JP2014053560A - Laser resonator and vertical resonator type surface-emitting laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser resonator capable of suppressing a deviation in a resonant wavelength even when a film thickness modulation DBR is used as a reflector for a vertical resonator type surface-emitting laser, and to provide a vertical resonator type surface-emitting laser composed of the laser resonator.SOLUTION: The laser resonator includes a configuration in which a first reflector and a second reflector, by using DBR, are arranged face to face and resonates with light of a wavelength λ. The first reflector and the second reflector are so configured that at least part of layers among multilayer films constituting a DBR at each of the reflectors includes a configuration in which an optical film thickness is modulated in thickness by being shifted from λ/4, and directions of the film thickness modulation of the DBR at each of the first reflector and the second reflector are opposed.

Description

  The present invention relates to a laser resonator and a vertical cavity surface emitting laser constituted by the laser resonator.

As a kind of semiconductor laser structure, a vertical cavity surface emitting laser (hereinafter referred to as VCSEL) is known.
The VCSEL is configured by sandwiching the upper and lower sides of an active layer (light emitting layer) between a pair of reflecting mirrors. The light emitted from the active layer is repeatedly reflected by the upper and lower reflecting mirrors, resonates at a specific wavelength, is amplified by the active layer, and oscillates. Part of the light passes through the reflecting mirror and is extracted from the surface, and operates as a surface emitting laser.
Generally, a distributed Bragg reflector (DBR) composed of a multilayer film of a semiconductor or a dielectric is used as a reflective mirror of a VCSEL.
A typical DBR is designed such that two types of layers having different refractive indexes are alternately stacked, and assuming that the wavelength of a predetermined light is λ, the thickness of each layer is an optical film thickness λ / 4. . Here, the optical film thickness is obtained by multiplying the thickness of a certain layer of the multilayer film constituting the DBR by the refractive index of the material constituting the layer.
By adopting such a configuration, the light reflected at the interface of each layer interferes and strengthens, and a high reflectance close to 100% can be obtained.

Recently, however, a VCSEL using a so-called film thickness modulation DBR in which the optical film thickness of each layer is intentionally shifted from λ / 4 has been proposed.
For example, in Patent Document 1, the optical film thickness of a material having a relatively large light absorption loss among the materials constituting the DBR is made thinner than λ / 4, and the optical film thickness of a material having a relatively small light absorption loss is disclosed. A VCSEL formed thicker than λ / 4 is disclosed.
By adopting such a configuration, it is possible to reduce the light absorption loss while maintaining the high reflectivity of the DBR, thereby realizing a highly efficient VCSEL.
In addition to these, in order to improve heat dissipation and to suppress the occurrence of cracks due to differences in thermal expansion coefficient and lattice constant, a film thickness modulation DBR is proposed that is used as a VCSEL reflecting mirror. (For example, Patent Documents 2 and 3)

Japanese Patent Laid-Open No. 3-225585 US Pat. No. 6,720,585 JP 2003-107241 A

As described above, when the film thickness modulation DBR is used in a vertical cavity surface emitting laser (VCSEL), various characteristics can be improved. However, in this case, as described below, there arises a problem that the resonance wavelength is deviated from the design value.
The above patent document describes the characteristics of the film thickness modulation DBR alone, but does not describe any optical characteristics of the resonator formed by combining them. As a result of our investigation, when a thickness-modulated DBR is used, a resonator with the desired optical characteristics can be obtained even if the resonator is designed with the same design guidelines as when a conventional DBR is used. The problem of not being found.

This problem will be specifically described based on the calculation results.
FIG. 7 is a schematic diagram showing an example of a vertical resonator structure using a film thickness modulation DBR. In order to become a 1λ resonator at a wavelength λ = 400 nm,
Above and below the GaN film 710 having an optical film thickness of 400 nm,
A dielectric DBR 700 in which 7 pairs of SiO ₂ layers 701 and Nb ₂ O ₅ layers 702 are alternately stacked;
A semiconductor DBR 720 in which 20 pairs of AlN layers 721 and GaN layers 722 are alternately stacked;
A resonator sandwiched between the GaN substrates 730 is formed on the GaN substrate 730.
In order to simplify the drawing, the DBR is illustrated with a smaller number of pairs than actual.

