Speckles in images formed by a laser beam are interference noise arising due to coherent nature of laser radiation. As the number of projection systems using laser light sources increases, new methods and devices appear for reducing the contrast of speckle-noise and thereby suppressing it. Taking into account the long-standing nature of the problem and the continuing relevance of its solution, this review discusses modern practical methods and devices—despecklers, used to reduce the speckle contrast and to achieve high quality of projected images. The review discusses the nature of the speckles appearance, considers their statistical properties, and describes the measurement of the speckle contrast. The requirements for despecklers are considered, the most important of which are speckle suppression efficiency, simplicity of design and compactness, low power consumption and low optical loss. The characteristics of different types of despecklers, including diffusers, devices based on electroactive polymers, optical waveguides, colloidal solutions and liquid crystals, as well as orthogonal phase matrices and diffraction gratings proposed as despeckers, are examined in detail. It is shown that despecklers with decorrelation of the phase front based on the mirror deformation and optical fiber have less light losses. Electro-optical-liquid crystal despecklers do not have mechanically deformable or moving elements that reduce the reliability and durability of operation, and are more compact and simple in design. A comparative table of characteristics of the most effective despecklers in which the speckle contrast is reduced to ten percent or less, with an indication of their advantages and disadvantages, is given.
Despite the obvious advantages of laser light sources (wide color spectrum, high brightness, long lifetime, directivity, high optical power, etc.) in comparison with traditional lamps and light-emitting diodes (LEDs), the presence of light spots—speckles in the image generated by a laser prevents widespread use of lasers in projection systems. Speckles are interference noise and arise due to the coherent nature of laser radiation [
Speckle-noise in the observed images can be reduced by averaging it directly on the screen or by destroying the phase relationships in the laser beam, that is, before projecting the images onto the screen. The averaging of the speckle structure on the screen is achieved using a rapidly moving (for example, rapidly rotating) light-scattering diffuser [
It is clear that a screen with mechanical movement is not the best solution of the problem. The speckle suppression device—despeckler (
Many other methods and devices for suppressing speckle structures are known as well that have appeared in recent years. Taking into account the continuing urgency of the problem, below a review of modern practical methods and devices is presented for effectively reducing speckle contrast in a laser image and thereby suppressing speckle-noise in it.
Experiments have shown that a despeckler can be recognized as effective, which is capable of providing a speckle pattern contrast of less than 3% for still images [
Laser radiation has a high degree of temporal coherence (monochromaticity) and spatial coherence [
objects, such as, for example, a rough surface or a transparent medium with a refractive index varying in space, as a result of the interference of laser beams, the intensity of the reflected or transmitted wave is characterized by large spatial fluctuations due to the overlap of many out-of-phase scattering or blurring functions, each of which corresponds to its point (its heterogeneity) of the object.
In
The resulting field amplitude (
U ( P 0 ) = ∑ n = 1 N a n exp ( i Δ φ n ) = A ( P 0 ) exp ( i φ ( P 0 ) ) , (1)
where N is the total number of independent sources of secondary waves on the illuminated area of a surface [
In this case, if the roughness or spatial inhomogeneities have a random character over the area of the object, then the resulting interference spots will be located randomly in the observation plane also (
Thus, a speckle structure is a random interference pattern, which is formed due to mutual interference of coherent waves with a random set of intensities and/or random phase shifts [
An aggregate of a large number of elementary light sources having phases and amplitudes statistically independent of each other is a diffuse scattering source. It is assumed that the illuminated section of the diffuse scatterer is much larger than the area of an individual elementary source, and the phases are uniformly distributed in the interval (−π ... +π). Speckle fields formed during scattering of radiation on such diffuse scattering sources are called developed speckle fields, and speckle structure is called developed speckle structure.
Since we do not know the detailed microscopic structure of a complex wave front leaving a scattering object, we use statistical approach to the question of the speckle structure properties. One of the important statistical characteristics
of the speckle structure is the intensity distribution density I in a certain point in the image.
