An Optimized Vapor Phase Doping Process to Fabricate Highly Yb-Doped Large Core Fibers

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 17, SEPTEMBER 1, 2015 3533

An Optimized Vapor Phase Doping Processto Fabricate Large Core Yb-Doped Fibers

Maitreyee Saha, Student Member, OSA, Atasi Pal, Member, OSA, Mrinmay Pal, Member, OSA,Chandan Guha, and Ranjan Sen, Member, OSA

Abstract—The paper demonstrates a standardized process of va-por phase doping to fabricate large core Yb–doped preforms withlonger useful length in reproducible manner. The optimization ofthe process led to successful achievement of Yb-doped core thick-ness of 4.5 mm (in 14.8 mm of preform diameter) by depositingup to 30 number of core layers with controlled amount of gener-ated precursor vapors. The influence of the process parameterswas studied rigorously to enhance the useful preform length up to380 mm. A combination of Yb and Al in different proportions wasdoped into the core with uniform dopant concentration along thelength by adjusting few process parameters efficiently. The Al2 O3concentration up to the level of 17.8 mol% has been achieved suc-cessfully which resulted in NA of 0.31. This is the highest everdoping of Al in passive fibers by any modified chemical vapor de-position process. The Yb2 O3 content in the active fibers is as highas 0.47 mol%.

Index Terms—Optical fibers, optical fiber fabrication, opticalfiber lasers, optical fiber measurements, rare earth compounds,vapor deposition, Ytterbium.

I. INTRODUCTION

F IBER laser has revolutionized the solid state laser technol-ogy due to large surface to volume ratio resulting better

thermal management and total elimination of thermal lensing.Their unique properties, specifically, the output power stabil-ity and unparalleled beam quality at high output power haveincreased fast penetration in all sectors of industrial, strategicand scientific applications [1], [2]. Rare–earth (RE) doped opti-cal fiber, an active medium to provide gain in fiber laser, playsan important role in differentiating the laser performance, thepower scaling capabilities, stability and cost. Among the rangeof RE dopants used in fiber lasers, so far, Ytterbium (Yb) hasshown excellent power scaling with output power exceedingmultikilowatt level because of the broadband absorption spec-trum of Yb+3 which enables multi-pump or multi-wavelengthpumping schemes facilitating power scaling through low costunstable pumps [3], [4]. Additionally, the simple two-level sys-tem of Yb+3 provides efficient lasing around 1 μm eliminating

Manuscript received January 7, 2015; revised April 13, 2015; accepted May31, 2015. Date of publication June 5, 2015; date of current version August 3,2015. This work was supported by the Department of Electronics and Infor-mation Technology, India and GLASSFIB Project of Council of Scientific andIndustrial Research, India. One of the authors, Maitreyee Saha wishes to thankCSIR, India for providing Research Fellowship.

M. Saha, A. Pal, M. Pal, and R. Sen are with the Fiber Optics and Pho-tonics Division, Central Glass and Ceramic Research Institute, Jadavpur,Kolkata 700032, India (e-mail: maitreyee.cgcri@gmail.com; atasi@cgcri.res.in;mpal@cgcri.res.in; rsen@cgcri.res.in).

C. Guha is with the Chemical Engineering Department, Jadavpur University,Jadavpur, Kolkata 700032, India (e-mail: cguha2003@yahoo.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2015.2442226

excited-state absorption, multi-phonon non-radiative decay andconcentration quenching [2]–[5]. The high power fiber laserdemands short lengths of large mode area (LMA) active fiberand small cladding to core ratios to maximize pump absorp-tion and minimize length dependent nonlinear effect. In orderto achieve suitable gain in laser resonator with shorter lengthof active fiber, highly Yb-doped fiber operated in the singlemode or low mode regime is necessary. For making highly Yb-doped fibers co-dopants like phosphorous (P) and/or aluminum(Al) are required at higher concentration to increase RE solubil-ity, preventing clusters and phase separation [6], [7]. But theseco-dopants, in addition, increases the core refractive index (RI),formation of central dips in RI and “star-like” defect at the core–clad boundary [6], [8] which can deteriorate the beam qualityas well as the power stability of the fiber laser.

