JP2007266654A - Frequency variable light source, and frequency calibrating system and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a frequency variable light source which has a wide frequency variable range and can vary the frequency of an optical pulse sequence with high accuracy, and a frequency calibrating system and its method using an optical pulse generated in the frequency variable light source. <P>SOLUTION: An optical frequency comb generator 11 generates an optical frequency comb having a plurality of optical frequency components at a predetermined frequency interval. A frequency variable optical band pass filter 12 has a transmission band smaller than the frequency interval of the optical frequency component in the optical frequency comb, and the transmission band is variable. A filter sweeping circuit 14 continuously sweeps the transmission band of the frequency variable optical bandpass filter 12, using a reference signal of a highly accurate constant frequency to be output from an oscillator 13. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a frequency variable light source that generates an optical pulse train whose frequency changes with time, and a frequency calibration system and method using an optical pulse train emitted from the frequency variable light source.

  The optical pulse train generated by the frequency variable light source is used as a reference frequency in optical communication by, for example, a frequency multiplex transmission system (wavelength multiplex transmission system), or used for high resolution frequency analysis. Hereinafter, an example of a conventional frequency variable light source that generates such an optical pulse train will be described. FIG. 9 is a block diagram showing a conventional frequency variable light source, and FIG. 10 is a diagram showing an optical pulse train generated by the conventional frequency variable light source.

  As shown in FIG. 9, the conventional variable frequency light source 100 includes a frequency reference light source 101, an optical switch 102, an optical loop circuit 103, and a timing control circuit 104. The frequency reference light source 101 emits continuous light having a single frequency, and the optical switch 102 pulses the continuous light from the frequency reference light source 101. The optical loop circuit 103 includes an optical multiplexer / demultiplexer 105, an optical frequency shifter 106, an optical delay line 107, and an optical amplifier 108. The optical loop circuit 103 circulates the optical pulses from the optical switch 102 and adjusts the frequency of each optical pulse. Change. The timing control circuit 104 controls the optical switch 102 and the optical frequency shifter 106 of the optical loop circuit 103 to control the generation timing of the optical pulse and the timing of changing the frequency.

  In the above configuration, when the control signal S101 (width Tw, period T) shown in FIG. 10A is output from the timing control circuit 104 to the optical switch 102, the continuous light from the frequency reference light source 101 in the optical switch 102 has a width. The optical pulse is Tw, and this optical pulse is input to the optical loop circuit 103. Note that the control signal S102 (width Ts) shown in FIG. 10B is output from the timing control circuit 104 to the optical frequency shifter 106 at the same timing as when the control signal S101 is output from the timing control circuit 104.

The optical frequency shifter 106 shifts the frequency of light by Δf when the control signal S102 from the timing control circuit 104 is input. Therefore, the frequency of the optical pulse input to the optical loop circuit 103 changes by Δf at the optical frequency shifter 106 every time it circulates in the optical loop circuit 103. The intensity of the optical pulse decreases due to attenuation or the like every time it circulates through the optical loop circuit 103, but the decrease in intensity is compensated by the optical amplifier 108. Therefore, from the external output terminal connected to the optical multiplexer / demultiplexer 105, as shown in FIGS. 10C and 10D, the intensity is constant and the time interval of each optical pulse is Td. An optical pulse train whose frequency changes by Δf for each pulse is output. For details of the conventional frequency variable light source, see, for example, Patent Document 1 below.
JP-A-10-190629

  By the way, in the conventional variable frequency light source 100 described above, the optical pulse that circulates in the optical loop circuit 103 is amplified by the optical amplifier 108, but the spontaneous emission light generated by the optical amplifier 108 also circulates in the optical loop circuit 103 and is amplified. Is done. For this reason, the optical pulse that circulates in the optical loop circuit 103 is affected by the noise caused by the amplified spontaneous emission light, and the number of circulations cannot be increased. As a result, the variable range of the frequency is converted into the wavelength. There was a problem that it was limited to about several nm. Even if the influence of the noise can be eliminated, the frequency band of the optical amplifier 108 provided in the optical loop circuit 103 is only a few tens of nanometers in terms of wavelength, and in principle, the frequency range is beyond this range. Cannot be changed.

