JP3339030B2 - Optical frequency reference generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention applies to the generation of light which can be used as a frequency reference. In particular, the present invention relates to an apparatus for generating a stable and accurate frequency reference light over a wide frequency band. The present invention can be used particularly for stabilizing an optical frequency in a laser spectrometer, an optical measurement device, or an optical frequency multiplex communication device.

[0002]

2. Description of the Related Art Generally, 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 frequency fluctuations or uncertainties in the frequency setting are as follows:
When performing high-precision spectroscopic measurement using laser light, or performing coherent optical communication in which information is transmitted with the frequency or phase of light, etc., the performance of the system is significantly limited.

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

However, when an appropriate absorption spectrum does not exist at a target optical frequency, a technique for stabilizing a relative frequency between the target optical frequency and a frequency of an absorption line relatively close to the target optical frequency has been developed. More is needed.

[0005] Such a technique is called an optical frequency synthesizer, and generally includes a master light source that stabilizes an absolute frequency by using an absorption spectrum, a slave light source that oscillates light of a desired frequency, and a master light source. It comprises a device for generating a relative frequency reference for stabilizing the relative frequency with the slave light source. For such a technology, see "Coherent Optical Quantum Optics" by Motoichi Otsu
I am familiar with Asakura Shoten.

As a method of generating a relative frequency reference, it has been proposed 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 a microwave frequency electric signal is as follows.
Very high-precision relative frequency stabilization is possible by combining with a 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 is at most 20 GHz. It is about. The method using the resonance peak of the Fabry-Perot interferometer can realize a relative frequency setting range of sub THz or more, but it is difficult to set the absolute frequency of the resonance peak with high accuracy. In the method using the external modulation sideband component, the reference standard can be directly obtained 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:
Although it can be overcome by using a multi-stage laser to construct a chain or by using a wideband frequency conversion using a nonlinear optical effect, there is a drawback that the configuration is further complicated.

As described above, there has been no suitable optical frequency synthesizing technique having a simple configuration, a wide relative frequency setting band, and high frequency stability until recently. Recently, K. Shimizu et al., Applied
Optics., Vol. 32, No. 33, pp. 6718-6726, 1993 and
Vol.33, No.15, pp.3209-3219, 1994
No. 232540 proposes to use a lightwave frequency conversion ring circuit. This makes it possible to suppress the relative frequency between the master light source and the slave light source 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 an optical frequency conversion ring circuit. In this conventional example, an absolute frequency stabilized light (primary frequency reference) from a frequency reference light source 1 is pulsed by an optical pulse modulator 2 and input to an optical ring circuit via an optical directional coupler 3. The optical ring circuit has an optical amplifier 4,
The optical amplifier 4 amplifies an optical wave propagating around the optical ring circuit. The optical narrow-band filter 12 includes an optical narrow-band filter 12, an optical delay element 6, an optical frequency shifter 7, and an optical switch 8. The generated spontaneous emission noise light is removed, the optical delay element 6 delays the light wave propagating through this 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 and off. I do. Here, the optical frequency shifter 7 and the optical switch 8 are shown as separate elements, but these can be realized by one acousto-optical element.

In this configuration, if the pulse width is set to be shorter than the round-trip time of the optical ring circuit and the amplification gain of the optical amplifier 4 is adjusted so as to compensate for the loss of the optical ring circuit, the inside of the optical ring circuit will be reduced. The frequency of the pulse light propagating in the orbit is swept accurately with respect to time in a stepwise manner. 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-turn pulse light generated in this way as a secondary frequency reference. An example in which the absolute frequency of the slave light source is actually stabilized is reported in Shimizu et al., IEICE Technical Report, "Laser Quantum Electronics", February 1995.

