Patent Publication Number: US-2023136573-A1

Title: Optical ad converter and optical receiver

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2021-177000, filed on Oct. 28, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical analog-to-digital (AD) converter and an optical receiver. 
     BACKGROUND 
     An optical receiver has to have a function of converting information included in received signal light into a digital electric signal. An existing optical receiver amplifies an analog electric signal obtained by converting received signal light by a light receiving element, and then converts the analog electric signal into a digital electric signal by using an analog-to-digital converter (ADC) realized by an electric circuit. 
     As a technique related to an optical receiver, for example, there is a technique in which received multiplexed signal light is polarization-separated by a polarization separator and a 90-degree hybrid circuit, signal light of an in-phase component and a quadrature component is converted into an electric signal by a photoelectric converter, the electric signal is amplified by an amplifier, and the amplified signal is output to an ADC. There is a technique in which received signal light is polarization-separated by a polarization division unit, converted into an electric signal by an optical-electric conversion unit, and signal distortion of wavelength dispersion is compensated by a dispersion compensation unit. 
     For example, there is a successive approximation register (SAR) type AD converter that sequentially compares an analog signal of information included in signal light of an analog signal respectively in N bits, converts the analog signal into a digital signal, and outputs the digital signal. 
     Japanese Laid-open Patent Publication No. 2020-198637 and Japanese Laid-open Patent Publication No. 2017-5551 are disclosed as related art. 
     Successive-approximation ADC, [online], Wikipedia, [searched on Sep. 6, 2021], Internet &lt;URL:https://en.wikipedia.org/wiki/Successive-approximation_ADC&gt; and TUTORIAL ON SUCCESSIVE APPROXIMATION REGISTERS (SAR) AND FLASH ADCS, Maxim Technical Documents tutorials 1080, [Searched on Sep. 6, 2021], Internet &lt;URL:https://www.maxi mintegrated.com/en/design/technical-documents/tutorials/1/1080.html&gt; are also disclosed as related art. 
     SUMMARY 
     According to an aspect of the embodiments, an optical analog-to-digital (AD) converter includes, wherein the optical AD converter converts an analog signal of information included in inputted signal light into a digital signal, and is formed of N stages corresponding to a number N of bits of the digital signal, optical waveguides configured to respectively guide the signal light, base light obtained by branching local light, and reference light obtained by branching the local light, a light receiver configured to detect and compare light levels of the signal light and the reference light, and output a binary comparison result, and an optical modulator configured to variably control a light level of the base light, based on the binary comparison result, in each stage of the N stages, wherein an output variably controlled of the optical modulator is multiplexed with the reference light of a next stage. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a circuit diagram illustrating an optical AD converter according to an embodiment; 
         FIG.  2 A  is an explanatory diagram of a configuration of an internal circuit of the optical AD converter (part 1); 
         FIG.  2 B  is an explanatory diagram of a configuration of the internal circuit of the optical AD converter (part 2); 
         FIG.  2 C  is an explanatory diagram of a configuration of the internal circuit of the optical AD converter (part 3); 
         FIG.  2 D  is an explanatory diagram of a configuration of the internal circuit of the optical AD converter (part 4); 
         FIG.  2 E  is an explanatory diagram of a configuration of the internal circuit of the optical AD converter (part 5); 
         FIG.  2 F  is an explanatory diagram of a configuration of the internal circuit of the optical AD converter (part 6); 
         FIG.  3 A  is a diagram illustrating a configuration example of an existing SAR type AD converter; 
         FIG.  3 B  is an explanatory diagram of an operation example of the existing SAR type AD converter; 
         FIG.  3 C  is an explanatory diagram of an operation example of the optical AD converter of the embodiment; 
         FIGS.  4 A and  4 B  are a comparison diagram of power consumption between the existing technique and the embodiment; 
         FIG.  5    is an explanatory diagram of delay amount setting of a delay unit disposed in the optical AD converter; 
         FIGS.  6 A and  6 B  are diagrams illustrating a schematic configuration example of an optical modulator; 
         FIGS.  7 A to  7 D  are diagrams illustrating power and an output example of a digital signal of converted light and reference light by the optical AD converter; 
         FIG.  8    is a diagram illustrating another configuration example of a light receiving unit of the optical AD converter; 
         FIG.  9    is a circuit diagram illustrating an optical AD converter for IQ modulation; 
         FIGS.  10 A and  10 B  are explanatory diagrams of an operation example of the optical AD converter for IQ modulation; 
         FIG.  11    is a circuit diagram illustrating another configuration example of the optical AD converter for IQ modulation; 
         FIG.  12    is an explanatory diagram of a branching ratio of intensity of light to be incident on a light receiving element of the optical AD converter for IQ modulation; 
         FIG.  13    is a diagram illustrating a configuration example of an optical receiver (part 1); 
         FIG.  14    is a diagram illustrating a configuration example of the optical receiver (part 2); 
         FIG.  15    is a diagram illustrating a configuration example of the optical receiver (part 3); and 
         FIG.  16    is a diagram illustrating a configuration example of the optical receiver (part 4). 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In order to perform AD conversion in an optical receiver, signal light is converted into an analog electric signal by a light receiving element and then converted into a digital electric signal by an ADC of an electric circuit, and thus power consumption of the AD conversion in the optical receiver has increased. In order to cope with high density and large capacity of optical transmission, low-speed ADCs have to be parallelized, and also in this respect, power consumption has increased. 
     Hereinafter, an embodiment of a technique capable of reducing power consumption by AD conversion including an optical circuit is described in detail with reference to the drawings. 
     Embodiment 
       FIG.  1    is a circuit diagram illustrating an optical AD converter according to an embodiment. An optical AD converter  100  uses an optical circuit for AD conversion of information included in received signal light. The optical AD converter  100  directly receives signal light (converted light), propagates the signal light through an internal optical circuit, and performs analog-digital conversion. 
