Patent Publication Number: US-2004051939-A1

Title: Optical passive components and bi-directional amplifier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application is related to U.S. Patent Application entitled Reflection-Type Polarization-Independent Optical Isolator, Optical Isolator/Amplifier/Monitor, and Optical System, by Simon Cao, attorney docket number 1398.1001/GMG, U.S. Ser. No. 09/437,791, filed Nov. 10, 1999 and incorporated herein by reference.  
     [0002] This application is also related to U.S. Patent Application entitled Nonlinear Interferometer for Fiber Optic Dense Wavelength Division Multiplexer Utilizing a Phase Bias Element to Separate Wavelengths in an Optical Signal, by Simon Cao, filed on Feb. 10, 1999, U.S. Ser. No. 09/247,253, incorporated herein by reference.  
     [0003] This application is related to U.S. application entitled Bi-Directional Polarization-Independent Optical Isolator, attorney docket number 1398.1002, U.S. Ser. No. 09/438,043 by Simon Cao, filed Nov. 10, 1999 and incorporated by reference herein.  
     [0004] This application is further related to U.S. application entitled Dense Wavelength Division Multiplexer Utilizing an Asymmetric Pass Band Interferometer, U.S. Ser. No. 09/388,350, by Simon X. F. Cao and Xiaoping Mao, filed Sep. 1, 1999 and incorporated by reference herein. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0005] 1. Field of the Invention  
       [0006] This invention relates to optical passive components used in conjunction with a bi-directional amplifier system capable of simultaneously and independently providing optical amplification to light-wave data channels propagating in opposite directions through an optical fiber.  
       [0007] 2. Description of the Related Art  
       [0008] The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to augment the amount of actual fiber-optic cable as well as to utilize the bandwidth of existing fiber-optic cable more efficiently. The latter practice of increasing the carrying capacity of existing fiber cable is sometimes referred to as the creation of “virtual fiber” and is clearly more cost effective than adding real fiber.  
       [0009] One of the ways in which “virtual” fiber is created is through the practice of Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.” To increase data carrying capacity in a given direction, the number of such channels or bands must be increased.  
       [0010] Additionally, it is desirable to use existing fiber for bi-directional communications. More particularly, through the use of WDM, a single optical fiber may be used to transmit, both simultaneously and independently, both eastbound (northbound) as well as westbound (southbound) data.  
       [0011] This bi-directional data-carrying capability of optical fiber increases the need for additional channels still further. However, since all the channels (wavelength bands) must reside within specific low-loss wavelength regions determined by the properties of existing optical fiber, increased channel capacity requires increased channel density. Thus, as the need for increased data carrying capacity escalates, the demands on WDM optical components-to transmit increasing numbers of more closely spaced channels with no interference or “crosstalk” between them and over long distances-becomes more severe.  
       [0012] Optical amplifiers are important components of fiber-optic communication systems. Traditionally, signal regeneration has been accomplished through the use of repeaters, which are combinations of demultiplexers, receivers, signal recovery electronics, transmitters (light sources together with optical modulators), and multiplexers. In a repeater, the signal for each channel is recovered electronically and transmitted anew. Unfortunately, the complexity and cost of repeater-based systems becomes unwieldy with increase in the number of channels of WDM systems. Optical amplifier systems have therefore become attractive alternatives to repeaters. Erbium-doped fiber amplifier (EDFA) systems have become especially popular owing to their gain characteristics near the 1.5 μm (micrometer) transmission band of conventional optical fiber.  
       [0013] EDFA systems are generally associated with sets of set of so-called “optical passive components” which perform various signal and laser pump beam combination, separation, and re-direction functions. Also included in the set of optical passive components are optical isolators, generally one disposed to either side of the optical gain element (Er-doped fiber), which guard against amplification and subsequent transmission of backward-propagating signals. An example of a conventional EDFA system  100  including the various optical passive components is shown in FIG. 1. More particularly, FIG. 1 shows an amplifier of the prior art, which includes a basic EDFA block diagram. The EDFA system  100  shown in FIG. 1 is a unidirectional amplifier.  
       [0014] In FIG. 1, optical isolators, such as isolator  101  and isolator  102 , are disposed to either side of an Er-doped fiber  103 . At least one pump laser, such as co-pump laser  104  and/or counter-pump laser  105 , are generally required to generate the fluorescence in the Er-doped fiber  103 , which leads to amplification. These pump lasers  104 ,  105  generally have wavelengths of 980 or 1480 nm whereas the signal(s) has a wavelength near 1500 nm. Generally, at least two lasers are used such that one laser-the co-pump laser  104 -directs its light along the Er-doped fiber  103  in the same direction as signal propagation and the other laser-the counter-pump laser  105 -directs its light in the direction opposite to signal propagation.  
       [0015] Because of the differences in wavelengths between signal(s) and laser(s), wavelength-division multiplexers/demultiplexers (WDM&#39;s) such as input WDM  106  and output WDM  107  are required. Input WDM  106  combines the co-pump laser light together with the signal light such that both propagate in the same direction through the Er-doped fiber  103 . Likewise, output WDM  107  combines the counter-pump laser light together with the signal light such that the two lights propagate in opposite directions through the Er-doped fiber  103 . Furthermore, input WDM  106  and output WDM  107  remove any residual counter-pump light or co-pump light, respectively, from the system.  
       [0016] Other optical passive components, such as bandpass filter or isolator  108 A and bandpass filter or isolator  108 B, may be present to prevent laser light of the opposite laser from entering each respective pump laser. Finally, signal taps, such as input tap  109  and output tap  110 , may be present so as to sample small proportions of the input and output signals, respectively. These samples of input and output signals may be directed to separate respective photo-detectors such as photo-detector  111 A and photo-detector  111 B so as to monitor the amplifier system performance using comparison and control logic  112 .  
       [0017] Bi-directional lightwave communications systems are those in which signal lights are carried in both directions within individual optical fibers. In the current state of the art, separate amplifiers are used for eastbound (northbound) and westbound (southbound) communications channels as shown in the prior art band bi-directional amplifier  200  shown in FIG. 2. The counter-propagating signals are respectively separated and recombined on either side of the pair of optical amplifiers.  
       [0018] For instance, in FIG. 2, if the “blue” or relatively short wavelength band  201  shown as solid lines represents westward propagating signals and the “red” or relatively long wavelength band  202  shown as dash-dot lines represents eastward propagating signals, then these two signals are separated and recombined by WDMs  203 A and  203 B. Between the two WDMs  203 A,  203 B, the blue and red signals propagate on separate physical optical fiber sub-paths  204  and  205 , respectively, but to either side of each WDM, the westbound blue and eastbound red signals co-propagate along the same physical fiber pathways  211  and  212 . Each of the fiber sub-paths  204  and  205  contains its own amplifier system,  206  and  207 , respectively.  
       [0019] Optional second amplifiers  208  and  209  may be placed in each of the fiber sub-paths and the locations between each of the resulting sequential amplifiers  206  and  208  or  207  and  209  corresponds to mid-stage access ports  210 A and  210 B in the blue and red sub-paths, respectively. Generally, each of the optical amplifier systems,  206  and  207  and, optionally,  208  and  209 , shown in FIG. 2, comprises all the basic components illustrated in FIG. 1 and possibly others. In particular, the amplifier  206  (and optionally  208 ) contains optical isolators that only permit westward light propagation and the amplifier  207  (and optionally  209 ) contains optical isolators that only permit eastward light propagation.  
       [0020] One example of the wavelength constitution of co-propagating bi-directional signals is illustrated in FIG. 3, which corresponds to a band bi-directional amplifier. In FIG. 3, as an example, the “blue” band  301  and the “red” band  302  occupy separate wavelength regions each wholly contained within the well-known fiber transmission band  303  centered near a wavelength of 1.55 μm. For instance band  301  might represent the wavelength constitution of the westbound signal channel(s)  201  of FIG. 2 while band  302  might represent the wavelength constitution of the eastbound signal channel(s)  202 . This type of bi-directional lightwave transmission scheme is termed “band bi-directional” transmission herein. Other types of band bi-directional transmission schemes are possible. For instance, the “blue” band might correspond to all or a portion of the 1.3 μm fiber transmission band while the “red” band might correspond to all or a portion of the 1.55 μm transmission band, etc.  
       [0021] Optical amplifiers are costly and complex components of optical data and telecommunications systems. The prior-art bi-directional optical amplification system shown in FIG. 2 uses two such amplifiers, effectively doubling the cost, complexity, and bulk relative to unidirectional transmission systems. This doubling of systems is necessitated by the fact that optical isolators, which are integral passive components of optical amplifiers, generally perform isolation in a unidirectional sense, regardless of the wavelength of light propagated through them.  
       SUMMARY OF THE INVENTION  
       [0022] Accordingly, it is an object of the present invention to create such advantages, as described herein, over the prior art through the disclosure of a set of optical passive components that together comprise a bi-directional amplifier.  
       [0023] An object of the present invention is to provide a bi-directional optical amplification system of reduced complexity, cost, and bulk relative to prior art bi-directional amplification systems.  
       [0024] Another object of the present invention is to provide a bi-directional optical transmission system which includes a bi-directional optical amplifier.  
       [0025] A further object of the present invention is to provide a bi-directional optical transmission system providing selective, bi-directional isolation of optical signals.  
       [0026] Yet another object of the present invention is to provide a bi-directional polarization independent optical isolator simultaneously transmitting at least two separate signal rays in opposite forward directions while simultaneously suppressing backward transmission of each signal ray in its respective reverse direction, and a corresponding method thereof.  
       [0027] Still another object of the present invention is to provide a method of asymmetric interleaved bi-directional wavelength multiplexed optical signal propagation in a light wave communications system, wherein a first set of channels included within a first set of bands propagates in a first direction, a second set of channels included within a second set of bands propagates in a second direction opposite to the first direction, the width of bands of the first set of bands is not equal to the width of bands of the second set of bands, and the first and second bands are interleaved with one another.  
