Optical AD Converter and Optical Receiver

Abstract

To reduce power consumption by AD conversion including an optical circuit.SOLUTION: An optical AD converter 100 converts an analog signal of information included in signal light to be input into a digital signal. The optical AD converter 100 has N stages corresponding to a number N of bits of the digital signal. Each of the N stages has: optical waveguides 101 to 103 that respectively guide the signal light, base light obtained by branching local light, and reference light obtained by branching the local light; a light receiver 110 that detects and compares light levels of the signal light and the reference light and outputs a binary comparison result as a digital value; and an optical modulator 120 that variably controls a light level of the base light on the basis of the comparison result of the light receiver 110. The modulation output of the optical modulator 120 is multiplexed with the reference light of a next stage so that each stage sequentially compares the signal light and modulation light and outputs the binary of the N-bit digital signal.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical AD converter and an optical receiver.

Background Art

[0002] In an optical receiver, a function of converting information carried on the received signal light into a digital electrical signal is required. In the current optical receiver, after amplifying the analog electrical signal obtained by converting the received signal light with a light receiving element, a digital electrical signal is obtained using an analog-to-digital converter (ADC: AD converter) implemented by an electrical circuit.

[0003] As a technology related to an optical receiver, for example, there is a technology in which received multiplexed signal light is polarization-separated by a polarization beam splitter and a 90-degree hybrid circuit, and the signal light of the in-phase component and the quadrature component is converted into an electrical signal by a photoelectric converter and output to an ADC after being amplified by an amplifier. Also, there is a technology in which received signal light is polarization-separated by a polarization division unit, converted into an electrical signal by a photoelectric conversion unit, and the signal distortion due to wavelength dispersion is compensated by a dispersion compensation unit (see, for example, Patent Documents 1 and 2 below).

[0004] For example, there is a SAR type AD converter that sequentially compares an analog signal of information included in the signal light of an analog signal for each of N bits and outputs a converted digital signal (see, for example, Non-Patent Documents 1 and 2 below).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0006]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] Conventionally, in order to perform AD conversion in an optical receiver, after converting signal light into an analog electrical signal with a light-receiving element, it is then converted into a digital electrical signal with an ADC in an electrical circuit, resulting in an increase in the power consumption of AD conversion in the optical receiver. Also, in order to cope with the high density and large capacity of optical transmission, it is necessary to parallelize low-speed ADCs, which also increases the power consumption in this regard.

[0008] In one aspect, the present invention aims to reduce power consumption through AD conversion including an optical circuit.

Means for Solving the Problems

[0009] According to one aspect of the present invention, there is provided an optical AD converter that converts an analog signal of information included in an input signal light into a digital signal. Each of N stages corresponding to the number of bits N of the digital signal includes an optical waveguide that respectively guides the signal light, a reference light obtained by branching a local light, and a reference light obtained by branching the local light, a light receiving unit that detects and compares the optical levels of the signal light and the reference light and outputs a binary comparison result as a digital value, and an optical modulator that variably controls the optical level of the reference light based on the comparison result of the light receiving unit. The modulation output of the optical modulator is multiplexed with the reference light of the next stage.

Effect of the Invention

[0010] According to one aspect of the present invention, there is an effect that power consumption can be reduced by AD conversion including an optical circuit.

Brief Description of the Drawings

[0011]

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Embodiments for Carrying Out the Invention

[0012] Hereinafter, with reference to the drawings, embodiments of the disclosed optical AD converter and optical receiver will be described in detail.

[0013] (Embodiment) FIG. 1 is a circuit diagram showing an optical AD converter according to the present invention. The optical AD converter 100 uses an optical circuit for AD conversion of information contained in the received signal light. The optical AD converter 100 directly receives the signal light (light to be converted), propagates it through the internal optical circuit, and performs analog-digital conversion.

[0014] Here, an existing SAR-type AD converter performs an AD conversion operation of sequentially comparing the analog input of an N-bit electrical signal performed only by an electrical circuit from the upper bits in order and outputting an N-bit digital output. The optical AD converter 100 performs the same AD conversion operation as the SAR-type AD converter, using an optical circuit and an electrical circuit, sequentially comparing the analog input of an N-bit signal light from the upper bits in order, and outputting an N-bit digital output.

[0015] The optical AD converter 100 receives the transmitted and received signal light (light to be converted) E sig and the local light (LO light) of the local oscillation light source is input.

[0016] As shown in FIG. 1, in the optical AD converter 100, a plurality of N stages (Stage1 to N) are arranged along the optical waveguide direction (X direction in FIG. 1) of light. Also, a plurality of groups of optical waveguides 101 to 103 (Group1 to 3) are arranged along the Y direction orthogonal to the X direction in FIG. 1. The number of stages N is arranged in the number of columns corresponding to the number of bits (resolution) of N bits (Bit1 to N, for example, 5 bits, 8 bits, etc.) when the information (analog signal) in the state carried on the light to be converted is digitally converted. The N bits consist of N bit columns from MSB to LSB.

[0017] The electrical circuit of the optical AD converter 100 includes a light receiving unit 110 arranged in each of the N stages. The light receiving unit 110 outputs bits of digital signals corresponding to each stage. The light receiving unit 110 includes a light receiving element 111 and a discriminator (comparator) 112. The light receiving element 111 is of a balanced (differential) type, and the signal light of the light to be converted E sig input to one side and the reference light E ref input to the other side are detected, and the light to be converted E sig and the reference light Eref Outputs an electrical signal of the difference.

[0018] Based on the output (electrical signal) of the light receiving element 111, the discriminator 112 compares the light intensity (light level) of the light to be converted E sig and the reference light E ref and outputs a binary digital signal (output 1 / 0) as the comparison result.

[0019] The optical circuit of the optical AD converter 100 includes optical waveguides 101 to 103 that propagate the signal light and an optical modulation unit (optical modulator) 120. Based on the input of the binary digital signal (electrical signal of output 1 / 0), which is the comparison result of the discriminator 112, the optical modulation unit 120 performs phase modulation on the reference light of Group 3 and outputs the modulated output E Mod to the optical waveguide 102 of Group 2 for multiplexing output. As will be described later, the optical modulation unit 120 performs modulation that is in-phase or out-of-phase with respect to the phase of the reference light E Mod with which the modulated output E ref is multiplexed. ref is used as a reference.

