JP2017017276A - Wavelength-variable laser, information acquisition device, and imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength-variable laser having a wide wavelength sweeping band by single element.SOLUTION: A wavelength-variable laser includes: a pair of reflection mirrors 102 and 113; active layers 105 and 106 arranged between the pair of reflection mirrors; and driving means deviating at least one of the pair of reflection mirrors. The active layers include a first active region 105 which profits for a first profit wavelength region and a second active region 106 which profits for a second profit wavelength region. The first profit wavelength region and second profit wavelength region are at least partially overlapped, and the reflection mirror has a reflection rate that a reflection spectrum can oscillate in the first and second profit wavelength regions. The wavelength-variable laser is constructed so that a first resonance optical and a second resonance optical which is derived from the first active region and the second active region, and can be oscillated is resonated in a resonator formed by the pair of reflection mirrors at a region where overlaps a wavelength region so as to resonate while phase synchronizing.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a wavelength tunable laser, an information acquisition device, and an imaging device.

  In recent years, tunable lasers have been actively studied. There are various types of wavelength tunable lasers, and one of them is a wavelength tunable laser using a MEMS (Micro Electrical Mechanical Systems) mechanism. For example, there is a type in which a MEMS mechanism is attached to a mirror of a surface emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser), and the wavelength can be swept by changing the resonator length by moving the mirror. The laser using the MEMS mechanism is characterized in that the sweep wavelength region is wider than other systems. Various MEMS driving methods have been proposed, such as those using electrostatic force and those using thermal distortion.

  Non-Patent Document 1 discloses a laser element using such a MEMS mechanism, in which a MEMS mechanism is added to one mirror of a VCSEL, and one mirror is moved up and down using electrostatic force. One application of a wavelength tunable laser using such a MEMS mechanism is an optical coherence tomography apparatus (OCT apparatus: Optical Coherence Tomography) mainly used for ophthalmologic diagnosis. There are various types of OCT, but a tunable laser is used in OCT called SS-OCT (Swept Source-OCT), which performs measurement while sweeping the wavelength of a light source at high speed. This has advantages such as high speed and high S / N ratio compared to other methods, and is attracting attention as the next generation OCT. In OCT, the wider the wavelength variable range, the more advantageous the depth resolution of the obtained image, which is advantageous.

Proceedings of SPIE Vol. 8276, 82760P (2012)

  In general, it can be said that a wavelength tunable laser has a wider application range and is excellent as a light source as the wavelength tunable range is wider. Therefore, development has been performed to obtain a wider tunable range. An object of the present invention is to provide a wavelength tunable laser or the like having a wide wavelength tunable range using mirror driving means such as a MEMS mechanism.

  The wavelength tunable laser according to the present invention includes a pair of reflecting mirrors, an active layer disposed between the pair of reflecting mirrors, and a drive unit that displaces at least one of the pair of reflecting mirrors. It is. The active layer includes a first active region having a gain in a first gain wavelength region and a second active region having a gain in a second gain wavelength region, wherein the first gain wavelength region and the first gain wavelength region The two gain wavelength regions overlap at least partially. The reflection mirror has a reflectance that allows the reflection spectrum to oscillate in the first gain wavelength region and the second gain wavelength region. Then, the oscillating first resonant light and second resonant light respectively derived from the first active region and the second active region are overlapped by the gain wavelength region, and the pair of reflecting mirrors. In the formed resonator, the resonators oscillate while resonating in phase with each other.

  According to the present invention, instead of using a method of separately driving a plurality of elements having different wavelength tunable ranges, the wavelength tunable range is widened by using appropriate phase synchronization means in one element, so that the wide A single laser element having a wavelength tunable range can be obtained.

FIG. 3 is a schematic cross-sectional view showing an element configuration of an embodiment of a wavelength tunable laser. The figure which shows the gain distribution of the active layer with respect to the wavelength of one Embodiment of a wavelength tunable laser. The figure which shows the relationship between the intensity distribution of the resonant light of a laser element, and the diameter of a current confinement part. The figure which shows the relationship between the energy of the active layer of an asymmetric quantum well structure, and a film thickness direction position. The figure which shows intensity distribution of the resonant light with respect to the wavelength of one Embodiment of a wavelength variable laser. The schematic cross section which shows the element structure of other embodiment of a wavelength variable laser. The figure which shows the light intensity distribution of the resonant light of the active layer vicinity of other embodiment of a wavelength variable laser. The figure which shows the pattern of the phase adjustment layer introduction of other embodiment of a wavelength variable laser. 1 is a schematic top view showing an element configuration of a first embodiment of the present invention. 1 is a schematic cross-sectional view showing an element configuration of a first example of the present invention. FIG. FIG. 5 is another schematic cross-sectional view showing the element configuration of the first embodiment of the present invention. The figure which shows the example of the gain distribution in the active layer of an asymmetric quantum well structure. The schematic cross section which shows the element structure of the 2nd Example of this invention. FIG. 6 is another schematic cross-sectional view showing the element configuration of the second embodiment of the present invention. The schematic cross section which shows the element structure of the 3rd Example of this invention. The figure which shows the structure of embodiment of a wavelength sweep type | mold optical coherence tomography apparatus.

  In the following embodiments and examples, in a single-element tunable laser having a wide wavelength sweep band, a plurality of active regions are designed so that at least a part of the gain wavelength region overlaps. A laser element capable of continuous wavelength sweeping in a wide band can be obtained by phase-synchronizing oscillating resonant lights from a plurality of active regions in the overlapping region.