  As the frequency variable light source, in addition to the frequency variable light source using the optical loop circuit 103 described above, a frequency variable light source such as an external resonator type frequency variable light source, a distributed feedback laser light source, or a distributed Bragg reflection laser light source. Is realized. However, these frequency variable light sources have a problem that the required accuracy cannot be obtained because the accuracy of the frequency change rate can be obtained only in the wavelength order in terms of wavelength.

  The present invention has been made in view of the above circumstances, and includes a frequency variable light source having a wide frequency variable range and capable of changing the frequency of the optical pulse train with high accuracy, and an optical pulse train generated by the frequency variable light source. It is an object of the present invention to provide a frequency calibration system and method used.

In order to solve the above-described problem, the frequency variable light source of the present invention is a frequency variable light source (10, 20, 30) that generates an optical pulse train whose frequency changes with time. An optical frequency comb generator (11) that generates an optical frequency comb having a frequency variable optical bandpass filter that has a transmission band narrower than the frequency interval of the optical frequency component in the optical frequency comb, and the transmission band is variable (12) and a sweeping device (13, 14) for continuously changing the transmission band of the frequency variable optical bandpass filter.
According to this invention, the optical frequency comb generated by the optical frequency comb generator is swept under the control of the sweep device by the frequency variable optical bandpass filter having a transmission band narrower than the frequency interval of the optical frequency component. .
In the frequency variable light source of the present invention, the sweep device includes an oscillator (13) that outputs a reference signal having a constant frequency, and the frequency variable optical bandpass filter based on the reference signal output from the oscillator. And a filter sweep circuit (14) for linearly sweeping the transmission band.
The frequency variable light source of the present invention includes a first light receiver (22) that receives a part of an optical pulse having the optical frequency component that has passed through the frequency variable optical bandpass filter and outputs a light reception signal. The sweep device is detected by a phase comparator (23) for detecting a phase difference between the light reception signal output from the first light receiver and the reference signal output from the oscillator, and the phase comparator. And a first control circuit (24) for outputting a control signal for making the phase difference constant to the filter sweep circuit.
The frequency variable light source of the present invention includes a reference light source (32) that generates a reference light having a frequency included in a frequency band that can change the transmission band of the frequency variable optical bandpass filter, and the frequency variable light source. A multiplexer (31) for combining the optical pulse having the optical frequency component transmitted through the optical bandpass filter and the reference light from the reference light source; and receiving the light combined by the multiplexer. A second light receiver (33) for outputting a light reception signal and an optical pulse having the optical frequency component transmitted through the frequency variable optical bandpass filter based on the light reception signal output from the second light receiver. And a second control circuit (34) for controlling the optical frequency comb generator so that the optical frequency becomes a constant value with the frequency of the reference light source.
Furthermore, the frequency variable light source of the present invention is characterized in that the reference light source generates light whose frequency does not vary as the reference light.
In order to solve the above problems, a frequency configuration system according to the present invention is a frequency calibration system that calibrates a reference frequency in an optical device that handles light, and is connected to an optical fiber network (N1 to N6) and the optical fiber network. The optical apparatus (40) and the frequency variable light source (50) according to any one of the above connected to the optical fiber network are provided.
In order to solve the above-mentioned problem, the frequency configuration method of the present invention is a frequency calibration method for calibrating a reference frequency in an optical device (40) that handles light, and is generated from the frequency variable light source (50) described above. Transmitting the optical pulse train to the optical fiber network, and calibrating a reference frequency of the optical apparatus using the optical pulse train transmitted via the optical fiber network. .

According to the present invention, there is an effect that it is possible to provide a frequency variable light source that has a wide frequency variable range and can change the frequency of the optical pulse train with high accuracy.
Further, according to the present invention, there is an effect that the frequency of the optical pulse train from the frequency variable light source can be changed with reference to the frequency of the reference light.
Furthermore, according to the present invention, there is an effect that a reference frequency in a plurality of optical devices can be easily configured.

  Hereinafter, a variable frequency light source and a frequency calibration system and method according to embodiments of the present invention will be described in detail with reference to the drawings.

[Frequency variable light source]
<First Embodiment>
FIG. 1 is a block diagram showing a schematic configuration of a variable frequency light source according to the first embodiment of the present invention. As shown in FIG. 1, the frequency variable light source 10 of the present embodiment includes an optical frequency comb generator 11, a frequency variable optical bandpass filter 12, an oscillator 13, and a filter sweep circuit 14. The oscillator 13 and the filter sweep circuit 14 correspond to the sweep device according to the present invention.

