JP2003258733A - Multilevel light intensity modulation circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the amplitude distortion of a plurality of intermediate level values caused by conversion from a multilevel electric signal to a multilevel optical modulated signal. <P>SOLUTION: The multilevel light intensity modulation circuit is provided with a light distribution means for distributing an optical carrier wave to (n) sequences, (n) light intensity modulators for respectively modulating the intensity of optical carrier waves of the (n) sequences with (n) binary electric signals and outputting the binary optical modulated signals of (n) sequences, optical phase adjusting means for matching the phases of the optical carrier waves of the (n) sequences distributed by the light distribution means or of the binary optical modulated signals of the (n) sequences outputted from the (n) light intensity modulators, a light intensity adjusting means for setting the light intensity of the optical carrier waves of the (n) sequences distributed by the light distribution means or of the binary optical modulated signals of the (n) sequences outputted from the (n) light intensity modulators approximately to 1:2:...:2<SP>n-1</SP>, and a photocoupling means for coupling the binary optical modulated signals of the (n) sequences obtained bvia the optical phase adjusting means and the light intensity adjusting means for outputting a 2<SP>n</SP>-level optical modulated signal. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multilevel light intensity modulation circuit (or device) for generating a multilevel light modulation signal.

[0002]

2. Description of the Related Art In a wavelength division multiplexing transmission system in which it is desired to improve frequency utilization efficiency due to a demand for an increase in transmission capacity in an optical communication system, band suppression of an optical spectrum is important for arranging a plurality of wavelength channels at narrow intervals. Has become a problem. As a means for solving this, there is a method of suppressing the spectrum spread by making the signal multi-valued and lowering the bit rate accordingly. For example, when a signal is transmitted with 2 ⁿ amplitude levels set, the same amount of information can be transmitted at a bit rate of 2/2 ⁿ and the spectrum band is about 2 as compared with the conventional binary optical intensity modulation method. It can be suppressed by ²ⁿ times.

FIG. 7 shows an example of the configuration of a conventional multi-valued (four-valued) light intensity modulation circuit (the light source 66 is not included in the circuit).
(See Non-Patent Document 1 below). In FIG.
Binary electric signals having the same power are input to the two input terminals 61 and 62. The power of the one binary electric signal is attenuated to 1/2 by the attenuator 63 and the coupler 64 is used.
A four-valued electric signal is generated by combining the signals. The four-valued electric signal is applied to the optical intensity modulator 65, and the optical carrier output from the light source 66 is intensity-modulated to generate a four-valued optical modulation signal.

FIGS. 8A to 8D show eye patterns of respective signals in a simulation example in which a four-valued electric signal and a four-valued optical modulation signal are generated from two two-valued electric signals. The two binary electric signals shown in FIGS. 8A and 8B are
By electrically coupling by the system as shown in FIG. 7, the four-valued electric signal shown in FIG. 8 (c) is obtained, and by using this, the intensity modulation of the optical carrier is performed to obtain the four-valued electric signal as shown in FIG. 8 (d). A four-valued optical modulation signal can be obtained.

The following is given as the above and other prior art document information. Among them, Patent Documents 1 and 2 specifically describe the present invention in the description of "means for solving problems" described later. It will be described as a comparison with other configurations. Further, Non-Patent Document 2 relates to an embodiment described later and is given as a conventional reference example of a configuration using a variable attenuator such as a Mach-Zehnder interferometer.

[0006]

[Non-Patent Document 1] S. Walklin et al.,
"Tenji BS 4 array ASK lightwave system (A 10Gb / s 4-ary ASK Lightwa
veSystem) ”, ECOC97 Proceedings, (UK), IE
E, September 1997, No. 448, p. 255-258

[Patent Document 1] JP-A-63-5633

[Patent Document 2] Japanese Patent Laid-Open No. 10-209961

[Non-Patent Document 2] Kuninori Hattor
i) et al., PLC-Based Optical Add / Drop Switch with PLC-Based Optical Add / Drop Switch
with Automatic Level Control ”, Journal of Lightwa
ve Technology), (USA), IEEE, 1999, 12
Mon, Vol. 17, No. 12, p. 2562-1571

[0007]

By the way, a Mach-Zehnder type optical intensity modulator is widely used as the optical intensity modulator 65.
FIG. 9A shows the response characteristic when used for the value intensity modulation. As is apparent from the figure, the amplitude distortion of the mark (1) and space (0) levels of the binary electric signal is suppressed, and a good binary optical modulation signal can be obtained. However, if the intensity modulation of the optical carrier is performed by the four-valued electric signal, the result shown in FIG.
As shown in, although the level 0 and level 3 amplitude distortions are suppressed, there is a problem that the level 1 and level 2 amplitude distortions expand.

