JP2007079074A - Optical isolator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical isolator capable of setting so that a forward loss may become the minimum related to a specified wavelength, and a backward loss may become the maximum related to other specified wavelengths in the case of using the optical isolator in a wavelength multiple communication system, and then, capable of enhancing the degree of freedom in design. <P>SOLUTION: In the optical isolator comprising an incident side glass polarizer 5, an exit side glass polarizer 7 and a Faraday rotator constituted of garnet crystal 6 and a magnet 8 for applying a magnetic field on the garnet crystal 6, provided that the rotating angle of the Faraday rotator in the case the wavelength is λ1 is expressed by θ1, and the rotating angle in the case the wavelength is λ2 is expressed by θ2, the optical isolator is set so as to satisfy θ1+θ2=90°±5°, and also, so that an angle difference between the polarization axes of the incident side polarizer and the exit side polarizer may be θ1±5°. The optical isolator can be set so that the forward loss may become the minimum related to the specified wavelength and that the backward loss may become the maximum related to other specified wavelengths, then, the degree of freedom in design can be enhanced. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical isolator that is an optical component used in an optical communication system, an optical measurement device, or the like.

  In an optical communication system or an optical measurement system, when reflected light from the middle of an optical transmission path returns to a semiconductor laser as a light source, oscillation becomes unstable or noise increases. In order to block the return light, an optical isolator is used.

  FIG. 5 is a perspective view showing a basic structure of a conventional optical isolator. As shown in the following Patent Document 1 and Non-Patent Document 1, the garnet crystal 16 that constitutes a Faraday rotator, a magnet 8 that applies a magnetic field thereto, an incident-side polarizer 15, and an output-side polarizer 17 are included. The incident light 9 in the forward direction passes through the incident side polarizer 15 whose polarization axis is aligned with the polarization direction of the light source, and the polarization direction is rotated by 45 degrees in the garnet crystal 16, and the polarization axis is aligned with the rotated polarization direction. Most of the optical power is emitted from the exiting polarizer 17.

  On the other hand, the incident light 10 from the reverse direction passes through the output-side polarizer 17 and is opposite to the incident light 9 whose polarization direction is the forward direction in the garnet crystal 16 (for example, the forward direction is the direction of travel. If the rotation is clockwise, the reverse direction is counterclockwise), and the incident light 9 in the forward direction and the polarization direction are orthogonal to each other. The loss of

  In general, since the optical isolator has a wavelength dependency on the rotation angle of the Faraday rotator of the garnet crystal to be used, the forward loss and the reverse loss also have wavelength dependency. Normally, the forward loss is set to be the smallest and the backward loss is set to be the largest at the center wavelength of the light source used. That is, as described above, the rotation angle of the Faraday rotator is set to 45 ° at the center wavelength, and the angle difference between the polarization axes of the entrance side and exit side polarizers is set to 45 °. The peak of forward loss and reverse loss is The difference between the forward loss and the reverse loss at the same wavelength, that is, the isolation, is set to be the maximum.

  An example of the wavelength characteristics of the forward and reverse losses designed with the center wavelength λ0 = 1550 nm is shown in FIGS. 6 (a) and 6 (b), respectively.

Japanese Patent Laid-Open No. 11-12596 NEC TOKIN TECHNICAL REVIEW, vol. 31, p. 7-9

  In recent years, in an optical communication system, it is common to use a wavelength multiplexing communication system in order to expand communication capacity, and a multi-wavelength light source is also used as a light source. In such a case, as described above, a normal optical isolator is always set to have a minimum forward loss and a maximum reverse loss with respect to a specific wavelength therein, so that the degree of freedom is limited. That is, a certain wavelength component has a small output, so it is desirable to reduce the forward loss, but another wavelength component has a large reflected light in the system, or a light source of that wavelength easily generates noise with respect to the reflected light. For this reason, it is not possible to meet the demand for increasing the reverse loss.

  That is, there is a problem that the degree of freedom in designing the forward loss and the reverse loss with respect to the wavelength when the optical isolator is used in the system cannot be obtained. Therefore, an object of the present invention is to minimize the forward loss for a specific wavelength and maximize the reverse loss for other specific wavelengths when used in a wavelength division multiplexing communication system. An object of the present invention is to provide an optical isolator that can be set and has a high degree of design freedom.

  In order to solve the above problems, in the present invention, in an optical isolator composed of an incident-side and output-side polarizer and a Faraday rotator, the rotation angle when the wavelength of the Faraday rotator is λ1 is θ1, and the wavelength is λ2. When the rotation angle is θ2, θ1 + θ2 = 90 ° ± 5 ° is satisfied, and the angle difference between the polarization axes between the incident side and outgoing side polarizers is set to be θ1 ± 5 °. .

