Abstract:
By employing a high-repeatability optical switch that transmits input optical power selectively either to the standard or the unit under test (UUT), OPHASE presents a system for performing a rapid, repeatable comparison between the standard and the UUT. Further, the selective routing of beam traveling through one of the two output fibers that are coupled to the switch either to the standard or the UUT enables the elimination of much of the system uncertainty by enabling initial characterization of the ratio, R p , and inequivalence, I m , between the power outputs of the multiple output fibers coupled to the switch. This characterization is accomplished by using an angled interface which is constructed so as to allow simultaneous coupling of the multiple output fibers to the angled interface and enable the power readout of all the output fibers at the standard. R p  and I m  are then used to calculate the correction factor that reduces the total uncertainty level in the subsequent calibration of the unit under test.

Description:
DEDICATORY CLAUSE 
     The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to me of any royalties thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     With the recent rapid advances in technologies such as lasers, electro-optics, fiber optics and other similar photometric sciences, there has grown a requirement for high-accuracy, fast yet simple and inexpensive optical power measurement and calibration techniques. 
     A currently available optical power measurement device known as Electrically Calibrated Pyroelectric Radiometer (ECPR) is practical and highly accurate. It has a + or −1% uncertainty rate and, unlike most other optical power measurement devices, is wavelength-independent over a broad spectral range. Further, it is designed to respond to power change. Therefore, incoming continuous wave optical power, to be incident on the ECPR, must be chopped or AC-modulated when making optical power measurements. The National Institute of Standards and Technology (NIST) uses the ECPR as its optical power transfer calibration standard. 
     The substitution method is employed by many calibration techniques. It consists of comparing the unit under test (UUT) with the chosen standard. The UUT is said to be transfer-calibrated by the standard. Naturally, each transfer-calibration that is made further down the measurement process from the standard adds uncertainty. Therefore, it is desirable to reduce the number of intermediate steps in the measurement chain. One way to achieve such a reduction is to use a higher-level standard, rather than some other intermediate standard, at the location where the UUT is to be calibrated. 
     Many laboratories which need to calibrate their UUT&#39;s already have fiber-coupled optical power sources at the several wavelengths required to perform optical power transfer-calibration by the substitution method. At present, these laboratories typically use an intermediate standard to transfer-calibrate the UUT. What is needed is a low-cost and easy-to-use means to utilize ECPR or other higher-level standard in conjunction with fiber-coupled optical power sources, optical attenuation and other equipment already existing in these laboratories to perform the necessary calibration. 
     SUMMARY OF THE INVENTION 
     By employing a high-repeatability optical switch  11  that can transmit input optical power selectively either to the standard or the unit under test (UUT), OPHASE presents a means for performing a rapid, repeatable comparison between the standard and the UUT, thereby eliminating the requirement for expensive optical power monitoring/compensation systems or reference detectors to obtain optical power stability. Further, the selective outing of beam traveling through one of the output fibers that are coupled to the witch either to the standard or the UUT eliminates much of the system uncertainty by enabling initial (i.e. prior to testing any UUT) characterization of the inequivalence, I m , and the ratio, R p , between the power outputs of the multiple output fibers coupled to the switch. This characterization is accomplished by using angled interface  45  which allows simultaneous coupling of the multiple output fibers to the angled interface so as to enable the power readout of all the output fibers at the standard. R p  and I m  are then used to calculate the correction factor that reduces the total uncertainty level in the subsequent measurement of the UUT. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates the use of OPHASE in a collimated beam calibration configuration. 
     FIG. 2 is a detailed frontal view of standard head alignment cap. 
     FIG. 3 is a detailed side view of the fifth and the sixth optical fiber connectors, each having a collimating lens  36 . 
     FIG. 4 illustrates the use of OPHASE in a connectorized fiber calibration configuration. 
     FIG. 5 is a detailed side view of the fifth and the sixth optical fiber connectors, each having a typical connector ferrule  39 . 
     FIG. 6 is a top view of the standard head alignment cap with the angled interface. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One area of notable concern in any effort to measure and calibrate, no matter what is the parameter being measured (example: voltage, current, optical power) is the instability, over time, of output of the source supplying the parameter in question. Because of the instability which may be caused by long-term drift and short-term fluctuation, the degree of measurement certainty is adversely affected when a conventional substitution method is used. All optical power sources have some level of drift and fluctuation that need to be taken into consideration if overall measurement uncertainty is to be reduced. 
