JP4837617B2 - Jitter measuring apparatus and calibration method thereof - Google Patents

Description

  The present invention relates to a jitter measuring apparatus for evaluating jitter of an optical phase modulation signal obtained by phase modulation (for example, DPSK, DQPSK, etc.) of an optical carrier wave in a coherent optical communication system and a phase calibration method thereof.

  In long-distance optical communication, a phase modulation system (for example, DPSK, DQPSK, etc.) is superior to an on-off keying (OOK) system, and an optical communication system using an optical phase modulation signal is desired.

  Also in the optical communication system using this optical phase modulation signal, the data signal cannot be normally transmitted when the phase fluctuation (jitter) of the data signal becomes large as in the on-off keying (OOK) system.

  For this reason, even in an optical communication system using an optical phase modulation signal, it is necessary to measure in advance the jitter characteristics of the data transmission system and the devices constituting them.

FIG. 8 shows the configuration of a jitter measuring device 50 that measures the jitter of an optical OOK signal. Photodetector 5 1 converts the measured data signals DR, which is the optical OOK signal to an electrical signal input to the A / D converter 52.

  The A / D converter 52 sequentially digitally converts the data signal to be measured DR converted into the electric signal in synchronization with the sampling clock signal e, and stores the obtained data in the waveform memory 53 sequentially as waveform data.

  The display unit 54 overwrites and displays the waveform data read from the waveform memory 53.

  Here, if the pattern of the data signal DR is random, the display unit 54 displays a waveform in which the rising edge and the falling edge intersect with each other as shown in FIG.

  This waveform is called an eye pattern. When the jitter of the measured data signal DR is large, the width W of the intersection between the rising edge and the falling edge of the eye pattern increases.

  Therefore, the jitter amount of the data signal DR to be measured can be grasped from the width W of the intersection of the eye patterns displayed on the display unit 54. The jitter calculator 55 calculates the jitter amount from the waveform data sequentially read from the waveform memory 53.

  A specific method for calculating the amount of jitter will be described using a scope scan method.

FIG. 5 shows a one-dimensional histogram of each time point in the time axis direction in the eye pattern and the intersection width W. This one-dimensional histogram shows a statistical estimate of the probability distribution function of jitter, and the standard deviation of the distribution is the jitter execution value Jrms. Total jitter Jp-p
Is given by the time distance between the extremes of the one-dimensional histogram.

Total jitter Jp-p is caused by uncorrelated random jitter RJ due to physical random processes such as temperature, and spurious events such as intersymbol interference, crosstalk, sub-frequency distortion or power switching. And deterministic jitter DJ having periodicity can be separated and evaluated.

  FIG. 6 is a diagram for explaining a method of calculating random jitter RJ or deterministic jitter DJ from a one-dimensional histogram of time points in an eye pattern.

  First, two separate Gaussian distribution functions are fitted to the edges on both sides of the one-dimensional histogram of time points.

Here, the Gaussian distribution function is given by equation (1).
Aexp [− (t−μ) 2 / 2σ2] (1)
Here, A is a normalization coefficient, t is a time variable, μ is an average value, and σ is a variance value.

RJ and DJ are given by the equations (2) and (3), respectively, assuming that the left and right Gaussian distributions are (μL, σL) and (μR, σR), respectively.
RJ = (σR + σL) / 2 (2)
DJ = μR−μL (3)
By using such an evaluation method, it is important to divide the jitter into random jitter RJ and deterministic jitter DJ and grasp the jitter in order to ascertain and reduce or eliminate the cause of the jitter.

The jitter evaluation of the optical OOK signal has been described above. However, even in an optical communication system using an optical phase modulation signal, it is necessary to measure in advance the jitter characteristics of the data transmission system and the devices constituting them.
Japanese Patent No. 3790741

  However, the configuration of FIG. 8 cannot measure the jitter of the optical phase modulation signal.

  It is an object of the present invention to provide a jitter measuring apparatus that can solve such problems and can evaluate jitter of a phase modulation signal.

In order to solve the above-described problem, in the jitter measuring apparatus according to claim 1 of the present invention, a bit delay unit (2f) is included in one of the two arms, and an optical carrier wave is a phase of a data signal as input light. Upon receiving the modulated optical phase modulation signal, the light is demultiplexed into two lights passing through the respective arms, and the two lights passing through the respective arms are combined and interfered with each other so that the two phases 180 of each other. A bit delay interferometer (2) that outputs different light intensity conversion signals, and at least one of the two light intensity conversion signals output from the bit delay interferometers are converted into electric signals. An optical waveform measurement unit (7, 15, 20, 30) for digitally converting a signal waveform and storing the waveform data, and jitter of the light intensity conversion signal from the waveform data stored in the optical waveform measurement unit Serial optical phase jitter calculating unit that calculates a jitter modulation signal and a (8), the bit delay interferometer of the two either one predetermined phase arm (phi) optical phase delay device which gives the (2j) And a phase control means (12) for controlling the optical path length of the optical phase delay device by a phase control signal (c), and the relationship between the phase control signal and the phase difference between the two arms of the bit delay interferometer. A control memory (12a) for storing phase control data, a calibration light source (3) for outputting unmodulated reference light at the same wavelength as the optical phase modulation signal, and any one of the measurement mode and the calibration mode A mode designating means (11) for outputting a mode designating signal for designating the optical phase modulation signal, the reference light and the mode designating signal, and when the mode designating signal designates a measurement mode An optical switch (1) for outputting the optical phase modulation signal to the bit delay interferometer and outputting the reference light to the bit delay interferometer when the mode designation signal designates a calibration mode; Phase calibration processing means (13) for calculating the relationship between the phase control signal and the level of the waveform data, and when the mode designation signal designates a calibration mode, the phase control means In addition to changing the optical path length of the phase delay device, the phase calibration processing means calculates the phase control data from the level of the phase control signal and the waveform data, and stores it in the control memory.

  Also, in the jitter measuring apparatus according to claim 2 of the present invention, in the jitter measuring apparatus according to claim 1 described above, the electrical waveform measuring unit outputs each of the two light intensity conversion signals output from the bit delay interferometer. A balanced receiver (4) that receives the signals by the two light receivers (4a, 4b) and converts them into electric signals, subtracts the obtained outputs and outputs an electric signal waveform, and is output from the balanced receiver. And an electrical waveform measuring unit (6) for digitally converting the electrical signal waveform and storing the obtained waveform data.

