JPH0980365A - Waveguide type optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration in polarized wave extinction rate due to the mixture of a polarized wave of TM mode when a light signal is modulated by using a polarized wave of TE mode. SOLUTION: On a lithium niobate substrate 42 of the waveguide type optical device 41, a waveguide 43 is formed. Above the waveguide 43, electrodes 45 and 46 are formed across an SiO2 film. With end surfaces of the waveguide substrate 42, an input-side optical fiber 47 and an output-side optical fiber 48 are optically coupled respectively. The output terminal of a driving circuit 49 is connected between the two electrodes 45 and 46. The modulation degree of the driving circuit 49 is set to <=30%. An unillustrated polarization controller is connected to the input-side optical fiber 47 and a polarized wave of TE mode is made incident. On the surface of the waveguide substrate 42 nearby its incidence side, an oblique crossing waveguide 51 is arranged crossing the waveguide 43 and the polarized wave of TM mode is removed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical device used, for example, in optical communication, and more particularly to a waveguide type optical device adapted to use a TE mode when modulating an optical signal. .

[0002]

2. Description of the Related Art A waveguide type optical device uses a substrate having an electro-optical effect. A portion having a high refractive index is formed as an optical waveguide in this substrate, and an electrode for applying a voltage is formed above or near the waveguide. By using this electrode to apply an electric field and change the refractive index, the phase and intensity of light can be modulated and the optical path can be switched.

Among the ferroelectric materials, lithium niobate (LiNbO ₃ ) which exhibits a relatively high electro-optical effect is generally used for the substrate of such a waveguide type optical device. In this waveguide type optical device, a titanium (Ti) film is formed on a lithium niobate substrate, patterned into a desired waveguide pattern, and then thermally diffused at a high temperature of about 1000 ° C. for several hours to form a waveguide. It is carried out. After that, a buffer layer of silicon dioxide (SiO ₂ ) is formed on this, and an electrode is formed on the upper surface of the buffer layer by a metal film, whereby a device element is formed.

This waveguide type optical device is disclosed in Japanese Patent Application Laid-Open No. 58-58
It is possible to integrate a circuit portion having a function of modulating light and a function of switching an optical path on a substrate, as seen in Japanese Patent Laid-Open No. 91426 and Japanese Patent Laid-Open No. 3-57448.
Since the circuit part having such a function can operate at high speed, an external modulator or OTDR (Optical Time Do
Main Reflectmeter: An optical pulse tester or an optical fiber analyzer) is being developed as a switch for switching the optical path. The OTDR is, for example, a measuring device that measures a breaking point or a propagation loss of an optical fiber, and an optical switch is used for its configuration. The higher the switching speed of the optical switch, the more accurately the fracture position can be measured.

FIG. 3 shows the structure of a Mach-Zehnder type high-speed optical modulator, which has been studied for research and development and practical application. Lithium niobate substrate (waveguide substrate) 12 of Mach-Zehnder type optical modulator 11
A waveguide 13 is formed on the top. This waveguide 13
Is the two branch parts 13A, 13B on the left and right and the central part 2
It has parallel waveguide portions (arms) 13C and 13D of the book.

FIG. 4 is a sectional view of the Mach-Zehnder type optical modulator taken along the line AA '. Two waveguide pairs 13C and 13D are formed near the surface of the waveguide substrate 12,
On top of these, the entire surface of the entire waveguide substrate 12 is
The O ₂ film 14 is formed. On the upper part of the SiO ₂ film 14, electrodes 15 and 16 of a metal layer made of chromium and gold are formed corresponding to the waveguide pairs 13C and 13D.

The input side optical fiber 17 or the output side optical fiber 1 is provided on both end faces of the waveguide substrate 12 in FIG.
8 are optically coupled. Also, the two electrodes 15,
An output terminal of a drive circuit 19 for applying a voltage between them and inputting a signal is connected between 16.

FIG. 5 shows a state in which a voltage is applied to the Mach-Zehnder type optical modulator by a drive circuit. By applying this voltage, a vertical electric field is generated in the waveguide pair 13C and 13D formed in the waveguide substrate 12 as indicated by the arrow. In this state, the refractive index of the waveguide pair 13C and 13D changes due to the electro-optic effect of lithium niobate.

