JP2527878B2 - Optical differentiator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical differentiator which outputs an optical pulse when light is incident with a simple structure.

[0002]

2. Description of the Related Art As a device for detecting a light incident and outputting a short pulse when the light is incident, a photodiode is used as a detector, and an electrical short circuit such as one-shot triggering from the detection signal is used. A device that outputs a pulse can be considered. When it is desired to obtain an optical pulse instead of an electrical short pulse, it is conceivable to amplify the electrical short pulse and drive the laser diode to obtain the optical pulse.

[0003]

According to the above-mentioned method, when the light is detected and the short light pulse is output when the light is incident, an electric process is performed in the middle of the light. The output of the pulse will be delayed. That is, the operation speed becomes slow. In addition, the circuit configuration is complicated, and it is very difficult to integrate in consideration of application to, for example, an optical IC.

[0004]

In order to solve the above-mentioned problems, the optical differentiator of the present invention comprises a first light receiving element whose absorption wavelength end is shifted to a short wavelength side when the reverse bias voltage becomes small, and a predetermined light receiving element. And a second light receiving element whose absorption wavelength end is shifted to the long wavelength side in accordance with the magnitude of the reverse bias voltage. Means for applying a reverse bias voltage in parallel to the first and second light receiving elements via a current limiting element (resistor, constant current source, etc.), and incident light to either the first or second light receiving element. Is output as light differentiated through the other.

The first light receiving element is a light receiving element whose absorption wavelength end is shifted to the short wavelength side when the reverse bias voltage becomes small due to the QCSE effect, and the second light receiving element is the Wannie.
It may be characterized in that it is a light receiving element whose absorption wavelength end is shifted to the long wavelength side according to the magnitude of the reverse bias voltage by the r Stark effect.

[0006]

In the optical differentiator of the present invention, the wavelength of the incident light is the first
In the range in which the absorption wavelength end of the light receiving element changes and the range in which the absorption wavelength edge of the second light receiving element changes, the incident light is absorbed by the first light receiving element, so that the first light receiving element transmits light. An electric current flows. Due to the voltage drop of the current limiting element due to this current, the reverse bias voltage of the first light receiving element becomes small.
As a result, the absorption wavelength edge of the first light receiving element shifts to the short wavelength side. When the absorption wavelength edge becomes smaller than the wavelength of the incident light, the incident light passes through the first light receiving element.

On the other hand, the reverse bias voltage of the second light receiving element in parallel with the first light receiving element also decreases, and the absorption wavelength end of the second light receiving element shifts to the long wavelength side. When the absorption wavelength edge becomes longer than the wavelength of the incident light, the incident light is absorbed by the second light receiving element. In this way, the element that absorbs the incident light is switched from the first light receiving element to the second light receiving element, and the incident light is transmitted with the timing difference. As a result, short pulsed light can be obtained.

When an element having the QCSE effect and the Wannier Stark effect is used, the above-mentioned operation is performed by using the quantum effect.

[0009]

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

FIG. 1A shows an optical differentiator of the present invention, and FIG. 1B shows its equivalent circuit. This optical differentiator is formed monolithically and is connected in parallel so that a reverse bias voltage is applied to the light receiving element sections D1 and D2, and the light receiving element section D2 is on the optical path of the light transmitted through the light receiving element section D1. It is scented. The resistor R is the light receiving element section D
1 and D2 are protective resistors for limiting the current when they are energized. The optical input P _in is the light receiving elements D1 and D2.
And is output as a light output P _OUT .

The light receiving element portions D1 and D2 are pin photodiodes, and an MQW structure (MUTI-QUANTUM WEL
L, quantum well structure), and a barrier layer of MQW structure is provided in this i layer. As shown in FIG. 2A, the barrier layer of the light receiving element portion D1 is in a state in which the barrier layer is thick and electrons are localized in the well to create a level (well level) in the well. When a reverse bias voltage is applied to such a light receiving element section D2, the quantum level changes and the effective band gap decreases. This is known as the quantum well Stark effect (QCSE effect), and is a phenomenon also observed in general semiconductors. Accordingly, the light absorption characteristics of the light receiving element section D2 are
The change in (a) is shown. In this figure, the reverse bias voltage is “V ₁ > V ₂ > V ₃ ”, and the absorption wavelength end changes to the shorter side as the voltage is reduced.

On the other hand, the barrier layer of the light receiving element portion D2 is formed as shown in FIG.
As shown in (b), it is formed narrower than the spread of the wave of the electron, and the electron transits to the adjacent well by the tunnel effect. Therefore, the electrons interfere with each other to form a subband. Such a light receiving element section D2
When a reverse bias voltage is applied to, the potential structure also changes, but in this case, the electrons do not tunnel and the subband splits. As a result, the electron energy level rises and blue shifts (Wann
ier Stark effect). That is, the light absorption characteristics of the light receiving element section D2 are changed as shown in FIG. Light receiving element D
At the absorption wavelength edge of 2, the absorption wavelength edge changes to the longer side as the voltage is reduced.