  Also, in the jitter measuring apparatus according to claim 3 of the present invention, in the jitter measuring apparatus according to claim 1 described above, the electrical waveform measuring unit is one of the two light intensity conversion signals output from the bit delay interferometer. One is received by the light receiver (5a) and converted into an electrical signal, and a single receiver (5) that outputs an electrical signal waveform, and the electrical signal waveform output from the single receiver is digitally converted and obtained waveform data And an electrical waveform measuring section (6) for storing

  In the jitter measuring apparatus according to claim 4 of the present invention, in the jitter measuring apparatus according to claim 1, the electrical waveform measuring unit includes an optical pulse generator (7e) for outputting an optical pulse signal, and the two light beams. A nonlinear optical crystal (7a) that synthesizes one of the intensity conversion signals and the optical pulse signal and outputs the intensity correlation signal as a sum frequency light, and converts the sum frequency light into an electric signal to generate an electric pulse. A light receiver (7b) for outputting a signal, an A / D converter (7c) for converting the pulse signal into a digital signal, and a waveform memory (7d) for storing the digital signal are included.

In the jitter measuring apparatus according to claim 5 of the present invention, in the jitter measuring apparatus according to claim 1, the optical waveform measuring unit receives the light intensity conversion signal and the electrical sampling clock signal (e, d), and receives the sampling clock. The electroabsorption optical modulator (20a) that changes the transmittance of the light intensity conversion signal (Px) according to the signal by the electroabsorption effect, and the outgoing light (Py) transmitted through the electroabsorption optical modulator is converted into an electric signal. A light receiver (20b) for converting, an A / D converter (20c) for converting the electric signal into a digital signal, and a waveform memory (20d) for storing the digital signal are included.

In the jitter measuring apparatus according to claim 6 of the present invention, in the jitter measuring apparatus according to claim 1,
The optical waveform measurement unit receives the short optical pulse generator (30e) that outputs the sampling optical pulse (Ps), the optical intensity conversion signal and the sampling optical pulse, and determines the transmittance of the optical intensity conversion signal as the sampling light. An optical gate means (30a) that is controlled in accordance with the incidence of a pulse, a light receiver (30b) that converts light (Py) emitted from the optical gate means into an electrical signal, and an A / D that converts the electrical signal into a digital signal. A converter (30c) and a waveform memory (30d) for storing digital signals are included.

Further, in the jitter measuring apparatus according to claim 7 of the present invention, in the jitter measuring apparatus according to claim 6 , the optical gate means saturablely absorbs the transmittance of the light intensity conversion signal in accordance with the incidence of the sampling optical pulse. And an electroabsorption optical modulator (31a) that varies depending on the characteristics.

The jitter measuring apparatus according to claim 8 of the present invention is the jitter measuring apparatus according to claim 6 , wherein the optical gate means receives the first optical port (31a ₁ ) for receiving the light intensity conversion signal and the sampling optical pulse. An electroabsorption optical modulator (31a) having two optical ports (31a ₂ ) and changing the transmittance of the light intensity conversion signal by saturable absorption characteristics in response to the incidence of the sampling optical pulse; The sampling optical pulse received from the pulse generator is incident on the second optical port of the electroabsorption optical modulator, and the light emitted from the second optical port is an optical path (31b ₁ ) that receives the sampling optical pulse. Includes an optical coupler (31b) that emits light to a different optical path (31b ₃ ).

Further, in the jitter measuring apparatus according to claim 9 of the present invention, in the jitter measuring apparatus according to claim 6 , the optical gate means allows the transmittance for the light intensity conversion signal to be adjusted by the carbon nanotube in accordance with the incidence of the sampling light pulse. And a carbon nanotube saturable absorption element (32a) that is changed according to a saturated absorption characteristic.

Furthermore, in the jitter measuring device according to claim 1 0 of the present invention, the jitter measuring apparatus according to claim 6, the optical gate means receives the sampling optical pulse and the first optical port that receives light intensity conversion signals (32a ₁₎ The second optical port (32a ₂ ) has a carbon nanotube disposed on the optical path connecting the first optical port and the second optical port, and the transmittance for the light intensity conversion signal according to the incidence of the sampling optical pulse. And a sampling light pulse received from the short optical pulse generator is incident on the second optical port of the carbon nanotube saturable absorption element. The optical path (32b) is different from the optical path (32b ₁ ) from which the light emitted from the second optical port receives the sampling optical pulse. ₃ ) and an optical coupler (32b) that emits light to ( ₃ ).

Furthermore, in the jitter measuring device according to claim 1 of the present invention, the jitter measuring apparatus according to claim 6, the optical gate means, photodetector for converting converts the sampling optical pulses into electrical pulses and (33b), the light receiver And an electroabsorption type optical modulator (33a) that is arranged in the vicinity of the base plate and changes the transmittance of the light intensity conversion signal by the electroabsorption effect according to the electric pulse.
According to a twelfth aspect of the present invention, there is provided a calibration method for a jitter measuring apparatus, wherein the jitter measuring apparatus includes a bit delay device (2f) in one of two arms, and an optical carrier wave is data as input light. In response to the optical phase modulation signal (P) phase-modulated by the signal, the optical signal is demultiplexed into two lights passing through the respective arms, and the two lights passing through the respective arms are combined to interfere with each other. A bit delay interferometer (2) that outputs two light intensity conversion signals (Px) and at least one of the two light intensity conversion signals output from the bit delay interferometer are converted into electrical signals, and is obtained. An optical waveform measurement unit (7, 15, 20, 30) for digitally converting the waveform of the electrical signal and storing the waveform data; and jitter of the light intensity conversion signal from the waveform data stored in the optical waveform measurement unit. A jitter calculating section (8) for calculating the jitter of the optical phase modulation signal, and changing the phase difference between the two arms in a state in which the unmodulated reference light is input to acquire the waveform data. A step of calculating a predetermined function representing a relationship between the phase difference of the two arms and the level of the waveform data from the phase difference of the two arms and the level of the waveform data; and based on the calculated function And calibrating the phase difference between the two arms.

