1. Field of the Invention
This invention relates to gyroscopes and more particularly to improvements in laser gyroscopes.
2. The Prior Art
One of the most dramatic recent developments in optical technology is the laser gyroscope, which combines the properties of the optical oscillator, the laser, and general relativity to produce an integrating rate gyroscope.
The laser gyroscope measures path differences of less than 10.sup.-.sup.6 A, and frequency changes of less than 0.1 Hz (a precision of better than one part in 10.sup.15) in order to read rotation rates of less than 0.1.degree. per hour. The conventional instrument is simply a laser that has three or more reflectors arranged to enclose an area. The three mirrors, together with the light-amplifying material in the laser path, comprise an oscillator (laser). In fact, there are two oscillators, one that has energy travelling clockwise, and one that has energy travelling counterclockwise around the same physical cavity.
The frequencies at which these oscillators operate are determined by the optical path length of the cavity they travel. In order to sustain oscillation, two conditions must be met: The gain must be equal to unity at some power level set by the amplifying medium, and the number of wavelengths in the cavity must be an exact integer (that is, the phase shift around the cavity must be zero). If the first condition is to be achieved, the laser frequency must be such that the amplifying medium has sufficient gain to overcome the losses at the reflectors and other elements in the laser path. In addition, the wavelength must be an exact integer for the path around the cavity. This last condition actually determines the oscillation frequency of the laser.
When the enclosed ring is rotated in inertial space, the clockwise and counterclockwise paths have different lengths. The path difference in these two directions causes the two oscillators to operate at different frequencies. The difference is proportional to the rate at which the ring is rotating since path difference is proportional to inertial rotation rate. The readout of the gyroscope is accomplished by monitoring the frequency difference between the two lasers.
The laser gyroscope assembly consists of the gain media, the reflectors defining the path and enclosed area, and a readout device for monitoring the difference between the two oscillators.
For measuring small length changes the use of an optical oscillator was proposed, in which the cavity dimensions and lengths determined the oscillation frequency. In this manner a small length change is transformed into an easily measured frequency difference between oscillators. The laser gyroscope uses two oscillators at high frequencies (3-5 .times. 10.sup.14 Hz). Exact frequencies are determined by the length of the two cavities, one for clockwise and the other for counterclockwise travelling radiation.
The laser oscillator operates at light frequencies and, as in all oscillators, it must have a gain mechanism arranged in such a way that the losses are compensated for. It must also operate at a frequency controlled so that the phase shift for a trip around the cavity is zero.
In addition to the oscillator conditions of gain and loss, the condition of zero phase shift must also exist. Another way of saying this is that the number of wavelengths in the cavity must be equal to an integer. In the laser gyroscope this integer is several millions. Many frequencies will satisfy these conditions of zero phase, but they are separated by an amount equal to c/L (the speed of light, c, divided by the total length of the cavity, L). For a total length of one meter the frequency separation is 300 MHz.
The length differences in the two paths due to inertial rotation rates cause a difference in the frequency of these two oscillators, whereas physical changes in length caused by temperature, vibration, etc., do not cause frequency differences. The fundamental boundary condition is that the laser wavelength, .lambda., must be equal to an integer fraction of the optical length around the cavity. Stated another way, the length of the cavity is equal to an integral number of wavelengths. N is an integer typically in the range of 10.sup.5 to 10.sup.6, and EQU L = N.lambda. 1
a length change of .DELTA. L will cause a wavelength change ##EQU1## The corresponding frequency change, .DELTA. f, is given as ##EQU2## Therefore, given small length differences .DELTA. L and reasonable cavity lengths L, the operating frequency should be as high as possible.
The relation between inertial input rates, .omega., and apparent length change .DELTA. L has been given as ##EQU3##
The relation between .DELTA.f and .omega., in terms of the gyroscope size and wavelength, is determined by substituting Eq. (3) into Eq. (4), giving ##EQU4##
This concept forms the basis for the recent development in conventional laser gyroscopes wherein the apparent change in length of the laser cavity of a ring laser manifests itself as a shift in the laser frequency and the development of a beat frequency between counterdirectionally travelling laser radiation. Beat frequency is measurable to provide an indication of the rate of angular rotation of the laser cavity about an axis.
Additional useful discussion of some of the basic theories involved in the laser gyroscope may further be found in IEEE SPECTRUM "The Laser Gyro," Joseph Killpatrick, pages 44-55 (October 1967).
From the foregoing relationship, equation 5, it is readily observable that in order to increase the beat frequency, .DELTA. f, for a particular rotation rate, it is necessary to increase the area, A.
Increasing the size of the area circumscribed by a ring laser cavity has certain limits as far as practical application of the laser gyroscope is concerned. In particular, enlarging the area circumscribed by a laser cavity machined from a solid block of quartz becomes impractical beyond the current laser gyroscope which is generally commercially fabricated from a solid block of quartz. In this event, three individual ring laser cavities are machined in the quartz, one for each of the three axes to give the laser gyroscope a three-dimensional capability. Quartz is the preferred material of construction because of its low coefficient of thermal expansion and, therefore, for larger sizes of quartz structure the cost becomes increasingly greater.
Other limitations include such factors as, for example, accommodating the large gyroscope size in the vehicle in which it is placed, temperature fluctuations experienced by the gyroscope support structure (thus the preference for quartz), and other changes caused by local support disturbances such as microseisms, etc. These factors are of significance since the improved laser gyroscope of this invention is also useful as a velocity measuring device and for the measurement of extremely small rotational rates and rotation rate changes, for example, those experienced in the measurement of polar wobble, earth tides, continental drift, length of day variations, etc.
With respect to these lower rotation rates, another important consideration is an effect referred to in the art as "pulling" which is the phenomena experienced when the beat frequency is less than about 100 Hertz (Hz). Pulling manifests itself in fluctuating frequencies independent of the rotation rate and is an inherent deficiency of the conventional laser gyroscopes. Pulling can be minimized by either increasing the area circumscribed by the laser cavity as previously discussed or by externally increasing the rotation rate by a known amount and thereafter compensating for the known rotation rate when determining the true rotation rate.
In view of the foregoing, what is needed is another and relatively simple device to increase the effective area counter-directionally circumscribed by the laser radiation to thereby provide an increased measurable difference between the clockwise and counterclockwise laser radiation per unit of angular rotation of the area.