Patent Description:
Along with the recent rapid development of automatic control and autonomous navigation technologies, a demand for improving in accuracy of the current position of a moving body has been increasing every year. As the autonomous navigation technology, a GNSS (Global Navigation Satellite System) and an INS (Inertial Navigation System) are known.

As a sensor for use in the INS, a fiber optic gyroscope (FOG) is known (see, for example, Patent Document <NUM>). The fiber optic gyroscope is a rotation angular velocity sensor utilizing the Sagnac effect of light. The fiber optic gyroscope uses a fiber optic coil and has advantages of having no moving element, being smaller in size than conventional mechanical gyros, and being maintenance free and thus has attracted attention.

A phase difference Δϕ generated according to the rotation angular velocity of the fiber optic gyroscope is calculated by multiplying an angular velocity Ω by a scale factor (SF) as a coefficient. That is, the coefficient of the angular velocity Ω in the right side of the below described equation for the phase difference Δϕ is called as a scale factor.

Wherein, R is the radius of the fiber optic coil, N is the number of turns of the fiber optic coil, λ is the wavelength, and c is the light speed.

The scale factor corresponds to the ratio between angular velocity and an output signal and, as understood from Numeral <NUM>, the scale factor is subject to variations in wavelength λ. Unless the scale factor is temporally stabilized, the phase difference fluctuates even under a constant angular velocity, resulting in fluctuation of a sensor output. As a result, no matter how high sensitivity (output signal) is, the Allan deviation which is the index of the accuracy of the gyroscope deteriorates as the time elapses. Thus, to improve the Allan deviation, it is necessary to enhance the stability of the scale factor.

There is, for example, Patent Document <NUM> as a stabilized light source for driving a fiber optic gyroscope. The purpose of Patent Document <NUM> is to solve spectral asymmetry between orthogonal axes of wavelength that may cause instability of the scale factor. Patent Document <NUM> relates to a symmetrical wavelength multiplexor configured to mitigate the spectral asymmetry between orthogonal axes of wavelength so as to reduce scale factor errors.

Further, as a sensor similar to the fiber optic gyroscope, a ring laser gyroscope is known (see, for example, Patent Document <NUM>). Patent Document <NUM> describes an optical system having a laser configured to generate light having a first laser spectrum with a first linewidth, a waveform generator configured to produce a noise waveform, and an electro-optic phase modulator in optical communication with the laser and in electrical communication with the waveform generator. The electro-optic phase modulator is configured to receive the light having the first laser spectrum, to receive the noise waveform, and to respond to the noise waveform by modulating the light to produce light having a second laser spectrum with a second linewidth broader than the first linewidth. Patent Document <NUM> describes a symmetrical wavelength multiplexor (SWM) that includes a first port that receives a depolarized beam of light of a first center wavelength for a first wavelength range. A second port of the SWM receives a second center wavelength of light for a second wavelength range that is greater than the first wavelength range. A third port of the SWM provides a substantially symmetrical wavelength output to drive a fiber optic gyroscope (FOG) in accordance with the second center wavelength. The depolarized beam of light of the first center wavelength travels from the first port to the second port of the SWM and light of the second center wavelength travels from the second port to the third port of the SWM. Patent Document <NUM> describes a resonator fiber optic gyroscope that comprises a master laser device that emits a reference optical signal, a first slave laser device that emits a clockwise optical signal, and a second slave laser device that emits a counter-clockwise optical signal. A resonator ring cavity is in communication with the master laser device and the slave laser devices. A sine wave generator is coupled to the resonator ring cavity and outputs a common cavity modulation frequency comprising in-phase and quadrature signals. A laser stabilization servo receives a clockwise reflection signal that includes the common cavity modulation frequency from the resonator ring cavity. A modulation stripper coupled to the servo receives the in-phase and quadrature signals, receives a net error signal from the servo, demodulates the net error signal at the common cavity modulation frequency, and transmits a stripper signal to the servo to remove the signal at the common cavity modulation frequency.

As described above, it is necessary to enhance stability of the scale factor in order to improve the accuracy of the gyroscope. However, a fiber optic gyroscope like the one disclosed in Patent Document <NUM> has a scale factor with low stability.

Further, a stabilized light source like the one disclosed in Patent Document <NUM> has a good scale factor stability but has a narrow laser light bandwidth, so that when used as a light source for a fiber optic gyroscope, performance degradation due to such as light backscattering and polarization coupling, in the fiber optic coil cannot be avoided. To avoid such light backscattering or polarization coupling in the fiber optic coil, a broadband laser light needs to be used; in this case, however, the center frequency (center wavelength) is not stabilized to result in instability of the scale factor.

Further, a ring laser gyroscope like the one disclosed in Patent Document <NUM> has a good and highly accurate Allan deviation to a certain degree but is large in size and expensive. Further, there is also a demand for gyroscopes with higher accuracy.

In view of the above situations, an object of the present invention is to provide a light source device for a fiber optic gyroscope capable of broadening the bandwidth of a laser light and improving the stability of a scale factor and a fiber optic gyroscope using the same.

The invention provides light source devices for a fiber optic gyroscope as defined in claim <NUM> and claim <NUM>, which include: a laser light source that emits a laser light of a predetermined frequency; a stabilizing part that stabilizes the predetermined frequency of the laser light emitted from the laser light source; and a bandwidth broadening part that makes the laser light stabilized by the stabilizing part into a light having a continuous broadband spectrum.

The stabilizing part may lock the laser light emitted from the laser light source to a reference frequency source for stabilization.

