JPH04163406A - Waveguide type optical coupler - Google Patents

Abstract

PURPOSE:To eliminate the polarization dependence without necessity of eliminating double refraction of respective channel waveguides by arranging a polarization separation circuit and two input waveguides for horizontal polarized light and for vertical polarized light. CONSTITUTION:The wavelength multiple signal light from a transmission line 9 are separated into a horizontal polarization (TE) light component and a vertical polarization (TM) light component by means of a polarization separation circuit 10, and through polarization holding fibers 11, 12 the TE polarization light component and the TM polarization light component pass through the first input waveguide 2 and the second input waveguide 3 respectively, leading to incidence on an input side slab waveguide 4. The light diffuses because no horizontal confinement exists within the slab waveguide, and waves are guided into a plurality of channel waveguide 6. After receiving the required phase difference at an array waveguide, the light is emitted to the output side slab waveguide, and converges in the vicinity of the center of curvature. The converging position is obtained from a different output waveguide 8 based on the wavelength according to the phase difference received at the array waveguide.

Description

Detailed Description of the Invention: In the field of optical communications, a method for increasing communication capacity by loading multiple signals onto light of different wavelengths and transmitting them through a single optical fiber (wavelength division). (multiplex transmission method) is being considered. In this system, an optical multiplexer/demultiplexer that multiplexes or demultiplexes lights of different wavelengths plays an important role.The present invention relates to eliminating polarization dependence in this optical multiplexer/demultiplexer.

BACKGROUND ART In recent years, attempts have been made to dramatically increase the amount of transmission by increasing the degree of multiplexing in wavelength division multiplexing transmission systems. In reality, a multiplexer/demultiplexer that can multiplex and demultiplex multiple signal lights with a wavelength #3 interval of about 1 nanometer or less is required. However, with conventional optical multiplexers and demultiplexers using diffraction gratings, the usable diffraction orders are limited and sufficient dispersion cannot be obtained.
It was impossible to keep the distance below 7 meters.

One known effective method to solve the above problem is to use an arrayed waveguide diffraction grating ('Arry
ed-waveguide grating for
wavelength division multi
/ demultiplexer with nano
meterresolution'; Electr
onics Letters, vol, 26. pp81
-88.1990 and special crime Hei 1-65588).

FIG. 6 is a plan view showing the configuration of a leading wave circuit of an optical multiplexer/demultiplexer using the above-mentioned arrayed waveguide type diffraction grating. Board 5
6, an input waveguide 57, an input slab waveguide 58, an array waveguide 59, a channel waveguide 60 forming the array waveguide, an output slab waveguide 61, and a plurality of output waveguides 62-i (i= ], 2, 3...) are connected and arranged sequentially.

The input slab waveguide is fan-shaped with its center of curvature near the connection point with the input waveguide, and the output slab waveguide is
It is fan-shaped with its center of curvature near the connection point with the I circuit.

The wavelength-multiplexed signal light introduced from the input waveguide enters the input side slab gi and a plurality of channel waveguides 60 which are wide waveguides and constitute an array waveguide in the same phase. Each channel waveguide 60 differs from adjacent channel waveguides by a certain amount Δ. The light emitted from the array waveguide 59 is focused near the pressure waveguide, which is the center of curvature. When propagating through the array waveguide, the light is subjected to a phase difference corresponding to ΔI, so the focusing position changes depending on the wavelength. The output waveguides are arranged at intervals corresponding to the wavelength multiplexing interval, and there are separate output waveguides 62-i (i=], 2°3, . . . for each wavelength.
).

