1. Introduction

Optical microstructures on the optical fiber tips become of interest in developing highly efficient and compact fiber-optic systems. Conventional fiber-based optical systems still rely on bulky optical components such as lenses, mirrors, filters, and gratings. These optical elements are much larger than a single optical fiber, making the miniaturization of fiber-optic systems difficult and requiring precise optical alignment. Recently, functional optical elements on a fiber-top surface provide a new function for compact fiber-optic systems with high performance. For instance, microlens enhances coupling efficiency between laser diode and optical fibers [1, 2], ball lens provides side-focusing [3], and micro-prisms on fiber bundles deflect an incident beam into different directions [4]. Besides, polymer-lens on hollow optical fiber and coreless silica fiber assembly generates Bessel beam [5], dielectric phase masks enable the beam shaping of doughnut or top-hat intensity distribution [6], and spiral phase plates generate optical vortices [7]. These functional optical fibers can serve as a key element for diverse fiber-optic applications such as all-fiber imaging probes, optical scanning probes, or optical fiber tweezers [8–12].

The microfabrication of functional fiber tips has been actively achieved by using chemical etching [13], mechanical polishing [14], focused ion beam (FIB) [15], or two photon lithography (TPL) [16, 17]. Chemical etching and mechanical polishing allow a simple and cost effective fabrication but they have some technical limitations in design flexibility. In addition, FIB and TPL provide high resolution and arbitrary patterning but still require high cost with low throughput. As a result, a rapid and cost-effective method with high design flexibility is still in need for the microfabrication of functional fiber tips.

Here we report a facile method for direct integration of functional optical microstructures on a fiber facet. The microfabrication combines photoresist coating and programmable DMD based photolithography. This method allows rapid prototyping and low-cost fabrication with spatial linewidth down to 2.2 μm on the top surface of a single optical fiber. The shapes and sizes of microstructures were precisely controlled on a fiber top surface by spatially modulating UV light with DMD. A fiber ferrule assisted method also provided the direct spin-coating of photoresist on the fiber top surface as well as the easy separation of an optical fiber from the ferrule. Moreover, the functional optical microstructures on a fiber tip were optically evaluated after coupling a red laser (637 nm) beam.

2. Experimental setup and microfabrication on a wafer

A schematic diagram and photograph of the experimental setup are shown in Fig. 1. The maskless UV exposure system was developed using a DMD as a spatial light modulator (1024 x 768 arrays of 13.68 x 13.68 μm micrromirrors, Texas Instruments), UV light emitting diodes (LED) sources (single LED, 365 nm in center wavelength, and 120 mW power), UV lenses, UV mirrors, optical microscope with a 40x objective lens (0.5 in NA, 5 mm in focal length, and 1 mm in working distance), and three-axis stage for precise alignment of an optical fiber at the final image plane. A white light LED source and CCD image sensor were also employed for monitoring the reflected patterns on the fiber top surface. The illumination part consisted of UV LED, a condenser lens, and a mirror, where the condenser lens (f = 250.0 mm, diameter 2 inch) collimated UV light and the mirror was precisely positioned considering the height and tilt angle of the DMD between incident and reflected rays. UV pattern was spatially modulated by the individual micromirrors of DMD, demagnified to the aperture size of the microscope through two convex lens (f = 150.0 mm, 250.0 mm, diameter 2 inch), and injected into the upright optical microscope. The objective lens projected the modulated UV pattern on a fiber facet with a demagnification factor of approximately 57. The photoresist coated optical fiber was placed at the working distance of the objective lens. The exposure area was 245 μm x 185 μm, which can be enlarged by using a low-magnification objective lens for patterning different size of an optical fiber.

 

Fig. 1 (a) Schematic diagram of DMD based maskless lithography for patterning functional optical elements on a fiber facet. (i) Optical image of optical fiber ferrule assembly, coated by a SU-8 photoresist layer. The coated photoresist has uniform area over the fiber region. (ii) A UV pattern is illuminated on the photoresist layer on an optical fiber through a 40x objective lens. (b, c) Photograph of the experimental set up.

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The optical performance of the DMD based photolithographic system was first evaluated by patterning functional optical elements on a silicon wafer. Three different types of a positive tone photoresist (AZ1512, AZ Electronic Materials), a negative tone photoresist (SU-8 2002, MicroChem), and a black SU-8 photoresist (GMC 1040, Gersteltec), serving as an optical absorber, were prepared on a silicon wafer, respectively. The DMD can spatially modulates UV beam pattern for various microstructures without photomasks. Figure 2 shows some examples of microstructures such as line arrays, checker board, hole arrays, concentric ring elements, and Fresnel zone plate lens. The DMD based maskless lithography provides the spatial resolution of 2.2 μm within the FOV of 245 μm x 185 μm.

 

Fig. 2 The 2D fabrication results using DMD based maskless lithography on wafer. The micro fabrication was performed with a 2 μm negative tone photoresist ((a), (b) SU-8 2002), 1.2 μm positive tone photoresist ((c) AZ1512), 3 μm negative tone photoresist ((d), (e) black SU-8). Optical images and SEM images of diverse microstructures such as (a) line arrays, (b) checker board, (c) hole arrays, (d) concentric ring elements, and (e) Fresnel zone plate lens. The fabrication results show high spatial resolution with the minimum line/space of 2.2 μm.

