JPH01217416A - Optical modulating element - Google Patents

Abstract

PURPOSE:To attain rapid modulation at a low voltage even when the intensity of incident light is >=0.1mW by setting up a difference between incident photon energy and modulation waveguide forbidden band width energy to >=50meV to suppress the deterioration of a modulation voltage and band width due to the increment of incident light intensity and setting up the length of the title element also to a previously fixed length. CONSTITUTION:When incident intensity of incident light into an optical waveguide is >=0.1mW, the forbidden band width energy of the optical waveguide layer is >=50meV and the ratio of energy difference between forbidden band width energy and incident light energy to the length of optical modulating element determined by the length from the incident end face of the optical waveguide up to its projection end face is 10meV/mm-250meV/mm. Consequently, the optical modulating element capable of executing low voltage and rapid modulation almost without generating the increment of the modulation voltage or the reduction of the band width even when the incident light intensity is increased from 0.1mW practical level to several mW or more can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a light modulation element that modulates light incident from the outside.

(Conventional technology and its problems) Optical fiber communication technology has progressed by taking advantage of the ultra-low loss properties of optical fibers and the ultra-broadband properties that light inherently has, and has enabled transmission of increasingly longer distances and larger capacities. Research is underway worldwide. Nowadays, the loss of optical fibers has reached its theoretical limit, and research into increasing transmission speed and capacity has become particularly important.

Currently, a method of directly modulating a semiconductor laser is generally used as a technique for rapidly turning on and off optical signals. However, in the direct modulation method, since the current of the semiconductor laser, which is the oscillation element, changes rapidly, the oscillation wavelength fluctuates greatly over time, and as a result, the oscillation spectral width becomes abnormally large compared to the spectral width of the modulation band. It will spread. Therefore, in long-distance or high-speed transmission, good transmission characteristics cannot be obtained because the received optical pulses are distorted due to the large influence of wavelength dispersion of the optical fiber. Therefore, in order to avoid such problems, a method has been recently considered in which the output of the semiconductor laser is held constant and high-speed modulation is performed using an external optical modulation element.

As optical modulators, optical modulators using ferroelectric materials such as LiNbO3 and optical modulators that can be monolithically integrated with single wavelength semiconductor lasers such as DFB lasers have been proposed. Electro-absorption optical modulators, which apply an electric field to a wave path and modulate the intensity by electro-absorption effects, are considered the most promising.

FIG. 1 is a perspective view of a conventional electroabsorption light modulation element. On an n-type InP substrate 1, an n--InGaAsP modulation waveguide layer 2, a mesa-shaped P-type 1nP cladding layer 3, and a p-type InP substrate 1 are formed.
A type 1nGaAsP cap layer 4 is laminated, and a p-type electrode 5 and an n-type electrode 6 are each made of p-type InGa.
It is formed so as to be in contact with the AsP camp layer 4 and the n-type InP substrate 1. In this optical modulation element, light is incident on the InGaAsP modulation waveguide layer 2, and the negative voltage applied to the p-type electrode 5 and the positive voltage applied to the n-type electrode 6 are changed, and the absorption coefficient of the InGaAsP modulation waveguide layer 2 is changed. By changing , the intensity of the emitted light can be modulated. In an electroabsorption modulation element, it is important to be able to perform modulation at a low voltage, to be able to perform high-speed modulation, and to have small spectral broadening during high-speed modulation. Until now, InGaA
The closer the incident light photoenergy hv is to the forbidden band energy E of the sP modulation waveguide layer 2, the larger the change in absorption coefficient can be obtained at low voltage. It was believed that this would enable high-speed modulation and suppress spectral broadening. Therefore, conventionally, it was thought that a high-performance optical modulation element could be realized by focusing only on the energy difference ΔE, (E, -hν) between the two and setting the energy difference ΔE9 to 30 to 40 meV. However, in conventional optical modulation elements, when the incident light intensity is about 100 μm or less, the modulation voltage decreases.

