1. Introduction

Connecting optical fibers to microscopic devices, such as photonic integrated circuits, with high precision presents a significant challenge. The solutions presented to date [1–7] require advanced processing of substrate material (etching, cutting, drilling), and lack versatility. In-situ characterization and positioning are difficult to achieve in most subtractive manufacturing techniques. Laser two-photon 3D-printing techniques such as Direct Laser Writing (DLW) have been developed to the point when several commercial systems are available [8–11], offering excellent parameters: repeatability, sub-micron positioning precision with significantly lower barrier of entry than more traditional pattering techniques. Photoresists are available with a wide spectrum of properties [4, 12–20], e.g. refractive index, stiffness, magnetic properties, bio-compatibility, thermal expansion coefficient or electric conductivity. Moreover, fabricating molds with this technique [21–23] allows for replicating 3D-printed objects without direct access to the DLW printer. All of these features make DLW a great solution for rapid prototyping of micro-optical and photonic devices.

In this paper, we present a compact, high-precision fiber connector, fabricated on a planar substrate with laser two-photon additive manufacturing.

2. Methods

The micro-connector is fabricated on a glass substrate using commercially available laser lithography system (Photonic Professional, Nanoscribe GmbH). Tightly focused 780 nm femtosecond laser beam triggers polymerization inside a droplet of liquid UV-curable resin (IP-DIP, Nanoscribe GmbH) via two-photon absorption process (TPP), occurring only in a limited volume of the laser focus – the voxel. In our setup the voxel is an ellipsoid of revolution with major and minor axes equal to 0.4 μm and 1.2 μm respectively. The voxel positioning accuracy is limited by the precision of the piezo stage (P-563, Physik Instrumente GmbH & Co.), well below 50 nm. Complex free-form 3D structures can be produced by relative movement of substrate (resin) and voxel. The resin acts as both photosensitive material and immersion medium for ×100, NA = 1.3 objective (EC Plan-Neofluar, Zeiss) and the laser beam is focused inside the resin without passing through the substrate. This enables printing on non-transparent materials – an important feature for semiconductor (e.g. silicon or GaAs) substrates.

To characterize the positioning precision of the micro-connector, a series of insert-remove cycles was performed with a single-mode optical fiber with the mode field diameter equal (5.0 ± 0.5) μm @ 850 nm and cladding diameter of (125 ± 1) μm (780HP, Thorlabs). Laser diode with wavelength of 980 nm was coupled into the fiber for visualization. The fiber was mounted on a 10 cm long arm to a flexure XYZ positioning stage (NanoMax TS, Thorlabs). While the stage is specified to provide the positioning accuracy better than 1 μm, the differential micrometers were operated manually during the fiber approach and insertion procedure. Moreover, the arcuate cross talk between the stage Z axis (parallel to the fiber axis) is specified to result in up to 5 μm displacement in the X and Y axes. The fiber connector was designed to accommodate this initial misalignment, as described in detail in the following section.

After each insertion, the relative position of the fiber core and the DLW-printed cross-hair was determined by machine vision analysis of the images from the optical microscope (Eclipse Ti-U, Nikon, ×60, NA = 0.7 S Plan Fluor, Nikon, USB camera DFK23UX236, The Imaging Source GmbH) (see Fig. 1(e)). The linear scale calibration of the images was done by performing 2D Discrete Fourier Transform analysis of 500 grooves/mm transmission diffraction grating image. For pattern recognition, we used sub-pixel accuracy algorithm from National Instruments LabVIEW VISION Module machine vision package. The photoluminescence sample used in thermal durability tests was a 10 nm CdTe quantum well embedded in 50 nm (Cd,Mg)Te barriers, on a 5.5 μm CdTe buffer deposited on a GaAs (100) substrate.

 

Fig. 1 (a) SEM image of the DLW 3D-printed optical fiber micro-connector consisting of three flexible pylons, connected to a circular pedestal, standing on a planar substrate surface. (b) Single-mode optical fiber inserted into the connector (for the fiber insertion movie see Visualization 1). The fiber cladding has a diameter of 125 μm. (c) An isometric scheme of the fiber micro-connector. (d) Cross-sectional view of the micro-connector pylon. The docking drogue cone directs the fiber towards the target position. The alignment collar fixes the fiber at the correct position. The thickness of the waist determines the stiffness and thus accuracy of the connector [6,18]. (e) Transmission optical microscope bottom view of the connector with the fiber surface resting on pylons’ base. Light emerging from the core is visible as a spot with a nearly-Gaussian distribution inside the cross-haired circle. Red light is coupled into the fiber cladding to enhance visibility.

Download Full Size | PDF

3. Results and discussion

The DLW-printed fiber micro-connector consists of three pylons (see Fig. 1) and has three-fold rotational symmetry with respect to the axis perpendicular to the substrate, passing through its center. The angular width of a single pylon is 25°. During insertion, the optical fiber first comes into contact with the docking drogue cone (see Fig. 1(d)), which helps to reach the correct final position of the fiber core. In the next phase, the fiber reaches the alignment collar – the vertical part of the pylon that has the inner diameter equal to the outer diameter of the optical fiber.

