JPH07113711B2 - Semiconductor waveguide type optical switch - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical switch that opens and closes an optical signal in an optical transmission line in the fields of optical communication and optical information processing.

[Prior art and its problems]

With the development of optical communication systems in recent years, systems that provide unprecedented new functions and services have been considered.
One of the devices required for such a system is an optical switch using a semiconductor, which has the possibility of ultra-high-speed switching, low-voltage operation, small size, and easy integration. Among them, an optical switch using the Franz-Keldysh effect, which has relatively excellent performance in terms of downsizing and voltage reduction, can be considered. However, this optical switch also has the following drawbacks. Hereinafter, a waveguide type optical switch using the Franz-Keldysh effect in a single guide layer will be described.

The Franz-Keldysh effect is the effect that the fundamental absorption edge moves to the long wavelength side in response to the application of an electric field. The band-cap wavelength λ _g of the waveguide layer of the switch is set slightly shorter than the wavelength λ of the light source. The waveguide layer (guide layer) referred to here is a semiconductor layer through which light propagates. When the electric field is not applied to the waveguiding layer, λ is longer than λ _g , so the propagating light is not absorbed, but when an electric field is applied and the basic absorption edge spreads to the long wavelength side up to λ or more, the corresponding light Absorption of. By utilizing this effect, it is possible to manufacture a gate switch that controls absorption of light by an electric field.

Fig. 4 shows the change in absorption coefficient due to the Franz-Keldysh effect as a function of the band cap energy E _g and the photon energy of the incident light. Energy difference from Is shown on the horizontal axis. This Figure 4 is "Applied Physics Letters (Appl.Phys.Lett.34 (1979) 744)"
It is a quotation of what is described in. Here, the case where E = 0 V / cm and 5 × 10 ⁴ V / cm is shown. When an optical gate switch is actually manufactured by using the Franz-Keldysh effect, the wavelength λ of the light source is set near the long wavelength of the band-cap wavelength λ _g of the switch, that is, When is set to a small value, as can be seen from Fig. 4,
The shape of the fundamental absorption edge does not cut sharply at E _g , and has a large tail on the long wavelength side, so E = 0 V / cm
In the case of, too, the absorption is large, and the waveguide loss becomes large as a switch. For example In that case, the waveguide loss at that time will be a very large value of 50 dB / cm. Therefore, in order to reduce the waveguide loss, the energy difference between the bandgap energy and the photon energy of the incident light Must be above 80-100 meV. However When is increased, the change in absorption coefficient with respect to a certain change in the electric field becomes small, and a sufficient extinction ratio cannot be obtained.
As described above, in the optical gate switch using the Franz-Keldysh effect in the single guide layer, the one having sufficient performance such as low loss, low potential, and high extinction ratio due to the skirting of the basic absorption edge of the crystal. It was hard to get.

In order to solve the problem of skirting of the fundamental absorption edge, when a multilayer structure (hereinafter referred to as a multiple quantum well structure) in which layers with different forbidden band widths of 500 Å or less are alternately laminated is used for the guide layer, Waveguide-type optical modulation is considered (1985, National Electro-Communications Society General Conference S3-4). FIG. 5 shows an example. n ⁺ -GaAs n by molecular beam epitaxy on the substrate 21 ⁺ -Al _0.3 Ga _0.7 As cladding layer 22 1.
5 μm growth, and then n ^-- GaAs / AlGaAs multiple quantum well guide 23 as GaAs quantum well layer 100Å, Al _0.2 Ga _0.8 As barrier layer 30
16 cycles of 0Å, and p ⁺ −Al _0.3 Ga _0.7 As clad layer 24 of 1.5
A μm, p ⁺ -GaAs cap layer 25 is grown, ohmic electrodes 26 and 27 are attached to the n-side and p-side to form a stripe-shaped high-mesa structure waveguide. The optical absorption spectrum of such a multiple quantum well structure has a stepwise change in the density of states,
Due to the quantum size effect such as the appearance of a sharp exciton peak, the wavelength at the fundamental absorption edge becomes sharper and sharper. Further, when an electric field is applied to this multi-quantized well structure, the absorption edge moves to the long wavelength side as the exciton peak moves to the long wavelength side, and the tail ends. Utilizing this fact, an optical gate switch that realizes modulation of light absorption by an electric field in the vicinity of the long wavelength side of the absorption edge is realized also in the multiple quantum well structure waveguide. In this structure, the absorption edge is steeper than in the case of a single-layer waveguide, so the wavelength of the light source can be made closer to the absorption edge, and high performance such as low voltage and high extinction ratio is expected. it can. However, even in this case, there are still the following problems. The absorption edge with excitons in the multiple quantum well structure is steeper than the basic absorption edge in the case of a single layer, but it is not an ideal state. There is a skirt that cannot be removed at the absorption edge due to the fluctuation of the interface of the multi-quantum well layer and the influence of impurities. In particular, the influence increases as the wavelength of the incident light approaches the absorption edge. Therefore, even if a multiple quantum well structure waveguide is used, it is very difficult to bring the wavelength of incident light close to the absorption edge to achieve a low voltage and a high extinction ratio and further reduce the waveguide loss. .