     The OPHASE system offers a cost-effective means to overcome a large portion of the time-dependent optical source instability by making comparison measurements between the standard and UUT with a very short time interval (seconds) between the measurements. This is achieved by using high-repeatability optical switch  11  that selectively routes optical power to the standard and the UUT via a first optical path and a second optical path, respectively, where additionally the second optical path is movable so as to extend selectively between the switch and the calibration standard or between the switch and the UUT. After performing initial characterization of the output power levels from the two paths at the standard and the calculation of the optical power ratio, R p , and corresponding power correction factor, the second path can be quickly moved to extend between the switch and the UUT, thus effectively eliminating any real concern about the power source instability. Further, because comparison measurements are performed quickly and easily, additional sets of measurements can be made rapidly to collect further data. This data can then be used to average out any system random uncertainty that may be due to fluctuation of the power source, non-repeatability of the switch and/or the ECPR standard. 
     Referring now to the drawing wherein like numbers represent like parts in each of the several figures, the structure and operation of the OPHASE system are presented in detail. 
     FIG. 1 shows the use of OPHASE in a collimated beam calibration configuration. In it, continuous wave (CW) optical beam of a given wavelength λ and power emanates from source  1  and travels, via third output fiber  37 , to third optical fiber  3 , the third output fiber and the third optical fiber being coupled to each other by third bulkhead connector  2 . This input beam continues to travel through third optical fiber  3  to reach fourth bulkhead connector  4  where it is transmitted by seventh optical fiber connector  5  and fourth output fiber  38  to first collimating lens  6 . The first collimating lens converts the CW fiber signal to a collimated free space CW signal and transmits the signal to focusing lens  7  which brings the free space collimated beam into the core of fifth output fiber  8 . The optical axes of first collimating lens  6  and focusing lens  7  are maintained in alignment by air gap assembly  9 . Within the air gap assembly is chopper  10 , positioned between the focusing lens and the first collimating lens, that upon selected activation converts the CW signal to a modulated (changing) signal at the frequency, duty cycle and waveform required by ECPR  32 , standard detector  28  and first electrical cable  30 . The optical beam, whether modulated or not as required by the chosen calibration standard, enters switch  11  via fifth output fiber  8  and is routed therefrom selectively either to first output fiber  12  or second output fiber  13 . As illustrated in FIG. 1, the first and second output fibers comprise, respectively, the first optical path and the second optical path. 
     The first optical path further comprises first and second optical fiber connectors  14  and  18 , and first bulkhead connector  16  coupled between the first and second optical fiber connectors. From second optical fiber connector  18 , first optical fiber  20  extends to be terminated with fifth optical fiber connector  22  which has, mounted to one end thereof, second collimating lens  36  as shown in FIG.  3 . In a like manner, the second optical path further comprises third and fourth optical fiber connectors  15  and  19 , and second bulkhead connector  17  coupled between the third and fourth optical fiber connectors. From fourth optical fiber connector  19 , second optical fiber  21  extends to be terminated with sixth optical fiber connector  23  which also has, mounted to one end thereof, a third collimating lens identical to second collimating lens  36  shown in FIG.  3 . In the connectorized fiber calibration configuration depicted in FIG. 4, the first and second optical paths are constituted in exactly the same way as in the collimated beam calibration configuration of FIG. 1 except that fifth and sixth optical fiber connectors  22  and  23  each has mounted thereto a standard connector ferrule  39  as detailed in FIG. 5 instead of the second and third collimating lenses. 
     In order to perform precise calibration of the UUT, initial compensation factors must be determined for any differences between the power outputs of the two optical paths (R p ) and maximum inequivalence (I m ) between the two inputs to ECPR detector  28  via the two optical paths prior to taking measurements of any UUT  33  for calibration. The difference between the power outputs of the two optical paths may be caused by the nature of optical power source  1  and/or switch  11  putting unequal power into first and second output fibers  12  and  13  as a function of wavelength λ whereas any inequivalence between the two inputs to ECPR detector  28  may be the result of geometrical considerations or non-uniformity of ECPR detector  28  itself. 
     FIG. 6 illustrates how R p  and power correction factor are determined in OPHASE. Fifth and sixth optical fiber connectors  22  and  23  are both connected simultaneously to ECPR detector  28  via head alignment cap (HAC)  26  which has incorporated onto it angled interface  45 . The two surfaces of the angled interface have mounted thereto first and second half-bulkhead connectors  24  and  25  and to these first and second half-bulkhead connectors fifth and sixth optical fiber connectors  22  and  23  are removably coupled. In this way, the power output of the first and second optical paths are directed to the same area on ECPR detector  28 . After the connection of fifth and sixth optical fiber connectors  22  and  23  to first and second half-bulkhead connectors  24  and  25 , respectively, the optical power levels of the two optical paths are measured with the ECPR by using switch  11  to select the routing of the optical signal through either first or second output fibers  12  or  13 . The resulting optical power levels P low  and P high  will be different by some amount according to the specific optical switch employed in the system, wavelength X and any intermediate attenuation differences between switch  11  and fifth and sixth optical fiber connectors  22  and  23 . These resulting power levels are used to calculate the power ratio R p  for the configurations in FIG.  1  and FIG.  4 : 
     
       
         R p  (collimated beam)=P high /P low    (eq. 1)  
       
     
     
       
         R p  (connectorized fiber)=P high /P low    (eq. 2)  
       
