JP2009044377A - Optical phase modulation evaluation apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately measure an inter-symbol phase difference even for a phase modulated light modulated in phase with a high transmission rate data signal. <P>SOLUTION: A phase-modulated incident light S is branched into two light signals. The first phase-modulated light Sa is incident to a first delay interference unit 30 for conversion into a first intensity converted light Sa4 having an amplitude depending on a cosine value of the inter-symbol optical phase difference. The second phase-modulated light Sb is incident to a second delay interference unit 40 for conversion into a second intensity converted light Sb4 having an amplitude depending on a sine value of the inter-symbol optical phase difference. The intensity converted lights Sa4, Sb4 are incident to optical sampling units 60, 70 to conduct the sampling with the optical pulses Sp1, Sp2 for sampling having the period equal to an integer multiple of the symbol clock or the period different by offset time from such integer multiple of the symbol clock. Thereafter, the inter-symbol phase difference is obtained based on the value obtained by photoelectric conversion of the optical pulses Sa5, Sb5 obtained. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique for accurately evaluating phase-modulated light that has been phase-modulated by a high-speed data signal.

  An optical signal modulated with a data signal is used for various types of communication. In recent years, instead of intensity modulation for modulating the intensity of light with a data signal, phase modulation (DPSK: differential for modulating the phase of light with a data signal) Phase shift keying, DQPSK: differential π / 4 phase shift keying, etc.) are realized, and an apparatus for evaluating the optical phase modulation is required.

  As an apparatus that evaluates light phase-modulated by a data signal, the phase-modulated light to be evaluated is split into two, and one symbol (1 bit in the DPSK system, 2 bits in the DQPSK system) between the two lights In some cases, the evaluation processing is performed after converting the phase-modulated light into the intensity-converted light by combining the signals with the delay given (for example, Patent Document 1).

JP-A-6-021891

  FIG. 13 shows a schematic configuration of this type of optical phase modulation evaluation apparatus. The phase-modulated light S to be evaluated, which is phase-modulated by the DPSK method, is transmitted by the duplexer 2 of the delay interference unit 1 to the light S1, S2 is demultiplexed and emitted to the optical paths L1 and L2, respectively. The light S2 is delayed by one symbol (in this case, 1 bit) by the delay device 3 inserted in the optical path L2, and the delayed light S3 is The light S1 is incident on the multiplexer 4 and multiplexed.

  It is assumed that the optical conditions (length, refractive index, etc.) of the two optical paths L1 and L2 are the same except for the delay device 3, and no phase difference of light due to the difference in the optical path occurs.

  Here, in the DPSK system, the phase is changed by π when the data becomes “0”, and is modulated so that the phase does not change when the data is “1”.

  Therefore, for example, each time the phase-modulated light S is phase-modulated with the data signal of FIG. 14A by the DPSK method and the phase of the light S1 becomes “0” as shown in FIG. 14B. When π changes, the light S3 is combined with the light S1 in a state of being delayed by 1 bit with respect to the light S1, as shown in FIG. 14C, and the combined light is a period T1, T2, T5,... Strengthen each other, and the reverse phase periods T3, T4 weaken each other.

  For this reason, the light S4 emitted from the multiplexer 4 becomes intensity-converted light as shown in FIG. 14 (d), and data “1”, a predetermined value during a period when the intensity of the intensity-converted light S4 is larger than a predetermined value. By assigning data “0” to a smaller period, the modulation data in FIG. 14A can be reproduced.

  The intensity-converted light S4 emitted from the delay interference unit 1 enters the photoelectric converter 6, is converted into an electric signal E having a level corresponding to the intensity, is input to the A / D converter 7, and is converted into digital data. It is converted to D and input to the arithmetic processing unit 9.

  Based on the input data, for example, the arithmetic processing unit 9 demodulates the original data, and performs a predetermined evaluation operation (for example, calculation of a bit error rate) on the demodulated data, thereby improving the quality of the phase-modulated light S. Can be evaluated.

  However, the optical phase modulation evaluation apparatus having the above configuration has a problem that the inter-symbol phase difference as an evaluation element of the optical phase modulation light cannot be measured correctly.

  That is, in the case of the DPSK method, the theoretical value of the inter-symbol phase difference is 0 or π, but the actual optical signal phase difference fluctuates due to the influence of noise or the like. It is necessary to evaluate the quality of the optical signal.

