Sol-gel-based doped granulated silica for the rapid production of optical fibers

April 2 2014 922 WSPCGuidelines-IJMPB S0217979214420107

International Journal of Modern Physics BVol 28 No 12 (2014) 1442010 (20 pages)ccopy World Scientific Publishing Company

DOI 101142S0217979214420107

Sol-gel-based doped granulated silica for the

rapid production of optical fibers

Valerio Romanolowast and Soenke Pilz

Applied Fiber Technology ALPS

Bern University of Applied SciencesPestalozzistrasse 20

Burgdorf CH3400 Switzerlandlowastvalerioromanobfhch

Dereje Etissa

Fiber and Fiber Lasers Institute of Applied Physics University of Bern

Sidlerstrasse 5 Bern CH3012 Switzerland

derejeetissaiapunibech

Received 8 December 2013Accepted 16 December 2013Published 11 March 2014

In the recent past we have studied the granulated silica method as a versatile and costeffective way of fiber preform production We have used the sol-gel technology combinedwith a laser-assisted remelting step to produce high homogeneity rare earth or transitionmetal-activated microsized particles for the fiber core For the fiber cladding pure orindex-raised granulated silica has been employed Silica glass tubes appropriately filledwith these granular materials are then drawn to fibers eventually after an optionalquality enhancing vitrification step The process offers a high degree of compositionalflexibility with respect to dopants it further facilitates to achieve high concentrationseven in cases when several dopants are used and allows for the implementation of fibermicrostructures By this ldquorapid preform productionrdquo technique that is also ideally suitedfor the preparation of microstructured optical fibers several fibers have been producedand three of them will be presented here

Keywords Optical fibers preform large mode area active fibers

PACS numbers 4255Wd 4255Xi 4270-a 4281Bm 4281Cn 4281Dp 4265Ky

1 Introduction

New fiber concepts such as photonic crystal fibers or large core active fibers have

increased the importance of special fiber production1ndash5 and new applications can

benefit from the existence of diversification in production technology

In the recent past we have studied two versatile cost effective and in some effects

new ways of fiber preform production

1442010-1

V Romano S Pilz amp D Etissa

One is the sol-gel method that allows to produce doped silica optical materials

of high purity at temperatures well below the 2000C required by pure silica when

processed with standard methods6ndash9 the other one is the granulated silica method

that allows to assemble the preforms in a simple form with tubes and doped or pure

sand1011

It has been shown that the manufacturing of fiber preforms can be done at a

high quality level also with the very simple sol-gel technology If one can afford

the intrinsically high OH content of the products derived with this technology one

can benefit from other aspects such as homogeneity of the dopant distribution Fur-

thermore with its versatility the sol-gel method complementarily satisfies very well

some of the needs of these new fiber concepts mainly in the case where the addi-

tion of dopants eg rare earths is desired The sol-gel process starts from liquid

metal alkoxide precursors at room temperature and enables to reach high dopant

concentrations with considerable less effort than traditional glass production meth-

ods In addition the process offers a high degree of compositional flexibility with

respect to dopants as well as matrix components it further facilitates to achieve

high homogeneity mdash down to the molecular level mdash even in cases when several

dopants at the precursor level are involved

However if one uses sol-gel simply to coat the interior of the preform tubes

indeed one does exploit the compositional freedom offered by the method but there

is no advantage with respect to structural flexibility as required by new microstruc-

tured fiber concepts

This further flexibility is offered by the other alternative preform production

technology that we have been studying namely the granulated silica method This

method starts from coarse grains of pure silica mixed with dopant powder and co-

dopant powder in general dopants are rare earth oxides and co-dopants aluminum

or germanium oxide This method offers unprecedented flexibility with respect to

the geometry of the fibers Even more it can be regarded as a ldquofiber rapid proto-

typingrdquo technique as it can be used to draw fibers without vitrification step directly

from silica tubes appropriately filled with coarse granulated silica (gt 100 microm grains)

and the corresponding dopants and co-dopants as fine powders This versatility has

its price depending on the dopant levels losses in the range of 1ndash5 dBm at 632 nm

are obtained These losses are due to microbubbles and glass inhomogeneities as a

result of incomplete diffusion This leads to scattering in the drawn fibers How-

ever the flexibility of the method is so attractive that several process variants

and additional techniques have been developed12ndash17 among others to obtain more

homogeneous glass

One way is to use fine silica powder and add the dopants as liquids This method

leads to homogeneously doped glasses and one of its variants is known under the

name ldquoRepusil (TM)rdquo18 However one of the advantages of using coarse grains

namely the possibility of easy and efficient ldquoin processrdquo evacuation from the top of

the preform is lost

1442010-2

Sol-gel-based doped granulated silica

A second efficient way is the homogenization by means of stack-and-draw

technique19 This method yields scattering losses as low as 01 dBm at 1100 nm

wavelength

The technique that we are studying allows us to draw the fibers from coarse

grains and still obtain homogeneous glass in the fiber method (a) is to use itera-

tive meltingremilling on the granulated silicadopants mix (b) is to produce first

a base material already homogeneously doped by the needed dopants by sol-gel

technique and then undergo the same iterative melting remilling procedure as in

method (a)

