Threshold of a Symmetrically Pumped Distributed ... - IEEE Xplore

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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 8, AUGUST 2008

Threshold of a Symmetrically Pumped Distributed Feedback Fiber Laser With a Variable Phase Shift Yuri O. Barmenkov, Alexander V. Kir’yanov, Pere Pérez-Millán, José Luis Cruz, and Miguel V. Andrés, Member, IEEE

Abstract—In this paper, we study, both theoretically and experimentally, the threshold characteristics of a distributed feedback fiber laser that depend on the value of a phase shift introduced into the fiber Bragg grating structure. We show that as the phase shift possesses a noticeable birefringence, the laser oscillates at any phase shift value. We also reveal that the laser threshold is different for the cavity eigen polarizations and depends on the phase shift value. We derive a simple analytical formula to phase shift; this calculate the laser threshold in the case of formula can be utilized to estimate a minimal threshold value for the laser with certain active fiber and Bragg grating parameters. The developed theory allows us to fairly model the experimentally measured dependence of the laser threshold on induced phase shift value.

Fig. 1. DFB FL structure. FBG1 and FBG2 are two equal phase Bragg gratings written in an active fiber and divided by a short-length defect introducing a variable spatial phase shift. The cavity length (L ) is taken as a sum of FBGs’ lengths (L ).

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In the present work, we study, both theoretically and experimentally, the threshold characteristics of a DFB FL that are dependent on the value of a phase shift introduced into the DFB structure. We show that if the phase shift is notably “birefringent,” i.e., different for two orthogonal polarizations, lasing in the DFB FL can be attainable at any phase shift value, and the laser threshold is different for each of the cavity eigen polarizations. A simple analytical formula to calculate the threshold of a DFB FL is derived in the case where the phase shift is , which is the minimal threshold value for the laser with certain fiber gain and grating strength.

Manuscript received July 27, 2007. This work was supported in part by CONACyT Grant 47029, México, and in part by the Ministerio de Educación y Ciencia under Grant TEC2005-07336-C02-01 and Grant PCI2005-A7-0209, Spain. Y. O. Barmenkov and A. V. Kir’yanov are with the Centro de Investigaciones en Optica, 37150 Leon, Mexico (e-mail: [email protected]; [email protected]). P. Pérez-Millán, J. L. Cruz, and M. V. Andrés are with the Departamento de Física Aplicada—ICMUV, Universidad de Valencia, E46100 Burjassot (Valencia), Spain (e-mail: [email protected]; [email protected], miguel.andres@ uv.es). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2008.923555

II. EXPERIMENTAL SETUP AND RESULTS In experiments, we studied the threshold characteristics of a DFB FL. The laser was built using the standard procedure [13] of exposing a preliminary hydrogenised Er-doped photosensitive fiber to UV-light through a phase mask, thus writing a uniform FBG in it. A variable phase shift was introduced into the FBG by gluing a central (4-mm) fiber segment to a small rod of magnetostrictive alloy, which in turn was subject to a permanent magnet field. The phase shift was varied within an interval of several ’s by changing the distance between the rod and the magnet. The active fiber (DF1500L-980) had a nonsatucm rated (weak-signal) absorption coefficient and a full-saturated gain coefficient cm , both measured at a wavelength of 1532 nm. The DFB cavity length cm, and the FBG efficiency at Bragg wavelength, was nm, was 41.3 dB, yielding a coupling coefficient cm . The FBG written in the active fiber was checked to be uniform throughout its length. The reflection coefficient of each of the two FBG-halves (FBG1 and FBG2 on Fig. 1), separated by the phase defect, was measured to be the same with high accuracy (the measurements were performed on a slight elongation of the fiber piece containing only one of the FBG halves, what allowed the testing of each part separately).

Index Terms—Distributed feedback fiber laser (DFB FL), erbium-doped fiber, laser threshold, polarization state, variable phase shift.

