Abstract:
This invention relates to invention relates to a method for filtering and a variable slope optical filter for in-line use with an optical amplifier signal. The filter has a wavelength response that is substantially linear in slope within a band of operation wavelengths of the amplifier; the slope of the filter in an operating band of wavelengths is between zero and a only one of a positive or negative number; furthermore the filter has an amplitude response that has an opposite and counter slope as a function of wavelength to that of a gain tilt of the amplifier within the operation band of wavelengths. Relative movement between the filter and the optical amplifier signal is provided to vary the slope of the filter by means of varying the amplitude wavelength response of the filter.

Description:
This application claims benefit of provisional app. No. 60/095,646 filed Aug. 7, 1998. 
    
    
     This invention relates to a variable gain tilt control system for use with an optical amplifier, and more particularly with a rare earth doped optical fiber amplifier. 
     BACKGROUND OF THE INVENTION 
     Optical amplifiers and particularly erbium doped optical fiber amplifiers are nearly ubiquitous in optical transmission systems, particularly in the field of telecommunications. Erbium doped fiber amplifiers (EDFAs) have high polarization insensitive gain, low cross talk between signals of different wavelengths, good saturation output power, and a noise figure close to the fundamental quantum limit. The excellent noise characteristics allow hundreds of these amplifiers to be cascaded to cover spans of thousands of kilometers of optical fibre. EDFAs as opposed to electronic repeaters are also transparent to data rate, signal format, and wavelength over a limited range, making them useful for wavelength multiplexed (WDM) communication systems that simultaneously transmit a large number of signals using different wavelength bands for each signal. 
     Notwithstanding these generally excellent characteristics, a disadvantage associated with EDFAs is their narrow spectral width and uneven gain band. The useful telecommunications window of an EDFA is approximately 20-30 nm wide, while an ideal amplifier would have a flat spectral gain across the full spectrum which extends from approximately 1520 nm to 1570 nm. The peak wavelength of the erbium gain spectrum varies from about 1530 nm to about 1535 nm depending upon the host glass material. FIG. 1 shows the characteristic gain spectrum of a particular conventional EDFA where it can be seen that the gain as a function of wavelength varies; this variation will be referred to hereinafter as gain ripple. Numerous techniques have been published for widening and flattening the gain spectrum (i.e. reducing the ripple) and include for example co-doping an erbium-doped silica glass fibre with Al 2 O 3 ; changing the host glass material itself; using various forms of attenuating filters to reduce the gain at the emission peak; and, constructing hybrid devices having two or more different types of serially connected erbium doped fibre and actively adjusting pump conditions independently in each fibre section to compensate for the different gain slopes of each fibre. 
     In addition to the aforementioned problems and solutions associated with minimizing gain ripple, another significant problem exists to which there have been no simple, inexpensive, and practicable solutions. This other significant problem solved by this invention relates to improving dynamic gain tilt. The term dynamic gain tilt as used hereafter means the variation in gain at one wavelength as a result of changing the gain at any other wavelength via a change in input EDFA operating conditions. Although the techniques described above for minimizing gain ripple can provide a relatively flat spectrum in a specified wavelength band for a specific set of input optical powers and wavelength, the gain equalization performance degrades rapidly when the gain is changed (change in average population inversion levels) from the nominal conditions by changing the input power to the amplifier. One reported solution to this problem is allegedly achieved by a hybrid fibre device having cascaded amplifying stages with different gain spectra and an equal number of pump sources to allow the gain spectra of the individual stages to be effectively tuned independently so that when the total gain is changed, the relative contribution of each stage can be adjusted to arrive at the desired gain, with a resulting gain spectrum having a minimal amount of spectral distortion over the selected wavelength band. As an example, an erbium doped fibre having a positive gain slope may be combined with a different erbium doped fibre having a negative gain slope such that the hybrid device has a nearly flat gain at specific input power conditions. However, if the overall gain of the hybrid device must be changed, the gain slope of each of the constituent states will generally change at different rates when the pump power input to one of the stages is changed. In order to achieve good compensation at the new operating point, the relative gain of each of the constituent gain stages must be readjusted to make the gain slopes compensate each other. In implementing this type of amplifier, one skilled in the art would likely cascade two or more different erbium doped fibre compositions and provide a separate pump source for each amplifying stage at an end of each stage so as to minimize the number of splices and make it as convenient as possible to independently control the pump power to each stage. However, this technique for reducing or improving dynamic gain tilt requires a complex control scheme during operation in which the total power of multiple pump sources must be coordinated in order to realize gain slope compensation over a range of different gains (i.e. to change input power while maintaining a fixed target output power). 
