We report on the progress of our efforts to apply silicon nitride photonic integrated circuits (PIC) to the miniaturization of optical coherence tomography (OCT) with the goal of facilitating its widespread use in ophthalmology at the point of care. In particular, we highlight the design and optical characterization of photonic building blocks allowing the realization of a silicon nitride PIC-based multi-channel swept-source OCT system in the 1060 nm wavelength region. Apart from waveguide structures, these building blocks include 3D-printed microlenses on the PIC end facets for efficient light coupling to and from the PIC.
We present a miniaturized optical coherence tomography (OCT) setup based on photonic integrated circuits (PIC) for the 850 nm range. We designed a 512-channel arrayed waveguide grating (AWG) on a PIC for spectral domain OCT (SD-OCT) that is co-integrated with PIN-photodiodes and analog-to-digital-converters on one single chip. This image sensor is combined with all the necessary electronics to act as a camera. It is integrated into a fiber-based OCT system, achieving a sensitivity of >80dB and various samples are imaged. This optoelectronic system will allow building small and cost-effective OCT systems to monitor retinal diseases.
The paper shows concepts of optical splitting based on three dimensional (3D) optical splitters based on multimode interference principle. This paper is focused on the design, fabrication and characterization of 3D MMI splitter with formed output waveguides based on IP-Dip polymer for direct application on optical fiber. The MMI optical splitter was simulated and fabricated using direct laser writing process. Output characteristics were characterized by highly resolved near-field scanning optical microscope (NSOM) and compared with 3D MMI splitter without output waveguides.
We present 256-channel, 25-GHz AWG designed for ultra-dense wavelength division multiplexing. For the design two in-house developed tools were used: AWG-Parameters tool for the calculation of input design parameters and AWGAnalyser tool, used to evaluate the simulated transmission characteristics. The AWG structure was designed for AWG central wavelength of 1550 nm and simulated with PHASAR tool from Optiwave. To keep the size of AWG structure as small as possible the number of waveguides in the phased array was tested. The simulations show that there is a certain minimum number of phased array waveguides necessary to reach sufficient AWG performance. After optimization, the AWG structure reached 10 cm x 11 cm in size and satisfying optical properties.
This paper describes two different designs of 1×8 passive optical splitters. The first splitter consists of cascade arranged directional waveguide branches (Y-branch splitter) with (0.8×0.16) µm2 waveguide cross-section. The second splitter is based on multimode interference occurring in a large MMI coupler, which uses a self-imaging effect for beam propagation, exhibiting the same waveguide core size as a Y-branch splitter. The waveguide channel profile, used in both approaches, is based on a silicon nitride material platform, with a refractive index of core being nc = 1.925 and a refractive index of cladding ncl = 1.4575. The splitters are designed as a planar structure for a medical operating wavelength 850 nm. Design, simulation, and optimization of passive optical components are performed by a commercial photonic software tool BeamPROP simulation engine by RSoft Photonics Suite tool, employing beam propagation method. This work aims to find the minimum physical dimensions of the designed splitters with the satisfactory optical performance. According to the minimum insertion loss and minimum non-uniformity, the optimum length of the splitters is determined. Finally, the optical properties of splitters for both approaches are discussed and compared with each other.
This paper describes two different designs of 1×128 passive optical splitters for telecommunication applications. The first splitter consists of cascade arranged directional waveguide branches (Y-branch splitter) with a standard waveguide core size (6x6) μm2. The second splitter is based on multimode interference occurring in a large MMI coupler, which uses a self-imaging effect for beam propagation, exhibiting the same waveguide core size as a Y-branch splitter. The waveguide channel profile, used in both approaches, is based on a standard silica-on-silicon material platform, with refractive index of core being nc = 1.456 and a refractive index of cladding ncl = 1.445. The splitters are designed and simulated at a telecommunication operating wavelength, λ = 1550 nm. Except for the lengths of the used Y-branches and MMI coupler, design parameters such as port pitch between the waveguides and simulation parameters for all splitters are considered fixed. Design, simulation, and optimization of passive optical components are performed by a commercial photonic software tool BeamPROP from Optiwave, which uses the Beam Propagation Method. This work aims to find the minimum physical dimensions of the designed splitters with a satisfactory optical performance. According to the minimum insertion loss and minimum non-uniformity, the optimum length of the splitters is determined. Finally, the optical properties of splitters for both approaches are discussed and compared with each other.