(First embodiment)
A first embodiment of the present invention will be described. FIG. 1 is a conceptual diagram showing an element configuration of a first embodiment of a surface-emitting type wavelength tunable laser. FIG. 1 shows a schematic cross section showing the main configuration of the entire device. Reference numeral 101 denotes a substrate. A lower reflecting mirror 102 and a lower cladding layer 103 are provided on the substrate in a direction perpendicular to the substrate, and a lower spacer layer 104 is laminated thereon. On top of that, an active layer including a plurality of active regions is stacked, and the first active layer 105 as the first active region and the second active layer as the second active region in the same plane. It is divided into 106 and stacked. Further thereon, an upper spacer layer 107 and an upper cladding layer 108 are laminated in this order. A current confinement layer 109 for confining the drive current is provided in the upper clad layer 108, and a first current confinement portion 110 and a second confinement portion 111 are formed. The first current confinement part 110 and the second current confinement part 111 are located above the first active layer 105 and the second active layer 106, respectively. An upper reflection mirror 113 is provided above the upper cladding layer 108 with an air gap 112 interposed therebetween. The upper reflecting mirror 113 is supported by a MEMS beam structure (not shown), and has a movable mirror structure that moves up and down in response to external force such as electrostatic attraction. The detailed structure of the MEMS beam structure is shown in FIGS. 9-1 to 9-3.

  The wavelength tunable laser of the present embodiment is a surface-emitting type wavelength tunable laser having a resonator formed between a pair of reflecting mirrors. This is a MEMS VCSEL in which the resonator length is changed and the oscillation wavelength is changed by moving at least one of the pair of mirrors up and down by the MEMS beam structure as described above. By controlling the external force applied to the movable mirror in the MEMS structure, the wavelength of the oscillation light can be arbitrarily changed.

  In the wavelength tunable laser according to the present embodiment, the first gain wavelength region derived from the first active layer (the wavelength region having a gain equal to or greater than the threshold gain) and the second gain wavelength region derived from the second active layer. The laser oscillates at an oscillation wavelength in any wavelength region (including a region where they overlap). In this embodiment, the oscillation light in the first gain wavelength region is emitted from the first current confinement unit 110 from the first active layer, and the oscillation light in the second gain wavelength region is emitted from the second active layer. Originated and emitted from the second current constriction 111. Normally, elements that oscillate from different active layers behave as individual laser elements, but by adopting the configuration of the present embodiment, it is possible to drive the same wavelength variable laser element. Furthermore, in this embodiment, it is possible to obtain a wide variable wavelength range in one element by partially overlapping the gain wavelength region of the first active layer and the gain wavelength region of the second active layer.

(principle)
Hereinafter, the principle of the wavelength tunable laser according to this embodiment will be described. FIG. 2 is a schematic diagram showing the gain distribution of each active region of the wavelength tunable laser according to the present embodiment. 201 is a curve representing the first gain wavelength region due to the first active layer, 202 is a curve representing the second gain wavelength region due to the second active layer, and 203 is a gain necessary for oscillation (threshold gain). A line 204 represents a gain wavelength region in an overlapping portion of gains equal to or higher than the threshold gain. Since each active layer oscillates with a gain equal to or higher than the threshold gain, oscillation derived from the first resonance light that can be oscillated by the first active layer occurs in the first gain wavelength region, and the second gain wavelength region. Then, oscillation derived from the second resonance light that can be oscillated by the second active layer occurs. In a wavelength region (also referred to as an overlapping wavelength region) where the gain is equal to or higher than the threshold gain, oscillation derived from both resonance lights can occur. Ordinarily, in the oscillation occurring in this overlapping wavelength region, the phases of the laser beams are not aligned, and each oscillation behaves as if it were oscillating as a separate laser. With respect to the separately oscillated laser beams, the phase of the laser beams can be used to align the respective phases, thereby creating a situation where the two lasers oscillate as a single laser. In the present embodiment, specifically, the distance between the first and second current confinement portions 110 and 111 of the current confinement layer in which the two laser beams oscillate is made closer to the wavelength order of the laser beam, thereby A phase synchronization phenomenon can be caused.

  Usually, a laser using this phase synchronization (also referred to as phase-locked laser) is used for phase-locking laser beams having the same oscillation wavelength and operating as a high-power single laser. In the present embodiment, the wavelength region of the active layer is intentionally shifted and partially overlapped, and the phase synchronization phenomenon occurs only in the overlapping wavelength region. Thus, the same laser is operated even in the overlapping wavelength region. In this way, if the overlapping wavelength region of gain is kept small, a laser having a wide wavelength region, which is almost the sum of the original wavelength regions in the respective active layers, can be realized as a single element.

  The oscillation wavelength can be strictly controlled by the resonator length. For example, when the resonator length is controlled by moving the upper reflection mirror 113 by the MEMS beam structure so that the oscillation wavelength gradually changes from the short wavelength side to the long wavelength side, the first current confinement part 110 first Short wavelength light oscillates. When the oscillation wavelength is lengthened and the gain overlapping wavelength region 204 is reached, oscillation occurs from both the first and second current confinement portions 110, but here also operates as a single laser due to the phase synchronization phenomenon described above. When the wavelength is further increased and the gain wavelength region of the second active layer 106 is reached, the second current confinement portion 111 oscillates. Thus, since it functions as a single laser in all gain wavelength regions, it is possible to obtain a laser device having a wide band as a whole.

(Design method)
FIG. 3 is a schematic diagram showing a state of the optical amplitude distribution (intensity distribution) of the resonance light in the present embodiment. 301 represents the amplitude distribution of the first resonance light, and the shaded portion 302 represents the diameter of the first current confinement portion 110. A shaded area 303 represents the diameter of the second current confinement portion 111, and a numeral 304 represents the amplitude distribution of the second resonance light. In order to generate phase synchronization, it is necessary to place the resonant lights close to each other so that the lights are coupled to each other. In the present embodiment, it is the position of the current confinement layer 109 that determines the mode field of the resonant light. In the first and second current confinement portions 110 and 111 of the current confinement layer 109, a current flows only in those portions and a gain is generated. Usually, the confinement layer itself has a low refractive index in a high refractive index semiconductor layer. Therefore, optical confinement occurs due to a difference in refractive index. Therefore, the current confinement layer 109 can be regarded as a kind of waveguide structure by gain waveguide or refractive index waveguide. The shaded portions 302 and 303 in FIG. 3 are images of the waveguide.

