Optical transmission device and transmission method

Abstract

This optical transmitter comprises: a splitter that splits a first electrical signal into a second electrical signal and a third electrical signal; a phase shifter that shifts the phase of the third electrical signal by 180 degrees; a first optical phase modulator that modulates the phase of output light from a first laser diode on the basis of the second electrical signal; a second optical phase modulator that modulates the phase of output light from a second laser diode on the basis of the third electrical signal; an optical multiplexer that multiplexes the output light from the first optical phase modulator and the output light from the second optical phase modulator to separate into first output light and second output light; and a light intensity modulator that modulates the intensity of output light from a third laser diode on the basis of a synthesized component obtained by synthesizing a first heterodyne detection signal and a second heterodyne detection signal which are outputted from a connection point between an anode of either a first photodiode or a second photodiode and a cathode of the other of the first and second photodiodes, the first photodiode converting the first output light into a heterodyne detection signal and the second photodiode converting the second output light into a heterodyne detection signal.

Description

Technical Field

[0001] The present invention relates to an optical transmission device and a transmission method.

Background Art

[0002] As a network system for distributing video to subscriber homes, for example, an FTTH (Fiber to the Home) type CATV (Cable Television) system is known. FIG. 5 shows an example of the network configuration of a conventional FTTH type CATV system 1. As shown in FIG. 5, the FTTH type CATV system 1 includes, for example, a head end 100 (Head-End), an optical transmission device 200 (Tx), an amplifier 300 (V-OLT), and an optical reception device 400 (V-ONU) installed in each subscriber home or the like.

[0003] The head end 100 receives the radio wave carrying the video signal transmitted from the broadcasting station via a ground transmission tower, an artificial satellite, or the like, and performs adjustments such as amplification on the received radio wave. Then, the head end 100 outputs an electrical signal based on the video signal to the optical transmission device 200. The optical transmission device 200 converts the acquired electrical signal into an optical signal and sends the optical signal to an optical transmission path constructed by optical fibers. The optical transmission path is divided into a section of a relay network 500 and a section of an access network 600.

[0004] The relay network 500 is a communication network that connects between the optical transmission device 200 and the access network 600. In the relay network 500, when the transmission distance reaches a long distance or the like, amplifiers 300 functioning as relay amplifiers are configured in multiple stages. Each amplifier 300 (V-OLT) sends the amplified optical signal to another amplifier 300 in the subsequent stage, or to a device such as an optical reception device 400 (V-ONU) within the section of the access network 600, or branches the optical signal by an optical coupler and sends it to both another amplifier 300 in the subsequent stage and a device within the section of the access network 600.

[0005] On the one hand, the access network 600 is a communication network that connects between the relay network 500 and each optical receiver 400 that terminates an optical signal. In the access network 600, in order to distribute the optical signal output from the relay network 500 to the optical receivers 400 installed in a plurality of subscriber homes, generally, a PON (Passive Optical Network) configuration is applied. Further, as shown in FIG. 5, for the purpose of compensating for losses associated with the distribution of the optical signal by the PON configuration and losses associated with the branching of the optical signal by the amplifier 300, an amplifier (access amplifier) (not shown) may also be used in the access network 600.

[0006] In the conventional FTTH type CATV system 1 having the above network configuration, for example, an FM (Frequency Modulation) batch conversion method is used as the optical transmission method (see, for example, Non-Patent Document 1). In this FM batch conversion method, the optical transmitter 200 receives the electrical signals of the frequency-division multiplexed multi-channel videos output from the head end 100, and collectively converts the electrical signals into a broadband frequency modulation (FM) signal of one channel. Further, the optical transmitter 200 converts the converted FM signal into an optical signal by intensity modulation and transmits it to the optical transmission line. On the other hand, when the optical receiver 400 receives an optical signal from the optical transmission line, the optical receiver 400 converts the optical signal into an electrical FM signal and further demodulates it. Thereby, the optical receiver 400 can extract the electrical signals of the frequency-division multiplexed multi-channel videos.

