JP2008104141A - Optical phase modulation evaluation apparatus and calibration method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical phase modulation evaluation apparatus capable of measuring a relative inter-bit phase difference of an optical phase modulation signal by arbitrarily changing an optical path length of an optical phase retarder provided in one of arms of a bit delay interferometer and giving different two optical phase differences ϕ between two arms of the bit delay interferometer. <P>SOLUTION: The optical phase modulation evaluation apparatus includes: a bit delay interferometer 12 which is configured by including a bit retarder 2f and an optical phase retarder 2j in an arm 2d and converts an optical phase modulation signal into a light intensity conversion signal; an optical waveform measuring unit 15 which converts the light intensity conversion signal output from the bit delay retarder 2 into an electric signal and stores waveform data thereof in a memory 6b; a phase control means 12 which arbitrarily controls an optical path length of the optical phase retarder 2j so as to give different phase differences ϕ<SB>1</SB>, ϕ<SB>2</SB>between two arms 2c and 2d; and a signal processing means 8 for determining a relative inter-bit phase difference of the optical phase modulation signal on the basis of two waveform data items, corresponding to the different optical phase differences ϕ<SB>1</SB>, ϕ<SB>2</SB>, read from the memory 6b. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical phase modulation evaluation apparatus that evaluates an optical phase modulation signal in which an optical carrier wave in a coherent optical communication system is phase modulated (for example, DPSK, DQPSK, etc.) by a data signal. The present invention relates to an optical phase modulation evaluation apparatus capable of measuring a phase difference between bits of a phase modulation signal with high accuracy and a phase calibration method thereof.

  Conventionally, as an optical phase modulation evaluation apparatus for evaluating an optical phase modulation signal in which an optical carrier wave is phase-modulated by a data signal, the optical phase modulation signal is detected by performing optical phase detection of the optical phase modulation signal with one bit delay interferometer. There are some which evaluate the phase modulation characteristics of (see, for example, Patent Document 1).

  FIG. 9 shows a schematic configuration of this type of optical phase modulation evaluation apparatus. An optical phase modulation signal, which is light to be measured by the optical phase modulation evaluation apparatus, is generated by phase-modulating an optical carrier wave with a data signal, and is input to a bit delay interferometer 2 as an optical phase detector. The bit delay interferometer 2 is composed of a Mach-Zehnder interferometer using an optical waveguide, and the optical phase modulation signal input from the port 2a and the light passing through the arm 2c by the demultiplexing unit 2b and the arm 2d (bit delay device) The light passing through the arm 2c and the light passing through the arm 2d are combined and interfered by the combining unit 2e. Thereby, the phase change of the optical phase modulation signal is converted into the change of the optical intensity, and two optical intensity conversion signals whose phases are different from each other by 180 ° (π) are output from the two ports 2g and 2h, respectively. 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 light receiver (PD) 20 photoelectrically converts the light intensity conversion signal output from the port 2g of the bit delay interferometer 2 and outputs an electrical signal. The signal processing unit 21 demodulates the data signal from the electrical signal output from the light receiver 20, and performs error rate measurement and waveform observation using the demodulated waveform obtained thereby to evaluate the optical phase modulation signal. .
JP-A-6-21891

  However, such a conventional optical phase modulation evaluation apparatus has a problem in that it cannot measure the relative inter-bit phase difference (hereinafter referred to as relative inter-bit phase difference) of the optical phase modulation signal.

Here, the reason will be described using mathematical expressions. That is, the electric field intensity of the optical phase modulation signal that is phase-modulated with the relative inter-bit phase difference Δφ _mod and is input to the port 2 a of the bit delay interferometer 2 is expressed by equation (1).

E = E ₀ · exp {j (ωt + Δφ _mod )} (1)
Further, the optical phase modulation signal is demultiplexed into light that passes through the arm 2c and light that passes through the arm 2d by the demultiplexing unit 2b, and the light and arm that have passed through the arm 2c when each is input to the multiplexing unit 2e. Respective electric field intensities E _a and E _{b and} light intensities P _a and P _b of light passing through 2d are expressed by equations (2) to (5), respectively.

E _a = A _a · exp {j (ωt + φ _a )} (2)
E _b = A _b · exp {j (ωt + φ _b )} (3)
P _a = │E _a・ E _a ^* │ (4)
P _b = │E _b・ E _b ^* │ (5)
Here, phi _a light phase difference in the arm 2c, the phi _b is an optical phase difference in the arm 2d. E _a ^* is _a conjugate complex number of E _a , and E _b ^* is a conjugate complex number of E _b . In the equations (2) and (3), the relative inter-bit phase difference Δφ _mod in the equation (1) is omitted for easy understanding.

  The light intensity P of the combined light in the combining unit 2e is obtained as in Expression (6) using Expressions (2) and (3) above.

P = (E _a + E _b ) · (E _a ^* + E _b ^* )
^{_{= A a 2 + A b 2}} +2 · A a · A b · cos (φ a -φ b) (6)
Then, when the electric field strength is A _a = A _b = 1/2 and the above-described relative inter-bit phase difference Δφ _mod is taken into consideration, the light intensity P of the combined light (interference light) is expressed by equation (7). Further, when the optical phase difference (φ _a −φ _b ) between the arm 2c and the arm 2d (referred to as “between two arms as appropriate)” is expressed by φ, the equation (7) becomes the equation (8).

P = 0.5 + 0.5 cos (Δφ _mod + φ _a −φ _b ) (7)
P = 0.5 + 0.5 cos (Δφ _mod + φ) (8)
Further, the optical phase difference phi _b of the optical phase difference phi _a of the arm 2d of the arm 2c is equal (φ _{a =} φ _b), i.e., when the optical phase difference phi = 0 between the two arms, the combined light intensity P Becomes the equation (9).

