JP2001285257A - Optical transmitter, and wavelength multiplex optical transmission system using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmitter that uses modulators connected in parallel to modulate the same optical signal, synthesized modulation results, and transmits the synthesized optical signal with a sufficient signal power by suppressing interference among the optical signals caused at wave synthesis. SOLUTION: An optical demultiplexer section 110 demultiplexes an optical signal outputted from a light source 100 into n-sets of signals, which receive intensity modulation at 1st n-th intensity modulation sections 210-1 to n on the basis of 1st to n-th input signals 1 to n. An optical multiplexer section 120 multiplexes the modulated optical signals and the multiplexed signal is transmitted. First to n-th delay control sections 300-1 to n control a propagation time of the 1st n-th intensity modulation sections 210-1 to n so that the propagation time of each optical signal after demultiplexing from the optical demultiplexer section 110 until the optical multiplexer section 120 is nearly equal to each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical transmitter for transmitting an optical signal modulated based on a plurality of frequency multiplexed signals, and a wavelength multiplexed optical transmission system using the same.

[0002]

2. Description of the Related Art In optical fiber transmission, a method for transmitting information using an intensity-modulated optical signal has been widely adopted. Methods for intensity-modulating an optical signal include a direct modulation method in which an optical signal is generated and modulation is performed simultaneously, and an external modulation method in which an optical signal is generated and then modulated. In the direct modulation method, a phenomenon that the oscillation wavelength is simultaneously modulated at the same time as the intensity modulation is performed, that is, a so-called wavelength chirp occurs due to the structure and physical properties of the semiconductor laser used as the light source. This wavelength chirp is based on the dispersion characteristics of an optical fiber as a transmission line and the gain (or loss) of an optical amplifier, an optical filter, etc.
In combination with the wavelength dependence of the above, the transmission signal is distorted. For this reason, for example, when a light source having a wavelength of 1.55 μm which can use an optical amplifier is used and a single mode fiber having a zero dispersion wavelength of 1.3 μm is used as a transmission line, the wavelength chirp is in principle. Is preferably used. On the other hand, in the external modulation method, the non-linearity at the time of photoelectric conversion is inferior to the direct modulation method, so that the waveform distortion of the optical transmission line increases. Therefore, in the external modulation method, the distortion characteristic of the optical transmitter is a dominant factor that determines the transmission characteristic. From these, the direct modulation method is used for short-distance transmission, and the external modulation method is used for long-distance transmission.
Each is used favorably.

As described above, an external modulation method is often adopted for long-distance transmission. However, the external modulation method has a problem that it is difficult to increase the transmission capacity (that is, the number of transmission channels) due to the limitation of the distortion characteristic of the optical transmitter.

In order to solve this problem, an optical transmitter combining a direct modulation method and an external modulation method is disclosed in
65247. FIG.
247 is a block diagram illustrating a configuration of an optical transmitter according to H.247.
The optical transmitter 19 includes a light source 101, an optical branching unit 110, first and second intensity modulation units 200-1 and 200-2, and an optical multiplexing unit 1.
20. The optical transmitter 19 is connected to the photoelectric conversion unit 30 via the optical fiber 20.

A third input signal 1- which is a frequency multiplexed signal
3 is input to the light source 101. The light source 101 outputs an optical signal whose intensity is modulated by the direct modulation method based on the third input signal 1-3. The optical signal output from the light source 101 is split into two in the optical splitter 110 and supplied to the first and second intensity modulators 200-1 and 200-2, respectively. First and second input signals 1-1 and 2 which are frequency multiplexed signals are input to first and second intensity modulators 200-1 and 200-2, respectively. The first and second intensity modulators 200-1 and 200-2 output the first and second input signals 1
An optical signal intensity-modulated by the external modulation method is output based on -1, 2. First and second intensity modulator 200
The optical signals output from -1, 2 are multiplexed in the optical multiplexing unit 120, and then transmitted through the optical fiber 20,
The signal is input to the photoelectric conversion unit 30. The photoelectric conversion unit 30 includes:
The input optical signal is converted into an output signal 2 which is an electric signal and output.

In the optical transmitter 19, the total frequency multiplexing number N of the electric signal is defined as the sum of the frequency multiplexing numbers N1, N2, and N3 of the first to third input signals 1-1 to 1-3. Let M be the total number. For example, since the optical transmitter 19 illustrated in FIG. 14 includes the light source 101 for direct modulation and the first and second intensity modulation units 200-1 and 200-2 for external modulation, M = 3. JP-A-8-65247 states that according to this optical transmitter, the frequency multiplicity per optical modulator is reduced to N / M, so that the influence of intermodulation distortion caused by the non-linear characteristics of each optical modulator. It is stated that a single light source can be used to transmit an input signal having a frequency multiplex number exceeding the limit of the frequency multiplex number of each optical modulator.

[0007]

However, as in the above-mentioned prior art, in an optical transmitter in which intensity modulators are connected in parallel and modulate the same optical carrier, interference generated when multiplexing optical signals is caused. Is a problem. This interference is determined by the relative phase relationship between the optical signals, that is, the difference in the propagation times of the optical signals. For example, in the optical transmitter 19 shown in FIG. 14, when the difference between the propagation times of the optical signals passing through the two intensity modulators is 180 degrees in terms of the phase component of the optical signal, the optical power component becomes Are canceled and suppressed, and signal transmission cannot be performed. In addition, since this optical transmitter performs intensity modulation by combining a direct modulation method and an external modulation method, there is also a problem that these multiplication components become second-order distortion and adversely affect transmission.

Accordingly, the present invention provides a high-quality optical transmission system with a small number of waveform distortions, in which a plurality of optical modulators are connected in parallel to increase the transmission capacity, suppress interference generated when multiplexing optical signals, and reduce waveform distortion. The purpose is to provide a vessel. Another object of the present invention is to provide a high-density and large-capacity wavelength-division multiplexing optical transmission system by using such an optical transmitter to increase the number of frequency multiplexing per optical signal. .

[0009]

A first invention is an optical transmitter for transmitting an optical signal modulated based on an input signal, comprising: a light source for outputting an optical signal; An optical splitter for splitting the split optical signal into a plurality of optical signals,
A plurality of optical modulators each for modulating each optical signal after branching, an optical multiplexer for multiplexing and transmitting the optical signals output from each of the optical modulators, and the optical multiplexer from the optical branching unit And a delay control unit for controlling the propagation time of each optical signal up to.

According to the first aspect, the optical signal output from the single light source is modulated in parallel using the plurality of optical modulators, and the propagation of each optical signal from the branch to the multiplexing is performed. Since the time is controlled, it is possible to suppress the interference of the optical signal generated at the time of the multiplexing and transmit the input signal with high quality.

In a second aspect based on the first aspect, the delay control section converts a difference in propagation time of each optical signal from the optical branch section to the optical multiplexing section into a phase component of the optical signal. Each of them is controlled so as to be almost 0 degrees.

According to the second aspect of the invention, the phase difference of the optical signal from the branch to the multiplex is substantially 0 degree.
The electric signal power that can be transmitted can be maximized.

[0013] In a third aspect based on the first aspect, the delay control section is configured so that a difference in propagation time of each optical signal from the optical branching section to the optical multiplexing section corresponds to a coherent length. It is characterized in that it is controlled to be longer.

According to the third aspect, since the difference between the propagation times of the optical signals is longer than the time corresponding to the coherent length, stable modulation characteristics can be obtained without interference of the optical signals. .

In a fourth aspect based on the first aspect, the delay control section controls a propagation time of an optical signal in each of the optical modulation sections.

According to the fourth aspect, by directly controlling the propagation time of the optical signal in the optical modulator,
The propagation time of an optical signal can be controlled without providing a new mechanism for delaying the optical signal.

In a fifth aspect based on the first aspect, each of the optical signal paths from the optical branching section to the optical multiplexing section further includes a plurality of delay sections for delaying the optical signal. The control unit controls the propagation time of the optical signal in each of the delay units.

According to the fifth aspect of the invention, by providing the delay section for delaying the optical signal, the propagation time of the optical signal can be increased with a large degree of freedom, for example, by increasing the difference in the propagation time of each optical signal. Can be controlled.

According to a sixth aspect based on the first aspect, each of the optical modulators modulates each of the branched optical signals based on the frequency multiplexed signal supplied thereto.

According to the sixth aspect, a large amount of information can be transmitted by transmitting a signal including many frequency components.

