JPH04198913A - Electric field absorption type semiconductor modulator - Google Patents

Abstract

PURPOSE:To prevent the faults, such as deterioration in the characteristics of the optical modulator by a fluctuation in refractive index and absorptivity, fluctuation in the oscillation wavelength of a DFB laser, decrease in output, and deterioration of single wavelength oscillation, by increasing the substantial thickness of a light absorption layer gradually from an input side toward an output side, by which the absorption quantity of light is nearly uniformized over the entire part of the light absorption layer. CONSTITUTION:A diffraction grating 9 which is the same in the width and period of peaks but is made gradually shallower in the depth thereof from the input side toward the output side is formed between an InP substrate 1 and GaInAsP light guide layer 3 of the optical modulator. The equiv. thickness of the light guide layer 3 decreases as well. Then, the equiv. thickness of the light absorption layer is gradually increased from the input side toward the output side and the confinement of the light in the light absorption layer 10 on the input side is lessened and the absorptivity of the light is increased. The absorption quantity of the light is thereby nearly uniformized over the entire part of the light absorption layer 10, by which the fluctuation in the refractive index of the optical modulator and the fluctuation in the absorptivity are prevented and the faults, such as fluctuation in the oscillation wavelength of the DFB laser, decreases in the output and deterioration of the single wavelength oscillation, are eliminated.

Description

[Detailed Description of the Invention] [Summary] Regarding an electroabsorption semiconductor optical modulator, it is possible to prevent fluctuations in the refractive index and absorption rate of the optical modulator, and to improve the DFB.
In order to eliminate obstacles such as fluctuations in laser oscillation wavelength, decrease in output, and deterioration of single wavelength oscillation, the thickness of the equivalent absorption layer is made to gradually increase from the input side to the output side. In this way, the modulation effect is made almost uniform throughout the absorption layer.

In this case, a diffraction grating is used between the substrate and the light guide layer, in which the width and period of the peaks are constant and the depth of the peaks becomes gradually shallower from the input side to the output side, or the width and depth of the peaks are By forming a diffraction grating whose period is constant and whose period gradually becomes smaller from the input side to the output side, the thickness of the equivalent absorption layer becomes gradually thicker from the input side to the output side. You can do it like this.

[Industrial application field]

The present invention relates to an electroabsorption semiconductor optical modulator.

In recent years, optical fiber communication with large transmission capacity has come into widespread use, and in this technical field C2, the current supplied to the semiconductor laser device is determined by (1). Direct intensity modulation that turns on and off at a cycle of 6 gigahinotes/second is in practical use.

However, this direct intensity modulation has the disadvantage of causing fluctuations in the oscillation wavelength of the laser beam as the modulation frequency increases. An external modulation method is adopted in which the output light is oscillated with the output of the optical fiber, and the output light is turned on/off to be modulated by an electro-absorption semiconductor modulator O.

1. Prior Art; FIG. 5 is a schematic diagram of a device in which a conventional electro-absorption semiconductor modulator and a semiconductor laser as a light source are integrated.

In this figure, 1 is an InP substrate, 2 is a diffraction grating, and 3 is a diffraction grating.
4 is a light guide layer, 4 is an active layer, 5 is a crat layer, 6 is an ohmic contact layer, 7, 8, 12 is an electrode, 10 is a light absorption layer, and 11 is a non-reflection coat.

This common layer is made of Ga on the lnP substrate which becomes the crat layer.
1nAsP optical guide layer 3, Ga1nAsP active layer 4, I
An nPn contact layer 5, a Ga1nAsP ohmic contact layer 6 are formed, an optical diffraction grating 2 is formed between the substrate 1 and the optical guide layer 3, and electrodes 7 and 8 are formed. laser) and GaInAs on InP substrate I.
P optical kite layer 3, GaInAsP optical absorption layer 10, InP
A Ga1nAsP atomic contact layer 6 is formed between the substrate 1 and the optical fiber layer 3, a diffraction grating 9 having a period different from the diffraction grating 2 of the DFB laser is formed, and electrodes 7 and 12 are formed. It is constructed by integrating an optical modulator formed with a.

In this device, the laser light generated by the DFB laser is transmitted to the right through the light absorption layer 10 and the optical kite layer 3 of the optical modulator, and passes through the anti-reflection coating 11 consisting of iW L, S + N y, S + ○. However, since the absorption coefficient of the absorption layer 10 is controlled by the modulation signal voltage applied between the electrodes 7 and 12 of the modulator, the intensity of the output light is changed by 2fN. be done.

However, in this conventional electro-absorption semiconductor optical modulator, in the process of absorbing light from the input side to the output side of the light absorption layer, near the input side, Since the absorbed light intensity is overwhelmingly greater than the light intensity absorbed near the output side, a large amount of heat is generated on the input side.

