Search

A tunable wavelength operation of a linear cavity Thulium-doped fiber laser based on the use of a fiber optical loop mirror with high birefringence (Hi-Bi) fiber in the loop is experimentally demonstrated.

Nonlinear polarization dynamics of single and coupled solitons in twisted fibers is experimentally and numerically studied. The output polarization for coupled solitons evolves toward a circular polarization and shows a jump at linear input ellipticity.

We experimentally demonstrated a stable, wavelength-tunable fiber laser using a polarization-maintaining, double-clad Er:Yb doped fiber amplifier in the cavity. The output wavelength is tunable over the range from 1535 to 1567 nm using a fixed grating and the dichroic mirror placed on a rotational mount; under rotation of the dichroic mirror the tuning ratio of 50 nm/deg was found. We studied the wavelength tuning range dependence on the amplifier fiber length and achieved a maximal output power of 850 mW. This configuration can be Q-switched for high peak power and its narrow bandwidth is suitable for nonlinear optics applications, such as parametric teraherthz generator.

We demonstrate experimentally the operation of a linear cavity dual-wavelength fiber laser using a polarization maintaining fiber Bragg grating (PM-FBG) as an end mirror that defines two closely spaced laser emission lines. The PM-FBG is also used to tune the laser wavelengths. The total tuning range is ∼8 nm. The laser operates in a stable dual-wavelength mode for an appropriate adjustment of the cavity losses for the generated wavelengths. The high birefringence (Hi-Bi) fiber optical loop mirror (FOLM) is used as a tunable spectral filter to adjust the losses. The FOLM adjustment was performed by the temperature control of the Hi-Bi fiber.

We experimentally demonstrate simultaneous Tm3+ passive Q-switched (PQS) and Ho3+ gain-switched laser operations at 1888.8 and 2021.2 nm, respectively, in a single-cavity all-fiber laser. The PQS operation of the Tm3+ laser is based on the use of a high-concentration holmium-doped fiber as a fiber saturable absorber. Then the Tm3+ laser emission is used as a pulsed pump source to achieve Ho3+ gain-switched pulses. A high birefringence fiber optical loop mirror used as a spectral filter allows the tuning of both Tm3+ and Ho3+ laser emissions.

We experimentally demonstrate a fine adjustment of cavity loss by Sagnac loop for a dual wave-length generation. The single or dual wavelengths are obtained by controlling the losses on both cavities through a fiber optical loop mirror (FOLM). Wavelength separation on the dual laser is 0.98 nm. The dual or single wavelength is obtained by changes in temperature in the order of 10−1°C around the maximum in the FOLM. Also, we investigate energy fluctuations on signal level saturation effect in the cavity through different pumping power that act on the EDF, where we note that from the 60-mW pumping begins to generate dual-wavelength and 80-mW stabilizes.

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.

The operation of a power-symmetric NOLM using the nonlinear polarization rotation, is investigated. We designed our device with a symmetrical coupler, highly twisted fiber and a QWR located asymmetrically in the loop.

We report an all-fiber dual-wavelength actively Q-switched pulsed laser. The linear cavity is limited by two fiber Bragg gratings with central wavelengths of 1538.3 nm and 1548 nm respectively in one side and Sagnac interferometer (SI) in the other side. The pump power is coupled into the doped fiber through a beam combiner. The SI used to equalize generated laser lines competition by cavity losses adjustment is formed by high birefringence (Hi-Bi) fiber in the loop. The SI spectrum wavelength displacement adjustment is performed by temperature variations applied on the Hi-Bi fiber loop with a temperature controller. The Q-switch operation is achieved at pulse repetition rate of 120 kHz with average pulsed output power of 404 mW and pulse duration of 470 ns.

We propose in this work a technique for determining the amplitude distribution of a wave packet containing a large number of short optical pulses with different amplitudes. The technique takes advantage of the fast response of the optical Kerr effect in a fiber nonlinear optical loop mirror (NOLM). Under some assumptions, the statistics of the pulses can be determined from the energy transfer characteristic of the packet through the NOLM, which can be measured with a low-frequency detection setup. The statistical distribution is retrieved numerically by approximating the solution of a system of nonlinear algebraic equations using the least squares method. The technique is demonstrated numerically in the case of a packet of solitons.

