Effects of elevating temperature and high- temperature

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Effects of elevating temperature and high- temperature

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Effects of elevating temperature and hightemperature annealing upon state-of-the-art of yttia-alumino-silicate fibers doped with Bismuth
D. Ramirez-Granados,1 A.V. Kir’yanov,1,3,6 Y.O. Barmenkov,1 A. Halder,2 S. Das,2 A. Dhar,2 M.C. Paul,2,7 S.K. Bhadra,2 S.I. Didenko,3 V.V. Koltashev,4 and V.G. Plotnichenko4,5
1Centro de Investigaciones en Optica, Loma del Bosque 115, Col. Lomas del Campestre, Leon 37150, Mexico 2Fiber Optics and Photonic Division, Central Glass & Ceramic Research Institute-CSIR, 196, Raja S.C. Mullick
Road, Kolkata-700 032, India 3National University of Science and Technology (MISIS), Leninsky Avenue 4, Moscow 119049, Russia 4Fiber Optics Research Center of the Russian Academy of Sciences, Vavilov Street 38, Moscow 119991, Russia 5Moscow Institute of Physics and Technology, Institutskii per. 9, Dolgoprudny, Moscow Region, 141700, Russia
[email protected] [email protected]
Abstract: We report an experimental analysis of attenuation and fluorescence (at low-power 750-nm excitation) spectra’ transformations in yttria-alumino-silicate fiber doped with Bismuth (Bi), which occur at higher than room, but not exceeding 700°C, temperatures. As well, we address impact of elevating temperature upon the fiber’s basic characteristics, such as fluorescence/resonant-absorption saturation, fluorescence lifetime, and pump-light backscattering, given by the presence of Bi-Al related active centers (BACs). The experimental data reveals dramatic impact of heating and high-temperature annealing in excess of 500…550°C on the fiber’s state-of-the-art, expressed as significant rise of resonant absorption, enhancement of BACs NIR fluorescence, and reduction of scattering loss. In the meantime, such microscopic parameters of the fiber as BACs fluorescence lifetime and saturation power are found to be kept almost unchanged in its post-annealed state as compared to the pristine one. Possible mechanisms responsible for the phenomena and advantages of utilizing temperature-treated fiber of such type for lasing/amplifying purposes are discussed.
©2016 Optical Society of America
OCIS codes: (060.2290) Fiber materials; (160.2750) Glass and other amorphous materials; (300.6250) Spectroscopy, condensed matter.
References and links
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Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 486

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35. Y. Zhao, L. Wondraczek, A. Mermet, M. Peng, Q. Zhang, and J. Qiu, “Homogeneity of bismuth-distribution in bismuth-doped alkali germanate laser glasses towards superbroad fiber amplifiers,” Opt. Express 23(9), 12423– 12433 (2015).
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1. Introduction
Nowadays, Bismuth (Bi) doped silica fibers (BDFs) attract considerable attention as they exhibit broadband near-infrared (NIR) fluorescence, covering the spectral region that extends from ~1.1 to 1.8 μm, when pumped at wavelengths ranging from ~450 to ~1500 nm, which is an invaluable property for applications in next-day telecom systems. Many compact, versatile, and simple in assembling coherent light sources (lasers and amplifiers) based on BDFs for this, low-loss transmission, window of silica, not matched by any other “active” fibers, have been reported so far. Depending on core-glass chemical composition, BDFs demonstrate diversity of types of fluorescing Bi-related active centers (further – BACs) with a variety of spectral signatures in visible (VIS) and NIR [1–9]. In particular, strong fluorescent ability of BACs, formed in alumino-silicate glasses and fibers and, thus, associated with the presence of Aluminum (Al), is usually observed within the ~1100…1400-nm spectral range at ~1.0…1.1μm pumping, i.e. at the operation wavelengths of well-developed high-power Ytterbium fiber lasers. However, the nature of BACs responsible for the broadband NIR emission in BDFs, including the ones with alumino-silicate core-glass, is still disputed; moreover, attributing of such-type BACs is controversial. Apparently, absence of clarity of the nature of BACs formed in alumino-silicate fibers and misunderstanding of the mechanisms defining their functionality at pumping into different absorption bands of BACs limit further progress in the area.
A separate point of interest regarding BDFs is impact of heating above room temperature (25°C, further – RT) and pump wavelength’s variation upon their absorptive and fluorescent properties; see e.g. Refs [1,4,9–15]. To-date, different kinds of the effect of temperature on basic parameters of BDFs with different core compositions and light sources built on their base have been reported. It also deserves mentioning that BACs NIR fluorescence lifetime, measured in BDFs having different core-glasses and different doping levels, varies from ~0.5 μs to ~1.0 μs at RT [1]. Furthermore, NIR fluorescence of BACs formed in alumino-silicate fibers drops at increasing temperature, the effect known as temperature quenching.
On the other hand, spectral transformations with temperature’s variation, adherent to the presence of BACs in alumino-silicate fibers, were poorly addressed in the literature on BDFs. As known to authors, the effect of temperature in such-type BDFs was under scope in very few publications only [9–12,14–16], whilst the results there reported are somewhat contradictory. In virtue of this, the main motivation of the current work was to highlight details of high-temperature-related phenomena in BDFs of such type, viz. in yttria-aluminosilicate (YAS) BDFs, for shading more light on the matter. Note that YAS-BDFs is a new material, invented recently [17], although Bi-doped YAS bulk glass was reported earlier [18].
Hereafter, we report the data of an experimental study of thermally-induced changes occurring in the resonant-absorption and NIR fluorescence bands of Bi-Al related BACs at low-power 750-nm excitation in YAS-based BDFs, moderately (see below) doped with Bi, at temperatures ranged from RT to 700°C. These fibers were drawn using a standard drawing tower from nano-engineered YAS-based preforms, obtained through the Modified Chemical Vapor Deposition (MCVD) method employed in conjunction with the Solution-Doping (SD) technique. A comprehensive analysis of the raw preforms and final fibers is provided in [17].

