JP3773880B2 - Distributed feedback semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser that outputs light used for optical communication and optical measurement, and more particularly to a distributed feedback semiconductor laser that outputs light of a single wavelength.
[0002]
[Prior art]
In a semiconductor laser that outputs light used for optical communication or optical measurement, for example, as shown in FIG. 13A, an active layer 2 and a clad layer 3 are stacked on a semiconductor substrate 1. When a voltage is applied between the lower surface and the upper surface of the cladding layer 3, the light 4 is output from the end surface of the active layer 2. However, when this light 4 is verified in detail, it can be regarded as a set of a plurality of lights each having a slightly different wavelength λ, as shown in FIG.
[0003]
So single wavelength λ₀As shown in FIG. 13C, a distributed feedback (DFB) semiconductor laser in which a diffraction grating 5 is formed in the emission direction of the light 4 at a position adjacent to the active layer 2 is used. Has been advocated. In the distributed feedback semiconductor laser in which the diffraction grating 5 is incorporated in this way, if the equivalent refractive index of the optical waveguide is n and the grating interval is d, of the light having a large number of wavelengths λ generated in the active layer 2 Λ is a single wavelength λ that satisfies the condition of λ = 2nd₀(= 2nd) light 4a is to be output.
[0004]
However, as shown in FIG. 13C, the distributed feedback semiconductor laser is in principle single in the case where the diffraction grating 5 formed therein is a uniform phase continuous type along the light emission direction. Wavelength λ₀As shown in FIG. 13D, the wavelength λ that satisfies the condition of λ = 2nd is not realized.₀Is not output and this wavelength λ₀Another wavelength λ₊₁, Λ_-1Is output.
[0005]
In order to eliminate such inconvenience, a “single mode oscillation” can be achieved by forming a phase shift structure called a λ / 4 shift structure in the middle of the diffraction grating 5 to shift the phase of light by λ / 4. Realized.
[0006]
However, the diffraction grating 5 having a phase shift structure in the middle cannot be manufactured in a batch exposure operation process by a laser interference exposure method that is simple and excellent in mass productivity, and generally takes a long time using an electron beam lithography apparatus. The actual situation is that a manufacturing method for drawing is adopted. On the other hand, Japanese Patent No. 1781186 (Japanese Patent Publication No. 4-67356) proposes a technique for obtaining an effect equivalent to that of a λ / 4 shift structure even in a diffraction grating manufactured by a laser interference exposure method.
[0007]
That is, in the proposed distributed feedback semiconductor laser, as shown in FIG. 14, the first and second diffraction grating waveguides 6a and 6b and the first and second diffraction grating waveguides 6a and 6b are formed below the active layer 2, respectively. A flat coupling waveguide 7 that couples the diffraction grating waveguides 6a and 6b is formed as an integral structure on the same surface. The respective diffraction gratings of the first and second diffraction grating waveguides 6a and 6b are matched in phase so as to form a part of a virtual single diffraction grating using the laser interference exposure method. Formed.
[0008]
The coupled waveguide 7 has a phase of the transmitted light that is an integral multiple of π when the coupled waveguide 7 has the same structure as the first and second diffraction grating waveguides 6a and 6b. The transmission characteristic is shifted from
[0009]
In this proposal, “the phase of light transmitted from the first diffraction grating waveguide 6a to the second diffraction grating waveguide 6b is shifted from an integer multiple of π”, but the distributed feedback semiconductor laser manufactured is manufactured. The structure having a high probability of generating light of a single wavelength is as follows: “The phase of light transmitted from the first diffraction grating waveguide 6a to the second diffraction grating waveguide 6b is a half-odd multiple (π / It is clear that the structure is “2, 3π / 2, 5π / 2,.
[0010]
[Problems to be solved by the invention]
However, the distributed feedback semiconductor laser having the structure shown in FIG. 14 still has the following problems to be solved.
[0011]
First, the length L of the coupling waveguide 7 having a function of shifting the phase of light transmitted from the first diffraction grating waveguide 6a to the second diffraction grating waveguide 6b from an integer multiple of π will be examined in detail.
[0012]
In general, the phase of the light transmitted through the coupling waveguide 7 in the coupled waveguide 7 is shifted as compared with the case where there is a diffraction grating due to the difference in the structure of the optical waveguide such as the presence or absence of the diffraction grating. This is because the propagation constant corresponding to the propagation speed is slightly different. The light propagation constant is determined by the equivalent refractive index n felt by the transmitted light.
[0013]
Referring to FIG. 15, the difference between the light propagation constant of the first and second diffraction grating waveguides 6 a and 6 b and the propagation constant of the coupling waveguide 7 is the equivalent refractive index n of the coupling waveguide 7.₁And the average equivalent refractive index n of the first and second diffraction grating waveguides 6a and 6b₀Difference from (n₀-N₁). Furthermore, this equivalent refractive index difference (n₀-N₁) Is the thickness h of the coupled waveguide 7₁And an average thickness h considering the depth of the grooves existing between the gratings of the first and second diffraction grating waveguides 6a and 6b.₀Difference from (h₀―H₁).
[0014]
That is, in order to make the amount of phase shift of the light transmitted through the coupling waveguide 7 exactly half-odd times (π / 2, 3π / 2, 5π / 2...), The difference (h₀―H₁) Must be accurately controlled.
[0015]
However, in the conventional distributed feedback semiconductor laser shown in FIG. 14, the difference in cross-sectional shape between the first and second diffraction grating waveguides 6a and 6b and the coupling waveguide 7 is fixed.₀-N₁) Is a constant value.
[0016]
As a result, the amount of phase shift is the light wavelength λ.₀Is 2π, the coupled waveguide length is closest to the half-odd multiple of one of a plurality of half-odd multiples of π (π / 2, 3π / 2, 5π / 2,...). It is necessary to set L, and this coupled waveguide length L cannot be arbitrarily selected.
