Optical signal generation device and optical signal generation method

Abstract

An optical modulator according to the present invention is a Mach-Zehnder interferometer wherein a light is branched and inputted to first and second Mach-Zehnder-type modulators and wherein lights outputted by the respective ones of the first and second Mach-Zehnder-type modulators are combined to generate a QAM signal. First and second drive-signal-applied electrodes change the optical path lengths of first and second arms of the first Mach-Zehnder-type modulator in accordance with first and second drive signals, respectively, which are multivalued electric signals. Third and fourth drive-signal-applied electrodes change the optical path lengths of first and second arms of the second Mach-Zehnder-type modulator in accordance with third and fourth drive signals, respectively, which are multivalued electric signals. First to fourth drive signal generating units generate and apply the first to fourth drive signals to the first to fourth drive-signal-applied electrodes, respectively. The first to fourth drive signals are mutually independent addition signals.

Description

[Technical Field]

[0001] The present invention relates to an optical signal generation device and an optical signal generation method. [Background technology]

[0002] In high-speed, high-capacity optical transmission systems, optical QAM (Quadrature Amplitude Modulation), which incorporates information into multi-level signals, particularly optical phase and optical intensity, is widely used. To increase transmission capacity, it is crucial to maximize the multi-level nature of the optical QAM signal.

[0003] IQ optical modulators are widely used to generate optical QAM signals. Figure 7 shows the structure of an IQ optical modulator 90 and its drive system used in a conventional optical modulation signal generation device 9. Typically, an IQ optical modulator is configured in a nested structure in which two Mahtzehnder modulators are placed on each of the two arms of another Mahtzehnder interferometer. The IQ optical modulator 90 shown in Figure 7 has a configuration in which one Mahtzehnder modulator 91 is placed on each of the two arms of a Mahtzehnder interferometer 20.

[0004] Continuous wave (CW) light is input to the input terminal of the IQ optical modulator 90. The branching section 21 of the Mahatzehnder interferometer 20 branches the input CW light and outputs each of the branched CW lights to the Mahatzehnder modulators 91 of each arm. One of the two Mahatzehnder modulators 91 is used to generate an in-phase optical signal, and the other is used to generate a quadrature-phase optical signal. In this application, the former is referred to as the I signal Mahatzehnder modulator 91a, and the latter as the Q signal Mahatzehnder modulator 91b. Furthermore, the output optical field of the I signal Mahatzehnder modulator 91a and the Q signal Mahatzehnder modulator 91b are each E I ,E Q This is defined as follows: The photoelectric field E of the Mahatzehnder modulator 91a for the I signal. I and the photoelectric field E of the Mahtzehnder modulator 91b for the Q signal QThese signals are combined by the multiplexer 22 of the Mahatzehnder interferometer 20 to generate an optical QAM signal.

[0005] Here, precise adjustment is required for the optical path difference between the two arms of the two Mahatzehnder modulators 91 and the optical path difference between the two arms of the Mahatzehnder interferometer 20. That is, if the wavelength of the CW light is λ, then at the moment when no drive signal is applied, the optical path difference ΔL of the two arms of the I signal Mahatzehnder modulator 91a is λ / 2, and the optical path difference ΔL of the two arms of the Q signal Mahatzehnder modulator 91b is also λ / 2, and E I and E Q The optical path difference ΔL between the two arms of the Mahtzehnder interferometer 20 that combines the two signals is λ / 4. The optical path difference referred to here is not the difference in geometrical optical path length. The optical phase shift in the multiplexing section 22 of the Mahtzehnder interferometer 20, and the delay caused by the change in refractive index of the arms that occurs when a drive signal is applied to each Mahtzehnder modulator 91 are also treated as optical path differences.

[0006] Next, we will describe the state in which a drive signal is applied to each Mahatzehnder modulator 91. The optical path difference between the two arms of each Mahatzehnder modulator 91 changes, and the interference intensity also changes, resulting in optical modulation.

[0007] In order to drive the Mahtzehnder modulator 91, electrodes are required to apply the drive signal. There are several types of electrode arrangements, but in this application, we will describe the case where the Mahtzehnder modulator 91 is a dual-drive type Mahtzehnder modulator.

[0008] To apply a drive signal to the input light, a first I-signal drive electrode 12a-1 is positioned on one of the two arms of the I-signal Mahatzehnder modulator 91a, and a second I-signal drive electrode 12a-2 is positioned on the other arm. Opposing signals are applied to the first I-signal drive electrode 12a-1 and the second I-signal drive electrode 12a-2. Here, these are each V Ip ,V In This is how it is written. In the case of a lithium niobate type optical modulator, the drive signal takes the form of positive and negative voltages, VIp = -V In In the case of a semiconductor modulator, V Ip and V In An offset voltage is added to both of them to make it only a positive voltage or only a negative voltage. In either case, V Ip , V In changes moment by moment according to the pattern of the signal to be generated. In the prior art, when the optical path length of one arm increases due to V Ip , the optical path length of the other arm decreases due to V In , and when the optical path length of one arm decreases due to V Ip , the optical path length of the other arm increases due to V In (see, for example, Patent Document 1).

[0009] V Ip and V In In generating V Ip and V In , first, the DAC (Digital Analog Converter) 93a for the I signal generates a multi-valued electrical signal. The output of the DAC 93a for the I signal is amplified by the differential amplifier 94a for the I signal, and V Ip and V In [[ID=3​​​​​​​​​​​​​​​​​​​​​​​​​​​​ The same configuration is used for the Q signal Mahatzehnder modulator 91b. That is, the Q signal DAC 93b generates a multi-level electrical signal, and the Q signal differential amplifier 94b amplifies the output from the Q signal DAC 93b to V Qp ,V Qn Generates V Qp =-V Qn That is the case.

[0011] Here, when the DAC93a for the I signal and the DAC93b for the Q signal generate independent electrical signals of value n, the number of symbols in the optical QAM signal generated by the IQ optical modulator 90 is n. 2 This is the result.

