Optical signal generation device

Abstract

An optical signal generation device (1) comprises: a first drive signal generation unit that generates a first drive signal having a first voltage amplitude; a second drive signal generation unit that generates a second drive signal having a second voltage amplitude; and a Mach-Zehnder optical modulator (2). The Mach-Zehnder optical modulator has a demultiplexing unit (21) that demultiplexes continuous wave light into first demultiplexed light and second demultiplexed light, a first drive electrode (23-1) that generates, from the first demultiplexed light, a first optical electric field the optical phase of which is modulated by modulating, in accordance with the first drive signal, the optical path length of a first arm (22-1) for transmitting the first demultiplexed light, a second drive electrode (23-2) that generates, from the second demultiplexed light, a second optical electric field the optical phase of which is modulated by modulating, in accordance with the second drive signal, the optical path length of a second arm (22-2) for transmitting the second demultiplexed light, a delay unit (24) that adjusts the optical path difference between the optical path length of the first arm and the optical path length of the second arm on the basis of the wavelength of the continuous wave light, and a multiplexing unit (25) that multiplexes the first optical electric field and the second optical electric field. Each of the maximum value of the first voltage amplitude and the maximum value of the second voltage amplitude is greater than Vπ and less than or equal to 2 Vπ, where Vπ is the half-wavelength voltage of the Mach-Zehnder optical modulator.

Description

Optical signal generation device

[0001] This invention relates to an optical signal generation device.

[0002] High-speed, high-capacity optical transmission systems widely utilize multi-level signals such as optical quadrature amplitude modulation (optical QAM) signals. In optical QAM signals, information is transmitted by modulating the optical phase and optical intensity. To improve transmission capacity, the magnitude of the multi-level function of the optical QAM signal is crucial. Furthermore, uniform distances between symbols in the constellation are also important.

[0003] In-phase and quadrature (IQ) optical modulators are widely used to generate optical QAM signals. Generally, an IQ optical modulator has a nested structure where two Mach-Zehnder type optical modulators are mounted on two arms of a Mach-Zehnder interferometer. In the following, the interferometer that forms the outer shell of this nested structure is defined as the parent Mach-Zehnder interferometer. That is, each of the two arms of the parent Mach-Zehnder interferometer is assumed to have a child Mach-Zehnder type optical modulator.

[0004] Figure 12 shows a first example of the configuration of an optical signal generation device. The optical signal generation device 100a comprises a Mach-Zehnder optical modulator 201, a data generation unit 301, a signal DAC 401, and a signal differential amplifier 501. The Mach-Zehnder optical modulator 201 comprises a demultiplexer 202, two arms 203, two drive electrodes 204, a delay unit 205, and a multiplexer 206. Here, the drive electrode 204-1 is provided on arm 203-1. The drive electrode 204-2 is provided on arm 203-2. The delay unit 205 is provided on arm 203-2, downstream of the drive electrode 204-2.

[0005] The data generation unit 301 generates numerical data representing each voltage of the multi-level digital signal. The signal DAC 401 (Digital-to-Analog Converter) generates an n-level digital signal (electrical signal) with n-level discrete voltages based on this numerical data. The signal DAC 401 inputs the n-level digital signal with n-level discrete voltages to the signal differential amplifier 501. The signal differential amplifier 501 amplifies the voltage of the n-level digital signal. The signal differential amplifier 501 inputs the amplified n-level digital signal as a first drive signal to the drive electrode 204-1. The signal differential amplifier 501 also applies a second drive signal, which is the differential signal of the first drive signal, to the drive electrode 204-2.

[0006] In the optical signal generation device 100a, a Mach-Zehnder type optical modulator 201 generates modulated light. Here, continuous wave (CW) light is input to the input terminal of the Mach-Zehnder type optical modulator 201. The demultiplexer 202 demultiplexes the input continuous wave light into a first demultiplexed light and a second demultiplexed light.

[0007] The optical path difference "ΔL" between arm 203-1 and arm 203-2 is precisely adjusted according to the wavelength "λ" of the continuous wave light. Here, when the first drive signal is applied to the drive electrode 204-1 from the signal differential amplifier 501, the refractive index of a part of arm 203-1 having the drive electrode 204-1 changes, and the optical path length changes, so the delay of the first demultiplexed light also changes. When the second drive signal is applied to the drive electrode 204-2 from the signal differential amplifier 501, the refractive index of a part of arm 203-2 having the drive electrode 204-2 changes, and the optical path length changes, so the delay of the second demultiplexed light also changes. These delays affect the optical path difference "ΔL". In addition, the optical phase shift in the multiplexing section 206 also affects the optical path difference "ΔL". Therefore, the optical path difference "ΔL" is not determined solely by the geometric difference in the length of the optical waveguides.

[0008] The optical path difference "ΔL" may be adjusted so that it becomes "λ / 4" or "-λ / 4" when no drive signal is applied to the drive electrode. In other words, the optical path difference "ΔL" may be adjusted so that the Mach-Zehnder interferometer is biased to a quad point.

[0009] The optical path difference "ΔL" may be adjusted so that it becomes 0 when no drive signal is applied to the drive electrode. In other words, the optical path difference "ΔL" may be adjusted so that the Mach-Zehnder interferometer is biased to the peak point.

[0010] The optical path difference "ΔL" may be adjusted so that it becomes "λ / 2" when no drive signal is applied to the drive electrode. In other words, the optical path difference "ΔL" may be adjusted so that the Mach-Zehnder interferometer is biased to the null point.

[0011] In the following, unless otherwise specified, the Mach-Zehnder interferometer is biased to the null point. In the optical signal generator 100a, the delay unit 205 biases the Mach-Zehnder type optical modulator 201 (Mach-Zehnder interferometer) to the null point.

[0012] In optical modulators using lithium niobate, the drive signal voltage can be either positive or negative. In contrast, in semiconductor optical modulators, the drive signal voltage may have to be either positive or negative. In this case, an offset voltage is added to the drive signal voltage. In the following, unless otherwise specified, lithium niobate is used as an example of the modulator, and no offset voltage is added to the drive signal voltage.

[0013] There are two types of optical modulators using lithium niobate: one that simultaneously modulates each demultiplexed light with a single drive signal for both arms, and another that modulates the demultiplexed light with different drive signals for each arm. In the following, we will use the type of optical modulator that modulates the demultiplexed light with different drive signals for each arm.

[0014] The differential amplifier 501 for signals uses a voltage of "V" １ The drive signal and the voltage "V２ Generate the drive signal of "". The voltage "V １ " and the voltage "V ２ " The relationship is "V ２ = -V １ ". Thus, the voltages of each drive signal are opposite.

[0015] The differential amplifier 501 for signals applies the drive signal of the voltage "V １ " to the drive electrode 204-1. As a result, the refractive index of the arm 203-1 changes partially, and the optical path length is modulated. The modulated optical electric field "E １ " is input to the multiplexing section 206.

[0016] The differential amplifier 501 for signals applies the drive signal of the voltage "V ２ " to the drive electrode 204-2. As a result, the refractive index of the arm 203-2 changes partially, and the optical path length is modulated. The modulations applied to the arm 203-1 and the arm 203-2 are opposite. The modulated optical electric field "E ２ " is input to the delay section 205 and then input to the multiplexing section 206.

[0017] When the optical path difference ΔL between the arm 203-1 and the arm 203-2 is equal to the wavelength "λ" of the continuous light wave, the optical phase difference caused by ΔL corresponds to "2π". The delay section 205 increases the optical path length so that ΔL becomes equal to half of the wavelength of the continuous wave "λ / 2" when no drive signal is applied to each drive electrode, and delays the phase of the optical electric field "E ２ " by π radians with respect to the optical electric field "E １ ". The multiplexing section 206 multiplexes the optical electric field "E １ " output from the drive electrode 204-1 and the optical electric field "E ２ " output from the delay section 205. The multiplexing section 206 outputs the multiplexed optical electric field "E １ +E ２ " as the modulated light.

[0018] The optical electric field "E １ " is expressed as in Equation (1). Also, the optical electric field "E ２ " is expressed as in Equation (2).

[0019] E １ = exp(iωｃ t+iθ+iπ / 2) / 2…(1)

[0020] E ２ =exp(iω ｃ t-iθ-iπ / 2) / 2...(2)

[0021] Here, "ω ｃ " " represents the optical angular frequency of continuous wave light. "t" represents time. "θ" represents the delay in the phase of the optical electric field caused by each driving signal. Voltage "V １ = -V ２ When "= 0" holds true, the phase delay "θ" is defined as 0. In equations (1) and (2), the " / 2" on the right-hand side indicates that when no drive signal is applied and "θ" is 0, the optical phase difference between the first and second demultiplexed light signals, which are demultiplexed from the continuous wave light, is set to π. Note that variables representing the intensity of the continuous wave light and the loss of the arm (optical waveguide) are omitted in each equation because they have little influence.

[0022] Combined optical field (modulated light) "E １ +E ２ The relative intensity of " is expressed as shown in equation (3).