     An analog input of an N-bit electric signal, which is performed by an existing SAR type AD converter only with an electric circuit, is sequentially compared in order from the higher-order bit, and an AD conversion operation of performing an N-bit digital output is performed. The optical AD converter  100  performs an AD conversion operation similar to that of the SAR type AD converter by using the optical circuit and the electric circuit, sequentially compares analog inputs of N-bit signal light in order from the higher-order bit, and performs the N-bit digital output. 
     Signal light (converted light) E sig  that is transmitted and received and local light (LO light) of a local oscillation light source are input to the optical AD converter  100 . 
     As illustrated in  FIG.  1   , in the optical AD converter  100 , a plurality of N stages (Stages 1 to N) is arranged along an optical waveguide direction (X direction in  FIG.  1   ). Each of optical waveguides  101  to  103  of a plurality of groups (Groups 1 to 3) is arranged along a Y direction orthogonal to the X direction in  FIG.  1   . The number N of stages that are arranged is the number of strings corresponding to the number of bits (resolution) of N-bit (Bit 1 to Bit N, for example, 5-bit, 8-bit, or the like) in a case where information (analog signal) included in the converted light is digitally converted. N-bit includes N-bit-strings of MSB to LSB. 
     The electric 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 a digital signal corresponding to each stage in bits. The light receiving unit  110  includes a light receiving element  111  and a discriminator (comparator)  112 . The light receiving element  111  that is a balanced-type (differential-type) light receiving element detects the converted light E sig  input to one side and signal light of reference light E ref  input to the other side and outputs an electric signal of the difference between the converted light E sig  and the reference light E ref . 
     Based on an output (electric signal) of the light receiving element  111 , the discriminator  112  compares the magnitude of the light intensity (light level) of the converted light E sig  and the reference light E ref . As a comparison result, the discriminator  112  outputs a binary digital signal (output 1/0). 
     The optical circuit of the optical AD converter  100  includes the optical waveguides  101  to  103  that propagate signal light and an optical modulation unit (optical modulator)  120 . Based on the input of the binary digital signal (output 1/0 electric signal) which is a comparison result of the discriminator  112 , the optical modulation unit  120  multiplexes and outputs a modulation output E Mod  obtained by performing phase modulation on base light of Group 3 to the optical waveguide  102  of Group 2. As will be described later, the optical modulation unit  120  performs modulation in phase or in opposite phase with the reference light E ref  with reference to the reference light E ref  with which the modulation output E Mod  is multiplexed. 
     The converted light E sig  input to the optical AD converter  100  is branched and output to each of the N stages via the optical waveguide  101  of Group 1. The optical waveguide  101  of Group 1 guides the converted light E sig  in which N-bit information is included to N stages (Stages 1 to N). The converted light E sig  of Group 1 is input to the light receiving units  110  of N stages with the same light intensity. 
     LO light input to the optical AD converter  100  is branched into the reference light E ref  and base light E LO . The optical waveguide  103  of Group 3 branches and outputs the base light E Lo  to the optical modulation units  120  respectively provided in the N stages (Stages 1 to N). The base light of Group 3 is input to the optical modulation units  120  of N stages at the same light intensity. 
     The optical waveguide  102  of Group 2 guides the reference light E ref  so as to sequentially pass through Stages 1 to N of N stages. The optical modulation unit  120  provided in each of the N stages multiplexes the modulation output E Mod  with the reference light E ref  of Group 2 and supplies the resultant to the next stage. For example, when the light intensity of the reference light of Group 2 output by Stage 1 is set as E′ reff , the light intensity of the reference light of Stage 2 is changed to E″ ref  by multiplexing the modulation output E Mod  in which the phase is modulated by the optical modulation unit  120  of Stage 1. 
     A delay device τ ( 130 ) that delays signal light is disposed over the optical waveguides  101  to  103  of respective groups (Groups 1 to 3). The delay device τ ( 130 ) matches the timing of the signal light of the respective groups (Groups 1 to 3) of the respective N stages (Stages 1 to N). Details of the matching of timing by the delay device τ ( 130 ) will be described later. 
       FIG.  2 A  to  FIG.  2 F  are explanatory diagrams of a configuration of an internal circuit of the optical AD converter. A configuration and an operation example of the optical AD converter  100  along the guided wave of the signal light will be described with reference to these drawings. As illustrated in  FIG.  2 A , the converted light E sig  that is input to the optical AD converter  100  is branched from the optical waveguide  101  of Group 1 in Stage 1 at the top, and is input to the light receiving element  111  of the light receiving unit  110 . Also in the latter Stages 2 to N, the converted light E sig  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 converted light E sig  is input to the light receiving units  110  of the N stages (Stages 1 to N) at the same light intensity. 
     As illustrated in  FIG.  2 B , 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 base light of the optical waveguide  103  of Group 3. Sequentially through Stages 1 to N, the reference light E ref  of the optical waveguide  102  of Group 2 is branched and input to the light receiving element  111  of the light receiving unit  110  of each of Stages 1 to N. As described above, the reference light E ref  may have different light intensities for every stage of Stages 1 to N based on the modulation output E Mod  of the optical modulation unit  120 . 
     The base light of the optical waveguide  103  of Group 3 is branched and input respectively to the optical modulation units  120  of Stages 1 to N. The base light is input to the optical modulation units  120  of N stages (Stages 1 to N) at the same light intensity. 
     As illustrated in  FIG.  2 C , the light receiving unit  110  provided in each of Stages 1 to N performs photoelectric conversion on the converted light E sig  and the signal light of the reference light E ref  by the light receiving element  111 . The discriminator  112  of the light receiving unit  110  digitally outputs a bit corresponding to Stage 1 as a comparison result obtained by comparing the magnitude of light reception levels of the converted light E sig  and the reference light E ref . The discriminator  112  digitally outputs a digital signal (1/0) of Bit 1 (MSB). 
