Free Propagation Region Research Articles

Crosstalk reduction for Arrayed waveguide gratings on Silicon-on-Insulator platform

Ultracompact silicon-based arrayed waveguide gratings (AWGs) with low loss and low crosstalk are essential for on-chip optical interconnect and miniaturized spectroscopic analysis systems. In our previous research, we demonstrated the crosstalk reduction by utilizing constant projection period for phased array waveguide on the tangent line to the grating pole to reduce the imaging error induced by phased array waveguide end point positions [1] and using deep etching waveguides at the interface of free propagation region to further decrease the footprint of phase region of an AWG to reduce fabrication imperfections induced phase error [2]. Here we first study the effect of curved waveguides in the phased array of 16 × 400 GHz AWGs on the adjacent crosstalk of output channels. According to the AWGs made in our fabrication platform and the testing results of five dies from one wafer at different locations, the AWGs with equal length of single-mode curved waveguides have an average adjacent crosstalk level of ∼ 4 dB smaller than those with equal length of narrow single-mode waveguides composed of straight and curved waveguides in their phased array. Based on the design of equal curved waveguide length in phased array, in combination with our reported approaches on crosstalk reduction, 16 × 200 GHz, 8 × 800 GHz and 8 × 1250 GHz AWGs are fabricated. For characterization, five dies including all designs from different locations of the fabricated wafer are tested. Experimental results show that among these five dies, the best adjacent crosstalk levels are less than − 21.97 dB, −25.72 dB, −24.83 dB and the worst adjacent crosstalk levels are less than − 20.51 dB, −25.03 dB, −24.34 dB for 16 × 200 GHz, 8 × 800 GHz and 8 × 1250 GHz AWGs with FPR angle of 120°, respectively, within the passband width of 25 % channel spacing. The on-chip insertion losses for all output channels of these fabricated AWGs are less than 3 dB.

Low Loss 1 × 16/40 Flat Type Beam Splitters on Thin Film Lithium Niobate Using Photolithography Assisted Chemo‐Mechanical Etching

AbstractIntegrated photonic devices based on thin film lithium niobate (TFLN) have attracted great attention due to their excellent performance. In this work, a flat type TFLN 1×N beam splitter is designed by adjusting the widths of tapered waveguides between free propagation region and arrayed waveguides. Two chips with 16 and 40 output ports, respectively, are manufactured with the femtosecond laser photolithography assisted chemo‐mechanical etching technology (PLACE). The excess losses are measured ≈1.43 and 1.94 dB, respectively. In theory, the flat‐type beam splitter for a single‐mode structure can maintain the flat intensity distribution within a 300 nm wavelength range. Experimentally, different types of output intensity distribution such as tilted or M‐shaped distributions can be obtained with the multimode structure by varying the position of the lensed fiber when the input light is TM‐polarized. This work explores an efficient way for the development of multichannel optical beam splitters.

On-chip multi-beam frequency shifter through sideband separation.

In this paper, we introduce a novel method to realize a multi-beam optical frequency shifting component for photonic integrated circuits, utilizing an array of parallel optical modulators and a free-propagation region (FPR), such as a slab waveguide-based star coupler. This component generates multiple optical beams with different frequency shifts, making it suitable for various systems, such as multi-beam laser Doppler vibrometry (LDV). We thoroughly elaborate on the working principle of the component through theoretical analysis and demonstrate that by applying periodic wave-like modulation in the modulator array, the discrete harmonic content of the light can be selectively directed to different outputs based on the delay between consecutive modulators. A design comprising a 16-element modulator array and 5 outputs will be presented. Simulations show that this design can generate and collect 5 different harmonics (-2, -1, 0, +1, +2) in the different outputs with a side band suppression ratio of 20 dB to 30 dB for each output. Our proposed design is just one possibility and the component can be modified and optimized for specific applications.

Flat optical phased array receiver incorporating an on-chip metalens concentrator.

We propose and design a flat optical phased array (OPA) receiver that consists of a grating antenna, a free-propagation region (FPR) incorporating an on-chip metalens concentrator (OCMC), and an output port of a tapered waveguide. By concatenating the OCMC-integrated FPR with the antenna, the proposed OPA allows light coupled at a slanted ψ angle to be conveyed to the output, thereby resolving the challenges of phase-controlled light detection. To impose a space-dependent phase on the incident light from the antenna, the OCMC is constructed by laterally arranging subwavelength slot meta-atoms with varying slot lengths, which are created in the core layer of a slab and uniformly quantized at 16 phase levels. Hence, without the aid of phase modulators, the light beam emerging from the grating antenna can be focused on the output port through angle-tolerant coupling along the lateral direction. The miniaturized OCMC was confirmed to play a pivotal role in achieving enhanced in-plane coupling efficiency over the field of view.

