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In this paper, the performance of a switchable multi-wavelength Er–Yb-doped fibre ring laser incorporating a comb filter is described. The comb filter was developed by using the double-pass Mach–Zehnder interferometer with a Sagnac interferometer (DMZI–SI) arranged in series, a 10-m-long PANDA-type polarisation-maintaining optical fibre, and polarisation controllers (PCs). The laser output can be switched from 1536 to 1548 nm (C-band) via careful adjustment of the PCs. A side-mode separation ratio, greater than 28 dB, was obtained to facilitate stable single-, double-, quadruple-, and quintuple-wavelength emissions, with spectral power and wavelength fluctuations of 0.47–1.06 dB and 0.01–0.06 nm, respectively. The measurements were performed for 60 min at 25 °C. The proposed comb-filter configuration facilitates the double-clad fibre ring laser to realise switchable multi-wavelength emission. Experimental results reveal that the proposed laser demonstrates high stability and significantly improved performance compared to the existing DMZI–SI filter-based systems. This indicates their potential for use in applications involving optical sensing, communication systems, and microwave generation.

Supercontinuum (SC) generation directly from noise-like pulse (NLP) emission is demonstrated in a mode-locked thulium-doped all-fiber laser with nonlinear polarization rotation. The laser supports NLP emission at a repetition rate of 812.79 kHz, producing optical pulses of 2 ns pulse duration and 400 nm of optical bandwidth, which expands from 1800 nm to 2200 nm at floor level and covers the entire thulium band. The SC source presents a high dynamic range, higher than 50 dB, a high degree of spectral flatness, without significant spectral variations or pump remnants, and an output average power of 20.25 mW. Moreover, our proposal avoids the inclusion of external components and devices, thus preserving the robustness and simplicity in a single all-fiber system.

We introduce a photochemical alternative procedure for uric-acid crystals degradation by controlled exposure of pulsed near infrared radiation from a ring fiber laser. The laser provides a wide spectrum from 1050 nm to 1120 nm, this spectrum exhibits a burst pulsed response with a pulse packet frequency rate of 210 kHz and an internal pulse frequency of 29 MHz. To demonstrate the technique, uric acid crystals were prepared synthetically and exposed under different radiation conditions, then by using an effective digital image processing the samples are before and after compared. Results confirm it is possible to exponentially reduce the number of crystals by an adequate combination of energy and number of pulses. The technique offers the possibility to reduce up to 50 % of the initial uric acid in a long period of safety wavelength radiation.

In this work we study statistically the internal structure of noiselike pulses generated by a passively mode-locked fiber laser. For this purpose, we use a technique that allows estimating the distribution of the amplitudes of the sub-pulses in the bunch. The technique takes advantage of the fast response of the optical Kerr effect in a fiber nonlinear optical loop mirror (NOLM). It requires the measurement of the energy transfer characteristic of the pulses through the NOLM, and the numerical resolution of a system of nonlinear algebraic equations. The results yield a strongly asymmetric distribution, with a high-amplitude tail that is compatible with the existence of extreme-intensity sub-pulses in the bunch. Following the recent discovery of pulse-energy rogue waves and spectral rogue waves in the noiselike pulse regime, we propose a new way to look for extreme events in this particular mode of operation of mode-locked fiber lasers, and confirm that rogue wave generation is a key ingredient in the complex dynamics of these unconventional pulses.

In this paper, we carry out a numerical study on nonlinear and chaotic effects produced by the propagation of hyperbolic secant optical pulse in standard fibre. We use the fourth-order Runge–Kutta in the interaction picture (RK4IP) method to analyse the pulse interaction in the fibre medium, this method was employed due to its potential to reduce the numerical error while simultaneously allow increasing the step size. During the study, the input peak power was varied up hundreds of kW trough 1 km of standard single-mode fibre (SMF-28); as a consequence, it was possible to observe the generation of nonlinear and chaotic effects on the final spectrum. These numerical results show the spectral and temporal evolution of energy in a 3D rendering, here, it was possible to demonstrated break frequencies and fragmentation of packets of energy by changing the pump power. Finally, we show that this analysis can be used in subsequent studies employing complex dynamic pulses (noise-like pulses, NLPs). Keywords: Fibre optics, Nonlinear fibre optics, Optical solitons, Generalized nonlinear Schrödinger equation

