JP2012155238A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator with simple constitution, at low cost, easy to control, and with low loss.SOLUTION: An optical modulator for converting an input electric signal to a multiple value signal includes: a light branch circuit 102 for branching an incident signal light beam; phase modulators 104a, 104b for modulating phases of propagation light beams according to an electric signal; and a light merge circuit 106 for merging output from the phase modulators 104a, 104b. It is set that at least one of the light branch circuit 102 and the light merge circuit 106 has aspherical branch ratio or merge ratio, and intensities of signal light beams guided to the output side from the phase modulators 104a, 104b through the light merge circuit 106 are different to each other.

Description

  The present invention relates to an optical modulator that converts an electrical signal into an optical signal and transmits the optical signal.

  In order to increase the capacity of a WDM optical communication system, it is useful to increase the transmission rate per wavelength. Here, when the symbol rate transmitted to the optical transmission line is increased without changing the modulation method, the allowable residual dispersion amount is inversely proportional to the square of the symbol rate, so that the chromatic dispersion tolerance of the optical transmission line is reduced. There was a problem. In addition, it is necessary to execute electrical signal processing at high speed, which increases the cost of analog electrical components.

For this reason, in recent years, active research has been conducted to increase the capacity of the system by increasing the signal multiplicity per symbol without increasing the symbol rate.
As a method for increasing the signal multiplicity, for example, a QPSK method for doubling the transmission capacity by assigning two values (multiplicity 2) to one symbol, or a transmission by assigning four values (multiplicity 4) to one symbol. Multi-level modulation schemes such as 16QAM scheme and 16APSK scheme for quadrupling the capacity are known.

  Usually, when performing such multilevel modulation, an I / Q modulator is used as an optical modulator. The I / Q modulator is also called an orthogonal modulator, and is a modulator that can independently generate orthogonal optical electric field components (I channel and Q channel), and is a Mach-Zehnder (MZ) modulator. It has a special configuration in which are connected in parallel.

For example, when executing QPSK modulation, a Dual Parallel MZ modulator (DPMZM: Dual Parallel Mach-Zehnder Modulator) in which two MZ modulators are connected in parallel is used (for example, refer to Patent Document 1 below).
When 16QAM modulation is performed, a method of driving DPMZM with an electric multilevel signal is used (for example, see Non-Patent Document 1 below).

Here, the operation of the conventional DPMZM will be described.
FIG. 11 is a schematic diagram showing the configuration of a conventional DPMZM.
In FIG. 11, 1101 is an input waveguide, 1102 is an optical branch circuit, 1103a and 1103b are connection waveguides, 1104a and 1104b are optical branch circuits, 1105a, 1105b, 1105c and 1105d are phase modulators, and 1106a and 1106b are optical confluences. Circuits 1107a and 1107b are phase adjustment regions, 1108 is an optical converging circuit, and 1109 is an output waveguide.

  In FIG. 11, structures 1110a and 1110b surrounded by wavy lines indicate Mach-Zehnder modulators, respectively. The configuration shown in FIG. 11 forms a so-called parent Mach-Zehnder interferometer in which two Mach-Zehnder modulators 1110a and 1110b are arranged in parallel, and two Mach-Zehnder modulators 1110a and 1110b are incorporated in a part of the arm. It is called Dual Parallel MZ Modulator (DPMZM).

  The signal light first enters from the input waveguide 1101 and is branched into two at a branching ratio of 1: 1 by the optical branch circuit 1102 and is connected to the Mach-Zehnder modulator 1110a and the Mach-Zehnder modulator 1110b via the connection waveguide 1103a and the connection waveguide 1103b. Led to. The Mach-Zehnder modulator 1110a includes a second optical branch circuit 1104a having a symmetrical distribution ratio with a branch ratio of 1: 1, phase modulators 1105a and 1105b, and an optical combining circuit 1106a.

  The optical merging ratio of the optical merging circuit 1106a is set so that the ratio of the intensity of light incident from the phase modulator 1105a and the phase modulator 1105b becomes the ratio of the output intensity as it is, that is, from the phase modulator 1105a and the phase modulator 1105b. When the incident signal light intensities are equal, they have characteristics such that they are output from the junction circuit at an equal intensity ratio. Hereinafter, such characteristics are referred to as “1: 1 confluence ratio”.

  The Mach-Zehnder modulator 1110b includes an optical branch circuit 1104b having a branch ratio of 1: 1, phase modulators 1105c and 1105d, and an optical merge circuit 1106b having a merge ratio of 1: 1. Each of the Mach-Zehnder modulators 1110a and 1110b is push-pull driven.

  The signal lights modulated by the Mach-Zehnder modulators 1110 a and 1110 b are incident on the phase adjustment regions 1107 a and 1107 b, and after the phase relationship is adjusted, are combined by the optical combining circuit 1108 and output from the output waveguide 1109. As the optical combining circuit 1108, an optical combining circuit having an optical combining ratio of 1: 1 is usually used. In the phase adjustment regions 1107a and 1107b at this time, the phases are relatively adjusted so that the phase difference between both signal lights when combined by the optical combining circuit 1108 is π / 2.

As an example of optical multilevel modulation, constellation will be described first by taking quaternary QPSK modulation as an example.
FIG. 12 is a diagram for explaining a constellation of a conventional optical modulator. 12A shows the locus of the optical electric field modulated by the push-pull drive Mach-Zehnder modulator 1110a, and FIG. 12B shows the constellation of DPMZM.

  In the Mach-Zehnder modulator 1110a, the phase modulators 1105a and 1105b are driven by differential signals. In the case of a phase modulator, the signal light has a constant intensity and the phase angle changes according to the applied voltage. Considering the case of a QPSK signal, the drive electric signal is a binary signal having an amplitude of a half-wave voltage Vπ.

For this reason, the electric field of the signal light emitted from the phase modulator 1105a reciprocates between the I ₀ point and the I ₁ point in accordance with the drive electric signal in the form of tracing the upper half of the semicircle in FIG. To do. On the other hand, considering the phase modulator 1105b, since the Mach-Zehnder modulator 1110a is push-pull driven, the phase modulator 1105b is driven by the differential signal of the phase modulator 1105a.

Therefore, the electric field of the signal light emitted from the phase modulator 1105b reciprocates between the I ₀ point and the I ₁ point in accordance with the drive electric signal in the form of tracing the lower half of the semicircle in FIG. . Therefore, since the electric field of the signal light output from the optical combining circuit 1106a is a combination of the outputs from the phase modulators 105a and 105b, the points I ₀ and I ₁ are linearly passed through the origin 0 in FIG. The point reciprocates between the points according to the drive electric signal.

  Similarly, since the Mach-Zehnder modulator 1110b is push-pull driven by the differential signal, the trace of the output signal becomes a straight line passing through the origin 0. The signal lights output from the two Mach-Zehnder modulator 1110a and Mach-Zehnder modulator 1110b are adjusted in the phase adjustment regions 1107a and 1107b so that the relative phase difference between them is π / 2, and the optical combining circuit 1108 Are added together.

  Therefore, the trace of the electric field of the signal light output from the Mach-Zehnder modulator 1110a and the trace of the electric field of the signal light output from the Mach-Zehnder modulator 1110b are added in an orthogonal manner, and the output waveguide The final constellation of the signal light output from 1109 is four points that are 90 degrees apart from each other as shown in FIG. This is the constellation of QPSK in quaternary modulation. When the relative phase difference is adjusted to be π / 2 and added by the optical combining circuit 1108, a principle loss of 3 dB occurs.

Next, 16-value modulation will be described.
When 16-value modulation is performed using DPMZM, the drive electric signal is a quaternary signal having a maximum amplitude of a half-wave voltage Vπ. At this time, the electric field of the signal light emitted from the phase modulator 1105a divides the phase angle between 0 and π into three, so that the upper half of the semicircle in FIG. , B, c, and d are reciprocated according to the drive electric signal.

For the phase modulator 1105b as well, the lower half of the semicircle in FIG. The electric field of the signal light output from the optical converging circuit 1106a is linear between the points I ₀ , I ₁ , I ₂ , and I _{3 in a} straight line passing through the origin 0 in FIG. The form to trace to.

That the distance from the origin 0 is different at each of these points indicates that the electric field amplitude or the light intensity is different in each state. Here, in order to make the interval between the points I ₀ , I ₁ , I ₂ , and I ₃ constant, it should be noted that the phase angle intervals, that is, ∠a0b, ∠b0c, and ∠c0d are not constant. . In other words, the quaternary electric drive waveform is required to be four values at unequal intervals.

  Similarly, since the Mach-Zehnder modulator 1110b is push-pull driven by a differential signal, the output signal is traced linearly between four points. The signal lights output from the two Mach-Zehnder modulator 1110a and Mach-Zehnder modulator 1110b are adjusted in the phase adjustment regions 1107a and 1107b so that the relative phase difference between them is π / 2, and the light combining circuit 1108 Add together.

