1. Introduction

While design and fabrication of electronic micro-circuitry has evolved into mature technology, with spectacular advances in miniaturization and integration, in parallel there is a race towards reliable photonic chips, with light propagating in (near- or sub-wavelength) waveguides and put to work at various optical and opto-electronic components [1,2]. One of the challenges in photonic chip development is integration of nano- and micro-optics with bulk optical components and optical fibers are natural candidates for light delivery from and to external devices. Yet, typical dimensions of single mode fibers – tens of micrometers – do not match the scale of photonic components and assemblies. Aligning the fibers with respect to optical micro-elements is not easy either; to this end, we have demonstrated a three-dimensional (3D) laser printed micro-connector for repeatable docking of fibers with sub-micrometer accuracy to photonic structures, e.g. semiconductor quantum dot light sources [3]. Three-dimensional direct laser writing (DWL) has been also recently used to fabricate mirrors and lenses that guide light on a photonic chip [4].

Here, we demonstrate a fundamentally different approach – Photonic-Chip-on-Tip (PCT): rather than connecting optical fibers to photonic micro-devices, we fabricate the latter directly on fiber tips, thus circumventing problems of scale mismatch, alignment and assembly. 3D DWL was used to print a simple circuit with in- and out-coupling prisms onto single mode fiber cores and, as an example of a compound photonic device, we have chosen a whispering gallery mode (WGM) micro-resonator coupled to a free-standing, tapered waveguide (Fig. 1).

 

Fig. 1 Computer-aided design (CAD) visualization (a,b) and realization (c,d) of a simple Photonic-Chip-on-Tip. In- and out- coupling prisms (orange), tapered waveguide (red) and WGM micro-resonator (purple) have been fabricated on a single mode fiber bundle (blue) using 3D two-photon laser photolithography. The fiber diameter is 125 µm and the ring resonator diameter is 40 µm. Panels (c) and (d) are scanning electron microscope (SEM) photographs with computer-added colors.

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2. Methods

Two single mode telecom fibers (SMF-28, Corning) were cleaved, cleaned, aligned and glued together under an optical microscope. Thus prepared fiber bundle was mounted into a custom-made holder in a commercial 3D two-photon lithography workstation (Photonic Professional, Nanoscribe GmbH). With the fiber bundle, the resin-glass interface can be found in the same way as for standard planar glass substrates (plates). Coupling weak red light into the fibers at the other end allows for positioning their cores with the necessary precision.

Direct laser writing is an attractive fabrication technology for prototyping integrated photonics in the visible and near infrared: transparent materials (resins) are available, the resolution readily allows for fabrication of near- and sub-wavelengths elements and complex structures with overall dimensions of up to hundreds of microns can be printed in reasonabletime. Various elements have been laser-written on a single optical fiber tip: lenses [5], lens assemblies [6,7], phase plates [8], micro-structured antireflective coatings [9] and optical microphones [10]. These structures were limited to rather simple geometries, were printed on a single fiber core and usually light was only delivered to the structure via this single fiber core.

Diffraction gratings are typically used to couple light, either from free space beams or from optical fibers, to photonic structures, especially if the central wave-vector is designed to change direction by 90 degrees [11–13]. Gratings have one advantage – they can be fabricated with two-dimensional photolithography – and a number of intrinsic limitations: they work over a finite range of wavelengths and are very sensitive to the input beam angle. Photonic wire bonding was also demonstrated as a feasible technique of coupling between waveguides and integrated optical circuits [14,15]. We exploit the 3D printing capabilities of two-photon laser photolithograpy and use total internal reflection in prisms instead to couple light from and into the single mode fibers. Our photonic chip assembly has two 12 µm high 45/90° prisms. The photo-curable resin (IPL, Nanoscribe GmbH) refractive index of 1.5 at 1550 nm [16] provides total internal reflection over a large spectral and angular bandwidth.

The two prisms are connected with a waveguide of varying cross section. At the input fiber end, the waveguide has 30 µm long coupling taper that has been designed taking into account the tradeoff between maximizing light throughput from the input fiber and the space constrains of the fiber platform (Fig. 2(b)). The taper is similar to those used in two-dimensional silicon-on-insulator photonic devices [17–19], but in our design it is a 3D, free-standing structure [20]. In the main part, the waveguide has 3 × 3 µm² cross-section, tapering to 2 × 1 µm² near the WGM resonator disc (Fig. 2(a)).

