Patent Publication Number: US-2022239052-A1

Title: Doped Fiber Amplifier Having Pass-Through Pump Laser

Description:
BACKGROUND OF THE DISCLOSURE 
     In a doped fiber amplifier, an optical signal is transmitted through a doped fiber. At the same time, ions in the doped fiber are energized using pump light, which is provided at a different wavelength from a pump laser diode. Photons of the optical signal interact with the energized ions, causing the ions to give up some of their energy in the form of photons at the same wavelength as the photons of the optical signal, with the ions returning to a lower energy state. The optical signal is thereby amplified as it passes through the doped fiber. 
     For example, an Erbium doped fiber amplifier (EDFA) can be used in an optical fiber link to amplify signals at low loss in a 1550-nm wavelength range of the fiber. In the EDFA, a short length (few meters) of the optical fiber is doped with the rare-earth element erbium. A pump laser injects light into the erbium-doped fiber at a given wavelength to excite the erbium ions in the fiber. Energy is transferred to the optical signal passing through the fiber when the excited ions return to an unexcited state. The wavelength to be amplified can be in the 1550-nm range, and the wavelength of the pump laser can be 980 and/or 1480 nm. 
     In many fiber amplifiers, multiple pump laser injections are required within the optical topology. In addition, there are more requirements where multiple EDFA are required in a single physical location. Convention is to use a single pump laser chip per injection point or per gain stage for cost efficiency, but there are common cases where more than one pump laser may be beneficial. This chip may be individually packaged or multiple chips may be included in one pump laser package. This works well, but there are always limits on the smallest size that a fiber amplifier can be due to the number of optical components required in a gain stage. In addition, costs will be higher the more optical components are needed. 
     The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     SUMMARY OF THE DISCLOSURE 
     According to the present disclosure, a fiber amplifier is operable with an electric drive signal for amplifying signal light having a signal wavelength. The apparatus comprises a laser diode and a doped filer. The laser diode has an active section and has an input facet and an output facet. The input and output facets are in optical communication with the signal light and are configured to pass the signal light through the laser diode from the input facet to the output facet. The active section is configured to generate pump light in response to injection of the electrical drive signal into the active section. The pump light has a pump wavelength different from the input wavelength. 
     The fiber is doped with an active dopant. The fiber is in optical communication with the signal light and is in optical communication with at least a portion of the pump light from the laser diode. For example, the doped fiber can be downstream of the laser diode&#39;s output facet in a co-pumping arrangement, or the doped fiber can be upstream of the laser diode&#39;s input facet in a backward pumping arrangement. Other configurations are possible. Either way, the pump wavelength of the pump light is configured to interact with the active dopant of the fiber and thereby amplify the signal light. 
     According to the present disclosure, a method is used with signal light having a signal wavelength. The method comprises: receiving the signal light at an input facet of a laser diode; passing the signal light from the input facet through the laser diode to an output facet; generating pump light in an active section of the laser diode by injecting an electrical drive signal into the active section of the laser diode, the pump light having a pump wavelength different from the signal wavelength; transmitting the signal light and at least a portion of the pump light to a doped fiber amplifier in optical communication with the laser diode; and amplifying the signal light by interacting the pump light with the doped fiber amplifier. 
     The foregoing summary is not intended to summarize each potential embodiment or every aspect of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an apparatus having a doped fiber amplifier and a pass-through pump laser according to the present disclosure. 
         FIG. 2A  illustrates a portion of the apparatus of  FIG. 1  that includes the pass-through laser diode and input and output optical fibers. 
         FIG. 2B  illustrates a portion of the apparatus of  FIG. 1  that includes the pass-through laser diode and input and output optical fibers. 
         FIG. 3  illustrates an end-section of a pass-through laser diode. 
         FIG. 4A  illustrates a fiber amplifier having a doped fiber and one configuration of a pass-through pump laser. 
         FIG. 4B  illustrates a fiber amplifier having a doped fiber and another configuration of a pass-through pump laser. 
         FIG. 4C  illustrates a fiber amplifier having a doped fiber and yet another configuration of a pass-through pump laser. 
         FIG. 5  illustrates a process for operating a fiber amplifier system having a pass-through laser diode according to the present disclosure. 
         FIGS. 6A-6C  illustrate fiber amplifiers having laser diodes arranged in various pumping directions relative to one or more doped fibers. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
       FIG. 1  illustrates a fiber amplifier system  100  having a fiber amplifier  102  with a pass-through laser diode  200  according to the present disclosure. The laser diode  200  is disposed in a path  105  of input or primary signal light S and is configured to pass signal light S received at an input facet through the laser diode  200  to an output facet, while also generating and outputting pump light P. Together, the signal light S and at least a portion of the pump light P are transmitted from the output facet into the path  105  toward a doped fiber  108 . For example, at least a portion of the input signal light S that reaches the input end facet of the laser diode  200  and passes into the laser diode  200 , where the input signal light S is combined with at least some of the pump light P added by the laser diode  200 . The combined portions of signal and pump light S+P reach the output end facet to be transmitted through the end facet to an optical fiber of the path  105  to then propagate to the doped fiber  108 . 
     As disclosed herein, configuring the laser diode  200  to receive the input signal light S and to output the received input signal light S and the added pump light P from one end facet may eliminate the need for some of the conventional components required for a pump laser of a fiber amplifier. Namely, there is no need for a coupler, such as a dichroic coupler, to be used to couple pump light into the optical path of input signal light. 
     The fiber amplifier  102  of the present disclosure uses the pass-through laser diode  200  having an active section positioned between two end facets, each of which has low reflectivity. For example, each of the end facets may have an anti-reflection (AR) coating. Details related to a two-facet laser diode that can be used for the pass-through laser diode  200  are disclosed in co-pending U.S. application Ser. No. 16/947,643, filed 11 Aug. 2020 and entitled “DUAL OUTPUT LASER DIODE,” which is incorporated herein by reference in its entirety. 
