1. Field of the Invention
The present invention relates to a laser. In particular, the present invention relates to a ring laser and a ring laser type optical gyro for detecting rotation.
2. Related Background Art
A gyro is a sensor for detecting an angular velocity of a moving object. The gyro can be used for controlling the posture of an aircraft and a robot, detecting the position in car navigation, detecting sideways sliding of a car, preventing hand-shaking of a silver-salt camera, a digital camera, and a video camera, etc.
As a gyro, a mechanical gyro provided with a rotor and an oscillator, and an optical gyro are known. In particular, the optical gyro is capable of performing instantaneous start-up and has a large dynamic range, so that the optical gyro is bringing about innovation in the field of a gyro technique. Examples of the optical gyro include a ring laser type gyro, an optical fiber gyro, and a passive ring resonator gyro. Among them, the development of the ring laser type gyro using a gas laser was first launched, and it has already been put into practical use in an aircraft and the like. Recently, as a small ring laser type gyro with high precision, a semiconductor laser gyro integrated on a semiconductor substrate is also proposed. For example, Japanese Patent Application Laid-Open No. High 5-288556 describes this type of gyro. In this manner, a ring laser is important in terms of application to a gyro. In order to increase a signal/noise ratio as a gyro, it is desirable to minimize the number of oscillation modes.
However, in a conventional ring laser type gyro, restriction of the number of oscillation modes has not been studied well. In particular, a semiconductor laser has a large gain bandwidth (i.e., about 10 nm), so that a resonant mode in this bandwidth may oscillate. That is, a semiconductor ring laser is likely to oscillate in a multi-mode. In order to restrict the number of oscillation modes, the number of resonant modes present in the gain bandwidth may be decreased. However, a resonant optical filter such as a Fabry-perot resonator and a diffraction grating cannot be used for the following reason. If a resonant optical filter is used, a forward running wave and a backward running wave are coupled, and the coupling strength of counterpropagating laser lights becomes strong; as a result, the oscillation of one mode is suppressed. This phenomenon is known as lock-in, which causes a problem particularly in the case where the difference in oscillation frequency of counterpropagating laser lights is small (e.g., in the case where the rotation speed is small). Therefore, there is a demand for a ring laser capable of restricting the number of oscillation modes under the condition that a forward running wave and a backward running wave are not coupled.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a ring laser and a ring laser type gyro capable of restricting the number of oscillation modes so as not to cause lock-in.
In order to solve the above-mentioned problem, according to a first aspect of the present invention, a ring laser includes multiple paths with different optical path lengths, wherein at least part of the multiple paths is optically coupled, and a propagating direction of laser light propagating in an optical resonator is not reversed by the multiple paths.
In order to achieve the above-mentioned object, according to a second aspect of the present invention, in the first aspect of the invention, a ring laser has a structure such that counterpropagating laser lights having different oscillation frequencies are present in an optical resonator.
In order to achieve the above-mentioned object, according to a third aspect of the present invention, in the second aspect of the invention, a ring laser has a structure such that at least part of the multiple paths is provided with a mechanism for giving a loss difference or a net gain difference with respect to the laser lights counterpropagating in an optical resonator.
In order to achieve the above-mentioned object, according to a fourth aspect of the present invention, in the third aspect of the invention, a ring laser has a structure such that the mechanism for giving a loss difference or a net gain difference with respect to the laser lights counterpropagating in the optical resonator is a taper-shaped optical waveguide having an asymmetric shape.
In order to achieve the above-mentioned object, according to a fifth aspect of the present invention, in the third aspect of the invention, a ring laser has a structure such that the mechanism for giving a loss difference or a net gain difference with respect to the laser lights counterpropagating in the optical resonator is composed of an optical component including a polarizer.
In order to achieve the above-mentioned object, according to a sixth aspect of the present invention, in any one of the above aspects 1 to 5, a ring laser includes multiple electrodes capable of electrically and independently controlling at least part of the multiple paths.
In order to achieve the above-mentioned object, according to a seventh aspect of the present invention, in the first aspect of the invention, a ring laser has a structure such that only laser lights propagating in one propagating direction is present in the optical resonator.
In order to achieve the above-mentioned object, according to an eighth aspect of the present invention, in the seventh aspect of the invention, a ring laser has a structure such that at least part of the multiple paths is provided with a mechanism for giving a loss difference or a net gain difference with respect to laser lights counterpropagating in the optical resonator.
In order to achieve the above-mentioned object, according to a ninth aspect of the present invention, in the seventh aspect of the invention, a ring laser has a structure such that the mechanism for giving a loss difference or a net gain difference with respect to laser lights counterpropagating in the optical resonator is a taper-shaped optical waveguide.
In order to achieve the above-mentioned object, according to a tenth aspect of the present invention, in the seventh aspect of the invention, a ring laser has a structure such that the mechanism for giving a loss difference or a net gain difference with respect to laser lights counterpropagating in the optical resonator is composed of an optical component including a polarizer.
