J A Beswick

C Rizzo

Structure of the photon and magnetic field induced birefringence and dichroism

Keywords:

In this letter we show that the dichroism and ellipticity induced on a linear polarized light beam by the presence of a magnetic field in vacuum can be explained in the framework of the de Broglie's fusion model of a photon. In this model it is assumed that the usual photon is the spin 1 state of a particle-antiparticle bound state of two spin 1/2 fermions. The other S = 0 state is referred to as the second photon. On the other hand, since no charged particle neither particles having an electric dipole are considered, no effect is predicted in the presence of electric fields and this model is not in contradiction with star cooling data or solar axion search.

Very recently an experimental observation of optical activity of vacuum in the presence of a magnetic field has been reported [1] by the PVLAS collaboration. The observed results could not be explained in the framework of the standard Quantum ElectroDynamics (QED), and the existence of light pseudoscalar spinless bosons of the same nature of the Peccei and Quinn axion [2] has been suggested (see e.g. [3]). This explanation however is in contradiction with other existing experimental data. In particular, the particle needed to justify the PVLAS results should be largely produced in the star core by interaction of photons with plasma electric fields. Such a particle should escape because of its very low coupling with matter, and induce a fast cooling of stars at a level already excluded by astrophysical observations [START_REF] Raffelt | Stars as Laboratories for Fundamental Physics[END_REF]. Moreover, CAST experiment [START_REF] Zioutas | [END_REF] devoted to detect solar axions by conversion in a magnetic field, has already excluded the existence of such a particle in the range of mass and coupling constant necessary to give the PVLAS effect. To get rid of this contradiction more exotic solutions have been proposed. In particular, the existence of a massive paraphoton which would couple with the standard photon [6] and with the axionlike particle (ALP), the photon-initiated real or virtual production of pair of low mass millicharged particles [7], and the existence of an ultralight pseudo-scalar particle interacting with two photons and a scalar boson and the existence of a low scale phase transition in the theory [8].

In this letter we show that dichroism and ellipticity induced on a linear polarized light beam by the presence of a magnetic field in vacuum can be predicted in the framework of the de Broglie's fusion model of a photon [9]. In this model it is assumed that the usual photon is the spin 1 state of a particle-antiparticle bound state of two spin 1/2 fermions. The other S = 0 state is referred to as the second photon. The mass of the usual photon is supposed to be zero or negligible.

In particular, we show that taken the spin-spin coupling and the interaction with an external magnetic field proportional to s 1 .s 2 and B.(s 1s 2 ), respectively, magnetic induced birefringence and dichroism are obtained with a (ǫ.B) 2 pseudo-scalar symmetry where ǫ is the polarization of the photon (as usual ǫ is defined by the direction of the electric field). Thus both dephasing and absorption appear for linearly polarized light parallel to the external applied magnetic field.

On the other hand, since no charged particle neither particles having an electric dipole are considered, no effect is predicted in the presence of electric fields and this model is not in contradiction with star cooling data or solar axion search.

We consider the photon as composed of a spin 1/2 particle and its antiparticle. The spin Hamiltonian is assumed to be approximate by

H 0 = - ∆ 2 s 1 .s 2 (1)
with ∆ > 0. The ground state eigenstates are then given by:

|S=1, M z =1 = |↑, ↑ ; |S=1, M z = -1 = |↓, ↓ |S=1, M z =0 = 1 √ 2 |↑, ↓ + |↓, ↑ (2)
with energy E 1 = -∆/4, corresponding to the ordinary photon γ 1 . The second photon γ 0 is then given by the excited singlet state

|S=0, M z =0 = 1 √ 2 |↑, ↓ -|↓, ↑ (3)
with energy E 0 = (3/4) ∆. The energy difference between the two photons γ 1 and γ 0 is then given by ∆.

We assume the particle/antiparticle have magnetic moments m 1 = (β µ B / ) s 1 and m 2 = -µ 1 . Thus the total magnetic moment m = (β µ B / ) (s 1s 2 ) has zero average value for the γ 1 photon and m z = β µ B for the γ 0 photon.

In the presence of a magnetic field B along Oz we shall have

V = (β µ B B/ ) (s 1z -s 2z ) (4)
The only non-zero matrix element of V is

S=1, M z =0|V |S=0, M z =0 = β µ B B (5)
After diagonalisation

|Ψ 1,0 = cosθ |S=1, M z =0 + sinθ |S=0, M z =0 |Ψ 0,0 = -sinθ |S=1, M z =0 + cosθ |S=0, M z =0 (6) with tan(2 θ) = 2β µ B B/∆ (7)
and eigenvalues

E 1 = E 1 + E 0 2 - ∆ 2 1 + tan 2 (2 θ) E 0 = E 1 + E 0 2 + ∆ 2 1 + tan 2 (2 θ) (8)
with the new energy difference

∆ = ∆ 1 + tan 2 (2 θ) (9)
The ordinary photon γ 1 can be described by a linear combination of the two helicity states |S=1, M k = ± 1 where M k is the projection of the spin angular momentum in the direction of propagation of the photon. Let first note that if the γ 1 is propagating along the direction of the magnetic field, only |S=1, M z = ± 1 will be involved and no effect is expected. Consider now a γ 1 propagating along Oy and linearly polarized in the direction of Oz. We shall have

|ǫ z = - 1 √ 2 |S=1, M y =1 -|S=1, M y = -1 (10)
but

|S, M y = Mz=0,±1 |S, M z d S Mz ,My (π/2) ( 11
)
where the d S Mz ,My are the Wigner d-functions. Using

d 1 1,±1 (Θ) = 1 2 (1 ± cos Θ) d 1 0,±1 (Θ) = ± 1 2 √ 2 sin Θ d 1 -1,±1 (Θ) = 1 2 (1 ∓ cos Θ) (12)
we get from ( 10) and ( 11)

|ǫ z = -|S=1, M z =0 (13)
and this state will be affected by the magnetic field through its coupling to the |S=0, M z =0 state. We note in passing that in the case of a linear polarization along the Ox axis we have

|ǫ y = 1 √ 2 |S=1, M y =1 + i |S=1, M y = -1 = 1 √ 2 |S=1, M z =1 + i |S=1, M z = -1 (14)
and this state will not be affected by the magnetic field.