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.

An all-fiber curvature laser sensor by using a novel modal interference in-fiber structure is proposed and experimentally demonstrated. The in-fiber device, fabricated by fusion splicing of multimode fiber and double-clad fiber segments, is used as wavelength filter as well as the sensing element. By including a multimode fiber in an ordinary modal interference structure based on a double-clad fiber, the fringe visibility of the filter transmission spectrum is significantly increased. By using the modal interferometer as a curvature sensitive wavelength filter within a ring cavity erbium-doped fiber laser, the spectral quality factor Q is considerably increased. The results demonstrate the reliability of the proposed curvature laser sensor with advantages of robustness, ease of fabrication, low cost, repeatability on the fabrication process and simple operation.

An experimental study of an all-fiber tunable thulium-doped fiber laser based on an abrupt-tapered in-fiber interferometer is presented. A microfiber filter with length of 6 mm and diameter of 20 μm is used to achieve single laser wavelength tuning in a range of 19.4 nm and dual-wavelength laser operation at 1761.8 and 1793.4 nm with a channel spacing of 31.6 nm. The abrupt-tapered structure allows multi-modal interference at the air-cladding interface. The proposed in-fiber interferometer exhibits characteristics of low cost and simple fabrication, making it suitable for practical applications in wavelength filtering and wavelength selection in all-fiber lasers.

Actively Q-switched laser operation of a narrow linewidth compact fiber laser based on an Er–Yb double-clad fiber is presented. The laser linewidth as a function of the repetition rate and the Q-switched pulses characteristics for different pump powers are experimentally analyzed. Stable Q-switched laser operation with spectral laser linewidth of 73 pm in a repetition rate range from 90 to 270 kHz is obtained. The minimum pulse duration of 178 ns, maximum peak power of 30.5 W, and maximum pulse energy of 5.4 µJ are observed. The maximum average power reached is 1.1 W.

A compact, highly sensitive optical fiber displacement and curvature radius sensor is presented. The device consists of an adiabatic bi-conical fused fiber taper spliced to a single-mode fiber (SMF) segment with a flat face end. The bi-conical taper structure acts as a modal coupling device between core and cladding modes for the SMF segment. When the bi-conical taper is bent by an axial displacement, the symmetrical bi-conical shape of the tapered structure is stressed, causing a change in the refractive index profile which becomes asymmetric. As a result, the taper adiabaticity is lost, and interference between modes appears. As the bending increases, a small change in the fringe visibility and a wavelength shift on the periodical reflection spectrum of the in-fiber interferometer is produced. The displacement sensitivity and the spectral periodicity of the device can be adjusted by the proper selection of the SMF length. Sensitivities from around 1.93 to 3.4 nm/mm were obtained for SMF length between 7.5 and 12.5 cm. Both sensor interrogations, wavelength shift and visibility contrast, can be used to measure displacement and curvature radius magnitudes.

We report on a ring cavity, multi-wavelength, passive Q-switched erbium–ytterbium double cladding fiber laser based on the use of an unpumped segment of Tm-doped fiber acting as a saturable absorber for passive Q-switched pulse generation and a wavelength filter for multi-wavelength laser generation. By performing pump power variations from 1.6 to 9.8 W, stable Q-switched laser pulses are observed in a repetition rate from 135.8 to 27.5 kHz at room temperature. With a maximal repetition rate of 135.8 kHz, the minimum pulse duration of 430 ns is obtained. The maximal average output power of 2.2 W is reached with a pump power of 9.8 W. The maximum pulse energy was 16.4 µJ and the average output power slope efficiency is ~24.8%. The obtained results demonstrate a laser performance with extended range of high repetition rate and improved stability.

We present an experimental method for straight forward dual wavelength Erbium doped fiber linear cavity laser characterization based in laser line spectrum behavior due to the Hi-Bi FOLM transmission spectrum wavelength displacement by temperature variations in the fiber loop. The laser operation is for a single and dual mode, obtained through the adjustment of the cavity losses by the Sagnac interferometer spectrum wavelength displacement due to the temperature variation of the fiber loop. The method allow determine the laser operation from a single emission line and a two emission lines simultaneously through the Sagnac transmittance spectrum optical power variations measurement due to wavelength spectrum shifting for each laser wavelength generated separately and overlapping these obtained spectrums.

