Novel Laser Systems
The understanding of the ultra-fast dynamics of semiconductor materials is extremely important in order to develop and improve next generation photonic sources. We investigate the intrinsic dynamical properties of a wide variety of materials including InAs/GaAs based QDs and GaInNAs/GaAs QWs emitting near 1.3 and 1.55 micron. This work allows revealing the underlying physical mechanisms, which govern dynamics of photonics sources and allow the improvement of fundamental characteristics of devices that will be used in a wide range of applications including telecommunications, bio-photonics and imaging. It also provides valuable information for the existing research groups based in both CIT and Tyndall National Institute and enhances the collaboration with other institutions.
These dynamical properties are important because they affect the modulation and saturation behaviour of the semiconductor devices. Hence, knowledge of the relaxation of the complete optical gain and index after a sub-picosecond light pulse is essential to assess the potential of current semiconductor materials for photonic systems applications.
We use pump-probe spectroscopy technique, which provides direct, time domain, measurement of gain and refractive index nonlinearities in optical waveguides with sub-picosecond resolution.
Pump probe spectroscopy experiments use a heterodyne detection method, which allows us to distinguish between co-polarized pump and probe beams. Since the technique is independent on polarization, the polarization anisotropy present in semiconductor amplifiers, where transition probabilities are governed by polarization selection rules, may be studied. It also provides gain and refractive index recovery measurements under true operating conditions, which is essential for the assessment of the optical devices to be used in ultrafast signal processing.
In this experiment, a high intensity short pump pulse is used to excite the investigated sample by modifying various carrier populations. It leads to the changing of the optical properties of the sample, which can be measured as a change of the transmission (gain), or phase (refractive index) of the low intensity short probe pulse.
In order to follow the gain and refractive index recoveries, the delay between pump and probe pulses is changed and the time resolution of the measurement is determined mainly by duration of the pulses. Both pump and probe pulses follow the reference pulse, which is used for detection purposes.
For more details on the pump-probe technique, refer to the Techniques page.
Photonic Crystals are composed of a periodic, strong variation of the refractive index. For wavelengths, roughly twice the periodicity of the photonic crystal structure, strong diffraction occurs that gives rise to what are known as photonic bandgaps- wavelengths at which the propagation of light is forbidden. The photonic bandgap is a powerful tool to control light on the wavelength scale and inhibit spontaneous emission and realise high-reflecting omni-directional mirrors amongst many other phenomena. Photonic crystals can be fabricated from a range of materials- e.g. semiconductors such as silicon or gallium arsenide or from di-electrics such as silicon nitride, using semiconductor mass fabrication tools.
Hybrid photonic crystal lasers are comprised of an indium phosphide gain chip coupled to a silicon based photonic crystal (PhC) chip. The PhC is typically operated as a resonant mirror that wavelength selective feedback into the gain chip to setup the laser cavity (resonant feedback). By appropriately designing the waveguide and the PhC cavity, the reflectivity of the coupled system can be controlled and in this way, the characteristics of the laser can be defined. The hybrid laser configuration decouples the optimisation of the gain material from the task of wavelength selection, giving advantages in terms of performance and of cost. The host of functions that can integrated into the laser cavity is giving rise to a new family on on-chip lasers.
Swept Source Lasers
Swept Source Lasers are a key components of the modern spectral-domain Optical Coherence Tomography (OCT) systems. Widespread implementation of OCT in various areas such as medical diagnostics and industrial inspection would not be possible without constant improvement of those sources. Currently available swept source lasers are typically based on complex rapidly tunable wavelength selective components in their cavity, such as Micro-Ectro-Mechanical System (MEMS) filters. Even though MEMS based tunable lasers offer high scanning speed and long coherence length, the use of mechanical elements leads to non-linearity, hysteresis and limit the OCT image quality. It also affects the cost of the final device. Further progress of OCT technology requires swept source lasers working at specific, application dependent wavelength, which are in addition compact and low-cost and don’t suffer from wavelength sweep non-linarites. CAPPA works on development of the low cost, compact, semiconductor swept source lasers based on slots etched along the ridge. That type of laser does not require any complex regrowth and expensive E-beam lithography steps during fabrication. Such devices have been already implemented in OCT systems showing very good performance in terms of image quality, scanning range and axial resolution.
Mid Infrared Lasers
Photonic sensors are key elements in a wide range of applications: security, safety, space, defence, healthcare, multimedia, scientific instruments and others. They provide the means to evaluate samples in-situ and can detect and quantify chemical substances, without the need to chemically prepare or damage the sample in question. Such sensors therefore avoid the use of consumables required by many chemical and electrical sensors, making optical sensors ideal for Process Analytic Technologies, environmental monitoring, breath sensing, and the pharmaceutical and petrochemical industries. Practical sensing systems require CW emission in single spectral mode. Nowadays, only GaSb-based interband-cascade lasers (ICL) allow covering the 3 – 5 µm spectral range with CW emission above RT and power consumption compatible with the development of battery powered photonic sensors. This wavelength range is particularly important since it includes the fundamental absorption lines of various gases of high interest from an industrial and environment point of view (methane, ethane) but also for daily life (nitric oxide, carbon oxide, formaldehyde). Several types of ICLs based on loss coupling have recently demonstrated single mode emission. Still, these approaches are expensive because they require extensive e-beam lithography. CAPPA works on innovative approaches that have not been implemented with ICLs yet such as the slotted laser.