Next, the operation of this optical differentiator will be described.

First, by adjusting the voltage V and the resistance R of the bias power source, the absorption wavelength end of the light receiving element portion D2 at the time of bias is shorter than the absorption wavelength end of the light receiving element portion D1, and the light receiving element with respect to the change of the reverse bias voltage. The range of the absorption wavelength end of the portion D2 and the range of the absorption wavelength end of the light receiving element portion D2 are set to overlap with each other. At this time, the reverse bias voltage is the voltage V ₁ .

When the optical input P _in of the wavelength λ ₀ changes in a pulse shape as shown by the dotted line _in FIG.
Is absorbed by 1. As the light intensity increases, the current flowing through the light receiving element section D1 increases, and the reverse bias voltage of the light receiving element sections D1 and D2 decreases from the voltage V ₁ due to the voltage effect of the resistor R.

When the reverse bias voltage becomes close to the voltage V ₂ ,
The absorption wavelength end of the light receiving element portion D1 becomes close to the wavelength λ ₀ , and the optical input P _in starts to pass through the light receiving element portion D1. On the other hand, the absorption wavelength end of the light receiving element portion D2 is also in the vicinity of the wavelength λ ₀ , the optical input P _in passes through the light receiving element portion D2 and is output as the optical output P _OUT , and part of it is absorbed (the light indicated by the solid line in FIG. 4). Output P _OUT rising). As a result, a current starts to flow in the light receiving element portion D2, the reverse bias voltage becomes smaller, the absorption wavelength end of the light receiving element portion D2 becomes longer than the wavelength λ ₀ , and the optical input P
_in is absorbed by the light receiving element portion D2. This eliminates the light output P _OUT (falling of the light output P _{OUT in} the solid line in FIG. 4) and finally the reverse bias voltage is balanced at the voltage V ₃ .
In this equilibrium state, the light input P _in disappears and the light receiving element D
When the photocurrent of 2 disappears, the reverse bias voltage becomes the initial state of the voltage V ₁ .

In this way, the positive feedback operation is performed by the reverse bias voltage and the change of the absorption wavelength end, and it operates as a differentiating circuit which outputs a short optical pulse when light is incident. Since this operation is due to the quantum effect, the rise and fall of the light pulse is very steep. In addition, the circuit configuration is simple without adding an external amplifier,
It can be integrated and becomes very small.

The present invention is not limited to the above-mentioned embodiment, but various modifications can be made.

For example, since the resistor R is for current limiting,
Alternatively, a constant current source composed of transistors and FETs may be used. Further, although a positive voltage bias is applied, a negative voltage bias may be applied by changing the polarity. Further, as shown in FIG. 1, the light is incident on each layer of the light receiving element portion in a direction parallel to each layer, but the light may be incident in a vertical direction (vertical direction in FIG. 1). In addition, the light receiving element section D
A mirror or the like may be inserted between 1 and D2.
Then, although the light is incident from the light receiving element portion D1, the light may be incident from the light receiving element portion D2. Even in this case, the same operation is performed.

[0020]

As described above, according to the optical differentiator of the present invention,
Since the incident light is transmitted at the timing difference at which the element that absorbs the incident light is switched from the first light receiving element to the second light receiving element, a short pulsed light can be obtained at the timing when the incident light enters. In particular, it can be realized with a very simple configuration with a small number of components.

Further, when the second element having the QCSE effect and the Wannier Stark effect is used, the operation is performed by the quantum effect, so that the operation speed becomes very high.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment.

FIG. 2 is a band diagram of the i layer.

FIG. 3 is a light absorption characteristic diagram.

FIG. 4 is an output waveform diagram.

[Explanation of symbols]

 D1, D2 ... Light receiving element section, R ... Resistor.

Claims (2)

(57) [Claims] 1. A first light-receiving element whose absorption wavelength edge shifts to a shorter wavelength side when the reverse bias voltage becomes smaller, and an absorption wavelength edge shorter than the first light-receiving element at a predetermined reverse bias voltage. A second light receiving element whose absorption wavelength end is shifted to the long wavelength side according to the magnitude of the reverse bias voltage, and a reverse bias voltage is applied in parallel to the first and second light receiving elements via a current limiting element. And an optical differentiator, wherein the incident light on either the first or second light receiving element is output as light differentiated through the other. 2. The first light receiving element is a light receiving element whose absorption wavelength end is shifted to a short wavelength side when the reverse bias voltage is reduced by the QCSE effect, and the second light receiving element is the Wannier Stark effect. 2. The optical differentiator according to claim 1, which is a light receiving element whose absorption wavelength end is shifted to the long wavelength side according to the magnitude of the reverse bias voltage.