[0020] The light to be converted E sig input to the optical AD converter 100 is branched and output to N stages through the optical waveguide 101 of Group 1. The optical waveguide 101 of Group 1 guides the light to be converted E sig carrying N-bit information to each of the N stages (Stage1 to N). The light to be converted E sig of Group 1 is input to the light receiving units 110 of the N stages with the same light intensity.

[0021] The LO light input to the optical AD converter 100 is branched into the reference light E ref and the reference light E LO The optical waveguide 103 of Group 3 branches and outputs the reference light E LO to the optical modulation units 120 provided in each of the N stages (Stage1 to N). The reference light of Group 3 is input to the optical modulation units 120 of the N stages with the same light intensity.

[0022] The optical waveguide 102 of Group 2 guides the reference light E ref to sequentially pass through Stages 1 to N of N stages. Here, the optical modulation units 120 provided in each of the N stages combine the modulated output E Mod with the reference light E ref of Group 2 and supply it to the next-stage. For example, when the light intensity of the reference light of Group 2 output from Stage 1 is E’ ref , the light intensity of the reference light of Stage 2 becomes E” Mod by the combination of the modulated output E ref phase-modulated by the optical modulation unit 120 of Stage 1.

[0023] Also, on the optical waveguides 101 to 103 of each group (Group 1 to 3), a delay device τ(130) for delaying the signal light is arranged. The delay device τ(130) synchronizes the timing of the signal light of each group (Group 1 to 3) at each of the N stages (Stages 1 to N). Details of the timing synchronization by the delay device τ(130) will be described later.

[0024] Figures 2A to 2F are explanatory diagrams of the configuration of the internal circuit of the optical AD converter. Using these figures, the configuration and operation example of the optical AD converter 100 along the waveguide of the signal light will be described. As shown in Figure 2A, the light E sig to be converted input to the optical AD converter 100 is branched from the optical waveguide 101 of Group 1 at the first Stage 1 and input to the light receiving element 111 of the light receiving unit 110. Also, at the subsequent Stages 2 to N, the light E sig to be converted is branched from the optical waveguide 101 of Group 1 and input to the light receiving element 111 of the light receiving unit 110. The light E sig to be converted is input to the light receiving units 110 of the N stages (Stages 1 to N) with the same light intensity.

[0025] As shown in Figure 2B, the LO light input to the optical AD converter 100 is branched into the reference light of the optical waveguide 102 of Group 2 and the reference light of the optical waveguide 103 of Group 3. The reference light E refIt is sequentially input to the light receiving elements 111 of the light receiving units 110 of each of Stages 1 to N via Stages 1 to N. The reference light E ref is, as described above, the modulation output E of the optical modulation unit 120 Mod and can have different light intensities for each of Stages 1 to N.

[0026] Also, the reference light of the optical waveguide 103 of Group 3 is branched and input to the optical modulation units 120 of Stages 1 to N respectively. The reference light is input to the optical modulation units 120 of N stages (Stages 1 to N) with the same light intensity.

[0027] As shown in FIG. 2C, the light receiving unit 110 provided in each of Stages 1 to N photoelectrically converts the signal light of the converted light E sig and the reference light E ref . Also, the discriminator 112 of the light receiving unit 110 digitally outputs the bit corresponding to Stage 1 as a comparison result of comparing the received levels of the converted light E sig and the reference light E ref . The discriminator 112 digitally outputs a digital signal (1 / 0) of Bit1 (MSB).

[0028] The discriminator 112 compares the received levels of the converted light E sig and the reference light E ref , and when the converted light E sig > the reference light E ref , it outputs Bit1 = 1. Also, when the converted light E sig < the reference light E ref , it outputs Bit1 = 0. When the converted light E sig = the reference light E ref , it outputs Bit1 output = 0.

[0029] The output of the electrical signal of the discriminator 112 is output to the optical modulation unit 120. The reference light of Group 3 is branched and input to the optical modulation unit 120. The comparison result of the converted light E sig output by the light receiving unit 110 (discriminator 112) and the reference light E ref is input to the optical modulation unit 120.

[0030] As shown in Fig. 2D, based on the input comparison result (output 1 / 0), the optical modulation unit 120 performs modulation output E on the reference light of Group 3 Mod by multiplexing the reference light E ref used as a reference. With respect to the phase of this reference light E ref , modulation is performed such that the phase is in-phase or anti-phase. For example, if the comparison result of the discriminator 112 is output 1, the optical modulation unit 120 performs phase modulation that is in-phase with the phase of the reference light E ref . Also, if the comparison result is output 0, the optical modulation unit 120 performs phase modulation that is anti-phase with the phase of the reference light E ref . In this way, the optical modulation unit 120 switches the phase of the signal light according to the comparison result 1 / 0, and outputs the modulation output E Mod of the comparison result in Stage 1.

[0031] As shown in Fig. 2E, the modulation output E Mod of the optical modulation unit 120 is multiplexed and input to the reference light of Group 2. As a result, for example, corresponding to the modulation output E Mod of the optical modulation unit 120 in Stage 1, the intensity of the reference light supplied to the next-stage Stage 2 becomes variable. When the reference light in Stage 1 is E' ref , the intensity of the reference light in Stage 2 is varied to E” Mod by the multiplexing of the modulation output E ref .

[0032] For example, when the comparison result of the discriminator 112 in Stage 1 is output 1, the modulation output E Mod of the optical modulation unit 120 becomes in-phase, and the optical level of the reference light E ref in Stage 2 increases (for example, 1.5 times). On the other hand, when the comparison result of the discriminator 112 in Stage 1 is output 0, the modulation output E Mod of the optical modulation unit 120 becomes anti-phase, and the optical level of the reference light E ref in Stage 2 decreases (for example, 0.5 times).

[0033] Regarding the configurations of Stage2 to StageN-1, they are the same as that of Stage1. As a result, Stage2 to StageN-1 each perform digital outputs of N bits (Bit2 to N-1) in the same manner as Stage1.

[0034] As shown in Fig. 2F, a light receiving unit 110 (light receiving element 111 and discriminator 112) is provided in StageN. In the light receiving unit 110 of StageN, the converted light E of Group1 sig and the reference light E of Group2 after passing through StageN-1 ref are input. The light receiving unit 110 (discriminator 112) of StageN performs a digital output of BitN.

[0035] Here, the configuration and operation example of an existing SAR type AD converter will be described. Fig. 3A is a diagram showing a configuration example of an existing SAR type AD converter, and Fig. 3B is an explanatory diagram of an operation example of an existing SAR type AD converter. Also, Fig. 3C is an explanatory diagram of an operation example of the optical AD converter of the embodiment.

