THORLABS EVOA1550F User Manual page 13

Electronic Variable Optical Attenuators
Starting from the left of Figure 5: The light in the input fiber is coupled directly into a MEMS-based variable optical
attenuator (VOA), and the attenuated light is sampled by a low-percentage tap. The tap output is sent into a
photodiode whose signal is used by the digital output power display, the Power Lock control loop, and the SMA
monitor output. The attenuation value can be controlled by the manual power adjustment knob, the Power Lock
control loop, or the SMA modulation input.
When the power is switched off, the EVOA defaults to a mid-range attenuation whose value is independent of the
knob position.
4.2 Powering & Charging the Unit
The EVOA is powered by an internal rechargeable Li-ion battery which provides approximately 300 hours of
continuous operation on a full charge. The battery's internal circuits prevent full discharge after the Low Battery
indicator appears on the display. To charge the unit, use either the 2.1 mm coaxial power cord that is included
with the EVOA, or use a USB mini-B cable and charger (not included). If the device continues to be used while the
Low Battery indicator is active, it will eventually be shut down by the battery's safety circuits.
The 2.1 mm coaxial power cord will provide 1 A of charging current and hence the fastest charging time. In
contrast, the charging current provided by a USB cable is up to 500 mA, as determined by the power source. For
example, some USB chargers (e.g., for phones or tablets) will provide a maximum of 500 mA, while others (e.g.,
those on a PC) will be as low as 100 mA. All charging rates are sufficient to power the device while simultaneously
charging the battery, but charging will take longer at lower currents.
Two LEDs on the side utility panel indicate the charging status of the unit. A yellow LED indicates that charging is
in progress, while a green LED indicates that charging is complete. If both LEDs are lit, there is a battery fault. If
this occurs, unplug the charger and try again. Please contact Thorlabs' tech support (techsupport@thorlabs.com)
if this occurs repeatedly.
When the power is switched on, the attenuation will be set at the value determined by the position of the manual
power adjustment knob. If this value is not known, we recommend rotating it completely counterclockwise before
switching the power on. This will ensure maximum attenuation.
When the power is switched off, a voltage remains on the internal VOA, such that there will still be mid-range
attenuation even when the power is off. This is a precautionary feature that uses virtually no current, as the EVOA
is an ultra-low current device. This feature does not drain the battery meaningfully faster than the battery itself
would lose charge otherwise.
4.3 Fiber Connections
The fiber patch cable input and output bulkheads (see Figure 2) are both FC style. They may be ordered with PC-
or APC-style internal connectors:
Page 10
Figure 5 Basic Architecture of EVOA
Chapter 4: Operating Instructions and Details
000-104-213