WO2014051096A1

WO2014051096A1 - Optical modulator and optical modulation method

Info

Publication number: WO2014051096A1
Application number: PCT/JP2013/076385
Authority: WO
Inventors: 孝知伊藤; 篠崎　稔; 志展矢澤
Original assignee: 住友大阪セメント株式会社
Priority date: 2012-09-28
Filing date: 2013-09-27
Publication date: 2014-04-03
Also published as: JPWO2014051096A1

Abstract

This optical modulator is provided with a generating unit and a beam synthesizing unit. The generating unit is provided with a first generating unit and a second generating unit. The first generating unit is configured in such a manner as to provide modulation to a prescribed phase, generating a phase-modulated beam having a first light intensity. The second generating unit is configured in such a manner as to provide modulation to a prescribed phase, generating a phase-modulated beam having a second light intensity that differs from the first light intensity. The beam synthesizing unit is configured in such a manner that the two phase-modulated beams generated by the generating unit are emitted in polarization directions that are perpendicular to each other, and polarization synthesis is performed on the two phase-modulated beams, and an optical modulation signal is outputted.

Description

Optical modulator and optical modulation method

The present invention relates to an optical modulator that outputs a quadrature amplitude modulation (QAM) signal.
This application claims the priority based on Japanese Patent Application No. 2012-215675 for which it applied to Japan on September 28, 2012, and uses the content here.

QAM modulators are known as multi-level modulation type modulators (see, for example, Patent Documents 1 and 2).

In the modulator described in Patent Document 1, a loss part (6 dB) is provided in the arm output part on one side, and a 16QAM signal is obtained by superimposing QPSK signals having an output difference of 6 dB. Moreover, the structure which formed the branch part of input / output as a planar optical waveguide (PLC) on a glass substrate, and this glass substrate was optically connected with the lithium niobate substrate was employ | adopted. On the other hand, also in the optical modulator described in Patent Document 2, the optical power ratio is adjusted to ¼ by providing an optical power adjustment unit on one arm and providing an optical attenuation of −6 dB. Further, the power branching ratio of light is preferably 2: 1 in the configuration of Patent Document 1 and 1: 1 in the configuration of Patent Document 2, but in reality, it is rare that power splits ideally in the optical branching section. Depends on manufacturing error. For this reason, the variation due to the power branching ratio of the light and the adjustment due to the attenuation in the optical power adjustment unit result in the loss of light.
If the balance between the power branching ratio of these lights and the QPSK modulation unit is lost, the desired constellation figure of the QAM signal cannot be obtained, and the amplitude and phase distance between symbols is lost and the reading accuracy of the QAM signal is lowered, resulting in transmission. This leads to a decrease in distance.

In the modulators described in Patent Documents 1 and 2, the power branching ratio of the light branching portion varies, and the optical signal is attenuated in the arm on one side, so that the optical loss is large.

Japanese Unexamined Patent Publication No. 2009-94988 Japanese Unexamined Patent Publication No. 2009-244682

An embodiment of the present invention provides an optical modulator and an optical modulation method configured to suppress optical loss.

An optical modulator according to an aspect of the present invention is a generation unit including a first generation unit and a second generation unit, wherein the first generation unit is modulated to a predetermined phase and has a first light intensity. Configured to generate phase-modulated light, and wherein the second generation unit is configured to generate phase-modulated light that is modulated to a predetermined phase and has a second light intensity different from the first light intensity. The generation unit and the two phase-modulated lights generated by the generation unit are set to polarization directions perpendicular to each other, and the two phase-modulated lights are combined to output a modulated signal light. A photosynthesis unit configured as described above.

In the optical modulator of one aspect of the present invention, the light combining unit is configured to convert a polarization direction of the first phase modulated light out of the two phase modulated lights, and the polarization rotation A polarization beam splitter configured to combine the first phase-modulated light whose polarization direction is converted by the element and second phase-modulated light different from the first phase-modulated light;

In the optical modulator of one aspect of the present invention, the generation unit includes an optical branching unit configured to branch input light into a pair of branched lights having different light intensities, and the pair of branches branched by the optical branching unit. And a phase modulation unit configured to output the first phase modulated light and the second phase modulated light.

In the optical modulator according to one aspect of the present invention, the optical branching unit includes a beam splitter.

In the optical modulator of one aspect of the present invention, the generation unit is configured to branch the input light into first branched light, second branched light, third branched light, and fourth branched light. The first branch light, the second branched light, the third branched light, and the fourth branched light, respectively, and the first branched light and the second branched light are combined to produce the first phase modulated light. A phase modulation unit configured to output light, combine the third branched light and the fourth branched light, and output the second phase modulated light.

