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Past Projects

An optical position-clamp with predictive control

We have developed a predictive control-algorithm which overcomes some of the limitations of our previous proportional real-time control strategy. With this new algorithm the resonance peak due to the loop-delay is smaller, which allows us to use higher gains and achieve higher effective trap stiffness.

See our November 2009 APL paper:
H. Ojala, A. Korsbäck, A.E. Wallin, and E. Haeggström, “Optical position-clamping with predictive control”, Applied Physics Letters 95 181104 (2009). (doi:10.1063/1.3257693)

Real-Time optical force-clamp

We are developing an FPGA-based real-time optical force-clamp (see video below).

This video shows two optically trapped polystyrene beads as seen through a 100x bright-field microscope. The optical traps are indicated by cross-hairs. A DNA-molecule (invisible) is tethered between the beads. The scale-bar on the right is 20 micrometers long. The yellow-trace shows the force set-point. When the feedback-control is activated the FPGA steers the upper trap using AODs in order to achieve the set-point force on the lower bead. The actual force on the lower bead is shown in blue. The distance between the the two beads is indicated in green.

See our 2009 SPIE conference paper:
Anders E. Wallin, Heikki Ojala, Gabija Žiedaitė, Linda Degerth, Dennis Bamford, and Edward Hæggström, High-resolution optical tweezers for investigating DNAbinding/translocating molecular motors, Proc SPIE 7400 (2009) (doi:10.1117/12.826470)

Real-Time feedback control of optical tweezers

“FPGA-Based Real-Time Feedback Control of Optical Tweezers” application note on National Instruments’ website. We are employing an FPGA-based data acquisition card which allows real-time control of trap position and stiffness. A description of this appplication won the Life Science & Biotechnology categor of the Paper Contest at NIWeek 2007.


A.E. Wallin, H. Ojala, E. Haeggström, and R. Tuma, “Stiffer optical tweezers through real-time feedback control“, Applied Physics Letters 92 (22) 224104 (2008) (doi:10.1063/1.2940339)

Anders E. Wallin, Heikki Ojala, Anders Korsbäck, Edward Haeggström, Roman Tuma, “Real-time control of optical tweezers”, SPIE Optics & Photonics, Vol. 6644 Optical Trapping and Optical Micromanipulation IV, San Diego, California, 26 – 30 August 2007. ( doi:10.1117/12.737269 )

Power spectral density (PSD) of bead position fluctuation during position-clamp feedback control with increasing gain from zero (top trace, blue) to 24.8 (bottom trace, black). The PSD becomes increasingly non-lorentzian with increasing gain and shows a resonance peak around 12 kHz. The experimental data is well fit by a theoretical PSD derived in our paper (solid lines). Inset shows effective trap stiffness as a function of feedback gain.

2007: Monte Carlo simulation


We have developed a Monte Carlo simulation for modeling a typical molecular motor(MM) experiment. in which a microsphere (radius r), tethered through a WLC, is held in a force-clamp (F_opt) while the WLC contour length (L) is successively shortened with a step length (dL). The end-to-end distance (d) is measured and from the position vs. time trace the step length of the molecular motor is determined. Our simulation can predict the SNR for different experimental parameters.

Anders E. Wallin, Ari Salmi and Roman Tuma “Step Length Measurement – Theory and Simulation for tethered Bead Constant-Force Single Molecule Assay”, Biophysical Journal, August 2007 Vol 93 No 3 (doi:10.1529/biophysj.106.097915)
(also presented as a poster at the International Biophysics Congress 2005, August 2005, Montpellier, France)

Older projects

Photoacoustic generation of surface acoustic waves

Elastic waves can be generated using various pulse lasers e.g. Nd:YAG (1.06um). The high intensity of the laser beam heats the surface of the sample causing thermal expansion which in turn generates elastic waves i.e. ultrasound.



When combined with some optical detection method e.g. knife edge, interferometer a totally remote ultrasonic measurement method is achieved.

For more information, please contact:

Juha Aaltonen
Jyrki Stor-Pellinen, Vaisala

Ultrasonic measurement of paper thickness

To measure thickness of a thin film or a foil without touching the sample in real time is essential in many industrial applications. Normally small pieces of transparent samples are taken under a microscope where the thickness can be estimated. Interferometric systems are also available but they are expensive. Paper industry needs information of the profile of the paper during manufacturing process. Measuring area in paper mills is very large, so the measurement has to be fast.

A new reliable and fast ultrasonic measurement system (see figure) has been studied. This system uses ultrasound of 20 kHz to 40 kHz and pulsed sound of 7 kHz to 15 kHz. A wide range of thin samples have been tested from 2 µm Mylar to 100 µm paper with good accuracy. The other advantage of this system is also that it can be used with samples that are not transparent. The optimum sound frequency is chosen depending of the sample thickness: the massive the sample, the lower the frequency. Both electrostatic and piezo ceramic transducers have been used and both have advantages and disadvantages and can only be used within narrow bandwiths. The sound pulse system consists of a loudspeaker and a pair of microphones.