     The discriminator  112  compares the magnitude of the light reception levels of the converted light E sig  and the reference light E ref , and outputs Bit 1=1 when the converted light E sig &gt;the reference light E ref . When the converted light E sig &lt;the reference light E ref , Bit 1=0 is output. When the converted light E sig =the reference light E ref , Bit 1 output=0 is output. 
     An output of the electric signal of the discriminator  112  is output to the optical modulation unit  120 . The base light of Group 3 is branched and input to the optical modulation unit  120 . The comparison result between the converted light E sig  and the reference light E ref  output by the light receiving unit  110  (discriminator  112 ) is input to the optical modulation unit  120 . 
     As illustrated in  FIG.  2 D , the optical modulation unit  120  performs modulation on the base light of Group 3 based on the comparison result (output 1/0) input thereto such that the modulation is in phase or in opposite phase with the reference light E ref  with reference to the reference light E ref  with which the modulation output E Mod  is multiplexed. For example, when the comparison result of the discriminator  112  is the output 1, the optical modulation unit  120  performs phase modulation in phase with reference light E ref . When the comparison result is the output 0, the optical modulation unit  120  performs phase modulation in opposite phase with the reference light E ref . As described above, the optical modulation unit  120  switches the phase of the signal light in accordance with the comparison result 1/0, and outputs the modulation output E Mod  of the comparison result in Stage 1. 
     As illustrated in  FIG.  2 E , the modulation output E Mod  of the optical modulation unit  120  is multiplexed with and input to the reference light of Group 2. Accordingly, for example, the intensity of the reference light supplied to Stage 2 of the next stage is varied in accordance with the modulation output E Mod  of the optical modulation unit  120  of Stage 1. Assuming that the reference light of Stage 1 is E′ ref , the intensity of the reference light of Stage 2 is changed to Fref by multiplexing the modulation output E Mod . 
     For example, when the comparison result of the discriminator  112  of Stage 1 is the output 1, the modulation output E Mod  of the optical modulation unit  120  is in phase, and the light level of the reference light E ref  of Stage 2 increases (for example, 1.5 times). On the other hand, when the comparison result of the discriminator  112  of Stage 1 is the output 0, the modulation output E Mod  of the optical modulation unit  120  is in opposite phase, and the light level of the reference light E ref  of Stage 2 decreases (for example, 0.5 times). 
     The configurations of Stage 2 to Stage N−1 are similar to that of Stage 1. Accordingly, Stage 2 to Stage N−1 respectively perform digital output of N-bits (Bit 2 to Bit N−1) in the same manner as Stage 1. 
     As illustrated in  FIG.  2 F , the light receiving unit  110  (light receiving element  111  and discriminator  112 ) is provided in Stage N. The converted light E sig  of Group 1 and the reference light E ref  of Group 2 that has passed through Stage N−1 are input to the light receiving unit  110  of Stage N. The light receiving unit  110  (discriminator  112 ) of Stage N performs digital output of Bit N. 
     A configuration and an operation example of an existing SAR type AD converter will be described.  FIG.  3 A  is a diagram illustrating a configuration example of an existing SAR type AD converter, and  FIG.  3 B  is an explanatory diagram illustrating an operation example of an existing SAR type AD converter. In addition,  FIG.  3 C  is an explanatory diagram illustrating an operation example of the optical AD converter of the embodiment. 
     As illustrated in  FIG.  3 A , in a case where signal light is subjected to AD conversion, the SAR type AD converter  300  is used. A light receiving element  301 , a transresistance (transimpedance) amplifier (TIA)  302 , and a deserializer (Des)  303  are disposed at a previous stage of the SAR type AD converter  300 . 
     The light receiving element  301  of differential-type performs photoelectric conversion on a pair of signal lights obtained by multiplexing received signal light and LO light of a light source, and amplifies the pair of signal lights by the TIA  302 . An analog signal of the TIA  302  is parallelized by the deserializer  303  and input to the SAR type AD converter  300 . 
     The SAR type AD converter  300  includes a successive approximation register (SAR)  310 , a digital-to-analog converter (DAC)  311 , a sample and hold (S/H) circuit  312 , and a discriminator  313 . The sample and hold circuit  312  holds a predetermined voltage V in  input thereto. The voltage V in  is a voltage corresponding to the value of the N-bit digital signal included in the received signal. The discriminator  313  compares V in  with an output of the DAC  311  and outputs a comparison result to the successive approximation register (SAR)  310 . 
     In  FIG.  3 B , the horizontal axis indicates time and the vertical axis indicates voltage. As illustrated in  FIG.  3 B , the MSB of the successive approximation register (SAR)  310  is initialized to 1 in an initial state and is supplied to the DAC  311 . The DAC  311  supplies an analog signal V DAC  corresponding to a digital code (Vref/2) to the discriminator  313 . 
     The discriminator  313  compares V DAC  of the higher-order bit (MSB) and V in , and when the voltage of V DAC  does not exceed V in , the output of bit 1 is kept at 1, and when the voltage of V DAC  exceeds V in , the bit (Bit 1) is reset (0) with respect to the SAR  310 . 
     After that, the SAR  310  sets the next lower-order bit (Bit 2) to 1, performs the same test, and continues the same processing until the test is completed for all N bits of the SAR  310 . As described above, the SAR  310  sequentially compares V DAC  and V in  for N bits bit by bit, and thus digitally outputs N-bit values each of which is obtained by digitally approximating the sampled input voltage V in . 
     As illustrated in  FIG.  3 C , the optical AD converter  100  of the embodiment also performs an operation similar to that of the existing SAR type AD converter  300 . The horizontal axis in  FIG.  3 C (a) indicates N stages, and the vertical axis indicates voltage. The converted light E sig  has a predetermined voltage corresponding to the N-bit value. For each Bit in the order of Stages 1 to N of N stages, the optical AD converter  100  outputs the digital signal (1/0) corresponding to the comparison result between the converted light E sig  and the reference light E ref . 
