Selffocussing phase transmission grating for an integrated optical microspectrometer

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Abstract

An integrated optical microspectrometer in slab waveguides is presented for broadband spectroscopy in the visible wavelength range. Monomode silicon oxinitride (SiON) waveguides on silicon substrates are structured to produce a novel selffocussing waveguide transmission grating with a waveguide to air index transition endface. Regarding reflection gratings such transmission gratings have four-times larger facettes which is linked to reliable fabrication by standard thin film technology as well as to an exceptional high spectral dispersion. Combined with further functional optic elements, a compact microspectrometer is presented for long-term stable operation.

The selffocussing and dispersion properties of the transmission grating are developed theoretically by phase matching and evaluated successively with electromagnetic theory. The spectral and spacial condition of constructive interference is generated in the waveguide plane at a focus length of only 13 mm yielding a miniaturized, integrated optical, chirped ‘grism’ — grating prism — a specially designed planar imaging optic with structured waveguide endfaces results in a compact and monolithical core. The fabrication sequence based on standard thin film technology consists of a single deposition and etching process which allows an economic manufacturing. The realized microspectrometer exhibits high efficiency >60% in the first diffraction order and a spectral range of 350–650 nm with a spectral resolution of 9 nm.

Introduction

Much emphasis has been drawn in recent years to develop different kinds of diffraction gratings for wavelength division devices. On one hand, integrated optics have an outstanding importance in multichannel communication systems due to the planar design flexibility: Passive devices like arrayed waveguide gratings (AWG) are developed to produce at high diffraction orders (>10) optical channel separation of less than 2 nm spectral bandwidth at 1.55 μm [1], [2], [3]. Joined to monomode optical fibers, insertion losses less than 2 dB are aspired as well as negligible birefringence of less than 0.2 nm TE–TM spectral shift and a flat-top spectrum. Up to 128 spectral channels in silica waveguides consumes substantial substrate area in the waveguide branch which is translated to costs of several $10 per channel [4], [5]. But such devices enables to increase substantially the information density by a higher number of optical channels in already existing fiber transmission systems.

On the other hand, integrated optics enables economic mass products for sensor applications satisfying demands for industrial process monitoring and control [6]. In particular in the consumer market, low fabrication costs are fundamental conditions to establish thin film technology for sensor devices. Surface relief gratings such as wavelength selective couplers diffract and focus light simultaneously in and out of the waveguide plane but are limited to applications with a narrow spectral bandwidth [7], [8].

Broadband imaging devices need high performance imaging of multiple wavelengths which can be accomplished by a transfer of well-known concave reflection diffraction pattern to planar waveguides [9]. Due to downscaling of such conventional spectrometers — keeping the diffraction properties and hence the grating period constant, — miniaturized spectrometers exhibit in general a larger spectral bandwidth of approximately 10 nm. Such reflection gratings are usually optimized for flatfield design to obtain broadband focus imaging on a planar photodetector.

The LIGA-spectrometer as such an economic selffocussing reflection grating is fabricated by hot embossing of polymer waveguides (PMMA) of LIGA-made masters [10], [11]. An exceptional high structuring resolution of <0.25 μm is needed across the entire waveguide thickness of 100 μm in order to achieve diffraction efficiencies of approximately 10%. Caused by waveguide inhomogenities during embossing, significant stray light generates cross talk resp. reduces detection accuracy in particular in spectral ranges of low light intensity. Due to additional optical losses in PMMA in the UV range, the useful spectral range is indicated between 380–780 nm.

Our approach is to develop an integrated optical microspectrometer for the visible and near UV wavelength range (Table 1). Highly transparent SiON waveguides are utilized for the planar optical system which are deposited by LPCVD batch processing [12], [13]. Due to the selffocussing phase transmission grating and the imaging optic in reflection, the complete spectrometer is integrated in a monolithical and compact ‘waveguide core’. In comparison to reflection gratings, transmission gratings with an appropriate refractive index transition of n1n2=0.5 have a 4–5 times larger grating facette [14]. Thus, designed for a blaze wavelength of λblaze=500 nm in the first diffraction order, such gratings have facette widths of approximately 1 μm. Drawing attention to this grating design, structuring can be carried out reliably by standard thin film technology of UV-lithography and plasma etching.

Section snippets

Periodic phase transmission gratings in SiON slab waveguides

A periodic non-focussing phase transmission grating is evaluated theoretically in order to obtain both by classical phase matched diffraction as well as by solving Maxwell’s equation basic diffraction properties. Successively, such patterns are transferred to a selffocussing approach described in Section 3.

