JPH08262515A - Optical frequency reference generator - Google Patents

Abstract

PURPOSE: To generate light utilizable as a secondary frequency reference standard of high frequency stability and set likelihood over a wide relative frequency range by subjecting frequency standard signal which is a primary frequency reference standard to frequency shift. CONSTITUTION: An optical ring circuit is composed by connecting an optical amplifier 4, an optical delay element 6, an optical frequency shifter 7 and an optical switch 8 to an annular form. This device shifts the frequency by repetitively and cyclically supplying pulse light in this optical ring circuit. An optical modulator 5 and an optical resonator 9 are inserted into the optical ring circuit. The sum of the modulation frequency of this optical modulator 9 and the frequency shift quantity of the optical frequency shifter 7 is set substantially equal to the resonance peak intervals of the optical resonator 9.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is used to generate light which can be used as a frequency reference. In particular, it relates to a device that generates a frequency reference light with high stability and accuracy over a wide frequency band. INDUSTRIAL APPLICABILITY The present invention can be utilized particularly for stabilizing an optical frequency in a laser spectroscopic device, an optical measuring device, or an optical frequency multiplex communication device.

[0002]

2. Description of the Related Art Normally, the oscillation frequency of light output from a laser light source fluctuates due to subtle fluctuations in the external environment such as temperature and instability inherent in the laser itself. Such fluctuations in frequency or indeterminacy in frequency setting are
When performing highly accurate spectroscopic measurement using laser light, or performing coherent optical communication in which information is added to the frequency or phase of light, communication performance is significantly limited.

Therefore, conventionally, in order to stabilize the optical frequency and fix the absolute frequency, a method of referring to an absorption spectrum of an atom or a molecule has been used. According to this method, the frequency can be stabilized relatively easily when a proper absorption spectrum exists at the target optical frequency.

However, when a proper absorption spectrum does not exist at the target optical frequency, a technique for stabilizing the relative frequency between the target optical frequency and the frequency of the absorption line relatively close to the target optical frequency has been proposed. Further required.

Such a technique is called optical frequency synthesis, and generally, a master light source whose absolute frequency is stabilized by using an absorption spectrum, a slave light source which oscillates light of a target frequency, and a master light source are used. It is composed of a device for generating a relative frequency reference standard for stabilizing the relative frequency with respect to the slave light source. For this kind of technology, refer to Genichi Otsu, "Coherent Photon Optics".
Detailed in Asakura Shoten.

As a method of generating a relative frequency reference standard, it has been proposed so far to use a microwave frequency electric signal, a resonance peak of a Fabry-Perot interferometer, or an externally modulated sideband component of output light of a master light source. ing. The method using microwave frequency electrical signals is
It is possible to stabilize the relative frequency with extremely high accuracy by combining with the phase locking loop, but the relative frequency setting range is limited by the band of the receiver that detects the beat frequency of the master light and the slave light, and at most 20 GHz. It is a degree. The method using the resonance peak of the Fabry-Perot interferometer can realize a relative frequency setting range of sub THz or higher, but it is difficult to set the absolute frequency of the resonance peak with high accuracy. The method using the external modulation sideband component can directly obtain the reference reference in the form of a light wave, but the setting range of the relative frequency is at most 2 depending on the modulation variable band of the external modulator.
It is limited to about 0 GHz. These technical constraints are
Further, it can be overcome by constructing a chain using multi-stage lasers or using wideband frequency conversion using a nonlinear optical effect, but there is a drawback that the configuration becomes more complicated.

As described above, until recently, there was no suitable optical frequency synthesis technique having a simple structure, a wide relative frequency setting band, and high frequency stability. Recently, K. Shimizu et al., Applied has been proposed as a technique that can solve the above problems relatively easily.
Optics., Vol.32, No.33, pp.6718-6726, 1993 and the same.
Vol.33, No.15, pp.3209-3219, 1994 and JP-A-5
In Japanese Patent No. 232540, it has been proposed to use a lightwave frequency conversion ring circuit. As a result, the relative frequency between the master light source and the slave light source can be suppressed to a frequency fluctuation within 1 MHz over the range of 0 to 120 GHz.

FIG. 7 is a block diagram showing a conventional optical frequency reference generator using a lightwave frequency conversion ring circuit. In this conventional example, the absolute frequency stabilizing light (primary frequency reference) from the frequency reference light source 1 is pulsed by the optical pulse modulator 2 and input to the optical ring circuit via the optical directional coupler 3. The optical amplifier 4 in the optical ring circuit,
The optical narrow band filter 12, the optical delay element 6, the optical frequency shifter 7 and the optical switch 8 are provided, the optical amplifier 4 amplifies the optical wave propagating in the optical ring circuit, and the optical narrow band filter 12 is provided in the optical amplifier 4. The generated spontaneous emission noise light is removed, the optical delay element 6 delays the light wave propagating through the optical ring circuit, the optical frequency shifter 7 gives a constant frequency shift to the light wave, and the optical switch 8 is turned on / off. I do. Although the optical frequency shifter 7 and the optical switch 8 are shown as separate elements here, they can be realized by a single acousto-optic element.