The laser light source may be a continuous laser light source that emits a continuous light.

The laser light source may be a pulse laser light source that emits a light of pulse-like spectra equally spaced at predetermined intervals.

In this case, the bandwidth broadening part may include a white noise modulation part to which the laser light stabilized by the stabilizing part is inputted and which performs frequency modulation using white noise for making a continuous spectrum with a modulation width equal to or more than a predetermined interval of the pulse-like spectra.

In the light source device defined in claim <NUM>, the bandwidth broadening part includes: an optical comb generation part to which the laser light stabilized by the stabilizing part is inputted and which emits a light having a plurality of spectra equally spaced at predetermined intervals with a predetermined frequency as a center; and a white noise modulation part to which the light emitted from the optical comb generation part is inputted and which performs frequency modulation using white noise for making a continuous spectrum with a modulation width equal to or more than a predetermined interval of a plurality of spectra; or a white noise modulation part to which the laser light stabilized by the stabilizing part is inputted and which performs frequency modulation using white noise for making a continuous spectrum with a predetermined modulation width; and an optical comb generation part to which the light emitted from the white noise modulation part is inputted and which emits a light having a plurality of spectra equally spaced at predetermined intervals that is equal to or less than a predetermined modulation width by using white noise, with a predetermined frequency as a center.

In the light source device defined in claim <NUM>, the bandwidth broadening part includes: a parametric down-conversion part that emits a photon pair using the laser light stabilized by the stabilizing part as excitation light; or an optical comb generation part to which the laser light stabilized by the stabilizing part is inputted and which emits a light having a plurality of spectra equally spaced at predetermined intervals with a predetermined frequency as a center; and a parametric down-conversion part that emits a photon pair using the light emitted from the optical comb generation part as excitation light.

A fiber optic gyroscope according to the present invention using the above light source device for a fiber optic gyroscope includes: an optical circulator that separates between the light having a continuous broadband spectrum from the bandwidth broadening part and an interference light obtained by recombining a counterclockwise light and a clockwise light that have passed through a fiber optic coil; a multifunctional integrated optical circuit including a polarizer to which the light having a continuous broadband spectrum is inputted from the bandwidth broadening part through the optical circulator and which allows a single polarized light to pass therethrough, a Y-branch/recombiner that branches the light from the polarizer and makes the resultant lights enter both ends of the fiber optic coil and recombines counterclockwise light and clockwise light that have passed through the fiber optic coil as interference light, a first phase modulator that modulates the light to enter one end of the fiber optic coil, and a second phase modulator that modulates the light to enter the other end of the fiber optic coil; a phase modulation signal generator that generates, in order to reduce thermal phase noise, a phase modulation signal for performing phase modulation at integral multiples of an eigenfrequency of the fiber optic coil to the first and second phase modulators; an optical detector to which the interference light obtained by recombining the counterclockwise light and clockwise light that have passed through the fiber optic coil is inputted from the optical circulator and which detects a light intensity signal of the interference light; and a synchronous detector that uses the phase modulation signal from the phase modulation signal generator as a reference signal to synchronously detect the light intensity signal detected by the optical detector and outputs the detected light intensity signal as a detection signal of input angular velocity to the fiber optic coil.

Further, a fiber optic gyroscope according to the present invention using the above light source device for a fiber optic gyroscope includes: an optical circulator that separates between the light having a continuous broadband spectrum from the bandwidth broadening part and interference a light obtained by recombining a counterclockwise light and a clockwise light that have passed through a fiber optic coil; a multifunctional integrated optical circuit including a polarizer to which the light having a continuous broadband spectrum is inputted from the bandwidth broadening part through the optical circulator and which allows a single polarized light to pass therethrough, a Y-branch/recombiner that branches the light from the polarizer and makes the resultant lights enter both ends of the fiber optic coil and recombines counterclockwise light and clockwise light that have passed through the fiber optic coil as interference light, a first phase modulator that modulates the light to enter one end of the fiber optic coil, and a second phase modulator that modulates the light to enter the other end of the fiber optic coil; a phase modulation signal generator that generates, in order to reduce relative intensity noise, a phase modulation signal for performing phase modulation at odd multiples of an eigenfrequency of the fiber optic coil to the first and second phase modulators; an optical detector to which the interference light obtained by recombining the counterclockwise light and clockwise light that have passed through the fiber optic coil is inputted from the optical circulator and which detects a light intensity signal of the interference signal; a reference light detector that detects a reference light intensity signal of light having a continuous broadband spectrum from the bandwidth broadening part; and a synchronous detector that uses the phase modulation signal from the phase modulation signal generator as a reference signal to synchronously detect a sum signal of the light intensity signal detected by the optical detector and the reference light intensity signal output from the reference light detector and outputs the detected sum signal as a detection signal of input angular velocity to the fiber optic coil.