Further, since the response characteristic of the Mach-Zehnder type optical intensity modulator is non-linear, in order to equalize the level intervals of the four-level optical modulation signal, level 1 and level 2 are set in advance.
It is necessary to generate a four-valued electric signal with a narrow interval. This is also the case when the intensity modulation is performed by a multilevel electrical signal of four or more levels, and the problems of level interval setting of intermediate level and amplitude distortion cannot be avoided. The present invention has been made in view of the above-mentioned circumstances, and is a multilevel optical intensity capable of suppressing amplitude distortion of a plurality of intermediate level values that occurs when converting a multilevel electrical signal into a multilevel optical modulation signal. An object is to provide a modulation circuit.

[0009]

SUMMARY OF THE INVENTION Therefore, according to the present invention, an optical distribution means for distributing an input optical carrier into an n-series (n is an integer of 2 or more) and the n-series optical carriers are respectively input. A plurality of optical intensity modulators, each optical intensity modulator intensity-modulating the optical carrier with an input binary electrical signal to output a binary optical modulation signal; N series of 2 output from the n light intensity modulators
Optical phase adjusting means for providing an optical phase difference between the value optical modulation signals,
Light intensity adjusting means for giving a light intensity difference between the n-series binary light modulation signals output from the n light intensity modulators, and the light intensity adjusting means and the light intensity adjusting means. an optical coupling means for coupling the n-series binary optical modulation signals and outputting the 2 ^n- level optical modulation signals, wherein the optical phase difference and the optical intensity difference are for generating the 2 ^n- level optical modulation signals. There is provided a multi-valued light intensity modulation circuit which is preset.

The optical phase adjusting means may be placed on the input side or the output side of at least one of the n light intensity modulators. The light intensity adjusting means may also be provided at the input side or the output side of at least one of the n light intensity modulators. As a typical example, the light intensity adjusting means is (n-
1) The intensity of the input light of the series is 1/2, 1/4, ..., 1 /
It is configured to be attenuated to 2 ^n-1 .

Further, as a typical example, n = 2, the light intensity distribution ratio of the light distribution means is 1: 1, and the light intensity adjustment means has a light intensity ratio between two series of binary optical modulation signals. 2: 1 ± 8
It is set to%, by the optical phase adjusting means,
The optical phase difference between the two series of binary optical modulation signals is set to 90 degrees ± 3%, and the optical coupling means combines the two series of binary optical modulation signals into a four-level optical modulation signal. This is the configuration to generate.

The present invention also provides an optical distribution means for distributing an input optical carrier into n series (n is an integer of 2 or more), and said n
N optical intensity modulators to which optical carriers of a series are respectively inputted, each optical intensity modulator intensity-modulates the optical carrier by an inputted binary electric signal and outputs a binary optical modulation signal. And an optical phase adjusting unit for providing an optical phase difference between the n-series binary optical modulation signals output from the n optical intensity modulators, and the optical phase adjusting unit. An optical coupling means for coupling the n-series binary optical modulation signals obtained by the above, and outputting a 2 ^n- valued optical modulation signal, the distribution ratio of the optical distribution means and the coupling ratio of the optical coupling means, and the optical The phase difference provides a multilevel optical intensity modulation circuit that is preset for generating the 2 ^n- level optical modulation signal. The optical phase adjusting means may be placed on the input side or the output side of at least one of the n light intensity modulators.

As a typical example, n = 2, the light intensity distribution ratio of the light distribution means is a: 1, the light intensity combination ratio of the light coupling means is b: 1, and a · b = 2. The optical phase adjusting means is arranged at the input side or the output side of either of the two binary intensity modulating means satisfying ± 8%, and the optical phase adjusting means is provided between two series of binary optical modulating signals. The optical phase difference is set to 90 ° ± 3%, and the optical coupling means has a configuration for coupling the two series of binary optical modulation signals to generate a four-level optical modulation signal. In each of the multilevel light intensity modulation circuits described above, the light intensity modulator may be integrated and formed on a lithium niobate (LN) substrate, and a Mach-Zehnder type light intensity modulator may be used as the light intensity modulator.

