Laser Diode Design Principals

Laser Diode Design Principals

The impressive technological advances that resulted in semiconductor diode laser technologies within the last decade can be grouped roughly into four areas: higher optical output power, higher single transverse mode and diffraction-limited output, increased range of lasing wavelengths, and significantly improved reliability (see Part II). Most noteworthy commercial demonstrations in high-power continuous wave (cw) outputs of single-emitter and multi-element emitter laser products, as an example, within the 980 nm band, include 0.75 W ex-fiber for single spatial mode, narrow-stripe emitters, 12 W for tapered master oscillator power amplifier emitters with single-mode, diffraction-limited operation, 25 W for standard 100 μm wide aperture single-emitter devices, and 1000 W quasi-cw for standard 1 cm multi-element linear laser arrays with nearly diffraction-limited beams.

The introduction of novel design approaches including strained quantum wells and quantum cascade structures, plus the advanced maturity of material systems such as compounds predicated on GaN, CdS, and GaSb, have significantly extended the operating wavelength array of semiconductor lasers through the visible spectrum in to the ultraviolet regime right down to about 0.375 μm from the short-wavelength side and far into the infrared regime with cw operation within 3–10 μm at room temperature, and beyond 10 μm up to 300 μm at operating temperatures around 77 K in the very long wavelength side. Compressively-strained InGaAs/AlGaAs quantum well lasers emitting in the 980 nm band are typical examples of lasers with wavelengths, which lattice-matched quantum well structures cannot deliver.

The basic device structure is made of a rectangular parallelepiped of an immediate bandgap semiconductor, usually a III–V compound semiconductor such as for instance GaAs, incorporating a forward-biased, heavily doped p–n junction to deliver the optical gain medium in a resonant optical cavity. The operating principle of a semiconductor laser requires the gain medium to be pumped with some external power source, either electrical or optical, to produce and continue maintaining a nonequilibrium distribution of charge carriers, which has to be large enough make it possible for a population inversion when it comes to generation of optical gain. Pumping realized by optical excitation of electron–hole pairs is generally only very important to the rapid characterization regarding the quality associated with the laser material without electrical contacts.

The more technologically important technique, however, is direct electrical pumping using a forward-biased semiconductor diode with a heavily doped p–n junction at the center of all state-of-the-art semiconductor injection lasers, that is, diode lasers. The Fermi levels in these heavily doped and so degenerate n- and p-type materials lie into the conduction and valence band, respectively. With no bias voltage applied, the quasi-Fermi levels are identical across the p–n junction at thermal equilibrium aided by the conduction and valence bands bent. In this steady state, further diffusion of electrons and holes throughout the p–n boundary is opposed because of the built-in potential (diffusion potential) resulting from the depletion layer or space-charge region formed by the negatively charged acceptors and positively charged donors regarding the p- and n-sides, respectively.

In contrast to stimulated absorption discussed above, the different interactions mixed up in stimulated emission process are reversed, that is, an event photon stimulates the recombination of an electron–hole pair by simultaneously generating the emission of a brand new photon. It is a positive gain mechanism resulting in the amplification of radiation, considering that the stimulated photons, that are aligned in direction and phase into the incident photons, are emitted into the incident radiation field leading to a solid coherent optical emission.

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