Wavelength converter and fiber optic transmission system

1. A wavelength converter comprising:

an input port configured to input a signal light and a pump light;

an output port configured to output a converted light having a wavelength component different from the signal light and from the pump light;

a nonlinear optical fiber that has a nonlinear optical effect;

a polarization beam splitter configured to split incident light into a first polarized wave and a second polarized wave;

a first polarization controller provided between the polarization beam splitter and a first end of the nonlinear optical fiber a second polarization controller provided between the polarization beam splitter and a second end of the nonlinear optical fiber; and

a control circuit configured to monitor a characteristic of the converted light and control a first amount of rotation of the first polarization controller and a second amount of rotation of the second polarization controller,

wherein in an optical loop connecting the polarization beam splitter, the first polarization controller, the nonlinear optical fiber, and the second polarization controller, the first polarized wave and a first component of the pump light travel through the nonlinear optical fiber in a first direction, and the second polarized wave and a second component of the pump light travel through the nonlinear optical fiber in a second direction opposite to the first direction, and

wherein the control circuit is connected to monitor to monitor power of the converted light output from the optical loop, and controls a polarization state of the converted light in the first direction with the first polarization controller and a polarization state of the converted light in the second direction with the second polarization controller based on the power of the converted light output from the optical loop.

2. The wavelength converter as claimed in claim 1 , wherein the nonlinear optical fiber includes a first nonlinear optical fiber and a second nonlinear optical fiber,

wherein the first polarization controller is provided between the polarization beam splitter and the first nonlinear optical fiber, and the second polarization controller is provided between the polarization beam splitter and the second nonlinear optical fiber.

3. The wavelength converter as claimed in claim 2 , comprising:

a polarizer provided between the first nonlinear optical fiber and the second nonlinear optical fiber.

4. The wavelength converter as claimed in claim 1 , comprising:

an optical multiplexer/demultiplexer provided outside the optical loop and optically connected to the polarization beam splitter,

wherein the optical multiplexer/demultiplexer has a first port configured to input the signal light, a second port configured to input the pump light, a third port configured to guide the signal light and the pump light to the polarization beam splitter, and a fourth port configured to extract and output the converted light from an output of the optical loop.

5. The wavelength converter as claimed in claim 1 , wherein the polarization beam splitter has a first port configured to input the signal light, a second port configured to input the first polarized wave into the optical loop in the first direction, a third port configured to input the second polarized wave into the optical loop in the second direction, and a fourth port configured to output the converted light from the optical loop.

6. The wavelength converter as claimed in claim 5 , wherein the optical loop has a first input port configured to input the first component of the pump light into the optical loop in the first direction, and a second input port configured to input the second component of the pump light into the optical loop in the second direction.

7. The wavelength converter as claimed in claim 1 , comprising:

an optical multiplexer/demultiplexer provided outside the optical loop and optically connected to the polarization beam splitter,

wherein the optical multiplexer/demultiplexer has a first port configured to input the signal light, a second port configured to guide the signal to the polarization beam splitter, and a third port configured to output the converted light, and

wherein an output light of the optical loop travels from the polarization beam splitter, through the second port, and is incident on the optical multiplexer/demultiplexer at which the converted light is separated and output from the third port.

8. The wavelength converter as claimed in claim 1 , wherein the control circuit controls the first polarization controller and the second polarization controller so as to maximize a power of the converted light output from the optical loop.

9. A wavelength converter comprising:

an input port configured to input a signal light and a pump light;

an output port configured to output a converted light having a wavelength component different from the signal light and from the pump light;

a nonlinear optical fiber that has a nonlinear optical effect;

a polarization beam splitter configured to split incident light into a first polarized wave and a second polarized wave;

a first polarization controller provided between the polarization beam splitter and a first end of the nonlinear optical fiber; and

a second polarization controller provided between the polarization beam splitter and a second end of the nonlinear optical fiber,

wherein in an optical loop connecting the polarization beam splitter, the first polarization controller, the nonlinear optical fiber, and the second polarization controller, the first polarized wave and a first component of the pump light travel through the nonlinear optical fiber in a first direction, and the second polarized wave and a second component of the pump light travel through the nonlinear optical fiber in a second direction opposite to the first direction,

wherein the second polarization controller is configured to bring a polarization direction of light output from the polarization beam splitter and incident on the nonlinear optical fiber in the second direction to be matching with an optical axis determined by birefringence generated in the nonlinear optical fiber, and

wherein the first polarization controller is configured to bring a polarization axis of the light output from the nonlinear optical fiber in the second direction to be matching with a first axis of the polarization beam splitter.

