Datasheet: LSR-3000 Series Update: 06.08.2012 Copyright 2012 Optotune Laser Speckle Reducer LSR-3000 Series Speckle noise from a laser-based system is reduced by dynamically diffusing the laser beam. A diffuser is bonded to a thin elastic membrane, which includes four independent electo-active polymer electrodes that induce a circular oscillation of the diffuser in x- and y-direction. The LSR-3000 Series integrates fully certified drive electronics powered through a single micro-usb connector. Two sizes are available: the LSR-3005 and the LSR-3010 that respectively exhibit a clear aperture of 5 mm and 10 mm. The following table outlines the specification of Optotune s LSR-3000 Series. The diffusion angle can be adapted on demand. Mechanical specifications LSR-3005 LSR-3010 Clear aperture 5 10 mm Outer diameter 41 48 mm Thickness 8.8 8.8 mm Weight 24.43 32.84 g Electrical specifications 1 Power supply (micro-usb interface) 5 5 VDC Power consumption 310 310 mw Optical specifications Diffusion angle (FWHM) 1 /10 /20 1 /10 /20 Transmission spectrum 2 see figures 2&3 see figures 2&3 Damage threshold 2 >3 >3 W/cm 2 Oscillation frequency ~300 ~180 Hz Oscillation amplitude ~400 ~400 µm Thermal specifications Storage temperature [-40,+85] [-40,+85] C Operating temperature [-30,+85] [-30,+85] C Figure 1 (a-c) show the effect of the LSR-3005 on a laser spot (= 650 nm, P = 5 mw). (a) Without LSR (b) with LSR OFF (c) with LSR ON Figure 1: CCD images of a laser spot without and with LSR-3005 on. 1 100-230 VAC to 5 VDC micro-usb power supply provided by Optotune 2 The standard LSR comes with polycarbonate diffusers. On request, custom diffusers made of proprietary polymers are available, which offer a higher transmission range and a damage threshold of 300 W/cm 2. Furthermore, it is possible to build LSRs with coated glass diffusers of similar size and weight. Page 1 of 4
Datasheet: LSR-3000 Series Update: 06.08.2012 Copyright 2012 Optotune The following two figures show the transmission spectrum of the LSR-3000 Series with standard polycarbonate diffusers and coated BK7 cover glasses. In addition to the dynamic diffuser, standard products ship with a 1 static diffuser to increase speckle reduction efficiency. Transmission [%] 100 90 80 70 60 50 40 30 LSR-3000 VIS, 1 dynamic, no static diffusor LSR-3000 VIS, 10 dynamic, no static diffusor 20 LSR-3000 VIS, 20 dynamic, no static diffusor LSR-3000 VIS, 1 dynamic and 1 static diffusor 10 LSR-3000 VIS, 10 dynamic and 1 static diffusor LSR-3000 VIS, 20 dynamic and 1 static diffusor 0 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 Wavelength [nm] Figure 2: Transmission spectrum of the LSR-3000 series with VIS-coated cover glasses (0 angle of incidence) 100 90 80 70 Transmission [%] 60 50 40 30 LSR-3000 NIR, 1 dynamic, no static diffusor LSR-3000 NIR, 10 dynamic, no static diffusor 20 LSR-3000 NIR, 20 dynamic, no static diffusor LSR-3000 NIR, 1 dynamic and 1 static diffusor 10 LSR-3000 NIR, 10 dynamic and 1 static diffusor LSR-3000 NIR, 20 dynamic and 1 static diffusor 0 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 Wavelength [nm] Figure 3: Transmission spectrum of the LSR-3000 series with NIR-coated cover glasses (0 angle of incidence) Page 2 of 4
Datasheet: LSR-3000 Series Update: 06.08.2012 Copyright 2012 Optotune Figure 4 shows the dimensions of the LSR-3005 (left) and of the LSR-3010 (right). The housing exhibits a M4 thread in order to facilitate the integration of the LSR on an optical table. The power supply interface is made through a micro-usb connector. The LSR-3000 series is also available without housing (see datasheet of LSR- OEM) Figure 4: Mechanical drawing of the LSR-3005 (left) and of the LSR-3010 (right) (unit: mm) Ordering information for standard products The LSR-3000 can be delivered with two sizes of clear aperture, three different diffusing angles (circular) and two types of cover glasses. When ordering, please refer to these specifications as follows: LSR-30CA--AR CA = Clear aperture (05 or 10 mm) = Diffusion angle (1, 10 or 20 ) AR= Antireflection coating (VIS or NIR 3 ) Example: LSR-3005-20-VIS refers to a speckle reducer of 5 mm aperture with a diffusion angle of 20 and VIS coated cover glasses. Custom products Optotune offers customized versions of the LSR. This datasheet only contains variations of the LSR-3000 Series products. For LSRs with substantially smaller or larger apertures, please contact sales@optotune.com. Diffusers In principle, any type of diffuser can be used as long as size and weight are similar to the standard polycarbonate diffuser. Optotune has several types of polycarbonate diffusers in stock (circular: 0.5, 2, 3, 5, 30, 3 By standard, the NIR version is only available for the LSR-3005 Page 3 of 4