I. INTRODUCTION T PRESENT, distributed feedback (DFB) fiber lasers (FLs) attract much interest owing to their applicability to optical communications, fiber sensors and spectroscopy. DFB FLs are typically single-mode devices that have narrow line width [1], high signal-to-noise ratio [2], and relatively low cost. They have been shown to operate both in the continuous-wave and -switching [3], [4] regimes. A DFB FL is usually implemented by writing, with the use of UV light, a long fiber Bragg grating (FBG) inside a rare-earth-doped fiber, and subsequently introducing a spatial phase shift into the grating. So far, a lot of research has been conducted on DFB FLs towards optimization of their parameters [5], [6], treatment of the cavity’s standing mode profile [7], [8], performance limitations [9], intensity and frequency noise features [2], [10], analysis of the fundamental and higher mode thresholds [11], polarization characteristics [12], etc.

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BARMENKOV et al.: THRESHOLD OF A SYMMETRICALLY PUMPED DFB FL

Fig. 2. (a) Transmittance spectra of the DFB laser cavity measured at random incidence polarization. (b) Superposition of the transmittance spectra for two eigen polarization states of the cavity. Zero detuning corresponds to the Bragg wavelength. The spectra were measured at pump power below the laser threshold.

Thus, once pumped from both FBG ends (see below), completely equal conditions of interaction of light with the active medium to the left and to the right side of the phase defect were held. The laser cavity transmittance spectrum was measured using a tunable, narrow-line, single-frequency semiconductor -pm interval around the Bragg wavelength laser, scanning nm. The experimental transmittance spectrum of the “cold” cavity Fig. 2(a) shows two peaks spaced by 5.5 pm, which correspond to eigen polarization states of the cavity. The presence of these peaks in the spectrum is due to birefringence of the active fiber that appears upon writing the FBG in the photosensitive fiber [14], [15] and subsequently introducing a phase shift into the cavity (this last source for birefringence has recently been observed in DFB lasers with UV- and stress-induced phase shifts [12], [16]). The phase defect birefringence is confirmed by separate measurements of the transmittance spectra of the cavity at the two eigen polarizations [see Fig. 2(b)]: it is seen that the distance between the peaks, when one superposes the spectra edges, is about 3.5 pm. The revealed proof of spatial homogeneity of the FBG pieces forming the DFB FL cavity (see Fig. 1) and the knowledge of fine polarization features of the cavity readily seen from the plotted spectra (Fig. 2) allowed us to properly address the laser threshold characteristics in the laser modeling (see Section III). The DFB FL shown in Fig. 1 was symmetrically pumped nm by the light from opposing sides at wavelength launched from a standard fibered semiconductor laser after a 50/50 power division using a fiber multiplexer. The symmetric pumping scheme [17] provides the conditions of a quasi-homogeneous interaction of pump light with the DFB structure and allows one to avoid possible influence of inhomogeneous heating of the active fiber and, therefore, to reduce inhomogeneities in the refractive index distribution (heating arises from the Stokes loss, the excited-state absorption, and the Auger up-conversion in the Er-doped fiber [18], [19]). The dependence of the DFB FL threshold on the applied phase shift is presented in Fig. 3. The phase shift (in terms of

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Fig. 3. Dependence of the DFB FL threshold on detuning from the Bragg wavelength ( = 1532:6 nm). The experimental values are normalized to the minimal threshold value (2.5 mW).

