JPH0632328B2 - Semiconductor optical bistable device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor optical bistable device that can be applied to an optical logic device used in a high-speed optical computer, an optical discrimination amplifier used in an all-optical repeater for optical communication, and the like. Is.

[Prior art]

The basic structure of the optical bistable element is the Fabry-Perot interferometer widely used in the optical field. The Fabry-Perot interferometer is a device in which, as shown in FIG. 1, two semi-transparent mirrors (half mirrors) 11 and 12 are arranged parallel to each other with a predetermined distance therebetween. Here, the semi-transparent mirror 11
A space between the first and the second portions is called a cavity 13, and a substance that allows light to pass through is placed therein. In order to realize optical bistability, the substance in the resonator 13 must be a substance having a nonlinear refractive index.

When laser light is focused on such a non-linear refraction material,
The refractive index of the substance changes depending on the intensity of laser light.
By changing the refractive index of the substance in the resonator 13, the resonance wavelength changes, and thus the phase relationship of light can be changed. Naturally, the transmittance also changes accordingly. Incident light 14 is incident on the resonator 13 from the outside through one of the semi-transparent mirrors 11, and a part of the light is reflected as reflected light 15 without entering the resonator 13, and a part of the light in the resonator 13 is a semi-transparent mirror. It is emitted from 12 as transmitted light 16.

FIG. 2 shows the relationship between the optical path length given by the product of the length of the substance in the resonator 13 and its refractive index, and the transmittance of the interferometer.
In the case of FIG. 1, since the resonator 13 has a fixed length, this can be regarded as the relationship between the refractive index and the transmittance. N is a positive integer, λ
Is a wavelength, and the optical path length is equal to an integral multiple of a half wavelength of the incident light 14, that is, In such cases, the interference is most constructive, and the transmitted light 16 becomes maximum. In FIG. 2, the transmittance is small between the peaks and hardly changes. As the optical path length approaches an integral multiple of a half wavelength, the transmittance gradually increases up to a certain optical path length, but increases sharply beyond that.

Since the substance in the resonator 13 is a non-linear refractive substance, the refractive index, that is, the optical path length can be changed by changing the intensity of the incident laser light. Therefore, first, the intensity of the incident light 14 is selected so that the refractive index of the substance has an optical path length in a region deviating from the peak in FIG. In this state, the transmittance is low.
Here, when the incident light intensity gradually increases, the refractive index and the optical path length gradually change, and the transmittance slightly increases.

When the incident light 14 reaches a certain light intensity, the refractive index of the substance and the light intensity in the resonator 13 become a positive feedback (returning) state in which they increase each other, and the internal interference is strengthened by the change in the refractive index. As the degree increases and the light intensity further increases, the refractive index changes more and more. When the incident light intensity reaches a certain value, the mutual strengthening between the change in the refractive index and the increase in the internal light intensity becomes very strong, and enters the peak region where the transmittance sharply increases in FIG. The transmittance is 1 at this peak.

Next, the incident light intensity is reduced. First, in the peak state, even if the incident light 14 is weakened to some extent, the transmittance is not immediately close. This is because the light intensity inside the resonator 13 is sufficient to maintain the refractive index and the optical path length corresponding to the maximum transmittance. Even further reduction of the incident light intensity results in only a slight decrease in transmittance. However, when the light intensity reaches a certain value, the refractive index and the light intensity in the resonator 13 mutually decrease. That is, a slight decrease in the incident light intensity causes a change in the refractive index to decrease the light intensity in the resonator 13, and a change in the intensity changes the refractive index, so that the transmittance sharply decreases.