Further, a fiber optic gyroscope according to the present invention using the above light source device for a fiber optic gyroscope includes: an optical circulator that separates between the light having a continuous broadband spectrum from the bandwidth broadening part and an interference light obtained by recombining a counterclockwise light and a clockwise light that have passed through a fiber optic coil; a multifunctional integrated optical circuit including a polarizer to which the light having a continuous broadband spectrum is inputted from the bandwidth broadening part through the optical circulator and which allows a single polarized light to pass therethrough, a Y-branch/recombiner that branches the light from the polarizer and makes the resultant lights enter both ends of the fiber optic coil and recombines counterclockwise light and clockwise light that have passed through the fiber optic coil as interference light, a first phase modulator that modulates the light to enter one end of the fiber optic coil, and a second phase modulator that modulates the light to enter the other end of the fiber optic coil; a phase modulation signal generator that generates, in order to reduce relative intensity noise, a phase modulation signal for performing phase modulation at even multiples of an eigenfrequency of the fiber optic coil to the first and second phase modulators; an optical detector to which the interference light obtained by recombining the counterclockwise light and clockwise light that have passed through the fiber optic coil is inputted from the optical circulator and which detects a light intensity signal of the interference light; a reference light detector that detects a reference light intensity signal of light having a continuous broadband spectrum from the bandwidth broadening part; and a synchronous detector that uses the phase modulation signal from the phase modulation signal generator as a reference signal to synchronously detect a difference signal between the light intensity signal detected by the optical detector and the reference light intensity signal detected by the reference light detector and outputs the detected difference signal as a detection signal of input angular velocity to the fiber optic coil.

The phase modulation signal generator generates, in order to reduce thermal phase noise and relative phase noise, a phase modulation signal for performing phase modulation at integral multiples of the eigenfrequency of the fiber optic coil to the first and second phase modulators of the multifunctional integrated optical circuit.

Here, the synchronous detector may synchronously detect a phase modulation signal from the phase modulation signal generator, whereby the synchronous detector further outputs a signal canceling a phase difference between the counterclockwise light and the clockwise light generated by input angular velocity to the fiber optic coil, to the phase modulation signal generator, as a feedback control signal for feedback control of the phase modulation signal generator.

The light source device for a fiber optic gyroscope according to the present invention has advantages of being capable of broadening the bandwidth of the laser light and enhancing stability of the scale factor.

Hereinafter, embodiments for practicing the present invention will be described with illustrated examples. Alight source device for a fiber optic gyroscope according to the present invention is a device for driving a fiber optic gyroscope <NUM> having a fiber optic coil. The fiber optic gyroscope <NUM> is a sensor utilizing the Sagnac effect. The Sagnac effect is a phenomenon in which the length of an optical path appears to change due to movement of the fiber optic coil as the optical path. The fiber optic gyroscope <NUM> uses a fiber optic coil having a length of, for example, <NUM>. The fiber optic gyroscope <NUM> to be driven is not particularly limited in structure, and the light source device for a fiber optic gyroscope according to the present invention can drive any existing or future fiber optic gyroscope.

<FIG> is a schematic block diagram for explaining the configuration of the light source device for a fiber optic gyroscope according to the present invention. As illustrated, the light source device for a fiber optic gyroscope according to the present invention is mainly constituted by a laser light source <NUM>, a stabilizing part <NUM>, and a bandwidth broadening part <NUM>. That is, the light source device for a fiber optic gyroscope according to the present invention uses the stabilizing part <NUM> to stabilize the laser light from the laser light source <NUM>, uses the bandwidth broadening part <NUM> to broaden the bandwidth of the laser light, and drives the fiber optic gyroscope <NUM>. The following describes in detail the configurations of the above constituent elements.

The laser light source <NUM> emits a laser light of a predetermined frequency. The laser light from the laser light source <NUM> may be a continuous wave (CW) light. For example, a semiconductor laser, a solid-state laser, a gas laser, and a dye laser, etc. are available as the laser light source <NUM>. The wavelength of the laser light emitted from the laser light source <NUM> is not limited to a specific one. That is, the laser light source <NUM> may emit at least a laser light of a <NUM> wavelength at which light propagation loss of the optical fiber is small or a <NUM> wavelength which is the second harmonic thereof.

Incidentally, the laser light source of the light source device for a fiber optic gyroscope according to the present invention is not limited to a continuous wave laser light source and may be a pulse laser light source that emits a light of pulse-like spectra equally spaced at predetermined intervals. A specific example may be an optical comb light source.

The stabilizing part <NUM> stabilizes a predetermined frequency (wavelength) of the laser light emitted from the laser light source <NUM>. In the light source device for a fiber optic gyroscope according to the present invention, the stabilizing part <NUM> may lock the laser light emitted from the laser light source <NUM> to a reference frequency source for stabilization. Specifically, when the predetermined frequency is a <NUM> wavelength, the laser light may be stabilized with the stability of frequency in the order of kHz or MHz (up to about <NUM> ppm). The frequency spectral line width of the laser light is originally narrow to a certain degree, so that the stabilizing part <NUM> of the light source device for a fiber optic gyroscope according to the present invention may enhance the long-term frequency stability (i.e., relatively slow fluctuation) of the laser light. Therefore, it is not always necessary to reduce short-term phase noise and the like with high accuracy so as to narrow the frequency spectral line width. The configuration of the stabilizing part <NUM> will be described later more specifically.

The bandwidth broadening part <NUM> makes the laser light stabilized by the stabilizing part <NUM> into a light having a continuous broadband spectrum. The bandwidth broadening part <NUM> may function to convert the laser light into, for example, light having a plurality of spectra continuously arranged with equally spaced at predetermined intervals with a predetermined frequency as a center. Specifically, when the predetermined frequency is a <NUM> wavelength, a frequency band is broadened in THz order (nm order in wavelength). The resultant light is a light having continuous broadband spectrum obtained by conversion based on the stabilized laser light, and thus the center frequency (center wavelength) thereof is also stabilized. Accordingly, the scale factor is also stabilized. The configuration of the bandwidth broadening part <NUM> will be described later more specifically.

