A gauge for measuring flatness by utilizing the interference of light is composed of a block having a reference or standard surface of a high flatness and surfaces intersecting the reference surface. Three supporting members are fixedly secured to said surfaces in an appropriate arrangement. Each of three pins which is anchored to respective supporting member has a pointed top portion jut beyond the reference surface. The object or specimen to be tested is rested on said top portions of the three pins to support the specimen with the tested surface extending substantially in parallel with said reference surfaces in opposing position.

An optical filter having broad resonant frequency passbands for filtering an optical input including a plurality of multiplexed optical wavelengths into a first set of transmitted wavelengths and a second set of reflected wavelenghts. The filter has first and second inner mirrors separated substantially by an inner spacer, a first outer mirror separated from the first inner mirror substantially by a first outer spacer, and a second outer mirror separated from the second inner mirror substantially by a second outer spacer. The inner mirrors have reflectivity which is greater than the reflectivity of the outer mirrors, and are formed from layers of alternating high and low refractive index materials. The optical thicknesses of the spacers are substantially identical, and are sized to align the resonant frequency passbands of the filter with the first set of transmitted wavelengths. A method of making the filter using solid spacer(s) is disclosed wherein a plurality of spacers are formed from a single wafer of material. Also disclosed is a method of making a filter wherein the interfaces between filter segments are within the inner mirrors of the filter.

The rugged filter/switch cavity includes at least a two section low thermal expansion glass tubular frame, each section with a highly polished edge for abutting engagement and rigid gripping by electrostatic attraction to maintain the cavity integrity. Each frame section supports a multi-layer low loss thin film mirror in opposing relation to define the cavity gap. One mirror is fixed in the first section and the other mirror is carried by a diaphragm portion of the other section for axial movement toward and away from the first mirror to tune the cavity to different frequencies. Coaxially aligned optical fibers, affixed to each section by flexible epoxy, guide light from a broadband source including, one or more signals, into the first section, through the first mirror and back and forth between mirrors until the cavity is resonant to a signal to be selected which signal then exits the cavity via the second mirror and the other optical fiber. The cavity is tuned by piezo electric transducers moving the diaphragm to change the gap. A predetermined potential applied to the transducers will select the desired filter frequency. To switch between signal frequencies, control means capable of exhibiting or storing potentials or control signals similarly selects the gap setting corresponding to the frequencies to be switched.

A novel tuned coupled-microresonator filter with a known transfer function. This transfer function may be used, along with the filter's output signal, to extract the original input signal. The quality factor of manufacturable microresonators typically exceeds 10.sup.6 and permits the attainment of exceptionally high resolution in a small, physically-rugged, environmentally-protected package. In addition, the properties of the resonator and coupling optics lend themselves to low cost production.