Hence, fabrication of Yb-doped fibers with varied designs,compositions and high Yb+3 concentration attracts a lot of re-search interests. The aim was to establish a standardized, re-producible fabrication process in order to improve the com-positional homogeneity of RE-doped glass and consequentlyincrease the laser output powers. A number of methods werefollowed to incorporate Yb+3 ions into silica glass, most ofwhich are alterations of the conventional modified chemicalvapor deposition (MCVD) process. Two of the most commontechniques are solution doping method [9], [10] and vapor phasedoping technique [11]–[18]. Till date, solution doping methodis the most popular due to its simple operational scheme andwide range of selectivity of dopants. However, it suffers frompoor repeatability and variation in dopants concentration [19],especially when higher concentrations are desired. Moreover,it is difficult to control RI precisely and also reached the limitregarding large core (LC) size [20] which is essential for laserfibers. On this technological backdrop, vapor phase doping tech-nique based on high temperature sublimation of RE chelate com-pounds [11] is gaining much importance. The high volatility ofthe RE chelates at moderate temperature ranges [21] evolvessufficient amount of RE ions to be incorporated during corelayer formation. In addition, since the incorporation of REsand co-dopands occurs simultaneously with the deposition ofsilica, there is the possibility for homogeneous dopants distri-bution in the silica network which leads to significantly less REclustering. However, till date, very few fibers are made by thismethod [11]–[18] due to many critical challenges of the pro-cess, viz. decomposition of RE compounds prior to the reactionzone, blocking of delivery lines due to condensation of precursormaterials [22], [23], variation in dopants concentration over thelength of the preforms [15],[22] etc. This paper reports a system-atic investigation on process parameters of vapor phase doping

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3534 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 17, SEPTEMBER 1, 2015

Fig. 1. Schematic diagram of MCVD system with vapor delivery unit.

technique for fabrication of highly Yb-doped optical fibers inreliable manner. Process parameters were optimized to depositmultiple numbers of “in-situ” sintered core layers in repro-ducible manner for fabricating LC Yb-doped preforms/fibers.This process can readily be adapted for the commercial produc-tion of RE-doped optical fibers of targeted dopant concentrationswith different waveguide designs and compositions.

II. EXPERIMENTS

A. Preform Fabrication Through Vapor Phase Doping1

Preforms, doped with Ytterbium Oxide (Yb2O3) and Alu-minium Oxide (Al2O3), were fabricated by using OFC–12MCVD system (make: Nextrom Technology, Finland). This sys-tem is specially designed with high temperature vapor deliveryunit for delivering solid RE precursors in vapor phase. Fig. 1presents the schematic diagram of this system.

The MCVD system consists of i) one normal gas cabinet(NGC) unit for delivering Silicon Tetrachloride (SiCl4) andother RI modifying dopants along with Oxygen (O2) and He-lium (He) gases and ii) one high temperature cabinet (HTC)unit for delivering Aluminium Chloride (AlCl3) and RE chelatecompounds in vapor phase. There are three separate deliverylines for transporting i) liquid precursors from NGC and solidprecursors of ii) Al and iii) REs from HTC separately. There isa unique rotary union and two ribbon burners for maintainingsteady flow condition of the generated precursor vapors. All thedelivery lines and rotary union are provided with heaters, insu-lators, temperature and pressure sensors to maintain high levelof accuracy of the flowing condition of delivered vapors up tothe reaction zone. There is a high precision pyrometer and adiameter controlling unit provided with feedback control loopsfor auto control of temperature and pressure of the substratetube respectively.

The preform fabrication process started with the depositionof pure silica (SiO2) cladding layers in a Heraeus F-300 silicatube of diameter 28/24 or 20/17 mm with a length of 500 mm,which was followed by SiO2-Al2O3-Yb2O3 sintered core layerdeposition in vapor phase. The Yb chelate compound, Yb(thd)3[thd = 2,2,6,6-tetramethyl–3,5-heptanedionate] (Strem Chemi-cals, USA; 99.9% Yb) and anhydrous AlCl3 (Alfa Aesar, USA;

1Part of the preform fabrication technique is applied for intellectual propertyprotection through PCT.

99.999% Al) were used as precursor materials for Yb and Al,respectively. O2 was used as carrier gas for reactants deliveredfrom NGC while He was used as carrier gas for transportingAl and Yb compounds from HTC unit. The solid precursor ma-terials were heated within individual sublimators to transforminto their respective vapors which were then transported to thereaction zone by passing controlled amount of preheated carriergas through the sublimators. To identify the optimum sublimatortemperature, the temperature of sublimators was varied in therange of 120–170 and 200–240 °C for Al and Yb, respectively.He gas flow rates were regulated in the range of 8–280 and60–250 sccm for Al and Yb respectively to achieve specifieddopant concentrations. The total flow rate of gas mixture waskept in the range of 1–1.5 liter/min (depending upon the tubesize), with additional He in the range of 15–30% of the totalflow to facilitate sintering. The dopants delivered from HTCwere allowed to mix with O2 and SiCl4 only before the reactionzone, to prevent premature oxidation of the precursor materials.The delivery lines and rotary union were kept at an elevatedtemperature (>200 °C) and ribbon burners was adjusted in therange of 200–330 °C.