In the optical modulator according to one aspect of the present invention, the generation unit condenses the optical branching unit configured to split the input light into a pair of branched lights having different light intensities, and the pair of branched lights. And a phase modulation section configured to phase-modulate the pair of condensed light beams and output the first phase-modulated light and the second phase-modulated light, respectively.

The light modulation method according to one aspect of the present invention generates two phase-modulated lights that are modulated to have a predetermined phase and have different light intensities, and the two phase-modulated lights thus generated are polarized perpendicular to each other. The two phase-modulated lights are combined by polarization and modulated signal light is output.

In the light modulation method of one aspect of the present invention, outputting the modulated signal light includes converting a polarization direction of the first phase modulated light out of the two phase modulated lights and converting the polarization direction. Polarization synthesis of the first phase modulated light and the second phase modulated light different from the first phase modulated light.

In the light modulation method according to one aspect of the present invention, the generation of the two phase-modulated lights includes branching the input light into a pair of branched lights having different light intensities and the pair of the branched branched lights. Respectively phase-modulating and outputting the first phase-modulated light and the second phase-modulated light.

In the light modulation method of one aspect of the present invention, generating the two phase-modulated lights branches the input light into a first branched light, a second branched light, a third branched light, and a fourth branched light. The first branched light, the second branched light, the third branched light, and the fourth branched light, respectively, and the first branched light and the second branched light are combined to form the first phase modulated light. Outputting light, combining the third branched light and the fourth branched light, and outputting the second phase modulated light.

In the light modulation method of one aspect of the present invention, the generation of the two phase-modulated lights includes branching the input light into a pair of branched lights having different light intensities, and collecting the pair of branched lights by a rod lens. Illuminating and phase-modulating each of the pair of condensed branched lights and outputting the first phase-modulated light and the second phase-modulated light.

According to one aspect of the present invention, in a QAM modulator, two phase-modulated lights that are modulated to a predetermined phase and have different light intensities have polarization directions that are perpendicular to each other, and two phases that have the polarization directions perpendicular to each other. By combining the modulated light with polarization, light loss can be suppressed as compared with a method in which loss is caused in one arm.

1 is a plan view of an optical modulator according to a first embodiment. The figure which shows the optical modulator which concerns on 2nd Embodiment. The figure which shows the optical modulator which concerns on 3rd Embodiment.

(First embodiment)
Hereinafter, embodiments of one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a plan view of the optical modulator according to the first embodiment. The optical modulator 100 of the first embodiment outputs a quadrature amplitude modulation (QAM) signal.

The optical modulator 100 includes an optical branching unit 110, a microlens array 120, and an optical waveguide element 130. On the light input side of the optical modulator 100, an optical fiber collimator 15 for allowing collimated light to enter the optical modulator 100 is disposed. An optical fiber 11 having a proximal end connected to a laser light source (not shown) is connected to the optical fiber collimator 15. An optical fiber 12 is connected to the light output side of the optical modulator 100, and the optical fiber 12 is connected to the processing device 17.

The light branching unit 110 has a configuration in which a beam splitter 112 and a mirror 113 arranged in parallel to each other are provided inside the glass substrate 111. The beam splitter 110 splits the incident laser beam into two laser beams by the beam splitter 112, and emits the laser beam transmitted through the beam splitter 112 and the laser beam reflected by the beam splitter 112 and the mirror 113. . In the case of the first embodiment, the beam splitter 112 is set to a transmittance of 80% and a reflectance of 20% for the laser light input from the optical fiber collimator 15. The mirror 113 is a total reflection mirror. An antireflection film (AR coating) or the like may be formed on the light incident surface or light exit surface of the light branching unit 110.

The microlens array 120 includes a rectangular parallelepiped transparent base 121 and two

microlenses

122 and 123 formed on one surface of the base 121 (surface on the light branching portion 110 side). The optical axes of the two

microlenses

122 and 123 are arranged coaxially with the optical axes of the two laser beams emitted from the optical branching unit 110. The surface of the base 121 opposite to the

microlenses

122 and 123 is optically bonded to the optical waveguide element 130. The base 121 has a thickness corresponding to the focal length of the

microlenses

122 and 123. The laser light incident on the

microlenses

122 and 123 is condensed on the input end of the optical waveguide formed on the side end surface of the optical waveguide element 130.

