JP2024058613A - Surface emitting laser, laser device, detection device, moving body, and method for driving surface emitting laser - Google Patents

Abstract

[Problem] To provide a surface-emitting laser, a laser device, a detection device, and a method for driving a surface-emitting laser that can obtain short-pulse light with reduced tailing. [Solution] The surface-emitting laser has an active layer, a plurality of reflecting mirrors that face each other across the active layer, and an electrode pair that is connected to a power supply device and can inject a current into the active layer, and defines a period during which a current is injected by the power supply device as a current injection period, and a period after the current injection period during which the value of the current injected into the active layer is lower than the current value during the current injection period as a current reduction period, and when the current value injected during the current injection period is a first current value, laser oscillation occurs at least once during the current injection period, and when the current value injected during the current injection period is a second current value that is higher than the first current value, laser oscillation occurs at least once during the current reduction period. [Selected Figure] Figure 6

Description

The present invention relates to a surface-emitting laser, a laser device, a detection device, a moving body, and a method for driving a surface-emitting laser.

Laser safety standards for human eyes are classified into eye-safe classes and are stipulated in IEC. 60825-1Ed. 3 (Japanese standard JIS C 6802 equivalent). In order to use distance measuring devices in various environments, it is desirable to meet the Class 1 standards, which do not require safety measures or warnings. One of the standards for Class 1 is the upper limit of average power. In the case of pulsed light, the peak output, pulse width, and duty ratio are converted to average power and compared with the standard value. Since the allowable peak output increases as the pulse width of the pulsed light becomes shorter, a laser light source with a high peak output and a short pulse width is useful for achieving both high accuracy and long distances in TOF (Time Of Flight) sensors while satisfying eye safety.

Methods for achieving short pulses of 1 ns or less include gain switching, Q switching, and mode locking. Gain switching is a method for achieving pulse widths of 100 ps or less by utilizing the relaxation oscillation phenomenon. Since this can be achieved simply by controlling the pulse current, the configuration is simpler than Q switching or mode locking.

However, because gain switching utilizes the relaxation oscillation phenomenon, it is easy for a series of multiple pulses to be output after the leading pulse. Or, after the relaxation oscillation subsides, a tail light with a wide pulse width is likely to be output. These phenomena are undesirable in practical applications. For example, when detecting in Geiger mode using a single photon avalanche diode (SPAD), only the highest peak output is the sensing target, and if there are multiple pulses other than the target pulse, they become noise, and the tail light is unnecessary energy, which is disadvantageous from the perspective of eye safety.

There is room for further study on surface-emitting lasers that can generate short pulses of light with reduced tailing.

The present invention aims to provide a surface-emitting laser, a laser device, a detection device, a moving body, and a method for driving a surface-emitting laser that can obtain short pulse light with reduced tailing.

According to one aspect of the disclosed technology, the surface-emitting laser has an active layer, a plurality of reflecting mirrors facing each other across the active layer, and an electrode pair connected to a power supply device and capable of injecting a current into the active layer. The period during which the current is injected by the power supply device is defined as a current injection period, and the period after the current injection period during which the current value injected into the active layer is lower than the current value during the current injection period is defined as a current reduction period. When the current value injected during the current injection period is a first current value, the surface-emitting laser oscillates at least once during the current injection period, and when the current value injected during the current injection period is a second current value that is higher than the first current value, the surface-emitting laser oscillates at least once during the current reduction period.

The disclosed technology makes it possible to obtain short pulsed light with reduced tailing.

1 is a cross-sectional view showing a surface-emitting laser according to a first embodiment. 2 is a cross-sectional view showing an oxidized constriction layer and its vicinity in the first embodiment. FIG. FIG. 11 is a cross-sectional view showing an oxidized constriction layer and its vicinity in a reference example. FIG. 1 is an equivalent circuit diagram showing a circuit used for actual measurement. FIG. 13 is a diagram showing the results of actual measurements for a reference example. FIG. 11 is a diagram showing actual measurement results for the first embodiment. 11A and 11B are diagrams illustrating differences in distribution of electric field intensity and transmission refractive index due to structures. 11A and 11B are diagrams showing changes in distribution of electric field intensity and transmission refractive index over time. FIG. 13 is a diagram showing a simulation result for the carrier density and threshold carrier density in a reference example. FIG. 13 is a diagram showing a simulation result regarding the optical output in the reference example. FIG. 11 is a diagram illustrating an example of a function used in a simulation for the first embodiment. FIG. 11 is a diagram showing a simulation result regarding the optical output in the first embodiment. 5A to 5C are diagrams illustrating simulation results of carrier density, threshold carrier density, photon density, and lateral optical confinement factor in the first embodiment. FIG. 14 is an enlarged view of a portion of FIG. 13 . 11A and 11B are diagrams illustrating examples of actual measurement results and simulation results of pulsed light. FIG. 1 is a diagram showing a first model used in a simulation. FIG. 13 is a diagram showing a cross-sectional profile of the electric field intensity distribution in the fundamental mode. FIG. 13 is a diagram showing the calculation results of the relationship between the optical confinement factor and the thickness of the oxidized confinement layer and the diameter of the non-oxidized region. FIG. 13 is a diagram showing the calculation results of the relationship between the thickness of the oxidized confinement layer and the optical confinement factor for the first model. FIG. 13 is a diagram showing a second model used in the simulation. FIG. 13 is a diagram showing a third model used in the simulation. FIG. 13 is a diagram showing calculation results of the relationship between the amount of decrease in refractive index and the optical confinement factor for the second model and the third model. FIG. 13 is a diagram showing a fourth model used in the simulation. FIG. 13 is a diagram showing the calculation results of the relationship between the thickness of the oxidized region at a position 3 μm away from the boundary to the outside and the optical confinement factor for the fourth model. FIG. 13 is a diagram showing the relationship between the current confinement area and the peak optical output. 6A and 6B are diagrams for explaining pulsed light oscillation characteristics when the pulse current value is large. 6A and 6B are diagrams for explaining pulsed light oscillation characteristics when the pulse current value is small. FIG. 11 is a cross-sectional view showing a surface-emitting laser according to a second embodiment. FIG. 13 is a diagram illustrating a laser device according to a third embodiment. FIG. 4 is a diagram showing the relationship between the duty ratio and the peak output of pulsed light. FIG. 13 is a diagram showing a distance measuring device according to a fourth embodiment. FIG. 13 is a diagram showing a moving body according to a fifth embodiment.

Embodiments of the present disclosure will be described below with reference to the accompanying drawings. Note that in this specification and drawings, components having substantially the same functional configurations may be denoted by the same reference numerals to avoid redundant description.

First Embodiment
First, a first embodiment will be described. The first embodiment relates to a surface emitting laser. Fig. 1 is a cross-sectional view showing a surface emitting laser according to the first embodiment.

The surface-emitting laser 100 according to the first embodiment is, for example, a vertical cavity surface-emitting laser (VCSEL) that employs oxide confinement. The surface-emitting laser 100 has an n-type GaAs substrate 110, an n-type distributed Bragg reflector (DBR) 120, an active layer 130, a p-type DBR 140, an oxide confinement layer 150, an upper electrode 160, and a lower electrode 170.

In this embodiment, light is emitted in a direction perpendicular to the surface of the n-type GaAs substrate 110. Hereinafter, the direction perpendicular to the surface of the n-type GaAs substrate 110 may be referred to as the vertical direction, and the direction parallel to the surface of the n-type GaAs substrate 110 may be referred to as the horizontal direction or in-plane direction.

The n-type DBR 120 is on the n-type GaAs substrate 110. The n-type DBR 120 is, for example, a semiconductor multilayer reflector formed by stacking multiple n-type semiconductor films. The active layer 130 is on the n-type DBR 120. The active layer 130 includes, for example, multiple quantum well layers and barrier layers. The active layer 130 is included in the resonator. The p-type DBR 140 is on the active layer 130. The p-type DBR 140 is, for example, a semiconductor multilayer reflector formed by stacking multiple p-type semiconductor films. In the resonator, the active layer 130 is provided at a position closer to the antinode than the middle between the antinode and the node of the standing wave of the oscillating light. When the active layer 130 is provided at a position that is the antinode of the standing wave, the light emission efficiency is highest.

