JP3036610B2 - Polarization direction switch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization direction switch for switching the polarization direction required for measuring the polarization characteristics of a passive optical circuit component such as an optical waveguide circuit component. Is mainly used for evaluation of products in the manufacture of waveguide type optical components for optical communication or optical information processing.

[0002]

2. Description of the Related Art In a passive optical circuit component such as an optical waveguide circuit, it is very important to eliminate the polarization dependence of the component.

For example, a silica-based optical waveguide circuit component manufactured by depositing a silica-based glass on the surface of a Si substrate has a practical passive-type component due to its good matching with an optical fiber and small optical loss. Practical application as optical circuit components is expected. The most important issues of such passive optical circuit components are:
"The characteristic does not change depending on the polarization state of the input light." This is because most of the optical fibers already laid at present are single mode optical fibers, and the polarization state of the propagating light is not stable due to disturbance of the optical fiber. Therefore, if the passive optical circuit component for branching or signal processing the propagating light has polarization dependence, its function will not be stable depending on the polarization state of the input light.

However, in general, an optical waveguide formed on a substrate surface has birefringence having a principal axis in a direction parallel to or perpendicular to the substrate surface due to anisotropy of its structure. That is, polarization dependence exists between TE polarized light having an electric field component in a direction parallel to the substrate surface and TM polarized light having an electric field component in a direction perpendicular to the substrate surface. The biggest problem for putting a waveguide type optical component into practical use is to eliminate this polarization dependence.

As a method for eliminating the polarization dependence of a waveguide type optical component, a method of laser trimming a stress applying film, a method of loading a different kind of thin film, and the like have been proposed. However, in order to eliminate the polarization dependence with high accuracy, a measuring device that accurately and easily evaluates the polarization dependence is required.

Conventionally, there have been the following two methods for evaluating the polarization characteristics of a waveguide optical component.

(Prior Art 1) A method in which a polarization maintaining optical fiber with a thin film polarizer is used as an input fiber, and this is mechanically rotated to evaluate the polarization characteristics of the waveguide type optical component.

(Prior Art 2) A polarization maintaining optical fiber is used as an input fiber, and a half-wave plate is inserted into the optical fiber and the angle is mechanically rotated. A method for evaluating wave characteristics.

FIG. 1 shows a schematic configuration of a polarization characteristic measuring apparatus adopting the polarization characteristic evaluation method of the above-mentioned prior art. Here, 20 is a light source having a wavelength of 1.3 μm, and 21 is a light source having a wavelength of 1.5 μm.
5 μm light source, 43 and 44 are polarization maintaining optical fibers, respectively 3 is an optical channel selector, 42 is a polarization maintaining optical fiber with a thin film polarizer having a thin film polarizer marketed under the name of “Ramipol” (trademark) , And 41 are optical fiber rotating devices. 25 is a waveguide type optical component as a sample to be measured, 45 is a single mode optical fiber, and 31 is an optical power meter for measuring the optical insertion loss of the sample to be measured. In this conventional apparatus, a polarization maintaining optical fiber 42 with a Ramipole thin film polarizer is used as an input fiber of the sample 25 to be measured. Thus, the characteristics of the sample 25 to be measured can be evaluated by defining the input polarization state to the sample 25 to be measured as the direct polarization. further,
By rotating the input optical fiber 42 by the optical fiber rotating device 41, the dependence of the sample 25 to be measured on the input polarization can also be evaluated.

The above-mentioned Ramipole (multilayer thin film polarizer) is composed of a metal thin film and a dielectric thin film (S
iO ₂ ) is obtained by alternately stacking about 200 layers of 70 μm alternately and cutting it to a thickness of about 30 μm. When light passes through this thin film, light having an electric field component parallel to the metal thin film is absorbed, and light having an electric field component perpendicular to the metal thin film is transmitted. As a result, the thin film functions as a polarizer. The advantages of this thin film polarizer are that it is compact and functions as a polarizer in a wide wavelength range from 1.3 μm to 1.6 μm. As shown in FIG. 2, by connecting the direction of the metal film of the thin-film polarizer 53 and the direction of the main stress of the polarization maintaining optical fibers 51 and 52 together, the output end face of the polarization maintaining optical fiber 52 is always linearly polarized. Wave can be output.

FIG. 3 shows a schematic configuration of a polarization characteristic measuring apparatus employing the polarization characteristic evaluation method of the prior art 2. Here, 61 is a light source, 62 is a collimating lens, 63 is 1 /
A two-wavelength plate, 64 is a condenser lens, and 65 is a polarization maintaining optical fiber. In this conventional apparatus, a half-wave plate 63 is inserted into an input optical fiber 65 of a sample 25 to be measured, and
By rotating the angle of the half-wave plate 63, the input polarization state of the sample 25 to be measured is changed, and its dependence is measured.

