Transmission of information by the use of 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. Fibers are substituted for other transmission media and all signal processing is done electronically, resulting in lowered cost and high quality digital transmission.
As fiber optic applications technology develops direct optical processing of signals without conversion to electronic signals will be required. Optical fiber systems will be applied in computer networks, for example, in multiple access computer networks. Such applications require optical fiber devices such as amplifiers, multiplex/demultiplexes, splitters, couplers, filters, equalizers, switches and other optical signal processors. An economical low-loss, easily and reproducibly manufactured single-mode optical fiber filter, the design of which can be adapted to a desired bandwidth, FSR and finesse is an important component for such fiber optic systems. A fiber Fabry-Perot (FFP) interferometric filter is such a component.
FFPs which possess optical properties suitable for telecommunication applications have been described. These FFPs consist of two highly reflective, preferably plane-parallel mirrors, forming the optical cavity through at least a portion of which, in most cases, a length of single-mode optical fiber extends. This basic 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.
The transmission characteristics of a typical FFP of length, l.sub.c, have been described, for example see the description in C. M. Miller U.S. Pat. No. 5,212,745. An FFP is tuned between successive resonance maxima by, for example, changing the optical cavity length, l.sub.c. (Alternatively, tuning of the 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.
J. Stone and L. W. Stulz (1987) Elect. Lett., 23(15):781-783 described three configurations of FFP interferometric filters. A Type I FFP is a long cavity FFP having mirrors deposited at the ends of a continuous fiber. Stretching the fiber, for example using piezoelectric transducers (PZTs), tunes the bandwidth (BW) over the free spectral range (FSR). A Type II FFP is a gap resonator, formed between two opposed mirror-ended fiber ferrules. Since there is no optical fiber inside the optical cavity, signal loss increases significantly with cavity length and the useful cavity length of this type of FFP is less than about 5 .mu.m. A Type III FFP has an internal fiber-containing waveguide positioned between two fiber ferrules each of which has a fiber end. The optical cavity is formed between a mirrored-end of one ferrule and a mirror at the end of the waveguide remote from the mirror-ended ferrule and the fiber of the waveguide is within the optical cavity. There is also at least one fiber gap within the cavity, for example between the waveguide and the mirror-ended ferrule. The length of the fiber gap can be varied to tune the filter.
The type III FFP is generally better suited to telecommunication applications than either of the other types of FFPs.
The ferrule components and waveguide of Type II and III FFPs must be axially aligned to high precision in order to minimize transmission loss. Type II and III FFPs tuned using PZTs to change cavity length are the subject of U.S. Pat. No. 4,861,136. Elaborate alignment brackets and fixtures were necessary to change cavity length without detriment to fiber alignment.
U.S. Pat. No. 5,062,684 describes tunable or fixed FFP filters in which the resonance cavity is formed by two wafered ferrules with mirrors embedded between the wafer and the ferrule and axially disposed optical fibers. The two ferrules are positioned in the filter configuration with mirrors opposed and the optical fibers of the ferrules aligned. The resonance cavity formed between the embedded mirrors contains a fiber gap between the wafered ends of the ferrules. The ferrule combination is held in alignment by an alignment fixture which can include PZTs to change the cavity length and tune the filter. A support fixture useful for holding a FFP ferrule assembly in axial alignment which includes PZTs for changing the cavity length and means for adjustment of alignment is described in EP patent application 0 457 484.
C. M. Miller U.S. Pat. No. 5,212,746 describes improved fixed or tunable single-wafered ferrule FFP filters in which an optical cavity is formed between a mirror-ended ferrule and a wafered ferrule with an embedded mirror.
G. F. De Veau and C. M. Miller U.S. Pat. No. 4,545,644 describe a rotary mechanical splice ferrule alignment fixture. 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 and the fibers of the ferrules aligned therein using a rotary alignment technique as described in the patent. The "flat" portions on the alignment rods provide an alignment fixture offset necessary for rotary alignment. Once alignment is adjusted it is maintained by establishing a multi-point (preferably three-point) pressure contact of the alignment rods with the ferrule. 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.
In prior art FFPs tuned with PZTs, the fibers in a ferrule assembly are aligned by adjusting the relative tightness of set screws around the circumference of one or both of the ferrule holders. These screws contact the ferrule directly or indirectly via an intermediate sleeve. See EP patent application 0 457 484, U.S. patent 19-91, 20-91, 2-92 and 24-93.
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. This can represent a drift of a full FSR (free spectral range) over 15.degree. C. See C. M. Miller and F. J. Janniello (1990) Electronics Letters 26:2122-2123.
Control circuitry can be used with PZT-tuned FFPs to lock the filter onto a 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). These systems can require control voltage swings of several tens of volts to compensate for variations of cavity length with temperature. A high voltage power supply capable of providing 60 volts was needed to maintain a wavelength lock over a temperature range of about 30.degree. C. (Fishman et al. supra).
Miller and Janniello (1990) supra describe passive temperature compensation of PZT-tuned FFPs using aluminum blocks. Since PZTs require a higher voltage at higher temperature to maintain a given length, cavity length effectively decreases with increasing temperature (with constant voltage). Thus, a PZT-tuned FFP has a negative temperature coefficient. 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. Passive compensation significantly reduced the voltage requirements for FFP locking circuits so that +/- 12 volt power supplies, such as are conventionally employed in computer systems, can be employed for locking.
C. M. Miller et al. U.S. patent application Ser. No. 07/929,836, now allowed, reports that it is important to use controlled thicknesses of positive temperature coefficient adhesives, such as epoxy, when constructing FFPs to obtain consistent temperature compensation. This application also described ferrule holders for use in PZT-tuned FFPs in which the temperature coefficient of the FFP can be adjusted after its construction by changing the points of contact between the ferrule and its holder. This technique significantly improved the production yield of highly accurate, passively compensated FFPs significantly reducing over or under compensation of the FFPs. PZT-tunable FFPs which display wavelength drift less than 1 FSR/100.degree. C. (-25.degree. C. to 85.degree. C.) and less than 1 dB insertion loss over the same temperature range have been constructed. These FFPs, however, employ set screw adjustment for fiber alignment. This method requires more time and skill to achieve good alignment and is less accurate and stable than the rotary alignment method.
Thus, there is a need in the art for alignment fixtures and component ferrule holders for PZT-tunable FFPs having improved means for fiber alignment which retain excellent thermal and optical properties.
In some applications, such as for mounting on circuit boards, it is desirable to minimize the size of an FFP. Circuit board application require FFPs less than or equal to about 6.8 mm in height, due to standard board spacing. Thus, there is a general need in the art for FFPs which can be miniaturized for such applications.