According to a seventh aspect based on the first aspect, each of the optical modulators modulates the intensity of each of the branched optical signals based on the input signal supplied thereto.

According to the seventh aspect, in the optical transmitter employing the intensity modulation, it is possible to suppress the decrease of the optical carrier component due to the interference of the optical signal and to obtain the stable modulation characteristic.

In an eighth aspect based on the seventh aspect, each of the optical modulators is a Mach-Zehnder type modulator having two optical waveguides to which an input signal is applied, and Wherein the difference in the propagation time of the optical signal in the optical waveguide is approximately 90 degrees in terms of the phase component of the optical signal.

According to the eighth aspect of the invention, the use of a Mach-Zehnder modulator as an intensity modulation unit suppresses a decrease in an optical carrier component due to interference of an optical signal, thereby obtaining a stable modulation characteristic. .

According to a ninth aspect, in the first aspect, each of the optical modulators performs single sideband modulation on each of the branched optical signals based on the input signals supplied thereto.

According to the ninth aspect, in the optical transmitter employing the single sideband modulation, a decrease in the optical carrier component due to the interference of the optical signal can be suppressed, and a stable modulation characteristic can be obtained.

According to a tenth aspect, there is provided an optical transmitter for transmitting an optical signal modulated based on an input signal. The optical transmitter outputs an optical signal, and converts the optical signal output from the light source into a plurality of optical signals. An optical splitter for splitting, a carrier output unit for outputting an optical signal containing a carrier component based on one split optical signal, and a plurality of optical modulators for modulating each of the other split optical signals And an optical multiplexing unit that multiplexes and transmits optical signals output from the carrier output unit and each of the optical modulation units, and controls a propagation time of each optical signal from the optical branching unit to the optical multiplexing unit. And a delay control unit.

According to the tenth aspect, an optical signal output from a single light source is modulated in parallel by using a plurality of optical modulators and multiplexed with an optical signal containing a carrier component. Since the propagation time of each optical signal from branching to multiplexing is controlled, it is possible to suppress interference of optical signals generated at the time of multiplexing and transmit an input signal with high quality.

According to an eleventh aspect, in the tenth aspect,
The delay control unit converts the difference in propagation time of each optical signal from the optical branching unit to the optical multiplexing unit into a phase component of the optical signal, and between the optical signals passing through the optical modulation units. In both cases, the control is performed so that the angle between the optical signal passing through each of the optical modulators and the optical signal passing through the carrier output unit is substantially 90 degrees.

According to the eleventh aspect, the phase difference of the optical signal from the branch to the multiplexing is approximately 0 degrees between the optical modulation units, and approximately 90 degrees between the optical modulation units and the carrier output unit. Therefore, the electric signal power that can be transmitted can be maximized.

According to a twelfth aspect, in the tenth aspect,
The delay control unit controls a propagation time of an optical signal in each of the optical modulators.

According to the twelfth aspect, by directly controlling the propagation time of the optical signal in the optical modulator, a mechanism for newly delaying the optical signal is not provided.
The propagation time of the optical signal can be controlled.

According to a thirteenth aspect, in the tenth aspect,
Each of the optical signal paths from the optical branching unit to the optical multiplexing unit via each of the optical modulators further includes a plurality of delay units for delaying an optical signal, and the delay control unit includes Is characterized by controlling the propagation time of the optical signal at

According to the thirteenth aspect, by providing the delay unit for delaying the optical signal, the propagation time of the optical signal can be increased with a large degree of freedom, for example, by increasing the difference in the propagation time of each optical signal. Can be controlled.

According to a fourteenth aspect, in the tenth aspect,
Each of the optical modulators modulates each of the branched optical signals based on the frequency multiplexed signal supplied thereto.

According to the fourteenth aspect, a large amount of information can be transmitted by transmitting a signal including many frequency components.

According to a fifteenth aspect, in the tenth aspect,
Each of the optical modulators performs a phase modulation on each of the branched optical signals based on the input signal supplied thereto.

According to the fifteenth aspect, in the optical transmitter employing the phase modulation, a decrease in the optical carrier component due to the interference of the optical signal can be suppressed, and a stable modulation characteristic can be obtained.

According to a sixteenth aspect, in the tenth aspect,
Each of the optical modulators performs carrier suppression modulation on each of the branched optical signals based on the input signal supplied thereto.

According to the sixteenth aspect, in the optical transmitter employing the carrier suppression modulation, it is possible to suppress the decrease of the optical carrier component due to the interference of the optical signal and to obtain a stable modulation characteristic.

According to a seventeenth aspect, in the sixteenth aspect,
Each of the light modulators has an input signal applied to one of them.
A Mach-Zehnder modulator having two optical waveguides, wherein a difference between propagation times of optical signals in the two optical waveguides is approximately 180 degrees in terms of a phase component of the optical signal.

According to the seventeenth aspect, the use of a Mach-Zehnder type modulator as the carrier suppression modulator can suppress the decrease of the optical carrier component due to the interference of the optical signal and obtain a stable modulation characteristic. it can.

According to an eighteenth aspect, in the tenth aspect,
The carrier output unit may output an optical signal with a delay.

According to the eighteenth aspect, an optical signal including a carrier component can be generated by a simple configuration circuit.

According to a nineteenth aspect, in the tenth aspect,
The carrier output unit may intensity-modulate an optical signal based on the supplied input signal.

According to the nineteenth aspect, by using the modulator as the carrier wave output unit, it is possible to generate an optical signal including a carrier wave component and at the same time further increase the multiplicity of the optical signal.

According to a twentieth aspect, there is provided an optical transmitter for transmitting an optical signal modulated based on an input signal. The optical transmitter outputs an optical signal, and splits the optical signal output from the light source into two. An optical branching unit, based on input signals each having a different frequency band, two intensity modulation units for intensity-modulating each optical signal after branching, and an optical signal output from each of the intensity modulation units, A polarization multiplexing unit that multiplexes and transmits the signals so that the polarization planes are orthogonal to each other.

According to the twentieth aspect, by combining the intensity-modulated optical signals such that the polarization planes are orthogonal to each other, it is possible to obtain a stable modulation characteristic without interference of the optical signals. Can be.

[0049] A twenty-first invention is a wavelength multiplexing optical transmission system for transmitting a plurality of optical signals modulated based on an input signal, wherein a plurality of optical transmitters each transmitting an optical signal having a different wavelength from each other. And an optical multiplexing unit that wavelength-multiplexes the optical signal transmitted from each of the optical transmitting units, and an optical fiber that transmits the optical signal wavelength-multiplexed in the optical multiplexing unit,
From the optical signal transmitted through the optical fiber, an optical demultiplexing unit that extracts a plurality of optical signals having the same wavelength as the optical signal transmitted from each of the optical transmission units, and is extracted in the optical demultiplexing unit. An optical-to-electrical conversion unit that converts each optical signal into an electric signal, and each of the optical transmission units, a light source that outputs an optical signal, and an optical branching unit that branches the optical signal output from the light source into a plurality of light signals A plurality of optical modulators each for modulating each optical signal after branching, an optical multiplexer for multiplexing and transmitting the optical signals output from each of the optical modulators, and the optical multiplexer from the optical branching unit And a delay control unit that controls the propagation time of each optical signal up to that point.

A twenty-second invention is a wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated on the basis of an input signal, wherein the plurality of optical transmitters each transmit an optical signal having a different wavelength from each other. And an optical multiplexing unit that wavelength-multiplexes the optical signal transmitted from each of the optical transmitting units, and an optical fiber that transmits the optical signal wavelength-multiplexed in the optical multiplexing unit,
From the optical signal transmitted through the optical fiber, an optical demultiplexing unit that extracts a plurality of optical signals having the same wavelength as the optical signal transmitted from each of the optical transmission units, and is extracted in the optical demultiplexing unit. An optical-to-electrical conversion unit that converts each optical signal into an electrical signal; each of the optical transmission units includes a light source that outputs an optical signal; and an optical branch that splits the optical signal output from the light source into a plurality of optical signals. And the optical signal after one branch,
A carrier output unit that outputs an optical signal containing a carrier component,
A plurality of optical modulators each for modulating each optical signal after the other branching; an optical multiplexing unit for multiplexing and transmitting the optical signals output from the carrier output unit and the optical modulators; and A delay control unit that controls a propagation time of each optical signal from the branching unit to the optical multiplexing unit.

