Microresonators As Building Blocks For VLSI Photonics

Microresonators As Building Blocks For VLSI Photonics Alfred Driessen, Douwe H. Geuzebroek, Hugo J.W.M. Hoekstra, Henry Kelderman, Edwin J. Klein, Dion J.W. Klunder, Chris G.H. Roeloffzen, Freddy S. Tan, Integrated Optics MicroSystems, MESA+, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

Evgueni Krioukov and Cees Otto, Biophysical Techniques Group, MESA+, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

Henkjan Gersen, Niek F. van Hulst, Laurens Kuipers Applied Optics Group, MESA+, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Abstract. In the last years much effort has been taken to arrive at optical integrated circuits with high complexity and advanced functionality. For this aim high index contrast structures are employed resulting in photonic wires in conventional index guiding waveguides or in photonic bandgap structures. In both cases the number of functional elements within a given chip area can be enhanced by several orders of magnitude: VLSI photonics. In this talk optical microresonators are presented as promising basic building blocks for filtering, amplification, modulation, switching and sensing. Active functions can be obtained by monolithic integration or a hybrid approach using materials with thermo-, electro- and opto-optic properties and materials with optical gain. Examples are mainly taken from work at MESA+.

INTRODUCTION After several years of declining activities in the field of optical communications new optimistic voices can be heard triggered by the recent massive introduction of broadband access. At the recent European Conference on Optical Communication (ECIO '03) in Rimini statistics1 were presented of meanwhile 80 Million broadband access points based mainly on cable and (A)DSL with an increase of 60% in the last year. In Japan, even Fiber To The Home (FTTH) is a reality with about half a Million subscribers. The up to now installed communication hardware seems still to be CP709, Microresonators as Building Blocks for VLSI Photonics: International School of Quantum Electronics, 39th Course, edited by F. Michelotti, A. Driessen, and M. Bertolotti © 2004 American Institute of Physics 0-7354-0184-5/04/$22.00

1 Downloaded 28 Mar 2006 to 159.226.100.210. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

sufficient for the currently required bandwidth of the network, but will become unsatisfactory within a few years. Looking a little more ahead, let’s say 10 to 15 years, optical techniques will largely be employed in the METRO- and Access networks. The optical systems and devices needed then will operate at Gbit/s and will be compact to allow complex optical routing and data processing. Reliability will be an important issue as no trained personnel is available in a home or small enterprise environment. These optical systems will be low-cost, as the end user can not share costs, and can be classified as consumer photonics. The only answer to these challenges will be very large scale integrated (VLSI) photonics in close analogy with the electronic VLSI electronic circuits. Table I. summarizes the challenges and targets for these photonic components. TABLE 1. Challenges And Targets For VLSI Photonics. Challenge For Target time fs frequency response THz frequency selectivity 10 GHz transmission speed Tbit/s wavelength selectivity 10 pm critical dimensions nm complexity circa 10 000 elements/chip costs 5 $/chip

The photonic components meeting these requirements have to be compact, allowing active functions like switching and amplification and will make use of nanotechnology. Similar to the transistor in the electronic world one would like to have also a basic building block in photonics. Structures with optical feedback, for example resonators, could fulfill this role. There are principally two approaches for compact waveguiding structures. In both, light is propagating in a channel due to reflections at the 'walls'. In the traditional case confinement is obtained by total reflection at the interface between the high reflection index core and the low index cladding. In the case of high contrast (∆n>0.5) structures single mode waveguides with a width of a few hundreds of nm can be obtained: photonic wires. In the second approach reflection is similar to a Bragg grating due to high index contrast periodic structures (∆n > 1). These devices are called photonic bandgap structures or photonic crystals, an analogy taken from electronic levels in solid state physics. In both cases the resulting electro-magnetic field can have the same dimensions vertical to the propagation direction in the order of hundreds of nanometers, and also the same extreme small bending radius in the order of a micrometer. In both structures also high Q resonators can be realized. With respect to the maturity of the technology, index guiding devices will be earlier in production, as there is an evolutionary path from the current low-index to the future high-index devices. Photonic bandgap structures follow a revolutionary approach as there is no low-index alternative even not to the expense of concessions to larger size. Both approaches are in the realm of nanophotonics as the optical materials have to be dimensioned with nanometer precision.


