Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-07-14
2002-12-31
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S018000
Reexamination Certificate
active
06501879
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical coupler with a new geometry and a new principle of action. The optical coupler provides spectrally selective coupling of light to or from a waveguide, such as an optical fibre.
TECHNICAL BACKGROUND
Spectrally selective optical couplers, also known as channel drop filters, are utilised for extraction of a single wavelength channel from a broadband optical signal, or for insertion of a single wavelength channel into a broadband optical signal. Typically, spectrally selective couplers are used in wavelength division multiplexed optical communications systems for adding and dropping a single wavelength channel.
Channel drop filters have previously been implemented as dual-waveguide couplers. The article “Narrow-Band Optical Channel-Dropping Filter”, Journal of Lightwave Technology, vol. 10, no. 1, January 1992 (Haus et al.) describes an optical channel-dropping filter comprising a first and a second waveguide, the first of which contains a &lgr;/4 shifted distributed feedback (DFB) resonator. Light propagating in the second waveguide is coupled to the first waveguide by evanescent coupling between the two waveguides. Only one wavelength of light is resonant in the first waveguide, and consequently only that wavelength of light is efficiently coupled to the first waveguide. By making the &lgr;/4 shifted DFB resonator asymmetric (i.e. the grating is longer on one side of the &lgr;/4 shift), light can be coupled out of the DFB resonator.
However, prior art channel drop filters have some significant drawbacks and limitations. The filters are difficult to manufacture, due to the fact that very precise placement of the waveguides is required, in order to obtain a reliable evanescent coupling. Furthermore, the prior art filters are difficult to control. The coupling strength and the coupled wavelength is, to a large extent, fixed once the device is assembled. Also, each filter needs to be of a certain size in order to achieve the necessary feedback. In particular, when a number of channels are to be dropped separately (e.g. when constructing a demultiplexer), the device needs to be quite large. Yet another problem with the prior art filters is that they are difficult to implement in a fibre configuration, since the evanescent coupling between the waveguides needs to be very accurate. Any perturbation of either of the waveguides can cause large uncontrolled changes in performance.
SUMMARY OF THE INVENTION
The present invention provides an optical coupler with a new geometry and a new principle of action, which eliminates, or at least alleviates, the aforementioned problems in the prior art.
An optical coupler according to the present invention comprises an optical waveguide, preferably an optical fibre, in which there is provided a deflector for deflecting at least some of the light, propagating in said waveguide, into an external resonator. By external resonator, it is meant that the resonator is defined by mirrors that are arranged outside the optical waveguide. The external resonator is arranged to be resonant to a predetermined wavelength. The resonance of the predetermined wavelength in the external resonator causes an energy build-up of said wavelength in said resonator. In effect, the coupling of the resonant wavelength is rendered much stronger than what is obtained by the deflector alone. Owing to this, the deflector can be made very weak, deflecting only a small fraction of the light in the waveguide. The resonant wavelength can, in a very advantageous way, be coupled out of the external resonator. Light of a wavelength that is not resonant in the external resonator is essentially unperturbed by the coupling, since only a very small fraction of the light in the waveguide is deflected by the deflector.
A general insight, forming a basis for the present invention, is that coupling of light is much more efficient and spectrally selective when performed resonantly. This fact is utilised in the present invention by arranging an external resonator outside an optical waveguide. In the waveguide, there is arranged a deflector, deflecting light into the external resonator. Consequently, light being resonant in the external resonator is more strongly coupled (by the deflector) between the waveguide and the resonator.
Hence, a very weak coupling factor can be used if the coupling is made in connection with a resonance to the wavelength at issue for coupling. Wavelengths not exhibiting resonance in the coupling region are essentially unperturbed, due to the very weak coupling factor for non-resonant wavelengths.
In one aspect, the present invention provides a spectrally selective optical coupler, comprising a waveguide with a deflector. The deflector is operative to deflect light from the light guiding structure of the waveguide into an external resonator, which external resonator is defined by two mirrors provided on opposite sides and outside of said light guiding structure. Light can be coupled out of the external resonator by either of the mirrors having a slightly reduced reflectivity, or by either of the mirrors being provided with an aperture of strongly reduced reflectivity (as compared to the reflectivity of the same mirror off said aperture).
In a preferred embodiment, the deflector comprises a periodic refractive index modulation in the light guiding structure of the waveguide. In the most preferred embodiment, this periodic refractive index modulation is a tilted optical Bragg grating. The tilted optical Bragg grating forms, effectively, a deflector comprising a cascaded series of very weak reflectors, each of which couples light into a single resonant mode in the external resonator.
In another aspect, the present invention provides a spectrally selective optical coupler that is tuneable, thus allowing coupling of different wavelengths at different instants by tuning said coupler. This is obtained by at least one of the mirrors defining the external resonator being adjustable, as to distance between the mirrors or to resonator angle with respect to the light guiding structure of the waveguide. By tilting the external resonator with respect to the light guiding structure, the inventive optical coupler can also be made to pass all wavelengths.
In yet another aspect, the present invention provides optical couplers with external resonators, in which the propagation direction of light is essentially perpendicular with respect to the light guiding structure of the waveguide. Thus, the spectrally selective optical couplers according to the present invention can easily be cascaded, in order to achieve coupling of different wavelengths at different positions. This is obtained by arranging a plurality of optical couplers in series, each coupler of said plurality of optical couplers being resonator to a different wavelength.
It will be appreciated by those skilled in the art that the features of the present invention governing the coupling of light out of an optical waveguide are equally applicable to the coupling of light into an optical waveguide, since the latter is merely the time reversal of the former.
According to the invention, the deflector that is used to deflect light into the external waveguide can be wavelength discriminating. However, the spectral selectivity of the deflector need not be very high, since spectral selectivity is mainly achieved by means of the external resonator. Nevertheless, in some cases, it might be desirable to have a degree of spectral selectivity in the deflector. This is easily achieved by using a deflector comprising a tilted or a transversally asymmetric Bragg grating, which is adapted for deflection of a predetermined wavelength in a predetermined direction. Generally, the deflected wavelength and its deflection angle are determined by the period of the Bragg grating.
In a transversally asymmetric Bragg grating, the amplitude (modulation depth) is lower at one edge of the grating (radially) than at the opposite edge. This means that when light is reflected against the grating it will have
Asseh Adel
Bergman Mikael
Henriksson Anders
Sahlgren Bengt
Sandgren Simon
Birch & Stewart Kolasch & Birch, LLP
Proximon Fiber Optics AB
Rojas Omar
Sanghavi Hemang
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