Recently, there has been a renewed interest in coherent optical detection. The reasons for this are: a) coherent optical
receivers achieve high receiver sensitivities; b) multilevel modulation formats can be detected very efficiently; c)
optical WDM systems with high spectral efficiency can be implemented; and d) preservation of the optical phase
allows electrical equalizers to efficiently compensate optical channel impairments. These advantages of coherent
optical detection over direct detection can be used to overcome some of the obstacles that limit the data capacity and
the reach of current direct detection systems, both fiber and
free-space based. The essential part of the coherent optical receiver is the optical local oscillator (LO) laser. It has to provide a high
optical output power with low linewidth and low relative intensity noise (RIN). With a widely tunable LO laser a
frequency-agile receiver can be constructed.
To determine the best candidates for tunable LO lasers, different laser technologies are discussed in terms of output
power, power variation, electrical power dissipation, switching time, control leads, package dimensions, tuning
range, linewidth and RIN.
A heterodyne receiver to detect 2.5 Gb/s and 10 Gb/s signals has been implemented with a standard distributed feed
back (DFB) laser. Upgrades of the coherent receiver with a widely tunable LO will be presented. Experimental
comparison of the LO lasers and their impact on the receiver sensitivity will be shown.