  The inter-symbol phase difference is a relative phase difference between the above-described light beams S1 and S3. As described above, if the optical conditions of the optical paths L1 and L2 are equal and no phase difference due to the optical path difference occurs, 2 The two lights S1 and S3 are expressed as follows.

S1 = Aa · exp [j (ωt + φa)]
S3 = Ab · exp [j (ωt + φa + φm)]
Where Aa and Ab are the amplitude of each light, ω is the frequency of the phase-modulated light S, φa is the phase generated by propagating through the optical paths L1 and L2, and φm is the intersymbol phase.

The intensity P of the light S4 obtained by combining the two lights S1 and S3 is
P = (S1 + S3). (S1 ^* + S2 ^* )
= Aa ² + Ab ² + 2 · Aa · Ab · cos (φm)
However, S1 ^* and S2 ^* are conjugate complex numbers of S1 and S2.

Here, assuming that Aa = Ab = A, the intensity P of the light S4 is
P = 2A ² [1 + cos (φm)]
It becomes.

  The expression of the intensity P changes in a cosine function in the range of phase difference −π to π as shown in FIG. 15, and there are two phase differences that give the intensity X, φm1 and φm2. Cannot be specified, and the inter-symbol phase difference cannot be obtained correctly.

  Further, in the optical phase modulation evaluation apparatus having the above configuration, when the speed of the data signal that modulates the phase-modulated light S becomes high (for example, several tens of GHz or more), the intensity of the light S4 also changes at high speed. The response characteristic of the converter becomes insufficient, and the intensity of the light S4 cannot be accurately measured.

  An object of the present invention is to provide an optical phase modulation evaluation apparatus that can solve these problems all at once and can correctly measure the inter-symbol phase difference even for phase modulated light modulated by a high-speed data signal. It is said.

In order to achieve the above object, an optical phase modulation evaluation apparatus according to claim 1 of the present invention includes:
Demultiplexing means (21) for demultiplexing phase-modulated light (S) phase-modulated at a predetermined symbol rate into first phase-modulated light (Sa) and second phase-modulated light (Sb);
The first phase-modulated light is demultiplexed by the demultiplexing means (31), one of them is delayed by one symbol by the delay unit (32), and is multiplexed with the other by the multiplexing means (33). A first delay interference unit (30) that converts the phase-modulated light into first intensity-converted light (Sa4) having an amplitude corresponding to the cosine value of the optical phase difference between the symbols of the first phase-modulated light;
The second phase-modulated light is demultiplexed by the demultiplexing means (41), one of which is delayed by one symbol by the delay unit (42), and the other of the phase-demultiplexed and demultiplexed phase-modulated light The optical phase of any one of the phase-modulated light is shifted by π / 2 by the phase shifter (43) and multiplexed by the multiplexing means (44), and the second phase-modulated light is symbolized by the second phase-modulated light. A second delay interference unit (40) for converting into second intensity converted light (Sb4) having an amplitude corresponding to the sine value of the optical phase difference between
Sampling optical pulse generating means (50) for generating a sampling optical pulse having a period equal to an integer multiple of a symbol clock of the phase-modulated light or a period different from the integer multiple by a predetermined offset time; ,
A first optical sampling unit (60) for sampling the first intensity-converted light by the sampling optical pulse;
A second optical sampling unit (70) for sampling the second intensity-converted light by the sampling optical pulse;
A first photoelectric converter (81) that receives the first pulsed light emitted from the first optical sampling unit;
A second photoelectric converter (82) for receiving the second pulsed light emitted from the second optical sampling unit;
A first A / D converter (83) for sampling an output signal (Ea) of the first photoelectric converter with a sampling signal synchronized with the sampling optical pulse and converting it into a digital value;
A second A / D converter (84) for sampling the output signal (Eb) of the second photoelectric converter with a sampling signal synchronized with the sampling optical pulse and converting it into a digital value;
An operation for obtaining a phase shift amount between symbols of the phase-modulated light based on the output value (Da) of the first A / D converter and the output value (Db) of the second A / D converter. And a processing unit (90).

An optical phase modulation evaluation apparatus according to claim 2 of the present invention is the optical phase modulation evaluation apparatus according to claim 1,
The first delay interference unit and the second delay interference unit have a demultiplexing unit, a multiplexing unit, and a delay unit in common.

An optical phase modulation evaluation apparatus according to claim 3 of the present invention is the optical phase modulation evaluation apparatus according to claim 1 or 2,
The first optical sampling unit and the second optical sampling unit are
The intensity-converted light and the sampling light pulse are incident on the nonlinear optical crystal (161), and the intensity-converted light is sampled.

