-
A chip was fabricated containing two blocks of thermal waveguides, as illustrated in Fig 2a. The athermal overwrite segments feature 3 mm long adiabatic tapered terminations, where the pulse energy is ramped in a linear fashion from 0 – 200 nJ. to ensure a smooth transition. The chip contains two blocks of waveguides, corresponding to two respective characterisation experiments. All experimental measurements, hereon, were performed using robotic 6-axis active alignment for coupling the launch and collector single-mode fibres to the chip.
-
This block is comprised of straight waveguides which feature multi-pass segments of incrementally shortened length, ranging from 8 mm to 2 mm, ending with an unmodified reference waveguide; this is analogous to a cutback measurement. The linear propagation loss of the composite waveguide was obtained by measuring the insertion loss of all waveguides in this block and extracting the slope of the linear relationship between insertion loss and composite segment length. The y-axis intercept of the loss curve represents the aggregate contribution of coupling losses and the scattering losses associated with the athermal termination tapers. A comparison of measured mode-field profiles is shown in Fig. 1. Coupling losses were obtained from the overlap integral of the imaged fibre and thermal waveguide modes, hence, from the subtraction, the termination taper losses were calculated. The linear propagation losses of the thermal waveguides are measured simply by taking the average of the insertion loss of the 4 reference waveguides, subtracting the coupling loss and dividing by the length of the chip. These data are summarised in Table 1.
Fig. 1 Measured mode-fields of a standard single-mode fibre, a thermal waveguide and a composite waveguide guiding at a wavelength of 1550 nm, imaged with an InGaAs SWIR camera.
Loss Component Magnitude Unit Linear Propagation - Thermal 0.18 dB cm−1 Linear Propagation - Composite 0.50 dB cm−1 Coupling Mode-Field Mismatch 0.06 dB Athermal Termination 0.18 dB Table 1. Waveguide linear loss components.
The excess loss of the athermal termination tapers is due to localized glass damage in the low power region of the taper. This loss component also varies between 0.10 and 0.18 dB. This variance arises from high sensitivity to the precise transverse positioning of these damage effects, relative to the waveguide core. For rigor, the worst-case value of 0.18 dB is used for subsequent simulation.
-
The second block contains sets of waveguides featuring s-bends based on two concatenated circular arcs with a total y-dimension side-step of 500 µm over a prescribed length L in the x-dimension (see Fig. 2a), which is varied from 8 mm to 2 mm in 0.2 mm increments. Circular arcs were selected since they feature constant curvature, the radius of which, scales proportionately to L, equating to a bend radius scan spanning 32.1 mm to 2.1 mm. For each value of L, two waveguides are studied. The first is a purely thermal modification which serves as a reference. The second waveguide features a composite modification segment spanning the length of the s-bend and a 3 mm adiabatic transition region at each end. The total insertion loss was decomposed in order to extract the underlying pure bend-loss component, which is plotted as a function of bend radius in Fig. 2b). The composite waveguide offers clear advantages in tight bending scenarios, which are sufficient to outweigh the associated loss penalties caused by scattering and the taper losses. Subtracting the coupling losses and the contributions of the straight waveguide lead-in segments, and considering the insertion loss of only the s-bends, the abscissa at which the thermal and composite regimes have equal insertion loss occurs at a bend radius of 9.8 mm. For a 500 µm side-step, the measured abscissa corresponding to a 1.0 dB cut-off threshold for s-bend insertion loss occurs at a bend radius of 3.0 mm for the composite waveguide and 9.15 mm for the thermal waveguide, In other words, the minimum x-dimension length required to achieve 1.0 dB insertion loss s-bend with a 500 µm side-step was reduced from approximately 4.2 mm to 2.4 mm, a reduction of 43%. Next, considering specifically the extracted bend-loss component, the 1.0 dB cm−1 cut-off occurs at a bend radius of 11.0 mm and 3.5 mm, for the thermal and composite waveguides, respectively, a 69% improvement which compares favourably with the state-of-the-art previously reported at 10 mm, using an integrated microcrack technique22.
Fig. 2 a Schematic diagram of the multi-pass experiment chip. The upper block of waveguides iterates the minimum radius of curvature across a series of circular arc s-bends, in order to characterise the evolution of the bend-loss component. The lower block is a set of straight waveguides featuring composite segments of decreasing length, in order to measure linear propagation losses. The red lines denote regions of composite modification, where the waveguide core has been overwritten with an athermal modification. b Plot showing the simulation and measurement of the bend-loss component (i.e. excluding linear propagation and coupling losses) for both, conventional thermal waveguides and the novel composite waveguide for progressively smaller radii of curvature at 1550 nm wavelength.