     The pass-through laser diode  200  disclosed herein can be used with a number of fiber amplifier systems  100 , such as an erbium-doped fiber amplifier (EDFA) system or other systems. For example, the fiber amplifier  102  can include one or more glass fibers  108  that are doped with rare earth ions. Some example dopants include erbium, neodymium, ytterbium, praseodymium, bismuth, holium, or thulium. For these dopants, pump light P from the laser diode  200  propagates through the fiber core of the doped fiber  108  along with the primary signal S to be amplified, and the pump light P provides energy to the active dopant. 
     A maximum optical power density within the laser diode  200  may be reduced by a factor of about two because no pump light P or relatively little pump light P is reflected at one end facet back to the other as occurs in conventional laser diodes having an AR coating at one end facet and an HR coating at the other. However, it should be noted that the reduced maximum optical power density of the pass-through laser diode  200  according to the present disclosure may have increased reliability when compared to laser diodes having AR and HR coatings at opposite end facets. 
     Depending on the desired implementation, the pass-through laser diode  200  may have equal or unequal reflectivities at the opposing facets. For example, AR coatings of unequal reflectivity can be used at the end facets, such as a first AR coating with a reflectivity of 1% at a first end facet and a second AR coating with a reflectivity of 0.75% at a second end facet. Alternatively or additionally, a waveguide of the laser diode  200  can be structured to have different transmissivities at or near the two end facets, such as a transmissivity of 99.5% for a portion of the waveguide near the first end facet and a transmissivity of 99% for a portion of the waveguide near the second end facet. These percentage values are only exemplary and can be varied depending on the implementation. 
     Alternatively or additionally, first and second portions of the laser diode  200  can be controlled differently. For example, the laser diode  200  may include a first anode and cathode electrically coupled to a first portion of the laser diode  200  and can include a second anode and cathode electrically coupled to a second portion of the laser diode  200 . An etched mirror, a distributed feedback (DFB) mirror, or other reflective structure may be formed in the laser diode  200  between the first and second portions to at least partially isolate optical communication of the pump light P from one of the first and second portions to the other. Accordingly, the first and second portions of the laser diode  200  may be independently operated while being integrally formed in a single structure. 
     To monitor operation, the fiber amplifier system  100  can include an input optical tap  104  communicating with an input photodiode  122  connected to a controller  120 . Likewise, an output optical tap  112  can communicate with an output photodiode  124  connected to the controller  120  at the output end. A first optical isolator  106  can be used between the laser diode  200  and the input optical tap  104 , and a second optical isolator  110  can be used between the laser diode  200  and the doped fiber  108 . In general, the fiber amplifier  102  is configured to receive pass-through optical signal light S as input and is configured to output amplified signal light SA that is an amplified version of the optical signal light S. 
     In more detail, the pass-through optical signal light S is received at the input tap  104 . This signal light S can be generated by a suitable source (not shown), depending on the implementation. A small portion (e.g., 2%) of the signal light S can be directed by the input tap  104  to the input photodiode  122 , which can measure optical power of the optical signal light S. A remainder (e.g., 98%) of the pass-through optical signal light S passes through the input tap  104  and the first isolator  106  to the laser diode  200 . The first isolator  106  may prevent or at least reduce back reflection. 
     The pass-through optical signal light S has a first input wavelength λ In . The laser diode  200  pumps pump light P at a second pump wavelength λ Pump  selected to provide optical amplification to the corresponding optical signal light S operating at Ain in the presence of a specific rare-earth dopant within the doped fiber  108 . The dopant may be erbium, ytterbium, or other dopant. When the dopant is erbium, for example, the wavelength λ Pump  of the pump light P emitted by the laser diode  200  may be about 980-nanometers (nm) (e.g., 970-nm to 990-nm). The pump light P at the pump wavelength λ Pump  of about 980-nm can be configured to provide amplification in the doped fiber  108  to the optical signal light S when its wavelengths Ain is about 1550-nm, such as wavelengths in the C band (˜1528 nm to 1568 nm), or about 1590-nm, such as wavelengths in the L band (˜1568 nm to 1625 nm). 
     The laser diode  200  outputs the optical signal light S combined with at least a portion the pump light P to the doped fiber  108 . Ideally, all or at least most of the pump light P is injected into the doped fiber  108 , but the amount may be a proportion of the total possible power that the laser diode  200  can emit as some of the 980-nm pump light P may be lost. The pump light P at the pump wavelength λ Pump  energizes ions in the doped fiber  108 , and the signal light S at the input wavelength λ In  interacts with the energized ions. In particular, photons of the signal light S at the input wavelength λ In  stimulate emission of photons from the energized ions at the input wavelength λ In  to generate the amplified signal light SA. 
     The amplified signal light SA passes through the second isolator  110  and can pass to the output tap  112 . If used, the output tap  112  directs a small portion of the amplified signal light SA to the output photodiode  124 , which can measure optical power of the amplified signal light SA. The remainder of the amplified signal light SA passes through the output tap  112  and is output from the fiber amplifier  102 . 
     The controller  120  can control one or more laser drivers  126 A-B for the laser diode  200 . In particular, the controller  120  can monitor the input and output signals using the input and output photodiodes  122  and  124  and can control the one or more laser drivers  126 A-B, which apply electrical drive signal(s) to the laser diode  200  as directed by the controller  120 . In turn, the electrical drive signal(s) may dictate the optical power of the pump light P emitted by the laser diode  200 . For example, the laser  200  may emit pump light P with an optical power that is proportional to or has some other defined relationship to current of the electrical drive signal(s). 
     During operation, the controller  120  can compare the optical power of the signal light S (measured by the input photodiode  122 ) to the optical power of the amplified signal light SA (measured by the output photodiode  124 ) to determine gain of the fiber amplifier  102 . If the gain is above or below a target gain, the one or more laser drivers  126 A-B can adjust the electrical drive signal(s) to increase or decrease the gain of the fiber amplifier  102 . As discussed in more detail later, the laser diode  200  can include two portions that may be independently controlled by a corresponding one of the laser drivers  126 A-B to independently control gain in the fiber amplifiers  102 . 
