Fluidic micro-lens with variable focal length

Now a day, Variable focal length is a necessary attribute in many optical applications like: video camera, optical scanner, high-definition project, optical communication, and optical and biomedical inspection instruments. Some different approaches to fabricate variable focal length lens are: using liquid crystal, electro-wetting, tunable fluidic method… On the other hand, polymers are being essential materials in development of micro-devices: micro optical, micromechanical, micro-fluidic systems…( low cost, amenability to high-throughput molding, wide range of physical and chemical properties). A highly successful example is soft lithography based on micro-molding of PDMS. So, in this research, we try to use PDMS micro-molding to fabricate tunable fluidic micro lens. The purposes of this research are: design, fabricate and characterize the tunable fluidic micro-lens so that focal length, numerical aperture and resolution of these lens can be controlled by changing applied pressure.

Tunable fluidic micro-lens fabricating process:

1. Master fabricating: Using photolithography technique with SU8 photo resist

2. PDMS molding: PDMS casting, baking (with and with out pressurizing) and peeling off (master was silanized).

3. PDMS thin film was made by PDMS spincoating and baking (n-Hexane was used to reduce PDMS viscosity).

4. PDMS mold was bonded together with (or not) PDMS thin film, Glass substrate, PDMS blocks and Fluidic inlet (Si tube) by High Frequency Generator or Plasma Surface Treatment method and Epoxy Resin.

5. Tunable fluidic micro-lens was connected with Syringe pump and Pressure sensor and checked by Micro-scope, CCD camera, Stereo-scope. The relation between Focal length, Numerical Aperture, Shape changing, Contact angle of lens with Applied Pressure was studied.

Results :

Tunable fluidic micro-lens was fabricated by two ways: with PDMS thin film and without PDMS thin film (by using Pressurizing in PDMS casting and baking).

The relation between PDMS thin film thickness and Spincoating speed was studied in two ways: with and without 25% n-Hexane in PDMS mixture.

Fabricated Tunable fluidic micro-lens can be controlled by Pressurizing.

Fig1. Tunable fluidic micro-lense

Fig 2. Microscope checking results: Lens shape and Optical image changing

Fig 3. CCD camera checking results: Contact angle of lens changing by Pressurizing control

Fig 4. Curves of Relation between Focal length, Numerical Aperture and Pressure

F= 104.65251*e(-P/3.75465) + 140.35602*e(-P/0.31918) -1.46859 NA = 0.0242.P + 0.0873

Fig 5. Curves and functions between Focal length, Numerical Aperture and Pressure after fitting