JPH085874A - Optical element module - Google Patents

Abstract

PURPOSE:To obtain an optical element module which permits arranging of a lens near an optical element and is short in the length of a bonding wire by forming the front end of a strip line to a tapered shape. CONSTITUTION:A coated fiber 3b of the microstrip line 3 and an electrode of an optical modulator 1 are connected by the bonding wire 4. The front end of a substrate 3a of the microstrip line 3 is formed to the tapered shape made finer toward the optical modulator 1. The microstrip line 3 formed to the tapered shape in such a manner is used as a transmission line. A modulation signal propagates in the microstrip line 3 and is impressed on the electrode of the optical modulator 1 via the bonding wire 4. Since the front end of the microstrip line 3 is formed to the tapered shape, arranging of the lenses 6a, 6b near the end face of the waveguide of the optical modulator 1 is possible without increasing the length of the bonding wire 4. The input and output beams of the waveguide are guided to the lenses 6a, 6b without shielding these beams.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element module, which optically couples an optical fiber to an optical element such as a semiconductor laser or an optical modulator through a lens and transmits a modulation signal through a strip line and a bonding wire. In the optical element module for sending to the optical element, the lens can be arranged close to the optical element and the wire bonding length is shortened. According to the present invention, high optical coupling efficiency can be realized without impairing the modulation characteristic even when the optical element is modulated at high speed.

[0002]

2. Description of the Related Art With the increase in transmission speed of optical communication systems, there is an increasing demand for wider bandwidth of optical modulation circuits. High-speed operation of 10 Gb / s has already been realized by directly modulating a semiconductor laser, but in recent years, a band of 40 GHz or more has been reported by a semiconductor electro-absorption optical modulator, which will be applied to the next-generation ultra-high-speed optical communication system. Is expected to be applied. An optical fiber is optically coupled to the input / output end faces of the waveguide of these ultra-high-speed operable optical elements.
When realizing a fixed optical element module, some problems to be overcome remain.

In the electro-absorption optical modulator, in general, the confinement of light in the direction perpendicular to the electric field is strengthened in order to improve the modulation efficiency. The cross-sectional shape of the waveguide is rectangular, and the cross section of the output beam shape of the waveguide is elliptical. This tendency is
It becomes remarkable in the electro-absorption optical modulator. On the other hand, since the spot shape of the optical fiber is circular, when the waveguide and the optical fiber are optically coupled, a large coupling loss occurs due to the difference in spot shape.

Here, if the spot size of the optical fiber is ω _f and the spot sizes of the waveguide in the x and y directions are ω _x and ω _y , the coupling efficiency is expressed by the following equation. η = 4 / (ω _x / ω _f + ω _f / ω _x ) · (ω _y / ω _f + ω _f / ω _y ) ... (1) Here, the maximum coupling efficiency is ω _f = (ω _x ω _y ) ^{1 / 2} ... (2), and η _max = 4 / {(ω _x / ω _f ) ^1/2 + (ω _f / ω _x ) ^1/2 } ² (3).

As is clear from the equation (3), even if the spot size of the optical fiber is converted and optimized by the optical system,
Inevitably, the coupling efficiency is reduced due to the distortion of the beam in the waveguide of the optical element. The thickness of the waveguide can be controlled to a thickness of about several atomic layers by the crystal growth technique, while the width of the waveguide depends on the etching technique, and even if the dry etching technique is used for the buried type waveguide. It is difficult to realize a waveguide width on the order of submicron. Therefore, if the confinement of light in the direction perpendicular to the electric field is increased in order to improve the modulation efficiency as in the electro-absorption optical modulator, the coupling loss increases, and there is a gap between the modulation efficiency and the optical coupling efficiency. Generally, it can be said that there is a trade-off relationship. In order to improve the coupling efficiency in coupling the waveguide of such an optical element with an optical fiber, it is necessary to insert an optical system for converting the aspect ratio of the waveguide output beam and making the beam shape close to a circle. .