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Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 488

As follows, we investigate absorption and fluorescence spectra’ transformations in these BDFs as well as changes in other basic characteristics of the fibers (fluorescence and resonant-absorption saturation, fluorescence lifetime, pump backscattering at 750-nm excitation, etc.), mainly caused by the presence in core-glass of Bi related specie, in function of temperature.
The data reported below were collected at varying temperature between RT and 700°C. It deserves mentioning that, as was checked by means of numerous repetitions of heating/cooling and annealing cycles applied to BDF samples, a trend of partial losing mechanical rigidness (unavoidable at treating any silica fiber at high temperature) becomes detectable in our fibers at temperatures exceeding 680…700°C. Thus, a technical limit of our experiments was chosen to be 700°C.
The results reported in this study reveal a dramatic effect of elevating temperature upon enhancing “state-of-the-art” of the YAS-based BDFs, termed here as significant rise of resonant absorption and NIR fluorescence in the characteristic bands of Bi-Al related BACs during and posterior to high-temperature treatment of the fibers. Possible mechanisms, underlying this – unusual on a first glance – phenomenon, are briefly discussed. It is worth mentioning that, as shown below, in YAS-BDFs annealed at high (over 500…550°C) temperature the changes in the absorption/fluorescence spectra have a non-reversible character, although such microscopic parameters of post-annealed fibers as NIR fluorescence lifetime and fluorescence saturation power are preserved almost unchanged. This fact deserves attention as a valuable property of high-temperature treated BDFs of such or similar [15] type, permitting enhancement of laser/amplifying potential of the fibers. On the contrary, at lower temperatures – ranged from RT to 500…550°C – all found modifications in the BDFs’ basic properties are virtually reversible (with no hysteretic behavior observed), allowing, as shown in [16], their use for effective sensing temperature.
We handled in experiments a couple of YAS-based BDFs, both having a relatively high but differing by few-times Bi-doping level, which permits drawing some aspects of the effect of Bi content upon temperature-related phenomena.
2. Fibers characterization and experimental techniques employed
2.1. Fabricating route and basic properties of YAS based BDFs
In experiments were tested the YAS-glass based fibers, labeled hereafter Bi-1 and Bi-2, moderately doped with Bi2O3 (~0.6 and ~1.0% wt., respectively) and co-doped with Al2O3 (~8.0 wt. %), Y2O3 (~2.5 wt. %), and P2O3 (in small amount, ~0.2 wt. %). Note that doping levels given here were measured at the preform stage [17] but could be subject to deviations after drawing the fibers (certainly, this applies to Bi content as Bi is known as a very volatile species). Our choice to handle BDFs doped with Bi and Al in a relatively high degree (providing a high concentration of Bi-Al related BACs), was defined by necessity to use short fiber pieces in experiments. In turn, the latter is a requirement of homogeneous heating of fiber samples, ensuring reliability and reproducibility of the collected data.
Doping the fibers with Al2O3 solved a few tasks simultaneously: apart from being a prerequisite for creating Bi-Al related BACs, Al permits engineering of the fiber’s numerical aperture (NA) without adding such precursors as Germanium (Ge) and enhances chemical durability of the core-glass, which is important for applications. Adding Y2O3 into the coreglass targets facilitating of the radiative transitions between the electronic levels of BACs (phonon energy of Y2O3 is one of the lowest cutoffs among oxides [18]) and, eventually, enhances the fluorescent ability of BACs (note that, as compared to [18], doping our fibers with Y2O3 was much lower). Adding a small amount of P2O5 into the YAS glass aims its softening and accessing phase-separation of nano-sized areas, enriched with Bi [17].
At fabricating Bi doped preforms, un-sintered SiO2-P2O5 soot layers were deposited at 1500 ± 10°C and then soaked in aqueous solution, comprising Bi(NO3)3, Al(NO3)3, HNO3,