     U.S. Pat. No. 5,764,406 in the name of Newhouse et al. entitled Hybrid Optical Amplifier Dynamic Gain Tilt incorporated herein by reference, describes a system wherein an erbium doped fibre amplifier device has a dynamic gain tilt that is less than the gain tilt of any of the constituent fibres. The hybrid device has at most one less pumping source than the number of constituent waveguides of the device. The hybrid device automatically provides a change in the pump distribution among the constituent doped waveguide sections so as to achieve a readjustment of the relative gains of the constituent sections. In one embodiment, this invention provides constituent EDFs of different co-dopant compositions that provides an automatic change in the pump distribution or partitioning among the constituent EDF sections so as to achieve a readjustment of the relative gains of the constituent EDF sections. 
     Although the &#39;406 patent appears to achieve its intended function, it is a relatively costly and is a complicated solution to dynamically controlling gain tilt. 
     Most known solutions for correcting for dynamic gain tilt have an associated power loss (approximately 5 dB) and furthermore, increased power is required for extra pumping. Another deleterious result of these systems is an increased resulting noise. 
     It is an object of this invention to provide a novel filter and method of use of such filter that can be placed in-line within an optical amplifier for varying the gain tilt dynamically as the power of the input signal is varied. 
     It is another object of the invention to provide an inexpensive filter for use in controlling tilt gain of an optical amplifier. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided, a variable slope optical filter for in-line use with an optical amplifier signal, the filter being disposed at an input side, an output side, or within the optical amplifier, the filter having a wavelength response that is substantially linear in slope within a band of operation wavelengths of the amplifier, the slope of the filter in said band of wavelengths being between zero and a only one of a positive or negative number, the filter for passing a band of wavelengths having a centre wavelength λc and having an amplitude response that has an opposite and counter slope as a function of wavelength to that of the amplifier&#39;s gain tilt within the operation band of wavelengths; and, 
     a means for providing relative movement between the filter and the optical amplifier signal to vary the slope of the filter within the band of operation wavelengths. 
     It should be noted, that relative movement may be actual displacement of one of the filter and the input port or, alternatively, by varying the relative angle between the input port and the filter. 
     In accordance with the invention there is further provided, an optical amplifier having coupled thereto, an optical filter for dynamic and variable gain tilt control for use with the amplifier, the filter in a predetermined wavelength band having a width of at least 10 nm where gain tilt control is required, having a centre wavelength λc and an amplitude response that has an opposite and counter slope as a function of wavelength to that of the amplifier within the predetermined wavelength band; and, 
     a controller and movement mechanism for varying the slope of the filter in response to a control within the predetermined band, the output response of the filter having a slope throughout the predetermined wavelength band which is always only one of negative and positive. 
     In accordance with the invention there is further provided, an optical filter for dynamic and variable gain tilt control of the signal, the filter in a wavelength band of operation where tilt control is required, having a centre wavelength λc and an amplitude response that has slope of an opposite sign as a function of wavelength to that of the amplified optical signal prior to passing through the optical filter within the predetermined wavelength band; and, 
     means for relatively moving at least one of the beam and the filter to vary the slope of the filter in a substantially linear manner within the predetermined band as the beam passes therethrough, the output response of the filter having a slope throughout the predetermined wavelength band which only negative or positive. 
     In accordance with another aspect of the invention, a method for controlling gain tilt of an amplified beam is provided, comprising the steps of: 
     relatively moving at least one of a filter having two different regions and a beam to be filtered by the filter, each region of the filter having a different amplitude response as a function of wavelength, said relative movement being performed in a controlled manner to vary the power of the beam with wavelength, so as to correct for undesired gain tilt of the amplifier, at least one of the regions of the filter having a response that is substantially sloped to offset tilt, the filter being moved such that the beam propagates through portions of both regions of the filter, the proportion of the beam propagating through the two distinct regions being varied to dynamically correct for undesired gain tilt of the amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will now be described in conjunction with the drawings, in which: 
     FIG. 1 is a graph of gain (dB) versus wavelength (nm) for a typical erbium doped amplifier; 
     FIG. 2 a  is a schematic diagram of an optical filter that functions in transmission in accordance with the invention wherein the filter is disposed between a pair of collimating/focusing lenses. 
