Patent EP-0418509-A2 (EP)
Inventor: HART MICHAEL G
IPC: G01M11/00
Filed: 1990-07-25
Abstract: A lightwave component analyzer comprising at least an internal optical receiver and preferably also comprising an internal optical source which are selectively connectable by switches configurable by means of an instrument controller for calibration and performance of electro-optical, opto-electrical, and optical measurements. The switches are arranged in a switch matrix. The configurable switch matrix is connected by the instrument controller in response to selection of a measurement by a user to facilitate calibration of, and test measurements of devices under test with, the lightwave component analyzer. Transmission measurements of E'O devices can be corrected for source match errors, and transmission measurements of O E devices can be corrected for load match errors, in addition to frequency response and cross-talk error correction. Response and match error correction provides for improved measurements of test devices with an electrical port having an impedance different from that of the measurement system impedance. 
     As an example, a laser diode can have an electrical input impedance of 5 ohms, for example, which is significantly lower than the measurement system impedance of 50 ohms. This poor impedance match results in standing waves between the system's rf source and the input port of the laser under test. A large systematic error is seen as a periodic ripple in the measurement. The actual transmission frequency response does not contain this ripple. Response and match error correction provides a more accurate measurement by removing source match error. 
     Another example is a photodiode with an electrical output impedance of 500 ohms. In this example, the test device has an impedance larger than the system impedance. This also results in a large systematic error which manifests itself as a ripple in the measurement. Response and match error correction in accordance with one embodiment of the invention processes the measured data to remove load match errors. 
     These two forms of response and match error correction can be thought of as subsets of full two-port. twelve-term error correction. Due to the nature of electro-optical devices, the reverse transmission is assumed to be zero. No correction is made for the match at the optical port. and so the optical port match is assumed to be ideal. The full two-port error correction equations reduce to require only two measured parameters instead of four. Response and match error correction of E 0 devices requires five correction coefficients. while error correction of O.E devices requires six coefficients. 
     The equation to error-correct E O devices is:       
     The equation to error-correct O/E devices is:     where
     Edf is forward directivity;   Edr is reverse directivity;   Erf is forward reflection tracking;   Err is reverse reflection tracking;   Esf is forward source match;   Esr is reverse source match;   Exf is isolation;   Elf is load match; and   Etf is transmission tracking.   
     The EO response and match calibration steps are as follows. Five steps are needed. 
     The first step is to measure a short on electrical test port 18,. The second step is to measure an open on electrical test port 18 1 . The third step is to measure a load on electrical test port 18 1  . The data measured during these steps are used to calculate Esf. Erf, and Edf. The fourth step is to measure isolation, which is used as Exf. Finally, the fifth step is to measure electrical and optical through connections, which are used to calculate Etf. The above five steps can be performed in any order. 
     The O E response and match calibration steps are as follows. Nine steps are needed. 
     The first step is to measure a short on electrical test port 18,. The second step is to measure an open on electrical test port 18,. The third step is to measure a load on electrical test port 18,. The data measured during these first three steps are used to calculate Elf. The fourth step is to measure a short on electrical test port 18 2 . The fifth step is to measure an open on electrical test port 18 2 . The sixth step is to measure a load on electrical test port 18 2 . The data measured during the fourth through sixth steps are used to calculate Esr, Err, and Edr. The seventh step is to measure isolation, which is used as Exf. The eighth step is to measure an electrical through, which is used to calculate Elf. Finally, the ninth step is to measure electrical and optical through connections, which are used to calculate Etf. The above nine steps can be performed in any order. 
     Edf, Edr, Erf, Err, Esf, Esr, Exf, Elf, and Etf can be calculated based on the preceding measurements, as is well known. See, for example, Vector Measurements of High Frequency Networks , Hewlett-Packard Company Part No. 5958-0387 (April 1, 1989), pages 3-5 to 3-16. The reflection coefficients may also be used for one-port error correction of S11 for E/O devices and S22 for O/E devices. 
     The measurement of calibration standards and test device parameters is controlled by the analyzer as specified in the firmware. The calculation of error correction coefficients and the computation of the error correction algorithms using these coefficients is also in the analyzer firmware.