JPH0621571A - Bistable semiconductor laser - Google Patents

Abstract

PURPOSE:To reduce the minimum switching light intensity necessary to generate quenching, in a bistable semiconductor laser performing total light input operation. CONSTITUTION:The title bistable semiconductor laser consists of the following; a bistable laser part having a multiple quantum well structure as an active layer, two or more optical waveguides intersecting the laser part, and a saturable absorption region 4A and a gain quenching region 4B which are formed at the intersecting parts. In the bistable semiconductor laser, the optical waveguides 1B, 2B which have the gain quenching region 4B are larger than the optical wavequides 1A, 2A which have the saturable absorption region 4A.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bistable semiconductor laser capable of optical logic / optical switching operation which can be used in optical switching / optical repeaters expected to constitute optical communication / optical information systems. .

[0002]

2. Description of the Related Art Bistable semiconductor lasers having hysteresis in current-to-optical output characteristics and optical input / output characteristics have features such as a high-speed optical switch, optical logic operation, and optical memory, and therefore have a function of forming an optical communication / optical information system. Expected as a device.

FIG. 16 shows a conventional structural view (surface view) of a bistable semiconductor laser (Japanese Patent Laid-Open No. 3-66189) having two or more optical waveguides intersecting the bistable laser section at a right angle or at an angle. . Although two optical waveguide portions are provided for simplification, it is possible to operate in the same manner even if the number of optical waveguide portions is two or more. Further, although the crossing angle between the optical waveguide and the bistable laser is set to be a right angle, the same discussion can be made when crossing at other angles. 42A and 43A are optical waveguides A, 42B and 43B are optical waveguides B, 44A to 44C are gain regions of bistable lasers, 45A is a saturable absorption region, 45A
B is a gain quenching region, 46A to 46D, 47A to
Reference numeral 47D is a groove formed for the purpose of electrically insulating the optical waveguide from the saturable absorption region / gain quenching region / gain region. FIG. 17 shows a cross section of the optical waveguide portion AA 'of FIG. Reference numerals 48A and 49A are electrodes for injecting a current in order to give a gain to the optical waveguide to reduce the loss, 50A is a p-side electrode in the saturable absorption region, 52 is a p ⁺ -InGaAs cap layer, and 53 is p-type InP. The cladding layer 54 is an MQW waveguide layer (InGaA
s well layer, InGaAsP or InP barrier layer), 55 is an n-type InP cladding layer, and 56 is an n-type I
An nP substrate, 57 is an n-side electrode, 58 is a lens, and 59 is external injection light. The grooves 51A and 51C are formed by removing a part of the InGaAs cap layer and the p-type InP clad layer by chemical etching or the like, or by proton implantation or G
a FIB (Focused Ion Beam) injection is performed to increase the resistance. FIG. 18 shows a cross section of the bistable laser section BB ′ of FIG.
A to 60C are p-side electrodes in the gain region, 61A is a p-side electrode in the saturable absorption region, and 61B is a p-side electrode in the gain quenching region. The method of forming the grooves 62B, 62D, 63B, 63D is the same as that of the grooves 51A, 51C. In this example, the optical waveguide layer has the same structure as the active layer of the bistable laser, but this is because the waveguide or laser can be formed within the same crystal growth. In order to compensate for the loss, it is necessary to pass a current that does not oscillate in the waveguide.

FIG. 19 shows the relationship between the intensity of externally injected light and the intensity of output light when light is externally injected into the elements shown in FIGS. Since switching and gain quenching can be performed for the same injection light wavelength, the memory operation can be performed by all-optical input by adjusting the injection light intensity.

[0005]

However, in general, gain quenching is a phenomenon in which already injected carriers are deprived of by light, and is larger than saturable absorption in which carriers are excited to cause absorption saturation. External injection light is required. Further, when light is injected into the bistable laser portion in the orthogonal direction as shown in FIG. 16, the interaction length of light is as short as several μm as defined by the ridge structure width, and the light required for quenching is There is a problem that the intensity needs to be one digit or more as compared with the case of switching.

