Abstract:
An all-fiber, optically-tunable, narrowband optical loop-mirror filter ( 10 ) that makes use of a loop-mirror ( 24 ). The optical filter ( 10 ) includes a coupler ( 12 ), including two input fiber ports ( 14, 16 ) and two output fiber ports ( 18, 20 ) that are connected together to form the loop-mirror ( 24 ). A saturable medium ( 22 ) is positioned in the loop-mirror ( 24 ), and can be either a saturable absorber medium or a saturable gain medium. A potentially broadband optical input signal to be filtered and a pump are applied to the input ports ( 14, 16 ) of the coupler ( 12 ). The counter propagating signals in the loop-mirror ( 24 ) generated by the input signal and the pump create two standing wave interference patterns in the saturable medium ( 22 ). Depending on whether the input signal and the pump are applied to the same input port ( 14, 16 ) or different input ports ( 14, 16 ) determines whether the two standing wave interference patterns are in phase with each other or Ξ radians out of phase with each other. If the saturable medium ( 22 ) is a saturable gain medium, then the peaks of the pump standing wave interference pattern bleach the gain of the filter ( 22 ) at the peak locations, preventing amplification of the input signal at these locations. Likewise, if the saturable medium ( 22 ) is a saturable absorber medium, the peaks of the pump interference pattern bleach the absorption of the saturable medium ( 22 ) at the peak locations, leaving rest of the saturable medium ( 22 ) to provide saturation. Therefore, the filter ( 10 ) can be either a bandpass filter or a notch filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an all-fiber, narrowband optical filter and, more particularly, to a dynamic all-fiber, loop-mirror optical filter that provides filtering by a pump-induced saturable gain grating or a saturable absorber grating. 
     2. Discussion of the Related Art 
     Optical filters are an integral part of most optical systems. All-fiber, optically-tunable filters, including narrowband optical filters are beneficial in many of these optical systems, for example, in optical spectrum analyzers, RF signal processing in analog optical communications links, subcarrier multiplexing and demultiplexing in optical networks, subcarrier removal in digital optical networks where subcarriers are used for signaling, subcarrier signal processing in subcarrier-multiplexed optical fiber communications, ASE-noise removal, and channel routing and monitoring in wavelength-division-multiplexed (WDM) communication networks. All-fiber filters are important to reduce insertion losses in the optical system, and reduce the size and weight of the system. 
     Fiber Fabry-Perot filters, thin-film dielectric interference filters, conventional fiber Bragg gratings, acousto-optic tunable filters, and arrayed-waveguide grating routers represent the current technology available to provide optical filtering. These techniques and approaches, however, all suffer from one or more drawbacks, including the difficulty to provide bandwidths less than 1 GHz, operation only at fixed wavelengths or over a limited tuning range, requirements for mechanical or temperature tuning, inherent temperature sensitivity, non-fiber design, and finite free spectral range or periodicity. These various optical filtering techniques, as well as other known optical filtering techniques, are discussed and compared in the article by D. Sadot and E. Boimovich, “Tunable Optical Filters For Dense WDM Networks,” IEEE Communications Magazine, December 1998, pgs. 50-55. 
     Investigations have been previously performed in the art using low-concentration erbium-doped fibers as a saturable medium to attempt to develop bandpass optical filters based on both saturable gain and saturable absorber gratings. See for example, S. J. Frisken, “Transient Bragg Reflection Gratings in Erbium-Doped Fiber Amplifiers,” Optics Letters, Vol. 17, pp. 1776-1778, Dec. 15, 1992; B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, “Nonlinear four-wave mixing in erbium-doped fiber amplifiers,” Electronics Letters, Vol. 29, pp. 1858-1859, Oct. 14, 1993; and B. Fischer, J. L. Zyskind, J. W. Sulhoff, and D. J. DiGiovanni, “Nonlinear wave mixing and induced gratings in erbium-doped fiber amplifiers,” Optics Letters, Vol. 18, pp. 2108-2110, Dec. 15, 1993. However, the results proposed in these papers are limited, and complicated system configurations prevent their practical application. 