In the jitter measuring apparatus according to the first aspect of the present invention, the jitter of the optical intensity conversion signal output from the bit delay interferometer is measured, and the obtained jitter is evaluated as the jitter of the optical phase modulation signal. The jitter amount of the signal can be accurately evaluated. In addition, the optical phase difference φ between the two arms of the bit delay interferometer is calibrated using an unmodulated reference light having the same wavelength as the optical phase modulation signal (light to be measured). Jitter can be measured with high accuracy even in an environment where the optical path difference (optical phase difference φ) between the two arms of the bit delay interferometer changes due to measurement at the optical wavelength, temperature change, or the like.

  In the jitter measuring apparatus according to the second aspect of the present invention, since a balanced receiver is used for the optical waveform measuring unit and two optical intensity conversion signals output from the bit delay interferometer are received in a balanced manner, The light receiving sensitivity can be improved.

  In the jitter measuring apparatus according to claim 3 of the present invention, a single receiver is used for the optical waveform measuring section, and one of the two light intensity conversion signals output from the bit delay interferometer is received. The configuration can be simplified and the price can be reduced.

  In the jitter measuring apparatus according to claim 4 of the present invention, since the nonlinear optical element is used for the optical waveform measuring unit, the jitter amount of the optical phase modulation signal from which the characteristics of the receivers of the balanced receiver and the single receiver are removed is obtained. be able to.

In the jitter measuring apparatus according to the fifth aspect of the present invention, since the electroabsorption optical modulator having a transmission loss smaller than that of the nonlinear optical element is used for the optical waveform measuring unit, the sampling efficiency is high and the balanced receiver is used. The jitter amount of the optical phase modulation signal from which the characteristics of the receiver of the single receiver are removed can be obtained.

In the jitter measuring apparatus according to the sixth aspect of the present invention, since the short optical pulse whose line width is narrower than the electric pulse is used as the sampling signal, it can cope with sampling of a high modulation signal.

In the jitter measuring apparatus according to the seventh and eighth aspects of the present invention, since the electroabsorption optical modulator having less transmission loss than the nonlinear optical element is used for the optical waveform measuring unit, the sampling efficiency is high, and Thus, the jitter amount of the optical phase modulation signal that can be used for sampling a high modulation signal can be obtained.

In the jitter measuring apparatus according to claim 9 and claim 1 0 of the present invention, since to use a carbon nanotube saturable absorber element in the optical waveform measuring section, to retain the absorption of essential electroabsorption modulator DC The power waveform is not required and the configuration of the optical waveform measuring unit can be simplified.

  In the jitter measuring apparatus according to claim 4 of the present invention, in the optical waveform measuring unit, the short optical pulse is converted into the electric pulse by the high-speed PD, and the electric pulse is incident on the electroabsorption optical modulator close to the high-speed PD. Since the transmittance of the intensity conversion signal is controlled, it is possible to obtain the jitter amount of the optical phase modulation signal that can cope with sampling of a high modulation signal.

Embodiments of the present invention will be described below.
[First Embodiment]
The configuration of the jitter measuring apparatus according to the first embodiment of the present invention is shown in FIG. An optical phase modulation signal that is light to be measured by the jitter measuring apparatus is generated by phase-modulating an optical carrier wave with a data signal, and is input to an optical switch (optical SW) 1. The calibration light source 3 outputs unmodulated reference light having the same wavelength as the optical phase modulation signal to the optical switch 1. This reference light is used to calibrate the optical phase difference φ (see equation (8)) between the two arms of the bit delay interferometer 2. The optical switch 1 is controlled by a mode designation signal a that designates either the measurement mode or the calibration mode. When the mode designation signal a is in the measurement mode, the optical phase modulation signal is used. Output to the bit delay interferometer 2. The mode designation signal a is output from the mode designation means 11 based on a command from an operation unit or the like (not shown).

  First, the contents related to the case where the mode designation signal a designates the measurement mode and the optical phase modulation signal is input to the bit delay interferometer 2 via the optical switch 1 will be described.

The bit delay interferometer 2 is composed of a Mach-Zehnder interferometer using an optical waveguide. The optical phase modulation signal input from the port 2a is separated from the light passing through the first arm 2c by the branching unit 2b. The light that passes through the arm 2d (including the bit delay device 2f and the optical phase delay device 2j) is demultiplexed, and the light that passes through the arm 2c and the light that passes through the arm 2d are combined by the multiplexing unit 2e. Wave and interfere. Thereby, the change in the phase of the optical phase modulation signal is converted into the change in the optical intensity, and two optical intensity conversion signals whose phases are different from each other by 180 ° (π) are converted into two ports, the third port 2g and the fourth Each is output from the port 2h to the balanced receiver 4.

  The optical phase delay 2j is controlled by phase control data c output from the phase control means 12 so as to give an optical phase difference φ = 0 between the two arms of the bit delay interferometer 2.

  By the way, the phase control means 12 receives the mode designation signal a, and when the mode designation signal a designates the measurement mode, the phase control means 12 stores the phase control data c corresponding to the optical phase difference φ = 0 in the control memory. The phase of the optical phase delayer 2j is controlled by reading out from 12a. Further, when the mode designation signal a designates the calibration mode, the phase control means 12 outputs the phase control data c and sequentially changes the optical path length of the optical phase delay device 2j. The trigger d is output every time the optical path length of the optical phase delay 2j is sequentially changed by the phase control data c. The phase control data c and the trigger d are input to a phase calibration processing unit 13 and a switch 14 described later, respectively.

  Here, the two light intensity conversion signals P1 and P2 output from the bit delay interferometer 2 described above are expressed by mathematical expressions. When the optical phase difference designation signal b designates the optical phase difference φ = 0 and the optical phase delay 2j is controlled to give the optical phase difference φ = 0 between the two arms, the equation (4) Since φ = 0 in FIG. 5, the light intensity conversion signal P1 output from the port 2g and the light intensity conversion signal P2 output from the port 2h are expressed by equations (5) and (6), respectively.

