Background

Semiconductor diode lasers are becoming more and more attractive in comparison with conventional gas lasers and solid-state lasers. Progress in various aspects of semiconductor technology over the last two decades has led to major improvement in diode laser performance (wider spectral range, much lower threshold currents, much higher output power and single-mode operation) and a simultaneous reduction in price. This trend will certainly continue. In particular, the short-wavelength end of the visible spectrum is becoming available commercially due to progress in GaN-based devices. Right now, commercial diode lasers are available in the 0.4-0.52 μm range (InGaN/GaN system), 0.63-0.69 μm range (InGaP/InGaAlP system), the 0.75-0.85 μm range (GaAs/AlGaAs system), the 0.9-1.1 μm range (InGaAs/GaAs system) and the 1.2-1.6 μm range (InGaAsP/InP system). Recently the InGaAsSb lasers grown on GaSb covered the 2-2.6 μm region. These spectral ranges are mainly determined by the energy gaps of the corresponding materials.

Hydrostatic pressure reduces the lattice constant of the crystal and shifts the electronic energy bands. In III‑V semiconductors, the direct bandgap increases with pressure at a rate of about 100 meV/GPa, implying that the emission wavelength of a diode laser subjected to pressure will shift to lower values. For a 2 GPa pressure system we can achieve 200 meV increase of the bandgap. Similarly, lowering of the temperature of the laser diode increases the bandgap (by 80 – 100 meV in the temperature range from 300K down to 80K). This translates to different wavelength tuning ranges according to the relation λ*E ≈1,24μm*eV. For example, we demonstrated pressure tuning from 2400nm down to 1700nm (InGaAsSb/AlGaAsSb lasers), from 1550nm down to 1300nm (InGaAsP/InP lasers) and from 970nm to 840nm (InGaAs/GaAs lasers).

The Institute of High Pressure Physics (UNIPRESS) has specialised for over 35 years in the construction and application of large volume gas and liquid cells. The purpose of our group is to develop a compact pressure system working in the 2 GPa range, specifically designed for pressurising diode lasers. At the same time, we need to adjust the construction and the mounting of the diode lasers for high-pressure tuning. Presure tuning turned out to be effective for wavelengths above 800 nm. For wavelengths in the 630 – 800 nm range pressurized lasers have to be cooled down to keep threshold currents low. Therefore we developed different cooling systems for laser diodes under pressure which enabled us to reach yellow emission down to 575 nm. We also developed cooling systems for the laser diode (without applying pressure) which allow to tune the emission in the 80-100 meV range.

The linewidth of the laser diode can be narrowed down by using external resonators. Therefore we developed external resonators compatible with pressure/temperature tuning. For the applications where both the beam quality and high power are important we demonstrated wide range tuning of tapered lasers.

The possibility of substantial tuning of the emission wavelength makes the pressure-tuned diode laser an attractive and cheap alternative to the dye lasers or sapphire lasers which have been widely used for monitoring air pollution (LIDAR) and for gas detection.

The pressure-tuned diode laser could also be useful in diagnostics of telecommunication systems, e.g. for measurements of the dispersion in optical fibers. Tunable lasers have also important medical applications in photodynamic therapy, dermatology and ophthalmology. For the diode-pumped solid-state lasers, a tunable pump might be valuable.

We develop two types of pressure-tuned devices. One is a quasi-continuously tuned laser diode coupled to external grating (in Littrow or Littman configuration). This has been demonstrated e.g. in the 980 – 840 nm range and in the 1575 – 1225 nm range. The second type of device is pressure (and temperature) tuned high power laser emitting at a fixed wavelength. Since high power laser diodes are commercially available at some specific wavelengths (like 980 nm, 808 nm, 650 nm) our tuning can shift this wavelength to some other value, required by the user. In particular, we demonstrated this type of tuning for red lasers (645 nm) shifting their emission down to 575 nm (using high pressure and low temperature).

Pressure and temperature are very useful variables for characterizing laser diodes; we change the bandgap and the effective masses in the active layer, we change the depth of the quantum well (in the case of indirect barriers), we can change the mounting-induced strain in the device etc. Therefore a substantial part of our work is devoted to laser diode characterization using pressure and temperature.