An integrated tunable interferometer controlled by liquid diffusion in polydimethylsiloxane

We demonstrated an integrated tunable interferometer in Polydimethylsiloxane (PDMS). In contrast to most on-chip interferometers which require complex fabrication, our design is realized by conventional soft lithography fabrication. The optical path difference occurs during propagation across a fluid-fluid interface. The diffusion level of the two miscible liquids which is controlled by liquid flow rates provides tunability. Different ratio of two liquid flow rates result in the interference spectral shift. Interference peak numbers are varied with flow rate ratio of two liquids. Mutual diffusion between two liquids changes the profile of the refractive index across the fluidic channel. The two arms structure of our design provides convenience for sensing and detection in biology system. This device not only offers the convenience for microfluidic networks but also paves the way for sensing in chemical microreactors. © 2012 Optical Society of America OCIS codes: (230.3990) Micro-optical devices; (160.5470) Polymers; (130.3120) Integrated optics devices. References and links 1. D. Psaltis, S. R. Quake, and C. Yang, “Developing optofluidic technology through the fusion of microfluidics and optics,” Nature 442(7101), 381–386 (2006). 2. C. Monat, P. Domachuk, and B. J. Eggleton, “Integrated optofluidics: A new river of light,” Nat. Photonics 1(2), 106–114 (2007). 3. N. T. Nguyen, “Micro-optofluidic Lenses: A review,” Biomicrofluidics 4(3), 031501 (2010). 4. Z. Li and D. Psaltis, “Optofluidic dye lasers,” Microfluid. Nanofluid. 4(1–2), 145–158 (2008). 5. D. B. Wolfe, D. V. Vezenov, B. T. Mayers, G. M. Whitesides, R. S. Conroy, and M. G. Prentiss, “Diffusioncontrolled optical elements for optofluidics,” Appl. Phys. Lett. 87(18), 181105 (2005). 6. J. G. Cuennet, A. E. 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We experimentally demonstrated a tunable interferometer controlled by diffusion.Diffusion at the interface between two streams of liquids with different refractive indices, a controllable concentration and corresponding refractive index gradient are brought by laminar flow.This device represents a fresh approach for tunability, and it takes advantage of different diffusion degree between two miscible liquids, which changes the phase difference between the optical paths.The flow rate ratio determined the length scale for diffusion and the refractive index.The two parallel channels are convenient for sampling.More importantly, the design allows for more stable laminar flows.As we all know, micro total analysis systems include the processes of sampling, analysis, waste treatment.One promising application of the device in biotechnology can be controllable real-time micro-reactors.

Experiment
Figure 1(a) shows the schematic of an optofluidic interferometer.The DI water and ethylene glycol were injected into the chip using syringe pumps (PHD2000, Harvard Apparatus).The chip was observed under an inverted microscope (IX51, Olympus).Micrographs of the microlenses and inserted fibers are shown in the Figs.1(b) and 1(c).Experimental setups are presented by Figs.1(d) and 1(e).The device with a height of 128 µm was fabricated with PDMS via conventional soft-lithography [23,24].The PDMS chip consists of two fluid inputs and a fluid output.Two miscible liquids were infused by syringe pumps via two inlets.Solution 1 was de-ionized (DI) water (n = 1.33), solution 2 was ethylene glycol (n = 1.43).Amplified Spontaneous Emitting (ASE) served as light source with wavelength ranges from 1528nm to 1573nm.We adopted the Erbium Doped Fiber Amplifier (EDFA) to amplify the incident light.Light was coupled to the input optical fiber and collimated by the first PDMS micro-lens.The collimated light travelled through the device and was focused into the output optical fiber by the second PDMS micro-lens.Interference curves are recorded by an optical spectrum analyzer (AQ6370C) with the resolution of 0.02nm.Inserted fibers are single mode with 9 µm cores and a numerical aperture of 0.14.Two arms were symmetrical in design.Light was launched into the straight channel and propagates along the straight line instead of split into the left and right arms because the interference phenomenon also emerged when two arms were full of high refractive index solution.