[0010]

As described above, by using the lightwave frequency conversion ring circuit, the relative frequency setting range can be expanded to about 200 GHz. However, since the acousto-optic element is used as a frequency shifter, the frequency shift amount per operation is 100 to 2 times.
Limited to 00 MHz. For this reason, several hundred GHz
In order to obtain such a large frequency shift, the number of rounds of about 1000 is required. However, this number of rounds is limited by the transmission bandwidth of the optical filter inserted into 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 during which a reference pulse light having a specific frequency shift can be output becomes long. In this case, the interval at which the frequency control of the slave light source is performed becomes longer accordingly, 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 to provide an optical frequency reference generator that generates frequency reference light having excellent frequency stability and setting accuracy over a wide relative frequency range.

An optical frequency reference generator according to the present invention comprises a frequency reference light source having a stabilized oscillation frequency, an optical pulse modulator for modulating continuous light output from the frequency reference light source into pulse light, and a light wave circulating light source. An optical ring circuit for coupling the pulsed light output from the optical pulse modulator to the optical ring circuit, and an optical coupling means for extracting the output light from the optical ring circuit. An optical amplifier for compensating for the loss of the optical ring circuit, an optical delay element for giving a delay to the propagation light to adjust its circulating time, an optical frequency shifter for giving a predetermined frequency shift to the propagation light, An optical switch for intermittent light is inserted, and an optical frequency reference generator having synchronization control means for synchronously controlling the optical pulse modulator and the optical switch is provided. Further, an optical modulator for modulating the phase or intensity of the propagating light and an optical resonator for spectrum-shaping the modulation component of the propagating light are inserted, and the frequency of one resonance peak of the optical resonator is set to the frequency reference light source. Is adjusted so as to match the optical frequency of the output light, and the half width of the resonance peak of the optical resonator is set to be equal to or less than the frequency shift amount of the optical frequency shifter. The sum of the modulation frequency and the frequency shift amount of the optical frequency shifter is set substantially equal to the resonance peak interval of the optical resonator.

The optical frequency shifter and the optical switch can be constituted by one acousto-optical element.

[0015] 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. The control means includes means for splitting continuous light output from the frequency reference light source and entering the optical resonator, means for monitoring continuous light output from the optical resonator, and monitoring output of the monitoring means. Means for controlling the resonance frequency of the optical resonator based on the information. In particular, the means for impinging includes means for coupling continuous light to the optical ring circuit in a direction opposite to the lightwave circulating around the optical ring circuit, the means for monitoring includes means for splitting continuous light from the optical ring circuit, An optical isolator for removing continuous light from the circuit may be provided.

[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 pulse 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 into the optical ring circuit, and the amplification gain G of the optical amplifier is adjusted to compensate for the loss of the optical ring circuit. If the optical pulse width is set to be equal to the rounding time of the optical ring circuit, the input optical pulse can be multiplexed. Further, the spontaneous emission noise light component generated in the optical amplifier is almost removed along with the circuit excluding the vicinity of the resonance peak frequency of the optical resonator. Further, the synchronous control means synchronously controls the optical pulse modulator and the optical switch in the optical ring circuit, so that the multiplexed round of the input pulse light can be periodically repeated.

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, the amount of frequency shift per circuit can be increased.

Further, since the number of rounds required for wideband frequency conversion is very small, frequency conversion over a wider range than before can be performed without being affected by accumulation of spontaneous emission noise light due to multiple rounds.

By using the frequency-converted pulse light generated as described above as a secondary frequency reference, it is possible to stabilize the frequency of the slave light source over a wide frequency range. Further, the time interval at which the frequency reference pulse light is supplied at this time can be greatly shortened as compared with the related art, 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 having an oscillation frequency stabilized, an optical pulse modulator 2 for modulating continuous light output from the frequency reference light source 1 into pulse light, and an optical ring circuit for circulating light waves.
The optical ring circuit includes a directional coupler 3 for coupling the pulsed light output from the optical pulse modulator 2 and extracting the output light from the optical ring circuit. An optical amplifier 4 for compensating for the loss of the optical ring circuit, an optical delay element 6 for giving a delay to the propagation light to adjust its circulating time, an optical frequency shifter 7 for giving a predetermined frequency shift to the propagation light, An optical switch 8 for interrupting 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 modulating component of the propagating light are further inserted into the optical ring circuit, and the modulation frequency of the optical modulator and the frequency of the optical frequency shifter are inserted. The sum with the shift amount is set substantially equal to the resonance peak interval of the optical resonator 9. A 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 using, for example, an absorption line of acetylene molecules in a wavelength band of 1.5 μm, and outputs continuous light. The optical pulse modulator 2 includes, for example, an acousto-optic pulse modulator, and modulates continuous light output from the frequency reference light source 1 into pulse light. The optical directional coupler 3 has four ports 31 to 3
4, the pulse light input from the port 31 is coupled to the optical ring circuit via the port 33, and the optical ring circuit circulates around the optical ring circuit and halves the intensity of the light input to the port 34 from the port 33 again. The other half is output from the port 32 as an external output while being guided to the circuit.

