Abstract:
A fiber optic isolator device is used by fiber optic systems operating at more than one wavelength. The device may be inserted anywhere within the fiber network. The fiber optic device permits the separation of the wavelengths so that an optical isolator module can isolate a first wavelength without significantly affecting the second wavelength. This device is useful isolating a communications signal at 1.55 μm while avoiding significant losses for an optical time domain reflectometry signal, for example at 1.3 μm.

Description:
This application is a divisional of application Ser. No. 09/505,077, filed Feb. 16, 2000. The application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed generally to a fiber optic isolator, and more particularly to a fiber optic isolator for fiber optic systems operating at multiple wavelengths. 
     BACKGROUND 
     Optical fibers find many uses for directing beams of light between two points. Optical fibers have been developed to have low loss, low dispersion, and polarization maintaining properties and can also act as amplifiers. As a result, optical fiber systems find widespread use, for example in optical communication applications. 
     It is not uncommon for an optical fiber system to support the transport of light at two or more wavelengths. For example, the communications signal propagating along the fiber may have a wavelength of, or be within a wavelength range centered at, about 1.55 μm, while a diagnostic signal may also be sent along the fiber, having a wavelength of approximately 1.3 μm. The diagnostic signal may be, for example, an optical time domain reflectometry (OTDR) signal. Other wavelengths that may be used in the same fiber as the communications signal include a pump signal for pumping an optical amplifier. For example, where the optical signal is at about 1.55 μm, the pump signal may be at about 980 nm for pumping an erbium-doped fiber amplifier, or at about 1.48 μm for pumping a fiber Raman amplifier. 
     It is common to isolate a fiber signal source from a fiber amplifier by placing an isolator between the two. However, the isolator may introduce loss to the other wavelength component propagating within the fiber. For example, an isolator positioned between the transmitter and a fiber amplifier transmits both the optical communications signal and the OTDR signal in the forward direction. In the reverse direction, the isolator introduces large losses for the optical communications signal. However, the isolator also introduces significant losses for the OTDR signal in the reverse direction, which interferes with the ability to use OTDR as an effective diagnostic tool in a fiber system. 
     Therefore, there is a need to provide an isolator that is effective at introducing high losses at one wavelength in the reverse direction, but which introduces little loss to a second wavelength in the reverse direction. 
     In other situations, it may be desired to provide isolation at two wavelengths. However, the bandwidth over which an isolator is effective is limited. Where the two wavelengths are separated by more than the effective bandwidth of the isolator, only one of the wavelengths is isolated effectively. 
     Therefore, there is a need for a fiber optic isolator that can operate effectively for two wavelengths having a relatively wide separation. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention relates to an isolator device for use in fiber optic systems that operate with light at more than one wavelength. The isolator device may be inserted anywhere within the fiber network. One particular embodiment of the invention permits the separation of the wavelengths so that an optical isolator module can operate on that separated wavelength without operating on the other wavelength component or components. The different wavelengths may then be recombined. In another embodiment of the invention, different wavelengths may be combined into a single fiber, with an optical isolator module being disposed to operate on one of the wavelengths. 
     One particular embodiment of the invention is a fiber optic isolator device having a first optical fiber optically coupled to transmit light at first and second wavelengths along a first optical path. A wavelength separator is disposed on the first optical path and is adapted to direct light at the first wavelength along a second optical path and light at the second wavelength along a third optical path different from the second optical path. A wavelength combiner is optically coupled to combine light propagating along the second and third optical paths into a fourth optical path and a second optical fiber optically coupled to the fourth optical path. A first optical isolator module is disposed along the second optical path between the wavelength separator and the wavelength combiner to transmit light at the first wavelength passing from the first fiber to the second fiber, and to substantially block light at the first wavelength from passing from the second fiber to the first fiber. 
     Another embodiment of the invention is a fiber optic device that has wavelength separating means for separating a light beam into a first light beam containing light at a first wavelength and a second light beam containing light at a second wavelength different from the first wavelength, the first and second beams respectively propagating along first and second beam paths. Optical isolating means is disposed on the first beam path for passing light in the first wavelength from the wavelength splitting means to the wavelength combining means, and for blocking light at the first wavelength from passing from the wavelength combining means to the wavelength splitting means. The device also includes wavelength combining means for combining light propagating in the first direction along the first beam path, and light propagating in the first direction along the second beam path into a single output beam. 
     Another embodiment of the invention is a fiber optic device that has first and second optical fibers optically coupled via first and second optical paths respectively to a wavelength combiner. Light at a first wavelength from the first optical fiber is combined with light at a second wavelength from the second fiber at the wavelength combiner to form a combined output beam. A third optical fiber is coupled via a third optical path to receive the combined output beam from the wavelength combiner. A first optical isolator module is positioned on the first optical path to pass light at the first wavelength from the first optical fiber to the wavelength combiner, and to substantially block light at the first wavelength from passing from the wavelength combiner to the first optical fiber. 