With the thus configured light source device for a fiber optic gyroscope according to the present invention, the wavelength of the laser light is stabilized to thereby enhance the stability of the scale factor. Then, the laser light can be broadened in bandwidth with the stabilized scale factor, so that it is possible to avoid performance degradation due to such as light backscattering and polarization coupling in the fiber optic coil.

Next, a specific example of the stabilizing part of the light source device for a fiber optic gyroscope according to the present invention is described. <FIG> is a schematic block diagram for explaining a specific example of the stabilizing part of the light source device for a fiber optic gyroscope according to the present invention. In the drawing, the same reference numerals as those in <FIG> denote the same parts. <FIG> is graphs schematically illustrating a state where the frequency of the laser light is stabilized by the stabilizing part of the light source device for a fiber optic gyroscope according to the present invention; <FIG> illustrating frequency characteristics with respect to the time before stabilization, and <FIG> illustrating frequency characteristics with respect to the time after stabilization. As illustrated in <FIG>, the stabilizing part <NUM> serves as a feedback circuit for locking the laser light emitted from the laser light source <NUM> to a reference frequency source for stabilization. The illustrated feedback circuit uses a second harmonic generation (SHG) part <NUM> to convert the wavelength into <NUM>/<NUM> of the original frequency and then locks the resultant laser light to an atomic cell <NUM> as the reference frequency source, followed by feedback to the laser light source <NUM>. The SHG part <NUM> may be a nonlinear crystal that converts the wavelength into, for example, <NUM>/<NUM> of the original frequency. The atomic cell <NUM> may be a cell enclosing a rubidium (Rb) atomic gas, when, for example, a wavelength of <NUM> is used.

Specifically, as the laser light source <NUM>, a semiconductor laser light source that outputs a continuous wave (CW) light of a <NUM> wavelength is used. The frequency characteristics of the laser light from the semiconductor laser light source is illustrated in <FIG>. As illustrated, the laser light from the semiconductor laser light source relatively slowly fluctuates in frequency. The wavelength of this laser light is converted into <NUM> by the SHG part <NUM>. The resultant laser light of <NUM> is made to pass through the atomic cell <NUM> enclosing the Rb atomic gas to be separated using various absorption spectroscopy methods such as linear absorption spectroscopy and saturated absorption spectroscopy. The spectra thus obtained are used to perform feedback to the semiconductor laser light source. This stabilizes the frequency of the laser light from the laser light source <NUM> by means of the Rb atoms of the atomic cell <NUM> as the reference frequency source. That is, as illustrated in <FIG>, long-term frequency stability is enhanced.

Incidentally, the stabilizing part <NUM> of the light source device for a fiber optic gyroscope according to the present invention is not limited to the one that uses the above SHG and atomic spectroscopy. For example, absorption lines for molecules such as C<NUM>H<NUM> and HCN exist around <NUM>, so that it is also possible to stabilize the frequency of the laser light from the laser light source <NUM> by using a glass cell enclosing these molecular gases to separate the laser light of a <NUM> wavelength according to various absorption spectroscopy methods such as linear absorption spectroscopy and saturated absorption spectroscopy. That is, the stabilizing part <NUM> may stabilize the frequency of the laser light by separating the laser light using one enclosing, in a glass cell, atoms or molecules for which the absorption lines exist in a desired frequency band as the reference frequency source.

Besides, an optical resonator is applicable as the stabilizing part <NUM>. That is a Fabry-Perot optical resonator constituted by two mutually facing mirrors. This optical resonator resonates with a light of a specific frequency, so that feedback is performed using the optical resonator as the reference frequency source to lock the frequency of the light. Thus, any existing or future type as long as it can stabilize a predetermined frequency of the inputted laser light are applicable as the stabilizing part <NUM> of the light source device for a fiber optic gyroscope according to the present invention.

Next, a specific example of the bandwidth broadening part of the light source device for a fiber optic gyroscope according to the present invention is described. <FIG> is a schematic block diagram for explaining a specific example of the bandwidth broadening part of the light source device for a fiber optic gyroscope according to the present invention. In the drawing, the same reference numerals as those in <FIG> denote the same parts. <FIG> is graphs schematically illustrating the frequency spectrum of the laser light processed by the bandwidth broadening part of the light source device for a fiber optic gyroscope according to the present invention; <FIG> illustrating a frequency spectrum in an output from an optical comb generation part, and <FIG> illustrating a frequency spectrum in an output from a white noise modulation part. The bandwidth broadening part <NUM> makes the laser light stabilized by the stabilizing part <NUM> have a broadband spectrum and a continuous spectrum.

In the example illustrated in <FIG>, the bandwidth broadening part <NUM> is constituted by an optical comb generation part <NUM> and a white noise modulation part <NUM>. The optical comb generation part <NUM> is to which the laser light stabilized by the stabilizing part <NUM> is inputted, and emits a light having a plurality of spectra equally spaced at predetermined intervals with a predetermined frequency as a center. That is, when the stabilized laser light is inputted, the optical comb generation part <NUM> outputs light having a plurality of highly stable spectra equally spaced intervals on the left and right sides with a predetermined frequency as a center, as illustrated in <FIG>. Since the laser light stabilized in frequency is inputted, the spectra of the generated optical comb are also highly stabilized. Incidentally, any existing or future type as long as it can generate an optical comb are applicable as the optical comb generation part <NUM>.