We assume that the magnetic field is switch-on between t = 0 and t = τ = L/c, where L is the field length (of the order of 1 m in PVLAS experiment). We shall have |ψ(0) = -|1, 0 = -cos θ |Ψ 1,0 + sin θ |Ψ 0,0 where from now on the kets correspond to |S, M z . At time τ

|ψ(τ ) = -cos θ e -i E1 τ / |Ψ 1,0 (15)
+ sin θ e -i E0 τ / |Ψ 0,0

which in terms of the non-perturbed kets |1, 0 and |0, 0 , will be given by

|ψ(τ ) = -cos 2 θ e -i E1 τ / + sin 2 θ e -i E0 τ / |1, 0 -cos θ sin θ e -i E1 t/ -e -i E0 τ / |0, 0 (17)
This can be written as

|ψ(τ ) = -e -i (E1+E0) τ /2 cos(∆ τ /2 ) + i cos(2θ) sin(∆ τ /2 ) |1, 0 + i sin(2θ) sin(∆ τ /2 ) |0, 0 (18)
From (18) the probability to produce |0, 0 is P γ1→γ0 = | 0, 0|ψ(τ ) | 2 which gives

P γ1→γ0 = tan 2 (2θ) 1 + tan 2 (2θ) sin 2 ∆ 1 + tan 2 (2θ) τ /2
(19) with tan 2 (2θ) given by (7).

In the limit where 2β µ B B ≪ ∆, tan 2 (2θ) ≪ 1, and

P γ1→γ0 ≃ β µ B B τ 2   sin ∆ τ /2 ∆ τ /2   2 (20)
Thus, when ∆ τ /2 ≪ 1, P γ1→γ0 does not depend on ∆.

In an apparatus like the PVLAS one where a linearly polarized laser passes through a region where a magnetic field pointing at 45 degrees with respect to light polarization plane is present, such a conversion probability will show as a linear dichroism giving an apparent rotation of the polarization plane ρ = 1 2 P γ1→γ0 . It is worth to stress that standard QED [10] does not predict any dichroism for light propagating in vacuum in the presence of a magnetic field.

As for the phase of the |1, 0 , this is given by

φ 1 = -(E 1 + E 0 ) τ /2 + arctan cos(2θ) tan(∆ τ /2 ) (21)
On the other hand, for the Ox polarization the phase is φ 1 = E 1 τ / . The phase difference between γ 1 states for polarization along and perpendicular to the magnetic field is then given by

δφ ≡ φ 1 -φ 1 = arctan cos(2θ) tan(∆ τ /2 ) -∆ τ /2
(22) Expanding this function in powers of θ around zero, we found

δφ = β µ B B ∆ 2 ∆ τ -sin(∆ τ / ) (23)
Again, in the case of an apparatus like the PVLAS one, this dephasing will show as an ellipticity ǫ = δφ/2 acquired by the polarized beam passing through the magnetic field region. Ellipticity is associated to the existence of a birefringence by the formula

ǫ = π L λ (n -n ⊥ ) ( 24
)
where λ is the light wavelength, and n and n ⊥ are the indexes of refraction for light polarized parallel and perpendicular with respect to the magnetic field, respectively. Thus, in the framework of our model a vacuum will show an apparent magnetic birefringence

(n -n ⊥ ) = λ 2π c τ β µ B B ∆ 2 ∆ τ -sin(∆ τ / )
(25) that depends on the time the photon stays in the magnetic field region. Standard QED predicts that a vacuum is a magnetic birefringent medium showing a (n -n ⊥ ) ≃ 4 × 10 -24 B 2 where B is given in Tesla. That only depends on the value of fundamental constants and the square of the magnetic field intensity [10]. QED also predicts that a corresponding effect exists in the presence of an electric field, such an effect is absent in the framework of our model. We note that our formulas for the conversion probability and the dephasing are equivalent to the ones obtained in the axion case [11] since axion-photon coupling can be also treated as a two level system [12]. Our ∆ corresponds to the ratio m 2 a /ω and β to g aγγ , where m a is the axion mass, ω the photon energy, and g aγγ the axionphoton coupling constant. The mass and coupling constant associated to the ALP needed to explain PVLAS results, m a ≈ 10 -3 eV and g aγγ ≈ 3 × 10 -6 GeV -1 [1], have been chosen by comparing the dichroism signal of PVLAS with limits published by the BRFT collaboration in 1993 [13]. In fig. 1 we show the corresponding graph following equation (20). We have assumed as usual that the measured effect is simply the effect predicted by this formula multiplied by the number of passages in the magnetic field due of the presence of optical cavities. Dotted line represents the lower border of the parameters plane forbidden by BRFT results at a 2σ level, while full line represents the PVLAS signal.

The main difference between our model and the axion model is that in our case the optical effects do not depend on the photon energy. Thus, in our case, the oscillations between the two states of the hamiltonian only depend on the time the γ 1 stay in the magnetic field i.e. the length of the magnetic field region. In the axion case the oscillations depend on the length divided by the photon energy ω. Oscillations can therefore be avoided by choosing higher energy photons for longer magnets, and that was the case of BRFT collaboration with respect to PVLAS collaboration. Eventually, this explains why in our case the allowed window for the parameters that could explain the PVLAS dichroism is larger that in the axion case treated in ref. [1].

In our model the mixing between the ordinary photon γ 1 and the second photon γ 0 only appears in a magnetic field. This will not affect the energy balance and star evolution, but should be important in the case of photon emission from neutron stars which show magnetic fields as high as 10 9 T. This is anyway an important issue also for ALP (see e.g. ref. [14], and [15]).

Since the fusion model assumes structure of the photon, we should expect excited states associated to internal motion and presumably non zero mass of the constituents. However, if the masses and the spatial dimension of the photon are very small, the first excited state can be very high up in energy.

In conclusion, once the PVLAS signal will be confirmed, the exotic but simple de Broglie's fusion model for the photon can provide an explanation for this signal that is not in contradiction with star observation or solar axion search. On the other hand, experiments testing the propagation of light in the presence of a magnetic field in terrestrial laboratories or by astrophysical observations can put more and more stringent limits on its free parameters.

FIG. 1 :

 1 FIG. 1: Comparison between PVLAS signal and BRFT limits

ACKNOWLEDGEMENTS

We thanks E. Massó for very helpful discussion and carefully reading our manuscript.


