JP7710146B2 - Optical transmitting device and transmitting method - Google Patents

Description

The present invention relates to an optical transmitting device and a transmitting method.

As a network system for distributing video to subscribers' homes, for example, an FTTH (Fiber to the Home) CATV (Cable Television) system is known. FIG. 5 shows an example of the network configuration of a conventional FTTH CATV system 1. As shown in FIG. 5, the FTTH CATV system 1 is configured to include, for example, a head end 100 (Head-End), an optical transmitting device 200 (Tx), an amplifier 300 (V-OLT), and an optical receiving device 400 (V-ONU) installed in each subscriber's home, etc.

The headend 100 receives radio waves carrying video signals transmitted from broadcast stations via terrestrial transmission towers or artificial satellites, and performs adjustments such as amplification on the received radio waves. The headend 100 then 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 of optical fiber. The optical transmission path is divided into a section for the relay network 500 and a section for the access network 600.

The relay network 500 is a communication network that connects the optical transmission device 200 and the access network 600. In the relay network 500, when the transmission distance is long, the amplifiers 300 that function 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, sends it to equipment such as an optical receiving device 400 (V-ONU) in 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 equipment in the section of the access network 600.

On the other hand, the access network 600 is a communication network that connects the relay network 500 and each optical receiving device 400 that terminates the optical signal. In the access network 600, a PON (Passive Optical Network) configuration is generally applied to distribute the optical signal output from the relay network 500 to the optical receiving devices 400 installed in multiple subscriber homes. Furthermore, as shown in FIG. 5, an amplifier (access amplifier) not shown may also be used in the access network 600 for the purpose of compensating for losses associated with the distribution of optical signals by the PON configuration and losses associated with the branching of optical signals by the amplifier 300.

In the conventional FTTH CATV system 1 having the above-mentioned 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 frequency-multiplexed multi-channel video electrical signal output from the headend 100 and converts the electrical signal into a single-channel wideband frequency modulation (FM) signal in a batch. Furthermore, the optical transmitter 200 converts the converted FM signal into an optical signal by intensity modulation and sends it to the optical transmission path. On the other hand, when the optical receiver 400 receives an optical signal from the optical transmission path, it converts the optical signal into an electrical FM signal and further demodulates it. As a result, the optical receiver 400 can extract the frequency-multiplexed multi-channel video electrical signal.

Here, an example of the configuration of a conventional optical transmission device 200b will be described. FIG. 6 shows the configuration of the optical transmission device 200b (Tx) disclosed in Non-Patent Document 2. Frequency-multiplexed multi-channel electrical signals are input to the optical transmission device 200b separately 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 rear stage of a laser diode 21 (LD), and phase-modulates the optical signal. On the other hand, electrical signal B is split into two by a splitter 8, and then directly modulates the optical signal by the laser diode 21 and the laser diode 22. The optical signal is directly modulated by the input signal, resulting in frequency chirping (i.e., frequency modulation). At this time, as shown in FIG. 6, by using a phase shifter 9 on one of the electrical signal B split by the splitter 8, electrical signals with opposite phases to each other can be input to the laser diode 21 and the laser diode 22, respectively. This suppresses the remaining intensity modulation components.

The optical signals output from the laser diode 21 and the laser diode 22 are multiplexed by the optical multiplexer 4 and input to the photodiode 51 (PD). The photodiode 51 performs optical heterodyne detection, and an FM signal centered on a frequency equal to the difference in the oscillation frequencies of the laser diode 21 and the laser diode 22 is output from the photodiode 5. The FM signal is input to the optical intensity modulator 6, which intensity-modulates the output light from the transmission laser diode 23. The signal light generated by the intensity modulation is transmitted to the optical receiving device 400 via the transmission fiber.

Here, we will explain the roles of the laser diode 21 and the laser diode 22 when modulating the electrical signal A. Figure 13 is a block diagram showing an example of the configuration of a conventional optical transmission device 200. However, components that are not directly related to the modulation of the electrical signal A are omitted.

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

Here, t is 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 when unmodulated. Electric signal _φ1 (t) is an electric signal input from electric signal input terminal 11 (however, this electric signal is the same as the electric signal input to optical phase modulator 31 because there is no loss or phase shift between electric signal input terminal 11 and optical phase modulator 31). m is a constant representing the modulation efficiency of optical phase modulator 31.

