186
4 Light Sources for Fiber Links
σ =
λ − λ 0
√
2ln2
=
0.195 nm
√
2ln2
= 0.166 nm
Example 4.14 Consider a double-heterostructure edge-emitting Fabry-Perot
AlGaAs laser, which emits at 900 nm. Suppose that the laser chip is 300 μm long
and the refractive index of the laser material is 4.3. (a) How many half-wavelengths
span the region between the Fabry-Perot mirror surfaces? (b) What is the spacing
between the lasing modes?
Solution (a) From Eq. (4.40) the number of half-wavelengths that span the region
between the Fabry-Perot mirror surfaces is
m =
2nL
λ
=
2(4.3)(300 μm)
0.90 μm
= 2866
(b) From Eq. (4.46) the spacing between the lasing modes is
λ =
900 × 10
−9 m
2
2
300 × 10 −6 m
(4.3)
= 0.314 nm
Drill Problem 4.7 A GaAs laser operating at 900 nm has a 300-μm length
and a refractive index n = 3.58. Use Eq. (4.45) to show that the frequency
separation of the resonant modes is 140 GHz.
4.3.5 Structures and Radiation Patterns of Laser Diodes
A basic requirement for efficient operation of laser diodes is that, in addition to transverse optical confinement and carrier confinement between heterojunction layers, the
current flow must be restricted laterally to a narrow stripe along the length of the
laser. Numerous novel methods of achieving this, with varying degrees of success,
have been proposed, but all strive for the same goals of limiting the number of lateral
modes so that lasing is confined to a single filament, stabilizing the lateral gain, and
ensuring a relatively low threshold current. The most common devices are called
index-guided lasers. If a particular index-guided laser supports only the fundamental
transverse mode and the fundamental longitudinal mode, it is known as a singlemode laser. Such a device emits a single, well-collimated beam of light that has an
intensity profile that is a bell-shaped Gaussian curve.
When designing the width and thickness of the optical cavity, a tradeoff must
be made between current density and output beam width. As either the width or
4 Light Sources for Fiber Links
σ =
λ − λ 0
√
2ln2
=
0.195 nm
√
2ln2
= 0.166 nm
Example 4.14 Consider a double-heterostructure edge-emitting Fabry-Perot
AlGaAs laser, which emits at 900 nm. Suppose that the laser chip is 300 μm long
and the refractive index of the laser material is 4.3. (a) How many half-wavelengths
span the region between the Fabry-Perot mirror surfaces? (b) What is the spacing
between the lasing modes?
Solution (a) From Eq. (4.40) the number of half-wavelengths that span the region
between the Fabry-Perot mirror surfaces is
m =
2nL
λ
=
2(4.3)(300 μm)
0.90 μm
= 2866
(b) From Eq. (4.46) the spacing between the lasing modes is
λ =
900 × 10
−9 m
2
2
300 × 10 −6 m
(4.3)
= 0.314 nm
Drill Problem 4.7 A GaAs laser operating at 900 nm has a 300-μm length
and a refractive index n = 3.58. Use Eq. (4.45) to show that the frequency
separation of the resonant modes is 140 GHz.
4.3.5 Structures and Radiation Patterns of Laser Diodes
A basic requirement for efficient operation of laser diodes is that, in addition to transverse optical confinement and carrier confinement between heterojunction layers, the
current flow must be restricted laterally to a narrow stripe along the length of the
laser. Numerous novel methods of achieving this, with varying degrees of success,
have been proposed, but all strive for the same goals of limiting the number of lateral
modes so that lasing is confined to a single filament, stabilizing the lateral gain, and
ensuring a relatively low threshold current. The most common devices are called
index-guided lasers. If a particular index-guided laser supports only the fundamental
transverse mode and the fundamental longitudinal mode, it is known as a singlemode laser. Such a device emits a single, well-collimated beam of light that has an
intensity profile that is a bell-shaped Gaussian curve.
When designing the width and thickness of the optical cavity, a tradeoff must
be made between current density and output beam width. As either the width or