Optofluidic Variable Optical Attenuator

Radhakrishnan Ranjini, Amrutha K Adiyodi, Murukeshan Vadakke Matham,* and Nguyen Nam-Trung

Department of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 39798, Singapore

*Corresponding author: MMurukeshan@ntu.edu.sg

Abstract- A variable optical attenuator was designed and fabricated based on optofluidic technology. Desired optical attenuation was achieved due to the refractive index modulation of the fluid. Here fluid used is a nematic liquid crystal which possesses high molecular order together with fluidity. The methodology adopted is to change the molecular alignment of the fluid by allowing it to flow in a rectangular channel with suitable flow rate. The effect of absorption and scattering from the liquid is also demonstrated for the selected wavelength.

I. INTRODUCTION

Variable optical attenuator (VOA) is an optical element applied for controlling the light intensity. In recent years, the technology of microelectromechnical (MEMS) systems is widely used for the fabrication of VOA [1-3]. In MEMS- based attenuators, several mirrors or shutters are used to achieve the desired attenuation. Another methodology used is with voltage driven liquid crystal based devices [4-5]. In comparison with MEMS technology, liquid crystal based VOA has several advantages such as low optical loss and simple fabrication processes. The main drawback with LC based devices is its slower response time since there is a trade off between response time and dynamic range provided by the LC devices. Hence, there is a need of developing VOA with simpler fabrication and much higher response time.

A new emerging field, Optofluidics, which is the integration of optics and fluidics made it possible to fabricate many optical components such as lasers [6-7], sensors [8], lens [9] etc with large tunability, reconfigurability and adaptability. The term “Optofluidics”, coined in 2003 and in the last seven years this terminology has been widely explored in the realization of many lab- on- a chip applications.

In this context, here we proposes and illustrates a novel method of attaining desired power attenuation on an optofluidic platform using nematic liquid crystal (NLC). Liquid crystal is a new thermodynamic state of matter which possesses high order in molecular arrangement even in the liquid form. The average orientation of the molecules in liquid crystal, which are generally rod shaped or disk like

can be described by means of a unit vector field n which is known as the director. There has been reports on the rheological studies based on the director reorientation in nematic liquid crystals by the application of voltage [10], oscillatory shear [11] and in Poiseuille flow [12]. In the present work we have adopted the features of LC such as birefringence (refractive index modulation), and the high crystalline nature for the design and fabrication of VOA. The working principle of the presented VOA is based on controlling the laser power by changing the refractive index of the liquid medium (NLC), applying an external shear. An external force or shear is applied on the liquid medium by flowing it in a rectangular channel with suitable flow rate.

II. EXPERIMENT DESIGN

Figure 1 shows the schematic diagram of the experimental set up used for the illustration of the proposed VOA.

Fig.1. Schematic diagram of the experimental set up used for the illustration of the proposed VOA.

Argon-ion laser of wavelength 514 nm was used for the experiment. The output power was monitored using a power meter which is connected to a computer for data analysis. The nematic liquid crystal, 4-pentyl-4’-cyanobiphenyl

Inflow

0.5 mm

1 mm 2.5 cm

Flow

LaserPower meter

Polarizer

Outflow

Analyzer

InflowSyringe pump

(Merck Chemical Co) was introduced into the flow cell without further purification. Flow cell was connected to a syringe pump using silicone tubing. Liquid crystal (5CB) was injected through the silicone tube into the flow cell and kept without any disturbance in order to cease the flow completely. The influence of different flow rates on the transmitted laser power was monitored. Absorption spectrum of NLC was also analyzed.

A. Fabrication of the channel

Fig 2: Flow channel

A rectangular flow channel of 2.5 cm length, 1 mm width and 0.5 mm height was fabricated using microscopic glass substrate and used as the flow cell in our experiment as shown in figure 2. All the sides of the channel were made by glass substrate of refractive index 1.52. Reducing connectors were used to connect silicone tubing of outer diameter of 0.3 cm and inner diameter of 0.2 cm to the inlet of the channel. The channel was cleaned before introducing the liquid.

III. RESULTS AND DISCUSSION

Figure 3 illustrates normalized power variation with respect to the applied flow rate. Here the effect of flow rates from 1 ml/hr to 50 ml/hr with an increment of 5 ml/hr is demonstrated. It can be seen that the transmitted power decreases as the flow rate increases.

0 10 20 30 40 500.0

0.2

0.4

0.6

0.8

1.0

No

rmal

ized

Po

wer

Flow rate (ml/hr)0.5 mm

Glass slide 1

Glass slide 2

ass slide 4

Glass slide 3

Gl

1 mm

2.5 cm

Fig.3. Variation of transmitted power with flow rates of 1 ml/hr to 45 ml/hr

From the UV-vis absorption spectrum as shown in figure 4, it is observed that the effect of absorption in the power attenuation can be neglected. Based on the initial experiments we have done, it can be inferred that, the effect of scattering and birefringence have significant role in the transmitted power through the NLC in the flow cell. When the flow is introduced, the scattering of incident light take place due to the refractive index modulation in the LC medium under external perturbation. Change in the refractive index is related to the birefringence of the medium which can be expressed by the relation given below (equation 1).

Inlet

(1)

Where n� is the birefringence of the medium and andare the change in the refractive index of the medium

known as extraordinary and ordinary refractive indices. Therefore, the variation in the transmitted power upon the application of flow rate can be a combined effect of scattering and birefringence. In future we are interested to know the percentage contribution of these effects on the power attenuation.

en

0n

e 0n = n - n�

2.5 cm

Flow InletReducing connector

1 mm

FlowFlow0.5 mm

Silicone tube

Fig.4. UV- vis absorption spectrum of NLC

IV. CONCLUSION

In summary, a novel design of VOA based on optofluidic technology has been demonstrated. Here we have integrated LC properties and optofluidic techniques to achieve power attenuation. From the experimental results, it can be inferred that the flow rate (shear rate) applied on the NLC medium has a significant effect on the transmitted power. Therefore by efficiently controlling the applied shear, desired power attenuation can be achieved.

ACKNOWLEDGMENT

The authors acknowledge the financial support received through MOE-ARC 3/08 and NTU, Singapore for the research student support.

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