An optical phase modulation evaluation apparatus according to claim 4 of the present invention is the optical phase modulation evaluation apparatus according to claim 1 or 2,
The first optical sampling unit and the second optical sampling unit are
A multiplexing means (165) for multiplexing the intensity-converted light and the sampling light pulse;
An electroabsorption optical modulator (164) for receiving the light emitted from the multiplexing means;
The electric field absorption modulator has a high absorptance during a period when the sampling optical pulse is not incident, and the electric field so that the absorptivity of the electroabsorption modulator decreases when the sampling optical pulse is incident. A DC power source (166) for applying a predetermined DC voltage to the absorption modulator;
And a wavelength filter (167) for extracting the intensity-converted light component from the light emitted from the electroabsorption modulator.

An optical phase modulation evaluation apparatus according to claim 5 of the present invention is the optical phase modulation evaluation apparatus according to claim 1 or 2,
The first optical sampling unit and the second optical sampling unit are
The intensity-converted light and the sampling light pulse are incident on the carbon nanotube element (170), and the intensity-converted light is sampled.

  As described above, in the optical phase modulation evaluation apparatus of the present invention, the first delay interference unit that converts the first phase modulated light into the first intensity converted light having the amplitude corresponding to the cosine value of the optical phase difference between the symbols, And a second delay interference unit that converts the second phase modulated light into a second intensity converted light having an amplitude corresponding to the sine value of the optical phase difference between the symbols, and is emitted from these two delay interference units. The intensity-converted light is sampled with an optical pulse of an integer multiple of the symbol clock or an optical pulse with a period that differs from the integer multiple by the offset time, and the optical pulse signal obtained by the sampling is photoelectrically converted, Since the phase difference is obtained, the phase difference between symbols can be obtained accurately even with high-speed modulated light.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows the configuration of an optical phase modulation evaluation apparatus 20 to which the present invention is applied.

  The optical phase modulation evaluation apparatus 20 demultiplexes the phase-modulated light S, which has been phase-modulated at a predetermined symbol rate, into a first phase-modulated light Sa and a second phase-modulated light Sb by using a demultiplexer 21, and the first phase The modulated light Sa enters the first delay interference unit 30, and the second phase modulated light Sb enters the second delay interference unit 40.

  The first delay interfering unit 30 is inserted into the demultiplexer 31 that divides the first phase-modulated light Sa into two light beams Sa1 and Sa2 and emits them to the optical paths La1 and La2, respectively, and one optical path La2 side. A delay 32 that gives Sa2 a delay corresponding to one symbol (corresponding to one bit in the case of the DPSK system), a light Sa3 emitted from the delay 32 and a light Sa1 in the other optical path La1 are combined and And a multiplexer 33 for emitting the light obtained by the multiplexing as the first intensity-converted light Sa4.

  The optical conditions (substantial optical path lengths) of the optical paths La1 and La2 excluding the delay unit 32 of the first delay interference unit 30 are equal, and the delay amount of the delay unit 32 matches the cycle of the symbol clock. In the case of the DPSK method, the light Sa1 and Sa3 combined by the multiplexer 33 are theoretically adjusted so that they are completely combined in phase or in phase.

  The second delay interfering unit 40 is inserted on the side of one optical path Lb2 and the demultiplexer 41 that branches the second phase-modulated light Sb into the two light beams Sb1 and Sb2, and outputs them to the optical paths Lb1 and Lb2, respectively. A delay unit 42 that gives a delay equivalent to one symbol (corresponding to one bit in the case of the DPSK system), and a phase shifter 43 that shifts the optical phase of the light Sb3 emitted from the delay unit 42 by π / 2, A multiplexer 44 that multiplexes the light Sb3 ′ emitted from the phase shifter 43 and the light Sb1 on the other optical path Lb1 and emits the light obtained by the combination as the second intensity converted light Sb4. Have.

  The optical conditions (substantial optical path lengths) of the two optical paths Lbl and Lb2 excluding the delay device 42 and the phase shifter 43 of the second delay interference unit 40 are the optical paths of the first delay interference unit 30. It is equal to La1 and La2, and the delay amount of the delay unit 42 coincides with the cycle of the symbol clock, and the phase shift amount of the phase shifter 43 is exactly π / 2 with respect to the wavelength of the phase-modulated light S. In the case of the DPSK method, it is assumed that the light Sb1 and Sb3 ′ combined by the optical unit 44 are adjusted so as to be combined with a completely different phase by π / 2.

