1,352 research outputs found
Retrieval of phase relation and emission profile of quantum cascade laser frequency combs
The major development recently undergone by quantum cascade lasers has
effectively extended frequency comb emission to longer-wavelength spectral
regions, i.e. the mid and far infrared. Unlike classical pulsed frequency
combs, their mode-locking mechanism relies on four-wave mixing nonlinear
processes, with a temporal intensity profile different from conventional
short-pulses trains. Measuring the absolute phase pattern of the modes in these
combs enables a thorough characterization of the onset of mode-locking in
absence of short-pulses emission, as well as of the coherence properties. Here,
by combining dual-comb multi-heterodyne detection with Fourier-transform
analysis, we show how to simultaneously acquire and monitor over a wide range
of timescales the phase pattern of a generic frequency comb. The technique is
applied to characterize a mid-infrared and a terahertz quantum cascade laser
frequency comb, conclusively proving the high degree of coherence and the
remarkable long-term stability of these sources. Moreover, the technique allows
also the reconstruction of electric field, intensity profile and instantaneous
frequency of the emission.Comment: 20 pages. Submitted to Nature Photonic
High-speed Modelocked Semiconductor Lasers And Applications In Coherent Photonic Systems
1.55-µm high-speed modelocked semiconductor lasers are theoretically and experimentally studied for various coherent photonic system applications. The modelocked semiconductor lasers (MSLs) are designed with high-speed (\u3e5 GHz) external cavity configurations utilizing monolithic two-section curved semiconductor optical amplifiers. By exploiting the saturable absorber section of the monolithic device, passive or hybrid mode-locking techniques are used to generate short optical pulses with broadband optical frequency combs. Laser frequency stability is improved by applying the Pound-Drever-Hall (PDH) frequency stabilization technique to the MSLs. The improved laser performance after the frequency stabilization (a frequency drifting of less than 350 MHz), is extensively studied with respect to the laser linewidth (~ 3 MHz), the relative intensity noise (RIN) (\u3c -150 dB/Hz), as well as the modal RIN (~ 3 dB reduction). MSL to MSL, and tunable laser to MSL synchronization is demonstrated by using a dual-mode injection technique and a modulation sideband injection technique, respectively. Dynamic locking behavior and locking bandwidth are experimentally and theoretically studied. Stable laser synchronization between two MSLs is demonstrated with an injection seed power on the order of a few microwatt. Several coherent heterodyne detections based on the synchronized MSL systems are demonstrated for applications in microwave photonic links and ultra-dense wavelength division multiplexing (UD-WDM) system. In addition, efficient coherent homodyne balanced receivers based on synchronized MSLs are developed and demonstrated for a spectrally phase-encoded optical CDMA (SPE-OCDMA) system
Baseband Radio over Fiber Aided Millimeter-Wave Distributed Antenna for Optical/Wireless Integration
A Baseband Radio Over Fiber (BROF) architecture is proposed, where upto four Radio Frequency (RF) carriers can be generated, while using the heterodyne photo-detection of only two optical signals. This proposed BROF architecture has a star-like structure and it is composed of six Radio Access Units (RAUs), where data is transmitted from the Central Unit (CU) to the Base Station (BS) and from the BS to the RAU over a distance of 20 Km and 0.3 Km, respectively, at a rate of 768 Mbps. The performance of the system supporting four carrier frequencies drops by at most 1dB, at a BER of 10-9, compared to conventional heterodyne photo-detection
External Cavity Mode-locked Semiconductor Lasers For The Generation Of Ultra-low Noise Multi-gigahertz Frequency Combs And Applications In Multi-heterodyne Detection Of Arbitrary Optical Waveforms
The construction and characterization of ultra-low noise semiconductor-based mode-locked lasers as frequency comb sources with multi-gigahertz combline-to-combline spacing is studied in this dissertation. Several different systems were built and characterized. The first of these systems includes a novel mode-locking mechanism based on phase modulation and periodic spectral filtering. This mode-locked laser design uses the same intra-cavity elements for both mode-locking and frequency stabilization to an intra-cavity, 1,000 Finesse, Fabry-Pérot Etalon (FPE). On a separate effort, a mode-locked laser based on a Slab-Coupled Optical Waveguide Amplifier (SCOWA) was built. This system generates a pulse-train with residual timing jitter o
Photonic Filtering for Applications in Microwave Generation and Metrology
This work uses the photonic filtering properties of Fabry-Perot etalons to show improvements in the electrical signals created upon photodetection of the optical signal. First, a method of delay measurement is described which uses multi-heterodyne detection to find correlations in white light signals at 20 km of delay to sub millimeter resolution. By filtering incoming white light with a Fabry-Perot etalon, the pseudo periodic signal is suitable for measurement by combining and photodetecting it with an optical frequency comb. In this way, optical data from a large bandwidth can be downconverted and sampled on low frequency electronics. Second, a high finesse etalon is used as a photonic filter inside an optoelectronic oscillator (OEO). The etalon\u27s narrow filter function allows the OEO loop length to be extremely long for a high oscillator quality factor while still suppressing unwanted modes below the noise floor. The periodic nature of the etalon allows it to be used to generate a wide range of microwave and millimeter wave tones without degradation of the RF signal
