JPH11307874A - Optical isolator, distributed feedback laser and optical integrated element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain high directivity, by comprising a waveguide having a second- or higher-order diffraction grating asymmetric in section and reflectivity different at both sides of the waveguide to a radiation mode light emitted from the waveguide. SOLUTION: The optical isolator has second- or higher-order diffraction grating 10, esp., a waveguide structure having a second-order diffraction grating 10 emits a radiation mode light to a substrate perpendicularly to the guiding direction and reverse direction, the second-order diffraction grating 10 has an asymmetric sectional shape in the traveling direction, i.e., a blaze angle land hence the radiation mode at the substrate 1 side becomes strong and weak in either the right or left traveling direction of the light wave, depending on the blaze angle. When the guided light runs from the right to the left, the radiation mode light is mostly radiated to the substrate 1 and absorbed by an absorption layer 30, not returning to the waveguide. When running from the left to the right, the radiation mode light is mostly radiated upwards reverse to the substrate 1 and returnes by a reflection structure 20, and thus a high directivity can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an optical isolator,
The present invention relates to a distributed feedback laser and an optical integrated device. More specifically, the present invention relates to a waveguide-type optical isolator, a distributed feedback laser, and a monolithically integrated optical integrated device, which are compact, have high directivity, and have good light coupling.

[0002]

2. Description of the Related Art In a distributed feedback laser (DFB laser), a diffraction grating is provided along a waveguide, and a Bragg from the diffraction grating is provided.
g) Laser oscillation is generated by utilizing diffracted light for optical feedback. Since the oscillation longitudinal mode is selected by the period of the diffraction grating, there is an advantage that 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 that disclose optical isolators include, for example,
Ryoichi Ito and Michiharu Nakamura, "Semiconductor Laser", Baifukan, I
SBN4-563-03437-1, C3055, P6
386E, 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, D
It was manufactured as a separate part from the FB laser, and it was necessary to adjust and assemble these optical axes so as to be assembled. Also,
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.

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.

FIG. 8 is a conceptual perspective view illustrating a schematic configuration of the optical isolator. In the optical isolator shown in the figure, the diffraction grating 111 is arranged obliquely with respect to the waveguide direction on the semiconductor substrate S, and the refractive index n ₂ on one side of the waveguide structure is lowered to be asymmetric, and the return light Are dissipated to the n ₂ 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. Also,
Since n ₂ layers having different refractive indices must be separately grown, the manufacturing process becomes complicated. Further, when a diagonally formed diffraction grating is used, the coupling as a DFB laser is deteriorated.

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 devices have high directivity, are compact, have good coupling with an optical fiber or a DFB laser, are easily manufactured, and are integrated with each other. It is in.

[0008]

That is, an optical isolator according to the present invention includes a waveguide having a second-order or higher diffraction grating having an asymmetric cross-sectional shape, and has a reflectance with respect to radiation mode light emitted from the waveguide. It is characterized in that it is configured differently on both sides of the waveguide.

The optical isolator and the distributed feedback laser of the present invention are provided on one side of a waveguide having a second or higher order diffraction grating having an asymmetric cross-sectional shape.
A reflection structure for returning radiation mode light emitted from the waveguide to the waveguide, and a reflection structure provided on the other side of the waveguide to return radiation mode light emitted from the waveguide to the waveguide. And a non-reflection structure that is not provided. Here, the distributed feedback laser further includes an active layer.

[0010] Here, the reflection structure is provided apart from the waveguide so as not to affect the waveguide mode light that seeps out of the waveguide and propagates.
It is characterized in that the waveguide mode light is provided apart from the waveguide so as not to affect the guided mode light that leaks out from the waveguide and propagates.

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.

Alternatively, the non-reflection structure is a non-reflection coat for the radiation mode light.

Further, the reflection structure is a Bragg multilayer reflection film for the radiation mode light.

Further, the diffraction grating has a plurality of different periods.

Further, the waveguide is characterized in that the effective refractive index is not constant.

Further, the diffraction grating has at least one phase shift.

