We present both design and experimental results for an As 2S 3 grating coupler on a thin film LiNbO 3 substrate. A basic grating coupler structure is designed with coupling efficiency of 53% to a single mode fiber. A maximum simulated coupling efficiency of 78.8% is achieved, assuming a polymer bonding process. The basic structure was fabricated, and the coupling efficiency was measured to be at least 23.4% at 1540 nm. Some of the loss may be attributable to non-grating sources, such as waveguide tapers and testing fiber tails. A grating cavity was then measured using the grating couplers. The cavity waveguide propagation loss was 2.0 dB/cm. For a 400 nm thick As 2S 3 on 500 nm thin film LiNbO 3 on insulator, the confinement factor in the LiNbO 3 crystal is 82.3% when the As 2S 3 waveguide width is 400 nm, showing that As 2S 3-on-thin film LiNbO 3 is an excellent candidate for thin film electro-optic applications.
Lithium niobate (LiNbO3) has been widely used in high-speed modulators and phased arrays due to the excellent electro-optic properties of LiNbO3 material [
Thin film LiNbO3 furthers the advantage over traditional LiNbO3 by confining mode in vertical direction. Thin film LiNbO3 is produced through ion implantation, to create crystal damage at the desired depth of bulk LiNbO3 wafer, followed by wet etching or thermal annealing [
Arsenic trisulfide (As2S3) waveguide on LiNbO3 crystal is a hybrid platform for both near-infrared (near-IR) and mid-infrared (mid-IR). With certain waveguide width and thickness values, optical mode inside the waveguide partially overlaps with LiNbO3 crystal, enabling EO applications. Since standard semiconductor fabrication techniques can be applied to As2S3 patterning, different As2S3 structures including sidewall Bragg grating [
Coupling light between this platform and the external light source and detector should be carefully designed, in order to avoid excessive mode mismatch loss. In order to couple light into and out of different As2S3 structures, a grating coupler can be used. Reference [
Light diffraction in a grating coupler follows phase matching condition. The goal of grating coupler design is to maximize the coupling efficiency between the first order light diffraction at the coupler output and a single-mode fiber (SMF) mode. Equation (1) is the grating coupler equation that builds connection between the first order diffraction angle θ and the effective refractive index of the grating coupler neff [
Λ = λ / ( n e f f − n c sin θ ) . (1)
Effective refractive index of a grating coupler is determined by the material refractive index and thickness of each layer, as well as grating coupler DC. In simulation, we use 500 nm as LiNbO3 thin film thickness and 2 μm as SiO2 layer thickness, which is used in device fabrication.
There are more than one set of parameters satisfies (1), each of which results in a first diffraction field. Fimmprop (from Photon Design Inc.) is used to simulate grating coupler output field, and Python scripting is used to calculate and maximize the overlapping integral between the grating coupler output field and the Gaussian approximation in a SMF. The coupling angle is set to 12˚. The
Material | Refractive index at 1550 nm |
---|---|
As2S3 | 2.4373 |
LiNbO3 (e) | 2.1376 |
SiO2 | 1.4440 |
Si | 3.4777 |
cladding index is set to 1.444, since SiO2 is used as cladding material. The target wavelength is 1550 nm. After optimization, grating coupler period is 850 nm, its thickness is 400 nm and its DC is 0.59. In order to further increase coupling efficiency, DC in the first several periods of the grating coupler can be tweaked (also named apodization) in order to make the grating coupler output field more resembles Gaussian approximation in a SMF [
The above design methodology can be carried out targeting mid-infrared application. The optimized mid-infrared structure would have different grating period, duty cycle, and material thickness compared to above design.
Further improvements on top of the basic grating coupler structure in terms of coupling efficiency to a SMF require substantially changing the structure. One way is to introduce a Bragg reflector between substrate and grating coupler to reduce substrate leakage [
Since additional layers are required between LiNbO3 thin film and substrate, this improved grating coupler consists of two parts: the first part is Bragg reflector layers on a substrate; the second part is similar to the basic grating coupler
structure. A bonding step brings these two parts together.
Benzocyclobutene (BCB) can be used as the bonding polymer. BCB is widely applied to low temperature wafer level bonding, with good adhesion, easy fabrication, repeatable thickness and refractive index control [
Optical simulation of the improved grating coupler structure is carried out in Fimmprop. After optimization, the coupling efficiency is 78.8% at 1550 nm for TE polarization. The design parameters are listed in
Parameter | Value |
---|---|
As2S3 thickness in Bragg reflector | 130 nm |
SiO2 thickness in Bragg reflector | 340 nm |
BCB thickness | 2 μm |
As2S3 thickness for grating layer | 500 nm, 100 nm |
Thin film X-cut LiNbO3 thickness | 500 nm |
Grating coupler period | 850 nm |
Grating coupler duty cyclea | 0.94, 0.82, 0.70, 0.59 |
Number of periods | 2, 2, 2, 16 |
a. For the first two periods, DC = 0.94; for the next two periods, DC = 0.82; then the next two DC = 0.70; for the rest 16 periods, DC = 0.59.
from the efficiency peak. Spectrum symmetry can be further optimized by tweaking grating period.