     In another embodiment of the invention, a fiber optic device includes a first optical fiber optically coupled via a first optical path to a wavelength separator to transmit light to the wavelength separator. The wavelength separator is arranged to separate light received from the first optical fiber into components at first and second wavelengths. A second optical fiber is coupled via a second optical path to the wavelength separator to receive light at the first wavelength. A third optical fiber is coupled via a third optical path to the wavelength separator to receive light at the second wavelength. A first isolator module is positioned on the second optical path to transmit light at the first wavelength from the wavelength separator to the second optical fiber and to substantially block transmission of light at the first wavelength from the second optical fiber to the wavelength separator. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
     FIG. 1 schematically illustrates a fiber optic communications system; 
     FIG. 2A schematically illustrates a multiwavelength isolator device according to an embodiment of the invention; 
     FIG. 2B schematically illustrates an in-line isolator device; 
     FIG. 3 schematically illustrates a polarization based wavelength separator; 
     FIGS. 4A and 4B schematically illustrate a first embodiment of an isolator module; 
     FIG. 5A schematically illustrates a second embodiment of an isolator module; 
     FIG. 5B illustrates the relative position of different polarization states at different points throughout the second embodiment of the isolator module illustrated in FIG. 5A; 
     FIG. 6 schematically illustrates an embodiment of a multiwavelength isolator device according to the present invention; 
     FIG. 7A schematically illustrates another embodiment of a multiwavelength isolator device according to the present invention; 
     FIG. 7B schematically illustrates a terminal isolator device; 
     FIGS. 8-11 schematically illustrate additional embodiments of a multiwavelength isolator device according to the present invention; 
     FIGS. 12 and 13 schematically illustrate embodiments of a multiple wavelength separator/combiner device according to the present invention; 
     FIG. 14 schematically illustrates an embodiment of a multiwavelength isolator device according to the present invention; and 
     FIG. 15 schematically illustrates an embodiment of a multiwavelength isolator device providing isolation at one wavelength and circulation at another wavelength. 
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention is applicable to optical fiber systems, and is believed to be particularly suited to optical fiber communication systems in which light propagates along the fiber at more than one wavelength. 
     A fiber system  100  operating at more than one wavelength is illustrated in FIG.  1 . The fiber system  100  includes a fiber optic communications channel  104  coupled between a transmitter  102  and a receiver  106 . The transmitter  102  includes a light source  108  operating at a first wavelength, λ1. For example, the light source  108 , may generate a communications signal at approximately 1.55 μm, or may generate a band of individual wavelengths at about 1550 nm, such as a multiplexed optical communications signal. 
     A diagnostic light source  112 , for example an optical time domain reflectometer (OTDR), includes a light source  114  operating at a second wavelength λ2, for example 1.3 μm. Light from the OTDR is combined with light from the transmitter in a combiner  116  and launched into the fiber channel  104 . 
     The fiber channel  104  includes an amplifier section  110 , for example an erbium-doped fiber amplifier. A pump laser  118 , operating at a third wavelength, λ3, is coupled via a fiber coupler  120  to the fiber channel  104 . Where the amplifier section  110  is an erbium-doped fiber amplifier, the third wavelength is typically around 980 nm. Where the amplifier section is a stimulated Raman amplifier, the third wavelength may be approximately 1.47 μm. 
     A first isolator  122  is positioned on the fiber channel  104  before coupler  120  to prevent the propagation of amplified, backscattered signals to the transmitter  108 . A second isolator  124  may be positioned at the output end of the amplifier section  110  to prevent feedback of signals into the amplifier that may reduce the amplifier gain for the forward travelling communications signal at λ1. 
     Light at two wavelengths passes through the first isolator  122 , at λ1 and λ1 and λ2. The isolator  122  ideally has negligible loss in the forward direction for signals at λ1, and has a high loss for signals at λ1 in the reverse direction. Also, the isolator  122  should permit the OTDR signal at λ2 to pass with negligible loss in both the forward and the backward directions. To achieve such operation, the isolator  122  may be may be of the type discussed below. 
     Light of at least two wavelengths passes through the second isolator  124 . Like the first isolator  122 , the second isolator  124  ideally transmits the OTDR signal at λ2 with negligible loss in both directions, while passing the communications signal at λ1 with negligible loss only in the forward direction, and substantially blocks light at λ1 from passing in the backwards direction. 
     One particular embodiment of a fiber optic isolator device operating at more than one wavelength is schematically illustrated in FIG.  2 A. The device  120  has two fibers  202  and  204  that couple to the external fiber optic system. The device  200  is in an “in-line” configuration, having one fiber at each end. An advantage of this configuration is that the overall width of the device package is small. A view of the “in-line” package is illustrated in FIG. 2B, illustrating the device housing  240 , with the two fibers  202  and  204  attached at either end of the housing  240 . 
