Page 166 - An Introduction to Microelectromechanical Systems Engineering

Fiber-Optic Communication Devices 145

Partially
reflective Nonreflective
facet facet Reflective mirror
Auxiliary output Laser diode Collimating lens
(front facet) (gain medium)
First diffractive order
Output (back facet)
θ
Incident beam
Thermoelectric
cooler
Grating
Pivot point (a)

θ 1
θ 2

(b)
Figure 5.8 (a) Illustration of the Littman-Metcalf external cavity laser configuration. Light from
the laser diode is collimated and diffracted by a grating acting as a wavelength filter. Lasing occurs
only at one wavelength, whose diffraction order is reflected by the mirror back into the cavity. (b)
Rotating the mirror around a virtual pivot point changes the wavelength and tunes the laser.

collimating lens, then diffracts on the grating. The mirror reflects back into the cav-
ity and the gain medium only one wavelength whose diffracted beam is exactly per-
pendicular to the mirror. This wavelength (and corresponding diffraction angle θ)
depends strictly on the grating pitch as well as the relative angle of the mirror with
the diffraction grating. In actuality, because the diffraction grating has finite disper-
sion [16], the linewidth of the reflected wavelength is broadened to a few picome-
ters. The output of the laser is typically the main (undiffracted) order reflecting from
the grating, but an auxiliary output can be taken from the partially reflective facet of
the laser diode.
The Littman-Metcalf configuration utilizes a fixed grating but rotates the reflec-
tive mirror to tune the laser to a different wavelength [Figure 5.8(b)]. It is the rota-
tion of the mirror that achieves both tuning operations simultaneously: it selects a
different diffracted wavelength from the grating, and it modulates the physical
length of the cavity. By appropriately selecting a virtual pivot point [17], the dimen-
sional change of the cavity length can be such that an integral number of new half
wavelengths can fit within the cavity—note that a rotation about a virtual pivot
point is geometrically equivalent to a rotation about a real pivot point and a linear
translation. A poor choice of pivot point or misalignment can cause serious