Patent Publication Number: US-2020304205-A1

Title: Optical test apparatus

Description:
FIELD OF THE INVENTION 
     The present disclosure relates generally to optical fiber test apparatuses, and more particularly to test apparatuses for in-line bi-directional testing of one or more selected wavelengths of light travelling through an optical system. 
     BACKGROUND OF THE INVENTION 
     Measuring optical power levels of an operating optical communications system requires the use of an inline power meter capable of measuring the power of individual wavelength components of the optical signal travelling in the optical fiber by sampling a portion of the total signal. Such optical communications systems may be, e.g., passive optical networks (“PON”), like those used in fiber to the “X” (FTTX, where “X” may be: H=home, C=curb, N=node, P=premises, etc.) networks, optical local area networks (“OLANs”), and coarse wavelength division multiplexing (“CWDM”) links, among others. 
     Existing apparatuses for measuring optical power levels in operating optical communications systems tend to have complex and bulky optical structures for signal sampling and spectral power detection. As a result, excessive optical loss may occur within these optical structures and may limit the dynamic range of the power measurement. 
     Accordingly, improved optical fiber test apparatuses are desired in the art. In particular, optical fiber test apparatuses that are scalable to measure various numbers of different wavelength components within the two-way signal traffic would be advantageous. Additionally, optical fiber test apparatuses that measure the optical power levels without compromising the signal quality and integrity of the two-way signal traffic in the operating optical communications system would be advantageous. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In accordance with one embodiment, an optical test apparatus is provided. The optical test apparatus includes a collimator comprising a dual-fiber ferrule and a collimating lens. The optical test apparatus also includes an optical filter optically aligned with the collimator, a first optical fiber pigtail connected to the collimator at a first bore of the dual-fiber ferrule, a second optical fiber pigtail connected to the collimator at a second bore of the dual-fiber ferrule, and a photodetector in optical communication with the collimator. The optical test apparatus defines a reflection path from the first optical fiber pigtail to the optical filter and from the optical filter to the second optical fiber pigtail and a transmission path from the first optical fiber pigtail through the optical filter to the photodetector. 
     In accordance with another embodiment, an inline multi-wavelength power measurement apparatus is provided. The inline multi-wavelength power measurement apparatus includes a first optical test apparatus and a second optical test apparatus serially connected to the first optical test apparatus. The first optical test apparatus includes a collimator comprising a dual-fiber ferrule and a collimating lens. The first optical test apparatus also includes an optical filter having a first passband and optically aligned with the collimator of the first optical test apparatus, a first optical fiber pigtail connected to the collimator of the first optical test apparatus at a first bore of the dual-fiber ferrule of the first optical test apparatus, a second optical fiber pigtail connected to the collimator of the first optical test apparatus at a second bore of the dual-fiber ferrule of the first optical test apparatus, and a photodetector in optical communication with the collimator of the first optical test apparatus. The first optical test apparatus defines a reflection path from the first optical fiber pigtail of the first optical test apparatus to the optical filter having the first passband and from the optical filter having the first passband to the second optical fiber pigtail of the first optical test apparatus and a transmission path from the first optical fiber pigtail of the first optical test apparatus through the optical filter having the first passband to the photodetector of the first optical test apparatus. The second optical test apparatus includes a collimator comprising a dual-fiber ferrule and a collimating lens. The second optical test apparatus also includes an optical filter having a second passband different from the first passband and optically aligned with the collimator of the second optical test apparatus, a first optical fiber pigtail connected to the collimator of the second optical test apparatus at a first bore of the dual-fiber ferrule of the second optical test apparatus, a second optical fiber pigtail connected to the collimator of the second optical test apparatus at a second bore of the dual-fiber ferrule of the second optical test apparatus, and a photodetector in optical communication with the collimator of the second optical test apparatus. The second optical test apparatus defines a reflection path from the first optical fiber pigtail of the second optical test apparatus to the optical filter having the second passband and from the optical filter having the second passband to the second optical fiber pigtail of the second optical test apparatus and a transmission path from the first optical fiber pigtail of the second optical test apparatus through the optical filter having the second passband to the photodetector of the second optical test apparatus. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures. 
