JP2006106362A - Optical frequency synthesizer and wavelength converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical frequency synthesizer, the accuracy of which an optical frequency of output light is defined with accuracy of a microwave synthesizer compared with an optical frequency of input light, and also provide a wavelength converter. <P>SOLUTION: The optical frequency synthesizer comprises: an optical frequency shifter 2; an optical loop circuit 1 including the optical frequency shifter 2 inside; a multiplexer 3 to synthesize orbiting light and reference light; and an optical output circuit 4 to output prescribed light to the outside of the optical loop circuit 1. Furthermore, the optical frequency synthesizer is equipped with: an optical interference circuit 5 to take out orbiting light beams with the number of orbiting K and L (K, L are zero or natural numbers, K≠L) from the optical output circuit 4 and to make them interfere with each other; a photoelectric conversion means 6 to output an electric signal of the interference light; a phase comparator 7 to output an error signal corresponding to a phase difference between an input microwave reference frequency signal and the electric signal; a loop filter 9 to filter a low frequency component of the error signal; and a voltage control oscillator to control an oscillation frequency with the filtered error signal and to drive the optical frequency shifter 2 based on the oscillation frequency. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical frequency synthesizer and a wavelength converter, and more particularly, to a broadband and highly accurate optical frequency synthesizer and wavelength converter required in the fields of optical communication and optical measurement.

  In recent years, optical communication systems capable of transmitting large volumes of information at high speed have attracted attention because of the desire to transmit large volumes of information with the advancement of information technology. In such an optical communication network, it is indispensable to effectively use the band, and in order to enable the band to be used effectively, it is required to enable wavelength conversion in a wide band.

  In such wavelength conversion, a light source that stabilizes the wavelength of a laser beam, such as a semiconductor laser or a helium neon laser, is used as an optical frequency reference light source in the optical region, for an absorption line of gas such as acetylene or cyanide. Yes. These have sufficient optical frequency pricing and error evaluation of their absorption lines, and have sufficient accuracy for industrial applications.

  However, these evaluated absorption lines are only tens of at most, and cannot be an arbitrary optical frequency reference light source. Therefore, an optical frequency grid is used in which one of a plurality of longitudinal modes of mode-locked laser light is frequency-synchronized with the above-described absorption line, so that the longitudinal mode interval is the grid interval. This mode-locked light is a light short pulse in the time domain and has a large light peak power. Thus, an ultra-wideband optical grid has been obtained by using the nonlinearity of an optical fiber or the like to widen the spectrum band.

  However, in a mode-locked laser, the mode-locked frequency (longitudinal mode interval) can change only at most several percent of the optimum frequency determined by the effective laser cavity length, so it is difficult to generate an arbitrary optical frequency. As a solution to this problem, a technique has been proposed in which the continuous light is shortened by an optical modulator and then the above-mentioned short pulse light is incident on an optical nonlinear material that induces an optical nonlinear phenomenon to increase the bandwidth. However, when the repetition frequency of the optical pulse is greatly changed, the pulse characteristics such as the optical pulse width change, which causes a problem that the intensity of each spectral component varies.

  Also disclosed is a wavelength conversion device capable of extending the optical frequency grid generation range to ± 1.8 THz or more by operating the optical frequency shifter in an optical loop circuit and performing optical frequency shift many times. (See Patent Document 1 and Non-Patent Document 1). In such a wavelength converter, when a single sideband optical modulator (also referred to as an SSB optical modulator) is used as an optical frequency shifter, the optical frequency shift amount can be arbitrarily selected up to about several tens of GHz. is there. Therefore, an arbitrary optical frequency can be generated, and a broadband optical frequency sweep is also possible. For example, when the drive frequency of the optical frequency shifter is swept by 20 GHz from 5 GHz to 25 GHz, the optical frequency can be swept continuously for 1 THz with respect to 50 rounds of light for the optical loop circuit.

JP 2004-37805 A A. Takada et al, “Wavelength Converter Operating on Strict Frequency Grid Using a Single Side Band Optical Modulator in a Circulating Loop” OFC2003 FP5, 2003 O.Tadanaga et al, “Variable Optical Frequency Shifter Using Multiple Quasi-Phase-Matched LiNbO3 Wavelength Converters” OFC2003 paper FP3

  However, in the optical loop circuit including the optical frequency shifter described above, the output light suffers an optical delay compared to the input light by the product of the loop length of the optical loop circuit and the number of times the light passes through the optical loop circuit. Further, the optical circulation delay time, which is the optical delay time, varies depending on the temperature change of the optical loop circuit. Therefore, the optical frequency of the emitted light varies depending on the optical circulation delay time.