In the following we will concentrate on microresonators2 based on high index contrast waveguding structures and will show that they are promising building blocks for VLSI photonics. In section 2 we give an overview of the basic principles of microresonators illustrated in section 3 by detailed photonic scanning tunneling microscope (PSTM) measurements. Thereafter the microresonator as optical filter is presented. In section 5 a complete subsystem is described that currently is investigated in the European NAIS collaboration followed by first examples of microresonators in a new application field, optical sensing. Finally a summary with short conclusions are given. The examples presented are mainly taken from work at the MESA+ Institute at the University of Twente. References to other important work can be found in other parts of these proceedings3.

BASIC PRINCIPLE OF MICRORESONATORS

intensity [a.u.]

An optical microresonator is an integrated optics structure with optical feedback that allows a variety of functions like wavelength filter, optical switch or optical transistor4. Fig.1.a. gives a top view of such a device with two adjacent single mode port waveguides. Light enters at Iin and couples in part to the resonator. The rest of the power goes to Ithrough. Within the ring resonator the light propagates in a whispering gallery mode and couples partly to the output waveguide Idrop. After further propagation within the ring the light couples partly to Ithrough. Depending on the phase of the light after a roundtrip constructive or destructive interference will occur within the ring or Ithrough. In the case of constructive interference the resonator is on-resonance and the light coupled to Ithrough has a phase shift of 180º with respect to Iin. Fig.1.b. gives schematically the resulting normalized spectral response of a loss-less, symmetric resonator to a constant power input signal with changing wavelength or alternatively changing phase in the resonator or changing effective index. The power Ithrough is always equal to Iin with exception near to the resonance, in that case Idrop = Iin.

phase [a.u. ] or wavelength [a.u.] or effective index [a.u.]

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FIGURE 1. Microresonator with two adjacent waveguides serving as in- and output port; (a) topview; (b) schematic spectral response to a constant input intensity.


Pthrough/Pin [dB] Pdrop/Pin [dB]

The performance of the microresonator is characterized by its Lorentzian lineshape of the resonance lines in the drop port that allows for a small 3dB bandwidth ∆λ3dB but admits a poor rejection in the off-resonance condition. This becomes especially clear in a logarithmic plot, see Fig.2. Also shown are the free spectral range (FSR) and the cross talk level of -24 dB. A relative measure for the selectivity of the resonator is the finesse F = FSR/ ∆λ3dB. The quality factor Q is given by Q = λ/∆λ3dB, the cavity ringdown time τcav = λ Q/2πc and the average number of roundtrips m of photons in a resonator m = F/2π.

∆λ-3dB

FSR 1550 1560 wavelength [nm]

FIGURE 2. Example of the power in the drop and through port of a loss-less microresonator with F ~ 100 as a function of wavelength..

For the design of a microresonator the field coupling constant κ between the port waveguides and the ringresonator plays an essential role. In a loss-less resonator with an infinite unloaded finesse the port waveguides introduce the load determined only by the coupling constant(s) κ. If the resonator has a certain propagation loss α, the resulting finesse, the normalized power in the drop channel Pdrop/Pin and the normalized power in the cavity waveguide Pcav/Pin are largely varying functions of κ, see Fig. 3. For wavelength selectivity the finesse is the most relevant parameter and accordingly small coupling constants should be chosen. With regard to the drop efficiency even small losses will reduce Pdrop completely for a weakly coupled resonator. Therefore highly coupled structures should be employed, which have, however, a reduced F and wavelength selectivity. For applications where high cavity fields are desired, e.g. all-optical data processing, Pcav shows a maximum at low losses and small κ. There are principally two solutions for the positioning of the adjacent waveguides with respect to the resonator: horizontal or vertical arrangement. Fig. 4. gives schematically the geometries. In the vertical arrangement, Fig.4.a., a two-step lithographic process is needed. The coupling constant is mainly determined by the thickness and refractive index of the intermediate layer and the relative offset of the underlying waveguide with respect to the ring. This approach allows also for an optimized independent choice for ring and port waveguides.