The white noise modulation part <NUM> is to which the light emitted from the optical comb generation part <NUM> is inputted, and performs frequency modulation using white noise for making a continuous spectrum with a modulation width equal to or more than a predetermined interval of a plurality of spectra. That is, when an optical comb having stabilized spectra as illustrated in <FIG> is inputted, the white noise modulation part <NUM> performs frequency modulation with white noise so as to fill the interval between the plurality of spectra to obtain a continuous spectrum (uniform spectrum) as illustrated in <FIG>. Thus, the white noise modulation part <NUM> can generate a light of a continuous spectrum over a broad bandwidth. Incidentally, any existing or future type as long as it can perform frequency modulation using white noise are applicable as the white noise modulation part <NUM>.

As described above, in the light source device for a fiber optic gyroscope according to the present invention, the spectra of the optical comb are highly stabilized, and a predetermined frequency (wavelength) is kept stable. Specifically, when a predetermined frequency is a <NUM> wavelength, the laser light is stabilized with the stability of frequency in the order of kHz or MHz (up to about <NUM> ppm). As a result, the stability of the scale factor is enhanced. Then, in a state where the stability of the scale factor is enhanced, the stabilized laser light is spread by the optical comb such that a plurality of spectra is continuously arranged at even intervals over a broad bandwidth, and the plurality of spectra is made into a continuous spectrum by white noise, with the result that the light is broadened in bandwidth. Specifically, when the frequency of the laser light from the laser light source <NUM> is a <NUM> wavelength, for example, the frequency band is broadened in the order of THz (nm order in wavelength). Thus, the light source device for a fiber optic gyroscope according to the present invention can improve the scale factor while preventing performance degradation due to such as light backscattering and polarization coupling in the fiber optic coil.

Incidentally, in the example illustrated in <FIG>, the optical comb is generated, and then a uniform spectrum is obtained using white noise; however, the present invention is not limited to this. <FIG> is a schematic block diagram for explaining another specific example of the bandwidth broadening part of the light source device for a fiber optic gyroscope according to the present invention. In the drawing, the same reference numerals as those in <FIG> denote the same parts. <FIG> is graphs schematically illustrating the frequency spectrum of the laser light processed by the bandwidth broadening part of the light source device for a fiber optic gyroscope according to the present invention; <FIG>Aillustrating a frequency spectrum in an output from the white noise modulation part, and <FIG> illustrating a frequency spectrum in an output from the optical comb generation part. In the example illustrated in <FIG>, contrary to the example illustrated in <FIG>, frequency modulation is performed by the white noise modulation part <NUM> first, and then a plurality of spectrization is performed by the optical comb generation part <NUM>. That is, the bandwidth broadening part <NUM> is constituted by the white noise modulation part <NUM> and optical comb generation part <NUM>. The white noise modulation part <NUM> is to which the laser light stabilized by the stabilizing part <NUM> is inputted, and performs frequency modulation using white noise for making a continuous spectrum with a predetermined modulation width. That is, when the stabilized laser light is inputted, the white noise modulation part <NUM> performs frequency modulation using white noise for making a continuous spectrum as illustrated in <FIG> with a predetermined modulation width thereby achieving a continuous spectrum (uniform spectrum) having a predetermined modulation width.

And the optical comb generation part <NUM> is to which the light emitted from the white noise modulation part <NUM> is inputted, and emits a light having a plurality of spectra equally spaced at predetermined intervals that is equal to or less than a predetermined modulation width by using white noise, with a predetermined frequency as a center. That is, when the laser light having a continuous spectrum with a predetermined modulation width as illustrated in <FIG> is inputted, the optical comb generation part <NUM> outputs light having a plurality of spectra equally spaced intervals on the left and right sides with a predetermined frequency of the inputted laser light as a center, as illustrated in <FIG>. As a result, as in the example illustrated in <FIG>, it is possible to generate light of a continuous spectrum over a broad bandwidth.

Next, another specific example of the light source device for a fiber optic gyroscope according to the present invention is described using <FIG> is a schematic block diagram for explaining another example of the light source device for a fiber optic gyroscope according to the present invention. In the drawing, the same reference numerals as those in <FIG> denote the same parts. The illustrated bandwidth broadening part <NUM> is constituted by a parametric down-conversion part <NUM>. The parametric down-conversion part <NUM> uses a nonlinear optical crystal and emits a photon pair using the laser light stabilized by the stabilizing part <NUM> as excitation light. Here, a specific description will be made of a case where the output frequency of the light source device for a fiber optic gyroscope according to the present invention is set to <NUM> at which the propagation loss of an optical fiber is small. As the laser light source <NUM>, a semiconductor laser that can emit a laser light of <NUM> is used. In this case, the stabilizing part <NUM> may perform feedback using only the atomic cell <NUM>. That is, an absorption spectroscopy method, etc. is used to lock the <NUM> light to the atomic cell <NUM> enclosing an Rb atomic gas, and the obtained spectrum is used to perform feedback to the semiconductor laser.

The thus stabilized laser light is inputted to the parametric down-conversion part <NUM>. The parametric down-conversion part <NUM> generates a photon pair with the stabilized laser light as excitation light. That is, the laser light inputted to the parametric down-conversion part <NUM> has a continuous spectrum (uniform spectrum) spreading to the left and right with <NUM>/<NUM> frequency (twice in wavelength) of a predetermined frequency of the inputted laser light. Specifically, when a light of stabilized wavelength of <NUM> is inputted to the parametric down-conversion part <NUM>, the frequency band of the laser light of <NUM> wavelength is broadened in the order of THz (nm order in wavelength). This allows light having a continuous broadband spectrum to be obtained.