The optical waveguide device 130 includes a substrate 131 and an optical waveguide and an electrode formed on the substrate 131. In the case of the first embodiment, the substrate 131 is a lithium niobate (LiNbO 3) substrate. As the substrate 131, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used.

The light modulator 140 has nested Mach-

Zehnder waveguides

134 and 135. The nested Mach-Zehnder waveguide 134 includes

phase modulation units

141 and 142. The nested Mach-Zehnder waveguide 135 includes

phase modulation units

143 and 144. Each of the phase modulation units 141 to 144 includes a Mach-Zehnder waveguide and an electrode.

Two

input waveguides

132 and 133 extend from the side edge of the substrate 131 joined to the microlens array 120. The input waveguides 132 and 133 are respectively connected to nested Mach-

Zehnder waveguides

134 and 135 having two arms.

Phase modulation units

141 and 142 are provided on the respective arms of the nested Mach-Zehnder waveguide 134.
A bias electrode part 134 a is provided on the output end side of the nested Mach-Zehnder waveguide 134. Each of the arms of the nested Mach-Zehnder waveguide 135 is provided with

phase modulators

143 and 144. On the output end side of the nested Mach-Zehnder waveguide 135, a bias electrode portion 135a is provided.

The phase modulators 141 to 144 perform bi-phase modulation (BPSK) on the input optical signal and output it. The

phase modulators

141 and 142 are set so that their phase changes are orthogonal to each other. The

phase modulators

143 and 144 are also set so that their phase changes are orthogonal to each other. The nested Mach-

Zehnder waveguides

134 and 135 each constitute a QPSK (four-phase phase shift keying) optical modulator. The optical signals modulated by the nested Mach-

Zehnder waveguides

134 and 135 are output through the

output waveguides

136 and 137, respectively.

The optical waveguide device 130 has a microlens array 145 on the

output waveguides

136 and 137 side. The microlens array 145 includes a rectangular parallelepiped transparent base 146 and two

microlenses

147 and 148 formed on one surface of the base 146 (the surface on the photosynthesis unit 150 side). The optical axes of the two

microlenses

147 and 148 are arranged coaxially with the optical axes of the two laser beams emitted from the

optical output waveguides

136 and 137, respectively. The surface of the base 146 opposite to the

microlenses

147 and 148 is optically bonded to the optical waveguide element 130. The base 146 has a thickness corresponding to the focal length of the

microlenses

147 and 148. The laser light incident on the

microlenses

147 and 148 is output to the light combining unit 150.

The light combining unit 150 includes a half-wave plate 151 as a polarization rotation element that rotates the polarization direction by 90 °, and a polarization beam splitter 152 provided with a beam splitter 154 and a mirror 153 that are arranged in parallel with each other inside the glass substrate. And an optical fiber collimator 16 for coupling light. In addition, you may use elements other than said half-wave plate 151 as a polarization rotation element. The half-wave plate 151 is emitted by rotating the polarization direction of the laser light emitted from the microlens 147 by 90 °. The mirror 153 reflects the laser light emitted from the half-wave plate 151 toward the beam splitter 154 side. The beam splitter 154 reflects the laser light reflected by the mirror 153 and transmits the laser light emitted from the microlens 148. For this reason, the polarization beam splitter 152 combines the two laser beams without causing interference. The two laser beams emitted from the polarization beam splitter 152 are input to the optical fiber collimator 16 and combined. In this manner, the optical signals QPSK1 and QPSK2 output from the nested Mach-

Zehnder waveguides

134 and 135 are combined in the optical combining unit 150 so as to maintain the intensity ratio thereof, and 16QAM-compatible signal light is generated. The two optical signals QPSK1 and QPSK2 that generate a 16QAM compatible signal are input to the optical fiber collimator 16 to be coupled to the optical fiber 12 and transmitted to the processing device 17 via the optical fiber 12. The processing device 17 performs processing for obtaining a 16QAM signal based on the 16QAM compatible signal.

Next, the optical modulator 100 of the first embodiment having the above configuration will be described.
Laser light supplied through the optical fiber 11 and expanded to a predetermined diameter by the optical fiber collimator 15 is incident on the optical modulator 100. Incident light is incident on the beam splitter 112 of the optical branching unit 110. The beam splitter 112 transmits 80% of incident light and reflects 20%.

The light transmitted through the beam splitter 112 is incident on the microlens 122 and is collected by the microlens 122 at the input end of the input waveguide 132. On the other hand, the light reflected by the beam splitter 112 is reflected by the mirror 113, enters the microlens 123, and is collected by the microlens 123 at the input end of the input waveguide 133.

