Joe Seregelyi

Generation and distribution of a wide-band continuously tunable millimeter-wave signal with an optical external modulation technique

A new technique to generate and distribute a wide-band continuously tunable millimeter-wave signal using an optical external modulator and a wavelength-fixed optical notch filter is proposed. The optical intensity modulator is biased to suppress the odd-order optical sidebands. The wavelength-fixed optical notch filter is then used to filter out the optical carrier. Two second-order optical sidebands are obtained at the output of the notch filter. A millimeter-wave signal that has four times the frequency of the microwave drive signal is generated by beating the two second-order optical sidebands at a photodetector. Since no tunable optical filter is used, the system is easy to implement. A system using an LiNbO/sub 3/ intensity modulator and a fiber Bragg grating filter is built. A stable and high spectral purity millimeter-wave signal tunable from 32 to 50 GHz is obtained by tuning the microwave drive signal from 8 to 12.5 GHz. The integrity of the generated millimeter-wave signal is maintained after transmission over a 25-km standard single-mode fiber. Theoretical analysis on the harmonic suppression with different modulation depths and filter attenuations is also discussed.

Optical generation and distribution of continuously tunable millimeter-wave signals using an optical phase modulator

In this paper, we propose an approach to generate and distribute two wide bands of continuously tunable millimeter-wave (mm-wave) signals using an optical phase modulator and a fixed optical notch filter. We demonstrate theoretically that the odd-order electrical harmonics are cancelled and even-order electrical harmonics are generated at the output of a photodetector when the optical carrier is filtered out from the phase-modulated optical spectrum. Analysis shows that dispersion compensation is required in order to maintain the suppression of the odd-order electrical harmonics, in order to eliminate signal fading of the generated electrical signal when the optical signal is distributed using conventional single-mode optical fiber. It is experimentally demonstrated that, when the electrical drive signal is tuned from 18.8-25 GHz, two bands of mm-wave signals from 37.6 to 50 GHz and from 75.2 to 100 GHz with high signal quality are generated locally and remotely. This approach does not suffer from the direct current (dc) bias-drifting problem observed when an optical intensity modulator is used.

Phase-Noise Analysis of Optically Generated Millimeter-Wave Signals With External Optical Modulation Techniques

In this paper, the phase-noise performance of optically generated electrical signals based on external optical modulation techniques is investigated theoretically and experimentally. Mathematical models are developed to represent perturbations on the transmitted optical signal caused by the phase fluctuations of the electrical drive signal applied to the external modulator and the optical carrier that feeds the external modulator. Closed-form expressions of the power spectral density (PSD) for the electrical signals, generated both locally and remotely, are derived. The calculated PSD of the locally generated electrical signal indicates that its phase noise is determined only by the phase noise of the electrical drive signal. The PSD of the remotely generated signal shows that its spectral quality is also affected by the chromatic dispersion of the fiber and the optical carrier linewidth. An experimental setup that can generate a millimeter-wave (mm-wave) signal, continuously tunable from 32 to 60 GHz using an electrical drive signal tunable from 8 to 15 GHz, is built. The spectra of the generated millimeter-wave signal are measured for both locally and remotely generated electrical signals, with optical carriers of different linewidths. The theoretical results agree with the experimental measurements

Discriminator-Aided Optical Phase-Lock Loop Incorporating a Frequency Down-Conversion Module

A new discriminator-aided optical phase-lock loop (OPLL) incorporating a frequency down-conversion module to generate a low phase noise, highly frequency-stable, and frequency-tunable microwave signal is proposed and demonstrated. The inclusion of the frequency down-conversion module allows the use of lower frequency components in the control circuits with a reduced cost. In addition, the new design allows continuous frequency tuning of the generated microwave signal. The down-conversion concept is presented, along with a phase noise analysis detailing the contributions to the phase noise from the reference sources. An OPLL based on the proposed configuration is implemented. The phase noise performance, as well as the frequency stability is experimentally studied

A True Time Delay Beamforming System Incorporating a Wavelength Tunable Optical Phase-Lock Loop

A true time delay (TTD) beamforming system incorporating a wavelength tunable optical phase-lock loop (OPLL) module is proposed and experimentally demonstrated. In the proposed system, instead of using a high-frequency intensity modulator to modulate the optical carrier with an RF signal, we use two laser diodes (LDs) that are phase locked to generate an RF signal, which is then sent to a fiber Bragg grating (FBG) prism to produce different time delays. Since no optical intensity modulator is used, the system can operate at much higher frequencies with a reduced cost. In addition, the use of only two wavelengths eliminates the power-penalty problem caused by chromatic dispersion. In the proposed approach, the wavelengths from the two LDs are phase-locked using a frequency-discriminator-aided OPLL. A TTD beamforming system, using the OPLL in combination with an FBG prism to achieve tunable time delays, is investigated. Experimental time-delay results are provided.

