Ultra-broadband polarisation beam splitters and rotators based on 3D-printed waveguides
doi: 10.37188/lam.2023.022
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Abstract: Multi-photon lithography has emerged as a powerful tool for photonic integration, allowing to complement planar photonic circuits by 3D-printed freeform structures such as waveguides or micro-optical elements. These structures can be fabricated with a high precision on the facets of optical devices and enable highly efficient package-level chip–chip connections in photonic assemblies. However, plain light transport and efficient coupling is far from exploiting the full geometrical design freedom offered by 3D laser lithography. Here, we extended the functionality of 3D-printed optical structures to manipulation of optical polarisation states. We demonstrate compact ultra-broadband polarisation beam splitters (PBSs) that can be combined with polarisation rotators and mode-field adapters into a monolithic 3D-printed structure, fabricated directly on the facets of optical devices. In a proof-of-concept experiment, we demonstrate measured polarisation extinction ratios beyond 11 dB over a bandwidth of 350 nm at near-infrared telecommunication wavelengths around 1550 nm. We demonstrate the viability of the device by receiving a 640 Gbit/s dual-polarisation data signal using 16-state quadrature amplitude modulation (16QAM), without any measurable optical-signal-to-noise-ratio penalty compared to a commercial PBS.
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Figure 1. Concept of a 3D-printed polarisation beam splitter and rotator in an integrated optical assembly (not drawn to scale). The structure connects a rotationally symmetric single-mode fibre (SMF) with degenerate polarisation states (red and blue arrows) to a photonic integrated circuit (PIC) with highly polarisation-sensitive waveguides. As an example of high practical interest, we illustrate a dual-polarisation receiver for coherent communications, in which data signals in orthogonal polarisation states are split and independently detected using a pair of coherent optical receivers (Coh. Rx) which are fed by a joint local oscillator (LO). The polarisation beam splitter (PBS) and the polarisation rotators (PR) can be merged with additional 3D freeform waveguide elements such as mode-field adapters to form a single monolithic structure. This structure can be fabricated in a single exposure step by high-resolution 3D-laser lithography, thereby offering the freedom to adapt the geometry of the 3D-printed structure to the positions of the various optical device facets.
Figure 2. Concept and design of 3D-printed waveguide-based PBS. a 3D model of the PBS, comprising an input waveguide port with a circular cross section and a pair of output waveguide ports with rectangular cross sections of high aspect ratio. The two orthogonally polarised modes at the input port are denoted as $ {E}_{H}^{\left(I\right)} $ and $ {E}_{V}^{\left(I\right)} $, whereas $ {E}_{H}^{\left(V\right)} $ refers to the horizontally and $ {E}_{V}^{\left(V\right)} $ to the vertically polarised mode at the vertical output V, while $ {E}_{V}^{\left(H\right)} $ denotes the vertically and $ {E}_{H}^{\left(H\right)} $ the horizontally polarised mode at the horizontal output H. The PBS consists of three segments denoted by A, B, and C. Within Segment A, the circular cross section at the input port is adiabatically morphed into a cross-shaped cross section. Within Segment B, the structure can be represented by two spatially overlapping partial waveguides WGH and WGV with high-aspect-ratio rectangular cross sections, which are gradually separated to drag the strongly guided eigenmodes into the two distinct waveguides at the input of Segment C. The 3D rendering of the structure also depicts the simulated electric field distribution for a horizontally polarised excitation $ {E}_{H}^{\left(I\right)} $ at the input port at a wavelength of 1550 nm. The PBS exhibits full geometrical symmetry with respect to a plane that is oriented at 45° between the horizontal and the vertical direction, see Inset 1. The refractive index of the 3D-printed PBS core region amounts to nPBS = 1.53, and the cladding material is air, nclad = 1. b Electric field plots (|E|) of the fundamental modes for both polarisations at all three ports of the PBS, calculated for a wavelength of 1550 nm. The arrows indicate the orientation of the dominant transverse component of the electric field. The strongly guided target modes $ {E}_{H}^{\left(H\right)} $ and $ {E}_{V}^{\left(V\right)} $ at the horizontal and vertical output exhibit a higher effective index and a stronger confinement to the rectangular core than the undesired modes $ {E}_{V}^{\left(H\right)} $ and $ {E}_{H}^{\left(V\right)} $. c Simulated wavelength dependence of the squared magnitudes of complex scattering parameters (S-parameters) and the reciprocal of the polarisation extinction ratio (1/PER) of the PBS on a logarithmic scale. The transmission is better than –2.0 dB with a maximum of approximately –1.55 dB near λ = 1550 nm. The reciprocal of the polarisation extinction ratio (1/PER) and the spurious coupling $\big| $ S $ {}_{{E}_{V}^{\left(H\right)}{E}_{H}^{\left(I\right)}}\big|^{2}=\big| $ S ${}_{{E}_{H}^{\left(V\right)}{E}_{V}^{\left(I\right)}}\big|^{2} $, $\big| $ S $ {}_{{E}_{H}^{\left(H\right)}{E}_{V}^{\left(I\right)}}\big|^{2}=\big| $ S ${}_{{E}_{V}^{\left(V\right)}{E}_{H}^{\left(I\right)}}\big|^{2} $, and $\big| $ S ${}_{{E}_{H}^{\left(V\right)}{E}_{H}^{\left(I\right)}}\big|^{2}=\big| $ S ${}_{{E}_{V}^{\left(H\right)}{E}_{V}^{\left(I\right)}}\big|^{2} $ between input and output modes are below –16 dB over the 400 nm wide wavelength range. These parameters can be further reduced for smaller wavelength ranges. Details on how to extract the PER from the simulations can be found in Supplementary Information Section S3.