[0017]
In the distributed feedback semiconductor laser having the first and second diffraction grating waveguides 6a and 6b separated from each other in the light emitting direction, the following problem occurs when the coupling waveguide length L cannot be arbitrarily selected.
[0018]
As shown in FIG. 16, when the coupled waveguide length L is short, the intensity distribution of the light generated in the distributed feedback semiconductor laser has a substantially chevron waveform. As a result, the output light intensity P of the light 4 emitted from both end faces of this distributed feedback semiconductor laser.₀Decreases. As a result, output reduction and line width deterioration are likely to occur due to spatial hole burning.
[0019]
On the contrary, when the coupling waveguide length L is long, not only the size and shape of the entire distributed feedback semiconductor laser is increased, but the number of elements that can be manufactured with one wafer is decreased, and the unit manufacturing cost of the elements is increased.
[0020]
The present invention has been made in view of such circumstances, and the coupling layer length can be set to an arbitrary length. As a result, the light intensity of the output light can be easily increased, and the output light can be increased. An object of the present invention is to provide a distributed feedback semiconductor laser capable of narrowing the spectral line width and further improving the single-mode property of the output light.
[0021]
[Means for Solving the Problems]
  The present invention relates to a semiconductor substrate and above the semiconductor substrate.PlacedIn the light exit directionAcross the bonding layerFirst and second diffraction grating layers disposed apart from each other, and the first and second diffraction grating layersAnd bonding layerThe present invention is applied to a distributed feedback semiconductor laser including an active layer disposed above and a cladding layer disposed above the active layer.
[0022]
  In order to solve the above problem, in the distributed feedback semiconductor laser of the present invention,The first diffraction grating layer is composed of a first diffraction grating having a plurality of gratings arranged at a predetermined grating spacing d, and the second diffraction grating layer is a grating equal to the grating spacing of the first diffraction grating. It is composed of a second diffraction grating having a plurality of gratings arranged at intervals d. In addition, a phase shift amount Δd exists between the grating arrangement of the first diffraction grating and the grating arrangement of the second diffraction grating.
  The length L of the coupling layer in the light emission direction and the equivalent refractive index n of the first and second diffraction grating layers ₀ And the equivalent refractive index n of the coupling layer ₁ Difference from (n ₀ -N ₁ ) Obtained by adding the light phase shift amount ΔΦ corresponding to the phase shift amount Δd to the product (B · L) of the light propagation constant difference B obtained based on The phase shift amount Δd is set so that the total phase shift amount Φ (= B · L + ΔΦ) generated is π / 2. As a result, the length L of the coupling layer can be arbitrarily set.
[0023]
In the distributed feedback semiconductor laser configured as described above, the first diffraction grating and the second diffraction grating, which are disposed apart from each other in the light emitting direction, have virtually the same grating interval (pitch). Has a phase but is not continuous. That is, even if the first diffraction grating layer and the second diffraction grating layer are continuous without being separated from each other, the presence of the spatial phase shift causes the first diffraction grating layer and the second diffraction grating layer to be The phase of the light propagating through the diffraction grating layer is shifted by this set phase shift amount.
[0024]
As described above, the first diffraction grating layer and the second diffraction grating layer are separated from each other via the coupling layer in which the diffraction grating is not formed, so that the first diffraction grating layer and the second diffraction grating layer are disposed. The phase of light propagating through the layers can be shifted. The amount of phase shift of the light is determined by the difference in cross-sectional shape between the first and second diffraction grating layers and the coupling layer (coupling waveguide) and the length L of the coupling layer.
[0025]
As described above, the phase shift amount of light propagating through the first diffraction grating layer and the second diffraction grating layer is close to a half odd number multiple of π (π / 2, 3π / 2, 5π / 2...). It only has to be. Therefore, the spatial phase shift amount between the first diffraction grating and the second diffraction grating and the difference in cross-sectional shape between the first and second diffraction grating layers and the coupling layer are adjusted to propagate. It is possible to set a plurality of coupling layer lengths L that allow the phase shift amount Φ of the light to be exactly half-odd times (π / 2, 3π / 2, 5π / 2,...) Of π. That is, the length L of the coupling layer of this distributed feedback semiconductor laser can be set almost arbitrarily.
[0026]
Therefore, for example, by setting the length L of the coupling layer to be long, spatial hole burning can be suppressed and light can be generated with high efficiency at the time of high current injection. Therefore, the high efficiency of this distributed feedback semiconductor laser Can be achieved.
[0027]
The term “phase shift” as used herein refers to a structure in which the amount of phase shift of light due to the propagation constant difference between the diffraction grating region and the flat coupling layer is deviated from an ideal π / 2. Introduced to complement it. Therefore, it is not limited to π / 2 or the amount in the vicinity thereof. When priority is given to increasing the output of the device or narrowing the spectral line width, it is desirable to make the length of the flat coupling layer as long as possible, and the phase shift amount of light caused by the difference in propagation constant is larger than π / 2. In this case, it can be canceled by introducing a negative phase shift in the diffraction grating.
[0028]
On the other hand, when giving priority to mass production at a low price, it is desirable to increase the number of chips (yield) that can be taken out from one wafer. For this purpose, the chip length must be short. Since the length of the coupling layer cannot exceed the chip length, if the amount of phase shift of the light due to the propagation constant difference is less than π / 2, this is introduced by introducing a positive phase shift into the diffraction grating. Can be supplemented. This creates a degree of freedom in device design that can meet various requirements.
[0029]
  In another invention, a semiconductor substrate, an active layer disposed above the semiconductor substrate, and above the active layerArranged,In the direction of light emissionAcross the bonding layerFirst and second diffraction grating layers disposed apart from each other, and the first and second diffraction grating layersAnd bonding layerThe present invention is applied to a distributed feedback semiconductor laser including a cladding layer disposed above the semiconductor layer.