In this paper a linear cavity Erbium doped fiber (EDF) laser based in a fiber Bragg grating (FBG) and a fiber optical loop mirror with a high birefringence fiber in the loop (Hi-Bi FOLM) is used as a strain sensor. The Fabry-Perot cavity is formed by the FBG and the Hi-Bi FOLM, used as a measurement system of strain variations produced on the FBG, used as a strain sensor device. Usually, fiber laser sensor experimental setups determine the measured variable magnitude by using of an optical spectrum analyzer (OSA). Hi-Bi FOLM transmission spectrum wavelength displacement by fiber loop temperature variations measurement can be an attractive application exploiting the characteristics of FOLM transmission spectrum behavior due to Hi-Bi fiber loop temperature variations to determine the FBG strain applied through the maximal optical power monitoring by simple use of a photodetector and a temperature meter.

A ring cavity Erbium doped fiber laser with generated laser line wavelength widely tuned in a range of 39nm from 1555nm to 1593nm by the use of a Sagnac interferometer is presented

In general, the propagation of light in a twisted, birefringent fiber evolves with a period and phase change non-linearly dependent on the initial polarization state. We show both analytically and numerically that a critical value for the ratio of twist rate to birefringence can be chosen so that the period and nonlinear phase change are no longer sensitive to the input polarization state. This makes possible the design of a input-state independent Sagnac interferometer for mode-locked laser applications.

We experimentally investigate an all-fiber passively mode-locked laser generating ps pulses. The experimental setup is a figure eight fiber laser configuration, including a power-symmetric Nonlinear Optical Loop Mirror (NOLM) with highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by polarization asymmetry between the counter-propagating beams in the loop. We used a Quarter-Wave Retarder in the loop to break the polarization symmetry. Using a polarizer beam-splitter cube as the NOLM output we got the best quality output pulses from the laser. At this output, we are monitoring the output pulse polarization component which is parallel to the input NOLM component. We achieved stable generation of ∼25 ps pulses at the repetition frequency of 0.78 MHz with milliwatts average output power. The mode-locked laser ran in stable operation for hours.

We study experimentally the behavior of a dual wavelength Erbium doped fiber laser based on superimposed fiber Bragg gratings (SI-FBGs) and a loop mirror. The linear cavity is formed by a Hi-Bi Fiber Optical Loop Mirror (HBFOLM) and a SI-FBG. Three pairs of SI-FBGs were used to tune the dual wavelength laser at different wavelength ranges. The SIFBGs were placed on a mechanical mount that allow compression, both lines are shifted simultaneously. The separation of SI-FBGs is 5nm, and the set of three SI-FBGs permit to cover almost the entire spectrum of erbium from 1529nm to 1556nm approximately. The HBFOLM is used as a spectral tunable filter, which adjusts the losses by controlling the temperature in the Hi-Bi fiber, in order to stabilize the dual wavelength emission of the laser.

We show that the cavity quality for generated wavelengths in dual-wavelength laser must be substantially different for stable dual wavelength operation. In experiments the linear cavity included two FBGs and a fiber optical loop mirror (FOLM) with a high-birefringence fiber in the loop. The reflection spectrum of the FOLM shifts through temperature control of the hi-bi fiber loop that allows an adjustment of the ratio between FOLM reflections at generated wavelengths. The dual wavelength operation required an appropriate adjustment of this ratio. We measured the ratio necessary for dual wavelength operation at different wavelength separation. Wavelength separation is tuned by axial tension applied to one of FBG in the range 0.6 - 3 nm. Shorter wavelength always required higher reflection, and ratio between short wavelength reflection and long wavelength reflection increased strongly with wavelength separation. The tolerance of dual wavelength generation to reflection adjustment was measured to be about 1%.

