Surface light-emitting laser

Abstract

To provide a surface light-emitting laser which is improved in modulation bandwidth improved.SOLUTION: A surface light-emitting laser 1 comprises a main resonator 10 and a plurality of external resonators 30. The main resonator 10 has a VCSEL structure 12, and also has an electrode for modulation and an emission window for laser beam. Each external resonator 30 has a VCSEL structure in which an active layer mutually connects with the main resonator 10, and is laterally coupled to the main resonator 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a surface-emitting semiconductor laser, and more particularly to achieving higher speeds or higher power outputs. [Background technology]

[0002] Conventionally, the single-wavelength output of surface-emitting lasers has been limited to the milliwatt level. If watt-level high-output operation becomes possible, a wide range of applications will become possible, such as wavelength sweeping light sources for optical coherence tomography (OCT), light sources for medium- to long-distance optical communications, light sources for laser radar (LIDAR) mounted on automobiles, drones, robots, etc., monitoring systems, automatic inspection equipment at manufacturing sites, and laser dryers for printers.

[0003] Patent Document 1 discloses a vertical-cavity surface-emitting laser (VCSEL) having a main cavity and an external cavity that are laterally coupled. In this technology, the main cavity and the external cavity have the same cross-sectional structure, and therefore their cavity lengths, i.e., resonance wavelengths, are the same.

[0004] According to the VCSEL of Patent Document 1, high-speed modulation is possible by feeding back light from an external resonator to a main resonator. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6240429 Summary of the Invention [Problem to be solved by the invention]

[0006] As a result of studying the VCSEL described in Patent Document 1, the present inventors have come to recognize the following problems.

[0007] The VCSEL in Patent Document 1 can widen the bandwidth compared to when there is no external resonator, enabling high-speed modulation, but because the resonant wavelengths of the main resonator and the external resonator are essentially the same (specifically, the wavelength difference Δλ is about 1 nm), single-mode oscillation is unstable and there is room for improvement in the noise level. Furthermore, to achieve single-mode oscillation, the opening of the oxide confinement must be miniaturized to about several μm, which increases the current density, posing a major reliability problem.

[0008] The present invention has been made in light of these circumstances, and one exemplary purpose of an embodiment of the present invention is to provide a surface-emitting laser with an improved modulation bandwidth, and another exemplary purpose is to provide a surface-emitting laser that suppresses multi-mode oscillation and achieves a single mode even for a relatively large oxide confinement opening, and / or has improved noise characteristics. [Means for solving the problem]

[0009] One aspect of the present invention relates to a surface-emitting laser, which includes a main cavity having a VCSEL (vertical cavity surface-emitting laser) structure and including a modulation electrode and an exit window for a laser beam, and multiple external cavities, each of which has a VCSEL structure in which the main cavity and an active layer are continuous, and which are laterally coupled to the main cavity.

[0010] According to this embodiment, by providing multiple external resonators, slow light is fed back from the multiple external resonators to the main resonator, so the modulation bandwidth can be further expanded compared to when a single external resonator is provided.

[0011] The sizes of the multiple external resonators may be different. This allows the phase of the feedback slow light for each external resonator to be optimized using the size of the external resonator as a parameter. If the external resonator is rectangular, its size can be understood as the resonator length in the propagation direction of the slow light and the width perpendicular to that. If the external resonator is circular, its size can be understood as its radius. If the external resonator is rectangular with a diagonal in the propagation direction of the slow light, its size can be understood as the length of the diagonal. If the main resonator or external resonator has an oxidation confinement structure, its size is determined based on the oxidation opening diameter.

[0012] The multiple external resonators may have different coupling coefficients with the main resonator, which allows the intensity of the slow light fed back (i.e., the feedback ratio) to be optimized for each external resonator using the coupling coefficient as a parameter.

[0013] The multiple external resonators and the main resonator may have different lateral sizes, which allows stable single-mode oscillation to be achieved even if the main resonator is large, thereby achieving high-power operation, high reliability, and low-noise characteristics.

[0014] The multiple external cavities may have different cavity lengths in the vertical direction. By individually optimizing the cavity lengths in the vertical direction of the multiple external cavities, the performance of the surface-emitting laser as a whole can be improved.

