SURFACE-EMITTING LASER

1. FIELD OF THE INVENTION

The present invention relates to a surface-emitting semiconductor laser, and particularly to a surface-emitting semiconductor laser with increased high-power output.

2. DESCRIPTION OF THE RELATED ART

With conventional surface-emitting lasers, increasing the single-wavelength output has been limited to the mW level. If such a surface-emitting laser could be improved to be capable of providing watt-class high-power output, this would allow various kinds of applications to be developed. Examples of such applications include: wavelength scanning light sources for optical coherence tomography (OCT); light sources for medium-to-long-distance optical communication; laser radar (LIDAR) light sources to be mounted on a vehicle, drone, robot, or the like; monitoring systems; automatic inspection apparatuses employed at a manufacturing site; laser dryers employed in a printer; etc.

NON-PATENT LIST
Non-Patent Document 1

A. Haglund, “Single Fundamental-Mode Output Power Exceeding 6 mW From VCSELs With a Shallow Surface Relief,” IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, February 2004.

Non-Patent Document 2

Jean-Francois Seurin et al., “High-power vertical-cavity surface-emitting lasers for solid-state laser pumping,” Vertical-Cavity Surface-Emitting Lasers XVI, edited by Chun Lei, Kent D. Choquette, Proc. of SPIE Vol. 8276, 2012.

Non-Patent Document 3

Kazuyoshi Hirose, et. al., “Watt-class high-power, high-beam-quality photonic-crystal lasers,” NATURE PHOTONICS, VOL 8, p. 406 May 2014.

Non-Patent Document 4

Toshikazu Shimada, et. al., “Lateral integration of vertical-cavity surface-emitting laser and slow light Bragg reflector waveguide devices,” APPLIED OPTICS, Vol. 53, No. 9, p. 1766, March 2014.

Non-Patent Document 5

M. Nakahama, “Lateral integration of MEMS VCSEL and slow light amplifier boosting single mode power,” IEICE ELEX, vol. 9, no. 6, pp. 544-551, 2012.

In order to provide a surface-emitting laser with such high output, a surface-machined structure designed to suppress high-order mode oscillation has been proposed (Non-patent document 1). However, there is a limit to increasing the area size up to 10 micrometers or less, and such an arrangement is not capable of providing an output exceeding 10 mW. In a case of employing an array structure (Non-patent document 2) in which a great number of surface-emitting lasers are two-dimensionally integrated, such an arrangement is capable of providing high output of 10 W or more. However, individual elements cannot be configured with a uniform phase and uniform wavelength. This leads to a problem of a wide oscillation spectrum width, a problem of a large beam divergence angle, and a problem in that such a beam cannot be focused even if a lens is used.

In a case in which a surface-emitting laser is configured using a two-dimensional photonic crystal (Non-patent document 3), such an arrangement supports watt-class high-power output and a high-quality beam. However, such an arrangement requires a semiconductor to have a fine cyclic structure as its internal structure, which is a problem from the viewpoint of manufacturing and reliability.

In order to solve such problems, the present inventors have proposed a surface-emitting laser with a light amplification function having a structure in which a VCSEL (vertical-cavity surface-emitting laser) and a slow light SOA (semiconductor optical amplifier) are arranged in the lateral direction of a substrate (Non-patent documents 4 and 5). Such a surface-emitting laser described in Non-patent document 4 provides a maximum light output of 6 mW, and is not capable of providing watt-class output.

SUMMARY OF THE INVENTION

The present invention has been made in view of such a situation. Accordingly, it is an exemplary purpose of an embodiment of the present invention to provide a surface-emitting laser with high-power output.

An embodiment of the present invention relates to a surface-emitting laser. The surface-emitting laser comprises: an output unit having an oblong-shaped VCSEL (vertical-cavity surface-emitting laser) structure; and a driving circuit structured to inject a current that is larger than an oscillation threshold value into the VCSEL structure so as to maintain an oscillation state. The output unit is structured such that a coherent seed light is received via one end of the VCSEL structure in a longitudinal direction, such that the seed light propagates as a slow light through the VCSEL structure in a longitudinal direction while being reflected multiple times in the VCSEL structure in a vertical direction, and such that an output light is extracted from an upper surface of the VCSEL structure.