     Having an understanding of a fiber amplifier system  100  of the present disclosure, discussion turns to further details of a fiber amplifier  102  of the present disclosure. 
       FIG. 2A  illustrates a portion of the fiber amplifier  102  of  FIG. 1  that includes the pass-through laser diode  200 , an input optical fiber  130 A, and an output optical fiber  130 B. The input and output optical fibers  130 A-B may include, be included in, or correspond to the optical path  105  of  FIG. 1 . 
     As illustrated in  FIG. 2A , the laser diode  200  includes two end facets  230 A-B spaced apart from each other. The input end facet  230 A has low reflectivity at least for the pass-through signal&#39;s wavelength Δ In  (or multiple wavelengths or a range of wavelengths associated with the pass-through signal wavelength λ In ). The input facet  230 A further has high reflectivity at least for the laser diode&#39;s operational wavelength λ Pump  (or multiple wavelengths or a range of wavelengths associated with the laser diode&#39;s operational wavelength λ Pump ). This can be achieved using a high reflection mirror  134 A on the input end facet  230 A. For example, the mirror  134 A can include a high reflection (HR) facet coating having multiple thin film layers that include materials of different refractive indices and that have a thickness fractioned to a wavelength of interest. 
     Meanwhile, the output end facet  230 B has low reflectivity for multiple wavelengths or a range of wavelengths, such as those wavelengths for the laser diode&#39;s operational wavelength λ Pump  and the pass-through signal wavelength Δ In . The reflectivities referenced may be (or may include) reflectivity for a single wavelength, multiple wavelengths, or a range of wavelengths, such as an operational wavelength range of the laser  200 . The operational wavelength range may include wavelengths suitable for pump light (P), such as wavelengths of about 980-nm or other wavelengths. In some configurations, the operational wavelength range may be from 970-nm to 990-nm, or from 975-nm to 985-nm, or other suitable range. 
     The input and output optical fibers  130 A-B are positioned so that each of the corresponding end facet  230 A-B is optically coupled to the corresponding optical fiber  130 A-B. For example, the first end facet  230 A is optically coupled to the input optical fiber  130 A, and the second end facet  230 B is optically coupled to the output optical fiber  130 B using suitable forms of optical coupling. For example, each optical fiber  130 A-B may be optically aligned to the corresponding end facet  230 A-B and positioned sufficiently close to the corresponding end facet  230 A-B so that light is properly coupled one to the other. Alternatively or additionally, one or more optical elements, such as one or more lenses or other optical elements, may be positioned between the end facet  230 A-B and the optical fiber  130 A-B. Various types of optical coupling can be used. 
     The output optical fiber  130 B may include a fiber Bragg grating (FBG)  132 B formed therein. The FBG  132 B may be configured for one or more wavelengths. The FBG  132 B may be configured to reflect a portion, e.g., 2-4%, of the pump light (P) back to the laser diode  200 . The FBG  132 B may be configured to reflect back a predetermined wavelength or multiple predetermined wavelengths which may “lock” the laser diode  200  to the predetermined wavelength(s) so that the laser diode  200  exhibits stable lasing at the predetermined wavelength(s). 
     The FBG  132 B may be configured to reflect back one or more wavelengths (e.g., 974-nm and 976-nm), but can pass the higher wavelength λ Pump  of the signal light (S). The reflected light may be coupled through the second end facet  230 B into the laser  200  where it interacts generally with a second portion  234 B of the laser  200  such that the second portion  234 B of the laser  200  is locked to both 974-nm and 976-nm. 
     More generally, the FBG  132 B may lock corresponding first or second portion  234 A,  234 B of the laser  200  to one or multiple predetermined wavelength(s). In other arrangements, the laser diode  200  itself may include a DFB structure to lock the laser  200  to a predetermined wavelength(s) so that the FBG  132  may be omitted. 
     In some configurations, the FBG  132 B forms a fiber cavity with the laser diode  200 , where the FBG  132 B provides sufficient reflectivity to ensure lasing of the laser diode  200 . Alternatively or additionally, the laser diode  200  may include a ridge structure as described with respect to  FIG. 3 . Roughness of the ridge structure, thermal induced refractive changes, or gain induced refractive changes along the length of the laser diode  200  may reflect and scatter light generated in the laser diode  200  sufficiently to build up the optical field and ensure lasing of the laser diode  200 . In some arrangements, the laser diode  200  may have a higher threshold or gain for lasing than other lasers in view of the low reflectivity at the end facets  230 A-B. 
     Either way, the laser diode  200 , the reflectivity of the input facet  230 A, the FBG  132 B, and the like are transparent to the higher wavelength λ In  of the input signal light S that passes through an active section of the laser diode  200 . In this way, the pass-through input signal light S at the higher wavelength λ In  and at least a portion of the pump light P at the lower wavelength λ Pump  can be transmitted through the output facet  230 B to propagate onward to the fiber amplifier ( 102 ). As already disclosed in one implementation, the pass-through wavelength λ In  can be about 1550-nm and the pump wavelength λ Pump  can be 980-nm, when the fiber amplifier ( 102 ) uses an erbium doped fiber ( 108 ). 
     The arrangement in  FIG. 2A  is well suited for a pump and amplifier topology in which the pass-through pump has the gain medium downstream of the pump. Details of such an arrangement are described below with respect to  FIG. 4A , for example. Other arrangements are possible. 
     As another example,  FIG. 2B  illustrates a portion of the fiber amplifier  102  of  FIG. 1  that includes another configuration of the pass-through laser diode  200 , the input optical fiber  130 A, and the output optical fiber  130 B. The input and output optical fibers  130 A-B may include, be included in, or correspond to the optical path  105  of  FIG. 1 . 