detuning from the Bragg wavelength ) was taken as an average of the ones corresponding to the cavity eigen polarization states (e.g., the detuning that relates to the cavity transmittance spectrum shown in Fig. 2(a) was taken as 1.2 pm). The FL threshold was fixed at the moment when, instead of the noisy pattern, a relaxation frequency peak appears in the laser output RF spectrum [20]. It is seen that the minimal threshold of the laser is observed near zero detuning of phase shift from the Bragg wavelength. pm , threshold changes Within the broad detuning range -pm values, weakly. When detuning approaches the threshold rapidly increases. Upon further increase of the detuning, over 16 pm, the next transmittance peaks appear at the opposite slopes of the cavity transmittance spectrum, that leads to a repetition of the law plotted in Fig. 3. As the phase shift induced into the laser cavity by the magnetostrictive transducer is sensitive to temperature variation, the phase shift increases with the pump power, owing to the fiber heating at high pump levels. As a result, the lasing wavelength moves towards the right of the cold cavity resonant wavelength. Our study of the laser polarization properties was limited to 5 over the threshold. We observed small excess values that the polarization of lasing depends on the detuning sign: the laser oscillates at one of the two eigen polarization states of the cavity, and it hops when detuning changes from negative to positive and vice versa. Near zero detuning, lasing takes place at the two orthogonal polarizations simultaneously. III. THEORY A DFB FL usually consists of a comparatively long (5–20 cm in length) uniform FBG written in a rare-earth-doped (active) fiber with a short-length defect inside, which introduces a phase shift of a certain (usually ) value. An analysis of the DFB FL is simplified in the case of a symmetric laser cavity (see Section II), where a variable phase defect is localized at the cavity center and divides the cavity into two equal gratings FBG1/ FBG2. Using the coupling waves’ equations for the two counterpropagating waves A and B (see Fig. 1) [21] and bearing in mind

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Therefore, a DFB FL oscillates at any phase shift if the fiber gain is high enough to allow the FBG reflectivity to reach 100%. Notice that a similar behavior demonstrated for a semiconductor laser [23], [24]; it is accepted that 100%-reflectivity of an active FBG is in fact the laser threshold criterion [23]. Fig. 4(b) shows the dependencies of the FBG power reflectivity calculated using formula (1) for different fiber gain values and for the reflectivity phase at zero gain (the reflectivity phase is changed a little within 20-22-pm interval). As seen, FBG can reach 100%-reflectivity at the detuning ranging from 18 pm to 18 pm. The detuning is linked to the phase shift, , as (2) where is the phase shift of the wave reflected on the FBG, and is the phase shift introduced to the traveling waves due to the presence of spatial phase shift . The threshold gain is related to the threshold pump power, . Implying that a three-level model for the active medium (Er fiber) holds, one can get the following formula for net gain in the fiber: (3)

Fig. 4. (a) DFB FL cavity transmittance spectra at different values (see labels cm L cm and  : cm above the curves) of phase shift; L R : . (b) Reflectivity R of a FBG written in an active fiber (left scale) and reflectivity phase at zero gain (right scale) versus detuning. The gain values are : ; : ; 0 (3); 0.02 (4) and 0.04 (5) cm (the values attainable for DF1500L-980 fiber used in the experiments).

(

= 98 7%) 00 04(1) 00 02(2)

= 5 ( = 10 )

= 0 573

the active fiber gain, one can find (see [22] and [23]) the amplitude reflectivity of each of the gratings (FBG1/FBG2) forming the DFB FL cavity as

and are the normalized populations of Er ions where and are, respectively, the in the ground and excited states, fiber unsaturated (no pump) absorption and fully saturated (very high pump rates) gain, both at the lasing wavelength (nearby ), is the pump power, and is the ( is the modal area saturation power at pump wavelength at , is the Plank constant, is the pump photon frequency, is the ground-state absorption cross section of Er ions at , is the decay time of the metastable level , and is the overlap factor at ). Notice that in formula (3) all losses (on scattering, etc.), rather than saturable loss given by the presence of Er ions, are ignored as the former are normally much lower than . is reached at the threshold Because the threshold gain , formula (3) can be rewritten as follows: pump power (4)