In the arrayed waveguide diffraction grating, the following equation is satisfied: n, ds1nθhalfneΔL=mλ (]). Here, n is the effective refractive index of the slab waveguide, n is the effective refractive index of the channel waveguide 60 constituting the array waveguide, and d is the effective refractive index of the channel waveguide 60 at the connection with the output slab waveguide. The period of the waveguide spacing of the array waveguide, θ is the diffraction angle of the diffracted light in the output (1117 slab waveguide), △L is the optical path length difference between the channel waveguides constituting the array waveguide, m (integer) is the diffraction The order, λ, is the wavelength. Near the center wavelength λ, the diffraction order m is m = ne△L/λ. (2) At this time, in the range where sin θ, COS θ; From equation (1), the dispersion is given by: In the arrayed waveguide grating, the dispersion is proportional to the optical path length difference ΔL. Therefore, by setting ΔL large, it can be compared with the conventional diffraction grating. There is a feature that allows you to obtain an incomparably large variance.

Furthermore, all leading wave circuits can be fabricated all at once on a 3-inch substrate, making it easier to mass produce, with stable characteristics, and at a lower cost than the bulk type, which is fabricated by assembling lenses and diffraction gratings. It is also advantageous in that respect.

As described above, the arrayed waveguide type diffraction grating has features superior to conventional diffraction gratings in producing an optical multiplexer/demultiplexer for wavelength division multiplexing transmission with a narrow wavelength interval.

``Invention or problem to be solved'' However, the optical multiplexer/demultiplexer shown in Figure 6 has a drawback of having a large polarization dependence. This is due to the fact that the channel waveguides constituting the array waveguide have birefringence. That is, in the second equation, the effective refractive index of the channel waveguide for horizontally polarized (TE) light ne,T):l,
Since neaT□ for vertically polarized (TM) light is different, T
The center wavelengths of E and TM do not match.

For example, in the case of a silica-based waveguide fabricated by the flame deposition method, the birefringence value B−l n e(rt+ n cfTMl
l =4X10-'. At this time, the shift in the center wavelength is 043 nm from equation (2). A wavelength shift of 0.43 nm is fatal for demultiplexing light multiplexed with a wavelength interval of 1 nm.

The birefringence of channel waveguides varies depending on the materials used and the manufacturing method. Of course, anisotropic crystals may have birefringence, but even glass-based waveguides that are originally supposed to be isotropic may have birefringence. In the case of a silica-based waveguide fabricated on a silicon substrate using the flame deposition method, which is the most practical method,
The reason for the birefringence is that the thermal expansion coefficients of silicon and quartz glass differ by one order of magnitude.

During manufacture, there is a high temperature of over 1000 degrees, and when the temperature drops from there to room temperature, thermal stress occurs and birefringence occurs. Therefore, in order to remove the birefringence of the waveguide after fabrication, a method was developed in which the quartz glass near the waveguide was removed to form a stress release groove to remove the compressive stress applied to the glass.

However, with this method, the process is complicated and the dimensions of the grooves need to be delicately adjusted, making it difficult to completely eliminate stress. ]It is almost impossible to remove birefringence by applying this method to all of the hundreds of channel waveguides that make up an array waveguide, even if we are targeting a book waveguide.

It is difficult to eliminate birefringence not only in silica-based waveguides but also in channel waveguides in general, and the resulting polarization dependence has been a cause of deterioration in the characteristics of optical multiplexers and demultiplexers.

Therefore, the present invention aims to solve the problem of eliminating polarization dependence in an arrayed waveguide type opening grating, and to realize a more practical optical multiplexer/demultiplexer for wavelength division multiplex transmission.

"Means for Solving the Problems - As mentioned above, it is difficult to remove the birefringence of a channel waveguide. Therefore, in the present invention, without removing the birefringence, an array waveguide type We propose a new optical circuit configuration to eliminate the polarization dependence of a diffraction grating optical multiplexer/demultiplexer.

The optical multiplexer/demultiplexer of the present invention has two input waveguides, one for TE polarization and one for TM polarization, compared to an optical multiplexer/demultiplexer using a conventional arrayed waveguide type diffraction grating. It is configured,
It is characterized in that each polarized wave component enters the input slab waveguide from different positions.