[0007] Here, an example of the configuration of the conventional optical transmission device 200b will be described. FIG. 6 shows the configuration of an optical transmission device 200b (Tx) disclosed in Non-Patent Document 2. Frequency-division multiplexed multi-channel electrical signals are separately input to the optical transmission device 200b for each band. Here, electrical signal A is input to the electrical signal input terminal 11, and electrical signal B is input to the electrical signal input terminal 12. Electrical signal A is input to an optical phase modulator 31 connected to the subsequent stage of a laser diode 21 (LD) to phase-modulate the optical signal. On the other hand, electrical signal B is branched into two by a splitter 8 and then directly modulates the optical signal with laser diodes 21 and 22. When the optical signal is directly modulated by the input signal, frequency chirping occurs (i.e., frequency modulation is performed). At this time, as shown in FIG. 6, by using a phase shifter 9 for one of the electrical signals B branched by the splitter 8, electrical signals having opposite phases can be respectively input to the laser diode 21 and the laser diode 22. Thereby, the remaining intensity modulation component is suppressed.

[0008] The optical signals output from the laser diode 21 and the laser diode 22 are multiplexed by an optical multiplexer 4 and input to a photodiode 51 (PD). In the photodiode 51, optical heterodyne detection is performed, and an FM signal centered on a frequency equal to the oscillation frequency difference between the laser diode 21 and the laser diode 22 is output from the photodiode 5. The FM signal is input to an optical intensity modulator 6 to intensity-modulate the output light from the transmission laser diode 23. The signal light generated by the intensity modulation is transmitted to the optical reception device 400 through an optical fiber.

[0009] Here, the roles of the laser diode 21 and the laser diode 22 when modulating the electrical signal A will be described. FIG. 13 is a block diagram showing an example of the configuration of the conventional optical transmission device 200. However, the description of the components not directly related to the modulation of the electrical signal A is omitted.

[0010] At the input terminal of the photodiode 51, the electric field E1(t) of the output light from the optical phase modulator 31 and the electric field E2(t) of the output light from the laser diode 22 can be expressed by the following equations (1) and (2), respectively.

[0011]

Equation

[0012]

Equation

[0013] Here, t is the time. E1(t) and E2(t) are the instantaneous values of the electric field at time t. E1 and E2 are the maximum values of the electric field. ω1 and ω2 are the angular frequencies of the electric field without modulation. The electrical signal φ1(t) is the electrical signal input from the electrical signal input terminal 11 (however, since there is no loss or phase shift change between the electrical signal input terminal 11 and the optical phase modulator 31, it is the same as the electrical signal input to the optical phase modulator 31). m is a constant representing the modulation efficiency of the optical phase modulator 31.

[0014] Based on these optical inputs, heterodyne detection is performed in the photodiode 51. As a result, the current I(t) of the output electrical signal (heterodyne detection signal) is expressed by the following equation (3).

[0015]

Equation

[0016] Here, “<···>” is an operator that performs an averaging process according to the band of the photodiode 51. That is, this operator gives an averaged value to the component that varies at a frequency of ω1 or ω2 or higher.

[0017] Here, as is clear from the above equation (1), the role of the laser diode 21 is to output a carrier light for superimposing the electrical signal φ1(t). Also, as is clear from the above equation (2), the role of the laser diode 22 is to output local light emission for coherently receiving the electrical signal φ1(t).

[0018] As shown in the above equation (3), in the current I(t) output as a result of heterodyne detection, phase modulation is performed in a form proportional to the electrical signal φ1(t) input to the optical phase modulator 31.

Prior Art Documents

Non-Patent Documents

[0019]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0020] For the two laser diodes 21 and 22 for directly modulating the electrical signal B, from the viewpoint of improving the distortion characteristics of the signal, a very high linearity is required between the bias current and the oscillation frequency. For this reason, there is a problem that the sorting cost of each laser diode becomes very high. To solve this problem, a method of processing all electrical signals only by phase modulation without performing direct modulation in the laser diodes 21 and 22 can be considered.

[0021] FIG. 7 is a diagram showing a configuration example of an optical transmission device 200c configured to process all electrical signals only by phase modulation. The configuration of the optical transmission device 200c is a configuration in which the electrical signal input terminal 12, the distributor 8, and the phase shifter 9 are omitted from the configuration of the conventional optical transmission device 200b shown in FIG. 6 described above.

[0022] Here, when the input power of the electrical signal A is small at the electrical signal input terminal 11, the signal power becomes small with respect to the noise generated from the photodiodes 51 and 52. As a result, the signal-to-noise ratio decreases and the signal quality deteriorates (degradation of noise characteristics). On the other hand, when the input power of the electrical signal A is large at the electrical signal input terminal 11, distortion occurs due to an amplifier (not shown) disposed between the electrical signal input terminal 11 and the optical phase modulator 31, and the signal quality deteriorates (degradation of distortion characteristics). Against such a background, in an optical transmission device that performs FM batch conversion, it has been a problem to simultaneously improve the noise characteristics and the distortion characteristics.