  For this reason, the trace of the electric field of the signal light output from the Mach-Zehnder modulator 1110a and the trace of the electric field of the signal light output from the Mach-Zehnder modulator 1110b are added in an orthogonal manner. The final constellation of the signal light output from the waveguide 1109 is 16 points arranged in a lattice shape orthogonal to each other as shown in FIG. This is a 16QAM constellation which is 16-value modulation.

  Also in this case, when the relative phase difference is adjusted to be π / 2 and added by the optical combining circuit 1108, a principle loss of 3 dB occurs. Further, since the output of the Mach-Zehnder modulator 1110a and the output of the Mach-Zehnder modulator 1110b change the light intensity at the change of the bit and the bit, respectively, the output from the optical converging circuit 1108, which is the addition, also changes the change of the bit. There is a feature that changes greatly.

JP-T-2004-516743

Lutz Mole, four others, “Polarization Multiplexed 20 Gbaud Square 16QAM Long-Haul Transmission over 1120 km using EDFA Amplification”, EcoC 2009, 2009. Paper 8.8.4

However, when the conventional DPMZM modulator is used, there is a problem in that the number of MZ modulators increases, so that the cost increases and the number of bias control points increases.
In addition, since the multi-value intervals of the multi-value electric signal must be set at unequal intervals and a high-resolution digital-analog converter (DAC) must be used, there is a problem that the cost increases. It was.

Furthermore, there is also a problem that a principle loss of 3 dB occurs due to the DPMZM configuration in which the phases of the two Mach-Zehnder modulators are orthogonalized and merged.
In addition, since the output light intensity fluctuates at the bit transition, there is a problem of non-linearity of the transmission fiber such as cross phase modulation.

  In light of the above, an object of the present invention is to provide an optical modulator that is simple in configuration, low in cost, easy to control, and low loss.

An optical modulator according to a first invention for solving the above-described problem is
An optical modulator that converts an input electrical signal into a multilevel optical signal,
Optical branching means for branching incident signal light;
At least two phase modulation means connected to the output side of the optical branching means for modulating the phase of propagating light according to the electrical signal;
Optical merging means for merging outputs from the at least two phase modulation means,
Furthermore, at least one of the light branching means and the light joining means has an asymmetric branching ratio or joining ratio,
The intensity of the signal light guided to the output side from the at least two phase modulation means through the optical combining means is set to be different from each other.

An optical modulator according to a second invention for solving the above-described problem is
An optical modulator that converts an input electrical signal into a multilevel optical signal,
Optical branching means for branching incident signal light;
An optical merging means for merging the branched signal light;
At least two waveguides connecting the optical branching means and the optical combining means constituting the Mach-Zehnder interferometer;
A phase modulation means for each of the at least two waveguides;
Furthermore, at least one of the light branching means and the light joining means has an asymmetric branching ratio or joining ratio,
The intensity of the signal light passing through the phase modulation means and guided to the output side through the optical converging means is set to be different from each other according to the propagation path of the signal light.

An optical modulator according to a third aspect of the present invention for solving the above problem is the first aspect of the invention or the second aspect of the invention.
The intensity of the signal light guided to the output side is different from each other by 1/4 times.

An optical modulator according to a fourth invention for solving the above-described problems is
An optical modulator that converts an input electrical signal into a multilevel optical signal,
Optical branching means for splitting the incident signal light into two parts with a branching ratio of 2: 1;
Two phase modulation means connected to the output side of the first optical branching means for modulating the phase of propagating light according to the electrical signal;
Optical merging means for merging the outputs from the two phase modulation means at a merging ratio of 2: 1;
The optical branching unit and the optical combining unit are connected so that the intensity of the signal light guided from the two phase modulation units to the output side through the optical combining unit is 4: 1.

An optical modulator according to a fifth aspect of the present invention for solving the above problem is the optical modulator according to any one of the first to fourth aspects of the invention,
Phase adjustment means for adjusting the phase of the laser oscillation light to be propagated is connected to all of the at least two phase modulation means or the remaining phase modulation means except for one of the at least two phase modulation means. It is characterized by being.

An optical modulator according to a sixth aspect of the present invention for solving the above problem is the optical modulator according to any one of the first to fifth aspects of the invention,
Output from the at least two phase modulation means via the optical merging means to all of the at least two phase modulation means or to the remaining phase modulation means excluding one of the at least two phase modulation means. Phase adjusting means for adjusting the phase of the signal light propagating so that the phase difference of the signal light guided to the side becomes an integer multiple of 0 degree or 90 degrees is connected.

An optical modulator according to a seventh aspect of the present invention for solving the above problem is the optical modulator according to any one of the first to sixth aspects of the invention,
Intensity adjusting means for adjusting the intensity of the propagating laser oscillation light is connected to all of the at least two phase modulating means or the remaining phase modulating means excluding one of the at least two phase modulating means. It is characterized by being.

An optical modulator according to an eighth aspect of the present invention for solving the above problem is the optical modulator according to the seventh aspect of the present invention.
As the intensity adjusting means, an absorbing means for absorbing propagating signal light or an amplifying means for amplifying propagating signal light is connected.

An optical modulator according to a ninth aspect of the present invention for solving the above problem is the optical modulator according to the seventh aspect of the invention,
A Mach-Zehnder interferometer is connected as the intensity adjusting means.

An optical modulator according to a tenth aspect of the present invention for solving the above problems is the optical modulator according to the first to ninth aspects of the invention.
The optical branching means is one of a multimode interference coupler, a funnel coupler, a directional coupler, and a Y-shaped optical combiner.

An optical modulator according to an eleventh aspect of the present invention for solving the above problems is the optical modulator according to the first to tenth aspects of the invention.
The optical combining means is any of a multimode interference coupler, a funnel type coupler, a directional coupler, and a Y-shaped optical combiner.

  According to the present invention, it is possible to provide an optical modulator having a simple configuration, low cost, easy control, and low loss.

It is the schematic diagram which showed the structure of the optical modulator which concerns on the 1st Example of this invention. It is a figure for demonstrating the constellation of the optical modulator which concerns on the 1st Example of this invention. It is the schematic diagram which showed the example of a structure of the optical branching means which has an asymmetric branching ratio, or the optical joining means which has an asymmetrical joining ratio which concerns on this invention. It is the schematic diagram which showed the structure of the optical modulator which concerns on the 2nd Example of this invention. It is a figure for demonstrating the constellation of the optical modulator which concerns on the 2nd Example of this invention. It is the schematic diagram which showed the structure of the optical modulator which concerns on the 3rd Example of this invention. It is the schematic diagram which showed the structure of the optical modulator which concerns on the 4th Example of this invention. It is a figure for demonstrating the constellation of the optical modulator which concerns on the 4th Example of this invention. It is the schematic diagram which showed the structure of the optical modulator which concerns on the 5th Example of this invention. It is the schematic diagram which showed the structure of the optical modulator which concerns on the 6th Example of this invention. It is the schematic diagram which showed the structure of conventional DPMZM. It is a figure for demonstrating the constellation of the conventional optical modulator.

  Hereinafter, embodiments for implementing an optical modulator according to the present invention will be described with reference to the drawings.

The first embodiment of the optical modulator according to the present invention will be described below.
The optical modulator according to this embodiment is characterized in that multilevel modulation is realized based on an MZ modulator in which two phase modulators are connected in parallel.

Here, before describing the optical modulator according to the present embodiment, the branching ratio of the optical branching circuit and the converging ratio of the optical combining circuit will be defined.
When light is incident on the optical branch circuit and p ₁ %, p ₂ %,... P _n % are output to a plurality of output ports (for example, n ports), p ₁ : p ₂ : ..: _Pn is defined as the branching ratio. When n = 2, if 50% is output to both ports, the branching ratio is 50: 50 = 1: 1 symmetrical branch, 20% for the first port and 80% for the second port. For example, the branching ratio is 20: 80 = 1: 4.

On the other hand, as for the merge ratio, there are a plurality of input ports, p ₁ % of the light incident on the _first waveguide is output, and p ₂ % of the light incident on the second waveguide is output. When p _n % of the light incident on the nth waveguide is output, p ₁ : p ₂ :...: P _n is defined as the merge ratio. Therefore, when a certain optical branch circuit is used from the reverse direction to form an optical merge circuit, the branch ratio and the merge ratio coincide.

FIG. 1 is a schematic diagram illustrating a configuration of an optical modulator according to the present embodiment.
In FIG. 1, 101 is an input waveguide, 102 is an optical branching circuit which is an optical branching means having an asymmetric branching ratio, 103a and 103b are connection waveguides, 104a and 104b are phase modulators, and 105a and 105b are connection waveguides. , 106 are optical merging circuits which are optical merging means having an asymmetric merging ratio, and 107 indicates an output waveguide.

