Optical Gas-Phase Frequency References Based on Photonic Crystal Technology

Optical frequency references are devices providing well-defined and stable optical frequency responses to incoming radiation for applications such as high-precision spectroscopy and optical fibre communications. To stabilise the emission frequency of lasers, which drifts with time mostly because of fluctuations in temperature and mechanical vibrations, atomic and molecular optical transitions can be used since they show precise and well-defined frequency responses to incoming radiation. However from a practical standpoint conventional gas cell devices cannot be easily integrated into existing optical systems because of their bulky dimensions. To replace conventional absorption cells, photonic crystal fibres filled with gas-phase material are promising devices owing to their robustness, reliability, and portable characteristics. In addition they can be directly embedded into existing optical systems and they perform well in harsh environments. In this experimental study, optical gas-phase frequency references based on photonic crystal technology are realised. The gas-sensing properties of different photonic crystal fibre samples are studied and the long-term stability and reliability of fibre gas cells are demonstrated. In addition an analytical model predicting the gas-filling time in photonic crystal fibres (PCF) is developed and can be applied to any type of fibre, fibre geometry, or length. Then fibre gas cells filled with acetylene gas, a recognised frequency reference gas-phase material, have been prepared to conduct fundamental research on slow & fast light generation in optical fibres to verify the possibilities of slow light in enhancing light-matter interaction. The group velocity of light is controlled by modifying the material and structural dispersive properties of the PCF absorption cells through stimulated Brillouin scattering and cavity ring resonators, respectively. We could demonstrate that material slow light has no impact on the molecular absorption effect whereas structural slow light has an impact on the absorption efficiency scaling linearly with the group index. Such radically different responses to slow light suggest that group velocity is not the universal physical quantity scaling light-matter interaction, and that the optical absorption of molecules is more closely related to the velocity of the electromagnetic energy. Finally the impact of slow light on the molecular absorption efficiency is also evaluated in dispersion-engineered photonic crystal waveguides. We demonstrate that in planar photonic crystal waveguides the field enhancement and its evanescent fraction have more impact on the absorption efficiency than the reduction of the group velocity of light.

Generation and application of dynamic gratings in optical fibers using stimulated Brillouin scattering

