JP2012028570A - Super-luminescent diode and oct device using the same as light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a super-luminescent diode capable of suppressing an excess electric current due to return light generated near its reflection end surface and being utilized without using an isolator, and an OCT device using the same as a light source.SOLUTION: There is provided a super-luminescent diode comprising a ridge structure formed by etching a laminated body including an active layer at a prescribed width and depth from the top face of the laminated body, in which natural emission light generated by applying a prescribed voltage to an upper surface electrode and lower surface electrode formed on the upper surface and lower surface of the laminated body, respectively, is guided by the ridge structure, and the guided natural emission light is emitted from an emission end face side. In the diode, the upper surface electrode includes a structure divided into a first electrode and a second electrode, and means for suppressing an excess current provided between the first electrode and the second electrode suppresses the excess current due to return light generated near a reflection end face opposite to the emission end face side.

Description

  The present invention relates to a super luminescent diode and an OCT apparatus using a super luminescent diode as a light source.

A super luminescent diode (SLD) is known as a high-intensity light source that emits amplified spontaneous emission (ASE) and has low coherence.
For example, a device configuration as disclosed in Patent Document 1 is provided. Hereinafter, this superluminescent diode is referred to as SLD.
As an application example, this SLD is used as a light source for SD (spectral domain) -OCT (optical coherence tomography) that acquires an optical tomographic image at high speed.

Here, an outline of a general SLD in the conventional example will be described. FIG. 5A is a schematic plan view for explaining an outline of a general SLD in the conventional example, and FIG. 5B is a schematic diagram of a current density distribution in the driving state of the SLD.
5A and 5B, 500 is an SLD, 501 is an electrode, 507 is amplified spontaneous emission light, 508 is an emission end face, 509 is a reflection end face, 510 is a ridge structure, 511 is a current density, and 512 device. Length.
The SLD includes a ridge structure 510 formed by etching with a predetermined width and depth from the upper surface of the stacked body including the active layer.
For example, in the case of an AlGaAs system of 0.8 μm band, when a voltage of about 1.8 V is applied to the upper electrode (501) and the lower electrode (not shown) formed on the upper and lower surfaces of the laminate, A current flows to the pn junction of this, and spontaneous emission light is generated. The generated spontaneous emission light is amplified while propagating inside the device, and the amplified spontaneous emission light 507 is extracted from the emission end face 508.
As described above, spontaneous emission light is amplified inside the SLD. The same applies to incident light from outside the element.

At this time, the return light of the amplified spontaneous emission light 507 reaches the reflection end face 509 opposite to the emission end face 508, and when the inside is in a strongly excited state, the light is absorbed more than usual near the end face. Heat generation and current concentration are likely to occur, and the probability that the end face is broken increases.
As shown in FIG. 5B, the return light is generally amplified approximately 1000 times inside the SLD in the figure, and reaches the rear reflection end face 509.
The reflecting end surface during operation is in a state where the light density is very high, and reflecting this, the current density is high in the vicinity of the end surface as shown in FIG. In particular, the reflection end face 509 having a higher reflectance than the emission end face 508 is remarkable. The energy of the amplified return light is added thereto and the vicinity of the end surface becomes high temperature, so that an excessive current flows locally, and the reflection end surface is easily destroyed by DLD (dark line defect) or the like.
In particular, when an SLD is mounted on SD-OCT, an expensive isolator element is required between the optical system and the SLD in order to prevent end face destruction of the element due to return light from the optical system in the OCT.
As a method for preventing the end face of the semiconductor light emitting element from being destroyed, for example, a window structure as in Patent Document 2 is known.

JP-A-6-188509 JP-A-7-58402

However, even when the window structure as described above is used, there is a limit to driving conditions that can prevent end face destruction.
In particular, when SLD is applied to SD-OCT, the light intensity level of the return light is high and end face destruction is likely to occur.

  In view of the above-described problems, the present invention suppresses excess current due to return light generated in the vicinity of a reflection end face, and makes it possible to use a superluminescent diode without using an isolator and an OCT using a superluminescent diode as a light source. The purpose is to provide a device.