In the present work, we investigated the polarization-imbalanced non-linear optical loop mirror (NOLM) for compressing the soliton spectrum as well as removing the sidebands. In the passive mode-locked soliton fiber laser, the spectra of solitons are usually accompanied with dispersive waves that can form spectral sidebands. The sidebands are the result of a constructive interference between the soliton and dispersive waves that take place at frequencies in which the phase difference is 2π. These sidebands may contain a significant part of the energy of the output spectrum, it is important first clean the spectrum by removing the sidebands and then compress the spectrum to have an optimal soliton profile. Our NOLM consists of a nearly symmetrical coupler with a 51/49 coupling ratio, and a fiber loop which consists of two 50-m pieces of SMF-28 fibers twisted in opposite directions. The maximum transmission of the NOLM at high power was 34%. At the NOLM output we reached a spectral compression of more than 50% for different values of input power. Although the sidebands around 1555 nm and 1570 nm were totally cleaned, nearly 5% of the sidebands were remaining around 1560 nm and 1567 nm. In order to get stronger spectral compression, the signal at the NOLM output was first amplified then sent into 1 km of SMF-28. For minimum input power the full width at half maximum (FWHM) bandwidth of the spectrum decreased from 5.5 nm to 0.67 nm. For higher input power the FWHM decreased from 4.23 nm to 0.59 nm.

We study experimentally the polarization of the radiation resulting from the pulse break-up process in a SMF-28 twisted fiber. The fiber twist causes circular birefringence and also mitigates the linear birefringence. The twisted fiber may be considered for nonlinear effect as fiber without linear birefringence, which allows the investigation of polarization properties which cannot be studied in common fibers because of the random residual birefringence. We found that the polarization of the formed solitons is more stable when the input pump polarization has elliptical polarization with big angle of ellipticity. At input polarization close to linear we observed that the polarization ellipticity angle tends to be higher than the polarization ellipticity angle of the input pump. The fluctuation of the polarization grows when the input polarization approaches to the linear.

We report the efficient and stable operation at 1549 nm of an actively Q-switched fiber laser based on an Er 3+ /Yb 3+ doped double-clad single mode fiber. It operates with a repetition rate from 45 to 120 kHz and pulse duration from 34 to 80 ns; the output pulse shape and peak pulse intensity are stable over hours of operation. For a repetition rate of 120 kHz and a maximum pump power of 8.1 W we obtained an average output power of 4.0 W with an overall efficiency of 50% and with a minimum pulse duration of 34 ns.

This paper covers a numerical analysis of supercontinuum spectrum generation in a piece of standard fiber by using as the pump noise-like pulses produced by a passively mode-locked fiber laser. An experimental study was also carried out, yielding results that support the numerical results. In the numerical study we estimated that the spectral extension of the generated supercontinuum reaches ~ 1000 nm, and that it presents a high flatness over a region of ~ 220 nm (1630 nm-1850 nm) when we use as the pump noise-like pulses with a wide optical bandwidth (~ 50 nm) and a peak power of ~ 2 kW. Experimentally, the output signal spectrum extends from ~ 1530 nm to at least 1750 nm and presents a high flatness over a region of 1640 nm to 1750 nm for the same value of numerical input power, 1750 nm being the upper limit of the optical spectrum analyzer. The numerical analysis presented here is thus an essential part to overcome the severe limitation in measuring capabilities and to understand the phenomena of supercontinuum generation, which is mainly related to Raman self-frequency shift. Finally, this work demonstrates the potential of noise-like pulses from a passively mode-locked fiber laser for broadband spectrum generation.