[0015] The multiple external cavity VCSEL structure may include a phase adjustment layer, which allows the resonant wavelength λ2 of the external cavity to be longer than the resonant wavelength λ1 of the main cavity. By controlling the thickness of the phase adjustment layer, the difference Δλ between the resonant wavelengths of the external cavity and the main cavity can be increased from a few nanometers to a few tens of nanometers, enabling stable lateral coupling.

[0016] The phase adjustment layer may be a semiconductor layer. The phase adjustment layer may be a dielectric (insulator) layer. When a dielectric multilayer film is used to form a phase adjustment layer, the thickness of each layer of the multilayer film must be controlled. However, by using a semiconductor layer or a dielectric layer, it is only necessary to control the thickness of a single layer, and the phase adjustment layer can be easily formed using a general semiconductor process.

[0017] The surface-emitting laser may further include an electrode formed in a region adjacent to the main cavity. At least one of the plurality of external cavities may be configured using reflection at the boundary of the electrode.

[0018] The external cavity may be formed by using reflection at the boundary between the electrodes. In this case, the resonant wavelengths of the main cavity and the external cavity may be the same. This allows the application of a normal surface-emitting laser manufacturing process.

[0019] Another aspect of the present invention also relates to a surface-emitting laser. The surface-emitting laser includes a main cavity having a VCSEL structure and including a modulation electrode and an exit window for a laser beam, and at least one external cavity having a VCSEL structure in which the main cavity and an active layer are continuous and laterally coupled to the main cavity. The at least one external cavity and the main cavity have different resonant wavelengths.

[0020] This makes it possible to suppress multi-mode oscillation and improve noise characteristics due to the effect of the composite resonator between the main resonator and the external resonator.

[0021] In this specification, the terms up and down, lateral, horizontal and vertical directions are used for convenience and are unrelated to the directions in actual operation.

[0022] Any combination of the above components or conversion of the present invention between methods, devices, etc. are also valid aspects of the present invention. [Effects of the Invention]

[0023] According to an embodiment of the present invention, it is possible to improve the modulation bandwidth of a surface-emitting laser, and also to suppress multimode oscillation and / or improve noise characteristics. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a diagram schematically illustrating a surface-emitting laser according to a first embodiment. [Figure 2] 1A and 1B are diagrams illustrating the operation of a surface-emitting laser. [Figure 3] FIG. 10 is a diagram showing the modulation bandwidth (simulation results) of a surface-emitting laser. [Figure 4] FIG. 10 is a diagram showing the relative intensity noise (simulation results) of a surface-emitting laser. [Figure 5] 5(a) and 5(b) are diagrams showing the modulation bandwidth of the DTCC (simulation results). [Figure 6] FIG. 10 is a diagram showing the modulation bandwidth of an STCC (simulation results). [Figure 7] 7(a) and (b) show the measurement results of the modulation bandwidth of the DTCC and STCC samples. [Figure 8] 8(a) and 8(b) are a perspective view and a plan view of a surface-emitting laser according to one embodiment. [Figure 9] 9(a) to 9(c) are plan views of surface-emitting lasers according to modified examples. [Figure 10] 10(a) to 10(d) are plan views of a surface-emitting laser 1A according to a modified example. [Figure 11] FIG. 10 is a cross-sectional view of a surface-emitting laser according to a second embodiment. [Figure 12] 12 is a diagram showing the oscillation spectra (measurement results) of the main cavity and the external cavity of the surface-emitting laser of FIG. 11. FIG. [Figure 13] FIG. 2 is a diagram showing the spectrum of the output beam of a surface-emitting laser. [Figure 14] 1A and 1B are diagrams showing far-field and near-field patterns of the output of a surface-emitting laser. [Figure 15]15(a) and 15(b) are schematic cross-sectional and plan views of a surface-emitting laser according to the third embodiment. [Figure 16] 16(a) to 16(c) are plan views of surface-emitting lasers according to modified examples. [Figure 17] 17(a) is a diagram showing the measurement results of the modulation bandwidth in Example 3, and FIG. 17(b) is a diagram showing the measurement results of the oscillation spectrum in Example 3. In FIG. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing are designated by the same reference numerals, and redundant descriptions will be omitted where appropriate. Furthermore, the embodiments are illustrative and do not limit the invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0026] (Embodiment 1) Fig. 1 is a diagram schematically illustrating a surface-emitting laser 1A according to embodiment 1. The surface-emitting laser 1A includes a main resonator 10 and a plurality of external resonators 30. Fig. 1 illustrates an example in which there are two external resonators 30, which are denoted as 30_1 and 30_2, respectively.