A twenty-third invention is a wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated on the basis of an input signal, wherein the plurality of optical transmitting units each transmit an optical signal having a different wavelength from each other. And an optical multiplexing unit that wavelength-multiplexes the optical signal transmitted from each of the optical transmitting units, and an optical fiber that transmits the optical signal wavelength-multiplexed in the optical multiplexing unit,
From the optical signal transmitted through the optical fiber, an optical demultiplexing unit that extracts a plurality of optical signals having the same wavelength as the optical signal transmitted from each of the optical transmission units, and is extracted in the optical demultiplexing unit. An optical-to-electrical conversion unit that converts each optical signal into an electric signal, wherein each of the optical transmission units is a light source that outputs an optical signal, and an optical branching unit that splits the optical signal output from the light source into two. , Which modulates the intensity of each of the branched optical signals based on input signals having different frequency bands.
And a polarization multiplexing unit that multiplexes the optical signals output from the intensity modulation units so that their polarization planes are orthogonal to each other and transmits the multiplexed optical signals.

According to the twenty-first to twenty-third aspects, in the wavelength division multiplexing optical transmission system, the number of channels that can be transmitted per wavelength can be increased, and larger capacity optical transmission can be realized.

A twenty-fourth invention is a wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated based on an input signal, wherein the plurality of optical transmission units each transmit an optical signal having a different wavelength from each other. An optical signal transmitted from each of the optical transmission units, a polarization multiplexing unit that multiplexes the optical signals having adjacent wavelengths such that the polarization planes are orthogonal to each other,
An optical fiber for transmitting the optical signal multiplexed in the polarization multiplexing unit, and a plurality of optical fibers having the same wavelength as the optical signal transmitted from each of the optical transmitters, from the optical signal transmitted through the optical fiber. An optical demultiplexing unit that extracts an optical signal, a polarization filter that separates each optical signal extracted in the optical demultiplexing unit into two orthogonal polarization components, and outputs one of the polarization components, An optical-to-electrical conversion unit that converts each optical signal that has passed through the polarization filter into an electric signal, wherein each of the optical transmission units branches a light source that outputs an optical signal and a plurality of optical signals that are output from the light source. An optical branching unit, a plurality of optical modulation units each modulating each optical signal after branching, an optical multiplexing unit for multiplexing and transmitting the optical signals output from each optical modulation unit, and the optical branching unit Control the propagation time of each optical signal from the section to the optical multiplexing section. Characterized in that it comprises a delay control unit.

A twenty-fifth aspect of the present invention is a wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated based on an input signal, wherein the plurality of optical transmission units each transmit an optical signal having a different wavelength from each other. An optical signal transmitted from each of the optical transmission units, a polarization multiplexing unit that multiplexes the optical signals having adjacent wavelengths such that the polarization planes are orthogonal to each other,
An optical fiber for transmitting the optical signal multiplexed in the polarization multiplexing unit, and a plurality of optical fibers having the same wavelength as the optical signal transmitted from each of the optical transmitters, from the optical signal transmitted through the optical fiber. An optical demultiplexing unit that extracts an optical signal, a polarization filter that separates each optical signal extracted in the optical demultiplexing unit into two orthogonal polarization components, and outputs one of the polarization components, An optical-electrical conversion unit that converts each optical signal that has passed through the polarization filter into an electric signal, wherein each of the optical transmission units includes a light source that outputs an optical signal, and a plurality of optical signals that are output from the light source. An optical splitter for splitting the optical signal into a signal, a carrier output unit for outputting an optical signal containing a carrier component based on one optical signal after the split, and a plurality of optical modulators each modulating each optical signal after the other split A modulator, the carrier output unit and each of the optical modulators An optical multiplexing unit that multiplexes and outputs the optical signals output from the optical multiplexing unit, and a delay control unit that controls a propagation time of each optical signal from the optical branching unit to the optical multiplexing unit. .

According to the twenty-fourth or twenty-fifth aspect, in a wavelength division multiplexing optical transmission system, the number of channels that can be transmitted per wavelength can be increased, and larger capacity optical transmission can be realized. In addition, optical signals having adjacent wavelengths are multiplexed so that their polarization planes are orthogonal to each other, and a polarization filter is applied after demultiplexing. Transmission can be realized.

[0056]

(First Embodiment) FIG. 1 is a block diagram showing a configuration of an optical transmitter according to a first embodiment of the present invention. The optical transmitter 11 according to the present embodiment includes the light source 1
00, optical branching unit 110, first to n-th intensity modulation units 210
-1 to n, first to n-th delay control units 300-1 to 300-n, and an optical multiplexing unit 120. The optical transmitter 11 has a first
To multiplex and transmit optical signals modulated based on the nth input signals 1-1 to n.

The first to n-th intensity modulation sections 210-1 to 210-n receive the first to n-th input signals 1-1 to n including a plurality of frequency components, respectively. The frequency bands of the first to n-th input signals 1-1 to n are different from each other. The input signal to the optical transmitter 11 is not limited to one having the above characteristics,
It may be a signal containing only a single frequency component,
MA (Code Division Multiple)
(Access) signal may be transmitted in overlapping frequency bands.

As shown in FIG. 1, an optical signal output from the light source 100 is branched into n optical signals in an optical splitter 110, and is input to first to n-th intensity modulators 210-1 to 210-n, respectively. The n-branched optical signals are intensity-modulated in the first to n-th intensity modulators 210-1 to 210-n based on the first to n-th input signals 1-1-n respectively. 120 is input. The optical multiplexing unit 120 multiplexes the input optical signal and guides the multiplexed optical signal to the optical fiber 20. Optical fiber 20
Is converted into an output signal 2 which is an electric signal in the photoelectric conversion unit 30. In this way, an electric signal whose intensity changes according to the first to n-th input signals 1-1 to n is obtained.

The optical transmitter 11 controls each of the n-branched optical signals so that the propagation times from the optical branching unit 110 to the optical multiplexing unit 120 are substantially equal. Therefore, the optical transmitter 11 controls the first to n-th delay controllers 300-1 to 300-1 to the first to n-th intensity modulators 210-1 to 210-n, respectively.
n is provided to control the propagation time of each optical signal. 1st to nth
The parameter values for controlling the propagation times of the optical signals in the first to n-th intensity modulation units 210-1 to 210-n are set in the delay control units 300-1 to 300-n, respectively. When the n-branched optical signals pass through the first to n-th intensity modulators 210 to n, respectively, the first to n-th delay controllers 30
Delay by a time determined by parameter values set in 0-1 to n. In order to determine this parameter value, a method of determining the parameter value by obtaining the propagation time of each optical signal in advance or monitoring a part of the optical signal power multiplexed in the optical multiplexing unit 120 and monitoring the power There is a method of dynamically switching the parameter value so that the maximum value is obtained.

For example, for the optical transmitter 11 having two intensity modulators, the parameter values set in the first and second delay controllers 300-1 and 300-2 are determined as follows. . The angular frequency of the optical signal output from the light source 100 is ω, and the angular frequencies of the first and second input signals 1-1 and 2 are ω ₁ and ω ₂ , respectively, and the first and second intensity modulators 2
The amplitude of the modulation signal in 10-1 and 10-1 is m. Further, the difference between the propagation times of the two optical signals from the optical branching unit 110 to the optical multiplexing unit 120 is converted into a phase component of the optical signal, and is set to θ. At this time, the optical signal S (t) output from the optical multiplexing unit 120 is approximated by the following equation (1).

[0061]

(Equation 1)

When this optical signal is converted into an electric signal in the photoelectric conversion unit 30, the extracted electric signal has a low-frequency component (S) of the square of S (t) as shown in the following equation (2). _low (t)
² ).

[0063]

(Equation 2)

Therefore, the level of the extracted electric signal changes depending on θ, and becomes maximum when θ = 0. Therefore, for the first and second delay control units 300-1 and 300-2, θ = 0, that is, the propagation time of the two optical signals from the optical branching unit 110 to the optical multiplexing unit 120 is By converting the phase component into 0 degree and controlling it to be 0 degrees, the level of the electric signal extracted from the photoelectric conversion unit 30 can be maximized. Similarly, also when n is 3 or more, the first to n-th delay control units 300-1 to 300-1
n, by controlling the propagation time of each optical signal from the optical branching unit 110 to the optical multiplexing unit 120 to be 0 degree in terms of the phase component of the optical signal. The extracted electrical signal level can be maximized.

