/e2 intensity radius. ω is a function of the distance, z, from a point of minimum radius called the beam waist.
Higher-order modes, designated TEMmn, where m>0 and/or n>0, have a more complicated mathematical description. Briefly, the TEM10 mode can be described as a set of headlights, that is, two spots side-by-side. The TEM01 is similar but rotated by 90°. The TEM11 mode has four spots—essentially two sets of headlights, one on top of the other. Note that all three of these modes have a null point (or zero energy) at the center of the beam. This is characteristic of any mode with an odd index. Modes that have both indices (m and n) being even always have energy at the center of the beam. The mode index can be determined by counting the number of null regions along a particular direction, either x or y.
Higher-order modes have larger spatial areas than do the fundamental mode. Hence an internal body aperture can be used to against them. That is, an aperture of the correct size adds little loss to the fundamental mode, but adds measurable loss to higher-order modes whose beams are spatially offset from the beam of the fundamental mode. The higher the mode numbers, the more loss added. So, for example, in one type of device, the internal apertures add about 10 ppm loss to the fundamental mode and 100 ppm loss to the TEM01 and TEM10 modes.
If, however an aperture diameter were scaled to a larger RLG aperture diameter, then it would be very difficult to build any hardware, since the alignment would be very difficult to achieve with that small of an aperture (e.g., on the order of 0.032″). But a slightly larger aperture diameter provides less than 1 ppm loss for the TEM01 and TEM10 modes. The result is that these modes lase very well and one must attempt to reduce them with an external aperture placed in front of the LIM (Laser Intensity Monitor) sensor.
The LIM aperture discriminates against higher order modes and allows the PLC loop to lock onto the fundamental mode as it is presented to the LIM detector as the most intense mode. It is fortunate that higher order modes lase at a different frequency than does the fundamental. For example, the TEM10 mode lases approximately mid-way between one fundamental mode and another (representing path lengths that differ by one optical wavelength). Hence if the PLC loop is controlling on a fundamental mode, the TEM10 mode will not lase. The TEM01, the TEM20 and the TEM02 modes all are within approximately 0.1 modes from the fundamental. Again, when the loop is controlling on the fundamental mode none of these three will lase.
Thus, the lowest, higher-order transverse modes are discriminated against because they are separated in frequency from the fundamental modes, and other higher-order modes are discriminated against primarily by using the internal body aperture and secondarily by frequency separation.
The problem is that the frequency separation between the fundamental mode and any higher-order mode is a function of mirror curvature. The higher the mode indices, the stronger the function.
The wedge (the mirror having the readout detector on it—which is a second mirror distinguished from the first transducer mirror attached to the cofired driver) is flat, so this does not pose a problem. A curved third mirror's curvature is specified to a narrow range in which there are likely no problems. The piezoelectric drive transducer is nominally flat, but in fact, actually changes curvature.
Problems arise when a higher-order transverse mode has no frequency separation between itself and the fundamental mode. In that situation, the only potential solution is the aperture, and if the aperture doesn't provide sufficient discrimination (i.e., loss), then the higher order mode dominates and essentially causes error in the rate output.
There are 10 transverse modes with indices m and n under 10 which resonate with the fundamental as the transducer radius of curvature changes from −0.2 m to −2.0 m. One of these is the TEM41 mode. Since it has one odd index, it has an intensity null at the center, so when it lases, one would expect the LIM to drop.
For co-fired multi-layer data, FIG. 9 shows an LIM plot (versus temperature) from a run that had bad rate output. Specifically, it shows an LIM v. Temperature LIM plot of LBA with 4-layered cofired driver without pads. Note the LIM spikes upon return from a hot temperature, between the range of 145° F. and 160° F.
The bad rate data was coincident with the LIM spikes observed at temperatures between 145° F. and 160° F. on the return from a hot temperature. Note that these spikes are not simply single data points, but involve a somewhat gradual decrease, followed by an increase.
FIGS. 10 and 11 show mode scans from the same gyro in the same thermal region. The FIG. 10 graph is a mode scan of LBA with 4-layer cofired ceramic. The left most 0—0 mode shows evidence of “multi-moding.” As compared to FIG. 11, here the higher order transverse mode has moved somewhat to the right side of the mode. FIG. 11 is a graph showing a Mode Scan of LBA with 4-layer cofired ceramic. The left most 0—0 mode shows evidence of “multi-moding.”
The left most mode of FIG. 10 indicates that it is lower in intensity than the rest and that there is actually a slight drop at the center. In FIG. 11, the same mode shows an anomaly slightly to the right of the peak. The difference between FIGS. 10 and 11 is that they were taken at different times during which the temperature was dropping. In fact, if one observed the oscilloscope during this period, one could see the anomaly move across the fundamental mode from left to right.
What appears to be happening is that in the condition of FIG. 11, the radius of curvature of the transducer is exactly correct to allow a transverse mode to have exactly the same frequency as the fundamental; hence the transverse mode lases and decreases the intensity of the fundamental mode. It lases because the internal body aperture is not small enough to totally discriminate against this mode. Since the energy of the transverse mode is more spread out spatially than that of the fundamental, the LIM sensor receives less energy through the LIM aperture and therefore the LIM voltage drops. In the condition of FIG. 11, the transducer curvature has changed somewhat as has the frequency separation between the transverse and the fundamental modes. The transverse mode still lases, but only when the PLC loop is detuned sufficiently.
Note that the strength of the LIM drop most probably is a function of the internal body aperture (and the beam alignment within the aperture). If the aperture was larger (and it wouldn't have to be much larger), the higher-order mode would lase more strongly, the fundamental mode would lase less strongly and the LIM would drop further. This cautions against allowing the body aperture to become much larger.
The reason the above effect more readily occurs with co-fired piezoelectric transducer drivers is because the co-fired drivers change the normal curvature of the transducer. Two reasons for this change are 1) stronger coupling between the ceramics and mirror, and 2) the increased “non-flatness” of the co-fired drivers.