The upper electrode 160 contacts the upper surface of the p-type DBR 140. The lower electrode 170 contacts the lower surface of the n-type GaAs substrate 110. The pair of upper electrode 160 and lower electrode 170 is one example of an electrode pair. However, the position of the electrodes is not limited to this, and may be in any position where a current can be injected into the active layer. For example, an intra-cavity structure may be used in which the electrodes are placed directly on the spacer layer of the resonator, rather than via the DBR.

The p-type DBR 140 includes, for example, an oxidized constriction layer 150. The oxidized constriction layer 150 contains Al. The oxidized constriction layer 150 includes an oxidized region 151 and a non-oxidized region 152 in a plane perpendicular to the light emission direction. The oxidized region 151 has an annular planar shape and surrounds the non-oxidized region 152. The non-oxidized region 152 is composed of a p-type AlAs layer 155 and two p-type Al _0.85 Ga _0.15 As layers 156 that sandwich the p-type AlAs layer 155 in the vertical direction. The oxidized region 151 is composed of AlO _x . The refractive index of the oxidized region 151 is lower than that of the non-oxidized region 152. For example, the refractive index of the oxidized region 151 is 1.65, the refractive index of the p-type AlAs layer 155 is 2.96, and the refractive index of the p-type Al _0.85 Ga _0.15 As layer 156 is 3.04. In plan view, the inner part of the inner edge of the oxidized region 151 of the mesa 180 is an example of a high refractive index region, and the outer part of the inner edge of the oxidized region 151 of the mesa 180 is an example of a low refractive index region. Note that a p-type Al _x Ga _1-x As layer (0.70≦x≦0.90) may be provided instead of the p-type Al _0.85 Ga _0.15 As layer 156. In this embodiment, the p-type DBR 140, the active layer 130, and the n-type DBR 120 constitute the mesa 180. However, in this embodiment in which the current confinement region is formed by oxidation confinement, it is sufficient that at least the oxidized confinement layer 150 and the semiconductor layer located above the oxidized confinement layer 150 are formed in a mesa shape. Furthermore, by forming at least the active layer to be included in the mesa, it is possible to prevent light generated in the active layer from leaking laterally.

Here, the oxidized constriction layer 150 will be described in detail. Figure 2 is a cross-sectional view showing the oxidized constriction layer and its vicinity in the first embodiment.

2, the oxidized region 151 has an annular outer region 153 and an annular inner region 154 in plan view. The outer region 153 is exposed to the side of the mesa 180. The outer region 153 is a region whose thickness changes so that the contact surface of the surface is located outside the oxidized region 151 in cross-sectional view, and the inner region 154 is a region whose thickness changes so that the contact surface of the surface is located inside the oxidized region 151 in cross-sectional view. The inner region 154 is located inside the outer region 153. The thickness of the inner region 154 matches the thickness of the outer region 153 at the boundary with the outer region 153, and becomes thinner as it approaches the center of the mesa 180. The inner region 154 has a tapered shape that gradually becomes thicker from the inner edge to the boundary with the outer region 153 in cross-sectional view. The non-oxidized region 152 is located inside the outer region 153. A part of the non-oxidized region 152 sandwiches the inner region 154 in the vertical direction. Another part of the non-oxidized region 152 is inside the inner edge of the inner region 154 in a plan view. For example, the thickness of the non-oxidized region 152 is 35 nm or less. The thickness of the outer region 153 may be greater than the thickness of the non-oxidized region 152. In this disclosure, the thickness of the non-oxidized region 152 refers to the thickness of the portion closer to the center of the mesa 180 than the inner edge of the oxidized region 151 (the inner edge of the inner region 154). For example, the distance from the side of the mesa 180 to the inner edge of the oxidized region 151 is in the range of approximately 8 μm to 11 μm.

The oxidized region 151 is formed by, for example, oxidizing and constricting a p-type AlAs layer and a p-type Al _0.85 Ga _0.15 As layer. For example, the oxidized region 151 can be formed by oxidizing the p-type AlAs layer and the p-type Al _0.85 Ga _0.15 As layer in a high-temperature water vapor environment. Even if the same p-type AlAs layer and the p-type Al _0.85 Ga _0.15 As layer are oxidized, the structure of the oxidized constriction layer obtained from the p-type AlAs layer and the p-type Al _0.85 Ga _0.15 As layer may differ depending on the oxidation conditions. Therefore, even if the layer that becomes the oxidized constriction layer 150 by oxidation, for example, the p-type AlAs layer and the p-type Al _0.85 Ga _0.15 As layer, have the same structure before oxidation, the oxidized constriction layer 150 having the oxidized region 151 and the non-oxidized region 152 may not be obtained depending on the oxidation conditions.

Here, the effects of the first embodiment will be explained in comparison with the reference example. Figure 3 is a cross-sectional view showing the oxidized constriction layer and its vicinity in the reference example.

In the reference example, the oxidized constriction layer 150 has an oxidized region 951 and a non-oxidized region 952 instead of the oxidized region 151 and the non-oxidized region 152. The oxidized region 951 has an annular planar shape and surrounds the non-oxidized region 952. The non-oxidized region 952 is composed of a p-type AlAs layer 955 and two p-type Al _0.85 Ga _0.15 As layers 956 that sandwich the p-type AlAs layer 955 in the vertical direction. The oxidized region 951 is composed of AlO _x . The oxidized region 951 has an annular outer region 953 and an annular inner region 954 in a plan view. The outer region 953 is exposed to the side surface of the mesa 180. The thickness of the outer region 953 is constant in the in-plane direction. The inner region 954 is located inside the outer region 953. The thickness of the inner region 954 is equal to the thickness of the outer region 953 at the boundary with the outer region 953, and becomes thinner toward the center of the mesa 180. The inner region 954 has a tapered shape that gradually becomes thicker from the inner edge toward the boundary with the outer region 953 in a cross-sectional view. The non-oxidized region 952 is located inside the outer region 953. A part of the non-oxidized region 952 sandwiches the inner region 954 in the vertical direction. Another part of the non-oxidized region 952 is located inside the inner edge of the inner region 954 in a plan view. For example, the distance from the side surface of the mesa 180 to the inner edge of the oxidized region 951 is in the range of about 8 μm to 11 μm. The thicknesses of the oxidized region 951 and the non-oxidized region 952 are equal to the thickness of the oxidized constriction layer 150.

First, we will explain the results of actual measurements for the first embodiment and the reference example. Figure 4 is an equivalent circuit diagram showing the circuit used for the actual measurements.

In this circuit, a resistor 12 for monitoring current is connected in series to a surface-emitting laser 11 corresponding to the first embodiment or the reference example. A voltmeter 13 is connected in parallel to the resistor 12. The light output from the surface-emitting laser 11 is received by a wideband high-speed photodiode and converted into a voltage signal, which is then observed by an oscilloscope.

Figure 5 shows the results of measurements for the reference example. Figure 5(a) shows the results of measurements when the pulse current width is about 2 ns, Figure 5(b) shows the results of measurements when the pulse current width is about 9 ns, and Figure 5(c) shows the results of measurements when the pulse current width is about 17 ns. The magnitude of the bias current and the amplitude of the pulse current are the same in the measurements of Figures 5(a) to (c). Figure 5 shows the current flowing through resistor 12 connected to the surface-emitting laser of the reference example and the optical output measured with a high-speed photodiode. The current flowing through resistor 12 can be calculated using voltmeter 13.

As shown in FIG. 5, in the reference example, regardless of the width of the pulse current, a pulsed light is output immediately after the pulsed current is injected, and thereafter, an equilibrium state is reached until the injection of the pulsed current is stopped, and a constant tail light is output. The leading pulsed light is caused by relaxation oscillation, which is a typical gain switching drive. Even if the pulse width is changed, the timing at which the pulsed light is generated does not change. This is because the pulsed light generated by relaxation oscillation is generated immediately after the carrier density in the laser resonator exceeds the threshold carrier density. In order to suppress the output of the tail light, it is possible to stop the current injection immediately after the pulsed light is output. However, since the time width of the pulsed light due to relaxation oscillation is 100 ps or less, when the magnitude of the current is as large as 10 A or more, it is difficult to stop the current injection in a time of 100 ps or less immediately after the pulsed light is output.