P = 0.5 + 0.5 cos (Δφ _mod ) (9)
Incidentally, the light intensity P expressed by the equation (9) is a light intensity conversion signal output from the port 2g or the port 2h. Therefore, if the light intensity P represented by the expression (9) is represented by the expression (10) as the light intensity conversion signal P ₁ output from the port 2g, the expression from the port 2h is represented by the expression (11). A light intensity conversion signal P ₂ having a phase difference of 180 ° (π) is output.

P ₁ = 0.5 + 0.5 cos (Δφ _mod ) (10)
P ₂ = 0.5−0.5 cos (Δφ _mod ) (11)
As a result, when the light intensity conversion signal P ₁ represented by the expression (10) is input to the light receiver 20 and subjected to photoelectric conversion, and thereafter the offset power is canceled, the light intensity I given by the expression (12) is obtained. _An electric signal representing _α is output to the signal processing unit 21.

I _α ∝0.5 cos (Δφ _mod ) (12)
Therefore, the signal processing unit 21 obtains the relationship between the relative inter-bit phase difference Δφ _mod and the light intensity I _α in advance based on the above equation (12), so that the relative inter-bit phase difference Δφ is obtained from the light intensity I _{α. Although} it seems that _mod can be measured, as shown in FIG. 4, two relative bit phase differences Δφ _mod correspond to one light intensity I _α , and one of them cannot be specified. Therefore, the relative bit phase difference Δφ _mod cannot be measured.

  The present invention provides an optical phase delay device in one arm of one bit delay interferometer as an optical phase detector of an optical phase modulation signal, and arbitrarily changes the optical path length of the optical phase delay device, thereby causing a bit delay. By providing two different optical phase differences φ between the two arms of the interferometer, it is possible to solve these problems and to measure the relative inter-bit phase difference of the optical phase modulation signal with high accuracy. An object of the present invention is to provide a phase modulation evaluation apparatus and a phase calibration method thereof.

In order to solve the above-mentioned problem, in the optical phase modulation evaluation apparatus according to claim 1 of the present invention, a predetermined optical phase difference φ is provided between one of two arms and a bit delay (2f) and the two arms. And an optical phase delay device (2j) controlled to provide an optical phase modulated signal (P) in which an optical carrier wave is phase-modulated with a data signal as input light and passes through the respective arms. A bit delay interferometer (2) that demultiplexes into two lights and multiplexes and interferes with the two lights passing through the respective arms (Px) to output two light intensity conversion signals; and the bit delay interference An optical waveform measuring unit (7, 15,) that converts at least one of the two light intensity conversion signals output from the meter into an electric signal, digitally converts the obtained electric signal waveform, and stores the waveform data. 25, 30) and Said predetermined optical phase difference phi is such that at least two different first optical phase difference phi ₁ and a second optical phase difference phi _2, is stored in advance in the control memory (12a), said predetermined Phase control means (12) for reading out phase control data for controlling the optical path length of the optical phase retarder corresponding to the optical phase difference φ and arbitrarily controlling the optical path length of the optical phase retarder; Based on the first waveform data when the optical phase difference φ is the first optical phase difference φ ₁ and the second waveform data when the optical phase difference φ ₂ is the second optical phase difference φ _2. And signal processing means (8) for obtaining a phase difference between the relative bits of the signal.

  Also, in the optical phase modulation evaluation apparatus according to claim 2 of the present invention, in the optical phase modulation evaluation apparatus according to claim 1 described above, the optical waveform measurement unit includes the two optical intensities output from the bit delay interferometer. Each of the converted signals is received by two light receivers (4a, 4b) and converted into an electric signal, and a balanced receiver (4) that subtracts the obtained outputs and outputs an electric signal waveform; The electrical signal waveform output from the receiver is digitally converted, and an electrical waveform measurement unit (6) for storing the obtained waveform data is included.

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

  The optical phase modulation evaluation apparatus according to claim 4 of the present invention is the optical phase modulation evaluation apparatus according to claim 1, wherein the optical waveform measurement unit includes an optical pulse generator (7 e) that outputs an optical pulse signal. A non-linear 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, and converts the sum frequency light to an electric signal. A light receiver (7b) for outputting an electrical pulse 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. I made it.

  The optical phase modulation evaluation apparatus according to claim 5 of the present invention is the optical phase modulation evaluation apparatus according to claim 1, wherein the optical waveform measurement unit includes a light intensity conversion signal and an electrical sampling clock signal (e, d). In response to the sampling clock signal, the electroabsorption optical modulator (25a) that changes the transmittance of the light intensity conversion signal (Px) by the electroabsorption effect, and the outgoing light transmitted through the electroabsorption optical modulator ( A light receiver (25b) that converts Py) into an electrical signal, an A / D converter (25c) that converts the electrical signal into a digital signal, and a waveform memory (25d) that stores the digital signal are included.

  The optical phase modulation evaluation apparatus according to claim 6 of the present invention is the optical phase modulation evaluation apparatus according to claim 1, wherein the optical waveform measurement unit outputs an optical pulse generator (Ps) for outputting a sampling optical pulse (Ps). 30e), a light gate means (30a) that receives the light intensity conversion signal and the sampling light pulse, and controls the transmittance of the light intensity conversion signal according to the incidence of the sampling light pulse; A light receiver (30b) that converts the emitted light (Py) into an electrical signal, an A / D converter (30c) that converts the electrical signal into a digital signal, and a waveform memory (30d) that stores the digital signal. I made it.

  In the optical phase modulation evaluation apparatus according to claim 7 of the present invention, in the optical phase modulation evaluation apparatus according to claim 6 described above, the optical gate means transmits the light intensity conversion signal in response to the incidence of the sampling optical pulse. An electroabsorption optical modulator (31a) that changes the rate according to the supersaturated absorption characteristic is included.

