In-line optical isolator

An in-line optical isolator comprising:

• a first polarization separation element, a Faraday rotator made of a bismuth-substituted rare-earth iron garnet crystal film, and a second polarization separation element, the first polarization separation element, the Faraday rotator, and the second polarization separation element being placed in that order along an optical axis;
• crystal heat sinks in contact with two opposite surfaces of the bismuth-substituted rare-earth iron garnet crystal film;
• a first optical fiber collimator which is placed at a forward-beam incident side of the first polarization separation element and which includes a collimating lens and a first optical fiber;
• a second optical fiber collimator which is placed at a forward-beam exit side of the second polarization separation element and which includes a collimating lens and a second optical fiber, the second optical fiber being connected to a fiber amplifier;
• and
• an edge filter placed between the second optical fiber collimator and the second polarization separation element in a way that a normal to a filter plane is inclined with respect to the optical axis, the edge filter transmitting light which is emitted from the first optical fiber and has a wavelength equal to an oscillation wavelength and reflecting light which is emitted from the first optical fiber and has wavelengths shorter than the oscillation wavelength.