An Erbium-Ytterbium co-doped doublecladfiber Q-switch wavelength tunable laser based on self Q-switching principle is proposed in a simple ring cavity configuration. This laser doesn’t employ any component like polarization controller, fiber bragg grating, un-pumped fiber section, Mach-Zehnder interferometer and saturable absorber as is usually used to produce Q-switch lasers. This laser is successfully tuned in the range 1535-1550nm by using OTF-320 manually tunable optical filter. We observe a stable Q-switch laser in the range 1535 to 1550nm, with pulse energy of 17nJ at 1445 nm. An increase in repetition rate is observed from 31.53 to 35.64 KHz with increase in the filter wavelength from 1535 to 1550 nm.
the inner cladding and 1200 dB/m in the core. The in- ner cladding has an octagonal shape with 125 µm aver- age diameter and the core has the diameter of 6 µm and numerical aperture of 0.13. The outer cladding is made from low refractive index polymer. The laser output of the Fabry-Perot laser was obtained from perpendicularly cleaved fiber ends and in the case of ring YDFL the output was obtained from a fused optical fiber coupler. Usually, the 80% branch of the coupler was used for the output. A pigtailed optical fiber isolator was inserted into the ring cavity to ensure unidirectional propagation of the laser sig- nal. The fiber isolator was the main lossy element in the cavity and as such it was placed behind the output cou- pler where the laser signal is minimal. Such a position also helped to prevent damage of the isolator by eventual self- Q-switched pulses. This configuration is commonly used for high-power fiber-ring lasers . In the experiments, we used one multimode-pump laser diode pigtailed with mul- timode fiber of 105 µm core diameter. Unless otherwise stated, the temperature of the pump laser diode was stabi- lized to 25 ◦ C and its central wavelength at maximum out-
From a large number of EYDFs fabricated in house, we selected a fiber with large pump absorption for the experiments described here. The high pump absorption allowed for a good efficiency for fibers as short as 1.4–2 m, even in this cladding-pumped configuration. Short fibers are attractive for mitigating various nonlinear effects, such as stimulated Brillouin scattering. We measured the absorption in a 1-m-long piece of fiber and found it to be 2.7 dB at the absorption peak at around 975 nm and 2.0 dB at 915 nm. It was measured with a white light source that filled the NA and the aperture of the inner cladding. We further measured the erbium core absorption at the 1535-nm peak to 60 dB/m. The pump beam in an actual EYDFL is typically launched under different conditions, which modifies the absorption somewhat. Much more important, though, is that different pump modes (of the inner cladding) are absorbed at different rates. In a double-cladfiber with a rare-earth doped core centered in a circular inner cladding, a large number of pump modes have a poor overlap with the core. The absorption of such pump modes will be poor. For efficient absorption of all pump light, we need to scramble the pump modes somehow, especially when working with short fibers for which mode-scrambling would otherwise be negligible. See . In our laser experiments, we did scramble the modes by bending the fiber into a figure-eight. However, in each end approximately 0.2 m of the fiber remained essentially straight. This reduced the overall mode-scrambling, and thus the total pump absorption, especially with short fibers. Fig. 1 illustrates the absorption spectra, measured with and without bending the fiber to a figure-eight. By bending the fiber, we increased the small-signal absorption of the 1-m-long fiber to 4.9 dB at 915 nm and to 13.4 dB/m at the 975-nm peak. Still, for Yb in phosphosilicate, the peak absorption at 975 nm is typically more than five times larger than the absorption at 915 nm, which would be even more than in Fig. 1. In a fiber with a core centered in a circular inner cladding, some modes overlap significantly with the core, and are almost completely absorbed over 1 m, at both 975 and 915 nm. However, most of the
In this paper, new configuration is proposed, investigated and analyzed. Fiberlaser and fiber am- plifier are merged in one configuration which called Erbium Doped FiberLaser and Amplifier (EDFLA) or integrating EDFL and EDFA in one design. The configuration has three main outputs; one for laser, second for amplifier and the third it can be used for on/off output. A surprising phe- nomenon was remarked during the operation of this configuration. The amplification is abolished during the lasing state and appeared again if the lasing is stopped or the switch is off. The differ- ence between outputs with the lasing and without lasing has 30 dB EDFA gain value; it is a good sign that this optical configuration can be used as an on/off integrated optical device for fiber la- ser.