[0012] Figure 8 shows the photoelectric field E when n is 4 at 100% swing. I ,E Q And the photoelectric field E output from the IQ optical modulator 90 I +E Q This is a diagram showing and on the complex plane. Figure 8(a) is E I Figure 8(b) shows E Q Figure 8(c) shows E I +E Q This shows that the vertical and horizontal axes represent E at 100% swing. I and E Q It is standardized so that the value is a maximum of ±2. I +E Q A 16-value QAM is generated by the vector sum of E. Figure 8(c) shows E I +E Q Of the symbols, the one furthest from the origin is 2x2 0.5 It is located at this position. It should be noted here that, in the case of a 100% swing, the 16-value symbols are not evenly distributed. This is due to the sinusoidal characteristics of the Mahatzehnder interferometer. To evenly distribute each symbol, the drive amplitude is usually set to less than 100% swing (see, for example, Non-Patent Document 1).

[0013] Figure 9 shows the photoelectric field E when n is 4, with the drive amplitude reduced to half (50% swing). I , E QAnd the photoelectric field E output from the IQ optical modulator 90 I +E Q This is a diagram showing and on the complex plane. Figure 9(a) is E I Figure 9(b) shows E Q Figure 9(c) shows E I +E Q This is shown in Figure 9(c). From Figure 9(c), it can be seen that each symbol of the 16-value QAM is almost equal. However, compared to Figure 8(c), each symbol of the QAM is closer to the origin, and as a result the optical power of the optical QAM signal is reduced. In other words, the modulation loss is increased.

[0014] Up to this point, the explanation has assumed that the symbols of the QAM signal are arranged in a grid, and the probability of each symbol's existence has not been specifically mentioned. Symbols far from the origin of the complex plane have high light intensity and are prone to degradation due to nonlinear optical effects within the optical transmission path. Therefore, configurations have been proposed in which symbols are not placed at positions far from the origin of the complex plane, or the probability of symbols located far from the origin being selected is reduced. In such configurations, it is already known that it is desirable for the probability distribution of the existence of symbols with respect to the distance from the origin to be a Gaussian distribution (see, for example, Non-Patent Document 2). [Prior art documents] [Patent Documents]

[0015] [Patent Document 1] Patent No. 5261779 [Non-patent literature]

[0016] [Non-Patent Document 1] H. Kawakami et al., “Auto bias control and bias hold circuit for IQ-modulator in flexible optical QAM transmitter with Nyquist filtering,” OpticsExpress, Vol.22, No.23, pp.28163-28168, 2014. [Non-Patent Document 2] Zhen Qu et al., “On the Probabilistic Shaping and Geometric Shaping in Optical Communication Systems,” IEEE Access, vol.7, pp.21454-21464, 2019. [Overview of the Initiative] [Problems that the invention aims to solve]

[0017] The conventional technology shown in Figures 7-9 presents the following problems. First, increasing the number of levels requires increasing the resolution of the I signal DAC and the Q signal DAC. However, maintaining the linearity of the DAC and increasing the resolution while maintaining a high operating speed presents difficulties in circuit design.

[0018] Next, even at 100% swing, the magnitude of the optical field is limited, as shown in Figure 8. Therefore, the optical power of the optical QAM signal is limited, and the optical signal-to-noise ratio is also limited. In particular, as shown in Figure 9, when the drive amplitude is reduced to increase linearity, the modulation loss increases, and the optical signal-to-noise ratio deteriorates further.

[0019] Furthermore, in conventional technology, in order to make the probability distribution of the N symbols in the generated N-value QAM signal approximate a Gaussian distribution, the resolution M of the DAC is set high, and the number of M that can be generated is also high. 2The problem was that it required complex processing, such as making N selections from the positions of individual symbols (where M > N), or increasing the frequency of selecting symbols near the origin when mapping data to symbols.

[0020] In view of the above circumstances, the present invention aims to provide an optical signal generation device and an optical signal generation method that can generate optical QAM signals having more symbols without using a drive system that is difficult to manufacture. [Means for solving the problem]

[0021] An optical signal generation apparatus according to one aspect of the present invention includes an optical modulator which is a Mahatzehnder interferometer that splits light, inputs the split light to a first Mahatzehnder optical modulator and a second Mahatzehnder optical modulator, and combines the light output by the first Mahatzehnder optical modulator and the light output by the second Mahatzehnder optical modulator to generate a QAM signal; a first drive signal application electrode that changes the optical path length of the first arm of the first Mahatzehnder optical modulator according to a first drive signal which is a multi-level electrical signal; a second drive signal application electrode that changes the optical path length of the second arm of the first Mahatzehnder optical modulator according to a second drive signal which is a multi-level electrical signal; and the first arm of the second Mahatzehnder optical modulator The device comprises a third drive signal application electrode that changes the optical path length according to a third drive signal which is a multi-level electrical signal; a fourth drive signal application electrode that changes the optical path length of the second arm of the second Mahatzehnder type optical modulator according to a fourth drive signal which is a multi-level electrical signal; a first drive signal generation unit that generates the first drive signal and applies it to the first drive signal application electrode; a second drive signal generation unit that generates the second drive signal and applies it to the second drive signal application electrode; a third drive signal generation unit that generates the third drive signal and applies it to the third drive signal application electrode; and a fourth drive signal generation unit that generates the fourth drive signal and applies it to the fourth drive signal application electrode, wherein the first drive signal, the second drive signal, the third drive signal, and the fourth drive signal are mutually independent multi-level signals.