Datasheet: LSR-3000 Series Update: 06.08.2012 Copyright 2012 Optotune 40, elliptical: on demand e.g. 40 x0.1 ) and can also produce diffusers of a proprietary polymer material, which offers a higher transmission range (see Figure 5) and a damage threshold of 300 W/cm 2. Furthermore, it is possible to use specialty coated glass diffusers if they are light enough. If necessary, it is also possible to remove the membrane from the optical path. The following table summarizes these options: Diffuser type Polycarbonate (standard) Optotune proprietary polymer Glass diffusers (typical) Thickness 250 um 150 300 um As thin as possible Density 1.2 g/cm 3 1.2 g/cm 3 2.2 g/cm 3 Weight 5 mm aperture 10mm aperture Transmission range Damage threshold 8.5 mg 28.5 mg 5.1 10.1 mg 17.1 34.2 mg up to 20 mg up to 70 mg 350 1550 nm 240 2500 nm 170 12000 nm (depending on material) > 3W/cm 2 > 300 W/cm 2 1 kw/cm 2 (depending on material) Please note that diffusers cannot be exchange after the LSR has been produced. Also, when using customized diffusers it might be necessary to select an appropriate cover glass to match the transmission range. Figure 5: Comparison of standard polycarbonate diffusers and Optotune s proprietary diffuser material Cover glasses Cover glasses can be customized with respect to material and antireflection coating. The standard cover glasses are VIS or NIR coated BK7. If the LSR is to be integrated into a clean environment, then the cover glasses can be omitted. Frequency The oscillation frequency is set to the measured resonant frequency of the LSR during production. With the current standard electronics it is not possible to change that frequency after production. However, Optotune can provide customized electronics to control both voltage and frequency of the electro-active polymer. Page 4 of 4
Application Note: Transmissive Laser Speckle Reducer LSR-3000 & LSR-OEM Series Page 1 of 11
Speckle pattern Lasers have the unique characteristic to provide high power and low divergent light. Besides, due to its working principle, laser light exhibits a high degree of coherence that enables, e.g. efficient interference processes. Although this characteristic is widely used in many scientific systems, it leads to a significant drawback for applications that use a light detector. When shining a surface, laser light is scattered by each corrugation points of the illuminated surface. Each of these scattered points may be described as secondary coherent light sources. If the corrugation depth is of the order of the laser wavelength, local interferences occur such that a random intensity pattern also known as speckle pattern is observed. Figure 1 shows an image and the corresponding intensity profile of a speckle pattern. (a) (b) Figure 1 (a) Image of a speckle pattern on a CCD camera (b) Measured intensity profile on a horizontal axis through the spots center. This non-uniform intensity distribution puts significant constraints on light detectors that exhibit local saturation points. Besides, this pattern may disturb the human eye. Speckle reduction Speckle contrast The speckle contrast S is defined as the standard deviation of the intensity I i within a certain area normalized by its mean value I mean as shown in Eq. 1 hereunder. (1) with (2) The speckle contrast varies between 0 and 1, 0 representing a homogenous beam without speckles. Page 2 of 11