         FIG. 1  illustrates an optical test apparatus in accordance with one or more example embodiments of the present disclosure. 
         FIG. 2  illustrates an optical test apparatus in accordance with one or more additional example embodiments of the present disclosure. 
         FIG. 3  illustrates an optical test apparatus in accordance with one or more further example embodiments of the present disclosure. 
         FIG. 4  provides an example spectral reflectance/transmittance curve of an exemplary bandpass optical filter as may be incorporated in one or more example embodiments of the present disclosure. 
         FIG. 5  illustrates an inline multi-wavelength power measurement apparatus in accordance with one or more example embodiments of the present disclosure, the inline multi-wavelength power measurement apparatus comprising a plurality of optical test apparatuses. 
         FIG. 6  illustrates an inline multi-wavelength power measurement apparatus in accordance with one or more additional example embodiments of the present disclosure, the inline multi-wavelength power measurement apparatus comprising a plurality of optical test apparatuses. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     As used herein, terms of approximation, such as “generally,” or “about” include values within ten percent greater or less than the stated value. When used in the context of an angle or direction, such terms include within ten degrees greater or less than the stated angle or direction. For example, “generally vertical” includes directions within ten degrees of vertical in any direction, e.g., clockwise or counter-clockwise. 
     Referring now to  FIGS. 1 through 3 , various embodiments of optical test apparatus  10  in accordance with the present disclosure are provided. The test apparatus  10  may, for example, include a first input (IN  1 )  12  and a second input (IN  2 )  14 . The first and second inputs  12  and  14  may each be connected to an operating optical communications system to receive light signals, e.g. infrared light, travelling through the system for transmission through the other components of the test apparatus  10 . The inputs  12  and  14  may each be or include, for example, a universal connector interface or an FC connector (i.e., ferrule connector). Suitable FC connectors may include, for example, FC/UPC and FC/APC connectors. 
     Alternatively, however, other suitable optical connectors or splices may be utilized. The light signals may include light at one or more suitable predetermined wavelengths and/or light signals travelling in opposing directions. For example, as illustrated in  FIGS. 1 through 3  and described in more detail below, light signals travelling through the optical system may include a first light signal  200  travelling in a first direction and a second light signal  300  travelling in a second direction opposite the first direction. The optical test apparatus  10  may split the first light signal  200  into a first reflected portion  202  and a first transmitted portion  204  and may split the second light signal  300  into a second reflected portion  302  and a second transmitted portion  304 . 
     The test apparatus  10  may further include at least one collimator, for example, a first collimator  60 . The collimator  60  generally collimates, or aligns, light being transmitted therethrough. For example, in exemplary embodiments as shown, the collimator includes a lens  62 . The lens may, in some embodiments, be a graded-index lens. Alternatively, other suitable glass lenses or lenses formed from other suitable materials may be utilized. Collimator  60  may, in some embodiments, further include a ferrule  64 . The ferrule  64  may be connected to and/or otherwise optically aligned with an end of the lens  62 , as shown. In exemplary embodiments, the ferrule  64  may be a dual fiber ferrule, and thus define two parallel bores or channels  66  extending therethrough. Each channel  66  may accommodate an optical fiber therein for connection to the collimator  60 , as discussed herein. Alternatively, the ferrule  64  may include one, three or more channels  66 . 
     In some embodiments, a protective sleeve  68  may additionally be provided, and the collimator  60  may be housed in the protective sleeve  68 . The sleeve  68  may further align the collimator  60  with other components of the test apparatus  10  as discussed herein. 