In order to achieve the above-mentioned object, according to an eleventh aspect of the present invention, in any one of the above aspects 7 to 10 of the present invention, a ring laser includes multiple electrodes capable of electrically and independently controlling at least part of the multiple paths.
In order to achieve the above-mentioned object, according to a twelfth aspect of the present invention, in a method for driving a ring laser, at least part of the multiple electrodes is electrically and independently controlled.
In order to achieve the above-mentioned object, according to a thirteenth aspect of the present invention, a gyro apparatus includes: a ring laser in any one of the aspects 1 to 6 of the invention; a power source of driving of the ring laser; an apparatus for measuring a difference in oscillation frequencies of laser lights; and an apparatus for controlling the power source of driving in accordance with the difference in oscillation frequency.
In order to achieve the above-mentioned object, according to a fourteenth aspect of the present invention, in the thirteenth aspect of the invention, a gyro apparatus includes an apparatus for measuring the difference in oscillation frequencies of laser lights counterpropagating in an optical resonator from a change in a current, a voltage, or impedance of the ring laser.
In order to achieve the above-mentioned object, according to a fifteenth aspect of the present invention, in the thirteenth aspect in the invention, a gyro apparatus includes: a photodetector for simultaneously receiving laser lights counterpropagating in the optical resonator; and an apparatus for measuring the difference in oscillation frequencies of the laser lights from an output of the photodetector.
In order to achieve the above-mentioned object, according to a thirteenth aspect of the present invention, a gyro apparatus includes: an apparatus for measuring the difference in oscillation frequencies of laser lights counterpropagating in the optical resonator from a change in a current, a voltage, or impedance of the ring laser; a photodetector for simultaneously receiving the laser lights counterpropagating in the optical resonator; and an apparatus for measuring the difference in oscillation frequencies of the laser lights from an output of the photodetector.
In order to achieve the above-mentioned object, according to a seventeenth aspect of the present invention, in any one of the above aspects 13 to 16 of the invention, a gyro apparatus includes a frequency-voltage conversion circuit.
In order to achieve the above-mentioned object, according to an eighteenth aspect of the present invention, in any one of the above aspects 13 to 16 of the invention, a gyro apparatus includes a frequency counter.
In order to achieve the above-mentioned object, according to a nineteenth aspect of the present invention, a method for driving a gyro apparatus controls the power source of driving so that the difference in oscillation frequencies of the laser lights becomes constant while the gyro apparatus in any one of the above aspects 13 to 18 stands still.
In order to achieve the above-mentioned object, according to a twentieth aspect of the present invention, a method for driving a gyro apparatus controls the power source of driving so that the difference in oscillation frequencies of the laser lights becomes constant while the gyro apparatus in any one of the aspects 13 to 18 is rotated.
In order to achieve the above-mentioned object, according to a twenty-first aspect of the present invention, in any one of the above aspects 19 to 20 of the invention, the power source of driving supplies an electric power to multiple electrodes of the ring laser.
In order to achieve the above-mentioned object, according to a twenty-second aspect of the present invention, in the twenty-first aspect of the invention, the power source of driving supplies an electric power to at least part of the multiple electrodes of the ring laser so that the electrodes are electrically independent of each other.
In order to achieve the above-mentioned object, according to a signal detection method of a gyro apparatus of a twenty-third aspect of the present invention, in the gyro apparatus of any one of the above aspects 13 to 18, at least one of a change in a current, a voltage or impedance of the ring laser, an output of the photodetector, a signal for controlling the power source of driving, and an output current or an output voltage of the power source of driving is used as an output signal of a gyro.
In order to achieve the above-mentioned object, according to a signal detection method of a gyro apparatus of a twenty-fourth aspect of the present invention, in the twenty-third aspect of the invention, multiple signals of a change in a current, a voltage or impedance of the ring laser, an output of the photodetector, a signal for controlling the power source of driving, and an output current or an output voltage of the power source of driving are subjected to statistical processing, and an obtained signal is used as an output signal of a gyro.
In order to achieve the above-mentioned object, according to a twenty-fifth aspect of the present invention, a ring waveguide of a ring laser includes a connecting waveguide for optical coupling at least two portions of the ring waveguide.
In order to achieve the above-mentioned object, according to a twenty-sixth aspect of the present of the invention, a ring waveguide includes an asymmetric taper portion, and the connecting waveguide is optically coupled to the ring waveguide in a portion other than the asymmetric taper portion.
In order to achieve the above-mentioned object, according to a twenty-seventh aspect of the present invention, in a ring waveguide, the asymmetric taper portion has a portion protruding at a right angle from one point of a side wall of the ring waveguide.
In order to achieve the above-mentioned object, according to a twenty-eighth aspect of the present invention, a ring waveguide includes an asymmetric taper portion, wherein a part of the asymmetric taper portion is a non-waveguide region that does not guide laser light, and the non-waveguide region does not cross the asymmetric taper portion or a side wall of the ring waveguide.
In order to achieve the above-mentioned object, according to a twenty-ninth aspect of the present invention, in a ring waveguide, the asymmetric taper portion has a portion protruding at a right angle from one point of a side wall of the ring waveguide.