The numbers of deposited core layers were between 2 and30 to achieve the desired core thickness in fiber, keeping SiCl4flow rate constant at 0.58 gm/min. Main burner temperature wasmaintained near 1850 °C to limit decomposition of Yb precursorat upstream end of the reaction zone. Carriage traverse speedwas fixed at 125 mm/min to achieve well sintering condition ofdeposited soot layer. The purging steps (vapor delivery lines areflushed with preheated He through vent lines) were also tunedto ensure steady flow condition for solid precursor materials.Subsequent to deposition of the core layers, the substrate tubewas collapsed into solid rod in oxidizing atmosphere followingsoft collapsing technique.

B. Characterization of Fabricated Preforms/Fibers

Fibers, with diameter of 125 ± 0.5 μm, were drawn from thedifferent parts of the preforms by using a Fiber Drawing Tower(Heathway, USA) to characterize their properties. The RI profile(RIP) of the preforms was measured by using Preform analyzer(Photon Kinetics, PK 2600) at every 30 mm along the length ofthe preform to check the uniformity. The RIPs of the fibers weremeasured at Fiber analyzer (EXFO, NR–9200). The absorptionspectrum was recorded by using Bentham spectrometer and theYb concentration was estimated from the absorption peak at915 nm determined by “cut-back” method. Dopants concentra-tion was also evaluated through Electron Probe Micro Analysis(EPMA) which was performed on polished preform samples of2 mm thickness. EPMA results showed uniform dopants distri-bution at the radial direction of the preform and matched wellwith the measured values in fibers. All the optical characteriza-tions were performed at around 25 °C.

III. RESULTS AND DISCUSSIONS

A. Optimization of the Process Parameters

As described in the earlier reported results, this process suffersfrom poor repeatability [12],[22], “clogged” delivery lines due

SAHA et al.: OPTIMIZED VAPOR PHASE DOPING PROCESS TO FABRICATE LARGE CORE YB-DOPED FIBERS 3535

to condensation of precursor materials [22], [23], shorter usablepreform length [13], [14], lower RE concentrations [12], [13],non-uniform RE distribution along the length of the preform [15]and higher OH− content in the fibers [11], [18]. So, the primetarget of this study was to demonstrate a standardized processtechnology to fabricate LC, low numerical aperture (NA) andhigh Yb–doped fibers for laser applications with higher repro-ducibility. From the initial stage of the experiments, depositionof multiple numbers of core layers in reproducible manner hasbeen tried. But it was a pretty complicated task to ensure uniformflowing condition of the generated precursors’ vapor beyond 8–10 deposited layers. Several challenges have been faced duringthe course of process optimization, such as condensation ofthe solid precursor materials, which resulted in blockage of thevapor delivery lines; decomposition of chelate precursor com-pounds, which resulted in variation of RE concentration overthe length of the preform; evaporation of dopants during col-lapsing step, which resulted in central dip in RIP; non-circularcore formation etc. However, after systematic optimization ofthe process parameters, it has been possible to deposit up to 30numbers of core layers without any interruption, which resultedin 4.5 mm of core diameter. The process parameters which havemajor influences on the fabricated preforms’ characteristics arediscussed below:

1) Ribbon Burner Temperature: It has been observed that thetemperature of the front ribbon burner is the most crucial pa-rameter to maintain the uniformity of dopants distribution alongthe length of the preform. The temperature of the front ribbonburner should be sufficient enough that the RE precursor ma-terials can be transported without condensation in the deliverylines. But, if temperature is >330 °C, decomposition of chelatecompound occurs in the silica delivery tube before reaching thereaction zone and elemental carbon deposition is also observed.On the other hand, when temperature is <200 °C, condensationof chelate compound and recrystallization of AlCl3 occurs inthe silica delivery tube. Rear burner temperature should also beadjusted properly in the stated range, to avoid condensation ofunreacted precursor materials in the collector tube.

2) Main Burner Temperature: At early stage, core layer depo-sition temperature was maintained at around 1950 °C followingthe prior arts. It was however noticed that pre-reaction of theprecursor materials is taking place before reaching the reactionzone. This secondary reaction zone moves adjacently with themain burner, maintaining almost same distance along the sub-strate tube. This observation indicated that with the increase innumber of deposited layers for LC preforms, the wall of thesubstrate tube is not getting enough time to be cooled before thenext core layer deposition. This is resulting in decomposition ofchelate compound prior to the reaction zone and uncontrolledpre-reaction of precursor materials is taking place which re-sulted in non-uniform dopant concentration along the length ofthe preform. Afterwards main burner temperature was decreasedto 1850 °C while the starting position of the burner was fixed to10 cm apart from the front ribbon burner.