Therefore, first, a linearly polarized state is created by the polarization maintaining optical fiber with a thin film polarizer 42 described above, and the linearly polarized light is collimated by a collimating lens 62 into a parallel beam in the air. The light passes through the wavelength plate 63, is further narrowed down by the condenser lens 64, and enters the polarization maintaining optical fiber 65. The polarization state at the output end of the polarization maintaining optical fiber 65 can be changed by rotating the half-wave plate 63.

[0013]

The characteristic of the measuring apparatus of the prior art 1 is that the polarization characteristic of the sample to be measured can be evaluated in a wide light wavelength range. However, in this measuring device, the input fiber 42 must be rotated in order to measure the polarization characteristics. At this time, there is a problem that it is difficult to accurately evaluate the polarization characteristics of the sample 25 to be measured due to the positional shift of the input fiber 42 which necessarily occurs.

Further, in the measuring device of the prior art 2, it is not necessary to move the input fiber 65 when switching the state of input polarization to the sample 25 to be measured. Therefore, since the optical axis of the input light does not change, if the optical loss when the half-wavelength plate 63 is rotated is measured in advance, the sample 25 to be measured can be measured.
It is possible to precisely evaluate the polarization characteristics of the. However,
In this measuring device, since the wavelength plate 63 is used, there is a problem that the wavelength of the light source that can be used is limited by the wavelength plate 63.

Therefore, an object of the present invention is to solve the problems of the prior arts 1 and 2.
The problems in the conventional polarization characteristic evaluation method of the waveguide type optical component as shown in (1) are solved, and the polarization characteristic of the waveguide type optical component is changed to a wide optical wavelength ranging from 1.3 μm to 1.6 μm, for example. An object of the present invention is to provide a polarization direction switcher which can be evaluated within a range and precisely.

[0016]

In order to achieve the above object, a polarization direction switch of the present invention comprises an N × 2 optical space switch, a 2 × 2 polarization maintaining optical coupler, and two , And one of the two polarization-maintaining optical fibers is attached by being twisted by 90 degrees with respect to the other fiber.

Further, the present invention provides, as an embodiment thereof,
The × 2 polarization maintaining optical coupler is a waveguide type wavelength-independent coupler having a structure of a Mach-Zehnder optical interferometer, and a thin film polarizer is inserted in each of the two polarization maintaining optical fibers. It is characterized by.

[0018]

As described above, the polarization direction switch of the present invention comprises an N (arbitrary positive integer) × 2 optical space switch, a 2 × 2 polarization maintaining optical coupler, and two And a polarization-maintaining optical fiber, one of which is twisted 90 degrees with respect to the other. Therefore, the polarization direction incident on the optical coupler differs by 90 degrees depending on which of the two polarization maintaining optical fibers has propagated. Therefore, the polarization direction of the output light from the 2 × 2 polarization maintaining optical coupler can be rotated by 90 degrees by switching the N × 2 optical space switch. Since this polarization direction switch uses a twist of a polarization maintaining optical fiber as a polarization rotation mechanism, there is no wavelength dependency.

Therefore, in the polarization characteristic evaluation apparatus using the polarization direction switch according to the present invention, when switching the polarization direction of the input light, an optical component (sample to be measured) to be evaluated.
Since there is no relative position change between the optical fiber and the input optical fiber, it is possible to evaluate the precise polarization characteristics of the sample to be measured, and there is no wavelength dependence in the switching of the polarization direction. The polarization characteristics can be evaluated.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 4 shows a schematic configuration of a polarization direction switch according to an embodiment of the present invention. The polarization direction switch of this example is
As a polarization maintaining optical fiber with three Ramipole thin film polarizers connected to a light source (not shown), as an optical channel selector using a reflecting mirror as a 3 × 2 optical space switch, as a 2 × 2 polarization maintaining optical coupler Waveguide Independent Coupler 1
2 and two polarization maintaining optical fibers 1 with Ramipole thin film polarizers connecting these 3 and 12. The configuration of the Ramipol (trade name) thin film polarizer 13 is the same as that shown by reference numeral 53 in FIG.