[0022] An optical signal generation apparatus according to one aspect of the present invention comprises a Mahatzehnder optical modulator that branches light, inputs the branched light to a first arm and a second arm, and combines the light output from the first arm and the light output from the second arm to generate a QAM signal; a first drive signal application electrode that changes the optical path length of the first arm according to a first drive signal which is a multi-level electrical signal; a second drive signal application electrode that changes the optical path length of the second arm according to a second drive signal which is a multi-level electrical signal; a first drive signal generation unit that generates the first drive signal and applies it to the first drive signal application electrode; and a second drive signal generation unit that generates the second drive signal and applies it to the second drive signal application electrode, wherein the first drive signal and the second drive signal are mutually independent multi-level signals.

[0023] A method for generating an optical signal according to one aspect of the present invention includes: a branching step of branching light and inputting the branched light to a first Mahatzehnder-type optical modulator and a second Mahatzehnder-type optical modulator; a first drive signal generation step of generating a first drive signal which is a multi-level electrical signal; a second drive signal generation step of generating a second drive signal which is a multi-level electrical signal; a third drive signal generation step of generating a third drive signal which is a multi-level electrical signal; a fourth drive signal generation step of generating a fourth drive signal which is a multi-level electrical signal; a first application step of changing the optical path length of the first arm of the first Mahatzehnder-type optical modulator by a first drive signal application electrode in accordance with the first drive signal; and the second arm of the first Mahatzehnder-type optical modulator The device includes a second application step of changing the optical path length by a second drive signal application electrode in accordance with the second drive signal; a third application step of changing the optical path length of the first arm of the second Mahatzehnder optical modulator by a third drive signal application electrode in accordance with the third drive signal; a fourth application step of changing the optical path length of the second arm of the second Mahatzehnder optical modulator by a fourth drive signal application electrode in accordance with the fourth drive signal; and a combination step of combining the light output by the first Mahatzehnder optical modulator and the light output by the second Mahatzehnder optical modulator to generate a QAM signal, wherein the first drive signal, the second drive signal, the third drive signal, and the fourth drive signal are mutually independent multi-level signals.

[0024] One embodiment of the present invention is an optical signal generation method comprising: a branching step of branching light and inputting the branched light to a first arm and a second arm of a Mahatzehnder type optical modulator; a first drive signal generation step of generating a first drive signal which is a multi-level electrical signal; a second drive signal generation step of generating a second drive signal which is a multi-level electrical signal; a first application step of changing the optical path length of the first arm by a first drive signal application electrode in accordance with the first drive signal; a second application step of changing the optical path length of the second arm by a second drive signal application electrode in accordance with the second drive signal; and a combination step of combining the light output from the first arm and the light output from the second arm to generate a QAM signal, wherein the first drive signal and the second drive signal are mutually independent multi-level signals. [Effects of the Invention]

[0025] This invention makes it possible to generate optical QAM signals with more symbols without using a difficult-to-manufacture drive system. [Brief explanation of the drawing]

[0026] [Figure 1] This figure shows the configuration of the optical modulation signal generation device according to the first embodiment. [Figure 2] This figure shows the optical electric field in the IQ optical modulator of the first embodiment. [Figure 3] This figure shows the optical electric field in the IQ optical modulator of the second embodiment. [Figure 4] This figure shows the optical electric field in the IQ optical modulator of the third embodiment. [Figure 5] This figure shows the optical field in an IQ optical modulator of a variation of the third embodiment. [Figure 6] This figure shows the optical electric field in the IQ optical modulator of the fourth embodiment. [Figure 7] This diagram shows the configuration of a conventional optical modulation signal generation device. [Figure 8] This diagram shows the optical field in a conventional IQ optical modulator. [Figure 9] This diagram shows the optical field in a conventional IQ optical modulator. [Modes for carrying out the invention]

[0027] Embodiments of the present invention will be described in detail below with reference to the drawings. Note that the same parts are denoted by the same reference numerals in multiple drawings, and their descriptions are omitted. This embodiment relates to an optical signal generation device that generates an optically modulated signal using an external modulator. This embodiment aims to provide a technology that generates an optical QAM signal with more symbols using a lower-resolution DAC, while suppressing the influence of the sine wave characteristics of the modulator and reducing modulation loss. Furthermore, this embodiment aims to provide a technology that allows the probability distribution of symbols with respect to distance from the origin in the complex plane to be freely set, even in a low-resolution DAC.

[0028] (First embodiment) Figure 1 shows the configuration of the optical modulation signal generation device 1 in the first embodiment. The optical modulation signal generation device 1 is an example of an optical signal generation device. In Figure 1, the same parts as those in the conventional optical modulation signal generation device 9 shown in Figure 7 are denoted by the same reference numerals. The optical modulation signal generation device 1 includes an IQ optical modulator 10 and a drive system for the IQ optical modulator 10. The drive system includes a first I signal DAC 13a-1, a second I signal DAC 13a-2, a first Q signal DAC 13b-1, a second Q signal DAC 13b-2, a first I signal amplifier 14a-1, a second I signal amplifier 14a-2, a first Q signal amplifier 14b-1, and a second Q signal amplifier 14b-2.

[0029] The IQ optical modulator 10 is configured with one optical modulator 11 on each of the two arms of the Mahtzehnder interferometer 20. The optical modulator 11 is a Mahtzehnder type optical modulator. The optical modulator 11 used to generate the in-phase optical signal is called the I signal optical modulator 11a, and the optical modulator 11 used to generate the quadrature-phase optical signal is called the Q signal optical modulator 11b. Here, we will explain using the case where the optical modulator 11 is a dual-drive type Mahtzehnder optical modulator as an example. In this case, the I signal optical modulator 11a and the Q signal optical modulator 11b have the same configuration as the I signal Mahtzehnder modulator 91a and the Q signal Mahtzehnder modulator 91b shown in Figure 7. Of the two arms of the I signal optical modulator 11a, a first I signal drive electrode 12a-1 is positioned on one arm, and a second I signal drive electrode 12a-2 is positioned on the other arm. Of the two arms of the Q signal optical modulator 11b, a first Q signal drive electrode 12b-1 is positioned on one arm, and a second Q signal drive electrode 12b-2 is positioned on the other arm.