     An optical filter  70  may be optically aligned with the first collimator  60 . In some embodiments, for example as illustrated in  FIG. 1 , the optical filter  70  may be spaced from the first collimator  60 , e.g., the optical filter  70  may be attached to a second collimator  90  as depicted in  FIG. 1  and described in more detail below. Further, embodiments where the optical filter  70  is spaced from the first collimator  60  may also include a beam splitter  72  which may be optically aligned with the first collimator  60 , e.g., the beam splitter  72  may be optically aligned with and connected to the first collimator  60  as illustrated in  FIG. 1 . In some embodiments, for example as illustrated in  FIGS. 2 and 3 , the optical filter  70  may be optically aligned with and connected to the first collimator  60 . For example, the optical filter  70  may be in contact with and connected to an end of the first collimator  60 . The optical filter  70  may be optically aligned with an end of the lens  62  opposite the end to which the ferrule  64  is connected. In embodiments wherein a protective sleeve  68  is utilized, the optical filter  70  may be housed within the protective sleeve  68 . In various embodiments, the optical filter  70  may be attached to either the first collimator  60  or the second collimator  90 , or positioned independently and aligned between the first collimator  60  and the second collimator  90 . 
     Any suitable optical filter  70  may be utilized. For example, in some embodiments, the optical filter  70  may be or include prisms, gratings, optical thin film coating, a combination of these elements, or any other suitable components singly or in combination, which can filter out a single wavelength, a cluster of wavelengths, or a continuous wavelength band from the light signals  200  and  300 . In some embodiments, the optical filter  70  may be a bandpass filter, such as a narrow band filter. For example, a narrowband filter may have a passband of about 20 nm or less, such as about 15 nm or less, such as about 10 nm or less, such as about 5 nm or less, such as about 2.0 nm or less, such as about 1.5 nm or less, such as about 1.0 nm or less, such as about 0.5 nm or less. In one example, the optical filter  70  may have a passband of about 0.4 nm centered at about 1550 nm for dense wavelength division multiplexing (DWDM) signal power measurement. Another example is a CWDM filter with center wavelength at about 1490 nm and approximately 15 nm bandwidth, which may be used for measuring the power of a PON network downstream signal. 
     Test apparatus  10  may further include various components for facilitating the transmission of light from the optical system with which the test apparatus  10  is connected, such as via the inputs  12  and  14 , to and through the test apparatus  10 . Optical test apparatus  10  may further include various components to facilitate the transmission of light from the test apparatus  10  to a measuring equipment such as a power meter or a photodetector which may be connected to and/or integrated with the test apparatus  10 . These light transmissions may be provided through the first collimator  60  and via the optical filter  70  as discussed herein. 
     For example, test apparatus  10  may include a first optical fiber pigtail  80 . The first optical fiber pigtail  80  may be connected, such as at a first end thereof, to the first input  12 . Test apparatus  10  may further include a second optical fiber pigtail  82 . The second optical fiber pigtail  82  may be connected, such as at a first end thereof, to the second input  14 . In exemplary embodiments, the first optical fiber pigtail  80  and second optical fiber pigtail  82  are single-mode optical fibers. Further, in exemplary embodiments, each optical fiber pigtail  80 ,  82  may have a standard optical fiber core diameter and outer cladding diameter, i.e., 9 micrometers and 125 micrometers respectively. In other exemplary embodiments, each optical fiber pigtail  80 ,  82  may have any suitable core diameter and outer cladding diameter. 
     As illustrated in  FIGS. 1-3 , the first optical fiber pigtail  80  may be optically aligned and connected, e.g., at a second end thereof, to the first collimator  60 . For example, the first optical fiber pigtail  80  may extend through a channel  66  of the ferrule  64  and be connected to the lens  62 . Accordingly, the first optical fiber pigtail  80  may be connected to and between the first input  12  and the first collimator  60 . 
