Mode profile imaging and loss measurement for uniform and tapered single-mode 3D waveguides in diffusive photopolymer

Abstract

We demonstrate single-mode uniform and parabolically tapered three-dimensional waveguides fabricated via direct-write lithography in diffusion-based photopolymers. Modulation of the writing power is shown to compensate Beer-Lambert absorption in the single-photon initiator and to provide precise control of modal tapers. A laminated sample preparation is introduced to enable full 3D characterization of these modal tapers without the need for sample polishing which is difficult for this class of polymer. The accuracy and repeatability of this modal characterization is shown to allow precise measurement of propagation loss from single samples. These testing procedures are used to demonstrate single-mode waveguides with 0.147 dB/cm excess propagation loss and symmetrical tapers up to 1:2.5 using 1.5 microwatts of continuous write power.

Design concepts for diffusive holographic photopolymers

“…Consumption of photoinitiator leads to an unwanted decrease in sensitivity with dose. This effect can be compensated by scheduling of successive exposes or dynamic modulation of a rastered writing focus, or it can be mitigated by using a high photoinitiator concentration, while at the same time ensuring that the recording wavelength is sufficiently far onto a low‐absorptivity shoulder that the overall Beer‐Lambert absorption is still tolerably low for a given media thickness.…”

Monolithic integration of optical waveguide and fluidic channel structures in a thiol-ene/methacrylate photopolymer

“…A waveguide-only device demonstrates the ability of the polymer to develop low loss waveguides. This low loss is most likely achieved because of the lack of edge scatter in the waveguide due to the gradient index response of the polymer at the edges of the waveguide, which is typical of holographic photopolymers [11]. A connectorized refractometer device, roughly 1 cm 3 in volume, demonstrates the ability of the polymer to create lithographically aligned waveguides across a fluidic channel in a compact planar geometry.…”

Transport-of-intensity-based phase imaging to quantify the refractive index response of 3D direct-write lithography

“…At large exposure times and intensities, the index response saturates. The saturation Δn of the 10 wt% monomer formulation of 3 7 10 − × is half that of the 30 wt% monomer formulation at 2 1.4 10 − × , indicating the material sensitivity and maximum obtainable Δn are limited by the initial monomer concentration. To support this hypothesis, photoinitiator consumption has been calculated to be negligible over the given exposure times and intensities (see Appendix D).…”

“…Direct laser writing (DLW) optical lithography is a common technique used to create micronscale three-dimensional (3D) refractive index structures deeply buried within a photosensitive medium [1][2][3]. The primary applications of DLW have been 3D routed waveguides for interconnects to integrated optoelectronic circuits [3][4][5][6][7], 3D diffractive optics that provide greater control over the multidimensional coherence function than their 2D counterparts [8], and 3D optical data storage that extends the capacity of traditional surface recording [9,10]. Proper design for such applications demands the ability to understand this light/matter interaction, which includes characterizing the optical properties of both the focused writing beam and photosensitive material, and measuring the magnitude and shape of the resulting index response.…”

“…To circumvent this issue, formulations typically mix high and low refractive index components whose fractional composition, and thus refractive index, is modified via mass transport when exposed to patterned illumination, but left nominally unchanged by the uniform post-processing cure [17]. The high photo-sensitivity of these two-component photopolymers, and their ease of processing, have led to applications in holography, 3D data storage, self-written waveguides [18,19], and laser direct-write lithography of waveguides [7,20,21]. Although the holographic community has invested significant effort both into developing a rational design for the material and understanding the index response to sinusoidal illumination [22][23][24][25][26], little has been done to quantitatively predict how these two-component photopolymer materials respond to a focused beam.…”