[0036] As shown in Fig. 3A, when performing AD conversion on signal light, an SAR type AD converter 300 is used. Also, in front of the SAR type AD converter 300, a light receiving element 301, a TIA (Transresistance Amplifier) 302, and a deserialiser (Des) 303 are arranged.

[0037] The differential type light receiving element 301 photoelectrically converts a pair of signal lights after multiplexing the received signal light and the LO light of the light source, and amplifies it with TIA 302. The analog signal of TIA 302 is parallelized by deserialiser 303 and input to SAR type AD converter 300.

[0038] The SAR type AD converter 300 includes an SAR (Successive Approximation Register) 310, a DA converter (DAC) 311, a sample and hold (S / H) circuit 312, and a discriminator 313. The sample and hold circuit 312 holds the input predetermined voltage V in and holds it. The voltage V inis a voltage corresponding to the value of an N-bit digital signal carried on the received signal. The discriminator 313 compares V in with the output of the DAC 311 and outputs the comparison result to the successive approximation register (SAR) 310.

[0039] The horizontal axis in FIG. 3B is time and the vertical axis is voltage. As shown in FIG. 3B, in the initial state, the most significant bit (MSB) of the successive approximation register (SAR) 310 is initialized to 1 and supplied to the DAC 311. The DAC 311 supplies an analog signal V DAC corresponding to the digital code (Vref / 2) to the discriminator 313.

[0040] The discriminator 313 compares the V DAC of the upper bit (MSB) with V in . If the voltage of V DAC does not exceed V in , the output of bit 1 remains 1. If the voltage of V DAC exceeds V in , this bit (Bit1) is reset (0) to the SAR 310.

[0041] After that, the SAR 310 sets the next lower bit (Bit2) to 1 and performs the same test, and continues the same process until all N bits of the SAR 310 are tested. In this way, the SAR 310 sequentially compares each of the N bits with V DAC and V in , and digitally outputs the value of each of the N bits obtained by digitally approximating the sampled input voltage V in .

[0042] And, as shown in FIG. 3C, the optical AD converter 100 of the embodiment also performs the same operation as the existing SAR type AD converter 300. The horizontal axis in FIG. 3C(a) is each of the N stages and the vertical axis is voltage. The optical signal to be converted E sig has a predetermined voltage corresponding to the value of N bits. The optical AD converter 100 compares the optical signal to be converted E sig with the reference optical signal E ref bit by bit in the order of Stage1 to N of the N stages.Outputs a digital signal (1 / 0) according to the comparison result with

[0043] And the discriminator 112 of Stage1 is the converted light E sig > reference light E ref to output Bit1 = 1. At this time, as shown in Fig. 3C(b), the modulation output E of the optical modulation unit 120 of Stage1 Mod is in phase, and the optical level of the reference light E” of Stage2 ref is increased by ΔE1 (for example, 1.5 times).

[0044] After this, in Stage2, the converted light E sig and the reference light E after passing through Stage1 ref are input. The light receiving unit 110 (discriminator 112) of Stage2 performs a digital output of Bit2 in the same manner as Stage1. The discriminator 112 of Stage2 is the converted light E sig < reference light E ref to output Bit2 = 0. At this time, as shown in Fig. 3C(b), the modulation output E of the optical modulation unit 120 of Stage2 Mod is in antiphase, and the optical level of the reference light E” of Stage2 ref is decreased by ΔE2 (for example, 0.5 times). In this way, the optical AD converter 100 performs sequential comparison for each bit and outputs a digital output for N bits, similar to the existing SAR type AD converter 100.

[0045] Generally, the output voltage of the TIA302 is not linear with respect to the input voltage and tends to saturate in the region where the output voltage is close to the upper limit. In order to prevent this influence, it is necessary to use a TIA302 with a higher voltage output and a wider linear output voltage region. However, when such a TIA302 with good linearity is used, the power consumption increases in exchange for the linearity. This also occurs in the DAC311 of the SAR type AD converter 300.

[0046] On the other hand, in the optical AD converter 100 of the embodiment, the optical circuit and the electric circuit are appropriately arranged and configured. Here, corresponding to the subtraction / addition and the signal latch functions of the existing successive approximation register (SAR) type SAR AD converter 300, in the embodiment, based on the determination result of the previous stage, modulation is performed in phase or in antiphase with respect to the phase of the reference light, which is the reference light for multiplexing the modulation output of the optical modulation unit 120, and then multiplexed and interfered with the reference light of the subsequent stage. Also, in the embodiment, the LO light is supplied to each stage in separate groups as the reference light and the reference light, and by comparing the light to be converted with the reference light by the light receiving unit 110 of each stage, the equivalent of the latch function of the S / H circuit 302 is realized.

[0047] When comparing the existing AD converter 300 with the optical AD converter 100 of the embodiment, corresponding to the light receiving element 301 at the forefront of the existing AD converter 300, in the optical AD converter 100 of the embodiment, the light receiving elements 111 are arranged in each of the N stages. Also, the TIA 302 used in the existing AD converter 300 is not used in the optical AD converter 100 of the embodiment, and discriminators 112 are arranged in each of the N stages. Also, the Des 303 used in the existing AD converter 300 is not used in the optical AD converter 100 of the embodiment.

[0048] In this way, by including an optical circuit inside, the optical AD converter 100 of the embodiment can eliminate the linearity required for the linear electronic circuits necessary for AD conversion using the existing SAR type AD converter 300, such as the linearity required for the TIA 302 and the DAC 311. As a result, according to the embodiment, the power consumption can be reduced compared to the prior art.

[0049] Also, since the existing AD converter 300 itself is slow, as shown in FIG. 3A, it can handle high-speed processing by arranging the AD converters 300 in parallel after the Des 303. On the other hand, in the embodiment, the parallel arrangement of the existing SAR type AD converter 300 composed of an electric circuit is not required, and the power consumption can be reduced.

[0050] Figure 4 is a comparison diagram of power consumption between the existing technology and the embodiment. Figure 4(a) shows the existing SAR type AD converter 300, and Figure 4(b) shows the optical AD converter 100 of the embodiment. The power consumption of the existing technology shown in Figure 4(a) is 2 pJ / Bit in the TIA302 part and 8 pJ / Bit in the SAR type AD converter 300 part. In the existing technology, linearity is required for TIA302 and DAC311, so the power consumption increases. As a result, in the existing technology, the overall power consumption is 2 + 8 = 10 pJ / Bit per bit.

