Patent Application
20050271336
The present invention generally relates to optical devices for telecommunications or other applications, and more particularly concerns a material composition for coupling two optical components together, the resulting device and a method for making this device.
Light injection into optical waveguide, such as an optical fiber or planar waveguide, is widely used in many photonic applications ranging from medicine to telecommunications. The origin of the light to be injected may be, for example, a semiconductor chip or another waveguide. Very often, the difference of modal compositions or numerical apertures of two elements to be coupled makes it difficult to efficiently couple the light radiation, thus resulting into significant optical losses.
Traditional methods of coupling light into an optical fiber use micro-lens systems positioned between a light source or an input waveguide and the fiber. Typical examples of these types of system are shown in U.S. Pat. No. 5,215,489, entitled METHOD OF MAKING AN OPTICAL SEMICONDUCTOR MODULE, which issued on Jun. 1, 1993 to Nakamura; in Canadian patent No. 2,159,136, entitled OPTICAL FIBER ASSEMBLY, which issued on Jul. 3, 2001 to Takahashi; and in Canadian patent No 1,113,762, entitled OPTICAL COUPLER FOR CONNECTING A LIGHT SOURCE TO AN OPTICAL TRANSMISSION LINE, which issued on Dec. 8, 1981 to Balliet. Generally, light emerging from the light-source component passes through a micro-lens system and crosses several optical surfaces between media with different refractive indices, which introduces undesirable reflection losses. In addition, the required micro-optical elements, and the need for fine micro positioning of these elements, significantly decrease the performance and reliability of the device and increase its cost, particularly for small diameter components such as single mode fibers, etc.
A method for manufacturing a lens at the end of an optical fiber was described in U.S. Pat. No. 6,415,087, entitled POLISHED FUSED OPTICAL FIBER END FACE which issued on Jul. 2, 2002, to Yang et al. This technique creates a hyperbolic end face surface with an intermediate frustum region that is preferably polished prior to fusing tip. This simplifies the assembly process; however, the complexity of its fabrication limits its practical use.
A more interesting technique for coupling together two dissimilar optical elements uses the principle of self-focusing of light travelling in photopolymerizable materials. This well-known phenomenon was discovered by Askaryan in the early 1960's. The photosensitive polymerizing properties of such a material are used to create a light-guiding channel therein upon illumination of light with Gaussian form transfers intensity distribution. The refractive index n of the cured region being higher than that of the non-cured region, the light itself builds a gradient index lens and gets self-collimated along its propagation direction, thus creating a self-written channel of optical guiding.
The various curves of
These self-collimating phenomena have been widely used by many authors for mutual light trapping, including the case of optical beams emerging from two opposed optical fibers. The use of self-created polymer elements at the end of glass fibers, such as focusing and collimating tip, was described in an article entitled INTEGRATION OF POLYMER ELEMENTS AT THE END OF OPTICAL FIBERS BY FREE-RADICAL PHOTOPOLYMERIZATION, published in Synthetic metals, 124 (2001) 29-31, by Ecoffet et al. Frisken also described the use of photopolymerizable materials to create fiber uptapers in an article entitled LIGHT-INDUCED OPTICAL WAVEGUIDE UPTAPERS, Optics Letters, Vol. 18, No.13, pp. 1035-1037 (1993). In this work a UV curable epoxy is used between the two optical components to be coupled.
In another example, the article entitled QUASI-SOLITONIC BEHAVIOR OF SELF-WRITTEN WAVEGUIDES CREATED BY PHOTOPOLYMERIZATION, published in Optics Letters, Vol. 27, No.20, p. 1782, describes the experimental coupling of two optical fibers that are separated up to 1 cm distance.
Such solitonic couplers could be very useful for connecting two waveguides, which have significant difference of mode field diameters since these couplers can play the role of a tapered transition zone with adiabatic mode converting capability. This approach is even more important when two fibers made from quite different materials (for example silica and chalcogenide glasses) must be connected. In this case, the melting temperatures being very different for those two materials, traditional thermal splicing devices cannot be used to connect them and the solitonic coupler could be the best solution. Finally, this approach is much more tolerant on the spatial and angular positioning of fibers, which can significantly increase the efficiency of coupling.