     In exemplary embodiments, e.g., as shown in  FIGS. 1 through 3 , the second optical fiber pigtail  82  may be connected, such as at the second opposite end thereof, to the first collimator  60 . For example, the second optical fiber pigtail  82  may extend through a channel  66  of the ferrule  64  and be connected to the lens  62 . Accordingly, the second optical fiber pigtail  82  may be connected to and between the second input  14  and the first collimator  60 . Such connections may facilitate the various light transmissions therethrough as discussed herein. 
     In embodiments such as the example embodiment illustrated by  FIG. 1  where the beam splitter  72  is provided, the beam splitter  72  may be a glass, an optical film coating, or a cubic. As is generally understood, the beam splitter  72  may transmit a portion of light received by the beam splitter  72  therethrough, and may reflect another portion of the received light. In exemplary embodiments, the beam splitter  72  may reflect 50% of the light and transmit 50% of the light. Alternatively, the beam splitter  72  may reflect between 40% and 60% of the light and transmit between 60% and 40% of the light, such as reflect between 45% and 55% of the light and transmit between 55% and 45% of the light. In further embodiments, the beam splitter  72  may reflect about eighty percent (80%) of the light or more, such as about ninety percent (90%) of the light or more, such as about ninety-five percent (95%), and may transmit about twenty percent (20%) of the light or less, such as about ten percent (10%) of the light or less, such as about five percent (5%) of the light. In other alternative embodiments, any suitable relative percentages of transmitted and reflected light may be utilized. 
     In embodiments where the beam splitter  72  is not included, e.g., as illustrated in  FIGS. 2 and 3 , the optical filter  70  may both filter and split the received light signals  200  and  300 . For example, the optical filter  70  may be a bandpass filter having a spectral reflectance/transmittance curve similar to the example curves shown in  FIG. 4 . The passband may be centered at wavelength λ 0 , and may be a theoretically rectangular passband, as illustrated in  FIG. 4 . The bandwidth is Δλ. Within the passband, a first portion of light from the first input  12  and the second input  14  with the percentage of α % passes the filter, while a second portion of the light with the percentage of (100−α) % is reflected by the optical filter  70 . Outside the passband, all light is reflected. Thus, for example, the reflected portions  202  and  302  may include one hundred percent (100%) of the light signals  200  and  300  outside of the passband and one hundred minus alpha percent (100−α) % of the light signals  200  and  300  within the passband, and the transmitted portions  204  and  304  may include α % of the light signals  200  and  300  within the passband. In various embodiments, the percentage a may include any of the percentages described above with respect to the beam splitter  72 . For example, a may be about five. In one example, the bandpass filter can be centered at about 1550 nm with approximately 0.4 nm passband, and have a sampling ratio of about 5% (e.g., alpha may be about five) for inline dense wavelength division multiplexing (DWDM) signal power measurement. The foregoing percentages, bandwidths, and centers are provided by way of example only. In practice, the practical filtering spectrum may deviate from the foregoing examples due to various factors such as light loss, manufacturing tolerances, and other reasons. 
     Referring now to  FIGS. 1 and 2 , in some embodiments, test apparatus  10  includes a second collimator  90 . The collimator  90  generally collimates, or aligns, light being transmitted therethrough. The second collimator  90  may further facilitate the various transmissions of light as discussed herein. For example, in exemplary embodiments as shown, the collimator includes a lens  92 . The lens may, in some embodiments, be a graded-index lens. Alternatively, other suitable glass lenses or lenses formed from other suitable materials may be utilized. Collimator  90  may, in some embodiments, further include a ferrule  94 . The ferrule  94  may be connected to and/or otherwise optically aligned with an end of the lens  92 , as shown. In exemplary embodiments, the ferrule  94  may be a dual fiber ferrule, and thus define two parallel bores or channels  96  extending therethrough. Each channel  96  may accommodate an optical fiber therein for connection to the collimator  90 , as discussed herein. Alternatively, the ferrule  94  may include one, three or more channels  96 . 