High Resolution and Ultra-Compact On-Chip Spectrometer Using Bidirectional Edge-Input Arrayed Waveguide Grating

Two Edge-input arrayed waveguide gratings (AWGs) with star couplers of Confocal and Rowland configurations, respectively, are experimentally investigated and the one with the Rowland configuration shows better performance in terms of insertion loss and crosstalk. Based on the optimized Rowland configuration, two 64 × 0.4 nm ultra-compact on-chip spectrometers using bidirectional AWGs with two side inputs and different lengths of free propagation region (FPR) are designed and fabricated. Due to the utilization of two side inputs, the footprint of AWG spectrometer is significantly reduced compared to the conventional one center-input AWG spectrometer with the same resolution. The reduced footprint contributes to the performance improvement for an AWG spectrometer on high index contrast platforms, which is verified in our fabricated spectrometers on the silicon-on-insulator platform. The demonstrated ultra-compact on-chip spectrometer has large potential applications in miniaturized spectral analysis systems on a chip.

Compact ultrabroad-bandwidth cascaded arrayed waveguide gratings.

Here, we present a compact, high-resolution, and ultrabroad-bandwidth arrayed waveguide grating (AWG) realized in a silicon nitride (Si3N4) platform. The AWG has a cascaded configuration with a 1×3 flat-passband AWG as the primary filter and three 1×70 AWGs as secondary filters (i.e. 210 output channels in total). The primary AWG has 0.5-dB bandwidth of 45 nm over 190 nm spectral range. The ultrabroad-bandwidth is achieved by using an innovative design that is based on a multiple-input multi-mode interference (MMI) coupler placed at the entrance of the first free propagation region of the primary AWG. The optical bandwidth of the cascaded AWG is 190 nm, and the spectral resolution is 1 nm. The overall device size is only 1.1 × 1.0 cm2. Optical loss at the central channel is 4 dB, which is 3 dB less than a conventional design with the same bandwidth and resolution values but using a primary filter with Gaussian transfer function. To the best of our knowledge, this is the first demonstration of an ultrabroad-bandwidth cascaded AWG on a small footprint. We also propose a novel low-loss (∼ 0.8 dB) design using a small AWG instead of an MMI coupler in the primary filter part, which can be used in applications where the light intensity is very weak, such as Raman spectroscopy.

Athermal echelle grating filter in silicon-on-insulator using a temperature-synchronized input.

Athermal design of integrated photonic devices can reduce the need for active temperature stabilization and consequently the energy required to operate photonic integrated circuits. For silicon photonic filters such as AWGs which employ wire or ridge waveguides, temperature insensitivity can be achieved using cladding materials with negative thermo-optic coefficients. On the other hand, in echelle grating filters the inteference takes place in the slab free-propagation region, and therefore the modal overlap with the cladding is small, rendering this method ineffective. In this work we present an approach to design an athermal echelle grating filter exploiting a temperature-synchronized Mach-Zehnder interferometer as input. This reduces the spectral shift over a temperature range of 20 K to less than ±45 pm compared to the 1.6 nm shift for the same echelle grating with a conventional waveguide input. Furthermore, the proposed design relies exclusively on a standard fabrication process for silicon-on-insulator photonic devices and exhibits a good tolerance to fabrication uncertainties.

Silicon-based cyclic arrayed waveguide grating routers with improved loss uniformity

We present silicon-on-insulator (SOI)-based cyclic arrayed waveguide grating routers (AWGRs) with improved channel loss uniformity in the full free spectral range (FSR) by using dual-tapered auxiliary waveguides between the ends of arrayed waveguides. The field distribution at the image plane of the output free propagation region (FPR) is adjusted by the auxiliary waveguides and a flat-top shape can be obtained by appropriate optimization. 8×8 and 15×15 AWGRs with optimized auxiliary waveguides, having a channel spacing of 3.2 nm and 1.6 nm, respectively, are fabricated and the measured loss non-uniformity is less than 0.5 dB, significantly improved by at least 1.5 dB compared with conventional AWGRs.

Methods for Low Crosstalk and Wavelength Tunability in Arrayed-Waveguide Grating for On-Silicon Optical Network

Arrayed waveguide gratings (AWGs) are used to accomplish complex functionalities in dense wavelength division multiplexing networks, as well as in signal processing, measurement, characterization or sensing. In this work, we discuss the design and the experimental characterization of a 6 × 400 GHz channel-spaced AWG to be used as wavelength multiplexer/demultiplexer in a silicon-on-insulator based optical transport network. We adopted a whiskered-shaped star coupler design to investigate how the reduction in the reflectivity of the free propagation region boundaries affects the overall performances of the AWG. We demonstrated 2.9 dB insertion loss, which does not include fiber coupling loss, and ( $-24.4\,\pm \,0.1$ ) dB crosstalk for a design realized with a single etching technique. Moreover, we demonstrated an efficient fine-tuning (7.5 nm/W) of the spectral position of the AWG output channels.