[0023] The present invention has been made in view of the above technical background, and an object thereof is to provide a technique capable of simultaneously improving the noise characteristics and the distortion characteristics.

Means for Solving the Problem

[0024] One aspect of the present invention is a distributor that distributes a first electrical signal input to an electrical signal input unit into a second electrical signal and a third electrical signal, a phase shifter that phase-shifts the third electrical signal by 180 degrees, a first optical phase modulator that phase-modulates the output light from a first laser diode based on the second electrical signal output from the distributor, a second optical phase modulator that phase-modulates the output light from a second laser diode based on the third electrical signal output from the phase shifter, a wavelength multiplexer that multiplexes the output light from the first optical phase modulator and the output light from the second optical phase modulator and demultiplexes the multiplexed output light into a first output light and a second output light, a first photodiode that converts the first output light from the wavelength multiplexer into a first heterodyne detection signal, a second photodiode that converts the second output light from the wavelength multiplexer into a second heterodyne detection signal, and an optical intensity modulator that intensity-modulates the output light from a third laser diode based on a composite component of the first heterodyne detection signal and the second heterodyne detection signal output from a connection point between the anode of one of the first photodiode and the second photodiode and the cathode of the other.

[0025] One aspect of the present invention includes a distribution step of distributing a first electrical signal input to an electrical signal input unit into a second electrical signal and a third electrical signal, a phase shift step of phase-shifting the third electrical signal by 180 degrees, a first optical phase modulation step of phase-modulating output light from a first laser diode based on the second electrical signal, a second optical phase modulation step of phase-modulating output light from a second laser diode based on the third electrical signal phase-shifted by 180 degrees, a multiplexing step of multiplexing the output light phase-modulated by the first optical phase modulation step and the output light phase-modulated by the second optical phase modulation step, and demultiplexing the multiplexed output light into a first output light and a second output light, a first conversion step of converting the first output light multiplexed by the multiplexing step into a first heterodyne detection signal by a first photodiode, a second conversion step of converting the second output light multiplexed by the multiplexing step into a second heterodyne detection signal by a second photodiode, and an optical intensity modulation step of intensity-modulating output light from a third laser diode based on a composite component of the first heterodyne detection signal and the second heterodyne detection signal output from a connection point between an anode of one of the first photodiode and the second photodiode and a cathode of the other.

Advantages of the Invention

[0026] According to the present invention, in an optical transmission device that performs FM batch conversion, it is possible to simultaneously improve noise characteristics and distortion characteristics.

Brief Description of the Drawings

[0027]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

[0028] Hereinafter, embodiments of the present invention will be described. Note that since the system configuration of the network system in each of the embodiments described below is the same as the network configuration of the conventional FTTH type CATV system 1 shown in FIG. 5 described above, the description will be omitted.

[0029] <Embodiment> Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0030] [Configuration of Optical Transmitter] Hereinafter, the configuration of the optical transmitter according to the embodiment of the present invention will be described. FIG. 1 is a block diagram showing the configuration of the optical transmitter 200a according to the embodiment of the present invention.

[0031] As shown in FIG. 1, the optical transmitter 200a includes an electrical signal input terminal 11, a distributor 8, a phase shifter 9, three laser diodes (laser diodes 21 to 23) (LDs), two optical phase modulators (optical phase modulators 31 to 32), a 2-input 2-output optical combiner 41, two photodiodes (photodiodes 51 to 52) (PDs), an optical intensity modulator 6, and an optical signal output terminal 7.

[0032] The electrical signal A is taken into the interior of the optical transmission device 200a from the electrical signal input terminal 11 and input to the input terminal of the distributor 8. The distributor 8 divides the input electrical signal A into two. One of the electrical signals A divided by the distributor 8 is input to the input terminal of the optical phase modulator 31. The other of the electrical signals A divided into two is input to the input terminal of the phase shifter 9. The electrical signal A input to the phase shifter 9 is phase-shifted by 180 degrees and then input to the input terminal of the optical phase modulator 32.

[0033] The light output from the laser diode 21 is phase-modulated by the electrical signal A in the optical phase modulator 31. Also, the light output from the laser diode 22 is phase-modulated by the phase-shifted electrical signal A in the optical phase modulator 32. These phase-modulated lights are combined by a 2-input 2-output optical combiner 41, then demultiplexed, and input to the photodiode 51 and the photodiode 52, respectively.