A thin film X-cut LiNbO3 sample (from NanoLN) is cleaned using a Q-tip with acetone, methanol and isopropyl alcohol (IPA). This sample has 500 nm X-cut thin film LiNbO3 on top of 2 μm SiO2 on Si substrate. We sputter 400 nm As2S3, 130 nm SiO2 and 40 nm Ti onto the sample. Then electron beam lithography (EBL) is performed for pattern definition. 2.25% PMMA solution is used as positive EBL resist. The exposure dosage is 350 μC/cm2. After EBL, the sample is developed in fresh developer that is made from 40 mL IPA and 20 mL DI water. O2 Plasma ashing is then carried out to remove the residue PMMA resist at corner areas. 30 nm of Ti is deposited through e-beam evaporation, followed by a lift-off step in acetone.
Reactive ion etching (RIE) is performed to remove the metal layer and As2S3 outside pattern areas. In order to make sure As2S3 patterns are preserved, the sample needs to be taken out and checked under a microscope several times throughout RIE. After RIE, 100 nm SiO2 is sputtered as a protective layer. A complete fabrication flow chart is shown in
In comparison, the improved structure is much more complicated.
After fabrication, the grating coupler is tested with a vertical fiber coupling setup. A source and a detector are integrated in a LUNA optical vector analyzer (OVA). The LUNA not only separates two polarization directions (TE and TM), but also provides time domain impulse response.
Parameter | Basic structure | Improved structure |
---|---|---|
Deposition layers | 4 | 10 |
Requires BCB bonding | No | Yes |
Requires controlled etching | No | Yes |
Requires substrate removal | No | Yes |
Coupling efficiency | 53.0% | 78.8% |
coupler and the waveguide, there is a 200 μm long linear taper, with the wider tip of 12 μm connecting the grating coupler and the narrower tip of 1 μm connecting the waveguide. The grating coupler period is 0.85 μm. Due to the fabrication imperfection, the peak transmission wavelength is shifted to the shorter wavelength range. As a rough estimation, the propagation loss at 1540 nm is 2.0 dB/cm. Assuming that the mode conversion loss in both tapers is negligible and the waveguide loss is wavelength-independent, the lowest fabricated grating coupler loss is 6.3 dB at 1540 nm. Compared to the experimental result in [
The difference between measured grating coupler loss and simulation result is due to several factors: first, the overall insertion loss includes the SMF tails that are used to couple the OVA source/detector to the grating couplers. Since it is not convenient to isolate the fiber tails from the device under test (DUT), the surface qualities of those fiber tails and light propagation loss inside them would result in higher calculated grating coupler loss. Similarly, the tapers’ loss might also be one contributing factor to the overall loss. In terms of fabrication, surface roughness caused by topology of the grating coupler can also result in lower coupling efficiency than theoretical prediction. During measurement, any small angular offset of the sample orientation would cause additional loss.
In order to improve the grating coupler performance, the grating period and DC need to be tweaked from the design values based on the current experimental results, since the actually materials and their refractive indices are off from the material database to some extent. Longer taper could be included in order to further reduce mode-conversion loss.
The basic grating coupler can be used to measure As2S3 grating cavity structure on thin film LiNbO3. With electrode patterned on both sides of the cavity waveguide, it becomes EO tunable filters. Grating cavity reflection response can be extracted by truncation of the time domain impulse reflection (including grating coupler), followed by inverse Fourier transformation.
Optical confinement factor in LiNbO3 layer is a key parameter for EO tuning applications. Higher confinement factor in LiNbO3 layer means more effective EO tuning, since LiNbO3 crystal is the only EO tunable material in this structure. By decreasing As2S3 waveguide width, confinement factor in thin film LiNbO3 can be effectively increased, as shown in
Fitting parameter | Value |
---|---|
Coupling coefficient | 0.85 mm−1 |
Effective refractive index | 2.11 |
Group refractive index | 2.6 |
Waveguide loss | 2.0 dB/cm |
As2S3 grating coupler on thin film LiNbO3 is designed in order to open the door to multiple EO applications with thin film LiNbO3. A basic grating coupler structure is fabricated and the experimental results show 23.4% coupling efficiency while the simulation is at 53.0%. The efficiency difference is due to fabrication imperfection, testing fibers, and misalignment during measurement. An improved grating coupler is designed, and the theoretical coupling efficiency can be as high as 78.8% at 1550 nm with 60 nm bandwidth.
As2S3 waveguide propagation loss of 2.0 dB/cm is calculated through TMM fitting a grating cavity measurement result enabled by the As2S3 grating coupler. A simulation of As2S3 waveguide cross-section on a 500 nm thin film LiNbO3 result indicates as high as 82.3% optical confinement in LiNbO3 layer if the As2S3 cavity waveguide width is reduced to 0.4 μm. This means that high EO tuning efficiency can be achieved on thin film LiNbO3 layer, once an electric field is properly applied across the cavity waveguide section.
The authors would thank Dr. Long Chang from The University of Houston Nanofabrication Facility for valuable discussion on E-beam lithography.
Zhang, C. and Madsen, C.K. (2018) Demonstration of an As2S3 Grating Coupler on Thin Film LiNbO3. Optics and Photonics Journal, 8, 111-121. https://doi.org/10.4236/opj.2018.84011