       [0028] To accomplish those objects, the present invention is a bi-directional polarization independent optical amplifier system simultaneously transmitting two separate signal rays in opposite forward directions while simultaneously suppressing backward transmission of each of the two separate signal rays in its respective reverse direction. The bi-directional polarization independent optical amplifier system of the present invention comprises a bi-directional polarization independent optical isolator of the present invention. The bi-directional polarization independent isolator of the present invention comprises a birefringent polarization element, a reciprocal rotation element, a non-reciprocal rotation element, a reflective element, and a lens.  
       [0029] The birefringent polarization element separates each of the two separate signal rays into components thereof upon a first traverse therethrough and selectively re-combines the components of the two separate signal rays into two respective output signal rays upon a second traverse therethrough. The -reciprocal optical rotation element and the non-reciprocal optical rotation element together selectively rotate the direction of the plane of polarization of both of the components of each of the two separate signal rays depending upon the transmission direction of the each of the two signal rays. The reflective element reflects the components of each of the two separate signal rays and selectively rotates both of the components of one of the two separate signal rays. The lens collimates and directs the components of the two separate signal rays traveling in a forward direction onto the reflective element, and focuses the reflected components onto the birefringent polarization element.  
       [0030] The two separate signal rays are separated from each other based upon their respective wavelengths.  
       [0031] More particularly, the two separate signal rays are separated into two, respective and separate wavelength bands.  
       [0032] Alternatively, the two separate signal rays include two sets of wavelengths, each of the two sets of wavelengths including a plurality of wavelengths, such that wavelengths of the two signal rays are alternatingly interspersed with each other.  
       [0033] In addition, the present invention is a method of bi-directional optical amplification of signal rays within an optical communications system. The method of the present invention comprises simultaneously inputting into a bi-directional polarization independent optical isolator of the present invention of the optical communications system two separate signal rays of the optical communications system. The bi-directional polarization independent optical isolator of the present invention causes simultaneous transmission of the two separate signal rays in opposite forward directions through an optical gain element and simultaneously suppresses backward transmission of each of the two separate signal rays in its respective reverse direction.  
       [0034] Moreover, the present invention includes a method of asymmetric interleaved bi-directional wavelength multiplexed optical signal propagation in a light wave communications system, in which a first set of channels included within a first set of bands propagates in a first direction and a second set of channels included within a second set of bands propagates in a second direction opposite to the first direction, the width of bands of the first set of bands is not equal to the width of bands of the second set of bands, and the first and second bands are interleaved with one another.  
       [0035] These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0036] The objects and features of the present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:  
     [0037]FIG. 1 is a basic block diagram of an optical fiber amplifier showing the assembly of conventional optical passive components.  
     [0038]FIG. 2 is a basic block diagram of a prior-art system of optical amplifiers utilized for separate amplification of component sub-signals of a bi-directional optical transmission system.  
     [0039]FIG. 3 is a schematic graph of an arrangement of wavelength constitution of optical transmission bands in a band-bi-directional lightwave transmission system.  
     [0040] FIGS.  4 A-C are schematic illustrations of configurations of optical transmission bands in symmetrically and asymmetrically interleaved wavelength-division multiplexed bi-directional lightwave transmission systems.  
     [0041]FIG. 5A and 5B are side views of two embodiments of an integrated set of optical passive components comprising a band bi-directional and interleaved bi-directional amplifier system, respectively, according to the present invention.  
     [0042]FIG. 6 is a side view of the central mirror and waveplate assembly of an integrated set of optical components for a band bi-directional amplifier system showing ray paths therethrough.  
     [0043]FIG. 7 is a graph of the preferred reflectivity curves, in reflectivity against wavelength, for the two reflective elements of a single-stage band bi-directional optical isolator in a bi-directional amplifier system.  
     [0044]FIG. 8 is a side view of a non-linear interferometer for use in the optical passive components of the interleaved bi-directional amplifier of the present invention showing the paths of signal light and laser pump light.  
     [0045]FIGS. 9A through 9D are end views of the fiber (port) configuration within both the front and rear ferrules of both the East and West passive component sets of a bi-directional amplifier system in accordance with the present invention.  
     [0046]FIG. 10A and 10B are side views of bounding ray paths of pump beams and of westbound light signals within the East and West set of optical passive components, respectively.  
     [0047]FIG. 11A and 11B are side views of bounding ray paths of pump beams and of eastbound light signals within the West and East set of optical passive components, respectively.  
     [0048]FIG. 12A and 12B are side views of the operation of the East bi-directional optical passive components in a bi-directional amplifier system showing central ray paths for light signals of the westbound channels in each of the two principal polarization states propagating east-to-west and west-to-east, respectively.  
     [0049]FIG. 12C and 12D are side views of the operation of the East bi-directional optical passive components in a bi-directional amplifier system showing central ray paths for light signals of the eastbound channels in each of the two principal polarization states propagating west-to-east and east-to-west, respectively.  
     [0050]FIG. 13A and 13B are side views of the operation of the West bi-directional optical passive components in a bi-directional amplifier system showing central ray paths for light signals of the eastbound channels in each of the two principal polarization states propagating west-to-east and east-to-west, respectively.  
     [0051]FIG. 13C and 13D are side views of the operation of the West bi-directional optical passive components in a bi-directional amplifier system showing central ray paths for light signals of the westbound channels in each of the two principal polarization states propagating east-to-west and west-to-east, respectively.  
     [0052]FIG. 14 is a basic block diagram of a bi-directional fiber amplifier system for two light signals propagating in opposite directions in a single fiber optic line showing the assembly of optical passive components according to the present invention.  
     [0053]FIG. 15 is a flowchart showing signal ray transmission through the bi-directional polarization independent optical amplifier of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0054] In the description of the present invention, it is to be noted that like parts are designated by like reference numerals throughout in the accompanying drawings.  
     [0055] Before the preferred embodiments of the present invention are described beginning with reference to FIGS. 5A and 5B, a brief description of interleaved bi-directional transmission is presented with reference to FIGS.  4 A- 4 C.  
     [0056] FIGS.  4 A- 4 C illustrate a more complex form of bi-directional transmission, herein termed “interleaved bi-directional” transmission, which occurs in an interleaved bi-directional amplifier. In interleaved bi-directional transmission, one set of channels propagates in one direction along an optical fiber while the remaining set of channels propagates in the other direction, wherein the first and second sets are interleaved with each other. For instance, in FIG. 4A, the even-numbered or first set of channels  401  denoted by darkly shaded rectangles outlined by solid lines, for example, might comprise the westbound channels, whereas the odd-numbered or second set of channels  402  denoted by lightly shaded rectangles outlined by dashed lines, for example, might comprise the eastbound channels. The aforementioned solid and dashed lines schematically represent the optical pass bands and isolation bands for the first and second channel sets, respectively.  
     [0057]FIG. 4A illustrates symmetric interleaving of wavelength division multiplexed channels, wherein the pass bands of the first set  401  of channels comprise a band width, w 1 , the pass bands of the second set  402  of channels comprise a band width w 2  substantially equal to w 1 , and channels of the first set and the second set are interleaved or alternate with one another. FIGS. 4B and 4C schematically illustrate two examples of asymmetric interleaving of wavelength division multiplexed channels, wherein the two band widths of the two channel sets, w 1 , and w 2 , are not identical to one another and the pass bands of one (or the other) of the interleaved sets may comprise more than one channel. Shaded rectangles in each of FIG. 4A and 4B schematically illustrate the widths and positions of conventional wavelength division multiplexed channels. Thus, as shown in FIG. 4B, each of the pass bands of the first set  401  and second set  402  of channels encompasses or comprises three conventional channels and a single conventional channel, respectively. In FIG. 4C, each of the pass bands of the first set  401  and second set  402  of channels encompasses or comprises seven conventional channels and a single conventional channel, respectively.  
     [0058] Each pass band of either channel set may comprise multiple conventional channels, as shown in FIG. 4B and 4C, or else may comprise a single channel of non-conventional band width. In either symmetric or asymmetric interleaving, pass bands of the first and second channel sets are interleaved with one another and one channel set propagates in a first direction and the other channel set propagates in a second direction opposite to the first direction. Such symmetric and asymmetric wavelength division channel interleaving, as illustrated in FIGS.  4 A-C are disclosed in a co-pending U.S. Patent Application titled Dense Wavelength Division Multiplexer Utilizing An Asymmetric Pass Band Interferometer, which is incorporated herein by reference.  
     [0059] Functionally, the set of channels  401  corresponds to the blue channel  301  of FIG. 3 and the westbound signal  201  of FIG. 2 while the set of channels  402  corresponds to the red channel  302  and the eastbound signal  202 .  
     [0060] Moreover, and as discussed herein above, a problem with the prior art bi-directional optical amplification system shown in FIG. 2 is that a doubling of the number of components, particularly the costly and complex optical amplifiers, over that of unidirectional optical systems is required. To overcome this problem of prior-art optical isolators, a bi-directional polarization independent optical isolator is disclosed in the above-mentioned co-pending U.S. Patent Application entitled Bi-Directional Polarization-Independent Optical Isolator. These bi-directional polarization independent optical isolators have the property such that the direction in which isolation occurs depends upon wavelength. Clearly, the incorporation of such bi-directional optical isolators into an optical amplifier system would obviate the need for two amplifiers in bi-directional photonic systems and would facilitate the development of a bi-directional amplifier. Such a development would have the advantage of reduced cost, bulk, and complexity as compared to existing bi-directional optical amplification systems and would have the additional advantage of facilitating the incorporation of bi-directional amplification into existing fiber-optic cable.  