Jean-Bernard Duchez

Franck Mady

Yasmine Mebrouk

Mourad Benabdesselam

Simultaneous effects of photo-and radio-darkening in ytterbium-doped aluminosilicate fibres

Keywords: Fiber optics, Radiation effects on optical elements, devices and systems

We present original characterizations of photo-radiodarkening in ytterbium-doped silica optical fibres submitted to the simultaneous action of the pump and of an ionizing radiation. We present the interplay between both radiations, showing e.g. that the pump is able to darken or bleach the fibre depending on the ionizing dose. The photo-resistance of the fibre is shown to play a crucial role on its radioresistance, and that photo-resistant fibres should be also radio-resistant in low dose rate conditions. All the results are thoroughly explained by a physical model presented in a separate submission by Mady et al. (this conference).

INTRODUCTION

Ytterbium-doped silica optical fibres (YDF) may suffer from twofold degradation when operated in amplifying conditions and harsh environments. Excess optical losses are induced by external ionizing radiations (radio-darkening or RD), but also, possibly, due to the pump itself (photodarkening, PD). PD and RD of YDF have been well characterized already [1], but by distinct communities. Possible interplay between PD and RD has neither been characterized nor modelled, though pumping and ionizing radiations should be at play simultaneously in some operation conditions, as in space-based applications. We present an experimental study of simultaneous photo-radiodarkening (PRD). The results show that the PD level is of first importance in the RD resistance. A complementary work by Mady's et al., also submitted in this conference, presents a model that explains all of the present results.

EXPERIMENTAL METHODS

The measurement bench is based on a pump-probe scheme [2] with contra-propagating pump (977 nm, ∼230 mW) and visible probe (633 nm, a few mW). Customized wavelength division multiplexers (OZ-optics) are used for coupling pump and probe lights into the fibre under test (FUT). Two YDF FUT were compared: K10 (0.9% Al 2 O 3 , 0.3% Yb 2 O 3 , NA = 0.11), fabricated and drawn in our lab, which is not optimized to resist against PD nor RD, and a commercial one (nLight Yb1200-4/125, NA=0.2), that offers low PD levels. The FUTs consisted in ∼2 cm long uncoated samples. They were irradiated in-line, under pumping or not, by an X-ray generator (Cu anode, 45 kV).

RESULTS

The effects of PD and PRD in the K10 sample are shown in Fig. 1. When the FUT is in-core pumped, the pump output power decays due to PD, but the PD level tends to a stable value. Such a PD equilibrium was already observed, carefully examined by Jetschke et al. [3], and shown to be determined by the pump power. Hence the pump is not only responsible for PD, but also for a photobleaching (PB) effect, which balances PD at long pumping time. The same K10 fibre (fresh sample) was submitted to a PRD run: it was irradiated by X-rays (22,7 krad(SiO 2 ). min -1 , 340 krad(SiO 2 )) during pumping and then, after the end of the X-ray irradiation, left to the action of the pump.

In the PRD stage, the FUT output power decays very rapidly. Then the YDF is obviously photo-bleached by the pump: the output power moves back up, tending to the same equilibrium level than pure PD. The pump is therefore capable of bleaching also the PRD, which is essentially due to RD. This demonstrates a basic interplay between PD and RD and highlights the crucial role of the PD equilibrium level (PDEL). It is not surprising that the pump bleaches both PD and RD, because our team showed that trappedstates responsible for PD and RD in YDF are identical [4].

Similar experiments were done at various doses (K10, fresh samples). Results are displayed in Fig. 2 for pump and probe wavelengths. Log-log scales are used to enlarge the PRD region. For the 2 highest doses, the result is the same as in Fig. 1: PRD followed by pump-induced PB. When Xrays are stopped at 45 krad, the output power level is still above the PDEL and the pump is responsible for further degradation, i.e. PD. All plots tend to the PDEL. Therefore the primary action of the pump (PD or PB) depends on the ionizing dose. Similar behaviours are always found for the pump and the probe, even if degradation is more important in the visible range. To highlight the pump effect during PRD, same doses were given with and without pumping during X-ray irradiation (RD and PRD conditions) for the K10 and Yb1200 fibres.

Results are shown in Fig. 3, now in terms of the excess absorption coefficient. Two doses have been imparted: one is small (30 krad) to keep the radio-induced degradation below the PDEL, the other is high (450 krad) to exceed this level. At the same pump input power the Yb1200 fibre has a much lower PDEL than our K10 fibre. It is also found to present a better X-ray resistance (the 2 fibres show RD or PRD levels in similar proportion with respect to their PDEL). At low dose, below the PDEL, both fibres are less darkened in RD than in PRD conditions. Then, the pump accelerates degradation. For the dose above the PDEL, RD yields more degradation than PRD so the pump slows-down the degradation. The pump has hence the same PD or PB effect during as after PRD, according to the dose. Because of its PB action, the pump will mitigate RD at high doses. Indeed the PRD will also reach a stationary state above the PDEL at long irradiation time, thus freezing the PRD level despite the continuous irradiation. Decreasing the dose rate will drop this PRD equilibrium closer to the PDEL. In very low dose rate conditions the PRD level will be minimal and equal to the PDEL. RD will then develop very slowly so the pump will bleach the fibre "adiabatically" as soon as the PRD level will exceed the PDEL (PRD saturation at the PDEL). To confirm this point, we gave a same high dose (900 krad) to the K10 and Yb1200 fibres, at decreasing mean dose rates. To illustrate PB by the pump, we did not use lower constant dose rates, but impart the dose in 1, 3 and 6 fractions by observing a 75 min PB time between each fraction. Results are displayed in Fig. 4. For both fibres, the mean PRD well tend towards an equilibrium value that decreases with the mean dose rate. We could not reach very low dose rate as those typically encountered in space (too long experiments), but the validated model presented in the submission by Mady et al., which reproduce our experiments, enabled us to extrapolate the observed behaviour to such low dose rates where the PRD is well found to saturate at the PDEL. These outcomes have major consequences on the understanding and assessment of the YDF resistance for applications concerned with PRD.

Fig. 1 .

 1 Fig. 1. Decay of the normalized output power at 977 nm for the K10 fibre, for pure PD and PRD.

Fig. 2 .

 2 Fig. 2. PD and PRD of K10 at various doses at 977 nm (a) and 633 nm (b).

Fig. 3 .