It is an object of the present invention to provide a bistable semiconductor laser having an all-optical input operation, which has a structure for reducing the light intensity of the minimum switching required to cause quenching. .

[0007]

In order to solve the above-mentioned problems, a bistable semiconductor laser of the present invention comprises a bistable laser section having a multiple quantum well structure as an active layer and two bistable laser sections intersecting the laser section. In the bistable semiconductor laser having the above-mentioned optical waveguide and having a saturable absorption region and a gain quenching region at the intersection, the optical waveguide having the gain quenching region as a part thereof is the saturable absorption region. Is larger than the optical waveguide that has

[0008]

According to the present invention, a saturable absorption region in which almost no current is injected and the amount of absorption is controlled by a change in applied voltage is controlled, and a gain quenching region into which current is injected are provided on the same substrate. Since the optical waveguide having a region as a part thereof is larger than the optical waveguide having a saturable absorption region as a part thereof, the minimum switching light intensity for quenching can be reduced.

[0009]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a surface view of an embodiment of a bistable semiconductor laser according to the present invention. Although two optical waveguide portions are provided for simplification, it is possible to operate in the same manner even if the number of optical waveguide portions is two or more. Further, although the crossing angle between the optical waveguide and the bistable laser is set to be a right angle, the same discussion can be made when crossing at other angles. Optical waveguides 1A and 2A are provided. Reference numerals 1B and 2B are optical waveguides B, and the width thereof is set larger than that of the optical waveguide A. In order to prevent the absorption of incident light in the optical waveguide and obtain the amplification effect, the optical waveguide 1
Current is injected into the regions A, 2A and 2A, 2B. Three
A to 3C are gain regions of a bistable laser, 4A is a saturable absorption region, 4B is a gain quenching region, 5A to 5D, 6A.
Reference numerals 6D to 6D are grooves formed for the purpose of electrically insulating the optical waveguide from the saturable absorption region / gain quenching region / gain region. The sectional structures of the bistable laser section and the optical waveguide section in the figure are basically the same as those in FIGS. FIG. 2 shows the relationship between the optical output and the injection current (hereinafter referred to as the gain current) injected into the gain region of the bistable laser portion. The oscillation wavelength of the bistable laser is located on the longer wavelength side than the two-dimensional exciton absorption peak wavelength due to the bandgap reduction effect. Here, when the applied voltage (expressed as V _A and V _B , respectively) to the saturable absorption region or the gain quenching region is changed from the forward bias to the reverse bias, the two-dimensional excitons of the two-dimensional excitons are generated by the quantum confined Stark effect. The absorption peak wavelength shifts to the long wavelength side, and the absorption amount increases. Therefore, if the applied voltage to the saturable absorption region or the gain quenching region is reduced, the characteristic curve,
As shown by and, the characteristics show that both the oscillation threshold current and the hysteresis width increase. At this time, the two-dimensional excitons play a role as a saturable absorber. Here, the applied voltages V _A and V _B of the saturable absorption region 4A and the gain quenching region 4B are values (V ₁ and V ₂ ) in which the current-optical output characteristics of the bistable laser show a hysteresis like a characteristic curve.
Set to. V ₁ is a value at which absorption saturation occurs due to external injection light, and V ₂ is a value at which gain quenching occurs due to external injection light. Generally, V ₁ is lower than the rising voltage of the current-voltage characteristic (current Is not injected), and the value is equal to or higher than the rising voltage (area where current is injected). In the figure, the applied voltages in the saturable absorption region and the gain quenching region are set so as to cause hysteresis for the purpose of performing the memory operation.
Now, when the gain current of the bistable laser is set to I _g ,
The light input / output characteristics are shown in FIG. The incident light intensities L _inA and L _inB to _each optical waveguide are shown by rectangles in the figure. The same figure (a)
Shows the light input / output characteristics of the bistable laser portion with respect to the light injected into the optical waveguide A, and FIG. 7B shows the light input / output characteristics of the bistable laser portion with respect to the light injected into the optical waveguide B. Since the width of the optical waveguide B is larger than that of the optical waveguide A, the crossing portion between the bistable laser portion and the optical waveguide portion, that is, the coupling length of the externally injected light can be made longer, so that the threshold value of the quenching light intensity can be increased. It is decreasing as shown by the arrow. Here, the width of the optical waveguide A is 1 to 3 μm, and the width of the optical waveguide B is 5 to 10 μm.
m. If the width of the optical waveguide B is 10 μm or more, the oscillation threshold of the bistable laser section is increased, and the effect of increasing the optical waveguide width is reduced. FIG. 4 shows an operation example of this element under the setting conditions of FIG. First, the bistable laser causes switching by the input light L _inA to the optical waveguide A, and becomes an oscillating state. Since the set value I _g of the gain current is inside the hysteresis, the bistable laser maintains the oscillation state even after the input light disappears. Next, input light L _inB to the optical waveguide B
Causes quenching, causing the bistable laser to transition to the non-oscillating state. Similar to the switching operation, the bistable laser maintains the non-oscillation state even after the input light disappears. L _out is the optical output of the bistable laser.