P = 0.5 + 0.5cos (Δφmod + φ) (4)
P1 = 0.5 + 0.5 cos (Δφ mod) (5)
P2 = 0.5−0.5 cos (Δφ mod) (6)
The optical path lengths of the arms 2c and 2d of the bit delay interferometer 2 need to be kept constant. For this purpose, the optical waveguide is designed to be short so that the optical path length does not change by temperature control. In addition, the bit delay unit 2f has an optical path length of the arm 2d corresponding to the delay amount so that the delay amount (delay time difference) between the two arms becomes a delay amount (time) corresponding to one bit of the data signal. Is longer than the optical path length of the arm 2c. The delay amount T is given by equation (15).

T = c / (f · n) (7)
Here, c is the speed of light in vacuum, f is the frequency of the data signal, and n is the refractive index of the optical waveguide.

  The balanced receiver 4 includes light receivers (PD) 4a and 4b and a subtractor 4c, and receives two light intensity conversion signals output from the two ports 2g and 2h of the bit delay interferometer 2, respectively. 4a and 4b receive and photoelectrically convert them, and subtract their outputs with a subtractor 4c. As a result, the two light intensity conversion signals P1 and P2 are received in a balanced manner, and the output electric signal waveform is output to the electric waveform measuring unit 6. The light intensity Pα when the two light intensity conversion signals, that is, the light intensity conversion signals P1 and P2 expressed by the above-described expressions (5) and (6) are received in a balanced manner, is expressed by the expression (8). Thus, the offset power is canceled and the light intensity is doubled. Further, the electric signal waveform of the balanced reception output represents the light intensity Iα given by the equation (9).

Pα = P1-P2 = cos (Δφmod) (8)
Iα∝cos (Δφmod) (9)
The electric waveform measuring unit 6 is composed of an A / D converter 6a and a waveform memory 6b, and an electric signal waveform input from the balanced receiver 4 is synchronized with a sampling clock signal e by the A / D converter 6a. The digital data is sequentially converted, and the obtained waveform data is sequentially stored in the waveform memory 6b. The sampling clock signal e is input from the sampling clock generator 10 through the switch 14. The balanced receiver 4 and the electric waveform measuring unit 6 constitute an optical waveform measuring unit 15.

  The jitter calculator 8 calculates the jitter amount based on the waveform data sequentially output from the waveform memory 6b by the same calculation method as the conventional jitter calculator 25 of FIG.

  The display 9 displays the eye pattern by overwriting the waveform data read from the waveform memory 6b, and also displays the jitter amount calculated by the jitter calculator 8.

  When the sampling clock generator 10 measures (evaluates) the optical phase modulation signal in synchronization with the clock signal of the data signal described above, the clock signal of the data signal or the electric signal waveform output from the balanced receiver 4 The signal recovered from the clock is output as the sampling clock signal e, and when measuring the optical phase modulation signal asynchronously with the clock signal of the data signal, the clock signal generated by itself is output as the sampling clock signal e. To do.

  The switch 14 is controlled by a mode designation signal a that designates either the measurement mode or the calibration mode. When the mode designation signal a designates the measurement mode, the sampling clock signal input from the sampling clock generator 10 e is outputted to the A / D converter 6a, and when the mode designation signal a designates the calibration mode, the trigger d inputted from the phase control means 12 is outputted to the A / D converter 6a.

  Next, the mode designation signal a designates the calibration mode, and the reference light (unmodulated signal having the same wavelength as the optical phase modulation signal) output from the calibration light source 3 is passed through the optical switch 1 to the bit delay interferometer 2. The contents related to the case of being input to will be described.

  The optical phase delay device 2j of the bit delay interferometer 2 calculates the phase control data c output from the phase control means 12 in the calibration mode, that is, the optical phase difference φ between the two arms in the bit delay interferometer 2. It is controlled by the phase control data c to be changed. As a result, the optical path length of the optical phase delay 2j is sequentially changed, and the optical phase difference φ is also sequentially changed. As a result, the light intensity P of the combined light in the combining unit 2e of the bit delay interferometer 2 is Δφmod = 0 in the above-described equation (4) (re-displayed), so that the phase change of the optical phase delay 2j occurs. The light intensity conversion signal PC1 output from the port 2g and the light intensity conversion signal PC2 output from the port 2h are expressed by equations (10) and (11), respectively.

P = 0.5 + 0.5cos (Δφmod + φ) (4)
PC1 = 0.5 + 0.5cosφ (10)
PC2 = 0.5−0.5cosφ (11)
The light intensity PC when the light intensity conversion signals PC1 and PC2 represented by the equations (10) and (11) are balancedly received by the balanced receiver 4 is represented by the equation (12). The electric signal waveform of the balanced reception output represents the light intensity IC given by the equation (13).

PC = PC1-PC2 = cosφ (12)
IC∝cosφ (13)
The electric waveform measuring unit 6 sequentially converts the electric signal waveform input from the balanced receiver 4 into digital data in the waveform memory 6b sequentially by the A / D converter 6a in synchronization with the trigger d. Remember. The trigger d is input from the above-described phase control unit 12 via the switch 14, and the optical phase delay of the bit delay interferometer 2 is determined by the phase control data c output from the phase control unit 12 in the calibration mode. Each time the phase of the optical signal is sequentially changed by changing the optical path length of the device 2j.

  The phase calibration processing unit 13 changes the waveform data sequentially read from the waveform memory 6b of the optical waveform measuring unit 15 and the optical path length of the optical phase delay 2j input from the phase control unit 12 to change the phase of the optical signal. Is used to calibrate the optical phase difference φ between the two arms of the bit delay interferometer 2 based on the phase control data c for sequentially changing the maximum / minimum level detecting means 13a and level-phase calculating means 13b. And phase control data calculation means 13c.

  That is, the maximum / minimum level detection means 13a is the waveform data (corresponding to the IC of the above-mentioned formula (13)) sequentially read from the waveform memory 6b of the optical waveform measurement section 15 and the light output from the phase control means 12. In response to the phase control data c for sequentially changing the phase of the optical signal by changing the optical path length of the phase delay unit 2j, the adjacent maximum level Lmax and minimum level Lmin of the waveform data as shown in FIG. To detect.

