Bistable element

A bistable element (100) comprising: a multi-mode interference optical waveguide (1), has two ports on one edge face (1a) thereof, and has one port on the other edge face (1b) thereof; a first group of optical waveguides (2), and each of which is composed of two optical waveguides each having one edge face connected to each port arranged on the one edge face (1a) side of the multi-mode interference optical waveguide (1); and a second group of optical waveguides (3), and each of which is composed of one optical waveguide having one edge face connected to each port arranged on the other edge face (1b) side of the multi-mode interference optical waveguide (1). The multi-mode interference optical waveguide (1) has a saturable absorption region (22) where the absorption coefficient is reduced to cause the saturation of the amount of absorbed light when the intensity of incident light becomes high.

FIELD

The present invention relates to a bistable element having an optical waveguide structure, and more particularly relates to such a bistable element in which a wide operating current range (i.e., a wide hysteresis window) can be obtained in a bistable operation.

BACKGROUND

Recently, as an element having an optical memory function which is necessary for realization of all-optical routers and so forth, a bistable element, above all, a semiconductor bistable element, is investigated.

Saying a word “semiconductor bistable element”, semiconductor bistable elements based on a variety of principles are reported. In particular, as a representative example in which a relatively stabilized bistable operation is realized, a semiconductor bistable element, which utilizes a structure having two different optical waveguide paths in a multi-mode interference optical waveguide, is reported by Patent Document 1 (JP-2003-084327 A) and Non-Patent Document 1 (M. Takenaka and Others, “Multimode Interference Bistable Laser Diode”, IEEE Photonics Technology Letters, Vol. 15, No. 8, pp. 1035-1037).

According to this semiconductor bistable element, although it is reported that a superior operation can be obtained as a bistable element, there are a problem (1) that a hysteresis window in a bistable operation (which is referred to as a bistable hysteresis window hereinafter) is narrow (a current must be set within a range less than several percentages of the bistable operation current), and a problem (2) that the element per se is not suitable to integration due to the fact that a full length of the element is too long.

Patent Document 2 (JP-2008-250110 A) and Non-Patent Document 2 (H. A. Bastawrous and Others, “A Novel Active MMI Bi-Stable Laser Using Cross-Gain Saturation Between Fundamental and First Order Modes”, Proceedings of The 34th European Conference on Optical Communication (ECOC 2008, Brussels, Belgium), P. 2. 15, pp 81-82, September 2008) report a bistable element in which aforesaid problems have been solved.

In particular, in Non-Patent Document 2, it has been proved that a wide operating current condition, in which a bistable hysteresis window was about 10% of the operating current, and which could not be achieved by now, can be obtained.

SUMMARY

However, in the conventional bistable element, although the wide operating current condition, which could not be achieved by now, can be obtained, the bistable hysteresis window is merely on the order of 10% of the operating current at the most. Thus, when a future high-density integration is taken into consideration, there is still a problem that a further wide hysteresis window must be obtained. In particular, in the conventional bistable element, when a high-density integration is attempted, due to manufacturing errors, a bistable operation cannot be necessarily obtained at the same operating current value in all components of an integrated device, and thus it is difficult to carry out control at the same operating current. For this reason, in an integrated device including conventional bistable elements, it is necessary to set different operating currents in the respective elements (see:FIG. 17(a)), and thus there is a problem that it is difficult to put such an integrated device to practical use. On the other hand, when a further hysteresis window can be obtained, it is possible to set a common operating current in correspondence to deviation, which is derived from manufacturing errors, in operating currents for obtaining bistable operations in respective bistable elements (see:FIG. 17(b)).

The present invention has been developed to solve the aforesaid problems, and aims at providing a bistable element in which not only can miniaturization be further carried out in comparison with a conventional bistable element, but also it is possible to obtain a further wide bistable hysteresis window.

A bistable element according to the present invention comprises: a multi-mode interference optical waveguide provided on a substrate, and having a number M of ports formed in one end face thereof (M is integers more than one), and a number N of ports formed in another end face (N is integers more than zero, and less than M or equal to M); a first group of optical waveguides provided on the substrate, and including a number M of optical waveguides, each of which has one end face connected to a corresponding port formed in the one end face of the multi-mode interference optical waveguide; and a second group of optical waveguides provided on the substrate, and including a number N of optical waveguides, each of which has one end face connected to a corresponding port formed in the other end face of the multi-mode interference optical waveguide, wherein the multi-mode interference optical waveguide includes a saturable absorbing region in which saturation in an amount of absorbed light is caused by the fact that an absorbing coefficient is reduced as an intensity of incident light becomes larger.

In the bistable element according to the present invention, not only can miniaturization be further carried out in comparison with a conventional bistable element, but also it is possible to obtain a further wide bistable hysteresis window.

DETAILED DESCRIPTION

First Embodiment

As shown inFIG. 1, a bistable element100includes a multi-mode interference optical waveguide1as stated hereinafter, a first group of optical waveguides2, and a second group of optical waveguides3, which are integrated in a substrate10.

The multi-mode interference optical waveguide1is provided on the substrate10, and is defined as an active optical waveguide having a number M of ports formed in one end face1athereof (M is integers more than one), and a number N of ports formed in the other end face1b(N is integers more than zero, and less than M or equal to M). Also, the multi-mode interference optical waveguide1includes a main exciting region21for exciting an active layer by supplying a bias current between an external electrode provided on a front surface of the substrate10(which is referred to as a front surface electrode hereinafter) and an external electrode provided on a rear surface of the substrate10(which is referred to as a rear surface electrode hereinafter), and a saturable absorbing region22in which saturation in an amount of absorbed light is caused by the fact that an absorbing coefficient is reduced as an intensity of incident light becomes larger. The saturable absorbing region22maintains a saturable absorbing state until a laser oscillation is started, but electrons and active holes are sufficiently created in the saturable absorbing region22after the laser oscillation is started, and thus loss is reduced in the saturable absorbing region22, resulting in continuation of the laser oscillation.

Note, in this embodiment, although the multi-mode interference optical waveguide1having the two ports in the one end face1aand the one port in the other end face1bis explained, the present invention is not limited to only the multi-mode interference optical waveguide1having this number of ports. In particular, the multi-mode interference optical waveguide1according this embodiment is formed as a generally-rectangular interference region having a length of the waveguide (referred to as a waveguide length hereinafter) of about 135 □m, which is along a light guiding direction, and which is set in accordance with the below-mentioned formula (1), and a waveguide of about 7.4 μm.

Also, in this embodiment, a part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, is defined as a region in which a zero-order mode light and a first-order mode light defined as light propagating modes are not superimposed over each other. For example, the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, is defined as a region having a region width which is equal to the waveguide width of the multi-mode interference optical waveguide1, and a region length which is equal to a partial waveguide length of the multi-mode interference optical waveguide1measured from the one end face1ato a predetermined location, with the part of the saturable absorbing region22being the region of the multi-mode interference optical waveguide1measured from the one end face1ato the predetermined location. In particular, in this embodiment, the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, has the region length of about 29 □m (25□m except for the below-mentioned electrical separating groove4), which is measured from the one end face1aof the multi-mode interference optical waveguide1to the predetermined location. Note that the region length of the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, i.e., the distance, which is measured from the one end face1aof the multi-mode interference optical waveguide1to the predetermined location, may be set based on characteristics of the bistable element100and the waveguide length (and the waveguide width) of the multi-mode interference optical waveguide1. For example, it may be set so as to be equal to or less than a half of the waveguide width length of the multi-mode interference optical waveguide1, whereby it is possible to secure the main exciting region21so as to contribute to the laser oscillation. Also, for example, in a case where the region length of the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, i.e., the distance, which is measured from the one end face1aof the multi-mode interference optical waveguide1to the predetermined location, is set so as to be equal to or more than the waveguide width of the multi-mode interference optical waveguide1, it is possible to obtain a desirable wide bistable hysteresis window, as mentioned hereinafter.