On the other hand, an optical integrated device according to the present invention is characterized in that any one of the above-mentioned optical isolators and any one of the above-mentioned distributed feedback lasers are monolithically integrated.

Further, the present invention is characterized in that an external modulator is further integrated.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides an optical isolator having high directivity by using a diffraction grating of second order or higher having an asymmetric cross-sectional shape and a reflection structure disposed on one side thereof. That is, in a waveguide structure having a second-order or higher diffraction grating, guided light is generally dissipated as a radiation mode. However, if it is reflected back to the waveguide, the loss due to dissipation is reduced. Blaze gratings, ie
When 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 reflective 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.

On the other hand, when the guided light travels in the opposite direction,
The radiation mode on the dissipation side without the reflection structure increases, and the radiation mode on the reflection structure side decreases. Therefore, the loss for the traveling wave in the opposite direction increases. In this way, an optical isolator having a different loss depending on the traveling direction of the guided light can be realized. This optical isolator has a DF
Since it can be formed as an extension of the diffraction grating of B laser, DF
It also has an advantage that it can be easily monolithically integrated with a B laser or an external modulator.

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 an optical isolator according to a 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 cladding layer 2, an InGaAsP layer 4, a p-type InP cladding 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.

The absorption layer 30 may be made of any material having a high absorption coefficient for radiation mode light. For example, the absorption layer 30 can be formed by using a semiconductor material having a smaller band gap than the waveguide layer 4.

On the surface of the waveguide layer 4, a second-order diffraction grating 1 having a sawtooth asymmetric cross-sectional shape in the illustrated direction is provided.
0 is formed. Here, the “order” of the diffraction grating is
It 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 comparable 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 has a feature that the fabrication is much easier. On the other hand, the reflection structure 20
For example, it can be formed by a Bragg multilayer reflective film in which two types of derivative films having different refractive indexes are alternately laminated. By appropriately selecting the refractive index and the thickness of each layer of such a Bragg multilayer reflective film, a reflective structure having a high reflectance for radiation mode light can be obtained.

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 formed on the n-type InP substrate 1 by 2.5 μm.
m. 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 waveguide 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.

Next, an InGaAsP layer 4 is grown to about 0.3 μm. The crystal growth so far is performed continuously. Waveguide layer 4
The surface has a blaze angle (blaze angle)
angle), a sawtooth asymmetric second-order diffraction grating 10
Are formed to constitute a waveguide. As a method for 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.

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
Although the p-type is used in consideration of integration with the DFB laser, the n-type may be used when only the optical isolator is used alone.

Finally, a reflective structure 20 composed of, for example, a derivative multilayer film is deposited and formed. The reflectance of the reflection 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 waveguide mode, and does not affect the waveguide mode light. Thus, the optical isolator of FIG. 1 is completed.

Next, the operation mechanism of the optical isolator of the present invention will be described. The optical isolator of FIG.
It has a diffraction grating of higher order. In particular, a waveguide structure having a second-order diffraction grating has a substrate side perpendicular to the waveguide direction.
It emits radiation mode light in both directions, de) and the superstrate side. further,
In the optical isolator shown in FIG. 1, the cross-sectional shape of the secondary diffraction grating is asymmetric in the traveling direction, that is, the blaze angle.
Having. 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 either the left or right. When the traveling direction of the light wave is reversed, the distribution ratio between the substrate side and the opposite side is also reversed.

More specifically, in FIG. 1, when the guided light travels from right to left, most of the radiation mode light is emitted toward the substrate 1 as indicated by the 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.

On the other hand, when the guided light travels from left to right, most of the radiation mode light is emitted upward, opposite to the substrate, as indicated by arrow A. This light is reflected by the reflection structure 2
The light is reflected by 0 and returns to the waveguide structure as shown by 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 the waveguide characteristics in an asymmetrical diffraction grating.
In other words, FIG.
ting-Coupled Radiation in GaAs: GaAlAs lasers and Wa
veguides-II: Blazing Effects "with IEEE Journ
al of Quantum Electronics, Vol.QE-12, pp.4494-4499,
It is a graph figure published in 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 figure is the asymmetry (blaz
e) and δ = △ / Λ =
0.5 corresponds to a diffraction grating having a symmetrical cross-sectional shape. As shown in the inset, the guided light travels from left to right and δ = △ /
When Λ = 1.0, it can be quantitatively understood that the radiation mode () to the substrate side becomes strong and the radiation mode () to the opposite side becomes extremely small.