The semiconductor DBR 720 is generally designed such that the optical film thickness of the AlN layer 721 and the GaN layer 722 is λ / 4, and the total optical film thickness of one cycle is λ / 2.
Here, the calculation is performed for the case where the optical film thickness of each layer of the semiconductor DBR 720 is shifted from λ / 4 to be a film thickness modulation DBR.
In this calculation, the optical film thickness in one cycle is kept at λ / 2 in order to suppress the decrease in reflectance. For example, when the thickness of the AlN layer is reduced, the thickness of the GaN layer is increased accordingly.

FIG. 8 shows the result of calculating the reflection spectrum when a plane wave is incident on the structure shown in FIG. 7 from the upper side of the figure.
The change in resonance wavelength was calculated when the optical film thickness of the AlN layer 721 constituting the semiconductor DBR 720 was changed in the range of 0.15λ to 0.35λ.
The dip wavelength at which the reflectivity rapidly decreases corresponds to the resonance wavelength.
The film thickness of the dielectric DBR 700 is not modulated, and the optical film thickness of each layer is λ / 4.
In the case where the semiconductor DBR is not subjected to film thickness modulation (the optical film thickness of the AlN layer and the GaN layer is 0.25λ), the resonance wavelength is 400 nm as designed, but when the film thickness modulation is applied, the degree of modulation is increased. Accordingly, the resonance wavelength shifts.
If the thickness of the AlN layer is reduced and the thickness of the GaN layer is increased, the resonance wavelength shifts to the short wavelength side. Conversely, if the thickness of the AlN layer is increased and the thickness of the GaN layer is reduced, the resonance wavelength is increased. Shift to the side.
As described above, if the reflection mirror of the VCSEL is simply replaced from the general DBR to the film thickness modulation DBR, the resonance wavelength is deviated from the design value.

  In view of the above problems, the present invention provides a laser resonator capable of suppressing a shift in resonance wavelength even when the film thickness modulation DBR is used as a reflector of a vertical cavity surface emitting laser, and the laser resonator An object of the present invention is to provide a vertical cavity surface emitting laser constituted by:

A laser resonator according to the present invention is a laser resonator having a configuration in which a first reflecting mirror and a second reflecting mirror by DBR are arranged to face each other, and resonates light having a wavelength λ.
In the first reflecting mirror and the second reflecting mirror, at least a part of the multilayer film constituting the DBR in each reflecting mirror is modulated by shifting the optical film thickness from λ / 4. Having a configuration,
The first reflecting mirror and the second reflecting mirror are configured such that the direction of the film thickness modulation of the DBR in each reflecting mirror is reversed.
The vertical cavity surface emitting laser of the present invention includes the above-described laser resonator, and includes an active layer between the first reflecting mirror and the second reflecting mirror in the laser resonator. It is characterized by.

  According to the present invention, it is possible to realize a laser resonator capable of suppressing a shift in resonance wavelength even when a film thickness modulation DBR is used, and a vertical cavity surface emitting laser constituted by the laser resonator. Can do.

It is a cross-sectional schematic diagram explaining the structural example of the laser resonator in embodiment of this invention. It is a graph which shows the film thickness modulation | alteration of DBR and the relationship of a reflectance in the laser resonator of embodiment of this invention. It is a graph which shows distribution of the refractive index and light intensity in the laser resonator of embodiment of this invention. It is a cross-sectional schematic diagram explaining the structural example of the laser resonator in embodiment of this invention. It is a cross-sectional schematic diagram which shows the layer structure of VCSEL in Example 1 of this invention. It is a graph which shows the resonant wavelength of VCSEL in Example 1 of this invention. It is a schematic diagram for demonstrating the subject which a prior art has. It is a graph for demonstrating the subject which a prior art has. It is the schematic diagram and graph for demonstrating the phase of the reflected light of film thickness modulation DBR in this invention. It is the schematic diagram and graph for demonstrating the phase of the reflected light of film thickness modulation DBR in this invention. It is a schematic diagram for demonstrating the phase of the reflected light of DBR in this invention.

