JP2017139255A - Surface emission laser element and atomic oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow for generation of high quality laser light, even when multiple surface emission lasers, having wavelength adjustment regions of different thickness, are formed in one surface emission laser element.SOLUTION: A surface emission laser element includes a lower reflector 22, an upper reflector 24 placed at a position closer to the laser light exit than the lower reflector 22, a resonator 23 placed between the lower reflector 22 and upper reflector 24, and resonating the light, and wavelength adjustment regions 25 formed in the upper reflector 24, and adjusting the wavelength of the light. The wavelength adjustment region 25 includes a first wavelength adjustment layer 61 containing indium, and a second wavelength adjustment layer 62 not containing indium, and the first wavelength adjustment layer 61 is thinner than the second wavelength adjustment layer 62.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a surface emitting laser element and an atomic oscillator.

  As the laser light source, a surface emitting laser element in which a plurality of surface emitting lasers (VCSEL: Vertical Cavity Surface Emitting Laser) are arranged in a plane is used. A surface emitting laser is a semiconductor laser that emits light in a direction perpendicular to a substrate surface. A surface emitting laser is advantageous in terms of cost reduction, power consumption, miniaturization, two-dimensional integration, and the like, compared to an edge emitting semiconductor laser.

  For example, in a surface emitting laser in which a resonator is disposed between upper and lower Bragg reflectors, a wavelength adjustment region including one or more wavelength adjustment layers is formed in the Bragg reflector, and the thickness of the wavelength adjustment region is changed. Thus, there is a technique for adjusting the wavelength of laser light (Patent Document 1). By forming a plurality of surface emitting lasers having different wavelength adjustment regions on one surface emitting laser element, it becomes possible to emit laser light having a plurality of wavelengths in one surface emitting laser element.

  A surface emitting laser is usually formed in a mesa shape by etching after various semiconductor layers are epitaxially grown on a substrate by MOCVD (Metal Organic Chemical Vapor Deposition) method, MBE (Molecular Beam Epitaxy) method or the like. At this time, when a plurality of surface-emitting lasers having different wavelength adjustment regions are formed in one surface-emitting laser element as described above, the etching depth is uneven among the plurality of surface-emitting lasers. Prone. If the etching depth is non-uniform, the remaining film thickness of the resonator becomes non-uniform, and the quality of the laser beam may be reduced.

  The present invention has been made in view of the above, and an object of the present invention is to improve the quality of laser light of a surface emitting laser element in which a plurality of surface emitting lasers having different thicknesses of wavelength adjustment regions are formed.

  In order to solve the above-described problems and achieve the object, the present invention includes a lower reflecting mirror, an upper reflecting mirror disposed nearer to a laser beam exit than the lower reflecting mirror, and the lower reflecting mirror. A resonator disposed between the upper reflector and resonating light; and a wavelength adjustment region formed in the upper reflector for adjusting the wavelength of the light, wherein the wavelength adjustment region comprises indium. A surface-emitting laser element including a first wavelength adjustment layer including a second wavelength adjustment layer not including indium, wherein the thickness of the first wavelength adjustment layer is smaller than the thickness of the second wavelength adjustment layer It is.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to improve the quality of the laser beam of the surface emitting laser element in which the several surface emitting laser from which the thickness of a wavelength adjustment area | region differs was formed.

FIG. 1 is a cross-sectional view showing the configuration of the surface emitting laser according to the first embodiment. FIG. 2 is a top view showing the configuration of the surface emitting laser element according to the first embodiment. FIG. 3 is a diagram illustrating a specific configuration example of the first surface emitting laser, the second surface emitting laser, and the third surface emitting laser according to the first embodiment. FIG. 4 is a graph showing the relationship between the remaining film thickness of the resonator and the characteristic change rate. FIG. 5 is a diagram illustrating an example of the wavelength adjusting region etching method according to the first embodiment. FIG. 6 is a diagram showing resist patterning and selective etching according to the first example of the first embodiment. FIG. 7 is a diagram showing resist patterning and selective etching according to a second example of the first embodiment. FIG. 8 is a diagram illustrating the position of the abdominal node in the longitudinal mode of the wavelength adjustment region of the first surface emitting laser according to the first embodiment. FIG. 9 is a diagram illustrating a replacement position of the wavelength adjustment region in the upper Bragg reflector according to the first embodiment. FIG. 10 is a diagram illustrating the position of the abdominal node in the longitudinal mode of the wavelength adjustment region of the first surface emitting laser according to the second embodiment. FIG. 11 is a diagram illustrating a replacement position of the wavelength adjustment region in the upper Bragg reflector according to the second embodiment. FIG. 12 is a diagram illustrating a configuration of an atomic oscillator according to the third embodiment. FIG. 13 is a diagram illustrating an example of a structure of atomic energy levels related to CPT. FIG. 14 is a diagram illustrating an example of a side band generated when laser light is modulated. FIG. 15 is a diagram illustrating an example of the relationship between the modulation frequency and the amount of transmitted light.