Considering the above-mentioned waveguide, the resonant light in the constricted structure can also be regarded as a kind of guided light, and the theory of waveguide coupling can be applied when discussing the coupling between the resonant lights. . Specifically, the degree of coupling changes according to how much the skirt portion of the resonant light in FIG. 3 overlaps the adjacent waveguide. In this embodiment, the length of the bottom of the light is determined using parameters such as the resonator length of the laser element, the refractive index of the semiconductor constituting the element, the thickness of the current confinement layer 109, the diameter of the constrictions 110 and 111, and the like. The overlap, that is, the degree of coupling can be controlled by the length of the bottom and the distance between the waveguides. As an index representing the degree of coupling, there is a coupling coefficient as described in the literature (ELECTRONICS LETERS 21st June 1990 Vol. 26 No. 13 pp 896 (1990)). Another document (IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-21, NO. 5, pp458 (1985)) shows that stable phase synchronization is realized when this coupling coefficient has a value of 10 ⁻⁵ or more. Reporting.

For example, the resonator length is 3.5λ, the refractive index of the semiconductor portion is 3.4, the refractive index of the oxide layer (current confinement layer) is 1.5, the layer thickness is 0.0286λ, the confinement portion is circular and the diameter is 2. Consider the case of 86λ. First, the optical amplitude distribution of the resonant light is calculated considering the case where the resonant light exists alone. Assuming that the distance between the constricted portions is 1.43λ, assuming that the amplitude distributions of the respective lights are close to each other, calculating the overlap integral between the light amplitude distribution and the waveguide structure portion, the coupling efficiency is Can be obtained. In the case of the above conditions, the coupling efficiency is calculated to be 9 × 10 ⁻⁵ . Here, λ indicates a wavelength within ± 20% from the center wavelength of the region covered by the two gain wavelength regions. More preferably, it refers to a wavelength in the range of ± 10%, more preferably ± 5%. Since the coupling coefficient is 10 ⁻⁵ or more, it is considered that stable phase synchronization is realized.

(Laser type)
In this embodiment, the laser element has a VCSEL configuration. However, for example, an external resonator type laser configuration in which a gain chip is installed in the resonator as an active medium and a resonator is formed by an external mirror can be used. is there. In this case, a wavelength tunable function can be provided by arranging a tunable bandpass filter having a narrow band inside the resonator.

(material)
In this embodiment, the material used is a GaAs / InGaAs / GaAsP group III-V group compound semiconductor. In addition, III-V group semiconductors such as GaAs / AlGaAs series and InGaAsP / InP series, III-V nitride semiconductors such as GaN / AlGaN / InGaN, and II-VI group semiconductors such as ZnSe can also be used. The same material can be used for a gain chip as an active medium.

(Type of active layer)
The active layer structure that can be used in this embodiment will be described. The active layer in the present embodiment has a structure in which the first active layer and the second active layer are arranged in the same plane. This active layer is obtained by growing separate active layers in the same plane. Since the first and second resonance lights are emitted from the first and second active layers, respectively, it is necessary to provide a desired active layer corresponding to a position where light having a desired wavelength is desired. As a specific arrangement, for example, the boundary portion of the active layer is positioned between the respective resonant lights, and the resonant light and the active layer are made to correspond to each other. As a configuration for changing the current injection amount for each constriction, which will be described later, for example, an insulating layer extending in the vertical direction between the current confinement layer 109 and the active layer in FIG. 1 may be formed along such a boundary. .

  The two active regions forming the first and second gain wavelength regions may be active layers having different structures, but they may be formed of asymmetric quantum wells having the same structure in order to manufacture the active regions more simply and at low cost. it can. FIG. 4 is a schematic diagram showing the spatial position (x) and energy (E) of the asymmetric quantum well structure. 401 is a well layer and 402 is a barrier layer. “Asymmetric” is an expression for “symmetric” multi-quantum wells, where each well layer or barrier layer has a different well width and / or depth, or different barrier layer width and / or barrier layer height. ”And“ the strain amount of the well layer and / or the barrier layer is different ”. That is, an asymmetric quantum well structure layer satisfies at least one of these conditions. For example, as shown in FIG. 4, a plurality of different active layers are asymmetrically stacked in the layer thickness direction of the substrate as opposed to a “symmetrical” multiple quantum well structure composed of only the same well layer. This is called an asymmetric multiple quantum well. This asymmetric quantum well active layer has the property that the gain distribution varies greatly depending on the carrier injection density and the carrier distribution. By utilizing this, for example, by adopting a configuration in which the injection current can be changed in conjunction with changing the resonator length in accordance with the oscillation wavelength, the substantial gain felt by the first and second resonant lights can be realized. A difference can be made to produce an effect. The change in the injection current to the active layer is performed by increasing or decreasing the diameters of the two constriction portions or changing the injection current to the element. Details will be described with reference to the example of FIG.

(Granting method)
In the present embodiment, a method of applying gain to the active layer uses current injection through a current confinement portion provided in the vicinity of the active layer. The narrowed portions 110 and 111 can be formed by using a method such as selective oxidation of a semiconductor layer by water vapor or the like, or high resistance by ion implantation. It is also possible to adopt a light excitation type (see the example of FIG. 12) in which excitation light is condensed and irradiated from an external light source, and laser oscillation is performed at the condensing part. In this case, the current confinement structure is not necessarily required, and the size of the excitation light irradiation spot performs the same function as the current confinement portion. The waveguide structure can be regarded as a gain waveguide type waveguide structure because gain occurs but is not confined by the refractive index.