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 light modulation modulator 5 is, for example, Ti-diffused LiNb.
It is composed of an O ₃ waveguide and gives 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. In a phase modulator using a Ti-diffused LiNbO ₃ waveguide, high-speed phase modulation up to a modulation frequency of about 20 GHz is possible. The optical delay element 6 is composed of, for example, an optical fiber and gives a delay to the modulated pulse light from the optical modulator 5. The optical frequency shifter 7 is constituted by, for example, an acousto-optic frequency shifter, and gives a fixed frequency shift to the modulated pulse 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 performs on / off control of the modulated pulse light. Although the optical switch 8 can be constituted by an element separate from the optical frequency shifter 7,
When an acousto-optic frequency shifter is used as the optical frequency shifter 7, its function can be shared. Optical resonator 9
Is, for example, a Fabry-Perot resonator has a number of resonant peaks which are arranged at a constant frequency interval f _s. 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 to the port 34 of the optical directional coupler 3,
An optical ring circuit is configured.

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

In this configuration, by adjusting the gain of the optical amplifier 4 so as to compensate for the loss in the optical ring circuit, it is possible to multiplex the phase modulated pulse signal light. In addition, by setting the one round trip time of the optical ring circuit to be equal to the width of the input pulse signal light, it becomes possible to regard the multiple round pulse light train from the optical ring circuit as pseudo continuous light. Further, by controlling the optical pulse modulator 2 and the optical switch 2 synchronously by the synchronous control device 11, it becomes possible to repeatedly generate a multi-circular pulse light train.

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

[0026] Here, f ₀ the optical frequency of the output light of the frequency reference source 1, the fundamental modulation frequency of the optical modulator 5 and f _m,
Furthermore, assuming the use of acousto-optic frequency shifter as an optical frequency shifter 7 represents an optical frequency shift amount and -f _A0. Further, the frequency interval of the resonance peak of the optical resonator 9 is f _s
And then, the half-value transmission bandwidth of each resonance peak and f _B. Here, the half-value transmission bandwidth f _B of the optical resonator 9 is sufficiently smaller than the resonance peak interval f _s, and is about the size f _A0 of the frequency shift amount in the optical frequency shifter 7. Specifically, the values of f _B , f _s , and f _A0 are respectively 10
Assume 0 MHz, 10 GHz, and 120 MHz. These values are values that can be sufficiently realized by the optimal design of the optical resonator.

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

[0028]

(Equation 1) The spectrum of the signal pulse light having the optical frequency f ₀ input to the optical ring circuit is subjected to phase modulation by the optical modulator 5, and as shown in FIG.
₀ of carrier component and an optical frequency f ₀ + f _m, 1-order modulation sideband components with f ₀ -f _m, optical frequency _{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 pulse 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, it is possible to make the amplitude ratio of the carrier component, the primary modulation sideband component, and the secondary modulation sideband component approximately 1: 2: 1, as shown in FIG. 2B.

Next, the signal pulse light that has been subjected to the high-speed phase modulation by the optical modulator 5 passes through the optical frequency shifter 7, and has a constant frequency shift 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 zero-order component (carrier component) and other modulation sideband components deviate from the position of the resonance peak.