However, several important problems remain to be solved in this approach. One of the most important problems is related to the fact that the light curing must be spatially non-uniform to create the gradient-index guiding channel. Following the initial intensity distribution of the polymerizing beam, the guiding channel can possess, for example, a Gaussian transverse distribution of the refractive index of the cured material, therefore high in the center and low at the borders. Such a structure can guide light with specific modal and dispersion characteristics. However, it is then absolutely necessary to preserve this spatially non-uniform refractive index distribution as stable as possible in time. Unfortunately, it is well known that the chemical reaction of polymerization will evolve in the rest of the material, and that it must be fixed somehow to avoid the channel erasure. A uniform photoexposition may polymerize the rest of the material, but it will significantly reduce the modulation depth of the refractive index n, such as illustrated by profile 14′ on
A second problem to be solved is related to the fact that the light curable materials commercially available up to today are traditionally sensitized to the UV or visible light only. Significant progress was reported in U.S. Pat. No. 6,398,981, entitled PHOTOPOLYMERIZABLE COMPOSITION SENSITIVE TO LIGHT IN A GREEN TO INFRARED REGION OF THE OPTICAL SPECTRUM, which issued on Jun. 4, 2002, to Galstian et al. It should however be noted that even for this composition, sensitivity is almost negligible (for one photon excitation) beyond 900 nm. At the same time, the single mode fibers traditionally used in the telecommunication industry are multi-mode at wavelengths below 900 nm (they have cut-off wavelengths above 1300 nm). This means that the photopolymerizing light, with wavelength below 900 nm, may be guided in these fibers in multiple modes. The intensity distribution of the output fiber thus will not necessarily have a single maximum, such as for example a Gaussian form, but will rather possess multiple maxima and minima of intensity transversally to the propagation direction. This will generate multiple channels of self-trapping or filamentations, and a corresponding degradation of the coupling efficiency.
Accordingly, there is still a need for a simple and practical method for optically connecting a light between two optical components, such as a light emitting device and a waveguide structure that is both efficient and stable.
In accordance with a first aspect of the present invention, there is provided a material composition for creating a light guiding channel between first and second optical components. The material composition includes a first material system having polymerizing properties responsive to light of a wavelength within a pre-determined wavelength range. Polymerization of this first material system creates the light guiding channel. A second material system is also provided. The second material system is miscible with the first material system prior to polymerization thereof, but substantially non-miscible with this first material system after polymerization thereof. The second material system has a refractive index smaller than the refractive index of the first material system after polymerization thereof.
In accordance with another aspect of the present invention, there is also provided a method for creating a light guiding channel between first and second optical components. This method includes the following steps:
In accordance with yet another aspect of the present invention, there is also provided an optical device including first and a second optical components in close proximity and in alignment with each other, and an optical coupler disposed between the first and second optical components. The coupler is made of a material composition including a first material system having polymerizing properties responsive to light of a wavelength within a pre-determined wavelength range, and a second material system. The second material system is miscible with the first material system prior to polymerization thereof, and substantially non-miscible with the first material system after polymerization thereof. The second material system has a refractive index smaller than the refractive index of the first material system after polymerization thereof. The coupler has a light guiding channel therethrough created by polymerization of the first material system therealong.
Advantageously, the present invention provides a specific liquid material solution containing a reactive, that is, an optically curable material and a low refractive index material that is miscible with the initial liquid solution, while being diffused (and repulsed) from the cured region during the curing process. The curing regime of the whole material system and mobility of the non-reactive material should be specifically chosen to allow them the spatial diffusion, and therefore the repulsion from cured regions. The passive (non-reactive) character of this material is preferably chosen in a way to prevent the polymerization propagation across the regions filled by this material. The waveguide created by light curing then adopt a specific “W-shaped” transverse profile of refractive index and is very stable over time.
Other features and advantages of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.
The present invention provides a material composition, a method and an apparatus for the efficient and stable coupling radiation between two optical components, particularly for components of dissimilar natures of light transmitting characteristics. For example, the present invention may be used for coupling a light-emitting source, such as vertical cavity surface emitting lasers, traditional edge emitting diode lasers, any other appropriate laser diode or an optical waveguide, to another optical waveguide such as an optical fiber or planar waveguide.
With reference to
The first material system 22 has polymerizing properties which are photosensitive, that is, that they are responsive to light in a predetermined wavelength range, preferably in the UV-NIR spectra. In the preferred embodiment, the first material system 22 is a complex material system including photosensitive and polymerizing compounds. The second material system 24 may be polymerizable or not. As mentioned above, the first and second material systems are selected so that they are well miscible before the polymerization of the first material system. A second requirement for this selection is the non miscibility, or at least strongly reduced miscibility of the polymer 22′ formed by the polymerization of the first material system 22, with the second material system 24. This means that a repulsion of the compounds of the second material system 24 from the polymerized zones 22′ of the first material system is obtained. This phenomenon is illustrated in
Another characteristic of the material solution of the invention is that the second material system 24 has a lower refractive index than the polymerized compounds of the first material system. In the case where the second material system is also polymerizable, the final refractive index after its polymerization should still be smaller than the refractive index of the polymerized first material system. The resulting non-uniform spatial distribution 30 of the refractive index across the coupling region is shown in
The first and second material systems 22 and 24 described above may be selected among a large variety of commercially available products. In order to obtain the photosensitive properties of the first material system, the multifunctional monomers/oligomer defining the polymerizable compounds are preferably sensitized from visible to infrared region with a photoinitiating system, consisting of a sensitizer (preferably cyanine dyes), an electron donor as initiator comprising preferably heavy atom (i.e. Br—, I—, B— or Fe— containing compound), and a coinitiator substance (preferably tertiary aromatic amines). In the preferred embodiment, the second material system includes an additive compound having the necessary lower refractive index. This compound may be organic. Several non-limitative examples for the choice of these materials may be found in the following non-exhaustive list:
Monomers:
Photoinitiators: cyanine dyes such as IR-140, IR-132, IR-143, IR-786 etc., Rose Bengal, Methylen blue, etc. (all from Aldrich);
Initiators (electron donors): i.e. heavy atom (Br—, B—, I— or Fe—) containing compound —CBr4, CHBr3, CHI3, etc. (all from Aldrich);
Coinitiators (electron donors): Ethyl-DiMethyl-Amino-Benzoate (EDMABzt), (from Aldrich), Benzophenone, Reactive Amine Coinitiator (CN383) (from Sartomer).