[0034] Heterodyne detection is performed respectively in the photodiode 51 and the photodiode 52. The electrical signals (heterodyne detection signals) output respectively from the photodiode 51 and the photodiode 52 are input to the input terminal of the optical intensity modulator 6.

[0035] On the other hand, the light output from the laser diode 23 is intensity-modulated in the optical intensity modulator 6 based on the combined component of the electrical signals (heterodyne detection signals) output respectively from the photodiode 51 and the photodiode 52. The intensity-modulated signal light is output from the optical signal output terminal 7 to the outside of the optical transmission device 200a.

[0036] FIG. 2 is a diagram showing the configuration of a balanced receiver according to an embodiment of the present invention. The above two photodiodes 51 and photodiode 52 constitute a balanced receiver. As a method of configuring a balanced receiver, for example, as shown in FIG. 2(a), there is a method of connecting the anode of the photodiode 51 and the cathode of the photodiode 52 and outputting a current from the connection point (note that a constant voltage source or ground connected to the cathode of the photodiode 51 and the anode of the photodiode 52 is not shown in FIG. 15(a)).

[0037] Also, as another method of configuring a balanced receiver, for example, as shown in FIG. 15(b), there is a method of connecting the cathode of the photodiode 51 and the anode of the photodiode 52 and outputting a current from the connection point (note that a constant voltage source or ground connected to the anode of the photodiode 51 and the cathode of the photodiode 52 is not shown in FIG. 15(b)).

[0038] Heterodyne detection is performed on the photodiode 51 and the photodiode 52, respectively. The electrical signals (heterodyne detection signals) output from the photodiode 51 and the photodiode 52 are combined and then input to the input terminal of the optical intensity modulator 6. On the other hand, the light output from the laser diode 23 is intensity-modulated in the optical intensity modulator 6 based on the combined component of the electrical signals (heterodyne detection signals) output from the photodiode 51 and the photodiode 52, respectively. The intensity-modulated signal light is output from the optical signal output terminal 7 to the outside of the optical transmission device 200a.

[0039] Note that when the optical transmission device 200h drives the optical phase modulator 31 and the optical phase modulator 32, or the optical intensity modulator 6, it may adjust the amplitude or bias of the electrical signals input from the respective electrical signal input terminals and control the modulation amount. Therefore, an amplifier, an attenuator, and a bias tee may be used in the optical transmission device 200h.

[0040] At the input end of the optical multiplexer 41, the electric fields E'1(t) of the output light of the optical phase modulator 31 and E'2(t) of the output light of the optical phase modulator 32 can be expressed by the following equations (4) and (5) respectively (details will be explained in the addendum described later).

[0041]

Equation

[0042]

Equation

[0043] Here, t is the time. E'1(t) and E'2(t) are the instantaneous values of the electric field at time t. (√2·E1 + √2·e1(t)) and (√2·E2 + √2·e2(t)) are the amplitudes of the electric field. Among these, √2·E1 and √2·E2 are constants representing the output electric field amplitudes of the laser diodes 21 and 22.

[0044] On the other hand, √2·e1(t) and √2·e2(t) are variables representing the fluctuations of the output electric field amplitudes of the laser diodes 21 and 22 at time t.

[0045] ω1 and ω2 are the angular frequencies of the electric field without modulation. The electrical signal φ1(t) is the electrical signal input from the electrical signal input terminal 11 (here, whether the terms including φ1(t) in equation (4) or (5) are added or subtracted respectively depends on the presence or absence of the phase shifter 9). m is a constant representing the modulation efficiency of the optical phase modulator 31 and the optical phase modulator 32.

[0046] In the optical multiplexer 41, generally, when the light input from terminal a (or terminal b) is split and output from terminals A and B, the phases of the output lights have a relative difference of 90 degrees.

[0047] Therefore, the electric field at the input terminal of the photodiode 51 for the light transmitted through the optical combiner 41 and output from the terminal A is the component E A1 derived from the optical phase modulator 31 and the component E A2 derived from the optical phase modulator 32, which can be expressed by the following equations (6) and (7) respectively (details will be described in the addendum below).

[0048]

Equation

[0049]

Equation

[0050] As a result of the relative 90-degree difference in the phase between the output lights, the phase of the electric field of the light transmitted from the terminal a to the terminal A does not change (cos in Equation (4) ⇒ cos in Equation (6)), and the phase of the electric field of the light transmitted from the terminal b to the terminal A changes by 90 degrees (-sin in Equation (5) ⇒ cos in Equation (7)).