FIG. 6 shows the relationship between the applied voltage and the optical output of the waveguide type optical device. Both electrodes 1
The applied digital signal that does not apply voltage to 5 and 16 is 0V
In this state, the light incident from the input side optical fiber 17 is once branched by one branching portion 13A and the other branching portion 13B.
To join. Since there is no phase difference in these lights,
Except for propagation loss and branching loss, the same light is output from the optical fiber 1
8 will be output. On the other hand, both electrodes 15,
Increasing the voltage applied during 16 begins to cause a phase difference in the light. As a result, if a voltage is applied between the electrodes 15 and 16 so that the phases of the two lights are just inverted, this light is not output.

Based on this principle, both electrodes 15,
When the voltage applied to 16 is turned on / off, the light can be modulated. An optical modulator that modulates light based on such an operating principle is called a Mach-Zehnder type optical modulator.

The Mach-Zehnder type optical modulator is conventionally
It is used for high speed modulation of 0 Gb / sec (sec) or higher. In order to cope with such high speed modulation, it is desirable that the modulation voltage V is as low as possible. Therefore, the TM mode has been conventionally used as the incident light. Here, the TM mode refers to a mode of a waveguide having no magnetic field component in the wave propagation direction. On the other hand, a waveguide mode having no electric field component in the wave propagation direction is called a TE mode. However, the modulation factor is, for example, 3
When the modulation is set at a low value such as less than 0%, it is not always necessary to use the TM mode modulation. On the contrary, when the modulation is performed in the TM mode, the variation tolerance (tolerance) of the modulation voltage becomes small and the control of the modulation voltage becomes strict.

Further, when the TM mode is adopted, compared with the case where the TE mode is adopted, reduction of insertion loss and simplification of the waveguide structure are hindered.

Therefore, Japanese Laid-Open Patent Publication No. 5-150199 proposes that the polarization of the light incident on the waveguide is in the TE mode only when the degree of modulation is low. First, the theory will be explained.

(A) Polarization State of Incident Light and Modulation Voltage Vπ and Variation Tolerance In the Mach-Zehnder type optical modulator, as described with reference to FIG. 6, the voltage is not applied (V = 0) and the applied voltage is Vπ. Voltage modulation occurs between states. The applied voltage Vπ is generally obtained by the following equation (1).

Vπ = λG / 2n ³ γΓL (1) where λ: wavelength of light G: gap between electrodes n: refractive index of waveguide γ: electro-optic constant Γ: overlap integral of electric field and light L: electrode length

Normally, in the modulation of light by the Mach-Zehnder type optical modulator, it is necessary to obtain the maximum and minimum of the light output (this difference is called the extinction ratio), so the modulation voltage is "0" and "Vπ". Two values of "" are applied to turn the light output on and off. Also,
From the formula (1), in order to reduce the value of Vπ, in addition to the wavelength λ of light as a parameter associated with the structure, the gap G between the electrodes, the overlap integral Γ between the electric field and the light, and the electrode length L, the refractive index n And the electro-optic constant γ also need to be considered. In general, a Z-cut-lithium niobate substrate is often used as the waveguide substrate so that the electro-optic constant γ can be large.

Under certain conditions, the wavelength λ of the light, the gap G between the electrodes, the overlap integral Γ between the electric field and the light, and the electrode length L.
When the waveguide structure is determined by fixing the
Is determined depending on whether the polarization of incident light is TM mode or TE mode. In this case, each applied voltage Vπ
Is expressed by the following equation (2) or equation (3).

Vπ = λG / 2n ₀ ³ γ ₁₃ ΓL (2) (in TE mode) Vπ = λG / 2n _e ³ γ ₃₃ ΓL (3) (in TM mode)

When a Z-cut lithium niobate substrate is used, the following equation (4) is established. 3n ₀ ³ γ ₁₃ ≈n _e ³ γ ₃₃ (4) Therefore, the TM mode is normally used for the polarization of the light incident on the optical modulator.

