The Optical Communications Reference

type 2 grating. The grating then remains unchanged for

a longer irradiation. The energy of the irradiation beam is

about 6 mJ. The spectrum of the final grating is shown in

Fig. 3.2.27. The stop band is less than 0.5 nm, and the

extinction ratio is 28 dB.

The unique feature of the POF grating is its large

thermal and mec hanical tuning ranges. Fig. 3.2.28a shows

that a change of temperature over 508 C shifts the Bragg

wavelength by about 8 nm, and Fig. 3.2.28b shows

a mechanical strain can tune the wavelength by 10 nm

[15]. More importantly, these tunings are linear, and no

hysteresis is observed. These features can be utilized for

constructing very sensitive sensing devices, such as cur-

rent sensors and acousti c sensors.

3.2.7 Segmented cladding POF

Photonic crystal fibers (PCFs; also called holey fibers)

have received intense study recently (see Chapter 3).

These fibers have been made in both silica and polymer

form [16]. The common feature of these fibers is that

longitudinal holes are distributed either randomly or

regularly over the cross-section of the fiber. One of the

distinguishing characteristics of holey fiber is its large

wavelength range fo r single-mode operation. This is based

on the principle of differential leakage of the higher order

modes in the fiber. As long as the effective index of the

cladding is less than the mode index of the fundamental

mode and larger than the mode indices of all higher order

modes, all the higher order modes will eventually leak

away at different rates provided the fiber is long enough,

leaving the single mode in the core. Based on this prin-

ciple, it is possible to design new fibers without holes that

also possess large wavelength range single-mode opera-

tion. An interesting design is the segmented cladding fiber

shown in Fig. 3.2.29 [17]. In this fiber, the core material

fans out into the cladding in the form of spokes, thus

segmenting the cladding. A polymer version of this fiber

has been fabricated with four segments, as shown in

Fig. 3.2.30 [18]. Its core has a diameter of 20 mm, and the

overall fiber diameter is 200 mm.

0

0.2

0.4

0.6

0.8

Refractive index modulation (1e-3)

1

Type II Grating

Type I Grating

0 1020304050607080

Time t(min)

90 100 110 120

Fig. 3.2.26 Build-up of Bragg gratings in single-mode POF.

UV beam

phase mask

POF under UV

exposure

silica optical fiber

beam dumper

silica prism

ASE source

OSA

beam

dumpe

r

Fig. 3.2.25 Writing of Bragg gratings in single-mode POF. ASE,

amplified spontaneous emission.

−35

−30

−25

−20

−15

−10

−5

0

5

1569 1571 1573 1575 1577

Wavelength (nm)

Transmission (dB)

Fig. 3.2.27 Spectrum of Bragg grating in single-mode POF.

121

Polymer optical fibers CHAPTER 3.2