FIG. 6 is an explanatory diagram of the light absorption process of a conventional electro-absorption semiconductor optical modulator.

The symbols in this figure are: 3-1 is the optical fiber layer near the input side;
3-2 is the light guide layer near the output side, Ll is the light intensity distribution near the input side, and L2 is the light intensity distribution near the output side. It is the same.

As shown in this figure, since the thickness of the waveguide is the same near the input side and near the output side, there is no change in the position of the peak of the light intensity distribution in the thickness direction on both sides, and the absolute value of the light intensity is Since the light is gradually absorbed and attenuated by the light absorption layer IO, the intensity of light propagating through the light absorption layer 10 gradually decreases from the input side to the output side as shown by the area of the shaded part in the drawing.

Therefore, there is a large imbalance in the heat generated by the absorption of this light in the direction in which the light travels, causing fluctuations in the refractive index and absorption rate, which not only deteriorate the modulation characteristics but also In a device that integrates a laser and a modulator, heat is conducted to the DFB laser side, creating areas with uneven refractive index, etc. in the DFB laser, resulting in fluctuations in the oscillation wavelength, decrease in output, and Failures such as deterioration of single wavelength oscillation may occur.

In particular, variations in the propagation constant make the moat unstable,
Laser light that should have a single wavelength becomes multiple wavelengths, or wavelength fluctuations occur, which seriously impedes wavelength-multiplexed optical communications.

In view of the drawbacks of the conventional optical modulator, the present invention aims to improve the refraction of the optical modulator by equalizing the absorption of light in the optical absorption layer 10 of the electro-absorption semiconductor optical modulator in the direction in which the light travels. The purpose of this invention is to prevent fluctuations in the rate and absorption rate, and eliminate obstacles such as fluctuations in the oscillation wavelength of the DFB laser, reduction in output, and deterioration of single wavelength oscillation.

(Means for Solving the Problems) In the electro-absorption semiconductor optical modulator C according to the present invention, the thickness of the equivalent absorption layer is gradually increased from the input side to the output side. , the modulation effect was made almost uniform throughout the absorption layer.

In this case, there is a diffraction grating between the substrate and the optical power layer, in which the width and period of the peaks are constant and the depth of the peaks gradually becomes shallower from the input side to the output side, or By forming a diffraction grating that has a constant depth and a period that gradually decreases from the input side to the output side, the equivalent absorption layer thickness can be gradually increased from the input side to the output side. You can make it happen.

1 Effect: In the electro-absorption semiconductor optical modulator S, if the thickness of the equivalent absorption layer is gradually increased from the input side to the output side, the absolute value of the transmitted light is large on the input side. Light confinement in the light absorption layer becomes smaller, the light absorption rate decreases, and light confinement increases on the output side where it is absorbed in the light absorption layer on the input side and the absolute value of transmitted light decreases. As a result, the light absorption rate increases, so that the amount of light absorbed is substantially uniform throughout the light absorption layer.

In order to reduce light confinement on the input side and increase it on the output side, the thickness of the absorption layer can be made gradient or stepwise, gradually increasing from the input side to the output side. However, it is not easy to form such a structure by crystal growth.

To facilitate manufacturing, by forming a diffraction grating in the light absorption layer and changing the width, depth, or period of the peaks, the equivalent thickness of the absorption layer can be oriented from the input side to the output side. It is conceivable to gradually increase the thickness.

However, if a diffraction grating is directly imprinted on the light absorption layer, defects will occur in the crystallinity of the light absorption layer and the absorption characteristics will deteriorate. It is preferable to form a diffraction grating between the light guide layer and the outer and outer layers.

:Example] Embodiments of the present invention will be described below with reference to the drawings.

(1) First Embodiment FIGS. 1(a) and 1(b) are schematic configuration diagrams of a first embodiment of the present invention.

The symbols in this figure are the 5th one, except that 9 is the diffraction grating.
They correspond to those described with the same reference numerals in the figures.

When the optical modulator of this embodiment is integrated with a semiconductor laser as a light source, as shown in FIG. 1(a), an InP substrate 1
A GaInAsP optical guide layer 3, a GaInAsP active layer 4, an InP cladding layer 5, and a GaInAsP ohmic contact layer 6 are formed thereon, a diffraction grating 2 is formed between the substrate 1 and the optical guide layer 3, and electrodes 7 and 8 are formed. The formed DFB laser and the Ga1nAsP light guide layer 3 are formed on the InP substrate 1.
, Ga1nAsP light absorption layer 10,! nP cladding layer 5,
A Ga1nAsP omic contact layer 6 is formed, a diffraction grating 9 different from the diffraction grating 2 of the DFB laser is formed between the substrate 1 and the optical guide layer 3, and an optical modulator in which electrodes 7 and 12 are formed is integrated. It consists of

In this device, the laser light generated by the DFB laser is transmitted to the light absorption layer 10 of the optical modulator and the light guide layer 3.
The light propagates to the right through the inside, passes through the anti-reflection coating 11 made of SiN, 510X, and is emitted from the electrodes 7 and 12.
Similarly, the intensity is modulated by the modulation signal voltage applied during this period.