At the end of both procedures granulated silica with the desired coarse granu-

lometry in the range of above some 100 microm is obtained where already the sin-

gle grains are homogeneously doped The main advantage of the sol-gel-based

method is given by the low temperature (in general room temperature) at which the

dopants are mixed into the later glass material This can be beneficial for the con-

trolled addition of dopants with low evaporation temperatures eg phosphorous

Furthermore it has been reported that silica powder made by the sol-gel method

has much lower impurities compared with natural quartz powder which is commonly

used for silica glass melting20

In its simplest implementation for standard fibers a silica glass tube forming the

future core region of the fiber preform is filled with the mix of variant (a) or with

the doped granuli of variant (b) This tube is mounted in the center of a larger tube

forming the future cladding The empty space between the two tubes is filled with

SiO2 powder After evacuating and preheating at temperatures close to 1000C in

the drawing tower the preform is drawn to a fiber The method is most attractive

if one wants to implement more complicated fiber geometries including multicore

or microstructured fibers Furthermore preforms with large cores where layer by

layer MCVD deposition can be the most time consuming part of the process can

be built much faster with the production time virtually not depending on the size

of the fiber core

Admittedly if no measures to reduce the content of contaminants OH-groups

etc are taken especially at high RE concentrations above 1 at the background

losses in such fibers can be high (of the order of 1 dBm) This undesired effect may

also be increased by unwanted recrystallization taking place in one of the processing

steps Although these losses might not be of main concern in special applications

like lasers and amplifiers where only short pieces of fiber in the order of some

meters are needed we are attempting to reduce them as much as possible at least

in the core of the fibers Our strategy hereto is to mill and remelt the part of the

materials that will build the core

Applications that can immediately benefit from the granulated silica method are

either those where one wants to incorporate many different dopants into the core of

the fiber or applications where one wants to obtain a fiber with a microstructure

that cannot be obtained in a straight forward way by other techniques

1442010-3

Also a good control of the index step between core and cladding is typical for

the granulated silica method this makes the method interesting for large mode

area active fibers where a small index contrast is needed This can be achieved

by simultaneously mixing aluminum and phosphorous into the core and in this

way partly cancelling their index raising effect on one side and taking profit of the

beneficial effect of phosphorous with respect to photodarkening on the other side

In this article we will

(i) introduce the granulated silica method in both variants (variant a is the one

when using a mixture of single powders containing the oxides and variant b when

using coarse powder doped already at the single grain level) and

(ii) present three examples of fibers obtained by one of the variants of the granulated

silica method namely a single core broadband fiber a multicore fiber with the single

cores doped with different rare earths and an ytterbium Yb3+-doped and Al2O3-

P2O5-co-doped large core fiber

2 The Granulated Silica Method

In its simplest way for the production of standard fibers the granulated silica method

is implemented in the following way (Fig 1) a silica glass tube forming the future

core region of the fiber preform is filled with a mixture of silica powder of coarse

granulometry (gt 100 microm) and fine dopant powder (lt 10 microm grain diameter) This

tube is mounted in the center of a larger tube forming the future cladding The

empty space between the two tubes is filled with undoped SiO2 powder After

evacuating and preheating at temperatures close to 1000C in the drawing tower

the preform is drawn to a fiber as described in Ref 14

The coarseness of the powder is necessary to ensure that evacuation from the

top of the preform can be efficiently done during preheating and drawing

This method allows to draw fiber prototypes in a very short time even if

nonstandard sizes and shapes are desired It also allows the intermixing of pre-

Fig 1 Arrangement of the granulated silica method

1442010-4

Fig 2 Left multicore fiber preform right fiber with three neodymium-doped cores

Fig 3 Cutback measurement result for a Nd3+ (1 at) Al3+(10 at) silica glass fiber Theresulting extinction length of 58 m 1e corresponds to losses of 075 dBm633 nm (Ref 14)

fabricated doped or undoped rods serving as regions with different index or

dopants

In such a way one can implement complicated fiber geometries including multi-

core or microstructured fibers (Fig 2)

Furthermore preforms with large cores where layer by layer MCVD deposition

can be the most time consuming part of the process can be built much faster with

the production time virtually not depending on the size of the fiber core

However fibers fabricated as described above suffer from significant higher

scattering losses when compared to fibers produced with the conventional MCVD

technique Typical values for losses are in the range between 075 dBm and 5 dBm

at 633 nm (Fig 3) for ldquoall granulated silica variantsrdquo if no other measures are taken