 3 Fig. 3. Excess loss Δα at 977nm, 30 and 450 krad doses, for K10 (a) and Yb1200 (b). Pump effect during PRD.

Fig. 4 .

 4 Fig. 4. Excess loss Δα in K10 (a) and Yb1200 (b) after a 900 krad X-ray dose given in 1, 3 and 6 fractions.


Yasmine Mebrouk

Franck Mady

Mourad Jean-Bernard Duchez

Mourad Benabdesselam

Colour centres

Keywords: Oral, Radiation effects optical fibres, thermoluminescence, silica, radiation

INTRODUCTION

Optical properties of erbium-doped silica-based fibres are significantly affected by ionizing irradiation [1]. Radiation leads to the conversion of some native defects into colour centres (CC) by electron or hole trapping [2] and gives rise to the so-called radiation-induced absorption (RIA). Understanding the RIA development under irradiation is the main key for assessing its resistance in harsh environments. This paper shows that, additionally to the RIA, the gain of erbium-doped amplifiers is damaged by the loss in Er 3+ ions under irradiation. By the mean of thermal annealing experiments, we correlate the recovery of Er 3+ ions to the well-known silica defects, especially the non-bridging oxygen hole centre (NBOHC) absorbing at 2 eV [3] and AlE' centre with its absorption band at 4 eV [4].

EXPERIMENTAL

Irradiations were carried out with a 45 kV X ray tube. Absorption measurements were performed by means of a PerkinElmer Lambda 1050 spectrophotometer. Both irradiations and absorption measurements have been carried out at room temperature (RT). The RIA is calculated as the difference between the post-and pre-irradiation absorption coefficient spectra (in mm -1 ). TSL measurements were performed with a Harshaw TLD 3500 reader. This technique is a powerful tool to study the thermal relaxation processes, especially in rare earth doped fibres [5]. TSL consists of photons emitted upon radiative recombination of 'detrapped' carriers with carriers of opposite polarity on reduced/oxidized centres (recombination centres). After radiation exposure, samples are heated up linearly (2 °C s -1 ) from RT up to 600 °C and carriers are thermally released from traps of increasing depths. The TSL is plotted as a function of temperature to give 'glow curves' that reflect the energy distribution of trapped states generated by irradiation. Glow curves exhibit peaks. Each trapping level gives rise to a single peak (the deeper the trap, the higher the peak temperature). TSL spectra acquired at any temperature with an optical multi-channel analyzer allow the identification of photoionized luminescent centres. Samples are preform discs with 0.5 mm core radius and 1 mm thick fabricated in our laboratory by modified chemical vapour deposition (MCVD) and solution doping. Undoped silica (SiO 2 ), aluminum doped silica (SiO 2 :Al), Er-doped aluminosilicate (SiO 2 :Al,Er) and Er-doped silica (SiO 2 :Er) samples are considered in this study.

RESULTS

Normalized TSL glow curves obtained after X irradiation are shown in Fig. 1. They present three main peaks between 100 and 150 °C (peak 1), in the 200-250 °C range (peak 2) and between 350 and 450 °C (peak 3). For the SiO 2 :Er, SiO 2 and SiO 2 :Al samples, the second peak is not intense but it contributes to the broadening of the 100 °C first peak. None of the TSL peak can be attributed specifically to Al or Er, but the Al-Er co-doping apparently enhances the third peak (near 400 °C). TSL spectra from Er-containing samples always consist of the Er 3+ luminescence lines. It is the case of the SiO 2 :Er perform, for which the visible TSL spectrum is shown in the inset of Fig. 1. This luminescence is observed throughout the temperature range (RT-600 °C). The thermal relaxation therefore involves the recombination of detrapped carriers to form Er 3+ ions. Accordingly, the irradiation must be responsible for a valence change in Er 3+ ions (presumably forming Er 2+ ions and hence trapped holes). As a result, Er 3+ ions are involved in the darkening and bleaching processes. This is a novel statement, since the Er implication was dismissed to date [6]. Therefore the amplifier performance will be dropped not only due to the colour centres (darkening) absorbing signal and pump, and so damaging the gain, but also because of the loss in Er 3+ ions, which will reinforce the gain degradation. Measurements done in the NIR range (not presented here) show the same reduction in the Er 3+ absorption around 1530 nm. The Er 3+ density is well reduced by irradiation and recovered by recombination under thermal stimulation. For a 10 m long fibre initially doped with 10 19 cm -3 Er 3+ ions and a 300 mW pump input power, a 20% decrease in the Er 3+ ion density results in a > 50% gain drop at 1530 nm (out of RIA). The lower part of Fig. 3 displays the recovery peaks, i.e. derivatives of the upper part plots (in % of the total RIA at each wavelength). These recovery peaks clearly correlates to the TSL curve, showing the Er 3+ ions partially recover at each TSL peak, i.e. when carriers are progressively released from deeper TSL trap. In the inset of Fig. 3 we plotted (in % of the total RIA) the recovery peaks at 2 (NBOHC) and 4 eV (AlE'). NBOHC mainly recover around 400°C. Hence the third TSL peak can be related to NBOHC relaxation. AlE' centres recover at 150°C and rather correlate to the first glow peak (100-150 °C). Since NBOHC and AlE' are trapped-hole centres, it appears that TSL peaks rather correspond to holes release. This finding supports the proposal that Er 2+ ions are formed together with trapped holes.

Behaviours of the other RIA bands and their involvement in the darkening/bleaching mechanisms will be reported at the conference.

Fig. 1 .

 1 Fig. 1. Normalized thermoluminescence intensity after X rays exposure. Inset: Spectrum analysis of erbium doped samples TSL Core-absorption spectra for the SiO 2 :Al,Er sample is plotted in Fig. 2. The related RIA (in mm -1 ) is shown in inset. Dashed curves correspond to absorption after thermal bleaching from RT up to 600 °C, by steps of 50° (T stop = 50, 100, …, 600 °C). The absorption returns down to its initial value as T stop is increased, thus showing that the darkening process is fully reversible. The annealing is done at the same heating rate as TSL measurements (2 °C/s) to allow the comparison of the T stop values with the TSL temperature scale. Er 3+ ground state absorption transitions to 2 G 11/2 and 4 S 3/2 levels are located at 378 and 521 nm respectively.

Fig. 2 .

