JPH04128714A - Optical controller - Google Patents

Abstract

PURPOSE:To easily obtain the optical controller which is small in size and highly reliable by laminating an optical function element and an electric circuit on a single substrate. CONSTITUTION:On the optical crystal substrate having the optical function element where propagated light in an optical waveguide 2 is controlled electrically, the electric circuit which performs electric control over the optical function element is formed, and a buffer layer 11 is interposed at least at the place where the optical waveguide 2 and the metal clad part 10 of the electric circuit are superposed one over the other. This buffer layer 11 is made of a material which has a lower refractive index than the substrate and has effect for reducing the propagation loss at the metal clad part 10 until the loss can be ignored as compared with the propagation loss of the optical waveguide 2 itself. Consequently, the optical controller which is small in size on the whole and easily manufactured and has superior reliability is obtained.

Description

[Detailed description of the invention]

[0001]

[Industrial application field]

The present invention relates to an optical control device used for, for example, an optical modulator or an optical switch in an optical communication system. [0002]

[Conventional technology]

Optical communication systems include IN5 (integrated network service) communication, OA (office automation), F
It is being applied in a wide variety of fields, including computer data networks such as A (factory automation), and mobile communications such as aircraft and ships. In recent years, three-dimensional waveguide type optical functional elements that can propagate light along a predetermined pattern formed on or inside a substrate have been widely used as optical functional elements in optical communication systems. It is used for processing such as intensity modulation, single frequency modulation, and phase modulation of input light. [0003] By the way, optical functional elements are provided as various devices by being combined with electric circuits for control. Automatic gain control (AGC) in the receiving section of optical communication systems
) is also an example. This is a control performed to achieve stable reception operation, and conventionally, external optical components such as beam splitters are used to perform feedforward control of optical functional elements depending on the intensity of input light or output light. Feedback control is being performed. [0004] FIG. 4 is a block diagram schematically showing a control system when performing feedforward control on an optical functional element. In this control system, at the same time, a portion of the input light enters the optical functional element 33 via the beam splitter 31 and the condensing lens 32, and at the same time, a portion of the input light enters the optical functional element 33 through the beam splitter 31 and the condenser lens 32. 31, the light is also incident on the photodetector 34. The light incident on the photodetector 34 is converted into an electric signal (feedforward control signal) according to its intensity, further amplified by an amplifier 35, and sent to the optical functional element 33. The optical functional element 33 determines the amount of processing for incident light according to the electrical signal. According to this control system, even if the intensity of the incident light fluctuates, stable output light that has undergone a certain amount of processing is always sent to the light output end P via the collimator lens 36. It can be obtained from ut. [0005] FIG. 5 is a block diagram showing a control system when performing feedback control on the optical functional element. In this control system, the light input/output terminals in FIG. 4 described above are reversed, and the light incident on the photodetector 34 is converted into an electric signal (feedback control signal) according to its intensity. [0006] However, in these control technologies, it is troublesome to align the optical axis between external optical components such as beam splitters and lenses and optical functional elements, and the electric circuit for generating electrical signals naturally Since it will be placed on a different substrate from the substrate on which the functional elements are formed, high integration cannot be expected. This will be explained with reference to FIG. [0007] This optical control device includes a Matsuhatsu Enda interferometer type optical modulator (
Hereinafter, it will be referred to as an MZ modulator. ) and an electric circuit for performing feedback control thereof are interconnected. The optical functional element 40 is made of lithium niobate (L 1Nb
O3) On the substrate 41, the incident light is transmitted to the light input end P and to the light output end P. A waveguide 42 is formed that guides the wave to ut, and the waveguide 42 is branched into two at a midpoint, and electrodes 43a and 43b combined in a comb shape are provided on both sides and in the middle of each branched waveguide 42a and 42b. It is arranged as follows. Optical output end P. The optical signal output from ut enters the light receiving element 45 via the optical fiber 44, where it is converted into an electrical signal and then input to the electrical circuit 46. The electric circuit 46 performs predetermined processing such as amplification on a portion of the electric signal input from the light receiving element 45 to generate a control signal, which is then sent to the electrode pads connected to the electrodes 43a and 43b again. 47a and 47b. As a result, the potentials of the electrodes 43a and 43b are adjusted, and the phase modulation of the light propagating through each branch waveguide is controlled. In this way, feedback control using an external electric circuit is possible while monitoring the optical signal output from the MZ modulator. [0008] On the other hand, the applicant of the present application previously disclosed in Japanese Unexamined Patent Publication No. 63-71827 that a sub-waveguide is formed by arranging a sub-waveguide partially parallel to the main waveguide of an optical functional element (in this case, an optical variable attenuator). An optical control device is disclosed in which a feedforward control signal or a feedback control signal is extracted by a directional coupler. However, according to this technology, although the part that extracts part of the guided light as monitor light is formed on the same substrate as the optical functional element, it is necessary to generate an appropriate electrical signal based on the extracted monitor light. circuitry still resides on the external board. SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a light control device that can be manufactured using easy manufacturing techniques, is small, and has high reliability. [0009]

