Passive Fiber Research Articles

Investigations on the Extreme Frequency Shift of Phosphosilicate Random Fiber Laser

Random fiber lasers (RFLs), which employ random distributed feedback (RDFB) provided by the intrinsic Rayleigh scattering (RS) along the passive fiber, have gained wide attention in the last decade. A major advantage of Raman gain based RFL is wavelength flexibility, and the so-called cascaded Raman conversion can extend the operating wavelength considerably. However, frequently used silica fibers often need multiple Raman shifts to reach the final desired wavelength due to its peak frequency shift of ~13.2 THz. A good alternative is the phosphosilicate fiber which has a gain peak at ~39.8 THz. Nevertheless, the simultaneous existence of silica-related and phosphorus-related gain peaks in the phosphosilicate fiber intensifies the gain competition, further limiting the power scaling and the enhancement of spectral purity, even making it difficult to reach the desired wavelength. In this article, we experimentally explored the influences of the pump wavelength and bandwidth on the lasing performance of a phosphosilicate RFL, and measured the extreme frequency shift of the phosphorus-related Raman gain peak. Besides, by the aid of a modified power balance model, we qualitatively simulated the impacts of pump spectrum and boundary condition on the laser output. The simulation agrees well with the experimental results, and may also explain the parasitic lasing in the prior reports of phosphosilicate RFLs. As a result, the extreme frequency shift of phosphosilicate RFL is measured to be ~1 THz. And through optimizing the pump wavelength and pump bandwidth, 99.24% high spectral purity random lasing at 1237 nm with 33.1 W output power is obtained. This work may help to reveal the gain competition in phosphosilicate RFL and guide the future laser design.

Beam cleaning effects in multimode GRIN-fiber Raman lasers and amplifiers

Raman conversion in multimode GRIN fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation, which offers opportunities to create amplifiers and lasers of new type based on directly 9xx-nm diode pumped passive GRIN fibers, which may operate at almost any wavelength in transmission window (0.95-2 micron) of silica fibers. Here we review experimental results and physical mechanisms leading to the beam quality improvement (beam cleaning) in multimode Raman lasers and amplifiers, including effects of Raman gain, fiber Bragg gratings and Rayleigh backscattering feedback in GRIN fibers.

Investigation of speckle suppression beyond human eye sensitivity by using a passive multimode fiber and a multimode fiber bundle.

A method of speckle suppression without any active device is expected for pico-projectors. The effectiveness of the passive method of speckle reduction using a single multimode fiber and a multimode fiber bundle was actually measured and theoretically analyzed. The dependences of the speckle contrast and speckle suppression coefficient on the parameters of multimode fiber and projection systems were investigated. Our results shown that the efficiency of speckle suppression was limited because only the radial direction of the objective lens aperture was used. An improvement using both of the radial and azimuthal directions of the objective lens aperture is required.

Electron beam deposited silver (Ag) saturable absorber as passive Q-switcher in 1.5- and 2-micron fiber lasers

We demonstrate a passive Q-switching fiber laser using silver (Ag) as saturable absorber. The Ag-SA was fabricated using electron beam deposition technique onto a polyvinyl alcohol (PVA) thin-film and possessed a low modulation depth of 2% at 1.5-micron wavelength. The Ag-SA then was incorporated into two fiber laser cavities, 1.5- and 2-micron. At 1.5-micron, a laser with average output power of 3.8 mW was produced, with narrowest pulse of 3.1 μs. Besides, 2-micron able to produce high pulse energy of 161.7 nJ at pump power of 498 mW. Our experimental work suggest that the Ag saturable absorber is an effective solution for Q-switched generation in erbium-doped and thulium-doped fiber laser.

Analysis of loose bunches of soliton molecules in passive mode-locked fibre laser using time-stretching technology

Loose bunches of soliton molecules (SMs) in a passive mode-locked fiber laser with a WTe2 saturable absorber are observed using the photonic time-stretching (TS) technique. Due to the large temporal separations between the SMs, the TS profiles of the loose bunches appear as the intensity superposition of the spectral interference fringes of the adjacent SMs. The bandwidth limitations and the electronic system sampling rate contribute to the loss of phase information between adjacent SMs and the formation of the intensity overlap profiles. The phase difference and pulse width are estimated using a simulated annealing algorithm.

Tellurium-nanorod-based saturable absorber for an ultrafast passive mode-locked erbium-doped fiber laser.