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):

Here, "<...>" is an operator that performs averaging processing according to the band of the photodiode 51. That is, this operator gives an average value to components that vary at angular frequencies equal to or greater than _ω1 or _ω2 .

As is clear from the above formula (1), the role of the laser diode 21 is to output carrier light for superimposing the electrical signal φ ₁ (t), and as is clear from the above formula (2), the role of the laser diode 22 is to output local light for coherently receiving the electrical signal φ ₁ (t).

As shown in the above equation (3), the current I(t) output as a result of heterodyne detection is phase-modulated with a value proportional to the electrical signal φ ₁ (t) input to the optical phase modulator 31.

"ITU-T J.185: Transmission equipment for transferring multi-channel television signals over optical access networks by frequency modulation conversion," International Telecommunication Union, June 2012. Toshiaki Shimoba et al., "Optical Video Distribution Technology Using FM Sweep Conversion Method," Institute of Electronics, Information and Communication Engineers, IEICE Technical Report, CS2019-84, IE2019-64, December 2019

The two laser diodes 21 and 22 used to directly modulate the electrical signal B require very high linearity between the bias current and the oscillation frequency in order to improve the distortion characteristics of the signal. This poses the problem that the selection cost for each laser diode becomes very high. In order to solve this problem, a method is considered in which all electrical signals are processed only by phase modulation without performing direct modulation in the laser diodes 21 and 22.

Figure 7 shows an example of the configuration of an optical transmitter 200c that is configured to process all electrical signals using only phase modulation. The configuration of the optical transmitter 200c is the same as that of the conventional optical transmitter 200b shown in Figure 6 above, except that the electrical signal input terminal 12, the distributor 8, and the phase shifter 9 are omitted.

Here, when the input power of the electrical signal A at the electrical signal input terminal 11 is small, the signal power is small relative to the noise generated by the photodiodes 51 and 52. This reduces the signal-to-noise ratio and deteriorates the signal quality (deterioration of noise characteristics). On the other hand, when the input power of the electrical signal A at the electrical signal input terminal 11 is large, distortion occurs due to the amplifier (not shown) arranged between the electrical signal input terminal 11 and the optical phase modulator 31, and the signal quality deteriorates (deterioration of distortion characteristics). Against this background, the challenge is to simultaneously improve the noise characteristics and distortion characteristics in an optical transmission device that performs FM batch conversion.

The present invention has been made in consideration of the above-mentioned technical background, and aims to provide technology that can simultaneously improve noise characteristics and distortion characteristics.

One aspect of the present invention includes a splitter that splits a first electrical signal input to an electrical signal input section 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 phase-modulates output light from a first laser diode based on the second electrical signal output from the splitter, a second optical phase modulator that phase-modulates output light from a second laser diode based on the third electrical signal output from the phase shifter, and a phase modulator that combines the output light from the first optical phase modulator and the output light from the second optical phase modulator and converts the combined output light into the first output light and the second output light. an optical multiplexer that splits the first output light from the optical multiplexer into a first heterodyne detection signal, 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, 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 an anode of one of the first photodiode and a cathode of the other of the first photodiode and the second photodiode.

One aspect of the present invention includes a distribution step of distributing a first electrical signal input to an electrical signal input section into a second electrical signal and a third electrical signal, a phase shift step of shifting the phase of the third electrical signal by 180 degrees, a first optical phase modulation step of phase modulating an output light from a first laser diode based on the second electrical signal, a second optical phase modulation step of phase modulating an output light from a second laser diode based on the third electrical signal shifted by 180 degrees, and an optical 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 splitting the multiplexed output light into a first output light and a second output light. a first conversion step of converting the first output light combined in the optical combining step into a first heterodyne detection signal by a first photodiode; a second conversion step of converting the second output light combined in the optical combining 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 a cathode of the other of the first photodiode and the second photodiode.

The present invention makes it possible to simultaneously improve noise and distortion characteristics in an optical transmitting device that performs batch FM conversion.