As mentioned earlier, in the conventional type, due to the birefringence of the channel waveguide that constitutes the array waveguide, the light focusing position is
There was a drawback that the light and TM light were different. Therefore, in the optical multiplexer/demultiplexer of the present invention, this positional shift is taken advantage of, the input signal light is rounded up, the polarization separation circuit separates the TE polarization component and the TM polarization component, and each is placed at a different position (first (equivalent to positional deviation) from which it enters the input slab waveguide. Then, due to the reciprocity of the light, the TE polarized light and the TM polarized light are focused at the same position. In other words, polarization dependence is eliminated.

E Example d FIGS. 1(A) and 1(B) show a configuration diagram of a waveguide type optical self-wavelength device using an arrayed waveguide type diffraction grating as a first example of the present invention. . Polarization separation circuit 105 consists of a waveguide circuit having two polarization maintaining fibers 11 and 12.2 input waveguides. The waveguide circuit is on the board 1, T
An input waveguide 2 for E polarization, an input waveguide 3 for TM polarization, an input slab waveguide 4, an array waveguide 5, an output slab waveguide 7, and an output waveguide 8 are connected and arranged in sequence. .

The array waveguide 5 is composed of channel waveguides 6 having different lengths. At the connecting portions between the slab waveguides 4 and 7 and the array waveguide 5, the width of each channel waveguide 6 is widened to connect the slab waveguides without gaps and reduce connection loss. The taper of the wide portion is preferably as gentle as possible to prevent radiation loss; for example, in the case of a quartz waveguide, the taper may be about 1/250. The input slab waveguide is fan-shaped with its center of curvature near the connection piece with the input waveguide 2.3. The output slab waveguide is fan-shaped with the center of curvature near the connection piece with the output waveguide 8-i (i = 1. 2. 3).

First, the flow of signal light in this embodiment will be explained in order. The wavelength-multiplexed signal light from the transmission line 9 is separated into a TE polarization component and a T~1 polarization component by a polarization separation circuit 10, and the TEW wave is sent to the first input guide through polarization-maintaining fibers 1 and 12. The waveguide 2 is connected to the second input waveguide 3, and the TM component is connected to the second input waveguide 3.
and enters the input slab waveguide 4. Since there is no horizontal confinement within the slab waveguide, the light spreads and is guided into the plurality of channel waveguides 6. After receiving the desired phase difference through the array waveguide, the light is emitted to the output slab skin and focused near the center of curvature. To be precise, the focusing position is determined according to the phase difference received by the array waveguide, and the light is extracted from different output waveguides 8 depending on the wavelength.

Next, when explaining in detail using mathematical formulas the mechanism for eliminating polarization dependence without the need to eliminate birefringence according to this example, we will take into consideration the fact that the incident position is different, and we will use the above ((
1) Change the formula as follows.

n, d sinθ, + n, d sinθa +
n c△L=mλ (4) Here, the diffraction angle θ at the end is
θ. Then, the incident angle is newly set to 01.

Further, the explanation will be made while comparing the conventional type with reference to FIGS. 3 and 4. FIG. 3 is a diagram for explaining the polarization dependence of a conventional optical multiplexer/demultiplexer. FIG. 4 is a diagram for explaining a method for eliminating polarization dependence of an optical multiplexer/demultiplexer according to the present invention.

Here, in the conventional type and the optical multiplexer/demultiplexer according to the present invention,
The basic configuration is the same except for the polarization dependence cancellation method.
nsiDingE+-1, 4530-, nefTEl-1
, 4510 silica-based waveguide, wavelength 1550 μm
TE light, incident angle θ, = 00, diffraction angle θ = Q O
So that d=25 μm, △L=106.8 μm
, m= ] 00, and the focal length f=18.162 mm. Also, the birefringence value of the channel waveguide is 4
It is assumed that n eiTMl=1, 4 v +4 at X10-'.

In the conventional type shown in FIG. 3, the input waveguide 30 has an incident angle θ1
It is connected to the input side slab waveguide 31 at the position =00. Output angle θ of the TE acid component of the light incident from the input waveguide. The light is focused in the direction 36 of −〇° and taken out from the output waveguide 35-2.