     As illustrated again in  FIG. 2B , the laser diode  200  includes the two end facets  230 A-B spaced apart from each other. Each of the end facets  230 A-B has low reflectivity. The reflectivity may be or include reflectivity for a single wavelength, multiple wavelengths, or a range of wavelengths, such as an operational wavelength range of the laser  200 . The operational wavelength range may include wavelengths suitable for pump light (P), such as wavelengths of about 980-nm or other wavelengths. In some configurations, the operational wavelength range may be from 970-nm to 990-nm, or from 975-nm to 985-nm, or other suitable range. 
     The input and output optical fibers  130 A-B are positioned so that each of the corresponding end facet  230 A-B is optically coupled to the corresponding optical fiber  130 A-B. For example, the first end facet  230 A is optically coupled to the input optical fiber  130 A, and the second end facet  230 B is optically coupled to the output optical fiber  130 B using suitable forms of optical coupling. For example, each optical fiber  130 A-B may be optically aligned to the corresponding end facet  230 A-B and positioned sufficiently close to the corresponding end facet  230 A-B so that light is properly coupled one to the other. Alternatively or additionally, one or more optical elements, such as one or more lenses or other optical elements, may be positioned between the end facet  230 A-B and the optical fiber  130 A-B. Various types of optical coupling can be used. 
     In this configuration, each of the optical fibers  130 A-B may include a fiber Bragg grating (FBG)  132 A-B formed therein. The input FBG  132 A may be configured for a first wavelength, while the output FBG  132 B may be configured for the first wavelength and a second wavelength. The FBGs  132 A-B may be configured to reflect a portion, e.g., 2-4%, of the pump light (P) back to the laser diode  200 . Each FBG  132 A-B may be configured to reflect back a predetermined wavelength or multiple predetermined wavelengths which may “lock” the laser diode  200  to the predetermined wavelength(s) so that the laser diode  200  exhibits stable lasing at the predetermined wavelength(s). The FBGs  132 A-B may be configured to reflect back the same or different predetermined wavelength(s), to cause the laser diode  200  to emit pump light (P) from the output end facet  230 B at at least two predetermined wavelengths. 
     For example, the first FBG  132 A may be configured to reflect back a first wavelength of 974-nm, but can pass the higher wavelength of the signal light (S). The reflected light may be coupled through the first end facet  230 A into the laser  200  where it interacts generally with a first portion  234 A of the laser  200  such that the first portion  234 A of the laser  200  is locked to 974 nm. 
     The second FBG  132 B may be configured to reflect back both the first wavelength of 974-nm and a second wavelength of 976-nm, but can pass the higher wavelength of the signal light (S). The reflected light may be coupled through the second end facet  230 B into the laser  200  where it interacts generally with a second portion  234 B of the laser  200  such that the second portion  234 B of the laser  200  is locked to both 974-nm and 976-nm. 
     More generally, each FBG  132 A-B may lock the corresponding first or second portion  234 A,  234 B of the laser  200  to one or multiple predetermined wavelength(s). In other arrangements, the laser diode  200  itself may include a DFB structure to lock the laser  200  to a predetermined wavelength(s) so that the FBGs  132  may be omitted. 
     In some configurations, each of the FBGs  132 A-B forms a fiber cavity with the laser diode  200 , where the FBGs  132 A-B provides sufficient reflectivity to ensure lasing of the laser diode  200 . Alternatively or additionally, the laser diode  200  may include a ridge structure as described with respect to  FIG. 3 . Roughness of the ridge structure, thermal induced refractive changes, or gain induced refractive changes along the length of the laser diode  200  may reflect and scatter light generated in the laser diode  200  sufficiently to build up the optical field and ensure lasing of the laser diode  200 . In some arrangements, the laser diode  200  may have a higher threshold or gain for lasing than other lasers in view of the low reflectivity at the end facets  230 A-B. 
     Either way, the laser diode  200 , the FBGs  132 A-B, and the like are transparent to the higher wavelength λ In  of the input signal light S that passes through an active section of the laser diode  200 . In this way, the pass-through input signal light S at the higher wavelength λ In  and at least a portion of the pump light P at the lower wavelength λ Pump  can be transmitted through the output facet  230 B to propagate onward to the fiber amplifier ( 102 ). As already disclosed in one implementation, the pass-through wavelength λ In  can be about 1550-nm and the pump wavelength λ Pump  can be 980-nm, when the fiber amplifier ( 102 ) uses an erbium doped fiber ( 108 ). 
     The arrangement in  FIG. 2B  is well suited for a pump and amplifier topology in which the pass-through pump has the gain medium upstream and downstream of the pump. Details of such an arrangement are described below with respect to  FIG. 4B , for example. 
       FIG. 3  shows an end-section of a pass-through laser diode  200  according to the present disclosure. The laser diode  200  may include, be included in, or correspond to any of the laser diodes disclosed herein. The end-sectional view of  FIG. 3  is in a plane that is parallel to end facets ( 230 A-B) of the laser diode  200  and perpendicular to a light emission direction of the laser diode  200 . The light emission direction is in and out of the page in  FIG. 3 , and this direction is also referred to as a longitudinal direction. 
     As illustrated in  FIG. 3 , the laser diode  200  includes various epitaxial layers, such as a substrate  202 , a lower cladding layer  204 , a lower waveguide layer  206 , an active layer  208 , an upper waveguide layer  210 , an upper cladding layer  212 , a cathode  214 , and an anode  216 . The laser diode  200  may include additional or different layers or elements than illustrated in  FIG. 3  in other arrangements. The end facets ( 230 A-B) of the laser  200  may be formed in the epitaxial layers, e.g., by cleaving through the epitaxial layers. 
     The configuration of  FIG. 3  includes the active layer  208  with multiple quantum wells (MQWs) embedded in the lower and upper waveguide layers  206 ,  210  and surrounded by the lower and upper cladding layers  204 ,  212 . These cladding layers  204 ,  212  are configured to confine the optical mode in a transversal direction, e.g., vertically in  FIG. 3 . 
