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The mathematical model of a speckle-suppression method based on two Barker code-type diffractive optical elements (DOEs) moving in orthogonal directions is developed. The analytic formulae for speckle suppression efficiency are obtained. The model indicates that the one pair of DOEs can be used for laser beams of different colors. The speckle contr...
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Citations
... To achieve this goal, it is essential to quantitatively analyze the speckle [7][8][9][10][11][12], including the classification of speckle [7] and the calculation of speckle contrast [9][10][11], which is very important either for the suppression of speckle or the utilization of speckle features for extracting information such as in bio-oximetry [12]. In recent years, many speckle reduction methods have been investigated to eliminate the influence of speckle on projected images [13][14][15][16][17][18][19][20][21], including the use of special screen [13,18], diffractive optical element [14,15,22], diffuser [16], multimode fiber [19] and liquid light guide [20]. Different methods of speckle suppression are usually divided into the introduction of wavelength [22][23][24], angular [15,22], and polarization diversities. ...
... In recent years, many speckle reduction methods have been investigated to eliminate the influence of speckle on projected images [13][14][15][16][17][18][19][20][21], including the use of special screen [13,18], diffractive optical element [14,15,22], diffuser [16], multimode fiber [19] and liquid light guide [20]. Different methods of speckle suppression are usually divided into the introduction of wavelength [22][23][24], angular [15,22], and polarization diversities. In addition, speckle can also be reduced by temporal averaging, for instance, by moving the screen [21,25]. ...
... In recent years, many speckle reduction methods have been investigated to eliminate the influence of speckle on projected images [13][14][15][16][17][18][19][20][21], including the use of special screen [13,18], diffractive optical element [14,15,22], diffuser [16], multimode fiber [19] and liquid light guide [20]. Different methods of speckle suppression are usually divided into the introduction of wavelength [22][23][24], angular [15,22], and polarization diversities. In addition, speckle can also be reduced by temporal averaging, for instance, by moving the screen [21,25]. ...
Speckle is a significant challenge for laser imaging systems, as it degrades the image quality. In this study, an improved theoretical model is established to describe the speckle features in free-space optical path. The model quantitatively defines the relationship between speckle contrast and five parameters: wavelength, screen surface roughness, light-spot diameter, incidence angle, and observation angle. Subsequently, the theoretical results are experimentally verified. This study enhances the theory of speckle suppression in free-space optical path, and thus enriches the existing speckle suppression theory. The speckle features based on free-space surface-scattered fields have the potential for applications related to non-imaging optics.
... It has been demonstrated that the usage of two-dimensional (2D) DOE is more effective in speckle reduction than 1D DOE. A simplified formula for the speckle contrast of a 2D DOE with code length Ncode is given as [34]: ...
The speckle phenomenon produced by coherent waves interfering with each other is undesirable in laser imaging systems. For each of the laser speckle reduction methods in the literature, it is difficult to reduce speckle to an extremely low level (<3%) and also ensure good image quality. Therefore, a compound speckle reduction method based on the combination of a vibrating multimode fiber and a tracked moving flexible DOE loop is proposed and demonstrated for the first time. We have experimentally demonstrated the effectiveness of the proposed compound method, which can reduce the speckle contrast to 1.96% and obtain good spot quality. The relationship between the time-averaging effect of the speckle patterns from a vibrating multimode fiber and from a tracked moving DOE loop is discussed thoroughly. Our experimental results are in good agreement with Goodman’s speckle theory. We expect that the compound speckle reduction method we proposed will have promising potential for applications in laser imaging systems.
... Many approaches have been proposed to reduce speckle, such as a moving diffuser [8], screen movement [9], or a rotating light pipe [10], etc. Among them, the most popular method is to use moving diffraction optical elements (DOEs) [11,12], which enables efficient reduction in speckle, but the use of active moving elements will increase the power consumption and reduce the reliability of the systems. ...