For example, if the temperature of an optical loop circuit with a loop length of 2 meters changes by 1 ° C. per second, the optical frequency of the light that circulates the optical loop circuit 50 times is shifted by about 670 Hz with respect to the light having a wavelength of 1.5 microns. To do. Here, the linear expansion coefficient of the optical fiber constituting the loop was about 10 ⁻⁵ / ° C. If the optical spectrum line width of the input reference light is about 10 KHz or more, this degree of optical frequency shift is not a problem. However, when the microwave / millimeter wave is generated as a beat by photoelectric conversion using a photodiode or the like by interfering with the output light and the input reference light, the above-described optical frequency shift is directly applied to the microwave / millimeter wave. There was a problem that it led to a frequency shift.

  Therefore, an optical frequency synthesizer with a stable beat frequency between output lights or between input light and optical loop output light is required even when fluctuations in loop delay occur between the input light and output light to the optical loop circuit. It is said that.

  The present invention has been made in view of such problems, and the object of the present invention is to provide light with an accuracy that the optical frequency of the output light can be defined by the accuracy of the microwave synthesizer compared to the frequency of the input light. The object is to provide a frequency synthesizer and a wavelength converter.

  In order to achieve the above object, the present invention provides an optical frequency shifter for shifting the optical frequency of input light by a predetermined shift amount, and the optical frequency shifter therein. Including an optical loop circuit, a multiplexer that multiplexes light circulating around the optical loop circuit and input light from outside the optical loop circuit, and one or more specific lights among the combined light A light output circuit that outputs light of a frequency outside the optical loop circuit, and outputs output light in a wide-band optical frequency range by shifting the optical frequency of the input light by the predetermined shift amount many times. In the frequency synthesizer, the circulating light output from the optical output circuit and circulated the optical loop circuit a predetermined number of times K and the circulating light circulated the optical loop circuit a predetermined number of times L (K and L are zero or natural) An optical interference circuit that picks up and outputs K ≠ L), photoelectric conversion means for converting the interference light output from the optical interference circuit into an electrical signal, and an input for inputting a microwave reference frequency signal A phase comparison that compares the phase of the microwave reference frequency signal input from the input port with the electrical signal output from the photoelectric conversion means and outputs an error signal according to the phase difference And a voltage-controlled oscillator that controls the oscillation frequency based on the error signal output from the phase comparator and drives the optical frequency shifter based on the oscillation frequency.

  According to a second aspect of the present invention, in the first aspect of the present invention, the error signal output from the phase comparator is further filtered, and means for passing a predetermined frequency component is further provided.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the optical frequency shifter is a single sideband (SSB) optical modulator.

  The invention according to claim 4 is the invention according to claim 1 or 2, wherein the optical frequency shifter includes an optical nonlinear element made of an optical nonlinear material, excitation light of the nonlinear material, and incident light to the optical frequency shifter. And an optical multiplexing means for combining the optical nonlinear element with the optical nonlinear element.

  According to a fifth aspect of the present invention, the optical frequency synthesizer according to any one of the first to fourth aspects is provided.

  As described above, according to the present invention, the shift amount of the optical frequency of the optical frequency shifter is controlled according to the error signal based on the phase difference between the microwave reference frequency signal and the electrical signal output from the photoelectric conversion means. Therefore, the light output from the optical loop circuit can be output light in which the fluctuation of the optical frequency due to the fluctuation of the optical circulation delay time is suppressed. Therefore, it is possible to output the output light with the accuracy that can be defined by the accuracy of the microwave synthesizer compared to the frequency of the input light.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings described below, components having the same function are denoted by the same reference numerals, and repeated description thereof is omitted.
FIG. 1 is a configuration diagram of an optical frequency synthesizer according to an embodiment of the present invention.
In FIG. 1, reference numeral 1 denotes an optical loop circuit in which a loop is formed by means for transmitting light such as an optical fiber or an optical waveguide, and the optical frequency of the input light is shifted by a predetermined shift amount Δν. An optical frequency shifter 2 is provided for output. Therefore, the optical frequency of the light that circulates the optical loop circuit 1 N times is shifted by approximately N × Δν.

  The optical loop circuit 1 has a plurality of input ports, and combines light input from the input ports and outputs the combined light to the same output port (also called the output port of the multiplexer 3). A multiplexer 3 is provided. The first input port of the plurality of input ports of the multiplexer 3 is connected to the optical frequency shifter 2 through means for transmitting light (also referred to as optically connected). The second input port of the input port is an input port for inputting reference light. With such a configuration, it is possible to multiplex light that circulates in the optical loop circuit 1 (also simply referred to as circulated light) and reference light that is input light from the outside of the optical loop circuit.

