In multiplexed optical communication networks a single optical fiber typically carries multiple independent data channels, with each data channel assigned to a different optical wavelength.  Such networks are referred to as wavelength division multiplexed (WDM) networks.  As signals propagate through the network, data in different channels may be separated using various kinds of optical frequency filters, e.g. a deinterleaver. 
Optical frequency interleavers/deinterleavers are widely recognized as key components enabling the rapid expansion of WDM networks to higher channel counts and narrower channel spacing while preserving inter-channel cross-talk performance, in combination with existing demultiplexer technologies.  Because of the periodic frequency nature of the International Telecommunications Union (ITU) grid, interleavers/deinterleavers tend to be constructed from combinations of one or more interferometric structures, e.g. etalons, Mach-Zehnder interferometers, and Michelson interferometers.  The desirable features of interleavers/deinterleavers include a flat-topped passband and high isolation in the stop-band. 
One form of interleaver/deinterleaver includes a conventional Michelson interferometer (MI).  A Michelson interferometer includes a beamsplitter for separating an input optical signal into two component parts and for directing the component parts along separate, perpendicular arms of the device.  A reflecting mirror is positioned at the end of each arm for redirecting the components back to the beamsplitter for recombination.  This type of interferometer provides a linear phase response, dependent on the optical path difference between the two arms of the interferometer.  The linear phase response generates a rounded passband with no chromatic dispersion. 
Another form of interleaver/deinterleaver, referred to as a Michelson Gires-Tournois interferometer (MGTI), is a Michelson interferometer in which the mirror of one arm is replaced by a Gires-Tournois (GT) etalon, which is disclosed in U.S. Pat.  No.  6,304,689 issued Oct. 16, 2001 to Benjamin Dingel et al.  The GT etalon perturbs the linear phase response of the interferometer and produces a non-linear phase response that generates a flat-topped passband that is desired in telecommunication systems. 
Yet another form of interleaver/deinterleaver is disclosed in U.S. Pat.  No.  6,252,716 issued Jun. 26, 2001 to Reza Paiam, in which both arms of a Michelson interferometer have a GT etalon.  A particularly desirable flat top response function is observed when the optical path difference is one half, or multiples of one half, the GT cavity length. 
In order to achieve the desired phase condition, the optical path difference must be accurate to within 1 micron.  Typically, beamsplitters available for use in interferometers do not provide this kind of accuracy, and therefore create an optical path mismatch because the divided sub-beams do not travel through equal amounts of solid material.  Accordingly, phase tuning of the device is provided by the incorporation of a tuning plate in the air gap of one arm of the interferometer.  The tuning plate introduces flexibility in the optical path length of one arm of the interferometer by providing a variable amount of glass/air that the beam of light has to pass through.  Tuning plates of this type have been disclosed in U.S. Pat.  No.  6,275,322 issued Aug. 14, 2001 to Kuochou Tai et al, which is incorporated herein by reference.  However, the Tai et al device relates to tuning the optical cavity length of a GT etalon by adjusting a tuning plate inside the etalon cavity.  The present invention relates to tuning the optical path difference between the two arms of an interferometer over a wide range of temperatures.  According to the present invention, the adjustment of the spectral response to the ITU grid can be accomplished by angle tuning the incident beam of light.  In these instances, optimum interference, and hence optimum isolation performance and optimum insertion loss, is observed. 
In order for the device to be completely athermal, the amount of glass, or other transparent solid, should be identical in both arms.  However, the introduction of a tuning plate also introduces temperature sensitivity.  For example, the thickness and refractive index of the tuning plate will generally change with changes in temperature, thus affecting both the optical path length difference and the relative amount of glass, or other transparent solid, in the two arms of the interferometer.  In particular, the presence of the tuning plate generally reduces the air gap of the arm accommodating the tuning plate over a range of practical temperatures. 
It is an object of the present invention to eliminate any optical path mismatch due to a beamsplitter in an interferometer.  It is another object of the present invention to provide an athermal gap for use in optical devices, such as interferometers, to maintain a constant optical path length difference over a practical range of temperatures.