In bulk form silica is a poor nonlinear medium. However in form of fiber, power confinement could be quite high due to its small cross-sectional area and very long interaction length. These two factors may lead to large intensities and hence nonlinear effects. The regenerator spacing becomes larger due to introduction of EDFAs. This elevated optical fiber nonlinearities to an important system design issue. Nonlinear effects in optical fibers occur due to (i) change in the refractive index of the medium with optical intensity and, (ii) inelastic- scattering phenomenon. The power dependence of the refractive index is responsible for the Kerr-effect. Depending upon the type of input signal, the Kerr-nonlinearity manifests itself in three different effects such as Self-Phase Modulation (SPM), Cross-Phase Modulation (CPM) and Four-Wave Mixing (FWM). At high power level, the inelastic scattering phenomenon can induce stimulated effects such as Stimulated Brillouin-Scattering (SBS) and Stimulated Raman-Scattering (SRS). The nonlinear scattering effects in optical fibers are due to the inelastic scattering of a photon to a lower energy photon. The energy difference is absorbed by the molecular vibrations or phonons in the medium (Agrawal, 1995). In other words one can state that the energy of a light wave is transferred from one wave to another wave, which is at a higher wavelength (lower energy). The energy difference appears in form of phonons.
Harvesting energy from ambient vibration source is a promising method for providing a continuous source of power especially for low power micro electro mechanical system (MEMS). Thus, multiple methods were proposed in order to overcome the linear resonant generator system. This paper presents experimental results on two modes (softening and bi-stable) the use of magnets for improving the functionality of energy harvesting device under constant input displacement. Two set of the experiments were conducted. The quasi-static measurement was conducted to investigate the system stiffness and the dynamic measurement was conducted to investigate the performance of the response across a frequency range. The results for four configurations which operate in attractive and repulsive mode with a fixed gap between the magnets are investigated. The result shows that there is a wider bandwidth for the device operating in the softening mode. By placing with the double attractive stationary magnets, the energy harvester shows the most effective softening non-linear system.
magnesium alloys WE43 and ZE41. The resulting micro- structure was analysed and some mechanical properties were determined. Nonetheless, most of these previous works focussed mainly on revealing the inﬂuences of laser processing parameters upon microstructure with much eﬀort spent on the characterisation and identiﬁcation of the phases present in the laser-clad coating. Notwithstanding the con- tributions of these studies, it is considered that our under- standing on the solidiﬁcation behaviour and phase evolution in laser cladding of protective layers on Mg-based materials is still far from satisfactory, especially in modelling the evolution of microstructure. With this background in mind, the present research aims to study the solidiﬁcation behaviour and phase evolution in the laser cladding of aluminium on magnesium substrate using the constitutional supercooling theory and the columnar-to-equiaxed transition model.
as a hardfacing material for the surface modification of iron alloys. 4) On the laser surface cladding where a mixed pow- der layer of raw materials on an iron substrate was dissolved by a laser beam, vertical cracks (Fig. 1) were observed in the surface chromium carbide layer of the specimens. The laser melting procedure involved three scanning schemes. In the first scan (Scan A), the laser beam was scanned in one direc- tion at 1 mm pitch on the surface of the mixed powder. At this stage, many pores were observed. Then, the specimen was rotated by 90 ◦ and the carbide surface layer was simi- larly remelted with laser beam scanning at 1 mm pitch (Scan B). The remelting process was repeated after an additional ro- tation of 90 ◦ (Scan C). Since no crack was observed in the car- bide layer after Scan A, the crack shown in Fig. 1 was thought to have been generated during the remelting process of Scan B or C. The crack was not observed along the carbide/iron substrate interface, and it is known that the laser-clad surface carbide layer has excellent resistance to abrasion over short wear distances. However, there is still a danger that abrasion debris promotes further abrasion when the abrasion distance becomes longer. In the present work, the mechanism of ver- tical crack generation in the laser-clad carbide hardfacing of iron alloys was investigated on the basis of stress analysis.
The most well-known and commonly use optical fiber is the single mode fiber (SMF). An SMF only allows one mode of light to propagate along the fiber channel. There are also several types of optical fiber which had been introduced into market for various purposes. FMF is an optical fiber which can allow more than one mode of light wave to propagate into the channel. Most recent years, there has been a growing interest in the development of FMF for mode division multiplexing (MDM), which is founded to be a promising solution for scaling the data-carrying capacity of networks system . FMFs are demonstrated as a good compromise as they are sufficiently resistant to mode coupling compared to standard multimode fiber. A standard multimode fiber has larger core diameter as compare with SMF. In terms of dispersion and loss, they have the same performance as SMF. Due the absence of mode interaction it is possible to use this fiber in the single-mode operation where all the data is carried in only one of the spatial modes throughout the fiber. An experiment for single-mode operation was carried out by splicing SMF to both ends of a 35-km-long FMF at 1310 nm. After 35 km of transmission, no modal dispersion or excess loss was observed .