     FIG. 2 b  is a graph of the output response of a portion of the optical filter shown in FIG. 2 a;    
     FIG. 2 c  is a front functional view of the optical filter of FIG. 2 a;    
     FIG. 3 a  is a schematic diagram of an alternative embodiment of a optical filter that functions in transmission in accordance with the invention; 
     FIG. 3 b  is a graph of the output response of a portion of the optical filter shown in FIG. 3 a;    
     FIG. 3 c  is a diagram of the filter shown in FIG. 3 a  wherein the relative position of the beam and the filter has changed; 
     FIG. 3 d  is a graph of the output response of the filter of FIG. 3 c;    
     FIG. 4 a  is a side view of a GRIN lens optically coupled with a movable split filter, in accordance with the invention; 
     FIG. 4 b  is a graph of the spectral output response for the attenuating portion of the filter shown in FIG. 4 a  that is coated with thin film dichroic layers; 
     FIG. 5 a  is a side view of an alternative embodiment of a filter in accordance with the invention; 
     FIGS. 5 b  and  5   c  are output responses for two different portions of the filter shown in FIG. 5 a;    
     FIG. 6 a  is an alternative embodiment of the invention wherein 3 cascaded movable filters in accordance with this invention are provided; 
     FIG. 6 b  is a graph of the output response of the three filters shown in FIG. 6 a;    
     FIG. 7 is a detailed side view of a transmissive filter in accordance with the invention; 
     FIG. 8 is a detailed side view of a reflective filter in accordance with the invention; 
     FIG. 9 a  is a detailed view of an etalon filter in accordance with an aspect of this invention wherein one of the etalon reflective end faces has a continually varying reflectivity (finesse) along a substantial portion of one of its end faces; 
     FIG. 9 b  is a graph of wavelength versus amplitude for the etalon filter shown in FIG. 9 a , wherein three plots are shown for the filter in three different positions with respect to an input beam of light. 
     FIG. 9 c  is a detailed view of an etalon filter in accordance with an aspect of this invention wherein one of the etalon reflective end faces has a discretely varying reflectivity (finesse) along a portion of one of its end faces; 
     FIG. 9 d  is a detailed view of an etalon filter having variable reflectance surfaces at each end; 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1 the gain spectrum of a typical EDFA amplifier is shown wherein it can be seen that the gain as a function of wavelength varies. 
     For a wide variety of glass hosts, rare earth doped optical amplifiers and particularly EDFAs are effectively homogeneously broadened, and the overlap of dopant ions with the signal modes is nearly wavelength independent. As such, the gain spectrum, here being the small signal gain that would be measured by a weak probe signal while the operating point of the amplifier is fixed, is constrained to a single parameter family of wavelength dependencies. Therefore if gain of the amplifier at some reference wavelength is changed due to a change in input (i.e., pump and/or signal power) the amplifier gain at other wavelengths will change by well defined amounts that likely are to be different from the amount of gain change at the reference wavelength. The wavelength dependence of the amplifier gain change as of result of an input change is referred to herein as dynamic gain tilt. Hence, the dynamic gain tilt is a distortion of the amplifier gain spectrum for operating conditions that differ from the operating point for which the amplifier was designed. 
     As it was pointed out in the background of this invention, filters have been provided heretofore for offsetting and flattening the gain spectrum of conventional EDFAs. However providing such a fixed filter does not provide a solution to controlling distortion due to dynamic gain tilt a condition that varies with input power of the input optical signal. 
