It is not the intent of this chapter to fully cover the optical setup details. However, some common issues and concerns will be discussed. More general information of fiber optic measurement setups can be found elsewhere (e.g., [3]-[4]).
Optical Linearity
The linearity of the characterized photodiode directly affects the accuracy of the measurement and the optical input power at which RF photocurrent will remain linear is important. One way to check that the measurement path is still linear is to normalize the S21 plot against itself and increase the optical power to see if there is a gross sensitivity (the absolute level will move, but the important question is if the shape of the response changes with optical power). For example, set the optical laser power to 4 or 5 dBm with the setup like that of Figure: General 2-port E/O or O/E Measurement Setup. Store the resulting S21 to memory (under the Display/View Trace menu) and then view DataMemMath (data divided by memory). Now increase the laser power in 1 dB steps until some compression (in this case, meaning that the deviation from flatness is visible on this scale) is seen in the normalized plot on the scale of 2 dB/div. At that point, decrease the optical power level until it is out of compression. Make sure that the maximum DC current for the photodiode is not exceeded.
It is often useful to maximize optical power to improve dynamic range as long as the linearity of the DUT or the system optical components is not compromised. Every 1 dB increase in optical power will improve dynamic range by 2 dB due to the power-law detection characteristics of the photodiodes.
Laser Power and Photodetector Bias Sequencing
Always make sure the photodiode is biased properly before turning the laser on. Improper bias or no bias can degrade photodiode performance and can also result in damage. Instructions on handling and biasing of the photodiode are shipped with the characterized photodiode accessory. Always observe ESD precautions as these devices are very sensitive to static discharge.
Optical Fiber Lengths
The measurement setup will typically require optical fibers to interconnect optical components with different connectors. For example, a modulator with an FC/PC connector at the output will require an optical patch cord to adapt to the FC/APC connector on the input of the characterized photodiode accessory. Optical fibers have negligible frequency-dependent loss over microwave modulation bandwidths. Thus, adding short lengths of optical patch cords to the setup does not affect the accuracy of transfer function measurements. To avoid certain polarization-related issues, it is recommended to keep the patch cord length under 10 m. The use of patch cords appropriate for the wavelength being used is recommended, as multi-mode propagation is possible in some cases and may result in some measurement instability.
Modulator Bias Control
Lithium niobate modulators are generally biased using a modulator bias controller (MBC) to control the operating point of the modulator. When biased in quadrature, the input RF signal linearly modulates the optical carrier. Note that when an MBC is applied, it must be designed for small signal operation. The default power from the Port 1 test port is –10 dBm for the MS4647B when equipped with Options 51, 61 or 62. This level results in a modulation depth of <10% for many commercially available modulators. If a different model VNA or a different option configuration is being used, it may be required to drop the VNA power level from the default setting to ensure that the modulator is operating linearly. Different technology modulators may have different linearity limits so it may be required to consult the manufacturer of that device.
A DC power supply can be used in place of an MBC. However, the stability of the measurement may be degraded due to drift in the modulator’s bias point.
These concepts apply to the MN4775X converter; the unit defaults to quadrature bias maintained with a closed-loop dithering system (a small modulation is added to the bias and the detected response to that modulation used for feedback). The dithering frequency is 3 kHz by default, but can be changed (should generally be higher than the current measurement IF bandwidth). There are cases when trace noise, on a fine scale, can be improve by reducing dither amplitude or turning it off, but, again, stability will be affected. Other bias states (modulator full on, full off...) may be useful for certain measurements (particularly DC).
Connector Care
It is important to establish proper cleaning procedures when connecting fiber optic devices together. Fiber optic cores are made of glass and can easily be scratched or chipped if care is not taken. The connectors found on the MN4765X are of the FC/APC type. APC (Angled Physical Contact) is chosen to help minimize back-reflection. The 8° angle at the end-face of the APC connector has an optical return loss of better than 50 dB. DFB lasers require large amounts of reflection isolation to function properly. The optical return loss from a common PC connector can 30 dB or worse, depending upon the polish and cleanliness of the connector.
The following are some tips to help ensure quality connections:
• Always clean connectors after every connection.
• Use a fiber optic scope often to ensure there are no defects on the connector end face that can cause damage to other connectors.
• Use insertable patch cords for expensive devices that require many connections.
• Always use a cloth that is free of fiberglass to clean the connectors. If necessary, use alcohol to remove stubborn dirt and oil. Thoroughly remove any alcohol residue before reconnecting.
• Avoid using any oils for connecting two cables together. Oils are messy and very difficult to clean up.
• Optical connectors do not need torque. Some connections are better when the two fibers are barely touching. Tightening the connector too much will result in higher insertion loss, more reflection, and in some cases damage to the connector.
• Always observe proper mating to APC connectors. Connecting APC to PC connectors will damage the connectors.
Specification Items
In the ME7848A system Technical Data Sheet, there are a number of parameters unique to the O/E, E/O, and O/O classes of measurements. Repeatability is defined to include trace noise and any short-term instability in optical connections. Because of the relative low received signal level to the VNA (often < -40 dBm), trace noise is generally worse than in a traditional VNA measurement with the same IF bandwidth and averaging.
Dynamic range or noise floor is defined with losses of one or both optical converters included (depending on which measurement is being referenced). As an example, if one is measuring an E/O device, the dynamic range or noise floor metric of interest includes effects of both the VNA and the O/E module. The noise floor can be expressed in dBm terms (power delivered to the instrument) or in absolute responsivity terms of the DUT (A/W or W/A).