3) Purging Steps: Purging steps during preform fabricationhave been optimized on the basis of carrier gas flow rate andduration. All the vapor delivery lines were flushed through vent

TABLE ICOMPARISON OF RESULTS OF SUBLIMATOR TEMPERATURE (PASSIVE FIBERS)

Preform Id Temp. (in °C) Al2 O3 Conc. (in mol%) NA

RE_1 155 8.3 0.21RE_2 150 6.4 0.18RE_3 145 5.1 0.16RE_4 140 3.3 0.13RE_5 130 2.0 0.10RE_6 120 0.5 0.05

TABLE IICOMPARISON OF RESULTS OF SUBLIMATOR TEMPERATURE (ACTIVE FIBERS)

Preform Id Temp. (in °C) Al2 O3 Conc. (in mol%) Yb+ 3 Conc. (in ppm) NA

RE_7 200 3.1 2600 0.13RE_8 210 3.2 3300 0.13RE_9 220 3.1 3600 0.13RE_10 240 2.9 5300 0.13

line with dry He gas at higher temperature and flow rate. Thisstep has proved to be effective to eliminate contamination invapor delivery lines (if any) and allowed deposition of highernumbers of core layers consistently without any interruption.Further, it is also reveled from the results that systematic purginghelped to minimize OH– content in the fibers.

4) Sublimator Temperature: To identify the optimum sub-limator temperature to attain desired dopant concentration, 10numbers of core layers were deposited in a set of preform runs ata fixed carrier gas flow rate with varied sublimator temperatures.The AlCl3 sublimator was tuned in the range of 120–170 °C withconstant He flow rate of 25 sccm. It has been observed duringthe experiments that the vapor delivery lines are obstructed dueto massive condensation of precursor material when AlCl3 sub-limator temperature is kept above 168 °C. Moreover, unsinteredcore formation and phase separation have also been observedwhen sublimator temperature is kept beyond 160 °C. From theseobservations it has been concluded that as vapor pressure ofAlCl3 is significantly high over 160 °C, it is difficult to main-tain desirable dopant concentration in glass and so, maximumAlCl3 sublimator temperature was restricted to 155 °C at whichfully sintered and circular core could be obtained. But as perthe requirement of low NA fibers, the sublimator temperaturewas decreased further and from the obtained results (as shownin Table I), working temperature of AlCl3 sublimator has beenselected as 120–140 °C.

For identification of Yb(thd)3 sublimator temperature, it wastuned in the range of 200–240 °C with constant He flow rate of100 sccm with fixed AlCl3 sublimator temperature of 140 °C andflow rate of 25 sccm. From the results (as shown in Table II),it can be noticed that Al2O3 concentration is almost same ineach fiber at around 3 mol% while Yb+3 concentration is inthe range of 2600–5300 ppm depending upon sublimator tem-perature. From these results, depending upon the requirementof Yb concentration mostly in the range of 3000–4000 ppm inlaser fibers, 220 °C has been selected as working temperature

3536 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 17, SEPTEMBER 1, 2015

Fig. 2. Effect of carrier gas flow rate on Al2 O3 concentration (passive fiber).

Fig. 3. Effect of carrier gas flow rate on NA (passive fiber).

for Yb+3 sublimator. It has been observed that Yb concentrationin fibers at this sublimator temperature is highly satisfactory.

5) Carrier Gas Flow Rate: After establishing steady flowcondition for multiple core layers, next focus was on the influ-ence of carrier gas flow rates on dopant concentrations. Severalexperiments were performed, keeping the Al and Yb sublima-tor temperatures fixed at 140 and 220 °C, respectively. Initially,passive fibers were fabricated by regulating He flow rate in therange of 18–280 sccm for determining Al2O3 concentration asshown in Fig. 2. Then the study was extended to active fibers.Two series of experiments were performed: i) for determiningAl2O3 concentration—He/AlCl3 flow rate was regulated in therange of 8–93 sccm, keeping Yb(thd)3 flow rate constant at0.076 gm/min and ii) for Yb2O3 concentration determination,He/ Yb(thd)3 flow rate was tuned in the range of 60–250 sccm,maintaining AlCl3 flow rate stable at 0.005 gm/min.

From the results of passive fibers, it is evident that making offibers with Al2O3 concentration up to the level of 17.8 mol%is possible which resulted in NA of 0.31 as shown in Fig. 3.As per authors’ knowledge, it is the highest ever Al-doping insilica optical fibers by any MCVD process. This curve indicates,Al2O3 concentration reaches its saturation level after increasingcarrier gas flow rate over 280 sccm. In an effort to increase Al2O3concentration further in the preform, carrier gas flow rate wasset to 350 sccm. But, phase separation has been observed above

Fig. 4. Effect of carrier gas flow rate on NA for (a) active fibers and (b) passivefibers.

