We report the design and fabrication of three-dimensional integrated mode couplers operating in the C-band. These mode-selective couplers were inscribed into a boro-aluminosilicate photonic chip using the femtosecond laser direct-write technique. Horizontally and vertically written two-core couplers are shown to allow for the multiplexing of the LP11a and LP11b spatial modes of an optical fiber, respectively, with excellent mode extinction ratios (25-37 + dB) and low loss (~1 dB) between 1500 and 1580 nm. Furthermore, optimized fabrication parameters enable coupling ratios close to 100%. When written in sequence, the couplers allow for the multiplexing of all LP01, LP11a and LP11b modes. This is also shown to be possible using a single 3-dimensional three-core coupler. These integrated mode couplers have considerable potential to be used in mode-division multiplexing for increasing optical fiber capacity. The three-dimensional capability of the femtosecond direct-write technique provides the versatility to write linear cascades of such two- and three-core couplers into a single compact glass chip, with arbitrary routing of waveguides to ensure a small footprint. This technology could be used for high-performance, compact and cost-effective multiplexing of large numbers of modes of an optical fiber.
© 2014 Optical Society of America
Mode-division multiplexing (MDM) stands as one of the final frontiers for increasing optical fiber capacity [1–4]. The idea is that each mode of a few-mode fiber can be used as an individual data channel, so that an N-mode fiber will have roughly the same capacity as N single-mode fibers . Although simple in principle, this approach carries some considerable technological challenges, including digital signal processing to unravel mode crosstalk, the inline amplification of several modes and the topic of this paper, the actual excitation and detection of the individual modes at either end [1–9]. The difficulty with multiplexing and demultiplexing fiber modes stems from the fact that their intensity distributions overlap and so standard spatial filters cannot be used. The most common techniques employed for exciting and detecting spatial modes in experimental demonstrations of few-mode fiber networks have relied on the use of lossy bulk free-space optics [1, 2]. In recent years, however, research has pushed for the development of simpler, fiber/waveguide-based mode multiplexers that could make spatial mode-division multiplexing more practical. Simple one-to-one mode mapping fiber/waveguide-based solutions include asymmetric Y-junctions [5–7], dissimilar-core photonic lanterns [8, 9], long-period gratings , tapered velocity couplers [11–14], and the focus of this paper, asymmetric mode-selective couplers [4, 15–19].
Mode-selective couplers typically involve a higher-order mode in a fiber core coupling to the fundamental mode of another closely-positioned fiber core, or vice versa . The mode-selective functionality is achieved by matching the modal propagation constants, for instance by using different core diameters. The theory of these interference-based devices has recently been delineated [4, 19], and a few attempts at making such devices have been reported [15–18]. However the fabrication techniques used have invariably provided limited foreseeable scalability or reproducibility, and have focused exclusively on two-core devices.
If both orientation states (e.g. LP11a and LP11b) of an asymmetric mode (e.g. LP11) are to be excited or detected separately, an inline pair of two-core devices with appropriate angular offset is required . Alternatively, a single three-core device can be used . However, the fabrication techniques used to date are not suited to the realization of such configurations. The ability to distinguish between orthogonal modal orientations is of high importance in coherent MDM where each degenerate orientation state is used as a unique data channel. In this case, the one-to-one mode mapping characteristic of such couplers facilitates the equalization of mode-dependent effects such as differential group delay and mode-dependent loss/gain .
In this paper we demonstrate the fabrication of two- and three-core C-band mode couplers, written into a photonic chip using the femtosecond laser direct-write technique [21, 22]. This represents the first realization of three-dimensional non-tapered mode couplers in a photonic chip. It also represents an extension of previous research in which three-dimensional tapered mode couplers (operating in the 600 to 1000 nm waveband) were successfully fabricated using the same technique . It also relates to prior research into spot-based couplers fabricated using direct-write, but which only achieved one-to-many mode mapping . The unprecedented versatility afforded to the femtosecond direct-write technique accommodates for the inscription of intricate three-dimensional architectures of two- and three-core couplers with arbitrary orientations, otherwise not possible using planar technologies . This approach could therefore be used to successively multiplex (or demultiplex) all the symmetric and asymmetric modes of a few-mode optical fiber . The technique also provides the flexibility for arbitrary routing of the input (or output) waveguides, such that a large number of modes could be addressed in a single compact chip. In the following sections we present a proof of concept of the two- and three-core coupler building blocks of such envisioned photonic architectures.