-
It was not feasible to directly measure the transverse refractive index profile of the composite waveguide, due to limitations associated with the respective measurement techniques. Measurement of thermal waveguides is achieved using the refracted near-field technique (RNF)26; however this method is unsuitable for studying the athermal modifications due to the presence of highly scattering microvoids in the region of negative index change. Conversely, quantitative phase imaging (QPI)27, which measures optical pathlength differences, is unsuitable for studying thermal waveguides due to the requirement for a short modification length. This is problematic because in the thermal regime it is not possible to fabricate a perfect waveguide termination, instead, the transient dissipation of the plasma results in rounded off and tapered structures, for which no valid measurement can be recorded. Importantly, both techniques measure the refractive index distribution in the visible range (632 nm for Rinck RNF profiler, 600 nm for the Phasics QPI camera), while the structures under test are intended to guide at 1550 nm. It was found that this leads to an overestimation of refractive index change. Hence, it was necessary to extract the refractive index contrast via comparative study of the experimentally measured bend losses and simulation (RSOFT FEMsim), based on the finite element method. This is achieved by stepping the refractive index delta of the simulation until agreement is reached by the simulated and experimentally measured bend loss curves.
Bend loss is extremely sensitive to refractive index contrast. Consequently, the bend loss curve represents a very robust tool for extracting the index contrast. For example, the peak positive index change of the thermal waveguide profile is 8.5 × 10−3 according to the RNF measurement. However, from the bend-loss curve, the true value is revealed to be 6.5 × 10−3, a 30% overestimation. Since the composite index profile cannot be measured directly with either of the two techniques, a synthetic approximation was crafted by additively combining the respective profiles of the two underlying modifications in MATLAB. The refractive index addition is not one-to-one, but instead equivalent to the thermal waveguide plus approximately 44% of the athermal index change. This value was determined via iterative simulation where the contribution of the athermal modification is progressively increased until good agreement with measured data occurred. Further study is required to determine the physical explanation for the incomplete addition; we note that the thermal inscription pass has changed the structure and composition of the glass in the focal volume and the incident beam is being focused through the existing waveguide which influences the focal intensity distribution. The final value for the real positive index contrast (relative to the bulk material) was found to be 1.12 × 10−2, or 0.8%. The respective transverse refractive index profiles are shown in Fig. 3. The polarization dependence of the composite waveguides has not been explicitly studied yet. However, the birefringence of athermal multiscan waveguides and annealed thermal waveguides, have been previously measured to be on the order of 2.5 × 10−4 and 1.2 × 10−6, respectively, in boro-aluminosilicate glass28,29.
Low bend loss, high index, composite morphology ultra-fast laser written waveguides for photonic integrated circuits
- Light: Advanced Manufacturing 5, Article number: (2024)
- Received: 07 September 2023
- Revised: 06 January 2024
- Accepted: 16 January 2024 Published online: 14 March 2024
doi: https://doi.org/10.37188/lam.2024.009
Abstract: We demonstrate a novel, composite laser written 3D waveguide, fabricated in boro-aluminosilicate glass, with a refractive index contrast of 1.12 × 10−2. The waveguide is fabricated using a multi-pass approach which leverages the respective refractive index modification mechanisms of both the thermal and athermal inscription regimes. We present the study and optimisation of inscription parameters for maximising positive refractive index change and ultimately demonstrate a dramatic advancement on the state of the art of bend losses in laser-written waveguides. The 1.0 dB cm−1 bend loss cut-off radius is reduced from 10 mm to 4 mm, at a propagation wavelength of 1550 nm.
Research Summary
Bending the Rules: Composite High Index Laser Written Waveguides
Microscopic surgery on laser written waveguides results in boosted refractive index contrast, leading to a dramatic improvement in light confinement when turning tight corners. 3D ultrafast laser written photonic devices fill a vital role in enabling future fibre networks and next-generation computing applications by bridging the gap between silicon and optical fibre. A key challenge is the enormous difference in feature size between these two media. Addressing this requires dramatic improvements in the miniaturisation of laser written circuits. The limiting factor is bend loss, which determines the minimum radius of curvature. Andrew Ross-Adams and the Macquarie University team report on the development of a novel composite laser written waveguide which combines two distinct physical mechanisms of refractive index change and achieves a 2.5x improvement to the minimum bend radius compared to previous literature.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article′s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article′s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.