280 J. Opt. Soc. Am. B/Vol. 20, No. 2 /February 2003 Dvoyrin et al.

Effective room-temperature luminescence inannealed chromium-doped silicate optical fibers

Vladislav V. Dvoyrin, Valery M. Mashinsky, Vladimir B. Neustruev, and Evgeny M. Dianov

Fiber Optics Research Center, General Physics Institute, Russian Academy of Sciences, 38 Vavilov Street,119991 Moscow, Russia

Alexei N. Guryanov and Andrei A. Umnikov

Institute of Chemistry of High-Purity Substances, Russian Academy of Sciences, 49 Tropinin Street, 603600 NizhniiNovgorod, Russia

Received March 21, 2002; revised manuscript received July 29, 2002

The influence of annealing at 1000 °C on the absorption and luminescence spectra of chromium in alumino-silicate and gallium silicate fibers at temperatures of 77 and 300 K is investigated. The spectra of initiallyweak luminescence with a maximum at 1100 nm and a quantum yield of approximately 1025 at 300 K shift tothe 790–880-nm range in annealed fibers. After annealing the quantum yield increases by as much as 10%.The luminescence lifetime is found to be approximately 20 ms. The luminescence changes are explained by arearrangement of the nearest environment and recharge of chromium ions (Cr41 to Cr31). The results ob-tained hold promise for the creation of tunable and ultrashort pulse fiber lasers. © 2003 Optical Society ofAmerica

OCIS codes: 300.6280, 060.2320, 060.0060.

1. INTRODUCTIONCreation of broadband tunable light sources in thenear-IR spectral range (especially in the vicinity of 1.3mm), which would be compatible with the standard com-munication optical fibers, is a topical problem of modernfiber optics. Semiconductor and Raman devices as wellas Pr31-doped fibers have not yet solved this problem. Inthis connection, interest in chromium-doped silicateglasses has recently accelerated.1–8 According to the lit-erature, the Cr41 ion in crystals and glasses of variouscomposition has a broad absorption band in the visibleand near-IR regions and a broad luminescence band thatcovers the optical communication windows (1.3–1.55mm).1–5,9–14 The Cr31 ion also has a broad absorptionband in the visible region and a luminescence band in thenear-IR region that stretches to 1.2 mm.6–8,11–14 Thus,chromium-doped silicate glasses can be candidate materi-als for the creation of tunable lasers, amplifiers, and ul-trashort pulse lasers with broadband pumping. How-ever, certain technological restrictions are imposed on themanufacture of optical glass fibers with suitable chro-mium luminescent properties. As far as we know, therehave been only two papers devoted to the manufacture ofaluminosilicate core optical fibers with Cr41 ions and tothe observation of feeble luminescence in such fibers at 77K.1,2 Based on previous research the first observation ofCr41 luminescence with a quantum yield of approxi-mately 1025 at room temperature in aluminosilicate andgallium silicate fibers was reported.5

Here we study the influence of high-temperature an-nealing of fibers with an aluminosilicate and gallium sili-cate core, which contains a magnesium additive as a

0740-3224/2003/020280-04$15.00 ©

charge compensator, on the degree of the oxidation stateand luminescent properties of chromium ions.

2. SAMPLES FABRICATION ANDPROCESSINGThe fiber preforms were made by modified chemical-vapordeposition method. Aluminum was incorporated into thecore glass from the gas phase. The other elements, Mg,Cr, and Ga (instead of aluminum), were incorporated bythe solution technique. The concentrations of the dop-ants are given in Table 1. Concentration C of Al2O3 inthe core was determined from the preform refractive-index profile Dn(r) by use of the ratio Dn/C ' 23 1023 (mol. %)21.15 The content of Ga, Mg, and Crwas specified in gram ions of the respective metal per 1liter of solution (g-i/l). Few-mode fibers (Dn5 0.008–0.018, core diameter of approximately 15 mm)were drawn from the preforms. Annealing of the fiberswas carried out at 1000 °C for 3–8 h in an electric furnacein air.