     In embodiments wherein a protective sleeve  68  is utilized, the second collimator  90  may be housed in the protective sleeve  68 . 
     As illustrated, the second collimator  90  may be spaced from the first collimator  60 , such as along a longitudinal axis  100 . Accordingly, a longitudinal gap  102  may be defined between beam splitter  70  and collimator  90 . The collimator  90  may further be optically aligned with the optical filter  70  and, in some embodiments, the beam splitter  72 . Light travelling from the first collimator  60  to the second collimator  90  or vice versa may be transmitted across this gap  102 . The gap  102  may be determined to obtain a desired light transmission efficiency. In exemplary embodiments, the gap may be less than or equal to 20 millimeters, such as less than or equal to 15 millimeters, such as between 3 millimeters and 20 millimeters, such as between 4 millimeters and 15 millimeters, such as between 5 millimeters and 10 millimeters. The second collimator  90  may further have an opposite orientation along the longitudinal axis  100  to the first collimator, such that the lenses  62 ,  92  face each other and are proximate each other relative to the ferrules  64 ,  94 . 
     As may be seen in  FIGS. 1 and 2 , test apparatus  10  may further include, for example, a first output (OUT 1   λ  where lambda represents the wavelength(s) of light passing the optical filter  70 )  16  and a second output (OUT 2   λ )  18 . The first output  16  may be connected to a first photodetector  11  and the second output  18  may be connected to a second photodetector  13 . Connecting each output  16  and  18  to a respective photodetector  11  and  13  permits measurement of optical power for selected wavelength components in the signals  200  and  300 . If the outputs  16  and  18  are connected to a spectral characterization apparatus, such as an optical spectrum analyzer, the spectrum of the signals  200  and  300  within the passband of the filter  70  may be measured as well. 
     In these embodiments, test apparatus  10  may further include a third optical fiber pigtail  84 . The third optical fiber pigtail  84  may be connected, such as at a first end thereof, to the first output  16 . Test apparatus  10  may further include a fourth optical fiber pigtail  86 . The fourth optical fiber pigtail  86  may be connected, such as at a first end thereof, to the second output  18 . In exemplary embodiments, the third optical fiber pigtail  84  and fourth optical fiber pigtail  86  are single-mode or multi-mode optical fibers having any suitable core size. Further, in exemplary embodiments, each optical fiber pigtail  84 ,  86  may have an optical fiber core that is greater, i.e. greater in cross-sectional diameter, than the cores of the first and second optical fiber pigtails  80  and  82 . For example, each optical fiber pigtail  84 ,  86  may have an optical fiber core and outer cladding diameter that are 62.5 micrometers and 125 micrometers respectively while the first and second optical fiber pigtails  80  and  82  may, as mentioned above, have a core diameter of 9 micrometers, e.g., when the first and second optical fiber pigtails  80  and  82  include single-mode optical fibers, or may have a core diameter of 50 micrometers, e.g., when the first and second optical fiber pigtails  80  and  82  include multi-mode optical fibers. As another example, the first and second optical fiber pigtails  80  and  82  may each have a core diameter of 9 micrometers and the third and fourth optical fiber pigtails may each have a core diameter of 50 micrometers. Advantageously, the third and fourth optical fiber pigtails  84  and  86  having such larger cores capture a significant amount of light therein with reduced perturbation by modal variations along the optical paths. 
     In these embodiments, the third optical fiber pigtail  84  may be connected, such as at a second end, to the second collimator  90 . For example, the third optical fiber pigtail  84  may extend through a channel  96  of the ferrule  94  and be connected to the lens  92 . Accordingly, the third optical fiber pigtail  84  may be connected to and between the first output  16  and the second collimator  90 . The fourth optical fiber pigtail  86  may be connected to the second collimator  90 . In some embodiments, as illustrated in  FIGS. 1 and 2 , the fourth optical fiber pigtail  86  may be connected, i.e. at a second end thereof, to the second collimator  90 . For example, the fourth optical fiber pigtail  86  may extend through a channel  96  of the ferrule  94  and be connected to the lens  92 . Accordingly, the fourth optical fiber pigtail  86  may be connected to and between the second output  18  and the second collimator  90 . Such connections may facilitate the various light transmissions therethrough as discussed herein. 
