JP2734214B2 - 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 which enables ultra-high-speed optical communication and optical signal processing.

[0002]

2. Description of the Related Art In optical information processing and optical communication, it is necessary to perform optical control such as optical modulation or optical operation in order to put a signal on light. In these systems that are currently in practical use, a method of performing control using electric signals (electric-light control method) is used. That is, light modulation and light calculation are performed by directly modulating a semiconductor laser as a light source with a current, or by using a change in a refractive index or absorption due to application of a voltage to a semiconductor or a dielectric material.

In this control method using electric signals, the processing speed is limited by the speed of the element itself or the speed mismatch between the electric signal and the light to be controlled, and it is extremely difficult to make the processing speed about sub-nanosecond or more. . Therefore, a so-called light-light control method in which light modulation or light operation is performed by an optical signal is being studied as a method for overcoming the processing speed limit of the conventional electric-light control method. This light-light control method has a feature that there is a possibility that an ultra-high speed can be achieved because there is no limitation by the CR time constant.

[0004]

The operating characteristics of an optical element that controls light with light largely depend on the optical nonlinear material used for the optical element. The magnitude of the optical nonlinearity of a material is
It is expressed as the value of χ ⁽³⁾ for that material. The characteristics required of the optical element include a low power property of being operable with a low optical power, in addition to the high speed property as described above. In order to realize these, a nonlinear optical material that has a short relaxation time τ for optical excitation (high speed) and a large χ ⁽³⁾ (low power) is required. However, a nonlinear optical material having a large χ ⁽³⁾ generally shows a large value of τ. If the same material is used under different conditions such as the light wavelength, the physical phenomena involved in the optical nonlinearity are different. However, the physical phenomena showing a large χ ⁽³⁾ also generally show a large τ. A detailed description of this can be found in R.A. A. Fisher, Optical Phase Conjugation (Op
physical phase conjugation)
(Academic Press, 1983) in Chapter 10.

[0005] Among various combinations of non-linear materials and non-linear phenomena, it is known that a method of generating electron / hole plasma by light in a semiconductor material exhibits a large χ ⁽³⁾ . This is a phenomenon in which the control light excites electrons in the valence band into the conduction band, resulting in a change in the refractive index of the semiconductor (band filling effect). However, the relaxation time of this phenomenon is 10 ^-9 seconds or more even in the case of a direct transition type semiconductor such as GaAs, which is insufficient for applications such as ultra-high-speed signal processing (optical phase conjugation). , Chapter 10 (supra)).

[0006]

Accordingly, the i-layer having the pin structure is a nonlinear semiconductor waveguide exhibiting optical nonlinearity, and a high reverse bias electric field is applied to the pin structure. By sweeping the electron / hole plasma generated in the nonlinear waveguide to the outside of the waveguide, it is conceivable to shorten the relaxation time effectively. However, in this case, since the band is discontinuous at the heterojunction surface for forming the waveguide, carriers are accumulated on the discontinuous surface. Due to the so-called pile-up effect, there is a problem that carrier sweeping is not performed efficiently even when a high electric field is applied.

[0007]

A non-linear optical element according to the present invention.
Is an i-type semiconductor with a narrow energy band gap.
N-type and p-type semiconductor layers with larger band gaps
With pin-type double heterostructure semiconductor waveguide sandwiched
Then, an electron / hole plasma is excited in the i-layer of the semiconductor waveguide by photoexcitation.
Induces the band-feeling effect by generating
The p-layer and the n-layer have electrodes, and a reverse bias voltage is applied.
To guide the electron / hole plasma at high speed
Non-linear optical element that sweeps out of the road,
N-type high impurity concentration outside the hetero surface near the n-layer side of the structure
Layer and the hetero surface near the p layer side of the pin hetero structure
Forming a p-type high impurity concentration layer on the outside.
You. The high impurity concentration layer includes a delta (δ) doped layer,
It is characterized in that it is a corner-doped layer, a Gaussian-doped layer or a uniformly doped layer over a larger area.

[0008]

The description of the third-order optical nonlinear constant χ ^{(3) due} to the photoexcitation of electrons from the valence band to the conduction band in a semiconductor is given in D. A. B. Given by Miller et al. (D.
A. B. Miller et al., Optics Communications, Vol. 35, No. 2, pp. 221-226, 1980
Year). Equation (11) of this document gives χ ⁽³⁾ , but this notation clearly has a typographical error. Therefore, when χ ⁽³⁾ is obtained by essentially the same method as in the same document, the following equation 1 is obtained.