FIG. 7 shows the relationship between the applied voltage Vπ and the extinction ratio when modulation is performed in the TE mode and the TM mode. From this figure, it can be seen that when the optical modulator is used with a reduced degree of modulation, it is preferable to use the TE mode, which has a wide variation tolerance of the modulation voltage.

(B) Polarization state of incident light and insertion loss FIG. 8 shows the directions of the electric field in the waveguide in both the TM mode and the TE mode. The same figure (a) is TE
The case of the mode and the case of the TM mode are shown in FIG. 9B, and the arrows 21 and 22 represent the electric field components of the guided light. When the TM-mode polarized light propagates through the waveguide, the propagation loss becomes large because the propagation path has an asymmetric structure as seen from FIG. On the contrary, when the TE-mode polarized light propagates in the waveguide, the propagation path has a symmetrical structure as is clear from FIG.
Propagation loss can be suppressed smaller than that in TM mode propagation.

From the above, the following conclusions can be drawn for the Mach-Zehnder type optical modulator. TE mode propagation: Small insertion loss. Applied voltage Vπ is large TM mode propagation: Large insertion loss. Applied voltage Vπ is small

Therefore, when the optical modulator is used with a reduced degree of modulation, the TE mode can be used to obtain the following improvements and merits. The variation tolerance of the modulation voltage is widened. Insertion loss is lower than in TM mode.

FIG. 9 shows an example of a waveguide type optical device proposed in Japanese Unexamined Patent Publication No. 5-150199. The light emitted from the light source 32 enters the waveguide type optical device 31 as a TE mode polarized wave 34 through the polarization controller 33. Waveguide type optical device 31
Is formed with a waveguide 35 that branches into two parallel waveguide pairs on the way and joins again, and electrodes 36 and 37 are formed above the waveguide pair. These electrodes 36, 3
A voltage for modulation is applied to drive signal 7 from drive circuit 38. The modulated light 39 is output from the output side of the waveguide type optical device 31.

[0025]

In such a conventional waveguide type optical device 31, since the polarization controller 33 causes the incident light to be in the TE mode, it is possible to increase the variation tolerance of the modulation voltage. Insertion loss can be reduced.

Such a conventional waveguide type optical device 31
Then, only the TE component is made incident. For this purpose, generally, the polarization extinction ratio of the light source 32, that is, the component ratio of the TE mode polarization and the TM mode polarization is increased,
The light source 32 and the end face of the waveguide 35 are optically connected by a polarization maintaining fiber (PMF). In this case, it is necessary to stably maintain the main axis of the polarization maintaining fiber vertically or horizontally with respect to the waveguide 35. In this way, it is theoretically possible to enter only the TE component into the waveguide 35.

However, the polarization extinction ratio may be deteriorated due to the angle adjustment of the main axis, the fiber, or an external factor. Here, the external factor means, for example, temperature change, application of stress to the polarization maintaining fiber, or the like. When the polarization extinction ratio deteriorates, polarization of TM mode in addition to TE mode is incident, resulting in distortion of the modulation waveform and deterioration of transmission quality.

Therefore, an object of the present invention is a waveguide type which can prevent deterioration of the polarization extinction ratio due to mixing of TM mode polarization when the optical signal is modulated using the TE mode polarization. It is to provide an optical device.

[0029]

According to a first aspect of the present invention, (a) a waveguide substrate having a waveguide formed therein and having at least a pair of electrodes in the vicinity of the waveguide, and (b) an incident side of the waveguide. And (c) a TM mode component for removing the TM mode component of the light incident from the polarization incidence means arranged on the waveguide of the waveguide substrate. The removing means is provided in the waveguide type optical device.

That is, according to the first aspect of the invention, the TE mode polarized wave is mainly incident on the waveguide and the TM
The polarization of the mode is removed by the TM mode component removing means to prevent the deterioration of the polarization extinction ratio.

According to the second aspect of the invention, (a) a waveguide substrate having a waveguide and at least a pair of electrodes in the vicinity of the waveguide, and (b) a signal having a relatively shallow modulation degree on the pair of electrodes. And (c) a polarization incident means arranged on the incident side of the waveguide for mainly injecting a TE mode polarized wave, and (d) a polarization incident means arranged on the waveguide of the waveguide substrate. The waveguide type optical device is provided with a TM mode component removing means for removing the TM mode component of the light incident on the optical waveguide device.