In the configuration example disclosed in the above-mentioned Patent Document 1 (Japanese Patent Laid-Open No. 63-5633, "Optical multilevel communication system"), a plurality of binary optical signals are generated by using different light sources and then these are generated. Although they are combined, since the optical phase relationship between different light sources is random, it is virtually impossible to adjust the phase, and interference noise occurs when they are combined. On the other hand, in the present invention, since one optical carrier is branched to generate a plurality of optical binary signals, the phase difference between the respective signals becomes constant. Therefore, each phase difference is adjustable, and interference noise does not occur at the time of combining. Furthermore, amplitude distortion can be suppressed to a minimum by setting the optical phase difference and the light intensity ratio to appropriate values in advance.

As described above, in the configuration example disclosed in Patent Document 1, since a light source corresponding to the number of multi-valued levels is required, temperature and power must be controlled for each light source, which makes control complicated. At the same time, an installation space is required for each light source.
On the other hand, in the present invention, only one light source is required regardless of the number of multi-valued levels, so the system configuration is simple and the control is easy. Further, in the conventional configuration example, since direct modulation is performed, chirp (transient fluctuation of light wavelength) occurs as the modulation speed increases. On the other hand, in the present invention, since external modulation is performed, chirp is less likely to occur, and it can be applied to speeding up.

Further, the above-mentioned Patent Document 2 (Japanese Patent Laid-Open No. 10-20996)
In the system described in No. 1), when the electric signal has the amplitude distortion, the amplitude distortion is also projected / expanded to the output optical signal (see the above-mentioned "related technology"). On the other hand, in the present invention, even if the electric signal has amplitude distortion, it is possible to obtain an output waveform that suppresses the distortion.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 shows a first embodiment of a multilevel light intensity modulation circuit of the present invention. However, the light source 10 is not a constituent element of the modulation circuit. Similarly, in the following embodiments, parts other than the light source are constituent elements of the modulation circuit. Further, the light intensity modulation circuit may be referred to as a light intensity modulation device.

In the figure, the optical carrier wave output from the light source 10 is divided into two light intensities by the optical distributor 11, and is input to the light intensity modulators 12a and 12b, respectively. On the other hand, the light intensity modulators 12a, 12b have two input terminals 14a, 1
Binary electric signals of equal light intensity are input from 4b, and the optical carrier is intensity-modulated. One of the two binary optical modulation signals generated here is the optical phase / intensity adjusting means 1
The signal is input to 6, and the phase is adjusted to +90 degrees or −90 degrees (that is, orthogonal to each other) with respect to the other signal, and the intensity is adjusted to ½.

As a result, the optical intensity ratio of the two binary optical modulation signals is set to 2: 1 ± 8%, and the optical coupler 17 couples the optical intensity ratio to 4: 1.
It is output as a value light modulation signal. Further, in FIG. 1, the optical phase / intensity adjusting means 16 is arranged at the latter stage of the light intensity modulator 12b, but the same operation can be performed if it is arranged at the former stage of the light intensity modulator 12b. Further, the optical phase / intensity adjusting means 16 may be arranged before or after the optical intensity modulator 12a.

Here, the reason why the phase difference between the two binary optical modulation signals is set to 90 degrees will be described. When n = 2 in the 2 ^n- valued light intensity modulation circuit, the electric field intensity of each path is defined as shown in FIG. The output electric field is defined as follows.

[Equation 1] Therefore, the electric field strength is defined as follows.

[Equation 2]

Since E ₁ or E ₂ is 0 in level 1 and level 2, the output optical power is constant regardless of the phase difference from the above equation, but in level 3 the optical coupler 17 outputs two binary optical modulation signals. Are coupled, the output power fluctuates due to the phase difference between the optical signals at the time of coupling. From the above equation, it can be seen that the optical power of level 3 varies sinusoidally with respect to the phase difference. In order for the level intervals between the levels to be equal, the above formula and FIG.
The phase difference should be more than ± 90 °.

