JPH03291617A - Integrated type optical modulator - Google Patents

Abstract

PURPOSE:To improve the coupling efficiency, to execute the modulation at a high speed and to make the wavelength chirping width small by forming an optical waveguide layer of the same structure and the same composition having light emission and absorption functions of light from a quantum well layer and a barrier layer having an energy gap being higher by 20 - 80meV than a quantum level of the quantum well layer. CONSTITUTION:In a laser area I and an optical modulator area II, an optical waveguide layer 3 of the same structure and the same composition having light emission and absorption functions of light is provided. Also, the optical waveguide layer 3 consists of single or plural quantum well layers 13, and a barrier layer 14 having an energy gap being higher by 20 - 80meV than a quantum level of the quantum well layer 13. Also, in the vicinity of the optical waveguide layer 3 of the laser area I, a diffraction grating 7 is provided, and on the laser area I, and on the optical modulator area II, an electrode 9 for injecting a current, and an electrode 10 for applying an electric field are formed independently, respectively. In such a way, optical coupling of 100% is obtained, and also, it can be formed by single crystal growth, therefore, the manhour is small and easy and the yield is enhanced, and moreover, high speed modulation can be executed, and the wavelength chirping width becomes small.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an integrated type integrated feedback semiconductor laser and optical modulator used in the fields of optical communication, optical measurement, and optical information processing, in which a distributed feedback semiconductor laser and an optical modulator are integrated on the same substrate. This invention relates to optical modulators.

[Conventional technology]

In recent years, long-distance and large-capacity optical fiber communications have been rapidly increasing. When performing high-speed, long-distance transmission in the several Gb/s band using optical fiber communication, if a change in wavelength due to light intensity modulation, so-called wavelength chirping, occurs on the light source side, the wavelength dispersion of the optical fiber causes a change in the wavelength after transmission. distortion of the optical waveform occurs, resulting in transmission errors. Therefore G b /s
In order to realize high-speed, long-distance optical fiber communication,
A signal light source with small wavelength chirping during modulation is strongly required.

Distributed feedback semiconductor laser (hereinafter referred to as DF) is used as such a light source.
An integrated optical modulator that integrates an optical modulator (referred to as a BLD) and an optical modulator is being researched. An electroabsorption optical modulator is an example of a light modulator capable of high-speed modulation. Since the modulation band is determined by the size of the parasitic capacitance, the speed can be increased by reducing it.

As an example of this, a device that integrates a DFBLD and an electroabsorption optical modulator is the Institute of Electronics, Information and Communication Engineers, Photon and Quantum Electronics Study Group, Technical Research Report, Vol. 89, No. 87, 0QE89-30 (
June 1989>. 5Gb/
It is possible to perform high-speed modulation of s, and the wavelength chirping width has been suppressed to 0.16, which is less than 1/10 of that of conventional methods. In this integrated optical modulator, a direct current is injected into the DFBLD to operate at a constant output, and the output light is intensity-modulated by an electro-absorption optical modulator. Here, when an electric field is applied to the absorption layer of the optical modulator, the increase in absorption of light with energy smaller than the bandgap of the absorption layer (Franz-Kjeldetsch effect) is utilized.Therefore, the bandgap of the absorption layer is the same as that of the DFBLD. It is larger than the band gap of the active layer.This band gap difference ΔEg is 33 to 86 me in conventional examples including the above-mentioned document.
It is V. This value is determined in consideration of the fact that when ΔEg is large, the operating voltage required for modulation becomes high, and when ΔEg is small, the insertion loss of the optical modulator becomes large.

In the conventional example, the active layer in the laser region where the DFBLD is formed and the absorption layer in the optical modulator region where the optical modulator is formed have different compositions (or energy band gaps), so there is a refraction gap between the two regions. The optical coupling efficiency was low at 50 to 60%, and about 1/3 of the light was scattered light.For this reason, the fiber coupling input was low and the scattered light was large, so fiber and integrated type This was an obstacle when coupling the optical modulator.Also, since the active layer in the laser region and the absorption layer in the optical modulator region have different compositions, it is difficult to connect them laterally without any step by selective growth. This is difficult, requires a large amount of man-hours in device manufacturing, and reduces yield.

[Problem to be solved by the invention]

An object of the present invention is to provide an excellent integrated optical modulator that is easy to manufacture, has a high yield, has good coupling efficiency between the laser region and the optical modulator region, is capable of high-speed modulation, and has a small wavelength chirping width. It is in.

