Abstract:
An optical power meter including a mechanical interface that establishes a predetermined air gap, while avoiding physical contact with the sensitive area of the DUT. The mechanical interface is formed such that the test instrument contacts the DUT in the non-sensitive region over an area large enough to establish contact pressure that is well within the strength of the DUT&#39;s material. Accordingly, the non-contacting optical element enables optical power to be collected and relayed with a quantifiable and repeatable power loss. A high-NA, large area optical element is used to collect and relay optical power accurately while maintaining low sensitivity to axial or radial alignment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from U.S. Provisional Patent Application No. 61/838,593, entitled “Optical Power Measurement Device” filed Jun. 24, 2013, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to an optical power measurement device, and in particular to a small form factor optical power measurement device with a test ferrule for engaging a device under test without directly contacting optically sensitive areas. 
       BACKGROUND 
       [0003]    Direct measurement of optical power from cable terminations situated within bulkheads or devices is not feasible with conventional optical power meters (OPMs) because conventional OPMs are too large to fit in the allowed space. A typical solution is to relay power to an OPM via a “reference cable”, which makes physical contact at the device under test (DUT) and is coupled to an optical sensor at the OPM. Reference cables, however, are prone to breakage, loss and contamination. They further add uncertainty and complexity to the measurement process. Moreover, physical contact between the reference cable and the DUT increases the risk of damaging or contaminating the DUT. In view of the foregoing, there are significant problems and shortcomings with current technologies in direct measurement of optical power measurement devices. 
       SUMMARY OF THE INVENTION 
       [0004]    Accordingly, the present invention relates to an optical power measurement device for measuring optical power of a source of light from a device under test (DUT) comprising: a test instrument having a longitudinal optical axis and including: an optical element for transmitting test light, formed from at least a portion of the source of light; and a ferrule surrounding the optical element; and a photodetector for measuring the test light optically coupled to the optical element; wherein the ferrule includes: a first contact surface for abutting against a second contact surface on the DUT, parallel to the first contact surface, and a non-contact surface spaced from an optically transmitting area of the DUT by an air gap when the first and second contact surfaces are abutting, the non-contact surface including an optically receptive area formed by an end of the optical element for receiving the test light from an optically transmitting area of the DUT. 
         [0005]    Another aspect of the present invention relates to an optical power measurement device for measuring optical power of a source of light from a device under test (DUT) comprising: a test instrument having a longitudinal optical axis and including a ferrule; and a photodetector mounted within the ferrule for measuring test light, formed from at least a portion of the source of light; wherein the ferrule includes: a first contact surface for abutting against a second contact surface on the DUT, parallel to the first contact surface, and a non-contact surface spaced from an optically transmitting area of the DUT by an air gap when the first and second contact surfaces are abutting, the non-contact surface including an optically receptive area formed by an end of the optical element for receiving the test light from an optically transmitting area of the DUT. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0007]      FIG. 1   a  depicts a cross-sectional view of an optical power meter (OPM) of the present invention engaging a device under test (DUT); 
           [0008]      FIG. 1   b  depicts an enlarged view of area A from  FIG. 1   a;    
           [0009]      FIG. 1   c  depicts an isometric view of the end of the probe from the OPM of  FIG. 1   a;    
           [0010]      FIG. 2   a  depicts a cross-sectional view of an alternate embodiment of an optical power meter (OPM) of the present invention engaging a device under test (DUT); 
           [0011]      FIG. 2   b  depicts an enlarged view of area A from  FIG. 2   a;    
           [0012]      FIG. 2   c  depicts an isometric view of the end of the probe from the OPM of  FIG. 2   a;    
           [0013]      FIG. 3  depicts an isometric view of a probe from an alternate embodiment of an optical power meter of the present invention, including an enlarged view of the end; 
           [0014]      FIG. 4  depicts an isometric view of a probe from an alternate embodiment of an optical power meter of the present invention, including an enlarged view of the end; 
           [0015]      FIG. 5  depicts an isometric view of a probe from an alternate embodiment of an optical power meter of the present invention, including an enlarged view of the end; 
           [0016]      FIG. 6  depicts a cross-sectional view of a probe from an alternate embodiment of an optical power meter of the present invention; 
           [0017]      FIG. 7  depicts a cross-sectional view of a probe from an alternate embodiment of an optical power meter of the present invention; and 
           [0018]      FIG. 8  depicts a cross-sectional view of a probe from an alternate embodiment of an optical power meter of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    With reference to FIGS  1   a,    1   b  and  1   c,  a test instrument, generally indicated at  1 , of the present disclosure is comprised of two main components, one mechanical and one optical. The mechanical component may be a unique configuration of the instrument&#39;s ferrule geometry, which enables the system to maintain a consistent and controllable air gap AG between the test instrument  1  and a device under test (DUT)  2  in the most sensitive region of the DUT  2 . Accordingly, a test ferrule  3  may be provided with an end face configuration, such that there may be near planar contact between the test instrument  1  and the DUT  2  in at least one mating non-sensitive region, thereby limiting stress/pressure at an optical interface. 