The light introduced into the input waveguide 132 is branched at the input end of the nested Mach-Zehnder waveguide 134, modulated by the

phase modulation units

141 and 142, and then applied to the bias electrode unit 134a by a predetermined voltage. The phase difference is adjusted to be π / 2. The light whose phase difference has been adjusted is combined at the output end of the nested Mach-Zehnder waveguide 134 to become an optical signal QPSK1. The optical signal QPSK 1 is collimated by the microlens 147, the polarization direction is converted by 90 ° by the half-wave plate 151 of the light combining unit 150, and then reflected by the mirror 153 to the beam splitter 154.

The light introduced into the input waveguide 133 is branched at the input end of the nested Mach-Zehnder waveguide 135, modulated by the

phase modulation units

143 and 144, and then applied to the bias electrode unit 135a by a predetermined voltage. The phase difference is adjusted to be π / 2. The light whose phase difference has been adjusted is synthesized at the output end of the nested Mach-Zehnder waveguide 135 to become an optical signal QPSK2. The optical signal QPSK2 is collimated by the microlens 148 and output to the beam splitter 154 without changing the polarization direction.

The optical signal QPSK 1 and the optical signal QPSK 2 are combined without interference in the beam splitter 154 and input to the optical fiber collimator 16 for polarization combining.

In the case of the first embodiment, the light introduced into the input waveguide 133 is light having an intensity of 20% branched by the beam splitter 112. Therefore, the optical signal QPSK2 output from the nested Mach-Zehnder waveguide 135 has a power that is ¼ that of the optical signal QPSK1 generated from light having an intensity of 80%. Then, in the optical combining unit 150, the optical signal QPSK1 and the optical signal QPSK2 having the above power ratio are combined to generate a 16QAM compatible signal.

The power ratio (4: 1) of the optical signals QPSK 1 and QPSK 2 may be slightly shifted due to a manufacturing error of the optical modulator 100. For example, the ratio may be 3: 1 or 5: 1. The ratio can also be adjusted by the transmittance and reflectance of the beam splitter 112 in the light branching section 110.

As described above, according to the first embodiment, the two optical signals QPSK1 and QPSK2 that are modulated to have a predetermined phase and have different light intensities have the polarization directions perpendicular to each other, and the two lights having the polarization directions perpendicular to each other. By combining the signals QPSK1 and the optical signal QPSK2 with polarization, optical loss can be suppressed as compared with a method in which loss is caused in one arm.

In the optical modulator 100 of the first embodiment, the laser beam is split at a predetermined power ratio in the optical branching unit 110, and these laser beams are introduced into the optical waveguide device 130 through the microlens array 120. ing. In the optical branching unit 110, optical loss hardly occurs when the laser beam is split, so that the optical loss is greatly reduced compared to the conventional configuration in which the optical signal power adjusting unit is provided on the lithium niobate substrate. be able to.

Specifically, when the incident laser beam is split into two and each generates a QPSK signal and the intensity of one QPSK signal is adjusted to ¼, the intensity of the output 16QAM compatible signal is 62.5% with respect to the emitted laser light. For such a configuration, the optical modulator 100 according to the first embodiment is improved by 1.6 dB (37.5%) only by the theoretical optical loss.

Further, the power adjustment unit as described above in the conventional configuration is formed on the lithium niobate substrate, whereas in the first embodiment, a QPSK modulator that does not include the power adjustment unit and a light combining unit are provided. That's fine. Therefore, the substrate 131 made of lithium niobate or the like can be reduced in size with respect to the configuration in which the power adjustment unit is provided.

Further, in the optical modulator 100 according to the first embodiment, the laser light branched by the light branching unit 110 is condensed by the

microlenses

122 and 123 onto the

input waveguides

132 and 133 and introduced into the optical waveguide device 130. . Therefore, there is no difficulty in manufacturing as in the case where optical waveguides formed on different substrates are connected, and there is no problem of damage due to a difference in expansion coefficient between substrates or light loss due to misalignment.

(Second Embodiment)
Next, a second embodiment will be described with reference to FIG.
FIG. 2 is a diagram illustrating an optical modulator according to the second embodiment.
In the following embodiment, the same code | symbol is attached | subjected to the same component as previous embodiment, and the detailed description about them is simplified or abbreviate | omitted.

As shown in FIG. 2, the optical modulator 200 of the second embodiment includes an optical branching unit 110A, a microlens array 120A, and an optical waveguide element 130A.