FIG. 6 is a diagram showing the results of actual measurements for the first embodiment. FIG. 6(a) shows the results of actual measurements when the pulse current value injected into the surface-emitting laser 100 is 10 mA. The pulse width of the pulse current at this time is 15 ns. FIG. 6(b) shows the results of actual measurements when the pulse current value injected into the surface-emitting laser 100 is 15 mA. The pulse width of the pulse current at this time is also 15 ns. The magnitude of the bias current and the width of the pulse current are both the same. The area of the non-oxidized region 152 in both oxidized constriction layers 150 is 40 μm ^2. FIG. 6(c) shows the results of actual measurements when the pulse current value injected into the surface-emitting laser 100 is 15 mA. The pulse width of the pulse current at this time is 2.5 ns. The area of the non-oxidized region 152 in the oxidized constriction layer 150 of the surface-emitting laser 100 is 80 μm ² . 6 shows the current flowing through the resistor 12 connected to the surface-emitting laser 100 (11) according to the first embodiment and the optical output measured by a high-speed photodiode. The current flowing through the resistor 12 can be calculated using a voltmeter 13.

As shown in FIG. 6(a), when the pulse current value injected into the surface-emitting laser 100 is relatively small, a single pulse of light is not generated over the injection period of the pulse current, while multiple short pulses of light 61, 62, 63, etc. are output. In contrast, as shown in FIG. 6(b), when the pulse current value injected into the surface-emitting laser 100 is relatively large, a single short pulse of light 71' is output immediately after the injection of the pulse current is reduced. Also, in FIG. 6(c), a single short pulse of light 71'' is output immediately after the injection of the pulse current is reduced. In FIG. 6(a), a short pulse of light 71 with a smaller intensity than the short pulses of light 61, 62, 63, etc. is also observed immediately after the injection of the pulse current is reduced. In FIG. 6(b), a light output (short pulse of light 61') is observed during the injection period of the pulse current, but the intensity of the short pulse of light 71' immediately after the injection of the pulse current is reduced is greater than the intensity of the short pulse of light 61'. In FIG. 6(c), no light is output during the current injection period. Here, in all cases of FIG. 6(a), FIG. 6(b), and FIG. 6(c), almost no tail light is observed after the pulse light is output. Also, the optical pulse width of each of the short pulse lights 61, 62, 63, etc. shown in FIG. 6(a) is approximately 50 ps. The number of pulse lights can be controlled by changing the current pulse width. Also, the optical pulse width of the short pulse light 61' shown in FIG. 6(b) is approximately 50 ps. In this way, the oscillation characteristics of short pulse light less than 100 ps can be changed by the pulse current value injected into the surface-emitting laser 100.

If the optical output is due to gain switching, multiple short pulses of light oscillated periodically as shown in FIG. 6(a) are not generated. In addition, as shown in FIG. 6(b) and FIG. 6(c), pulsed light is not output in response to a decrease in the injection of pulsed current. Therefore, it can be said that the optical output from the surface-emitting laser 100 in the first embodiment is not normal gain switching that utilizes the relaxation oscillation phenomenon.

Here, the period during which a pulse current is injected into the surface-emitting laser 100 may be referred to as a "current injection period." Also, the period after the current injection period during which the current value injected into the surface-emitting laser 100 is lower than that during the current injection period may be referred to as a "current reduction period."

As in the case of FIG. 6(a), the current value for outputting pulsed light at least once during the current injection period of the surface-emitting laser 100 according to the first embodiment may be referred to as the "first current value." At this time, during the current reduction period, there are cases where pulsed light is not output, and cases where pulsed light with a lower intensity than the pulsed light during the current injection period is output. The range of the first current value may be, for example, a current value less than 15 mA.

In contrast, as in the case of FIG. 6(b), a current value that causes at least one pulsed light to be output during the current reduction period by injecting a pulsed current having a current value greater than the first current value into the surface-emitting laser 100 according to the first embodiment is sometimes referred to as the "second current value." In this case, during the current injection period, there are cases where no pulsed light is output, and cases where pulsed light with a lower intensity than the pulsed light during the current reduction period is output. Note that when the current value injected during the current injection period is the second current value, the pulsed light output during the current injection period is an example of the "first light." Also, the pulsed light output during the current reduction period is an example of the "second light."

As such, the mechanism and manner of light output are clearly different between the first embodiment and the reference example. This difference is explained as follows.

In a surface-emitting laser, the laser light propagates in the cavity in a direction perpendicular to the oxide constriction layer. Therefore, the thicker the oxide constriction layer, the longer the equivalent waveguide length that depends on the refractive index difference, and the greater the lateral light confinement effect. When a DBR including an oxide constriction layer is considered as an equivalent waveguide structure, when the equivalent refractive index difference is large as shown in FIG. 7(a), the electric field intensity distribution of the laser light is concentrated near the center. In contrast, when the equivalent refractive index difference is small as shown in FIG. 7(b), the electric field intensity distribution of the laser light spreads to the surrounding oxidized region. Comparing the first embodiment and the reference example, in the first embodiment, the oxide constriction layer 150 includes the inner region 154, so that the equivalent refractive index difference is small in the first embodiment. Therefore, in the reference example, the electric field intensity distribution of the laser light is concentrated near the center as shown in FIG. 7(a), whereas in the first embodiment, the electric field intensity distribution of the laser light spreads to the oxidized region 151 as shown in FIG. 7(b).

Here, the lateral optical confinement coefficient is defined as the ratio of the "integral strength of the electric field in the same radial region as the current passing region" to the "integral strength of the electric field in the lateral cross section passing through the center of the surface-emitting laser element," and is given by formula (1). Here, a corresponds to the radius of the current passing region, and Φ represents the direction of rotation about the axis of rotation perpendicular to the substrate.

Next, we will explain the model of the phenomenon that occurs when the injection of the pulse current is stopped. When the pulse current is being injected, the oxide confinement layer causes the current path to concentrate near the center of the mesa, resulting in a high carrier density. At this time, in the non-oxidized region with high carrier density, the carrier plasma effect occurs, which reduces the refractive index. The carrier plasma effect is a phenomenon in which the refractive index decreases in proportion to the free carrier density. For example, according to the literature "Kobayashi, Soichi, et al. "Direct frequency modulation in AlGaAs semiconductor lasers." IEEE Transactions on Microwave Theory and Techniques 30.4 (1982): 428-441," the amount of change in the refractive index is expressed by formula (2). Here, N is the carrier density.

Figure 8 shows a schematic of the equivalent refractive index and electric field intensity distribution during the period when the pulse current is injected (Figure 8(a)) and the period after the injection of the pulse current is stopped (Figure 8(b)). During the period when the pulse current is injected, the carrier plasma effect acts in a direction that cancels the equivalent refractive index difference (n1-n0) caused by the oxidized confinement layer, and the equivalent refractive index difference becomes (n2-n0). When the injection of the pulse current is reduced in this state, the carrier plasma effect disappears and the equivalent refractive index difference returns to (n1-n0). As a result, the photons that had spread to the periphery of the mesa are collected in the center of the mesa, and the photon density in the non-oxidized region increases. In other words, the lateral optical confinement changes to a strong state. When the injection of the pulse current is stopped, the carriers accumulated in the resonator decrease over the carrier life time. However, if the lateral optical confinement becomes strong before the carrier density completely decays, stimulated emission begins, and the accumulated carriers are consumed all at once, and pulsed light is output. The period during which the pulse current is injected is an example of a current injection period, and the period during which the injection of the pulse current is stopped and decreases is an example of a current decrease period.

The results of verifying the above model through simulation are shown below. The rate equations for carrier density and photon density are shown in equations (3) and (4).