[0051] On the other hand, the power consumption of the optical AD converter 100 of the embodiment shown in Figure 4(b) is 55 fJ / Bit in the light receiving unit 110 (discriminator 112) per stage and 1 pJ / Bit in the optical modulation unit 120. When the N-bit output is 5, the overall power consumption is (0.055 + 1) × 5 = 5.3 pJ / Bit. Thus, according to the embodiment, power consumption can be reduced compared to the existing technology.

[0052] Figure 5 is an explanatory diagram of the delay amount setting of the delay unit arranged in the optical AD converter. The horizontal axis of Figure 5 is time, and the vertical axis shows each signal light that guides the optical waveguides 101 to 103 of Group 1 to 3 in the optical AD converter 100. The light E to be converted that guides the optical waveguide 101 of Group 1 sig and the reference light E that guides the optical waveguide 103 of Group 3 LO both have the same delay time τ because they do not have an optical circuit.

[0053] Here, the light receiving element 111 of the light receiving unit 110 has a delay time τ PD for photoelectric conversion, and the discriminator 112 has a delay time τ Disc for comparison and judgment. Also, the optical modulation unit 120 has a delay time τ Mod for the modulation operation. The delay time τ1 in the light receiving unit 110 is the delay time τ PD of the light receiving element 111 + the delay time τ Disc of the discriminator 112. Also, the delay time τ2 of the optical modulation unit 120 = τ ModTherefore, inside the optical AD converter 100, in the optical waveguide 102 of Group 2 of the light receiving unit 110 and the optical modulation unit 120, a delay time τ1 + τ2 of the optical waveguide occurs.

[0054] Correspondingly, a delay device τ(130) having a delay corresponding to the delay time τ(=τ1 + τ2) corresponding to the delay time τ1 + τ2 is arranged on the optical waveguide 101 of Group 1 and the optical waveguide 103 of Group 3 of each stage. Thereby, the timing of the signal light input to each stage can be matched (to the same timing) among each of Group 1 to 3 (optical waveguides 101 to 103). As shown in FIG. 1, a delay device τ(130) for fine adjustment may also be arranged on the optical waveguide 102 of Group 2.

[0055] FIG. 6 is a diagram showing a schematic configuration example of an optical modulator. FIG. 6(a) describes the function of the optical modulation unit 120 for one stage. Further, as shown in FIG. 6, the delay devices τ(130) of each of Group 1 to 3 (optical waveguides 101 to 103) are formed by spirally forming a part of the optical waveguide, so that the guided wavelength can be increased to obtain a predetermined delay time τ.

[0056] The optical modulation unit 120 includes, for example, a pair of PN units 601, a pair of heaters 602, a photodetection unit (MPD) 603, and a controller 604. The PN unit 601 has two interference parts in which a pair of electrodes are arranged along each of the branched optical waveguides.

[0057] The controller 604 variably controls the applied voltage to the pair of electrodes based on the output V i (1 / 0) of the discriminator 112 to change the interference state of the interference part. As a result, the optical modulation unit 120 performs phase modulation on the reference light E LO in Group 3 (optical waveguide 103), and outputs a modulation output E Mod in phase or antiphase. The heater 602 adjusts the temperature of the PN unit 601. The MPD 603 monitors the output of the optical modulation unit 120 and outputs it to the controller 604.

[0058] The controller 604 is the output V of the discriminator 112i Based on (1 / 0), for example, when V i = 1, maximize the power, and when V i = 0, perform control to output a modulation output E that minimizes the power of the modulation output Mod The controller 604 performs temperature adjustment by the heater 602 based on the monitor output of the MPD 603, and changes the interference state in the PN section 601 to output an in-phase or anti-phase modulation output E Mod

[0059] Fig. 6(b) shows the orthogonal axes of I(Re)-Q(Jm). The controller 604 adjusts the temperature by the heater 602 so that on this IQ axis, the power (electric field strength) E ref of the reference light E propagating on the optical waveguide 102 of Group 2 i and the power F i when V = 0, and the power F i when V = 1 are located on the same straight line i

[0060] Fig. 7 is a diagram showing the power of the light to be converted and the reference light by the optical AD converter, and an output example of the digital signal. Figs. 7(a) to (d) show bit sequences with different values when the digital signal included in the light to be converted is 8 bits. In this case, the optical AD converter 100 has N = 8 (Stage1 to 8). The horizontal axis of each figure is the stage number, and the vertical axis is the electric field strength

[0061] As shown in Fig. 7(a), when all the values of the digital output are 0 "00000000", the electric field strength of the light to be converted E sig is the lowest (0), and as shown in Fig. 7(d), when all the values of the digital output are 1 "11111111", the electric field strength of the light to be converted E sig is the highest

[0062] In the case of Fig. 7(a), the optical AD converter 100 performs digital output for each bit by sequentially comparing the light to be converted E sig with the reference light E ref At this time, for each process in Stage1 to 8, the reference light E ref ​​The electric field strength converges and decreases toward the electric field strength (0) of the light to be converted, E sig .

[0063] Also in FIG. 7(d), in the optical AD converter 100, in Stages 1 to 8, the light to be converted, E sig is sequentially compared with the reference light, E ref to perform digital output for each bit. At this time, for each process in Stages 1 to 8, the electric field strength of the reference light, E ref converges and increases toward a predetermined electric field strength of the light to be converted, E sig .

[0064] Also in the cases of FIGS. 7(b) and 7(c), in the optical AD converter 100, in Stages 1 to 8, the light to be converted, E sig is sequentially compared with the reference light, E ref to perform digital output for each bit. At this time, for each process in Stages 1 to 8, the electric field strength of the reference light, E ref converges toward a predetermined electric field strength of the light to be converted, E sig . The convergence operation in the optical AD converter 100 shown in FIGS. 7(a) to 7(d) is the same as the operation of the existing AD converter 300 (see FIG. 3A, etc.).

[0065] FIG. 8 is a diagram showing another configuration example of the light receiving unit of the optical AD converter. As described above, in FIG. 1 and the like, the light receiving unit 110 is composed of the light receiving element 111 and the discriminator 112, but it is not limited thereto. The light receiving unit 110 shown in FIG. 8 includes a pair of light receiving elements (PD) 801, a pair of TIAs 802, and a comparator 803 that compares the outputs of the pair of TIAs 802. Even in such a configuration, the light to be converted, E sig can be compared with the reference light, E ref , and the comparison result can be output to the optical modulation unit 120.

