Optical device

Abstract

To provide an optical device capable of obtaining light having a desired oscillation wavelength by improving the linearity of the oscillation wavelength with respect to a drive voltage.SOLUTION: An optical device according to an embodiment includes a first reflector, a second reflector, an elastic support that supports the second reflector, and a piezoelectric element provided in the elastic support. In the optical device according to the embodiment, a driving voltage is applied to the piezoelectric element to elastically deform the elastic support, thereby changing a distance between the first and second reflectors, and changing the oscillation wavelength of light by a light emitting unit.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to an optical device.

Background Art

[0002] In recent years, research on autonomous driving and driving assistance systems for automobiles has advanced, and in-vehicle radars as sensing means have attracted attention. Among in-vehicle radars, frequency-modulated continuous wave (FMCW) radars are widespread as radars that can measure the distance and relative speed to an object and as radars using a heterodyne method that can detect weak reflected waves, and development of electromagnetic wave directivity and antenna miniaturization has progressed. Under such circumstances, research on FMCW LiDAR (Light Detection and Ranging) aiming for improved directivity, miniaturization, and low power consumption by replacing electromagnetic waves with laser light has also been advanced. The light source of FMCW LiDAR needs to sweep the wavelength with respect to time, and a MEMS (Micro Electro Mechanical Systems) type tunable laser that changes the wavelength by directly modulating the resonator length is known.

[0003] For example, Patent Document 1 describes a tunable laser that emits tunable laser light with an output power spectrum over a radiation wavelength band having a central wavelength and an average radiation power. This tunable laser includes an optical resonator including first and second mirrors, a gain region interposed between the first and second mirrors, a gap tuning region, and a MEMS drive mechanism for adjusting the gap, the MEMS drive mechanism including a deformable dielectric membrane that is transparent over the wavelength band and is mounted on a rigid support structure. The MEMS drive mechanism has a membrane stress value in the range of 100 to 1000 MPa. The frequency response of the MEMS drive mechanism has substantially increased damping due to a squeeze film damping effect controlled by a central plate having a diameter larger than 50 μm and not exceeding the actuator diameter. The free spectral range (FSR) of the optical resonator exceeds 5% of the central wavelength. The tunable laser operates in a substantially single longitudinal and transverse mode over the wavelength band. The MEMS drive mechanism has a wavelength tuning frequency response with a 6 dB bandwidth exceeding about 1 kHz.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in a conventional MEMS-type tunable laser, since electrostatic attraction is used to modulate the resonator length, that is, the distance between two opposing mirrors in parallel, the linearity (straightness) of the drive voltage and the displacement amount is insufficient, and it is difficult to obtain a desired wavelength. Furthermore, since the initial value of the resonator length cannot be accurately adjusted to an interval that is an integral multiple of the oscillation wavelength during manufacturing, there is a problem that it is necessary to apply a high voltage to the movable mirror part during oscillation to move the mirror to a resonator length that satisfies the oscillation conditions.

[0006] The present invention has been made based on the above problems, and an object thereof is to provide an optical device capable of improving the linearity of the oscillation wavelength with respect to the drive voltage and obtaining light having a desired oscillation wavelength.

Means for Solving the Problems

[0007] The optical device of the present embodiment includes a light emitting part including an active layer and a first reflecting part, a second reflecting part, a frame-shaped fixed support part surrounding the second reflecting part, and a connection between the second reflecting part and the fixed support part, and supports the second reflecting part with a through hole interposed between the second reflecting part and the fixed support part. A plurality of elastic support part, and the A plurality of elastic support part each of a piezoelectric element provided therein, and a movable mirror element including the same, and a bonding layer provided at a position overlapping the fixed support part and bonding the light emitting part and the movable mirror element. The plurality of elastic support portions are provided around the second reflecting portion and spaced apart from each other, and the piezoelectric element is not provided on the second reflecting portion. By the elastic deformation of the elastic support part due to the drive voltage applied to the piezoelectric element, the distance between the first reflecting part and the second reflecting part is changed, and the oscillation wavelength of the light generated in the active layer is changed. A plurality of This is a feature.

Effects of the Invention

[0008] According to the present invention, it is possible to provide an optical device capable of improving the linearity of the oscillation wavelength with respect to the drive voltage and obtaining light having a desired oscillation wavelength.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Embodiments for Carrying Out the Invention

[0010] With reference to FIGS. 1 to 16, the tunable laser (optical device) 10 of the present embodiment will be described in detail. In the following description, the X direction, Y direction, and Z direction are based on the directions of the arrow lines drawn in the figures. Also, in this specification, "drive voltage" may be read as "drive signal".

[0011] FIG. 1 is a diagram showing an example of the overall configuration of the tunable laser 10 of the present embodiment.

[0012] The tunable laser 10 includes a Half-VCSEL (Vertical Cavity Surface Emitting Laser) element 100 as a light emitting part, a movable mirror element 200, and a bonding layer 300 for maintaining (limiting) the relative positional relationship between the Half-VCSEL element 100 and the movable mirror element 200. The Half-VCSEL element 100 and the movable mirror element 200 have a rectangular shape extending in the XY plane, and the bonding layer 300 has a rectangular frame shape connecting the peripheral parts of the XY plane of the Half-VCSEL element 100 and the movable mirror element 200 to the upper surface part and the lower surface part in the Z direction.

[0013] The Half-VCSEL element 100 includes a first mirror (first reflecting part) 400, a semiconductor substrate 500, and an antireflection film 600, which are laminated in order from the upper layer to the lower layer in the Z direction. The Half-VCSEL element 100 includes an electrode 101, an electrode 102, and a wiring 103 for electrically connecting the electrode 101 and the electrode 102.

[0014] The movable mirror element 200 includes a fixed support part 700 having a rectangular frame shape extending in the XY plane, and a rectangular second mirror (second reflecting part) 800 supported at the central part of the XY plane of the fixed support part 700. The second mirror 800 is supported by the fixed support part 700 in a manner that it can approach and separate from the first mirror 400 from the reference facing position with the first mirror 400 defined by the bonding layer 300 (the details will be described later).