Here, the contents of each character in the formulas (3) and (4) are as follows:
N: carrier density [1/cm ³ ]
S: photon density [1/cm ³ ]
i(t): Injection current [A]
e: elementary charge [C]
V: Resonator volume [cm ³ ]
τ _n (N): Carrier lifetime [s]
v _g : group velocity [cm/s]
g(N, S): Gain [1/cm]
Γ _a : Optical confinement factor τ _p : Photon lifetime [s]
β: spontaneous emission coupling coefficient g ₀ : gain coefficient [1/cm]
ε: Gain suppression coefficient N _tr : Transparent carrier density [1/cm ³ ]
η _i : current injection efficiency α _m : cavity mirror loss [1/cm]
h: Planck constant [Js]
ν: Frequency of light [1/s]

The gain g(N, S) is expressed by equation (5).

The optical confinement coefficient Γ _a is defined as the product of the optical confinement coefficient in the lateral direction Γ _r and the optical confinement coefficient in the vertical direction Γ _z as shown in formula (6).

The threshold carrier density N _th is expressed by the formula (7).

The threshold current I _th and the threshold carrier density N _th have the relationship expressed by equation (8).

The relationship between the optical output P and the photon density S from the resonator is given by equation (9).

Here, the results of a simulation for the reference example will be described. For the reference example, the lateral optical confinement factor _Γr was set to 1, and the simulation was performed by inputting the current monitor waveform shown in Fig. 5. The simulation results for the carrier density N and threshold carrier density _Nth are shown in Fig. 9, and the simulation results for the optical output are shown in Fig. 10.

As shown in Figures 9 and 10, immediately after about 5 ns from the injection of the pulse current, the carrier density N exceeds the threshold carrier density _Nth , and pulsed light due to relaxation oscillation is output. After that, an equilibrium state is reached and a constant tail light is output. In this way, the simulation has obtained results close to the actual measurement results shown in Figure 5.

Next, the results of the simulation for the first embodiment will be described. For the first embodiment, the lateral optical confinement factor _Γr is set to less than 1, and the lateral optical confinement factor _Γr is set to a function that decreases with an increase in carrier density N, and the current monitor waveform shown in FIG. 6 was input to perform the simulation. The reason why the lateral optical confinement factor _Γr is set to the above function is to take into account the influence of the refractive index change due to the carrier plasma effect. FIG. 11 is a diagram showing an example of the function. The simulation results of the optical output are shown in FIG. 12.

As shown in Figure 12, pulsed light is output when the injection of the pulsed current is stopped. In this way, the simulation produced results close to the actual measurement results shown in Figure 6.

In order to analyze this result in detail, the simulation results of the carrier density N, threshold carrier density _Nth , photon density S, and lateral optical confinement factor _Γr under the condition of a pulse width of 2.5 ns are shown in Fig. 13. Fig. 13(a) shows the simulation results of the carrier density N, threshold carrier density _Nth , and photon density S, and Fig. 13(b) shows the simulation result of the lateral optical confinement factor _Γr .

Since the lateral optical confinement factor _Γr is a function of the carrier density N, the lateral optical confinement factor _Γr decreases in the range of 3 ns to 5.5 ns when the pulse current is injected. In this range, the threshold carrier density _Nth increases as the lateral optical confinement factor _Γr decreases, and since N< _Nth , stimulated emission is unlikely to occur and the photon density S does not increase. When the injection of the pulse current begins to decrease at 5.5 ns, the lateral optical confinement factor _Γr increases again, and in the process, the photon density S is generated in a pulsed manner. A graph in which the time axis is enlarged in the range of 5 ns to 6 ns in FIG. 13 is shown in FIG. 14.

When the injection of the pulse current starts to decrease at about 5.5 ns, the carrier density N starts to decrease. At the same time, the lateral optical confinement factor _Γr also increases, and the threshold carrier density _Nth decreases. Since the threshold carrier density _Nth decreases faster than the carrier density N, there is a time when N> _Nth in the process of the carrier density N decreasing. During this time, the photon density S first increases due to spontaneous emission, and when the photon density S increases to a certain extent, stimulated emission becomes dominant and the photon density S increases rapidly. At the same time, the carrier density N decreases rapidly, and when N< _Nth again, the photon density also decreases rapidly.

In this way, the simulation was able to reproduce the phenomenon in which pulsed light is output when the injection of pulsed current is stopped.

The difference between FIG. 6(b) and FIG. 6(c), i.e., the presence or absence of optical output at the start of current injection, may be influenced by the characteristics of the light-emitting element of the surface-emitting laser 100 and the characteristics of the injected current pulse. The characteristics of the light-emitting element are, for example, the strength of oscillation suppression, and the characteristics of the current pulse are, for example, the shape of the current pulse. It is believed that the closer to an ideal square wave without spike-like overshoot, the more the oscillation at the start of current injection is suppressed.

The rise time of the pulsed light becomes shorter when the threshold carrier density _Nth decreases faster than the carrier lifetime. In other words, according to equation (6), the rise time becomes shorter as the lateral optical confinement factor _Γr increases faster. The decay time of the pulsed light depends on the photon lifetime. Examples of actual measurement results and simulation results of the pulsed light are shown in Figure 15. Figure 15(a) shows the actual measurement results, and Figure 15(b) shows the simulation results.

In this specification, the optical pulse width is defined as the time width at which the output peak value of the pulsed light is 1/ ^e2 . The obtained optical pulse width is 86 ps in the actual measurement result of FIG. 15(a) and 81 ps in the simulation result of FIG. 15(b). Here, e is the base of the natural logarithm (Napier's constant). According to this model, the optical pulse width is shorter than the injected pulse current and can be shortened without being limited by the time width of the injected pulse current.

In the actual measurement results of Fig. 6(a) to (c), a pulsed light with an optical pulse width (time width at which the peak value is 1/ ^e2 ) of 80 to 110 ps was obtained in all cases. In the case of Fig. 6(a), in addition to the pulsed light with an optical pulse width of 80 to 110 ps, an optical pulse with an optical pulse width of about 110 to 200 ps was also included. On the other hand, in the actual measurement results of the reference example (Fig. 5(a) to (c)), a certain amount of tail light is output after the pulsed light, so that the peak value does not become 1/e2 or ^less until the current injection is stopped. In other words, since the optical pulse width (time width at which the peak value is 1/ ^e2 ) depends on the width of the pulse current, it is difficult to obtain an optical pulse width on the order of picoseconds.

In this embodiment, a continuous train of optical pulses is unlikely to occur after the output of pulsed light occurs during the current reduction period. This is because when pulsed light is generated, the injection of pulsed current is reduced, making relaxation oscillation unlikely to occur.

In addition, tail light is less likely to occur after the pulsed light is output. This is because the injection of pulsed current is reduced after the pulsed light is output, making it difficult for the carrier density to increase.

In addition, when the current value injected during the current injection period is the second current value, the pulsed light is output immediately after the injection of the pulsed current is stopped, so the output timing of the pulsed light can be controlled arbitrarily.

In addition, the width of the optical pulse generated by the first embodiment is shorter than the width of the injected pulse current. Since there is no need to shorten the pulse current width even when a large current is applied, it is less susceptible to the effects of parasitic inductance.

By arranging a plurality of surface-emitting lasers 100 according to the first embodiment in parallel to form a surface-emitting laser array and simultaneously outputting pulsed light, it is possible to obtain a larger optical peak output. The current injected into the surface-emitting laser array is larger than the current injected into a single surface-emitting laser 100, but since the width of the optical pulse output by the surface-emitting laser 100 is narrower than the width of the injected pulse current, it is possible to output an optical pulse with a small width.

The pulse width of the light output from the surface-emitting laser 100 according to the first embodiment is not limited, but is, for example, 1 ns or less, preferably 500 ps or less, and more preferably 100 ps or less.