Radio-over-Fibre access for sustainable Digital Cities

Pervasive broadband access will transform cities to the net social, environmental and economic benefit of the e-City dweller as did the introduction of utility and transport network infrastructures. Yet without action, the quantity of greenhouse gas emissions attributable to the increasing energy consumption of access networks will become a serious threat to the environment. This paper introduces the vision of a ‘sustainable Digital City’ and then considers strategies to overcome economic and technical hurdles faced by engineers responsible for developing the information and communications technology (ICT) network infrastructure of a Digital City. In particular, ICT energy consumption, already an issue from an operating cost perspective, is responsible for 3 % of global energy consumption and is growing unsustainably. A grand challenge is to conceive of networks, systems and devices that together can cap wireless network energy consumption whilst accommodating growth in the number of subscribers and the bandwidth of services. This paper provides some first research directions to tackle this grand challenge. A distributed antenna system with radio frequency (RF) transport over an optical fibre (or optical wireless in benign environments) distribution network is identified as best suited to wireless access in cluttered urban environments expected in a Digital City from an energy consumption perspective. This is a similar architecture to Radio-over-Fibre which, for decades, has been synonymous with RF transport over analogue intensity-modulated direct detection optical links. However, it is suggested herein that digital coherent optical transport of RF holds greater promise than the orthodox approach. The composition of the wireless and optical channels is then linear, which eases the digital signal processing tasks and permits robust wireless protocols to be used end-to-end natively which offers gains in terms of capacity and energy efficiency. The arguments are supported by simulation studies of distributed antenna systems and digital coherent Radio-over-Fibre links.

A beat-frequency tunable dual-mode fiber-Bragg-grating external-cavity laser

A dual-mode external cavity laser employing two spatially separated fiber Bragg gratings has been investigated. We experimentally demonstrated that, without utilizing polarization separation techniques, a closely spaced (/spl sim/0.3 nm) dual-mode emission can be stabilized if a specific in-phase relation between the external feedbacks and the gain chip facet residual feedback is satisfied at both lasing wavelengths. The radio-frequency (RF) beat signal generated by the dual-mode optical signal had a typical extinction ratio of 25 dB above the noise floor and a 3-dB linewidth of approximately 2 MHz. The RF beat signal had a frequency tuning range of 200 MHz, where the dual-single-mode spectrum can be maintained.

Numerical Study of Dual Mode Generation Using a Sampled-Grating High-Order Quantum-Dot Based Laterally-Coupled DFB Laser

We propose a fabrication-friendly dual-mode laser source based on a sampled surface-grating, quantum-dot (QD), third-order, and laterally-coupled distributed feedback (LC-DFB) laser composed of alternating grating and Fabry-Perot sections. The dynamic behavior of this device is investigated through numerical modelling, and mode spacing in the millimeter-wave domain (60 GHz) was achieved. We extended a time-domain travelling-wave algorithm, including Streifers terms, to numerically study the dynamic behavior of the modified high-order LC-DFB lasers. We also incorporated an active QD region via a set of rate equations that considers both in homogeneous broadening because of spatial distribution of QD and homogeneous broadening because of the scattering or polarization dephasing rate. It was found that stable dual mode operation in the millimeter-wave range can be achieved with a dual-side-mode-suppression-ratio as high as ~ 50 dB.

Microwave signal generation using an erbium-doped external cavity laser

The increasing demand for broadband mobile communications has generated interest in exploring new frequency bands and modifying network structures. In such systems, photonic technologies can bring both cost reduction as well as an increase in performance, mainly due to the low-loss properties of optical fibers. An optical source capable of producing tunable, high-quality microwave/mm-wave signals would be of great interest not only in such communications systems, but in fiber sensors and numerous other applications as well. One potentially cost-effective method to fabricate such a system is via optical heterodyning. In this approach, the difficulties in generating a high-quality signal are two-fold. The first issue is in maintaining a specific frequency difference (i.e. microwave signal) between the lasers for an extended period of time. The second is in narrowing the inherent linewidth of the laser from the MHz values typically produced by conventional semiconductor lasers, down to values practical for a communications system. Both of the above requirements are facilitated by the newly developed doped-fiber, external cavity laser (DFECL), which offers relatively stable single-longitudinal-mode operation in addition to narrow linewidth operation. This paper will demonstrate frequency locking of a DFECL using a delay-line discriminator. The RF linewidth, initially 10-15MHz, is reduced to levels conducive to optical PLL locking. Optical power levels are approximately -3 dBm and unamplified microwave power output levels are typically -35 dBm, depending on photodetector responsivity. Carrier-to-noise ratios are generally 40-45 dB. The physical mechanisms underlying the observed laser dynamics are discussed, including laser-to-fiber alignment and thermal fluctuations.

Effects of amplified spontaneous emission noise of optical amplifiers on the phase noise of optically generated electrical signals

The applications of optical amplifiers such as erbium-doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs) are inevitable in most optical transmission links. These optical amplifiers employed in a transmission link will provide amplification to the optical signals to be transmitted, at the same time the amplifiers will also add amplified spontaneous emission (ASE) noise to the amplified optical signals. In radio-over-fiber systems, optical links are used to distribute high quality radio frequency (RF) signal, microwave signal or millimeter-wave (mm-wave) signal over optical fiber for low loss long-distant transmission. In this paper, the effects of amplified spontaneous emission (ASE) noise of optical amplifiers on the quality of the optically generated electrical signals are theoretically studied.