[0030]
  In order to solve the above problem, in the distributed feedback semiconductor laser of the present invention,The first diffraction grating layer is composed of a first diffraction grating having a plurality of gratings arranged at a predetermined grating spacing d, and the second diffraction grating layer is a grating equal to the grating spacing of the first diffraction grating. It is composed of a second diffraction grating having a plurality of gratings arranged at intervals d.
In addition, a phase shift amount Δd exists between the grating arrangement of the first diffraction grating and the grating arrangement of the second diffraction grating.
  The length L of the coupling layer in the light emission direction and the equivalent refractive index n of the first and second diffraction grating layers ₀ And the equivalent refractive index n of the coupling layer ₁ Difference from (n ₀ -N ₁ ) Obtained by adding the light phase shift amount ΔΦ corresponding to the phase shift amount Δd to the product (B · L) of the light propagation constant difference B obtained based on The phase shift amount Δd is set so that the total phase shift amount Φ (= B · L + ΔΦ) generated is π / 2. As a result, the length L of the coupling layer can be arbitrarily set.
[0031]
In the distributed feedback semiconductor laser of the previous invention, the first and second diffraction grating layers are disposed below the active layer, whereas in the distributed feedback semiconductor laser to which the present invention is applied, the active A first diffraction grating layer and a second diffraction grating layer are disposed above the layer. Other configurations are similar to the distributed feedback semiconductor laser of the previous invention.
[0032]
Accordingly, it is possible to achieve substantially the same operational effects as the distributed feedback semiconductor laser of the previous invention.
[0033]
In the case where a diffraction grating layer is disposed below the active layer, it is not necessary to create a re-growth interface on the p side when using an n-type semiconductor substrate, preventing deterioration of characteristics due to Si pileup. it can.
[0034]
When a diffraction grating layer is provided above the active layer, the grating interval (pitch) of the diffraction grating can be set after the active layer is grown first. Furthermore, the number of times of growth can be reduced by one.
[0035]
In another invention, in the distributed feedback semiconductor laser of each invention described above, the coupling efficiency κ of the first diffraction grating.₁And the coupling efficiency κ of the second diffraction grating₂Are set to different values.
[0036]
The coupling efficiency κ in the diffraction grating is an index indicating the degree of coupling between the forward wave and the backward wave of light propagating through the diffraction grating. A high coupling efficiency κ means that the forward wave while light travels on the diffraction grating. The degree of conversion from to the backward wave is strong, and the low coupling efficiency κ indicates that the degree of conversion from the forward wave to the backward wave while the light travels on the diffraction grating is weak.
[0037]
For example, the larger the ratio of the volume of the grating in the diffraction grating in the waveguide to the volume occupied by the part other than the grating including the gap, the lower the coupling efficiency κ, and the refractive index of the grating and the gap in the diffraction grating are included. The smaller the difference from the refractive index of the portion other than the grating, the lower the coupling efficiency κ.
[0038]
Coupling efficiency κ in the diffraction grating of the first diffraction grating layer₁And the coupling efficiency κ in the diffraction grating of the second diffraction grating layer₂Is different from the reflectance of light in the first diffraction grating layer and the reflectance of light in the second diffraction grating layer. As a result, it is possible to make the amount of light output from the end face on the first diffraction grating layer side different from the amount of light output from the end face on the second diffraction grating layer side. Therefore, the intensity of light output from one end face of the distributed feedback semiconductor laser can be intentionally increased as compared with the intensity of light output from the other end face.
[0039]
In the distributed feedback semiconductor laser of another invention, the semiconductor substrate is formed of InP, the first and second diffraction grating layers are formed of InGaAsP, the active layer is formed of a material containing InGaAsP, and the cladding layer Is made of InP.
[0040]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a schematic cross-sectional view showing a schematic configuration of the distributed feedback semiconductor laser according to the first embodiment of the present invention.
[0041]
A first diffraction grating layer 12a made of n-type InGaAsP, a coupling layer 13 and a second diffraction grating layer 12b made of n-type InGaAsP are formed on the upper surface of the semiconductor substrate 11 made of n-type InP. The first diffraction grating layer 12a includes a first diffraction grating 16a including a plurality of gratings 14 and a plurality of gaps 15 existing between the gratings. On the other hand, the second diffraction grating layer 12b is composed of a second diffraction grating 16b composed of a plurality of gratings 14 and a plurality of gaps 15 existing between the gratings. The coupling layer 13 that fills the region between the first and second diffraction grating layers 12 a and 12 b is formed of the same material as the semiconductor substrate 11.
[0042]
As shown in FIG. 2, the gratings 14 and the gaps 15 of the first and second diffraction gratings 16a and 16b have the same shape, and the grating interval d is set equal. However, the phase of the grating array is spatially shifted by a distance of Δd between the first and second diffraction gratings 16a and 16b. Specifically, when it is assumed that the coupling layer 13 is not present and the first diffraction grating 16a extends to the second diffraction grating 16b as shown by a dotted line in FIG. The phase of the grating arrangement of the grating 16b is shifted by Δd compared to the phase of the grating arrangement of the first diffraction grating 16a. This phase shift amount Δd can be arbitrarily set independently of the length L of the coupling layer 13 during the manufacturing process of the distributed feedback semiconductor laser.
[0043]
Further, the refractive index of each grating 14 constituting the first and second diffraction grating layers 12 a and 12 b is set higher than the refractive index of the semiconductor substrate 11.