     Each fiber  202  and  204  is terminated by a respective collimating lens  206  and  208  to reduce coupling losses between the two fibers  202  and  204 . Light  210  propagating from the first fiber  202  may contain one or more wavelength components. For the present discussion, it is assumed that two wavelength components are present, namely λ1 and λ2. Light at λ1 may be, for example, a communications signal at 1.55 μm, while the light at λ2 is an OTDR signal at 1.3 μm. The second wavelength component may also be, for example, pump light for an amplifier, such as 1.48 μm or 980 nm. The second wavelength component may be separated from the first wavelength component by at least 20 nm. 
     In the particular embodiment shown, it is desired that an optical device  212  operate only on the light at one wavelength, λ1, while the optical device  212  does not operate on the light at λ2. Accordingly, the wavelength components λ1 and λ2 are separated by a wavelength separator  214  into two components  216  and  218  respectively, propagating along different optical paths. The first component  216 , at λ1, is transmitted by the wavelength separator  214  and propagates through the first free space region  220  between the wavelength separator  214  and the wavelength combiner  222 . The first wavelength component  216  passes through the optical device  212  positioned in the free space region  220 . 
     The second wavelength component  218  is directed from the wavelength separator  214  along a path different from the path of the first wavelength component  216 . A guiding prism  224  has two reflective surfaces  226  and  228  that direct the second wavelength component  218  to the wavelength combiner  222 , where the first and second wavelength components  216  and  218  are combined into a single output beam  230  that propagates to the second collimating lens  208  and is focused into the second fiber  204 . The prism  224  may be, for example, a roof-top prism. 
     The figure illustrates only light passing from the first fiber  202  to the second fiber  204 . It will be appreciated that light may also pass from the second fiber  204  to the first fiber  202 . However, the isolator module  212  prevents light at λ1 from passing back into the first fiber  202  from the second fiber  204 . Since the light at λ2 bypasses the isolator module  212 , the light at λ2 may pass from the second fiber  204  to the first fiber  202 . 
     The wavelength separator  214  is any device that separates the light beam  210  into two wavelength components. In the particular embodiment illustrated, the wavelength separator is a beamsplitter cube  231  having a dichroic reflector  232  that transmits light at one wavelength, λ1, and reflects light at the other wavelength, λ2. The wavelength combiner  222  may be the same device as the wavelength separator  214 , only operating in reverse. In the embodiment shown, the wavelength combiner  222  is a beamsplitter cube  233  that includes a dichroic reflector  234  that transmits the light at λ1, and reflects the light at λ2. 
     Other types of wavelength separator and combiner may also be employed. For example, the wavelength separator may be a dispersing prism that separates light at different wavelengths into different paths. Such a prism also operates as a wavelength combiner, wherein light at different wavelengths introduced into the prism at selected angles emerges at the same angle. Another type of wavelength separator and/or combiner may be a diffraction grating. 
     The wavelength separator and/or combiner may also operate on a principle that depends on the polarization of light, for example as illustrated in FIG.  3 . The separator  314  is formed from a birefringent material. The single light beam  310  propagates within the separator to the reflecting surface  332 . The first wavelength component  316  propagates in a first polarization, with an associated first refractive index n1. The second wavelength component  318  propagates through in a second polarization, orthogonal to the first polarization, with an associated second refractive index n2, where n2&gt;n1. The surface  332  is cut at such an angle that the second wavelength component  318  is totally internally reflected, whereas the first wavelength component is transmitted at the surface  332 . It will be appreciated that such a polarization dependent device may also be used to combine light of different wavelengths, having different polarizations. 
     The guiding prism  224  is provided to direct the second wavelength component  218  from the wavelength separator  214  to the wavelength combiner  222 . It will be appreciated that other components may also be provided to serve this function. For example, two separate mirrors placed at the positions of the reflecting surface  226  and  228  of the prism may be used as a substitute for the prism  224 . 
     The isolator module  212  may be any suitable type of isolator module that allows passage of light at one wavelength in the forward direction, but prevents passage of light in the backwards direction. 
     One particular embodiment of isolator module  400  is illustrated in FIGS. 4A and 4B. The isolator module is described in U.S. Pat. No. 4,548,478, incorporated herein by reference. The isolator module includes two birefringent crystals  402  and  404  that are wedge shaped. The wedge of the second crystal  404  is oriented in a direction opposite the wedge of the first crystal  402 . 
     A non-reciprocal polarization rotator  406  is disposed between the two birefringent crystals  402  and  404 . The non-reciprocal polarization rotator  406  may be a Faraday rotator or any other suitable optical element that non-reciprocally rotates the polarization of light passing therethrough. 
     The isolator module  400  is positioned between first and second fibers  410  and  412  and respective first and second collimating lenses  414  and  416 . The passage of light from the first fiber  410  to the second fiber  412  is illustrated in FIG. 4A, while the passage of light from the second fiber  412  to the first fiber  410  is illustrated in FIG.  4 B. 
