Integrated-optic fourier transform spectrometer using twin waveguide arrays
A Fourier transform spectrometer implemented on a photonic integrated circuit (PIC) is provided. An input optical signal waveguide carries an input optical signal to be analyzed to an on-chip Y branch splitter to split the input signal equally to carry coupled-optical signals related to the input optical signal into twin array waveguides of a first waveguide array and a second waveguide array. The optical signal from the first waveguide array and the optical signal from the second waveguide array intersect at a preset intersecting angle at an output plane of the PIC such that the spectral and spatial resolutions of the interferogram of the spectrometer are determined by the intersecting angle.
1 . A Fourier transform spectrometer implemented on a photonic integrated circuit (PIC), comprising:
an input optical signal waveguide carrying an input optical signal to be analyzed;
an on-chip Y branch splitter configured to split the input optical signal into first and second optical signals;
a first waveguide array configured to receive the first optical signal;
a second waveguide array configured to receive the second optical signal; wherein the first waveguide array and the second waveguide array terminate at an output plane of said PIC at a preset non-zero intersecting angle, such that optical fields output from said first and second waveguide arrays overlap in a slab waveguide region to generate a spatial interference fringe pattern forming an interferogram; and,
said intersecting angle determining a spectral resolution and a spatial resolution of the interferogram.
2 . The Fourier transform spectrometer of claim 1 , wherein the optical signal from the first waveguide array and said optical signal from said second waveguide array intersect at a preset intersecting angle to generate interference of the two optical signals from the output of the two intersecting waveguide arrays resulting in an interferogram at the output plane of said PIC, said interferogram processed to obtain the spectrum by taking Fourier transform.
3 . The Fourier transform spectrometer of claim 1 , wherein said photonic integrated circuit (PIC) is a planar waveguide circuit (PLC).
4 . The Fourier transform spectrometer of claim 1 , further comprising a lower clad-mode region and an upper clad-mode region incorporated within said Y branch splitter to deflect undesirable radiated leakages.
5 . The Fourier transform spectrometer of claim 1 , further comprising a segmented taper region to efficiently couple the optical power from a slab region to array waveguides on each side.
6 . The Fourier transform spectrometer of claim 1 , wherein a DC offset of said interferogram is calculated using a smoothing algorithm such as least squares regression analysis.
7 . The Fourier transform spectrometer of claim 1 , further comprising an alignment waveguide for PIC packaging.
8 . The Fourier transform spectrometer of claim 1 , wherein said intersecting angle is between five to seven degrees.
9 . The Fourier transform spectrometer of claim 1 , further comprising a thermo-optic (TO) micro-heater at an input waveguide of one of said twin waveguide arrays and after said Y branch splitter to perform spectral interpolation to compute the spectrum without a magnifying lens.
10 . The Fourier transform spectrometer of claim 1 , further comprising a detector positioned to capture said interferogram at said output plane.
11 . The Fourier transform spectrometer of claim 1 , further comprising a camera detector array with a suitable pixel pitch for receiving output optical signals from the PIC output through an imaging magnifying lens.
12 . The Fourier transform spectrometer of claim 1 , further comprising multiple PICS, each having a transform spectrometer as recited in claim 1 .
13 . A Fourier transform spectrometer system comprising:
a photonic integrated circuit (PIC) comprising the Fourier transform spectrometer of claim 1 ;
a detector array optically coupled to the output plane of the PIC and configured to capture the interferogram; and
one or more processors configured to apply a Fourier transform to the captured interferogram to determine a spectrum of the input optical signal.
14 . The system of claim 13 , wherein the one or more processors are further configured to estimate and remove a DC offset from the interferogram using a smoothing algorithm.
15 . The system of claim 13 , further comprising an imaging lens disposed between the output plane of the PIC and the detector array.
16 . The system of claim 13 , wherein the detector array comprises a camera detector array having a pixel pitch selected to sample the spatial interference fringe pattern.
17 . An integrated spectrometer device comprising:
(a) a photonic integrated circuit (PIC) comprising:
(i) an input slab region and an output slab waveguide region;
(ii) first and second waveguide arrays that connect the input slab region to the output slab waveguide region;
(iii) wherein the first and second waveguide arrays terminate at the output slab waveguide region at a preset non-zero intersecting angle to launch optical fields into the output slab waveguide region to generate a spatial interference fringe pattern forming an interferogram; and
(iv) a segmented taper region disposed between the input slab region and the first and second waveguide arrays, the segmented taper region configured to efficiently couple optical power from the input slab region to the first and second waveguide arrays; and
(b) a detector array optically coupled to the output slab waveguide region and configured to capture the interferogram.
18 . The device of claim 17 , further comprising an alignment waveguide integrated on the PIC for packaging alignment, the alignment waveguide being optically decoupled from the first and second waveguide arrays.
19 . The device of claim 17 , further comprising a thermo-optic micro-heater disposed to apply a controllable phase shift to at least one of the split optical signals to perform spectral interpolation.
20 . The device of claim 17 , wherein the preset intersecting angle is between five degrees and seven degrees.