110 A of optical branching parts have the structure by which the

beam splitters

114, 115, and 116 and the mirror 113 which were arrange | positioned mutually parallel inside the glass base material 111 were provided. The optical branching unit 110A splits the incident laser light into four laser lights by the beam splitters 114 to 116, and emits them to the outside. An antireflection film (AR coating) or the like may be formed on the light incident surface or the light exit surface of the light branching portion 110A.

From the optical branching unit 110A, laser light that has passed through the beam splitter 114, laser light that has been reflected by the beam splitter 114 and the beam splitter 115, laser light that has passed through the beam splitter 115 and then has been reflected by the beam splitter 116, and After passing through the beam splitter 116, the laser beam reflected by the mirror 113 is emitted.

In the case of the second embodiment, the beam splitter 114 is set to have a transmittance of 40% and a reflectance of 60% with respect to incident light. The beam splitter 115 is set to have a transmittance of 33% and a reflectance of 67% with respect to incident light. The beam splitter 116 is set to have a transmittance of 50% and a reflectance of 50% with respect to incident light. The mirror 113 is a total reflection mirror.

The microlens array 120A includes a rectangular parallelepiped transparent base 121, and four

microlenses

124, 125, 126, and 127 formed on one surface of the base 121 (the surface on the light branching portion 110A side). The optical axes of the four microlenses 124 to 127 are arranged coaxially with the optical axes of the four laser beams emitted from the optical branching unit 110A.

The surface of the base 121 opposite to the microlenses 124 to 127 is optically bonded to the optical waveguide element 130A. The base 121 has a thickness corresponding to the focal length of the microlenses 124 to 127. The laser light incident on the microlenses 124 to 127 is focused on the input end of the optical waveguide formed on the side end face of the optical waveguide element 130A.

The optical waveguide element 130A includes a substrate 131, and an optical waveguide and an electrode formed on the substrate 131. An optical modulation unit 140A is formed by these optical waveguides and electrodes.

The optical modulation unit 140A includes four phase modulation units 141 to 144.
Four input waveguides 231 to 234 extend from the side edges of the substrate 131 joined to the microlens array 120A. The input waveguide 231 is connected to the input end of the phase modulation unit 141. The input waveguide 232 is connected to the input end of the phase modulation unit 142. The input waveguide 233 is connected to the input end of the phase modulation unit 143. The input waveguide 234 is connected to the input end of the phase modulation unit 144.

The output side of the

phase modulators

141 and 142 is connected at the light combining point 247. The output sides of the

phase modulators

143 and 144 are connected at the light combining point 248.

phase modulators

143 and 144 are also set so that their phase changes are orthogonal to each other.

In the case of the second embodiment, the

phase modulators

141 and 142 and the light combining point 247 constitute a QPSK optical modulator, and the

phase modulators

143 and 144 and the light combining point 248 constitute a QPSK optical modulator. Has been. The optical signals modulated by the respective optical modulators are output to the light combining unit 150 via the

microlenses

147 and 148 of the microlens array 145 disposed on the

output waveguides

136 and 137 side.

The optical waveguide element 130A has a microlens array 145 on the

output waveguides

136 and 137 side, as in the first embodiment. The microlens array 145 includes a base 146 and two

microlenses

147 and 148. The optical axes of the two

microlenses

optical output waveguides

136 and 137, respectively. The surface of the base 146 opposite to the

microlenses

147 and 148 is optically bonded to the optical waveguide element 130A.

The light combining unit 150 includes a half-wave plate 151, a polarizing beam splitter 152, and an optical fiber collimator 16 as in the first embodiment. The light combining unit 150 combines the optical signals QPSK1 and QPSK2 so as to maintain the intensity ratio, and generates 16QAM compatible signal light. The two optical signals QPSK1 and QPSK2 that generate a 16QAM compatible signal are input to the optical fiber collimator 16 to be coupled to the optical fiber 12 and transmitted to the processing device 17 via the optical fiber 12. The processing device 17 performs processing for obtaining a 16QAM signal based on the 16QAM compatible signal.

In the optical modulator 200 of the second embodiment having the above-described configuration, the laser light is incident on the beam splitter 114 of the optical branching unit 110A from the optical fiber collimator 15. The beam splitter 114 transmits 40% of incident light and reflects 60%.

The light that has passed through the beam splitter 114 enters the microlens 124 and is collected by the microlens 124 at the input end of the input waveguide 231. On the other hand, the light reflected by the beam splitter 114 enters the beam splitter 115. The beam splitter 115 transmits 33% of incident light and reflects 67%.