The present invention provides a super luminescent diode configured as follows and an OCT apparatus using the super luminescent diode as a light source.
The superluminescent diode of the present invention comprises a ridge structure formed by etching at a predetermined width and depth from the upper surface of a laminate including an active layer,
A superstructure that guides spontaneous emission light generated by applying a predetermined voltage to the upper surface electrode and the lower surface electrode formed on the upper and lower surfaces of the laminated body by the ridge structure, and emits the spontaneous emission light from the emission end face side. A luminescent diode,
The upper surface electrode comprises a structure divided into a first electrode and a second electrode,
Means for suppressing excess current is provided between the first electrode and the second electrode, and the means for suppressing excess current causes the return light generated in the vicinity of the reflection end face in the direction opposite to the emission end face side. It is characterized by suppressing excess current.
Moreover, the OCT apparatus of this invention is equipped with the light source comprised by the above-mentioned superluminescent diode, It is characterized by the above-mentioned.

  According to the present invention, an excess current due to return light generated near the reflection end face is suppressed, and a superluminescent diode that can be used without using an isolator and an OCT apparatus using a superluminescent diode as a light source are realized. can do.

FIG. 3 is a schematic plan view illustrating the configuration of an SLD having a ridge structure according to an embodiment of the present invention and Example 1. Sectional drawing explaining the structure of the laminated body containing the active layer which comprises SLD in Example 1 of this invention. FIG. 6 is a schematic plan view for explaining the configuration of an SLD having a ridge structure in Embodiment 2 of the present invention. The figure explaining the structural example of SD-OCT provided with the light source comprised by the superluminescent diode in Example 3 of this invention. It is a schematic diagram explaining the structure of the general SLD in a prior art example, (a) is the schematic top view, (b) is the schematic diagram of the current density distribution in the drive state.

A configuration example of an SLD (super luminescent diode) in an embodiment of the present invention will be described with reference to FIG.
The SLD 100 according to this embodiment includes a ridge structure 110 formed by etching with a predetermined width and depth from the upper surface of a stacked body including an active layer.
By applying a predetermined voltage to the upper and lower electrodes formed on the upper and lower surfaces of the laminate, for example, when a voltage of about 1.8 V is applied, a current flows to the pn junction inside the element, and spontaneous emission light is generated. To do.
The generated spontaneous emission light is amplified while being propagated by the ridge structure through the waveguide, and the amplified spontaneous emission light 107 is emitted from the emission end face side 108.
Thus, spontaneous emission light is amplified inside the SLD. The same applies to incident light from the outside. When light is incident on the incident end face from the outside, the return light is strongly amplified and reaches the reflection end face vicinity 109 in the direction opposite to the exit end face side.
When the inside is in a strong excited state, heat generation and current concentration occur due to light absorption near the end face, and the end face is destroyed.

In order to cope with these, in the SLD 100 of the present embodiment, the upper surface electrode is divided into a first electrode 101 and a second electrode 102, and reflection is performed between the first electrode and the second electrode. Means are provided for suppressing excess current due to return light generated in the vicinity of the end face.
Specifically, an independent second electrode 102 is provided in the vicinity of the reflection end face of the SLD 100, and the drive current of the portion is limited by the constant current diode 104 configured as means for suppressing the excess current connected thereto. .
Thereby, it is possible to prevent the reflection end face of the SLD from being broken by return light from a system such as SD-OCT without using an isolator.
In this embodiment, in addition to the constant current diode 104, an adjustment resistor 103 is further provided on the first electrode side 101, and the voltage applied to the second electrode 102 can be adjusted in advance according to the light intensity level of the return light. Can be configured.
At this time, the adjustment resistor and the constant current diode can be configured to adjust the voltage applied to the first electrode 101 and the second electrode 102 and to make the driving current density constant.
Further, as a means for suppressing the excessive current, instead of the constant current diode 104, a resistance element whose resistance value can be adjusted in advance so that the reflection end face is not destroyed by the light intensity level of the return light is configured. Can do.
In addition, the laminate including the active layer can be made of an AlGaAs material.

Hereinafter, embodiments of the present invention will be described.
[Example 1]
As Example 1, a manufacturing method of a stacked structure including an active layer constituting an SLD having a ridge structure will be described with reference to FIGS.
In FIG. 2, 201 is a p-type GaAs contact layer, 202 is a p-type Al _0.4 Ga _0.6 As cladding layer, and 203 is an Al _0.2 ga _0.8 As / GaAs quantum well.
_Reference numeral 204 denotes an n-type Al _0.4 Ga _0.6 As clad layer, and 205 denotes an n-type GaAs buffer and a wafer.
Reference numeral 206 denotes a p-type electrode layer, and 207 denotes an n-type electrode layer.
First, an n ⁺ GaAs wafer 205 is prepared as a substrate for forming an SLD. Epitaxial growth is performed to form a high-quality device layer.
As an epitaxial wafer, an n ⁺ -GaAs wafer 205 having a doping concentration of about 7 × 10 ¹⁸ cm ⁻³ is epitaxially grown as follows.
After removing the surface oxide film before crystal growth, GaAs205 is grown by about 1 μm at a high temperature of about 650 ° C. by a crystal growth apparatus such as MOCVD.

Next, crystal growth is performed in order to form the layer structure of the SLD 100.
As the cladding layer shown in FIG. 2, an n-type Al _0.4 Ga _0.6 As layer 204 is grown by about 1 μm with a carrier concentration of about ³ × 10 ¹⁷ cm ⁻³ .
Next, an Al _0.2 Ga _0.8 As / GaAs quantum well layer (MQW) 203 is grown as an active layer by about 0.4 μm.
The MQW 203 has a structure in which a barrier layer Al _0.2 Ga _0.8 As has a layer thickness of 80 nm and a well layer GaAs has a thickness of 20 nm, and this is alternately crystal-grown.
Next, a p-type Al _0.4 Ga _0.6 As layer 202 of about 5 × 10 ¹⁷ cm ^{−3 is formed} as an upper clad layer of about 1 μm, and finally a 1 × 10 ¹⁹ cm ⁻³ p ⁺ GaAs layer 201 is formed of about 50 nm as a contact layer. To do.
In this way, a laminate structure including an active layer is formed on the substrate.

Next, a process for forming the ridge structure 110 and the end face will be described.
In FIG. 1, a ridge structure 110 having a width of 4 μm is formed in a laminated structure at an angle of about 5 degrees with respect to an angle orthogonal to the reflection end face 109 formed by cleaving the crystal such as <110>.
A resist having a stripe pattern, which is a source of the ridge, is formed on the substrate surface with a width of 4 μm on the upper surface of the multilayer structure at the above angle.
Next, an area without resist is etched to a depth of about 1 μm by a dry etching apparatus using chlorine gas, and then the resist is removed. The Al _0.2 Ga _0.8 As / GaAs quantum well layer (MQW) 203 in FIG. 2 is the etched surface. Next, an insulating film is formed on the etching surface by using polyimide except the contact layer 206 on the surface of the ridge.
Next, the wafer is cleaved by a scribing device so that the device length is 2 mm and the width is 0.7 mm.
Next, the reflective coating of the end face is performed. The emission end face 108 and the reflection end face 109 are deposited with SiO ₂ and TiO ₂ by a sputtering apparatus to adjust the reflectance. The reflectivities of the emission end face 108 and the reflection end face 109 are about 0% and 1%, respectively.

Next, as an electrode formation process, Au / Ge / Ni is vapor-deposited as the n-type electrode layer 207 on the back surface of the substrate in the multilayer structure, and then annealed in nitrogen at about 400 degrees to form an ohmic electrode.
Next, when the electrodes are formed on the upper surface of the laminated structure, the electrodes for forming the first electrode 101 and the second electrode 102 as shown in FIG. In this region, the contact layer is selectively etched.
Etching is performed using a resist pattern formation and a sulfuric acid-based etchant.
Next, in order to form two electrodes, first, after patterning with a resist, Ti / Au is vacuum-deposited to form the p-type electrode layer 206 for forming the two electrodes, and then the resist is lifted off. To form two electrically separated electrodes.
Finally, as a wiring process, as shown in FIG. 1, an adjustment resistor is wired to the first electrode 101, and a constant current diode 104 is wired to the second electrode 102.
The adjustment resistor 103 and the constant current diode 104 are connected to the upper portion of the first electrode 101 and the second electrode 102 by wire bonding of a gold wire via an electrode pad on the submount.