According to this element structure, since the switching section and the quenching section are in different regions, there is no need to select a wavelength that causes two operations at the same time, and the tolerance for the wavelength of the input light is correspondingly large. .

When the end faces of the optical waveguides A and B are formed as cleavage planes and a current is injected into the electrodes of the optical waveguides to the extent that a gain is generated, the wavelength dependence of the minimum switching / quenching light intensity is shown in FIG. As described above, the narrow wavelength sensitivity characteristic corresponding to the resonator mode of the optical waveguide is shown. On the other hand, when an antireflection coating is formed on the end face or an absorber is formed on part of the optical waveguide,
As shown in FIG. 6, a flat wavelength sensitivity characteristic can be obtained.

Other operation examples of the same element are shown in FIGS. L _out is the optical output of the bistable laser. In the case of FIG. 7, two input lights L _inA1 and L _inA2 are simultaneously injected into the optical waveguide A of FIG. Here, the applied voltage of the saturable absorption region 4A existing in the middle of the optical waveguide A is set to a voltage value at which the bistable laser causes a switching operation by the injection light from the outside. Input light intensity L _in shown in FIG.
Then, the bistable laser does not oscillate when there is no incident light in the two optical waveguides of FIG. 1 or when light is incident on only one of them. The so-called AND operation in which the bistable laser oscillates can be realized only when two incident lights are simultaneously injected into the optical waveguide (2 × L _in ). Next, in order to stop the oscillation of the oscillated bistable laser, since the gain current is set inside the hysteresis, the input light L _inB may be injected into the optical waveguide B.

Further, as shown in FIG. 8, the two incident light intensities L _inA1 and L _inA2 to the optical waveguide A are set to be _equal to or higher than the threshold value of the light input / output characteristic, and at least one of the two inputs is input. In this case, a so-called OR operation in which the bistable laser oscillates can be performed.

9 and 10 show a case where two input lights are coupled to the optical waveguide B instead of the optical waveguide A. Here, the applied voltage of the gain quenching region 4B existing in the middle of the optical waveguide B is set to a voltage value at which the bistable laser causes the quenching operation by the injection light from the outside.
Similarly to FIGS. 7 and 8, NAND operation,
An OR operation can be realized. However, in FIG. 9 and FIG.
Similarly, the gain current is set within the hysteresis, and the input light L _inA is injected into the optical waveguide A in order to return the bistable laser whose oscillation has stopped to the oscillation state again.

