Miniaturised multi-plane light converters via laser-written geometric phase holograms

TL;DR

The paper demonstrates miniaturised, glass-embedded MPLCs fabricated by femtosecond laser writing of cascaded geometric phase holograms. Phase planes are implemented as two nanograting layers separated by $44\mu\text{m}$ to form spatially varying half-wave plates, imprinting a Pancharatnam-Berry phase on circularly polarized light. Proof-of-concept devices include a 3-mode Hermite-Gaussian sorter in a volume of $<0.5\,\text{mm}^3$ and a 10-mode sorter in a volume of $<0.8\,\text{mm}^3$, realized at a pixel pitch of $2\mu\text{m}$. Experimental results show reasonable agreement with simulations but higher cross-talk for the 10-mode device, highlighting needs to improve input-beam fidelity and per-plane efficiency (currently about $20\%$). The work offers a compact, passive alternative to SLM-based MPLCs and enables rapid prototyping of monolithic photonic processors for communications, imaging, and quantum photonics.

Abstract

Multi-plane light converters (MPLCs) are an emerging 3D beam shaping technology capable of deterministically mapping a basis of input spatial light modes to a new basis of output modes. The ability to perform such spatial reformatting operations has many future applications in both classical and quantum photonics, spanning from optical communications to photonic computing and imaging. MPLCs are intricate optical systems consisting of a cascade of inverse-designed diffractive optical elements, typically separated by free-space. In this work we investigate the fabrication of miniaturised fully-encapsulated transmissive MPLCs within a glass chip using single-step direct laser writing. Our approach relies on the formation of femto-second laser induced birefringent nanogratings with a spatially controllable fast axis orientation. The glass chip is internally patterned with layers of these nanogratings to create multiple geometric phase holograms which imprint controllable phase patterns onto circularly polarised light propagating through them. We experimentally demonstrate two proof-of-concept glass-embedded 700x700x2000 micrometer cubed MPLCs: a 3-mode and a 10-mode Hermite-Gaussian mode sorter. We discuss the fabrication challenges and future improvements of these devices. Our work plots a path towards the rapid prototyping of robust monolithic MPLC technology.

Figures (3)

• Figure 1: Concept. (a) A schematic of a monolithic $2$-plane MPLC laser-written inside a silica glass chip. Here the MPLC is shown sorting the energy carried by $3$ HG transverse spatial modes incident from the left into focussed spots in different locations on the right. In our experiments the incident and output spatial modes are centred on a common axis -- they are shown displaced vertically here for clarity. The two inverse-designed phase planes are visible inside the glass chip. (b) A schematic of our custom-built direct laser writing system to fabricate miniaturised MPLCs. The incoming fs-beam is linearly polarised. A half wave-plate held in a motorized rotation mount controls the orientation ($\theta$) of the linear polarisation of the beam, after which the beam is focussed into the glass substrate by an objective lens. The glass is translated relative to the writing beam using 3D translation stages whose motion (along Cartesian directions $x$, $y$, and $z$) is synchronised with the laser emission and the HWP orientation. (c) Our MPLC inverse-design algorithm yields the optimised phase profiles for each plane, and this is then mapped to the writing orientation of the nanogratings, which corresponds to the local fast-axis of the geometric phase holograms. (d) 60X optical images of a laser-written geometric phase hologram: in bright field and imaged through two crossed linear polarisers. The hologram has 4 phase levels, although it appears binary when imaged between crossed polarisers. Here the pixel size is $10$$\upmu$m, and the scale bar is $100$$\upmu$m.
• Figure 2: Laser-written computer generated hologram and $\bm{3}$-mode HG mode sorter. (a) Demonstration of a single laser-written $1$$\times$$1$ mm$^2$ geometric phase hologram designed to project an image (tropical island scene -- target image shown in inset) into the far-field. We see the glass slide patterned with various laser-written test structures in the foreground. The hologram under test is illuminated with a Gaussian beam, and the image is projected onto a piece of white card in the background, after passing through a polarisation filter. (b) Jointly inverse-designed phase profiles of the two planes of the $3$-mode HG mode sorter. Upper panel: first hologram; Lower panel: second hologram. (c) Top row: experimentally measured intensity images of input HG modes; Middle row: simulated far-field outputs of ideal $3$-mode HG mode sorter; Bottom row: experimentally measured outputs of the laser-written sorter. The location of the output channels is marked by pink circles, with the target output channel highlighted; the central violet circle marks the point around which the channels are arranged. (d) Simulated and experimentally measured coupling matrices describing the relative intensity transmitted into each output channel as the MPLC is illuminated with each input mode in turn. The sum of each column is normalised to $1$. The mean off-diagonal intensity value of the simulated [experimental] coupling matrix is 0.02 [0.2].
• Figure 3: $\bm{10}$-mode HG mode sorter. (a) Top row: experimentally measured intensity images of input HG modes; Middle row: simulated far-field outputs of ideal $10$-mode HG mode sorter; Bottom row: experimentally measured outputs of the laser-written sorter. The location of the output channels is marked by pink circles, with the target output channel highlighted; the central violet circle marks the point around which the channels are arranged. (b) Simulated and experimentally measured coupling matrices describing the relative intensity transmitted into each output channel as the MPLC is illuminated with each input mode in turn. The sum of each column is normalised to $1$. The mean off-diagonal intensity value of the simulated [experimental] coupling matrix is 0.13 [0.58].