  As shown in FIG. 7, the level-phase calculating means 13b determines the position associated with the phase control data c of the maximum level Lmax and the minimum level Lmin as the optical position between the two arms in the bit delay interferometer 2, respectively. The level L of the waveform data (corresponding to IC) is expressed by the equation (14) as 0 and π of the phase difference φ.

L = (Lmax + Lmin) / 2 + {(Lmax−Lmin) / 2} cos φ (14)
The phase control data calculation means 13c obtains the level L with respect to the optical phase difference φ from the above equation (14), and associates the obtained level L with the phase control data c to obtain the optical phase difference φ and the phase control data c. The relationship between the correlated optical phase difference φ and the phase control data c is stored in the memory 12 a of the phase control means 12. That is, the respective phase control data c corresponding to 0, π / 4, π / 2, 3π / 4, π, etc. of the optical phase difference φ are obtained and stored in the memory 12a.

  In the measurement mode of the first embodiment, the case where the optical phase delay 2j is controlled so that the optical phase difference φ between the two arms of the bit delay interferometer 2 is 0 and π / 2 will be described. However, the present invention is not limited to this, and any two optical phase differences φ1 and φ2 can be used as long as the phase advance and delay can be determined.

  In the calibration mode of the first embodiment, the reference light output from the calibration light source 3 is used as the bit delay interferometer 2 in order to calibrate the optical phase difference φ between the two arms of the bit delay interferometer 2. However, the present invention is not limited to this, and the optical phase modulation signal as the light to be measured may be unmodulated. In that case, the optical switch 1 and the calibration light source 3 are unnecessary.

In the first embodiment, a Mach-Zehnder interferometer is used as the bit delay interferometer 2, but the present invention is not limited to this, and a Michelson interferometer may be used.
[Second Embodiment]
The configuration of the jitter measuring apparatus according to the second embodiment of the present invention is shown in FIG. In the first embodiment shown in FIG. 1, the two light intensity conversion signals output from the bit delay interferometer 2 are received by the balanced receiver 4, but in the second embodiment, from the bit delay interferometer 2. One of the two output light intensity conversion signals is received by the single receiver 5. Therefore, mainly the operation of the single receiver 5 when the mode designation signal a designates the measurement mode will be described.

  The single receiver 5 includes a light receiver (PD) 5a and an offset circuit 5b. The light intensity conversion signal P1 output from the port 2g of the bit delay interferometer 2 is received and photoelectrically converted by the light receiver 5a. The offset power included in the output is canceled by the offset circuit 5 b, and the output electric signal waveform is output to the electric waveform measuring unit 6. The relationship between the light intensity conversion signal P1 output from the port 2g of the bit delay interferometer 2 and the light intensity Iα represented by these electric signal waveforms is as follows: 15).

P1 = 0.5 + 0.5 cos (Δφ mod) (5)
Iα∝0.5cos (Δφmod) (15)
Instead of the light intensity conversion signal output from the port 2g of the bit delay interferometer 2, the light intensity conversion signal P2 output from the port 2h may be used. In this case, the relationship between the light intensity conversion signal P2 output from the port 2h and the light intensity Iα represented by these electric signal waveforms is expressed by the expressions (6) (reprinted) and (16). It becomes.

P2 = 0.5−0.5 cos (Δφ mod) (6)
Iα∝−0.5cos (Δφmod) (16)
The single receiver 5 and the electrical waveform measurement unit 6 constitute an optical waveform measurement unit 15.
[Third Embodiment]
The configuration of the jitter measuring apparatus according to the third embodiment of the present invention is shown in FIG. In the first embodiment shown in FIG. 1, the two light intensity conversion signals output from the bit delay interferometer 2 are received by the balanced receiver 4 of the optical waveform measurement unit 15, but in the third embodiment, One of the two light intensity conversion signals output from the bit delay interferometer 2 is received by the optical waveform measuring unit 7. Therefore, the operation of the optical waveform measurement unit 7 when the mode designation signal a designates the measurement mode will be mainly described.

  The optical waveform measurement unit 7 includes a nonlinear optical crystal 7a, a light receiver 7b, an A / D converter 7c, a waveform memory 7d, an optical pulse generator 7e, and polarization direction controllers 7f and 7g. A sampling optical pulse train f (angular frequency ωSAM) having a repetition period equal to the sampling clock signal e generated from the optical pulse generator 7e and a light intensity conversion signal (angular frequency ωDATA) input from the port 2g of the bit delay interferometer 2 are nonlinear. When incident on the optical crystal 7a, an optical pulse signal having a sum angular frequency ωSAM + ωDATA (hereinafter referred to as SFG) is generated by a second-order nonlinear optical effect.

  Polarization direction controllers 7f and 7g generate light intensity conversion signal ωDATA input from port 2g and optical pulse generator 7e, respectively, in order to cause phase matching in nonlinear optical crystal 7a and generate sum frequency light ωSAM + ωDATA. The polarization direction of the sampling light ωSAM is controlled to an optimum direction. For example, if the nonlinear optical crystal 7a is a “type 1” crystal material, the light intensity conversion signal ωDATA is controlled so that the polarization directions of the sampling light ωSAM coincide with each other. If the nonlinear optical crystal 7a is a “type 2” crystal material, the light intensity conversion is performed. Control is performed so that the polarization directions of the signal ωDATA and the sampling light ωSAM are orthogonal.

  The optical pulse signal ωSAM + ωDATA generated from the nonlinear optical crystal 7a is photoelectrically converted by the light receiver 7b to output an electric pulse signal. Then, the A / D converter 7c sequentially converts the electric signal waveform formed by the peak value of the electric pulse signal in synchronization with the sampling clock signal e, and the obtained waveform data is sequentially stored in the waveform memory 7d. Remember. As a result, the waveform data input to the jitter calculation unit 8 or the phase calibration processing unit 13 is sequentially read from the waveform memory 7d. The optical pulse is output from the optical pulse generator 7e, and the sampling clock signal e is input from the sampling clock generator 10 through the switch 14.

