JPS6049888B2 - optical demultiplexer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical demultiplexer constructed by connecting two optical waveguides with different electrical lengths, with their input ends connected by a polarizer and their output ends connected by an analyzer. The present invention relates to a demultiplexer for optical communications that makes it possible to increase the frequency band utilization rate by sharpening the rise characteristic.

A conventional optical demultiplexer of this kind is composed of a polarizer 1, an analyzer 2, and a birefringent crystal 3 disposed between them, as shown in FIG. 1A.

After the light propagating through the incident optical waveguide 4 passes through the polarizer 1, it has polarization in the P direction in the coordinate system shown in FIG. 1B. The analyzer 2 has the function of separating this polarized light into two mutually orthogonal polarized light components, and also directs the optical axis Z and the direction X perpendicular to this Z axis in a direction forming 45 degrees with respect to the P direction. The light propagated through the birefringent crystal 3 is directed in a direction that separates the polarized light component parallel to P and the polarized light component perpendicular to P. At this time, light containing a number of signals having center frequencies of frequencies Fl, f2, f3, . . . , F2n passes through the polarizer 1, becomes polarized light in the P direction, and propagates through the birefringent crystal 3. When the length e of the birefringent crystal 3 is determined as shown in the following equation, the odd-numbered center frequencies Fl, f3, . . . of the light exiting the birefringent crystal 3 are
F2. ．． The polarization direction of light including −1 is parallel to P, and the polarization direction of light including even-numbered center frequencies F2, f4, . . . , F2n is perpendicular to P. Here, C is the speed of light in vacuum, Δf is the difference between adjacent center frequencies, N8 is the refractive index for ordinary rays, and NO is the refractive index for extraordinary rays.
Next, when the polarized light components parallel and perpendicular to P are separated and taken out from the output optical waveguides 5 and 6 by the analyzer 2, the odd numbered branch center frequencies Fl, f3,..., F2n-1 and the even numbered The center frequencies F2, fl, . . . , F2n are taken out from separate output optical waveguides and operate as an optical demultiplexer.
At this time, since the ordinary ray and the extraordinary ray propagating through the birefringent crystal 3 have different refractive indices, their phases will be as shown in FIG. By the way, their phase difference is an integral multiple of π. Therefore, the frequency response of the optical amplitude output to the output optical waveguides 5 and 6 is as shown in FIG. 2B. However, in an optical demultiplexer with such a configuration, the phase difference between the ordinary ray and the extraordinary ray is
As shown in FIGS. 2A and 2B, only the center frequency within one channel frequency interval is an integral multiple of π, and the frequency response of the light amplitude taken out from the three output optical waveguides is only sinusoidal. For this reason, the rise characteristic of the frequency response was slow and the frequency band utilization rate was poor. The present invention solves the above-mentioned drawbacks of the prior art.An object of the present invention is to provide an optical demultiplexer that sharpens the amplitude rise characteristic and increases the frequency band utilization rate.

The key point of the configuration of the present invention to achieve this object is to install a periodic delay circuit in at least one arbitrary position of two optical waveguides with different electrical lengths connected by a polarizer and an analyzer in an optical demultiplexer. By inserting a stepwise change in at least one of the phases of the light propagating through two optical waveguides with different electrical lengths, we can change the phase of the light propagating through two optical waveguides with different electrical lengths. The point is that the amplitude rise characteristic is sharpened by rapidly changing the phase difference in the middle of the demultiplexed frequency. Next, an embodiment of the present invention will be described in detail with reference to the drawings.

3A and 3B are conceptual diagrams showing one embodiment of the present invention, in which 1 is a polarizer, 2 is an analyzer, 3 is a birefringent crystal, 4 is an input optical waveguide, and 5 and 6 are output optical waveguides. , 7 are periodic delay circuits, and the periodic delay circuit 7 is installed at the end of the birefringent crystal 3 so as to effectively act on at least one of the ordinary ray and the extraordinary ray propagating through the birefringent crystal 3. Ru. The arrangement of other components is the same as that of the optical demultiplexer in the direction shown in FIG. The operation will be explained below.

