Self-phase modulation | EPFL Graph Search

Self-phase modulation (SPM) is a nonlinear optical effect of light–matter interaction. An ultrashort pulse of light, when travelling in a medium, will induce a varying refractive index of the medium due to the optical Kerr effect. This variation in refractive index will produce a phase shift in the pulse, leading to a change of the pulse's frequency spectrum. Self-phase modulation is an important effect in optical systems that use short, intense pulses of light, such as lasers and optical fiber communications systems. Self-phase modulation has also been reported for nonlinear sound waves propagating in biological thin films, where the phase modulation results from varying elastic properties of the lipid films. Theory with Kerr nonlinearity The evolution along distance z of the equivalent lowpass electric field A(z) obeys the nonlinear Schrödinger equation which, in absence of dispersion, is: :\frac{dA(z)}{dz} = -j\gamma \left| A(z)\right|^2 A(z) with j the im

Regenerative similariton laser

Camille Sophie Brès, Thibault Emmanuel North

Self-pulsating lasers based on cascaded reshaping and reamplification (2R) are capable of initiating ultrashort pulses despite the accumulation of large amounts of nonlinearities in all-fiber resonators. The spectral properties of pulses in self-similar propagation are compatible with cascaded 2R regeneration by offset filtering, making parabolic pulses suitable for the design of a laser of this recently introduced class. A new type of regenerative laser giving birth to similaritons is numerically investigated and shows that this laser is the analog of regenerative sources based solely on self-phase modulation and offset filtering. The regenerative similariton laser does not suffer from instabilities due to excessive nonlinearities and enables ultrashort pulse generation in a simple cavity configuration. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Amer Inst Physics2016

Mode interference and UV-induced mode conversion in few-mode fibers

Soham Basu

Resonant and non-resonant effects in few-mode fibers have found use in versatile devices. From sensing perspective, extensive research has been done on fringe sensitivity of nonresonant two-mode interference. Resonant mode converter gratings are promising candidates for the fields of mode division multiplexing and active devices based on few-mode fibers. This thesis explores multi-parameter sensing using two effects of two-mode interference (TMI), namely fringe shift and shift of group-velocity equalization (GVE) wavelength. The following sensing parameters were achieved: a) TMI fringe shift can be used for measuring the effect of changes in temperature and strain on intermodal dispersion. For exposure of the fiber with a scanning laser spot of constant velocity, the change in intermodal dispersion due to such particular exposure can also be measured using TMI fringes. b) GVE wavelength shift can be used for straightforward sensing of temperature and strain, without suffering from any dependency on the fiber length like TMI fringes. Using the shift of GVE wavelength during exposure of the full fiber with a scanning laser spot, the change in core index can be estimated due to a simple model. This allows measurement of small index changes due to irradiation, which is not possible with FBGs due to imperceptible strength of FBGs for small index changes and lower sensitivity. Combining the GVE and FBG resonance wavelengths provides a method for differentiating strain and temperature. The GVE wavelength also has an approximately three times higher temperature sensitivity compared to FBG resonance at similar wavelengths. The strain sensitivity of GVE and FBG resonance wavelengths are of similar magnitude but of opposite sign. TMI fringe shift can also be used to measure the extra phase added between two modes by each mark of laser irradiation. Two methods have been developed for completely characterizing the intermodal dispersion, which were verified experimentally for the LP01-LP02 dispersion. Intermodal and intramodal reflection peaks of two excited modes from an FBG written in the few-mode fiber can be used to approximately estimate the offset of the intermodal dispersion of the pristine fiber. Combining this with TMI phase unwrapping provides estimate of complete intermodal dispersion including offset. Measuring the resonance wavelength for a mode converter written using marks already characterized using TMI provides a precise estimate of the offset of the intermodal dispersion of the pristine fiber. From the estimate of extra phase added by each mark and intermodal dispersion of the pristine fiber, the intermodal dispersion experienced by resonantly interacting modes in an MC can be predicted. Fabrication of broadband mode converter gratings at any desired wavelength is an ongoing field of research. Different strategies were evaluated for making broadband MCs: a) Partial core irradiation with a tightly focussed laser spot was simulated with simple approximate models. In was concluded that the losses might be too high due to poor mode overlaps. b) It was determined using simulations that phase-shifted mode converter gratings can provide broader than 35 nm bandwidth at 20 dB conversion strength using reasonable irradiation parameters. The method was verified by achieving 41 nm bandwidth at 16 dB conversion strength.