  The signal light first enters from the input waveguide 101, is branched into two by the optical branch circuit 102, and is guided to the phase modulator 104a and the phase modulator 104b via the connection waveguide 103a and the waveguide 103b.

  The branching ratio of the optical branching circuit 102 is set to be 2: 1, and the input signal light has an intensity of 2/3 (√ (2/3) in the electric field) in the connection waveguide 104a. 1/3 in intensity (√ (1/3) in electric field) is output to the waveguide 103b. The phase modulators 104a and 104b are each driven by an independent electric drive signal. Therefore, in this case, the dual drive Mach-Zehnder modulator (DDMZM: Dual Drive MZM) is distinguished from a push-pull drive modulator using a differential signal.

Here, the multilevel modulation operation of DDMZM will be described.
FIG. 2 is a diagram for explaining the constellation of the optical modulator according to the present embodiment.
When 16-value modulation is performed using DDMZM, the drive electrical signal is a 4-value signal whose maximum amplitude is 1.5 times the half-wave voltage. Assuming that the phase modulator 104a is modulated by a quaternary electrical signal, the electric field of the signal light emitted from the phase modulator 104a is divided into three parts between phase angles 0 and 1.5π. In the form of tracing on the circumference of FIG. 2 (a), it reciprocates between points A, B, C, and D having different angles by 90 degrees according to the drive electric signal.

  Here, in FIG. 2A, the position inclined 45 degrees with respect to the IQ coordinate, that is, the point A is made to correspond to the reference point, that is, the phase angle 0. In the optical modulator according to the present embodiment, the phase angle differences necessary for making the intervals of the A, B, C, and D corner points equal are all 90 degrees constant. Differences in the amplitude of the electric drive signal are also equally spaced.

  Similarly, assuming that the phase modulator 104b is modulated by a four-valued electrical signal that is 1.5 times the half-wave voltage, the electric field of the signal light emitted from the phase modulator 104b has a phase angle of 0 to 1. .5π is divided into three, so that the electric field locus similar to the output from the phase modulator 104a as shown in FIG.

  The signal lights output from the phase modulators 104a and 104b are combined in the optical combining circuit 106 via the connection waveguides 105a and 105b. The optical converging ratio of the optical converging circuit 106 is set to be 2: 1, and the connection method is 2/3 of the intensity of the signal light incident from the connection waveguide 105a (√ (2/3) in electric field). However, 1/3 of the signal light intensity incident from the connection waveguide 105b (√ (1/3) in the electric field) is output from the output waveguide 107.

  As a result, the electric field of the signal light from the input waveguide 101 through the first path of the optical branch circuit 102, the connection waveguide 103a, the phase modulator 104a, the connection waveguide 105a, and the optical converging circuit 106 is input to the input waveguide 101. Compared with the incident signal light, √ (2/3) × √ (2/3) = 2/3.

  On the other hand, the electric field of the signal light from the input waveguide 101 through the second path of the optical branch circuit 102, the connection waveguide 103b, the phase modulator 104b, the connection waveguide 105b, and the optical converging circuit 106 is √ (1/3) Since x√ (1/3) = 1/3, the signal light output from the optical converging circuit 106 has electric field amplitudes of the signal light passing through the first path and the signal light passing through the second path. The difference is 3 dB, that is, the intensity difference is 6 dB.

  At this time, the trace of the electric field of the signal light output from the first path including the phase modulator 104a and the trace of the electric field of the signal light output from the second path including the phase modulator 104b are output waveguides. Since it is added so that an intensity difference of 6 dB is generated when it is output from the output 107, the final constellation of the signal light output from the output waveguide 107 is indicated by ● in FIG. As shown in the figure, four points A, B, C, and D arranged on a large circumference are centered on four points, and four points are arranged on a small circumference having a half radius of the large circle. It becomes the point.

  For example, assuming that the constellation of the signal light output from the phase modulator 104a is point B shown in FIG. 2B, the electric field of the signal light output from the phase modulator 104b is added. The trace reciprocates between a, b, c, and d on the circumference of a small radius centered on point B. This is easy to understand considering the addition of electric field vectors.

Here, the reason why the radius of the small circle is ½ of the radius of the large circle will be briefly described.
As described above, the signal light that has passed through the phase modulator 104a and the signal light that has passed through the phase modulator 104b are merged by the optical merge circuit 106, and the intensity difference when output from the output waveguide 107 is 6 dB. ing. Since the constellation diagram of FIG. 2 is a diagram showing the trace of the electric field of light, the electric field intensity difference 3 dB corresponding to the optical intensity difference 6 dB, that is, ½ of the radius of the large circle is the radius of the small circle.

  Here, returning to the description of the constellation, the same discussion as the discussion regarding the B point described above also holds for the A point, the C point, and the D point in FIG. Therefore, the constellation in this embodiment has 16 constellations in which four points are arranged on a small circumference having a half radius of a large circle with A, B, C, and D in FIG. It can be understood that this is a point.

  When the constellation in this embodiment is compared with the constellation of the conventional example shown in FIG. 12 (d), the constellation of 16QAM, which is 16-value modulation similar to the case where the conventional quadrature modulator is used in this embodiment, is obtained. It turns out that it is obtained. That is, by using the configuration shown in the present embodiment, it is possible to realize an optical modulator for optical multilevel modulation with a simple configuration, low cost, easy control, and low loss.

Here, in this embodiment, it is pointed out that the 3 dB excess loss that is essentially generated when the conventional quadrature modulator is used does not occur.
As described above, the electric field amplitude of the signal light from the input waveguide 101 through the first path of the optical branch circuit 102, the connection waveguide 103a, the phase modulator 104a, the connection waveguide 105a, and the optical converging circuit 106 is √ (2 / 3) × √ (2/3) = 2/3.

  On the other hand, the electric field amplitude of the signal light passing from the input waveguide 101 through the second path of the optical branching circuit 102, the connecting waveguide 103b, the phase modulator 104b, the connecting waveguide 105b, and the optical combining circuit 106 is √ (1/3) ) × √ (1/3) = 1/3.

When the electric fields of these two signal lights are added in the same phase, the electric field amplitude is 2/3 + 1/3 = 1, and the light intensity is 1 ² = 1 by the square of the electric field amplitude. It can be understood that the electric field or light intensity of the signal light incident on the waveguide 101 is output to the output waveguide 107 without loss.

  In the configuration of the optical modulator according to the present embodiment, since the light intensity is constant for both the signal light that has passed through the first path and the signal light that has passed through the second path, As compared with the case of using the conventional quadrature modulator, the intensity fluctuation of the modulated output optical signal is reduced, and the modulated optical signal generated by the configuration of the optical modulator according to the present embodiment is There is an advantage that it is not easily affected by non-linearity.

  The configuration of the optical branching circuit 102 that is the optical branching unit and the optical combining circuit 106 that is the optical combining unit used in the present embodiment is not particularly limited, and a Y branching circuit, a directional coupler, a funnel type coupler, A similar effect can be obtained by using any configuration such as an MMI coupler using the interference effect of a multimode waveguide.

  For example, the MMI coupler having the shape shown in FIG. 3A can perform an asymmetric branching operation. In FIG. 3A, 301a and 301b are input waveguides, 302 is a multimode interference region, and 303a and 303b are branch output waveguides.

  As shown in FIG. 3A, the branching ratio can be adjusted by changing the shape of the multimode interference region 302 from a rectangle. Therefore, the optical modulator according to the present invention can be used as an optical branching unit / optical combining unit having an asymmetric branching ratio / merging ratio.

It is also possible to configure an optical branching means having an asymmetric branching ratio using a normal optical branching circuit / optical converging circuit having a symmetrical branching ratio. An example is shown in FIG.
In FIG. 3B, 304 is an input waveguide, 305 is a symmetrical optical branch circuit having a branching ratio of 1: 1, 306 is a connecting waveguide, 307 is a phase adjustment region, 308 is two inputs and two outputs, and a branching ratio is 1. 1 symmetric multiplexer / demultiplexer circuit, that is, a normal 3 dB coupler, and reference numerals 309 and 310 denote branch output waveguides.

  The input waveguide 304, the symmetric optical branch circuit 305, the connection waveguide 306, the phase adjustment region 307, the symmetric coupler circuit 308, and the branch output waveguides 309 and 310 constitute a Mach-Zehnder interferometer. The signal light incident from the waveguide 304 is output to the output waveguide 309 or the output waveguide 310 or both the output waveguide 309 and the output waveguide 310 according to the phase state in the phase adjustment region 307. .

  Accordingly, by appropriately controlling the phase in the phase adjustment region 307, it becomes possible to branch light into the branch output waveguide 309 and the branch output waveguide 310 at a desired branch ratio, and an optical branch having an asymmetric branch ratio. It can be used as a means.