[0066] (Other Embodiments) Next, another embodiment of the optical AD converter will be described. In the following description, an optical AD converter for IQ modulation that receives polarization multiplexed signal light will be described.

[0067] FIG. 9 is a circuit diagram showing an optical AD converter for IQ modulation. In the optical AD converter 900 for IQ modulation shown in FIG. 9, the same components as those of the optical AD converter 100 described above are denoted by the same reference numerals. Note that only Stage1 of the N stages is shown in FIG. 9, and the other Stages 2 to N are the same as those in FIG. 1.

[0068] The optical AD converter 900 for IQ modulation is provided with a 90° hybrid circuit 901 for the input optical signal to be converted, and IQ separation is performed by the 90° hybrid circuit 901. And the optical waveguide 101 of Group1 has optical waveguides 101a and 101b for IQ respectively corresponding to this IQ separation. In this embodiment, the optical waveguide 101 of Group1 for the optical signal to be converted is separated into IQ and further separated (optical branched) into an X polarization wave and a Y polarization wave, and has a total of four optical waveguides.

[0069] Light receivers 110a and 110b are respectively provided in the optical waveguides 101a and 101b. The light receiver 110a has a pair of light receiving elements 111a and a comparator 112a. The comparator 112 can be composed of an amplifier that does not require linearity.

[0070] Also, the optical AD converter 900 for IQ modulation branches the input LO optical signal, inputs one of the branched signals to the 90° hybrid circuit 901, and inputs the other branched signal to the optical waveguide 102 of Group2 and the optical waveguide 103 of Group3. The optical waveguide 102 of Group2 and the optical waveguide 103 of Group3 are branched into a reference optical signal for IQ and a reference optical signal. The optical waveguide 102 of Group2 for the reference optical signal is branched into two for IQ. The optical waveguide 103 of Group3 for the reference optical signal is branched into two for the light receivers 110I and 110Q of IQ, and further branched into two for the optical modulation units 120I and 120Q of IQ.

[0071] Regarding the I - processing unit 902I, the reference optical signal of the optical waveguide 103I of Group3 and the reference optical signal of the optical waveguide 102I of Group2 are combined and then branched again and input to the light receiver 110I. The light receiver 110I includes a pair of differential light receiving elements 111I and a comparator 112I.

[0072] Also, the reference light of the optical waveguide 103I in Group 3 is input to the optical modulation unit 120I. The modulation output of the optical modulation unit 120I is multiplexed with the optical waveguide 102I in Group 2. A phase shifter (PS) 903I is provided on the optical waveguide 102I in Group 2. Also, Inc in the figure corresponds to the controller 604 (see FIG. 6). The controller 604 monitors the output of the comparator 110I and controls the phases of the reference light and the reference light to be in phase.

[0073] Then, the output of the comparator 112a and the output of the comparator 112I are input to the comparator 905I. The comparator 905I compares the output of the comparator 112a and the output of the comparator 112I, and digitally outputs the I component of the comparison result.

[0074] The Q processing unit 902Q has the same configuration as the I processing unit 902I. In the Q processing unit 902Q, the reference light of the optical waveguide 103Q in Group 3 and the reference light of the optical waveguide 102Q in Group 2 are input to the light receiving unit 110Q.

[0075] The reference light of the optical waveguide 103Q in Group 3 is input to the optical modulation unit 120Q, and the modulation output of the optical modulation unit 120Q is multiplexed with the optical waveguide 102Q in Group 2. Then, the output of the comparator 112b and the output of the comparator 112Q are input to the comparator 905Q. The comparator 905Q compares the output of the comparator 112b and the output of the comparator 112Q, and digitally outputs the Q component of the comparison result.

[0076] FIG. 10 is an explanatory diagram of an operation example of an IQ modulation optical AD converter. In FIG. 10(a), only the light receiving unit 110b and the Q processing unit 902Q are shown, and the delay element τ(130) and the phase adjustment are omitted. The optical field vector E of the light to be converted input to the IQ modulation optical AD converter 900 S , and the LO light has an electric field vector E LO . FIG. 10(b) shows the electric field intensity on the IQ axis.

[0077] In this case, as the output of the 90° hybrid circuit 901, one of the differential light-receiving elements 111b of the light-receiving unit 110b detects the electric field vector E LO +iE S and the other of the light-receiving elements 111b detects the electric field vector E LO -iE S The comparator 112b extracts the projection of iE S in the direction of E LO ×|E LO | (E S * E LO -E S E LO * , * is the complex conjugate) and outputs it to the comparator 905Q as the comparison result.

[0078] On the other hand, the comparator 112Q of the light-receiving unit 110Q of the Q processing unit 902Q extracts the projection of iE ref in the direction of E LO ×|E LO | (E ref * E LO -E ref E LO * ) and outputs it to the comparator 905Q as the comparison result.

[0079] By adjusting the phase of the controller 604 of the Q processing unit 902Q, as shown in Fig. 10(b), E LO and E ref are oriented in the same direction. As a result, the comparator 905Q can output the digital value of the Q component of bit 1.

[0080] Fig. 11 is a circuit diagram showing another configuration example of the IQ-modulated optical AD converter. The IQ-modulated optical AD converter 1100 shown in Fig. 11 is a configuration example that reduces the number of comparators 112a, 112b, 905I, and 905Q used in the IQ-modulated optical AD converter 900 shown in Fig. 9. In Fig. 11, the same components as those in Fig. 9 are labeled with the same reference numerals.

[0081] As shown in Fig. 11, only the light-receiving elements 111a and 111b are provided in the light-receiving units 110a and 110b, and the comparators 112a and 112b described in Fig. 9 are not provided. Also, in the I processing unit 902I and the Q processing unit 902Q, only the light-receiving elements 111I and 111Q are provided, and the comparators 112I and 112Q are not provided.

[0082] The output of the light-receiving element 111a of the I component and the output of the light-receiving element 111I are input to the discriminator 1101I. The output A of the light-receiving element 111a is I1 - I2, and the output B of the light-receiving element 111I is I3 - I4. Thus, the discriminator 1101I can output the digital value of the I component of bit 1 as (I1 - I2) - (I3 - I4).

[0083] Similarly, the output of the light-receiving element 111b of the Q component and the output of the light-receiving element 111Q are input to the discriminator 1101Q. Thus, the discriminator 1101Q can output the digital value of the Q component of bit 1.

[0084] According to the configuration example of Fig. 11, two discriminators for comparison can be provided per stage, reducing the number of comparators compared to the six comparators 112 (112a, 112b, 112I, 112Q, 905I, 905Q) used in the configuration example of Fig. 9, and power consumption can be reduced.