         [0020]    The test ferrule  3  is shaped to engage and receive an optical signal from a DUT ferrule  4 , which may have an end face polished at an angle, e.g. at 8°, from normal (APC), i.e. from a plane perpendicular to an optical axis of the DUT  2 . Accordingly, the generally circular end face of the test ferrule  3  may comprise: 1) a first planar contact surface  6  parallel to the end face of the DUT ferrule  4 , for mating with a planar non-sensitive area  7  of the end face of the DUT ferrule  4 , and 2) a second non-contact surface  8  sloping away at an acute angle from the first contact surface  6  defining a wedge-shaped air gap AG, e.g. 5° to 15°, but ideally at an 8° angle, between an optically transmitting sensitive area  9  of the end face of the DUT ferrule  4  and an optical receptive section  10  of an optical element  28  in the end face of the test ferrule  3 . In the illustrated first embodiment, the first contact surface  6  may be polished at the same angle from the second non-contact surface  8 , as the end face of the DUT ferrule  4  is from normal, e.g. 8°. Ideally, the second non-contact surface  8  may be flat, i.e. perpendicular to the longitudinal optical axis of the DUT  2  and the test ferrule  3 ; however, both the first and second surfaces  6  and  8 , respectively, could be angled, e.g. 5° to 15°, relative to the normal or flat surface, as long as the appropriate gap AG is provided between the sensitive area  9  and the optical receptive section  10 . 
         [0021]    With reference to  FIGS. 2   a ,  2   b  and  2   c , a test instrument  11  includes a test ferrule  13 , which may be shaped to function with a DUT  12 , including a DUT ferrule  14 , which has an end face polished normal (perpendicular) to the DUT&#39;s longitudinal center axis (LA) (Flat). Accordingly, the generally circular end face of the test ferrule  13  comprises: 1) a planar first contact surface  16  parallel to the end face of the DUT ferrule  14 , for mating with a planar non-sensitive area  17  of the end face of the DUT ferrule  14 , and 2) a second non-contact surface  18  sloping away at an acute angle from the first contact surface  16  defining a wedge-shaped air gap AG, e.g. 5° to 15°, but ideally at an 8° angle, between an optically transmitting sensitive area  19  of the end face of the DUT ferrule  14  and an optical receptive section  20  of the optical element  28  in the end face of test ferrule  13 . In the illustrated embodiment, the second non-contact surface  18  may be at an angle from the first contact surface  16 , e.g. 5° to 15°, ideally 8°, providing the appropriate air gap AG between the sensitive area  19  and the optically receptive section  20 . Ideally the first contact surface  16  may be flat, i.e. perpendicular to the longitudinal optical axis LA of the DUT  14  and the test ferrule  13 ; however, both the first and second surfaces  16  and  18 , respectively, could be angled, e.g. 5° to 15°, relative to the normal or flat surface, as long as the appropriate gap AG is provided between the sensitive area  19  and the optical receptive section  20   
         [0022]    In each case, the end face of the test instrument&#39;s ferrule  3  and  13  may be shaped such that there is a near-planar contact in the contact regions  6 / 7 ,  16 / 17  beginning outside the sensitive area  9 / 19 , e.g. at a radius approximately 75 um to 200 um, ideally 125 microns from the longitudinal center axis of the DUT  2 / 12  or outside the core and cladding region of fiber under test. The sensitive area  9 / 19  of the DUT  2 / 12  ends at a radius of 62.5 microns from longitudinal axis, i.e. center. The sensitive area  9 / 19  of the DUT  2 / 12  may be defined, in the illustrated embodiments of  FIGS. 1   a  and  2   a , as the portion of the DUT  2 / 12  that may be comprised of optical fiber (core and/or cladding). The non-sensitive area may be defined as the area comprised of a mechanical ferrule  13 / 14 , often made of zirconia. 
         [0023]    The optically receptive section  10 / 20 , i.e. the optical element  28 , of the test instrument  1 / 11  employs a relatively larger (e.g. more than 2×, preferably more than 5×, more preferably more than 10×) diameter compared to the optically sensitive area  9 / 19  of the DUT  2 / 12 , and a higher (at least 2×) numerical aperture (NA) to efficiently relay at least a portion of the optical power to a photodiode  30  that may be optically coupled at a distance beyond the mechanical constraints of the DUT  2 / 12 . The diameter and NA of the optical receptive section  10 / 20  may be a function of the air gap AG used in the design. The optical element  28  used must relay a consistent percentage, e.g. 50% to 95%, ideally between 85% and 95%, of the DUT&#39;s optical power to be effectively used for power measurement. In the preferred embodiment, the optical element  28  used for the optical receptive section  10 / 20  is a 0.39 NA, 300-micron core, step-index silica fiber; however, other optical elements are within the scope of the invention. As described above, the test fiber&#39;s ferrule  3 / 13  may be shaped and polished to establish near planar contact in the non-sensitive region  6 / 7  and  16 / 17 , i.e. at a radius of  125  microns from the optical center of the DUT  12  when the DUT  12  may be a single mode optical fiber. The air gap AG of the preferred embodiment between the optically transmitting area  9 / 19  and the optically receptive section  10 / 20  may be between 10 um and 25 um, preferably between 15 um and 20 um, and ideally 18 microns. Relayed optical power is emitted at the termination of the optical element  28  in free space to the photo diode  30 . The photo diode  30  may be part of an electronic circuit which interprets the output of the optical element  28  to accurately display the power transmitted to it. 