#251792 © 2016 OSA

Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 489

and Y(NO3)3, for 45 min. This was done using a specially designed SD setup, permitting control over the solution’s flow rate at dipping and draining-out stages. The function of P2O5 was to achieve nucleation for increasing phase separation besides generation of Bi-rich nanoparticles in the core area during porous layers’ deposition, while that of HNO3 was to stop the formation of colloidal BiONO3. Adding Bi into YAS core-glass was fulfilled through SD, followed by suitable thermal treatment. Subsequent to the SD step, dehydration and oxidation of porous layers were made at 900…1000°C. Sintering was conducted gradually by increasing the MCVD burner’s temperature from 1500 to 2000°C, followed by collapsing of the tubes. Final fibers were drawn from the preforms using a standard drawing tower.
The major part of the data, presented below (in subsections 3.1 and 3.2), relate to Bi-1 fiber, lower than Bi-2 doped with Bi, since the temperature-induced effects were revealed to be more spectacular in Bi-1 fiber. Meanwhile, we provide (in subsection 3.3) a brief resume of the results obtained with Bi-2 fiber for comparison and, on the other hand, for emphasizing a general character of the temperature-related phenomena in YAS-BDFs.
The three resonant-absorption bands detected in Bi-1 fiber, labelled hereafter I (~750 nm), II (~1000 nm), and III (~500 nm), refer to spectrum 1 in Fig. 1, are the characteristic signatures of BACs, formed in alumino-silicate glasses (YAS glass certainly belongs to this type of glass since Y, at low doping, has the role of a glass modifier). When pumped at 1.06 µm, the fiber demonstrates broadband NIR fluorescence (centered at ~1.15 µm and spanned over ~250…300 nm) and, also, up-conversion emissions (UC) [7], peaked at ~630 and ~750 nm; see spectrum 2 in Fig. 1. The other peaks, at ~1450 and ~860 nm, and the shoulder at the left side of the ~700-nm band of BACs (at ~660 nm) do not relate to Bi doping but, rather (see Ref [17].), stem from the 1st, 2nd and 3rd cutoffs of the fiber, labelled in Fig. 1 as λC1, λC2, and λC3, correspondingly. NA of Bi-1 fiber was measured (at the preform stage) to be ~0.12; so, the fiber demonstrated essentially multimode properties in the spectral range under scope. Note that the ~700-nm and (especially) ~500-nm absorption bands appear on the background that gets elevated towards shorter wavelengths (the well-known fact for Bi-Al related BACs). 1.06-µm pump power, saturating absorption in band II, was estimated to be ~2.5…3 mW.

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Fig. 1. Spectra of absorption (curve 1) and fluorescence at 1.06 µm pumping (curve 2) of Bi-1 fiber, measured at RT; in both cases short pieces (2…5 cm) of the fiber were under tests. The cutoff features, shown by the vertical red arrows, are addressed in the text.
Bi-2 fiber possesses of very similar to Bi-1 properties, e.g., it has comparable waveguiding characteristics (say, λC1~1.5 µm) and almost equal absorption and fluorescence saturation powers. On the other hand, it demonstrates more intensive resonant-absorption bands I to III and stronger NIR fluorescence at both 1.06-µm [17] and 750-nm pumping. But, in overall, the absorption/fluorescence spectra are nearly identical for Bi-1 (see Fig. 1) and Bi-2 fibers. More information about these two BDFs can be found in Ref [17]. where they are referred to as Bi-B and Bi-A, respectively.

