Wide and tunable spectral asymmetry between narrow and wide facet outputs in a tapered quantum-dot superluminescent diode.

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.

High-power quantum-dot superluminescent tapered diode under CW operation.

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.

Double-pass amplification of picosecond pulses with a tapered semiconductor amplifier.

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.

Picosecond pulse amplification up to a peak power of 42 W by a quantum-dot tapered optical amplifier and a mode-locked laser emitting at 1.26 µm.

We experimentally study the generation and amplification of stable picosecond-short optical pulses by a master oscillator power-amplifier configuration consisting of a monolithic quantum-dot-based gain-guided tapered laser and amplifier emitting at 1.26 µm without pulse compression, external cavity, gain- or Q-switched operation. We report a peak power of 42 W and a figure-of-merit for second-order nonlinear imaging of 38.5 W2 at a repetition rate of 16 GHz and an associated pulse width of 1.37 ps.

High-power quantum-dot tapered tunable external-cavity lasers based on chirped and unchirped structures.

A high-power tunable external cavity laser configuration with a tapered quantum-dot semiconductor optical amplifier at its core is presented, enabling a record output power for a broadly tunable semiconductor laser source in the 1.2 - 1.3 µm spectral region. Two distinct optical amplifiers are investigated, using either chirped or unchirped quantum-dot structures, and their merits are compared, considering the combination of tunability and high output power generation. At 1230 nm, the chirped quantum-dot laser achieved a maximum power of 0.62 W and demonstrated nearly 100-nm tunability. The unchirped laser enabled a tunability range of 32 nm and at 1254 nm generated a maximum power of 0.97 W, representing a 22-fold increase in output power compared with similar narrow-ridge external-cavity lasers at the same current density.

Two-state semiconductor laser self-mixing velocimetry exploiting coupled quantum-dot emission-states: experiment, simulation and theory.

We exploit the coupled emission-states of a single-chip semiconductor InAs/GaAs quantum-dot laser emitting simultaneously on ground-state (λ(GS) = 1245 nm) and excited-state (λ(ES) = 1175 nm) to demonstrate coupled-two-state self-mixing velocimetry for a moving diffuse reflector. A 13 Hz-narrow Doppler beat frequency signal at 317 Hz is obtained for a reflector velocity of 3 mm/s, which exemplifies a 66-fold improvement in width as compared to single-wavelength self-mixing velocimetry. Simulation results reveal the physical origin of this signal, the coupling of excited-state and ground-state photons via the carriers, which is unique for quantum-dot lasers and reproduce the experimental results with excellent agreement.

Tapered InAs/InGaAs quantum dot semiconductor optical amplifier design for enhanced gain and beam quality.

In this Letter, a design for a tapered InAs/InGaAs quantum dot semiconductor optical amplifier is proposed and experimentally evaluated. The amplifier's geometry was optimized in order to reduce gain saturation effects and improve gain efficiency and beam quality. The experimental measurements confirm that the proposed amplifier allows for an elevated optical gain in the saturation regime, whereas a five-fold increase in the coupling efficiency to a standard single mode optical fiber is observed, due to the improvement in the beam quality factor M² of the emitted beam.

Coherence function control of Quantum Dot Superluminescent Light Emitting Diodes by frequency selective optical feedback.

Low coherent light interferometry requires broad bandwidth light sources to achieve high axial resolution. Here, Superluminescent Light Emitting Diodes (SLDs) utilizing Quantum Dot (QD) gain materials are promising devices as they unify large spectral bandwidths with sufficient power at desired emission wavelengths. However, frequently a dip occurs in the optical spectrum that translates into high side lobes in the coherence function thereby reducing axial resolution and image quality. We apply the experimental technique of frequency selective feedback to shape the optical spectrum of the QD-SLD, hence optimizing the coherence properties. For well-selected feedback parameters, a strong reduction of the parasitic side lobes by a factor of 3.5 was achieved accompanied by a power increase of 40% and an improvement of 10% in the coherence length. The experimental results are in excellent agreement with simulations that even indicate potential for further optimizations.

High-power, low-divergence, linear array of quasi-diffraction-limited beams supplied by tapered diodes.

We describe for the first time to our knowledge the performance for a linear array of tapered laser diodes with both fast- and slow-axis collimation using a microlens for fast-axis collimation and a laser-written phase plate for slow-axis collimation and correction of the residual fast-axis errors from lens aberrations, thermal lensing, astigmatism, pointing errors, and other wavefront distortions. The phase plate leads to M(2) factor reductions of 1.5 for the lensed array following the fast axis and 2.6 for the whole bar following the slow axis.