However, in this embodiment, the InP substrate 1 of the optical modulator
A diffraction grating 9 is formed between the Ga1nAsP light guide layer 3 and the diffraction grating 9, which has the same peak width and period, but whose depth gradually becomes shallower from the input side to the output side. A feature is that the equivalent thickness of layer 3 is reduced.

This diffraction grating 9 has a structure adopted to adjust the thickness of the equivalent light absorption layer, and does not use the original function of diffracting light, but on the contrary, it prevents human-powered light from being diffracted. Because of this necessity, it is necessary to use a diffraction grating with a period different from that of the DFB laser.

Regarding the equipment currently in use, DFB
When the oscillation wavelength of the laser is 1.55 μm, the period of the diffraction grating is 240 nm, so avoid using integral multiples of that, and use 300 to 430, which is about 1.2 to 1.8 times that.
It is desirable to use a diffraction grating with a period on the order of nm.

Note that the thickness of the light absorption layer is approximately 0.15 to 0.30 μm.

FIG. 1(b) shows a case where the optical modulator of the present invention is formed alone.

In this example, both the light entrance and exit surfaces are Q (non-reflective).
11 is formed.

FIG. 2 is an explanatory diagram of the light absorption process of the electro-absorption semiconductor optical modulator according to the first embodiment of the present invention.

Symbols used in this figure: 9-1 is the diffraction grating near the input side, 9-2 is the diffraction grating near the output side. handle.

As shown in this figure, the width and period of the peaks of the diffraction grating 9-1 near the input side and the diffraction grating 9-2 near the output side are the same, but their depth is It gradually becomes smaller along the direction Q, and the average, that is, the light guide layer 3
The effective thickness of C2 also decreases as shown by the dashed line.

Examples of the refractive index of each layer constituting this electro-absorption semiconductor optical modulator are 3°42 for the light absorption layer IO, 3.3 for the optical guide layer 3, and 3°17 for the InP substrate 1. Since the thickness of the light guide layer 3, which has a larger refractive index than the InP substrate 1, gradually decreases along the direction of light propagation, the position of the peak of the light intensity distribution gradually shifts to the center of the light absorption layer 10. ,
The amount of light absorbed in the light absorption layer 10 is approximately uniform from the input side to the output side, as shown by the area of the shaded area in the drawing.

Therefore, the heat generated by absorption of light is balanced,
Problems that occur in conventional optical modulators, such as deterioration of optical modulator characteristics due to fluctuations in refractive index and absorption rate, fluctuations in oscillation wavelength of DFB laser, decrease in output, and deterioration of single wavelength oscillation. does not produce

In the present embodiment, the explanation has been made regarding the case where the optical power layer or the light absorbing layer is located below the light absorbing layer, but the same effect can be achieved even if the light absorbing layer is located above the light absorbing layer.

(2) Second embodiment FIG. 3 is a schematic configuration diagram of a second embodiment of the present invention.

The reference numerals in this figure correspond to those described with the same reference numerals in FIG.

The optical modulator of this example has Ga on an In, P substrate 1.
1nAsP optical guide layer 3, Ga1nAsP active layer 4, I
A DFB laser including an nP ground layer 5, a Ga1nAsP atomic contact layer 6, a diffraction grating 2 between the substrate 1 and the optical guide layer 3, and electrodes 7 and 8;
GaInAsP light guide layer 3 on nP substrate 1, Ga1n
AsP light absorption layer 10. InP crat layer 5, Ga1nA
An optical modulator in which an sP optical contact layer 6 is formed, a diffraction grating 9 which is different from the diffraction grating 2 of the DFB laser is formed between the substrate l and the optical power layer 3, and electrodes 7 and 12 are formed. This is the same as the optical modulator of the first embodiment shown in FIG.

In this device, the laser light generated by the DFB laser is transmitted to the light absorption layer 10 of the optical modulator and the light guide layer 3.
The light propagates to the right through the inside, passes through the anti-reflection coating 11 made of 5iNX and 5IOX, and is emitted from the electrodes 7 and 12.
Similarly, the intensity is modulated by the modulation signal voltage applied during this period.