     First, with respect to FIG. 4A, light  420  diverges from the first fiber  420  and is collimated by the first collimating lens  414 . The collimated light enters the first birefringent crystal  402 . Light passing through the first crystal  402  as an ordinary wave, labeled “o”, propagates as a first ray  422  in a first direction, while light passing through the first crystal  402  as an extraordinary wave, labeled “e”, propagates as a second ray  424  in a second direction different from the first direction. The first ray  422  is refracted at the angled surface  421  of the first crystal  402 . The second ray  424  is incident on the angled surface  421  at a smaller angle of incidence than the first ray  422 , and is refracted to a lesser extent. The second ray  424  may be normally incident on the angled surface  421 . 
     The first and second rays  422  and  424  pass through the non-reciprocal polarization rotator  406 , where the polarization of each ray is rotated through approximately 45°. The first and second rays  422  and  424  then propagate to the second birefringent crystal  404 . The optical axis of the second birefringent crystal  404  is rotated 45° relative to the optical axis of the first birefringent crystal  402 . Therefore, the first ray  422  passes through the second birefringent crystal  404  as an ordinary wave, while the second ray  424  passes through the second birefringent crystal as an extraordinary wave. 
     The two rays  422  and  424  emerge from the second birefringent crystal mutually parallel and are focused by the second collimating lens  416  into the second fiber  412 . Thus, irrespective of the polarization of the light  420  transmitted by the first fiber  410 , the light  420  is transmitted to the second fiber  412 . 
     Next, we examine the propagation of light from the second fiber  412  to the first fiber  410  with reference to FIG.  4 B. Light  430  diverges from the second fiber  412  and is collimated by the second collimating lens  416 . The collimated light enters the second birefringent crystal  404 . Light passing through the second birefringent crystal  404  as an ordinary wave, labeled “o”, propagates as a first ray  432  in a first direction, while light passing through the second crystal  404  as an extraordinary wave, labeled “e”, propagates as a second ray  434  in a second direction different from the first direction. The first ray  432  is refracted at the angled surface  436  of the second crystal  404 . The second ray  434  is incident on the angled surface  436  at a smaller angle of incidence than the first ray  432 , and is refracted to a lesser extent. The second ray  434  may be normally incident on the angled surface  436 . 
     The first and second rays  432  and  434  pass through the non-reciprocal polarization rotator  406 , where the polarization of each ray is rotated through approximately 45°. However, since the rays  432  and  434  are propagating in the opposite direction to the rays  422  and  424 , the handedness of the polarization rotation is different. The first and second rays  432  and  434  then propagate to the first birefringent crystal  402 . The optical axis of the first birefringent crystal  402  is rotated 45° relative to the optical axis of the second birefringent crystal  404 . However, the direction of this relative rotation is opposite the direction of polarization rotation. Therefore, the first ray  432 , having passed through the second crystal as an ordinary ray, passes through the first birefringent crystal  402  as an extraordinary wave, marked “e.” Also, the second ray  434 , having passed through the second crystal  404  as an extraordinary ray, passes through the first birefringent crystal  402  as an ordinary wave, marked “o”. 
     In the forward direction, the two wedged birefringent crystals operate as a complementary prism pair, so that light exiting from the second crystal is parallel to the direction in which light entered the first crystal  402 . In other words, the deviation caused by one wedge is compensated for by the other wedge. However, since light propagating in the backward direction passes through one crystal as an “o” ray and in the other crystal as an “e” ray, the two wedged crystals  402  and  404  do not act as a complementary prism pair, and the two rays  432  and  434  emerge from the first birefringent crystal  402  in different directions. Accordingly, neither ray  432  nor ray  434  is focused by the first collimating lens to the first fiber  410 . Thus, irrespective of the polarization of the light  430  transmitted by the second fiber  412 , the light  430  is not transmitted to the first fiber  410 . Therefore, the isolator module  400  is effective as a polarization insensitive isolator. 
     Another embodiment of an isolator module  500  is illustrated in FIG.  5 A. This isolator module  500  is described in detail in U.S. Pat. No. 5,262,892, incorporated herein by reference. The isolator module  500  includes three birefringent crystals  502 ,  504  and  506 . A first non-reciprocal polarization rotator  508  is disposed between the first and second crystals  502  and  504 , and a second non-reciprocal polarization rotator  510  is disposed between the second and third crystals  504  and  506 . The optical elements  502 ,  504 ,  506 ,  508  and  510  are substantially flat. 
     FIG. 5B illustrates the manner in which orthogonal polarization states are transmitted through the isolator module  500 , by showing the relative displacement of the different polarization components, at the respective points marked A-F, as the light progresses through the isolator module  500 . The line marked (I) illustrates the polarization states as the light propagates from left to right, as viewed from the right hand side of the figure. 
