1. Introduction

Multicore optical fibers (MCFs) consisting of either a one- or two-dimensional array of guiding cores have found applications in a number of fields including sensing and lasers [1–4]. Due to the close proximity and geometrical arrangement of the cores, the coupling of light to and from MCFs can pose a significant problem. To date, a number of fan-out devices that allow each core of an MCF to be addressed individually have been proposed [4, 5], but only the fan-out described in [4] has been demonstrated practically. As described in [4], the fan-out fabrication was complex, requiring the use of HF acid to etch standard fibers which were then arranged and glued together in a hollow square capillary. Clearly, an integrated fan-out device whose fabrication could readily be altered to allow the coupling of light to and from MCFs with any core geometry would be of great interest to the MCF field.

It is now well known that ultrafast-lasers can be used to create high quality optical waveguides in many dielectric materials including both glasses [6, 7] and crystals [8]. By focusing femto- or picosecond pulses of sub-bandgap radiation inside a bulk dielectric material, optical energy can be deposited in the focal region by means of multi-photon absorption, tunneling ionization and avalanche ionization [9]. The deposited optical energy can induce a highly localized structural modification of the substrate material which, under the correct fabrication conditions, can result in a positive refractive index change. This refractive index modification can then be used to directly inscribe optical waveguides in a direct-write fashion by translating the material in three dimensions through the focus. In this paper we use ultrafast-laser waveguide-inscription to successfully fabricate a fan-out device that allows each core of a 2×2 core array MCF to be addressed individually by one of four single mode fibers held in 4×1 fiber V-groove array (FVA).

2. Fan-out fabrication

To fabricate the fan-out device we used a regeneratively amplified Ti:Sapphire laser system (Spectra-Physics Hurricane) emitting a linearly polarized train of 200 µJ pulses at a repetition rate of 5 kHz. The central wavelength of the laser radiation was 800 nm. At the output of the laser the pulse duration was measured to be ≈130 fs (FWHM). These pulses were subsequently compressed to ≈100 fs using an in-house adaptive optics setup. Recently, Ams et al have shown that circularly polarized light results in waveguides with lower propagation losses [10]. Consequently, after pulse compression the polarization of the beam was adjusted to circular using a zero-order quarter waveplate. To control the cross sectional shape of the fabricated waveguides we used the slit method described in [11]. Accordingly, the ≈2.5 mm diameter (at 1/e² intensity points) collimated laser beam was passed through a 300 µm slit placed approximately 6 cm before a 0.42 NA, ×50 long working distance microscope objective (Mitutoyo M-Plan NIR). To fabricate the waveguides, the pulse train was focused to a depth of ≈200 µm below the surface of a multi-component, predominantly silica glass slide (Menzel-Gläser Extra-white Electroverre-Glass) that was mounted on automated Newport ILS linear travel x-y-z translation stages, each with a resolution of 100 nm and repeatability of ≈200 nm. The translation stages were controlled using a Newport ESP 300 control module. Waveguides were created by translating the substrate material in three dimensions through the focus of the laser beam. All waveguides were fabricated using a translation speed of 400 µm.s^-1 and pulse energy of 1.3 µJ, as measured after the objective.

 

Fig. 1. Graphical representation of the fabricated fan-out device. Each waveguide is numbered and referred to in the text. The axes shown are directly related to those shown in Fig. 2(a) and Fig. 3.

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Figure 1 shows a graphical representation of the fabricated fan-out device in which each of the four waveguides consists of three linear sections connected end-to-end. At each end of the middle section a 2.5 mm long “run-in” section was placed to directly couple with the MCF or FVA cores. The run-in sections align with the y-axis translation stage in the fabrication setup thereby requiring only motion in one axis — this helps reduce waveguide propagation loss by minimizing stage jitter. Each waveguide was fabricated separately by inscribing consecutively the first run-in section, followed by the middle section, and lastly the second run-in section, thus removing any stitch errors between the sections. At the MCF coupling end of the fan-out the four run-in sections were arranged in a 50 µm×50 µm array to match the known core geometry of the MCF (fabricated by Fibercore Limited), as shown in Fig. 2. At the FVA coupling end of the fan-out the four run-in sections were arranged in a one-dimensional array with a 250 µm spacing to match the core spacing of the FVA. The depth of the FVA run-in array was set to match the depth of the MCF run-in sections for waveguides 2 and 3 shown in Fig. 1 and Fig. 2. Consequently, for waveguides 1 and 4 the middle connecting section required a 50 µm translation in the z axis, a 350 µm translation in the x axis and a 20 mm translation along the y-axis. For waveguides 2 and 3, the middle section required no z-axis translation but a 100 µm translation along the x-axis and a 20 mm translation along the y-axis was necessary. After waveguide fabrication the sample was polished prior to characterization. The final device length was 25.0 mm along the y-axis.