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Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 490

2.2. Experimental equipment and techniques
In all experiments described below lengths of Bi-1 and Bi-2 samples were varied between 1 and 10 cm. The experimental arrangements and equipment are specified as follows.
The experimental setup employed at measurements of the fibers’ absorption and NIR fluorescence is sketched in Fig. 2(a). BDF samples were illuminated by a white-light (WL) source (Yokogawa AQ4305), at measuring transmission and absorption (after re-calculating measured transmission into absorption, in dB/m) spectra, or were pumped by a 750-nm LED with single-mode fiber output (Exalos), at measuring fluorescence spectra. The LED had a rather broad emission spectrum (see Fig. 2(c)) and output power, tunable up to 4.5 mW. It is seen that the LED’s operation wavelength matches the right slope of absorption band I of BACs (see Fig. 1). The LED’s output was connected to a delivering SMF-28 fiber, spliced with a Bi-doped fiber, the latter placed into an electric oven; its output was spliced with another piece of SMF-28 fiber, connected to either a Ge photo-detector (Newport, model 2033, 200-kHz bandwidth), at fluorescence lifetime measurements, or an optical spectrum analyzer (OSA) (Ando AQ-6315A), at spectral measurements.
The BDFs’ optical transmission spectra were measured employing the cutback method at turning OSA to 2…5-nm resolution. The spectra were recorded before (at RT), at each step of heating and cooling (i.e. at a temperature increase/decrease within the interval RT…700°C), and after completing thermal treatment (normally, after 24 h., which ensured a sample’s coreglass relaxation), again at RT. In some of the figures below, the difference spectra – in terms of induced absorption (IA) – are provided, which were obtained after subtraction of the original attenuation spectra of a pristine BDF sample (taken prior to passing it through a heating/cooling cycle) from the ones obtained during or after treating at certain temperature. This allowed a straightforward view on the “net” spectral absorption changes, or IA, within the absorption bands I, II, and III, belonging to Bi-Al related BACs.
At fluorescence lifetime measurements, a NIR fluorescence signal was captured after fast switching pump-light off; the LED’s power before pump’s blocking was fixed at the maximum (~4.5 mW). To diminish pump background in the measured signal, a long-pass optical filter with cut-on wavelength at ~1000 nm (Thorlabs, model FEL1000) was placed between a BDF sample and photo-detector. The resolution of the measurements was ~1.5 μs.

Fig. 2. Experimental arrangements employed at measurements of (a) absorption and fluorescence, including lifetime, and (b) back-fluorescence and backscattering (crosses indicate splices); (c) normalized LED’s emission spectrum; (d) multiplexer transmission spectra between the port spliced with SMF-28 fiber (refer to 2(b)) and the port used as output 1 (curve 1) and the port used for connecting with LED (curve 2).
When studying backscattering and backward fluorescence at increasing/decreasing temperature, a fused 50:50 fiber multiplexer for 750 nm was spliced between the LED and first piece of SMF-28 fiber, see Fig. 2(b). In Fig. 2(d), we demonstrate the multiplexer’s spectral response in VIS-NIR. Output 1 of the multiplexer was connected with OSA and its unused output was inserted into a cuvette with oil, refractive index of which matches the one of silica (to diminish back-reflection of pump light). The same action and with the same

#251792 © 2016 OSA

Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 491

purpose was done in these experiments with output of the first SMF-28 fiber. This permitted high-accuracy detection of changes versus temperature in backscattering and backward fluorescence in situ.
Regarding the fluorescence measurements, we limited ourselves by collecting the data obtained at exciting BDFs at ~750 nm (the data obtained at moderate-power 1060-nm pumping will be reported elsewhere). Regarding the absorption measurements, we paid most of attention to the changes arising at high temperatures in BACs resonant-absorption bands I and II (these two are most suitable to get lasing), because the ones arising in band III are more complex and need a more comprehensive study.
3. Experimental results and discussion
3.1. Changes in absorptive properties of Bi-1 fiber during and posterior to heating
Here, basic results obtained from the measurements of absorption spectra of Bi-1 fiber during and after treatment at temperatures, ranged from RT to 700°C, are reported.
In main frames of Figs. 3 and 4, we demonstrate, for a 6-cm Bi-1 sample, the temperature dependences of IA in BACs bands I, II, and III (viz. in their maxima) during (Fig. 3) and posterior to (Fig. 4) heating/cooling, whilst the overall spectral responses to thermal treatments are exemplified, in either case, in the figures’ insets.
Hereafter we denote as current temperature T the one, measured during a sample’s heating (or cooling). In this case, the term IA (T) means a current value of additional absorption established in the sample during the process, at T. In turn, we designate hereafter as annealing temperature T* the maximal one, up to which a sample was heated. In this case, the term IA (T*) applies to additional absorption measured at RT after completing a cycle of heating the sample to T*, followed by 1…2-h. annealing, posterior spontaneous cooling down to RT, and thermal relaxation during 24 h.

