Optical trapping system with a DA AOD – optical design and interface

interface

Multiple trapping can only be realized in an optical trapping system by time multiplexing [80,81] or space multiplexing [82,83] of single traps. Time multiplexed optical traps, realized by acousto-optic technology is one of the popular ways to produce optical landscapes [76]. In this work, such a DA-AOD based optical trapping setup was used for creating optical landscape inside a microfluidic chip.

In order to implement this, an DA-AOD based optical trapping system was constructed around an inverted biological microscope, a schematic of which is given in Fig. 21. A Yb-YAG fiber laser (IPG Photonics) with wavelength 1064 nm and maximum power output 6 W was used to build this trapping setup. The laser beam was relayed to an inverted biological microscope (Nikon). As can be seen in Fig. 21, the output from the laser was made to pass through a half wave plate (HWP1) followed by a polarizing beam splitter (PBS). This was to split the power of the laser beam so that the power for trapping can be lowered even when the output from the laser is set above 3 W, where the power output of the laser would be more stable. The beam in-turn passed through another half wave plate (HWP2) which modifies the polarization state of the beam in order to ensure maximum efficiency for DA-AOD. Mirror M1 was kept at the conjugate plane of the back aperture of the objective as can be seen from Fig. 21. Hence this mirror can be used as a steering mirror to position the optical trap at the desired location of the field of view by tilting it.

Fig. 21: Schematic diagram of an optical tweezer setup equipped with DA-AOD. A laser beam of wavelength 1064 nm was relayed to an inverted biological microscope after expanding to fill the back aperture of the objective. (M-mirror; L-lens; HWP-half wave plate; PBS – polarizing beam

splitter; BB-beam blocker; AOD-dual axis acousto-optic deflector; f-focal length of the lens)

The combination of lenses L1 and L2 formed a telescopic system to expand the beam to match the beam diameter to the DA-AOD aperture. The clear aperture of DA-

AOD was 8 mm. However 6 mm was the manufacture recommended beam diameter to reduce aberrations. The beam diameter at the output from the laser was ~3 mm. Hence the telescopic system was formed with lenses with focal length in the ration 1:2. The expanded beam was passed through DA-AOD (IntraAction Corp.). DA-AOD was mounted on a three axis translation stage for precise positioning. The DA-AOD consisted of two orthogonally oriented tellurium dioxide crystals for achieving deflection in two lateral axes. The DA-AOD was positioned at the conjugate plane of the back aperture of the objective which is Fourier plane of the sample plane.

The beam coming out of DA-AOD was then relayed to the back aperture of the objective with a 1:1 telescope as the diameter of the back aperture of the objective was ~6 mm. In this telescopic system mirror M3 was positioned at the back focal plane of lens L4 so that M3 would be at the Fourier plane of the back aperture of the objective. Hence tilting M3 would help to position the beam entering into the objective so as to ensure that the back aperture of the objective was overfilled. The microscope was equipped with a Kohler illumination in transmission mode. The image was collected using the same objective that was used for trapping and images were recorded using a CCD camera (Basler). A photograph of this system is given in Fig. 22.

Fig. 22: A photograph of the DA-AOD based trapping setup. (M-mirror; L-lens; HWP-half wave plate; BE-beam expander; PBS-polarizing beam splitter)

4.2.1 DA-AOD controller

The DA-AOD had a response time of the order of milliseconds. The position of the trapping beam may be deflected using the DA-AOD by giving analog signals with a particular frequency to the DA-AOD crystals. The deflection angle had a linear relationship with the frequency of the signal sent to the DA-AOD crystal. In the DA- AOD system we used, the deflection bandwidth of the DA-AOD crystal was 8 MHz, with a central frequency of 27 MHz. There were two crystals in the DA-AOD to achieve deflection in X and Y directions. In order to deflect the beam to a specific position in the sample plane, a pair of frequencies corresponding to the two co-ordinates in the sample plane was given to the pair of DA-AOD crystals oriented orthogonal to each other.

In order to create a stable optical landscape at the sample plane, it was essential to have a system to drive the two crystals in DA-AOD in a synchronous fashion. We used a digital frequency synthesizer (Gooch & Housego) for driving the DA-AOD which was capable of synchronously triggering both of the crystals within the DA-AOD. The digital frequency synthesizer (DFS) accepts the frequency information as 30 bit binary data and generates analog signals with a frequency corresponding to the number specified in the digital sequence. There were two sets of signal generators within the DFS to address the two crystals in the DA-AOD. It was possible to use a latch signal to simultaneously trigger the two DA-AOD crystals with a new set of coordinates. This ensured generation of stable and reproducible optical landscapes at the sample plane.

The DFS accepts the 30 bit binary word as 30 bits through a 37 pin D-Sub adaptor. Each binary bit in the 30 bit word had to be sent through one of the 37 wires of the cable. We used a LabVIEW (National Instruments) interface to generate the 30 bit word. This word was then sent to a first in first out (FIFO) buffer of a Field Programmable Gate Array (FPGA) based data acquisition (DAQ) board optimized for superior accuracy at fast sampling rates (National Instruments). At the DAQ board each of the binary word was split into different bits and through a pair of 68 pin shielded I/O connector block these bits were sent to DFS. Fig. 23 shows the flowchart of the program that sends signal to DFS for controlling DA-AOD.

Fig. 23: Flow chart detailing the program for sending digital signals to DFS for generating analog signals to control DA-AOD.

4.2.2 LabVIEW interface

While the previous sub-section explained the details of the program that runs in the background to generate optical landscape using DA-AOD, this sub-section looks into the front end interface of this optical trapping system. A LabVIEW interface was constructed for controlling this optical trapping system equipped with DA-AOD. Fig. 24 shows a screenshot of the front end interface.

The functionalities of this interface can be classified into three. One section controls the camera. This section has a region for real-time observation of the sample plane. Along with controls to adjust gain and exposure of the camera, this section also has functionalities to record video with specified frame rates. The next section was for DA- AOD control. This section can be used to send a new pattern to DA-AOD. The pattern can be imported from images drawn with other drawing software (eg. MS paint). Also

this interface has a third section which is a drawing board which can be used to draw desired patterns to be sent to DA-AOD.

Fig. 24: Front end interface of to generate optical landscape using DA-AOD based optical trapping system.

As can be seen in Fig. 24, the LabVIEW based interface is equipped with drawing board and recording functions which make it suitable to create desired optical landscape and to observe and record it in real-time.