Furthermore, if a slight loss is allowed, it is possible to provide an optical branching means having an asymmetric branching ratio by a simple method. FIG. 3C shows the configuration.
In FIG. 3C, 311 is an input waveguide, 312 and 313 are symmetrical optical branch circuits with a branching ratio of 1: 1, 314 and 315 are waveguides for branch output, and 316 is a waveguide.

  The signal light incident from the input waveguide 311 is branched into two by the symmetric optical branch circuit 312, and ½ optical power is guided to the branch output waveguide 314, and the remaining ½ optical power is guided to the symmetric optical branch circuit 313. It is burned.

  The signal light incident on the symmetric optical branch circuit 313 is branched into two by the symmetric optical branch circuit 313, and ½ optical power is guided to the branch output waveguide 315, and the remaining ½ optical power is guided to the waveguide 316. .

  Considering the intensity of the signal light output to the branch output waveguide 315, the intensity is 1/2 × 1/2 = 1/4 compared to the signal light incident from the input waveguide 311. It can be seen that the branching ratio between the branching output waveguide 314 and the branching output waveguide 315 is 2: 1 when compared with the branching output waveguide 314 that outputs ½ optical power. At this time, the signal light output from the waveguide 316 is not used.

  Therefore, when the signal light output from the waveguide 316 is considered as a loss, the configuration of FIG. 3C is an optical branching unit having an asymmetric branching ratio with respect to the branching output waveguide 314 and the branching output waveguide 315. Can think.

  Since the optical branching unit operates as an optical merging unit if the entrance and the outlet are reversed, the configurations described in FIGS. 3B and 3C all have an asymmetric branching ratio as they are. Needless to say, it can be used as 106.

  Further, even when the configuration shown in FIG. 3C is used as the optical branching unit 101 and the optical combining unit 106, the loss caused by discarding the signal light output from the waveguide 316 is not caused by the conventional quadrature modulator. Therefore, the low-loss optical multilevel modulation modulator, which is a feature of the optical modulator according to the present invention, is realized.

The structure and material of the Mach-Zehnder modulator used in the configuration of the optical modulator according to the present embodiment is not particularly limited, and is typically a dielectric material typified by LiNbO ₃ , InP-based, or GaAs-based. Compound materials, semiconductor materials such as Si, polymer materials, etc. All material systems that can normally be used as modulators, in other words, voltage application, current injection, heating, cooling, light irradiation, etc., external control By taking the configuration of the optical modulator according to the present embodiment for all material systems in which the refractive index can be varied by the above, the effects as described above can be expected.

  The waveguide structure is not particularly limited, and a similar effect can be expected by adopting this configuration as long as the waveguide structure is capable of propagating signal light, such as a buried structure, a ridge structure, and a high mesa structure. .

The second embodiment of the optical modulator according to the present invention will be described below.
FIG. 4 is a schematic diagram illustrating the configuration of the optical modulator according to the present embodiment.
In FIG. 4, 401 is an input waveguide, 402 is an optical branching circuit which is an optical branching means having an asymmetric branching ratio, 403a and 403b are connection waveguides, 404a and 404b are phase modulators, and 405a and 405b are connection waveguides. , 406 are optical combining circuits which are optical combining means having an asymmetric combining ratio, 407 is an output waveguide, and 408a and 408b are phase adjustment regions which are phase adjusting means.

  The operating principle of the optical modulator according to the present embodiment is the same as that of the optical modulator according to the first embodiment shown in FIG. 1 except for the point relating to the operation of the phase adjustment regions 408a and 408b. 401, an optical branch circuit 402, connection waveguides 403a and 403b, phase modulators 404a and 404b, connection waveguides 405a and 405b, an optical merge circuit 406, and an output waveguide 407, respectively, an input waveguide 101, an optical branch circuit 102, By replacing the connection waveguides 103a and 103b, the phase modulators 104a and 104b, the connection waveguides 105a and 105b, the optical converging circuit 106, and the output waveguide 107, the operation can be explained.

  That is, the signal light first enters the input waveguide 401, is branched into two by the optical branch circuit 402, and is guided to the phase modulator 404a and the phase modulator 404b via the connection waveguide 403a and the connection waveguide 403b. The phase modulators 404a and 404b are each driven by an independent electric drive signal.

  As in the first embodiment, the drive electric signal is a quaternary signal whose maximum amplitude is 1.5 times the half-wave voltage. Assuming that the phase modulator 404a is modulated by a quaternary electric signal, the electric field of the signal light emitted from the phase modulator 404a is divided into three parts between phase angles 0 and 1.5π. Therefore, similarly to the optical modulator according to the first embodiment, the driving electric signals are provided between points A, B, C, and D having different angles by 90 degrees while tracing the circumference of FIG. Make a round trip accordingly.

  Also in the optical modulator according to the present embodiment, the difference in phase angle necessary for making the intervals of the A, B, C, and D corner points equal is constant by 90 degrees, and therefore, the necessary four values are obtained. Differences in the amplitude of the electric drive signal are also equally spaced.

  Similarly, assuming that the phase modulator 404b is modulated by a four-value electric signal that is 1.5 times the half-wave voltage, the electric field of the signal light emitted from the phase modulator 404b has a phase angle of 0 to 1. .5π is divided into three, so that the electric field locus similar to the output from the phase modulator 404a can be obtained.

  The signal lights output from the phase modulators 404a and 404b are combined in the optical combining circuit 406 via the connection waveguides 405a and 405b. The optical converging ratio of the optical converging circuit 406 is set to be 2: 1, and the connection method is 2/3 of the signal light intensity incident from the connection waveguide 405a (√ (2/3) in the electric field) ), And 1/3 of the signal light intensity incident from the connection waveguide 405b (√ (1/3) in the electric field) is output from the output waveguide 407.

  As a result, the electric field of the signal light from the input waveguide 401 through the first path of the optical branch circuit 402, the connection waveguide 403 a, the phase modulator 404 a, the connection waveguide 405 a, and the optical combining circuit 406 is input to the input waveguide 401. Compared with the incident signal light, √ (2/3) × √ (2/3) = 2/3.

  On the other hand, the electric field of the signal light from the input waveguide 401 through the second path of the optical branch circuit 402, the connection waveguide 403b, the phase modulator 404b, the connection waveguide 405b, and the optical merge circuit 406 is √ (1/3) Since x√ (1/3) = 1/3, the signal light output from the optical converging circuit 406 includes the electric field amplitude of the signal light passing through the first path and the signal light passing through the second path. The difference in the electric field amplitude is 3 dB, that is, the intensity difference is 6 dB.

  At this time, the trace of the electric field of the signal light output from the first path including the phase modulator 404a and the trace of the electric field of the signal light output from the second path including the phase modulator 404b are output waveguides. When output from 407, they are added so that an intensity difference of 6 dB occurs.

  For this reason, the final constellation of the signal light output from the output waveguide 407 is indicated by ● in FIG. 2B, as in the optical modulator according to the first embodiment shown in FIG. 16 points having four points arranged on a small circumference having a half radius of the large circle, each centered on four points A, B, C, and D arranged on the large circumference, Become. That is, by using the configuration of the optical modulator according to the present embodiment, it is possible to realize a modulator for optical multilevel modulation that has a simple configuration, low cost, easy control, and low loss.

Here, the function of the phase adjustment regions 408a and 408b will be described.
If the element is ideally manufactured, it is possible to realize a modulator for optical multilevel modulation with the configuration described in the first embodiment. However, when an element is actually manufactured, a small number of the waveguides constituting the arm region of the Mach-Zehnder interferometer respectively composed of the connection waveguide 403a or the connection waveguide 403b to the connection waveguide 405a or the connection waveguide 405b are used. The manufacturing error becomes the optical path length difference between both arms, the optical path length difference is generated between the first path and the second path described above, and the signal light passing through the phase modulator 404a and the phase modulator 404b is optical. There is a possibility that a phase difference may occur when the merging circuit 406 adds them.

  In the optical modulator according to the present embodiment, the phase difference regions 408a and 408b are provided in the arm of the Mach-Zehnder interferometer, thereby making it possible to adjust the phase difference and improving the fabrication tolerance and yield of the device. .

  That is, when there is no phase adjustment region 408a, 408b, the signal light constellation via the phase modulator 104a or the phase modulator 404a and the signal light constellation via the phase modulator 104b or the phase modulator 404b There is a possibility that the relative phase angle shifts, and the total constellation map may be tilted without the 16 circles arranged in a lattice pattern as shown in FIG.

  Therefore, in the optical modulator according to the present embodiment shown in FIG. 4, the light is incident on the optical combining circuit 406 by either the phase adjustment region 408a or the phase adjustment region 408b, or by both the phase adjustment region 408a and the phase adjustment region 408b. By adjusting the relative phase difference of the signal light, a lattice-like constellation map as shown in FIG. 5B can be realized.