Therefore, the intensity Pa of the first intensity-converted light Sa4 emitted from the first delay interference unit 30 is the same as that of the conventional device described above.
Pa = 2 · A ² [1 + cos (φm)] (1)
As shown in FIG. 2, the intensity Pa changes in a cosine function around 2 · A ² with respect to the change in the inter-symbol phase difference φm.

On the other hand, since the second delay interference unit 40 is multiplexed with a phase shift of π / 2 by the phase shifter 43 in addition to the delay by the delay unit 42, the second delay interference unit 40 The intensity Pb of the emitted second intensity converted light Pb4 is:
Pb = 2 · A ² [1 + cos (φm + π / 2)]
= 2 · A ² · [1 + sin (φm)] (2)
As shown in FIG. 2, the intensity Pb changes sinusoidally around 2 · A ² with respect to the change in the inter-symbol phase difference φm.

Thus, for example, as shown in FIG. 2, Pa = 2 two phases giving X FM1, if φm2 is present, if the intensity Pb center value 2 · A ² or less, the inter-symbol phase can be identified with FM1 If the intensity Pb is larger than the center value 2 · A ² , the intersymbol phase can be specified as φm2.

Further, when Pa = X = 4A ² , the intersymbol phase can be specified as 0, and when Pa = X = 0, the intersymbol phase can be specified as π (or −π).

  However, the actually obtained intensity values are the output values Ia and Ib of the photoelectric converter, and the output values Ia and Ib are proportional to the incident light intensity.

  Therefore, the relationship between the output value of the photoelectric converter and the inter-symbol phase difference at an arbitrary incident light intensity is obtained, and the output value and incident light intensity of the photoelectric converter obtained when the unknown phase-modulated light is incident. From this, the phase between symbols can be specified.

  As described above, it is desirable to share the optical system of the two delay interference units 30 and 40 as a configuration example for matching the optical conditions of the two delay interference units 30 and 40.

  For example, as shown in FIG. 3, the phase-modulated light S is divided into a first phase-modulated light Sa and a second phase along a parallel optical axis by a duplexer 21 composed of a half mirror 21a and three flat mirrors 21b to 21d. Divided into modulated light Sb, it enters a half mirror 101 having an inclination of 45 ° with respect to both optical axes.

  The first phase-modulated light Sa is divided into light Sa1 that is reflected by the half mirror 101 and light Sa2 that is transmitted through the half mirror 101, and one of the lights Sa1 is along an optical axis shifted in parallel by the orthogonal mirror 102. Is then folded back to the half mirror 101.

  The light Sa2 enters the delay device 110, the light Sa3 delayed by one symbol by the delay device 110 enters the orthogonal mirror 103, and is orthogonal to the optical axis of the light Sa1 reflected by the orthogonal mirror 102. The first intensity converted light Sa4 is obtained by folding back to the half mirror 101 along the optical axis to be combined with the light Sa1.

  On the other hand, the second phase modulated light Sb incident on the half mirror 101 in parallel with the first phase modulated light Sa is divided into light Sb1 reflected by the half mirror 101 and light Sb2 transmitted through the half mirror 101. The light Sb1 is folded back to the half mirror 101 by the orthogonal mirror 102.

  The light Sb2 enters the delay device 110, the light Sb3 delayed by one symbol is turned back by the orthogonal mirror 103, and the optical phase is shifted by π / 2 by the phase shifter 43. Then, the light Sb3 ′ emitted from the phase shifter 43 is incident on the half mirror 101 along the optical axis perpendicular to the optical axis of the light Sb1 turned back by the orthogonal mirror 102, and is combined with the light Sb1. Second intensity converted light Sb4 is obtained.

  In this configuration example, the half mirror 101 also serves as the duplexer 31 and the multiplexer 33 of the first delay interference unit 30 and the duplexer 41 and the multiplexer 44 of the second delay interference unit 40. Since the delay device 110 also serves as the delay devices 32 and 42, it is easy to equalize the optical conditions, and the configuration itself can be simplified.

  Moreover, you may comprise as FIG. In this configuration, the phase-modulated light S is divided into a first phase-modulated light Sa and a second phase-modulated light Sb along a parallel optical axis by a duplexer 21 composed of a half mirror 21a and three flat mirrors 21b to 21d. Then, the light enters the half mirror 201 having an inclination of 45 ° with respect to both optical axes.