(First embodiment)
FIG. 1 is a cross-sectional view showing the configuration of the surface emitting laser 11 according to the first embodiment. The surface emitting laser 11 includes a semiconductor substrate 21, a lower Bragg reflector 22 (lower reflector), a resonator 23, an upper Bragg reflector 24 (upper reflector), a wavelength adjustment region 25, a contact layer 26, an upper electrode 27, and a lower portion. An electrode 28, an etching region 29, a protective layer 30, and a polyimide layer 31 are included.

  A lower Bragg reflector 22 is formed on a semiconductor substrate 21 made of n-GaAs or the like. The lower Bragg reflector 22 is a dielectric film made of oxide, nitride, fluoride, or the like, and is formed by alternately laminating layers made of semiconductor materials having different refractive indexes (low refractive index layer and high refractive index layer). It is formed by. A resonator 23 including an active layer is formed on the lower Bragg reflector 22.

  An upper Bragg reflector 24 is formed on the resonator 23. A wavelength adjustment region 25 is formed in the upper Bragg reflector 24. The upper Bragg reflector 24 is a dielectric film made of oxide, nitride, fluoride, or the like, and is formed by alternately laminating layers (low refractive index layers and high refractive index layers) made of semiconductor materials having different refractive indexes. It is formed by. The wavelength adjustment region 25 according to the present embodiment is formed by replacing the high refractive index layer of the upper Bragg reflector 24.

  A contact layer 26 is formed on the upper Bragg reflector 24. An upper electrode 27 is formed on the contact layer 26. The upper electrode 27 is formed so that a part of the contact layer 26 is exposed, and a portion where the contact layer 26 is exposed serves as a laser light emission port 32. A lower electrode 28 is formed on the lower surface side of the semiconductor substrate 21.

  The surface emitting laser 11 is formed in a mesa shape by the etching region 29. The etching region 29 is formed by etching the upper Bragg reflector 24 (including the wavelength adjustment region 25) and the resonator 23 around the emission port 32 in the stacking direction. The depth of the etching region 29 is formed so that the spacer layer of the resonator 23 remains by a predetermined thickness.

  A protective layer 30 and a polyimide layer 31 are formed in the etching region 29. The protective layer 30 is a layer mainly composed of SiN or the like, and has a function of protecting the current confinement layer (selective oxidation region) exposed by etching.

  By supplying a current to the upper electrode 27 of the surface emitting laser 11 having the above-described configuration, light having a wavelength adjusted by the wavelength adjustment region 25 is amplified by the resonator 23 and emitted from the emission port 32 as laser light. .

  FIG. 2 is a top view showing the configuration of the surface emitting laser element 1 according to the first embodiment. The surface emitting laser element 1 includes a first light emitting region 41, a second light emitting region 42, a third light emitting region 43, a fourth light emitting region 44, a first electrode pad 45, a second electrode pad 46, a second 3 electrode pads 47 and a fourth electrode pad 48.

  In the first light emitting region 41, a plurality of surface emitting lasers 11A are arranged in an array. In the second light emitting region 42, a plurality of surface emitting lasers 11B are arranged in an array. In the third light emitting region 43, a plurality of surface emitting lasers 11C are arranged in an array. In the fourth light emitting region 44, a plurality of surface emitting lasers 11D are arranged in an array. The first electrode pad 45 supplies a current to each surface emitting laser 11A in the first light emitting region 41. The second electrode pad 46 supplies a current to each surface emitting laser 11B in the second light emitting region 42. The third electrode pad 47 supplies a current to each surface emitting laser 11 </ b> C in the third light emitting region 43. The fourth electrode pad 48 supplies a current to each surface emitting laser 11D in the fourth light emitting region 44.

  Each of the surface emitting lasers 11A, 11B, 11C, and 11D basically has the same configuration as that of the surface emitting laser 11 shown in FIG. 1, but the thickness of the wavelength adjustment region 25 is different. Therefore, the wavelengths of the laser beams emitted from the surface emitting lasers 11A, 11B, 11C, and 11D (each of the light emitting regions 41 to 45) are different from each other. By controlling which electrode pads 45 to 48 are supplied with current by a predetermined control unit, it becomes possible to emit laser light having a desired wavelength. The configuration shown in FIG. 2 is an example, and the configuration of the surface emitting laser element 1 is not limited to this. For example, the number, shape, position, and the like of the light emitting regions 41 to 44 and the electrode pads 45 to 48 are matters that should be appropriately designed.

  FIG. 3 is a diagram illustrating a specific configuration example of the first surface emitting laser 11A, the second surface emitting laser 11B, and the third surface emitting laser 11C according to the first embodiment. Here, three types of surface emitting lasers 11A to 11C will be described as an example.

