Abstract:
An integrated optical-amplification module includes a housing member, a first input optical terminal configured to receive an optical signal, a second input, optical terminal that can receive a pump light, and an output optical terminal that can output a combined optical signal comprising at least a portion of the optical signal and a portion of the pump light. The integrated optical-amplification module also includes an optical combiner fixedly installed relative to the housing member. The optical combiner can receive the pump light and the optical signal and an optical prism fixedly installed relative to the housing member. The optical combiner can merge the pump light and the optical signal to form the combined optical signal. The optical prism can direct at least a portion of the optical signal through free space to the optical combiner.

Description:
BACKGROUND 
     The present disclosure relates to optical-amplification modules for high-power optical applications. 
     The invention of optical fiber amplifier is a significant milestone in fiber communication history. Before the appearance of fiber amplifier, the repeaters in fiber communication system had been implemented using opto-electrical and electro-optical transmitters. The optical communication systems were complicated, inefficient, expensive, and sometimes unpredictable. Subsequent research efforts have been devoted to all-optic repeaters such as Raman amplifier, semiconductor amplifier, and Rare-earth doped fiber amplifier. Fiber amplifiers have demonstrated superior performance including high gain, high saturation output power, and low noise levels. These advantages have has made fiber amplifiers key components in fiber communication systems. 
     Conventional optical fiber amplifiers can be implemented in different pumping configurations. Referring to  FIG. 1A , a fiber laser system  100  includes an input optical fiber  11 , an isolator  12 , an optical coupler  13 A, a fiber amplifier  15 , another isolator  16 , a gain flatting filter device  17 , and an output optical fiber  18 . The isolator  12 , the optical coupler  13 A, the fiber amplifier  15 , the isolator  16 , and the gain flatting filter device  17  can be sequentially coupled by optical fibers. The fiber amplifier  15  can be implemented by an erbium doped fiber. The optical coupler  13 A can be implemented by a wavelength division multiplexer (WDM). The input optical fiber  11  is configured to receive a signal laser beam at a wavelength λ 1 . The optical coupler  13 A is coupled to a fiber  14  that is configured to receive a pump laser beam at a wavelength λ 2 . The fiber amplifier  15  can amplify the signal laser beam using the energy provided by the pump laser beam received from the upstream direction. Since the pump laser beam is coupled into the fiber amplifier  15  from the upstream direction relative to the fiber amplifier  15 , the optical coupler  13 A can be said to be in a forward pumping mode. The gain flatting filter device  17  is used to flatten or smoothen out signal intensities over a specified wavelength range and to ensure uniform gains in different wavelength channel. 
     In another laser pumping configuration, referring to  FIG. 1B , the fiber laser system  110  includes an optical coupler  13 B positioned downstream relative to the fiber amplifier  15 . The fiber amplifier  15  can amplify the signal laser beam using the energy provided by the pump laser beam from the downstream direction. The fiber laser system  110  can be said to have a backward pumping mode. 
     In another laser pumping configuration, referring to  FIG. 1C , the fiber laser system  120  includes an optical coupler  13 A positioned upstream relative to the fiber amplifier  15  and an optical coupler  13 B positioned downstream relative to the fiber amplifier  15 . Pump laser beams wavelength λ 2  can be respectively coupled in to the optical coupler  13 A and the optical coupler  13 B via the optical fibers  14 A and  14 B. The fiber amplifier  15  can amplify the signal laser beam using the energy provided by the pump laser beam received from the optical fiber  14 A in the upstream direction, and by the energy provided by the pump laser beam from the optical fiber  14 B in the downstream direction. The fiber laser system  120  thus has a bidirectional pumping mode which includes both a forward laser pump and a backward laser pump. 
     Different optical components in the above described fiber laser systems  100 ,  110 ,  120  are typically connected by fiber splicing. Drawbacks of these conventional laser systems include: complex configuration, low reliability due to too many splicing joints, inflexibility, and high cost. These conventional laser systems also suffer large insertion losses. The insertion loss for each fiber joint can be in the range of 0.1-0.2 dB. The described conventional laser systems each have 10 or more fiber joints, which can generate more than 1 dB optical power loss. As fiber network is becoming increasingly more complex, reliability and cost become more important. There is a need for a laser system and/or components for laser systems which are highly reliable, and low cost, and easy to be integrated into an optical device. 