In this configuration, if the pulse width is set to be shorter than one round time of the optical ring circuit and the amplification gain of the optical amplifier 4 is adjusted so as to compensate the loss of the optical ring circuit, the inside of the optical ring circuit is adjusted. The frequency of the pulsed light propagating in the orbit is swept stepwise and accurately with respect to time. As a result, a large frequency shift is obtained after multiple rounds. The absolute frequency of the slave light source can be stabilized by using the frequency of the multi-cycle pulsed light thus generated as the secondary frequency reference standard. An example of actually stabilizing the absolute frequency of a slave light source is reported in Shimizu et al., "Laser Quantum Electronics", February 1995, report by IEICE.

[0010]

As described above, by using the light wave frequency conversion ring circuit, the relative frequency setting range can be expanded to about 200 GHz. However, since the acousto-optic device is used as a frequency shifter, the frequency shift amount per time is 100 to 2
Limited to 00 MHz. Therefore, several hundred GHz
In order to obtain the above-mentioned large frequency shift, the number of rounds of about 1000 times is required. However, this number of rounds is limited by the transmission bandwidth of the optical filter inserted in the optical ring circuit and the accumulation of spontaneous emission noise light generated in the optical amplifier, and it is actually impossible to realize a frequency shift of 200 GHz or more. It was difficult.

Furthermore, in order to obtain a large relative frequency, it is necessary to increase the number of rounds, and the time interval at which the reference pulse light having a specific frequency shift can be output becomes long. In this case, the interval of frequency control of the slave light source is correspondingly lengthened, so that the absolute frequency stability of the slave light cannot be increased.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide an optical frequency reference generator which generates a frequency reference light with excellent frequency stability and setting accuracy over a wide relative frequency range.

[0013]

An optical frequency reference generator according to the present invention comprises a frequency reference light source whose oscillation frequency is stabilized, and optical pulse modulation for modulating continuous light output from the frequency reference light source into pulsed light. Optical ring circuit for circulating an optical wave, and optical coupling means for coupling the pulsed light output from the optical pulse modulator to the optical ring circuit and for extracting the output light from the optical ring circuit. Is an optical amplifier that amplifies propagating light to guarantee the loss of the optical ring circuit, an optical delay element that delays propagating light and adjusts its orbiting time, and a predetermined frequency shift to propagating light. An optical frequency shifter and an optical frequency reference generator, in which an optical switch for connecting and disconnecting propagating light is inserted, and including an optical pulse modulator and a synchronous control means for synchronously controlling the optical switch, An optical modulator that modulates the phase or intensity of the propagating light and an optical resonator that spectrum-shapes the modulation component of the propagating light are further inserted in the ring circuit, and the modulation frequency of the optical modulator and the frequency shift of the optical frequency shifter are inserted. It is characterized in that the sum of the quantities is set to be substantially equal to the resonance peak interval of the optical resonator.

The optical frequency shifter and the optical switch can be constructed by one acousto-optic device.

The optical frequency reference generator of the present invention may further comprise control means for stabilizing the frequency of one resonance peak of the optical resonator to the optical frequency from the frequency reference light source. This control means splits the continuous light output from the frequency reference light source into the optical resonator, enters the optical resonator, monitors the continuous light output from the optical resonator, and monitors the monitoring output of the monitoring means. Based on the resonance frequency of the optical resonator. In particular, the means for entering includes means for coupling the continuous light into the optical ring circuit in the opposite direction of the light waves circulating in the optical ring circuit, and the means for monitoring includes means for branching the continuous light from the optical ring circuit. An optical isolator that removes continuous light from the circuit may be included.

[0016]

After the absolute frequency stabilized continuous light from the frequency reference light source used as the master light source is pulsed by the optical pulse modulator, the pulsed light is input to the optical ring circuit by the optical coupling means. An optical amplifier, an optical modulator, an optical delay element, an optical frequency shifter, an optical switch and an optical resonator are inserted in the optical ring circuit, and the amplification gain G of the optical amplifier is adjusted to compensate the loss of the optical ring circuit. Moreover, if the optical pulse width and the one-cycle time of the optical ring circuit are set to be equal to each other, it becomes possible to multiple-cycle the input optical pulse. Further, the spontaneous emission noise light component generated in the optical amplifier is almost removed along with the circulation except near the resonance peak frequency of the optical resonator. Further, by synchronously controlling the optical pulse modulator and the optical switch in the optical ring circuit by the synchronous control means, it is possible to periodically repeat the multiple rounds of the input pulsed light.

