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Exulus® Spatial Light Modulator
1920 x 1080 HD Resolution
Image Generated by SLM Using Computer-Generated Hologram Projection. For Details, See the App Note Tab
3840 x 2160 4K Resolution
Thorlabs' Exulus® Spatial Light Modulators (SLMs) employ Liquid Crystal on Silicon (LCoS) technology to produce high-resolution, high-speed reflective phase modulation with individually addressable pixels. This phase control is highly stable with minimal fluctuations, minimal crosstalk with adjacent pixels, and can produce phase shift values up to 4.7pi at 532 nm (EXULUS-HD1) and 2pi at 633 nm (EXULUS-HD1 and EXULUS-4K1). These SLMs provide far more pixels than lower-order phase modulators such as segmented or deformable mirrors. The Exulus SLMs provide spatial light modulation in two dimensions for applications such as optical trapping, beam steering and shaping, femtosecond pulse shaping, adaptive optics, imaging applications, and holography.
Each Exulus includes a built-in SLM panel with independent horizontal and vertical tilt adjustment of ±3.2°. Locking rings are installed to fix the adjustment settings, as well as provide extra stability. Customized versions of the EXULUS-HD1(/M) or EXULUS-4K1(/M) with the panel separated from the control unit are also available; contact Tech Support for details. Both Exulus SLMs have a standard retardance mode for 2pi phase stroke at 633 nm, while the EXULUS-HD1 also has an extended mode for 4.7pi phase stroke at 532 nm.
Exulus SLMs are driven by an HDMI signal and operate as a general HD or 4K screen. They are bundled with a software GUI that provides complete control over the device. Different driving modes are supported by the software, including full frame, image input, video input, Fresnel lens, diffraction, and computer-generated holography (CGH). The CGH mode also allows tilting and focusing effects to be overlaid onto a pattern. The GUI enables quick switching between operating modes, as well as allowing images, videos, and patterns to be uploaded to the panel. For more details on the operating modes, please see the Software and App Note tabs.
All Exulus SLMs are shipped in a carrying case along with HDMI-to-HDMI and HDMI-to-DisplayPort cables as well as a mini-USB cable for connecting to a PC. Also included are a 12 V power supply with location-specific power cord and a HKTS-5/64 hex key thumbscrew for adjusting the horizontal and vertical tilt adjusters.
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The back panel provides USB and HDMI ports for connecting the SLM to a PC as well as the 12 V power supply input and an on/off switch. A female SMA port for a trigger output (3.3 V TTL) is present on the EXULUS-4K1(/M) only.
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The Exulus SLMs feature two bottom-located 8-32 (M4) taps for post mounting. Two taps located on each side of the housing provide additional mounting options. Four 4-40 taps around the SLM panel allow a 30 mm cage system to be attached.
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The Exulus SLMs are provided with a magnetic cover; when the SLM is in use the cover can be attached to either side of the housing as shown in the photo to the left.
When a repeating, linear phase pattern is displayed on the SLM, it will function similarly to a blazed diffraction grating. The diffraction efficiency is the power in the first-order of the diffraction pattern divided by the zero-order when the phase of the SLM is set to zero across the panel. These measurements were made at 635 nm for several test patterns with varying phase steps, effectively creating gratings with varying line spacing (denoted as line pair / mm). The patterns used are plotted below, while the measured results for both the EXULUS-HD1 and EXULUS-4K1 SLMs are plotted to the right.
Phase Patterns Used to Measure Diffraction Efficiency
The Exulus® SLMs come with a software interface that provides complete control of the SLM panel as well as device settings. Users can input a specific phase level (gray level) over the entire panel, import a custom image, produce a computer generated holography (CGH) pattern, and other patterns such as Fresnel lenses and diffraction gratings. All the patterns can be saved into a sequence list and played with a predefined interval of 16.7 ms (60 Hz frame rate, EXULUS-HD1) or 33.3 ms (30 Hz, EXULUS-4K1) in standard mode. The EXULUS-HD1 also features a triple mode; if RGB images are used then the RGB channels will be played in succession for an overall frame rate of 180 Hz. The software also supports video input at 1080p or 4K resolution with the H.264 video codec (supported file formats: MP4, M4V, and MOV).