In the first embodiment, the thickness of the oxidized region 151 at a position 3 μm outward from the inner edge of the inner region 154, that is, at a position 3 μm from the tip of the boundary between the non-oxidized region 152 and the oxidized region 151, is preferably 2 times or less than the thickness of the non-oxidized region 152. For example, when the thickness of the non-oxidized region 152 is 31 nm, the thickness at a position 3 μm outward from the inner edge of the inner region 154 is preferably 62 nm or less, and may be 54 nm. When the distance (oxidation distance) from the side surface of the mesa 180 to the inner edge of the oxidized region 151 is in the range of 8 μm to 11 μm, the distance of 3 μm corresponds to 28% to 38% of the oxidation distance. When the thickness of the oxidized region 951 and the thickness of the non-oxidized region 952 were measured at a position 3 μm outward from the inner edge of the oxidized region 951 during the actual measurement of the above reference example, the former was 79 nm and the latter was 31 nm, and the former was 2.55 times the latter. The inventors compared and evaluated elements with various oxide confinement structures and found that when the ratio is 2 or less, the lateral optical confinement factor _Γr becomes small, making it easier to obtain high-output, short-pulse light without tailing.

Here, we will explain the results of an optical mode simulation performed using actual measured values of the first embodiment and the reference example. In the optical mode simulation, the electric field intensity distribution of the eigenmode can be calculated by setting the refractive index of each region in a rotationally symmetric layered structure model. In this optical mode simulation, calculations were performed under cold cavity conditions that do not include the effects of heat generation due to current flow.

Figure 16 shows the first model used in the simulation. The first model has an n-type DBR 20, an active layer region 30, and a p-type DBR 40. The active layer region 30 is located on the n-type DBR 20, and the p-type DBR 40 is located on the active layer region 30.

The active layer region 30 has a lower spacer layer 31, a quantum well layer 32, and an upper spacer layer 33. The lower spacer layer 31 is on the n-type DBR 20, the quantum well layer 32 is on the lower spacer layer 31, and the upper spacer layer 33 is on the quantum well layer 32.

The p-type DBR 40 has multiple low-refractive index layers 41, multiple high-refractive index layers 42, and an oxidized constriction layer 50. The lowest low-refractive index layer 41 (41A) is on the upper spacer layer 33. The oxidized constriction layer 50 is on this low-refractive index layer 41A. The oxidized constriction layer 50 includes an oxidized region 51 and a non-oxidized region 52 in a plane perpendicular to the light emission direction. The oxidized region 51 has a ring-shaped planar shape and surrounds the non-oxidized region 52. The non-oxidized region 52 is made of AlAs. The second low-refractive index layer 41 (41B) is on the oxidized constriction layer 50. A high-refractive index layer 42 and another low-refractive index layer 41 are alternately stacked on this low-refractive index layer 41B. In this model, the thickness of the oxidized region 51 is constant and does not have a tapered shape.

The optical thickness of the active layer region 30 is the oscillation wavelength λ, the sum of the thicknesses of the low refractive index layer 41A, the oxidized constriction layer 50, and the low refractive index layer 41B is 3λ/4, and the thickness of the high refractive index layer 42A is λ/4.

First, the relationship between the optical confinement coefficient and the thickness of the oxidized constriction layer 50 will be described. FIG. 17 shows the cross-sectional profile of the electric field strength distribution of the fundamental mode in a structure with a diameter of the non-oxidized region of 5 μm. FIG. 17(a) shows the cross-sectional profile when the thickness of the oxidized constriction layer 50 is 20 nm, and FIG. 17(b) shows the cross-sectional profile when the thickness of the oxidized constriction layer 50 is 40 nm. The optical confinement coefficient can be estimated as the ratio of the electric field strength in the oxidized constriction layer 50 to the sum of the electric field strength in the entire region. In the structure with the oxidized constriction layer 50 having a thickness of 20 nm, the electric field strength distribution spreads laterally compared to the structure with the oxidized constriction layer 50 having a thickness of 40 nm, and the proportion of the electric field present outside the non-oxidized region 52 is large, so the optical confinement coefficient is small.

Next, the relationship between the optical confinement coefficient and the thickness of the oxidized confinement layer 50 and the diameter of the non-oxidized region 52 will be described. FIG. 18 is a diagram showing the calculation results of the relationship between the optical confinement coefficient and the thickness of the oxidized confinement layer and the diameter of the non-oxidized region. The horizontal axis in FIG. 18 indicates the diameter of the non-oxidized region 52, and the vertical axis indicates the optical confinement coefficient. FIG. 18 shows the results of the optical confinement coefficient calculated when the thickness of the oxidized confinement layer 50 is in the range of 20 nm to 60 nm and the diameter of the non-oxidized region 52 is in the range of 3 μm to 9 μm. When the thickness of the oxidized confinement layer 50 is 40 nm or more and the diameter of the non-oxidized region 52 is 5 μm or more, the optical confinement coefficient tends to saturate at about 0.9 or more. This tendency corresponds to the fact that in the range where the optical confinement coefficient is smaller than the saturation value, the horizontal spread of the electric field intensity distribution becomes large as shown in FIG. 17(a), and the proportion of the electric field intensity outside the oxidized region is large.

So far, we have shown the calculation results for the relationship between the thickness of the oxidized constriction layer 50 and the diameter of the non-oxidized region 52 and the optical confinement coefficient. Next, we will show the results of calculating the electric field strength distribution when the refractive index near the tip of the oxidized region 51 is intentionally reduced.

The region 60 for reducing the refractive index is a region with a thickness of about 200 nm above the non-oxidized region 52 (AlAs layer), which corresponds to the lowest pair of p-type DBR 40, and a region with a thickness of about 300 nm below the non-oxidized region 52 (AlAs layer), which includes the upper spacer layer 33, the quantum well layer 32, and the lower spacer layer 31. The reason why the lower region is larger than the upper region is because it is assumed that the lower region is closer to the active layer region 30 and has a larger region with a higher carrier density. The radial region for changing the refractive index is the same range as the non-oxidized region 52.

Figure 19 shows the calculation results of the relationship between the thickness of the oxidized constriction layer and the optical confinement coefficient for the first model. The thickness of the oxidized constriction layer 50 was set to four levels ranging from 30 nm to 60 nm, and the amount by which the refractive index n was reduced (reduction in the refractive index n) ranged from 0 to approximately 0.02. The horizontal axis in Figure 19 indicates the reduction in the refractive index n, and the vertical axis indicates the optical confinement coefficient. The greater the reduction in the refractive index n, the lower the refractive index in the region 60. When the thickness of the oxidized constriction layer 50 is 60 nm, the optical confinement coefficient is hardly reduced even if the refractive index is significantly reduced, but it can be seen that the optical confinement coefficient is more likely to decrease with the thickness of the oxidized constriction layer 50, even if the reduction in the refractive index n is small.

Next, the results of a simulation using a model closer to the first embodiment or the reference example will be described. FIG. 20 is a diagram showing the second model used in the simulation, and FIG. 21 is a diagram showing the third model used in the simulation. The second model is closer to the first embodiment, and the third model is closer to the reference example. As with the first model, the electric field intensity distribution was calculated for the second and third models when the refractive index near the tip of the oxidized region was intentionally reduced.

Figure 22 is a diagram showing the calculation results of the relationship between the amount of reduction in the refractive index and the optical confinement coefficient for the second and third models. The amount of reduction in the refractive index n was in the range of 0 to about 0.01. The horizontal axis on the lower side of Figure 22 indicates the amount of reduction in the refractive index n, and the vertical axis indicates the optical confinement coefficient. As shown in Figure 22, in the second model (Figure 20) simulating the first embodiment, when the refractive index is reduced to about 0.01, the optical confinement coefficient, which was 0.7 before the reduction, is reduced to 0.1. On the other hand, in the third model (Figure 21) simulating the reference example, when the refractive index is reduced to about 0.01, the optical confinement coefficient, which was 0.9 before the reduction, is reduced only to 0.7. Therefore, since the amount of change in the optical confinement coefficient with respect to the reduction in the refractive index is large in the second model, it can be said that pulsed light output due to a sudden change in the optical confinement coefficient can be realized.