The relationship between the light intensity conversion signal P1 output from the port 2g of the bit delay interferometer 2 and the light intensity Iα represented by these electric signal waveforms, and the output from the port 2h of the bit delay interferometer 2 The relationship between the light intensity conversion signal P2 described above and the light intensity Iα represented by these electric signal waveforms is the same as that of the single receiver 5 according to the second embodiment.
[Fourth Embodiment]
FIG. 9 shows the configuration of the jitter measuring apparatus according to the fourth embodiment of the present invention. Only the configuration of the optical waveform measurement units 15 and 20 is different from that of the first embodiment, and the configuration part other than the optical waveform measurement unit and the method of calibrating the jitter measurement apparatus are the same as those of the first embodiment, and thus description thereof is omitted. .

  In the fourth embodiment, the light intensity conversion signal Px is sampled by applying the electroabsorption characteristic of the electroabsorption optical modulator.

The electroabsorption optical modulator 20a has two optical ports 20a ₁ and 20a ₂ for entering and exiting light and a power supply terminal 20a ₃ for applying an electric field to the optical path between the _two optical ports. It has an electro-absorption characteristic that changes the absorption rate of incident light in accordance with the size of.

When the light intensity conversion signal Px output from the bit delay interferometer 2 is input to the optical port 20a ₁ and the sampling clock signal e of the sampling clock generator 10 is given to the power supply terminal 20a ₃ , the power It decreased absorption rate with respect to the light intensity conversion signal Px only while the pulse to the terminal 20a ₃ is added (transmittance increases), and is output as the sampling optical pulse signal Py from the optical port 20a _2. The optical pulse signal Py is converted into an electrical pulse signal by the light receiver 20b, and is sequentially converted into a digital signal in synchronization with the sampling clock signal e by the A / D converter 20c, and then stored in the waveform memory 20d as waveform data. Is done.
[Fifth Embodiment]
The configuration of the jitter measuring apparatus according to the fifth embodiment of the present invention is shown in FIGS. Only the configuration of the optical waveform measurement units 15 and 30 is different from that of the first embodiment, and the configuration part other than the optical waveform measurement unit and the method of calibrating the jitter measurement apparatus are the same as those of the first embodiment, and thus description thereof is omitted. .

  The fifth embodiment is the same as the fourth embodiment in that the light intensity conversion signal Px is sampled by an ON / OFF operation to obtain an optical pulse signal Py synchronized with the sampling clock signal e, but the sampling clock signal e Is different from the fourth embodiment in that a short light pulse Ps converted by a short light pulse generator 30e is applied to the optical gate means 30a and sampling is performed with the light pulse. Therefore, since sampling can be performed with a short optical pulse having a narrower line width than the electrical pulse e, it is possible to cope with sampling of a modulation signal as high as several tens of grams.

  FIG. 11 shows the configuration of the optical gate means 30a of this embodiment. In the present embodiment, sampling of the light intensity conversion signal Px is performed by applying the saturable absorption characteristic of the electroabsorption optical modulator 31a.

The optical gate means 30a includes an electroabsorption modulator 31a, an optical circulator 31b, and a DC power source 31e. Electroabsorption modulator 31a is in the optical path between the two optical ports _31a 1, 31a ₂ and Ryohikari ports for input and output light has a power supply terminal 31a ₃ applying an electric field, the optical port 31a ₁ a DC voltage as shown input light intensity conversion signal Px high absorption rate in a state given from the DC power supply 31e to the power supply terminal 31a _3, the short optical pulse Ps over port _31b 1, 31b ₂ of the optical circulator 31b When input to the optical port 31a ₂ , due to the saturable absorption characteristic of the electroabsorption modulator 31a, the absorptance with respect to the light intensity conversion signal Px is reduced only when the short optical pulse Ps is input (the transmittance increases). ), sampled optical pulse signal Py is outputted from the optical port 31a _2. The optical pulse signal Py is input to the light receiver 30b via the optical ports 31b ₁ and 31b ₂ of the optical circulator 31b.
[Sixth Embodiment]
The configuration of the optical gate means 30a of the sixth embodiment of the present invention is shown in FIG. Only the configuration of the gate means 30a is different from that of the fifth embodiment, and the configuration other than the gate means 30a and the method of calibrating the jitter measuring apparatus are the same as those of the fifth embodiment, and the description thereof will be omitted.

  In the sixth embodiment, the light intensity conversion signal Px is sampled by applying a saturable absorption characteristic of a carbon nanotube (CNT: Carbon Nanotube). The wavelength indicating the saturable absorption characteristic of the carbon nanotube is determined by the diameter thereof, and is prepared in accordance with the frequency range of the sampled light (Japanese Patent Laid-Open No. 2003-121892).

In FIG. 12, the optical gate means 30a includes a CNT saturable absorption element 32a and an optical circulator 32b. The CNT saturable absorption element 32a includes two optical ports 32a ₁ and 32a ₂ for entering and exiting light and a carbon nanotube disposed on the optical path between the two optical ports. While inputting light intensity conversion signals Px from bit delay interferometer 2 into the optical ports 32a _1, port _32b of the optical circulator 32b to the optical port 32a ₁ 1, 32b ₂ a short optical pulse Ps light port 32a ₂ through , The absorptance with respect to the light intensity conversion signal Px is lowered only when the short light pulse Ps is being inputted (the transmittance is increased) due to the saturable absorption characteristic of the CNT saturable absorber 32a, and sampling is performed. optical pulse signal Py has is output from the optical port 32a _2.

  In FIG. 12, the signal flow is described using a directional optical circulator as means for entering and exiting the optical pulse signal Py and the short optical pulse Ps. However, the present invention is not limited to this, and there is no directionality 2 × Other means such as a type 2 optical coupler can be used as long as the optical signal can be multiplexed / demultiplexed.

  The CNT saturable absorption element of the sixth embodiment is the same as the electroabsorption modulator 31a of the fifth embodiment in that sampling is performed with a short light pulse Ps, but a DC power supply 31e that provides a high absorption rate is unnecessary. There is an advantage that the configuration of the optical gate means can be simplified.

FIG. 13 shows an example of the internal structure of the CNT saturable absorber 32a. The two optical ports 32a ₁ and 32a ₂ form a collimating system with the collimating lenses 32a ₄ and 32a ₅ respectively, and a glass plate 32a ₅ in which carbon nanotubes are laminated on one side is arranged on the optical path of collimated light that is spatial light. ing.

