We report an experimental study of low-frequency (~10 kHz) self-pulsing of the output intensity in a highconcentration erbium-doped fiber laser. We suggest that the fast intensity fluctuations caused by multimode and polarization instabilities play the role of an external noise source, leading to low-frequency auto-oscillations through a coherence resonance scenario. © 2010 Optical Society of America OCIS codes: 060.3510, 060.2410, 140.3500, 140.1540 Output intensity self-pulsing in high-concentration erbium-doped fiber lasers (HC EDFLs) has been extensively studied for its importance in communications, reflectometry, distributed fiber-optic sensing, biomedicine, etc. [1][2][3][4][5][6][7]. For a long time, the presence of clustered erbium ions and their role as saturable absorber was regarded as the only possible mechanism responsible for self-pulsing at frequencies of 10-100 kHz [7]. However, detailed microscopic study of erbium-doped glasses by means of x-ray-absorption fine structure spectroscopy (XAFS) has revealed a short-range coordination order of erbium ions, rather than pair clustering [8]. It was also found that suppression of the short-range order leads to improved characteristics of high-concentration erbiumdoped fiber amplifiers and lasers. The suppression can be realized by increasing the solubility of erbium in the host matrix (codoping by Al [9]) or by changing the deposition process (direct nanoparticle deposition [10,11]). Because self-pulsing was observed in HC EDFLs even with these types of fibers, the pair-clusters approach cannot explain this effect [4,5]. On the other hand, experiments and theory indicate that the pump-tosignal intensity noise transfer (PSINT) can significantly contribute to low-frequency self-pulsing [4,5]. However, PSINT decreases as the pump current increases, and therefore this process can induce self-pulsing only near the lasing threshold [1,2]. To reveal a mechanism that can drive self-pulsing with pump currents beyond the lasing threshold, we test and experimentally prove the concept of coherence and anticoherence resonance (CR and ACR) [12][13][14]. The details are reported in this Letter. The experimental setup used to study self-pulsing in EDFL is shown in Fig. 1. The laser cavity consists of a high-concentration (c Er ¼ 3:7 10 25 ions=m 3 ) erbiumdoped fiber (5 m, Liekki Er40-4/125), with a Faraday mirror (R ¼ 90%) and a fiber Bragg grating (λ peak ¼ 1556 nm, Δλ 3 dB ¼ 0:2 nm, R ¼ 86%) as reflectors. According to manufacturing data [10] and previous experimental results [11], the short-range coordination order of erbium ions is suppressed and no saturable absorption is present. Total intracavity losses are 5 dB. The Faraday mirror rotates the plane of polarization of the reflected beam by π=2 at λ ¼ 1550 nm. To pump the erbium-doped fiber, we used a 978 nm laser diode (LD) with a tunable current source (200-600 mA). The pump current at the lasing and self-pulsing threshold was 240 mA.To characterize the polarization instability as a possible source of high-fre...

Abstract:We report results on experimental and theoretical characterisation of self-pulsing in high concentration erbium doped fibre laser which is free from erbium clusters. Unlike previous models of self-pulsing accounting for pair-induced quenching (PIQ) on the clustered erbium ions, new model has been developed with accounting for statistical nature of the excitation migration and upconversion and resonance-like pumpto-signal intensity noise transfer. The obtained results are in a good agreement with the experimental data. PACS

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We report on high power issues related to the reliability of fibre Bragg gratings inscribed with an infrared femtosecond laser using the point-by-point writing method. Conventionally, fibre Bragg gratings have usually been written in fibres using ultraviolet light, either holographically or using a phase mask. Since the coating is highly absorbing in the UV, this process normally requires that the protective polymer coating is stripped prior to inscription, with the fibre then being recoated. This results in a time consuming fabrication process that, unless great care is taken, can lead to fibre strength degradation, due to the presence of surface damage. The recent development of FBG inscription using NIR femtosecond lasers has eliminated the requirement for the stripping of the coating. At the same time the ability to write gratings point-by-point offers the potential for great flexibility in the grating design. There is, however, a requirement for reliability testing of these gratings, particularly for use in telecommunications systems where high powers are increasingly being used in long-haul transmission systems making use of Raman amplification. We report on a study of such gratings which has revealed the presence of broad spectrum power losses. When high powers are used, even at wavelengths far removed from the Bragg condition, these losses produce an increase in the fibre temperature due to absorption in the coating. We have monitored this temperature rise using the wavelength shift in the grating itself. At power levels of a few watts, various temperature increases were experienced ranging from a few degrees up to the point where the buffer completely melts off the fibre at the grating site. Further investigations are currently under way to study the optical loss mechanisms in order to optimise the inscription mechanism and minimise such losses.