[Means to solve the problem]

The integrated optical modulator of the present invention includes a laser region and an optical modulator region integrated on a semiconductor substrate, and the laser region and the optical modulator region have the same structure and the same composition and have light emission and absorption functions. The optical waveguide layer is composed of a single or multiple quantum well layers and a barrier layer having an energy gap higher than the quantum level of the quantum well layer by 20 to 80 m e V, A diffraction grating is provided near the optical waveguide layer in the laser region, an electrode for current injection is formed above the laser region, and an electrode for applying an electric field is independently formed above the optical modulator region. It is characterized by

[Effect]

In this integrated optical modulator, the laser region and the optical modulator region have the same composition and structure, and a continuous waveguide layer is used from the laser region to the optical modulator region, so the boundary between the laser region and the optical modulator region is 100% optical coupling is obtained with no scattered light at the end. In addition, since it can be formed by two times of crystal growth, it is easy and has a high yield with a small number of man-hours. Light emission in the laser region is performed by quantum wells, and the oscillation wavelength is approximately determined by the pitch of the diffraction grating, so the absorption coefficient of the quantum well at the oscillation wavelength is relatively small, and the proportion of the quantum well in the optical waveguide layer That is, since the optical confinement coefficient in the quantum well layer can be controlled relatively freely, the optical waveguide used in the present invention can be used as a low-loss optical waveguide that sufficiently suppresses absorption in the quantum well layer. . Furthermore, since the barrier layer in the optical waveguide of the present invention has a bandgap that is several + meV larger than the oscillation wavelength of the laser, it functions as a very transparent optical waveguide when no electric field is applied to the optical modulator region. However, when an electric field is applied, Franz
Due to the Kjelditssch effect, light from the laser region is absorbed. Furthermore, since it is an electro-absorption type optical modulator, high-speed modulation is possible and the wavelength chirping width can be reduced. In this way, a high-yield, low-loss, high-speed, low-chirp integrated optical modulator can be obtained.

〔Example〕

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

FIGS. 1(a), (b), and (c) show an embodiment of an integrated optical modulator according to the present invention. FIG. 1(a) is a perspective view thereof, FIG. 1(b) is a sectional view taken along line AA' along the optical axis along which light propagates, and FIG. 1(c) is an enlarged view of the optical waveguide layer. First, the manufacturing process will be described.

First, a diffraction grating 7 is formed partially on the n-InP substrate 1 (the part that will become the laser region 2) by interference exposure method and chemical etching method. The period of the diffraction grating 7 is equal to the oscillation wavelength of 1.
The number of people was 2420 so that the thickness was 55 μm. Thereafter, a 0.2 μm thick n-InGaAsP cladding layer 2 with a wavelength composition of 1.20 μm, a 0.3 μm optical waveguide layer 3 having light emission and absorption functions, and a 0.1 μm p-InP cladding layer 4 are then applied to the entire surface.
A p-InGaAs cap layer 5 having a thickness of 0.5 μm is sequentially formed by MOVPE. The optical waveguide layer 3 includes a 1-InGaAs quantum well layer 13, as shown in FIG. 1(C).
and i-I n G a A with a wavelength composition of 1.45 μm.
It consists of 1.4 sP barrier layers, and here the structure is such that two quantum well layers of 70 layers are sandwiched between barrier layers of 950 layers. Next, after forming a mesa stripe 8 including the optical waveguide layer 3 in a direction perpendicular to the diffraction grating 7 by a chemical etching method,
A Fe-doped high-resistance In2 layer 6 is grown using the MOVPE method except for the upper part of the mesa stripe 8. The width of the mesa stripe 8 in the optical waveguide layer 3 is 1.5 to 2.0 μm, and the height is 2 μm. Separated electrodes 9.10 are formed on the surfaces of the laser region I and the optical modulator region (2), and electrodes 11 are formed on the substrate 1 side, and in order to obtain electrical insulation between the electrodes 9.10, A groove having a depth up to the middle of the p-InP cladding layer 4 is provided between them. The length between the electrodes 9 and 10 is 50 μm, and the electrical resistance therebetween is several MΩ or more. By cleavage, the laser region length is 300 μm and the optical modulator region length is 150 μm.
After cutting out to a size of μm, a non-reflection coating film 12 is applied to the end face on the side of the optical modulator region. In this way, an integrated optical modulator is completed.

Next, the operation of the integrated optical modulator of this example will be explained. In this embodiment, laser oscillation can be obtained by injecting current into the laser region through electrode 9. At this time, the wavelength is 1.55 from the period of the diffraction grating and the thickness of the quantum well.
A single spectral oscillation in μm was obtained. The oscillation threshold current was about 30 mA. The light oscillated in the laser region travels through the optical waveguide layer and is guided directly to the optical modulator region. In the present invention, since the laser region and the optical modulator region are optically connected by the same optical waveguide layer 3, the coupling efficiency here is 100%.

