A novel laser diode wavelength stabilisation technique for use in high resolution spectroscopy

Tuneable diode laser absorption spectroscopy (TDLAS) based gas sensors are widely used for trace gas detection for their high selectivity and sensitivity. The laser source used in TDLAS requires a narrow line width in the order of 10s of MHz, with a wavelength stability multiple orders lower than the molecular absorption line width, which is, for example, 4.1GHz (38pm) for an air broadened methane line. TDLAS requires the use of a laser diode with a long term wavelength stability of better than 10% of the absorption line width of the target gas species. The wavelength stability of the laser is highly temperature dependent as the wavelength increases with increasing temperature. Therefore, control of the temperature of the laser diode is vital for stabilising the laser emission wavelength. In this thesis, a novel method has been proposed to measure and stabilise the temperature of a laser diode. The laser diode emission wavelength was stabilised by using its measured junction voltage in a control feedback loop. In order to determine the junction voltage, a series resistance correction term was identified, which was the novel part of this wavelength stabilisation technique. The laser diode junction and forward voltages were calculated from the forward voltage drop of the laser diode at measured at various operating temperatures. The laser diode series resistance was measured dynamically and was subtracted from the forward voltage to calculate the junction voltage. Both the forward voltage and series resistances were found to be temperature dependent. This method was investigated for its short term (~ 5minute) and long term (~ 1 hour) wavelength stability and was compared with other available methods. The laser diode wavelength stability attained using this method has been also investigated at various ambient temperatures (10-40 °C). ...[cont.

Semiconductor Laser Dynamics

This is a collection of 18 papers, two of which are reviews and seven are invited feature papers, that together form the Photonics Special Issue “Semiconductor Laser Dynamics: Fundamentals and Applications”, published in 2020. This collection is edited by Daan Lenstra, an internationally recognized specialist in the field for 40 years

High efficiency and high frequency resonant tunneling diode sources

Terahertz (THz) technology has been generating a lot of interest due to the numerous potential applications for systems working in this previously unexplored frequency range. THz radiation has unique properties suited for high capacity communication systems and non-invasive, non-ionizing properties that when coupled with a fairly good spatial resolution are unparalleled in its sensing capabilities for use in biomedical, industrial and security fields. However, in order to achieve this potential, effective and efficient ways of generating THz radiation are required. Devices which exhibit negative differential resistance (NDR) in their current-voltage (I – V) characteristics can be used for the generation of these radio frequency (RF) signals. Among them, the resonant tunnelling diode (RTD) is considered to be one of the most promising solid-state sources for millimeter and submillimeter wave radiation, which can operate at room temperature. However, the main limitations of RTD oscillators are producing high output power and increasing the DC-to-RF conversion efficiency. Although oscillation frequencies of up to 1.98 THz have been already reported, the output power is in the range of micro-Watts and conversion efficiencies are under 1 %. This thesis describes the systematic work done on the design, fabrication, and characterization of RTD-based oscillators in monolithic microwave/millimeter-wave integrated circuits (MMIC) that can produce high output power and have a high conversion efficiency at the same time. At the device level, parasitic oscillations caused by the biasing line inductance when the diode is biased in the NDR region prevents accurate characterization and compromises the maximum RF power output. In order to stabilise the NDR devices, a common method is the use of a suitable resistor connected across the device, to make the differential resistance in the NDR region positive. However, this approach severely hinders the diode’s performance in terms of DC-to-RF conversion efficiency. In this work, a new DC bias decoupling circuit topology has been developed to enable accurate, direct measurements of the device’s NDR characteristic and when implemented in an oscillator design provides over a 10-fold improvement in DC-to-RF conversion efficiency. The proposed method can be adapted for higher frequency and higher power devices and could have a major impact with regards to the adoption of RTD technology, especially for portable devices where power consumption must be taken into consideration. RF and DC characterization of the device were used in the realization on an accurate large-signal model of the RTD. S-parameter measurements were used to determine an accurate small-signal model for the device’s capacitance and inductance, while the extracted DC characteristics where used to replicate the I-V characteristics. The model is able to replicate the non-stable behavior of RTD devices when biased in the NDR region and the RF characteristics seen in oscillator circuits. It is expected that the developed model will serve in future optimization processes of RTD devices in millimeter and submillimeter wave applications. Finally, a wireless data transmission link operating in the Ka-band (26.5 GHz – – 40 GHz) using two RTDs operating as a transmitter and receiver is presented in this thesis. Wireless error-free data transfer of up to 2 gigabits per second (Gbit/s) was achieved at a transmission distance of 15 cm. In summary, this work makes important contributions to the accurate characterization, and modeling of RTDs and demonstrates the feasibility of this technology for use in future portable wireless communication systems and imaging setups

