JP2012027161A - Frequency characteristic calibration method of conversion efficiency in photoelectric conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light source used for heterodyne detection having a characteristic demanded as a heterodyne detection light source, with a simple device.SOLUTION: A light source system for heterodyne detection includes a light source 11 and a light modulator 12 for generating two-tone light corresponding to light entering from the light source 11 and a modulation signal. The light modulator 12 has a Mach-Zehnder waveguide 14. The two-tone light output from the light modulator 12 is f±nf(n: integral number; for example, n=1, 2 or 3) when setting a frequency of light entering from the light source 11 as fand setting a frequency of modulation signal as f. Then, the intensity of fcomponent output from the light modulator 12 becomes lower than the intensity of f+nfcomponent by 20 dB or more. Driving of light modulator under the conditions results in lowering of carrier components and acquisition of two-tone signal having the uniform intensity.

Description

  The present invention relates to a method for calibrating frequency characteristics of conversion efficiency in a photoelectric conversion device.

  An electro-optical (Electrical to Optical, E / O) conversion device represented by an optical modulator and a directly modulated semiconductor laser, and an opto-electrical (Optical to Electrical, O / E) conversion device represented by a photodiode (PD) are digital. It is a key component of optical communication systems and fiber radio (Radio On Fiber, ROF) systems. As a main performance index of these devices, there is O / E and E / O conversion efficiency in direct current (DC) called “responsibility”.

  In recent years, the digital optical communication system has been remarkably increased in speed as it exceeds 100 Gbps per wave, and ROF has been reported on research on systems using carriers with higher frequencies one after another. For this reason, it is very important to accurately measure not only DC responsivity but also frequency characteristics in the frequency region including the millimeter wave band to the terahertz band.

  Currently, in order to measure the frequency characteristics of O / E and E / O devices (photoelectric conversion devices), it is common to use an optical component analyzer (LCA) manufactured by Agilent (O. Funke, “The value of”). traceable S-parameter charac- terization of electro-optical components, "EN-Genius Network, test & measurement ZONE, November 2008.). The LCA combines a radio frequency (RF) network analyzer with an optical test set incorporating E / O and O / E devices whose frequency characteristics are calibrated. Therefore, the LCA can measure frequency characteristics calibrated for all combinations of E / O, O / E, and O / O as well as E / E input / output. Here, “calibrated frequency characteristics” means that the data is traceable to a metrology standard managed by the National Institute of Standards and Technology. The power of unmodulated continuous wave light and RF signal is linked to the primary standard of current and voltage by comparing with the heat generation of the heater using a thermal detector such as a calorimeter.

  The National Institute of Standards and Technology (NIST) has established a measurement standard for photoelectric conversion based on the heterodyne method and provides a calibration service using it (P. D. Hale and C. M. Wang, “Calibration service of”. optoelectronic frequency response at 1319 nm for combined photodiode / RF power sensor transfer standards, “NIST Special Publication, 250-51, Non-patent Document 1”, Non-patent Document 1, Non-patent Document 1 The heterodyne method utilizes the fact that when two light waves of the same power / same polarization with a determined frequency difference are received, 100% intensity modulation is applied at a frequency equal to the frequency difference of the two light waves when viewed from the time axis. Therefore, it is based on the fact that the conversion efficiency can be accurately calculated from the input optical power and the output RF signal power. In order to realize such two light waves, NIST proposes a complex system in which two LD-pumped Nd: YAG lasers are phase-locked to determine the frequency difference and the intensity and polarization are automatically controlled. Yes. NIST provides a calibration service using this system. Agilent uses this service to calibrate its internal standard PD and use it to calibrate commercial measuring instruments.

  By the way, when the measurement accuracy of LCA is examined, values under various conditions are listed (Agilent Technologies Inc., “N4373C 67 GHz Single-Mode Lightwave Analyzer Analyzer for 40 / 100G electro-optical” Data Sheet, pp. 8-11, Feb. 2008.). According to this report, the absolute uncertainty in measuring the frequency response characteristics of O / E and E / O conversion efficiency is surprisingly around ± 2 dB. On the other hand, the repeatability of measurement is as excellent as about ± 0.02 dB in a low frequency range of 0.7 GHz or less and about ± 0.3 dB in a high frequency range of 20 GHz or more, which is 10 times as excellent. In the case of E / E, the accuracy is that of the RF network analyzer, which is also as high as about ± 0.2 dB. The reason for this is that the RF network analyzer is assumed to be calibrated by the user using a calibration kit immediately before measurement, but O / E and E / O are calibrated only when a calibration service for the measuring instrument is requested. It is presumed that calibration cannot be performed before daily measurement. In this case, typically about a year has passed since the last calibration, so even if measurement reproducibility is excellent, it seems difficult to maintain absolute accuracy.

  First, the principle of the heterodyne method based on the photoelectric conversion standard and the characteristics required for the light source will be described.

  Consider an analysis model of two light waves. The two light waves are named “+1” and “−1”, respectively, and their power and frequency difference are defined as follows.

Where P ₊₁ and P ₋₁ are the power of each light wave, P _opt is the total light power, δ is the power difference between the two light waves, ω ₊₁ and ω ₋₁ are the frequencies of each light wave, and ω _RF is the frequency difference. . At this time, the instantaneous optical power is calculated as follows.

  When this light wave is square-detected with a PD having a conversion efficiency κ and the direct current is cut, an RF current is obtained and is calculated as follows.

  The average power consumed by this RF current with a load of 50Ω is calculated as follows:

  From the equations (3) and (4), the conversion efficiency κ is as follows.

  If the power difference δ between the two light waves is 0, the modulation factor is 100% and the conversion efficiency κ is as follows.

  Only optical power and RF power are measured, and both can be traceable.

The light source used in the heterodyne method described so far requires the following four characteristics. That is, the heterodyne method, following the light source satisfying the characteristics entered into the measured PD to determine the optical electric conversion efficiency from the output RF signal strength P _RF and incident light intensity Popt.
(1) Only two light waves.
(2) The frequency difference can be set to any RF frequency.
(3) Two light waves have the same polarization.
(4) Two light waves have the same intensity.

  Of these conditions, (3) is necessary to make the heterodyne efficiency 100%.

  In order to realize a light source for photoelectric conversion standards having the characteristics listed above, it has been proposed to use two LD-pumped Nd: YAG lasers (Non-Patent Document 1).

  Since this apparatus can control each laser individually, the conditions of (1) only two light waves, (3) the same polarization, and (4) the same intensity can be realized relatively easily. However, this device (2) is difficult to set an arbitrary frequency difference. In this device, the condition of (2) is realized by forming an optical phase-locked loop. However, since it is necessary to use an LD-pumped Nd: YAG laser with a nominal line width of about kHz, the calibration wavelength is Limited to 1319 nm.

P. D. Hale and C. M. Wang, “Calibration service of optoelectronic responsive 99 at Nr for combined phodiode / RF power sensor transport and RF power sensor transport.

  The method of calibrating frequency characteristics of a photodetector disclosed in Japanese Patent Application Laid-Open No. 2005-14785 requires two intensity modulators. Therefore, in order to execute this method, it is necessary to synchronize the two modulators. In addition, the apparatus used in this method is complicated because it has two intensity modulators.

  Therefore, an object of the present invention is to provide a light source used for heterodyne detection having characteristics required as a heterodyne detection light source with a simple apparatus.

  Another object of the present invention is to provide a light source having a mechanism for automatically controlling the bias voltage in the above-described apparatus.

  Another object of the present invention is to realize a simple photoelectric conversion standard that can maintain an absolute uncertainty of about ± 0.1 dB over several years. If such a photoelectric conversion standard can be realized, it is considered that measurement accuracy equivalent to that of the RF network analyzer can be easily achieved by distributing it to users widely like a calibration kit for RF network analyzers and making calibration just before measurement. It is done.