     The laser diode  200  can also include a mesa or ridge structure  218  to confine the optical mode in a lateral direction, e.g., horizontally in  FIG. 3 . The ridge structure  218  with lower and upper waveguide layers  206 ,  210  and lower and upper cladding layers  204 ,  212  forms a waveguide that extends longitudinally, e.g., in and out of the page in  FIG. 3 , between end facets ( 230 A-B) of the laser diode  200 . In the pass-through laser diode  200 . This waveguide is configured to guide the input signal light (S) and to guide the pump light (P) generated by the laser diode  200  longitudinally. 
     The active layer  208  may extend longitudinally for all or a portion of a length (e.g., in and out of the page in  FIG. 3 ) of the laser diode  200 . Alternatively or additionally, the anode  216  may extend longitudinally for all or a portion of the length of the laser diode  200  and the anode  216  may have a region in which current is injected (referred to as a current injection region) that may extend longitudinally for all or a portion of a length of the anode  216 . A length of the current injection region may determine a longitudinal extent of stimulated emission of pump light (P) within the laser diode  200 . 
     As disclosed herein, a portion of the laser diode  200  that extends longitudinally along the length of the active layer  208 , the length of the anode  216 , or the length of the current injection region of the anode  216  may be referred to as an “active section” of the laser diode  200 . For the pass-through of the input signal light (S), the active section of the laser diode  200  extends longitudinally from one end facet to the other. 
     The cathode  214  and the anode  216  are electrically coupled to opposite sides of the active section. In the example of  FIG. 3 , the cathode  214  and the anode  216  are electrically coupled in particular to a bottom and top of the active section of the laser diode  200 . A laser driver, such as the laser driver  126  of  FIG. 1 , may be coupled to the anode  216  to inject an electrical drive signal into and through the laser diode  200  to the cathode  214 . The electrical drive signal may cause electrons and holes to be injected from opposite sides into the active layer  208  where they recombine via stimulated emission to generate photons for the pump light (P). 
     Having an understanding of a pass-through laser diode  200  used in a fiber amplifier  102  as discussed above, discussion now turns to particular configurations. 
       FIG. 4A  is a top schematic view of a laser diode  200  arranged for pass-through in a fiber amplifier  102  according to the present disclosure. As illustrated in  FIG. 4A , the laser diode  200  includes an input facet  230 A, an output facet  230 B, and an active section  220  positioned between the facets  230 A-B. 
     In general, the active section  220  is configured to generate pump light P that propagates toward each of the facets  230 A-B. The pump light P is generated by the active section  220  in response to injection of an electrical drive signal into the active section  220 . A cathode  214  and an anode  216  are electrically coupled to opposite sides, e.g., a top and a bottom, of the active section  220  to inject the electrical drive signal into the active section  220  between the cathode  214  and the anode  216 . 
     The input and output facets  230 A-B have low reflectivity at least for the pass-through signal wavelength λ In , while the input facet  230 A has a high reflectivity at least for the operational wavelength λ Pump  of the diode  200 . In an example, the reflectivity at each of the end facets  230 A-B is achieved by cleaving the laser diode  200  from a wafer of lasers and forming an appropriate AR/HR coating on the cleaved end facets  230 A-B. As noted previously with reference to  FIG. 2A , the input end facet  230 A can include an HR coating  134 A to reflect the wavelength λ Pump  of the pump light (P), but to pass the higher wavelength λ In  of the signal light (S). 
     The input end facet  230 A is configured to transmit a portion, such as a majority, of the pass-through input signal light S (at the input wavelength λ In ) into the active section  220 , and the output end facet  230 B is configured to transmit a portion, such as a majority, of that input signal light S along with a portion, such as half or more, of the pump light P at a pump wavelength λ Pump  out of the active section  220 . 
     For example, the end facet  230 B may be configured to transmit at least 95%, 97%, or 99% of the light that reaches the end facets  230 A-B through the end facet  230 A-B. In these and other configurations, the end facet  230 B may have a reflectivity less than 1%. The reflectivity may be or include reflectivity for a single wavelength, multiple wavelengths, or a range of wavelengths such as an operational wavelength range of the laser diode  200 . The operational wavelength range of the laser diode  200  may be the same as or different than other operational wavelength ranges described herein. The output end facet  230 B is configured to transmit a portion of the pump light P generated by the active section  220  as well as a majority of the input signal light S that reaches the output end facet  230 B through the end facet  230 B. 
     In some embodiments as already noted, the reflectivity of the input end facet  230 A is different than the reflectivity of the output end facet  230 B for given wavelengths. Accordingly, the optical power of light at the end facets  230 A-B may be different. For example, the laser diode  200  may be configured so that more of the pump light P is output at the output end facet  230 B than the input facet  230 A in arrangements where the doped fiber  108  is arranged toward the output end facet  230 B, such as shown here in  FIG. 4A . 
     The active section  220  includes a waveguide  225  that extends between the end facets  230 A-B. The waveguide  225  may include the waveguide layers and the like as described with respect to  FIG. 3 . A first portion  234 A of the waveguide  225  near the input end facet  230 A may have a first transmissivity at least for the pump wavelength λ Pump , and a second portion  234 B of the waveguide  225  near the output end facet  230 B may have a second transmissivity at least for the pump wavelength λ Pump . The first and second transmissivities may each be (or may include) transmissivity for a single wavelength, multiple wavelengths, or a range of wavelengths, such as the operational wavelength range of the laser diode  200 . 
     The first and second transmissivities of the first and second portions  234 A,  234 B of the waveguide  225  may be the same or different. The first and second transmissivities may depend on materials and structure of the first and second portions  234 A-B of the waveguide  225 . Accordingly, the materials or structure of the first and second portions  234 A-B of the waveguide  225  may be selected to output light with equal or different optical power from the end facets  230 A-B, as desired. 