In many fields of physics, and in optics in particular, one has to deal with complex random variables, which are the sum of many small “elementary” complex contributions. The role of such complex numbers is often played by the so-called phasors [
The total complex wave amplitude (resulting phasor), formed due to scattering on a set of small independent scatterers, is the sum of random phasors and is defined as:
A = A e i θ = 1 N ∑ n = 1 N a n e i φ n (2)
where an is the amplitude, and φn are the phases of each elementary wave, N is the number of complex contributions (complex phasors).
To simplify the analysis, the following assumptions are made [
• The amplitude an and the phase φn of a specific elementary wave are statistically independent of each other.
• The amplitude an and the phase φn of any elementary wave are statistically independent of the amplitudes and phases of all other elementary waves (uncorrelated).
• The phases are evenly distributed in the interval (−π, π).
The intensity I and the amplitude A are related as follows:
I = A 2 (3)
when N is infinite, the joint probability density function of amplitude and phase can be expressed as:
P A , θ ( A , θ ) = A 2 π σ 2 exp ( − A 2 2 σ 2 ) . (4)
Then the probability density function of the amplitude is the Rayleigh distribution:
P A ( A ) = A σ 2 exp ( − A 2 2 σ 2 ) (5)
Considering that
P I ( I ) = P A ( I ) | d A d I | = 1 2 I P A ( I ) , (6)
the corresponding probability density function of the intensity for I > 0 can be represented as:
P I ( I ) = I σ 2 exp ( − I 2 σ 2 ) 1 2 I = I 2 σ 2 exp ( − I 2 σ 2 ) (7)
The values of this distribution are easy to find by direct integration, and a result is
I q ¯ = ( 2 σ 2 ) q q ! (8)
It can be seen that the average intensity I ¯ is equal to 2 σ I . Therefore, the equivalent statement is that the q-th result is defined as follows:
I q ¯ = I ¯ q q ! , (9)
and the probability density function can be rewritten as:
P I ( I ) = ( 1 I ¯ ) exp ( − I I ¯ ) . (10)
Speckles with such intensity distribution are called fully developed speckles. Dispersion and standard deviation of the intensity for the current case are determined as
I 2 ¯ = 2 I ¯ 2 , σ I 2 = 2 I ¯ 2 , σ I = I ¯ . (11)
An important characteristic of speckles, characterizing the degree of noise in a speckle pattern (speckle structure), is speckle contrast, which is defined as the ratio of the standard deviation of intensity fluctuation to the mean value of the intensity C = σ/ [
C = σ I I ¯ , (12)
that is, the speckle contrast characterizes the depth of spatial modulation of the scattered radiation and shows how strong the intensity fluctuations in the speckle sample are compared to the average intensity.
In this case, the signal-to-noise ratio is the reciprocal value of the speckle contrast and is defined as:
S N = 1 C = I ¯ σ I . (13)
Speckle contrast values range from 0 to 1 (or from 0% to 100%). Therefore, at C = 1, the intensity fluctuations of the fully developed speckles are of the same order of magnitude as the average value. In practice, developed speckle fields with the maximum possible value of speckle contrast are observed when wide laser beams are diffracted on a highly rough surface or a strongly scattering transparency [
Depending on the optical scheme for observing speckle patterns, objective and subjective speckles are distinguished [
A subjective speckle pattern occurs when a scattering surface is displayed on a screen due to an optical system. Thus, if we photograph a speckle picture with a camera, we will get a subjective picture, since its parameters will already depend on the lens of the camera. In this case, the detailed structure of the subjective speckle pattern depends on the parameters of the imaging optical system and its location. The optical scheme of the formation of a subjective spectrum pattern is shown in
When the same CCD is used, and a size of objective and subjective speckles is the same, the speckle contrast measured in the image plane, is in good agreement with that when measured in free space [
If a rough surface is illuminated by coherent light, the intensity distribution in the resulting speckle pattern depends on the properties of the rough surface, the direction of incidence and observation of the laser beam, the temporal and spatial coherence of radiation, as well as on the polarization and phase distribution of the light wave field scattered by the object [
For M uncorrelated speckle structures, the contrast of the resulting speckle pattern can be reduced in accordance with the law C = σ / 〈 I 〉 = 1 / M [
When creating time-varying independent speckle patterns, one should take into account the integration time of the human eye, which is about 30 ms. During measuring speckles, the camera integration time should be comparable to the eye integration time.