[0009]

In the above equation e is the electron charge, h is Planck's constant, m _o is the dipole moment between the electron effective mass, p is the valence band and the conduction _{band, ω g = E g / (} h / 2
π), E _g are the band gap, μ is the reduced mass relative to the effective mass of electrons and pores, and T ₁ and T ₂ are the longitudinal and transverse relaxation times of the electron, respectively. Further, here, the angular frequency omega ^s of the angular frequency omega _p and the signal light of the pump light to extend the analysis of the above references (control light) (controlled light) is also considered differ. In addition, since the nonlinear refractive index change is considered here, only the real part of χ ⁽³⁾ was obtained. That is, the magnitude of the change in the refractive index of a substance due to light excitation is given by the following equation 2 using the nonlinear refractive index.

N = n ₀ + n ₂ I (2) where n ₀ indicates a linear refractive index, and n ₂ I indicates a change in the refractive index due to the light intensity I. This n ₂ is called a non-linear refractive index, and has a relationship with the above χ ^{(3) according} to the following formula 3 (unit is a cgs system).

[0012]

From the above, the nonlinear refractive index change caused by photoexcitation of electrons from the valence band to the conduction band in a semiconductor can be interpreted as follows. When a semiconductor is irradiated with light having a wavelength whose energy is larger than the band gap, a large number of electrons are excited from the valence band to the conduction band with light absorption.
However, as electrons accumulate in the conduction band, excitation from the valence band to the conduction band becomes more difficult (band-filling effect). This effect is expressed as χ ⁽³⁾ . And χ
The real part of ⁽³⁾ is related to the refractive index of the semiconductor through Equations 2 and 3. Here, the speed of excitation from the valence band to the conduction band by light absorption is extremely high (several hundred f
s). That is, it can be considered that the nonlinear refractive index appears at the moment when light enters the semiconductor. When this nonlinear phenomenon is applied to an optical device, the operation speed of the optical device is limited because carriers that have already been excited do not disappear immediately after the light irradiation is stopped. This depends on the magnitude of T ₁ (longitudinal relaxation time or carrier recombination time) in Equation 1. Since T ₁ is 10 ns or more even in a semiconductor of direct transition, χ ⁽³⁾ cannot follow a fluctuation in light intensity of 0.1 GHz or more.

In order to solve the above-mentioned problem, attention is paid to the fact that the non-linearity appears only in a (spatial) portion where carriers are excited. Usually, the speed limit of this kind of nonlinear phenomenon is the carrier recombination time as described above. However, if the carriers (electrons or holes) are swept out of the optical path of the signal light by any method, the signal light is no longer affected by the carriers, and the nonlinearity disappears in about the carrier sweep time. In order to sweep carriers spatially, a DC electric field (or a magnetic field) may be applied to the semiconductor. For example, in the case of high-purity undoped GaAs, when the electric field strength is several KV / cm, the electron The drift speed can be 10 ⁷ cm / s or more even at room temperature. If the optical path width is about 1 μm, the time required for sweeping electrons is about 10 ps. In this way, the optical device can operate at about 100 GHz.

The simplest and most reliable method of applying the DC electric field is to form a semiconductor having a non-linearity of undoped by sandwiching the semiconductor with p- and n-doped semiconductors to form a so-called pin structure. Is formed, and a reverse bias electric field may be applied thereto. In this case, care must be taken that the i-layer shows a non-linear property and is made of a semiconductor having a higher refractive index than the p- and n-layers, and that the i-layer becomes a guide layer of the waveguide. In this way, a DC electric field can be efficiently applied to the nonlinear waveguide. However, in the case of the waveguide type nonlinear element, the waveguide layer is made to coincide with the light absorbing layer to obtain more waveguide characteristics. Therefore, a hetero structure having a narrow band gap is used for the waveguide layer.

If there is a hetero structure in the pin structure,
Carriers are trapped in the band discontinuity plane (ie, the edge of the waveguide layer), and despite the application of an electric field,
Carriers do not go out of the waveguide layer smoothly. In order to solve this problem, in the nonlinear optical element of the present invention, n
The n-type is provided outside the hetero surface closer to the layer, and the p-type
A high impurity concentration layer of a mold type, for example, a delta doped layer is provided to reduce the size of the band discontinuity.

[0017]

Next, a non-linear optical element of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram of one embodiment of the present invention. The device is an n ⁺ -GaAs substrate (N
d−10 ¹⁹ / cc) on the n-AlGaAs layer 2 (N _d ~
10 ¹⁷ / cc, Ga composition ratio x = 0.4) is 4 μm, i
The AlGaAs cladding layer 3 (x to 0.4) is 1.0,
0.5 μm of i-GaAs waveguide layer 4 and i-AlGaAs
0.15 μm clad layer 5 (x to 0.4), p-Al
GaAs6 (N _a -10 ^{17 /} cc, x = 0.4)
The rib layer 6 of the waveguide is formed by MBE growth with a thickness of 85 μm and etching. Waveguide layer is i-G
In aAs4, the rib forms a waveguide mode in the i-GaAs4 layer below the rib. In the i-AlGaAs 5 layer, a 10 ¹⁴ / cm ² p-type delta-doped layer 7 is formed at a distance of 50 A (angstrom) from the heterojunction with the i-GaAs layer 4, and the same as in the i-AlGaAs layer 3. An n-type delta-doped layer 8 having a similar surface concentration is formed at the position.