Reduction factor At a microscopic level, the speckle ratio depends on The wavelength of the laser light The state of polarization of the laser light These two parameters are well defined with laser light and contribute together with the quality of the illuminated surface to the speckle process. At a fixed wavelength and state of polarization, the better the quality of the illumination surface, the smaller is the speckle contrast. Alternatively, for a fixed surface quality, the larger the wavelength, the lower is the speckle contrast. At a macroscopic level, the speckle ratio depends on: The diffusion angle of the LSR The numerical aperture of the detection system The reduction factor is in this last case of:, where is the diffusion angle and is the numerical aperture of the detection system. Reduction efficiency For a reference optical system, the reduction efficiency R may be described as the ratio between speckle contrast without LSR S and the speckle contrast with LSR S LSR as follow: (3) For instance a reduction of the speckle ratio from 0.5 to 0.2 provides a reduction efficiency of 4 db. : working principle The laser speckle reducer from Optotune is based on a dynamic process. The speckle pattern is moved at a sufficiently high frequency and amplitude such that the detection system integrates the speckle pattern over time as a uniform light distribution. The LSR consists of a diffuser bonded on a polymer membrane that includes four independent dielectric elastomer actuators (DEAs). Under activation, the surface of the electrodes increases and causes a motion of the rigid diffuser in the membrane plane. The four independent electrodes are used to obtain displacement of the diffuser in both directions of the x- and y-axis, as shown in Figure 2. In case of the LSR-3000, the control signals of the four electrodes (x1, y1, x2 and y2) have the same amplitude and frequency, but with a phase shift of 90 in between. This controlling profile of the electrical signals driving the electrodes generates a circular motion of the diffuser. The moving frequency is optimal when reaching the mechanical resonance frequency of the system und such provides the highest speckle reduction. Page 3 of 11
Figure 2: Illustration of four independent DEAs to move the rigid diffuser (blue circle) in the plane of the membrane. The equilibrium (no voltage applied on the electrodes) position of the diffuser is represented by the dashed circle. (a) The x1 and y1 electrodes are activated, the diffuser moves in positive x- and y-direction. In the pictures (b), (c) and (d) the analog effect as described for all four different states of the diffuser. After reaching state (d), the cycle continues with position (a). A dedicated driving electronic that provides the optimal electrical control signal is integrated in the LSR-3000 and energized by a 5V Micro-USB power supply. Measurement of the speckle reduction Reference setup In our reference setup, the LSR is positioned in a collimated laser beam of 5 mm as shown in Figure 3. Figure 3 : Reference setup for measuring speckle reduction. Laser: He-Ne, P=20mW, λ=632.8nm, linearly polarized. Beam expander: 15x. Objective: Computar, T4Z2813 CS IR. Camera: Mightex Systems, Monochrome 1.3MP CMOS, MCE-B013-US USB2.0. The laser light is expanded up to a 5mm diameter collimated beam by a beam expander. An attenuator at the input of the beam expander controls the laser power. Likewise an aperture can be used at the output of the beam expander to precisely control the illumination beam size and minimize stray light on the screen at the very end of the bench. The transmissive LSR is positioned in the collimated beam after the aperture and an image of the laser spot on the screen is made by the camera. Page 4 of 11
Figure 4 shows typical speckle images that are obtained without any LSR (a), with the LSR in a static mode (b) and with the LSR in a dynamic mode (c). The colored lines show the horizontal plane in which the intensity profiles are measured and depicted in Figure 5. (a) (b) (c) Figure 4: Typical images of the speckle contrast measured on the reference setup (a) without LSR in the collimated path (b) with the LSR in a static mode (c) with the LSR in a dynamic mode. The colored lines show the cut planes that refer to Figure 5. Figure 5 shows the corresponding intensity profile of the images of Figure 4 in a horizontal plane. Figure 5 Measurement of the speckle contrast on a horizontal plan as depicted in Figure 4. In red, without any LSR, in yellow with the LSR in a static mode and in blue with the LSR in a dynamic mode. The same characterization method is applied to all our standard LSR and the results are introduced in the following section. Page 5 of 11