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100 200 300 400 500 600 700

Current temperature T , oC

Fig. 3. Dependences of IA establishing in Bi-1 fiber on current temperature T at heating from RT to 700°C, measured in resonant-absorption bands I, II, and III (represented in main-frame by three blue curves, labelled accordingly). Inset demonstrates the changes in absorption spectra of Bi-1 fiber during heating, “snapshotted” at T = 405°C (grey curve 1), 625°C (green curve 2), and 700°C (magenta curve 3). Lengths of Bi-1 fiber samples were 6 cm. The dashed and dotted-dashed lines guide to the eye slopes of the dependences as per the presence of the two stages (see text).
It is seen from Fig. 3 that attenuation in all bands monotonously grows with temperature and that two well-definable stages in the process can be segregated: see blue curves in the main frame of Fig. 3. At the first stage (from RT up to 500…550°C), absorption growth in the bands is steep and moderate, whereas at the second one (starting when T overcomes 550°C and continues up to our limit, 700°C), IA rises with a much bigger slope versus T. An IAincrease at both stages seem to be uncommon, because, as it would be expected, at higher

#251792 © 2016 OSA

Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 492

temperatures peak absorption ought to decrease (which happens, as we checked using the same experimental arrangement, at heating Erbium doped fibers). Thus, an opposite IA-trend, observed in resonant-absorption bands I, II, and III of Bi-1 fiber at elevating T, is remarkable. Furthermore, the presence of two distinct stages in the process deserves emphasizing as they are probably “switched” by different mechanisms, relating to some transformations happening with BACs and/or their surrounding in the core-glass.
The IA-spectra “snapshotted” at heating Bi-1 sample up to different T, 425°C (curve 1), 605°C (curve 2), and 700°C (curve 3), demonstrate certain deviations from the absorption spectrum of pristine Bi-1 fiber, measured prior to heating (Fig. 1). Some of them are seemingly “imperfections”, arising due to the effect of cutoffs (λC2 and λC3) upon the differential spectra appearance, but others, for instance, a dip within band III, marked as “!” (also reported in [11]), seem to be the features relating to complex nature of BACs formed in YAS-BDFs.
Figure 4 demonstrates another unusual effect observed in Bi-1 fiber, never recorded in earlier studies with BDFs. We plot in this figure the temperature dependences of IA in bands I, II, and III (relative to attenuations in these bands in pristine state of the fiber, i.e. before heating), obtained after turning the fiber to completed cycles of heating to certain temperature T* and posterior cooling to RT.

Induced absorption (IA), dB/m Absorption, dB/m

70 AFTER HEATING, COOLING, AND RELAXATION

(MEASURED AT RT )