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coupling constant K FIGURE 3. Finesse (logarithmic sale), normalized drop power and normalized cavity power as a function of the field coupling constant K for a microresonator of radius 25 pm (taken from ref. 4).

Critical in the vertical arrangement is the alignment of the two lithographic steps, where a precision within 100 nm is needed. In the case of horizontal coupling (see Fig. 4.b) only a single lithographic step with a single mask is needed. The coupling is mainly determined by the width w of the gap between the straight and bent waveguides and demands nanometer precision in the case of high rehctive index contrasts. There is reduced design flexibility as core layer and core thickness should be identical.

FIGURE 4. The two basic geometries for microresonators with port waveguides: (a) vertical arrangement; @) lateral arrangement. The structure in this case is fabricated in silicon-based technology, with the index of refractionof SiOz and Si3N41.45 and 2.0 respectively.

In order to reduce the losses of the whispering gallery modes in the resonator, one has to work with high index contrast materials. A cladding layer instead of air cladding will always reduce the local index contrast but, surprisingly enough, for a certain index range the presence of a cladding may reduce the bend losses as well5.


From Fig. 5.a., it can be seen that the bend losses have a minimum for a cladding index around 1.3. This minimum is attributed to the increase in the effective lateral contrast due to the presence of an upper cladding resulting in a maximum lateral contrast for a cladding index somewhat larger than the thermal oxide buffer layer. At larger cladding index the upwards leakage of the bend mode, see Fig. 5.c., becomes increasingly dominant causing the steep increase in Fig. 5.a. If one includes also losses due to surface scattering the decreased index contrast will be even more favorable and the bend loss minimum will shift even to higher values of the cladding index.

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(a) FIGURE 5. Influence of the index of the cladding layer for a Si3N4 based resonator: (a) theoretical curve showing minimum loss at an cladding index of ~ 1.35 together with two experimental points obtained with air and PMMA cladding, (b) the calculated field for air cladding; (c) calculated field for PMMA cladding.

In Fig. 1.b. the spectral response of a microresoantor as a function of wavelength has already been given. Just by changing the wavelength, the effective index or the phase light can be directed either to the drop or the through port. In this way the device performs as a filter or space switch. There is another mode. If one considers a single resonance line, the amplitude and width is determined by the roundtrip losses, see Fig. 6. By changing the losses and consequently reducing the Q-factor, light can effectively be switched between the two drop ports. From the foregoing it is clear that the microresonator can carry out a large number of optical functions. It can be used as a compact filter with high resolution. For Wavelength Divison Multiplexing (WDM) applications the unused input of the second port waveguide with the drop port can be considered as the add port. In this way an ultra-compact add-drop node can be realized. Switching of light -the optical analogue of a relais - can be done by changing the phase in the resonator by thermal, mechanical or electro-optical means. Electro-optic switching is especially attractive as with materials like polymers, modulation of signals exceeding 0.1 THz can be achieved6. Working with all-optical materials allowing the interaction of light by light, an optical transistor can be realized allowing bi-stability, light amplification, wavelength conversion and logic AND or OR functions7.


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FIGURE 6. The drop power of a microresonator as a function of phase shift for different values of roundtrip losses, c.q finesse..

The second mode of optical switching by changing the Q factor of the resonator can be used in mechano-optical and opto-optical structures. Working with materials with optical gain, wavelength conversion devices and coherent light sources and ultracompact ring-lasers can be realized. Finally a quite different application is feasible. Exploiting the high sensitivity of high finesse cavities to small index changes, ultracompact optical sensors8 or sensor arrays can be realized.

PHOTONIC SCANNING TUNNELING MICROSCOPE MEASUREMENTS Imaging of microresonators under working conditions is a challenging task because of the small dimensions and the restricted access under operation. The best picture that standard optical microscopy can obtain is given in Fig. 7, see also Fig. 1.a) for a SEM picture.

FIGURE 7. Optical microscope picture of a vertically coupled ringresonator: (a) standard microscope; (b) IR picture taken at ~ 1550 nm with the resonator on resonance.


Much better spatial resolution and consequently insight can be obtained with a photonic scanning tunneling microscope (PSTM). In a collaboration between the IOMS and the OT group of MESA+ PSTM measurements on microresonators have been carried out with continuous wave as well as fs light sources. In a PSTM the image is obtained by scanning a metal coated fiber tip with a small opening below 100 nm across the device under test. The distance of the fiber tip is held constant to some tens of nm in order to probe the evanescent tail of the mode profile of the device. The resolution of a PSTM is determined by the fiber tip opening and the scanning precision that can be well below 10 nm.

FIGURE 8. PSTM measurements of a microdisk resonator with a diameter of 50 µm, above the images linescans are given of the white dotted line in the image: (A) topographical view obtained in the AFM mode, clearly the non-ideal geometry of the disk can be seen; (B) PSTM image of the same area as (A); (C) detail view of the mode beat pattern; (D) increased detail of the mode beat pattern showing among others interference counter propagating modes with a beat length of 190 nm.

Balistreri et al.9,10 carried out extended PSTM measurements of a Si-based resonator disk with a spatial resolution below 50 nm, see Fig. 8.

FSR

FIGURE 9. Linescan of a PSTM oscillating in radial direction at the edge of the microdisk of Fig. 8. Below the intensity [a.u.] as a function of wavelength at two positions (dotted line in the PSTM scan) are given.


They observed several radial modes and mode conversion to counter propagating modes and found good agreement with model calculations, see also Klunder et al.1 Y2 The spectral response of the microresonator could also be measured by scanning the tip along a line and changing the wavelength. In this way the FSRs of the individual modes could be determined and the beating phenomena analyzed. As an example in Fig. 9 a wavelength spectrum while scanning along the white arrow in the top of Fig. 8.A) is given. This scan allows an analysis of the different radial modi, as for any position the FSR can directly be obtained. In a theoretical paper Hagness et al.13 simulated by a Finite Difference Time Domain calculation the propagation of fs pulses in a microresonator. They observed ballistic transport of the light pulse through the waveguide and the resonators. The experimental confirmation of the quasi particle behavior of fs pulses could be given by H. Gersen et al.14 who used a PSTM in an interferometer set up with a delay line to image the propagating pulse. The technique15,16 allows measuring the amplitude as well as the phase of the propagating pulse with nm and fs resolution. Fig. 10 gives PSTM images of a fs pulse just entering a microresonator with a diameter of 50 um. The resolution of the phase images is such that the individual maxima and minima of the propagating wave can be seen. Note also the scatter centers in the waveguides that act as source of circular waves.

FIGURE 10. PSTM images of fs light pulses propagating in microresonator with a diameter of 50 um above: just entering the resonator; left: amplitude image; right: phase image.

THE MICRORESONATOR AS FILTER The strong wavelength dependent performance of a microresonator can be conveniently exploited in ultra-compact optical filters17. As a single port device it has

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wavelength [nm] wavelength [nm] FIGURE 11. Experimental wavelength response of the through port of a microresonator (diameter 50 um); left: global scan (1530 -1550 nm); right: enlargement of the resonance peak at 1550 nm.


the functionality of a notch filter. Fig. 11 gives an example of Si-based devices with lateral coupling having a diameter of 50 µm. Its high finesse, 180 at 1510 nm allows a notch as narrow as 44 pm (~ 6 GHz) at a finesse of 8 nm. This demonstrates the combination of very high spectral resolution and a footprint area on the chip well below 0.01 mm2. Using two port waveguides the resonator can be used as a natural ADD-DROP filter for WDM applications. Due to the small footprint and the low on-chip insertion loss (off resonance below 0.1 dB) resonators can conveniently arranged in large filter fabrics resulting, for example in a compact optical cross connects18. As mentioned before the Lorentzian lineshape of the resonator response suffers from relative large side wings of the resonance peak. In a digital world one would prefer a boxshape response having within a certain range a constant value and zero outside. In this way, a signal at a certain wavelength, having perhaps small deviations, can be routed completely to the desired channel. The ideal boxshape can be approximated by working with higher order filters19. Principally one can work in a parallel or serial geometry. In the serial configuration, light from the input waveguide is coupled to a series of resonators directly coupled to each others, and leaves the last resonator at the drop port. As the resonator rings are directly coupled, high demands are put on the device technology. In the parallel configuration, see Fig. 12 for an example, the input and output waveguides are connected to all resonators lying on a row. There is, however, no direct coupling between the resonators. This can technologically easier be realized, but the phase of the interconnecting waveguides has to be controlled very carefully to avoid undesired interference effects in the drop port.