  The optical frequency comb generator 11 generates an optical frequency comb having a plurality of optical frequency components arranged in a comb shape at predetermined frequency intervals. Here, the internal configuration of the optical frequency comb generator 11 will be described. FIG. 2 is a block diagram showing an example of the configuration of the optical frequency comb generator 11. As shown in FIG. 2, the optical frequency comb generator 11 includes a mode-locked laser 16, an optical amplifier 17, and a high optical nonlinear optical fiber 18.

  The mode-locked laser 16 emits a laser beam composed of a plurality of optical frequency components whose frequency interval is f (for example, several KHz). This frequency interval f is the interval of the longitudinal mode of the mode-locked laser 16. Note that the frequency band of laser light emitted from the mode-locked laser 16 is much narrower than the frequency band of the optical frequency comb generated by the optical frequency comb generator 11. The optical amplifier 17 is, for example, an erbium-doped optical fiber amplifier (EDFA), and amplifies the laser light emitted from the mode-locked laser 16 with a predetermined amplification factor. Here, the frequency band that can be amplified by the optical amplifier 17 may be about the frequency band of laser light emitted from the mode-locked laser 16, for example.

  The high optical nonlinear optical fiber 18 is an optical fiber having high nonlinearity, and generates supercontinuum light from the laser light amplified by the optical amplifier 17 by the nonlinear optical effect. The supercontinuum light is composed of a plurality of optical frequency components having the same frequency interval as the frequency interval f of the laser light emitted from the mode-locked laser 16, and the number of frequency bands of the laser light emitted from the mode-locked laser 16. The frequency band is about 100 times (for example, several hundred nm in terms of wavelength). Since the frequency interval f of the optical frequency component of the supercontinuum light is the longitudinal mode interval of the mode-locked laser 16 as described above, the frequency stability of the oscillator (not shown) driving the mode-locked laser 16 is set. It can be controlled with extremely high accuracy up to the level. The high optical nonlinear optical fiber 18 outputs this supercontinuum light as the above optical frequency comb.

  Returning to FIG. 1, the frequency-variable optical bandpass filter 12 has a transmission band having a predetermined width (frequency transmission band) and transmits a part of the optical frequency comb from the optical frequency comb generator 11. Specifically, the frequency variable optical bandpass filter 12 has a transmission band that is narrower than the frequency interval f of the optical frequency component in the optical frequency comb generated by the optical frequency comb generator 11. Only the frequency component is transmitted. The frequency variable optical bandpass filter 12 can change (sweep) the transmission band at high speed in the frequency domain without changing the width of the transmission band. As the frequency variable optical bandpass filter 12, for example, an etalon type filter represented by an optical fiber Fabry-Perot etalon, an acoustooptic effect type tunable filter, a dielectric multilayer filter, a grating filter, or the like can be used.

  The oscillator 13 generates a reference signal having a constant frequency with high accuracy. The frequency (cycle) of the reference signal is set according to the cycle for changing the frequency of the laser beam output from the variable frequency light source 10. For example, when the frequency of the laser beam output from the variable frequency light source 10 is changed with the period T, the period of the reference signal is also set to T. The filter sweep circuit 14 continuously changes (sweeps) the transmission band of the frequency variable optical bandpass filter 12 based on the reference signal from the oscillator 13. Specifically, for example, the transmission band of the frequency variable optical bandpass filter 12 is linearly swept so that the center frequency of the transmission band of the frequency variable optical bandpass filter 12 continuously changes by a constant frequency per unit time. To do.

  Next, the operation of the variable frequency light source 10 having the above configuration will be described. FIG. 3 is a diagram for explaining the operation of the variable frequency light source according to the first embodiment of the present invention. When the laser light is emitted from the mode-locked laser 16 shown in FIG. 2, the laser light is amplified by the optical amplifier 17 and then enters the high optical nonlinear optical fiber 18, thereby generating supercontinuum light. It is output from the optical frequency comb generator 11 as an optical frequency comb. This optical frequency comb is composed of a plurality of optical frequency components having a frequency interval f as shown in FIG.