Here, an optical modulator and an optical resonator are inserted in the optical ring circuit, and the sum of the modulation frequency and the frequency shift amount in the optical frequency shifter is set to be equal to the resonance peak interval of the optical resonator. By doing so, it is possible to increase the frequency shift amount per revolution.

Further, since the number of orbits required for wideband frequency conversion is extremely small, it is possible to perform frequency conversion over a wider range than before without being affected by the accumulation of spontaneous emission noise light associated with multiple orbits.

By using the frequency-converted pulsed light thus generated as the secondary frequency reference standard, it becomes possible to stabilize the frequency of the slave light source over a wide frequency range. Further, the time interval at which the frequency reference pulsed light is supplied at this time can be greatly shortened as compared with the conventional technique, and the frequency can be stabilized with high accuracy.

[0020]

1 is a block diagram showing an optical frequency reference generator according to a first embodiment of the present invention. This device includes a frequency reference light source 1 whose oscillation frequency is stabilized, an optical pulse modulator 2 which modulates continuous light output from the frequency reference light source 1 into pulsed light, an optical ring circuit which circulates a light wave,
The optical ring circuit is provided with a directional coupler 3 for coupling the pulsed light output from the optical pulse modulator 2 and for extracting the output light from the optical ring circuit. The optical ring circuit amplifies the propagating light and outputs the amplified light. An optical amplifier 4 that guarantees the loss of the optical ring circuit, an optical delay element 6 that delays propagating light to adjust its orbiting time, an optical frequency shifter 7 that gives a predetermined frequency shift to propagating light, and a propagation An optical switch 8 for connecting and disconnecting light is inserted, and a synchronization control device 11 for synchronously controlling the optical pulse modulator 2 and the optical switch 8 is provided.
In this embodiment, an optical modulator 5 for modulating propagating light and an optical resonator 9 for spectrum shaping the modulation component of propagating light are further inserted in the optical ring circuit, and the modulation frequency of the optical modulator and the frequency of the optical frequency shifter are inserted. The sum of the shift amount is set to be substantially equal to the resonance peak interval of the optical resonator 9. The drive signal source 10 is connected to the optical modulator 5.

The frequency reference light source 1 is a light source whose absolute frequency is stabilized by utilizing the absorption line of the acetylene molecule in the wavelength band of 1.5 μm, and outputs continuous light. The optical pulse modulator 2 is composed of, for example, an acousto-optical pulse modulator, and modulates the continuous light output from the frequency reference light source 1 into pulsed light. The optical directional coupler 3 has four ports 31 to 3
4, the pulsed light input from the port 31 is coupled to the optical ring circuit via the port 33, and the optical ring circuit is circulated to halve half the intensity of the light input to the port 34 from the port 33 again. While leading to the circuit, the other half is output from the port 32 as an external output.

The optical amplifier 4 in the optical ring circuit is composed of, for example, an erbium-doped optical fiber or a semiconductor laser amplifier, and amplifies the signal pulse light input to the optical ring circuit. The optical modulator 5 is, for example, Ti diffused LiNb.
It is composed of an O ₃ waveguide and applies high-speed modulation to the output signal light of the optical amplifier 4. Hereinafter, a phase modulator will be described as an example of the optical modulator 5. The phase modulator using the Ti diffused LiNbO ₃ waveguide is capable of high-speed phase modulation up to a modulation frequency of about 20 GHz. The optical delay element 6 is composed of, for example, an optical fiber, and delays the modulated pulsed light from the optical modulator 5. The optical frequency shifter 7 is composed of, for example, an acousto-optical frequency shifter, and gives a constant frequency shift to the modulated pulsed light. The frequency shift amount of the acousto-optic frequency shifter is about 100 to 200 MHz,
Its stability is high. The optical switch 8 controls ON / OFF of the modulated pulsed light. Although this optical switch 8 can be constituted by an element separate from the optical frequency shifter 7,
When an acousto-optical frequency shifter is used as the optical frequency shifter 7, the function can be shared. Optical resonator 9
Is, for example, a Fabry-Perot resonator, which has a large number of resonance peaks arranged at a constant frequency interval f _s . The spontaneous emission noise light generated in the optical amplifier 4 is significantly reduced by passing through the optical resonator 9. By inputting the output light of the optical resonator 9 into the port 34 of the optical directional coupler 3,
An optical ring circuit is constructed.

The drive signal source 10 is a highly stable signal source for driving the optical modulator 5, and supplies an electric signal having a constant frequency.