On the other hand, the TM acid component, θ from equation (4). -10.0674
°, the light is focused at a position 37 shifted by 21.4 μm, and cannot be extracted from the output waveguide 36-2.
This will cause excessive losses.

Therefore, as shown in FIG. 4, if the TE light is input from the input waveguide 38 at the θ knee of 0° and the 7M light is input from the input waveguide 39 at 01--00674°, then from equation (4), the TE light and θ for both the TM destination and the TM destination. −〇〇. Therefore, it becomes possible to extract the TE light and the 7M light from the same output waveguide 45-2, and polarization dependence is eliminated. At this time, of course, the wavelength dispersion is no different from that of the conventional arrayed waveguide type diffraction grating, and the light with a wavelength of 1.550 μm is extracted from the output waveguide 45-2, and the light with a wavelength of 1551 μm is extracted from the output waveguide 45-3. It can be done.

In this example, birefringence in the slab waveguide was not taken into account for the following reason. In an array waveguide type diffraction grating with a large optical path length difference ΔL, most of the phase difference occurs in the array waveguide, so n
The birefringence of , n.

This is a value that can be ignored compared to the birefringence of .

Even if n and birefringence of the quartz slab waveguide caused by the stress from the substrate are on the same level as n, the center wavelength shift is approximately several pm (1 pm-○, OO1 nm), which is similar to that in the case of ne. It is about 1/100th, and has almost no effect on the wavelength of 1 nm. Of course, it is obvious that this point must be taken into consideration when designing an optical multiplexer/demultiplexer with wavelength spacing or pm order.

In either case, whether it is the sum of the effects of birefringence in both the channel waveguide and the slab waveguide, or whether it appears as a misalignment of the focusing position in the output slab waveguide, the input waveguide should be adjusted accordingly. The feature of the present invention is that it is possible to correct the polarization dependence at the position of .

Furthermore, in order to suppress moat dispersion, it is desirable that the channel waveguides constituting the array waveguide satisfy a single moat condition. This is because when a plurality of propagation motes exist, the difference in effective refractive index between the motes causes a shift in the focusing position, as in the case of birefringence. However, in order to reduce scattering loss due to processing of waveguide side walls and radiation loss in curved sections, a multimode waveguide that is wider than a single mode waveguide is often required. . In this case, take measures to prevent propagation modes other than the main propagation mode (usually the zero-order mode) from occurring, such as at the connection point between a straight section and a curved section, or at the connection point between two curves with different directions of curvature. For example, it is also possible to use a multimode waveguide. The curved portion is shifted outward from the center of gravity of the guided light or the geometric center of gravity of the waveguide (the center of the waveguide).

When connecting curved waveguides, it is known that the generation of unnecessary motes can be suppressed by connecting the curved waveguides so that the center of gravity of the guided light coincides with the center of gravity rather than the geometric center of gravity, and such a method is appropriate.

FIG. 2(A) and FIG. 2(B) show a configuration example of a waveguide type optical multiplexer/demultiplexer using an arrayed waveguide type diffraction grating as a second embodiment of the present invention. On the substrate, a polarization separation circuit 22, a first input waveguide 23, a second input waveguide 24,
Input side slab waveguide 25, array waveguide 26, channel waveguide 27 forming the array waveguide, output side slab waveguide 28, output waveguide 29-i (i=1.2.3 ・)
Arrange them in sequence and connect them.

The polarization separation circuit 22 consists of a first channel waveguide 14, a second channel waveguide 15, and first and second 3 dB couplers (directional couplers) 16 and 19, and has birefringence in the middle. A first optical path 17 and a second optical path 18 are included. Further, the second optical path 18 includes a birefringence adjuster 26 using an amorphous silicon stress imparting film, and the first optical path 1
At 7, a phase shifter 21 using an amorphous υcon stress imparting film is installed. The lengths of the optical paths 17 and 18, the birefringence adjuster 2o, and the phase shifter 21 are set so that the TE light is transmitted to the first input waveguide 23 and the TM light is transmitted to the second input waveguide 24. be.