  The optical frequency comb emitted from the optical frequency comb generator 11 is incident on the frequency variable optical bandpass filter 12, and a part of the optical frequency comb passes through the frequency variable optical bandpass filter 12 and is output to the outside of the frequency variable light source 10. The Here, in FIG. 3A, a rectangular region denoted by reference numeral Tr indicates a transmission band that transmits the optical frequency on which the frequency variable optical bandpass filter 12 is incident. As shown in the figure, the transmission band Tr is set to be narrower than the frequency interval f of the optical frequency component in the optical frequency comb generated by the optical frequency comb generator 11, so that the frequency variable optical bandpass filter 12 is optical frequency comb. Only one of the optical frequency components contained in is transmitted at a time. That is, the plurality of optical frequency components of the optical frequency comb do not pass through the frequency optical bandpass filter 12 at a time.

  For example, simultaneously with the emission of the laser beam from the mode-locked laser 16, a reference signal with a constant frequency with high accuracy is output from the oscillator 13 to the filter sweep circuit 14. This reference signal is, for example, a sine wave signal as shown in FIG. 3B, and its period T is set to a period for changing the frequency of the laser light output from the variable frequency light source 10. The reference signal output from the oscillator 13 is output to the filter sweep circuit 14. The filter sweep circuit 14 continuously changes (sweeps) the transmission band of the frequency variable optical bandpass filter 12 based on the reference signal from the oscillator 13.

  Specifically, as shown in FIG. 3C, the filter sweep circuit 14 is configured so that, for example, the center frequency of the transmission band Tr of the frequency variable optical bandpass filter 12 continuously changes by a constant frequency per unit time. The transmission band Tr of the frequency variable optical bandpass filter 12 is linearly swept. Then, for each period T of the reference signal, the center frequency of the transmission band Tr is changed to the sweep start frequency, and thereafter, the center frequency of the transmission band Tr of the frequency variable optical bandpass filter 12 is similarly swept in a sawtooth manner. The filter sweep circuit 14 sweeps the center frequency of the transmission band Tr, for example, in the range of several tens to several hundreds of nm in terms of wavelength.

  When the frequency variable optical bandpass filter 12 is swept by the filter sweep circuit 14, the intensity is constant from the frequency variable light source 10 as shown in FIGS. 3 (d) and 3 (e). An optical pulse train whose time interval is Td and whose frequency changes by Δf (= f) is output for each optical pulse.

  As described above, the variable frequency light source 10 according to the first embodiment of the present invention has a plurality of optical frequency components arranged in a comb shape at a predetermined frequency interval f, and the number of frequency bands occupied by the optical frequency components is several. An optical frequency comb having a wide wavelength of 100 nm is generated from the optical frequency comb generator 11, and this optical frequency comb is a frequency optical bandpass filter whose transmission band is set narrower than the frequency interval f of the optical frequency component of the optical frequency comb. 12 is used for sweeping, the optical pulse frequency variable range can be expanded. In addition, since the optical frequency components of the optical frequency comb are arranged at a high frequency with the frequency interval f, it is possible to generate an optical pulse train whose frequency changes with high accuracy for each pulse.

Second Embodiment
FIG. 4 is a block diagram showing a schematic configuration of a frequency variable light source according to the second embodiment of the present invention. In FIG. 4, the same components as those shown in FIG. 1 are denoted by the same reference numerals. As shown in FIG. 4, the frequency variable light source 20 of the present embodiment includes an optical branching device 21, a light receiver, in addition to the optical frequency comb generator 11, the frequency variable optical bandpass filter 12, the oscillator 13, and the filter sweep circuit 14. 22, a phase comparator 23, and an operational amplifier circuit 24. The light receiver 22 corresponds to the first light receiver according to the present invention, and the operational amplifier circuit 24 corresponds to the first control circuit according to the present invention.

  The optical branching device 21 branches a part of the light transmitted through the frequency optical bandpass filter 12. The remainder that is not branched by the optical splitter 21 is output to the outside of the variable frequency light source 20. Note that the branching rate of the optical branching device 21 (the intensity of the light branched by the optical branching device 21) may be arbitrary, but in order to prevent a decrease in the intensity of the light (optical pulse train) output to the outside of the frequency variable light source 20, The branching rate of the optical branching device 21 is preferably as small as possible. The light receiver 22 receives the light branched by the light splitter 21 and outputs a light reception signal.

  The phase comparator 23 detects the phase difference between the light reception signal output from the light receiver 22 and the reference signal output from the oscillator 13 and outputs the detection result. Based on the detection result of the phase comparator 23, the operational amplifier circuit 24 filters a control signal that makes the phase difference between the light reception signal output from the light receiver 22 and the reference signal output from the oscillator 13 constant. Output to.