 2 Fig. 2. Core-absorption spectra before (black) and after (red) X-ray irradiation for the Al-Er-doped preform. Inset: radiation induced absorption We calculated the optical intensity OI (OI = FWHM ⋅ maximum intensity) of each band for pristine, irradiated and all intermediate T stop spectra. The results are shown on the upper part of Fig. 3. OI of Er 3+ absorption bands in the UV-visible range decreases by more than 20% after irradiation and then recovers with increasing T stop .

Fig. 3 :

 3 Fig. 3: Up : OIs of 521 nm and 378 nm Er 3+ ion absorption bands. Down : Thermal recovery at 2.38 and 3.28 eV and for 2 and 4 eV in the inset. Points represent the annealed absorption in % of the total RIA.


Keywords: Optical communications, Radiation effects, Thermally Stimulated Luminescence, Ytterbium-doped silica optical fibers

Radiation effects in ytterbium-doped silica optical fibers are still poorly studied despite their importance in spacebased applications as optical inter-satellite links. We investigate such effects by coupling thermally stimulated luminescence and optical absorption measurements. Connections between traps populated by X ray irradiations and the radiation-induced optical absorption (RIOA) are established. The color centers responsible for RIOA are proved to correspond to reduced forms of defects related to dopants. Holes released upon reduction processes are shown to be trapped on defects of the silica matrix.

I. INTRODUCTION

HE radiation-induced degradation of rare-earth-doped fiber amplifiers or lasers has become a crucial concern for the space industry. Radiation effects on erbium (Er)doped fibers (EDF) has been quite widely investigated for about 10 years (see e.g. [START_REF] Caussanel | Extrapolation of radiation-induced EDFA gain degradation at space dose rate[END_REF], [START_REF] Girard | Proton-and gamma-induced effects on erbium-doped optical fibres[END_REF]) because EDF amplifiers constitute the standard technology for optical communications at 1550 nm. By comparison, the radiation resistance of ytterbium-doped fibers (YDF) is still much less explored (see e.g. [START_REF] Tortech | Radiation effects on Yb-and Yb/Er-doped optical fibres: A micro-luminescence study[END_REF]), although YDF provide a solution of choice to make the high output power fiber lasers required for optical intersatellite links (OISL). Considering the poor attention paid so far in the study of radiation effects on YDF, this work focuses on this problem. The original techniques and procedures we present could be followed for Er-doped fibers. The performances of irradiated YDF are reduced due to the formation of color centers which absorption bands spread from ultra-violet (UV) to visible and have significant tails overlapping the near-infra-red (NIR) absorption spectrum of Yb 3+ ions. This results in excess optical losses, notably at pump and gain wavelengths. Basically, color centers are formed due to the ionization of the valence band or of preexisting centers associated with band-gap energy levels, as optically active ions, e.g. ytterbium (Yb), defects, dopants.... Manuscript received July 2, 2010. Authors are with the University of Nice-Sophia Antipolis, Laboratoire de Physique de la Matière Condensée (LPMC), Centre National de la Recherche Scientifique (CNRS), UMR 6622, Nice, France (corresponding author: +33 492 076 330; fax: +33 492 076 754; e-mail: mady@unice.fr).

Since carriers released upon ionization are trapped on other defects, one has still to elucidate whether the color centers correspond to the ionized centers (i.e. reduced or oxidized centers), or on the contrary to populated traps.

In order to advance this basic question, we follow an original approach based on the coupling of thermally stimulated luminescence (TSL) and optical absorption measurements. TSL peaks reveal the trapping levels populated by irradiation. On one hand, some of these traps are found to be connected to the radiation-induced optical absorption (RIOA), because thermal detrapping from these levels is accompanied by RIOA recovery. On the other hand, our experiments demonstrate that populated traps are not the RIOA color centers that correspond to the reduced form of centers related to the core dopants (ytterbium or aluminum). Holes released in the reduction process will be shown to be trapped on intrinsic defects of the silica matrix.

II. MATERIALS AND METHODS

A. Samples

At LPMC, we have a long experience in fabricating silica optical fibres by modified chemical vapour deposition (MCVD) and solution-doping techniques [START_REF] Dussardier | Luminescent ions in silicabased optical fibers[END_REF]. We therefore have the capacity to fabricate samples on demand, notably to test the radiation response in the presence of each single dopant, e.g. rare-earth or aluminium (Al). This also allows us to follow the defects related to the RIOA from the commercial substrate tube down to the fiber, via the preform stage.

Samples consist of ~4 mm radius and 1-2 mm thick slices of fiber preforms. They have a central doped core (doped MCVD-deposited silica layers) surrounded by the cladding (undoped deposited layers plus substrate tube). Samples with Yb-free aluminosilicate core (referred to as AS), Yb-doped Al-free silicate core (S:Yb), Yb-doped alumino-silicate cores (AS:Yb) and Yb-free germanosilicate core (GS) have been considered, as well as fibers drawn from the AS:Yb preforms.

B. Methods

Samples were irradiated with X rays (45 kV) at ~0.25 krad(SiO 2 ) min -1 in a 30-50 krad dose range. UV radiations from a deuterium lamp were also used to check sub-gap excitation processes. Then, interferential filters allowed us to tune the photon energy (for instance 5.4 and 4.95 eV).

Radiation effects in ytterbium-doped silica optical fibers: traps and color centers related to the radiation-induced optical losses Franck Mady, Mourad Benabdesselam, Yasmine Mebrouk and Bernard Dussardier T Absorption measurements have been performed with a Varian Cary 500 spectrophotometer. The RIOA is calculated as the difference between the preform core transmission before and after irradiation (not dimensioned). We note RIOA to discriminate between our measurements and the so-called radiation-induced attenuation (RIA) measured in dB/m. TSL measurements have been done with a laboratory-made apparatus which has already given rise to numerous publications (see e.g. [START_REF] Benabdesselam | Thermal quenching investigation in CVD diamond by thermally stimulated conductivity and luminescence[END_REF]). Below we remind the reader of the TSL basics (see e.g. [START_REF] Mady | Dimensionless study of the total response of thermally stimulated luminescence[END_REF] for details).

C. TSL principle and benefits

As already mentioned, irradiation is responsible for electron-hole pair generation and ionization of pre-existing centers. Free electrons or holes released in bands are trapped on defect gap levels. After radiation exposure, one has therefore ionized centers and populated traps. Then, the sample is heated up at constant rate (1 °C s -1 ). Trapped carriers are progressively released by the thermal stimulation. If a detrapped carrier recombines with an ionized luminescent center of opposite polarity, characteristic photons are emitted. The TSL signal corresponds to these photons, collected by an optical multi-channel analyzer (OMA).