The optical frequency of each modulation sideband component and the optical resonator 9
FIG. 3A shows the relationship with the resonance frequency. The deviation of the optical frequency of the 0th order component (carrier component) and the modulation sideband component from the resonance frequency is -2f for the -1st order component, 0th order component (carrier component), + 1st order component, and + 2nd order component, respectively.
_{A 0} , −f _A0 , 0, f _A0 , and therefore suffers a large transmission loss when passing through the optical resonator 9 except for the +1 order component. Assuming that the half width at half maximum of the resonance peak is 50 MHz and the frequency shift f _A0 = −120 MHz by the optical frequency shifter 7, the transmission coefficient T + 1 for the +1 order component
Is _{1, the} transmission coefficients with respect to the amplitudes of the −1st-order component, the 0th-order component, and the + 2nd-order component are T ₋₁ = 1/8 and T ₀ =
１ ／, T ₊₂ = about １ ／.

Therefore, as shown in FIG.
The final amplitude ratio with respect to the first-order component, the zero-order component, the + 1st-order component, and the + 2nd-order component is about 1: 1: 8: 1, and the ratio of the + 1st-order component to the total light intensity of the input pulse light is 95.
%become. Thus, by going around the optical ring circuit, if the presence of a small sideband components separately, the optical frequency of the input pulse light is enabled be regarded as shifted by f _m -f _A0.

As described above, by adjusting the resonance peak frequency interval of the optical resonator 9, the modulation frequency of the optical modulator 5, and the amount of frequency shift in the optical frequency shifter 7 according to the equation 1, The frequency shift amount per round for the + 1st order modulation component is several GHz to 10 GHz.
It can be set as large as possible, and the intensity of other modulation components can be suppressed.

The 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 is taken out from the port 32 as an external output.
The rest is input to the optical amplifier 4 again via the port 33. In this way, optical amplification, phase modulation, frequency shift,
By repeating the spectrum shaping and branching, the optical frequency of the signal pulse light shifts the resonance frequency peak one after another with 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 rounds.

Here, the change in the amplitude ratio of each modulated component of the phase-modulated pulsed light accompanying the multiple rounds is analyzed to clarify that other components are suppressed with respect to the + 1st-order component. Here, considering the -1st-order component, the 0th-order component, the 1st-order component, and the 2nd-order component as the modulation sideband components, the spectrum change accompanying the circling of the signal pulse light is expressed by the following equation.

[0036]

(Equation 3)

[0037]

(Equation 4) n is the number of rounds (in FIG. 1, it is defined that the round is started when the optical amplifier 4 exits), E is a belttle indicating the amplitude of light, and j = 1, 2, 3, and 4 are: +2
An index indicating the next, + 1st, 0th, and -1st 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 normalized by assuming that the transmission coefficient at the resonance peak frequency is 1, and the effect of the loss due to the insertion and branching of each optical component is represented by the loss L of the optical ring circuit. G is the amplification gain of the optical amplifier 4 in each round, and is controlled so as to compensate for the loss of the optical ring circuit.

Since 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, it 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 round can be calculated by the following equation.

[0040]

(Equation 6) Here, the eigenvalues λ ₁ , λ ₂ , λ ₃ , and λ ₄ of the matrix T and the corresponding unit eigenvectors are calculated using the formulas e ₁ , e ₂ , e ₃ , and e _{4 according} to Equation 6. Specifically, a format in which the amplitude vector of the input pulse signal light is expanded by the unit eigenvector is used.

[0041]

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

[0042]

(Equation 8) Here, assuming that the largest eigenvalue is λ ₁ , the repetition of the circulation reduces the projected components to the eigenvectors e ₁ , e ₂ , e ₃ , and e _4, 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, the ratio of the maximum eigenvalue λ ₁ of the matrix T to other eigenvalues becomes large, and in the eigenvector e ₁ corresponding to λ ₁ , the component of j = 2 corresponding to the + 1st-order modulation component is different from the other components. By optimizing the modulation depth in the optical modulator 5 and the transmission characteristics of the optical resonator 9 so that the signal pulse light becomes sufficiently large, the signal pulse light maximizes the intensity of the + 1st-order modulation component and the other modulation components The circling is repeated while maintaining the suppressed and substantially constant spectrum, and the optical frequency of the + 1st-order modulation component can shift the resonance peak of the optical resonator 9 one by one.