Optical sampling and metrology using a soliton-effect compression pulse source
A low jitter optical pulse source for applications including optical sampling and optical
metrology was modelled and then experimentally implemented using photonic
components. Dispersion and non-linear fibre effects were utilised to compress a periodic
optical waveform to generate pulses of the order of 10 picoseconds duration, via
soliton-effect compression. Attractive features of this pulse source include electronically
tuneable repetition rates greater than 1.5 GHz, ultra-short pulse duration (10-15 ps), and
low timing jitter as measured by both harmonic analysis and single-sideband (SSB)
phase noise measurements. The experimental implementation of the modelled
compression scheme is discussed, including the successful removal of stimulated
Brillouin scattering (SBS) through linewidth broadening by injection dithering or phase
modulation. Timing jitter analysis identifies many unwanted artefacts generated by the
SBS suppression methods, hence an experimental arrangement is devised (and was
subsequently patented) which ensures that there are no phase modulation spikes present
on the SSB phase noise spectrum over the offset range of interest for optical sampling
applications, 10Hz-Nyquist. It is believed that this is the first detailed timing jitter study
of a soliton-effect compression scheme. The soliton-effect compression pulses are then
used to perform what is believed to be the first demonstration of optical sampling using
this type of pulse source.
The pulse source was also optimised for use in a novel optical metrology (range
finding) system, which is being developed and patented under European Space Agency
funding as an enabling technology for formation flying satellite missions. This new
approach to optical metrology, known as Scanning Interferometric Pulse Overlap
Detection (SIPOD), is based on scanning the optical pulse repetition rate to find the
specific frequencies which allow the return pulses from the outlying satellite, i.e. the
measurement arm, to overlap exactly with a reference pulse set on the hub satellite. By
superimposing a low frequency phase modulation onto the optical pulse train, it is
possible to detect the pulse overlap condition using conventional heterodyne detection.
By rapidly scanning the pulse repetition rate to find two frequencies which provide the
overlapping pulse condition, high precision optical pulses can be used to provide high
resolution unambiguous range information, using only relatively simple electronic detection circuitry. SIPOD’s maximum longitudinal range measurement is limited only
by the coherence length of the laser, which can be many tens of kilometres. Range
measurements have been made to better than 10 microns resolution over extended
duration trial periods, at measurement update rates of up to 470 Hz. This system is
currently scheduled to fly on ESA’s PROBA-3 mission in 2012 to measure the intersatellite
spacing for a two satellite coronagraph instrument.
In summary, this thesis is believed to present three novel areas of research: the first
detailed jitter characterisation of a soliton-effect compression source, the first optical
sampling using such a compression source, and a novel optical metrology range finding
system, known as SIPOD, which utilises the tuneable repetition rate and highly stable
nature of the compression source pulses
Radio over fibre distribution systems for ultra-wide band and millimetre wave applications
Short range wireless technology such as ultra-wideband (UWB) and 60 GHz millimetre wave (mm-wave) play a key role for wireless connectivity in indoor home, office environment or large enclosed public areas. UWB has been allocated at the frequency band 3.1-10.6 GHz with an emission power below -41.3 dBm. Mm-wave signals around 60 GHz have also attracted much attention to support high-speed data for short range wireless applications. The wide bandwidth and high allowable transmit power at 60 GHz enable multi-Gbps wireless transmission over typical indoor distances. Radio-over-fibre (RoF) systems are used to extend the propagation distance of both UWB and mm-wave signals over hundred of meters inside a building. UWB or mm-wave signals over fibre can be generated first at the central office before being distributed to the remote access points through optical fibre.
In this work, we investigate two new techniques to generate and distribute UWB signals. These techniques are based on generating Gaussian pulse position modulation (PPM) using a gain switched laser (GSL). The simulation and experimental results have been carried out to show the suitability of employing gain switching in UWB over fibre systems (UWBoF) to develop a reliable, simple, and low cost technique for distributing UWB pulses. The second part of this work proposes two configurations for optical mm-wave generation and transmission of 3 Gbps downstream data based on GSL. We investigate the distribution of these two methods over fibre with wireless link, and demonstrate the system simplicity and cost efficiency for mm-wave over fibre systems. Both configurations are simulated to verify our obtained results and show system performance at higher bit rates. In the third part, we generate phase modulated mm-waves by using an external injection of a modulated light source into GSL. The performance of this system is experimentally investigated and simulated for different fiber links
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