  The present invention basically uses an optical system including a Mach-Zehnder waveguide under predetermined conditions without using two separate light sources to create two-tone light for heterodyne detection. , Based on the knowledge that two-tone light that satisfies the specifications required for heterodyne detection can be easily generated.

The first aspect of the present invention is a heterodyne including a light source 11 and an optical modulator 12 that receives output light from the light source 11 and generates two-tone light corresponding to the light incident from the light source 11 and a modulation signal. The present invention relates to a light source system for detection. Here, the optical modulator 12 is an optical modulator having a Mach-Zehnder waveguide 14. Two-tone light output from the optical modulator 12, the frequency of the light incident from the light source 11 and _{f 0,} if the frequency of the modulation signal is a _{f m, f 0 ± nf m} (n is an integer, eg, n = 1, 2 or 3). Then, the intensity of the _{f 0} component output from the optical modulator 12 _is suppressed more than 20dB with respect to the intensity of the f 0 + nf _m components. By driving the optical modulator under such conditions, it is possible to obtain a two-tone signal having a uniform intensity with suppressed carrier components.

Conventionally, a technique for obtaining a carrier-suppressed double-sideband modulated signal using an optical modulator having a Mach-Zehnder waveguide 14 has been known. However, the main purpose of this modulation technique is to suppress the carrier wave, and it was not intended to align the strength of the double sideband modulation signal. As described above, the present invention suppresses unnecessary carrier components and third-order harmonic components by adjusting the phase difference of the Mach-Zehnder waveguide and driving the optical modulator under a driving condition that suppresses the carrier by 20 dB or more. In addition to this, it is based on the knowledge that the two intensities of the upper sideband modulation signal and the lower sideband modulation signal can be made uniform. In a preferred embodiment of this heterodyne detection light source system, the drive is controlled by a drive condition that the maximum value of the voltage amplitude of the modulation signal is limited to not less than 0.2 times and not more than twice the half-wave voltage. Note that n = 1 is preferable. That is, as will be described later, when n = 1, the carrier component f ₀ is suppressed to 20 dB or less compared to LSB or USB, so that the intensity difference between USB and LSB can be controlled to be small. When the intensity of the modulation signal applied to the optical modulator 12 is not less than 0.1 times and not more than 2 times the half-wave voltage of the optical modulator 12, particularly the intensity of the USB and LSB can be controlled small. Therefore, it is preferable.

  In a preferred embodiment of this heterodyne detection light source system, a wavelength variable light source is used as the light source 11. By using a variable wavelength light source as the light source 11, the frequency range that can be calibrated can be expanded. Since the present invention uses a light source for heterodyne detection including an optical modulator having a Mach-Zehnder waveguide 14, even if the frequency of the carrier is changed by using a wavelength tunable light source as the light source 11, the light is output. The balance of the two tone signals can be properly maintained.

  In a preferred embodiment of the light source system for heterodyne detection, a sub Mach-Zehnder waveguide 15 is provided on one arm of the Mach-Zehnder waveguide 14. In this aspect, the Mach-Zehnder waveguide 14 preferably has an electric field control Y-branch waveguide at one or both of the branching portion and the combining portion. By the control electric field, the intensity imbalance of the optical signal propagating through both arms of the Mach-Zehnder waveguide 14 can be easily corrected.

  Since the sub-Mach-Zehnder waveguide 15 is provided in one arm, the intensity imbalance of the optical signal propagating through both arms of the Mach-Zehnder waveguide 14 can be easily corrected.

  A preferred embodiment of this heterodyne detection light source system is an optical modulator 12 in which the optical modulator 12 has sub-Mach-Zehnder waveguides 15 and 16 in both arms of the Mach-Zehnder waveguide 14, respectively. In this way, the optical modulator 12 having the Bach-Zehnder waveguides 15 and 16 on both arms is relatively easy to manufacture because of its symmetrical structure. A preferred embodiment of the light source system for heterodyne detection has an electric field control Y-branch waveguide at the branching portion or the multiplexing portion of the Mach-Zehnder waveguide 14 of the optical modulator 12 or both. By the control electric field, the intensity imbalance of the optical signal propagating through both arms of the Mach-Zehnder waveguide 14 can be easily corrected.

A preferred embodiment of the light source system for heterodyne detection includes a photodetector 13 on which light output from the optical modulator 12 is incident, a control device 18 for analyzing the frequency characteristics of the light detected by the photodetector 13, Is further included. Then, the control device 18 controls the bias voltage source of the optical modulator 12 based on the intensity information of the f ₀ component and the f ₀ ± nf _m (n is an integer) component included in the light detected by the photodetector 13. And a first bias adjusting means for issuing a command for changing the bias voltage applied to the optical modulator 12.

  Since this system has the first bias adjusting means, the bias voltage applied to the optical modulator can be automatically adjusted. The preferred n is 1 above. Since this system can be configured up to twice the frequency region of the modulation signal, it is particularly effective for calibration at high frequencies. An example of high frequency is the millimeter wave band.

  WO2009-110039 pamphlet discloses a bias adjustment method for a Mach-Zehnder interferometer. This bias adjustment method evaluates the MZ interferometer without using the zeroth-order component.

A preferred embodiment of the light source system for heterodyne detection includes a photodetector 13 on which light output from the optical modulator 12 is incident, a control device 18 for analyzing the frequency characteristics of the light detected by the photodetector 13, Further included. Then, the control unit 18 based on the intensity information f _m component and 2f _m component light detector 13 is included in the light detected with respect to a bias voltage source of the optical modulator 12, applied to the optical modulator 12 And a second bias adjusting means for issuing a command for changing the bias voltage.

  Since this system has the second bias adjusting means, it is effective when the modulation frequency is low. This system is also effective when it is not easy to separate carrier components.

  A preferred embodiment of the light source system for heterodyne detection includes a photodetector 13 on which the light output from the optical modulator 12 is incident, and a control device 18 for analyzing the frequency characteristics of the light detected by the photodetector 13. In addition. And the control apparatus 18 has a 1st bias adjustment means and a 2nd bias adjustment means.

  This system has a first bias adjusting means and a second bias adjusting means. For this reason, this system may switch the automatic bias adjustment to the first bias adjusting means or the second bias adjusting means in accordance with the frequency region to be calibrated. Then, this system can calibrate in a wide frequency range from a low frequency range to a millimeter wave range.

  ADVANTAGE OF THE INVENTION According to this invention, the light source used for the heterodyne detection provided with the characteristic requested | required as a heterodyne detection light source with a simple apparatus can be provided.