Although the granulated-silica method targets optical fibers for applications

where only a few meters of optical fiber are needed eg highly doped active fibers

for high power fiber lasers amplifiers and sensing applications losses above 1 dBm

in most cases are too high Certainly the variant with prefabricated rods for the

ldquosensitiverdquo regions (eg the core) brings a significant improvement in scattering

losses but then much of the freedom in composition and structure are lost

1442010-5

Fig 4 Left Multi-mode fiber produced without melting and milling (strongly over-exposed)Right Multi-mode fiber produced with melting and milling applied twice The white lines indicatethe position of the fiber The green fluorescence is from the 4S32 rarr 4I153 transition of Er3+

and is excited by energy transfer upconversion (ETU) pumping with a diode laser operated at975 nm ETU in this case can occur only if groups of ions cluster and get close enough Thisindicates either a very high concentration of Er3+ ions or clustering at low concentrations (as isthe case here)

Furthermore in this most simple implementation the mixing of dopants into

silica is based on diffusion processes and especially at high doping levels (gt 01

at) incomplete diffusion can produce inhomogeneous glass and lead to scattering

21 Homogeneization by iterative CO2 laser remelting and milling

To homogenize the material the granulated silica and dopants mixture is iteratively

milled and remelted with a CO2 laser with the procedure described in Ref 17 After

this CO2 laser processing step optically clear pieces of remolten doped silica are

obtained Furthermore eventual residual impurities are burnt away The remolten

pieces are then milled to a coarse powder and used for the fiber core production

The effect of iterative remelting and milling is shown in Fig 4

22 Homogenization by sol-gel production of granulated silica

The sol-gel technology is a valid complement for the production of fiber preforms

It is attractive for several reasons (i) the materials can be produced at low tem-

peratures (ii) since the process starts from the liquid phase at room temperature

the dopants (eg RE nitrides or chlorides as well as aluminum and phosphorus)

are homogeneously dissolved into the precursors This allows for high dopant con-

centrations (up to several at)

The doped liquids can then be used in several ways to produce fiber preforms

one way is to coat the inner walls of silica tubes collapse them and subsequently

draw the fibers in a standard way

This route has been explored by several authors and leads to active fibers with

acceptable background losses As an example in Ref 21 a sol-gel derived Yb-doped

and Al-codoped fiber with a slope efficiency of 64 and tunable emission from

1033ndash1108 nm is described Its background losses at 1100 nm are only 31 dBkm

1442010-6

Fig 5 Production of the doped granulated SiO2 by direct mixing of oxides (left route) or sol-gelmethod (right route)

Considering that with the reported Yb-concentration of 1 at only some meters

of active fiber are needed for laser operation this value has to be considered as

very good

Another route the one interesting in this context is to produce homogeneously

doped bulk material

23 Combination of sol-gel method with the granulated

silica method

In the context of the granulated silica method the sol-gel route is adopted to prepare

doped bulk material that can be used as a starting point for the granulated silica

preparation In this case only one meltingmilling step is necessary as the material

is homogeneously doped However the material is porous and CO2-Laser melting

gives clear glass pellets These are then milled to coarse granulated silica that can

be used for the rapid variant of the fiber drawing method (Fig 5)

An active fiber drawn with core material produced by using sol-gel derived

coarse granulated silica and only one meltingmilling iteration to obtain a coarse

powder lowered the fiber losses to 035 dBm at 633 nm wavelength22

3 Example Fibers

The active fibers that we present here to illustrate the granulated silica method

very well subsume the potential of dopants compositional freedom and structural

1442010-7

flexibility of the granulated silica method we discuss two broadband emission fibers

and an Yb3+-doped fiber that can be used as laser fiber One of the two broadband

fibers shown has multiple cores

31 Broadband fibers

Broadband light sources have become indispensable for a multitude of applications

among them are spectroscopy microscopy sensing or medical diagnosis23ndash25 Many

of the applications rely on the very short coherence length which is a consequence

of the broad spectral distribution and which may be as short as a few microns

Usually employed broadband light sources are thermal light sources light emit-

ting diodes super luminescence diodes amplified spontaneous emission and super-

fluorescent fiber sources femtosecond oscillators or white light sources based on

nonlinear continuum generation Other sources such as very long Raman fiber

lasers have been investigated but are not as widespread While most light sources

have bandwidths of less than 100 nm some are as broad as a couple of hundred

nm Because of their superior beam quality and high spatial coherence fiber based

sources most prominently super-fluorescent rare earth doped or highly nonlinear

fibers are often preferred to other sources A further important characteristic is

the output power of a light source Generally the broadest bandwidth but also the

lowest output power is reached with spontaneous emission Amplified spontaneous

emission has a higher power but shows some narrowing of the spectra depending on

the degree of amplification Finally the highest output power is reached with laser

emission but in continuous wave operation this comes at the cost of a considerably

reduced bandwidth Nevertheless even in the case of cw laser activity laser emis-

sion can cover a range of 50 nm in the case of a Nd3+ Al3+ glass fiber or 75 nm for

a Yb3+ Al3+ glass fiber The broadest bandwidths and the highest output powers

however are undoubtedly reached with standalone mode-locked oscillators or with

subsequent continuum generation but at the expense of high costs

311 Single core broadband fiber doped with rare earths and Bismuth

We report on a fiber realized by the granulated silica method where we targeted the