  That is, even when a relative optical path length difference or phase difference occurs between the plurality of arms of the Mach-Zehnder interferometer due to a manufacturing error or the like, the configuration of the optical modulator according to the present embodiment is stable. Thus, multi-level modulation operation can be performed.

  The phase adjustment method in the phase adjustment regions 408a and 408b is not limited to the method of aligning the phases in which the relative phases coincide with each other so that the points A and a are located at the lower right as shown in FIG. c) to (e), when the directions of the large circle and the small circle are different from each other by 90 degrees, 180 degrees, and 270 degrees, that is, the relative phase difference between them becomes an integral multiple of 90 degrees. Even in this case, it can be understood that the 16 circles are similarly arranged neatly in a lattice pattern, and the same operation is possible.

  There are no particular restrictions on the structure of the phase adjustment region 408a and the phase adjustment region 408b. It may be the same as the phase modulator 404a and the phase modulator 404b. In addition, the effect of the present invention can be realized with any structure as long as the phase changes. As the phase adjusting means, any means such as current injection, local temperature adjustment by a heater, electric field application, light irradiation, or the like can be used.

  As described above, the configuration of the optical modulator according to the present embodiment is simple in configuration, low in cost, easy to control, and has a conventional quadrature modulation even in the case where a manufacturing error in element production occurs. Thus, it is possible to realize a low-loss optical multilevel modulation modulator that does not cause the 3 dB principle loss inherently occurring in the optical device.

  In the present embodiment, the case where the phase adjustment region is connected to the phase modulators, which are all phase modulation means, has been described, but one of the outputs from the phase modulation means can be used without phase adjustment. .

  For example, the phase adjustment region 408b connected to the phase modulator 404b is used in a state where no phase change occurs, and in the phase adjustment region 408a connected to the phase modulator 404a, the relative light of the signal light incident on the optical combining unit 406 is compared. It is possible to adjust the general phase relationship. That is, the phase adjustment region 408b is not necessary in principle. That is, in principle, it is sufficient for the operation of the optical modulator according to the present embodiment if the phase adjustment region 408a is provided in the remainder excluding one of the phase modulation means.

  However, the configuration in which the phase adjustment regions are connected to all the phase modulation means as shown in this embodiment has excellent geometric symmetry of the optical circuit, so both arms of the Mach-Zehnder interferometer It is also pointed out that the optical path length difference is less likely to occur, the wavelength characteristics are excellent, the individual elements constituting the optical circuit are easily laid out, and the circuit is easy to arrange.

  In the present embodiment, the configuration in which the phase modulation unit and the phase adjustment region are arranged in this order with respect to the propagation direction of the signal light has been described. However, the arrangement order is not limited to this, and the order may be changed. There is no problem, and exactly the same operation can be realized. Moreover, the order does not need to be the same order about all the arms which comprise a Mach-Zehnder interferometer, and each may be arrange | positioned in a different order.

  The configuration of the optical branch circuit 402 and the optical combining circuit 406 in the optical modulator according to the present embodiment is not particularly limited as in the case of the optical modulator according to the first embodiment. The same effect can be obtained by using any configuration such as a circuit, a directional coupler, a funnel type coupler, and an MMI coupler using the interference effect of a multimode waveguide.

The structure and material of the Mach-Zehnder modulator used in the configuration of the optical modulator according to the present embodiment is not particularly limited, and is a dielectric material represented by LiNbO ₃ , InP-based, GaAs-based. All materials that can be used as a normal modulator, such as compound semiconductor materials, semiconductor materials such as Si, polymer materials, etc., in other words, voltage application, current injection, heating, cooling, light irradiation, etc. The effects described above can be expected by adopting the configuration of the optical modulator according to the present embodiment for all material systems in which the refractive index can be varied by control.

  Also, the waveguide structure is not particularly limited, and the configuration of the optical modulator according to the present embodiment is not limited as long as the waveguide structure allows signal light to propagate through a buried structure, a ridge structure, a high mesa structure, or the like. Similar effects can be expected.

A third embodiment of the optical modulator according to the present invention will be described below.
FIG. 6 is a schematic diagram illustrating the configuration of the optical modulator according to the present embodiment.
In FIG. 6, 601 is an input waveguide, 602 is an optical branch circuit, 603a and 603b are connection waveguides, 604a and 604b are phase modulators, 605a and 605b are connection waveguides, 606 is an optical converging circuit, and 607 is an output guide. Waveguides 608a and 608b are phase adjustment regions, and 609a and 609b are electroabsorption regions which are light intensity adjusting means.

  The operating principle of the optical modulator according to the present embodiment is the same as that of the optical modulator according to the second embodiment shown in FIG. 4 except for the point relating to the operation of the electroabsorption regions 609a and 609b which are light intensity adjusting means. The input waveguide 601, the optical branch circuit 602, the connection waveguides 603a and 603b, the phase modulators 604a and 604b, the connection waveguides 605a and 605b, the optical converging circuit 606, the output waveguide 607, and the phase adjustment regions 608a and 608b. , Input waveguide 401, optical branch circuit 402, connection waveguides 403a and 403b, phase modulators 404a and 404b, connection waveguides 405a and 405b, optical converging circuit 406, output waveguide 407, and phase adjustment regions 408a and 408b, respectively. The operation can be explained by reading as follows.

  In the configuration of the optical modulator according to the present embodiment, in addition to the configuration of the optical modulator according to the second embodiment, electroabsorption regions 609a and 609b which are intensity adjusting means are arranged on both arms constituting the Mach-Zehnder interferometer. Has been. The function of the optical branching circuit 602 which is an optical branching unit due to a manufacturing error or the like, the variation of the branching ratio and the combining ratio of the optical combining circuit 606 which is an optical combining unit, the connection waveguides 603a and 603b, and the phase modulators 604a and 604b. Depending on the loss variation in the phase adjustment regions 608a and 608b, the optical waveguide 601 to the optical branch circuit 602, the connection waveguide 603a, the phase modulator 604a, the phase adjustment region 608a, the electric field absorption region 609a that is the intensity adjustment means, the connection guide The signal light that has passed through the first path of the waveguide 605a and the optical converging circuit 606, the optical waveguide from the input waveguide 601 to the optical branching circuit 602, the connection waveguide 603b, the phase modulator 604b, the phase adjustment region 608b, and the electric field absorption that is the intensity adjustment means. The signal light is transmitted through the second path of the region 609b, the connection waveguide 605b, and the optical converging circuit 606. Even if the difference in amplitude is 3 dB, that is, the intensity difference is deviated from 6 dB, the desired intensity difference can be set to 6 dB by adjusting the intensity with the electroabsorption region 609 a or the electroabsorption region 809 b as the intensity adjusting means. Is possible.

  Therefore, the configuration of the optical modulator according to the present embodiment is a feature of the optical modulator according to the first or second embodiment, the configuration is simple, low cost, easy to control, and the conventional quadrature modulation. The manufacturing tolerance can be increased while maintaining the effect of low loss that does not cause the 3 dB principle loss inherently occurring in the vessel.

  Further, if a slight excess loss is allowed, only one of the optical branching circuit 602 and the optical combining circuit 606 may have an asymmetric optical branching ratio or optical combining ratio, and the other has a symmetrical optical branching ratio or optical combining ratio. Even when a simple configuration is used, it is possible to set a desired intensity difference of 6 dB by adjusting the intensity with the electric field absorption region 609a or the electric field absorption region 609b, which is an intensity adjustment means, thereby realizing a desired operation. It is possible.

  In this embodiment, the configuration in which the phase modulation unit, the phase adjustment region, and the intensity adjustment unit are arranged in this order with respect to the propagation direction of the signal light has been described. However, the arrangement order is not limited to this, and the order is not limited to this. The same operation can be realized without any problem even if they are replaced. Moreover, the order does not need to be the same order about all the arms which comprise a Mach-Zehnder interferometer, and each may be arrange | positioned in a different order.

  In the present embodiment, the case where the phase adjustment regions 608a and 608b are connected to the phase modulators 604a and 604b, which are all phase modulation means, has been described. However, as described in the second embodiment, the phase modulation means One of the outputs can be used without phase adjustment.

  For example, the phase adjustment region 608b connected to the phase modulator 604b is used in a state where no phase change occurs. In the phase adjustment region 608a connected to the phase modulator 604a, the relative signal light incident on the optical converging means 606 is used. It is possible to adjust the general phase relationship. That is, the phase adjustment region 608b is not necessary in principle. That is, in principle, it is sufficient for the operation of the optical modulator according to the present embodiment if the phase adjustment region 608a is provided in the remainder excluding one of the phase modulation means.

  However, the configuration in which the phase adjustment regions 608a and 608b are connected to all the phase modulation means as shown in the present embodiment has excellent geometric symmetry of the optical circuit, so the Mach-Zehnder interferometer The optical path length difference between the two arms is less likely to occur, the wavelength characteristics are excellent, the layout of the individual elements constituting the optical circuit is easy, and the circuit is easy to arrange. The same as in the case of the modulator.