     The effectiveness of bandpass filters based on pump-induced saturable absorber gratings has been demonstrated for laser linewidth narrowing and a theory to explain its effect has been developed. See, for example, M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, “Narrow-linewidth, singlemode erbium-doped fiber laser with intracavity wave mixing in saturable absorber,” Electronics Letters, vol. 30, pp. 648-649, Apr. 14, 1994; M. Horowitz, R. Daisy, B. Fischer, and J. Zyskind, “Linewidth-narrowing mechanism in lasers by nonlinear wave mixing,” Optics Letters, vol. 19, pp. 1406-1408, Sep. 15, 1994; Y. Cheng, J. T. Kringlebotn, W. H. Loh, R. I. Laming, and D. N. Payne, “Stable single-frequency traveling-wave fiber loop laser with integral saturable-absorber-based tracking narrow-band filter,” Optics Letters, vol. 20, pp. 875-877, Apr. 15, 1995; and M. Horowitz, R. Daisy, and B. Fischer, “Filtering behavior of a self-induced three-mirror cavity formed by intracavity wave mixing in a saturable absorber,” Optics Letters, vol. 21, pp. 299-301, Feb. 15, 1996. 
     What is needed is an all-fiber, optically-tunable, narrowband optical filter that does not suffer from the drawbacks mentioned above. It is therefore an object of the present invention to provide such a filter. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, an all-fiber, optically-tunable narrowband optical filter is disclosed that makes use of a loop-mirror. The optical filter includes a coupler having two input fiber ports and two output fiber ports, where the output ports are connected together to form the loop-mirror. An optical saturable medium is positioned in the loop-mirror, which is either a saturable absorber medium or a saturable gain medium. A potentially broadband optical signal light to be filtered and a pump light are applied to the input ports of the coupler. The counter-propagating light waves in the loop generated by both the input signal light and the pump light create a signal standing wave interference pattern and a pump standing wave interference pattern in the saturable medium. Whether the input signal and the pump are applied to the same input port or different input ports determines whether the two standing wave interference patterns are in phase with each other or π radians out of phase with each other, as set by the operation of the loop-mirror. 
     If the saturable medium is a saturable gain medium, then the peaks, or nodes, of the pump interference pattern bleach the gain of the saturable medium at the peaks, preventing amplification of the input signal at those locations. Likewise, if the saturable medium is a saturable absorber medium, the peaks of the pump interference pattern bleach the absorption of the saturable medium at the peaks, preventing absorption of the input signal at those locations. Therefore, depending on whether the saturable medium is a saturable absorber medium or a saturable gain medium, and whether the input signal interference pattern and the pump interference pattern are in phase or π radians out of phase with each other, the filter is a bandpass filter or a notch filter. 
     Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view of a dynamic fiber loop-mirror filter employing a pump-induced saturable gain or saturable absorber grating, according to an embodiment of the present invention; 
     FIG. 2 is a schematic plan view of a dynamic fiber loop-mirror filter employing a pump-induced saturable gain grating, according to another embodiment of the present invention; 
     FIG. 3 is an experimental set-up incorporating the dynamic fiber loop-mirror filter shown in FIG. 1, according to the invention; 
     FIG. 4 is a graph with response in dB on the vertical axis and signal frequency offset from the pump frequency in GHz on the horizontal axis, showing the theoretical response of a bandpass loop-mirror filter employing a saturable absorber grating for three different erbium-doped fiber lengths, according to the invention; 
     FIG. 5 is a graph response in dB on the vertical axis and signal frequency offset from the pump frequency in MHz on the horizontal axis, showing the theoretical response of a notch loop-mirror filter employing a saturable gain grating for three different erbium-doped lengths, according to the invention; 
     FIG. 6 is a graph with response in dB on the vertical axis and signal frequency offset from the pump frequency in MHz on the horizontal axis, showing the measured response of a tunable notch loop-mirror filter employing a saturable gain grating for three different pump wavelengths, according to the invention; and 
     FIG. 7 is a graph with response in dB on the vertical axis and signal frequency offset from the pump frequency in MHz on the horizontal axis, showing the measured response of a tunable bandpass loop-mirror filter employing a saturable absorber grating for three different pump wavelengths, according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a fiber loop-mirror filter employing one of either a saturable gain grating or a saturable absorber grating is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
     FIG. 1 is a schematic view of a fiber, loop-mirror filter (LMF)  10 , according to the invention, that includes a coupler  12 , such as a 50/50 or 3 dB coupler. The coupler  12  includes a first fiber optic input cable  14  and a second fiber optic input cable  16 , and a first fiber optic output cable  18  and a second fiber optic output cable  20 . The output cable  18  is the direct propagation route for light from the input cable  14  through the coupler  12  and the output cable  20  is the coupled route for light from the input cable  14  through the coupler  12 . Likewise, the output cable  20  is the direct propagation route for light from the input cable  16  through the coupler  12  and the output cable  18  is the coupled route for light from the input cable  16  through the coupler  12 . The first and second output cables  18  and  20  are connected to opposite ends of a saturable medium  22  to define a closed loop  24 . 
