System and method for fabricating components of precise optical path length

A method of providing sets of interrelated glass elements for use in DWDM applications, wherein the sets of elements provide an athermal interferometric function with a selected frequency periodicity, comprising the steps of:

• calculating preliminary target values for at least two different glasses which establish the athermal condition and the frequency periodicity desired for the interferometric function;
• initially fabricating lengths of glass elements of the different materials to slightly greater dimensions than the target lengths;
• generating a sinusoidally varying transmission response using the initially fabricated glass elements in different delay paths;
• calculating a temperature compensated frequency periodicity from the sinusoidally varying transmission function;
• and
• adjusting the lengths of sets of glass elements to final target lengths having selected frequency periodicity and substantially athermal response at a center wavelength.

The method of fabricating microoptic elements of precise optical path lengths for use in optical communication systems employing differential delays in interferometric processes in which frequency periodicity and stable temperature response at a center wavelength must be precise, comprising the steps of:

• fabricating at least two different types of precursor optical elements that can each be formed into multiple microoptic elements, the precursor elements having preliminary thicknesses which are oversized relative to values calculated on the basis of nominal index of refraction, thermal expansion coefficient and thermooptic properties for each precursor optical element;
• separating individual precursor microoptic elements from each precursor optical element;
• employing precursor microoptic elements of at least two different types of delay elements in a multi-frequency interferometric measurement under varying temperature conditions;
• calculating final target thicknesses needed for stable temperature response and
• removing corrective amounts of material from each of the precursor optical elements to provide final thicknesses needed for selected frequency periodicity and stable temperature response.

A method of measuring the optical path lengths of optical elements for use in an interferometric application comprising the steps of:

• establishing a first target frequency periodicity of a first optical element;
• separating an input optical beam into two beam paths, delaying the optical signal in the first beam path by inserting the first optical element, while separately delaying the optical signal in the second beam path, and interferometrically re-combining the two beam paths into a combined beam path;
• determining the first initial frequency periodicity of the combined beam path carried by a single polarization of the interfering optical signals;
• adjusting the length of the first optical element to provide a first final frequency periodicity within a closer tolerance to the first target frequency period than that of the first initial frequency periodicity.

A method of providing shared glass elements for use in DWDM applications, wherein the shared elements are to provide an athermal interferometric function with a selected frequency periodicity, comprising the steps of:

• calculating target values for two different glasses which establish the athermal condition and desired frequency periodicity;
• providing glasses of the different materials that are initially fabricated to slightly greater lengths than the target lengths;
• determining the sinusoidal transmission variation resulting from the differential delay of one beam through a first glass element in a first delay path in relation to a different media in the other delay path;
• adjusting the length of the first glass element to a close approximation of the target length;
• using a second glass element having a predetermined initial length greater than a second target length in the second delay path while retaining the first glass element in the first delay path;
• determining the interferometric frequency variations resulting from differential delay of the beams in the different glass elements in the first and second paths;
• adjusting the second glass element to a close approximation of the target length.

The method of providing optical elements of precise path length and optical delay characteristics for use in DWDM interleaving components operating at selected frequency periodicity within selected frequency bands, comprising the steps of:

• generating a variable wavelength beam of monochromatic light in the frequency band of interest;
• varying the beam wavelength sequentially to provide a test beam that varies with time incrementally in frequency across the frequency band of interest;
• inputting the test beam to a polarization interferometer as a beam which is split into two substantially parallel beams of orthogonal polarization, directed through separate delay paths and then recombined into a single output beam producing a wavelength dependent, intensity modulated output;
• interposing in one delay path a first glass delay element of slightly greater thickness than the calculated target thickness, the calculation based on the indices of refraction, thermal expansion coefficients, and thermooptic coefficients of the glasses, to provide both the correct frequency periodicity and low drift of the center wavelength with temperature when combined with one or more additional glass elements in an interleaving component;
• employing a different delay media in the other delay path;
• measuring the amplitude output at each wavelength from the polarization interferometer;
• applying a least squares fit calculation to the measured amplitude output, to extract measured frequency period and phase from the fit;
• using the measured frequency period to ascertain the needed reduction in thickness of the first glass element to provide an element with frequency period accurate to within a given frequency period tolerance;
• reducing the thickness of the first glass element to the target thickness until the desired frequency accuracy is achieved;
• interposing a second glass element of typical index of refraction, thermal expansion and thermooptic coefficient which is of an initial thickness slightly greater than the calculated target thickness needed for the athermal interleaver stage in the second delay path, while maintaining the first glass element in the other parallel delay path;
• repeating the sequence of steps of inputting beams sequentially at incrementally varying wavelengths, employing the polarization interferometer to derive wavelength dependent, intensity modulated beams, calculating the frequency periodicity and determining the thickness reduction needed in the second glass element;
• reducing the thickness of the second glass element to the target thickness to establish the target periodicity to within the frequency period tolerance;
• and
• repeating the same steps as for the second element for any additional glass elements.