JP5195726B2 - Laser apparatus and manufacturing method of reflection film - Google Patents

Description

  The present invention relates to a method for manufacturing a reflective film, and a laser device using the reflective film.

  A reflection film having a multilayer structure in which two or more kinds of thin films having an optical film thickness of ¼ of the wavelength of light to be reflected is laminated is known. By forming the reflection film having the multilayer film structure on the end face of the optical resonator of the laser diode, it is possible to reduce the threshold value and increase the output.

Basic parameters that serve as indices of laser characteristics of a semiconductor laser device having a pair of resonator end faces include threshold gain, external differential quantum efficiency, front-to-back ratio, and slope efficiency. The threshold gain g _th is
g _th = α _i + (1 / L) ln (1 / (R _f R _r ) ^1/2 )
Is defined. Here, α _i is the internal loss in the optical resonator, L is the resonator length, and R _f and R _r are the reflectivities of the front end face and the rear end face, respectively.

The external differential quantum efficiency η _d is
η _d = η _i × ln (1 / R) / (α _i L + ln (1 / R))
Is defined. Here, η _i is the internal quantum efficiency, and R = R _f = R _r is assumed.

The front-to-back ratio r is
r = ((1- _Rf ) / (1- _Rr )) * ( _Rr / _Rf ) ^1/2
Is defined.
The slope efficiency S _d is
S _d = 1.24 × η _d / λ
Is defined. Here, λ is an oscillation wavelength.

As can be seen from the above definition, when the reflectances R _f and R _r decrease, the external differential quantum efficiency η _d and the slope efficiency S _d improve, but the threshold gain g _th increases. That is, the threshold current increases. In particular, in a high temperature operating environment, the light output characteristics may be impaired due to an increase in threshold current.

Usually, the characteristic evaluation of a laser diode is performed in the atmosphere or an inert gas atmosphere. However, in actual operation, the laser diode is mounted on a mounting substrate and then covered with a resin or the like. When the reflection end face of the optical resonator is covered with resin, the reflectance decreases and the threshold gain g _th increases. For this reason, it is difficult to evaluate the light output characteristics in an actual use environment.

  An object of the present invention is to provide a method of manufacturing a reflective film and a laser device capable of evaluating characteristics of a semiconductor laser device under conditions close to the actual use environment.

According to one aspect of the invention,
A laser medium having an oscillation wavelength λ [nm] having two reflecting surfaces defining an optical resonator and an effective refractive index n ₀ ;
A first layer having a thickness d ₁ [nm] made of silicon oxide formed on at least one reflecting surface of the laser medium;
A second layer having a thickness d ₂ [nm] made of silicon having a refractive index n _Si formed on the surface of the first layer;
A third layer having a thickness d ₃ [nm] made of silicon oxide formed on the surface of the second layer, and having an effective refractive index n ₀ of 3.18 or more and 3.28 or less, The film thickness d ₁ is
(0.11-9.2 × 10 ⁻³ R + 2.2 × 10 ⁻⁴ R ² ) λ / 1.45 ± 15
And the film thickness d ₂ is
(−8.7 × 10 ⁻³ + 3.5 × 10 ⁻³ R−1.2 × 10 ⁻⁵ R ² ) × (−3.6 + 17 / n _Si ) λ ± 15
And the film thickness d ₃ is
(0.23-4.9 × 10 ⁻³ R + 7.7 × 10 ⁻⁵ R ² ) λ / 1.45 ± 15
In the above formula, there is provided a laser device in which R [%] in the above formula is 15 or more and 30 or less.