For more information, please contact:

Reijo Vuohelainen, Ph. D., Mikkeli AMK, reijo.vuohelainen “at”

Magneto-acoustic storage correlator

Magnetic media have a memory property to record and preserve information. The majority of modern signal recording is based on the preservation of the state of magnetization in ferromagnetic films or tapes as a local magnetization distribution. The recorded signal can be read several times, or or it can erased or re-written by exposing the magnetized recording material to a magnetic field that deletes the formerly recorded distribution.

In magnetic materials, the magnetic and acoustic phenomena are connected by magnetostriction. When a magnetic medium is subjected to a mechanical stress and a magnetic field, simultaneously, the magnetization distribution, the domain structure is different from that is the resulting structure with the stress-free case. As a consequence, in a several magnetic solids, there occurs a phenomenon called long-term acoustic wave memory. This appears as an elastic response when a piece of suitable material is subjected to a magnetic reading pulse, and an elastic signal has been applied before to the memory. The phenomenon has been observed in a wide variety of materials.

We are studying the magnetoacoustic memory phenomenon and its applications to signal processing by the developement of a tool that we call the magnetoacoustic memory correlator. The correlator is very promising in view to applications because it operates in real time, in other words the correlation is available once the two signals have been written. Basically, the correlator consists of a ferrite rod with a shear transducer at one end, with a pick-up coil around the ferrite.



The operation is based on the nonlinear interaction of the magnetic and acoustic wave in the correlator medium. An acoustic reading signal applied to the correlator with a stored magnetization distribution generates a magnetic field that is their convolution, that for its part is proportional to the correlation between the stored signal and the reading signal. We have used the correlator in 2.5 MHz to 50 MHz NDT applications for a better signal-to-noise ratio. The curve above is a signal from pulse-echo measurement. The above curve that is the same signal compressed with the correlator indicates two reflections, a and b.

Ultrasonic tracking device for portable electronics applications

We have developed an acoustic hand position input device using airborne ultrasound. Pulse-echo triangulation method is used to obtain a 3D hand position estimate. This is achieved with a low-cost device employing three pairs of cheap ultrasonic transducers.


To achieve good controllability of the pointer, we have used noise insensitive and stable time-of-flight measurements. Moreover, the refresh rate is high enough to allow averaging and to provide potential for recognizing gesture-based commands.
Prototype’s resolution is good enough to generate accurate position estimates. The developed method provides good potential for 3D pointer steering and for recognizing gesture-based commands in portable devices or on large information screens.

Noninvasive detection of microbes in commercial food products using ultrasound


Using ultrasonic characterization techniques and statistical data analysis we investigate the possibilities of characterizing prepackaged food for occurrence of micro-organisms.

We have been able to detect a palette of, in industrial food processes, commonly met anaerobe and aerobe bacteria, yeasts and mildue. Our concept makes it possible to monitor the whole batch with incubation times shorter than those in conventual plating. The products investigated represent different categories of viscosity and homogeneity.

The research is a part of an international Eureka (European Union) project for developing end test measurement techniques in food industry. The participing nations are Sweden, France, Holland and Finland.

Confocal Scanning Laser Microscope

The confocal scanning laser microscope (CSLM) has proven to be very useful tool for obtaining images having high resolution in the axial direction since out-of-focus signals make only a weak contribution to the detected signal. The unique depth discrimination property makes the CSLM useful in noninvasive medical and industrial inspection and in many other applications. In our laboratory we are mostly using the CSLM for measuring the thickness of transparent or semi-transparent films and sheets. We have also measured the dimensions and internal structure of optical fibers. In these cases one-dimensional imaging is adequate. The optics of the CSLM are used to obtain information from a single point and thus do not directly produce an image. To obtain a one-dimensional image of specimen, the focal point of the CSLM is scanned in the axial direction. The schematic of the reflective mode CSLM used in our laboratory is shown in Fig. 1.


Light from a HeNe laser is directed through a spatial filter. The collimated beam filling the aperture of an objective, O1, is focussed on a specimen. Light reflected from the specimen is collected by the objective and directed by a beamsplitter to a second objective, O2, which focusses the light on a pinhole or a slit. The signal is detected by photodiode or a photomultiplier tube. The axial movement of of O1 is executed with an electromagnetic scanner. The experimental setup also enables the scanning of the specimen with a piezoelectric actuator in the axial direction and with a stepper motor driven translation stage in the radial direction.




The figure above shows a one-dimensional image of a cylindrical plano-convex lens, which is examined by using the CSLM from the curved side. Reflection from the front surface gives the first and the strongest peak of the signal. Passing the front surface the light suffers refraction, and due to the cylindrical geometry of the surface it now has two focal points. Both of the focal points give their own reflections and consequently their own peaks to the recorded signal. These peaks can be separated using slit detectors as shown in Figs 2 b and c.

Fibre Optic Confocal Microscope

The CSLM is made using bulk components. We have also constructed a fibre optic version which is shown below. In this device the HeNe gas laser is replaced with a 650 nm laser diode, which has a single mode fibre pigtail. The beamsplitter is replaced with a 2×1 single mode fibre coupler and the detector is attached directly to the fibre. The output light from the fibre is collimated and then focused by the scanning objective.

By using fibre optics the adjustment of the CSLM is made very easy: the collimating and the focusing objectives are the only components which need adjustment, and even that is not critical. The single mode optical fibres also have the benefit that they shape the light so that it has almost perfect intensity profile making it easier to maintain the diffraction limited operation of the microscope.