Free-space Optical Data Transmission Research Articles

A dual-modality optical system for single-pixel imaging and transmission through scattering media

It is well recognized that it is difficult to develop an optical system to retrieve effective information when dynamic and turbid water exists in an optical channel. It could be more challenging to incorporate dual or multiple modalities in one optical system. In this Letter, we report a dual-modality optical system for single-pixel imaging (SPI) and transmission through scattering media. A series of mutually-orthogonal random illumination patterns are designed and adopted to realize high-resolution image recovery in SPI. The data to be transmitted are also encoded into random illumination patterns in a differential way, and high-fidelity free-space optical data transmission can be simultaneously realized. Experimental results validate feasibility of the proposed optical system and its high robustness against scattering. The developed dual-modality optical system realizes high-resolution SPI and high-fidelity data transmission in scattering media using only one set of realizations, offering an efficient implementation with reduced power and equipment requirements. The proposed method is promising toward the development of an integrated system with multiple modalities for optical information retrieval, especially in dynamic scattering media.

Physically-Secured Ghost Diffraction and Transmission

A new approach to realizing physically-secured ghost diffraction and transmission is proposed in this letter. A series of random 2D arrays of numbers are used as optical information carriers to transmit original data, e.g., analog signals or images as ghosts. Computer-generated magnification factors are applied for optical data encoding, and physically-generated scaling factors are generated with absorptive filters in free-space optical data transmission. The series of computer-generated magnification factors and physically-generated scaling factors serves as security keys, and is explored to realize high-fidelity and high-security free-space optical data (ghost) transmission. It is experimentally demonstrated that the proposed method is feasible and effective in different environments, i.e., without or with scattering media. The proposed physically-secured ghost diffraction scheme offers a new research perspective on secured optical information (e.g., analog signal) transmission in free space.

Optical data transmission through highly dynamic and turbid water using dynamic scaling factors and single-pixel detector.

Free-space optical data transmission through non-static scattering media, e.g., dynamic and turbid water, is challenging. In this paper, we propose a new method to realize high-fidelity and high-robustness free-space optical data transmission through highly dynamic and turbid water using a series of dynamic scaling factors to correct light intensities recorded by a single-pixel bucket detector. A fixed reference pattern is utilized to obtain the series of dynamic scaling factors during optical data transmission in free space. To verify the proposed method, different turbidity levels, different strengths of water-flow-induced turbulence and a laser with different wavelengths are studied in optical experiments. It is demonstrated that the proposed scheme is robust against water-flow-induced turbulence and turbid water, and high-fidelity free-space optical information transmission is realized at wavelengths of 658.0 nm and 520.0 nm. The proposed method could shed light on the development of high-fidelity and high-robustness free-space optical data transmission through highly dynamic and turbid water.

High-fidelity temporally-corrected transmission through dynamic smoke via pixel-to-plane data encoding.

We propose a new approach for high-fidelity free-space optical data transmission through dynamic smoke using a series of 2D arrays of random numbers as information carriers. Data to be transmitted in dynamic smoke environment is first encoded into a series of 2D arrays of random numbers. Then, the generated 2D arrays of random numbers and the fixed reference pattern are alternately embedded into amplitude-only spatial light modulator, and are illuminated to propagate through dynamic smoke in free space. Real-time optical thickness (OT) is calculated to describe temporal change of the properties of optical wave in dynamic smoke environment, and transmission noise and errors caused by dynamic smoke are temporally suppressed or corrected. Optical experiments are conducted to analyze the proposed method using different experimental parameters in various scenarios. Experimental results fully verify feasibility and effectiveness of the proposed method. It is experimentally demonstrated that irregular analog signals can always be retrieved with high fidelity at the receiving end by using the proposed method, when average optical thickness (AOT) is lower than 2.5. The proposed method also shows high robustness against dynamic smoke with different concentrations. The proposed method could open up an avenue for high-fidelity free-space optical data transmission through dynamic smoke.

Atmospheric turbulence forecasting using two-stage variational mode decomposition and autoregression towards free-space optical data-transmission link

Free space optical communication (FSOC) is a promising technology for satellite-to-earth communication systems, where vector beams, especially orbital angular momentum (OAM), can further increase the capacity of the optical link. However, atmospheric turbulence along the path can introduce intensity scintillation, wavefront aberrations and severe distortion of spatial patterns, leading to data degradation. Forecasting atmospheric turbulence allows for advanced scheduling of satellite-to-earth data transmission links, as well as the use of adaptive optics (AO) to compensate for turbulence effects and avoid data transmission link performance degradation. Therefore, atmospheric turbulence forecasting is critical for practical applications. In this work, we proposed a hybrid atmospheric turbulence forecasting model based on a two-stage variational mode decomposition (TsVMD) and autoregression model. The variational mode decomposition (VMD) algorithm is first used, to our best knowledge, to denoise the observed atmospheric turbulence dataset, and then is used again to decompose the datasets into several intrinsic mode functions (IMFs). Finally, the autoregression model is used to predict each IMF independently. And the predictions of each IMF are combined to obtain the final atmospheric turbulence predictions. Experiments employing the observed turbulence datasets and two additional methodologies were carried out to verify the performance of the proposed model. The experimental results show that the performance of the proposed model is much superior to that of the comparative methods.