According to another aspect of the invention,
Providing an optical medium having a reflective surface and having a refractive index n ₀ of 3.18 or more and 3.28 or less;
Determining the wavelength λ and reflectance R [%] of the light to be reflected;
On the reflective surface of the optical medium, a first layer made of silicon oxide and having a thickness d ₁ [nm], the thickness d ₁ being
(0.11-9.2 × 10 ⁻³ R + 2.2 × 10 ⁻⁴ R ² ) λ / 1.45 ± 15
Forming a first layer within a range of
On the surface of the first layer, a second layer having a thickness d ₂ [nm] made of silicon having a refractive index n _Si , the thickness d ₂ being
(−8.7 × 10 ⁻³ + 3.5 × 10 ⁻³ R−1.2 × 10 ⁻⁵ R ² ) × (−3.6 + 17 / n _Si ) λ ± 15
Forming a second layer within the range of
On the surface of the second layer, a third layer made of silicon oxide and having a thickness d ₃ [nm], the thickness d ₃ being
(0.23-4.9 × 10 ⁻³ R + 7.7 × 10 ⁻⁵ R ² ) λ / 1.45 ± 15
And a step of forming a third layer that falls within the range is provided.

  When the film thicknesses of the first to third layers are selected so as to satisfy the above formula, the difference between the reflectance when arranged in the air and the reflectance after resin sealing is reduced. Can do.

  Even when the external medium of the reflective film is replaced, the fluctuation range of the reflectance with respect to light of a specific wavelength can be reduced. When this reflection film is applied to the end face of the optical resonator of the laser diode, the threshold value after resin sealing can be predicted with high accuracy by measuring the threshold value in the atmosphere.

It is sectional drawing of the optical apparatus by a 1st Example. It is sectional drawing of the semiconductor laser apparatus by a 2nd Example. It is a graph which shows the temperature characteristic of the fluctuation range of the threshold value before and behind resin sealing of the semiconductor laser device by the 2nd example and a comparative example. It is sectional drawing of the optical apparatus by a 3rd Example. It is a graph which shows the film thickness of the multilayer film used for the reflective film of the optical apparatus by a 3rd Example. It is a graph which shows the temperature characteristic of the fluctuation range of the threshold value before and behind resin sealing of the semiconductor laser device to which the reflective film according to the third example and the comparative example is applied.

FIG. 1 is a sectional view of a reflective film according to a first embodiment of the present invention. A reflective film 5 having a laminated structure is formed on the reflective surface of the optical medium 1 having a refractive index n ₀ . The reflective film 5 is formed by alternately stacking k sets of first layers 2 having a refractive index n ₁ and second layers 3 having a refractive index n ₂ on the surface of the uppermost second layer 3. It has a laminated structure in which _one third layer 4 is formed. Here, k is a positive integer.

When the wavelength of light to be reflected is λ, the film thickness d ₁ of the _first layer 2 is
d ₁ = (λ / 4 + (λ / 2) × N ₁ ) / n ₁ (N ₁ is 0 or a positive integer) (65)
It is. The film thickness d ₂ of the second layer 3 is
d ₂ = (λ / 4 + (λ / 2) × N ₂ ) / n ₂ (N ₂ is 0 or a positive integer) (66)
It is.

A method for designing the reflective film 5 so that the reflectance of the external medium (medium in contact with the third layer 4) is ns ₁ and ns ₂ is equal will be described. The film thickness d ₃ of the third layer 4 is
d ₃ = d + (λ / 2n ₁ ) × N ₃ (N ₃ is 0 or a positive integer) (67)
as well as,

(K is 0 or a positive integer)
Have been selected to satisfy.

  The reflectance R [%] of the reflective film 5 with respect to light of wavelength λ is

It is expressed. Here, ns is the refractive index of the external medium in contact with the third layer 4.

If the film thickness of the third layer 4 is set so as to satisfy Expression (68), as can be seen from Expression (69), the reflectance and the refractive index when the refractive index of the external medium is ns ₁ are obtained. Is equal to the reflectivity when ns ₂ . For example, when ns ₁ = 1 and ns ₂ is set equal to the refractive index of the external medium when the reflective film 5 is actually used, the reflectivity of the reflective film 5 in the atmosphere or in an inert gas is actually It becomes equal to the reflectance when using.

  For this reason, the reflectance at the time of actual use can be predicted with high accuracy by conducting an evaluation experiment of the reflectance in the atmosphere. Note that it is difficult to form the first layer 2, the second layer 3, and the third layer 4 so that these film thicknesses are equal to the ideal film thickness obtained by the above calculation. . Actually, even when the thickness of each layer is different from the ideal thickness by about ± 20%, a good effect will be obtained. Therefore, in this specification, the “film thickness” of a thin film includes a film thickness that is increased or decreased by ± 20% from an ideal film thickness given by a calculation formula.