Pulse breakup and the formation of a bunch of solitons are the principal processes at the initial stage of the supercontinuum generation using long pulses for pumping. Most investigations use the measurement of the output spectrum to characterize the development of the supercontinuum. The extraction of an individual soliton or a group of solitons with similar parameters from the bunch can reveal details that are usually hidden when only the output spectrum is measured. Earlier we have studied the NOLM including a twisted fiber and a quarter wave retarder (QWR) in the loop. Its operation is based on the nonlinear polarization rotation effect. We showed that this NOLM is stable to changes of environmental conditions, and allows simple and predictable changes of its characteristics. In previous works we demonstrated its application for mode-locked lasers, pedestal suppression, or retrieval of a pulse shape. In this work we demonstrate that the NOLM is a viable device for the investigation of pulse breakup process and soliton formation. The operation principle is based on the fact that the NOLM has a maximum transmission for the solitons with specific durations while solitons with shorter and longer durations are strongly rejected. The duration associated with high transmission depends on the NOLM length and can also be changed by amplification of the solitons before entering the NOLM. By an appropriate choice of the NOLM parameters and the amplification of the bunch of solitons, the extraction of the solitons with selected parameters is possible.

We demonstrated a wavelength-tunable fiber laser using a double-clad Er:Yb doped fiber into the cavity. The output wavelength is tunable from 1535 to 1567 nm. We achieved a maximal output power of 850 mW.

An all fiber based compact phase-locking fiber laser design is proposed by using a Talbot mirror fiber device (TMFD). We numerically demonstrate that periodically placed fibers lasers can be phased together using a large mode area (LMA) fiber element. The LMA fiber with a partial reflector at one end acts as a Talbot mirror, when the input beam to the LMA fiber travels one round trip in the LMA, it will self image back at the fibers' input.

Wavelength-tunable sources are important for applications such as fiber optics sensing, medical instrumentation, and optical communication. A tunable source design can use a Fabry-Perot (FP) cavity configuration with a tunable filter inserted into the system. In this work we experimentally demonstrated a wavelength-tunable fiber laser by the use of a PM-Er:Yb double-clad fiber placed in a FP cavity formed by two Dichroic Mirrors (DM). Our pump laser wavelength was 976 nm and the maximal output power from this laser that we can introduce to the fiber was 20 W. To get wavelength tunability we included a Diffractive Grating (DG) with 600 l/mm in our laser cavity. The zero order reflection from the DG is the output coupler and the first order diffracted beam is reflected back into the fiber using a DM. We included a half-wave plate and a polarizer in the cavity to increase the stability of the continuous wave laser operation and also to impose a linear polarization at the laser output. With this configuration we can tune the wavelength from 1535 to 1567 nm by rotating a DM at a rate of 0.02 degrees/nm. With our configuration wide tunability was possible only when we used fiber lengths between 2.5 and 3.6 m. The maximal output power was 850 mW using 3.6 m of fiber length. We believe that a Q-switched extension of this source can be used in nonlinear optics applications, such as Terahertz generation.

We report numerically and experimentally analysis of optic fiber Sagnac interferometer and fine adjustment of cavity loss by the use of the FOLM with a hi-bi fiber in the loop. Changes in transmittance profile amplitude and wavelength shift are caused by the whirl effect in the connectors of a coupler with ports output connected to a birefringent fiber. Also the experimental demonstration of dual wavelength operation of a fiber laser through fine adjustment of cavity loss, using a Fiber Optical Loop Mirror (FOLM) with a high-birefringence fiber in the loop. The reflection and transmission of the FOLM presents a sinusoidal wavelength dependence which can be shifted by controlling the temperature of the hibi fiber. A temperature change of the hi-bi fiber by 0.1°C causes a measurable change in the ratio between the reflectance for the wavelengths R(λ 1 )/R(λ 2 ). Using this adjustment be able to change the generation mode from single wavelength to stable dual wavelength generation with equal powers for λ 1 and λ 2 or to stable dual wavelength generation with unequal powers at λ 1 and λ 2 . The change of the ratio between the FOLM reflection R(λ1)/R(λ2) allows the investigation of tolerance of dual wavelength generation on the ratio between cavity loss. Was found that for the switch from a single wavelength emission at λ 1 to single wavelength emission at λ 2 the ratio R(λ 1 )/R(λ 2 ) has to be changed by the order of magnitude of 10 -2 . This value shows the tolerance of the dual wavelength laser to the cavity loss adjustment.