     SUMMARY 
     In a general aspect, the present invention relates to an integrated optical-amplification module that includes a housing member; a first input optical terminal that can receive an optical signal; a second input optical terminal that can receive a pump light; an output optical terminal that can output a combined optical signal comprising at least a portion of the optical signal and a portion of the pump light; an optical combiner fixedly installed relative to the housing member, wherein the optical combiner can receive the pump light and the optical signal, wherein the optical combiner can merge the pump light and the optical signal to form the combined optical signal; and an optical prism fixedly installed relative to the housing member and being separated from the optical combiner and the first input optical terminal by free space, wherein the optical prism can receive the optical signal and direct at least a portion of the optical signal to the optical combiner. 
     In another general aspect, the present invention relates to an integrated optical-amplification module that includes a housing member; a first input optical terminal that can receive a first optical signal; a second input optical terminal that can receive a first pump light; a first output optical terminal that can output a first combined optical signal comprising at least a portion of the first optical signal and a portion of the first pump light; a first optical combiner fixedly installed relative to the housing member, wherein the first optical combiner can receive the pump light and the optical signal wherein the first optical combiner can merge the pump light and the optical signal to form the combined optical signal; a first prism fixedly installed relative to the housing member and being separated from the first optical combiner and the first input optical terminal by free space, wherein the first optical prism can receive the first optical signal and direct at least a portion of the first optical signal to the first optical combiner; a third input optical terminal that can receive a second optical signal; a fourth input optical terminal that can receive a second pump light; a second output optical terminal that can output a second combined optical signal comprising at least a portion of the second optical signal and a portion of the second pump light; a second optical combiner fixedly installed relative to the housing member, wherein the second optical combiner can receive the second pump light and the second optical signal, wherein the second optical combiner can merge the second pump light and the second optical signal to form the second combined optical signal; and a second prism fixedly installed relative to the housing member and being separated from the second optical combiner and the third input optical terminal by free space, wherein the second optical prism can receive the second optical signal and direct at least a portion of the second optical signal to the second optical combiner. 
     In another general aspect, the present invention relates to an integrated optical-amplification module that includes a housing member; a first optical prism fixedly installed relative to the housing member, wherein the first optical prism can include a first surface configured to receive an optical signal from free space and to allow the optical signal to refract through the first optical prism; and a second surface that can receive a pump light from free space and to merge at least a portion of the optical signal refracted through the first optical prism and at least a portion of the pump light to output an merged light in free space; and an amplifier that can amplify the portion of the optical signal in the merged light in response to the portion of the pump light in the merged light to produce an amplified light. 
     Implementations of the system may include one or more of the following. The optical signal can travel in free space at least a portion of the distance between the first input optical terminal and the optical prism. The first input optical terminal, the second input optical terminal, and the output optical terminal can be fixed to a same side of the housing member. The integrated optical-amplification module can further include an optical isolator positioned between the first input optical terminal and the optical prism, wherein the optical isolator can substantially uni-directionally pass the optical signal from the first input optical terminal to the optical prism. A surface of the optical prism can be coated with a polarization-insensitive thin film. The integrated optical-amplification module can further include a dual core optical collimator that can direct the pump light from the second input optical terminal to the optical combiner and to direct the combined optical signal from the optical combiner to the output optical terminal. The integrated optical-amplification module can further include a photo detector coupled to the optical prism, wherein the photo detector can capture at least a portion of the optical signal and produce an electric signal in response to the optical signal. Embodiments may include one or more of the following advantages. The disclosed systems and methods provide a compact, integrated, and lower cost optical module for optical-amplification applications. Compared to conventional systems, the disclosed optical-amplification module can have much lower insertion loss, higher reliability, is easier for system integration and modularization, and more convenient for system management and optimization. The disclosed optical-amplification includes an assembly of optical components separated by free space and can meet a wide range of applications. Another advantageous feature of the disclosed systems and methods is that the optical-amplification module can provide loading multiplexing, and flexible management of the optical signal and the pump light. 
     An advantage of the described integrated optical-amplification module is that it can be used in combination with different types of amplifiers. For example, fiber amplifiers having different doping elements and different lengths can be used to receive the combined optical signal. The optical amplification can be conducted at different wavelengths. Another advantage of the described integrated optical-amplification module is that it is compact and easy to use. The optical prism folds the optical paths to allow the input and output optical terminals to be positioned on the same side of the housing member, which reduces foot print and allow easy handling. Another advantage of the described integrated optical-amplification module is that it is flexible. Multiple of optical-amplification modules can be configured in parallel for separate optical amplifications or connected in series for a common optical amplification. Yet another advantage of the described integrated optical-amplification module is that multiple optical components can be packaged and tested in factory, which eliminates time and cost for assembling and testing during device integration. 