In particular, as stated above, in the bistable element100according to this embodiment, the waveguide length L1of the multi-mode interference optical waveguide1is about 135 μm, and the waveguide width L2of the multi-mode interference optical waveguide1is about 7.4 μm. Also, the region length L2of the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, is about 29 μm. Thus, a percentage of the region length L2of the part of the saturable absorbing region22to the waveguide length L1of the multi-mode interference optical waveguide1is 21.5% (=29/135μ 100), and this result fulfills the conditions as mentioned above by way of example, whereby not only can a desirable wide bistable hysteresis as mentioned hereinafter be obtained, but also it is possible to secure the main exciting region21.

The electrical separating groove4is formed by removing a part of the below-mentioned contact layer16so as to cross the lengthwise direction of the multi-mode interference optical waveguide1. Note that the electrical separating groove16according to this embodiment is provided in the multi-mode interference optical waveguide1so as to extend from the boundary between the main exciting region21and the saturable absorbing region22toward the side of the saturable absorbing region22by a distance of about 4 μm. The first group of optical waveguides2includes the number M of optical waveguides, each of which has one end face connected to a corresponding one of the formed in the one end face1aof the multi-mode interference optical waveguide1, and each of the optical waveguides included in the first group2is defined as an active optical waveguide included in the saturable absorbing region22. Also, the other end face of each of the optical waveguides included in the first group2is defined as a light-incidence face and/or a light-emission face.

Note, in this embodiment, since reference is made by way of example to the multi-mode interference optical waveguide1having the two ports formed in the one end face1athereof, two optical waveguides (a first optical waveguide2aand a second optical wave guide2b) are included in the first group of optical waveguides2. Also, the first and second optical waveguides2aand2bare perpendicularly connected to the one end face1aof the multi-mode interference optical waveguide1so as to be juxtaposed with each other.

In particular, each of the first and second optical waveguides2aand2baccording to this embodiment is defined as a generally-rectangular linear waveguide having a waveguide length of about 65 μm, and a waveguide width of about 2.7 μm. Also, the optical waveguides (the first and second optical waveguides2aand2b) included in the first group2forms the other or remaining part of the saturable absorbing region22, with each of the optical waveguides, which form the other part of the saturable absorbing region22, having a region width and a region length which are respectively equal to the waveguide width and the waveguide length of the optical waveguide concerned. In short, each of the first and second optical waveguides2aand2bhas no part included in the main exciting region21, and forms only the other part of the saturable absorbing region22.

The second group of optical waveguides3includes the number N of optical waveguides, each of which has one end face connected to a corresponding one of the formed in the other end face1bof the multi-mode interference optical waveguide1, and each of the optical waveguides included in the second group2is defined as an active optical waveguide. Also, the other end face of each of the optical waveguides included in the second group3is defined as a light-incidence face and/or a light-emission face.

Note, in this embodiment, since reference is made by way of example to the multi-mode interference optical waveguide1having the one port formed in the other end face1bthereof, one optical waveguide (a third optical waveguide3a) is included in the second group3. Also, the third optical waveguide3ais positioned so as to be opposite to the first waveguide2aincluded in the first group2, with the multi-mode interference optical waveguide1being intervened therebetween. In short, the third optical waveguide3ais defined as an optical waveguide in which a zero-order mode light and a first-order mode light, each of which is defined as a light propagating mode, are allowed.

In particular, the third optical waveguide3aaccording to this embodiment is defined as a generally-rectangular linear waveguide having a waveguide length of about 90 μm, and a waveguide width of about 2.7 μm. Also, the third optical waveguide3ahas no part included in the saturable absorbing region22, and forms only the part of the main exciting region21.

Accordingly, in the stable element100according to this embodiment, the saturable absorbing region22is defined by a part of the multi-mode interference optical waveguide1and the first group of optical waveguides2, which continuously extend as the saturable absorbing region22, with a full length of the saturable absorbing region22being about 94 μm (90 μm except for the electrical separating groove4). Also, in the stable element100according to this embodiment, the main exciting region21is defined by the other part of the multi-mode interference optical waveguide1and the second group of optical waveguides3.

Note, the other part of the multi-mode interference optical waveguide1and the second group of optical waveguides3(i.e., the third optical waveguide3a), which form the main exciting region21, have the same layer structure. Also, the part of the multi-mode interference optical waveguide1and the first group of optical waveguides2(i.e., the first and second optical waveguides2aand2b), which form the saturable absorbing region22, have the same layer structure. In particular, in this embodiment, the multi-mode interference optical waveguide1, the first group of optical waveguides2(i.e., the first and second optical waveguides2aand2b) and the second group of optical waveguides3(i.e., the third optical waveguide3a) have the same layer structure except for the electrical separating groove4in the multi-mode interference optical waveguide1.

As shown inFIG. 2(a), concretely, a cross-sectional structure of the main exciting region21is defined as a ridge structure in which a buffer layer11composed of an n-InP-based material forming an n-type semiconductor, a light-emitting layer12composed of an InGaAsP/InGaAsP-based material and defined as an active layer for realizing a bistable element featuring a long wavelength region (a 1.55 □m region), a first clad layer13composed of an p-InP-based material forming a p-type semiconductor, an etching stopper layer14composed of a p-InGaAsP-based material forming a p-type semiconductor, a second clad layer15composed of a p-InP-based material forming a p-type semiconductor, a contact layer16composed of a p-InGaAs-based material forming a p-type semiconductor are laminated in order on a substrate10composed of an n-InP-based material. Also, as shown inFIG. 2(b), in comparison with the cross-sectional structure of the main exciting region21, a cross-sectional structure of the saturable absorbing region22corresponds to one in which the contact layer16is partially removed from a region forming the electrical separating groove4(which is referred to as a separating groove region22ahereinafter).

As shown inFIG. 2(a), the aforesaid ridge structure corresponds to one in which the contact layer16and the second clad layer15are partially removed from a non-waveguide region by an etching process.

In particular, in this embodiment, the buffer layer11is about 100 nm in film thickness; the light-emitting layer12is 100 nm in film thickness; the first clad layer13is 200 nm in film thickness; the etching stopper layer14is 10 nm in film thickness; the second clad layer15is 800 nm in film thickness; and the contact layer16is 150 nm in film thickness.

Note, in the bistable element100according to this embodiment, although the multi-mode interference optical waveguide1, the first group of optical waveguides2and the second group of optical waveguide3have the ridge structure, the present invention is not necessarily limited to this ridge structure, and may be applied to, for example, a buried structure, a high-mesa structure and so forth.

Also, although the light-emitting layer12according to this embodiment is defined as a usual light-emitting layer featuring an SCH (separate confinement hetero-structure) and an MQW (multi-quantum well), the present invention is not necessarily limited to only this usual light-emitting layer. For example, the light-emitting layer may feature a strained layer multiple quantum well or may be defined as a bulk light-emitting layer.

Also, although the light-emitting layer12according to this embodiment features the 1.55 μm wavelength region, the present invention is not necessarily limited to only this wavelength region. For example, the light-emitting layer may feature a 1.3 μm wavelength region, or a visual light region.

Further, although the InGaAsP/InGaAsP-based material is used for the light-emitting layer12according to this embodiment, the present invention is not necessarily limited to only this material. For example, an InGaAlAs-based material may be used for it, or it is possible to freely select a material so as to accord with the wavelength region.

