Faraday rotator

A variable optical attenuator for variably controlling a level of an optical signal, comprising:

• a polarizer made of a birefringent crystal and arranged on an incoming side of the optical signal;
• an analyzer made of a birefringent crystal and arranged on an outgoing side of the optical signal;
• and
• a Faraday rotator inserted between the polarizer and the analyzer, wherein Faraday rotation is caused by a first magnetic field applied to a magneto-optical crystal of the Faraday rotator, a Faraday rotation angle is controlled by a second magnetic field over an entire variable strength range of the second magnetic field, and the magneto-optical crystal is positioned in such a manner that a direction of a combined magnetic field of the first and second magnetic fields, except for a direction of the first magnetic field, is variable intermediately between an easy magnetization axis and hard magnetization axis of the magneto-optical crystal.

A variable optical attenuator for variably controlling a level of an optical signal, comprising:

• a polarizer made of a birefringent crystal and arranged on an incoming side of the optical signal;
• a reflector element for reflecting the optical signal;
• and
• a Faraday rotator inserted between the polarizer and the reflector element, wherein Faraday rotation is caused by a first magnetic field applied to a magneto-optical crystal of the Faraday rotator, a Faraday rotation angle is controlled by a second magnetic field over an entire variable strength range of the second magnetic field, and the magneto-optical crystal is positioned in such a manner that a direction of a combined magnetic field of the first and second magnetic fields, except for a direction of the first magnetic field, is variable intermediately between an easy magnetization axis and hard magnetization axis of the magneto-optical crystal.

An optical shutter for shutting down an optical output, comprising:

• a polarizer made of a birefringent crystal and arranged on an incoming side of an optical signal;
• an analyzer made of a birefringent crystal and arranged on an outgoing side of the optical signal;
• a Faraday rotator inserted between the polarizer and the analyzer, wherein Faraday rotation is caused by a first magnetic field applied to a magneto-optical crystal of the Faraday rotator, a Faraday rotation angle is controlled by a second magnetic field over an entire variable strength range of the second magnetic field, and the magneto-optical crystal is positioned in such a manner that a direction of a combined magnetic field of the first and second magnetic fields, except for a direction of the first magnetic field, is variable intermediately between an easy magnetization axis and hard magnetization axis of the magneto-optical crystal;
• and
• a driving circuit for switching a current value thereof between a value at which an amount of optical attenuation is at a minimum and a value at which the amount of optical attenuation is at a maximum.

An optical shutter for shutting down an optical output, comprising:

• a polarizer made of a birefringent crystal and arranged on an incoming side of an optical signal;
• a reflector element for reflecting the optical signal;
• a Faraday rotator inserted between the polarizer and the reflector element, wherein Faraday rotation is caused by a first magnetic field applied to a magneto-optical crystal of the Faraday rotator, a Faraday rotation angle is controlled by a second magnetic field over an entire variable strength range of the second magnetic field, and the magneto-optical crystal is positioned in such a manner that a direction of a combined magnetic field of the first and second magnetic fields, except for a direction of the first magnetic field, is variable intermediately between an easy magnetization axis and hard magnetization axis of the magneto-optical crystal;
• and
• a driving circuit for switching a current value thereof between a value at which an amount of optical attenuation is at a minimum and a value at which the amount of optical attenuation is at a maximum.

A variable optical equalizer for equalizing a gain, comprising:

• a polarization separator element for separating input light into ordinary light and extraordinary light;
• a polarization plane coincidence control element for causing polarization planes of the two separated beams of light to coincide with each other;
• a first Faraday rotator for rotating a polarization angle of the two beams of light whose polarization planes have been made to coincide with each other, wherein Faraday rotation is caused by a first magnetic field applied to a magneto-optical crystal of the first Faraday rotator, a Faraday rotation angle is controlled by a second magnetic field over an entire variable strength range of the second magnetic field, and the magneto-optical crystal is positioned in such a manner that a direction of a combined magnetic field of the first and second magnetic fields, except for a direction of the first magnetic field, is variable intermediately between an easy magnetization axis and hard magnetization axis of the magneto-optical crystal;
• a wavelength-dependent transmittance characteristic varying element for imparting a wavelength-dependent transmittance characteristic corresponding to the rotation of the polarization angle;
• a second Faraday rotator for causing a rotation of the polarization angle which is opposite in direction to the rotation of the polarization angle caused by the first Faraday rotator and which has an absolute value equal to that of the rotation of the polarization angle caused by the first Faraday rotator, wherein Faraday rotation is caused by a first magnetic field applied to a magneto-optical crystal of the second Faraday rotator, a Faraday rotation angle is controlled by a second magnetic field over an entire variable strength range of the second magnetic field, and the magneto-optical crystal is positioned in such a manner that a direction of a combined magnetic field of the first and second magnetic fields, except for a direction of the first magnetic field, is variable intermediately between an easy magnetization axis and hard magnetization axis of the magneto-optical crystal;
• a phase difference control element for compensating for a phase difference between P-polarized light and S-polarized light caused in the wavelength-dependent transmittance characteristic varying element;
• and
• a polarization plane restoration element for combining the ordinary light and the extraordinary light.