As described above, in the conventional structure, a waveguide type optical switch using electric field application as a control means has not been obtained with excellent performance in terms of operating voltage, extinction ratio and loss.

[Object of the Invention]

The object of the present invention is to eliminate the drawbacks of the waveguide type optical switch that controls the light propagating through the guide layer by applying an electric field as described above, and is compact and suitable for integration, and operates at a low voltage.
It is to provide a waveguide type optical switch with a high consumption ratio and low loss.

[Structure of Invention]

The present invention relates to a semiconductor waveguide type optical switch for controlling guided light propagating in a waveguide layer by applying an electric field, wherein the waveguide layer exists in a single layer and at least one of upper and lower sides of the single layer. And a multiple quantum well layer to
It has a means for applying an electric field perpendicularly to the hetero interface of the multiple quantum well layer, and the absorption peak wavelength of the exciton of the ground level of the multiple quantum well structure is set near a short wavelength with respect to the wavelength of the modulated light. For the modulated light, the average refractive index of the multiple quantum well layer is slightly smaller than the refractive index of the single layer before the electric field is applied, and the average refractive index of the multiple quantum well layer is smaller than that of the single layer by applying the electric field. It is a semiconductor waveguide type optical switch having a refractive index that increases and an absorption coefficient of the multiple quantum well layer when an electric field is applied is higher than that before the application of an electric field.

[Principle of Invention]

The present invention has solved the problems of the prior art by adopting the above configuration. The principle of the present invention will be described with reference to FIG. As described above, a sharp exciton peak is seen near the absorption edge in the absorption spectrum of the multiple quantum well. Further, there is a Kramers-Kronig relationship between the absorption and the refractive index, and the presence of the exciton peak causes the refractive index spectrum to change greatly near the exciton peak wavelength. The states of the absorption spectrum and the refractive index spectrum are shown in FIGS. 2 (a) and 2 (b), respectively. Next, the absorption and the refractive index when an electric field is applied to the multiple quantum well will be described. When an electric field is applied to the multiple quantum well, the exciton peak moves to the long wavelength side and its half-width widens. As the exciton peak moves to the long wavelength side, the refractive index spectrum also moves to the long wavelength side. The states of the absorption spectrum and the refractive index spectrum at that time are shown in FIG.
It shows in (d). When the exciton peak wavelength in the absence of an electric field is set near the short wavelength of the modulated light, the absorption coefficient is increased by 10 ³ to 10 ⁴ cm ^-1 and the refractive index is increased by about 10 ^-2 by applying the electric field. Therefore, by applying both the large change in absorption coefficient and the change in refractive index due to the electric field to the waveguide type optical switch, a high performance waveguide type optical switch, which is unprecedented in terms of waveguide loss, operating voltage, extinction ratio, etc. Is obtained.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view of a waveguide type optical switch showing one embodiment of the present invention. In this example, GaAs / AlGaA
A description will be given of the one using the s-based semiconductor material.

The structure of this waveguide type optical switch will be described together with its structure method. n ⁺ -AlGaAs layer 2 by molecular beam epitaxy on the n ⁺ -GaAs substrate 1, n ^- -AlGaAs single layer 3, n ^- -GaAs / AlG
An aAs multiple quantum well layer 4 and ap ⁺ -AlGaAs layer 5 are grown. Next, in order to form a three-dimensional guide, both sides of the portion to be the waveguide are etched to reach the n ⁺ -AlGaAs layer 2, and both sides of the waveguide are subjected to the liquid phase epitaxy method to form the n ^-- AlGaAs buried layer 6 Then, the stripe electrode 7 is attached to the p side and the electrode 8 is attached to the n side.

Here, the thickness and composition of each of the well layer and the barrier layer of the multiple quantum well layer 4 are (GaAs, Al _0.5 Ga _0.5 As) = (85Å, 95Å), which is 12 periods, and the composition of the single layer 3 is Al _0.15 Ga _0.85. As, with a layer thickness of 0.2 μm, n ⁺ -AlGaAs layer 2, p ⁺ -AlGaAs layer 5 and n ^-- A
The composition of the lGaAs buried layer 6 is Al _0.3 Ga _0.7 As, n ⁺ and p ⁺
The layer thickness of the AlGaAs layers 2 and 5 is set to about 1 to 2 μm. In this way, a three-dimensional waveguide including the single layer 3 and the multiple quantum well layer 4 is formed. Ohmic contact is formed by using gold / zinc for the p-side electrode 7 and gold / germanium for the n-side electrode 8. The p-side electrode and the n-side electrode 8 are the multiple quantum well layers 4
Means for applying an electric field perpendicularly to the hetero interface.
The manufacturing process described above is merely an example, and the present invention is not limited to this process. The multiple quantum well layer may be grown by using a vapor phase growth method, a metal organic material method or the like, and the buried layer may be formed by a vapor phase growth method. The three-dimensional waveguide may have a rib shape or a grooved shape. Furthermore, the multiple quantum well layer may be formed under a single layer,
Also, multiple quantum well layers may be formed above and below a single layer.

