Chemical Physics Letters 384 (2004) 16–19

www.elsevier.com/locate/cplett

Optimized combination of Ho3þ and sulfide glassfor U-band fiber-optic amplifiers

Tae Hoon Lee a, Jong Heo a,*, Yong Gyu Choi b, Bong Je Park b, Kyong Hon Kim b

a Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong,

Nam-gu, Pohang, Kyungbuk 790-784, South Koreab Basic Research Laboratory, Electronics and Telecommunications Research Institute, Yuseong, P.O. Box 106, Daejeon 305-600, South Korea

Received 29 August 2003; in final form 29 November 2003

Published online:

Abstract

Spectroscopic properties of 1.6 lm emission from Ho3þ-doped sulfide glass were evaluated for their potential use as U-band

fiber-optic amplifiers. Emission from the 5I5 ! 5I7 transition showed a peak wavelength of 1670 nm and a full width at half

maximum of �65 nm with a cross-section of �7.1� 10�21 cm2. Ho3þ concentration in glass should be limited to 0.08 mol% due to

energy transfers between Ho3þ ions which lead to a decrease in the 5I5 level lifetime. Co-doping of Tb3þ effectively decreased

terminal 5I7 level lifetimes via the energy transfer of Ho3þ:5I7 !Tb3þ:7F0; 1; 2.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

Several types of fiber-optic amplifiers have been

proposed to cover the entire wavelength window wheresilica-based line fibers show a low transmission loss. In

addition to EDFAs (Erbium-Doped Fiber Amplifiers)

being used for the conventional 1.5 lm band, several

other fibers doped with Pr3þ or Tm3þ are being inves-

tigated extensively for use in the 1.3 and 1.4 lm bands,

respectively [1–3]. On the other hand, works concerning

the development of 1.6 lm-band amplifiers are scare.

Tm3þ ion was the first active dopant proposed but wasnot pursued further due to large amplified spontaneous

emission (ASE) centered at the 1.8 lm wavelength re-

gion [4]. The emission at �1.6 lm was also observed in

Pr3þ ions [5]. The lifetime of the fluorescing level (3F3,3F4) and the quantum efficiency of the emission were

�210 ls and �65%, respectively [6]. However, this

transition suffers from multiphonon relaxation because

of the small energy gap between the fluorescing level andthe next lower-lying level (�1350 cm�1). Therefore, the

* Corresponding author. Fax: +82-54-279-5872.

E-mail address: jheo@postech.ac.kr (J. Heo).

0009-2614/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2003.12.009

emission was observed only in the selenide glass host

[5,6].

In this report, we propose the combination of Ho3þ

and sulfide glass as a new candidate for amplification inthe 1.6 lm region (U-band). Even though 1.6 lmemission from Ho3þ has been already reported [7], no

suggestion has been made concerning potential use for

fiber-optic amplification. Furthermore, to the best of the

author�s knowledge, no further work has been reported

on the spectroscopic properties of this emission. There-

fore, it is necessary to examine the potential of this new

rare-earth/glass combination for amplifiers in this rela-tively unexplored wavelength band.

2. Experimental

We prepared sulfide glass with a nominal composi-

tion of Ge30Ga2As8S60 (at.%). Ho3þ and/or Tb3þ ions

were doped into the glass in varying concentration.

Purities of all the starting materials were higher than

99.99%. To prevent contamination from the atmo-

spheric environment, those were weighed inside the

glove box under the inert atmosphere. Weighed mate-rials were sealed in the silica ampoules and then melted

T. H. Lee et al. / Chemical Physics Letters 384 (2004) 16–19 17

at 950 �C for 12 h using a rocking furnace for homo-

geneous mixing. The melt were subsequently quenched

in water and annealed at near the glass transition tem-

perature. Specimens were then cut and polished for

spectroscopic measurements.Polished specimens were excited using a 900 nm

pump light from a Ti-sapphire laser pumped by an Arþ

laser. The absorption cross-section at the excitation

wavelength (900 nm) was 8.75� 10�22 cm2 while the

peak of the absorption is located at 897 nm with

the cross-section of 9.20� 10�22 cm2. Other details on

the optical measurement can be found in our previous

reports [8,9].

