JPH1070312A - Super luminescent diode - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To improve the absorption efficiency in the absorption region of a superluminescent diode, reduce the influence of the reflection on the end face on the absorption region side, and narrow the emission angle. SOLUTION: A super luminescent diode is:
A first absorption region whose absorption region is in contact with the excitation region;
A second absorption region in contact with the absorption region, and the width of the optical waveguide is increased at least on the side of the second absorption region in the first absorption region as the distance from the excitation region increases,
Further, the optical axis of the optical waveguide is bent at the boundary between the first absorption region and the second absorption region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superluminescent diode which can emit incoherent light having a large intensity and a small radiation angle, which is useful as an optical fiber gyro, a light source for an optical measuring instrument, and the like.

[0002]

2. Description of the Related Art In a superluminescent diode capable of extracting high-power incoherent light from an end face of an active layer, a gain is generated in the active layer by current injection from the outside, and light generated in the active layer is further reduced. Amplify in the stripe direction without oscillating in Fabry-Perot mode. Therefore, it is important to suppress laser oscillation in the Fabry-Perot mode. Since the characteristics of the active layer as an optical waveguide, or the dimensions and electrical characteristics of the entire device can be considered in the same way as a semiconductor laser diode, in order to suppress the Fabry-Perot mode even under high current injection, light reflection at the device end face is required. It is important to reduce the rate as much as possible. For this reason, various countermeasures such as a non-reflection coating on the end face, installation of a non-excitation region, oblique etching of the end face, and a window structure by embedding the end face have been implemented, and effective results have been realized.

FIG. 8 shows an example of a conventional super luminescent diode. In this conventional example, an absorption region II is provided in contact with an active region I, and a waveguide (active layer) of the active region I is provided.
In this example, a bent waveguide 2 is connected to the side opposite to the emission end face of the light-emitting device 1.

The active layer 1 receives light from an electrode 3 formed thereon and generates light, and the generated light is amplified in the active layer and emitted from one end face on which an antireflection film 4 is provided. I do. Since the antireflection film cannot completely prevent the reflection of light, the reflected light is bent through the active layer 1 and enters the waveguide 2. When the intersection angle (θ) between the end face 5 on the absorption region side and the optical axis of the bent waveguide 2 is set to a critical angle or more, all the reflected light propagating through the bent waveguide exits from the end face 5 and enters the bent waveguide. Does not return (Nagai et al., Appl.
Phys. Lett. 54, pp. 1719-1721, 1989). However, since the radius of curvature R of the curved waveguide 2 needs to be 500 μm or more in order to propagate the guided light in the active region, the length of the absorption region II must be 200 μm or more to realize the above structure. This has the disadvantage that the element length is increased and the lateral width of the element is increased.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the conventional device, improves the absorption efficiency in the absorption region, reduces the influence of the reflection on the end surface on the absorption region side, and furthermore, A super luminescent diode that has a narrower emission angle than before and that can connect high-power incoherent light to an output optical fiber or output optical waveguide element with high efficiency and high reliability. The purpose is to:

[0006]

In order to achieve the above-mentioned object, a superluminescent diode according to the present invention comprises an excitation region, an absorption region adjacent to the excitation region on a side opposite to a light exit surface, and In a waveguide type superluminescent diode having an optical waveguide penetrating an excitation region and the absorption region, a first absorption region in which the absorption region is in contact with the excitation region and a second absorption region in contact with the first absorption region Region, and the width of the optical waveguide is the first width.
At least on the side of the second absorption region in the absorption region, the distance increases from the excitation region, and the optical axis of the optical waveguide is bent at the boundary between the first absorption region and the second absorption region. It is characterized by having.

Preferably, the lateral width of the optical waveguide increases in the second absorption region as the distance from the first absorption region increases.

Light propagating in the optical waveguide in a direction from the first absorption region to the second absorption region is emitted without radiation at the boundary between the first absorption region and the second absorption region. , Can be incident on the optical waveguide in the second absorption region.

It is preferable that at least one of the lateral width and the thickness of the optical waveguide is reduced at least in a portion of the excitation region near the light emitting end face as approaching the light emitting end face.

It is preferable that a portion of the optical waveguide close to the light emitting end face has a gain at an emission wavelength, and a portion of the optical waveguide close to the light emitting end face is transparent at an emission wavelength.