3. EXPERIMENTALThe optical loss and luminescence spectra and lumines-cence lifetimes in the initial and annealed fibers weremeasured at temperatures of 77 and 300 K (the spectra ofthe annealed fibers were measured only at room tempera-ture). The total loss spectra in the fibers were measuredby the cutback technique. We estimated the scatteringloss by comparing the light intensity scattered from a fi-ber through its lateral surface and the output light inten-

2003 Optical Society of America

Dvoyrin et al. Vol. 20, No. 2 /February 2003 /J. Opt. Soc. Am. B 281

sity in the 476–676-nm spectral range and by taking intoaccount the geometric factor. We measured the lumines-cence spectra using excitation with Kr1 laser (l 5 476-,520-, 568-, 647-, and 676-nm) radiation. We correctedthe spectra by taking into account the setup of the spec-tral sensitivity function.

Because of a high loss in Cr-doped fibers and for thesake of convenience, we spliced (by use of an arc dis-charge) a length of the fiber tested (from 15 to 40 cm) andtwo pieces of a low-loss germanosilicate fiber. For mea-surements at 77 K the length of the tested fiber was im-mersed into a vessel filled with liquid nitrogen. The lu-minescence lifetime was measured under excitation by cwKr1 electro-optically modulated laser radiation at 647nm. The time resolution of the setup was approximately4 ms. The luminescence quantum yield was determinedas the ratio of luminescence quanta number to absorbedquanta number. The laser excitation light was measuredat the output end of a fiber by a calorimeter. On the ba-sis of this measurement and using an absorption spec-trum we calculated the power launched into the fiber andits absorbed part. The luminescence power was mea-sured by the integration of its spectrum, having takeninto account (1) the capture of the luminescence by the fi-ber aperture, (2) the reabsorption of the luminescencealong the fiber length, (3) the transmission of the mono-chromator, and (4) the absolute calibration of a germa-nium photodiode. The error of such luminescence quan-tum yield determination is estimated as approximately50%, caused mainly by the error in determination of theeffective aperture of luminescence radiation captured bythe fibers and by the error in calibration of the photodi-ode. The relative (among the fibers) accuracy is esti-mated to be ;20%.

4. RESULTSA. Initial SamplesThe spectra of optical loss in initial fibers FF-3, FF-4, andFF-6 are shown in Figs. 1, 2, and 3, respectively. Thespectrum of the FF-1 fiber with the highest Cr content isquite similar to FF-4 fiber (Fig. 2), the maximum absorp-tion at 780 nm reaching 110 dB/m. The scattering loss inall the samples did not exceed 0.1 dB/m; therefore, ab-sorption was the main loss factor.

All the spectra of the initial fibers have a complexstructure and are qualitatively similar. The compositeabsorption band with a maximum at approximately 800nm can be assigned to Cr41 ions in a tetrahedralenvironment.1–4,9–14,16 The absorption at wavelengths

Table 1. Concentration of Aluminum Oxidein Core Glass and Ion Concentration

of Ga, Mg, Cr in a Salt Solution

SampleAl2O3

(mol. %)Ga

(g-i/l)Mg

(g-i/l)Cr

(g-i/l)

FF-1 8 — — 0.098FF-3 — 13.5 — 0.078FF-4 7.5 — 0.81 0.068FF-6 6.9 — 10 0.02

shorter than 370 nm is apparently caused by Cr61 ions.16

In the spectra of initial FF-3, FF-4 absorption, possiblycaused by Cr31 ions, is appreciable at wavelengthsshorter than 690 nm.7,9,16 Decreasing the temperature to77 K leads to only a small shift of the bands to shorterwavelengths (as shown in Fig. 2).

