Remote opto-chemical sensing with extreme sensitivity: design, fabrication and performance of a pigtailed integrated optical phase-modulated Mach–Zehnder interferometer system
Introduction
There is a growing need for sensitive chemical sensors in areas such as process technology, health care, environmental control, biotechnology, etc. 1, 2. For many applications these sensors should show a high resolution over a wide dynamic range, they should be very sensitive, selective, fast, small and cheap, and should be suited for remote sensing. An attractive option fulfilling all these requirements is offered by integrated optical (IO) sensors [3]. In these optical chips the chemical parameter to be sensed, the measurand, influences in the sensing region directly or by means of a chemo-optical transduction layer the propagation properties of a guided light beam propagating through the chip. Using appropriate optical circuitry, these changes of propagation properties can be converted into a change of the optical output power. In turn, this is measured using a photo detector and appropriate electronics. Generally, selectivity is provided by the chemo-optical transduction layer, alternatively called the sensitive layer. This layer contains receptor units, that selectively associate with the chemical entities of the measurand. In addition, this layer can concentrate the measurand molecules and enhance the optical effects of their presence.
Using identical optical chips and electronics, the concentration of a large variety of measurands can be determined, simply by applying the appropriate transduction layer. Such sensor families can imply immunosensors having antibody proteins as receptor molecules [4], gas sensors (e.g., CO2, SO2) with silicates containing functional organic groups [5], etc.
Mostly the relevant changes of optical parameters occur in a region just outside the core layer of the waveguide, the evanescent field region (see Fig. 1). Hence, such chemical IO sensors are often called evanescent field sensors. The changing optical parameters imply whether the refractive index n, the absorption coefficient α, or luminescence parameters. In the refractive and absorptive sensors the parameter change expresses itself as a change of the (complex) phase velocity of the guided beam and can be quantitatively expressed as the change of the (complex) effective refraction index Ñeff of the propagating mode. A pure index change manifests itself as a change of the real part of the effective index, an absorption change as a change of its imaginary part. In the refractive sensors (purely Δn) the read out of the change ΔNeff can be based on mode coupling, as is the case in surface plasmon resonance (SPR) sensors 6, 7, 8 and grating sensors 9, 10, or on the related change of the phase. The latter form the base for the interferometric sensors such as the difference interferometer (polarimeter) 11, 12 and the Mach–Zehnder Interferometer (MZI) 13, 14, 15, 16, 17, 18.
For all these types of Δn-based sensors, the minimal detectable change of the effective refractive index ΔNeff has been derived 3, 19 to be:with Lint the “interaction length” of the guided wave with the analyte, λ is the wavelength used, and the factor g, depending on the applied experimental measurement techniques, is generally in the order of 1–5×10−3 [3].
Although very good results are obtained with both the SPR sensor and the grating coupler concept, the interferometric sensors are generally supposed to have the highest sensitivity potential 3, 17, 19. This is mainly based on the possibility of using large interaction length values resulting in enhanced sensitivity (see Eq. (1)). A typical value of Lint is 1 cm for the MZI, being (at least) an order of magnitude larger than the typical values of 1 mm for the grating coupler and 10–100 μm for the SPR sensor. Amongst the interferometric sensors it is only the MZI sensor that contains an easily accessible reference arm (see Fig. 2). When used properly, this reference arm can make the sensor (nearly) insensitive to many perturbing effects 19, 20. The resulting large intrinsic stability results in an improved sensor resolution, making the MZI sensor very attractive [4].
The high potential of the integrated MZI has initiated a large amount of research. Recently much progress has been made, resulting in various promising applications 13, 14, 15, 16, 17, 18, 20, 21, 22. Nevertheless, until now the IO MZI is seldom applied for commercial sensor applications. The reason may be, that coping with intrinsic factors, such as fringe order ambiguity, directional ambiguity and sensitivity fading, just as with extrinsic disturbing factors such as light source and temperature fluctuations and in addition the influence of technological imperfections, will lead to rather complicated optical chips and processing electronics.
In this paper, we will describe the design, the realisation and the performance of a complete MZI system, including both the optical chip and the electronics. The design objectives will not be directed to obeying a given set of quantitative specifications but will show a more qualitative nature, such as a low detection limit and a high resolution over a wide range. Also the system should be incorporated into a fibre network, hence amongst others a low input power is wished. The area of the optical chip has to be kept small, while there is a strong wish to keep the technology as simple as possible implying a small number of technological steps only.
An analysis of the MZI (Section 2) enables the transformation of the general objectives into more operational ones. A functional structure is proposed and requirements are set to the individual functions. In Section 3, a novel serrodyne type of modulation principle will be introduced. After a discussion of the choice of the materials and the (available) technologies (Section 4) the individual functions are designed, optimising them with respect to their requirements (Section 5). In Section 6the separate steps for the fabrication of the optical chip are given. Both the individual components and the complete sensing system are tested and the results are discussed (Section 7). Finally the prospects of this new device are given (Section 8), followed by a summary of the main conclusions (Section 9).