The resonator 23 is disposed between the lower Bragg reflector 22 and the upper Bragg reflector 24. The resonator 23 includes a lower spacer layer 51, an active layer 52, and an upper spacer layer 53. The lower spacer layer 51 and the upper spacer layer 53 are layers mainly composed of, for example, Al _0.2 Ga _0.8 As. The active layer 52 is a layer including, for example, a GaInAs quantum well layer / GaInPAs barrier layer. In addition, the component which comprises the upper and lower spacer layers 51 and 53 and the active layer 52 is not restricted above.

  The wavelength adjustment region 25 is formed in the upper Bragg reflector 24. As shown in FIG. 3, the thicknesses T of the wavelength adjustment regions 25 of the surface emitting lasers 11A to 11C are different from each other.

  The wavelength adjustment region 25 of the first surface emitting laser 11A includes a lower phase adjustment layer 55, an upper phase adjustment layer 56, a first wavelength adjustment layer 61, and a second wavelength adjustment layer 62. The wavelength adjustment region 25 of the first surface emitting laser 11A includes two first wavelength adjustment layers 61 and one second wavelength adjustment layer 62. From the bottom, the lower phase adjustment layer 55, The first wavelength adjustment layer 61, the second wavelength adjustment layer 62, the first wavelength adjustment layer 61, and the upper phase adjustment layer 56 are laminated in this order.

  The wavelength adjustment region 25 of the second surface emitting laser 11B includes a lower phase adjustment layer 55, an upper phase adjustment layer 56, a first wavelength adjustment layer 61, and a second wavelength adjustment layer 62. The wavelength adjustment region 25 of the second surface emitting laser 11B includes one first wavelength adjustment layer 61 and one second wavelength adjustment layer 62. From the bottom, the lower phase adjustment layer 55, The first wavelength adjustment layer 61, the second wavelength adjustment layer 62, and the upper phase adjustment layer 56 are laminated in this order.

  The wavelength adjustment region 25 of the third surface emitting laser 11 </ b> C includes a lower phase adjustment layer 55, an upper phase adjustment layer 56, and a first wavelength adjustment layer 61. The wavelength adjustment region 25 of the third surface emitting laser 11C includes a single first wavelength adjustment layer 61. From the bottom, the lower phase adjustment layer 55, the first wavelength adjustment layer 61, and the upper phase adjustment are arranged. The layers 56 are stacked in this order.

  As described above, the thickness T of the wavelength adjustment region 25 is adjusted by changing the number of layers of the first wavelength adjustment layer 61 or the second wavelength adjustment layer 62.

Lower phase adjusting layer 55 and the upper phase adjusting layer 56 is a layer for adjusting the phase of the light to be amplified by the resonator 23, for example a layer mainly composed of _p-Al 0.1 _{Ga 0.9} As . In addition, the component which comprises the upper and lower phase adjustment layers 55 and 56 is not restricted above.

  The first wavelength adjustment layer 61 and the second wavelength adjustment layer 62 are layers that adjust the wavelength of light amplified by the resonator 23. The first wavelength adjustment layer 61 is a layer containing indium, for example, a layer containing AlGaInP as a main component. The second wavelength adjustment layer 62 is a layer that does not contain indium, for example, a layer mainly composed of AlGaAs.

  As shown in FIG. 3, the thickness T <b> 1 of the first wavelength adjustment layer 61 is smaller than the thickness T <b> 2 of the second wavelength adjustment layer 62. Since indium is a difficult-to-etch material, the first wavelength adjustment layer 61 containing indium has a greater influence on the formation of the etching region 29 than the second wavelength adjustment layer 62 not containing indium. Therefore, as in the present embodiment, by making the thickness T1 of the first wavelength adjustment layer 61 thinner than the thickness T2 of the second wavelength adjustment layer 62, adverse effects on etching due to indium can be suppressed. It becomes possible. When a plurality of surface emitting lasers 11A to 11D having different thicknesses T of the wavelength adjustment region 25 are formed in one surface emitting laser element 1, the depth (resonator) of the etching region 29 of each of the surface emitting lasers 11A to 11D is formed. The remaining film thickness of 23 is likely to be non-uniform. However, as described above, by reducing the ratio of the thickness T1 of the first wavelength adjustment layer 61 to the thickness T of the entire wavelength adjustment region 25, the unevenness of the depth of the etching region 29 is suppressed. Is possible.

  As shown in FIG. 3, in this embodiment, the lower end portions of the etching regions 29 of the surface emitting lasers 11 </ b> A to 11 </ b> C are all contained in the lower spacer layer 51 of the resonator 23. When the lower end portion of the etching region 29 reaches the lower Bragg reflector 22 or stays in the active layer 52, the variation of the light amplification effect by the resonator 23 increases, and the quality of the surface emitting laser element 1 is increased. Is prone to decline.

  FIG. 4 is a graph showing the relationship between the remaining film thickness of the resonator 23 and the characteristic change rate. As the characteristic change rate is larger, the characteristics of the emitted laser light are more likely to vary. Therefore, even when the lower end portion of the etching region 29 is accommodated in the lower spacer layer 51, it is preferable to design so as to ensure a predetermined remaining film thickness. In the example shown in FIG. 4, it can be said that the remaining film thickness of the resonator 23 (lower spacer layer 51) is preferably 200 nm or more.