[0027] The main cavity 10 has a VCSEL (vertical cavity surface emitting laser) structure 12, and also has a modulation electrode (not shown) and an exit window 20 for extracting the laser beam LB to the outside. The VCSEL structure 12 includes an active layer 14, a lower DBR layer 16, and an upper DBR layer 18.

[0028] Each of the external cavities 30_1 and 30_2 has a VCSEL structure 32. The VCSEL structure 32 includes an active layer 34, a lower DBR layer 36, and an upper DBR layer 38. The active layer 34 of the VCSEL structure 32 is formed continuously with the active layer 14 of the VCSEL structure 12, and each of the external cavities 30_1 and 30_2 is laterally coupled to the main cavity 10.

[0029] The coupling coefficient between the main resonator 10 and the external resonator 30_1 is represented as η1, and the coupling coefficient between the main resonator 10 and the external resonator 30_2 is represented as η2. The resonant wavelength of the main resonator 10 is represented as λ1, and the resonant wavelengths of the external resonators 30_1 and 30_2 are represented as λ 2_1 , λ 2_2 The size of the main resonator 10 is denoted as W, and the sizes of the external resonators 30_1 and 30_2 are denoted as Lc1 and Lc2, respectively.

[0030] In one embodiment, the resonant wavelength λ of at least one external resonator 30_1, 30_2 2_1 , λ 2_2 is different from the resonant wavelength λ1 of the main resonator 10, and is preferably λ 2_1 , λ 2_2 >λ1. More preferably, λ 2_1 and λ1, the difference Δλ1 and λ 2_2 The difference Δλ2 between λ and λ1 is greater than 3 nm and may be on the order of 5 nm or greater. 2_1 ≠λ 2_1 It may be ≠λ1.

[0031] In one embodiment, the sizes Lc1 and Lc2 of at least one of the external resonators 30_1 and 30_2 are different from the size W of the main resonator 10 (Lc1≠W, Lc2≠W). When two (or more) external resonators 30_1 and 30_2 are provided, Lc1≠Lc2 may be satisfied.

[0032] Furthermore, when two or more external resonators 30 are provided, the coupling coefficient with the main resonator 10 may be individually optimized for each external resonator 30, and η1≠η2 may be satisfied.

[0033] The above is the basic configuration of the surface-emitting laser 1A. Next, its operation will be explained. First, the principle of expanding the modulation band by using an external resonator will be explained.

[0034] 2 is a diagram for explaining the operation of the surface-emitting laser 1 A. Here, for simplicity of explanation, attention is focused on only one external resonator 30.

[0035] A modulation signal is applied to the modulation electrode of the main resonator 10. A drive signal may be applied to the control electrode of the external resonator 30.

[0036] The main cavity 10 operates as a normal VCSEL, and the laser light is amplified in the active layer 34 while traveling back and forth between the lower DBR layer 16 and the upper DBR layer 18, and is extracted to the outside through the exit window 20.

[0037] Because the main resonator 10 and the external resonator 30 are coupled, part of the laser light generated in the main resonator 10 leaks into the external resonator 30. Inside the external resonator 30, the light injected from the main resonator 10 is multiple-reflected between the lower DBR layer 36 and the upper DBR layer 38 as shown by the dashed-dotted line (i), and slowly propagates in the direction shown by the solid line (ii) (this is called slow light propagation), is reflected at the end of the external resonator 30 (iii), and returns to the main resonator 10 (iv). Part of the returned slow light is then fed back to the main resonator 10.

