JPH06194604A - Optical isolator with fiber - Google Patents

Abstract

PURPOSE:To provide the optical isolator with fibers which can ameliorate the insertion loss over the entire part of an amplifier by decreasing the number of the optical parts on the optical path within an optical fiber amplifier. CONSTITUTION:This optical isolator with the fibers which do not depend on polarized light consist of polarizing elements 3 to 5, Faraday rotors 1, 2 and permanent magnets 6, 7 for impressing magnetic fields, are disposed with condenser lenses 8, 9 for coupling incident and exit light at both ends thereof and are respectively connected with optical fibers 11, 12. At least either of the optical fibers 11, 12 are fibers for optical amplification. These fibers for optical amplification are added with a rare earth element. Er, Pr or Nd is preferable as this rare earth element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator with a fiber used as a component of an optical fiber amplifier in the fields of optical communication and optical measurement.

[0002]

2. Description of the Related Art In a large-capacity optical communication system using an optical fiber, in order to amplify an optical signal that is attenuated during transmission, conventionally, the optical signal is first converted into an electric signal, amplified, and then amplified again. Electrical amplifiers that convert to optical signals have been used. However, in recent years, the development of an optical fiber amplifier that amplifies an optical signal as it is without converting the optical signal into an electric signal has been advanced.

An optical fiber amplifier is a device that guides an optical signal to be amplified to an optical fiber doped with a rare earth element such as Er, injects a laser beam for excitation, and amplifies the original optical signal. An optical fiber amplifier used in a large-capacity optical communication system can usually obtain an amplification factor of about 30 dB. The optical fiber amplifier has characteristics such as a high C / N ratio as compared with the conventional electric optical signal amplifier, and is expected to replace the electric optical signal amplifier in the future.

The main components of an optical fiber amplifier are an optical fiber having a core doped with a rare earth element, a semiconductor laser for pumping, an optical coupler for synthesizing signal light and pumping light, spontaneous emission light and extra pumping light. Examples include narrow band optical filters for cutting and optical isolators.

In an optical fiber amplifier, an optical isolator is an essential element in terms of its structure. This is because if the return light enters the laser light source for excitation, the oscillation of the laser light source is disturbed and becomes noise, which is amplified inside the optical fiber and interferes with the signal light. Also,
Depending on the configuration of the optical fiber amplifier, the signal light after amplification enters the optical fiber for transmission as return light and interferes with the signal light before amplification attenuated by long-distance transmission and the waveform of the optical signal Another optical isolator is typically required to prevent disturbing the. That is, the optical fiber amplifier requires a total of two or more optical isolators at the emission end of the excitation laser light source, the optical fiber connection end on the incident side, and the like. Here, at least the optical isolator at the optical fiber connection end on the incident side is required to be a polarization-independent type. This is because the signal light inside the optical fiber is generally considered to be elliptically polarized light, and therefore, when a polarization-dependent optical isolator is used, a large attenuation of the signal light cannot be avoided. In a large-capacity optical communication system using optical fibers, the amount of attenuation at the time of transmission of an optical isolator must be minimized in order to prevent transmission loss of signal light even a little. Is considered unusable.

As described above, the polarization-independent optical isolator is an essential component for an optical fiber amplifier, and it is expected that the demand for the optical isolator will be greatly increased by putting the amplifier into practical use. Then, it has the following problems.

[0007]

A conventional polarization-independent optical isolator generally has a box body 1 as shown in FIG.
Polarization-independent optical isolator main body housed in 0, single mode fibers 102, 10 connected to both ends thereof
3 and lenses for optical coupling and the like. However, since the conventional polarization-independent optical isolator is not supposed to be used for an optical fiber amplifier, the optical fibers connected to both ends thereof are often ordinary single mode fibers. Therefore, in order to connect this to the optical fiber amplifier, it was necessary to provide connectors on both ends of the fiber of the optical isolator and connect it to the optical fiber amplifying fiber via the connectors. In this case, the transmitted light is partially attenuated by the optical fiber and the connectors at both ends. Therefore, it has been desired to develop a polarization-independent optical isolator dedicated to an optical fiber amplifier that does not need to connect the optical isolator via an optical connector.

An object of the present invention is to provide an optical isolator with a fiber which can reduce the number of optical components on the optical path in an optical fiber amplifier and improve the insertion loss of the entire amplifier.

[0009]

According to the present invention, there are two or more polarizing elements, one or more Faraday rotators and one or more permanent magnets. A polarization-independent fiber-attached optical isolator in which an optical lens is arranged and optical fibers are connected to each other, wherein at least one of the optical fibers is an optical amplifying fiber To be

Further, according to the present invention, in the optical isolator with a fiber, an optical isolator with a fiber is obtained in which the optical amplification fiber is an optical fiber doped with a rare earth element.

Further, according to the present invention, there is provided an optical isolator with a fiber, wherein the rare earth element added to the optical amplification fiber is Er.

Further, according to the present invention, in the optical isolator with a fiber, an optical isolator with a fiber is obtained, in which the rare earth element added to the optical amplification fiber is Pr.

Further, according to the present invention, in the optical isolator with a fiber, an optical isolator with a fiber is obtained in which the rare earth element added to the optical amplification fiber is Nd.

[0014]

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

1 and 2 show one embodiment of the present invention. The polarization-independent optical isolator shown in this embodiment has two Faraday rotators 1 and 2 as its constituent elements.
It has two and three polarizing elements 3, 4, and 5, two magnetic field applying permanent magnets 6 and 7, and two sets of condenser lenses 8 and 9 for coupling. The condenser lens 8, the polarization element 3, the Faraday rotator 1, the polarization element 4, the Faraday rotator 5, and the condenser lens 9 are sequentially arranged inside the case 10. The magnetic field permanent magnets 6 and 7 apply magnetic fields to the Faraday rotators 1 and 2, respectively.