     
     Head alignment cap  26  is constructed so that the spots resulting on ECPR detector  28  from two beams incident thereon through the two optical paths are essentially of equal size. However, to minimize the inequivalence between them, any inequivalence that may be present can be accurately measured by first attaching fifth and sixth optical fiber connectors  22  and  23  to the first and second half-bulkhead connectors, respectively, and measuring the outputs; then disconnecting the fifth and sixth optical fiber connectors and reconnecting them to the first and second half-bulkhead connectors, but in reverse order this time, and again measuring the outputs. The second and third collimating lenses, mounted at the end of the fifth and sixth optical fiber connectors, respectively, as shown in FIG. 3, are designed to allow easy attachment and detachment of the same to and from half-bulkhead connectors. It is noted also that, instead of disconnecting and reconnecting the fifth and sixth optical fiber connectors, they can be left in place and the head alignment cap itself rotated by 180° from its initial position. This method is simpler and may actually result in a more accurate measurement of inequivalence. Maximum inequivalence is calculated as follows: 
     
       
         I m =[R p —R p  (180°)]/[R p +R p  (180°)]/2   (eq. 3)  
       
     
     where R p  (180°) is the power ratio with the fifth and sixth optical fiber connectors reversed at the first and second half-bulkhead connectors. The closer I m  is to “0”, the smaller is the inequivalence between the inputs onto ECPR detector  28 . 
     Following the above calculations of R p  and I m , UUT  33  can be calibrated by comparing its measurement results against the ECPR. To do this, whichever optical fiber connector, either  22  or  23 , that has P low  is disconnected from HAC  26  and projected to UUT detector  29  via connection to third half-bulkhead connector  43 , held in place by supporting fixture  27 , while the other optical fiber connector remains connected to the HAC. Thereafter switch  11  is selected to route incoming optical signal to the ECPR detector  28  where the power output P ECPR  is measured when the optical signal is converted to an electrical signal and is transmitted, via first electrical cable  30 , to ECPR read-out in ECPR  32  which then displays the electrical signal as a numerical value indicating the absolute optical power radiating on the ECPR detector  28 . Next, the switch is selected to route the optical signal to the UUT where the power output P UUT  is measured when the optical signal is converted to an electrical signal and is transmitted, via second electrical cable  31 , to UUT read-out in UUT  33  which, in turn, displays the electrical signal as a numerical value indicating the absolute optical power radiating on the UUT detector  29 . 
     Subsequent to the readings of initial power outputs of the ECPR and the UUT, the previously calculated R p  is used to correct for the difference in P ECPR  and P UUT : 
     
       
         P UUT  (corrected)=R p ×P UUT    (eq. 4)  
       
     
     The values for P ECPR  and P UUT  (corrected) can then be directly compared to determine the initial absolute power correction at a given power level between the ECPR and the UUT: 
     
       
         Initial Power Correction=P ECPR −P UUT  (corrected)   (eq. 5)  
       
     
     If I m  is non-negligible, i.e. it has a significant effect on the overall uncertainty level of the calibration process, this impact can be minimized by factoring one minus half of the maximum inequivalence value in the initial power correction to yield the final power correction: 
     
       
         Final Power Correction  
       
     
     
       
         (P ECPR −P UUT  (corrected))×(1−|(I m /2)|)   (eq. 6)  
       
     
     Use of eq.6 reduces the uncertainty in the calibration process that is due to inequivalence by a factor of two having the same error reduction effect as taking the median of a number of measurements. 
     Adding the initial power correction value to any measured P UUT  results in the UUT reading the same as the ECPR at the power level and wavelength of the input CW optical beam, thereby yielding a unit under test that has been calibrated against the standard. 
     OPHASE can also be used to provide an effective means for calibrating irradiance, obviating the traditional requirements that the input beam of known irradiance be uniform and large enough in diameter to overfill the apertures of the standard and the unit under test. The collimated beam, in the collimated beam calibration configuration of FIG. 1, is typically gaussian in beam profile. Its optical power in watts is measured with the ECPR standard, P ECPR . The effective aperture radius of the UUT; R eff , is measured with conventional means that allows calculation of the effective area of the UUT aperture, A eff . The derived irradiance is given by: 
     
       
         Ir=P ECPR /A eff    (eq. 7)  
       
     
     which has the required units of W/cm 2 . Knowing the irradiance to the UUT from this method permits calibration without having to overfill the aperture or be concerned about the beam uniformity. One way to imagine how this works is to think of the incoming collimated beam as being virtually expanded. This virtual, expanded beam will have exactly the same optical power as the real beam and exactly the same area as the UUT aperture effective area. It will have perfect irradiance uniformity because the optical power is evenly distributed over the effective area. 
     In summary, OPHASE system provides a fast, reliable and a very accurate means for calibrating units under test against a pre-selected standard such as ECPR. The system is also inexpensive because it can be used with existing fiber-coupled optical sources that are commonly found in many laboratories that require such calibration. By bringing a highly accurate standard such as ECPR to be utilized directly in transfer-calibration of the units under test, OPHASE reduces the number of or eliminates intermediate standards, thereby increasing the level of accuracy in the calibration process. Once initial measurements for R p  and I m  are completed and connection is made to the UUT, OPHASE allows a “hands-off” calibration system that can easily be automated. 
     Although a particular embodiment and form of this invention has been illustrated, it is apparent that various modifications and embodiments of the invention may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure. Accordingly, the scope of the invention should be limited only by the claims appended hereto.