 

Fig. 2 Coupling prisms and waveguide design. The waveguide (red) has a 3 × 3 µm² square cross-section with the 30 µm long in-coupling taper and a 2 × 1 µm² micro-ring resonator coupling taper (a). Calculated coupling efficiency as a function of the in-coupling taper length L (b). At around 30 µm the coupling saturates and this length has been chosen for the PCT. The non-monotonic behavior arises from the interplay between diffraction and adiabatic transition along the taper. Calculated lowest mode profiles in the waveguide: in the main section (c) and in the coupling taper (d). (e) Calculated modulus of the electric field for a light pulse (from top left down): propagating in the in-coupling fiber, reflecting from the prism, propagating in the taper and entering the waveguide.

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Direct laser writing has been successfully used to fabricate micro-disc resonators with Q as high as 10⁵ [21,22]. Our micro-disc resonator has the diameter of 40 µm, the thickness of  2 µm and is supported by a solid cylinder in the middle. From the SEM images the RMS surface roughness of the micro-disc has been estimated to be below 50 nm. One of the challenges in the ring resonator coupling to the waveguide is maintaining the small distance between the two elements, so that their evanescent fields overlap [23].

3. Results and discussion

The transmission spectrum between the input and output fibers was measured with a tunable CW semiconductor laser (OSICS 100T, Yenista Optics) scanned between 1510 and 1610 nm central wavelength with the line width below 10 pm and an infrared power meter (1830-R, Newport Corporation). The WGM resonance peaks are visible, with very low noise, spaced every 2.16 THz, in agreement with the calculated resonator FSR. From fitting the Lorentz profile to the resonance curve the resonant Q factor was retrieved to be 245 (Fig. 3(a)). This low value results most likely from very strong coupling between the waveguide and the micro-ring – a well-known problem in the 3D micro-resonators design resulting from the material (resin) mechanical properties and processing. The main source of loss is the sub-optimal coupling into the output fiber – this can be addressed e.g. with a free form mirror integrated into the out-coupling prism [4].

 

Fig. 3 Measured transmission between the input and output fibers as a function of the laser wavelength (a). Lorentz profile (red dashed line) was fitted to one of the minima. Measured resonance position shift as a function of temperature (b).

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Using the WGM resonance wavelength shift with temperature the photonic chip assembly has been calibrated to work as a temperature sensor between the room temperature and 60 °C (Fig. 3(b)).

4. Conclusion

We have presented what may become a new approach in photonic and micro-optics integration: a compound circuit fabricated directly on optical fibers. Our simple setup has one input and one output fibers, but the concept can be scaled up and we have successfully glued up to seven fiber bundles as a platform for direct laser writing. Micromachining (milling) of the fibers themselves, either with laser/chemical etching [24] or focused ion beam (FIB) [25,26], combined with DWL, may open up even more degrees of freedom in the Photonic-Chip-on-Tip fabrication.

The photonics-on-tip can be further developed with custom-made fibers having a number of optical cores as well as hollow channels to deliver liquids or gasses, thus enabling another development: a Lab-on-Tip. One such fiber is presented in Fig. 4: it has been designed as a universal platform for integrating photonic and microfluidic compound circuitry, readily integrated with external light sources, detectors and liquid/gas connectors. It has six single mode optical cores, six multimode optical cores and six hollow channels and tapers by a factor of 10 in size from the other end where it can connect to light sources (laser, diodes), detectors and external pumps.

 

Fig. 4 SEM photo of a cleaved end of a multi-core fiber developed as a universal platform for compound photonic/micro-fluidic Lab-on-Tip devices (with added colors). There are six single-mode optical cores (green), six multi-mode optical cores (blue) and six hollow channels (purple) within the 250 µm diameter fiber (a). Scale bar is 50 µm long. A Lab-on-Tip designed for the fiber in (a) with two fluid input channels, mixer and WGM resonator for chemical sensing (b).

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