Also, in the optical phase modulation evaluation apparatus according to claim 8 of the present invention, in the optical phase modulation evaluation apparatus according to claim 6 described above, the optical gate means includes the first optical port (31a ₁ ) that receives the light intensity conversion signal and sampling. An electroabsorption optical modulator (31a) that has a second optical port (31a ₂ ) for receiving the optical pulse for use and changes the transmittance of the light intensity conversion signal by the saturable absorption characteristic in response to the incidence of the sampling optical pulse. ), And the sampling optical pulse received from the optical pulse generator is incident on the second optical port of the electroabsorption optical modulator, and the light emitted from the second optical port is received by the sampling optical pulse ( 31b ₁ ) and an optical coupler (31b) that emits to a different optical path (31b ₃ ).

  The optical phase modulation evaluation apparatus according to claim 9 of the present invention is the optical phase modulation evaluation apparatus according to claim 6, wherein the optical gate means transmits the light intensity conversion signal in response to the incidence of the sampling optical pulse. A carbon nanotube supersaturated absorption element (32a) in which the rate is changed by the supersaturated absorption characteristic of the carbon nanotube is included.

In the optical phase modulation evaluation apparatus according to claim 10 of the present invention, in the optical phase modulation evaluation apparatus according to claim 6 described above, the optical gate means includes a first optical port (32a ₁ ) that receives the light intensity conversion signal and sampling. A second optical port (32a ₂ ) that receives the optical pulse for use, and a carbon nanotube disposed on the optical path connecting the first optical port and the second optical port, and the light intensity according to the incidence of the sampling optical pulse The carbon nanotube supersaturated absorption element (32a) that changes the transmittance for the converted signal according to the supersaturated absorption characteristic of the carbon nanotube, and the sampling optical pulse received from the optical pulse generator enter the second optical port of the carbon nanotube supersaturated absorption element. together, different from the light emitted from the second optical port optical path which receives the sampling optical pulse and (32 b ₁₎ And to include an optical coupler for emitting the optical path (32b ₃₎ (32b).

  The optical phase modulation evaluation apparatus according to claim 11 of the present invention is the optical phase modulation evaluation apparatus according to claim 6, wherein the optical gate means converts the sampling optical pulse into an electrical pulse for conversion into an electrical pulse ( 33b) and an electroabsorption optical modulator (33a) that is disposed in the vicinity of the light receiver and changes the transmittance of the light intensity conversion signal by the electroabsorption effect according to the electric pulse. .

Further, in the optical phase modulation evaluating device according to claim 12 of the present invention, in any one of the optical phase modulation evaluating device according to claim 1 to 11 described above, the first optical phase difference phi ₁ is 0 and the The optical phase difference φ _{2 of 2} was set to π / 2.

  Moreover, in the optical phase modulation evaluation apparatus according to claim 13 of the present invention, in the optical phase modulation evaluation apparatus according to any one of claims 1 to 12, the signal processing means applies the first and second waveform data. A relative inter-bit phase difference histogram of the optical phase modulation signal is calculated from the relative inter-phase phase difference of the optical phase modulation signal obtained based on the above.

  Moreover, in the optical phase modulation evaluation apparatus according to claim 14 of the present invention, in the optical phase modulation evaluation apparatus according to any one of claims 1 to 13, the signal processing means applies the first and second waveform data. The constellation of the optical phase modulation signal is calculated from the phase difference between the relative bits of the optical phase modulation signal obtained based on the above.

  In the optical phase modulation evaluation apparatus according to claim 15 of the present invention, in the optical phase modulation evaluation apparatus according to any one of claims 1 to 14, the signal processing means includes the first and second waveform data and Each time the first and second waveform data are read from the phase difference table (8a) that stores and holds the relationship between the relative phase difference of the optical phase modulation signal and the waveform memory of the optical waveform measurement unit. Sequentially refer to the phase difference table to obtain a phase difference calculation means (8b) for obtaining a phase difference between the relative bits of the optical phase modulation signal, and a plurality of phase differences between the relative bits sequentially output from the phase difference calculation means. A phase difference histogram calculating means (8c) for obtaining a relative inter-bit phase difference histogram of the optical phase modulation signal, and the relative inter-bit phase difference sequentially output from the phase difference calculating means. Based on, and a constellation calculating means (8d) determining the constellation of the optical phase modulation signal.

In the optical phase modulation evaluation apparatus according to claim 16 of the present invention, a mode designation signal for designating one of the measurement mode and the calibration mode is output in the optical phase modulation evaluation apparatus according to any one of claims 1 to 15 described above. Receiving mode designation means (11), a calibration light source (3) for outputting unmodulated reference light having the same wavelength as the optical phase modulation signal, the optical phase modulation signal, the reference light and the mode designation signal. When the mode designation signal designates a measurement mode, the optical phase modulation signal is input to the bit delay interferometer, and when the mode designation signal designates a calibration mode, And an optical switch (1) for inputting reference light to the bit delay interferometer, and the phase control means, when the mode designation signal designates the measurement mode, the optical path length of the optical phase delay device The If the mode designation signal designates the calibration mode, the optical path length of the optical phase delay device is changed by one cycle, and the mode designation signal designates the calibration mode. to, in response to sequential waveform data read from the optical waveform measurement unit, detects the maximum level L _max and the minimum level L _min of adjacent of said waveform data, detected said maximum level L _max and said minimum level L _min A predetermined function representing a relationship between the phase control data and the level L of the waveform data is obtained, and based on the obtained predetermined function, the phase control data and the phase of the optical phase delayer are Phase calibration processing means (13) for calculating the relationship and storing it in the control memory.

  Further, in the optical phase modulation evaluation apparatus according to claim 17 of the present invention, in the optical phase modulation evaluation apparatus according to claim 16 described above, the predetermined function is represented by the following expression.

L = (L _max + L _min ) / 2 + {(L _max −L _min ) / 2} cos φ
The calibration method for an optical phase modulation evaluation apparatus according to claim 18 of the present invention is a method for calibrating the optical phase modulation evaluation apparatus according to any one of claims 1 to 15, wherein the unmodulated reference light is input. Changing the delay amount of the optical phase delay unit (2j) to obtain waveform data read from the electrical waveform measuring unit; and determining the optical phase delay unit from the delay amount of the optical phase delay unit and the level of the waveform data Calculating a predetermined function representing a relationship between a delay amount of the delay time and a phase difference between the two arms of the bit delay interferometer (2), and calibrating the optical phase delay device based on the calculated function; It is provided with.