  Next, the operation of the variable frequency light source 20 in the above configuration will be described. When the laser light is emitted from the mode-locked laser 16 shown in FIG. 2, the laser light is amplified by the optical amplifier 17 and then enters the high optical nonlinear optical fiber 18 as in the first embodiment. Supercontinuum light is generated and output from the optical frequency comb generator 11 as an optical frequency comb.

  The optical frequency comb emitted from the optical frequency comb generator 11 enters the frequency variable optical bandpass filter 12, and a part of the optical frequency comb passes through the frequency variable optical bandpass filter 12. The light that has passed through the frequency variable optical bandpass filter 12 is incident on the optical branching device 21 and partly branched, and the rest is output to the outside of the frequency variable light source 20. The light branched by the branching device 21 is received by the light receiving device 22, and a light receiving signal corresponding to the light branched by the branching device 21 is output from the light receiving device 22.

  When the light reception signal from the light receiver 22 is input, the phase comparator 13 detects the phase difference between this light reception signal and the reference signal from the oscillator 13 and outputs the detection result. The detection result is input to the operational amplifier circuit 24. Based on the detection result of the phase comparator 23, the operational amplifier circuit 24 filters a control signal that makes the phase difference between the light reception signal output from the light receiver 22 and the reference signal output from the oscillator 13 constant. Output to. The filter sweep circuit 14 continuously changes (sweeps) the transmission band of the frequency variable optical bandpass filter 12 based on the reference signal from the oscillator 13.

  FIG. 5 is a diagram for explaining the operation of the variable frequency light source according to the second embodiment of the present invention. The frequency optical bandpass filter 12 is controlled by the filter sweep circuit 14, but the center frequency of the transmission band does not change linearly with respect to the level of the control signal of the filter sweep circuit 14 as shown in FIG. Sometimes. That is, even if the filter sweep circuit 14 attempts to change the center frequency of the transmission band of the frequency optical bandpass filter 12 linearly, the control signal of the filter sweep circuit 14 is actually changed from the characteristics of the frequency optical bandpass filter 12. It is conceivable that the center frequency of the transmission band is deviated from the level.

  At this time, the frequency component of the optical frequency comb output from the optical frequency comb generator 11 is not swept by the frequency optical bandpass filter 12 at a constant time interval. For this reason, as shown in FIG. 5B, the time interval of each optical pulse forming the optical pulse train output from the frequency variable light source 20 is not constant, and the time change of the frequency also changes as shown in FIG. 5C. Become nonlinear.

  The frequency variable light source 20 according to the second embodiment of the present invention shown in FIG. 4 has the configuration shown in FIG. 4 in order to eliminate such problems. That is, a part of the light transmitted through the frequency optical bandpass filter 12 is received by the light receiver 22, and the phase difference between the light reception signal obtained thereby and the reference signal output from the oscillator 13 is compared by the phase comparator 23. The operational amplification circuit 24 outputs a control signal for making the phase difference constant to the filter sweep circuit 14. Thus, even if there is a deviation in the characteristics of the frequency optical bandpass filter 12, the center frequency of the transmission band of the frequency optical bandpass filter 12 can be linearly changed with respect to the level of the control signal of the filter sweep circuit 14. It is possible to generate an optical pulse train whose frequency changes with high accuracy.

<Third Embodiment>
FIG. 6 is a block diagram showing a schematic configuration of a frequency variable light source according to the third embodiment of the present invention. In FIG. 6, the same components as those shown in FIG. 1 are denoted by the same reference numerals. As shown in FIG. 6, the frequency variable light source 30 of the present embodiment includes an optical frequency divider 31, an absolute light in addition to the optical frequency comb generator 11, the frequency variable optical bandpass filter 12, the oscillator 13, and the filter sweep circuit 14. A frequency stabilized light source 32, a light receiving circuit 33, and an operational amplifier circuit 34 are included. The absolute optical frequency stabilized light source 32 corresponds to the reference light source according to the present invention, the light receiving circuit 33 corresponds to the second light receiver according to the present invention, and the operational amplifier circuit 34 corresponds to the second control circuit according to the present invention. Correspond.