Frequencies Fl, f2,...F from the incident optical waveguide 4
When light containing 2n is incident, the basic waveguide order of the light and the operation of each component circuit are the same as in the conventional optical demultiplexer shown in FIG. Therefore, only the different points will be described. The fundamental difference is the phase characteristics of the light that exits the birefringent crystal 3 and enters the periodic delay circuit. The phase is delayed at regular intervals by the periodic delay circuit 7, and in cases where the periodic delay circuit 7 acts only on one optical waveguide, it acts on both waveguides as shown in Figure 4A. In this case, the phase of the light undergoes a stepwise change as shown in FIG. 5A. For this reason,
The phase difference between the light propagated through two optical waveguides is an integer multiple of π, which is a maximum of 3 times in the case of Figure 4 and a maximum of 5 times in the case of Figure 5, per channel frequency interval of the splitter. Compared to a duplexer that only sets the value to an integer multiple of π once, the phase difference can evenly approach the value of π over a wider frequency band. Therefore, when the analyzer 2 synthesizes the light propagating through the two optical waveguides, the frequency band in which the light is in phase or out of phase becomes wider, and the amplitude characteristics become as shown in Figure 4B or Figure 5B. It has an extremely sharp rise. Figure 6 is the 4th
The rise characteristics of the frequency response are calculated and shown for each of the cases shown in FIGS. In FIG. 6, the solid line indicates the case of FIG. 5, the dotted line indicates the case of FIG. 4, and the dashed line indicates the case of FIG. 2. FIG. 7 shows a case where an optical fiber having an elliptical core is used in place of the birefringent crystal 3 shown in FIG. 3 to form two optical waveguides with different electrical lengths.

In Figure 7, 8 is a fiber with an elliptical core, 9 is an elliptical core,
10 is the long axis of the elliptical core, and 11 is the short axis of the elliptical core.
If the fiber 8 with an elliptical core is installed by rotating the long axis 10 of the elliptical core 9 so that it becomes approximately 45 polarized with respect to the polarization plane polarized by the polarizer 1 in Fig. 3, this P polarized light will be equivalent to The two orthogonal polarization components PS (5P') with polarization parallel to the long axis 10 and the short axis 11 are propagated in the fiber 8 having an elliptical core.Now, the propagation constants of these two orthogonal polarization components are There is a difference in the propagation constant between the Ps polarization component and the Pe polarization component because it is proportional to the ellipticity (ratio of major axis to minor axis) of the core.Therefore, at the output end of the fiber 8 having an elliptical core, there is a phase difference. As a result, the same effect as the birefringent crystal 3 shown in Fig. 3 can be obtained. This is an example in which a digital delay circuit is added.

12 is a polarizing film, 13 is a resonator using a single or multilayer dielectric film, 14 is a total reflection film, 15 is incident light, and 16 is output light.

The polarizing film 12 is arranged so that the Ps polarized component of the incident light 16 is completely reflected, and all the Pf polarized component is transmitted. The resonator length of the resonator 13 must be selected such that the periodic interval of its resonant frequency is equal to the demultiplexing frequency interval of the entire duplexer. Further, the polarizing film 12, the resonator 13, and the total reflection film 14 are installed at an angle of θ0 with respect to the incident light 15. Now, when the incident light 15 enters, the Ps polarized component of the incident light 15 is completely reflected by the polarizing film 12 and becomes the output light 16. On the other hand, the P' polarized light component passes through the polarizing film 12 and causes a resonance phenomenon in the resonator 13 at a period equal to the demultiplexing frequency interval of the optical demultiplexer (7). Since the phase is delayed by π at the resonance point, Its phase characteristic exhibits a step-like change as shown in FIG. The 13 periodic delay circuits shown in Figure 8 are used to cause a periodic delay in the phase of light propagating through one of two optical waveguides with different electrical lengths. This is an example in which periodic delay circuits are added to both of two optical waveguides having different electrical lengths among the periodic delay circuits 7 shown.

The configuration and basic principle are as shown in Figure 8.
Since this is the same as the delay circuit, only the differences will be described. That is, since there is no polarizing film 13 compared to FIG. . For this reason, Ple. , p, and the phase of both the polarized light components undergoes a stepwise change as shown in FIG. At this time, if the permittivity of the dielectric material constituting the resonator 13 and the incident angle 0 are appropriately selected, a Brieuster angle exists for the P' polarized light component, so the Pe polarized light component is smaller than the P, polarized light component. It is strongly coupled to the resonator 13. Therefore, by adjusting the degree of this coupling, the step-like change that occurs in the phase of both can be set arbitrarily to some extent. By utilizing this fact, the phase difference can be made an integral multiple of π at any frequency within one channel frequency interval, and a phase relationship as shown in FIG. 5 can be set. As explained above, according to the present invention, by inserting a circuit that periodically causes a delay in at least one of two optical waveguides having different electrical lengths, at least one of the light propagated through the 27 optical waveguides can be By adding step-like changes to the phase characteristics of This has the advantage that the rise and fall characteristics of the response can be sharpened.