As the temperature increases linearly, carriers are released from traps of increasing depths. Plots of the spectrallyintegrated TSL intensity as a function of temperature thus exhibit peaks. In principle, each trapping level gives rise to a single peak (the deeper the trap, the higher the peak temperature). The TSL analysis provides two types of characterizations at once. First, the TSL peak plots reflect the non-luminescent traps populated by the radiation. Second, the characteristic emissions shown by the TSL spectra reveal the luminescent centers that have been ionized. By emptying traps and recombining carriers, the TSL readout resets the radiation effects while pointing out the traps and luminescent centers altered by the radiation. TSL is therefore a powerful method to investigate ionizing dose effects. It is at least as efficient as photo-luminescence-like techniques often used to characterize the formation of luminescent centers [START_REF] Tortech | Radiation effects on Yb-and Yb/Er-doped optical fibres: A micro-luminescence study[END_REF].

III. TSL FEATURES OF PREFORMS AND FIBERS

The typical TSL peak plots obtained after X irradiation from two AS:Yb preforms and fibers and from the GS preform are shown in Fig. 1. TSL plots have been normalized with respect to their maximum because fiber and preform samples have different shapes and volumes so intensity comparisons are not relevant. At this stage, the existence (or not) of peaks and the position of these peaks are pretty informative.

The AS:Yb preforms of Fig. 1(a) and (b) have been made from different commercial substrate tubes (tubes 1 and 2 respectively) but according to the same drawing process. The silica of tube 2 is known to be of much higher quality than that of tube 1. This was clearly confirmed by TSL measurements. Tube 1 exhibits a large TSL signal similar to that of the preform (Fig. 1(a)), revealing a large amount of defects. The TSL of tube 2 was found to be very weak, close to the noise level (not visible in Fig. 1(b)).

Irradiations and TSL readout normally affect both core and cladding regions of preforms or fibers. In Fig. 1(a), the TSL of the AS:Yb preform from tube 1 is obviously dominated by the signal brought by the poor quality tube which is found again in the preform cladding. We therefore cut a small sample out of the AS:Yb preform core to examine the TSL of this tubefree region. In the corresponding plot of Fig. 1(a), the continuously increasing luminescence above 400°C is not TSL signal. It corresponds to the black body radiation from the resistive sample furnace which increases as T(K) 4 . TSL signals from other samples combining core and cladding parts were intense so the black body contribution to the total light is not visible. By contrast, the TSL from the core alone was found to be weak, exceeded by the black body radiation above 400 °C. The same problem appeared for the AS:Yb preform from tube 2 in Fig 1(b). Its TSL plot has been cut off at 400°C for clarity.

TSL measurements reveal three complex peaks probably made up of sub-peaks: one in the 100-125°C range (hereafter referred to as peak 1), one around 225-250°C (peak 2), and one between 400 and 450°C (peak 3). This shows that the ionizing radiation populates three distinguishable groups of trapping levels. When peaks 1 and 3 are markedly present in the substrate tube, they are still predominant in the AS:Yb preform and fiber. For the AS:Yb preform from tube 1, peak 2 appears when considering the preform core only. Its intensity is then rather comparable to that of peak 1. Peak 3 may be present in the core, but hidden by the black body emission. Contrary to tube 1, tube 2 has negligible TSL. The TSL signals of Fig. 1(b) are therefore mainly related to the MCVDdeposited silica layers, as for the core sample in Fig. 1(a). The TSL of the AS:Yb preform from tube 2 is very similar to that of the core of the AS:Yb preform from tube 1 (similar peaks 1 and 2, weak TSL signal and >400 °C region altered by the black body radiation). In the absence of the large TSL brought by the tube, the effect of the fiber drawing process is also clear in Fig. 1(b): the fiber TSL peaks exhibit a better resolution than that of the preform and sub-peaks of the complex structure of peaks 1-3 appear partially. It seems that some traps present in the preform have been annealed by the drawing process. Note that Peak 3 is strongly present in the fiber, even if it could not be observed in the preform.

If peak 2 is only observed for MCVD-deposited silica layers, its presence is not due to the core dopants, namely Yb or Al. As seen from the inset of Fig. 1(b), the Yb-free and Alfree GS preform is characterized by the same TSL peaks 1-3 as AS:Yb samples. In fact, the 3 main TSL peaks are observed in tube 1 and all fibers and preforms independently of the presence of Al, Yb or Ge. We therefore conclude that the ionizing radiation populates intrinsic silica matrix defects.

At last, let us mention that irradiation of AS:Yb samples with UV photons at 5.4 and 4.95 eV produce the same TSL peaks and thus populate the same traps as X irradiation, even if relative peaks intensities are modified. This confirms that sub-gap ionization mechanisms are at play. Various experiments allowed us to demonstrate that Yb 3+ ions are responsible for this sub-gap absorption, but this point is not discussed in detail in this extended summary.

IV. THE RIOA AND ITS THERMAL RECOVERY

A. Degradation and core darkening

The optical transmission of preform samples were measured before and after irradiation (right before the TSL readout) in order to obtain the RIOA. Fig. 2 displays the RIOA obtained for the AS:Yb preform made from tube 1 after the sample was given a 15 and a 30 krad(SiO 2 ) X ray dose. The measurement at 15 krad(SiO 2 ) has been made in the core and in the cladding for comparison. RIOA plots obtained for the AS:Yb preform made from tube 2 have similar shapes, showing a bit more of radiation-induced core absorption in the visible region for the same doses. The core RIOA differs from that of cladding. First, degradation is much more important in the core. Second, radiation-induced absorption bands are cleary specific in the core where they strongly affect the visible and NIR regions. This specific core absorption is precisely responsible for the performance degradation of fiber amplifiers and lasers.

When the dose exceeds ~30 krad(SiO 2 ), the visible part of the RIOA becomes apparent to the naked eyed because the preform cores darken. The photos shown in Fig. 2 well illustrate the darkening is specific to the core. The core consists of MCVD-deposited silica layers, just as the surrounding cladding which remains transparent. Darkening is therefore due to the presence of the core dopants, Yb or Al. Actually, photographs of Fig. 2 demonstrate that darkening is obtained either in the Yb-free aluminosilicate (AS) or the Alfree Yb-doped silica (S:Yb) core. Of course, RIOA patterns measured in these cores are different. Nevertheless, they both have significant band-tails in the NIR region and the fact remains that the presence of each single dopant, Yb or Al is able to produce degradation.