The above phenomenon is shown by the relationship between the intensity of the incident light 14 and the intensity of the transmitted light 16, as shown in FIG.
It is a loop consisting of. A graph of this type is called a hysteresis loop. First, in the stable region (OFF state) 17, when the intensity of the incident light 14 is increased from a small value, the intensity of the transmitted light 16 initially increases only slightly. However, when the intensity of the incident light 14 reaches a certain value, the curve sharply rises and the transmitted light becomes extremely large. This rising portion is the switch-on operation area 18. Next, when the light intensity is decreased from this state, the curve follows a different path. At the beginning, it is a graph in which the transmitted light remains large even when the incident light is reduced. This region is the stable region (ON state) 19.

However, when the incident light decreases to a certain value, the curve sharply moves downward, and the transmitted light is greatly reduced even if the incident light is slightly weakened. This fall is the switch-off operation area 20.

Further, according to the configuration of FIG. 1, not only the bistable characteristic but also the
The differential gain can also be obtained. Assuming that the phase change of light when passing through the resonator once in the limit of incident light 0 is φ ₀ , φ ₀ is changed and the input / output characteristics of the light input power Pi and the output power Po are Figure 4 (A), (B), (C),
It changes in the order of (D), (E), and (F). Here, Fig. 4
(D), (E), and (F) show bistable characteristics, FIGS. 4 (A) to (C) show differential characteristics, and sometimes FIG. 4 (C) shows remarkable differential gain. .

Optical bistability can be achieved by using gas lasers and solid-state lasers, but considering the application to optical computers and optical communication devices, it is a device based on a semiconductor laser that is easy to miniaturize and integrate. It is desirable to realize it.

Figure 5 shows Nakai et al. "Optical Bistabi-lity in S
emiconductor Laser Amplifier ”IOOC'83,30, B1-2 (Jun
(e, Tokyo) presents a structural example of a semiconductor optical bistable device that has already been announced. In this semiconductor optical bistable element 20, an active layer (light emitting region) 23 of the optical bistable element is formed between semiconductor layers 21 and 22. Junctions of the semiconductor layers 21 and 22 having different compositions are formed on both sides of the active layer 23, and the cleavage planes at both ends perpendicular to the junction are incident surfaces 2.
4 and the exit surface 25. Here, the refractive index of the active layer 23 is set to be higher than the refractive indexes of the semiconductor layers 21 and 22 on both sides thereof. From the bias circuit 26, a direct current is transmitted via the stabilizing coil 27 to the optical bistable element 20.
And the current level thereof is controlled to be equal to or lower than the laser oscillation threshold of the optical bistable element 20. When the injection current into the optical bistable element 20 is sufficiently larger than the laser oscillation threshold value, laser oscillation is obtained in the active layer 23.

Explaining the correspondence with FIG. 1, the incident surface 24 is a semi-transmissive mirror 1.
The emission surface 25 corresponds to the semi-transmissive mirror 12, and the active layer 23 corresponds to the cavity 13. here,
The wavelength of the incident light beam 14 is the optical bistable element 2 with no input.
Longer than 0 resonance wavelength. The beam diameter of the incident light beam 14 is narrowed down by a condenser lens (collimator lens) 28,
It couples to the active layer 23 on the incident surface 24 side and propagates in the active layer 23. As the light incident on the active layer 23 increases, the injection carrier density in the active layer 23 decreases, as a result, the refractive index of the active layer 23 increases, and the resonant wavelength of the optical bistable element 20 shifts to the long wavelength side. . In this way, when the resonant wavelength of the optical bistable element 20 approaches the wavelength of the incident light beam 14,
As shown in FIG. 3, the light intensity of the output light 16 radiated to the outside from the active layer 23 on the emission surface 25 side suddenly passes from the OFF state 17 to the switch ON operation region 18 and then to the ON state 19 thereof.
Transition to. The output light is taken out as a parallel output light beam 16 by the condenser lens 29.

In the ON state 19, even if the power of the incident light 14 is increased, the amplification factor of the optical bistable element 20 is lowered, so that the power level of the output light 16 is almost constant.

Using the theoretical formula, the power level P of the incident light beam 14
The relationship expressed by the following equations (1) to (5) is approximately established between i and the power level Po of the emitted light beam 16.