Taking advantage of this advantage, in optical frequency multiplexing transmission,
Since the spacing between each channel can be narrowed, even with the same frequency bandwidth, you can have more channels than before! 5. The utilization rate can be increased.

[Brief explanation of the drawing]

Figures 1A and B are conceptual diagrams of the configuration of a conventional optical demultiplexer;
Figures A and B are phase and amplitude characteristic diagrams of a conventional optical demultiplexer.
3A and 3B are conceptual diagrams of the configuration of the optical JO demultiplexer according to the present invention, and FIGS. 4A and 4B are conceptual diagrams of the phase and amplitude when using the periodic delay circuit of the optical demultiplexer according to the present invention, Figure 5A
and B are phase and amplitude characteristic diagrams when another periodic delay circuit is used in the optical demultiplexer according to the present invention, FIG. 6 is an amplitude characteristic diagram of the conventional optical demultiplexer and the optical demultiplexer according to the present invention, and FIG. FIG. 8 is an explanatory diagram of a fiber having an elliptical core according to the present invention, FIG. 8 is a diagram showing one embodiment of a periodic delay circuit, and FIG. 9 is a diagram showing another embodiment of the periodic delay circuit. 1 is a polarizer, 2 is an analyzer, 3 is a birefringent crystal, 4 is an input optical waveguide, 5 and 6 are output optical waveguides, 7 is a periodic delay circuit,
8 is a fiber with an elliptical core, 9 is an elliptical core, 10 is the long axis of the ellipse, 11 is the short axis of the ellipse, 12 is a polarizing film, 13 is a resonator, 14 is a total reflection film, 15 is incident light, 16 is a This is the emitted light.

Claims (1)

[Claims] 1. An input end of a birefringent crystal that provides two optical waveguides with different electrical lengths for the incident optical waveguide, the ordinary ray, and the extraordinary ray,
The output ends of the two optical waveguides made of the birefringent crystal and the output optical waveguide are connected by an analyzer, and the output ends of the two optical waveguides made of the birefringent crystal are connected by sandwiching polarizers oriented so that the intensities of both light beams are approximately equal.
A periodic delay circuit is added to at least one of the two optical waveguides, the delay amount of which increases and decreases repeatedly at approximately the same period as the demultiplexing period that is alternately repeated on the frequency axis. optical demultiplexer. 2. As the periodic delay circuit, the 2 circuits having different electrical lengths may be used.
A polarizer, a resonator made of a single or multilayer dielectric film, and a total reflection film are installed in the order of proximity to the incident optical waveguide, tilted to the propagation axis of the optical waveguide, and the polarizer includes two polarizers having different electrical lengths. The optical demultiplexer according to claim 1, characterized in that the optical demultiplexer is installed so that only one side of the light propagating through the optical waveguide is almost completely reflected and the other side is almost completely transmitted. . 3. As the periodic delay circuit, a resonator using a single or multilayer dielectric film and a total reflection film are installed in the order of proximity to the incident optical waveguide, and both are connected to the propagation of the two optical waveguides having different electrical lengths. Claim 1, characterized in that the optical waveguide is installed at an angle so that the degrees of coupling between the two beams propagating through the two optical waveguides having different electrical lengths and the resonator are different from each other. The optical demultiplexer according to item 1. 4. The input end of an optical fiber having an elliptical cross-section core that provides an input optical waveguide and two optical waveguides with different electrical lengths for ordinary and extraordinary rays is parallel to each of the long axis and short axis of the optical fiber. Two light beams with similar polarization are connected by sandwiching a polarizer oriented so that the intensity of both light beams is approximately equal,
The output end of the two optical waveguides made of the birefringent crystal and the output optical waveguide are connected by an analyzer, and at least one of the two optical waveguides is connected alternately on the frequency axis. An optical demultiplexer characterized in that a periodic delay circuit is added in which the amount of delay is repeatedly increased and decreased at approximately the same cycle as the repeated demultiplexing cycle.