2 (a) to 2 (e), two binary electric signals (FIGS. 2 (a) and 2 (b)) and two binary optical modulation signals (FIG. 2) obtained by simulation relating to the present embodiment.
(C), (d)), The eye pattern of a 4-value | binary optical modulation signal (FIG.2 (e)) is shown. In the present embodiment, the light intensity modulator 12
Since a and 12b are light intensity modulation by a binary electric signal, as shown in FIGS. 2 (c) and 2 (d), amplitude distortion at the mark (1) and space (0) levels can be suppressed, and then The phase and light intensity of the two binary light modulation signals are adjusted and combined. Therefore, as shown in FIG. 2E, the amplitude distortion of each level of the four-level optical modulation signal can be suppressed.

Here, the binary electric signals in the simulations of FIGS. 2A to 8E and FIGS. 8A to 8D have the same code sequence, and the eye opening deterioration is 0.5 dB.
Is. The eye opening deterioration of the four-valued optical modulation signal at this time is
It is 0.2 dB in the method of the present invention, while it is 2.2 dB in the conventional method.
It became B, and it was confirmed that the waveform deterioration was significantly improved. However, regarding the eye opening deterioration of the four-valued optical modulation signal, the eye opening degree is obtained between the levels (levels 0-1, 1-2, 2-3), and the one having the smallest eye opening degree is set as the worst value. It was calculated as log ₁₀ (worst value of eye opening / (1/3)) [dB].

Next, in the conventional four-valued light intensity modulation circuit described in FIG. 7, an output four-valued light modulation signal is supplied to the modulation circuit of this embodiment with a two-valued electrical signal that causes eye opening deterioration of 0.52 dB. By inputting an error to the intensity adjustment amount and the phase difference and calculating the eye opening deterioration, the eye opening deterioration was calculated and compared with the conventional method, and the range (permissible error) in which the present embodiment obtained better results was obtained.
First, regarding the intensity adjustment, as is clear from the graph of FIG. 12, the attenuation amount of the light intensity (the ratio when the original intensity is 1) is 0.5, which is the optimum value, and the deterioration of the eye opening is minimized.
As the distance from the optimum value increases (that is, as the error increases), the eye opening deterioration increases. When the amount of attenuation is between 0.46 and 0.54, the method of the present embodiment causes less deterioration of the eye opening than the conventional method. That is, if the error is within ± 8% from the optimum attenuation amount, the method of the present embodiment is more effective.

Regarding the phase adjustment, as is clear from the graph of FIG. 13, when the phase change amount is 90 °, the optimum value is 90 °, and the deterioration of the eye opening is minimized. growing. When the amount of attenuation is between 87 and 93 °, the method of the present embodiment causes less deterioration of the eye opening than the conventional method. That is, if the error is within ± 3% from the optimum phase change amount, the method of the present embodiment can obtain a superior effect.

(Second Embodiment) FIG. 3 shows a second embodiment of the multilevel light intensity modulation circuit of the present invention. The feature of this embodiment is that the function of the optical phase / intensity adjusting means 16 in the first embodiment is the same as the optical phase adjusting means 21 and the optical intensity adjusting means 22.
The optical intensity modulators 12a and 12b are respectively arranged in the subsequent stages. The positions of the light phase adjusting means 21 and the light intensity adjusting means 22 are set to the light intensity modulators 12a, 1a.
It may be before or after 2b, or may be arranged on the side of one light intensity modulator.

As the optical phase adjusting means 21, a structure for adjusting the optical path length, a structure for performing phase adjustment by applying a phase adjusting bias to the optical phase adjusting device, or the like can be used. The light intensity adjusting means 22 may be, for example, a fixed attenuator that attenuates the light intensity to ½, and a variable attenuator such as a Mach-Zehnder interferometer (see Non-Patent Document 2 above) is used to control the bias voltage. The light intensity may be adjusted by. Alternatively, a light intensity bifurcating means such as an optical coupler may be used.

(Third Embodiment) FIG. 4 shows a third embodiment of the multilevel light intensity modulation circuit of the present invention. The feature of this embodiment is that, instead of the light distributor 11, the light intensity adjusting means 22 and the optical coupler 17 in the second embodiment, the distribution ratio a:
The optical distributor 31 of 1 and the optical coupler 32 of coupling ratio b: 1 are used. Here, a and b are constants that satisfy a · b = 2 (± 8%). As a result, the light intensity modulator 12
It is possible to obtain a four-level optical modulation signal by adjusting the light intensity ratio of the two binary optical modulation signals output from a and 12b to 2: 1 ± 8% and combining them. The position of the optical phase adjusting means 21 may be either one of the light intensity modulators 12a and 12b and either before or after.