  The adjustment of the thickness T of the wavelength adjustment region 25 is performed by selectively etching the first wavelength adjustment layer 61 or the second wavelength adjustment layer 62 and adjusting the number of layers 61 and 62. In the present embodiment, since the thickness T1 of the first wavelength adjustment layer 61 and the thickness T2 of the second wavelength adjustment layer 62 are different, the wavelength adjustment region depends on which of the two layers 61 and 62 is etched. 25 changes in thickness T. Further, when the upper phase adjustment layer 56 is formed, there is a difference in the quality of the upper phase adjustment layer 56 between the case where the crystal is grown from the first wavelength adjustment layer 61 and the case where the crystal is grown from the second wavelength adjustment layer 62. May occur.

  The above problems can be solved by an etching method for the wavelength adjustment region 25. FIG. 5 is a diagram illustrating an example of the etching method of the wavelength adjustment region 25 according to the first embodiment. In the etching method of this example, the first wavelength adjustment layer 61 and the second wavelength adjustment layer 62 are paired, and etching is performed for each pair. For example, when the thickness T1 of the first wavelength adjustment layer 61 is 6 nm and the thickness T2 of the second wavelength adjustment layer 62 is 9 nm, the thickness of one pair is 15 nm. By etching every pair, the resonance wavelength can be adjusted by 1 nm. Such an etching method is not necessarily executed, and the first wavelength adjustment layer 61 or the second wavelength adjustment layer 62 may be left alone.

  Below, the manufacturing method of the surface emitting laser 11 is illustrated. Each layer constituting the surface emitting laser 11 is formed by MOCVD method or MBE method. When each layer of the wavelength adjustment region 25 is grown by MOCVD or the like, the resonance wavelength on the wafer is measured. The number of layers of the first wavelength adjustment layer 61 and the second wavelength adjustment layer 62 in the wavelength adjustment region 25 is adjusted so that the measured resonance wavelength becomes the target wavelength. The adjustment of the number of layers is performed by repeating the resist patterning and the selective etching until the predetermined number of layers are obtained after the layers 61 and 62 are laminated to the predetermined number.

  FIG. 6 is a diagram showing resist patterning and selective etching according to the first example of the first embodiment. In this example, the layer structure of each wavelength adjustment region 25 of the first surface-emitting laser 11A, the second surface-emitting laser 11B, and the third surface-emitting laser 11C is illustrated as a method shown in FIG. It is. In this example, the resist patterning is first performed so that the resist 70 is not formed on the second surface emitting laser 11B in the second light emitting region 42 and the third surface emitting laser 11C in the third light emitting region 43. 1 wavelength adjustment layer 61 is selectively etched (etching 1). Thereafter, resist patterning is performed so that the resist 70 is not formed on the third surface emitting laser 11C, and then the second wavelength adjustment layer 62 is selectively etched (etching 2). Thereafter, the resist 70 is removed (resist removal).

  FIG. 7 is a diagram showing resist patterning and selective etching according to a second example of the first embodiment. Similar to the first example shown in FIG. 6, this example also illustrates a method of setting the layer structure of each wavelength adjustment region 25 to the state shown in FIG. 3. In this example, first, after resist patterning is performed so that the second surface emitting laser 11B in the second light emitting region 42 is not formed with the resist 70, the first wavelength adjustment layer 61 is selectively etched (etching 1). Thereafter, resist patterning is performed so that the resist 70 is not formed on the third surface emitting laser 11C, and then the first wavelength adjustment layer 61 and the second wavelength adjustment layer 62 are selectively etched in order (etching 2). Thereafter, the resist 70 is removed (resist removal).

  For example, a mixed solution of sulfuric acid, hydrogen peroxide, and water can be used as an etching solution of GaAsP (the same applies to GaAs). As a GaInP etching solution, for example, a mixed solution of hydrochloric acid and water can be used.

  On the first wavelength adjustment layer 61 or the second wavelength adjustment layer 62 formed as described above, the upper phase adjustment layer 56, the upper Bragg reflector 24 (above the wavelength adjustment region 25), and the contact layer 26 are provided. Crystals are grown in order. Thereafter, each layer is etched to the depth of the resonator 23 (etching region 29 is formed) to form a mesa. As the etching for forming the mesa, a dry etching method can be used. The shape of the mesa is not particularly limited. For example, the shape of the mesa may be a circular shape, an elliptical shape, a rectangular shape, a polygonal shape, or the like when viewed from above.

