Feedback-Based Imaging Enables Miniature Scanning Confocal Microscopes
Daniel S. Burgess
As part of their work toward the development of a handheld scanning confocal microscope, researchers at Institut Femto-ST (Franche-Comté Electronique Mecanique Thermique et Optique — Sciences et Technologies) at Université de Franche-Comté in Besançon, France, have demonstrated a detection scheme that relies on optical feedback in a vertical-cavity surface-emitting laser (VCSEL). Although the approach ultimately may not be as sensitive as other laser-based methods that monitor optical power modulation, the investigators suggest that it promises a simpler setup, resulting in microscopes hundreds of times smaller than comparable instruments today and in arrayed devices for the interrogation of microfluidic systems in the biosciences.
The detection scheme dispenses with an external detector by directly monitoring the feedback-induced changes in the voltage of a vertical-cavity surface-emitting laser under a fixed injection current. Doing so enables the miniaturization of the confocal microscope system. Courtesy of Christophe Gorecki. ©OSA.
Christophe Gorecki, a Centre National de la Recherche Scientifique research director at the institute, said that scanning confocal microscopes are invaluable tools for a variety of imaging applications in the biological sciences but that they are too bulky and heavy for specialized in situ and in vivo tasks. His team thus is working to realize a measuring head that comprises a VCSEL, which acts as both the illumination source and detector, and two microelectromechanical actuators with integrated microlenses, which act as a doublet objective and a three-axis scanner. An array of these millimeter-scale microscopes, he explained, could be used with lab-on-a-chip systems for applications such as DNA recognition and cancer diagnosis.
Voltage-based detection has the potential to enable the desired miniaturization, the researchers believe. In the method, backscattered laser radiation induces changes in the voltage in a VCSEL that are interpreted to produce an image of the sample. These changes are measured directly under a fixed injection current and in the presence of a mechanical oscillation of the distance between the laser output and the sample.
To demonstrate the viability of the voltage imaging, the scientists produced 2-D images of a gold film deposited on glass. A 780-nm VCSEL from Avalon Photonics of Zurich, Switzerland, served as the illumination source/detector, and commercially available components from piezosystem jena GmbH of Jena, Germany, enabled X-Y scanning and Z-axis oscillation. For comparison, optical images of laser power modulation also were generated, using a silicon photodiode from Thorlabs Inc. of Newton, N.J.
To demonstrate the detection scheme, 2-D images were produced of a gold film on a glass substrate. For comparison, optical images were produced without and with Z-axis modulation, as illustrated in (a) and (b), respectively. Voltage-based images were obtained with Z-axis modulation at two values of laser threshold current, as illustrated in (c) and (d). In all cases, the fringes are the result of the inclination of the sample to the imaging head.
Gorecki said that the most significant hurdle to overcome in realizing either a handheld instrument or a microscope array involves the monolithic integration of microlenses with the proposed microelectromechanical actuators. He noted, however, that the researchers have devised a potential solution using molded glass lenses and anodic bonding to silicon membranes. They are finalizing the design of an X-Y stage that employs electrostatic comb drives, and they then will focus on achieving a 3-D microelectromechanical scanning system.
Optics Express, April 17, 2006, pp. 3396-3405.
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