That is, according to the second aspect of the invention, since the TE mode polarized wave is incident on the waveguide to reduce the modulation degree of the signal, the fluctuation tolerance of the modulation voltage is widened, and the insertion loss is higher than that of the TM mode. Becomes lower. Further, in the present invention, the TM mode component removing means is used to remove the TM mode polarized waves when they are mixed, so that the TM mode polarized waves are mixed from the polarized wave incident means due to temperature fluctuation or the like. Even when the input is input as, it is possible to prevent the deterioration of the polarization extinction ratio.

The invention according to claim 3 is characterized in that the TM mode component removing means is one or a plurality of intersecting waveguides arranged in parallel with each other so as to intersect with the waveguide formed on the waveguide substrate. There is. Since the polarization extinction ratio can be prevented from being deteriorated by such a simple structure, it is possible to simplify the configuration of the polarization incidence unit and reduce the cost. Of course, other TM mode removing means may be used.

According to a fourth aspect of the invention, the crossed waveguides are inclined by 15 ° in the direction in which light propagates through the waveguides. As a result, it is possible to efficiently remove the TM mode polarized wave.

The invention according to claim 5 is characterized in that the modulation degree of the modulating means is 30% or less. When such a shallow modulation is performed, the fluctuation tolerance of the modulation voltage becomes small in the TM mode and the control of the modulation voltage becomes strict. However, when the TE mode is adopted, this problem is solved, the insertion loss is reduced, and the waveguide structure is reduced. Is advantageous in terms of simplification.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to embodiments.

FIG. 1 shows a waveguide type optical device which realizes a Mach-Zehnder type optical modulator in one embodiment of the present invention. A waveguide 43 is formed on the lithium niobate substrate (waveguide substrate) 42 of the waveguide type optical device 41 of this embodiment. The waveguide 43 includes two waveguide pairs (arms) 4 which are parallel to each other as in the case described in FIG.
It has 3C and 43D. Above these, metal layer electrodes 45 and 46 made of chromium and gold are formed via a SiO ₂ film (not shown). An input side optical fiber 47 or an output side optical fiber 48 is optically coupled to both end faces of the waveguide substrate 42. Also, two electrodes 4
An output end of a drive circuit 49 for applying a voltage and inputting a signal for modulation is connected between 5 and 46. Since shallow modulation is performed in this embodiment, the modulation degree is 30%.
It is set as follows.

Although not shown, a polarization controller 33 for emitting a TE mode polarized wave is connected to the input side optical fiber 47, and the light emitted from the light source 32 is connected to the polarization controller 33. It is supposed to be incident.

The waveguide type optical device 41 of this embodiment is also used.
On the surface of the waveguide substrate 42 near the incident side,
Three diagonally intersecting waveguides 51 are arranged so as to intersect with 3. The diagonal crossing waveguide 51 is inclined by 15 ° with respect to the propagation direction of the waveguide 43. The diagonal crossing waveguide 51 is used to remove this component when the TM mode polarized wave propagates in the waveguide 43.

FIG. 2 shows a comparison between the ambient temperature and the polarization extinction ratio of the waveguide type optical device of this embodiment and the conventional waveguide type optical device shown in FIG.
In the conventional waveguide type optical device shown in FIG. 9, the polarization extinction ratio fluctuates with temperature as shown by one characteristic curve 53, but in the waveguide type optical device of the present embodiment, the oblique cross waveguide 51 is Since the TM-mode polarization component is removed, the polarization extinction ratio becomes constant independent of temperature, as shown by the other characteristic curve 54. Moreover, due to the elimination of the TM-mode polarization, the polarization extinction ratio becomes much higher than the conventional one.