The current confinement layer made of AlAs exposed on the side surface of the mesa is changed to an insulating layer made of Al _x O _y which is a selective oxidation region by heat treatment in water vapor. Thereby, it is possible to form a current confinement structure that restricts the current path only to the current confinement region made of non-oxidized AlAs at the center. Thereafter, the protective layer 30 is formed on the insulating layer, and the polyimide layer 31 is formed on the protective layer 30, thereby planarizing the upper surface of the surface emitting laser 11. At this time, the protective layer 30 and the polyimide layer 31 are once formed on the contact layer 26. Thereafter, the protective layer 30 and the polyimide layer 31 on the contact layer 26 are removed, and an upper electrode 27 serving as a p-side electrode is formed on the contact layer 26. If SiN is used as the protective layer 30, the side and bottom surfaces of the layer containing Al that is easily corroded and exposed by etching when the mesa is formed can be protected with a dielectric, so that the reliability can be improved.

  FIG. 8 is a diagram illustrating the position of the abdominal node in the longitudinal mode of the wavelength adjustment region 25 of the first surface-emitting laser 11A according to the first embodiment. The first wavelength adjustment layer 61 and the second wavelength adjustment layer 62 are semiconductor layers. The refractive index of the semiconductor layer is significantly lower than that of the dielectric layer. Therefore, when the upper Bragg reflector 24 above the wavelength adjustment region 25 is a dielectric, the interface 81 between the wavelength adjustment region 25 and the upper Bragg reflector 24 needs to be arranged at the antinode position of the longitudinal mode. is there. This is because when the interface 81 is arranged at the node position, light is reflected at the interface 81 in a reverse phase. On the other hand, when the upper Bragg reflector 24 above the wavelength adjustment region 25 is a semiconductor, the wavelength adjustment function is not impaired by disposing the wavelength adjustment layers 61 and 62 at the node positions of the longitudinal mode. Reduction of the threshold current and the like can be realized. This is due to scattering loss caused by the difference between the refractive index 3.3 of GaInP included in the first wavelength adjustment layer 61 and the refractive index 3.5 of GaAsP included in the second wavelength adjustment layer 62. This is because the influence is reduced by arranging the wavelength adjustment layers 61 and 62 at the node positions.

  When the upper Bragg reflector 24 below the wavelength adjustment region 25 is a semiconductor, doping is necessary to make the wavelength adjustment region 25 electrically conductive. At this time, the electrical resistance increases due to a hetero spike or the like generated in the band structure of the wavelength adjustment layers 61 and 62. Therefore, in order to reduce the electrical resistance, it may be necessary to perform high concentration doping only on the wavelength adjustment layers 61 and 62. However, if high concentration doping is applied to the antinode position in the longitudinal mode, light absorption increases, and problems such as an increase in oscillation threshold threshold current and a decrease in slope efficiency may occur. Therefore, by arranging the wavelength adjusting layers 61 and 62 at the node positions, even if the wavelength adjusting layers 61 and 62 are subjected to high concentration doping, the electric resistance is not increased without causing an increase in oscillation threshold current, a decrease in slope efficiency, or the like. Can be reduced. As the electrical resistance decreases, the maximum light output increases.

  FIG. 9 is a diagram showing a replacement position of the wavelength adjustment region 25 in the upper Bragg reflector 24 according to the first embodiment. The lower Bragg reflector 22 and the upper Bragg reflector 24 according to the present embodiment have a structure in which low refractive index layers 85 and high refractive index layers 86 are alternately stacked. The wavelength adjustment region 25 according to the present embodiment is formed by replacing the high refractive index layer 86.

Both the low refractive index layer 85 and the high refractive index layer 86 are layers mainly composed of n-AlGaAs. The difference between the low refractive index layer 85 and the high refractive index layer 86 is in the Al content. The Al content of the low refractive index layer 85 is relatively large, and the Al content of the high refractive index layer 86 is relatively small. The low refractive index layer 85 is a layer containing, for example, n-Al _0.9 Ga _0.1 As as a main component. The high refractive index layer 86 is a layer containing, for example, n-Al _0.1 Ga _0.9 As as a main component. Each of the low refractive index layer 85 and the high refractive index layer 86 has an optical thickness of λ / 4, for example. The lower Bragg reflector 22 includes, for example, 35.5 pairs of a low refractive index layer 85 / a high refractive index layer 86. The upper Bragg reflector 24 between the resonator 23 and the wavelength adjustment region 25 is composed of, for example, five pairs of a low refractive index layer 85 and a high refractive index layer 86.

As shown in FIG. 9, the wavelength adjustment region 25 includes a lower phase adjustment layer 55, wavelength adjustment layers 61 and 62, and an upper phase adjustment layer 56. Similar to the high refractive index layer 86, the lower phase adjustment layer 55 and the upper phase adjustment layer 56 are layers mainly composed of p-AlGaAs having a relatively small Al content. The lower phase adjustment layer 55 and the upper phase adjustment layer 56 may be layers mainly composed of, for example, n-Al _0.16 GaAs.

