Four-photon mixing stimulated fluorescence in silica-based optical fibers

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Four-photon mixing stimulated fluorescence in silica-based optical fibers Bertrand Poumelleca*, 1-lervé Février’~ S R5

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Abstract A kind of fluorescence around 620 650. 440 450 and ~90 420 nni is achiesed in silica-based optical hheis h~high

power pumping at 0.532 pm. They are independent of the mde~prohle hut depend on the doping. Lifetime measurements show that all emissions have the same time behavior as the pump. These lines are often accompanied h~four photon rni\ing so we suggest the amplification of this nonlinear process

I. Introduction

interpretation is not complete, nor is a lumines-

Various point defects are produced during the manufacturing of a monomode optical fiber. e.g. dangling bonds, oxygen vacancies or aliovalent species such as reduced germanium. They remain after the process together with a stress field produced by the rapid cooling of the drawing. They give rise to nonlinear properties such as two-photon addition (TPA), second harmonic generation (SHG). nonlinear photoconduetion, photogalvanic effect. and also to up-converted light generations. When a fiber is irradiated with IR or visible light, it generates light with energy larger than the pump. This is common in silica based optical fibers~it is only necessary to turn the power up enough. This phenomenon has been very often accounted for by a wave mixing process. Here, we show that this *

2. Experimental details The Nd: YAG laser is the pump source. It has been used with a KDP doubler. A delas line and athe streak as a detector to measure lOpscamera event. Because of theallow glass one window of this camera, the The accessible rangeall was to 350 720nm. fibers were madelimited from the MCVD process at Alcatel Alsthom Recherche

3. Experimental observations 3.1.

Waie/engt/i

spectra

The generation of blue light from green light is common but difficult to obtam. The required

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cence process taken alone. So, we propose a combined process which we call four-photon mixing stimulated fluorescence.

Elsescer Science B.V All rights iesersed

S

B. Poumel/ec, H. Ferrier

Journal of Luminescence 60&61 (1994) 676 680

pump is about 82 116ps m, depending on the cornponent and the fiber. For the blue emission (440—450 nm), the time delay is about 48 64 ps m with respect to the pump. For the red emission (620 680 nm), the time delay is 24 to 36 ps m. More extensive results can be found in Ref. [1]. A part of these observations is easily explained by the linear optical properties of the fiber. This is made in the next section

power density approaches the damage threshold. In Fig. 1, besides the central emissions surrounding the pump and composed of the stimulated Raman scattering and parametric combination, a blue band appears at around 450 nm. On the band, a line is enhanced at 445 nm. In the red range, another band lies from 650 to 680 nm, again with several lines enhanced. Computations show that they are connected to the blue line by four-photon mixing. In other Ge-doped fibers, we again find the series of stimulated Raman scattering and also a violet luminescence. Lines appear in the near infrared which might be conjugated with it.

3.3. Analysis The fact that components are delayed or advanced with respect to the pump according to the wavelength indicates that they do not occur at the output end of the fiber. Computation has been performed using a step index profile with a core radius of 3 ~.tmand an index difference of 8 x 10 We deduced first that for a given wavelength the intermodal dispersion between the LPOI and a, the higher mode, is around 10 ps/rn. This difference is scaled with the index profile difference, and contributes to the appearance of several components for a given wavelength. In addition, we cannot exclude self- and cross-phase modulation [2]. The second point is that we have found a positive delay for the blue emission and a negative one for

3.2. Time spectra Typical results are shown in Fig. 2 for an Nddoped, Ge-doped fiber. No significant difference is detected regarding the rare earth doping. Time spectra at all wavelengths for all fibers that we have tested appear to be more or less Gaussian, similar to the one of the pump itself. Sometimes. the pulse exhibits a larger width because several components appear at the same wavelength (see fiber 8147 at 384 nm: 63 Ps, i.e. 25 ps m). Otherwise, the widths are close to 35 ps. For the violet emission (380 420nm), the time delay with respect to the

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Time in ps

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the red one with respect to the pump. The difference in group delay between 400 and 532 nm IS about lOOps m. Between 450 and 532 nrn. this difference reaches SOps rn. If we take into account the modal dispersion between 680 and 532 nm. we find 25 Ps m. All these estimates are in good agreement with the observations and have confirmed th~it all lines are created at the input end of the fiber,

Another point is the presence of near infrared lines accompanying the deep blue lines or of red lines accompanying the blue lines. This is explainable by four-photon mixing, hut computation shows that the central line is not always the pump. Sometimes Raman lines or parametric lines close to the pump play the role of the central line in FPM.