Optical Gas Sensing: Media, Mechanisms and Applications

Optical gas sensing is one of the fastest developing research areas in laser spectroscopy. Continuous development of new coherent light sources operating especially in the Mid-IR spectral band (QCL—Quantum Cascade Lasers, ICL—Interband Cascade Lasers, OPO—Optical Parametric Oscillator, DFG—Difference Frequency Generation, optical frequency combs, etc.) stimulates new, sophisticated methods and technological solutions in this area. The development of clever techniques in gas detection based on new mechanisms of sensing (photoacoustic, photothermal, dispersion, etc.) supported by advanced applied electronics and huge progress in signal processing allows us to introduce more sensitive, broader-band and miniaturized optical sensors. Additionally, the substantial development of fast and sensitive photodetectors in MIR and FIR is of great support to progress in gas sensing. Recent material and technological progress in the development of hollow-core optical fibers allowing low-loss transmission of light in both Near- and Mid-IR has opened a new route for obtaining the low-volume, long optical paths that are so strongly required in laser-based gas sensors, leading to the development of a novel branch of laser-based gas detectors. This Special Issue summarizes the most recent progress in the development of optical sensors utilizing novel materials and laser-based gas sensing techniques

Yb-based femtosecond oscillators for high-power amplification

Motivated by the industrial requirement for multi-100-W sub-ps pulses, this thesis describes the development of a high-average-power master-oscillator power-amplifier (MOPA) based on seeding an Yb:YAG planar waveguide amplifier using an Yb-based modelocked oscillator operating at 1030 nm. The scope of the research presented includes both the development of the seed oscillators and of the high-power amplifier. Two end pumped Yb:KYW oscillators were demonstrated, one with Brewster-Brewster crystal geometry and another with a novel plane-Brewster crystal, permitting a simple pumping arrangement and which provided superior efficiency than Brewster-Brewster geometry. In both oscillators, one or more Gires-Tournois interferometer (GTI) mirrors were used in the cavity to compensate for the large amount of positive dispersion from the crystal. Both systems were modelocked using semiconductor saturable absorber mirrors. The oscillators demonstrated here produced amongst the highest average powers reported from end-pumped systems to date, generating up to 4.5 W in the nearinfra-red region in the form of 500-fs pulses, with a repetition frequency of 53 MHz. The study of oscillator performance was extended to include a comparison between Yb:KYW and Yb:YAG lasers constructed in similar configurations, both based on plane-Brewster crystal geometries and dispersion compensated using GTI mirrors. The Yb:YAG system provided 700-fs pulses, compared to 500-fs pulses obtained using Yb:KYW, with the average power produced being 2.88 W for Yb:YAG, and 2.42 W for Yb:KYW, despite significantly better CW performance being observed with Yb:YAG. Due to their near-infrared wavelengths, high average powers and sub-ps pulse durations, both systems showed potential as seed lasers for high-power Yb:YAG amplifiers. A MOPA system was developed around an Yb:YAG planar waveguide amplifier seeded by the Yb:KYW femtosecond laser based on a Brewster-Brewster crystal geometry and operating at 53 MHz repetition frequency. With single-sided pumping and five passes of the gain waveguide, the Yb:YAG amplifier provided 700-fs pulses with average powers of 50 W at 1030 nm. With the extension to double-sided pumping and the use of toroidal mirrors to achieve seven passes, the amplifier produced 780-fs pulses with average powers of 255 W. A numerical simulation of the amplifier identified gain narrowing as the dominant pulse-shaping mechanis