An erbium-doped fiber laser cavity design is demonstrated to produce radially and azimuthally polarized beams. A c-cut calcite crystal is set within a three-lens telescope system in the laser cavity. Due to the birefringence of the crystal, radial and azimuthal polarizations are focused to different foci. The ray behavior through the crystal is discussed in details with birefringent ray tracing. By translating a collimation lens, one can select either the radial or the azimuthal polarization to lase in the cavity. The maximum output power obtained is about 140 mW for both polarizations.

We numerically demonstrate that periodically placed fibers can be phased together using a large mode area fiber element. The LMA fiber element is a Talbot mirror that images the reflected beams at the fibers’ input.

We present a theoretical analysis of pulse propagation and self-focusing in a gain-guided (GG) fiber amplifier. A weak pulse is launched in the GG fiber when the input pulse reaches a critical power the pulse begins to collapse in the transverse direction. By using different input powers the transmission characteristics are changed. We add coupling to a single-mode fiber at the output end and study pulse dispersion and energy.

We examine the transmission characteristics of a NOLM device using a symmetrical coupler, highly twisted fiber, and a quarter-wave (QW) retarder plate introducing a polarization asymmetry in the loop. We demonstrate high dynamic range with controllable transmissivity, and good stability over long times. We experimentally study the transmission behavior for different input polarization states and distinguish between different polarization components of the output beam. Experiments are in good agreement with our theoretical approach previously published. Appropriate choice of the input and output polarizations allows a very high dynamic range. The adjustment of the QW retarder and input polarization enables tuning the critical power over a wide range.

We report experimental observations of stimulated Raman scattering in a standard fiber using a directly modulated DFB semiconductor laser amplified by two erbium-doped fibers. The laser pulse width was variably controlled on a nanosecond-scale; the laser emission was separated into two distinct regimes: an initial transient peak regime, followed by a quasi steady-state plateau regime. The transient leading part of the pump pulse containing fast amplitude modulation generated a broadband Raman-induced spectral shift through the modulation instability and subsequent intra-pulse Raman frequency shift. The plateau regime amplified the conventional Stokes shifted emission expected from the peaks of the gain distribution. The output signal spectrum at the end of a 9.13 km length of fiber for the transient part extends from 1550 nm to 1700 nm for a pump pulse peak power of 65 W. We found that the Raman-induced spectral shift is measurable about 8 W for every fiber length examined, 0.6 km, 4.46 km, and 9.13 km. All spectral components of the broadband scattering appear to be generated in the initial kilometer of the fiber span. The Stokes shifted light generation threshold was higher than the threshold for the intra-pulse Raman-induced broadened spectra. This fact enables the nonlinear spectral filtering of pulses from directly modulated semiconductor lasers.

We investigated numerically the transmission of a set of solitons through a nonlinear optical loop mirror (NOLM) and found that under some conditions a single soliton can be extracted. We analyzed the evolution of relatively long (20-50 ps) and no very strong (10 W) pulses. The results show that the input pulse duration and the amplification of the solitons resulting from pulse breakup play important role to extract the highest quality pulse. The transmitted pulses correspond to fundamental solitons with subpicosecond durations. We believe that the proposed method can be useful for the study of nonlinear phenomena in optical fibers.

We experimentally demonstrated a wavelength-tunable passively mode-locked Erbium-doped figure-eight fiber laser. Wide tunability is achieved using a high-birefringence (hi-bi) fiber Sagnac loop. The filter transmission function is controlled by selecting the hi-bi fiber loop length. The output pulses are wavelength tunable over a range from 1525 nm to 1555 nm. The FWHM of the autocorrelation trace is about 3.1 ps and the pulse spectrum has a FWHM of 1.5 nm. The pulse temporal and spectral widths remain constant over the tunable range.

We experimentally investigate picosecond pulse generation in an all-fiber mode-locked laser based on a nonlinear polarization asymmetry in a fiber loop configuration. The laser generates 9 ps pulses at a 0.78 MHz repetition frequency.