(1) , with being the where detuning from the grating Bragg wavelength , is the fiber gain, is the coupling coefficient ( is the phase grating amplitude defined as the fundamental spatial harmonic is the amplitude of the refractive index perturbation), and modal refractive index. The power reflectivity of each grating . can be obtained as Fig. 4(a) shows the dependence of power transmittance of the cavity with zero gain on the induced phase shift value (detuning from the Bragg wavelength). It is seen that the transmittance of the phase shift. peaks exist at all possible values

Fig. 5 shows how the threshold power depends on the spatial phase shift value. The plotted curves were calculated using formulas (1) and (4) at . It is seen that a laser with cavity length cm and FBG1/FBG2 reflectivity (this value corresponds to the DFB (without gain) FL studied experimentally) reaches threshold within the phase 0.1 1.9 . At increasing the cavity length shift range or 20 cm), the laser oscillates at any (see curves for value of the phase shift. The minimal threshold pump power at any cavity length is obtained at phase shift, i.e., at zero detuning from the Bragg wavelength. Notice that the theory presented is applicable as well for a nonsymmetric DFB FL where the threshold condition holds ( and are the reflection coefficients of the FBG’s segments surrounding a phase defect).

BARMENKOV et al.: THRESHOLD OF A SYMMETRICALLY PUMPED DFB FL

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Fig. 5. Dependences of the normalized threshold pump, P =P , on spatial : cm and R : (solid curves). Cavity phase shift ' at g length is 5 (1), 10 (2), 15 (3), and 20 (4) cm. Dotted lines show the phase shift limits for lasing, which correspond to the detunings for which the condition g holds. g

= 0 042

= 98 7%

=

Fig. 6. Dependences of the threshold gain on the gratings’ reflection coefficient L is labeled on each at various cavity lengths. The cavity length L curve; the phase shift is  .

(

=2 )

At phase shift, one can get an analytical formula for the laser threshold pump power. This formula is useful for estimating the minimal laser threshold, provided the active fiber gain and FBG strength are known. Taking into account the in, applying the Taylor transform, and limiting to equality the first order of smallness, we obtain the FBG power reflection coefficient [see (1)] as (5) Notice that if the condition does not hold (the case of a heavily doped fiber), one needs to account for the subsequent term of the Taylor transform. on the Formula (5) can be rewritten for threshold gain assumption that reflectivity of the FBG1 and FBG2 is 100%

(6) where

is the reflection coefficient at zero gain . At (the case of a long DFB structure or very strong grating), formula (6) is reduced (see [23]) to (7) For the active fiber used in the experiment, the calculated values of the threshold gain and pump power are cm and mW. The last value is slightly different from the experimentally measured in-fiber pump power (2.5 mW); this difference seems to arise from a neglecting of the amplified spontaneous emission and Er excited state absorption [18], and possible fiber nonresonant loss. The dependencies of the threshold gain on gratings’ reflectivity at various DFB FL lengths and phase shift, calculated using formula (6), are shown in Fig. 6. It is seen that a symmetric DFB FL with a cavity length of 10 cm reaches threshold when the FBGs forming the cavity have reflectivity exceeding 90%;

Fig. 7. Filled circles: experimentally measured laser threshold. Curves: results of modeling of the DFB FL threshold: Solid curve 1 corresponds to an “ideal” cavity, without taking into account phase shift birefringence; dotted curves 2a-2b and 3a-3b correspond to a “real” cavity, where two orthogonal polarizations are observed and phase shift birefringence is taken to be 0.5 rad. The other cavity parameters used in modeling are the same as they were experimentally measured. The threshold values are normalized to the ones at  phase shift.

it occurs when the parameter is bigger than 1.83 (notice here that the threshold gain values, found by using formulas (6) 10 ). As the cavity length increases, the and (7), differ by “threshold” reflectivity decreases and vice versa. If the phase shift introduced into the cavity is different from , the minimal FBG reflectivity, at which lasing threshold is reached, increases (see Fig. 5). IV. COMPARISON OF THE EXPERIMENTAL AND MODELING RESULTS AND DISCUSSION We have demonstrated that under certain conditions a DFB FL can oscillate in a rather broad range of phase shift values. Fig. 7 shows a direct comparison of the experimental and modeled dependencies of the laser threshold on phase shift value. The experimental data (filled circles), shown in Fig. 7, are the

