Development of Optical Fiber Gratings for WDM Systems
by Ikuo Ota *, Toshiaki Tsuda *, Atsushi Shinozaki *, Shigehito Yodo *,Toshihiko Ota *, Takashi Shigematsu *2 and Yasuhiro Ibusuki *3

Full-scale implementation of wavelength-division multiplexing (WDM) has
focused attention on optical fiber gratings, capable of multiplexing/demultiplexingof multiple wavelengths. Furukawa Electric has carried out basic R & D on optical fiber gratings,
developing products for WDM systems and a variety of other applications. This paper introduces
the basic theory of optical fiber gratings and describes manufacturing techniques. It also summarizes developmental results with emphasis on gratings for WDM system applications.

ABSTRACT1.

INTRODUCTION

Optical fiber gratings make use of the photo-refractive
effect discovered by Hill et al. in 1978, whereby the refractive index of an optical fiber is increased by exposure toultraviolet light. 1) At that time, the grating depended oninterference between the forward-propagating wave incident on the fiber and the backward-propagating wave
resulting from reflection at the end face of the fiber. Thus
the Bragg wavelength depended on the wavelength of thelaser light input to the fiber, making it impossible to form agrating of the desired wavelengths. Later, however, amethod was proposed by Meltz et al. whereby the fiber
was irradiated laterally with UV light, producing in the corea grating of the desired period. 2) It further became possible, by treating the fiber with high-pressure hydrogen, toincrease its photosensitivity, 3) accelerating R&D to thepoint where commercially viable products are now available.
Over the past few years, the Internet has led the rapid
increase in demand for fiber-optic communications, andmajor strides have been made in WDM systems toincrease the transmission capacity of existing lines. In thisarea, fiber gratings are being used in filtering devices for
multiplexing/demultiplexing in WDM systems, gain equalizers for Erbium-doped fiber amplifiers (EDFAs), and in
the external cavity lasers, used to stabilize light-source
wavelength.
This paper describes the principle of the optical fiber
grating and techniques for its manufacture, and introducessome products applying fiber gratings in WDM systems.

2.

PRINCIPLE OF THE OPTICAL FIBERGRATING

2.1 Overview
The optical fiber grating is a device that creates a periodic
perturbation in the refractive index of the fiber core,reflecting only the specific wavelength corresponding tothat period.
This periodic perturbation in the refractive index isbrought about by forming an interference fringe on thefiber by means of lateral irradiation with UV light, and by
writing that pattern in the fiber by the photo-refractive
effect.
There are two methods that are commonly used to produce the interference fringe. The first is known as theholographic method (see Figure 1), in which UV light ispassed through a beam splitter and reflected by a mirror
to form an interference fringe at the point of intersection. A
grating of the required characteristics can be obtained bymaking minute adjustments to the angle of the mirror, but
this method requires a light source of high coherency andis easily influenced by the vibration of the optics, etc.
The other method is known as the phase-mask method,in which fine slits are made in a phase mask mounted ona silica substrate by exposing it to an electron beam or the
like. For light passing through the phase mask (see Figure2), the 0-order light traveling straight ahead is almost
entirely suppressed, so that the interference produced by
the ±1-order light can be formed on the fiber. The relation-

Beam splitter

Mirror
*Opto-Technology Lab., R & D Div.
*2 Production Technology Development Center, Plants andFacilities Dept.
*3 Production Engineering Sec., FITEL Products Div.

Fiber

Figure 1

Holographic method

35

wide range of applications including band rejection filters,
and since their reflection point differs according to thewavelength (see Figure 3) they could also serve as dispersion compensators. 4)

Phase mask

Figure 2

λLONG

Figure 3

Phase-mask method

λSHORT

Principle of dispersion-compensating gratingship between the period of the phase mask ΛMASK and theperiod of the fiber grating ΛFBG may be shown as

ΛMASK = 2ΛFBG...(1)

Thus when it comes to producing fiber gratings of differing wavelengths, this method has the disadvantage ofrequiring several masks according to the period of thegrating, making for higher cost than with the holographic
method. However it achieves more stable characteristics
and is better suited to mass production, and is therefore
the method generally adopted for the manufacture of optical fiber gratings.
The optical characteristics of fiber gratings are determined by three parameters: the magnitude of the perturbation in the refractive index, the period of the grating, andits length. The magnitude of refractive index perturbationand grating length in the main greatly influence
reflectance and bandwidth. The period of the grating, on
the other hand, determines the center wavelength, and it
is also possible, by varying its magnitude in the longitudinal direction, to realize various types of gratings.
Gratings having a period that is constant in the longitudinal direction are referred to as "uniform", and they generally have a steep wavelength characteristic with sharp
peaks. Depending on grating length, bandwidths of 0.1 nm
or less are possible, and such devices are widely used for
add-drop and narrow-band WDM applications. The center
wavelength may be represented by

λB = 2nΛ
where: λB is the Bragg wavelength.

