JP3595677B2 - Optical isolator, distributed feedback laser and optical integrated device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical isolator, a distributed feedback laser, and an optical integrated device. More specifically, the present invention relates to a waveguide type optical isolator, a distributed feedback type laser, and a monolithically integrated optical integrated device, which are compact, have high directivity, and have good light coupling.
[0002]
[Prior art and its problems]
In a distributed feedback laser (DFB laser), a diffraction grating is provided along a waveguide, and laser oscillation is generated by using Bragg diffracted light from the diffraction grating for optical feedback (optical feedback). It is characterized by making it. Since the oscillation longitudinal mode is selected by the period of the diffraction grating, there is an advantage that a single longitudinal mode (single longitudinal mode) oscillation is possible. The DFB laser is utilized as a light source for high-speed optical communication and measurement via an optical fiber, taking advantage of such advantages.
[0003]
However, the DFB laser has a disadvantage that it is weak to return light. In other words, there is a problem that the oscillation condition of the single longitudinal mode, which has been oscillating, is disturbed by external returning light. Therefore, the oscillation wavelength fluctuates (referred to as a wavelength chirp), and in the worst case, instability such as a jump in a longitudinal mode occurs.
[0004]
Therefore, the DFB laser requires an optical isolator. Many commonly used optical isolators use a Faraday element. Documents disclosing the optical isolator include, for example, Ryoichi Ito and Michiharu Nakamura, “Semiconductor Laser”, Baifukan, ISBN4-563-03437-1, C3055, P6386E, p. 276-277. However, such a conventional optical isolator is difficult to monolithically integrate with a DFB laser device because the material is different from a DFB laser. Therefore, it was manufactured as a separate component from the DFB laser, and it was necessary to adjust and assemble these optical axes so as to be assembled. Further, these optical isolators have a problem that they require a magnetic field, are large in size, are expensive, and have a high cost of a module equipped with a DFB laser.
[0005]
In order to solve such a problem, a completely different type of optical isolator that can be integrated on a semiconductor substrate has been proposed. As a document which discloses this type of optical isolator, for example, JP-A-8-179142 can be cited.
[0006]
FIG. 8 is a conceptual perspective view illustrating a schematic configuration of the optical isolator. In the optical isolator shown in the figure, a diffraction grating 111 is arranged on a semiconductor substrate S obliquely to a waveguide direction, and a refractive index n on one side of a waveguide structure. ₂ And the asymmetry is reduced, and the return light is n ₂ Dissipated to the side as a radiation mode.
However, this configuration has the following disadvantages. That is, since the waveguide structure is left-right asymmetric, the output beam also has a left-right asymmetric distribution, and the coupling with the optical fiber is deteriorated. In addition, n having different refractive indices ₂ Must be separately crystal-grown, which complicates the manufacturing process. Furthermore, when a diagonally formed diffraction grating is used, the coupling as a DFB laser deteriorates.
[0007]
The present invention has been made based on the recognition of such a problem. That is, an object of the present invention is to provide an optical isolator, a distributed feedback laser, and an optical integrated device in which these have high directivity, are compact, have good coupling with an optical fiber or a DFB laser, are easy to manufacture, and are integrated with each other. It is in.
[0008]
[Means for Solving the Problems]
That is, the optical isolator of the present invention has a cross-sectional shape. Serrated A waveguide having a second or higher order diffraction grating; Either on the top or bottom A reflecting structure provided to return radiation mode light emitted from the waveguide to the waveguide; On the other side above or below A non-reflection structure that prevents radiation mode light emitted from the waveguide from returning to the waveguide.
[0009]
The distributed feedback laser of the present invention has an active layer and a cross-sectional shape. Serrated A waveguide having a second-order or higher diffraction grating; Either on the top or bottom A reflecting structure provided to return radiation mode light emitted from the waveguide to the waveguide; On the other side above or below A non-reflection structure that prevents radiation mode light emitted from the waveguide from returning to the waveguide.
[0010]
Here, the reflection structure is provided apart from the waveguide so as not to affect the guided mode light that exudes and propagates from the waveguide, and the non-reflection structure exudes from the waveguide. It is characterized by being provided apart from the waveguide so as not to affect the propagating waveguide mode light.
[0011]
Further, the non-reflection structure is made of a semiconductor material having a band gap smaller than that of the semiconductor material forming the waveguide.
[0012]
Alternatively, the non-reflection structure is a non-reflection coat for the radiation mode light.
