JPH06208145A - Method for generating short optical pulse - Google Patents

Abstract

PURPOSE:To provide a method for generating short optical pulse having the excellent characteristic in thee point of pulse width, density of pulse line to be generated and chirping. CONSTITUTION:In the case where voltage is not applied between electrodes 1, 7, when the incident light at a constant intensity is made to enter from a port (1) of a wave-guide, the light is made to go out from a port (2). In the case where a predetermined voltage is applied between the electrodes 1, 7, when the incident light at a constant intensity is made to enter form the port (1), both of refractive index change (DELTAn.GAMMA) and the quenching ratio of thee wave-guide are changed simultaneously. Consequently, value of the voltage to be applied is previously set at a value more than the predetermined value to generate the refractive index change and the absorption, and this absorption is utilized to sharply shape the optical pule. The optical pulse, which is thereby obtained, is taken off from a port (3).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of generating a so-called short light pulse having a short pulse width.

[0002]

2. Description of the Related Art A short light pulse generation technique is indispensable and important for realizing a high-speed optical transmission system.

FIG. 6 is a schematic perspective view showing the structure of a pulse generator for explaining an example of a conventional method of generating a short optical pulse. In FIG. 6, 1 is a gold electrode, 2 is InGa
As cap layer, 3 p-InP clad, 4 InG
aAs (7.6 nm) / InAlAs (5 nm) MQW
It is the core, and its exciton peak is set to 1.47 μm. 5 is n-InP clad, 6 is n-In
A P substrate and 7 are n-side electrodes.

The short optical pulse generator shown in FIG. 6 generates short optical pulses by injecting light of a constant intensity into a semiconductor intensity modulator and cutting out a part of the light with the semiconductor intensity modulator. Is. In this example, a multiple quantum well (MQW) optical modulator is used as the semiconductor optical modulator.

Next, the operating principle will be described in detail with reference to FIGS. 7 and 8. First, FIG. 7 is a graph showing the light transmission characteristics with respect to the applied voltage of the semiconductor optical modulator. When a voltage is applied, the absorption wavelength of excitons shifts to the long wavelength side, so that light is absorbed and the intensity of transmitted light is attenuated (reference on absorption by excitons: Kono et al., IEEE Journal of Quantum).
Electronics, vol. QE-28, p
p. 224-230, 1992). Therefore, when light having a constant intensity is incident on the semiconductor optical modulator and an electric pulse as shown in FIG. 8A is applied to the semiconductor optical modulator,
An optical pulse as shown in FIG. 8B can be cut out. Since a light pulse is cut out by applying a voltage, a sine wave may be used as the electric pulse.

[0006]

By the way, as can be seen from FIG. 7, in order to generate light with a short pulse width, a sharp absorption characteristic with respect to the applied voltage is required. In order to obtain a sharp absorption characteristic with respect to the applied voltage, it is necessary to increase the electric field strength E in the non-doped layer (MQW core 4 in FIG. 6) of the semiconductor optical modulator. When the applied voltage is V, the internal electric field strength E is given by the following equation.

[0007]

## EQU00001 ## E = V / d (1) where d is the thickness of the non-doped layer. As can be seen from the formula (1), it is necessary to reduce the thickness d of the non-doped layer in order to increase the internal electric field strength E. However, since the electric capacitance C is inversely proportional to the thickness d of the non-doped layer, the electric capacitance C increases when the thickness d of the non-doped layer is reduced. Electrical capacitance is C
As is well known, the 3 dB optical modulation band Δf in the case of

[0008]

## EQU2 ## Δf = 1 / (2πRC) (2) Here, R is a terminal resistance.

As can be seen from equation (2), the optical modulation band Δf becomes smaller as the electrical capacitance C becomes larger.

Therefore, in the conventional short optical pulse generation method,
When the thickness d of the non-doped layer is reduced for the purpose of shortening the pulse width, electrical response characteristics deteriorate, and it is difficult to make the pulse width extremely short in the end. There was a problem that it was difficult to occur. Furthermore, in this method, since the light is absorbed by using the intensity modulator, the wavelength fluctuation of the light resulting from the chirping or the absorption is unavoidable, and the pulse width of the light is widened while being transmitted into the fiber. There was also the problem of being lost.

