From Understanding Lasers

9.10.2 Bandwidth and Modes

Earlier sections have mentioned the oscillating modes of various types of diode lasers, but it is worth looking at them again and

how they affect the spectral bandwidth of diode lasers. Spectral bandwidth is important in fiber-optic systems, because the slight variation in refractive index with wavelength can cause pulses to stretch out when traveling through long lengths of fiber, an effect called dispersion.

High-power diode lasers and arrays of many diode laser stripes oscillate in many modes, and the range of wavelengths they emit is usually limited to a nanometer or less, which is fine for most applications.

Narrow-stripe diode lasers typically oscillate in only one transverse mode, so they may be called "single-mode lasers," but Fabry Perot lasers can oscillate in more than one longitudinal mode, and hop between modes when operating conditions change. The modes are 0.2 to 0.7 nm apart, and the range of wavelengths they emit is too broad for high-speed fiber-optic networks.

DFB, DBR, and external-cavity lasers limit laser oscillation to a single stable longitudinal mode, giving them the extremely narrow bandwidth required for high-speed fiber-optic networks.

The short cavity of VCSELs limits them to oscillating in a single longitudinal mode, but their bandwidth is not as narrow as DFB or DBR lasers, which internally stabilize the output wavelength. Large-aperture VCSELs oscillate in multiple transverse modes.

Products & Services
Fiber Optic Transceivers
Fiber optic transceivers include both a transmitter and a receiver in the same component.
Fiber Lasers
Fiber lasers use optical fibers doped with low levels of rare-earth halides as the lasing medium to amplify light.
Fiber Optic Test Sources
Fiber optic test sources review the performance of a system by injecting light through the fibers.
Lasers are devices that produce intense beams of monochromatic, coherent radiation. The word "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation.
Fiber Optic Cables
Fiber optic cables are composed of one or more transparent optical fibers enclosed in protective coverings and strength members. Fiber optic cables are used to transmit "light" data.

Topics of Interest

9.11 DIODE LASER MATERIALS AND WAVELENGTHS Earlier in this chapter you learned that the wavelengths emitted by diode lasers depended on the composition of the semiconductor, and learned which families...

The simplest type of diode laser resonator is the Fabry Perot cavity introduced in Chapter 3, a linear cavity with flat mirrors on the two ends. In a semiconductor laser, the linear cavity is the...

9.14 WHAT HAVE WE LEARNED? Semiconductor lasers are often called diode lasers or laser diodes. Diodes are two-terminal electronic devices that conduct current in one direction. Electrons in the...

9.6.2 Distributed Feedback Lasers Applications such as high-speed data transmission in fiber optics require limiting laser emission to a narrower range of wavelengths than possible with a Fabry Perot...

9.7.1 Vertical-Cavity Surface-Emitting Lasers (VCSELs) Vertical-cavity surface emitting lasers (VCSELs) get their name because their resonant cavities are vertical, perpendicular to the active layer,...