Consecutive uncorrelated speckle patterns can be created by a moving (often rotating) diffuser-scatterer through which a laser beam passes. This is the earliest and most widely used speckle suppression method. Such diffusers can generate many uncorrelated speckle patterns. Reduction of contrast and suppression of speckles is achieved in them by averaging speckle structures temporarily.
In 2013, a despeckler was announced, which was an optically rough diffuser with a certain scattering angle [
Decorrelation object | Instant averaging | Sequential averaging |
---|---|---|
Temporal coherence | Broadband light source; Many light sources of different wavelengths | Dynamic wavelength variation (e.g. tunable laser) |
Spatial coherence | Using a spatially expanded light source; Dividing a light source into spatially incoherent sub-sources by breaking temporal coherence; Simultaneous illumination of different areas of the object with several incoherent light sources | Movement (scanning) of a coherent light spot |
Wave field direction | Simultaneous illumination with multiple light sources of different directions | Dynamic changing the angular spectrum |
Polarization | Splitting a light source into two temporary incoherent tracks with linear polarization changes | Dynamic changing the state of polarization |
Phase front | Dynamic diffuser; Vibrating multimode fiber |
the rotation speed, the greater the temporary degree of freedom M. By choosing a suitable diffuser scattering angle, the speckle contrast was reduced to 4%.
In [
A despeckler similar in principle [
Power Technology, Inc. [
Suppression of speckles in such systems is accompanied with a deterioration in the quality of the light beam and an increase in its divergence. In addition, purely mechanically moving devices are less reliable and durable, and are difficult to be compact.
A vibrating diffuser is more compact (
The company Optotune (Switzerland) proposed a speckle suppression method [
The diffuser is mounted on an elastic membrane made of electroactive polymer, which contracts and thickens, depending on the presence of voltage on the electrically conductive coatings attached to it, and thereby moves the diffuser
forth and back. A different speckle pattern is generated for each individual diffuser position. During the integration time, the superposition of speckle structures results in homogenization of the observed light pattern [
Electroactive polymers are known as “artificial muscles”. Flexible electrically conductive surfaces are attached to the top and bottom of a thin elastomeric film. When voltage is applied to the electrodes, they both attract each other and thus compress the elastic film (
The maximum reduction in speckle contrast achieved with an LSR is determined by its optical system. The greatest reduction occurs at high frequencies and high vibration amplitudes, in high-resolution diffuser samples and at long integration times. As in all methods with diffusers, the increase in the divergence of the laser beam should be within acceptable limits, and this increase is more significant for very shallow diffuser structures. The efficiency of LSR application increases with decreasing a diffuser surface roughness, since the smaller the grain size, the more completely uncorrelated images are within the diffuser swing. Unfortunately, as the roughness size decreases, the laser radiation divergence increases.
One of the effective ways to obtain time-consistent uncorrelated speckle structures is the use of multimode optical fibers. The superposition of contributions from different fiber modes on the output surface forms a speckle pattern that can change over time as a result of fiber bending or vibration. The paper [
Dimensions, mm | 17 × 17 × 3.8 |
---|---|
Weight (only LSR/with electronics), g | 1.44/2.55 |
Power supply unit (micro-USB interface), VDC, V | 5 |
Power consumption (with standard electronics), mW | 310 |
Electrode capacity, pF | 75 |
Working temperature, ˚C | –30 … +85 |
Speckle structure contrast, С, % | 2 |
Fiber | Core size, μm | Cladding diameter, μm | NA | Fiber type |
---|---|---|---|---|
A | 115 | 200 | 0.22 | Silica core/Silica cladding |
B | 210 | 300 | 0.57 | Silica core/Polymer cladding |
C | 250 | 350 | 0.57 | Heptagonal core Silica core/Polymer cladding |
D | 480 | 500 | 0.50 | Plastic Optical Fiber |
study. Fibers A, B, and C had a round core, while the fiber C had a polyhedral core. The size of the 7-angles fiber core C was measured through the diameter of the inserted circle.