FIG. 1 is a block diagram showing the configuration of the photodetector system. FIG. 2 is a diagram illustrating a configuration example of the optical modulator. FIG. 3 is a flowchart showing a process example of a method for calibrating the characteristics of a photodetector. FIG. 4 is a block diagram of the heterodyne detection light source system of the present invention. FIG. 5 is a block diagram showing a second embodiment of the light source system for heterodyne detection. FIG. 6 is a block diagram showing a third embodiment of the heterodyne detection light source system. FIG. 7 is a block diagram showing a fourth embodiment of the heterodyne detection light source system. FIG. 8 is a block diagram showing a fifth embodiment of the light source system for heterodyne detection. FIG. 9 shows a flowchart for bias adjustment in the fifth embodiment. FIG. 10 is a block diagram showing a sixth embodiment of the heterodyne detection light source system. FIG. 11 shows a flowchart for bias adjustment in the sixth embodiment. FIG. 12 is a block diagram showing a seventh embodiment of the heterodyne detection light source system. FIG. 13 shows a flowchart for bias adjustment in the seventh embodiment. FIG. 14 is a block diagram showing a seventh embodiment of the heterodyne detection light source system. FIG. 15 is a block diagram showing an eighth embodiment of the heterodyne detection light source system. FIG. 16 shows a method for realizing a photoelectric conversion standard using a Mach-Zehnder (MZ) type optical modulator. FIG. 17 shows the structure of a high extinction ratio MZ optical modulator. FIG. 18 shows an example of an optical spectrum during operation of the high extinction ratio MZ interferometer. FIG. 19 shows an RF signal spectrum after square detection with a PD. FIG. 20 shows the calibration result of the frequency response characteristic. FIG. 21 is a graph replaced with a drawing showing an optical spectrum at a PD input point. FIG. 22 shows the RF spectrum near 10 GHz. FIG. 23 shows the RF spectrum near 20 GHz. FIG. 24 shows the RF spectrum near 5 GHz. FIG. 25 is a graph replaced with a drawing showing an optical spectrum at a PD input point. FIG. 26 shows the RF spectrum near 10 GHz. FIG. 27 shows the RF spectrum near 5 GHz. FIG. 28 is a graph replaced with a drawing showing an optical spectrum at a PD input point. FIG. 29 shows an RF spectrum near 10 GHz. FIG. 30 is a graph replaced with a drawing showing an optical spectrum at a PD input point. FIG. 31 shows an RF spectrum near 10 GHz. FIG. 32 shows an RF spectrum near 5 GHz. FIG. 33 is a diagram showing a system including DPMZM. FIG. 34 shows an example of an output signal from the DSB-SC modulator. FIG. 35 shows the frequency characteristic measurement results of the two light wave levels and the extinction ratio of the carrier wave and the harmonic wave. As shown in FIG. 36A, the branching ratio is controlled using an electric field generated by a DC voltage applied to the DC electrode. FIG. 36B shows an optical modulator having MZ. FIG. 37 shows the spectral profile of the time domain of the OOK signal. FIG. 38 shows the output of DSB-SC modulation using this apparatus.

  Hereinafter, the photodetector system will be described. FIG. 1 is a block diagram showing the configuration of the photodetector system. As shown in FIG. 1, the photodetector system includes a light source 11, an optical modulator 12, a photodetector 13, and a control device 18. The optical modulator 12 has sub-Mach-Zehnder waveguides 15 and 16 in both arms of the main Mach-Zehnder waveguide 14, respectively. Further, the control device 18 analyzes the frequency characteristic of the photodetector 13 corresponding to the two-tone light, and detects the detected value of the optical signal detected by the photodetector 13 according to the analyzed frequency characteristic of the photodetector 13. Has a calibration unit 19 for calibrating.

  The photodetector system of the present invention is a system that can evaluate the characteristics of the photodetector and perform calibration based on the evaluated characteristics. Thus, the photodetector system of the present invention can include any photodetector. Since the photodetector is used as a decoder in optical information communication, the photodetector system of the present invention can be used as a decoder.

  The light source 11 may be a signal in optical information communication. Since the photodetector system of the present invention evaluates the characteristics of the photodetector, the light source 11 for evaluating the characteristics of the photodetector may be a light source according to the actually used communication mode.

  The optical modulator 12 includes sub Mach-Zehnder waveguides 15 and 16 in both arms of the main Mach-Zehnder waveguide 14 respectively. Such an optical modulator is already known. An example of this optical modulator is disclosed in Japanese Patent Application Laid-Open No. 2007-065240.

  FIG. 2 is a diagram illustrating a configuration example of the optical modulator. As shown in FIG. 2, this optical modulator has a main Mach-Zehnder waveguide 28. The main Mach-Zehnder waveguide 28 includes an optical signal input section 22, a branch section 23 where the optical signal branches, a first arm 24 which is a waveguide through which the optical signal branched from the branch section 23 propagates, Two arms 25, a combining unit 26 that combines the optical signals output from the first arm 24 and the second arm 25, and a light that outputs the optical signal combined by the combining unit. A signal output unit 27.

  The optical modulator includes a first sub Mach-Zehnder waveguide 29 provided on the first arm 24 and having a first electrode (electrode A) 31. Further, the optical modulator includes a second Mach-Zehnder waveguide 30 provided on the second arm 25 and having a second electrode (electrode B) 32.

  A preferred embodiment of this optical modulator is provided so as to be along at least a part of the waveguide between the output part and the multiplexing part of the first sub Mach-Zehnder waveguide 29 in the main Mach-Zehnder waveguide 28. The first main Mach-Zehnder electrode 33a is provided. The optical modulator is provided so as to extend along at least a part of the waveguide between the output part and the multiplexing part of the second Mach-Zehnder waveguide 30 in the main Mach-Zehnder waveguide 28. 2 main Mach-Zehnder electrodes 33b.

  In the optical modulator of this aspect, the phase of the optical carrier signal or the specific higher-order optical signal included in the output signal from the first sub Mach-Zehnder waveguide 29 and the output signal from the second Mach-Zehnder waveguide 30 is 180 °. A controller is provided for adjusting the voltage applied to the first main Mach-Zehnder electrode 33a and the second main Mach-Zehnder electrode 33b so as to be shifted. Thereby, the optical modulator of this aspect can achieve DSB-SC modulation.

  A more preferable aspect of this optical modulator is provided so as to extend along at least a part of the waveguide between the output portion and the multiplexing portion of the first sub Mach-Zehnder waveguide 29 in the main Mach-Zehnder waveguide 28. The light intensity correction mechanism 51 is provided. The optical modulator is a light provided along at least a part of the waveguide between the output part and the multiplexing part of the second Mach-Zehnder waveguide 30 in the main Mach-Zehnder waveguide 28. And an intensity correction mechanism 52. This optical modulator may include either or both of the light intensity correction mechanism 51 and the light intensity correction mechanism 52.

  Then, one or both of the light intensity correction mechanism 51 and the light intensity correction mechanism 52 is an optical carrier signal that travels from the first arm 24 to the multiplexing unit 26 so as to suppress the optical carrier signal output from the output unit 27. And the intensity of the optical carrier signal traveling from the second arm 25 to the multiplexing unit 26 are adjusted. Alternatively, either one or both of the light intensity correction mechanism 51 and the light intensity correction mechanism 52 is a specific unit that proceeds from the first arm 24 to the combining unit 26 so as to suppress a specific high-order optical signal output from the output unit 27. The intensity of the high-order optical signal and the intensity of the specific high-order optical signal traveling from the second arm 25 to the multiplexing unit 26 are adjusted.

  By obtaining two-tone light using the optical modulator 12 capable of obtaining a high extinction ratio in this way, two-tone light with uniform intensity can be obtained. For this reason, the photodetector system of the present invention can appropriately calibrate the characteristics of the photodetector.

  In a preferred embodiment, the optical modulator 12 is a carrier-suppressed double sideband (DSB-SC) modulator 12. By obtaining a DSB-SC signal using the above optical modulator, two-tone light with uniform intensity can be obtained. As a result, the characteristics of the photodetector can be calibrated with a simple system. Two-tone light means an optical signal in which the intensity of two frequency components is stronger than the intensity of other frequency components. An example of the two-tone light is a DSB signal (preferably, a DSB-SC signal in which a carrier wave is suppressed).

  As described above, the photodetector 13 is a device that detects the intensity of light used in optical information communication. As will be described later, in the present invention, the frequency characteristics, polarization characteristics, wavelength response characteristics, etc. of the photodetector can be evaluated. Furthermore, the present invention can provide a system that can calibrate the value measured by the photodetector based on the evaluated characteristics.

  The control device 18 analyzes the frequency characteristic of the photodetector 13 corresponding to the two-tone light detected by the photodetector 13. That is, the intensity of two-tone light is essentially the same. On the other hand, the sensitivity of the photodetector may differ depending on the frequency. Therefore, even two-tone light that should originally have the same intensity may be measured to have different intensities depending on the photodetector. The control device 18 evaluates such frequency response.

  The control device includes an input / output unit, a control unit, a calculation unit, and a storage unit, and each element can exchange information via a bus or the like. And when information is input, a control part reads the control program memorize | stored in the memory | storage part. Then, using the input information and the information stored in the storage unit, the calculation unit performs calculation processing. Then, the obtained calculation result is stored in the storage unit and output from the output unit.