The intensity modulator used for the optical transmitter 11 includes a Mach-Zehnder type, an electro-absorption type, and an external modulator of a semiconductor amplifier. FIG. 2 is a block diagram showing an example of a detailed configuration of the optical transmitter 11 including two Mach-Zehnder type external modulators. The Mach-Zehnder type external modulator splits an optical signal into two, phase-modulates one optical signal based on an input signal, and then performs intensity modulation by causing these optical signals to interfere with each other. In FIG. 2, the first and second intensity modulators 210-1 and 210-2 are both Mach-Zehnder type external modulators. The first intensity modulator 210-1 includes a first optical splitter 211-1, a first phase modulator 212-1, a first optical transmission line 213-1, and a first optical multiplexer 214. -1. The second intensity modulator 210-2 includes the same elements as the first intensity modulator 210-1.

The optical signal split into two in the optical splitter 110 is split again in the first and second optical splitters 211-1 and 211-1 and 21-1 included in the first and second intensity modulators 210-1 and 210-1. Thus, first to fourth optical signals 215 to 218 are obtained. The first optical signal 215 is transmitted to the first phase modulator 212
Only the phase phi ₁ based on the first input signal 1-1 is an electric signal in -1 is phase modulated. Second optical signal 216
Are not phase-modulated, and the first optical transmission line 213-
Pass 1 Phase-modulated first optical signal 215 and second optical signal 216 passing through first optical transmission line 213-1
Are multiplexed in the first optical multiplexing unit 214-1.
At this time, the first phase modulation unit 21 converts the difference between the propagation times of the two optical signals into θ ₁ in terms of the phase component of the optical signal.
A bias voltage based on the parameter value set in the first delay control unit 300-1 is applied to 2-1.

On the other hand, the third optical signal 217 is phase-modulated by the phase φ ₂ in the second phase modulator 212-2 based on the second input signal 1-2 which is an electric signal. The fourth optical signal 218 passes through the second optical transmission line 213-2 without being phase-modulated. The phase-modulated third optical signal 217 and the fourth optical signal 218 that has passed through the second optical transmission line 213-2 are multiplexed in the second optical multiplexing unit 214-2. At this time, the two so that the difference in propagation time of the optical signal becomes theta ₂ in terms of the phase component of the optical signal, to the second phase modulator section 212-2, a second delay control unit 300
A bias voltage based on the parameter value set to -2 is applied.

First and second optical multiplexing sections 214-1, 2
Are multiplexed in the optical multiplexing unit 120. At this time, as described below, the propagation time of the optical signal in the first and second intensity modulators 210-1 and 210-2 is controlled by the first and second delay controllers 300-1 and 300-2. The angular frequency of the optical signal output from the light source 100 is ω, and the difference between the propagation times of the second optical signal 216 and the fourth optical signal 218 from the optical branching unit 110 to the optical combining unit 120 is the phase component of light. Is converted to α, the optical multiplexing unit 1
The optical signal S (t) output from 20 is represented by the following equation (3).

[0069]

(Equation 3)

[0070] When this optical signal is converted into an electric signal in the photoelectric conversion unit 30, the phase phi ₁ the component I ₁ of the electrical signal taken out (t) and the phase phi ₂ components I ₂ (t) is S
By calculating the low frequency components of the square of (t), they are expressed by the following equations (4) and (5), respectively.

[0071]

(Equation 4)

The Mach-Zehnder type external modulator is generally used in a state where θ ₁ = θ ₂ = π / 2. Therefore, α = 0, that is, the difference between the propagation times of the second optical signal 216 and the fourth optical signal 218 from the optical branching unit 110 to the optical multiplexing unit 120 is 0 in terms of the phase component of the optical signal. Control, the I ₁ (t) of the electric signal level extracted from the photoelectric conversion unit 30
And I ₂ (t) can both be maximized.

As described above, according to this embodiment, the optical signals output from a single light source are intensity-modulated in parallel by using a plurality of intensity modulators, and the propagation of each optical signal from branch to multiplexing is performed. Control the time. As a result, interference of optical signals generated at the time of multiplexing is suppressed, and deterioration of transmission characteristics due to the occurrence of interference is suppressed, so that a frequency multiplexed signal having more channels than the limit of one intensity modulator can be obtained with high quality. Can be transmitted.

(Second Embodiment) FIG. 3 shows a second embodiment of the present invention.
It is a block diagram which shows the structure of the optical transmitter which concerns on 1st Embodiment. The optical transmitter 12 according to the present embodiment includes a light source 100, an optical branching unit 110, and first to n-th single sidebands (S
(Single Side Band; hereinafter, referred to as SSB) Modulating units 220-1 to 220-n, and an optical multiplexing unit 120, and compared with the optical transmitter 11 according to the first embodiment,
In place of the first to n-th intensity modulation sections 210-1 to 210-n,
It is characterized by including n-th SSB modulators 220-1 to 220-n. Among the components of the present embodiment, the same components as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

As shown in FIG. 3, the optical signals n-branched in the optical branching unit 110 are the first to n-th SS signals, respectively.
The signals are input to B modulation sections 220-1 to 220-n. The n-branched optical signals are first to n-th input signals 1-1 to n, respectively.
, And is input to the optical multiplexing unit 120. The optical signal that has passed through the optical fiber 20 is converted into an output signal 2 that is an electrical signal in the photoelectric converter 30.
In this way, an electric signal whose intensity changes according to the first to n-th input signals 1-1 to n is obtained.

The first to n-th SSB modulators 220-1 to 220-n
Performs SSB modulation on each of the n-branched optical signals based on the first to n-th input signals 1-1 to n, respectively. FIG.
(A) and (b) are the schematic diagrams of the spectrum of the intensity-modulated optical signal and the SSB-modulated optical signal, respectively. Intensity modulated optical signal includes three frequency components of the carrier f _c, the upper wave (f _c + f _m) and the lower wave (f _c -f _m). In contrast, SSB modulated optical signal, for example, 2 of the carrier f _c and the upper wave (f _c + f _m)
Contains only one frequency component. As described above, since the SSB modulation suppresses either the lower side wave or the upper side wave component of the transmission signal and transmits the signal, it has a characteristic that the dispersion tolerance of the transmission path is high.

The optical transmitter 12, like the optical transmitter 11 according to the first embodiment, converts each of the n-branched optical signals into
The propagation time from the optical branching unit 110 to the optical multiplexing unit 120 is controlled so as to be substantially equal in terms of the phase component of the optical signal. Therefore, the optical transmitter 12 is provided with the first to n-th delay controllers 300-1 to 300-n for the first to n-th SSB modulators 220-1 to 220-n, respectively. Control the time. For example, the first to n-th delay control units 300-1
To control the propagation time of each optical signal by controlling the bias voltage applied to the first to n-th SSB modulators 220-1 to 220-n. Accordingly, it is possible to suppress interference that occurs when optical signals are multiplexed in the optical multiplexing unit 120, and to reduce deterioration in signal quality.

As described above, according to the present embodiment, even when the SSB modulation method having a high dispersion tolerance of the transmission line is used, the interference of the optical signal generated at the time of the multiplexing is suppressed, and the transmission characteristic accompanying the occurrence of the interference is suppressed. , It is possible to transmit a frequency multiplexed signal having the number of channels equal to or larger than the limit of one SSB modulator with high quality.

Note that each of the optical transmitters 11 and 12 according to the first and second embodiments converts an n-branched optical signal into n
It is assumed that the light is modulated by the optical modulators.
Only (n-1) of the n-branched optical signals may be modulated, and the remaining one optical signal may not be modulated. According to such an optical modulator, the propagation time of each optical modulator can be easily controlled on the basis of the propagation time of an unmodulated optical signal.

(Third Embodiment) FIG. 5 shows a third embodiment of the present invention.
It is a block diagram which shows the structure of the optical transmitter which concerns on 1st Embodiment. The optical transmitter 13 according to this embodiment includes a light source 100, an optical branching unit 110, first to n-th phase modulation units 230-1 to 230-1.
n, first to n-th delay control units 300-1 to 300-n, and delay unit 3
10 and an optical multiplexing unit 120. Optical transmitter 13
Generates and transmits an optical signal whose intensity changes in accordance with an input signal by multiplexing a phase-modulated optical signal and an optical signal including a carrier component output from the delay unit 310. It is characterized by. Among the components of the present embodiment, the same components as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

As shown in FIG. 5, the optical signal output from the light source 100 is (n + 1) -branched by the optical branching unit 110. The n optical signals are phase-modulated in the first to n-th phase modulation units 230-1 to 230-n based on the first to n-th input signals 1-1 to n, respectively.
20. The remaining one optical signal is supplied to the delay unit 31
After passing through 0, it is input to the optical multiplexing unit 120. The optical multiplexing unit 120 multiplexes these (n + 1) optical signals and guides them to the optical fiber 20. In the optical transmitter 13, the intensity modulation of the optical signal is performed by a combination of each of the first to n-th phase modulation units 230-1 to 230-n and the delay unit 310. In this way, an electric signal whose intensity changes according to the first to n-th input signals 1-1 to n is obtained.