Speckle-free imaging using a multimode fiber has been widely used for imaging systems. Generally, previous work has assumed that all the propagating modes of the fiber are uniformly excited, but the modal power distribution is actually affected by excitation conditions. Here, we propose the utilization of a modal analysis method to study the dependence of the speckle contrast on the modal power distribution by changing the tilt angle of the Gaussian beam and on the group delay time difference caused by different fiber lengths. The results of numerical simulations and experiments show that, with an increase in the tilt angle of the Gaussian beam, the modal power is transferred to higher-order modes and the maximum delay difference between excitation modes becomes larger. Therefore, the inter-mode interference effect is effectively weakened, and the speckle contrast is significantly reduced. The increase in fiber length will also make the delay difference between excitation modes larger and thus the speckle contrast is decreased. For the larger tilt angle of the Gaussian beam, only a shorter optical fiber is required to reduce the speckle contrast significantly. Our work further promotes the use of a multimode fiber to produce speckle-free patterns in laser imaging systems.
... Було показано [9,10], що за певних умов автокореляційна функція колімованого лазерного пучка, що пройшов через 2D ДОЕ на основі коду Баркера, може бути спрощена до добутку двох вузьких 1D автокореляційних функцій, кожна з яких залежить від однієї координати. Такими умовами є: за інтервал часу, який дорівнює часовій роздільності ока, ДОЕ переміщуються на один період ДОЕ-структури (і на NM періодів), в ортогональному напрямку (де N -довжина коду, М являє собою будь-яке позитивне ціле число) числова апертура об'єктива повинна бути достатньо великою, щоб зібрати практично все світло пучка, що продифрагував на ДОЕ (кут розбіжності дифрагованого променя повинен бути менше, ніж вхідна числова апертура лінзи системи освітлення). ...
... Ці дві 1D автокореляційні функції є автокореляційними функціями 1D Баркер-коду [9] (рис. 4). ...
... Greater angle diversity, independent of laser beam coherence, can be achieved by using an active diffuser [10] or diffractive optical elements (DOEs) [11][12][13][14][15][16][17][18][19][20] . Active DOEs based on an electronically switching liquid crystal panel are an attractive concept, but the switching frequency speed is not large enough to be applied in laser projectors, although several research groups have recently obtained encouraging results [19,20] proposed [10] . ...
... Active DOEs based on an electronically switching liquid crystal panel are an attractive concept, but the switching frequency speed is not large enough to be applied in laser projectors, although several research groups have recently obtained encouraging results [19,20] proposed [10] . And a regular DOE structure based on a pseudorandom sequence has been proposed too for efficient speckle suppression [14][15][16][17] . In this case, a linear shift of the DOEs or rotation of quasi-spiral DOEs was proposed to achieve speckle suppression below the sensitivity of the human eye. ...
... To fully develop and demonstrate an efficient technological solution for speckle suppression, which is suitable for the miniaturization of laser projectors, this paper reports a proposed theory for determining the optimal structure parameters of the DOE loop. The theory is further developed on the basis of our previous theoretical approach [15,16] . According to the proposed theory, speckle suppression effects are simulated with different DOE parameters, and the results are discussed in detail. ...
... J. I. Trisnadi firstly proposed the speckle reduction method using Hadamard phase diffuser 14,15 . Later, more researches about this topic 16,17 and its extensions 18,19 were presented. However, most of these methods need either a vibration system or a line scan system, which is cumbersome for laser display system. ...
In this paper, a solution for speckle reduction using phase plate array (PPA) and lens array (LA) in a motionless way is proposed. The specially designed PPA is composed of sub-phase plates, which are constituted by phase patterns formed by Hadamard sub-matrices. Each component of the proposed optical system should satisfy the stated relationships. The incident laser beam will be incoherent after passing through PPA, and superpose on the screen under the action of LA and main lens. Speckle reduction can be achieved by the averaging of the incoherent speckle patterns. Because of abandoning the mechanical movement, it will be suitable for laser displays and images.
... Laser displays provide a wide color gamut and higher contrast [3][4][5]. Laser imaging devices with high resolution can be used in a number of applications, such as microscopy [1,6], photolithography [2], and displays [3][4][5][7][8][9][10][11][12][13][14][15]. ...
... Nowadays, a number of works describe successful methods for speckle suppression in laser-based imaging devices [1,5,[7][8][9][10][11][12][13][14][15]. Many of them utilize a moving diffractive optical element (DOE) with a binary phase modulating structure [7,9,[10][11][12][13]. ...
... Nowadays, a number of works describe successful methods for speckle suppression in laser-based imaging devices [1,5,[7][8][9][10][11][12][13][14][15]. Many of them utilize a moving diffractive optical element (DOE) with a binary phase modulating structure [7,9,[10][11][12][13]. That DOE can be performed on the base of Hadamard matrices [7], Barker codes [8,9], maximum length sequences (M sequences) [12,16], or other codes [13,17] having a sharp autocorrelation function with low side-lobe levels. ...