Next, a principle for obtaining a wide bistable hysteresis window in the bistable element100according to this embodiment will explained below.

Before the principle of the present invention is explained, an operating principle of a conventional bistable element200in an all-optical flip-flop, which is disclosed in Patent Document 1 as a multi-mode interference optical waveguide type bistable element having two ports in each of opposite end faces thereof, is explained with reference toFIG. 3(a).

Note, as shown inFIG. 3(a), the conventional bistable element200includes a multi-mode interference optical waveguide201having a generally rectangular shape in a plan view, a first optical waveguide202a, a second optical waveguide202b, a third optical waveguide203aand a fourth optical waveguide203bwhich are joined to the multi-mode interference optical waveguide201. Also, the first and second optical waveguides202aand202bare joined to one end face201aof the multi-mode interference optical waveguide201, and the third and fourth optical waveguides203aand203bare joined to the other end face201bof the multi-mode interference optical waveguide201, which is opposite to the one end face201athereof. Also, the first optical waveguide202aand the third optical waveguide203aare substantially aligned with each other, and the second optical waveguide202band the fourth optical waveguide203bare substantially aligned with each other.

As shown inFIG. 3(a), in general, the operating principle of the bistable element200is based on the fact that (1) there are two optical waveguide paths (i.e., optical waveguide paths211and212inFIG. 3(a)), in which laser-lights can be simultaneously oscillated, in the multi-mode interference optical waveguide201, and that (2) the oscillation is selected in only one of the two optical waveguide paths in accordance with a cross-gain difference between the two optical waveguide paths in the multi-mode interference optical waveguide201.

For example, on the condition that a certain constant operating current is previously injected into the active optical waveguide (i.e., the multi-mode interference optical waveguide201, the first optical waveguide202a, the second optical waveguide202b, the third optical waveguide203aand the fourth optical waveguide203b), when a laser light is input from an outside to a port221, one of the optical waveguide paths211and212, i.e., the optical waveguide path211having the port221as an input terminal is selected as an optical waveguide path for laser light oscillation. In this time, a gain on the one optical waveguide path211in the multi-mode interference optical waveguide201becomes larger in comparison with a gain on the other optical waveguide path212, resulting in the laser light oscillation being obtained in only the one optical waveguide path211.

Next, in this condition, when a laser light is input to a port222, the other optical waveguide path212having the port222as an input terminal is selected as a waveguide path for laser light oscillation. In this time, the gain on the other optical waveguide path212in the multi-mode interference optical waveguide201becomes larger in comparison with the gain on the one optical waveguide path211, resulting in the laser light oscillation being obtained in only the other optical waveguide path212, and thus the laser light oscillation in the one optical waveguide path211is stopped by way of compensation. Like this, in the conventional bistable element200, the two bistable states are created.

Nevertheless, a region231, in which the two optical waveguide paths, i.e., the optical waveguide paths211and212are intersected with each other, merely occupies the multi-mode interference optical waveguide201at a very small ratio. Since the conventional bistable element200merely utilizes the cross-gain difference in the very small region, there is a problem that it is not possible to obtain a wide bistable hysteresis window.

Then, an operating principle of a conventional bistable element300, which is disclosed in Patent Document 2 as a multi-mode interference optical waveguide type bistable element having two ports in one end face thereof and one port in the other end face thereof, is explained with reference toFIG. 3(b).

Note, as shown inFIG. 3(b), the conventional bistable element300includes a multi-mode interference optical waveguide301having a generally rectangular shape in a plan view, a first optical waveguide302a, a second optical waveguide302band a third optical waveguide303awhich are joined to the multi-mode interference optical waveguide301. Also, the first and second optical waveguides302aand302bare joined to one end face301aof the multi-mode interference optical waveguide301, and the third and fourth optical waveguide303ais joined to the other end face301bof the multi-mode interference optical waveguide301, which is opposite to the one end face301athereof. Also, the first optical waveguide302aand the third optical waveguide303aare substantially aligned with each other, and the second optical waveguide302bare parallel to the first and third optical waveguides.

This conventional bistable element300features the following two stable modes as a light propagating mode:

When a laser light is input from an outside to a port321, a zero-order mode light enters the multi-mode interference optical waveguide301from the second optical waveguide302b, travels along a cross path in the multi-mode interference optical waveguide301, and then radiates from a port332of the third optical waveguide303a(as indicated by a waveguide path311shown inFIG. 3(b)).

When a laser light is input from an outside to a port323, a first-order mode light enters the multi-mode interference optical waveguide301from the first optical waveguide302a, travels along a generally linear path in the multi-mode interference optical waveguide301, and then radiates from the port332of the third optical waveguide303a(, as indicated by a waveguide path312shown inFIG. 3(b)).

Like this, the third optical waveguide303ais constituted so that the zero-order mode light and the first-order mode light can exist as standing waves. Also, the multi-mode interference optical waveguide301features a region (i.e., a cross-gain restraint region331) in which the zero-order mode light and the first-order mode light are integrated with and superimposed over each other.

As is apparent from the foregoing, an operation principle of the bistable element100according to this embodiment is based on the fact that (1) a generally crank-shaped optical waveguide path (which corresponds to the optical waveguide path311shown inFIG. 3(b)) and a generally linear optical waveguide path (which corresponds to the optical waveguide path312shown inFIG. 3(b)) are utilized as two optical waveguide paths in which laser-lights can be simultaneously oscillated, and that (2) the oscillation is selected in only one of the two optical waveguide paths in accordance with a cross-gain difference between the two optical waveguide paths.

In particular, according to the present invention, it is possible to utilize the superimposition of the two optical waveguide paths311and312in the wide region (i.e., the cross-gain restraint region331). In short, in the multi-mode interference optical waveguide1, a phenomenon, in which respective self interference images occur in a cross direction on a zero-order mode and a bar direction on a first-order mode, respectively, is utilized when a waveguide length LMMI of the multi-mode interference optical waveguide1is represented by the following formula:
NF (Numerical Formula) 1
LMMI=3Lπ/4≈nrWe2/λ  (1)

Herein: Lπ, is a beat length; Weis an effective width of the optical waveguide; nris a refraction of the waveguide region, and λ is a wavelength.

Nevertheless, even in the structure shown inFIG. 3(b), there is a region which does not belong to the cross-gain restraint region331, i.e., a region (i.e., a non-cross-gain restraint region332) in which the two optical waveguide paths (the optical waveguide paths311and312) are not superimposed with each other, and this means that a further wide bistable hysteresis window can be obtained by improving such a region.

Thus, the following facts (1) and (2) have been found by the recent researches of the inventors:

(1) The two optical waveguide paths in the saturable absorbing region do not serve as the cross-gain restraint region. That is, the bistable hysteresis window can be controlled by the superimposition of the two optical waveguide paths.

(2) Although a part of the multi-mode interference optical waveguide forming the active optical waveguide is defined as the saturable absorbing region, it is possible to obtain an active multi-mode interference effect.

The aforesaid fact (1) means that it is possible to obtain a further large cross-gain restraint effect by extending the region (i.e., the non-cross-gain restraint region), in which the superimposition of the two optical waveguide paths is not obtained, as the saturable absorbing region as large as possible, so that a ratio, at which the main exciting region is occupied by the superimposition region of the two optical waveguide paths (i.e., the cross-gain restraint region), is increased. That is, before the wide bistable hysteresis window can be obtained, the region (i.e., the non-cross-gain restraint region), in which the two optical waveguide paths are not theoretically superimposed over each other, must be extended as the saturable absorbing region as large as possible.