1 is a block diagram showing a configuration of an optical transmitting device 200a according to an embodiment of the present invention. 1 is a block diagram showing an example of the configuration of a balanced receiver according to an embodiment of the present invention. 4 is a flowchart showing an operation of the optical transmitting device 200a according to the embodiment of the present invention. 1 is a diagram for explaining the phase relationship between two output lights in a two-input, two-output optical multiplexer 41. FIG. 1 is a block diagram showing an example of a network configuration of an FTTH CATV system. FIG. 1 is a block diagram showing an example of a configuration of a conventional optical transmitting device 200b. FIG. 1 is a block diagram showing an example of a configuration of a conventional optical transmitting device 200c.

Hereinafter, an embodiment of the present invention will be described. Note that the system configuration of the network system in each embodiment described below is similar to the network configuration of the conventional FTTH CATV system 1 shown in FIG. 5 described above, so the description will be omitted.

<Embodiment>
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[Configuration of optical transmitter]
1 is a block diagram showing the configuration of an optical transmission device 200a according to an embodiment of the present invention.

As shown in FIG. 1, the optical transmitting device 200a includes an electrical signal input terminal 11, a distributor 8, a phase shifter 9, three laser diodes (laser diodes 21-23) (LD), two optical phase modulators (optical phase modulators 31-32), a two-input, two-output optical multiplexer 41, two photodiodes (photodiodes 51-52) (PD), an optical intensity modulator 6, and an optical signal output terminal 7.

Electrical signal A is taken into the optical transmitting device 200a from the electrical signal input terminal 11 and input to the input terminal of the distributor 8. The distributor 8 splits the input electrical signal A into two. One of the two electrical signals A split by the distributor 8 is input to the input terminal of the optical phase modulator 31. The other of the two electrical signals A 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.

The light output from the laser diode 21 is phase-modulated by the electrical signal A in the optical phase modulator 31. 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 multiplexed by the two-input, two-output optical multiplexer 41, then split and input to the photodiode 51 and the photodiode 52, respectively.

Heterodyne detection is performed in the photodiode 51 and the photodiode 52. The electrical signals (heterodyne detection signals) output from the photodiode 51 and the photodiode 52 are input to the input terminals 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 composite component of the electrical signals (heterodyne detection signals) output from the photodiodes 51 and 52. The intensity-modulated signal light is output from the optical signal output terminal 7 to the outside of the optical transmitting device 200a.

Figure 2 is a diagram showing the configuration of a balanced receiver according to an embodiment of the present invention. The two photodiodes 51 and 52 constitute a balanced receiver. As a method for constructing a balanced receiver, for example, as shown in Figure 2(a), there is a method in which the anode of photodiode 51 and the cathode of photodiode 52 are connected and a current is output from the connection point (note that a constant voltage source or ground, etc. connected to the cathode of photodiode 51 and the anode of photodiode 52 are not shown in Figure 15(a)).

Another method of constructing a balanced receiver is to connect the cathode of photodiode 51 and the anode of photodiode 52 and output a current from the connection point, as shown in Figure 15 (b) (note that a constant voltage source or ground, etc. connected to the anode of photodiode 51 and the cathode of photodiode 52 are not shown in Figure 15 (b)).

Heterodyne detection is performed in the photodiode 51 and the photodiode 52. 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. Meanwhile, the light output from the laser diode 23 is intensity modulated in the optical intensity modulator 6 based on the composite component of the electrical signals (heterodyne detection signals) output 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.

In addition, when the optical phase modulator 31 and the optical phase modulator 32, or the optical intensity modulator 6 are driven, the optical transmission device 200h may adjust the amplitude or bias of the electrical signals input from each electrical signal input terminal to control the amount of modulation. For this reason, an amplifier, an attenuator, and a bias tee may be used in the optical transmission device 200h.

The electric field _E'1 (t) of the output light from the optical phase modulator 31 and the electric field _E'2 (t) of the output light from the optical phase modulator 32 at the input end of the optical multiplexer 41 can be expressed by the following equations (4) and (5), respectively (details will be explained in the appendix below).

Here, t is time. _E'1 (t) and _E'2 (t) are 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. Of these, √2· _E1 and √2· _E2 are constants that represent the output electric field amplitudes of the laser diodes 21 and 22.

On the other hand, √2·e ₁ (t) and √2·e ₂ (t) are variables representing the fluctuations of the output electric field amplitudes of the laser diode 21 and the laser diode 22 at time t.

