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Planar and Channel Waveguides
Excerpt from Field Guide to Lasers
When a thin layer with a slightly increased refractive index is fabricated on top of some crystal or glass, it functions as a planar waveguide. Light injected into the waveguide layer with a small enough divergence will be guided perpendicular to the layer and unguided in the other direction. The waveguide layer may also be embedded between two cladding layers.
A channel waveguide (with guidance in both directions) has a guiding structure in the form of a stripe with a finite width. This may be a ridge on top of the cladding structure or an embedded channel. The latter has more symmetric waveguide modes.
Channel waveguides are extensively used, for example, within various laser diodes. Small, low-power LDs have a single-mode channel waveguide with transverse dimensions of, for example, 1 µm. Broad area LDs have a wider channel that supports multiple modes in the horizontal direction. Diode bars (diode arrays) contain an array of broad-stripe diode structures.
Larger channel and planar waveguides made from rareearth doped dielectric materials are used for high-power waveguide lasers and amplifiers. Pump light may be injected along the amplified beam, from the side, or from the top. Even in situations with strong heating, the waveguide may help to stabilize a single-mode laser beam with high beam quality.