  The optical loop circuit 1 has a plurality of output ports, and outputs means for outputting light having a specific optical frequency or a plurality of optical frequencies out of the combined light (reference light and circulating light). The light output circuit 4 is provided. The input port of the optical output circuit 4 is optically connected to the output port of the wavelength multiple polymerization wave circuit 3. The first output port of the plurality of output ports of the optical output circuit 4 is optically connected to the optical frequency shifter 2.

  The second output port of the output port is connected to an optical interference circuit 5 as means for causing the input light to interfere. With such a configuration, the output light from the optical output circuit 4 and the optical loop circuit 1 that circulates the optical loop circuit 1 a predetermined number of times K (also referred to as “circular light with the frequency K”) and the optical loop circuit 1 the predetermined number of times L The circulated light (also referred to as circulated light having the number of laps L) interferes with the optical interference circuit 5 and is output from the first output port of the optical interference circuit 5. The light output from the optical output circuit 4 and not interfered by the optical interference circuit 5 is the final output light and is output from the second output port of the optical interference circuit 5. In addition, by adjusting the optical interference circuit 5, a part of the light related to the interference in the optical interference circuit 5 can be output from the second output port without interference. Here, K and L are zero or a natural number, and K ≠ L. When K or L = 0, the corresponding circulating light is the reference light.

  Reference numeral 6 denotes a photoelectric conversion means 6 that converts input light into electrical output. When the interference light output from the first output port of the optical interference circuit 5 is input, the photoelectric conversion means 6 generates and outputs a beat signal corresponding to the beat of the optical frequency caused by the interference for the interference light. To do.

  Reference numeral 7 denotes a phase comparator 7 which has a plurality of input ports, compares the phases of signals input from two of the plurality of input ports, and outputs an error signal based on the phase difference. is there. The first input port of the plurality of input ports of the phase comparator 7 is connected to the photoelectric conversion means 6 via a means for transmitting electricity such as a conducting wire (also referred to as being electrically connected). The second input port is electrically connected to the microwave reference frequency oscillator 8. The second input port may be connected to an input terminal for inputting a microwave reference frequency signal.

  A loop filter 9 is electrically connected to the output port of the phase comparator 7 as means for filtering the input error signal and passing it through a predetermined frequency band. The loop filter 9 may be a low pass filter that filters low frequency components. The loop filter 9 has an oscillation frequency controlled by the error signal filtered by the loop filter 9, and a voltage controlled oscillator (VCO) 10 as means for driving the optical frequency shifter 2 based on the controlled oscillation frequency. Are electrically connected. The voltage controlled oscillator 10 generates a frequency Δf for controlling the optical frequency shift amount of the optical frequency shifter 2 based on the error signal output from the loop filter 9.

With such a configuration, the reference light having the optical frequency ν _i input from the second input port of the multiplexer 3 is shifted many times by circling the optical loop circuit 1, and has a wide optical frequency range. Output light. At this time, the difference between the optical frequencies is the shift amount Δν, and the circulating light with the frequency K and the circulating light with the frequency L are extracted and interfered, and the interfered output light and the microwave reference frequency of the frequency f _M Since the optical frequency shifter is controlled by the voltage controlled oscillator based on the comparison result with the signal, fluctuations in the optical frequency due to fluctuations in the optical circulation delay time of the circulating light can be suppressed. Therefore, an optical frequency synthesizer having an arbitrary optical frequency with high accuracy equivalent to that of the microwave frequency signal source over several terahertz can be configured. It is also possible to obtain a short optical pulse source having a variable repetition frequency.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited to the following Example, It cannot be overemphasized that it can change variously in the range which does not deviate from the summary.

Example 1
In the present embodiment, an optical frequency synthesizer and a wavelength converter operating in a wavelength of 1.5 microns will be described with reference to FIG.
In FIG. 2, a single sideband optical modulator (SSB optical modulator) 11 is used as the optical frequency shifter 2. SSB optical modulators have been developed with optical waveguides that combine Mach-Zehnder interferometers on lithium niobate crystals and planar traveling-wave electrodes on the branches of each interferometer. Broadband SSB modulation up to about 25 GHz (frequency shift width (shift amount Δν)) is possible. Since the electrical signal for driving the SSB optical modulator 11 requires sinusoidal signals having a phase difference of 90 degrees, the 90-degree hybrid 12 having a phase difference of 90 degrees is used. The conversion efficiency from the input optical power to the SSB optical modulator 11 to the shifted optical power is about 10 dB.

  Such an SSB optical modulator 11 is provided in the optical loop circuit 1 looped by an optical fiber. The SSB optical modulator 11 is electrically connected to the SSB optical modulator 11 so that the 90-degree hybrid 12 can input two sine wave signals having a phase difference of 90 degrees to the SSB optical modulator 11. ing.

  In the optical loop circuit 1, an optical amplifier 13 is optically connected to the SSB optical modulator 11. The optical amplifier 13 is necessary to compensate for the conversion efficiency of the SSB optical modulator 11 and the loss of passive optical components in the optical loop circuit 1, and an erbium-doped optical amplifier or the like is applicable.

A four-port optical circulator 14 is optically connected to the optical amplifier 13. An optical fiber is connected to the first end of the optical circulator 14, from which reference light having a predetermined frequency ν _i is input. An optical amplifier 13 is optically connected to the second end of the optical circulator 14, and a fiber Bragg grating (FBG) having a reflection center optical frequency substantially the same as the optical frequency ν _i of the reference light is interposed between them. 15 is arranged. The SSB optical modulator 11 is optically connected to the third end of the optical circulator 14, and the FBGs 16a to 16c are disposed therebetween. The reflection center wavelengths of the FBGs 16a to 16c are set to approximately ν _i , ν _{i + 1} , and ν _{i + N} (N: an integer of 2 or more), respectively. Further, the reflectivities of the FBGs 16a and 16b are set to about 10% or less. Here is a _{ν i + k = ν i +} kf s. However, k is a natural number equal to or less than N, and f _s is an optical frequency shift amount caused by making one round of the optical loop circuit 1. An optical circulator 17 is connected to the fourth end of the optical circulator 14.

  In such a connection configuration, the light incident on the first end of the optical circulator 14 is emitted only from the second end, the light incident on the second end is emitted only from the third end, The light incident on the third end is emitted only from the fourth end.

That is, the input frequency reference light from the optical frequency [nu _i is the optical circulator 14 is emitted upward (second end) of FIG. 2, it is reflected by FBG15 having the same reflection center optical frequency between the reference light frequency [nu _i. The light returning to the optical circulator 14 is emitted to the right side (third end) in FIG. The FBGs 16a to 16b are arranged on the third end side. However, since the reflectivities of the FBGs 16a and 16b are set to about 10% or less, they are emitted from the third end of the optical circulator 14. Most of the light having the optical frequency ν _i passes through these FBGs 16 a to 16 c, and the optical frequency is shifted to approximately ν _{i + 1} by the SSB optical modulator 11. On the other hand, the light having the optical frequency ν _i reflected by the FBG 16a enters from the third end of the optical circulator 14 and is emitted downward (fourth end) in FIG.

The above-described light shifted to ν _{i + 1} passes through the FBG 15 and is emitted to the right side (third end) in FIG. 2 by the optical circulator 14. After that, when the circulating light having the optical frequency ν _{i + N} = ν _i + Nf _s (formula 1) is incident on the FBG 16c after N turns, the circulating light is reflected by the FBG 16c, and the light It is emitted downward (fourth end) in FIG. 2 by the circulator 14. In addition, during the N rounds, a part of the light having the optical frequency ν _{i + 1} is reflected by the FBG 16b and is output downward (fourth end) in FIG.

An optical circulator 14 is optically connected to the first end of the three-port optical circulator 17, and an optical fiber is coupled to the second end. The optical fiber includes an FBG 18a and 18b is arranged. The reflection center wavelengths of the FBGs 18a and 18b are set to approximately ν _i and ν _{i + 1} , respectively. The final output light is output from the second end of the optical circulator 17. A photodiode (PD) 19 is disposed on the third end side of the optical circulator 17 so that light emitted from the third end is incident on the photodiode 19.

  In such a connection configuration, the light incident on the first end of the optical circulator 17 is emitted only from the second end, and the light incident on the second end is emitted only from the third end.

That is, the light of the optical frequencies ν _i and ν _{i + 1} emitted from the optical circulator 14 is emitted to the right side (second end) in FIG. 2 by the optical circulator 17 and reflected by the FBGs 18a and 18b to become interference light. . Next, the reflected light (interference light) is emitted downward (third end) in FIG. 2 by the optical circulator 17. This interference light enters the photodiode 19 and is converted into a beat signal.

On the other hand, the light having the optical frequency ν _{i + N} emitted from the optical circulator 14 is emitted to the right side (second end) in FIG. 2 by the optical circulator 17 and passes through the FBGs 18a and 18b to become the final output light.

An amplifier 20 is electrically connected to the photodiode 19, and the phase comparator 7 is electrically connected to the amplifier 20. The phase comparator 7 has two input ports. The amplifier 20 is electrically connected to the first input port, and the microwave reference frequency having a frequency f _M = 25 GHz is connected to the second input port. A microwave reference frequency oscillator 8 that oscillates a signal is electrically connected. The phase comparator 7 compares the beat signal amplified by the amplifier 20 with a microwave reference frequency signal having a frequency of 25 GHz, and generates and outputs an error signal corresponding to the frequency difference between them.

  A loop filter 9 is electrically connected to the output port of the phase comparator 7. The loop filter 9 filters and outputs the low frequency component of the error signal output from the phase comparator 7. In addition, a voltage controlled oscillator (VCO) 10 is electrically connected to the loop filter 9, and a 90 degree hybrid 12 is electrically connected to the voltage controlled oscillator 10. The voltage controlled oscillator 10 oscillates an electric signal having a frequency Δf based on the error signal input from the loop filter 9 and controls the SSB optical modulator 11.

  The phase comparator 7 can be a microwave balanced mixer or the like. In order to transmit the low-frequency component of the error signal, a low-pass filter having an appropriate cutoff characteristic can be applied.

Now, since the loop length of the optical loop circuit 1 has changed due to environmental temperature fluctuations, the light that has circulated around the loop has a small optical frequency shift caused by fluctuations in loop delay time in addition to the optical frequency shift by the SSB optical modulator 11. Suffer. The amount is
LN (β + δn / δT) (dT / dt) / λ
It becomes. Here, L is the loop length of the optical loop circuit 1, N is the number of turns, β is the linear expansion coefficient of the fiber constituting the loop, δn / δT is the change in effective refractive index per unit temperature change of the fiber, ( dT / dt) is the temperature variation per unit time of the fiber, and λ is the wavelength of the circulating light.

  As described above, in a loop having a loop length of 2 meters that changes by 1 ° C. per second, light in a 1.5 micron band that circulates about 50 times is shifted by several hundred hertz. This amount is when the line width of the incident optical frequency reference light is kilohertz or more and can be ignored when the output light is used alone, but the original incident reference light and the output light interfere with each other, and the microwave / millimeter In application fields where waves are generated, the frequency and phase fluctuate.

  Therefore, in the present embodiment, the driving frequency of the SSB optical modulator 11 is controlled based on the above error signal so as to cancel out the optical frequency fluctuation due to the optical loop delay fluctuation, thereby using the incident reference light. Output light having an optical frequency difference with an accuracy equivalent to the accuracy of the microwave reference frequency signal is obtained.

Specifically, out of the light emitted downward (fourth end) in FIG. 2 by the optical circulator 14, the light having the optical frequency ν _i (non-circular light) (reference light) and ₁ of the optical frequency ν _{i + 1} . The light that has circulated only once is extracted by the FBGs 18 a and 18 b and the optical circulator 17, and those beats are generated by the photodiode 19. The beat is the sum of the optical frequency shift amount (Δf) by the SSB optical modulator 11 and the optical frequency shift due to the loop circulation delay time fluctuation.

  The phase difference between the beat signal and the microwave reference frequency signal from the microwave reference frequency oscillator 8 is compared by the phase comparator 7, and a voltage controlled oscillator (VCO) 10 is obtained by the low frequency component of the obtained error signal. To control. As described above, the VCO 10 can control the SSB optical modulator so as to reduce the fluctuation of the optical frequency due to the fluctuation of the optical circulation delay time based on the error signal which is the comparison result of the phase comparator 7. The influence of the optical delay can be automatically reduced every time the circulating light goes around the optical loop circuit 1. Therefore, the final output light can have the same accuracy and stability as the microwave reference frequency signal.

  That is, in this configuration, the sum of the optical frequency shift amount by the BBS optical modulator 11 controlled by the VCO 10 and the optical frequency shift due to the optical loop delay time variation of the optical loop circuit 1 becomes the frequency of the microwave reference frequency signal. It is a phase locked loop (PLL) that works like this.

Therefore, the frequency difference between the optical frequency of the output light of the optical frequency synthesizer according to the present embodiment and the optical frequency of the reference light that is the input light is the microwave reference frequency signal (f _M = 25 GHz in this embodiment). Equivalent accuracy is guaranteed. Since the absolute frequency accuracy depends on that of the input light, a gas absorption line stabilized light source whose accuracy has been evaluated for error may be used as the light source of the input light (reference light).

When the optical frequency synthesizer according to the present embodiment is incorporated in a micro / millimeter wave generator, the reflectance of the FBG 18a disposed on the right side (second end side) of the optical circulator 17 is reduced, and the optical frequency ν _i Is output so as to be output together with the light of the optical frequency ν _{i + N} , and the beats of the light of two frequencies emitted from the second end of the optical circulator 17 may be obtained by a photoelectric converter such as a broadband PD. . In addition, the reference frequency signal is swept from 5 GHz to 25 GHz, and the 12-round light is photoelectrically converted by an ultra-wideband PD such as a single traveling carrier PD (UTC-PD), so that a millimeter wave having a frequency from about 60 GHz to about 300 GHz is obtained. Can be generated continuously.