     The discriminator  112  of Stage 1 outputs Bit 1=1 from the converted light E sig &gt;the reference light E ref . At this time, as illustrated in  FIG.  3 C (b), the modulation output E Mod  of the optical modulation unit  120  of Stage 1 is in phase, and the light level of the reference light E″ ref  of Stage 2 is increased by ΔE 1  (for example, 1.5 times). 
     After that, the converted light E sig  and the reference light E ref  after passing through Stage 1 are input to Stage 2. The light receiving unit  110  (discriminator  112 ) of Stage 2 performs digital output of Bit 2 in the same manner as Stage 1. The discriminator  112  of Stage 2 outputs Bit 2=0 from the converted light E sig &lt;the reference light E ref . At this time, as illustrated in  FIG.  3 C (b), the modulation output E Mod  of the optical modulation unit  120  of Stage 2 is in opposite phase, and the light level of the reference light E″ ref  of Stage 2 is decreased by ΔE 2  (for example, 0.5 times). As described above, similarly to the existing SAR type AD converter  100 , the optical AD converter  100  performs bit-by-bit sequential comparison and performs digital output for N bits. 
     Generally, an output voltage of the TIA  302  is not linear relative to an input voltage, and the output voltage tends to be saturated in a region close to an upper limit. To suppress this influence, it is requested that the TIA  302  with higher voltage output and wider range of linear output voltage is used. However, in a case where the TIA  302  having good linearity is used, power consumption increases in exchange for linearity. This also similarly occurs in the DAC  311  of the SAR type AD converter  300 . 
     By contrast, the optical AD converter  100  of the embodiment is configured such that the optical circuit and the electric circuit are appropriately disposed. Corresponding to the subtraction and addition and the function of the signal latch included in the existing SAR type AD converter  300  of the successive approximation (SAR) method, in the embodiment, based on the determination result of the previous Stage, the reference light with which the modulation output of the optical modulation unit  120  is multiplexed is used as a reference, modulation in phase or in opposite phase is performed on the phase of the reference light, and the reference light is multiplexed and interfered with reference light in the latter stage. According to the embodiment, the LO light is supplied to each stage as separate groups of the base light and the reference light, and the converted light is compared with the reference light in the light receiving unit  110  of each stage, thereby realizing a function equivalent to the latch function of the S/H circuit  312 . 
     Comparing the existing AD converter  300  and the optical AD converter  100  of the embodiment, in the optical AD converter  100  of the embodiment, the light receiving element  111  is disposed in each of the N stages corresponding to the light receiving element  301  at the forefront stage of the existing AD converter  300 . The TIA  302  used in the existing AD converter  300  is not used in the optical AD converter  100  of the embodiment, and the discriminator  112  is disposed in each of the N stages. The Des  303  used in the existing AD converter  300  is not used in the optical AD converter  100  of the embodiment. 
     As described above, since the optical AD converter  100  of the embodiment includes the optical circuit therein, the linearity requested in a linear electronic circuit, for example, the TIA  302  and the DAC  311 , which is requested in the AD conversion using the existing SAR type AD converter  300 , may be omitted. Accordingly, according to the embodiment, it is possible to reduce power consumption as compared with the existing technique. 
     Further, since the existing AD converter  300  itself is slow, as illustrated in  FIG.  3 A , high-speed processing is supported by arranging the AD converter  300  in parallel in the subsequent stage of the Des  303 . By contrast, in the embodiment, the parallel arrangement of the existing SAR type AD converter  300  including the electric circuit is not requested, and thus power consumption may be reduced. 
       FIGS.  4 A and  4 B  are a comparison diagram of power consumption between the existing technique and the embodiment.  FIG.  4 A  illustrates the existing SAR type AD converter  300 , and  FIG.  4 B  illustrates the optical AD converter  100  of the embodiment. Power consumption by the existing technique illustrated in  FIG.  4 A  is 2 pJ/bit for the TIA  302  part and 8 pJ/bit for the SAR type AD converter  300  part. According to the existing technique, since the TIA  302  and the DAC  311  have to have linearity, power consumption increases. As a result, in the existing technique, the entire power consumption is 2+8=10 pJ/bit per bit. 
     By contrast, the power consumption of the optical AD converter  100  of the embodiment illustrated in  FIG.  4 B  is 55 fJ/bit for the light receiving unit  110  (discriminator  112 ) and  1  pJ/bit for the optical modulation unit  120  per one stage. When the N-bit output is 5, the overall power consumption is (0.055+1)×5=5.3 pJ/bit. As described above, according to the embodiment, it is possible to achieve lower power consumption than that of the existing technique. 
       FIG.  5    is an explanatory diagram of delay amount setting of a delay unit arranged in the optical AD converter. The horizontal axis in  FIG.  5    indicates time, and the vertical axis indicates signal light guided through the optical waveguides  101  to  103  of Groups 1 to 3 respectively in the optical AD converter  100 . Neither the converted light E sig  that is guided through the optical waveguide  101  of Group 1 nor the base light E Lo  guided through the optical waveguide  103  of Group 3 has the optical circuit, and thus have the same delay time τ. 
     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 determination. The optical modulation unit  120  has a delay time τ Mod  in the modulation operation. A delay time τ 1  in the light receiving unit  110  is equal to the delay time τ PD  of the light receiving element  111 +the delay time τ Disc  of the discriminator  112 . A delay time τ 2  of the optical modulation unit  120 =Tmod. For this reason, in the optical AD converter  100 , a delay time τ 1 +T 2  of optical guiding is generated in the optical waveguide  102  of Group 2 of the light receiving unit  110  and the optical modulation unit  120 . 
