Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-03-09
2001-03-13
Palmer, Phan T.H. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S028000, C385S123000, C385S124000
Reexamination Certificate
active
06201910
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical fiber components for optical telecommunication systems, and more specifically to a fiber Bragg grating with offset equivalent mirror plane and to a method of manufacturing such gratings.
BACKGROUND OF THE INVENTION
The use of fiber Bragg gratings in components for optical telecommunication systems such as lasers, amplifiers, filters, add-drop multiplexers, wavelength multiplexers/demultiplexers, etc. has been known for some time. A review of the use of fiber Bragg gratings as components of optical telecommunication systems is found for instance in the papers “Lightwave Applications of Fiber Bragg Gratings”, by C. R. Giles, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, pp. 1391 et seq., and “Fiber Gratings in Lasers and Amplifiers”, by J. Archambault and S. G. Grubb, ibid., pp. 1379 et seq.
In particular, in applications in wavelength division multiplexing systems it is necessary to have devices capable of separating the various channels. For this purpose it is possible to use gratings of which the reflection spectrum presents a peak that is, insofar as possible, narrow and free of side lobes.
When fiber Bragg gratings are used to make one or both the reflecting elements that delimit a resonant cavity of a component, e.g. a Fabry-Perot cavity laser, to be used in such systems, one encounters problems linked to the cavity length. This length depends, as is well known, on the position of the so-called equivalent mirror plane of the grating. The equivalent mirror plane is the plane where a mirror would have to be positioned in order that a pulse sent by a source and reflected by the mirror returns to the source in the same time the pulse sent into the grating would take to return. The distance between the equivalent mirror plane and the input end of the grating constitutes the equivalent length of the grating. The length of a resonant cavity that makes use of fiber Bragg gratings is therefore represented by the distance between the equivalent mirror plane of the grating and the other reflecting element of the cavity (if only one such element is made by a grating) or between the equivalent mirror planes of the two gratings (if both reflecting elements are made by gratings). Now, if the linewidth of the laser is to be kept limited, the length of the cavity cannot be shorter than a certain minimum length, which is determined by manufacturing requirements; on the other hand, the longer the cavity, the shorter the distance between the modes and hence the harder the separation between the different modes.
The gratings proposed until now have a modulation of the refractive index which, as a function of the length of the grating, presents a symmetrical profile with respect to the central point of the grating. In these symmetrical gratings the equivalent mirror plane is placed substantially at the center of the grating, if the latter is a low-reflecting grating, and is located in a more advanced position towards one end if the grating is a highly reflecting grating. “Low-reflecting” indicates a value of reflectivity such that, when the grating is used as the reflecting element of the cavity, the radiation fraction exiting the cavity is sufficient for practical uses (typically, a reflectivity of the order of 70% in a laser); “highly reflecting” indicates a reflectivity of practically 100% or very close to this value. A highly reflecting grating could be used as one of the reflecting elements of the cavity, thereby reducing its length, provided the other reflecting element presents a sufficiently high transmission factor. In the case of a cavity with only one reflecting element made by a grating, the latter is positioned in correspondence with the output end and the use of a highly reflecting grating under such conditions is clearly inconceivable. In the case of a cavity where both reflecting elements are made by gratings (in the example, the cavity of an all-fiber laser), the use of a highly reflective grating does not solve the problem of obtaining a narrow band with a very reduced length of the cavity, both because the spectral line of those gratings is in any case relatively wide, and because one of the gratings should be a low-reflecting grating and hence would present a high equivalent length.
SUMMARY OF THE INVENTION
The aforesaid problems are solved by the grating according to the present invention, which presents both a narrow reflectivity spectrum, free of secondary lobes, and a reduced equivalent length.
More specifically, a grating is provided that presents a non uniform, asymmetrical profile of modulation of the refractive index in the direction of the length, which profile is represented by a curve that has a minimum and substantially zero value, with substantially horizontal tangent, in correspondence with one end of the grating, and rises gradually and monotonically until a maximum value, also with a substantially horizontal tangent, is reached in correspondence with the other end of the grating, where the curve returns to the minimum value with substantially vertical slope.
Preferably such a curve has a trend represented by one of the following functions:
y
=exp (−
x
2
) (i.e. a Gaussian function),
y=
sin
2
x,
y=
tanh
x.
An asymmetrical modulation profile like the one provided according to the invention effectively guarantees that the equivalent mirror plane is moved forward, in proximity with the maximum of the modulation profile of the refractive index, as is readily apparent when applying the description provided in L. A. Coldren, S. W. Corzine: “Diode Lasers and Photonic Integrated Circuits”, Wiley & Sons, 1995. In a practical embodiment of the invention, in a grating with length of about 1 cm and reflectivity of the order of 70%, with a half-Gaussian modulation profile, the equivalent mirror plane was positioned about 2.5 mm from the end closer to the modulation maximum; by way of comparison, a conventional grating of the same length and similar reflectivity, with symmetrical Gaussian profile of the refractive index modulation, would have an equivalent length of the order of 5 mm, thus substantially double.
A grating such as the one described can therefore be advantageously employed with a resonant cavity, to form one or both the reflecting elements that delimit the cavity. Moreover, tests carried out have demonstrated that there are no secondary peaks and that the reflection band is narrow.
To make a grating such as the one described, the conventional techniques for writing gratings into optical fibers are used. A review of such techniques can be found in the paper “Fiber Bragg Grating Technology Fundamentals and Overview”, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, pp. 1263 et seq. According to the invention, in order to obtain the refractive index modulation described above when writing the grating by using a phase mask, the diaphragm used to generate the intensity distribution of the writing radiation on the phase mask must be such as to create an asymmetrical distribution, corresponding to the desired profile of the refractive index modulation. Hence the diaphragm will be such as to intercept half the beam and to create, with reference to the exemplary functions mentioned above, a distribution corresponding to the part included between the minimum and the maximum of a Gaussian curve or of a curve of the type sin
2
x, tanh x, etc.
REFERENCES:
patent: 5066133 (1991-11-01), Brienza
patent: 0 805 365 (1997-11-01), None
patent: 03246510 (1991-11-01), None
Fiber Gratings in Lasers and Amplifiers, Jean-Luc Archambault and Stephen G. Grubb, 1997, pp. 1378-1390.
Fiber Bragg Grating Technology Fundamentals and Overview, Kenneth O. Hill and Gerald Meltz, Aug. '97, pp. 1263-1276.
Lightwave Applicatiosn of Fiber Bragg Gratings, C. R. Giles, pp. 1391 to 1404, Aug. 8, 1997.
On the Use of Tapered Linearly Chirped Gratings . . . , Javier Martu et al, Feb. 1997, pp. 179 to 187.
Asymmetrically Apodised Linearly-Chirped Fibre Bragg . . . , XP-00210788
Boschis Laura
Gnazzo Angelantonio
Rossotto Oriana
Tallone Luigi
Dubno Herbert
OTC-Optical Technologies Center S.r.l.
Palmer Phan T.H.
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