region above 1000 nm for emission26 The envisaged use of the fiber is in the field

of broadband amplification To that end the rare earths erbium and neodymium as

well as the transition element bismuth were chosen as active dopants Aluminum

was added to enhance the solubility of the rare earths

This fiber was fabricated using the technique of dry granulated oxides5 and was

produced at the IAP (Institute of Applied Physics University of Bern) The doped

core-area of the fiber is composed of a mixture of pure granulated silica (SiO2) rare

earth oxides (Er2O3 Nd2O3) and metal oxides (Bi2O3 Al2O3) Bismuth- erbium-

and neodymium-oxide were chosen to optimize the generation of fluorescence in the

spectral range between 1000 and 1700 nm27ndash32

1442010-8

The composition of the core-mixture consisted of 9857 at of SiO2 13 at

of Al2O3 01 at of Bi2O3 002 at Er2O3 and 001 at Nd2O3

Aluminum is used to increase the refractive index of the core compared to the

cladding and to improve the solubility of the dopant material28 Another benefit of

Aluminum is that it prevents the rare earth ions from clustering

Co-doping with 13 at leads to a refractive index step (∆n) smaller than

00046 and to a numerical aperture NAcore smaller than 0115 This core-mixture

powder was melted and vitrified with the aid of a CO2-laser After the vitrification

the mixture was roughly milled The procedure of melting vitrifying and milling was

repeated three times all in all to increase the homogeneity The resulting vitrified

material mixture was filled into a silica tube with an inner diameter of 17 mm

and an outer diameter of 21 mm This preform was drawn to a fiber-rod with a

diameter of approximately 24 mm This fiber-rod was placed and centered in a

second 17 mm by 21 mm silica tube and became the fiber-core of the active fiber

The remaining space of the second preform was filled up with undoped gran-

ulated silica (nSiO2 = 145) Together with the walls of the first and second silica

tubes the undoped granulated silica became the cladding This preform was drawn

to a fiber with 125 microm cladding diameter and asymp 255 microm core diameter (Fig 6)

The fiber was coated with a low refractive index acrylate (ncoat = 1389) to

achieve also a waveguide structure for the cladding which results in a double-clad

fiber (DCF) The total fiber diameter including the coating was asymp 400 microm and the

numerical aperture of the cladding given by its refractive index and the index of

Fig 6 (a) Geometry and refractive indices of the active double-clad fiber structure (b) micro-

scope image of the fiber end when injecting a white light source (c) 800 nm pump light and(d) fluorescence distribution of the fiber by imaging the fiber end onto a CCD

1442010-9

the coating material resulted to be 041 a good value for pumping with diodes of

large numerical aperture

Numerical aperture of the core

A transversal refractive index scan of an uncoated fiber piece is shown in Fig 7

The refractive index difference between the core and the cladding is ∆nB asymp 00046

for fiber B leading to a numerical aperture of 012

Spectral emission and output power

First we excited a 996 m long active fiber piece with 376 W at 800 nm Figure 8

shows the measured spectral shape from 1000 up to 1700 nm In the spectrum four

main peaks will identified The peak at 1060 nm can be chalked up to the transition4F32 rarr

4I112 of the Nd3+6 The Nd3+ is also responsible for the peak at 1333 nm

the associated transition is 4F32 rarr4I132 Er

3+ produces the peak at 1531 nm

corresponding to the 4I132 rarr4I152 transition Bi3+ is responsible for the peak

around 1100 nm The emitted power PoutIR in the spectral range above 1000 nm

was measured to 65925 microW when pumped with 376 W

At 976 nm pump wavelength there is no absorption from neodymium whereas

the absorption cross-section of Er3+ is stronger at 976 nm than at 800 nm Bi3+

also absorbs at 976 nm32

If we compare both spectra generated by 800 nm and 976 nm pump diodes we

observe that the emission from Er3+ is stronger for pumping at 976 nm (Fig 8)

This is in good agreement with the smaller absorption cross-section for 800 nm

excitation

Both Nd3+ peaks vanish for 976 nm pumping due to the lack of absorption at

976 nm The Bi3+ peak is broader for 976 nm excitation compared with 800 nm

Fig 7 Refractive index profile of the broadband fiber the difference between core and claddingis sim 00046

1442010-10

Fig 8 (Color online) Spectral emission of 996 m of fiber pumped at 800 nm (blue curve) andat 976 nm (green curve)

The photoluminescence power both from the core and the cladding PoutIR was

about 14 mW when pumped with 343 W at 976 nm

32 Broadband emission from a multi-core multi-dopant fiber

This fiber described in more detail in Ref 33 demonstrates nicely the ease with

which different dopants and a multicore structure can be combined This multi-

core multi-dopant fiber when pumped with a single pump source around 800 nm

emits a more than one octave-spanning fluorescence spectrum ranging from 925 nm

to 2300 nm The fiber preform is manufactured from granulated oxides and the

individual cores are doped with five different rare earths ie Nd3+ Yb3+ Er3+

Ho3+ and Tm3+ The same preform is appropriately drawn to obtain two different

sizes of the cores (16 microm resp 5 microm)