Low-loss single-mode operation in silicon multi-mode arrayed waveguide grating with a double-etched inverse taper structure

We investigate the single-mode operation in a silicon arrayed multi-mode waveguide grating. By introducing a double-etched structure at the boundary of a star coupler with inverse-tapered waveguides (WGs), the suppression of the mode-coupling between adjacent arrayed multi-mode WGs, and the adiabatic optical mode conversion between a FPR (free propagation region) and arrayed WGs can be achieved with a reduced phase error of the arrayed WGs. The fabricated four-channel Si arrayed waveguide grating (AWG) demonstrates a good performance level, such as a low insertion loss of 0.61 dB (0.73 dB) and low adjacent crosstalk less than −31.3 dB (−32.2 dB) for transverse-electric (TE) (transverse-magnetic, TM) polarization, in the range of ∼1300 nm wavelength.

Performance improvement for silicon-based arrayed waveguide grating router

We analyze the impact of aberration on spectral performance of silicon-based arrayed waveguide grating (AWG) router with the conventional design using a constant pitch along the grating circle for the array waveguides near the free propagation region (FPR), and simulation results show that due to existence of large aberration, side lobes occur in spectral responses of peripheral output channels for the center input light while more serious side lobes appear in most output channels within a free spectral range (FSR) for the edge channel input. Therefore, there is a high crosstalk in conventional N × N silicon AWG, which is very detrimental for router applications. In order to address it, a simple design with a constant projected period on a line tangent to the grating at its pole for the array waveguides near the FPR is proposed, and aberrations of all output wavelengths within a FSR are kept at a rather low level both for the center and edge input. Then we fabricate two kinds of AWG routers with the conventional and proposed design respectively on a SOI wafer, and experimental results show that spectral responses of the AWG router with the proposed design are significantly improved compared to those obtained in the conventional design, especially for the edge channel input.

High-Resolution Arrayed-Waveguide-Gratings in Astronomy: Design and Fabrication Challenges

A comprehensive design of a folded-architecture arrayed-waveguide-grating (AWG)-device, targeted at applications as integrated photonic spectrographs (IPS) in near-infrared astronomy, is presented. The AWG structure is designed for the astronomical H-band (1500 nm–1800 nm) with a theoretical maximum resolving power R = 60,000 at 1630 nm. The geometry of the device is optimized for a compact structure with a footprint of 5.5 cm × 3.93 cm on SiO 2 platform. To evaluate the fabrication challenges of such high-resolution AWGs, effects of random perturbations of the effective refractive index (RI) distribution in the free propagation region (FPR), as well as small variations of the array waveguide optical lengths are numerically investigated. The results of the investigation show a dramatic degradation of the point spread function (PSF) for a random effective RI distribution with variance values above ∼ 10 - 4 for both the FPR and the waveguide array. Based on the results, requirements on the fabrication technology for high-resolution AWG-based spectrographs are given in the end.

Echelle grating based mode demultipexer for vertical mode-division multiplexing.

We have proposed a mode multi/demultiplexer based on the echelle grating suitable for vertical multimode bus waveguide. Compared with the lateral multimode waveguide, the vertical scheme has much smaller bending radius which leads to compact system size. The guided modes in the multimode waveguide are able to be separated simultaneously by the echelle grating since they experiences different propagation constants in the free propagation region. Due to the high performance of the grating itself, the theoretical modal crosstalk can be as low as around -40dB. Moreover, we demonstrate the possibility to integrate WDM together with MDM by using only one echelle grating, which is favorable in real applications considering the compact system setup and simple design rules.

Low-crosstalk Si arrayed waveguide grating with parabolic tapers.

A silicon arrayed waveguide grating (AWG) with low channel crosstalk was demonstrated by using ultra-short parabolic tapers to connect the AWG's free propagation regions and single-mode waveguides. The tapers satisfied the requirements of low-loss mode conversion and lower channel crosstalk from the coupling of neighboring waveguides in the AWGs. In this work, three different tapers, including parabolic tapers, linear tapers, and exponential tapers, were theoretically analyzed and experimentally investigated for a comparison of their effects when implemented in AWGs. The experimental results showed that the AWG with parabolic tapers had a crosstalk improvement up to 7.1 dB compared with the others. Based on the advantages of parabolic tapers, a 400-GHz 8 × 8 cyclic AWG with 2.4 dB on-chip loss and -17.6~-25.1 dB crosstalk was fabricated using a simple one-step etching process. Its performance was comparable with that of existing AWGs with bi-level tapers, which require complicated two-step etching fabrication processes.