  The first phase-modulated light Sa is divided into light Sa1 that is reflected by the half mirror 201 and light Sa2 that is transmitted through the half mirror 201, and one of the lights Sa1 is a flat plate mirror arranged in parallel to the half mirror 201. The light is incident on 202, reflected at right angles, and emitted to the half mirror 203 along the optical axis parallel to the light Sa2.

  The light Sa2 is incident on the delay device 110, and the light Sa3 delayed by one symbol by the delay device 110 is incident on the flat mirror 204 arranged parallel to the half mirror 201 and reflected at a right angle. Then, the light is incident on the half mirror 203 along the optical axis orthogonal to the optical axis of the light Sa1 reflected by the flat mirror 202, and is combined with the light Sa1 to obtain the first intensity-converted light Sa4.

  On the other hand, the second phase-modulated light Sb incident on the half mirror 201 in parallel with the first phase-modulated light Sa is divided into light Sb1 reflected by the half mirror 201 and light Sb2 transmitted through the half mirror 201. Similar to the light Sa1, Sb1 enters the flat mirror 202, is reflected at a right angle, and is emitted to the half mirror 203 along the optical axis parallel to the light Sb2.

  The light Sb2 enters the delay device 110, and the light Sb3 delayed by one symbol by the delay device 110 enters the phase shifter 43. Then, the light Sb 3 ′ emitted from the phase shifter 43 is incident on the flat mirror 204, reflected at right angles, and reflected to the half mirror 203 along the optical axis perpendicular to the optical axis of the light Sb 1 reflected by the flat mirror 202. Incident light is combined with the light Sb1 to obtain the second intensity-converted light Sb4.

  In the configuration example of FIG. 4, the half mirror 201 is also used as the duplexers 31 and 41, the delay device 110 is also used as the delay devices 32 and 42, and the half mirror 203 is also used as the multiplexers 33 and 44. Similarly to the above configuration example, it is easy to equalize the optical conditions, and the configuration itself can be simplified.

  In order to obtain the intersymbol phase φm as described above, it is necessary to measure the intensities of the intensity-converted lights Sa4 and Sb4 obtained by the two delay interference units 30 and 40. In the configuration where the intensity is obtained by directly entering the converter, the response speed of the photoelectric converter is insufficient with respect to the intensity-converted lights Sa4 and Sb4 obtained from the light phase-modulated at a high speed with a clock of several tens of GHz or more. The signal corresponding to the waveform cannot be obtained accurately.

  Therefore, in the present invention, the equivalent time sampling by the optical pulse is performed on the intensity-converted lights Sa4 and Sb4.

  That is, the sampling optical pulse generator 50 generates a predetermined value for an integer N times the symbol clock period Tc of the phase-modulated light S (in the case of the DPSK method, the clock period of the data signal that modulates the phase-modulated light S). A sampling optical pulse Sp having a period Ts that is different by the offset time ΔT (which may be an integer multiple of the symbol clock period when a series of waveform information is not required) is generated, and the first optical signal is passed through the demultiplexer 51. This is given to the optical sampling unit 60 and the second optical sampling unit 70.

  The configuration of the sampling optical pulse generator 50 is arbitrary as long as it can generate a narrow sampling optical pulse with a specified period Ts.

  FIG. 5 shows an example of this. A stable signal Ra having a period Ts (frequency Fs) designated by the arithmetic processing unit 90 is generated by a reference signal generator 50a having a synthesizer configuration and input to a multiplier 50b. The output signal Rb is input to the optical modulator 50c, and the continuous light Scw emitted from the light source 50d is modulated to generate an optical pulse Spa having a period Ts / M. The pulse width of the optical pulse Spa is narrowed to 1 / M compared to the case where the continuous light Scw is directly modulated with the signal Ra.

  Then, this optical pulse Spa is thinned to 1 / M by the optical gate circuit 50e to generate an optical pulse Spb having a period Ts, which is incident on the dispersion reducing fiber 50f to further narrow the pulse width, and the sampling optical pulse Sp To be emitted.

  The sampling optical pulse Sp is branched into two by the duplexer 51, one of which is incident on the first optical sampling unit 60 and the other is incident on the second optical sampling unit 70.

  The first optical sampling unit 60 and the second optical sampling unit 70 sample the intensity-converted light beams Sa4 and Sb4 with the sampling optical pulses Sp1 and Sp2, respectively, and output the optical pulse signals Sa5 and Sb5 obtained by the sampling. Each is emitted.