[0038] When the electric field injected from the main resonator 10 to the external resonator 30 is E(t), the electric field injected back from the external resonator 30 to the main resonator 10 can be expressed as E(t-τ). τ is the delay time for the light injected into the external resonator 30 to return after slow light propagation, τ=2·Lc(n g / c) It is expressed as n g is the group index of refraction of slow light in a medium relative to the speed of light c in a vacuum, typically n g >30.

[0039] By feeding back the slow-light feedback light E(t-τ) to the main cavity 10 in antiphase with the electric field E(t) inside the main cavity 10, the effective differential gain of the entire surface-emitting laser 1A increases, thereby increasing its relaxation oscillation frequency and widening the bandwidth. The coupling coefficient η and cavity size Lc described above are design parameters for the amount of feedback and the feedback phase. Therefore, by individually optimizing the coupling coefficient η and / or size Lc for each of the multiple external cavities 30, the modulation bandwidth of the entire surface-emitting laser 1A can be expanded. Furthermore, a peak occurs due to the photon-photon resonance effect associated with lateral resonance in the external cavity 30, which increases the modulation sensitivity in the high-frequency range. This increases the relaxation oscillation frequency and broadens the modulation bandwidth.

[0040] 3 is a diagram showing the modulation bandwidth (simulation results) of the surface-emitting laser 1A. The horizontal axis represents the frequency (modulation frequency) of the modulation signal applied to the modulation electrode, and the vertical axis represents the response gain. The diagram shows the modulation bandwidth when the number N of external resonators 30 is changed from 0 to 1 to 2. A surface-emitting laser with N=1 is also called an STCC (Single Transverse Coupled Cavity), and a surface-emitting laser with N=2 is also called a DTCC (Double Transverse Coupled Cavity).

[0041] The parameters of the STCC with N=1 are λ1=~850nm, W=4μm, Lc1=10μm, η1=0.96. The parameters of the DTCC with N=2 are λ1=~850nm, W=4μm, Lc1=7μm, η1=0.9, Lc2=8μm, η2=0.7.

[0042] When N=1, the 3 dB bandwidth is about 40 GHz, whereas when N=2, the 3 dB bandwidth can be further extended to 90 GHz. Note that the fact that the frequency bandwidth can be improved by increasing the number of external resonators 30 is not obvious to those skilled in the art, but was discovered independently by the present inventors.

[0043] Figure 4 shows the relative intensity noise (simulation results) of the surface-emitting laser 1A. The horizontal axis represents the bias current Ib. The parameters of the STCC with N=1 are λ1=850 nm, W=4 μm, Lc1=15 μm, and η1=0.6. The parameters of the DTCC with N=2 are λ1=850 nm, W=4 μm, Lc1=15 μm, η1=0.6, Lc2=25 μm, and η2=0.9. The noise characteristics of the conventional surface-emitting laser with N=0 are the best, while the noise characteristics of the STCC with N=1 are significantly worse. The DTCC with N=2 achieves noise characteristics comparable to those of the conventional surface-emitting laser.

[0044] 5(a) and (b) are diagrams showing the modulation bandwidth (simulation results) of the DTCC. Fig. 5(a) shows the dependency of the coupling coefficient η1 between the external resonator 30_1 and the main resonator 10, and Fig. 5(b) shows the dependency of the coupling coefficient η2 between the external resonator 30_2 and the main resonator 10. The parameters are λ1=850 nm, W=4 μm, Lc1=7 μm, and Lc2=8 μm.

[0045] As can be seen from FIGS. 5(a) and 5(b), by individually optimizing the coupling coefficients η1 and η2 of the multiple external resonators 30, the modulation bandwidth can be expanded.

[0046] Figure 6 shows the modulation bandwidth (simulation results) of the STCC. Figure 6 shows the dependence of the coupling coefficient η between the external resonator 30 and the main resonator 10. The parameters are λ1 = 850 nm, W = 4 μm, and Lc = 10 μm. When the coupling coefficient η is changed in the STCC, a peak due to the photon-photon resonance effect appears at a specific frequency, but this peak does not contribute significantly to improving the 3 dB bandwidth. These simulation results confirm that the STCC has limitations in terms of improving the modulation bandwidth, and that the DTCC is advantageous.