Another problem in modularizing the high-speed modulation optical element is that the electric circuit for modulation limits the space near the input / output end face of the waveguide of the optical element. This is the fact that the degree of freedom in constructing an optical coupling system is reduced. Bonding wires, ribbons, etc. are usually used to connect the core of the transmission line that transmits the modulation signal to the electrodes of the optical element.However, if the wire length becomes long, the capacitance, resistance value, and parasitic inductance of the wire may cause It causes a roll-off of the modulation characteristic. Therefore, it is desirable to make the wire length as short as possible.

The transfer function when the inductance of the wire is L, the capacitance value of the optical element is C _P , and the termination resistance value is R is represented by the following equation. H (ω) = {1- (ω / ω ₀ ) ² } ² + {ω / (ω ₀ / Q)} ^{2 1/2} (4) where ω ₀ is the cutoff frequency, and expressed. ω ₀ = {(R _L + R) / RCL} ^1/2 (5) Q = {RCL (R _L + R)} ^1/2 / {L + (R _L CR)} (6) where R _L is The characteristic impedance of the transmission line, which is usually 50Ω. For example, when the parasitic capacitance of the electro-absorption optical modulator is 0.15 pF, the inductance of the bonding wire is 0.2 nH, and R is 50Ω, the cutoff frequency is about 18 GHz.

Generally, the optical coupling method between the optical fiber and the input / output end face of the waveguide of an optical element such as a semiconductor laser or a semiconductor optical modulator uses a tapered spherical fiber (see FIG. 9).
(See FIG. 10) and a method of performing optical coupling using one or more lenses (see FIG. 10).

In the former method, the tip of the optical fiber is processed into a spherical shape so as to have a lens function, and the mode field diameter of the beam of the optical fiber is expanded or reduced so as to match that of the waveguide of the optical element. Explaining with a concrete example based on FIG. 9,
The input / output end faces of the waveguide of the optical modulator 1 and the end faces of the tapered spherical fibers 2a and 2b are opposed to each other. The microstrip line 3 is composed of a dielectric substrate 3a and a core wire 3b for transmitting a modulation signal, and the electrode of the optical modulator 1 and the core wire 3b are connected by a bonding wire 4.

The method shown in FIG. 9 has an advantage that the number of parts at the time of manufacturing a module is reduced and the working efficiency and the yield at the time of manufacturing a module are improved. Further, when a modulation signal is applied to a semiconductor laser, a semiconductor optical modulator, etc., the modulation circuit often occupies a space near the input / output end face of the optical element. In such a case, as shown in FIG. A spherical fiber 2 with a diameter of about 100 μm
Since a and 2b can be inserted into a narrow space, it is possible to design a module that does not affect the modulation circuit.

However, as described above, the beam shape of the waveguide of the semiconductor laser and the semiconductor optical modulator is an ellipse reflecting the shape of the waveguide, and the major axis is several μm and the minor axis is several minutes. It is about one size. On the other hand, the spot shape of the single mode fiber is circular and its diameter is 5 μm.
In order to realize the optimum optical coupling state, it is necessary to further reduce the spot size of the fiber to match that of the optical element waveguide. In this case, the optimum tip diameter is the weighted average of the spot sizes in the longitudinal and lateral directions of the waveguide as shown in equation (3), which is a few μm, and the optimum tip position of the waveguide and the tip of the fiber. The distance between them is similar to this. In this way, when using a spherical fiber, the distance between the fiber and the waveguide end face becomes very close at the optimum coupling point, and the tip of the fiber and the waveguide end face may come into contact during alignment and damage the both. There is. For this reason, there is an attempt to increase the focal length (working length) by enlarging the core diameter at the tip of the fiber. Further, when the spherical fiber is used, the tolerance in the direction perpendicular to the optical axis at the optimum coupling position is extremely small, which becomes a major factor of lowering the yield in manufacturing the module. Furthermore, the decrease in coupling efficiency with time due to environmental changes cannot be ignored.

On the other hand, it is 2 when optical coupling is performed by a lens.
With a confocal configuration that combines one or more lenses,
Tolerance in the direction perpendicular to the optical axis can be relaxed by enlarging the spot sizes of both the waveguide and the fiber to perform optical coupling. Explaining in a concrete example based on FIG. 10, the input / output end faces of the waveguide of the optical modulator 1 and the end faces of the optical fibers 5a and 5b are opposed to each other, and further, between the optical fiber 5a and the optical modulator 1 and between the optical fibers. A lens 6 is provided between the fiber 5b and the optical modulator 1.
a and 6b are arranged.

As described above, in the semiconductor optical modulator, since the light is strongly confined in the direction perpendicular to the electric field of the waveguide in order to improve the modulation efficiency, the beam shape becomes largely distorted. Is a factor that greatly reduces the coupling efficiency with a perfect circle optical fiber. When optical coupling is performed using a lens, an optical component that converts the aspect ratio of the beam (for example, a cylindrical lens, prism, etc.) can be inserted between the lens and the optical fiber, and the beam shape of the waveguide can be a perfect circle. The coupling efficiency can be greatly improved by approaching to.

However, at present, the lens size is about 0.5 mm for a spherical lens and 2 for an aspherical lens.
Since it is about mm, it becomes difficult to dispose the lens near the end face of the waveguide when the circuit for modulation is mounted as described above. If a lens is inserted here, the bonding wire length becomes long as shown in FIG. 10, and a situation often occurs in which the modulation characteristic is deteriorated. For this reason, it has been difficult to incorporate an optical system using a lens when modularizing an optical element that operates at high speed, although it has many merits.

[0015]

When manufacturing a module of an optical element such as a semiconductor laser or a semiconductor optical modulator, if optical coupling is performed by an optical system using a lens, the coupling efficiency is high and the tolerance at the time of alignment is high. It is expected to have merits such as large size. However, when a circuit for modulation is mounted, it is difficult to construct a module with an optical system using a lens because the space near the input / output end face of the waveguide is greatly limited.

In view of the above-mentioned conventional technique, it is an object of the present invention to provide an optical element module in which a lens can be arranged near the optical element and a bonding wire length is short.
According to the present invention, it is possible to realize an optical element module with a high yield without impairing the modulation characteristics when it is necessary to apply a high-speed modulation signal to the optical modulation element. In other words, the present invention realizes an optical element module having high optical coupling efficiency without impairing the modulation characteristics when it is necessary to mount a circuit for modulation such as a semiconductor laser or a semiconductor optical modulator. It is an object.

[0017]

The structure of the present invention which solves the above-mentioned problems is such that the input / output end face of the waveguide of the optical element and the end face of the optical fiber are opposed to each other via a lens, and the core wire of the strip line is further provided. Also, in the optical element module for transmitting the modulated signal to the optical element via the wire, the end portion of the strip line on the optical element side has a tapered shape that narrows toward the optical element.

[0018]

As described above, the optical element module of the present invention is characterized in that the strip line for applying the modulation signal is tapered. A bonding wire is used to connect the strip line and the electrode of the optical element, but if the wire length becomes long, the modulation characteristic is rolled off due to the parasitic inductance of the wire, and the modulation characteristic is deteriorated. Therefore, it is desirable to make the wire length as short as possible. However, when the lens is incorporated in the conventional modulation circuit, the wire length becomes long and the modulation characteristic deteriorates. In the present invention, the tip end of the strip line is tapered to secure a space for mounting the lens, so that an optical system using the lens can be realized without increasing the wire length connecting the core wire of the strip line and the electrode of the optical element. Can be configured.

[0019]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. As shown in the figure, in this embodiment, the input / output end faces of the waveguide of the optical modulator 1 and the end faces of the optical fibers 5a and 5b are opposed to each other, and the space between the optical fiber 5a and the optical modulator 1 Lenses 6a and 6b are arranged between the fiber 5b and the optical modulator 1. Further, the core wire 3b of the microstrip line 3
And the electrodes of the optical modulator 1 are connected by the bonding wires 4. And the substrate 3 of the microstrip line 3
The tip of a has a tapered shape that becomes thinner toward the optical modulator 1.

In the microstrip line 3, a core wire 3b is formed by depositing a metal such as gold or copper on the upper surface of a substrate 3a made of alumina, sapphire, quartz or the like, and a ground electrode is formed on the lower bottom surface. It was done. Lens 6a, 6b
As the lens, various lenses such as an aspherical lens and a spherical lens are used. As the bonding wire 4, a gold wire, a gold ribbon or the like is preferably used. Optical fiber 6a,
As 6b, a single mode fiber, a polarization maintaining fiber, etc. are preferably used.