  When the wavelength adjustment layers 61 and 62 are composed of three layers of the first wavelength adjustment layer 61 (p ++ GaInP) / the second wavelength adjustment layer 62 (p ++ GaAsP) / the first wavelength adjustment layer 61 (p ++ GaInP), the wavelength adjustment is performed. The optical thickness of the entire region 25 is 3λ / 4. At this time, the optical thickness from the lower end of the wavelength adjustment region 25 to the center position in the stacking direction of the second wavelength adjustment layer 62 at the center is λ / 2. Further, when the optical thickness of the entire wavelength adjustment region 25 is 5λ / 4, the optical thickness from the lower end of the wavelength adjustment regions 61 and 62 to the central portion of the central second wavelength adjustment layer 62 in the stacking direction. Becomes λ / 2 or λ. When the optical thickness of the entire wavelength adjustment region 25 is 7λ / 4, the optical thickness from the lower end portion of the wavelength adjustment region 25 to the central portion of the central second wavelength adjustment layer 62 in the stacking direction is λ. / 2, λ, or 3λ / 2.

  That is, when λ is the wavelength of the laser beam, N is an integer of 1 or more, and M is an integer of 1 to N, the optical thickness of the wavelength adjustment region 25 is calculated by (2N + 1) × λ / 4. . The optical thickness from the lower end of the wavelength adjustment region 25 (the end on the resonator 23 side) to the center position in the stacking direction is calculated from the lower end of the wavelength adjustment region 25 by M × λ / 2.

  In many cases, the thicknesses T1 and T2 of the wavelength adjustment layers 61 and 62 are usually 5 nm or more due to the limitations of the film formation technique. The adjustment of the number of the wavelength adjustment layers 61 and 62 is usually intended to finely adjust the oscillation wavelength due to a film formation error. Therefore, the optical thickness per layer of the wavelength adjustment layers 61 and 62 is 0. .1λ or less is appropriate.

  As described above, by replacing the high refractive index layer 86 of the upper Bragg reflector 24 with the wavelength adjustment region 25, it is not necessary to use a crystal containing a large amount of Al as the material of the wavelength adjustment layers 61 and 62. Thereby, oxidation, corrosion, etc. can be prevented at the time of selective etching of the wavelength adjusting layers 61, 62, and the reliability of the surface emitting laser element 1 can be improved.

  As described above, according to the present embodiment, the thickness T1 of the first wavelength adjustment layer 61 containing indium having a large influence on etching is larger than the thickness T2 of the second wavelength adjustment layer 62 not containing indium. Thinly formed. Thereby, even when a plurality of surface emitting lasers 11A to 11D having different thicknesses T of the wavelength adjusting region 25 are formed in one surface emitting laser element 1, variation in the depth of the etching region 29 is suppressed. Therefore, it becomes possible to generate high-quality laser light.

  Other embodiments will be described below with reference to the drawings. However, the same or similar portions as those in the first embodiment may be denoted by the same reference numerals and the description thereof may be omitted.

(Second Embodiment)
FIG. 10 is a diagram illustrating the position of the abdominal node in the longitudinal mode of the wavelength adjustment region 91 of the first surface emitting laser 11A according to the second embodiment. FIG. 11 is a diagram showing a replacement position of the wavelength adjustment region 91 in the upper Bragg reflector 24 according to the second embodiment.

  The wavelength adjustment region 91 according to the present embodiment is formed by replacing the low refractive index layer 85 of the upper Bragg reflector 24.

The wavelength adjustment region 91 according to the present embodiment includes a lower phase adjustment layer 95, wavelength adjustment layers 97 and 98, and an upper phase adjustment layer 96. Similar to the low refractive index layer 85, the lower phase adjustment layer 95 and the upper phase adjustment layer 96 are layers mainly composed of p-AlGaAs having a relatively high Al content. The lower phase adjustment layer 95 and the upper phase adjustment layer 96 may be layers mainly composed of, for example, n-Al _0.7 Ga _0.3 As.

The first wavelength adjustment layer 97 is, for example _p ++ _(Al 0.7 _{Ga 0.3)} is a layer composed mainly of _{0.5 In} 0.5 P. The second wavelength adjustment layer 98 is a layer containing, for example, p ++ Al _0.7 Ga _0.3 As as a main component. When the wavelength adjustment layers 97 and 98 are composed of three layers of the first wavelength adjustment layer 97 / the second wavelength adjustment layer 98 / the first wavelength adjustment layer 97, the optical thickness of the entire wavelength adjustment region 91 is 3λ / 4. At this time, the optical thickness from the lower end of the wavelength adjustment region 91 to the central position in the stacking direction of the second wavelength adjustment layer 98 at the center is λ / 4. When the optical thickness of the entire wavelength adjustment region 91 is 5λ / 4, the optical thickness from the lower end portion of the wavelength adjustment region 91 to the central portion in the stacking direction of the central second wavelength adjustment layer 98 is λ. / 4 or 3λ / 4. When the optical thickness of the entire wavelength adjustment region 25 is 7λ / 4, the optical thickness from the lower end portion of the wavelength adjustment region 91 to the central portion in the stacking direction of the central second wavelength adjustment layer 98 is λ. / 4, 3λ / 4, or 5λ / 4.