[0051] Similarly, the electric field at the input terminal of the photodiode 52 for the light transmitted through the optical combiner 41 and output from the terminal B is the component E B1 derived from the optical phase modulator 31 and the component E B2 derived from the optical phase modulator 32, which can be expressed by the following equations (8) and (9) respectively.

[0052]

Equation

[0053]

Equation

[0054] As a result, the phases of the output lights have a relative difference of 90 degrees. Therefore, the phase of the electric field of the light transmitted from terminal a to terminal B changes by 90 degrees (cos in Equation (4) ⇒ sin in Equation (8)), and the phase of the electric field of the light transmitted from terminal b to terminal B does not change (-sin in Equation (5) ⇒ -sin in Equation (9)).

[0055] Here, in photodiode 51, heterodyne detection is performed based on the light inputs represented by Equations (6) and (7). As a result, the current I A (t) of the output heterodyne detection signal is represented by the following Equation (10) using Equations (6) and (7).

[0056]

Equation

[0057] Here, “<···>” is an operator that performs an averaging process according to the band of photodiode 51. That is, this operator gives an averaged value to the components that vary at angular frequencies of ω1 or ω2 or higher. Since e1(t) and e2(t) are minute, the values of e1(t) 2 , e2(t) 2 , and e1(t)e2(t) are set to 0.

[0058] Similarly, in photodiode 52, heterodyne detection is performed based on the light inputs represented by Equations (8) and (9). As a result, the current I B (t) of the output heterodyne detection signal is represented by the following Equation (11) using Equations (8) and (9).

[0059]

Equation

[0060] Here, “<···>” is an operator that performs an averaging process according to the band of the photodiode 52. That is, this operator gives an averaged value to the component that varies at a frequency of ω1 or higher or ω2 or higher. Since e1(t) and e2(t) are very small, e1(t) 2 , e2(t) 2 , and the values of e1(t)e2(t) are assumed to be 0.

[0061] The currents I A (t) and I B (t) output from the photodiode 51 and the photodiode 52 respectively are combined as described above to form an electric signal of the current I(t), and then input to the optical intensity modulator 6. This current I(t) (heterodyne detection signal) is represented by the following equation (12).

[0062]

Equation

[0063] Here, the reason why I A (t) and I B (t) are combined with opposite signs to each other is that the photodiode 51 and the photodiode 52 have a balanced receiver configuration.

[0064] As is clear from the comparison of the above equations (10), (11), and (12), for the configuration with only one photodiode, the DC components E1 2 , E2 2 and the components E1e1(t) and E2e2(t) caused by fluctuations disappear from the output electric signal. In particular, the components E1e1(t) and E2e2(t) caused by fluctuations become signal degradation factors as noise. Therefore, according to the optical transmission device 200a according to the embodiment, there is an effect that the noise characteristics and the distortion characteristics are improved by the disappearance of the components E1e1(t) and E2e2(t).

[0065] Also, as is clear from the comparison between the above equations (3) and (12), for a configuration with only one photodiode, the effective modulation efficiency increases from m to 2m with a configuration having two photodiodes forming a balanced receiver. That is, the power of the electrical signal A required to perform the same modulation as before is halved. Therefore, according to the optical transmission device 200a according to the embodiment, the distortion generated by an amplifier (not shown) disposed between the electrical signal input terminal 11 and the optical phase modulator 31 is reduced, and there is an effect that the distortion characteristics are improved.

[0066] As a prior art using a balanced receiver, a configuration is known in which one signal light and one local oscillation light (unmodulated light) are multiplexed / demultiplexed by an optical multiplexer / demultiplexer, and detection is performed while removing noise (relative intensity noise) generated from the local oscillation light (Reference: Katsumi Ishitani, "Advances in Coherent Optical Communication Technology", Journal of the Optical Society of Japan, Vol. 38, No. 5, p. 239, May 2009). However, this reference does not disclose a configuration in which two signal lights are multiplexed / demultiplexed and detection is performed while removing noise (E1e1(t) and E2e2(t)) generated from both of the two signal lights, as in the optical transmission device 200a in the embodiment, nor is there any suggestion regarding such a configuration and effect.

