JP2000241775A - Optical modulator, light source for optical communication and module for optical communication - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate an excellent optical transmission waveform of a high extinction ratio. SOLUTION: An incident light waveguide part 600, a branch part 500, a first electric field absorption type modulator part 400A, a second electric field absorption type modulator part 400B, a multiplexer part 300 and an output light waveguide part 200 are formed respectively on a substrate 100. The first electric field absorption type modulator part 400A and the second electric field type modulator part 400B are provided with a difference between the absorption end wavelengths of the absorption layers of respective semiconductor electric field absorption type modulator parts 400A, 400B (1.495 μm, 1.510 μm) so that the absorption end wavelengths of the absorption layers are different from each other, or when the same modulation voltage is applied to first, second respective semiconductor electric field absorption type modulator parts 400A, 400B, a phase difference between two light waves propagating through respective electric field type modulator parts 400A, 400B becomes π[rad]. The extinction ratio when a high voltage is applied is increased while keeping an extinction characteristic when a low voltage is applied compared with the optical modulator constituted of a single electric field absorption type modulator part, and an excellent optical modulation waveform and the high extinction ratio are realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator and an optical communication light source, which are important elements in an optical communication system, and more particularly to an electric field absorption type having a large extinction ratio and capable of generating a good transmission light modulation waveform. And an optical communication light source and an optical communication module using the same.

[0002]

2. Description of the Related Art In recent years, with the increase in speed and distance of an optical communication system, the problem of a conventional direct modulation method using a semiconductor laser is becoming apparent. That is, in the direct modulation method of the semiconductor laser, wavelength chirping occurs at the time of modulation, thereby deteriorating the waveform after fiber transmission. This phenomenon becomes more conspicuous as the signal transmission speed increases and the transmission distance increases. In particular, this problem is serious in a system using the existing 1.3 μm zero-dispersion fiber. Even if an attempt is made to extend the transmission distance by using a light source in the 1.55 μm wavelength band where the fiber propagation loss is small, the dispersion caused by chirping. The transmission distance is limited by the restriction. This problem can be remedied by causing the semiconductor laser to emit light at a constant light output and employing an external modulation method in which the light emitted from the semiconductor laser is modulated by an optical modulator different from the semiconductor laser. Therefore, the development of external optical modulators has been activated. As the external modulator, one using a dielectric material such as LiNbO ₃ and one using a semiconductor such as InP or GaAs can be considered. Other optical devices such as a semiconductor laser and an optical amplifier and electronic devices such as FETs are considered. Expectations for semiconductor optical modulators are increasing because they can be integrated with circuits and can be easily reduced in size and voltage.

A semiconductor optical modulator utilizes the effect of shifting the absorption edge to a longer wavelength side by applying an electric field, such as the Franz-Keldysh effect of a bulk semiconductor and the Stark effect of quantum confinement in a multiple quantum well. There are a semiconductor electroabsorption type optical modulator that modulates intensity by changing a coefficient, and a Mach-Zehnder type optical modulator that uses a refractive index change caused by an electro-optic effect (Pockels effect) or a quantum confined Stark effect of a bulk semiconductor.

A Mach-Zehnder optical modulator generates a phase difference between light waves due to a change in refractive index in principle, and turns on / off the output light by an interference effect at the time of multiplexing. In order to obtain a predetermined refractive index change at a practical operating voltage, LiNb
A waveguide length of several cm to about 10 cm is required for a dielectric such as O ₃ . Even when a semiconductor material is used, an element length of about 1 mm to 1 cm is required as a whole. However, the extinction characteristic of the Mach-Zehnder optical modulator with respect to the applied voltage is such that the degree of extinction is also halved at half the amplitude applied voltage, and the cross point of the optical output eye pattern with respect to the NRZ signal voltage is also located substantially at the center. An eye pattern is obtained.