     Prior to entry into the first crystal  502 , the light beam  512  is composed of both polarization states, as indicated at position A. Upon entry into the first crystal  502 , the light beam  512  is split into its ordinary and extraordinary components (A-B). The separated polarization components pass through the first non-reciprocal polarization rotator  508 , where each component is subjected to a polarization rotation of 45° in the clockwise direction (C). The two components then pass into the second birefringent crystal  504 , where the extraordinary component is displaced (D). Both polarization components are rotated by another 45° in the clockwise direction in the second non-reciprocal polarization rotator  510  (E). The two polarization components are combined into a single output beam  514  by the third crystal  506  (F), which is directed to the output fiber of the dual wavelength isolator fiber device. 
     Now consider light travelling in the reverse direction, for which the positions of the different polarization components are illustrated in line (II), as viewed from the left side of the figure. The light from the output fiber may be in a mixed polarization state (F). The ordinary and extraordinary polarization components are separated in the third crystal  506  (E). The separated polarization components are rotated by 45° in the clockwise direction by the second non-reciprocal polarization rotator  510  (D). The extraordinary polarization component is displaced on passage through the second birefringent crystal  504  (C). Both polarization components are rotated another 45° in the clockwise direction by the first non-reciprocal polarization rotator  508  (B). The polarization component that was not translated on passage through the second birefringent crystal  504  is translated upon passage through the first birefringent crystal  502  (A). Neither polarization component is returned to the optical axis  520  in the reverse direction, and so neither component is focused back into the input fiber by the collimating lens, and so the isolator module  500  is effective as a polarization insensitive isolator module. 
     It will be appreciated that other designs of isolator module may be employed: there is no intention to limit the type of isolator module used by the examples shown here. Other examples of isolator module that are applicable to the present invention are discussed in U.S. Pat. Nos. 5,237,445 and 5,262,892, both of which are incorporated by reference. 
     Another particular embodiment of a fiber optic device operating at more than one wavelength is illustrated in FIG.  6 . The device  600  has two fibers  602  and  604  that couple to the external fiber optic system. The device  600  is in a “terminal” configuration, having both fibers  602  and  604  at the same side of the package. An advantage of this configuration is that the length of the space required by the device is less than that for the “in-line” configuration of FIG. 2, since the fibers connecting to the device have a limited radius of curvature. 
     Each fiber  602  and  604  is terminated by a respective collimating lens  606  and  608  to reduce coupling losses between the two fibers  602  and  604 . Light  610  propagating from the first fiber  602  contains two wavelength components, λ1 and λ2, which are separated by a wavelength separator  614  into two components  616  and  618  respectively, propagating along different optical paths. The first component  616 , at λ1, is reflected by the wavelength separator  614  and propagates through the free space region  620  between the wavelength separator  614  and the wavelength combiner  622 . The first wavelength component  616  passes through the isolator module  612  positioned in the free space region  620 . 
     The second wavelength component  618  is directed from the wavelength separator  614  along a path different from the path of the first wavelength component  616 . A guiding prism  624  has two reflective surfaces  626  and  628  that direct the second wavelength component  618  to the wavelength combiner  622 , where the first and second wavelength components  616  and  618  are combined into a single output beam  630  that propagates to the second collimating lens  608  and is focused into the second fiber  604 . 
     Another particular embodiment of a fiber optic device operating at more than one wavelength, and in a “terminal” configuration, is illustrated schematically in FIG.  7 A. Two fibers  702  and  704  couple the device  700  to the external fiber system. A view of the “terminal” package is illustrated FIG. 7B, illustrating the housing  750  with the fibers  702  and  704  connecting at the same end. 
     The two fibers  702  and  704  share a single collimating lens  706 , which is typically a gradient index (GRIN) lens. The output beam  708  from the first fiber  702  exits collimated from the collimating lens  706 , but propagating at an angle relative to the axis  707  of the lens  706 . A parallelizing optic  710  may be used to divert the beam  708  to be parallel to the axis  707 . The parallelizing optic  710  may be, for example, a lens or a prism. 
     The collimated beam  708  is incident on a wavelength separator  714 , which, in this particular embodiment, is a dichroic mirror on a substrate. The wavelength separator  714  reflects the first wavelength component  716  at λ1, and transmits the second wavelength component  718  at λ2. The two wavelength components are recombined at the wavelength combiner  722 , which may be a dichroic mirror on a substrate. The first wavelength component  716  at λ1 passes through the isolator module  712  between the wavelength separator and combiner  714  and  722 , providing isolation at λ1. 
     The second wavelength component  718  at λ2 is directed by a reflecting surfaces  726  and  728  of a prism  724  to the wavelength combiner  722 , where the two different wavelength components  716  and  718  are recombined. The second wavelength component  718  may pass through one or more isolator modules  740  and  742  before recombining with the first wavelength component  716 . 
     Another particular embodiment of a fiber optic isolator device  800  providing isolation at more than one wavelength is illustrated in FIG.  8 . The device  800  employs similar components for separating and combining the different wavelength components as in the device  800 , but in an “in-line” configuration. Two fibers  802  and  804  couple the device  800  to the external fiber system. The two fibers  802  and  804  each have a respective collimating lens  806  and  808 , which may be a GRIN lens. 
     The collimated output beam from the first fiber  802  is incident on a wavelength separator  814 , which, in this particular embodiment, is a dichroic mirror on a substrate. The wavelength separator  814  transmits the first wavelength component  816  at λ1, and reflects the second wavelength component  818  at λ2. The two wavelength components  816  and  818  are recombined at the wavelength combiner  822 , which may be a dichroic mirror on a substrate. The first wavelength component  816  passes through the first isolator module  812  positioned between the wavelength separator and combiner  814  and  822 . 
     The second wavelength component  818  at λ2 is directed by a reflecting surfaces  826  and  828  of a prism  824  to the wavelength combiner  822 , where the two different wavelength components  816  and  818  are recombined. The second wavelength component  818  may pass through one or more isolator modules  840  and  842  operating at λ2, before recombining with the first wavelength component  816 . 
     Another particular embodiment of a multiwavelength isolator device  900  operating is illustrated in FIG.  9 . This embodiment uses a different type of wavelength separator and combiner and has a “terminal” configuration, and provides isolation at two wavelengths. 
     Two fibers  902  and  904  couple the device  900  to the external fiber system. The two fibers  902  and  904  share a single, collimating lens  906  and a parallelizing optic  910  in a manner as described above with regard to FIG.  7 . 
     The collimated beam  908  output from the first fiber  902  is incident on a wavelength separator  914 , which, in this particular embodiment, includes a beamsplitter cube  916  and a turning prism  917 . A dichroic reflector  915  in the beamsplitter cube  916  reflects the first wavelength component  920  at λ1 and transmits the second wavelength component  921  at λ2. The reflecting face  918  of the turning prism  917  reflects the second wavelength component  921  to be parallel to the first wavelength component  920 . 
     The two wavelength components  920  and  921  are recombined at the wavelength combiner  922 , which is similar to the wavelength separator, having a beamsplitting cube  923  and a turning prism  925 . The reflecting face  924  of the turning prism  925  reflects the second wavelength component  921  so that it recombines with the first wavelength component  920  at the dichroic reflector  926  of the beamsplitting cube  923 . The combined output beam  930  passes through the parallelizing optic  910  and is focused by the collimating lens  906  into the second fiber  904 . 
     This arrangement provides a free space propagation region between the wavelength separator  914  and the wavelength combiner  922  which allows the placement of a first isolator module  932  in the path of the first wavelength component  920  and a second isolator module optical  934  in the path of the second wavelength component  921 . 
     Another particular embodiment of a multiwavelength isolator device  1000  is illustrated in FIG.  10 . This embodiment uses a wavelength separator and combiner that are similar to those described above in FIG. 9, but is arranged in an “in-line” configuration. This embodiment also provides isolation at both wavelengths. 
     Two fibers  1002  and  1004  couple the device  1000  to the external fiber system. The two fibers  1002  and  1004  each have respective collimating lenses  1006  and  1008 . The collimated beam  1010  output from the first fiber  1002  is incident on a wavelength separator  1014 , which reflects the first wavelength component  1016  at a dichroic surface and reflects the second wavelength component  1018  at an internal prism surface  1017 . 
     The two wavelength components  1016  and  1018  are recombined at the wavelength combiner  1022 , which is similar to the wavelength separator  1014 . The second wavelength component  1018  is reflected at a prism surface  1024 , and recombined with the first wavelength component  1016  at a dichroic reflector  1026  which transmits the first wavelength component  1016  and reflects the second wavelength component  1018 . The combined output beam  1030  propagates to the collimating lens  1008  and is focused into the second fiber  1004 . 
     This arrangement provides a free space propagation region between the wavelength separator  1014  and the wavelength combiner  1022  which allows the placement of a first isolator module  1032  in the path of the first wavelength component  1016  and a second isolator module  1034  in the path of the second wavelength component  1018 . 
     Another particular embodiment of a multiwavelength isolator device  1100  is illustrated in FIG.  11 . This embodiment uses a wavelength separator and combiner that are similar to those described above in FIG. 9, but is arranged in a “corner” configuration, in which one coupling fiber is positioned at an angle relative to the other, unlike the previously described embodiments where the coupling fibers are substantially parallel. This arrangement may be useful in applications where the fiber arrangements of the “in-line” or “terminal” configurations are unsuitable, for example because of limited availability of space to accommodate the minimum bending radius of a fiber. Isolation may be provided at both wavelengths. 
     Two fibers  1102  and  1104  couple the device  1100  to the external fiber system. The two fibers  1102  and  1104  each have respective collimating lenses  1106  and  1108 . The collimated beam  1110  output from the first fiber  1102  is incident on a wavelength separator  1114 , which transmits the first wavelength component  1116  at a dichroic reflector  1115 . The second wavelength component  1118  is reflected at the dichroic reflector to a reflecting surface  1117 , in this case an internally reflecting prism surface, which reflects the second wavelength component  1118  to a reflecting surface  1124  of the wavelength combiner  1122 . 