In this embodiment, the polarization separation circuit is configured by a Matsuhatsu Enter interferometer, but the present invention is not limited to this, and even if other polarization separation circuits are used,
It is clear that a similar effect can be obtained.

The feature of this embodiment is that productivity and stability of operation are improved by arranging the polarization separation circuit and the array waveguide diffraction grating on the same substrate 13. The effect of eliminating polarization dependence in this embodiment is similar to that in the first embodiment.

FIG. 5(A) shows the third input method for each polarization according to the present invention.
An embodiment of the present invention, which includes first and second channel waveguides 47, 48, a first directional coupler 49, a second directional coupler 52, a first optical path 50, a second optical path 51, It is composed of a birefringence adjuster 53 and a phase shifter 54. The configuration and operation of the polarization separation circuit are similar to those in the first embodiment. A feature of this embodiment is that the second directional coupler 52 is directly connected to the input slab waveguide 55.

Depending on the waveguide fabrication process, the birefringence value of the channel waveguides constituting the array waveguide may be larger than that of the previous example.
Sometimes it's small. In the previous example, if the input position shift between the TE light and the TM destination was 214 μm, for example, if the birefringence value was half, the required spacing between the input waveguides for TE polarization and TM polarization is It is approximately 10 μm. Furthermore, the distance between the two input waveguides may become narrower depending on the ratio of focal lengths, which will be described later.

When the input waveguide spacing becomes narrower and coupling occurs, it is difficult to independently input TE polarized waves and TM polarized waves. Therefore, it is better to connect the directional coupler 52 directly to the input slab waveguide 55 as shown in FIG. 5(A). The graph in FIG. 5(B) shows the second directional coupler 5.
2 and the input side slab waveguide 55, the TE
It shows the intensity distribution of light and TM light. The degree of coupling, length, and spacing of the directional coupler need to be designed so that the center of the intensity of the E light and the TM tip is separated by the desired input position distance, as shown in the graph.

The effect of eliminating polarization dependence in this embodiment is the same as in the first and second embodiments.

In each embodiment, the input slab waveguide and the output slab waveguide are designed to have the same size, but it is not necessary. If the focal distance (curvature radius) of the input and output slab waveguides is not 1:1, the image of the diffracted light at the end of the output slab waveguide (output waveguide side) or the input slab waveguide Since the waveguide end (input waveguide side) is expanded or contracted according to the ratio of focal lengths, if a design is applied accordingly, the effect of eliminating polarization dependence will be the same as in the previous embodiment. For example, if the focal length of the input slab waveguide is 2
0 mm and the focal length of the output slab waveguide is 10 mm, the size of the image on the output side is half that of the input side. Therefore,
The input position interval for each polarized wave may be designed to be twice the focal position deviation of the TE-TM on the output side. Of course, at this time, the width of the condensing light also changes, so it is necessary to design the aperture widths of the input and output waveguides according to the focal length ratio.

Furthermore, in each of the examples, a quartz-based waveguide fabricated according to a flame deposition method is used; however, the present invention is not limited to this material system, and may include multi-component glass-based waveguides, plastic-based waveguides, and It is noted that the guide waveguide configuration of the present invention can be applied to various waveguide systems such as lithium niobate waveguides and compound semiconductor waveguides.

Effects of the Invention Normally, in a leading wave circuit that utilizes wavelength dispersion, the birefringence of the waveguide causes polarization dependence.

Removal of this requires a great deal of effort, leading to decreased productivity and high costs, making it impossible to realize a waveguide type optical multiplexer/demultiplexer suitable for practical use.

As explained above, in the waveguide type optical multiplexer/demultiplexer of the present invention, the array waveguide is It is possible to eliminate polarization dependence without having to remove the birefringence of the individual channel waveguides that make up the waveguide. In other words, the present invention has the feature that it is possible to eliminate polarization dependence at the design stage of the leading wave circuit without increasing the complicated process of removing birefringence. This provides an independent waveguide type optical multiplexer/demultiplexer.