(Fourth Embodiment) FIG. 5 shows a fourth embodiment of the multilevel light intensity modulation circuit of the present invention. In the figure,
The optical carrier output from the light source 10 is divided into three light intensities by the optical distributor 41, and the light intensity modulators 12a and 12a,
b, 12c. On the other hand, the light intensity modulator 12a,
12b, 12c have three input terminals 14a, 14b,
Binary electric signals having the same light intensity are input from 14c, and the optical carrier is intensity-modulated.

The binary optical modulation signals output from the optical intensity modulators 12b and 12c are optical phase / intensity adjusting means 16a and 1a.
6b adjusts the phase and the light intensity is 1/2, 1
It is attenuated to / 4. These signals are transmitted to the light intensity modulator 12
The optical coupler 42 together with the binary optical modulation signal output from a
Are combined and output as an 8-level optical modulation signal.

An example of the phase difference provided by the phase adjusting means in this case is shown below. In a configuration in which the optical coupler 42 first couples the outputs of the optical intensity modulator 12a and the optical phase / intensity adjusting means 16a, and then couples them with the optical phase / intensity adjusting means 16b (shown in FIG. 14, (See the schematic configuration of the optical coupler 42 in this case), and the output light phase of the optical phase / intensity adjusting means 16a is 90 ° as the relative phase when the output light phase of the light intensity modulator 12a is 0 °. The output light phase of the light intensity phase / intensity adjusting means 16b is 8 from the output level 0 to 7.
The value is dynamically set to 180 ° when the value is 3, 90 ° when the value is 5, and 135 ° when the value is 7. Such dynamic setting will be described in more detail below. Table 1
FIG. 6 is a diagram showing a relationship between a plurality of output levels of an 8-value optical modulation signal and an adjusted phase amount.

[0032]

[Table 1]

In Table 1, in the first and second columns, each output level of three output signals (that is, three binary optical modulation signals from the optical intensity modulator 12a and the optical phase / intensity modulating means 16a and 16b). Shows the output level value at. In the third column, the level value and phase at each output level of the combined signal obtained by combining the signal from the optical intensity modulator 12a and the signal from the optical phase / intensity modulating means 16a by the optical coupler 42. Is shown.

This combined signal and optical phase / intensity adjusting means 16b
When the output signals from are further combined, the phases of both signals must be quadrature for the level spacing between each level to be equal. This condition is the optical phase / intensity adjusting means 16
b is satisfied when the above phase values 180 °, 90 °, and 135 ° are assigned to level 3, level 5, and level 7, respectively. Here at other levels, the combined signal or optical phase
Since the output level value of either one of the outputs of the intensity adjusting means 16b is 0, it is not necessary to consider the phase difference.

The optical phase / intensity adjusting means 16a, 16
The position of b may be either before or after the light intensity modulators 14b and 14c, or may be separately arranged to the optical phase adjusting means and the light intensity adjusting means as in the second embodiment. Further, as the optical distributor 41 and the optical coupler 42, those which can set a predetermined distribution ratio and a predetermined coupling ratio as shown in the third embodiment may be used. In particular, the three binary optical modulation signals are 4:
If they are combined at a ratio of 2: 1, the light intensity adjusting means becomes unnecessary.

In general, the optical distributor 41 and the optical coupler 42.
N distribution and n coupling are used as the above, n optical intensity modulators and optical phase / intensity adjusting means are combined, phase adjustment is performed according to the output value, and the light intensity is 1: 2: ...: 2 ^n- By setting to ¹ , it is possible to generate a 2 ^n- valued optical modulation signal from ⁿ binary-valued optical modulation signals. Each level interval of the ^2n- valued light modulation signal is input to the light intensity adjusting means (n-1).
It is set by adjusting the light intensity of the input light of the series.

(Fifth Embodiment) FIG. 6 shows a fifth embodiment of the multilevel light intensity modulation circuit of the present invention. The feature of this embodiment is that in the configuration of the second embodiment for generating a four-valued optical modulation signal, the optical distributor 11, the Mach-Zehnder type optical intensity modulators 52a and 52b, and the optical phase adjusting means 21 are provided on the LN substrate 51. Forming the light intensity adjusting means 22 and the optical coupler 17,
It is where they are connected by an optical waveguide.

