1. Diode laser The diode or semiconductor laser Diode lasers are extremely compact (0.3×0.2×0.1 mm), high efficiency (up to 40%), tuneable lasers, which have low power consumption Energy levels are not discrete Emission is from recombinant radiation occurring when electrons in the conduction band recombine with holes in the valence band Applying a bias voltage changes the energy levels to give a population inversion. Photon energy related to band gap, E g =E C – E V Emission wavelengths range from 375 nm to 30 μm Active medium is a p-n junction:

2. Diode laser In general, the polished ends of the laser are partially reflective and serve as the cavity mirrors Typical diode laser materials are GaAs, InGaAs, InGaAsP, AlGaAs with emission from 400 to 1600 nm. Lead salt lasers have emission in the mid-IR. Owing to the short cavity, the diode laser beam is more highly diverging than other lasers

3. Diode laser These modes are separated by At low powers, multiple longitudinal modes are emitted Δν = c / 2nd (n is the index of refraction) Tuning wavelength is accomplished by changing the current or temperature. “mode hops”: sudden changes in wavelength as the resonant longitudinal mode changes in the cavity.

4. Diode laser Mode hops are problematic if a significant tuning range is desired. A larger tuning range can be achieved by increasing the length of the cavity and including an wavelength selective element. Such a system is known as an external cavity diode laser (ECDL). For example, detecting gases based on their rotational vibrational transitions An advantage of current tuning is that the wavelength can be modified at kHz to MHz rates – this high frequency modulation is useful for improving the signal-to-noise ratio in many experiments. ECDL configurations: