We demonstrate amplification (> 18 dB) of 200-fs pulses in a quantum-dot (QD) semiconductor amplifier. Our measurements have shown that such QD devices can provide amplification of femtosecond pulses over a spectral range that exceeds 100 nm.

Room temperature, tunable, external-cavity short-wavelength InAs/AlSb quantum cascade laser (QCL) is reported. Wavelength tuning of 85 nm for the spectral range between 3190 nm and 3275 nm has been achieved by rotating the diffraction grating forming the external cavity. To suppress lasing inside the QCL cavity, its ridge was tilted by 7° at the external cavity end. The optimal tilting angle of the laser ridge was chosen by careful consideration of the return losses of the TM-polarized waveguide mode from the diffraction grating in a quasi-Littrow configuration and the Fabry-Pérot feedback from the tilted laser facet. No antireflection coating was used.

Multimode high-power laser diodes suffer from inefficient beam focusing, leading to a focal spot 10–100 times greater than the diffraction limit. This inevitably restricts their wider use in ‘direct-diode’ applications in materials processing and biomedical photonics. We report here a ‘super-focusing’ characteristic for laser diodes, where the exploitation of self-interference of modes enables a significant reduction of the focal spot size. This is achieved by employing a conical microlens fabricated on the tip of a multimode optical fibre using 3D laser nano-printing (also known as multi-photon lithography). When refracted by the conical surface, the modes of the fibre-coupled laser beam self-interfere and form an elongated narrow focus, usually referred to as a ‘needle’ beam. The multiphoton lithography technique allows the realisation of almost any optical element on a fibre tip, thus providing the most suitable interface for free-space applications of multimode fibre-delivered laser beams. In addition, we demonstrate the optical trapping of microscopic objects with a super-focused multimode laser diode beam thus rising new opportunities within the applications sector where lab-on-chip configurations can be exploited. Most importantly, the demonstrated super-focusing approach opens up new avenues for the ‘direct-diode’ applications in material processing and 3D printing, where both high power and tight focusing is required.

Quasi-phase-matching is an important and widely-used technique in nonlinear optics enabling efficient frequency up-conversion. However, since its introduction almost half a century ago, this technique is well developed for near infrared (IR) but is intrinsically limited in spectral tunability in the visible range by the strict conditions set by the spatial modulation which compensates the momentum mismatch imposed by the dispersion. Here, we provide a fundamental generalization of quasi-phase-matching based on the utilization of a significant difference in the effective refractive indices of the high- and low-order modes in multimode waveguides. This concept enables to match the period of poling in a very broad wavelength range and opens up a new avenue for an order-of-magnitude increase in wavelength range for frequency conversion from a single crystal. Using this approach, we demonstrate an all-room-temperature continuous-wave (CW) second harmonic generation (SHG) with over 60 nm tunability from green to red in a periodically-poled potassium titanyl phosphate (PPKTP) waveguide pumped by a single broadly-tunable quantum-dot laser diode.

A transfer-matrix method has been developed for cylindrical waves. Expressions are obtained for reflection and transmission coefficients of cylindrical light waves by an isolated cylindrical boundary...