  A preferred example of the control device 18 includes a calibration unit 19. The calibration unit 19 is an element for calibrating the detection value of the optical signal detected by the photodetector 13 according to the analyzed frequency characteristic of the photodetector 13.

  FIG. 3 is a flowchart showing a process example of a method for calibrating the characteristics of a photodetector. As shown in FIG. 3, first, the output light from the light source 11 enters the optical modulator 12 (step 101). Two-tone light is output from the optical modulator 12 (step 102). Two-tone light is input to the photodetector 13 (step 103). Information on the two-tone light detected by the photodetector 13 is input to the control device 18 (step 104). The control device 18 analyzes the frequency characteristic of the photodetector 13 corresponding to, for example, two-tone light (step 105). Then, the calibration unit 19 calibrates the detection value of the optical signal detected by the photodetector 13 according to the frequency characteristic of the photodetector 13 analyzed by the control device 18 (step 106).

The photodetector system evaluates the frequency characteristics of the photodetector at a plurality of frequencies in the same manner as the examples of f _t1 and f _t2 described above. And the calibration value according to the response characteristic in each frequency is memorize | stored in a memory | storage part. In addition, when the detection is actually performed using the photodetector, the calibration value is read from the storage unit using the actually measured frequency value. Then, the measured value and the calibration value are multiplied to obtain the strength after calibration. In this way, the detection value can be obtained with high accuracy even for a wide range of frequencies. If the frequency value is discrete, the calibration value between them may be linearly interpolated.

The center frequency (f _O ) of the light output from the light source 11 is changed. Then, the change in the intensity of the frequency component corresponding to the optical two-tone signal accompanying the change in the center frequency (f _O ) of the light output from the light source 11 is evaluated. Thereby, the wavelength characteristic of the photodetector 13 is evaluated. The calibration method is the same as above.

  The polarization of the two-tone signal may be changed. Then, the change in the intensity of the frequency component corresponding to the two-tone signal accompanying the change in the polarization of the two-tone signal is evaluated. Thereby, the polarization characteristic of the photodetector 13 is evaluated. The calibration method is the same as above. That is, the photodetector also knows the polarization state. Alternatively, information regarding the polarization state of the input signal is input to the control device. The control device stores a calibration value corresponding to the polarization state. At the time of actual measurement, a calibration value is read out based on information on the polarization state, and multiplied by the actual measurement value to obtain a detection value after calibration.

A modulation signal having a _first frequency (f ₁ ) is applied to the first sub Mach-Zehnder waveguide 15 and a modulation signal having a second frequency (f ₂ ) is applied to the second sub-Mach-Zehnder waveguide 16. You may apply. The bias voltage applied to the main Mach-Zehnder waveguide is suppressed so that a frequency (f ₁ + f ₂ ) component corresponding to the sum of the first frequency (f ₁ ) and the second frequency (f ₂ ) is suppressed. adjust. In this way, good two-tone light with uniform intensity can be obtained as demonstrated by the examples described later. Therefore, according to this method, the characteristics of the photodetector can be appropriately calibrated.

A modulation signal (f ₁ ) having a _first frequency is applied to the two sub Mach-Zehnder waveguides 15 and 16. Then, the second frequency modulation signal (f ₂ ) is applied to the main Mach-Zehnder waveguide. Then, the bias voltage of the main Mach-Zehnder waveguide has an optical phase difference of 90 degrees which is twice the frequency component of the first frequency modulation signal (f ₁ ), and the second frequency modulation signal (f ₂ ) Control is performed so that the phase difference between double frequency components is 90 degrees. In this way, good two-tone light with uniform intensity can be obtained as demonstrated by the examples described later. Therefore, according to this method, the characteristics of the photodetector can be appropriately calibrated.

  FIG. 4 is a block diagram of the heterodyne detection light source system of the present invention. As shown in FIG. 4, this system includes a light source 11, an optical modulator 12 that receives output light from the light source 11, and generates two-tone light according to the light incident from the light source 11 and a modulation signal. including. The optical modulator 12 is an optical modulator having a Mach-Zehnder waveguide 14. Reference numeral 21 in FIG. 4 denotes a modulation signal generator for applying a modulation signal to the optical modulator 12. Reference numeral 22 denotes a bias voltage generator for applying a bias voltage to the optical modulator 12. Reference numeral 23 denotes a photodetector. Here, the photodetector 23 may be a photoelectric conversion device that is a target for calibrating frequency characteristics. The modulation signal generator 21 and the bias voltage generator 22 are connected to a control device described later. The voltage applied to the optical modulator can be changed in accordance with a control command from the control device.

In this optical modulator 12, the intensity of the applied modulation signal (f _m ) is preferably 0.1 to 2 times the half-wave voltage (Vπ) of the optical modulator 12. In order to suppress the influence of noise generated in a laser or an optical amplifier with an emphasis on conversion efficiency, the intensity of the modulation signal (f _m ) applied to the half-wave voltage (Vπ) of the optical modulator 12 is It is preferably 1 to 2 times. Therefore, for example, high conversion efficiency is required, and it is preferable to control the system using an optical amplifier so that the intensity of the modulation signal falls within the above range. On the other hand, in order to suppress unnecessary sidebands, for example, the intensity of the applied modulation signal (f _m ) is preferably less than or equal to 1 time with respect to the half-wave voltage (Vπ) of the optical modulator 12, and is preferably 0.3 times. The following is more preferable, and may be 0.2 times or less. For example, in a system in which the intensity of the secondary component or the tertiary component is strong, it is preferable to control in such a range. When the intensity of the applied modulation signal (f _m ) is higher than the half-wave voltage (Vπ) of the optical modulator 12, the intensity of unnecessary light such as high-order harmonics is lost when the intensity of the modulation signal is high. . On the other hand, when the intensity of the modulation signal is weak, the signal intensity is also weakened and the S / N ratio may be deteriorated. From such a viewpoint, the intensity of the applied modulation signal (f _m ) may be 0.1 to 1.6 times the half-wave voltage (Vπ) of the optical modulator 12. The intensity of the applied modulation signal may be 1 to 1.6 times the half-wave voltage (Vπ) of the optical modulator 12, or 1.2 to 1.6 times. It may be 1.2 times or more and 1.5 times or less. For example, Japanese Patent No. 386682 discloses an invention for obtaining a half-wave voltage and a chirp parameter of an optical modulator from a spectrum distribution of an MZ type optical modulator. Therefore, the half-wave voltage (Vπ) of the optical modulator may be obtained using the technique disclosed in this publication. Moreover, you may employ | adopt suitably the adjustment method disclosed by WO2009-110039 pamphlet.

Two-tone light output from the optical modulator 12, the frequency of the light incident from the light source 11 and _{f 0,} if the frequency of the modulation signal is a _{f m, f 0 ± nf m} (n is an integer, eg, n = 1, 2 or 3). Then, the intensity of the _{f 0} component output from the optical modulator 12 _is suppressed more than 20dB with respect to the intensity of the f 0 + _{f m} component. By driving the optical modulator under such conditions, it is possible to obtain a two-tone signal having a uniform intensity with suppressed carrier components.

  An example of a light source is a continuous light (CW) light source.

  The optical modulator 12 has a Mach-Zehnder waveguide 14. The branch portion of the Mach-Zehnder waveguide 14 is preferably an electric field control Y branch. The electric field control Y branch can change the branching ratio by generating an electric field at the branching portion.

  FIG. 5 is a block diagram showing a second embodiment of the light source system for heterodyne detection. This heterodyne detection light source system uses a wavelength tunable light source as the light source 11. By using a wavelength tunable light source as the light source 11, the wavelength range that can be calibrated can be expanded. Wavelength tunable light sources are known. Therefore, in this aspect, a known variable wavelength light source can be appropriately employed.

