Transmission of information by light over optical fibers is widely used in long-haul telecommunication systems. Optical signals are generated, transported along optical fibers and detected to regenerate the original electronic signal with as little change as possible. Fiber transmission media are combined with electronic signal processing resulting in lowered cost and high quality digital transmission.
In general, fiber optic systems require fiber connectors for precise alignment of optic fiber ends within the system. One technique for making fiber connections employs fiber ferrules which are cylindrical, typically glass, elements supporting optic fibers in an axially passageway with a fiber end flush with one end of the ferrule. Connections are made by aligning ferrules with fiber ends opposed in an alignment fixture.
Fiber optic system applications require direct optical processing of signals without conversion to electronic signals necessitating optical signal processors such as amplifiers, multiplex/demultiplexes, splitters, couplers, filters, equalizers, and switches adapted for use with optical fibers. Economical, low-loss, easily and reproducibly manufactured, single-mode optical fiber filters, adaptable to a desired bandwidth, FSR and finesse, are important components for such fiber optic systems. A fiber Fabry-Perot (FFP) interferometric filter is such a filter.
FFPs with optical properties suitable for telecommunication applications employing fiber ferrules to create optical cavities have been described. These FFPs contain fiber ferrule assemblies with aligned optical fiber extending through the ferrules forming an optically continuous path containing a tunable optical cavity. Two highly reflective, preferably plane-parallel mirrors, in the ferrule assembly transverse to the fiber path create the optical cavity. Ferrules are supported in precision fixtures with fibers aligned to maximize signal transmission through the assembly. This design eliminates the need for collimating and focusing lenses, improves stability and optical performance and makes the FFPs compatible with single-mode optical fibers and other fiber devices.
C. M. Miller U.S. Pat. No. 5,212,745, for example, describes the transmission characteristics of a typical FFP. An FFP is tuned between successive resonance maxima by, for example, changing the optical cavity length, l.sub.c. (Alternatively, tuning of an FFP can be accomplished by changing the index of refraction, n.) The bandwidth (BW) is the full width at half maximum. The finesse of the filter, F=FSR/BW, can be measured experimentally by measuring the ratio of FSR to BW from the transmission curves generated by varying l.sub.c with constant wavelength, .lambda.. Measuring F in this manner accounts for all non-dispersive losses including mirror absorption, diffraction and alignment losses. If .lambda. is varied to generate transmission curves, dispersive properties of the mirrors, fibers, and cavity modes are also included in the measured FSR.
Fixed-wavelength and tunable-wavelength FFPs having aligned ferrule assemblies have been described. Changing the distance between optic fiber ends in the cavity or stretching an optical fiber in the cavity tunes the wavelength. Tuning can be accomplished by controlled temperature variation of the FFP (temperature-tuned) or by changing the relative position of the ferrule elements, without destroying alignment, for example, by electromechanical means such as piezoelectric transducers.
Ferrule configurations for FFP filters having either a continuous fiber or a fiber gap in the optical cavity have been described. (J. Stone and L. W. Stulz (1987) Elect. Lett., 23(15):781-783). FFP configurations having a fiber gap are preferred for telecommunications applications. The Stone and Stulz Type III FFP configuration has an internal fiber-containing waveguide positioned between opposed faces of two fiber ferrules each of which has a fiber end. A mirrored-end of one ferrule and a mirrored-end of the waveguide remote from that ferrule form the optical cavity such that the fiber of the waveguide is within the optical cavity. The length of the fiber gap between the waveguide and the mirror-ended ferrule can be varied to tune the filter. U.S. Pat. No. 5,062,684 describes FFPs having two wafered ferrules, each having a wafer waveguide rigidly bonded to a mirror-ended ferrule, to form an optical cavity between the two embedded mirrors. The optical cavity in this configuration contains a tunable fiber gap between the wafered ends of the ferrules. U.S. Pat. No. 5,212,746 describes a single-wafered ferrule FFP configuration in which the optical cavity is formed by aligning a mirror-ended ferrule and a wafered ferrule with an embedded mirror.
