Abstract:
A multireflector fiber optic filter apparatus, wherein the transmittance and reflectance spectra are periodic in frequency, the apparatus comprising an etalon with N equally spaced reflectors wherein the transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: ΔνFSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the medium between the reflectors, L=separation between said reflectors, and N is an integer=3,4,5, . . . In a further aspect, an optical circulator is connected to the etalon and an optical fiber is connected to the optical circulator for reflected output.

Description:
CROSS REFERENCE TO RELATED PROVISIONAL PATENT APPLICATION 
     This non-provisional patent application is related to provisional patent application No. 60/259,894 filed on Jan. 5, 2001. Applicant hereby claims the benefit of the related provisional application and the entire provisional application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a multireflector fiberoptic filter apparatus and method. In particular, the invention relates to a multireflector fiberoptic filter apparatus and method, wherein the transmittance and reflectance spectra are periodic in frequency, including an etalon with N equally spaced lossless reflectors with a specified period spacing between them. 
     BACKGROUND OF THE INVENTION 
     The Fabry-Perot interferometer (FPI), sometimes called the Fabry-Perot etalon, consists of two mirrors separated by a distance L. Since its invention about 100 years ago, the bulk-optics version of the FPI has been widely used for high-resolution spectroscopy. In the early 1980&#39;s, fiber optic FPIs consisting of mirrored fiber ends separated by an air gap were introduced as a filter technology for optical communications. 
     The simple, compact fiber FPI filters have proven very useful in lightwave communications, but suffer from a fundamental drawback: the gradual (6 dB/octave) dropoff of transmittance with optical frequency away from the transmittance peak. A filter with a flatter in-band response and greater out-of-band rejection would make it possible to increase the channel density and decrease the filter-related power penalty in dense wavelength division multiplexing (WDM) systems. 
     WDM is widely used in fiber optic communication to increase the data capacity of an optical fiber. Currently,  16 ,  32 , or more data channels are transmitted in parallel on a single mode fiber using different optical carrier frequencies for each channel. To combine and separate these channels, a variety of frequency-selective components have been developed, including multilayer dielectric coatings, fiber Bragg gratings, arrayed waveguide gratings, and Mach-Zehnder chains. None of these techniques satisfies industry requirements for low cost, high-speed tunability, low optical insertion loss, and small size. 
     Thus, there is a need in the art for providing an apparatus and method for a fiber optic filter that is inexpensive to produce and that has high-speed tunablity, low optical insertion loss and is small in size. 
     SUMMARY OF THE INVENTION 
     Accordingly, the multireflector fiberoptic filter apparatus and method of the present invention includes an etalon with N equally spaced reflectors, wherein the transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: (Δν)FSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the medium between the reflectors; L=the separation between reflectors; and N is an integer=3,4,5, . . . . 
     In other aspects of the invention, the etalon is comprised of a serial arrangement of single mode optical fibers of a common length L, each with a reflector deposited on one end. In other aspects of the invention, the single mode optical fibers are aligned in tubes or in V-grooves formed on a silicon substrate, and are interfaced to one another end-to-end by means of one from a group including optical cement, epoxy, fusion splicing, or refractive-index-matching liquid. 
     In another aspect of the invention, the reflectors are multilayer quarterwave stacks comprising alternating layers of high and low index dielectric materials. In another aspect of the invention, the dielectric materials are chosen from a group including TiO2 and SiO2. 
     In yet another aspect of the invention, the reflectors are mirrors. In a further aspect of the invention, an optical circulator is connected to the etalon and an optical fiber is connected to the optical circulator for reflected output. In another aspect of the invention, a plurality of etalons are connected in series. In another aspect of the invention, a strain inducing device is attached to the etalon. In a further aspect of the invention, the strain inducing device is a piezoelectric element attached along the length of the etalon. In another aspect of the invention, delay sections are added between etalons. 
     In another embodiment of the invention, a fiber optic system contains a first input fiber, a second transmitted output fiber and a third reflected output fiber, connected to a multireflector etalon for separating equally spaced frequency channels into two groups—one transmitted and the other reflected. In said system, the transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: (Δν)FSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the optical fiber medium between the reflectors; and L=the separation between reflectors. Said etalon is comprised of the serial arrangement of single mode fibers of a common length L separated by N reflectors, N=3,4,5, . . . , and is connected to the second transmitted output fiber. The transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: (Δν)FSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the medium between the reflectors. An optical circulator is connected to the first input fiber, the etalon, and the third reflected output fiber. 
     In another aspect of the invention, a plurality of etalons is provided in series. In a further aspect of the invention, fiber delay sections are added between the etalons. 
     In another embodiment of the invention, in a single mode optical fiber in which optical frequency channels are equally spaced, a method for wavelength division multiplexing is provided where a subgroup of the equally spaced frequency channels are selected for transmission and the remainder are reflected wherein the transmittance and reflectance spectra are periodic in frequency, the method including the step of providing an etalon with N equally spaced reflectors. The transmittance and reflectance spectra of said etalon are periodic in optical frequency with a period given by the formula: (Δν)FSR=c/(2ngL), where c=the free space speed of light; ng=the group refractive index for the light propagating in the optical fiber medium between the reflectors. A single mode optical fiber is connected to the etalon. The etalon is connected to an output transmission fiber and equally spaced optical frequency channels are transmitted from the optical fiber to the etalon. 
