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.

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Fiber Lasers
Fiber lasers use optical fibers doped with low levels of rare-earth halides as the lasing medium to amplify light.
Fiber Optic Transceivers
Fiber optic transceivers include both a transmitter and a receiver in the same component.
Helium Neon Lasers
Helium neon (HeNe) lasers have an emission that is determined by neon atoms by virtue of a resonant transfer of excitation of helium. They operate continuously in the red, infrared and far-infrared regions and emit highly monochromatic radiation.
Fiber Optic Test Sources
Fiber optic test sources review the performance of a system by injecting light through the fibers.
Lasers
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.

Topics of Interest

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