In the present invention, a reflection structure is provided only on one side of such an asymmetrical diffraction grating. If there is a reflection structure, 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.

Since the guided light traveling in the opposite direction is equivalent to the case where △ / Λ = 1, the radiation mode () 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.

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 a small loss. An extremely compact waveguide type optical isolator having high direction selectivity can be realized.

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 plane of the drawing have different compositions (different refractive indexes) on both side surfaces.
Had to be done. 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. In addition, a waveguide mode NFP (near field image: near fie
ld pattern) is also symmetrical, and has the advantage that optical coupling to a fiber or the like is extremely easy.

Next, a second embodiment of the present invention will be described. FIG. 3 is a conceptual sectional view illustrating a main configuration 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, respectively.

The difference from the optical isolator of the above-described first embodiment is as follows: 1) a reflection structure 20 is provided below the diffraction grating 10 instead of the absorption layer 30; On the upper side, an anti-reflection coat (AR) 40 is provided instead of the reflection structure 20. Here, the reflection structure 20 is, for example, a high reflection DBR (Distribute) having a multilayer structure of semiconductor crystal layers.
d Bragg Reflector: a distributed reflector. Further, as the anti-reflection coat 40, for example, １ ／
Wavelength film, that is, the wavelength of the radiation mode light is λ and the refractive index is n
Then, a dielectric thin film having a thickness of λ / 4n can be used. 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.

In the present embodiment, the reflection and absorption functions are only reversed up and down of the diffraction grating 10, and the principle of operation is the same as in the first embodiment. In other words, the guided light traveling from right to 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.

On the other hand, the guided light traveling from left to right in the figure is radiated 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, light emitted from the diffraction grating 10 is emitted to the outside without being reflected,
The component returning to the diffraction grating 10 is extremely small.

After all, in the optical isolator of FIG.
The waveguide loss for the guided light component traveling from left to right in the drawing is high, and the guided loss for the guided light component for the guided light component traveling from right to left is low. In this way, directivity 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.

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, the diffraction grating 10 'is provided, and the absorption structure 40 is provided above the diffraction grating 10', and the 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, the same reference numerals are given to the respective components, and the detailed description is omitted.

The feature of this embodiment is that the diffraction grating 1
0 'is that it has two types of periods of the periodic lambda ₁ and the period lambda _2. In this manner, two types of guided light beams 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.

Further, if 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 in the case where 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 axial direction by the phase shift effect.

Although not shown, by introducing three or more different periods into the diffraction grating, it is possible to cope with 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.

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.

Next, a fourth embodiment of the present invention will be described. FIG. 5 is a conceptual cross-sectional view illustrating a main configuration 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.

The present embodiment is different from the diffraction grating 10 ″ in that a portion where the grating period is discontinuously shifted, that is, a phase shift 11 is provided. The amount of shift of the phase shift 11 is, for example, It can be set to λ / 4 with respect to the guided wavelength λ of the guided wave .. By providing the phase shift 11, it is possible to control the axial profile of the resonator of the guided light and the radiation mode light. Become.

FIG. 5 shows an isolator waveguide that has good transmission of right-handed guided light and large attenuation in the opposite direction. After passing through the phase shift, the light wave and the diffraction grating 10
, The attenuation factor increases. Phase shift 1
1 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 from 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.

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.

Next, a fifth embodiment of the present invention will be described. FIG. 6 shows a DFB according to the embodiment of the present invention.
FIG. 3 is a conceptual sectional view illustrating a configuration of a main part of a laser. 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.