3. Results and discussion

Fig. 1 shows the 1.6 lm band emission from the

Ho3þ:5I5 ! 5I7 transition after corrected for instrumen-

tal spectral response. The emission peak was centered at

�1670 nm with a full width at half maximum intensityof �65 nm. The measured lifetime ðsmÞ of the emitting

level and quantum efficiency ðgÞ of the transition were

�1.33 ms and �57%, respectively. It is noteworthy that

this lifetime is about six times longer than that of the

Pr3þ:(3F3,3F4) level in selenide glass [6]. Calculated

branching ratio of the transition was �51% and the

emission cross-section ðreÞ calculated from the Fuch-

tbauer–Ladenburg relation was �7.1� 10�21 cm2. Thus,the product of the emission cross-section and lifetime

ðresmÞ proportional to optical gain, was 9.44� 10�28m2

sec.

The advantage of using a sulfide glass host can be

envisaged as follows. The energy gap between the fluo-

rescing level (5I5) and the next lower-lying level is small

with approximately 2600 cm�1. Therefore, 1.6 lmemission is strongly influenced by multiphonon relaxa-tion. Specifically, in heavy metal fluoride glasses with a

1550 1600 1650 1700 1750

10

5

0

En

erg

y(x

103 cm

-1)

5I4

5I5

5I6

5I7

5I8

Em

issi

onin

tens

ity(a

.u.)

Wavelength (nm)

Fig. 1. 1.67 lm emission spectrum from Ho3þ in sulfide glass. Inset

shows the corresponding electronic transition.

phonon energy of �500 cm�1, quantum efficiency of the

transition is less than 5%, the value of which is much

smaller than that in sulfide [10]. Furthermore, the value

of resm was only one eighth of that for our sulfide sys-

tem [10]. On the other hand, since the quantum effi-ciency in sulfide glass is still relatively low, it is expected

that selenide glass can provide higher quantum efficiency

since it has low phonon vibration energy (�200 cm�1).

However, the UV-side absorption edge of selenide

glasses is located near 800 nm with a tail extending be-

yond 900 nm [5]. Therefore, in addition to the loss of

pump energy, the non-radiative energy transfer from the

Ho3þ:5I5 level to a conduction band is also possibleand can lead to lifetime quenching [11]. This explanation

can support the fact that the measured 5I5 level lifetime

in selenide glass was only �1.0 ms.

The lifetime of the 5I5 level in Ho3þ decreased sharply

as the amount of Ho3þ in the glass exceeded 0.08 mol%

(Fig. 2). This concentration dependence is most proba-

bly due to the two energy transfer (ET) processes, i.e.,

(5I5,5I8 ! 5I7,

5I7) and (5I5,5I5 ! 5I8,

5F1). During the(5I5,

5I8 ! 5I7,5I7) process (ET-1), a portion of Ho3þ

ions excited to the 5I5 level transfer their energy to

nearby Ho3þ ions in the ground state and both end up

staying at the 5I7 level. Net result of this process is an

increase in the 5I7 level population at the expense of the5I5 level. This energy transfer is evidenced from the de-

crease in integrated intensity ratio between 1.67 and

2.01 lm (I1:67/I2:01) emissions with increasing Ho3þ

concentration (Fig. 3). In addition, a type ET-2 resonant

energy transfer of (5I5,5I5 ! 5I8,

5F1) can also occur

when Ho3þ ions in the 5I5 level accept the energy of

other Ho3þ ions in the same level and are further excited

to the 5F1 level [12]. The ET-1 process is a typical cross

relaxation process involving a phonon-assisted energy

transfer similar to that among Tm3þ ions [13] while

ET-2 is a resonant-type cooperative upconversion pro-cess. Since ET-2 process requires two nearby ions both

0.0 0.1 0.2 0.3 0.40.90

1.05

1.20

1.35

1.50

Ho3+ concentration (mol%)

Mea

sure

d lif

etim

e (m

s)

Fig. 2. Changes in measured lifetimes of the 5I5 level with Ho3þ

concentration.