In the first embodiment of the present invention, the tapered microstrip line 3 is used as a transmission line. The modulation signal propagates through the microstrip line 3 and is applied to the electrode of the optical modulator 1 via the bonding wire 4. Since the tip of the microstrip line 3 is tapered, the lenses 6a and 6b can be arranged near the waveguide end face of the optical modulator 1 without increasing the length of the bonding wire 4. Further, there is an effect that the input and output beams of the waveguide can be guided to the lenses 6a and 6b without being blocked.

The characteristic impedance of the microstrip line 3 is determined by the dielectric constant and thickness of the material of the substrate 3a and the width of the core wire 3b. Since the width of 3a becomes narrow, the characteristic impedance changes, and a part of the modulation signal may be reflected at the tip. Therefore, in order to evaluate the transmission characteristics of the microstrip line 3 according to the first embodiment of the present invention, three types of strip lines having a tip angle of 30 ° and 45 ° and a conventional type (vertical cut type) were manufactured, and termination characteristics were obtained. Was measured.

The material of the substrate 3a of the microstrip line 3 is alumina, the substrate thickness is 0.635 mm, and the length is 3.
The width is 9 mm and the width is 3.0 mm. The characteristic impedance of the line is 50Ω. The core wire 3b is formed by vacuum-depositing a nickel chrome thin film and a gold thin film and then gold-plating the thin film. The gold film thickness is 2 μm. A chip resistor was used as the terminating resistor, the electrode of the resistor and the core wire 3b of the strip line were connected by a bonding wire 4, and the return loss was measured by a network analyzer.

FIG. 2 (a) shows the S11 frequency characteristic of a conventional vertical cut type strip line, and FIGS. 2 (b) and 2 (c) show the S11 frequency characteristic of a microstrip line with tip angles of 30 ° and 45 °, respectively. Indicates. In each of the strip lines, the return loss was -10 dB or less in the measured frequency range up to 26.5 GHz, and the deterioration of the frequency characteristic due to the taper of the tip of the strip line 3 was not observed. .

FIG. 3 shows a second embodiment of the present invention. In the second embodiment, the coplanar strip line 7 is used and its tip is tapered. Moreover, the substrate 7a that serves as a grounding portion
Not only the core wire 7b but also the core wire 7b is narrowed to have a tapered shape, so that the effect of reducing the change in the characteristic impedance is obtained. Incidentally, in the first embodiment shown in FIG. 1, the core wire 3b
Since only the substrate 3a is constricted without changing the width of the strip line, the characteristic impedance of the tip portion of the strip line 3 may change slightly. However, in the second embodiment, such a fear disappears.

The other parts of the second embodiment have the same structure as the first embodiment. In the second embodiment as well, similar to the first embodiment, not only a space for mounting the lens can be secured, but also the beam emitted from the end face of the waveguide is not blocked by the strip line to the lens. Can be guided.

Next, an MQW electroabsorption type optical phase modulator module according to a third embodiment of the present invention is shown in FIG.
This will be described with reference to FIG. 5, which is an enlarged side view, and FIG. 6, which shows the optical system.

5 and 6, 10 is an electroabsorption type optical phase modulator, 11a and 11b are aspherical lenses, and 12a and 1a.
Reference numeral 2b is a polarization-maintaining optical fiber whose tip is obliquely polished, 13 is a laminated ceramics capacitor, 14 is a chip capacitor, 15 is a terminating resistor, 4 is a bonding wire, and 1 is a microstrip line.