[0015] There is a space between the first mirror 400 and the second mirror 800, and the space is filled with vacuum or gas. The active layer 105 (to be described later with reference to FIG. 2), which is the light emitting source of the Half-VCSEL element 100, exists between the first mirror 400 and the second mirror 800.

[0016] The tunable laser 10 sweeps the wavelength of the light by the Half-VCSEL element 100 and changes the Z-direction distance (gap) G between the first mirror 400 and the second mirror 800 to change the oscillation wavelength of the light by the Half-VCSEL element 100. In FIG. 1, the upper end of the arrow indicating the distance G is drawn from the second mirror 800, and the lower end of the arrow indicating the distance G is drawn from slightly above the first mirror 400, but this is due to reasons of drawing convenience (equivalent to the lower end of the arrow indicating the distance G being drawn from the first mirror 400). The structure for changing the Z-direction distance G between the first mirror 400 and the second mirror 800 will be described in detail later.

[0017] FIG. 2 is a diagram showing an example of the detailed structure of the Half-VCSEL element 100.

[0018] The Half-VCSEL element 100 has a semiconductor substrate 500 laminated on the upper surface of an antireflection film 600. The semiconductor substrate 500 is composed of a semiconductor substrate such as an n-GaAs substrate, for example. A first mirror 400 is laminated on the upper surface of the semiconductor substrate 500.

[0019] The first mirror 400 constitutes a semiconductor multilayer film mirror having a first semiconductor layer 410, a second semiconductor layer 420, and a third semiconductor layer 430 laminated in order from the upper layer to the lower layer. Note that the number and arrangement of the semiconductor layers constituting the first mirror 400 have degrees of freedom (not limited to those exemplified here), and various design changes are possible.

[0020] The semiconductor layers (for example, the first semiconductor layer 410, the second semiconductor layer 420, and the third semiconductor layer 430) constituting the first mirror 400 may include, for example, a low refractive index layer made of n-Al0.9Ga0.1As and a high refractive index layer made of n-Al0.2Ga0.8As. Between the semiconductor layers (refractive index layers) constituting the first mirror 400, in order to reduce the electrical resistance, a composition gradient layer with a thickness of, for example, 20 nm, whose composition gradually changes from one composition to the other, may be provided. The film thickness of each semiconductor layer (each refractive index layer) is preferably set to an optical thickness of λ / 4, including half of the adjacent composition gradient layer, where λ is the oscillation wavelength. Note that when the optical thickness is λ / 4, the actual thickness D of the layer is D = λ / 4n (where n is the refractive index of the medium of the layer).

[0021] On the upper surface of the semiconductor substrate 500, in addition to the first mirror 400 or as a part of the components of the first mirror 400, an electrode 101, an electrode 102, a wiring 103, a spacer layer 104, an active layer 105, a selective oxidation layer 106, a contact layer 107, an insulating layer 108, a mesa 109, a groove 110, an opening 111, and an opening 112 are formed.

[0022] The spacer layer 104 is composed of, for example, an undoped AlGaInP layer and is formed at the boundary layer between the semiconductor multilayer mirror of the first mirror 400 and the active layer 105. The portion including the spacer layer 104 and the active layer 105 is also referred to as a resonator structure (resonator region), and is set to have an optical thickness of 1 wavelength (λ), including half of the adjacent composition gradient layer. Two spacer layers 104 are formed spaced apart in the Z direction, and the active layer 105 is formed between the two spacer layers 104. The active layer 105 is, for example, an active layer with a three quantum well structure having three quantum well layers and four barrier layers. For example, each quantum well layer is an InGaAs layer, and each barrier layer is an AlGaAs layer.

[0023] The selective oxidation layer 106 is composed of an oxidized region 106A and a non-oxidized region 106B. For example, the selective oxidation layer 106 made of p-AlAs with a thickness of 30 nm is inserted into the semiconductor multilayer mirror of the first mirror 400. The insertion position can be, for example, within the pair of the second high refractive index layer and low refractive index layer counted from the spacer layer 104. Note that the selective oxidation layer 106 may include layers such as a composition gradient layer or an intermediate layer above and below, and here the actually oxidized layer is also referred to as the selective oxidation layer 106 together.

[0024] The contact layer 107 is composed of, for example, a p-GaAs layer and is formed on the semiconductor multilayer mirror of the first mirror 400.

[0025] A mesa 109 and a groove 110 are formed by etching away a part of the semiconductor multilayer mirror of the first mirror 400 (for example, the first semiconductor layer 410, the second semiconductor layer 420, and the third semiconductor layer 430), the spacer layer 104, the active layer 105, and the contact layer 107.

[0026] The insulating layer 108 is composed of, for example, SiN, SiON, SiO2, etc. and covers the mesa 109. An opening 111 for exposing a part of the contact layer 107 of the mesa 109 is formed in the insulating layer 108. The opening 111 is formed at a position overlapping the non-oxidized region 106B of the selective oxidation layer 106 in plan view.

[0027] An electrode 101 electrically connected to the contact layer 107 through the opening 111 is formed on the insulating layer 108 of the mesa 109. As the electrode 101, for example, a laminated film obtained by laminating titanium (Ti) / platinum (Pt) / gold (Au) in this order from the insulating layer side can be used.

[0028] The insulating layer 108 covers the groove 110. An opening 112 for exposing a part of the semiconductor substrate 500 is formed in the insulating layer 108.

[0029] An electrode 102 is formed on the insulating layer 108 of the groove 110 and is electrically connected to the contact layer 107 through the opening 112. As the electrode 102, for example, a laminated film in which a germanium alloy (AuGe) / nickel (Ni) / gold (Au) is laminated in order from the side of the semiconductor substrate 500 can be used.

[0030] The wiring 103 electrically connects the electrode 101 and the electrode 102. As the wiring 103, for example, a laminated film in which titanium (Ti) / platinum (Pt) / gold (Au) is laminated in order from the side of the semiconductor substrate 500 can be used.