In this configuration, the gain of the optical amplifier 4 is adjusted so as to compensate for the loss in the optical ring circuit, so that the phase-modulated pulse signal light can be multiplexed. Further, by setting the one-cycle time of the optical ring circuit to be equal to the width of the input pulse signal light, the multiple-cycle pulse light train from the optical ring circuit can be regarded as pseudo continuous light. Further, by synchronously controlling the optical pulse modulator 2 and the optical switch 2 by the synchronization control device 11, it becomes possible to repeatedly generate a multi-cycle pulsed optical train.

2 and 3 are diagrams for explaining the operation of this embodiment, and are diagrams for explaining the combined effect of phase modulation, frequency shift and spectrum shaping by the optical ring circuit.

Here, the optical frequency of the output light of the frequency reference light source 1 is f ₀ , and the basic modulation frequency of the optical modulator 5 is f _m ,
Furthermore, assuming that an acousto-optical frequency shifter is used as the optical frequency shifter 7, the optical frequency shift amount is represented by -f _A0 . Further, the frequency interval of the resonance peak of the optical resonator 9 is set to f _s
And the half width of the half-value transmission band of each resonance peak is f _B.
Here, the half bandwidth f _B of the optical resonator 9 is sufficiently smaller than the resonance peak interval f _s, and the optical frequency shifter 7
It is assumed that the frequency shift amount is about f _A0 . Specifically, 50 MHz, 10 GHz, and 120 MHz are assumed as the values of f _B , f _s , and f _A , respectively. These values are values that can be sufficiently realized by the optimum design of the optical resonator.

Under the above setting, as shown in FIG. 2A, one frequency of the resonance peak of the optical resonator 9 (its position is shown by a broken line in FIG. 2) is set to the frequency f of the master light source.
Adjust to match ₀ . Further, the sum f _m −f _A0 of the basic modulation frequency f _m of the optical modulator 5 and the frequency shift amount −f _A0 in the optical frequency shifter 7 is equal to the resonance peak frequency interval f _s of the optical resonator, f _m Alternatively, adjust f _s , or both.

[0028]

[Equation 1] The spectrum of the signal pulsed light having the optical frequency f ₀ input to the optical ring circuit is subjected to phase modulation by the optical modulator 5, so that as shown in FIG.
₀ carrier wave component and primary modulation sideband components having optical frequencies f ₀ + f _m , f ₀ −f _m , optical frequencies f ₀ + 2f _m , f ₀ −
Secondary modulation sideband component having a 2f _m, so further with higher order modulation sidebands component. Here, the amplitude of each modulation side wave component is expressed as follows using the Bessel function J.

[0029]

[Equation 2] Here, A is the amplitude of the input pulsed light, and δ is the modulation depth of the optical modulator 5. J _i is an i-th order Bessel function, and its shape is shown in FIG. Here, the modulation depth is δ = 2
2B, the amplitude ratio of the carrier wave component, the first-order modulation sideband component, and the second-order modulation sideband component can be adjusted to about 1: 2: 1.

Next, the signal pulsed light subjected to the high-speed phase modulation by the optical modulator 5 passes through the optical frequency shifter 7 and, as shown in FIG.
Receive f _A0 . Optical frequency of the + 1st order modulation side band component by which proceeds to the next frequency of the resonance peak of _{f 0 + (f m -f A0} ) = f 0 + f s , and the just the optical resonator 9. On the other hand, the frequencies of the 0th-order component (carrier component) and other modulation sideband components deviate from the resonance peak position.

Optical frequency of each modulation sideband component and optical resonator 9
The relationship with the resonance frequency of is shown in FIG. The deviations of the optical frequencies of the 0th-order component (carrier component) and the modulation sideband component from the resonance frequency are −2f for the −first-order component, the 0th-order component (carrier component), the + first-order component, and the + second-order component, respectively.
_A0 , _-fA0 , 0, _fA0 . Therefore, except for the + first-order component, a large transmission loss occurs when passing through the optical resonator 9. Assuming that the full width at half maximum of the resonance peak is 50 MHz and the frequency shift f _A0 by the optical frequency shifter 7 is −120 MHz, the transmission coefficient T + 1 for the + 1st order component is T + 1.
When 1 is set to 1, the transmission coefficients for the amplitudes of the −1st order component, the 0th order component, and the + 2nd order component are T ₋₁ = 1/8 and T ₀ =
_¼ , T ₊₂ = about _¼ .

Therefore, as shown in FIG.
The final amplitude ratio for the first-order component, the 0th-order component, the + first-order component, and the + second-order component is about 1: 1: 8: 1, and the ratio of the + 1st-order component in the total light intensity of the input pulsed light is 95.
%become. Therefore, it is possible to consider that the optical frequency of the input pulsed light shifts by f _m −f _A0 , except for the existence of a small sideband component, by making one round in the optical ring circuit.