     As further shown in  FIG. 4A , the pass-through pump laser diode  200  is used in a fiber amplifier system having a fiber amplifier  102  with a doped fiber  108 . The input facet  230 A is in optical communication with the input signal light S and is configured to receive the input light S, such as from an input fiber  130 A. The active section  220  of the diode  200  is configured to pass the input light S from the input facet  230 A to the output facet  230 B. The active section  220  is configured to generate pump light P in response to injection of the electrical drive signal into the active section  220 . The pump light P has a pump wavelength λ Pump  different from the input wavelength λ In  that passes through the diode  200 . 
     The fiber amplifier  102  has a fiber portion  130 B and the doped fiber  108 . The fiber portion  130 B is in optical communication with the output facet  230 B and is configured to receive the input light S and at least a portion of the pump light P from the output facet  230 B. The pump wavelength λ Pump  of the pump light P is configured to interact with the doped fiber  108  in the manner disclosed herein. 
     In another configuration,  FIG. 4B  is a top schematic view of a laser diode  200  arranged for pass-through in a fiber amplifier  102  according to the present disclosure. As illustrated in  FIG. 4B , the laser diode  200  includes an input facet  230 A, an output facet  230 B, and an active section  220  positioned between the facets  230 A-B. 
     The fiber amplifier  102  has the laser diode  200  and doped fibers  108 A-B arranged in forward and backward pumping directions in which a first portion of the pump light P co-propagates with the primary signal light S through one doped fiber  108 B and in which a second portion of the pump light P propagates against the primary signal light S through another doped fiber  108 A. As noted, the laser diode  200  allows the primary signal light S being amplified to pass-through. As also noted, the laser diode  200  generates a pump light P that may pass in both directions. Yet, the laser diode  200 , the reflectivities of its facets ( 230 A-B), the transmissivities of with waveguide ( 220 ), its multiple active regions, etc. can be configured to pass more, less, or equal portions of pump light P in forward and backward directions as desired. This is merely schematically shown in  FIG. 4B . In any event, the pump light P directed to the doped fiber  108  can amplify the signal light S. 
     In general, the active section  220  is configured to generate pump light P that propagates toward each of the facets  230 A-B. The pump light P is generated by the active section  220  in response to injection of an electrical drive signal into the active section  220 . A cathode  214  and an anode  216  are electrically coupled to opposite sides, e.g., a top and a bottom, of the active section  220  to inject the electrical drive signal into the active section  220  between the cathode  214  and the anode  216 . 
     The input and output facets  230 A-B have low reflectivity. In an example, the low reflectivity at each of the end facets  230 A-B is achieved by cleaving the laser diode  200  from a wafer of lasers and forming an AR coating on the cleaved end facets  230 A-B. 
     The input end facet  230 A is configured to transmit a portion, such as a majority, of the pass-through input signal light S (at the input wavelength Δ In ) into the active section  220 , and the output end facet  230 B is configured to transmit a portion, such as a majority, of that input signal light S along with a portion, such as half or more, of the pump light P at a pump wavelength λ Pump  out of the active section  220 . 
     For example, the end facets  230 A-B may be configured to transmit at least 95%, 97%, or 99% of the light that reaches the end facets  230 A-B through the end facet  230 A-B. In these and other configurations, the end facets  230 A-B may have a reflectivity less than 1%. The reflectivity may be or include reflectivity for a single wavelength, multiple wavelengths, or a range of wavelengths such as an operational wavelength range of the laser diode  200 . The operational wavelength range of the laser diode  200  may be the same as or different than other operational wavelength ranges described herein. The output end facet  230 B is configured to transmit a portion of the pump light P generated by the active section  220  as well as a majority of the input signal light S that reaches the output end facet  230 B through the end facet  230 B. 
     In some embodiments and as already noted, the reflectivity of the input end facet  230 A is different than the reflectivity of the output end facet  230 B. Accordingly, the optical power of light at the end facets  230 A-B may be different. For example, the laser diode  200  may be configured so that more of the pump light P is output at the output end facet  230 B than the input facet  230 A in arrangements where the doped fiber  108  is arranged toward the output end facet  230 B. 
     The active section  220  includes a waveguide  225  that extends between the end facets  230 A-B. The waveguide  225  may include the waveguide layers and the like as described with respect to  FIG. 3 . A first portion  234 A of the waveguide  225  near the input end facet  230 A may have a first transmissivity at least for the pump wavelength Δ Pump , and a second portion  234 B of the waveguide  225  near the output end facet  230 B may have a second transmissivity at least for the pump wavelength λ Pump . The first and second transmissivities may each be or include transmissivity for a single wavelength, multiple wavelengths, or a range of wavelengths such as the operational wavelength range of the laser diode  200 . 
     The first and second transmissivities of the first and second portions  234 A,  234 B of the waveguide  225  may be the same or different. The first and second transmissivities may depend on materials and structure of the first and second portions  234 A-B of the waveguide  225 . Accordingly, the materials or structure of the first and second portions  234 A-B of the waveguide  225  may be selected to output light with equal or different optical power from the end facets  230 A-B, as desired. 
     As further shown in  FIG. 4B , the pass-through pump laser diode  200  of  FIG. 4B  is used in a fiber amplifier system having a fiber amplifier  102  with a doped fiber  108 . The input facet  230 A is in optical communication with the input signal light S and is configured to receive the input light S, such as from an input fiber  130 A. The active section  220  of the diode  200  is configured to pass the input light S from the input facet  230 A to the output facet  230 B. The active section  220  is configured to generate pump light P in response to injection of the electrical drive signal into the active section  220 . The pump light P has a pump wavelength λ Pump  different from the input wavelength λ In  that passes through the diode  200 . 
     The fiber amplifier  102  has a fiber portion  130 B and the doped fiber  108 . The fiber portion  130 B is in optical communication with the output facet  230 B and is configured to receive the input light S and at least a portion of the pump light P from the output facet  230 B. The pump wavelength λ Pump  of the pump light P is configured to interact with the doped fiber  108  in the manner disclosed herein. 