We propose and analyse numerically a novel dispersion-imbalanced non-linear optical loop mirror (NOLM) scheme allowing simultaneous compression and amplitude noise reduction of chirped ultrashort-pulsed signals. An all-fibre distributed Gires–Tournois etalon (DGTE) made of two uniform fibre Bragg gratings is inserted asymmetrically in the loop, and is used both to dispersion imbalance the NOLM and to compress the pulses. The results show that the output pulses are accompanied by side lobes, which originate from the no-uniformity of the DGTE dispersion spectrum. We analyse the formation of these side lobes and show that, in the case of highly chirped input pulses, side lobes are strongly reduced by the NOLM architecture. In this case, the device operates well even when the signal optical spectrum extends over an entire free spectral range of the DGTE. We believe that this work will be useful for the design of all-optical regeneration schemes for highly chirped data streams in optical transmission systems.

In this paper we study theoretically and experimentally a wavelength-tuneable Sagnac birefringence filter. The device is a Sagnac interferometer including a symmetric fibre coupler and a length of high-birefringence fibre in the loop. A wave retarder is inserted at each end of the birefringent fibre for absolute wavelength tuning. We show theoretically that wavelength tuning through wave plate orientation ensuring constant amplitude of the filtering function is possible only if a minimum of two wave retarders are included in the setup. The position of the transmission peaks then varies linearly with the angle of one of the retarders and can be adjusted over one entire channel spacing. This happens only when a quarter-wave retarder and a half-wave retarder are used, if the former is oriented at 45° with respect to the fibre birefringence axes, while the orientation of the latter serves as the adjustment parameter. The theoretical predictions are confirmed by the experimental results.

We analyse numerically the capabilities of a power-symmetric nonlinear optical loop mirror (NOLM) in the ultrashort pulse regime for high-quality amplitude regeneration of an optical signal. The device, which operates through nonlinear polarisation rotation, includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. For particular adjustments of the retarder orientation, and a circularly polarised input beam, the output energy characteristic flattens near the switching energy, a property that can be used to eliminate large amplitude fluctuations in an optical signal. The group velocity mismatch between polarisation components introduced by twist is mitigated by the interplay between anomalous dispersion and the nonlinear Kerr effect, although strong twist should be avoided as it still introduces substantial pulse distortion. Contrary to other designs, where a plateau characteristic requires a large power imbalance between the counter-propagating beams, both pulses in the present scheme can be simultaneously close to fundamental solitons, which allows a substantial widening of the plateau for particular pulse parameters. Good quality, nearly transform-limited pulses are obtained in this case at the NOLM output. The device is applicable for the regeneration of ultrafast data streams in which the signal-to-noise ratio is severely deteriorated.

We propose and study numerically a fibre-based scheme designed to generate a step-like optical decision function for ultrafast data processing applications. The setup includes two main sections disposed in series: a Nonlinear Optical Loop Mirror with an output polariser on the one hand, and a circularly birefringent fibre span with a quarter wave retarder at the input and a polariser at the output on the other hand. Both sections operate through nonlinear polarisation rotation, and the switching characteristic is controlled through the light ellipticity, which can be adjusted by setting properly the orientation of the input polarisation, and by rotating the wave retarder and the two polarisers. For proper adjustment of these four parameters, a step-like optical decision function with ∼10 dB/dB steepness is predicted. In the frame of all-optical regeneration, the setup allows eliminating ∼76% peak-to-peak amplitude fluctuations on the marks and ∼30% relative power on the spaces. We believe that this architecture should be considered for the design of ultrafast transmission networks and in the frame of all-optical computing applications.

A numerical study of a fibre-based scheme for the regeneration of ultrashort-pulsed optical signals is presented. The setup is made of a power-symmetric Nonlinear Optical Loop Mirror (NOLM) followed by a polariser. The NOLM operates through nonlinear polarisation rotation, and includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. When the orientations of the linear input polarisation and of the output polariser are properly adjusted, the output energy characteristic flattens at high power, a property that can be used to eliminate large amplitude fluctuations on the logical ones of an optical signal. When the input pulse parameters closely match those of fundamental solitons or of stable elliptically polarised solitary waves, a wide and flat plateau is obtained, allowing the reduction of ∼30% amplitude fluctuations to less than 1%. Very large amplitude fluctuations beyond 50% can also be reduced down to a few %. Although the output pulses are slightly chirped, they are free of pedestal, thanks to the zero low-power transmission of the NOLM, which also allows the simultaneous regeneration of logical zeros. We believe that this setup will be useful for the regeneration of highly degraded signals in future ultrafast transmission networks.