[0031] In this embodiment, the case where the Half-VCSEL element 100 is used as the "light emitting part" has been illustrated and described. However, it is also possible to use an LD (Laser Diode) or an LED (Light Emitting Diode) as the "light emitting part". Further, an EEL (Edge Emitting Laser) may be used as the "light emitting part". Furthermore, the "light emitting part" may be a single light source or a plurality of light sources (for example, a VCSEL array light source) that emit light simultaneously. That is, there is freedom in the specific aspect of the "light emitting part", and various design changes are possible.

[0032] Here, in a conventional MEMS type tunable laser, an electrostatic attraction force is used to modulate the resonator length, that is, the distance between two opposing mirrors in parallel. Therefore, the linearity (straightness) of the drive voltage (drive signal) and the displacement amount is insufficient, and it is difficult to obtain a desired wavelength. Furthermore, since the initial value of the resonator length cannot be accurately adjusted to an interval that is an integer multiple of the oscillation wavelength during manufacturing, there is a problem that it is necessary to apply a high voltage to the mirror movable part during oscillation to move the mirror to a resonator length that satisfies the oscillation conditions.

[0033] In a MEMS tunable laser using electrostatic attraction, the reason why the driving voltage (driving signal) and the resonator length do not have a linear relationship is due to the fact that the position of the mirror on the MEMS side is proportional to the square of the driving voltage. The position of the mirror on the MEMS side is determined under the condition that the electrostatic attraction generated by the potential difference and the restoring force of the spring, which is the driving structure of the MEMS, balance in addition to the initial gap between the two mirrors when no voltage is applied. When the driving voltage is increased, the electrostatic attraction, which is the force that attracts each other between the two mirrors, increases, and the resonator length becomes shorter. When the voltage range is wide, the displacement amount of the mirror deviates from the linear relationship. To solve this problem, a method of pre-distorting the driving voltage so that the wavelength change is linear with respect to the driving voltage can be considered. However, due to variations during device manufacturing and temperature changes, the initial mirror gap (1 to 2 μm) changes, making it difficult to achieve ultra-high-precision initial gap control. Therefore, with a driving voltage pre-distorted under static conditions, linearity cannot be achieved in the change of the resonator length.

[0034] Furthermore, when the gap between the two mirrors is displaced by more than 1 / 3 of the initial gap, a pull-in effect occurs where the electrostatic attraction generated by the potential difference always exceeds the restoring force of the spring, which is the driving structure of the MEMS, and the films are attracted to each other, so that the function of the MEMS tunable laser cannot be exerted. In other words, the mirror driving range in the MEMS tunable laser using electrostatic attraction is limited to less than 1 / 3 of the initial gap.

[0035] In this embodiment, the above problems of the MEMS tunable laser using electrostatic attraction are regarded as important technical issues, and a structure in which a piezoelectric element is formed on a spring is used as a driving source for the MEMS side mirror. In this structure, by utilizing the linearity between the applied voltage and the volume reduction amount of the piezoelectric element, the resonator length is linearly modulated with respect to the voltage. When an electrode is formed so as to sandwich a piezoelectric element on a meander-shaped spring connected to the mirror on the MEMS side and a voltage is applied, the volume of the piezoelectric element is reduced in a linear relationship with the voltage. In response to the stress in the in-plane direction of the film formation generated by this reduction, the meander-shaped spring deforms in the plane perpendicular direction. At this time, the linearity is maintained between the stress and the deformation amount of the piezoelectric element. Furthermore, since the spring is connected to the mirror, the spring deformation amount and the mirror displacement amount are maintained in a linear relationship, and as a result, the mirror can be driven in a linear relationship with respect to the voltage. Furthermore, since this relationship does not depend on the initial value of the resonator length, the resonator length can be controlled with high precision for electrostatic drive type MEMS.

[0036] More specifically, in this embodiment, an elastic support portion 900 that supports the second mirror 800 and a piezoelectric element 1000 provided on the elastic support portion 900 are formed on the fixed support portion 700 of the movable mirror element 200. By applying a driving voltage to the piezoelectric element 1000 to elastically deform the elastic support portion 900, the distance between the first mirror 400 and the second mirror 800 is changed, and the oscillation wavelength of light by the Half-VCSEL element 100 is changed.

[0037] Here, by integrally forming the first mirror 400 and the Half-VCSEL element 100 as a "first substrate" and integrally forming the second mirror 800, the elastic support portion 900, and the piezoelectric element 1000 as a "second substrate", the configuration of the tunable laser 10 can be simplified. For example, the Half-VCSEL element 100 can constitute the "first substrate", and the movable mirror element 200 can constitute the "second substrate".

[0038] FIG. 3 is a plan view showing a first example of the support structure of the second mirror 800 (support structure of the MEMS side mirror). FIG. 4 is a cross-sectional view taken along line A-A of FIG. 3.

[0039] As shown in FIG. 3, the second mirror 800 is supported in a floating state with a through hole 700X interposed in the central portion of the fixed support portion 700 extending in the XY plane. Connection ends 810 and 820 protrude from the upper and lower sides of the second mirror 800, and connection portions 710 and 720 protrude from the upper and lower sides of the fixed support portion 700. The connection end 810 and the connection portion 710 are connected by an elastic support portion 910, and the connection end 820 and the connection portion 720 are connected by an elastic support portion 920. The elastic support portions 910 and 920 are provided spaced apart around the second mirror 800.