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same as those in Fig. 3 (Section II), while the horizontal axis is rescaled from detuning (in picometers) to induced phase shift (in radians). The modeling data (solid curve 1 in Fig. 7) are obtained by using the theory (Section III), where all the input parameters are the ones obtained experimentally for the FBG and the active Er fiber, and birefringence of the phase shift is not accounted for (Section II). It is seen that this theoretical curve fits well the experimental data; however, notice that the experimentally measured phase shifts’ range for the FL threshold is wider than what is predicted by the theory. Even a better agreement between the experiment and the theory can be attained if one corrects the modeling data, taking into account the birefringence of the phase defect [the gap between the transmittance peaks, see Fig. 2(b)]. This gap (3.5 pm) corresponds [see Fig. 4(b)] to the difference in the FBG reflecand gives rise to the phase shift tivity phase of rad. According to the above birefringence of made estimates, the birefringence (see Fig. 7) can be taken into account by equidistantly shifting solid curve 1 by a half of the towards the left phase shift value (giving curve 2a-2b) and the right (giving curve 3a-3b). These new curves correspond to the threshold condition for the two orthogonal (eigen) polarizations. The resultant (solid) curve 2a-3a describes the real laser threshold, which is the lowest one for both the polarizations. Thus, the conclusion is that our laser oscillates at one of the two orthogonal polarizations, depending on the phase shift value. Furthermore, a variation of phase shift by changing the external magnetic field allows one to control the polarization of a DFB FL.