The light reflected by the beam splitter 115 enters the microlens 125 and is collected by the microlens 125 at the input end of the input waveguide 232. On the other hand, the light transmitted through the beam splitter 115 enters the beam splitter 116. The beam splitter 116 transmits 50% of incident light and reflects 50%.

The light reflected by the beam splitter 116 enters the microlens 126 and is collected by the microlens 126 at the input end of the input waveguide 233. On the other hand, the light transmitted through the beam splitter 116 is reflected by the mirror 113, enters the microlens 127, and is collected by the microlens 127 at the input end of the input waveguide 234.

The light introduced into the input waveguides 231 and 232 is modulated by the

phase modulation units

141 and 142, and then adjusted to a predetermined phase difference (π / 2) by applying a voltage to the bias electrode unit 134a. Thereafter, the signal is combined at the light combining point 247 to be an optical signal QPSK 1 and output to the light combining unit 150.

The light introduced into the input waveguides 233 and 234 is modulated by the

phase modulation units

143 and 144, and then adjusted to a predetermined phase difference (π / 2) by applying a voltage to the bias electrode unit 135a. Thereafter, the signals are combined at the light combining point 248 to become an optical signal QPSK 2 and output to the light combining unit 150.

In the case of the second embodiment, the light introduced into the input waveguides 231 and 232 is light having an intensity of 40% with respect to the light input from the optical fiber collimator 15, respectively. On the other hand, the light introduced into the input waveguides 233 and 234 is light having an intensity of 10%. Therefore, the optical signal QPSK2 generated at the light combining point 248 has a power of 1/4 with respect to the optical signal QPSK1.

In the optical combining unit 150, the optical signal QPSK1 and the optical signal QPSK2 having the above power ratio are combined so as to maintain the intensity ratio thereof, thereby generating a 16QAM compatible signal. The two optical signals QPSK1 and QPSK2 that generate a 16QAM compatible signal are input to the optical fiber collimator 16 to be coupled to the optical fiber 12 and transmitted to the processing device 17 via the optical fiber 12. The processing device 17 generates a 16QAM signal based on the 16QAM compatible signal.

Note that the power ratio (4: 1) of the optical signals QPSK1 and QPSK2 may be slightly shifted due to a manufacturing error of the optical modulator 200. For example, the ratio may be 3: 1 or 5: 1. The power ratio can also be adjusted by the transmittance and reflectance of the beam splitter in the light branching section 110A.

The optical modulator 200 of the second embodiment is configured to split the laser light into four at the optical branching section 110A and introduce each laser light into the phase modulation sections 141 to 144 by the microlens array 120A. That is, the branching point of the laser beam provided in the nested Mach-

Zehnder waveguides

134 and 135 in the optical modulator 100 according to the first embodiment is provided outside the substrate as the optical branching unit 110A in the second embodiment. Yes.

This makes it possible to reduce the number of light branch points that cause optical loss in the optical waveguide, and tolerate variations in characteristics caused by manufacturing errors of the optical waveguide. In addition, an optical modulator with less optical loss can be obtained as compared with the first embodiment. In addition, since a waveguide structure for optical branching is unnecessary, the substrate 131 made of lithium niobate or the like can be reduced in size.

Also in the optical modulator 200 of the second embodiment, the laser beam branched by the optical branching unit 110A is condensed on the input waveguides 231 to 234 by the microlenses 124 to 127 and introduced into the optical waveguide device 130A. Yes. Therefore, there is no difficulty in manufacturing as in the case where optical waveguides formed on different substrates are connected, and there is no problem of damage due to a difference in expansion coefficient between substrates or light loss due to misalignment.

In the second embodiment described above, as in the first embodiment, the two optical signals QPSK1 and QPSK2 that are modulated to have a predetermined phase and have different light intensities are made to have mutually perpendicular polarization directions, and the polarization directions are By combining the two optical signals QPSK1 and QPSK2 that are perpendicular to each other, the optical loss can be suppressed as compared with a system that causes a loss in one arm.

(Third embodiment)
Next, a third embodiment will be described with reference to FIG.
FIG. 3 is a diagram illustrating an optical modulator according to the third embodiment.
In the following embodiment, the same code | symbol is attached | subjected to the same component as previous embodiment, and the detailed description about them is simplified or abbreviate | omitted.

As shown in FIG. 3, the optical modulator 300 of the third embodiment includes an optical branching unit 110, a rod lens 120B, and an optical waveguide element 130B. The optical branching unit 110 is common to the first embodiment.

