June 30, 2017
As network transmission speeds increase to meet the demand for high-bandwidth services, the testing requirements associated with optical modules, photo detectors, optical receivers, and similar devices have become more complex. To that end, the test tool box for engineers designing optical components and high-speed networks should include a vector network analyzer (VNA), which can be a strong complement to more traditional optical test solutions such as a BERT and optical spectrum analyzer (OSA).
Traditionally used to verify performance of electrical devices and networks, the VNA provides some distinct advantages for optical engineers compared to lightwave component analyzers (LCAs) in high-speed designs. In today’s post, we will discuss the benefits a VNA can bring when testing optical components for current and next-generation networks.
VNAs have traditionally been used to measure transmission parameters in both magnitude and phase in the microwave and millimeter wave (mmWave) frequency bands in the form of S-parameters. High-speed optical digital signals have similar frequency content and bandwidth as high-speed electrical digital signals measured by VNAs. Therefore, VNAs can be effective in characterizing today’s optical components, as long as the signal can be converted between the electrical and optical mediums. Figure 1 shows the typical optical spectrum of a laser modulated at a 20 Gb/s data rate.
To leverage the inherent S-parameter benefits that a VNA can provide, it is important to have it be part of an efficient process to convert between the electrical and optical mediums. To do this effectively, a laser with an optical modulator (E/O) is required on one side and a calibrated photodiode (O/E) on the other.
Importance of Calibration
Because this process involves measuring magnitude and phase of signals, the VNA-based solution needs to be calibrated to establish reference planes for the measurements. Being able to de-embed components is advantageous in moving the measurement planes from the electrical to the optical realm. Understanding this usefulness, Anritsu VNAs, such a VectorStar® and ShockLine™, have these native capabilities, eliminating the need for additional software.
A VNA-based test solution for optical module verification is shown in figure 2, which features VectorStar. A laser and optical modulator provide electrical stimuli to the optical signal from the VNA RF output. The resulting optical signal will be at a wavelength determined by the laser modulated with an electrical sinusoidal CW signal, which should be stepped in frequency over a desired bandwidth. The optical signal is converted back to an RF signal using a photodiode that is measured by the VNA. The receiver at the RF input of the VNA is tuned to the RF frequency with which the optical signal is modulated.
The configuration in figure 2 samples the RF output signal and compares it with the measured RF input signal to determine an S21, which is the ratio of RF in/RF out with both magnitude and phase terms, for each measurement. The VectorStar architecture is advantageous in this application, as it allows the VNA to be tuned to a very narrow instantaneous bandwidth for a high signal-to-noise (S/N) measurement.
Conducting E/O Measurements
If a photodiode with a characterized O/E transfer function in magnitude and phase is used, it can be de-embedded from the measurement. This process accounts for the photodiode response, allowing measurements of optical transmitters and modulators to be conducted without the photodiode. The resulting E/O measurement is an accurate characterization of these devices.
De-embedding the photodiode permits its optical response and variation over the measurement bandwidth to be considered. The result is electrical-to-optical characteristics, such as bandwidth, aptitude flatness, phase linearity and group delay, can be accurately measured. Electrical return loss of the optical modulator’s RF input can also be measured.
Laser/optical modules can be used to achieve a simple, high-performance characterization of photo receivers and photodiodes when combined with the MN4765B O/E Calibration Module and de-embedding tools. These products feature low RIN DFB laser sources and high-bandwidth electro-optic modulators with excellent flatness and low ripple. There is automatic bias control to lock the operating point of the modulator independent of operating conditions.
Conducting O/E Measurements
The configuration shown in Figure 3 measures O/E devices. Anritsu provides a characterization file with each O/E calibration module, including the one shown in the figure, that allows the photodiode to be removed from the measurement. This file is obtained using a primary standard characterized by the National Institute of Standards (NIST). Because the characterized photodiode can be de-embedded, an engineer can use it to determine the magnitude and phase performance of the optical modulator used in the O/E measurement.
Using a transfer function file, the performance of the now-characterized optical modulator can be de-embedded by inserting the unknown photodiode in place of the characterized one to measure performance. This measurement’s uncertainty is slightly higher than the E/O measurements due to the second level of de-embedding.
Anritsu O/E calibration modules can be used to perform these O/E measurements with the VectorStar and ShockLine VNAs. The MN4765B modules are available in different wavelengths with various RF bandwidths to support industry requirements. When used with Anritsu VNAs, error-corrected transfer function, group delay and return loss measurements of E/O and O/E components and subsystems can be made with high accuracy.
For more information on E/O and O/E measurement techniques and VNA calibration processes, download Anritsu’s application note entitled Electrical-to-Optical and Optical-to-Electrical (E/O and O/E) Converter Measurements.