Incidentally, when the parametric down-conversion part <NUM> as illustrated in <FIG> is used, the laser light source <NUM> may be a continuous laser light source that emits a continuous light and may alternatively be a pulse laser light source that emits a light of pulse-like spectra equally spaced at predetermined intervals. In the similar manner to the continuous light, a pulse laser light is locked to a predetermined frequency by the stabilizing part <NUM> for stabilization, whereby the pulse-like spectra are each stabilized.

Also, when the pulse laser light source that emits a light of pulse-like spectra is used as the laser light source <NUM> in the light source device for a fiber optic gyroscope according to the present invention, the bandwidth broadening part <NUM> is not limited to the parametric down-conversion part <NUM> illustrated in <FIG>. That is, in place of the parametric down-conversion part <NUM>, the white noise modulation part <NUM> may be used as the bandwidth broadening part <NUM>. The pulse-like spectra equally spaced at predetermined intervals from the pulse laser light source are stabilized by the stabilizing part <NUM> and inputted to the white noise modulation part <NUM>. Then, the white noise modulation part <NUM> uses white noise to perform frequency modulation with a modulation width equal to or more than a predetermined interval of pulse-like spectra to obtain a continuous spectrum (uniform spectrum).

Further, using <FIG>, still another specific example of the light source device for a fiber optic gyroscope according to the present invention is described. <FIG> is a schematic block diagram for explaining still another example of the light source device for a fiber optic gyroscope according to the present invention. In the drawing, the same reference numerals as those in <FIG> denote the same parts. The illustrated bandwidth broadening part <NUM> is constituted by the optical comb generation part <NUM> and parametric down-conversion part <NUM>. That is, the bandwidth broadening part <NUM> has a configuration combining the optical comb generation part <NUM> illustrated in <FIG> and the parametric down-conversion part <NUM> illustrated in <FIG>. A specific description will be made of a case where the output frequency of the light source device for a fiber optic gyroscope according to the present invention is <NUM> at which the propagation loss of an optical fiber is small. As the laser light source <NUM>, a semiconductor laser that can emit a laser light of <NUM> is used. In this case, the stabilizing part <NUM> may perform feedback using only the atomic cell <NUM>. That is, an absorption spectroscopy method, etc. is used to lock the light of <NUM> of the laser light source <NUM> to the atomic cell <NUM> enclosing an Rb atomic gas, and the obtained spectrum is used to perform feedback to the semiconductor laser.

The thus stabilized laser light is inputted to the optical comb generation part <NUM>. The optical comb generation part <NUM> emits a light having a plurality of spectra equally spaced intervals on the left and right sides with a predetermined frequency of the inputted stabilized laser light as a center. Then, the light of a plurality of spectra equally spaced intervals is inputted to the parametric down-conversion part <NUM>, and the parametric down-conversion part <NUM> generates a photon pair with the stabilized laser light as excitation light. That is, the laser light inputted to the parametric down-conversion part <NUM> has a continuous spectrum (uniform spectrum) spreading to the left and right with <NUM>/<NUM> frequency (twice in wavelength) of a predetermined frequency of the inputted laser light. Specifically, when the <NUM> light of a plurality of spectra equally spaced intervals is inputted to the parametric down-conversion part <NUM>, the frequency band of the laser light of <NUM> wavelength is broadened in the order of THz (nm order in wavelength). This allows light having a continuous broadband spectrum to be obtained.

In the example illustrated in <FIG>, the light is pulsed by the optical comb generation part <NUM>, so that conversion efficiency by the parametric down-conversion part <NUM> is enhanced as compared to the example illustrated in <FIG>.

Next, a fiber optic gyroscope suitable for the light source device for a fiber optic gyroscope according to the present invention. The light source device for a fiber optic gyroscope according to the present invention is not limited in use only to a specific fiber optic gyroscope but is versatile. However, when being used in combination of a fiber optic gyroscope to be described below, intensity noise associated with broadening of bandwidth can be reduced. In addition, thermal phase noise caused by heat of a fiber optic coil can be reduced.

As described above, according to the light source device for a fiber optic gyroscope of the present invention, light from a light source can be broadened in bandwidth, thereby allowing enhancement of stability of the scale factor. On the other hand, broadening of bandwidth may cause intensity noise due to mutual interference between lights of different frequencies. This noise is referred to as "RIN (Relative Intensity Noise)".

As a conventional technique to reduce RIN, there is a method in which two fiber optic coils having the same length are prepared, and a reference light is inputted to one of them to measure intensity noise to detect intensity correlation of a light source, based on which influence of RIN is reduced. This method uses two fiber optic coils having the same length and is thus costly. Further, it is difficult to prepare fiber optic coils having exactly the same characteristics.

As another conventional technique to reduce RIN, there is a method in which utilizing a fact that a light that has passed through a fiber optic coil is delayed more than a light that has not passed through the fiber optic coil by a time taken to propagate through the fiber optic coil, the intensities of the two lights are summed so as to reduce the intensity noise. The light source device for a fiber optic gyroscope according to the present invention can be applied to a fiber optic gyroscope having a configuration that can reduce RIN.

Also, the optical fiber of the fiber optic coil is formed of quartz, and the atoms constituting quartz are in thermal motion at room temperature. The thermal motion of the atoms causes a variation in the refractive index of the optical fiber, so that phase noise is superimposed on the light that has passed through the fiber optic coil. This noise is referred to as "Thermal Phase Noise".