In the initial fibers (except for FF-6), luminescence ofCr41 ions was observed at room temperature under laserexcitation in the 568–676-nm wavelength range. The lu-minescence is presented by a broadband (peak at 1100nm, FWHM of approximately 300 nm) that is assigned toa 3T2—3A2 transition of Cr41 ions in a tetrahedralenvironment.1,3,4,9–14 At T 5 77 K, the luminescence in-tensity grows by more than 1 order of magnitude withoutan essential change in spectrum shape. Under excitationat 470- and 520-nm wavelengths the luminescence wasnot detected. The luminescence quantum yield that didnot depend (within experimental accuracy) on the excita-tion wavelength is given in Table 2. The luminescencespectra of the initial fibers were published elsewhere.5

B. Annealed SamplesThe absorption spectra of annealed fibers FF-3 and FF-4are also shown in Figs. 1 and 2. The absorption spec-

Fig. 1. Absorption spectra of initial and annealed FF-3 fiber(SiO2 –Ga2O3 –Cr).

Fig. 2. Absorption spectra of initial and annealed FF-4 fiber(SiO2 –Al2O3 –MgO–Cr) measured at temperatures of 300 and 77K.

282 J. Opt. Soc. Am. B/Vol. 20, No. 2 /February 2003 Dvoyrin et al.

trum of annealed fiber FF-1 was qualitatively similar toFF-4. It is seen that the annealing process changes theabsorption spectra considerably. However, in alumino-silicate fiber with Mg (FF-6) the absorption changes afterannealing were undetectable within experimental accu-racy. The annealing-induced changes in the spectra ofFF-1, FF-3, and FF-4 correspond to a decrease of the Cr61

and Cr41 ions concentration and an increase of the con-centration of Cr31 ions in the octahedral environment asis manifested by the absorption bands of Cr31 ions at 430and 640 nm. A preliminary estimation has shown that inthe FF-3 gallium silicate fiber the absorption of Cr41 de-creased after annealing by as much as three or four timesand in the FF-4 aluminosilicate fiber by as much as two tothree times. The Cr41 ion in FF-6 fiber with a ratherhigh Mg concentration appeared the most stable.

After annealing, all the fibers except FF-6 had inten-sive luminescence at room temperature with the spectrashown in Fig. 4. The maximum position for FF-1 is atapproximately 880 nm, for FF-4, at 850 nm, and for FF-3,at 790 nm. The emission bands have a width of approxi-mately 200 nm and a long-wavelength tail up to 1300 nm.The band shape does not really depend on the excitationwavelength. The quantum yield increased strongly and,at 647-nm excitation, achieved 10% (see Table 2).

The luminescence lifetime in the annealed fibers isgiven in Table 3. Because of a relatively long system re-sponse connected with a low luminescence intensity un-der cw laser excitation we do not draw a conclusion aboutthe character of luminescence decay during the first 10ms. But in the 10–100-ms time range the decay curves

Fig. 3. Absorption spectra of initial FF-6 fiber(SiO2 –Al2O3 –MgO–Cr).

Table 2. Luminescence Quantum Yield in InitialSamples and After Annealinga

SampleInitial

T 5 300 KInitial

T 5 77 KAnnealed

T 5 300 K

FF-1 3 3 1025 2 3 1024 0.015FF-3 2 3 1025 3 3 1024 0.03FF-4 3 3 1025 5 3 1024 0.10

a Excitation at 647 (nm).

were well fitted by the exponential functions with timeconstants from Table 3. A detailed discussion of lumines-cence lifetime demands more precise measurements.However, we do note that luminescence lifetime increaseswhen the temperature is lowered and when the peakwavelength of luminescence decreases. We did not mea-sure the luminescence lifetimes in the initial fibers be-cause of the low luminescence intensity.

Thus, combining the features of luminescence in an-nealed fibers allows us to conclude that luminescence iscaused basically by Cr31 ions.6–8,11–14

5. DISCUSSIONTo explain the influence of high-temperature annealingon absorption and luminescence spectra, we suggest thefollowing hypothesis.