The optical waveguide has a butt joint structure at a portion near the light emitting end face, and a joint surface of the butt junction is inclined with respect to an optical axis of the optical waveguide.

[0012] An electrode may be provided in the absorption region.

Here, the superluminescent diode has a waveguide region between the excitation region and the light emitting end face, and the optical waveguide of the waveguide region has an effective band gap of the active layer. It is composed of a semiconductor layer larger than the band gap of the active layer of the optical waveguide in the excitation region or a semiconductor waveguide layer having a multiple quantum well structure.

It is preferable that at least one of the light emitting end face and the end of the second absorption region has a window structure.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an embodiment of the present invention will be briefly described with reference to FIG. In the present invention, the first absorption region IIA in which the absorption region II contacts the excitation region I
And two regions of a second absorption region IIB that is in contact with the first absorption region IIA. An optical waveguide (an optical waveguide 1 in an excitation region, an optical waveguide 2A in a first absorption region, and an optical waveguide in a second absorption region) formed of an active layer formed on a semiconductor substrate through an excitation region and an absorption region. A waveguide 2B) is formed, the width of the optical waveguide 2A is increased in the first absorption region as the distance from the excitation region increases, and the optical axis of the optical waveguide is increased by the first absorption region IIA and the second absorption region IIB.
It is bent at the border.

The spontaneous emission light amplified in the excitation region is emitted from the light emitting end on which the anti-reflection film 4 is formed, but a part of the light is reflected by the anti-reflection film and returns to the excitation region. enter. The reflected light is absorbed in the first and second absorption regions. In these regions, the width of the waveguide is wide, so that the light is well confined in the lateral direction, and thus the light is efficiently absorbed. Further, when the width of the waveguide is increased in these regions, the waveguide takes a tapered shape, and the equivalent refractive index of the waveguide gradually changes at the boundary between the respective regions. No reflection occurs.

Further, since the optical waveguides inside the first and second absorption regions form a fixed angle, the light reflected at the end of the second absorption region and returned returns to the first absorption region. The ratio of coupling to the internal optical waveguide is reduced, and reflected light is effectively suppressed.

Since the reflected light is suppressed by the above-described operation, it is not necessary to use a curved waveguide having a large radius of curvature as in the conventional example, so that the size of the entire device can be reduced.

The angle formed by the center line of the optical waveguide inside the first absorption region and the center line of the optical waveguide inside the second absorption region, and the angle θ shown in FIG. Is larger than the total reflection angle at the interface between the core and the cladding of the optical waveguide, the light incident from the optical waveguide in the first absorption region to the optical waveguide in the second absorption region is emitted without being emitted. 2
The light propagates through the optical waveguide in the absorption region. If radiated light is generated, it propagates through the cladding without being absorbed, is reflected by the end face at the end of the absorption region, and a part thereof is undesirably re-coupled to the optical waveguide. However, such a problem does not occur in the present invention. (Incidentally, the total reflection angle is defined as the angle between the normal to the interface between the media having different refractive indices and the incident angle of light when total reflection occurs.) In addition, the output side is tapered. By providing an optical waveguide having a small width or a small thickness, the spot size of light at the output end face increases, so that a narrow exit angle can be realized.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring again to FIG. 1, a first embodiment of the superluminescent diode according to the present invention will be described. FIG. 1 is a schematic top view of the first embodiment.

A waveguide structure composed of the optical waveguides 1, 2A and 2B is formed by depositing an active layer and a clad layer surrounding the active layer on a semiconductor substrate such as InP by a usual crystal growth method and a photolithographic method. it can. The optical waveguide 1 in the excitation region I has a single mode structure. An electrode 3 for current injection is formed above the excitation region, and a bottom electrode (not shown) is provided on the bottom surface of the element, and is kept at the ground potential. An anti-reflection film 4 is formed on the emission-side end face by an ordinary method. Further, an electrode 6 is also formed above the absorption region.