It should be noted that, in the present specification, for convenience, the up-and-down direction, the horizontal direction, and the vertical direction are defined independent of the directions defined in the actual operation.

With this embodiment, the output unit having the VCSEL structure is operated as an amplifier that amplifies a seed light externally input in a state in which the output unit having the VCSEL structure is laser-oscillated. This allows high-power output to be provided.

Also, the wavelength λ1 of the seed light and the oscillation wavelength λ2 provided by the VCSEL structure of the output unit may be designed to satisfy a relation λ1≠λ2. This arrangement is capable of preventing the light coupled with the end (coupling end) of the output unit from being emitted again via the coupling end.

Also, a seed light source structured to generate the seed light and the output unit may be integrated adjacent to each other in the longitudinal direction such that they share the VCSEL structure. This allows the surface-emitting laser to be manufactured with a further reduced size and a further reduced cost.

Also, the wavelength λ1 of the seed light and the oscillation wavelength λ2 provided by the VCSEL structure may be designed to satisfy a relation λ1<λ2. This improves a function (isolation) for suppressing the occurrence of return light that propagates from the output unit to the seed light source. This provides improved beam quality.

Also, the VCSEL structure of the seed light source and the output unit may comprises an air gap layer. Also, the air gap layer on the seed light source side may be structured to have a variable thickness that can be controlled by means of a micromachined structure. This provides the relation λ1<λ2.

The VCSEL structure of the seed light source and the output unit may be structured such that there is a difference between the number of layers between the seed light source side and the output unit side. More specifically, an upper DBR (Distributed Bragg Reflector) of the VCSEL structure of the output unit may be structured to have a greater number of layers than those of the upper DBR of the VCSEL structure of the seed light source. This provides the relation λ1<λ2.

Also, the VCSEL structure of the seed light source may comprise a low-refractive-index layer. This provides the relation λ1<λ2.

Also, the seed light source may have a coupled resonance structure. This provides the relation λ1<λ2.

Also, the output unit may be formed such that it is bent in a zig-zag manner. This arrangement requires only a further reduced area to provide higher-power output.

Also, an optical confinement layer that forms the active-layer VCSEL structure may be structured to have a refractive index that is smaller than an average refractive index of the upper DBR and the lower DBR. This arrangement is capable of cutting off the waveguide mode due to total reflection.

It should be noted that any desired combinations of the aforementioned components or representation of the present invention may be mutually substituted between a method, apparatus, and so forth, which are also effective as an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:

FIG. 1 is a cross-sectional diagram showing a surface emitting laser according to an embodiment;

FIG. 2 is a diagram showing input/output characteristics of an output unit of the surface emitting laser shown in FIG. 1;

FIG. 3 is a diagram showing a measurement system employed in an experiment;

FIG. 4A is a diagram showing amplification characteristics of the output unit, FIG. 4B is a diagram showing a spectrum of the output light, and FIG. 4C is a diagram showing the beam angle and the beam width;

FIG. 5 is a diagram showing results of simulation of the amplification characteristics of the output unit;

FIG. 6 is a cross-sectional diagram showing a surface emitting laser according to a first embodiment;

FIG. 7 is a cross-sectional diagram showing a surface emitting laser according to a second embodiment;

FIG. 8 is a cross-sectional diagram showing a surface emitting laser according to a third embodiment;

FIG. 9 is a plan view showing a surface emitting laser according to a fourth embodiment;

FIG. 10 is a layout diagram showing a surface emitting laser according to a fifth embodiment;

FIG. 11 is a cross-sectional diagram showing a surface emitting laser according to a sixth embodiment;

FIG. 12A and FIG. 12B are diagrams each showing results of simulation of the surface-emitting laser shown in FIG. 11; and

FIG. 13 is a diagram showing results of simulation of the surface-emitting laser shown in FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Description will be made below regarding the present invention based on preferred embodiments with reference to the drawings. The same or similar components, members, and processes are denoted by the same reference numerals, and redundant description thereof will be omitted as appropriate. The embodiments have been described for exemplary purposes only, and are by no means intended to restrict the present invention. Also, it is not necessarily essential for the present invention that all the features or a combination thereof be provided as described in the embodiments.