The rod lens 120B has a curved lens surface. In the rod lens 120B, a flat surface opposite to the lens surface is optically bonded to the side end surface of the optical waveguide element 130B. Two laser beams branched and emitted by the light branching unit 110 are incident on the lens surface of the rod lens 120B. The laser light incident on the rod lens 120B is focused on the input end of the optical waveguide formed on the side end surface of the optical waveguide element 130B.

The optical waveguide element 130B includes a substrate 131, and an optical waveguide and an electrode formed on the substrate 131. The optical modulator 140B is formed by these optical waveguides and electrodes.

The light modulation unit 140B has substantially the same configuration as that of the light modulation unit 140 according to the first embodiment, and is different in that it includes

input waveguides

236 and 237 that intersect each other. The input waveguides 236 and 237 each have an input end on a side end surface of the substrate 131 to which the rod lens 120B is bonded, intersect each other at a position extending inward from the end edge of the substrate 131, and then each include a nested Mach-Zehnder guide. It is connected to the

waveguides

134 and 135.

The optical waveguide element 130B has the microlens array 145 on the

output waveguides

136 and 137 side as in the first embodiment. The microlens array 145 includes a base 146 and two

microlenses

147 and 148. The optical axes of the two

microlenses

optical output waveguides

136 and 137, respectively. The surface of the base 146 opposite to the

microlenses

147 and 148 is optically bonded to the optical waveguide element 130B.

In the optical modulator 300 of the third embodiment having the above-described configuration, the laser light emitted from the optical fiber collimator 15 enters the beam splitter 112 of the optical branching unit 110. The beam splitter 112 transmits 80% of incident light and reflects 20%.

The light that has passed through the beam splitter 112 is incident on the rod lens 120B, and is condensed on the input end of the input waveguide 237 by the rod lens 120B. The light introduced into the input waveguide 237 is input to the nested Mach-Zehnder waveguide 135.

On the other hand, the light reflected by the beam splitter 112 is reflected by the mirror 113, then enters the rod lens 120B, and is condensed at the input end of the input waveguide 236 by the rod lens 120B. The light introduced into the input waveguide 236 is input to the nested Mach-Zehnder waveguide 134.

Therefore, in the optical modulator 300 of the third embodiment, the intensity ratio of the light input to the nested Mach-

Zehnder waveguides

134 and 135 is opposite to that of the first embodiment.

The light introduced into the nested Mach-Zehnder waveguide 134 is branched at the input end, modulated by the

phase modulation units

141 and 142, and then applied with a voltage to the bias electrode unit 134a to obtain a predetermined phase difference (π / 2). It is adjusted to become. After that, the signal is synthesized at the output end of the nested Mach-Zehnder waveguide 134 to become an optical signal QPSK 1, and is output to the optical synthesis unit 150 through the microlens 147.

The light introduced into the nested Mach-Zehnder waveguide 135 is branched at the input end, modulated by the

phase modulation units

143 and 144, and then applied with a voltage to the bias electrode unit 135a to obtain a predetermined phase difference (π / 2). It is adjusted to become. After that, the signal is synthesized at the output end of the nested Mach-Zehnder waveguide 135 to become an optical signal QPSK 2, which is output to the optical synthesis unit 150 via the microlens 148.

In the case of the third embodiment, since the light introduced into the input waveguide 236 is light having an intensity of 20% branched by the beam splitter 112, the optical signal QPSK1 output from the nested Mach-Zehnder waveguide 134 is In the nested Mach-Zehnder waveguide 135, the power is reduced to ¼ with respect to the optical signal QPSK2 generated from light having an intensity of 80%.

Then, in the optical combining unit 150, the optical signal QPSK1 and the optical signal QPSK2 having the above power ratio are combined so as to maintain the intensity ratio thereof, thereby generating a 16QAM compatible signal. The two optical signals QPSK1 and QPSK2 that generate a 16QAM compatible signal are input to the optical fiber collimator 16 to be coupled to the optical fiber 12 and transmitted to the processing device 17 via the optical fiber 12. The processing device 17 generates a 16QAM signal based on the 16QAM compatible signal.

The power ratio (1: 4) of the optical signals QPSK1 and QPSK2 may slightly deviate due to a manufacturing error of the optical modulator 300, and the power ratio is adjusted until the output signal reaches the polarization beam combiner. There is no need. For example, the ratio may be 1: 3 or 1: 5. The power ratio can also be adjusted by the transmittance and reflectance of the beam splitter 112 in the optical branching section 110, and these can be easily changed by exchanging parts to be used, unlike the branching section of the waveguide. .