321 Fabrication procedure

The geometry of the design is shown in Fig 9(a) Seven differently doped cores are

arranged in honeycomb geometry with six cores surrounding the central core With

the goal to cover an emission band ranging from about 900 nm to over 2 microm five

different trivalent rare earth ions have been chosen ie Nd3+ Ho3+ Er3+ Tm3+

and Yb3+ They are well suited because all can be excited with a single pump

wavelength around 800 nm27

Although Yb3+ has its maximum absorption at 977 nm the transition is so

broad that even at 800 nm the absorption is sufficiently high The emission spectra

cover the range from 925 nm (Nd3+4F52 rarr4I112) to 2100 nm (Ho3+5I7 rarr

With the Nd3+ Er3+ and Tm3+ absorption cross-sections from Refs 34 and 35

concentrations of 0018 at 0342 at and 0235 at were determined and used

For Ho3+ and Yb3+ a concentration of 1 at was chosen In order to facilitate

easy identification of the differently doped cores two are doped with Nd3+

1442010-11

Fig 9 (a) Arrangement of the seven differently doped cores Images of the rear fiber end with(b) all cores pumped (c) the two Nd3+-doped cores pumped and (d) a single Nd3+-doped corepumped

The granulated silica preform from which the fibers were drawn were produced

as follows in a first step the preform is assembled by closely stacking seven silica

tubes of 3 mm by 5 mm diameter in the center of a bigger 17 mm by 21 mm silica

tube and by filling the remaining space with undoped granulated silica of typically

400 microm grain size as described in detail in Ref 36 Each inner tube is filled with

a mixture of granulated silica the appropriate concentration of rare earth oxide

and aluminium oxide Aluminium prevents the rare earth ions from clustering and

raises the index of refraction to facilitate guiding of light The aluminium oxide

concentration corresponds to 7 at of Al3+ with respect to silicon The preform

is preheated at approximately 1400C evacuated for two hours and then drawn

at a temperature of about 1850C to a fiber with a diameter of 124 mm In a

second step this fiber is then packed in the center of a larger silica tube (17 mm

by 21 mm) and the remaining space is again filled with undoped granulated silica

After preheating and evacuation the final preform is drawn to a fiber with diameters

ranging from 145 mm to 051 mm corresponding to core diameters of 16 microm to

5 microm respectively Thus all cores are multi-mode except for the smallest diameter

of 5 microm where the limit for single-mode operation for all wavelengths of interest

is reached

Fiber emission

In the first experiment a 21 cm long fiber with 16 microm large core is selected and

the seven cores are either pumped simultaneously or individually The pump light

1442010-12

Fig 10 Spectra of individually pumped 16 microm large cores (a) Yb3+ (b) Nd3+ (c) Ho3+ and(d) Tm3+ When pairs of cores are pumped simultaneously mixed spectra are observed (e) Nd3+

and Er3+ and (f) Nd3+ and Yb3+

around 800 nm stems from an argon ion laser pumped Ti sapphire system with a

maximum pump power of 400 mW

The emission from the different cores is assigned to Yb3+ (2F52 rarr2F72)

Nd3+(4F32 rarr4 I112) Er

3+ (4I112 rarr4I152) Ho

3+ (5I5 rarr5I8 or 5I6 rarr

and Tm3+ (3H5 rarr3H6 or 3H4 rarr

3H6) respectively In Fig 9(c) only the two

Nd3+-doped cores are pumped by aiming the pump light at the upper half of the

core area Finally Fig 9(d) demonstrates the controlled excitation of only a single

core (Nd3+) by focusing and aiming the pump light properly

The measured spectra of individually as well as simultaneously pumped cores are

shown in Fig 10 When only the Yb3+-doped core is pumped [Fig 10(a)] the well-

known fluorescence spectrum of Yb3+ is observed with the prominent sharp feature

of the 2F52 rarr 2F72 transition at 977 nm The spectrum of the Nd3+-doped core

[Fig 2(b)] shows the 4F32 rarr 4I112 transition at 1060 nm and the fluorescence

around 930 nm stems from the 4F32 rarr 4I92 transition The emission spectra of

the Ho3+-doped core with the 5I7 rarr 5I8 transition centered at 2113 nm and of

the Tm3+-doped core with the 3F4 rarr 3H6 transition around 18 microm are shown

1442010-13

Fig 11 Fluorescence spectrum emitted from a fiber with 5 microm large cores The spectral inten-sities are as measured that is without correcting for efficiencies etc The line at 1954 nm labeledwith ldquordquo is assigned to a second-order peak of the 977 nm Yb3+ emission

in Figs 10(c) and 10(d) The spectra depicted in Figs 2(e) and 2(f) are measured

with two cores being simultaneously excited Figure 10(e) shows the Er3+4I132rarr4I152 transition centered at 1532 nm and the Nd3+4F32 rarr