Further, in this example, as each element, a magnetic garnet film is used as the Faraday rotators 1 and 2, rutile parallel plates are used as the polarizing elements 3, 4 and 5, and SmCo permanent magnets are used as the magnetic field applying permanent magnets 6 and 7. I'm using it. The condenser lenses 8 and 9 are made of glass, and their surfaces are coated with a non-reflective coating corresponding to the wavelength of the transmitted signal light. Each of these elements is fixed in the case 10 by fixing means. Two optical fibers 11 and 12 are connected to both ends of the case 10 with adhesives 13 and 14 for inputting and outputting signal light. At least one of these optical fibers 11 and 12 (the optical fiber 12 in the embodiment shown in FIGS. 1 and 2) is an optical fiber having a function of amplifying light doped with a rare earth element such as Er. And the total length is also the length required for optical fiber amplification. The Er is mixed in the host glass at a ratio of 5 to 100 ppm. Further, connectors 15 and 16 for connection into the optical fiber amplifier are attached to the respective ends of the optical fibers 11 and 12 as needed.

The polarization-independent optical isolator having the above-described structure is used in the optical fiber 12 having a fiber amplification function.
Is directly attached, and there is no need to incorporate an optical fiber or a connector for connection that is not directly involved in amplification, unlike the conventional optical isolator. Further, the above-mentioned connector at the fiber end is arranged at the connection point to the demultiplexer / multiplexer, which is a component in the optical fiber amplifier, or at the connection point at the end of the optical fiber amplifier. The number of constituent elements on the optical path will not increase. Therefore, by using the polarization independent optical isolator having the shape of the embodiment, the number of elements on the optical path in the optical fiber amplifier can be reduced in any case as compared with the conventional case. The insertion loss can be improved.

In the above embodiments of the optical isolator, if only one of the optical fibers to be connected is an optical fiber having an amplifying function to which a rare earth element such as Er is added, the need for a general optical fiber amplifier should be satisfied. You can However, depending on the optical fiber amplifier, it may be necessary to install an optical isolator in the middle of the rare earth-doped optical fiber in order to improve the amplification factor. In such a case, the above-mentioned rare earth-doped optical fiber must be connected to both ends of the polarization independent isolator. However, even in this case, compared to the case where the conventional optical isolator is used, the connector and the optical fiber on the optical path can be partially omitted, so that the insertion loss of the optical fiber amplifier can be improved. Further, as shown in FIG. 3, both the optical fibers 11 and 12 may be optical amplification fibers. The above embodiments of the optical isolator are used for optical communication in which the wavelength of the signal light is in the 1.55 μm band.

Next, the wavelength dispersion of the silica-based optical fiber is zero.
An example using a signal light in the 1.31 μm band will be described. This embodiment has the same components with the reference numerals of FIGS. 1 and 2. The optical fiber 12
The rare earth element added to this is Pr or Nd.
Is. This Pr or Nd is 5 for the host glass.
It is mixed in a ratio of 0 to 1000 ppm. In this embodiment, when the Nd-doped optical fiber 12 is used, the oscillation wavelength of the excitation laser is 0.78 μm, 1.06 μm or 1.32 μm. When the optical fiber 12 doped with Pr is used, the oscillation wavelength of the excitation laser is 1.02 μm. In this case, although the rotation angle of the Faraday rotator varies depending on the transmission wavelength, the optical isolator with a fiber of the present invention ensures that the reverse insertion loss becomes 0 for the light of 1.31 μm which is the wavelength of the signal light. It is set. This is because the loss of the signal light in the optical isolator is minimized and the return light in the 1.31 μm band that particularly adversely affects the laser on the oscillation side is prevented from entering the transmission optical fiber.

[0020]

The optical isolator according to the present invention can reduce the number of optical components on the optical path in an optical fiber amplifier incorporating the optical isolator to improve the insertion loss of the entire amplifier. Therefore, according to the present invention, since the amplification factor of the optical fiber amplifier is expected to be improved, it is possible to improve the installation interval of the amplifier within the optical communication line, and as a result, a corresponding cost reduction in the field of optical communication. Can be expected.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of the present invention.

FIG. 2 is an enlarged sectional view showing one embodiment of the present invention.

FIG. 3 is a perspective view showing another embodiment of the present invention.

FIG. 4 is a perspective view showing a conventional optical isolator with a fiber.

[Explanation of symbols]

 1, 2 Faraday rotator 3, 4, 5 Polarizing element 6, 7 Magnetic field applying permanent magnet 8, 9 Condensing lens 11, 12 Optical fiber

Claims (5)

[Claims] 1. An optical fiber comprising at least two polarizing elements, at least one Faraday rotator and at least one permanent magnet, both ends of which are provided with a condenser lens for coupling incoming and outgoing light. A polarization-independent optical isolator with a fiber, wherein at least one of the optical fibers is a fiber for optical amplification. 2. The optical isolator according to claim 1, wherein the optical amplification fiber is an optical fiber doped with a rare earth element. 3. The optical isolator with a fiber according to claim 2, wherein the rare earth element added to the optical amplification fiber is Er. 4. An optical isolator with a fiber according to claim 2, wherein the rare earth element added to the optical amplification fiber is Pr. 5. The optical isolator with a fiber according to claim 2, wherein the rare earth element added to the optical amplification fiber is Nd.