The waveguide-type wavelength-independent coupler 12 includes:
The one that is manufactured so as to have a binding rate of approximately 50% over 2 μm to 1.6 μm is used. This coupler has already been Jinguji et al., “Mach-Zehnde”
r Interferometer Type Opt
ical Waveguide Coupler wi
th Wavelength-flattened C
coupling Ratio. "(Mach-Zehnder interferometer type optical waveguide coupler having wavelength smoothing coupling rate) E
electron Lett. vol. 26, no. 1
7, p. 1326, published in 1990, a buried single mode glass waveguide 4,5,6,7,8,9 on a silicon substrate using flame deposition, photolithography and reactive ion etching. Was produced. At this time, the core glass of this waveguide is doped with germanium so that the refractive index is 0.3% higher than that of the surrounding clad glass. The pattern is a Mach-Zehnder interferometer, which has two directional couplers 10, 11, two asymmetric length arm waveguides 6, 7, and two input / output waveguides 4, 5, 8 respectively. , 9
It consists of

FIG. 5 shows the characteristics of the waveguide type wavelength-independent coupler 12 shown in FIG. From this figure, it can be seen that the waveguide type wavelength-independent coupler 12 has 50
% Can be seen. Further, since the waveguide type wavelength-independent coupler 12 has birefringence in the waveguide, there is no mode conversion between the TE polarization and the TM polarization.

In the polarization maintaining optical fibers 1 and 2 with the Ramipole thin film polarizer, only light having an electric field component in the main stress direction of the polarization maintaining optical fiber is transmitted. 2 connecting the channel selector 3 and the waveguide type wavelength-independent coupler 12
In the polarization maintaining optical fiber 1 with a Ramipole thin-film polarizer, one fiber is twisted by 90 degrees as compared with the other fiber and attached as shown by reference numerals A and B in FIG. I have.

Next, the operation of switching the polarization direction in the embodiment of FIG. 4 will be described. Arbitrary light from a light source (not shown) passes through the polarization-maintaining optical fiber 2 with a Ramipole thin-film polarizer and is incident on the optical channel selector 3 in a linearly polarized state. In the optical channel selector 3, the incident light can be output to one of the two output ports by moving the internal reflecting mirror mechanically (mechanically). At this time, the polarization mode in the waveguide-type wavelength-independent coupler 12 differs depending on which Ramipole thin-film polarizer-attached polarization-maintaining optical fiber 1 is output. However, in the waveguide-type wavelength-independent coupler 12, the coupling ratio is always 50% regardless of the light wavelength. Therefore, the waveguide-type wavelength-independent coupler 1 does not require any waveguide through the polarization-maintaining optical fiber 1 with the Ramipole thin-film polarizer. The output light intensities of the dependent couplers 12 are equal.

Therefore, the waveguide type wavelength-independent coupler 12
As for the output light, the polarization direction can be switched by changing the optical channel selector 3 without changing the light intensity. In addition, since this polarization direction switch does not use a wave plate or the like, it has a characteristic that it has no wavelength dependency. Actually, the coupling ratio of the waveguide type wavelength-independent coupler 12 slightly deviates from 50%. However, there is no problem because this can be corrected by taking a reference described later in advance.

FIG. 6 shows a schematic configuration of a polarization characteristic measuring apparatus according to the present invention incorporating the polarization direction switch of FIG.
In a passive optical circuit component, its polarization dependence and wavelength dependence must be reduced as much as possible. For that purpose, an apparatus for accurately measuring the polarization characteristics in a wide wavelength range is required. FIG. 6 shows a measurement system according to the present invention for accurately measuring the polarization characteristics of a waveguide optical component in a wide wavelength range.

The polarization characteristic measuring apparatus of this embodiment has a wavelength of 1.3 μm.
m, a 1.55 μm wavelength light source 21 and a white light source 22, a polarization direction switch 23 having the configuration of FIG. 4 according to the present invention, a sample to be measured (waveguide type optical component) 25 and an input. A polarization maintaining optical fiber 26, an automatic alignment device 24 for positioning the optical axis of the output single mode optical fiber 27, an optical power meter 30 for measuring the amount of reference light, and light insertion of the sample 25 to be measured. An optical power meter 31 for measuring a loss, an optical spectrum analyzer 32 for measuring a spectrum of transmitted light of the sample 25 to be measured, a channel selector 33 for switching an optical path depending on which of the measurements 31 and 32 is performed, and It comprises a microcomputer 34 for managing these operations. 28 and 29 are single mode optical fibers.

Next, a specific method of measurement in this embodiment will be described step by step with reference to FIG.

First, a reference is taken. Therefore, the sample 25 to be measured is removed, and the input / output fibers 26, 2 are removed.
7 is directly applied, and the difference between both outputs of the optical power meter 30 and the optical power meter 31 is measured using the microcomputer 34. The measurement of this reference is performed using I ₁ -O ₁ and I _1- of the input / output terminals of the 3 × 2 optical channel selector 3.
O ₂ , I ₂ -O ₁ , I ₂ -O ₂ , I ₃ -O ₁ , I ₃ -O
_{This is performed} for each of the six states where the two are connected. This is for compensating the wavelength-dependent characteristics and the polarization-dependent characteristics of the waveguide-type wavelength-independent coupler 12 and the optical channel selector 3.