The rigidity of the connector can be adjusted during design by varying the number of pylons, angular width of each pylon and the waist thickness. To best reproduce the requirements of fast prototyping, we used the default settings of the slicer and the 3D laser printer software (DeScribe and NanoWrite respectively, Nanoscribe GmbH), and we chose the minimum number of pylons – three. Although significant improvement of resolution [24], structure smoothness [25] and mechanical properties [19] can be achieved with advanced parameter optimization, for most applications of the fiber connector this basic, rapid approach was sufficient, as we demonstrate below.

The results of the displacement measurements between the fiber core and the optical cross-hair for 46 consecutive insert-remove cycles are presented in Fig. 2. The accuracy of the optical fiber core positioning equals (77 ± 14) nm and (114 ± 16) nm for the X and Y axis respectively. These values are well below typical core-clad concentricity, as specified by the fiber manufacturer (⩽ 0.5 μm). The insertion losses originating from misalignment and separation, calculated for a Gaussian beam emerging from e.g. an on-chip waveguide and coupled into the optical fiber docked in the micro-connector, are below −0.005 dB [26–30].

 

Fig. 2 (a) Relative displacements between the fiber core and the cross-hair mark for 46 measurements (fiber insert-remove cycles). (b, c) Histograms of relative positions for X and Y axis. The data were fitted with Gaussian distribution (black solid line). Full widths at half maximum (FWHM) are equal to (77 ± 14) nm and (114 ± 16) nm for the X and Y axis respectively and are represented by the gray area in the background.

Download Full Size | PDF

The DLW technique is particularly well suited for positioning the fiber connector in-situ on the substrate. This is possible since in most DLW systems the same microscope objective is used for focusing the IR beam, visual inspection of the substrate and 3D printing. With light coupled into a photonic device, the desired position of the connector can be determined.

Another advantage of the DLW-printed optical fiber connector system is alignment-free coupling. This feature is equally important in both fast prototyping and batch manufacturing, where lower requirements of fiber pre-alignment reduce both docking time and costs of precise positioning equipment [1,3–5]. In our micro-connector design, the docking drogue cone slope angle is equal to 23°, which corresponds to ±17 μm of the fiber position pre-alignment. Larger tolerances of the initial fiber position can be obtained using the entire workspace volume available in modern DLW systems (typically exceeding 200 × 200 × 200 μm³), hence docking drogue cone of arbitrary size and shape can be fabricated. Still, even with the current design of the docking drogue cone, the typical insertion time is below 3 minutes.

The durability of the device was tested using a micro-connector printed on semiconductor molecular beam epitaxy grown quantum well sample (see Methods section). After completing the docking procedure, the optical fiber was fixed with a drop of UV-curable epoxy (NOA65, Norland Products Inc.). The sample with thus attached fiber (see Fig. 3(c)) was then subjected to five thermal cycles between room temperature and 4.2 K, each consisting of 7 minutes of cooling, 5 minutes in liquid helium and 4 minutes of warming-up back to room temperature. Every time when the sample was in liquid helium, the photoluminescence signal was collected (see Fig. 3(a)). The total integrated photoluminescence intensity after each cycle remained constant (standard deviation equal to 1.9 %, Fig. 3(b)), which indicates that our fiber connector is suitable for cryogenic applications. Additionally, no sign of aging was observed after storing the device under ambient conditions over the course of several months.

 

Fig. 3 Cryogenic cycling of a quantum well sample with an optical fiber attached to the surface after being plugged into the fiber connector. (a) Series of 5 photoluminescence spectra acquired at 4.2 K during repetitive cycling the sample between room temperature and liquid helium. (b) Integrated, normalized photoluminescence intensities of the spectra from (a) having a standard deviation of 1.9 %. (c) Photograph of the sample with an optical fiber glued to the surface (black pyramid). Copper protective housing is attached to a 1.5 m-long metal tube (“cane”), used to immerse the sample in a liquid helium dewar.

Download Full Size | PDF

4. Conclusions

In this paper we presented DLW 3D-printed optical fiber connector which allows for fiber positioning with 120 nm precision. The fiber micro-connector can withstand several tens of fiber insertion attempts as well as multiple thermal cycling. Adaptation of existing design of micro-connector for different applications is possible. For faster docking (larger initial misalignment), the docking drogue cone angle can be changed, while for increased final positioning precision the number of pylons and/or their stiffness may be increased. Previous studies showed that common photoresists [31,32] can be printed on different substrates: glasses, plastics, metals and a broad range of commonly used semiconductors. Thus, the DLW 3D-printed optical fiber connector can find applications [33–39] in integrated photonic circuits, waveguides, microfluidic systems, bio-compatible biological systems, semiconductor single photon emitters, quantum dots, thin layers of semiconducting transition-metal dichalcogenides and many more.