PhD Engineering: High brightness laser diode with 2D optical beam steering

The School of Engineering of the University of Glasgow is seeking a highly motivated graduate to undertake an exciting 3.5-year PhD project entitled ‘High brightness laser diode with 2D optical beam steering’.

High power, single-frequency and quasi-single-spatial-mode semiconductor lasers operating at wavelengths around 1.55 µm are essential components for many applications such as Raman pumps for fibre communication systems, spectroscopy, remote sensing, free-space communications, eye-safe laser-based radar (LIDAR), and wavelength conversion in nonlinear materials. Recently techniques have been developed based on seeding arrays of semiconductor optical amplifiers (SOAs) from a single laser, to generate output beams that can be combined coherently. However, the coherent beam combination (CBC) diodes reported before have had shortcomings, such as overall system size and cost, and require complex optical architectures.

We will use a simple, scalable monolithically integrated approach comprising a tuneable 1.55 µm distributed feedback (DFB) laser feeding several stages of multi-mode interference couplers (MMIs) and semiconductor optical amplifiers (SOAs) to deliver a high power beam with a low divergence angle with a clear coherent interference far-field pattern (FFP).
The Brightness of a laser beam is defined as
B=P/(AΩ)
where P is the optical output power, A the emitting area and Ω the solid angle into which the power is emitted. It is impossible to increase the Brightness by combining beams from separate sources using passive optical components, but optical amplifiers do increase brightness. At high powers, the light in semiconductor lasers and amplifiers degrades into multi-mode beams, and single mode operation can only be sustained to a few Watts at best. However, if we use a parallel array of amplifiers and feed them from a common source, the light in the amplifiers will be mutually coherent. The individual beams can now be combined into a single beam using passive optical components, made possible because of the mutual coherence of the light in the amplifiers. The Brightness will therefore be increased.

The phase of the light at output of an optical amplifier can be controlled by changing the drive current to the amplifier. We can therefore control the direction of the main beam in the far-field in one dimension. To scan the beam in a second direction, we will tune the wavelength of the laser diode and couple the light out of the array using a diffraction grating. Two-dimensional beam-steering can therefore be achieved where the axis parallel to the waveguides is tuned with wavelength and the axis perpendicular to the waveguides is tuned by phase-tuning the individual channel.

The final innovative and important step is to stabilize of the output power and particularly the phase of the output from the SOA array. This motivates the development of electronic control system to maintain the required relative phase in each amplifier and so stabilize the output laser power into a single lobed beam. The control system is therefore required to change iteratively the set of drive currents to the amplifier stages to maximise output power. The amplifier gain is largely set by a fixed DC bias, with small changes imposed by the control system. The effective controlled gain change is due to a phase modulation of the amplifier gain which results in a periodic variation in total output power for each driven amplifier due to interference control of the mode structure in the combiner. We will implement simple algorithms to give the required control.

The aims of this project:
- Fibre coupled power of >500 mW at 1550 nm from a DFB laser with ultra-narrow linewidth (<100 kHz) and high beam quality (i.e. beam quality factor M2 close to unity)
- InP photonic integrated circuit for either 1D or 2D beam steering as a eye-safe laser based radar
- Develop electronic control system to stabilize the output power and tune the phase of the output SOA arrays