FIG. 2 shows a cross-sectional view of the semiconductor laser device according to the second embodiment using the reflective film according to the first embodiment. A platform 13 is disposed in the outer frame 10 having an open top. The platform 13 is constituted by a silicon substrate, for example. On the surface of the platform 13, a laser diode 8 and a photodiode 14 are mounted. The laser diode 8 is an InGaAsP / InP-based Fabry-Perot laser device having an oscillation wavelength of 1.3 μm, for example. The equivalent refractive index n ₀ of this optical resonator is 3.23.

Reflective films 5A and 5B according to the first embodiment are formed on both end faces of the optical resonator of the laser diode 8. The first layer 2 and the third layer 4 in FIG. 1 are made of SiO ₂ , the refractive index n ₁ is 1.45, the second layer 3 is made of Si, and the refractive index n ₂ is 3.8. The SiO ₂ film and the Si film can be formed by, for example, ion-assisted vapor deposition, plasma enhanced chemical vapor deposition, thermal chemical vapor deposition, or sputtering.

  Laser light transmitted through the reflective film 5B and emitted backward enters the photodiode 14. By measuring the output signal of the photodiode 14, the oscillation state of the laser diode 8 can be monitored.

  A part of the laser light transmitted through the reflection film 5 </ b> A and emitted forward enters the optical fiber 12. The optical fiber 12 is placed on the surface of the platform 13, and its position is fixed by a press plate 15. The optical fiber 12 passes through the side surface of the outer frame 10 and is led out of the outer frame 10. A portion of the optical fiber 12 that penetrates the outer frame 10 is protected by a holder 11.

  Sealing resin 16 covers the ends of the photodiode 14, the laser diode 8, and the optical fiber 12. The sealing resin 16 is formed of, for example, a silicone resin. The refractive index of the silicone resin is 1.38. The opening of the outer frame 10 is closed with a lid 17. A plurality of signal input / output terminals 18 are attached to the bottom of the outer frame 10.

From the equations (44) and (45), the film thickness d ₁ of the first layer 2 in FIG. ₁ is 224 nm, and the film thickness d ₂ of the second layer ₂ is 86 nm. Here, N ₁ = N ₂ = 0. When ns ₁ = 1 and ns ₂ = 1.38 in the equation (68), cos ² Δ = 0.395 is obtained. From this, the film thickness d ₃ of the third layer 4 is determined to be 127 nm as an example. At this time, from the equation (69), the reflectance R [%] is 76.7%.

  FIG. 3 shows the change in threshold value before and after resin sealing of the laser diode according to the second embodiment as a function of operating temperature. The horizontal axis represents the operating temperature in units of ° C., and the vertical axis represents the fluctuation range of the threshold value before and after sealing with silicone resin in unit%. The solid line a in the graph indicates the threshold fluctuation width of the laser diode according to the second embodiment, and the solid line b indicates the laser diode using the reflective film without the third layer 4 shown in FIG. Indicates the threshold fluctuation range.

  When the reflective film according to the first embodiment is used, the threshold fluctuation width is 5% or less. On the other hand, when the third layer is not provided, the threshold fluctuation range is about 20 to 45%. Thus, by using the reflective film of the first embodiment, the threshold fluctuation range before and after resin sealing can be reduced. In particular, it can be seen that the effect is high when the operating temperature is high.

  This is because the reflectance in the atmosphere of the reflective film according to the first embodiment is substantially equal to the reflectance after resin sealing. When the third layer 4 shown in FIG. 1 is not used, since the reflectance in the air is different from the reflectance after resin sealing, the threshold value greatly fluctuates before and after resin sealing. When the reflective film according to the first embodiment is used, the threshold value of the laser diode can be evaluated in the atmosphere, and the threshold value after resin sealing can be predicted with high accuracy.