(Upper and lower mirrors)
In the present embodiment, it is necessary to widen the high reflection band of the pair of mirrors as in the gain wavelength region. For example, a semiconductor multilayer film made of compound semiconductor AlAs / GaAs (50 pairs) functions as a mirror having a reflectance of 99% or more in a wavelength band of 100 nm or more when the center wavelength is 1060 nm. If the multilayer mirror is formed of a dielectric, the reflection wavelength region can be further expanded. In recent years, it has been reported that a structure in which a grating structure is provided at a pitch of about half the wavelength on a high refractive index layer having a thickness of about 1/5 of the wavelength also functions as a mirror having a single broadband reflectance. ing. This mirror is called HCG (High Contrast Grating) and can also be used in this embodiment. A resonator can be configured by arbitrarily combining the mirrors listed above.

(MEMS system)
As the MEMS structure for moving the mirror, an electrostatic force driving method, a driving method using a piezoelectric element, a thermal driving method using a difference in thermal expansion coefficient, or the like can be used.

(Control of wavelength overlap region)
A secondary function of the laser element in this embodiment will be described. FIG. 5 is a schematic diagram showing the relationship between the wavelengths of the first and second resonance lights and the optical output. Reference numeral 501 is a curve representing the light emission intensity from the first resonance light, 502 is a curve representing the light emission intensity from the second resonance light, and 503 is a curve representing both combined light emission intensities. As shown in FIG. 5, in the overlapping wavelength region of the light emission intensity, the respective light intensities are integrated and the intensity is increased. Therefore, as a whole laser element, a combined light intensity as shown in 503 is obtained. The shape of the curve 503 can be changed by controlling the size of the overlapping wavelength region of the light emission intensity. When the overlapping wavelength region is wide, it is possible to obtain a shape in which 503 has a high strength at the center of the wavelength region and a low strength at the end. In addition, when the overlapping wavelength region is narrowed, the light emission intensity of each resonance light is overlapped at a weak portion (bottom portion of the light emission intensity), and the light intensity with less unevenness can be obtained over the entire wavelength region. . If the overlapping region is further narrowed so that it overlaps only in the wavelength region at the bottom of the skirt, it is possible to obtain two combined light intensities.

(Production method)
The laser element in the present embodiment can be manufactured using a normal semiconductor laser manufacturing technique (epitaxial crystal growth, lithography, dry etching, vapor deposition, lift-off, water vapor oxidation, ion implantation, etc.).

(Second Embodiment)
A second embodiment of the present invention will be described. FIG. 6 is a conceptual diagram showing the element configuration of the second embodiment. In the present embodiment, all components from the substrate 601 to the upper reflection mirror 613 are the same as those in the first embodiment. In the present embodiment, unlike the first embodiment, the first and second active layers are not on the same plane but are separated in different planes, and the upper side of the substrate 601 is the first active layer. The layer 605 and the lower side are the second active layer 606. Furthermore, in this embodiment, in the resonator on the first current confinement portion 610 side, there are phase adjustment layers on both sides of the active layer. That is, the upper phase adjustment layer 615 is provided on the upper side, and the lower phase adjustment layer 614 is provided on the lower side.

(Effect of 2nd Embodiment)
The effect brought about by the phase adjustment layer will be described with reference to FIG. FIG. 7 is a schematic conceptual diagram showing the positional relationship between the resonant light and the active layer in the vicinity of the active layer inside the resonator. The left side of FIG. 7 represents the upper side with respect to the substrate, and the right side represents the lower side with respect to the substrate. The first active layer 605 and the second active layer 606 are displaced in the vertical direction as shown in FIG. On the other hand, in the curve 703 representing the intensity distribution of the first resonance light and the curve 704 representing the intensity distribution of the second resonance light, the positions of the magnitudes (mountains and valleys) are the positions of the active layers 605 and 606. Deviation is the same as the deviation. This is because the position of the resonant light in the vicinity of the active layer is substantially shifted by the upper and lower phase adjustment layers in FIG. In the present embodiment, the upper phase adjustment layer 615 has a longer optical path length and the lower phase adjustment layer has a shorter optical path length on the first resonance light side than the second resonance light side. Therefore, the upper resonator length is shorter and the lower resonator length is longer on the second resonant light side than on the first resonant light side. It is assumed that the change is equal between the part of the resonator length that is substantially longer and the part that is shorter. The position of the resonance light in the vicinity of the active layers 605 and 606 is shifted by this change, and as a result, the first resonance light 703 is transferred to the first active layer 605 and the second resonance light 704 is changed to the second resonance light 704. A mountain is located in the second active layer 606. Accordingly, the first resonant light 703 comes to feel mainly the gain of the first active layer 605, and the second resonant light 704 comes to feel mainly the gain of the second active layer 606, and the active layers are separated vertically. Even if it is done, the same effect as the first embodiment can be obtained.

  The conditions of the phase adjustment layer in this embodiment will be described below. Changes in the optical path length of the second resonant light with respect to the first resonant light are denoted by Δu and Δd, respectively, above and below the active layer. Then, in this embodiment, Δu + Δd = Nλ / 2 (N: integer, | Δu |, | Δd | is greater than or equal to the resonator length of the upper resonator and the lower resonator so as not to change the wavelength of the resonant light. Must be). Δu = Nu · λ / 2 + δ, Δd = −Nd · λ / 2−δ (Nu, Nd: integer, 0 <| δ | <λ / 2). From Δu and Δd ≠ 0, Thus, it is necessary that both resonant lights always have an optical path difference. In particular, when N = 0, that is, Δu = −Δd, the necessary optical path length difference values Δu and Δd can be reduced without making them λ / 2 or more, respectively. Furthermore, it is preferable that the first active layer has a crest position with respect to the first resonance light, the second active layer has a node position, and the second resonance light has an inverse relationship with these, It is preferable that Δu = λ / 4 and Δd = −λ / 4. In this case, it is preferable that the distance between the first and second active layers is also λ / 4 apart in terms of the optical path length.