Further, according to the aforesaid fact (2), it has been found that it is possible to obtain the active multi-mode interference effect in the active optical waveguide in which the part of the multi-mode interference optical waveguide is defined as the saturable absorbing region, which has not been proved by now. Thus, it is possible to theoretically obtain the substantially complete superimposition region (i.e., the cross-gain restraint region) between the two optical waveguide paths in the main exciting region, by defining the region (i.e., the non-cross-gain restraint region), in which the superimposition of the two optical waveguide paths is not almost obtained, as the saturable absorbing region in the multi-mode interference optical waveguide.

In the bistable element100according to this embodiment designed based on the above-mentioned principle, as shown inFIG. 4(a), it was possible to obtain the bistable hysteresis featuring 93 mA, which corresponds to more than ten times larger than 8 mA of the bistable hysteresis window (see:FIG. 4(b)), which is reported by Non-Patent Document 2.

Note, in this embodiment, although the waveguide length of the multi-mode interference optical waveguide1is set based on the above-mentioned formula (1), it is unnecessary to precisely carry out the setting of the waveguide length. If the waveguide length of the multi-mode interference optical waveguide1is set within the □10% range of a value obtained from the formula (1), this setting is applicable to the present invention.

Also, in this embodiment, although the port321is selected as the input port for the zero-order mode light, and although the port323is selected as the input port for the first-order mode light, the present invention is not limited to only this combination of the input ports. For example, if a port322may be selected as the input port for the zero-order mode light, and if the323may be selected as the input port for the first-order mode light, this combination of the input ports is applicable to the present invention. Also, if the port321is selected as the input port for the zero-order mode light, and if the port322is selected as the input port for the first-order mode light, this combination of the input ports is applicable to the present invention. Further, it is possible to select the port322as the input port for both the zero-order mode light and the first-order mode light. In this case, the port321may be selected as the output port for the zero-order mode light, and the port323may be selected as the output port for the first-order mode light.

Next, with reference toFIG. 2,FIGS. 5 to 8, a production method of the bistable element100according to this embodiment will now be explained.

First, an n-In film31(the buffer layer11), a 1.55 μm wavelength region InGaAsP/InGaAsP film32(the light emitting layer12), a first p-InP film33(the first clad layer13), a p-InGaAs film34(the etching stopper layer14), a second p-InP film35, and a p-InGaAs film36are grown in order on a usual n-InP substrate10by using an MOCVD (metal organic chemical vapor deposition) process (FIG. 5(a)).

Then, a mask37for an etching process is formed on the p-InGaAs film36by a photolithography process using a stepper (i.e., a lens reduction projection aligner) so as to conform with a plane configuration of the multi-mode interference optical waveguide1, the first group of optical waveguides2and the second group of optical waveguides3shown inFIG. 1(FIG. 5(b)).

With using the mask37, the p-InGaAs film36to be defined as the contact layer16, and the second p-InP film35to be defined as the second clad layer15are subjected to a dry etching process using an ICP (inductively coupled plasma) process, so that the disused portions of the p-InGaAs film36and the second p-InP film35(on which the mask37is not formed) are removed, resulting in the formation of the ridge structure in the cross-sectional configuration (FIG. 5(c)).

Thereafter, the mask37, which lies on the contact layer16, is removed by using an chemical solution and an ashing process (FIG. 2(a)).

Note, production steps stated hereinafter are directed to a separation of the ridge structure, including the multi-mode interference optical waveguide1, the first group of optical waveguides2and the second group of optical waveguides3, into the main exciting region21and the saturable absorbing region22.

Then, a mask38for an etching process is formed on the contact layer16and the etching stopper layer14by the photolithography process using the stepper so as to conform with the plane configuration, except for the electrical separating groove4, shown inFIG. 1(FIG. 6(a)).

With using the mask38, the contact layer16is subjected to a wet etching process using a sulfuric-acid-based etching solution so that the contact layer16is removed from a part (corresponding to the separating groove region22a) of the multi-mode interference optical waveguide1, at which the electrical separating groove4is to be defined (FIG. 6(b)).

Thereafter, the mask38, which lies on the contact layer16and the etching stopper layer14except for the separating groove region22a, is removed by using the chemical solution and the ashing process (FIG. 2(b)).

Then, a mask40for an etching process is formed on the SiO2 film39by the photolithography process using the stepper so as to conform with both a plane configuration except for the multi-mode interference optical waveguide1, the first group of optical waveguides2and the second group of optical waveguides3and a plane configuration of the electrical separating groove4shown inFIG. 1(FIG. 7(a) andFIG. 8(b)).

Thereafter, with using the mask40, the SiO2 film39, which lies on the multi-mode interference optical waveguide1except for the separating groove region22a, the first group of optical waveguides2and the second group of optical waveguides3, is subjected to a wet etching process by using an etching solution containing a BHF (buffered hydrofluoric acid), so as to be removed therefrom (FIG. 7(b) andFIG. 8(c)).

Then, the mask40, which lies on the area except for the first group of optical waveguides2and the second group of optical waveguides3, and on the separating groove region22a, is removed by using the chemical solution and the ashing process (FIG. 7(c) andFIG. 8(d)).

Then, a photoresist layer, which is not shown in the drawings, is formed on the contact layer16by the photolithography process using the stepper so as to confirm with a plane configuration of the multi-mode interference optical waveguide1except for the separating groove region22a, the first group of optical waveguides2and the second group of optical waveguides3shown inFIG. 1.

Then, a Ti/Pt/Au electrode material layer, which is to be defined as an external electrode (i.e., a front electrode) for exciting the active layer by supply of a bias current, is formed over the substrate10by using an electron-beam evaporation process.

Thereafter, the photoresist layer, which is not shown in the drawing, is removed together with the Ti/Pt/Au electrode material lying thereon, by a lift-off process using an chemical solution.

Note, in this embodiment, although the front surface electrode is formed on the contact layer16in the main exciting region21and the saturable absorbing region22except for the separating groove region22a, the front surface electrode may be formed on only the main exciting region21if an electrical connection is established between the main exciting region21and the saturable absorbing region22. Also, the front surface electrode may be formed on all the area of the substrate10except for the saturable absorbing region22(including the separating groove region22a). Optionally, the front surface electrode may be formed on the main exciting region21and the separating groove region22a. Further, the front surface electrode may be formed on all the area of the substrate10except for the saturable absorbing region22(excluding the separating groove region22a).

On the other hand, a front surface electrode may be formed on the saturable absorbing region22so that a bias current can be injected into the saturable absorbing region22. With this arrangement, when a sufficient bistable hysteresis window can be obtained, it is possible to lower an operating current at the sacrifice of the bistable hysteresis window by injecting the bias current into the saturable absorbing region22.

Thereafter, the rear surface of the substrate10, in which no optical waveguide is formed, is polished, and a Ti/Pt/Au layer, which is not shown in the drawing, is formed on all the rear surface of the substrate10as an external electrode (a rear surface electrode) for exciting the active layer by supply of a bias current, using the electron-beam evaporation process.

Then, the substrate10, on which a plurality of bistable elements100are formed, is cut along the boundaries between the adjacent bistable elements100so that the individual bistable elements100are separated from each other, whereby it is possible to obtain the individual bistable elements as shown inFIG. 1.

Note, in the production method according to this embodiment, although the stepper is used in the photolithography process, the production method is not necessarily limited to only the use of the stepper. For example, a vector scan electron exposure system may be applied to the production method.

Also, in the production method according to this embodiment, although the thermal CVD process is used for the formation of the SiO2 film39, for example, a plasma CVD process or a sputtering process may be applied to the production method.

