Special Feature

Tellurite Fiber Raman Amplifiers
Atsushi Mori† and Hiroji Masuda
Abstract
NTT Photonics Laboratories is developing optical amplifiers capable of amplifying multiple signal
channels across a wide wavelength region of 100 nm or more. This is achieved through the use of a newly
developed fiber Raman amplifier that employs tellurite fiber.

1. What is a fiber Raman amplifier?
The fiber Raman amplifier (FRA) differs in principle from rare-earth-doped fiber amplifiers in that it
uses molecular vibration and an optical-scattering
phenomenon in the fiber material that acts as anamplification medium. Figure 1 compares the operating principles of the FRA and the erbium (Er3+)doped fiber amplifier (EDFA). With the FRA, a pump
† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: a-mori@aecl.ntt.co.jp

light of frequency v is scattered by the thermal vibration with frequency Ω of a Si-O (or Ge-O) moleculeand is consequently emitted with a frequency of ν–Ω.
Making use of this energy-conversion phenomenon,
the pump light stimulates the occurrence of Raman
scattering to perform Raman amplification. The signal that can be amplified has a wavelength that islonger than that of the pump light by only the molecular vibration frequency Ω (i.e., Stokes shift: 100 nm
in the 1500-nm band (Fig. 1(a)). In contrast, an EDFA
utilizes the absorption or stimulated emission
between the energy levels in an Er 3+ ion, whichmeans that the wavelength of the amplifiable signal(b) Er3+-doped fiber amplifier(a) Fiber Raman amplifierΩPhononΩ

Photonν–Ω4∆n: 2.5%
Loss: 0.6 dB/km
Pump wavelength:1460 nm

3210100 nm
050 100 150 200 250
Stokes shift (nm)Emission cross section (×10–21 cm2)

Raman gain coefficient (W–1 km–1)

ν1480-nm
pumpingO–+–980-nmpumping

Si – + –1530-nmstimulated
emission
Er3+

64
20
1450

1500

1550

1600

1650

Wavelength (nm)Fig. 1. Comparison of operating principles. (a) fiber Raman amplifier and (b) Er3+-doped fiber amplifier.
Here, ∆n is the refractive index difference.

Vol. 2 No. 12 Dec. 2004

51

Special Feature

Gain by λ1

Pump light•••λN

O –+–•••λ2 λ1Wavelength

Fig. 2. Multi-wavelength pumping technique for
wideband Raman amplifiers.

corresponds to the difference between those energy
levels (1530 nm), as shown in Fig. 1(b). Comparing
these two principles, we can see that the FRA isadvantageous in that it allows the transmission fiber
itself to be used as the amplification medium (distributed amplification) and enables signals of any wavelength to be amplified if we select the pump lightwavelength. In particular, using a distributed FRA isa very robust approach that can improve the signal-tonoise ratio (SNR), and in combination with an EDFA,
it can be used to increase the interval between inline
repeaters and the transmission distance between electrical regenerators in terrestrial high-speed/largecapacity trunk systems and undersea long-haul systems [1].
In recent years, a multi-wavelength pumping technique has been used in FRAs to achieve wideband
amplification (Fig. 2). With this technique, the optical fiber used for Raman amplification is pumped
with multi-wavelength light and a wide amplification
band is achieved by overlaying the Raman gain spectra thus obtained. In addition, the intensity of thepump light for each wavelength can be adjusted toflatten the resulting gain. The gain-band expansionthat can be achieved by multi-wavelength pumping isdetermined by the magnitude of the Stokes shift. A
large Stokes shift is therefore desirable for a wideband FRA using multi-wavelength pumping. The
maximum amplification bandwidth of an FRA is consequently about 100 nm for silica fiber. Research continues on optical amplifiers with bandwidths inexcess of 80 nm, which cannot be achieved by rareearth-doped fiber amplifiers [2].
2. Advantages of tellurite FRA
When a silica fiber is used as the amplification
medium, the seamless wavelength region that can beamplified is limited in principle to a bandwidth of 100
nm in the 1500-nm band. At NTT Photonics Labora52

