Design and fabrication of on-fiber diffractive elements for fiber-waveguide coupling by means of e-beam lithography
Introduction
Diffractive optical elements (DOEs) are optical elements which modify the wave field by diffraction. They allow the conversion of the incident wave into a reflected or transmitted wave which, after free-space propagation, has a desired distribution of its amplitude, phase or polarization. If the wave fields and their propagation are represented as digital data, computer generated DOEs carrying out complex optical tasks (impossible to be implemented with traditional refractive optics) can be calculated. Other advantages of the DOEs are their size and weight. They are smaller and lighter than the traditional refractive optics, allowing the compactness of the systems in which they are used. From the working principle point of view, there are two principal types of DOEs: amplitude diffractive elements (ADEs) and phase diffractive elements (PDEs). ADEs, which change only the amplitude of the incident wave, have a very low efficiency and hence are rarely used. Since they act only on the phase of the incident wave, PDEs are instead characterized by high efficiency values, the transformations of the wave field being carried out without energy loss. PDEs that generate two-dimensional arrays of light spots (fan-out elements), desired continuous intensity distributions (beam shaping elements) or control the phase in the frequency domain (correlation filters) already represent key components in many optical signal processing and communication systems [1].
The coupling efficiency represents the main problem that arises when an optical fiber should be coupled with an optical waveguide. First, one must be able to transform highly divergent light from the fiber into a collimated beam usable with bulk optics. Second, one must be able to transform the collimated beam into a tightly focused spot that matches the properties of the waveguides. Third, one must be able to precisely align optics to effectively couple light from one medium to another [2].
In this paper, we propose fiber-waveguide coupling by means of a phase diffractive element realized in a polymeric material coated on top of the fiber. The role of this element is to focus and shape the beam exiting the fiber into a desired intensity distribution at the waveguide input. Since the DOE is realized on top of the fiber, the beam does not propagate free-space before entering the coupling optics, thus avoiding the collimation problem. The alignment is also easier than with an independent coupling optics, since the fiber and the DOE are already aligned during the fabrication process. In addition, with the DOE one can imagine an arbitrary intensity distribution at the waveguide input and not only an elliptical pattern as obtained with classical coupling optics.
We have used two approaches to calculate the PDE’s phase function: ray-tracing and iterative algorithms (Section 2). The phase function has been implemented by multilevel profiling the polymeric material coated on top of the fiber. The steps of the profiling process are described in Section 3. Experimental results obtained for the coupling of an optical fiber with a rectangular waveguide are presented in Section 4 and conclusions are discussed in the final section.
Section snippets
Design of the phase diffractive element
We assume a monochromatic Gaussian beam travelling inside the fiber (the wavelength is 1.55 μm, the diameter of the fiber core is 10 μm). The coupling element, PDE attached on top of the fiber, has to modify the phase of the exiting beam in such a way that a specified intensity distribution is obtained at the entrance of the optical waveguide placed at a certain distance (100 μm) from the fiber. We consider a uniformly filled-in rectangle (8×5 μm) for the intensity distribution at the entrance
Fabrication process overview
Electron Beam Lithography (EBL) is currently used as a powerful tool in order to obtain 3D continuous profiles [5] of arbitrary complexity with very high resolution. Computer generated profiles can be fabricated by means of e-beam writing on a suitably sensitive resist. Depending on the electron dosage delivered per unit area, after the resist development it is possible to obtain zones having different resist thickness. Phase modulation is achieved by a multilevel profiling of the resist coated
Results of the experimental optical fiber-waveguide coupling
To verify the coupling efficiency using our PDE fabricated directly on the top of the fiber, we have used the experimental set-up illustrated in Fig. 5. The main components are: tunable laser (Tektronix, model LPB1100), single-mode fiber with PDE fabricated on its top-end, magnification lens, vidicon camera. The laser output is a fiber output which allows an easy junction with our fiber, with reduced losses. For a better measurement of the intensity pattern produced by the PDE in the waveguide
Conclusions
Fiber-waveguide coupling through a PDE fabricated directly on the top of the fiber is proposed in this paper. The optical characterisation has successfully validated this technique. However, the gain of the coupling is still small because small differences between the calculated and the fabricated PDE profile affect dramatically the beamshaping. To avoid this problem, a better control of the fabrication process is required. In the future work, a more accurate AFM analysis of the fabricated PDE
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