According to the present invention, if a polarization-independent optical multiplexer/demultiplexer or a waveguide type suitable for mass production and one with a narrow wavelength spacing is put into practical use, high-density wavelength division multiplexing with a wavelength spacing of 1 nanometer or less will be possible. This makes it possible to realize transmission systems, and is expected to have an immeasurable effect in increasing the capacity of optical communication systems.

[Brief explanation of drawings]

FIGS. 1(A) and (B) are block diagrams of a first embodiment of the waveguide type optical multiplexer/demultiplexer according to the present invention, and FIGS. 2(A) and (B) are the waveguide type optical multiplexer/demultiplexer according to the present invention. A configuration diagram of a second embodiment of the waveguide, FIG. 3 is an explanatory diagram of polarization dependence in a conventional arrayed waveguide type diffraction grating, and FIG. Diagram for explaining polarization dependence pre-determination method, Figure 5 (A)
5 shows the configuration V of the third embodiment of the polarization sub-input method in the waveguide optical multiplexer/demultiplexer according to the present invention, and FIG. 5(B) shows the connecting portion between the second directional coupler and the input slab waveguide. TE in
FIG. 6, a graph showing the intensity distribution of light and the TM destination, is a configuration diagram of a waveguide type optical multiplexer/demultiplexer using a conventional arrayed waveguide type diffraction grating. 1/Recon board 2 First input waveguide 3 Second input waveguide 4 Input end slab waveguide 5 Array waveguide 6 Channel waveguide 7 forming the array waveguide Output side slab waveguide 8 ・Output waveguide 9 Transmission line 10 Polarization separation circuit 11 Polarization maintaining fiber 12 Polarization maintaining fiber 13 Substrate 14 - First channel waveguide 15 Second channel waveguide 16 First directional coupling device 17, first optical path 18, second optical path 19, second directional coupler 20 - birefringence adjuster 21, phase shifter 22...polarization separator 23, first input waveguide 24, second input Waveguide 25 Input end slab waveguide 26 ・Array waveguide 27 ・Channel waveguide 28 configuring the array waveguide ・Output side slab waveguide 29 ・Output guide? Skin path 30 ・Input waveguide 31 ・Input side slab waveguide 32 ・Signal light 33 propagating in the input side slab waveguide ・・Array waveguide 34 ・・Output side slab waveguide 35 ・Output side slab waveguide 36 ・TE polarized diffracted light 37 TM polarized diffracted light 38 ・First manual waveguide 39 ・・Second input waveguide 40 ・Input side slab waveguide 41 ・・TE polarized input light 42− - TM polarized input light 43 - Array waveguide 44 - Output slab waveguide 45 - Output waveguide 46 - Diffracted light 47 - First channel waveguide 48 - Second channel waveguide Channel waveguide 49 ] Directional coupler 50 First optical path 51 - Second optical path 52 Second directional coupler 53 ・Birefringence adjuster 54 ... Phase shifter 55 ・Input side slab Waveguide 56 Substrate 57 Input skin guide 58 Input slab waveguide 59 - Array waveguide 60 Channel waveguide 61 forming the array waveguide Output slab waveguide 62 Output waveguide Applicant: Ehon Telegraph and Telephone Co., Ltd.

Claims (2)

[Claims] (1) An arrayed waveguide type diffraction grating consisting of a substrate, an input waveguide, an input slab waveguide, an array waveguide, an output slab waveguide, and a plurality of output waveguides arranged on the substrate. In a waveguide type optical multiplexer/demultiplexer using A waveguide type optical multiplexer/demultiplexer characterized in that the waveguide and the input waveguide for vertically polarized light are provided at different positions depending on the birefringence of the slab waveguide and the array waveguide. (2) In the waveguide optical multiplexer/demultiplexer according to the first claim,
A polarization separation circuit is arranged on the same substrate, and the horizontal polarization output from the polarization separation circuit is connected to the input waveguide for horizontally polarized light, and the vertical polarization output is connected to the input waveguide for vertically polarized light. A waveguide type optical multiplexer/demultiplexer characterized by: