Olson Technology MUSCLE-EM55X - Manual (Page 9)

OTOT-EM55X/XL Optical Transmitter Rev. x1 www.olsontech.com
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lor the CNR/CSO/CTB performance to the used frequency plan and the requirements of the customer by using a dif-
ferent input level.
Via a high-pass circuit the electrical RF-signal is applied to the input of a predistortion circuit, which is foreseen to lin-
earize the squared sine wave transmission function of the electro optical modulator. The predistortion circuit is re-
quested to minimize 3rd order intermodulations (CTB = composite triple beat). The output signal of the predistortion
circuit is amplified to proper input level for the electro optical modulator, to establish a sufficient modulation depth of
the optical output signal.
The central core of the transmitter is the electro optical modulator working as a Mach-Zehnder-interferometer. The
light from the laser diode is coupled into an optical strip waveguide. An integrated optical splitter divides the light into
two identical portions, which are phase modulated by an RF signal applied to the electrodes of the modulator. The
concept of the electrodes results in a push pull phase modulation of both branches. Following the modulating section
the signals of both arms are combined and interfere. The interference of the phase modulated signals results in an am-
plitude modulation of the output light signal, which is available on both outputs of the combiner.
The necessary CW input light for the modulator is produced by a DFB laser diode working with a wavelength between
1550 and 1560nm. There are two control loops for operating the laser diode at constant optical output power as well as
at constant temperature by means of a thermoelectric cooler. The OTOT-EM55XL has been designed for DWDM ap-
plications and allows tuning the operation frequency (wavelength) by ±100GHz in steps of 50GHz. The laser diode
operating current is monitored in order to detect an increase of the initial value due to ageing of the laser diode. The
increase should not exceed 30%. The temperature of the laser diode is controlled by measuring the required drive cur-
rent for the thermoelectric cooler. At 90% of the available cooler drive current and/or >130% of the initial laser diode
operating current a B-grade alarm is generated which indicates a warning. At 100% cooler drive current the laser diode
operating current is switched off to protect the laser diode against irregular temperature conditions and an A-grade
alarm is generated indicating a severe malfunction. Both types of alarms are causing the corresponding LED on the
front plate of the optical transmitter to emit. In case of a B-grade alarm the yellow LED is lighting since the unit is still
working properly, however close to its limits. In case of an A-grade alarm the red LED is emitting. The messages to
the network management system are of course more detailed. They include the actual values of the currents and tem-
perature as well as alarm flags.
To suppress the Stimulated Brillouin Scattering (SBS) the wavelength spectrum of the optical signal is broadened. Two
technologies are used:
• Broadening the optical spectrum by modulating the laser diode operating current with a microwave signal
• Broadening the optical spectrum by driving an optical phase modulator with a microwave signal
These circuits are mandatory to avoid stimulated Brillouin scattering (SBS) in optical fibres and allow operation with
optical amplifiers feeding at least +13dBm of optical power into standard single mode fibres. For the XL-version of
the transmitter all microwave signals can be adjusted in amplitude via the push-buttons on the front panel to optimize
the SBS and SPM (self phase modulation) performance.
The coupling of light from the laser diode into the modulator is performed by using a polarization maintaining optical
fiber. The optical modulator provides two optical outputs. The signal of one of these outputs is tapped to an InGaAs
photodiode. The electrical signal of this photodiode is evaluated for two reasons:
1) To supervise a proper working of the CW laser diode. In case of optical output power drop of 2dB of nominal power
a B-grade alarm (=warning alarm) is generated, in case of optical output power 0dBm an A-grade alarm (=urgent
alarm) is generated.
2) A detector circuit measures CSO and CTB distortions to optimise the bias point of the electro optical modulator. For
a proper operation of the detection circuit at least two TV carriers with a frequency spacing of 24MHz have to be pre-
sent. Using this standard software setting of the detection scheme all known European and Chinese frequency plans