[0013]
The reflection structure is a Bragg multilayer reflection film for the radiation mode light.
[0014]
Further, the diffraction grating has a plurality of different periods.
[0015]
Further, the waveguide is characterized in that the effective refractive index is not constant.
[0016]
Further, the diffraction grating has at least one phase shift.
[0017]
On the other hand, an optical integrated device of the present invention is characterized in that any one of the above-described optical isolators and any one of the above-described distributed feedback lasers are monolithically integrated.
[0018]
Further, the present invention is characterized in that an external modulator is further integrated.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
SUMMARY OF THE INVENTION The present invention provides an optical isolator having high directivity by using a second or higher order diffraction grating having a sawtooth cross-sectional shape and a reflection structure disposed on either the upper or lower side. That is, generally, in a waveguide structure having a second-order or higher diffraction grating, guided light is dissipated as a radiation mode. However, if it is reflected back to the waveguide, the loss due to dissipation is reduced. When a blazed diffraction grating, that is, a diffraction grating having an asymmetric cross-sectional shape, is used, the radiation mode becomes extremely strong in any of the traveling directions of the guided light. Therefore, providing a reflection structure in this direction reduces the loss. On the opposite side, even if the radiation mode is dissipated without providing the reflection structure, the loss is small because the radiation mode is originally small.
[0020]
On the other hand, when the guided light travels in the opposite direction, the number of radiation modes on the dissipation side where no reflection structure is provided increases, and the number of radiation modes on the reflection structure side decreases. Therefore, the loss for the traveling wave in the opposite direction increases. In this manner, an optical isolator having a different loss depending on the traveling direction of the guided light can be realized.
Since this optical isolator can be formed as an extension of the diffraction grating of a DFB laser, it also has an advantage that it can be easily monolithically integrated with a DFB laser or an external modulator.
[0021]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a conceptual diagram illustrating a schematic configuration of the optical isolator according to the first embodiment of the present invention. That is, the drawing shows a cross-sectional structure along the waveguide of the optical isolator. In the optical isolator of the present embodiment, an InGaAs absorption layer 30, an n-type InP clad layer 2, an InGaAsP layer 4, a p-type InP clad layer 5, a p-type InGaAsP layer 6, and a reflection structure 20 are formed on an n-type InP substrate 1. It has a configuration in which the layers are sequentially stacked.
[0022]
The absorption layer 30 only needs to be a material having a high absorption coefficient for radiation mode light, and can be formed by using, for example, a semiconductor material having a smaller band gap than the waveguide layer 4.
[0023]
On the surface of the waveguide layer 4, a secondary diffraction grating 10 having a sawtooth asymmetric cross-sectional shape in the illustrated direction is formed. Here, the “order” of the diffraction grating is determined by the relationship between the period of the diffraction grating and the wavelength of light that causes diffraction by Bragg reflection. For example, a first-order diffraction grating has a first-order period corresponding to the wavelength of light. The second-order diffraction grating has twice the period of the first-order diffraction grating. For example, in an InGaAsP / InP-based optical element having a wavelength band of 1.3 μm, the period of the primary diffraction grating is about 0.2 μm. The processing accuracy required for the formation is equivalent to 0.1 μm, and it is difficult to control the depth. On the other hand, the period of the second-order diffraction grating is 0.4 μm, which also has a feature that the fabrication is much easier.
On the other hand, the reflection structure 20 can be formed by, for example, a Bragg multilayer reflection film in which two types of derivative films having different refractive indexes are alternately stacked. By appropriately selecting the refractive index and the thickness of each layer of such a Bragg multilayer reflective film, it is possible to obtain a reflective structure having a high reflectance for radiation mode light.
[0024]
The outline of the manufacturing process of the optical isolator of FIG. 1 is as follows. First, an InGaAs absorption layer 30 for radiation mode light is grown to a thickness of 2.5 μm on the n-type InP substrate 1. Subsequently, the n-type InP cladding layer 2 is grown to a thickness of about 1 μm. This is a thickness that does not affect the guided mode. That is, the absorption layer 30 is provided at a distance that does not affect the seepage of the guided mode light into the cladding layer.