Through

Input

Drop FIGURE 12. Photograph of a three-ring filter (diameter 50 µm) in the parallel configuration.

The spectral response of the multi-ring devices depends on the geometrical parameters and absorption of the individual rings, the coupling constants between port waveguides and ring, and the waveguide losses. In the vertical arrangement the coupling constants are mainly determined by the thickness of the intermediate layer, which is equal for all resonators. Only small variations are possible by applying a small horizontal offset between ring and resonator. Tan et al.20 developed a model that on the basis of the design parameters allows simulating the filter response. Fig.13.a. gives a comparison of results with a not yet fully optimized three-ring device together with the simulation model. In Fig. 13.b. the simulated optimized result is given showing the improved pass band and the reduced rejection band in comparison with a single ring device, see Fig. 2.


Normalized drop power [dB]

Normalized drop power [dB]

Wavelength [µm]

Wavelength [µm]

FIGURE 13. Spectral response of a three-ring filter; (a.) experimental data for the not yet optimized device shown in Fig. 12 (dots) with fit by the simulation model (line); (b) simulated response of an optimized device.

When explaining the basic principle of the microresonator, mention was made of the phaseshift of 180 º introduced when light at resonance leaves the resonator by the through port. This strongly wavelength dependent phaseshift can be exploited in interferometric devices like a Mach Zehnder Interferometer (MZI) to tailor the response function. Instead of the cosine response function of a MZI one often prefers a flattened shape with steep flanks. Roeloffzen21 studied in detail a passband flattened MZI filter realized in SiON22 technology. In his design, see Fig. 14.a., a ringresonator with a FSR equal to a single MZI fringe is placed adjacent to one of the MZI branches. Several heaters are used to trim the two 3dB couplers of the MZI and the phase of the microresonator. As a result the desired pass band flattened response is obtained at both outputs of the MZI.

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In optical networks the polarization state of the light in the fibers is a slowly varying function of time depending on stress, temperature, mechanical movement and other factors. Any optical device therefore should be polarization independent. High index contrast waveguiding structures, like the microresonators, show a strong polarization dependent behavior that only in very specific cases can be mitigated by careful design. Another possibility is polarization diversity, where both polarization


states are split and processed individually. Klunder and al.23 applied this in a proposal for a WDM add-drop filter based on two (or four) identical ringresonators.

FIGURE 15. Proposed scheme for a polarization independent add-drop filter based on microresonators.

Light with an arbitrary polarization state is entering at S(in) and passes along microresonator MR1. This resonator drops only TE light at the specific wavelength to S(6), the rest passes via connection L2(2) to a polarization converter PC1 and L1(2) to the second microresonator MR2, which is identical to MR1. Here the TE signal at the specific wavelength (the former TM light at that wavelength) is dropped to S(drop). On the other branch, the dropped TE signal S(6) is guided to the second polarization converter, becomes thus TM and is combined with the TE signal at S(drop). In this way both polarization states of the desired wavelength are dropped; all other wavelengths are brought to S(thr). In addition, light at the resonance wavelength that enters in a TE mode at S(add) is carried eventually as a TM mode to S(through).

