Cost-efficient OFDM generation at 60-GHz by heterodyne technique with direct modulation and envelope detector

A cost-effective radio over fiber system to up-convert and transmit multigigabit signals at 60 GHz is presented. A low intermediate frequency OFDM signal is used to directly modulate a laser, which is combined with an independent unmodulated laser. The generated millimeter wave frequency can be adjusted by tuning the frequency separation between the lasers. Since no external modulator is required, this technique is low-cost and it is easily integrable in a single chip. In this paper, we present numerical results showing the feasibility of generating an IEEE 802.15.3c compliant 3.5-Gbps 60-GHz OFDM. We show that received signal quality is not limited by the lasers' linewidth but by the relative intensity noise. © 2013 IEEE.

Normal dispersion fibre-enhanced nonlinear optical loop mirror for 2R regeneration and phase margin improvement

A novel all-optical regeneration technique using loop-mirror intensity-filtering and nonlinear broadening in normal-dispersion fibre is described. The device offers 2R-regeneration function and phase margin improvement. The technique is applied to 40Gbit/s return-to-zero optical data streams.

Normal dispersion fibre-enhanced nonlinear optical loop mirror for 2R regeneration and phase margin improvement

A novel all-optical regeneration technique using loop-mirror intensity-filtering and nonlinear broadening in normal-dispersion fibre is described. The device offers 2R-regeneration function and phase margin improvement. The technique is applied to 40Gbit/s return-to-zero optical data streams.

Non-metallic cold-cathode electron emission from composite metal-insulator microstructures

A detailed investigation has been undertaken into a field-induced electron emission (FIEE) mechanism that occurs at microscopically localised `sites' on uncoated, dielectric-coated and composite-coated metallic cathodes. An optical imaging technique has been used to observe and characterize the spatial and temporal behaviour of the populations of emission sites on these cathodes under various experimental conditions, e.g. pulsed-fields, gas environment etc. This study has shown that, for applied fields of 20MVm^-1, thin dielectric (750AA) and composite metal-insulator (MI) overlayers result in a dramatic increase in the total number of emission sites (typically 30cm^-2), and hence emission current. The emission process has been further investigated by a complementary electron spectroscopy technique which has revealed that the localised emission sites on these cathodes display field-dependent spectral shifts and half-widths, i.e. indicative of a `non-metallic' emission mechanism. Details are also given of a comprehensive investigation into the effects of the residual gas environment on the FIEE process from uncoated Cu-cathodes. This latter study has revealed that the well-known Gas Conditioning process can be performed with a wide range of gas species (e.g. O_2, N_2 etc), and furthermore, the degree of conditioning is influenced by both a `Voltage' and `Temperature' effect. These experimental findings have been shown to be particularly important to the technology of high-voltage vacuum-insulation and cold-cathode electron sources. The FIEE mechanism has been interpreted in terms of a hot-electron process that is associated with `electroformed' conducting channels in MI, MIM and MIMI surface microstructures.

Fiberised set-up for eye length measurement

We describe a configuration consisting of two fiberised Michelson interferometers and show that, with an optical balanced technique and suitable electronics, the signal corresponding to coherent interference in the first interferometer can be sufficiently attenuated in the channel processing the signal from the second interferometer. In this way it is possible to display simultaneous reflectograms of two different regions in the eye (e.g. cornea and retina) and infer the eye length from these measurements.

Fiberised set-up for eye length measurement

We describe a configuration consisting of two fiberised Michelson interferometers and show that, with an optical balanced technique and suitable electronics, the signal corresponding to coherent interference in the first interferometer can be sufficiently attenuated in the channel processing the signal from the second interferometer. In this way it is possible to display simultaneous reflectograms of two different regions in the eye (e.g. cornea and retina) and infer the eye length from these measurements.

A strain sensing system using a novel optical fibre Bragg grating sensor and a synthetic heterodyne interrogation technique

A potential low cost novel sensing scheme for monitoring absolute strain is demonstrated. The scheme utilizes a synthetic heterodyne interrogation technique working in conjunction with a linearly chirped, sinusoidally tapered, apodized Bragg grating sensor. The interrogation technique is relatively simple to implement in terms of the required optics and the peripheral electronics. This scheme generates an output signal that has a quasi-linear response to absolute strain with a static strain resolution of ~±20 με and an operating range of ~1000 με.

Liquid level sensing by use of digital formatted optical spectrum spreading technique

We propose a novel technique for optical liquid level sensing. The technique takes advantage of an optical spectrum spreading technique and directly measures liquid level with a digital format. The performance of the sensor does not suffer from changes of environmental variables and system variables. Due to its distinct measurement principle both high resolution and a large measurement range can be achieved simultaneously.