0.0 0.1 0.2 0.3 0.4

0.2

0.4

0.6

0.8In

tens

ity r

atio

(I1.

67/I 2.

01)

Ho3+ concentration (mol%)

Fig. 3. Integrated emission intensity ratios between the 1.67 and

2.01 lm emission in Ho3þ-doped sulfide glass.

18 T. H. Lee et al. / Chemical Physics Letters 384 (2004) 16–19

excited to the 5I5 level at the same time, the energytransfer efficiency depends on the excitation pump

power. However, no distinguishable change in 5I5 level

lifetime was observed while increasing pump power up

to 800 mW. It implies that ET-2 process is not a major

source of cross relaxation in our glasses.

It is necessary to maintain continuous population

inversion between the fluorescing and terminal levels

in the amplification system. In our glass, however, the5I7 level lifetime (�7.1 ms) is considerably longer than

that of the 5I5 level (�1.33 ms), which makes our

system self-terminating. We co-doped the second rare-

earth ions to depopulate the terminal 5I7 level fol-

lowing the approach used for Tm3þ[9]. Examination

of the Ho3þ energy level structure led us to co-dope

Tb3þ ions as acceptors. Fig. 4 shows the Tb3þ con-

centration dependence on both Ho3þ:5I5 and 5I7 levellifetimes. As the concentration of Tb3þ increased to

0.3 mol%, the lifetime of the 5I7 level decreased from

7.1 ms to 1.8 ms due to the efficient energy transfer

from the Ho3þ:5I7 to Tb3þ:7F0; 1; 2 levels. The spectral

overlap between the emission (Ho3þ:5I7 ! 5I8) and the

0.0 0.1 0.2 0.3

0.0

1.5

3.0

4.5

6.0

7.5

Mea

sure

dlif

etim

e(m

s)

Tb3+ concentration (mol%)

5I5

5I7

Fig. 4. Effect of Tb3þ concentration on measured lifetimes of the

Ho3þ:5I5 and 5I7 levels.

absorption (Tb3þ:7F6 ! 7F0; 1; 2) spectra supports the

possibility of energy transfer. On the other hand, the5I5 level lifetime showed only a minimal change with

increasing Tb3þ concentration. Since the lifetimes of

the Ho3þ:5I7 level for the Tb3þ co-doped glasses werestill longer than that of the 5I5 level, we investigated

the population density to confirm the population in-

version between the Ho3þ:5I5 and 5I7 levels. From the

result of the analysis using rate equations, the popu-

lation inversion could be achieved when the concen-

tration of Tb3þ exceeded 0.05 mol%. Therefore, it

appears to be possible to realize gain in the U-band

wavelength region provided that the optical loss in theglass or fiber can be properly controlled. We are

preparing the detailed result of this analysis to be

published elsewhere.

4. Summary

In summary, we investigated the spectroscopic

properties of 1.6 lm band emissions from Ho3þ doped

into sulfide glass. The peak emission wavelength was

located at 1670 nm with a cross-section of �7.1� 10�21

cm2. The emitting level lifetime was 1.33 ms. Two energy

transfer processes were responsible for the concentration

quenching of the 5I5 level lifetime and the optimum

Ho3þ concentration was 0.08 mol%. Up to 0.3 mol%

Tb3þ ions were added to reduce the Ho3þ:5I7 level life-

time from 7.1 ms down to 1.8 ms, providing an efficient

population inversion between the 5I5 and 5I7 levels.

Sulfide glass doped with Ho3þ and Tb3þ is a promisingcandidate for future U-band fiber-optic amplifiers.

Acknowledgements

This work was financially supported by the Ministry

of Information and Communication, Republic of

Korea.

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