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Theory of Operation
Both the internal laser power level and wavelength are stabilized by feedback
control circuits. The power level is controlled with a laser power supply that is
adjusted in response to a photodetector in the laser module.
The wavelength of the internal laser is internally calibrated, or it can be adjusted
over a small range using the front panel l control. The wavelength is controlled
by varying the temperature of the laser diode with a thermoelectric cooler and
thermistor temperature sensor.
Modulator. The modulator is an electro-optic transducer that modulates the
optical input in response to an electrical modulation signal. The modulator used
is a titanium-diffused, lithium niobate modulator designed for 10 Gb/s operation.
The modulation is done with a Mach-Zehnder technique, where the electric fields
from the modulation input interact with the optical signal as it propagates down
the optical waveguide, causing interferometer effects. The use of the Mach-Zeh-
nder modulator results in high-performance optical modulation with low chirp.
The electrical interface to the modulator requires both DC control and RF signal
inputs. The DC control input to the modulator is designed to maintain a
quadrature bias condition for optimal output signal quality. The quadrature bias
point is automatically controlled with a bias feedback control circuit that adjusts
for bias drift with temperature and time. The RF signal input to the moduator
requires a modulator driver amplifier to decrease the required drive level of the
electrical modulation input.
The modulator driver in the transmitter is a limiting amplifier with gain control
capability. The limiting response of the modulator driver gives a flat output
signal for a relatively broad input amplitude range. The gain control feature of
the modulator driver allows for variable control of the optical output extinction
ratio. The modulator driver also features an internal bias control circuit that
allows for eye pattern crossing point adjustment. This crossing point adjustment
is only available from the remote port.
DC Power Supply
The AC line voltage from the rear panel is fed to the power supply input,
through the rear-panel power switch. The DC power supply is a switching supply
that converts the line voltage (100 – 240 VAC ±10%) to DC supply voltages that
are used by the circuitry on the transmitter board.
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OI1125 E/O Transmitter Instruction Manual
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