Macular damage in Behçet's disease [La maculopathie dans la maladie de Behçet]

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Multimode Interference Spectrometer

Faramarz Farahi

Department of Physics and Optical Science, University of North Carolina, 9201 University City Blvd., Charlotte, NC 28223, USA.

ffarahi@uncc.edu ABSTRACT

Designs and simulations for a class of integrated photonic lightwave circuit spectrometers with low manufacturing tolerance are presented. The designs are applicable to all material platforms, including, but not limited to, silicon, glass, polymer, sol-gel, and gallium arsenide.

Keywords: integrated optics, planar lightwave circuits, spectrometer 1. INTRODUCTION

Portable/handheld spectroscopic testing instruments are playing increasingly important role in many fields such as optical communications, networking, and optical sensors. Manny of the desktop optical spectrum analyzers use scanning gratings. Due to the presence of moving parts, the repeatability of these systems cannot be guaranteed and the mechanical wearing will unavoidably affect their measurement accuracy so that these equipments require recalibration.

Therefore, a spectrometer that does not require any moving part, so the measurement repeatability and device reliability are ensured would be desirable.

Current state-of-the-art integrated spectrometers are based on Arrayed Waveguide Gratings (AWG) or incorporation of diffraction grating at a planar structure [1 ,2]. These devices are complex and the technology is not conducive to low cost applications. In another example, in a Lab-on-a-chip application a polymer based embossed grating has been incorporated in a system with a micro-fluidic channel [3] .This, like any other diffraction grating, requires long path to achieve wavelength resolution.

2. MULTIMODE INTERFERENCE SPECTROMETER DESIGN 2.1 MM! Spectrometer Design

This spectrometer is intended to be a planar lightwave circuit (PLC) device in that it can be directly integrated onto a planar substrate using any of the PLC platforms reported to date. A few examples of PLC platforms include silicon, silica glass, sapphire, gallium arsenide, indium phosphide, silicon oxynitride, polymers, and hybrid organic/inorganic sol-gels. ^The spectrometer design is based on a multimode interference (MMI) device [4, 5],_which,in general, comprises N input single mode waveguides, a large multimode slab waveguide region, and M output single mode waveguides. The basic idea is to split or combine the N input channels into M output channels, where it is not necessarily required for N to be greater than M. The dimensions of the multimode slab region, the index of refraction of the waveguide cladding, and

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the index of refraction of the waveguide core region are all chosen so that the input waveguides self-image with the number of output channels desired for the power splitter or combiner. See reference 4 for more detail regarding the theory and applications ofMMI devices.

A scanning electron micrograph of a multimode interference coupler that we have fabricated is shown in figure 1 . ^This device is a 1 x 4 power splitter. This particular device is designed to have a single input of 1 550nmwavelength and split the power equally, with minimal power loss, to each output waveguide. A simulation of the MMI performance for 1 x 5_splitteris shown in figure 2. The simulation shows the distribution of the electric field inside the MMI device as it propagates along the device. The dimensions of the slab region in this particular design are 70 pm wide and 2 1 50 pm long. In this particular design, the field intensity is equally divided into five output waveguides. As figure 2 shows the intensities of the output channels do not exhibit strong dependency on wavelengths.

Figure 1 . SEMimage of an MMI power splitter.

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Figure2. Left: Distribution of the electric field inside the MMI device as it propagates along the device. Right: Intensities of output waveguide versus wavelength.

In this paper a new spectrometer based on MMI is presented. This application of MMI has not been explored before. In fact, it is widely believed that one of the advantages of MMI splitter is its relative lack of sensitivity to wavelength change. The design presented in this paper, however, would allow us to use the multi-mode interference to identify wavelengths of input light.