[0030] With the above configuration, the first I signal drive electrode 12a-1 and the second I signal drive electrode 12a-2 of the I signal optical modulator 11a are supplied with independent drive signals V I1 and V I2 These are each added. The drive signal V is applied to the first I signal drive electrode 12a-1. I1 This is generated by amplifying the output of the first I signal DAC 13a-1 with the first I signal amplifier 14a-1. The drive signal V is applied to the second I signal drive electrode 12a-2. I2 This is generated by amplifying the output of the second I signal DAC 13a-2 with the second I signal amplifier 14a-2.

[0031] The same applies to the optical modulator 11b for the Q signal. That is, the drive signal V applied to the first drive electrode 12b-1 for the Q signal. Q1 This is generated by amplifying the output of the first Q signal DAC 13b-1 with the first Q signal amplifier 14b-1. The drive signal V is applied to the second Q signal drive electrode 12b-2.Q2 This is generated by amplifying the output of the second Q signal DAC 13b-2 with the second Q signal amplifier 14b-2. Therefore, in this embodiment, one IQ optical modulator 10 is used to generate V I1 , V I2 , V Q1 , V Q2 A total of four drive signals are applied, and these are mutually independent multi-level signals.

[0032] Here, we will explain the operation of the IQ optical modulator 10. The IQ optical modulator 10 receives CW light as input. The branching section 21 of the Mahatzehnder interferometer 20 branches the input CW light, outputs one of the branched beams to the I signal optical modulator 11a, and outputs the other branched beam to the Q signal optical modulator 11b.

[0033] The branching section 111a of the I signal optical modulator 11a branches the light input from the branching section 21, outputs one of the branched beams to the arm where the first I signal drive electrode 12a-1 is located, and outputs the other branched beam to the arm where the second I signal drive electrode 12a-2 is located. The first I signal drive electrode 12a-1 receives the drive signal V output by the first I signal amplifier 14a-1. I1 By changing the optical path length of the arm, the second I signal drive electrode 12a-2 receives the drive signal V output by the second I signal amplifier 14a-2. I2 This changes the optical path length of the arm. The multiplexer 112a of the I signal optical modulator 11a combines the light transmitted through each of the two arms and outputs the combined light.

[0034] Similarly, the branching section 111b of the Q signal optical modulator 11b branches the light input from the branching section 21, outputs one of the branched beams to the arm where the first Q signal drive electrode 12b-1 is located, and outputs the other branched beam to the arm where the second Q signal drive electrode 12b-2 is located. The first Q signal drive electrode 12b-1 receives the drive signal V output by the first Q signal amplifier 14b-1. Q1 This changes the optical path length of the arm, and the second Q signal drive electrode 12b-2 receives the drive signal V output by the second Q signal amplifier 14b-2. Q2Change the optical path length of the arm. The multiplexing section 112b of the Q-signal optical modulator 11b multiplexes the lights transmitted through the two arms respectively and outputs the multiplexed light.

[0035] The multiplexing section 22 of the Mach-Zehnder interferometer 20 multiplexes the light output from the multiplexing section 112a of the I-signal optical modulator 11a and the light output from the multiplexing section 112b of the Q-signal optical modulator 11b, and outputs the optical QAM signal generated by the multiplexing from the IQ optical modulator 10.

[0036] When no drive signal is applied at all, the optical path difference ΔL between the two arms of the I-signal optical modulator 11a and the optical path difference ΔL between the two arms of the Q-signal optical modulator 11b are λ / 2, and the optical path difference ΔL between the two arms of the Mach-Zehnder interferometer 20 is λ / 4. Let the output optical electric field of the I-signal optical modulator 11a be E I , and the output optical electric field of the Q-signal optical modulator 11b be E Q . The output optical electric field from the IQ optical modulator 10 is E I + E Q .

[0037] Here, assume that each of the drive signals V I1 , V I2 , V Q1 , V Q2 is all 4-valued and has a 50% swing. FIG. 2 is a diagram showing the optical electric field E I , E Q obtained at this time and the optical electric field E I + E Q output from the IQ optical modulator 10 on the complex plane. FIG. 2(a) shows E I , FIG. 2(b) shows E Q , and FIG. 2(c) shows E I + E Q .

[0038] Since there is no anti-phase property of the drive signal, the optical electric field E I and the optical electric field E Q are not arranged in a straight line on the complex plane. As shown in FIG. 2(a), the optical electric field E IThere are 16 possible combinations, but some overlap on the complex plane, resulting in 13 symbols. As shown in Figure 2(b), the photoelectric field E Q Similarly, 13 symbols are generated for this as well. As shown in Figure 2(c), the optical field E output from the IQ optical modulator 10 I +E Q In this case, the number of independent symbols reaches 169. The output of each DAC, DAC13a-1 for the first I signal, DAC13a-2 for the second I signal, DAC13b-1 for the first Q signal, and DAC13b-2 for the second Q signal, is 4-level, and while using a conventional IQ optical modulator 10, the number of multi-levels increases dramatically compared to the result of 16 multi-levels in the conventional configuration shown in Figure 9. These 169 symbols can be used to operate as a 169-level QAM, or the use of adjacent symbols can be prohibited to operate as a 64-level QAM. In this case, the drive system outputs a drive signal with a value corresponding to the symbols used in operation.

[0039] As shown in Figure 1, the optical modulation signal generation device 1 may have a mapping unit 5, either internally or externally, that maps data to the symbols to be used. The mapping unit 5 controls the drive system to generate drive signals for generating the symbols mapped to the data.