[5] V. C. Lauridsen, J. H. Povlsen, and P. Varming, “Optimising erbiumdoped DFB fibre laser length with a respect to maximum output power,” Electron. Lett., vol. 35, pp. 300–302, 1999. [6] K. Yelen, L. M. B. Hickey, and M. N. Zervas, “A new design approach for fiber DFB lasers with improved efficiency,” IEEE J. Quantum Electron., vol. 40, no. 6, pp. 711–720, Jun. 2004. [7] E. Rønnekleiv, M. Ibsen, M. N. Zervas, and R. I. Laming, “Characterization of fiber distributed-feedback lasers with an index-perturbation method,” Appl. Opt., vol. 38, pp. 4558–4565, 1999. [8] S. Foster and A. Tikhomirov, “Experimental and theoretical characterization of the mode profile of single-mode DFB fiber lasers,” IEEE J. Quantum Electron., vol. 41, no. 6, pp. 762–766, Jun. 2005. [9] K. H. Ylä-Jarkko and A. B. Grudinin, “Performance limitations of high-power DFB fiber lasers,” IEEE Photon. Technol. Lett., vol. 15, no. 2, pp. 191–193, Feb. 2003. [10] G. A. Cranch, M. A. Englund, and C. K. Kirkendall, “Intensity noise characteristics of erbium-doped distributed-feedback fiber lasers,” IEEE J. Quantum Electron., vol. 39, no. 12, pp. 1579–1586, Dec. 2003. [11] S. W. Løvseth and E. Rønnekleiv, “Fundamental and higher order mode thresholds of DFB fiber lasers,” J. Lightw. Technol., vol. 20, no. 3, pp. 494–501, Mar. 2002. [12] E. Rønnekleiv, M. Ibsen, and G. J. Cowle, “Polarization characteristics of fiber DFB lasers related to sensing applications,” IEEE J. Quantum Electron., vol. 36, no. 6, pp. 656–664, Jun. 2000. [13] A. Othonos and K. Kalli, Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing. Norwood, MA: Artech House, 1999, pp. 43–43. [14] T. Erdogan and V. Mizrahi, “Characterization of UV-induced birefringence in photosensitive Ge-doped silica optical fibers,” J. Opt. Soc. Amer. B, vol. 11, pp. 2100–2105, 1994. [15] H. Renner, “Effective-index increase, form birefringence and transition losses in UV-side-illuminated photosensitive fibers,” Opt. Exp., vol. 9, pp. 546–560, 2001. [16] J. L. Philipsen, M. O. Berendt, P. Varming, V. C. Lauridsen, J. H. Povlsen, J. Hubner, M. Kristensen, and B. Palsdottir, “Polarisation control of DFB fibre laser using UV-induced birefringent phase-shift,” Electron. Lett., vol. 34, pp. 678–679, 1998. [17] S. A. Babin, D. V. Churkin, A. E. Ismagulov, S. I. Kablukov, and M. A. Nikulin, “Single frequency single polarization DFB fiber laser,” Las. Phys. Lett., vol. 4, pp. 428–432, 2007. [18] Y. O. Barmenkov, A. V. Kir’yanov, and M. V. Andrés, “The resonant and thermal changes of refractive index in a heavily doped erbium fiber pumped at wavelength 980 nm,” Appl. Phys. Lett., vol. 85, pp. 2466–2468, 2004. [19] A. V. Kir’yanov, Y. O. Barmenkov, and N. N. Il’ichev, “Excited-state absorption and ion pairs as sources of nonlinear losses in heavily doped erbium silica fiber and erbium fiber laser,” Opt. Exp., vol. 13, pp. 8498–8507, 2005. [20] Y. O. Barmenkov and A. V. Kir’yanov, “Pump noise as the source of self-modulation and self pulsing in erbium fiber laser,” Opt. Exp., vol. 12, pp. 3171–3177, 2004. [21] H. Kogelnik and C. V. Shank, “Coupled-wave theory of distributed feedback lasers,” J. Appl. Phys., vol. 43, pp. 2327–2335, 1972. [22] T. Erdogan, “Fiber grating spectra,” J. Lightw. Technol., vol. 15, no. 8, pp. 1277–1294, Aug. 1997. 2 distributed [23] S. L. McCall and P. M. Platzman, “An optimized feedback laser,” IEEE Quantum Electron., vol. QE-21, no. 12, pp. 1899–1985, Dec. 1985. [24] G. Björk and O. Nilsson, “A new exact and efficient numerical matrix theory of complicated laser structures: Properties of asymmetric phaseshifted lasers,” J. Lightw. Technol., vol. LT-5, no. 1, pp. 140–146, Jan. 1987.

V. CONCLUSION In the present work, we investigate, both experimentally and theoretically, the threshold of a DFB FL as the dependence of a phase shift introduced into the laser cavity. We show that a phase shift induced by a magnetostrictive transducer possesses a noticeable birefringence that explains the oscillation of a DFB FL at any value of the phase shift, with the light polarization being dependent on its value. Using the coupled waves’ equations, we derive a simple analytical formula to calculate the threshold value for a symmetric DFB FL with an induced phase shift and demonstrate usefulness of this formula for modeling a real DFB FL. The controllable phase shift technique can be used for polarization control and fine wavelength tuning of a DFB FL. REFERENCES [1] W. H. Loh, B. N. Samson, L. Dong, G. J. Cowle, and K. Hsu, “High performance single frequency fiber grating-based erbium:ytterbiumcodoped fiber lasers,” J. Lightw. Technol., vol. 16, no. 1, pp. 114–118, Jan. 1998. [2] E. Rønnekleiv, “Frequency and intensity noise of single frequency fiber Bragg grating lasers,” Opt. Fiber Technol., vol. 7, pp. 206–235, 2001. [3] P. Pérez-Millán, J. L. Cruz, and M. V. Andrés, “Active -switched distributed feedback erbium-doped fiber lasers,” Appl. Phys. Lett., vol. 87, pp. 011104–011104, 2005. [4] M. Delgado-Pinar, A. Díez, J. L. Cruz, and M. V. Andrés, “Single-frequency active -switched distributed fiber laser using acoustic waves,” Appl. Phys. Lett., vol. 90, pp. 171110–171110, 2007.