Results and discussion
The incident light propagates along the straight line.Half of the collimated beam travelled through the upper region of the straight channel while the other half travelled through lower region of the straight channel.The phase difference between them will cause the constructive interference, which satisfies , ( 1, 2,3...), nd m m where λ is the wavelength of the incident light and n ∆ is refractive index difference .d is the length of light propagation path.m is a positive integer.From the above equation, when d is fixed, the number of the interference peak is increasing with n ∆ .
Here, we controlled flow rates of two liquids to achieve a different degree of mixing to produce a different refractive index gradient.n ∆ is varied with different refractive index gradient.In other words, the output intensity will be dependent on the dynamics of diffusion between two liquids.Different from previous researches of others group, our work has advantages of high detection sensitivity and convenience, moreover, free of need to exchange the type of liquid in the experiment process.Figure 2(a) plots the interference curves at flow rate ratios of Q water : Q ethylene glycol = 15µl/min:3µl/min and Q water : Q ethylene glycol = 10µl/min:3µl/min.This plot demonstrates the flow rate ratio changes refractive index gradient which results in a center wavelength drift of 0.35nm.A program was written in COMSOL to calculate the refractive index gradient at different flow rates.Flesh color represents the DI-water and the carmine represents the ethylene glycol.Color bar describes refractive index distribution.From the simulation results in Figs.2(b) and 2(c), the diffusivity decreases with lager flow rate ratio.Figure 5 gives micrographs of the different location of the interface and the corresponding refractive index distribution.It can be observed that the interference was controlled by flow rates of the two miscible liquids.Refractive index gradient manipulate the optical path difference.Our device detects small variation of the refractive index difference and has no restriction for refractive index of samples.It also can be used to measure the reaction degree of two liquids in biochemistry in terms of the peak numbers.In theory, the results can be optimized by larger flow rate ratio.However, it will lead to the instability of the laminar flow interface.
The device has many applications, such as tunable filter, real-time micro-reactor and sensor.The experimental results show that it has a sensitivity of 139 nm per refractive index unit (RIU).The key parameter sensitivity needs to be optimized if it acts as a sensor.Acting as an optical switch, the response speed needs to be improved, because the stability of laminar flow takes several seconds when the flow rate changes.

Conclusion
This letter describes a tunable optofluidic interferometer controlled by liquid diffusion.Several nanometers wavelength drift was achieved in our experiment.It is relatively easy to vary the refractive index difference n ∆ and avoid changing the liquid types.In contrast to most microfluidic interferometers, our device features exact and easy controllability and simple structure.Tunable method in our experiment is simple and direct.The peak numbers of the interference curves are increasing with the flow rate ratio.Such an interferometer will pave the way for microfluidic components that used for biochemical tests in fully integrated and highly compact sensing system.

Fig. 1 .
Fig. 1.(a) Configuration of the interferometer.(b) and (c)Micrographs of the inserted fibers and micro-lenses.(d) and (e) Part of chip in experiment.

Fig. 2 .
Fig. 2. (a) Interference curves at flow rates of Qwater: Qethylene glycol = 15µl/min:3µl/min and Qwater: Qethylene glycol = 10µl/min:3µl/min.(b) and (c) Simulation results of the diffusion at the flow rate ratios of Qwater: Qethylene glycol = 10µl/min:3µl/min and Qwater: Qethylene glycol = 15µl/min:3µl/min.The interference phenomena are more and more obvious as the flow rate ratio increased.Refractive index gradient distributions are given in the Figs.2(b) and 2(c).From the simulation results, maximum refractive index spans are about 0.05 and 0.06 at the flow rate ratios of Q water : Q ethylene glycol = 10µl/min:3µl/min and Q water : Q ethylene glycol = 15µl/min:3µl/min, respectively.Interference phenomenon is weakened as the refractive index span decreased.

Figure 3 (
Figure 3(a) depicts the interference curves at the flow rates of Q water : Q ethylene glycol = 25µl/min:5µl/min, Q water : Q ethylene glycol = 25µl/min:3µl/min, Q water : Q ethylene glycol = 25µl/min:2µl/min.Figures 3(b)-3(d) are the corresponding refractive index gradient under the different flow rate ratios, respectively.From the Fig. 3(a), there is an about 5nm wavelength shift by changing the flow rate ratio.Compare Fig. 2(a) with Fig. 3(a), the peak numbers are increasing with the flow rate ratio.In the other words, the refractive index difference n ∆ is increasing with the lager flow rate ratio.When the flow rate ratio continues to increase, the number of interference peak increased to four which is presented in the Fig. 4(a).Figures 4(b)-4(d) denote the refractive index distribution at the flow rates of Q water : Q ethylene glycol = 38µl/min:6µl/min, Q water : Q ethylene glycol = 35µl/min:2.5µl/min,Q water : Q ethylene glycol = 40µl/min:0.5µl/min.