FFP ferrules and waveguides require high precision axial alignment to minimize transmission loss. U.S. Pat. No. 4,861,136 describes FFPs tuned using piezoelectric transducers (PZTs). Elaborate alignment brackets and fixtures were necessary to change cavity length without detriment to fiber alignment. EP patent application 0 457 484 describes another alignment fixture for PZT-tuned FFPs in which ferrules are aligned by adjusting the relative tightness of set screws, which directly contact a ferrule around its circumference. U.S. Pat. Nos. 5,212,745 and 5,289,552 describe alignment fixtures which provide for adjustment of fiber alignment by use of set screws which contact a ferrule directly or indirectly via an intermediate sleeve.
G. F. De Veau and C. M. Miller U.S. Pat. No. 4,545,644 describes a rotary mechanical slice fiber ferrule alignment fixture for making fiber connections. This fixture comprises a plurality, typically three, alignment rods held within a spring bracket. At least one of the alignment rods, preferably two in a three-rod splice, includes a "flat", as defined in that patent, extending along the rod from one end for a substantial fraction of the length of the rod. Ferrules are inserted into the splice, with fiber ends opposed. The fibers are aligned by rotating the ferrules relative to each other in the splice fixture by a rotary alignment technique as described in the patent. The "flat" portions on the alignment rods provide an alignment fixture offset said to be necessary for rotary alignment. Once fiber alignment is adjusted for maximum signal transit, it is maintained by establishing a multi-point (preferably three-point) pressure contact of the alignment rods with the ferrule using a spring clip. C. M. Miller U.S. Pat. No. 5,212,745 describes a temperature tunable FFP which employs a rotary mechanical splice fixture. The rotary mechanical splice fixture has not been used in FFPs tuned using PZTs.
Signal loss due to wavelength drift and increased insertional loss as a function of temperature can be a significant problem in FFPs. An uncompensated FFP, like that of U.S. Pat. No. 5,062,684 or EP application 0 457 484, can exhibit a relatively large change in cavity length with temperature, of the order 0.05 .mu.m/.degree.C. representing a full FSR (free spectral range) drift over 15.degree. C.
Since PZTs require a higher voltage at higher temperature to maintain a given length, cavity length in PZT-tuned FFPs effectively decreases with increasing temperature (with constant voltage) and these FFPs have negative temperature coefficients. Addition of a material, like aluminum, having a positive temperature coefficient in series with the PZTs compensates for the negative temperature coefficient of the PZTs. C. M. Miller and F. J. Janniello (1990) Electronics Letters 26:2122-2123. In addition, as reported in U.S. Pat. No. 5,212,745, the use of controlled thicknesses of positive temperature coefficient adhesives, such as epoxy, when constructing FFPs is important to achieve consistent temperature compensation. PZT-tuned FFPs with appropriate control circuitry can be locked on to a desired wavelength over a wide temperature range (I. P. Kaminow (1987) Electronics Letters 23:1102-1103 and D. A. Fishman et al. (1990) Photonics Technology Letters pp. 662-664). To compensate for large FFP temperature variations of cavity length, wavelength control systems can require high voltage power supplies capable of providing 60 volts to maintain a wavelength lock over a temperature range of about 30.degree. C. (Fishman et al. supra). Passive temperature compensation can significantly reduce the voltage requirements for FFP locking circuits so that +/-12 volt power supplies, such as are conventionally employed in computer systems, can be used.
U.S. patent application Ser. No. 08/135,679 describes PZT-tuned FFPs that can be adjusted after their construction by providing ferrule holders and alignment fixtures designed to allow the points of contact between the ferrule along its length and its holder to be changed. This technique significantly improves the production yield of highly accurate, passively compensated FFPs by significantly reducing over or under compensation of the FFPs. These FFPs employ set screw adjustment for fiber alignment.
U.S. patent application Ser. No. 08/161,702 describes ferrule holders for alignment fixtures and FFPs having ferrule passageways shaped to include flats allowing fibers to be aligned by the ferrule rotary alignment techniques of U.S. Pat Nos. 4,545,644 and 5,212,745 and allowing ferrule alignment to be retained by three-point contact in the holder. These FFPs require less time and skill to achieve good alignment and are more accurate and stable than set screw alignment methods.
Ferrule holders and alignment fixtures in the U.S. Ser. No. 08/161,702 application combine rotary ferrule alignment, controlled epoxy thicknesses and adjustable temperature compensation. That patent application also describes the use of ferrule holders made of metal alloys having relatively low thermal expansion coefficients in combination with these other means of temperature compensation. The present invention provides improved designs for fiber ferrule connectors and ferrule alignment fixtures combining these beneficial features.