[0023] | E １ +E ２ | ２ =1 / 2-cos(2θ) / 2...(3)

[0024] When no drive signal is applied to the drive electrode, i.e., when the voltage "V" is not applied. １ = -V ２ When "θ" is 0, the phase delay "θ" is 0, and therefore the modulated light disappears. The larger the absolute value of the phase delay "θ", the higher the intensity of the modulated light. When the absolute value of the phase delay "θ" is π radians (voltage "V") １ = -V ２ " is "V π / 2" or "-V π When "V" is 2, the intensity of the modulated light is at its maximum. π This represents the half-wavelength voltage of a Mach-Zehnder interferometer.

[0025] Voltage "V" １ " and voltage "V ２The absolute value of each of the '' is "V π If it is greater than " / 2", the absolute value of the phase delay "2θ" exceeds π radians, so the intensity of the modulated light decreases as the absolute value of the phase delay "θ" increases. For this reason, in the optical signal generator 100a, the voltage "V １ " and voltage "V ２ Each upper limit of "" is "V π The voltage is set to "V" (V) １ " and voltage "V ２ Each lower limit of '' is "-V π It is set as " / 2". Therefore, the maximum value of the voltage amplitude (total width) of each of the two drive signals is "V π It is defined as follows: The voltage amplitude of each of the two drive signals is the half-wavelength voltage "V π The term "100% swing rate" is sometimes used to describe a situation where a swing rate equals 100%.

[0026] As mentioned above, in semiconductor optical modulators, the drive signal voltage "V" １ " and voltage "V ２ An offset voltage may be applied to each of these. In this case, the offset voltage is the voltage "V １ The voltage subtracted from " and the offset voltage are the voltage "V ２ The voltage subtracted from " is set to be inversely related, and the maximum voltage amplitude of each of the two drive signals is set to "V π It is stipulated that "

[0027] Photoelectric field "E" １ +E ２ The optical phase of " is expressed as shown in equation (4), including the optical phase of the modulation component and the optical phase of the carrier component.

[0028] E １ +E ２ =exp(iω ｃ t) {exp(+iθ+iπ / 2)+exp(-iθ−iπ / 2)} / 2...(4)

[0029] Photoelectric field "E" １ +E ２ The optical phase of " " can be expressed as shown in equation (5) by omitting the optical phase of the carrier and describing only the modulation component.

[0030] E １+E ２ = {exp(+iθ+iπ / 2)+exp(-iθ−iπ / 2)} / 2...(5)

[0031] Here, "exp(+iθ+iπ / 2)" is the photoelectric field "E １ This term originates from ". Also, "exp(-iθ-iπ / 2)" is the optical field "E ２ This term originates from "[...]".

[0032] Figure 13 shows a first example of an optical field. Figure 13 shows the optical field "E １ " and the photoelectric field "E ２ " and the photoelectric field "E １ +E ２ An example of this is shown in complex vector representation. Voltage "|V" expressed as absolute value １ | <V π Under the condition of " / 2", the voltage "V １ " and voltage "V ２ The "E" changes, so the photoelectric field "E" changes. １ The imaginary component "Im" of " is always positive, and the photoelectric field "E ２ The imaginary component "Im" of " is always a negative value. Also, the voltage "V" １ = -V ２ Under the condition that the voltage "V" １ " and voltage "V ２ The "E" changes, so the photoelectric field "E" changes. １ +E ２ " has only a real number component, "Re".

[0033] Figure 14 shows the first example of a constellation. In this example, the voltage "V" １ = -V ２ " and the voltage expressed as an absolute value "|V １ | <V π Under the condition " / 2", the drive signal is a four-level digital signal (electrical signal) with four discrete voltages, and the voltage is "V １ " and voltage "V ２ Each of these is "±V" in this context. π / 2" and "±V" π The voltages were set to take four values, including "V / 6". The intervals between these voltages were equal to "V π It is / 3.

[0034] In the constellation illustrated in FIG. 14, four symbols appear. The light intensity of a symbol is proportional to the square of the distance from the origin of the complex plane to that symbol. Also, as described above, when the voltage “V １ = -V ２ ”, is “V π / 2” or “-V π / 2”, the intensity of the modulated light is maximized. In this case, the absolute value of the intensity of the optical electric field is 1 in the notation of Equation (5).

[0035] Since the voltages of the respective drive signals are opposite, “V １ = -V ２ ”, as illustrated in FIG. 14, the imaginary component disappears. For this reason, the optical phase can only take two values of 0 or π. This property has the effect of suppressing unnecessary changes in the optical phase of the modulated light when a multi-valued optical intensity modulation signal or a binary optical phase modulation signal is generated. However, since the optical electric field “E １ + E ２ ” has no imaginary component, the optical signal generation device 100a cannot generate an optical QAM signal.

[0036] FIG. 15 is a diagram showing a second example of the configuration of an optical signal generation device. The optical signal generation device 100b includes two data generation units 301, two signal DACS 401, two signal differential amplifiers 501, and an IQ optical modulator 601. The IQ optical modulator 601 includes two Mach-Zehnder optical modulators 201, a demultiplexer 602, two arms 603, a delay device 604, and a multiplexer 605. Here, the Mach-Zehnder optical modulator 201-1 is provided in the arm 603-1. The Mach-Zehnder optical modulator 201-2 is provided in the arm 603-2. The delay device 604 is provided in the arm 603-2 at the subsequent stage of the Mach-Zehnder optical modulator 201-2.

[0037] The demultiplexer 602 demultiplexes the input continuous light wave. The demultiplexer 602 inputs one of the demultiplexed continuous light waves to the Mach-Zehnder optical modulator 201-1 via the arm 603-1. The demultiplexer 602 inputs the other of the demultiplexed continuous light waves to the Mach-Zehnder optical modulator 201-2 via the arm 603-2.

[0038] The data generation unit 301-1 generates numerical data representing each voltage of the digital signal for the Mach-Zehnder optical modulator 201-1 and transmits it to the signal DAC 401-1. Based on the received numerical data, the signal DAC 401-1 generates an n-value digital signal (electrical signal) with discrete voltages of n values ​​and inputs it to the signal differential amplifier 501-1. The signal differential amplifier 501-1 amplifies the voltage of the n-value digital signal. The signal differential amplifier 501-1 applies the amplified n-value digital signal as a first drive signal to the first drive electrode of the Mach-Zehnder optical modulator 201-1. The signal differential amplifier 501-1 applies a second drive signal, which is the differential signal of the first drive signal, to the second drive electrode of the Mach-Zehnder optical modulator 201-1.

[0039] The data generation unit 301-2 generates numerical data representing each voltage of the digital signal for the Mach-Zehnder optical modulator 201-2 and transmits it to the signal DAC 401-2. Based on the received numerical data, the signal DAC 401-2 generates an n-value digital (electrical signal) with discrete voltages of n values ​​and inputs it to the signal differential amplifier 501-2. The signal differential amplifier 501-2 amplifies the n-value digital signal. The signal differential amplifier 501-2 applies the amplified n-value digital signal as a third drive signal to the third drive electrode of the Mach-Zehnder optical modulator 201-2. The signal differential amplifier 501-2 applies a fourth drive signal, which is the differential signal of the third drive signal, to the fourth drive electrode of the Mach-Zehnder optical modulator 201-2. Here, the data sequence generated by the data generation unit 301-2 is an uncorrelated data sequence independent of the data sequence generated by the data generation unit 301-1.

[0040] The Mach-Zehnder type optical modulator 201-1 has an optical field "E １ +E ２ The signal " is input to the multiplexer 605. The Mach-Zehnder type optical modulator 201-2 uses the optical field "E ３ +E ４The signal " is input to the delay unit 604. The delay unit 604 adjusts the optical phase difference between the light output from the Mach-Zehnder type optical modulator 201-1 and the light output from the Mach-Zehnder type optical modulator 201-2 based on the wavelength of the continuous wave light. That is, the delay unit 604 lengthens the optical path length of arm 603-2 so that the optical path difference between arm 603-1 and arm 603-2 when all drive signals are zero becomes one-quarter of the wavelength of the continuous wave "λ / 4", thereby increasing the optical field "E ３ +E ４ The optical phase of the optical field "E1 + E2" is delayed by "π / 2" radians. In this way, in the optical signal generation device 100b, the delay device 604 biases the parent Mach-Zehnder interferometer that constitutes the IQ optical modulator 601 to the quad point.

[0041] The multiplexer 605 receives the optical field "E" output from the Mach-Zehnder type optical modulator 201-1. １ +E ２ " and the optical field "E" output from the delay unit 604 ３ +E ４ The combiner 605 combines the combined optical field "(E １ +E ２ ) + (E ３ +E ４ This is output as modulated light.

[0042] The voltage "V" of the first drive signal output from the signal differential amplifier 501-1 to the Mach-Zehnder type optical modulator 201-1. １ " and the voltage of the second drive signal "V ２ " is the voltage "V １ = -V ２ " and the voltage expressed as an absolute value "|V １ |≦V π The condition "V / 2" is satisfied. The voltage of the third drive signal output from the signal differential amplifier 501-2 to the Mach-Zehnder type optical modulator 201-2 is "V ３ " and the voltage of the fourth drive signal "V ４ " is the voltage "V ３ = -V ４ " and the voltage expressed as an absolute value "|V ３ |≦V π The condition " / 2" is met.