The optical transmitter 13 transmits an optical signal passing through the first to n-th phase modulators 230-1 to 230-n among the (n + 1) -branched optical signals, from the optical branching unit 110 to the optical multiplexing unit 120. Control is performed so that the propagation time to the optical signal becomes substantially equal in terms of the phase component of the optical signal. In addition, the optical transmitter 13
The difference between the propagation time and the propagation time of the optical signal passing through the delay unit 310 is controlled so as to be approximately 90 degrees in terms of the phase component of the optical signal. For this reason, the optical transmitter 13
The first to n-th delay control units 300-1 to 300-n are provided for each of the first to n-th phase modulation units 230-1 to 230-n to control the propagation time of each optical signal. For example, the first to n-th delay control units 300-1 to 300-n include the first to n-th phase modulation units 2
The propagation time of each optical signal is controlled by controlling the bias voltage applied to 30-1 to 30-n. As a result, as will be described later, the transmittable electric signal power can be maximized, and the secondary distortion generated after photoelectric conversion can be minimized.

Next, in the optical transmitter 13, the first to n-th
Are obtained as the output signal 2 and the design conditions for the optimal parameter values. The angular frequency of the optical signal output from the light source 100 is ω, and the phase components added in the first to n-th phase modulators 230-1 to 230-n are φ _k (k = 1 to n), respectively. In addition, for the optical signal passing through the first to n-th phase modulation units 230-1 to 230-n and the optical signal passing through the delay unit 310, the difference in the propagation time from the optical branching unit 110 to the optical multiplexing unit 120 is calculated. And converted into the phase components of θ _k (k
= 1 to n). At this time, the optical signal S (t) output from the optical multiplexing unit 120 is represented by the following equation (6).

[0084]

(Equation 5)

When this optical signal is converted into an electrical signal in the photoelectric conversion unit 30, the maximum electrical signal to be extracted may be set to θ _k = π / 2 or −π / 2. The first to n-th input signals are sine waves having an angular frequency ω _k , and the optical modulation degree after photoelectric conversion is _mk , β _k = _mk × π /
Assuming that 2, the electric signal level I _k (t) ² is expressed by the following equation (7).

[0086]

(Equation 6) In the above equation (7), J ₁ (z) is a first-order Bessel function.

Next, the branching ratio of the optical signal in the optical branching unit 110 will be described. First to n-th phase modulation units 230
The optical powers of the optical signals input to −1 to n are equal, and the optical power of the optical signal passing through the delay unit 310 is equal to the optical power of the optical signal passing through the first to n-th phase modulation units 230-1 to 230-n. Let it be x times the sum of the power. In consideration of the branching ratio of the optical signal, Equation (6) is expressed by the following Equations (8) to (10).

[0088]

(Equation 7)

The low-frequency component S _low (t) ^{2 of the} square of S (t) expressed by the above equation (8) becomes maximum when x = 1. At this time, the first to n-th phase modulation units 230-1 to 230-n
The branching ratio of the optical signal between the optical signal and the delay unit 310 is α ₀ ² : α ₁ ² = 1
/ N: 1.

As described above, according to the present embodiment, the combination of the phase modulation unit and the delay unit modulates the optical signal output from a single light source into an optical signal having a variable intensity, from branching to multiplexing. To control the propagation time of each optical signal. As a result, it is possible to suppress interference of optical signals generated at the time of multiplexing, suppress deterioration of transmission characteristics due to occurrence of interference, and transmit a large number of frequency multiplexed signals with high quality.

(Fourth Embodiment) FIG. 6 shows a fourth embodiment of the present invention.
It is a block diagram which shows the structure of the optical transmitter which concerns on 1st Embodiment. The optical transmitter 14 according to the present embodiment includes a light source 100, an optical branching unit 110, first to n-th carrier wave suppression modulation units 240-
1 to n, the first to n-th delay control units 300-1 to 300-n, the delay unit 310, and the optical multiplexing unit 120, and compared with the optical transmitter 13 according to the third embodiment, the first The first to n-th carrier wave suppression modulation sections 240-1 to 240-n are provided in place of the first to n-th phase modulation sections 230-1 to 230-n. Among the components of the present embodiment, the same components as those of the third embodiment are denoted by the same reference numerals, and description thereof is omitted.

As shown in FIG. 6, among the (n + 1) -branched optical signals in the optical branching unit 110, n optical signals are:
The first to n-th carrier suppression modulators 240-1 to 240-n perform carrier suppression modulation based on the first to n-th input signals 1-1 to n, respectively, and are input to the optical multiplexer 120. The remaining one optical signal passes through the delay unit 310,
The signal is input to the optical multiplexing unit 120. In the optical transmitter 14, the first
To n-th carrier wave suppression modulation sections 240-1 to 240-n and delay section 31
The intensity modulation of the optical signal is performed by a combination with 0.
Thus, an electric signal whose intensity changes according to the first to n-th input signals 1-1 to n is obtained.

The optical transmitter 14 comprises a first to n-th carrier wave suppression modulator 240-1 of each of the (n + 1) -branched optical signals.
Is controlled so that the propagation time from the optical branching unit 110 to the optical multiplexing unit 120 of the optical signal passing through n is substantially equal in terms of the phase component of the optical signal. Also, the optical transmitter 1
4 indicates that the difference between this propagation time and the propagation time of the optical signal passing through the delay unit 310 is approximately 9 in terms of the phase component of the optical signal.
It is controlled to be 0 degrees. For this reason, the optical transmitter 14
Are the first to n-th delay control units 300-1 to 300-n for the first to n-th carrier wave suppression modulation units 240-1 to 240-n, respectively.
To control the propagation time of each optical signal. For example, the first
The -nth delay control units 300-1 to 300-n control the propagation time of each optical signal by controlling the bias voltage applied to the first to nth carrier wave suppression modulation units 240-1 to 240-n.
In this way, similarly to the third embodiment, the electric signal power that can be transmitted can be maximized, and the secondary distortion generated after photoelectric conversion can be minimized.

The first to n-th carrier suppression modulation sections 240-1
To n perform carrier suppression modulation on n optical signals of the (n + 1) -branched optical signals based on the first to n-th input signals 1-1 to n, respectively. FIG. 7A is a schematic diagram of the spectrum of the optical signal output from the first to n-th carrier wave suppression modulation sections 240-1 to 240-n. These optical signal includes a signal component of the upper side band (f _c + f _m) and the lower wave (f _c -f _m), it does not contain a signal component of the carrier f _c shown by the dashed line.

The delay section 310 is, for example, an optical waveguide that delays an optical signal by substantially the same time as the first to n-th carrier wave suppression modulation sections 240-1 to 240-n. FIG. 7B is a schematic diagram of the spectrum of the optical signal output from the delay unit 310. The optical signal includes only the signal component of the carrier f _c. The first to n-th input signals 1-1 to n can be obtained even if the optical signal subjected to carrier suppression modulation shown in FIG. 7A is transmitted and converted into an electric signal by square-law detection in the photoelectric conversion unit 30. Can not. Therefore, the first to n-th input signals 1 to 1 are combined by combining the optical signal subjected to the carrier suppression modulation shown in FIG. 7A and the optical signal containing the carrier component shown in FIG. 7B. An optical signal that can restore 1 to n is created.

FIG. 8 is a block diagram showing an example of a detailed configuration of the optical transmitter 14 provided with two carrier suppression modulators. In FIG. 8, first carrier wave suppression modulation section 240-1
Is the first as in the case of the intensity modulator 210-1 shown in FIG.
, An optical branching unit 241-1, a first phase modulating unit 242-1,
First optical transmission line 243-1 and first optical multiplexing unit 2
44-1. Also, the second carrier wave suppression modulation section 24
0-2 includes the same elements as the first carrier suppression modulation section 240-1. However, the first and second carrier wave suppression modulation sections 240-1 and 240-2 are controlled such that the difference between the propagation times of the two-branched optical signal becomes 180 degrees in terms of the phase component of the optical signal.