The light intensity conversion signal Px emitted from the optical port 32a ₁ is projected onto the carbon nanotubes on the front surface of the glass plate 32a ₅ , and the transmittance is controlled by the short light pulse Ps projected from the back surface, so that the light is converted into the light pulse signal Py. and it is emitted from the port 32a _2.

  The CNT saturable absorption element 32a is not limited to the structure shown in FIG. 13, and a type using a polymer solution in which carbon nanotubes are dispersed and mixed, an optical fiber containing carbon nanotubes in the core, or the like is used. be able to.

  Further, instead of entering the short light pulse Ps from the subsequent stage of the CNT saturable absorber 32a, it is possible to perform sampling by entering from the previous stage as shown in FIG.

In the optical gate means of FIG. 12, the optical circulator is arranged at the subsequent stage of the CNT saturable absorbing element 32a. However, in the configuration of FIG. 14, the optical coupler 32c arranged at the preceding stage of the CNT saturable absorbing element 32a The short light pulse Ps is combined and incident on the CNT saturable absorber 32a. Of the optical pulse signal Py and the short optical pulse Ps transmitted through the CNT saturable absorption element 32a, the short optical pulse Ps unnecessary for jitter analysis is removed by the filter 32d.
[Seventh Embodiment]
The configuration of the optical gate means 30a of the seventh embodiment of the present invention is shown in FIG. Only the configuration of the gate means 30a is different from that of the fifth embodiment, and the configuration other than the gate means 30a and the method of calibrating the jitter measuring apparatus are the same as those of the fifth embodiment, and the description thereof will be omitted.

  In the seventh embodiment, sampling is performed by applying the electroabsorption characteristic of the electroabsorption optical modulator 33a. The short light pulse Ps is converted into an electric pulse h by the high-speed PD 33b, and the transmittance of the light intensity conversion signal Px incident on the electroabsorption optical modulator 33a is controlled by the electric pulse h. Although the transmittance of the light intensity conversion signal Px is controlled by the electroabsorption characteristic, it is the same as in the fourth embodiment. However, the high-speed PD 33b is arranged close to the electroabsorption optical modulator 33a (for example, the same) Since it can be sampled with an electric pulse h having a line width as close as possible to the narrow line width of the short light pulse Ps), it can be used for sampling of a modulation signal as high as several tens of G. it can.

The figure which shows the structure of 1st Embodiment of this invention. The figure which shows the structure of 2nd Embodiment of this invention. The figure which shows the structure of 3rd Embodiment of this invention. Illustration showing eye pattern Diagram showing the relationship between eye pattern and jitter Diagram for explaining the scope scan method Diagram for explaining optical delay device calibration method The figure which shows the structure of the jitter measuring device of an optical OOK signal. The figure which shows the structure of 4th Embodiment of this invention. The figure which shows the structure of 5th Embodiment of this invention. The figure which shows the structure of the optical gate means in 5th Embodiment of this invention. The figure which shows the structure of the optical gate means in 6th Embodiment of this invention. The figure which shows the structure of the CNT saturable absorption element in 6th Embodiment of this invention. The figure which shows the other structure of the optical gate means in 6th Embodiment of this invention. The figure which shows the structure of the optical gate means in 7th Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical switch (optical SW), 2 ... Bit delay interferometer, 2a, 2g, 2h ... Port, 2b ... Demultiplexing part, 2c, 2d ... Arm, 2e ... Multiplexer, 2f ... bit delay device, 2j ... optical phase delay device, 3 ... light source for calibration, 4 ... balanced receiver, 4a, 4b, 5a, 7b, 20b, 30b, 51 ... Light receiver (PD), 4c ... Subtractor, 5 ... Single receiver, 5b ... Offset circuit, 6 ... Electrical waveform measuring unit, 6a, 7c, 20c, 30c, 52 ... A / D converter, 6b, 7d, 20d, 30d, 53 ... waveform memory, 7, 15, 20, 30 ... optical waveform measurement unit, 7a ... nonlinear optical crystal, 7e, 30e ... Optical pulse generator, 7f, 7g ... polarization direction controller, 8 ... jitter calculator, 9, 54 ..Display, 10... Sampling clock generator, 11... Mode designation means, 12... Phase control means, 13... Phase calibration processing means, 13 a. 13b: level-phase calculation means, 13c: phase control data calculation means, 14 ... switch, 30a ... optical gate means, 20a, 31a, 33a ... electroabsorption optical modulator, 31b , 32b, 32c, optical coupler, 32a, CNT saturable absorption element, 31e, DC power supply, Ps, short light pulse, Px, light intensity conversion signal.

Claims (12)