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Frame Tab in Standard Frame Rate Mode: Setting a specific gray level from 0 to 255 will set the entire panel to a certain phase level.
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Frame Tab in Triple Frame Rate Mode (EXULUS-HD-1): Each of three successive frames (at 180 fps) can have a different gray level.
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File Tab: Upload a user-defined pattern in PNG, JPEG, or BMP format.
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CGH Tab: Convert a user-defined image into a holographic pattern. For more details, please see the App Note tab.
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Pattern Tab for Fresnel Lens Generation: Sets the SLM panel to focus the reflected light at a user-selected wavelength and focal length.
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Pattern Tab for Diffraction Grating Generation: Sets the SLM panel to diffract the reflected light with user-selected wavelength, deviation angle, and grating rotation angle. For more infromation about the diffraction efficiency of different patterns, please see the Specs tab.
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Video Tab: User-uploaded video can be played on the SLM panel. In standard mode, grayscale video can be played at a 60 Hz (EXULUS-HD1) or 30 Hz (EXULUS-4K1) frame rate. In triple mode (EXULUS-HD1 only), a color video will play the R, G, B channels in succession at a 180 Hz frame rate.
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Figure 1: A schematic diagram of the SLM LCoS panel.
Two-dimensional spatial light modulators (SLMs) offer individually addressable pixels of phase shift. Thorlabs' Exulus® 2D SLM is fabricated with liquid crystal on silicon (LCoS) technology, which is based on display technology. This allows it to provide far more pixels than lower-order phase modulators such as segmented or deformable mirrors. Also, the phase shift of one pixel has little crosstalk with other pixels. Therefore, the Exulus 2D SLM is perfect for many beam manipulation applications including optical trapping, beam steering and shaping, femtosecond pulse shaping, adaptive optics, imaging applications, and holography.
The main component of the Exulus® is the 2D SLM LCoS panel. Figure 1 shows a basic schematic of the LCoS panel. The liquid crystal layer is sandwiched between the top transparent and conductive ITO electrode, and the reflective electrode on the bottom. Each pixel on the SLM panel corresponds to individually addressable electrodes on the bottom. Together with the ITO layer on top, an electric field is built up by applying a voltage between the two electrodes. The liquid crystal modules line up according to the direction and strength of the electric field. Since liquid crystal is a birefringent material, the alignment of the liquid crystal molecules in turn controls the retardance or phase shift of each pixel. A wavefront that is incident on the panel reflects with its phase or wavefront being shifted according to the signal that is sent to the SLM panel. With a properly calculated pattern on the SLM panel, the reflected wavefront results in different optical effects in the far field. These effects typically include diffraction, tilt, focus, and holographic image formation.
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Figure 2: Setup for CGH using the EXULUS-HD1 SLM. In this case, the focusing effect added to the CGH is set to a 100 000 mm focal length.
We show here a holographic projection as one of the applications of the Exulus SLM. Figure 2 shows a typical setup required to realize a 2D holographic projection using the EXULUS-HD1. A collimated laser beam is incident on the SLM panel; best results for holographic projection are obtained using a beam size just smaller than Ø7 mm. The incident beam passes through a polarizer and a half-wave plate such that it is polarized at 45°, the direction of the optical axis of the panel. The target projection image is first converted into a computer generated holography (CGH) pattern that is calculated by the bundled software (accessed through the CGH tab in the software). The output beam is separated from the incident beam with a beam splitter. Within the SLM software, a focusing effect is added to the CGH pattern; if you do not desire to have the SLM provide focusing, set the focal length to an extremely long value. In this example, we have set the focal length to 100 000 mm (at the laser wavelength of 635 nm), checked the "Invert Image" box to set the image outline to be bright in the projected pattern, and set the position to "Fit" so that the entire image is visible on the preview. Since CGH relies on far-field diffraction, a set of imaging lenses is required to produce a sharp holographic image on a screen (in this example, first lens: f = 50 mm, second lens: f = 75 mm) . Figure 3 shows the holographic image to be projected and the corresponding CGH pattern, which is calculated using the bundled software. The resulting holographic projection is shown in Figure 4.
