Optical waveguides – Accessories – Splice box and surplus fiber storage/trays/organizers/ carriers
Reexamination Certificate
1999-02-02
2003-04-08
Chang, Audrey (Department: 2872)
Optical waveguides
Accessories
Splice box and surplus fiber storage/trays/organizers/ carriers
C385S123000, C385S105000, C242S614000, C057S204000
Reexamination Certificate
active
06546180
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a coiled optical assembly made of a long optical fiber and a fabricating method for the same; and, in particular, to a dispersion compensator for reducing the wavelength dispersion of an optical fiber transmission line in the wavelength band of 1.55 &mgr;m, and a fabricating method for the same.
2. Related Background Art
Long-distance, large-capacity transmission is possible in the wavelength band of 1.55 &mgr;m by means of an optical amplifier operable in the wavelength band of 1.55 &mgr;m utilizing an optical fiber doped with erbium (Er) which is a rare-earth element. However, when transmission is carried out in the wavelength band of 1.55 &mgr;m by using a single-mode optical fiber whose zero-dispersion wavelength is in the 1.3-&mgr;m band (1.3 SMF), which is employed in conventional optical transmission lines, then a large wavelength dispersion occurs and distorts optical signals, there by deteriorating the signal quality. As a consequence, when carrying out transmission in the 1.55-&mgr;m band with the use of 1.3 SMF, its wavelength dispersion is required to be kept low. Known as one of techniques therefor is a method using a dispersion-compensating optical fiber (DCF) having a large wavelength dispersion with a polarity opposite to that of the dispersion of 1.3 SMF, so as to cancel the wavelength dispersion in the 1.55-&mgr;m band.
In a technique employed for carrying out such dispersion compensation with DCF in a conventional long-distance transmission line, a dispersion compensator having a compact size in which a long DCF is wound around a bobbin is installed at every repeater station.
SUMMARY OF THE INVENTION
Nevertheless, since a wide-band DCF has a large bending loss in general, it may yield a large transmission loss in the 1.55-&mgr;m band, i.e., transmission wavelength band, when formed into a coil having a small diameter. This bending loss can be reduced when the coil has a larger diameter so that its number of turns is reduced. Increasing the coil diameter, however, is unfavorable in that the dispersion compensator accordingly becomes larger.
Also, the dispersion compensator is often used together with an optical amplifier using an erbium-doped optical fiber. In this case, the temperature of the dispersion compensator increases due to the heat from a pumping laser within the optical amplifier, so that the bobbin may thermally expand. As a result, distortions may occur in the wound wide-band DCF, thereby increasing the transmission loss. Using a material with less thermal expansion in the bobbin can reduce the transmission loss in such a high-temperature environment. However, materials having low coefficients of thermal expansion, such as silica glass, ceramics, special alloys, and the like, are hard to process or expensive.
Therefore, it is an object of the present invention to provide an optical assembly accommodating therein a long optical fiber in a compact fashion, which yields less increase in transmission loss upon such bending or heat; and a method of making the same.
In order to achieve the above-mentioned object, the inventors carried out various studies using the wide-band DCF, results of which will be discussed here.
FIG. 1
is a sectional view of a DCF constituting an optical fiber coil studied. As shown in
FIG. 1
, the DCF employed has an optical fiber
11
made of glass and two coating layers
13
,
15
, each made of a resin, formed around the optical fiber
11
.
FIG. 2
shows the refractive index profile of this DCF. The glass portion
11
is a double-cladding type DCF whose core portion has a diameter a of 2.65 &mgr;m, depressed cladding portion has a diameter b of 7.58 &mgr;m, and outside diameter c is 100 &mgr;m. The primary coating layer
13
has a thickness d of 20 &mgr;m. The secondary coating layer
15
has a thickness e of 20 &mgr;m. The outside diameter f of the fiber is 180 &mgr;m. The relative refractive index differences &Dgr;+, &Dgr;− of the core portion and depressed cladding portion with respect to the refractive index of the outer cladding portion were set to 2.1% and −0.4%, respectively. At 20° C., the Young's modulus of the primary coating layer
13
was 0.06 kgf/mm
2
, and that of the secondary coating layer
15
was 65 kgf/mm
2
. The wavelength dispersion and wavelength dispersion slope of this DCF were −100 ps
m/km and −0.29 ps
m
2
/km at the wavelength of 1.55 &mgr;m, respectively, whereas its transmission loss was 0.40 dB/km.