  With respect to the arrangement of the phase adjustment layer, variations other than those shown in this embodiment will be described with reference to FIG. FIG. 8 is a conceptual diagram for simply representing the position of the phase adjustment layer in the resonator, and shows an example of a part of the adjustment layer to be introduced. FIG. 8 shows a schematic diagram of a resonator sandwiched between a lower reflection mirror 602 and an upper reflection mirror 613. Since Δu and Δd ≠ 0, the phase adjustment layer is provided above and below the active layers 605 and 606, respectively, and the phase adjustment layer +615 has a phase adjustment layer that increases the optical path length with respect to the adjacent resonance light. The adjustment layer to be shortened is the phase adjustment layer 614. The adjustment layer located on the left side of the resonator represents the adjustment layer acting on the first resonance light, and the adjustment layer located on the right side represents the adjustment layer acting on the second resonance light.

  In the present embodiment, the upper phase adjustment layer is positive and the lower phase adjustment layer is negative with respect to the first resonant light. However, as in the example on the left side of FIG. It is also possible to replace it. In addition, as in the center example, the same effect can be obtained by inserting a phase adjustment layer in which the signs are changed above and below the active layer for both resonance lights. However, in this case, since the adjustment layer is included at the position of both resonance lights, the relative shift amount of the resonance light of each other is double that when the adjustment layer is inserted only on one side. Therefore, the same effect can be obtained by setting the adjustment amount so that the optical path length difference is half that of the case where the adjustment layer is provided only on one side. In addition, since it is sufficient if the optical path length difference is provided between the resonant lights, the same type of resonator is used for the upper and lower resonators of the first resonant light and the second resonant light as in the right example (two pluses in the example). ) It is also possible to put an adjustment layer. Also in the center and right examples in FIG. 8, it is possible to use one in which the adjustment layers are switched upside down or one in which the sign is inverted.

  A specific example of the phase adjustment layer actually used will be described. Examples of the phase adjustment layer + include a high refractive index semiconductor layer whose composition is changed in the same family in the compound semiconductor layer constituting the resonator, a different high refractive index semiconductor, and a dielectric. As the phase adjusting layer, a dielectric such as an oxide formed in a compound semiconductor or an air layer can be used. Furthermore, for a surface emitting laser element having an air gap in the resonator, it is possible to form a phase adjustment layer by cutting the semiconductor at the boundary between the semiconductor and the air gap to form a recess.

  The operation is as follows. The upper reflection mirror 613 is moved in the current injection state. Thereby, the light of the first or / and second resonance light having the wavelength at which the peak position reaches the first or second active layer oscillates with a large gain. This wavelength changes according to the change in the resonator length due to the movement of the upper reflecting mirror 613. In the overlapping wavelength region, the first and second resonant lights oscillate in phase synchronization. That is, when the resonance wavelength of the resonator length is a wavelength in the non-overlapping wavelength region of the gain distribution of the first active layer, the first resonance light of the wavelength is located at a substantially peak position in the first active layer. Oscillates. In addition, when the resonance wavelength of the resonator length is a wavelength in the non-overlapping wavelength region of the gain distribution of the second active layer, the second resonance light of the wavelength has a substantially peak position in the second active layer. Oscillates. Further, when the resonance wavelength of the resonator length is a wavelength in the overlapping wavelength region of the gain distribution of the first and second active layers, the first and second resonance lights having the wavelength are respectively the first and second resonance lights. The active layer oscillates with an approximate mountain position. At this time, the current confinement part is formed so that the first and second resonance lights are phase-synchronized. The phase adjustment layer (part) is provided as described above.

(First embodiment)
A first embodiment of the present invention will be described. FIG. 9A is a schematic top view showing the element structure of this example, FIG. 9-2 is an AA ′ schematic cross-sectional view showing the element structure of this example, and FIG. It is a BB 'schematic sectional view showing element structure. In this embodiment, the laser element has a doubly-supported MEMS VCSEL structure. Since the structure from the substrate 901 to the upper reflecting mirror 913 is the same as that of the first embodiment, the explanation thereof is omitted. In this embodiment, a back electrode 914 is provided on the back surface of the substrate of the element, an intermediate electrode 915 is provided on the upper cladding layer 908, and an upper electrode 917 is provided on the upper reflecting mirror 913. In FIG. 9C, reference numeral 918 denotes the boundary between the semicircular first and second active layers 905 and 906. The resonance light defined by the first current confinement part 910 is the first resonance light, and the resonance light defined by the second current confinement part 911 is the second resonance light.

  The back electrode 914 and the intermediate electrode 915 are electrodes for driving the VCSEL portion of the laser element. In this embodiment, the MEMS portion is driven by applying an electrostatic force, and the intermediate electrode 915 and the upper electrode 917 are used to drive the MEMS portion. In this embodiment, the intermediate electrode 915 is the ground. The air gap 912 is formed by removing a part of the sacrificial layer by an etching process, and the remaining sacrificial layer 916 serves as a column supporting a beam portion including the upper reflecting mirror 913. In this embodiment, the laser element has a mesa structure, and the side wall is covered with a passivation film (not shown).

The center oscillation wavelength λ of the laser of this example is 1100 nm. The material constituting each part is n-type GaAs for the substrate 901, n-type for the lower and upper cladding layers 903 and 908, and p-type Al _0.7 Ga _0.3 As. The lower and upper spacer layers 904 and 907 are undoped Al _0.25 Ga _0.75 As, the current confinement layer 909 is AlO _x , and the first and second current confinement portions 910 and 911 are p-Al _0.98 Ga. _0.02 As, the sacrificial layer 916 is undoped GaAs. The first active layer 905 has a three-layer multiple quantum well structure in which the well layer / barrier layer is In _0.27 Ga _0.73 As / Ga _0.22 As _0.75 P, and the second active layer 906 has a well The layer / barrier layer has a three-layer multiple quantum well configuration of In _0.30 Ga _0.70 As / Ga _0.28 As _0.72 P. The thickness of each well layer / barrier layer is 8 nm / 10 nm. The lower reflection mirror 902 is a multilayer mirror in which 30 pairs of λ / 4-thick semiconductor films of n-GaAs / AlAs are stacked, and the upper reflection mirror 913 is a 40-pair stacked mirror of p-GaAs / AlAs. The resonator length is an optical path length of 3.5λ, and the optical path length of the semiconductor material portion is 2.25λ.