For fiber A, the dependences of speckle structures on the vibration frequency, as well as on the conditions of the laser beam incidence and the fiber bending radius, were investigated.
The efficiency of reducing the speckle contrast using a multimode fiber depends on the mode of the light beam propagation in it. There were two types of modes: meridional and oblique. The meridional modes in a fiber are associated with the passage through its central axis. The oblique modes propagate through the fiber on helical trajectory, without passing through its central axis. Accordingly, both modes of rays propagation were considered: through the center of the fiber (meridional modes) and with an offset of 50 μm from the center of the core (oblique modes).
The efficiency of speckle-noise suppression was also evaluated for various types of fibers, the results of which are shown in
various types of fibers. Near-field patterns were observed for the multimode fiber B with a core diameter of 210 μm and a numerical aperture NA = 0.57. For the input of the laser beam, the NA value was 0.10.
Fiber B, like A, had a round core and was wound into 1 ring with a bend radius of 35 mm. The observed diagrams for vibration off and on conditions are shown in
For multimode fiber C with a heptagonal core, the observed patterns and intensity linear profiles for vibration off and on are shown in
Although plastic fiber has a high speckle reduction ability, it is not suitable for
the purpose of effective reducing the speckle contrast. The first reason is the large numerical aperture. Due to the high sensitivity of such fibers to stresses, modes were mixed in them, and the numerical aperture of the output beam became almost equal to the numerical aperture of the fiber. However, higher output numerical aperture requires higher NA optics, which is more complex and expensive. The second reason is the high losses in optical signal transmission in a plastic fiber, much higher than in a quartz fiber. There is also a third reason, which is that, a plastic fiber can only be used at low laser power because its heating and low thermal stability.
The speckle contrast value obtained using the round fiber was 0.056, and using the heptagonal core fiber was 0.067. However, in opinion of the authors, a fiber with a non-circular core shape can be more effective in reducing speckle contrast, since it does not require the use of the fiber bending effect and, thereby, allows creating a more compact despeckler.
It should be noted that often even small turbulences in the air surrounding the fiber, or insignificant changes in its temperature, are enough to change the speckle pattern on the output surface.
In [
Macrofiber composite (
In order to increase damage resistance and flexibility with respect to monolithic ceramics, aligned rectangular piezoceramic rods (bonded with a matrix adhesive) are sandwiched between layers of an adhesive matrix material and a polyimide film. The electrodes are attached to a film that transfers the applied voltage directly to the strip rods. The adhesive binds the drive components together. When voltage is applied to the electrodes of the device, the device is compressed due to the piezoelectric effect. The MFC generates vibrations as a result of the periodic contractions and stretches.
The experimental setup with a low coherent light source and using a combination of MMF and MFC is shown in
Fiber | Length, m | Core diameter, μm | Fiber diameter, μm | NA | Refractive index of core/cladding |
---|---|---|---|---|---|
A | 0.4 | 1470 | 1500 | 0.51 | 1.492/1.402 |
B | 0.4 | 400 | 970 | 0.22 | 1.457/1.439 |
Speckle pictures are captured by a CCD camera without an imaging lens attached. The CCD camera has a resolution of 1280 × 1024 pixels with a pixel size of 5.2 µm × 5.2 µm. The multifunction generator generates sinusoidal voltages that are amplified by the high-voltage amplifier for driving the MFC and cause the fiber to oscillate periodically.
The dynamic speckle patterns generated by the MMC vibration were averaged over the exposure time by the CCD video camera or the observer’s eye (reaction of 30 ms).
