Complex Optical Spectrum Analysis
Since their origins, optical transmission
systems have been improving in terms of
transmission capacity at an amazing pace. The
introduction of WDM enabled a huge increase
of capacity but actual networks are saturated
and need to increase their capacity while
maintaining their costly DWDM infrastructure.
The only way out has been increasing the
spectral efficiency of the optical signals, which
have been reluctant to abandon the traditional
on-off keying format, in which the power of an
optical carrier was modulated to have light (a
logical ‘1’) or darkness (a logical ‘0’). In addition
to being spectrally very inefficient (OOK signals
occupy a spectral bandwidth of twice their data
rate), they are also suboptimal against
chromatic dispersion and many non-linear
effects. In this scenario, more advanced
modulation formats, most of them inherited
from radio communications, that combine
power and phase to encode information are
being rapidly introduced. 40G systems already
experienced a strong introduction of DPSK
systems, and much more complex modulation
formats are being proposed for 100G networks.
The use of the optical phase to encode information has given a boost for the development of new measurement techniques, a step beyond the technology of traditional optical oscilloscopes. For most of them that work in the time domain, the measurement of the phase has become a challenge. This evolution of the transmission systems has generated a demand for new technologies to enable the characterization and performance evaluation of these signals and subsystems.
Phase Modulation Characterization
The measurement of the phase shift keyed
(PSK) signals is normally performed by using
balanced detection following a Mach-Zehnder
differential interferometer (MZDI). For
modulations with more symbols, such as
quadrature phase-shift keying (QPSK), two
signals must be acquired, the in-phase (I) and
the in-quadrature signals (Q). Considering the
fact that BOSA PHASE provides the possibility
of directly obtaining the instantaneous Phase,
an alternative representation is possible: the
phase eye diagram, which comprises all the
information in a very intuitive graph.
Optical Signal-to-Noise (OSNR) Measurement
OSNR is one of the key parameters to measure the quality of the
signals in a DWDM system. Its value is related to the BER of the channel and
represents a direct way to monitor the status of the network.
Due to the evolution of new optical networks, OSNR measurement has become a new challenge. The addition of ROADMs to the existing infrastructure, plus the increasing data bit rates, has made the standard measurement processes obsolete.
Two "in-band" OSNR measurement techniques can be supported using the optical spectrum analyzer. Spectral measurements of a 40Gbps P-DPSK commercial transmission signal permit direct measurement of the noise level made accessible through the polarization nulling method. This method takes advantage of the fact that the signal under investigation has a defined state of polarization, while the noise is unpolarized. Polarization nulling provides a maximum and minimum power value: at the maximum, the noise below the signal corresponds to half the power of the total depolarized noise. In the corresponding minimum position the noise level can be measured directly. An application note, referenced below, can provide detailed measurement information.