     If the saturable medium  22  was removed from the loop  24  and the output cables  18  and  20  were directly connected, the loop  24  would define a loop-mirror, where an optical input signal on one of the input cables  14  and  16  is coupled across the coupler  12 , and passes through the loop  24  and back through the coupler  12  to be a reflection of the input signal on the same cable  14  or  16 . Particularly, an optical input signal applied to the input cable  14  passes directly through the coupler  12  to the output cable  18 , and is also coupled by the coupler  12  into the output cable  20 . This optical coupling causes the input signal on the output cable  20  to be phase shifted by π/2 relative to the input signal propagating on the output cable  18 . When the counter propagating signals traveling through the loop  24  return to the coupler  12 , the same coupling and phase shifting occurs. The π/2 phase shifted signal enters the coupler  12  from the output cable  18 , passes directly into the input cable  14  with the π/2 phase shift, and is coupled into the input cable  16  with a π phase shift relative to the input signal. Likewise, the signal enters the coupler  12  through the output cable  20 , passes directly to the input cable  16  with no phase shift, and is coupled into the input cable  14  with a π/2 phase shift. Therefore, the two signals entering the input cable  14  constructively interfere with each other when they exit the coupler  12  and travel back through cable  14 . The two signals entering the input cable  16  are phase shifted π radians apart, and destructively interfere so that essentially no signal is on the input cable  16 . 
     The saturable medium  22  can be either a saturable gain medium or a saturable absorber medium, according to the invention. In one example, the saturable medium  22  is a length of an erbium-doped optical fiber, but can be any saturable optical medium suitable for the purposes described herein. When the saturable medium  22  is operating as a saturable absorber medium, light propagating therethrough is absorbed by the saturable medium  22  when the intensity of the light is below a predetermined threshold intensity that is determined by the doping concentration and physical limitations of the saturable medium  22 . When the light exceeds the predetermined intensity, the absorber material of the saturable medium  22  saturates or “bleaches”, making it transparent and causing light to pass through unabsorbed. When the saturable medium  22  is operating as a saturable gain medium, a filter activation signal of a certain frequency is applied to the saturable medium  22  to cause the dopant ions to be excited to higher energy states. When an optical signal passes through the saturable medium  22 , the photons in the signal interact with the dopant ions, which causes other duplicate photons to be released from the medium by stimulated emission, and causes the ions to return to a lower energy state. As more photons are released, more photons are available to drive the stimulated emission, thus increasing the optical intensity of the signal. The frequency of the activation signal is dependent on the dopants in the saturable medium. The intensity of the optical signal increases to a predetermined maximum gain level supported by the saturable gain medium. 
     If an input signal is coupled into the loop  24  through one of the input cables  14  or  16 , an input signal standing wave interference pattern is created in the loop  24  by the counter-propagating input signal waves. If a second optical signal is coupled into the loop  24  through one of the input cables  14  or  16 , referred to herein as a pump, a second set of counter-propagating waves is generated that creates a pump standing wave interference pattern in the loop  24 . The standing wave interference patterns are sinusoidal waves that are fixed relative to the loop  24 . The pump interference pattern defines an optical grating in the saturable medium  22 . 
     In this discussion, the input signal can be a broadband signal that is to be filtered, and the pump is a narrowband optical signal which creates the saturable gain or saturable absorber grating that does the filtering. If the pump has an amplitude significantly greater than the amplitude of the input signal, its interference pattern will not be disturbed by the interference pattern created by the input signal counterpropagating waves. Although the input signal is possibly broadband and thus creates multiple standing waves in the loop  24  at its various frequencies, the input signal standing wave interference pattern referred to herein is the standing wave interference pattern generated by the frequencies of the input signal that coincide with the frequencies of the pump. 
     The π/2 phase shift generated by the coupler  12  and the operation of the loop-mirror as discussed above, creates a known phase relationship between the two standing wave interference patterns that provides filtering. By selecting which of the two input cables  14  or  16  is used to input the pump and the input signal into the loop  24 , the interference patterns created by the two sets of counter-propagating waves are either in phase or π radians out of phase with each other. The relative phase between the input signal and the pump at the loop input does not effect this relationship. As will be discussed below, the phase relationship between the input signal standing wave interference pattern and the pump standing wave interference pattern causes optical filtering to occur. 