Organic Additives: Propylen Oxide (PPO), Ethylen Oxide, Ethyl Formate, Amyl acetate, etc. (all from Aldrich).
For a more precise example, the above-mentioned compounds were mixed in the following proportion (with respect to the total weight of the material composition):
The obtained mixture has been successfully used for the creation of light-induced W-type waveguides.
A non-limitative example of a composition having been used to create such a waveguide is the following:
It can be noted that, in addition to their stability, the above-mentioned W-type waveguides may have very useful optical properties, such as anomalous dispersion, a controllable fundamental mode diameter and cut-off wavelengths. More details on waveguides of this type may be found in the article entitled CHARACTERISTICS OF A DOUBLY CLAD OPTICAL FIBER WITH A LOW-INDEX INNER CLADDING published in IEEE Journal of Quantum Electronics, 1974, QE-10(12), pp. 879-887 by Shojiro Kawakami and Shingeo Nishida.
In accordance with another aspect of the present invention, and with reference to
The method includes a first step of bringing the two optical components 42 and 44 in proximity to each other and in alignment, so that the light beam exiting one component will propagate in the general direction of the other. It is an advantageous aspect of this method that it has a high tolerance for imprecision in this positioning, both in the longitudinal distance 45 between the extremities of the two components, and in the transverse and angular mismatches of the cores 46 and 48 of the coupled fibers 42 and 44.
A material composition 50 as described above is provided between the extremities of the two components. In practice, a hollow housing 52 is preferably provided for giving rigidity to the resulting coupler. The extremities of the optical fibers are preferably glued to this housing and the material solution injected therein.
A first polymerizing light beam 54 is outputted from the first optical component 42 in the material solution, towards the second. Similarly, a second polymerizing light beam is outputted from the second optical component 44, towards the first. The wavelength of each polymerizing light beam is selected to be within the polymerizing wavelength range for the selected materials. It is understood that each polymerizing light beam may be polychromatic, and that both light beams need not have the same spectral profile. The created channel may have reflective (and channel selective) properties if the two beams used are coherent and create reflective grating along with the principal channel formation. However, for only the channel formation, the coherence of those beams is preferably very low.
As explained above, the effect of light of a proper wavelength in the material solution according to the invention will be to generate therein a light guiding channel through polymerization of the polymerizable compounds in the first material system. The polymerization of the first material system may be directly induced by absorption of the first and second polymerizing light beams in the material composition, or indirectly induced by heat release. The self-generated light guiding paths from both optical fibers will meet along the way to create one light guiding channel coupling the two fibers. As also explained above, the transverse refractive index profile in the coupling region will be W-shaped.
In the preferred embodiment, the photopolymerizable materials in the first material system are sensitive to the UV-NIR spectra. As previously mentioned, this spectral region is below the cut-off wavelength of commonly used optical fibers for telecommunication applications, and the resulting transmitted light beam will show multiple maxima. In order to solve this issue, there are at least two possible ways: 1. use of polymerizing light with longer wavelength (e.g., 1550 nm) and material compositions that are sensitive to these wavelengths (single or multiple photo absorption to support the long wavelength induced polymerization) or 2. using a mode converter 58 in the path of first polymerizing light beam 54 propagating in the core 46 of the first optical fiber 42. The converter 58 will change the modal composition of the fiber 42 and optimize the coupling efficiency. It can, for example, simply strip (remove) the undesired modes or transfer the energy from the undesired modes to the modes that are preferred for the coupling.
It will be noted that the advantages provided by mode converters are also important in the case where the materials used may be polymerized by two (or more) photon absorption. Even in this case, the modal adaptation and control (using various methods) before recording the guiding channel will increase the coupling efficiency for desired modes.
Referring to
Referring to
According to the illustrated embodiment of
The present invention also provides an optical device including first and second optical components, and a coupler therebetween made of a material composition as described above. It will be understood that any such device resulting from one of the embodiments of a method as described above is considered to be within the scope of the present invention.
Of course, numerous modifications could be made to the embodiments described above without departing from the scope of the present invention as defined in the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| 60571251 | May 2004 | US |