  FIG. 6 is a block diagram showing a third embodiment of the heterodyne detection light source system. In this embodiment, a sub Mach-Zehnder waveguide 15 is provided on one arm of the Mach-Zehnder waveguide 14. As shown in FIG. 6, the Mach-Zehnder waveguide 14 has a branching point where light from the light source branches into two arms. The branching ratio at the branching point is designed so that the arm having the sub Mach-Zehnder waveguide 15 is larger. In this way, by combining the bias voltage applied to the sub Mach-Zehnder waveguide 15 provided in one arm by intentionally asymmetricing the branching ratio of the optical signal, the multiplexing unit of the two arms is adjusted. The intensity of light propagating through each arm can be adjusted to be equal. In this aspect, the Mach-Zehnder waveguide 14 preferably has an electric field control Y-branch waveguide at one or both of the branching portion and the combining portion. By the control electric field, the intensity imbalance of the optical signal propagating through both arms of the Mach-Zehnder waveguide 14 can be easily corrected. Since the sub-Mach-Zehnder waveguide 15 is provided in one arm, the intensity imbalance of the optical signal propagating through both arms of the Mach-Zehnder waveguide 14 can be easily corrected.

  When the arm provided with the sub Mach-Zehnder waveguide 15 is the first arm and the arm not provided with the sub-Mach-Zehnder waveguide 15 is the second arm, the example of the branch point is that the light intensity is 10: 1 to 1: 1. What separates into 10: 9 (preferably 3: 2 to 10: 9) is mentioned.

  FIG. 7 is a block diagram showing a fourth embodiment of the heterodyne detection light source system. This embodiment is an optical modulator 12 in which the optical modulator 12 has sub-Mach-Zehnder waveguides 15 and 16 in both arms of the Mach-Zehnder waveguide 14, respectively. As described above, the optical modulator 12 having the Bach-Zehnder waveguides 15 and 16 on both arms is relatively easy to manufacture because of its contrasting structure. Such an optical modulator is known as disclosed in, for example, International Publication WO2009-110039. It is preferable that the Mach-Zehnder waveguide 14 of the optical modulator 12 has an electric field control Y-branch waveguide at the branching portion or the multiplexing portion or both. By the control electric field, the intensity imbalance of the optical signal propagating through both arms of the Mach-Zehnder waveguide 14 can be easily corrected.

FIG. 8 is a block diagram showing a fifth embodiment of the light source system for heterodyne detection. A preferred embodiment of the light source system for heterodyne detection includes a photodetector 13 on which light output from the optical modulator 12 is incident, a control device 18 for analyzing the frequency characteristics of the light detected by the photodetector 13, Is further included. Note that any optical modulator disclosed in this specification can be adopted as the optical modulator 12 in this embodiment. The same applies to the following. Then, the control device 18 controls the bias voltage source of the optical modulator 12 based on the intensity information of the f ₀ component and the f ₀ ± nf _m (n is an integer) component included in the light detected by the photodetector 13. And a first bias adjusting means for issuing a command for changing the bias voltage applied to the optical modulator 12.

  FIG. 9 shows a flowchart for bias adjustment in the fifth embodiment. In the example shown in FIG. 8, first, the bias power supply is turned on, a bias voltage is applied to the optical modulator 12, and sweeping is performed. Then, the bias voltage is set so that the detection value of the photodetector (optical power meter 1) that measures the total light output intensity from the optical modulator 12 becomes the smallest value. Thus, the optical modulator 12 is set to operate at a so-called null bias point where the carrier wave is most suppressed.

Next, the modulation signal source is turned on, and a modulation signal (RF signal f _m ) is applied to the optical modulator 12. In general, when a modulation signal is applied, the null bias point slightly shifts, so that the bias voltage applied to the optical modulator 2 is swept a little again so that the detection value of the photodetector (optical power meter 2) becomes the smallest value. , Fine-tune the bias voltage. Here, the optical power meter 2 measures the light intensity of only the carrier wave component extracted by the optical bandpass filter out of the output from the optical modulator 12. As a result, the optical modulator 12 is accurately set to the null bias point and operates as a carrier-suppressed double sideband modulator. In this state, the total light intensity (in fact f ₀ ± f _m is the primary, the other components are largely suppressed.) Measurement and the detection value of the optical power meter 1 being measured, the light intensity of the carrier wave only The carrier wave suppression ratio is calculated from the detected value of the optical power meter 2 and is confirmed to be, for example, 20 dB or more.

In this state, was determined by the total light intensity P _opt was calculated from the detection value of the optical power meter _1, and the only component of the internal 2f _m output from the calibration PD detected by the RF power meter is taken out by the RF bandpass filter with P _RF, obtaining the calibration factor. The calibration factor is as described above. For this reason, this system stores P _opt and P _RF obtained by the photodetector in the memory, reads these values from the memory during the arithmetic processing, and calculates κ according to an arithmetic program or arithmetic circuit for obtaining κ. Find the value. An example of an expression for obtaining κ is κ = (P _RF ) ^1/2 / (5P _opt ) as described above.

Thus, the calibration coefficients in 2f _m is determined, to determine the calibration coefficients for the appropriate new f _m, and outputs the information to the system controller. This system controller controls the RF-BPF transmission region and also controls the high-frequency signal (RF signal) source of the optical modulator.

Since this system has the first bias adjusting means, the bias voltage applied to the optical modulator can be automatically adjusted to maintain a high carrier suppression ratio during calibration measurement. In addition, since this system calibrates twice the frequency of the modulation signal, it is particularly effective for calibration at a high frequency. An example of high frequency is the millimeter wave band. On the other hand, this first bias adjustment means assumes that only the carrier component can be separated by the optical bandpass filter, and the calibration frequency is low and the carrier component (f ₀ ) and the modulated wave component (f ₀ ) are obtained by the optical bandpass filter. When (± nf _m ) cannot be separated (typically when the calibration frequency is 10 GHz or less), there is a risk of malfunction.

FIG. 10 is a block diagram showing a sixth embodiment of the heterodyne detection light source system. This aspect further includes a photodetector 13 on which the light output from the optical modulator 12 is incident, and a control device 18 for analyzing the frequency characteristics of the light detected by the photodetector 13. Then, the control unit 18, based on the intensity information f _m component and 2f _m component light detector 13 is included in the light detected with respect to a bias voltage source of the optical modulator 12, applied to the optical modulator 12 And a second bias adjusting means for issuing a command for changing the bias voltage.

  FIG. 11 shows a flowchart for bias adjustment in the sixth embodiment. In the example shown in FIG. 10, first, the bias power supply is turned on, a bias voltage is applied to the optical modulator 12, and sweeping is performed. Then, the bias voltage is set so that the detection value of the photodetector (optical power meter) that measures the total light output intensity from the optical modulator 12 becomes the smallest value. Thus, the optical modulator 12 is set to operate at a so-called null bias point where the carrier wave is most suppressed.

Next, the modulation signal source is turned on, and a modulation signal (RF signal f _m ) is applied to the optical modulator 12. In general, when a modulation signal is applied, the null bias point slightly shifts, so that the bias voltage applied to the optical modulator 2 is swept a little again so that the detection value of the high frequency detector (detector 1) becomes the smallest value. Fine-tune the voltage. Here, the detector 1 measures the intensity of the _fm component taken out by the mixer and the low-pass filter in the output from the PD to be calibrated. component (f ₀₎ and a first modulation wave component (f ₀ ± f _m) 2 square-law detection with the output, is used as an index representing the intensity of the carrier. As a result, the optical modulator 12 is accurately set to the null bias point and operates as a carrier-suppressed double sideband modulator. In this state, the detection value of the detector 1 measuring the output obtained by squaring the carrier wave component (f ₀ ) and the primary modulation wave component (f ₀ ± f _m ) and the primary modulation wave components (f ₀ -f _m and the f _{0 +} f _m) from the square-law detection with the detected value of to which the detector 2 measures the output, to calculate the carrier suppression ratio, previously confirmed that for example more than 20dB.