In the second embodiment, SiO ₂ and Si are used as the first and second layers 2 and 3 in FIG. 1, but other materials such as Al, Si, Ti, Zn, Mg, or Li are used. Oxides, nitrides, or fluorides may be used. When the reflective film is formed on the end face of the optical resonator of the laser diode, it is preferable that the first layer that is in direct contact with the end face is formed of an insulating material.

  In the second embodiment, the Fabry-Perot laser has been described as an example. However, the reflective film according to the first embodiment is applied to other laser diodes such as a distributed feedback laser diode and a distributed Bragg reflection laser diode. It is also possible.

Since the refractive index of a normal material is 1 or more, both ns ₁ and ns ₂ in formula (68) are 1 or more. In general, the refractive index of a reflective film material that can be used in the oscillation wavelength region of a laser diode is 4 or less. For this reason, in general,
1 ≦ (ns ₁ × ns ₂ ) ≦ 16 (70)
it is conceivable that.

  From this condition and equation (68),

as well as

Is obtained. That is, when the film thickness of the third layer 4 in FIG. 1 is d ₃ , the film thickness d ₃ is restricted so as to satisfy the expressions (67), (71), and (72). For example, if k = 1, n ₀ = 3.23, n ₁ = 1.45, n ₂ = 3.8,
49 nm ≦ d ≦ 138 nm, or 311 nm ≦ d ≦ 411 nm (73)
Is obtained.

In the second embodiment, the reflection film applied to the reflection end face of the laser diode has been described. However, the reflection film according to the first embodiment is formed on the reflection surface of the optical medium having the refractive index ns ₁ other than the laser diode. May be. At this time, the reflective film is covered with an optical medium having a refractive index of ns ₂ . When the reflection film is applied to a laser diode, the oscillation wavelength of the laser diode corresponds to the wavelength of light to be reflected by the reflection film. When the reflection film is formed on the reflection end face of the optical medium, the wavelength of light to be reflected by the reflection film can be specified by the following method.

The optical film thicknesses of the first layer 2 and the second layer 3 shown in FIG.
λ / 4 + (λ / 2) × N (N is 0 or a positive integer) (74)
It is. Here, the optical film thickness means a film thickness obtained by multiplying the actual film thickness by the refractive index of the film. The film thicknesses of the first layer 2 and the second layer 3 constituting the reflection film are measured, and the optical film thickness is obtained. The wavelength λ is specified by variously changing N in the equation (74) with respect to the optical film thicknesses of the first and second layers. At this time, N regarding the first layer and N regarding the second layer may not be equal.

When the wavelength to be reflected is specified, a suitable film thickness d ₃ of the third layer 4 shown in FIG. 1 can be obtained from Expression (68) and Expression (67) in which ns ₁ = 1 is substituted. . In the reflective film formed in this way, the reflectance in an atmosphere having a refractive index of 1, for example, in the atmosphere, and the reflectance in a medium having a refractive index of ns ₂ are equal. For this reason, the reflectance in the medium having the refractive index ns ₂ can be predicted with high accuracy by evaluating the reflectance in the atmosphere.

  Next, the configuration of the optical apparatus according to the third embodiment of the present invention will be described. The reflective film of the first embodiment basically includes a stack of films having a thickness of ¼ of the wavelength of light of interest. The reflective film according to the third embodiment has a three-layer structure, and the thickness of each film is determined without sticking to ¼ of the wavelength.

FIG. 4 shows a cross-sectional view of the optical device according to the third embodiment. A first layer 21, a second layer 22, and a third layer 23 are stacked on the surface of the optical medium 20. The reflective film 24 is constituted by three layers of the first to third layers 21 to 23. The optical medium 20 is a laser diode having an equivalent refractive index of 3.23 and an oscillation wavelength of 1.31 μm, and the first layer 21 and the third layer 23 are made of SiO ₂ having a refractive index of 1.45, The layer 22 is made of silicon having a refractive index of 3.8.

  The film thicknesses of the first to third layers 21 to 23 are changed variously, and the reflectance with respect to light having a wavelength of 1.31 μm and a wavelength in the vicinity thereof when the reflective film 24 having the three-layer structure is placed in the atmosphere; Then, it was obtained by calculation when it was coated with a resin having a refractive index of 1.38.