Next, the operation in the optical modulator area will be explained. Second
Figures (a) and (b) show the band structure in the optical modulator region in order to explain the modulation operation in the optical modulator region. ) shows the case when an electric field is applied. The light with a wavelength of 1.55 μm guided to the optical modulator region passes through the optical waveguide layer 3 and is extracted as an optical output 15 when no reverse bias is applied to the electrode 10. At this time, as shown in the band structure of FIG. 2(a), the absorption loss in the optical modulator region for light of 1.55 μm is mainly due to the quantum well layer 13 in the optical waveguide layer 3. The influence of the barrier layer 14 on absorption loss is very small because its wavelength composition is 1.45 μm and the energy difference is about 55 meV. Further, in the case of a quantum well laser, the absorption coefficient in the quantum well at the oscillation wavelength is generally 200 to 300c+n-', but in the present invention, the quantum well layer 1 in the optical waveguide layer 3
3, that is, the confinement coefficient of guided light in the quantum well is 4.
%, the absorption coefficient as a waveguide is 30 to 40 dB/cm, and considering the optical modulator region length of 150 μm, the effect of absorption loss due to the quantum well layer 13 is about 0.5 dB. It turns out it's small. Therefore, the optical output 1 when no reverse bias is applied to the electrode 10
5, mW order can be obtained.

Next, consider a case where a reverse bias is applied to the electrode 10.

The band structure at this time is shown in FIG. 2(b). In this case, since an electric field is applied to the entire optical waveguide layer 3, the potential of the barrier layer 14 is also deformed. As a result, light with a wavelength of 1.55 μm is absorbed due to the Franz Kjelde90 Issis effect in the barrier layer. Here, since the difference from the bandgap energy of the barrier layer is about 55 m e V, an increase in absorption coefficient of 500 cm -' or more can be obtained with an electric field strength of 1×10 5 V/σ. Therefore, by applying a voltage of 3 V to the electrode 10, an extinction ratio of 20 dB or more can be obtained.

As explained above, in this example, the coupling efficiency between the laser region and the optical modulator region is 100%, and the optical output from the end face of the optical modulator region is approximately several mW, and the operating voltage is 3 V or less and the optical power is 20 dB or more. An integrated optical modulator that can obtain intensity modulation with an extinction ratio can be realized, and since it uses the Franz-Kjelditssch effect, it has low chirping and high-speed modulation characteristics, and it is also possible to perform good modulation at 2.4 Gb/s. It is.

The embodiment shown here is an example, and the thickness of the quantum well layer, the number of calendars, the composition of the barrier layer, the thickness of each layer, etc. are not limited to the embodiments, as long as similar effects can be obtained. Regarding materials, not only I n Ga A s P type but also I
nGaAlAs or the like may also be used.

〔Effect of the invention〕

As explained in detail above, in the integrated optical modulator according to the present invention, since the laser region and the optical modulator region have continuous optical waveguide layers with the same structure, a high coupling efficiency of 100% can be obtained, and Since selective growth is not required in the process, there are advantages in that the number of man-hours for manufacturing the device is small and the yield is high. Further, the optical waveguide layer used in the present invention has a small absorption loss, so a high optical output can be obtained, and furthermore, a high extinction ratio can be obtained at a low voltage due to the Franz-Kjelditssch effect.

Furthermore, since this device exhibits almost no chirping during modulation, it is ideal as a light source for long-distance ultra-high capacity optical fiber communications in the Gb/s band.

[Brief explanation of drawings]

FIGS. 1(a), (b), and (c) are diagrams showing an embodiment of the integrated optical modulator of the present invention, in which (a) is a perspective view and (b) is a sectional view taken along line AA'. , (C) is an enlarged cross-sectional view of the optical waveguide layer. Figure 2(a). (b) shows the band structure in the optical modulator region in order to explain the modulation operation of the device of the present invention, and (a)
(b) is a diagram when no electric field is applied, and (b) is a diagram when an electric field is applied. In the figure, 1 is an n-InP substrate, 2 is an n-InGaAs
P cladding layer, 3 optical waveguide layer, 4 p-InP cladding layer, 5 1) InGaAs cap layer, 6 high resistance InP layer, 7 diffraction grating, 8 mesa stripe, 9,1
0.11 is an electrode, 12 is a non-reflective coating film, 13 is an i-I nGaAs quantum well layer, and 14 is an i-I n
G a A s P barrier layer, 15 is the optical output.

Claims (1)

[Claims] It consists of a laser region and an optical modulator region integrated on a semiconductor substrate, and the laser region and the optical modulator region have an optical waveguide layer having the same structure and composition and having a light emission and absorption function, The optical waveguide layer includes a single or multiple quantum well layers and a barrier layer having an energy gap 20 to 80 meV higher than the quantum level of the quantum well layer, and a diffraction grating is provided near the optical waveguide layer in the laser region. An integrated optical modulator, characterized in that an electrode for current injection is formed above the laser region, and an electrode for applying an electric field is formed above the optical modulator region, respectively.