The formation of an optical trap using a single-beam laser requires focusing the laser with high Numerical Aperture (NA) so that the gradient force compensates the scattering force (see Section 2.1.1). Under this condition, it is very difficult to collect all the deflected light that emerges from the optical trap. Thus it is not possible to perform a measurement of force based on the change of the linear momentum of the light. The Minitweezers consist of two counter-propagating infrared laser beams that form a single optical trap. The advantage of using counter-propagating beams is that the scattering forces cancel out. This allows to reduce the NA of the lasers (i.e., the diameter of the laser beam) and all the exiting light can be collected. Now, the use of two low-NA laser beams with high-NA focusing lenses is the key to collect and measure the change in the light momentum.
Apart from having a calibration independent of several experimental conditions (bead size, index of refraction, etc.) there are other advantages in the double-beam optical trap. First, the laser beams do not need to be highly focused, which minimizes the effect of spherical aberration of the lenses. A lens with spherical aberration focuses the marginal rays of a laser beam more tightly than the rays near the optical axis, which produces a blurred focal point. Since the intensity of a low NA laser beam is concentrated near the optical axis, such beam is less affected by spherical aberration. Moreover, a low focused beam has a longer focal distance, which makes possible to focus the laser beam deeper inside the fluidics chamber. It allows to reduce the hydrodynamic effects of the boundaries of the fluidics chamber (i.e., the coverslips) on the particle trapped in the optical trap. Apart from the optical considerations, the use of low focused lasers reduces the heating of the medium by infrared absorption. Another advantage is that the instrument can operate in the two-traps mode. On the other hand, the double-beam is quite difficult to implement because it requires an accurate alignment of the laser beams. In fact, small misalignments produce optical traps that induce non-uniform forces on the trapped particles. An accurate alignment is achieved by electronically assisted feedbacks. Feedbacks such as autoalign read the PSD measurements and lightly reposition the optical traps in order to cancel any misalignment between the focuses of the laser beams.