Letters

Photonic Crystal Fiber
Katsusuke Tajima, Kenji Kurokawa, Kazuhide Nakajima,and Izumi Sankawa†
Abstract
Photonic crystal fiber (PCF) is a promising candidate as a future transmission medium because it has
unique features that are not available with conventional single-mode fiber. This article describes recent
important progress on PCF.

1. Introduction
Optical fiber has become a vital element of theinfrastructure supporting the continuously expanding
IT (information technology) society. Although it has
mainly been used in backbone networks, its application has now been extended to access networks. However, the optical characteristics of conventional single-mode fiber (SMF), where dopants are doped into
silica glass to form a waveguide structure, limit theoptical communication capacity at currently usedwavelengths. There has been a strong need for a new
type of fiber with which to construct more economical networks based on an ultrahigh-capacity backbone.
Recently, optical fibers called “holey fibers” havereceived increasing attention because they provide
transmission characteristics that are superior to thoseof conventional single-mode fiber. One such holeyfiber is index guiding photonic crystal fiber (PCF)[1], which has dozens of small air holes arranged likea crystal lattice, as shown in Fig. 1. As with conventional single-mode fiber, light can propagate along
the fiber by a mechanism based on total internalreflection.
In this article, we outline recent progress on PCF
especially for transmission purposes.
2. Unique features of PCF
PCF has several unique features that make it suitable for ultrawide-band and ultralong-distance trans-HoleCoreCore(light trapped inside)

CladdingFig. 1. Cross-sectional photograph of photonic crystal fiber.
† NTT Access Network Service Systems Laboratories
Tsukuba-shi, 305-0805 Japan
E-mail: sankawa@ansl.ntt.co.jp

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Lettersmission. One of its most interesting properties is its
endlessly single-mode characteristics, which could
open a new optical transmission window ranging
continuously from the visible to near-infrared wavelength regions. This particularly wide wavelength
window cannot be obtained with conventional singlemode fiber. Therefore, it should be possible to greatly increase the transmission capacity of PCF.
In addition, the intrinsic loss of PCF is less than thatof conventional single-mode fiber. This is because
PCF is composed solely of pure silica glass without
any dopants. This is an advantage because the use ofdopants often increases the scattering loss. This feature makes PCF a promising candidate as a long-distance transmission medium. However, if we are to use
it as a transmission medium, we must fabricate long
lengths of low-loss PCF.

ing the optical attenuation caused by surface roughness and OH absorption. As a result of these efforts,the loss now approaches the intrinsic loss of silicabased fiber (0.14 dB/km).
Until recently, the maximum available length of
PCF was limited to a few kilometers, which meantthat it would have been necessary to concatenate
many segments to construct a long transmission line.
Recent work on the fiber drawing process and the useof a large PCF preform led to the production of an
approximately 10-km-long PCF in September 2003
[3] and a 100-km-long, low-loss (0.3 dB/km) PCF inMarch 2005 [4].
4. Mechanical reliability of PCF
A PCF cabling process is needed if PCF is to be
used as a transmission medium. Therefore, highmechanical reliability is required. The fact that PCF
contains air holes has led to the belief that its tensile
strength must be lower than that of conventional single-mode fiber. However, it has been found that PCF
with an optimized structure is actually stronger thanconventional single-mode fiber [5]. The tensile
strengths of a PCF with 90 holes and a conventional
single-mode fiber are compared in Fig. 3. The tensile
strength of the PCF is 62 N and that of the singlemode fiber is 58 N. The existence of the holes prevents cracks from growing. There appears to be an
optimal hole size, hole pitch, and number of holes for

3. Low-loss and long-length PCF
The main loss factors for PCF are confinement loss,
intrinsic silica loss due to scattering and absorption,and extrinsic loss caused by contamination and surface roughness. Unfortunately, the extrinsic loss
dominates the optical attenuation of current PCF.
Efforts to reduce attenuation are shown in Fig. 2. InMarch 2003, NTT Access Network Service Systems
Laboratories established a world record of 0.37dB/km at 1550 nm [2] and then broke this record sixmonths later with a loss of 0.28 dB/km [3] by reduc1000

240 dB/km: University of Southampton (UK)80 dB/km: University of Bath (UK)100

Loss (dB/km)

3.2 dB/km: NTT (Japan)1
1 dB/km: NTT (Japan)2100.58 dB/km: Blazephotonics (UK)0.37 dB/km:NTT (Japan)2

10.14 dB/km: Intrinsic loss of silica glass

0.28 dB/km:NTT (Japan)20.1
1999

2000

1: NTT Network Innovation Labs.
2: NTT Access Network Service Systems Labs.