[Means to solve the problem]

An optical control device according to the present invention is proposed to achieve the above-mentioned object, and includes an optical functional element on an optical crystal substrate having an optical functional element, in which light propagating through an optical waveguide is electrically controlled. An electric circuit for performing electrical control is formed, and a buffer layer is interposed at least in a portion where the optical waveguide and the metal cladding portion of the electric circuit overlap. [0010]

[Effect]

The optical control device of the present invention is compact as a whole, easy to manufacture, and has high reliability because the optical functional element and the electric circuit for electrically controlling it are integrally formed on the same substrate. It is excellent. What makes such integration possible is the buffer layer interposed at least in the region where the optical waveguide and the metal cladding portion of the electric circuit overlap. The buffer layer is made of a material with a lower refractive index than the substrate, and has the effect of reducing the propagation loss due to the metal cladding portion to a negligible level compared to the propagation loss of the optical waveguide itself. Therefore, due to the presence of this buffer layer, even if electrical circuits are stacked, there is no substantial effect on the operation of the optical functional element. Such a light control device does not require any special technology to manufacture, and can be manufactured with high reliability and productivity using normal manufacturing techniques applied in the field of manufacturing semiconductor devices. [0011]

【Example】

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. [0012] Example 1 In this example, copper was deposited on a lithium niobate (LiNb03) substrate on which an MZ modulator was formed as an optical functional element, through a buffer layer made of a silicon oxide layer selectively formed. This is an example of an optical control device in which a metal cladding portion made of a printed wiring pattern is formed, and necessary electrical components are connected to the metal cladding portion. [0013] FIGS. 1 and 2 show an example of the configuration of the light control device of this embodiment. FIG. 1 is a schematic perspective view of this light control device. Input light incident on the lithium niobate substrate 1, which is an optical crystal substrate, from the optical input end P is transmitted to the optical output end P. An optical waveguide 2 is formed which guides waves up to U, and on the optical waveguide 2 there is an optical input end P and a first output end P from the side. An MZ modulator 3 is formed toward the U side. Here, as the lithium niobate substrate 1, 3
0mm (horizontal) x 10mm (vertical) x 1mm (thickness) (
7) An X-cut plate was used. In the above MZ modulator 3, the optical waveguide 2 is symmetrically branched into two branch waveguides 2a and 2b in the middle, and the guided light propagating through each branch waveguide 2a and 2b is phase-modulated by the electro-optic effect. After that, they are superimposed again to perform amplitude modulation. [0014] Further, the light control device has a light input end P and a light output end P from the side. A predetermined modulation is performed on the input light n during the period up to ut, and the result can be used for feedback control to perform AGC operation. This operation is made possible by the distributor 4, photodiode 5, and the like. In the splitter 4, a branching waveguide 2C is formed which branches from the optical waveguide 2 at a predetermined branching angle θ, and the guided light propagating through the optical waveguide 2 is determined according to the magnitude of the branching angle θ. The distribution ratio allows the light to be extracted as monitor output light. The branching angle θ is approximately selected in the range of 1 to 10°. A diffraction grating (not shown) for emitting guided light toward the substrate surface is formed at the terminal end of the branch waveguide 2C, and a photodiode 5 is disposed on the diffraction grating (not shown). [0015] Further, on the lithium niobate substrate 1, the MZ
Various wiring and electrode portions are formed for controlling the operation of the modulator 3 and for configuring an electric circuit for AGC. These are thin films of copper and lithium niobate substrates.
Since the metal cladding part 1 is deposited on the metal cladding part 1 by printed circuit technology, it will be collectively referred to as the metal cladding part 1 in this specification.
0, and is represented by a diagonal line in FIG. First, in the MZ modulator 3, each branch waveguide 2a
, 2b are provided with electrode portions 6a and 6b combined in a comb shape on both sides and in the middle thereof, and a predetermined DC voltage is supplied from an external power source (not shown) via electrode pads 7a and 7b for power connection. are set to the ground potential and the positive potential, respectively. At this time, the refractive index of the lithium niobate substrate 1 changes due to the electric field generated in accordance with the potential, so that the phase of the light propagating through each branch waveguide 2a, 2b is advanced or delayed. On the other hand, the metal cladding part 1 constituting the electric circuit for AGC