 

Fig. 2. Transmission mode optical micrographs of (a) the MCF coupling end of the fabricated fan-out and (b) the end of the MCF fiber for which the fan-out device was designed. The axes shown in (a) are directly related to those shown in Fig. 1 and Fig. 3.

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3. Fan-out device characterization

3.1 Guiding properties of each fan-out waveguide

Initially, the guiding properties of the fabricated waveguides were investigated by imaging each end of the fan-out onto a distance and power calibrated IR-Vidicon camera while coupling 1.55 µm light into each waveguide at the opposite end. In this way, the fabricated waveguides were found to be single mode at 1.55 µm, with an average mode field diameter (MFD) of 15.8 µm and 11.8 µm in the x- and z-axis respectively, as measured at the 1/e² points of the intensity profile. As a typical example, Fig. 3 shows the near-field image of the 1.55 µm mode from the MCF coupling end of waveguide 4. Shown in red directly above and to the right of the image are the normalized intensity profiles of the waveguide mode. For comparison the blue dotted line shows the measured intensity profiles of the 1.55 µm mode from Corning SMF-28 (C-SMF-28) fiber. All the measured waveguide MFDs lay within ±1.5 µm of the average value. Since all waveguides were fabricated using the same process parameters, the differences in the measured MFDs are thought to be primarily the result of the ≈±1.0 µm accuracy in the MFD measurement itself. The increased depth of the MCF coupling end run-in sections for waveguides 1 and 4 did not have any noticeable effect on the measured MFD. Using a calibrated optical microscope operating in transmission mode, the cross sectional size of the waveguides was measured to be ≈13.5 µm and ≈10.2 µm in the x- and z-axis respectively. Because the size of the cross-sectional size of the waveguides along the z-axis is close to the 8.2 µm core diameter of C-SMF-28, and since the intensity profile of the waveguide mode along the z-axis is very similar to that of C-SMF-28, we conclude that the refractive index contrast of the waveguides is comparable to the 0.36 % refractive index contrast of C-SMF-28 [12].

 

Fig. 3. Near-field image of the 1.55 µm mode emitted from the MCF coupling end of fan-out waveguide 4. Shown in red directly above and to the right of the image are the normalized intensity profiles of the waveguide mode. For comparison, the blue dotted line shows the measured intensity profiles of the 1.55 µm mode for Corning SMF-28 fiber. The axes shown are directly related to those shown in Fig. 1 and Fig. 2(a).

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The performance of each waveguide was investigated separately by directly butt-coupling C-SMF-28 fibers to either end of the waveguide using index matching gel to reduce Fresnel reflections. Using a broadband amplified spontaneous emission (ASE) source the insertion losses (ILs) in the 1.55 µm spectral region for each of the four waveguides were measured to be 5.4, 3.3, 3.1 and 5.0 dB for waveguides 1, 2, 3 and 4 respectively, the error in the IL measurements is estimated as ≈0.2 dB due to alignment accuracy. Due to the nature of the measurement however, the true IL cannot be higher than measured. Using the technique described in [13], the waveguide to C-SMF-28 fiber coupling loss was measured to be 0.5 dB ±0.2 dB. We therefore attribute 1.0 dB±0.4 dB of the measured ILs to coupling losses, and as a result, the total loss due to propagation, cross talk and bend losses is calculated to be 4.4, 2.3, 2.1 and 4.0 dB±0.6 dB for waveguides 1, 2, 3 and 4 respectively. The ILs measured for waveguides 2 and 3 are within error of the 3.3 dB IL measured for a straight 25.0 mm long waveguide fabricated using the same process parameters. Consequently, we conclude that the bend and cross-talk losses for waveguides 2 and 3 are negligible. It is our belief that the higher ILs measured for waveguides 1 and 4 are due a combination of increased bend losses (due to the larger angular misalignment between the linear sections that form the total waveguide) and increased propagation losses (since waveguides 1 and 4 require the sample to be translated using all three translation stages simultaneously). Using a linearly polarized tunable laser and polarization controller to scan all polarization states, the polarization dependent loss was measured to be <0.12 dB for all four waveguides. By coupling the broadband ASE signal into each waveguide and measuring the light emerging from that waveguide and its neighbors the cross-talk was measured to be <-26 dB throughout the 1.54–1.6 µm spectral region.