  As the sampling elements of the optical sampling units 60 and 70, nonlinear optical crystals, optical gate devices, electroabsorption optical modulators, carbon nanotubes (CNT), or the like can be used.

  FIG. 6 uses a nonlinear optical crystal 161, and the polarization states of the intensity-converted light Sa4 (or Sb4) and the sampling light pulse Sp1 (or Sp2) are set to predetermined polarization directions by the polarization controllers 162 and 163, respectively. Then, the light enters the nonlinear optical crystal 161 having polarization dependency, and emits the optical pulse signal Sa5 (or Sb5) having a sum frequency generated by the nonlinearity.

  FIG. 7 uses an electroabsorption optical modulator 164. The intensity-converted light Sa4 (or Sb4) and the sampling optical pulse Sp1 (or Sp2) are combined by the multiplexer 165 to be electroabsorption type. The light is incident on the light modulator 164. The electroabsorption optical modulator 164 has a high light absorptance during the period when the sampling light pulse Sp1 (or Sp2) is not incident, and absorbs the light when the sampling light pulse Sp1 (or Sp2) is incident. Since a predetermined DC voltage is applied from the DC power source 166 so as to reduce the rate, the intensity-converted light Sa4 (or Sb4) passes only at the timing when the sampling light pulse Sp1 (or Sp2) is incident. Therefore, the optical pulse signal Sa5 (or Sb5) can be obtained by extracting only the wavelength component of the intensity-converted light Sa4 (or Sb4) from the emitted light by the wavelength filter 167.

  FIG. 8 uses a carbon nanotube element (hereinafter referred to as a CNT element) 170. In the CNT element 170, a CNT material made of a tube-like carbon crystal is applied to, for example, a glass or resin substrate surface, mixed with a polymer solvent and solidified into a plate shape, or the core portion contains the CNT material. This is a saturable absorption device having an extremely fast recovery time using a fiber or the like as an element member, and receives intensity-converted light Sa4 (or Sb4) at one end side through an optical circulator 171 and from the other end side through an optical circulator 172. The optical pulse signal Sa5 (or Sb5) is obtained from the optical circulator 172 by reducing the absorptance with respect to the intensity-converted light Sa4 (or Sb4) only when the sampling optical pulse Sp1 (or Sp2) is incident. 173 is a terminator). In this example, the sampling light pulse and the intensity-converted light are incident on the CNT element 170 from opposite directions, but both light are incident in the same direction from one end and emitted from the other end. It is also possible to obtain an optical pulse signal by extracting only the wavelength component of the intensity-converted light from the obtained light using a wavelength filter.

  The above configuration is the same for the two optical sampling units 60 and 70. For example, the intensity-converted lights Sa4 and Sb4 shown in FIGS. 9A and 9B and the sampling optical pulse Sp1 shown in FIG. , Sp2 are subjected to optical pulse signals Sa5 and Sb5 as shown in (d) and (e) of FIG.

  The optical pulse signals Sa5 and Sb5 are incident on the photoelectric converters 81 and 82, respectively, and converted into electric pulse signals Ea and Eb as shown in (f) and (g) of FIG. 83 and 84.

  The A / D converters 83 and 84 receive the sampling signals Es1 and Es2 synchronized with the sampling optical pulses Sp1 and Sp2, sample the respective peak values of the electric pulse signals Ea and Eb, and display the sample values as shown in FIG. Are converted into digital data values Da and Db as shown in (h) and (i) of FIG.

  The arithmetic processing unit 90 stores the data values Da and Db in the internal memory (not shown) as the instantaneous intensity data of the intensity converted light Sa4 (or Sb4), and the above-described method, that is, the two data values Da and Db, The inter-symbol phase φm is identified from the relationship between the output value of the photoelectric converter and the inter-symbol phase difference at an arbitrary incident light intensity obtained, and the histogram, constellation, and time waveform of the inter-symbol phase difference (eye Pattern).

  In order to perform such processing, the arithmetic processing unit 90 includes a phase difference table 91, an inter-symbol phase difference calculating unit 92, a histogram calculating unit 93, a constellation calculating unit 94, and an eye pattern calculating unit 95. .

  In the phase difference table 91, the data relating the output value of the photoelectric converter at the arbitrary incident light intensity described above, and the inter-symbol phase difference φm and the magnitudes Da and Db of the electric pulse signals Ea and Eb are related. Data (data in FIG. 2) is stored in advance, and the inter-symbol phase difference calculating unit 92 obtains the inter-symbol phase difference φm of the incident phase-modulated light S based on such information.