In the device shown in FIG. 1, when a voltage is applied to the electrode 3 and a current is injected into the active layer in the excitation region I, spontaneous emission light is generated due to the recombination of carriers, and the generated light is emitted from the excitation region I. The light is amplified while traveling through the optical waveguide, and is output from the light emitting end on which the antireflection film 4 is formed. At this time, an antireflection film having perfect non-reflection characteristics cannot be actually manufactured, and a certain amount of reflected light is re-input to the excitation region. Therefore, the present invention employs a method of providing the first and second absorption regions IIA and IIB and preventing laser oscillation due to resonance of reflected light. In order to increase the absorption efficiency in the absorption region, it is effective to increase the waveguide width of the absorption region and increase the light confinement ratio. However, when the waveguide width increases stepwise, the equivalent refractive index of the waveguide structure becomes smaller. The rate of change is large, causing reflection. Therefore, in the present invention, the width of the optical waveguide 2A in the first absorption region IIA is gradually increased from the connection portion of the excitation region I with the optical waveguide 1 to the connection portion of the second absorption region IIB with the optical waveguide 2B. For example, the reflection is reduced by widening to a required width in a linear function, a quadratic function, an exponential function, or a combination thereof. However, the width may not be widened over the entire first absorption region, but may be constant near the excitation region. Further, the second absorption region IIB
, The width of the optical waveguide 2B is constant or preferably widened toward the end face 5. In this embodiment, the optical waveguide 2
The width of B is extended linearly, but may be extended in a curved shape. Further, the center line 2b of the optical waveguide 2B is inclined with respect to the center line 1a of the optical waveguide 1 in the excitation region (the center line 2a of the optical waveguide 2A in the first absorption region). Due to such a shape of the optical waveguide 2B in the second absorption region, the reflected light incident on the optical waveguide 2B is absorbed while exciting a higher-order propagation mode. The reflected light that has not reached the rear end face 5 of the device without being absorbed in the second absorption area is reflected by the end face 5 and again enters the second absorption area, but the optical waveguide 2B is inclined with respect to the end face 5 Therefore, the reflectance is low, and the re-incident light is weakened. Here, if the angle formed by the optical axis of the reflected light propagating toward the end face 5 through the optical waveguide 2B and the normal to the end face 5 is increased, the amount of reflected light entering the optical waveguide 2B is further reduced. The light is reflected by the end face 5 and reaches the excitation region while being absorbed again in the second absorption region and the first absorption region.
However, at this time, the reflected light has a higher-order propagation mode, whereas the optical waveguide 1 in the excitation region is a single mode, so that the coupling ratio is small. Through the above process,
Even if the reflected light returns from the rear end face 5 of the device, the intensity of light incident on the optical waveguide 1 in the excitation region is very weak, and laser oscillation is effectively suppressed.

A general problem of the laser oscillation suppression method using the first and second absorption regions described above is that, when the output power is high, photocarriers are generated in the absorption region and the absorption efficiency is reduced. It is. In the present invention, in addition to the above-described structure,
By providing an independent electrode 6 on the absorption region and setting this electrode 6 to a ground potential or a potential that causes a reverse bias with respect to the pn junction, photocarriers can be effectively extracted and high output power can be obtained. It is possible to prevent a decrease in absorption efficiency at the time. In order to set the electrode 6 in the absorption region to the ground potential, it is only necessary to form a short circuit with the bottom electrode of the element. At this time, the absorption region can be used as a photodiode for monitoring the output light power of the device. That is, by inserting an ammeter between the electrode 6 and the bottom electrode or in a circuit connecting the electrode 6 and an external power supply, the amount of photocarriers generated in the absorption region can be measured as a current value. Therefore, in the super luminescent diode of the present invention, the output power can be monitored without externally attaching a photodiode, or a control circuit for constant output operation is constituted by using the electrode 6 in the absorption region. It is possible to

Due to the synergistic effects of the components described above, the superluminescent diode of the present invention not only suppresses laser oscillation but also reduces the ripple of the spectrum of emitted light.

FIG. 2 is a schematic top view of a second embodiment of the superluminescent diode according to the present invention.

In the present embodiment, the optical waveguide 1 in the excitation region comprises a portion 1A having a constant width on the absorption region side and a portion 1B on the emission end side. The width of the optical waveguide 1B decreases in a tapered shape toward the emission end side. With such a structure, the electric field confinement of light gradually becomes weaker toward the light emitting end, and the field distribution in the fundamental propagation mode is widened. As a result, the light emitting angle of the output light becomes very narrow. for that reason,
High-efficiency connection to the output optical fiber or output optical waveguide element without using a lens is possible, and the tolerance for positional deviation in connection is greatly increased. As a result, a marker for positioning in mounting is used. Can be used. Also, the reliability of the connection part is dramatically improved.

Further, there is an advantage that the antireflection film works more effectively due to the effect that the wavefront of the propagating light at the light emitting end approaches a plane wave.

