Constant envelope modulation formats for optical communications

To 
ompare system performan
e the sensitivity has been evaluated: here sensi-tivity is meant as the OSNR level required to obtain a BER (Bit Error Rate) equalto 10−6.As far as CPFSK format is 
on
erned, a 10 Gbit/s intensity dire
tmodulatedlaser has been employed. At the re
eiver an asymmetri
 Ma
hZehnder interferom-eter and a balan
ed photodiode have been used. Moreover, we had to apply also anele
tri
 amplier and a lowpass lter.Dire
t modulation is a 
heaper te
hnique, but has a disadvantage: the frequen
yresponse is nonuniform and low frequen
ies are penalized. Some solutions to thisproblem already exist, but with this work a simpler one is suggested. We propose toemploy an 8B/10B line 
oding that needs simple digital operations on the transmit-ted sequen
e: thanks to it, the low frequen
y 
omponents are emptied. This 
oding
an dete
t errors, but a 25 % overhead is introdu
ed.The ee
tiveness of this te
hnique has been fully proved with experiments: theresults of this work have been submitted and a

epted for a presentation at 10◦Convegno Nazionale sulle Te
ni
he Fotoni
he nelle Tele
omuni
azioni (FOTONICA2007).Moreover, a simulation system for CPFSK transmission has been prepared andoptimized.The transmitter for POLSK modulation, instead, is based on a CW (ContinuousWave) laser and a LiNbO3 phase modulator having the input pigtail rotated by45◦ with respe
t to the standard orientation for phase modulation; the re
eiver is adierential demodulator built as the previous one.Finally, the DPSK employs a normal phase modulator; the dierential re
eiveris the same used for POLSK format. XVIII