Contrast Enhancing Microscopy

We demonstrate an all-optical, marker-free technique for bright field microscopy that enhances contrast, removes noise, and sharpens edges. The method relies on phase conjugation (the nonlinear reversal of a wavefront) and multiple transmission of light through an object. The setup is self-aligned, works at low power levels, and can be integrated easily with existing microscope and optical systems. In addition, the technique removes unwanted speckle and interference aberrations that are responsible for the unpopularity of coherent light sources in bright field microscopy.

A simplified experimental setup is shown in Figure 1. We demonstrate the method in the simplest configuration, with a double-pass microscope using a coherent light source.


Figure 1 - Simplified experimental setup : Collimated laser light at 532nm is focused onto a sample. The outgoing beam is sent to a phase conjugation mirror, and a self-aligned back-propagating copy of the signal beam undergoes a second transmission through the sample. A CCD camera collects the double-pass image.

Figure 2. shows experimental results on a prepared slide with a slice of corn stem. We compare a single-pass (normal transmission) image, a phase-conjugated image, and a double-pass, phase-conjugated image. For comparison, Fig. 2(d). shows a single-pass picture that has been numerically adjusted to the same amount of contrast; noise, speckle, and diffraction effects are all worse than the double-pass case.

Figure 2 - Experimental results shows the advantages of all-optical phase-conjugation contrast enhancement : Contrast is enhanced as speckle noise and aberration (ringing) from out-of-focus layers is reduced. This technique enables the acquisition of fast, speckle free, coherent images, and is particularly useful for wide-field digital holography.

In conclusion, we have demonstrated an imaging technique that enhances contrast by using multiple passes through an object. Using a phase-conjugate mirror, images in each pass are self-aligned with the sample and automatically corrected for aberrations, speckle, and diffraction. Since the nonlinear element is separated from the sample, the technique is compatible with all existing microscopy setups can be implemented easily as an add-on. In addition, the nonlinearity itself enables further all-optical image processing capabilities, such as edge enhancement, amplification of low-level signals, wider field of view, more selective depth of field, resolution improvement, and noise control.

Related Publications

Contrast Enhancement by Double Pass Phase Conjugation Microscopy
N. C. Pegard and J. W. Fleischer,Frontiers in Optics, post-deadline paper,PDPA10, (2010)