We report the generation of a stable and broadband optical frequency comb featuring 28 THz bandwidth, sustained by a single 80 fs cavity soliton recirculating in a fiber Fabry-Pérot resonator. This large spectrum is comparable to frequency combs obtained with microresonators operating in the anomalous dispersion regime. Thanks to the compact design and the easy coupling of the resonator, cavity solitons can be generated in an all-fiber experimental setup with a continuous wave pumping scheme.
We report the generation of optical frequency combs in fiber Fabry-Perot resonators operating in the normal dispersion regime. Thanks to the compact design and the easy coupling of the resonator, switching waves can be generated in an all-fiber experimental setup employing a pulsed pumping scheme. The influence of dispersion is thoroughly discussed, revealing the potential to create a frequency comb spanning a 15 THz bandwidth through the utilization of a flattened low dispersion cavity. The experimental results are in good agreement with the theory and the numerical simulations.
Numerical studies on Kerr frequency comb generation with vertically-coupled whispering-gallery-mode (WGM) Si3N4 resonators are presented. These resonators include a frequency-dependent access coupler and are characterized by a free spectral range (FSR) of 220 GHz. We present numerical simulations based on the Ikeda map that allows implementation of complex-valued frequency-dependent and non-reciprocal access coupler transfer matrix in the simulation of Kerr comb in the cavities modelled by Arlotti et al. We use a Runge-Kutta 4 Interaction picture (RK4IP) method with adaptive step-size control as developed by Balac et al. to circumvent the numerical burden added by this modelling approach and successfully simulate Kerr comb generation using an approach that accurately models any optical cavity that can be considered as spatially one-dimensional regardless of its quality factor, finesse or dispersive properties which comes in useful in this study when access coupling properties degrade the resonator quality factor.
We experimentally investigate the impact of relevant parameters such as dispersion regime, and coupling ratio between the two loops on the phase noise performances of a 10 GHz coupled optoelectronic oscillator (COEO). The setup is based on a mode-locked semi-conductor laser at 1.55μm combined to a classical OEO. Optimization of these parameters leads to ultra-low phase noise at close-to-carrier frequencies (-100 dBc/Hz at 100 Hz and -125 dBc/Hz at 1 kHz).
Over the last two decades, integrated whispering-gallery-mode resonators have been increasingly used as the basic building blocks for selective filters, high-sensitivity sensors, and as nonlinear converters. In the latter two cases, optimum performance is achieved when the intra-cavity power or the resonance feature contrast are maximum. For devices with transversely singlemode resonator and access waveguides, the above-mentioned conditions are obtained when the system is critically coupled i.e. when the coupler power transfer rate corresponds to the single-pass intra-cavity loss. Designing coupled resonators for which critical-coupling is maintained over a large spectral range is therefore attractive to facilitate sensing or nonlinear frequency conversion.
In this paper, we theoretically show, using a generic model based on the universal description of the device spectral characteristics and a coupled-mode theory treatment of the coupling section, that access-waveguide-coupled resonators can exhibit a wideband critical-coupling bandwidth when their constitutive resonator and access waveguides are different i.e. when they are phase-mismatched. To illustrate this, we have calculated the spectral response of Si3N4/SiO2 racetrack resonators and have found that, when the coupler beat-length becomes achromatic, the device critical-coupling bandwidth is expanded by more one order of magnitude compared to their phase-matched counterpart.
A Microwave domain characterization technique is proposed to measure the optical properties of high quality factor optical resonators, featuring a very high precision in frequency which can be as good as 1 Hz. It aims to acquire a full knowledge of the complex transfer function (amplitude and phase) characterizing these resonators. It is shown that the amplitude response gives access to the measure of several parameters like the free spectral range and the quality factor. Moreover the phase transition at the resonance is used to define the coupling regime and to calculate the resonator parameters: transmission coefficient and intra-cavity losses.
We present the stabilization of the beatnote of an Er,Yb:glass Dual Frequency Laser at 1.53 μm with optical fiber delay
lines. Instead of standard optoelectronics oscillators, this architecture does not need RF filter and offers a wide tunability
from 2.5 to 5.5 GHz. Thank to a fine analysis of the laser RIN to phase noise conversion in the photodiodes, the expected
RF-amplifiers noise limit is reached with a phase noise power spectral density of -25 dBc/Hz at 10 Hz (respectively -110
dBc/Hz at 10 kHz) from the carrier over the whole tuning range. Implementation of a double fiber coil architecture
improves the oscillator spectral purity: the phase noise reaches a level of -35 dBc/Hz at 10 Hz (respectively -112 dBc/Hz
respectively 10 kHz) from the carrier.