Next, the operation of the waveguide type optical switch of this embodiment will be briefly described. Light incident on the n ^-- AlGaAs single layer 3 is incident on the electrode 7
When no reverse bias voltage is applied between the electrode and the electrode 8, that is, n ⁻ −
When no electric field is applied to the GaAs / AlGaAs multiple quantum well layer 4, the n ⁻ -AlGaAs single layer 3 having the highest refractive index is mainly guided. Since this layer has a very low loss with respect to the incident light, the light propagates as it is and can be taken out as the emitted light 10. n ^{- -}
When an electric field is applied to the GaAs / AlGaAs multiple quantum well layer 4, the average refractive index increases as the exciton peak of the multiple quantum well layer 4 moves, and becomes higher than the refractive index of the single layer 3. Then, the incident light 9 mainly propagates through the multiple quantum well layer 4. The absorption coefficient increases as the refractive index of the multiple quantum well layer 4 increases. Therefore, the incident light 9 is largely absorbed while propagating through the multiple quantum well layer 4. Therefore,
The emitted light 10 is extinguished and cannot be taken out. In this way, a waveguide type optical switch utilizing the change in the refractive index and the absorption coefficient due to the electric field becomes possible.

Further, this waveguide type optical switch will be described in detail with reference to FIGS. In FIG. 2, (a) and (b) are respectively an absorption spectrum and a refractive index spectrum when an electric field is not applied to the multiple quantum well layer 4, and (c) and (d) are each the multiple quantum well layer 4. 2 is an absorption spectrum and a refractive index spectrum when an electric field is applied to the. Third
In the figure, (a) and (b) are the refractive index distribution of the waveguide and the field distribution of the guided light when no electric field is applied, and (c) and (d) are the cases when an electric field is applied. And the field distribution of the guided light.

In the absorption spectrum of FIG. 2 (a), the wavelength of the incident light is λ ₃ = 0.84 μm And In the GaAs / AlGaAs multiple quantum well structure described above, n
= 1 and the transition wavelength between the electron and the heavy hole is λ ₁ , and the exciton peak wavelength is λ ₂ , they are near the short wavelength of the incident light wavelength λ ₃ , and λ ₁ is ~ 0.82.
μm and λ ₂ are up to 0.83 μm. While receiving this multiple quantum well layer 4 is alpha ₁ absorption at the wavelength lambda _3, the absorption of alpha ₁ is sufficiently small compared to the absorption becomes Ekishitonpiku. Further, in this composition, the average refractive index of the multiple quantum well layer 4 is n ₁ = 3.48, the refractive index of the single layer 3 is n ₃ = 3.51, and the n ⁺ and p ⁺ -AlGaAs layers 2 with respect to the wavelength λ ₃ . , 5 has a refractive index n
₄ = 3.4. The refractive index distribution at that time is shown in FIG. Also, the field distribution of the guided light at that time is shown in FIG. 3 (b), 11 is a portion that absorbs α _{1 in} the region of the multiple quantum well layer 4, and 12 is most of the absorption in the region of the single layer 3. It is the part that does not receive Incident light is guided mainly in the portion of the single layer 3 having the highest refractive index n ₂ = 3.51 and the seeping into the multiple quantum well layer 4 is slight. Moreover, since the absorption coefficient α ₁ in the exudation portion is relatively small, the waveguide loss as a whole is sufficiently smaller than that of the conventional waveguides which are all composed of multiple quantum well layers.

Next, a case where an electric field is applied to the multiple quantum well layer 4 will be described. The absorption spectrum at that time is shown in FIG. In this case, when a reverse bias voltage of about 4 V is applied, the electric field strength is 5 × 10 ⁴ to 10 ⁵ V / cm, and at that time, the exciton peak is λ ₂ = 0.83 μm to λ ₄ = 0.
About 20 nm to 85 μm, converted to energy and moved to the low energy side of 30-40 meV. Therefore, the wavelength of incident light λ ₃ = 0.
The absorption coefficient α ₂ for 84 μm is much larger than that of α ₁ and is about 10 ⁴ cm ⁻¹ . As the entire absorption spectrum including the exciton peak moves to the long wavelength side by the application of the electric field, the refractive index spectrum also moves to the long wavelength side as shown in FIG. 2 (d). Therefore, the wavelength λ ₃
= 0.84 μm, the average refractive index of the multiple quantum well layer 4 is
When no electric field was applied, n ₁ = 3.48, whereas when an electric field was applied, n ₂ = 3.55, which was large, and the refractive index of the single layer 3 was n ₃ = 3.51 or more. The refractive index distribution is shown in FIG. The field distribution of light at that time is shown in FIG. 3 (d), and 13 is the region of the multiple quantum well layer 4 with α ₂
Is a region that absorbs a very large amount of ~ 10 ⁴ cm ^-1 , and 14 is a region that hardly absorbs in the region of the monolayer 3. Incident light is mainly guided through the part of the multiple quantum well layer 4 having the highest refractive index n ₂ = 3.55. Therefore, most of the guided light is absorbed by the multiple quantum well layer, resulting in a large extinction ratio.

From the above, the waveguide type optical switch with this structure has very little waveguide loss, operates at a low voltage (5 V or less), and has a large extinction ratio. It is also possible, and although not mentioned in particular, the element length is 1 mm or less and it can be sufficiently manufactured.

The wavelength of the incident light, the composition of each layer of AlGaAs, the layer thickness, the layer thickness of each GaAs / AlGaAs multiple quantum well, etc. used here are merely examples, and the change in the absorption coefficient due to the electric field and the refractive index. The present invention is not particularly limited to the embodiment as long as the same effect as that of the embodiment can be obtained due to changes or the like.

Further, as the semiconductor material, not only the GaAs / AlGaAs-based material but also InGaAsP / InP, InGaAs / InAlAs, etc. may be used.

〔The invention's effect〕

As described in detail above, according to the present invention, the waveguide loss can be significantly reduced, and the operating voltage, the extinction ratio, and the conductivity can be reduced as compared with the conventional waveguide type optical switch in which the multiple quantum well layer is used as the waveguide. In terms of wave loss, a high-performance guided-wave optical switch becomes possible, which will contribute to the realization of future optical functional devices, optical circuits, or optical communication and optical information processing systems that integrate and systemize them. Is.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the waveguide type optical switch according to the present invention, and FIG. 2 is a diagram for explaining the operation principle of the waveguide type optical switch of the present invention. FIGS. 2 (c) and 2 (a) and 2 (b) are views showing the absorption spectrum and the refractive index spectrum of the multiple quantum well layer when no electric field is applied, respectively.
FIG. 3D is a diagram showing an absorption spectrum and a refractive index spectrum of the multiple quantum well layer when an electric field is applied, and FIG. 3 is a diagram for explaining an embodiment of the waveguide type optical switch according to the present invention. 3 (a) and 3 (b) respectively show the refractive index distribution of the waveguide and the field distribution of the guided light when no electric field is applied, and FIGS. 3 (c) and 3 (d) show the electric field. FIG. 4 is a diagram showing a refractive index distribution of a waveguide and a field distribution of guided light when applied, FIG. 4 is a diagram for explaining a Franz-Keldysh effect in a single layer structure, and FIG. 5 is a conventional multiple quantum well. It is a figure for explaining a waveguide type optical switch which made only a layer into a guide layer. 1,21 ...... n ⁺ -GaAs substrate 2 ...... n ⁺ -AlGaAs layer 3 ...... n ^-- AlGaAs single layer 4 ...... n ^-- GaAs / AlGaAs multiple quantum well layer 5 ...... p ⁺ -AlGaAs layer 6 ... … AlGaAs buried layer 7,8,26,27 …… Electrode 9 …… Incoming light 10 …… Emitting light 11,13 …… The part that guides the multiple quantum well layer by the field distribution of guided light 12,14 …… Waveform light field distribution that guides a single layer 22 …… n ⁺ −AlGaAs cladding layer 23 …… n ⁻ −GaAs / AlGaAs multiple quantum well guide 24 …… p ⁺ −AlGaAs cladding layer 25 …… p ⁺ −GaAs Cap layer

Claims (1)

[Claims] 1. A semiconductor waveguide type optical switch for controlling guided light propagating in a waveguide layer by applying an electric field, wherein the waveguide layer is a single layer and at least one of upper and lower sides of the single layer. Existing multiple quantum well layers, having means for applying an electric field perpendicular to the hetero interface of the multiple quantum well layers, and the absorption peak wavelength of excitons at the ground level of the multiple quantum well structure is modulated. The wavelength of light is set near a short wavelength, and the average refractive index of the multiple quantum well layer is slightly smaller than the refractive index of the single layer before the electric field is applied to the modulated light. By having a refractive index higher than that of the single layer, and the absorption coefficient of the multi-quantum well layer when an electric field is applied is higher than that before the electric field is applied. Guide Waveguide type optical switch.