EPFL2020

Brillouin Echoes for Advanced Distributed Sensing in Optical Fibres

Stella Foaleng Mafang

Brillouin scattering is particularly efficient and attractive for the implementation of strain and temperature distributed sensing in optical fibres. Recently a trend has been observed that modern advanced applications require a substantial step towards better spatial resolution, while preserving temperature/strain precision over a long range. For this purpose the state of the art does not satisfy all these requirements. In this thesis we present a radically new approach named Brillouin Echoes distributed sensing (BEDS) that allows covering these requirements. In the first part, we propose an updated configuration of the classical existing Brillouin sensor for time domain analysis allowing drastic noise reduction. Then we investigate the limitations (due to non-linear effects) of the classical Brillouin sensor in terms of long distance range measurements. The identified nonlinear effects are pump depletion due to SBS itself, self-phase modulation (SPM), modulation instability (MI), which occurs only in fibres presenting an anomalous dispersion at the pump wavelength and Raman scattering (RS). We propose the modeling of the pump depletion effect to obtain analytical expressions that are useful for the proper design of a BOTDA sensor and for the determination of a very small depletion. The model confirmed by experimental measurements is informative on the conditions maximizing the depletion effect; therefore a standard configuration can be defined to test the value of the depletion in the set-up. Furthermore, we demonstrate that SPM-induced spectral broadening can have a significant effect on the measured effective gain linewidth. Modeling and experiments have undoubtedly demonstrated that the effective gain linewidth can easily experience a two-fold increase in standard conditions when the pulse intensity profile is Gaussian. We showed that the problem can be practically circumvented by using a clean rectangular pulse with very sharp rising and falling edges. The theoretical and experimental analysis of the undesirable effects of MI and forward RS in distributed BOTDA sensors systems gives a simplified expression to predict the critical power for a given distance range. MI turns out to be the dominant nonlinear limitation since it shows the lowest critical power, but it is less critical since it can be avoided to a wide extent by using the fibre in the normal dispersion spectral region such as a DSF in the C-band. On the other hand Raman scattering can be avoided only by limiting the optical pump power and therefore is the ultimate nonlinear limitation in a distributed sensing system. Under similar conditions RS shows a critical power ∼5 times larger than MI. In the second part, we present the new approach Brillouin echo distributed sensing (BEDS) which has proved to be a powerful solution to realize sub-metric spatial resolutions in Brillouin distributed measurements. We have demonstrated both theoretically and experimentally that an optimized configuration is reached when the optical wave is π-phase shifted. The experimental tests have shown a spatial resolution down to 5 cm, with a clear margin for further improvement down to a real centimetric spatial resolution. This optimized configuration produce the best contrast independently of the pulse intensity, with a factor 2 of improvement compared to other techniques based on the same approach (dark pulse, bright pulse). This extends the dynamic range by 3 dB, which corresponds in standard loss conditions to a 5 km extension of the sensing range. An analytical developed model has proved to be an excellent tool not only for optimizing the pumping scheme but also in post-processing the measured data. Finally the potentialities of BEDS technology provide solutions in real contexts. Using the BEDS technology in landslide monitoring at laboratory scale, for the first time it became possible to observe the failure propagation in laboratory scale with an accurate precision. Furthermore, using BEDS we have proposed and demonstrated the possibility of mapping geometrical structure fluctuations along a photonic crystal fibre (PCF). Both long- and short-scale longitudinal fluctuations in the Brillouin frequency shift have been identify and quantify. Observation of Brillouin linewidth broadening in PCF fibre through distributed measurement of the Brillouin gain spectrum using BEDS has allowed fundamental understanding of SBS in PCF fibre and in their design in view of applications to optical-strain/temperature sensing.

EPFL2011