In this state, by using the P _RF obtained from the detector 2 which detect only component of the internal 2f _m output from the optical power total light intensity P _opt was calculated from the detection value of the _meter, and the calibration PD, calibration Find the coefficient. The calibration factor is as described above. Here, the detector 2 and the output of the calibration PD, a signal in which the frequency of the modulating signal is doubled by multiplying by the mixer by doubler, and detects only the component of 2f _m by transmitting the low-pass filter. For this reason, this system stores P _opt and P _RF obtained by each detector in a memory, reads these values from the memory at the time of arithmetic processing, and calculates κ according to an arithmetic program or arithmetic circuit for obtaining κ. Find the value. An example of an expression for obtaining κ is κ = (P _RF ) ^1/2 / (5P _opt ) as described above.

Thus, the calibration coefficients in 2f _m is determined, to determine the calibration coefficients for the appropriate new f _m, and outputs the information to the system controller. This system controller controls a high frequency signal (RF signal) source of the optical modulator.

  Since this system has the second bias adjusting means, the bias voltage applied to the optical modulator can be automatically adjusted to maintain a high carrier suppression ratio during calibration measurement. This system is effective when the carrier wave and the primary modulation wave component are detected by electrical means without using an optical bandpass filter, and the calibration frequency is low. On the other hand, this second bias adjustment means requires a doubler that doubles the modulation frequency, and the upper limit of the calibration frequency is limited by its characteristics.

  FIG. 12 is a block diagram showing a seventh embodiment of the heterodyne detection light source system. FIG. 13 shows a flowchart for bias adjustment in the seventh embodiment.

The mode shown in FIG. 12 further includes a photodetector 13 on which the light output from the optical modulator 12 is incident, and a control device 18 for analyzing the frequency characteristics of the light detected by the photodetector 13. And the control apparatus 18 has a 1st bias adjustment means and a 2nd bias adjustment means. This system has means for selecting the first bias adjusting means or the second bias adjusting means depending on the magnitude of ω _RF .

  This system has a first bias adjusting means and a second bias adjusting means. For this reason, this system may switch the automatic bias adjustment to the first bias adjusting means or the second bias adjusting means in accordance with the frequency region to be calibrated. Then, this system can calibrate in a wide frequency range from a low frequency range to a millimeter wave range.

  FIG. 14 is a block diagram showing a seventh embodiment of the heterodyne detection light source system. In this example, a system including an optical modulator, a photodetector, an RF amplifier, a 1/2 frequency divider, and an OEO (optoelectronic oscillator) in which an RF filter is connected in a ring shape is used. In this configuration, since a frequency twice as high as the frequency for modulating the optical modulator 12 is output from the photodetector, a ½ frequency divider is used. By using this RF signal, it is possible to easily detect the fundamental wave and the double wave component included in the output of the photo-detector that is the object of measurement, and it is possible to automatically control the bias voltage when detecting a low frequency. In this mode, the bias voltage can be automatically controlled basically in the same manner as the mode shown in FIG.

  FIG. 15 is a block diagram showing an eighth embodiment of the heterodyne detection light source system. In this example, a system including an optical modulator, a photodetector, an RF amplifier, a 1/2 frequency divider, and an OEO (optoelectronic oscillator) in which an RF filter is connected in a ring shape is used. In this configuration, since a frequency twice as high as the frequency for modulating the optical modulator 12 is output from the photodetector, a ½ frequency divider is used. By using this RF signal, it is possible to easily detect the fundamental wave and the second harmonic component contained in the output of the photodetector to be measured, and the second bias control means with high accuracy is provided. In addition, a first bias adjusting means using an optical bandpass filter is also provided. By switching appropriately according to the frequency region to be calibrated, it is highly accurate in a wide frequency region from a low frequency region to a millimeter wave region. The bias voltage can be adjusted. In this mode, the bias voltage can be automatically controlled basically in the same manner as the mode shown in FIG.

  FIG. 16 shows a method for realizing a photoelectric conversion standard using a Mach-Zehnder (MZ) type optical modulator. The MZ modulator is operated with the null point biased, and the light from the single mode light source is modulated in the carrier-suppressed double sideband (DSB-SC) mode. At this time, the intensity of the modulation signal applied to the optical modulator is set to be 0.1 to 1.6 times the half-wave voltage of the optical modulator.

Then, it is possible to realize an optical spectrum in which the upper sideband (USB) and the lower sideband (LSB) are mainly used and the other optical waves are suppressed. The frequency difference between these two light waves can be arbitrarily set as twice the modulation frequency, which is advantageous for calibration in the high frequency band. Since the incident polarization is preserved by matching the crystal axis of lithium niobate (LiNbO ₃ ) of the substrate, the same polarization can be easily realized. However, it is difficult to guarantee ((4) the same intensity) (for two light waves), which is a condition required for a light source for heterodyne detection. In particular, when the calibration frequency is low, it is difficult to separate the two light waves by optical means, and the power of each light wave cannot be measured independently.

It is known that the operating characteristics of the LN-MZ optical modulator are in good agreement with the analysis result of an ideal two-wave interference model. Here, a push-pull type MZ optical modulator driven by a CW RF signal oscillating at a frequency ω _RF as shown in FIG. 16 is considered. The analysis model uses a highly versatile model that includes all four main parameters: optical power unbalance between both arms, bias phase difference, chirp, and skew. Subscripts “1” and “2” represent quantities related to the upper arm and the lower arm, respectively. At this time, the output electric field amplitude from the MZ optical modulator can be expressed as follows.

Here, φ _B1 , φ _B2 , and φ _B are bias phases, η is an optical power imbalance, φ ₁ , φ ₂ , φ are skews, A ₁ , A ₂ , A, α _A are parameters related to chirp. Yes, defined by

  The quantities related to the carrier, USB, and LSB are identified by the subscripts “0”, “+1”, and “−1”, and the respective powers are calculated as follows.

Here, φ _B = π, that is, a state in which the MZ optical modulator is biased to the null point, is as follows.

At this time, the carrier power P0 takes the minimum value, and the USB and LSB powers P _{+ 1} and P- ₁ are equal. This means that, regardless of the values of the three parameters, optical power imbalance η, chirp α _A , and skew φ, the following three states are ideally equivalent.
• The MZ light modulator is biased to the null point.
・ Carrier power is minimized.
-There is no difference between USB and LSB power.

Therefore, the intensity of the modulation signal applied to the optical modulator is 0.1 to 2 times the half-wave voltage of the optical modulator. Under these conditions, ideally, if the bias is controlled so as to be the minimum value by monitoring the carrier power, the powers of the main two light waves, USB and LSB, become equal.

  In actual experiments, it is difficult to always set the bias point to the null point. Even if the null point is set at the initial setting, the bias point is gradually shifted over time due to the DC drift characteristic of the LN, and it can be guaranteed that the carrier is kept below a certain level at most. Degree. When the bias point is deviated, if the skew remains, a power difference as shown in the following equation is generated between the USB and the LSB.

If the skew is unknown, in order to reduce the power difference δ is required to suppress the carrier power P _0. Therefore, in the following embodiments, the high extinction ratio MZ light modulator having the bias automatic adjustment function described above is used. FIG. 17 shows the structure. Also in this case, the intensity of the modulation signal applied to the optical modulator was adjusted to be 0.1 to 2 times the half-wave voltage of the optical modulator. A sub-MZ is provided on both upper and lower arms of a normal MZ optical modulator, and the optical power of each arm can be adjusted from the outside by control voltages DCV1 and V2. In a normal MZ optical modulator, the optical power imbalance is determined by manufacturing errors such as the branching ratio of the input / output Y branch and the loss of each arm, and the carrier suppression ratio is limited to about 20 dB. However, in the high extinction ratio MZ interferometer, it is possible to easily obtain a high carrier suppression ratio of 40 dB or more by matching the powers of both arms with high accuracy at V1 and V2 and setting the phase to be opposite phase with V3. After that, it was confirmed that it could be maintained at 30 dB or more even if it deteriorated during measurement for about 1 hour.

