303 research outputs found
Towards Hollow-Core-Fiber Delivery of Broadband Mid-Infrared Light for Remote Spectroscopy
No full textWe describe progress towards fiber-delivered broadband mid-IR light for multi-species spectroscopy in inaccessible environments. Water and HCl are resolved after propagating 3-μm light through five meters of hollow-core fiber, illustrating the technique's potential.</p
Hollow-Core-Fibre-Delivered Attenuated Total Internal Reflection Heterodyne Spectroscopy
No full textAttenuated total internal reflection Fourier-transform infrared (ATR-FTIR) spectroscopy is a commonly used modality for measuring the spectra of highly absorbing liquids and solids. Previously, we combined broadband OPO-based mid-IR spectroscopy [1] with the use of a non-resonant hollow-core fibre (HCF) [2] to demonstrate heterodyne detection in a way that achieved common mode rejection of absorption features unique to the fibre transmission path [3]. We now extend this approach to enable simple, fibre-delivered ATR spectroscopy
Continuous ultraviolet to blue-green astrocomb
No full textBroadband UV-green generation is achieved from a 1-GHz Ti:sapphire frequency comb using a Zn-indiffused, grating-engineered MgO:PPLN ridge waveguide. Etalon-filtering produces a 30 GHz astrocomb from 390–480 nm, with modes resolvable by an Echelle-prism spectrograph
Ti:sapphire frequency combs for dual-comb distance metrology
No full textThis thesis presents research developing Ti:sapphire lasers for applications in precision
absolute distance metrology. Dual Ti:sapphire frequency combs were developed and
characterised, each typically generating 64 fs pulses at a wavelength of 780 nm and a
pulse repetition rate of 513 MHz. Electronic phase stabilisation using f-to-2f
interferometry and direct pump power modulation achieved phase slips of 234 mrad
(124 mrad) for the carrier-envelope offset frequencies of the probe (local-oscillator) comb
in observation times of 1 second. The comb mode spacings were stabilised with cavity
length feedback, achieving a phase noise of 4 mrad in one second for each laser.
The developed Ti:sapphire dual-comb system was evaluated for distance
metrology. When both combs were fully locked, absolute distance metrology was
demonstrated for distances of up to 1.6 m, corresponding to ambiguity ranges of up to
order 6. Time-of-flight precision of < λ/4 was achieved in an averaging time of 1 s,
allowing handover to interferometric precision, which achieved a precision of 2 nm after
an averaging time of 4 s. Cross-calibration of distance measurements using a
100-nm-precision delay stage allowed a direct measurement of the group velocity of air
to an uncertainty (0.0026%) consistent with values from established atmospheric
dispersion models. Modifying the repetition-rates of probe and local-oscillator combs was
shown to potentially extend the effective ambiguity range up to 12.385 km.
Methods of reducing the cost and complexity of a Ti:sapphire comb were explored
by constructing a Ti:sapphire frequency comb directly pumped by 462 nm and 520 nm
laser diodes. The laser was modelocked using a commercial saturable absorber and
generated 90 mW average power. Using piezoelectric feedback to the laser cavity length,
and current modulation of one of the pump diodes, the laser was fully phase-stabilised to
achieve the first example of a directly diode-pumped Ti:sapphire laser with fully
stabilised repetition rate and carrier envelope offset frequencies. This comb achieved a
phase slip of 860 mrad for the 10 MHz carrier envelope offset frequency and 54 mrad for
the 79 MHz repetition frequency, each over 1 second. Pulses with durations of 54 fs were
generated at a central wavelength of 803 nm. Single walled carbon nanotubes were
explored as potential saturable absorbers. A device was fabricated using spin coating and
was shown to achieve modelocking at pump powers as low as 545 mW
Environmentally stable, near-infrared, mode-locked fibre lasers for industrial applications
No full textMode-locked laser systems that show robust operation in the presence of environmental instabilities are desirable for numerous applications in the defence sector
and beyond. In this thesis I present the development of two such systems based on
fibre-laser architecture and developed for applications in radar and lidar.
