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A wide spectral asymmetry between the wide and narrow facets of a two-section tapered quantum dot (QD) superluminescent diode (SLD) emitting around 1240 nm was observed and investigated. This asymmetry, as characterized by the mismatch in the center wavelengths of the wide and narrow facet spectra, was found to be tunable and had some dependence on the magnitude of the difference in current densities applied to each section of the device. A maximum spectral mismatch of 14 nm was observed when the current density difference between the two SLD sections was 1.5 kAcm2.This spectral asymmetry presents an unexplored degree of freedom which could be exploited via multiplexing from a single device to optimize spectral bandwidth. Furthermore, potentially useful output powers of up to 50 mW were observed from the narrow facet of the SLD, which could again be exploited via single device multiplexing to increase output power, with little to no cost to spectral bandwidth. The experimental findings were analyzed using a rate-equation based QD model considering the QD ensemble inhomogeneous broadening, the multilayer chirped active material, the spatial distribution of the QD carriers and the spectral and spatial distribution of the photons in the SLDs. The numerical simulations were able to predict the asymmetric output powers extracted from the SLD facets, mainly to due to different equivalent material losses experienced by the forward and backward fields in the weakly gain guided tapered device. Simulations were also able to predict the spectral distribution of the optical fields at the output facets.

Combining the output of two or more separate, distinct superluminescent diodes (SLDs) via spectral multiplexing has the potential to produce very wide optical spectra which could prove beneficial in imaging applications1. Likewise, the implementation of a multi-section contact layout also allows for optimization of the spectral bandwidth and output power through the independent tuning of the current density in each section2. Here, we present the first comparative investigation of the output from both facets of a tapered multi-section SLD. A significant spectral asymmetry between the output of both facets was observed and was found to be widely tunable through the variation of the biasing conditions in both sections of the device. These results highlight an as yet untapped extra degree of freedom in multi-section superluminescent diodes which could be exploited to engineer the spectral bandwidth of such devices through the multiplexing of both outputs of a single device.

A high-power quantum-dot superluminescent diode is demonstrated under continuous-wave operation, with an output power of 137.5 mW and a corresponding spectral bandwidth of 21 nm. This represents not only the highest output power, but also a record-high power spectral density of 6.5 mW/nm for a CW-operated superluminescent diode in the 1.1 - 1.3 μm spectral region, marking more than a 6-fold increase with respect to the state of the art. The two-section contact layout of the reported device introduces additional degrees of freedom, which enable a wide tunability of the bandwidth and power depending on the desired application. A maximum bandwidth of 79 nm was recorded, with an output power of 1.4 mW. The high-power continuous-wave operation of this device would be particularly relevant for continuous, high-speed, high-sensitivity spectroscopy, imaging and sensing applications, as well as in optical communications.

A wide spectral asymmetry between the front and rear facets of a tapered chirped quantum dot multi-section superluminescent diode is reported. The spectral asymmetry between the two facet outputs was found to be tunable and highly dependent on the bias asymmetry between the two contact sections, with a spectral mismatch of up to 14 nm. Numerical simulations confirmed a relationship between this spectral asymmetry and the non-uniform filling of the quantum dots’ confined states when different current densities are applied to the device electrodes. The results from this investigation open up an additional degree of freedom for multi-section superluminescent diodes, which could pave the way for optical bandwidth engineering via multiplexing the spectral output from both facets, using only a single device.

A novel passively mode-locked semiconductor edge-emitting laser is reported, based on an AlGaAs multi-quantum-well structure. Ultra short pulses are generated at 752 nm, with a 19.4-GHz repetition rate and a pulse duration of 3.5 ps.

A deep-red semiconductor monolithic mode-locked laser is demonstrated. Multi-section laser diodes based on an AlGaAs multi-quantum-well structure were passively mode-locked, enabling the generation of picosecond optical pulses at 752 nm, at pulse repetition rates of 19.37 GHz. An investigation of the dependence of the pulse duration as a function of reverse bias revealed a predominantly exponential decay trend of the pulse duration, varying from 10.5 ps down to 3.5 ps, which can be associated with the concomitant reduction of absorption recovery time with increasing applied field. A 30-MHz-tunability of the pulse repetition rate with bias conditions is also reported. The demonstration of such a compact, efficient and versatile ultrafast laser in this spectral region paves the way for its deployment in a wide range of applications such as biomedical microscopy, pulsed terahertz generation as well as microwave and millimeter-wave generation, with further impact on sensing, imaging and optical communications.

We demonstrate the first semiconductor mode-locked lasers for ultrashort pulse generation at the 760 nm waveband. Multi-section laser diodes based on an AlGaAs multi-quantum-well structure were passively mode-locked, resulting in the generation of pulses at around 766 nm, with pulse durations down to ~4 ps, at pulse repetition rates of 19.4 GHz or 23.2 GHz (with different laser cavity lengths of 1.8 mm and 1.5 mm, respectively). The influence of the bias conditions on the mode-locking characteristics was investigated for these new lasers, revealing trends which can be ascribed to the interplay of dynamical processes in the saturable absorber and gain sections. It was also found that the front facet reflectivity played a key role in the stability of mode-locking and the occurrence of self-pulsations. These lasers hold significant promise as light sources for multi-photon biomedical imaging, as well as in other applications such as frequency conversion into the ultraviolet and radio-over-fibre communications.

Double-pass amplification of picosecond pulses is demonstrated and compared with single-pass amplification. This was achieved using a two-section tapered semiconductor optical amplifier with a chirped quantum-dot active region and a mode-locked laser diode as a seed. Across the range of biasing conditions common to both configurations, an enhancement in signal gain of up to 7 dB and output power by a factor of 4.1 was seen in the double-pass amplifier, compared to the single-pass. Only marginal increases in pulse duration were observed in the double-pass regime compared to the single-pass amplifier, meaning that enhancements in output power were well translated into peak power. Furthermore, the two-section contact layout of the SOA allowed the pulse duration to be optimised for a given fixed output power, giving additional flexibility to the amplifier. These results demonstrate the suitability of this simple and versatile technique, which could become the new standard in amplification of ultrashort pulses.