In solid-state oxides of aluminum, gallium, and chro-mium, cations are incorporated more preferably in the 13oxidation state (trivalent) in an octahedral environmentof the nearest oxygen atoms. However, in the silicateglass network that contains aluminum (or gallium) (espe-cially with a low concentration), these atoms can replaceSi atoms and hence be stabilized in the tetrahedral envi-ronment. As this takes place, these atoms form nega-tively charged complexes @AlO4/2#2 (or @GaO4/2#2).16–18

The Cr ions in the second coordination sphere of an alu-minum (or gallium) ion can work as charge compensatorsand contribute to stabilization of aluminum (or gallium)in the tetrahedral site. Simultaneously, this charge com-pensation also favors stabilization of Cr41 in the meta-stable tetrahedral site.1,2,16 Such a situation seems to berealized during the fiber drawing process with fast glassquenching.

Fig. 4. Luminescence spectra of annealed fibers FF-1, FF-3, andFF-4 (lexc 5 647 nm). The peak at 1294 nm is due to the excit-ing light in the second order of the monochromator transmission.

Table 3. Luminescence Lifetime tin Annealed Fibers

Samplet, ms

T 5 300 Kt, ms

T 5 77 K

FF-1 19 21FF-3 26 28FF-4 18 23.5

Dvoyrin et al. Vol. 20, No. 2 /February 2003 /J. Opt. Soc. Am. B 283

In the annealing process, relaxation and switching ofstressed chemical bonds seem to occur (in the short-orderrange of the glass network). On a larger scale, such pro-cesses as liquation and crystallization are probable,3

which apparently results in rearrangement of the nearestenvironment of Al, Ga, and Cr atoms. Thus, aluminumand gallium acquire a more favorable octahedral environ-ment, and the compensating charge returns to chromiumions that decrease its oxidation state to 13 and alsochange its nearest environment to the octahedral one.This hypothesis is confirmed by the considerable decreaseof absorption of Cr41 ions, partial annealing of short-wavelength absorption by Cr61 ions, growth of absorptionbands in the regions of 430 and 650 nm (belonging toCr31), and the occurrence of intense luminescence. Be-sides, as a result of structural relaxation, the parametersof the crystalline field of the chromium ion are also likelyto change, which will result in a shift of its energy statesand transition probabilities.

A large concentration of more effective charge compen-sators (for example, Mg) result in Cr41 ions that enter theglass network in such a way that silicon and magnesiumatoms are in the second coordination sphere. Such clus-ters could be more stable and not subject to essential re-arrangement at 1000 °C. Such behavior of Cr41 ions isobserved in the spectra of FF-6.

6. CONCLUSIONAbsorption and luminescence properties of modifiedchemical-vapor deposition aluminosilicate and galliumsilicate fibers doped with gallium, chromium, and magne-sium by the solution technique were studied. Initial (as-made) fibers show weak room-temperature luminescenceof Cr41 ions.

After the fibers were annealed at 1000 °C, an intenseluminescence arose, with a lifetime in the 18–26-msrange, peaks in the 790–880-nm range, and a width of ap-proximately 200 nm with a long-wavelength tail thatstretched to 1.3 mm. The luminescence is caused mainlyby Cr31 ions that form as a result of structural rearrange-ment of the nearest environment of Cr41 ions. The lumi-nescence quantum yield of Cr31 ions reaches ;10%.

The wideband and sufficiently high quantum yield ofthe luminescence at room temperature opens possibilitiesfor the creation of broadband fiber lasers and amplifiersin the near-IR (700–1050-nm) range on the basis ofchromium-containing silica glass.

ACKNOWLEDGMENTSThe authors thank M. V. Yashkov and N. N. Vechkanov(Institute of Chemistry of High-Purity Substances, Rus-sian Academy of Sciences) for their help in making thepreforms and fibers.

The research was supported in part by the RussianFoundation for Basic Research, grants 00-15-96650 and02-02-16501.

V. V. Dvoyrin’s e-mail address is vlad@fo.gpi.ru.

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