[0045] As described above, by inserting a high-speed optical modulator 5 and an optical resonator 9 to lightwave frequency conversion ring circuit, the frequency shift amount in the modulation frequency f _m and the optical frequency shifter 7 of the phase modulation f _A0 Is set to be equal to the resonance peak interval f _s of the optical resonator 9, the frequency shift amount f _s per round can be expanded to about several GHz to 10 GHz.

Furthermore, since the number of rounds required for wideband frequency conversion is significantly reduced, frequency conversion over several THz is possible without being affected by accumulation of spontaneous emission noise light accompanying multiple rounds. By using the frequency-converted pulsed light generated in this manner as a secondary frequency reference, it is possible to stabilize the frequency of the slave light source over a wide frequency range. Also, at this time, the time interval at which the frequency reference pulse light is supplied is significantly reduced as compared with the method according to the related art, so that highly accurate frequency stabilization becomes possible.

Next, as a more specific example, equal to the sum of the frequency shift amount f _A0 of the modulating frequency f _m and the optical frequency shifter 7 of the optical modulator 5 is the resonance peak interval f _s of the optical resonator 9 (number 1), the half width f of the resonance peak of the optical resonator 5
_B 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 matches the frequency f ₀ of the input pulse signal light from the frequency reference light source 1. A description will be given of the case where the settings are made as described above. In this case, the optical frequency of the main component of the modulated pulse light that is circulated undergoes a frequency shift equal to the resonance peak interval f _s per round.

[0048] Hereinafter, 10.120GHz the modulation frequency f _m by the optical modulator 5, the frequency shift amount f _A0 by optical frequency shifter 7 -120MHz, the frequency interval f _s of the resonance peak in the optical resonator 9 10.0 GHz, for the case where the half-value width f _B of the resonance peak was assumed 100 MHz, analyzes the change in the frequency spectrum due to the orbiting of the quantitative phase modulated pulse signal light, the frequency of the pulse signal light with the circulation is gradually being stepped sweep Indicates that

The modulation depth in the optical modulator 5 is adjusted so that the amplitude of the 0th-order modulation component and the amplitude of the secondary modulation component are each と き when the amplitude of the primary modulation component is 1.
Therefore, the Bessel function normalized by the amplitude of the primary modulation component is as follows for each component.

[0050]

(Equation 10) Assuming that 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 the 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 resonator length. This formula is shown in Maruzen, "Basics of Optoelectronics" by A.Yariv. Finesse F 1
When set to 00, the value of the half width / interval of the resonance peak is 1
00 MHz / 100 GHz = 1/100. Therefore, assuming that the transmission ratio to the + 1st-order modulation component is 1, -1
The transmission ratios for the second, zeroth and + 2nd order modulation sideband components are:
According to the equation of the formula 11, the following is obtained.

[0052]

(Equation 12) Here, the modulation sideband components above +3 and below -2 are ignored because the amplitude is small and the transmittance of the Fabry-Perot resonator is small, so that the −1st order component, the 0th order component, the + 1st order component, and the + 2nd order component Four components are considered. At this time, the matrix T representing the effect of the optical ring circuit on the sideband component ratio of the phase modulation signal is as follows from Equations 4, 10, and 12 respectively.

[0053]

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

[0054]

[Equation 14] Accordingly, the corresponding unit eigenvectors are as follows, with the third decimal place rounded off.

[0055]

(Equation 15) Here, in the eigenvector e ₁ corresponding to the eigenvalue λ ₁ ,
It can be seen that most 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) To find the amplitude vector after n rounds, a matrix T (Equation 1)
Substituting 3) and the input amplitude vector (Equation 16) into the equation of Equation 6 describing the multiplexing, 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 projected components to e ₃ and e ₄ are sufficiently smaller than the projected components to e ₁ . Thus, in after some extent orbiting, component ratio of the amplitude vector of the phase modulated pulse signal light is kept at a constant value given by the following eigenvector e _1.

