Friday, February 24, 2012


DFB: Distributed Feedback. Feedback is essential to sustaining laser oscillation. In a Fabry-Perot laser, that feedback comes from light on the laser transition reflected back into the resonator by the cavity mirrors. Distributed feedback comes from a source distributed through the laser cavity, which in practice means a grating that scatters light back into the laser medium.

The spacing of the lines in the grating determines the wavelength at which the feedback is strongest, and light amplification by stimulated emission concentrates emission at that wavelength. For a grating with line spacing D in a material with refractive index n, the peak wavelength l is

where m is the order of the grating, usually 1 or 2. That means that for a first-order grating in InGaAsP, with n=3.4, a grating period of 228 nm is needed to generate 1550 nm light.

Distributed feedback is used mostly in semiconductor diode lasers, where parallel lines etched in the active layer form a conventional diffraction grating (see figure). In fiber lasers, distributed feedback is produced by fabricating fiber Bragg gratings -- alternating regions of high and low refractive index perpendicular to the fiber axis -- in the optically pumped fiber. Note that distributed Bragg reflector (DBR) lasers are not considered DFB lasers because the feedback comes from gratings outside the gain region; DBR cavities are used for both diode and fiber lasers.

FIGURE. Distributed feedback laser. (From Jeff Hecht, Understanding Fiber Optics, 5th edition)

The big advantage of distributed feedback is its ability to stabilize lasers so they emit in a fixed-narrow range of frequencies, and DFB diode lasers were crucial for high-speed fiber-optic systems. A typical Fabry-Perot diode oscillates on multiple longitudinal modes spread across a few nanometers, but a temperature-stabilized DFB laser can prevent mode-hopping and limit oscillation to a single longitudinal mode with megahertz linewidth. DFB diode lasers provide the narrow linewidth essential for dense wavelength-division multiplexing (DWDM) and high-speed transmission. DFB lasers also can be tuned over limited ranges, and provide narrow-line emission for sensing and other demanding applications. can be used in other applications such as sensing.

Sunday, February 19, 2012


LASER: Light Amplification by Stimulated Emission of Radiation
In 1957, Gordon Gould began his notes on the feasibility of a LASER by spelling out the acronym: light amplification by stimulated emission of radiation. The acronym, like the concept itself, was inspired by the maser, the laser's microwave counterpart, invented earlier by Charles Townes. Gould's catchy acronym gives the gist of the idea, but is not a full definition.

Light sources from candles to LEDs emit light spontaneously, when excited atoms or molecules release energy on their own, so the photons are not identical and travel in various directions. Stimulated emission occurs when a photon stimulates an excited atom to emit an identical photon, traveling in the same direction and coherent with the first. The effect amplifies the light of the first photon, and the additional photon also can stimulate emission, producing a beam of identical photons.

However, the acronym glosses over an essential point -- a laser also requires a resonant cavity to generate a beam. A pair of mirrors facing each other bounce light back and forth, so it oscillates through a laser medium, amplifying the light that emerges through the output mirror as the laser beam. Oscillation makes the laser beam directional, coherent and monochromatic, conditions considered essential for a laser.

Townes and Arthur Schawlow saw the laser as a variant of the maser, and in 1958 they described it as an "optical maser" in a pioneering paper. The terms "laser" and "optical maser" competed for public favor, as the two sides competed for patent rights and credit for the invention. Schawlow was quick to point out that Gould's choice was a poor one because the device needed an oscillator in order to generate a beam. He said that meant the "laser was a loser," because it should be spelled out as "Light Oscillation by Stimulated Emission of Radiation."

Schawlow had a point, but laser won the popularity race. In time, Townes won a Nobel Prize, and Gould got rich from his patents. But Schawlow got the most laughs.