One of the conventional methods for reducing reflection is to incline the light emission direction from the emission end face. In this conventional method, however, the light emission direction changes due to refraction at the emission end face. The law becomes difficult. It is important that the angle between the light emitting direction and the emitting end face be perpendicular as in the present invention.

FIG. 3A is a schematic top view of a third embodiment of the superluminescent diode according to the present invention.
(B) is a partially enlarged view thereof.

In the present embodiment, a waveguide region III is provided on the emission end side of the excitation region I.
2, an optical waveguide 7 having a tapered shape and decreasing in width toward the emission end side is formed in the same manner as the optical waveguide 1B of the embodiment shown in FIG. The active layer forming the optical waveguide 7 is a semiconductor layer having a composition with a larger effective band gap than the semiconductor layer of the active layer forming the optical waveguide 1 or has a multiple quantum well structure.

Since the structure of this embodiment has a greater degree of freedom in design and manufacture than the structure of FIG. 2, a narrower emission angle can be obtained. However, in this structure, a new reflection point may be formed at a connection point between the active layer (optical waveguide 1) in the excitation region and the active layer (optical waveguide 7) in the waveguide region. In principle, if the matching of the equivalent refractive index at the connection between the optical waveguide 1 and the optical waveguide 7 is sufficient, the reflection is minimized, but the reflection is caused by an axial deviation or a process fluctuation which cannot be avoided in actual design. May occur. In this embodiment, as shown in FIG. 3B, by tilting the light propagating through the interface 8 of the coupling portion from the optical axis of the light, the optical waveguide 1 in the excitation region can be produced even when reflection occurs.
It is possible to eliminate the effects by radiating the radiation without recombination.

FIG. 4 shows a fourth embodiment of the superluminescent diode according to the present invention. FIG. 4 (a) is a schematic perspective view, FIG. 4 (b) is a sectional view of the excitation region I, and FIG. (C) is a cross-sectional view of the exit end face of the waveguide region III. In FIG. 4A, the illustration of the current injection electrode and the electrode on the absorption region is omitted.

In this embodiment, similarly to the embodiment shown in FIG. 3, the excitation region I, the absorption region II, and the waveguide region II
I, the absorption region II is composed of the first absorption region IIA and the second absorption region IIA.
And the absorption region IIB.

A ridge 12 is formed on an n-type InP substrate 11, and an excitation region I and an absorption region I are formed on the ridge 12.
In I, the MQs forming the optical waveguide on the ridge portion 12
W active layer 13 is made of InGa in waveguide region III.
An AsP optical waveguide layer 14 is formed so as to be connected to the MQW active layer 13. MQW active layer 13 and InGaAsP
N-type InP current blocking layer 15 with optical waveguide layer 14 interposed
Are formed, and a p-type InP layer 16 is deposited so as to cover them entirely. An antireflection film 17 is provided on the end surface on the emission side. On the upper surface of the excitation region I, a current injection electrode 19 is provided via an InGaAsP contact layer 18. Although not shown, the electrode 6 described above is similarly provided on the upper surface of the absorption region II. On the bottom surface of the n-type InP substrate 11, a bottom electrode 20 is provided. The structure of the present embodiment is similar to the embodiment of FIG. 3, but the width of the optical waveguide layer 14 in the waveguide region is constant, and the thickness decreases in a tapered manner toward the emission end face. Optical waveguide layer 14
A selective growth technique by MOVPE (metal organic chemical vapor deposition) was used to form a taper in the thickness direction.

The dimensions of each part are such that the length L1 of the excitation region is 40
0 μm, the length L2 of the absorption region is 155 μm, the length L3 of the waveguide region is 300 μm, and the width of the optical waveguide layer 14 is MQW.
The same as the width of the active layer 13, 1.5 μm,
Is 0.3 μm, the thickness of the optical waveguide layer 14 is 0.3 μm at the connection with the MQW active layer 13, and 0.1 μm at the emission end face. The maximum width of the end of the optical waveguide in the second absorption region is 7.5 μm.

FIG. 5, FIG. 6, and FIG. 7 show the characteristics of the above-described superluminescent diode actually produced.
FIG. 5 is a characteristic diagram showing an example of a current-light output characteristic, FIG. 6 is an example of an output light spectrum characteristic (absorption region electrode is ground potential), and FIG. 7 is a characteristic diagram showing an example of a far-field pattern characteristic of emitted light. Each shows good characteristics. In FIG. 5, when the absorption region electrode is open, a large output is obtained, and when the absorption region electrode is at the ground potential, the output slightly decreases, but as shown in FIG. The characteristics can be selected according to the purpose of use. The far-field pattern of the emitted light shown in FIG. 7 shows an epoch-making narrow emission angle characteristic in which the full width at half maximum is about 10 degrees as compared with the conventional element. It was confirmed that a highly efficient connection of -1.9 dB was obtained.

In the above embodiment, the element structure by combining the InP-based material and the pn buried structure has been described as an example. However, the superluminescent diode of the present invention can be applied to other material-based materials such as GaAs-based material and other conductive materials. Needless to say, it can be realized by a combination of a wave structure, for example, a rib type, a ridge, and the like. In particular, in the case of a ridge-type waveguide structure, since the electric field distribution of light at the emission end is widened and the emission light has a narrow emission angle, the width of the optical waveguide is reduced toward the emission end face, or the thickness of the optical waveguide is reduced. It is effective to reduce the height toward the emission end face. A window layer made of a semiconductor layer having a smaller refractive index than the effective refractive index of the active layer constituting the optical waveguide may be formed on the end surface on the light emission side, the rear end surface of the element, or both.

[0038]

As described above, according to the present invention,
A super luminescent diode that can emit high-power, high-quality incoherent light with small spectral ripple and that can be easily, efficiently, and reliably connected to an optical fiber or optical waveguide device for optical output can do.

The super luminescent diode according to the present invention has the following effects.

(1) In a method of providing an absorption region having a waveguide width set wider than that of the excitation region and preventing laser oscillation, the absorption region is divided into two, and the waveguide width in the first absorption region is reduced. By gradually increasing the width to a required width from the connection portion with the tapered shape in the lateral direction, it is possible to prevent the occurrence of reflection due to the mismatch of the equivalent refractive index at the connection portion.

If the waveguide is bent without a lateral taper in the first absorption region, reflection may occur when the bent portion is deformed by process disturbance. This structure is not affected by this.

(2) In the second absorption region, the waveguide width is constant or further widened toward the rear end facet of the element, and the light guiding direction is an extension axis of the light guiding direction in the excitation region. By being bent in a direction different from the above, reflected light incident on the second absorption region is efficiently absorbed while exciting a higher-order propagation mode. Further, since the waveguide is inclined with respect to the end face, the reflectance at the rear end of the element is reduced. Further, the remaining weak reflected light reaches the excitation region while being absorbed again in the second absorption region and the first absorption region. However, the reflected light has a higher-order propagation mode, whereas the reflected light has a higher propagation mode. Since the waveguide is single mode, its coupling ratio is small. Laser oscillation is effectively suppressed by the above synergistic effect.

(3) By setting the electrode independently provided in the absorption region to the ground potential or the reverse bias potential, it is possible to avoid a problem that the absorption efficiency in the absorption region is reduced when the output power is high, and the effect of the above-mentioned item can be avoided. Since a further synergistic effect can be obtained, the superluminescent diode of the present invention not only suppresses laser oscillation but also reduces ripples in the spectrum of emitted light.

(4) An electrode provided independently in the absorption region can be used as a photodiode for monitoring the output light power of the device. Therefore, it is possible to monitor output power without externally attaching a photodiode, or to configure a control circuit for constant output operation.

(5) Since the output light has a very narrow emission angle,
High-efficiency connection can be made to the output optical fiber or output optical waveguide element without using a lens, and the tolerance for positional deviation in connection is very large.
In mounting, a passive alignment method using alignment markers can be used. Further, since the connection portion is not affected by a slight displacement of the connection portion caused by long-term use, the reliability of the connection portion and, consequently, the reliability of the module are greatly improved.

Further, there is an advantage that the antireflection film works more effectively due to the effect that the wavefront of the propagating light at the light emitting end approaches a plane wave.

(6) Even at the connection point between the active layer in the excitation region and the semiconductor tapered waveguide layer in the waveguide region, the coupling surface is inclined from the optical axis, so that even when reflection occurs, the reflected light is returned to the excitation region. It can be released without binding, and the effect on properties can be eliminated.