  FIG. 5 shows the condition that the reflectance when arranged in the atmosphere is substantially equal to the reflectance when resin-sealed, and that the reflectance varies relatively little when the wavelength of the target light varies. The film thickness combinations satisfying the above are shown as a function of the reflectance R [%]. The horizontal axis represents the reflectance R in unit%, and the vertical axis represents the film thickness in unit nm. Solid lines a1, a2, and a3 in the figure indicate the film thicknesses of the first, second, and third layers 21, 22, and 23, respectively.

  In general, the reflectance at the interface between the cleavage plane of a laser diode and air is about 30%. Usually, the reflective film applied to the laser diode is set to have a reflectance equal to or lower than the interface between the cleavage plane and the air. For this reason, the upper limit of the reflectance in FIG. 5 is set to 30%. Further, a desired solution could not be obtained in a region where the reflectance was 15% or less. That is, when a reflective film having a three-layer structure is used, the design value of the reflective film is preferably 15% or more. For this reason, the lower limit of the reflectance in FIG. 5 is set to 15%.

When the film thickness d ₁ (curve a1) of the first layer 21 is approximated by a quadratic expression of the reflectance R [%],
d ₁ = (0.11-9.2 × 10 ⁻³ R + 2.2 × 10 ⁻⁴ R ² ) λ ₀ / n ₁ (75)
It becomes. Here, λ ₀ is the wavelength of the target light, that is, 1.31 μm, and n ₁ is the refractive index of the first layer 21, that is, 1.45. Incidentally, it is considered that the thickness d ₁ is approximately proportional to the wavelength lambda _0, representing the thickness d ₁ as a primary expression of the wavelength lambda _0.

Similarly, the film thickness d ₂ (curve a2) of the _second layer 22 is
d ₂ = (− 8.7 × 10 ⁻³ + 3.5 × 10 ⁻³ R−1.2 × 10 ⁻⁵ R ² ) × (−3.6 + 17 / n ₂ ) λ ₀ (76)
Is approximated by Here, n ₂ is the refractive index of the second layer 22, that is, 3.8. The term (−3.6 + 17 / n ₂ ) is a term derived from the same graph as FIG. 5 obtained by changing the refractive index n ₂ from 3.6 to 3.85.

Actually, the refractive index of a silicon film formed by plasma enhanced chemical vapor deposition, sputtering, or the like varies within a range of approximately 3.6 to 3.85 due to variations in film forming conditions. Therefore, it is preferable to determine the film thickness of the second layer 22 by substituting the refractive index n ₂ suitable for the actual film formation conditions into the equation (76).

The film thickness d ₃ (curve a3) of the _third layer 23 is
d ₃ = (0.23-4.9 × 10 ⁻³ R + 7.7 × 10 ⁻⁵ R ² ) λ ₀ / n ₃ (77)
Is approximated by Here, n ₃ is the refractive index of the third layer 23, that is, 1.45.

Further, when the film thicknesses d _{1 to} d ₃ increase or decrease within a range of ± 15 nm (the range indicated by the broken line in FIG. 5) around the values obtained from the equations (75) to (77), Fluctuates within a range of about ± 3%. For example, when the film thickness is set so that the reflectance is 25%, the reflectance varies between 22% and 28% due to the fluctuation of the film thickness of about 15 nm. This variation in reflectivity is within an acceptable range.

Further, the respective film thickness d ₁ to d _3, when increased or decreased order of ± 15 nm from the target value, the difference between the reflectance in the case where the laser diode was sealed reflectivity and a resin sealing of when placed in the air, most 2 The calculation result was about%. On the other hand, when a single-layer reflective film is used, the difference is 10%. That is, even when the film thicknesses d _{1 to} d ₃ increase or decrease within a range of ± 15 nm, a sufficient effect of reducing the difference between the reflectance in the air and the reflectance after resin sealing is expected. For example, when it is desired to set the reflectance to 26%, preferable film thicknesses d ₁ , d ₂ , and d ₃ are 28.2 nm or less, 66.2 to 96.2 nm, and 121.2 to 151.2 nm, respectively. .