G = exp [(g-α) · l ₁ ] (2) φ _o = 2πn ₁ l ₁ / λ (5) where r _i is the reflectance of the incident surface 24 and r _o is the exit surface 25.
Reflectance, g is saturated gain factor, alpha is the absorption coefficient of the active layer 23, l ₁ is the length of the active layer 23, g _o is the small signal input gain,
P _s is the saturated output power, and φ is the phase cyst of the light wave when it passes through the active layer 23 once. φ _o is an initial value of the phase shift from the resonance state, and is determined by the resonance wavelength of the optical bistable element 20 in the non-input light state and the wavelength of the incident light beam 14. n
₁ is the refractive index of the active layer 23 in the non-input light state. b is the non-waveguide ratio, which is the ratio of the decrease in the real part of the refractive index to the increase in the imaginary part of the refractive index. It is also assumed here that the incident light beam 14 is 100% coupled to the active layer 23.

FIG. 6 is a diagram showing the relationship between the incident light power P _i and the outgoing light power P _o in the GaAlAs optical bistable device according to the conventional structure shown in FIG. 5, and these powers P _i and P _o are
It is shown normalized by the saturation value Ps of the optical output when the above laser is operated as an amplifier. This relationship is described by K. Otsu Ka and S. Kobayashi in “Optical B
istability and Nonlinear Resonance in a Resonant-t
ype Semiconductor Laser Amplifier ”, Electronics L
Already announced in etters 31 st March 1983 Vol.19 NO.7 PP.262-263. In that case, (1) to (5) in equation _{b = 3, g o = 48cm} -1, d = 20cm -1, l 1 = 300μm, R i
= R _o = 0.3. Here, when φ _o is changed, the optical input / output characteristics change, Or It can be seen from FIG. 6 that the bistable characteristic is obtained, while the differential gain is obtained when φ _o = N _x . Here, N is a positive integer.

In the above configuration, φ _o changes if the wavelength λ of the incident light is changed, but φ _o cannot be controlled for the incident light of an arbitrary wavelength λ, and the selection or control of the bistable characteristic or the differential gain cannot be performed. The drawback was that it could not be done.

〔Purpose〕

An object of the present invention is to eliminate these drawbacks and to provide a semiconductor optical bistable device capable of freely selecting and controlling a differential gain and a bistable characteristic with respect to incident light of an arbitrary wavelength. It is in.

[Structure of Invention]

In order to achieve such an object, in the present invention, an active layer and a waveguide layer that is optically coupled to the active layer and has an absorption edge wavelength shorter than that of the active layer are provided, and the active layer and the waveguide are independently provided with a current. Is injected into the active layer, a current below the laser oscillation threshold is flown into the active layer, and the refractive index of the waveguide layer is changed by the plasma effect by changing the injection current. The signal light is input to the resonator, and the signal light is output from the Fabry-Perot resonator without photoelectrically returning the signal light.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 7 shows an embodiment of the present invention, in which parts corresponding to those in FIG. 5 are designated by the same reference numerals. A direct current is injected from the bias circuit 26 into the optical bistable element 20 through the stabilizing coil 27, and the current level thereof is controlled to be equal to or lower than the laser oscillation threshold of the optical bistable element 20.

In the present invention, the waveguide layer 33 is arranged between the semiconductor layers 31 and 32, and the waveguide layer 33 is optically coupled to the active layer 23. Here, the waveguide layer 33 is formed of a semiconductor having low loss with respect to the incident light beam 14, and the semiconductor layers 31 and 32 are configured so that current can be injected into the waveguide layer 33 independently of the active layer 23.

Here, it is preferable that the active layer 23 and the waveguide layer 33 are integrated on the same substrate as a semiconductor multilayer structure to form an optical bistable element in the form of an optical integrated circuit.

