Fiber sheath flat v1

Simplified sibling of sheath_v1 without the on-chip lens / segmented scaffold.

This is a simplified sibling of ../sheath_v1. It keeps the passive, self-aligning Nanoscribe fiber sheath, but removes the on-chip lens and the segmented lens scaffold.

The intended stack is:

                              ___________
                            /             \         <- funnel
                           |               |           ID 124 -> 250, wall 20 um
                           |               |           height 50 um
                            \             /
                            +-------------+
                            |   fiber     |        <- sheath top
                            |   (125 OD)  |           ID 124, OD 250
                            |             |           height 750 um
                            |             |
                            +-------------+
                            |             |        <- sheath bottom
                            |             |           ID 130, OD 250
                            |             |           height 50 um
                            +-------------+
       . . . o . . . . . . . o . . . . . . . o   <- 3 bumps (orbit r=50, hemi r=3)
       =============================================  <- membrane (2 um, OD 250)
       =============================================  <- PEC / substrate
       z = 0

The fiber is pushed into the sleeve and lands on three 3 um-tall index-matching bumps. The membrane is printed directly at the chip plane. There is no refractive lens in this version.

Files

• generate_geometry.py - emits three STL files plus geometry_params.json.
• simulate_optics.py - runs a Tidy3D FDTD simulation of the flat membrane, bump/glue layer, fiber, and PEC stack, then writes a timestamped Markdown report in reports/.
• stl/region1_membrane_bumps.stl - high-resolution writing region: r <= 75 um, z in [-1, 5] um. Contains the central membrane region and the three bumps.
• stl/region2_inner_sheath.stl - no-stitching writing region: r <= 75 um, z >= 5 um. Contains the inner wall of the sheath bore.
• stl/region3_outer_support.stl - outer membrane, bulk sleeve support, and funnel infrastructure.
• stl/combined_sheath_flat_v1.stl - all three regions concatenated into one inspectable STL.

Write order on the Nanoscribe tool: region 1 first (bumps and membrane center), region 2 second (no stitching across the optical bore), region 3 last (outer support that may be stitched freely).

Geometry parameters (microns / degrees)

part	parameter	value
Membrane	thickness	2.0
Membrane	outer diameter	250
Bumps	hemisphere radius	3.0
Bumps	orbit radius / angles	50 / 30, 150, 270
Sheath gap	angular	30 deg
Sheath wall	angular span	45..375 deg
Sheath bottom	ID / OD / height	130 / 250 / 50
Sheath top	ID / OD / height	124 / 250 / 750
Funnel	ID bottom -> top	124 -> 250
Funnel	wall thickness / height	20 / 50
Total height	top of funnel above PEC	852

Indices at 780 nm: IP-S resin n = 1.51, air n = 1.0. Fiber is modeled as a Thorlabs 630HP-like step-index fiber at 780 nm: 125 um cladding, 3.5 um core, NA = 0.12, n_core = 1.4583, n_clad = 1.4533.

Running it

# from this directory
pip install trimesh manifold3d scipy tidy3d

python generate_geometry.py
python simulate_optics.py

simulate_optics.py submits one FDTD job to the Tidy3D cloud per invocation. Each run writes a fresh reports/report_<timestamp>.md; re-runs never overwrite a prior report. Override knobs via environment variables:

variable	default	meaning
SHEATH_WL0	0.780	center wavelength (um)
SHEATH_BW	0.06	spectral bandwidth (um)
SHEATH_NWL	31	number of wavelength samples
SHEATH_MIN_STEPS	9	minimum grid cells per wavelength
SHEATH_RUN_TIME	6e-13	FDTD run time, seconds
SHEATH_SIMXY	10.0	transverse FDTD window (um)
SHEATH_FIBLEN	3.0	fiber stub length above the face (um)
SHEATH_NOTES	""	free-form notes appended to the report

Simulation scope

FDTD is expensive, so the simulation covers only the central optical stack, not the full 852 um sleeve. The simulated stack is:

z <= 0      : PEC
z = 0..2    : resin membrane
z = 2..5    : resin / index-matching glue bump zone
z = 5..     : fiber core + cladding

The mechanical bumps live at r = 50 um, outside the compact FDTD window. Their central optical effect is modeled as a uniform resin layer between the membrane and the fiber face.

Coupling efficiency is computed from the returned fiber-mode amplitude:

CE(f) = |amps("+", mode_index=0, f)|^2

The source is normalized to unit launched modal power.

Results

Sheath flat v1 - Local Preflight Report

• Generated: 2026-05-27T13:23:34
• Status: local geometry/preflight only
• FDTD cloud solve: not run

This report was generated with SHEATH_DRY_RUN=1. It verifies the simplified flat membrane stack locally and writes the same geometry figures used by the full FDTD report, but it does not solve the fiber mode or upload the simulation to Tidy3D.

Structure geometry

level	z (um)
PEC substrate / membrane bottom	0.00
Top of membrane	2.00
Top of bumps / fiber face	5.00
Sheath top end	802.0
Funnel top	852.0

Material indices: resin (IP-S) n = 1.51, fiber core n = 1.4583, fiber cladding n = 1.4533.

Local simulation object

• Transverse window: 10.00 x 10.00 um
• Vertical extent: z in [-0.62, 8.62] um
• Grid: auto with 9 cells per wavelength at lambda0
• Boundary conditions: PML in x, y, z; PEC structure at z <= 0
• Symmetry: (1, -1, 0), Ey-polarized HE11 mode
• Run time configured for full FDTD: 0.60 ps

Full FDTD run

Run without SHEATH_DRY_RUN=1 to submit the mode solve and FDTD job to Tidy3D and generate coupling-efficiency plots and field figures.

Raw data

• Geometry parameters: ../geometry_params.json
• STL files: ../stl/

Downloads

• region1_membrane_bumps.stl
• region2_inner_sheath.stl
• region3_outer_support.stl
• combined_sheath_flat_v1.stl

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Source: nanophotonic_devices/fiber_collection_optics/sheath_flat_v1/