  That is, when λ is the wavelength of the laser beam, N is an integer of 1 or more, and M is an integer of 0 or more and (N−1) or less, the optical thickness of the wavelength adjustment region 91 is (2N + 1) × λ / 4. Is calculated by Further, the optical thickness from the lower end of the wavelength adjustment region 91 to the center position in the stacking direction is calculated by (2M + 1) × λ / 4.

As described above, when the low refractive index layer 85 of the upper Bragg reflector 24 is replaced with the wavelength adjustment region 91, the combination of the first wavelength adjustment layer 97 and the second wavelength adjustment layer 98 is, for example, ( Al _0.7 Ga _0.3) becomes and _0.5 an in _0.5 P and _Al 0.7 _{Ga 0.3} as. The refractive index of (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P and the refractive index of Al _0.7 Ga _0.3 As are both about 3.1. Therefore, according to the second embodiment, in addition to the effect of suppressing the variation in the depth of the etching region 29, the effect of reducing the scattering loss in the wavelength adjustment region 91 can be obtained.

(Third embodiment)
FIG. 12 is a diagram illustrating a configuration of an atomic oscillator 101 according to the third embodiment. The atomic oscillator 101 is an oscillator using a CPT (Coherent Population Trapping) system that excites atoms by light obtained by modulating the laser light emitted from the surface emitting laser element 1 according to the first or second embodiment. is there. The atomic oscillator 101 includes a light source 111, a collimating lens 112, a λ / 4 wavelength plate 113, an alkali metal cell 114 (encapsulation unit), a photodetector 115 (detection unit), and a modulator 116 (modulation unit).

  The light source 111 is a device that includes the surface-emitting laser element 1 according to the first or second embodiment and emits light obtained by modulating the laser light emitted from the surface-emitting laser element 1. The alkali metal cell 114 is filled with Cs atomic gas as an alkali metal. In CPT, for example, the transition of the D1 line of Cs can be used. The photodetector 115 is a device that detects the amount of light transmitted through the alkali metal cell 114. The photodetector 115 is configured using, for example, a photodiode. The modulator 116 is a device that modulates the laser light emitted from the surface emitting laser element 1 based on the detection result of the photodetector 115.

  Light emitted from the light source 111 and adjusted by the collimating lens 112 and the λ / 4 wavelength plate 113 is irradiated to the alkali metal cell 114. The light excites Cs electrons. The amount of light transmitted through the alkali metal cell 114 is detected by the photodetector 115. The detection signal of the photodetector 115 is fed back to the modulator 116. The modulator 116 modulates the laser light of the surface emitting laser element 1 in the light source 111 based on the feedback.

  FIG. 13 is a diagram illustrating an example of a structure of atomic energy levels related to CPT. When electrons are excited from the two ground levels to the excited level simultaneously, electromagnetically induced transmission in which the light absorption rate decreases occurs. The surface emitting laser element 1 according to the present embodiment emits a laser beam having a carrier wave wavelength close to 894.6 nm. The carrier wave wavelength can be tuned by changing the temperature or output of the surface emitting laser element 1. When the temperature and output are increased, the carrier wavelength shifts to the longer wavelength side. It is preferable to use a temperature change for tuning the carrier wavelength. The temperature dependence of the wavelength is about 0.05 nm / ° C.

  FIG. 14 is a diagram illustrating an example of a side band generated when laser light is modulated. By modulating the laser light, sidebands are generated on both sides of the carrier wavelength. In this example, a state is shown in which the frequency difference between the carrier wave wavelength and the sideband is 4.6 GHz so as to correspond to 9.2 GHz which is the natural frequency of the Cs atom.

  FIG. 15 is a diagram illustrating an example of the relationship between the modulation frequency and the amount of transmitted light. The amount of light passing through the excited Cs atom gas is maximized when the sideband frequency difference matches the natural frequency difference of Cs atoms. The modulator 116 modulates the laser light emitted from the surface emitting laser element 1 of the light source 111 so that the light amount detected by the photodetector 115 maintains the maximum value. The modulation frequency obtained at this time is a stable value because the natural frequency of the Cs atom is extremely stable. The acquired modulation frequency is output to the outside and used as a reference frequency or the like. In addition, when the transition of the wavelength 894.6 nm of the Ds line of Cs is used, the wavelength range of the light emitted from the light source 111 needs to be 894.6 nm ± 1 nm. That is, light having a wavelength in the range of 893.6 nm to 895.6 nm is required.