Let us now consider a slightly different configuration as depicted in figure 3 .In this particular configuration the output channels do not have the same distance from the input channel however, the design is symmetric. The electric field distribution for this configuration is given in the same figure. The simulation reveals (figure 3) that the wavelength dependency of this device is very

1.3 1.4 1.6 I .7

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different from that shown in figure 1 .The wavelength dependency of the central waveguide is the same as what was expected from figure 1 however, the two other waveguides have anti-phase behavior. This characteristic could be further studied by considering other possible variations of the MMI design. For example, a configuration in which the output channels distances from the input channel increase as we go from right to left as shown in figure 4. The electric field distribution for this device and the simulations shown in figure 4 demonstrate a much higher sensitivity to wavelength change relative to the previous cases.

4000 020

—100 0 100

x (jm) ^1.5 free_space_wave'ength^1.6 ^1.7

Figure3. Left: A symmetric configuration. Middle: Electric field distribution. Right: Intensity of three output channels vs. wavelength.

For example, in the region when the middle channel signal decreases (red squares) the signal of the channel at the far right (pink triangles) increases. In this region the function (S 1 —_52)/(51 +

52) is very wavelength sensitive. Similarly at the lower wavelengths region, signals from channels 1 and 2 _(blue circles and green squares) decrease with increase in wavelength while signal from channel 4 increases.

....

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40 0 40

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x (nm) tree_space_wavelength

Figure4. Left: A non-symmetric MMI configuration. Middle: Electric field distribution. Right: Intensity of five output channels vs. wavelength.

0

2.2 Performance Simulations

We define the absolute wavelength accuracy as the maximally allowed wavelength errors centered at 0 error. For our MMI spectrometer, the measurement wavelength errors must be within the limits of [-AA, +AA], that is, over the operating wavelength range we have,

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,

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where A2' =Xs - _2c is the wavelength error, Xs is the peak wavelength reported by the MMI spectrometer and ?c is the wavelength measured by a calibrated wavelength meter. The maximum value of (IA?I) ^is the absolute value of the maximum wavelength error over the operating wavelength range.

The performance of the MMI spectrometer presented in figure 4 was evaluated using Beam Propagation Method (BPM) simulations (BeamProp, RSoft, Ossining, NY). An absolute wavelength accuracy of 0.5 nm could be achieved with this particular configuration.

3. CONCLUSION

We have presented a class of integrated photonic lightwave circuit spectrometers. These spectrometers could be made using a large range of materials such as silicori—glass, polymer, sol- gel, and gallium arsenide. The low temperature processing parameters of some of these materials like hybrid sol-gel and polymers make them ideal for direct integration with optoelectronics and electronics integrated circuits for combined optical electrical functionality on a single process wafer.

Variety of designs can be used to increase the wavelength sensitivity in a particular wavelength region. Applications of these spectrometers are broad, and in particular this can be used for channel monitoring, and sensors that require some form of wavelength measurement or identification. Example of such sensors are those based on fiber Bragg gratings, fluorescence based sensors, etc.

4. REFERENCES

[1] T. Kamalakis and T. Sphicopoulos, "An Efficient Technique for the Design of an Arrayed- Waveguide Grating With Flat Spectral Response" J. Light. Technol. 19, 1716-1725, 2001.

[2] H. Staerk, A. Wiessner, C. Muller, and J. Mohr, "Design considerations and performance of a spectro- streak apparatus applying a planar LIGA microspectrometer for time-resolved ultrafast fluorescence spectroscopy"fleview ofScientfIc Instruments 67, 2490-2495, 1996

[3] C. Maims ,^T.G.Harvey ,^P.Summersgill ,^P.R.Fielden and N.J. Goddard "Embossed polymer leaky waveguide devices for spectroscopic analysis" Analyst, 2001, 126, 1293 —¹²⁹⁷

[4] L.B. Soldano and E.C.M. Pennings, "Optical Multi-Mode Interference Devices Based on Self- Imaging: Principles and Applications", J. Light. Technol. 13, 615-627 (1995).

[5] 5. Nagai, B. Morishima, H. Inayoshi, and K. Utaka, "Multimode Interference Photonic Switches (MIPS)", J. Light. Technol. 20, 675-68 1 (2002).

[6] P. J. Moyer, A. Pridmore, and F. Farahi, "Multimode Interference Switch Design with Loose Manufacturing Tolerance", Opt. Eng. Accepted for publication.

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