Mach-Zehnder type optical intensity modulators 52a, 5
The binary electric signals having equal light intensities are input to the input terminals 2a and 14b (reference numeral 500 indicates an electrode to which the modulation signal is applied) from the input terminals 14a and 14b, and the optical carriers are split into two by the optical splitter 11. Intensity is modulated. Bias terminal 53
The DC biases applied from a and 53b are
When the electric signals input from the input terminals 14a and 14b are zero, the Mach-Zehnder type optical intensity modulators 52a and 52b
The output light intensity of is set to almost zero.

The optical phase adjusting means 21 has a structure for adjusting the optical path length or a structure for performing the phase adjustment by applying the phase adjusting bias to the optical phase adjusting device (in this case, the phase adjusting bias is supplied from the terminal 55a). Applied). Further, in the light intensity adjusting means 22, when a variable attenuator such as a Mach-Zehnder interferometer is used, the light intensity is adjusted by controlling the bias voltage (in this case, an intensity adjusting bias is applied from the terminal 55b).

[0040]

According to the present invention, a binary optical modulation signal is generated from each of a plurality of binary electrical signals, and the phases and light intensities of the plurality of binary optical modulation signals are adjusted and combined, A multilevel light modulation signal can be generated. This makes it possible to suppress the amplitude distortion of the mark and space levels in the binary optical modulation signal and suppress the amplitude distortion of all level values of the multilevel optical modulation signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of a multilevel light intensity modulation device of the present invention.

FIG. 2 is a block diagram of a binary electric signal according to the first embodiment;
It is a figure which shows the production | generation process of a value light modulation signal.

FIG. 3 is a block diagram showing a second embodiment of a multilevel light intensity modulation device of the present invention.

FIG. 4 is a block diagram showing a third embodiment of a multilevel optical intensity modulator of the present invention.

FIG. 5 is a block diagram showing a fourth embodiment of a multilevel light intensity modulation device of the present invention.

FIG. 6 is a block diagram showing a fifth embodiment of a multilevel optical intensity modulator of the present invention.

FIG. 7 is a block diagram showing a configuration example of a conventional multi-level (four-level) light intensity modulator.

FIG. 8 is a diagram showing a process of generating a four-valued optical modulation signal from a two-valued electrical signal in the conventional configuration.

FIG. 9 is a diagram showing input voltage versus output light intensity characteristics of a Mach-Zehnder type light intensity modulator.

FIG. 10 is a diagram showing the definition of input / output electrolysis of two binary optical modulation signal paths.

FIG. 11 is a graph showing the relationship between the optical power of each level and the phase difference.

FIG. 12 is a graph showing a relationship between an error of optical attenuation and deterioration of an eye opening.

FIG. 13 is a graph showing the relationship between the error in the amount of phase change and the deterioration of the eye opening.

14 is a diagram showing a schematic configuration example of an optical coupler 42 in FIG.

[Explanation of symbols]

10 light sources 11,41 Optical distributor 12a-12c Light intensity modulator 14a, 14b input terminals 16 Optical phase / intensity adjusting means 17,42 Optical coupler 21 optical phase adjusting means 22 Light intensity adjusting means 31 Optical distributor (distribution ratio a: 1) 32 optical coupler (coupling ratio b: 1) 51 LN board 52a, 52b Mach-Zehnder type optical intensity modulator

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G02F 1/03 505 G02F 1/035 1/035 H04B 9/00 L H04B 10/02 M 10/06 10 / 142 10/152 10/18 (72) Inventor Mitsuhiro Teshima 2-3-1, Otemachi, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (72) Inventor Noboru Kochio 2-3, Otemachi, Chiyoda-ku, Tokyo No. 1 F-Term in Nippon Telegraph and Telephone Corporation (reference) 2H079 AA02 AA12 AA13 BA01 BA03 CA04 DA03 EA05 FA03 GA01 GA03 5K102 AA01 AA61 AH24 AH26 KA19 KA42 PH02 PH42 PH49 PH50 RB01

Claims (13)