Hereinafter, a laser resonator (hereinafter referred to as a resonator) and a vertical cavity surface emitting laser (VCSEL) according to an embodiment of the present invention will be described.
First, terms in this specification are defined.
In this specification, the substrate side of the element is defined as the lower side, and the side opposite to the substrate is defined as the upper side.
In this specification, when referring to the thickness of a layer, unless otherwise specified, it means an optical film thickness, not a physical film thickness.
In the present specification, of the two types of layers constituting the DBR, a layer made of a material having a relatively low refractive index is a low refractive index layer, and a layer made of a material having a relatively high refractive index is a high refractive index layer I will call it.
In this specification, when light is incident on the DBR, the first layer and the second layer are referred to in order from the light incident side.
In general, the DBR is configured by alternately laminating first layers and second layers. When a resonator is formed by making a pair of DBRs face each other, light is assumed to enter the DBR from the inside of the resonator, and the first layer and the second layer are sequentially formed from the inside of the resonator.
In this specification, an optical film thickness of each layer constituting the DBR is referred to as a non-modulated DBR.
In the present specification, the film thickness modulation means shifting the optical film thickness of each layer constituting the DBR from λ / 4. In the following, the thickness-modulated DBR is referred to as a film thickness modulation DBR.
In this specification, when the optical film thickness of each layer constituting the DBR is λ / 4, the case where the optical characteristics are equivalent to the optical film thickness λ / 4 is also included. Specifically, the case where the optical film thickness is an odd multiple of λ / 4 is included.
Here, the direction of film thickness modulation in this specification is defined.
For convenience, the direction of film thickness modulation when the first layer of each DBR is made thinner than the optical film thickness λ / 4 is negative (−), and conversely, the first layer is formed from the optical film thickness λ / 4. The direction of film thickness modulation when the thickness is increased is defined as positive (+).
Further, when the signs indicating the direction of film thickness modulation of the pair of DBRs forming the resonator are different, for example, the direction of film thickness modulation of the upper DBR is negative and the direction of film thickness modulation of the lower DBR is positive. In this case, the direction of film thickness modulation of the upper and lower DBRs is reversed.

Next, the contents considered by the present inventor regarding the cause of the above-described problem will be described.
The above-described resonance wavelength shift is caused by the phase shift of the reflected light depending on the degree of film thickness modulation of the DBR.
9A and 9B, the influence of the degree of DBR film thickness modulation on the phase of reflected light will be described.
The DBR shown in FIG. 9A is the same as the semiconductor DBR 720 below the laser resonator shown in FIG.
In FIG. 9A, the light reflection occurs only on the upper surface 725 of the semiconductor DBR 720, but actually light reflection occurs not only on the upper surface 725 but also on the interface of each layer. The synthesized light becomes the reflected light of the entire DBR. In the calculation in this specification, the phase difference between the incident light and the reflected light (synthetic wave) on the DBR upper surface 725 is obtained by calculation, and the difference is regarded as a phase shift at the time of reflection.