As described above, the resonance peak frequency interval of the optical resonator 9, the modulation frequency of the optical modulator 5, and the frequency shift amount in the optical frequency shifter 7 are adjusted according to the equation (1). Frequency shift amount per revolution for + 1st order modulation component is several GHz to 10 GHz
It is possible to set the value as large as possible, and it is possible to suppress the intensity of other modulation components.

A signal pulse having the modulation component amplitude ratio shown in FIG. 3B is input to the port 34 of the optical directional coupler 3 and a part of the signal pulse is taken out from the port 32 as an external output.
The rest is input again to the optical amplifier 4 via the port 33. In this way, optical amplification, phase modulation, frequency shift,
By repeating spectrum shaping and branching, the optical frequency of the signal pulse light shifts the resonance frequency peak one after another, accompanied by some small modulation sideband components. For example, if f _m −f _A0 = 10 GHz is set, a frequency shift of 1 THz is possible in 100 turns.

Here, the change in the amplitude ratio of each modulation component of the phase-modulated pulsed light due to multiple turns is analyzed, and it is clarified that other components are suppressed with respect to the + 1st-order component. Here, if the −1st-order component, the 0th-order component, the 1st-order component, and the 2nd-order component are taken into consideration as the modulation sideband component, the change in the spectrum due to the circulation of the signal pulse light is expressed by the following equation.

[0036]

(Equation 3)

[0037]

[Equation 4] n is the number of revolutions (in FIG. 1, it is defined that the revolution is started at the time of leaving the optical amplifier 4), E is a beltrepresenting the amplitude of light, and j = 1, 2, 3, and 4, respectively. +2
It is an index showing the second-order, + first-order, zero-order, and −1st-order modulation sideband components. The matrix T represents the combined effect of spectrum shaping by the phase modulation 5, the frequency shifter 7, and the optical resonator 9, and is obtained by combining the value of the Bessel function corresponding to each modulation sideband component and the transmission coefficient of the optical resonator 9. The transmission coefficient is standardized with the transmission coefficient at the resonance peak frequency being 1, and the effect of the loss due to the insertion of each optical component and the optical branch is represented by the loss L of the optical ring circuit. Further, G is the amplification gain of the optical amplifier 4 in each round, and is controlled so as to compensate the loss of the optical ring circuit.

The amplitude vector of the input pulse signal in the first round is adjusted in advance so that the input optical frequency coincides with one resonance peak position of the optical resonator 9, and can be expressed by the following equation. .

[0039]

(Equation 5) Further, by using the equation (3), the amplitude vector of the pulse signal light in the (n + 1) th circulation can be calculated by the following equation.

[0040]

(Equation 6) Here, the equation (6) is calculated by using the eigenvalues λ ₁ , λ ₂ , λ ₃ , λ ₄ of the matrix T and the corresponding unit eigenvectors using e ₁ , e ₂ , e ₃ , e ₄ . Specifically, the form in which the amplitude vector of the input pulse signal light is expanded by the unit eigenvector is used.

[0041]

(Equation 7) Using this formula, the formula of the equation 6 is expressed as follows.

[0042]

(Equation 8) Here, when the maximum eigenvalue is λ ₁ , the projection components on the eigenvectors e ₁ , e ₂ , e ₃ , and e ₄ are reduced by repeating the circulation, and the amplitude vector of the pulse signal light converges.

[0043]

[Equation 9] Therefore, after a certain number of rounds, the amplitude vector of the pulse signal light becomes a constant multiple of the eigenvector e ₁ corresponding to the maximum eigenvalue λ ₁ of the matrix T.

Therefore, in the eigenvector e ₁ corresponding to λ ₁ in which the ratio of the maximum eigenvalue λ ₁ of the matrix T to other eigenvalues is large and the component of j = 2 corresponding to the + 1st order modulation component is different from the other components. By optimizing the modulation depth of the optical modulator 5 and the transmission characteristics of the optical resonator 9 so that the signal pulse light is sufficiently large in comparison, the signal pulsed light has the maximum intensity of the + 1st-order modulation component and other modulation components. It becomes possible to repeat the orbit while keeping the suppressed almost constant spectrum, and the optical frequency of the + 1st order modulation component can move the resonance peak of the optical resonator 9 one by one.

As described above, the high-speed optical modulator 5 and the optical resonator 9 are inserted in the light wave frequency conversion ring circuit, and the modulation frequency f _{m for} phase modulation and the frequency shift amount f _A0 in the optical frequency shifter 7 are inserted. It is possible to expand the frequency shift amount f _s per revolution to several GHz to 10 GHz by setting the sum of the above and the resonance peak interval f _s of the optical resonator 9 to be equal.