In this paper, we propose and demonstrate ultrafast Te nanorods as a saturable absorber (SA) for producing mode locking from an erbium-doped fiber laser for the first time, to the best of our knowledge. The Te nanorods were fabricated by a simple green chemical method with energy conservation and without a purification process. The morphology and structure measurements confirm uniform Te nanorods with a constant aspect ratio. The synthesized SA has a saturation intensity and modulation depth of $25.44\, {\rm MW/cm}^{2} $25.44MW/cm2 and 4%, respectively. By integrating the proposed SA into an erbium-doped all fiber-based ring cavity, the mode-locked fiber laser was readily generated. The conventional soliton pulses of ${3.56}\;{\rm ps}$3.56ps pulse width were obtained at 1566.7 nm central wavelength and a pulse repetition rate of 1.87 MHz. The results show that the moderate saturable-absorption characteristics of Te nanorods have superior performance in the ultrafast optics field, which is eligible in many applications, such as optical communications.

Analysis of Seepage in a Laboratory Scaled Model Using Passive Optical Fiber Distributed Temperature Sensor

Seepage is the key factor in the safety of dikes and earth-fill dams. It is crucial to identify and localize the seepage excesses at the early stages before it initiates the internal erosion process in the structure. A proper seepage monitoring system should ensure a continuous and wide area seepage measurement. Here, continuous monitoring of seepage at the laboratory-scale is achieved by a passive optical fiber Distributed Temperature Sensing (DTS) system. An experimental model was designed which consists of initially unsaturated sand model, water supply, seepage outflow, optical fiber DTS system, and water and air temperature measurement. Initially, the sand temperature was higher than the temperature of the seepage water. An optical fiber DTS system was employed with a high-temperature resolution, short sampling intervals and short time intervals for temperature monitoring in the sand model. In the system, the small variation in the temperature due to groundwater flow was detected. The numerical analysis was conducted for both the seepage process and the heat transfer progression in the sand model. The results of the heat flow simulation were evaluated and compared with the measured temperature by the optical fiber DTS. Obvious temperature reduction was obtained due to seepage propagation in the sand. The rate of temperature reduction was observed to be dependent on the seepage flow velocity.

2 kW high-efficiency Raman fiber amplifier based on passive fiber with dynamic analysis on beam cleanup and fluctuation.

In this paper, we study the power scaling in high power continuous-wave Raman fiber amplifier employing graded-index passive fiber. The maximum output power reaches 2.087 kW at 1130 nm with an optical conversion efficiency of 90.1% (the output signal power versus the depleted pump power). To the best of our knowledge, this is the highest power in the fields of Raman fiber lasers based merely on Stokes radiation. The beam quality parameter M2 improves from 15 to 8.9 during the power boosting process, then beam spot distortion appears at high power level. This is the first observation and analysis on erratic dynamic properties of the transverse modes in high power Raman fiber amplifier.

Real-time wavelength routing based on Bragg reflection with integrated forward Raman for long-reach networks

We experimentally demonstrate the all-optical efficient technique for dynamic full-duplex wavelength routing and reach extension through adoption of passive fibre Bragg grating (FBG), energy efficient vertical cavity surface emitting lasers (VCSELs) and high gain forward Raman pump. In this study, two VCSEL channels at 1 549.62 nm and 1 550.01 nm are directly modulated with a 8.5 Gbit/s data signal each, and transmitted over 25.5 km fibre to a passive single mode FBG-based wavelength routing node. The precise wavelength selectivity of the FBG is exploited to achieve all-optical real-time wavelength routing of the 1 549.62 nm VCSEL channel with a maximum insertion loss of 22.1 dB. The 1 550.01 nm channel is transmitted through the FBG with a maximum penalty of 1.03 dB. By utilizing the 7.2 dB flat gain of a forward Raman pump, the routed 1 549.62 nm VCSEL channel is transmitted over 76.7 km fibre with a maximum penalty of 3.84 dB at bit error rate (BER) of 5−5. Our technique is all-optical, power efficient as it employs passive FBG and circulators and has low cross talk allied to precise wavelength selectivity of the technique.

Анализ эффективности методов защиты от атак активного оптического зондирования на волоконные системы квантового распределения ключей в спектральном диапазоне 1260-1650 nm

In this paper, the results of research of spectral features of several passive fiber optic components are presented. There are spectra of isolators, circulators, spectral filters, and WDM, that are used in quantum key distribution (QKD) systems, and contribute to the protection of these systems against active optical probing attacks. We also provide an estimate of the degree of isolation required to protect the QKD system from Trojan-horse attacks. Based on the isolation requirements and measurement data we analyzed the efficiency of the QKD system protection through the use passive isolating components. As an example, we have shown the achievement of the required isolation level (150 dB) in the considered spectral range in the system presented in the article.