[0040] (Second embodiment) The second embodiment will be described focusing on the differences from the first embodiment. The configuration of the optical modulation signal generation device in the second embodiment is the same as that of the optical modulation signal generation device 1 shown in Figure 1. However, V I1 , V I2 , V Q1 , V Q2 The amplitudes of the four types of drive signals are 100%. Figure 3 shows the optical field E obtained in the IQ optical modulator 10 of this embodiment in this case. I , E Q And the photoelectric field E output from the IQ optical modulator 10 I +E Q This is a diagram showing and on the complex plane. Figure 3(a) is E I Figure 3(b) shows E Q Figure 3(c) shows E I +EQ This indicates.

[0041] As shown in Figure 3(c), at 100% swing, the unequal spacing between symbols becomes noticeable, similar to the example of the conventional optical field shown in Figure 8. However, because the number of symbols is sufficiently large, it is possible to generate a QAM signal with symbols arranged at equal intervals by prohibiting the use of closely spaced symbols. What is noteworthy in Figure 3(c) is that, unlike Figure 8(c), the optical field E I +E Q In this case, the position of the symbol furthest from the origin is 2x2 0.5 This means exceeding E on the complex plane. I and E Q This is because the angle formed with the other element is not necessarily a right angle, and can be an acute angle. Therefore, even when using a conventional IQ optical modulator 10, it is possible to increase the number of levels and reduce the modulation loss.

[0042] Another point worth noting in Figure 3(c) is the photoelectric field E I +E Q In this case, the probability of a symbol existing decreases as the distance from the origin increases. This means that the probability of an optical signal having a high instantaneous intensity decreases. Therefore, when the IQ optical modulator 10 generates 169 QAM values ​​using all 169 symbols, it can reduce optical noise originating from nonlinear optical effects in the transmission path.

[0043] Furthermore, when the IQ optical modulator 10 selects 64 symbols from these 169 symbols to generate a 64-value QAM optical signal, the selection may be configured such that the probability distribution of the 64 symbols with respect to their distance from the origin approaches a Gaussian distribution. The drive system outputs a drive signal to generate the selected symbols. The mapping unit 5 may control the drive system to map the data to the selected symbols and generate a drive signal to generate these symbols.

[0044] (Third embodiment) The third embodiment will be described focusing on the differences from the embodiments described above. In the configuration of the optical modulation signal generation device described in the embodiments described above, when two natural numbers M and N satisfy M≧N, E is expressed on the complex plane. I +E Q By selecting N symbols from the M possible symbols and mapping data to them, an N-value QAM signal is generated. However, no explanation was given for the remaining MN symbols.

[0045] When applying a Nyquist filter to an optical QAM signal, or when using pre-emphasis to correct waveform degradation on the optical transmission path or waveform distortion in electrical circuits, it is necessary to appropriately generate the optical field of the intermediate transition state during the process of changing from one symbol to another. In other words, in order to generate an N-value optical QAM signal using such complex processing, it is essential to be able to generate more types of optical fields than N. Ideally, while the electrical drive signal is discrete and has a finite number of multiple values, it is desirable that the optical phase and optical intensity of the output optical field be selectable to arbitrary values ​​in an analog-like manner. The optical modulation signal generation device of this embodiment has an overwhelmingly large number of types of optical phase and optical intensity of the generated optical signal compared to the number of multiple values ​​of the drive signal. Therefore, it can achieve a state close to the ideal conditions described above.

[0046] The configuration of the optical modulation signal generation device in the third embodiment is the same as that of the optical modulation signal generation device 1 shown in Figure 1. However, V I1 ,V I2 ,V Q1 ,V Q2 The amplitudes of the four types of drive signals are set to 100%, and each drive signal has 8 values. Figure 4 shows the optical field E obtained in the IQ optical modulator 10 of this embodiment in this case. I , E Q And the photoelectric field E output from the IQ optical modulator 10 I +E Q This is a diagram showing and on the complex plane. Figure 4(a) is E I Figure 4(b) shows E Q Figure 4(c) shows E I +E Q This indicates.

[0047] As shown in Figure 4, if the coordinates on the horizontal and vertical axes are within ±2, then E I +E Q The symbols are arranged very densely. Therefore, it can be seen that optical phase and optical intensity can be generated in an analog-like manner.

[0048] (Variation of the third embodiment) When two natural numbers M and N satisfy the condition M > N, E can be expressed in the complex plane. I +E Q By selecting more than N symbols from the M possible symbols and mapping data to them in a many-to-one manner, the optical modulation signal generator can also generate an N-value QAM signal. In this case, it is necessary to strictly define the correspondence rules, but there are the advantages shown below. The configuration of the optical modulation signal generator in this embodiment is the same as that of the optical modulation signal generator 1 of the first embodiment shown in Figure 1.

[0049] The first I signal amplifier 14a-1, the second I signal amplifier 14a-2, the first Q signal amplifier 14b-1, and the second Q signal amplifier 14b-2 are signal amplifiers that generate drive signals. When the output waveform of a signal amplifier is observed at a specific moment, whether its amplitude is large or small depends on the pattern of the signal to be generated. Because there are limits to the linearity of a signal amplifier, if the output changes large, the waveform of the generated drive signal is prone to distortion. Conversely, if the output of the signal amplifier remains almost constant for a long period of time, the waveform of the drive signal is also prone to distortion due to the low-frequency cutoff of the amplifier. However, as in this embodiment, by performing a many-to-one mapping, it is possible to select symbols so that the amount of change in the output waveform of the signal amplifier is always of an appropriate magnitude.

[0050] Furthermore, by performing many-to-one mapping, it becomes possible to prioritize the selection of symbols close to the origin of the complex plane in transmission lines with large optical nonlinear effects, and conversely, to prioritize the selection of symbols farther from the origin of the complex plane in transmission lines with large background noise.

[0051] For example, the mapping unit 5 monitors the output waveforms of the first I signal DAC 13a-1, the second I signal DAC 13a-2, the first Q signal DAC 13b-1, and the second Q signal DAC 13b-2, and selects a symbol to use from among multiple symbols corresponding to the data so that the amount of change in the output waveform is within a predetermined appropriate range. Alternatively, the mapping unit 5 monitors the transmission line and selects a symbol to use from among multiple symbols corresponding to the data based on the optical nonlinear effects or background noise of the transmission line. The mapping unit 5 controls the drive system to generate a drive signal to generate the selected symbol. In this way, the correspondence between data and symbols may be changed depending on the conditions of the transmission line and the signal pattern.