[0043] In this case, the photoelectric field "E" １ " is expressed as shown in equation (6). Photoelectric field "E ２ " is expressed as shown in equation (7). Photoelectric field "E ３ " is expressed as shown in equation (8). Also, the photoelectric field "E ４ This can be expressed as shown in equation (9).

[0044] E １ =exp(iω ｃ t + iθ １２ +iπ / 2) / 4...(6)

[0045] E ２ =exp(iω ｃ t - iθ １２ -iπ / 2) / 4...(7)

[0046] E ３ =exp(iω ｃ t + iθ ３４ +iπ) / 4...(8)

[0047] E ４ =exp(iω ｃ t - iθ ３４ ) / 4 …(9)

[0048] Here, "θ" １２ The first drive signal output from the signal differential amplifier 501-1 causes the Mach-Zehnder type optical modulator 201-1 to generate an optical field "E" １ The delay that occurs in the phase of " and the second drive signal output from the differential amplifier 501-1 to the Mach-Zehnder type optical modulator 201-1 cause the optical field "E ２ This represents the delay that occurs in the phase of "θ". ３４ The Mach-Zehnder type optical modulator 201-2 is subjected to a third drive signal output from the signal differential amplifier 501-2, which generates an optical field "E" ３ The delay that occurs in the phase of " and the fourth drive signal output from the differential amplifier 501-2 to the Mach-Zehnder type optical modulator 201-2 cause the optical field "E ４ This represents the delay that occurs in the phase of "[ ]".

[0049] Figure 16 shows a second example of the constellation. In Figure 16(a), the first drive signal, the second drive signal, the third drive signal, and the fourth drive signal are each four-level digital signals (four-level electrical signals) with four discrete voltage values. Sixteen symbols are distributed in a grid pattern in the complex plane.

[0050] In Figure 16(b), the first, second, third, and fourth drive signals are each 16-value digital signals (16-value electrical signals) with discrete voltages of 16 values. 256 symbols are distributed in a grid pattern in the complex plane.

[0051] The higher the level of the drive signal, the more exponentially the number of symbols in the constellation increases. However, even with high level of level, the upper limit of the length of one side of the constellation is 1. Therefore, the upper limit of the distance from the origin of the complex plane to the symbols is (1 / 2). ０．５ "

[0052] By the way, in the constellation illustrated in Figure 16, the distance between symbols is not uniform. The distance between symbols is, for example, wider near the origin of the complex plane and narrower at each corner of the constellation. This is due to the sinusoidal characteristics of the Mach-Zehnder optical modulator. The uneven distance between symbols degrades the signal quality. To prevent the degradation of signal quality, the optical signal generator 100b uses a voltage expressed as an absolute value, "|V １ | and the voltage expressed as an absolute value |V ３ The upper limit of "|" is "V π It needs to be kept to around 4 / 4. In other words, the swing rate needs to be kept to around 50%.

[0053] Furthermore, both the Mach-Zehnder type optical modulator 201-1 and the Mach-Zehnder type optical modulator 201-2 are biased to the null point. If they are biased to a point other than the null point, the constellation will not be symmetrical with respect to the origin of the complex plane, but will be biased to a specific quadrant. A state in which the constellation is biased to a specific quadrant is not desirable for the modulated light.

[0054] In optical signal generators 100a and 100b, the voltages of the respective drive signals are reciprocal. In contrast, in Patent Document 1, the voltages of the respective drive signals are not reciprocal. Therefore, in Patent Document 1, the "θ" in equation (1) and the "θ" in equation (2) are different values, and thus equations (3) through (9) do not hold. Below, we will explain the configuration using non-reciprocal drive signals as described in Patent Document 1 and its operation.

[0055] Figure 17 shows a third example of the configuration of an optical signal generation device. Specifically, Figure 17 shows an example of the configuration of an optical signal generation device in which a single Mach-Zehnder optical modulator is driven by the drive system described in Patent Document 1. The optical signal generation device 100c comprises a Mach-Zehnder optical modulator 201, two data generation units 301, two signal DACs 401, and two signal amplifiers 701. The Mach-Zehnder optical modulator 201 comprises a demultiplexer 202, two arms 203, two drive electrodes 204, a delay unit 205, and a multiplexer 206. Here, the drive electrode 204-1 is provided on arm 203-1. The drive electrode 204-2 is provided on arm 203-2. The delay unit 205 is provided on arm 203-2, downstream of the drive electrode 204-2.

[0056] The data generation unit 301-1 generates numerical data representing each voltage of the digital signal for the drive electrode 204-1 and transmits it to the signal DAC 401-1. Based on the received numerical data, the signal DAC 401-1 generates an n-value digital signal (electrical signal) with n discrete voltages. The signal DAC 401-1 inputs the n-value digital signal with n discrete voltages to the signal amplifier 701-1. The signal amplifier 701-1 amplifies the voltage of the n-value digital signal. The signal amplifier 701-1 applies the amplified n-value digital signal as the first drive signal to the drive electrode 204-1.

[0057] The data generation unit 301-2 generates numerical data representing each voltage of the digital signal for the drive electrode 204-2, independently of the data generation unit 301-1, and transmits it to the signal DAC 401-2. Based on the received numerical data, the signal DAC 401-2 generates an n-value digital signal (electrical signal) with discrete voltages of n values ​​and inputs it to the signal amplifier 701-2. The signal amplifier 701-2 amplifies the voltage of the n-value digital signal. The signal amplifier 701-2 applies the amplified n-value digital signal as a second drive signal to the drive electrode 204-2. The voltage of the first drive signal "V" １ " and the voltage of the second drive signal "V ２ The relationship with "V １ = -V ２ It doesn't have to be "."

[0058] Figure 18 shows a second example of the photoelectric field. Figure 18 shows the photoelectric field "E １ " and the photoelectric field "E ２ " and the photoelectric field "E １ +E ２ An example of this is shown in complex vector representation. Voltage "|V" expressed as absolute value １ |≦V π / 2", and the voltage expressed as an absolute value "|V ２ |≦V π Under the condition of " / 2", the voltage "V １ " and voltage "V ２ The "E" changes, so the photoelectric field "E" changes. １ The imaginary component "Im" of " is always positive, and the photoelectric field "E ２ The imaginary component "Im" of " is always a negative value.

[0059] However, unlike the optical signal generator 100a, the optical signal generator 100c generates the voltage of the first drive signal "V １ " and the voltage of the second drive signal "V ２ The relationship with "E" does not have to be reciprocal. For this reason, the optical field "E" １ +E ２ " can have not only a real component "Re" but also an imaginary component "Im". As a result, "|V １ |≦V π / 2", and the voltage expressed as an absolute value "|V ２ |≦Vπ Under the condition of " / 2", the photoelectric field "E １ +E ２ The complex vector of " can take any angle from 0 to 2π with respect to the real axis.

[0060] Thus, the voltage "|V" is expressed as an absolute value. １ "|" and the voltage expressed as an absolute value "|V" ２ Each upper limit of | is "V π The voltage is " / 2" and expressed as an absolute value "|V １ "|" and the voltage expressed as an absolute value "|V" ２ If the | values ​​are independent, the optical signal generator 100c can generate any optical phase. In other words, in the optical signal generator 100c, the swing rate is set to 100%, and the limitation of reciprocity of the drive signals is eliminated, making it possible to generate any optical phase.

[0061] Figure 19 shows a third example of the constellation. In the optical signal generator 100c, the voltage "V １ The range of " is "-V π From " / 2" to "+V π The range is up to " / 2" (the range of 100% swing rate). Similarly, the voltage "V ２ The range of " is "-V π From " / 2" to "+V π This range is up to " / 2" (the range of a 100% swing rate).

[0062] In the optical signal generator 100c, even though an IQ optical modulator is not used, the optical field "E １ +E ２ The symbol has an imaginary component, and the symbols are distributed in the imaginary axis direction of the complex plane. However, while the real component of each symbol is distributed in the range of ±1, the imaginary component of each symbol is distributed in a range narrower than ±0.5. To solve this problem, Patent Document 1 uses a configuration similar to that of the optical signal generator 100c in an IQ optical modulator.

[0063] Figure 20 shows a fourth example of the configuration of an optical signal generation device. Specifically, Figure 20 shows an example of the configuration of an optical signal generation device that drives an IQ optical modulator with the drive system described in Patent Document 1. The optical signal generation device 100d comprises four data generation units 301, four signal DACs 401, four signal amplifiers 701, and an IQ optical modulator 601. The IQ optical modulator 601 comprises two Mach-Zehnder type optical modulators 201, a demultiplexer 202, two arms 603, a delay unit 604, and a multiplexer 605. Here, the Mach-Zehnder type optical modulator 201-1 is provided on arm 603-1. The Mach-Zehnder type optical modulator 201-2 is provided on arm 603-2. The delay unit 604 is provided on arm 603-2, downstream of the Mach-Zehnder type optical modulator 201-2.

