Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
1999-03-09
2001-05-01
Kim, Robert H. (Department: 2877)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S102000, C372S006000, C372S096000, C372S092000, C372S097000, C372S049010
Reexamination Certificate
active
06226311
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to sources for optical telecommunication systems and, more specifically, to a laser with an external cavity made by an optical fiber Bragg grating. Such devices are known in the art as HDBR (Hybrid Distributed Bragg Reflector) lasers or Fiber Grating (FG) lasers.
Such devices generally comprise a Fabry-Perot semiconductor active element (laser diode) with a terminal facet bearing an anti-reflection coating, coupled to a grating written on a length of optical fiber tapered at one end and positioned with the tapered end adjacent to the facet bearing the anti-reflection coating. As described in the literature, lasers of this kind are particularly well suited for use as sources of pump radiation, single mode sources for optical telecommunication systems, in particular wavelength division multiplexing systems, mode locked lasers for the generation of pulses within a wide frequency range, etc.
BACKGROUND OF THE INVENTION
For a review of the applications of such lasers, reference can be made for instance to the papers “Lightwave Applications for Fiber Bragg Gratings”, C. R. Giles, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, pages 1391 et seq., and “Fiber Gratings in Lasers and Amplifiers”, by J. Archambault and S. G. Grubb, ibid. pages 1379 et seq.
It is well known that some of the characteristics of the aforesaid lasers are linked to the overall length of the cavity, which in a hybrid laser is given by the sum of the length of the cavity of the active element, of the distance between the anti-reflection coated facet and the tip of the tapered fiber end and lastly of the length of the fiber portion between the tip and the equivalent mirror plane of the grating. The equivalent mirror plane, as is well known, is the plane wherein 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. In particular, the shorter the cavity of the laser, the greater the modulation band obtainable and the better the mode separation. It is evident that the attainment of good characteristics in terms of modulation band and mode separation is of particular interest for the use of lasers as sources for telecommunication systems.
The conventional low-reflecting Bragg gratings (with output reflectivity of the order of 70% currently used to form the external cavity of hybrid lasers have a profile of modulation of the refractive index that is symmetrical with respect to the central point in the grating, thus giving rise to an equivalent mirror plane positioned substantially at the center of the grating. On the other hand highly reflective gratings—with substantially 100% reflectivity—cannot be used, for the external cavity of the laser even if they would in themselves have an equivalent mirror plane offset towards one end because they would not allow sufficient power in the fiber.
The gratings used for these applications have a length of the order of a centimeter and thus the length of the external cavity constitutes nearly the entirety of the length of the whole cavity, since the active element has a cavity length of the order of 200 &mgr;m. The use of conventional gratings may then give rise to a cavity length that is not sufficiently limited to obtain satisfactory characteristics for the laser. One could think of reducing the drawback by writing the grating in the end portion of the fiber, but this gives rise to additional problems when the fiber is fastened by means of resins onto the support of the module. It is evident that one of the fastening points must be in correspondence with the end portion, to guarantee the constant alignment between the active element and the fiber, and under such conditions the resin interacts with the grating. Experience has shown that the resin, upon curing, causes alternations in the structure of the grating, thus rendering the solution unfeasible.
SUMMARY OF THE INVENTION
These problems are solved by the laser according to the invention, where the external cavity makes use of a fiber grating with such a profile of modulation of the refractive index as to give a reduced equivalent grating length while maintaining a low reflectivity. In this way the overall length of the cavity is sufficiently limited.
A laser module is provided comprising a Fabry-Perot cavity active element with a facet treated with an anti-reflection coating and an external cavity made by a low reflectivity optical fiber grating. This grating presents a profile of modulation of the refractive index that is non-uniform and asymmetrical in the direction of the length, and is such as to give rise to a position of the equivalent mirror plane that is offset towards one end of the grating, and the grating is mounted in such a way that this latter end is the grating end closest to the laser. In a preferred embodiment, this profile of modulation is represented by a curve which has minimum and substantially null value, with substantially horizontal tangent, in correspondence with the grating end that is farther away from the laser, and then rises gradually and monotonically to a maximum value, also with substantially horizontal tangent, which is reached in correspondence with the other end of the grating, where the curve returns to the minimum value with a substantially vertical slope.
By wave of a nonlimiting example, the profile of modulation of the refractive index, in the portion with gradual variation, can have a trend represented by one of the following functions: y=exp(−x
2
), y=sin
2
x, y=tanh x.
REFERENCES:
patent: 5845030 (1998-12-01), Sasaki et al.
patent: 5870417 (1999-02-01), Verdiell et al.
patent: 5905745 (1999-05-01), Grubb et al.
Meliga Marina
Paoletti Roberto
Scofet Marco
Tallone Luigi
Agilent Technologie,s Inc.
Dubno Herbert
Flores Ruiz Delma R
Kim Robert H.
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