  The optical combiner / branch 31 branches part of the light transmitted through the frequency optical bandpass filter 12 and combines one of the branched light and the reference light emitted from the absolute optical frequency stabilized light source 32. The remainder of the light that has passed through the frequency optical bandpass filter 12 that has not been branched by the optical coupler 31 is output to the outside of the variable frequency light source 30. Note that the branching rate of the optical combiner / branch 31 (the intensity of the light branched by the optical combiner / branch 31) may be arbitrary, but in order to prevent the intensity of the light (optical pulse train) output to the outside of the frequency variable light source 30 from decreasing. The branching rate of the optical combiner / branch 31 is preferably as small as possible.

  The absolute optical frequency stabilized light source 32 emits reference light having a frequency that is within the sweep range of the frequency variable optical bandpass filter 12 because the frequency does not change or the frequency fluctuation is extremely small. The absolute light frequency stabilized light source 32 includes, for example, acetylene gas, hydrogen cyanide gas, or the like, and emits reference light whose frequency is fixed to molecular absorption lines of these gases. The light receiving circuit 33 receives the light combined by the optical multiplexer / demultiplexer 31 and outputs a light reception signal. The light receiving circuit 33 is configured to be able to detect a frequency band equal to or lower than the frequency change rate of the optical pulse train output from the frequency variable light source 30 (Δf (= f) shown in FIG. 3E). Yes. Here, when the frequency of the light transmitted through the frequency variable optical bandpass filter 12 is close to the frequency of the reference light emitted from the absolute optical frequency stabilized light source 32, a beat signal is output as the light receiving signal. .

  The operational amplifier circuit 34 controls the optical frequency comb generator 11 based on the light reception signal output from the light reception circuit 33. Specifically, when a beat signal is output as a light receiving signal from the light receiving circuit 33, the optical frequency comb output from the optical frequency comb generator 11 is set in the frequency domain so that the beat signal has a constant value. Shift. Thus, the predetermined frequency component of the optical pulse transmitted through the frequency variable optical bandpass filter 12 is changed to a stable known frequency having a certain difference from the frequency of the reference light emitted from the absolute optical frequency stabilized light source 32. can do. A control signal from the operational amplifier circuit 34 is input to the mode-locked laser 16 shown in FIG. 2, and the oscillation frequency of the mode-locked laser 16 is controlled.

  Next, the operation of the variable frequency light source 30 in the above configuration will be described. FIG. 7 is a diagram for explaining the operation of the variable frequency light source according to the third embodiment of the present invention. When the laser light is emitted from the mode-locked laser 16 shown in FIG. 2, the laser light is amplified by the optical amplifier 17 and then enters the high optical nonlinear optical fiber 18 as in the first embodiment. Supercontinuum light is generated and output from the optical frequency comb generator 11 as an optical frequency comb. The optical frequency comb emitted from the optical frequency comb generator 11 enters the frequency variable optical bandpass filter 12, and a part of the optical frequency comb passes through the frequency variable optical bandpass filter 12. The light that has passed through the frequency variable optical bandpass filter 12 is incident on the optical combiner / branch 31 and partly branched, and the rest is output to the outside of the frequency variable light source 30.

  Similarly to the first embodiment, for example, simultaneously with the emission of laser light from the mode-locked laser 16, a reference signal with a constant frequency with high accuracy is output from the oscillator 13 to the filter sweep circuit. Based on this reference signal, the filter sweep circuit 14 continuously changes (sweeps) the transmission band of the frequency variable optical bandpass filter 12. As a result, as shown in FIGS. 7A and 7B, the frequency variable light source 30 has a constant intensity, the time interval of each optical pulse is Td, and the frequency is Δf ( = F) An optical pulse train that changes only by f is output.

  Here, in this embodiment, the light branched by the optical multiplexer / demultiplexer 31 (the light transmitted through the frequency variable optical bandpass filter 12) is combined with the reference light emitted from the absolute optical frequency stabilized light source 32. Light is received by the light receiving circuit 33. When the frequency difference between the light branched by the optical multiplexer / demultiplexer 31 and the reference light from the absolute optical frequency stabilized light source 32 is larger than Δf (= f) shown in FIG. No light reception signal is output from 33. This is because, as described above, the light receiving circuit 33 is configured to be able to detect a frequency band equal to or lower than the frequency change rate (Δf) of the optical pulse train output from the variable frequency light source 30.