[0085] Fig. 12 is an explanatory diagram of the branching ratio of the light intensity incident on the light-receiving elements of the optical AD converter for IQ modulation. In Fig. 12, the branching ratio of the differential pair of light-receiving elements 111 in the optical AD converter 1100 shown in Fig. 11 will be described as an example.

[0086] In Fig. 12, the pair of branching ratios in the light-receiving element 111a is a1, and the pair of branching ratios in the light-receiving element 111a is a1. For the light-receiving element 111I of the I processing unit 902I, the branching ratio of the reference light of one Group 3 is a1, and the branching ratio of the reference light of the other Group 2 is b1. Also, the branching ratio of the reference light of Group 3 for the optical modulation units 120I and 120Q is c1, and the light intensity attenuation amount is d. e is the light intensity of the reference light of Group 2.

[0087] In this case, the branching ratio a1 is set based on the following formula (1). Also, the branching ratio b1 is set based on the following formula (2). And the branching ratio c1 is set based on the following formula (3).

[0088] [Number]

[0089] [Number]

[0090] [Number]

[0091] The optical AD converter 1100 for IQ modulation has N stages (Stage1 to N). If the same branching ratio is set in each stage, the optical intensity decreases as the stage number increases in the subsequent stages. In response to this, by the above setting, the attenuation in each stage, particularly the branching ratio in the light receiving element 111 to which the modulation output of the optical modulation unit 120 is input, is appropriately set. Thereby, the optical intensities of the signal light (light to be converted, reference light, and reference light) incident on each of the N stages (Stage1 to N) can be made uniform.

[0092] (Configuration example of optical receiver) Figs. 13 to 16 are diagrams showing configuration examples of an optical receiver. A configuration example of an optical receiver including the above-described optical AD converters 100 and 1100 will be described. In each figure, the same reference numerals are given to the components having the same configuration as the above configuration.

[0093] WDM (Wavelength Division Multiplexing) light is input to the optical receiver 1300 shown in Fig. 13. The WDM light is input to the wavelength separation unit 1350 via an optical fiber amplifier (EDFA: Erbium Doped Fiber Amplifier) 1310. Local light (LO light) of the local oscillation light source 1320 is input to the wavelength separation unit 1350.

[0094] The local oscillation light source 1320 includes a plurality of light sources (LDs) 1321 with different wavelengths, a multiplexer 1322 that multiplexes the light of the LDs with multiple wavelengths, and a semiconductor optical amplifier (SOA). The LO light output by the local oscillation light source 1320 is optically amplified by the EDFA 1330, branched by the fiber coupler 1331, and input to the wavelength separation unit 1350.

[0095] The wavelength separation unit (Demux) 1350 separates and outputs the input WDM light and LO light by wavelength, and outputs them to a plurality of receiving units 1360 with different wavelengths. The received signal (light to be converted) input to the receiving unit 1360 of a certain wavelength (one channel) is input to the 90° hybrid circuit 901 via the variable optical attenuator (VOA). The MPD 1362 monitors the input light to be converted and variably controls the VOA 1361.

[0096] The light receiving unit 110 described above is provided at the subsequent stage of the 90° hybrid circuit 901. The light receiving unit 110 includes a pair of light receiving elements 111 for I and Q and a discriminator 112. Although not shown, the above-mentioned optical modulation unit 120 is connected to the light receiving element 111 and the discriminator 112. The digital output in bit units output by the discriminator 112 is input to the DSP (Digital Signal Processor) 1363 as a data processing unit. The DSP 1363 processes the digital signal after AD conversion, extracts the information contained in the light to be converted, and outputs it.

[0097] Then, by performing reception processing for each different wavelength by a plurality of light receiving units 1360, reception processing for each wavelength (multiple channels) of the WDM light is performed.

[0098] Also, in the example of FIG. 13, an IQ modulation unit 1370 is connected to the output of the wavelength separation unit 1350 for a plurality of light receiving units 1360. The modulation outputs of the IQ modulation units 1370 of the plurality of light receiving units 1360 are multiplexed by the wavelength multiplexing unit (Mux) 1380 and can be externally output as a WDM signal.

[0099] FIG. 14 shows a configuration example of the one-channel receiver 1360. In addition to the optical AD converter 100 described above, it includes a 90-degree hybrid circuit 901, a local oscillation light source 1320, and a DSP 1363, and constitutes a one-channel receiver 1360 for a WDM signal.

[0100] As described above, the one-channel receiver 1360 consists of N stages (Stage1 to N), guides the optical signal to be converted and the LO light of the local oscillation light source 1320 between each stage, and outputs the bit values of the corresponding digits of the digital signal for each stage. Stage1 to N each include a light receiving unit 110 and an optical modulation unit 120.

[0101] The IQ digital signals output from each stage (Stage1 to N) are output to the DSP 1363. The DSP 1363 processes the digital signal after AD conversion, extracts the information contained in the optical signal to be converted, and outputs a two-valued N-bit digital signal from MSB to LSB.

[0102] FIG. 15 shows a configuration example of the single-polarization receiver 1500. In the wavelength separation unit (Demux) 1350 of the single-polarization receiver 1500, a plurality of one-channel receivers 1360 are connected for each channel. The plurality of one-channel receivers 1360 each output the information contained in the optical signal to be converted as a multi-bit digital signal.

[0103] FIG. 16 shows a configuration example of the WDM receiver 1600. A polarization multiplexed WDM signal is input to the WDM receiver 1600, and polarization X and polarization Y are separated and output by the polarization splitter 1601. The outputs of each polarization X and Y are respectively output to a single-polarization receiver 1500. Each single-polarization receiver 1500 outputs the information contained in the optical signal to be converted as a multi-bit digital signal for each of the plurality of channels.

[0104] Based on the outputs of a plurality of single-polarization receivers 1500 for polarization X and polarization Y, the DSP 1602 outputs information for polarization X and polarization Y of the optical signal to be converted as a multi-bit digital signal. Note that the DSP 1602 may be integrated with the DSP 1363 included in the single-polarization receiver 1500 (one-channel receiver 1360) for processing.

[0105] The optical AD converter according to the embodiment can be applied to various receivers that receive signal light by polarization multiplexing such as WDM communication and various modulation methods and perform AD conversion.