2001
Year

2002

2003

Fig. 2. Progress in reducing the loss of PCF.

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Letters

higher-tensile-strength PCFs. However, before PCF
can be employed as a practical transmission infrastructure, the long-term stability of its mechanicaland optical characteristics must be studied.

transmission experiments. NTT Access Network Service Systems Laboratories performed the first successful 19-channel wavelength division multiplexing
transmission experiment (190 Gbit/s) through a 5km-long PCF in the 850–1550-nm wavelength region[6]. The measured bit error rate (BER) performancein the 850-, 1310-, and 1550-nm bands is shown inFig. 4. There was no noticeable BER degradation in

5. Application as transmission medium

Failure probability (%)

The new low-loss PCF has been tested in several

100
80
6050
4030
201086543SMF
90-hole PCF
No. of samples: 50
Gauge length: 200 mm
Strain speed: 12.5%/min.

2140

50

60
Tensile strength (N)7080

Gauge length: fiber length towhich tensilestress is applied.

Fig. 3. Probability plot of tensile strength test.

Frequency(THz)

200

250

300

350

400450Single-mode region of conventional fiber
L, C, S bands

Single-mode region of PCF
Wavelength(nm)1700

Bit error rate (log (BER))

1500
–3

1300

1100

1550-nm bandbtb
ch1ch2ch3ch4
ch5
ch6
ch7
ch8
ch9ch10
ch11
ch12
ch13
ch14
ch15
ch16

–4

1000
–3

900

1310-nm band
btb
1309
1313

–4

800
–3

–6

–8

–8

–10

–10

–10

–12

–12

–8

–20

–16

–12

850-nm band
btb
853

–4

–6

–6

700

–12
–20 –16–12

Received power (dBm)

–15

–13

–11
btb: back-to-backFig. 4. Ultrawide-band transmission experiment.

Vol. 3 No. 8 Aug. 2005

59Letterssion are shown in Fig. 5. A BER of less than 10–11
was successfully obtained without any power penalty. The output pulse was slightly wider than the input
pulse, but no BER degradation was observed because
the output pulse width was much narrower than thebit slot. Thus, power-penalty-free dispersion-managed soliton transmission over 100 km was achieved.
These experimental results show that low-loss PCF
is a promising candidate as a long-distance transmission medium.

any of the transmission bands. These results showthat the single-mode high-bit-rate transmission
potential of PCF was confirmed experimentally overa wide wavelength range from 850 to 1550 nm, whichcorresponds to a bandwidth of 160 THz. We can
expect that an endlessly single-mode PCF will provide an ultrawide band of several hundred terahertz
for future optical communication systems with anultralarge capacity on the petabit-per-second order.
Moreover, NTT Access Network Service Systems
Laboratories has reported the first optical soliton
transmission experiment with a 100-km-long andlow-loss (0.3 dB/km) PCF at 10 Gbit/s [4]. Theexperimental setup, BER performance, and autocorrelation traces before and after the 100-km transmis-

Recent progress on PCF has shown its potential asa high-bit-rate, large-capacity, and long-distanceLN
modulator

OBPF EDFA
Mode-locked
fiber laser

6. Future directions

PCF

DCF

PC
EDFA

EDFA

PCF

Pulse pattern generator
Receiver OBPF DCF

PCF

EDFA DCF

DCF
PCF

Error
detector
EDFA
OBPF: optical band pass filter
LN: lithium niobate

EDFA: erbium-doped fiber amplifier
DCF: dispersion compensation fiber

PC: polarization controller(a) Experimental setup–3

10 Gbit/s
Bit error rate (log (BER))–4btb
100 km–5
Input

Output–6
20.6 ps∆t = 14.6 ps–7

29.8 ps∆t = 21.1 ps

–8
–9
–10
–11
–12
–24

Time delay 20 ps/div
–22
–20
–18
–16
Received power (dBm)–14∆t: pulse width obtained from the full width at
half maximum of the autocorrelation trace(b) Experimental results

Fig. 5. Long-distance and nonlinear optical transmission experiment.