In the optical phase modulation evaluation apparatus according to the first aspect of the present invention, an optical phase delay is provided in one arm of the bit delay interferometer, and the optical path length of the optical phase delay is arbitrarily changed, so that the bit delay interferometer Since two different optical phase differences φ ₁ and φ ₂ are given between the two arms, the phase difference between the relative bits of the optical phase modulation signal can be measured. Therefore, it is possible to accurately evaluate the modulation state of the optical phase modulation signal obtained by phase modulation (for example, DPSK, DQPSK, etc.) of the optical carrier wave in the coherent optical communication system by the data signal.

  In the optical phase modulation evaluation apparatus according to claim 2 of the present invention, a balanced receiver is used for the optical waveform measurement unit, and the two light intensity conversion signals output from the bit delay interferometer are balanced and received. The light receiving sensitivity of the measurement can be improved.

  In the optical phase modulation evaluation apparatus according to claim 3 of the present invention, a single receiver is used for the optical waveform measurement unit, and one of the two light intensity conversion signals output from the bit delay interferometer is received. Therefore, the device configuration can be simplified and the price can be reduced.

  In the optical phase modulation evaluation apparatus according to claim 4 of the present invention, since the nonlinear optical element is used in the optical waveform measurement unit, the relative bits of the optical phase modulation signal from which the characteristics of the receivers of the balanced receiver and the single receiver are removed. The interphase difference can be obtained.

  In the optical phase modulation evaluation apparatus according to claim 5 of the present invention, an electroabsorption optical modulator having a transmission loss smaller than that of the nonlinear optical element is used for the optical waveform measurement unit, so that the sampling efficiency is high and the balance is balanced. The phase difference between the relative bits of the optical phase modulation signal from which the characteristics of the receivers of the receiver and the single receiver are removed can be obtained.

  In the optical phase modulation evaluation apparatus according to the sixth aspect of the present invention, since the 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 optical phase modulation evaluation apparatus according to claims 7 and 8 of the present invention, an electroabsorption optical modulator having a transmission loss smaller than that of the nonlinear optical element is used for the optical waveform measurement unit. In addition, the relative inter-bit phase difference of the optical phase modulation signal can be obtained by sampling at a high frequency.

  In the optical phase modulation evaluation apparatus according to the ninth and tenth aspects of the present invention, since the carbon nanotube supersaturated absorption element is used in the optical waveform measuring section, the electroabsorption optical modulator is used in the method using the electroabsorption optical modulator. A DC power source required to keep the absorption rate constant is not necessary, and the configuration of the optical waveform measuring unit can be simplified as compared with a method using an electroabsorption optical modulator.

  In the optical phase modulation evaluation apparatus according to claim 11 of the present invention, in the optical waveform measurement unit, the optical pulse is converted into an 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 light intensity conversion signal is controlled, it is possible to obtain the relative inter-bit phase difference of the optical phase modulation signal that can cope with sampling of a high modulation signal.

In the optical phase modulation evaluation apparatus according to claim 12 of the present invention, the first and second optical phase differences φ ₁ and φ ₂ in the bit delay interferometer are set to 0 and π / 2, respectively, from the memory of the optical waveform measurement unit. Since the phase difference between the read first and second waveform data is set to π / 2 as shown in FIG. 5, the minimum portion and the maximum portion of the light intensity change rate are combined. Thus, the relative bit phase difference can be measured with high accuracy.

  In the optical phase modulation evaluation apparatus according to the thirteenth and fifteenth aspects of the present invention, a relative bit phase difference histogram of the optical phase modulation signal can be obtained from the relative bit phase difference of the optical phase modulation signal.

  In the optical phase modulation evaluation apparatus according to claims 14 and 15 of the present invention, the constellation of the optical phase modulation signal can be obtained from the phase difference between the relative bits of the optical phase modulation signal.

  In the optical phase modulation evaluation apparatus according to the sixteenth and seventeenth aspects of the present invention, the optical level between the two arms of the bit delay interferometer is obtained by using unmodulated reference light having the same wavelength as the optical phase modulation signal (light to be measured). Since the phase difference φ is calibrated, the relative bit can be measured 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 various measured light wavelengths or temperature changes. The phase difference can be measured with high accuracy.

Embodiments of the present invention will be described below.
[First Embodiment]
The configuration of the optical phase modulation evaluation apparatus according to the first embodiment of the present invention is shown in FIG. The same elements as those in the conventional optical phase modulation evaluation apparatus are denoted by the same reference numerals. An optical phase modulation signal, which is light to be measured by the optical phase modulation evaluation 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, and the optical phase modulation signal input from the port 2a and the light passing through the arm 2c by the demultiplexing unit 2b and the arm 2d (bit delay device) 2f and the optical phase delay device 2j), the light passing through the arm 2c and the light passing through the arm 2d are combined and interfered by the multiplexing unit 2e. Thereby, the phase change of the optical phase modulation signal is converted into the change of the light intensity, and two light intensity conversion signals whose phases are different from each other by 180 ° (π) are transferred from the two ports 2g and 2h to the balanced receiver 4 respectively. Output.

  The optical phase delay device 2j controls the phase so as to give a predetermined optical phase difference φ (see equation (8)) between the two arms of the bit delay interferometer 2, for example, φ = 0 and φ = π / 2. It is controlled by the phase control data c output from the means 12.