4I112 transition The

combined excitation of the Nd3+- and the Yb3+-doped core yields the spectrum

in Fig 2(f) Thus each core excited separately shows the expected fluorescence

proving that the ions are properly ionized and well embedded in the glass matrix

In other words fabricating cores from granulated oxides seems feasible at least for

all the rare earth ions used in this experiment

In the second experiment the fiber with 5 microm large cores is used The smaller core

size allows for homogeneous pumping of all cores without changing the optical setup

Figure 11 shows the spectrum for a pump wavelength of 804 nm The spectrum

indicates contributions from all five rare earths

Note each core emits its spectrum independently and largely undisturbed from

the others The width of the resulting spectrum corresponds to one octave plus a

major third and nearly completely covers the wavelength range from 925 nm to

about 2300 nm Only below 1300 nm there is a gap between the emission of Nd3+

and Tm3+ The spectral gap may be filled with eg Bi3+ which shows fluores-

cence at 1250 nm when pumped at 800 nm30 The relative spectral distribution

can be considerably modified if the pump wavelength is changed While a longer

wavelength eg about 820 nm favors the Yb3+ fluorescence and reduces the Nd3+

fluorescence a shorter wavelength eg 786 nm strongly favors Tm3+ and restrains

the fluorescence from the other ions That is the overall spectral distribution can

be manipulated within certain limits by a judicious choice of the relative rare earth

ion concentrations and the pump wavelength The efficiency of the broadband light

1442010-14

source is mostly limited by the small solid angle that overlaps with the numerical

aperture of the fiber and which is roughly in between 01 and 1

33 Yb3+-doped granulated silica fiber with sol-gel derived

powder core

The granulated silica method is very attractive for the production of large mode

area fibers one can achieve precise small index contrasts between core and cladding

simultaneously with large cores

Fiber material

The active fiber core material composed of SiO2 P2O5 Al2O3 and Yb2O3 is pre-

pared by the sol-gel method from tetraethyl orthosilicate (TEOS Si(OC2H5)4)

trimethyl phosphate (TMP (CH3)3PO4) aluminium nitrate (Al(NO3)3 middot 9H2O)

and hydrated Yb3+ chloride (YbCl3 middot 6H2O 9999 from S Aldrich) Ethanol and

de-ionized water were used as solvents and hydrochloric acid (HCl) as catalyst The

glass matrix concentration of the above input precursor materials were in the molar

percentage ratio of SiO2 Al (NO3)3 middot 9H2O TMP YbCl3 middot 6H2O = 947 30 20

03 This results in a phosphorus to aluminium Al ratio of 06 which increases the

refractive index of the doped fiber core compared to the undoped cladding as well

as suppressing clustering28 In addition to the optical active ion of the Yb3+ the

also contributes to the refractive index contrast of the core and cladding which is

essential for the guidance of light based on total internal reflection

Production of doped granulated core material by the sol-gel method

The solution was prepared in separate stages First TEOS was dissolved in ethanol

and thoroughly stirred at room temperature until clear solution was formed

Second TMP Al(NO3)3 middot 9H2O and YbCl3 middot 6H2O were dissolved in ethanol and

then added to the solution containing TEOS This mixture was then stirred at

450 rpm In the beginning the mixture of the two solutions is a milky solution

which turns into a clear stable solution due to the stirring In the next step 18

mole of di-ionized water (D minus IH2O) was added Subsequently 01 mole of HCl

was added to keep the doped silica sol at a pH value lower than 6 The sol was

stored in a loosely closed glass container and heated to an elevated temperature of

70Cndash150C The gelation occurred after 36 h resulting in a xerogel-material This

material was then slowly dried in an oven by increasing the temperature at a rate

of 2Cmin until a temperature of 300C In the next step the material was heated

to a temperature of 1200C in order to completely burn out any organic residues

Subsequently the material was sintered at a temperature of 1500Cndash1600C for 3 h

To achieve a more uniform distribution of the matrix elements in the host glass

the mixture was filled in a graphite box and exposed to a CO2-laser beam of 600 W

(beam diameter of ca 2 cm) for about 25 s With this procedure we produced so

1442010-15

called pellets with a diameter of ca 2 cm To get a uniform vitrification of the

pellets the back side was also irradiated in a similar way The produced pellets

were then milled with a planetary small ball milling machine (with three 14 cm

and three 09 cm zirconium balls) for 5 min Iterative melting and milling was

applied (at least three times) until the produced pellets appeared homogenous and

transparent Finally the pellets were milled to coarse grained powder and sieved

to a size of 100ndash200 microm The size of the powder grains is important parameter

for the evacuation process of the preform during the fiber drawing process The

concentration ratio of the active core fiber preform is 3 mol Al2O3 2 mol

P2O5 and 03 mol Yb2O3

Sintering Milling and CO2-Laser melting to improve glass quality

Xerogel which is produced from the standard sol-gel process is very porous and

is therefore extremely prone to adsorb water and impurities which in turn results