Further, since the number of rounds required for wideband frequency conversion is significantly reduced, frequency conversion over several THz is possible without being affected by the accumulation of spontaneous emission noise light associated with multiple rounds. By using the frequency-converted pulsed light thus generated as the secondary frequency reference standard, it becomes possible to stabilize the frequency of the slave light source over a wide frequency range. Further, at this time, the time interval at which the frequency reference pulse light is supplied is significantly shortened as compared with the method according to the related art, so that the frequency can be stabilized with high accuracy.

Next, as a more specific example, the sum of the modulation frequency f _m of the optical modulator 5 and the frequency shift amount f _A0 of the optical frequency shifter 7 is equal to the resonance peak interval f _s of the optical resonator 9 (number 1), the degree and frequency shift amount f _A0 of the optical frequency shifter 7 is a half-value width f _B of the resonance peak of the optical resonator 5 same or is at less, one frequency is frequency reference source of the resonance peak of the optical resonator 5 A case where the frequency is set to match the frequency f ₀ of the input pulse signal light of 1 will be described. In this case, the optical frequency of the main component of the modulated and revolving pulsed light undergoes a frequency shift equal to the resonance peak interval f _s per revolution.

Hereinafter, the modulation frequency f _m by the optical modulator 5 is 10.120 GHz, the frequency shift amount f _A0 by the optical frequency shifter 7 is -120 MHz, and the frequency interval f _s of the resonance peak in the optical resonator 9 is 10.0 GHz. When the half-value width f _B of the resonance peak is assumed to be 100 MHz, the change in the frequency spectrum accompanying the circulation of the phase-modulated pulse signal light is quantitatively analyzed, and the frequency of the pulse signal light is swept stepwise along with the circulation. Indicates that.

The modulation depth in the optical modulator 5 is adjusted so that the amplitudes of the 0th-order modulation component and the 2nd-order modulation component are 1/2 when the amplitude of the 1st-order modulation component is 1.
Therefore, the Bessel function standardized by the amplitude of the primary modulation component is as follows for each component.

[0050]

[Equation 10] If a Fabry-Perot resonator is used as the optical resonator 9, its transmission characteristic (transmission amplitude / incident amplitude) is given by the following equation when finesse is defined by F:

[0051]

[Equation 11] Here, λ is the light wavelength, and n and l are the refractive index of the medium and the cavity length. This equation is shown in Maruzen, "The Basics of Optoelectronics" by A. Yariv. Finesse F 1
When set to 00, the value of the half-value width / interval of the resonance peak is 1
00 MHz / 100 GHz = 1/100. Therefore, if the transmission ratio for the + 1st order modulation component is 1, -1
The transmission ratios for the 2nd, 0th, and + 2nd order modulation sideband components are
The following equations are obtained from the equations of the equation (11).

[0052]

(Equation 12) Here, the modulation sideband components of +3 or more and −2 or less are ignored because they have small amplitudes and small transmittances of the Fabry-Perot resonator, and the −1st-order component, 0th-order component, + 1st-order component, + 2nd-order component The four components are considered. At this time, the matrix T representing the effect that the optical ring circuit has on the sideband component ratio of the phase-modulated signal is as follows from the equations 4, 10 and 12.

[0053]

(Equation 13) Here, j = 1, 2, 3, and 4 are + secondary and +1 respectively.
It is an index indicating the 2nd, 0th, and −1st order modulation components. Matrix T
The eigenvalues λ ₁ , λ ₂ , λ ₃ , and λ ₄ of are as follows.

[0054]

[Equation 14] Therefore, the corresponding unit eigenvectors are as follows, rounding off the decimal point to the third place.

[0055]

(Equation 15) Here, in the eigenvector e ₁ corresponding to the eigenvalue λ ₁ ,
It can be seen that most of the light intensity is concentrated on j = 2, that is, the + 1st-order modulation component.

The normalized amplitude vector for the input pulse signal can be transformed as follows by expanding the equation of Equation 5 with the eigenvector given by Equation 15.

[0057]

[Equation 16] In order to obtain the amplitude vector after n rounds, the matrix T (equation 1
Substituting 3) and the input amplitude vector (Equation 16) into the equation of Equation 6 that describes the multi-turn, and transforming using the eigenvalue (Equation 14) and the eigenvector (Equation 15), the following equation is obtained.

[0058]

[Equation 17] After four or five rounds, the eigenvector e ₂ ,
The projection components on e ₃ and e ₄ are sufficiently smaller than the projection components on e ₁ . Therefore, after the circuit has circulated to some extent, the component ratio of the amplitude vector of the phase-modulated pulse signal light is maintained at a constant value given by the next eigenvector e ₁ .