Fiber-based lasers have been extensively researched during the past decade because of their high efficiency, compactness and excellent beam quality. The output power of fiber lasers has been increased significantly over the last few years by adapting the cladding pumping technology, and is now competing with conventional bulk solid-state laser in applications such as micro-machining, welding and material processing. In particular, the cladding pumped Yb-doped fiberlaser has already reached kW levels at around 1.1µm with a nearly diffraction- limited output beam [1,2]. In contrast, the power scaling of a three-level laser still remains a technological challenge because of the competing unwanted four (or quasi four) level laser transition which has the lower threshold , and becomes even more difficult in the case of the cladding pumped fiberlaser owing to the relatively low pump absorption which requires a relatively long device length as compared to core pumped fiberlaser. For example, Nd-doped alumino-silicate fibers have two strong emission bands; a three-level around 930 nm and a four-level one centered at 1060 nm. While Nd-doped 1060 nm lasers are relatively easy to realize due to the absence of ground-state absorption (GSA), the interest in this is rather modest because of the superiority of Yb-doped fiber lasers emitting at the same wavelength. However, the 0.9µm Nd-doped fiberlaser is still attractive for applications such as blue generation by frequency doubling  and water sensing . Similarly, the Yb-doped fibers consist of two broad emission bands at 0.98 µm and 1.03 – 1.1µm respectively. The band at ~ 0.98 µm corresponds to a three-level laser transition and requires a much higher level of the population inversion and also suppression of the other laser transitions between 1.03µm and 1.1µm. Increasing the output power of such fiber lasers is of interest as they have applications as a pump source for erbium-doped fiber amplifiers and fiber DFB lasers .
in spite of a number of known difficulties with using such a short wavelength for detection of this gas. Thus the overlap of several absorption lines from fa- miliar environmental gases in the 1.5 μ m wavelength range points to the use of absorption bands in a more distinctive region of the spectrum. In this work, wavelengths around 2 μ m have been chosen to enable more specific detection where, for example, the ab- sorption of CO 2 over the 2 μ m wavelength band is 1,000 times stronger than that of the 1.5 μ m region . The broad fluorescence spectra of the thulium ion in silica glass allows the fabrication of a widely tun- able fiberlaser with tuning range from 1.7 to 2.1 μ m, thus providing an excellent overlap with the strong absorption lines that are suitable for CO 2 measure- ment . Furthermore, a number of groups have de- monstrated tunability of the Tm laser emission in the near IR by using bulk components, such as grat- ings and birefringent tuning plates [4,5]. Recently a tunable Tm-doped fiber ring laser based on a Fabry – Perot filter (from Micron Optics) has been reported in the work of Geng et al. . Employing a large wave- length tuning range obtained through the relaxation and compression of the FBG is a well-established technique for the wavelength range of 1.5 μ m [7 – 9]. However, for longer operating wavelengths of around 2 μ m, the bending loss is higher for standard com- mercial photosensitive fibers (typically with a cutoff wavelength at ∼1 . 3 μ m), thus limiting tuning cap- ability through compression-relaxation. Addition- ally, in order to achieve a stable laser output, it is important to ensure an equal strain be transferred to the FBG pair. Despite the constraints in tuning range, a compact “ all-fiber ” laser system is a real boon for “ in-the-field ” use for environmental monitor- ing. Such a laser system has the advantage of being more robust, making it more rugged and reliable for installation and maintenance.
Due to the evolution of triple-play communication system, there is a splendid growth of internet and data traffic. Therefore, huge bandwidth or transmission capacity is demanded in telecommunication field to keep a good momentum with the growth of broad communication system . Through development of wavelength division multiplexing system, a broad transmission capacity up to TB/s could be achieved. Unfortunately, as the number of channels demanded in the system increase, the number of light sources also increases and this arising the cost and complexity of the system. For this reason, a multiwavelength fiberlaser technology is beneficial in acting as a single gain medium of light source that can provide several number of channels simultaneously . A multiwavelength fiberlaser with erbium- doped fiber amplifier as the gain medium gives a challenge to researchers in developing and demonstrating it due to its homogeneous broadening behaviour in room temperature which causes all atoms to have the same gain spectrum and emerge a mode competition among the wavelengths whereby dominant wavelength will suppress other wavelengths and hence preventing the desired multiple wavelength output [3-4].