         [0024]    Alternatively, the optical element  28  may be comprised of the photo diode  30  and some sort of optical relaying element, e.g. fiber or lens, or the optical element  28  may consist of the photo diode  30  only, without need of any optical relaying element. In these cases, an electrical signal from the photo diode  30 , encased in the test instrument ferrule  3 / 13 , may be transmitted to a control device, e.g. hardware and software, electrically connected to the photodiode  30 . 
         [0025]    Within the two components of the design, there exist alternate embodiments that would achieve the same result and are within the scope of the same invention. With reference to  FIGS. 3 to 5 , the air gap AG may be produced by means of a slot or undercut defining the non-contact surface. With reference to  FIG. 3 , a test instrument  31  includes a test ferrule  33  surrounding the optical element  28 . A generally circular end face of the test ferrule  33  includes a diametrically extending rectangular slot  39  defining the sensitive area, including the optically receptive section  40  of the optical element  28 . The bottom surface of the slot  39  has a flat planar surface, perpendicular to the longitudinal optical axis of the optical element  28  and the DUT  12 , and is spaced from first planar contact areas  36   a  and  36   b,  on either side thereof, by vertical walls defining the desired gap AG between the DUT  12  and the optical receptive section  40  at the end of the optical element  28 . The flat planar contact areas  36   a  and  36   b,  e.g. forming segments of a circle, are for contacting the second contact areas on the end face of the DUT, e.g. two diametrically-opposed, separate planar sections of the contact area  17  on the flat DUT  12 , illustrated in  FIGS. 2   a  and  2   b.    
         [0026]    In an alternative embodiment, illustrated in  FIG. 4 , a test instrument  41  includes a test ferrule  43  surrounding the optical element  28 . The sensitive area may be defined by a slot  49  with a curved or concave lower face, which also includes optical receptive section  50  at the end of the optical element  28 . The flat planar contact areas  36   a  and  36   b  form segments of a circle, as above, provide at least one contact surface for abutting against the corresponding contact area  17  on the DUT  12 , e.g. two diametrically-opposed, separate planar sections of the contact area  17  on the flat DUT  12 , illustrated in  FIGS. 2   a  and  2   b.    
         [0027]    With reference to  FIG. 5 , a test instrument  61  includes a test ferrule  63  surrounding the optical element  28 . The generally circular endface of the test ferrule  63  includes a circular recessed area defining a sensitive area  59 , which includes an optical receptive section  60  of the optical element  28 , surrounded by annular-shaped first contact surface  66 . The gap AG between the DUT  12  and the optical receptive section  60  at the end of the optical element  28  may be defined by the annular vertical wall between the recessed area  59  and the annular shaped contact surface  66 . The annular flat planar contact surface  66  provides a contact surface for abutting against the corresponding contact area  17  on the DUT  12 , e.g. an annular contact area  17  completely surrounding the sensitive area  19  of the DUT  12 . 
         [0028]    With reference to  FIGS. 6 ,  7  and  8 , the optical element  28  may take several forms other than an optical fiber. With reference to  FIG. 6 , a test ferrule  83  includes an optical element  28 ′ comprised of a plurality of relay lenses  85 , space apart in a chain along the longitudinal axis of the optical element  28 ′ mounted in the test ferrule  83  for transmitting light to an optical fiber, optically coupled to the photodiode  30 , or directly to the photodiode  30 . 
         [0029]    The embodiment illustrated in  FIG. 7  includes a test ferrule  93  with the optical element  28 ″ comprised of a ball lens  95  for focusing light into an optical fiber  96  for transmission to the photodiode  30 .  FIG. 8  illustrates an embodiment in which an optical element  28 ′″ is comprised of a grin lens  101  mounted within a test ferrule  103  for transmitting light to an optical fiber, optically coupled to the photodiode  30 , or to the photodiode  30  directly. 
         [0030]    In the optical design, the optical elements  28  may be substituted by an equivalent system including (but not limited to), a GRIN lens or lenses, a ball lens/fiber combination, or a series of relay optics functioning in free space. 
         [0031]    During use, the first contact surface  6 ,  16 ,  36   a,    36   b  and  66  are manually brought into abutment with the second contact surface  7 ,  17  providing a predefined and consistent distance for the air gap AG between the optically transmitting sensitive area  9 ,  19 ,  39 ,  49  and  59 , and the optical receptive section  10 ,  20 ,  40 ,  50  and  60 , enabling light to be transmitted across the air gap AG at a predetermined loss, e.g. 5% to 15%, into the optical element  28  for transmission to and measurement (optical power) by the photodiode  30 . 
         [0032]    The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.