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Figure 4: CGH projection of an image using the setup shown in Figure 2 and the image shown in Figure 3. The central bright spot is a zero-order diffraction spot due to the gaps between the SLM pixels.
b. The corresponding CGH pattern generated by the Exulus software.
Figure 3: a. The image used to generate the holographic projection.
c. The CGH settings tab in the Exulus software.
Effects of Focusing and Tilt
Due to the fill factor of the SLM panel, there is a small gap between the pixels. This in turn causes higher diffraction orders and a high-energy zero-order spot which is unaffected by the SLM, but inherently exists at the output. The center bright spot often overlaps with the holographic projection on the same image plane, as is visible in Figure 4. It is highly preferable to remove this zero-order-spot in many applications.
In order to further enhance the holographic image, one can adjust the focusing effect added to the CGH pattern. This causes the CGH projection to focus itself without the requirement of imaging lenses. Since the zero-order spot is not affected by the focusing parameter, the CGH projection will form an image while the zero-order spot remains collimated at the original beam size. Figure 5 shows the focusing effect that is added to the CGH as well as the final processed CGH pattern sent to the SLM panel; in this example, the focal length was changed to 100 mm in the software; the other software settings remained unchanged.
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Figure 6: Setup used for removing the focused zero-order spot from the CGH projection. Note that the lenses are in different locations than in Figure 2.
If the imaging lenses used in the first example above are inserted into the beam path again, then the center bright spot can be made to diverge while the holographic projection is refocused. This can be accomplished by following these steps:
The experimental setup is shown in Figure 6; note the lenses are in different positions than they are in Figure 2. The resulting holographic projection is shown in Figure 7.
Additionally, the Exulus software allows an X and/or Y tilt to be added to the CGH pattern; this can be used to displace the CGH projection from the central zero-order spot.
Optical Tweezers Application
In optical tweezers systems, an SLM can be used to generate several focused spots at different locations in the sample volume. By using the video or sequence features in the Exulus software package, a moving pattern of focal points can be generated to move trapped particles within a sample volume. In the video to the lower right, several trapped beads are moved continuously in a circle. The tweezers system incorporating the EXULUS-HD1 is shown in Figure 8.
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Figure 8: An optical tweezers system incorporating an EXULUS-HD1 SLM.
Video showing trapped particles moving due to the changing SLM pattern.
Damage Threshold Data for Thorlabs' Exulus Spatial Light Modulator
The specifications to the right are measured data for Thorlabs' Exulus Spatial Light Modulator.
Laser Induced Damage Threshold Tutorial
The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.
Thorlabs' LIDT testing is done in compliance with ISO/DIS11254 and ISO 21254 specifications.
The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm2 (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.
According to the test, the damage threshold of the mirror was 2.00 J/cm2 (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.
Continuous Wave and Long-Pulse Lasers
When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) . Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions. Additionally, when pulse lengths are between 1 ns and 1 µs, LIDT can occur either because of absorption or a dielectric breakdown (must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.
Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a large PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.
In order to use the specified CW damage threshold of an optic, it is necessary to know the following:
The power density of your beam should be calculated in terms of W/cm. The graph to the right shows why expressing the LIDT as a linear power density provides the best metric for long pulse and CW sources. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).
Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):
While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application.
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.
As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.
Pulses shorter than 10-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism . In contrast, pulses between 10-7 s and 10-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.
When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:
The energy density of your beam should be calculated in terms of J/cm2. The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e2 beam.
Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately . A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm2 at 1064 nm scales to 0.7 J/cm2 at 532 nm):
You now have a wavelength-adjusted energy density, which you will use in the following step.
Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT . For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.
The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:
Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10-9 s and 10-7 s. For pulses between 10-7 s and 10-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.
