Demonstration of Directly Nanoimprinted Silica-Titania Large-Size Vertical Grating Couplers for Multichannel Photonic Sensor Development

Abstract

The article discusses the design, fabrication, and experimental evaluation of a large-area vertical grating coupler (VGC) enabling simultaneous coupling of multiple input optical beams. The presented VCG was fabricated by direct nanoimprinting of a grating pattern in a non-hardened SiO_X:TiO_Y waveguide (WG) film. The WG film was deposited on a glass substrate using a combination of the sol-gel method and the dip-coating technique. The fabrication process allowed precise control of the waveguide film thickness and refractive index, as well as the VGC geometry. The relevance of the process was proved by a demonstration of optical coupling of multiple quasi-parallel input beams via the VGC to the WG layer. To make this possible, a dedicated optical coupling system was designed, including a polymer microlens array and optical fiber array positioned in a V-groove. This opens promising perspectives on using the proposed structure for the fabrication of low-cost multichannel optical sensor chips, as highlighted in the article's final section.

Keywords: multichannel photonic sensors; on-chip optical signal distribution; silica-titania waveguides; sol-gel; vertical grating coupler.

Figure 1

Schematic of sensor system with integrated photonic transducer.

Figure 2

A schematic of the side view of a waveguide with a sensing opening in the top cladding layer, allowing the ambient environment to interact with a light signal guided by the waveguide.

Figure 3

Schematic representations of exemplary layouts of integrated photonics devices for sensory applications: (A) Young interferometer; (B) Mach–Zehnder interferometer; (C) ring resonator.

Figure 4

Technological characteristics of SiO_X:TiO_Y WG film thickness and refractive index as function of substrate withdrawal speed from sol, demonstrating possibility of fine-tuning of SiO_X:TiO_Y WG film properties.

Figure 5

Schematic representation of fabrication process for SiO_X:TiO_Y directly patterned VGCs: (i) sol formation and ageing process; (ii) dip-coating deposition; (iii) imprinting process. Acronyms: H—hydrolysis; C—condensation; D—drying.

Figure 6

A schematic of the multichannel coupling scheme via a single large-area VGC. FA—optical fiber array; LA—microlens array; VGC—vertical grating coupler imprinted on SiO_X:TiO_Y waveguide film.

Figure 7

A photograph of the optical fiber transmission head composed of four single-mode optical fibers attached to an array of V-grooves.

Figure 8

A schematic representation of the geometrical arrangement of the optical fiber and the lens in the system, showing the dependence between the lens and fiber parameters.

Figure 9

A schematic of the kinoform lens profile used in the lens design; h_min and h_max stand for the minimum and maximum depths of the kinoform shape.

Figure 10

An optical microscope image of a PMMA kinoform lens fabricated at CEZAMAT from the lens array used in the experiments.

Figure 11

Characteristics showing dependence of WG film’s guided-mode effective index n_eff and homogeneous sensitivity S_H on WG film thickness d for WG film refractive index n_f = 1.81.

Figure 12

SEM images of the fabricated VGC.

Figure 13

Simulations of the resonant coupling angle vs. the VGC period for both fundamental modes and two pairs of measured values for the WG film’s thickness and refractive index.

Figure 14

Schematics of the experimental setup for multichannel optical signal coupling to the VGC. DAQ—data acquisition card; GEN—sine wave generator; LD—laser diode; LDCC—laser diode current controller; LTS—linear translational stage; MLA—array of microlenses; MM PCS—multimode plastic-clad silica; MRS—motorized rotational stage; OFTH—optical fiber transmission head; PC—personal computer; PD₁, PD₂—photodiodes; S₁, S₂, S₃—single-mode fiber optic splitters.

Figure 15

An image of the photonic chip in a chip holder illuminated with two quasi-parallel beams at the optimal coupling angle.

Figure 16

Angle transmission spectra collected at the output facets of the photonic chip for upper and lower light streaks (Figure 13).

Figure 17

Images of the photonic chip illuminated at different positions on the VGC at the optimal coupling angle.

Figure 18

An image of the photonic chip illuminated at the optimal coupling angle with two optical beams separated by 1 mm.

Figure 19

An image of the photonic chip illuminated at the optimal coupling angle with three optical beams.