     [0061] The present invention is a bi-directional optical amplifier system including the above-mentioned bi-directional polarization independent optical isolators.  
     [0062] Two embodiments of the present invention, each of which comprises an integrated set of optical components for use in a bi-directional optical amplifier system, are shown in FIG. 5A and 5B, respectively. A first embodiment of the present invention is shown in FIG. 5A, and a second embodiment of the present invention is shown in FIG. 5B.  
     [0063]FIG. 5A shows a side view of an integrated set of optical components comprising a bi-directional amplifier system  500 A of the present invention suitable for use in a band bi-directional amplifier system. More particularly, FIG. 5A shows a band bi-directional amplifier  500 A of the present invention for use in a band bi-directional system.  
     [0064]FIG. 5B is a side view of an embodiment of the present invention  500 B suitable for use in an interleaved bi-directional amplifier system. More particularly, FIG. 5B shows an interleaved bi-directional amplifier  500 B of the present invention for use in an interleaved bi-directional system.  
     [0065] In FIG. 5A, reference numeral  501  is a front four-fiber holder or ferrule. Contained within and secured to or by front ferrule  501  are two sets of fibers  502 A and  502 B and  503 A and  503 B, respectively. Fibers  502 A and  502 B are on the logical west side of the device whereas fibers  503 A and  503 B are on the logical east side of the device. The end faces of the front ferrule and fibers contained therein are cut at an angle of approximately 8° to prevent backward propagation of reflected light and are polished flat. Also, in FIG. 5A, reference numeral  504  is a birefringent walk-off plate, and reference numerals  505  and  506  are a λ/2 (half-wave) plate and Faraday rotator, respectively, which rotate the polarization planes of linearly polarized light passing therethrough by 45° counter-clockwise (CCW) in a reversible and non-reversible fashion, respectively. Magnets  507 A and  507 B (FIG. 5A) provide the magnetic field under which the magneto-optic effect of Faraday rotator  506  is actuated. The system  500 A also contains a front focusing lens  508 , a reflector/waveplate assembly  509  and a rear focusing lens  510  disposed in this order. Also shown in FIG. 5A, at the end of device  500 A opposite to the front four-fiber ferrule  501 , is a second identical rear four-fiber ferrule  511 . The rear four-fiber ferrule  511  contains, in a fashion similar to the assembly of front ferrule  501 , four additional rear fibers  512 A,  512 B,  513 A, and  513 B. The ferrule  511  is aligned such that the four fibers  512 A,  512 B,  513 A and  513 B are directly opposite to fibers  502 A,  502 B,  503 A and  503 B, respectively. The end faces of rear ferrule  511  and the fibers contained therein are cut flat and polished and face in a direction towards the front ferrule  501 . Disposed adjacent to the end face of rear ferrule  511  and the fibers contained therein, in the direction facing front ferrule  501 , is a second birefringent walk-off plate  514  which is of similar material and thickness to front birefringent plate  504  and which is aligned such that its optic axis is approximately symmetrically disposed about a vertical plane with respect to the optic axis of birefringent element  504 . Disposed adjacent to birefringent element  514 , in the direction facing front ferrule  501  are a pair of rear half-wave plates  515 A and  516 A or  515 B and  515 C and a rear Faraday Rotator  516 B or  516 C. The two rear half-wave plates  515 A and  516 A or  515 B and  515 C and the rear Faraday Rotator  516 B or  516 C have optical properties such that they rotate the polarization planes of linearly polarized light passing therethrough by 45° in a reversible and non-reversible fashion, respectively. Magnets  517 A and  517 B (FIG. 5A) provide the magnetic field under which the magneto-optic effect of Faraday rotator  516 B or  516 C is actuated. The exact positions and rotation directions, either clockwise (CW) or counterclockwise (CCW), of the rear half-wave plates and the rotation direction of the rear Faraday rotator are illustrated in detail in FIG. 9B and FIG. 9D and depend upon the usage of the bi-directional amplifier system  500 A or  500 B as either a logical East device or a logical West device, as described in more detail herein following.  
     [0066]FIG. 5B is a side view of an embodiment of the present invention  500 B suitable for use in an interleaved bi-directional amplifier system. All components in system  500 B are identical and in similar locations to those in system  500 A except for the replacement of reflector/waveplate assembly  509  by a new element, selective pass through non-linear interferometer  519 . The details of the construction and optical properties of reflector/waveplate assembly  509  of the bi-directional amplifier system  500 A are shown in FIG. 6 and FIG. 7. The details of the construction and optical properties of the selective pass-through non-linear interferometer assembly  519  of bi-directional amplifier system  500 B are shown in FIG. 8.  
     [0067] As described in greater detail below, both assembly  509  and  519  have the property that, upon reflection therefrom, they rotate the direction of the polarization plane of one signal channel or set of signal channels by 90° while maintaining constant the polarization plane direction of the other channel or set of channels. In assembly  509  (FIG. 5A), the polarization plane directions of the shorter wavelength (or “blue”) channel or set of channels is rotated by 90° around the propagation axis whereas the polarization plane direction of the longer wavelength (or “red”) channel or set of channels is not rotated. Likewise, in assembly  519  (FIG. 5B), the polarization plane directions of every second (or “even”) channel is rotated by 90° around the propagation axis whereas the polarization plane direction of the remaining (or “odd”) channel or set of channels is not rotated. As discussed in greater detail below, through these respective properties, the assembly  509  enables the system  500 A to be used as a band bi-directional set of optical passive components and the assembly  519  enables the system  500 B to be used as an interleaved bi-directional set of optical passive components.  
     [0068] The mirror/waveplate assembly  509  included in the bi-directional amplifier system  500 A is shown in side view in FIG. 6. The assembly  509  includes four elements-a red reflector (mirror)  602 , a first λ/4 (quarter-wave) plate  604 , a blue reflector (mirror)  606  and a second compensating λ/4 plate  608  all disposed in this order in the direction away from the front lens  508 . Also, the optical axes of the first λ/4 plate  604  and the second λ/4 plate  608  are each disposed in a plane perpendicular to the direction of light propagation therethrough, at right angles to one another, and at a 45° angle relative to the polarization plane orientations of sub-signal lights passing therethrough.  
     [0069]FIG. 7 shows idealized mirror reflectivity curves for a reflector/waveplate assembly in a bi-directional amplifier system of the present invention. More particularly, FIG. 7 shows examples of idealized mirror reflectivity curves of the preferred variation of reflectivity against wavelength for the “blue” reflector  606  and the “red” reflector  602 , respectively, of the reflector/waveplate assembly  509  of the bi-directional amplifier system  500 A, under the assumption that the “red” band and “blue” band as discussed above are as defined in FIG. 3. It is also assumed that all light that is not reflected by a given reflector is transmitted therethrough although, in practice, a small percentage of light will be absorbed therein. The reflectivity of the red reflector  602  is chosen so as to reflect and transmit preferably approximately 95% and approximately 5%, respectively, of the light signal in the red band while transmitting preferably 100% of light of shorter wavelengths. Likewise, the reflectivity of the blue reflector  606  is chosen so as to reflect and transmit preferably approximately 95% and approximately 5%, respectively, of the light signal in the blue band while transmitting preferably 100% of light of shorter and longer wavelengths.  
     [0070] Although the reflectivity of the blue reflector  606  and of the red reflector  602  are illustrated, respectively, by specific curves in FIG. 7, one of ordinary skill in the art will readily recognize that, without departing from the scope or spirit of the present invention, the wavelength definitions of the red and of the blue bands may be chosen differently, and that for such alternate definitions, the reflectivity curves of the two reflectors may need to be varied from those shown in FIG. 7, as appropriate. Furthermore, the relative physical positions of the red and blue reflectors may be interchanged and simultaneous associated changes of the reflectivity curves and/or band propagation directions may be made without changing the basic functionality or the spirit or scope of the present invention.  
     [0071] The reflected and transmitted portions of light of various wavelengths during interaction with reflector/waveplate  509  are shown in FIG. 6 together with their linear polarization directions. Because of the chosen reflectivities of reflectors  602  and  606 , substantially 100% of the light of wavelengths shorter than the blue band passes completely through the reflector/waveplate assembly  509 . Furthermore, approximately 95% of the signal light in the red band does not pass through the first λ/4 plate  604 , the blue reflector  606  or the second λ/4 plate  608 . Furthermore, approximately 95% of the light in the blue band passes through the red reflector  602  and the first λ/4 plate  604  twice—once before and once after reflection off the blue reflector  606 —but does not pass through the second λ/4 plate. Furthermore, a proportion equivalent to approximately 5% of the signal light comprising the red band and also comprising the blue band passes completely through all four elements comprising the reflector/waveplate assembly  509 .  
     [0072] As shown in FIG. 6, the approximately 95% of the light comprising the red band that is reflected by red reflector  602  does not experience a change in the direction of its polarization. Furthermore, the approximately 95% of the light comprising the blue band that is reflected by blue reflector  606  experiences a 90° rotation of the direction of its polarization. This is because this reflected blue-band light passes twice through the first λ/4 plate  604  whose optical axes are disposed at a 45° angle relative to the incoming light polarization direction. The light that passes completely through the assembly  509 —that is, pump laser light of wavelengths shorter than the blue band as well as approximately 5% proportions of the signal light of the red and blue bands that are transmitted through reflectors  602  and  606 , respectively—does not experience any net rotation of the direction of its polarization plane. This is because all such light must pass through both the first λ/4 plate  604  as well as the second λ/4  608  plate and the optical axes of the first and second λ/4 plates are disposed at right angles to one another. Thus, the second λ/4 plate  608  exactly compensates any birefringent retardance imposed upon these transmitted lights by the first λ/4 plate  604 .  