[0097] When the phase modulation amount to be applied to the optical signal at the suppressed carrier modulation section and phi _1, an optical signal S output from the suppressed carrier modulation section (t) is expressed by the following equation (11).

[0098]

(Equation 8)

The optical multiplexing section 120 adds the optical signal passing through the delay section 310 to this optical signal. The difference between the propagation times of the optical signal that has passed through the carrier suppression modulation section and the optical signal that has passed through the delay section is 90 degrees in terms of the phase component of the optical signal. Therefore, the multiplexed optical signal S (t) is represented by the following equation (12). In this way, it is possible to obtain a modulated signal whose intensity changes according to the first to n-th input signals 1-1 to n.

[0100]

(Equation 9)

As described above, according to the present embodiment, even when the carrier suppression modulator is used, it is possible to suppress the interference of the optical signals generated at the time of multiplexing, and to suppress the deterioration of the transmission characteristics due to the occurrence of the interference. Many frequency multiplexed signals can be transmitted with high quality.

In the third and fourth embodiments, the delay control is not performed on the delay section 310. However, the delay section 310 may be provided with a delay control section. In the third and fourth embodiments, the delay unit 310 is used to generate an optical signal including a carrier component.
However, instead of this, a phase modulation unit or an intensity modulation unit that performs phase modulation or intensity modulation on an optical signal based on an input signal may be used. When such an optical modulator is used, the difference in propagation time between the first to n-th optical modulators and the added optical modulator is converted into a phase component of the optical signal to be 90 degrees. By controlling, the electric signal power that can be transmitted can be maximized. In addition, since one phase modulation unit or one intensity modulation unit is added, the number of frequency multiplexes that can be transmitted increases as compared with the case where a delay unit is used.

(Fifth Embodiment) FIG. 9 shows a fifth embodiment of the present invention.
It is a block diagram which shows the structure of the optical transmitter which concerns on 1st Embodiment. The optical transmitter 15 according to the present embodiment includes a light source 100, an optical branching unit 110, first to n-th intensity modulation units 210-1 to 210-1.
n, an optical transmitter according to the first embodiment, including first to n-th delay control units 300-1 to 300-n, first to n-th delay units 320-1 to 320-n, and an optical multiplexing unit 120 Compared to 11,
The first to n-th delay units 320-1 to 320-n are provided. Among the components of the present embodiment, the same components as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

In FIG. 9, first to n-th delay units 320
−1 to n are the optical branching unit 110 to the optical multiplexing unit 1
On the path of n optical signals up to 20, for example,
It is provided between the n-th intensity modulation units 210-1 to 210-n and the optical multiplexing unit 120. First to n-th delay units 320-1 to 320-n
Are the first to n-th delay control units 300-1 to 300-n, respectively.
The optical signal is delayed and output according to the control from. By providing a delay unit that operates independently of the intensity modulation unit in this way, such as increasing the difference in the propagation time of each optical signal,
The propagation time of an optical signal can be controlled with a large degree of freedom.

In the optical transmitter 15, as in the first embodiment, the propagation time of each optical signal from the optical branching unit 110 to the optical multiplexing unit 120 becomes 0 degree in terms of the phase component of the optical signal. In this way, the photoelectric conversion unit 3
The electric signal level extracted from 0 can be maximized. As a result, interference of optical signals generated at the time of multiplexing is suppressed, and deterioration of transmission characteristics due to the occurrence of interference is suppressed. Can be transmitted. Also, in the optical transmitters 12 to 14 according to the second to fourth embodiments, similarly to the optical transmitter 15 shown in FIG. 9, a delay unit is provided on the optical signal path, and the delay control unit A configuration for controlling time can be adopted.

In the optical transmitter 15, the first to n-th delay units 320-1 to 320-n are constituted by using optical fibers or the like, so that the propagation time of the optical signal is lengthened.
The difference in the propagation time of the optical signal from 0 to the optical multiplexing unit 120 can be increased. Taking advantage of this function, the first to n-th delay units 320-1 to 320-n are connected to the optical branching unit 1 for the optical transmitter using the intensity modulation unit or the SSB modulation unit.
The difference between the propagation times from 10 to the optical multiplexing unit 120 may be controlled so as to be equal to or longer than the time corresponding to the coherent length. In such an optical transmitter, since the correlation of the optical signals is not maintained when the signals are multiplexed, interference of the optical signals can be avoided. Therefore, there is an effect that it is not necessary to precisely control the propagation time of the optical signal in each modulation unit.

(Sixth Embodiment) FIG. 10 is a block diagram showing a configuration of an optical transmitter according to a sixth embodiment of the present invention. The optical transmitter 16 according to the present embodiment includes a light source 100,
Optical branching unit 110, first and second intensity modulating units 210-
1, 2, and a polarization multiplexing unit 130, and a polarization multiplexing unit 130 is provided instead of the optical multiplexing unit 120 as compared with the optical transmitter 11 according to the first embodiment. I do. Among the components of the present embodiment, the same components as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

As shown in FIG. 10, the optical signal output from the light source 100 is split into two by an optical splitter 110,
First and second intensity modulators 210-1 and 210-1, 2 respectively
Is input to Each of the two branched optical signals is the first
The signal is intensity-modulated based on the second input signals 1-1 and 2 and is input to the polarization multiplexing unit 130. The first and second intensity modulators 210-1 and 210-2 may be any of Mach-Zehnder type, electroabsorption type, and other optical transmitters as long as they perform intensity modulation.

The polarization multiplexing section 130 multiplexes the inputted two optical signals such that the polarization planes are orthogonal to each other, and guides the multiplexed optical signals to the optical fiber 20. The optical signal that has passed through the optical fiber 20 is
The signal is converted into an output signal 2 which is an electric signal in the photoelectric conversion unit 30. In this way, an electric signal whose intensity changes according to the first to n-th input signals is obtained.

As described above, according to this embodiment, the polarization multiplexing section 130 multiplexes the optical signals so that the polarization planes are orthogonal to each other, so that the two optical signals do not interfere on the receiver side. Received. Therefore, there is an effect that it is not necessary to precisely control the propagation times of the optical signals output from the two intensity modulators.

(Seventh Embodiment) FIG. 11 is a block diagram showing a configuration of a wavelength division multiplexing optical transmission system according to a seventh embodiment of the present invention. This wavelength multiplexing optical transmission system
The optical transmitter includes first to m-th optical transmitters 40-1 to 40-m, an optical multiplexing unit 50, an optical fiber 20, an optical demultiplexing unit 60, and first to m-th photoelectric conversion units 30-1 to 30-m. This wavelength division multiplexing optical transmission system multiplexes the first to m-th input signals 3-1 to m using the optical fiber 20 to the first to m-th output signals 2-1 to m, respectively.

The first to m-th optical transmitters 40-1 to 40-m are any of the optical transmitters 11 to 16 according to the first to sixth embodiments. Each of these optical transmitters has a first
To m-th input signals 3-1 to m. 1st to mth
Are composed of a plurality of frequency multiplexed signals. The input signal is divided into n signals in each of the optical transmitters, and is then input to n optical modulators in each of the optical transmitters. For example, the first to m-th optical transmitters 40-1 to 40-m are optical transmitters 11 according to the first embodiment.
, The first to m-th input signals 3-1 to m are
The signals are respectively input to first to n-th intensity modulators 210-1 to 210-n inside the first to m-th optical transmitters 40-1 to 40-m. The first to m-th optical transmitters 40-1 to 40-m are respectively
A light source is provided inside and light signals output from the light source are
The signal is modulated based on the m-th input signals 3-1 to m and output.
Here, the wavelengths of the optical signals output from the first to m-th optical transmitters 40-1 to 40-m are all different from each other.

The optical signals output from the first to m-th optical transmitters 40-1 to 40-m pass through the optical fiber 20 after being wavelength-multiplexed in the optical multiplexing unit 50. The wavelength-division multiplexed light that has passed through the optical fiber 20 is split by the optical demultiplexing unit 60 into m optical signals for each wavelength of the optical signals output from the first to m-th optical transmitters 40-1 to 40-m. Is done. The m divided optical signals are respectively referred to as first to m-th photoelectric conversion units 30-1.
Is converted to an electric signal. In this way,
The first to m-th output signals 2-1 to m are obtained.