In the case of a general unmodulated DBR in which layers having an optical film thickness of λ / 4 are stacked, the phase shift of reflected light at the wavelength λ is 0 or π. When the refractive index of the incident side medium is larger than the refractive index of the first layer of the DBR, the phase shift is 0. Conversely, when the refractive index is smaller than the refractive index of the first layer, the phase shift is π.
The incident side medium of the semiconductor DBR 720 shown in FIG. 9A is assumed to be GaN (refractive index 2.54). The first layer is an AlN layer 721 (refractive index 2.09), and the refractive index of the incident side medium is larger than that of the first layer, so that the phase shift of the reflected light is 0 when no film thickness modulation is applied. is there.
When film thickness modulation is applied to the DBR, the phase of the reflected light is shifted according to the degree of modulation.
FIG. 9B shows the result of calculating the relationship between the degree of film thickness modulation of the semiconductor DBR 720 in FIG. 9A and the phase shift of the reflected light.
In this calculation, the optical film thickness of one period of the semiconductor DBR 720 is kept at λ / 2.
From FIG. 9B, it can be seen that the phase of the reflected light changes in proportion to the degree of film thickness modulation. When the thickness of the AlN layer 721 as the first layer is reduced and the thickness of the GaN layer 722 as the second layer is increased, the phase shifts in the negative direction, and conversely, the thickness of the AlN layer 721 is increased. If the thickness of the GaN layer 722 is reduced, the phase is shifted in the positive direction. When the phase at the time of light reflection by the DBR changes, the effective optical path length expands and contracts.
When the phase shifts in the positive direction, this is equivalent to an increase in the optical path length, and when the phase shifts in the negative direction, this is equivalent to a reduction in the optical path length.
As a result, it is considered that the resonance wavelength shift as shown in FIG. 8 occurs although the actual resonator length is not changed.

FIG. 10 shows another calculation result for explaining in more detail the mechanism of the phase shift of the reflected light in the film thickness modulation DBR (how the phase shifts in what configuration).
When the low refractive index layer is arranged first when viewed from the light incident side, that is, when the first layer is the low refractive index layer (FIG. 10A), the high refractive index layer is arranged first. In other words, for the two DBR structures when the first layer is a high refractive index layer (FIG. 10B), the phase of the reflected light when a plane wave is incident was calculated.
In addition, the direction of the arrow in a figure represents the incident direction of light.
The parameters used for the calculation are as follows: the refractive index of the incident side and the outgoing side is 1.5, the refractive index of the low refractive index layer is 1.0, the refractive index of the high refractive index layer is 2.0 or 3.0, and DBR The number of pairs was 10.
In this case as well, the optical film thickness in one period of DBR was kept at λ / 2, and the calculation was performed by changing the film thickness ratio.

FIG. 10C shows a calculation result of the structure shown in FIG. 10A, and FIG. 10D shows a calculation result of the structure shown in FIG. 10B.
The graphs show the cases where the refractive index of the high refractive index layer is 2.0 and 3.0, respectively.
Comparing FIG. 10C and FIG. 10D, the phase shift is proportional to the degree of film thickness modulation. However, the sign of the graph is opposite.
In FIG. 10C, the phase shift increases as the thickness of the low refractive index layer increases. In FIG. 10D, the phase shift decreases as the thickness of the low refractive index layer increases. This can be understood such that the phase is positive when the film thickness of the layer arranged in front of the light incident side is increased, and the phase is negative when the film thickness is decreased.
Further, it can be seen from the comparison between the cases where the refractive index of the high refractive index layer is 2.0 and 3.0, the magnitude of the inclination of the graph depends on the refractive index of the layer constituting the DBR.

This mechanism will be explained using a simple model.
FIG. 11 is a schematic diagram showing the state of reflection when a plane wave is incident on the DBR 1100.
The first layer 1101 and the second layer 1102 are arranged from the light incident side, and thereafter, the first layer, the second layer, the first layer, and so on. Are alternately stacked with the same period.
Here, in order to simplify the theory, only the light reflected only once at each interface is handled ignoring multiple reflection.
Considering the light reflected at the upper interface 1105 of the DBR 1100 as a reference, the phase difference of the light reflected at each interface and returning to the interface 1105 will be considered.
The phase difference between the light reflected by the interface 1105 and the light reflected by the interface 1115 and returning to the interface 1105 is
Optical thickness of first layer × 2 × 2π / λ + phase shift upon reflection π
It is expressed.
When the optical thickness of the first layer is λ / 4, the phase difference is 2π, and there is no substantial phase difference. When the optical thickness of the first layer is smaller than λ / 4, the phase difference is smaller than 2π, and when it is larger than λ / 4, the phase difference is larger than 2π.
The phase difference between the light reflected by the interface 1105 and the light reflected by the interface 1125 and returning to the interface 1105 is
(Optical film thickness of first layer + optical film thickness of second layer) × 2 × 2π / λ
It is expressed.