2. Design and fabrication
The mode-selective couplers were designed using the tried and tested Finite Element and Beam Propagation Methods. The slight asymmetries in the refractive index profiles of femtosecond direct-written waveguides , were taken into account during the numerical design. This core-ellipticity lifts the LP11a/LP11b mode degeneracy and hence the phase-matching conditions for the horizontal and vertical two-core couplers are different. The femtosecond laser pulse energies used for waveguide inscription, the interaction lengths and/or core-to-core separations therefore require appropriate adjustment. The key features of the horizontal and vertical two-core mode couplers are shown in Fig. 1(a). The core diameters (and corresponding laser pulse energies) were chosen such that the propagation constants of the fundamental LP01 modes in the horizontal and vertical single-mode (SM) cores matched those of the respective orthogonal LP11a and LP11b modes in the multimode (MM) core. The coupler dimensions shown were optimized via both numerical simulations and experimental trials. At both the input and output of the mode couplers cosine bends were used to spatially separate the waveguides. Interaction/coupler lengths between 0 and 8 mm were tested for the horizontal and vertical couplers, whereas the core-to-core spacing was 16 µm for both devices. The mode couplers were all fabricated using a high repetition rate Ti:sapphire oscillator (Femtolasers FEMTOSOURCE XL500, 800 nm, 5.1 MHz, < 50 fs). The circularly polarized laser beam was focused 170 µm below the top surface of a 25 mm long boro-aluminosilicate glass chip (Corning Eagle2000) using a 100 × 1.4 NA oil immersion objective.
The glass chip was translated using high precision XYZ air-bearing stages (Aerotech) at a constant velocity of 250 mm/min. The various core diameters were realized by adjusting the pulse energy using a computer controlled attenuator (Aerotech ADR75 rotation stage and half-wave plate/polarizer arrangement). The multimode waveguides were inscribed with a pulse energy of 215 nJ, whereas the horizontal and vertical single-mode waveguides were written with 105 nJ and 131 nJ pulse energy, respectively. These values were optimized with multiple trials to achieve phase-matching. After all the devices were written, the glass chip was thermally post-annealed to simplify the refractive index profiles of the waveguides. This process results in cores of positive index change surrounded by a depressed ring [24–26].
3. Experimental results
The mode couplers were characterized by launching light into the single-mode waveguides and imaging the end-face of the glass chip using an infrared camera (FLIR SC7000). The light was launched via a single-mode fiber (Corning SMF-28e) connected to a tunable C-band laser (Santec TSL-210). Coupling ratios which quantify the total power transfer from the single-mode waveguide to the multimode waveguide were determined based on the ratios of integrated intensities imaged by the camera. Mode extinction ratios (defined as the power ratio of LP01 and LP11 modes) were calculated based on the near-field profiles , for which the null of the LP11 mode in the multimode core provides an estimate of LP01 contamination. The insertion losses were determined by comparing the integrated intensities of the input fiber and the coupler output. The coupling ratios achieved for the horizontal and vertical couplers near 1550 nm were around 50% and 100%, respectively. The reduced coupling ratio of the horizontal coupler is associated with a drift of the laser between the initial parameter scan used to determine the pulse energy for perfect phase-matching, and the inscription of the final chip. As shown in Fig. 2(a)-(b) the wavelength-dependence also closely fits analytic models . The spectral response of the couplers is relatively flat, with only a small variation in coupling performance observed across the entire C-band. The relatively broadband performance would suit the application of wavelength-division multiplexing. The mode extinction ratios achieved are shown in Fig. 3(a) and peak at 32 ± 2 dB and 37 dB for the horizontal and vertical couplers, respectively. The latter is a lower bound, set by the dynamic range limit of the measurement. These values compare favorably even with state of the art mode couplers fabricated using planar technologies or the fused-taper approach [16, 17]. The insertion losses were low at 1.2 ± 0.1 dB for both the horizontal and vertical couplers, and they were largely independent of wavelength.