  The configuration of the optical branching circuit 602 and the optical combining circuit 606 used in the optical modulator according to the present embodiment is not particularly limited as in the case of the optical modulators according to the first and second embodiments. It is possible to obtain the same effect by using any configuration such as a Y-branch circuit, a directional coupler, a funnel type coupler, and an MMI coupler using the interference effect of a multimode waveguide. It is self-explanatory.

The structure and material of the Mach-Zehnder modulator used in the configuration of the optical modulator according to the present embodiment is not particularly limited, and is a dielectric material represented by LiNbO ₃ , InP-based, GaAs-based. Compound semiconductor materials represented by, semiconductor materials such as Si, polymer materials, etc., all material systems that can be used as normal modulators, in other words, voltage application, current injection, heating, cooling, light irradiation, etc. The effects described above can be expected by adopting the configuration of the optical modulator according to the present embodiment for all material systems whose refractive index can be varied by external control.

  The waveguide structure is not particularly limited, and a similar effect can be expected by adopting this configuration as long as the waveguide structure is capable of propagating signal light, such as a buried structure, a ridge structure, and a high mesa structure. .

The fourth embodiment of the optical modulator according to the present invention will be described below.
The optical modulator according to the present embodiment shows a case where the number of arms of the Mach-Zehnder interferometer or the number of phase modulation means is 3 in the optical modulator according to the second embodiment shown in FIG. .

FIG. 7 is a schematic diagram illustrating the configuration of the optical modulator according to the present embodiment.
In FIG. 7, 701 is an input waveguide, 702 is an optical branch circuit, 703a, 703b, and 703c are connection waveguides, 704a, 704b, and 704c are phase modulators, 705a, 705b, and 705c are connection waveguides, and 706 is an optical confluence. A circuit, 707 is an output waveguide, 708a, 708b, and 708c are phase adjustment regions, and 709a, 709b, and 709c are electroabsorption regions that are light intensity adjusting means.

  In this embodiment, the branching ratio of the optical branching means 702 is asymmetric, and the branching ratios to the connection waveguides 703a, 703b, and 703c are set to 4: 2: 1, respectively. The confluence ratio of the optical confluence means 706 is asymmetric, and the confluence ratios from the connection waveguides 705a, 705b, and 705c are set to be 4: 2: 1, respectively.

  The principle of operation of the optical modulator according to the present embodiment is as follows: Mach-Zehnder interferometer arm and phase modulators 704a, 704b, 704c which are phase modulation means disposed on the arm, and electroabsorption region 709a which is intensity adjustment means. , 709b, 709c and phase adjustment regions 708a, 708b, 708c are the same as those of the optical modulator according to the third embodiment shown in FIG. 6, input waveguide 601, optical branch circuit 602, connection waveguides 603 a and 603 b, phase modulators 604 a and 604 b, connection waveguides 605 a and 605 b, optical converging circuit 606, output waveguide 607, phase adjustment region 608 a, 608b and electroabsorption regions 609a and 609b are connected to an input waveguide 701, an optical branch circuit 702, and connection waveguides 703a and 703, respectively. 703c, phase modulators 704a, 704b, 704c, connection waveguides 705a, 705b, 705c, optical combining circuit 706, output waveguide 707, phase adjustment regions 708a, 708b, 708c, electroabsorption regions 709a, 709b, 709c. It is possible to understand the operation.

  That is, as in the first to third embodiments, considering the operation when driving with a quaternary signal whose maximum amplitude is 1.5 times the half-wave voltage Vπ, the input waveguide 701, the optical branch circuit 702, the connection conductor The constellation of the signal light output through the first path via the waveguide 703a, the phase modulator 704a, the phase adjustment region 708a, the electroabsorption region 709a, the connection waveguide 705a, the optical converging circuit 706, and the output waveguide 707 is Based on the same principle as that of the optical modulator according to the third embodiment shown in FIG. 6, the circles arranged at equal intervals on the circumference having a large radius r1 as shown in FIG. 8A are obtained.

  Since the branching ratio of the optical branch circuit 702 is 4: 2: 1, the ratio of branching to the first path, the input waveguide 701, the optical branch circuit 702, the connection waveguide 703b, the phase modulator 704b, and the phase adjustment The ratio of branching to the second path via the region 708b, the electroabsorption region 709b, the connection waveguide 705b, the optical converging circuit 706, and the output waveguide 707 is 4: 2 = 2: 1, as shown in FIG. This is the same as the optical modulator according to the third embodiment.

  Also, since the confluence ratio of the optical confluence circuit 706 is 4: 2: 1, the confluence ratio from the first path and the second path is also 4: 2 = 2: 1, and the third ratio shown in FIG. Since it is the same as the optical modulator according to the embodiment, the signal light passing through the first path and the signal light passing through the second path are recorded in the output waveguide 707 with an intensity difference of 6 dB. It will be.

  Therefore, the constellation when the signal light output from the first path and the signal light output from the second path are added is the radius at which the constellation output from the first path is arranged. Since the constellation is equidistantly arranged on a small circumference having a radius r2 that is ½ of the circle of r1, the signal light passing through the first path and the signal light passing through the second path The phase difference is adjusted to 0 degrees, 90 degrees, 180 degrees, or 270 degrees, that is, the relative phase difference is an integral multiple of 90 degrees in the phase adjustment area 708a or the phase adjustment area 708b. The constellation of the signal light synthesized by adding up becomes 16 points arranged on the circumference of the radius r2 centered on the circle as shown by the circles in FIG. 8B. Shown in 6 It was the same as the optical modulator according to the third embodiment.

  In this embodiment, the input waveguide 701, the optical branch circuit 702, the connection waveguide 703c, the phase modulator 704c, the phase adjustment region 708c, the electroabsorption region 709c, the connection waveguide 705c, the optical converging circuit 706, the output waveguide are further added here. The signal light output through the third path passing through the waveguide 707 is added.

  The connection waveguide 703c, the phase modulator 704c, the phase adjustment region 708c, the electroabsorption region 709c, and the connection waveguide 705c function as the connection waveguide 703a, the phase modulator 704a, the phase adjustment region 708a, the electroabsorption region 709a, and the connection waveguide. Since 705a and the connection waveguide 703b, the phase modulator 704b, the phase adjustment region 708b, the electroabsorption region 709b, and the connection waveguide 705b are similar, the signal light constellation via the third path also has a circle with a radius r3. It can be easily understood that the four points are arranged at equal intervals on the circumference.

  Here, since the branching ratio of the optical branching circuit 702 is 4: 2: 1, the ratio of branching to the first path and the ratio of branching to the third path are 4: 1, and the optical merging Since the merge ratio of the circuit 706 is also 4: 2: 1, the merge ratio from the first path and the third path is also 4: 1. Therefore, the signal light passing through the third path is Compared with the signal light passing through the path, the light is output to the output waveguide 707 at 1/16, that is, with an intensity difference of 12 dB.

  Therefore, it can be understood that the constellation radius r3 of the signal light passing through the third path is 6 dB down, that is, 1/4 of r1 in the electric field corresponding to the intensity difference of 12 dB. Further, in the phase adjustment region 708c, the phase difference between the signal light passing through the first path and the signal light passing through the third path is one of 0 degree, 90 degrees, 180 degrees, and 270 degrees, that is, mutual By adjusting the relative phase difference to be an integral multiple of 90 degrees, the constellation of the combined signal light passing through each of the three paths is as indicated by ● in FIG. The radius r3 is centered on 16 points ◎ arranged on the circumference of the radius r2 with the four points arranged on the circumference of the radius r1 centered at the origin, respectively. It can be seen that it is represented by 64 points arranged on the edit.

  Here, the second path is automatically adjusted by adjusting the phase difference between the signal light passing through the first path and the signal light passing through the third path to be an integral multiple of 90 degrees. Needless to say, the phase difference between the signal light passing through and the signal light passing through the third path is also an integral multiple of 90 degrees. Therefore, the adjustment of the relative phase difference of the signal light passing through the third path may be performed on either the signal light passing through the first path or the second path.

  The constellation shown in FIG. 8C is a 64QAM constellation in which 64 points are arranged in a lattice pattern at equal intervals, and the structure of the optical modulator according to the present embodiment enables 64QAM modulation. I know that there is.

  In this embodiment, since the electroabsorption regions 709a, 709b, and 709c, which are light intensity adjusting means, are arranged in each arm of the Mach-Zehnder interferometer, the light branching circuit 702 and the light combining means are the light branching means. When the branching ratio and merging ratio of a certain optical merging circuit 706 deviate from the setting due to manufacturing errors or the like, or due to the propagation loss difference of the arms of the Mach-Zehnder interferometer, the difference in intensity of the signal light propagating through each arm is predetermined. If the intensity ratio is deviated from this ratio, the intensity ratio can be set to a desired value by the electroabsorption regions 709a, 709b, and 709c, which are intensity adjusting means, so that a QAM modulator with high fabrication tolerance is provided. It becomes possible. In the case where the optical modulator according to the first to third embodiments is used, the electroabsorption regions 709a, 709b, and 709c, which are light intensity adjusting means, are not necessary in principle when ideally manufactured. Same as the case.