In this paper, the authors discussed the enhanced possibilities offered by this device when a single polarisation component is selected at its output. Applications include all-optical signal processing, regeneration, and self-starting passive mode locking of fibre lasers.

We report experimental results that demonstrate the mode-locking operation of a figure eight fiber laser based on a symmetrical nonlinear optical loop mirror (NOLM) with a twisted low-birefringence fiber in the loop. The mode-locking operation is achieved by nonlinear polarization rotation in the NOLM, where the counterpropagating beams accumulate a differential nonlinear phase shift when they have different polarizations. By adjusting a single quarter wave (QW) retarder in the NOLM it was possible to change the NOLM transmission behavior from a maximum to a minimum at low input power. Self-starting mode-locking operation was found for only specific settings of the QW retarder. The pulse repetition frequency was around 0.8 MHz. The mode-locked laser operated stably for hours and the adjustment procedure was straightforward. We achieved stable generation of picosecond pulses with milliwatts of average output power.

We investigated spectral broadening in a standard fiber using a nanosecond directly modulated DFB laser (λ=1549 nm), amplified by a two stage Erbium-doped fiber amplifier. The amplifier provided amplification of 2-mW peak power input pulses to 100-W peak power output pulses. In other hand, the directly modulation of DFB lasers caused transient oscillations at the beginning of pulses. In our case pulses consisted of a 2-ns transient part followed by a steady-state plateau. We used a monochromator to measure the spectrum at the fiber output. A fast photodetector was placed at the monochromator output and pulse shapes were measured for different wavelengths. This technique allowed the separate measurement of different parts in output pulses spectrum. We used the SMF-28 fiber with the standard dispersion of 20 ps/nm-km for our wavelength. We made measurements of the output spectra for three fiber lengths: 0.6-km, 4.46-km and 9.15-km; finding that the initial transient part of a pulse shows supercontinuum generation whereas the plateau results in conventional Raman amplification of this supercontinuum.

We report an original noise-like pulse dynamics observed in a figure-eight fiber laser, in which fragments are continually released from a main waveform that circulates in the cavity. Particularly, we report two representative cases of the dynamics: in the first case the released fragments drift away from the main bunch and decay over a fraction of the round-trip time, and then vanish suddenly; in the second case, the sub-packets drift without decaying over the complete cavity round-trip time, until they eventually merge again with the main waveform. The most intriguing result is that these fragments, as well as the main waveform, are formed of units with sub-ns duration and roughly the same energy.

We report a self-Q-switched Erbium-Ytterbium-doped double cladding fiber ring laser with dual wavelength or tunable single wavelength operations. A Sagnac interferometer with a high birefringence fiber in the loop was used for the wavelength tuning of the single line operation and cavity loss adjustment for dual wavelength laser operation. Single wavelength laser operation for a pump power of 421 mW tunable in a range of 1561.4 nm to 1569.8 nm and dual wavelength laser operation at 1561.1 nm and 1571.4 nm with equal output powers are presented.

We report an experimental study of the noise-like pulses generated by a ~300 m long passively mode-locked erbium-doped figure-eight fibre laser. Non-self-starting mode locking yields the formation of ns scale bunches of sub-ps pulses. Depending on birefringence adjustments, noise-like pulses with a variety of temporal profiles and optical spectra are obtained. In particular, for some adjustments the Raman-enhanced spectrum reaches a 10 dB bandwidth of ~130 nm. For the first time to our knowledge, we extract information on the inner structure of the noise-like pulses, using a birefringent Sagnac interferometer as a spectral filter and a nonlinear optical loop mirror as an intensity filter. In particular we show that the different spectral components of the bunch are homogeneously distributed within the temporal envelope of the bunch, whereas the amplitude and/or the density of the sub-pulses present substantial variations along the envelope. In some cases, the analysis reveals the existence of an intermediate level of organization in the structure of the noise-like pulse, between the ns bunch and the sub-ps inner pulses, suggesting that these objects may be even more complex than previously recognized.