2.3 Mode Coupling Theory
If we let A(z) be the amplitude of the forward-propagating
wave and B(z) be the amplitude of the backward-propagating wave, the electrical field distribution E(z) in the fiber
may be represented byE(z) = A(z) exp(-iβz)+B(z) exp(iβz)...(3)

A(z) and B(z) will also satisfy the mode coupling equationdA=-iκ B exp(2iδ z)
dz
dB=iκ A exp(-2iδ z)dz...(4)

where κ is a coupling coefficient and δ = β -π/Λ.

...(2)

Since the effective refractive index n of the mode andthe grating period Λ vary with respect to temperature anddistortion, monitoring the center wavelength can provide atemperature and distortion sensor.
Gratings with a period that varies in the longitudinal
direction, on the other hand, are referred to as "chirped"
gratings. Since the reflected wavelength is represented in
Equation (2), a continuous change in period appears as acontinuous change in reflected wavelength, and a broadband waveform can be obtained. Chirped gratings have a36

2.2 The Photo-reactive EffectAs was stated above, an optical fiber grating is a device inwhich an area with an increased refractive index is formed
periodically in the fiber core. This higher index can beachieved easily, merely by irradiating the fiber with UVlight, but the mechanism by which this occurs is still not
fully understood. Several models have been proposed,
however, and it may well be that they interact. Here we
propose to introduce the Kramers-Kronig model as the primary one among these interacting mechanisms. 5)
The spectrum of germanium-doped fiber has an absorption band at around 240 nm, so that the nearby area isbleached by the UV irradiation, and instead of this anabsorption band, which is thought to be due to the GeE'
center, is produced on the shorter-wavelength side. It isby means of this change in the absorption spectrum that
the Kramers-Kronig relationship explains the increase in
the refractive index.
Further, because the amount of perturbation of therefractive index increases during the manufacture of optical fiber gratings, a method is frequently adopted wherebyhydrogen is added to the fiber in high concentrations priorto UV irradiation. The role of the hydrogen has been insufficiently explored, but it is thought to contribute to the activation of the reaction and the fixation of defects.

Solving Equation (4) for the vicinity of the peak, we thenhave(

A(z)exp(-iδ z)
A(0)=M
B(z)exp(iδ z)
B(0)

) ( )

M=

(

...(5)

cosh(γ z)-i δγ sinh(γ z)-i κ
γ sinh(γ z)iκ
γ sinh(γ z)cosh(γ z)+i δγ sinh(γ z))...(6)

where: γ = √
κ 2 -δ 2

Furukawa Review, No. 19. 2000

κ 2 sinh 2 (γ L)
γ 2 +κ 2 sinh 2 (γ L)...(7)

Then by substituting γ = κ in Equation (7), reflectance atpeak wavelength becomes
R = tanh2(κ L )3.

...(8)OPTICAL FIBER GRATINGS FOR
OADMs

WDM communication systems require an optical add-drop
multiplexer (OADM) to add to (or drop from) the line a
specified signal wavelength. Since the optical fiber grating
is a device with a steep wavelength characteristic andgood wavelength selectivity, it may be considered as
applicable to OADM systems. Figure 4 shows an example
in combination with a circulator. The reflected wavelength
λ1 of the grating is dropped from the IN port to the DROP
port and signal λ1 from the ADD port is reflected by thegrating and added to the OUT port line.
There are two characteristics that may be adduced as
essential for an optical fiber grating for OADM applications: 1) a steep spectrum without ripples in the transmission and reflection characteristics; and 2) lower loss in thetransmission band.
When the refractive index perturbation of the grating is
uniform in the longitudinal direction of the fiber (see Figure5(a)), the reflection spectrum has, on either side of themain peak, a plurality of peaks known as side-lobes,
which can be suppressed (Figure 5(b)) by a techniqueknown as apodization, in which refractive index perturbation is reduced on both sides of the grating. 6) Figures 6
shows calculated values for the reflection spectrum when
not apodized, and Figure 7 shows analogous values when
the Gaussian distribution is apodized. Apodization can be