[0040] The elastic support portion 910 has a first arm portion 911 extending to the right from the connection portion 710, a folded-back portion 912 folded downward from the first arm portion 911, and a second arm portion 913 extending to the left from the folded-back portion 912 and connected to the connection end 810 (having a meander structure). The left half of the first arm portion 911 is provided with a piezoelectric element 1011, which becomes a displacement portion 911A that elastically deforms when a driving voltage is applied to the piezoelectric element 1011. The right half of the first arm portion 911 is not provided with a piezoelectric element 1011 and becomes a displacement defining portion 911B that does not elastically deform when a driving voltage is applied to the piezoelectric element 1011. The right half of the second arm portion 913 is provided with a piezoelectric element 1013, which becomes a displacement portion 913A that elastically deforms when a driving voltage is applied to the piezoelectric element 1013. The left half of the second arm portion 913 is not provided with a piezoelectric element 1013 and becomes a displacement defining portion 913B that does not elastically deform when a driving voltage is applied to the piezoelectric element 1013. Thus, the elastic support portion 910 has the first arm portion 911 and the second arm portion 913 that extend adjacent to each other, and the folded-back portion 912 that connects the ends of the first arm portion 911 and the second arm portion 913 in the extending direction. The piezoelectric element 1011 is provided on the first arm portion 911, and the piezoelectric element 1013 is provided on the second arm portion 913. Further, the piezoelectric element 1011 and the piezoelectric element 1013 are provided at positions shifted (at different positions, not arranged in a line) in a direction orthogonal to the extending direction of the first arm portion 911 and the second arm portion 913 (the vertical direction in the figure).

[0041] The elastic support portion 920 has a first arm portion 921 extending rightward from the connecting portion 720, a folded-back portion 922 folded upward from the first arm portion 921, and a second arm portion 923 extending leftward from the folded-back portion 922 and connected to the connection end 820 (having a meander structure). The left half of the first arm portion 921 is provided with a piezoelectric element 1021, which becomes a displacement portion 921A that elastically deforms when a driving voltage is applied to the piezoelectric element 1021. The right half of the first arm portion 921 is not provided with a piezoelectric element 1021 and becomes a displacement defining portion 921B that does not elastically deform when a driving voltage is applied to the piezoelectric element 1021. The right half of the second arm portion 923 is provided with a piezoelectric element 1023, which becomes a displacement portion 923A that elastically deforms when a driving voltage is applied to the piezoelectric element 1023. The left half of the second arm portion 923 is not provided with a piezoelectric element 1023 and becomes a displacement defining portion 923B that does not elastically deform when a driving voltage is applied to the piezoelectric element 1023. Thus, the elastic support portion 920 has the first arm portion 921 and the second arm portion 923 that extend adjacent to each other, and the folded-back portion 922 that connects the ends of the first arm portion 921 and the second arm portion 923 in the extending direction. The piezoelectric element 1021 is provided on the first arm portion 921, and the piezoelectric element 1023 is provided on the second arm portion 923. Also, the piezoelectric element 1021 and the piezoelectric element 1023 are provided at positions shifted (at different positions, non-aligned positions) in a direction orthogonal to the extending direction of the first arm portion 921 and the second arm portion 923 (the vertical direction in the figure).

[0042] As shown in FIG. 4, the fixed support portion 700 is configured by laminating a support layer 730, an oxide insulating layer 740, a silicon active layer 750, and an insulating layer 760 in order from the lower layer side to the upper layer side in the Z direction.

[0043] The displacement defining portion 911B and the displacement defining portion 921B are configured by laminating a silicon active layer 750 and an insulating layer 760 in order from the lower layer side to the upper layer side in the Z direction (having a laminated structure partially common to the fixed support portion 700).

[0044] The displacement portion 913A (including the piezoelectric element 1013) and the displacement portion 923A (including the piezoelectric element 1023) constitute an actuator in which the piezoelectric element 1000 is formed on the upper surface of the silicon active layer 750. The piezoelectric element 1000 is configured by laminating a lower electrode 1000A, a piezoelectric material 1000B, and an upper electrode 1000C in order from the lower layer side to the upper layer side in the Z direction. By applying a driving voltage to the lower electrode 1000A and the upper electrode 1000C, the displacement portions 913A and 923A are elastically deformed. An insulating layer 760 and a protective film 1100 are provided on the upper surface of the piezoelectric element 1000 (upper electrode 1000C).

[0045] In the present embodiment, by applying a driving voltage to the piezoelectric elements 1011, 1013, 1021, and 1023 to elastically deform the elastic support portions 910 and 920, the distance between the first mirror 400 and the second mirror 800 can be changed, and the oscillation wavelength of light by the Half-VCSEL element 100 can be changed. That is, the second mirror 800 as a movable mirror takes over the displacement generated by the combination of the actuator (piezoelectric elements 1011, 1013, 1021, 1023) on the meander structure, the displacement defining portions 911B, 913B, 921B, 923B, and the folding portions 912 and 922 via the connection ends 810 and 820, and can translate in the Z direction while maintaining the parallelism between the fixed support portion 700 and the second mirror 800 as a movable mirror. This translation in the Z direction is executed by applying a driving voltage to the lower electrode 1000A and the upper electrode 1000C of the actuator to generate a potential difference.

[0046] FIG. 5 is a plan view showing a second example of the support structure of the second mirror 800 (support structure of the MEMS side mirror). FIGS. 6 and 7 are perspective views and side views depicting the driving state of the second mirror 800 in the support structure of FIG. 5.

[0047] In the second example, in addition to the elastic support portions 910 and 920 located on the right half of the fixed support portion 700 extending in the XY plane, elastic support portions 930 and 940 located on the left half of the fixed support portion 700 extending in the XY plane are provided. The elastic support portions 910, 920, 930, and 940 are provided spaced apart around the second mirror 800. In this way, by providing the four elastic support portions 910, 920, 930, and 940 in a symmetric shape with respect to the four corners of the fixed support portion 700 extending in the XY plane, the parallelism of the surfaces of the fixed support portion 700 and the second mirror 800 as the movable mirror can be more reliably maintained.

