JP2715864B2 - Nonlinear optical element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-linear optical element, and more particularly to a non-linear optical element used as a light control element used in fields such as optical fiber communication and optical information processing.

[0002]

2. Description of the Related Art In order to increase the speed of an optical fiber communication system or an optical information processing system, it is indispensable to increase the speed of operation of an element for controlling light. Conventionally, a method of performing light control using an electric signal (electric-light control) has been used in a light control element. In recent years, a method of performing light control using light (light -Light control).

[0003] For example, as a development example by the same applicant as the present application, Japanese Patent Application No. 4-341863 discloses a nonlinear optics having a means for applying an electrostatic field to an optical waveguide portion exhibiting a nonlinear refractive index change by light absorption. An element is described. In the optical waveguide portion of this element, the control light is absorbed, and a non-linear refractive index change is caused by the band filling effect due to the excited carriers. The signal light propagating through the optical waveguide is phase-modulated by the nonlinear refractive index change. In the case of the nonlinear refractive index change due to the band-filling effect, the time required for its manifestation is very short (1 ps or less), but the time required for its disappearance depends on the lifetime of the excited carriers (usually 1n).
s or more), which hinders high-speed operation of the device. Therefore, in the device described in Japanese Patent Application No. 4-31863, the recovery of the nonlinear refractive index change is speeded up by applying an electrostatic field to the optical waveguide and sweeping the excited carriers out of the optical waveguide. ing. The operating speed of the device is limited by the time required for carriers excited in the optical waveguide to travel outside the optical waveguide due to the electrostatic field. However, when the carriers are swept, the applied electrostatic field is screened by the spatially separated electrons and holes to some extent, and the electric field strength is reduced. This decrease in the intensity of the applied electrostatic field lowers the traveling speed of the carrier and hinders high-speed operation of the device. Therefore, the aforementioned Japanese Patent Application No. 4-341863 is disclosed.
In the device described in the above item, the decrease in the applied electrostatic field strength due to the electric field screening is suppressed by increasing the area of the electrode used for applying the electric field and increasing the electric capacity of the device.

FIG. 2 shows an example of a nonlinear optical element having the above characteristics. On a GaAs substrate 2 doped with 10 ¹⁸ cm ^{−3 of} Si, a 2 μm-thick Al _x Ga _{1 -x} As (x = 0.18) doped with 10 ¹⁸ cm ^{−3 of} Si.
07) Lower cladding layer 3, non-doped thickness 0.5
μm GaAs core layer 4, non-doped with a thickness of 0.2
μm of _{Al x Ga 1 - x As (} x = 0.07) upper cladding layer 5, the Be 10 ^{1 8} cm ^{- 3} doped thickness _{0.6μm Al x Ga 1 - x As} (x = 0.07 2) An upper cladding layer 6 and a 0.2 μm-thick GaAs cap layer 7 doped with Be at 10 ¹⁸ cm ⁻³ are sequentially laminated. Further, a height of 0.1 mm is obtained by an etching process.
A stripe having a width of 9 μm and a width of 4 μm is formed. An SiO ₂ insulating film 8 and an electrode 9 are laminated on the element surface.
Are in ohmic contact with the GaAs cap layer 7. The ohmic electrode 1 is also formed on the back surface of the substrate 2.

[0005] A reverse bias voltage is applied between the electrodes 1 and 9, whereby an electrostatic field is applied to the GaAs layer 4. The control light pulse absorbed by the optical waveguide and the signal light propagating through the optical waveguide are incident on this element. Carriers are generated in the optical waveguide by absorption of the control light, causing a nonlinear refractive index change. The generated carriers are swept out of the optical waveguide by an electrostatic field applied to the optical waveguide. That is, electrons and holes are swept to the lower cladding and the upper cladding, respectively. As a result,
The nonlinear index change disappears. The signal light is phase-modulated by the change in the refractive index. High-speed light control is performed by a series of operations as described above.

[0006]

By the way, the change in the non-linear refractive index accompanying the generation of carriers in a semiconductor increases as the carrier density increases, but the rate of increase in the non-linear refractive index decreases as the carrier density increases. That is, the nonlinear refractive index change tends to be saturated as the carrier density increases. Therefore, the phase shift amount of the same magnitude can be obtained with a lower total number of carriers in a case where the carriers are uniformly distributed, than in a case where the carriers have a non-uniform region in which a region having a large carrier density and a region having a small carrier density exist on the waveguide. Will be done. That is, operation with low control light pulse energy becomes possible.

However, in the device shown in FIG. 2, the carrier density generated in the optical waveguide portion monotonously decreases along the propagation direction as shown in FIG. Although the carrier density increases near the entrance of the optical waveguide, an efficient change in refractive index cannot be obtained due to saturation of the nonlinear effect. Therefore, it is disadvantageous in terms of low energy operation.

It is an object of the present invention to utilize not only a carrier sweep by an electrostatic field, and to suppress a decrease in an applied electrostatic field during the carrier sweep by increasing an element capacity to enable high-speed operation, but also to realize a high speed operation. A nonlinear optical element that enables low-energy operation by changing the intensity of the optical waveguide along the propagation direction of the optical waveguide and suppressing the non-uniformity of the carrier density generated in the optical waveguide along the propagation direction. To provide.

[0009]

According to the present invention, there is provided a nonlinear optical element comprising: a core layer which is a semiconductor material which causes a nonlinear refractive index change due to light absorption; and a cladding layer joined to the core layer. A waveguide-type nonlinear optical element having a confinement structure formed therein and having a means for realizing the light confinement structure also in a direction parallel to the layer, comprising a plurality of electrodes arranged on the waveguide along the propagation direction, The apparatus is characterized in that there is provided a means for applying an electrostatic field to each of the electrodes in which the electric field intensity of each region monotonically increases from the incident side to the emission side.