However, in this embodiment B, the width and depth of the peaks of the diffraction grating 9 formed between the InP substrate 1 and the Ga InAsP light guide layer 3 of the optical modulator are the same, but from the input side It is characterized in that the period becomes gradually smaller toward the output side, so that the equivalent thickness of the light guide layer 3 decreases.

Also in this embodiment, in order to prevent input light from being diffracted by the diffraction grating of the optical modulator, it is desirable to use a diffraction grating having a period different from that of the diffraction grating of the DFB laser.

Regarding the equipment currently used, when the oscillation wavelength of the DFB laser is 1.55 μm, the period of the diffraction grating 2 is 240 nm, or 3 times 1.2 to 1.8.
It is desirable to use a diffraction grating with a period varying within the range of 0.000 to 4300 m.

However, even if a part of the period becomes an integral multiple of the period of the diffraction grating of the DFB laser, there will be no particular adverse effect on the overall operation since only that one point will match.

FIG. 4 is an explanatory diagram of the light absorption process of the electro-absorption optical modulator according to the second embodiment of the present invention.

The numbers used in this figure correspond to those explained in FIG.

As shown in this figure, the diffraction grating 9 near the input side
-1 and the period of the diffraction grating 9-2 near the output side gradually becomes smaller in the light traveling direction S, and the average, that is, the equivalent thickness of the light guide layer 3,
It gradually becomes thinner.

Since an example of the refractive index of each layer constituting this electro-absorption semiconductor optical modulator is as described for the first embodiment, the thickness of the optical guide layer 3 having a larger refractive index than that of the InP substrate 1 is The current distribution gradually decreases along the traveling direction of the light, and the peak position of the current distribution gradually shifts to the center of the light absorption layer 10, and the amount of light absorbed in the light absorption layer 10 is indicated by the area of the shaded part in the drawing. It is almost uniform from the input side to the output side so that the

Therefore, as in the first embodiment, the heat generated by absorption of light is balanced, and the problems that occur in conventional optical modulators do not occur.

(Effects of the Invention) As described above, according to the present invention, the effective thickness of the light absorption layer gradually increases from the input side to the output side. The absorption rate of light in the light absorption layer decreases, and the absolute value of the transmitted light decreases as it is absorbed in the light absorption layer on the input side.The light absorption rate increases on the output side, so the amount of light absorbed by the light absorption layer decreases. The overall S is almost uniform, the heat generated by light absorption is balanced, the characteristics of the optical modulator are deteriorated due to fluctuations in the refractive index and absorption rate, the DFB Rayleigh oscillation wavelength is fluctuated, the output is decreased, and the Does not cause problems such as deterioration of wavelength oscillation.

[Brief explanation of the drawing]

1(a) and 1(b) are schematic configuration diagrams of the first embodiment of the present invention, FIG. 2 is an explanatory diagram of the light absorption process of the electroabsorption optical modulator of the first embodiment of the present invention, Figure 2 is a schematic diagram of the second embodiment of the present invention, Figure 4 is an explanatory diagram of the light absorption process of the electro-absorption optical modulator of the second embodiment of the present invention, and Figure 5 is the conventional electro-absorption modulation. FIG. 6 is a schematic diagram of a device in which a device and a semiconductor laser as a light source are integrated. FIG. 6 is an explanatory diagram of the light absorption process of a conventional electroabsorption optical modulator. 1-1 nP substrate, 2-diffraction grating, 3-light guide layer,
4-active layer, 5-cladding layer, 6-ohmic contact layer, 7.8.12-electrode, 9-diffraction grating, 9-1--
Diffraction grating near input side, 9-2 Diffraction grating near output side, 10 light absorption layer, 11 - non-reflection coat, Ll light intensity distribution near input side, L2 - light intensity distribution near output side Patent applicant Fujitsu Limited Patent Attorney Akira Aitani Attorney Patent Attorney Hiroshi Watanabe - 12 electrodes Figure 1 Figure 2 DFB Ray Ray Optical Modulator Schematic diagram of the second embodiment of the present invention Figure 4

Claims (3)

[Claims] (1) An electro-absorption semiconductor optical modulator in which the thickness of the equivalent absorption layer is gradually increased from the input side to the output side to make the modulation effect substantially uniform over the entire absorption layer. (2) By forming a diffraction grating between the substrate and the light guide layer, the width and period of the peaks are constant, and the depth of the peaks gradually becomes shallower from the input side to the output side. An electro-absorption semiconductor optical modulator with a nearly uniform effect over the entire absorption layer. (3) By forming a diffraction grating between the substrate and the light guide layer, the width and depth of the peaks are constant, and the period gradually decreases from the input side to the output side, thereby improving the modulation effect. An electro-absorption semiconductor optical modulator with a nearly uniform absorption layer throughout.