An environmentally stable, 800 nm, laser operating at 7.2 GHz was developed
with a timing jitter of 26 fs (integrated from 10 Hz to 10 MHz) for photonic assisted analogue to digital conversion. A novel method of active stabilisation of the
Mach-Zehnder Modulator was demonstrated, which allowed turn-key operation and
maintained low jitter mode-locking for > 50 hours. When combined with stabilisation of the cavity length, timing jitters of < 42 fs (10 Hz to 10 MHz) can be achieved
for a temperature range of 0 oC to 50 oC.
A 1.5 µm, passively mode-locked erbium fibre laser was developed for non-line-of-sight lidar applications. The laser had a native repetition rate of 22.6 MHz and an
average power of 0.6 mW. The laser output was pulse-picked and amplified to provide
a repetition rate of 11.3 MHz and an average power of 354 mW. An innovative
strategy was demonstrated to mitigate Q-switching instabilities. The laser was
validated to be robust against vibration, shock and temperature, as required for
its intended application
Femtosecond optical parametric oscillator frequency combs for coherent pulse synthesis
No full textCoherent pulse synthesis takes as its objective the piecewise assembly of a sequence of identical broadband pulses from two or more mutually-coherent sequences of narrowband pulses. The requirements for pulse synthesis are that the parent pulses share the same repetition frequency, are phase coherent and have low mutual timing jitter over the required observation time.
The work carried out in this thesis explored the requirements for broadband coherent pulse synthesis between the multiple visible outputs of a synchronously pumped femtosecond optical parametric oscillator. A femtosecond Ti:sapphire laser was characterised and used to pump a PPKTP-based OPO that produced a number of second-harmonic and sum-frequency mixing outputs across the visible region. Using a novel lock-to-zero CEO stabilisation technique, broadband phase coherence was established between all the pulses on the optical bench, producing the broadest zero-offset frequency comb to date. Employing a common optical path for all the pulses provided common-mode rejection of noise, ensuring less than 150 attoseconds of timing jitter between the pulses over a 1 second observation window. The parent pulses were compressed and their relative delays altered in a quasi-common path prism delay line, allowing pulse synthesis at a desired reference plane
Development and characterisation of a near-infrared femtosecond optical parametric oscillator frequency comb
No full textThis thesis describes a 280 MHz MgO:PPLN-based optical parametric oscillator (OPO)
synchronously pumped by a 50 fs Ti:sapphire laser to produce ultrafast pulses in the
near-infrared. The OPO tuned over a wavelength range from 1450 - 1550 nm and
1624 - 1750 nm for the signal and idler respectively. The carrier-envelope-offset (CEO)
frequency of the signal pulses was stabilised to a 10 MHz reference frequency without
f-2f self-referencing, with an RMS phase variation of 0.56 rad over an observation time
of 1 second. The relative intensity noise was measured for the CEO-stabilised OPO
over an observation time of 64 seconds as 0.04%.
The repetition frequency of the OPO was stabilised to 280 MHz using a frequency
synthesiser at the eighth harmonic (2.24 GHz). This locking loop had an RMS phase
variation of 0.98 mrad over a 1 second observation time. The CEO- and repetition
frequencies were then locked simultaneously to a synthesiser referenced to a
Rb-disciplined source, to generate a fully stabilised 1.5 μm frequency comb with an
absolute uncertainty in comb mode position of 110 Hz. The upper limit for the
fractional instability for a comb mode at 200 THz was found to be 2 x 10-11, limited by
the stability of the Rb reference.
A five-fold increase in comb mode spacing to 1.4 GHz was demonstrated with the
stabilised frequency comb. This was achieved using a passive filter cavity, stabilised to
a transmission peak via dither locking. The FWHM bandwidth of the optical spectrum
for the filtered frequency comb was reduced to 8 nm, however no increase in comb
linewidth was observed.