As shown on the upper horizontal axis of FIG. 22, when converted from the formula (2) for estimating the carrier plasma effect, a decrease in the refractive index n of 0.006 corresponds to a carrier density N of 1.5×10 ¹⁸ [1/cm ³ ], and a decrease in the refractive index n of 0.010 corresponds to a carrier density N of 2.5×10 ¹⁸ [1/cm ³ ]. FIG. 11 shows a function in which the optical confinement factor decreases when the carrier density N is in the range of 5.0×10 ¹⁸ [1/cm ³ ] to 1.5×10 ¹⁹ [1/cm ³ ]. The range of the carrier density N is different between FIG. 11 and FIG. 22, which is considered to be because the target of the change in the refractive index is a wide region including the top and bottom of the quantum well layer in FIG. 11, whereas the target of the change in the refractive index is a wide region including the top and bottom of the quantum well layer. It is assumed that the carrier density in the wide region above and below the quantum well layer spreads due to lateral diffusion or the like. Assuming that the carrier density in the region with the reduced refractive index is approximately one order of magnitude smaller than the carrier density in the quantum well layer, the carrier density range shown in FIG. 11 can be considered equivalent to the carrier density range shown in FIG. 22.

Next, regarding the first embodiment, the results of a simulation of the relationship between the thickness of the oxidized region at a position 3 μm from the boundary between the non-oxidized region 52 and the oxidized region 51 and the optical confinement coefficient are shown. FIG. 23 is a diagram showing a fourth model used in the simulation. In the fourth model, the oxidized region 51 has a first region 51A, a second region 51B, and a third region 51C. The first region 51A, the second region 51B, and the third region 51C have annular planar shapes. The first region 51A is located outside the non-oxidized region 52, the second region 51B is located outside the first region 51A, and the third region 51C is located outside the second region 51B. The non-oxidized region 52 also has an AlAs layer 55 and two AlGaAs layers 56 that sandwich the AlAs layer 55 in the vertical direction.

The thickness of the AlAs layer 55 is 30 nm. The thickness of the first region 51A is 30 μm, the thickness of the third region 51C is T [μm], and the thickness of the second region 51B is T/2 [μm]. The radial widths of the first region 51A and the second region 51B are both 1.5 μm. The thickness of the oxidized region 51 at a position 3 μm outward from the boundary 59 between the non-oxidized region 52 and the oxidized region 51, i.e., the boundary between the non-oxidized region 52 and the first region 51A, is the thickness of the third region C.

Figure 24 shows the calculation results of the relationship between the thickness of the oxidized region at a position 3 μm outward from the boundary for the fourth model and the optical confinement coefficient. Figure 24 shows the calculation results when the refractive index does not decrease and when the refractive index decreases by 0.006. The horizontal axis in Figure 24 indicates the thickness of the oxidized region 51 at a position 3 μm outward from the boundary 59, and the vertical axis indicates the optical confinement coefficient.

24, when the thickness of the oxidized region 51 at a position 3 μm away from the boundary 59 is 60 nm or less, the decrease in the optical confinement coefficient is larger than when the thickness exceeds 60 nm. In other words, when the thickness of the oxidized region 51 at a position 3 μm away from the boundary 59 is twice the thickness of the AlAs layer 55 or less, the decrease in the optical confinement coefficient is larger than when the thickness exceeds twice the thickness. Therefore, in the region where the thickness of the oxidized region 51 at a position 3 μm away from the boundary 59 is twice the thickness of the AlAs layer 55 or less, the change in the optical confinement coefficient with respect to the change in the refractive index is large, so that it can be said that the pulsed optical output due to the sudden change in the optical confinement coefficient according to the first embodiment can be realized. The actual measured value of the thickness of the oxidized region 151 at a position 3 μm away from the inner edge of the inner region 154 of the sample produced according to the first embodiment was 54 nm. The actual measured value of the thickness of the oxidized region 951 at a position 3 μm away from the inner edge of the oxidized region 951 of the sample produced according to the reference example was 79 nm.

The area of the non-oxidized region 152 in plan view (current confinement area) is desirably ¹²⁰ μm2 or less. As a result of the inventors' comparative evaluation of elements with various non-oxidized regions 152, it was found that when the non-oxidized region 152 exceeds ¹²⁰ μm2, the phenomenon of outputting pulsed light immediately after the injection of the pulsed current is stopped is less likely to occur. It was also found that a smaller non-oxidized region 152 makes it easier to obtain pulsed light with a high peak output. Figure 25 shows the measurement results of peak light output for samples with non-oxidized region areas in the range of 50 ^μm2 to ¹²⁰ μm2.

In addition, in the surface-emitting laser 100 according to the first embodiment, the oscillation characteristics of the pulsed light can be changed as follows depending on the injected pulsed current value. That is, the oscillation characteristics of the pulsed light can be changed depending on whether the injected pulsed current value is within a first current value range or is a second current value that exceeds the first current value.

There are two main types of carriers to be considered in the carrier plasma effect: carriers accumulated in the active layer 130 and carriers present in the path of the injected current. In the oxide confinement type surface emitting laser 100, the oxide confinement layer 150 confines the current injected into the active layer 130. Therefore, in the carrier density distribution after passing through the oxide confinement layer 150, the carrier density at the center of the element is higher than that at the peripheral portion. In addition, the carrier distribution at this time is not uniform, and when the injected pulse current value is small, the current value flowing through the tip of the oxidized region 151 of the oxide confinement layer 150 is large, whereas as the pulse current value increases, the current value flowing through the non-oxidized region 152 located at the center of the element increases. Therefore, when the pulse current value is large, the photon distribution at the center of the element including the non-oxidized region 152 is considered to decrease due to the carrier plasma effect. That is, when the pulse current value is relatively large, the effect of decreasing the optical confinement coefficient Γ _r in the lateral direction is large, and the photon density at the center of the element is sufficiently decreased. As a result, the threshold carrier density in laser oscillation increases, and laser oscillation is suppressed during the injection of the pulse current. On the other hand, at least one short pulse light is obtained in synchronization with the falling edge of the pulse current. In the case of the second current value, which is larger than the first current value, the effect of suppressing laser oscillation is higher than that in the case of the first current value, and since a sufficient number of carriers are accumulated, laser oscillation occurs at a high peak output as soon as the current injection is stopped.

On the other hand, when the pulse current value is small, the carrier density distributed in the center of the element is reduced, so the effect of lowering the lateral optical confinement factor Γ _r is small, and the decrease in the photon density in the center of the element is small, compared to when the pulse current value is large. That is, the threshold carrier density does not increase so much. Therefore, the carrier density accumulated in the active layer 130 during the injection of the pulse current eventually reaches the threshold carrier density. This causes stimulated emission, and oscillation of a short pulse light occurs during the current injection period. Furthermore, the oscillation of the short pulse light consumes the carriers accumulated in the active layer 130, and photons in the resonator also disappear according to the output of the short pulse light, but during the injection period of the pulse current, carriers are accumulated again due to the carrier plasma effect, and the oscillation of the pulse light is repeated. As a result, multiple short pulse lights are output during the current injection period.

The difference in the oscillation characteristics of the pulsed light will be described in detail with reference to Fig. 26 and Fig. 27. First, Fig. 26 shows the time changes of the carrier density N (solid line), threshold carrier density _Nth (dashed line), and current value I (dotted line) of the pulsed current in the active layer 130 when the pulsed current value is large. In contrast, Fig. 27 shows the time changes of the carrier density N (solid line), threshold carrier density _Nth (dashed line), and current value I (dotted line) of the pulsed current in the active layer 130 when the pulsed current value is small.

26 and 27, regardless of the magnitude of the pulse current value, after the pulse current is injected into the surface-emitting laser 100 according to the first embodiment, the carrier density N in the active layer 130 increases. Here, when the pulse current value is large, as shown in FIG. 26, the pump rate of carriers to the active layer 130 is large, and the rate of increase in the carrier density N is larger than when the pulse current value is small. Furthermore, when the pulse current value is large, the carrier density N distributed in the center of the element is high, so the lateral optical confinement factor _Γr becomes smaller. As a result, the effect of increasing the threshold carrier density _Nth is enhanced.

