from __future__ import annotations
from collections.abc import Callable
import numpy as np
import numpy.typing as npt
import gdsfactory as gf
from gdsfactory.path import transition_adiabatic
from gdsfactory.typings import CrossSectionSpec
def neff_TE1550SOI_220nm(w: float) -> float:
"""Returns the effective index of the fundamental TE mode for a 220nm-thick core with 3.45 index, fully clad with 1.44 index.
Args:
w: width in um.
Returns:
effective index.
"""
adiabatic_polyfit_TE1550SOI_220nm = np.array(
[
1.02478963e-09,
-8.65556534e-08,
3.32415694e-06,
-7.68408985e-05,
1.19282177e-03,
-1.31366332e-02,
1.05721429e-01,
-6.31057637e-01,
2.80689677e00,
-9.26867694e00,
2.24535191e01,
-3.90664800e01,
4.71899278e01,
-3.74726005e01,
1.77381560e01,
-1.12666286e00,
]
)
return np.poly1d(adiabatic_polyfit_TE1550SOI_220nm)(w) # type: ignore
[docs]
@gf.cell
def taper_adiabatic(
width1: float = 0.5,
width2: float = 5.0,
length: float = 0,
neff_w: Callable[[float], float] = neff_TE1550SOI_220nm,
alpha: float = 1,
wavelength: float = 1.55,
npoints: int = 200,
cross_section: CrossSectionSpec = "strip",
) -> gf.Component:
"""Returns a straight adiabatic_taper from an effective index callable.
Args:
width1: initial width.
width2: final width.
length: 0 uses the optimized length, and otherwise the optimal shape is compressed/stretched to the specified length.
neff_w: a callable that returns the effective index as a function of width
- By default, will use a compact model of neff(y) for fundamental 1550 nm TE mode of 220nm-thick core with 3.45 index, fully clad with 1.44 index. Many coefficients are needed to capture the behaviour.
alpha: parameter that scales the rate of width change.
- closer to 0 means longer and more adiabatic;
- 1 is the intuitive limit beyond which higher order modes are excited;
- [2] reports good performance up to 1.4 for fundamental TE in SOI (for multiple core thicknesses)
wavelength: wavelength in um.
npoints: number of points for sampling.
cross_section: cross_section specification.
References:
[1] Burns, W. K., et al. "Optical waveguide parabolic coupling horns." Appl. Phys. Lett., vol. 30, no. 1, 1 Jan. 1977, pp. 28-30, doi:10.1063/1.89199.
[2] Fu, Yunfei, et al. "Efficient adiabatic silicon-on-insulator waveguide taper." Photonics Res., vol. 2, no. 3, 1 June 2014, pp. A41-A44, doi:10.1364/PRJ.2.000A41.
npoints: number of points for sampling
"""
xs = gf.get_cross_section(cross_section)
layer = xs.layer
assert layer is not None
# Obtain optimal curve
x_opt, w_opt = transition_adiabatic(
width1, width2, neff_w=neff_w, wavelength=wavelength, alpha=alpha
)
# Resample the points
from scipy import interpolate # type: ignore
w_opt_interp = interpolate.interp1d(x_opt, w_opt)
if not length:
length = x_opt[-1]
x = np.linspace(0, length, npoints)
w: npt.NDArray[np.float64] = w_opt_interp(x)
# Stretch/compress x
x_array = np.linspace(0, length, npoints) * (1 + length - x_opt[-1])
y_array = w / 2
c = gf.Component()
c.add_polygon(
list(zip(x_array, y_array)) + list(zip(x_array, -y_array))[::-1], layer=layer
)
# Define ports
c.add_port(
name="o1",
center=(0, 0),
width=width1,
orientation=180,
cross_section=cross_section,
layer=layer,
)
c.add_port(
name="o2",
center=(length, 0),
width=width2,
orientation=0,
cross_section=cross_section,
layer=layer,
)
xs.add_bbox(c)
return c
if __name__ == "__main__":
c = taper_adiabatic(width1=0.5, width2=5, cross_section="rib_bbox")
c.show()