0, electrode portions 9a and 9b for connecting the terminals of the photodiode 5 and the inverting amplifier 8, and a wiring portion for taking out the output of the inverting amplifier 8 and enabling input/output to the transistor 13 are formed. ing. [0016] Further, in the above optical control device, at least the portion where the optical waveguide 2 and the branch waveguides 2a, 2b and the metal cladding portion 10 overlap (the portion surrounded by a broken line circle in FIG. 1) has a buffer layer 11 between them. A configuration with intervening is adopted. FIG. 2 shows a schematic side view of the light control device to make the positional relationship of the buffer layer 11 clear. However, this diagram does not correspond to specific parts in FIG. 1, but only shows the upper and lower stacking relationships of the lithium niobate substrate 1, the optical waveguide 2, the buffer layer 11, and electrical components such as the photodetector 5 and the inverting amplifier 8. This is a schematic explanation. Furthermore, it does not indicate that the actual optical waveguide 2 and buffer layer 11 are formed over the entire surface of the substrate. [0017] The buffer layer 11 is provided to block light (evanescent light) leaking from the waveguide toward the surface of the substrate, thereby increasing light propagation efficiency. Therefore, a transparent dielectric film or a transparent conductor film having a refractive index sufficiently lower than that of lithium niobate is used as the material. Silicon oxide, alumina, etc. can be used as the transparent dielectric film, and ITO (indium-tin oxide) can be used as the transparent conductor film. [0018] Here, the reason why the buffer layer 11 is not formed over the entire surface of the substrate is as follows. The buffer layer 11
is usually formed by sputtering, electron beam evaporation, or the like. However, since the buffer layer 11 formed in this way is in an oxygen-deficient state, when a DC voltage is applied from the outside, ion movement occurs in the buffer layer.
Ions are absorbed by the negative electrode, canceling out the externally applied electric field. Therefore, a so-called DC drift phenomenon occurs in which the output light intensity decreases or increases over time even when a constant DC voltage is applied. If a DC drift phenomenon occurs between the electrodes, optical control becomes impossible, and short circuits occur particularly when the buffer layer is made of conductive ITO. For this reason, the buffer layer 11
is selectively formed at or near a location where the metal cladding portion 10 and the optical waveguide 2 overlap, and is not formed between any electrodes. In the simplest case, the patterns of the metal cladding part 10 and the buffer layer 11 may be the same, and this configuration was also adopted in this embodiment. [0019] Each electrode section and each wiring section of the metal cladding section 10 formed through the buffer layer 11 as described above include a photodiode 5, an inverting amplifier 8, a chip resistor 12a12b,
Electrical components such as 12c and transistor 13 are appropriately connected by soldering or the like to form a closed circuit. Here, when using an NPN) transistor with a grounded emitter type as the transistor 13, the base B of the transistor 13 is connected to the output terminal of the inverting amplifier 8 via the chip resistor 12b, and the emitter E is grounded for power supply connection. side electrode pad 7
In the wiring part connected to a, and the collector C is a chip resistor 1.
2C to the positive side electrode pads 7b for power supply connection. When using a PNP transistor, the polarity of the electrode pads 7a and 7b for power supply connection is reversed to that described above. [0020] An outline of the AGC operation of such a light control device is as follows. First, the light incident on the light input end P is transmitted to the electrode portion 6a.
, 6b depending on the potential difference between the branch waveguides 2a. While propagating through n 2b, it receives a predetermined phase modulation due to the electro-optic effect, and when superimposed again, it receives a predetermined amplitude modulation. Most of the light that has undergone such modulation is at the optical output end P. ut
A part of the light is guided to the photodiode 5 according to the distribution ratio determined by the branching angle θ. The light incident on the photodiode 5 is converted into a current, amplified by the inverting amplifier 8, and input to the base B of the transistor 13. Here, for example, if the light received by the photodiode 5 is excessive and the base current becomes large, the collector current increases and the voltage drop across the chip resistor 12a becomes small, and as a result, the potential of the electrode portion 6b becomes high. Become. If the amount of light received by the photodiode 5 is too small, the operation is the opposite. [0021] Next, an example of a method for manufacturing such a light control device will be described. First, on an X-cut lithium niobate substrate 1, a strife pattern made of titanium is formed by photolithography according to the formation pattern of the optical waveguide 2 and the branch waveguides 2a, 2b, and 2c, and the strife pattern is heated to around 1000°C. The optical waveguide 2 and branch waveguides 2a, 2b, and 2c were formed by thermal diffusion treatment. Here, when the pattern width of the stripe pattern was about 6 μm, an optical waveguide with a width of about 10 μm was formed. [0022] Next, a silicon oxide layer that will become the buffer layer 11 and copper that will become the metal node part 10 are sequentially deposited on the entire surface of the lithium niobate substrate 1 by means such as sputtering or CVD, and are formed into a predetermined pattern. Through