In the microwave domain and among many other advantages, optics represents an elegant solution to increase the
quality Q factor in a system. Different types of optical resonators lead to Q factors above 109, and these resonators can
be used as an alternative to optical delay lines to set up the frequency in optoelectronic oscillators (OEO). However,
microwave-optics is also a complex field, and if the use of optical resonators in high spectral purity frequency
generation systems like OEO has been already demonstrated, many aspects of these OEOs are still incompletely
understood, especially the contribution to the oscillator phase noise of the different optical and microwave elements
used in the oscillator system. In order to improve the phase noise of a fiber ring resonator based OEO, this oscillator has
been theoretically studied in term of white frequency noise. In this paper, we present a theoretical study that has lead us
to optimize a fiber ring resonator and the experimental phase noise results obtained for an OEO based on an optimized
optical resonator. The OEO thermal stability is also investigated in this paper.
Microwave optical systems for frequency generation are described in this paper. The goal is to reach high spectral
purity in the microwave frequency range using ultra high Q optical resonators. The resonators investigated are of two
types : resonant (passive) fiber rings and WGM tridimensional resonators. They all feature ultra high optical Q factors, in
excess of 108 or 109 near 1550 nm. These resonators also sustain a large number of optical resonances, and the
microwave signal is stabilized on two (or more) resonances of this optical comb. Different problems have to be
overcome in order to reach a functional system, such as : resonator design and coupling, laser stabilization on a
resonance, overall system design, noise optimization... This paper gives an overlook on these problems, and on some
solutions we found to work towards a compact and efficient microwave opto-electronic oscillator (OEO). A first result is
presented on a 10 GHz OEO based on a resonant fiber ring.
Various circuits dedicated to high spectral purity signal transmission using fiber optics are presented. Three application types are investigated: signal transmission of ultra stable oscillators at 10 MHz, IF distribution at 874 MHz and microwave synthesized signals at 3.5 GHz. The receiver circuit is an optically synchronized oscillator, which provides a good signal conditioning far from the carrier while maintaining the high input signal quality close to the carrier
Phase noise of microwave free running sources has always been an important problem in various applications. This noise generates an increased bit error rate in a telecommunication link and degrades the sensitivity of a radar (particularly in the case of Doppler or FM-CW radar). Reducing this noise contribution is a difficult challenge for microwave engineers and circuit designers. The main contributor to this noise is well known to be the microwave transistor and finally an improvement of the oscillator phase noise will result from an optimization of the transistor phase noise. The 10 kHz to 1 MHz offset frequency range is the most important frequency range for many microwave oscillators applications. An improvement of the transistor (or oscillator) phase noise in this frequency range cannot be obtained without a good knowledge of the noise mechanisms involved in the device. In this frequency range, two different mechanisms may be at the origin of the phase noise. The first one involves the conversion to high frequencies of the transistor baseband noise (or 1/f noise) through the devices nonlinearities. The second one is due to the direct superposition of the transistor high frequency noise. This noise is simply added to the carrier, and this contribution may be described using the amplifier noise figure. In this paper, the evidence of the transistor high-frequency noise contribution in residual phase noise data is demonstrated. This behavior is observed in several bipolar devices in which the low-frequency noise contribution has been carefully minimized using an optimized bias network. Then, the phase noise behavior is correlated to nonlinear noise figure measurements. This study has been carried on numerous different microwave transistors, including FET and bipolar devices. An increase of the noise figure with the microwave signal level has been observed in each case.
Phase noise in microwave transistors is studied both theoretically and experimentally using residual phase noise measurements. The experimental approach allows the exploration of many interesting features of phase noise generation in these devices, such as the dependence of phase noise versus microwave power or transistor low frequency loading, meanwhile nonlinear simulation is still necessary to optimise the microwave load and the whole oscillator circuit. The different behaviours described are illustrated in various microwave circuits, and particularly dielectric resonator oscillators, with some of them featuring state of the art performance.
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