In FIG. 7, the bias circuit 34 is connected to the stabilizing coil 3
A direct current is injected into the waveguide layer 33 via 5. By controlling the direct current by the bias circuit 34,
The refractive index of the waveguide layer 33 is controllable, and the coupling surface 36 of the waveguide layer 33 with the active layer 23 allows highly efficient coupling with almost no reflection of the incident light 14. The optical bistable device 21 in which the active layer 23 and the waveguide layer 33 are integrated has a semiconductor multilayer structure. Here, the waveguide layer 3
3 is changed, the refractive index n of the waveguide layer 33 is changed.
_The change of ₂ is due to the plasma effect.

Here, the plasma effect means that the refractive index changes over anomalous dispersion by injecting a current (carrier) into the semiconductor, and this refractive index change is generally represented by the following formula (6).

In the formula, N is the number of carriers, n is the refractive index, ε is the dielectric constant, and m _c
Is the effective mass of the electron, and ω is the angular frequency of the incident light.

In an example of a specific configuration of the element having the above configuration,
The active layer 23 is formed of p-type Al _0.05 Ga _0.05 As, and the waveguide layer 33 is constituted by Al _0.35 Ga _0.65 As having an absorption edge wavelength shorter than the wavelength of light generated from this active layer 23. . As a result, the light generated in the active layer 23 can be guided without being absorbed much in the waveguide layer 33.

Further, the semiconductor layers 21 and 31 are integrally formed, the semiconductor layers 22 and 32 are integrally formed, and the former is p-type Al _0.5 Ga _0.5 As having a lower refractive index than the active layer 23 and the waveguide layer 33.
Layer, the latter of which is an n-type Al _0.5 Ga _0.5 As layer,
By sandwiching the active layer 23 and the waveguide layer 33 by these two layers, light can be confined in the active layer 23 and the waveguide layer 33, and further, the electrons injected into the active layer 23 can prevent such a hetero barrier. Therefore, the active layer 23 can be confined.

The active layer 23 and the waveguide layer 33 are directly coupled at the coupling portion 36. Each thickness of the active layer 23 and the waveguide layer 33 is set to an appropriate thickness so that the transmission refractive index and the electrolytic distribution of the active layer 23 and the waveguide layer 33 can be matched. By doing so, the reflection at the joint 36 is reduced, and
A highly efficient coupling of about 8% can be performed.

In addition, by separately providing electrodes to the semiconductor layers 21 and 22 and 31 and 32 for the active layer 23 and the waveguide layer 33, currents can be independently injected into the active layer 23 and the waveguide layer 33, respectively. can do.

The light input / output characteristics in the embodiment of the optical bistable device having the above-described structure according to the present invention are approximated by the above-mentioned formulas (1), (2), (3) and (4) and then formula (7). Can be expressed as

φ _o = 2π (n ₁ l ₁ + n ₂ l ₂ ) / λ (7) where n ₂ is the refractive index of the waveguide 33 and l ₂ is the waveguide layer 3
3 in length. Active layer 23 and waveguide layer 33 are 100%
It is assumed that they are coupled and that there is no loss in the waveguide layer 33.

According to the present invention, since the refractive index n ₂ of the waveguide layer 33 is changed by changing the current injected into the waveguide layer 33, φ _o is changed as can be seen from the equation (7). As a result, it becomes possible to select and control the bistable characteristic and the differential gain.

The increment Δφ _{o of} φ _o and the increment Δ of the refractive index n ₂ of the waveguide layer 33.
n ₂ has the following relationship.

FIG. 8 shows such a relationship between Δφ _o and Δn ₂ . Here, l ₂ = 200 μm and λ = 0.84 μm. When Δφ _o changes, as shown in FIG. 6, a bistable characteristic and a differential gain can be obtained corresponding to the value of φ _o . Further, φ _o changes even if the wavelength λ of the incident light changes, but since the value of φ _o can be freely selected by controlling n ₂ , the selection and control of the bistable characteristic and the differential gain can be performed. It will be possible.