     In a further aspect of the method, an optical circulator is connected between the single mode optical fiber and the etalon and a reflected output channel is connected to the optical circulator. In another aspect of the method, a tuning device is added to the etalon. In a further aspect, a plurality of etalons are provided and fiber delay sections are included between the etalons. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiment, the appended claims and the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of the multireflector fiberoptic filter apparatus of the present invention in a single mode optical waveguide; 
     FIG. 2 is a schematic illustrating the ideal transmittance and reflectance spectra for a filter designed for use in wavelength division multiplexing in which optical frequency channels are equally spaced; 
     FIG. 3 is a schematic illustrating the calculated transmittance spectra for three cases: N=2, R 1 =R 2 =0.962; N=4, R 1 =R 4 =0.695, R 2 =R 3 =0.984; N=6, R 1 =R 6 =0.448, R 2 =R 5 =0.941, R 3 =R 4 =0.981; 
     FIG. 4 is a schematic illustrating the use of the multireflector filter apparatus of the present invention in conjunction with an optical circulator to separate equally spaced frequency channels into two groups-one transmitted by the filter and the other reflected; 
     FIG. 5 is a schematic illustrating the configuration of the present invention for simultaneously switching equally spaced frequency channels; 
     FIG. 6 is a schematic illustration of tuning a multimirror etalon of the present invention to transmit (upper plot) or reflect (lower plot) all of the equally spaced frequencies in a WDM system; 
     FIG. 7 is a schematic illustration of the calculated plot of reflectance vs. frequency for multireflector etalons for the following three cases: N=2 and mirror reflectances R 1 =R 2 =0.0245; N=4 and mirror reflectances R 1 =R 4 =0.0523, R 2 =R 3 =0.3418; and N=6 with mirror reflectances R 1 =R 6 =0.039, R 2 =R 5 =0.389, and R 3 =R 4 =0.689; and 
     FIG. 8 is a schematic illustration of the present invention utilized to effect a variable delay line for WDM application. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention is illustrated by way of example in FIGS. 1-8. The multireflector fiberoptic filter  10  of the present invention is illustrated in FIG.  1 . Reflectances of the individual equally spaced reflectors  12  as well as the separation between them are regarded as design variables which can be chosen to control the spectral dependence of the transmitted light wave  14  and the reflected light wave  16  amplitudes from the etalon  18 . A discussion of such design variables is described in the document “Design of Multireflector Resonant Bandpass Filters for Guided Wave Optics.” written by applicant and incorporated herein by reference. 
     Referring now to FIG. 2, the ideal transmittance and reflectance spectra for a filter  10  designed for use in wavelength division multiplexing in which optical frequency channels are equally spaced is illustrated. As illustrated, in operation, the filter  10  of the present invention selects a subgroup of equally spaced frequencies for transmission; the other frequency channels are reflected. 
     Multireflector etalons  10  as illustrated in FIG. 1 can be implemented by a serial arrangement of single mode fibers  20 , each of which has a mirror  12  deposited on one end. Prior to mirror deposition the fibers  20  are mass-polished to a common length L. The mirrors  12  could be multilayer quarter-wave stacks comprising alternating layers of high- and low-index dielectric materials, such as TiO2 and SiO2. Further, the mirrored fibers  20  can be aligned in tubes  22  or in Si V-grooves and joined end-to-end with optical cement or epoxy. Alternatively, the mirrored fiber  20  can be permanently joined together by fusion splicing. 
     For any etalon  18  with equally spaced reflectors  12 , the transmittance and reflectance spectra are periodic in frequency, with the period—known as the free spectral range (Δν)FSR—given by: 
     (Δν)FSR=c/(2ngL), 
     with c the free space speed of light=2.998×108 m/s, ng the group refractive index of the waveguide mode, and L the separation between reflectors. For example, if the waveguide is a single mode optical fiber  20  with ng=1.46, and L=1 mm, then (Δν)FSR=102.7 Ghz. Idealized transmittance T and reflectance R spectra for a filter  10  for use in wavelength division multiplexing (WDM) is shown in FIG.  2 . The square profiles for the spectra ensure that crosstalk between WDM channels is minimized. 
     Calculated filter transmittance spectra for N=2, 4, and 6 are shown in FIG.  3 . In each case the mirror reflectances are chosen to give alternating spectral regions of high and low transmittance such that the transmittance is &lt;−20 dB (i.e., &lt;1%) over a spectral region of width equal to ⅛ (12.5%) of the free spectral range. (The calculations are further described in the applicant&#39;s article “Design of Multireflector Resonant Bandpass Filters for Guided Wave Optics.”) The plots of FIG. 3 show that increasing the number of reflectors  12  makes it possible for the spectral characteristics of a multireflector etalon  18  to approach more closely to the ideal characteristics of FIG.  2 . 