     For this reason, the delay device τ ( 130 ) having a delay of the delay time τ (=τ i +τ 2 ) corresponding to the delay time τ i +τ 2  is disposed over the optical waveguide  101  of Group 1 and the optical waveguide  103  of Group 3 in each Stage. Accordingly, the timing of the signal light input to each Stage may be matched (at the same timing) between respective Groups 1 to 3 (optical waveguides  101  to  103 ). As illustrated in  FIG.  1   , the delay device τ ( 130 ) for fine adjustment may also be disposed over the optical waveguide  102  of Group 2. 
       FIGS.  6 A and  6 B  are diagrams illustrating a schematic configuration example of the optical modulator.  FIG.  6 A  illustrates functions of the optical modulation unit  120  for one stage. As illustrated in  FIG.  6 A , the delay device τ ( 130 ) of each of Groups 1 to 3 (optical waveguides  101  to  103 ) is formed by forming a part of the optical waveguide in a spiral shape, and thus it is possible to increase a waveguide length and obtain a predetermined delay time τ. 
     The optical modulation unit  120  includes, for example, a pair of PN units  601 , a pair of heaters  602 , an optical detection unit (MPD)  603 , and a controller  604 . The PN units  601  include two interference portions including a pair of electrodes arranged along respective branched optical waveguides. 
     Based on the output V i (1/0) of the discriminator  112 , the controller  604  variably controls voltages applied to the pair of electrodes to change the interference state of the interference portion. As a result, the optical modulation unit  120  performs phase modulation on the base light E LO ) of Group 3 (optical waveguide  103 ) and outputs the modulation output E Mod  in phase or in opposite phase. The heater  602  adjusts the temperature of the PN units  601 . The MPD  603  monitors an output of the optical modulation unit  120  and outputs the monitored output to the controller  604 . 
     Based on the output V i (1/0) of the discriminator  112 , the controller  604  performs control so as to output the modulation output E Mod , for example, in which power is maximized when V i =1, and power of the modulation output is minimized when V i =0. Based on the monitor output of the MPD  603 , the controller  604  performs temperature adjustment by the heater  602  and changes the interference state in the PN unit  601  thereby to output the modulation output E Mod  in phase or in opposite phase. 
       FIG.  6 B  illustrates an orthogonal axis of I (Re)−Q (Jm). At this IQ-axis, the controller  604  performs temperature adjustment by the heater  602  such that the power F i  of V i =0 and V i =1 is located in the same straight line on the line between the center and the power (electric field intensity) E i  of the reference light E ref  guided over the optical waveguide  102  of Group 2. 
       FIGS.  7 A to  7 D  are diagrams illustrating power and an output example of a digital signal of converted light and reference light by the optical AD converter.  FIGS.  7 A to  7 D  illustrate bit strings in which a digital signal included in the converted light have different values in 8 bits. In this case, the optical AD converter  100  has N=8 (Stages 1 to 8). The horizontal axis in each figure indicates the number of stages, and the vertical axis indicates the electric field intensity. 
     As illustrated in  FIG.  7 A , when all the values of the digital output are 0 “00000000”, the electric field intensity of the converted light E sig  is the lowest (0), and as illustrated in  FIG.  7 D , when all the values of the digital output are 1 “11111111”, the electric field intensity of the converted light E sig  is the highest. 
     In the case illustrated in  FIG.  7 A , the optical AD converter  100  sequentially compares the converted light E sig  with the reference light E ref  in Stages 1 to 8 to perform digital output for each bit. At this time, for each processing in Stages 1 to 8, the electric field intensity of the reference light E ref  decreases so as to converge to the electric field intensity (0) of the converted light E sig . 
     Also in  FIG.  7 D , the optical AD converter  100  sequentially compares the converted light E sig  with the reference light E ref  in Stages 1 to 8 to perform digital output for each bit. At this time, for each processing in Stages 1 to 8, the electric field intensity of the reference light E ref  increases so as to converge to a predetermined electric field intensity of the converted light E sig . 
     Also in the case illustrated in  FIGS.  7 B and  7 C , the optical AD converter  100  sequentially compares the converted light E sig  with the reference light E ref  to perform digital output for each bit in Stages 1 to 8. At this time, for each processing in Stages 1 to 8, the electric field intensity of the reference light E ref  converges to a predetermined electric field intensity of the converted light E sig . Convergence operations in the optical AD converter  100  illustrated in  FIGS.  7 A to  7 D  are similar to the operation of the existing AD converter  300  (see  FIG.  3 A  and the like). 
       FIG.  8    is a diagram illustrating another configuration example of the light receiving unit of the optical AD converter. As described above, although the light receiving unit  110  is configured by the light receiving element  111  and the discriminator  112  in  FIG.  1    and the like, the configuration is not limited thereto. The light receiving unit  110  illustrated in  FIG.  8    includes a pair of light receiving elements (PD)  801 , a pair of TIAs  802 , and a comparator  803  that compares outputs of the pair of TIAs  802 . Also in such a configuration, it is possible to compare the converted light E sig  and the reference light E ref  and output the comparison result to the optical modulation unit  120 . 
     Other Embodiments 
     Next, other 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. 
       FIG.  9    is a circuit diagram illustrating an optical AD converter for IQ modulation. Constituent elements of an optical AD converter  900  for IQ modulation illustrated in  FIG.  9    that are similar to those of the optical AD converter  100  described above are denoted by the same reference signs.  FIG.  9    illustrates only Stage 1 of the N stages, and the other Stages 2 to N are the same as those illustrated in  FIG.  1   . 
     The optical AD converter  900  for IQ modulation is provided with a 90-degree hybrid circuit  901 , and performs IQ separation on input converted light by the 90-degree hybrid circuit  901 . The optical waveguide  101  of Group 1 includes optical waveguides  101   a  and  101   b  corresponding respectively to the IQ separation. According to this embodiment, the optical waveguide  101  of Group 1 for converted light is subjected to IQ separation and separated (optical branched) into an X-polarized wave and a Y-polarized wave, and has a total of four optical waveguides. 