A bit delay unit (2f) is included in one of the two arms, and receives an optical phase modulation signal (P) in which an optical carrier wave is phase-modulated with a data signal as input light. A bit delay interferometer (2) that demultiplexes the two light passing therethrough and multiplexes and interferes with the two lights passing through the respective arms to output two light intensity conversion signals (Px);
An optical waveform measuring unit (converting at least one of the two light intensity conversion signals output from the bit delay interferometer into an electric signal, digitally converting the obtained electric signal waveform, and storing the waveform data) 7, 15, 20, 30),
A jitter measuring apparatus and a jitter calculating unit that calculates (8) the jitter of the light intensity conversion signal from the waveform data stored in the optical waveform measurement unit as jitter of the optical phase modulation signal,
An optical phase delay (2j) that gives a predetermined phase (φ) to one of the two arms of the bit delay interferometer;
Phase control means (12) for controlling the optical path length of the optical phase delay device by a phase control signal (c);
A control memory (12a) for storing phase control data indicating the relationship between the phase control signal and the phase difference between the two arms of the bit delay interferometer;
further,
A calibration light source (3) for outputting unmodulated reference light at the same wavelength as the optical phase modulation signal;
Mode designation means (11) for outputting a mode designation signal for designating either the measurement mode or the calibration mode;
The optical phase modulation signal, the reference light, and the mode designation signal are received, and when the mode designation signal designates a measurement mode, the optical phase modulation signal is output to the bit delay interferometer, and the mode An optical switch (1) for outputting the reference light to the bit delay interferometer when the designation signal designates a calibration mode;
Phase calibration processing means (13) for calculating the relationship between the phase control signal and the level of the waveform data;
When the mode designating signal designates the calibration mode, the phase control means changes the optical path length of the optical phase delay device, and the phase calibration processing means computes the level of the phase control signal and the waveform data. A jitter measuring apparatus, wherein the phase control data is calculated from and stored in the control memory. The optical waveform measuring unit is
Each of the two light intensity conversion signals output from the bit delay interferometer is received by two light receivers (4a, 4b) and converted into electric signals, respectively, and the obtained outputs are subtracted to generate an electric signal. A balanced receiver (4) for outputting a waveform;
The jitter measuring apparatus according to claim 1, further comprising: an electric waveform measuring unit (6) for digitally converting the electric signal waveform output from the balanced receiver and storing the obtained waveform data. The optical waveform measuring unit is
A single receiver (5) for receiving any one of the two light intensity conversion signals output from the bit delay interferometer by an optical receiver (5a) and converting the signal into an electric signal, and outputting an electric signal waveform;
The jitter measuring apparatus according to claim 1, further comprising: an electric waveform measuring unit (6) for digitally converting the electric signal waveform output from the single receiver and storing the obtained waveform data. The optical waveform measuring unit is
An optical pulse generator (7e) for outputting an optical pulse signal;
A nonlinear optical crystal (7a) that synthesizes one of the two light intensity conversion signals and the light pulse signal and outputs the intensity correlation signal as sum frequency light;
A light receiver (7b) for converting the sum frequency light into an electric signal and outputting an electric pulse signal;
An A / D converter (7c) for converting the pulse signal into a digital signal;
The jitter measuring apparatus according to claim 1, further comprising a waveform memory (7d) for storing the digital signal. The optical waveform measuring unit is
Electroabsorption optical modulation that receives the light intensity conversion signal and the electrical sampling clock signals (e, d) and changes the transmittance of the light intensity conversion signal (Px) by the electroabsorption effect according to the sampling clock signal A vessel (20a);
A light receiver (20b) for converting the emitted light (Py) transmitted through the electroabsorption optical modulator into an electrical signal;
An A / D converter (20c) for converting the electrical signal into a digital signal;
The jitter measuring apparatus according to claim 1 , further comprising a waveform memory (20d) for storing the digital signal . The optical waveform measuring unit is
A short optical pulse generator (30e) for outputting a sampling optical pulse (Ps);
Optical gate means (30a) for receiving the light intensity conversion signal and the sampling light pulse, and controlling the transmittance of the light intensity conversion signal according to the incidence of the sampling light pulse;
A light receiver (30b) for converting light (Py) emitted from the optical gate means into an electrical signal;
An A / D converter (30c) for converting the electrical signal into a digital signal;
The jitter measuring apparatus according to claim 1, further comprising a waveform memory (30d) for storing the digital signal . The optical gate means includes
7. An electroabsorption optical modulator (31a) that changes the transmittance of the light intensity conversion signal by saturable absorption characteristics in accordance with the incidence of the sampling light pulse. The jitter measuring apparatus according to 1 . The optical gate means includes
A first optical port (31a ₁ ) that receives the light intensity conversion signal; and a second optical port (31a ₂ ) that receives the sampling light pulse, and the light intensity according to incidence of the sampling light pulse. An electroabsorption optical modulator (31a) for changing the transmittance of the converted signal by saturable absorption characteristics;
The sampling optical pulse received from the short optical pulse generator enters the second optical port of the electroabsorption optical modulator, and the light emitted from the second optical port receives the sampling optical pulse. The jitter measuring apparatus according to claim 6 , comprising an optical coupler (31b) that emits to an optical path (31b ₃ ) different from the optical path (31b ₁ ) . The optical gate means includes
A carbon nanotube saturable absorption element (32a) that changes the transmittance of the light intensity conversion signal according to the saturable absorption characteristic of the carbon nanotube in response to the incidence of the sampling light pulse is provided. The jitter measuring apparatus according to claim 6 . The optical gate means includes
A first optical port (32a ₁ ) that receives the light intensity conversion signal, a second optical port (32a ₂ ) that receives the sampling optical pulse, and an optical path that connects the first optical port and the second optical port. A carbon nanotube saturable absorption element (32a) that changes the transmittance for the light intensity conversion signal according to the saturable absorption characteristics of the carbon nanotube in response to the incidence of the sampling light pulse,
The sampling optical pulse received from the short optical pulse generator is incident on the second optical port of the carbon nanotube saturable absorption element, and the light emitted from the second optical port is received by the sampling optical pulse. The jitter measuring apparatus according to claim 6 , comprising an optical coupler (32b) that emits to an optical path (32b ₃ ) different from the optical path (32b ₁ ) . The optical gate means includes
A light receiver (33b) for converting the sampling light pulse into an electric pulse;
And an electroabsorption optical modulator (33a) which is disposed in the vicinity of the photoreceiver and changes the transmittance of the light intensity conversion signal by an electroabsorption effect in accordance with the electric pulse. The jitter measuring apparatus according to claim 6 . A method for calibrating a jitter measuring apparatus,
The jitter measuring apparatus is
A bit delay unit (2f) is included in one of the two arms, and receives an optical phase modulation signal (P) in which an optical carrier wave is phase-modulated with a data signal as input light. A bit delay interferometer (2) that demultiplexes the two light passing therethrough and multiplexes and interferes with the two lights passing through the respective arms to output two light intensity conversion signals (Px);
An optical waveform measuring unit (converting at least one of the two light intensity conversion signals output from the bit delay interferometer into an electric signal, digitally converting the obtained electric signal waveform, and storing the waveform data) 7, 15, 20, 30),
A jitter calculating section (8) for calculating jitter of the light intensity conversion signal as jitter of the optical phase modulation signal from waveform data stored in the optical waveform measuring section;
With the input of unmodulated reference light, changing the phase difference between the two arms to obtain the waveform data;
Calculating a predetermined function representing the relationship between the phase difference between the two arms and the level of the waveform data from the phase difference between the two arms and the level of the waveform data;
And a step of calibrating the phase difference between the two arms based on the calculated function .

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