Fig. 5. Effect of carrier gas flow rate on Yb2 O3 concentration with Al2 O3concentration at around 3 mol%.

this high doping level. The core of this preform also becamenon-circular.

From the active fiber fabrication runs, it has been observedthat both Al2O3 and Yb2O3 concentrations increase almost lin-early with carrier gas flow rates. Fibers of Al2O3 concentrationin the range of 1.3–7.7 mol% have been fabricated proficientlyby pre-calculating the He gas flow rates to achieve the targetedNA. Fig. 4 presents the NA of active fibers which is in therange of 0.08–0.2 while the dotted curve is the predicted trendof NA due to pure Al2O3 doping with identical AlCl3 flowrates as in active fibers. It also has been observed that Al2O3concentration in active fibers decreases with increment of Ybcontent as compared to passive fibers which is similar in natureas reported by Lindner et al. [18]. Fig. 5 presents Yb+3 concen-tration which has been achieved in the range of 1350–9500 ppm.But from the experimental results, it is reveled that obtainingYb concentrations close to the targeted value is not possibleby pre-calculating the carrier gas flow rate only since RE pre-cursors are much prone to decomposition and/or condensationand needs longer residential time to produce saturated vaporsas compared to Al precursor. Further, the RE concentration alsodepends upon Al/RE ratios in the inlet gas mixture due to theco-operative mechanism [24]. Thus, linear variation of Yb con-centration is practically not feasible without considering these

SAHA et al.: OPTIMIZED VAPOR PHASE DOPING PROCESS TO FABRICATE LARGE CORE YB-DOPED FIBERS 3537

Fig. 6. Micrograph of core-clad interface of a highly doped preform.

points. The highest Yb+3 concentration in the fiber, so far, is1.6 mol% which has been achieved by increasing the sublimatortemperature to 240 °C and by adjusting the other experimentalparameters. But it is not under the scope of this paper.

Fig. 6 presents microscopic view of the core–clad boundary ofa high NA active preform whose core is doped with 6.5 mol%of Al2O3 and 0.28 mol% of Yb2O3 . From the photograph itis evident that the preform is devoid of any “star-like” imper-fection at the core–clad interface which is always observed inpure silica clad preforms made by solution doping techniquefor Al2O3 concentration >2 mol% [8] and is a result of Al2O3rich phase separation which occurs due to the localized con-centration variation of Al in porous soot layer [25]. This resultindicates significant improvement in compositional homogene-ity in terms of Al2O3 networking in silica matrix. Moreover, theprimary crystalline phase separation due to formation of mullitefor Al2O3 content >4 mol% [26], is significantly absent in va-por phase doping method as core layer deposition takes place athigher temperature with respect to unsintered soot layer whichdissolves these crystals to form amorphous glass matrix.

6) Number of Deposition Passes: As the target was to fab-ricate LC preforms, a series of preform runs were carried outunder different conditions to deposit higher numbers of core lay-ers. Up to 25 numbers of core layers have been deposited in asubstrate tube of diameter 20/17 mm, keeping SiCl4 , AlCl3 andYb(thd)3 flow rates constant at 0.58, 0.005 and 0.076 gm/min,respectively. From the results of these experiments, a gradualincrement in core thickness has been observed which is propor-tional to numbers of core layer deposition passes. Till now, thelargest achieved core diameter is 40 μm in a fiber of 125 μmoverall diameter. Fibers of core diameters 11.6, 18.6, 27 and34.5 μm have been fabricated by varying the numbers of corelayer passes as 2, 5, 10 and 18. These results are plotted in Fig. 7.The curve indicates that initially the core diameter increases pro-portionally with the number of passes, but beyond 20 number oflayers, the core thickness tends to decrease and higher numberof deposition passes are required to increase the core diameterfurther. However, entry taper zone of the LC preforms has been

Fig. 7. Influence of numbers of core layer passes on core diameter.

limited to 5–6 cm at maximum as compared to the long taperrange of∼20 cm as observed and reported by Lindner et al. [18].

7) Soot Box Pressures: As the aim was to deposit highernumbers of core layers, special attention was given to restrictcollapsing of the substrate tube during core layer deposition.It has been found that when pressure inside the tube is <0.45torr, diameter of the tube decreases slowly with every pass andwhen pressure inside the tube is >0.85 torr, diameter of the tubeswells up. During collapsing also pressure inside the substratetube has been maintained above 0.25 torr to restrict non-circularcore formation.