The refractive index difference between AlGaAs and GaAs is 5%
On the other hand, the change in the refractive index due to doping is so small as to be negligible (1% or less), so that the delta doping hardly affects the optical waveguide characteristics. As a result, a high light confinement coefficient Γ-0.6 unique to the heterostructure was achieved, which is important for a nonlinear element. On the other hand, the delta-doped layers 7 and 8 have a large effect on the behavior when a reverse bias is applied between the electrode 9 and the n ⁺ -GaAs ^{1 and} photocarriers are generated in the i-GaAs 4 (waveguide layer) by the excitation light. Effect. This is shown in FIG.

FIG. 2A is a band diagram of the device of FIG. 1 when a reverse bias electric field is applied. For reference, FIG. 2 shows a band diagram of the device of FIG. 1 in the case where the delta doped layers 7 and 8 are not provided.
(B). In the figure, the central i-GaAs layer corresponds to i-GaAs4 (waveguide layer) in FIG. Excitation of the electron / hole plasma in the same layer by the guided light changes the refractive index of the waveguide, and a light-light control element can be configured by using this. The refractive index change is connected during the time when the electron / hole plasma remains in the waveguide. However, the present device uses the i-layer having the pin structure as a waveguide, and applies a reverse bias to the waveguide. By quickly sweeping the plasma, a fast response speed (falling of the nonlinear refractive index) is to be obtained. However, without the delta-doped layer, carriers are piled up due to band discontinuity of the heterojunction for forming the waveguide, as is apparent from FIG. 2B. Of course, the carriers accelerated by the high electric field jump into the band discontinuity, but a certain amount of carriers are caught between the heterojunctions, which delays the fall time of the nonlinear refractive index change.

On the other hand, in the case of the nonlinear optical element having the delta-doped layer according to the present invention, as shown in FIG. 2 (a), the band discontinuity which causes pileup of electrons and holes is eliminated by delta doping. Has been peeped. For this reason, most of the electrons and holes generated by the excitation light are quickly swept out of the waveguide layer by the applied electric field, so that the nonlinear refractive index change also falls at a speed corresponding to the applied electric field,
High-speed response is possible.

Although the embodiment of the nonlinear optical element of the present invention has been described above, the present invention is not limited to this embodiment. In the present embodiment, band discontinuity is reduced by delta doping. However, any doping profile that reduces band discontinuity may be used. For example, triangular doping, Gaussian doping, or a wider range of uniform doping.

In the present embodiment, a GaAs-based semiconductor is taken as an example, but the same example can be applied to any semiconductor. At wavelengths important in optical communication, InGaAsP and I
It goes without saying that the present invention can be applied to an nGaAs-based element.

In this embodiment, the number of band discontinuous surfaces is one. However, in the case of a more complicated structure, a plurality of pairs of band discontinuous surfaces appear. Even in this case, the present invention can be applied as it is.

[0024]

As described above, according to the present invention, it is possible to reduce the carrier pileup due to band discontinuity and increase the response speed of the nonlinear optical element.

[Brief description of the drawings]

FIG. 1 is a schematic view of one embodiment of the present invention.

FIG. 2 is an energy band diagram for explaining the operation of the present invention. FIG. 2A is a band diagram when a reverse bias electric field is applied to the nonlinear optical element of FIG. 1, and FIG. 2B is a band diagram of an element without a delta doped layer.

[Explanation of symbols]

Reference Signs List 1 n ⁺ -GaAs substrate 2 n-AlGaAs layer 3 i-AlGaAs cladding layer 4 i-GaAs waveguide layer 5 i-AlGaAs cladding layer 6 p-AlGaAs rib layer 7 p-type delta-doped layer 8 n-type delta-doped layer 9 electrode

Claims (1)

(57) [Claims] 1. An i-type semiconductor having a narrow energy band gap.
The body is made of n-type and p-type
Semiconductor waveguide with pin-type double heterostructure sandwiched between conductor layers
And an electron / hole pump in the i-layer of the semiconductor waveguide by photoexcitation.
Creates a feeling of band by generating plasma
Induced, the p-layer and the n-layer have electrodes, and have a reverse bias voltage.
The electron / hole plasma at high speed by applying
A nonlinear optical element that sweeps out of a waveguide,
N-type high impurity outside the hetero surface near the n-layer side of the terrorist structure
Concentration layer and heterostructure near the p-layer side of the pin heterostructure
Features a p-type high impurity concentration layer formed outside the surface
Nonlinear optical element.