E/O & O/E measurements using the Anritsu 37300C series VNA The following note describes the set-up and calibrations required to make E/O and O/E measurements using the Lightning VNA and a transfer standard. This method can be applied to characterize Optical Modulators (E/O) and Photodiode Receivers (O/E). In the following example, E/O measurements are made on a 40 Gb/s NRZ optical modulator. Required Equipment: Anritsu VNA 37397C (65 GHz), 37377C (50 GHz), 37369C (40 GHz), or 37347C (20 GHz) DFB Laser (1550 nm) Photodiode (characterized) Modulator (DUT) Power Supply Figure 1. E/O or O/E Measurement Setup Figure 1 shows the standard set-up for making E/O or O/E measurements.
In the following E/O measurement example, Transfer Function (Bandwidth) and Reflection measurements will be made on an optical modulator using this setup. The laser source supplies the optical input to the modulator under test. The microwave signal from the VNA Port 1 modulates the optical carrier. The photodiode converts the optical signal back to electrical for measurement by the VNA. The calibration required for the E/O measurement is conducted in the following manner. First, a 12-term calibration is performed on the VNA over the frequency range of interest. Note that the characterized photodiode response should be available over the entire band of interest. Then the internal VNA application software de-embeds the Photodiode response from the 12-term calibration. This step moves the calibration planes to the electrical input and optical output of the modulator. Once the calibration is done, S21 (transmission parameter) measures the transfer function, i.e. bandwidth of the modulator, and S11 (reflection parameter) measures the return loss of the modulator. Measurement Steps: 1. Perform a 12-term calibration on the VNA over the frequency range of interest, with reference planes at the DUT input and the photodiode output as shown in Fig. 1. 2. Set-up the measurement as shown in Figure 1. 3. Press the APPLICATION menu button on the VNA s front panel. Select E/O Measurement. Follow the instructions in the help menu (Figure 2) to load the 12- term electrical calibration (from either the hard drive or floppy drive). 4. Load the characterization file of the transfer standard (photodiode reference). The de-embedding is performed immediately after the file is read into the VNA (from either the hard drive or floppy drive). The VNA is now ready for E/O measurements. The S21 data is a measurement of the transfer function of the modulator, while the S11 data is the modulator s electrical return loss.
Figure 2: E/O Measurement Menu
Figure 3 shows the Return loss (S11) and Transfer function (S21) measurement of a 40 Gb/s NRZ modulator. The 3-dB bandwidth of the modulator is approximately 24 GHz. Figure 3. Measurement of a 40 Gb/s modulator O/E Measurements O/E measurements of a photodiode receiver can similarly be made using the O/E Measurement function inside the APPLICATION menu of the VNA (Figure 4). In this case, a characterized optical modulator will be required. The modulator can be characterized using the same photodetector reference that was used for E/O measurements. The software creates an s2p file, which can then be read into the VNA when making the O/E measurement. Once the modulator data is read into the VNA, the 12-term calibration references planes are shifted to the input and output of the O/E DUT. Measurement of a photodiode detector using this technique is shown in Figure 5, with a comparison to the measurement performed using the optical heterodyning technique.
Figure 4: O/E Measurement Menu Figure 5. Measurement of a Photoreceiver showing Correlation between the traditional Heterodyne and the VNA techniques
For additional information, please contact: ARNO PETTAI OR AMAR GANWANI ANRITSU COMPANY MMD MARKETING 490 Jarvis Drive Morgan Hill, CA (408) 778 2000 email: arno.pettai@anritsu.com or amar.ganwani@anritsu.com
Appendix A: Creating a characterization (*.s2p) file from vendor supplied data This appendix guides the user through the process of creating an.s2p file that can be read into the 37300C VNA for de-embedding the response of the transfer standard used (Modulator or Photodiode). Creating the.s2p file Microsoft Excel is the recommended application for creating and storing the.s2p file containing the characterization data. A sample.s2p file is available for reference (contact Anritsu MMD Marketing for a copy). The file format is the standard.s2p format that includes the four S-parameters (see section below for details on the.s2p format). The transfer function data supplied by the vendor should be copied into the columns designated for S21 data and the frequencies into the FREQ column. Once the data has been entered, the file should be saved as an.s2p file (as shown below). Saving the.s2p file in excel
S2p file format.snp data files are ASCII text files in which data appears line by line, one line per data point, in increasing order of frequency. Each line of data consists of a frequency value and one or more pairs of values for the magnitude and phase of each S-parameter at that frequency. Values are separated by one or more spaces, tabs or commands. Comments are preceded by an exclamation mark (!). Comments can appear on separate lines, or after the data on any line or lines. The general format consists of: An option line Data lines Comments The Option line contains the specifications of the data i.e. the frequency units, the normalizing impedance and the measured parameter (S, Y, Z etc.) Format: # <frequency unit> <parameter> <format> <R n> where, # The delimiter that tells the program you are specifying these parameters frequency unit parameter format R n The set of units desired (GHz, MHz, KHz, Hz) The parameter desired (S, Y or Z for S1P components; S, Y, Z, G, or H for S2P components; S for S3P or S4P components) The format desired (DB for db-angle, MA for magnitude-angle, or RI for real-imaginary) The reference resistance in ohms, where n is a positive number of ohms (the real impedance to which the parameters are normalized) The default option line for component data files is: # GHZ S MA R 50 for Y-parameters with Real Imaginary data, this option line header will change to # GHz Y RI R 50
The data lines contain the data of interest. Data for all four S-parameters will be listed on a single line for a particular frequency point. The format is: [Frequency] [S11] [S21] [S12] [S22] Example: 0.040000000 1.426492E-04 18.642 0.139900000 2.840961E-03 22.262 (freq.) S11 (mag.) S11(angle) Finally, the comment lines begin with a!. They can be inserted at any point in an S2P file and are ignored by the application program. S2P file example:!! DATE 04/11/2002 13:16 PAGE 1! Photodiode receiver characterization NRZ-40G # GHz S MA R 50.00! FREQ S11M S11A S21M S21A S12M S12A S22M S22A 0.040000000 1.426492E-04 18.642 3.008963E-04 87.040 1.618370E-04 27.317 2.708149E-04 129.056 0.139900000 2.840961E-03 22.262 6.185992E-03-114.026 2.539190E-03 125.940 5.977178E-03-7.691 0.239800000 3.072268E-03 97.851 7.423908E-03 109.019 3.147963E-03-88.205 7.634960E-03-75.071 0.339700000 1.887820E-02 81.836 3.814256E-02-135.753 1.632677E-02-27.669 3.348942E-02 117.818 0.439600000 2.173782E-02 142.412 4.595363E-02 72.973 1.962434E-02 107.429 3.946349E-02 40.979 0.539500000 2.634556E-02-62.971 5.206176E-02 31.589 2.257828E-02-22.787 4.638838E-02 70.663 0.639400000 3.060178E-02-103.325 6.416773E-02 120.463 2.732290E-02 5.656 5.685493E-02-124.645......................