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Letters

transmission medium. From the practical point ofview, several feasibility studies will be undertaken on
PCF as a future communications medium. These willinclude investigating its transmission characteristics
and reliability over wide wavelength regions andtechnologies for testing, connecting, and cabling it.
References
[1]

[2]
[3]
[4]

[5]

[6]

“Research Advances Rapidly on a New Breed of Optical Fiber thatCould Lead to Ultrahigh-capacity Communications and Cost Reductions,” NTT Technical Review, Vol. 2, No. 2, pp. 89-90, 2004.
K. Tajima, J. Zhou, K. Nakajima, and K. Sato, “Ultra low loss andlong length photonic crystal fiber,” Tech. Digest of OFC 2003, PD1.
K. Tajima, J. Zhou, K. Kurokawa, and K. Nakajima, “Low water peak
photonic crystal fibers,” Tech. Digest of ECOC 2003, Th4.1.6.
K. Kurokawa, K. Tajima, J. Zhou, K. Nakajima, T. Matsui, and I.
Sankawa, “Penalty-free dispersion-managed soliton transmission
over 100 km low loss PCF,” Tech. Digest of OFC 2005, PDP21.
J. Zhou, K. Tajima, K. Kurokawa, K. Nakajima, and I. Sankawa,“High tensile strength photonic crystal fiber,” Tech. Digest of OFC
2004, WI2.
K. Nakajima, J. Zhou, K. Tajima, K. Kurokawa, C. Fukai, and I.
Sankawa, “Ultra wide band 190 Gbit/s WDM transmission over along length and low loss PCF,” Tech. Digest of OFC 2004, PDP23.

Katsusuke Tajima
Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received the B.E. degree in electrical engineering from Science University of Tokyo,
Tokyo in 1980 and the M.S. and Ph.D. degrees inelectronics engineering from the University ofTokyo, Tokyo in 1982 and 1985, respectively.
He joined NTT Ibaraki Electrical Communication Laboratory in 1985 and engaged in researchon the transmission characteristics of opticalfibers. His current research includes the design ofoptical fibers, new fiber materials especially for
low-loss transmission media, and nonlinear
effects in optical fibers. He is a member of theInstitute of Electronics, Information and Communication Engineers (IEICE) of Japan.

Kenji Kurokawa
Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in physics from the University of Tokyo, Tokyoin 1985, 1987, and 1997, respectively. He joined
NTT Ibaraki Electrical Communication Laboratories in 1987 and has been engaged in researchon ultrashort optical pulse generation and nonlinear effects in optical fibers. He is a member ofIEICE, the Japan Society of Applied Physics(JSAP), and the Physical Society of Japan (JPS).

Kazuhide Nakajima
Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received the M.S. and Ph.D. degrees inelectrical engineering from Nihon University,Chiba in 1994 and 2005, respectively. In 1994,
he joined NTT Access Network Systems Laboratories, Tokai, Ibaraki, where he has been engaged
in research on optical fiber design and related
measurement techniques. He is a member ofIEICE.

Izumi Sankawa
Group Leader, Senior Research Engineer, NTTAccess Network Service Systems Laboratories.
He received the B.S. and Dr.Eng. degrees inelectrical engineering from Waseda University,Tokyo in 1979 and 1991, respectively. He joined
the Ibaraki Electrical Communication Laboratories, Nippon Telegraph and Telephone Public
Corporation (now NTT), Ibaraki in 1979 and has
been engaged in research on optical fiber splicing, testing technologies, and optical fiber measurement and surveillance systems. He is a member of IEICE, JSAP, and JPS.

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