reducing laser speckle with el ectroactive poly mer … reducing...reducing laser speckle with el...

Reducing laser speckle with electroactive polymer actuators Chauncey Graetzel*a, Marcel Sutera, Manuel Aschwandena

aOptotune Switzerland AG, Bernstrasse 388, 8953 Dietikon, Switzerland

ABSTRACT

Laser light sources have inherent advantages in terms of brightness and low beam divergence. However, the coherence of lasers causes speckle to form on the target. In this paper, we show how electroactive polymers can provide a compact and energy-efficient solution to reduce laser speckles. The design parameters are discussed. The speckle reducers are characterized both mechanically and optically. Finally, a case-study is presented, illustrating the implementation of the speckle reducer into a compact laser projector system.

Keywords: Laser speckle reduction, electro-active polymers, laser projectors

1. INTRODUCTION 1.1 Lasers as light sources

Lasers are bright and unidirectional, two properties that are sought after in many optical systems. When compared to conventional illuminations systems, laser have both a broader colour spectrum and a higher lifetime. Lasers also inherently come with a high degree of coherence, a property that is widely used in many scientific systems such as interferometers. In applications where lasers are purely used as a light source, however, optical coherence leads to a significant drawback in the form of speckles. On microscopically rough optical surfaces such as a wall or a cinema screen, local interferences occur which are observed as a grainy pattern of spots. This effect causes noise in projected images and reduces the resolution of measurement systems (1).

1.2 Methods to reduce speckles

Several approaches can be used to tackle the problem of speckles. Moving diffusers destroy the temporal and spatial coherence of the laser and average out the speckle pattern and are an efficient way to reduce speckle in an optical projection system. However, for common rotating systems the required mechanics behind this principle limit miniaturization and are susceptible to faults due to continuous friction. Alternatively, laser light can be passed through a vibrating glass fiber or a bundle of fibers of different lengths to destroy the coherence of the laser, although the practical implementation of these solutions in small projectors is challenging. A mechanically stable and compact solution can be realized using broadband lasers, i.e. lasers with lower coherence. However, this comes at the cost of lower specific brightness, one of the primary arguments to use lasers in the first place. 1.3 Electro-active polymers

Optotune has developed a laser speckle reducer by combining the efficient speckle reduction method moving diffuser with Electro-active polymers (EAP) as actuators technology to enable minimal mechanical friction and wear.

Electro-active polymers consist in an electrically isolating (dielectric) elastomer where each side has been coated with a conductive sheet (2). By applying voltage across the sheets, electrostatic forces compress the elastomer which in turn expands laterally. Due to the large elongations attainable with elastomers, the lateral expansion can be quite big given the actuator size.

For the speckle problem, the large travel range obtainable from an EAP actuator is key, as it allows a compact yet effective means to reduce speckles. Another advantage of using electro-active polymers is energy efficiency. Due to its electrostatic nature, only small amounts of power are required to drive the actuator. Ohmic losses occur only during charge and discharge of the capacitors, and a minor amount of leakage currents happens through the polymer itself. Compared to rotating actuators, the EAPs are near frictionless, guaranteeing high energy efficiency and long lifetime.

Invited Paper

Electroactive Polymer Actuators and Devices (EAPAD) 2015, edited by Yoseph Bar-Cohen, Proc. of SPIE Vol. 9430, 943004 · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2086088

Proc. of SPIE Vol. 9430 943004-1

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a)

c) rb)

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The biggest advantage of all may be integration: the electro-active polymer is not only the actuator, but functions also as the support structure for the optical diffuser and the compliant element of the resonant mass-spring-damper system. By fulfilling three roles with one element, the complexity and size of the whole system is elegantly minimized.

2. DESIGN The principle of operation of the EAP-based speckle reducer is conceptually described in Figure 1. A circular diffuser is suspended to a transparent membrane. The membrane is coated with four conductive areas surrounding the diffuser and a ground electrode on the backside. The actuators are activated in a round-robin fashion, generating a circular motion of the diffuser in the center of the membrane. By driving the actuators at the natural mechanical resonant frequency of the system, the amplitude of motion of the diffuser can be maximized, which in turn maximizes the speckle reduction.

Figure 1. Laser speckle reduction concept. A diffuser (black) is suspended on an elastomer membrane (white). The membrane is coated on both sides with conductive layers that form four electrodes (light gray). By applying voltage to individual electrodes (dark gray), the diffuser is moved away from the electrode due to the lateral expansion of the compressed elastomer. A) Position at rest. B) Vertical diffuser motion. C) Diagonal motion. D) Horizontal motion.

An embodiment of the design is depicted in Figure 2. The system is powered by a 5V input voltage from a micro-USB cable. The printed circuit board (PCB) contains the control electronics that delivers the signals to the four independent electrodes. 2.1 Maximizing speckle reduction efficiency

Essentially, the efficiency of the speckle reduction is maximized when the incoming beam travels over many diffuser structures during the integration period. Four main parameters contribute to an efficient speckle reducer:

1. The motion speed of the diffuser

2. The diffuser’s structure

3. The exposure time of the observer/camera

4. The optical system layout (beam diameter, position of the LSR, f-number, additional optics)



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4. CAASE STUDYY: IMPLEMMENTATIONN IN A LASSER PROJEECTOR 4.1 Integrattion of diffuseer-based specckle reducers

Whenever a ddiffuser is inttroduced in ann optical systeem, the outgoiing beam willl diverge. To avoid beam ddivergence, ann additionally llens can be inttroduced into the system (seee Figure 5).



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Electro activethe electro acelement for oother approacconcurrently.We strongly solutions can

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5. DISCUS

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projector using

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ONCLUSIO

hat allow tightn, mechanical to be significroles, it can b

of electroactivmaterial sciencomises.

must be founde applied to mdesign, characchnology.

components.

SR speckle redu

ON

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ng to fulfill alln commerciadnd mechanica

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his applicationd elastic returne compared tol requirements

dl applicationsal engineering

The Optotuneors all the waynd presented a

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6. REFERENCES [1] J. Goodman, "Some fundamental properties of speckle," J. Opt.Soc. Am., vol. 66, no. 11, 1976. [2] Y. Bar-Cohen, Electroactive Polymer (EAP) Actuators as Artificial Muscles: Reality, Potential, and Challenges,

SPIE, 2004. [3] Roelandt, S. et al, "Standardized speckle measurement method matched to human speckle

perception in laser projection systems," Optics Express 8770, vol. 20, no. 8, 2012. [4] Optotune, "Optotune LSR-3000 Datasheet," 09 09 2014. [Online]. Available:

http://www.optotune.com/images/products/Optotune%20LSR-3000%20Series.pdf. [5] Optotune, "LSR application note," 27 10 2014. [Online]. Available:

http://www.optotune.com/images/products/Optotune%20application%20note%20LSR.pdf. [6] Optotune, "LSR for speckle free laser projection," October 2013. [Online]. Available:

http://www.optotune.com/images/products/Optotune%20application%20note%20for%20speckle%20free%20laser %20projection.pdf.



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