_ω1 and _ω2 are angular frequencies of the electric field when unmodulated. Electric signal _φ1 (t) is an electric signal input from electric signal input terminal 11 (here, whether a term including _φ1 (t) is added or subtracted in equation (4) or equation (5) depends on the presence or absence of phase shifter 9). m is a constant representing the modulation efficiency of optical phase modulator 31 and optical phase modulator 32.

In the optical multiplexer 41, generally, when 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.

Therefore, the electric field at the input end of the photodiode 51 of the light that passes through the optical multiplexer 41 and is output from terminal A can be expressed by the following equations (6) and (7), respectively, for the component E _A1 (t) derived from the optical phase modulator 31 and the component E _A2 (t) derived from the optical phase modulator 32 (details are explained in the appendix below).

As a result of the relative phase difference of 90 degrees between the output lights, the phase of the electric field of the light transmitting from terminal a to terminal A does not change (cos in equation (4) ⇒ cos in equation (6)), but the phase of the electric field of the light transmitting from terminal b to terminal A changes by 90 degrees (-sin in equation (5) ⇒ cos in equation (7)).

Similarly, the electric field at the input end of the photodiode 52 of the light that passes through the optical multiplexer 41 and is output from terminal B can be expressed by the following equations (8) and (9), respectively, for the component E _B1 (t) derived from the optical phase modulator 31 and the component E _B2 (t) derived from the optical phase modulator 32.

As a result of the relative phase difference of 90 degrees between the output lights, the phase of the electric field of the light transmitting from terminal a to terminal B changes by 90 degrees (cos in equation (4) ⇒ sin in equation (8)), while the phase of the electric field of the light transmitting from terminal b to terminal B does not change (-sin in equation (5) ⇒ -sin in equation (9)).

Here, in the photodiode 51, heterodyne detection is performed based on the optical input expressed by equations (6) and (7). As a result, the current I _A (t) of the output heterodyne detection signal is expressed by the following equation (10) using equations (6) and (7).

Here, "<...>" is an operator that performs averaging processing according to the band of the photodiode 51. In other words, this operator gives an average value to components that vary at angular frequencies equal to or greater than _ω1 or _ω2 . Note that, 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.

Similarly, in the photodiode 52, heterodyne detection is performed based on the optical input expressed by equations (8) and (9). As a result, the current I _B (t) of the output heterodyne detection signal is expressed by the following equation (11) using equations (8) and (9).

Here, "<...>" is an operator that performs averaging processing according to the band of the photodiode 52. In other words, this operator gives an average value to components that vary at angular frequencies equal to or greater than _ω1 or _ω2 . Note that, 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.

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 become an electrical signal of current I(t), which is then input to the optical intensity modulator 6. This current I(t) (heterodyne detection signal) is expressed by the following equation (12).

Here, the reason why I _A (t) and I _B (t) are combined with mutually opposite signs is because the photodiode 51 and the photodiode 52 have a balanced receiver configuration.

As is clear from the comparison of the above formulas (10), (11) and (12), the configuration with two photodiodes and a balanced receiver, as opposed to the configuration with only one photodiode, causes the DC components E ₁ ² and E ₂ ² and the components E ₁ e ₁ (t) and E ₂ e ₂ (t) caused by fluctuations to disappear from the output electrical signal. In particular, the components E ₁ e ₁ (t) and E ₂ e ₂ (t) caused by fluctuations become noise and cause signal degradation. Therefore, according to the optical transmission device 200a according to the embodiment, there is an effect that the noise characteristics and distortion characteristics are improved by eliminating the components E ₁ e ₁ (t) and E ₂ e ₂ (t).

Also, as is clear from a comparison of the above equations (3) and (12), the effective modulation efficiency increases from m to 2m by using a balanced receiver with two photodiodes compared to a configuration with only one photodiode. In other words, the power of the electrical signal A required to perform the same modulation as in the conventional method is halved. Therefore, according to the optical transmission device 200a of the embodiment, the distortion generated by the amplifier (not shown) arranged between the electrical signal input terminal 11 and the optical phase modulator 31 is reduced, and the distortion characteristics are improved.