B. Thermal recovery

After reading out the TSL of the AS:Yb samples at 1°C s -1 up to 600°C we observe that: (i) further TSL readouts do not provide any TSL signal, unless the sample is re-irradiated, (ii) the optical transmission of the non-irradiated sample cores is totally recovered. This indicates that: (i) a single TSL readout empties all the traps responsible for the 3 main TSL peaks, and (ii) The core recovery follows the trap emptying.

Thermal recovery was examined according to the following procedure: after the optical transmission of the pristine AS:Yb preform core was measured, AS:Yb samples were irradiated with X rays. The transmission of the irradiated core was measured and then the sample was heated up to T stop = 100 °C at 1°C s -1 . The new core transmission was measured after cooling at room temperature. Then, the sample was heated up to T stop = 150°C and the new core transmission was measured, etc… The partial recovery of the core transmission within 50 °C brackets was monitored by considering T stop = 200, 250,… up to 600°C. Fig. 3 presents the results of this experiment and shows the progressive recovery at the successive T stop values.

The difference between each couple of consecutive transmission plots gives the recovered transmission within each 50 °C bracket as a function of wavelength. The values obtained for three selected UV, visible and NIR wavelengths (310 or 350, 550 and 800 nm) are plotted in figure 4. They are given in % of the total RIOA at the respective wavelengths.

Two peaks of thermal recovery exist for the core transmission of AS:Yb samples. A main peak located around 450°C and a secondary one at ~250 °C. These recovery peaks are in very good correspondence with TSL peaks 2 and 3 observed for the same samples (Fig. 1). There is strong evidence that detrapping from traps related to the TSL peak #2 and 3 leads to RIOA recovery. By contrast, release from traps associated with the TSL peak at ~100 °C (peak 1) does not induce significant recovery.

V. DISCUSSION

Irradiation populates traps, thus allowing the observation of TSL peaks, and induces RIOA. The TSL readout results in detrapping and perfect recovery of the optical transmission. If the connection of RIOA with populated traps is established, only the traps of TSL peaks 2 and 3 are directly involved in degradation. Our experiments show that populated traps correspond to intrinsic silica defects, present in the core and cladding of alumino-silicate (AS, Yb-doped or not) samples. However, RIOA and darkening require Yb or Al doping since only the core of AS:Yb, AS or S:Yb samples darkens. In addition, the 3 typical traps are also populated by irradiation in the germanosilicate (GS) sample which is not observed to darken, even at high doses. As a consequence, populated traps are not the color centers responsible for the RIOA. The latter therefore correspond to pre-existing centers ionized by irradiation. If each populated trap was a color center with characteristic RIOA bands, thermal annealing of a given band would correspond to detrapping from the associated trap and the location of recovery peaks would be strongly dependent on wavelength. By contrast, recovery plots of Fig. 4 consist of the two same peaks, regardless of the RIOA wavelength, indicating that detrapping from trap groups 2 and 3 contributes similarly to RIOA annealing at any wavelength. As a consequence: (i) RIOA annealing is due to recombination on ionized centers upon carrier detrapping from traps 2 and 3, (ii) traps of TSL peaks 2 and 3 are of the same nature. The TSL spectra, not presented in this summary, show that TSL from AS:Yb samples contain the luminescence of Yb 3+ ions. We conclude that Yb 3+ ions are altered by radiation to produce Yb 2+ ions with related absorption patterns and trapped holes. Thus, we are dealing with hole traps. These traps provide receptor sites for carriers released by ionization. Their presence is therefore a main concern for the radiation resistance of Al-or Yb-doped silica, as that of YDF. The RIOA is due to the creation of Yb-or Al-related centers that cannot be removed. Controlling the presence of the receptor traps is then an important route of YDF radiation hardening. The ability to follow the evolution of traps from the tube to the fiber is then crucial. . Thermal recovery of the core transmission for the AS:Yb preform from tube 1 and tube 2 (inset) at three selected wavelengths as a function of temperature. For clarity, plots are displayed as scatter graphs but rather correspond to histograms. For instance, the ordinate of a point at 400 °C should be understood as the percentage of annealed absorption between 350 and 400 °C at the indicated wavelength.

Fig. 1 .

 1 Fig.1. Spectrally-integrated TLS intensity as a function of temperature for (a) AS:Yb from tube 1, preform and fiber (b) AS:Yb from tube 2, preform and fiber. Inset: TSL plot from a GS preform made from tube 1 exhibiting the same peak groups as AS:Yb samples. Main peaks are numbered in (a).

Fig. 2 .

 2 Fig.2. Radiation-induced optical absorption (RIOA) in the core and cladding of the AS:Yb preform sample made from tube 1. The vertical scale gives the difference, in points, between the optical transmission (in %) before and after irradiation. Insets: photographs of the AS and S:Yb preform samples after X irradiation (dose ~50 krad(SiO 2 )).

Fig. 3 .

 3 Fig.3. Optical transmission of the AS:Yb preform sample (from tube 2) as a function of wavelength, before irradiation, after X irradiation and then after heating up the sample from room temperature to the Tstop temperatures indicated on plots.


Antoine Danchin

Bacteria are not Lamarckian

In order to separate between the two opposite interpretations (instruction vs selection) it was necessary to find as a starting mutation, chosen for its reversion potential, a mutation (i) that was unable to generate true revertants (e. g. a deletion rendering impossible the restoration of original activity) and (ii) that could manifest similar growth properties on different growth media, so that influence of the growth medium could be assayed for (a) induction of outgrowing mutants and (b) induction of mediumoriented mutations. Inactivation of the adenylyl cyclase gene of Escherichia coli fulfils such requirements. For this reason I used the deletion mutant cyaΔ854 (8) that renders bacteria unable to ferment a variety of carbon sources. In particular cya deficient mutants are unable to ferment maltose, lactose, melibiose, glycerol or mannitol, even when plated on rich media. As a consequence a cya mutant yields white colonies on McConkey plates supplemented with these carbon sources (but, as a control, form red colonies on such plates supplemented with glucose). I therefore investigated the fate of colonies obtained on McConkey plates supplemented with glycerol, melibiose, rhamnose, lactose, maltose or mannitol, after one day at 37°C followed by several days at room temperature (18-22°C).