We present experimental verification of tree-dimensional (3D) 1x4 Y-branch splitter based on IP-Dip polymer as a core and polydimethylsiloxane (PDMS) Sylgard 184 as a cladding. The splitter was designed to operate in a wavelength region around 1550 nm. The design parameters of the splitter were optimized according to required optical properties and technological limitations. Based on the simulation results, the 3D Y-branch splitter was realized using direct laser writing lithography. Cladding of the splitter was prepared by PDMS pouring and curing. The measurements were performed by coupling optical signal into the splitter using standard SM fiber. By intensity monitoring of CCD camera we successfully documented splitting of the input optical signal into four output signals.
In this paper, we report our recent advances in the development of CMOS compatible PECVD silicon nitride waveguide based photonic key building blocks for optical coherence tomography (OCT) in the 850 nm wavelength region and for biosensing applications in the visible domain around 650 nm wavelength. We discuss the design and experimental verification of compact low-loss waveguide bends, broadband couplers with varying power splitting ratios required for PIC based OCT systems, arrayed waveguide gratings for spectral domain OCT, and a monolithically integrated optically pumped organic solid-state laser for low-cost biosensing applications.
A swept source optical coherence tomography (SS-OCT) system with the interferometer engine being a photonic integrated circuit (PIC) has been developed. Furthermore, an Arrayed Waveguide Grating (AWG), representing a grating on a PIC, for spectral domain OCT (SD-OCT) has been integrated in a fiber-based OCT system. With measured sensitivities of ~87 dB (SS-OCT) and ~80 dB (SD-OCT), scattering tissue imaging becomes feasible for OCT-on-chip systems. In this study, we present two OCT-on-chip systems and first results of biological tissue imaging in-vivo and exvivo.
In this paper we present the size reduction of a 160-channel, 50-GHz Si3N4 based AWG-spectrometer. The spectrometer was designed for TM-polarized light with a central wavelength of 850 nm applying our proprietary “AWG-Parameters” tool. For the simulations of AWG layout, the PHASAR photonics tool from Optiwave was used. The simulated results show satisfying optical properties of the designed AWG-spectrometer. However, such high-channel count AWG features large size. To solve this problem we designed a special taper enabling the reduction of AWG structure by about 15%, while keeping the same optical properties. The technological verification of both AWG designs is also presented.
The most common application of optical Y-splitters is their use in FTTx networks. It allows several customers to share the same physical medium, bringing high-speed networking, digital television and telephone services to residences using fiber-optic cables. The task of the optical splitters in such FTTH networks is to split one optical signal in many identical signals bringing for example the same TV signal in different households. Of course, the more buildings can be served by one optical splitter the lower are the installation costs. Therefore, the special attention is paid mainly to the design of high channel optical splitters presenting the serious challenge for the professional designers. In this paper a new Y-branch shape is proposed for 1×32 Y-branch splitter ensuring better splitting properties compared to the one recommended by ITU, in terms of their performance in transmission systems using wavelength division multiplexing.
We present the design of 20-channel, 50-GHz Si3N4 based AWG applying our proprietary AWG-Parameters tool. For the simulations of the AWG layout we used PHASAR photonics tool from Optiwave. The simulated transmission characteristics were then evaluated applying our AWG-Analyzer tool. We studied the influence of one of the design parameters – the separation between input/output waveguides, dx on the channel crosstalk. The results show that there is some minimum waveguide separation necessary to keep the crosstalk between transmitting channels low. The AWGs were designed for TM-polarized light with a central wavelength of 850 nm. They will later be used in a photonic integrated circuit dedicated to medical diagnostic imaging applications.