  FIG. 18 shows an example of an optical spectrum during operation of the high extinction ratio MZ interferometer. It can be seen that the carrier is suppressed by 50.6 dB with respect to the LSB. FIG. 19 shows an RF signal spectrum after square detection with a PD. The span is only 10 kHz with respect to the center frequency of 30 GHz on the horizontal axis, and it can be seen that an extremely high purity RF signal is obtained without using an optical phase-locked loop. It has also been confirmed that the signal intensity does not change even if the optical fiber is moved slightly, and that the two incident light waves are considered to be in the same polarization state.

Table 1 shows the main specifications of the two types of PD used in the calibration experiment. PDs whose 3 dB bands were significantly different from 20 GHz and 50 GHz were selected. FIG. 20 shows the calibration result of the frequency response characteristic. Both types of PDs have been measured to have a 3 dB bandwidth as specified. On the other hand, the DC responsiveness value is indicated by a dotted line in FIG. In the experiment, the worst value of the power difference between USB and LSB was 0.18 dB. During this experiment, the carrier was suppressed by at least 30 dB or more. In this case, the power difference between USB and LSB is calculated to be within 0.001 dB at worst. However, the measurement accuracy of the light intensity of the optical spectrum analyzer used for the measurement is only about 0.4 dB, and the measured power difference between USB and LSB is considered to be an error of the measuring instrument. In general, the operation of LN Mach-Zehnder optical modulators is known to be in good agreement with theory, and the difference between USB and LSB is considered to be kept sufficiently small below the error of the measuring instrument.

RF2 tone by DPMZM (Two-tone)
In order to evaluate the non-linear characteristics of the RoF system receiving system, the generation of a two-tone RF signal was examined when detecting with an ideal PD. When a Two-tone signal is added to an ordinary intensity modulator as an electrical input, a RoF signal including the nonlinearity of the modulator itself is generated. Here, single tone signals of different frequencies f1 and f2 are added to the sub-MZs of DPMZM (a Mach-Zehnder interferometer having two Mach-Zehnder interferometers in parallel), respectively, and an ideal Two-tone for PD output. Got a signal. DSB-SC modulation was performed in each sub-MZ, and an optical Two-tone signal was generated.

  The two-tone signals derived from each MZ were combined at the main MZ. Since the optical circuit is linear, no new frequency component is generated here. A total of four spectral components were obtained as optical outputs, USB and LSB of modulation frequencies f1 and f2, respectively. In the square detection with PD, two spectral components 2f1, 2f2, and f1 + f2 are generated. However, by controlling the optical phase (bias) in the main MZ, the f1 + f2 component was suppressed and a Two-tone signal composed of 2f1 and 2f2 components was obtained. The basic principle is that the beats of the f1 USB and f2 LSB and the f2 USB and f1 LSB beats can be reversed in phase by adjusting the bias of the main MZ. Since the signals applied to the sub-MZ are each multiplied by two, in general, when a signal modulated by QAM or the like is input, the PD output is not an appropriate modulation signal, but the phase is limited to between 0 and 180 degrees. In the case of modulation, the phase change is doubled, and the phase modulation signal is in the range of 0-360 degrees at the PD output. Using this principle, we can expect to synthesize wideband FDM and OFDM signals that are difficult to generate in electrical circuits.

  NEL NLK1554BTZ-A 80128 0deg 149.7 mA was used as a light source. The output of the light source was 11.67 dBm (the output end of the polarization controller). After passing through a polarization controller, an optical amplifier (EDFA (OPREL)) and a band pass filter (BPF 5 nm (applied photoelectric)) input to an FSK modulator (TFSK1.5-10-PSN 182376), PD ( NEL KEPD 2525VPG).

  The RF input was controlled by two Rohde & Schwartz SMF-100A units. The bias control was controlled using the applied photoelectric E12069. The optical spectrum (at the PD input point) was measured with an ANDO optical spectrum analyzer. PD output was measured using an Agilent E4448A spectrum measurement device.

  Measurements were performed as follows. First, the bias of the FSK modulator was adjusted. The bias was adjusted while measuring with an optical spectrum analyzer.

  The bias of the first sub-MZ (MZa) was set to Null. In this setting, the RF output of the MZa was turned on and the RF input of the second sub MZ (MZb) was turned off. Next, the bias of MZa was adjusted so that the second-order and zeroth-order sidebands were minimized (the zeroth order does not become zero because it contains components from MZb). After that, the MZb bias was set to null. Next, RF was input to both MZa and MZb, and the bias of MZa and MZb was adjusted to minimize the secondary sideband (corresponding to temperature changes due to simultaneous RF input). Thereafter, the bias of the main MZ (MZc) was adjusted to minimize the carrier. The above procedure was repeated as appropriate.

  The PD output was monitored with an RF spectrum analyzer and fine-tuned to minimize the f1 + f2 component. Mainly adjust the bias of MZc. MZa and MZb bias are also used for fine adjustment, but when they were changed greatly, they were checked again with an optical spectrum analyzer.

  The RF power was adjusted so that the intensity of the primary sideband became equal by adjusting the optical spectrum analyzer. In addition, the RF spectrum analyzer was used for adjustment. The 2f1 and 2f2 components were adjusted to be equal. However, if the RF power is changed, the suppression ratio of the f1 + f2 component also fluctuates, so fine adjustment was also made to match the bias voltage (mainly MZc). 5.00 GHz (27.47 dBm modulator input point) was used as f1, and 5.01 GHz (27.24 dBm) was used as f2.

  FIG. 21 is a graph replaced with a drawing showing an optical spectrum at a PD input point. FIG. 22 shows the RF spectrum near 10 GHz. FIG. 23 shows the RF spectrum near 20 GHz. FIG. 24 shows the RF spectrum near 5 GHz.

  From FIG. 22, it was confirmed that the desired component Two-tone was obtained in the vicinity of 10 GHz. The suppression ratio of the unnecessary component f1 + f2 is 30 dB or more. Quadruple 20.00GHz and 20.04GHz are also generated. Nearly 5 GHz fundamental wave should be suppressed in principle, but about -60 dBm is generated. It can be seen that about 10 dB is suppressed compared to the desired component (second harmonic) shown in FIG.

  The experiment was performed in the same manner as in Example 1 with optical power: modulator output -2.97 dBm, PD input 6.45 dBm, f1: 5.00 GHz (27.40 dBm), and f2: 6.00 GHz (27.49 dBm). FIG. 25 is a graph replaced with a drawing showing an optical spectrum at a PD input point. FIG. 26 shows the RF spectrum near 10 GHz. FIG. 27 shows the RF spectrum near 5 GHz.

  Also in this case, good two-tone light could be obtained.

Example 4 RF Two-tone signal generation by DPMZM (SSB-like) Here, a single tone signal of frequency f1 is added to each sub-MZ of DPMZM with a phase difference of 90 degrees, and a single frequency f2 is added to the main MZ. An ideal two-tone signal was obtained for the PD output. DSB-SC modulation was performed at each sub-MZ to generate a frequency 2f1 component. In the main MZ, 2f1 + f2,2f1-f2 components are generated, but if the bias is set to 90 degrees optical phase difference, 2f1 and 2f2 components are suppressed at the PD detection output, and it consists of 2f1 + f2,2f1-f2 components It became a two-tone signal.

  NEL NLK1554BTZ-A 80128 0deg 149.7mA was used as the light source. The output was 11.67 dBm (the output end of the polarization controller). It was input to the FSK modulator (TFSK1.5-10-P SN 182376) via the polarization controller. After passing through an optical amplifier (EDFA (OPREL)) and a bandpass filter (BPF 5nm (applied photoelectric)), it was input to PD (NEL KEPD 2525VPG).