Te – + –Gain by λ2Ω’Ω’
Phononν

Raman gain coefficient (W–1 km–1)

Optical power

Gain by λN

Photonν–Ω’60∆n: 2.2%
50 Loss: 20 dB/km
Pump wavelength:40 1460 nm
3020170 nm

100050 100 150 200 250 300
Stokes shift (nm)Fig. 3. Stimulated Raman scattering of tellurite FRA.

tories, we have successfully extended this amplification bandwidth to 170 nm by using tellurite fiber(with tellurium dioxide as its main component) as theamplification medium (Fig. 3). This is possible
because the vibration frequency Ω’ caused by thethermal vibration of a Te-O molecule is larger than
the thermal vibration Ω of a Si-O (or Ge-O) molecule. Figure 3 shows the stimulated Raman scattering
spectrum for a tellurite FRA. We compared this spectrum with that of Raman scattering in a dispersion
compensation fiber (DCF), of which the silica fiber
shown in Fig. 1(a) is one type. This silica fiber, which
has a Stokes shift of 100 nm in the 1500-nm band, hasa spectrum with a unimodal shape. In contrast, the tellurite fiber has a Stokes shift 1.7 times as large (=170nm) and a bimodal spectrum. Moreover, tellurite
glass has a nonlinear susceptibility about one order ofmagnitude greater than that of silica glass, so its
Raman gain coefficient (scattering intensity) is about
16 times larger. These features show that tellurite
fiber is more promising than silica fiber for making
an ultra-wideband FRA with a shorter fiber lengthand fewer pump wavelengths.
Figure 4(a) shows the configuration of a tellurite
FRA using the multi-wavelength pumping technique.
This amplifier pumps a 250-m tellurite fiber moduleusing laser diodes (LDs) operating at four different
wavelengths. The fiber length here is about one-tenththat of commonly used silica DCF (about 3 km). We
confirmed that this amplifier achieves wideband
amplification across the 160-nm wavelength regionof 1490–1650 nm with a gain of greater than 10 dB
NTT Technical Review

Special Feature

30Gain, NF (dB)Tellurite fiber
250 m
CouplerIsolator

20Gain1510P2P11430 nm

1410 nm

51420 nm0Pump lightsource160 nm
G > 10 dB, NF < 10 dB

25NF
1480

1460 nm(a)1520 1560 1600
Wavelength (nm)NF: noise figure

1640

(b)

Fig. 4. Multi-wavelength pumping tellurite FRA. (a) configuration and (b) gain characteristics.

and a noise figure of less than 10 dB.
The gain excursion (∆G) is rather large
with a maximum value of 10 dB andthe gain flatness (∆G/G, where G is thelowest gain) is about 100% [3], [4].
3. Issues in WDM transmission
applications

Distributedamplifier section

Discrete amplifier section
Tellurite
fiber 1
200 m

SMF
80 km

P51390 nm
1400 nm
1455 nmTellurite
fiber 2150 m

Silica DCF
10 km

P1

P21430 nm 1410 nm

P3

P41455 nm

1390 nm
1430 nmGain equalizer (GEQ)