  In this embodiment, it goes without saying that the optical frequency of the light finally output can be controlled by setting the reflection center frequency of the FBG 16c to a frequency obtained by changing N in Equation 1 as desired.

In this embodiment, it is important that the beat signal to be compared with the microwave reference frequency signal includes the shift amount of the optical frequency due to the fluctuation of the optical circulation delay time that occurs when the loop goes around, and the FBGs 16a and 16b. , 18a and 18b are not essential to set the reflection center frequencies to ν _i and ν _{i + 1} , respectively. That is, the reflection center frequencies of the FBGs 16a, 16b, 18a, and 18b are set so that the optical frequency difference between the reflection center frequencies of the FBGs 16a and 18a and the reflection center frequencies of the FBGs 16b and 16b becomes the optical frequency shift amount f _s. That's fine.

  As described above, by using continuous light (CW light) whose accuracy of oscillation light frequency such as gas absorption line stabilized laser light is evaluated as light input to the apparatus according to the present embodiment, the apparatus according to the present embodiment is The case where it is used as an optical frequency synthesizer is described. By inputting an optical signal modulated by a data signal or the like to the apparatus according to the present embodiment, an optical signal in which only the optical frequency is converted can be obtained as the output light of the apparatus according to the present embodiment. In this case, the apparatus according to the present embodiment operates as a wavelength conversion apparatus.

(Example 2)
As shown in FIG. 3, the present embodiment is a configuration of the optical frequency synthesizer according to the first embodiment, in which the FBG 16c is removed, and light having a plurality of optical frequencies that circulate in the loop as output light is simultaneously extracted from the same port. Thus, a short light pulse output is obtained. That is, the optical coupler 21 having no frequency dependency is optically connected to the third end of the optical circulator 14, and the FBGs 16a and 16b are disposed therebetween. Further, a photodiode 19 is arranged on the fourth end side of the optical circulator 14, and light emitted from the fourth end enters the photodiode 19.

  The SSB optical modulator 11 is optically connected to the first output port of the optical coupler 21. In addition, an optical fiber is connected to the second output port of the optical coupler 21, and the final output light is output therefrom.

In such a configuration, in the same manner as in Example 1, and the reference light optical frequency [nu _i, part of the circulating light of optical frequency [nu _{i + 1} interferes been reflected by FBG16a and 16b, the optical circulator 14 And is input to the photodiode 19 to become a beat. Next, in the same manner as in the first embodiment, the SSB optical modulator 11 is controlled so as to cancel the optical frequency variation due to the optical loop delay variation based on the phase comparison between the beat and the microwave reference frequency signal.

In this embodiment, since the optical coupler 21 has no frequency dependence, the input light is branched regardless of the frequency and is output from the two output ports. The light output from these two output ports includes the light from the reference light having the optical frequency ν _i to the circulating light having the number of rotations N, that is, all the optical frequencies circulating around the reference light and the optical loop circuit 1. Of light. Therefore, in the present embodiment, the reference light and the light of all the optical frequencies circulating around the optical loop circuit 1 can be used as the final output light. At this time, the final output light can be light in which the influence of fluctuation of the optical frequency due to the loop delay is suppressed.

  In addition, since the optical frequency of each circulating light including the optical phase is locked by the PLL configuration of the present embodiment, the light circulating in the optical loop is a short optical pulse when observed in the time domain. Therefore, the final output light is also a short light pulse.

(Example 3)
The present invention can be realized by using not only an SSB optical modulator but also an optical nonlinear element as an optical frequency shifter. As the optical nonlinear element, a quasi phase matching optical nonlinear element that satisfies the phase matching condition between the light waves over a long element length (longitudinal direction of the element) by periodically inverting the optical axis of the optical crystal can be used. (See Non-Patent Document 2). In Non-Patent Document 2, a lithium niobate waveguide (also simply referred to as PPLN) in which the optical axis of an optical crystal is inverted at a predetermined period is used as a quasi phase matching optical nonlinear element. In this embodiment, as in Non-Patent Document 2, PPLN can be used as the optical nonlinear element.