     [0073]FIG. 8 shows a selective pass-through non-linear interferometer assembly. More particularly, FIG. 8 is a side view of the selective pass-through non-linear interferometer  519  used in the second embodiment of the present invention, showing the paths of signal light and laser pump light. The non-linear interferometer  519  shown in FIG. 8 is a modified version of an invention disclosed in the above-mentioned co-pending U.S. Patent Application entitled Nonlinear Interferometer for Fiber Optic Dense Wavelength Division Multiplexer Utilizing a Phase Bias Element to Separate Wavelengths in an Optical Signal and in the above-mentioned co-pending U.S. Patent Application entitled Dense Wavelength Division Multiplexer Utilizing An Asymmetric Pass Band Interferometer. The non-linear interferometer  519  comprises a first (or front) glass plate  802  whose second (or rear) surface hosts a first partially reflective coating  804 , a second (or rear) glass plate  806  whose first (or front) surface hosts a second partially reflective coating  808 , an inner wave plate  810  between the two coated glass plates  802  and  806 , an outer wave plate  812  adjacent to the front surface of glass plate  802 , a first compensating wave plate  814  and a second compensating wave plate  816 . The first compensating wave plate  814  and the second compensating wave plate  816  are disposed to the rear of second glass plate  806  and, in the example shown, are identical to outer wave plate  812  and to inner wave plate  810 , respectively, except that the directions of the optic axes of first compensating wave plate  814  and second compensating wave plate  816  are at right angles to those of outer wave plate  812  and inner wave plate  810 , respectively. The inner wave plate  810 , outer wave plate  812 , first compensating wave plate  814 , and second compensating wave plate  816  comprise a birefringent optical material such as single-crystal quartz.  
     [0074] The selective pass-through non-linear interferometer  519  is modified from that disclosed in Dense Wavelength Division Multiplexer Utilizing An Asymmetric Pass Band Interferometer through incorporation of the two compensating wave plates. The first and second partially reflective coatings  804  and  808  have reflectivity values of r 1  and &gt;99%, respectively, for wavelengths of light comprising signal bands and values of 0% for wavelengths of light shorter than those of the signal bands. Furthermore, the outer wave plate  812  and first compensating wave plate  814  both comprise a birefringent differential optical retardance value of L 1  (in units of length), and the inner wave plate  810  and second compensating wave plate  816  both comprise a birefringent differential optical retardance value of L 2 .  
     [0075] Because of the reflectivity characteristics of second reflective coating  808 , a proportion equivalent to &gt;99% of any signal light ray entering non-linear interferometer  519  from the front side will be reflected as shown in FIG. 8. The remaining &lt;1% proportion of said signal light ray as well as 100% of all light rays of wavelengths shorter than those comprising signals will pass completely through non-linear interferometer  519 . The operation of the non-linear interferometer  519  is such that, of the reflected portion of the signal light, linearly polarized signal light having a wavelength corresponding to every one of a first set of bands (for instance, even-numbered bands) will be reflected with a 90° rotation of its plane of polarization whereas linearly polarized signal light of wavelengths corresponding to the remaining bands interleaved with the first set (for instance odd-numbered bands) will be reflected without change in polarization. The light that passes completely through the non-linear interferometer  519 —that is, pump laser light of wavelengths shorter than the wavelengths of signal lights as well as the &lt;1% proportions of the signal light that are transmitted through the partially reflective coating  808 —does not experience any net rotation of the direction of its polarization plane. This is because all such light must pass through both the inner wave plate  810  and the second compensating wave plate  816  as well as through both the outer wave plate  812  and the first compensating wave plate  814 .  
     [0076] The specific configuration of interleaved bands, as exemplified by FIGS.  4 A- 4 C, is determined by the choice of the values of the parameters L 1 , L 2  and r 1  associated with interferometer  519 . For instance, to obtain the symmetrically interleaved configuration illustrated in FIG. 4A, in which one set of bands comprises odd channels and the other set comprises even channels, these three parameters are set at λ/8, λ/4, and 18.5%, respectively. To obtain the three-to-one and seven-to-one asymmetric interleaving configurations illustrated in FIGS.  4 B- 4 C, these three parameters are set at 3λ/16, λ/8, and 32.5% and 7λ/32, λ/16 and 56.3%, respectively. Other choices of parameter sets lead to different interleaved band configurations than those illustrated in FIGS.  4 A-C, and the invention is not intended to be limited to these three particular examples.  
     [0077] The first  814  and second  816  compensating wave plates in the selective pass-through non-linear interferometer  519  together compensate any changes in polarization of light transmitted completely through the elements  802 - 812 . Since the thickness and material of the inner wave plate  810  is identical to that of the second compensating wave plate  816  wherein the optical axes of two said wave plates are disposed at right angles to one another and to the light propagation direction, then any birefringent differential optical retardance imposed upon lights transmitted through the inner wave plate  810  is exactly negated during subsequent transmission through the second compensating wave plate  816 . Likewise, since the thickness and material of the outer wave plate  812  is identical to that of the first compensating wave plate  814  wherein the optical axes of two said wave plates are disposed at right angles to one another and to the light propagation direction, then any birefringent differential optical retardance imposed upon lights transmitted through the outer wave plate  812  is exactly negated during subsequent transmission through the first compensating wave plate  814 . In this fashion, for any light transmitted completely through the selective pass-through non-linear interferometer  519 , the polarization state of the light after such transmission is identical to that before the transmission (FIG. 8). One of ordinary skill in the art will, however, recognize that, without departing from the scope or spirit of the present invention, such polarization state compensation may be accomplished by different choices and arrangements of optical elements or that such compensation may even be neglected entirely, provided that accompanying changes are made in the optical elements disposed to the front of rear four-fiber ferrule  511 .  
     [0078] In operation, each embodiment of the current invention is used in pairs, comprising both a logical East device and a logical West device. The two devices of each pair are disposed at alternate sides of a bi-directional amplifier system in accordance with the present invention. Furthermore, each device is a set of optical passive components in accordance with either embodiment of the current invention.  
     [0079]FIGS. 9A through 9D show an input/output port configuration of a bi-directional amplifier, and, more specifically, the input/output port configuration of the bi-directional amplifier shown beginning with FIG. 10A. More particularly, FIGS. 9A through 9D show end views of the fiber or port configuration associated with both the front and rear ferrules of both logical East passive component device  902  and a logical West passive component device  904 . The configurations of the front ports are identical for both East and West devices  902  and  904 . However, the configuration of the rear port varies depending upon whether the device is used on the logical East side or the logical West side of a bi-directional amplifier system.  
     [0080] As illustrated in FIG. 9A and FIG. 9C, the four fibers or ports associated with the front ferrule of either a logical East device or a logical West device themselves comprise two logical West fibers and two logical East fibers. As shown in FIG. 9A, the logical West ports (fibers) of the East device  902 , Port E 1   905  and Port E 2   906 , are disposed to one side of the interior of the front four fiber ferrule  501  and the logical East ports (fibers) of the East device  902 , Port E 3   907  and Port E 4   908 , are disposed to the other side of the interior of ferrule  501 . As shown in FIG. 9C, the logical West ports (fibers) of the West device  904 , Port W 1   913  and Port W 2   914 , and the logical East ports (fibers) of the West device  904 , Port W 3   915  and Port W 4   916 , have dispositions similar to those in the East device  902 . Also as shown in FIGS. 9A, 9C,  5 A and  5 B, a 45°-counterclockwise-rotating λ/2 plate  505  is disposed adjacent to the two logical West Ports, Port E 1   905  and Port E 2   906  or Port W 1   913  and Port W 2   914 , in the direction of the rear lens  510 . Also as shown in FIGS. 9A, 9C,  5 A and  5 B, a 45°-counterclockwise rotating Faraday rotator  506  is disposed adjacent to the two logical East Ports, Port E 3   907  and Port E 4   908  or Port W 3   915  and Port W 4   916 , in the direction of the rear lens  510 .  
     [0081] As also illustrated in FIG. 9B, the set of Ports (fibers) within the rear four-fiber ferrule  511  of the East device  902  comprises Ports (fibers) E 5   909 , E 6   910 , E 7   911  and E 8   912  disposed such that these ports are exactly opposite to and facing the Ports E 1   905 , E 2   906 , E 3   907  and E 4   908 , respectively. Three separate optical elements, a first 45°-counterclockwise-rotating λ/2 plate  515 A, a second 45°-counterclockwise-rotating λ/2 plate  516 A, and a 45°-clockwise-rotating Faraday rotator  516 B are disposed adjacent to Ports E 6   910 , E 7   911  and E 8   912 , respectively, in the direction of the rear lens  510 . As further illustrated in FIG. 9D, the set of Ports (fibers) within the rear four ferrule  511  of the West device  904  comprises Ports (fibers) W 5   917 , W 6   918 , W 7   919  and W 8   920  disposed such that these ports are exactly opposite to and facing the Ports W 1   913 , W 2   914 , W 3   915  and W 4   916 , respectively. Three separate optical elements, a 45°counterclockwise-rotating λ/2 plate  515 B, a 45°-clockwise-rotating λ/2 plate  515 C, and a 45°-counterclockwise-rotating Faraday rotator  516 C are disposed adjacent to Ports W 5   917 , W 6   918  and W 8   920 , respectively, in the direction of the rear lens  510 .  
     [0082] A summary of the operation of the optical passive components of the present invention as utilized in conjunction with a bi-directional amplifier system of the present invention is provided in FIGS. 10A, 10B,  11 A and  11 B. A summary of the optical paths of logical westbound (FIGS. 10A and 10B) and logical eastbound (FIGS. 11A and 11B) signals is represented. The eastbound and westbound signals each comprise one or the other of the two wavelength bands utilized in conjunction with the band bi-directional device,  500 A, which is the first embodiment of the present invention. Alternatively, the eastbound and westbound signals each comprise one of the two interleaved sets of channels utilized in conjunction with the interleaved bi-directional device,  500 B, which is the second embodiment of the present invention.  