[0064] The data generation unit 301-1-1 generates numerical data representing each voltage of the digital signal for the Mach-Zehnder optical modulator 201-1, independently of the other data generation units 301, and transmits it to the signal DAC 401-1-1. The signal DAC 401-1-1 generates an n-value digital (electrical signal) with discrete voltages of n values ​​based on the received numerical data and inputs it to the signal amplifier 701-1-1. The signal amplifier 701-1-1 amplifies the voltage of the n-value digital. The signal amplifier 701-1-1 applies the amplified n-value digital as the first drive signal to the first drive electrode of the Mach-Zehnder optical modulator 201-1.

[0065] The data generation unit 301-1-2 generates numerical data representing each voltage of the digital signal for the Mach-Zehnder optical modulator 201-1, independently of the other data generation units 301, and transmits it to the signal DAC 401-1-2. The signal DAC 401-1-2 generates an n-value digital (electrical signal) with discrete voltages of n values ​​based on the received numerical data and inputs it to the signal amplifier 701-1-2. The signal amplifier 701-1-2 amplifies the voltage of the n-value digital. The signal amplifier 701-1-2 applies the amplified n-value digital signal as a second drive signal to the second drive electrode of the Mach-Zehnder optical modulator 201-1.

[0066] The data generation unit 301-2-1 generates numerical data representing each voltage of the digital signal for the Mach-Zehnder optical modulator 201-2, independently of the other data generation units 301, and transmits it to the signal DAC 401-2-1. Based on the received numerical data, the signal DAC 401-2-1 generates an n-value digital signal (electrical signal) with discrete voltages of n values ​​and inputs it to the signal amplifier 701-2-1. The signal amplifier 701-2-1 amplifies the voltage of the n-value digital signal. The signal amplifier 701-2-1 applies the amplified n-value digital signal as a third drive signal to the third drive electrode of the Mach-Zehnder optical modulator 201-2.

[0067] The data generation unit 301-2-2 generates numerical data representing each voltage of the digital signal for the Mach-Zehnder optical modulator 201-2, independently of the other data generation units 301, and transmits it to the signal DAC 401-2-2. The signal DAC 401-2-2 generates an n-value digital signal (electrical signal) with discrete voltages of n values ​​based on the received numerical data and inputs it to the signal amplifier 701-2-2. The signal amplifier 701-2-2 amplifies the voltage of the n-value digital signal. The signal amplifier 701-2-2 applies the amplified n-value digital signal as a fourth drive signal to the fourth drive electrode of the Mach-Zehnder optical modulator 201-2.

[0068] Figure 21 shows a fourth example of the constellation. In Figure 21, the first drive signal, the second drive signal, the third drive signal, and the fourth drive signal are each four-level digital signals (four-level electrical signals) with four discrete voltage values.

[0069] In the optical signal generator 100d, the voltage "V" １ The range of " is "-V π From " / 2" to "+V π The range is up to / 2 (the range of 100% swing rate). Voltage "V ２ The range of "V" and the voltage "V" ３ The range of "V" and the voltage "V" ４ The same applies to the range of ''. Here, the voltage of each drive signal is determined independently of the other drive signals.

[0070] In the optical signal generator 100d, the upper limit of the distance from the origin of the complex plane to the symbol is "(1 / 2) ０．５ It exceeds "[ ]". Furthermore, in the optical signal generator 100d, the number of symbols has increased dramatically compared to the optical signal generator 100b (16 symbols). The light intensity of each symbol is proportional to the square of the magnitude of the optical field (distance from the origin). Therefore, the further the symbols are placed from the origin, the more the modulation loss of IQ optical modulation can be reduced.

[0071] International Publication No. 2024 / 079894

[0072] However, the optical signal generator 100c has a problem in that the distribution of symbols in the real axis direction and the distribution of symbols in the imaginary axis direction are non-uniform in the constellation. Although this problem is resolved in the optical signal generator 100d, the distance of each symbol from the origin does not reach the theoretical limit, and there is room for further expansion. In other words, there is a problem in that the modulation loss cannot be reduced to the theoretical minimum value.

[0073] In view of the above circumstances, the present invention aims to provide a technology that enables the generation of an optical QAM signal having a constellation in which symbols are arranged substantially equally in both the real axis and the imaginary axis directions, using a single Mahatzehnder optical modulator. Furthermore, the present invention also aims to provide a technology that enables the generation of an optical QAM signal having a constellation in which symbols are arranged further from the origin compared to the conventional technology, using an IQ optical modulator, thereby reducing modulation loss.

[0074] One aspect of the present invention is an optical signal generating apparatus comprising: a first drive signal generating unit that generates a first drive signal having a first voltage amplitude; a second drive signal generating unit that generates a second drive signal having a second voltage amplitude; and a Mach-Zehnder type optical modulator, wherein the Mach-Zehnder type optical modulator comprises: a demultiplexing unit that demultiplexes continuous wave light into first demultiplexed light and second demultiplexed light; a first drive electrode that generates a first optical electric field with modulated optical phase from the first demultiplexed light by modulating the optical path length of a first arm that transmits the first demultiplexed light in accordance with the first drive signal; and the optical path length of a second arm that transmits the second demultiplexed light, The Mach-Zehnder optical modulator includes a second drive electrode that generates a second optical field with modulated optical phase from the second demultiplexed light by modulating it in accordance with the second drive signal, a delay unit that adjusts the optical path difference between the optical path length of the first arm and the optical path length of the second arm based on the wavelength of the continuous wave light, and a multiplexing unit that combines the first optical field and the second optical field, wherein the first drive signal and the second drive signal are mutually independent multi-level digital or analog signals, and the maximum value of the first voltage amplitude and the maximum value of the second voltage amplitude each represent the half-wavelength voltage of the Mach-Zehnder optical modulator V π In that case, V π Larger, and 2V π The following is an optical signal generating device.

[0075] One aspect of the present invention is an optical signal generating apparatus comprising: a demultiplexer; a first drive signal generating unit that generates a first drive signal having a first voltage amplitude; a second drive signal generating unit that generates a second drive signal having a second voltage amplitude; a third drive signal generating unit that generates a third drive signal having a third voltage amplitude; a fourth drive signal generating unit that generates a fourth drive signal having a fourth voltage amplitude; a first Mach-Zehnder type optical modulator; a second Mach-Zehnder type optical modulator; a delay unit; and a multiplexer, wherein the demultiplexer demultiplexes continuous wave light, and the first Mach-Zehnder type optical modulator demultiplexes the demultiplexed continuous wave light A first demultiplexer that demultiplexes one of the signals into a first demultiplexer and a second demultiplexer; a first drive electrode that generates a first optical electric field with modulated optical phase from the first demultiplexer by modulating the optical path length of a first arm that transmits the first demultiplexer according to the first drive signal; a second drive electrode that generates a second optical electric field with modulated optical phase from the second demultiplexer by modulating the optical path length of a second arm that transmits the second demultiplexer according to the second drive signal; and a first drive electrode that adjusts the optical path difference between the optical path length of the first arm and the optical path length of the second arm based on the wavelength of the continuous wave light. The second Mach-Zehnder optical modulator has a delay unit and a first multiplexing unit that combines the first optical field and the second optical field, and the second Mach-Zehnder optical modulator has a second demultiplexing unit that demultiplexes the other of the demultiplexed continuous wave light into a third demultiplexed light and a fourth demultiplexed light, a third drive electrode that generates a third optical field with modulated optical phase from the third demultiplexed light by modulating the optical path length of the third arm that transmits the third demultiplexed light in accordance with the third drive signal, and a fourth optical field with modulated optical phase by modulating the optical path length of the fourth arm that transmits the fourth demultiplexed light in accordance with the fourth drive signal The device comprises a fourth drive electrode generated from wave light, a second delay unit that adjusts the optical path difference between the optical path length of the first arm and the optical path length of the second arm based on the wavelength of the continuous wave light, and a second multiplexer that combines the third and fourth optical fields, wherein the delay unit adjusts the optical phase difference between the light output from the first Mach-Zehnder type optical modulator and the light output from the second Mach-Zehnder type optical modulator based on the wavelength of the continuous wave light, and the multiplexer combines the combined result of the first and second optical fields with the combined result of the third and fourth optical fields.Each of the first drive signal, the second drive signal, the third drive signal, and the fourth drive signal is an independent multi-level digital or analog signal, and the maximum value of the first voltage amplitude and the maximum value of the second voltage amplitude are such that the half-wavelength voltage of the first Mach-Zehnder optical modulator is V. πａ In that case, V πａ Larger, and 2V πａ The following applies, where the maximum value of the third voltage amplitude and the maximum value of the fourth voltage amplitude are equal to the half-wavelength voltage of the second Mach-Zehnder type optical modulator V πｂ In that case, V πｂ Larger, and 2V πｂ The following is an optical signal generating device.

[0076] The present invention makes it possible to generate modulated light with high signal quality using an optical modulator composed of a Mach-Zehnder interferometer.