in high background losses Some improvement can be obtained by a temperature

treatment at up to 1500Cndash1600C This leads to a consolidation and to a reduced

material porosity After this stage the sintered doped xerogel is milled and melted

by a CO2-Laser The pellets obtained with this method are very transparent and

suitable for fiber drawing

Preform assembling and fiber drawing

The fabricated active powder material was filled into a silica tube with closed

bottom with a outer diameter of 12 mm and an inner diameter of 4 mm The size

of the dopant grains for active core material is about 100ndash200 microm The preform was

evacuated to a pressure of 10minus3 mbar In the next step the preform was preheated

to a temperature about 1000C for 30 min in order to outgas the powder followed

by a preheating stage at 1500C for another 30 min Finally the oven temperature

Fig 12 Microscope image of the cleaved fiber end fabricated by sol-gel method

1442010-16

Fig 13 XRD patterns of Yb-doped and P-Al-co-doped silica glass prepared by sol-gel and gran-ulated silica methods (a) sol-gel powder was sintered at the temperature of 1600C for 3 h and(b) sintered sol-gel powder after drawn to optical fiber and (c) granulated oxides powder repeatedlymelted by CO2-laser and milled and drawn to optical fiber

was raised to about 1900C to draw a fiber from the preform The preform was

drawn to a fiber with a diameter of about 250 microm (as shown in Fig 12)

Crystallinity test of core material before and after fiber drawing

Since the preform was thermally treated we investigated whether noticeable crystal

formation occurred during the cooling-down process To detect the exact crystalline

phase formation during the heat treatment X-ray diffraction (XRD) measurements

were preformed (Fig 13) As shown in Fig 13(a) material prepared by the sol-gel

method and heated at 1600C for 3 h shows several crystalline peaks After drawing

the peaks disappear [Fig 13(b)] showing that the material is fully amorphous as

shown by the broad band around 23C The vitrification occurs probably because

of the fast cooling rate when the fiber leaves the furnace in the drawing process

Refractive Index Profile of sol-gel derived granulated silica fiber and element

distribution

From the measured refractive index profile (Fig 14) the average index difference

with respect to the undoped silica was ∆n = 53 middot10minus3 and the numerical aperture

(NA) of this fiber was calculated to be 012 The standard deviation of the variation

of the profile was 4 middot 10minus4 and no central dip is visible

In Fig 15 the distribution of Al P and Yb in the fiber core detected by electron

microprobe analysis is shown Fiber core materials were randomly taken for analysis

The doping level of the fiber core is almost comparable with the starting precursors

The variation of Al and P ions along the fiber core was observed with the standard

deviation of 012 and 009 One can see that Yb ions are uniformly distributed

having a standard deviation of 001 Though there is a slight variation of average

doping level in the central core region the fiber consists of the original composition

1442010-17

Fig 14 Refractive index profile

Fig 15 Average elements distribution along the active fiber core measured by electron probemicroanalysis (EPMA)

of the dopant incorporated in the sol-gel preparation This shows that the material

prepared using sol-gel method is stable to exposure at high temperatures

Attenuation measurement of the fiber core using cut-back method

To measure the scattering loss of this Yb3+ doped aluminophosphosilicate active

optical fiber we used a HendashNe laser (6328 nm) to avoid absorption in the range

of 800ndash1100 nm After injecting light into the core we observed regions with barely

visible scattering separated by periodically strong scattering centers 10 cm to 1 m

apart Several cut-back measurements revealed scattering losses of 035 dBm in

fiber pieces between the strong scatterers

1442010-18

4 Conclusions

We have shown some of the work done in our laboratories at the Institute of

Applied Physics of the University of Bern in collaboration with the Applied Fiber

Technology Laboratory of the Bern University of Applied Sciences on the

improvement of the granulated silica technique as a powerful method for the

rapid production of fiber preforms and fibers The examples shown namely

the broadband emission fiber and the multicore fiber clearly reveal the potential

of the method with respect to compositional and structural flexibility for special

fibers The Yd3+-doped LMA fiber with piecewise 035 dBm losses at 633 nm

(corresponding to about 01 dBm at 1100 nm if Rayleigh scattering is assumed)

shows that the method will be useful also for fiber laser applications The method

is to be regarded as very powerful for the production of active photonic crystal

fibers and leakage channel fibers as it allows to easily match the refractive indices

of differently doped fiber core and cladding

Actual work is being devoted to further reduce the scattering losses to below

02 dBm at 633 nm

Acknowledgments

We are very thankful to Thomas Feurer Willy Luthy Loredana Dilabio Ruth

Renner Bettina Wilhelm Martin Locher Martin Neff Manuel Ryser for active

help in drawing and discussions This work has been carried on during the last

10 years and has involved many technicians and students of the IAP whom we

also thank

It has been partly financially supported by the Swiss Commission for the

Encouragement of Scientific Research CTI under project 78642 and the Swiss

National Science Foundation under grant 200020-1216631

References

1 J C Knight Nature 424 847 (2003)2 T A Birks J C Knight and P St J Russell Opt Lett 22 961 (1997)3 P St J Russell J Lightwave Technol 24(12) 4729 (2006)4 F Poli A Cucinotta and S Selleri Photonic Crystal Fibers Properties and Applica-

tions Springer Series in Materials Science Vol 102 ISBN 1-40206-325-3 (2007)5 A Bjarklev J Broeng and A S Bjarklev Photonic Crystal Fibers (Kluwer Academic