As described above, the tellurite
Pump light source
FRA has a number of attractive feaFig. 5. Configuration of hybrid tellurite/silica FRA with distributed silica
tures. It provides a gain of greater thanRaman amplification.
10 dB across 160 nm and can cover thelow-loss region of transmission fiber
across the S, C, and L bands (See Fig. 2 of thethe S band, as shown in Fig. 4(b).
overview article on page 36). However, when we use(5) Signal power transfer caused by stimulated
a tellurite FRA as an inline repeater in an actual
Raman scattering between optical signals inwavelength division multiplexing (WDM) system,transmission fiber: When WDM signals
we must deal with the following problems.
extending over a 100-nm-or-wider band are(1) 20 dB gain: The repeater span in a 1500-nminput into a transmission fiber, S-band signal
band optical transmission system is generally
power is converted into L-band signal power
80 km, and the span loss of about 20 dB includdue to stimulated Raman scattering. This pheing the required margin must be compensatednomenon causes attenuation of the S-band sigfor by gain.
nal power.
(2) Gain flatness: When an FRA is used as a linear
Below, we describe how we overcame these probrepeater in a multiple-repeater WDM transmislems. For the first three problems, we examined thesion system, gain excursion accumulates. As adifferences between the gain spectra of tellurite andresult, the maximum transmission distance andsilica FRAs and configured a discrete hybrid (tellurite
transmission quality are determined by theand silica) FRA that can achieve both dispersion
channel with the lowest optical SNR, so gain
compensation and gain flattening for linear repeaters
flatness is very important.
[5]. For the other two problems, we configured a dis(3) Dispersion compensation: Although it is comtributed/discrete hybrid FRA that adds a distributed
mon to insert a DCF in an amplifier to compenFRA with a high SNR centered on the S band to thesate for dispersion in the transmission fiber, it
discrete hybrid FRA (Fig. 5).
remains unclear whether DCF can be successThis optical amplifier has a two-stage configuration
fully inserted in a tellurite FRA.
with the distributed FRA section as the first stage and(4) Noise figure degradation on the short-wavethe discrete FRA section as the second stage. The dislength side: The noise figure of a tellurite FRA
crete FRA section itself consists of three stages condeteriorates in the short wavelength region ofnected in series with tellurite fiber for the first andVol. 2 No. 12 Dec. 2004

53

Special Feature

130 nm

3530

Total

Gain (dB)

252015Silica
Tellurite

1050–51460

1500

1540
Wavelength (nm)

1580

1620

Fig. 6. Gain characteristics of hybrid tellurite/silica FRA.
127 nm3530

Total gain
with GEQ

Total gain
without GEQ

Gain (dB)2520Loss15

Discrete FRA105Distributed FRA

0–5
1460

1500

1540

1580

1620

Wavelength (nm)Fig. 7. Gain characteristics of hybrid tellurite/silica FRA with distributed silica Raman amplification.

third stages and silica DCF for the second stage. A
gain equalizer (GEQ) was placed between the secondand third stages (between the DCF and second tellurite fiber). The distributed FRA section consists of 80
km of single-mode fiber (SMF) as transmission fiber,
which is backwardly pumped by LDs operating at
three wavelengths. The pump wavelengths and powers were determined to give this amplifier a low noise
figure and high output as well as a flat gain spectrum.
Figure 6 shows the individual gain spectra of the tellurite and silica FRA and their total combined gain
spectrum (without gain equalization). The gain
achieved by the silica FRA compensated for the gain
dips that occur in the tellurite FRA. As a result, the130-nm gain band (1478–1608 nm) indicated in thefigure exhibits a minimum gain of 20.9 dB and amaximum gain of 31.4 dB. The gain excursion is consequently 10.5 dB, but the use of a gain equalizer
could flatten this gain without degrading the noise
figure or the output power. We were therefore able toimprove the gain flatness ∆G/G to about 50%.
54

Figure 7 shows the gain spectra of the discrete anddistributed FRAs and the total gain spectrum withand without gain equalization. The figure also shows
the loss for 80 km of SMF. The distributed Ramangain of this amplifier is about 10 dB in the shortwavelength amplification region (1480–1500 nm)
while the discrete Raman gain is greater than 15 dB
across a wide wavelength region of 1500–1610 nm.
The total gain spectrum after equalization indicatesan average gain of 22 dB across the 127-nm wavelength region of 1480–1607 nm. For the 80-km SMFloss (15.5–18 dB), a system margin of about 3 dB canbe obtained in the same band.
Finally, Fig. 8 shows the noise figure spectrum forthe discrete hybrid FRA and the equivalent noise figure for the discrete/distributed hybrid FRA. The“equivalent noise figure” is computed as the net noisefigure (defined as the loss between the input end of
the 80-km SMF and the amplifier output) minus theSMF loss in dB units. The noise figure of the discrete
FRA degrades to more than 8 dB in the short waveNTT Technical Review

Special Feature

14Noise figure (dB)

1210Discrete hybrid FRA

864201460

Discrete/distributed hybrid FRA
1500

1540
Wavelength (nm)

1580

1620

Fig. 8. Noise figure spectra of discrete hybrid FRA and discrete/distributed hybrid FRA.