  By the way, the phase control means 12 receives the optical phase difference designation signal b (designating the optical phase difference φ) and the mode designation signal a inputted from an operation unit or the like (not shown), and receives the mode designation signal a. When the measurement mode is designated, the phase control data c corresponding to the optical phase difference φ designated by the optical phase difference designation signal b is read from the control memory 12a to control the phase of the optical phase delay 2j. . That is, when the optical phase difference designation signal b designates the optical phase difference φ = 0, the phase control data c corresponding to φ = 0 and the optical phase difference φ = π / 2 are designated. Reads out the phase control data c corresponding to φ = π / 2 from the control memory 12a and controls the optical path length of the optical phase delay 2j. In addition, when the mode designation signal a designates the calibration mode, the phase control means 12 changes the optical phase difference φ between the two arms in the bit delay interferometer 2 by at least one period of the reference light. The phase control data c that can be generated is output, and the optical path length of the optical phase delay 2j is sequentially changed. 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 output from the bit delay interferometer 2 described above are expressed by mathematical expressions based on the same conditions as described in the section “Problems to be Solved by the Invention”. When the optical phase difference designation signal b designates the optical phase difference φ = 0 and the optical phase delay device 2j is controlled to give the optical phase difference φ = 0 between the two arms, the above (8 ) since the phi = 0 in formula (shown again), the light intensity conversion signal _{P 2} output from the optical intensity conversion signal _{P 1} and the port 2h is output from the port 2g, respectively above (10), (11) It becomes a formula (repost). When the optical phase difference designation signal b designates the optical phase difference φ = π / 2 and the optical phase delay 2j is controlled to give the optical phase difference φ = π / 2 between the two arms. Is φ = π / 2 in the above equation (8), the light intensity conversion signal P ₃ output from the port 2 g and the light intensity conversion signal P ₄ output from the port 2 h are (13), (14)

P = 0.5 + 0.5 cos (Δφ _mod + φ) (8)
P ₁ = 0.5 + 0.5 cos (Δφ _mod ) (10)
P ₂ = 0.5−0.5 cos (Δφ _mod ) (11)
P ₃ = 0.5 + 0.5 cos (Δφ _mod + π / 2) (13)
P ₄ = 0.5−0.5 cos (Δφ _mod + π / 2) (14)
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) (15)
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 are received in a balanced manner, and the output electric signal waveform is output to the electric waveform measuring unit 6. These two light intensity conversion signals, that is, the light intensity conversion signals P ₁ and P ₂ expressed by the above-described expressions (10) and (11) and the light intensity expressed by the above-described expressions (13) and (14), respectively. The respective light intensities P _α and P _β when the converted signals P ₃ and P ₄ are received in a balanced manner are expressed by the expressions (16) and (17), respectively, and the offset power is canceled and the light intensity is reduced. Doubled. In addition, the electric signal waveform of each balanced reception output represents the light intensity I _α given by the equation (18) and the light intensity I _β given by the equation (19).

P _α = P ₁ −P ₂ = cos (Δφ _mod ) (16)
_{P β = P 3 -P 4 =} cos (Δφ mod + π / 2) (17)
I _α ∝cos (Δφ _mod ) (18)
I _β ∝cos (Δφ _mod + π / 2) (19)
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.

In the signal processing means 8, the two waveform data sequentially read from the waveform memory 6b of the optical waveform measuring unit 15, that is, the optical phase difference designation signal b designates the optical phase difference φ = 0, and the optical phase delay device 2j Waveform data when the optical phase difference φ = 0 is controlled between the two arms and the optical phase difference designation signal b designates the optical phase difference φ = π / 2, and the optical phase delay 2j Based on the waveform data when the optical phase difference φ = π / 2 is controlled between the two arms, the relative bit phase difference Δφ _mod due to the phase modulation of the optical phase modulation signal is obtained, and the relative The inter-bit phase difference histogram and the constellation are obtained, and includes a phase difference table 8a, a phase difference calculation means 8b, a phase difference histogram calculation means 8c, and a constellation calculation means 8d.

That is, the phase difference table 8a is a relationship between two waveform data (corresponding to I _α and I _β ) read from the waveform memory 6b of the optical waveform measuring unit 15 and the relative inter-bit phase difference Δφ _mod of the optical phase modulation signal. That is, as shown in FIG. 5, the relationship represented by the above equations (18) and (19) is measured in advance and stored in a table. When the relative inter-bit phase difference Δφ _mod is obtained from the relationship between the equations (18) and (19), as can be seen from FIG. 5, I _α and I _β are shifted from each other by π / 2. The minimum portion and the maximum portion of the change rate of the light intensity are combined, and the measurement accuracy of the relative inter-bit phase difference Δφ _mod is improved.

The phase difference calculating means 8b sequentially refers to the phase difference table 8a every time two pieces of waveform data (corresponding to I _α and I _β ) are read from the waveform memory 6b of the optical waveform measuring section 15, and outputs the optical phase modulation signal. A relative inter-bit phase difference Δφ _mod is obtained.

The phase difference histogram calculation unit 8c obtains a relative inter-bit phase difference histogram as shown in FIG. 7 of the optical phase modulation signal based on the plurality of relative inter-bit phase differences Δφ _mod sequentially output from the phase difference calculation unit 8b. .

The constellation calculating means 8d obtains a constellation of the optical phase modulation signal as shown in FIG. 8 based on the relative inter-bit phase difference Δφ _mod sequentially output from the phase difference calculating means 8b.

  The display 9 displays a relative inter-bit phase difference histogram and a constellation of the optical phase modulation signal output from the signal processing means 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 retarder 2j of the bit delay interferometer 2 has the phase control data c output from the above-described 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 phase control data c that is changed at least by one period of the reference light. 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 Δφ _mod = 0 in the above-described equation (8) (re-displayed), so that the phase change of the optical phase delay 2j occurs. Te, the light intensity conversion signal _{P C2} output from the optical intensity conversion signal _{P C1} and port 2h is output from the port 2g, respectively (20) and (21).

P = 0.5 + 0.5 cos (Δφ _mod + φ) (8)
P _C1 = 0.5 + 0.5 cosφ (20)
P _C2 = 0.5−0.5cosφ (21)
Then, this (20), the light intensity _{P C} when the light intensity conversion signal _{P C1, P C2} is balanced receiver with balanced receiver 4 of the formula (21) may be expressed by (22) The The electrical signal waveform of the output of the balanced receiver represents the light intensity I _C given by (23).