[0077] This figure shows an example of the basic configuration of the optical signal generator in the first embodiment. This figure shows an example of the optical field in the first embodiment. This figure shows a second example of the optical field in the first embodiment. This figure shows an example of the configuration of the optical signal generator in the first embodiment. This figure shows an example of a constellation in the first embodiment. This figure shows an example of a constellation in a modified version of the first embodiment. This figure shows an example of the configuration of the optical signal generator in the second embodiment. This figure shows an example of the optical field in the second embodiment. This figure shows an example of a constellation in the second embodiment. This figure shows an example of a constellation in a modified version of the second embodiment. This figure shows an example of the hardware configuration of the control device in each embodiment. This figure shows an example of the configuration of the optical signal generator. This figure shows an example of the optical field in the first embodiment. This figure shows an example of a constellation. This figure shows an example of the configuration of the optical signal generator. This figure shows an example of the optical field in the first embodiment. This figure shows an example of a constellation. This figure shows an example of the configuration of the optical signal generator. This figure shows an example of a constellation

[0078] Embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) Figure 1 is a diagram showing an example of the basic configuration of an optical signal generation device in the first embodiment. The optical signal generation device 1 includes a Mach-Zehnder type optical modulator 2. The Mach-Zehnder type optical modulator 2 includes a demultiplexer 21, two arms 22, two drive electrodes 23, a delay unit 24, and a multiplexer 25. Here, the drive electrode 23-1 is provided on arm 22-1. The drive electrode 23-2 is provided on arm 22-2. The delay unit 24 is provided on arm 22-2, downstream of the drive electrode 23-2.

[0079] In the optical signal generation device 1, the Mach-Zehnder type optical modulator 2 generates modulated light from continuous wave light. Here, continuous wave light is input to the input terminal of the Mach-Zehnder type optical modulator 2. The demultiplexer 21 demultiplexes the input continuous wave light into a first demultiplexed light and a second demultiplexed light.

[0080] The optical path difference "ΔL" between arm 22-1 and arm 22-2 is precisely adjusted according to the wavelength "λ" of the continuous wave light. In the optical signal generation device 1, the delay unit 24 biases the Mach-Zehnder type optical modulator 2 (Mach-Zehnder interferometer) to the null point.

[0081] The drive electrode 23-1 has a voltage "V" １ The first drive signal "V" is applied. The drive electrode 23-2 is subjected to the voltage "V" ２ The second drive signal of "V" is applied. In the optical signal generator 1, the voltage "V" is applied. １ " and voltage "V ２ Each upper limit of "" is "V π The voltage is set as "V １ " and voltage "V ２ Each lower limit of '' is "-V π It is determined that the maximum voltage amplitude (total width) of each drive signal is "2V π This is defined as follows: In the optical signal generator 1, the swing rate is greater than 100% and less than or equal to 200%. Below, unless otherwise specified, the swing rate is 200%.

[0082] Figure 2 shows a first example (example of action) of the photoelectric field in the first embodiment. １" is derived from Arm 22-1 (first arm). Optical field "E ２ " originates from arm 22-2 (second arm). Figure 2 shows the optical field "E １ " and the photoelectric field "E ２ " and the photoelectric field "E １ +E ２ An example of this is shown using complex vector representation.

[0083] In Figures 13 and 18, the photoelectric field "E" １ The imaginary component of " is limited to positive, and the photoelectric field "E ２ The imaginary component of "" was limited to negative values. In contrast, in this embodiment, the voltage "|V" is expressed in absolute value. １ | <V π " and the voltage expressed as an absolute value "|V ２ | <V π Under the condition that the voltage "V" １ " and voltage "V ２ The "E" changes, so the photoelectric field "E" changes. １ " and the photoelectric field "E ２ Each of these can take any positive or negative imaginary component, and in the complex plane, each complex vector can point in any direction.

[0084] For example, as illustrated in Figures 2(b) and 2(d), the photoelectric field "E １ The direction of the complex vector "E" and the optical field "E" ２ It is possible to make the direction of the complex vectors of " the same along the imaginary axis. For example, Figure 2(b) corresponds to the figure in Figure 2(e) with the real and imaginary axes swapped. For example, Figure 2(d) corresponds to the figure in Figure 2(c) with the real and imaginary axes swapped.

[0085] As a result, in generating optical QAM signals, the optical signal generator 1 can distribute the symbols of the constellation almost equally in the real axis direction and the imaginary axis direction.

[0086] Figure 3 shows a second example (example of operation) of the photoelectric field in the first embodiment. Figure 3(a) shows the voltage "V １ = -V π / 4" and voltage "V ２ = +3V πIn the state of " / 4", the photoelectric field "E １ The direction of the complex vector "E" and the optical field "E" ２ The direction of the complex vector "E" and the optical field "E" １ +E ２ This represents the direction of the complex vector of "V". In Figures 3(a) to 3(e), the voltage "V" １ " is voltage "+V π The voltage changes by 4 units, and the voltage "V" ２ " is the voltage "-V π It changes by 4 units. Figure 3(e) shows the voltage "V １ = +3V π / 4" and voltage "V ２ = -V π In the state of " / 4", the photoelectric field "E １ The direction of the complex vector "E" and the optical field "E" ２ The direction of the complex vector "E" and the optical field "E" １ +E ２ This represents the direction of the complex vector.

[0087] In Figures 3(a) to 3(e), the photoelectric field "E １ +E ２ Even if the length of the complex vector of "" changes, the optical field "E １ +E ２ The angle "φ" of the complex vector is π / 4 or 5π / 4 radians with respect to the real axis. This property allows for the suppression of unwanted changes in the optical phase of the modulated light when generating multi-level optical intensity modulated signals or binary optical phase modulated signals.

[0088] Furthermore, in the conventional technology shown in Figure 13, the photoelectric field "E" １ +E ２ While the angle of the vector "" with respect to the real axis can only be 0 or π radians, the optical signal generator 1 uses the voltage "V １ " and voltage "V ２ By changing the ratio of "E", the photoelectric field "E" １ +E ２ It is possible to modulate the light intensity while keeping the angle of the complex vector with respect to the real axis to one of two values, "φ" or "φ + π", for any value "φ".

[0089] Figure 4 shows an example of the configuration of an optical signal generation device in the first embodiment. The optical signal generation device 1a comprises a Mach-Zehnder optical modulator 2, two data generation units 3, two signal DACs 4, and two signal amplifiers 5. The Mach-Zehnder optical modulator 2 comprises a demultiplexer 21, two arms 22, two drive electrodes 23, a delay unit 24, and a multiplexer 25. Here, the drive electrode 23-1 is provided on arm 22-1. The drive electrode 23-2 is provided on arm 22-2. The delay unit 24 is provided on arm 22-2, downstream of the drive electrode 23-2.

[0090] The data generation unit 3-1 generates numerical data representing each voltage of the digital signal for the drive electrode 23-1, independently of the data generation unit 3-2, and transmits it to the signal DAC 4-1. Based on the received numerical data, the signal DAC 4-1 generates an n-value digital signal (electrical signal) with discrete voltages of n values ​​and inputs it to the signal amplifier 5-1. The signal amplifier 5-1 amplifies the voltage of the n-value digital signal. The signal amplifier 5-1 applies the amplified n-value digital signal as the first drive signal to the drive electrode 23-1. This modulates the optical path length of the arm 22-1.

[0091] The data generation unit 3-2 generates numerical data representing each voltage of the digital signal for the drive electrode 23-2, independently of the data generation unit 3-1, and transmits it to the signal DAC 4-2. Based on the received numerical data, the signal DAC 4-2 generates an n-value digital signal (electrical signal) with discrete voltages of n values ​​and inputs it to the signal amplifier 5-2. The signal amplifier 5-2 amplifies the n-value digital signal. The signal amplifier 5-2 applies the amplified n-value digital signal as a second drive signal to the drive electrode 23-2. This modulates the optical path length of the arm 22-2.

[0092] The data generation unit 3 may also include a signal DAC 4. If the data generation unit 3 includes a signal DAC 4, the data generation unit 3 inputs the generated n-value digital signal to the signal amplifier 5.

[0093] The voltage of the first drive signal "V" １ " and the voltage of the second drive signal "V ２The relationship with " is "V １ = -V ２ It does not have to be ". In the optical signal generator 1a, the voltage is "V １ The range of " is "-V π " to "+V π This is the range up to " (the range of a 200% swing rate). Voltage "V" ２ The same applies to the range of ''. Here, the voltage of each drive signal is determined independently of the other drive signals.

[0094] The drive electrode 23-1 has a voltage "V" １ The first drive signal "V" is applied from the signal amplifier 5-1. A voltage "V" is applied to the drive electrode 23-2. ２ The second drive signal "V" is applied from the signal amplifier 5-2. In the optical signal generator 1a, the voltage "V" is applied. １ " and voltage "V ２ Each upper limit of "" is "V π The voltage is set as "V １ " and voltage "V ２ Each lower limit of '' is "-V π It is determined that the maximum voltage amplitude (total width) of each drive signal is "2V π It is stipulated that "

[0095] The photoelectric field "E" modulated by the driving electrode 23-1 １ The following is input to the multiplexing unit 25. The optical field "E" modulated by the driving electrode 23-2 ２ The signal is input to the delay unit 24 and then to the multiplexing unit 25. The delay unit 24 lengthens the optical path length of arm 22-2 so that the optical path difference between arm 22-1 and arm 22-2 when all drive signals are zero becomes half the wavelength of the continuous wave "λ / 2", thereby increasing the optical field "E ２ The phase of the optical field "E" １ The optical field "E" is delayed by π radians. The multiplexing section 25 is configured to delay the optical field "E" １ " and the photoelectric field "E ２ The combined wave section 25 combines the combined optical field "E １ +E ２ This is output as modulated light.