In the optical modulator 300 of the third embodiment, the laser beam is split at a predetermined power ratio in the optical branching unit 110, and these laser beams are introduced into the optical waveguide element 130B via the rod lens 120B. . Even when the rod lens 120B is used, the same effects as those of the first embodiment can be obtained.

In the third embodiment described above, as in the first and second embodiments described above, the two optical signals QPSK1 and QPSK2 that are modulated to a predetermined phase and have different light intensities are set to have mutually perpendicular polarization directions. By combining the two optical signals QPSK1 and QPSK2 whose polarization directions are perpendicular to each other, the optical loss can be suppressed as compared with a method in which a loss is caused in one arm.

In the above-described embodiment, a method of generating 16-level QAM-compliant signal light has been shown by taking quaternary phase-modulated light using a QPSK modulator as two phase-modulated lights having different light intensities. It is also possible to apply other methods for generating multilevel modulated light. When a 6-level phase modulator having a phase shift amount of π / 3 is used, 36-level QAM-compatible signal light can be generated.

100, 200, 300 Optical modulator 110 Optical branching unit 120

Microlens array

130, 130A, 130B Optical waveguide element 141-144 Phase modulation unit 145 Microlens array 150 Photosynthesis unit 151 Half-wave plate 152 Polarization beam splitter 153 Mirror 154 Beam splitter

Claims

A generating unit including a first generating unit and a second generating unit, wherein the first generating unit is configured to generate phase-modulated light that is modulated to a predetermined phase and has a first light intensity; The generation unit, wherein the second generation unit is configured to generate phase-modulated light that is modulated to a predetermined phase and has a second light intensity different from the first light intensity, and the generation unit A light combining unit configured to cause the two phase-modulated lights generated by the above to have polarization directions perpendicular to each other and to combine the two phase-modulated lights with polarization to output modulated signal light. Modulator.
The photosynthesis unit
A polarization rotation element configured to convert a polarization direction of the first phase modulation light of the two phase modulation lights;
A polarization beam splitter configured to combine the first phase modulated light whose polarization direction is converted by the polarization rotation element and the second phase modulated light different from the first phase modulated light. The optical modulator according to claim 1.
The generator is
A light branching section configured to branch the input light into a pair of branched lights having different light intensities;
The phase modulation unit configured to phase-modulate the pair of branched lights branched by the light branching unit and output the first phase modulated light and the second phase modulated light, respectively. Light modulator.
The optical modulator according to claim 3, wherein the optical branching unit includes a beam splitter.
The generator is
A light branching unit configured to branch the input light into a first branched light, a second branched light, a third branched light, and a fourth branched light;
The first branched light, the second branched light, the third branched light, and the fourth branched light are respectively phase modulated, and the first branched light and the second branched light are combined to output the first phase modulated light. The optical modulator according to claim 2, further comprising: a phase modulation unit configured to combine the third branched light and the fourth branched light and output the second phase modulated light.
The generator is
A light branching section configured to branch the input light into a pair of branched lights having different light intensities;
A rod lens configured to collect the pair of branched lights;
The optical modulator according to claim 2, further comprising: a phase modulation unit configured to phase-modulate each of the collected pair of branched lights and output the first phase-modulated light and the second phase-modulated light.
Generating two phase-modulated lights modulated to a predetermined phase and having different light intensities,
An optical modulation method in which the generated two phase-modulated lights are polarized in directions perpendicular to each other, and the two phase-modulated lights are combined by polarization to output modulated signal light.
To output the modulated signal light,
Converting the polarization direction of the first phase modulated light of the two phase modulated lights;
The light modulation method according to claim 7, further comprising: combining the first phase-modulated light whose polarization direction has been converted and second phase-modulated light different from the first phase-modulated light.
Generating the two phase-modulated lights
Branching input light into a pair of branched lights having different light intensities;
The light modulation method according to claim 8, further comprising: phase-modulating each of the pair of branched light beams to output the first phase-modulated light and the second phase-modulated light.
Generating the two phase-modulated lights
Branching input light into first branched light, second branched light, third branched light, and fourth branched light;
The first branched light, the second branched light, the third branched light, and the fourth branched light are respectively phase modulated, and the first branched light and the second branched light are combined to output the first phase modulated light. The light modulation method according to claim 8, further comprising: combining the third branched light and the fourth branched light and outputting the second phase modulated light.
Generating the two phase-modulated lights
Branching input light into a pair of branched lights having different light intensities;
Condensing the pair of branched lights by a rod lens;
The optical modulator according to claim 8, further comprising: phase-modulating each of the pair of condensed branched lights and outputting the first phase-modulated light and the second phase-modulated light.