[0047] 7(a) and (b) show the measurement results of the modulation bandwidth of the DTCC and STCC samples. The design parameters of the DTCC and STCC are as follows: DTCC λ1=850nm W=4μm, Lc1=10μm, Lc2=9μm, η1=0.25, η2=0.25

[0048] STCC λ1=850nm W=4μm, Lc1=4μm, η1=0.25

[0049] The measurement results show the same tendency as the simulation results, and it can be confirmed that the modulation bandwidth can be expanded by connecting multiple external resonators 30.

[0050] Next, the cross-sectional shape of the surface-emitting laser 1A will be described. Figures 8(a) and 8(b) are a perspective view and a plan view of the surface-emitting laser 1A according to one embodiment.

[0051] In this embodiment, the main resonator 10 and the external resonators 30_1 and 30_2 are rectangular and coupled at one side. The above-mentioned coupling coefficients η1 and η2 can be adjusted using parameters such as the shape, width, length, and equivalent refractive index of the active layer in the coupling portion 40. The equivalent refractive index may be controlled by the amount of impurity doping or the material. The phase τ of the feedback light can be designed based on the lengths Lc1 and Lc2 of the external resonators 30_1 and 30_2, respectively.

[0052] As shown in FIG. 8(a), the main cavity 10 and the external cavities 30_1 and 30_2 have VCSEL structures 12, 32_1 and 32_2, respectively, in which the active layer is continuous. The VCSEL structure and material may be any known technology and are not particularly limited, but an example will be described. For example, the semiconductor substrate 50 is a III-V group semiconductor and may be a GaAs substrate. An electrode (not shown) may be formed on the back surface of the semiconductor substrate 50. The lower DBR layer 16 (36) is made of Al doped with silicon, which is an n-type impurity. 0.92 Ga 0.08 As layer and Al 0.16 Ga 0.84 It has a laminated structure of As layers (AlGaAs = aluminum gallium arsenide) and has a reflectivity of nearly 100%.

[0053] The active layer 14 (34) is In 0.2 Ga 0.8 The active layer 14 (34) has a multi-quantum well structure of As / GaAs (indium gallium arsenide / gallium arsenide). For example, the active layer 14 (34) may have a three-layer quantum well structure. On both sides of the multi-quantum well structure, undoped Al 0.3 Ga 0.7 A lower spacer layer and an upper spacer layer are formed, each of which is an As layer.

[0054] The upper DBR layer 18 (38) can be formed of a semiconductor layer, a dielectric multilayer film, or a combination thereof. For example, the upper DBR layer 18 (38) can be formed of carbon-doped Al 0.92 Ga 0.08 As layer and Al 0.16 Ga 0.84 Alternatively, the main resonator 10 may have a laminated structure of As layers (AlGaAs = aluminum gallium arsenide). Since the upper DBR layer 18 of the main resonator 10 needs to extract the laser beam to the outside, the number of layers is determined so that the reflectance is less than 100%. On the other hand, in the external resonators 30_1 and 30_2, the upper DBR layer 38 is designed to have a reflectance of substantially 100% so that the laser beam does not leak to the outside. The top surfaces of the external resonators 30_1 and 30_2 may be covered with a metal wiring layer.

[0055] A modulating electrode 42 is formed on the top surface of the main resonator 10, and control electrodes 44, 46 are formed on the top surfaces of the external resonators 30_1, 30_2. A current confinement layer (oxidized layer) 48 is provided on the main resonator 10 and the external resonators 30_1, 30_2. The current confinement layer 48 can be formed by selective oxidation and includes an oxidized region 48b along the periphery and a non-oxidized region 48a surrounded by the oxidized region 48b. Depending on the shape of the current confinement layer 48, the effective sizes of the main resonator 10 and the external resonators 30_1, 30_2 can be controlled, and the coupling efficiencies η1, η2 can also be controlled.

[0056] 9(a) to 9(c) are plan views of a surface-emitting laser 1A according to a modified example. As shown in FIG. 9(a), the main resonator 10 and the external resonator 30 are rectangular (quadrilateral), and are arranged so that their diagonals are aligned in the propagation direction of the slow light, and are joined at their vertices. In FIG. 9(b), similar to FIG. 9(a), the main resonator 10 and the external resonator 30 are rectangular or diamond-shaped and joined at their vertices, but a connecting portion 52 is provided between the main resonator 10 and the external resonator 30. The connecting portion 52 also has a VCSEL structure, similar to the main resonator 10 and the external resonator 30.