Publishers Boston Dordrecht London 2003) ISBN1-4020-7610-X6 J MacChesney Cer Trans 95 65 (1999)7 F Wu et al J Mater Res 9(10) 2703 (1994)8 D Michel et al Tunability of a Nd3+ Al3+ Sol-Gel Glass Fiber Laser in EPS-QEOD

Europhoton Conference on Solid-state and Fiber Coherent Light Sources LausanneSwitzerland August 20ndashSeptember 3 Europhysics Conference Abstracts 28C Fib-10055 (2004)

9 M Locher V Romano and H P Weber Rare-earth doped waveguides produced bythe sol-gel method Opt Lasers Eng 43 341 (2005)

1442010-19

10 C Pedrido Method for fabricating an optical fiber preform for fabricating an opticalfiber optical fiber and apparatus Patent Nr WO 2005102946 A1

11 C Pedrido Optical fiber and its preform as well as method and apparatus for fabri-cating them Patent Nr WO 2005102947 A1

12 J Ballato and E Snitzer Appl Opt 34(30) 6848 (1995)13 M Neff V Romano and W Luethy Opt Mater 31 247 (2008)14 R Renner-Erny L Di Labio and W Luthy Opt Mater 29(8) 919 (2008)15 M Leich et al Opt Lett 36(9) 1557 (2011)16 A Langner et al Comparison of silica-based materials and fibers in side- and end-

pumped fiber lasers Proc SPIE 7195 71950Q-1 (2009)17 B Wilhelm V Romano and H P Weber J Non-Cryst Solids 328(1) 192 (2003)18 A Langner et al Proc SPIE 6873 687311 (2008)19 Velmiskin and V Vladimir et al Active material for fiber core made by powder-

in-tube method subsequent homogenization by means of stack-and-draw techniquein Proc SPIE Vol 8426 (2012) doi10111712922188

20 M Tomozawa D-L Kim and V Lou J Non Cryst Solids 296 102 (2001)21 U Pedrazza V Romano and W Luthy Opt Mater 29(7) 905 (2007)22 D Etissa et al Rare earth doped optical fiber fabrication by standard and sol-gel

derived granulated oxides in Proc SPIE Vol 8426 (2012) doi1011171292262923 M Jacquemet and N Picque et al Opt Lett 32(11) 1387 (2007)24 T R Corle and G S Kino Confocal Scanning Optical Microscopy and Related Imag-

ing Systems (Academic Press US 1996)25 S Martin-Lopez M Gonzalez-Herraez and A Carrasco-Sanz et al Meas Sci Tech-

nol 17(5) 1014 (2006)26 S Pilz et al Infrared broadband source from 1000 nm to 1700 nm based on an

erbium neodymium and bismuth doped double clad fiber ALT Proceedings Vol 1(2012) doi1012684alt173

27 A A Kaminskii Laser Crystals Their Physics and Properties 1st edn (SpringerUS 1981)

28 M J Digonnet Rare-earth-doped fiber lasers and amplifiers 2nd edn (CRC PressUS 2001)

29 M Peng et al Opt Lett 30(18) 2433 (2005)30 T Suzuki and Y Ohishi Appl Phys Lett 88(19) 191912 (2006)31 M Neff Metal and transition metal doped fibers PhD thesis University of Bern

(2010)32 E M Dianov and V V Dvoyrin et al Quantum Electron 35 1083 (2005)33 L Di Labio et al Broadband emission from a multicore fiber fabricated with gran-

ulated oxides Appl Opt 47(10) 1581 (2008)34 P Tosin W Luthy and H P Weber Determination of the spectral absorption in

silica samples with known rare earth dopant concentration in Proc 9th CIMTEC-

World Ceramics Congress and Forum of New Materials (Florenz Italy June 14ndash19 1998)

35 S Zemon et al IEEE Photon Technol Lett 3(7) 621 (1991)36 L Di Labio et al Novel technology to fabricate mixed multi-core fiber lasers in

standard and air-clad configuration in 2nd EPS-QEOD Europhoton Conference (PisaItaly 10ndash15 September 2006) Vol 30J

1442010-20

Introduction

The Granulated Silica Method

Homogeneization by iterative CO2 laser remelting and milling

Homogenization by sol-gel production of granulated silica

Combination of sol-gel method with the granulated silica method

Example Fibers

Broadband fibers

Single core broadband fiber doped with rare earths and Bismuth

Broadband emission from a multi-core multi-dopant fiber

Fabrication procedure

Yb3++++-doped granulated silica fiber with sol-gel derived powder core

Conclusions