0-10

Loss (dB)R=

done relatively easily by varying the amount of UV irradiation either mechanically or optically while the grating isbeing written.
A further problem is excessive loss on the shorter wavelength side of the main peak in the transmission spectrum.
Figure 8 shows the transmission spectrum when a grating
is written in an ordinary single-mode fiber. This is due tocoupling from the waveguide mode to the backward-propagating cladding mode. This may be overcome by suppressing the coupling itself and by shifting the region inwhich the excessive loss occurs outside the waveband
being used. In either case it is achieved by modifying the
profile of the fiber. As methods that suppresses the cou-

-20

-30

-40
1545

1547

1549

1551

1553

1555

Wavelength (nm)Figure 6

Reflection spectrum of uniform grating (calculated)

0-10

Loss (dB)

The reflectance R of a fiber grating with a length of L
can be obtained by substituting z = L and B(L) = 0 in
Equations (5) and (6), as

-20

-30

-40
1545

1547

1549

1551

1553

1555

Wavelength (nm)
IN

FBG

OUT

Figure 7
λ1
DROP

Figure 4

λ1
ADD

Reflection spectrum of apodized grating (calculated)0

Schematic of OADM with fiber Bragg grating
Loss (dB)→ Perturbation→ Perturbation

-5

-15

Longitudinal direction

Longitudinal directiona) Uniform gratingb) Apodized grating

Figure 5-10

Refractive index perturbation of uniform andapodized gratings-201550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560

Wavelength (nm)Figure 8

Transmission spectrum of fiber grating written in asingle-mode fiber37

0Loss (dB)

-10

-20

-30

-40
1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562

Wavelength (nm)Figure 9

Photo 1

Temperature-compensated fiber grating

Transmission and reflection spectra of fiber grating
written in a high-NA fiber

FBG

Material A
Material B
Linear expansion coefficient: A<B

Figure 10 Temperature-compensated package composed oftwo materials such that the coefficient of linear
expansion of A is less than that of B

Photo 2OADM module

0.8

Shift in wavelength (nm)

0.6
0.4

W/o temperature compensation

0.01 nm/°C

With temperature compensation

0.2
0.0
0.001 nm/°C

-0.2
-0.4
-0.6
-0.8
-40 -30 -20 -10

0

10

20

30

40

50

60

70

80

90

Temperature (°C)

Figure 11 Temperature dependence of center wavelength oftemperature-compensated package

pling itself, a Ge co-doped cladding fiber 7) and a Wshaped index fiber 8) have been proposed. To shift thepeak outside the waveband used, a high-NA fiber is effective. 9) By using a high-NA fiber of proprietary design, weare producing gratings that are unaffected by the cladding
mode 12 nm and more from the main peak.
Figure 9 shows transmission and reflection spectra
when a high-NA fiber was used. A KrF excimer laser wasused as the UV light source, and the phase-mask methodof manufacture was used. There was no excessive loss inthe transmission band, and gratings with steep wavelength characteristics were obtained.
Another important point in relation to WDM systems,
given the small intervals between wavelengths, is wave-

38

length control technology. With ordinary fiber gratings, thechange in center wavelength due to temperature amounts
to about 0.01 nm/°C. Any temperature-caused divergenceof center wavelength from grid wavelength will lead todegradation of transmission characteristics, so that thecenter wavelength of the grating should remain fixed at
the grid wavelength irrespective of temperature.
The temperature-dependence of the center wavelength
may be attributed to the fact that n and Λ in Equation (2)increase with rises in temperature. The temperaturedependence of n is inherent in the material of which thefiber is made and is difficult to change, but the temperature-dependence of Λ is comparatively easily suppressed.
Thus if it has an inverse relationship to temperature, it cancancel out the increase in n to provide a constant center
wavelength.
To achieve such an inverse relationship, we may use apackage made of a material with a negative coefficient oflinear expansion 10) or, as shown in Figure 10, of materials
with different coefficients. 11)
Figure 11 shows the temperature-dependence of thecenter wavelength for a grating having a package made oftwo different materials. The temperature-dependence is0.001 nm/°C or less, a variation that can be ignored at thetemperature range in which OADMs are used.
Photo 1 shows a fiber grating having a temperaturecompensated package, and Photo 2 shows an OADM
module combining a circulator. The module measures
only 108 x 80 x 8.5 mm, making it extremely compact andconvenient.