8) Collapsing Process: As LC preforms are much prone to benon-circular, collapsing process has been optimized precisely toobtain good quality preforms. It has been further observed thatthe collapsing steps are also very crucial to get circular preformwith uniform dopant distribution at the radial direction. Whenlimited number (<3) of steps has been used, the preform tendsto be non-circular with non-uniform dopant distribution at theradial direction. To eliminate this difficulty, soft collapsing tech-nique has been adapted with >7 numbers of forward collapsingpasses. It permits improved geometrical characteristics of thepreform and also proved to be extremely effective to avoid cen-tral dip due to dopants evaporation. The main burner temperaturehas been increased in step wise manner from 2050 to 2250 °C byregulating H2 flow rate and carriage traverse speed. The diameterof the annular space has been reduced below 0.5 mm in the finalforward collapsing pass to avoid non-circular core formation.

B. Demonstration of Small Core (SC) Fiber

Fabrication of SC preforms was comparatively easier as onlytwo core layers have been deposited in a substrate tube of diam-eter 20/17 mm in reproducible manner. For this case higher Ybconcentration, longer usable preform length with good unifor-mity of dopant concentration along the length of the preformshave been targeted. The bubbler and sublimators’ temperaturesand carrier gas flow rates have been calibrated accordingly tosupply of SiCl4 , AlCl3 and Yb(thd)3 at 0.58, 0.005 and 0.076gm/min, respectively.

The maximum length of SC preforms has been achieved, sofar, is 430 mm with diameter of 10.6 mm and NA of ∼ 0.11. The

3538 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 17, SEPTEMBER 1, 2015

Fig. 8. RIP and cross section (Inset) of the SC preform.

Fig. 9. Dopants distribution in the core region of SC preform.

TABLE IIICOMPARISON OF RESULTS ON FIBER CHARACTERISTICS AT THE TWO ENDS

OF THE PREFORM SAMPLES

Sample SC_1 Sample LC_1

Parameters Delivery End Collector End Delivery End Collector End

Core Dia. (in μm) 11.6 11.6 33.0 33.0NA 0.11 0.11 0.11 0.11Al2 O3 Conc. (in mol%) 1.70 1.67 1.60 1.65Yb2 O3 Conc. (in mol%) 0.47 0.465 0.20 0.21

RIP of a typical preform is shown in Fig. 8 and inset showingcross section of the preform. The core diameter was around11.6 μm out of 125 μm of overall fiber diameter. The fabricatedfiber corresponds to 1.7 mol% of Al2O3 and 0.47 mol% ofYb2O3 in the core with good uniformity along the length ofthe preform. From the absorption spectrum curves it has beenwitnessed that the variation of Yb concentrations at the twoends of the preform is almost negligible (<1%) and RIP alongthe length of the preform yielded exactly the same value. TheEPMA results as shown in Fig. 9 depicted uniform dopantsdistribution at the radial direction of the preform and the dataare in good agreement with the values measured in the fibers.The dopant concentrations at the two ends of the preform havebeen presented in Table III.

The background loss of the fiber is ∼21 dB/km at 1200 nm,which is comparatively much lower than the previous reportedresults of Yb–doped fibers [17],[22], considering the Yb2O3

Fig. 10. RIP and cross section (Inset) of the LC preform.

concentration >0.4 mol%. The OH– absorption centered at1380 nm indicates ∼2.3 ppm of water in the fiber which is no-tably lower than the reported results [11],[18]. This is significantresult considering that no dehydration step has been employed,but the purging condition was optimized systematically.

C. Demonstration of LC Fiber

Fabrication of LC preforms was comparatively much diffi-cult than the SC one, as it needs steady flow conditions of soliddopant precursors for multiple core layers. For this case, tem-peratures and flow rates of the carrier gases had to be calibratedjudicially to supply the dopants in constant amount in each passas per the requirement of the fiber specifications. The purgingsteps before starting the core layer deposition were a crucialfactor to ensure steady flow condition and to minimize decom-position of RE precursors upstream of the reaction zone. Thecollapsing steps were also very important for LC preforms as itusually tends to be non-circular. The toughest task of this studywas to maintain uniform dopant concentration throughout thelength of the preform, as both the decomposition and/or con-densation causes variation of dopant concentration in radial aswell as longitudinal directions. So, the main challenge was tocontrol both simultaneously.

But all the above stated steps and parameters have been op-timized accordingly to fabricate good quality preforms. For LCruns, up to 30 numbers of core layers have been deposited in asubstrate tube of diameter 28/24 mm, keeping SiCl4 , AlCl3 andYb(thd)3 flow rates constant at 0.78, 0.008 and 0.046 gm/min,respectively. The bubbler and sublimators’ temperatures, carriergas flow rates and purging condition have been maintained toachieve unremitting flowing condition of solid precursor materi-als. Unlike the preforms made by solution doping method, coreof the preforms made by vapor phase doping process cannot bedemarked by deposited layers.