[0059]

(Equation 18) In this case, about 94% of the light intensity is added to the + 1st-order modulation sideband component.
Are concentrated, and the relative intensity ratios of the +2 order component, the 0 order component, and the −1 order component to the +1 order component are −17 dB and −1, respectively.
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 pulse light becomes 10G.
Hz, so that a frequency shift of 1 THz is possible after 100 rounds. At this time, if the time per round is set to 10 μs, it becomes possible to supply the frequency-based reference pulse light having a frequency shift amount of 1 THz at a cycle of 1 ms.

FIG. 6 is a block diagram showing an optical frequency reference generator according to a 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, an optical directional coupler 21 that branches continuous light output from the frequency reference light source 1 and an optical directional coupler 22 that splits the branched light into the optical resonator 9.
And a light directional coupler 23 and a photodetector 25 for monitoring continuous light output from the optical resonator 9. The resonance frequency of the optical resonator 9 is controlled based on the detection output of the photodetector 25. The control device 26 is provided. Further, an optical isolator 24 is provided to remove continuous light used for stabilizing 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 using, for example, absorption lines of acetylene molecules 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, branches a part of the continuous light, and guides the branched light to the optical directional coupler 22. The optical pulse modulator 2 includes, for example, an acousto-optical pulse modulator, and modulates continuous light from the frequency reference light source 1 via the optical directional coupler 21 into pulse light. The optical directional coupler 3 has four ports 31 to
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 circulates around the optical ring circuit and halves the intensity of the light input to the port 34 from the port 33 again. The other half is output from the port 32 as an external output while being guided to the circuit.

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 light modulation modulator 5 is, for example, Ti-diffused LiNb.
It is composed of an O ₃ waveguide and gives 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 gives a delay to the modulated pulse light from the optical modulator 5. The optical frequency shifter 7 is constituted by, for example, an acousto-optic frequency shifter, and gives a fixed frequency shift to the modulated pulse light. The optical switch 8 performs on / off control of the modulated pulse light. The light that has passed through the optical switch 8 is converted into an optical isolator 24 and an optical directional coupler 23.
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.
Have a number of resonance peaks. 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 the signal to 4, an optical ring circuit is formed. The drive signal source 10 is a highly stable signal source for driving the optical modulator 5, and supplies an electric signal of a constant frequency.

The optical directional coupler 22 is branched by the optical directional coupler 21 to stabilize one resonance peak frequency of the optical resonator 9 to the frequency of the frequency stabilizing 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 is input to the optical resonator 9. The optical directional coupler 23 extracts the continuous output light from the optical resonator 9 and guides the continuous light to the photodetector 25. Part of the output continuous light from the optical resonator 9 propagates in the opposite direction to the optical ring circuit.
It is removed by the optical isolator 24. Photodetector 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 fluctuation of the external environment can be easily avoided. It becomes 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, it is possible to multiplex the modulated pulse signal light.
In addition, by setting the round trip time of the optical ring circuit to be equal to the width of the input pulse signal light, the multiple round pulse light train from the optical ring circuit can be regarded as pseudo continuous light. Further, by controlling the optical pulse modulator 2 and the optical switch 2 synchronously by the synchronous control device 11, it becomes possible to repeatedly generate a multi-circular pulse light train.

[0066] In this configuration, as in the first embodiment, modulation of the optical modulator 5 frequency f _m and the resonance peak interval f _s of the sum of the frequency shift amount f _A0 of the optical frequency shifter 7 is the optical resonator 9 equally, the degree half-value width f _B of the resonance peak of the optical resonator 5 is a frequency shift amount f _A0 of the optical frequency shifter 7 same or is at less, one frequency is frequency reference source of the resonance peak of the optical resonator 5 The frequency is set so as to match the frequency f ₀ of the input pulse signal light from No. 1. Thus, the main component of the optical frequency of the pulse light circulating modulated is subjected to frequency shift equal to one circuit per resonance peak interval f _s. Further, by performing feedback control, the resonance peak frequency of the optical resonator 5 is stabilized,
The stable operation of the optical ring circuit becomes possible.