Section snippets
The principle
In a MZI (see Fig. 2), the incoming light is divided over two branches. Both branches are combined again, that combination leading to the interference of the individual branch beams. This results into an output power Pout that depends on the phase difference Δϕb of both beams in the combination area. It is that phase difference that will be made dependent on the measurand concentration.
Conversion of a chemical concentration into a phase difference
A cross-section of the sensing waveguide along the propagation direction has been given in Fig. 1. In that
(De)modulation principle
In Section 2, it has been concluded to apply an AC modulation scheme and to implement an electro-optic modulator in each of both interferometer branches. Many modulation principles are known to increase the sensitivity of the integrated interferometer 22, 23. A survey of the most relevant ones in case of electro-optical phase modulation has been given in Ref. [23]. Here we propose a novel serrodyne type of modulation method based on the detection of quadrature points. The principle is
Materials and technology
To realise the passive IO circuits, the SiON technology, as developed in our laboratory [26], is well-suited. By varying the O/N ratio the refractive index can be set between n=1.46 (SiO2) and 2.01 (Si3N4). The specific value is controlled by the gas flow ratios, when depositing these layers by low-pressure chemical vapour deposition (LPCVD) or plasma-enhanced chemical vapour deposition (PECVD) technologies. The waveguiding multi-layer structure (see Fig. 1) commonly consists of a SiO2
Fabrication
The masks contain both stand alone individual components and complete MZI systems. The latter are located in groups of five, the lateral dimensions of such a unit corresponding to 55×3 mm. This chip area is used as a standard for packaging. In the following, the most important wafer processing steps, as elucidated in Fig. 16, will be briefly described.
(1) Starting point of the optical chip is a highly conducting (0.010–0.018 Ω cm) 3-in. silicon 〈100〉 wafer.
(2) Thermal wafer oxidation (at
Testing results and discussion
In this section results of testing both the individual components and the complete sensing system will be presented. Individual components will be treated in the following sequence: straight channel waveguide, Y-junction, TE/TM polariser, fibre-to-chip coupling function, modulator, sensing unit and tapers. The section ends with a discussion of the phase resolution and long-term stability as obtained with the complete system. An application example is given.
Prospects
The MZI realised here shows a better short-term phase resolution (≤1×10−4×2π) and long-term stability (≤3×10−4×2π/h) than those reported in literature 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 21, 22. This mainly stems from the improved chip design and the introduction of an effective modulation principle. These figures can be improved even more by doing the following.
⋅ Increasing the interaction length, now being 4 mm only, to larger values. This however can only be obtained at the cost of the
Summary
We have demonstrated the design, the fabrication and the testing of an extremely sensitive MZI sensing system, consisting of a laser, an IO chip and processing electronics. The system can be used as a stand alone system but can also be inserted into a fibre network. The system shows a short-term (minutes) phase resolution ≤1×10−4×2π and, in a not thermostated room, a long-term (“drift”) stability of 1–3×10−4×2π/h. These values are equivalent to a short-term refractive index resolution of ≤5×10−8
Acknowledgements
These investigations were supported by the Dutch Innovative Research Program (IOP) Electro-Optics. The authors want to thank Dr.Ir. T.S.J. Lammerink for the realisation of the electronic circuitry, Dr.Ir. T.J. Ikkink for the theoretical signal aspects on the MZI, Ir. B. Middelbos for the work on the electro-optic modulation, Ing. T. Andringa for the fabrication of the optical chips, Ing. R. Wijn for the device and electronics measurements, Ing. A. Hollink, I. Jentink and Ing R. Wijn for the
René G. Heideman (1965) obtained his PhD in Applied Physics at the University of Twente in 1993. At the same University, he worked for several years as post-doc. His research areas were the development of new photonic devices and bio-chemical sensors. In 1997, he joined Twente MircoProducts as product specialist on IO-MST devices. Currently, he works at Mierij Meteo as head of the R&D-Integrated Optics section.Paul V. Lambeck (1939) is an associate professor of Materials Science at the
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René G. Heideman (1965) obtained his PhD in Applied Physics at the University of Twente in 1993. At the same University, he worked for several years as post-doc. His research areas were the development of new photonic devices and bio-chemical sensors. In 1997, he joined Twente MircoProducts as product specialist on IO-MST devices. Currently, he works at Mierij Meteo as head of the R&D-Integrated Optics section.Paul V. Lambeck (1939) is an associate professor of Materials Science at the University of Twente, The Netherlands. He is one out of two groupleaders in the Lightwave Device Group of the MESA Institute of this University. His research areas are integrated optical devices and photonic circuits for application in the optical telecommunication and sensors. At present, his research focuses on integrated optical switches, amplifiers. MOEMS and chemo-optical sensors.
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Currently working at Meteo Instruments, Weltevreden 4c, 3731 AL de Bilt, Netherlands.