[0106] The optical AD converter according to the embodiment described above is an optical AD converter that converts an analog signal of information included in the input signal light into a digital signal. Each of the N stages corresponding to the number of bits N of the digital signal includes an optical waveguide that guides the signal light, a reference light obtained by branching the local light, and a reference light obtained by branching the local light, a light receiving unit that detects and compares the optical levels of the signal light and the reference light and outputs a binary comparison result as a digital value, and an optical modulator that variably controls the optical level of the reference light based on the comparison result of the light receiving unit. The modulation output of the optical modulator is multiplexed with the reference light of the next stage.

[0107] Also, the optical AD converter is an optical AD converter that converts an analog signal of information included in the input signal light into a digital signal, has N stages corresponding to the number of bits N of the digital signal, and the N stages each branch-input the signal light and include a group 1 optical waveguide composed of one or more waveguides, a group 2 optical waveguide composed of one or more waveguides that guide the reference light with one of the branched local lights as the reference light, and a group 3 optical waveguide composed of one or more waveguides that guide the reference light with the other of the branched local lights as the reference light. Further, it includes a light receiving unit that detects and compares the optical levels of the signal light and the reference light and outputs a binary comparison result as the digital value of the stage, and an optical modulator that is branched and input with the reference light guided by the group 3 optical waveguide and variably controls the optical level of the reference light by modulation based on the comparison result of the light receiving unit. And the group 2 optical waveguide multiplexes the modulation output of the modulation unit onto the reference light and guides it as the reference light for the next stage, thereby outputting an N-bit digital signal by N stages.

[0108] Thereby, sequential comparison can be performed bit by bit in the same manner as in the existing AD converter. In the existing AD converter, linearity is required for the TIA, DAC, etc. of the electric circuit, resulting in an increase in power consumption. In contrast, the optical AD converter of the embodiment does not use a TIA, DAC, and S / H circuit. Also, since the sequential processing circuit includes not only an electric circuit but also an optical circuit, and a comparator of the electric circuit is arranged in each of the plurality of N stages that perform sequential comparison, the electric circuit such as the comparator does not require linearity and can perform comparison processing, achieving low power consumption.

[0109] Also, the optical AD converter may be configured to include a light receiving unit having a pair of differential light receiving elements that detect the signal light and the reference light, and a comparator that compares and outputs the optical levels of the signal light and the reference light based on the differential output of the pair of light receiving elements. While the existing AD converter provides the light receiving unit externally, the optical AD converter of the embodiment provides the light receiving unit for each internal stage, and since the comparator of the light receiving unit does not require linearity, low power consumption can be achieved.

[0110] Also, in the optical AD converter, when the optical level of the reference light is higher than that of the signal light, the comparator outputs a value of 1, and otherwise outputs a value of 0. Thus, a binary digital output can be performed for each bit with a simple comparator configuration.

[0111] Also, the optical AD converter sets a predetermined branching ratio for one side and the other side of the light receiving element to equalize the input levels to N stages. If the same branching ratio is set for the N stages, the light intensity will decrease in the subsequent stages. Regarding this point, by appropriately setting the attenuation in each stage, particularly the branching ratio in the light receiving element where the modulation output of the optical modulation unit is input, the light intensities of the respective signal lights (the light to be converted, the reference light, and the comparison light) incident on each of the N stages can be equalized, enabling stable bit determination.

[0112] Also, in the optical AD converter, based on the comparison result of the light receiving unit, the optical modulator performs phase modulation in phase or in antiphase with respect to the phase of the reference light using the reference light. For example, when the output of the comparator is a value of 1, the optical modulator performs phase modulation in phase with the phase of the reference light, and when the output of the comparator is a value of 0, the optical modulator performs phase modulation in antiphase with the phase of the reference light. Thus, the optical modulator can control the light intensity of the reference light in the subsequent stage by simply switching the modulation output to in phase or in antiphase, and the modulation output of the modulation unit is multiplexed with the reference light in the subsequent stage.

[0113] Also, the optical AD converter may be provided with a delay device in the optical waveguide to align the timings of the signal light, the reference light, and the reference light. By setting an appropriate delay time in the delay device, the timings of each group of signal lights in each of the N stages can be aligned, enabling stable bit determination in each stage.

[0114] Also, in the optical AD converter, the optical modulator can be used to arrange a pair of electrodes along the branched optical waveguide and perform voltage control on the electrodes. In this way, an optical AD converter can be easily obtained by generally using an optical modulator.

[0115] Further, the optical AD converter may be configured such that, corresponding to the input to the optical signal of IQ modulation, the optical waveguide is branched separately for I and Q, and a 90° hybrid circuit is provided in the optical waveguide of the optical signal.

[0116] Further, the optical modulator can be configured to include the above-described optical AD converter, a local oscillation light source that generates local light, and a data processing unit that outputs information included in the optical signal after AD conversion by the optical AD converter. For example, a general-purpose local oscillation light source and a DSP as the data processing unit can be used to easily obtain an optical modulator.

[0117] Further, the optical modulator can be easily configured to include a polarization splitter that separates the signal light corresponding to the input of the polarization multiplexed WDM signal.

[0118] From these, according to the optical AD converter of the embodiment, an optical signal can be directly input and a digital electrical signal can be output, and power consumption can be reduced. In the embodiment, the addition and subtraction processing of data and signal latching performed by the electrical circuit of the existing AD converter are realized by the branching / combining of the optical circuit (optical waveguide) and the variable control of the optical intensity of the reference light by the optical modulator. Further, the optical circuit can also be miniaturized by miniaturization and integration of the optical waveguide.

[0119] Regarding the above-described embodiment, the following additional remarks are further disclosed.

[0120] (Additional Remark 1) An optical AD converter that converts an analog signal of information included in an input optical signal into a digital signal, each of the N stages corresponding to the number of bits N of the digital signal includes an optical waveguide that guides the signal light, a reference light obtained by branching the local light, and a reference light obtained by branching the local light, respectively, a light receiving unit that detects and compares the optical levels of the signal light and the reference light and outputs a binary comparison result as a digital value, and an optical modulator that variably controls the optical level of the reference light based on the comparison result of the light receiving unit. combining the modulation output of the optical modulator with the reference light of the next-stage stage; An optical AD converter characterized by the above.