In this embodiment, the thickness of the oxidized constriction layer (current confinement layer) 909 is 30 nm, the constrictions 910 and 911 are circular with a diameter of 3 μm, and the distance between the constrictions is 4.5 μm between the centers of the constrictions. is there. When the optical coupling coefficient is calculated based on these parameters, it becomes 0.9 × 10 ⁻⁴ , and the above 10 ⁻⁵ or more is obtained, so that phase synchronization can be obtained stably.

  This laser element is driven by current injection to use the entire gain wavelength region of the first active layer and the second active layer, and the overlapping wavelength region portion as a whole is one wavelength swept laser by phase synchronization. Function. When the position of the MEMS both-end supported beam is displaced and the wavelength is swept between the short wavelength side and the long wavelength side, the phase is also irregular due to the phase synchronization even when light reaches the overlapping part of the gain wavelength region. Will not change. In this way, a constant phase relationship can be maintained between both the resonant light of the first resonant light and the second resonant light.

As described above, the coupling coefficient needs to be 10 ⁻⁵ or more. However, it is preferable that the coupling coefficient is as small as possible if this condition is satisfied. When phase synchronization occurs, the effective refractive index of the resonant light changes, so the wavelength slightly changes with respect to the original wavelength. Since this change amount is proportional to the coupling coefficient, the smaller the coupling coefficient, the smaller. Therefore, it is preferable that the coupling efficiency is small in the range where phase synchronization occurs in order to suppress the wavelength jump during the sweep. In the device of this example, when the coupling coefficient is 10 ⁻⁵ , the expected wavelength jump can be suppressed to −0.01 nm or less. Considering application to an optical device, this value is a deviation that cannot be measured by the interferometer because the wavelength resolution (wave number resolution) of the wave number acquisition interferometer used in the device is about 0.1 nm. Even if the coupling coefficient is increased by a factor of 10, it does not become 0.1 nm or more. Therefore, it can be said that 10 ⁻⁴ is a sufficiently small coupling coefficient for OCT apparatus application. The influence of the wavelength jump can also be reduced, for example, by adjusting the optical path length by passing a current from the electrode provided for adjusting the optical path length of the resonator to the element.

  In this embodiment, the MEMS structure of the upper reflecting mirror portion 913 is a double-supported beam, but it can be a cantilever structure. In this embodiment, the upper reflecting mirror has a beam formed by the multilayer film reflecting mirror itself. However, a structure in which a multilayer film mirror or an HCG mirror is separately formed in a beam dedicated portion that is not a reflecting mirror is also possible. .

  In this embodiment, the first and second active layers having different compositions are spatially separated in the substrate in-plane direction to control the oscillation wavelengths from the respective current confinement portions 910 and 911. It is also possible to use such an asymmetric quantum well structure. This will be described below. FIG. 10 is a schematic diagram showing the gain distribution of the asymmetric quantum well structure layer. 1001 is a gain distribution in a high carrier injection region, and 1002 is a gain distribution in a low injection region. Reference numeral 1003 represents the center of the gain wavelength region. The asymmetric quantum well can realize a very wide gain wavelength region. However, as shown in FIG. 10, the gain on the short wavelength side becomes large in the high carrier injection region, and the gain on the long wavelength side in the low injection region. Has the property of becoming larger. Utilizing this property, control is performed to increase the current density when it is desired to oscillate at the shorter wavelength side and to decrease the current density when desired to oscillate at the longer wavelength side. By doing so, it is possible to make a difference in the gain felt by each resonance light. Such an asymmetric quantum well structure can also be used to construct at least one of the active regions.

  Specifically, when the two active layers are composed of a single common asymmetric quantum well, and the sweep is on the long wavelength side, the first constriction is injected low and the second constriction is high injection The gain on the long wavelength side felt by the first resonance light is increased, and the gain on the long wavelength side felt by the second resonance light is reduced. When the sweep is on the short wavelength side, the low injection and the high injection are switched. This can be realized by, for example, performing current injection into the two constricted portions with separate electrodes. In this way, by changing the injection level in accordance with the sweep wavelength, the wavelength dependence of the gain is controlled in real time, and the two active regions are made to be the same active layer that is not substantially spatially separated. An effect can be obtained. In this embodiment, since it is difficult to change the composition of the active layer between adjacent constricted portions, the ability to make the active layer the same has the merit of reducing the manufacturing difficulty.

(Second embodiment)
A second embodiment of the present invention will be described. FIG. 11A is a schematic cross-sectional view taken along a line corresponding to AA ′ in FIG. 9-1, showing the structure of the wavelength tunable laser element of this example, and FIG. It is a schematic cross section in the line corresponded to -B '. The top view of the present embodiment is the same as FIG. In this embodiment, the first active layer 1105 and the second active layer 1106 are divided into upper and lower parts. A first phase adjustment layer 1118 is formed below the first current confinement part 1110 and the first and second active layers, and the second current confinement part 1111 and the first and second active layers are formed. A second phase adjustment layer (part) 1119 is formed on the upper side of the first phase adjustment layer. Since the second phase adjustment layer is formed by making a depression in the cladding layer from the surface of the upper cladding layer 1108, it may be referred to as a phase adjustment unit.