[0044]
A buffer layer 17 made of the same material as the semiconductor substrate 11 is formed on the top surfaces of the first and second diffraction grating layers 12 a and 12 b and the coupling layer 13. The coupling layer 13 and the buffer layer 17 are integrally formed. An active layer 18 including a lower SCH layer, an MQW layer, and an upper SCH layer, each made of InGaAsP having an appropriate composition, is formed on the upper surface of the buffer layer 17. A clad layer 19 made of p-type InP is formed on the upper surface of the active layer 18.
[0045]
A p-electrode 20 is attached to the upper surface of the cladding layer 19, and an n-electrode 21 is attached to the lower surface of the semiconductor substrate 11. Further, antireflection films 22a and 22b are formed on the end surfaces of the active layer 18 from which the light 23 is emitted and the first and second diffraction grating layers 12a and 12b.
[0046]
A contact layer made of p-type InGaAsP or p-type InGaAs may be interposed between the clad layer 19 and the p-electrode 20.
[0047]
In the distributed feedback semiconductor laser according to the first embodiment configured as described above, when a voltage is applied between the p electrode 20 and the n electrode 21, the active layer 18 emits light having multiple wavelengths. Among the light beams having the wavelengths of 1 and 2, the grating spacing d of the first and second diffraction gratings 16a and 16b and the equivalent refractive index n of the first and second diffraction grating layers 12a and 12b.₀Single wavelength λ determined by₀Is selected and laser oscillation is performed.
[0048]
In this case, the phase of the forward wave and the backward wave of the light 23 is π due to the presence of the coupling layer 13 having the length L and the phase shift amount Δd between the first and second diffraction gratings 16a and 16b. Since it is shifted by a half-odd multiple (π / 2, 3π / 2, 5π / 2,...), A single wavelength λ₀Is output.
[0049]
Next, a method for setting the length L of the coupling layer 13 in the distributed feedback semiconductor laser of the first embodiment configured as described above will be described with reference to FIG.
[0050]
  Light 23Propagates the coupling layer 13 of length LOf the total produced in the processThe phase shift amount Φ is a value obtained by multiplying the length L of the coupling layer 13 by the difference B of the light propagation constant and the amount of light corresponding to the phase shift amount Δd between the first and second diffraction gratings 16a and 16b. It is a value obtained by adding the phase shift amount ΔΦ. As described above, the propagation constant difference B is equivalent to the equivalent refractive index n of the first and second diffraction grating layers 12a and 12b.₀And the equivalent refractive index n of the coupling layer 13₁Difference from (n₀-N₁) Function f (n₀-N₁). Therefore,Of light totalThe phase shift amount Φ is expressed by the following equation.
[0051]
Φ = L · B + ΔΦ = L · [f (n₀-N₁]] + ΔΦ
From this formula, the length L of the bonding layer 13 is represented by the following formula.
[0052]
L = (Φ−ΔΦ) / f (n₀-N₁)
The amount of phase shift Φ is the wavelength of light λ₀Is a half-odd multiple of π (π / 2, 3π / 2, 5π / 2,...), So that a half-odd multiple of an arbitrary π among a plurality of half-odd multiples of π is selected. , Phase shift amount Δd and equivalent refractive index difference (n₀-N₁) Is appropriately set, the length L of the coupling layer 13 of the distributed feedback semiconductor laser can be set almost arbitrarily.
[0053]
Therefore, as shown in FIG. 3, by setting the length L of the coupling layer 13 to be long, the central portion in the intensity distribution characteristic 24 of the light 23 generated in the distributed feedback semiconductor laser is substantially flat over a wide range. The light intensity distribution characteristic 24 as a whole can be made smooth, and spatial hole burning can be suppressed. Thereby, output saturation at the time of high current injection can be suppressed.
[0054]
As a result, as shown in FIG. 4, the output light intensity P of the light 23 emitted from both end faces of the distributed feedback semiconductor laser.₀The output light intensity P of the light 4 of the conventional distributed feedback semiconductor laser shown in FIG.₀The linear response in the light output characteristic 25 with respect to the applied current can be improved compared with the output characteristic 25a in the conventional distributed feedback semiconductor laser.
[0055]
Further, the coupling layer 13 existing between the first and second diffraction grating layers 12a and 12b is made of the same material as the buffer layer 17, and further constitutes the first and second diffraction grating layers 12a and 12b. The first and second diffraction gratings 16a and 16b have a plurality of gaps 15 penetrating from the upper surface to the lower surface perpendicular to the light emission direction. In general, the diffraction grating is composed of a plurality of gratings and a plurality of grooves existing between the gratings. In this embodiment, the grooves between the gratings 14 are formed by gaps 15 penetrating from the upper surface to the lower surface. ing.
[0056]
In this way, by forming the grooves between the gratings 14 with the gaps 15, the accuracy of the shape and dimensions when this diffraction grating is manufactured by etching is improved. That is, the equivalent refractive index n of the first and second diffraction grating layers 12a and 12b₀The variation between the elements is suppressed and stabilized at a constant value. As a result, the accuracy of the phase shift amount Φ is improved, and the wavelength stability of the output light 23 is further improved.
[0057]
In the distributed feedback semiconductor laser according to the first embodiment, since the first and second diffraction grating layers 12a and 12b are disposed below the active layer 18, the n-type semiconductor substrate is used. In addition, it is not necessary to form a regrowth interface on the p side, and deterioration of characteristics due to Si pileup can be prevented.
[0058]
(Second Embodiment)
FIG. 5 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to the second embodiment of the present invention. The same parts as those in the distributed feedback semiconductor laser according to the first embodiment shown in FIG.
[0059]
In the distributed feedback semiconductor laser according to the second embodiment, the coupling layer 13 existing between the first and second diffraction grating layers 12a and 12b is the same material as the first and second diffraction grating layers 12a and 12b. It is formed with. Furthermore, as in the first embodiment, the first diffraction grating 16a and the second diffraction grating 16b have a spatial phase shift amount Δd.