The laser illustrated in FIG. 6 is an example of the laser shown in FIG.
Has the same configuration as the optical isolator of the first embodiment shown in FIG. Therefore, the same elements as those in FIG. 1 are denoted by the same reference numerals, and 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 an MQW structure (Multiple Q).
A multi-layer high-efficiency structure such as uantum well (multiple quantum well) may be used.

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 an electrode. Above and below the semiconductor crystal layer, a p-side electrode 100 and an n-side electrode 200 are provided for conducting electricity.

In the DFB laser, guided lights traveling in both the left and right directions feedback each other via the diffraction grating 10 and oscillate by resonating. In the case of the present embodiment, the radiation mode loss of the guided light traveling rightward in the figure 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 is not so large at the left end face.

That is, according to the present embodiment, the slope efficiency on the output surface 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 that the single longitudinal mode performance is improved can be obtained.

Here, according to the present invention, in addition to the specific examples shown, DFB lasers corresponding to the configurations of the optical isolators according to the above-described second to fourth embodiments can be similarly provided. That is, the positional relationship between the absorption structure and the reflection structure is reversed, or the period of the diffraction grating is changed, or
By introducing a phase shift into the diffraction grating, DFs having effects corresponding to the various effects described above are provided.
A B laser is obtained.

Further, these DFB lasers can be used as surface emitting lasers. That is, when the light emitted 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 the output, the light output is not high, but the threshold value and other oscillation characteristics Can realize an extremely excellent surface emitting DFB laser. Such a surface-emitting type DFB laser can be easily integrated with the optical isolator of each of the above-described embodiments, and is an extremely compact and high-performance optical isolator.
An optical integrated device of a surface emitting laser can be realized.

Next, a sixth embodiment of the present invention will be described. FIG. 7 is a conceptual cross-sectional view illustrating a main configuration of the optical integrated device according to the embodiment of the present invention. The optical integrated device of this 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 modulator (EAM) are monolithically integrated. Corresponding to the specific example described above.

The DFB laser is driven by DC 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.

A first optical isolator OI1 is provided between the DFB laser and the EAM. The first optical isolator OI1 has a proton (H ⁺ ) irradiation region 300 and electrically insulates (isolates) the DFB and the EAM.
doing. Regarding the guided light, if the DFB laser and the EAM are not isolated, it is not preferable because the return light is input to the DFB laser and the light output and the wavelength fluctuate. According to this embodiment, the 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 the same simple process, and return light from the outside can be suppressed.

According to the present embodiment, the DFB laser and the first
The optical isolator, the electro-absorption type outer jacket modulator, and the second optical isolator can be integrated extremely compactly. In addition, since these elements share a waveguide, optical coupling between the elements can be sufficiently ensured. Furthermore, according to the present embodiment, the absorption layer 30 of the optical integrated device,
Since the diffraction grating 10 and the reflection structure 20 can be formed in a common process, a high-performance optical integrated device can be manufactured by a simple process.

The embodiment of the invention has been described with reference to examples. However, the present invention can be applied to various applications without departing from the gist of the present invention, and is not limited to these specific examples.

For example, the present invention can be applied to a GaAlAs / GaAs system, a GaInAlP / Ga
A similar effect can be obtained by applying the same to various material systems such as an As system and a GaN system.

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. 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 present invention is embodied in the form described above, and has the following effects.

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 and non-reflection structure is formed outside the waveguide structure. A waveguide type optical isolator can be configured. That is,
An optical isolator that is extremely compact and has high efficiency can be realized by the second-order or higher-order asymmetrical diffraction grating and the reflection structure and the absorption structure arranged on both 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 or the FFP (far-field image) of the emitted beam.

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.

Further, according to the present invention, by providing a phase shift to the diffraction grating, the distribution of the guided light and the emitted light can be controlled to optimize the light directionality and light emission characteristics.

Further, according to the present invention, it is possible to realize a DFB laser which is resistant to return light, has a high efficiency on the output side, and has a stable longitudinal mode.