The frequency dependences of the speckle contrast for fibers A and B are shown in
The highest efficiency of speckle suppression was achieved at resonance frequencies of about 20 Hz and about 80 Hz in the fiber A and at 60 Hz and 120 Hz in the fiber B. To evaluate the efficiency of speckle suppression depending on the control voltage, frequencies of 80 Hz for the fiber A and 120 Hz for the fiber B were chosen. Experimental dependences of speckle contrast and efficiency of speckle suppression on the control voltage for the fibers A and B are shown in
From the point of view of speckle suppression, the most effective excitation conditions were the following: 80 Hz, 150 V for the fiber A and 120 Hz, 150 V for the fiber B. Under these conditions, the speckle contrast decreased from 0.43 to 0.08 for the fiber A and from 0.48 to 0.19 for the fiber B. Consequently, the most efficient fiber was the fiber A, which had a higher numerical aperture.
In [
light modulator, for example, a liquid crystal one, which is a matrix of separately addressable liquid crystal cells. When controlling the electric voltage using a specially developed computer program, such a spatial light modulator (SLM) in real time can display various orthogonal matrices of phase shift of 0 and π corresponding to the Hadamard, Walsh or other orthogonal functions [
An example of a phase mask corresponding to the Hadamard matrix is given in [
The formation of the Hadamard matrix and the reduction of speckle contrast were demonstrated [
(GLV)—a one-dimensional spatial light modulator based on MEMS and with a scanner—galvanometer mirror for obtaining a two-dimensional image of 1080 × 1920 pixels on the screen. A diffuser was also used in the intermediate plane of the image to reduce the contrast of speckles. In this configuration, the value of the contrast of speckle structures was reduced to 8%.
The formation of binary phase matrices and their use for suppressing speckles was also tested in [
At present, purely phase modulation of light, which is in great demand in holographic applications, is most easily accomplished in SLMs-transmission type liquid crystal microdisplays using the electro-optical effect of controlled birefringence [
It should be admitted that the use of an electro-optical medium, divided onto light modulation channels (pixels), with an electronic addressing device for each channel, i.e. in fact, a whole display (microdisplay), strongly complicates and increases in price the optical speckle suppression system. Because of decreasing the working aperture (due to the presence of control electronic elements and the gaps between matrix elements) the light losses in such a device are quite large. Moreover, the periodic structure of the matrix introduces distortions of the wave front of coherent light, resulting in distortion of the image on a screen. Electronic control of the spatial light modulator is also a separate complex problem. Therefore, such a despeckler did not receive practical use.
Irish company Dyoptyka has developed phase randomized deformable mirror technology to improve illumination uniformity and reduce speckle contrast and other unwanted interference effects that can be observed in laser-illuminated displays [
The appearance of a deformable mirror in inactive and active states is shown in
For digital cinema projectors, the company has developed deformable mirrors with an active area of about 40 × 60 mm2, which can withstand up to 100 W of continuous optical power without destruction. A deformable mirror with an active area of about 3.0 × 4.5 mm2 and a power consumption of ≤30 mW has been developed for accompanying and built-in pico projectors. Piezoelectric actuators with a nominal service life of 100 thousand hours are used. The 99% reflectivity in the visible wavelength range was achieved using a dielectric mirror coating.
Surface deformations are within the elastic deformation of the mirror substrate and coatings and do not cause damage. The polarization of the radiation is also conserved.
The main characteristics of the Dyoptyka despeckler are shown in
To create uncorrelated speckle structures due to angular decorrelation, a mirror is used [
A photo of a prototype MEMS mirror with a two-dimensional deflection of the reflected laser beam and its design are shown in
In [
Dimensions, mm | 3.0 × 4.5 |
---|---|
Voltage, V | 5 |
Power consumption, mW | 100 |
Working temperature, ˚C | –40 … +85 |
Speckle structure contrast, С, % | 3 |
water. The particle density is 2 × 1010 cm−3. Such a solution acts as a diffuser moving without external energy. Really, due to Brownian motion in a solution at room temperature, uncorrelated speckle structures are generated rather quickly (for about 1 μs).