Outline

First, description will be made regarding an outline of a surface-emitting laser according to an embodiment. The surface-emitting laser includes an output unit having an oblong-shaped VCSEL (vertical-cavity surface-emitting laser) structure. The output unit operates in an oscillation state in which a current that is larger than an oscillation threshold value is injected. The output unit receives coherent seed light at one end of the VCSEL structure in the longitudinal direction. With the output unit, the light propagates as slow light in the longitudinal direction of the VCSEL structure while being reflected multiple times in the vertical direction. The output light is extracted via an upper surface of the VCSEL structure.

With the surface-emitting laser, by maintaining an oscillation state, such an arrangement is capable of providing high-efficiency optical amplification, thereby providing high-power output. Furthermore, by inputting coherent light having a single wavelength and uniform wavefronts as the seed light, this arrangement is capable of providing high-beam-quality output light having high-power output and uniform wavefronts.

Embodiments

FIG. 1 is a cross-sectional diagram showing a surface-emitting laser 1 according to an embodiment. The surface-emitting laser 1 has a structure in which a first surface-emitting laser (which will be referred to as the “seed light source 2” hereafter) and a second surface-emitting laser (which will be referred to as the “output unit 4” hereafter) are arranged in a lateral direction on a single semiconductor substrate. As described in the outline, the output unit 4 has an oblong-shaped VCSEL (vertical-cavity surface-emitting laser) structure 40. The output unit 4 may be designed to have a length on the order of 1000 times the length of the seed light source 2. The VCSEL structure 40 includes a lower DBR (Distributed Bragg Reflector) 26, an active layer 42, and an upper DBR 44.

The seed light source 2 has the same VCSEL structure 20 as that of the output unit 4. The seed light source 2 generates a coherent seed light L1. In the internal structure of the seed light source 2, the light is amplified by means of stimulated emission while being repeatedly reflected in the vertical direction. A part of the amplified light is coupled as the seed light L1 with one end (coupling surface 3) of the VCSEL structure of the adjacent output unit 4 in the longitudinal direction.

Specifically, the VCSEL structure 20 of the seed light source 2 includes the lower DBR 26, an active layer 22, and the upper DBR 24 formed on a semiconductor substrate 10. In order to provide an upper mirror of a vertical oscillator of the VCSEL structure 20 with a reflection ratio that is close to 100%, a high-reflection mirror 30 is preferably formed on the upper surface of the upper DBR 24. The high-reflection mirror 30 is preferably formed of a metal material such as gold (Au) or the like or otherwise is preferably configured as a dielectric multilayer film mirror.

A driving circuit 5 injects a current I_DRVthat is larger than an oscillation threshold value I_THinto the VCSEL structure 40 of the output unit 4 so as to operate the output unit 4 in an oscillation state. The output unit 4 receives the seed light L1 via its coupling surface 3. The seed light L1 propagates as a slow light in the longitudinal direction of the VCSEL structure 40 while being reflected multiple times in the vertical direction within the VCSEL structure. An output light L2 is extracted via the upper surface of the VCSEL structure 40. The upper reflecting face of a cavity of the output unit 4, i.e., the upper DBR 44 may be designed to have a reflection ratio on the order of 95% to 99%, for example.

If return light from the output unit 4 to the seed light source 2 occurs, this leads to mode disturbance in the seed light source 2. This leads to degraded beam quality of the seed light L1, resulting in degraded quality of the output light L2. Accordingly, the wavelength λ1 of the seed light L1 and the oscillation wavelength λ2 to be provided by the VCSEL structure of the output unit 4 are preferably designed such that the relation λ1≠λ2 holds true. In particular, in a case of employing the structure as shown in FIG. 1 in which the seed light source 2 and the output unit 4 are laterally integrated, the wavelengths are preferably designed such that the relation λ1≠λ2 holds true. This arrangement suppresses the occurrence of return light that propagates from the output unit 4 to the seed light source 2, thereby providing improved beam quality.