     Although the invention has been particularly shown and described with reference to multiple embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. 
         FIGS. 1A-1C  are schematic diagrams illustrating pumping configurations in conventional optical fiber amplifiers. 
         FIG. 2  is a schematic diagram of an optical-amplification module comprising optical components separated by free space. 
         FIG. 3  is a top view of an exemplified layout for an integrated optical-amplification module. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2 , an optical-amplification module  200  includes a collimator  211  configured to receive an optical signal  21  at wavelength λ 1  and to produce a collimated optical signal  21 A. The collimated optical signal  21 A passes through an isolator  22  to form optical signal  21 B that impinges on an optical multiplexer  24 . The isolator  22  can block undesirable backward lights and can minimize the interference between optical components, for example, amplified spontaneous emission (ASE) from an amplifier  27  (as described below) implemented by an erbium doped optical fiber. The optical multiplexer  24  can be implemented by an optical prism coated with films to make it insensitive to polarizations of the incident light, which can thus reduce polarization dependent loss (PDL). For example, surfaces S 1  and S 2  of the optical multiplexer  24  can be coated with polarization-insensitive films to reduce optical power loss related to directions of the polarizations. The polarization-insensitive films can also be configured to produce desirable splitting ratio between the intensities of the optical signal and pump light. A portion of the optical signal  21 B is reflected by surface S 1  of the optical multiplexer  24  to produce a reflected light  241 . The reflected light  241  can be received by a photo detector  23  that is configured to output an electric signal in response to intensity of the reflected light  241 . The photo detector  23  is used monitor the power of the optical signal. Another portion of the optical signal  21 B transmits through the optical multiplexer  24  to form a refraction light  242  which transmits surface S 2  of the optical multiplexer  24  to form a part of the light  243 . The intensities of the reflected light  241  and the refraction light  242  can be set by a predetermined splitting ratio. 
     A pump light  25  at wavelength λ 2  is collimated by a collimator  212  to form a collimated pump light  25 A that impinges on a optical multiplexer  26 . The pump light  25  can for example be a pump laser beam produced by a pump laser. Similar to the optical multiplexer  24 , the optical multiplexer  26  can be an optical prism coated with interference films to reduce polarization dependent optical losses. A portion of the collimated pump light  25 A transmits through the optical multiplexer  26  as a refracted light  262  which is not used in the amplification of the optical signal  21  and is desirably minimized by proper design of the coating on the surface S 3 . Another portion of the collimated pump light  25 A is reflected by surface S 3  of the optical multiplexer  26  to form a reflected pump light  261 . The reflected pump light  261  is reflected by surface S 2  of the optical multiplexer  24  to form a portion of the light  243 . The light  243  thus comprises the optical signal  21  at wavelength λ 1  and the pump light  25  at wavelength λ 2 . In other words, the light  243  is formed by multiplexing the optical signal and the pump light. The light  243  is coupled into an amplifier  27  by a collimator  213 . The amplifier  27  can be implemented by an erbium doped optical fiber capable of generating amplified spontaneous emission. The amplifier  27  amplifies the light  243  to form a light  28  which passes through a gain flattening filter (GFF)  29  to form an output light  30 . The amplifier  27  can be implemented by for example an erbium doped optical fiber. The gain flattening filter  29  can reduce gain variations across spectral bands at wavelengths. 
     Two or more of the collimators  211 - 213 , the isolator  22 , the optical multiplexers  24 ,  26 , the photo detector  23 , and the amplifier  27  can be separated by free space. As described below, one or more of the collimators  211 - 213 , the isolator  22 , the optical multiplexers  24 ,  26 , the photo detector  23 , and the amplifier  27  can be fixedly assembled in a housing member to form an integrated optical module. In the present invention, the term “free space” refers to a gap filled by air or other uniform medium (such as a gas or a liquid). The optical signal and the pump light can transmit between two or more of the above optical components in free space without the use of optical fibers. Comparing to conventional systems, the elimination of optical fibers for light transmissions between these components can significantly reduce insertion loss related to the coupling or splicing of optical fibers and improve performance. In some embodiments, the insertion loss of more than 1 dB in the conventional laser systems can be prevented. The optical-amplification module can also be miniaturized and reduce cost. 
     The described optical-amplification module can be assembled in an integrated optical module to reduce footprint, lower cost, and to decrease the time and cost for system integration and testing. Referring to  FIG. 3 , an integrated optical-amplification module  300  includes optical-amplification modules  370 ,  380 ,  390  installed in a rigid housing  301  and on a base  308  fixed to the rigid housing  301 . The rigid housing  301  can for example be made of a metallic or a plastic material. The base  308  can be made of a ceramic material. The optical-amplification modules  370 ,  380 ,  390  can be used in parallel for separate amplifications of different or the same input optical signals. The optical-amplification modules  370 ,  380 ,  390  respectively include optical collimators  311 ,  313 , and  315  for receiving optical signals. The optical collimators  311 ,  313 , and  315  are therefore input optical terminals. The optical-amplification modules  370 ,  380 ,  390  also respectively include optical collimators  312 A,  314 A, and  316 A for receiving pump light. The optical-amplification modules  370 ,  380 ,  390  also respectively include optical collimators  312 B,  314 B, and  316 B for outputting combined optical signals each including a mixture of the pump light and the optical signal. The optical collimators  311 ,  313 ,  315 ,  312 A,  314 A, and  316 A are input optical terminals. The optical collimators  312 B,  314 B, and  316 B are output optical terminals. 
     The optical-amplification module  370  includes a single fiber collimator  302 , an optical isolator  304 , an optical prism  307 , a photo detector  306 , an optical combiner  305 , and a dual fiber collimator  303 , which can be fixed to the base  308  by screws or adhesives such as epoxy. The optical prism  307  can be an optical prism coated with films to reduce its dependence on the polarizations of the incident light. The optical collimator  311  can be implemented by an optical fiber that goes through the rigid housing  301  to couple to the single fiber collimator  302 . The optical combiner  305  can be implemented by a WDM filter and in some applications, can be a gain flatten filter (GFF). 
     In operation, an optical signal is coupled into the optical collimator  311  and then into the single fiber collimator  302 . The optical signal passes through the optical isolator  304  and is directed to the optical prism  307 . The optical signal is reflected by total reflection by the optical prism  307  and to be coupled first into the optical combiner  305 . A portion of the optical signal can be coupled into the photo detector  306  for monitoring purpose. The photo detector  306  can produce an electric signal in response to the optical signal. 
     The optical collimator  312 A receives a pump light and couples it into the dual fiber collimator  303 . The optical combiner  305  receives the pump light from the dual fiber collimator  303  and the optical signal from the optical prism  307  (in opposite directions, thus backward pump light coupling). The optical combiner  305  combines the pump light and the optical signal to output a combined optical signal first through the dual fiber collimator  303  and then out of the optical collimator  312 B. The combined optical signal is input to an external amplifier to generate an amplified light. 
     The optical-amplification modules  370 ,  380 ,  390  can also be used in series, which in combination is used for a single optical amplification. The combined optical signal output from the optical collimator  312 B in the optical-amplification module  390  can be fed into the optical collimator  313  in the optical-amplification module  380 . Similarly, the combined optical signal output from the optical collimator  314 B in the optical-amplification module  380  can be fed into the optical collimator  315  in the optical-amplification module  370 . 
     An advantage of the described integrated optical-amplification module is that it can be used in combination with different types of amplifiers. For example, fiber amplifiers having different doping elements and different lengths can be used to receive the combined optical signal. The optical amplification can be conducted at different wavelengths. Another advantage of the described integrated optical-amplification module is that it is compact and easy to use. The optical prism folds the optical paths to allow the input and output optical terminals to be positioned on the same side of the housing member, which reduces foot print and allow easy handling. Another advantage of the described integrated optical-amplification module is that it is flexible. Multiple of optical-amplification modules can be configured in parallel for separate optical amplifications or connected in series for a common optical amplification. Yet another advantage of the described integrated optical-amplification module is that multiple optical components can be packaged and tested in factory, which eliminates time and cost for assembling and testing during device integration. 
     It should be understood that the described integrated optical-amplification modules can be implemented in other configurations without deviating from the spirit of the present invention. An integrated optical-amplification module can include one, two, or more optical-amplification modules that each can perform above described functions. One or more amplifiers can be included in the described integrated optical-amplification module. The layout and the materials of the various optical components can differ from the examples described above.