     The light receiving units  110   a  and  110   b  are provided in the optical waveguides  101   a  and  101   b , respectively. The light receiving unit  110   a  includes a pair of light receiving elements  111   a  and a comparator  112   a . The comparator  112  may be configured by an amplifier without requesting linearity. 
     Further, the optical AD converter  900  for IQ modulation branches the input LO light; one branch is input to the 90-degree hybrid circuit  901 , and the other branch is input to the optical waveguide  102  of Group 2 and the optical waveguide  103  of Group 3. The optical waveguide  102  of Group 2 and the optical waveguide  103  of Group 3 are branched into base light and reference light for IQ. The optical waveguide  102  of Group 2 for the reference light is branched into two for IQ. The optical waveguide  103  of Group 3 for the base light is branched into two for light receiving units  110 I and  110 Q of IQ, and is further branched into two for optical modulation units  120 I and  120 Q of IQ. 
     Describing an I processing unit  9021 , the base light of an optical waveguide  1031  of Group 3 and the reference light of an optical waveguide  1021  of Group 2 are multiplexed and then branched again, and are input to the light receiving unit  110 I. The light receiving unit  110 I includes a pair of differential-type light receiving elements  111 I and a comparator  112 I. 
     The base light of the optical waveguide  1031  of Group 3 is input to the optical modulation unit  120 I. A modulation output of the optical modulation unit  120 I is multiplexed into the optical waveguide  1021  of Group 2. A phase shifter (PS)  9031  is provided over the optical waveguide  1021  of Group 2. In the drawing, Inc corresponds to the controller  604  (see  FIG.  6 A ). The controller  604  monitors the output of the comparator  112 I and controls the phases of the base light and the reference light to be the same. 
     An output of the comparator  112   a  and an output of the comparator  112 I are input to a comparator  9051 . The comparator  9051  compares outputs of the comparator  112   a  and the comparator  112 I, and digitally outputs an I component of a comparison result. 
     Also, a Q processing unit  902 Q has the same configuration as that of the I processing unit  9021 . In the Q processing unit  902 Q, the base light of the optical waveguide  103 Q of Group 3 and the reference light of the optical waveguide  102 Q of Group 2 are input to the light receiving unit  110 Q. 
     The base light of the optical waveguide  103 Q of Group 3 is input to the optical modulation unit  120 Q, and a modulation output of the optical modulation unit  120 Q is multiplexed into the optical waveguide  102 Q of Group 2. An output of a comparator  112   b  and an output of a comparator  112 Q are input to a comparator  905 Q. The comparator  905 Q compares the outputs of the comparator  112   b  and the comparator  112 Q, and digitally outputs a Q component of a comparison result. 
       FIGS.  10 A and  10 B  are explanatory diagrams of an operation example of the optical AD converter for IQ modulation.  FIG.  10 A  illustrates only the light receiving unit  110   b  and the Q processing unit  902 Q, and omits illustration of the delay device τ ( 130 ) and phase adjustment. The converted light input to the optical AD converter  900  for IQ modulation has an electric field vector E S , and the LO light has an electric field vector E LO .  FIG.  10 B  illustrates the electric field intensity on the IQ axis. 
     In this case, as an output of the 90-degree hybrid circuit  901 , one of the differential-type light receiving elements  111   b  of the light receiving unit  110   b  detects an electric field vector E Lo +iE S , and the other of the light receiving elements  111   b  detects an electric field vector E LO −iE S . The comparator  112   b  extracts an iE LO  direction projection of E S ×|E LO | of illustrated in  FIG.  10 B  (E S *E Lo −E S E LO *, * is a complex conjugate), and outputs the extracted result to the comparator  905 Q as a comparison result. 
     On the other hand, the comparator  112 Q of the light receiving unit  110 Q of the Q processing unit  902 Q extracts an iE LO  direction projection of E ref ×|E LO | illustrated in  FIG.  10 B  (E ref *E LO −E ref E LO *), and outputs the extracted result to the comparator  905 Q as a comparison result. 
     As illustrated in  FIG.  10 B , E LO  and E ref  are oriented in the same direction by the phase adjustment of the controller  604  of the Q processing unit  902 Q. Accordingly, the comparator  905 Q may output the digital value of the Q component of the bit 1. 
       FIG.  11    is a circuit diagram illustrating another configuration example of the optical AD converter for IQ modulation. An optical AD converter  1100  for IQ modulation illustrated in  FIG.  11    is a configuration example in which the number of the comparators  112   a ,  112   b ,  9051 , and  905 Q used in the optical AD converter  900  for IQ modulation illustrated in  FIG.  9    is reduced. Constituent elements in  FIG.  11    that are similar to those illustrated in  FIG.  9    are denoted by the same reference signs. 
     As illustrated in  FIG.  11   , the light receiving units  110   a  and  110   b  are provided with only the light receiving elements  111   a  and  111   b , respectively, and the comparators  112   a  and  112   b  illustrated in  FIG.  9    are not provided. Also in the I processing unit  9021  and the Q processing unit  902 Q, only the light receiving elements  111 I and  111 Q are provided, and the comparators  112 I and  112 Q are not provided. 
     An output of the light receiving element  111   a  and an output of the light receiving element  111 I for the I component are input to the discriminator  1101 I. An output A of the light receiving element  111   a  is I 1 −1 2 , and an output B of the light receiving element  111 I is I 3 −1 4 . Accordingly, the discriminator  1101 I may output the digital value of the I component of the bit 1 for (I 1 −1 2 )−(I 3 −1 4 ). 
     Similarly, an output of the light receiving element  111   b  and an output of the light receiving element  111 Q for the Q component are input to the discriminator  1101 Q. Accordingly, the discriminator  1101 Q may output the digital value of the Q component of the bit 1. 
     According to the configuration example in  FIG.  11   , two discriminators for comparison may be provided for one stage, and the number of comparators may be reduced with respect to the six comparators  112  ( 112   a ,  112   b ,  112 I,  112 Q,  9051 ,  905 Q) used in the configuration example in  FIG.  9    and power consumption may be reduced. 