The above is the basic structure of the surface-emitting laser 1. Next, specific description will be made regarding several example configurations. The VCSEL structure and the materials may be designed using known techniques. Such an arrangement is not restricted in particular. Description will be made regarding an example thereof. For example, the semiconductor substrate 10 may be configured as a III-V family semiconductor substrate. Specifically, the semiconductor substrate 10 may be configured as a GaAs substrate. An n-side electrode (not shown) is formed on the back face of the semiconductor substrate 10. The lower DBR 26(46) has a layered structure in which an Al_0.92Ga_0.08As layer and an Al_0.16Ga_0.84As layer (AlGaAs is aluminum gallium arsenide), each of which has been doped with silicon as an n-type dopant, are alternately and repeatedly layered, which provides a reflection ratio in the vicinity of 100%.

The active layer 22(42) has a multiple quantum well structure comprising In_0.2Ga_0.8As/GaAs (indium gallium arsenide/gallium arsenide) layers. The active layer 22(42) may have a triple quantum well structure, for example. Furthermore, a lower spacer layer and an upper spacer layer, each of which is configured as an undoped Al_0.3Ga_0.7As layer, may be provided to both faces of the multiple quantum well structure, as necessary. The upper DBR 24(44) has a layered structure in which carbon-doped Al_0.92Ga_0.08As layers and Al_0.16Ga_0.84As layers (AlGaAs is aluminum gallium arsenide) are alternately and repeatedly layered.

Next, description will be made regarding the operation of the surface-emitting laser 1 shown in FIG. 1. When the seed light source 2 is oscillated, the seed light source 2 generates a light intensity distribution as indicated by reference numeral 100. A part of the light thus generated is emitted toward the output unit 4 side as the seed light L1. On the other hand, a current I that is larger than a threshold current I_THis injected into the output unit 4. Accordingly, the output unit 4 also comes to be in an oscillation state. When the seed light L1 is not coupled, as indicated by the line of alternately long and short dashes, spontaneous emission light generated by the output unit 4 and stimulated emission light generated based on the spontaneous emission light as a seed light are amplified while being reflected in the vertical direction. As a result, light L3 having a wavelength λ2 is emitted.

With the surface-emitting laser 1 shown in FIG. 1, instead of spontaneous emission light, oscillation with the seed light L1 as a seed coupled with the coupling surface 3 of the output unit 4 becomes dominant. Accordingly, the oscillation of the light L3 having the wavelength λ2 is suppressed. Furthermore, as shown in the drawing, the seed light L1 is amplified while being reflected multiple times in the vertical direction and propagating as a slow light toward the right side. The light L2 thus amplified is output from the upper surface of the output unit 4.

FIG. 2 is a diagram showing the input/output characteristics of the output unit 4 of the surface-emitting laser 1 shown in FIG. 1. The horizontal axis represents the intensity of the coupled light, i.e., the intensity of the seed light L1. The vertical axis represents the light output of the surface-emitting laser 1. As a comparison, the amplification characteristics provided by a conventional technique (Non-patent documents 4 and 5) are indicated by the dotted line. With conventional techniques, in order to provide the output light in proportion to the coupled light, a current that is smaller than the threshold current I_THis supplied to the slow light SOA. This limits the output light to a small level. In contrast, with the present embodiment, the output unit 4 is oscillated so as to provide a saturated gain with respect to the coupled-light intensity, thereby providing a high-power output operation.

In order to verify the amplification characteristics of the surface-emitting laser 1, only the output unit 4, which is a part of the surface-emitting laser 1, was manufactured, and the output characteristics thereof were measured. FIG. 3 is a diagram showing a measurement system employed in the experiment. Electrodes 50 are formed on the upper surface of the output unit 4 in order to allow a current to be injected. The seed light L1 received from a light source configured as an external component of the output unit 4 is input with an appropriate incident angle to the coupling surface 3 of the output unit 4. In the experiment, the seed light L1 was coupled with the output unit 4 via an optical fiber. The output light L2 is measured by means of a photodetector 6. A region interposed between the electrodes 50, which is a part of the output unit 4, contributes to amplification. The output unit 4 was manufactured to have a length L of 1 mm in the horizontal direction. The output angle of the output light L2 depends on the wavelength λ1 of the seed light L1. The output unit 4 provides an oscillation waveform λ2 of 980 nm.