[0059]

(Equation 18) In this case, about 9.4% of the light intensity in the + 1st-order modulation sideband
Are concentrated, and the relative intensity ratios of the + 2nd order component, the 0th order component, and the −1st order component to the + 1st order component are −17 dB and −1, respectively.
It becomes 4 dB and -17 dB. This is shown in FIG.

As described above, in this example, by making one round in the optical ring circuit, the optical frequency of the pulsed light is 10 G.
Hz shift, and as a result, a frequency shift of 1 THz becomes possible after 100 turns. At this time, when the time for one round is set to 10 μs, it becomes possible to supply the frequency reference pulsed light with the frequency shift amount of 1 THz in the cycle of 1 ms.

FIG. 6 is a block diagram showing an optical frequency reference generator according to the second embodiment of the present invention. This device differs from the first embodiment in that it has a control means for stabilizing the frequency of one resonance peak of the optical resonator 9 to the optical frequency from the frequency reference light source 1. That is, the optical directional coupler 21 that branches the continuous light output from the frequency reference light source 1 and the optical directional coupler 22 that inputs the branched light into the optical resonator 9.
And an optical directional coupler 23 and a photodetector 25 for monitoring continuous light output from the optical resonator 9, and controlling the resonance frequency of the optical resonator 9 based on the detection output of the photodetector 25. The control device 26 is provided. Further, an optical isolator 24 is provided to remove the continuous light used to stabilize the resonance peak frequency of the optical resonator 9 from the optical ring circuit.

The frequency reference light source 1 is a light source whose absolute frequency is stabilized by utilizing, for example, the absorption line of the acetylene molecule in the wavelength band of 1.5 μm, and outputs continuous light. The optical directional coupler 21 guides the continuous light output from the frequency reference light source 1 to the optical pulse modulator 2, and branches a part of the continuous light to the optical directional coupler 22. The optical pulse modulator 2 is composed of, for example, an acousto-optic pulse modulator, and modulates continuous light from the frequency reference light source 1 via the optical directional coupler 21 into pulsed light. The optical directional coupler 3 has four ports 31 to 31.
34, the pulsed light input from the port 31 is coupled to the optical ring circuit via the port 33, and the optical ring circuit is circulated to halve the intensity of the light input to the port 34 again from the port 33. While leading to the circuit, the other half is output from the port 32 as an external output.

The optical amplifier 4 in the optical ring circuit is composed of, for example, an erbium-doped optical fiber or a semiconductor laser amplifier, and amplifies the signal pulse light input to the optical ring circuit. The optical modulator 5 is, for example, Ti diffused LiNb.
It is composed of an O ₃ waveguide and applies high-speed modulation to the output signal light of the optical amplifier 4. The optical delay element 6 is composed of, for example, an optical fiber, and delays the modulated pulsed light from the optical modulator 5. The optical frequency shifter 7 is composed of, for example, an acousto-optical frequency shifter, and gives a constant frequency shift to the modulated pulsed light. The optical switch 8 controls ON / OFF of the modulated pulsed light. The light that has passed through the optical switch 8 is converted into the optical isolator 24 and the optical directional coupler 23.
Is guided to the optical resonator 9 via. The optical resonator 9 is, for example, a Fabry-Perot resonator, and has a constant frequency interval f _s.
It has a large number of resonance peaks lined up. The spontaneous emission noise light generated in the optical amplifier 4 is significantly reduced by passing through the optical resonator 9. The output light of the optical resonator 9 is passed through the optical directional coupler 22 to the port 3 of the optical directional coupler 3.
By inputting to 4, the optical ring circuit is constructed. The drive signal source 10 is a highly stable signal source for driving the optical modulator 5, and supplies an electric signal having a constant frequency.

The optical directional coupler 22 is branched by the optical directional coupler 21 in order to stabilize one resonance peak frequency of the optical resonator 9 to the frequency of the frequency-stabilized reference light from the frequency reference light source 1. The continuous light output from the frequency reference light source 1 is guided to the optical ring circuit in the opposite direction and input to the optical resonator 9. The optical directional coupler 23 takes out continuous light output from the optical resonator 9 and guides it to the photodetector 25. Although a part of the continuous light output from the optical resonator 9 propagates in the opposite direction to the optical ring circuit,
It is removed by the optical isolator 24. Photo detector 2
5 measures the light intensity, and the control device 26 controls the resonance frequency of the optical resonator 9 based on the detected light intensity. Specifically, the resonance frequency is adjusted by temperature control or voltage control. At this time, by performing feedback control so that the light intensity detected by the photodetector 25 is always maximized, the instability of the optical ring circuit due to the shift of the resonance peak frequency due to the change of the external environment can be easily avoided. It will be possible.