We present the design and simulation of 20-channel, 50-GHz Si3N4 based AWG using three different commercial photonics tools, namely PHASAR from Optiwave Systems Inc., APSS from Apollo Photonics Inc. and RSoft from Synopsys Inc. For this purpose we created identical waveguide structures and identical AWG layouts in these tools and performed BPM simulations. For the simulations the same calculation conditions were used. These AWGs were designed for TM-polarized light with an AWG central wavelength of 850 nm. The output of all simulations, the transmission characteristics, were used to calculate the transmission parameters defining the optical properties of the simulated AWGs. These parameters were summarized and compared with each other. The results feature very good correlation between the tools and are comparable to the designed parameters in AWG-Parameters tool.
We present the proportional method to correct the channel spacing between the transmitted output channels of an AWG. The developed proportional method was applied to 64-channel, 50 GHz AWG and the achieved results confirm very good correlation between designed channel spacing (50 GHz) and the channel spacing calculated from simulated AWG transmission characteristics.
Optical fibres are widely used in various applications as a medium for optical signals or optical transfer. This transport can be realized on long distance, compared to free space optics, which significantly extends reach of applications. Free space optics and fibre optics are combined in practice to yield the maximum performance of individual components forming a particular system. In such cases, light coupling from free space into fibres is required and it is frequently implemented with the use of lenses. An optical signal coupled into a fibre may also need certain modifications of spectral and spatial properties to allow its propagation down the fibre or reduce the amount of power carried in. The above requirement has been fulfilled by modifying surface of facets of photonic crystal fibres. By extrusion of a certain amount of host material from the surface, it is possible to obtain a structure resembling a thin film or an opaque layer for certain wavelengths. Several different structures of photonic crystal fibres and materials are considered to show influence of such thin-film on signal properties. This investigation is carried out in context of abilities of ablation of material from surfaces of photonic crystal fibres. Only certain shapes and geometrical arrangements can be considered. One of the goals is to specify, which of them are key for potential modification of spectral characteristics of photonic crystal fibres. The printed structures could potentially work like a thin-film ablation. Rigorous and versatile finite difference method has been employed to model propagation of light, its incidence onto a surface of the photonic crystal fibre, and subsequent propagation down the fibre. The simulations are carried on small pieces of photonic crystal fibres, with the length of tens of micrometres, due to well-known demands of the simulation technique on computational resources. Nevertheless, such a simplification is valid, since the structure is longitudinally uniform beyond the thin-film layer. However, this is aspect is not covered in the presented paper and it is our ongoing effort. Finally, the goal is to verify if the investigated structures can work as a slot waveguide.
It is well known that the main problem in the Y-branch splitting approach is the processing of the branching point where two waveguides start to separate. This is technologically very difficult; leading generally to an asymmetric splitting ratio causing non-uniformity of the split power over all the output waveguides. In this work we show that not only processing of branching points influences strongly splitting properties of the device but also the used waveguide structure itself. The standard low index waveguides have usually size of 6 μm x 6 μm ensuring on one side small coupling loses between fibers and waveguides and on the other side supporting mainly the single mode light propagation. However, our simulations showed that in the standard 6 μm x 6 μm waveguides is the presence of the first mode already so strong that it causes additional asymmetric splitting of the optical signals. To suppress the presence of the first mode we reduced the waveguide core size from 6 μm x 6 μm to 5.5 μm x 5.5 μm and 5 μm x 5 μm and this way were able to improve the uniformity of the split power over all the output waveguides by factor 3. Additionally, based on these results we were also able to reduce the size of the designed Y-branch to the half.
We present the design, simulation, evaluation, and technological verification of various low-index optical demultiplexers based on arrayed waveguide gratings (AWGs). When designing such optical demultiplexers, a set of input geometrical parameters must be first calculated. They are essential to create AWG layout that will be then simulated using commercial photonics tools. However, these tools do not support or support only partially such a fundamental calculation. Therefore, a new stand-alone tool called AWG-Parameters was developed, which strongly reduces the time needed for the design. From the calculated geometrical parameters, the AWG layouts were created and simulated using three commercial photonic tools: Optiwave, (Ottawa, Ontario, Canada), Apollo Photonics, (Ancaster, Ontario, Canada), and R-Soft, (Pasadena, California). The designs were also technologically verified. The simulated/measured transmission characteristics were evaluated by our newly developed AWG-Analyzer tool. This tool provides calculations of AWG transmission parameters, which are also missing in commercial photonic tools. Additionally, the tool provides clear definitions of calculated transmission parameters together with their textual and graphical representations. Finally, the transmission characteristics and parameters achieved from different photonic tools were compared with each other and discussed in detail. The simulated results were also compared with the measurements. Very good agreement was achieved between theoretical (AWG-Parameters tool), simulated (commercial photonic tools), and fabricated AWG transmission parameters.