FIG. 11 shows an embodiment in which a secondary diffraction grating is used as a structure for coupling the injected light 17 from the outside with the optical waveguide. Reference numeral 19 is a second-order diffraction grating, and the other configurations are the same as those in FIG. A part of the light incident in the direction perpendicular to the first-order diffraction grating is diffracted in the waveguide direction and coupled, and reaches the saturable absorption region / gain quenching region. The light causes absorption saturation in the saturable absorption region and gain quenching in the gain quenching region, and as a result, a switch ON / OFF operation occurs.

FIG. 12 shows an embodiment in which a 45-degree mirror is used as a structure for coupling the injected light 17 from the outside with the optical waveguide. 20 is a 45-degree mirror, and other configurations are the same as those in FIG.

FIG. 13 shows, as a structure for coupling the externally injected light 17 to the optical waveguide, a configuration in which the lens 16 is used to couple the light 17 perpendicularly to the crystal growth surface.

FIG. 14 has the same configuration as that of FIG.
16 'is a lens in which an InP substrate is monolithically formed by a method such as chemical etching.

FIG. 15 shows a structure in which the bistable semiconductor lasers described above are arrayed on the same substrate. 21A-21C, 28A-28C, 35A
~ 35C is the gain region of the bistable laser, 22A,
22B, 23A, 23B, 29A, 29B, 30A, 3
0B, 36A, 36B, 37A and 37B are optical waveguides, 24, 31 and 38 are saturable absorption regions, 2
5, 32, 39 are gain quenching regions, 2 respectively
Reference numerals 6, 27, 33, 34, 40, 41 are second-order diffraction gratings or 45-degree mirrors, respectively.

The Fabry-Perot type bistable semiconductor laser structure has been described above.
Alternatively, it goes without saying that the same effect can be realized with the DBR type.

Although the optical waveguide for injecting the external input light has been described as having the same MQW structure as the active layer of the bistable laser, the crystal forming the optical waveguide has a wavelength shorter than the emission wavelength of the bistable laser. It goes without saying that the same effect can be realized even with an InGaAsP quaternary mixed crystal having a band gap. In addition, after the optical waveguide has the same MQW structure as the active layer of the bistable semiconductor laser,
A silicon nitride film is formed only on the portion corresponding to the optical waveguide, and the rapid heating method is repeated (Miyazawa et al., Japane Jo.
internal of Applied Physics,
p. It is needless to say that the same effect can be realized even when a waveguide transparent to the oscillation wavelength of the bistable semiconductor laser is formed by mixing the MQW with L1039, 1989).

InGaAsP / InP as a crystal material
I have described the system, but InGaAs / In (Ga)
AlAs system, AlGaAs / GaAs system, InGaAs
/ GaAs strained superlattice system, InGaAs / InGaAsP
It goes without saying that the same effect can be realized even in the strained superlattice system.

[0024]

As described above, the present invention is a bistable semiconductor laser having two or more optical waveguides intersecting the bistable laser section at a right angle or at an angle, and has a gain quenching region in a part thereof. The minimum switching light intensity for quenching can be reduced by making the optical waveguide larger than the optical waveguide having a saturable absorption region in a part thereof.

[Brief description of drawings]

FIG. 1 shows the surface of an example of a bistable laser according to the present invention.

FIG. 2 is a gain current vs. optical output characteristic diagram of the bistable laser section of the present invention.

FIG. 3 is a light input / output characteristic diagram of a bistable laser unit.

FIG. 4 is a diagram showing an operation example of the present invention.

FIG. 5 is a diagram showing the relationship between the minimum switching input light intensity (switching and quenching) with respect to the input light wavelength in the case of end face cleavage.

FIG. 6 is a diagram showing the relationship between the minimum switching input light intensity (switching and quenching) with respect to the input light wavelength when the end face non-reflection coat or the absorber is provided in a part of the optical waveguide.

FIG. 7 is a diagram showing an operation example of the present invention.

FIG. 8 is a diagram showing an operation example of the present invention.

FIG. 9 is a diagram showing an operation example of the present invention.

FIG. 10 is a diagram showing an operation example of the present invention.