  Further, the histogram calculation means 93 obtains a histogram of the inter-symbol phase difference as shown in FIG. 10 based on the inter-symbol phase difference calculated by the inter-symbol phase difference calculation means 92 and displays it on the display unit 97. Let

  Further, the constellation calculating unit 94 obtains a constellation such as that shown in FIG. 11 based on the inter-symbol phase difference calculated by the inter-symbol phase difference calculating unit 92 and displays it on the display unit 97.

  Further, the eye pattern calculation unit 95 obtains a time waveform (eye pattern) of the inter-symbol phase difference as shown in FIG. 12, for example, based on the inter-symbol phase difference calculated by the inter-symbol phase difference calculation unit 92. Is displayed on the display unit 97. In order to obtain the time waveform data of the inter-symbol phase difference, as described above, the optical sampling unit performs optical sampling at a period different from the integer multiple of the symbol clock period by the offset delay time ΔT. The waveform data at timings substantially different by ΔT time is obtained, the inter-symbol phase difference is obtained for each time, and this is displayed on the time axis.

  Further, when acquiring information of phase positions having different waveforms, it is necessary to give the offset time ΔT as described above. However, when evaluating using data of the same phase position of the waveform, the offset time ΔT = Sampling may be performed at 0, that is, in the optical sampling unit at an integer multiple of the symbol clock period.

  The display modes of the histogram display, constellation display, and eye pattern display can be designated by operating the operation unit 96.

  Also, by specifying information such as the symbol rate and offset time of the phase-modulated light S by the operation unit 96, the arithmetic processing unit 90 obtains the period Ts of the sampling optical pulse, and the sampling optical pulse generator 50 is obtained. Set to

  In the optical phase modulation evaluation apparatus 20 of this embodiment, the accuracy of the inter-symbol phase is determined by the optical elements constituting the first delay interference unit 30 and the second delay interference unit 40 and the arrangement accuracy thereof, and the optical components thereof are optically modulated. Conditions are subject to change.

  Therefore, unmodulated light for calibration (phase difference between symbols is 0) with very little noise and a stable phase is incident at an appropriate interval, and the first delay is set so that the intensity of the first intensity-converted light Sa4 becomes maximum. By adjusting the interference unit 30 and adjusting the second delay interference unit 40 so that the intensity of the second intensity-converted light Sb4 is ½ of that, when the phase-modulated light to be measured is incident Highly accurate measurement can be performed.

  The delay interference units 30 and 40 are adjusted by, for example, inserting a phase shifter similar to the phase shifter 43 on the first delay interference unit 30 side and controlling the amount of phase shift from the outside. You can do it.

  As described above, the optical phase modulation evaluation apparatus 20 according to the embodiment converts the first phase modulated light Sa into the first intensity converted light Sa4 having an amplitude corresponding to the cosine value of the optical phase difference between the symbols. The interference unit 30 and the second delay interference unit 40 that converts the second phase-modulated light Sb into the second intensity-converted light Sb4 having an amplitude corresponding to the sine value of the optical phase difference between the symbols, and these 2 The intensity-converted light beams Sa4 and Sb4 emitted from the two delay interference units 30 and 40 are sampled by an optical pulse having a period different from the integer multiple of the symbol clock by an offset time ΔT with respect to the integral multiple of the symbol clock. Since the obtained optical pulse signal is photoelectrically converted to obtain the inter-symbol phase difference, the inter-symbol phase difference can be obtained accurately even for high-speed modulated light.

The figure which shows the structure of embodiment of this invention The figure which shows the relationship between the phase difference between symbols of embodiment, and a light reception output. The figure which shows the structural example of the common delay interference part of embodiment The figure which shows another structural example of the delay interference part shared by embodiment The figure which shows the structural example of the optical pulse generation part for sampling of embodiment The figure which shows the structural example of the optical sampling part of embodiment The figure which shows another structural example of the optical sampling part of embodiment The figure which shows another structural example of the optical sampling part of embodiment The figure which shows the operation example of embodiment The figure which shows an example of the histogram measurement result of embodiment The figure which shows an example of the constellation measurement result of embodiment The figure which shows an example of the eye pattern measurement result of embodiment Configuration diagram of conventional equipment The figure which shows the operation example of the conventional device The figure which shows the relationship between the phase between symbols of a conventional apparatus, and an output