60

90

5 after heating to 700 oC

III T* 80

I 50

70

4

after heating to 625 oC

60

40

50

40

after heating to 550 oC

30

30

3

20

2

10

20

0

1

after II heating to 500 oC

550 oC
I III

Bi-1 II 600 700 800 900 1000 1100

10

Wavelength, nm

0

0

100 200 300 400 500 600 700

Annealing temperature T * , oC

Fig. 4. Dependences of IA established in Bi-1 fiber at RT, as the result of completing cycles of heating/annealing/cooling to RT, on annealing temperature T*. As in Fig. 3, in main-frame are shown the data obtained in peaks of resonant-absorption bands I, II, and III (represented by three red curves, labelled accordingly). Inset demonstrates the changes in the resultant (after annealing) absorption spectra of Bi-1 fiber, “snapshotted” at T* = 500°C (olive curve 1), 550°C (blue curve 2), 625°C (green curve 3), and 700°C (magenta curve 4) (measured at RT). Black curve 5 in the inset presents, for comparison, the attenuation spectrum of pristine Bi-1 fiber at RT. Lengths of Bi-1 fiber samples were 6 cm. The dashed and dotted-dashed lines guide to the eye slopes of the dependences as per the presence of the two stages (see text).
On one hand, the result of heating Bi-1 to temperatures not exceeding 500…550°C, no IA is observed to rest after subsequent annealing. That is, the moderate changes in absorption in bands I, II, and III, arising during the first stage of heating up to 550°C (refer to Fig. 3), are reversible. On the other hand, if Bi-1 fiber was heated in excess of 550°C, IA in bands I, II, and III (after cooling and thermal relaxation), strongly increases with T*. IA reaches, at maximal T* = 700°C, ~70, ~50, and ~5 dB/m in bands I, III, and II, correspondingly (see the red curves in Fig. 4), i.e. extra-attenuation in these bands becomes comparable with attenuation, measured in pristine state of the fiber. That is, heating of Bi-1 fiber to temperatures higher than 550°C and posterior cooling to RT is an irreversible process, resulted in growth of resonant absorption in the BACs bands twice.
The “frozen” (i.e. taken at RT) IA-spectra, taken after applying whole cycles of heating/cooling, are exemplified in inset to Fig. 4, for a few annealing temperatures: T* =

#251792 © 2016 OSA

Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 493

500°C (curve 1), 550°C (curve 2), 625°C (curve 3), and 700°C (curve 4). It is seen that annealing at T* = 500°C does not led to any detectable IA in the fiber’s post-annealed state in neither band, whereas at heating to T* exceeding 550°C the effect of modifying the state-ofthe-art of the annealed fiber, in terms of resonant absorption, becomes dramatic. It is also seen from inset to Fig. 4 that rise of attenuation, say, in band I (curve 4) after heating Bi-1 to T* = 700°C is comparable with the initial attenuation in this band (see curve 5, showing the absorption spectrum of pristine Bi-1 fiber).
The attenuation spectra of a 2-cm Bi-1 sample, obtained for the interval 400…1400 nm and measured at 25°C before thermal treatment (curve 1) and after annealing at T* = 700°C (curve 2), are demonstrated in Fig. 5(a). Here, again (as in inset to Fig. 4) magenta curve 2 presents the result after applying a complete cycle of the fiber’s heating/cooling. The spectra were recorded using a very short Bi-1 sample to enable capturing changes in attenuation (IA) in all resonant-absorption bands, including the one centered at ~500 nm (III), characterized by maximal extinction. The vertical grey arrows in the figure guide to the eye a general trend of increasing attenuation in bands I, II, and III, whereas the vertical red ones designate spectral positions of the cutoff peaks (where, expectedly, almost no changes in attenuation are found as the result of high-temperature treatment).

Attenuation, dB/m

2 III λc3 I
100
1
11 dB/m
10

λc2 II

Bi-1 λc1

1.5 dB/m

(a)

(b)

1

400 600 800 1000 1200 1400
Wavelength, nm

Fig. 5. (a) Dependences of Bi-1 fiber’s attenuation, taken in its pristine (before thermal treatment, blue curve 1) and posterior to annealing at T* = 700°C (i.e. after completing a cycle of heating it to 700°C, followed by annealing, cooling to RT, and thermal relaxing), on annealing temperature T*. The BACs-related resonant-absorption bands I to III are labelled near the curves. Length of Bi-1 fiber sample tested was 2 cm. The specifications of the features represented by the vertical black / red arrows and by the dashed lines are addressed in the text. (b) Cross-sectional views of Bi-1 fiber (length, 10 cm) cuts, obtained at WL illuminations before (left photo) and after (right photo) completing the whole of thermal treatment: refer to spectra 1 and 2 in main-frame of the figure.
Spectra 1 and 2, when directly compared, allow noticing that the background loss is reduced as the result of high-temperature (in this case, at T* = 700°C) annealing of Bi-1 fiber. The dashed lines in the figure guide to the eye the feature: it is seen that in NIR the background loss is reduced by ~1.5 dB/m whereas in VIS, say, at ~400 nm, the loss reduction is much bigger, reaching ~11 dB/m.
In Fig. 5(b), we provide the cross-sectional images of Bi-1 fiber, obtained using a microscopy tool of the Vytran equipment under WL illumination before (left side) and after (right side) high-temperature (T* = 700°C) treatment. In both photos, are seen inner circles, being “replicas” of the over-cladding procedure (made at the fiber’s fabrication stage). It is also seen that the fiber’s glass takes, after annealing (see the right-side image), some “fogging”, mainly produced by WL guidance in core. We can hypothesize that this effect is produced by strengthening of the core-glass inhomogeneity after annealing, probably because of partial crystallization in areas enriched with Bi.
To get more information about the changes in the absorptive properties of Bi-1 fiber during and after heating to a high temperature, a comparative analysis of bands I and II was