Preforms with length up to 380 mm and diameter of 14.8 mmhave been produced with NA in the range of 0.11–0.12. Thecore diameter of the LC preforms was in the range of 25–38 μmout of 125 μm of fiber diameter. Fig. 10 shows RIP of a typ-ical LC preform while inset showing cross sectional view ofthe preform. The fiber corresponds to 1.6 mol% of Al2O3 and

SAHA et al.: OPTIMIZED VAPOR PHASE DOPING PROCESS TO FABRICATE LARGE CORE YB-DOPED FIBERS 3539

Fig. 11. Dopants distribution in the core region of LC preform.

Fig. 12. Lasing performance of the fabricated LC Yb-doped preform.

0.2 mol% of Yb2O3 in the core with good uniformity along thelength of the preform. From the absorption spectrum it has beenwitnessed that the variation of Yb concentrations at the two endsof the preform is quite small (<5%) and RIP also produced al-most same value along the length of the preform. The EPMAresults as shown in Fig. 11 depicted uniform dopants distribu-tion at the radial direction of the preform. The data regardingdopant concentrations at the two ends of the preform have beenpresented in the Table III. The background loss of the fiber isaround 60 dB/km at 1200 nm with OH– content of ∼ 4 ppm at1380 nm.

The resulting fibers showed low photodarkening effect andproficiently delivered 20 W of CW power with up to 77% oflasing efficiency at 1.06 μm emission wavelength as shown inFig. 12. The linear variation of laser output power with pumppower beyond threshold indicates opportunity of further powerscaling. The laser was in operation for 3 hours continuouslywithout any degradation in output power [27].

D. Verifying the Process Reproducibility

To check the reproducibility of the process, LC and SC pre-form fabrication runs were repeated keeping all the processconditions similar (such as reactant gas flow rates, depositiontemperature and pressure, collapsing condition, etc.) and theresults have been compared subsequent to characterization ofcorresponding fibers. The data of one such series has been pre-sented in Table IV.

TABLE IVCOMPARISON OF RESULTS ON PROCESS REPEATABILITY

Preform Id Core Dia. (in μm) NA Al2 O3 Conc. (in mol%) Yb2 O3 Conc. (in mol%)

LC_1 33.0 0.11 1.60 0.20LC_2 33.0 0.11 1.63 0.19SC_1 11.6 0.11 1.70 0.47SC_2 11.6 0.11 1.68 0.45

Fig. 13. Effect of inlet precursor concentration on final dopant concentrationin fibers (a) for Al and (b) for Yb.

From the results, it can be concluded that fabrication of pre-forms/fibers with almost identical characteristics is possible (re-peatability of dopants concentration as well as NA of the fiberis >90%), if all the process parameters are kept constant in theoptimized ranges. According to our observations, it is a wellrepeatable process with reproducibility >80%, which is prac-tically impossible for conventional solution doping method. Ithas also been noticed that the process reproducibility stronglydepends on the sublimator condition.

E. Dopant Incorporation Efficiency

From the experimental results, it has been observed thatboth Al2O3 and Yb2O3 concentrations almost linearly increasewith their precursor vapor concentration at inlet gas mixture.Fig. 13 presents the effect of inlet precursor concentration onfinal dopant concentration in glass preform for (a) Al precur-sor and (b) Yb precursor. The inlet AlCl3 concentration has

3540 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 17, SEPTEMBER 1, 2015

been regulated in the range of 0.15–1.67 mol% to achieveAl2O3 concentration of 1.3–7.7 mol% in active fibers, whileinlet concentration for Yb precursor has been tuned in the rangeof 0.5–1.24 mol% which resulted in Yb+3 concentration of1350–9500 ppm. From the results it is evident that incorpora-tion efficiency of Al+3 ions in silica network is much higherthan the Yb+3 ions.

The main achievements of the current study compared tothe previously reported results include: a) optimization of theprocess steps for fabricating SC as well as LC Yb–doped fibers,b) identification and proper control of the process parameterswhich have significant influence on the final fiber characteristics,c) better uniformity of RE concentration along the length of thepreform (variation in Yb2O3 concentration is <1% and <5% forSC and LC fibers, respectively), d) fabrication of longer lengthpreforms (useful preform length >85% and >75% of the depo-sition length for SC and LC preforms respectively), e) making ofLC preform (core size 4.5 mm) with uniform dopant distributionup to the length of 380 mm, f) highest concentration of Al2O3in the fabricated passive fibers (up to 17.8 mol% achieved)without any crystalline phase separation and/or core-cladinterface defect, g) lower OH− content in the fibers (restrictedaround 2.3 ppm without employing any dehydration steps), h)enhanced reproducibility (>80%) to fabricate a preform/fiberof given specification. The optimized process technology isnow suitable for making a preform/fiber of targeted design andRE-content in a reliable manner and can be readily adapted forcommercial production of RE doped laser fibers.