Results on standard products of the LSR-3000 Series LSR-3005-20 (5mm aperture, 20 diffusion angle, ~3um structure size) Reduction efficiency R = 15 db Figure 6: Measurement of the intensity profile: in red, without LSR (laser alone) and in blue with the LSR-3005-20. LSR-3005-10 (5mm aperture, 10 diffusion angle, ~20um structure size) Reduction efficiency R = 12 db Figure 7: Measurement of the intensity profile: in red, without LSR (laser alone) and in blue with the LSR-3005-10. LSR-3005-1 (5mm aperture, 1 diffusion angle, ~100um structure size) Reduction efficiency R = 6 db Figure 8: Measurement of the intensity profile: in red, without LSR (laser alone) and in blue with the LSR-3005-1. Page 6 of 11
Key parameters for efficient speckle reduction The resulting speckle reduction depends on a number of parameters including: Motion speed of the diffuser Diffuser structure Exposure time of the observer/camera Optical system layout (beam diameter, position of LSR, additional optics) Displacement Resonant frequency Diffuser structure size The higher the better Depends on: LSR size (larger = higher) Diffuser weight Maximum voltage Material parameters The higher the better Depends on LSR size (larger = lower) Diffuser weight Material parameters The smaller the better Various approaches: Random surface diffusers Diffractive optical elements (DOE) Figure 9: Summary of key parameters for efficient speckle reduction High motion speed The diffuser moves along a circle (or ellipse) due to the activation cycles of the electrodes. The main parameters of the actuation are: The motion amplitude (path perimeter L = 2π r ) The mechanical driving frequency f Together, these two parameters define the motion speed of the diffuser v=l f Example of LSR-3005: r=200um, f=300hz v=377mm/s The higher the motion speed of the diffuser, the more patterns can be overlapped during the exposure time of the observer. This parameter can be optimized for custom designs, but there are trade-offs to be made between motion amplitude, frequency, size of the LSR, weight of the diffuser and maximum voltage. Diffuser structures Speckle reduction efficiency is proportional to the number of structures passing under a point during exposure time. The addition of N uncorrelated speckle patterns yield a reduction of speckle contrast by 1/SQRT(N). The goal is thus to create as many uncorrelated speckle patterns as possible. Apart from moving the diffuser as much and as fast as possible (high motion speed), this can be influenced by optimizing the structure of the diffuser. As can be seen on page 6, the reduction efficiency is better with smaller structures. But that means higher angles, which again leads to a trade-off with beam divergence. Page 7 of 11
Optotune is working on optimized diffuser structures or the use of diffractive optical elements (DOEs) to achieve efficient speckle reduction at minimal increase of divergence. Exposure time Ideally, the frequency of the LSR is at least as high as the frame rate of the camera. In the case of the human eye, which has exposure time of about 17ms (60Hz), this is easy to achieve. An industrial camera, however, might run at higher rates. For example, the LSR-3005 runs at 300Hz, which corresponds to an exposure time of 3.3ms. If the exposure time of a camera is 1ms, then only about one third of the circular motion is captured, so the speckle reduction will be less. However, depending on the motion amplitude and the structure size of the diffuser, the reduction might still be very good. The LSR-3005-20 can still reduce the speckle contrast down to 3% for a 1ms exposure time as can be seen in Figure 10. Unfortunately at a MHz level, the speckle reduction process will not be efficient. Figure 10: Speckle contrast obtained by the LSR-3005-20 for an integration time from 1ms to 10ms How to optimise the integration of the LSR in a laser system For efficient laser speckle reduction, we advise to: position the LSR perpendicular to the optical axis illuminate the LSR by a collimated beam match the beam size that enter the LSR with its clear aperture ( 5mm diameter) Figure 11 illustrates the most straight-forward use of the LSR. The laser beam is collimated and its cross-section matches the clear aperture of the LSR. The zoom in Figure 11 shows the correct positioning of the LSR along the light path. The cone-shaped aperture corresponds to the light output. Page 8 of 11