An actively Q-switched double-wavelength Er/Yb fiber laser is experimentally demonstrated. The linear cavity is formed by a pair of fiber Bragg gratings on one side and a Sagnac interferometer (SI) with high birefringence fiber in the loop on the opposite side. A 3 m of double-cladding Er/Yb-doped fiber used as a gain medium is pumped by a 978 nm laser diode. The SI is used to adjust the internal cavity losses for simultaneous dual-wavelength laser generation. The adjustment is performed by temperature variations of the high birefringence fiber in the SI loop. The maximum average output power for the Q-switched laser operation in dual-wavelength mode was around 68 mW with a repetition rate of 40 kHz for 2 W of pump power. The minimum pulse duration was around 280 ns. The maximum pulse energy was 1.75 µJ.

The nonlinear optical loop mirror (NOLM) is used in application like optical switching and demultiplexing, all-optical active mode locking, passive mode locking, pedestal suppression, pulse shaping, etc. This device offers a versatile way to obtain a nonlinear transmission behavior through the nonlinear differential phase shift between the two interfering beams due to the self-phase modulation. Recently we discussed a NOLM device using a symmetrical coupler, highly twisted fiber, and a quarter-wave (QW) retarder introducing the polarization asymmetry in the loop. We have shown high contrast operation, flexibility of characteristics, and stability in time. In this report we analyze theoretically and study experimentally the transmission behavior for different input polarization considering as well different output polarizations. We propose a simple description of the NOLM transmission for right- and left-hand circular output polarization at different output polarization states. The nonlinear characteristics depend on the QW retarder plate angle, but also on the polarization state at the NOLM input. Experiments were carry out with the NOLM consisted of a 500-m length SMF-28 fiber with twist rate of 7 turns/meter. Experiments show a good agreement with our theoretical approach. Appropriate choice of the input and output polarizations allows very high contrast, at least higher than 5000. Our measurements were restricted by sensitivity of our detection system to measure low power pulses at conditions when theoretically transmission must be equal zero. Experiments show flexibility and stability of characteristics. Adjustment of the QW retarder and input polarization allows tuning the critical power over a wide range.

We report the investigation of stimulated Raman Scattering (SRS) in a standard fiber using a directly modulated DFB laser with wavelength equal to 1549 nm, amplified by a two stage Erbium-doped fiber amplifier. The amplifier provides amplification of 2-mW peak power input pulses to 100-W peak power output pulses. This setup provides a simple source of high power pulses for nonlinear investigation. However, the directly modulation of DFB lasers causes transient oscillations at the beginning of pulses. In our case pulses consisted of a 2-ns transient part followed by a steady-state plateau. We used a monochromator to measure the spectrum at the fiber output. A fast photodetector was placed at the monochromator output and pulse shapes were measured for different wavelengths. This technique allows the separate measurement of the spectrum of different parts of output pulses. We used the SMF-28 fiber with the standard dispersion of 20 ps/nm-km for our wavelength. We made measurements of the output spectra for four fiber lengths: 210 m, 600 m, 4.5 km, and 10 km. For short fiber lengths (210 m and 600 m) the spectra of the transient part and the plateau part were similar and manifest modulation instability (MI) and four-wave mixing (FWM). For 4.5-km long and 10-km long fibers we found that the transient part generates a continuous broadband spectrum while the plateau causes the Stokes wave with the spectrum maximum centered at 1656 nm.

We measure the transmission characteristics of the nonlinear optical loop mirror with a polarization asymmetry. Simple polarization adjustments enable the nonlinear control of the transmission for applications such as pedestal suppression, amplitude regularization, and mode-locking.

The fiber Raman amplifier employs the intrinsic properties of silica fiber to obtain the amplification, thus they use the transmission fiber as the amplification medium, where the gain is created along the transmission line. The amplification is realized by Stmulated Raman Scattering (SRS). This nonlinear process occurs when a sufficiently powerful pump is within the same fiber as the signal. In this paper, we showed experimental and numerical analysis of SRS in optical fibers. We obtain a continuous spectral when we plot the energy content in each Stokes sublines with the wavelength, which are self-pump between them. The numerical results are in agreement with the experimental results, just as the waveform in the time of optical fiber end and the energy is transferred from the signal pump to the Stokes sublines. With the simulations, we can obtain several parameters of this optical amplifier like the optical fiber effective length to obtain the amplification.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.