JPH08184789A - Polarization nondependent wavelength filter - Google Patents

Abstract

PURPOSE: To provide the polarization nondependent wavelength filiter which is high in efficiency, low in driving power, high in response speed and high in wavelength resolution, and can stably select specific wavelength from input light having an arbitrary polarization state. CONSTITUTION: The wavelength filter consists of a polarized wave separation area and a wavelength selection area consisting of two nearby active waveguides 11 and 12. In the polarized light separation area, an identical-directional grating 13 is formed and one of a TE mode and a TM mode as orthogonal polarization modes is coupled in the same direction between the two active waveguides 11 and 12. In the wavelength selection area, a DFB or DBR of grating 14 is formed and the TE Mode of one active waveguide 12 and the TM mode of the other active waveguide 11 are selected and transmitted with equal wavelength, so that wavelength selection can be performed irrelevantly to the polarization state of transmitted light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength filter for selecting an arbitrary wavelength used in optical wavelength division multiplexing communication, etc., so as to obtain a stable reception output even if the polarization state of the input changes. The present invention relates to a polarization-independent wavelength filter that can be used.

[0002]

2. Description of the Related Art Heretofore, some devices have been proposed for demultiplexing a wavelength-multiplexed optical signal. In particular,
A waveguide type wavelength filter based on a grating or a directional coupler is suitable for downsizing as compared with a bulk type wavelength filter. And other functional devices, for example,
It is easy to integrate with photodetectors and has a high degree of freedom in design. Furthermore, it is easy to control the central selection wavelength,
It can also be applied to applications that make use of the wavelength tunable feature. On the other hand,
The waveguide type wavelength filter has a problem that the transmission wavelength varies depending on the polarization state of the input light. Then, for example, Japanese Patent Laid-Open No. Sho 5
As in the wavelength filter presented in the specification of JP-A-7-168220, a polarization separation region for separating input light into each polarization component, a wavelength selection region for selecting a wavelength for each polarization component, and each wavelength selected region. The problem of polarization was solved by the integrated structure of the polarization combining area that combines the polarization components.

[0003]

However, in the wavelength filter as shown in the conventional example, since the TE-TM mode conversion by the optical crystal such as LiNbO ₃ is used as the wavelength selecting means, the wavelength resolution is high. It is relatively wide, about 1 nm, and it has been difficult to adapt to a high-density wavelength division multiplexing system that uses, for example, a wavelength-tunable DFB laser. Further, there are problems that the response speed is slow in the case where the TE-TM mode conversion is performed by the acousto-optic effect, and that a high drive voltage is required in the case where the electro-optic effect is used. Furthermore, since optical crystals have no gain for light,
The insertion loss of the wavelength filter itself was unavoidable.

Therefore, a first object of the present invention is to provide a polarization-independent wavelength filter capable of stably selecting a specific wavelength from input light having an arbitrary polarization state, with high efficiency, low power drive and high speed response. It is to provide with high speed and high wavelength resolution. An object of a second invention according to the present application is, in addition to the above first object, to greatly simplify the manufacturing process of the wavelength filter according to the present invention. In addition to the first object, the third object of the present application is to improve the coupling efficiency between the output light of the wavelength filter according to the present invention and the succeeding device, for example, a photodetector. The fourth object of the present application is, in addition to the first object, to further increase the polarization independence of the wavelength filter according to the present invention.

[0005]

In order to achieve the above first object, the present invention provides a wavelength filter comprising a polarization separation region and a wavelength selection region consisting of two active waveguides in close proximity to each other, A co-directional coupling grating is formed in the polarization separation region, and TE which is a polarization mode orthogonal to each other is formed.
One of a mode and a TM mode causes coupling in the same direction between the two active waveguides, and a DFB or DBR grating is formed in the wavelength selection region,
The TE mode in one active waveguide and the TM mode in the other active waveguide are selectively transmitted at the same wavelength, so that the wavelength can be selected regardless of the polarization state of the transmitted light.

In order to achieve the above second object, the present invention provides
In the above polarization independent wavelength filter, a TE mode in one active waveguide and a T mode in the other active waveguide
The thicknesses and / or widths of the two active waveguides are adjusted so that the M modes have equal propagation constants,
The DFB or DBR grating is common to the two active waveguides.

In order to achieve the third object, the present invention provides
In the above polarization independent wavelength filter, a polarization combining area is formed on the opposite side of the polarization separating area across the wavelength selecting area, and the polarization combining area has the same structure as the polarization separating area. It is characterized by consisting of.

In order to achieve the above-mentioned fourth object, the present invention provides
In the above polarization independent wavelength filter, the polarization mode transferred between the two active waveguides in the polarization combining region is different from the polarization mode transferred in the polarization separation region.