L-band dual wavelength passively Q-switched erbium-doped fiber laser based on tellurium oxide nanoparticle saturable absorber

We experimentally demonstrate a compact dual wavelength passive Q-switched fiber laser created by an erbium-doped fiber active medium using tellurium oxide nanoparticles (TeO2 NPs) as a new saturable absorber (SA) material. The saturable absorption properties of the prepared TeO2 -PVA thin film were systematically measured, and the modulation depth and saturation intensity were 8% and 90 MW cm−2, respectively. By integrating the proposed SA in an all-fiber ring cavity, a dual wavelength laser with 1568.02 and 1569.09 nm wavelengths was observed without adding spectral filters or polarization maintaining fibers to control the suppression of the mode competition. A frequency difference of 0.13 THz was obtained located in the terahertz waveband. In the meantime, a stable Q-switched regime with a maximal pulse repetition rate of 45.7 kHz, a shortest pulse width of 2.69 µs and maximum pulse energy up to 39.4 nJ was attained at a maximum available pump power of 275 mW. The experimental setup using the TeO2 NP SA represents a step forward in terahertz development and nonlinear optics.

Noise-Like Pulse Generation Using Polarization Maintaining Mode-Locked Thulium-Doped Fiber Laser With Nonlinear Amplifying Loop Mirror

We investigate noise-like pulse (NLP) generation in a mode-locked thulium-doped fiber laser with nonlinear amplifying loop mirror (NALM) and different net anomalous dispersions. The all-polarization maintaining (PM) fiber oscillator generates pulse trains with a frequency adjustable in the range of 4.06-20.4 MHz by utilizing different lengths of PM passive fiber in the NALM loop. An NLP with a coherence spike width of 232 fs was generated at 1993.6 nm with a 3-dB spectral width of 32.6 nm when the net cavity dispersion was -0.711 ps <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Optical fibres and fibre tapers with an array of Bragg gratings

The properties of optical fibres with an array of fibre Bragg gratings written directly during the fibre drawing are considered. Such optical fibres offer new possibilities for producing new types of active and passive fibre elements due to the enhanced back-reflected signal of the so-called artificial Rayleigh scattering, and are of interest for modelling physical phenomena associated with the statistics of reflecting centres in optical fibres, such as, e.g., localisation of photons. We studied fibres with an array of Bragg gratings written in photosensitive fibres, ordinary single-mode optical fibre of the SMF-28 type and in fibre with a core doped with erbium ions. To control the spectrum width of the reflected signal, we used chirped phase masks, as well as writing arrays of gratings in tapered optical fibres.

Frequency downconversion of low-power Raman laser using intracavity difference frequency mixing

We report efficient frequency downconversion of a continuous-wave low-power Raman laser using intracavity difference frequency mixing in a periodically poled lithium niobate (PPLN)-adopted optical parametric oscillator (OPO). The Raman laser was obtained using a 1060-nm fiber laser and 137-m passive fiber. Its central wavelength was fixed at 1111 nm with the power ranging from 320 mW to 5.984 W. The 1060- and 1111-nm pump beams were incident into the OPO at the same time. The high-power 1060-nm pump beam built parametric oscillation first, and the difference frequency generation (DFG) occurred between the low-power Raman laser and intracavity signal laser. The PPLN temperature was properly controlled at 45°C to ensure both the OPO and DFG processes synchronously satisfy phase matching conditions. Benefiting from intracavity high signal power, the Raman laser was successfully converted to the 3560-nm midinfrared radiation under every investigated pump power level. The maximum 3560-nm idler power reached 1.026 W, indicating a 17.4% pump-to-idler slope efficiency and about 15% optical-to-optical conversion efficiency. The comparative experiments also verified that the phase matching conditions were satisfied maximally at 45°C.

Q-switched Thulium-doped fibre laser using Bismuth (III) Telluride-based saturable absorber

We demonstrate a passive Q-switched thulium-doped fibre laser (TDFL) pulse generation using bismuth (III) telluride (Bi2Te3) thin film as saturable absorber. The Bi2Te3 was fabricated by a simple processing technique; simply by embedding the Bi2Te3 into the polyvinyl alcohol (PVA) film. By sandwiching 1 mm x 1 mm Bi2Te3 thin film between two fibre ferrules in a TDFL ring cavity, a stable Q-switching pulses train was generated. The repetition rate and pulse width were tuneable from 10.72 kHz to 39.9 kHz and 13.08 µs to 5.98 µs, respectively, as the pump power increased from 309 mW to 454 mW. The slope efficiency was 4.51%, while, the maximum pulse energy was calculated as 0.17 µJ.