Physically-secured high-fidelity free-space optical data transmission through scattering media using dynamic scaling factors.

In this paper, we propose a method of physically-secured high-fidelity free-space optical data transmission through scattering media using physically- and dynamically-generated scaling factors. Optical channel characteristics are explored, and scaling factors are physically and dynamically generated to serve as security keys in the developed free-space optical data transmission system. The generated dynamic scaling factors provide a security layer for free-space optical data transmission. To the best of our knowledge, it is the first time to physically and dynamically generate scaling factors in free-space optical data transmission system to realize data encryption. The scaling factors existing in free-space optical data transmission channel are physically and dynamically controlled by using two optical devices, i.e., variable beam attenuator (VBA) and amplitude-only spatial light modulator (SLM). Nonlinear and dynamic variation of scaling factors is realized in different free-space wave propagation environments. It is experimentally demonstrated that high security can be guaranteed in the developed physically-secured high-fidelity free-space optical data transmission system, since one random scaling factor is physically and dynamically generated for the transmission of each signal pixel value. In addition, the proposed physically-secured free-space optical data transmission scheme is robust to noise and scattering, and high-fidelity signals are retrieved at the receiving end. The proposed method could open up a new research perspective for the secured free-space optical data transmission.

Analog ghost hidden in 2D random binary patterns for free-space optical data transmission

We propose high-fidelity analog ghost diffraction and transmission through scattering media in free space using a series of 2D randomly-distributed binary patterns. The proposed method utilizes ghost diffraction to enable high-fidelity free-space optical transmission through scattering media. Any type of ghosts, e.g., analog signal, can be encoded into a series of 2D randomly-distributed binary patterns to serve as information carriers. After the generated 2D randomly-distributed binary patterns are sequentially embedded into spatial light modulator and are illuminated to propagate through scattering media in free space, a single-pixel detector is used to collect light intensity at the receiving end and high-fidelity signals can be retrieved without any complex post-processing algorithm. The proposed method possesses high robustness for high-fidelity free-space optical transmission through scattering media, and different wavelengths and different propagation distances can be flexibly used for free-space optical transmission. The method could open up an avenue towards many applications, e.g., free-space optical data transmission and communication.

Tunable phase of elliptical axicon for controlled spectral switching

Recently, spectral switching has attracted worldwide interest due to its ever-growing optical information processing applications. Over the past few decades, various light beam profiles have been extensively studied for attaining complete control over these spectral modulations. This paper has studied the spectral modulation in the partially coherent intensity profiles obtained from radially symmetric and elliptical axicon phases, respectively. Theoretical findings have been confirmed with experimental results. The study's outcomes conclude that the eccentricity of axicon phase plays a significant role in controlling the spectral switching at any desired position in the beam profiles. We have also proposed a free space optical data transmission model based on controlled spectral switching by an elliptical axicon phase with tunable eccentricity. This study might find applications in developing a convenient setup for free space optical communication.

Stable and Highly Efficient Free-Space Optical Wireless Communication System Based on Polarization Modulation and In-Fiber Diffraction

The performance of free-space optical data transmission is severely deteriorated by the fluctuation of optical signal intensity, which is resulted from the bias drift of electro-optical modulation devices. In order to improve the performance of free-space optical communication systems, a bias-drift avoiding, beam steered free-space optical wireless communication system is presented based on polarization modulation for the first time to the best of our knowledge. An in-fiber diffraction device based on a 45° tilted fiber grating (TFG) serves simultaneously as an in-fiber polarizer, free-space light emitter, and beam steerer in the proposed communication system, which endows the proposed system with the merits of highly efficient, compact, stable and bias-drift free. The usage of 45° TFG has three benefits: 1), it realizes polarization modulation preventing bias-drift due to its polarization sensitive feature; 2), it enables stable and highly efficient in-fiber compact free-space light emitter; and 3) it accomplishes free-space beam steering due to its wavelength-depending diffraction characteristic. A 2.1 m free-space optical wireless data transmission has been demonstrated with a data rate of 4.8 Gbps per beam using 1.2 GHz bandwidth OFDM signals. This proposal prevents the bias drift issue and maintains high stability. Hence, it can be applied in some harsh situations and confidential data transmission fields.

Degradation of signal to noise ratio in optical free space data links due to background illumination

In free space optical data transmission systems illumination of the receiver antenna by background radiation will decrease the signal to noise ratio. We derive expressions for that degradation both for direct and for heterodyne/homodyne receivers. Examples are given for cases where the sun, the moon, the earth, and Venus illuminate earth orbiting receivers operating at wavelengths of 0.85 microm, 1.3 microm, and 10.6 microm. Direct detection receivers will typically suffer a degradation of between 5 and 15 dB at lambda = 0.85 microm and lambda = 1.3 microm when illuminated by the sun. Heterodyne/homodyne receivers at 10.6 microm degrade more with sun radiation (typically 4 dB) than at the smaller wavelengths ( approximately 0.3 dB). The moon, earth, and Venus cause negligible reduction of signal to noise ratio.