  Now, as shown in FIG. 7B, assuming that the frequency of the reference light emitted from the absolute optical frequency stabilized light source 32 is f0, the frequency of the light branched by the optical multiplexer / demultiplexer 31 becomes the frequency f0 of the reference light. When they are close (when the difference between these frequencies is equal to or less than Δf), the light receiving circuit 33 outputs a beat signal as a light receiving signal. In the example shown in FIG. 7B, the beat signal shown in FIG. 7C is output when the optical pulses at the times t1 and t2 are generated. FIG. 7D is an enlarged view of a part of the beat signal.

  When a beat signal as a light receiving signal is output from the light receiving circuit 33, the operational amplifier circuit 34 outputs a control signal to the optical frequency comb generator 11, so that the beat signal from the light receiving circuit 33 becomes a constant value. The optical frequency comb output from the optical frequency comb generator 11 is shifted in the frequency domain. Specifically, the mode-locked laser 16 shown in FIG. 2 is controlled to control the oscillation frequency of the mode-locked laser 16. With this control, a predetermined frequency component that passes through the frequency variable optical bandpass filter 12 (the frequency component of the optical pulse at times t1 and t2 shown in FIG. 7B) is emitted from the absolute optical frequency stabilized light source 32. It can be fixed at a frequency shifted by a certain value from the frequency of the reference light.

  As described above, in this embodiment, a part of the light transmitted through the frequency optical bandpass filter 12 and the reference light emitted from the absolute optical frequency stabilized light source 32 are combined, and the difference between the frequencies is determined as a beat signal. The optical frequency comb generator 11 is controlled so that the beat signal becomes a constant value. As a result, one of the frequency components of the optical frequency comb output from the optical frequency comb generator 11 can be matched with the frequency of the reference light, whereby each frequency component of the optical frequency comb is referenced to the reference light. The frequency can be fixed. Alternatively, it can be fixed at a frequency shifted by a certain frequency. Therefore, the frequency of the optical pulse train can be changed with the frequency of the reference light as a reference.

<Fourth embodiment>
The frequency variable light source according to the fourth embodiment of the present invention has both the configuration of the frequency variable light source 20 according to the second embodiment shown in FIG. 4 and the configuration of the frequency variable light source 30 according to the third embodiment shown in FIG. Prepare. That is, in addition to the optical frequency comb generator 11, the frequency variable optical bandpass filter 12, the oscillator 13, and the filter sweep circuit 14, the optical branching device 21, the light receiver 22, the phase comparator 23, and the operational amplifier circuit 24, In this configuration, the optical combiner / branch 31, the absolute optical frequency stabilized light source 32, the light receiving circuit 33, and the operational amplifier circuit 34 are combined.

  With this configuration, even if there is a deviation in the characteristics of the frequency optical bandpass filter 12 as in the second embodiment, the center frequency of the transmission band of the frequency optical bandpass filter 12 is set to the level of the control signal of the filter sweep circuit 14. The frequency of the optical pulse train can be changed with reference to the frequency of the reference light emitted from the absolute optical frequency stabilized light source 32 as in the third embodiment.

[Frequency calibration system and method]
FIG. 8 is a block diagram showing a schematic configuration of a frequency calibration system according to an embodiment of the present invention. As shown in FIG. 8, the frequency calibration system of the present embodiment is connected to the optical fiber networks N1 to N6 connected to each other, the optical measuring device 40 connected to the optical fiber networks N1 to N6, and the optical fiber network N1. The frequency variable light source 50 is configured.

  The optical measuring device 40 is a kind of optical equipment that handles light, and is, for example, an optical spectrum analyzer, a wavelength meter, or the like. Here, the optical measuring instrument 40 is taken as an example of an optical instrument that handles light, but the optical instrument that handles light may be a wavelength variable light source (frequency variable light source) or the like in addition to the optical measuring instrument 40. The frequency variable light source 50 is any one of the frequency variable light sources 10, 20, and 30 according to the first to third embodiments of the present invention described above.

  In the above configuration, when an optical pulse train in which the time interval of each optical pulse is constant and the frequency changes for each optical pulse by a predetermined amount is emitted from the frequency variable light source 50, the optical pulse train is transmitted through the optical fiber networks N1 to N1. Transmitted to each of N6. Then, by calibrating the reference frequency of each optical measuring device 40 using the optical pulse train transmitted via the optical fiber networks N1 to N6, the reference frequencies of all the optical measuring devices 40 are adjusted to the frequency variable light source 50. Can match the frequency.