     As mentioned, the test apparatus  10  may be connected to and in line with an optical system, such as an optical communications system. Thus, in some embodiments, first light signal  200  travelling through the optical system is transmitted through the first optical fiber pigtail  80  as discussed. This light  200  is further transmitted from the first optical fiber pigtail  80  to and through the first collimator  60 , and from the first collimator  60  to the optical filter  70  and, in some embodiments, beam splitter  72 . As discussed, a reflected portion  202  of the light  200  may be reflected by the optical filter  70  or beam splitter  72  to the second optical fiber pigtail  82 , and through the second optical fiber pigtail  82  back to the optical system via the second input  14  as discussed and as illustrated in  FIGS. 1 through 3 . Further, in some embodiments, e.g., as shown in  FIGS. 1 and 2 , a transmitted portion  204  of the first light signal  200  may be transmitted through the optical filter  70  (and, in some embodiments, the beam splitter  72 ) to the second collimator  90 , and from the second collimator  90  to the third optical fiber pigtail  84  and first output  16 . As mentioned, the outputs  16  and  18  may each be connected to a respective photodetector  11  and  13 , thus, transmitted portion  204  may be further transmitted through the third optical fiber pigtail  84  and first output  16  to the respective photodetector  11  or  13 . Second light signal  300  may be transmitted from the optical system to and through the second optical fiber pigtail  82  via the second input  14 , and from the second optical fiber pigtail  82  to and through the first collimator  60 . A reflected portion  302  of the second light signal  300  may be transmitted from the optical filter  70  or beam splitter  72  back to the optical system via the first optical fiber pigtail  80 . A transmitted portion  304  of the second light signal  300  may further be transmitted from the first collimator  60  to and through the optical filter  70  and, in some embodiments, the beam splitter  72  to the second collimator  90 . The second transmitted portion  304  may then be transmitted from the second collimator  90  to and through the fourth optical fiber pigtail  86 , and from the fourth optical fiber pigtail  86  to the second output  18 . 
     Referring now specifically to  FIG. 3 , in some embodiments, optical test apparatus  10  may further include a photodetector  11  integrated into the test apparatus  10 . In exemplary embodiments, the photodetector  11  may be a photodiode. The photodetector may or may not include a flat or lensed window. In embodiments wherein a protective sleeve  68  is utilized, the photodetector  11  may be housed in the protective sleeve  68 . In this embodiment, the sampled and wavelength-filtered signal components from IN  1  and IN  2 , e.g., the first and second transmitted portions  204  and  304 , can be detected and converted into electric signals directly by the optical test apparatus  10 . In some embodiments, the photo detector  11  may be a multi-element detector, such that signal components from IN  1  or IN  2  can be detected separately. In other embodiments, the photodetector  11  may have only a single element, such that the total spectral power in IN  1  and IN  2 , i.e., P 1   λ  and P 2   λ , is detected. If the wavelength components from IN  1  and IN  2  are different, P 1   λ  and P 2   λ  can still be measured separately. In this embodiment, the optical filter  70  can be prisms, gratings, optical thin film coating, or any other suitable components. 
     As illustrated, the photodetector  11  may be spaced from the first collimator  60  and optical filter  70 , such as along a longitudinal axis  100 . Accordingly, a longitudinal gap  103  may be defined between optical filter  70  and photodetector  11 . Light travelling from the first collimator  60  to the photodetector  11  may be transmitted across this gap  103 . The gap  103  may be determined to obtain a desired responsivity. In exemplary embodiments, the gap may be less than or equal to 20 millimeters, such as less than or equal to 15 millimeters, such as between 3 millimeters and 20 millimeters, such as between 4 millimeters and 15 millimeters, such as between 5 millimeters and 10 millimeters. 