[0048] The elastic support portion 930 has a first arm portion 931 extending leftward from the connecting portion 710, a folded-back portion 932 folded downward from the first arm portion 931, and a second arm portion 933 extending rightward from the folded-back portion 932 and connected to the connection end 810 (having a meander structure). The right half of the first arm portion 931 is provided with a piezoelectric element 1031 and becomes a displacement portion 931A that elastically deforms when a driving voltage is applied to the piezoelectric element 1031. The left half of the first arm portion 931 is not provided with a piezoelectric element 1031 and becomes a displacement defining portion 931B that does not elastically deform when a driving voltage is applied to the piezoelectric element 1031. The left half of the second arm portion 933 is provided with a piezoelectric element 1033 and becomes a displacement portion 933A that elastically deforms when a driving voltage is applied to the piezoelectric element 1033. The right half of the second arm portion 933 is not provided with a piezoelectric element 1033 and becomes a displacement defining portion 933B that does not elastically deform when a driving voltage is applied to the piezoelectric element 1033. In this way, the elastic support portion 930 has the first arm portion 931 and the second arm portion 933 extending adjacent to each other, and a folded-back portion 932 connecting the ends of the first arm portion 931 and the second arm portion 933 in the extending direction. The piezoelectric element 1031 is provided on the first arm portion 931, and the piezoelectric element 1033 is provided on the second arm portion 933. Further, the piezoelectric element 1031 and the piezoelectric element 1033 are provided at positions shifted (at different positions, non-aligned positions) in a direction orthogonal to the extending direction of the first arm portion 931 and the second arm portion 933 (the vertical direction in the figure).

[0049] The elastic support portion 940 has a first arm portion 941 extending leftward from the connecting portion 720, a folded-back portion 942 folded upward from the first arm portion 941, and a second arm portion 943 extending rightward from the folded-back portion 942 and connected to the connection end 820 (having a meander structure). The right half of the first arm portion 941 is provided with a piezoelectric element 1041, which becomes a displacement portion 941A that elastically deforms when a driving voltage is applied to the piezoelectric element 1041. The left half of the first arm portion 941 is not provided with a piezoelectric element 1041 and becomes a displacement defining portion 941B that does not elastically deform when a driving voltage is applied to the piezoelectric element 1041. The left half of the second arm portion 943 is provided with a piezoelectric element 1043, which becomes a displacement portion 943A that elastically deforms when a driving voltage is applied to the piezoelectric element 1043. The right half of the second arm portion 943 is not provided with a piezoelectric element 1043 and becomes a displacement defining portion 943B that does not elastically deform when a driving voltage is applied to the piezoelectric element 1043. Thus, the elastic support portion 940 has the first arm portion 941 and the second arm portion 943 that extend adjacent to each other, and the folded-back portion 942 that connects the ends of the first arm portion 941 and the second arm portion 943 in the extending direction. The piezoelectric element 1041 is provided on the first arm portion 941, and the piezoelectric element 1043 is provided on the second arm portion 943. Further, the piezoelectric element 1041 and the piezoelectric element 1043 are provided at positions shifted (at different positions, non-aligned positions) in a direction orthogonal to the extending direction of the first arm portion 941 and the second arm portion 943 (the vertical direction in the figure).

[0050] In this embodiment, by applying a driving voltage to the piezoelectric elements 1011, 1013, 1021, 1023, 1031, 1033, 1041, 1043 to elastically deform the elastic support portions 910, 920, 930, 940, the distance between the first mirror 400 and the second mirror 800 can be changed, and the oscillation wavelength of light by the Half-VCSEL element 100 can be changed.

[0051] When a driving voltage is applied to the piezoelectric element, the volume of the piezoelectric element changes. Since the piezoelectric element and the arm portion are physically connected, in-plane stress is generated in the vicinity of the piezoelectric element in the arm portion. The arm portion to which the stress is applied elastically deforms to relieve the stress. At this time, if the dimensional ratio in the XY direction of the arm portion is made anisotropic and the thickness of the arm portion in the Z direction is made smaller than the dimensions in the XY direction, the behavior of the arm portion warping upward in the Z direction becomes dominant, and the piezoelectric element also warps in the Z direction while having a predetermined radius of curvature. The warp of the arm portion generated by the piezoelectric element is transmitted to the displacement regulating portion where the piezoelectric element is not formed. Although the displacement regulating portion bends in the Z direction due to the influence of gravity, it is sufficiently small compared to the warp of the piezoelectric element, so this can be ignored (equivalent to not elastically deforming).

[0052] FIG. 7 illustrates and enlarges the elastic support portion 920, but the elastic support portions 910, 930, and 940 also have the same configuration. In FIG. 7, the displacement Z1 in the Z direction of the folded portion 922 of the elastic support portion 920 can be increased (the radius of curvature can be decreased) by the deflection generated by the piezoelectric elements 1021 and 1023, or the displacement regulating portion 921B of the first arm portion 921 can be lengthened, or both can be implemented. The first arm portion 921 is inclined at an angle θ1 from the free state toward the folded portion 922, and immediately after being folded by the folded portion 922, the second arm portion 923 is inclined at an angle θ2 from the free state (the difference between the angle θ1 and the angle θ2 is the inclination angle of the first arm portion 921 and the second arm portion 923). When a piezoelectric element having the same dimensions as the first arm portion 921 is formed on the second arm portion 923 and driven, the inclination of the first arm portion 921 is canceled. Therefore, at the connection end 820 existing at the end of the second arm portion 923, the second mirror 800 as a movable mirror can be driven in the Z direction while maintaining the parallelism between the XY plane of the fixed support portion 700 and the second mirror 800 as a movable mirror before driving.

[0053] Note that the dimensions of the piezoelectric elements formed on the first arm portion 921 and the second arm portion 923 in the XY plane may or may not be the same. If they are not the same, it is necessary to adjust the voltage applied to each piezoelectric element so that the warpage generated in each piezoelectric element portion is the same.

[0054] The drive voltage applied to the piezoelectric element may be a DC voltage, or a sin wave, cos wave, or triangular wave whose voltage changes over time. Note that under the condition that the modulation frequency of the voltage and the resonance frequency of the movable mirror element are separated from each other, the linearity of the displacement amount with respect to the voltage is maintained as described above. The piezoelectric element is driven by applying a voltage, and there is a linear relationship between the voltage and the generated stress. Further, since there is also a linear relationship between the generated stress and the deformation amount of the arm portion, there is a linear relationship between the applied voltage and the displacement amount.