Figure 11: Optimal use and positioning of the LSR in a laser system Under this configuration, from the optical point of view, the LSR will expand the collimated beam with an angle that matches its diffusion angle (e.g. 20 FWHM value for the LSR-3005-20). In order to compensate for the diffusion angle, a collimation lens can be positioned downstream the LSR at a distance that matches its focal length in order to re-collimate the beam. This setup is illustrated in Figure 12. Figure 12: Optimal use of the LSR in a laser system with an optical system positioned upstream the LSR. Page 9 of 11
LSR in focal plane + homogenizer An alternative use of the LSR is to position it in the focal point of the laser. Acting as a point source, the beam can be very well collimated again. However, the diffuser structure becomes visible, which means a homogenizer is needed. The result is a speckle-free, collimated and homogeneous beam. LSR-3000 micro-lens array condensor field lens Laser screen Figure 13: Optical system layout with the LSR in the focal place of the laser, followed by a homogenizer LSR for use with DLP/LCOS micro displays In the following, three principle setups are described for the integration of the LSR into a projection system based on DLP or LCOS micro displays. Red Laser Collimation lenses Optotune LSR Beam homogenizer DLP/LCOS Projection Optics Green Laser Blue Laser Beam combiner Collecting lens Homogen, collimated, speckle free beam Figure 14: LSR is positioned before the micro display to illuminate the DLP/LCOS with speckle-free light Red Laser Collimation lenses Beam homogenizer DLP/LCOS Optotune LSR Projection Optics Green Laser Blue Laser Beam combiner Homogen, collimated beam Figure 15: The LSR positioned after the micro display in the image plane of the projection optics. The Image stays in focus thanks to minimal out-of-plain motion of the LSR Page 10 of 11
Red Laser Optotune LSR Beam combiner Beam homogenizer DLP/LCOS Projection Optics Green Laser Blue Laser Collimation optics Homogen, collimated, speckle free beam Figure 16: Each laser is despeckled separately. Glass diffusers with wavelength optimized coatings can be used. Trouble shooting The output beam does not exhibit any speckle reduction at all 1. Check that the power supply is on (blue light). 2. Check that a significant difference is obtained when the LSR is switched on (dynamic mode, Figure 4 (c)) compared to when the LSR is switched OFF (static mode, Figure 4 (b)) 3. To check if the diffuser is moving at all, place the diffuser into the focal point to make an image of the diffuser structure on the screen. Thanks to the magnification, the movement should be very apparent. 4. If none of the above solves your problem, this means the LSR is broken and should be sent back to Optotune for replacement. The output beam does not exhibit a sufficient speckle reduction 1. Try to optimize the speckle reduction by increasing the size of the input beam to match the size of the clear aperture of the LSR. 2. Try to optimize the position of the LSR perpendicular to the optical axis. 3. Try to increase the exposure time of your camera. 4. If none of the above solves your problem, this means the chosen LSR does not provide a sufficient speckle reduction ratio R for your application. A LSR with a larger diffusion angle should be used. Custom designs Optotune can design a dedicated LSR for your application, taking into account requirements for size, frequency and motion amplitude. It is also possible to integrate custom diffusers of your choosing. Feel free to ask for a feasibility study or a quote at: sales@optotutne.com. Page 11 of 11