We demonstrate theoretically and experimentally that efficient signal shaping operation can be obtained at moderate power by using the transmission characteristic of a power-symmetric nonlinear optical loop mirror (NOLM) including highly twisted fibre and operating through nonlinear polarisation rotation, when the circular polarisation state orthogonal to the input polarisation is selected at the NOLM output. By adjusting the angle of the quarter-wave retarder inserted in the loop, the phase bias of the transfer characteristic can be adjusted precisely to enable proper signal shaping for moderate values of input power, remaining well below switching power. The tolerance of the procedure to deviations of the input polarisation from the ideal circular case is investigated numerically. We demonstrate experimentally the capabilities of this setup for both power equalisation and extinction ratio enhancement. Finally, we show that this setup is also useful to shape ultrashort optical pulses from the relaxation oscillations of a DFB semiconductor laser. In comparison with other NOLM-based techniques, the proposed approach allows to reduce by a factor of 8–10 the peak power required for pulse shaping, for the same fibre length and Kerr coefficient.

We experimentally analyze the self-starting operation of a figure-eight mode-locked fiber laser. The design is based on a power-balanced nonlinear optical loop mirror (NOLM) with highly twisted low-birefringence fiber and a quarter-wave (QW) retarder in the loop. The NOLM operates by nonlinear polarization rotation. Self-starting mode-locking requires a careful adjustment of the NOLM low-power transmission, which is easily realized with our setup by adjusting the angle of the QW retarder. The laser is capable of generating ∼20 ps pulses at the fundamental repetition frequency of 0.78 MHz.

We propose a polarisation-maintaining NOLM switch design to be used as optical regenerator or wavelength converter in dense optical time-division multiplexed (OTDM) systems. The Sagnac loop is made of a piece of high birefringence fibre which is cut and cross-spliced in the middle. If pump and probe polarisations are linear and aligned in the co-propagating direction, the cross-splice ensures that the counter-propagating probe beam will be orthogonal to the pump, so that the parasitic cross-phase modulation between counter-propagating beams is minimised. This architecture also allows easy control of the optical phase bias, through squeezing a short section of the fibre, without any other modification of the setup. The performances of the proposed architecture are studied analytically and numerically, and compared with those of conventional schemes. It appears that, although the proposed setup reduces the interaction between counter-propagating beams only by a factor 3, it yields an extinction ratio improvement of a factor 10 or higher in comparison with conventional schemes. If there is substantial walkoff between pump and probe, a 10-fold reduction of the relative intensity noise of the emerging signal is also obtained when the mark ratio of the incoming data is variable.

We demonstrate theoretically, experimentally and numerically that a steep all-optical transfer characteristic can be obtained from a power-symmetric NOLM, including a quarter wave retarder and highly twisted fibre, followed by a polarizer. We first develop a theoretical analysis to show that, if the input polarisation is chosen linear, then for a correct choice of the wave plate and input polarisation angles the transmission grows from zero with a 4 dB/dB slope as power is increased, two times faster than can be obtained with a NOLM alone. An experimental study is then realised, which supports the theoretical results. Steep switching is demonstrated for reasonably low peak power levels that do not exceed 8 W, less than one third of the NOLM critical power. In addition, using the same setup we demonstrate the possibility to obtain a sinusoidal transmission characteristic that cancels out for a particular nonzero value of input power, which can be easily adjusted simply through rotation of the wave plate and output polarizer. This setup would be very useful in the frame of optical pulse shaping and signal processing applications, among others.

