Page 471 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers

transmitter and local oscillator (LO) depends on the modulation scheme and on the
bit rate. However, in general. coherent optical communications systems require
optical sources that are single frequency (sidemode suppression ratio >30 dB) and
narrow linewidth (6v - 100 kHzj. In addition, the LO laser must be tunable.
In the early 1980s, workers at British Telecom Research Laboratories used
ECLs to demonstrate the feasibility of coherent transmission over Iong-haul
fiber optic links. A PSK heterodyne experiment was reported in which a grating-
tuned ECL was used as the LO in a PSK heterodyne experiment, and a 1 S2-p~
HeNe laser was used as the transmitter [146]. A receiver sensitivity of -59 dBm
was obtained over a transmission path consisting of 109 km of optical fiber. In
another experiment, a pair of grating-tuned ECLs served as the transmitter and
LO in an FSK heterodyne experiment at 1.54 pm [11-71. A receiver sensitivity of
-55 dBm was measured over 200 km of optical fiber. In both experiments, the
intermediate frequency was stabilized by cavity-length tuning of the LO laser
using an intracavity silica tuning plate mounted on a galvanometer.
No coherent systems have yet been commercially deployed. A11 fiber optic
telecommunication systems in use today utilize on-off keying and direct detec-
tion. Compared to coherent technology. this is loosely analogous to the status of
spark-gap radio transmission prior to the commercial introduction of superhetero-
dyne receivers in 1924 [138]. As the demand for bandwidth increases and compo-
nent technology improves, it is likely that coherent transmission will become
commercially significant at some time in the future. Although it is likely that the
transmitters and LOs in coherent fiber optic telecommunication links %rill be
monolithically integrated tunable lasers such as DFB lasers. ECLs will undoubt-
edly be used in test instrumentation for the characteijzation of these monolithic
sources and in the maintenance of the installed coherent transmission links.

I 8. 7.2 l~aveleng~~-Division-Multiplexed Systems
Wavelength division multiplexing (WDM) increases the capacity of fiber
optic telecommunication links by transmitting at multiple-wavelength channels
to utilize the broad spectral transmission window of optical fiber. A demonstra-
tion of 1.37 terabit km/sec transmission capacity over 68 km of fiber was made
using 10 single-frequency DFB lasers [149]. To my knowledge, no WDM sys-
tem demonstrations using ECLs have been published. but recently there has been
activity to develop multiwavelength multistripe grating-cavity lasers specifically
for WDM applications [ 126-1 281.

18.2 Lightwave Testing and Measurement
The most important commercial use of ECLs is in instrumentation for the
testing and measurement of components for lightwave communications systems.
This is an ideal application for ECLs because performance is at a premium, and
the high value of the systems under test can support the relativelj. high cost of a