  The RF input was applied to the modulator via a Rohde & Schwarz SMF-100A (for sub-MZM) hybrid. The main MZM was driven and controlled using a Rohde & Schwartz SMR-20 (for main MZM). Bias control was performed using an applied photoelectric E12069. The optical spectrum (at the PD input point) was measured using an ANDO optical spectrum analyzer. The PD output was measured with an Agilent E4448A spectrum.

  Similarly to the above, the bias of the FSK modulator was adjusted. An RF signal was input to the sub MZM and the SSB operating conditions were set. As f1, 5.00 GHz (21.51 dBm MZa, 21.61 dBm MZb) was applied to the sub-MZ. On the other hand, 1 GHz (15.26 dBm MZc) was applied to the main MZ as f2.

  FIG. 28 is a graph replaced with a drawing showing an optical spectrum at a PD input point. FIG. 29 shows an RF spectrum near 10 GHz. From FIG. 29, it was confirmed that the desired component Two-tone was obtained in the vicinity of 10 GHz. The suppression ratio of the unnecessary component 2f1 was a few tens of dB.

  Next, as f1, 1.00 GHz (22.79 dBm MZa, 22.35 dBm MZb) was applied to the sub MZ. On the other hand, 10.00 GHz (14.13 dBm MZc) was applied to the main MZ as f2.

  FIG. 30 is a graph replaced with a drawing showing an optical spectrum at a PD input point. FIG. 31 shows an RF spectrum near 10 GHz. FIG. 32 shows an RF spectrum near 5 GHz. From FIG. 31, it was confirmed that the desired component Two-tone was obtained in the vicinity of 10 GHz.

  Next, a PD frequency characteristic calibration method using a high extinction ratio DSB-SC modulator was demonstrated. As the experimental system, the system shown in FIG. 33 was used. This system is a system including DPMZM. That is, the output light from the LD was DSB-SC modulated to generate two light waves. When the carrier wave was canceled, the intensity of the two light waves matched with high accuracy. FIG. 34 shows an example of an output signal from the DSB-SC modulator. In the figure, U1 means the primary component of USB, and L1 etc. mean the primary component of LSB. C means a carrier wave. In the example shown in FIG. 2, the carrier component remains. The intensity ratio between U1 and L1 is about 0.12 dB. The extinction ratio was 54 dB.

As an error factor of this method, there is a level difference between two light waves and the presence of harmonic components. If the extinction ratio is η (eta), the worst value Δ (delta) of the level error is Δ = 20 log ₁₀ (1-10 ^{(η / 20)} ). In order to realize 0.5 dB required for the photodetector, an extinction ratio of 25 dB or more is required.

  FIG. 35 shows the frequency characteristic measurement results of the two light wave levels and the extinction ratio of the carrier wave and the harmonic wave. From FIG. 35, it can be seen that an extinction ratio of 34 dB or more was obtained in the frequency region of 12 GHz to 35 GHz. That is, according to the present invention, it can be seen that a system capable of calibrating frequency characteristics can be provided by a simple apparatus.

Reference example
A DPMZ-type Ti diffusion waveguide was fabricated on the LN substrate of the electric field controlled Y-branch MZ interferometer X-cut, and a CPW (coplanar butterfly guide) electrode with a Y-branch electrode was formed through the SiO 2 buffer layer. FIG. 36A is a diagram showing an active Y-branching light intensity trimmer. Such a so-called Y-branch EO switch can control the branching ratio. As shown in FIG. 36A, the branching ratio is controlled using an electric field generated by a DC voltage applied to the DC electrode. FIG. 36B shows an optical modulator having MZ. By controlling the DC voltage, η (optical power imbalance) can be controlled and the extinction ratio can be increased. The half-wave voltage of this modulator was 3.2V. FIG. 37 shows the spectral profile of the time domain of the OOK signal. FIG. 38 shows the output of DSB-SC modulation using this apparatus. Also in this example, relatively good DSB-SC modulation can be achieved.

  The present invention can be used in the field of optical information communication equipment. In particular, the present invention can be preferably used as a system for ROF (fiber radio).

DESCRIPTION OF SYMBOLS 11 Light source 12 Optical modulator 13 Photo detector 14 Main Mach-Zehnder waveguide 15 1st sub Mach-Zehnder waveguide 16 2nd sub-Mach-Zehnder waveguide 18 Control apparatus

Claims (11)

A light source (11);
An optical modulator (12) that receives output light from the light source (11) and generates two-tone light according to the light incident from the light source (11) and a modulation signal;
Including
The light modulator (12) is a light modulator having a Mach-Zehnder waveguide (14);

Two-tone light output from said optical modulator (12), the frequency of the light incident from the light source (11) and _{f 0,} if the frequency of the modulation signal is a _{f m, f 0 ± nf m} (n is Integer),
Strength of _{f 0} component output from said optical modulator (12) _is suppressed more than 20dB with respect to the intensity of the f 0 + nf _m components,
Light source system for heterodyne detection.
The heterodyne according to claim 1, wherein the intensity of the modulation signal applied to the optical modulator (12) is 0.1 to 2 times the half-wave voltage of the optical modulator (12). Light source system for detection.
2. The light source system for heterodyne detection according to claim 1, wherein n is 1. 3.

The light source system for heterodyne detection according to claim 1, wherein the light source (11) is a wavelength variable light source.
The light source system for heterodyne detection according to claim 1, wherein one arm of the Mach-Zehnder waveguide (14) has a sub-Mach-Zehnder waveguide (15).
The light source system for heterodyne detection according to claim 5, wherein an electric field control Y-branch waveguide is provided in one or both of the branching section and the multiplexing section of the Mach-Zehnder waveguide 14.
The optical modulator (12) according to claim 1, wherein the optical modulator (12) is an optical modulator (12) having a sub Mach-Zehnder waveguide (15, 16) in each of both arms of the Mach-Zehnder waveguide (14). Light source system for heterodyne detection.
The light source system for heterodyne detection according to claim 7, wherein an electric field control Y-branch waveguide is provided in one or both of the branching section and the multiplexing section of the Mach-Zehnder waveguide 14.
A photodetector (13) on which the light output from the light modulator (12) is incident;
A control device (18) for analyzing frequency characteristics of light detected by the photodetector (13);
Further including
The control device (18)
Based on the intensity information of the f ₀ component and the f ₀ ± nf _m (n is an integer) component included in the light detected by the photodetector (13), the bias voltage source of the optical modulator (12) is 2. The light source system for heterodyne detection according to claim 1, further comprising first bias adjusting means for issuing a command for changing a bias voltage applied to the optical modulator.
A photodetector (13) on which the light output from the light modulator (12) is incident;
A control device (18) for analyzing frequency characteristics of light detected by the photodetector (13);
Further including
The control device (18)
Based on the intensity information f _m component and 2f _m component the photodetector (13) is included in the light detected with respect to the bias voltage source of said light modulator (12), said optical modulator (12) 2. The light source system for heterodyne detection according to claim 1, further comprising a second bias adjustment unit that issues a command for changing a bias voltage applied to.
A photodetector (13) on which the light output from the light modulator (12) is incident;
A control device (18) for analyzing frequency characteristics of light detected by the photodetector (13);
Further including
The control device (18)
Based on the intensity information of the f ₀ component and the f ₀ ± nf _m (n is an integer) component included in the light detected by the photodetector (13), the bias voltage source of the optical modulator (12) is A first bias adjusting means for issuing a command for changing a bias voltage applied to the optical modulator (12);
Based on the intensity information f _m component and 2f _m component the photodetector (13) is included in the light detected with respect to the bias voltage source of said light modulator (12), said optical modulator (12) 2. The light source system for heterodyne detection according to claim 1, further comprising a second bias adjustment unit that issues a command for changing a bias voltage applied to.

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