Explanation of symbols

  DESCRIPTION OF SYMBOLS 20 ... Optical phase modulation evaluation apparatus, 21 ... Demultiplexer, 30 ... First delay interference unit, 31 ... Demultiplexer, 32 ... Delayer, 33 ... Multiplexer, 40 ... First 2 delay interfering part, 41... Demultiplexer, 42... Delay, 43... Phase shifter, 44 ... multiplexer, 50 .. optical pulse generator for sampling, 51. 60... First optical sampling unit, 70... Second optical sampling unit, 81... First photoelectric converter, 82... Second photoelectric converter, 83. 84 ... second A / D converter, 90 ... arithmetic processing unit, 91 ... phase difference table, 92 ... inter-symbol phase difference calculating means, 93 ... histogram calculating means, 94 ... constellation Calculation means, 95 ... Eye pattern calculation means, 96 ... Operation part, 97 ... Display part

Claims (5)

Demultiplexing means (21) for demultiplexing phase-modulated light (S) phase-modulated at a predetermined symbol rate into first phase-modulated light (Sa) and second phase-modulated light (Sb);
The first phase-modulated light is demultiplexed by the demultiplexing means (31), one of them is delayed by one symbol by the delay unit (32), and is multiplexed with the other by the multiplexing means (33). A first delay interference unit (30) that converts the phase-modulated light into first intensity-converted light (Sa4) having an amplitude corresponding to the cosine value of the optical phase difference between the symbols of the first phase-modulated light;
The second phase-modulated light is demultiplexed by the demultiplexing means (41), one of which is delayed by one symbol by the delay unit (42), and the other of the phase-demultiplexed and demultiplexed phase-modulated light The optical phase of any one of the phase-modulated light is shifted by π / 2 by the phase shifter (43) and multiplexed by the multiplexing means (44), and the second phase-modulated light is symbolized by the second phase-modulated light. A second delay interference unit (40) for converting into second intensity converted light (Sb4) having an amplitude corresponding to the sine value of the optical phase difference between
Sampling optical pulse generating means (50) for generating a sampling optical pulse having a period equal to an integer multiple of a symbol clock of the phase-modulated light or a period different from the integer multiple by a predetermined offset time; ,
A first optical sampling unit (60) for sampling the first intensity-converted light by the sampling optical pulse;
A second optical sampling unit (70) for sampling the second intensity-converted light by the sampling optical pulse;
A first photoelectric converter (81) that receives the first pulsed light emitted from the first optical sampling unit;
A second photoelectric converter (82) for receiving the second pulsed light emitted from the second optical sampling unit;
A first A / D converter (83) for sampling an output signal (Ea) of the first photoelectric converter with a sampling signal synchronized with the sampling optical pulse and converting it into a digital value;
A second A / D converter (84) for sampling the output signal (Eb) of the second photoelectric converter with a sampling signal synchronized with the sampling optical pulse and converting it into a digital value;
An operation for obtaining a phase shift amount between symbols of the phase-modulated light based on the output value (Da) of the first A / D converter and the output value (Db) of the second A / D converter. An optical phase modulation evaluation apparatus having a processing unit (90).   2. The optical phase modulation evaluation apparatus according to claim 1, wherein the first delay interference unit and the second delay interference unit share a demultiplexing unit, a multiplexing unit, and a delay unit. The first optical sampling unit and the second optical sampling unit are
3. The optical phase modulation evaluation apparatus according to claim 1, wherein the intensity-converted light and the sampling light pulse are incident on the nonlinear optical crystal (161), and the intensity-converted light is sampled. The first optical sampling unit and the second optical sampling unit are
A multiplexing means (165) for multiplexing the intensity-converted light and the sampling light pulse;
An electroabsorption optical modulator (164) for receiving the light emitted from the multiplexing means;
The electric field absorption modulator has a high absorptance during a period when the sampling optical pulse is not incident, and the electric field so that the absorptivity of the electroabsorption modulator decreases when the sampling optical pulse is incident. A DC power source (166) for applying a predetermined DC voltage to the absorption modulator;
The optical phase modulation evaluation apparatus according to claim 1 or 2, further comprising a wavelength filter (167) for extracting the intensity-converted light component from the light emitted from the electroabsorption modulator. The first optical sampling unit and the second optical sampling unit are
The optical phase modulation evaluation apparatus according to claim 1 or 2, wherein the intensity converted light and the sampling light pulse are incident on the carbon nanotube element (170) to perform sampling on the intensity converted light.

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