Referring to FIG. 12, the effect of the wavelength division multiplexing optical transmission system according to the present embodiment will be described. FIGS. 12A and 12B are schematic diagrams showing the spectrums of optical signals output from the conventional optical transmitters and the optical transmitters 40-1 to 40-m, respectively. The trapezoidal bands shown in FIGS. 12A and 12B indicate the bandwidth of the optical branching unit 60. When an optical transmitter according to the related art is used, the number of transmission channels per wavelength is small, for example, a signal band is about 1 GHz. On the other hand, as the optical demultiplexing unit 60, an optical filter for high-density wavelength multiplexing (100 dB (0.8 nm)) (3 dB band: about several tens GHz) can be generally used. However, in the conventional optical transmitter having a small number of transmission channels per one wavelength, FIG.
As shown in (1), only a narrow range can be used compared to the bandwidth of the optical demultiplexing unit 60, and the utilization rate of the optical frequency is low. On the other hand, the optical transmitters 11 to 1 according to the first to sixth embodiments
When using No. 6, the number of channels that can be frequency-multiplexed is larger than that of the conventional optical transmitter, so that the 3 dB bandwidth of the optical demultiplexing unit 60 is effectively used as shown in FIG. ,
Optical frequency utilization efficiency can be improved.

As described above, according to the present embodiment, by using the optical transmitters according to the first to sixth embodiments,
The use efficiency of the optical frequency can be improved, and a larger capacity optical transmission can be performed.

(Eighth Embodiment) FIG. 13 is a block diagram showing a configuration of a wavelength division multiplexing optical transmission system according to an eighth embodiment of the present invention. This wavelength multiplexing optical transmission system
First to m-th optical transmitters 41-1 to m, a polarization multiplexing unit 70,
An optical fiber 20, an optical demultiplexing unit 60, first to m-th polarization filters 80-1 to 80-m, and first to m-th photoelectric conversion units 30-1 to 30-m are provided. This wavelength division multiplexing optical transmission system multiplexes the first to m-th input signals 3-1 to m using the optical fiber 20 to the first to m-th output signals 2-1 to m, respectively. Among the components of the present embodiment, the same components as those of the seventh embodiment are denoted by the same reference numerals, and description thereof is omitted.

The first to m-th optical transmitters 41-1 to 41-m are any of the optical transmitters 11 to 15 according to the first to fifth embodiments. These optical transmitters are the same as the first to m-th optical transmitters 40-1 to 40-m according to the seventh embodiment.
To output optical signals modulated based on the m-th input signals 3-1 to m. The optical signals output from the first to m-th optical transmitters 41-1 to 41-m are input to the polarization multiplexing unit 70.

The polarization multiplexing unit 70 receives a plurality of optical signals and multiplexes the optical signals such that the polarizations of the optical signals having adjacent wavelengths are orthogonal to each other. After passing through the optical fiber 20, the multiplexed optical signal is transmitted to the optical demultiplexing unit 60, where m optical signals are output for each wavelength of the optical signals output from the first to m-th optical transmitters 40-1 to 40-m. It is split into an optical signal. The demultiplexed optical signals are respectively sent to the first to m-th polarization filters 80-1.
To m, the first to m-th photoelectric conversion units 30-1
Is converted to an electric signal. The first to m-th polarization filters 80-1 to 80-m separate each optical signal into two orthogonal polarization components, and output only one of the polarization components.

As described above, according to the present embodiment, the optical transmitter according to the first to fifth embodiments uses the polarization multiplexing unit and the polarization filter to transmit an optical signal having an adjacent wavelength. Are combined so that their polarization planes are orthogonal to each other, and after demultiplexing, only a predetermined polarization component is extracted by a polarization filter. As a result, adjacent optical crosstalk between optical signals can be reduced, and higher quality or higher density optical transmission can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical transmitter according to a first embodiment of the present invention.

FIG. 2 is a block diagram illustrating a detailed configuration of the optical transmitter according to the first embodiment of the present invention.

FIG. 3 is a block diagram illustrating a configuration of an optical transmitter according to a second embodiment of the present invention.

FIG. 4 is a schematic diagram illustrating an optical signal spectrum in an optical transmitter according to a second embodiment of the present invention.

FIG. 5 is a block diagram illustrating a configuration of an optical transmitter according to a third embodiment of the present invention.

FIG. 6 is a block diagram illustrating a configuration of an optical transmitter according to a fourth embodiment of the present invention.

FIG. 7 is a schematic diagram illustrating an optical signal spectrum in an optical transmitter according to a fourth embodiment of the present invention.

FIG. 8 is a block diagram illustrating a detailed configuration of an optical transmitter according to a fourth embodiment of the present invention.

FIG. 9 is a block diagram illustrating a configuration of an optical transmitter according to a fifth embodiment of the present invention.

FIG. 10 is a block diagram illustrating a configuration of an optical transmitter according to a sixth embodiment of the present invention.

FIG. 11 is a block diagram illustrating a configuration of a wavelength division multiplexing optical transmission system according to a seventh embodiment of the present invention.

FIG. 12 is a schematic diagram illustrating an optical signal spectrum output from an optical transmitter according to a seventh embodiment of the present invention.

FIG. 13 is a block diagram illustrating a configuration of a wavelength division multiplexing optical transmission system according to an eighth embodiment of the present invention.

FIG. 14 is a block diagram illustrating a configuration of a conventional optical transmitter.

[Explanation of symbols]

 1-1 to n ... input signal 2-1 to m ... output signal 3-1 to m ... input signal 11, 12, 13, 14, 15, 16 ... optical transmitter 20 ... optical fiber 30-1 to m ... light Electrical conversion units 40-1 to m, 41-1 to m: Optical transmitter 50: Optical multiplexing unit 60: Optical demultiplexing unit 70: Polarization multiplexing unit 80-1 to n: Polarization filter 100: Light source 110 Optical branching section 120: Optical multiplexing section 130: Polarization multiplexing section 210-1 to n: Intensity modulation section 220-1 to n: SSB modulation section 230-1 to n: Phase modulation section 240-1 to n: Carrier suppression Modulation units 300-1 to 300n delay control units 310 and 320-1 to n delay units

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G02F 1/03 502 H04B 9/00 E 1/035 L H04B 10/152 M 10/142 10/04 10 / 06 10/02 10/18 F term (reference) 2H079 AA02 AA05 AA12 AA13 BA01 BA03 CA05 DA03 DA16 EA05 GA03 GA04 GA06 5K002 AA01 AA02 BA05 CA02 CA16 DA02

Claims (25)