[0009]

Embodiment 1 FIG. 1 is a plan view showing Embodiment 1, and a polarization-independent wavelength filter according to this embodiment has two active waveguides 11 and 12 which are close to each other (as will be described later). A waveguide including an active layer). The internal configuration is such that a polarization separation region and a wavelength selection region are arranged in this order from the input side of transmitted light. Transmitted light consists of two active waveguides 1
It is input from either 1 or 12. In this embodiment, the input is from the active waveguide 11. Two active waveguides 1
Since 1 and 12 have different thicknesses and / or widths (detailed later), they have strong asymmetry and coupling between transverse modes (for example, between the 0th order and the 1st order in any polarization mode). (Bonding) does not occur as it is. However, the grating 13 having a relatively rough cycle is formed in the polarization separation region, and only one of the TE mode and the TM mode, which is the polarization mode in the waveguide, is transmitted by the transmission light wavelength. In, the waveguides 11 and 12 are moved in the same direction. The polarization mode light that does not shift may be coupled at a wavelength far from the transmission light wavelength, but the distance from the transmission light wavelength to this wavelength is several tens of nanometers, so in the transmission light wavelength band , Its bond transfer can be ignored. As a result, the polarization separation function can be fulfilled. Of course, in this waveguide structure, the polarization components are conserved, so that the coupling between orthogonal polarization modes (TE mode and TM mode) does not occur.

In the wavelength selective region, the thicknesses and / or widths of the two active waveguides 11, 12 are adjusted to each other (which may be the same as or different from those in the polarization splitting region,
If they are the same, manufacturing will be simple. In the same case, the pitch of the grating in the polarization separation area may be set appropriately so that the desired function can be achieved. ), TM of the input side active waveguide 11 and one of TE modes and TE of the transition side active waveguide 12
The other of the TM mode and the TM mode has the same propagation constant (the same propagation constant does not cause coupling between the TE mode and the TM mode). Two active waveguides 11,
12 is a distributed feedback grating 14 with an equal period
And the light of each polarization mode propagating through the active waveguides 11 and 12 adjacent to each other is generated by the grating 1
A distributed feedback resonance is caused at a desired wavelength defined by 4 and strong amplification is performed, and the light is transmitted and output from the wavelength selection region. By detecting the output light, it is possible to stably extract and receive only the desired wavelength regardless of the polarization deviation or fluctuation of the input transmission light. At this time, the transmission wavelength band is generally 0.1 depending on the configuration of the distributed feedback grating.
nm or less.

[0011]

Second Embodiment FIG. 2 shows an example in which a polarization combining area is provided on the output side of the wavelength selection area. The configuration of the polarization combining region is the same as that of the polarization separating region, but here, only the polarization mode that is different from the polarization mode that has been coupled or shifted in the polarization separation region is coupled in the same direction at the transmission wavelength. So that the grating 15
Is set. With this configuration, the polarization multiplexing function can be achieved. In the polarization splitting region, as described in the first embodiment, the transmission wavelength band is the TE mode coupling wavelength band 3
1 (see the left part of FIG. 3), only the TE mode moves between the waveguides 11 and 12. In the wavelength selection region, as shown in the central portion of FIG.
2 selectively amplifies the same wavelength λc in the TE mode and the TM mode. In the polarization combining region, the transmission wavelength band coincides with the coupling wavelength region 32 of the TM mode as shown in the right part of FIG. 3, so that only the TM mode transits between the waveguides 11 and 12.

Here, the polarization modes different from each other in the polarization separation region and the polarization combining region are coupled and transferred.
The loss due to the transition between the TE mode and TM mode waveguides is
This is to make them equal throughout the element. However, if the gain in the wavelength selection region is larger than that in the TE mode, for example, there is also a method of shifting the TE mode twice to balance with the TM mode. According to the second embodiment, the output from the device is formed from one active waveguide (here, the waveguide 12), and the optical coupling to the detector is easy,
Moreover, the coupling efficiency can be improved.