[Claims] 1. An optical distribution unit for distributing an input optical carrier into n series (n is an integer of 2 or more), and n optical intensity modulators to which the n series optical carriers are respectively input. A light intensity modulator for intensity-modulating the optical carrier with an input binary electrical signal to output a binary optical modulation signal; and n optical intensity modulators. Optical phase adjusting means for giving an optical phase difference between the n-series binary optical modulation signals output, and an optical intensity difference between the n-series binary optical modulation signals output from the n optical intensity modulators. And an optical coupling means for coupling the n-series binary optical modulation signal obtained through the optical phase adjustment means and the optical intensity adjustment means and outputting a 2 ^n- value optical modulation signal. wherein the optical phase difference, the light intensity difference produces the 2 ⁿ value optical modulation signal Multilevel light intensity modulation circuit in which pre-set in order. 2. The multilevel light intensity modulation circuit according to claim 1, wherein the optical phase adjusting means is placed on an input side of at least one of the n light intensity modulators. 3. The multilevel light intensity modulation circuit according to claim 1, wherein the optical phase adjusting means is placed on an output side of at least one of the n light intensity modulators. 4. The multilevel light intensity modulation circuit according to claim 1, wherein the light intensity adjusting means is placed on an input side of at least one of the n light intensity modulators. 5. The multilevel light intensity modulation circuit according to claim 1, wherein the light intensity adjusting means is placed on an output side of at least one of the n light intensity modulators. 6. The multi-valued light intensity modulation circuit according to claim 1, wherein the light intensity adjusting means adjusts the light intensity of the (n-1) series input light to 1/2, 1/4, ..., 1 A multilevel light intensity modulation circuit configured to attenuate to 2 ^n-1 . 7. The multilevel light intensity modulation circuit according to claim 1, wherein n = 2, the light intensity distribution ratio of the light distribution means is 1: 1, and the light intensity adjustment means has two series. The optical intensity ratio between the two binary optical modulation signals is set to 2: 1 ± 8%, and the optical phase difference between the two series of binary optical modulation signals is 90 ° ± 3% by the optical phase adjusting means. %, And the optical coupling means is a multi-level optical modulation circuit configured to combine the two series of binary optical modulation signals to generate a four-level optical modulation signal. 8. The multilevel light intensity modulation circuit according to claim 1, wherein the multilevel light intensity modulation circuit is integrated and formed on a lithium niobate (LN) substrate, and a Mach-Zehnder type light intensity modulator is used as the light intensity modulator. Multilevel light intensity modulation circuit. 9. An optical distribution unit for distributing an input optical carrier into n series (n is an integer of 2 or more), and n optical intensity modulators to which the n series optical carriers are respectively input. A light intensity modulator for intensity-modulating the optical carrier with an input binary electrical signal to output a binary optical modulation signal; and n optical intensity modulators. An optical phase adjusting unit that gives an optical phase difference between the n-series binary optical modulation signals that are output and an n-series binary optical modulation signal that is obtained through the optical phase adjusting unit are combined to obtain a 2 ^n- value optical signal. An optical coupling unit that outputs a modulation signal, wherein the distribution ratio of the optical distribution unit, the coupling ratio of the optical coupling unit, and the optical phase difference are set in advance to generate the ^2n- valued optical modulation signal. A multi-valued light intensity modulation circuit. 10. The multilevel light intensity modulation circuit according to claim 9, wherein the optical phase adjusting means is placed on an input side of at least one of the n light intensity modulators. 11. The multilevel light intensity modulation circuit according to claim 9, wherein the optical phase adjusting means is placed on the output side of at least one of the n light intensity modulators. 12. The multilevel intensity modulation circuit according to claim 9, wherein n = 2, the light intensity distribution ratio of the light distribution means is a: 1, and the light intensity combination ratio of the light coupling means is b: 1, a · b = 2 ± 8% is satisfied, and the optical phase adjusting means is arranged on either the input side or the output side of the two binary intensity modulating means, and the optical phase adjusting means is The optical phase difference between the two series of binary optical modulation signals is 90 degrees ± 3%
The multi-level optical modulation circuit is configured such that the optical coupling means couples the binary optical modulation signals of the two series to generate a four-level optical modulation signal. 13. The multilevel light intensity modulation circuit according to claim 9, wherein the multilevel light intensity modulation circuit is formed integrally on a lithium niobate (LN) substrate, and a Mach-Zehnder type light intensity modulator is used as the light intensity modulator. Multilevel light intensity modulation circuit.