[0121] (Appendix 2) An optical AD converter that converts an analog signal of information included in an input signal light into a digital signal, having N stages corresponding to the number of bits N of the digital signal, each of the N stages branches and inputs the signal light, an optical waveguide group 1 composed of one or more optical waveguides, while branching the local light, using one of the branched local lights as reference light, an optical waveguide group 2 composed of one or more optical waveguides that guide the reference light, using the other branched local light as reference light, an optical waveguide group 3 composed of one or more optical waveguides that guide the reference light, a light receiving unit that detects and compares the optical levels of the signal light and the reference light and outputs a binary comparison result as the digital value of the stage, an optical modulator in which the reference light guided by the optical waveguide group 3 is branched and input, and the optical level of the reference light is variably controlled by modulation based on the comparison result of the light receiving unit; and having The optical waveguide group 2 combines the modulation output of the modulation unit with the reference light and guides it as the reference light of the next-stage stage, outputting an N-bit digital signal at the N stages, An optical AD converter characterized by the above.

[0122] (Appendix 3) The light receiving unit a pair of differential light receiving elements that detect the signal light and the reference light, a comparator that compares and outputs the optical levels of the signal light and the reference light based on the differential output of the pair of differential light receiving elements; The optical AD converter according to Appendix 1 or 2, characterized by having the above.

[0123] (Appendix 4) The comparator The optical AD converter according to appended note 3, wherein when the optical level of the reference light is higher than that of the signal light, a value 1 is output, and when it is not, a value 0 is output.

[0124] (Appended note 5) The optical AD converter according to any one of appended notes 1 to 4, wherein a predetermined branching ratio is set between one and the other of the light receiving elements to make the input levels to the N stages uniform.

[0125] (Appended note 6) The optical modulator The optical AD converter according to any one of appended notes 1 to 5, wherein based on the comparison result of the light receiving unit, phase modulation in phase or in antiphase with respect to the phase of the reference light is performed using the reference light.

[0126] (Appended note 7) The optical modulator When the output of the comparator is a value 1, phase modulation in phase with the phase of the reference light is performed, When the output of the comparator is a value 0, phase modulation in antiphase with the phase of the reference light is performed. The optical AD converter according to appended note 6, characterized by this.

[0127] (Appended note 8) The optical AD converter according to any one of appended notes 1 to 7, wherein a delay device for matching the timings of the signal light, the reference light, and the reference light is provided in the optical waveguide.

[0128] (Appended note 9) The optical modulator A pair of electrodes are arranged along the branched optical waveguide, and the voltage control for the electrodes is performed. The optical AD converter according to appended note 1 or 2, characterized by this.

[0129] (Appended note 10) The signal light of IQ modulation, The optical waveguide is branched by IQ, The optical AD converter according to appended note 1 or 2, characterized in that a 90° hybrid circuit is provided in the optical waveguide of the signal light.

[0130] (Appendix 11) An optical AD converter according to any one of Appendices 1 to 10, a local oscillation light source that generates the local light, and a data processing unit that outputs information included in the signal light after AD conversion by the optical AD converter, characterized by an optical receiver.

[0131] (Appendix 12) The signal light is a polarization multiplexed WDM signal, including a polarization splitter that separates the signal light and inputs it to the optical waveguide, characterized by the optical receiver according to Appendix 11.

Explanation of Reference Numerals

[0132] 100, 1100 Optical AD converter 101~103 Optical waveguide 110 Light receiving part 111 Light receiving element 112 Discriminator (comparator) 120 Optical modulation part 130 Delay device 602 Heater 604 Controller 900 Optical AD converter for IQ modulation 901 90° hybrid circuit 1300 Optical receiver 1320 Local oscillation light source 1360 Receiving part

Claims

1. An optical AD converter that converts an analog signal of information included in input signal light into a digital signal, comprising: Each of the N stages corresponding to the number of bits N of the digital signal: An optical waveguide that guides the signal light, a reference light branched from the local light, and the reference light branched from the local light, respectively; A light receiving unit that detects and compares the optical levels of the signal light and the reference light and outputs a binary comparison result as a digital value; An optical modulator that variably controls the optical level of the reference light based on the comparison result of the light receiving unit; The modulated output of the optical modulator is multiplexed with the reference light of the next stage; An optical AD converter characterized by the above.

2. An optical AD converter that converts an analog signal of information included in input signal light into a digital signal, comprising: Having N stages corresponding to the number of bits N of the digital signal; Each of the N stages: A group 1 optical waveguide that branches and inputs the signal light and is composed of one or more waveguides; A group 2 optical waveguide that branches one of the local lights as a reference light and guides the reference light, composed of one or more waveguides; A group 3 optical waveguide that branches the other of the local lights as a reference light and guides the reference light, composed of one or more waveguides; A light receiving unit that detects and compares the optical levels of the signal light and the reference light and outputs a binary comparison result as a digital value of the stage; An optical modulator that receives the reference light guided by the group 3 optical waveguide as a branched input and variably controls the optical level of the reference light by modulation based on the comparison result of the light receiving unit; The group 2 optical waveguide multiplexes the modulated output of the optical modulator with the reference light and guides it as the reference light of the next stage, Outputting an N-bit digital signal in the N stages; An optical AD converter characterized by the above.

3. The light receiving unit: A pair of differential light receiving elements that detect the signal light and the reference light; A comparator that compares and outputs the optical levels of the signal light and the reference light based on the differential output of the pair of differential light receiving elements; The optical AD converter according to claim 1 or 2, characterized by having the above.

4. The comparator: The optical AD converter according to claim 3, characterized in that it outputs a value 1 when the optical level of the signal light is higher than the optical level of the reference light, and outputs a value 0 otherwise.

5. The optical modulator: The optical AD converter according to any one of claims 1 to 4, wherein based on the comparison result of the light receiving unit, phase modulation in phase or in antiphase with respect to the phase of the reference light is performed using the reference light.

6. The optical modulator performs phase modulation in phase with the phase of the reference light when the output of the comparator is value 1, and performs phase modulation in antiphase with the phase of the reference light when the output of the comparator is value 0, The optical AD converter according to claim 4, characterized in that

7. The optical AD converter according to any one of claims 1 to 6, characterized in that a delay device for aligning the timings of the signal light, the reference light, and the reference light is provided in the optical waveguide.

8. The optical modulator arranges a pair of electrodes along the branched optical waveguide and performs voltage control on the electrodes. The optical AD converter according to claim 1 or 2, characterized in that

9. The signal light of IQ modulation, the optical waveguide is branched separately for I and Q, The optical AD converter according to claim 1 or 2, characterized in that a 90° hybrid circuit is provided in the optical waveguide of the signal light.

10. The optical AD converter according to any one of claims 1 to 9, a local oscillation light source for generating the local light, and a data processing unit for outputting information included in the signal light after AD conversion by the optical AD converter. An optical receiver, characterized in that

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