The diffraction grating pattern in waveguides is related to the lateral periodic refractive index transition by constructive interference condition of two adjacent rays, Fig. 1 [15], [16].dn2

Non-periodic selffocussing phase transmission grating

The design of the selffocussing phase transmission grating is based according to the periodic design on constructive interference condition at the refractive index transition. In order to obtain selffocussing properties in the waveguide plane, two different wavelengths have to interfere for parallel incident light in two different foci (Fig. 5). Hence, the position with the unknown parameters yi, zi of the grating facette i is given by two equations, each for the wavelength λ1 and λ2, Eq. (3)

Simulated selffocussing imaging

The grating calculation in Section 3 which is based on classical constructive interference expresses the selffocussing imaging insufficiently. Neither the facette width and the light intensity respectively, nor the radiation characteristic of the facettes are considered thoroughly. In addition, abberations of random errors by fabrication technology have to be taken into account as well as imaging properties of wavelengths far off λ1 and λ2. To overcome these problems, the farfield intensity I

Optical system

Since the selffocussing transmission grating is designed for parallel incident light, the divergent light which originates from the entrance slit of the spectrometer has to be collimated. In order to avoid imaging abberations of lenses, a parabolic mirror with a focus length of f=10 mm collimates the light with an off-axis alignment for total internal reflection. Considering technological feasibility of such a reflective optic, the imaging is not effected by the refractive index of the waveguide

Fabrication technology

The efficiency of the diffraction grating is linked to the structuring resolution of the waveguides. The smaller the edge radii the more light is transmitted into the desired order (Fig. 2). Hence, the essential fabrication development procedure comprising of deposition and structuring process is pointed out as the pattern transfer process into the SiON waveguides. In order to maximize the structuring resolution for a waveguide of 3 μm thickness, a plasma etching sequence is developed which uses

Performance of the integrated optical microspectrometer

The performance of the optical microspectrometer is characterized by the selffocussing transmission grating as well as the optical system. As a result of the optical ray forming, the light propagation within the waveguide core is presented in Fig. 13, highlighting four times total internal reflection. Additional stray light due to the reflection of four surfaces has not been detected.

The performance of the microspectrometer is related to different characterizing issues: The light throughput

Conclusion

An integrated optical microspectrometer is presented in particular for photometric applications in the visible wavelength range. The feasibility of manufacturing with standard thin film technology arise from a special designed non-periodic selffocussing transmission grating. Functional waveguide endfaces for ray forming result in a ‘monolithical’ spectrometer core. The fabrication sequence is carried out by a single deposition and structuring process which guarantees a high reproducibility at

Acknowledgements

The authors would like to thank the Deutsche Forschungsgemeinschaft for the support of this project.

Jörg Müller received the Dipl.-Ing, Dr.-Ing. and Habilitation degrees in subjects like MIS varactors, high-frequency mixers and IMPATT, p-i-n and fast photodiodes, while studying electrical engineering at the Technical University Braunschweig Germany. Since 1983 and after several years at Siemens AG, where he was head of microwave diode development and fabrication, he has been Professor of Electrical Engineering at the Technical University Hamburg-Harburg as Head of the Department of

References (27)

  • P.C. Clemens et al.

    Flat field spectrograph in SiO2–Si

    IEEE Photon. Technol. Lett.

    (1992)
  • P.C. Clemens, Optical phased array in SiO2/Si with adaptable center wavelength, in: Proceedings of 7th European...
  • M.K. Smit, Advanced devices for wavelength division multiplexing applications, in: Proceedings of ECIO 1999, 1999, pp....
  • K. Okamoto

    Fabrication of 128-channel arrayed waveguide grating multiplexer with 25 GHz channel separation

    Electron. Lett.

    (1996)
  • E.S. Koteles

    Integrated planar waveguide demultiplexers for high-density WDM applications

    Fiber Integr. Optics

    (1999)
  • E. Voges, Integrierte Optik auf Glas und Silizium für Sensoranwendungen, tm Technisches Messen 1991,...
  • R. Waldhäusl et al.

    Efficient coupling into polymer waveguides by gratings

    Appl. Opt.

    (1997)
  • D.S. Goldmann et al.

    Miniaturized spectrometer employing planar waveguides and grating couplers for chemical analysis

    Appl. Opt.

    (1990)
  • B.A. Capron et al.

    Design and performance of a multiple element slab waveguide spectrograph for multimode fiber-optic WDM systems

    J. Lightw. Technol.

    (1993)
  • B. Anderer, W. Ehrfeld, D. Münchmeyer, Development of a 10-channel wavelength division multiplexer/demultiplexer...
  • C. Müller, J. Mohr, Miniaturisiertes Spektrometersystem in LIGA-Technik, Ph.D. Thesis, Forschungszentrum Karlsruhe...
  • W. Gleine et al.

    Low-pressure chemical vapor deposition silicon-oxinitride films for integrated optics

    Appl. Opt.

    (1992)
  • D.T.C. Huo et al.

    SiO2 films by low pressure chemical vapor deposition using diethylsilane: processing and characterisation

    J. Vac. Sci. Technol. A

    (1991)
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    Jörg Müller received the Dipl.-Ing, Dr.-Ing. and Habilitation degrees in subjects like MIS varactors, high-frequency mixers and IMPATT, p-i-n and fast photodiodes, while studying electrical engineering at the Technical University Braunschweig Germany. Since 1983 and after several years at Siemens AG, where he was head of microwave diode development and fabrication, he has been Professor of Electrical Engineering at the Technical University Hamburg-Harburg as Head of the Department of Semiconductor Technology. He is engaged in thin film technology, SOI processes, integrated optics, high-temperature semiconductors and development of microsystems and processes.

    Dietmar Sander received the Dipl.-Ing. in electrical engineering and Dr.-Ing. degree in subjects like micro moulding, integrated optics, thin film technology from the Technical University of Hamburg-Harburg. At the Department of Semiconductor Technology he has been engaged for his PhD thesis in particular in microsystem technology. Since 1998 he is employed at the company Eppendorf Netheler Hinz GmbH for research and development of microfluidic systems and devices.

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