[0067]

As described above, the optical frequency reference generating apparatus of the present invention uses the frequency reference light (primary frequency reference standard) whose frequency has been stabilized using the absorption spectrum to several Ts.
Highly stabilized frequency reference light (secondary frequency reference or relative frequency reference) over a wide relative frequency range over Hz can be generated.

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

[Brief description of the 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 illustrating the operation of the embodiment.

FIG. 3 is a diagram illustrating the operation of the embodiment.

FIG. 4 is a diagram illustrating a Bessel function.

FIG. 5 is a diagram illustrating 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]

 Reference Signs List 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

Continuation of the front page (56) References JP-A-6-202181 (JP, A) JP-A-6-110103 (JP, A) JP-A-6-110102 (JP, A) JP-A-5-232540 (JP) JP-A-5-218550 (JP, A) JP-A-5-218549 (JP, A) JP-A-5-211366 (JP, A) JP-A-3-91726 (JP, A) 4-503868 (JP, A) Shimizu et al. , Applied Optics, Vol. 33, No. 15, p. 3209-3219 (1994) Shimizu et al. , Applied Optics, Vol. 32, No. 33, p. 6718-6726 (1993) Koji Tsuji et al., IEICE Technical Report, Vol. 94, no. 493 (OPE 94-108 to 126), p. 55-59, February 16, 1995 (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/00-7/00 H04B 10/00-10/28 H04J 14/00-14/02 JICST file (JOIS) WPI (DIALOG)

Claims (5)

(57) [Claims] 1. A frequency reference light source whose oscillation frequency is stabilized, an optical pulse modulator that modulates continuous light output from the frequency reference light source into pulse light, an optical ring circuit that circulates light waves, Optical coupling means for coupling the pulsed light output from the optical pulse modulator to the ring circuit and extracting the output light from the optical ring circuit. Optical amplifier that compensates for the loss of the optical signal, an optical delay element that delays the propagating light to adjust its orbit time, an optical frequency shifter that applies a predetermined frequency shift to the propagating light, and an optical switch that interrupts the propagating light Wherein the optical ring circuit further includes a synchronization control unit that controls the optical pulse modulator and the optical switch in synchronization. An optical modulator for modulating the propagating light and an optical resonator for spectrum shaping the modulation component of the propagating light are inserted, and the frequency of one resonance peak of the optical resonator is the optical frequency of the output light of the frequency reference light source. The half-width of the resonance peak of the optical resonator is set to be equal to or less than the frequency shift amount of the optical frequency shifter, and the modulation frequency of the optical modulator and the optical frequency are adjusted. An optical frequency reference generator, wherein the sum of the shift amount and the frequency shift amount of the shifter is set substantially equal to the resonance peak interval of the optical resonator. 2. The optical frequency reference generator according to claim 1, wherein said optical frequency shifter and said optical switch are constituted by one acousto-optic device. 3. The optical frequency reference generator according to claim 1, further comprising control means for stabilizing a frequency of one resonance peak of the optical resonator to an optical frequency from the frequency reference light source. 4. The control means includes means for branching continuous light output from the frequency reference light source and entering the optical resonator, and means for monitoring continuous light output from the optical resonator. 4. An optical frequency reference generator according to claim 3, further comprising means for controlling a resonance frequency of said optical resonator based on a monitoring output of said monitoring means. 5. The light incident means includes means for coupling the continuous light to the optical ring circuit in a direction opposite to a light wave circulating around the optical ring circuit, and the monitoring means comprises means for coupling the continuous light from the optical ring circuit to the optical ring circuit. 5. The optical frequency reference generator according to claim 4, further comprising an optical isolator that includes means for splitting light and removes the continuous light from the optical ring circuit.