High pulse energy near-infrared ultrafast optical parametric oscillators

A source-demand in the near- and mid-IR wavelength spectrum exists for various applications such as waveguide inscription, multiphoton imaging, and nonlinear spectroscopy. All of the applications seek for higher repetitions rates for faster processing speed, better signal to noise ratios or to improve the results for applications like laser waveguide inscription. This is in contrast to the high pulse energies, required to drive the nonlinear processes involved with these applications. Available systems are either based on low-energy, high-repetition-rate optical parametric oscillators or high-energy, low-repetition-rate optical parametric amplifiers. In this thesis a sources was developed that can bridge the wide gap between these two extremes, providing sufficient energy to drive nonlinear processes, with repetition rates in the MHz domain. This was achieved by introducing three techniques previously employed for energy scaling in laser cavities. Firstly an exchange from the conventionally used Ti:sapphire pump to a commercial high power Yb:fibre laser system readily scaled the usable pump energy. This was combined with a technique known as cavity-length extension, which allows a lowering of the cavity roundtrip time offering the build-up of pulses with increased energy. In a final stage, cavity-dumping on basis of an acousto-optic modulator was introduced into the a redesigned cavity. The combination of these three techniques, novel to synchronously pumped optical parametric oscillators, enabled the extraction of record-high pulse energies and peak power

High Energy, High Average Power, Picosecond Laser Systems To Drive Few-cycle Opcpa

The invention of chirped-pulse amplification (CPA) in 1985 led to a tremendous increase in obtainable laser pulse peak intensities. Since then, several table-top, Ti:sapphire-based CPA systems exceeding the 100 TW-level with more than 10 W average power have been developed and several systems are now commercially available. Over the last decade, the complementary technology of optical parametric chirped-pulse amplification (OPCPA) has improved in its performance to a competitive level. OPCPA allows direct amplification of an almost-octave spanning bandwidth supporting few-cycle pulse durations at center wavelengths ranging from the visible to the mid-IR. The current record in peak power from a table-top OPCPA is 16 TW and the current record average power is 22 W. High energy, few-cycle pulses with stabilized carrierenvelope phase (CEP) are desired for applications such as high-harmonic generation (HHG) enabling attoscience and the generation keV-photon bursts. This dissertation conceptually, numerically and experimentally describes essential aspects of few-cycle OPCPA, and the associated pump beam generation. The main part of the conducted research was directed towards the few-cycle OPCPA facility developed in the Laser Plasma Laboratory at CREOL (University of Central Florida, USA) termed HERACLES. This facility was designed to generate few-cycle pulses in the visible with mJ-level pulse energy, W-level average power and more than 100 GW peak power. Major parts of the implementation of the HERACLES facility are presented. The pump generation beam of the HERACLES system has been improved in terms of pulse energy, average power and stability over the last years. It is based on diode-pumped, solid-state amplifiers with picosecond duration and experimental investigations are presented in detail. A iii robust system has been implemented producing mJ-level pulse energies with ~100 ps pulse duration at kHz repetition rates. Scaling of this system to high power (\u3e30 W) and high peak power (50-MW-level) as well as ultra-high pulse energy (\u3e160 mJ) is presented. The latter investigation resulted in the design of an ultra-high energy system for OPCPA pumping. Following this, a new OPCPA facility was designed termed PhaSTHEUS, which is anticipated to reach ultra-high intensities. Another research effort was conducted at CELIA (Univeristé de Bordeaux 1, France) and aimed towards a previously unexplored operational regime of OPCPA with ultra-high repetition rates (10 MHz) and high average power. A supercontinuum seed beam generation has been established with an output ranging from 1.3 to 1.9 µm and few ps duration. The pump beam generation has been implemented based on rod-type fiber amplifiers producing more than 37 W average power and 370 kW peak power. The utility of this system as an OPCPA pump laser is presented along with the OPA design. The discussed systems operate in radically different regimes in terms of peak power, average power, and repetition rate. The anticipated OPCPA systems with few-cycle duration enable a wide range of novel experimental studies in attoscience, ultrafast materials processing, filamentation, LIBS and coherent contro