[0055] FIG. 8 is a diagram showing the relationship between the drive voltage and the resonator length of the tunable laser 10 of the present embodiment, and FIG. 9 is a diagram showing the relationship between the drive voltage and the resonator length of a conventional MEMS type tunable laser using electrostatic attraction. As shown in FIG. 8, according to the tunable laser 10 of the present embodiment, high linearity (linearity) is realized between the drive voltage and the resonator length. On the other hand, as shown in FIG. 9, in a conventional MEMS type tunable laser using electrostatic attraction, the position of the mirror on the MEMS side is proportional to the square of the drive voltage, and when the gap between the two mirrors is displaced by 1 / 3 or more of the initial gap, the linearity between the drive voltage and the resonator length becomes insufficient due to the influence of the pull-in effect in which the films are attracted to each other.

[0056] FIG. 10 is a plan view showing a third example of the support structure of the second mirror 800 (support structure of the MEMS side mirror).

[0057] In the third example, piezoelectric elements 1011X are arranged in the region of the displacement defining portion 911B of the first arm portion 911 where no piezoelectric element was provided in the first example, piezoelectric elements 1013X are arranged in the region of the displacement defining portion 913B of the second arm portion 913, piezoelectric elements 1021X are arranged in the region of the displacement defining portion 921B of the first arm portion 921, and piezoelectric elements 1023X are arranged in the region of the displacement defining portion 923B of the second arm portion 923. The piezoelectric elements 1011, 1013, 1021, 1023, 1011X, 1013X, 1021X, and 1023X are provided so as to be arranged in a direction (the vertical direction in the figure) orthogonal to the extending directions of the first arm portion 911 and the second arm portion 913 and the extending directions of the first arm portion 921 and the second arm portion 923.

[0058] In the third example, the piezoelectric elements are divided into a first set of piezoelectric elements 1011, 1013, 1021, 1023 and a second set of piezoelectric elements 1011X, 1013X, 1021X, 1023X. When a drive voltage is applied to the first set of piezoelectric elements, no drive voltage is applied to the second set of piezoelectric elements, and conversely, when a drive voltage is applied to the second set of piezoelectric elements, no drive voltage is applied to the first set of piezoelectric elements.

[0059] That is, in the third example, among the regions 911A and 911B of the first arm portion 911, the one to which the piezoelectric elements 1011 and 1011X are voltage-applied constitutes the displacement portion, and the one to which the piezoelectric elements 1011 and 1011X are not voltage-applied constitutes the displacement defining portion. Also, among the regions 913A and 913B of the second arm portion 913, the one to which the piezoelectric elements 1013 and 1013X are voltage-applied constitutes the displacement portion, and the one to which the piezoelectric elements 1013 and 1013X are not voltage-applied constitutes the displacement defining portion. Also, among the regions 921A and 921B of the first arm portion 921, the one to which the piezoelectric elements 1021 and 1021X are voltage-applied constitutes the displacement portion, and the one to which the piezoelectric elements 1021 and 1021X are not voltage-applied constitutes the displacement defining portion. Also, among the regions 923A and 923B of the second arm portion 923, the one to which the piezoelectric elements 1023 and 1023X are voltage-applied constitutes the displacement portion, and the one to which the piezoelectric elements 1023 and 1023X are not voltage-applied constitutes the displacement defining portion.

[0060] In the first example, a portion of the plurality of arm portions where no piezoelectric element is provided is used as the displacement defining portion, whereas in the third example, a portion of the plurality of arm portions where a piezoelectric element is provided but no drive voltage is applied is used as the displacement defining portion.

[0061] In the third example, even when a failure or malfunction occurs in the piezoelectric elements included in one of the first set and the second set of piezoelectric elements, by not applying a drive voltage to one set and applying a drive voltage to the other set, the tunable laser 10 can be used without being affected by the failure or malfunction of the piezoelectric elements.

[0062] FIGS. 11A and 11B are diagrams showing an example of wavelength sweeping by the tunable laser 10 of the present embodiment.

[0063] The laser oscillation wavelength by the tunable laser 10 is defined by the emission spectrum (i.e., wavelength distribution) inherent to the material constituting the active layer 105 and the resonator structure. The resonator structure is determined by the distance G in the Z direction between the first mirror 400 and the second mirror 800, and the refractive index and dimensions of the material present between the first mirror 400 and the second mirror 800. Although the active layer 105 emits light with the current injected through the electrodes of the Half-VCSEL element 100, the wavelength of the resonant light (oscillation light) changes according to the distance G in the Z direction between the first mirror 400 and the second mirror 800. FIG. 11A depicts a state where the resonant light (oscillation light) is directed downward in the Z direction, and FIG. 11B depicts a state where the resonant light (oscillation light) is directed upward in the Z direction.

[0064] Since the second mirror 800 as a movable mirror element can change its position in the Z direction, the distance G in the Z direction between the first mirror 400 and the second mirror 800 can be made variable, and the wavelength of the resonant light (oscillation light) can be modulated. In particular, in the present embodiment, by devising the support structure of the second mirror 800 using the elastic support portion 900 and the piezoelectric element 1000, the distance G in the Z direction between the first mirror 400 and the second mirror 800 can be controlled with high precision. Thereby, the linearity of the oscillation wavelength with respect to the drive voltage can be improved, and light with a desired oscillation wavelength can be obtained.

[0065] FIG. 12 shows the simulation results of the relationship between the length of the gap existing between the Half-VCSEL element 100 (first mirror 400) and the second mirror 800 and the oscillation wavelength. The oscillation light from the Half-VCSEL element 100 is emitted toward the mirror with the lower reflectivity among the first mirror 400 and the second mirror 800. Therefore, by adjusting the magnitude relationship of the reflectivities of the first mirror 400 and the second mirror 800, the emission direction of the oscillation light from the Half-VCSEL element 100 can be adjusted. For example, if the reflectivity of the first mirror 400 is made lower than that of the second mirror 800, the oscillation light is emitted downward in the Z direction (FIG. 11A), and if the reflectivity of the first mirror 400 is made higher than that of the second mirror 800, the oscillation light is emitted upward in the Z direction (FIG. 11B).