Therefore, an object of the present invention is to solve the above technical problems and to provide a short optical pulse generating method which is excellent in terms of pulse width, density of a generated pulse train and chirping.

[0012]

In order to achieve such an object, the invention according to claim 1 is a method of generating a short optical pulse for generating an optical pulse having a short pulse width, and a refractive index in a semiconductor optical waveguide. An electric pulse which causes a change and an absorption coefficient change is applied to the semiconductor optical waveguide.

According to a second aspect of the present invention, in the short optical pulse generating method according to the first aspect, one of the waveguide type optical switches having at least two optical waveguides is made incident and propagated. It is characterized by including a step of making the optical paths of a part of the transmitted light and the remaining propagating light excluding a part of the propagating light different.

According to a third aspect of the present invention, in the short optical pulse generating method according to the first aspect, a step of causing a part of the light incident on the semiconductor optical waveguide to be emitted from the emission side optical waveguide of the Mach-Zehnder type optical modulator. It is characterized by including.

[0015]

According to the present invention, a short light pulse is generated by using both the change in the refractive index and the change in the absorption coefficient, so that the thin non-doped layer as in the conventional embodiment is not necessary. As a result, the device for generating short light pulses can be operated at high speed, and the pulse width can be shortened to make the pulse train dense. Further, by allocating the change in the refractive index and the absorption, it becomes possible to generate a short optical pulse in which the amount of chirping is adjusted.

[0016]

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

(Embodiment 1) FIG. 1 is a schematic perspective view showing the structure of an example of a short light pulse generator for explaining the first embodiment of the short light pulse generation method of the present invention.

In the configuration of the short optical pulse generator shown in FIG. 1 used to describe this embodiment, the same components as those of the short optical pulse generator shown in FIG. 6 are designated by the same reference numerals. , The description is omitted.

In the short optical pulse generator shown in FIG. 1, a waveguide type directional coupling type optical switch having an MQW is used as an optical switch. In FIG. 1, 8 is InGaAl
It is an As (90 nm) / InAlAs (5 nm) MQW core, and its exciton peak is set to 1.44 μm. 9 is an i-InP side cladding.

Next, this embodiment will be described using the short optical pulse generator shown in FIG. First, when a certain intensity of incident light is made incident from the port of the waveguide, when no voltage is applied between the electrodes 1 and 7, the light is emitted to the port. When the voltage having the waveform shown is applied, the optical path is changed and the optical pulse having the waveform shown in FIG. 2B is emitted to the boat.

FIG. 3 is a graph showing changes in the refractive index and absorption coefficient (extinction ratio) of the waveguide with respect to the applied voltage. In FIG. 3, Δn is the change in refractive index, and Γ is the confinement coefficient within the MQW core. From FIG. 3, when a voltage is applied between the electrodes 1 and 7 in FIG. 1, a change in the refractive index of the waveguide (Δ
It can be seen that both n · Γ) and the extinction ratio change. Therefore, the value of the applied voltage is preset to a predetermined value or more so that a change in the refractive index and absorption occur, and the absorption is used to sharply shape the shape of the optical pulse. That is, since the present invention utilizes not only absorption but also the change in refractive index, it is not necessary to reduce the thickness of the non-doped layer as in the conventional method.

In the present invention, since both the change in the refractive index of the waveguide and the change in the absorption coefficient are used to generate a short optical pulse, unlike the conventional short optical pulse generating method using only absorption shown in FIG. It is not necessary to reduce the thickness d of the non-doped layer. Therefore, since the electric capacitance C is not increased, the response as the optical switch becomes faster as can be seen from the above formula (2). As a result, the pulse width can be shortened and the generated pulse train can be made dense. Further, in FIG. 1, there is also an advantage that the chirping is further reduced by attaching an electrode to the optical waveguide opposite to the optical waveguide on which the electrode is formed and performing the push-pull operation. Further, by allocating the change in the refractive index and the absorption, it is possible to generate a short optical pulse with the amount of chirping adjusted if necessary.

(Embodiment 2) FIG. 4 is a schematic perspective view showing the structure of an example of a short light pulse generator for explaining a second embodiment of the short light pulse generating method of the present invention. In this embodiment, a Mach-Zehnder optical modulator having an MQW as a Mach-Zehnder is used. 10 in FIG.
Is an electrical isolation groove.