     In yet another configuration,  FIG. 4C  is a top schematic view of another laser diode  200  arranged for pass-through in a fiber amplifier  102  of the present disclosure. As illustrated in  FIG. 4C , the laser diode  200  includes an input facet  230 A, an output facet  230 B, and an active section  220  positioned between the facets  230 A-B. The laser diode  200  additionally includes a waveguide  225 . The facets  230 A-B, the active section  220 , and the waveguide  225  are configured and operated in the same or similar manner as the corresponding components in other lasers described herein. 
     The laser diode  200  may additionally include a reflective structure  240  formed in the active section  220  between first and second portions  234 A-B of the active section  220 . The reflective structure  240  may be configured to optically isolate the first portion  234 A of the active section  220  at least partially from the second portion  234 B of the active section  220 . The reflective structure  240  may include an etched mirror, a DFB structure, or other suitable structure formed in the active section  220 . When implemented as a DFB structure, the reflective structure  240  may lock the laser  200  to a predetermined wavelength. 
     The placement of the reflective structure  240  within the active section  220  can divide the active section  220  into portions of equal or unequal length. For example, as illustrated in  FIG. 4C , the first portion  234 A can be shorter than the second portion  234 B. In general, greater active section length leads to greater optical power output, all other parameters being equal. Accordingly, this configuration is another option to provide equal or different unequal optical power at the end facets  230 A-B of the laser diode  200 , if desired. 
     The laser diode  200  may further include a first cathode and anode  214 ,  216  electrically coupled to the first portion  234 A of the active section  220  and may further include a second cathode and anode  214 ,  216  electrically coupled to the second portion  234 B of the active section  220 . In particular, the first cathode and anode  214 ,  216  may be electrically coupled to opposite sides (e.g., top and bottom) of the first portion  234 A of the active section  220 , and the second cathode and anode  214 ,  216  may be electrically coupled to opposite sides (e.g., top and bottom) of the second portion  234 B of the active section  220 . A first electrical drive signal may be injected through the first portion  234 A via the first cathode and anode  214 ,  216 , and a second electrical drive signal may be injected through the second portion  234 B via the second cathode and anode  214 ,  216 . Accordingly, while the first and second portions  234 A-B of the active section  220  are integrally formed in a single structure (e.g., an epitaxial structure of the laser diode  200 ), they may nevertheless be independently operated. 
     As further shown in  FIG. 4C , the pass-through pump laser diode  200  is used in a fiber amplifier system having the fiber amplifier  102  with the doped fiber  108 . The input facet  230 A is in optical communication with the input signal light S and is configured to receive the input light S, such as from an input fiber  130 A. The waveguide  225  of the active section  220  of the diode  200  is configured to pass the input signal light S from the input facet  230 A to the output facet  230 B. The active section  220  is also configured to generate pump light P at a pump wavelength λ Pump  in response to injection of the electrical drive signal into the active section  220 . As noted, the pump wavelength λ Pump  can be different from the input wavelength λ In  that passes through the diode  200 . 
     The fiber amplifier  102  has a fiber portion  130 B and the doped fiber  108 . The fiber portion  130 B is in optical communication with the output facet  230 B and is configured to receive the input signal light S and at least a portion of the pump light P from the output facet  230 B. The pump wavelength λ Pump  of the pump light P is configured to interact with the doped fiber  108  in the manner disclosed herein. 
     In arrangements disclosed herein, the structure of the waveguide  225  can be adjusted to achieve certain output ratios relative to the front and back fibres ( 130 A-B). The structure of the waveguide  225  can be modified to change output ratio. Accordingly, in either  FIG. 4B or 4C , the laser diode  200  including AR coating on both facets  230 A-B can have facet reflectivities adjusted to achieve certain input/output ratio relative to fibers  130 A-B. Additionally, or alternatively, the waveguide structure  225  can be adjusted to achieve certain input/output ratio from two fibers  130 A-B. Either way, the input signal light (S) is configured to pass through the active section  220 . 
       FIG. 5  illustrates a process  300  for operating a fiber amplifier system  100  having a pass-through laser diode  200  according to the present disclosure. In the following description, reference numerals for elements from other figures are used for understanding. The laser diode  200  may include any of the laser diodes  200  described herein. The fiber amplifier system  100  may include a system as disclosed previously or any other appropriate fiber amplifier system. 
     A primary signal light S is generated or is received from a source (not shown) depending on the implementation (Block  302 ). For example, the system  100  can be used in optical fiber communications, and the source of the primary signal light S may be used for communications over a fiber network. The system  100  can be used in other implementations, such as in laser material processing. The primary signal light S having a primary wavelength is to be amplified by the fiber amplifier  102  and is then to be used for the purposes of the implementation, such as fiber communications, laser material processing, and the like. 
     The primary signal light S is transmitted to the laser diode  200 , and a majority of the signal S that reaches the input end facet  230 A is passed through the input end facet  230 A to the diode&#39;s waveguide  225  (Block  304 ). 
     During operation, one or more electrical drive signals are injected into the active section  220  of the laser diode  200  via the one or more of anode  216  and cathode  214  (Block  306 ). As noted above, the drive signal may be injected using a cathode and an anode  214 ,  216 . Additionally, multiple electrical drive signal can be injected into different portions of the active section  200  using multiple cathode and anode arrangements  214 ,  216 . Moreover, the portions of the active section  220  may be optically isolated at least partially from the others by a reflective structure  240 . 
     In response to the injected electrical drive signal, the active section  220  generates pump light P having a pump wavelength λ Pump  (Block  308 ). As noted, the pump wavelength λ Pump  can be intended to interact with a doped fiber  108 . 