In this configuration, by adjusting the gain of the optical amplifier 4 so as to compensate for the loss in the optical ring circuit, the modulated pulse signal light can be multiplexed.
Further, by setting the one-cycle time of the optical ring circuit to be equal to the width of the input pulse signal light, the multiple-cycle pulse light train from the optical ring circuit can be regarded as pseudo continuous light. Further, by synchronously controlling the optical pulse modulator 2 and the optical switch 2 by the synchronization control device 11, it becomes possible to repeatedly generate a multi-cycle pulsed optical train.

In such a configuration, as in the first embodiment, the sum of the modulation frequency f _m of the optical modulator 5 and the frequency shift amount f _A0 of the optical frequency shifter 7 is the resonance peak interval f _s of the optical resonator 9. And the half-value width f _B of the resonance peak of the optical resonator 5 is equal to or less than the frequency shift amount f _A0 of the optical frequency shifter 7, and the frequency of one resonance peak of the optical resonator 5 is equal to The frequency is set to match the frequency f ₀ of the input pulse signal light of 1. As a result, the optical frequency of the main component of the modulated and revolving pulsed light undergoes a frequency shift equal to the resonance peak interval f _s per revolution. Further, by performing feedback control, the resonance peak frequency of the optical resonator 5 is stabilized, and the stable operation of the optical ring circuit becomes possible.

[0067]

As described above, the optical frequency reference generator of the present invention uses the frequency reference light (primary frequency reference reference) frequency-stabilized by using the absorption spectrum to obtain a number T.
A highly stabilized frequency reference light (secondary frequency reference or relative frequency reference) can be generated over a wide relative frequency range over Hz.

Further, by stabilizing the resonance frequency of the optical resonator in the optical frequency reference generator, more stable operation becomes possible.

[Brief description of drawings]

FIG. 1 is a block diagram showing an optical frequency reference generator according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the embodiment.

FIG. 3 is a diagram for explaining the operation of the embodiment.

FIG. 4 is a diagram illustrating a Bessel function.

FIG. 5 is a diagram for explaining effects of modulation, frequency shift, and spectrum shaping by an optical ring circuit.

FIG. 6 is a block diagram showing an optical frequency reference generator according to a second embodiment of the present invention.

FIG. 7 is a block diagram showing a conventional optical frequency reference generator.

[Explanation of symbols]

 1 frequency reference light source 2 optical pulse modulator 3, 21, 22, 23 optical directional coupler 4 optical amplifier 5 optical modulator 6 optical delay element 7 optical frequency shifter 8 optical switch 9 optical resonator 10 high frequency stable drive signal source 11 Synchronous control device 12 Optical narrow band filter 24 Optical isolator 25 Photodetector 26 Control device

Claims (5)

[Claims] 1. A frequency reference light source whose oscillation frequency is stabilized, an optical pulse modulator that modulates continuous light output from this frequency reference light source into pulsed light, an optical ring circuit that circulates a light wave, and this light. An optical coupling unit that couples the pulsed light output from the optical pulse modulator to the ring circuit and extracts the output light from the optical ring circuit, wherein the optical ring circuit amplifies the propagating light and Amplifier that guarantees the loss of light, an optical delay element that delays propagating light to adjust its orbiting time, an optical frequency shifter that gives a predetermined frequency shift to propagating light, and an optical switch that interrupts propagating light. In the optical frequency reference generator having a synchronization control means for synchronously controlling the optical pulse modulator and the optical switch, the optical ring circuit further includes: An optical modulator that modulates the propagating light and an optical resonator that shapes the modulation component of the propagating light are inserted, and the sum of the modulation frequency of the optical modulator and the frequency shift amount of the optical frequency shifter is the optical resonance. An optical frequency reference generator characterized in that it is set to be substantially equal to the resonance peak interval of the device. 2. The optical frequency reference generator according to claim 1, wherein the optical frequency shifter and the optical switch are formed by one acousto-optic device. 3. The optical frequency reference generator according to claim 1, further comprising control means for stabilizing the frequency of one resonance peak of the optical resonator to the optical frequency from the frequency reference light source. 4. The control means branches the continuous light output from the frequency reference light source into the optical resonator, and monitors the continuous light output from the optical resonator. 4. The optical frequency reference generator according to claim 3, further comprising means for controlling the resonance frequency of the optical resonator based on the monitoring output of the means for monitoring. 5. The means for injecting includes means for coupling the continuous light to the optical ring circuit in a direction opposite to the light waves circulating in the optical ring circuit, and the means for monitoring the continuous light from the optical ring circuit. The optical frequency reference generator according to claim 4, further comprising an optical isolator that includes means for branching light and removes the continuous light from the optical ring circuit.