Furukawa Review, No. 19. 2000

15

LD
FBG

AR
Lens

FerruleFigure 12 Structure of a pumping laser diode with fiber grating

Shift in wavelength (nm)HR

W/o fiber grating 380 pm/°C

0.08

10

With fiber grating -2.0 pm/°C

5

0

-5

-10
0

20

0.06

40

60

Diode temperature (°C)

With fiber grating

Figure 14 Temperature-dependence of peak wavelength oflaser diodes with and without fiber grating
0.04
15
0.02

0
1470

1475

1480

1485

1490

1495

1500

Wavelength (nm)Figure 13 Output spectra of laser diodes with and without fiber
grating

Shift in wavelength (nm)Output power (mW)W/o fiber gratingWith fiber grating

10

W/o fiber grating

5

0

-5

-10
0

4.

FIBER GRATINGS FOR LASER WAVELENGTH STABILIZATION

The pumping light source for EDFAs is provided by 1.48µm and 0.98-µm laser diodes. To achieve higher EDFA
output requires higher power from the pumping diode,together with the use of wavelength-division or polarization multiplexing.
To achieve wavelength-division multiplexing, it is essential that the peak wavelength of the pumping diode be stable with respect to the ambient conditions of use, and thatthe spectral band be narrowed.
Ordinary diodes lase by means of a Fabry-Perot resonator using the chip end face, so that other modes lase
simultaneously, leading to broadening of the spectrum. In
addition, perturbation of the refractive index of the activated region is produced by changes in injection current andtemperature, as a result of which the effective change incavity length is manifested as a change in peak wavelength of the laser.
If a fiber grating having a reflectivity of several percent
is arranged outside the pumping diode, lasing will occur
using the grating as an external cavity. In this case, lasing
will be due primarily to the fiber grating, and the peakwavelength will accord almost exactly with the center
wavelength of the fiber grating.
Since the temperature-dependent variation in the center
wavelength of the fiber grating is extremely small relative
to the variation in the peak wavelength referred to above,
the peak wavelength of a laser with a fiber grating will bestable with respect to temperature. It is also possible toobtain a narrower laser spectrum by reducing the bandwidth of the fiber grating.

200

400

600

800

Current (mA)

Figure 15 Current-dependence of peak wavelength of laser
diodes with and without fiber grating

Figure 12 shows the structure of a pumping laser diodewith a fiber grating. Due to the lenses at the antireflective
coating side of the diode, part of the light coupled to thefiber is reflected by the fiber grating forming an external
cavity with the highly reflective facet of the diode.
The fiber grating is produced by UV irradiation of a fiberthat has been treated with high-pressure hydrogen using a
KrF excimer laser. The area on which the grating is written
is protected by a hard package or by recoating. If a package is used, careful attention to the structure and materials used can result in gratings of even higher temperaturestability than those that are recoated.
Figure 13 shows the output spectrum of a 1.48-µm
pumping laser diode with a fiber grating (solid line) andwithout a grating (dotted line). Whereas the diode without
the grating has a broader output spectrum and a lower
peak level, it can be seen that in the diode provided with agrating, output power is pulled in to the grating making fora narrower band and higher output power.
Figure 14 shows the diode temperature dependence ofthe peak wavelength of a 1.48-µm pumping laser diodewith a fiber grating. Whereas for the diode without thegrating the peak wavelength shifts to the longer side as
diode temperature rises, there is virtually no influence oftemperature on the diode with a grating.
Figure 15 shows the change in peak wavelength due toinjection current. Here too it can be seen that in the diodewith a grating, peak wavelength remains stable.

39

5.

CONCLUSION

The results of a program to develop fiber gratings for
OADM applications and for stabilizing the wavelength ofpumping lasers in WDM systems has been reported.
The target wavelength characteristics for OADM gratings were achieved by optimization and apodization of thefiber. The use of temperature-compensated packages
made it possible to develop optical fiber gratings for use incommercial OADM systems. We have also commercialized an extremely compact OADM module, combining acirculator and fiber grating.
By incorporating a fiber grating in a pumping laser
diode, its effectiveness was confirmed, and pumping
lasers with fiber grating have been produced on a commercial basis.

40

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7) E.Delevaque et al.: OFC'95 PD5, 1995
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Manuscript received on June 24, 1999.

Furukawa Review, No. 19. 2000