If the optical film thickness of one period obtained by adding the optical film thickness of the first layer and the optical film thickness of the second layer is λ / 2, the phase difference of the reflected light at the interface 1105 and the interface 1125 is 2π. There is no substantial phase difference.
As described above, since the subsequent layers are formed by laminating the first layer and the second layer at the same period, the subsequent reflected light has the same relationship in terms of the phase difference.
After all, in the case of the film thickness modulation DBR in which the optical film thickness of one period is kept at λ / 2, whether the phase of the reflected light is shifted in the positive direction or in the negative direction compared with the unmodulated DBR is the first. It can be expressed only by the optical film thickness of one layer. As described above, when the optical film thickness of the first layer is thinner than λ / 4, the phase difference is smaller than 2π, and when it is thicker than λ / 4, the phase difference is larger than 2π.
Since the entire reflected light is a composite wave of reflected light from each interface, the influence on the whole varies depending on how much intensity the reflected light from each interface has. The intensity of reflected light at each interface depends on the reflectance of each interface. The reflectance of light at the interface depends on the refractive index before and after the interface. Therefore, when the refractive index of the material constituting the DBR changes, the magnitude of the phase shift changes as shown in FIG.
The phase shift of reflected light in the film thickness modulation DBR has been described above.

The present invention is based on the above findings.
The basic idea of the present invention is to cancel the phase shift caused by one DBR out of a pair of DBRs arranged so as to sandwich the resonator, by canceling the phase shift caused by the other DBR. Is to suppress the deviation.
In order to cancel out the phase shift, the sign (positive / negative) of the phase shift generated in the upper and lower DBRs may be reversed.
The case where the sign of the phase shift between the upper and lower DBRs is opposite and the absolute values are equal is most preferable because they completely cancel each other.
As a specific configuration for reversing the sign of the phase shift, the thickness modulation direction of the upper and lower DBRs may be reversed.

FIG. 1 shows an example of the structure and calculation result of a resonator to which the present invention is applied.
A resonator is formed on the substrate 130 so as to resonate at a wavelength λ = 1000 nm.
A DBR 100 that is a first reflecting mirror and a DBR 120 that is a second reflecting mirror are provided above and below a spacer layer 110 having a refractive index of 2.0, a physical thickness of 500 nm, and an optical thickness of 1λ. Resonators are formed by disposing them in opposition.
In the DBR 120, the first layer 121 is a layer having a refractive index of 1.0, the second layer 122 is a layer having a refractive index of 2.0, and the first layer and the second layer are alternately stacked for five periods. Is formed.
Similarly to the DBR 120, the DBR 100 is formed by forming the first layer 101 with a refractive index of 1.0 and the second layer 102 with a refractive index of 2.0, and alternately alternating the first layer and the second layer. Are stacked in five cycles.
For simplicity, the upper and lower DBRs are shown for only three periods.