In contrast, when the pulse current value is small, as shown in FIG. 27, the carrier density N present at the center of the element is low, and a decrease in the lateral optical confinement factor _Γr is prevented. Therefore, the photon density at the center of the element does not decrease, and the increase in the threshold carrier density _Nth is also small. As a result, during the current injection period, the carrier density N accumulated in the active layer 130 can exceed the threshold carrier density _Nth . This causes a short pulse of light to be output during the current injection period. Moreover, as shown in FIG. 27, this phenomenon is repeated, and multiple short pulses of light are output.

In this way, the surface-emitting laser 100 according to the first embodiment can oscillate a train of multiple pulsed lights during the current injection period when the injected pulse current value is small, while oscillating at least one pulsed light during the current reduction period when the pulse current value is large. However, multiple pulsed lights may be oscillated during the current reduction period.

Second Embodiment
Next, a second embodiment will be described. The second embodiment relates to a surface emitting laser. Fig. 28 is a cross-sectional view showing a surface emitting laser according to the second embodiment.

The surface-emitting laser 200 according to the second embodiment is a VCSEL equipped with a current confinement structure, for example, a Buried tunnel junction (BTJ). The surface-emitting laser 200 has an n-type GaAs substrate 110, an n-type DBR 120, an active layer 130, a p-type DBR 241, a BTJ region 250, a p-type DBR 242, an upper electrode 160, and a lower electrode 170.

The p-type DBR 241 is on the active layer 130. The p-type DBR 241 is, for example, a semiconductor multilayer reflector formed by stacking multiple p-type semiconductor films. The BTJ region 250 is on a part of the p-type DBR 241. The BTJ region 250 includes a p-type layer 251 and an n-type layer 252. The p-type DBR 242 is on the p-type DBR 241 and covers the BTJ region 250. The p-type DBR 242 is, for example, a semiconductor multilayer reflector formed by stacking multiple p-type semiconductor films. The p-type DBR 242, the p-type DBR 241, the active layer 130, and the n-type DBR 120 include a mesa 280. The BTJ region 250 is at the center of the mesa 280 in the plane.

The p-type layer 251 is on the p-type DBR 241, and the n-type layer 252 is on the p-type layer 251. The p-type layer 251 contains a higher concentration of p-type impurities than the p-type semiconductor film constituting the p-type DBR 241. The n-type layer 252 contains a higher concentration of n-type impurities than the n-type semiconductor film constituting the p-type DBR 242. For example, the thickness of the p-type layer 251 is 5 nm to 20 nm, and the thickness of the n-type layer 252 is 5 nm to 20 nm. In a plan view, the part inside the outline of the BTJ region 250 of the mesa 280 is an example of a high refractive index region, and the part outside the outline of the BTJ region 250 of the mesa 280 is an example of a low refractive index region.

The upper electrode 160 contacts the upper surface of the p-type DBR 242. The lower electrode 170 contacts the lower surface of the n-type GaAs substrate 110. The pair of the upper electrode 160 and the lower electrode 170 is an example of an electrode pair.

In the second embodiment, no current flows between the p-type DBR 241 and the p-type DBR 242 because of the reverse bias. A current flows between the p-type layer 251 and the n-type layer 252 due to a buried tunnel junction. Therefore, the current path between the upper electrode 160 and the lower electrode 170 is narrowed to the center of the mesa 280 where the BTJ region 250 is located. In addition, since the BTJ region 250 forms a step and is covered by the p-type DBR 242, the refractive index within the surface of the mesa 280 is high at the center and low around it. Therefore, the surface-emitting laser 200 has a lateral light confinement effect.

Therefore, in the second embodiment, it is possible to output pulsed light by injecting a pulse current similar to that in the first embodiment.

Third Embodiment
Next, a third embodiment will be described. The third embodiment relates to a laser device. Fig. 29 is a diagram showing a laser device according to the third embodiment.

The laser device 300 according to the third embodiment has the surface-emitting laser 100 according to the first embodiment and a power supply device 301 connected to the upper electrode 160 and the lower electrode 170 of the surface-emitting laser 100. The power supply device 301 injects a current into the surface-emitting laser 100.

It is preferable that the duty ratio of the current injection from the power supply device 301 is 0.5% or less. In other words, the current injection period and the current reduction period are repeated multiple times, and the ratio of the current injection period to the current reduction period is 0.5% or less. The duty ratio is the ratio of the time during which a current pulse is injected in a unit time. If the pulse current width is t [s] and the repetition frequency of the pulse current is f [Hz], the duty ratio corresponds to f x t (%). Figure 30 shows the relationship between the duty ratio and the peak output of the pulse light when the pulse current width is 2.5 ns.

As shown in FIG. 30, when the duty ratio exceeds 0.5%, the optical peak output tends to decrease. The following model is considered as the reason for this. First, as the duty ratio is increased, the amount of heat generated in the current confinement region (non-oxidized region 152) by the injected pulse current increases. This causes the temperature of the center where the current is concentrated to rise compared to the peripheral portion of the current confinement region, resulting in a temperature difference. As a result, the refractive index of the center of the current confinement region increases due to the thermal lens effect, and the optical confinement coefficient in the lateral direction increases. When the optical confinement coefficient in the lateral direction increases due to the thermal lens effect, the influence of the refractive index change caused by the carrier plasma effect caused by the increase and decrease of the pulse current becomes smaller. Therefore, the phenomenon of pulsed light being output immediately after the injection of the pulse current is stopped is less likely to occur. On the other hand, if the duty ratio is 0.5% or less, the influence of the refractive index change caused by the thermal lens effect becomes sufficiently small, and the refractive index change caused by the confinement structure becomes dominant, so it is considered that the peak output remains almost constant.

In addition, the surface-emitting laser 200 according to the second embodiment may be used instead of the surface-emitting laser 100 according to the first embodiment.

Fourth Embodiment
Next, a fourth embodiment will be described. The fourth embodiment relates to a distance measurement device. Fig. 31 is a diagram showing a distance measurement device according to the fourth embodiment. The distance measurement device is an example of a detection device.

The distance measurement device 400 according to the fourth embodiment is a distance measurement device using the TOF (Time of Flight) method. The distance measurement device 400 has a light emitting element 410, a light receiving element 420, and a drive circuit 430. The light emitting element 410 irradiates an emission beam (irradiation light 411) toward an object 450 to be measured. The light receiving element 420 receives reflected light 421 from the object 450 to be measured. The drive circuit 430 drives the light emitting element 410 and measures the round trip distance to the object 450 to be measured by detecting the time difference between the emission timing of the emission beam and the reception timing of the reflected light 421 by the light receiving element 420.

The light-emitting element 410 includes the surface-emitting laser 100 according to the first embodiment or the surface-emitting laser 200 according to the second embodiment. The light-emitting element 410 may include a plurality of the surface-emitting lasers 100 according to the first embodiment or the surface-emitting lasers 200 according to the second embodiment arranged in an array. The pulse repetition frequency is, for example, in the range from several kHz to several tens of MHz.

The light receiving element 420 is, for example, a photodiode (PD), an avalanche photodiode (APD), or a single-photon avalanche diode (SPAD). The light receiving element 420 may include a plurality of light receiving elements arranged in an array. The light receiving element 420 is an example of a detection unit.

In distance measurement using the TOF method, it is important to separate the signal from the object to be measured from the noise. When measuring an object that is farther away or has a lower reflectivity, it is preferable to obtain the signal from the object using a more sensitive light receiving element. However, using a more sensitive light receiving element increases the possibility of false detection of background light noise or shot noise. In order to separate the signal from the noise, it is possible to raise the threshold of the received light signal, but if the peak output of the emitted light beam is not increased accordingly, it will become difficult to receive the signal light from the object to be measured. However, the output of the emitted light beam is subject to restrictions imposed by laser safety standards.

The surface-emitting laser 100 according to the first embodiment or the surface-emitting laser 200 according to the second embodiment can output pulsed light with a pulse width of less than 100 ps. This is approximately 1/10 of the optical pulse width of several ns output by a conventional surface-emitting laser. According to the fourth embodiment, the shorter the pulse width of the pulsed light, the higher the peak output permitted by safety standards, so it is possible to achieve both high accuracy and long distance while satisfying eye safety.