[0060]
Therefore, the length L of the coupling layer 13 can be freely set by adjusting the phase shift amount Δd. Therefore, like the distributed feedback semiconductor laser of the first embodiment shown in FIG. 1, the central portion of the intensity distribution characteristic 24 of the light 23 generated in the distributed feedback semiconductor laser can be made almost flat over an arbitrary range. The entire light intensity distribution characteristic 24 can be made smooth, and spatial hole burning can be suppressed.
[0061]
(Third embodiment)
FIG. 6 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to the third embodiment of the present invention. The same parts as those in the distributed feedback semiconductor laser according to the first embodiment shown in FIG.
[0062]
In the distributed feedback semiconductor laser of the third embodiment, the coupling layer 13 existing between the first and second diffraction grating layers 12a and 12b is composed of two layers, a lower layer 13a and an upper layer 13b. Yes. The upper layer 13b is made of the same material as the upper buffer layer 17. The lower layer 13a is made of the same material as the first and second diffraction grating layers 12a and 12b.
[0063]
In this way, the lower layer 13a of the coupling layer 13 existing between the first diffraction grating layer 12a and the second diffraction grating layer 12b is the same material as the first and second diffraction grating layers 12a and 12b. Therefore, the equivalent refractive index does not change greatly at the interface between the first and second diffraction grating layers 12a and 12b and the coupling layer 13, and the reflection of light at this interface is reduced. The single mode property of the light 23 generated in this distributed feedback semiconductor laser can be further improved.
[0064]
Furthermore, as in the first embodiment, the first diffraction grating 16a and the second diffraction grating 16b have a spatial phase shift amount Δd. Therefore, the length L of the coupling layer 13 can be set almost arbitrarily as in the above-described embodiments.
[0065]
(Fourth embodiment)
FIG. 7 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to the fourth embodiment of the present invention. The same parts as those in the distributed feedback semiconductor laser according to the first embodiment shown in FIG.
[0066]
An active layer 18 including a lower SCH layer, an MQW layer, and an upper SCH layer, each made of InGaAsP having an appropriate composition, is formed on the upper surface of the semiconductor substrate 11 made of n-type InP. A buffer layer 17 made of the same material as that of the cladding layer 19 is formed on the upper surface of the active layer 18. A first diffraction grating layer 12 a made of p-type InGaAsP, a coupling layer 13, and a second diffraction grating layer 12 b made of p-type InGaAsP are formed on the upper surface of the buffer layer 17.
[0067]
The first and second diffraction gratings 16a and 16b constituting the first and second diffraction grating layers 12a and 12b are each composed of a plurality of gratings 14 and a plurality of gaps 15 existing between the gratings. Similarly to the first embodiment, the first diffraction grating 16a and the second diffraction grating 16b spatially have a phase shift amount Δd. The refractive index of each grating 14 constituting the first and second diffraction grating layers 12 a and 12 b is set higher than the refractive index of the semiconductor substrate 11.
[0068]
A clad layer 19 made of p-type InP is formed on the top surfaces of the first and second diffraction grating layers 12 a and 12 b and the coupling layer 13. The coupling layer 13b and the cladding layer 19 are integrally formed.
[0069]
A p-electrode 20 is attached to the upper surface of the cladding layer 19, and an n-electrode 21 is attached to the lower surface of the semiconductor substrate 11. Further, antireflection films 22a and 22b are formed on the end surfaces of the active layer 18 from which the single-mode light 23 is emitted and the first and second diffraction grating layers 12a and 12b.
[0070]
A contact layer made of p-type InGaAsP or p-type InGaAs may be interposed between the clad layer 19 and the p-electrode 20.
[0071]
In the distributed feedback semiconductor laser of the fourth embodiment configured as described above, when a voltage is applied between the p electrode 20 and the n electrode 21, the active layer 18 oscillates light having multiple wavelengths. Of the diffraction grating 16 and the equivalent refractive index n of the first and second diffraction grating layers 12a and 12b.₀Single wavelength λ determined by₀Is selected and laser oscillation is performed.
[0072]
In this case, due to the presence of the coupling layer 13 having a length L and the spatial phase shift amount Δd between the first and second diffraction gratings 16a and 16b, the phase of the light 23 is a half odd number (π / 2), 3π / 2, 5π / 2,...₀Is output. The length L in the propagation direction of the light 23 of the coupling layer 13 sandwiched between the first and second diffraction grating layers 12a and 12b is the same as that of the distributed feedback semiconductor laser of the first embodiment described above. It is determined by the spatial phase shift amount Δd between the second diffraction gratings 16 a and 16 b and the difference in cross-sectional shape between the first and second diffraction grating layers 12 a and 12 b and the coupling layer 13.
[0073]
Accordingly, the length L of the coupling layer 13 in the propagation direction of the light 23 can be set to an almost arbitrary value as in the distributed feedback semiconductor laser of the first embodiment described above, and therefore the first embodiment shown in FIG. The effects similar to those of the distributed feedback semiconductor laser of the form can be obtained.
[0074]
In the distributed feedback semiconductor laser according to the fourth embodiment, since the first and second diffraction grating layers 12a and 12b are disposed above the active layer 18, the first and second diffraction grating layers 12a and 12b are disposed from the active layer 18. Since up to two diffraction grating layers 12a and 12b can be formed by one growth, the number of times of growth can be reduced by one. Further, after the active layer 18 is first grown, the grating spacing (pitch) of the diffraction gratings in the first and second diffraction grating layers 12a and 12b can be set.
[0075]
(Fifth embodiment)
FIG. 8 is a schematic sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to the fifth embodiment of the present invention. The same parts as those of the distributed feedback semiconductor laser according to the fourth embodiment shown in FIG.