[0010]

FIG. 1 shows the structure of an embodiment of a nonlinear optical element according to the present invention. On a GaAs substrate 2 doped with 10 ¹⁸ cm ^{−3 of} Si, a 2 μm-thick Al _x Ga _{1 -x} As (x = 0.18) doped with 10 ¹⁸ cm ^{−3 of} Si.
07) Lower cladding layer 3, non-doped thickness 0.5
μm GaAs core layer 4, non-doped with a thickness of 0.2
μm of _{Al x Ga 1 - x As (} x = 0.07) upper cladding layer 5, the Be 10 ^{1 8} cm ^{- 3} doped thickness _{0.6μm Al x Ga 1 - x As} (x = 0.07 2) An upper cladding layer 6 and a 2 μm-thick GaAs cap layer 7 doped with Be at 10 ¹⁸ cm ⁻³ are sequentially laminated. Furthermore, 0.9μ height by etching process
m, a stripe having a width of 4 μm is formed. An SiO ₂ insulating film 8 is laminated on the element surface, and electrodes 9, 1
0 and 11 are formed. The optical waveguides below the electrodes 9, 10, and 11 are referred to as regions 12, 13, and 14, respectively.

A reverse bias voltage V ₁ , between electrodes 1 and 9, between electrodes 1 and 10, and between electrodes 1 and 11, respectively.
Takes V _2, V _3, a _{V 1 <V 2 <V 3} . Therefore, when the intensity of the electric field applied to the optical waveguide region 12, 13, 14 and _{E 1, E 2, E 3} , E 1 <E 2 <E 3
Becomes

The control light pulse absorbed by the optical waveguide and the signal light propagating through the optical waveguide are incident from the region 12 side of the element. Carriers are generated in the optical waveguide by absorption of the control light, causing a nonlinear refractive index change. The generated carriers are swept out of the optical waveguide by an electrostatic field applied to the optical waveguide. That is, electrons and holes are swept to the lower cladding and the upper cladding, respectively. As a result, the nonlinear refractive index change disappears. The signal light is phase-modulated by the change in the refractive index. High-speed light control is performed by a series of operations as described above.

Incidentally, the absorption coefficient of a semiconductor is expressed by Franz-Keldys.
h) The effect increases with an increase in electric field strength. V ₁
Since <V ₂ <V ₃ , the electric field intensities E ₁ , E ₂ and E ₃ applied to the optical waveguides in the regions 12, 13 and 14 are E ₁ <
E ₂ <E ₃ , and the absorption coefficients α ₁ , α ₂ , and α ₃ for the control light in the regions 12, 13, and 14 are α ₁ <α ₂ <α.
It becomes ₃ . Therefore, as shown in FIG. 3, the carrier density generated by the control light absorption is made uniform along the propagation direction, and is suppressed to a level where saturation of the nonlinear refractive index change does not occur. That is, the refractive index changes efficiently over the entire length of the waveguide, and light control with low energy becomes possible.

The nonlinear optical element having three electrodes on the waveguide has been described above by way of example, but the number of electrodes is not limited to three. Rather, the larger the number of electrodes, the more optimally the absorption coefficient of the optical waveguide below each electrode can be adjusted. Further, the present invention provides a GaAs-A
Similar effects can be obtained not only when an lGaAs-based material is used but also when another semiconductor material such as an InP-based material is used. Similar effects can be obtained not only when the core is bulk but also when the core has a large quantum well structure.

[0015]

As described above, in the nonlinear optical element of the present invention, a plurality of electrodes are provided on the optical waveguide, and the electrostatic field in each region of the optical waveguide is monotonously increased from the incident side to the emission side. The density of carriers generated by absorption of the control light pulse is made uniform along the propagation direction. This allows the non-linear optical element of the present invention to operate with low energy.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a structure of one embodiment of a nonlinear optical element of the present invention.

FIG. 2 is a perspective view showing a structure of a conventional nonlinear optical element.

FIG. 3 shows an embodiment of the nonlinear optical element according to the present invention.
FIG. 4 is a diagram illustrating a distribution of a carrier density in a propagation direction.

FIG. 4 is a diagram showing a distribution of carrier density in a propagation direction in a conventional nonlinear optical element.

[Explanation of symbols]

First electrode 2 Si-doped GaAs substrate 3 Si-doped AlGaAs lower cladding layer 4 non-doped GaAs core layer 5 of a non-doped AlGaAs upper cladding layer 6 Be-doped AlGaAs upper cladding layer 7 Be-doped GaAs cap layer 8 SiO ₂ insulating film 9 electrode 10 electrode 11 electrode 12 A region formed under the electrode 9 in the optical waveguide 13 A region formed under the electrode 10 in the optical waveguide 14 A region formed under the electrode 11 in the optical waveguide

Claims (1)

(57) [Claims] An optical confinement structure is formed in a direction perpendicular to a layer by a core layer, which is a semiconductor material which causes a nonlinear refractive index change by light absorption, and a cladding layer bonded to the core layer, and also in a direction parallel to the layer. A waveguide type nonlinear optical element having a means for realizing a light confinement structure, comprising a plurality of electrodes arranged on a waveguide along a propagation direction, and an electric field intensity in an optical waveguide portion below each of the electrodes is incident. A non-linear optical element having means for applying an electrostatic field monotonically increasing from a side to an output side to each of the electrodes.