JPS6247620A - Waveguide type optical switch - Google Patents

Abstract

PURPOSE:To make and break a light signal with a low voltage and low loss by forming a semiconductor optical waveguide into multiplex quantium well structure and largely changing absorptivity and refractive index according to the electric field to be impressed. CONSTITUTION:Incident light 9 on a single n<->-AlGaAs layer 3 conducts mainly in a layer 3 having the highest refractive index when a reverse bias voltage is not impressed between an electrode 7 and an electrode 8 and when an electric field is not impressed to the n<->-GaAs/AlGaAs multiplex quantum well layer 4. This layer 3 propagates the incident light as it is with a low loss. An exciton peak moves and the average refractive index increases when the electric field is impressed to the layer 4. The incident light 9 mainly propagates in the layer 4 and is absorbed by the increase of the refractive index. The light is thus made and broken with the low voltage and low loss when the electric field is controlled. The optical switch is miniaturized and the integration thereof is made easy.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is applicable to the field of optical communication and optical information processing.
This invention relates to an optical switch that opens and closes optical signals in an optical transmission line.

[Prior art and its problems]

With the recent development of optical communication systems, systems that provide new functions and services that have not existed before are being considered.

One of the devices required in such a system is an optical switch using a semiconductor, which has the potential of ultra-high-speed switching, low-voltage operation, small size, and easy integration. Among these, an optical switch using the Franz Keldytsch effect is considered, which has relatively excellent performance in terms of miniaturization and low voltage. However, this optical switch also had the following drawbacks. A waveguide optical switch using the Franz Keldytsch effect in a single guide layer will be described below.

The Franz Keldytsch effect is an effect in which the fundamental absorption edge shifts toward longer wavelengths when an electric field is applied. The bandgap wavelength λ9 of the waveguide layer of the switch is set to be 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 no electric field is applied to the waveguide layer, the propagating light is not absorbed because λ is longer than λ9, but when an electric field is applied and the fundamental absorption edge spreads to longer wavelengths beyond λ, the corresponding light Absorption occurs. By utilizing this effect, it is possible to fabricate a gate switch that controls light absorption using an electric field.

FIG. 4 shows the change in absorption coefficient due to the Franz Keldytsch effect, with the energy difference (E9-hω) between the band gap energy E9 and the photon energy hω of the incident light being plotted on the horizontal axis. This figure 4 is based on the Applied Physics Letters (Appl, Phys, Lett,
34 (1979) 744) J. Here E=OV/cm and 5
The case where X10'V/cm is shown. When an optical gate switch is actually manufactured using the Franz Keldytsch effect, if the wavelength λ of the light source is set to a value close to the long wavelength of the switch's bandgap wavelength λ, that is, (E, -hω) is small, as shown in Figure 4. As can be seen from the figure, the shape of the fundamental absorption edge does not cut sharply at E, but has a large tail on the long wavelength side, so it receives large absorption even when E = OV/cm. As a switch, the waveguide loss will be large. For example (Eg−h ω
)=4Qm e V, the waveguide loss at that time would be a very large value of 50 dB/cm. Therefore, in order to reduce the waveguide loss, the energy difference (E, -hω) between the band gap energy and the photon energy of the incident light must be 80 to 1QQmeV or more. However, if E, -hω) is increased, the change in absorption coefficient with respect to a certain change in electric field strength becomes small, making it impossible to obtain a sufficient extinction ratio. In this way, in an optical gate switch using the Franz Keldytsch effect in a single-layer guide layer, sufficient performance such as low loss, low voltage, and high extinction ratio can be obtained due to the tailing of the basic absorption edge of the crystal. It was difficult.

In order to solve this problem of the tailing of the basic absorption edge, a multilayer structure (hereinafter referred to as a multiple quantum well structure) in which layers with different forbidden band widths of 500 A or less are laminated alternately is used for the guide layer. A waveguide type optical modulator is being considered (1985 IEICE General Conference 33-4
). FIG. 5 is an example. n”-GaΔS substrate 21
n”-A I26 by molecular beam epitaxial method
．． 3G ao, tAS cladding layer 22 of 1.5 μm
Then, 100 GaAs quantum well layers are grown as n--GaAs/AβGaAs multiple quantum well guide 23, A
A βo, zGao, eAs barrier layer of 300 layers was grown for 16 cycles, and a p"Alo, 3G aa, 7A S cladding layer 24 of 1.5 μm Sp"-C+aAs cap layer 25 was grown, and ohmic electrodes 26 were formed on the n and p sides. 27 to form a waveguide with a striped high mesa structure. In the optical absorption spectrum of such a multi-quantum well structure, shortening of wavelength and steepening of the fundamental absorption edge are observed due to quantum size effects such as a step-like change in the density of states and the appearance of a sharp exciton peak. Furthermore, when an electric field is applied to this multi-quantum well structure, the exciton peak moves to the long wavelength side, and the absorption edge moves to the long wavelength side, making it tail. Taking advantage of this, an optical gate switch has been realized that modulates light absorption by an electric field near the long wavelength side of the absorption edge in a multi-quantum well structured 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 brought closer to the absorption edge, which is expected to lead to higher performance such as lower voltage and higher extinction ratio. can. However, even in this case, there were the following problems. Although the absorption edge with excitons in the multi-quantum well structure is steeper than the fundamental absorption edge in the case of a single layer, this is not an ideal situation. There is a tail at the absorption edge that cannot be removed due to fluctuations at the interface of the multiple quantum well layer, the influence of impurities, etc. In particular, the effect becomes greater as the wavelength of the incident light approaches the absorption edge. Therefore, even if a multi-quantum well structured waveguide is used, it is extremely difficult to bring the wavelength of the incident light closer to the absorption edge, lower the voltage, increase the extinction ratio, and further reduce the waveguide loss. .

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

[Purpose of the invention]

The purpose of the present invention is to eliminate the drawbacks of the conventional waveguide optical switch that controls light propagating through a guide layer by applying an electric field, as described above, to be small and suitable for integration, operate at low voltage, and have high extinction. The object of the present invention is to provide a waveguide type optical switch with relatively low loss.

[Structure of the invention]

The present invention provides a semiconductor waveguide optical switch in which guided light propagating through a guide layer is controlled by applying an electric field, wherein the guide layer is present in a single layer and at least one above and below the single layer. a multilayer structure in which semiconductor layers each having a thickness of less than 500 nm and semiconductor layers having a forbidden band width larger than the forbidden band width of the semiconductor layer are laminated alternately, and the layer is perpendicular to the heterointerface of the multilayer structure. means for applying an electric field, the absorption peak wavelength of excitons at the ground level of the multilayer structure is set near a short wavelength with respect to the wavelength of the modulated light, and the average of the multilayer structure is set with respect to the modulated light. The refractive index of the single layer is slightly smaller than that of the single layer.

[Principle of the invention]

The present invention has solved the problems of the prior art by adopting the above-described configuration. The principle of the present invention will be explained using FIG. 2. As mentioned above, the absorption spectrum of a multiple quantum well has a sharp exciton peak near the absorption edge. Furthermore, there is a Kramers-Kronig relationship between absorption and refractive index, and the presence of an exciton peak causes the refractive index spectrum to show a large change near the exciton peak wavelength. Their absorption spectra and refractive index spectra are shown in FIGS. 2(a) and 2(b), respectively. Next, we will discuss absorption and refractive index when an electric field is applied to a multiple quantum well. When an electric field is applied to a multiple quantum well, the exciton peak moves toward longer wavelengths and its half-width widens. As the exciton peak moves toward longer wavelengths, the refractive index spectrum also shifts toward longer wavelengths. The absorption spectrum and refractive index spectrum at that time are shown in Figure 2 (C) and
Shown in (d).

If the peak wavelength of excitons in the absence of an electric field is set near the shorter wavelength of the modulated light, the application of the electric field will increase the absorption coefficient by 10 3 to 10 cm and the refractive index by about 10 −2 . Therefore, by applying both the large absorption coefficient change and refractive index change caused by the electric field to a waveguide optical switch, a waveguide optical switch with unprecedented high performance 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 diagram showing one embodiment of the present invention. In this example, a case using a GaAs/AfGaAs semiconductor material will be explained, and FIG. 1 shows that the guide layer of the present invention is composed of a single layer and an adjacent multiple quantum well layer, and that guided light is guided by an electric field. A perspective view of a controlling waveguide optical switch is shown.

The structure of this waveguide type optical switch will be explained along with its manufacturing method. An n+-AβGaAs cladding layer 2.n- is formed on the n''-GaAs substrate 1 by molecular beam epitaxy.
-AβGaAs single layer3. n--GaAs/Aj7Ga
As multiple quantum well layer 4. A p''-AβCyaAs cladding layer 5 is grown.Next, in order to form a three-dimensional guide, both sides of the portion that will become the waveguide are etched so that n=-
The AβQaAs cladding layer 2 is dropped until it reaches the AβQaAs cladding layer 2, and both sides of the waveguide are coated with n-
--Aj! It is buried with a GaAs buried layer 6, and finally a stripe electrode 7 is attached to the p side and an electrode 8 is attached to the n side.

Here, the thickness and composition of each well layer and barrier layer of the multiple Doji well layer 4 are (GaAs, Aβo, 5Gao, 5As) = (
85 people, 95 people) and it is repeated 12 times, the composition of single layer 3 is Aβo, +sG ao, a5A S binding layer thickness is 0.2
The compositions of the cladding layer 25 and the buried layer 6 are Aβo, *Ga(1,7As), and the thickness of each cladding layer is about 1 to 2 μm.In this way, the single layer 3 and A three-dimensional waveguide consisting of a multiple quantum well layer 4 is formed.Ohmic contact is formed using gold and zinc for the p-side electrode 7 and gold and germanium for the n-side electrode 8.p
The side electrode and the n1.111 electrode 8 are the multi-quantum well layer 4
means for applying an electric field perpendicularly to the heterointerface of the

The manufacturing process described above is just an example and is not limited to this process. The multiple quantum well layer may be grown using a vapor phase growth method, a metal organic material method, etc., and the buried layer may be grown using a vapor phase growth method. Further, the shape of the three-dimensional waveguide may be a rib type or a grooved shape. Furthermore, the multiple quantum well layer may be formed under a single layer,
Further, multiple quantum well layers may be formed above and below the single layer.

Next, the operation of the waveguide optical switch of this example will be briefly described. The incident light 9 on the n--AfGaAs single layer guide 3 is generated when no reverse bias voltage is applied between the electrodes 7 and 8, that is, when no electric field is applied to the rr-CraAs/AβGaAs multiple quantum well guide 4. , the wave is guided through an n--AβGaAs single layer guide 3 having the highest refractive index.

Since this layer has very low loss for incident light, the light is transmitted as it is and can be extracted as output light 10. n--G
When an electric field is applied to the aAs/Aj7GaΔS multiple quantum well guide 4, the average refractive index increases as the exciton peak of the multiple quantum well 4 moves and becomes equal to or higher than the refractive index of the single layer guide 3.

Then, the incident light 9 comes to mainly propagate through the multiple quantum well guide 4. As the refractive index of the multiple quantum well 4 increases, the absorption coefficient also increases. Therefore, the incident light 9 undergoes significant absorption while propagating through the multiple quantum well guide 4. Therefore,
The emitted light 10 is extinguished and cannot be extracted. In this way, a waveguide type optical switch that utilizes changes in refractive index and absorption coefficient due to an electric field becomes possible.

Furthermore, this waveguide type optical switch will be explained in detail using FIGS. 2 and 3. In Figure 2, (a) and (d) are the absorption spectrum and refractive index spectrum, respectively, when no electric field is applied to the multiple quantum well layer, and (C) and (d)
) are the absorption spectrum and refractive index spectrum, respectively, when an electric field is applied to the multiple quantum well layer. In Figure 3, (a) and 3) are the refractive index distribution of the waveguide layer and the field distribution of the guided light, respectively, when no electric field is applied, and (C)
, (d) are the refractive index distribution of the waveguide layer and the field distribution of the guided light when an electric field is applied, respectively.

In the absorption spectrum of FIG. 2(a), the wavelength of the incident light is λ3=0.84 μm (h ω-1,47 e v). In the GaAs/AfGaAs multiple quantum well structure described above, if the transition wavelength between an n=1 electron and a heavy hole is λ1, and the peak wavelength of an exciton is 22, they are near the short wavelength of the incident light wavelength λ3, and λ1 is ~0.
82 μm, and λ2 is ~0.83 μm.

At wavelength λ3, this multi-quantum well layer experiences absorption α1, but the absorption α1 is sufficiently small compared to the absorption due to the exciton peak. In addition, in this composition, the average refractive index of the multi-quantum well layer for wavelength λ3 is n+
= 3.48, the refractive index of the single layer guide is n3 = 3.51,
The refractive index of the cladding layer is n=3.4. The refractive index distribution at that time is shown in FIG. 3(a). The field distribution of the guided light at that time is shown in Figure 3, etc., where 11 is the region of the multi-quantum well layer that receives absorption α1, and 12 is the region of the single layer guide that receives almost no absorption. be. The incident light is mainly guided through the single-layer guide portion with the highest refractive index of n2=3.51, and only a small amount of light leaks into the multi-quantum well layer.

Furthermore, since the absorption coefficient α1 in the seepage portion is relatively small, the overall waveguide loss is sufficiently smaller than that of a conventional waveguide layer in which all of the waveguide layers are composed of multiple quantum well layers.

Next, a case will be described in which an electric field is applied to the multiple quantum well layer. The absorption spectrum at that time is shown in FIG. 2(C). In this case, when a reverse bias voltage of about 4μ is applied, the electric field strength is 5X10' to 105V/cm, and the exciton peak is λ2 = 0.83μ.
It moves from m to λ4=0.85 μm by about 20 nm, 30 to 4 QmeV in terms of energy, toward the lower energy side.

Therefore, the absorption coefficient α2 for the wavelength λ3 = 0.84 μm of the incident light is very thick compared to α (~10'c
It will be about m-'. As the entire absorption spectrum including the exciton peak shifts toward longer wavelengths due to the application of an electric field, the refractive index spectrum also shifts toward longer wavelengths, as shown in FIG. 2(d). Therefore, wavelength λ3=0.84μ
The average refractive index of the multi-quantum well layer at m was n = 3.48 when no electric field was applied, whereas it increased to n = 3.55 when an electric field was applied. Therefore, the refractive index n3 of the single layer guide is 3.51 or more.

The refractive index distribution is shown in FIG. 3(C). Also, Figure 3 (
The field distribution of light at that time is shown in d), where 13 is the region of the multi-quantum well layer, which receives very large absorption with α2 of ~l Q 4 cm-1, and 14 is the region of the single-layer guide. This is the part that receives almost no absorption. The incident light is mainly guided through a portion of the multi-quantum well layer where n2=3.55 has the highest refractive index. 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 optical switch with this structure has very low waveguide loss, operates at low voltage (5V or less), has a large extinction ratio, and uses an electric field as a control means, so
The above-mentioned high-speed modulation is also possible, and although it was not mentioned in particular, it is sufficiently possible to manufacture the device with an element length of 1 mm or less.

In addition, the wavelength of the incident light used here, the composition of each layer of AβGaAs, the thickness of each layer of GaAs/AA'GaAs multiple quantum wells, etc. are just examples, and the changes in absorption coefficient due to electric field, The present invention is not particularly limited to the examples as long as effects equivalent to those of the examples can be obtained in terms of changes in refractive index, etc.

Furthermore, regarding semiconductor materials, not only GaAs/AβGaAs materials but also InGaAsP/InP, InC,
aAs/InAj! A material such as As may also be used.

〔Effect of the invention〕

As explained in detail above, according to the present invention, it is possible to significantly reduce waveguide loss, reduce operating voltage, extinction ratio, and It has become possible to create waveguide optical switches with high performance in terms of wave loss, and it will greatly contribute to the realization of future optical functional devices, optical circuits, and optical communication and optical information processing systems that integrate and systemize them. It is.

[Brief explanation 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 operating principle of the waveguide type optical switch according to the present invention. a) and (b) are diagrams showing the absorption spectrum and refractive index spectrum of the multiple quantum well layer when no electric field is applied, respectively, and Figure 2 (C) and (
d) is a diagram showing the absorption spectrum and refractive index spectrum of the multiple quantum well layer when an electric field is applied, respectively; FIG. 3 is a diagram for explaining an embodiment of the waveguide optical switch according to the present invention; FIGS. 3(a) and 3(b) are diagrams showing the refractive index distribution of the waveguide and the field distribution of guided light when no electric field is applied, respectively, and FIG. 3(C). (d) is a diagram showing the refractive index distribution of the waveguide and the field distribution of guided light when an electric field is applied, Figure 4 is a diagram to explain the Franz-Kjelditsch effect in a single layer structure, and Figure 5 is FIG. 2 is a diagram for explaining a waveguide optical switch using only a conventional multiple quantum well layer as a guide layer. 1.21 ...... n"-GaAs substrate 2.22 ...... n"-AβGaAs
Cladding layer 3 rr-AIGa
As single layer guide 4.23 ・−=−・−n−−Ga
As/AAGaAs multiple quantum well guide 5.24 ... River... p''-AβGaAs cladding layer 6 ...... AβGaAs buried layer?, 8.26.27... Electrode 9 ... ...... Incoming light 10 ...... Outgoing light 11, 13 ... River Portion that guides the multi-quantum well layer in the field distribution of guided light 12.14 - River... Part 25 that guides the wave in one layer... p”-GaAs cap layer agent Patent attorney Yoshiyuki Iwasa λ, enter, enter, X◆
λglue χz Xsχnowmi skin length λ
5 skin flea X(a)
(b) j skin flea λ 911
Agricultural input (C) (d) (a) (b) (C) (d) Figure 3 Figure 4 Figure 5 Procedural amendments voluntarily) July 26, 1985

Claims (1)

[Claims] (1) In a semiconductor waveguide optical switch in which guided light propagating through a guide layer is controlled by applying an electric field, the guide layer is a single layer and each layer exists on at least one of the upper and lower sides of the single layer. It is composed of a multilayer structure in which semiconductor layers thinner than 500 Å and semiconductor layers having a forbidden band width larger than the forbidden band width of the semiconductor layer are laminated alternately, and an electric field is applied perpendicularly to the heterointerface of the multilayer structure. the absorption peak wavelength of excitons at the ground level of the multilayer structure is set near a short wavelength with respect to the wavelength of the modulated light, and the average refraction of the multilayer structure with respect to the modulated light is A waveguide optical switch characterized in that the optical switch has a refractive index slightly smaller than the refractive index of the single layer.