The external view of the setup with the cuvette and its diagram are shown in
Speckle contrast measurements are made as follows. The laser beam is input from the free space into the colloidal solution from above, and in general, the fiber tip is placed at a distance L from the cuvette bottom, and the total thickness of the solution in the cuvette is t (
With the single-mode fiber tip at the top of the solution, the light is scattered mainly at the top part of the cuvette (
Speckle patterns were recorded at different distances from the cuvette bottom. Based on the contrast of the speckles, the number of independent speckle patterns and the decorrelation time were determined as a function of L (
Thus, the Brownian motion of colloidal particles results in both rapid temporal changes in the laser beam and angular diversity, which makes it possible to
rapidly reduce the speckle contrast to 3.2%. A significant advantage of using a colloidal solution for speckle-noise reducing is that there is no need for external power. However, to ensure stable operation over many years of service, it is necessary to control the chemical composition and electrostatic charge of the particles. In addition, a despeckler based on the use of colloidal solutions is quite difficult to make compact.
In the work of Taiwanese scientists [
A polymer network is required to stabilize and/or blocking LC molecules at different orientations in the layer (with different optical states), as well as to reduce the switching time and operating voltage. When a voltage was applied, a large depth of light modulation was obtained due to light scattering caused by a mismatch between the refractive indices of the reoriented LC layer and the polymer network. In essence, it is an electric field-activated diffuser that is switched off by switching off the voltage.
The results showed (see
Sample | Tmin, % | ΔTmax, % | Va, Tmin, V | Cmin | αmax, % | Va, Cmin, V |
---|---|---|---|---|---|---|
I | 43.0 | 42.8 | 5 | 0.050 | 47.6 | 3 |
II | 35.3 | 47.6 | 7 | 0.043 | 54.7 | 6 |
III | 40.0 | 39.2 | 10 | 0.052 | 54.7 | 6 |
IV | 38.1 | 44.8 | 10 | 0.025 | 74.7 | 10 |
of speckle-noise suppression (with a speckle contrast value of about 0.025) at low applied voltage values (≤10 V) was achieved in a wedge-shaped LC cell.
Paper [
To suppress speckle-noise at the P.N. Lebedev Physical Institute a method was proposed using an electro-optical cell with a ferroelectric liquid crystal (FLC), initially with a helical (spiral) structure [
In both types of FLC cells effective suppression of speckle-noise was, when specially formed bipolar two-frequency electric pulses were applied. Short-term application of an electric field to the FLC resulted in the formation of a structure of transition domains—spontaneously ordered regions, at the boundaries of which light scattering occurs. As a result of light scattering in the bulk of the FLC layer, structures with a random distribution of refractive index gradients were formed, which are the cause of spatially inhomogeneous (over the beam cross section) phase modulation of a laser beam and phase relation destroy in it [
Relative to FLC with helix structure, helix-free FLC provided such advantages [
In the despeckler prototype with a helix-free FLC the following material parameters were: spontaneous polarization РS = 40 nC/cm2, rotational viscosity γφ = 0.7 Poise, tilt angle of molecules in smectic layers Θ0 = 23˚ (at the temperature of 20˚C), temperature range of existence of ferroelectric (chiral smectic C*) phase from 2˚C to 75˚C [
A higher value of speckle contrast when using a helix-free FLC may be due to less accurate settings of the measurement parameters. It is important that a well-developed technology makes it possible to vary the size of the FLC despeckler aperture from fractions to several square centimeters with a thickness of 1 - 2 mm, including the control chip. Unfortunately, the despeckler, like all LCD devices, has limitations in the operating temperature range (approximately from zero to +60˚С).
In a few works the possibility of using diffraction gratings for speckle averaging was investigated [
In [
of different frequencies (
In [
Modulation of light is carried out due to the diffractive nano-relief created on the surface of a thin transparent dielectric layer at the appearance of ponderomotive forces in it when an electric voltage is applied. In the case when there is no electric voltage on electrodes, the gel-like layer does not change its shape (remains flat) and all incident rays are reflected from it as from a flat mirror. When a control signal is applied, a sinusoidal grating is formed on the gel-like layer, and the rays are diffracted on it according to the light diffraction law. Unfortunately, this type of despeckler also has large light losses and cannot be compact due to operating the diffraction grating only for reflection. In addition, the Doppler wavelength change and fatigue of the grating material are noticeable in the RLM.