Although both the modulation bandwidth and the spectral width exhibit good characteristics, it has become clear that as the intensity of the incident light increases beyond 0°11, the modulation voltage increases significantly and the bandwidth also decreases.

As mentioned above, in conventional electroabsorption optical modulators,
Low voltage modulation when the incident light intensity is low.

Although high-speed operation and narrow-spectrum operation are possible, when the incident light intensity is increased to a practical level of several mW, these characteristics deteriorate significantly.

(Object of the Invention and Features @) The present invention was made to solve the problems of the prior art described above. The purpose is to realize the device.

The present invention is characterized by setting the difference between the incident light photon energy and the modulation waveguide forbidden bandwidth energy to 50 meV or more to suppress deterioration of the modulation voltage and bandwidth due to an increase in the incident light intensity, and by predetermining the element length. By making the length
This is due to lowering the modulation voltage.

(Structure and operation of the invention) The present invention will be explained in detail below using the drawings.

(Example 1) FIG. 2 is a first example according to the present invention, and is a sectional view of a light modulation element.

The difference from the conventional structure is that an energy difference ΔE, (=E9-hν) is provided so that the forbidden band energy E9 of the n--InGaAsP optical waveguide layer 10 is 50 meV or more higher than the photon energy hν of the incident light, and Element length of modulation element. is 10 meV/mm≦ΔE, /L.

The length is set to satisfy ≦250 meV/mm.

Below, problems arising from the difference in the energy difference ΔE9 will be explained using FIGS. 3 and 4, which are experimental results of the present invention, and the element length L0 will be explained using FIG. 5 as well. In addition, in FIGS. 3 to 5, in order to clarify the difference between the present invention and the conventional method, the energy difference ΔE9 is 55 me
■ and ■ are attached to the characteristic diagram of the present invention at V and 50 meV,
Characteristic diagrams of conventional configurations with energy difference ΔEg of 40 meV and 30 meV will be described with ■ and ■.

First, the problems caused by the difference in energy ΔE9 will be explained in detail.

The present inventors discovered that the energy difference ΔE, (=E
9-hν) on the characteristics of the light modulation element when the incident light intensity is increased, using the light modulation element of the conventional configuration (Figure 1) and the configuration of the present invention (Figure 2). did. As a result, as in the conventional example described above, ΔE, = 30~
The modulation voltage and modulation bandwidth when set to 40 meV show good characteristics when the incident light intensity is about 0.1 mW or less, but as the incident light intensity increases to several mW, the modulation voltage decreases to about 2. It has been found that the modulation bandwidth increases to about twice as much, and the modulation bandwidth significantly deteriorates.

Figure 3 shows the experimental results according to the present invention, with an element length of 520 μm.
FIG. 7 is a characteristic diagram of the incident light intensity and the 20 dB extinction ratio voltage, showing the dependence of the voltage value giving the 20 dB extinction ratio of the element of m on the incident light intensity. The parameter ΔE9 (curves ■ to ■) in the figure is I
The energy difference between the forbidden band energy E9 of the nGaAsP optical modulation waveguide layers 10 and 2 and the incident light photon energy hv is E, (E, -hv). In the case of the conventional configuration of 8E, = 30 (curve ■), 40 meV (curve ■),
The 20 dB extinction ratio voltage increased as the incident light intensity increased, and when the incident light intensity reached a certain level of 3 to 4 mW, the increase in modulation voltage tended to be saturated. On the other hand, eight E q =
In the case of the present invention at 50 meV (curve ■) and 55 meV (curve ■), the increase in modulation voltage was almost linear.