Note that if the thickness of the first layer 21 is reduced by −15 nm from the target value, the thickness d ₁ may become 0 nm. However, since the first layer 21 is actually formed, the film thickness d ₁ does not actually become 0 nm but becomes thicker than 0 nm. In reality, the film thickness will be at least 2 nm or more.

In the third embodiment, as can be seen from FIG. 5, the possible range of the film thickness d ₁ of the first layer 21 is 40 nm or less. In the conventional multilayer reflective film, the thickness of each layer is determined on the basis of a quarter wavelength, so the thickness of each layer of the reflective film used in a general laser diode is 220 nm or more. It can be said that the film thickness of the first layer in contact with the optical medium is 40 nm or less is a great feature of the third embodiment.

  In the third embodiment, the case where the effective refractive index of the optical medium is set to 3.23 has been considered. However, it is preferable when the refractive index of the optical medium is within the range of 3.23 ± 0.05. The film thickness can be approximated by the above formulas (75) to (77).

In the third embodiment, the first layer 21 in contact with the optical medium is formed of SiO ₂ , the second layer 22 thereon is formed of silicon, and the third layer 23 thereon is formed of SiO ₂ . Formed. For other materials, a preferable combination of film thicknesses was obtained by calculation. Hereinafter, combinations of the film thicknesses of the optical devices according to the first to third modifications in the case where other materials are used will be described. The refractive index of the optical medium used in the first to third modifications is the same as that in the third embodiment.

First, a first modification of the third embodiment will be described. In the first modification, the first layer 21 and the third layer 23 in FIG. 4 are formed of aluminum oxide, and the second layer 22 is formed of silicon. The refractive indexes n ₁ and n ₃ of the first and third layers made of aluminum oxide were 1.72.

Suitable film thickness d ₁ of the first layer 21, a suitable thickness d ₃ of the second preferred thickness d ₂ of the layer 22, and third layer 23, respectively,
d ₁ = (1.7 × 10 ⁻³ + 1.1 × 10 ⁻³ R + 3.1 × 10 ⁻⁵ R ² ) λ ₀ / n ₁
d ₂ = (2.3 × 10 ⁻² + 3.5 × 10 ⁻³ R−5.6 × 10 ⁻⁵ R ² ) × (−1.4 + 8.9 / n ₂ ) λ ₀
d ₃ = (0.21−1.9 × 10 ⁻³ R + 2.1 × 10 ⁻⁵ R ² ) λ ₀ / n ₃ (78)
It becomes.

In this case, the allowable range of each film thickness is ± 15 nm of the target film thicknesses d _{1 to} d ₃ obtained from the above formula. The upper limit of the film thickness of the first layer is 60 nm. For example, suitable film thicknesses d ₁ , d ₂ , and d ₃ for setting the reflectance to 26% are 23.8 to 53.8 nm, 75.8 to 105.8 nm, and 117.5 to 147.5 nm.

Next, a second modification of the third embodiment will be described. In the second modification, the first layer 21 of FIG. 4 is formed of silicon oxide, the second layer 22 is formed of silicon, and the third layer 23 is formed of aluminum oxide. That is, n ₁ = 1.45, n ₂ = 3.6 to 3.85, and n ₃ = 1.72.

Suitable film thickness d ₁ of the first layer 21, a suitable thickness d ₃ of the second preferred thickness d ₂ of the layer 22, and third layer 23, respectively,
d ₁ = (− 3.1 × 10 ⁻⁵ + 3.6 × 10 ⁻³ R−3.5 × 10 ⁻⁵ R ² ) λ ₀ / n ₁
d ₂ = (3.5 × 10 ⁻² + 2.5 × 10 ⁻³ R-3.6 × 10 ⁻⁵ R ² ) × (−2.6 + 1.4 / n ₂ ) λ ₀
d ₃ = (0.21−1.9 × 10 ⁻³ R + 2.1 × 10 ⁻⁵ R ² ) λ ₀ / n ₃ (79)
It becomes.