In this resonator, the film thickness modulation directions of the first reflecting mirror and the second reflecting mirror are reversed. Specifically, in the DBR 100 as the first reflecting mirror, the thickness of the first layer 101 is reduced to 0.5 times λ / 4, and the thickness of the second layer 102 is set to 1 of λ / 4. The film thickness modulation direction is negative according to the definition in this specification.
On the other hand, in the DBR 120 as the second reflecting mirror, the thickness of the first layer 121 is 1.5 times as large as λ / 4, and the thickness of the second layer 122 is 0.5 times as large as λ / 4. The film thickness modulation direction is positive.
As described above, when the film thickness is modulated by DBR on one side, the resonance wavelength deviates from the designed value of 1000 nm.
For example, when only the DBR 120 is modulated in the same manner as the structure shown in FIG. 1 and the DBR 100 is an unmodulated DBR, the resonance wavelength is 1027 nm.
As shown in FIG. 1, by reversing the film thickness modulation direction of DBRs arranged opposite to each other, the influences are canceled out, and a resonator that resonates at 1000 nm as designed is obtained.

In the present invention, it is not necessary that the first reflecting mirror and the second reflecting mirror are composed of the same material DBR.
Rather, it may be preferable to be composed of different material systems. This is because if the same material system is used, it may be practically difficult to modulate the film thickness in the opposite direction.
For example, when the thickness-modulated DBR that aims to suppress cracks by using a thin semiconductor layer having a larger lattice mismatch with the substrate of the two types of semiconductor layers constituting the semiconductor DBR is used as the first reflector. think of.
In order to cancel out the phase shift generated in the first reflecting mirror with the second reflecting mirror, it is necessary to reverse the thickness modulation directions of the first and second reflecting mirrors.
When the first and second reflecting mirrors are made of the same semiconductor material system, the second reflecting mirror thickens the semiconductor layer having a large lattice mismatch with the substrate. As a result, cracks are likely to occur.
Here, if the second reflecting mirror is formed of the dielectric DBR, a configuration suitable for canceling the phases can be obtained without worrying about the occurrence of cracks.

In the present invention, the degree of modulation of the film thickness modulation DBR may be limited by the required reflectance or band.
FIG. 2 shows an example in which the relationship between film thickness modulation and reflectance in the DBR is calculated.
The reflectance is shown when a plane wave having a wavelength of 400 nm is incident on a semiconductor DBR in which 20 pairs of AlN layers and GaN layers are alternately stacked. In this calculation as well, the optical film thickness in one cycle of DBR was kept at λ / 2.
When the degree of film thickness modulation is small, the reflectance does not decrease so much, but when the modulation is applied to some extent, the reflectance decreases rapidly.
The allowable degree of film thickness modulation depends on the required reflectance.
For example, when a reflectance of 98% or more is required, the thickness of each layer can be modulated in the range of (0.25 ± 0.15) λ, and a reflectance of 99.5% or more is required. In this case, film thickness modulation can be performed in the range where the thickness of each layer is (0.25 ± 0.10) λ.

When a vertical cavity surface emitting laser is configured using the present invention, an active layer that is a light emitting layer is disposed in the resonator, but attention must be paid to the position of the active layer.
The phase shift at the time of reflection means that the light distribution at the resonator end face deviates from the antinodes and nodes.
As a result, the light distribution as a whole shifts to the upper side or the lower side as compared with the case where the unmodulated DBR is used.
In general, since the position of the active layer needs to be adjusted to the position of the antinode of the light distribution, it is desirable to estimate the deviation of the light distribution in advance and arrange the active layer in accordance with it.
As an example, FIG. 3 shows a refractive index distribution and a light distribution in the vicinity of the spacer layer 110 having the resonator structure shown in FIG.
The refractive index distribution is indicated by a solid line, the light intensity distribution is indicated by a broken line, and the center position of the spacer layer 110 in the thickness direction is indicated by a one-dot chain line. For convenience of illustration, FIG. 1 and FIG. 3 are 90 degrees different from each other.
1 corresponds to the left side of FIG. 3, and the lower side of FIG. 1 corresponds to the right side of FIG.