The linear increase in modulation voltage is due to the fact that the voltage applied to the light modulation element effectively decreases due to the voltage drop across the ohmic resistance due to the light absorption current, and this is due to the essential characteristic of absorption type light modulation elements. It is a characteristic. On the other hand, the rapid increase in modulation voltage observed when the conventional 8E9 is 30 to 40 meV is due to excess absorbed carriers that distort the band structure and weaken the effective electric field applied to the optical waveguide. This is thought to be due to the fact that

FIG. 4 shows experimental results for explaining the present invention in detail, and is a characteristic diagram of incident high intensity and band deterioration, showing the dependence of deterioration of frequency characteristics at high frequencies on incident light intensity using ΔEg as a parameter. In the case of the conventional structure with ΔE9 = 30 to 40 meV, as the incident light intensity increases, 2 to 3
Although a dB deterioration was observed, in the present invention where ΔE9 is 50 meV or more, the deterioration degree is 1 dB or less. Deterioration of 1 dB or less is a voltage drop in the ohmic resistance or termination resistor due to the absorption current as described above, so this is also considered to be an essential characteristic of the absorption type optical modulation element. Eight E, =
The deterioration due to 30 to 40 meV is caused by the fact that the electric field applied to the waveguide is inhibited by excessively absorbed carriers, and the externally applied voltage is not faithfully traced due to space charge effects and the like.

Conventionally, the modulation voltage is about 2V and the two frequency bandwidths are, for example,
When designing an optical modulation element with almost no spectral broadening above 5 GHz, 8E = 30 to 40 meV
, an element length of about 0.5 non is considered to be optimal, and has been actually produced. However, as is clear from FIGS. 3 and 4, the conventional energy difference ΔE.

(30 to 40 meV), it has been found that both the modulation voltage and the modulation bandwidth are significantly degraded when the incident light intensity is increased, which is a major problem.

Next, the relationship between the element length L0, modulation voltage, and frequency bandwidth will be explained. FIG. 5 shows experimental results according to the present invention for explaining the element length dependence of the modulation voltage (voltage for reducing the light intensity from 90% to 10%) when the incident light intensity is 5 mW. With current technology, it is possible to reduce the modulation voltage to about 4V. From Figure 5, 8E9 = 50
When meV (curve ■) and 55 meV (curve ■), the element length L0 is 0.2 mm and 0.5, respectively.
It may be set to mm or more. Therefore, ΔE, /L at that time is 2
50 meV/mm and 110 meV/body, and if the upper limit of 8E and /L is 250 meV/mm, the modulation voltage is 4
It can be seen that it is possible to make it approximately v.

The frequency bandwidth is almost determined by the modulation waveguide width, and when the modulation waveguide width is 1 μm, the upper limit of the length that can realize a bandwidth of 5 GHz or more is about 5M. Therefore, in order to obtain a high-speed optical modulator, 8E, /L≧10me
It is necessary to set it to V/in. Note that as the modulation waveguide width increases, 8E, /L also increases, and the modulation waveguide width becomes 5.
ΔE9/L in μm is 30 meV/mm.

As mentioned above, when the incident light intensity is 0.1 mW or more, 4
■In order to realize an optical modulation element with a bandwidth of 5 GHz or more with the following modulation voltage, △Eg≧50meV and 10meV/mm≦ΔE9/L O≦250m
eV/mm is a necessary condition. Therefore, the present invention provides this Δ
The light modulation element is configured to satisfy the conditions of E9 and ΔE9/LO.

(Example 2) FIG. 6 shows a second example according to the present invention, and is a sectional view of a light modulation element.

The difference from Example 1 is that in order to reduce absorption loss due to the built-in electric field, the n −-InGaAsP waveguide layer 1
InGaAsP waveguide layer 1 between 0 and p-InP layer 3
n--, which has the same conductivity type as 0 and has a layer thickness of approximately 0.2 μm.
It is at the point where InP Nll was inserted. Motoko is long. ll
If the width of l1[11 is 5 μm, an extinction ratio of 10 dB can be obtained at an operating voltage of 2.0 cm or less, and a 3 dB bandwidth of approximately 5 GHz can be achieved. Moreover, in the light modulation element of this example, 8E
Since the structure is such that 9≧50 meV 8E9/L = 50 meV/mm, even if the incident light intensity is increased, there is almost no increase in modulation voltage or decrease in modulation bandwidth.