  That is, in the optical isolator according to the present invention, the optimum wavelengths of the forward loss and the reverse loss (the wavelength with the smallest forward loss and the wavelength with the largest backward loss) are different.

  When the Faraday rotator constituting the optical isolator satisfies θ1 + θ2 = 90 ° and the angle difference of the polarization axis between the incident side and outgoing side polarizers is θ1, the wavelength λ1 incident from the forward direction of the optical isolator is The light passes through the incident-side polarizer aligned with the polarization direction, rotates the polarization direction by θ1 by the Faraday rotator, and passes through the output-side polarizer aligned with that direction, so that the loss is minimized.

  Here, based on the principle of the present invention, a deviation of about ± 5 ° with respect to the set angle is an allowable range that does not greatly affect the characteristics.

  On the other hand, the light having the wavelength λ2 incident from the opposite direction is rotated by θ2 by the Faraday rotator from the angle θ1 of the output-side polarizer. And the loss is maximized.

  As described above, the optical isolator according to the present invention has a minimum forward loss for a specific wavelength and a reverse loss for other specific wavelengths when used in a wavelength division multiplexing communication system. It can be set so as to be maximized, and there is an effect that the degree of freedom of design can be increased.

  Hereinafter, the best mode for carrying out the present invention will be described based on examples.

  FIG. 1 is a perspective view showing the structure of an embodiment of an optical isolator according to the present invention. The basic configuration is the same as that of the conventional optical isolator shown in FIG. 5, and a garnet crystal 6 constituting a Faraday rotator, a magnet 8 for applying a magnetic field thereto, an incident side glass polarizer 5 and an output side glass polarizer 7. It becomes by.

  In the present embodiment, the forward loss is minimized for incident light having a wavelength λ1 = 1400 nm, and the reverse loss is maximized for incident light having a wavelength λ2 = 1610 nm. Therefore, a Gd-based garnet crystal having a wavelength coefficient of −0.07 deg / nm is used as the garnet crystal 6 and the thickness thereof is set so that the Faraday rotation angle θ1 = 52 ° with respect to incident light having a wavelength λ1 = 1400 nm. Set. In this case, for incident light having a wavelength of λ2 = 1610 nm, the Faraday rotation angle is θ2 = 38 ° from the above wavelength coefficient, and the condition θ1 + θ2 = 90 ± 5 ° is satisfied.

  The incident light 9 in the forward direction of the wavelength λ1 passes through the incident-side glass polarizer 5 whose polarization axis is aligned with the polarization direction of the light source, and the polarization direction is rotated by 52 degrees in the garnet crystal 6, and in the rotated polarization direction. Most of the optical power is emitted from the exit side glass polarizer 7 with the polarization axis aligned. On the other hand, the incident light 10 from the opposite direction of the wavelength λ2 passes through the exit side glass polarizer 7 and is rotated by 38 degrees in the opposite direction to the incident light 9 having the forward polarization direction in the garnet crystal 6. The incident light 9 in the direction is orthogonal to the polarization direction, and most of the optical power is blocked by the incident-side glass polarizer 5, so that the loss in the reverse direction becomes large.

  FIG. 2A shows the wavelength characteristic of the forward loss in this example, and FIG. 2B shows the wavelength characteristic of the reverse loss. The forward loss for the light of each of the wavelengths λ1 and λ2 was 0.2 dB and 0.5 dB, respectively, and the reverse loss was 12 dB and 41 dB, respectively. That is, the forward loss can be reduced for incident light having a wavelength λ1 = 1400 nm, and the reverse loss can be increased for incident light having a wavelength λ2 = 1610 nm.

  Next, another embodiment of the optical isolator according to the present invention will be described. In an optical communication system, there is a system in which bidirectional communication is performed with one optical fiber using different wavelengths for transmission light sources of two opposing terminals in communication with a subscriber end. FIG. FIG. 2 shows a cross-sectional view of a bidirectional optical module which is an embodiment of an optical isolator according to the present invention used in a simple system.

  A laser diode (hereinafter referred to as LD) 20 that is a light source for transmission, a photodiode (hereinafter referred to as PD) 21 that is a photodetector for a wavelength of 1310 nm, and a transmission wavelength and a reception wavelength are separated. The bidirectional filter module is constructed by integrating the wavelength filter 23 and the optical fiber terminal 24. Further, in FIG. 3, the optical isolator according to the present invention is installed on the light incident / exit end face of the optical fiber terminal 24.