On the other hand, a semiconductor electroabsorption type optical modulator has a small element length of several hundred μm and a lower operating voltage than that of a Mach-Zehnder type optical modulator, but the extinction characteristic varies nonlinearly with applied voltage. However, at the center of the amplitude applied voltage, the extinction ratio greatly deviates from the center value. This gives N
The cross point of the light output eye pattern with respect to the RZ signal voltage is greatly shifted downward, and a good light output eye pattern cannot be obtained. In addition, if the applied voltage is adjusted to shift the cross point upward, the extinction ratio deteriorates. There is a problem that it does. Furthermore, since the extinction curve when a low voltage is applied is steeper than that of the Mach-Zehnder type optical modulator, the slight noise of the modulated electric signal is greatly amplified at the stage where it is converted into an optical signal, and the mark of the optical output eye pattern is obtained. There is a problem that noise such as jitter is superimposed on the level while the noise is enlarged.

In order to solve this problem, recently, an optical transmitter or the like using a built-in driving circuit having a cross-point adjusting function and using an electro-absorption modulator has been studied. An example of this is reported by Uto et al. In the 1998 IEICE Communications Society Conference Proceedings, p. 442 (Lecture No. B-10-120). In this example, the drive circuit changes the duty ratio of the input external modulation signal to apply a modulation voltage signal to the electroabsorption modulator. As in this example, in order to obtain a good optical output waveform using a semiconductor electro-absorption optical modulator, a complicated external drive circuit is required, and the transmission system configuration becomes complicated and cost increases. there were.

[0007]

As described above, the semiconductor electroabsorption optical modulator is advantageous in terms of miniaturization and low operating voltage. However, in the semiconductor electroabsorption optical modulator, as described above, The cross point of the light output eye pattern shifts downward due to the nonlinearity of the extinction characteristic with respect to the voltage application, and the external noise that is expanded to the mark level of the eye pattern is superimposed. Also, the cross point is applied so as to shift upward. When the voltage is adjusted, there is a problem that the extinction ratio deteriorates. Further, in the technology proposed to solve this problem, there is a problem that an external drive circuit or the like having a cross point adjusting function must be used, and the system configuration becomes complicated.

An object of the present invention is to provide an optical modulator and an optical communication system which can generate an optical output eye pattern having a high extinction ratio without performing cross point adjustment using a complicated external drive circuit or the like. It is to provide a light source.

[0009]

SUMMARY OF THE INVENTION An optical modulator according to the present invention comprises a first and a second semiconductor electroabsorption type for optically modulating guided light incident on an incident optical waveguide and branched at a branch portion. The absorption end wavelengths of the absorption layers in the modulator sections are different from each other, and the output lights of these modulator sections are multiplexed in the multiplexing section. When the same modulation voltage is applied to the first and second semiconductor electroabsorption modulators,
The phase difference between two light waves propagating through each electroabsorption modulator is π
In this case, a difference is provided in the absorption edge wavelength of the absorption layer of each of the semiconductor electroabsorption modulators so that [rad].

In the above-mentioned optical modulator, an optical waveguide other than the semiconductor electroabsorption modulator is formed on a quartz substrate, and the optical waveguide is multiplexed with a branch portion of the waveguide formed on the quartz substrate. The semiconductor electroabsorption modulator section is integrated in a hybrid between two optical waveguides between the section and the optical waveguide. Alternatively, the semiconductor electro-absorption type modulator section and the other optical waveguides may both be formed of a semiconductor material. Furthermore, the light absorption layer of the semiconductor electroabsorption modulator and the other optical waveguides are continuously formed without interruption, and the absorption edge wavelength of the light absorption layer of the semiconductor electroabsorption modulator and other As the absorption edge wavelength of the optical waveguide is different,
The structure is formed by the band gap control selective MOVPE method. In addition, the light absorption layer of the semiconductor electroabsorption modulator section has a multiple quantum well structure.

Further, in the light source for optical communication according to the present invention, a light emitting element for making light incident on the incident optical waveguide portion of the optical modulator according to the present invention is formed or installed on the same substrate as the optical modulator. It is characterized by being.

Further, the optical communication module of the present invention comprises:
Condensing means for optically coupling external input light to an incident optical waveguide portion of the optical modulator of the present invention, and optically coupling output light from an output optical waveguide of the optical modulator to an external optical fiber. The light collecting means for coupling is built in.
Alternatively, the light source for optical communication according to the present invention is configured to include a built-in condensing means for optically coupling output light from an output optical waveguide to an external optical fiber.