P _C = P _C1 −P _C2 = cosφ (22)
I _C ∝cosφ (23)
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 unit 13 a outputs waveform data (corresponding to I _C in the above equation (23)) sequentially read from the waveform memory 6 b of the optical waveform measurement unit 15 and the phase control unit 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 optical phase delay 2j, the maximum level L _max and the minimum level of the waveform data as shown in FIG. L _min is detected.

As shown in FIG. 6, the level-phase calculation means 13b determines the position associated with the phase control data c of the maximum level L _max and the minimum level L _min between the two arms in the bit delay interferometer 2, respectively. The level L of the waveform data (corresponding to I _C ) is expressed by equation (24) as 0 and π of the optical phase difference φ.

L = (L _max + L _min ) / 2 + {(L _max −L _min ) / 2} cos φ (24)
The phase control data calculation means 13c obtains the level L with respect to the optical phase difference φ from the above equation (24), 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 correlation between the correlated optical phase difference φ and the phase control data c is stored in the control 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 the obtained relationships are stored in the control 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 other two optical phase differences φ ₁ and φ ₂ 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 optical phase modulation evaluation apparatus of 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 output from the port 2g of the bit delay interferometer 2 is received by the light receiver 5a, and is photoelectrically converted. Is offset 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 above-described light intensity conversion signals P ₁ and P ₃ output from the port 2g of the bit delay interferometer 2 and the light intensities I _α and I _β represented by these electric signal waveforms is as described above (10 ), (13) formula (repost) and the above-described formula (12) (repost) and formula (25).

P ₁ = 0.5 + 0.5 cos (Δφ _mod ) (10)
P ₃ = 0.5 + 0.5 cos (Δφ _mod + π / 2) (13)
I _α ∝0.5 cos (Δφ _mod ) (12)
I _β ∝0.5 cos (Δφ _mod + π / 2) (25)
Instead of the light intensity conversion signal output from the port 2g of the bit delay interferometer 2, a light intensity conversion signal output from the port 2h may be used. In this case, the relationship between the light intensity conversion signals P ₂ and P ₄ output from the port 2h and the light intensity I _α and I _β represented by these electric signal waveforms is as described in (11), ( 14) Expression (repost), and Expressions (26) and (27).

P ₂ = 0.5−0.5 cos (Δφ _mod ) (11)
P ₄ = 0.5−0.5 cos (Δφ _mod + π / 2) (14)
I _α ∝−0.5 cos (Δφ _mod ) (26)
I _β ∝−0.5 cos (Δφ _mod + π / 2) (27)
Therefore, when such a single receiver 5 is used, the phase difference table 8a of the signal processing means 8 has a relationship between I _α represented by the above equation (12) and I _β represented by the equation (25), Alternatively, the relationship between I _α represented by the above equation (26) and I _β represented by equation (27) is measured in advance and stored in the table. The single receiver 5 and the electric waveform measuring unit 6 constitute an optical waveform measuring unit 15.
[Third Embodiment]
The configuration of the optical phase modulation evaluation apparatus of 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.

The sampling clock generation unit 10 supplies the sampling clock signal e to the optical waveform measurement unit via the switch 14, and the optical pulse generator 7e samples a sampling optical pulse train f (angular frequency ω _SAM ) having a repetition period equal to the sampling clock signal e. Is generated.

The sampling light pulse train f (angular frequency ω _SAM ) and the light intensity conversion signal (angular frequency ω _DATA ) input from the port 2 g of the bit delay interferometer 2 are incident on the nonlinear optical crystal 7 a and are caused by the second-order nonlinear optical effect. An optical pulse signal having a sum angular frequency ω _SAM + ω _DATA (Sum Frequency Generation) is generated.

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

The optical pulse signal ω _SAM + ω _DATA generated from the nonlinear optical crystal 7a is photoelectrically converted by the light receiver 7b to output an electrical 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 signal processing unit 8 or the phase calibration processing unit 13 is sequentially read from the waveform memory 7d.

The relationship between the light intensity conversion signal P ₁ output from the port 2 g of the bit delay interferometer 2 and the light intensity I _α represented by these electric signal waveforms, and the port 2 h of the bit delay interferometer 2. same as the above-described light intensity output converted signal P _2, the relationship between the light intensity I _alpha represented by these electrical signal waveforms, respectively, which constitute a single receiver 5 of the second embodiment described above from It is.
[Fourth Embodiment]
The configuration of the optical phase modulation evaluation apparatus according to the fourth embodiment of the present invention is shown in FIG. Only the configuration of the optical waveform measurement units 15 and 25 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 optical phase modulation evaluation apparatus are the same as those of the first embodiment. 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 25a has two optical ports 25a ₁ and 25a ₂ for entering and exiting light and a power supply terminal 25a 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 25a ₁ and the sampling clock signal e of the sampling clock generator 10 is supplied to the power supply terminal 25a ₃ , the power It decreased absorption rate with respect to the light intensity conversion signal Px only while the pulse to the terminal 25a ₃ is added (transmittance increases), and is output as the sampling optical pulse signal Py from the optical port 25a _2. The optical pulse signal Py is converted into an electric pulse signal by the light receiver 25b, and is sequentially converted into a digital signal in synchronization with the sampling clock signal e by the A / D converter 25c, and then stored in the waveform memory 25d 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 optical phase modulation evaluation apparatus are the same as those of the first embodiment, so that the description will be given. 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 the optical pulse Ps converted by the optical pulse generator 30e is applied to the optical gate means 30a and sampling is performed with the optical pulse. Therefore, since sampling can be performed with an optical pulse having a narrower line width than the electric pulse e, it is possible to cope with sampling of a modulation signal as high as several tens of G.

  FIG. 12 shows the configuration of the optical gate means 30a of this embodiment. In the embodiment of FIG. 12, the light intensity conversion signal Px is sampled by applying the saturable absorption characteristic of the electroabsorption optical modulator 31a.