Nikolay Primerov, Luc Thévenaz

Since the invention of optical fiber cable data transmission using light has become ubiquitous in the past years. Nonlinear optical effects, which are hardly noticed in our daily life, have become crucial and highly influential for the design and performance of advance high-capacity systems. Future optical systems will definitely require signal processing operations to be performed on data signals “on the fly” solely in the optical domain. Brillouin scattering is currently one of the most prominent nonlinear effects in optical fibers, as it has found many applications in various fields of photonics, such as slow light, new high coherent sources, filters, optical fiber sensing, etc. In this thesis we investigate a novel phenomenon based on Stimulated Brillouin Scattering, which is called dynamic Brillouin grating (DBG). Investigating the process of DBG generation in optical fibers we propose several new approaches to realize new tools for signal processing of the optical signal by using phonon-photon interactions. First an analytical model is proposed to describe localized DBG generation in optical fibers using three coupled equations for Stimulated Brillouin Scattering. Based on this model we can show how the grating is generated and which shape it has in time when two pump pulses are used with optical frequency difference equal to the Brillouin shift of the fiber. Moreover optical phase conjugation is also theoretically predicted for the optical wave reflected from DBG. Several applications of Dynamic Brillouin grating are identified and investigated. An optical signal delay line or buffer is proposed and experimentally demonstrated using DBG principle. Storage of an input optical pulse is achieved by changing the position of the localized DBG inside the fiber. Here DBG acts as a Bragg reflector for the optical pulse and change in DBG position defines the time-of-flight of the input and reflected pulse. Single pulse delays can be dynamically controlled with a buffer capacity ranging from several picoseconds to more than one microsecond. Such delays show that DBG based signal delay lines can easily outperform slow light based delay lines in terms of delay-bandwidth product. Optical delay of pulse trains is also achieved, but with maximum limited amount of delay governed by the acoustic wave decay constant, which in standard silica fibers is around 10-12 ns. Moreover microwave photonic signals can be also delayed using DBG demonstrating true time delay. In this configuration, the reflection bandwidth of the DBG defines the maximum operational radio frequency. Relation of the DBG bandwidth as a function of its length was measured and it was determined that DBG exhibits similar properties as weak uniform fiber Bragg grating (FBG). Another application of the DBG gratings is demonstrated by exploiting this phenomenon to realize microwave photonic filters. For the first time, two-, three- and four-tap microwave photonic filter configurations are realized based on DBG. Free spectral range and IV tcartsbA stop band central frequencies of the proposed microwave filters can be easily tuned by adjusting the properties and positions of DBGs. Dynamic gratings generated by amplitude modulated pump waves are used to realize optical differentiation, integration and time reversal of input optical signals. Performed experiments demonstrate that a DBG based signal processor performs operations on the complex amplitude of the input optical wave. The DBG based integrator offers high flexibility in terms of operational wavelength and variable integration time, which can be controlled by changing the DBG length. For the first time, new operations of true time reversal of several bit sequences are performed using dynamic Brillouin grating. To overcome the problem of the decay of the localized acoustic grating, a novel method based on phase modulation by pseudo-random bit sequence (PRBS) is proposed, which allows generation of localized and stationary DBG at random positions inside the fiber. Such a method opens up new possibilities for signal control, and for optical fiber sensing. Advantages and limitations of the PRBS technique in terms of signal-to-noise ratio are discussed. It is experimentally verified that PRBS generation of DBG is highly attractive for the operations on optical signals where time averaging is possible, such as in the fields of microwave photonics and distributed fiber sensing. On the other hand, it was experimentally verified that realization of the optical buffer for continuous digital data using the PRBS technique is difficult due to the substantial amount of instantaneous noise. A separate chapter of this thesis is devoted to the application of DBG to the field of distributed fiber sensing. A DBG based distributed temperature and stress sensor in polarization maintaining fibers has been realized, showing 1 cm spatial resolution, which is - to the best of our knowledge - first time, demonstrated using a pulsed-based Brillouin sensor. The demonstration is performed in a 20 m fiber, but the sensing range of the proposed DBG based sensor can be expanded up to several hundred meters. In addition, it is shown that the demonstrated spatial resolution is not the ultimate limit, and a finer spatial resolution can be achieved by simply reducing the probe pulse duration. Another Brillouin distributed temperature and stress sensor is proposed based on the DBG principle. Here DBG is generated by modulating the phases of a continuous Brillouin pump and probe waves by common PRBS with unipolar encoding and modulation speeds faster than phonon lifetime. By choosing the proper parameters of PRBS, only one localized and permanent dynamic Brillouin grating is placed inside the sensing fiber. Distributed measurements of temperature or strain are accomplished by scanning the position of the DBG inside the fiber and determining the Brillouin resonance condition at each position. Using this technique, a random access sensor with 1 cm spatial resolution is demonstrated over a 40 m standard single mode fiber. In the proposed scheme the spatial resolution can be easily changed by changing the bit duration of the PRBS, while the measurement range is controlled by the length of the PRBS sequence.