[0067] [Operation of Optical Transmission Device] Hereinafter, an example of the operation of the optical transmission device according to the embodiment of the present invention will be described. FIG. 3 is a flowchart showing the operation of the optical transmission device 200a according to the embodiment of the present invention.

[0068] The operation of the optical transmission device 200a shown in this flowchart starts when the electrical signal A is input to the electrical signal input terminal 11. The electrical signal input terminal 11 inputs the electrical signal A to the distributor 8. The distributor 8 distributes the electrical signal A into electrical signals A1 and A2, and then inputs the electrical signal A1 to the optical phase modulator 31 and the electrical signal A2 to the phase shifter 9. (Step S01).

[0069] The phase shifter 9 shifts the electrical signal A2 by 180 degrees and then inputs the electrical signal A2 to the optical phase modulator 32. (Step S02).

[0070] The optical phase modulator 31 phase-modulates the light output from the laser diode 21 with the electrical signal A1 (step S03). The optical phase modulator 32 phase-modulates the light output from the laser diode 22 with the electrical signal A2 (step S04).

[0071] The optical combiner 4 combines the output light from the optical phase modulator 31 and the output light from the optical phase modulator 32. Thereafter, the optical combiner 4 demultiplexes the combined output light into a first output light and a second output light. The optical combiner 4 outputs the first output light to the photodiode 51 and the second output light to the photodiode 52 (step S05).

[0072] The photodiode 51 performs heterodyne detection on the first output light output from the optical combiner 41 (step S06). Also, the photodiode 52 performs heterodyne detection on the second output light output from the optical combiner 41 (step S07).

[0073] The optical intensity modulator 6 intensity-modulates the light output from the laser diode 23 based on the combined component of the electrical signal (heterodyne detection signal) output from the connection point between the photodiode 51 and the photodiode 52 (step S08). The optical signal output terminal 7 outputs the intensity-modulated signal light to the relay network 500 (step S09). Thus, the operation of the optical transmission device 200 shown in the flowchart of FIG. 3 ends.

[0074] <Addendum> Hereinafter, with reference to FIG. 4, the phase relationship between the two output lights in the 2-input 2-output optical combiner 41 will be described. FIG. 4 is a diagram for explaining the phase relationship between the two output lights in the 2-input 2-output optical combiner 41.

[0075] Let T be the transfer function representing the relationship between the electric fields and phases of the two input lights and two output lights of the optical combiner 41. Here, due to symmetry, the changes in the electric field and phase when transmitting from "terminal a to terminal A" are the same as those when transmitting from "terminal b to terminal B". Similarly, the changes in the electric field and phase when transmitting from "terminal a to terminal B" are the same as those when transmitting from "terminal b to terminal A". Therefore, the transfer function T has symmetry as expressed by the following equation (13).

[0076]

Equation

[0077] Next, considering transmitting light from left to right in Fig. 4. Here, if the input optical electric fields to terminals a and b of the optical combiner 41-1 at point 1 are 1 and 0 respectively, the electric field at point 1 is expressed by the following equation (14).

[0078]

Equation

[0079] Furthermore, the output optical electric fields from terminals A and B of the optical combiner 41-1 (point 2), and the output optical electric fields from terminals A and B of the optical combiner 41-2 (point 3) are expressed as follows by equations (15) and (16) based on the above equations (13) and (14).

[0080]

Equation

[0081]

Equation

[0082] Here, if there is no loss in the optical multiplexers 41-1 and 41-2, the powers calculated by each of the equations (14) to (16) are the same, so the following equations (17) and (18) hold.

[0083]

Number

[0084]

Number

[0085] By squaring both sides of the above equation (17) and subtracting it from equation (18), the following equation (20) is obtained.

[0086]

Number

[0087]

Number

[0088] Here, if we set α = |α|exp(iθ α ) and β = |β|exp(iθ β ), the above equation (20) is transformed as follows into equation (21).

[0089]

Number

[0090] As a result, the phase relationship of the output optical electric fields from terminals A and B is derived as follows in equation (22).

[0091]

Number

[0092] Above, in the optical transmission device 200a in the embodiment, the reason why the optical phase depends on the input / output ends of the optical multiplexer 41-1 has been described.

[0093] As described above, the optical transmission device 200a according to the above-described embodiment multiplexes two phase-modulated optical signals by the optical multiplexer 41 and then demultiplexes them, and converts them into electrical signals by the configuration of the balanced receiver. By having such a configuration, the optical transmission device 200a can remove noise caused by fluctuations in the output electrical amplitude of the laser diode, and can improve the noise characteristics and distortion characteristics simultaneously.

