H. Okamura, "Shift lens external cavity diode laser for broad wavelength tuning and switching," Optics Letters, Vol. 35, Issue 8, pp. 1175-1177 (2010)

Purpose

Manipulation of particles is possible by laser. In this research we use two lasers of two different frequency to create moving interference fringes. The trapped particles is transported at a speed determined by the difference of the frequencies.

Frequency stabilization

The interference between two separate lasers can be observed only when the lasers are highly stabilized. The most commonly used technique for frequency stabilization is a frequency locking using absorption lines of atoms such as Rb and Cs is common. The drawbacks of this method is that the frequency cannot be locked to the wavelengths off the absorption lines, and that wavelength precision finer than the linewidths of absorption lines (for instance, up to 6 MHz in room temperature for Rb atoms) cannot be achieved. We employed a high resolution Fabry-Perot etalon for stabilizing the frequency differences. This offers much finer spectral resolution suitable for producing an interference fringe between the two lasers.

Introduction

Manipulating objects by light has been of great interest. Light have both energy and momentum; thus, actuators for manipulating objects can use either of them. In terms of utilizing the momentum of light, the optical tweezer was the first example. A focused laser beam can be used to manipulate small dielectric materials [1-3]. Manipulation of atoms is possible as well and can be applied to laser cooling [4]. Recently, Cizmar et al. has successfully showed a transportation of microparticles for a long distance (250μm) by generating standing waves via an interference of counter-propagating Bessel beams [5]. This is an extension of an optical tweezer called the ‘optical conveyor.’ In the optical conveyor, a linearly polarized beam was split by a polarization beam-splitter into two paths and injected counter-propagatingly on the submicron object to be conveyed. The standing wave created from the counter-propagating beams yielded nodes and antinodes. By changing the optical path length of one arm, they successfully moved submicron objects.

This scheme has some limitations that the translation of the objects is performed by mechanical translation of a mirror. This not only limits the traveling length and speed but also make it impossible to continuously move the fringe position. Here we propose a novel scheme to resolve the drawback using an interference of two laser beams having different frequencies. This requires no moving parts and eliminates all the limitations listed above.

External Cavity Laser Diode

It is a well-known fact that two lasers can interfere, however, it requires a highly-stable lasers. External cavity laser diodes is suitable for this purpose because of its narrow line width and frequency tunability. Our ECLD is a littro-type ECLD.

Experiment

The experiment is still on the stage of testing the high-resolution Fabry-Perot etalon. We hope to be able to actually move the fringes by the time of presentation. The original concept of optical conveyer belt is limited to trapping particles of submicron size, since it uses the conterpropagating laser beams [5]. However, if we suspend particles in water the conveyer belt might be able to move the solution as the whole, thus driving the water itself. This can be regarded a optically-driven pump. This is merely a possibility, but in any case for this type of applications a laser system that we described above will be the key component.

References:

[1] Ashkin, A., Phys. Rev. Lett. 24, 156-159 (1970).

[2] Ashkin, A., IEEE Journal of Selected Topics in Quantum Electronics 6, 841-856 (2000).

[3] Ashkin, A. et al., Opt. Lett. 11, 288-290 (1986).

[4] Chu, S. et al., Phys. Rev. Lett. 55, 48-51 (1985).

[5] Cizmar, T. et al., Appl. Phys. Lett. 86, 174101 (2005).

Noise reduction

The frequency has to be stable at least for several miliseconds in order to perform a transport experiment. According to the study of frequency noise spectrum of an ECLD, largest factor is due to mechanical vibrations of kHz range. The effect of this noise can be removed by making them a common-mode noise. Let A1 and A2 be the complex amplitudes of the two laser beams. We assume the fluctuations of the frequency f can be separated into Dfcommon and Df1 and Df2 and that Dfcommon is common to the two lasers. The propagation constant k is altered accordingly.

The resultant interference fringe is obtained as follows. One can see that the term of Dfcommon cancel out and does not affect the interference fringes.