[0076]
In the distributed feedback semiconductor laser according to the fifth embodiment, the coupling layer 13 existing between the first and second diffraction grating layers 12a and 12b is composed of two layers, a lower layer 13a and an upper layer 13b. ing. The upper layer 13b is formed of the same material as the upper cladding layer 19. The lower layer 13a is made of the same material as the first and second diffraction grating layers 12a and 12b. Further, not the gap 15 penetrating vertically between the gratings 14 of the first and second diffraction gratings 16a and 16b formed by the first and second diffraction grating layers 12a and 12b, but a groove 15a that does not penetrate. Yes.
[0077]
Therefore, in the distributed feedback semiconductor laser according to the fifth embodiment, all of the first diffraction grating layer 12a, the coupling layer 13, and the second diffraction grating layer 12b existing in the emission direction of the light 23 in the semiconductor element. It will be connected by the same substance over a region. Therefore, substantially the same operational effects as the distributed feedback semiconductor laser of the third embodiment shown in FIG. 6 can be obtained.
[0078]
(Sixth embodiment)
FIG. 9 is a schematic sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to the sixth embodiment of the present invention. The same parts as those of the distributed feedback semiconductor laser according to the fifth embodiment shown in FIG.
[0079]
In the distributed feedback semiconductor laser of the sixth embodiment, a gap 15 penetrates vertically between the gratings 14 of the first and second diffraction gratings 16a and 16b in the distributed feedback semiconductor laser of the fifth embodiment. Forming. Similar to the fifth embodiment, the first diffraction grating 16a and the second diffraction grating 16b spatially have a phase shift amount Δd.
[0080]
In this way, by forming the gaps 15 penetrating vertically between the gratings 14 of the first and second diffraction gratings 16a and 16b, the shape and dimensions when the diffraction gratings 16a and 16b are manufactured by etching. Improves accuracy.
[0081]
Further, since the length L of the coupling layer 13 can be set long by adjusting the phase shift amount Δd, the entire light intensity distribution characteristic 24 in the cloth feedback semiconductor laser can be made smooth, and the spatial hole Burning can be suppressed.
[0082]
(Seventh embodiment)
FIG. 10 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to the seventh embodiment of the present invention. The same parts as those in the distributed feedback semiconductor laser according to the first embodiment shown in FIG.
[0083]
In the distributed feedback semiconductor laser of the seventh embodiment, the coupling efficiency κ in the first diffraction grating 16a of the first diffraction grating layer 12a.₁And the coupling efficiency κ of the second diffraction grating layer 12b in the second diffraction grating 16b.₂Are set to different values.
[0084]
In the distributed feedback semiconductor laser according to the seventh embodiment, the volume occupied by a portion other than the grating 14 including the volume of the grating 14 and the gap 15 within one grating interval d of the diffraction grating 16a of the first diffraction grating layer 12a. Volume ratio v showing the ratio of₁And the volume ratio v in the diffraction grating 16b of the second diffraction grating layer 12b.₂Are set to different values.
[0085]
Since the gratings 15 of the diffraction gratings 16a and 16b and the gaps 15 of the first and second diffraction grating layers 12a and 12b are formed of the same material, it is necessary to consider the contribution of the refractive index. Each volume ratio v₁, V₂Are the coupling efficiencies κ of the first and second diffraction gratings 16a and 16b.₁, Κ₂Corresponding to
[0086]
Specifically, the first and second diffraction grating layers 12a and 12b that are spaced apart from each other with the coupling layer 13 in between have the same grating spacing (pitch) d, but the first and second The first and second diffraction gratings 16a and 16b formed by the diffraction grating layers 12a and 12b respectively have different shapes. Each of the diffraction gratings 16a and 16b is composed of one grating 14 and one gap 15 (or groove) at one grating interval d. The ratio of the volume and the volume occupied by the portion other than the lattice 14 including the gap 15 is defined as the volume ratio v as described above. The volume ratio v is determined by the width of the grating 14 in the direction coinciding with the traveling direction of the light 23, the height of the grating 14, the width of the grating 14 in the direction orthogonal to the traveling direction of the light 23, and the like.
[0087]
In the seventh embodiment of FIG. 10, the second diffraction grating layer 12b has a height in a direction that coincides with the traveling direction of the light 23 of the grating 14 in the diffraction grating 16a of the first diffraction grating layer 12a. The volume ratio v of the diffraction grating 16a of the first diffraction grating layer 12a is set by setting the height of the diffraction grating 16b in the direction matching the traveling direction of the light 23 of the grating 14 to a small value.₁And the volume ratio v of the diffraction grating 16b of the second diffraction grating layer 12b.₂Are different (v₁> V₂). Therefore, the coupling efficiency κ of the first diffraction grating 16a₁And the coupling efficiency κ of the second diffraction grating 16b₂Is different (κ₁> Κ₂).
[0088]
As a result, the reflectance of light in the diffraction grating 16a of the first diffraction grating layer 12a is different from the reflectance of light in the diffraction grating 16b of the second diffraction grating layer 12b. It is possible to make the amount of light 23a output from the end face on the first diffraction grating layer 12a side different from the amount of light 23b output from the end face on the second diffraction grating layer 12b side.
[0089]
Therefore, the intensity of light output from one end face of the distributed feedback semiconductor laser can be intentionally increased as compared with the intensity of light output from the other end face. In the seventh embodiment of FIG. 10, the intensity of the light 23b output from the end face on the second diffraction grating layer 12b side is greater than the intensity of the light 23a output from the end face on the first diffraction grating layer 12a side. Set high.