The thermal phase noise has a property of being reduced when being subjected to phase modulation at integral multiples of an eigenfrequency of the fiber optic coil. Thus, when the light source device for a fiber optic gyroscope according to the present invention is applied to a fiber optic gyroscope having a configuration in which thermal phase noise is reduced by synchronously detecting a light intensity signal with a phase modulation signal as a reference signal, the thermal phase noise can be reduced.

Hereinafter, a specific example of the fiber optic gyroscope suitably using the light source device for a fiber optic gyroscope according to the present invention is described. <FIG> is a schematic block diagram for explaining the configuration of a fiber optic gyroscope suitably using the light source device for a fiber optic gyroscope according to the present invention. In the drawing, the same reference numerals as those in <FIG>, etc. denote the same parts. The fiber optic gyroscope in the illustrated example can reduce the thermal phase noise. As illustrated, a fiber optic gyroscope <NUM> uses, as a light source, the above-described light source device (<NUM>, <NUM>, <NUM>) for a fiber optic gyroscope according to the present invention. That is, the fiber optic gyroscope <NUM> is driven by using a light having a continuous broadband spectrum generated by the light source device for a fiber optic gyroscope according to the present invention. The fiber optic gyroscope <NUM> is mainly constituted by an optical circulator <NUM>, a multifunctional integrated optical circuit <NUM>, a phase modulation signal generator <NUM>, an optical detector <NUM>, and a synchronous detector <NUM>. The multifunctional integrated optical circuit <NUM> is connected with a fiber optic coil <NUM>.

The optical circulator <NUM> separates between the light having a continuous broadband spectrum from the bandwidth broadening part <NUM> and an interference light obtained by recombining a counterclockwise light and a clockwise light that have passed through the fiber optic coil <NUM>. That is, the optical circulator <NUM> outputs the light from the bandwidth broadening part <NUM> to the multifunctional integrated optical circuit <NUM> to be described later and outputs the interference light returned thereto to the optical detector <NUM>. In the drawing, the light having a continuous broadband spectrum from the bandwidth broadening part <NUM> enters the optical circulator <NUM> from the left side thereof and is outputted from the right side thereof. Then, the returned interference light obtained by recombining the counterclockwise light and clockwise light that have passed through the fiber optic coil <NUM> enters the optical circulator <NUM> from the right side thereof and is outputted downward therefrom to the optical detector <NUM> to be described later.

The multifunctional integrated optical circuit <NUM> is constituted by a polarizer <NUM>, a Y-branch/recombiner <NUM>, a first phase modulator <NUM>, and a second phase modulator <NUM>. The polarizer <NUM> receives the light having a continuous broadband spectrum from the bandwidth broadening part <NUM> through the optical circulator <NUM>. Then, the polarizer <NUM> allows a single polarized light to pass therethrough. The Y-branch/recombiner <NUM> branches the light, i.e., the single polarized light, from the polarizer <NUM> and makes the resultant lights enter both ends of the fiber optic coil <NUM>. The lights entering the both ends of the fiber optic coil <NUM> become the counterclockwise light and clockwise light, respectively. Then the Y-branch/recombiner <NUM> recombines the counterclockwise light and clockwise light that have passed through the fiber optic coil <NUM> and outputs the resultant light to the optical circulator <NUM> as an interference light. The first phase modulator <NUM> modulates one of the incident lights which enters one end of the fiber optic coil <NUM>. For example, the light which becomes the clockwise light may be modulated. Also, the second phase modulator <NUM> modulates the other of the incident lights which enters the other end of the fiber optic coil <NUM>. For example, the light which becomes the counterclockwise light light may be modulated.

The phase modulation signal generator <NUM> is configured to generate a phase modulation signal so as to reduce thermal phase noise. The phase modulation signal generator <NUM> outputs the phase modulation signal to the first phase modulator <NUM> and second phase modulator <NUM> of the multifunctional integrated optical circuit <NUM>. The phase modulation signal generator <NUM> generates the phase modulation signal for the multifunctional integrated optical circuit <NUM> so as to perform phase modulation at integral multiples of the eigenfrequency of the fiber optic coil <NUM>. Specifically, the phase modulation may be performed to be at integral multiples of about <NUM> to <NUM> times of the eigenfrequency. This can reduce thermal phase noise.

The optical detector <NUM>, to which the interference light recombining the counterclockwise light and clockwise light that have passed through the fiber optic coil <NUM> from the optical circulator <NUM> is entered, detects the light intensity signal of the interference light.

The synchronous detector <NUM> uses the phase modulation signal from the phase modulation signal generator <NUM> as a reference signal to synchronously detect the light intensity signal detected by the optical detector <NUM> and outputs the detected light intensity signal as a detection signal of input angular velocity to the fiber optic coil <NUM>.

The thus configured fiber optic gyroscope using the light source device for a fiber optic gyroscope according to the present invention performs phase modulation at integral multiples of the eigenfrequency of the fiber optic coil <NUM> to thereby make it possible to reduce thermal phase noise.

Next, another specific example of the fiber optic gyroscope suitably using the light source device for a fiber optic gyroscope according to the present invention is described. <FIG> is a schematic block diagram for explaining the configuration of a fiber optic gyroscope suitably using the light source device for a fiber optic gyroscope according to the present invention. In the drawing, the same reference numerals as those in <FIG> denote the same parts, and overlapping description will be omitted. The fiber optic gyroscope in the illustrated example can reduce relative intensity noise (RIN). As illustrated, also the fiber optic gyroscope <NUM> in this example is driven by using a light having a continuous broadband spectrum generated by the light source device (<NUM>, <NUM>, <NUM>) for a fiber optic gyroscope according to the present invention. The fiber optic gyroscope <NUM> is mainly constituted by the optical circulator <NUM>, multifunctional integrated optical circuit <NUM>, a phase modulation signal generator <NUM>, optical detector <NUM>, a synchronous detector <NUM>, and a reference light detector <NUM>.

