The present invention relates generally to a housing for an optical coupler and, more particularly, to a housing for an optical fiber directional coupler.
An optical fiber directional coupler is used in optical fiber interconnection arrangements to couple electromagnetic waves from one of two or more optical fibers to another fiber in the group (alternatively, to couple one optical fiber to itself to form a loop). Such couplers have been used in optical communications, optical sensors, and fiber-optic gyros. One example is an optical fiber directional coupler formed by fusing and tapering two or more polarization maintaining optical fibers together. Fabrication of such a coupler generally involves aligning the principal birefringent axes of two or more polarization maintaining fibers, bringing them together, followed by heating those fibers to fuse and taper the fibers resulting in a fused and tapered region for coupling of optical power.
One type of polarization maintaining fiber includes a pair of stress applying parts having an optical core positioned parallel to, and between those parts. A glass cladding encases the stress applying parts and the optical core therein, with the cladding covered by a protective polymer jacket. The cladding must have an index of refraction less than that of the optical core to maintain total internal reflection within the core. The stress applying parts of the polarization maintaining fiber must have an index of refraction matched with that of the cladding to less than 0.2% of the cladding index in order to avoid higher order mode, or higher reflection angle, coupling. The cladding diameter needs to be small so that low loss and small sized coupler packages can be realized.
The stress applying parts create birefringent axes, a first principal axis and a second principal axis, along the fiber. Typically, the second principal axis is the intersection with a cross section of a fiber of a surface passing through the pair of stress applying parts and the optical core, and along which a propagating wave travels relatively slowly. The first principal axis, or fast axis, is rotated by 90.degree. with respect to the second principal, or slow axis, but also intersects the optical core. These axes can be identified by viewing a cross-section of the fiber under a microscope. Maintaining polarization in electromagnetic waves propagating through a coupler typically requires close alignment of the birefringent axes of the joined fibers.
The excess loss of the coupler, reflecting the net optical power loss due to the presence of the coupler, is defined as: EQU L=-10 LOG(P1+P2)/PT
where,
P1 is the direct transmission fiber output optical power.
P2 is the coupled transmission fiber output optical power.
PT is the total optical power coupled to the direct transmission fiber input.
The polarization extinction ratio of the coupler, indicating the coupling of a wave of one polarization propagating therethrough to the opposite polarization mode, is defined as: EQU ER=10 LOG(Pf/Ps)
where,
Pf is the output optical power detected along the first principal axis of the direct transmission fiber being tested or a principal axis of the coupled transmission fiber aligned therewith being tested, where polarized optical power is coupled to the input of a direct transmission fiber along the first principal axis.
Ps is the output optical power detected along the second principal axis of the direct transmission fiber being tested or a principal axis of the coupled transmission fiber aligned therewith being tested, where polarized optical power is coupled to the input of a direct transmission fiber along the first principal axis.
A well designed and fabricated coupler will have a low insertion loss and a high extinction ratio even after being positioned in a housing.
The optical signal processing performance of an optical fiber directional coupler in various environments typically depends upon the type of housing in which it is positioned for protection, and on the methods used to assemble the housed coupling. In a fused optical directional coupler, for instance, the fused and tapered portions of the coupler where the transfer of optical power takes place are structurally weak and sensitive to environmental conditions. The materials used in the housing for such a coupler must have thermal expansion properties as close as possible to the thermal expansion properties of the fused silica used in the making of optical fibers. The polarization extinction ratio and the transmissibility of the coupler can be degraded if the materials used in the housing subject the fibers to a non-uniform distribution of stresses either during the fabrication process, or thereafter during use due to changes in the environmental conditions in which it is used.
Quartz glass tubes have been used as a protective covering, and as a support, for the coupled portion in a fused optical coupler formed in a jacketless region of optical fibers. In such an arrangement, the coupled region is typically placed within the slotted quartz glass tube and epoxy is applied at the ends of the tubes to secure the optical fibers extending therefrom, and so the coupler, to the tube. However, difficulties arise in environments in which substantial shock or vibration occurs because of the resulting material movements of the coupled portion of the coupler suspended in the central open portion of the tube.
This problem has in part been overcome by placing the fibers within the bore of such a glass tube and then heating the mid region of the tube so assembled until it collapses about the fibers followed by stretching the tube to reduce the diameter thereof. This method places the glass tube in direct contact with the optical fibers and the coupled portion of the coupler, thereby providing rigid support to the coupled region. This, however, places stresses on the coupler causing losses and other difficulties. Similar stresses arise in a coupler formed by placing two optical fibers between a pair of glass substrates forcing the fiber together at the coupled region through direct contact with the substrates, thus forming rigid support for the coupled portion of the coupler. Therefore, an improved housing for optical fiber directional couplers is desired.