A <inline-formula> <tex-math notation="TeX">\(10 \times 10\) </tex-math></inline-formula>-Gb/s DFB-LD Array Integrated With PLC-Based AWG for 100-Gb/s Transmission

We describe a hybrid integration module of a 10 × 10-Gb/s distributed feedback laser diode array (DFB-LDA) and a planar light-wave circuit-based arrayed waveguide grating (AWG). For fabrication of the DFB-LDA, we adopted a selective area growth technique to tailor the channel gain properly, and E-beam lithography to accurately control the channel lasing wavelength and grating phase. For implementation of the AWG, we introduced a 2%-Δ structure to reduce the coupling loss between the AWG and DFB-LDA, and designed the tapered and parabolic waveguides at the junctions of the free propagation regions to widen the spectral passband width (i.e., a -1-dB spectral width of 3 nm). The developed module shows a side-mode suppression ratio of >45 dB, a clear eye opening with a dynamic extinction ratio of >4.4 dB at 10 Gb/s, and a power penalty of <;1.5 dB after a 2-km transmission for all channels.

Arrayed waveguide collimators for integrating free-space optics on polymeric waveguide devices.

Array-type optical devices are important for wavelength-division multiplexing optical communication system to achieve small footprint, mass production, and reliability. For fabricating transmitter module in an array configuration, it is difficult to achieve a passive alignment of isolator, collimating lens, and laser diode. To facilitate array isolator integration, a waveguide collimator is proposed in this work by using a low-contrast, large-core polymer waveguide. The diffraction of a guided mode propagating through a free-space region is suppressed by enlarging the guided mode. The fiber coupling loss due to the enlarged mode was overcome by incorporating an adiabatic taper structure. The excess loss of waveguide collimator including the loss through a 400-μm free-propagation region was less than 1.0 dB.

Low-loss Si_3N_4 arrayed-waveguide grating (de)multiplexer using nano-core optical waveguides

A 16-channel 200 GHz arrayed-waveguide grating (AWG) (de)-multiplexer is demonstrated experimentally by utilizing Si3N4 buried optical waveguides, which have 50 nm-thick Si3N4 cores and a 15 μm-thick SiO2 cladding. The structure with an ultra-thin core layer helps to reduce the scattering due to the sidewall roughness and consequently shows very low loss of about 0.4~0.8 dB/m. When using this type of optical waveguide for an AWG (de)multiplexer, there is no problem associated with gap refill using the upper-cladding material even when choosing a small (e.g., 1.0 μm) gap between adjacent arrayed waveguides, which helps to reduce the transition loss between the FPR (free-propagation region) and the arrayed waveguides. Therefore, the demonstrated AWG (de)multiplexer based on the present Si3N4 buried optical waveguides has a low on-chip loss. The fabricated AWG (de)multiplexer is characterized in two wavelength ranges around 1310 nm and 1550 nm, respectively. It shows that the crosstalk from adjacent and non-adjacent channels are about -30 dB, and -40 dB, respectively, at the wavelength range of 1310 nm. The Si3N4 AWG (de)multiplexer has a temperature dependence of about 0.011 nm/°C, which is close to that of a pure SiO2 AWG device.

Design of a Polarization-Insensitive Echelle Grating Demultiplexer Based on Silicon Nanophotonic Wires

A polarization-insensitive design of an echelle grating (or etched diffraction grating) demultiplexer based on silicon nanowires is proposed in the present letter, by introducing a polarization compensation area in its free propagation region. The polarization-dependent wavelength shift of the present device has been compensated at the whole spectral range. For a design with nine channels and 10-nm channel spacing, when the wavelength shift at the central channel 1550 nm is completely compensated, the wavelength shifts of the two edge channels are only 0.14 and 0.15 nm, which are acceptable.

Low Loss Star Coupler Concept for AWGs in Rib Waveguide Technology

We introduce a new concept for a tapered transition between waveguides and a free propagation region (FPR) for rib waveguide technology. The optical field is pressed downwards to the lower part of the guiding layer by tapering off the waveguide. The subsequent FPR is also etched down to the lower part of the guiding layer, which results in an adiabatic transition from the waveguide to the slab. Arrayed waveguide gratings, designed with this technique, were fabricated in silicon-on-insulator and showed a reduction in insertion loss of about 2.5 dB

Ultrasmall Overlapped Arrayed-Waveguide Grating Based on Si Nanowire Waveguides for Dense Wavelength Division Demultiplexing

A novel design for an ultrasmall arrayed-waveguide grating (AWG) demultiplexer based on a Si nanowire waveguide is presented. The two free propagation regions (FPRs) are overlapped so that the structure becomes more compact than the layout of a conventional design. Each half of an arrayed waveguide includes three straight sections and two bending sections, and all these sections are connected smoothly in sequence. Some recursive formulas for determining the geometrical parameters of the novel layout are derived. Numerical simulation results are given