  The frequency variable light source and the frequency calibration system and method according to the embodiment of the present invention have been described above. However, the present invention is not limited to the above embodiment, and can be freely changed within the scope of the present invention. For example, in the above embodiment, the optical frequency comb generator 11 having the configuration shown in FIG. 2 is used as a light source for generating supercontinuum light, but the present invention is not limited to such a light source.

It is a block diagram which shows schematic structure of the frequency variable light source by 1st Embodiment of this invention. 2 is a block diagram illustrating an example of a configuration of an optical frequency comb generator 11. FIG. It is a figure for demonstrating operation | movement of the frequency variable light source by 1st Embodiment of this invention. It is a block diagram which shows schematic structure of the frequency variable light source by 2nd Embodiment of this invention. It is a figure for demonstrating operation | movement of the frequency variable light source by 2nd Embodiment of this invention. It is a block diagram which shows schematic structure of the frequency variable light source by 3rd Embodiment of this invention. It is a figure for demonstrating operation | movement of the frequency variable light source by 3rd Embodiment of this invention. It is a block diagram which shows schematic structure of the frequency calibration system by one Embodiment of this invention. It is a block diagram which shows the conventional frequency variable light source. It is a figure which shows the optical pulse train produced | generated with the conventional frequency variable light source.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Frequency variable light source 11 Optical frequency comb generator 12 Frequency variable optical band pass filter 13 Oscillator 14 Filter sweep circuit 20 Frequency variable light source 22 Light receiver 23 Phase comparator 24 Operational amplifier circuit 30 Frequency variable light source 31 Optical multiplexer / demultiplexer 32 Absolute optical frequency Stabilized light source 33 Light receiving circuit 34 Operational amplifier circuit 40 Optical measuring device 50 Frequency variable light source N1 to N6 Optical fiber network

Claims (7)

In a frequency variable light source that generates an optical pulse train whose frequency changes with time,
An optical frequency comb generator for generating an optical frequency comb having a plurality of optical frequency components at a predetermined frequency interval;
A frequency variable optical bandpass filter having a transmission band narrower than the frequency interval of the optical frequency component in the optical frequency comb, and the transmission band being variable;
A frequency variable light source comprising: a sweeping device that continuously changes the transmission band of the frequency variable optical bandpass filter. The sweep device includes an oscillator that outputs a reference signal having a constant frequency;
The frequency variable light source according to claim 1, further comprising: a filter sweep circuit that linearly sweeps the transmission band of the frequency variable optical bandpass filter based on the reference signal output from the oscillator. A first light receiver that receives a part of an optical pulse having the optical frequency component that has passed through the frequency variable optical bandpass filter and outputs a light reception signal;
The sweep device includes a phase comparator that detects a phase difference between the light reception signal output from the first light receiver and the reference signal output from the oscillator;
The frequency variable light source according to claim 2, further comprising: a first control circuit that outputs a control signal that makes a phase difference detected by the phase comparator constant to the filter sweep circuit. A reference light source for generating reference light having a frequency included in a frequency band capable of varying the transmission band of the frequency variable optical bandpass filter;
A multiplexer that combines the optical pulse having the optical frequency component transmitted through the frequency variable optical bandpass filter and the reference light from the reference light source;
A second light receiver for receiving the light combined by the multiplexer and outputting a light reception signal;
Based on the light reception signal output from the second light receiver, the optical frequency of the optical pulse having the optical frequency component transmitted through the frequency variable optical bandpass filter becomes a constant value with the frequency of the reference light source. The frequency variable light source according to any one of claims 1 to 3, further comprising: a second control circuit that controls the optical frequency comb generator.   The frequency variable light source according to claim 4, wherein the reference light source generates light whose frequency does not vary as the reference light. In a frequency calibration system that calibrates the reference frequency in optical equipment that handles light,
An optical fiber network;
The optical equipment connected to the optical fiber network;
A frequency calibration system comprising: the variable frequency light source according to any one of claims 1 to 5 connected to the optical fiber network. In a frequency calibration method for calibrating a reference frequency in an optical instrument that handles light,
Transmitting the optical pulse train generated from the variable frequency light source according to any one of claims 1 to 5 to an optical fiber network;
Calibrating a reference frequency of the optical device using the optical pulse train transmitted through the optical fiber network.

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