     In various embodiments, e.g., as illustrated in  FIGS. 1 through 3 , the optical test apparatus  10  may define a first reflection path, e.g., the path of first reflected portion  202  as described above, and a first transmission path, e.g., the path of the first transmitted portion  204  as described above. The first reflection path may be at least partially defined by the first optical fiber pigtail  80 , the optical filter  70  and beam splitter  72 , and the second optical fiber pigtail  82 . For example, the first reflection path may extend from the first optical fiber pigtail  80  to the optical filter  70  and from the optical filter  70  (and/or beam splitter  72 ) to the second optical fiber pigtail  82 . The first transmission path may be at least partially defined by the first optical fiber pigtail  80 , the optical filter  70 , and the first photodetector  11 . For example, the first transmission path may extend from the first optical fiber pigtail  80  through the optical filter  70  (and/or beam splitter  72 ) to the first photodetector  11 . 
     The optical test apparatus  10  may further define a second reflection path, e.g., the path of second reflected portion  302  as described above, and a second transmission path, e.g., the path of the second transmitted portion  304  as described above. The second reflection path may be at least partially defined by the second optical fiber pigtail  82 , the optical filter  70  and beam splitter  72 , and the first optical fiber pigtail  80 . For example, the second reflection path may extend from the second optical fiber pigtail  82  to the optical filter  70  and from the optical filter  70  (and/or beam splitter  72 ) to the first optical fiber pigtail  80 . The second transmission path may be at least partially defined by the second optical fiber pigtail  82 , the optical filter  70  (and/or beam splitter  72 ), and a photodetector. For example, in embodiments such as those illustrated in  FIGS. 1 and 2 , the second transmission path may be partially defined by and may extend to the second photodetector  13 , e.g., the second transmission path may extend from the second optical fiber pigtail  82  through the optical filter  70 , the second collimator  90 , the fourth pigtail  86 , and to the second photodetector  13 . As another example, in embodiments such as the embodiment illustrated in  FIG. 3 , the second transmission path may extend from the second optical fiber pigtail  82  through the optical filter  70  to the photodetector  11 . 
     In these embodiments, the transmitted portions  204  and  304  may be transmitted to and through the first collimator  60 , and from the first collimator  60  to and through the optical filter  70 . Further, the transmitted portions  204  and  304  may be transmitted through the optical filter  70  to the photodetector(s)  11  and/or  13 . The transmitted light  204  and  304  may be converted by the photodetector(s)  11  and/or  13  to an electrical signal, and the electrical signal may be transmitted from the photodetector(s)  11  and/or  13  to a power monitor or a power meter. 
     Turning now to  FIGS. 5 and 6 , embodiments of the present disclosure may also include an inline multi-wavelength power measurement apparatus  400  or  500  comprising a plurality of optical test apparatuses  10 . The plurality of test apparatuses  10  may be any one or combination of the embodiments illustrated in  FIGS. 1  through  3 . Such test apparatuses  10  may be combined, as described in more detail below, to provide an inline multi-wavelength power measurement apparatus  400  or  500  where each test apparatus  10  of the plurality of test apparatuses  10  samples a portion of the light signals  200  and  300  at a particular wavelength distinct from the wavelength sampled by other test apparatuses  10  of the plurality of test apparatuses  10 . For example, the inline multi-wavelength power measurement apparatus  400  or  500  may include a first optical test apparatus  10  having an optical filter  70  with a first passband and a second optical test apparatus  10  having an optical filter  70  with a second passband different from the first passband. 