[0090]
In the seventh embodiment shown in FIG. 10, the reflectance of the antireflection film 22b formed on the end face on the second diffraction grating layer 12b side is set to reflect on the end face on the first diffraction grating layer 12a side. By setting it smaller than the reflectance of the prevention film 22a, the intensity of the light 23b output from the end face on the second diffraction grating layer 12b side is made higher. In this case, the antireflection film 22a formed on the end face on the first diffraction grating layer 12a side may be a highly reflective film or may remain a cleavage plane.
[0091]
It should be noted that the coupling efficiency κ of the diffraction grating 16a of the first diffraction grating layer 12a in the distributed feedback semiconductor laser of each of the embodiments of FIGS.₁And the coupling efficiency κ of the diffraction grating 16b of the second diffraction grating layer 12b₂Can be different.
[0092]
FIG. 11 shows the volume ratio v of the diffraction grating 16a of the first diffraction grating layer 12a.₁(Coupling efficiency κ₁) And the volume ratio v of the diffraction grating 16b of the second diffraction grating layer 12b₂(Coupling efficiency κ₁) And (v₁<V₂FIG. 4 is a diagram showing the steps up to the formation of diffraction gratings 16a and 16b in the method of manufacturing a distributed feedback semiconductor laser.
[0093]
In FIG. 11, the case where the first and second diffraction grating layers 12a and 12b are present below the active layer 18 will be described, as shown in the distributed feedback semiconductor laser of the seventh embodiment in FIG. .
[0094]
First, as shown in FIG. 11A, an n-type InGaAsP having a composition wavelength of 1.08 μm is formed on the semiconductor substrate 11 made of n-type InP as the first and second diffraction grating layers 12a and 12b with a thickness of 0. After the growth of 1 μm, an electron beam resist (for example, ZEP-520 manufactured by Nippon Zeon Co., Ltd.) is applied to the thickness of 0.2 μm.
[0095]
After the heat treatment, a diffraction grating pattern is drawn by an electron beam drawing apparatus. FIG. 11B shows a drawing pattern and drawing conditions in which the grating interval (pitch) d of the diffraction gratings 16a and 16b is 240 nm in this embodiment. The drawing condition of the diffraction grating portion is that the first diffraction grating layer 12a (left side in the figure) on the output side after chip processing has a large dose (for example, about 0.45 nC / cm), and the second diffraction grating on the opposite side. In the layer 12b, irradiation is performed with a small dose (about 0.3 nC / cm). The peripheral portion may be exposed by electron beam or ultraviolet irradiation.
[0096]
Then, the drawn diffraction grating pattern is developed. As shown in FIG. 11 (c), in this embodiment, the developed resist pattern has a gap 15 between the gratings 14 of the diffraction gratings 16a and 16b of about 140 nm when the dose is increased, and is less than about 140 nm. It is about 70 nm.
[0097]
In this state, etching is performed for about 20 seconds using an aqueous solution of saturated bromine water and phosphoric acid. Then, the cross-sectional shape shown in FIG. 11D is obtained, and the difference in the width of the lattice 14 is transferred to the difference in the height of the lattice 14 by heating in the subsequent crystal growth step.
[0098]
Thus, the volume ratio v of the diffraction grating 16a on the light exit side.₁Is small and the volume ratio v of the diffraction grating 16b on the opposite side₂Is large (v₁<V₂) A distributed feedback semiconductor racer having a structure is manufactured.
[0099]
On the other hand, as a method other than the dose modulation, it can be used that the etching rate during etching changes depending on the exposed area of the wafer surface. In other words, the etching rate is fast when the area of the wafer surface that is covered with an etching inhibitor such as a resist or dielectric film is large and the portion to be etched is small, and conversely, the etching is performed when etching a large area. The phenomenon in which the rate is slow is known in the above-mentioned saturated bromine water and phosphoric acid aqueous solutions. If this feature is used, it becomes possible to partially produce diffraction gratings having different volume ratios v by producing resist patterns as shown in FIGS. 12 (a), 12 (b), and 12 (c).
[0100]
As shown in FIG. 12A, when the etching is performed after removing the resist in the peripheral portion, the etching rate becomes slow.
[0101]
As shown in FIG. 12B, the etching rate can be controlled by changing the spread of the lattice pattern in the direction orthogonal to the light guiding direction (for example, 20 μm and 100 μm).
[0102]
Although the above description has been given for the case where a positive resist is used, the same applies to the case where a negative resist is used. In either case, instead of directly applying a resist to n-type InGaAsP, it is possible to use a selective etchant by dividing an etching into two stages by interposing an n-type InP layer having a thickness of about 30 nm, for example.
[0103]
In this case, the InP is partially removed by etching with an etchant containing saturated bromine water for about 5 seconds using the resist as an etching mask, and the exposed n-type InGaAsP is replaced with sulfuric acid using the n-type InP as an etching mask. A method of etching with an aqueous solution containing hydrogen oxide is also possible.
[0104]
In this method, since an aqueous solution containing sulfuric acid and hydrogen peroxide selectively etches only InGaAsP, etching is performed at the interface between the buffer layer and InP, particularly when a diffraction grating is formed above the active layer. Since the operation is stopped, it is possible to prevent an accident that the etching reaches the active layer by mistake.
[0105]
On the other hand, since the portion of the coupling layer 13 is exposed entirely, the etching rate is lowered, and even if the gap 15 in the first and second diffraction gratings 16a and 16b reaches InP even if it is etched, N type InGaAsP layer (13a) tends to remain thin. When it is desired to completely remove this layer (13a), the peripheral area is covered with a resist or the like as shown in FIG. 12C to reduce the exposed area and increase the etching rate.
[0106]
In any case, when being processed into a laser in the subsequent process, it is etched into a mesa shape having a width of about 2 to 3 μm along the emission direction of the light 23, so the outer region is in any state. There may be.
[0107]
【The invention's effect】
As described above, in the distributed feedback semiconductor laser of the present invention, the grating arrangement is spatially phase-shifted between the first and second diffraction gratings in the first and second diffraction grating layers. . Therefore, the length of the coupling layer can be set to an arbitrary length, the light intensity of the output light can be easily increased, the wavelength of the output light can be set to the target wavelength with high accuracy, and the output light can be further increased. Wavelength stability can be improved.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to a first embodiment of the present invention.
FIG. 2 is a view for explaining a method for setting the length of the coupling layer in the distributed feedback semiconductor laser according to the embodiment;
FIG. 3 is a view showing the relationship between the length of the coupling layer and the light intensity distribution characteristic in the distributed feedback semiconductor laser according to the embodiment;
FIG. 4 is a view showing light output characteristics of the distributed feedback semiconductor laser according to the embodiment;
FIG. 5 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to a second embodiment of the present invention.
FIG. 6 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to a third embodiment of the present invention.
FIG. 7 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to a fourth embodiment of the present invention.
FIG. 8 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to a fifth embodiment of the present invention.
FIG. 9 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to a sixth embodiment of the present invention.
FIG. 10 is a schematic cross-sectional view showing a schematic configuration of a distributed feedback semiconductor laser according to a seventh embodiment of the present invention.
FIG. 11 is a view showing a method of manufacturing a distributed feedback semiconductor laser.
FIG. 12 is a diagram showing each diffraction grating of a distributed feedback semiconductor laser.
FIG. 13 is a diagram for explaining the operating principle of a distributed feedback semiconductor laser.
FIG. 14 is a schematic cross-sectional view showing a schematic configuration of a conventional distributed feedback semiconductor laser.
FIG. 15 is a diagram for explaining a method for setting a coupled waveguide length in a conventional distributed feedback semiconductor laser;
FIG. 16 is a diagram for explaining problems of the conventional distributed feedback semiconductor laser;
[Explanation of symbols]
11 ... Semiconductor substrate
12a ... 1st diffraction grating layer
12b ... Second diffraction grating layer
13 ... Bonding layer
13a ... Lower layer
13b ... upper layer
14 ... lattice
15 ... Gap
15a ... groove
16a ... 1st diffraction grating
16b ... Second diffraction grating
17 ... Buffer layer
18 ... Active layer
19 ... Clad layer
20 ... p electrode
21 ... n electrode
22a, 22b ... antireflection film
23, 23a, 23b ... light
24: Light intensity distribution characteristics
25, 25a ... Output characteristics of light

Claims (4)

A semiconductor substrate (11), disposed above the semiconductor substrate, first and second diffraction grating layer disposed apart from each other across the bonding layer in the light emitting direction (23) (13) ( 12a, 12b), an active layer (18) disposed above the first and second diffraction grating layers and the coupling layer , and a cladding layer (19) disposed above the active layer. Distributed feedback semiconductor laser
The first diffraction grating layer is composed of a first diffraction grating (16a) having a plurality of gratings (14) arranged at a predetermined grating interval d,
The second diffraction grating layer is composed of a second diffraction grating (16b) having a plurality of gratings (14) arranged with a grating interval d equal to the grating interval of the first diffraction grating,
There is a phase shift amount Δd between the grating arrangement of the first diffraction grating and the grating arrangement of the second diffraction grating,
Wherein said length of outgoing direction of the light L of the binding layer first, the difference between the effective refractive index n ₀ of the second diffraction grating layer and the equivalent refractive index n ₁ of the coupling layer (n ₀ -n ₁₎ The process of light propagating through the coupling layer, obtained by adding the phase shift amount ΔΦ of the light corresponding to the phase shift amount Δd to the product (B · L) of the light propagation constant difference B obtained based on The phase shift amount Δd is set so that the total phase shift amount Φ (= B · L + ΔΦ) generated in step π / 2 becomes π / 2,
A distributed feedback semiconductor laser characterized in that the length L of the coupling layer can be arbitrarily set . The semiconductor substrate (11), the active layer (18) disposed above the semiconductor substrate, and the active layer (18) disposed above the active substrate are spaced apart from each other across the coupling layer (13) in the light (23) emission direction. And the first and second diffraction grating layers (12a, 12b), and the cladding layer (19) disposed above the first and second diffraction grating layers and the coupling layer. Distributed feedback semiconductor laser
The first diffraction grating layer is composed of a first diffraction grating (16a) having a plurality of gratings (14) arranged at a predetermined grating interval d,
The second diffraction grating layer is composed of a second diffraction grating (16b) having a plurality of gratings (14) arranged with a grating interval d equal to the grating interval of the first diffraction grating,
There is a phase shift amount Δd between the grating arrangement of the first diffraction grating and the grating arrangement of the second diffraction grating,
Wherein said length of outgoing direction of the light L of the binding layer first, the difference between the effective refractive index n ₀ of the second diffraction grating layer and the equivalent refractive index n ₁ of the coupling layer (n ₀ -n ₁₎ The process of light propagating through the coupling layer, obtained by adding the phase shift amount ΔΦ of the light corresponding to the phase shift amount Δd to the product (B · L) of the light propagation constant difference B obtained based on The phase shift amount Δd is set so that the total phase shift amount Φ (= B · L + ΔΦ) generated in step π / 2 becomes π / 2,
A distributed feedback semiconductor laser characterized in that the length L of the coupling layer can be arbitrarily set . Claim that bind efficiency (kappa ₁₎ of the first diffraction grating (16a), characterized in that said first coupling efficiency (kappa ₂₎ in the second diffraction grating (16b) and are set to different values 3. A distributed feedback semiconductor laser according to 1 or 2.   The semiconductor substrate is made of InP, the first and second diffraction grating layers are made of InGaAsP, the active layer is made of a material containing InGaAsP, and the cladding layer is made of InP. A distributed feedback semiconductor laser according to claim 1 or 2.

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