[0052] Figure 5 shows an example of symbol selection. In Figure 5, E is shown on the complex plane. I +E Q Two examples are shown in which 16 symbols are selected from a large number of possible symbols, and the IQ optical modulator 10 generates a QAM signal with the selected 16 values. Figure 5(a) shows four types of drive signals V, similar to those shown in Figure 4. I1 ,V I2 ,V Q1 ,V Q2 The amplitude of each swings to 100% (-V π / 2~+V π The drive signals are set to 8 values ​​( / 2) and each of these drive signals is given 8 values. 4 Only 4096 exist. Figure 5(b) shows 8 4 A first example is shown in which 16 combinations of drive signals are selected, and the IQ optical modulator 10 generates 16QAM using the selected 16 combinations. Here, in Figure 5(c), for the four symbols A to D included in the first quadrant of the complex plane shown in Figure 5(b), V I1 ,V I2 ,V Q1 ,V Q2 The values ​​that can be taken are shown in the table. In this table, the 8 values ​​of the drive signal are represented by 0 to 7, where 0 is -V π / 2, 7 is +V π Corresponds to / 2

[0053] Figure 5(d) shows 8 4 Figure 5(d) shows a second example in which 16 different combinations of drive signals are selected from those shown in Figure 5(b), and the IQ optical modulator 10 generates 16QAM using these selected combinations. In the example shown in Figure 5(d), the IQ optical modulator 10 generates 16QAM that is tilted by 45 degrees to show the degree of freedom in constellation generation, while reducing the maximum distance from the origin compared to the example shown in Figure 5(b). Figure 5(e) shows that for the six symbols A to F included in the first quadrant (including the boundary line) shown in Figure 5(d), V I1 ,V I2 ,V Q1 ,V Q2 The values ​​that can be taken are shown in a table, similar to Figure 5(c). As can be seen from Figure 5, the optical modulation signal generation device 1 of this embodiment can generate a wide variety of QAM signals with extreme flexibility from a drive signal with a level of at most 8 values.

[0054] (Fourth embodiment) In the embodiments described above, a conventional IQ optical modulator was used, and four independent drive signals were applied. However, the optical field E in Figures 4 and 3 I Focusing on this, the photoelectric field E I It can also be used as an irregular optical QAM signal. In this case, there is no need to use the IQ optical modulator 10, and a single Mahatzehnder optical modulator can be used.

[0055] Figure 6 shows the configuration of the optical modulation signal generation device 3 in the fourth embodiment. The optical modulation signal generation device 3 is an example of an optical signal generation device. The optical modulation signal generation device 3 comprises a Mahatzehnder type optical modulator 31, a first signal DAC 33-1, a second signal DAC 33-2, a first signal amplifier 34-1, and a second signal amplifier 34-2. A first signal drive electrode 32-1 is positioned on one of the two arms of the Mahatzehnder type optical modulator 31, and a second signal drive electrode 32-2 is positioned on the other arm. A drive signal V is applied to the first signal drive electrode 32-1. I1This is generated by amplifying the output of the first signal DAC 33-1 with the first signal amplifier 34-1. The drive signal V is applied to the second signal drive electrode 32-2. I2 This is generated by amplifying the output of the second signal DAC33-2 with the second signal amplifier34-2. Drive signal V I1 and V I2 These are mutually independent multi-level signals.

[0056] The branching section 311 of the Mahatzehnder-type optical modulator 31 splits the input CW light, outputs one of the split beams to the arm where the first signal drive electrode 32-1 is located, and outputs the other of the split beams to the arm where the second signal drive electrode 32-2 is located. The first signal drive electrode 32-1 receives the drive signal V output by the first signal amplifier 34-1. I1 The optical path length of the arm is changed, and the second signal drive electrode 32-2 receives the drive signal V output by the second signal amplifier 34-2. I2 The optical path length of the arm is changed by this. The multiplexer 312 of the Mahatzehnder type optical modulator 31 combines the light transmitted through each of the two arms and combines the optical field E I It outputs light.

[0057] The optical modulation signal generation device 3 is nothing more than the optical modulation signal generation device 1 of the first embodiment shown in Figure 1, with the Q signal optical modulator 11b and its drive system removed, and only the I signal optical modulator 11a receiving two types of drive signals. This configuration has the advantage that, although the number of multi-level signals is significantly reduced compared to other embodiments, it is possible to generate an optical QAM signal with a single Mahatzehnder type optical modulator.

[0058] The optical signal generation device of this embodiment makes it possible to generate a QAM signal with more symbols using a DAC with lower resolution compared to the conventional technology, while suppressing the influence of the sinusoidal characteristics of the IQ optical modulator and reducing modulation loss. Furthermore, the optical signal generation device of this embodiment makes it possible to freely set the probability distribution of symbol existence with respect to the distance from the origin of the complex plane, even with a DAC with low resolution.

[0059] According to the embodiment described above, the optical signal generation device comprises an optical modulator, first to fourth drive signal application electrodes, and first to fourth drive signal generation units. The optical signal generation device corresponds, for example, to the optical modulation signal generation device 1 of the embodiment. Also, for example, the optical modulator corresponds to the IQ optical modulator 10 of the embodiment. Also, for example, the first drive signal application electrode corresponds to the first I signal drive electrode 12a-1 of the embodiment, the second drive signal application electrode corresponds to the second I signal drive electrode 12a-2 of the embodiment, the third drive signal application electrode corresponds to the first Q signal drive electrode 12b-1 of the embodiment, and the fourth drive signal application electrode corresponds to the second Q signal drive electrode 12b-2 of the embodiment. Furthermore, for example, the first drive signal generation unit corresponds to the first I signal DAC 13a-1 and the first I signal amplifier 14a-1 of the embodiment, the second drive signal generation unit corresponds to the second I signal DAC 13a-2 and the second I signal amplifier 14a-2 of the embodiment, the third drive signal generation unit corresponds to the first Q signal DAC 13b-1 and the first Q signal amplifier 14b-1 of the embodiment, and the fourth drive signal generation unit corresponds to the second Q signal DAC 13b-2 and the second Q signal amplifier 14b-2 of the embodiment.