     Turning now to FIG. 2 a , an arrangement is shown in accordance with this invention, wherein a filter  10 , shown in detail in FIG. 2 c  has a first region  10   a  consisting of substantially light transmissive material such as clear glass and a second region  10   b  juxtaposed to the first region having a dichroic multi-layer interference filter disposed thereon. The second region  10   b  in a preferred embodiment has a sloped transmission response versus wavelength shown in FIG. 2 b . The dimension of the first and second region should be at least the size of the beam incident upon the filter. The term filter used within this specification should include any medium provides an output signal which alters a characteristic of light incident thereupon. In this embodiment, the filter is disposed between two graded index (GRIN) lenses  12  in a conventional manner, wherein the lenses are used to collimate light exiting the optical fibre  8   a , to provide a collimated beam at the input side of the filter  10 , and for focusing the collimated beam that has traversed the filter  10 , for coupling the beam into the optical fibre  8   b . Means for moving the filter  10  along a line coincident with arrows  14   a  and  14   b  are provided (not shown in FIG. 2 a ); such means can conveniently be provided in the form of a stepper motor or alternatively in the form of a piezo-electric actuator. Control circuitry and means for moving the filter in dependence upon the control signal are well known in the art and shall not be described. 
     In operation, the system in accordance with this invention functions in the following manner. When the filter  10  is disposed in a position such that the beam incident thereon passes entirely through the clear portion  10   a  of the filter  10 , the amplitude of the beam as a function of wavelength is essentially unchanged. Hence if the input beam has a signature or amplitude response such as the spectrum shown in FIG. 1, the output beam that has propagated through the clear portion of the filter  10   a  will essentially be unaffected and will have the same amplitude response as a function of wavelength. On the other hand, if an input beam has a positive gain tilt due to a change in power of the input beam, passing this beam through the negatively sloped portion  10   b  of the filter  10  lessens the effect of the positive gain tilt. Most importantly and paramount to this invention, the filter  10  is positionable and dynamically controlled by an actuator and control circuitry (not shown) such that the filter is positionable at locations along the line defined by the arrows  14   a  and  14   b . Hence, a beam incident upon the filter  10  passes through more or less of the portion  10   b  and less or more of the portion  10   a , respectively depending upon the position of the actuator. When a steeper sloped response is required the filter  10  is moved into a position such that less of the beam passes through the clear portion  10   a  of the filter  10  and more of the beam passes through the portion  10   b . By relatively varying position of the beam and the filter such that a greater portion of the beam passes through the region  10   b  than the region  10   a , the slope of the amplitude response with wavelength is augmented, thus affording a means of dynamically controlling for gain tilt. Conversely control of tilt in an opposite direction can be realized by lessening the portion of the beam that passes through the region  10   b  and increasing the portion that passes through the clear portion  10   a  of the filter  10 . This is illustrated more clearly by viewing FIGS. 3 a  to  3   d . FIG. 3 a  illustrates a response of the filter  10  and more particularly  10   b  as it would filter a beam of light  30  having a diameter of approximately 300 μm, traversing the filter through the portion  10   b , (as is shown in FIG. 3 b ). FIG. 3 c  illustrates a response of the same filter, for a same beam  30  traversing the filter (as is shown in FIG. 3 d ) through both regions  10   a  and  10   b . It is noted that the absolute value of the slope of the line  32   a  is greater than the absolute value of the slope of the line  32   b  since some of the light in FIG. 3 d  is passing through the clear portion of the filter. In the extreme case, where all of the light is passing through the clear portion  10   a  of the filter, the filters response is flat, and as more of the light passes through the portion  10   b , the absolute value of the slope increases to a maximum, in dependence upon the filter&#39;s characteristics. 
     Turning now to FIG. 4 a , an alternative embodiment of the invention is shown, wherein a first reflective filter  40  is shown having a first portion  10   a  that is completely reflective to all wavelengths, and a second portion  10   b  having a reflection response with wavelength shown in FIG. 4 b . This embodiment works in a similar manner to the embodiment shown in FIG. 2 a , however it is based on reflection rather than transmission. In both of the embodiments shown in FIGS. 2 a  and  4   a  a filter having a second portion with negative tilt is shown, however, alternatively a filter with portion with positive tilt can be provided in instances where this is required. 
     FIG. 5 a  illustrates an arrangement similar to that of FIG. 2 a , however the filter has a first portion  10   c  having a response with a positive slope, and a second portion having a response having a negative slope. Alternatively, a hybrid filter can be provided wherein a clear portion  10   a  is provided having at its extremities filters  10   b  and  10   c  as have been described. 