Fifth Embodiment
Next, the fifth embodiment will be described. The fifth embodiment relates to a moving body. FIG. 32 is a diagram showing an automobile as an example of a moving body according to the fifth embodiment. The distance measuring device 400 described in the fourth embodiment is provided on the upper front surface (for example, the upper part of the windshield) of an automobile 500 as an example of a moving body according to the fifth embodiment. The distance measuring device 400 measures the distance to an object 502 around the automobile 500. The measurement result of the distance measuring device 400 is input to a control unit of the automobile 500, and the control unit controls the operation of the moving body based on the measurement result. Alternatively, the control unit may display a warning on a display unit provided in the automobile 500 to a driver 501 of the automobile 500 based on the measurement result of the distance measuring device 400.

In this way, in the fifth embodiment, by providing the distance measurement device 400 to the automobile 500, the position of the object 502 around the automobile 500 can be recognized with high accuracy. The mounting position of the distance measurement device 400 is not limited to the upper front of the automobile 500, and it may be mounted on the side or rear. Also, in this example, the distance measurement device 400 is provided on the automobile 500, but the distance measurement device 400 may also be provided on an aircraft or ship. Also, it may be provided on a moving body that moves autonomously without a driver, such as a drone or a robot.

Although the preferred embodiments have been described above in detail, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

For example, aspects of the present invention are as follows.
<1> An active layer,
a plurality of reflecting mirrors facing each other with the active layer interposed therebetween;
A pair of electrodes connected to a power supply and capable of injecting a current into the active layer;
having
a period during which a current is injected by the power supply device is defined as a current injection period, and a period following the current injection period during which the value of the current injected into the active layer is lower than the current value during the current injection period is defined as a current reduction period,
when a current value injected during the current injection period is a first current value, laser oscillation occurs at least once during the current injection period;
When the current value injected during the current injection period is a second current value that is greater than the first current value, laser oscillation occurs at least once during the current reduction period.
Surface emitting laser.
<2> The surface-emitting laser according to <1>, wherein, when the current value injected during the current injection period is the first current value, laser oscillation is performed a plurality of times during the current injection period.
<3> The surface-emitting laser according to <1>, wherein, when the current value injected during the current injection period is the second current value, a first light is outputted by performing laser oscillation at least once during the current injection period, and a second light having an intensity greater than that of the first light is outputted by performing laser oscillation once during the current reduction period.
<4> The surface-emitting laser according to any one of <1> to <3>, wherein a time width that is 1/ ^e2 of an output peak value is set as an optical pulse width, and a single pulse light having an optical pulse width of 110 ps or less is emitted by at least one laser oscillation during the current injection period.
<5> A high refractive index region having a relatively high refractive index and a low refractive index region having a refractive index lower than that of the high refractive index region and surrounding the high refractive index region are provided in a plane perpendicular to the light emission direction,
The low refractive index region is formed by oxidation constriction,
The thickness of the high refractive index region is 35 nm or less;
The surface-emitting laser according to any one of <1> to <4>, wherein a thickness of the low refractive index region at a position 3 μm from an end of a boundary between the low refractive index region and the high refractive index region is equal to or less than twice a thickness of the high refractive index region.
<6> The surface-emitting laser according to <5>, wherein an area of a region surrounded by an end of a boundary between the low refractive index region and the high refractive index region in a plane perpendicular to a light emission direction is 120 μm ² or less.
<7> The surface emitting laser according to any one of <1> to <6>,
a power supply device connected to the electrode pair and configured to inject a current into the surface-emitting laser;
A laser device comprising:
<8> The laser device according to <7>,
a detection unit that detects light emitted from the surface emitting laser and reflected by an object;
Equipped with
Calculating the distance to the object based on the signal from the detection unit.
Detection device.
<9> A moving body including the detection device according to <8>.
<10> An active layer,
a plurality of reflecting mirrors facing each other with the active layer interposed therebetween;
A pair of electrodes connected to a power supply and capable of injecting a current into the active layer;
A method for driving a surface emitting laser having
a laser oscillation step of injecting the current from the electrode pair to cause the surface emitting laser to oscillate;
a period during which the current is injected by the power supply device is defined as a current injection period, and a period after the current injection period during which the value of the current injected into the active layer is lower than the current value during the current injection period is defined as a current reduction period,
The laser oscillation step includes:
when a current value injected during the current injection period is a first current value, oscillating the laser at least once during the current injection period;
performing laser oscillation at least once during the current reduction period when the current value injected during the current injection period is a second current value that is greater than the first current value;
A method for driving a surface emitting laser comprising the steps of:

100, 200 Surface emitting laser 120 n-type DBR
130 Active layer 140, 241, 242 p-type DBR
150 Oxidized constriction layer 151 Oxidized region 152 Non-oxidized region 160 Upper electrode 170 Lower electrode 180, 280 Mesa 250 BTJ region 251 p-type layer 252 n-type layer 300 Laser device 400 Distance measuring device 500 Automobile (mobile body)

U.S. Pat. No. 8,934,514

Claims (10)

An active layer;
a plurality of reflecting mirrors facing each other with the active layer interposed therebetween;
A pair of electrodes connected to a power supply and capable of injecting a current into the active layer;
having
a period during which a current is injected by the power supply device is defined as a current injection period, and a period following the current injection period during which the value of the current injected into the active layer is lower than the current value during the current injection period is defined as a current reduction period,
when a current value injected during the current injection period is a first current value, laser oscillation occurs at least once during the current injection period;
When the current value injected during the current injection period is a second current value that is greater than the first current value, laser oscillation occurs at least once during the current reduction period.
Surface emitting laser. The surface emitting laser according to claim 1, wherein when the current value injected during the current injection period is the first current value, the laser oscillates multiple times during the current injection period. The surface-emitting laser according to claim 1, wherein, when the current value injected during the current injection period is the second current value, the laser oscillates at least once during the current injection period to output a first light, and the laser oscillates once during the current reduction period to output a second light having a higher intensity than the first light. 3. The surface-emitting laser according to claim 1, wherein a time width at which an output peak value is 1/ ^e2 is set as an optical pulse width, and a single pulse light having an optical pulse width of 110 ps or less is emitted by at least one laser oscillation during the current injection period. a high refractive index region having a relatively high refractive index and a low refractive index region having a lower refractive index than the high refractive index region and surrounding the high refractive index region, in a plane perpendicular to the light emission direction;
The low refractive index region is formed by oxidation constriction,
The thickness of the high refractive index region is 35 nm or less;
3. The surface emitting laser according to claim 1, wherein a thickness of the low refractive index region at a position 3 μm from an end of a boundary between the low refractive index region and the high refractive index region is not more than twice a thickness of the high refractive index region. 6. The surface emitting laser according to claim 5, wherein an area of a region surrounded by an end of a boundary between the low refractive index region and the high refractive index region in a plane perpendicular to a light emission direction is 120 μm ² or less. A surface emitting laser according to claim 1 or 2;
a power supply device connected to the electrode pair and configured to inject a current into the surface emitting laser;
A laser device comprising: A laser device according to claim 7;
a detection unit that detects light emitted from the surface emitting laser and reflected by an object;
Equipped with
Calculating the distance to the object based on the signal from the detection unit.
Detection device. A moving object equipped with the detection device according to claim 8. An active layer;
a plurality of reflecting mirrors facing each other with the active layer interposed therebetween;
A pair of electrodes connected to a power supply and capable of injecting a current into the active layer;
A method for driving a surface emitting laser having
a laser oscillation step of injecting the current from the electrode pair to cause the surface emitting laser to oscillate;
a period during which the current is injected by the power supply device is defined as a current injection period, and a period after the current injection period during which the value of the current injected into the active layer is lower than the current value during the current injection period is defined as a current reduction period,
The laser oscillation step includes:
when a current value injected during the current injection period is a first current value, oscillating the laser at least once during the current injection period;
performing laser oscillation at least once during the current reduction period when the current value injected during the current injection period is a second current value that is greater than the first current value;
A method for driving a surface emitting laser comprising the steps of:

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