Next, the sample 25 to be measured is moved to the automatic alignment device 24.
Is set between the input and output fibers 26 and 27 of FIG. At this time, the input polarization maintaining optical fiber 26 is set so that the direction of the main stress is horizontal or vertical to the substrate surface of the sample 25 to be measured. In this state, the self-aligning device 24 is driven to align the three optical axes, and the optical axes of the input / output fibers 26 and 27 with respect to the sample 25 to be measured are fixed.

Next, light sources 20 and 21 and 2 according to the purpose are used.
2 and select one of the two polarizations (TE polarization, TM
The polarization (polarization) is measured by selecting the output O ₁ or O ₂ of the optical channel selector 3, respectively. However, this measurement is to measure the difference between the optical power meter 30 and the optical power meter 31. By compensating this measurement result with a reference measured in advance, a precise evaluation can be realized.

In the above, an example in which the waveguide type wavelength-independent coupler 12 is used as the polarization maintaining optical coupler has been described. Instead, a polarization maintaining optical fiber obtained by fusing two polarization maintaining optical fibers is used. Note that it is also possible to substitute a coupler.

Further, as the polarization maintaining optical coupler 12, it is possible to substitute a 2 × 2 optical space switch capable of switching the optical path while maintaining the polarization, for example, an optical channel selector using a reflecting mirror. Please note that.

[0035]

As described above, according to the present invention,
As a polarization rotation mechanism, a 90 degree twist of the polarization maintaining optical fiber is used, so there is no wavelength dependency, that is, a polarization direction switcher which can switch the polarization direction without being present in the optical wavelength. can get.

Therefore, by using the polarization characteristic measuring device using the polarization direction switch according to the present invention,
It is possible to manufacture waveguide type optical components used in a wide optical wavelength range from 1.3 μm to 1.6 μm, and to accurately evaluate polarization characteristics. Further, by this evaluation, it is possible to manufacture an optical waveguide circuit component whose polarization characteristics are strictly suppressed.

In other words, in the polarization characteristic evaluation apparatus using the polarization direction switch according to the present invention, when switching the polarization direction of the input light, the optical component to be evaluated (sample to be measured) and the input light Since there is no relative position change with the fiber, it is possible to evaluate the precise polarization characteristics of the sample to be measured.Because there is no wavelength dependence in switching the polarization direction, the polarization characteristics can be measured over a wide wavelength range. Sex can be evaluated.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a polarization characteristic device of a waveguide type optical component according to Prior Art 1.

FIG. 2 is a perspective view showing a configuration of a polarization maintaining optical fiber with a Ramipole thin film polarizer of FIG.

FIG. 3 is a schematic configuration diagram showing a polarization characteristic device for a waveguide-type optical component according to Prior Art 2.

FIG. 4 is a schematic configuration diagram showing a polarization direction switch according to one embodiment of the present invention.

FIG. 5 is a characteristic diagram showing a wavelength characteristic of a coupling rate of the waveguide type wavelength-independent coupler of FIG. 4;

6 is a schematic configuration diagram showing a configuration of a polarization characteristic measuring device in a wide wavelength range using the polarization direction switch of FIG. 4;

[Explanation of symbols]

1 Polarization-maintaining optical fiber with Ramipole thin-film polarizer 2 Polarization-maintaining optical fiber with Ramipole thin-film polarizer 3 Optical channel selector (3 × 2 optical space switch) 4, 5, 8, 9 Incorporation of waveguide-type wavelength-independent coupler Output waveguide 6,7 Arm waveguide of waveguide type wavelength independent coupler 10,11 3dB directional coupler of waveguide type wavelength independent coupler 12 Waveguide type wavelength independent coupler (2 × 2 polarization maintaining optical coupling) 13,53 Ramipole thin film polarizer 20 1.3 μm wavelength light source 21 1.55 μm wavelength light source 22 White light source 23 Polarization direction switcher of the present invention 24 Positioning of waveguide type optical component and input / output fiber Automatic alignment device 25 to be performed 25 Waveguide type optical component as sample to be measured 26 Polarization maintaining optical fiber 27, 28, 29 Single mode optical fiber 30 Light of reference light An optical power meter 32 the optical spectrum analyzer 33 channel selector 34 microcomputer for measuring the optical insertion loss of the optical power meter 31 the measurement sample for measuring the

Claims (2)

(57) [Claims] 1. An N × 2 optical space switch, a 2 × 2 polarization-maintaining optical coupler, and two polarization-maintaining optical fibers connecting them, and the two polarization-maintaining optical fibers Characterized in that one of the fibers is twisted by 90 degrees with respect to the other fiber. 2. The 2 × 2 polarization-maintaining optical coupler is a waveguide-type wavelength-independent coupler having a structure of a Mach-Zehnder optical interferometer, wherein each of the two polarization-maintaining optical fibers has a thin film polarization. The polarization direction switch according to claim 1, wherein a probe is inserted.