For example, when the optical input / output characteristic when no current is applied to the waveguide layer 33 is the bistable characteristic as the characteristic I in FIG. 9 depending on the wavelength and temperature conditions of the incident light, the waveguide layer 33 Current is injected into the waveguide layer 33 to change the refractive index n ₂ of the waveguide layer 33 to 8.4 × 10 ^−4.
When it is made smaller, the light input / output characteristic becomes the characteristic III in FIG. 9, showing a differential gain. Further, a current is injected into the waveguide layer 33, which is 1.68 as compared with the case where the refractive index n ₂ is not injected into the current.
If it is reduced by × 10 ^-3 , the optical input / output characteristics are those shown in Fig. 9.
II, showing bistable characteristics.

Also, depending on the wavelength of incident light and the temperature conditions, the waveguide layer 3
The optical input / output characteristic when current is not injected into 3 shows the characteristic III in FIG. 9, and when the differential gain is obtained, the waveguide layer 3
3 is injected to set the refractive index n ₂ of the waveguide layer 33 to 8.4.
If it is reduced by × 10 ^-4 , the optical input / output characteristics are those shown in Fig. 9.
It becomes II and becomes a bistable characteristic.

_{Here, l 2 = 200μm, λ =} 0.84μm, b = 3, g o = 48c
m ^-1 , α = 20 cm ^-1 , l ₁ = 300 µm, R _i = R _o = 0.3.

[Effect] As described above, in the present invention, by providing the waveguide layer and controlling the current injected into the waveguide layer, the bistable characteristic and the differential gain of the incident light of an arbitrary wavelength are controlled. It is possible to realize a small semiconductor optical bistable device that can be selected and controlled.

[Brief description of drawings]

1 is a diagram showing the concept of the Fabry-Perot interferometer, FIG. 2 is a diagram showing the relationship between the transmittance and the optical path length of the Fabry-Perot interferometer, and FIG. 3 is the hysteresis characteristic of the optical bistable element. Figures 4 (A) to 4 (F) show each optical input / output when the amount of phase change φ _o of the light when passing through the resonator once at the limit of incident light 0 is changed in 6 ways. Fig. 5 is a diagram showing the characteristics, Fig. 5 is a diagram showing a configuration example of a conventional semiconductor optical bistable element, and Fig. 6 is an optical input / output characteristic when φ _o is changed assuming a GaAlAs semiconductor optical bistable element. shows a seventh figure diagram showing an embodiment of the present invention, Figure 8 shows the increment [Delta] [phi _o of the n ₂ increment [Delta] n ₂ relationship phi _o figure 9 Figure practice of the present invention It is a figure which shows the optical input / output characteristic in an example. 14 ... Incident light beam, 16 ... Outgoing light beam, 20 ... Semiconductor optical bistable element, 21, 22 ... Semiconductor layer, 23 ... Active layer, 24 ... Incident end face, 25 ... Outgoing end face, 26 ...... Bias circuit, 27 …… Stabilizing coil, 28 …… Condenser lens, 29 …… Outgoing collimator lens, 31, 32 …… Semiconductor layer, 33 …… Waveguide layer, 34 …… Bias circuit, 35…. ... Stabilizing coil, 36 ... A coupling portion between the active layer 23 and the waveguide layer 33.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP 59-12421 (JP, A) JP 60-124982 (JP, A) JP 60-229387 (JP, A)

Claims (1)

[Claims] 1. An active layer and a waveguide layer which is optically coupled with the active layer and has an absorption edge wavelength shorter than that of the active layer are provided inside the Fabry-Perot resonator, and the active layer and the waveguide are provided in the active layer and the waveguide. Currents can be injected independently of each other, and a current of a level below the laser oscillation threshold value is flown into the active layer, and the refractive index of the waveguide layer is changed by the plasma effect by changing the injection current. None, a semiconductor optical bistable device characterized in that a signal light is input to the Fabry-Perot resonator, and the signal light is output from the Fabry-Perot resonator without photoelectrically returning the signal light. .