the mask formed in the above, unnecessary parts were removed by wet etching using a ferric chloride aqueous solution for copper and wet etching using buffered hydrofluoric acid for silicon oxide. In this way, by using a common mask during patterning, the buffer layer 11 was selectively formed directly under the metal cladding part 10. [0023] Next, a diffraction grating (not shown) for emitting monitor output light toward the substrate surface was formed at the terminal end of the branch waveguide 2C. This diffraction grating can be formed by forming layers of a material such as a titanium oxide layer with a higher refractive index than the lithium niobate substrate at a predetermined pitch, or by ion-implanting a material with a high refractive index such as potassium at a predetermined pitch. It can be formed by the following method. [0024] Next, the photodiode 5, the inverting amplifier 8, the chip resistors 12a, 12b, 12C, and the transistor 13 were connected by soldering or a combination of adhesive and wire bonding to complete the optical control device. [0025] Note that the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, a Y-cut plate can also be used as the lithium niobate substrate 1. In this case, the difference from the case where an X-cut plate is used is that the electrode part and the optical waveguide are formed by laminating each other in order to utilize the electric field component perpendicular to the substrate surface, but as described above, the metal clad part 10 and According to the method in which the buffer layer 11 is formed in the same pattern using a common mask, the light control device of the present invention can be constructed regardless of the surface orientation of the substrate. [0026] In forming the optical waveguide 2 and branch waveguides 2a, 2b, and 2c, proton exchange may be performed instead of titanium thermal diffusion. [0027] It is not necessary to provide a particular diffraction grating at the terminal end of the branch waveguide 2C of the distributor. In this case, the branch waveguide 2C disappears in the middle and is in a cut-off state, so the propagated light leaks from the terminal end to the front and back sides of the lithium niobate substrate 1 and is emitted to the outside. . However, if a diffraction grating is provided, the light receiving efficiency of the photodiode 5 can be significantly improved. [0028] As a constituent material of the metal cladding part 10, aluminum or the like can be used in addition to the above-mentioned copper. [0029] In addition to the above-mentioned MZ modulator 3, for example, a directional coupling type variable optical attenuator may be formed as the optical functional element. Furthermore, by changing the characteristics of the inverting amplifier 8 used together with the above-mentioned MZ modulator 3, it is possible to configure an optical bistable element, etc. in addition to the AGC. [0030] In addition to the above-mentioned asymmetric bifurcated waveguide, a directional coupler can also be formed as the distributor. However, in general, directional couplers have drawbacks such as the coupling coefficient changing sensitively due to manufacturing errors, making it difficult to match the coupling length and element length, and the evanescent effect changing due to temperature changes. Therefore, in order to realize stable AGC, the above-mentioned asymmetric bifurcated waveguide is more advantageous. [0031]Also, in the above embodiment, a feedback type AGC operation is performed, and the optical input end P and the optical output end P. It is also possible to perform feedforward type AGC operation by replacing ut. [0032] Embodiment 2 This embodiment is an example of an optical control device that incorporates a voltage amplification circuit on a substrate and performs optical intensity modulation according to an external modulation signal. This will be explained with reference to FIG. The configuration of the MZ modulator 23 in the optical control device of this example is the same as that described in Example 1, and includes branch waveguides 22a and 22b formed in the middle of the optical waveguide 22 in the lithium niobate substrate 21. Electrode parts 26a and 26b, which are combined in a comb shape, are arranged on both sides and in the middle part. The electrode portion 26a is connected to the ground side electrode pad 25a, and the electrode portion 26b is connected to the positive side electrode pad 25b via a chip resistor 27b, and a power supply voltage is applied between the picture electrode pads 25a and 25b. . Further, a transistor 24 is connected between the electrode parts 26a and 26b, and the voltage applied between each electrode part 26a and 26b is controlled according to a signal applied from the outside between the electrode pads 25a and 25c. being done. - As an example, the transistor 24 is an NPN transistor;
When using this as an emitter-grounded type, the base B of the transistor 24 is connected to the electrode pad 25C via a parallel circuit for frequency characteristic adjustment consisting of a chip resistor 27a and a capacitor 28, and the emitter E is connected to the chip resistor 27a.
7c to a ground side electrode pad 25a for power supply connection, and the collector C is connected to the electrode portion 26b and also to the positive side electrode pad 25b. [0033] In such a configuration, when the base current of the transistor 24 increases due to a signal voltage, for example, the collector current increases and the voltage drop across the chip resistor 27b decreases, and as a result, the potential of the electrode portion 26b increases. . [0034]