     [0083] For example, if FIGS. 10A, 10B,  11 A, and  11 B show a bi-directional device of the first embodiment of the present invention, used in a band bi-directional system, then FIGS. 10A and 10B would be directed to a “blue” band of input signal light traveling in the east-to-west direction and FIGS. 11A and 11B would be directed to a “red” band of input signal light traveling in the west-to-east direction. Similar features of the second embodiment of the present invention related to the interleaved bi-directional device  500 B would also be shown in FIGS. 10A, 10B,  11 A, and  11 B.  
     [0084] Each of the two counter propagating signals makes two consecutive traverses or passes through each of two devices, a logical East device  902  and a logical West device  904 , whereby each device comprises an integrated set of optical passive components consistent with either one of the embodiments of the present invention. An appropriate optical amplifier gain element (not shown), such as an Er-doped fiber, is disposed between the East device and the West device. Signal propagation through the logical East device is illustrated in FIG. 10A and FIG. 11B whereas signal propagation through the logical West device is illustrated in FIG. 10B and FIG. 11A.  
     [0085] Each of the figures, FIG. 10A, 10B,  11 A and  11 B, shows both of the two traverses of the particular signal-either the eastbound signal or westbound signal-through the particular device-either the East device or West device-illustrated therein. Each individual traverse comprises a diagonally opposed pair of ports or fibers-such as E 1   905  and E 4   908 , E 2   906  and E 3   907 , W 1   913  and W 4   916  or W 2   914  and W 3   915 —within the four-fiber ferrule utilized for input and output. The two traverses of each signal through each device are optically coupled one to another in a polarization-preserving fashion by an optical coupling means (not shown) which may comprise a set of mirrors, a retro-reflector, polarization preserving fibers, or the like. The optical coupling connects the input and output of Ports E 2   906  and E 4   908  and also of Ports W 2   914  and W 4   916 . Each signal traverse through each of the devices comprises one stage of optical isolation. Therefore, the two consecutive traverses of each signal through each device comprise double-stage optical isolation.  
     [0086] The present invention is explained using double-stage optical isolation. In addition, in the present invention, single-stage optical isolation (explained in detail in Bi-Directional Polarization-Independent Optical Isolator) may also be used. For example, in the present invention, an assembly of four parallel optical ports is optically coupled to the bi-directional polarization independent optical isolator such that corresponding input and output of each pass of the two separate signal rays through the isolator is coupled to a pair of diagonally disposed ports. The four ports are configured such that either single-stage bi-directional isolation is accomplished for each of two optical transmission lines or double stage bi-directional isolation is accomplished on a single optical transmission line.  
     [0087] In addition to simple signal propagation through each of the East and West devices, pump laser beams are injected or removed and small proportions of signals are monitored at various stages. These additional optical pathways are made possible by the reflective properties of the reflective elements within either the reflector/waveplate assembly  509 , or the non-linear interferometer,  519 , of the first and second embodiments,  500 A and  500 B, respectively, of the present invention. Each monitored signal or injected laser pump beam comprises a light beam that is transmitted through these reflective elements from front-to-back or from back-to-front, respectively. Thus, each monitored signal is input at one of the front ports and monitored at one of the rear ports and each laser pump beam is input at one of the rear ports and output at one of the front ports. Because of the image inversion properties of the pair of lenses,  508  and  510 , the separate members of the resulting pairs of ports are disposed diagonally relative to one another, as viewed end-on in FIGS. 9A through 9D. As discussed further below, the two laser pump beams propagate in opposite directions through the bi-directional amplifier system and the set of optics comprising the beam path of each of the pump laser beams within each device,  902  and  904 , comprises an optical isolator. Thus, back injection of the beam from one laser into the other laser is prevented, thereby preventing damage to either.  
     [0088] The full path of a westbound-propagating signal  1000  through the two devices is illustrated in FIGS.  10 A- 10 B. As shown in FIG. 10A, the westbound-propagating signal  1000  first enters the logical East device  902  through Port E 3   907 . As discussed in greater detail below, the signal  1000  is separated into two linearly polarized sub-signals,  1000 A and  1000 B, upon entering device  902 . Note that, to prevent a confusion of lines, such sub-signals are not distinguished from one another in FIGS. 10A, 10B,  11 A and  11 B. The greater proportion (&gt;95%) of each of the sub-signals  1000 A and  1000 B passes through only the front portion of device  902  (i.e., not including the rear lens  510  or components to the rear thereof) so as to re-emerge through Port E 2   906  of the front ferrule  501 . However, small proportions (&lt;5%) of each of these sub-signals, wherein such proportions comprise sub-signals  1000 E and  1000 F, are diverted to Port E 6   910  of the rear ferrule  511  for input monitoring purposes. These two sub-signals  1000 E and  1000 F are combined by the optics disposed to the front of Port E 6  so as to form the monitored signal  1000 I.  
     [0089] After emerging from Port E 2   906 , the two sub-signals  1000 A and  1000 B are optically transferred from Port E 2   906  to Port E 4   908  by the optical coupling means (not shown) so as to make a second traverse through the East device  902 . Because this coupling occurs in polarization preserving fashion, the separate identities of the two sub-signals  1000 A and  1000 B are maintained between the first and second traverses. Upon the second traverse through the East device  902 , the greater proportion (&gt;95%) of each of the sub-signals  1000 A and  1000 B passes through only the front portion of device  902  so as to finally emerge from said device through Port E 1   905  as the re-combined signal  1000 . Additionally, a westbound laser pump beam  1002 , comprising two linearly polarized sub-beams,  1002 A and  1002 B within device  902 , is injected into the device through Port E 8   912 . Note that, to prevent a confusion of lines, these sub-beams are not distinguished from one another in FIGS. 10A and 11B. The sub-beams  1002 A and  1002 B are re-combined into pump beam  1002  and emerge from the East device  902  at Port E 1   905  together with the signal  1000 .  
     [0090] After emerging from the logical East device  902 , the westbound signal  1000  propagates through the optical gain element (such as an EDF), together with the westbound  1002  and eastbound  1004  laser pump beams where it is amplified. As shown in FIG  10 B, the amplified westbound signal  1000  is then transferred from the optical gain element to the Port W 3   915  of the logical West device  904  so as to make a first traverse therethrough. Upon entering the West device  904 , the amplified signal  1000  is separated into two linearly polarized sub-signals,  1000 C and  1000 D. Note that, to prevent a confusion of lines, these sub-signals are not distinguished from one another in FIG. 10B. The greater proportion (&gt;95%) of each of the sub-signals  1000 C and  1000 D passes through only the front portion of device  904  so as to re-emerge through Port W 2   914 . Also, an eastbound laser pump beam  1004  is injected into the West device  904  through Port W 6   918  where it is immediately separated into sub-beams  1004 A and  1004 B. Note that, to prevent a confusion of lines, these sub-beams are not distinguished from one another in FIG. 10B. These sub-beams  1004 A and  1004 B propagate through the West device  904  so as to emerge from Port W 3   915  as a re-combined beam  1004  that is thenceforth transferred into the optical gain element.  
     [0091] After emerging from Port W 2   914 , the two sub-signals  1000 C and  1000 D are optically transferred from Port W 2   914  to Port W 4   916  by the optical coupling means (not shown) so as to make a second traverse through the West device  904 . Because this coupling occurs in polarization preserving fashion, the separate identities of the two sub-signals  1000 C and  1000 D are maintained between the first and second traverses. Upon the second traverse through the West device  904 , the greater proportion (&gt;95%) of each of the sub-signals  1000 C and  1000 D passes through only the front portion of device  904  so as to finally emerge from said device through Port W 1   913  as the re-combined amplified signal  1000 . At this point, the amplified signal  1000  exits the bi-directional amplifier system and re-joins the lightwave communications system in the logical westbound direction. Small proportions (&lt;5%) of each of the sub-signals  1000 C and  1000 D, wherein said proportions comprise sub-signals  1000 G and  1000 H, are diverted to Port W 5   917  of the rear ferrule  511  of device  904  for output monitoring purposes. The two sub-signals  1000 G and  1000 H are recombined by the optics disposed to the front of port W 5  so as to form the monitored signal  1000 J.  
     [0092] The full path of an eastbound-propagating signal  1100  through the two devices comprising the bi-directional optical amplifier system is illustrated in FIGS.  11 A-B. Note that, to prevent a confusion of lines, all pairs of sub-signals are not distinguished from one another in FIGS. 11A and 11B. The path of eastbound-propagating signal  1100  is exactly opposite to that of westbound-propagating signal  1000 . That is, as shown in FIG. 11A, signal  1100  enters the logical West device  904  through Port W 1   913  and is separated into two sub-signals  1100 A and  1100 B which make a first traverse through device  904  so as to emerge through Port W 4   916 . During this first traverse, small (approximately 5% or less) proportions of sub-signals  1100 A and  1100 B, comprising sub-signals  1100 E and  1100 F, respectively, are diverted to Port W 8   920  so as to be recombined into the input monitor signal  1100 I. Immediately after emerging from Port W 4 , the two sub-signals  1100 A and  1100 B are optically transferred, in a polarization-preserving fashion, into Port W 2   914  so as to make a second traverse through device  904 . These two sub-signals emerge from this second traverse through device  904  through Port W 3   915  as the re-combined signal  1100 . The signal  1100  is then transferred into the optical gain element where it is amplified.  