The operating principle of the second embodiment is as follows. Incident light of constant intensity is made incident on the Mach-Zehnder optical modulator. An electric pulse as shown in FIG. 5A is applied to this optical modulator from a signal source to the electrode 1 of the optical modulator.
Then, when the voltage of −V is applied, the phases of the lights propagating through both arms of the Mach-Zehnder optical modulator are deviated from each other by π, so that when they are combined, they cancel each other off.
It becomes a state. However, as shown in FIG. 5A, the electric pulse becomes 0 V at a certain moment, so that the light propagating through both arms at the moment is emitted in the same phase.
This in-phase light is obtained as an optical pulse having a short pulse width as shown in FIG. As in the case of the first embodiment, if the applied voltage is set to be equal to or higher than a predetermined value so that the absorption coefficient and the refractive index change, a short optical pulse having a short pulse width can be generated as in the first embodiment.

In the first embodiment, as an optical switch,
Although the method using the directional coupler type optical switch having the MQW has been described, the operating principle of the present invention does not depend on the structure of the optical switch, and a bulk semiconductor optical switch may be used instead of the MQW. Needless to say. In addition, as Mach-Zehnder in the second embodiment,
Although the method using the Mach-Zehnder having the MQW has been described, the operating principle of the present invention does not depend on the structure of the Mach-Zehnder, and it goes without saying that a bulk semiconductor Mach-Zehnder may be used instead of the MQW.

[0026]

As described above, according to the present invention, a short light pulse is generated by using both the change in the refractive index and the change in the absorption coefficient, so that a thin non-doped layer as in the prior art is not necessary.
As a result, the short light pulse generator can operate at high speed, and the pulse width can be shortened to make the pulse train dense.
Further, by allocating the change in the refractive index and the absorption, it is possible to generate a short optical pulse in which the amount of chirping is adjusted.

[Brief description of drawings]

FIG. 1 is a schematic perspective view showing a configuration example of a pulse generation device for explaining a first embodiment of a short optical pulse generation method of the present invention.

FIG. 2 (a) is a waveform diagram showing an electric pulse applied to the waveguide of the pulse generator in the first embodiment of the short optical pulse generating method of the present invention, and FIG. It is a wave form diagram which shows the generated optical pulse.

FIG. 3 is a diagram showing a change in a refractive index of a waveguide and an extinction ratio with respect to an applied voltage.

FIG. 4 is a schematic perspective view showing a configuration example of a pulse generation device for explaining a second embodiment of the short optical pulse generation method of the present invention.

5A is a waveform diagram showing an electric pulse applied to the waveguide of the pulse generator in the second embodiment of the short optical pulse generation method of the present invention, and FIG. It is a wave form diagram which shows the generated optical pulse.

FIG. 6 is a schematic perspective view showing the configuration of a pulse generator for explaining an example of a conventional method of generating a short optical pulse.

7 is a diagram showing a light transmission characteristic with respect to an applied voltage of a semiconductor optical modulator in the pulse generation device shown in FIG.

FIG. 8A is a waveform diagram showing an electric pulse applied to the waveguide of the pulse generator in the conventional short optical pulse generation method, and FIG. 8B is a waveform diagram showing the generated optical pulse. It is a figure.

[Explanation of symbols]

 1 Gold electrode 2 InGaAs cap 3 p-InP clad 4 InGaAs / InAlAsMQW core 5 n-InP clad 6 n-InP substrate 7 n-side electrode 8 InGaAlAs / InAlAsMQW core 9 i-InP side-clad 10 Separation groove

Claims (3)

[Claims] 1. A method for generating a short light pulse for generating a light pulse having a short pulse width, wherein an electric pulse for causing a change in a refractive index and a change in an absorption coefficient in the semiconductor optical waveguide is applied to the semiconductor optical waveguide. A method for generating short light pulses. 2. The short optical pulse generating method according to claim 1, wherein a part of the light which is incident on one of the optical waveguides of the waveguide type optical switch having at least two optical waveguides and propagates through the optical waveguide. And a step of making the optical paths of the remaining propagating light except a part of the propagating light different from each other. 3. The method for generating a short optical pulse according to claim 1, further comprising a step of causing a part of light incident on the semiconductor optical waveguide to be emitted from an emission-side optical waveguide of a Mach-Zehnder type optical modulator. A method for generating short light pulses.