an integrated tunable interferometer controlled by liquid

An integrated tunable interferometer controlled

by liquid diffusion in polydimethylsiloxane

Yun Zou,1,2

Zhenhua Shen,1,2

Xiang Chen,3 Ziyun Di,

4 and Xianfeng Chen

1,2,*

1Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China 2The State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong

University, Shanghai 200240, China 3Key Laboratory for Thin Film and Microfabrication of the Ministry of Education,

Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China 4The institute for Quantum Science and Engineering and Department of Physics & Astronomy,

Texas A&m University, College Station, Texas 77843, USA *[email protected]

Abstract: 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.

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#169750 - $15.00 USD Received 4 Jun 2012; revised 23 Jul 2012; accepted 23 Jul 2012; published 2 Aug 2012(C) 2012 OSA 13 August 2012 / Vol. 20, No. 17 / OPTICS EXPRESS 18931

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1. Introduction

Optofluidics, where optics and microfluidics are working together, is defined as a new filed

and technology [1,2]. Optofluidics provides unique optical properties such as optically smooth

interfaces, high thermo-optic coefficient, liquids with large variety of refractive index.

Compared with traditional rigid optical devices, optofluidic elements show features due to the

nature of the liquids which makes the device highly flexible, reconfigurable and real-time

tunable [3–8]. Various types of optofluidic interferometers for refractive index sensing have

been reported [9–15], such as Mach-Zehnder Interferometers (MZIs) [12,16], Young

interferometers [17–19], and Fiber Bragg gratings [20,21]. However, most of these devices

typically exploit interaction between liquid-air interface [22], liquid-PDMS interface [12], the

shortcomings of these devices are change of liquid types, and non-adjustable width and

location of the interface.

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.

2. 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 micro-

lenses 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 128µm 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 9µm 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.

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.

3. 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 mλ∆ = = (1)

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 Qwater: Qethylene glycol =

15µl/min:3µl/min and Qwater: Qethylene 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.


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 Qwater: Qethylene glycol = 10µl/min:3µl/min and Qwater: Qethylene glycol = 15µl/min:3µl/min,

respectively. Interference phenomenon is weakened as the refractive index span decreased.

Fig. 3. (a) Interference curves at flow rates of Qwater: Qethylene glycol = 25µl/min:5µl/min, Qwater:

Qethylene glycol = 25µl/min:3µl/min and Qwater: Qethylene glycol = 25µl/min:2µl/min. (b)-(d) Simulation

results of the diffusion at the flow rate ratios of Qwater: Qethylene glycol = 25µl/min:2µl/min, Qwater:

Qethylene glycol = 25µl/min:3µl/min and Qwater: Qethylene glycol = 25µl/min:5µl/min.

Figure 3(a) depicts the interference curves at the flow rates of Qwater: Qethylene glycol =

25µl/min:5µl/min, Qwater: Qethylene glycol = 25µl/min:3µl/min, Qwater: Qethylene 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 Qwater: Qethylene glycol =

38µl/min:6µl/min, Qwater: Qethylene glycol = 35µl/min:2.5µl/min, Qwater: Qethylene glycol =

40µl/min:0.5µl/min.


Fig. 4. (a) Interference curves at flow rates of Qwater: Qethylene glycol = 40µl/min:0.5µl/min, Qwater:

Qethylene glycol = 38µl/min:6µl/min and Qwater: Qethylene glycol = 35µl/min:2.5µl/min. (b)-(d)

Simulation results of the diffusion at the flow rates ratio of Qwater: Qethylene glycol =

38µl/min:6µl/min, Qwater: Qethylene glycol = 35µl/min:2.5µl/min and Qwater: Qethylene glycol =

40µl/min:0.5µl/min.

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.

Fig. 5. [1]-[4] Different locations of interface. (a)-(d) are the corresponding refractive index

distributions.


4. 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.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (Grant No.

61125503) and the Foundation for Development of Science and Technology of Shanghai

(Grant No. 11XD1402600, No. 10JC1407200).


an integrated tunable interferometer controlled by liquid

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