[0025]
Next, an InGaAsP layer 4 is grown to about 0.3 μm. The crystal growth so far is performed continuously. On the surface of the waveguide layer 4, a saw-tooth asymmetric second-order diffraction grating 10 having a blaze angle in the illustrated direction is formed to constitute a waveguide. As a method of forming such a diffraction grating, for example, after applying a resist to the surface of the waveguide layer 4 and performing EB (electron beam) exposure, the substrate is inclined and the surface is processed by an ion milling method. Finally, there is a method of finishing the surface with an appropriate solution (etchant). The depth of the diffraction grating thus obtained is about 0.1 μm.
[0026]
Next, a p-type InP cladding layer 5 is grown thereon to a thickness of about 1 μm or more. Further, a p-type InGaAsP layer 6 is grown for surface protection. These layers are p-type in consideration of integration with a DFB laser, but may be n-type when used only as an optical isolator alone.
[0027]
Finally, a reflective structure 20 made of, for example, a derivative multilayer film is deposited and formed. The reflectivity of the reflective structure 20 can be, for example, about 95%. Like the absorption layer 30, the reflection structure 20 is almost outside the exudation range of the guided mode, and does not affect the guided mode light. Thus, the optical isolator of FIG. 1 is completed.
[0028]
Next, the operation mechanism of the optical isolator of the present invention will be described.
The optical isolator of FIG. 1 has a second-order or higher diffraction grating. In particular, a waveguide structure having a second-order diffraction grating emits radiation mode light in both directions perpendicular to the waveguide direction, that is, in a substrate side and in a direction opposite to the substrate side. Further, in the optical isolator of FIG. 1, the cross-sectional shape of the secondary diffraction grating is asymmetric in the traveling direction, that is, it has a blaze angle. Then, the radiation mode on the substrate side becomes stronger or the radiation mode on the opposite side becomes stronger depending on the blaze angle with respect to the traveling direction of the light wave to one of the right and left. When the traveling direction of the light wave is reversed, the distribution ratio between the substrate side and the opposite side is also reversed.
[0029]
Specifically, in FIG. 1, when the guided light travels from right to left, most of the radiation mode light is emitted to the substrate 1 side as indicated by an arrow C. Since this light is absorbed by the absorption layer 30, it does not return to the waveguide, and becomes a waveguide loss as it is.
[0030]
On the other hand, when the guided light travels from left to right, most of the radiation mode light is emitted upward on the side opposite to the substrate as indicated by arrow A. This light is reflected by the reflection structure 20 and returns to the waveguide structure as shown by the arrow B. Therefore, the loss of the waveguide is small from left to right. The principle of this structure can be easily understood by a one-dimensional slab structure.
FIG. 2 is a graph showing a reference example of a waveguide characteristic in an asymmetrical diffraction grating. In other words, FIG. 1 is a diagram by Steffer et al. Entitled "Analysis of Grating-Coupled Radiation in GaAs: GaAlAs lasers and Waveguides-II: Blazing Effects Automotive." QE-12, pp. 4494-4499, 1976. In the figure, the horizontal axis represents the shape parameter δ of the diffraction grating, and the vertical axis represents the output of light emitted from the diffraction grating. Here, the shape parameter δ on the horizontal axis in the drawing represents the asymmetry (blaze) of the cross-sectional shape of the diffraction grating, and as can be understood from the inset, δ = △ / Λ = 0.5 is a symmetrical cross-sectional shape. Corresponds to a diffraction grating. As shown in the inset, when the guided light travels from left to right and δ = △ / Λ = 1.0, the radiation mode (3) to the substrate side becomes stronger and the radiation to the opposite side It can be quantitatively understood that the mode (1) becomes extremely small.
[0031]
In the present invention, a reflection structure is provided only on one side of such an asymmetrical diffraction grating. When the reflection structure is provided, the radiation mode returns to the waveguide structure, so that the loss of the waveguide structure is reduced. That is, the loss of the guided light traveling in this direction is reduced.
[0032]
Since the guided light traveling in the opposite direction is equivalent to the case where △ / Λ = 1, the radiation mode ((1)) in the opposite direction of the substrate becomes extremely large. Since there is no reflective structure in this direction, the loss of the waveguide structure increases.
[0033]
According to the present invention, due to the non-reciprocity of the direction of the guided light as described above, it is possible to realize a structure in which only one-way guided light (in this case, from left to right) is guided with less loss, and high direction selection An extremely compact waveguide-type optical isolator having the property can be realized.
