An optical transmission system transmits information from one place to another by way of a carrier whose frequency is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, or a lightwave signal. Such optical signals are commonly propagated in optical fibers.
In some systems, photonic devices are built into or include a segment of optical fiber. For example, a Bragg grating can be implemented in a segment of optical fiber by forming regions of periodically alternating refractive indices in the fiber segment through which an optical signal is propagated. This type of Bragg grating is commonly referred to as a fiber Bragg grating (FBG) and is typically used as a wavelength selective filter in fiber optic communication systems. For example, the FBG can be used to filter out a particular wavelength (known as the Bragg wavelength). The Bragg wavelength depends on the average or effective refractive index of the optical fiber segment and on distance between gratings of the alternating regions (i.e., the period). As is well known, the Bragg wavelength of a FBG is dependent on the temperature and the strain on the fiber segment containing the FBG.
Typically, the optical fiber segment containing the FBG is attached under strain to a package that can be mounted on a board or otherwise incorporated into a unit or assembly. As schematically illustrated in FIG. 1 (PRIOR ART), the optical fiber segment is attached to a conventional package at two attachment points, with the FBG section being between the attachment points.
In this example of a conventional package, an optical fiber 100 is attached to a package having portions 101 and 102. Optical fiber 100 is attached under strain to portions 101 and 102 using bonds 103 and 104. Bonds 103 and 104 are commonly solder, epoxy or other adhesive. A FBG (or other photonic device) can be implemented in the portion of optical fiber 100 between bonds 103 and 104. A screw 105 can be adjusted to vary the length between bonds 103 and 104, thereby adjusting the strain on the portion of optical fiber 100 between bonds 103 and 104. Precise control of the dimensions of the package and the length of the optical fiber between bonds 103 and 104 are needed to achieve the desired temperature compensation of the package.
As seen in FIG. 1, bonds 103 and 104 are relatively large so that optical fiber 100 is reliably attached to the athermal package. However, this relatively large size makes it difficult to determine the exact attachment point of optical fiber 100 to bond 103 (and bond 104) and tends to cause the exact attachment point to vary from bond to bond. As previously described, precise control of the length of the portion of optical fiber 100 between bonds 103 and 104 is needed to achieve the desired temperature compensation. Thus, the relatively large size of bonds 103 and 104 can undesirably cause variations in the performance of the athermal package.
Bonds 103 and 104 are typically implemented using adhesives (e.g. organic adhesive such as epoxy) or by soldering (e.g., glass solder or metal solder). However, organic adhesives can have reliability issues caused by ageing, temperature cycling, humidity, etc. Soldering tends to be complex (e.g., requiring the deposition of a metal on the optical fiber), which are generally undesirable in a large-scale manufacturing environment.