Significant advances in high-power diode pump lasers, refinement of power scaling and energy storage techniques, and fiber component fabrication, such as in-fiber Bragg gratings, are opening up the way to the development of active fiber systems that can deliver tens of watts of single-transverse and longitudinal-mode output power, millijoule pulse energies, and ultrashort pulses with peak powers in the 10-100 MW region. Furthermore, advances in nonlinear optical materials are permitting these high-power fiberlaser outputs to be efficiently converted to the visible and near-infrared (IR) spectral regions.
In the proposed configuration, a serially multiplexed array of FBG sensors consists of low reflectivity FBGs each at a different wavelength. This sequence of FBGs is repeated periodically along a single fiber. A low cost method of fabricating multiplexed FBG arrays is based on single pulse grating fabrication on the draw tower . Continued development of this technique has considerably improved the quality of the FBGs over the initial demonstrations and may make possible on-line fabrication of apodized FBGs at pre-determined wavelengths. Alternatively, other techniques based on strip and recoat  or by writing through the fiber coating  are currently available. The proposed configuration is also demonstrated to interrogate fiber Fabry-Pérot cavities (FFP), which provide greater than an order of magnitude increase in strain sensitivity. These can be fabricated in the same way as multiplexed arrays of FBG sensors.
We employ a photolithographically patterned thin dif- fractive lens with large aperture, fast response time, and a power-failure-safe configuration. Fig. 8 (a) compares the shape (phase profile) of a refractive lens (dashed line) with an ideal diffractive lens (dotted line). The diffractive lens is produced by removing the multiple 2π-phase retardation from the refractive lens, resulting in multiple Fresnel zones. The phase jump at each zone boundary is 2π for the design wavelength. To digitize the process, the continuous phase profile in each zone is divided into multiple sub- zones with a series of discrete phase levels (“staircase” structure, Fig. 8a). Diffraction efficiency increases by in- creasing the number of sub-zones reaching maximum val- ues of 40.5%, 81.1% and 95.0% for lenses with 2, 4, and 8
This constant loss in pulsed operation makes self-modelocking difficult at wavelengths corresponding to peaks in the CW transmission spectrum. In order to modelock the laser at these wavelengths, it is necessary to introduce an additional loss into the CW mode to ensure that pulsed operation is preferred over CW operation. In the diode-pumped CriLiSAF laser system, we found that self- modelocking was impossible at wavelengths coinciding with CW transmission peaks. When the laser modelocked, its spectrum was centred between two CW peaks; if it was tuned towards either peak then the self-modelocking dropped out. As one of the principal attractions of Cr:LiSAF as a laser material for self- modelocking is its wide tuneability, this behaviour was particularly unfortunate. Thus, although birefringence-induced loss is an interesting phenomenon, it is not desirable in a low-threshold laser, and it is particularly unwelcome in a self- modelocked low-tlireshold device. The obvious way to eliminate the effect is to rotate the crystal by the small angle (1 ® to 2 by which its c-axis is misaligned. Unfortunately, the rectangular cross-section of the Cr:LiSAF crystal used in the W-cavity and the design of the crystal mount made this rotation very difficult to achieve in practice. The low-threshold laser system described in Chapter 5, however, was engineered so that its crystal mount could be rotated about the long axis of the crystal. This enabled us to completely eliminate the modulation effects from the laser tuning curve, as will be discussed in the Chapter 5.
Threshold power has both its functionality and limitation. In the communication field, SBS limitation could be signal loss, power saturation and backward propagation . Hence, downside of Stoke wave is that it might saturates the amplifier and creates noise in the transmitter. Suppression of SBS can be done with several methods: one option would be to ensure the pump power is lower than the threshold power, the other option would be to increase the threshold power without dispersion penalty . On the contrary, SBS with low threshold power has its functionality in lasers and amplifiers . SBS has enormous potential in the application of sensing and optical communication. Low threshold power and short fiber length are two major indicators for a good sensor. In previous research, extensive experiments have been done to measure acoustic frequency shift across fibers. The focus is now on the threshold power therefore feasibility of different fibers in SBS applications are investigated . Chalcogenide (ChG) fiber which secured acoustic confinement in the fiber core shows a low