FIG. 11 is a diagram showing a cross-sectional structure of an optical waveguide portion of a bistable laser in which a second-order diffraction grating is formed on the same substrate.

FIG. 12 is a diagram showing a sectional structure of an optical waveguide portion of a bistable laser in which a 45-degree mirror is formed on the same substrate.

FIG. 13 is a diagram showing a cross-sectional structure of a bistable laser that uses a lens to couple externally injected light into a saturable absorption region (or a gain quenching region) in a direction perpendicular to a crystal plane.

FIG. 14 is a sectional view of a structure in which a lens is monolithically formed on an InP substrate.

FIG. 15 is an external view of an array structure of the present invention.

FIG. 16 is a diagram showing a surface structure of a conventional bistable laser.

FIG. 17 is a diagram showing a sectional structure of an optical waveguide portion of a conventional bistable laser.

FIG. 18 is a diagram showing a cross-sectional structure of a bistable laser portion of a conventional bistable laser.

FIG. 19 is a diagram showing light input / output characteristics of a conventional bistable laser.

[Explanation of symbols]

1A, 2A Optical waveguide A 1B, 2B Optical waveguide B 3A, 3B, 3C Gain region of bistable laser 4A Saturable absorption region 4B Gain quenching region 5A, 5B, 5C, 5D, 6A, 6B, 6C, 6D Optical waveguide -Gain region-Saturable absorption region-Groove for electrically separating gain-quenching region 7A, 8A Electrode of optical waveguide portion 9A Saturable absorption region electrode 9B Gain-quenching region electrode 10p ⁺ -InGaAs cap Layer 11 p-type InP clad layer 12 InGaAs / InP or InGaAs / In
GaAsP MQW active layer 13 n-type InP clad layer 14 InP substrate 15 n-side electrode 16 lens 16 ′ lens monolithically formed on an InP substrate 17 external injection light 18A, 18B, 18C gain region electrode 19 second-order diffraction grating 20 20 45 Degree mirror 21A, 21B, 21C, 28A, 28B, 28C, 3
Gain regions of 5A, 35B, 35C bistable lasers 22A, 22B, 23A, 23B, 29A, 29B, 3
0A, 30B, 36A, 36B, 37A, 37B Optical waveguide 24, 31, 38 Saturable absorption region 25, 32, 39 Gain quenching region 26, 27, 33, 34, 40, 41 External input light coupling part (2 Next-order diffraction grating or 45-degree mirror) 42A, 43A Optical waveguide A 42B, 43B Optical waveguide B 44A, 44B, 44C Bistable laser gain region 45A Saturable absorption region 45B Gain quenching region 46A, 46B, 46C, 46D, 47A , 47B, 4
7C, 47D optical waveguide, gain region, saturable absorption region,
Grooves for electrically separating the gain quenching region 48A, 49A Electrodes in the optical waveguide part 50A Electrodes in the saturable absorption region 51A, 51C Electrically connecting the optical waveguide, the gain region, the saturable absorption region, and the gain quenching region Groove for separation 52 p ⁺ -InGaAs cap layer 53 p-type InP clad layer 54 InGaAs / InP or InGaAs / In
GaAsP MQW active layer 55 n-type InP clad layer 56 InP substrate 57 n-side electrode 58 lens 59 external injection light 60A, 60B, 60C bistable laser gain region 61A saturable absorption region electrode 61B gain quenching region 62B, 62D, 63B, 63D Groove for electrically separating optical waveguide, gain region, saturable absorption region, and gain quenching region

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroyuki Tsuda 1-16 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (72) Takashi Kurokawa 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. A bistable laser section having a multiple quantum well structure as an active layer, and two or more optical waveguides intersecting with the laser section, and a saturable absorption region and gain quenching at the intersection. A bistable semiconductor laser having a region, wherein an optical waveguide having the gain quenching region in a part thereof is larger than an optical waveguide having the saturable absorption region in a part thereof.