[0096] Figure 5 shows an example of a constellation in the first embodiment. In the optical signal generator 1a, the voltage "V １ The range of " is "-V π " to "+V π This is the range up to " (the range of a 200% swing rate). Similarly, the voltage "V ２ The range of " is "-V π " to "+V π This is the range up to '' (the range of a 200% swing rate).

[0097] In Figure 5(a), the digital signal generated by the data generation unit 3 has a 4-level multi-level. In Figure 5(b), the digital signal has a 5-level multi-level. In Figure 5(c), the digital signal has a 16-level multi-level. As the multi-level multi-level of the digital signal increases, the number of symbols increases dramatically.

[0098] The degree of constellation degeneracy differs between the constellation illustrated in Figure 5(a) and the constellation illustrated in Figure 19. Therefore, the number of symbols in the constellation illustrated in Figure 5(a) is reduced compared to the constellation illustrated in Figure 19. However, despite the fact that the optical signal generator 1a does not use an IQ optical modulator, the symbols in Figure 5 are distributed almost equally in the real axis and imaginary axis directions.

[0099] As described above, the data generation unit 3-1, the signal DAC 4-1, and the signal amplifier 5-1 (first drive signal generation unit) generate the first voltage amplitude "-V π ≤ V １ ≤ V π The data generation unit 3-2, the signal DAC 4-2, and the signal amplifier 5-2 (second drive signal generation unit) generate a first drive signal with a second voltage amplitude "-V". π ≤ V ２ ≤ V π A second drive signal with the value " is generated. The maximum value of the first voltage amplitude and the maximum value of the second voltage amplitude, respectively, are used to set the half-wavelength voltage of the Mach-Zehnder optical modulator to "V". π When "V π Larger than " and "2V π The following is the statement.

[0100] In the Mach-Zehnder type optical modulator 2, the demultiplexer 21 demultiplexes continuous wave light into a first demultiplexed light and a second demultiplexed light. The drive electrode 23-1 (first drive electrode) modulates the optical path length of the arm 22-1 (first arm) that transmits the first demultiplexed light according to the first drive signal, thereby generating an optical phase-modulated optical electric field "E １ The first optical field (E) is generated from the first demultiplexed light. The driving electrode 23-2 (second driving electrode) modulates the optical path length of the arm 22-2 (second arm) that transmits the second demultiplexed light according to the second driving signal, thereby generating an optical phase-modulated optical field "E". ２ The second optical field (E) is generated from the second demultiplexed light. The delay unit 24 adjusts the optical path difference between the optical path length of the first arm and the optical path length of the second arm based on the wavelength "λ" of the continuous wave light. The multiplexing unit 25 generates the optical field "E" １ " and the photoelectric field "E ２ The combined wave section 25 combines the optical field "E". １ +E ２ This is output as modulated light. The first drive signal and the second drive signal are each independent multi-level digital signals.

[0101] The Mach-Zehnder optical modulator 2 is biased to the null point by the delay unit 24. When no electrical signal is applied to the first drive electrode and the second drive electrode, the optical path difference between the first arm and the second arm is adjusted to be half the wavelength of continuous wave light.

[0102] Thus, a single Mach-Zehnder optical modulator modulates the modulated light "E" almost equally in the real axis and imaginary axis directions. １ +E ２ This generates a QAM signal with a constellation in which symbols are distributed almost equally in the real and imaginary axes, using a single Mach-Zehnder optical modulator, even though an IQ optical modulator is not used.

[0103] (Modification of the First Embodiment) Figure 6 shows an example of a constellation in a modification of the first embodiment. In Figure 6(a), the multi-levelness of the digital signal generated by the data generation unit 3 is 4 levels. In Figure 5(b), the multi-levelness of the digital signal is 5 levels. In Figure 6(c), the multi-levelness of the digital signal is 16 levels. The swing rate is 200%, the same as in Figure 5. The number of symbols increases dramatically as the multi-levelness of the digital signal increases.

[0104] In a modified version of the first embodiment, the Mach-Zehnder optical modulator 2 is biased to the peak point instead of the null point. In the prior art, biasing to a point other than the null point had the drawback that the arrangement of the constellation would be biased to a specific quadrant. In contrast, in this embodiment, even if biased to a point other than the null point, the optical field "E １ " and the photoelectric field "E ２ Since each of these can take on any optical phase, the symbols are evenly distributed.

[0105] (Second Embodiment) In the second embodiment, the main difference from the first embodiment is that the optical signal generation device includes an IQ optical modulator. The second embodiment will be explained focusing on the differences from the first embodiment.

[0106] Figure 7 shows an example of the configuration of an optical signal generation device in the second embodiment. The optical signal generation device 1b comprises four data generation units 3, four signal DACs 4, four signal amplifiers 5, and an IQ optical modulator 6. The IQ optical modulator 6 comprises two Mach-Zehnder type optical modulators 2, a demultiplexer 7, two arms 8, a delay unit 9, and a multiplexer 10. Here, the Mach-Zehnder type optical modulator 2-1 is mounted on arm 8-1. The Mach-Zehnder type optical modulator 2-2 is mounted on arm 8-2. The delay unit 9 is mounted on arm 6-2, downstream of the Mach-Zehnder type optical modulator 2-2.

[0107] The demultiplexer 7 splits the input continuous wave light. The demultiplexer 7 inputs one of the split continuous wave light signals to the demultiplexer unit 21-1 via arm 8-1. The demultiplexer 7 inputs the other of the split continuous wave light signals to the demultiplexer unit 21-2 via arm 8-2.

[0108] The data generation unit 3-1-1 generates numerical data representing each voltage of the digital signal for the drive electrode 23-1-1 (first drive electrode), independently of the other data generation units 3, and transmits it to the signal DAC 4-1-1. The signal DAC 4-1-1 generates an n-value digital signal (electrical signal) with discrete voltages of n values ​​based on the received numerical data and inputs it to the signal amplifier 5-1-1. The signal amplifier 5-1-1 amplifies the voltage of the n-value digital signal. The signal amplifier 5-1-1 applies the amplified n-value digital signal as the first drive signal to the drive electrode 23-1-1 (first drive electrode).

[0109] The data generation unit 3-1-2 generates numerical data representing each voltage of the digital signal for the drive electrode 23-1-2 (second drive electrode), independently of the other data generation units 3, and transmits it to the signal DAC 4-1-2. The signal DAC 4-1-2 generates an n-value digital (electrical signal) with discrete voltages of n values ​​based on the received numerical data and inputs it to the signal amplifier 5-1-2. The signal amplifier 5-1-2 amplifies the voltage of the n-value digital signal. The signal amplifier 5-1-2 applies the amplified n-value digital signal as the second drive signal to the drive electrode 23-1-2 (second drive electrode).

[0110] The data generation unit 3-2-1 generates numerical data representing each voltage of the digital signal for the drive electrode 23-2-1 (third drive electrode), independently of the other data generation units 3, and transmits it to the signal DAC 4-2-1. The signal DAC 4-2-1 generates an n-value digital (electrical signal) with discrete voltages of n values ​​based on the received numerical data and inputs it to the signal amplifier 5-2-1. The signal amplifier 5-2-1 amplifies the voltage of the n-value digital signal. The signal amplifier 5-2-1 applies the amplified n-value digital signal as the third drive signal to the drive electrode 23-2-1 (third drive electrode).

[0111] The data generation unit 3-2-2 generates numerical data representing each voltage of the digital signal for the drive electrode 23-2-2 (fourth drive electrode), independently of the other data generation units 3, and transmits it to the signal DAC 4-2-2. The signal DAC 4-2-2 generates an n-value digital (electrical signal) with discrete voltages of n values ​​based on the received numerical data and inputs it to the signal amplifier 5-2-2. The signal amplifier 5-2-2 amplifies the voltage of the n-value digital signal. The signal amplifier 5-2-2 applies the amplified n-value digital signal as the fourth drive signal to the drive electrode 23-2-2 (fourth drive electrode).

[0112] The voltage of the first drive signal "V" １ " and the voltage of the second drive signal "V ２ The relationship with "V １ = -V ２ It does not have to be "V". The voltage of the third drive signal "V ３ " and the voltage of the fourth drive signal "V ４ The relationship with " is "V ３ = -V ４ It does not have to be ". In the optical signal generator 1b, the voltage "V １ The range of " is "-V π " to "+V π This is the range up to " (the range of a 200% swing rate). Voltage "V" ２ The range of "V" and the voltage "V" ３ The range of "V" and the voltage "V" ４ The same applies to each of the ranges of . Here, the voltage of each drive signal is determined independently of the other drive signals.