     As mentioned, the apparatus  10 ,  400 , and/or  500  disclosed herein may be configured to test or measure a first light signal  200  travelling in a first direction and a second light signal  300  travelling in a second direction opposite the first direction. In the example embodiments illustrated by  FIGS. 5 and 6 , the first and second light signals  200  and  300  each include N wavelength components, e.g., the light signals include multiple wavelengths λ 1 , λ 2 , etc., up to and including λ N . The ratio α of each test apparatus  10 , e.g. of the optical filter  70  or beam splitter  72  thereof, can be optimized for performing inline spectral power measurement, selected based upon the traffic signal conditions. 
     In some embodiments, as shown in  FIG. 5 , the inline multi-wavelength power measurement apparatus  400  may include a plurality of optical test apparatuses  10  whose inputs are serially connected. For example, as illustrated in  FIG. 5 , the second input (IN  2 )  14  of the second optical fiber pigtail  82  of one optical test apparatus  10  of the plurality of optical test apparatuses  10  may be directly connected to the first input (IN  1 )  12  of the first optical fiber pigtail  80  of an adjacent one of the plurality of optical test apparatuses  10 . The number of optical test apparatuses  10  can flexibly vary as needed. For example, as illustrated in  FIG. 5 , the inline multi-wavelength power measurement apparatus  400  may include N serially connected optical test apparatuses  10  for measuring power of the light signals  200  and  300  at each of the N wavelengths. As may be seen in  FIG. 5 , each test apparatus  10  may be configured to transmit light at one of the multiple wavelengths λ 1 , λ 2 , etc., up to and including λ N . For example, a first optical test apparatus  10  may transmit a first transmitted portion  204   1  at wavelength  3 A and a second transmitted portion  304   1  also at wavelength λ 1 , while a second optical test apparatus  10  may transmit first and second transmitted portions  2042  and  304   2  at wavelength λ 2 , etc. Additionally, where the first and second light signals  200  and  300  travel in first and second opposing directions, the first optical test apparatus  10  may be upstream of the adjacent second optical test apparatus  10  in the first direction while the first optical test apparatus  10  is downstream of the second optical test apparatus  10  in the second direction. 
     In some embodiments, as shown in  FIG. 6 , the inline multi-wavelength power measurement apparatus  500  may include a first wavelength division multiplexer  502  connected to the first optical fiber pigtail  80  of the first optical test apparatus  10 , such as by the first input  12  of the first optical fiber pigtail  80 , and a second wavelength division multiplexer  504  connected to the second optical fiber pigtail  82  of the second optical test apparatus  10 , such as by the second input  14  of the second optical fiber pigtail  82 . The wavelength division multiplexers  502  and  504  may each demultiplex one of the signals  200  and  300  and multiplex the other of the signals  200  and  300 , in particular the reflected portions  202  and  302  thereof. For example, as shown in  FIG. 6 , the first wavelength division multiplexer  502  demultiplexes the first light signal  200  into multiple components having differing wavelengths and directs various of the components of the first light signal  200  each to a corresponding test apparatus  10  configured to measure light of the appropriate wavelength (e.g., based on the bandpass of the optical filter  70  of the optical test apparatus  10 ). For example, a component of the first light signal  200  having a wavelength k may be directed to an optical test apparatus  10  having an optical filter  70  where the bandpass of the optical filter  70  includes, e.g., is centered on, the wavelength k. The first multiplexer  502  also multiplexes the reflected portions  302  from each of the plurality of optical test apparatuses  10 , such that the reflected portion  302  returned to the optical system includes multiple wavelength components (e.g., λ 1 , λ 2 , etc., up to and including λ N ). Similarly, the second multiplexer  504  multiplexes the first reflected portions  202  reflected from each of the plurality of optical test apparatuses  10  and demultiplexes the second signal  300  from the optical system. 
     Test apparatus  10  in accordance with the present disclosure may advantageously provide relatively non-complex optical structures which alleviate complexities in manufacturing and improved detectability while being modal insensitive. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.