We report on a design and simulation of silicon on insulator apodized surface diffraction grating fiber to chip coupler by
sub-wavelength structure which is compatible with 193 nm laser DUV lithography. The structure of designed fiber to
chip coupler consists of two parts: a relatively large tapered segment and a segment with the surface diffraction grating
having sub-wavelength structure. The first segment adjusts cross-section of silicon on insulator wire single mode
waveguide to standard single mode fiber diameter and the second one is designed for vertical coupling to the fiber. Four
types of surface diffraction grating apodization by sub-wavelength structure are designed and simulated. The simulation
of the fiber-to-chip coupler is performed by FDTD simulation method. The simulation results of coupling effects for
each apodization of surface diffraction grating are evaluated and compared with each other.
A new software tool and its application in the design of optical multiplexers/demultiplexers based on arrayed waveguide gratings is presented. The motivation for this work is the fact that when designing arrayed waveguide gratings a set of geometrical parameters must be first calculated. These parameters are the input for AWG layout that will be created and simulated using commercial photonic design tools. It is important to point out that these parameters influence strongly correct AWG demultiplexing properties and therefore have to be calculated very carefully. However, most of the commercial photonic design tools do not support this fundamental calculation. To be able to design any AWG, with any software tool and particularly to save the time needed for AWG design a new software tool was developed. The tool was already applied in various AWG designs and also technologically well-proven.
A new software tool, called AWG-Analyzer, is developed to evaluate the simulated/measured transmission characteristics
of optical multiplexers/demultiplexers based on arrayed waveguide gratings (AWG). The output of the
calculation is a set of the transmission parameters like: non-uniformity, adjacent channel crosstalk, non-adjacent
channel crosstalk, background crosstalk, insertion loss, polarisation dependent loss (PDL), etc. calculated for
each output channel first and then for the whole AWG - the worst case value of each parameter over all the output
channels. This set of the parameters is then taken as the AWG specification. The parameters are calculated for
a particular channel bandwidth (also known as the channel passband or ITU passband), that is also an input
parameter for the calculations. Additionally, the developed software tool, having a user friendly interface, offers
the help where all calculated transmission parameters are explained and exactly defined. The tool also includes
a brief overview about AWG functionality with a small animation and the information about various AWG types (CWDM and DWDM AWGs, Colourless AWGs).
Arrayed waveguide gratings (AWG) play a key role in dense wavelength division multiplexing (DWDM) systems. While
the standard channel count (up to 40) and standard channel spacing (100 GHz or 50 GHz) AWGs feature very good
transmission characteristics, increasing the channel counts and narrowing the channel spacings leads to a rapid increase
in the AWG size and this, in turn; causes the deterioration in optical performance like higher insertion loss and, in
particular, higher channel crosstalk. Channel crosstalk is a result of amplitude errors of the far field profile at the end of
the input coupler and phase errors appearing in the phased array as a result of possible effective index and geometrical
irregularities of the arrayed waveguides.
I our work we show that keeping the length of arrayed waveguides short and using specially-shaped couplers, both phase
and amplitude errors can be minimized and, as such, the channel crosstalk strongly improved. To demonstrate this effect,
we designed 256-channel, 25 GHz AWG with both, standard and also with specially-shaped couplers. The far field
distribution at the end of standard input coupler features strong amplitude errors causing the high channel crosstalk and
particularly very high background crosstalk. Applying the specially-shaped couplers led to elimination of amplitude
errors in the far field distribution and this had a positive effect on the transmission characteristics. The adjacent crosstalk
was improved by ~ 4 dB, non-adjacent crosstalk was improved by ~ 5 dB and background crosstalk by about 10 dB.
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