[0060] The optical modulator is a Mahatzehnder interferometer that splits light, inputs the split light to a first Mahatzehnder optical modulator and a second Mahatzehnder optical modulator, and combines the light output from the first Mahatzehnder optical modulator and the light output from the second Mahatzehnder optical modulator to generate a QAM signal. The first drive signal application electrode changes the optical path length of the first arm of the first Mahatzehnder optical modulator (of the two arms) according to the first drive signal, which is a multi-level electrical signal. The second drive signal application electrode changes the optical path length of the second arm of the first Mahatzehnder optical modulator (of the two arms) according to the second drive signal, which is a multi-level electrical signal. The third drive signal application electrode changes the optical path length of the first arm of the second Mahatzehnder optical modulator (of the two arms) according to the third drive signal, which is a multi-level electrical signal. The fourth drive signal application electrode changes the optical path length of the second arm of the second Mahatzehnder optical modulator according to the fourth drive signal, which is a multi-level electrical signal. The first drive signal generation unit generates a first drive signal and applies it to the first drive signal application electrode. The second drive signal generation unit generates a second drive signal and applies it to the second drive signal application electrode. The third drive signal generation unit generates a third drive signal and applies it to the third drive signal application electrode. The fourth drive signal generation unit generates a fourth drive signal and applies it to the fourth drive signal application electrode. The first drive signal generation unit, the second drive signal generation unit, the third drive signal generation unit, and the fourth drive signal generation unit each generate the first drive signal, the second drive signal, the third drive signal, and the fourth drive signal, which are multi-level signals independent of each other.

[0061] When natural numbers L, M, and N are M≧L≧N, the optical signal generator may generate a QAM signal with L values ​​on which N types of data are mapped, by matching L symbols selected from the M symbols that the optical electric field combined by the optical modulator can take on the complex plane with N types of data in a one-to-one or many-to-one relationship.

[0062] Furthermore, the L symbols may be selected such that the probability distribution of their existence with respect to the distance from the origin of the complex plane approaches a Gaussian.

[0063] Furthermore, the correspondence between L symbols and N types of data may be changed depending on the conditions of the transmission line or the signal pattern.

[0064] The optical signal generation device comprises a Mahatzehnder-type optical modulator, first and second drive signal application electrodes, and first and second drive signal generation units. For example, the optical signal generation device corresponds to the optical modulation signal generation device 3 of the embodiment, and the Mahatzehnder-type optical modulator corresponds to the Mahatzehnder-type optical modulator 31 of the embodiment. The first drive signal application electrode corresponds to the first signal drive electrode 32-1 of the embodiment, and the second drive signal application electrode corresponds to the second signal drive electrode 32-2 of the embodiment. Also, for example, the first drive signal generation unit corresponds to the first signal DAC 33-1 and the first signal amplifier 34-1, and the second drive signal generation unit corresponds to the second signal DAC 33-2 and the second signal amplifier 34-2. The Mahatzehnder optical modulator splits light, inputs the split light to a first arm and a second arm, and combines the light output from the first arm and the light output from the second arm to generate a QAM signal. The first drive signal application electrode changes the optical path length of the first arm according to the first drive signal, which is a multi-level electrical signal. The second drive signal application electrode changes the optical path length of the second arm according to the second drive signal, which is a multi-level electrical signal. The first drive signal generation unit generates the first drive signal and applies it to the first drive signal application electrode. The second drive signal generation unit generates the second drive signal and applies it to the second drive signal application electrode. The first drive signal generation unit and the second drive signal generation unit generate the first drive signal and the second drive signal, which are mutually independent multi-level signals.

[0065] Although embodiments of this invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and includes designs and the like that do not depart from the spirit of this invention. [Industrial applicability]

[0066] This invention can be applied to optical transmitters that generate optical QAM signals. [Explanation of Symbols]

[0067] 1, 3, 9 Optical Modulation Signal Generator 5. Mapping section 10.90 IQ Optical Modulator 11a Optical modulator for I signal 11b Q signal optical modulator 12a-1 First I signal drive electrode 12a-2 Second I signal drive electrode 12b-1 First Q signal drive electrode 12b-2 Second Q signal drive electrode 13a-1 DAC for the first I signal 13a-2 DAC for the second I signal 13b-1 DAC for the first Q signal 13b-2 DAC for the second Q signal 14a-1 Amplifier for the first I signal 14a-2 Second I signal amplifier 14b-1 Amplifier for the first Q signal 14b-2 Second Q signal amplifier 20. Mahatzehnder Interferometer Branching points 21, 111a, 111b, 311 22, 112a, 112b, 312 Multiplexing section 31. Mahatzehnder type optical modulator 33-1 First signal DAC 33-2 Second signal DAC 34-1 First signal amplifier 34-2 Second signal amplifier 91a Mahatzehnder modulator for I signal 91b Mahatzehnder modulator for Q signal 93a I signal DAC 93b Q signal DAC 94a Differential amplifier for I signals 94b Differential amplifier for Q signal