Further, in the production method according to this embodiment, although the ICP process is used as an etching process in the step in which the ridge structure is formed, the production method is not necessarily limited to only the use of the ICP process. For example, an RIE (reactive ion etching) process, a wet etching process or an NLD (magnetic neutral loop discharge) process may be applied to the production method.

Further, in the production method according to this embodiment, although the MOCVD process is used as the epitaxial growth process, the production method is not necessarily limited to only the MOCVD process. For example, an MBE (molecular beam epitaxy) process may be applied to the production method.

Also, in the production method according to this embodiment, although the lift-off process is used for the formation of the front surface electrode, the production method is not necessarily limited to the use of the lift-off process. For example, a method, in which a formation of an electrode pattern is carried out by using a photolithography process, and in which a disused Ti/Pt/Au electrode material is removed by using a milling process or the like, may be applied to the production method.

Also, in the bistable element100according to this embodiment, the electrical separating groove4is formed in the multi-mode interference optical waveguide1so that the main exciting region21and the saturable absorbing region22are isolated from each other. Nevertheless, if the bitable element is constituted so that the saturable absorbing region22cannot be supplied with a bias current, no formation of the electrical separating groove4is necessary, but it is considered that, for example, the contact layer16is removed from all the saturable absorbing region22.

In this case, it is considered that steps as shown inFIGS. 9 and 10are substituted for the steps shown inFIGS. 6 and 7.

First, a mask38for an etching process is formed on the contact layer16and the etching stopper layer14by the photolithography process using the stepper so as to conform with the plane configuration, except for the region to be defined as the saturable absorbing region22, shown inFIG. 1(FIG. 9(a)).

With using the mask38, the contact layer16is subjected to a wet etching process using a sulfuric-acid-based etching solution so as to be removed from the region to be defined as the saturable absorbing region22(FIG. 9(b)).

Thereafter, the mask38, which lies on the contact layer16and the etching stopper layer14except for the saturable absorbing region22, is removed by using the chemical solution and the ashing process (FIG. 9(c)).

Then, a mask40for an etching process is formed on the SiO2 film39by the photolithography process using the stepper so as to conform with a plane configuration except for the region to be defined as the main exciting region21shown inFIG. 1(FIG. 10(a) andFIG. 8(b)).

Thereafter, with using the mask40, the SiO2 film39, which lies on the region to be defined as the main exciting region21, is subjected to a wet etching process by using the etching solution containing the BHF (buffered hydrofluoric acid), so as to be removed therefrom (FIG. 10(b) andFIG. 8(c)).

Then, the mask40, which lies on the area except for the region to be defined as the main exciting region21, is removed by using the chemical solution and the ashing process (FIG. 10(c) andFIG. 8(d)).

Then, a photoresist layer, which is not shown in the drawings, is formed on the contact layer16of the region to be defined as the main exciting region21by the photolithography process using the stepper.

Then, a Ti/Pt/Au electrode material layer, which is to be defined as an external electrode (i.e., a front electrode) for exciting the active layer by supply of a bias current, is formed over the substrate10by the electron-beam evaporation process.

Thereafter, the photoresist layer, which is not shown in the drawing, is removed together with the Ti/Pt/Au electrode material lying thereon, by a lift-off process using an chemical solution.

As stated hereinbefore, in the bistable element100according to this embodiment, the multi-mode interference optical waveguide1includes the part of the saturable absorbing region22, with the part of the saturable absorbing region being defined as the region (i.e., the non-cross-gain restraint region) in which the zero-order mode light and the first-order mode light defined as light propagating modes are not superimposed over each other. Thus, the main exciting region21can be defined as the cross-gain restraint region in which the zero-order mode light and the first-order mode light can be sufficiently superimposed over each other, and thus there are merits or advantages that it is possible to obtain a very wide hysteresis window.

In particular, the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, is defined as a region having the region width which is equal to the waveguide width of the multi-mode interference optical waveguide1, and the region width which is equal to the partial waveguide length of the multi-mode interference optical waveguide1measured from the one end face1ato the predetermined location, with the part of the saturable absorbing region22being the region of the multi-mode interference optical waveguide1measured from the one end face1ato the predetermined location. Thus, there are merits or advantages that it is possible to easily set a separation between the main exciting region21and the saturable absorbing region22.

Note, in the bistable element100according to this embodiment, although the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, is defined as the region (i.e., the non-cross-gain restraint region) in which the zero-order mode light and the first-order mode light defined as light propagating modes are not superimposed over each other, the part concerned may be in the region (i.e., the cross-gain restraint region) in which the zero-order mode light and the first-order mode light are superimposed over each other.

Also, in the bistable element100according to this embodiment, although the first group of optical waveguides2forms the other part of the saturable absorbing region22, only the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, may be defined as the saturable absorbing region22without the first group of optical waveguides2being defined as the other part of the saturable absorbing region22.

In these cases, with the arrangement of the conventional bistable element in which only the part of the first group of optical waveguides2is defined as the saturable absorbing region22, since the saturable absorbing region22can be provided in the multi-mode interference optical waveguide1, it is unnecessary to lengthen the first group of optical waveguides2to secure the saturable absorbing region22, resulting in miniaturization of the bistable element100.

Nevertheless, the larger an area of the part of the saturable absorbing region22which is included in the multi-mode interference optical waveguide1, the wider the bistable hysteresis window. Thus, a value of an electric current for operating the bistable element100is increased so that an electric power consumption becomes larger in an integrated circuit device featuring the bistable element100. That is, there is a tradeoff between the widening of the bistable hysteresis window and the lowering of the electric power consumption in the integrated circuit device. Thus, preferably, the area of the part of the saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, should be suitably set in accordance with specifications of the integrated circuit device.

In particular, more preferably, a part of the saturable absorbing region22is defined in the non-cross-gain restraint region in the multi-mode interference optical waveguide1, and the other part of saturable absorbing region22should be secured in the first group of optical waveguides2so as to compensate a shortage of an area of the saturable absorbing region22. According to this arrangement, in the bistable element100, it is possible to obtain merits or advantages that not only can the electric power consumption of the integrated circuit device be suppressed, but also it is possible to widen the bistable hysteresis window while suppressing the length of the first group of optical waveguides2at the minimum.

Second Embodiment

FIG. 11(a) is a plan view showing an example of a schematic structure of a bistable element according to the second embodiment, andFIG. 11(b) is a cross-sectional view taken along the C-C′ line in the bistable element shown inFIG. 11(a). InFIG. 11, the same references as inFIGS. 1 to 10indicate similar elements or corresponding elements, and thus explanations for these elements are omitted.

A multi-mode interference optical waveguide1has two ports formed in one end face1athereof, and two ports formed in the other end face1bthereof. In particular, the multi-mode interference optical waveguide1is defined as a generally-rectangular interference region having a waveguide length of about 135 μm, which is set based on the above-mentioned formula (1), and a waveguide width of about 7.4 μm.

Also, two saturable absorbing regions22have respective parts which are included in the multi-mode interference optical waveguide1, and each of the respective parts of the saturable absorbing regions22is defined as a region in which a zero-order mode light and a first-order mode light defined as light propagating modes are not superimposed over each other.