[0034]
In a conventional optical isolator as illustrated in FIG. 8, the semiconductor layers for embedding the side surfaces of the waveguide in a direction perpendicular to the paper surface need to have different compositions (different refractive indexes) on both side surfaces. According to the present invention, there is no such need. That is, a stripe structure can be formed in exactly the same manner as in the related art. Therefore, the number of subsequent steps does not increase. Further, NFP (near field pattern) of the waveguide mode is also symmetrical, and has an advantage that optical coupling to a fiber or the like is extremely easy.
[0035]
Next, a second embodiment of the present invention will be described.
FIG. 3 is a conceptual cross-sectional view illustrating a configuration of a main part of an optical isolator according to a second embodiment of the present invention. Also in the present embodiment, a second-order or higher diffraction grating 10 is provided, and a reflection structure and an absorption structure are provided above and below it.
[0036]
The difference from the optical isolator of the first embodiment described above is that
1) a reflection structure 20 is provided below the diffraction grating 10 instead of the absorption layer 30;
2) an anti-reflection coat (AR) 40 is provided on the upper side of the diffraction grating instead of the reflection structure 20;
2 points. Here, the reflection structure 20 can be, for example, a high reflection DBR (Distributed Bragg Reflector) having a multilayer structure of semiconductor crystal layers. As the non-reflection coating 40, for example, a quarter-wave film, that is, a dielectric thin film having a thickness of λ / 4n when a wavelength of the radiation mode light is λ and a refractive index is n is used. it can. Elements other than these can be the same as those in the first embodiment described above with reference to FIG. 1, and thus the same reference numerals are given and detailed description is omitted.
[0037]
In the present embodiment, the reflection and absorption functions are only reversed up and down the diffraction grating 10, and the principle of operation is the same as in the first embodiment. That is, the guided light traveling from the right to the left in the drawing is radiated downward by the diffraction grating 10 as shown by the arrow A. Then, the light is reflected by the reflection structure 20, returns to the diffraction grating 10 as shown by the arrow B, and proceeds to the left.
[0038]
On the other hand, the guided light traveling from left to right in the figure is emitted upward by the diffraction grating 10 as shown by the arrow C. Since the non-reflection coating 40 provided on the upper side of the diffraction grating 10 has an extremely low reflectance, the light emitted from the diffraction grating 10 is emitted to the outside without being reflected, and the component returning to the diffraction grating 10 is extremely small.
[0039]
Eventually, in the optical isolator of FIG. 3, the waveguide loss for the guided light component traveling from left to right in the figure is high, and the waveguide loss for the guided light component traveling from right to left is low. Become. In this way, directionality is obtained and the device can operate as an optical isolator. Also in the present embodiment, the various effects described above with reference to FIG. 1 can be obtained similarly.
[0040]
Next, a third embodiment of the present invention will be described.
FIG. 4 is a conceptual sectional view illustrating a main configuration of an optical isolator according to a third embodiment of the present invention. Also in the present embodiment, a diffraction grating 10 'is provided, and an absorption structure 40 is provided above the diffraction grating 10' and a reflection structure 20 is provided below the diffraction grating 10 '. Since the basic configuration and the operation are the same as those of the second embodiment described above with reference to FIG. 3, each component is denoted by the same reference numeral and detailed description is omitted.
[0041]
A feature of the present embodiment is that the diffraction grating 10 ′ has a period Λ. ₁ And period Λ ₂ This is a point having two types of periods. In this manner, two types of guided light having different wavelengths corresponding to these periods can be handled as an optical isolator. That is, the dynamic range of the corresponding wavelength of the optical isolator can be expanded.
[0042]
Further, when the diffraction grating is provided at a place where the period of the diffraction grating is different along the axial direction of the waveguide, the same effect as when the phase shift is effectively provided can be obtained. Therefore, it is possible to control the profile of the guided light and the radiation mode light in the resonator axis direction by the phase shift effect.
[0043]
Although not shown, by introducing three or more different periods into the diffraction grating, it is possible to cope with the guided light having three or more different wavelengths as an optical isolator. Further, by continuously changing the period of the diffraction grating, it is possible to function as an optical isolator for guided light in a continuous wavelength range.
[0044]
Note that the same effect can be obtained by changing the effective refractive index of the waveguide along the axial direction instead of changing the period of the diffraction grating.
[0045]
Next, a fourth embodiment of the present invention will be described.
FIG. 5 is a conceptual sectional view illustrating a configuration of a main part of an optical isolator according to a fourth embodiment of the present invention. Since the optical isolator of the present embodiment has a configuration similar to that of the second embodiment described above with reference to FIG. 3, the same components are denoted by the same reference numerals and detailed description is omitted.