  In the present embodiment, the case where the number of arms of the Mach-Zehnder interferometer is 3 and the path of the signal light is 3 has been described. However, the same discussion holds even if the number of arms further increases, and the same operation is possible. . Generalizing the case where the number of arms is n (n ≧ 2), the intensity of the signal light that has passed through the kth (2 ≦ k ≦ n) path is equal to that of the signal light that has passed through the (k−1) th path. The branching ratio of the optical branching circuit 702 and the converging ratio of the optical combining circuit 706 may be set so that the intensity is ¼ (1/2 in electric field).

That is, if the branching ratio and the merging ratio are set to be 2 ^n-1 : 2 ^n-2 :...: 1, the intensity ratio of the signal light output to the output waveguide 707 is (2 ^{n −1} ) ² : (2 ⁿ⁻² ) ² :... (1) ^{2 and} the intensity of the signal light passing through the k-th path is 1 of the signal light passing through the (k−1) -th path. / 4.

Under such circumstances, the phase of the signal light passing through the k-th path is set to the phase adjustment regions 708a, 708b, and 708c so that the phase of the signal light passing through the other path is an integral multiple of 90 degrees. by adjusting a multi-level modulation of 4 ⁿ value 4 ⁿ QAM modulation can be realized.

That is, by using the configuration of the optical modulator according to the present embodiment, when the numbers of the arms of the Mach-Zehnder interferometer having n arms (n ≧ 2) are assigned in an arbitrary order, the arms are respectively arranged. The phase modulators 704a, 704b, and 704c, which are the phase modulation means, are each driven by a quaternary drive signal whose maximum amplitude is 1.5 times the half wavelength voltage, and the i th (1 ≦ i ≦ n) The optical branching circuit 702 as the optical branching unit and the optical combining circuit 706 as the optical combining unit 706 so that the intensity of the signal light passing through the path becomes 4 ^i-1 of the signal light intensity passing through the first arm. The branching ratio and the confluence ratio defined by the light intensity are set as 2 ^n-1 : 2 ^n-2 : ...: 1, and the phase of the signal light passing through the i-th path is set to other ^values . Phase is set to be an integral multiple of 90 degrees with respect to the signal light passing through the path. Sei regions 708a, 708b, a multi-level modulation of 4 ⁿ value 4 ⁿ QAM modulation can be achieved by phase adjustment at 708c.

  In this embodiment, the case where the electric field absorption regions 709a, 709b and 709c connected to the phase modulators 704a, 704b and 704c, which are phase modulation means, are used as the intensity adjustment means. It is not limited to the regions 709a, 709b, and 709c, and may use light absorption by current injection, may be an optical amplifier, and is connected to a Mach-Zehnder type optical attenuator circuit. Also good.

  If the configuration of the optical modulator according to the present embodiment is used, it is easy to avoid an excess loss of 3 dB, which is essentially generated when the quadrature modulator is used, based on the same discussion as when the number of arms is 2. Can understand.

  The optical branch circuit 702 used in the optical modulator according to the present embodiment is not particularly limited in the number of branches, and it is not always necessary to have one input and n outputs (n> 2). For example, even when a 1-input 2-output optical branch circuit is used, the optical branch circuits may be connected in multiple stages until a necessary number of branches is obtained. The same applies to the optical confluence circuit 707.

The fifth embodiment of the optical modulator according to the present invention will be described below.
FIG. 9 is a schematic diagram illustrating the configuration of the optical modulator according to the present embodiment.
In FIG. 9, 901 is an input waveguide, 902 is an optical branch circuit, 903a, 903b and 903c are connection waveguides, 904a, 904b and 904c are phase modulators, 905a, 905b and 905c are connection waveguides, and 906 is an optical confluence. A circuit, 907 is an output waveguide, 908a, 908b, and 908c are phase adjustment regions, and 909a, 909b, and 909c are electroabsorption regions that are light intensity adjusting means.

  In the configuration of the optical modulator according to the present embodiment, the optical branch circuit 902 is configured by multi-stage connection of the optical branch circuit 910 and the optical branch circuit 911, and the optical junction circuit 906 includes the optical branch circuit 912 and Except that the optical branch circuit 913 is configured by multistage connection, it is the same as the optical modulator according to the fourth embodiment shown in FIG.

  Therefore, the operation principle of the configuration of the optical modulator according to this embodiment is the same as that of the optical modulator according to the fourth embodiment shown in FIG. 7, and the input waveguide 701, the optical branch circuit 702 in FIG. Connection waveguides 703a, 703b, 703c, phase modulators 704a, 704b, 704c, connection waveguides 705a, 705b, 705c, optical converging circuit 706, output waveguide 707, phase adjustment regions 708a, 708b, 708c, electric field absorption region 709a , 709b, 709c, input waveguide 901, optical branch circuit 902, connection waveguides 903a, 903b, 903c, phase modulators 904a, 904b, 904c, connection waveguides 905a, 905b, 905c, optical converging circuit 906, output Waveguide 907, phase adjustment regions 908a, 908b, 908c, electroabsorption regions 909a, 909b By read as 909c, it is possible to understand its operation.

  In the present embodiment, the optical branching circuit 902 that is the optical branching means has a branching ratio of 4: 2: 1 to the connection waveguides 903a, 903b, and 903c, and the optical combining circuit 906 that is the optical combining means. Therefore, the branching ratios of the optical branching circuit 910 and the optical branching circuit 911 are 4: 3 and 2: 1, respectively, in order to set the merge ratio from the connection waveguides 905a, 905b, and 905c to 4: 2: 1. 1 and the merge ratios of the optical branch circuit 912 and the optical branch circuit 913 are also set to 4: 3 and 2: 1, respectively.

This is because when the 1-input 3-output optical branching means is configured by two-stage connection of the 1-input 2-output optical branching means, the branching ratio of the first-stage optical branching means is 2 ^3-1 : (2 ^{2 −1} +2 ^1-1 ) = 4: 3, the branching ratio of the second-stage optical branching means can be expressed as 2 ^2-1 : 2 ^1-1 = 2: 1, and the number of arms of the Mach-Zehnder interferometer Is generalized in the case of n (n ≧ 2), the i-th branching ratio counting from the input waveguide 901 side is set to the following formula (1), and the (i + 1) -th optical branching means Is connected to the waveguide having a branching ratio of the following formula (2).

  Similarly, with respect to the optical merging means, assuming that the i-th stage is closer to the output waveguide 907, the merging ratio of the i-th stage is set to the above equation (1), and the (i + 1) -th stage optical merging means is What is necessary is just to connect to the waveguide by which the confluence ratio is the above-mentioned formula (2) side.

Needless to say, even when the configuration of the optical modulator according to the present embodiment is used, an excess loss of 3 dB, which is essentially generated when the quadrature modulator is used, does not occur.
The configuration of the optical branching circuit and the optical combining circuit used in the optical modulator according to the present embodiment is not particularly limited. The interference effect of the Y branching circuit, the directional coupler, the funnel type coupler, and the multimode waveguide is not limited. The same effect can be obtained by using any configuration such as an MMI coupler using the.

In the configuration of the optical modulator according to the present embodiment, the structure and material of the Mach-Zehnder modulator to be used are not particularly limited, and a dielectric material such as LiNbO ₃ , InP-based, The effects as described above can be expected by adopting this configuration for all material systems that can be used as a normal modulator, such as compound semiconductor materials typified by GaAs, semiconductor materials such as Si, and polymer materials. . Also, the waveguide structure is not particularly limited, and the configuration of the optical modulator according to the present embodiment is not limited as long as the waveguide structure allows signal light to propagate through a buried structure, a ridge structure, a high mesa structure, or the like. Similar effects can be expected.

The sixth embodiment of the optical modulator according to the present invention will be described below.
The optical modulator according to this embodiment shows an example of the configuration of the optical branching circuit 902 in the optical modulator according to the fifth embodiment shown in FIG. The configuration of the optical modulator according to the present embodiment shows a configuration in which excess loss is smaller than that in the case of using a conventional quadrature modulator while using a symmetric optical branch circuit having a branching ratio of 1: 1 that is easy to manufacture.

FIG. 10 is a schematic diagram illustrating the configuration of the optical modulator according to the present embodiment.
In FIG. 10, 1001 is an input waveguide, 1002, 1003, 1004 are symmetrical optical branch circuits with a branching ratio of 1: 1, and 1005a, 1005b, 1005c, 1005d are output waveguides of the symmetrical optical branch circuits 1002, 1003, 1004, respectively. Show.
Now, the signal light incident from the input waveguide 1001 is branched into two by a symmetrical branch circuit 1002 having a branching ratio of 1: 1, and 1/2 power is output to the output waveguide 1005a and the remaining 1/2 power is two stages. The light enters the branch circuit 1003 of the eye.