[Example 2]
As Example 2, an example in which the resistance value is changed in advance so that the reflection end face is not destroyed by the light intensity level of the return light instead of the constant current diode will be described with reference to FIG. I will explain.
A resistance element 106 such as a chip resistor is used instead of the constant current diode wired to the second electrode 102 to limit the maximum current. The light intensity level of the return light is predicted in advance, and the resistance value is adjusted so that the current value does not break the end face.

[Example 3]
As a third embodiment, a configuration example of an OCT apparatus provided with a light source configured by the superluminescent diode of the present invention will be described.
Here, an example applied to SD-OCT will be described with reference to FIG.
4, 300 is SD-OCT, 301 is a sample, 302 is an SLD, 303 is a measurement system, 304 is a beam splitter, and 305 is a mirror.
The SD-OCT 300 irradiates the sample 301 with incoherent white light (= SLD) 302, causes the signal light obtained therefrom to interfere with the reference light by the beam splitter 304, and the wavelength by the diffraction grating inside the measurement system 303. Disperse.
This is acquired as wavelength information by a one-dimensional image sensor (line sensor) inside the measurement system 303.
In the SD-OCT 300, since the period of the interference fringes varies depending on the interference distance, the interference distance can be calculated by Fourier-transforming the signal of the line sensor and specifying the period. Based on this principle, the SD-OCT 300 acquires a three-dimensional image of the sample 301.
As shown in FIG. 4, the return light is generated by the reflection mirror 305 inside the OCT.
In the conventional example, in order to prevent the incidence of reflection, it is necessary to attach an isolator in front of the SLD. However, according to the configuration of this embodiment, the isolator is unnecessary.

100: SLD
DESCRIPTION OF SYMBOLS 101: 1st electrode 102: 2nd electrode 103: Adjustment resistance 104: Constant current diode 105: Applied voltage 106: Resistance element 107: Amplified spontaneous emission light 108: Output end surface 109: Reflection end surface 110: Ridge structure

Claims (7)

Comprising a ridge structure formed by etching at a predetermined width and depth from the upper surface of the laminate including the active layer;
A superstructure that guides spontaneous emission light generated by applying a predetermined voltage to the upper surface electrode and the lower surface electrode formed on the upper and lower surfaces of the laminated body by the ridge structure, and emits the spontaneous emission light from the emission end face side. A luminescent diode,
The upper surface electrode comprises a structure divided into a first electrode and a second electrode,
Means for suppressing excess current is provided between the first electrode and the second electrode, and the means for suppressing excess current causes the return light generated in the vicinity of the reflection end face in the direction opposite to the emission end face side. A super luminescent diode characterized by suppressing excess current.   2. The super luminescent diode according to claim 1, wherein the means for suppressing the excess current comprises a constant current diode. In addition to the constant current diode, an adjustment resistor is further provided on the first electrode side,
3. The superluminescent diode according to claim 2, wherein the adjustment resistor is configured such that a voltage applied to the second electrode can be adjusted in advance according to a light intensity level of the return light.   The adjustment resistor and the constant current diode are configured to be capable of adjusting a voltage applied to the first electrode and the second electrode to make a driving current density constant. Item 4. The superluminescent diode according to item 3.   The means for suppressing the excess current is configured by a resistance element whose resistance value can be adjusted in advance so as not to have a current value at which the reflection end face is destroyed by the light intensity level of the return light. Item 2. The superluminescent diode according to item 1.   The superluminescent diode according to any one of claims 1 to 5, wherein the stacked body including the active layer is made of an AlGaAs-based material.   An OCT apparatus comprising a light source configured by the superluminescent diode according to claim 1.

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