Using a laser light source in imaging devices provides a wide color gamut, high brightness, resolution, and efficiency. At the same time, it creates a speckle pattern that deteriorates the image quality. This paper is related to the application of the moving binary code diffractive optical element (DOE) for speckle suppression in a laser display. Analytical optimization of DOE parameters is made in this paper. The optimal DOE pitch is found analytically. Limitation of the DOE code length is shown, and the highest possible code length is estimated. Application of the compound Barker code for the DOE design is suggested. As an alternative, the M -sequence and minimum peak side-lobe codes are considered. The expected residual speckle contrast ratio after application of those codes is estimated for the number of codes. A comparison of the different codes is done, and recommendations for speckle suppression DOE design are provided.
... This conventional approach works on incoherent summation of several speckle patterns. Alternatively, significant reduction in the coherent noise is also achieved without mechanically moving the diffuser and by using diversity in parameters of the light beam, such as frequency, angular, spatial, polarization and DOE [5][6][7][8][9][10]. ...
Propagation of polarized monochromatic light through inhomogeneous media generates a noisy pattern known as a laser speckle. The speckle pattern obscures recovery of the polarimetric parameters (PPs) of the incident light. In this paper, we propose and demonstrate a technique to suppress the speckle and recover the desired PPs. This is implemented by analyzing the speckle pattern with the help of intensity measurement and introducing spatial averaging. In order to suppress the coherent noise and perform complete characterization of the source, we introduce four shot intensity correlation measurements. Utilizing this feature, experimental reconstruction of the PPs obscured by the diffuser is presented for different cases. The effects of the spatial averaging window on the coherent noise suppression and reconstruction of the PPs are also evaluated.
... The method based on DOE was firstly introduced in 1998 for speckle reduction in a laser projection system [11]. Thereafter, many works have been done to improve the DOE structures and optimize DOE activities for more effective speckle suppression and miniaturize the laser projection systems [12][13][14][15][16][17][18][19][20]. Two kinds of DOE structures, the one-sided 2D DOE (OSDOE) and the two-sided 1D DOE (TSDOE), have been examined for the speckle contrast and speckle suppression range [12][13][14][15]. ...
... Two kinds of DOE structures, the one-sided 2D DOE (OSDOE) and the two-sided 1D DOE (TSDOE), have been examined for the speckle contrast and speckle suppression range [12][13][14][15]. In addition, binary phase codes with pseudorandom sequences [13], Barker codes [16,[18][19] and the Hadamard matrix [10,17] have been developed for DOEs used in laser projection systems for speckle suppression. Furthermore, various activation methods, such as mechanical movements including linear shift, rotation and step-wise shift, as well as methods based on switching DOE structures, have been investigated for decorrelating laser beams [12][13][17][18][19][21][22]. ...
... In addition, binary phase codes with pseudorandom sequences [13], Barker codes [16,[18][19] and the Hadamard matrix [10,17] have been developed for DOEs used in laser projection systems for speckle suppression. Furthermore, various activation methods, such as mechanical movements including linear shift, rotation and step-wise shift, as well as methods based on switching DOE structures, have been investigated for decorrelating laser beams [12][13][17][18][19][21][22]. It was shown that DOE based on binary pseudorandom sequences can be realized as 1D and 2D DOE structures and both structures can use linear shift to achieve the best speckle suppression [18][19]. ...
We present the first general theoretical description of speckle suppression efficiency based on an active diffractive optical element (DOE). The approach is based on spectral analysis of diffracted beams and a coherent matrix. Analytical formulae are obtained for the dispersion of speckle suppression efficiency using different DOE structures and different DOE activation methods. We show that a one-sided 2D DOE structure has smaller speckle suppression range than a two-sided 1D DOE structure. Both DOE structures have sufficient speckle suppression range to suppress low-order speckles in the entire visible range, but only the two-sided 1D DOE can suppress higher-order speckles. We also show that a linear shift 2D DOE in a laser projector with a large numerical aperture has higher effective speckle suppression efficiency than the method using switching or step-wise shift DOE structures. The generalized theoretical models elucidate the mechanism and practical realization of speckle suppression.