[0094] Also, as described above, in the optical transmission device 200a according to the above-described embodiment, the optical phase modulator 31 is connected to the subsequent stage of the laser diode 21, and the optical phase modulator 32 is connected to the subsequent stage of the laser diode 22, respectively. Then, the optical transmission device 200a inputs an input signal whose phase is inverted by the phase shifter 8 to one of the optical phase modulators (the optical phase shifter 32 in this embodiment). The optical transmission device 200a multiplexes the light output from the optical phase modulator 31 and the optical phase modulator 31 by the optical multiplexer 41 and then demultiplexes it, and performs square-law detection by the photodiodes 51 and 52. By having such a configuration, the optical transmission device 200a can reduce the input signal voltage to the optical phase modulator 31 and the optical phase modulator 32. Thereby, even when the amplitude of the input signal to the optical phase modulator 31 and the optical phase modulator 32 increases due to, for example, addition of a ch (channel) of the input signal or increase in the bandwidth, the optical transmission device 200a can make it difficult to cause deterioration of the signal quality.

[0095] According to the above-described embodiment, the optical transmission device includes a distributor, a phase shifter, a first optical phase modulator, a second optical phase modulator, an optical multiplexer, a first photodiode, a second photodiode, and an optical intensity modulator. For example, the optical transmission device is the optical transmission device 200a in the embodiment, the distributor is the distributor 8 in the embodiment, the phase shifter is the phase shifter 9 in the embodiment, the first optical phase modulator is the optical phase modulator 31 in the embodiment, the second optical phase modulator is the optical phase modulator 32 in the embodiment, the optical multiplexer is the optical multiplexer 41 in the embodiment, the first photodiode is the photodiode 51 in the embodiment, the second photodiode is the photodiode 52 in the embodiment, and the optical intensity modulator is the optical intensity modulator 6 in the embodiment.

[0096] The above-described distributor distributes the first electrical signal input to the electrical signal input section into a second electrical signal and a third electrical signal. For example, the electrical signal input section is the electrical signal input terminal 11 in the embodiment, the first electrical signal is the electrical signal A in the embodiment, the second electrical signal is the electrical signal A1 in the embodiment, and the third electrical signal is the electrical signal A2 in the embodiment. The above-described phase shifter shifts the third electrical signal by 180 degrees.

[0097] The above-described first optical phase modulator phase-modulates the output light from the first laser diode based on the second electrical signal output from the distributor. For example, the first laser diode is the laser diode 21 in the embodiment. The second optical phase modulator phase-modulates the output light from the second laser diode based on the third electrical signal output from the phase shifter. For example, the second laser diode is the laser diode 22 in the embodiment.

[0098] The above optical multiplexer multiplexes the output light from the first optical phase modulator and the output light from the second optical phase modulator, and demultiplexes the multiplexed output light into a first output light and a second output light. For example, the first output light is the output light from terminal A of the optical multiplexer 41 in the embodiment, and the second output light is the output light from terminal B of the optical multiplexer 41 in the embodiment.

[0099] The above first photodiode converts the first output light from the optical multiplexer into a first heterodyne detection signal. The above second photodiode converts the second output light from the optical multiplexer into a second heterodyne detection signal. The above optical intensity modulator intensity-modulates the output light from the third laser diode based on the composite component of the first heterodyne detection signal and the second heterodyne detection signal output from the connection point between the anode of one of the first photodiode and the second photodiode and the cathode of the other. For example, the third laser diode is the laser diode 23 in the embodiment.

[0100] Note that the above optical transmission device may further include an amplifier that amplifies the first electrical signal or the second electrical signal when the first optical phase modulator is driven.

[0101] Note that the above optical transmission device may further include an amplifier that amplifies the first electrical signal or the third electrical signal when the second optical phase modulator is driven.

[0102] Note that the above optical transmission device may further include an attenuator that attenuates the first electrical signal or the second electrical signal when the first optical phase modulator is driven.

[0103] Note that the above optical transmission device may further include an attenuator that attenuates the first electrical signal or the third electrical signal when the second optical phase modulator is driven.

[0104] Incidentally, the above-described optical transmission device may further include a bias tee that adjusts the bias of the first electrical signal or the second electrical signal when the first optical phase modulator is driven.

[0105] Incidentally, the above-described optical transmission device may further include a bias tee that adjusts the bias of the first electrical signal or the third electrical signal when the second optical phase modulator is driven.