     FIGS. 5 b  and  5   c  show the output amplitude versus wavelength transmission responses for the filters  10   b  and  10   c  respectively. 
     In another embodiment of this invention, a series of cascaded filters of this type can be used as tunable gain flattening filters. FIG. 6 a  shows an embodiment according to the invention wherein three filters  62 ,  64 , and  66  are each tunable and designed to filter a different wavelength band. FIG. 6 b  illustrates the response for the three filters, and a dashed line in the graph illustrates the response of the individual filters. Less attenuation can be provided for any of the three wavelength bands by positioning any of the filters such that the beam traverses some or all of the non-attenuating portion of the filters. 
     In order to lessen unwanted effects of a difference in phase for part of the beam traversing the thin film dichroic filter compared with that portion of the beam traversing the clear glass substrate, the optical length of the two sections  10   a  and  10   b  can be matched to the appropriate tolerance. Turning now to FIG. 7, a side view of a transmissive filter in accordance with this invention is shown. Two glass blocks  70   a  and  70   b  are provided having an unattenuating substrate  72   a  and a thin film coated substrate  72   b . The inner meeting edges of the two substrates  72   a  and  72   b  are polished. Between the glass blocks and the substrates  72   a  and  72   b  is a refractive index matching epoxy  75 . 
     FIG. 8 shows a detailed side view of a reflective split filter wherein only a single glass lock is required. An index matching epoxy is disposed between the glass block  70   a  and reflective thin film coated substrate  82   b . Adjacent the substrate  82   b  is a glass substrate  82   a  having a reflective coating. 
     Turning now to FIG. 9 a , an etalon filter  94  is shown having an input end face  96  coated in such a manner as to have the reflectivity varying substantially linearly along its length in a direction from high reflectivity to lower reflectivity towards the bottom of the filter. The opposite end face  97 , in this example has a constant reflectivity coating along its length, however is not limited to such. The etalon can conveniently made of a single light transmissive block having two at least partially reflective end faces coated end faces. Since the input end face has a coating with a reflectivity which varies along the length thereof, by moving the light beam to be filtered and/or the lens, relatively, light will pass through the etalon such that the etalon will have a different finesse at different relative positions. By so doing, the slope of the output response of the etalon changes within a wavelength range of interest. Of course the free spectral range of the etalon must be chosen to provide a suitable window between two different adjacent zero sloping regions of the etalon, and of course the etalon must be tuned or selected to such that the sign (+/−) of the slope is suitable to offset the unwanted tilt of the amplifier to which it is coupled. Hence, a suitable fixed etalon or tunable etalon can be used. 
     FIG. 9 b  illustrates three output responses of the filter shown in FIG. 9 a  for light launched into three locations along the varying reflective end face. A first sinusoidal-like wave labeled  90   a  illustrates the amplitude output response versus wavelength for light launched into location A. The second sinusoidal-like wave labeled  90   b  illustrates the amplitude output response versus wavelength for light launched into location B; and the third sinusoidal-like wave labeled  90   c  illustrates the amplitude output response versus wavelength for light launched into location C. Since the amplitude of the three sinusoidal-like waves is significantly different, the corresponding slopes of each of the sinusoidal-like waveforms varies as well. By using this sloped part of the response as the useful working range of the filter, and varying the slope within this working range without varying the centre operating wavelength, a useful filter is provided for controlling unwanted gain tilt from an in-line amplifier. 
     FIG. 9 c  is similar to that of FIG. 9 a , however the input end face of the etalon has three different discrete portions essentially providing a three-stepped-finesse etalon. 
     FIG. 9 d  shows an etalon wherein each end face has a variable reflectance; here a beam of light incident at a location of the etalon would reflect between two surfaces having a same reflectance, however when the input port is moved by relatively moving at least one of the port and the etalon, the reflectance is varied. 
     Within this specification different regions of the filter may be distinct and different regions, or alternatively may be different regions having some common portions. For example relatively moving an input port and a variable reflectance face of the etalon described heretofore, is considered launching a beam into different regions of the etalon filter. 
     Of course, numerous other embodiments may be envisaged, without departing from the sprit and scope of the invention. For example, in the embodiments shown heretofore, the filter is shown and described to be movable, however, an embodiment wherein the beam is shifted can be realized.