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Yuri O. Barmenkov received the M.Sc. and Ph.D. degrees from the Leningrad State Technical University, Leningrad, Russia, in 1984 and 1991, respectively, all in radiophysics and electronics. He was with St. Petersburg State Technical University, Russia, from 1991 to 1996. Since 1996, he has been a Research Professor at the Centro de Investigaciones en Optica, Leon, Guanajuato, Mexico. He is a coauthor of more than 70 scientific papers and three patents. His main research activity includes fiber lasers, fiber optic sensors, and nonlinear optics.

BARMENKOV et al.: THRESHOLD OF A SYMMETRICALLY PUMPED DFB FL

Alexander V. Kir’yanov received the M.Sc. degree from the M.V. Lomonosov State University, Moscow, Russia, in 1986, and the Ph.D. degree in optics and laser physics from the A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, Russia, in 1995. He was with the A.M. Prokhorov General Physics Institute from 1987 to 1998. In 1988, he was a Visiting Scientist/Lecturer at the Central Research Institute for Physics, Hungary Academy of Sciences, Budapest; in 1996, at the Institute of Material Chemistry, University of Technology, Tampere, Finland; in 1997, at the Lawrence Livermore National Laboratory, Livermore, CA; and from 2001 to 2003, at the Imperial College, London, U.K. Since 1998, he has been a Research Professor at the Centro de Investigaciones en Optica, Leon, Guanajuato, Mexico. He is a coauthor of more than 100 scientific papers, a presenter of contributions at more than 50 international conferences, and a holder of three patents. His research interests are infrared solid-state and fiber lasers with passive -switching and passive mode-locking, phase conjugating in solid-state lasers, and nonlinear properties of thin films.

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Pere Pérez-Millán was born in Valencia, Spain, in 1977. He received the B.Sc. degree in physics from the University of Valencia, Spain, in 2001. He is a Predoctoral Researcher in the Department of Applied Physics, University of Valencia, Valencia, Spain. His research activity focuses on the design and fabrication of fiber Bragg gratings for application in sensors, fiber lasers, and microwave photonics systems.

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José Luis Cruz was born in Cuenca, Spain, in 1964. He received the M.S. and Ph.D. degrees in physics from the University of Valencia, Spain, in 1987 and 1992, respectively. Initially, his career focused on microwave devices for radar applications. Afterwards, he worked for two years in optical fiber fabrication at the University of Southampton, and currently is the Head Professor in the Department of Applied Physics, University of Valencia, Valencia, Spain, where he is conducting research on fiber lasers, microwave photonics and sensors.

Miguel V. Andrés was born in Valencia, Spain, in 1957. He received the B.Sc. and Ph.D. degrees in physics from the University of Valencia, Spain, in 1979 and 1985, respectively. Since 1983, he has successively served as Assistant Professor, Lecturer, and Professor in the Department of Applied Physics, University of Valencia, Valencia, Spain, where he founded the Optical Fiber Laboratory group. From 1984 to 1987 he was visiting, for several periods, at the Department of Physics, University of Surrey, U.K., as a Research Fellow. Until 1984, he was engaged in research on microwave surface waveguides. His current research interests include waveguide theory (inhomogeneous waveguides and microstructured optical fibers) and optical fiber devices and systems for microwave photonics, all-fiber lasers (active -switched Fabry–Perot and distributed feedback lasers) and sensor applications (optical fiber interferometers, evanescent field devices based on optical fiber tapers, in-fiber Bragg gratings, whispering-gallery modes of microcavities, and photonic crystal fibers).

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