[0113] The Mach-Zehnder type optical modulator 2-1 has an optical field "E １ +E ２ The signal " is input to the multiplexer 10. The Mach-Zehnder type optical modulator 2-2 uses the optical field "E ３ +E ４The signal " is input to the delay unit 9. The delay unit 9 adjusts the optical phase difference between the light output from the Mach-Zehnder optical modulator 2-1 and the light output from the Mach-Zehnder optical modulator 2-2 based on the wavelength of the continuous wave light. That is, the delay unit 9 lengthens the optical path length of arm 8-2 so that the optical path difference between arm 8-1 and arm 8-2 when all drive signals are zero becomes one-quarter of the wavelength of the continuous wave "λ / 4", thereby increasing the optical field "E ３ +E ４ The phase of the signal is delayed by π / 2 radians relative to the optical field "E1 + E2". In this way, in the optical signal generator 1b, the delay unit 9 biases the parent Mach-Zehnder interferometer that constitutes the IQ optical modulator 6 to the quad point.

[0114] The multiplexer 10 receives the optical field "E" output from the Mach-Zehnder type optical modulator 2-1. １ +E ２ " and the optical field "E" output from the delay unit 9 ３ +E ４ The combiner 10 combines the combined optical field "(E １ +E ２ ) + (E ３ +E ４ This is output as modulated light.

[0115] Figure 8 shows an example of the photoelectric field (example of action) in the second embodiment. Photoelectric field "E １ " is derived from arm 22-1-1 (first arm). Optical field "E ２ " is derived from Arm 22-1-2 (second arm). Optical field "E ３ " is derived from arm 22-2-1 (third arm). Optical field "E ４ The name " is derived from Arm 22-2-2 (the fourth arm).

[0116] Figure 8(a) shows the photoelectric field "E １ " and the photoelectric field "E ２ " and the photoelectric field "E １ +E ２ An example of this is shown in complex vector representation. The optical field "E １ The direction of the complex vector and the optical field "E" ２ The optical signal generator 1b aligns with the direction of the complex vector of "E" and generates an optical field "E"１ +E ２ It is possible to maximize the complex vector of "[ ]".

[0117] Figure 8(b) shows the photoelectric field "E ３ " and the photoelectric field "E ４ " and the photoelectric field "E ３ +E ４ An example of this is shown in complex vector representation. The optical field "E ３ The direction of the complex vector and the optical field "E" ４ The optical signal generator 1b aligns with the direction of the complex vector of "E" and generates an optical field "E" ３ +E ４ It is possible to maximize the complex vector of .

[0118] Figure 8(c) shows the photoelectric field "E １ +E ２ +E ３ +E ４ An example of this is shown in complex vector representation. The optical signal generator 1b generates an optical field "E １ +E ２ The direction of the complex vector and the optical field "E" ３ +E ４ The direction of the complex vector of "E" is matched, and the photoelectric field "E" １ +E ２ +E ３ +E ４ The magnitude of the complex vector can be set to the maximum magnitude that the IQ optical modulator can output. Furthermore, for the maximized complex vector, the phase of the optical field (the angle of the complex vector with respect to the real axis) can be set to any value.

[0119] Figure 9 shows a first example of a constellation in the second embodiment. In Figure 9(a), the multi-levelness of the digital signal generated by the data generation unit 3 is 4 levels. In Figure 9(b), the multi-levelness of the digital signal is 5 levels. In Figure 9(c), the multi-levelness of the digital signal is 16 levels. The higher the multi-levelness of the digital signal, the more dramatically the number of symbols increases. The optical signal generator 1b is capable of distributing symbols more densely compared to the optical signal generator 1a.

[0120] As described above, the data generation unit 3-1-1, the signal DAC 4-1-1, and the signal amplifier 5-1-1 (first drive signal generation unit) generate the first voltage amplitude "-V πａ ≤ V １ ≤ V πａ The data generation unit 3-1-2, the signal DAC 4-1-2, and the signal amplifier 5-1-2 (second drive signal generation unit) generate a first drive signal with a second voltage amplitude "-V". πａ ≤ V ２ ≤ V πａ A second drive signal is generated that has "V". πａ " represents the half-wavelength voltage of the Mach-Zehnder type optical modulator 2-1. The maximum value of the first voltage amplitude and the maximum value of the second voltage amplitude are respectively "V πａ Larger than " and "2V πａ The following is the statement.

[0121] The data generation unit 3-2-1, the signal DAC 4-2-1, and the signal amplifier 5-2-1 (third drive signal generation unit) generate the third voltage amplitude "-V πｂ ≤ V ３ ≤ V πｂ The data generation unit 3-2-2, the signal DAC 4-2-2, and the signal amplifier 5-2-2 (fourth drive signal generation unit) generate a third drive signal with a fourth voltage amplitude "-V". πｂ ≤ V ４ ≤ V πｂ A fourth drive signal is generated that has "V". πｂ " represents the half-wavelength voltage of the Mach-Zehnder type optical modulator 2-2. The maximum value of the third voltage amplitude and the maximum value of the fourth voltage amplitude, respectively, represent the half-wavelength voltage of the Mach-Zehnder type optical modulator as "V πｂ When "V πｂ Larger than " and "2V πｂ The following applies: The first drive signal, the second drive signal, the third drive signal, and the fourth drive signal are each independent multi-level digital signals.

[0122] The demultiplexer 7 demultiplexes the continuous wave light. In the Mach-Zehnder optical modulator 2-1 (first Mach-Zehnder optical modulator), the demultiplexer 21-1 (first demultiplexer) demultiplexes one of the demultiplexed continuous wave light into a first demultiplexed light and a second demultiplexed light. The drive electrode 23-1-1 (first drive electrode) modulates the optical path length of the arm 22-1-1 (first arm) that transmits the first demultiplexed light according to the first drive signal, thereby generating an optical phase-modulated optical electric field "E １ The optical phase-modulated optical field "E" is generated from the first demultiplexed light. The driving electrode 23-1-2 (second driving electrode) modulates the optical path length of the arm 22-1-2 (second arm) that transmits the second demultiplexed light according to the second driving signal, thereby generating the optical phase-modulated optical field "E". ２ The second demultiplexed light is used to generate the optical field "E". The delay unit 24-1 (first delay unit) generates the optical field "E" based on the wavelength "λ" of the continuous wave light. ２ The phase of the optical field "E" is delayed. The multiplexing section 25-1 (first multiplexing section) １ " and the photoelectric field "E ２ Combine the waves.

[0123] In the Mach-Zehnder type optical modulator 2-2 (second Mach-Zehnder type optical modulator), the demultiplexer 21-2 (second demultiplexer) demultiplexes the other of the demultiplexed continuous wave light into a third demultiplexed light and a fourth demultiplexed light. The drive electrode 23-2-1 (third drive electrode) modulates the optical path length of the arm 22-2-1 (third arm) that transmits the third demultiplexed light according to the third drive signal, thereby generating an optical phase-modulated optical electric field "E ３ The third optical field (E) is generated from the third demultiplexed light. The driving electrode 23-2-2 (fourth driving electrode) modulates the optical path length of the arm 22-2-2 (fourth arm) that transmits the fourth demultiplexed light according to the fourth driving signal, thereby generating the optical phase-modulated optical field "E ４ The (fourth optical field) is generated from the fourth demultiplexed light. The delay unit 24-2 (second delay unit) generates the optical field "E" based on the wavelength "λ" of the continuous wave light. ４ The phase of the optical field "E" is delayed. The multiplexing section 25-2 (second multiplexing section) ３ " and the photoelectric field "E ４ Combine the waves.

[0124] The delay unit 9 calculates the optical field "E" when all drive signals are zero, based on the wavelength "λ" of the continuous wave light.３ " and the photoelectric field "E ４ The combined wave result of "E ３ +E ４ The optical phase of "E1 + E2" is delayed by "π / 2", which corresponds to "λ / 4". In other words, the delay unit 9 biases the parent Mach-Zehnder interferometer to the quad point. The multiplexer 10 controls the optical field "E １ " and the photoelectric field "E ２ The combined wave result of " and the optical field "E ３ " and the photoelectric field "E ４ The combined wave result of " is combined with the combined wave. The combiner 10 uses the optical field "E １ ”, photoelectric field “E ２ ”, photoelectric field “E ３ " and the photoelectric field "E ４ The combined wave result of "E １ +E ２ +E ３ +E ４ This is output as modulated light.

[0125] The Mach-Zehnder optical modulator 2-1 is biased to the null point by the delay unit 24-1. When no electrical signal is applied to the first drive electrode and the second drive electrode, the optical path difference between the first arm and the second arm is adjusted to be half the wavelength of continuous wave light. The Mach-Zehnder optical modulator 2-2 is biased to the null point by the delay unit 24-2. When no electrical signal is applied to the third drive electrode and the fourth drive electrode, the optical path difference between the third arm and the fourth arm is adjusted to be half the wavelength of continuous wave light.