Further, according to the present invention, monolithic integration of a DFB laser or a waveguide type modulator and 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 the 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 shows an optical isolator D according to a sixth embodiment of the present invention.
It is sectional drawing which shows the monolithic integrated element of FB laser and a waveguide type modulator.

FIG. 8 is a perspective view showing an example of a conventional waveguide type optical isolator.

[Explanation of symbols]

Reference Signs List 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 (DFB laser, with p electrode in case of modulator) Contact layer) 10 Second-order diffraction grating (sawtooth-shaped cross section) 11 Phase shift 20 Reflection structure 21 End face 30 Radiation mode absorption layer 40 AR coat (for radiation mode) 41 End face AR coat 100 p-side electrode (DFB laser) 101 p-side Electrode (integrated DFB laser) 102 p-side electrode (integrated modulator) 200 n-side electrode 300 Proton (H ⁺ ) irradiation insulating region

Claims (19)

[Claims] 1. A waveguide having a second-order or higher diffraction grating having an asymmetric cross-sectional shape, wherein reflectivity for radiation mode light emitted from the waveguide is different on both sides of the waveguide. An optical isolator characterized by the above-mentioned. 2. A waveguide having a second-order or higher-order diffraction grating having an asymmetric cross-sectional shape, and provided on one side of the waveguide to return radiation mode light emitted from the waveguide to the waveguide. And a non-reflection structure provided on the other side of the waveguide to prevent radiation mode light emitted from the waveguide from returning to the waveguide. Optical isolator. 3. 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, and the non-reflection structure stains from the waveguide. 3. The optical isolator according to claim 2, wherein the optical isolator is provided away from the waveguide so as not to affect the guided mode light that is emitted and propagated. 4. The optical isolator according to claim 2, wherein said non-reflection structure is made of a semiconductor material having a smaller band gap than a semiconductor material forming said waveguide. 5. The optical isolator according to claim 2, wherein the non-reflection structure is a non-reflection coat for the radiation mode light. 6. The reflection structure according to claim 2, wherein said reflection structure is a Bragg multilayer reflection film for said radiation mode light.
6. The optical isolator according to any one of items 1 to 5, 7. The optical isolator according to claim 1, wherein said diffraction grating has a plurality of different periods. 8. The optical isolator according to claim 1, wherein an effective refractive index of said waveguide is not constant. 9. The optical isolator according to claim 1, wherein said diffraction grating has at least one phase shift. 10. An active layer, a waveguide having a second-order or higher-order diffraction grating having an asymmetric cross-sectional shape, and provided on one side of the waveguide to guide radiation mode light emitted from the waveguide. A reflection structure configured to return to the waveguide, and a non-reflection structure provided on the other side of the waveguide to prevent radiation mode light emitted from the waveguide from returning to the waveguide. A distributed feedback laser characterized by the following. 11. The reflection structure is provided apart from the waveguide so as not to affect the waveguide mode light that seeps out of the waveguide and propagates. 11. The distributed feedback laser according to claim 10, wherein the laser is provided apart from the waveguide so as not to affect the guided mode light emitted and propagated. 12. The distributed feedback laser according to claim 10, wherein said non-reflective structure is made of a semiconductor material having a band gap smaller than a semiconductor material forming said waveguide. 13. The apparatus according to claim 10, wherein the non-reflection structure is a non-reflection coat for the radiation mode light.
Or the distributed feedback laser according to 11. 14. The distributed feedback laser according to claim 10, wherein the reflection structure is a Bragg multilayer reflection film for the radiation mode light. 15. The distributed feedback laser according to claim 10, wherein said diffraction grating has a plurality of different periods. 16. The waveguide according to claim 10, wherein the effective refractive index of said waveguide is not constant.
15. The distributed feedback laser according to any one of 15. 17. The diffraction grating according to claim 10, wherein said diffraction grating has at least one phase shift.
The distributed feedback laser according to any one of the above. 18. An optical isolator according to any one of claims 1 to 9 and a distributed feedback laser according to any one of claims 10 to 17, which are monolithically integrated. Characteristic optical integrated device. 19. The optical integrated device according to claim 18, wherein an external modulator is further integrated.