As a prior art using a balanced receiver, a configuration is known in which an optical multiplexer/demultiplexer multiplexes/demultiplexes one signal light and one local oscillator light (unmodulated light), and detects the signal light while removing noise (relative intensity noise) generated from the local oscillator light (reference: Iwashita Masaru, "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 noise (E ₁ e ₁ (t) and E ₂ e ₂ (t)) generated from both of the two signal lights is removed while the signal lights are detected, as in the optical transmission device 200a in the embodiment, and there is no suggestion of the configuration or the effect of the configuration.

[Operation of the optical transmitter]
An example of the operation of the optical transmission device according to the embodiment of the present invention will be described below. Fig. 3 is a flowchart showing the operation of the optical transmission device 200a according to the embodiment of the present invention.

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

The phase shifter 9 shifts the phase of the electrical signal A2 by 180 degrees and then inputs the electrical signal A2 to the optical phase modulator 32 (step S02).

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

The optical multiplexer 4 multiplexes the output light from the optical phase modulator 31 and the output light from the optical phase modulator 32. The optical multiplexer 4 then splits the combined output light into a first output light and a second output light. The optical multiplexer 4 outputs the first output light to the photodiode 51 and the second output light to the photodiode 52 (step S05).

The photodiode 51 performs heterodyne detection on the first output light output from the optical multiplexer 41 (step S06). The photodiode 52 performs heterodyne detection on the second output light output from the optical multiplexer 41 (step S07).

The optical intensity modulator 6 intensity-modulates the light output by the laser diode 23 based on the composite 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). This completes the operation of the optical transmitter 200 shown in the flowchart of FIG. 3.

<Addendum>
Hereinafter, the phase relationship between the two output lights in the two-input two-output optical multiplexer 41 will be described with reference to Fig. 4. Fig. 4 is a diagram for explaining the phase relationship between the two output lights in the two-input two-output optical multiplexer 41.

Let T be the transfer function that represents the relationship between the electric field and phase of the two input lights and the two output lights of the optical multiplexer 41. Due to symmetry, the change in the electric field and phase when passing "from terminal a to terminal A" is the same as the change in the electric field and phase when passing "from terminal b to terminal B". Similarly, the change in the electric field and phase when passing "from terminal a to terminal B" is the same as the change in the electric field and phase when passing "from terminal b to terminal A". Therefore, the transfer function T has symmetry as expressed by the following equation (13).

Next, consider transmitting light from left to right in Figure 4. If the input optical fields to terminals a and b of optical multiplexer 41-1 at point 1 are 1 and 0, respectively, the electric field at point 1 is expressed by the following equation (14).

Furthermore, the output optical electric field (point 2) from each of terminals A and B of optical multiplexer 41-1, and the output optical electric field (point 3) from each of terminals A and B of optical multiplexer 41-2 are expressed as the following equations (15) and (16) based on the above equations (13) and (14).

Here, if there is no loss in optical multiplexer 41-1 and optical multiplexer 41-2, the power calculated by each of equations (14) to (16) will be the same, and the following equations (17) and (18) hold.

By squaring both sides of equation (17) above and subtracting it from equation (18), we obtain the following equation (20).

Here, by setting α=|α|exp( _iθα ) and β=|β|exp( _iθβ ), the above equation (20) is transformed into the following equation (21).

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

The above explains why the phase of light in the optical transmitting device 200a in the embodiment depends on the input/output terminals of the optical multiplexer 41-1.

As described above, the optical transmission device 200a according to the above embodiment multiplexes two phase-modulated optical signals using the optical multiplexer 41, then splits the signals and converts them into electrical signals using a balanced receiver configuration. With this configuration, the optical transmission device 200a can eliminate noise caused by fluctuations in the output electric field amplitude of the laser diode, and can simultaneously improve noise and distortion characteristics.

As described above, in the optical transmission device 200a according to the embodiment described above, the optical phase modulator 31 is connected to the rear stage of the laser diode 21, and the optical phase modulator 32 is connected to the rear stage of the laser diode 22. Then, the optical transmission device 200a inputs an input signal whose phase has been inverted by the phase shifter 8 to one of the optical phase modulators (the optical phase modulator 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 using the optical multiplexer 41, then splits the light, and performs square detection using the photodiode 51 and the photodiode 52. With this configuration, the optical transmission device 200a can lower the input signal voltage to the optical phase modulator 31 and the optical phase modulator 32. As a result, the optical transmission device 200a can make it difficult for degradation of signal quality to occur 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, the addition of a ch (channel) of the input signal or an increase in the band.