  That is, an optical isolator element in which the garnet crystal 26 constituting the Faraday rotator, the incident side glass polarizer 25, and the output side glass polarizer 27 are integrated is fixed to the light incident / exit end face of the optical fiber terminal 24 by a method such as adhesion. A magnet 28 for applying a magnetic field is fixed to the outside. Further, in the module, a lens 31 for collimating the light emitted from the LD 20, a lens for collimating the light incident on the optical fiber terminal 24 from the optical fiber terminal 24 and collimated from the optical fiber terminal 24 into the module. 32, and a lens 33 for making light from the optical fiber terminal 24 incident on the PD 21 is provided.

  In this embodiment, as the garnet crystal 26 used for the optical isolator, a Gd garnet having a wavelength coefficient of −0.07 deg / nm is used as in the above-described embodiment. Further, since the wavelength emitted from the LD is 1490 nm, the forward loss of the optical isolator is the minimum with respect to the light of 1490 nm, that is, λ1 is 1490 nm, and the backward loss of the return light to the LD 20 is used in this embodiment. So that the loss caused by the optical isolator when the light of 1310 nm received at the subscriber end is incident from the opposite direction of the optical isolator is about 6 dB or less. The wavelength λ2 that maximizes the directional loss was designed.

  As a result, λ2 = 1700 nm, and the thickness was set so that the Faraday rotation angle θ2 = 38 ° with respect to incident light of that wavelength. In this case, for incident light having a wavelength λ1 = 1490 nm, the Faraday rotation angle is θ1 = 52 ° from the above-described wavelength coefficient, and the condition θ1 + θ2 = 90 ± 5 ° is satisfied.

  The incident side glass polarizer 25 aligns the polarization axis with the polarization direction of the LD 20, and the exit side glass polarizer 27 aligns the polarization axis with the polarization direction rotated 52 ° therefrom.

  FIG. 4A and FIG. 4B show the forward loss characteristic and reverse loss characteristic of the isolator characteristic of the present embodiment, respectively. When 1490 nm light was incident from the forward direction, the forward loss was 0.2 dB, and when 1490 nm light was incident from the reverse direction, the reverse loss was about 12 dB. On the other hand, the loss due to the optical isolator when light of 1310 nm is incident on the bidirectional optical module from the optical fiber terminal 24 side, that is, the reverse loss with respect to the wavelength of 1310 nm is 6.5 dB. It was confirmed that

  At present, in an optical module incorporating an isolator, a pigtail type optical isolator in which a small optical isolator is installed on an end face of an optical fiber as shown in the above-mentioned literature is often used because of its small size and low cost. However, in the bidirectional optical module as in the present embodiment, this type cannot be used because it is necessary to transmit light bidirectionally at the optical fiber exit end. After all, conventionally, an optical isolator must be inserted between the LD 20 and the wavelength filter 23, and it is difficult to reduce the size and cost of the optical isolator portion.

  In contrast, in this embodiment, as described above, it is possible to use a pigtail type optical isolator for the bidirectional optical module by greatly changing the wavelength at which the forward loss is minimum and the wavelength at which the reverse loss is maximum. This makes it possible to reduce the size and price of the bidirectional optical module.

The perspective view which shows the structure of one Example of the optical isolator by this invention. The figure which shows the characteristic of the optical isolator by this invention. FIG. 2A is a diagram illustrating forward loss characteristics, and FIG. 2B is a diagram illustrating reverse loss characteristics. Sectional drawing of the bidirectional optical module which concerns on the Example of the optical isolator by this invention. The figure which shows the characteristic of the optical isolator shown in FIG. FIG. 4A is a diagram showing the forward loss characteristic of the optical isolator, and FIG. 4B is a diagram showing the reverse loss characteristic of the optical isolator. The perspective view which shows the structure of the conventional optical isolator. The figure which shows the characteristic of the conventional optical isolator. FIG. 6A is a diagram illustrating an example of forward loss characteristics, and FIG. 6B is a diagram illustrating an example of reverse loss characteristics.

Explanation of symbols

5,25 Incident-side glass polarizers 6, 16, 26 Garnet crystals 7, 27 Emission-side glass polarizers 8, 28 Magnet 9 Forward incident light 10 Incident light 15 from opposite direction Incident-side polarizer 17 Emission-side polarizer 20 LD
21 PD
23 wavelength filter 24 optical fiber terminal 31, 32, 33 lens

Claims (1)

  In an optical isolator composed of an entrance-side and exit-side polarizer and a Faraday rotator, when the rotation angle when the wavelength of the Faraday rotator is λ1 is θ1, and when the rotation angle when the wavelength is λ2 is θ2, , Θ1 + θ2 = 90 ° ± 5 °, and the optical isolator is set so that the angle difference between the polarization axes of the incident side and outgoing side polarizers is θ1 ± 5 °.

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