An example of manufacturing the wavelength filter according to the second embodiment will be described with reference to FIGS. This production example is Example 1.
Can also be applied to. On the n-InP substrate 41,
N-InP layer 42 which is a buffer layer, InGaAs / I
Active layer 43 (thickness 0.1 μm) made of nGaAsP superlattice
m), p-InGaAsP optical guide layer 44 (thickness: 0 .μ)
m) was grown, the optical guide layer 44 in the region to become the waveguide 12 on the transition side was selectively etched to have a thickness of 0.85 μm. Next, a grating 13 having a period of 97 μm is formed in the polarization separation region, a grating 15 having a period of 89 μm is formed in the polarization combining region by mask exposure / etching, and a period 0.24 μ is formed in the wavelength selection region by interference exposure / etching.
A m distributed feedback grating 14 was formed. That is, the active waveguide 1 is configured so that the desired operation can be performed with such a configuration.
The thicknesses and / or widths of 1 and 12 are set (the width is set in a process described later). Subsequently, a p-InP clad layer 45 and a p-InGaAs contact layer 401 were grown on the gratings 13, 14, and 15. Two ridge waveguides 46 and 47 having a width of 3 μm were produced by mask exposure / etching. Then, SiN _x
A passivation film 48 made of is formed, and the back of the ridge is exposed by the self-alignment method. Of the two active waveguides thus formed, the thicker one is the input-side waveguide 11
For the TM mode, the thinner one is T in the transition side waveguide 12.
By using it for the E mode, the Bragg wavelengths of the two modes are set to be equal in the waveguides 11 and 12 by the distributed feedback grating 14 having the same period.

In this way, since the propagation constants are made uniform by adjusting the size of the active waveguides, for example, the adjacent waveguides 11,
It is not necessary to make the period of the distributed feedback grating 14 different among the 12 units. Therefore, the manufacturing process can be significantly simplified. An Au / Cr electrode 49 divided into three is formed on the p-InP layer 45 in the wavelength selection region, and n
An Au / AuGeNi electrode 50 is formed on the -InP substrate 41 side.

Of the light input from the input side waveguide 11, T
The E component is coupled in the polarization separation region and moves to the adjacent waveguide 12. Waveguide 1 with TM component input as is
1 and a wavelength λc (= 1.55
Resonance occurs at (μm) and is amplified by the carriers injected through the electrode. The TE component propagates through the shifted waveguide 12, resonates at the wavelength λc in the wavelength selection region, and is amplified by the injected carriers. The amplification wavelength λc and the amplification degree are controlled by the amount of current injection into the three electrodes in the wavelength selection region. The TM components of the light wavelength-selectively amplified in the respective active waveguides 11 and 12 are coupled in the polarization combining region and move to the adjacent waveguide 12. As a result, the orthogonal polarization components are combined in the transition side waveguide 12. In the polarization splitting / combining region, it is attenuated because it propagates through the active waveguide. In order to prevent this, an electrode may be provided in the polarization splitting / combining region to inject a current. Further, the wavelength variable range by the injection current to the wavelength selection region is about 3-4 nm. The voltage required for driving is as low as about 1 V, and the response speed is as fast as about nsec.

[0016]

Third Embodiment In the third embodiment, as shown in FIG. 5, the waveguide core is etched in the entire region up to the substrate 41, and p-InP51, undoped InP52, n- are formed from the substrate side.
InP 53 is grown and a SiO ₂ passivation film 54 is formed.
By forming a film, a part other than the waveguide core was embedded. Thus, the waveguides 11 and 12 having the BH structure as shown in FIG.
Was configured. As compared with the ridge structure as shown in FIG. 4, the number of crystal growth increases, but the carrier injection efficiency improves,
Since there is no interface between SiN _x and the active layer / light guide layer,
It is characterized by low propagation loss and improved efficiency. The operation and the like are the same as in the above embodiment.

[0017]

[Embodiment 4] An example of manufacturing another polarization-independent wavelength filter according to the present invention will be described with reference to FIGS. This manufacturing example is also applicable to the first embodiment. On the n-GaAs substrate 61, n-GaAs layers 62, n serving as buffer layers
-AlGaAs cladding layer 63, GaAs / AlGaA
After growing the active layer 64 made of s superlattice and the p-AlGaAs optical guide layer 65, the optical guide layer 65 of one of the waveguides is thinned as in the second embodiment, and the period of 50 μm is set in the polarization separation region.
m grating 13 with a period of 45μ in the polarization combining area
m grating 15 was formed by mask exposure / etching, and distributed feedback grating 14 with a period of 0.24 μm was formed in the growth selection region by interference exposure / etching. Then, p-AlGaAs is formed on the optical guide layer 65.
The clad layer 66 and the p-GaAs contact layer 67 were grown. Ridge waveguide cores 11 and 12 having different thicknesses and widths were produced by mask exposure / etching.
Subsequently, SiN _x 70 was formed into a film, and the portion other than the waveguide core was embedded. The two active waveguides thus formed are used as the thicker and wider one 11 for the TM mode and the thinner and narrower one 12 for the TE mode. Is set so that the Bragg wavelengths of the two modes are equal in the respective waveguides 11 and 12.