According to the above-mentioned 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.

The distributor distributes a 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 electrical signal input terminal 11 in the embodiment, the first electrical signal is electrical signal A in the embodiment, the second electrical signal is electrical signal A1 in the embodiment, and the third electrical signal is electrical signal A2 in the embodiment. The phase shifter shifts the phase of the third electrical signal by 180 degrees.

The 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 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 laser diode 22 in the embodiment.

The optical multiplexer multiplexes the output light from the first optical phase modulator and the output light from the second optical phase modulator, and splits the combined output light into the first output light and the 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.

The first photodiode converts the first output light from the optical multiplexer into a first heterodyne detection signal. The second photodiode converts the second output light from the optical multiplexer into a second heterodyne detection signal. The optical intensity modulator intensity-modulates the output light from the 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 cathode of the other of the second photodiode. For example, the third laser diode is laser diode 23 in the embodiment.

In addition, the above-mentioned optical transmitting 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.

In addition, the above-mentioned optical transmitting 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.

In addition, the above-mentioned optical transmitting 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.

In addition, the above-mentioned optical transmitting 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.

In addition, the above-mentioned optical transmitting 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.

In addition, the above-mentioned optical transmitting 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.

The optical transmission device 200a in the above-mentioned embodiment may be partly or entirely 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 the recording medium may be read into a computer system and executed to realize the function. The term "computer system" here includes hardware such as an OS and peripheral devices. The term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, and storage devices such as hard disks built into a computer system. The term "computer-readable recording medium" may also include a medium that dynamically holds a program for a short period of time, 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, and a medium that holds a program for a certain period of time, such as a volatile memory inside a computer system that is a server or client in that case. The above-mentioned program may be a program for realizing part of the above-mentioned function, or may be a program that can realize the above-mentioned function in combination with a program already recorded in the computer system, or may be a program that is realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

The above describes in detail an embodiment of the present invention with reference to the drawings, but the specific configuration is not limited to this embodiment and also includes designs that do not deviate from the gist of the present invention.

1...FTTH type CATV system, 41...optical multiplexer, 51, 52...photodiode, 6...optical intensity modulator, 7...optical signal output terminal, 8...distributor, 9...phase shifter, 11, 12...electrical signal input terminal, 21, 22, 23...laser diode, 31, 32...optical phase modulator, 100...head end, 200a-c...optical transmitting device, 300...amplifier, 400...optical receiving device, 500...relay network, 600...access network

Claims (8)

a distributor that distributes a first electrical signal input to the electrical signal input unit 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 phase-modulates an output light from a first laser diode based on the second electric signal output from the distributor;
a second optical phase modulator that phase-modulates an output light from a second laser diode based on the third electric signal output from the phase shifter;
an optical multiplexer that multiplexes an output light from the first optical phase modulator and an 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 for converting the first output light from the optical multiplexer into a first heterodyne detection signal;
a second photodiode for converting the second output light from the optical multiplexer into a second heterodyne detection signal;
an optical intensity modulator that intensity-modulates an 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 cathode of the other of the first photodiode and the second photodiode;
An optical transmitting device comprising: The optical transmission device according to claim 1 , further comprising an amplifier that amplifies the first electrical signal or the second electrical signal when the first optical phase modulator is driven. 3. The optical transmission device according to claim 1, further comprising 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 , further comprising: 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 claim 1 , further comprising: 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 claim 1 , further comprising a bias tee that adjusts a 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 claim 1 , further comprising a bias tee that adjusts a bias of the first electrical signal or the third electrical signal when the second optical phase modulator is driven. a distribution step of distributing a first electrical signal input to the electrical signal input unit into a second electrical signal and a third electrical signal;
a phase shifting step of shifting the third electrical signal by 180 degrees;
a first optical phase modulation step of phase-modulating an output light from a first laser diode based on the second electrical signal;
a second optical phase modulation step of phase-modulating an output light from a second laser diode based on the third electrical signal that has been phase-shifted by 180 degrees;
an optical combining step of combining 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 splitting the combined 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 optical 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 optical multiplexing step into a second heterodyne detection signal by a second photodiode;
a light intensity modulation step of intensity-modulating an 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 a cathode of the other of the first photodiode and the second photodiode;
A transmission method comprising: