Portable system of the dual-pinhole off-axis holographic microscope

One of the key features of the dual-pinhole off-axis holographic microscope is lens-free and highly compact. But our system still occupies a quite large space on the optical table with a size of 35*40*20cm. Thus, we designed a new portable prototype which have a size of ~13*11*7cm and a weight of about 300g. The most part of this portable microscope is printed using 3D printing technique. And the total cost except for the digital camera is about 3000 RMB.

System design and setup

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Figure 1. (a) The schematic diagram and (b) the photo of the portable system

As shown in Fig. 1 (a), this microscope is mainly composed of a low-cost laser diode (SANWU, LD-T650H00, wavelength: 650nm, measured power: ∼ 120mW), a lithium battery, a dual-pinhole aperture (National aperture, diameter: 3µm, pinhole-pinhole distance: 1mm), a digital camera (XIMEA, MQ042MG-CM, resolution: 2048*2048, Pixel size: 5.5µm) and a 2D manual linear stage (Newport, consists of two M-MT-X and one M-MTAB2, travel: 1cm). The shell of this microscope was printed using 3D printing technique and was then sprayed to black in order to prevent laser beam leakage and lower the experimental noise received by the digital camera. The power of the laser is provided by the battery. The power of the camera, however, is provided by the computer through a USB 3.0 connection.

Experimental results

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Figure 2. (a) Hologram and (b) the reconstruction result of the portable system

In order to test the lateral resolution of our system, an amplitude resolution target (USAF 1951 T-22) was imaged using this portable microscope as displayed in Fig. 2. In this figure, elements with a line width of 1.7µm are resolved which implies that the resolution of our system is about 1.7µm.

Figure 3. Experimental results of the portable system. (a) Hologram. (b) Reconstructed phase image of the sample. (c) Measured surface profile of the sample.

Another hologram captured using our portable microscope is displayed in Fig. 3 (a). In this hologram, we imaged a pure phase object that is made up of polydimethylsiloxane (PDMS). The height difference of this phase object is calibrated as 120nm using the Alpha-Step surface profiler. Fig. 3 (b) shows the reconstructed phase image of this hologram. The surface profile of the sample could be easily calculated through multiplying a scaler by the unwrapped phase profile. Fig. 3 (c) demonstrates the resultant surface profile of the highlighted line in Fig. 3(b). Here, the measured height difference of the phase object using our system is 118.8nm, and the standard deviations of the three parts of the line are all below 15nm. This indicates our portable microscope possesses an axial accuracy of tens of nanometers.

We’ve submitted a paper to International Symposium of Optomechatronics Technology (ISOT 2015) based on this system, and it has been accepted.