     [0093] Referring now to FIG. 11B, after emerging from the optical gain element, the amplified eastbound signal  1100  then enters the East device  902  through Port E 1   905  and is then separated into two linearly polarized sub-signals  1100 C and  1100 D. The sub-signals  1100 C and  1100 D each make a first traverse through East device  902  and emerge from Port E 4   908 . Immediately thereafter, the two sub-signals  1100 C and  1100 D are optically transferred, in a polarization-preserving fashion, into Port E 2   906  so as to make a second traverse through device  902 . During this second traverse, small (approximately 5% or less) proportions of sub-signals  1100 C and  1100 D, comprising sub-signals  1100 G and  1100 H, respectively, are diverted to Port E 7   911  so as to be recombined into the output monitor signal  1100 J. The two sub-signals  1100 C and  1100 D emerge from this second traverse through device  902  through Port E 3   907  as the re-combined amplified signal  1100 . At this point, the amplified signal  1100  exits the bi-directional amplifier system and re-joins the lightwave communications system in the logical eastbound direction.  
     [0094] The detailed operation of either embodiment of the current invention as a bi-directional amplifier system is shown in FIGS.  12 A-D and FIGS.  13 A-D. FIGS.  12 A-D illustrate detailed light ray paths and polarization states for signals and laser pump beams within the logical East device of either a band bi-directional or an interleaved bi-directional amplifier system. FIGS.  13 A-D illustrate detailed light ray paths and polarization states for signals and laser pump beams within the complementary logical West device. In FIGS.  12 A-D and FIGS.  13 A-D, the disposition of components is as shown in either FIG. 5A or FIG. 5B except that a single element  580  is used to represent either the reflector/waveplate assembly  509  or the selective pass-through non-linear interferometer  519 . In the former case, the system is a band bi-directional amplifier system; in the latter case, it is an interleaved bi-directional amplifier system. The paths of signals through either such system are similar except that, in the band bi-directional system, westbound and eastbound signals comprise light of the “blue” band and “red” band, respectively, whereas, in the interleaved bi-directional system, these signals comprise light of the first set of interleaved bands and of the second set of interleaved bands, respectively.  
     [0095] In FIGS.  12 A-D and FIGS.  13 A-D, solid and dashed medium-weight lines with affixed directional arrows represent the respective paths of sub-signals comprising linearly polarized light rays having mutually perpendicular polarization plane orientations. Likewise, solid and dashed light-weight lines with affixed directional arrows represent the respective paths of small proportions of the aforementioned sub-signals utilized for intensity monitoring. Likewise, solid and dashed heavy-weight lines with affixed directional arrows represent the respective paths of sub-beams of pump laser light comprising linearly polarized rays having mutually perpendicular polarization plane orientations. Furthermore, double-barbed arrows contained within circles in FIGS.  12 A-D and FIGS.  13 A-D represent the orientations of polarization planes of linearly polarized light sub-signals or sub-beams as viewed from a fixed reference point at the left side of the respective diagram.  
     [0096]FIGS. 12A through 12D each show a bi-directional isolator, and, more particularly, an East bi-directional isolator. More particularly, FIG. 12A shows an East bi-directional isolator through which an east-to-west signal of the blue band (first interleaved set of bands) passes, an east-to-west pump laser light, and a blue signal input monitor. FIG. 12B shows an East bi-directional isolator which rejects a west-to-east traveling signal of the blue band (first interleaved set of bands) and which rejects a west-to-east traveling residual pump laser signal. FIG. 12C shows an East bi-directional isolator through which passes a west-to-east traveling signal of the red band (second interleaved set of bands), and which includes a red output monitor. FIG. 12D shows an East bi-directional isolator which rejects an east-to-west traveling signal of the red band (second interleaved set of bands).  
     [0097] More particularly, FIG. 12A and 12B are side views of the operation of the East bi-directional optical passive components  902  in a bi-directional amplifier system showing central ray paths for light signals of the westbound channels in each of the two principal polarization states propagating east-to-west and west-to-east, respectively, and FIG. 12C and 12D are side views of the operation of the East bi-directional optical passive components in a bi-directional amplifier system showing central ray paths for light signals of the eastbound channels in each of the two principal polarization states propagating west-to-east and east-to-west, respectively.  
     [0098] Also shown in FIG. 12A are central ray paths, within the East bi-directional optical passive components, for light, in each of two principal polarization states, of westbound pump laser light and monitored westbound signal light. Also shown in FIG. 12B are central ray paths, within the East bi-directional optical passive components, for light, in each of two principal polarization states, of residual eastbound pump laser light. Also shown in FIG. 12C are central ray paths, within the East bi-directional optical passive components, for light, in each of two principal polarization states, of monitored eastbound signal light.  
     [0099] In addition, FIG. 13A and 13B are side views of the operation of the West bi-directional optical passive components  904  in a bi-directional amplifier system showing central ray paths for light signals of the eastbound channels in each of the two principal polarization states propagating west-to-east and east-to-west, respectively, and FIG. 13C and 13D are side views of the operation of the West bi-directional optical passive components in a bi-directional amplifier system showing central ray paths for light signals of the westbound channels in each of the two principal polarization states propagating east-to-west and west-to-east, respectively.  
     [0100]FIG. 13A shows a West bi-directional isolator through which passes a west-to-east traveling signal of the red band (second interleaved set of bands), which includes a pump laser signal traveling west-to-east, and a red signal input monitor. Also shown in FIG. 13A are central ray paths, within the West bi-directional optical passive components, for light, in each of two principal polarization states, of eastbound pump laser light and monitored eastbound signal light.  
     [0101]FIG. 13B shows a West bi-directional isolator which rejects an east-to-west traveling signal of the red band (second interleaved set of bands) and which rejects a residual pump laser signal traveling east-to-west. Also shown in FIG. 13B are central ray paths, within the West bi-directional optical passive components, for light, in each of two principal polarization states, of residual westbound pump laser light.  
     [0102]FIG. 13C shows a West bi-directional isolator through which an east-to-west traveling signal of the blue band (first interleaved set of bands) passes, and which includes a blue signal output monitor. Also shown in FIG. 13C are central ray paths, within the West bi-directional optical passive components, for light, in each of two principal polarization states, of monitored westbound signal light.  
     [0103]FIG. 13D shows a West bi-directional isolator which rejects a west-to-east traveling signal of the blue band (first interleaved set of bands).  
     [0104] The front set of elements comprising the reflector/waveplate assembly  509  of the first embodiment,  500 A, together with all elements disposed to the front thereof comprises a band bi-directional polarization independent optical isolator. The front set of elements comprising the selective pass-through non-linear interferometer  519  of the second embodiment,  500 B, together with all elements disposed to the front thereof comprises an interleaved bi-directional polarization independent optical isolator. More specifically, because, in the current invention, the signals make two consecutive separate traverses through these isolators, the aforementioned front sets of elements comprise double stage band bi-directional and double stage interleaved bi-directional polarization independent optical isolators, respectively. The single stage and double stage band bi-directional polarization independent optical isolators and the single stage and double stage interleaved bi-directional optical isolators are embodiments of an invention disclosed in Bi-Directional Polarization-Independent Optical Isolator.  
     [0105] The aforementioned two bi-directional optical isolators have properties such that signals of a first set of wavelengths are allowed to propagate therethrough in a first direction but are prevented from propagating therethrough in the opposite or second direction and signals of a second set of wavelengths are allowed to propagate therethrough in the second direction but are prevented from propagating therethrough in the first direction. In the band bi-directional optical isolator, the first and second sets of wavelengths respectively comprise each one of two wavelength bands, such as a shorter wavelength or “blue” band and a longer wavelength or “red” band as illustrated in FIG. 3. Each such band may comprise one or more signal channels. In the interleaved bi-directional optical isolator, the first and second sets of wavelengths respectively comprise each one of two sets of interleaved bands within a multichannel system, such as is illustrated in FIGS.  4 A-C.  
     [0106] Together, FIG. 12 and FIG. 13 provide detailed information on the operation of each of the logical East device and the logical West device as a set of optical passive components for a bi-directional amplifier system. Furthermore, these figures also provide detailed information on the operation of the integrated system, comprising the sets of optical passive components together with an optical gain element, lasers, optical detectors and comparison and control logic, as a bi-directional amplifier system. A schematic block diagram of such a bi-directional amplifier system is illustrated, for instance, in FIG. 14.  
     [0107] As mentioned above, the front set of components of each device comprise bi-directional optical isolators. This front set of components comprises the pathway of the greater proportion of each signal propagating through each device and is identical between the East and West devices. The details of the operation of each bi-directional isolator are provided in Bi-Directional Polarization-Independent Optical Isolator. Briefly, however, FIG. 12A and FIG. 13C demonstrate the passage, through the logical East device and the logical West device, respectively, of westbound signals in the West direction. Also, FIG. 12B and FIG. 13D demonstrate the inhibited passage or rejection, within the logical East device and the logical West device, respectively, of westbound signals spuriously reflected or scattered backward in the east direction. Furthermore, FIG. 12C and FIG. 13A demonstrate the passage, through the logical East device and the logical West device, respectively, of eastbound signals in the east direction. Finally, FIG. 12D and FIG. 13B demonstrate the inhibited passage or rejection, within the logical East device and the logical West device, respectively, of eastbound signals spuriously reflected or scattered backward in the west direction.  
     [0108] The signal bi-directional optical isolator sections of each integrated set of optical passive components function through the separation, at a birefringent element, of incoming signals into physically separated sub-signals whose light rays have mutually perpendicular linear polarization directions, propagation of said sub-signals through both a reciprocal 45° rotation element and a non-reciprocal 45° rotation element, and propagation of said rotated sub-signals through a second or the same birefringent element. The birefringent element(s) separate unpolarized or randomly polarized light into two linearly polarized physically separated beams, an undeflected o-ray and a deflected e-ray. In the terminology used herein, reciprocal rotators have the property such that the direction of rotation about the axis of light propagation, either clockwise (CW) or counter-clockwise (CCW), is always the same when viewed facing the rotator towards the side at which the linearly polarized light beam enters the element. Conversely, non-reciprocal (non-reversible) rotators have the property such that the direction of polarization plane rotation about the axis of light propagation, either clockwise (CW) or counter-clockwise (CCW), is always the same when viewed facing the rotator from a fixed reference point in a fixed direction, regardless of the propagation direction of the light ray through the element. In the current invention, the λ/2 plates are reciprocal rotators and the Faraday rotation elements are non-reciprocal rotators.  