Here, the light having a continuous broadband spectrum from the bandwidth broadening part <NUM> may be separated by a beam splitter <NUM> or the like for also outputting to the reference light detector <NUM> to be described later apart from a light to be directed to the optical circulator <NUM>.

The phase modulation signal generator <NUM> is configured to generate a phase modulation signal so as to reduce relative intensity noise. The phase modulation signal generator <NUM> outputs the phase modulation signal to the first phase modulator <NUM> and second phase modulator <NUM> of the multifunctional integrated optical circuit <NUM>. The phase modulation signal generator <NUM> generates the phase modulation signal for the multifunctional integrated optical circuit <NUM> so as to perform phase modulation at odd multiples of the eigenfrequency of the fiber optic coil <NUM>. Specifically, the phase modulation may be performed to be at odd multiples of one time, three times, five times, etc. of the eigenfrequency. This can reduce relative intensity noise. At this time, when phase modulation is performed at odd multiples and integral multiples of the eigenfrequency, it is possible to reduce thermal phase noise in addition to relative intensity noise as in the example illustrated in <FIG>. That is, both the thermal phase noise and relative intensity noise can be reduced.

The reference light detector <NUM> detects a reference light intensity signal of the light having a continuous broadband spectrum from the bandwidth broadening part <NUM>. That is, the reference light detector <NUM> uses the light having a continuous broadband spectrum from the bandwidth broadening part <NUM> as a reference light to detect the reference light intensity signal thereof.

The synchronous detector <NUM> uses the phase modulation signal from the phase modulation signal generator <NUM> as a reference signal to synchronously detect a sum signal of the light intensity signal detected by the optical detector <NUM> and the reference light intensity signal detected by the reference light detector <NUM> and outputs the detected sum signal as a detection signal of input angular velocity to the fiber optic coil <NUM>.

The thus configured fiber optic gyroscope using the light source device for a fiber optic gyroscope according to the present invention performs phase modulation at odd multiples of the eigenfrequency of the fiber optic coil <NUM> to thereby make it possible to reduce relative intensity noise. Further, when phase modulation is performed at odd multiples and integral multiples of the eigenfrequency, it is possible to reduce thermal phase noise in addition to the relative intensity noise.

Incidentally, in this example, the phase modulation is performed at odd multiples, and the sum signal of the light intensity signal detected by the optical detector <NUM> and the reference light intensity signal detected by the reference light detector <NUM> is synchronously detected; alternatively, in place of this, a configuration may be possible, in which the phase modulation is performed at even multiples, and a difference signal between the light intensity signal detected by the optical detector <NUM> and the reference light intensity signal detected by the reference light detector <NUM> is synchronously detected. That is, the phase modulation signal generator <NUM> may generate the phase modulation signal for the multifunctional integrated optical circuit <NUM> so as to perform the phase modulation at even multiples of the eigenfrequency of the fiber optic coil <NUM>. Accordingly, the synchronous detector <NUM> may synchronously detect the difference signal between the light intensity signal detected by the optical detector <NUM> and the reference light intensity signal detected by the reference light detector <NUM>.

Here, the synchronous detector <NUM> illustrated in <FIG> or the synchronous detector <NUM> illustrated in <FIG> may use the phase modulation signal from the phase modulation signal generator <NUM> (<NUM>) as a reference signal to perform the synchronous detection. Further, a signal for canceling a phase difference between the counterclockwise light and the clockwise light generated by input angular velocity to the fiber optic coil <NUM> can be output to the phase modulation signal generator <NUM> (<NUM>) as a feedback control signal for feedback control of the phase modulation signal generator <NUM> (<NUM>). This can increase the dynamic range of the input angular velocity.

Claim 1:
A light source device for a fiber optic gyroscope (<NUM>) configured to drive a fiber optic gyroscope (<NUM>) having a fiber optic coil (<NUM>), the light source device comprising:
a laser light source (<NUM>) that emits a laser light of a predetermined frequency;
a stabilizing part (<NUM>) that stabilizes the predetermined frequency of the laser light emitted from the laser light source (<NUM>); and
a bandwidth broadening part (<NUM>) that makes the laser light stabilized by the stabilizing part (<NUM>) into a light having a continuous broadband spectrum;
wherein the bandwidth broadening part comprises an optical comb generation part (<NUM>) and a white noise modulation part (<NUM>);
wherein either:
the laser light stabilized by the stabilizing part (<NUM>) is inputted into the optical comb generation part (<NUM>), which emits a light having a plurality of spectra equally spaced at predetermined intervals with a predetermined frequency as a center, and the light emitted from the optical comb generation part (<NUM>) is inputted into the white noise modulation part (<NUM>), which performs frequency modulation using white noise for making a continuous spectrum with a modulation width equal to or more than a predetermined interval of a plurality of spectra; or
the laser light stabilized by the stabilizing part (<NUM>) is inputted into the white noise modulation part (<NUM>), which performs frequency modulation using white noise for making a continuous spectrum with a predetermined modulation width, and the light emitted from the white noise modulation part (<NUM>) is inputted into the optical comb generation part (<NUM>), which emits a light having a plurality of spectra equally spaced at predetermined intervals that are is equal to or less than a predetermined modulation width by using white noise, with a predetermined frequency as a center.