Page 391 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 3

1.3 Brief History of ECL Development
Several papers on external cavity lasers appeared in the early 1970s. Some of
these authors recognized a number of the basic issues of concern to the present-
day designer and user of ECLs. In the late 1970s several papers were also pub-
lished in the Soviet literature. The paper by Fleming and Mooradian in 1981 is
the earliest reference cited by many authors. since they were the first to stu@ the
spectral properties of ECLs in detail.
Considerable work was done in the early to mid-1980s at British Telecom
Research Laboratories, motivated by the prospect of using ECLs as transmitters
and local oscillators in coherent optical communication systems. In a similar
vein, the mid- to late 1980s saw a great deal of work at AT&T Bell Laboratories.
Eventually, the telecommunication companies realized that distributed feedback
lasers (DFBs) and distributed Bragg reflector lasers (DBRs) would better suit
their needs. The end of the 1980s and early 1990s saw growing interest in ECLs
as sources for spectroscopic work and in commercial fiber optic test equipment.

7.4 Scope of ECL Discussion
This chapter considers lasers operating in the strong-external-feedback
regime. This generally requires devices with facets that have dielectric anti-
reflection (AR) coatings or tilted-stripe devices where the light exits the facet at
the Brewster angle.
This chapter deals mainly ufith the design and continuous wave (cw) proper-
ties of laser diodes coupled to free-space external cavities using bulk optical
lenses, prisms, filters, and mirrors. Some treatment of integrated optic external
cavities is also given. We exclude the treatment of the important rnonolithically
tunable DFB and DBR lasers. The rationale for this is that the design of these
lasers is very specialized and their fabrication requires sophisticated equipment
that necessarily limits the number of organizations that can produce them.
Broadband tuning of DFB lasers over ranges comparable to ECLs has been
obtained [l]. However the linewidths of these lasers are 2 to 3 orders of magni-
tude broader than that obtainable with ECLs.
We also do not explicitly consider vertical-cavity surface-emitting diode
lasers (VCSELs). By their structure these lasers are well suited to Isw-cost, high-
density uses in computer networks, but their short active regions provide low
gain and require very high cavity Q to achieve oscillation. At present, vertical-
cavity lasers are limited to those materials systems that can be grown on GaAs
substrates. This has restricted the spectral coverage to wavelengths below 1 pm.
So far. the goal of the few published external-feedback studies on VCSELs is
from the point of view of their applications to optical signal processing and
optical communications. They have comparable feedback sensitivity [2] and
behave in agreement with theory developed for edge-emitting laser diodes [3].