     [0109] Additional properties of the present invention not previously disclosed in Bi-Directional Polarization-Independent Optical Isolator are the ability to inject a westbound pump laser beam into the system from the logical East device (FIG. 12A), the ability to block residual eastbound pump laser light from entering the East laser (FIG. 12B), the ability to inject an eastbound pump laser beam into the system from the logical West device (FIG. 13A), the ability to block residual westbound pump laser light from entering the West laser (FIG. 13B), the ability to monitor the input westbound signal at the East device (FIG. 12A), the ability to monitor the output eastbound signal at the East device (FIG. 12C), the ability to monitor the input eastbound signal at the West device (FIG. 13A), and the ability to monitor the output westbound signal at the West device (FIG. 13C).  
     [0110] The optical passive components of the present invention provide an additional bi-directional optical isolator function by preventing the entry of pump laser light from each laser into the other laser. The operation of this functionality at the East device is now described with reference to FIG. 5A, FIGS. 9A through 9D, FIG. 10A, FIG. 12A and FIG. 12B. Operation at the West device is similar and will not be re-described in detail.  
     [0111] Injection of westbound pump laser light  1002  into the system is first described. This injection occurs through Port E 8   912  in the rear four-fiber ferrule  511  of the East device  902 . This laser light immediately passes into and through the birefringent walk-off plate  514  that physically separates it into two linearly polarized sub-lights  1002 A and  1002 B. As shown in FIG.  12 A, the optical axis of the birefringent plate  514  is oriented such that sub-light  1002 A is a horizontally polarized o-ray whilst sub-light  1002 B is a vertically polarized e-ray during its respective traverse through plate  514 . The separation of sub-lights  1002 A and  1002 B is effected by the deflection of the e-ray, which is sub-light  1002 B. Immediately after leaving birefringent plate  514 , the sub-lights  1002 A and  1002 B pass through the Faraday rotator  516 B which, upon passage therethrough, imparts a 45° CW rotation (as viewed end-on from the left side of FIG. 12A) upon the direction of each of their polarization planes.  
     [0112] The two sub-lights  1002 A and  1002 B then pass towards and through the rear lens  510  that collimates them and directs them through either the mirror/waveplate assembly  509  or the selective pass-through non-linear interferometer  519  (both represented as element  580  in FIG. 12A) depending upon whether the device comprises the first or the second embodiment of the present invention, respectively. Because the sub-lights  1002 A and  1002 B are of a wavelength shorter than  1500  nm, and because of the wavelength dependent transmission characteristics of either the mirror/waveplate assembly  509  (FIG. 7) or of the non-linear interferometer  519  (FIG. 8), these sub-lights are transmitted completely therethrough without attenuation. Furthermore, because of the presence of the compensating plates, either λ/4 plate  608  (FIG. 6) or the first compensating wave plate  814  together with the second compensating wave plate  816  (FIG. 8), these sub-lights are transmitted through either mirror/waveplate assembly  509  or the non-linear interferometer  519  without a change in polarization. The sub-lights  1002 A and  1002 B then enter and pass through the front lens  508  which focuses and directs them towards and into the front λ/2 plate  505 . The front λ/2 plate  505  reversibly rotates directions of the planes of polarization of each of sub-lights  1002 A and  1002 B by 45° CCW about the direction of propagation or 45° CW as viewed from the left side of FIG. 12A since these sub-lights propagate from back to front.  
     [0113] After passing through the λ/2 plate  505 , the sub-lights  1002 A and  1002 B enter and pass through the front birefringent plate  504 . During their passage through front birefringent plate  504  as shown in FIG. 12A, the sub-lights  1002 A and  1002 B are vertically and horizontally polarized, respectively. Because the optic axis of front birefringent plate  504  is symmetrically disposed about a vertical plane with respect to that of rear birefringent plate  514 , then sub-light  1002 A and  1002 B pass through front birefringent plate  504  as a deflected e-ray and as an undeflected o-ray, respectively. Furthermore, since the thickness and material of birefringent plates  504  and  514  are identical, the deflection of sub-light  1002 A during its passage through front birefringent plate  504  is exactly equal and opposite to that of sub-light  1002 B during its passage through rear birefringent plate  514 . Thus, upon exiting front birefringent plate  504 , the two sub-lights  1002 A and  1002 B are re-combined so as to regenerate the laser light  1002 .  
     [0114] Because of the image reversing properties of the pair of lenses  508  and  510 , the regenerated laser light  1002  enters the front port or fiber-namely Port E 1   905 -that is diagonally disposed relative to the rear port-namely Port E 8   912 -from which it was injected.  
     [0115] As seen in FIG. 10A, the Port E 1   905  is directly optically coupled to the optical gain element. In this way, westbound pump laser light is injected into the optical gain element at the East device.  
     [0116] It is possible that residual eastbound (westbound) laser pump light  1004  ( 1002 ) originally injected at the West (East) device may enter the East (West) device through Port E 1  (W 3 ). The blocking of laser pump light  1004  from entering the laser of the East device is discussed below with reference to FIG. 12B. The discussion of the blocking of laser pump light  1002  from entering the laser port of the West device is similar and therefore omitted. By means of this blocking, each device of the present invention behaves as an optical isolator for the laser pump beams as well as for the signal lights.  
     [0117] As illustrated in FIG. 12B, any residual eastbound laser pump light  1004  entering the East device through Port E 1  immediately passes into and through the front birefringent walk-off plate  504  where it is physically separated it into two linearly polarized sub-lights  1004 C and  1004 D. As discussed above, the optical axes of the birefringent plate  504  are oriented such that sub-light  1004 C is a horizontally polarized o-ray whilst sub-light  1004 D is a vertically polarized e-ray during its respective traverse through plate  504 . The separation of sub-lights  1004 C and  1004 D is effected by the deflection of the e-ray, which is sub-light  1004 D. Immediately after leaving front birefringent plate  504 , the sub-lights  1004 C and  1004 D pass through the front λ/2 plate  505  which, upon passage therethrough, imparts a 45° CCW rotation upon the direction of each of their polarization planes. The two sub-lights  1004 C and  1004 D then pass towards and through the front lens  508  that collimates them and directs them through either the mirror/waveplate assembly  509  or the selective pass-through non-linear interferometer  519  (both represented as element  580  in FIG. 12B) depending upon whether the device is of the first or the second embodiment of the present invention, respectively. Because the sub-lights  1004 C and  1004 D are of a wavelength shorter than 1500 nm, then, as discussed above, these sub-lights are transmitted completely therethrough without attenuation. Furthermore, because of the presence of the compensating plates, either λ/4 plate  608  (FIG. 6) or the first compensating wave plate  814  together with the second compensating wave plate  816  (FIG. 8), these sub-lights are transmitted through either mirror/waveplate assembly  509  or the non-linear interferometer  519  without a change in polarization. The sub-lights  1004 C and  1004 D then enter and pass through the rear lens  510  which directs them towards and into the Faraday rotator  516 B which, upon passage therethrough, imparts a 45° CW rotation (as viewed end-on from the left side of FIG. 12B) upon the direction of each of their polarization planes. After passing through the Faraday rotator  516 B, the sub-lights  1004 C and  1004 D enter and pass through the rear birefringent plate  514 . During their passage through rear birefringent plate  514  as shown in FIG. 12B, the sub-lights  1004 C and  1004 D are horizontally and vertically polarized, respectively. Because the optic axis of rear birefringent plate  514  is symmetrically disposed about a vertical plane with respect to that of front birefringent plate  504 , then sub-light  1004 C and  1004 D pass through rear birefringent plate  514  as an undeflected o-ray and as a deflected e-ray, respectively. As shown in FIG. 12B, the pathways of the o-ray and e-ray within rear birefringent plate  514  are such that the physical separation between the sub-lights  1004 C and  1004 D increases upon passage therethrough and neither is directed into the Port E 8   912 . In this way, residual eastbound pump laser light is prevented from entering into the port-namely Port E 8   912 -from which westbound laser light is injected.  
     [0118] The optical passive components of the present invention provide both additional input and output monitoring capabilities of each of the counter-propagating signal light sets. As noted above in the discussion of laser light injection, these capabilities are possible because of the transmission characteristics of either the mirror/waveplate assembly  509  (FIGS.  6 - 7 ) or of the non-linear interferometer  519  (FIG. 8), which are both indicated as element  580  in FIGS.  12 A- 12 D and FIGS.  13 A- 13 D. Thus, a small proportion (&lt;5%) of each sub-signal is transmitted either element  509  or  519  so as to be intercepted by rear lens  510 . Furthermore, because of the presence of the compensating plates, either the λ/4 plate  608  (FIG. 6) or the first compensating wave plate  814  together with the second compensating wave plate  816  (FIG. 8), the transmitted proportions of the sub-signals do not incur a change in polarization during passage through element  509  or  519 .  
     [0119] The optical pathway of one monitor signal, namely the monitored proportion of the westbound signal obtained at the East optical passive components device, is described with reference to FIG. 12A. The discussions of the optical pathways of other monitor signals are similar and are therefore omitted. As shown in FIG. 10A and FIG. 12A, the un-amplified westbound signal  1000  enters the East device through Port E 3 . As shown in FIG. 12A, the signal  1000  first enters the first birefringent walk-off plate  504  where it is physically separated into two sub-signals  1000 A and  1000 B whose light rays have mutually orthogonal linear polarization plane orientations. As shown in FIG. 12A, the sub-signal  1000 A passes through element  504  as a horizontally polarized, undeflected o-ray and the sub-signal  1000 B passes through element  504  as a vertically polarized, deflected e-ray. As shown in FIGS. 5A, 9A and  12 A, these two sub-signals then pass through the Faraday rotator  506  which imparts a 45° CCW rotation about the propagation direction to each of their polarization planes. The sub-signals  1000 A and  1000 B then enter and pass through the front lens  508  which collimates them and directs them towards either the reflector/waveplate assembly  509  or the selective pass-through nonlinear interferometer  519 . As discussed above, the monitored proportion of each of these sub-signals passes through either element  509  or  519  (both represented as element  580  in FIG. 12A) without a change in polarization plane orientation and passes to and through the rear lens  510 .  