10. A wavelength converter comprising:

an input port configured to input a signal light and a pump light;

an output port configured to output a converted light having a wavelength component different from the signal light and from the pump light;

a nonlinear optical fiber that has a nonlinear optical effect;

a polarization beam splitter configured to split incident light into a first polarized wave and a second polarized wave;

a first polarization controller provided between the polarization beam splitter and a first end of the nonlinear optical fiber; and

a second polarization controller provided between the polarization beam splitter and a second end of the nonlinear optical fiber,

wherein in an optical loop connecting the polarization beam splitter, the first polarization controller, the nonlinear optical fiber, and the second polarization controller, the first polarized wave and a first component of the pump light travel through the nonlinear optical fiber in a first direction, and the second polarized wave and a second component of the pump light travel through the nonlinear optical fiber in a second direction opposite to the first direction,

wherein the wavelength converter has

an optical multiplexer/demultiplexer provided outside the optical loop and optically connected to the polarization beam splitter,

wherein the polarization beam splitter has a first port configured to input the signal light supplied from the optical multiplexer/demultiplexer, a second port configured to input the pump light, a third port configured to input the first polarized wave and the first component of the pump light into the optical loop in the first direction, and a fourth port configured to input the second polarized wave and the second component of the pump light into the optical loop in the second direction, and

wherein an output light of the optical loop is supplied from the first port to the optical multiplexer/demultiplexer, and the converted light is separated at and output from the optical multiplexer/demultiplexer.

11. The wavelength converter as claimed in claim 10 , comprising:

a third polarization controller connected to the second port of the polarization beam splitter and configured to control a polarization state of the pump light.

12. The wavelength converter as claimed in claim 11 , wherein the pump light is a linearly polarized light, and the third polarization controller is configured to control the linearly polarized light such that a linear polarization axis is at an angle of 45 degrees with respect to a transmitting axis of the second port of the polarization beam splitter.

13. A wavelength converter comprising:

an input port configured to input a signal light and a pump light;

an output port configured to output a converted light having a wavelength component different from the signal light and from the pump light;

a nonlinear optical fiber that has a nonlinear optical effect;

a polarization beam splitter configured to split incident light into a first polarized wave and a second polarized wave;

a first polarization controller provided between the polarization beam splitter and a first end of the nonlinear optical fiber; and

a second polarization controller provided between the polarization beam splitter and a second end of the nonlinear optical fiber,

wherein in an optical loop connecting the polarization beam splitter, the first polarization controller, the nonlinear optical fiber, and the second polarization controller, the first polarized wave and a first component of the pump light travel through the nonlinear optical fiber in a first direction, and the second polarized wave and a second component of the pump light travel through the nonlinear optical fiber in a second direction opposite to the first direction, and

wherein the polarization beam splitter, the first polarization controller, the nonlinear optical fiber, and the second polarization controller are connected by a single-mode optical fiber.

14. A wavelength converter comprising:

an input port configured to input a signal light and a pump light;

an output port configured to output a converted light having a wavelength component different from the signal light and from the pump light;

a nonlinear optical fiber that has a nonlinear optical effect;

a polarization beam splitter configured to split incident light into a first polarized wave and a second polarized wave;

a first polarization controller provided between the polarization beam splitter and a first end of the nonlinear optical fiber; and

a second polarization controller provided between the polarization beam splitter and a second end of the nonlinear optical fiber,

wherein in an optical loop connecting the polarization beam splitter, the first polarization controller, the nonlinear optical fiber, and the second polarization controller, the first polarized wave and a first component of the pump light travel through the nonlinear optical fiber in a first direction, and the second polarized wave and a second component of the pump light travel through the nonlinear optical fiber in a second direction opposite to the first direction, and

wherein the nonlinear optical fiber is wound around a reel that has a diameter of 1/2000 to 1/10000 of a length of the nonlinear optical fiber.

15. A fiber-optic transmission system comprising:

a first optical communication apparatus configured to convert an optical signal of a first wavelength band into an optical signal of a second wavelength band that is different from the first wavelength band using a wavelength converter, and output the optical signal of the second wavelength band into an optical transmission line; and

a second optical communication apparatus configured to receive the optical signal of the second wavelength band from the optical transmission line, and convert the optical signal of the second wavelength band into the optical signal of the first wavelength band using the wavelength converter,

wherein the wavelength converter includes

an input port configured to input a signal light and a pump light,

an output port configured to output a converted light having a wavelength component different from the signal light and from the pump light,

a nonlinear optical fiber that has a nonlinear optical effect,

a polarization beam splitter configured to split incident light into a first polarized wave and a second polarized wave,

a first polarization controller provided between the polarization beam splitter and a first end of the nonlinear optical fiber, and

a second polarization controller provided between the polarization beam splitter and a second end of the nonlinear optical fiber,

wherein in an optical loop connecting the polarization beam splitter, the first polarization controller, the nonlinear optical fiber, and the second polarization controller, the first polarized wave and a first component of the pump light travel through the nonlinear optical fiber in a first direction, and the second polarized wave and a second component of the pump light travel through the nonlinear optical fiber in a second direction opposite to the first direction.