     The two wavelength components  1116  and  1118  are recombined at the wavelength combiner  1122 , which is similar to the wavelength separator  1114 , except that the dichroic reflector  1126  reflects the first wavelength component  1116  and transmits the second wavelength component  1118 . The combined output beam  1130  propagates to the collimating lens  1108  and is focused into the second fiber  1104 . 
     This arrangement provides a free space propagation region between the wavelength separator  1114  and the wavelength combiner  1122  which allows the placement of a first isolator module  1132  in the path of the first wavelength component  1116  and a second isolator module  1134  in the path of the second wavelength component  1118 . 
     Another particular embodiment of a multiwavelength isolator device  1200  is illustrated in FIG.  12 . This embodiment is a variation of the embodiments illustrated in FIGS. 10 and 11, and may be used for separating or combining different wavelength components. This embodiment is arranged in a “corner” configuration, and may provide isolation at both wavelengths. 
     Three fibers  1202 ,  1204   a  and  1204   b  couple the device  1200  to the external fiber system. Each fiber  1202 ,  1204   a  and  1204   b  has a respective collimating lens  1206 ,  1208   a  and  1208   b . Like all the other embodiments described, light may pass through the device in both directions. Here, we initially describe passage of light from the first fiber  1202  to the other fibers  1204   a  and  1204   b . The collimated beam  1210  output from the first fiber  1202  is incident on a wavelength separator  1214 , which reflects the first wavelength component  1216  at a dichroic surface  1215  and reflects the second wavelength component  1218  at an internal prism surface  1217 . 
     The first wavelength component  1216  propagates through the first isolator module  1232  and passes to the second fiber  1204   a  via the collimating lens  1208   a . The second wavelength component  1218  is separated from the first wavelength component  1216  by reflection at the dichroic reflector  1215 . The second wavelength component  1218  is directed by the reflecting surface  1217  to the third collimating lens  1208   b  and the third fiber  1204   b . The second wavelength component  1218  may be directed via a first reflecting surface  1217  and a second reflecting surface  1224 . A second isolator module  1234  may be placed in the path of the second wavelength component  1218 . 
     It will be appreciated that the device  1200  may be used for separating wavelength components as just described, and also for combining wavelength components. For example, a first wavelength component propagating from the second fiber  1204   a  to the first fiber  1202  may be combined at the dichroic reflector  1215  with a second wavelength component propagating from the third fiber  1204   b . The combined beam, containing both wavelength components, propagates to the first fiber  1202 , where it is coupled to an external fiber system. 
     The different fibers may be positioned differently from the “in-line/corner” arrangement illustrated in FIG.  12 . For example, the third fiber  1204   b  may be positioned parallel to the second fiber  1204   a , as illustrated for the device  1300  in FIG.  13 . Here, the turning prism  1222  is omitted, so that the second wavelength component  1218  enters the third fiber  1204   b  in a direction parallel to the first wavelength component  1216 . 
     The “in-line” arrangement of FIG. 13 provides an advantage over the embodiment illustrated in FIG. 13 in that the overall package size for the device may be reduced. It should be appreciated that, instead of the two fibers  904   a  and  904   b  each having their respective collimating lens  908   a  and  908   b , the fibers  904   a  and  904   b  may be coupled to the wavelength combiner  914  via a single collimating lens and a parallelizing optic, for example as shown in FIG.  7 . 
     It will also be appreciated that many different configurations may be adopted for a fiber optic device that combines different wavelengths or separates different wavelengths. For example, the turning prism having the reflecting face  1217  may be omitted altogether, and the third fiber positioned to receive or direct the second wavelength component directly from or to the dichroic reflector  1215 . Also, different reflecting surfaces may be included in the fiber optic device  1300  so that the second and third fiber are both on the same side of the fiber optic device, for example in a “corner” configuration or in a “terminal” configuration. Furthermore, the second and third fibers may terminate on different sides of the fiber optic device, for example in a “T” configuration, with any of the fibers forming the base of the “T”, or in a mixed configuration, for with two fibers on the same side and the third fiber on a different side of the device  1300 . 
     Another particular embodiment of a multiwavelength isolator device  1400  is illustrated in FIG.  14 . This embodiment has some similarities to the embodiment illustrated in FIG. 11, but provides additional capabilities for inserting different isolator modules for at least one of the wavelength components. 
     Two fibers  1402  and  1404  couple the device  1400  to the external fiber system. The two fibers  1402  and  1404  each have respective collimating lenses  1406  and  1408 . The collimated beam  1410  output from the first fiber  1402  is incident on a wavelength separator  1414 , which transmits the first wavelength component  1416  and reflects the second wavelength component  1418  at a dichroic reflector  1415 . The second wavelength component  1418  is reflected by to reflectors  1417  and  1424  to the wavelength combiner  1422 . In this case, the reflectors  1417  and  1424  are internally reflecting prism surfaces, but may also be other types of reflectors. 