       FIG.  12    is an explanatory diagram of a branching ratio of the intensity of light to be incident on the light receiving element of the optical AD converter for IQ modulation. By using the optical AD converter  1100  for IQ modulation illustrated in  FIG.  11    as an example, the branching ratio of the light intensity with respect to the pair of differential-type light receiving elements  111  will be described with reference to  FIG.  12   . 
     In  FIG.  12   , a pair of branching ratios in the light receiving element  111   a  is a 1 , and a pair of branching ratios in the light receiving element  111   b  is a 1 . As for the light receiving element  111 I of the I processing unit  9021 , meanwhile, the branching ratio of the base light of Group 3 is a 1 , and on the other hand, the branching ratio of the reference light of Group 2 is b 1 . A branching ratio of the base light of Group 3 with respect to the optical modulation units  120 I and  120 Q is c 1 , and an attenuation of light intensity is d. The light intensity of the reference light of Group 2 is denoted by e. 
     In this case, the branching ratio a 1  is set based on the following formula (1). The branching ratio b 1  is set based on the following formula (2). The branching ratio c 1  is set based on the following formula (3). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     The optical AD converter  1100  for IQ modulation has N stages (Stages 1 to N), and when the same branching ratio is set in each stage, the light intensity decreases in the latter stages. For this reason, by the above-described setting, the attenuation in each stage, for example, 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. Accordingly, the light intensities of the signal light (converted light, base light, and reference light) incident on each of the N stages (Stages 1 to N) may be made uniform. 
     [Configuration Example of Optical Receiver] 
       FIG.  13    to  FIG.  16    illustrate a configuration example of an optical receiver. The configuration example of the optical receiver including the optical AD converters  100  and  1100  described above will be described. Constituent elements similar to those of the above-described configuration are denoted by the same reference signs in the drawings. 
     Wavelength division multiplexing (WDM) light is input to an optical receiver  1300  illustrated in  FIG.  13   . The WDM light is input to a wavelength demultiplexing unit  1350  via an erbium doped fiber amplifier (EDFA)  1310 . Local light (LO light) from a local oscillation light source  1320  is input to the wavelength demultiplexing unit  1350 . 
     The local oscillation light source  1320  includes a light source (laser diode (LD))  1321  in accordance with a plurality of wavelengths, a combiner  1322  that multiplexes light of the LDs having a plurality of wavelengths, and a semiconductor optical amplifier (SOA)  1323 . The LO light output from the local oscillation light source  1320  is optically amplified by an EDFA  1330 , branched by a fiber coupler  1331 , and input to the wavelength demultiplexing unit  1350 . 
     The wavelength demultiplexing unit (Demux)  1350  separates and outputs input WDM light and LO light by wavelength, and outputs the WDM light and the LO light to a plurality of receiving units  1360  by wavelength. A reception signal (converted light) input to the receiving unit  1360  of a certain wavelength (one channel) is input to the 90-degree hybrid circuit  901  via a variable optical attenuator (VOA)  1361 . A MPD  1362  monitors converted light input thereto, and variably controls the VOA  1361 . 
     In a subsequent stage of the 90-degree hybrid circuit  901 , the light receiving unit  110  described above is provided. The light receiving unit  110  includes the pair of light receiving elements  111  for I and Q and the discriminator  112 . Although not illustrated, the above-described optical modulation unit  120  is coupled to the light receiving element  111  and the discriminator  112 . A bitwise digital output of the discriminator  112  is input to a digital signal processor (DSP)  1363  as a data processing unit. The DSP  1363  performs data processing on a digital signal after AD conversion, extracts information included in the converted light, and outputs the extracted information. 
     The plurality of light receiving units  1360  performs reception processing by different wavelengths, thereby performing reception processing on the WDM light by wavelength (a plurality of channels). 
     Further, in the example illustrated in  FIG.  13   , an IQ modulation unit  1370  is coupled to an output of the wavelength demultiplexing unit  1350  in the plurality of light receiving units  1360 . A modulation output of the IQ modulation unit  1370  of the plurality of light receiving units  1360  is multiplexed by a wavelength multiplexing unit (Mux)  1380 , and may be output to the outside as a WDM signal. 
       FIG.  14    illustrates a configuration example of the 1-channel receiver  1360 . The 1-channel receiver  1360  for WDM signals includes the 90-degree hybrid circuit  901 , the local oscillation light source  1320 , and the DSP  1363 , in addition to the optical AD converter  100  described above. 
     As described above, the 1-channel receiver  1360  includes N stages (Stages 1 to N), guides the converted light and the LO light from the local oscillation light source  1320  between the stages, and outputs the bit value of digit corresponding to the digital signal for each stage. Stages 1 to N each include the light receiving unit  110  and the optical modulation unit  120 . 
     An IQ digital signal output from each stage (Stages 1 to N) is output to the DSP  1363 . The DSP  1363  performs data processing on a digital signal after AD conversion, extracts information included in the converted light, and outputs N-bit digital signals of MSB to LSB in the binary output. 
       FIG.  15    illustrates a configuration example of a 1-polarization receiver  1500 . In the 1-polarization receiver  1500 , a plurality of 1-channel receivers  1360  is coupled to the wavelength demultiplexing unit (Demux)  1350  by channel. The plurality of 1-channel receivers  1360  each outputs information included in the converted light as a digital signal of a plurality of bits. 
       FIG.  16    illustrates a configuration example of a WDM receiver  1600 . A polarization multiplexed WDM signal is input to the WDM receiver  1600 , and a polarized wave X and a polarized wave Y are separated and output by a polarization splitter  1601 . An output of each of the polarized waves X and Y is output to the 1-polarization receiver  1500 . Each of the 1-polarization receivers  1500  outputs information included in the converted light as a digital signal of a plurality of bits in accordance with the plurality of channels. 