In this case, the allowable range of each film thickness is ± 15 nm of the target film thicknesses d _{1 to} d ₃ obtained from the above formula. Moreover, the upper limit of the film thickness of the first layer is 40 nm. For example, suitable film thicknesses d ₁ , d ₂ , and d ₃ for setting the reflectance to 26% are 20.4 to 50.4 nm, 73.4 to 103.4 nm, and 117.5 to 147.5 nm.

Next, a third modification of the third embodiment will be described. In the third modification, the first layer 21 of FIG. 4 is formed of aluminum oxide, the second layer 22 is formed of silicon, and the third layer 23 is formed of silicon oxide. _{That, n 1 = 1.72, n 2} = 3.6~3.85, an n ₃ = 1.45.

Suitable film thickness d ₁ of the first layer 21, a suitable thickness d ₃ of the second preferred thickness d ₂ of the layer 22, and third layer 23, respectively,
d ₁ = (0.12-1.2 × 10 ⁻² R + 3.2 × 10 ⁻⁴ R ² ) λ ₀ / n ₁
d ₂ = (− 2.7 × 10 ⁻² + 3.4 × 10 ⁻³ R + 2.4 × 10 ⁻⁵ R ² ) × (−3.8 + 2.8 / n ₂ ) λ ₀
d ₃ = (0.23-4.9 × 10 ⁻³ R + 7.7 × 10 ⁻⁵ R ² ) λ ₀ / n ₃ (80)
It becomes.

In this case, the allowable range of each film thickness is ± 15 nm of the target film thicknesses d _{1 to} d ₃ obtained from the above formula. Moreover, the upper limit of the film thickness of the first layer is 50 nm. For example, suitable film thicknesses d ₁ , d ₂ , and d ₃ for achieving a reflectance of 26% are 0.9 to 30.9 nm, 69.6 to 99.6 nm, and 121.2 to 151.2 nm.

  As described above, when the combination of the materials of the first to third layers is changed, the preferable film thickness of each layer also changes. By obtaining a graph similar to FIG. 5 for various material combinations, it is possible to obtain a three-layer reflective film having a small difference between the reflectance before resin sealing and the reflectance after resin sealing.

  FIG. 6 shows the threshold values before and after resin sealing when the reflective film according to the third embodiment (including the first to third modifications) is applied to the laser diode shown in FIG. The fluctuation amount of the value is shown as a ratio to the threshold value before resin sealing. The horizontal and vertical axes are the same as those in FIG.

  The broken line group c in FIG. 6 shows the fluctuation range of the threshold when the three-layer reflective film according to the third embodiment is used, and the broken line group d is a case where the conventional single-layer reflective film is used. Indicates the fluctuation range of the threshold value. In the conventional case, the threshold value is increased by 20% or more by resin sealing. On the other hand, when the three-layer reflective film according to the third embodiment is used, the threshold fluctuation rate is ± 5% or less. Thus, by using the three-layer reflective film according to the third embodiment, it is possible to suppress fluctuations in the threshold value caused by resin sealing.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 1,20 Optical medium 2,21 1st layer 3,22 2nd layer 4,23 3rd layer 5,24 Reflective film 10 Outer frame 11 Holder 12 Optical fiber 13 Platform 14 Photodiode 15 Presser plate 16 Sealing Resin 17 Lid 18 Signal input / output terminal

Claims (4)