[0066] Also, bonding layers composed of a metal multilayer film made of a plurality of metals are formed on the Half-VCSEL element 100 (first mirror 400) and the second mirror 800, respectively. For example, by bonding the bonding layers on each element to each other by an atomic diffusion bonding method, a space can be secured between the first mirror 400 and the second mirror 800. Further, the active layer 105 of the Half-VCSEL element 100 is provided at the center of the resonator structure, which is a position corresponding to the antinode in the standing wave distribution of the electric field, so as to obtain a high stimulated emission probability.

[0067] FIG. 13 is a diagram corresponding to FIG. 1 showing a modified example of the tunable laser 10 of the present embodiment. In FIG. 13, a plurality of Half-VCSEL elements 100 are provided for one second mirror 800. Specifically, three Half-VCSEL elements 100X, 100Y, and 100Z adjacent to each other in the X direction in the XY plane are provided for one second mirror 800. Further, the first mirror 400, the semiconductor substrate 500, and the antireflection film 600 may be shared by the three Half-VCSEL elements 100X, 100Y, and 100Z. Note that the number and arrangement of the Half-VCSEL elements have degrees of freedom, and various design changes are possible. For example, two or four or more Half-VCSEL elements can be provided as a plurality of Half-VCSEL elements for the second mirror. That is, the number of Half-VCSEL elements is not limited to three, and any number of them may be provided as long as the second substrate has a plurality of light-emitting portions). Further, instead of / in addition to arranging a plurality of Half-VCSEL elements along the X direction or the Y direction in the XY plane with respect to the second mirror, a plurality of Half-VCSEL elements can also be arranged along the Z direction.

[0068] FIG. 14 is a diagram corresponding to FIG. 6 showing a modified example of the tunable laser 10 of the present embodiment. In FIG. 14, a movable mirror structure portion 800X for high-speed driving is added inside a second mirror 800 as a movable mirror. The length of the gap between the Half-VCSEL element 100 (first mirror 400) and the second mirror 800 joined via the bonding layer 300 changes depending on the usage environment (e.g., temperature) of the tunable laser 10. The change length of this gap is several μm, and the center value of the oscillation wavelength can be fixed by statically driving the meander structure with a voltage prepared (set) in advance according to the usage environment (e.g., temperature) of the tunable laser 10. Further, after fixing the center wavelength, the movable mirror structure portion 800X for high-speed driving is displaced in the Z direction by a modulation signal by several tens of nm, so that the center wavelength can be kept constant regardless of environmental changes, and the oscillation wavelength can be swept within a certain wavelength range. In this way, by providing the second mirror 800 having a relatively large driving range and the movable mirror structure portion 800X having a relatively small driving range (by dividing the functions), the gap between the Half-VCSEL element 100 (first mirror 400) and the second mirror 800 can be controlled with higher precision. For this reason, the linearity of the oscillation wavelength with respect to the driving voltage can be further improved, and light with a desired oscillation wavelength can be obtained.

[0069] For example, a piezoelectric element is provided in the movable mirror structure portion 800X for high-speed driving, and by applying a modulation signal in which the voltage continuously changes with time and the change is periodic to the element, the movable mirror structure portion 800X can be displaced in the Z direction at a driving speed of several tens of nm to several hundreds of nm in the MHz order.

[0070] Figs. 15A and 15B are diagrams showing an example of wavelength change when the temperature of a tunable laser changes. When the temperature of the tunable laser changes, for example, due to a change in the ambient temperature or heat generation of a module having this laser, if the materials constituting the movable mirror element and the Half-VCSEL element are different, the length of the gap alignment region between the above two elements changes due to the difference in the coefficient of thermal expansion. Further, since the refractive index of the materials (for example, semiconductor materials) constituting the resonators of the two elements also changes, as a result, the optical length of the resonator of the tunable laser fluctuates. By recording the wavelength fluctuation with respect to this temperature change and distorting the applied voltage to the piezoelectric element on the meander structure connected to the second mirror so that the wavelength becomes constant with respect to the temperature in advance, the wavelength fluctuation with respect to the temperature can be suppressed. This adjustment method is particularly useful when the amount of change in the length of the gap alignment region due to the temperature change is as large as several hundred nm. For example, for a change of the order described above, even if the movable mirror structure part 800X having a relatively small drive range is driven by a static voltage, a sufficient displacement amount cannot be obtained and the change amount cannot be absorbed. Therefore, after statically adjusting the position of the second mirror with respect to the temperature, by driving the movable mirror structure 800X having a relatively small drive range at high speed with a modulation signal such as a sine wave, the wavelength can be swept at high speed while maintaining the center wavelength with respect to the temperature change.

[0071] FIG. 16 is a diagram showing an example of a functional block diagram for realizing wavelength correction. For example, an element for measuring temperature (temperature measurement element) is arranged near the light source, and the temperature of the tunable laser or its vicinity during oscillation is measured. The storage unit stores the relationship between the wavelength and the optical output value and the temperature during tunable laser oscillation, for example, as a correlation formula or a table, and at the same time stores the relationship between the drive voltage of the piezoelectric element and the change amount of the oscillation wavelength. Based on the measured relationship between the temperature and the wavelength of the storage unit, the processing unit determines a correction voltage for which the oscillation wavelength remains constant or the optical output value becomes maximum with respect to the temperature change, and the drive signal source drives the piezoelectric element according to the voltage. The photoelectric conversion element converts the light emitted by the tunable laser into an electrical signal. This adjustment method is particularly useful when the change amount of the length of the air gap synchronization region due to temperature change is as large as several hundred nm. For example, for the change of the order described above, even if the movable mirror structure part 800X having a relatively small drive region is driven by a static voltage, a sufficient displacement amount cannot be obtained, and the change cannot be absorbed. Therefore, after the position of the second mirror is statically adjusted with respect to the temperature, the movable mirror structure part 800X having a relatively small drive region is driven at high speed with a modulation signal such as a sine wave, so that the wavelength can be swept at high speed while maintaining the center wavelength with respect to the temperature change.