MICRORESONATORS IN THE NAIS PROJECT In the EC funded NAIS (Next Generation Active Integrated Optic Subsystem) project24 a number of European academic and industrial groups from 7 countries collaborate to apply active and passive microresonator in an optical transceiver for the access network. The complete transceiver25 is depicted in Fig. 16, which shows the use of MR combinations in different functions like switches, modulators and filter arrays. For clarity, only a restricted number of up-stream and down-stream WDM channels are drawn. The single-chip transceiver needs only one fiber-chip connection and, hybrid attached with non-critical passive alignment, two photodiodes and one (superluminescent) LED. The downstream signal channels follow a polarization diversity circuit with polarization conversion in one of the two channels. In this way the two orthogonal polarized WDM signals are transformed to signals with the same polarization state that can be demultiplexed by two identical arrays of MRs optimized for a single polarization. The MR switches in the array have a centre wavelength only slightly shifted with respect to their neighbor so that any signal wavelength can be sampled by at least two MR switches. In addition the number of these MR switches in the array is sufficient to span the complete comb of incoming WDM signals. Switching ON or OFF the desired MR switch combinations can electronically


compensate any drift in the incoming wavelength or any drift in the centre wavelength of the MR switches. The selected WDM channels are eventually directed to the highspeed photodiodes.

Hybrid attached high speed photodiodes

Polarisation converter Polarisation splitter

fixed wavelength tap

Wavelength Mux Fiber connection to network

high speed switches Hybrid attached (super luminiscent) LED High speed modulators

non reflective absorbers

active ring resonator passive ring resonator

FIGURE 16. The proposed single-chip transceiver module for the NAIS project based on microresonators.

In the upstream direction the transmitter part starts from a high-intensity broadband source, e.g. a (superluminescent) LED. The first set of resonators serves as narrow-band filters to generate the up-stream WDM channels by spectral slicing. The following set consists of active MRs, which act as independently driven intensity modulators for each wavelength. The unused light in the ON-state of the MR is directed to a non-reflective absorber. With the last set of MRs the modulated signal of the WDM channels is coupled into the waveguide connecting to the fiber. The realization of the transceiver puts severe challenges in a broad range of disciplines. New materials have to be applied to obtain the high index contrast needed for the port waveguides and the resonator structures. For high speed modulation use is made of electro-optic polymers and electro-optic organic crystals. In order to design resonators with realistic geometries and birefringent layers one need simulation tools that currently are still not yet available. Even more challenging is the theoretical analysis of complete resonator arrays in the up- and downstream part of the transceiver resulting in predictions for parameters like cross-talk, bandwidth, wavelength response, Bit Error Rate, etc. The technology that allows the low-cost fabrication and packaging without need of trimming of individual components has still to be developed. There are encouraging results, for example the reproducibility of the radius of a series of microresonators on a chip, being 25 µm ± 10 nm. There is, however, still a long way before nanophotonics can be considered a standard technology. The characterization of the devices that is able to provide sufficiently detailed feedback to the design and realization activities ask for systems that can probe the propagation of light with a high resolution in space [nm], time [ps] and wavelength [pm]. In addition system evaluations have to be carried out and a route to economic mass production has to be explored.


At ECOC 03 an array of thermally tunable resonators26 has been demonstrated. Fig. 17 shows schematically the device, an array of wavelength selective switches, consisting of a set of four thermally tunable double rings each of which can select a specific wavelength band at λ1 to λ4 from the input port and conduct it to the common drop port.

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FIGURE 17. Microresonator based spectral slicer for 4 wavelengths based on a double rings in the serial configuration.

The device has been fabricated in SiON technology with thin film heaters on top of the cladding as local heat source for each individual microring. After bonding fiber pigtails have been added. Fig. 18.a. shows the complete device, where besides the single pigtailed device 7 other arrays are visible on the optical chip. In the enlargement, Fig. 18.b., two sets of double microresonators together with bondpads for the electrical connection to the heaters on top of the resonators are depicted.

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FIGURE 18. A pigtailed microresonator based spectral slicer; (a) overview; (b) enlargement (area circa 2 x 1 mm) showing microresonators and bondpads for the thin film heaters.