 R. M. Wood, Optics and Laser Tech. 29, 517 (1997).
In order to illustrate the process of determining whether a given laser system will damage an optic, a number of example calculations of laser induced damage threshold are given below. For assistance with performing similar calculations, we provide a spreadsheet calculator that can be downloaded by clicking the button to the right. To use the calculator, enter the specified LIDT value of the optic under consideration and the relevant parameters of your laser system in the green boxes. The spreadsheet will then calculate a linear power density for CW and pulsed systems, as well as an energy density value for pulsed systems. These values are used to calculate adjusted, scaled LIDT values for the optics based on accepted scaling laws. This calculator assumes a Gaussian beam profile, so a correction factor must be introduced for other beam shapes (uniform, etc.). The LIDT scaling laws are determined from empirical relationships; their accuracy is not guaranteed. Remember that absorption by optics or coatings can significantly reduce LIDT in some spectral regions. These LIDT values are not valid for ultrashort pulses less than one nanosecond in duration.
A Gaussian beam profile has about twice the maximum intensity of a uniform beam profile.
CW Laser Example
However, the maximum power density of a Gaussian beam is about twice the maximum power density of a uniform beam, as shown in the graph to the right. Therefore, a more accurate determination of the maximum linear power density of the system is 1 W/cm.
An AC127-030-C achromatic doublet lens has a specified CW LIDT of 350 W/cm, as tested at 1550 nm. CW damage threshold values typically scale directly with the wavelength of the laser source, so this yields an adjusted LIDT value:
The adjusted LIDT value of 350 W/cm x (1319 nm / 1550 nm) = 298 W/cm is significantly higher than the calculated maximum linear power density of the laser system, so it would be safe to use this doublet lens for this application.
Pulsed Nanosecond Laser Example: Scaling for Different Pulse Durations
As described above, the maximum energy density of a Gaussian beam is about twice the average energy density. So, the maximum energy density of this beam is ~0.7 J/cm2.
The energy density of the beam can be compared to the LIDT values of 1 J/cm2 and 3.5 J/cm2 for a BB1-E01 broadband dielectric mirror and an NB1-K08 Nd:YAG laser line mirror, respectively. Both of these LIDT values, while measured at 355 nm, were determined with a 10 ns pulsed laser at 10 Hz. Therefore, an adjustment must be applied for the shorter pulse duration of the system under consideration. As described on the previous tab, LIDT values in the nanosecond pulse regime scale with the square root of the laser pulse duration:
This adjustment factor results in LIDT values of 0.45 J/cm2 for the BB1-E01 broadband mirror and 1.6 J/cm2 for the Nd:YAG laser line mirror, which are to be compared with the 0.7 J/cm2 maximum energy density of the beam. While the broadband mirror would likely be damaged by the laser, the more specialized laser line mirror is appropriate for use with this system.
Pulsed Nanosecond Laser Example: Scaling for Different Wavelengths
This scaling gives adjusted LIDT values of 0.08 J/cm2 for the reflective filter and 14 J/cm2 for the absorptive filter. In this case, the absorptive filter is the best choice in order to avoid optical damage.
Pulsed Microsecond Laser Example
If this relatively long-pulse laser emits a Gaussian 12.7 mm diameter beam (1/e2) at 980 nm, then the resulting output has a linear power density of 5.9 W/cm and an energy density of 1.2 x 10-4 J/cm2 per pulse. This can be compared to the LIDT values for a WPQ10E-980 polymer zero-order quarter-wave plate, which are 5 W/cm for CW radiation at 810 nm and 5 J/cm2 for a 10 ns pulse at 810 nm. As before, the CW LIDT of the optic scales linearly with the laser wavelength, resulting in an adjusted CW value of 6 W/cm at 980 nm. On the other hand, the pulsed LIDT scales with the square root of the laser wavelength and the square root of the pulse duration, resulting in an adjusted value of 55 J/cm2 for a 1 µs pulse at 980 nm. The pulsed LIDT of the optic is significantly greater than the energy density of the laser pulse, so individual pulses will not damage the wave plate. However, the large average linear power density of the laser system may cause thermal damage to the optic, much like a high-power CW beam.