     The pass-through signal light S and the generated pump light P pass toward the output end facet  230 B. Because the facets  230 A-B have low reflectivity, some of the generated pump light P may travel out the input facet  230 A with another amount traveling out of the output facet  230 B. As noted above, techniques based on FBGs, different reflectivities, different transmissivities, and the like for the may be used for the laser diode  200  so that more of the pump light P travels out of the output facet  230 B. 
     A majority of the primary signal light S and portion of the pump light P that reaches the output facet  230 B passes through the facet  230 B and into the opposing fiber portion  130 B (Block  310 ). The light signals S+P then reach the doped fiber  108  where the primary signal light S is amplified by the interaction of the pump light P in the manner disclosed herein (Block  312 ). 
     In previous arrangements, the laser diode  200  and the doped fiber  108  are arranged in a forward pumping direction in which the pump light P co-propagates with the primary signal light S through the doped fiber  108 . Other arrangements can be used. Moreover, the fiber amplifiers  102  disclosed herein can be used as part of an amplifier chain in multiple stages. 
     As noted above with respect to  FIG. 4A , the laser diode  200  can be used in a co-pumping arrangement in which the pump light (P) is injected into a fiber amplifier that is downstream of the laser diode  200 . As noted with respect to  FIG. 4B , the laser diode  200  can be used in a dual-pumping arrangement in which the pump light is injected into fiber amplifiers that are upstream and downstream of the laser diode  200 . Other arrangements are possible. 
     In a primary arrangement,  FIG. 6A  illustrates a fiber amplifier  102  having a laser diode  200  and a doped fiber  108  arranged in a co-pumping direction in which the pump light P propagates with the primary signal light S through the doped fiber  108 , which is downstream of the laser diode  200 . As noted, the laser diode  200  allows the primary signal light S being amplified to pass-through. As also noted, the laser diode  200  generates pump light P that may pass in both directions. Yet, the laser diode  200 , the reflectivities of its facets ( 230 A-B), the transmissivities of with waveguide ( 220 ), its multiple active regions, etc. can be configured to pass more pump light P in one direction over the other. Namely, more of the pump light P can pass from the output facet ( 230 B) than from the input face ( 230 B). This is merely schematically shown in  FIG. 6A . In fact, an HR coating, such as discussed previously, at the input facet  230 A can allow passage at the higher wavelength λ In  of the signal light S, but may reflect at the lower wavelength λ Pump  of the pump light P. In any event, the pump light P directed to the doped fiber  108  can amplify the signal light S. 
     In alternative arrangement,  FIG. 6B  illustrates a fiber amplifier  102  having a laser diode  200  and a doped fiber  108  arranged in a backward pumping direction in which the pump light P propagates against the primary signal light S through the doped fiber  108 . As noted, the laser diode  200  allows the primary signal light S being amplified to pass-through. As also noted, the laser diode  200  generates pump light P that may pass in both directions. Yet, the laser diode  200 , the reflectivities of its facets ( 230 A-B), the transmissivities of with waveguide ( 220 ), its multiple active regions, etc. can be configured to pass more pump light P in one direction over the other. Namely, more of the pump light P can pass from the input facet ( 230 A) than from the output face ( 230 B). This is merely schematically shown in  FIG. 6B . In fact, an HR coating, such as discussed previously, at the output facet  230 B can allow passage at the higher wavelength λ In  of the signal light S, but may reflect at the lower wavelength λ Pump  of the pump light P. In any event, the pump light P directed to the doped fiber  108  can amplify the signal light S. 
     In yet another arrangement,  FIG. 6C  illustrates yet another fiber amplifier  102  having two laser diodes  200 A-B arranged in opposing forward and backward pumping directions relative to a doped fiber  108 . As noted, the laser diodes  200 A-B allow the primary signal light S being amplified to pass-through. As also noted, the laser diodes  200 A-B generate pump light P that may pass in both directions. Yet, the laser diodes  200 A-B, the reflectivities of its facets ( 230 A-B), the transmissivities of with waveguide ( 220 ), its multiple active regions, etc. can be configured to pass more, less, or equal portions of pump light P in forward and backward directions as desired. This is merely schematically shown in  FIG. 6C . In fact, HR coatings, such as discussed previously, at the input facet  230 A of the upstream diode  200 A and at the output facet  230 B of the downstream diode  200 B can allow passage at the higher wavelength λ In  of the signal light S, but may reflect at the lower wavelength λ Pump  of the pump light P. In any event, the pump light P directed to the doped fiber  108  can amplify the signal light S. 
     In the configurations of the fiber amplifier  102  disclosed herein, the laser diode  200  allows the primary signal light S being amplified to pass-through while also generating the pump light P. Accordingly, the fiber amplifiers  102  do not require the pump light P to be coupled with the input signal light S using a coupler, such as a dichroic coupler, which simplifies the system and can reduce issues associated with such couplers. 
     In the configurations of the fiber amplifier  102  disclosed herein, isolators  106  can be used as appropriate to reduce parasitic reflections that can cause parasitic laser oscillation or can damage the fibers. These isolators  106  can be Faraday isolators. 
     In the configurations of the fiber amplifier  102  disclosed herein, multiple pass-through laser diodes  200  can be used together in a chain along the optical path. For example, instead of having one laser diode  200  to provide backward direction of pump light P to the doped fiber  108  as in  FIG. 6B , multiple ones of the laser diodes  200  can be used, as long as considerations are made for the passage of pump light P from one of the diodes  200  through the active region of the other diodes  200 . Likewise, instead of having one laser diode  200  to provide backward and forward direction of pump light P to the doped fibers  108 A-B as in  FIG. 4B , multiple ones of the laser diodes  200  can be used, as long as considerations are made for the passage of pump light P from one of the diodes  200  through the active region of other diodes  200 . 
     The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. It will be appreciated with the benefit of the present disclosure that features described above in accordance with any embodiment or aspect of the disclosed subject matter can be utilized, either alone or in combination, with any other described feature, in any other embodiment or aspect of the disclosed subject matter.