Since the second branch circuit 1003 is also a symmetric branch circuit, the signal light is further branched into two, and as a result, the power of 1/2 × 1/2 = 1/4 is supplied to the output waveguide 1005b and the remaining 1/2 × 1. The power of / 2 = 1/4 is incident on the branch circuit 1004 at the third stage.
Since the third branch circuit 1004 is also a symmetric branch circuit, the signal light is further branched into two, resulting in 1/4 × 1/2 = 1/8 power to the output waveguide 1005c and the remaining 1/4 × 1. A power of / 2 = 1/8 is guided to the output waveguide 1005d.

  Now, focusing only on the output waveguides 1005a, 1005b, 1005c among the output waveguides 1005a, 1005b, 1005c, and 1005d, the branching ratio is 1/2: 1/4: 1/8 = 4: 2: 1. You can see that Therefore, when only the output waveguides 1005a, 1005b, and 1005c are used and the signal light output from the output waveguide 1005d is handled as a loss, the configuration of the optical modulator according to the present embodiment illustrated in FIG. 7 has an asymmetric branching ratio usable as the optical branching circuit 702 in the optical modulator according to the fourth embodiment shown in FIG. 7 and the optical branching circuit 902 in the optical modulator according to the fifth embodiment shown in FIG. It turns out that it can be used as an optical branching means.

  The optical branching means in the configuration of the optical modulator according to the present embodiment shown in FIG. 10 can be used as it is as an optical merging means by reversing left and right, so that the light according to the fifth embodiment shown in FIG. Needless to say, the optical combining circuit 906 in the modulator can be used.

Here, the excess loss when the configuration of the optical modulator according to the present embodiment shown in FIG. 10 is used will be discussed.
The total power output to the output waveguides 1005a, 1005b, and 1005c in the optical branching unit is 1/2 + 1/4 + 1/8 = 7/8. On the other hand, when this configuration is used as an optical merging means, the maximum is 7/8 as in the case of branching according to the so-called ray-reverse theorem. This is true when signal light is incident in the same phase from the three waveguides. At this time, the total transmission power of the modulator is 7/8 × 7/8 = 49/64, and the excess loss is about 1.2 dB. Compared with the excess loss of 3 dB when the conventional quadrature modulator is used. It can be seen that the loss can be reduced by 1.8 dB.

  As described above, either the optical branching unit or the optical combining unit, or both the optical branching unit and the optical combining unit are configured by a multistage connection of a symmetric optical branching circuit or a symmetric optical combining circuit, and the output port or the input port Even when a configuration that does not use one of them is used, a modulator for optical multilevel modulation that has a simple configuration, low cost, easy control, and low loss compared to a configuration using a conventional quadrature modulator is provided. It becomes possible to do.

  The present invention can be used, for example, in an optical modulator that converts an electrical signal into an optical signal and transmits the optical signal.

DESCRIPTION OF SYMBOLS 101 Input waveguide 102 Optical branch circuit 103a, 103b Connection waveguide 104a, 104b Phase modulator 105a, 105b Connection waveguide 106 Optical merge circuit 107 Output waveguide 301a, 301b Input waveguide 302 Multimode interference area | region 303a, 303b Branch output Waveguide 304 Input waveguide 305 Symmetric optical branch circuit 306 Connection waveguide 307 Phase adjustment region 308 Symmetric multiplexer circuit 309, 310 Branch output waveguide 311 Input waveguide 312, 313 Symmetric optical branch circuit 314, 315 Branch output Waveguide 316 Waveguide 401 Input waveguide 402 Optical branch circuits 403a and 403b which are optical branching means having asymmetric branching ratios Connection waveguides 404a and 404b Phase modulators 405a and 405b Connection waveguides 406 Optical converging circuit 407 Waveguide 408a, 408b Phase adjustment Tuning region 601 Input waveguide 602 Optical branch circuit 603a, 603b Connection waveguide 604a, 604b Phase modulator 605a, 605b Connection waveguide 606 Optical merge circuit 607 Output waveguide 608a, 608b Phase adjustment region 609a, 609b Electric field absorption region 701 Input Waveguide 702 Optical branch circuit 703a, 703b, 703c Connection waveguide 704a, 704b, 704c Phase modulator 705a, 705b, 705c Connection waveguide 706 Optical converging circuit 707 Output waveguide 708a, 708b, 708c Phase adjustment regions 709a, 709b, 709c Electric field absorption region 901 Input waveguide 902 Optical branch circuit 903a, 903b, 903c Connection waveguide 904a, 904b, 904c Phase modulator 905a, 905b, 905c Connection waveguide 906 Optical converging circuit 907 Output waveguide 908a, 908b, 908c Phase adjustment region 909a, 909b, 909c Electromagnetic absorption region 910, 911 Optical branch circuit 912, 913 Optical merge circuit 1001 Input waveguide 1002, 1003, 1004 Symmetric optical branch circuit 1005a, 1005b, 1005c, 1005d Waveguide 1101 Input waveguide 1102 Optical branch circuit 1103a, 1103b Connection waveguide 1104a, 1104b Optical branch circuit 1105a, 1105b, 1105c, 1105d Phase modulator 1106a, 1106b Optical merge circuit 1107a, 1107b Phase adjustment area 1108 Optical merge circuit 1109 Output guidance Waveguides 1110a and 1110b Mach-Zehnder modulators

Claims (11)

An optical modulator that converts an input electrical signal into a multilevel optical signal,
Optical branching means for branching incident signal light;
At least two phase modulation means connected to the output side of the optical branching means for modulating the phase of propagating light according to the electrical signal;
Optical merging means for merging outputs from the at least two phase modulation means,
Furthermore, at least one of the light branching means and the light joining means has an asymmetric branching ratio or joining ratio,
An optical modulator characterized in that the intensity of the signal light guided from the at least two phase modulation means to the output side through the optical converging means is set to be different from each other. An optical modulator that converts an input electrical signal into a multilevel optical signal,
Optical branching means for branching incident signal light;
An optical merging means for merging the branched signal light;
At least two waveguides connecting the optical branching means and the optical combining means constituting the Mach-Zehnder interferometer;
A phase modulation means for each of the at least two waveguides;
Furthermore, at least one of the light branching means and the light joining means has an asymmetric branching ratio or joining ratio,
An optical modulator characterized in that the intensity of the signal light passing through the phase modulation means and guided to the output side through the optical converging means is set to be different depending on the propagation path of the signal light. 3. The optical modulator according to claim 1, wherein the intensity of the signal light guided to the output side is different from each other by 1/4 times. An optical modulator that converts an input electrical signal into a multilevel optical signal,
Optical branching means for splitting the incident signal light into two parts with a branching ratio of 2: 1;
Two phase modulation means connected to the output side of the first optical branching means for modulating the phase of propagating light according to the electrical signal;
Optical merging means for merging the outputs from the two phase modulation means at a merging ratio of 2: 1;
An optical modulation characterized in that the optical branching means and the optical combining means are connected so that the intensity of the signal light guided from the two phase modulation means through the optical combining means to the output side is 4: 1 vessel. Phase adjustment means for adjusting the phase of the laser oscillation light to be propagated is connected to all of the at least two phase modulation means or the remaining phase modulation means except for one of the at least two phase modulation means. The optical modulator according to any one of claims 1 to 4, wherein the optical modulator is provided. Output from the at least two phase modulation means via the optical merging means to all of the at least two phase modulation means or to the remaining phase modulation means excluding one of the at least two phase modulation means. 6. A phase adjusting means for adjusting the phase of the signal light propagating so that the phase difference of the signal light guided to the side becomes an integer multiple of 0 degrees or 90 degrees is connected. The optical modulator according to any one of Items. Intensity adjusting means for adjusting the intensity of the propagating laser oscillation light is connected to all of the at least two phase modulating means or the remaining phase modulating means excluding one of the at least two phase modulating means. The optical modulator according to claim 1, wherein the optical modulator is a light modulator. 8. The optical modulator according to claim 7, wherein as the intensity adjusting means, an absorbing means for absorbing propagating signal light or an amplifying means for amplifying propagating signal light is connected. The optical modulator according to claim 7, wherein a Mach-Zehnder interferometer is connected as the intensity adjusting unit. The optical branching unit is any one of a multimode interference coupler, a funnel coupler, a directional coupler, and a Y-shaped optical combiner. Light modulator. 11. The device according to claim 1, wherein the optical combining unit is any one of a multimode interference coupler, a funnel type coupler, a directional coupler, and a Y-shaped optical combiner. Light modulator.

Images