#251792 © 2016 OSA

Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 494

made; see Fig. 6. We exemplify in this figure IA, established in these two bands, when Bi-1 fiber (6-cm in length) was heated to 500°C (a), 550°C (b), 625°C (c), and 700°C (d) (each of the figures was obtained at handling a pristine fiber sample for heating). The blue-color spectra in Fig. 6 were obtained at current (during heating) temperature T in the oven, whereas the red-color ones – at RT but after completing the cycles of heating/cooling at T* = 500°C, 550°C, 625°C, and 700°C. The black and grey arrows in the panels guide to the eye the spectral widths of bands I and II and the asterisks designate the spectral positions of the artifacts, located in proximity to cutoff λC2 (refer to Figs. 1 and 5(a)).

Induced absorption, dB/m Induced absorption, dB/m Induced absorption, dB/m Induced absorption, dB/m

70 Bi-1
60 50

T = 500oC T* = 500oC

40

30

* 20
10

I

0

-10

II (a)

600 700 800 900 1000

Wavelength, nm

70 Bi-1
60

50 40

I

T = 625oC T* = 625oC

30

* 20

10

II

0

-10

(c)

600 700 800 900 1000

Wavelength, nm

70 Bi-1
60 50

T = 550oC T* = 550oC

40

30
I 20
10
* 0
-10

II (b)

600 700 800 900 1000

Wavelength, nm

70 Bi-1 I
60 50

T = 700oC T* = 700oC

40

30

* 20
10

II

0

-10

(d)

600 700 800 900 1000

Wavelength, nm

Fig. 6. Examples of IA-spectra measured in the spectral interval, matching resonant-absorption bands I and II, in Bi-1 fibers, obtained at current temperatures at heating (T) and after applying cycles of heating/annealing/cooling, viz. at annealing temperatures T*. The blue and red spectra correspond to different T- and T*-values, exemplified by 500°C (a), 550°C (b), 625°C (c), and 700°C (d). Lengths of Bi-1 fiber samples were 6 cm. The black and grey arrows show IA-widths in bands I and II.
A few deserving attention observations can be made from Fig. 6. The first is that in the annealed state of fiber, after passing it through complete cycles of heating/cooling, IA in bands I and II (if presents, which only happens at T*>500°C: compare the red spectrum in panel (a) with the red ones in panels (b), (c), and (d)) is substantially bigger than during heating. The second is that, during heating fiber to lower temperatures (exemplified in Fig. 6 by blue spectra, taken at T = 500°C (a) and 550°C (b)), IA-bands appear on background, or offset, having negative value (with the negativity trend gaining towards shorter wavelengths). This also applies – as our other data show – to lower than 500°C temperatures. The third detail is that in the fiber’s heated state IA-bands are always wider than in their post-annealed state: compare the blue and red spectra in all panels of Fig. 6.
The last fact is not surprising, stemming from a common property of resonant-absorption lines widening at elevated temperature; however, the first two ones need a separate consideration. Concerning the second observation, we may propose that the negativity of the offset that the blue spectra in Fig. 6 obey (at lower T) originates from reduction of scattering loss with increasing temperature (a hypothesis in attempt to address this effect is given in subsection 3.2). Regarding the first observation, we think that rise of extinction in the resonant-absorption bands after annealing fiber at temperature higher than T* = 550°C (which is accompanied by rise of NIR fluorescence power of BACs; see subsection 3.2) has another reason. Probably, according to the literature data and to the Raman analysis made in subsection 4.1, its cause is “generating” of extra-BACs (refer again to Figs. 3 and 4) as the result of phase separation/core-glass transformation near/above the transition and

#251792 © 2016 OSA

Received 12 Oct 2015; revised 27 Nov 2015; accepted 18 Dec 2015; published 15 Jan 2016 1 Feb 2016 | Vol. 6, No. 2 | DOI:10.1364/OME.6.000486 | OPTICAL MATERIALS EXPRESS 495
FiberFigHeatingAbsorptionTemperature