Samarium (III) oxide thin film as a saturable absorber for the passively Q-switched Tm-doped fiber laser

We have demonstrated a passive Q-switched Thulium-doped fiber laser (TDFL using samarium oxide (Sm2O3) nanomaterial as saturable absorber (SA). The Sm2O3 based SA was fabricated by simply mixing the Sm2O3 powder solution with polyvinyl alcohol (PVA) solution. The homogeneously mixed solution was spread and dry to form a thin film. A piece of 1 mm x 1 mm of the SA thin film is sandwiched between two fiber ferrules and incorporated into a TDFL ring cavity for pulses generation. By controlling the loss and gain in the cavity, stable Q-switching operation was generated. The repetition rate was tunable from 17.62 kHz to 29.20 kHz by varying the pump power from 619 mW to 784 mW. The smallest pulse width of 3.54 µs and the highest pulse energy of 0.20 uJ were obtained at the highest pump power.

High-Temperature Sensitivity in Stimulated Brillouin Scattering of 1060 nm Single-Mode Fibers.

With the rapid advancement of Yb-doped fiber lasers (YDFL) whose output wavelength is near 1060 nm, passive fibers to carry the high optical power at the spectral range are also gaining significant importance. Stimulated Brillouin scattering (SBS) in the passive fibers connecting components in the lasers, especially, can set a fundamental limit in the power handling of YDFL systems. We experimentally analyzed SBS characteristics of passive single mode fibers (SMF) at a wavelength of 1060 nm. For two types of SMFs (Corning HI1060 and HI1060Flex), the Brillouin frequency (νΒ), its linewidth (ΔνΒ), and their variations with respect to the input laser power and the surrounding temperature were experimentally measured, along with the SBS threshold power (Pth). The optical heterodyne detection method was used to identify temperature-dependent SBS characteristics of fibers, and we found SMFs at λ = 1060 nm showed a temperature sensitivity in SBS frequency shift more than 40% higher than in conventional SMFs operating in C-band. Detailed procedures to measure the SBS properties are explained, and a new potential of 1060 nm SMF as a distributed temperature sensor is also discussed.

Active loss compensation in the resonant optical gyroscope with a "reflector": towards a cost-effective alternative for the navigational grade.

The performance of passive fiber optic gyroscopes involving ring resonators is limited mainly by the loss and finesse of the cavity. In this work, we show performance enhancement of the recently studied resonant fiber optic gyroscope with a "reflector" using active loss compensation. Our gyroscope does not require expensive ultra-narrow linewidth lasers, expensive lock-in detection methods, or polarization-maintaining fibers, which are mandatory for all standard resonant fiber optic gyroscopes. The performance of this gyroscope shows four-fold enhancement in the $Q$Q factor $ (2.2 \times {10^8}) $(2.2×108), compared to an earlier experimental setup involving losses. Enhanced sensitivity to rotation is experimentally demonstrated using loss compensation as well as tuning the embedded reflector in the resonator. A shot-noise-limited sensitivity of 0.03 deg/h is possible with this experimental realization. This work demonstrates that our gyroscope can provide a cost-effective alternative, even in the navigational grade.

Shaping of amplified beam from a highly multimode Yb-doped fiber using transmission matrix.

The transmission matrix of an ytterbium doped multimode fiber with gain was measured. It was shown to vary owing to the pump power level. Amplified beam focusing, beam steering and shaping were demonstrated using the measured matrix for input wavefront shaping, with an efficiency similar to the case of a passive fiber. The impact of weak gain saturation was lastly investigated.

Active–passive Q-switched fiber laser based on graphene microfiber

We report an active–passive Q-switched laser based on graphene-covered microfiber modulator. The graphene-covered microfiber not only serves as a passive saturable absorber in a single laser cavity, but also can be used as an all-optical modulation device to synchronize two pulses generated from different wavelengths. The laser can not only actively change the output frequency of the Q-switched pulse to achieve the repetition frequency reduction, but also compress the pulse and increase the peak power of the output pulse. We successfully achieve the output of the active–passive Q-switched pulse using this fiber laser with a repetition rate from 41.1 kHz to 50.5 kHz. The fiber laser has potential applications in simultaneous output of multi-wavelength pulse.