An additional experiment was carried out where an external cavity diode laser was
frequency-stabilised to a saturated absorption peak in Rb at 780.2 nm using dither
locking, providing an optical frequency reference for future OPO frequency combs
High pulse energy near-infrared ultrafast optical parametric oscillators
No full textA 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
Nonlinear microscopy for failure analysis of CMOS integrated circuits in the vectorial focusing regime.
No full textThis thesis focuses on the development of techniques for enhancing the spatial resolution and localisation precision in the sub-surface microscopy for failure analysis in semiconductor integrated circuits (ICs). Highest spatial resolutions are obtained by implementing solid immersion lenses (SIL), which provide unsurpassed numerical aperture (NA) for sub-surface microscopy. These high NA conditions mean that scalar diffraction theory is no longer valid and a vectorial focusing description should be applied to accurately describe the focal plane electric field distribution.
Vectorial theory predicts that under high NA conditions a linearly polarised (LP) light focuses to a spot that is extended along the electric field vector, but radially polarised (RP) light is predicted to form a circular spot whose diameter equals the narrower dimension obtained with linear polarisation. By implementing a novel liquid-crystal (LC) radial polarisation converter (RPC) this effect was studied for both two-photon optical-beam-induced current (TOBIC) microscopy and two-photon laser assisted device alteration (2pLADA) techniques, showing a resolution and localisation improvement using the RP beam. By comparing images of the same structural features obtained using linear, circular and radial polarisations imaging and localisation resolutions both approaching 100 nm were demonstrated. The obtained experimental results were in good agreement with modelling and were consistent with theoretically predicted behaviour. Certain artefacts were observed under radial polarisation, which were thought to result from the extended depth of focus and the significant longitudinal field component. In any application these effects must be considered alongside the benefits of the symmetric field distribution in the focal plane.
While SIL sub-surface microscopy offers unmatched spatial resolutions, it is prone to being severely degraded by aberrations arising from inaccurate dimensions of the SIL, imprecise substrate thickness or imperfect contact between SIL and substrate. It is in this context that techniques to identify and even mitigate aberrations in the system are important. A simple approach is demonstrated for revealing the presence of chromatic and spherical aberrations by measuring the two-photon autocorrelation of the pulses at the focal plane inside the sample. In the case of aberration free imaging, it was shown both theoretically and experimentally that the planes of the maximum autocorrelation amplitude and shortest pulse duration always coincide. Therefore, the optics of the imaging system can be first adjusted to obtain the minimum autocorrelation duration and then the wavefront of incident light modified to maximise the autocorrelation intensity, iterating this procedure until the positions of minimum pulse duration and maximum autocorrelation amplitude coincide
Hollow-Core-Fiber Delivery of Broadband Mid-Infrared Light for Remote Spectroscopy
No full textHigh-resolution multi-species spectroscopy is achieved by delivering broadband 3–4-μm mid-infrared light through a 4.5-meter-long silica-based hollow-core optical fiber. Absorptions from H37Cl, H35Cl, H2O and CH4 present in the gas within the fiber core are observed, and the corresponding gas concentrations are obtained to 5-ppb precision using a high-resolution Fourier-transform spectrometer and a full-spectrum multi-species fitting algorithm. We show that by fully fitting the narrow absorption features of these light molecules their contributions can be nulled, enabling further spectroscopy of C3H6O and C3H8O contained in a Herriott cell after the fiber. As a demonstration of the potential to extend fiber-delivered broadband mid-infrared spectroscopy to significant distances, we present a high-resolution characterization of the transmission of a 63-meter length of hollow-core fiber, fully fitting the input and output spectra to obtain the intra-fiber gas concentrations. We show that, despite the fiber not having been purged, useful spectroscopic windows are still preserved which have the potential to enable hydrocarbon spectroscopy at the distal end of fibers with lengths of tens or even hundreds of meters
- …