FIG. 4A is a diagram showing the amplification characteristics of the output unit 4. FIG. 4B is a diagram showing the spectrum of the output light L2. FIG. 4C is a diagram showing the beam angle and the beam width. FIG. 4A through FIG. 4C show the respective measurement results.

In FIG. 4A, the horizontal axis represents the intensity of the coupled light (seed light) L1, and the vertical axis represents the intensity of the output light L2. Here, an injection current of 180 mA is injected. It can be understood based on the gain saturation characteristics that the output unit 4 operates in an oscillation state. This arrangement is capable of acquiring the output light L2 exceeding 30 mW for a coupled light intensity of 1 mW. That is to say, this arrangement is capable of providing dramatically increased high-power output as compared with similar arrangements according to conventional techniques.

As shown in FIG. 4B, it has been confirmed that the output light L2 has a single wavelength with a narrow spectrum width. Furthermore, as shown in FIG. 4C, this arrangement is capable of providing high beam quality with a beam width on the order of 0.1 degrees without involving an optical system such as a lens configured to focus the output light L2.

As described above, it has been confirmed from the experimental results that the surface-emitting laser 1 including the output unit 4 configured to operate in an oscillation state is advantageous.

FIG. 5 is a diagram showing simulation results of the amplification characteristics of the output unit 4. The simulation results were calculated for an output unit 4 having a horizontal length L of 500 μm and an output unit 4 having a horizontal length L of 1000 μm. Furthermore, it is assumed that the radiation loss α_rtoward the upper side of the output unit 4=200 cm⁻¹, the coupled light intensity=1 mW, the number of well layers of the active region=5, the waveguide width of the output unit 4=10 μm, and the optical confinement factor Γ=6.36%. In this case, it can be understood that such an arrangement provides a high-power output operation of 8 W or more using an injection current of 10 A.

In the above-described experiment, it has been confirmed that such an arrangement provides an output of several dozen mW using an injection current on the order of 100 mA. However, it can be confirmed based on the simulation results that, by injecting a current of 1 A or more, this arrangement is capable of providing an output of several W.

Next, specific description will be made regarding an arrangement including the seed light source 2 and the output unit 4 having the same VCSEL structure 20 (40) that provides the relation λ1<λ2.

First Embodiment

FIG. 6 is a cross-sectional diagram showing a surface-emitting laser la according to a first embodiment. In the surface-emitting laser la, the VCSEL structures 20 and 40 of a seed light source 2a and an output unit 4a include respective air gap layers 28 and 48. The air gap layer 28 of the seed light source 2a side is configured to have a variable thickness that can be controlled by means of a micromachined structure, i.e., a MEMS (Micro-Electro-Mechanical Systems) structure. By changing the thickness of the air gap layer 28, the position of the high-reflection mirror 30 can be controlled. This allows the cavity length of the seed light source 2a to be changed, thereby allowing the oscillation wavelength λ1 to be reduced. It should be noted that, in the subsequent drawings, the driving circuit 5 is not shown.

Second Embodiment

FIG. 7 is a cross-sectional diagram showing a surface-emitting laser 1b according to a second embodiment. In the surface-emitting laser 1b, the number of layers that form the upper DBR 44 of the output unit 4b having the VCSEL structure 40 may be larger than that of the upper DBR 24 of the seed light source 2b having the VCSEL structure. The difference between the upper DBR 44 and the upper DBR 24 is represented by a phase control layer 52. The phase control layer 52 may be formed by means of selective growth. With the second embodiment, by increasing the cavity length of the output unit 4b, this arrangement is capable of providing the relation λ1<λ2.

Third Embodiment

FIG. 8 is a cross-sectional diagram showing a surface-emitting laser 1c according to a third embodiment. In the surface-emitting laser 1c, the seed light source 2c has a VCSEL structure 20 including a low-refractive-index layer 54. The low-refractive-index layer 54 is a part of the upper DBR 24, which can be formed by means of selective oxidation. By forming a part of the layers that form the upper DBR 24 with a low refractive index, this arrangement allows the effective cavity length of the seed light source 2c to be reduced, thereby providing the relation λ1<λ2.

