THORLABS EVOA1550F User Manual page 15

Electronic Variable Optical Attenuators
of resolution (the leftmost digit has a maximum value of 1). The display range will auto-scale for low or high input
power, and changes over at about 15 mW.
The digital display also has a Low Battery warning that indicates when the battery needs to be charged. The
warning appears while there is still sufficient charge remaining for the Li-ion battery not to degrade.
4.7 Modulation Input & Monitor Output Connections
The side utility panel (see Figure 4) provides two SMA connectors: an input for analog modulation and an output
for power monitoring.
The modulation input allows the user to control the attenuation by applying an AC or DC voltage of up to 4.9 V.
AC drive signals of up to 1 kHz can be used for high-speed modulation of the output power. Note that for the
modulation input to use the full attenuation range of the unit, the manual control knob must be fully rotated
clockwise to the maximum power (minimum attenuation) setting. This is because the external input simply adds
voltage to the internal VOA (shown in Figure 5), which is biased at almost 0 V at minimum attenuation.
The monitor output provides a voltage that is proportional to the output power. The conversion gain is
20 mV/mW, which results in a full-scale voltage of 4 V for 200 mW of output power. The monitor circuit has a
response bandwidth of 1 kHz minimum / 2 kHz typical.
4.8 Power Lock
Power Lock is a unique feature that locks the output power at a user-defined level. When this mode is active, the
EVOA will continuously adjust the attenuation in order to keep the output power constant as the input power
changes. This is ideal for use in long-term measurements where the optical power needs to be fixed.
When the Lock button is engaged (see Figure 2), the Power Lock control circuit will designate the output power
on the digital display as the output power setpoint. This circuit has a maximum bandwidth of 1 Hz.
Note that pushing the button may cause a slight offset in the output power, which is due to the resolution of the
sample-and-hold circuit. If the manual power adjustment knob is rotated while Power Lock is active, then
disengaging the button will cause the output power to jump to the value determined by the knob.
The indicator LED next to the Lock button will flash green intermittently when Power Lock is active and successfully
holding the output power constant. If the EVOA loses the lock, then the indicator light will flash red. This typically
indicates that the input power has changed significantly enough that the EVOA can no longer successfully
compensate.
If this occurs, but then the input power returns to the original range, the control circuit will reengage the lock and
the indicator will flash green. If the lock does not reestablish, then disengage the Lock button, rotate the manual
knob to the desired output power, and engage the button again.
Power Lock performs best when the input fluctuations are confined to within reasonable limits. In order for it to
maintain the output power at the setpoint and respond to changes in input power quickly, the input power must
always be greater than the sum of the output power and the EVOA insertion loss. As a rule of thumb, if the input
power is 2X the output power setpoint, the 1% settling time (i.e., the time until the output power returns to within
1% of the setpoint) is <0.5 seconds. For smaller ratios, the 1% settling time will increase, but the power will still
stabilize.
Chapter 5 Maintenance and Cleaning
The EVOA should not require regular maintenance. If necessary, the housing can be cleaned using a soft cloth
moistened with a mild glass cleaner. Do not use chemical solvents or harsh cleaning solutions on the display, and
do not spray cleaning solutions directly onto the unit.
Page 12
Chapter 5: Maintenance and Cleaning
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