In the above explanation, InGaAsP/InP materials were used as an example. , GaAs/GaAs series, AI InGaAs/InP series, and other materials can be similarly applied.

Furthermore, it is also possible to use a multiple quantum well layer made of these materials, and in that case, the forbidden band width used in the explanation becomes an effective forbidden band width determined by the quantum well level. Furthermore, although the stripe structure for stabilizing the transverse mode has been described using a strip loading type as an example, all conventional techniques such as a buried stripe structure and a ridge waveguide stripe structure are applicable.

(Effect of the invention) As described above, in the present invention, the energy difference ΔEg is
0 meV and ΔE,/L, is 10 meV or more and 2
Since the optical modulation element is configured to have a voltage of 50 meV/mm or less, even if the incident light intensity increases from the practical level of 0.1 mW to several meters or more, there is almost no increase in modulation voltage or decrease in bandwidth, resulting in low voltage , an optical modulation element capable of high-speed modulation can be realized. Furthermore, by inserting the InP layer 11 between the InGaAsP waveguide layer 10 and the p-InP layer 3, absorption loss due to the built-in electric field can be reduced. Therefore, it can be applied to ultra-high-speed optical fiber communications, etc., and its effects are extremely large.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a sectional view showing an example of a conventional electroabsorption light modulation device, FIG. 2 is a sectional view of a light modulation device according to a first embodiment of the present invention, and FIG. 3 , FIGS. 4 and 5 are characteristic diagrams of experimental results according to the present invention for comparing the characteristics of the present invention and a conventional light modulation element, and FIG. 6 is a second embodiment according to the present invention, FIG. 3 is a cross-sectional view of a light modulation element. 1-n"-1nP substrate, 2-...n--1nGaAs
P waveguide layer, 3・-p-InP layer, 4・”p I
nGaAsP cap layer, 5... p-side electrode, 6...
n-side electrode, 10...n--1nGaAsP waveguide layer, ll...n--InP layer. Patent applicant International Telegraph and Telephone Corporation

Claims (2)

[Claims] (1) An optical waveguide layer, p-type and n-type cladding layers having a smaller refractive index than the optical waveguide layer, and an electrode are provided on the substrate, and an electric field applied from the electrode to the optical waveguide layer causes the In an optical modulation element that performs optical modulation by changing an absorption coefficient for incident light of a constant intensity that enters an input end face of an optical waveguide, and extracts modulated light from an output end face of the optical waveguide, the input light enters the optical waveguide. When the intensity of the incident light is 0.1 mW or more, the forbidden band energy of the optical waveguide layer is 50 meV or more larger than the incident light energy, and
The ratio between the energy difference between the forbidden band energy and the incident light energy and the length of the optical modulation element determined by the length from the input end face to the output end face of the optical waveguide is 10 meV/mm.
A light modulation element characterized in that it is configured to have a voltage of at least 250 meV/mm. (2) An optical waveguide layer, p-type and n-type cladding layers having a smaller refractive index than the optical waveguide layer, and an electrode are provided on the substrate, and an electric field applied from the electrode to the optical waveguide layer causes the In an optical modulation element that performs optical modulation by changing an absorption coefficient for incident light of a constant intensity that enters an input end face of an optical waveguide, and extracts modulated light from an output end face of the optical waveguide, the input light enters the optical waveguide. When the intensity of the incident light is 0.1 mW or more, the forbidden band energy of the optical waveguide layer is 50 meV or more larger than the incident light energy, and
The ratio between the energy difference between the forbidden band energy and the incident light energy and the length of the optical modulation element determined by the length from the input end face to the output end face of the optical waveguide is 10 meV/mm.
or more and 250 meV/mm or less, and there is a forbidden band between the optical waveguide layer and the p-type cladding layer that is larger than the forbidden band width of the optical waveguide layer and smaller than or equal to the forbidden band width of the cladding layer. An optical modulation element characterized in that a semiconductor layer having a width and the same conductivity type as the optical waveguide layer is formed.