     [0120] In FIG. 12A, the monitored proportion  1000 E of sub-signal  1000 A and the monitored proportion  1000 F of sub-signal  1000 B are respectively shown as a thin dashed and a thin solid line. As shown in FIGS. 10A and 12A, these monitor sub-signals are focused and directed by rear lens  510  into and through the optical elements adjacent to Port E 6 . These elements are, in sequence, the rear λ/2 plate  515 A and the rear birefringent plate  514 . As shown in FIG. 12A, the rear λ/2 plate  515 A imparts a 45° CCW rotation to the polarization plane directions of each of the sub-signals  1000 E and  1000 F such that, after passing therethrough, these directions are vertical and horizontal, respectively. After passing through the λ/2 plate  515 A, the monitor sub-signals  1000 E and  1000 F then enter and pass through the rear birefringent plate  514 . Because the optic axis of rear birefringent plate  514  is symmetrically disposed about a vertical plane with respect to that of front birefringent plate  504 , then sub-signal  1000 E and  1000 F pass through rear birefringent plate  514  as a deflected e-ray and as an undeflected o-ray, respectively. Furthermore, since the thickness and material composition of birefringent plates  504  and  514  are identical, the deflection of sub-signal  1000 E during its passage through rear birefringent plate  514  is exactly equal and opposite to that of sub-signal  1000 B during its passage through front birefringent plate  504 . Thus, upon exiting rear birefringent plate  514 , the two monitor sub-signals  1000 E and  1000 F are re-combined so as to generate the monitor signal  1000 I. The monitor signal  1000 I then exits the device through Port E 6 .  
     [0121] In the foregoing discussions, the front birefringent plate  504  and the rear birefringent plate  514  have been illustrated as being of similar material and thickness and as being disposed such that their respective optic axes are symmetrically disposed about a vertical plane. The purpose of such conditions is so that sub-signals or separated optical rays separated in one such birefreingent plate are readily re-combined in the other such plate. However, one of ordinary skill in the art will readily recognize that such separation and re-combination can be accomplished by other choices of materials, thicknesses and orientations of said two birefringent plates, in various combinations. Such modifications are within the scope and spirit of the present invention.  
     [0122]FIG. 14 shows a bi-directional amplifier system, which includes an integrated isolator, monitor, and amplifier. More particularly, FIG. 14 is a basic block diagram of a band bi-directional Er-doped optical fiber amplifier (EDFA) system  1400  showing the assembly of optical passive components in accordance with the present invention. In FIG. 14, for purposes of example only, a westbound signal is shown as belonging to the “blue” band and an eastbound signal is shown as belonging to the “red” band. A logical West bi-directional optical passive component device  904  (or bi-directional polarization independent optical isolator) and a logical East bi-directional optical passive component device  902  (or bi-directional polarization independent optical isolator), both in accordance with the first embodiment of the present invention, provide the core of the bi-directional amplifier system  1400  shown in FIG. 14. An Er-doped optical fiber (EDF)  1406  disposed between the logical West and East devices is taken as the optical gain element.  
     [0123] With reference to FIGS. 10A, 10B,  11 A,  11 B and FIG. 14 and as described above, the EDF is optically coupled to Port W 3  of the West device  904  and to Port E 1  of the East device  902 . Mutually counter-propagating westbound signals  1407  of the blue band and eastbound signals  1409  of the red band are coupled from the logical West-side fiber  1408  of the light transmission system to the bi-directional amplifier system  1400  through Port W 1  of device  904 . These same signals are coupled from the logical East-side fiber  1410  of the light transmission system to the bi-directional amplifier system  1400  through Port E 3  of device  902 . An optional blue output tap  1412  and optional red input tap  1414 , which are optically coupled to Port W 5  and Port W 8 , respectively, of device  904 , direct a small monitored proportion of the output blue signal light and of the input red signal light to a first photo-detector  1416  and a second photo-detector  1418 , respectively. Likewise, an optional blue input tap  1420  and optional red output tap  1422 , which are optically coupled to Port E 6  and Port E 7 , respectively, of device  902 , direct a small monitored proportion of the input blue signal light and of the output red signal light to a third photo-detector  1424  and a fourth photo-detector  1426 , respectively. A logical West pump laser  1428  and a logical East pump laser  1430  provide the eastbound pump laser light  1432  and westbound pump laser light  1434 , respectively. These laser lights are optically coupled into the bi-directional amplifier system through Port W 6  of device  904  and Port E 8  of device  902 . These two laser lights are input to the Er-doped fiber  1406  through Port W 3  of device  904  and Port E 1  of device  902 .  
     [0124] The red-band signal light  1409  and the laser light  1432  propagate together in the eastbound direction through Er-doped fiber  1406  whilst the blue signal light  1407  and the laser light  1434  simultaneously propagate together in the westbound direction through this fiber. By this means, signal  1407  becomes amplified during its traverse through fiber  1406  from device  902  to device  904  and signal  1409  becomes amplified during its traverse from device  904  to device  902 . Because device  904  and device  902  comprise. signal optical isolator functions, device  904  and device  902  only permit transmission of signals  1407  and  1409 , respectively, in directions away from the Er-doped fiber  1406  so that spurious back-reflected signals do not become amplified. Also, device  904  and device  902  only permit transmission of signal  1409  and  1407 , respectively in directions towards the Er-doped fiber  1406  so that amplified signals do not exit the amplifier  1400  in incorrect directions. Furthermore, the eastbound laser light  1432  and the westbound laser light  1434  are prevented from entering the East pump laser  1430  and the West pump laser  1428 , respectively, so as to prevent damage to either of these lasers. East pump laser  1430  and West pump laser  1428  each operate upon propagating signal light in accordance with the description of co-pump laser  104  and counter-pump laser  105 , described herein above with reference to FIG. 1.  
     [0125] Finally, also shown in FIG. 14 is an optional comparison and control logic system  1436  which represents a set of electronic or computer systems together with decision-making software or firmware which monitors the electronic outputs of all the optional photo-detectors and controls the outputs of the two lasers  1428  and  1430  accordingly so as to obtain optimal amplification performance.  
     [0126]FIG. 15 shows a flowchart of the operation of an optical communications systems including the bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention. More particularly, FIG. 15 shows a method  2000  of bi-directional optical amplification of signal rays within an optical communications system.  
     [0127] The method  2000  of FIG. 15 simultaneously inputs  2002  into the bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention two separate signal rays from two different segments or spans of the optical communications system. These two segments are optically coupled to different ends, for instance, an end comprising a logical East bi-directional optical passive component device and an end comprising a logical West bi-directional optical passive component device, of the bi-directional polarization independent optical isolator and monitor/amplifier system. The bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention causes simultaneous transmission of the two separate signal rays in opposite forward directions through the optical gain element while simultaneously suppressing backward transmission of each of the two separate signal rays in its respective reverse direction.  
     [0128] The received input signal rays may be two separate signal rays separated into two, respective and separate wavelength bands.  
     [0129] Alternatively, the two separate signal rays may include two sets of wavelengths, each including a plurality of wavelengths, such that wavelengths of the two signal rays are alternatingly interspersed with each other. More particularly, then, the method of the present invention includes symmetric interleaved and asymmetric interleaved bi-directional wavelength multiplexed optical signal propagation in a light wave communications system including propagating a first set of channels included within a first set of bands in a first direction and a second set of channels included within a second set of bands in a second direction opposite to the first direction, in which the width of bands of the first set of bands either is (symmetric interleaved) or is not (asymmetric interleaved) equal to the width of bands of the second set of bands, and the first and second bands are interleaved with one another.  
     [0130] The bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention then separates  2004  the input signal rays into components, transmits the components through the bi-directional polarization independent optical isolator of the present invention, and monitors the power of the components. More particularly, the bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention monitors the optical power of each of the two signal rays both before its respective input into and after its respective output from the optical gain element.  
     [0131] The bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention re-combines  2006  the components traveling in the forward direction into output signal rays, while preventing the re-combination of the components traveling in the reverse direction. Subsequently the bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention simultaneously outputs  2008  the two separate signal rays into opposite ends of the optical gain element; both signal rays are simultaneously amplified within the optical gain element and an output end of the first of the two signal rays is an input end of the second of the two signal rays and an output end of the second of the two signal rays is an input end of the first of the two signal rays.  
     [0132] In addition, the bi-directional polarization independent optical isolator of the present invention transmits  2010  pump laser beams into the optical gain element to amplify the output signal rays. More particularly, the bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention transmits a pump laser beam through the optical gain element in a same direction as a first of the two signal rays and transmits another pump laser beam through the optical gain element in a same direction as a second of the two signal rays. Each of said pump laser beams is prevented by the bi-directional polarization independent optical isolator of the present invention from being output at the input of the other of the pump laser beams.  
     [0133] Finally, the bi-directional polarization independent optical isolator and monitor/amplifier system of the present invention outputs  2012  the output signal rays into the two segments or spans of the optical communications system such that the signal ray that was received from the first such segment is output to the second segment, and vice versa.  
     [0134] The present invention has been described with respect to the above-mentioned embodiments, but is not limited thereto.  
     [0135] The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention as described in the appended claims.