As shown in Table 1 after 8-10 days red papillae started to appear on individual colonies from several plates. Strain TP2000 (argH1 cyaΔ854 lacΔX74) was used in the first set of experiments. It carries a lactose deletion that was used as an internal control : no red papillae appeared after one month on McConkey plates supplemented with lactose. The overall number of papillae still grew steadily after one month on all media except lactose and glycerol. The case of glycerol plates is particular, because on such plates growth was found to continue at the low temperature, the colonies slowly increasing in size and displaying a rose, not a white colour. Individual papillae were purified on the original medium at low temperature, then analysed for their fermentation capacity on the other media. Most papillae isolated from maltose, melibiose or rhamnose plates were red on all three media, at low temperature as well as at 37°C (except as expected for melibiose, because of its natural thermosensitivity).

They failed to ferment mannitol. In sharp contrast the mannitol papillae were found to be red only on mannitol plates. As already stated no papillae were isolated on glycerol, indicating that papillae could only develop on non growing colonies, but the papillae isolated on maltose, melibiose or rhamnose were found to be red on glycerol at 37°C demonstrating additional capacity for fermenting various carbon sources.

The behaviour of the mutant papillae (which were stable, as shown by repeated streaking on similar plates) was reminiscent of the behaviour of the crp* allele of the crp gene, coding for a mutated receptor of cAMP that is able to activate transcription of catabolite sensitive operon in the absence of the mediator (9). However, because the original crp* mutant had been obtained only from heavily mutated bacteria (9, and A. Ullmann personal communication) the genetic background of the original strain was tested by repeating the experiment in a different background (strain TP7860 : arg his cyaΔ854 λ). It yielded similar results. Several P1 lysates were performed on clones isolated from independent plates, and were used to transduce the crp region of strain TP2339 (argH1 cyaΔ854 crpΔ39 lacΔX74), selecting for Mal + recombinants. All transductants were found to display the pleiotropic carbon positive phenotype, thus substantiating that the mutation is of the crp* type. In one experiment recovery of the wild type aroB marker (which is located near the crp gene) in a cya aroB background was used as a selection for P1 transductants : 38% of the transductants were found to have also recovered a pleiotropic carbon positive phenotype. Subsequently it was found that several different mutants of the cya gene (including a Mu insertion) yielded after some time on plates or in stabs bacteria that were able to ferment most if not all carbon sources that depended on the presence of an active CAP-cAMP complex for expression.

It was also found that a recA derivative of strain TP2000 generated red papillae on maltose, after a lag that was significantly longer (two weeks), demonstrating that recA is probably not involved in the generation of papillae, and suggesting that the lag depends on the growth rate of the bacteria : only those colonies that have completely stopped growing seem to be able to generate papillae.

Taken together these results indicate that on most such carbon sources a crp* allele can be obtained from papillae outgrowing dormant colonies from cya mutants. If the instructive paradigm was to be verified one would have expected to find mostly mutations located in the transcription control regions of the operons specific to each carbon source added to the plates. Indeed, in a few cases (especially on rhamnose plates), mutant types could be obtained, showing specificity towards the original carbon source added to the plates. The crp* result cannot be taken as a demonstration of an induction of specific mutations in a carbon source operon, unless one accepts that bacteria possess a magic foresight permitting them to identify this general control gene as the gene that will permit them to escape starvation.

The present experiments rule out the lamarkian interpretation of Cairns' experiments, and substantiate the recent theoretical analysis of Lenski and Mittler (9), but they do not give an explanation at the molecular level of the phenomenon that permits generation of papillae. In this respect it is worth noticing (as already remarked by Cairns or Hall) that papillae grew on colonies only after a certain delay (in a somewhat unpredictable fashion, generally more than five days in our case). In addition it seems important that only those colonies that stopped growing were able to generate papillae (see the glycerol plates experiments). In the same way, when using bacteria that harboured a plasmid permitting synthesis of a very small amount of cAMP (not sufficient to permit the colonies to be red or even rose), no papillae were found. Finally, there was an effect of the nature of the carbon source : on mannitol plates a large number of papillae were obtained, but they did not correspond to crp* mutants (no red colonies on the other carbon sources), but to some other mutation permitting expression of the mtl operon.

All this may suggest that, when in the stationary phase of growth for a significant period of time bacteria are able to enhance the overall mutagenic capacity of a subpopulation, permitting them to generate mutants, some of which are selected as capable of outgrowing the dormant (and probably dying) colony. This should be a further incentive to study gene expression in the late period of the stationary phase.

  de Régulation de l'Expression Génétique Département de Biochimie et Génétique Moléculaire 28 rue du Dr Roux -75724 PARIS CEDEX 15 Tel 33 (1) 45 68 84 42 Fax 33 (1) 45 68 89 48 E-Mail adanchin@pasteur.fr Instructive influence of environment on heredity has been a debated topic for centuries. Darwin's identification of natural selection coupled to chance variation as the driving force for evolution, against a formal interpretation proposed by Lamarck, convinced most scientists that environment does not specifically instruct evolution in an oriented direction. This is true for multicellular organisms. In contrast, bacteria were long thought of as prone to receive oriented influences from their environment, although much was in favour of the Darwinian route (1). In this context Cairns et al. raised a passionate debate by suggesting that bacteria generate mutations oriented by the environmental conditions (2). Several independent pieces of work subsequently demonstrated that mutations overcoming specific defects arised as a consequence of cultivation on specific media (3-7). Two diametrically opposed interpretations were proposed to explain these observations : either induction of mutations instructed by the environment (e.g. by a process involving a putative reverse transcription) or selection of variants among a large set of mutant bacteria generated when stress conditions are present. The experiments presented below indicate that the Darwinian paradigm is the most plausible.

Table I

 I Papillae on 1000 colonies after 15 days of strain TP2000 (cya-Δ854)

		Number of papillae	Pleiotropic sugar positive
	Maltose	32	+
	Melibiose		+
	Rhamnose		+
	Glycerol	no papillae, rose colonies	-
	Mannitol	>1000	-
	Lactose	no papillae, white colonies	-

Acknowledgements

I wish to thank Agnès Ullmann for her interest, and J. Mazé for her help in some experiments.