Fourth Embodiment

FIG. 9 is a plan view showing a surface-emitting laser 1d according to a fourth embodiment. In the surface-emitting laser 1d, the seed light source 2d has a coupled resonator structure. The coupled resonator can be designed by designing the shape of an oxidation opening 56. By controlling the interference conditions of the coupled resonator, and specifically, by providing a difference in the FSR (free spectrum range) between the two resonators, this arrangement is capable of modulating the wavelength provided by the seed light source 2d (Vernier effect), thereby providing the relation λ1<λ2.

Fifth Embodiment

As shown in FIG. 5, as the length L of the output unit 4 in the horizontal direction is increased, the output that can be extracted becomes higher. FIG. 10 is a layout diagram showing a surface-emitting laser le according to a fifth embodiment. An output unit 4e is laid out in a two-dimensional form. For example, the output unit 4e is configured in a zig-zag form, thereby providing an increase in the length L. As shown in FIG. 4C, this arrangement allows the output light L2 with a very small divergence angle to be output from the output unit 4e. Accordingly, by configuring the output unit 4e in a two-dimensional form, this arrangement is capable of generating a high-power output beam with a narrow divergence angle in a two-dimensional manner. Such a beam can be focused by means of an optical system 8 such as a lens, mirror, or the like, such that it is narrowed up to the diffraction limit. Such an arrangement can be expected to have many uses.

Sixth Embodiment

FIG. 11 is a cross-sectional diagram showing a structure of a surface-emitting laser if according to a sixth embodiment. In this embodiment, the optical confinement layer that forms the active layer 42 has a refractive index that is smaller than the average refractive index of those of the upper DBR layer 44 and the lower DBR layer 46. This arrangement is capable of cutting off the waveguide mode due to total reflection. By cutting off the waveguide mode, this arrangement is capable of suppressing parasitic oscillation in the horizontal direction due to the waveguide mode or suppressing energy consumption due to the growth of the amplified spontaneous emission light L4. As a result, by increasing the length of the surface-emitting laser, this arrangement allows the output light from the surface-emitting laser to be increased.

FIG. 12A and FIG. 12B are diagrams each showing a refractive index distribution and an electric field distribution provided by the surface-emitting laser shown in FIG. 11. FIG. 12B is an enlarged view of FIG. 12A. In FIG. 12A and FIG. 12B, the horizontal axis represents the relative position in the layered direction.

FIG. 13 is a diagram showing results of simulation of the optical confinement factor of the surface-emitting laser shown in FIG. 11. The horizontal axis represents the Al composition of the active region. The vertical axis represents the optical confinement factor. The optical confinement factor was calculated for (i) the light in the waveguide mode, and (ii) the light in the slow light mode.

By configuring the optical confinement layer to have a refractive index that is lower than the average refractive index of those of the upper DBR and the lower DBR, this arrangement is capable of cutting off the waveguide mode due to total reflection. For example, the simulation results show that, by configuring the optical confinement layer with an Al composition on the other of 0.55, this arrangement provides an optical confinement factor of almost zero in the waveguide mode. Furthermore, this arrangement allows the optical confinement factor to be maintained at a constant value of 4% (0.04) with respect to the seed light. This allows the amplified spontaneous emission light due to the waveguide mode to be suppressed, and allows the seed light to be amplified.

Seventh Embodiment

The seed light source 2 and the output unit 4 are not necessarily required to be integrated. Also, as shown in FIG. 3, the seed light source 2 and the output unit 4 may be configured as separate components.

Description has been made regarding the present invention with reference to the embodiments using specific terms. However, the above-described embodiments show only the mechanisms and applications of the present invention for exemplary purposes only, and are by no means intended to be interpreted restrictively. Rather, various modifications and various changes in the layout can be made without departing from the spirit and scope of the present invention defined in appended claims.

SURFACE-EMITTING LASER

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