The optical gate means 30a includes an electroabsorption optical modulator 31a, an optical circulator 31b, and a DC power supply 31e. The electroabsorption optical modulator 31a has two optical ports 31a ₁ and 31a ₂ for entering and exiting light, and a power supply terminal 31a ₃ for applying an electric field to the optical path between both optical ports, and the optical port 31a ₁ An optical pulse Ps via the ports 31b ₁ and 31b ₂ of the optical circulator 31b is applied in a state in which a DC voltage is applied from the DC power supply 31e to the power supply terminal 31a ₃ so that the light intensity conversion signal Px input to 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 (transmittance is increased) only while the optical pulse Ps is being input, sampling 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]
FIG. 13 shows the configuration of the optical gate means 30a according to the sixth embodiment of the present invention. 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 supersaturated absorption characteristic of a carbon nanotube (CNT: Carbon Nanotube). The wavelength showing the supersaturated absorption characteristic of the carbon nanotube is determined by its diameter, and is prepared in accordance with the frequency range of the sampled light (Japanese Patent Laid-Open No. 2003-121892).

In FIG. 13, the optical gate means 30a includes a CNT supersaturated absorption element 32a and an optical circulator 32b. The CNT supersaturated absorption element 32a has 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, a light pulse Ps to the optical ports 32a ₁ through the port _32b 1, 32b ₂ of the optical circulator 32b to the optical port 32a ₂ When input, due to the supersaturated absorption characteristics of the CNT supersaturated absorption element 32a, the absorptance with respect to the light intensity conversion signal Px decreases (transmittance increases) only while the optical pulse Ps is input, and the sampled optical pulse signal Py is outputted from the optical port 32a _2.

  In FIG. 13, the signal flow is described using a directional optical circulator as a means for inputting and outputting the optical pulse signal Py and the optical pulse Ps. However, the flow of the signal is not limited to this, and 2 × 2 having no directionality. Other means can be used as long as the optical signal can be multiplexed / demultiplexed, such as a type optical coupler.

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

FIG. 14 shows an example of the internal structure of the CNT supersaturated absorption element 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 light pulse Ps projected from the back surface, so that the optical port is converted into the optical port signal Py. It is emitted from 32a ₂ .

  The CNT supersaturated absorption element 32a is not limited to the structure shown in FIG. 14, but 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. Can do.

  Further, instead of entering the optical pulse Ps from the subsequent stage of the CNT supersaturated absorption element 32a, it is possible to perform sampling by entering from the previous stage as shown in FIG.

In the optical gate means of FIG. 13, the optical circulator is arranged at the subsequent stage of the CNT supersaturated absorption element 32a. However, in the configuration of FIG. Ps is multiplexed and enters the CNT supersaturated absorption element 32a. Of the optical pulse signal Py and the optical pulse Ps transmitted through the CNT supersaturated absorption element 32a, the optical pulse Ps unnecessary for jitter analysis is removed by the filter 32d.
[Seventh Embodiment]
The structure of the optical gate means 30a of 7th Embodiment of this 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 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 the sampling can be performed with the electric pulse h having a line width as close as possible to the narrow line width of the optical pulse Ps), the sampling of a modulation signal as high as several tens of G can be supported. .

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. Diagram showing the relationship between light intensity and relative phase difference between bits Diagram showing the relationship between light intensity and relative phase difference between bits Diagram for explaining optical phase difference calibration method Figure showing the relative bit phase difference histogram Diagram showing constellation The figure which shows schematic structure of a prior art example 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 supersaturated 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, 20, 25b, 30b・ ・ ・ Light receiver (PD), 4c ... Subtractor, 5 ... Single receiver, 5b ... Offset circuit, 6 ... Electrical waveform measuring unit, 6a, 7c, 25c, 30c ... A / D converter, 6b, 7d, 25d, 30d ... memory, 7, 25, 30 ... optical waveform measurement unit, 7a ... nonlinear optical crystal, 7e, 30e ... optical pulse generator, 7f 7g: Polarization direction controller, 8 ... Signal processing means, 8a ... Phase difference table, 8b Phase difference calculating means, 8c: Phase difference histogram calculating means, 8d: Constellation calculating means, 9: Display, 10: Sampling clock generator, 11: Mode specifying means, 12 ... Phase control means, 13 ... Phase calibration processing means, 13a ... Maximum / minimum level detection means, 13b ... Level-phase calculation means, 13c ... Phase control data calculation means, 14 ... Switch 15 optical waveform measuring unit 21 signal processing unit 30a optical gate means 20a 31a 33a electroabsorption optical modulator 31b 32b 32c Optical coupler, 32a ... CNT supersaturated absorption element, 31e ... DC power supply, Ps ... light pulse, Px ... light intensity conversion signal.

Claims (18)