An Au / AuGe electrode 71 divided into three parts is formed on the p-GaAs contact layer 67 in the wavelength selection region, and an Au / Cr electrode 72 is formed on the n-GaAs substrate side.
Is formed. The TE component of the light input from the input-side waveguide 11 is coupled in the polarization separation region, and moves to the adjacent waveguide 12. The TM component propagates through the input waveguide 11 as it is, and the wavelength λc (= 0.
Resonance occurs at 83 μm) and is amplified by the injected carriers. The TE component propagates through the transferred waveguide 12,
Resonance occurs at the same wavelength λc in the wavelength selection region, and is amplified by the injected carriers. The amplification wavelength λc and the amplification degree are controlled by the current injection amount into the three electrodes 71.
The polarization components combined in the polarization combining region are output side waveguide 12
Output from As a result, it is possible to configure a wavelength filter that can select an arbitrary wavelength regardless of the state of the input polarization.
In the polarization splitting and combining region, the active waveguide 11,
As it propagates 12, it is attenuated. In order to prevent this, current may be injected also in the polarization splitting / combining region as in the third embodiment.

In the above embodiment, the distributed feedback (DFB) grating is formed in the wavelength selection region, but the grating may be formed of either DFB or DBR.

[0020]

As described above, according to the present invention, in the polarization independent wavelength filter for selecting a certain specific wavelength channel in wavelength division multiplex transmission or the like, polarization separation / demultiplexing is performed.
As a synthesizing means, the strong polarization dependence of a directional coupler composed of two active waveguides is utilized, and the propagation constants of the two active waveguides are controlled so that the TE and TM modes are the same. By making the Bragg resonance at the wavelength of, the wavelength can be stably selected regardless of the polarization state of the transmitted light. According to the second invention of the present application, since the grating can have the same period over the two active waveguides,
This has the effect of significantly simplifying the manufacturing process of the polarization independent wavelength filter. Further, according to the third invention of the present application, since the output waveguide can be unified by applying the polarization combining region, the polarization independent wavelength filter and the subsequent device, for example, the photodetector are coupled. It has the effect of improving efficiency.
According to the fourth invention of the present application, there is an effect of further improving the polarization independence of the wavelength filter by making the polarization modes of coupling shift different between the polarization separation region and the polarization combining region. .

[Brief description of drawings]

FIG. 1 is a diagram for explaining the structure and operation principle of a first embodiment of a polarization independent wavelength filter according to the present invention.

FIG. 2 is a diagram for explaining the structure and operation principle of a second embodiment of the polarization independent wavelength filter according to the present invention.

FIG. 3 is a diagram for explaining the operation principle of the polarization independent wavelength filter according to the present invention.

FIG. 4 is a diagram illustrating a cross-sectional structure of a polarization independent wavelength filter according to a second embodiment of the present invention.

FIG. 5 is a diagram for explaining a sectional structure of a third embodiment of the polarization independent wavelength filter according to the present invention.

FIG. 6 is a diagram illustrating a sectional structure of a polarization independent wavelength filter according to a fourth embodiment of the present invention.

[Explanation of symbols]

 11 Input-Side Active Waveguide 12 Transition-Side Active Waveguide 13 Polarization Separation Grating 14 Wavelength Selection Grating 15 Polarization Combining Grating 31 TE Mode Coupling Wavelength Range 32 TM Mode Coupling Wavelength Range 41, 61 Substrate 42, 62 Buffer layer 43, 64 Active layer 44, 65 Optical guide layer 45, 63, 66 Cladding layer 401, 67 Contact layer 48, 54, 70 Passivation film 49, 50, 71, 72 Electrode 51, 52, 53 Waveguide embedded layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location // H01S 3/18

Claims (4)

[Claims] 1. A wavelength filter comprising a polarization separation region and a wavelength selection region, which are composed of two active waveguides close to each other, and in which the same direction coupling grating is formed. One of the TE mode and the TM mode, which are orthogonal polarization modes, causes coupling in the same direction between the two active waveguides, and DF is present in the wavelength selection region.
A polarization-independent wavelength filter, wherein a B or DBR grating is formed, and a TE mode in one active waveguide and a TM mode in the other active waveguide are selectively transmitted at the same wavelength. 2. The thickness and / or width of the two active waveguides are adjusted so that the TE mode in one active waveguide and the TM mode in the other active waveguide have equal propagation constants. , DFB or DBR grating is common to the two active waveguides, the polarization independent wavelength filter according to claim 1. 3. A polarization combining area is arranged on the opposite side of the polarization separating area across the wavelength selecting area, and the polarization combining area has the same structure as the polarization separating area. The polarization independent wavelength filter according to claim 1 or 2. 4. The polarization mode according to claim 3, wherein the polarization mode transferred between the two active waveguides in the polarization combining area is different from the polarization mode transferred in the polarization separation area. Wave independent wavelength filter.