[0106] Part or all of the optical transmission device 200a in the above-described embodiment may be realized by a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on this recording medium may be read into a computer system and executed to realize it. Here, the "computer system" is assumed to include hardware such as an OS and peripheral devices. Further, the "computer-readable recording medium" refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, or the like, and a storage device such as a hard disk built into the computer system. Furthermore, the "computer-readable recording medium" refers to a communication line such as a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, which dynamically holds the program for a short time, and also includes a volatile memory inside a computer system that becomes a server or a client in that case and holds the program for a certain period of time. Also, the above program may be for realizing a part of the above-described functions, and may further be realized in combination with a program already recorded in the computer system for realizing the above-described functions, or may be realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0107] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

Explanation of Reference Numerals

[0108] 1…FTTH type CATV system, 41…optical combiner, 51, 52…photodiodes, 6…optical intensity modulator, 7…optical signal output terminal, 8…distributor, 9…phase shifter, 11, 12…electrical signal input terminals, 21, 22, 23…laser diodes, 31, 32…optical phase modulators, 100…headend, 200a~c…optical transmission devices, 300…amplifier, 400…optical reception device, 500…relay network, 600…access network

Claims

1. A distributor that distributes a first electrical signal input to an electrical signal input unit into a second electrical signal and a third electrical signal, A phase shifter that phase-shifts the third electrical signal by 180 degrees, A first optical phase modulator that phase-modulates the output light from a first laser diode based on the second electrical signal output from the distributor, A second optical phase modulator that phase-modulates the output light from a second laser diode based on the third electrical signal output from the phase shifter, An optical multiplexer that multiplexes the output light from the first optical phase modulator and the output light from the second optical phase modulator, and demultiplexes the multiplexed output light into a first output light and a second output light, A first photodiode that converts the first output light from the optical multiplexer into a first heterodyne detection signal, A second photodiode that converts the second output light from the optical multiplexer into a second heterodyne detection signal, An optical intensity modulator that intensity-modulates the output light from a third laser diode based on a composite component of the first heterodyne detection signal and the second heterodyne detection signal output from a connection point between the anode of one of the first photodiode and the second photodiode and the cathode of the other, An optical transmission device comprising the above.

2. An amplifier that amplifies the first electrical signal or the second electrical signal when the first optical phase modulator is driven The optical transmission device according to claim 1, further comprising the above.

3. An amplifier that amplifies the first electrical signal or the third electrical signal when the second optical phase modulator is driven The optical transmission device according to claim 1 or 2, further comprising the above.

4. An attenuator that attenuates the first electrical signal or the second electrical signal when the first optical phase modulator is driven The optical transmission device according to any one of claims 1 to 3, further comprising the above.

5. An attenuator that attenuates the first electrical signal or the third electrical signal when the second optical phase modulator is driven The optical transmission device according to any one of claims 1 to 4, further comprising the above.

6. A bias tee that adjusts the bias of the first electrical signal or the second electrical signal when the first optical phase modulator is driven The optical transmission device according to any one of claims 1 to 5, further comprising the above.

7. A bias adjuster for adjusting a bias of the first electrical signal or the third electrical signal when the second optical phase modulator is driven The optical transmission device according to any one of claims 1 to 6, further comprising the bias adjuster. **Claim 8** A distribution step of distributing a first electrical signal input to an electrical signal input unit into a second electrical signal and a third electrical signal; A phase shift step of phase-shifting the third electrical signal by 180 degrees; A first optical phase modulation step of phase-modulating output light from a first laser diode based on the second electrical signal; A second optical phase modulation step of phase-modulating output light from a second laser diode based on the third electrical signal phase-shifted by 180 degrees; A multiplexing step of multiplexing the output light phase-modulated in the first optical phase modulation step and the output light phase-modulated in the second optical phase modulation step, and demultiplexing the multiplexed output light into a first output light and a second output light; A first conversion step of converting the first output light multiplexed in the multiplexing step into a first heterodyne detection signal by a first photodiode; A second conversion step of converting the second output light multiplexed in the multiplexing step into a second heterodyne detection signal by a second photodiode; An optical intensity modulation step of intensity-modulating output light from a third laser diode based on a composite component of the first heterodyne detection signal and the second heterodyne detection signal output from a connection point between an anode of one of the first photodiode and the second photodiode and a cathode of the other; A transmission method having the above steps.

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