[0046]
The present embodiment is different from the diffraction grating 10 ″ in that a portion where the period of the grating is discontinuously shifted in the diffraction grating 10 ″, that is, a phase shift 11 is provided. The phase shift 11 can control the axial profile of the waveguide light and the radiation mode light in the resonator.
[0047]
FIG. 5 shows an isolator waveguide that has good transmittance for right-going guided light and large attenuation in the opposite direction. After passing through the phase shift, the attenuation factor increases due to the phase difference between the light wave and the diffraction grating 10. The phase shift 11 is provided near the exit (right) of the isolator waveguide in FIG. Then, while the attenuation of the right-going guided light is small, the attenuation of the left-going guided light is large because the distance after passing the phase shift 11 is long. This is shown by the light intensity change indicated by the arrow in the figure. That is, the phase shift 11 can further enhance the effect of the isolation.
[0048]
It should be noted that the phase shift as introduced in the present embodiment can be similarly introduced into the optical isolators of the first and third embodiments to obtain the same effect.
[0049]
Next, a fifth embodiment of the present invention will be described.
FIG. 6 is a conceptual sectional view illustrating a main part configuration of the DFB laser according to the embodiment of the present invention. That is, the DFB laser of the present embodiment is a specific example in which the structure and operation principle of the optical isolator of each of the above-described embodiments are applied to a DFB laser.
[0050]
The laser illustrated in FIG. 6 has the same configuration as the optical isolator of the first embodiment illustrated in FIG. 1 as an example. Therefore, the same components as those in FIG. 1 are denoted by the same reference numerals, and the detailed description is omitted. However, the DFB laser of FIG. The active layer 3 has a role of light emission and a waveguide function. Here, the active layer 3 may be a single semiconductor layer, or may be a multilayer high-efficiency structure such as an MQW structure (Multiple Quantum Well: multiple quantum well).
[0051]
The reflection structure 20 is composed of a p-type semiconductor layer. Further, the p-type InGaAsP layer 6 has a role of improving contact with the electrode. Above and below the semiconductor crystal layer, a p-side electrode 100 and an n-side electrode 200 are provided for conducting electricity.
[0052]
In the DFB laser, guided lights traveling in both the left and right directions oscillate by feedback (feedback) to each other via the diffraction grating 10 and resonate. In the case of the present embodiment, the radiation mode loss of the guided light traveling rightward in the drawing is small and the light output from the right end face is increased by the same mechanism as the optical isolator of the first embodiment. Conversely, since the guided light traveling to the left has a large radiation mode loss, the output intensity does not increase so much at the left end face.
[0053]
That is, according to the present embodiment, the slope efficiency on the output side can be improved. In addition, since the laser itself includes an isolator function, the resistance to return light is also enhanced. The present embodiment is particularly effective in this regard, since the DFB laser has a characteristic that the oscillation condition is likely to be unstable due to the return light.
Furthermore, according to the present embodiment, since the gain difference of the threshold value between the longitudinal modes becomes large, the effect of improving the performance of the single longitudinal mode can be obtained.
[0054]
Here, according to the present invention, in addition to the illustrated specific examples, a DFB laser corresponding to the configuration of the optical isolator according to the above-described second to fourth embodiments can be similarly provided. In other words, by inverting the positional relationship between the absorption structure and the reflection structure, changing the period of the diffraction grating, or introducing a phase shift to the diffraction grating, effects corresponding to the various effects described above are obtained. A DFB laser having the above is obtained.
[0055]
Further, these DFB lasers can be used as surface emitting lasers. That is, when light that is transmitted through the reflection structure 20 or the absorption structure provided on either side of the diffraction grating and emitted to the outside is used as an output, the light output is not high, but the threshold and other oscillation characteristics Can realize a very excellent surface emitting DFB laser. Such a surface emitting DFB laser can be easily integrated with the optical isolator of each of the above-described embodiments, and an extremely compact and high performance optical integrated device of an optical isolator / surface emitting laser can be realized.
[0056]
Next, a sixth embodiment of the present invention will be described.
FIG. 7 is a conceptual sectional view illustrating a main part configuration of the optical integrated device according to the embodiment of the present invention. The optical integrated device of the present embodiment is monolithic, that is, a plurality of devices are integrated on the same substrate. In FIG. 7, the DFB laser according to the fifth embodiment described above as an example, the optical isolator according to the first embodiment, and a waveguide type electro-absorption external modulator (EAM) are monolithically integrated. Corresponding to the specific example described above.
[0057]
The DFB laser is DC-driven through the p-side electrode 101. A certain area of the absorption layer 7 is EAM. The EAM can modulate the absorption coefficient by applying a negative voltage to the p-side electrode 102. The guided light generated in the active layer 3 of the DFB laser is modulated according to the change in the absorption coefficient, and is output to the right in the drawing.
[0058]
A first optical isolator OI1 is provided between the DFB laser and the EAM. The first optical isolator OI1 has a proton (H ⁺ ) The irradiation area 300 is provided, and the DFB and the EAM are electrically insulated (isolated). As for the guided light, if the DFB laser and the EAM are not isolated, the return light is input to the DFB laser, and the light output and the wavelength are not preferable. According to this embodiment, an optical isolator can be easily integrated simply by removing the active layer between the DFB laser and the EAM. Further, the second optical isolator OI2 can be integrated on the output side of the EAM by a similar simple process, and external return light can be suppressed.
[0059]
According to this embodiment, the DFB laser, the first optical isolator, the electro-absorption type outer jacket modulator, and the second optical isolator can be integrated extremely compactly. Further, since these elements share a waveguide, optical coupling between the elements can be sufficiently ensured. Furthermore, according to the present embodiment, since the absorption layer 30, the diffraction grating 10, and the reflection structure 20 of the optical integrated device can be formed in a common process, a high-performance optical integrated device can be manufactured by a simple process.
[0060]
The embodiment of the invention has been described with reference to the examples. However, the present invention can be variously applied without departing from the gist of the present invention, and is not limited to these specific examples.
[0061]
For example, the present invention can be applied to various material systems such as a GaAlAs / GaAs system, a GaInAlP / GaAs system, and a GaN system in addition to the specific examples described above to obtain the same effect.
[0062]
Further, the positional relationship between the diffraction grating, the reflection structure, the absorption / non-reflection structure, and the like is not limited to the above-described vertical direction of the substrate. For example, a diffraction grating may be formed on a side surface of the waveguide, that is, a surface perpendicular to the substrate, and a reflection structure and an absorption structure may be arranged on both sides thereof. That is, in this case, the reflection structure, the diffraction grating, and the absorption structure are arranged in the in-plane direction of the substrate.
[0063]
【The invention's effect】
The present invention is embodied in the form described above, and has the effects described below.
[0064]
First, according to the present invention, without modifying the basic structure of the waveguide, the shape of the diffraction grating is devised, and a high-reflection, non-reflection structure is formed outside the waveguide structure, thereby providing a semiconductor waveguide type. An optical isolator can be configured. That is, an extremely compact and highly efficient optical isolator can be realized by the second-order or higher-order asymmetrical saw-tooth diffraction grating and the reflection structure and the absorption structure respectively arranged on the upper and lower sides thereof. Therefore, it is possible to optically couple with an optical fiber and various optical elements with extremely high efficiency without significantly changing the NFP (near-field image) of the waveguide and the FFP (far-field image) of the emitted beam.
[0065]
Further, according to the present invention, by changing the period of the diffraction grating, it is possible to easily expand the dynamic range of the wavelength and realize an optical isolator that operates on various wavelengths.
[0066]
Furthermore, according to the present invention, by providing a phase shift to the diffraction grating, it is possible to control the distribution of the guided light and the emitted light, and to optimize the directionality and the light emission characteristics of the light.
[0067]
Further, according to the present invention, it is possible to realize a DFB laser that is resistant to return light, has high efficiency on the output side, and has a stable longitudinal mode.
[0068]
Further, according to the present invention, monolithic integration of a DFB laser or a waveguide type modulator with an optical isolator becomes extremely easy. That is, the diffraction grating, the reflection structure, and the non-reflection structure can be configured in common, and the fabrication is easy. This eliminates the need for a separate optical isolator, thereby greatly reducing the cost of the optical module. In addition, since the elements share a waveguide, optical coupling between the elements can be sufficiently ensured.
[Brief description of the drawings]
FIG. 1 is a sectional view showing an optical isolator according to a first embodiment of the present invention.
FIG. 2 is a graph (from a known document) showing an example of a power distribution ratio of a radiation mode from an asymmetrical diffraction grating to a substrate side and an opposite side thereof.