[0072] As described above, the optical device of this embodiment includes a first reflecting portion, a second reflecting portion, an elastic support portion that supports the second reflecting portion, and a piezoelectric element provided on the elastic support portion. Further, by applying a drive voltage to the piezoelectric element to elastically deform the elastic support portion, the distance between the first and second reflecting portions is changed, and the oscillation wavelength of the light emitted by the light emitting portion is changed. That is, the mirror on the MEMS side for controlling the resonator length that determines the wavelength of light is connected to the meander-shaped elastic structure on which the piezoelectric element is formed. Thereby, the linearity of the oscillation wavelength with respect to the drive voltage can be improved, and light having a desired oscillation wavelength can be obtained.

[0073] Note that the present invention is not limited to the above-described embodiments, and various modifications can be made. In the above embodiments, the size, shape, function, etc. of the components illustrated in the accompanying drawings are not limited thereto, and can be appropriately changed within the range in which the effects of the present invention are exhibited. In addition, it can be appropriately changed and implemented as long as it does not deviate from the scope of the object of the present invention.

[0074] In the above embodiments, the elastic support portion is configured as a meander structure having two arm portions extending adjacent to each other and one folding portion connecting the ends of the two arm portions in the extending direction, and piezoelectric elements are provided on the two arm portions. However, the elastic support portion may be configured as a meander structure having three or more arm portions extending adjacent to each other and two or more folding portions connecting the ends of the three or more arm portions in the extending direction, and piezoelectric elements may be provided on the three or more arm portions. That is, the elastic support portion has a plurality of arm portions extending adjacent to each other and a folding portion connecting the ends of the plurality of arm portions in the extending direction, and the piezoelectric elements may be provided on the plurality of arm portions.

[0075] In the above embodiments, the case where two or four elastic support portions are provided at intervals around the second mirror is illustrated and described. However, the number of elastic support portions provided at intervals around the second mirror has a degree of freedom, and various design changes are possible. For example, at least two elastic support portions may be provided at intervals around the second mirror, or at least three elastic support portions may be provided.

Explanation of Reference Numerals

[0076] 10 Tunable Laser (Optical Device) 100 Half-VCSEL Element (Light Emitting Unit, First Substrate) 200 Movable Mirror Element (Second Substrate) 400 First Mirror (First Reflecting Portion) 800 Second Mirror (Second Reflecting Portion) 900, 910, 920, 930, 940 Elastic Support Portion 911, 921, 931, 941 First Arm Portion 911B, 921B, 931B, 941B Displacement Regulation Parts 913, 923, 933, 943 Second Arm Parts 913B, 923B, 933B, 943B Displacement Regulation Parts 912, 922, 932, 942 Folding Parts 1000, 1011, 1013, 1021, 1023, 1011X, 1013X, 1021X, 1023X, 1031, 1033, 1041, 1043 Piezoelectric Elements

Claims

1. An active layer, a first reflecting portion, a light emitting portion including the above, a second reflecting portion, a frame-shaped fixing support portion surrounding the second reflecting portion, a plurality of elastic support portions connecting the second reflecting portion and the fixing support portion and supporting the second reflecting portion with a through hole interposed between the second reflecting portion and the fixing support portion, a piezoelectric element provided on each of the plurality of elastic support portions, a movable mirror element including the above, a bonding layer provided at a position overlapping the fixing support portion and bonding the light emitting portion and the movable mirror element, having, the plurality of elastic support portions are provided around the second reflecting portion so as to be spaced apart from each other, the piezoelectric element is not provided on the second reflecting portion, by elastic deformation of the plurality of elastic support portions due to a driving voltage applied to the piezoelectric element, the distance between the first reflecting portion and the second reflecting portion is changed, and the oscillation wavelength of the light generated in the active layer is changed, an optical device characterized by the above.

2. a first substrate on which the first reflecting portion and the active layer are formed, a second substrate on which the second reflecting portion, the plurality of elastic support portions, and the piezoelectric element are formed, The optical device according to claim 1, characterized by having the above.

3. The first substrate has a plurality of the light emitting portions, The optical device according to claim 2, characterized by the above.

4. Each of the plurality of elastic support portions has a plurality of arm portions extending adjacent to each other and a folding portion connecting ends in the extending direction of the plurality of arm portions, The piezoelectric element is provided on the plurality of arm portions, The optical device according to any one of claims 1 to 3, characterized by the above.

5. The piezoelectric element is provided so as to be arranged in a direction orthogonal to the extending direction of the plurality of arm portions, The optical device according to claim 4, characterized by the above.

6. The piezoelectric element is provided at a position shifted in a direction orthogonal to the extending direction of the plurality of arm portions, The optical device according to claim 4, characterized by the above.

7. Each of the plurality of arm portions has a displacement defining portion that does not elastically deform when a driving voltage is applied to the piezoelectric element, The optical device according to any one of claims 4 to 6, characterized by the above.

8. The displacement defining portion is a portion of the plurality of arm portions where the piezoelectric element is not provided, The optical device according to claim 7, characterized by the above.

9. The displacement defining portion is a portion where the piezoelectric element is provided among the plurality of arm portions but no driving voltage is applied. The optical device according to claim 7, characterized in that.

10. At least three of the plurality of elastic support portions are provided spaced apart around the second reflection portion. The optical device according to any one of claims 1 to 9, characterized in that.

11. Each of the plurality of elastic support portions includes a first elastic support portion and a second elastic support portion. One connecting portion of the fixed support portion and one connection end of the second reflection portion are connected by the first elastic support portion and the second elastic support portion. The optical device according to any one of claims 1 to 10, characterized in that.

Images