In the optical modulator according to the present invention, the incident external light wave is split into two light waves at the branch part, and the light waves are incident on the first and second electro-absorption modulator parts having different absorption edge wavelengths. . When an in-phase signal voltage is applied to each of the electroabsorption modulators, the split lightwaves are absorbed by the modulators, respectively, and undergo phase changes of different magnitudes. And propagate to the output optical waveguide. At this time, the output light is extinguished together with the applied voltage due to the interference effect in the multiplexing part, in addition to the absorption in the respective electroabsorption modulators. When a low voltage is applied, the electric field absorption effect becomes larger than the interference effect, and the shape becomes similar to the extinction characteristic of each electroabsorption modulator. On the other hand, when the applied voltage is increased, the interference effect is increased, and a larger extinction can be obtained than in the electroabsorption modulator. By appropriately setting the absorption edge wavelength of each of the first and second electroabsorption modulators, the extinction is gradual when a low voltage is applied, and a large extinction ratio can be obtained with a predetermined applied voltage.
Thereby, a good light output eye pattern can be obtained without deteriorating the extinction ratio at the time of modulation.

In order to realize this structure, select MOVP
A band gap control method based on the E crystal growth method is used. This is based on the fact that if the width of the dielectric mask on both sides of the gap on the semiconductor substrate on which the light absorption layer is formed is partially changed, the absorption edge wavelength changes accordingly. By using this crystal growth method, it is possible to form an optical waveguide portion having different absorption edge wavelengths and an optical waveguide layer of an electroabsorption modulator portion in a single growth, and to repeat etching and crystal growth separately. No need. Therefore, the above-described optical modulator can be formed with good uniformity, reproducibility, and controllability.

Also, the optical modulator, the optical communication light source, and the optical communication module of the present invention are incorporated in transmitting means, and waveguide means for guiding output light from the transmitting means to the outside, The provision of the receiving means for receiving the output light from the waveguide means facilitates the construction of the optical communication system.

[0016]

Next, embodiments of the present invention will be described with reference to the drawings. (First Embodiment) FIG. 1 shows an InP-based multiple quantum well (MQW) as a first embodiment in which the present invention is applied to an optical modulator.
It is a perspective view of an embodiment applied to an electroabsorption type optical modulator. FIGS. 2A and 2B respectively show AA in FIG.
It is an expanded sectional view of a line and a BB line. An n-InP buffer layer (carrier concentration: 2 × 10 ¹⁷ cm ⁻³ ) 110 is laminated on an (100) -oriented n-InP substrate 100, and a selective band gap control MOVPE crystal growth method is selectively formed thereon. The incident optical waveguide section 600, the branch section 500,
A first electroabsorption modulator section 400A, a second electroabsorption modulator section 400B, a multiplexing section 300, and an output optical waveguide section 200 are formed respectively.

The first and second electroabsorption modulator sections 400A, 400 on the n-InP buffer layer 110
As shown in FIG. 2A, the n-InP cladding layer 120 (carrier concentration 5 ×
10 ¹⁷ cm ⁻³ ), an optical waveguide MQW core layer 130 (absorption edge wavelength: 1.350 μm), and a p-InP cladding layer 140 are selectively laminated by selective MOVPE growth, and p is further surrounded by these layers. -InP buried layer 1
Reference numeral 50 denotes a structure selectively formed by selective MOVPE growth.

On the other hand, as shown in FIG. 2B, the first and second electroabsorption modulator sections 400A and 400B
-N-InP layer 7 selectively on InP buffer layer 110
20 and 710, light absorbing layers 721 and 711, p-In
P layers 722 and 712 are sequentially stacked, respectively. At this time, the light absorption edge wavelength of each of the light absorption layers 721 and 711 of the first and second electroabsorption modulator sections 400A and 400B is 1.495 μm by using the selective band gap control MOVPE crystal growth method. , 1.510μ
m. Further, a p-Inp buried layer 703 is formed so as to surround these layers, and a p-InGaAs contact layer 702 (carrier concentration of 1 × 10 ¹⁹ cm ⁻³ ) and a Ti / Au electrode (p-side electrode) 701 are formed thereon. Are sequentially laminated, and Ti /
The structure is such that an Au electrode (n-side electrode) 512 is formed. A pad electrode 511 connected to the p-side electrode 701 is formed. Under the pad electrode 511, in order to realize a high-speed operation by reducing the element capacitance between the pad electrode 511 and the buffer layer 110. A polyimide film 410 is embedded.