Since the superluminescent diode according to the present invention has the above effects, it is easy to mount elements in an optical fiber gyro using a superluminescent diode as a light source, a portable OTDR device, an optical measuring instrument, and the like. The mounting cost has been reduced and the size of these devices has been reduced.

[Brief description of the drawings]

FIG. 1 is a schematic top view showing a first embodiment of the present invention.

FIG. 2 is a schematic top view showing a second embodiment of the present invention.

FIG. 3 is a schematic top view showing a third embodiment of the present invention.

FIG. 4 is a schematic top view and a sectional view showing a fourth embodiment of the present invention.

FIG. 5 is a characteristic diagram showing an example of a current-light output characteristic of the fourth embodiment.

FIG. 6 is a characteristic diagram showing an example of an emission light spectrum of the fourth embodiment.

FIG. 7 is a characteristic diagram illustrating an example of a far-field pattern of emitted light according to the fourth embodiment.

FIG. 8 is a schematic top view showing an example of a conventional super luminescent diode.

[Explanation of symbols]

I Excitation region II Absorption region IIA First absorption region IIB Second absorption region III Waveguide region 1 Optical waveguide in excitation region 1a Center line of optical waveguide in excitation region 2 Bend waveguide 2A Optical waveguide in first absorption region 2a center line of the optical waveguide in the first absorption region 2B center line of the optical waveguide in the second absorption region 2b center line of the optical waveguide in the second absorption region 3 electrode for current injection 4 anti-reflection film 5 end face of the rear end of the device 6 absorption Region electrode 7 Waveguide region optical waveguide 8 Interface between junction of optical waveguide in excitation region and optical waveguide in waveguide region 11 n-type InP substrate 12 ridge portion 13 MQW active layer 13 14 InGaAsP optical waveguide layer 15 n-type InP Current block layer 16 p-type InP layer 17 antireflection film 18 InGaAsP contact layer 19 current injection electrode 20 bottom electrode

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenji Kono 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Inventor Etsuo Noguchi 3-192-1, Nishishinjuku, Shinjuku-ku, Tokyo No. Nippon Telegraph and Telephone Corporation (72) Inventor Yoshio Itaya 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (10)

[Claims] 1. A waveguide type superluminescent diode having an excitation region, an absorption region adjacent to the excitation region on a side opposite to a light emission surface, and an optical waveguide penetrating the excitation region and the absorption region. The absorption region includes a first absorption region in contact with the excitation region and a second absorption region in contact with the first absorption region, and a width of the optical waveguide is at least a second absorption region in the first absorption region. On the side of the region, the distance increases from the excitation region, and the optical axis of the optical waveguide is
A superluminescent diode, which is bent at the boundary between the absorption region and the second absorption region. 2. The super-luminescent diode according to claim 1, wherein the width of the optical waveguide increases in the second absorption region as the distance from the first absorption region increases. 3. A light propagating in a direction from the first absorption region to the second absorption region in the optical waveguide is accompanied by radiation at a boundary between the first absorption region and the second absorption region. The super luminescent diode according to claim 2, wherein the light is incident on the optical waveguide in the second absorption region. 4. The optical waveguide according to claim 1, wherein at least one of a lateral width and a thickness of the optical waveguide is reduced at least in a portion of the excitation region near the light emitting end face as approaching the light emitting end face. 4. The superluminescent diode according to any one of 1 to 3. 5. The superluminescent diode according to claim 1, wherein a portion of the optical waveguide near the light emitting end face has a gain at an emission wavelength. 6. The superluminescent diode according to claim 1, wherein a portion of the optical waveguide close to the light emitting end face is transparent at an emission wavelength. 7. The optical waveguide according to claim 1, wherein the optical waveguide has a butt joint structure at a portion near the light emitting end face, and a joint surface of the butt junction is inclined with respect to an optical axis of the optical waveguide. 7. The superluminescent diode according to any one of 1 to 6. 8. The superluminescent diode according to claim 1, wherein an electrode is provided in said absorption region. 9. The superluminescent diode has a waveguide region between the excitation region and the light emitting end face, and the optical waveguide of the waveguide region has an effective band gap of an active layer of the excitation region. 9. The superluminescent diode according to claim 1, comprising a semiconductor layer larger than the band gap of the active layer of the optical waveguide in the region or a semiconductor waveguide layer having a multiple quantum well structure. 10. The superluminescent diode according to claim 1, wherein a window structure is provided on at least one of the light emitting end face and the end of the second absorption region.