FIG. 3A shows the calculation result of the resonator constituted by the film thickness modulation DBR on both the upper and lower sides shown in FIG. FIG. 3 (b) shows the calculation results for the resonator composed of unmodulated DBRs on both the upper and lower sides for comparison.
In FIG. 3B, since the antinode position of the light intensity distribution coincides with the center position of the spacer layer, the active layer may be disposed at the center of the spacer layer.
On the other hand, in FIG. 3A, the antinode position of the light intensity distribution does not coincide with the center position of the spacer layer. More specifically, the antinode position of the light intensity is closer to the lower DBR 120 in which the direction of film thickness modulation is positive.
As described above, when the resonator is formed using the film thickness modulation DBR, the light intensity distribution tends to be biased toward the DBR in which the film thickness modulation direction is positive, and a layer such as an active layer is changed to the light intensity distribution. If you want to put them together, you need to take them into account.

In the present invention, it is not necessary to uniformly modulate all the layers of the multilayer film constituting the DBR.
FIG. 4 shows a resonator structure in which only the upper two DBRs 400 and the upper two pairs 420 of the lower DBR are configured by the film thickness modulation DBR, and the lower part 421 of the lower DBR is configured by the non-modulation DBR.
A DBR in which only a part of layers is subjected to film thickness modulation like the lower DBR is referred to as a partial film thickness modulation DBR.
Even in the partial thickness modulation DBR, the phase of the reflected light changes according to the degree of the thickness modulation.
Accordingly, it is necessary to adjust the degree of film thickness modulation of the other DBR in accordance with this and cancel out the influence of the phase shift.

In the present invention, it is possible to use a structure called graded DBR in which the interface of each layer of DBR is not steep and the composition changes continuously.
In that case, the film thickness is estimated by regarding the center of the region (graded region) where the composition is continuously changed as a pseudo interface.
The present invention is not limited to a specific material system, and can be applied to any material system capable of forming a DBR. Examples of combinations of DBR materials include GaN / AlGaN, GaN / AlInN, InGaN / GaN, GaAs / AlGaAs, GaInAsP / InP, TiO ₂ / SiO ₂ , Ta ₂ O ₅ / SiO ₂ , and Nb ₂ O ₅ / SiO _2. Etc.

Examples of the present invention will be described below.
[Example 1]
As Example 1, a VCSEL to which the present invention is applied will be described with reference to FIG.
FIG. 5 is a schematic cross-sectional view showing the layer structure of the VCSEL in this example.
The VCSEL in this embodiment is formed of a nitride semiconductor and a dielectric, and is designed to oscillate at a wavelength λ = 400 nm in a vacuum.
A dielectric DBR 500 in which seven pairs of niobium pentoxide (Nb ₂ O ₅ ) layers 502 and silicon dioxide (SiO ₂ ) layers 501 are alternately stacked is formed as a reflecting mirror on the emission side of the resonator. On the non-emitting side, a semiconductor DBR 520 in which 11 pairs of AlN layers 521 and GaN layers 522 are alternately stacked is formed.

The semiconductor DBR 520 is a film thickness modulation DBR in which AlN is 0.125λ thinner than λ / 4 and GaN is 0.375λ thicker than λ / 4. The reason for modulating the thickness of the semiconductor DBR is to suppress the generation of cracks by reducing the thickness of AlN, which has a large lattice mismatch with the GaN substrate 530.
The dielectric DBR 500 is a film thickness modulation DBR in which the SiO ₂ layer 501 is 0.4λ thicker than λ / 4 and the Nb ₂ O ₅ layer 502 is 0.1λ thinner than λ / 4. Unlike the semiconductor DBR 520, the dielectric DBR 500 alone has no reason to modulate the film thickness, but the dielectric DBR 500 also modulates the thickness for the purpose of reducing the influence of the thickness modulation of the semiconductor DBR 520 on the resonance wavelength. .
In this embodiment as well, the optical film thickness of one DBR period is kept at λ / 2 during film thickness modulation.