One of the two arms includes a bit delay (2f) and an optical phase delay (2j) controlled so as to give a predetermined optical phase difference φ between the two arms. As an optical carrier wave, an optical phase modulation signal (P) obtained by phase modulation with a data signal is received and split into two lights passing through the arms, and the two lights passing through the arms (Px) are separated. A bit delay interferometer (2) that combines and interferes to output two light intensity conversion signals;
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, 25, 30),
Said predetermined optical phase difference phi is such that at least two different first optical phase difference phi ₁ and a second optical phase difference phi _2, is stored in advance in the control memory (12a), said predetermined Phase control means (12) for reading out phase control data for controlling the optical path length of the optical phase retarder corresponding to the optical phase difference φ and arbitrarily controlling the optical path length of the optical phase retarder;
Based on the first waveform data when the predetermined optical phase difference φ is the first optical phase difference φ ₁ and the second waveform data when the predetermined optical phase difference φ ₂ , the light An optical phase modulation evaluation apparatus comprising signal processing means (8) for obtaining a phase difference between relative bits of a phase modulation signal. 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, and the obtained outputs are subtracted to generate an electric signal waveform. A balanced receiver (4) that outputs
The optical phase modulation according to claim 1, further comprising: an electrical waveform measuring unit (6) for digitally converting the electrical signal waveform output from the balanced receiver and storing the obtained waveform data. Evaluation device. 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 optical phase modulation evaluation apparatus according to claim 1, further comprising: an electrical waveform measurement unit (6) for digitally converting the electrical 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 optical phase modulation evaluation apparatus according to claim 1, further comprising a waveform memory (7d) for storing the digital signal. (Fig. 10)
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 (25a);
A light receiver (25b) for converting the emitted light (Py) transmitted through the electroabsorption optical modulator into an electric signal;
An A / D converter (25c) for converting the electrical signal into a digital signal;
The optical phase modulation evaluation apparatus according to claim 1, further comprising a waveform memory (25d) for storing the digital signal. (Fig. 11)
The optical waveform measuring unit is
An 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 optical phase modulation evaluation apparatus according to claim 1, further comprising a waveform memory (30d) for storing the digital signal. (Fig. 12)
The optical gate means includes
7. The electroabsorption optical modulator (31a) that changes the transmittance of the light intensity conversion signal by a saturable absorption characteristic in response to the incidence of the sampling light pulse. The optical phase modulation evaluation apparatus described. (Fig. 12)
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 the saturable absorption characteristic;
An optical path through which the sampling optical pulse received from the 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 optical phase modulation evaluation apparatus according to claim 6, comprising an optical coupler (31b) that emits to an optical path (31b ₃ ) different from (31b ₁ ). (Fig. 13)
The optical gate means includes
The carbon nanotube supersaturated absorption element (32a) is configured to change the transmittance for the light intensity conversion signal according to the supersaturation absorption characteristic of the carbon nanotube in response to incidence of the sampling light pulse. 6. The optical phase modulation evaluation apparatus according to 6. (Fig. 13)
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 supersaturated absorption element (32a) that changes the transmittance with respect to the light intensity conversion signal according to the supersaturated absorption characteristic of the carbon nanotube in response to the incidence of the sampling light pulse,
The sampling optical pulse received from the optical pulse generator is incident on the second optical port of the carbon nanotube saturable absorber element, and the light emitted from the second optical port is received by the sampling optical pulse ( The optical phase modulation evaluation apparatus according to claim 6, comprising an optical coupler (32b) that emits to an optical path (32b ₃ ) different from 32b ₁ ). (Fig. 16)
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 optical phase modulation evaluation apparatus according to claim 6. The first is the optical phase difference phi ₁ is 0, and the optical phase modulation evaluating according to any of claims 1 to 11, the second optical phase difference phi _2, characterized in that a [pi / 2 apparatus.   The signal processing means calculates a relative inter-bit phase difference histogram of the optical phase modulation signal from the relative inter-bit phase difference of the optical phase modulation signal obtained based on the first and second waveform data. The optical phase modulation evaluation apparatus according to claim 1.   The signal processing means calculates a constellation of the optical phase modulation signal from a relative inter-bit phase difference of the optical phase modulation signal obtained based on the first and second waveform data. The optical phase modulation evaluation apparatus according to any one of 1 to 13. The signal processing means is
A phase difference table (8a) for storing and holding the relationship between the first and second waveform data and the phase difference between the relative bits of the optical phase modulation signal;
Phase difference calculating means for sequentially referring to the phase difference table each time the first and second waveform data are read from the waveform memory of the optical waveform measuring unit and calculating a relative inter-bit phase difference of the optical phase modulation signal. (8b)
Phase difference histogram calculation means (8c) for obtaining a relative inter-bit phase difference histogram of the optical phase modulation signal based on a plurality of the relative bit phase differences sequentially output from the phase difference calculation means;
The constellation calculation means (8d) for obtaining a constellation of the optical phase modulation signal based on the phase difference between the relative bits sequentially output from the phase difference calculation means. 14. The optical phase modulation evaluation apparatus according to claim 14. Mode designation means (11) for outputting a mode designation signal for designating either the measurement mode or the calibration mode;
A calibration light source (3) for outputting unmodulated reference light having the same wavelength as the optical phase modulation signal;
When the optical phase modulation signal, the reference light, and the mode designation signal are received and the mode designation signal designates a measurement mode, the optical phase modulation signal is input to the bit delay interferometer, and An optical switch (1) for inputting the reference light to the bit delay interferometer when the mode designation signal designates a calibration mode;
The phase control means controls the optical path length of the optical phase delay when the mode designation signal designates a measurement mode, and when the mode designation signal designates a calibration mode. , Changing the optical path length of the optical phase retarder by one cycle,
When the mode designation signal designates the calibration mode, the waveform data sequentially read from the optical waveform measurement unit is received, and the adjacent maximum level L _max and minimum level L _min of the waveform data are detected, Based on the detected _maximum level L _max and the minimum level L _min , a predetermined function representing a relationship between the phase control data and the level L of the waveform data is obtained, and based on the obtained predetermined function, The light according to any one of claims 1 to 15, further comprising phase calibration processing means (13) for calculating a relationship between phase control data and a phase of the optical phase delay device and storing it in the control memory. Phase modulation evaluation device. The optical phase modulation evaluation apparatus according to claim 16, wherein the predetermined function in the phase calibration processing unit is expressed by the following equation.
L = (L _max + L _min ) / 2 + {(L _max −L _min ) / 2} cos φ A method for calibrating the optical phase modulation evaluation apparatus according to claim 1,
With the input of unmodulated reference light, changing the delay amount of the optical phase delay (2j), obtaining waveform data read from the electrical waveform measurement unit,
A predetermined function representing the relationship between the delay amount of the optical phase delay device and the phase difference between the two arms of the bit delay interferometer (2) is calculated from the delay amount of the optical phase delay device and the level of the waveform data. Stages,
Calibrating the optical phase retarder based on the calculated function, and a calibration method for an optical phase modulation evaluation apparatus.

Images