Claims

1. A Mahatzehnder interferometer optical modulator that splits light, inputs the split light to a first Mahatzehnder optical modulator and a second Mahatzehnder optical modulator, and combines the light output by the first Mahatzehnder optical modulator and the light output by the second Mahatzehnder optical modulator to generate a QAM signal. A first drive signal application electrode changes the optical path length of the first arm of the first Mahatzehnder optical modulator in accordance with a first drive signal, which is a multi-level electrical signal. A second drive signal application electrode that changes the optical path length of the second arm of the first Mahatzehnder optical modulator in accordance with a second drive signal which is a multi-level electrical signal, A third drive signal application electrode changes the optical path length of the first arm of the second Mahatzehnder optical modulator in accordance with a third drive signal, which is a multi-level electrical signal. A fourth drive signal application electrode changes the optical path length of the second arm of the second Mahatzehnder optical modulator in accordance with a fourth drive signal, which is a multi-level electrical signal. A first drive signal generation unit generates the first drive signal and applies it to the first drive signal application electrode, A second drive signal generation unit generates the second drive signal and applies it to the second drive signal application electrode, A third drive signal generation unit generates the third drive signal and applies it to the third drive signal application electrode, The system includes a fourth drive signal generation unit that generates the fourth drive signal and applies it to the fourth drive signal application electrode, The first drive signal, the second drive signal, the third drive signal, and the fourth drive signal are mutually independent multi-level signals. When natural numbers L, M, and N satisfy M ≥ L ≥ N, the optical field obtained by combining the light output by the first Mahatzehnder optical modulator and the light output by the second Mahatzehnder optical modulator is mapped to the L symbols by assigning L symbols selected from the M symbols that can take place on the complex plane to N types of data in a one-to-one or many-to-one correspondence, thereby generating a QAM signal with an L value. Optical signal generation device.

2. A Mahatzehnder-type optical modulator that splits light, inputs the split light to a first arm and a second arm respectively, and combines the light output from the first arm and the light output from the second arm to generate a QAM signal, A first drive signal application electrode that changes the optical path length of the first arm in accordance with a first drive signal which is a multi-level electrical signal, A second drive signal application electrode that changes the optical path length of the second arm in accordance with a second drive signal, which is a multi-level electrical signal, A first drive signal generation unit generates the first drive signal and applies it to the first drive signal application electrode, The system includes a second drive signal generation unit that generates the second drive signal and applies it to the second drive signal application electrode, The first drive signal and the second drive signal are mutually independent multi-level signals. When natural numbers L, M, and N satisfy M ≥ L ≥ N, the optical field obtained by combining the light output from the first arm and the light output from the second arm is mapped to L symbols selected from M symbols that can take place on the complex plane, and N types of data are mapped to L symbols in a one-to-one or many-to-one correspondence with N types of data, so that a QAM signal with an L value is generated. Optical signal generation device.

3. L symbols are selected such that the probability distribution of the existence of the L symbols with respect to the distance from the origin of the complex plane approaches a Gaussian. The optical signal generating apparatus according to claim 1 or claim 2.

4. The correspondence between the L symbols and the N types of data is changed according to the conditions of the transmission line or the signal pattern. The optical signal generating apparatus according to claim 1 or claim 2.

5. A branching step in which light is branched and the branched light is input to a first Mahatzehnder type optical modulator and a second Mahatzehnder type optical modulator, A first drive signal generation step that generates a first drive signal which is a multi-value electrical signal, A second drive signal generation step, which generates a second drive signal that is a multi-value electrical signal, A third drive signal generation step that generates a third drive signal which is a multi-value electrical signal, A fourth drive signal generation step, which generates a fourth drive signal that is a multi-value electrical signal, A first application step in which the optical path length of the first arm of the first Mahatzehnder type optical modulator is changed by a first drive signal application electrode in accordance with the first drive signal, A second application step in which the optical path length of the second arm of the first Mahatzehnder type optical modulator is changed by the second drive signal application electrode in accordance with the second drive signal, A third application step in which the optical path length of the first arm of the second Mahatzehnder optical modulator is changed by a third drive signal application electrode in accordance with the third drive signal, A fourth application step in which the optical path length of the second arm of the second Mahatzehnder type optical modulator is changed by a fourth drive signal application electrode in accordance with the fourth drive signal, The system includes a combining step in which the light output by the first Mahatzehnder optical modulator and the light output by the second Mahatzehnder optical modulator are combined to generate a QAM signal. The first drive signal, the second drive signal, the third drive signal, and the fourth drive signal are mutually independent multi-level signals. When natural numbers L, M, and N satisfy M ≥ L ≥ N, the combined optical field in the combined step further comprises a mapping step in which L symbols selected from M symbols that can be taken on the complex plane by the combined optical field are mapped to N types of data in a one-to-one or many-to-one correspondence, thereby generating a QAM signal with an L value. Optical signal generation method.

6. A branching step in which light is split and the branched light is input to the first arm and the second arm of a Mahatzehnder type optical modulator, A first drive signal generation step that generates a first drive signal which is a multi-value electrical signal, A second drive signal generation step, which generates a second drive signal that is a multi-value electrical signal, A first application step in which the optical path length of the first arm is changed by the first drive signal application electrode in accordance with the first drive signal, A second application step in which the optical path length of the second arm is changed by the second drive signal application electrode in accordance with the second drive signal, The system includes a multiplexing step of combining the light output from the first arm and the light output from the second arm to generate a QAM signal. The first drive signal and the second drive signal are mutually independent multi-level signals. When natural numbers L, M, and N satisfy M ≥ L ≥ N, the combined optical field in the combined step further comprises a mapping step in which L symbols selected from M symbols that can be taken on the complex plane by the combined optical field are mapped to N types of data in a one-to-one or many-to-one correspondence, thereby generating a QAM signal with an L value. Optical signal generation method.

7. In the mapping step, L symbols are selected such that the probability distribution of the distances of the L symbols from the origin of the complex plane approaches a Gaussian. The optical signal generation method according to claim 5 or claim 6.

8. In the mapping step, the correspondence between the L symbols and the N types of data is changed according to the conditions of the transmission line or the signal pattern. The optical signal generation method according to claim 5 or claim 6.

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