Configurations, materials, and dimensions other than the active layer and the phase adjustment layer are the same as those in the first embodiment. In the active region, although the shapes are different, the materials and dimensions of the first and second active layers 1105, 1106 are the same as those in the first embodiment. The distance in the substrate vertical direction between both active layers is λ / 4 of the laser band center wavelength λ in terms of the optical path length, and 82 nm in terms of the actual thickness. The first phase adjustment layer 1118 is AlO _x and has a thickness of 174 nm. With this phase adjustment layer 1118, the optical path length difference of −λ / 4 can be given to the resonator length located in the second current confinement part 1111 in the resonator length located in the first current confinement part 1110. Is possible. Further, the thickness of the second phase adjustment layer (the depth of the phase adjustment unit 1119) is 116 nm, and the resonator located in the second current confinement portion is the resonator length located in the first current confinement portion. It is possible to add an optical path length difference of + λ / 4 to the length.

  Using the configuration as described above, the first resonance light located in the first current confinement portion 1110 is + λ // upper the active region than the second resonance light located in the second current confinement portion 1111. 4. An optical path length difference of −λ / 4 is formed below the active region. Therefore, as described in the second embodiment, the peaks and valleys of the resonance light can be shifted from each other. When the laser of this embodiment is driven, phase synchronization occurs only at the overlapping portion of the gain wavelength region, and a broadband wavelength tunable laser can be obtained. By using the configuration of this example, a laser element within the scope of the present invention can be realized in a form in which the active layer is divided into upper and lower parts.

(Third embodiment)
A third embodiment of the present invention will be described. FIG. 12 is a schematic cross-sectional view showing the structure of the wavelength tunable laser element of this example. This embodiment is characterized in that the laser is driven not by current injection but by photoexcitation. For this reason, the element of this embodiment does not have a narrowed portion of the injected current, but has an excitation light irradiation lens 1211 for irradiating the excitation light 1212 instead. Other configurations, materials, and dimensions are the same as those in the first embodiment.

  The excitation light irradiation lens 1211 can adjust the irradiation place by a lens deflection mechanism (not shown). Thereby, since the space | interval of 1st and 2nd resonance light can be adjusted freely, it becomes possible to control the coupling efficiency of resonance light freely. In addition, when an asymmetric quantum well structure is used for the active layer, adjustment of the excitation intensity according to the oscillation wavelength at the time of wavelength sweeping can be easily performed in this embodiment by changing the excitation light intensity.

  When the laser element of this embodiment is driven, gain wavelength regions are formed in the active layers in the light condensing part of the excitation light source, and the first resonance light and the second resonance light are obtained. By using them and moving the upper reflection mirror 1209, a wavelength tunable laser in a wide wavelength region can be realized. Examples of excitation light sources that can be used in this embodiment include laser light sources such as semiconductor lasers, solid-state lasers, and gas lasers, and high-power, non-laser light sources such as superluminescent diodes and flash lamps. Further, two independent light sources may be used as excitation light sources, or the light from one light source may be branched into two and the intensity of the two lights may be adjusted independently by an external modulation means. .

  The first to third embodiments are illustrative, and various conditions such as the structure, material, size, and shape of the laser element according to the present invention are not limited by the above embodiments.

(Third embodiment)
In this embodiment, an example of an information acquisition apparatus using the wavelength tunable laser of the above embodiment or example as a light source device will be described. The wavelength variable light source device can be used as a light source for optical communication or a light source for optical measurement. Furthermore, the wavelength tunable light source device can be used as a light source device of an information acquisition device that acquires information inside a measurement object in a non-invasive and non-destructive manner. Hereinafter, an optical coherence tomography apparatus (hereinafter referred to as an OCT apparatus) will be described with reference to FIG. 13 as an example of an information acquisition apparatus using the light source device of the present embodiment. This is a wavelength sweep type optical coherence tomographic imaging apparatus including the above-described wavelength tunable laser as a wavelength sweep laser.

  FIG. 13 is a schematic diagram showing an OCT apparatus according to this embodiment. The OCT apparatus includes at least a light source device 801, an interference optical system 802, a light detection unit 803, and an information acquisition unit 804 that acquires internal information of the measurement target. As the light source device 801, the wavelength tunable lasers of the above embodiments or examples can be used. Although not shown, the information acquisition unit 804 has a Fourier transformer. Here, the information acquisition unit 804 having a Fourier transformer is not particularly limited as long as the information acquisition unit 804 has a function of performing Fourier transform on the input data. An example is a case where the information acquisition unit 804 has a calculation unit, and this calculation unit has a function of performing Fourier transform. Specifically, this is a case where the arithmetic unit is a computer having a CPU, and this computer executes an application having a Fourier transform function. Another example is a case where the information acquisition unit 804 has a Fourier transform circuit having a Fourier transform function.

  The light emitted from the light source device 801 passes through the interference optical system 802 and is output as interference light having information on the object 812 to be measured. The interference light is received by the light detection unit 803. The light detection unit 803 may be a differential detection type or a simple intensity monitor type. Information on the time waveform of the intensity of the received interference light is sent from the light detection unit 803 to the information acquisition unit 804. The information acquisition unit 804 acquires the peak value of the time waveform of the intensity of the received interference light and performs Fourier transform to acquire information on the object 812 (for example, information on a tomographic image). Note that the light source device 801, the interference optical system 802, the light detection unit 803, and the information acquisition unit 804 can be arbitrarily provided.

  Hereinafter, a detailed description will be given of the process from when light is emitted from the light source device 801 until information inside the object to be measured is obtained. Light emitted from the light source device 801 passes through the fiber 805, enters the coupler 806, and is branched into irradiation light passing through the irradiation light fiber 807 and reference light passing through the reference light fiber 808. The coupler 806 can be configured with a single mode operation in the wavelength band of the light source, and the various fiber couplers can be configured with 3 dB couplers. Irradiation light passes through the collimator 809 to become parallel light and is reflected by the mirror 810. The light reflected by the mirror 810 is irradiated to the object 812 through the lens 811 and reflected from each layer in the depth direction of the object 812.