FIG. 4 is a diagram illustrating the configuration of the optical frequency shifter of the optical frequency synthesizer according to the present embodiment.
In FIG. 4, reference numeral 21 denotes a semiconductor laser that excites light (also referred to as first excitation light) having an optical frequency ν ₁ (wavelength λ ₁ ). The first excitation light oscillated from the semiconductor laser 21 enters the optical branching device 22. The first output port of the optical splitter 22 is optically connected to the first input port of the optical multiplexer 23, and the second output port is optically connected to the optical amplifier 24. Therefore, the first excitation light is branched to the optical multiplexer 23 and the optical amplifier 24 by the optical branching device 22. The optical amplifier 24 is optically connected to the first input port of the PPLN 25.

An optical fiber forming a loop of the optical loop circuit 1 is connected to the second input port of the PPLN 25, and incident light (also simply referred to as incident light) is incident on the optical frequency shifter from the second input port. Is done. The output port of the PPLN 25 is optically connected to the PPLN 29. The phase matching wavelength of the PPLN 25 is set to be approximately equal to the wavelength λ ₁ of the first excitation light. Therefore, the PPLN 25 generates the second harmonic when the first excitation light is incident from the optical amplifier 24 to the second input port. At this time, when incident light is incident from the first input port of the PPLN 25, the generated second harmonic plays the role of pump light, and the PPLN 25 has a difference frequency (= the optical frequency of the second harmonic−incident). Optical frequency of light).

  That is, the PPLN 25 is set to generate a second harmonic when the light oscillated from the semiconductor laser 21 is incident, and to generate a difference frequency between the second harmonic and the incident light.

On the other hand, reference numeral 26 denotes a semiconductor laser that excites light (also referred to as second excitation light) having an optical frequency ν ₂ (wavelength λ ₂ ). The second excitation light oscillated from the semiconductor laser 26 enters the optical branching device 27. The first output port of the optical splitter 27 is optically connected to the second input port of the optical multiplexer 23, and the second output port is optically connected to the optical amplifier 28. Therefore, the second excitation light is branched to the optical multiplexer 23 and the optical amplifier 28 by the optical branching device 27. The optical amplifier 28 is optically connected to the first input port of the PPLN 29.

The output port of the PPLN 25 is optically connected to the second input port of the PPLN 29, and an optical fiber forming a loop of the optical loop circuit 1 is connected to the output port of the PPLN 29. This output port The light emitted from the optical frequency shifter (also simply referred to as the emitted light) is emitted from. The phase matching wavelength of the PPLN 29 is set to be approximately equal to the wavelength λ ₂ of the second excitation light. Therefore, like the PPLN 25, the PPLN 29 generates a second harmonic when the second excitation light is incident, and generates a difference frequency by the second harmonic and the light emitted from the PPLN 25.

  That is, the PPLN 29 is set to generate a second harmonic when the light oscillated from the semiconductor laser 26 is incident, and to generate a difference frequency between the second harmonic and the light emitted from the PPLN 25. Yes.

  Reference numeral 23 denotes an optical multiplexer, which combines the first excitation light and the second excitation light incident from the optical branching units 22 and 27 and emits the combined light to the photodiode 30. . The photodiode 30 converts the multiplexed light emitted from the optical multiplexer 23 into a beat signal of the first excitation light and the second excitation light, and outputs the beat signal to the phase comparator 31.

  The phase comparator 31 has two input ports, the photodiode 30 is electrically connected to the first input port, and the 1/2 frequency divider is connected to the second input port. 32 is electrically connected. The 1/2 frequency divider 32 is electrically connected to the voltage controlled oscillator 10. When a signal having a frequency Δf is input from the voltage controlled oscillator 10, the electrical signal having Δf / 2 is supplied to the phase comparator 31. Output.

  The phase comparator 31 compares the phase of the input beat signal and the electric signal of Δf / 2, and outputs a second error signal based on the comparison result to the loop filter 33. The second error signal is input to the semiconductor laser 21 via the loop filter 33. An injection current for driving the semiconductor laser 21 is controlled based on the second error signal.

With such a configuration, the optical frequency shifter according to the present embodiment is configured. Hereinafter, a state in which the optical frequency of incident light is shifted using this optical frequency shifter will be described.
FIG. 5 is a diagram illustrating an example of the arrangement of incident light, outgoing light, and excitation light on the optical frequency axis according to the present embodiment.
First, the first excitation light (optical frequency ν ₁ ) from the semiconductor laser 21 is incident on the PPLN 25 to generate the second harmonic of the first excitation light having an optical frequency of 2ν ₁ . At this time, when incident light (optical frequency ν _i ) is incident on the PPLN 25, as can be seen from FIG. 5, the generated second harmonic wave having the optical frequency 2ν ₁ and the incident light having the optical frequency ν _i are generated in the PPLN 25. Difference frequency light (first difference frequency light) is generated at the optical frequency 2ν ₁ −ν _i .