     Generally, the optical filtering occurs because the peaks of the pump standing wave interference pattern bleach the absorption of the saturable medium  22  at their respective locations when the saturable medium  22  is a saturable absorber medium, and the peaks of the pump standing wave interference pattern bleach the gain in the saturable medium  22  at their respective locations when the saturable medium  22  is a saturable gain medium. The regions in the saturable medium  22  between the peaks of the pump standing wave interference pattern are still available to provide absorption for the saturable absorber case, and gain for the saturable gain case. By either removing or passing the frequencies of the input signal that coincide with the frequencies of the pump through the optical absorption or gain process discussed above, the LMF  10  can be either a bandpass filter or a notch filter. The gain or absorption of certain frequencies of the input signal standing wave interference pattern in the saturable medium  22  can be provided by adjusting the frequency of the pump to provide tuning of the filtering process. This tuning process does not require mechanical or temperature tuning, as necessary in the prior art filters. Because the length of the induced grating can be very long (meters), the bandwidth of the filtering process can be significantly narrowed. 
     If the saturable medium  22  is a saturable gain medium and the pump and the input signal enter the loop  24  through the same input cable, for example cable  14 , the input signal standing wave interference pattern created by the frequencies of the input signal that are the same as the frequencies of the pump will be in phase with the standing wave interference pattern created by the pump. In this case, the peaks of the standing wave interference pattern of these input signal frequencies experience the regions in the saturable medium  22  where the gain has been used by the pump interference pattern, and therefore these frequencies pass through the loop  24  with less gain. In other words, the peaks of the sinusoidal-shaped pump interference pattern only bleach those regions of the saturable medium  22  where the peaks are formed, leaving the areas between the peaks in the saturable medium  22  available to provide gain. 
     The frequencies of the input signal that deviate from the frequencies of the pump do not satisfy this phase and periodicity matching condition, and thus these frequencies will be amplified relative to the frequencies of the input signal that do match the pump. Therefore, the frequencies of the input signal that do match the frequencies of the pump are filtered from the input signal, thus creating a notch filter. The loop-mirror operation of the LMF  10  thus provides an amplified mirror copy of the input signal propagating in the opposite direction through the input cable  14 , with the frequencies of the input signal that match the frequency of the pump unamplified. Because the filtered output of the input signal and the pump will be output on the cable  14  in this example, and will have the same frequency, some form of modulation of the input signal is required to distinguish it from the pump. 
     If, however, the pump and the input signal enter the loop  24  through separate input cables  14  and  16 , the input signal standing wave interference pattern at the frequencies of the pump will be π radians out of phase with the pump standing wave interference pattern. In this case, the wavelengths of the input signal at the pump frequencies will align with the regions in the saturable medium  22  not effected by the peaks of the pump standing wave interference pattern. The frequencies of the input signal that match the frequencies of the pump are therefore amplified. As the frequencies of the input signal deviate from the pump frequencies, these input signal frequencies will begin to have more overlap with the regions where the pump has removed the gain of the saturable medium  22  and will not amplify as strongly. Thus, a bandpass filter is provided because only the frequencies of the input signal that match the frequencies of the pump are amplified. Therefore, if the input signal is applied to the input cable  14 , the loop-mirror operation of the LMF  10  will provide an amplified mirror copy of the input signal only at the frequencies matching the pump. 
     When the saturable medium  22  is a saturable absorber medium and the pump and the input signal enter the loop  24  through the same input cable  14 , where the interference patterns are in phase, the frequencies of the input signal that match the frequencies of the pump are not strongly absorbed by the saturable medium  22  because the pump has bleached the saturable medium  22  at the peaks of the pump interference pattern. As the frequencies of the input signal move away from the frequencies of the pump, these frequencies will be absorbed by the saturable medium  22 . Therefore, because the frequencies of the input signal that match the frequencies of the pump are not absorbed, the LMF  10  acts as a bandpass filter. 
     If the pump and the input signal are applied to the different input cables  14  and  16 , the portion of the input signal having frequencies the same as the pump frequencies are absorbed because the peaks of the input signal interference pattern are between the peaks of the pump interference pattern. Other frequencies of the input signal fall in the regions where the peaks of the pump and signal interference patterns are not synchronized and hence are not absorbed as strongly. In this embodiment, the LMF  10  acts as a notch filter. 
     A more complex filter can be synthesized and tailored by different means. The saturable gain parameters, including the saturation intensity and length, as well as the pump beam intensities, can change the filter parameters. In addition, by using a more complex pump, such as a multi-wavelength source with different and controllable intensities for each wavelength component, a complex and controllable filter could be created. 
