Abstract: An optical device incorporating an incident-side optical waveguide portion having an incident terminal port and an outgoing terminal port which are connected with a fiber array, and an outgoing-side optical waveguide portion having another outgoing terminal port. Between the incident-side and outgoing-side optical waveguide portions, there is disposed a wavelength division multiplexer for sending back an incident beam from the incident-side optical waveguide portion to the incident-side optical waveguide portion and permitting a part of the incident beam to pass therethrough. Between the wavelength division multiplexer and the outgoing-side optical waveguide portion, a polarization-independent type optical isolator is disposed. Between the wavelength division multiplexer and the incident-side optical waveguide portion, a first lens is disposed, and between the wavelength division multiplexer and the polarization-independent type optical isolator, a second lens is disposed.

Abstract: A surface treating method of single crystals by which single crystals for substrates having finished surfaces showing pit-free and atomic scale step structures are obtained by treating the {100}-plane surfaces of single-crystal SrTiO.sub.3 substrates by dissolving two-dimensional-lattice atomic layers forming the surfaces one layer by one layer by using a fluorine-based acidic solution (maintained at >35.degree. C. in temperature and <4 in pH) as a solution A and water as a solution B by alternately immersing the substrates in the substrates in the solution A and B.

Abstract: In an optical isolator, first, second and third birefringent crystals shaped in a flat plate are arranged in order in a forward direction in which a light beam advances and formed in ratios 1:1:1 in thickness. One magneto-optical member is interposed between the first and second birefringent crystals and another magneto-optical member may be arbitrarily disposed between the second and third birefringent crystals. Since the birefringent crystals are equal in thickness, the optical isolator can be readily produced with a high accuracy without much labor.

Abstract: An optical isolator having three or four birefringent crystals and two magneto-optic elements is disposed between two light waveguides to prevent reverse-directed light from returning to the light waveguide on the light-source side. Forward-directed light emitted from the light waveguide on the light-source side is allowed to properly enter into the other light waveguide, whereas the reverse-directed light returning reflectively toward the light waveguide on the light-source side is separated into two polarized component and undergoes polarization rotation while propagating in the optical isolator, to thereby advance out of the optical axis of the light-source side light waveguide.