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2

Extremely Short-External-Cavity Laser Diode

In this chapter, we deal with laser diodes closely aligned with a microstruc-
ture includinga diaphragm, a cantilever and a slider. New functional and
sophisticated microdevices are given by closely aligning or integrating the
microstructure with a laser diode, which does not require a lens system.

2.1 Background

Extremely short-external-cavity laser diodes (ESEC LDs) have been demon-
strated for monitoringreflectivity [2.1] and displacement [2.2]. Tunable LD
[2.3] are also demonstrated with an ESEC configuration. ESEC LDs include
desirable properties for optical communication, optical data storage, spec-
troscopy, and a variety of sensingand measurement systems.
A surface-emittingLD with a micromechanical reflector has been used for
tuned devices [2.4–2.6]. The structure is designed to have an air-gap of approx-
imately one wavelength. When a voltage is applied to the membrane reflector,
the electrostatic force reduces the air-gap, which changes the wavelength. An
edge-emitting laser diode is also applicable for the micromechanically tunable
laser [2.7,2.8]. By varying the external cavity length, the laser wavelength can
be easily changed. However, to date the experimental data of the ESEC LD
under various coupled conditions have been poor.
Some studies have reported special considerations for such a short-
external-cavity LD based on butt couplinginto an optical fiber [2.9]. In
a short-external-cavity LD system, the feedback light must return to an ap-
proximately 1-µm diameter LD aperture, although it is difficult in practice
to align the light beam with such micrometer-level accuracy, and it is more
difficult to maintain this accuracy for a longperiod against external envi-
ronmental perturbations [2.10]. One possible solution is automatic alignment
usingan air bearingslider which has the same flyingmechanism as that of a
magnetic hard disk drive (HDD).