【Effect of the invention】

In conventional optical control devices, the optical functional elements and the electrical circuits for controlling them had to be formed on separate substrates, making integration difficult and complicated to manufacture. . However, as is clear from the above description, according to the present invention, the optical functional element and the electric circuit are laminated on a single substrate, so that the conventional problems are solved at once. Moreover, its manufacture does not require any special technology. Therefore, a small and highly reliable light control device can be easily provided.

[Brief explanation of drawings]

FIG. 1 is a schematic perspective view showing a configuration example of a light control device to which the present invention is applied.

FIG. 2 is a schematic side view showing a configuration example of a light control device to which the present invention is applied.

FIG. 3 is a schematic perspective view showing another configuration example of a light control device to which the present invention is applied.

FIG. 4 is a block diagram schematically showing a conventional control system that performs feedforward control on an optical functional element using external optical components.

FIG. 5 is a block diagram schematically showing a conventional control system that performs feedback control on an optical functional element using external optical components.

FIG. 6 is a schematic perspective view showing a configuration example of a conventional optical control device in which an optical functional element and an electric circuit are separated.

[Explanation of symbols]

1.21 Lithium niobate substrate 2.22 ・Optical waveguides 2a, 2
b, 22a, 22b--branch waveguide 3.23
・MZ modulator 4
・Distributor 5 ・Photodiodes 6a, 6b, 26a, 26b--Electrode section 8
・Inverting amplifier 10
- Metal cladding part 1 buffer layer 12a. 12b. 12c. 27a. 27b. 7c Chip resistor 13゜transistor capacitor/optical input terminal

[Document core]

[Figure 1]

[Figure 2] drawing view

[Figure 3]

[Figure 4] 26a 26b

[Figure 5]

[Figure 6]

Claims (1)

[Claims] 1. An electrical circuit for electrically controlling the optical functional element is formed on an optical crystal substrate having an optical functional element by which light propagating through the optical waveguide is electrically controlled, An optical control device characterized in that a buffer layer is interposed at a portion where a wave path and a metal cladding portion of the electric circuit overlap.