3.4. CONTROL OF LONGITUDINAL MODES 99
Figure 3.14: Cross section of two index-guided semiconductor lasers: (a) ridge-waveguide struc-
ture for weak index guiding; (b) etched-mesa buried heterostructure for strong index guiding.
a result, permit strong mode confinement. Because of a large built-in index step, the
spatial distribution of the emitted light is inherently stable, provided that the laser is
designed to support a single spatial mode.
As the active region of a BH laser is in the form of a rectangular waveguide, spatial
modes can be obtained by following a method similar to that used in Section 2.2 for
optical fibers [2]. In practice, a BH laser operates in a single mode if the active-region
width is reduced to below 2μm. The spot size is elliptical with typical dimensions
2 × 1 μm^2. Because of small spot-size dimensions, the beam diffracts widely in both
the lateral and transverse directions. The elliptic spot size and a large divergence angle
make it difficult to couple light into the fiber efficiently. Typical coupling efficien-
cies are in the range 30–50% for most optical transmitters. A spot-size converter is
sometimes used to improve the coupling efficiency (see Section 3.6).
3.4 Control of Longitudinal Modes.....................
We have seen that BH semiconductor lasers can be designed to emit light into a single
spatial mode by controlling the width and the thickness of the active layer. However,
as discussed in Section 3.3.2, such lasers oscillate in several longitudinal modes simul-
taneously because of a relatively small gain difference (∼ 0 .1cm−^1 ) between neigh-
boring modes of the FP cavity. The resulting spectral width (2–4 nm) is acceptable for
lightwave systems operating near 1.3μm at bit rates of up to 1 Gb/s. However, such
multimode lasers cannot be used for systems designed to operate near 1.55μm at high
bit rates. The only solution is to design semiconductor lasers [36]–[41] such that they
emit light predominantly in a single longitudinal mode (SLM).
The SLM semiconductor lasers are designed such that cavity losses are different
for different longitudinal modes of the cavity, in contrast with FP lasers whose losses
are mode independent. Figure 3.15 shows the gain and loss profiles schematically for
such a laser. The longitudinal mode with the smallest cavity loss reaches threshold first