[Claims] An optical transmitter for transmitting an optical signal modulated based on an input signal, comprising: a light source that outputs an optical signal; an optical branching unit that branches an optical signal output from the light source into a plurality of light signals; A plurality of optical modulators each for modulating each optical signal after branching; an optical multiplexer for multiplexing and transmitting the optical signals output from each optical modulator; and the optical multiplexer from the optical branching unit An optical transmitter, comprising: a delay control unit that controls a propagation time of each optical signal to the optical transmitter. 2. The delay control unit according to claim 1, wherein a difference in propagation time of each optical signal from the optical branching unit to the optical multiplexing unit is converted to a phase component of the optical signal, and the difference is substantially zero. The optical transmitter according to claim 1, wherein the optical transmitter is controlled. 3. The delay control unit controls so that a difference in propagation time of each optical signal from the optical branching unit to the optical multiplexing unit is longer than a time corresponding to a coherent length. The optical transmitter according to claim 1, wherein 4. The optical transmitter according to claim 1, wherein the delay controller controls a propagation time of an optical signal in each of the optical modulators. 5. Each of the optical signal paths from the optical branching unit to the optical multiplexing unit further includes a plurality of delay units for delaying the optical signal, and the delay control unit includes a light control unit for controlling the light in each of the delay units. The optical transmitter according to claim 1, wherein the optical transmitter controls a signal propagation time. 6. The optical transmitter according to claim 1, wherein each of the optical modulators modulates each of the branched optical signals based on the frequency multiplexed signal supplied thereto. 7. The optical transmitter according to claim 1, wherein each of the optical modulators modulates the intensity of each of the branched optical signals based on the input signal supplied thereto. 8. The Mach-Zehnder modulator having two optical waveguides to each of which an input signal is applied, wherein each of the optical modulators has a propagation time of an optical signal in the two optical waveguides. The optical transmitter according to claim 7, wherein the difference is approximately 90 degrees in terms of a phase component of the optical signal. 9. The optical transmitter according to claim 1, wherein each of the optical modulators performs single sideband modulation on each of the branched optical signals based on an input signal supplied thereto. 10. An optical transmitter for transmitting an optical signal modulated based on an input signal, comprising: a light source that outputs an optical signal; and an optical branch that splits the optical signal output from the light source into a plurality of optical signals. A carrier output unit that outputs an optical signal including a carrier component based on one branched optical signal; a plurality of optical modulators each modulating each of the other branched optical signals; An output unit and an optical multiplexing unit that multiplexes and transmits optical signals output from each of the optical modulation units; and a delay control unit that controls a propagation time of each optical signal from the optical branching unit to the optical multiplexing unit. An optical transmitter comprising: 11. The delay control unit converts a difference in propagation time of each optical signal from the optical branching unit to the optical multiplexing unit into a phase component of an optical signal, and passes through each of the optical modulation units. The optical signal is controlled so as to be substantially 0 degrees between optical signals, and approximately 90 degrees between an optical signal passing through each optical modulation unit and an optical signal passing through the carrier wave output unit. The optical transmitter according to claim 10, wherein: 12. The delay control unit controls a propagation time of an optical signal in each of the optical modulators.
The optical transmitter according to claim 10. 13. An optical signal path from the optical branching unit to the optical multiplexing unit via each of the optical modulators,
The optical transmitter according to claim 10, further comprising a plurality of delay units for delaying the optical signal, wherein the delay control unit controls a propagation time of the optical signal in each of the delay units. 14. The optical transmitter according to claim 10, wherein each of the optical modulators modulates each of the branched optical signals based on the frequency multiplexed signal supplied thereto. 15. The optical transmitter according to claim 10, wherein each of the optical modulators modulates the phase of each of the branched optical signals based on the input signal supplied thereto. 16. The optical transmitter according to claim 10, wherein each of the optical modulators performs carrier suppression modulation on each of the branched optical signals based on the input signal supplied thereto. 17. Each of the optical modulators is a Mach-Zehnder modulator having two optical waveguides to each of which an input signal is applied, wherein each of the optical modulators has a propagation time of an optical signal in the two optical waveguides. The difference is approximately 18 in terms of the phase component of the optical signal.
17. The optical transmitter according to claim 16, wherein the angle is 0 degrees. 18. The optical transmitter according to claim 10, wherein the carrier output unit delays and outputs an optical signal. 19. The method according to claim 19, wherein the carrier output unit modulates the intensity of the optical signal based on the supplied input signal.
The optical transmitter according to claim 10. 20. An optical transmitter for transmitting an optical signal modulated based on an input signal, comprising: a light source for outputting an optical signal; and an optical splitter for splitting the optical signal output from the light source into two. Two intensity modulators for intensity-modulating each of the branched optical signals based on input signals having different frequency bands from each other; An optical transmitter, comprising: a polarization multiplexing unit that multiplexes and transmits orthogonally. 21. A wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated based on an input signal, wherein each of the plurality of optical transmitters transmits an optical signal having a different wavelength. An optical multiplexing unit that wavelength-multiplexes the optical signal transmitted from the optical transmitting unit, an optical fiber that transmits the optical signal wavelength-multiplexed in the optical multiplexing unit, and an optical fiber that is transmitted through the optical fiber. An optical demultiplexing unit that extracts a plurality of optical signals having the same wavelength as the optical signal transmitted from the transmission unit, and an opto-electric conversion unit that converts each optical signal extracted in the optical demultiplexing unit into an electric signal. A light source that outputs an optical signal; an optical branching unit that branches the optical signal output from the light source into a plurality of light signals; and a plurality of lights that modulate each of the branched optical signals. A modulator, and each of the light modulators An optical multiplexing unit that multiplexes and transmits the optical signal output from the tuning unit, and a delay control unit that controls a propagation time of each optical signal from the optical branching unit to the optical multiplexing unit. WDM optical transmission system. 22. A wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated based on an input signal, wherein each of the plurality of optical transmission units transmits an optical signal having a different wavelength. An optical multiplexing unit that wavelength-multiplexes the optical signal transmitted from the optical transmitting unit, an optical fiber that transmits the optical signal wavelength-multiplexed in the optical multiplexing unit, and an optical fiber that is transmitted through the optical fiber. An optical demultiplexing unit that extracts a plurality of optical signals having the same wavelength as the optical signal transmitted from the transmission unit, and an opto-electric conversion unit that converts each optical signal extracted in the optical demultiplexing unit into an electric signal. A light source that outputs an optical signal; an optical branching unit that splits the optical signal output from the light source into a plurality of optical signals; and a carrier wave based on one branched optical signal. Optical signal containing And a plurality of optical modulators, each of which modulates each optical signal after the other branching, and multiplexes and transmits the optical signals output from the carrier output unit and each of the optical modulators. A wavelength multiplexing optical transmission system comprising: an optical multiplexing unit; and a delay control unit that controls a propagation time of each optical signal from the optical branching unit to the optical multiplexing unit. 23. A wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated based on an input signal, wherein each of the plurality of optical transmission units transmits an optical signal having a different wavelength. An optical multiplexing unit that wavelength-multiplexes the optical signal transmitted from the optical transmitting unit, an optical fiber that transmits the optical signal wavelength-multiplexed in the optical multiplexing unit, and an optical fiber that is transmitted through the optical fiber. An optical demultiplexing unit that extracts a plurality of optical signals having the same wavelength as the optical signal transmitted from the transmission unit, and an opto-electric conversion unit that converts each optical signal extracted in the optical demultiplexing unit into an electric signal. Wherein each of the optical transmitters comprises: a light source that outputs an optical signal; an optical branching unit that splits the optical signal output from the light source into two, based on input signals each having a different frequency band from each other. After the branch Including two intensity modulators for intensity-modulating an optical signal, and a polarization multiplexing unit for multiplexing and transmitting the optical signals output from each of the intensity modulators such that their polarization planes are orthogonal to each other. 1. A wavelength division multiplexing optical transmission system. 24. A wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated based on an input signal, wherein each of the plurality of optical transmitters transmits an optical signal having a different wavelength. The optical signal transmitted from the optical transmitting unit, a polarization multiplexing unit that multiplexes an optical signal having an adjacent wavelength so that the polarization planes are orthogonal to each other, and multiplexed in the polarization multiplexing unit. An optical fiber that transmits an optical signal, and an optical demultiplexing unit that extracts, from the optical signal transmitted through the optical fiber, a plurality of optical signals having the same wavelength as the optical signal transmitted from each of the optical transmitters, Each of the optical signals extracted in the optical demultiplexing unit is orthogonalized by 2
A polarization filter that separates the two polarization components and outputs one of the polarization components; and a photoelectric conversion unit that converts each optical signal that has passed through the polarization filter into an electric signal. A light source that outputs an optical signal; an optical splitter that splits the optical signal output from the light source into a plurality of light sources; a plurality of optical modulators that respectively modulate each of the branched optical signals; An optical multiplexing unit that multiplexes and transmits an optical signal output from a modulation unit, and a delay control unit that controls a propagation time of each optical signal from the optical branching unit to the optical multiplexing unit. WDM optical transmission system. 25. A wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals modulated based on an input signal, wherein each of the plurality of optical transmitters transmits an optical signal having a different wavelength. The optical signal transmitted from the optical transmitting unit, a polarization multiplexing unit that multiplexes an optical signal having an adjacent wavelength so that the polarization planes are orthogonal to each other, and multiplexed in the polarization multiplexing unit. An optical fiber that transmits an optical signal, and an optical demultiplexing unit that extracts, from the optical signal transmitted through the optical fiber, a plurality of optical signals having the same wavelength as the optical signal transmitted from each of the optical transmitters, Each of the optical signals extracted in the optical demultiplexing unit is orthogonalized by 2
A polarization filter that separates the two polarization components and outputs one of the polarization components; and a photoelectric conversion unit that converts each optical signal that has passed through the polarization filter into an electric signal. A light source that outputs an optical signal, an optical branching unit that branches the optical signal output from the light source into a plurality of optical signals, and an optical signal that includes a carrier component based on the optical signal after one branch. A carrier output unit for outputting, a plurality of optical modulators each modulating each optical signal after the other branching, and multiplexing and transmitting the optical signals output from the carrier output unit and each of the optical modulators A wavelength multiplexing optical transmission system comprising: an optical multiplexing unit; and a delay control unit that controls a propagation time of each optical signal from the optical branching unit to the optical multiplexing unit.