For example, the part of one of the saturable absorbing regions22, which is included in the multi-mode interference optical waveguide1, is defined as a region having a region width which is equal to the waveguide width of the multi-mode interference optical waveguide1, and a region length which is equal to a partial waveguide length of the multi-mode interference optical waveguide1measured from the one end face1ato a predetermined location, with the part of the one saturable absorbing region22being the region of the multi-mode interference optical waveguide1measured from the one end face1ato the predetermined location. Also, the part of the other saturable absorbing region22, which is included in the multi-mode interference optical waveguide1, is defined as a region having a region width which is equal to the waveguide width of the multi-mode interference optical waveguide1, and a region length which is equal to a partial waveguide length of the multi-mode interference optical waveguide1measured from the other end face1bto a predetermined location, with the part of the other saturable absorbing region22being the region of the multi-mode interference optical waveguide1measured from the other end face1bto the predetermined location.

In particular, in this embodiment, the respective parts of the saturable absorbing regions22, which are included in the multi-mode interference optical waveguide1, have the same region lengths of about 29 μm (25 μm except for each of separating grooves4), which are measured from the respective end faces1aand1bof the multi-mode interference optical waveguide1to the predetermined locations.

A first group of optical waveguides2includes two optical waveguides (a first optical waveguide2aand a second optical waveguide2b). Also, the first and second optical waveguides2aand2bare perpendicularly connected to the one end face1aof the multi-mode interference optical waveguide1so as to be juxtaposed with each other.

In particular, each of the first and second optical waveguides2aand2baccording to this embodiment is defined as a generally-rectangular linear waveguide having a waveguide length of about 50 μm, and a waveguide width of about 1.5 μm. Also, the optical waveguides (the first and second optical waveguides2aand2b) included in the first group2forms the other or remaining part of the one saturable absorbing region22, with each of the optical waveguides, which form the other part of the one saturable absorbing region22, having a region width and a region length which are respectively equal to the waveguide width and the waveguide length of the optical waveguide concerned. In short, each of the first and second optical waveguides2aand2bhas no part included in the main exciting region21, and forms only the other part of the one saturable absorbing region22.

A second group of optical waveguides3includes two optical waveguides (a third optical waveguide3aand a fourth optical waveguide3b). Also, the third optical waveguide3ais positioned so as to be opposite to the first waveguide2aincluded in the first group2, with the multi-mode interference optical waveguide1being intervened therebetween. Also, the fourth optical waveguide3bis positioned so as to be opposite to the second waveguide2bincluded in the first group2, with the multi-mode interference optical waveguide1being intervened therebetween.

In particular, each of the third and fourth optical waveguides3aand3baccording to this embodiment is defined as a generally-rectangular linear waveguide having a waveguide length of about 50 μm, and a waveguide width of about 1.5 μm. Also, the optical waveguides (the third and fourth optical waveguides3aand3b) included in the second group3forms the other or remaining part of the other saturable absorbing region22, with each of the optical waveguides, which form the other part of the other saturable absorbing region22, having a region width and a region length which are respectively equal to the waveguide width and the waveguide length of the optical waveguide concerned. In short, each of the third and fourth optical waveguides3aand3bhas no part included in the main exciting region21, and forms only the other part of the other saturable absorbing region22.

Accordingly, in the stable element100according to this embodiment, the one saturable absorbing region22is defined by the first group of optical waveguides2and one part of the multi-mode interference optical waveguide1which is adjacent to the one end face1athereof, with the first group of optical waveguides2and the one part of the multi-mode interference optical waveguide1continuously extending as the one saturable absorbing region22. The other saturable absorbing region22is defined by the second group of optical waveguides3and another part of the multi-mode interference optical waveguide1which is adjacent to the other end face1bthereof, with the second group of optical waveguides3and the other part of the multi-mode interference optical waveguide1continuously extending as the other saturable absorbing region22. The respective saturable absorbing regions22on the sides of the first and second groups of optical waveguides2and3have the same region length of about 79 μm (75 μm except for the corresponding respective electrical separating groove4). Also, in the stable element100according to this embodiment, the main exciting region21is defined by the remaining part of the multi-mode interference optical waveguide1.

Note, since a production method of the bistable element100according to this embodiment is similar to the above-mentioned production method of the bistable element100according to the first embodiment except that a mask38is used to form the electrical separating groove4in the multi-mode interference optical waveguide on the side of the second group of optical waveguides3, and that an SiO2 film is left in the electrical separating groove4on the side of the second group of optical waveguides3, an explanation for the production method is omitted.

Next, reference is made to a reason why a steady bistable operating range (i.e., a wide bistable hysteresis window) is obtained by the bistable element100according to the second embodiment, and why the bistable element100can be miniaturized.

A operating principle of the bistable element100according to the second embodiment is basically similar to that of the above-mentioned bistable element100according to the first embodiment, and it is possible to theoretically obtain the substantially complete superimposition region (i.e., the cross-gain restraint region) between the two optical waveguide paths in the main exciting region21, by defining the regions (i.e., the non-cross-gain restraint region), in which the superimposition of the two optical waveguide paths is not almost obtained, as the saturable absorbing regions in the multi-mode interference optical waveguide1.

On the basis of this principle, in the second embodiment, both the side regions of the multi-mode interference optical waveguide1, in each of which the superimposition of the two optical waveguide paths is not almost obtained, are defined as the respective saturable absorbing regions22, whereby it is possible to obtain the wide bistable saturable window.

Note, the second embodiment is different from the first embodiment in that the multi-mode interference optical waveguide1has the two ports formed in the one end face1a, and the two ports formed in the other end face1b, and, except for merits or advantages obtained from this arrangement of the multi-mode interference optical waveguide1, the second embodiment features the merits or advantages similar to those of the first embodiment.

Third Embodiment

FIG. 12(a) is a plan view showing an example of a schematic structure of a bistable element according to a third embodiment; andFIG. 12(b) is a plan view showing another example of a schematic structure of a bistable element according to the third embodiment.FIG. 13(a) is a cross-sectional view corresponding to a cross-sectional view taken along the D-D′ line in the bistable element shown inFIG. 12, in which a epitaxial structure is produced by an MOCVD process;FIG. 13(b) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 13(a) in which a mask is formed;FIG. 13(c) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 13(b) in which a high-mesa structure formed by an etching process; andFIG. 13(d) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 13(c) from which the mask is removed.FIG. 14(a) is a cross-sectional view corresponding to a cross-sectional view taken along the E-E′ line in the bistable element shown inFIG. 12(a), in which a mask for an electrical separating groove is formed;FIG. 14(b) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 14(a) in which the electrical separating groove is formed by an etching process;FIG. 14(c) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 13(b) from which the mask is removed; andFIG. 14(d) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 14(c) in which an SiO2 film is formed by a thermal CVD process.FIG. 15(a) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 14(d) in which a mask is formed on the electrical separating groove;FIG. 15(b) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 15(a) from which the SiO2 film is removed by an etching process except for the electrical separating groove; andFIG. 15(c) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 15(b) from which the mask is removed.FIG. 16(a) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 13(d) in which an SiO2 film is formed by a thermal CVD process;FIG. 16(b) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 16(a) in which a mask is formed on an area except for an optical waveguide;FIG. 16(c) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 16(b) from which the SiO2 film on the optical waveguide is removed by an etching process except for the electrical separating groove; andFIG. 16(d) is a cross-sectional view corresponding to the cross-sectional view ofFIG. 16(c) from which the mask is removed. InFIGS. 12 to 16, the same references as inFIGS. 1 to 11indicate similar elements or corresponding elements, and thus explanations for these elements are omitted.

In a first group of optical waveguides2, comparing with a space between respective end faces of the two adjacent optical waveguides (each of which is defined as a boundary face between the multi-mode interference optical waveguide1and the optical waveguide concerned), a space between the respective other end faces of the two adjacent optical waveguides (each of which is defined as a light-incidence face and/or a light-emission face at a cut side face of a substrate10) is wider, and the other end faces of the two adjacent optical waveguides are substantially parallel to the cut side face of the substrate10. Also, at least one the two adjacent optical waveguides is defined as a curved waveguide (including either only a curved region or a curved region and a linear region) including a curved region (for example, see:FIG. 12). Note, in the first group of optical waveguides2, a lengthwise direction of end portions of the optical waveguides in the vicinity of the end faces thereof is substantially perpendicular to the end face1aof the multi-mode interference optical waveguide1. Nevertheless, if a light-incidence requirement is fulfilled at the boundary between the multi-mode interference optical waveguide1and the optical waveguides, the aforesaid lengthwise direction may not be perpendicular to the end face1aof the multi-mode interference optical waveguide1. On the other hand, a lengthwise direction of end portions of the optical waveguides in the vicinity of the other end faces thereof must be substantially perpendicular to the cut side face of the substance10so that each of the other end faces of the optical waveguides included in the first group2is defined as a reflecting face.

In a case where a second group of optical waveguides3includes at least two optical waveguides, comparing with a space between respective end faces of the two adjacent optical waveguides (each of which is defined as a boundary face between the multi-mode interference optical waveguide1and the optical waveguide concerned), a space between the respective other end faces of the two adjacent optical waveguides (each of which is defined as a light-incidence face and/or a light-emission face at a cut side face of a substrate10) is wider. To this end, at least one the two adjacent optical waveguides is defined as a curved waveguide (including either only a curved region or a curved region and a linear region) including a curved region (for example, see:FIG. 12(b)). Note, in the second group of optical waveguides3, a lengthwise direction of end portions of the optical waveguides in the vicinity of the end faces thereof is substantially perpendicular to the end face1bof the multi-mode interference optical waveguide1. Nevertheless, if a light-incidence requirement is fulfilled at the boundary between the multi-mode interference optical waveguide1and the optical waveguides, the aforesaid lengthwise direction may not be perpendicular to the end face1bof the multi-mode interference optical waveguide1. On the other hand, a lengthwise direction of end portions of the optical waveguides in the vicinity of the other end faces thereof must be substantially perpendicular to a corresponding cut side face of the substance10so that each of the other end faces of the optical waveguides included in the first group2is defined as a reflecting face.

Note, in this embodiment, as shown inFIG. 12(a), reference will be made to an example in which the first group of optical waveguides2includes two optical waveguide (i.e., the first and second optical waveguides2aand2b), and in which the second group of optical waveguides3includes one optical waveguide (i.e., the third optical waveguide3a) below.

Each of the first and second optical waveguides2aand2bincludes a generally S-shaped curved region and linear regions. In comparison with the space between the respective end faces of the optical waveguides (each of which is defined as the boundary face between the multi-mode interference optical waveguide1and the optical waveguide concerned), the space between the respective other end faces of the optical waveguides (each of which is defined as the light-incidence face and/or the light-emission face at the corresponding cut side face of a substrate10) is wider.

Note, the multi-mode interference optical waveguide1, the first optical waveguide2a, the second optical waveguide2band the third optical waveguide3ahave the same layer structure, and each of these optical waveguides is defined as a high-mesa waveguide. In particular, the multi-mode interference optical waveguide1has a waveguide length of about 140 μm, and a waveguide width of about 8 μm. Also, each of the first, second and third optical waveguides2a,2band3ahas a waveguide length of about 3 μm.

As shown inFIG. 13(d), a cross-sectional structure of these elements is defined as a high-mesa structure in which a buffer layer11composed of an n-InP-based material forming an n-type semiconductor, a light-emitting layer12composed of an InGaAsP/InGaAsP-based material and defined as an active layer for realizing a bistable element featuring a long wavelength region (a 1.55 μm region), a first clad layer13composed of an i-InP-based material forming an intrinsic semiconductor, a second clad layer15composed of a p-InP-based material forming a p-type semiconductor, a contact layer16composed of a p-InGaAs-based material forming a p-type semiconductor are laminated in order on the substrate10composed of an n-InP-based material.

As shown inFIG. 13(d), the high-mesa structure corresponds to one in which the contact layer16, the second clad layer15, the first clad layer13, the light-emitting layer12and buffer layer11are partially removed from a non-waveguide region by an etching process, with a part of the substrate10being simultaneously etched by the etching process.

Note, the light-emitting layer12is defined as a usual light-emitting layer featuring an SCH (separate confinement hetero-structure) and an MQW (multi-quantum well).

In particular, in this embodiment, the buffer layer11is about 100 nm in film thickness; the light-emitting layer12is 100 nm in film thickness; the first clad layer13is 100 nm in film thickness; the second clad layer15is 900 nm in film thickness; and the contact layer16is 150 nm in film thickness.

Next, with reference toFIGS. 13 to 16, a production method of the bistable element100according to this embodiment will now be explained.

First, an n-In film31, a 1.55 μm wavelength region InGaAsP/InGaAsP film32, an i-InP film43, a p-InP film45, and a p-InGaAs film36are deposited and laminated in order on a usual n-InP substrate10by using an MOVPE (metal-organic vapor phase Epitaxy) process (FIG. 13(a)).

Then, a mask37for an etching process is formed on the p-InGaAs film36by a usual photolithography process using a stepper (i.e., a lens reduction projection aligner) so as to conform with a plane configuration of the multi-mode interference optical waveguide1, the first group of optical waveguides2and the second group of optical waveguides3shown inFIG. 12a(FIG. 13(b)).

With using the mask37, the p-InGaAs film36to be defined as the contact layer16, the p-InP film45to be defined as the second clad layer15, the i-InP film43to be defined as the first clad layer13, the 1.55 μm wavelength region InGaAsP/InGaAsP film32to be defined as the light-emitting layer12, the n-InP film31to be defined as the buffer layer11are subjected to a dry etching process using an RIE (reactive ion etching) process, so that the disused portions of these films (on which the mask37is not formed) are removed, resulting in the formation of the high-mesa structure in the cross-sectional configuration (FIG. 13(c)). Note, as shown inFIG. 13(c), the etching reaches the surface of the substrate10so that the part of the substrate is etched, and the etched surface is indicated by reference10a.

Thereafter, the mask37, which lies on the contact layer16, is removed by using an chemical solution and an ashing process (FIG. 13(d)).

Note, production steps stated hereinafter are directed to a separation of the high-mesa structure, including the multi-mode interference optical waveguide1, the first group of optical waveguides2and the second group of optical waveguides3, into the main exciting region21and the saturable absorbing region22.

Then, a mask38for an etching process is formed on the contact layer16and the etched surface of the substrate10by the photolithography process using the stepper so as to conform with the plane configuration, except for the electrical separating groove4, shown inFIG. 12(a) (FIG. 14(a)).

With using the mask38, the contact layer16is subjected to a wet etching process using a sulfuric-acid-based etching solution so that the contact layer16is removed from a part (corresponding to the separating groove region22a) of the multi-mode interference optical waveguide1, at which the electrical separating groove4is to be defined (FIG. 14(b)).

Thereafter, the mask38, which lies on the contact layer16and the etched surface of the substrate10except for the separating groove region22a, is removed by using the chemical solution and the ashing process (FIG. 14(c)).

Then, a mask40for an etching process is formed on the SiO2 film39by the photolithography process using the stepper so as to conform with both a plane configuration except for the multi-mode interference optical waveguide1, the first group of optical waveguides2and the second group of optical waveguides3and a plane configuration of the electrical separating groove4shown inFIG. 12(a) (FIG. 15(a) andFIG. 16(b)).

Thereafter, with using the mask40, the SiO2 film39, which lies on the multi-mode interference optical waveguide1except for the separating groove region22a, the first group of optical waveguides2and the second group of optical waveguides3, is subjected to a wet etching process by using an etching solution containing a BHF (buffered hydrofluoric acid), so as to be removed therefrom (FIG. 15(b) andFIG. 16(c)).

Then, the mask40, which lies on the area except for the first group of optical waveguides2and the second group of optical waveguides3, and on the separating groove region22a, is removed by using the chemical solution and the ashing process (FIG. 15(c) andFIG. 16(d)).

Then, a photoresist layer, which is not shown in the drawings, is formed on the contact layer16by the photolithography process using the stepper so as to confirm with a plane configuration of the multi-mode interference optical waveguide1except for the separating groove region22a, the first group of optical waveguides2and the second group of optical waveguides3shown inFIG. 12(a).

Then, a Ti/Pt/Au electrode material layer, which is to be defined as an external electrode (i.e., a front electrode) for exciting the active layer by supply of a bias current, is formed over the substrate10by using an electron-beam evaporation process.

Thereafter, the photoresist layer, which is not shown in the drawing, is removed together with the Ti/Pt/Au electrode material lying thereon, by a lift-off process using an chemical solution.

Note, in this embodiment, although the front surface electrode is formed on the contact layer16in the main exciting region21and the saturable absorbing region22except for the separating groove region22a, the front surface electrode may be formed on only the main exciting region21if an electrical connection is established between the main exciting region21and the saturable absorbing region22. Also, the front surface electrode may be formed on all the area of the substrate10except for the saturable absorbing region22(including the separating groove region22a). Optionally, the front surface electrode may be formed on the main exciting region21and the separating groove region22a. Further, the front surface electrode may be formed on all the area of the substrate10except for the saturable absorbing region22(excluding the separating groove region22a).

On the other hand, a front surface electrode may be formed on the saturable absorbing region22so that a bias current can be injected into the saturable absorbing region22. With this arrangement, when a sufficient bistable hysteresis window can be obtained, it is possible to lower an operating current at the sacrifice of the bistable hysteresis window by injecting the bias current into the saturable absorbing region22.

Thereafter, the rear surface of the substrate10, in which no optical waveguide is formed, is polished, and a Ti/Pt/Au layer, which is not shown in the drawing, is formed on all the rear surface of the substrate10as an external electrode (a rear surface electrode) for exciting the active layer by supply of a bias current, using the electron-beam evaporation process.

Then, the substrate10, on which a plurality of bistable elements100are formed, is cut along the boundaries between the adjacent bistable elements100so that the individual bistable elements100are separated from each other, whereby it is possible to obtain the individual bistable elements as shown inFIG. 12(a).

Note, in the production method according to this embodiment, although the stepper is used in the photolithography process, the production method is not necessarily limited to only the use of the stepper. For example, a vector scan electron exposure system may be applied to the production method.

Also, in the production method according to this embodiment, although the thermal CVD process is used for the formation of the SiO2 film39, for example, a plasma CVD process or a sputtering process may be applied to the production method.

Also, in the production method according to this embodiment, although the MOVPE process is used as the epitaxial growth process in the production the high-mesa structure, the production method is not necessarily limited to only the MOVPE process. For example, an MBE (molecular beam epitaxy) process may be applied to the production method.

Further, in the production method according to this embodiment, although the RIE process is used as the etching process, the production method is not necessarily limited to only the RIE process. For example, an ICP process or the wet etching process may be applied to the production method.

Also, in the production method according to this embodiment, although the stepper is used in the photolithography process, the production method is not necessarily limited to only the use of the stepper. For example, a vector scan electron exposure system may be applied to the production method.

Also, in the production method according to this embodiment, although the lift-off process is used for the formation of the front surface electrode, the production method is not necessarily limited to the use of the lift-off process. For example, a method, in which a formation of an electrode pattern is carried out by using a photolithography process, and in which a disused Ti/Pt/Au electrode material is removed by using a milling process or the like, may be applied to the production method.

Note that the third embodiment is only different from the first and second embodiments in that at least one optical waveguide included in the first group and second groups2and/or3is defined as a curved optical waveguide, and thus features the same merits and advantages as in the first and second embodiments except for the below-mentioned merits or advantages obtained from such a curved optical waveguide.

In the bistable element100, it is intended that optical fibers are connected to the other end faces (each of which is defined as the light-incidence face and/or the light-emission face at the cut side face of a substrate10) of two adjacent optical waveguides included in the first group2or the second group3, and thus a distance between the centers of the other end faces of the two adjacent optical waveguides must be set so as to be at least a diameter of the optical fiber (e.g., 62.5 μm) so that an interference can be prevented at the connections of the two adjacent optical fibers to the other end faces of the two adjacent waveguides.

On the other hand, in the bistable element as shown inFIG. 1orFIG. 11(a), since each of the optical waveguides included in the first group2or the second group3is defined as a linear optical guide, the space between the one end faces between the two adjacent optical waveguides is substantially equal to that between the other end faces therebetween. Thus, when the bistable element100(the multi-mode interference optical waveguide) is miniaturized, the space between the other end faces of the two adjacent optical waveguides may be merely on the order of several microns.

For this reason, in the bistable element as shown inFIG. 1orFIG. 11(a), the optical fibers cannot be connected to the optical waveguides in such a manner that the respective other end faces of the optical waveguides abut against the light-incidence faces (the light-emission faces) of the optical fibers. Thus, it is necessary to establish an optical connection between the optical waveguides and the optical fibers by using either a lens system which is constituted so as to be suitable for a whole of the first group of optical waveguides2(the second group of optical waveguide or a microlens system which is suitable for each of the optical waveguide included in the first group2. Note, when either the lens system or the microlens system is used, not only a number of parts for constituting the bistable element is increased, but also it is necessary to carry out an adjustment of the lens system or the microlens system.

On the contrary, in the bistable element100according to this embodiment, at least one of the two adjacent optical waveguides is defined as the curved optical waveguide so that the space between the other end faces of the two adjacent optical waveguides can be more widened than that between the one end faces of the two adjacent optical waveguides (concretely, so that the distance between the centers of the other end faces of the two adjacent optical waveguides can be set to be at least the diameter of the optical fibers). Accordingly, the optical fibers can be connected to the optical waveguides in such a manner that the respective other end faces of the optical waveguides abut against the light-incidence faces (the light-emission faces) of the optical fibers, and thus it is possible to obtain merits or advantages that a connection structure between the bistable element100and the optical fibers can be simplified.

Note, when the bistable element100featuring the ridge structure is manufactured, a radius of curvature of the curved region in the curved optical waveguide must be on the order of several millimeters due to a restriction on manufacturing conditions. In this case, the distance measured from the one end face1a(the other end face1b) of the multi-mode interference optical waveguide1to a corresponding cue side face of the substrate10must be about 0.5 mm before the distance between the centers of the other end faces of the two adjacent optical waveguides can be set to be at least the diameter of the optical fibers, so that it is impossible to establish the bistable element100as an integrated element.

On the other hand, when the bistable element100featuring the high-mesa structure is manufactured, a radius of curvature of the curved region in the curved optical waveguide can fall within a range from 2 □m to 3 □m. In this case, the distance measured from the one end face1a(the other end face1b) of the multi-mode interference optical waveguide1to a corresponding cue side face of the substrate10can be about 30 must be about □m, so that it is possible to establish the bistable element100as an integrated element.

EXPLANATION OF REFERENCES

While various embodiments of the innovation have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the innovation as defined by the appended claims.