Figure 3. Characterisation of 3D-printed PBS using an infra-red (IR)-sensitive microscope. a Experimental setup: As test structures, we use a series of PBS that are 3D-printed on the facets of a single-mode fibre (SMF) array. Light at a wavelength of 1510 nm is fed to the devices by a laser and a subsequent polarisation controller. Light emitted from the PBS is characterised by an IR microscope equipped with polarisation filter (PF). b Scanning-electron microscope (SEM) images of a fabricated structure on the fibre array. A linear taper structure, shaded in red, is used at the input of the PBS to adapt the mode-field diameter of the SMF to the one of the PBS input. Within the PBS, which is shaded in green, the light is split into two orthogonal polarisations and emitted from the outputs (V and H) towards the IR microscope. Colours were added by image processing. c Recordings on the IR microscope for different combinations of input polarisation states, indicated by the different rows: Row 1 – vertical input polarisation only, Row 2 – horizontal input polarisation only, and Row 3 – both vertical and horizontal input polarisations. The columns correspond to the measurement of the radiated power without (Column 1) and with vertically and horizontally oriented polarisation filter (Columns 2 and 3, respectively) in the imaging path of the IR microscope. The output power of each port is estimated by integrating the measured intensity over the areas within the white circles, and a power ratio $ \Gamma $ in dB is calculated by dividing the larger by the smaller power. A top view of the PBS structure and the respective ‘active’ output port for each row is additionally illustrated in Column 1. The orientation of the polarisation axis of the PF is illustrated by the double arrows in the lower right-hand corner of the displays in Columns 2 and 3.
Figure 4. Experimental setup and results of proof-of-concept data transmission experiment. a Simplified experimental setup: The polarisation-division-multiplexed (PDM) 16QAM signal is fed to an SMF having a mode-field adapter and a 3D-printed polarisation beam splitter (PBS) on its facet. The PBS is additionally equipped with 3D-printed polarisation rotators (PR) in the form of twisted waveguides, which rotate the polarisations in both output ports to an identical direction. We simultaneously probe the two output signals by a fan-out structure that is 3D-printed on a second SMF array. The fan-out consists of two lenses and two pairs of total-internal-reflection (TIR) mirrors to adapt the 25 µm pitch of the PBS/PR outputs to the 127 µm pitch of the SMF in the array. The signals are subsequently decoded by a pair of commercial coherent receivers (Coh. Rx). To benchmark our device, we repeat the experiment by replacing the PBS/PR assembly and the fan-out by a commercial fibre-coupled PBS. b Measurement of the PER for both outputs: The PER is better than 11 dB in the wavelength range 1270−1620 nm, which was only limited by the tuning range of the underlying laser sources. c Constellation diagrams of received 80 GBd 16QAM signals for an optical signal-to-noise ratio (OSNR) of 36 dB. Upper row: experiment with our device. Lower row: experiment with the commercial PBS. d Bit-error-ratio (BER) vs. OSNR. Black: Theoretical curve for an ideal transmission system. Blue: Experiment with our 3D-printed PBS/PR assembly. Red: Experiment with the commercial PBS. Our device does not introduce an OSNR penalty with respect to the commercial PBS. At a BER of 1.25 × 10–2, which corresponds to the threshold of forward error correction with 15 % coding overhead, our transmission setup exhibits an implementation penalty of approximately 3 dB, see Supplementary Information Section S5 for details.
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