JP4105927B2 - Optical modulator element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical modulator element used for high-speed optical fiber communication, which converts an electrical signal into an optical ON / OFF signal.
[0002]
[Prior art]
FIG. 2 shows a structural example (a) of a conventional optical modulator element and a band structural example (b) thereof.
(a) is a basic structure of a semiconductor optical modulator device, in which a light absorption layer is formed on a lower clad layer (N-type clad layer) and an upper clad layer (P-type clad layer) formed in a stripe shape in the light guiding direction. Is sandwiched. The light absorption layer has a refractive index larger than both cladding layers and has a waveguide structure in which light is confined in the light absorption layer, and the stripe width is such a width that this waveguide becomes a single mode waveguide, for example, When an InP / InGaAsP material is used, the width is set to about 1 μm. The band gap energy of this light absorption layer is smaller than that of both clad layers and is slightly larger than the energy of light used when this device is incorporated in an optical communication system (generally 25 to It is set to have a large energy gap of about 45 meV).
[0003]
In the conventional structure, the well width in the light absorption layer (in this case, the whole light absorption layer is an MQW structure) is constant (L1 = L2 =) as shown in FIG. L3 = ... = Lm). When a reverse bias voltage is applied between the upper and lower P and N electrodes of this element, an electric field is applied to the MQW light absorption layer, and the absorption edge energy of light is reduced to the low energy side (high wavelength) mainly due to the quantum Stark effect. (The same phenomenon occurs due to the Franz-Keldish effect) and approaches the energy of light propagating in the waveguide, so that light absorption occurs and the amount of transmitted light decreases. By utilizing this phenomenon, it is possible to create an optical signal (light ON / OFF) that matches the electrical signal (voltage Hi / Low) (see, for example, Patent Document 1).
[0004]
[Patent Document 1]
JP-A-8-313853 Paragraph (0022)
[0005]
[Problems to be solved by the invention]
FIG. 3 shows changes in the amount of transmitted light when a reverse bias voltage (input electric signal) is applied between the upper and lower electrodes of a conventional optical modulator element.
This element functions by causing light absorption by utilizing the quantum Stark effect or Franz-Keldish effect, so that light absorption occurs rapidly as the reverse bias voltage increases, and almost no transmitted light is lost at a certain deep bias. Has characteristics.
Therefore, when an input electric signal with ripples is converted into an optical signal in the transmitted light amount curve of a conventional optical modulator element, the output optical signal on the Space side compresses the ripple of the electric signal as shown in FIG. S / N improves, but on the Mark side, since the reverse bias is shallow, the amount of transmitted light greatly fluctuates with a slight change in voltage, and the S / N deteriorates conversely. The phenomenon that will end up.
In order to prevent the increase in fluctuation on the Mark side, which is one drawback of this element, it is necessary to reduce the amount of light absorption change at a shallow reverse bias point as shown in FIG. Propose the structure to be realized.
[0006]
[Means for Solving the Problems]
In the present invention, by creating a structure in which the energy at the light absorption edge increases toward the P-type cladding layer, the light absorption curve becomes gentle in a region where the absolute value of the reverse bias voltage (input electric signal) is small. To do. In order to make such a structure of the light absorption edge, the well width is narrowed toward the P-type cladding layer side without changing the material composition for forming the well.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a structural example (a) and a band structural example (b) of the optical modulator element of the present invention.
In the conventional optical modulator element of FIG. 2, the well width of the light absorption layer is constant. However, in the present invention, the well width on the P-type cladding side becomes narrower toward the P-type cladding side ( L1 <L2 <L3 <... <Lm).
Since the absorption edge energy of the quantum well structure shifts to a higher energy side as the well width becomes narrower, the absorption edge energy becomes larger as the P-type cladding side is approached by using this structure (FIG. 1B). )reference).
When the reverse bias voltage applied to this structure is gradually increased, first, gradually from the interface between the P-type cladding and the MQW light absorption layer (P / N junction position) into the MQW light absorption layer toward the N-type cladding layer. Since the depletion layer expands and the reverse bias voltage applied to the device gives an electric field to this depletion layer, the absorption edge due to the quantum Stark effect (or Franz Keldish effect, or a combination thereof) within this depletion layer The lower energy side shift occurs. Therefore, in the absorption edge profile according to the structure of the present invention, light absorption in a region where the reverse bias is shallow (region corresponding to the Mark side voltage) is reduced, and a transmitted light amount profile as shown in FIG. 3 can be realized.
As a result, even when an electrical signal having a ripple is applied to the element, the ripple can be kept smaller than that of the conventional element as in the optical signal shown by the dotted line in FIG. 3, and the S / N on the Mark side as well as on the Space side. Can also be improved.
[0008]
【The invention's effect】
As explained above, even if there are ripples in the input electrical signal, the optical signal converted by this element can suppress not only the ripple compression on the Space side but also the ripple on the Mark side. It can be expected that the S / N of the converted optical signal is better than the S / N of the electrical signal.
[Brief description of the drawings]
FIG. 1 is a diagram showing a structural example (a) and a band structural example (b) of an optical modulator element of the present invention.
FIG. 2 is a diagram showing a structure example (a) of a conventional optical modulator element and a band structure example (b) thereof.
FIG. 3 is a diagram for explaining electrical signal-optical signal conversion in the conventional and optical modulator elements of the present invention.

Claims (1)

In an optical modulator element in which the entire absorption layer or a part on the P-type cladding layer side has a multiple quantum well structure (MQW),
By making the well width of the MQW structure narrower toward the P-type cladding layer, the energy at the light absorption edge is increased toward the P-type cladding layer to suppress the ripple of the output optical signal due to the ripple of the input electric signal. An optical modulator element characterized by improving the S / N ratio of the output optical signal.

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