FIG. 3
is a perspective view of a take-up bobbin
2
used for producing the optical fiber coil. Around the bobbin
2
made of aluminum having a body portion
20
with a diameter g of 100 mm, flanges
21
with a diameter of 200 mm, and a winding width k of 18 mm, the above-mentioned DCF having a fiber length of 10 km was wound at a winding pitch of 0.4 mm with a take-up tension of 40 gf, so as to produce the optical fiber coil.
The respective transmission characteristics of thus obtained optical fiber coil (type
1
) wound around the bobbin, the optical fiber coil (type
2
) loosened into a bundle form after being removed from the bobbin, and the optical fiber coil (type
3
) obtained after that of type
1
had been subjected to a predetermined heat treatment were measured and compared with each other.
FIG. 4
shows the heat cycle of the heat treatment applied to the optical fiber coil of type
3
. In this heat treatment, the optical fiber coil of type
1
was left for 1 hour at a temperature of 20° C., subsequently the temperature was raised at a rate of 1° C./minute until it reached 80° C., at which the optical fiber.coil was left for 1 hour, and then the temperature was lowered at a rate of 1° C./minute until it reached −40° C., at which the optical fiber coil was left for 1 hour. After this cycle was repeated once again, the optical fiber coil was finally maintained at 20° C. and left for 2 hours.
FIG. 5
is a graph comparing, at each wavelength, the transmission loss values of the optical fiber coils of types
1
and
2
with those of the optical fiber before being wound up. In the optical fiber coil of type
1
, a large transmission loss (1.7 dB at 1.55 &mgr;m) occurred in the 1.55-&mgr;m band, i.e., transmission wavelength band, and the transmission loss became greater as the wavelength was longer. It is due to the microbend loss occurring when the optical fiber is bent with a small curvature. By contrast, this microbend loss substantially disappeared from the optical fiber coil of type
2
. From these facts, the inventors have found that the transmission loss generated upon the winding of a coil is mainly caused by distortions in winding due to a multiplex winding, e.g., lateral pressures applied to each fiber from its adjacent fibers, which cause the optical fiber to bend, thereby generating a microbend loss upon coil winding. Hence, the inventors have concluded that an optical fiber coil having a low transmission loss can be produced if these lateral pressures are eliminated.
When the optical fiber coil of type
2
was heated to 70° C. and then its transmission loss was measured at a wavelength of 1.55 &mgr;m, the measured value was greater than that at 20° C. by 0.06 dB. This minute amount of change in transmission loss value is on a par with the value of 1.3 SMF reported in a literature (Tanaka et al., “TEMPERATURE DEPENDENCE OF INTRINSIC TRANSMISSION LOSS FOR HIGH SILICA FIBER,” European Conference on Optical Communication, pp. 193-196, 1987). Therefore, this transmission loss is considered to be the temperature-dependent loss inherent in the optical fiber, which is a value irrelevant to the lateral pressures.
FIG. 6
is a graph comparing, at each wavelength, the transmission loss values of the optical fiber coils of types
1
and
3
with those of the optical fiber before being wound up. In the optical fiber coil of type
3
, the amount of change in loss was improved over that of the optical fiber coil of type
1
, whereby its amount of change in transmission
Kobayashi Kohei
Koyano Yasushi
Onishi Masashi
Sasaoka Eisuke
Tanaka Shigeru
Chang Audrey
Curtis Craig
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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