FIG. 3 is a sectional view showing an optical isolator according to a second embodiment of the present invention.
FIG. 4 is a sectional view showing an optical isolator according to a third embodiment of the present invention.
FIG. 5 is a sectional view showing an optical isolator according to a fourth embodiment of the present invention.
FIG. 6 is a sectional view showing a DFB laser according to a fifth embodiment of the present invention.
FIG. 7 is a sectional view showing a monolithic integrated device of an optical isolator, a DFB laser, and a waveguide modulator according to a sixth embodiment of the present invention.
FIG. 8 is a perspective view showing an example of a conventional waveguide type optical isolator.
[Explanation of symbols]
1 n-type InP substrate
2 n-type InP cladding layer
3 InGaAsP laser active layer (including NQW structure)
4 InGaAsP waveguide layer
5 p-type InP cladding layer
6 p-type InGaAsP layer (contact layer with p-electrode for DFB laser and modulator)
10 Second order diffraction grating (sawtooth cross section)
11 Phase shift
20 Reflection structure
21 End face
30 radiation mode absorption layer
40 AR coating (radiation mode)
41 Edge AR Coating
100 p-side electrode (DFB laser)
101 p-side electrode (integrated DFB laser)
102 p-side electrode (integrated modulator)
200 n-side electrode
300 protons (H ⁺ ) Irradiation insulation area

Claims (18)

A waveguide having a second or higher order diffraction grating having a sawtooth cross section;
A reflection structure provided on one of the upper side and the lower side of the waveguide and returning radiation mode light emitted from the waveguide to the waveguide,
A non-reflection structure provided on the other side above or below the waveguide to prevent radiation mode light emitted from the waveguide from returning to the waveguide,
An optical isolator comprising: The reflection structure is provided apart from the waveguide so as not to affect the waveguide mode light that exudes and propagates from the waveguide,
2. The optical isolator according to claim 1, wherein the non-reflection structure is provided apart from the waveguide so as not to affect waveguide mode light that exudes from the waveguide and propagates. 3. 3. The optical isolator according to claim 1, wherein the non-reflection structure is made of a semiconductor material having a band gap smaller than a semiconductor material forming the waveguide. 4. The non-reflective structure, the optical isolator according to claim 1 or 2, wherein the non-reflective coating for the radiation mode light. The optical isolator according to any one of claims 1 to 4, wherein the reflection structure is a Bragg multilayer reflection film for the radiation mode light. The diffraction grating, the optical isolator according to any one of claims 1 to 5, wherein a plurality of different periods. The optical isolator according to any one of claims 1 to 6, wherein an effective refractive index of the waveguide is not constant. The diffraction grating, the optical isolator according to any one of claims 1 to 7, characterized in that it comprises at least one or more phase shift. An active layer;
A waveguide having a second or higher order diffraction grating having a sawtooth cross section;
A reflection structure provided on one of the upper side and the lower side of the waveguide and returning radiation mode light emitted from the waveguide to the waveguide,
A non-reflection structure provided on the other side above or below the waveguide to prevent radiation mode light emitted from the waveguide from returning to the waveguide,
A distributed feedback laser characterized by comprising: The reflection structure is provided apart from the waveguide so as not to affect the waveguide mode light that exudes and propagates from the waveguide,
10. The distributed feedback laser according to claim 9, wherein the non-reflection structure is provided away from the waveguide so as not to affect the waveguide mode light that permeates and propagates from the waveguide. . The distributed feedback laser according to claim 9 , wherein the non-reflection structure is made of a semiconductor material having a smaller band gap than a semiconductor material forming the waveguide. The distributed feedback laser according to claim 9 , wherein the non-reflection structure is a non-reflection coat for the radiation mode light. The distributed feedback laser according to any one of claims 9 to 12, wherein the reflection structure is a Bragg multilayer reflection film for the radiation mode light. The distributed feedback laser according to any one of claims 9 to 13, wherein the diffraction grating has a plurality of different periods. The distributed feedback laser according to any one of claims 9 to 14, wherein an effective refractive index of the waveguide is not constant. 16. The distributed feedback laser according to claim 9, wherein the diffraction grating has at least one phase shift. An optical isolator according to any one of claims 1 to 8 ,
An optical integrated device, wherein the distributed feedback laser according to any one of claims 9 to 16 is monolithically integrated. 18. The optical integrated device according to claim 17, wherein an external modulator is further integrated.

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