     The two wavelength components  1416  and  1418  are recombined at the wavelength combiner  1422 , which is similar to the wavelength separator  1414 , except that the first wavelength component  1416  is reflected at the dichroic reflector  1423  and the second wavelength component  1418  is transmitted through the dichroic reflector  1423 . The combined output beam  1430  propagates to the collimating lens  1408  and is focused into the second fiber  1404 . 
     This arrangement provides a different positions where isolator modules devices  1432 ,  1434 ,  1436  and  1438  may be inserted to operate on the respective wavelength components. 
     It should be appreciated that, where isolation is provided for two wavelength components in the embodiments described above, there may be isolation only at one wavelength. The other wavelength may be provided with any other type of optical device, such as an optical switch, modulator, filter, circulator, or the like. One particular embodiment of a fiber optic device  1500  operating at two wavelengths is illustrated in FIG.  15 . The device  1500  provides isolation at one wavelength and circulation at the other wavelength. A first fiber  1502  couples to an external fiber system. The output from the first fiber  1502  is collimated by the collimating lens  1510  and is parallelized by the parallelizing optic  1512 , for example in a manner as described with regard to the embodiment illustrated in FIG. 7. A second fiber  1506  is coupled to the external fiber system, and has a collimating lens  1507  for collimating light output from the second fiber  1506  and for focusing light into the second fiber  1506 . 
     Light  1511  at two wavelengths, λ1 and λ2, is transmitted from the first fiber  1502  into a wavelength separator  1514 , which may be a beamsplitter cube having a dichroic reflector  1515 . The first wavelength component  1516  is transmitted by the dichroic reflector  1515  to the circulator  1540 , and is transmitted to the wavelength combiner  1522 , which may be a beamsplitter cube having a dichroic reflector  1523 . The first wavelength component  1516  is transmitted through the dichroic reflector  1523 . The second wavelength component  1518  is reflected at the dichroic reflector  1515  and directed to the wavelength combiner  1522  along a path different from the first wavelength component  1516 , avoiding passage through the circulator  1540 . The second wavelength component  1518  may be reflected by first and second reflecting surfaces  1520  and  1521  to the wavelength combiner  1522 , where it is reflected by the dichroic reflector  1523 . The first wavelength component  1516  transmitted by the dichroic reflector  1523  and the second wavelength component  1518  reflected by the dichroic reflector  1523  form a combined output  1530  that propagates to the second fiber  1506 . The reflecting surfaces  1520  and  1521  may be internally reflecting prism surfaces, as illustrated, or front surface mirrors or the like. 
     An isolator module  1532  may be provided in the path of the second wavelength component  1518 , between the separator  1514  and the combiner  1522 . The isolator module  1532  permits passage of light at the second wavelength from the first fiber  1202  to the second fiber  1206 . 
     We now consider light, at the two wavelengths λ1 and λ2, passing from the second fiber  1506  back through the device  1500 . The reverse-propagating light is split into the two wavelength components at the combiner  1522 , by transmitting the first wavelength component  1516  through the dichroic reflector  1523  and reflecting the second wavelength component  1518 . The second wavelength component  1518  retraces the path via the reflector  1521  to the isolator module  1532 . The isolator module deviates the second wavelength component  1518  along a different path  1518   a  so that the light at the second wavelength does not return to the first fiber  1502  and does not pass to the third fiber  1508 . 
     The first wavelength component  1516 , however, is deviated by the circulator  1540  onto a different path  1516   a , which is transmitted through the dichroic reflector  1515 . The first wavelength  1516   a  component on the different path does not return to the first fiber, but passes through the parallelizing optic  1512  and the collimating lens  1510  to a third fiber  1508 . Thus, the device  1500  provides circulation it the first wavelength while providing isolation at the second wavelength. 
     The invention is not restricted to only those embodiments discussed above, but covers various modifications and changes from the specific embodiments. For example, the invention has been described with regard to two wavelength components. It will be appreciated that more than two wavelength components may be present. In such a case, one component may be separated from the other components, or different components may be separated out. For example, where three wavelength components are present, the component at λ1 may be separated from the other two components, λ2 and λ3. The two components λ2 and λ3 may then be separated from each other, or they may remain mixed. 
     It will also be appreciated that the isolator modules positioned within the multiwavelength isolator device may be orientated in different directions. Thus, a multiwavelength isolator device may permit passage of light at one wavelength through the device in one direction, while permitting light at a second wavelength to pass through the device in the opposite direction. 
     It will also be appreciated that more than one isolator module may be employed where only one was shown in the figures to provide a higher degree of extinction. For example, with regard to the embodiment illustrated in FIG. 6, the first wavelength component  616  may pass through more than one isolator module between the wavelength separator  614  and the wavelength combiner  622 . It will further be appreciated that many different configurations and arrangements of reflectors may be used in the multi-wavelength fiber optic device. 
     As noted above, the present invention is applicable to fiber optic systems and is believed to be particularly useful in systems that operate at more than one wavelength. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.