[0057] In FIG. 9(c), the main resonator 10 and the external resonator 30 are circular or elliptical and are partially coupled.

[0058] The number N of external resonators 30 is not limited to 2, but may be 3, 4, or more. FIGS. 10(a) to 10(d) are plan views of a surface-emitting laser 1A according to a modified example. In FIG. 10(a), three external resonators 30_1, 30_2, and 30_3 are connected to three sides of the main resonator 10. In FIG. 10(b), four external resonators 30_1 to 30_4 are connected to four sides of the main resonator 10. The external resonators 30 in FIGS. 10(a) and 10(b) are rectangular, with sides in the propagation direction of the slow light and in a direction perpendicular thereto. In FIG. 10(c), four external resonators 30_1 to 30_4 are connected to four vertices of the main resonator 10. The external resonator 30 is a rectangle whose diagonal is the propagation direction of the slow light. In FIG. 10(d), three circular external resonators 30_1 to 30_3 are connected to a circular main resonator 10.

[0059] (Embodiment 2) Fig. 11 is a cross-sectional view of a surface-emitting laser 1B according to embodiment 2. The surface-emitting laser 1B includes a main resonator 10 and one or more external resonators 30. Fig. 11 shows the case where N=1.

[0060] The main cavity 10 has a VCSEL structure 12 including an active layer 14, a lower DBR layer 16, and an upper DBR layer 18. An exit window 20 for extracting a laser beam is provided at the top of the main cavity 10 by setting the reflectance of the upper DBR layer 18 to less than 100%. A modulation electrode 22 is also formed around the exit window 20.

[0061] The external cavity 30 has a VCSEL structure 32, similar to the main cavity 10, and the VCSEL structure 32 includes an active layer 34, a lower DBR layer 36, and an upper DBR layer 38. The active layer 34 of the VCSEL structure 32 is continuous with the active layer 14 of the VCSEL structure 12 of the main cavity 10.

[0062] Since there is no need to extract a laser beam from the external cavity 30, no exit window is provided, and the reflectance of the upper DBR layer 38 may be set to 100%. A shielding portion such as a metal film may be formed on the top surface of the upper DBR layer 38.

[0063] The main cavity 10 and the external cavity 30 are optically coupled in the lateral direction via a common active layer.

[0064] In the second embodiment, the main resonator 10 and the external resonator 30 are configured to have different resonant wavelengths. Specifically, the main resonator 10 and the external resonator 30 have different effective optical path lengths (resonator lengths) in the vertical direction (depth direction). The effective optical path length can be controlled by the physical depth (length) and the refractive index.

[0065] 11, λ1<λ2, and the cavity length of the external resonator 30 is designed to be longer than the cavity length of the main resonator 10. For this reason, a phase adjustment layer 39 is provided in the external resonator 30. The phase adjustment layer 39 may be a semiconductor layer or a dielectric layer.

[0066] An example of a method for forming the phase adjustment layer 39 will now be described. The VCSEL structure 12 (32) is a half-VCSEL structure, and the upper DBR layer 18 (38) includes a semiconductor layer 18a (38a) and a dielectric multilayer film layer 18b (38b). In one embodiment, the phase adjustment layer 39 is formed on the entire upper surface of the semiconductor layer 18a (38a). In a subsequent wet etching process, the portion of the phase adjustment layer 39 that overlaps the semiconductor layer 18a is removed. Then, the dielectric multilayer film layers 18b and 38b are formed.

[0067] When the phase adjustment layer 39 is made of a semiconductor material, GaAs, Si, GaAlAs, InP, GaInAsP, GaAlInP, GaN, GaAlN, or the like can be used.

[0068] When the phase adjustment layer 39 is made of a dielectric material, SiO2, TiO2, Ta2O5, or the like can be used.

[0069] The phase adjustment layer 39 may also have a laminated structure of different semiconductor materials, a laminated structure of different dielectric materials, or a laminated structure of a semiconductor and a dielectric.