The waveguide type optical device 41 of this embodiment is used.
Has an insertion loss of 2.0 dB, which can be improved by 1.5 dB as compared with the insertion loss of 3.5 dB which is a conventionally used Mach-Zehnder type optical modulator using the TM mode. Further, the variation tolerance of the modulation voltage when the modulation degree is 10 ± 1% is ±
0.3V, but ± 0.9V in TE mode
Therefore, it can be expanded by 1.2V.

[0043]

As described above, according to the first aspect of the invention, the TM mode polarization is removed by the TM mode component removing means to prevent the deterioration of the polarization extinction ratio. Therefore, even when TM-mode polarized waves are mixed in due to changes in the environment, it is possible to stably operate in the TE-mode.

According to the second and fifth aspects of the invention, since the TE mode polarized wave is incident on the waveguide to reduce the modulation degree of the signal, the fluctuation tolerance of the modulation voltage is widened, and Insertion loss is lower than in TM mode. Further, since the TM mode component removing means is used to remove the TM mode polarized waves when mixed, the TM mode polarized waves are mixedly input from the polarized wave incident means due to temperature fluctuations or the like. In this case, the deterioration of the polarization extinction ratio can be prevented.

Further, according to the inventions of claims 3 and 4, one or a plurality of intersecting waveguides arranged in parallel with each other intersecting with the waveguide formed on the waveguide substrate are characterized. There is. Since the polarization extinction ratio can be prevented from being deteriorated by such a simple structure, it is possible to simplify the configuration of the polarization incidence unit and reduce the cost.

[Brief description of drawings]

FIG. 1 is a plan view of a waveguide type optical device that realizes a Mach-Zehnder type optical modulator according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram comparing the relationship between the environmental temperature and the polarization extinction ratio between the waveguide type optical device of this example and the conventional waveguide type optical device shown in FIG.

FIG. 3 is a plan view showing an example of a structure of a conventionally proposed Mach-Zehnder type high-speed optical modulator.

FIG. 4 is a sectional view taken along the line AA ′ of the Mach-Zehnder interferometer type optical modulator.

FIG. 5 is an explanatory diagram showing a state in which a voltage is applied from a drive circuit to the Mach-Zehnder interferometer type optical modulator.

FIG. 6 is a characteristic diagram showing a relationship between a voltage applied to a waveguide type optical device and an optical output.

FIG. 7 is a characteristic diagram showing a relationship between an applied voltage Vπ and an extinction ratio when modulation is performed in a TE mode and a TM mode.

FIG. 8 is an explanatory diagram showing directions of an electric field in a waveguide in both TM mode and TE mode.

FIG. 9 is a schematic configuration diagram showing an example of a waveguide type optical device proposed in JP-A-5-150199.

[Explanation of symbols]

 32 light source 33 polarization controller 41 waveguide type optical device 42 waveguide substrate 43 waveguide 45, 46 electrode 47 input side optical fiber 48 output side optical fiber 49 drive circuit 51 diagonal crossing waveguide

Claims (5)

[Claims] 1. A waveguide substrate in which a waveguide is formed and at least a pair of electrodes is provided in the vicinity of the waveguide, and polarized wave incidence means arranged on the incident side of the waveguide for mainly injecting a TE mode polarized wave. And a TM mode component removing unit arranged on the waveguide of the waveguide substrate to remove the TM mode component of the light incident from the polarization incidence unit. 2. A waveguide substrate in which a waveguide is formed and at least a pair of electrodes is provided in the vicinity of the waveguide, a modulation means for applying a signal having a relatively shallow modulation degree to the pair of electrodes, and the waveguide of the waveguide. Polarization incidence means arranged on the incident side for mainly injecting the TE mode polarization, and TM mode for eliminating the TM mode component of the light incident from the polarization incidence means disposed on the waveguide of the waveguide substrate. A waveguide type optical device comprising: a component removing unit. 3. The TM mode component removing means is one or a plurality of intersecting waveguides arranged in parallel with each other that intersect with the waveguide formed on the waveguide substrate. Alternatively, the waveguide type optical device according to claim 2. 4. The waveguide type optical device according to claim 3, wherein the crossed waveguides are inclined by 15 ° in a direction in which light propagates through the waveguides. 5. The waveguide type optical device according to claim 2, wherein the modulation degree of the modulation means is 30% or less.