A laser medium having an oscillation wavelength λ [nm] having two reflecting surfaces defining an optical resonator and an effective refractive index n ₀ ;
A first layer having a thickness d ₁ [nm] made of silicon oxide formed on at least one reflecting surface of the laser medium;
A second layer having a thickness d ₂ [nm] made of silicon having a refractive index n _Si formed on the surface of the first layer;
A third layer having a thickness d ₃ [nm] made of silicon oxide formed on the surface of the second layer, and an effective refractive index n ₀ of 3.18 or more and 3.28 or less, The film thickness d ₁ is
(0.11-9.2 × 10 ⁻³ R + 2.2 × 10 ⁻⁴ R ² ) λ / 1.45 ± 15
And the film thickness d ₂ is
(−8.7 × 10 ⁻³ + 3.5 × 10 ⁻³ R−1.2 × 10 ⁻⁵ R ² ) × (−3.6 + 17 / n _Si ) λ ± 15
And the film thickness d ₃ is
(0.23-4.9 × 10 ⁻³ R + 7.7 × 10 ⁻⁵ R ² ) λ / 1.45 ± 15
A laser device in which R [%] in the above formula is 15 or more and 30 or less. Providing an optical medium having a reflective surface and having a refractive index n ₀ of 3.18 or more and 3.28 or less;
Determining the wavelength λ and reflectance R [%] of the light to be reflected;
On the reflective surface of the optical medium, a first layer made of silicon oxide and having a thickness d ₁ [nm], the thickness d ₁ being
(0.11-9.2 × 10 ⁻³ R + 2.2 × 10 ⁻⁴ R ² ) λ / 1.45 ± 15
Forming a first layer within a range of
On the surface of the first layer, a second layer having a thickness d ₂ [nm] made of silicon having a refractive index n _Si , the thickness d ₂ being
(−8.7 × 10 ⁻³ + 3.5 × 10 ⁻³ R−1.2 × 10 ⁻⁵ R ² ) × (−3.6 + 17 / n _Si ) λ ± 15
Forming a second layer within the range of
On the surface of the second layer, a third layer made of silicon oxide and having a thickness d ₃ [nm], the thickness d ₃ being
(0.23-4.9 × 10 ⁻³ R + 7.7 × 10 ⁻⁵ R ² ) λ / 1.45 ± 15
And forming a third layer within the range of the method. A laser medium having an oscillation wavelength λ [nm] having two reflecting surfaces defining an optical resonator and an effective refractive index n ₀ ;
A first layer having a thickness d ₁ [nm] made of aluminum oxide formed on at least one reflecting surface of the laser medium;
A second layer having a thickness d ₂ [nm] made of silicon having a refractive index n _Si formed on the surface of the first layer;
A third layer having a thickness d ₃ [nm] made of aluminum oxide formed on the surface of the second layer, and an effective refractive index n ₀ of 3.18 or more and 3.28 or less, The film thickness d ₁ is
(1.7 × 10 ⁻³ + 1.1 × 10 ⁻³ R + 3.1 × 10 ⁻⁵ R ² ) × λ / 1.72 ± 15
And the film thickness d ₂ is
(2.3 × 10 ⁻² + 3.5 × 10 ⁻³ R−5.6 × 10 ⁻⁵ R ² ) × (−1.4 + 8.9 / n _Si ) λ ± 15
And the film thickness d ₃ is
(0.21-1.9 × 10 ⁻³ R + 2.1 × 10 ⁻⁵ R ² ) λ / 1.72 ± 15
A laser device in which R [%] in the above formula is 15 or more and 30 or less. Providing an optical medium having a reflective surface and having a refractive index n ₀ of 3.18 or more and 3.28 or less;
Determining the wavelength λ and reflectance R [%] of the light to be reflected;
On the reflective surface of the optical medium, a first layer made of aluminum oxide and having a thickness d ₁ [nm], the thickness d ₁ being
(1.7 × 10 ⁻³ + 1.1 × 10 ⁻³ R + 3.1 × 10 ⁻⁵ R ² ) × λ / 1.72 ± 15
Forming a first layer within a range of
On the surface of the first layer, a second layer having a thickness d ₂ [nm] made of silicon having a refractive index n _Si , the thickness d ₂ being
(2.3 × 10 ⁻² + 3.5 × 10 ⁻³ R−5.6 × 10 ⁻⁵ R ² ) × (−1.4 + 8.9 / n _Si ) λ ± 15
Forming a second layer within the range of
On the surface of the second layer, there is a third layer made of aluminum oxide and having a thickness d ₃ [nm], and the thickness d ₃ is
(0.21-1.9 × 10 ⁻³ R + 2.1 × 10 ⁻⁵ R ² ) λ / 1.72 ± 15
And forming a third layer within the range of the method.