The active region 540 is sandwiched between n-GaN 560 and p-GaN 550, and the optical thickness of the n-GaN 560, the active region 540, and the p-GaN 550 is designed to be one wavelength.
The active region 540 has a structure in which two quantum wells made of In _0.09 Ga _0.91 N with a thickness of 2.5 nm are arranged with a ud-GaN layer with a thickness of 7.5 nm interposed therebetween.
In order to show the effect of modulating the thickness of the dielectric DBR 500, FIG. 6 shows a calculation result showing the relationship between the degree of thickness modulation of the dielectric DBR 500 and the resonance wavelength.
The case where the optical film thicknesses of the SiO ₂ layer 501 and the Nb ₂ O ₅ layer 502 are both 0.25λ corresponds to unmodulated DBR. In this case, the resonance wavelength is 392.5 nm, which is shorter than the design wavelength.
The resonant wavelength can be brought close to the design wavelength by modulating the film thickness of the dielectric DBR 500 in an appropriate direction, and the film thickness of the SiO ₂ layer is 0.4λ and the film thickness of the Nb ₂ O ₅ layer as in this embodiment. When the thickness is 0.1λ, a VCSEL that resonates at a design wavelength of 400 nm is obtained.

Although the embodiments have been described above, the structure of the present invention is not limited to the described embodiments. The type, composition, shape, and size of the material can be changed as appropriate within the scope of the present invention.
In the above embodiment, a laser oscillation wavelength of 400 nm is shown, but operation at an arbitrary wavelength is possible by selecting an appropriate material and structure.
A plurality of surface emitting lasers of the present invention may be arranged on the same plane and used as an array light source.
The surface emitting laser of the present invention described above can also be used as a light source for drawing on a photosensitive drum of an image forming apparatus such as a copying machine or a laser printer.

100: First reflecting mirror (DBR)
101: First layer (of the first reflector) 102: Second layer (of the first reflector) 110: Spacer layer 120: Second reflector (DBR)
121: First layer (of the second reflecting mirror) 122: Second layer (of the second reflecting mirror) 130: Substrate

Claims (8)

A laser resonator having a configuration in which a first reflecting mirror and a second reflecting mirror by DBR are arranged to face each other, and resonates light having a wavelength λ,
In the first reflecting mirror and the second reflecting mirror, at least a part of the multilayer film constituting the DBR in each reflecting mirror is modulated by shifting the optical film thickness from λ / 4. Having a configuration,
The laser resonator, wherein the first reflecting mirror and the second reflecting mirror are configured such that the direction of the film thickness modulation of the DBR in each reflecting mirror is reversed. The first reflecting mirror is made of a dielectric;
The laser resonator according to claim 1, wherein the second reflecting mirror is made of a semiconductor. The partial film in the first reflecting mirror is configured such that the optical film is thinner than a thickness λ / 4,
3. The laser resonator according to claim 1, wherein the optical film of the partial layer of the second reflecting mirror is configured to be thicker than λ / 4.   4. The device according to claim 1, wherein the first reflecting mirror and the second reflecting mirror are configured such that an optical film thickness of one period in them is λ / 2. 5. Laser resonator. A DBR that is a first reflecting mirror and a DBR that is a second reflecting mirror are configured to face each other, and are laser resonators that resonate light having a wavelength λ,
In the first reflecting mirror and the second reflecting mirror, at least a part of the multilayer film constituting the DBR in each reflecting mirror is modulated by shifting the optical film thickness from λ / 4. Having a configuration,
The laser resonator, wherein the first reflecting mirror and the second reflecting mirror are configured such that signs of phase shifts at the time of reflection in each reflecting mirror are reversed.   6. The laser resonator according to claim 1, wherein at least one of the first reflecting mirror and the second reflecting mirror is made of a semiconductor containing GaN or AlN.   A laser resonator according to any one of claims 1 to 6, comprising an active layer between the first reflecting mirror and the second reflecting mirror in the laser resonator. A vertical cavity surface emitting laser characterized by the above.   8. The vertical cavity surface emitting according to claim 7, wherein the active layer is disposed at a position shifted from a center between the first reflecting mirror and the second reflecting mirror. laser.

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