3.2 Fan-out performance when coupling light between an FVA and an MCF

The performance of the fan-out when used to couple light between an FVA and an MCF was investigated as follows. Firstly, an FVA with four C-SMF-28 fibers each spaced by 250 µm core-to-core was directly butt coupled to the FVA side of the fan-out device using index matching gel and a manual x-y-z translation stage with pitch, roll and yaw angular adjustment. Coarse alignment of the FVA was performed by imaging the MCF coupling side of the fan-out onto an IR-Vidicon camera. A 60 cm length of silica MCF was then directly butt coupled to the MCF side of the fan-out device, again using index matching gel and a manual x-y-z translation stage with pitch, roll and yaw angular adjustment. Figure 4 shows a photograph of the fan-out with the MCF and FVA coupled to the left and right sides of the device respectively.

 

Fig. 4. Photograph of the fabricated fan-out device with the MCF and FVA coupled to the left and right sides of the device respectively.

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For the purpose of coupling light into the MCF the IL was defined as the difference in power between light emerging from the FVA and light emerging from the MCF. This is valid since the propagation loss over the 60 cm length of MCF was known to be negligible. When optimizing the coupling of light through each waveguide individually, the ILs were measured to be 5.9, 3.7, 3.4 and 5.1 dB for waveguides 1, 2, 3 and 4 respectively, slightly higher than the results obtained using C-SMF-28 fibers. Again by using the technique described in [13], it was confirmed that the increased ILs are due to higher waveguide-to-MCF fiber coupling losses of 0.8 dB±0.2 dB. To investigate this further, near field images of the MCF modes were obtained in the same manner as for the waveguide modes. Each core of the MCF was observed to be single mode at 1.55 µm, with an average MFD of 10.9 µm±1.0 µm, within error of the 10.4 µm±0.8 µm MFD of C-SMF-28 [12]. The exact shape of each MCF mode was however slightly different from core-to-core, with ellipticity ratios ranging from 1.1 to 1.3. The increased coupling losses are the result of the asymmetry of the MCF modes and, in particular, the rotational alignment of these modes with the elliptical waveguide modes. Since the increase in coupling loss was small we did not investigate this further.

To complete the characterization, the FVA and MCF were aligned to allow simultaneous coupling to all four MCF cores without further adjustment of the alignment. Under this alignment, the ILs were measured to be 6.5, 4.3, 3.9 and 5.4 dB for waveguides 1, 2, 3 and 4 respectively, an average of only 0.5 dB higher than that measured when the coupling to each waveguide was optimized individually.

4. Conclusions

In this paper we have presented a three dimensional fan-out device for MCF coupling applications. Although it could be concluded that the ILs presented here are too high for practical applications, particularly when compared to the ≈1 dB values reported for the fan-out fabricated by Flockhart et al. [4], the rugged construction and geometrical flexibility of ultrafast-laser inscribed fan-out devices may make them attractive for future applications. In previous studies by another group, fiber-to-waveguide coupling losses of ≈0.1 dB/facet [14] and propagation losses of ≈0.2 dB.cm^-1 were reported for ultrafast-laser inscribed waveguides [15]. Furthermore, the same group also demonstrated that ultrafast-laser inscribed waveguides can exhibit low loss bends (<0.1 dB) performing deviations similar to those required for the fan-out application [14]. Consequently, we conclude that with optimized fabrication significantly lower insertion losses should be possible. Future work will focus on reducing the ILs by a combination of using higher quality translation stages to reduce propagation losses, using properly designed waveguide bends to reduce bend losses and by precisely tuning the size of the fan-out waveguide modes to reduce coupling losses. Finally, we believe that fan-out devices fabricated in this way could be an enabling technology for future multi-core fiber applications.