Fig. 19 presents the spectral response of the device at the throughport. The array has been designed with a free spectral range of about 8nm and each individual resonator set to the same resonance if not externally heated. As can be seen, the fabrication tolerances lead to a variation in resonance wavelength within a band of 3 nm, corresponding to an accuracy of the linear length scale better than 2 x 10-3. As dip 3 and 4 are mainly overlapping only 3 dips per FSR are visible. Changing now the current through the heater of the resonator 2 (dip 2), the resonance dip can be changed nearly a full FSR (see Fig. 19). It appears that the silicon substrate is acting as a perfect heatsink as heating of a single microresonator does not influence its neighbor at a distance of about 1mm. When operating the heaters on top of the other resonators, a similar shift of its resonance can be obtained. In this way the device can be tuned efficiently to the desired wavelengths. As the heater devices are small, the experimentally determined time response is as high as 2 kHz.


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FIGURE 19. Spectral response at the through port of the device shown in Fig. 18 with a FSR of 8 nm. The grey shadowed dip corresponds to the microresonator with an active heater. With increasing heater current (from 0 to 15 mA) this dip shifts from 1552.5 to 1557.5 nm.

THE MICRORESONATOR AS OPTICAL SENSOR Optical sensing in general is based on changes on the optical properties of materials within the optical circuitry due to changes in the environment. Excluding sensors that in the sensing process generate light, one mostly tries to detect changes in the refractive index or the absorption at a certain wavelength. Using interferometric sensors, changes as low as 10-9 in the index of refraction can be measured27. Microresonators offer some unique properties for optical sensing as they are very sensitive to small changes in the refractive index. They are very small and the measuring volume can be below 1 pL making them ideal for Lab-on-a-chip applications. They are ideal for advanced sensor arrays that in combination with advanced data read-out could be used as optical nose or tongue. Optical sensor have in addition the big advantage that no electrical connection is needed making them ideal in high-risk environments or in medical applications.


Krioukov et al.8 demonstrated the feasibility of optical sensing by measuring the wavelength dependence of the scattered light of a microresonator that - for a small wavelength range - is proportional to the light power inside the resonator. In a practical design that power could be much easier be determined by measuring the intensity at the drop port. For the proof of principle they immersed the microresonator in glucose solution of various concentrations. Fig. 20 gives the result for pure water and a 0.5 and 1% glucose solution. In their not yet optimized set-up refractive index changes well below 10-4 could be detected. Working with high finesse resonators and advanced curve fitting, detection of changes as low as 10-9 becomes feasible. With further reduction of the resonator diameter the measuring volume could be even larger reduced so that single or at least few molecule detection would be possible. In a second paper Krioukov et al.28 demonstrated the use of optical microresonators for enhanced optical spectroscopy and sensing. Also here the small sensing volume make this kind of sensors attractive devices for high-sensitivity sensing and detection down to the single molecule level.

FIGURE 20. Scattering spectra recorded from a microresonator by fine laser tuning near a resonance with (a) water; (b) 0.5 % glucose, and (c) 1% glucose in the cladding.

CONCLUSIONS With the foregoing the basic principles and the potential of optical microresonators for a number of applications have been shown. It is, of course, still too early to speak about VLSI photonics, but microresonators can be considered as promising candidates for the basic building blocks needed in optical circuitry. Much work has still to be done in the field of new or improved materials that allow active optical functions, of better design tools for single devices as well systems, of better and more reliable technology and improved measurement methods. In this way the potential of nanophotonics can be gradually exploited. And finally system studies are needed that allow the application of complex optical circuits in communication, optical sensing and other upcoming fields.


The examples presented have mostly be taken from work carried out at the IOMS group of the MESA+ institute of the University of Twente, often in collaboration with other MESA+ groups.

ACKNOWLEDGMENTS The authors would like to thank a number of persons having contributed to the results described in this overview: Marcello L.M. Balistreri, Freek C. Blom, Henk Bulthuis, Lucy T.H. Hilderink, Anton J.F. Hollink, Paul V. Lambeck, Rene M. de Ridder, Gabriel Sengo, Remco Staffer, Kerstin Worhoff. For financial support the contributions of BTS (Senter), STW, FOM, NAIS (ECIST) are gratefully acknowledged.

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