This invention relates to an optical connector and, more particularly, to an optical connector incorporating a polarization-independent optical isolator to be provided between optical fibers in fiber optics communication systems and the like.
With the recent advances in optical communications that use semiconductor lasers as signal light sources, it has become possible to transmit signals at high speed and density exceeding several gigahertz. Among the various optical components used in such high-speed and density signal transmission is an optical isolator which prevents the reentrance of reflected light into semiconductor lasers.
Optical isolators are of two types, a polarization-dependent isolator which transmits only the light travelling in a specified direction of polarization and a polarization-independent isolator which transmits light in any direction of polarization. The second type of optical isolators are typically used in light amplifiers at repeaters in signal transmission systems and will be in great demand in the future.
FIG. 26 shows the construction of a typical example of the conventional polarization-independent optical isolator. Generally indicated by 410, the isolator comprises one Faraday rotator and three birefringent crystal plates.
In FIG. 26, the first to the third birefringent crystal plates are identified by 411, 412 and 413, respectively, and the Faraday rotator 414 is provided between the plates 411 and 412. A magnetic field parallel to the Z direction is applied to the Faraday rotator 414. The birefringent crystal plates 411, 412 and 413 are parallel plates prepared by polishing slices of a uniaxial crystal that have been cut in such a way that their C axis is at an angle with the surface. A ray of light incident normal to each of these parallel plates is separated into two components that are polarized in orthogonal crossed directions. The birefringent crystal plates 411, 412 and 413 have different thicknesses in the direction of light transmission and their ratio is 1:1/.sqroot.2:1/.sqroot.2. The plate 413 is such that its C axis coincides with the C axis of the plate 412 if the latter is rotated through 90.degree. about the Z axis. The Faraday rotator 414 is typically formed of a bismuth-substituted garnet and can rotate the direction of light polarization non-reciprocally through an angle of 45.degree.. Shown by 415 is a coupling lens for coupling the light to an optical fiber 416 or 417.
For the purpose of the following discussion, the direction of light travel is assumed to be "forward" if it is launched from the birefringent crystal plate 411 and "backward" if it is launched from the plate 413. Thus, the forward incident ray of light is indicated by 410f and the backward incident ray of light is indicated by 410b. When the incident light is separated into two components, those in the forward direction are indicated by f1 and f2 whereas those in the backward direction are indicated by b1 and b2. The direction of light travel is represented by the arrow.
FIG. 27 shows how light travels through the optical isolator when it is seen from the birefringent crystal plate 411. A part (1) of FIG. 27 refers to the case of forward light propagation and a part (2) of FIG. 27 refers to the case of backward light propagation; A-E correspond to the respective positions A-E in FIG. 26; the dots represent the positions of respective light components and the arrows represent the directions of planes of polarization. The plane of polarization is assumed to rotate in "+" direction if it rotates clockwise.
The operating principle of the optical isolator will now be described with reference to FIGS. 26 and 27. If the C axis of the birefringent crystal plate 411 is directed upward (along the Y axis), forward signal light 410f launched from the coupling lens 415 to be incident on the plate 411 is separated into two components f1 and f2 in orthogonal crossed directions of polarization (see at B in FIG. 27(1)). With their relative positions remaining the same, the components f1 and f2 have the respective planes of polarization rotated through +45 degrees by the Faraday rotator 414 and then enter the birefringent crystal plate 412 (see at C in FIG. 27(1)). The plate 412 is such that its C axis coincides with the C axis of the birefringent crystal plate 411 if the latter is rotated through -45 degrees, so the component f1 is refracted as an extraordinary component whereas the component f2 which is an ordinary component is not diffracted but simply transmitted through the plate 412 (see at D in FIG. 27(1)). The birefringent crystal plate 413 is such that its C axis coincides with the C axis of the plate 412 if the latter is rotated through +90 degrees, so the component f2 is refracted as an extraordinary component whereas the component f1 which is an ordinary component is simply transmitted through the plate 413 (see at E in FIG. 27(1)). Thus, the two components of polarization are recombined at point E and coupled to the optical fiber 416 by means of the coupling lens 415.
The backward light 410b, as far as it travels to point C, behaves in essentially the same way as the forward light 410f, except that due-to the non-reciprocity of the Faraday rotator 414, the incident light components b1 and b2 have their planes of polarization rotated through +45 degrees as seen in the forward direction before they are incident on the birefringent crystal plate 411 (see at B in FIG. 27(2)). As a result, the component b1 is refracted as an extraordinary component whereas the component b2 which is an ordinary component is simply transmitted through the plate 411 (see at A in FIG. 27(2)). Thus, the components b1 and b2 emerge from the birefringent crystal plate 411 in different positions than when the forward light was launched into the same plate 411 and, hence, they will not couple with the optical fiber 417, thereby insuring that the reflected light will be isolated from the semiconductor laser.
FIG. 28 shows the exterior appearance of a conventional polarization-independent optical isolator. The optical isolator generally indicated by 420 comprises an isolator portion 418 and a connector portion 419 at both ends. The isolator portion 418 has the components that are shown in FIG. 26 and which are adjusted and fixed within a case. The connectors 419 are connected to optical fibers in other transmission systems. The size of the optical isolator portion 18 may be about 7 mm in diameter and 45 mm long.
The conventional polarization-independent optical isolator comprising a plurality of birefringent polarizing plates and a single Faraday rotator has suffered from the following disadvantages.
(1) It contains many parts that need precise optical adjustments, so the number of steps involved in assembly is so great as to make it a cumbersome and time-consuming operation.
(2) When the optical isolator portion is to be coupled to optical fibers, the great number of its components increases the length of the space through which light propagates between fibers. In addition, the rays of light that entered in the forward direction will emerge at positions deviating from the axes of the incident rays; therefore, the positions that serve as guides for the coupling lenses and optical fibers at opposite ends cannot be uniquely determined and, hence, considerable labor is needed to achieve optical axial alignment.
(3) Coupling to other transmission systems is only accomplished by means of the connectors at opposite ends, so a large installation space is required to incorporate the optical isolator into a measuring instrument or communication equipment.
(4) The individual optical devices are provided normal to the optical fibers, so the reflected light from these optical devices will return to the optical fibers such as to deteriorate the system's reflection attenuation characteristics.
(5) If the optical isolator is to be provided on the exit side of an optical fiber amplifier, a separate wavelength filter operating over a narrow band of frequencies is necessary but then the construction of the amplifier becomes complicated.