A novel technique for polarisation mode dispersion measurements in optical fibres

A novel technique for determining the polarisation mode dispersion in optical fibres is described. The technique makes use of a sinusoidally frequency modulated source, and is applied to the measurement of the beat length of highly birefringent monomode fibre. The temporal delay between the two modes of the fibre is measured with a resolution of approximately ±0.6 ps.

All-optical passive 2R regeneration for N×40 Gbit/s WDM transmission using NOLM and novel filtering technique

We propose an all-optical passive 2R regeneration method for WDM (N×40 Gbit/s) dispersion-managed RZ transmission based on specially designed WDM guiding filters and in-line nonlinear optical loop mirrors. By system optimisation, the feasibility of 150 GHz-spaced × l6 channel transmission over 25,000 km of standard fibre is numerically demonstrated.

All-optical passive 2R regeneration for N × 40 Gbit/s WDM transmission using NOLM and novel filtering technique

We propose a passive all-optical 2R regeneration method for WDM (N×40 Gbit/s) dispersion-managed RZ transmission based on specially designed WDM guiding filters and in-line nonlinear optical loop mirrors. By system optimisation, the feasibility of 150 GHz-spaced × 16 channel 25000 km transmission over standard fibre is numerically demonstrated.

High birefringence fibre interrogating interferometer for optical sensing applications

We describe the use of high birefringence fibre forming a differential path interferometer for heterodyne fibre optic sensing applications. We firstly recover a low frequency strain amplitude of 1µe at 1Hz applied to a fibre Bragg grating sensor demonstrating a noise limited resolution of around 100ne/vHz. Secondly we interrogate a Mach-Zehnder interferometer sensor using the dual wavelength technique to detect a change in the Mach-Zehnder OPD of 200µm.

Nonlinear inverse synthesis technique for optical links with lumped amplification

The nonlinear inverse synthesis (NIS) method, in which information is encoded directly onto the continuous part of the nonlinear signal spectrum, has been proposed recently as a promising digital signal processing technique for combating fiber nonlinearity impairments. However, because the NIS method is based on the integrability property of the lossless nonlinear Schrödinger equation, the original approach can only be applied directly to optical links with ideal distributed Raman amplification. In this paper, we propose and assess a modified scheme of the NIS method, which can be used effectively in standard optical links with lumped amplifiers, such as, erbium-doped fiber amplifiers (EDFAs). The proposed scheme takes into account the average effect of the fiber loss to obtain an integrable model (lossless path-averaged model) to which the NIS technique is applicable. We found that the error between lossless pathaveraged and lossy models increases linearly with transmission distance and input power (measured in dB). We numerically demonstrate the feasibility of the proposed NIS scheme in a burst mode with orthogonal frequency division multiplexing (OFDM) transmission scheme with advanced modulation formats (e.g., QPSK, 16QAM, and 64QAM), showing a performance improvement up to 3.5 dB; these results are comparable to those achievable with multi-step per span digital backpropagation.

Receiver sensitivity in optical and microwave, heterodyne and homodyne systems

In this paper, we analyze the sensitivities of coherent optical receivers and microwave receivers. We derive theoretical limits of signal-to-noise ratio and bit error rate. By applying a generic approach to a broad range of receivers, we can compare their performance directly. Other publications have considered some of these receivers. However, their diverse nature obscures the big picture. Using our results as a unifying platform, previous publications can be compared and discrepancies between them identified.

Real-time high-resolution heterodyne-based measurements of spectral dynamics in fibre lasers

Conventional tools for measurement of laser spectra (e.g. optical spectrum analysers) capture data averaged over a considerable time period. However, the generation spectrum of many laser types may involve spectral dynamics whose relatively fast time scale is determined by their cavity round trip period, calling for instrumentation featuring both high temporal and spectral resolution. Such real-time spectral characterisation becomes particularly challenging if the laser pulses are long, or they have continuous or quasi-continuous wave radiation components. Here we combine optical heterodyning with a technique of spatiooral intensity measurements that allows the characterisation of such complex sources. Fast, round-trip-resolved spectral dynamics of cavity-based systems in real-time are obtained, with temporal resolution of one cavity round trip and frequency resolution defined by its inverse (85 ns and 24 MHz respectively are demonstrated). We also show how under certain conditions for quasi-continuous wave sources, the spectral resolution could be further increased by a factor of 100 by direct extraction of phase information from the heterodyned dynamics or by using double time scales within the spectrogram approach.