Next, the operation of the optical modulator according to the first embodiment will be described. The light wave (wavelength: 1.55 μm) incident on the incident optical waveguide 600 is split by the branching section 500 and is incident on the first electroabsorption modulator section 400A and the second electroabsorption modulator section 400B, respectively. The light is subjected to intensity modulation and phase modulation by the modulation voltage applied to the electrode 511, multiplexed by the multiplexing unit 300, and output from the output waveguide 200. FIG. 3 shows a voltage-extinction characteristic D3 of the first electroabsorption modulator 400A, a voltage-extinction characteristic D2 of the second electroabsorption modulator 400B, and a voltage-extinction characteristic D1 of the combined output light. . FIG. 4 shows a voltage-phase change characteristic ΔΦ1 by the first electroabsorption modulator 400A and a voltage-phase change characteristic ΔΦ2 by the second electroabsorption modulator 400B. FIG. 5 shows both electroabsorption modulators. Modulator section 4
The voltage dependence of the phase difference at 00A and 400B is shown. When a low voltage is applied, both electroabsorption modulator sections 400
Since the phase difference between A and 400B is small, the interference effect is also small, and the voltage-extinction characteristic of the output light is close to the extinction characteristic of both electroabsorption modulator sections 400A and 400B, but as the applied voltage increases, The phase difference between the two modulator sections 400A and 400B increases, and the interference effect increases, and an extinction ratio larger than the extinction ratio of the two electroabsorption modulator sections 400A and 400B is obtained.

As described above, the optical modulator of the present invention includes the first electroabsorption modulator section 400A and the second electroabsorption modulator section 400B having different absorption edge wavelengths of the absorption layer. After splitting the incident light, it passes through the first and second electroabsorption modulator sections 400A and 400B, respectively.
And by combining and outputting the output of each modulator section,
Compared to an optical modulator composed of a single electroabsorption modulator, it is possible to increase the extinction ratio when a high voltage is applied while maintaining the extinction characteristics when a low voltage is applied. A modulation waveform and a high extinction ratio can be realized.

The optical modulator of the present invention is not limited to the first embodiment. That is, in the above embodiment, the multiple quantum well structure is taken as the optical waveguide. However, the present invention is not limited to this, and is also effective in an optical modulator using a bulk semiconductor as an optical waveguide layer. It goes without saying that the shape, the absorption edge wavelength of each waveguide layer, the doping concentration and the like are not limited.

(Second Embodiment) FIG. 6 is a perspective view of a second embodiment in which the present invention is configured as a light source for optical communication. A DFB laser is monolithically applied to the optical modulator of the first embodiment. It is an example of an integrated light source for optical communication. That is,
The same parts as those in the first embodiment are denoted by the same reference numerals,
The description is omitted. Here, in addition to the configuration of the first embodiment, the DFB laser unit 625 has a structure selectively formed by the selective band gap control MOVPE crystal growth method. FIG. 7 shows a cross-sectional structure taken along line CC in FIG. The grating region 113 is partially formed only in the DFB laser portion 625 on the n-type substrate 100.
Is formed, and an n-layer 110 is stacked thereon. Further thereon, an n-type InP layer 313 (doping concentration 5 × 10 ¹⁷ cm ⁻³ ), an active layer 314 (bandgap wavelength 1.548 nm), and a p-type InP layer 315 are selectively formed.
(A doping concentration of 5 × 10 ¹⁷ cm ⁻³ ) are sequentially stacked, a p-type InP buried layer 316 is formed so as to surround these layers, and a p-type InP contact layer 318 (a doping concentration of 1 × 10 ¹⁷ cm ⁻³ ) is formed thereon. 10 ¹⁸ cm ⁻³ ), and a structure in which p-type Ti / Au electrodes 722 are stacked.
Further, a pad electrode 723 whose lower region is filled with a polyimide resin 410 is connected to the p-type Ti / Au electrode 724.