Problems can also be foreseen when using another polymer medium, not dynamic and thin, but bulk (the thickness is significantly greater than the wavelength), in which many diffraction gratings with different wave vectors are formed not electrically, but by light. A device based on a photopolymer medium is proposed for speckle suppression in [
Such a despeckler (beam shaper—beam corrector, as the authors called it) does not require control and can even serve as a matrix for replicating new products by pressing. However, it also has significant drawbacks: introducing additional noise from superimposed holograms; the criticality of speckle contrast to the exact compliance of the wavelength and angle of beam incidence; difficulties in creating a compact device and the problem of “fatigue” of the photopolymer medium during operation.
The search for methods of suppressing speckles in recent years has become especially relevant in connection with works of leading electronic companies (LG, Samsung, etc.) that develop the holographic displays and laser video projectors, both large-screen theatrical and compact pico-projectors placed inside a mobile phone and projecting video images (including television) onto a desktop or wall screen.
We considered above many methods and devices, which can effectively suppress speckle-noise and provide thereby for the observer an acceptable quality of images formed by a laser (with speckle contrast ≤ 10%). In particularly there were analyzed a vibrating diffuser; an electroactive polymer (elastic membrane) moving the diffuser; an optical fiber deformed by a piezoelectric vibrator; a mirror with phase randomization deformed by piezoceramic elements; a micro-mirror deflected in an electromechanical system; polymer and photopolymer medium; a colloidal solution with dispersed particles. As well we considered possible use of electro-optical-liquid crystal despeckers based on a light-scattering twist cell with a nematic LC stabilized by a polymer network and based on a cell with the ferroelectric LC implementing spatially inhomogeneous phase modulation of light.
The main characteristics of the described despecker developments for their comparison and assessing are summarized in
Operation mechanism and type of despeckler | Light scattering | Fhase | Combined | Light deflection | ||||
---|---|---|---|---|---|---|---|---|
Shifting diffuser LSR-5-17 | Switch-on diffuser PSLC | Colloidal solution | Opticalfiber and vibrator | Deformable mirror Dyoptyka | EO cell with helix-free LC | MEMS mirror | ||
Characteristics: | ||||||||
Dimensions and weight | хх | х | ххх | хх | хх | х | х | |
Power consumption | хх | х | - | хх | хх | х | х | |
Light losses | хх | хх | хх | х | х | х | хх | |
Complexity of electronics | хх | х | - | хх | хх | хх | хх | |
Complexity of optics | ххх | ххх | ххх | хх | хх | х | хх | |
Temperature requirements | - | ххх | хх | - | х | ххх | х | |
Difficulties in commercializ. | х | хх | ххх | хх | х | хх | хх | |
Speckle contrast value, % | 2 | 2.5 | 3.4 | 5.6 | 3 | 7 | 4.6 |
The data in
Of the considered despeckers with high efficiency of speckle-noise suppression, only devices from the companies Optotune (a diffuser on an electroactive polymer) and Dyoptyka (phase deformable mirror) have been brought to commercial use. Their speckle contrast values are 2% and 3%, respectively; these devices are compact, have low light losses, and they have no problems with operating temperature.
The developments of devices for suppressing speckle-noise based on the deflected mirrors and optical fibers are also effective. Despekers based on LCD cells have the same indicators. They also have low power consumption, low light losses and are compact, but they have limitations in temperature range. There is an interesting device based on a colloidal solution with dispersed particles, which does not require external energy and, accordingly, electronics for control at all, but it is difficult to make it compact and stable for a long time.
The problem of obtaining speckle-free images formed by a laser beam has been around for about 40 years. However, the data presented in the review indicate that this problem cannot be considered solved, and in the future one can expect the appearance of other methods and new materials and devices for speckle suppression.
This work was carried out within the framework of the Russia State Assignment No. 0023-2019-0002.
The authors declare no conflicts of interest.