     An optical circulator  24  in series with a multireflector fiber etalon  18  can be used to spatially separate transmitted frequency channels  14  from reflected channels  16 , as illustrated in FIG.  4 . From the input channel  26 , the light incident in one fiber passes through the circulator  24 , and one group  14  of equally spaced optical frequency channels is transmitted by the etalon  18 . The other frequency channels  16  are reflected by the etalon  18 , pass back through the circulator  24 , and exit through the third fiber, reflected output  16  . The function performed by the filter  10  of the present invention in this embodiment is sometimes termed “spectral slicing” or “deinterleaving”. 
     A circulator  24  in series with the multireflector fiber optic filter  10 , as in FIG. 5, can also be used for switching all the channels in a WDM system simultaneously, provided that the spacing between the frequency channels {νj} is equal to (Δν)FSR. Spectral dependence of reflectance for the two states of such a switch  10  are shown in FIG.  6 . In one state of the switch  10  (upper plot in FIG.  6 ), none of the light is reflected at any of the frequencies {νj}. In the other state, (lower plot in FIG. 6) all of the light is reflected in each of the channels. The spectra in FIG. 6 are the same except for a lateral translation in optical frequency of 0.5 (Δν)FSR. To achieve that frequency translation, it is necessary to produce a π-rad phase shift in Δφ, the round trip phase shift in the etalon, which is given by 
     Δφ=4πnLν/c, 
     Thus, tuning by 0.5 (Δν)FSR requires that 
     Δ(nL)=c/(4ν) 
     At a wavelength of 1550 nm, then ν=2.998×108/1550×10−9=1.93×1014 Hz, so Δ(nL)=388 nm. 
     The calculated dependence or reflectance on optical frequency is plotted in FIG. 7 for the cases that N=2, 4, and 6. For each value of N, mirror  12  reflectance values are chosen such that the refletance spectra approach the ideal shape of FIG. 6 as closely as possible. Thus, the cases represented in FIG. 7 correspond to designs for a device  10  for switching equally-spaced wavelength channels. It is important to note that the reflectance spectra approach the ideal spectrum of FIG. 6 more closely as N increases, 
     Again, FIG. 7, shows the calculated plot of reflectance vs. frequency for multireflector etalons  18  of the present invention for the following three cases: N=2 and mirror reflectances R 1 =R 2 =0.0245; N=4 and mirror reflectances R 1 =R 4 =0.0523, R 2 =R 3 =0.3418; and N=6 with mirror reflectances R 1 =R 6 =0.039, R 2 =R 5 =0.389, and R 3 =R 4 =0.689. In each case the mirror reflectances are chosen to give alternating spectral regions of high and low reflectance such that the reflectance is &lt;−20 dB (i.e., &lt;1%) over a spectral region of width equal to 20% of the free spectral range. 
     Importantly, applicant has determined that the filter  10  of the present invention as described above can be tuned by changing the optical length nL of the etalon  18  sections  20  between adjacent mirrors  12 . A change in nL causes a translation in the transmittance and reflectance spectra along the frequency axis. One way of changing nL is to strain (stretch) the fiber  20 . The strain can be produced by attaching (bonding) a piezoelectric element  28  along the length of the fiber etalon  18 , as shown in FIG. 1, such that an applied voltage causes a length change in the piezoelectric element  28 , inducing a longitudinal strain in the fiber sections  20  which comprise the etalon  18 . If L=1 mm, as in the preceding example, and n=1.46, then nL=1.46 mm and a strain ΔL/L=388×10−9/1.46×10−3=265 μstrain induced in the fiber will produce the π-rad phase shift required for switching. 
     Still further, applicant has determined that a plurality of these etalons  18  arranged in series with fiber delay sections  30  between them can be used to implement a variable delay line, as illustrated in FIG.  8 . If one of the etalons  18  is reflecting and the others are transmitting, the reflected optical signal will replicate the incident signal but be delayed. The greater the length of fiber traversed in the fiber delay section  30  before the light is reflected, the greater will be the delay. As indicated earlier, if (Δν)FSR equals the interchannel spacing in a WDM system, each etalon  18  will either transmit or reflect all of the wavelength channels. Thus, except for the effect of fiber dispersion, each of the channels will experience the same delay. 
     By means of this invention, then, a user is enabled, among other things and for the purposes of illustration only,: (1) to select (deinterleave) a set of equally spaced optical frequency channels from a larger group of optical frequencies propagating in a single mode fiber and (2) to switch light propagating in an optical fiber in a set of equally spaced optical frequency channels from one path to another. Further, a variable delay line for wavelength multiplexed optical signals can be implemented using a multiplicity of the filters of the present invention in a serial arrangement. 
     The description of the present embodiments of the invention have been presented for purposes of the illustration but are not to be exhaustive or to limit the invention to the form disclosed. Applicant&#39;s multireflector filter is extraordinarily well suited for use in fiberoptic communication systems. Obviously, any type of multireflector etalon now known or hereafter developed is encompassed within the scope of the invention. Many modifications and variations will be apparent to those of ordinary skill in the art. As such, while the present invention has been disclosed in connection with the preferred embodiment thereof, it should be understood that there may be other embodiment all within the spirit and scope of the invention as defined by the following claims.