[0070] Next, the advantages of the surface-emitting laser 1B will be explained. Fig. 12 shows the oscillation spectra (measurement results) of the main cavity 10 and the external cavity 30 of the surface-emitting laser 1B of Fig. 11. By adding the phase adjustment layer 39, λ1 and λ2 can be largely separated, and in this example, a wavelength difference of Δλ = λ2 - λ1 = 5 nm is obtained.

[0071] Fig. 13 is a diagram showing the spectrum of the output beam of the surface-emitting laser 1B. In the case of coupling a main resonator and a single external resonator, which have close resonant wavelengths, as disclosed in Patent Document 1, increasing the bias current makes single-mode oscillation unstable, resulting in oscillation at multiple wavelengths. In contrast, in the surface-emitting laser 1B of Fig. 11, multi-mode oscillation can be suppressed and single-mode oscillation can be maintained even if the bias current Ib is increased.

[0072] 14 shows the results of measuring the far-field and near-field patterns of the output of a surface-emitting laser 1B equipped with a main cavity and two external cavities. The beam profile was measured at bias currents Ib=6.5 mA, 10 mA, and 12.5 mA. The oxide aperture diameter is 7 μm × 8 μm. With a typical oxide confinement structure, it is difficult to obtain a single-peaked near-field pattern unless the oxide aperture diameter is 3 μm or less. However, with the surface-emitting laser 1B according to the second embodiment, single-mode operation can be achieved even with a large oxide aperture diameter of 7 μm × 8 μm.

[0073] The technique of embodiment 2 may be combined with the technique of embodiment 1. For example, in the surface-emitting laser 1A of Fig. 8, the resonance wavelength of at least one of the external resonators 30_1 and 30_2 may be made longer or shorter than the resonance wavelength of the main resonator 10. Specifically, a phase adjustment layer 39 may be inserted in at least one of the VCSEL structures 32_1 and 32_2 of the external resonators 30_1 and 30_2.

[0074] (Embodiment 3) 15(a) and (b) are a cross-sectional view and a plan view of a surface-emitting laser 1C according to embodiment 3. The surface-emitting laser 1C includes a main resonator 10 and one or more external resonators 30. Figures 15(a) and (b) show the case where N=2.

[0075] The main cavity 10 and the external cavity 30 have a VCSEL structure 60, with corresponding layers formed continuously. The VCSEL structure 60 includes a lower DBR layer 66, an active layer 64, an oxide layer (current confinement layer) 65, and an upper DBR layer 68. The upper DBR layer 68 includes a semiconductor DBR layer 68a and a dielectric DBR layer 68b.

[0076] 15(b), the oxide layer 65 includes an outer oxidized region 65b and a non-oxidized region 65a surrounded by the oxidized region 65b. The non-oxidized region 65a corresponds to the main resonator .

[0077] In addition, in a region adjacent to the main cavity 10 of the surface-emitting laser 1C in the lateral direction of the drawing, electrodes 70_1 and 70_2 are formed between the semiconductor DBR layer 68a and the dielectric DBR layer 68b. Slow light propagating in the lateral direction of the drawing is reflected by sides (electrode boundaries) E1 and E2 of the two electrodes 70_1 and 70_2. The external cavity 30 is configured by utilizing the reflection at the electrode boundaries.

[0078] In this configuration, the region sandwiched between the oxidation boundaries F1 and F2 of the non-oxidized region 65a is the main resonator 10, the region sandwiched between the oxidation boundary F1 of the non-oxidized region 65a and the side E1 of the electrode 70_1 is the external resonator 30_1, and the region sandwiched between the oxidation boundary F2 of the non-oxidized region 65a and the side E2 of the electrode 70_2 is the external resonator 30_2.

[0079] In the third embodiment, the main resonator 10 and the external resonator 30 may be configured to have the same resonant wavelength. Specifically, the main resonator 10 and the external resonator 30 may have the same layer structure in the vertical direction (depth direction) except for the electrodes and oxide layers.