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3. Fabrication of functional optical fibers and optical characterization

The microfabrication procedures of functional optical fibers include four main steps, i.e., the preparation of fiber-ferrule assembly, spin-coating of photoresist with precise thickness control and smooth surface over the entire fiber, DMD assisted UV exposure, and the extraction of an optical fiber from ferrule as described in Fig. 3. A multimode fiber (MMF, 200 μm core diameter) and a hybrid optical fiber comprising a single mode optical fiber (SMF, 6 μm core diameter) serially fusion-spliced with a coreless silica fiber (CSF) segment (125 μm diameter) were selected for the fabrication as shown in Fig. 3(a). The CSF length controls the distance between a SMF to optical microstructure, which can expand the mode field diameter [18, 19]. The fabricated hybrid optical fiber for beam-expanding is shown in Fig. 3(b). A fiber ferrule assisted method provide the direct spin-coating of photoresist on the fiber top surface as well as the easy separation of an optical fiber from the ferrule. Note that this method effectively protects the physical damage of an optical fiber during polishing and handling. In particular, the separation of an optical fiber from the ferrule assembly has also been successfully achieved as illustrated in Fig. 3(c). In detail, a stripped and cleaved optical fiber was inserted into a fiber-optic ferrule. The interstitial gap between optical fiber and ferrule was filled with positive photoresist (AZ-series, AZ Electronic Materials). The end of fiber-ferrule assembly had a sufficient bead of photoresist to support the end of the fiber during polishing. The photoresist was soft-baked to remove the solvent in a convection oven at 110 °C for 90 min. The ferrule fiber assembly was then polished with 9, 3, 1, and 0.03 μm polishing films in a sequence for a smooth surface. The uniform spin-coating of thin photoresist on a small fiber tip was obtained by using the fiber-ferrule assembly, which increases the spin-coating area as shown in Fig. 3(d). Otherwise, it would create non-planar and non-uniform coating on the top surface of an optical fiber due to surface tension. After assembling an optical fiber with the ceramic fiber-optic ferrule, the effective diameter was enlarged from 125 μm to 2.5 mm. Thin photoresist (SU-8, MicroChem) was directly spin-coated on the fiber-ferule assembly with a rotating chuck and soft-baked in an oven [20–22]. The spin coating process allows the repeated deposition of photoresist with different thickness ranging from 0.8 μm to 5 μm depending on the rotating speed (in the range 2500-6500 rpm, SU-8 photoresist). The fiber-top surface was then exposed by using the spatial modulation of UV light with DMD. The optical fiber was finally detached in acetone solution from the ceramic ferrule after the development process. This separation method can also be applied for different types of optical fibers such as single mode fiber, multimode fiber, and coreless silica fiber.

 

Fig. 3 (a, b) Hybrid optical fiber, i.e., a SMF serially concatenated with a CSF segment. (c) Preparation of separable fiber-ferrule assembly. (d) Schematic and photograph of rotating chuck for spin-coating photoresist on a fiber facet. (e) Maskless fabrication procedure for functional optical elements on the top surface of an optical fiber. (i) polished fiber ferrule assembly preparation, (ii) spin-coating of photoresist over the entire fiber (inset: multi-layer photoresist coating result), (iii) UV exposure by DMD-based maskless lithography, (iv) photolithographic definition of SU-8 resist on the fiber ferrule assembly, (v) extraction of the optical fiber from the ferrule.

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Diverse functional optical elements were finally fabricated on the top surface of optical fibers by using DMD-based maskless lithography. Figures 4(a)-4(c) and Figs. 4(g)-4(h) show the SEM and optical images for line, ring, hole, checker arrays, and characters on various optical fiber tips such as hybrid optical fiber (125 μm diameter), MMF (200 μm core diameter), and fiber bundle (two MMF), respectively. Figure 4(i) shows the SEM images of detached optical fiber tip. The fabricated optical fiber was easily separated from the ferrule in acetone solution. The experimental results also demonstrate that the functional optical elements still remain stable on the separated fiber top surfaces without any noticeable damage. Light distribution through the functional optical elements was further evaluated by using a fiber-coupled laser source (637 nm). Figures 4(d)-4(f) demonstrates the light distribution through their functional optical elements. The experimental results clearly show that functional optical elements on an optical fiber effectively modulate light distribution.

 

Fig. 4 (a-c) SEM of diverse functional optical elements on top surface of an optical fibers. (d-f) Light distributions at 637 nm wavelength. The optical characterization results shows effectively modulate light patterns. (g-h) Optical and SEM images of functional optical elements on top surface of a multimode fiber and a fiber bundle. (i) SEM images of detached optical fiber from the ferrule.

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4. Summary

In summary, this work provides a new method for fabricating optical microstructures on the top surface of an optical fiber. This method offers distinct advantages as a low-cost, simple, and rapid fabrication. The DMD based maskless exposure system has high spatial resolution with the minimum line/space of 2.2 μm for an exposed area of 245 μm x 185 μm. Diverse functional optical elements were successfully fabricated on different types of fiber-ferrule assembly. The fabricated optical fiber can be further separated in a simple manner from the ferrule for miniaturized all-fiber probe. This all-fiber functional probe effectively modulates the properties of fiber-coupled beam for integrated fiber-optic applications. This optical fabrication method provides a new direction for developing functional fiber-optic devices, particularly for in vivo biosensing or endomicroscopic imaging applications.