In the light source for optical communication having this configuration, the light wave emitted from the DFB laser section 625 is coupled to the incident optical waveguide 600 and modulated through the same process as in the first embodiment.
The light is emitted from the output waveguide 200. In this configuration, the DF
Since the B laser unit is monolithically integrated with the optical modulator of the first embodiment by the selective band gap control MOVPE crystal growth method, a small and low-cost light source for optical communication can be realized.

By using the optical modulator of the first embodiment and the optical communication light source of the second embodiment, it is possible to configure an optical communication module. For example, FIG.
The optical modulator 1 according to the first embodiment is mounted on a submount 17.
9 is an optical communication modulator module 18 in which optical fibers 13A and 13B are fixed on the optical axes on the input side and output side of the optical modulator 19 via aspheric lenses 12A and 12B, respectively. With this module, a good optical modulation transmission waveform with a high extinction ratio can be easily created. Similarly, although not shown, the optical communication light source module of the second embodiment is mounted on a submount, and an optical fiber is fixed to an optical output side of the optical mount via an aspheric lens, thereby forming an optical communication light source module. It becomes possible. By modularizing in this way, the optical modulator and the light source for optical communication can be applied to an optical communication system.

For example, FIG. 9 shows the optical communication module 18 shown in FIG.
FIG. 1 is a configuration diagram of a trunk optical communication system that uses an optical communication system. The transmission device 20 includes a light source 21 for inputting light to the optical communication modulator module 18 and a drive system 22 for driving the modulator module and the light source. The light from the light source 21 is converted into an optical signal by the optical communication modulator module 18,
Light receiving section 25 in receiving device 24 through optical fiber 23
Is detected by According to the optical communication system having this configuration, long-distance non-reproducing optical transmission with excellent reception sensitivity can be easily realized. This is based on the fact that the transmission light modulation waveform in the optical communication optical modulator module 18 used in the optical communication system of the present embodiment is good and a high extinction ratio can be obtained. Similarly, instead of the optical communication modulator module,
The light source module for optical communication can be applied to an optical communication system, in which case the light source 21 can be omitted.

[0026]

As described above, the optical modulator of the present invention has the following features.
The absorption edge wavelengths of the absorption layers in the first and second semiconductor electroabsorption modulators for optically modulating the guided light that is incident on the incident optical waveguide and branched in the branching unit are different from each other, or When the same modulation voltage is applied to the first and second semiconductor electroabsorption modulators, the phase difference between the two light waves propagating through each electroabsorption modulator is π [rad].
Thus, by providing a difference in the absorption edge wavelength of the absorption layer of each of the semiconductor electroabsorption modulator sections, a good optical transmission waveform with a high extinction ratio can be generated. Further, with the optical modulator of the present invention, a light source for optical communication and an optical communication module suitable for long-distance transmission can be configured, and further, an optical communication system using these can be constructed.

[Brief description of the drawings]

FIG. 1 is a perspective view of an optical modulator using an InP-based multiple quantum well structure according to a first embodiment in which the present invention is applied to an optical modulator.

FIG. 2 is an enlarged sectional view taken along line AA and line BB in FIG. 1;

FIG. 3 is a diagram illustrating voltage-quenching characteristics of the optical modulator according to the first embodiment.

FIG. 4 shows the voltage of the electro-absorption modulator of the first embodiment.
FIG. 4 is a diagram illustrating a phase change characteristic.

FIG. 5 is a diagram illustrating voltage dependence on a phase difference between two electroabsorption modulator units according to the first embodiment.

FIG. 6 is a perspective view of a light source for optical communication in which a DFB laser according to a second embodiment in which the present invention is applied to the light source for optical communication is monolithically integrated.

FIG. 7 is an enlarged sectional view taken along line CC of FIG. 6;

FIG. 8 is a schematic diagram of an optical communication module equipped with the optical modulator of the present invention.