[0126] In this way, the IQ optical modulator generates an optical QAM signal with a constellation distributed such that the maximum distance between the origin and each symbol reaches the maximum length that the IQ optical modulator can take. This makes it possible to reduce modulation loss during optical QAM signal generation using an IQ optical modulator having multiple nested Mach-Zehnder type optical modulators.

[0127] (Modification of the Second Embodiment) Figure 10 shows an example of a constellation in a modification of the second embodiment. In Figure 10(a), the multi-levelness of the digital signal generated by the data generation unit 3 is 4 levels. In Figure 10(b), the multi-levelness of the digital signal is 5 levels. In Figure 10(c), the multi-levelness of the digital signal is 16 levels. The higher the multi-levelness of the digital signal, the more dramatically the number of symbols increases.

[0128] In a modified version of the second embodiment, each of the Mach-Zehnder type optical modulator 2-1 and Mach-Zehnder type optical modulator 2-2 is biased to the peak point instead of the null point. Even if biased to a point other than the null point, the optical field "E １ " and the photoelectric field "E ２ " and the photoelectric field "E ３ " and the photoelectric field "E ４ Since each of these can take on any optical phase, the symbols are distributed almost evenly.

[0129] Compared to Figure 9(a), the symbol spacing in Figure 10(a) is uneven, but the constellation illustrated in Figure 10 approximates the constellation illustrated in Figure 9 as the multi-level complexity of the digital signal increases. In particular, the constellation illustrated in Figure 10(b) is identical to the constellation illustrated in Figure 9(b).

[0130] (Hardware Configuration) Figure 11 shows an example of the hardware configuration of the control device 11. The data generation unit 3 may be implemented as a processor 12 provided in the control device 11. The control device 11 may determine how to map data to each symbol of the generated QAM signal. The control device 11 may also perform filtering, such as a Nyquist filter, on the generated QAM signal.

[0131] The control device 11 is implemented as software by a processor 12, such as a CPU (Central Processing Unit) or DSP (Digital Signal Processor), executing a program stored in a storage device 13 and a memory 14 that have a non-volatile recording medium (non-temporary recording medium). The program may be recorded on a computer-readable recording medium. A computer-readable recording medium is a non-temporary recording medium such as a portable medium such as a flexible disk, magneto-optical disk, ROM (Read Only Memory), CD-ROM (Compact Disc Read Only Memory), or a storage device such as a hard disk or solid-state drive (SSD) built into a computer system. The communication unit 15 performs predetermined communication processing.

[0132] The control device 11 may be implemented using hardware including electronic circuits (or circuits) such as LSI (Large Scale Integrated Circuit), ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), or FPGA (Field Programmable Gate Array).

[0133] In the embodiments described above, an n-value digital signal is used as the drive signal to the drive electrode of the optical modulator. However, instead of an n-value digital signal, an analog signal with a continuously changing voltage may be used. That is, the first drive signal and the second drive signal may each be an independent analog signal. Furthermore, the first drive signal, the second drive signal, the third drive signal, and the fourth drive signal may each be an independent analog signal.

[0134] While 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.

[0135] The present invention is applicable to optical communication systems such as optical transmission systems.

[0136] 1, 1a, 1b... Optical signal generation device, 2... Mach-Zehnder type optical modulator, 3... Data generation unit, 4... Signal DAC, 5... Signal amplifier, 6... IQ optical modulator, 7... Demultiplexer, 8... Arm, 9... Delay unit, 10... Multiplexer, 11... Control device, 12... Processor, 13... Storage device, 14... Memory, 15... Communication unit, 21... Demultiplexer, 22... Arm, 23... Drive electrode, 24... Delay unit, 25... Multiplexer, 100 a, 100b, 100c, 100d... Optical signal generator, 201... Mach-Zehnder type optical modulator, 202... Demultiplexer, 203... Arm, 204... Drive electrode, 205... Delayer, 206... Multiplexer, 301... Data generation unit, 401... Signal DAC, 501... Signal differential amplifier, 601... IQ optical modulator, 602... Demultiplexer, 603... Arm, 604... Delayer, 605... Multiplexer, 701... Signal amplifier

Claims

1. An optical signal generating apparatus comprising: a first drive signal generating unit for generating a first drive signal having a first voltage amplitude; a second drive signal generating unit for generating a second drive signal having a second voltage amplitude; and a Mach-Zehnder type optical modulator, wherein the Mach-Zehnder type optical modulator includes: a demultiplexing unit for demultiplexing continuous wave light into first demultiplexed light and second demultiplexed light; a first drive electrode for generating a first optical electric field with modulated optical phase from the first demultiplexed light by modulating the optical path length of a first arm transmitting the first demultiplexed light in accordance with the first drive signal; a second drive electrode for generating a second optical electric field with modulated optical phase from the second demultiplexed light by modulating the optical path length of a second arm transmitting the second demultiplexed light in accordance with the second drive signal; a delay unit for adjusting the optical path difference between the optical path length of the first arm and the optical path length of the second arm based on the wavelength of the continuous wave light; and a multiplexing unit for combining the first optical electric field and the second optical electric field. The first drive signal and the second drive signal are each independent multi-level digital or analog signals, and the maximum value of the first voltage amplitude and the maximum value of the second voltage amplitude each represent the half-wavelength voltage of the Mach-Zehnder optical modulator V π In that case, V π Larger, and 2V π The following is an optical signal generating device.

2. The optical signal generating apparatus according to claim 1, wherein the Mach-Zehnder optical modulator is biased to a null point by the delay unit, and when no electrical signal is applied to the first drive electrode and the second drive electrode, the optical path difference between the first arm and the second arm is adjusted to be half the wavelength of the continuous wave light.

3. An optical signal generating apparatus comprising a demultiplexer, a first drive signal generating unit that generates a first drive signal having a first voltage amplitude, a second drive signal generating unit that generates a second drive signal having a second voltage amplitude, a third drive signal generating unit that generates a third drive signal having a third voltage amplitude, a fourth drive signal generating unit that generates a fourth drive signal having a fourth voltage amplitude, a first Mach-Zehnder type optical modulator, a second Mach-Zehnder type optical modulator, a delay unit, and a multiplexer, wherein the demultiplexer demultiplexes continuous wave light, the first Mach-Zehnder type optical modulator comprises a first demultiplexing unit that demultiplexes one of the demultiplexed continuous wave light into a first demultiplexed light and a second demultiplexed light, and a first drive electrode that generates a first optical electric field with modulated optical phase from the first demultiplexed light by modulating the optical path length of a first arm that transmits the first demultiplexed light according to the first drive signal, The second Mach-Zehnder optical modulator includes: a second drive electrode that generates a second optical field with modulated optical phase from the second delimited light by modulating the optical path length of the second arm that transmits the second delimited light according to the second drive signal; a first delay unit that adjusts the optical path difference between the optical path length of the first arm and the optical path length of the second arm based on the wavelength of the continuous wave light; and a first multiplexing unit that combines the first optical field and the second optical field. The second Mach-Zehnder optical modulator includes: a second demultiplexing unit that demultiplexes the other of the demultiplexed continuous wave light into a third demultiplexing light and a fourth demultiplexing light; a third drive electrode that generates a third optical field with modulated optical phase from the third demultiplexing light by modulating the optical path length of the third arm that transmits the third demultiplexing light according to the third drive signal; and a fourth drive electrode that generates a fourth optical field with modulated optical phase from the fourth demultiplexing light by modulating the optical path length of the fourth arm that transmits the fourth demultiplexing light according to the fourth drive signal. The device includes a second delay unit that adjusts the optical path difference between the optical path length of the first arm and the optical path length of the second arm based on the wavelength of the continuous wave light, and a second multiplexing unit that combines the third optical field and the fourth optical field, wherein the delay unit adjusts the optical phase difference between the light output from the first Mach-Zehnder optical modulator and the light output from the second Mach-Zehnder optical modulator based on the wavelength of the continuous wave light, and the multiplexer combines the combined result of the first optical field and the second optical field with the combined result of the third optical field and the fourth optical field.Each of the first drive signal, the second drive signal, the third drive signal, and the fourth drive signal is an independent multi-valued digital signal or analog signal, and each of the maximum value of the first voltage amplitude and the maximum value of the second voltage amplitude is the half-wave voltage of the first Mach-Zehnder type optical modulator set as V πａ When set as such, it is larger than V πａ and is not more than 2V πａ ; and each of the maximum value of the third voltage amplitude and the maximum value of the fourth voltage amplitude is the half-wave voltage of the second Mach-Zehnder type optical modulator set as V πｂ When set as such, it is larger than V πｂ and is not more than 2V πｂ ; an optical signal generating device.

4. The optical signal generating apparatus according to claim 3, wherein the first Mach-Zehnder optical modulator is biased to a null point by the first delay unit, and when no electrical signal is applied to the first drive electrode and the second drive electrode, the optical path difference between the first arm and the second arm is adjusted to be half the wavelength of the continuous wave light, and the second Mach-Zehnder optical modulator is biased to a null point by the second delay unit, and when no electrical signal is applied to the third drive electrode and the fourth drive electrode, the optical path difference between the third arm and the fourth arm is adjusted to be half the wavelength of the continuous wave light.

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