We investigate theoretically the operation of a novel, power-symmetric nonlinear optical loop mirror (NOLM) scheme, which relies on nonlinear polarisation rotation. The device is made of a symmetrical coupler, highly twisted fibre and a quarter wave plate (QWP) located asymmetrically in the loop, which can be rotated in a plane perpendicular to the fibre. Using a weak nonlinearity approach, the nonlinear NOLM transmission is accurately determined through a simple matrix description. For substantial twist, a simple though general analytical expression of the NOLM power transmission is found. This transmission strongly depends on the QWP angle, but also on the polarisation state at the NOLM input. This analysis allows to infer interesting properties of the switching characteristic, in particular its flexibility and, in some cases, its stability against small changes of input polarisation. The QWP angle and the input polarisation state are the control parameters through which the transmission can be adjusted in a versatile way, making the device attractive for various applications. Although the polarisation obtained at the NOLM output is power-dependent in general, we show that, for particular configurations, the output polarisation is constant with power. This is particularly interesting in the frame of polarisation-sensitive applications. We believe that the proposed model will be very useful to optimise the design and control of the proposed NOLM structure, which has the potential to play an important role in future ultrafast optical communication systems.

In this paper we report an experimental method to vary the transmission through a fiber optical loop mirror with a symmetrical coupler, highly twisted fiber and a quarter-wave (QW) retarder plate in the loop. We demonstrate that as the QW retarder plate is rotated it is possible to adjust the transmission over a wide range (from around 2% to around 82%) and that by adjusting the twist in the loop nearly ideal operation can be achieved. We provide a detailed theoretical analysis of the operating characteristics.

We investigate theoretically the operation of a versatile nonlinear optical loop mirror (NOLM) structure to be used in optical communication systems. The proposed device is a fibre Sagnac interferometer that includes a low-birefringence, highly twisted fibre and a quarter-wave plate retarder in the loop. We study, both analytically and numerically, the evolution of the intensity-dependent NOLM transmission for both output polarisation components, using different models for the NOLM. From this analysis, we propose an easy way to adjust the position of the NOLM maximum transmission, simply by tuning the angle of the retarder. This procedure is particularly useful for amplitude regularisation of an optical signal. We also demonstrate that, if a tuneable optical attenuator is inserted in the loop, the positions of both maximum and minimum transmission can be tuned separately, using a perfectly reproducible procedure. It is therefore possible to optimise the NOLM transmission for both pedestal and amplitude fluctuations removal in an optical pulse train. For a circular input polarisation, this procedure ensures the highest possible contrast between minimum and maximum transmission, and an output polarisation state that is linear and independent of the input power. Finally, we demonstrate that the transmission characteristic of this NOLM is robust to environmentally induced changes in the fibre birefringence. Thanks to its versatility, robustness and polarisation invariance, this device is thought to be of primary interest for applications such as passive mode locking, pulse compression and pedestal suppression, amplitude regularisation in harmonically mode-locked, rational-harmonically mode-locked or subharmonic synchronous mode-locked lasers, as well as damping of relaxation oscillations.

We investigate the influence of environmentally induced cavity length fluctuations on the energy and phase noises of an optical pulse train generated by an actively mode-locked erbium-doped fibre laser that is operated in the second-order rational harmonic mode-locking regime. Using a simple model that was recently proposed to describe this particular regime, we identify the specific nature of these energy and phase noises, and we demonstrate that their values averaged over two consecutive pulses in the train can be measured directly by implementing the time-domain demodulation technique. For a known cavity length variation, we compare the values of the average energy and phase fluctuations that are obtained both theoretically, using the model, and experimentally, through time-domain demodulation. This study brings out a mechanism of coupling between the energy and phase fluctuations that appears when cavity length fluctuations are at play. This coupling is caused by the periodic intracavity amplitude modulation and strongly depends on the average detuning of the cavity length. Through the measurement of the cross-correlation between energy and phase noises, we identify environmental perturbations as a major source of the noises that affect a pulse train generated through second-order rational harmonic mode locking.