The structure of the active layer of the electro-absorption optical phase modulator 10 is an InGaAlAs / InAlAs multiple quantum well structure, the active layer thickness is 0.4 μm, and the exciton absorption wavelength is 1.45 μm. The size of the waveguide is 300 μm and the width of the waveguide is 4 μm.
μm. The electro-absorption optical phase modulator 10 has a 50Ω terminating resistor 1 in order to achieve impedance matching with the transmission line.
5 is connected in parallel to the element. Further, when a bias voltage is applied to the element, a direct current flows through the terminating resistor 15 and the resistor generates heat, which may change the modulation characteristic of the modulator. Is connected to block the flow of direct current. Since the lower cutoff frequency increases as the capacitance of the capacitance element increases, it is desirable that the capacitance of the capacitance element be as large as possible. However, since it is generally difficult to obtain a capacitive element having a large capacity and excellent high-frequency characteristics, in this module, a multilayer ceramic capacitor 13 having a large capacity and a chip capacitor 14 having excellent high-frequency characteristics are connected in parallel to provide a wide band. It is trying to make it. The multilayer ceramic capacitor 13 has a capacitance of 0.1 μF, and the chip capacitor 14 has a capacitance of 100 μF.
pF.

The optical system of this modulator module is shown in FIG. The coupling optical system has a confocal configuration using two aspherical lenses. First lens 11a disposed near the end face of the waveguide
1, 11b-1 has a focal length of 0.5 mm and an NA value of 0.
5. On the other hand, the focal lengths of the second lenses 11a-2 and 11b-2 fixed to the obliquely polished polarization-maintaining fibers 12a and 12b are 1.81 mm and the NA value is 0.3. The microstrip line 1 having a taper tip angle of 45 ° was used as a transmission line.

FIG. 7 shows absorption fluctuation characteristics of the MQW electroabsorption type optical phase modulator module shown in FIGS. 4 and 5, and FIG. 8 shows modulation frequency characteristics thereof. As shown in both characteristics, in this optical phase modulator module, with respect to the TE polarized input optical signal,
The insertion loss at a wavelength of 1.55 μm was 9.2 dB, and 18 GHz was obtained as a −3 dB modulation band. As a result, it was confirmed that the present invention can realize a module having excellent modulation characteristics and high optical coupling efficiency.

Although an optical modulator is used as an optical element in each of the above embodiments, the present invention can be applied as it is even if a semiconductor laser is used. That is, in the case of an optical modulator, two optical fibers are optically coupled to one optical modulator, whereas in the case of a semiconductor laser, one optical fiber is optically coupled to one semiconductor laser. The configuration of other parts can be the same, only different.

[0034]

According to the present invention as specifically described in connection with the above embodiments, since the tip of the strip line is tapered, the lens can be arranged near the optical element and the length of the wire can be shortened. As a result, an optical element module having high optical coupling efficiency and wide band modulation characteristics can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a reflection characteristic of a strip line.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is a plan view showing a third embodiment of the present invention.

FIG. 5 is an enlarged side view showing a third embodiment.

FIG. 6 is a configuration diagram showing an optical system of a third embodiment.

FIG. 7 is a characteristic diagram showing a light absorption fluctuation amount of the third embodiment.

FIG. 8 is a characteristic diagram showing a modulation frequency characteristic of the third embodiment.

FIG. 9 is a configuration diagram showing a conventional technique using optical coupling with a spherical fiber.

FIG. 10 is a configuration diagram showing a conventional technique using optical coupling by a lens.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical modulator 2a, 2b Tapered spherical fiber 3 Microstrip line 3a Substrate 3b Core wire 4 Bonding wire 5a, 5b Optical fiber 6a, 6b Lens 7 Coplanar strip line 7a Substrate 7b Core line 10 Electroabsorption optical phase modulator 11a, 11b Aspherical lens 12a, 12b Polarization maintaining optical fiber 13 Multilayer ceramics capacitor 14 Chip capacitor

Claims (3)

[Claims] 1. An optical device in which an input / output end face of a waveguide of an optical device and an end face of an optical fiber are opposed to each other via a lens, and a modulated signal is sent to the optical device via a core wire and a wire of a strip line. In the module, an end of the strip line on the optical element side has a tapered shape that is narrowed toward an optical element. 2. The optical element module according to claim 1, wherein the strip line is a microstrip line, and the substrate of the microstrip line is tapered at the end on the optical element side. 3. The optical device according to claim 1, wherein the strip line is a coplanar strip line, and the substrate and the core wire of the coplanar strip line are tapered at the end on the optical device side. module.