     Based on a plurality of outputs of the 1-polarization receivers  1500  for the polarized wave X and the polarized wave Y, a DSP  1602  outputs information for the polarized wave X and the polarized wave Y of the converted light as a digital signal of a plurality of bits. The DSP  1602  may integrate processing with the DSP  1363  included in the 1-polarization receiver  1500  (1-channel receiver  1360 ). 
     The optical AD converter of the embodiment may be applied to various receivers that receive signal light by polarization multiplexing such as WDM communication and various modulation methods and perform AD conversion on the signal light. 
     The optical AD converter according to the embodiments described above is an optical AD converter that converts an analog signal of information included in input signal light into a digital signal, in which N stages corresponding to the number N of bits of the digital signal each include optical waveguides that respectively guide signal light, base light obtained by branching local light, and reference light obtained by branching the local light, a light receiving unit 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 that variably controls the light level of the base light based on the comparison result of the light receiving unit. A modulation output of the optical modulator is multiplexed with reference light in the next stage. 
     The optical AD converter is an optical AD converter that converts an analog signal of information included in input signal light into a digital signal, and includes N stages corresponding to the number N of bits of the digital signal, in which the N stages each include an optical waveguide of Group 1 that branches and inputs the signal light and is configured by one or more waveguides, an optical waveguide of Group 2 that is configured by one or more waveguides and guides, when one of branched local light is referred to as reference light, the reference light, and an optical waveguide of Group 3 that is configured by one or more waveguides and guides, when the other of the branched local light is referred to as base light, the base light. A light receiving unit that detects and compares light levels of the signal light and the reference light and outputs a binary comparison result as a digital value of the stage, and an optical modulator into which the base light guided by the optical waveguide of Group 3 is branched and input, and which variably controls the light level of the base light by modulation based on the comparison result of the light receiving unit are included. The optical waveguide of Group 2 multiplexes a modulation output of the above-described modulation unit with the reference light and guides the multiplexed signal as the reference light of the next stage, thereby outputting an N-bit digital signal by the N stages. 
     Accordingly, bit-by-bit sequential comparison may be performed as in the existing AD converter. The existing AD converter requests linearity for a TIA, a DAC, and the like of an electric circuit, resulting in increased power consumption. By contrast, the optical AD converter of the embodiment does not use a TIA, a DAC, and an S/H circuit. Further, since the circuit for the sequential processing includes not only the electric circuit but also the optical circuit, and comparators of the electric circuit are respectively disposed in the plurality of N stages that perform the sequential comparison, the electric circuit such as the comparator may perform the comparison processing without requesting linearity, and low power consumption may be achieved. 
     In the optical AD converter, the light receiving unit may be configured to include a pair of differential-type light receiving elements that detect the signal light and the reference light, and a comparator that compares and outputs the light levels of the signal light and the reference light based on the differential output of the pair of light receiving elements. Unlike the existing AD converter in which the light receiving unit is provided outside, the optical AD converter of the embodiment may achieve low power consumption because the light receiving unit is provided for each stage inside and linearity is not requested for the comparator of the light receiving unit. 
     In the optical AD converter, the comparator outputs a value 1 when the light level of the reference light is higher than the light level of the signal light, and outputs a value 0 in other cases. Accordingly, a binary digital output for each bit may be performed with a simple configuration of the comparator. 
     The optical AD converter sets a predetermined branching ratio for one and the other of the light receiving elements, and equalizes input levels to the N stages. By setting the same branching ratio in the N stages, the light intensity decreases in the latter stages. Regarding this point, by appropriately setting attenuation in each stage, for example, the branching ratio in the light receiving element to which the modulation output of the optical modulation unit is input, the light intensity of signal light (converted light, base light, and reference light) incident on each stage of the N stages may be made uniform, and the bit determination may be stably performed. 
     Further, in the optical AD converter, the optical modulator performs phase modulation in phase or in opposite phase with the reference light by using the base light based on the comparison result of the light receiving unit. For example, the optical modulator performs phase modulation in phase with the reference light when the output of the comparator has a value 1, and performs phase modulation in opposite phase with the reference light when the output of the comparator has a value 0. Accordingly, in the optical modulator, the modulation output of the modulation unit is multiplexed with the reference light in the next stage only by switching the modulation output to the same phase or the opposite phase, and the light intensity of the reference light in the next stage may be controlled. 
     In the optical AD converter, a delay device that matches the timing of the signal light, the reference light, and the base light may be provided in the optical waveguide. By setting an appropriate delay time in the delay device, it is possible to match the timing of the signal light of each group in each of the N stages, and to stably perform bit determination in each stage. 
     The optical AD converter may use an optical modulator that arranges a pair of electrodes along a branched optical waveguide and performs voltage control on the electrodes. As described above, it is possible to easily obtain the optical AD converter by using the optical modulator for general purposes. 
     The optical waveguide is branched by IQ in accordance with input of signal light for IQ modulation, the optical AD converter may provide a 90-degree hybrid circuit in the optical waveguide of the signal light. 
     The optical modulator may be configured to include the optical AD converter described above, a local oscillation light source that generates local light, and a data processing unit that outputs information included in signal light after AD conversion by the optical AD converter. For example, the optical modulator may be easily obtained by using a general-purpose local oscillation light source and a DSP as a data processing unit. 
     The optical modulator may be simply configured to include a polarization splitter that performs polarization separation on signal light in accordance with input of a WDM signal of the signal light that is polarization multiplexed. 
     For these reasons, according to the optical AD converter of the embodiment, it is possible to directly input the optical signal and output the digital electric signal, and thus low power consumption may be achieved. According to the embodiment, data addition and subtraction processing and signal latching performed by the electric circuit of the existing AD converter are realized by branching/multiplexing of the optical circuit (optical waveguide) and variable control of the light intensity of reference light by the optical modulator. The optical circuit may be reduced in size by miniaturization and integration of the optical waveguide. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.