The requirement for precise phase-matching along the entire interaction length of these directional couplers means that the fabrication tolerances are inherently low. This can, for instance, be inferred from Fig. 3(b), where the coupling ratio is shown as a function of interaction length for a series of horizontal devices written at two slightly different pulse energies. Varying the pulse energy for single-mode waveguide inscription by just a few percent results in dramatic decreases in the coupling ratio. Consequently, optimization of the coupler performance requires careful tuning of the inscription parameters.
As previously mentioned, a linear cascade of the horizontal and vertical two-core couplers can be used to independently excite or detect both of the LP11a and LP11b modes. Moreover, the LP01 mode could be excited or detected directly via the multimode core. All three modes could however be accessed, instead, by using a single three-core device similar to that shown in Fig. 1(b) . The proof of concept of such a device is shown in Fig. 4. The coupling ratios from the horizontal SM core to the LP11a mode, and the vertical SM core to the LP11b mode are shown in Fig. 4(b), and they peak at around 70% and 40% near 1580 nm, respectively. The reduction in coupling ratios was due to phase mismatch resulting from laser drift relative to the previous fabrication runs, and as previously shown is not attributed to the addition of a third waveguide [4, 13]. This result shows that with appropriate detuning, arbitrary tap-off mode couplers can be produced. The coupling ratios can be tuned by slight variation of the inscription parameters. The mode extinction ratios for this coupler exceeded the dynamic range limits of the measurements with a lower bound being 28–35 dB over the entire bandwidth. The cross-coupling between orthogonal single-mode waveguides was negligible with between 22 and 33 dB suppression for both horizontal and vertical injection. The insertion losses for horizontal and vertical injection were measured at 1.1 and 1.0 dB ( ± 0.1 dB), respectively.
The demonstration of this three-core mode coupler complements previous research into three-core tapered mode couplers fabricated using the same direct-write technique . However, the tapered mode couplers rely on the gradual adiabatic evolution of modes to achieve their functionality, and so inherently have larger footprints than the standard uniform couplers presented here, and this is especially true for modes of high order . In contrast, they have much higher fabrication tolerances and operate over much broader bandwidths . The latter feature is particularly valuable in the context of wavelength-division multiplexing. However, at present C-band three-dimensional tapered mode couplers have not yet been demonstrated. Although the mode couplers presented in this paper are not nearly as broadband, the fact that the interaction length does not dramatically scale with the mode-order means that large numbers of modes could in principle be multiplexed in a compact photonic chip and therein lies the main advantage of such couplers.
The experimental results presented in this paper extend previous research into mode multiplexers fabricated using planar technologies . Unlike with planar waveguides, the near-circular refractive index profiles of femtosecond direct-written waveguides are expected to allow for near-seamless interfacing with few-mode optical fiber. As shown, the three-dimensional capability of femtosecond direct-write also means that both orientation states of a given higher-order mode can be addressed. This is because the technique can accommodate for the fabrication of either three-core couplers or a cascade of two-core couplers with arbitrary angular offset , otherwise not possible using planar technologies.
In conclusion, this paper has demonstrated compact two- and three-core mode-selective couplers fabricated using the femtosecond laser direct-write technique. The couplers allow for the multiplexing of the LP01, LP11a and LP11b modes of an optical fiber over a bandwidth exceeding the C-band. It is shown that such mode couplers can exhibit mode extinction ratios in excess of 37 dB, coupling ratios as high as ~100%, and low insertion losses of around 1 dB. These couplers also represent the first one-to-one mode mapping C-band devices fabricated using the direct-write technique. This novel fabrication technique provides the versatility for fabricating intricate photonic mode-multiplexing architectures and in potentially large scales. This paper has provided the proof of concept of the mode coupler building blocks of such integrated mode multiplexers envisioned for the multiplexing and demultiplexing of many modes of an optical fiber, for future ultra-high bandwidth optical communication networks.
This research was supported by the Australian Research Council Centre of Excellence for Ultrahigh bandwidth Devices for Optical Systems (project CE110001018) and was performed in part at the OptoFab node of the Australian National Fabrication Facility using Commonwealth and NSW State Government funding. N. Riesen is supported by an ARC Laureate Fellowship awarded to T. M. Monro. S. Gross acknowledges a Macquarie University Research Fellowship.
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