[0080] In addition, the distance Lc1 (Lc2) between the boundary E1 (E2) of the electrode 70_1 (70_2) and the oxidation boundary F1 (F2) of the main resonator 10 should be designed to be about 3 μm or less so that light propagating laterally from the main resonator 10 is reflected.

[0081] 16(a) to 16(c) are plan views of a surface-emitting laser 1C according to a modified example. In Fig. 16(a), an electrode 70 is formed to surround a non-oxidized region 65a. The region surrounded by oxidation boundaries F1 to F4 functions as the main resonator 10. The region between oxidation boundary F1 and electrode boundary E1, the region between oxidation boundary F2 and electrode boundary E2, the region between oxidation boundary F3 and electrode boundary E3, and the region between oxidation boundary F4 and electrode boundary E4 function as four external resonators 30.

[0082] 16(b) is a combination of the third embodiment and the second embodiment. As for the vertical direction, as described in the third embodiment, electrodes 70_1 and 70_2 are formed at positions adjacent to the top and bottom of the main cavity 10, and two external resonators 30_1 and 30_2 are provided that utilize reflection from the boundary between the electrodes 70_1 and 70_2. As for the horizontal direction, an external resonator 30_3 having a different vertical resonance wavelength (λ1≠λ2) is connected using the mode of the second embodiment.

[0083] 16(c) is also a combination of the third embodiment and the second embodiment, in which electrodes 70_1 and 70_2 are formed at positions adjacent to the top and bottom of the main resonator 10, and two external resonators 30_1 and 30_2 are provided, utilizing reflection from the boundary between the electrodes 70_1 and 70_2. Furthermore, in the horizontal direction, using the aspect of the second embodiment, an external resonator 30_3 having a different vertical resonant wavelength is connected to a region adjacent to the right side of the main resonator 10 (λ1≠λ2). Furthermore, an external resonator 30_4 having a different vertical resonant wavelength is connected to a region adjacent to the left side of the main resonator 10 (λ1≠λ3).

[0084] Figure 17(a) shows the measurement results of the modulation bandwidth of the device with the structure shown in Figure 16(a). The size of the oxidized aperture (non-oxidized region) of the fabricated device is 8 μm × 8 μm, and an electrode aperture of 12 μm × 12 μm is formed around the periphery. Compared to a normal surface-emitting laser with a larger electrode aperture, a modulation bandwidth of 20 GHz is obtained, which is about twice as large.

[0085] Figure 17(b) shows the measurement results of the oscillation spectrum of the device with the structure of Figure 16(a). While multimode oscillation is observed in ordinary surface-emitting lasers with large electrode openings, single-mode operation is obtained over the entire current range.

[0086] The present invention has been described using specific terms based on the embodiments, but the embodiments merely illustrate the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the concept of the present invention defined in the claims. [Explanation of symbols]

[0087] 1. Surface-emitting laser 10 Main resonator 12 VCSEL structure 14 Active layer 16 Lower DBR layer 18 Upper DBR layer 20 Exit window 22 Modulation electrode 30 external resonator 32 VCSEL structure 34 Active layer 36 Lower DBR layer 38 Upper DBR layer 39 Phase Adjustment Layer

Claims

1. a main cavity having a VCSEL structure and having an exit window for a laser beam; an external cavity having a VCSEL structure in which an oxide layer is continuous with the main cavity, the external cavity being laterally coupled to the main cavity; the VCSEL structure has a continuous current confinement layer; The surface-emitting laser has the main cavity corresponding to the non-narrowed region and the external cavity provided in the narrowed region.

2. further comprising an electrode formed in a region adjacent to the main resonator; 2. The surface-emitting laser according to claim 1, wherein the external resonators are plural, and the plural external resonators are configured by utilizing reflection at the boundaries of the electrodes.

3. 3. The surface-emitting laser according to claim 1, wherein the external resonator and the main resonator have different vertical resonant wavelengths.

4. 4. The surface-emitting laser of claim 3, wherein the external cavity VCSEL structure includes a phase adjustment layer.

5. 4. The surface-emitting laser according to claim 1, wherein the external resonator and the main resonator have different coupling coefficients.

6. 2. The surface-emitting laser according to claim 1, wherein the main cavity and the external cavity have active layers that are continuous with each other.

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