  In the above embodiment, the wavelength of the Cs D1 line is used, but the wavelength of 852.3 nm of the Cs D2 line can also be used. Rb can also be used as the alkali metal. When Rb is used, the transition of the wavelength 795.0 nm of the D1 line or the wavelength 780.2 nm of the D2 line can be used. The material composition of the active layer 52 of the surface emitting laser 11 can be designed according to these wavelengths. When 87Rb is used, the laser beam may be modulated at 3.4 GHz, and when 85Rb is used, the laser beam may be modulated at 1.5 GHz. Even when these wavelengths (852.3 nm, 795.0 nm, or 780.2 nm) are used, the wavelength range of light emitted from the light source 111 needs to be these wavelengths ± 1 nm. That is, when the Cs D2 line is used, light having a wavelength in the range of 851.3 nm to 853.3 nm is required. When the Rb D1 line is used, light having a wavelength in the range of 794.0 nm to 796.0 nm is required. When the Rb D2 line is used, light having a wavelength in the range of 779.2 nm to 781.2 nm is required.

  According to the atomic oscillator 101 according to the present embodiment, since the surface-emitting laser element 1 according to the first or second embodiment is used as the light source 111, an accurate frequency is obtained based on high-quality laser light. It can be acquired.

  Although the embodiments of the present invention have been described above, the above-described embodiments are presented as examples and are not intended to limit the scope of the invention. The novel embodiment can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. This embodiment and its modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 Surface emitting laser element 11 Surface emitting laser 11A 1st surface emitting laser 11B 2nd surface emitting laser 11C 3rd surface emitting laser 11D 4th surface emitting laser 21 Semiconductor substrate 22 Lower Bragg reflector 23 Resonator 24 Upper part Bragg reflector 25, 91 Wavelength adjustment region 26 Contact layer 27 Upper electrode 28 Lower electrode 29 Etching region 30 Protective layer 31 Polyimide layer 32 Exit port 41 First light emitting region 42 Second light emitting region 43 Third light emitting region 44 4 light emitting region 45 first electrode pad 46 second electrode pad 47 third electrode pad 48 fourth electrode pad 51 lower spacer layer 52 active layer 53 upper spacer layer 55, 95 lower phase adjusting layer 56, 96 upper Phase adjustment layer 61, 97 First wavelength adjustment layer 62, 98 Second wavelength adjustment layer 70 Regis G 81 Interface 85 Low refractive index layer 86 High refractive index layer 101 Atomic oscillator 111 Light source 112 Collimate lens 113 λ / 4 wave plate 114 Alkali metal cell 115 Photo detector 116 Modulator

JP 2013-138176 A

Claims (10)

A lower reflector,
An upper reflecting mirror disposed at a position closer to the exit of the laser beam than the lower reflecting mirror;
A resonator disposed between the lower reflecting mirror and the upper reflecting mirror to resonate light;
A wavelength adjustment region that is formed in the upper reflector and adjusts the wavelength of the light;
With
The wavelength adjustment region includes a first wavelength adjustment layer containing indium and a second wavelength adjustment layer not containing indium,
The thickness of the first wavelength adjustment layer is thinner than the thickness of the second wavelength adjustment layer,
Surface emitting laser element. The upper reflector is configured by alternately laminating low refractive index layers and high refractive index layers,
The wavelength adjustment region is configured by replacing the high refractive index layer,
The surface emitting laser element according to claim 1. The upper reflector is configured by alternately laminating low refractive index layers and high refractive index layers,
The wavelength adjustment region is configured by replacing the low refractive index layer,
The surface emitting laser element according to claim 1. When λ is the wavelength of the laser beam, N is an integer of 1 or more, and M is an integer of 1 to N,
The optical thickness of the wavelength adjustment region is calculated by (2N + 1) × λ / 4,
The optical thickness from the lower end of the wavelength adjustment region to the center position in the stacking direction is calculated by M × λ / 2.
The surface emitting laser element according to claim 2. When λ is the wavelength of the laser beam, N is an integer of 1 or more, and M is an integer of 0 or more and (N−1) or less,
The optical thickness of the wavelength adjustment region is calculated by (2N + 1) × λ / 4,
The optical thickness from the lower end of the wavelength adjustment region to the center position in the stacking direction is calculated by (2M + 1) × λ / 4.
The surface emitting laser element according to claim 3. The optical thickness per layer of the first wavelength adjustment layer and the second wavelength adjustment layer is 0.1λ or less.
The surface emitting laser element according to claim 4 or 5. The first wavelength adjustment layer is mainly composed of AlGaInP,
The second wavelength adjustment layer is mainly composed of AlGaAs.
The surface emitting laser element according to claim 1. A plurality of wavelength adjustment regions having different thicknesses from each other;
Between the plurality of wavelength adjustment regions, the number of the first wavelength adjustment layer or the second wavelength adjustment layer is different,
The surface emitting laser element according to claim 1. A light source comprising the surface-emitting laser element according to claim 1;
An encapsulating part enclosing an alkali metal;
A detection unit for detecting the amount of light emitted from the light source and transmitted through the enclosing unit;
A modulation unit that modulates laser light emitted from the surface-emitting laser element so that the alkali metal maintains an electromagnetically induced transmission state based on the detected light amount;
An atomic oscillator comprising: The alkali metal is rubidium or cesium,
The atomic oscillator according to claim 9.

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