  On the other hand, the reference light is reflected by the mirror 814 through the collimator 813. In the coupler 806, interference light is generated by reflected light from the object 812 and reflected light from the mirror 814. The interfered light passes through the fiber 815, is collected through the collimator 816, and is received by the light detection unit 803. Information on the intensity of the interference light received by the light detection unit 803 is converted into electrical information such as a voltage and sent to the information acquisition unit 804. In the information acquisition unit 804, data on the intensity of the interference light is processed, specifically, Fourier transform is performed to obtain tomographic image information. The data on the intensity of the interference light to be Fourier-transformed is usually data sampled at equal wave intervals, but it is also possible to use data sampled at equal wavelength intervals.

  The obtained tomographic image information may be sent from the information acquisition unit 804 to the image display unit 817 and displayed as an image. Note that a three-dimensional tomographic image of the object 812 to be measured can be obtained by scanning the mirror 810 in a plane perpendicular to the direction in which the irradiation light is incident. The light source device 801 may be controlled by the information acquisition unit 804 via the electric circuit 818. Although not shown, the intensity of the light emitted from the light source device 801 may be monitored successively and the data may be used for amplitude correction of the signal of the intensity of the interference light.

  The OCT apparatus is useful in obtaining tomographic images in a living body such as an animal or a person in fields such as ophthalmology, dentistry, and dermatology. Information on a tomographic image of a living body includes not only a tomographic image of a living body but also numerical data necessary for obtaining a tomographic image. In particular, it is preferable that the measurement target is a fundus, tooth, or blood vessel of a human body and that it is used to acquire information about their tomographic images.

(Other embodiments)
The wavelength tunable laser according to the above-described embodiment or example can be used as a light source for optical communication or a light source for optical measurement in addition to the above OCT apparatus. A plurality of VCSEL structures to which the above embodiment is applied may be arranged on the same plane and used as an array light source.

  101: substrate, 102: lower reflecting mirror, 105: first active layer (first active region), 106: second active layer (second active region), 109: current confinement layer, 110: first Current confinement portion, 111: second current confinement portion, 113: upper reflection mirror

Claims (14)

A tunable laser having a pair of reflecting mirrors, an active layer disposed between the pair of reflecting mirrors, and a driving unit that displaces at least one of the pair of reflecting mirrors,
The active layer includes a first active region having a gain in a first gain wavelength region and a second active region having a gain in a second gain wavelength region,
The first gain wavelength region and the second gain wavelength region at least partially overlap;
The reflection mirror has a reflectance that allows a high reflection band to oscillate in the first gain wavelength region and the second gain wavelength region;
First oscillating light and second resonant light originating from the first active region and the second active region, respectively, are formed by the pair of reflecting mirrors in the overlapping region of the gain wavelength region. The tunable laser is configured to resonate in phase synchronization with each other. 2. The wavelength tunable laser according to claim 1, wherein the tunable laser is configured as a surface emitting laser capable of oscillating in the vertical direction by laminating the reflection mirror and the active layer in a direction perpendicular to the substrate. . The wavelength tunable laser according to claim 1, wherein the reflection mirror is a multilayer film mirror or an HCG mirror. 2 in the vicinity of the active layer in the order of the wavelength of the oscillation light so that the first resonant light and the second resonant light resonate in phase overlap with each other in the overlapping region of the wavelength regions. The wavelength tunable laser according to claim 1, wherein two current confinement portions are provided. Independently injecting current into the two current confinement portions to independently give gain to the first active region and the second active region, respectively, the first resonant light and the second resonant light are generated, respectively. The wavelength tunable laser according to claim 4, wherein the wavelength tunable laser is configured as described above. The light source is configured to irradiate at least one of the first active region and the second active region with light from an external light source so as to give a gain to the at least one active region. The wavelength tunable laser according to any one of claims 1 to 3. The wavelength tunable according to any one of claims 1 to 6, wherein at least one of the first active region and the second active region of the active layer is configured by an asymmetric quantum well structure layer. laser. The wavelength tunable laser according to any one of claims 1 to 7, wherein the first active region and the second active region of the active layer are configured by active layers having different compositions or structures. . 9. The wavelength tunable laser according to claim 1, wherein the first active region and the second active region of the active layer are formed in the same plane. The first active region and the second active region are provided in different planes, respectively, and the first resonant light and the second resonant light are respectively the first active region and the second active region. The wavelength tunable laser according to any one of claims 1 to 8, wherein gain is mainly obtained from the active region. Resonant light on both sides of the active layer so that the first resonant light and the second resonant light obtain gain mainly from the first active region and the second active region, respectively. The wavelength tunable laser according to claim 10, further comprising a phase adjustment layer for providing an optical path difference with respect to When changes in optical path length of the second resonant light with respect to the first resonant light, which can be applied by the phase adjustment layer, are Δu and Δd on both sides of the active layer, respectively, The wavelength tunable laser according to claim 10 or 11, wherein:
Δu + Δd = Nλ / 2 (N: integer, λ: center wavelength of laser band)
Δu = Nu · λ / 2 + δ, Δd = −Nd · λ / 2−δ (Nu, Nd: integer, 0 <| δ | <λ / 2) The tunable laser according to any one of claims 1 to 12,
And an information acquisition unit that acquires internal information of the measurement object. The tunable laser according to any one of claims 1 to 12,
An interference optical system for branching light from the wavelength tunable laser into irradiation light for irradiating the measurement object and reference light, and generating reflected light of the light irradiated on the measurement object and interference light by the reference light; ,
A light detector that receives the interference light;
An imaging apparatus comprising: an information acquisition unit that acquires information of the measurement object based on a signal from the light detection unit.

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