EPFL2013

Governing the speed of a light signal in optical fibers

Sang Hoon Chin

Dynamic control of the speed of a light signal, based on stimulated Brillouin scattering in optical fibers, was theoretically studied and also experimentally demonstrated as the core object of this thesis. To date, slow light based on stimulated Brillouin scattering has shown the unmatched flexibility to offer an efficient timing tool for the development of all-optical future router. Nevertheless, the seeming perfect Brillouin slow light suffered from three major obstacles: naturally narrow signal bandwidth, strong change of signal amplitude, and significant signal distortion. The essential contribution of this work has been mostly dedicated to resolve all those impairments so as to make Brillouin slow light a completely operating all-optical delay line for practical applications. Actually, high capability of tailoring the spectral distribution of the effective Brillouin resonance makes possible to resolve partially or completely all those problems. First of all, a broadband spectral window was passively obtained in between two Brillouin gain/loss resonances by simply appending two segments of fibers showing different Brillouin frequency shifts. The global Brillouin gain of the concatenated fibers manifests a gain/loss doublet resonance showing a broad window in between gain/loss peaks. In practice, this configuration has a crucial advantage that it removes the need of the pump modulation, generally used to create a polychromatic pump source. Therefore, a broadband Brillouin slow and fast light was simple realized with a reduced distortion. Secondly, the signal amplification or attenuation associated to the signal delay was completely compensated by superposing Brillouin gain and loss resonances with identical depth but different width. As a result, the Brillouin gain led to effectively a spectral hole in the center of the broadband absorption and opened a transparent window while the sharp change in the refractive index was preserve. This way it makes possible to realize zero-gain Brillouin slow light. This configuration was also exploited to produce Brillouin fast light with a total absence of signal loss, simply by swapping the spectral position of the two pumps. At last, a signal was continuously delayed through a Brillouin fiber delay line without any distortion. Due to the strong induced dispersion, pulse broadening is a major difficulty in all slow light systems and it is impossible to compensate such broadening using a linear system. Therefore, a conventional Brillouin slow light system was combined with a nonlinear optical fiber loop mirror that gives a nonlinear quadratic transmission. Using this configuration, the inevitable pulse broadening was completely compensated at the output of the loop and a signal was delayed up to one symbol without any distortion. Brillouin slow light systems were further studied in the spectral domain. For a given Brillouin resonance the spectrum of a light pulse was optimized to better match the Brillouin bandwidth. When the time envelope of a pulse was properly shaped, it was clearly observed that the spectral width of the pulse became minimized while preserving the pulse duration. This way the maximum time delay through Brillouin slow light could be enhanced for a fixed pulse width. Brillouin fast light was even realized in total absence of any pump source, which is a plain requirement for the generation of Brillouin slow or fast light. This self-generated delay line, key contribution of this thesis, relies on both spontaneous and stimulated Brillouin scattering in optical fibers. In this implementation, a light signal was strongly boosted above the so-called Brillouin threshold, so that most power of the signal was transferred to a backward propagating wave, namely the generation of amplified spontaneous Stokes wave. Since the center frequency of the intense Stokes wave is below the signal frequency by exactly Brillouin shift of the fiber used, this wave led to a Brillouin loss band centered at the signal frequency. Consequently, the signal experienced fast light propagation and the propagation velocity of the signal was self-controlled, simply by varying the signal input power. This technique has many practical advantages such as its high simplicity of the configuration and an invariant signal power in the output of this delay line. Additionally, this system self-adapts the signal bandwidth as the spectrum of the amplified Stokes wave matches the spectral distribution of the signal. An alternative method to generate all-optical delay line was proposed instead of slow light. This scheme makes use of the combination of wavelength conversion and group velocity dispersion. This type of delay line was mainly aimed at improving the storage capability of delaying element. The wavelength of a signal was simply and efficiently converted at a desired wavelength using cross gain modulation in semiconductor optical amplifiers. Then the converted signal was delivered to a high dispersive medium and arrived at the end of the medium with relative time delay due to the group velocity dispersion. A fractional delay of 140 was continuously produced through this delay line for a signal with a duration of 100 ps, preserving signal bandwidth and wavelength. The effect of slow light on linear interactions between light and matter was experimentally investigated to clarify the current scientific argument regarding slow light-enhanced Beer-Lambert absorption. It was predicted that real slowing of the light group velocity could enhance the molecular linear absorptions so as to improve the sensitivity of this type of sensing. However, the experimental results unambiguously show that material slow light (slow light in traveling wave media) does not enhance the Beer-Lambert absorption.

EPFL2009

Brillouin scattering in gas-filled hollow-core fibres

Flavien Gyger

Edward E. Hagenlocker and William G. Rado demonstrated Brillouin interaction in gas for the first time in 1965, right after the very first demonstrations of stimulated Brillouin scattering in solid materials. In their experiment, they used a megawatt pulsed ruby laser that they focused in a gas cell. Such a high optical power was required due to the weak efficiency of the interaction, as a result of the impossibility of confining a high optical intensity for a sufficiently long distance in free space. Thanks to the invention of the hollow-core photonic crystal fibres by Philip St.J. Russell in 1991, it is nowadays possible to tightly confine beams of light over distances of several kilometres. In such fibres, light beams are confined over an area of a few tens of square micrometres and propagate with small optical losses, that might reach those of standard optical fibres in the near future. The Brillouin gain being proportional to the light intensity and to the interaction length of the beams, its value is increased by many orders of magnitude in a hollow-core fibre, as compared with free-space experiments. As a consequence, the Brillouin interaction in a hollow-core fibre only requires optical power of a few tens of milliwatts. Additionally, the Brillouin gain shows a quadratic dependence on pressure. Hence, by pressurising the gas inside the hollow-core fibre, it is possible to further drastically increase the Brillouin gain, that can exceed the values of solid media (e.g. silica). It has to be noted that hollow-core optical fibres are ideal candidates for these kind of experiments since their hollow internal volume is extremely small and they seem able to sustain pressures of several thousand times the atmospheric pressure. The main objectives of this thesis are the demonstration of the quadratic dependence of the Brillouin gain on pressure, both theoretically and experimentally, and the use of this strong Brillouin interaction for practical applications. The first three chapters introduce a theoretical background, explaining light propagation in gases, acoustic waves and their interaction with light waves. The last three chapters give some experimental results to confirm the theoretical part as well as four applications of Brillouin scattering in gas-filled hollow-core fibres: amplification of an optical signal, slow light, gas Brillouin lasing and a novel, intrinsically strain-insensitive, distributed temperature sensor.

EPFL2020