In the PPLN 29, the second excitation light (optical frequency ν ₂ ) from the semiconductor laser 26 generates the second harmonic of the second excitation light having an optical frequency of 2ν ₂ . At this time, when light having an optical frequency of 2ν ₁ −ν _i is incident on the PPLN 29 from the PPLN 25, as can be seen from FIG. 5, the difference frequency light between the light and the second harmonic of the second excitation light (second (Difference frequency light) is generated at an optical frequency 2ν ₂ − (2ν ₁ −ν _i ) = ν _i +2 (ν ₂ −ν ₁ ). The difference frequency light having the optical frequency ν _i +2 (ν ₂ −ν ₁ ) is emitted from the PPLN 29 as outgoing light. That is, it is possible to obtain outgoing light whose optical frequency is shifted by twice the frequency difference between the first excitation light and the second excitation light.

  Here, in order to stabilize the optical frequency difference between the first excitation light and the second excitation light to the input microwave reference frequency signal, frequency offset locking is performed by a PLL circuit. That is, a part of the first excitation light and the second excitation light interfere with each other, and the beat frequency is Δf / 2 which is half the frequency (frequency Δf) for obtaining the same accuracy as the input microwave reference frequency signal. The injection current of the semiconductor laser 21 is controlled so as to lock. That is, the injection current to the semiconductor laser 21 is controlled based on the frequency Δf / 2 and the beat frequency from the photodiode 30 so as to cancel out the optical frequency fluctuation due to the optical loop delay fluctuation.

  In this embodiment, the amount of shift in the optical frequency shifter can be controlled by setting the phase matching wavelength of PPLN 25 and PPLN 29 and the excitation wavelength of semiconductor laser 21 and semiconductor laser 26 as desired. Needless to say.

  In the first and second embodiments, the optical frequency shifter according to the present embodiment can be used instead of the SSB optical modulator.

  As described in the first embodiment, it is possible to configure the wavelength converter using the optical frequency synthesizer described in the second and third embodiments.

It is a block diagram of the optical frequency synthesizer which concerns on one Embodiment of this invention. It is a block diagram of the optical frequency synthesizer which concerns on one Embodiment of this invention. It is a block diagram of the optical frequency synthesizer which concerns on one Embodiment of this invention. It is a figure which shows the structure of the optical frequency shifter of the optical frequency synthesizer which concerns on one Embodiment of this invention. It is a figure which shows an example of arrangement | positioning on the optical frequency axis of incident light, outgoing light, and excitation light based on one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical loop circuit 2 Optical frequency shifter 3 Multiplexer 4 Optical output circuit 5 Optical interference circuit 6 Photoelectric conversion means 7 Phase comparator 8 Microwave reference frequency oscillator 9 Loop filter 10 Voltage control oscillator

Claims (5)

An optical frequency shifter that shifts the optical frequency of the input light by a predetermined shift amount, an optical loop circuit that includes the optical frequency shifter inside, light that circulates through the optical loop circuit, and the outside of the optical loop circuit And a light output circuit for outputting light of one or more specific optical frequencies out of the light loop circuit out of the light loop circuit. In an optical frequency synthesizer that outputs an output light in a wide-band optical frequency range by shifting the optical frequency by a predetermined number of times,
The circulating light output from the optical output circuit that circulates the optical loop circuit a predetermined number of times K and the circulating light that circulates the optical loop circuit a predetermined number of times L (K and L are zero or natural numbers, ≠ L), an optical interference circuit that picks up, interferes and outputs,
Photoelectric conversion means for converting the interference light output from the optical interference circuit into an electrical signal and outputting the electrical signal;
An input port for inputting a microwave reference frequency signal is compared, and the phase of the microwave reference frequency signal input from the input port is compared with the electrical signal output from the photoelectric conversion means, and the phase difference is determined. A phase comparator that outputs a corresponding error signal;
An optical frequency synthesizer comprising: a voltage controlled oscillator that controls an oscillation frequency based on an error signal output from the phase comparator and drives the optical frequency shifter based on the oscillation frequency.   2. The optical frequency synthesizer according to claim 1, further comprising means for filtering an error signal output from the phase comparator to pass a predetermined frequency component.   3. The optical frequency synthesizer according to claim 1, wherein the optical frequency shifter is a single sideband (SSB) optical modulator. The optical frequency shifter is
An optical nonlinear element made of an optical nonlinear material;
3. The optical frequency synthesizer according to claim 1, further comprising: an optical multiplexing unit that combines the excitation light of the nonlinear material and the incident light to the optical frequency shifter and couples the combined light to the optical nonlinear element.   A wavelength converter comprising the optical frequency synthesizer according to claim 1.

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