550

500

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X(nm)

origin and of connection to structural defects of the fiber material. Several luminescences are discussed in the literature, f-or red luminescence, we get narrow lines in many fibers. The red luminescence is associated with a nonbonding oxygen hole center (NbOHC). It can be excited by 248 nm excimer laser radiation because of a 265 nm absorption band. The time decay is 22 tis [3]. For blue luminescence (440 450 nm), a 4Sf) nrn (2.75eV) fluorescence is detected in pure and dry SiO,. The excitation band is at 2SOnm (4.95eV) and the time decay is 10.2ms [4]. The structure suggested for the associated species is a silicon oxygen vacancy or a bi-coordinated silicon. f-or violet luminescence, the situation is not so clear-cut. In OH : SiO~ or in Ge: SiO,.

4. Discussion

a 38~ 413 nm (3.2 3.0 eV) band is excited either by direct absorption at 246 nm (5.05 eV) or by tv 5o

4.1. Re/ation.s with defects

photons at 488 or Sl4nm. It has been demonstrated that, in fact, several defects are in~oI~ed probably related to Ge reduced species. Time decay is of the order of 100 us [5].

The constancy of generation, whatever the pro file in the same region. is a proof of a material

B. Poumellec, H. Fé,rier Journal of Lumine,scence 60&61 (1994) 676 680

679

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4.2. Mechanisms

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From the discussion above, we think that the luminescence we invoke in our experiment can be produced by excitation in the 5eV band. All fluorescences that we have collected (red, blue and violet) are connected to a component in this band. It is reached by two-photon absorption from the

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Now, to understand strong emission we need to know a source and an amplification process. For defects or a nonlinear For the latter, itfrom can the former, it can be process. spontaneous emission be a simple monophoton process pumped by multiphoton absorption or a collective excitation called superfluorescence. Our experiments are consistent with an amplification process of a nonlinear process. As we have always observed conjugated lines by four-photon mixing (FPM), it is useful to investigate FPM-stimulated fluorescence or fluorescence-induced FPM in order to understand why the phase matching condition (generally eonsidered in nonlinear processes) appears to be not so important here, 4.3. Fluorescence-induced/our-photon mixing We have solved the coupled mode equations for FPM in the case when an amplification term is active. The calculations are described in [I]. Quoting A 1 and A2 (the amplitudes of the fields for the Stokes and anti-Stokes component), A/I (the phase mismatch), (the relevant third-order nonlinear coefficient for FPM process), ~ and ~2 (some amplification coefficients at the Stokes and anti-Stokes

where 2

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A~ c~ ~2 + iA/I and the power density is proportional to A2p. g is called the gain of the process. A first feature in this computation is the appearance of a real exponential term in the amplitude. A second feature is that there are two additional terms in the gain containing the difference of the amplification coefficients. Thus, the gain is maximized (1) when the phase mismatch is reduced (our computations show that this can be achieved by modal combinations) and (2) when the amplification coefficients are different. In this condition, the first generation to appear when the power is increased is the one which falls into fluorescence bands. The exact wavelength positions are defined by competition between overlap integrals and phase matching conditions. As a consequence, we observe structures which are fiber dependent on an almost bell-shaped background.

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5. Conclusion From the time dependence point of view, the above theory yields an amplified signal having exactly the time dependence of the source, i.e. almost the laser width. It is worth noting that the walk-off lengths between the three waves are of the order of 1’4 to 1 2 m. Hence, beyond this the FPM interaction does not occur. In contrast, the amplification can go on the length of the fiber without the FPM process, although it is less efficient. This explains most of the time behavior observed in the experiments.

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References

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fli

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]

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[~].1

P Harnaide and P Emplit, Opiic~LetS 14 l959) 659

41 1 i~iA

Sku)a, .1. Non ( r’,~aIIineSolid’, i 49 1997) ~() N Gur~anos, D 1) Gu’,io’,’,kii, F.M Diaiios. \ M Ma’,hinskii. V B Neu~true~ and \ F Khopin. So~ Ph~~ DokI ~7 l95~’)401