IV. CONCLUSION

The process technology of both LC and SC Yb–doped pre-forms has been demonstrated successfully through vapor phasedoping method. Several sets of preform fabrication runs weredesigned and performed to study the influence of process pa-rameters on the ultimate fiber characteristics. This systematicinvestigation has helped to overcome several existing problemsrelated to vapor phase doping. A number of good quality pre-forms of both SC and LC configurations were successfully fab-ricated with >80% repeatability. From the results, it can beconcluded that the process exhibits superior performance to theexisting solution doping method in terms of RE uniformity,elimination of “star-like” defect at the core–clad boundary andprocess reproducibility and is thus well suited to produce pre-forms/fibers of given specifications with almost same results.Although till now, only the incorporation of Yb ions into alu-minosilicate glass has been studied, but this process can readilybe adapted for other REs with different waveguide designs andcompositions.

ACKNOWLEDGMENT

The authors would like to thank the Director, CSIR-CGCRIand all the staff members of Fiber Optics and Photonics Divi-sion for their continuous support to carry out the work. Specialthanks are due to IPHT, Jena, Germany for carrying out EPMAmeasurements and characterizing the lasing performance of the

fibers and also thank to RRCAT, India for helping in lasingcharacterization.

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Maitreyee Saha received the M.Tech. degree in chemical engineering fromthe University of Calcutta, Kolkata, India, in 2009 and joined CSIR-CGCRI,Kolkata, India, as Research Fellow for pursuing the Ph.D. degree. The primeobjective of her thesis work is fabrication of rare-earth doped optical pre-forms/fibers through MCVD process coupled with vapor phase doping tech-nique. Her research interest includes optimization of fabrication conditions fordifferent glass compositions to improve the fiber performance and standardiza-tion of the process technology for making special designed fibers suitable forhigh power laser applications. She is a Student Member of Optical Society ofAmerica.

Atasi Pal received the B.E. degree in electronics and telecommunication en-gineering from Jadavpur University, Kolkata, India, in 2003 and received thePh.D. degree in measurement and instrumentation from City University Lon-don, U.K., in 2013. She has joined CSIR-CGCRI, Kolkata, India, in 2004 andcurrently working as a Scientist. Her research interest includes the design andcharacterization of specialty optical fiber and fiber laser for medical applicationand sensing. She is a Member of Optical Society of America.

Mrinmay Pal received the M.Tech. degree in optics and optoelectronics fromthe University of Calcutta, Kolkata, India, in 2000 and received the Ph.D. degreein engineering and technology from Jadavpur University, Kolkata, in 2010. Hejoined CSIR-CGCRI, Kolkata, India, in 2001 and currently working as a SeniorScientist on development of fiber lasers and amplifiers and supercontinuumsource for industrial and biomedical applications. His research interest alsoincludes ultrafast fiber laser technology and generation of novel laser sources atUV-VIS-IR regimes. He is a Member of Optical Society of America and LifeMember of Optical Society of India.

Chandan Guha received the M.E. degree in chemical engineering in 1980and the Ph.D. degree in engineering and technology from Jadavpur University,Kolkata India, in 1986. He is currently a Professor and the Head of ChemicalEngineering Department, Jadavpur University. He is having 30 years of teach-ing as well as industrial consultancy experience. His research activity includestransport phenomena, process simulation, CFD, two phase flow, phase changeproblems, reliability engineering, LOCA related problems and software devel-opment. He is a Life Member of Indian Institute of Chemical Engineers.

Ranjan Sen received the M.E. degree in chemical engineering in 1982 andreceived the Ph.D. degree in engineering and technology from Jadavpur Uni-versity, Kolkata, India, in 2005. He joined CSIR-CGCRI, Kolkata, India, in1983 and currently is the Head of Glass Division and Chief Scientist of FiberOptics and Photonics Division of this institute. His current research activityincludes specialty optical fibers for high power fiber laser, optical amplifier,interferrometric sensors etc., as well as the development of fiber-based com-ponents/devices for practical applications. He is having more than 25 yearsexperience in developing preforms/fibers of varied designs and compositions.In the area of glass science and technology, his research interest is primarilytoward specialty glass and glass ceramics for advanced applications. He is aMember of Optical Society of America and Life Member of Optical Societyof India, Indian Institute of Chemical Engineers, Indian Ceramic Society andMaterial Research Society of India.