length region corresponding to the S band and near1560 nm corresponding to the gain dip of the tellurite
FRAs. For the same wavelength regions, the equivalent noise figure is less than 4 dB, which is especially low, and for the entire flat region, it is less than 7dB. In particular, the low equivalent noise figure in
the short wavelength region was designed to suppress
the SNR degradation caused by stimulated Raman
scattering that occurs between optical signals in thisregion in high-speed large-capacity transmission systems.
We used this amplifier as a linear repeater in alarge-capacity WDM experiment (channel rate: 10Gbit/s; 313 channels). The results showed that thisamplifier could achieve error-free operation seamlessly across a 124-nm (1485–1609 nm) gain band.
This is the widest seamless amplification band yetreported [6].
4. Future developments
To make high-efficiency tellurite FRAs, we plan toresearch ways of reducing the loss in tellurite fiberand optimizing fiber parameters. We also aim todevelop amplifiers with even wider bandwidths andflatter gains by optimizing the multi-wavelength
pumping technique.

[4]

[5]

[6]

tellurite-based fiber Raman amplifier,” J. Lightwave Technol., Vol.
21, No. 5, pp. 1300-1306, 2003.
M. Yamada and M. Shimizu, “Ultra-wideband Amplification Technologies for Optical Fiber Amplifiers,” NTT Technical Review, Vol.
1, No. 3, pp. 80-84, 2003.
H. Masuda, A. Mori, K. Shikano, K. Oikawa, K. Kato, and M.
Shimizu, “Ultra-wide-band hybrid tellurite/silica fiber Raman amplifier,” OFC2002, ThB6, 2002.
H. Takara, H. Masuda, K. Mori, K. Sato, Y. Inoue, T. Ohara, A. Mori,
M. Kohtoku, Y. Miyamoto, T. Morioka, and S. Kawanishi, “Ultrawideband tellurite/silica fiber Raman amplifier and supercontinuum
lightwave source for 124-nm seamless bandwidth DWDM transmission,” OFC2002, PD-FB1, 2002.

Atsushi Mori
Senior Research Engineer, Photonic Integration Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees inphysics from Tohoku University, Sendai, Miyagi
in 1989 and 1991, respectively. He joined NTT
Opto-Electronics Laboratories in 1991, where hewas engaged in research on multicomponent
glass fiber including oxide, fluoride and chalcogenide glass fiber for optical fiber amplifiers.
Since 1995, he has mainly been engaged inresearch on wideband optical fiber amplifiers
using Er3+-doped tellurite fibers and on tellurite
fiber as a fiber Raman amplifier medium. He
received the 1996 JSAP Young Scientist Awardfor the Presentation of an Excellent Paper and the2000 IEICE Achievement Award. He is a member of the Institute of Electronics, Information
and Communication Engineers (IEICE) of Japan,
the Japan Society of Applied Physics, and thePhysical Society of Japan.

Hiroji Masuda

References
[1]

[2]

[3]

H. Masuda, S. Kawai, K.-I. Suzuki, and K. Aida, “75-nm 3-dB gainband optical amplification with erbium-doped fluoride fiber amplifiers and distributed Raman amplifiers in 9 × 2.5-Gbit/s WDM transmission experiment,” IOOC-ECOC’97, Vol. 5, PDP, pp. 73-76, Sep.
1997.
Y. Emori and S. Namiki, “100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM
high power laser diodes,” OFC’99, PD19, pp. 1-3, 1999.
A. Mori, H. Masuda, K. Shikano, and M. Shimizu, “Ultra-wide-band

Vol. 2 No. 12 Dec. 2004

Senior Research Engineer, High-speed Lightwave Transport Systems Research Group, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received the M.Sc. degree in physics from
Tokyo Institute of Technology, Tokyo in 1988.
He joined NTT Laboratories, Kanagawa, Japan,in 1988. He has been engaged in research on optical amplification technologies, and high-speed
and large-capacity transmission systems. He is amember of IEICE and IEEE.

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