     FIG. 2 shows an LMF  26  similar to the LMF  10  above, where the saturable medium  22  is a saturable gain medium  28  that is positioned in a loop-mirror  30  connected to a 3 dB coupler  32 . The LMF  26  operates in the manner discussed above for the saturable gain case. In this embodiment, a 980 nm or 1480 nm optical activation signal from, for example, a diode laser  34  is applied to a WDM coupler  36  to excite the gain medium  28  to provide the gain. A 980 nm or 1480 nm excitation signal is used in this embodiment because the saturable medium  28  is an erbium-doped fiber. Other saturable gain media may require other excitation wavelengths. Polarization controllers  38  and  40  are positioned in the loop  30  at opposite ends of the saturable medium  28  to allow maximization of the interference between the counter-propagating waves. Because the state-of-polarization of an optical signal propagating down a fiber tends to rotate, the polarization controllers  38  and  40  cause the counter-propagating waves to have the same polarization in the gain medium  28  so that when the waves interfere, the desired standing wave is created. Only one or no polarization controller may be required in certain embodiments. Alternately, polarization maintaining fibers can be used in the loop-mirror  30 , and the controllers  38  and  40  can be eliminated. A circulator  42  is positioned in one of the input fiber cables  44 , and couples the filtered signal received through the coupler  32  from the LMF  26 . Circulators of this type are well known in the art for removing one of counter-propagating signals. 
     FIG. 3 shows a plan view of an experimental setup for measurement of the characteristics of an LMF  48 , using a saturable absorber medium  50 . A tunable laser  52  emits a laser signal to an optical splitter  54  and a first split signal is applied to a modulator  56  that modulates the signal and applies it through a circulator  58  to one input port of a coupler  60 . A second split signal from the split  54  is applied to another input port of the coupler  60 . Because the input signals to the LMF  48  are applied to separate input ports, the two standing wave interference patterns will be n radians out of phase with each other, and the filter will be a notch filter. The length of the medium  50  is selected based on the desired bandwidth. The filtered output signal from the circulator  58  is applied to a detector  62  to detect the output signal, and a signal from the detector  62  is applied to a network analyzer  64 . The network analyzer  64  is used to make swept-frequency measurements and provides the RF drive signal to the modulator  56  to modulate the first split signal. The modulator  56  is driven at a relatively high power and the modulator bias is adjusted so all the power in the optical carrier is shifted into the sidebands. As the RF frequency input to the modulator  56  is swept, the sidebands probe the filter response. The magnitude of the sideband signals was adjusted to about 40 dB below the pump power. 
     FIG. 4 is a graph with filter response in dB on the vertical axis and signal frequency offset from pump frequency in GHz on the horizontal axis, showing the theoretical response of an LMF of the invention that employs a saturable absorber grating for three different erbium-doped fiber lengths. In this example, the input signal and the pump are applied to the same input port of the LMF so that the LMF acts as a bandpass filter. As is apparent, as the frequency of the input signal moves away from the frequency of the pump, the input signal is not amplified as strongly. 
     FIG. 5 is a graph with response in dB on the vertical axis and signal frequency offset from the pump frequency in MHz on the horizontal axis, showing the theoretical response for an LMF of the invention that employs a saturable gain grating for three different erbium-doped fiber lengths. In this example, the input signal and the pump are applied to the same input port of the LMF so that the LMF acts as a notch filter. As is apparent, as the frequency of the input signal moves away from the frequency of the pump, the input signal is increasingly amplified. 
     FIG. 6 is a graph with filter response in dB on the vertical axis and signal frequency offset from pump frequency in MHz on the horizontal axis that shows a measured response of a tunable LMF of the invention that employs a saturable gain grating filter for three different pump wavelengths. In this example, the input signal and the pump are applied to the same input port so that the LMF acts as a notch filter. As is apparent, as the frequency of the input signal moves away from the frequency of the pump for the three different pump wavelengths, the input signal is increasingly amplified. 
     FIG. 7 is a graph with filter response in dB on the vertical axis and signal frequency offset from pump frequency in MHz on the horizontal axis that shows a measured response for an LMF of the invention that employs a saturable absorber grating for three different pump wavelengths. In this example, the input signal and the pump are applied to the same input port so that the LMF acts as a bandpass filter. As is apparent, as the frequency of the input signal moves away from the pump frequency, the filtering input signal is increasingly attenuated. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.