FIG. 9 is a schematic diagram of an optical communication system using the optical communication module of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 n-InP substrate 200 output optical waveguide section 300 multiplexing section 400A first electroabsorption modulator section 400B second electroabsorption modulator section 500 branch section 600 input optical waveguide section 110 n-InP buffer layer 113 grating area 120 n-InP clad layer 130 optical waveguide MQW core layer 140 P-InP clad layer 150 P-InP buried layer 313 n-InP clad layer 314 active layer 315 p-InP clad layer 316 p-InP buried layer 318 p-InGaAs contact layer 410 Polyimide film 511 Pad electrode 512 n-side Ti / Au electrode 625 DFB laser part 701 p-side Ti / Au electrode 702 p-InGaAs contact layer 703 p-InP buried layer 710,720 n-InP clad layer 711,721 Multiple quantum well absorption layer 712, 722 p-InP cladding layer 723 Pad electrode 724 Ti / Au electrode 12A, 12B Aspheric lens 13A, 13B Optical fiber 17 Submount 18 Optical communication module 19 Optical modulator 20 Transmitter 21 Light source 22 Drive Circuit 23 Optical fiber 24 Receiver 25 Receiver

Claims (9)

[Claims] 1. An incident optical waveguide that guides incident light on a substrate, a branch portion that distributes guided light guided through the incident optical waveguide to two waveguides, and a branch portion that branches at the branch portion. A semiconductor electroabsorption modulator provided in each of the two waveguides, a multiplexing unit for multiplexing the output lights of the two waveguides, and a multiplexed light from the multiplexing unit. An optical modulator comprising an output optical waveguide and a semiconductor optical absorption modulator provided in the two waveguides, wherein the absorption end wavelengths of the absorption layers of the semiconductor electro-absorption modulators are different from each other. Light modulator. 2. An incident optical waveguide that guides incident light on a substrate, a branch portion that distributes guided light guided through the incident optical waveguide to two waveguides, and a branch portion that branches at the branch portion. A semiconductor electroabsorption modulator provided in each of the two waveguides, a multiplexing unit for multiplexing the output lights of the two waveguides, and a multiplexed light from the multiplexing unit. And an output optical waveguide that forms an output optical waveguide, wherein when the same modulation voltage is applied to each of the semiconductor electro-absorption modulator sections provided in the two waveguides, the electro-absorption modulation is performed. Propagating the body 2
2. The optical modulator according to claim 1, wherein a difference is provided between the absorption edge wavelengths of the absorption layers of the two semiconductor electroabsorption modulators so that the phase difference between the light waves becomes π [rad]. 3. An optical waveguide other than the semiconductor electroabsorption modulator section is formed on a quartz substrate, and an optical waveguide is provided between a branch section and a multiplexing section of the waveguide formed on the quartz substrate. 3. The optical modulator according to claim 1, wherein the semiconductor electro-absorption modulator section is integrated in a hybrid between the optical waveguides. 4. The optical modulator according to claim 1, wherein both the semiconductor electro-absorption modulator and the other optical waveguide are formed of a semiconductor material. 5. The light absorption layer of the semiconductor electro-absorption modulator section and the other optical waveguide are continuously formed without interruption, and the absorption end of the light absorption layer of the semiconductor electro-absorption modulator section is provided. 3. The optical modulator according to claim 1, wherein the optical modulator is formed by a band gap control selective MOVPE method so that the wavelength and the absorption edge wavelength of the other optical waveguides are different. 6. The light absorption layer of the semiconductor electroabsorption modulator section has a multiple quantum well structure.
An optical modulator according to any one of claims 1 to 5. 7. A light-emitting element for making light incident on an incident optical waveguide portion of the optical modulator according to claim 1, wherein the light-emitting element is formed or installed on the same substrate as the optical modulator. Light source for optical communication. 8. A condensing means for optically coupling an input light from the outside to an incident optical waveguide portion of the optical modulator according to claim 1, and from an output optical waveguide of the optical modulator. A light condensing means for optically coupling the output light to an external optical fiber. 9. An optical communication module comprising a light collecting means for optically coupling output light from an output optical waveguide of the optical communication light source according to claim 7 to an external optical fiber.