Packaging of electronic and photonic components requires high accuracy, which needs to be verified in the process of manufacturing and testing. Since very small misalignments and/or deformations can lead to unacceptable performance, a measurement approach is needed that would reveal displacements as small as a few tens of nanometers or less. In addition, misalignments and deformations occur both in the out-of-plane and in-plane directions, which may be very difficult to separate from each other. It was previously demonstrated that the two types of measurements can be implemented with different approaches: holographic interferometry for out-of-plane and Moiré interferometry for in-plane, but it is very desirable to have a single system with sufficient accuracy in both lateral and longitudinal directions. An optical technique developed by our group and presented in this paper is based on a holographic approach and combines the principles of holographic interferometry and phase modulating adaptive optics that could provide in-plane and out-of-plane measurements with high accuracy.
One of the main challenges in optoelectronic and photonic applications is the alignment accuracy after bonding of the
components is completed, especially if there are multiple single-mode alignments. Advanced applications often require
positioning accuracy of these components to be within a sub-micron range in order to avoid unacceptable signal
degradation. Also, there is a concern about the effects of certain processing techniques on component integrity. In all
cases, the extent to which a package is affected can only be evaluated by a measurement approach that allows detecting
misalignments/deformations on the order of 10nm. Unfortunately, many conventional techniques are virtually useless
when measurements are performed on diffuse objects, such as photonic packages. These limitations can be avoided using
holography, which facilitates recording and reconstruction of the optical waves reflected from any surface. In the process
of reconstruction it is possible to reproduce not only the amplitude of the reflected wave, but also its phase distribution,
which carries information about the distance to each point illuminated with light. An optical technique developed by our
group and presented in this paper is based on a holographic approach and combines the principles of holographic
interferometry and phase modulating adaptive optics.
Laser beam propagating through the atmosphere is distorted by atmospheric turbulence and platform vibrations, leading
to the reduction of received signals, beam pointing error, and bit-error rate (BER) degradation. Mathematical models of
atmosphere and platform vibrations were developed simultaneously to simulate the actual laser communication. A
realistic wavefront distortion generated by the Kolmogorov spectrum and McGlamery algorithm was applied with a
liquid crystal spatial light modulator (SLM). A disturbance signal implemented by a two-dimensional piezoelectric
steering mirror is applied to represent the platform jitter. Experimental results demonstrate how signal to noise ratio
(SNR), BER and pointing error change with the increase of atmospheric turbulence strength and vibration spectrum
bandwidth. This paper presents the modeling and measurement of effects of atmospheric turbulence and platform jitter.
The distortion compensation and tracking techniques can be tested based on the system.
KEYWORDS: Telecommunications, Optical tracking, Sensors, Laser communications, Signal to noise ratio, Robotics, Acquisition tracking and pointing, Signal detection, Control systems, Optical communications
Successful pointing, acquisition, and tracking (PAT) are crucial for the implementation of laser communication links
between ground and aerial vehicles. This technology has advantages over the traditional radio frequency communication,
thus justifying the research efforts presented in this paper. The authors have been successful in the development of a high
precision, agile, digitally controlled two-degree-of-freedom electromechanical system for positioning of optical
instruments, cameras, telescopes, and communication lasers. The centerpiece of this system is a robotic manipulator
capable of singularity-free operation throughout the full hemisphere range of yaw/pitch motion. The availability of
efficient two-degree-of-freedom positioning facilitated the development of an optical platform stabilization system
capable of rejecting resident vibrations with the angular and frequency range consistent with those caused by a ground
vehicle moving on a rough terrain. This technology is being utilized for the development of a duplex mobile PAT system
demonstrator that would provide valuable feedback for the development of practical laser communication systems
intended for fleets of moving ground, and possibly aerial, vehicles. In this paper, a tracking system providing optical
connectivity between stationary and mobile ground platforms is described. It utilizes mechanical manipulator to perform
optical platform stabilization and initial beam positioning, and optical tracking for maintaining the line-of-sight
communication. Particular system components and the challenges of their integration are described. The results of field
testing of the resultant system under practical conditions are presented.
Packaging of electronic and photonic components requires high accuracy, which has to be preserved not only in the
process of manufacturing, but also in the process of operation. Therefore, products that are built using microelectronic
components are subjected to extensive reliability testing. Shifts in alignment, both linear and rotational, could occur with
time or simply because of the temperature variations and the associated expansion/contraction of the materials.
Identifying where these problems occur and obtaining quantitative results with sub-micron accuracy could potentially be
achieved by photometric measurements. Unfortunately, many conventional techniques are virtually useless when
measurements are performed on diffuse objects, such as photonic packages. These limitations can be avoided using
holography, which facilitates recording and reconstruction of the optical waves reflected from any surface. In the process
of reconstruction it is possible to reproduce not only the amplitude of the reflected wave, but also its phase distribution,
which carries information about the distance to each point illuminated with light. An optical technique developed by our
group and presented in this paper is based on holographic interferometry. The main goal is to make this technique
suitable for reliability testing, which provides vital information for modifications in the design or the packaging process.
Laser communication systems are highly preferred for broadband applications. This technology uses higher regions of the spectrum, and offers unsurpassed throughput, information security, reduced weight and size of the components, and power savings. Unfortunately, small beam divergence requires precise positioning, which becomes very critical at high data rates. Complex motion patterns of the communicating platforms, vibrations, and atmospheric effects cause significant signal losses due to the pointing errors, beam wander, and other higher order effects. Mitigation of those effects is achieved by fast tracking, which can be successfully combined with signal modulation. In this work, we focus on the application of acousto-optic technology and its effect on communication performance. We present experimental results for a laser communication link affected by pointing distortions. These distortions are generated to emulate specific operation environments with particular spectral characteristics. The acousto-optic technology is used to build an agile tracking system combined with signal modulation in the same device to assure maximum signal reception, in spite of the harsh operational conditions. The received communication signal is recorded and statistically analyzed to calculate the bit error rates. This work presents synthesis of a tracking system and experimental results characterizing the communication performance under uncompensated pointing disturbance and with tracking.
Laser communication is viewed as an enabling technology with great potential in both long-range and “last-mile” applications, serving the needs of individual subscribers, corporate offices, military, and possibly deep-space missions. Indeed, lasers allow for concentration of energy within tightly focused beams and narrow spectral intervals, thus offering high throughput, information security, weight and size of components, and power requirements that cannot be matched by rf systems. A key challenge is that high data rates should be complemented by high-precision wide-bandwidth position control of a laser beam. In many applications, the ability to maintain a link is affected by the complex maneuvers performed by mobile communication platforms, resident vibrations, and atmospheric effects. This work is focused on the application of acousto-optic technology as an alternative to electromechanical devices, which addresses the tracking problems encountered under such conditions. With the realization that an acousto-optic Bragg cell is only a component of the entire system, which should perform complex tasks of pointing and tracking of the remote terminal, we present an attempt to consider this problem from the “systems” point of view. We present the developed electro-optical laboratory prototype, show synthesis of the control system, and feature experimental results that demonstrate excellent tracking performance.
The use of digital cameras and camcorders in prohibited areas presents a growing problem. Piracy in the movie
theaters results in huge revenue loss to the motion picture industry every year, but still image and video capture may
present even a bigger threat if performed in high-security locations. While several attempts are being made to
address this issue, an effective solution is yet to be found. We propose to approach this problem using a very
commonly observed optical phenomenon. Cameras and camcorders use CCD and CMOS sensors, which include a
number of photosensitive elements/pixels arranged in a certain fashion. Those are photosites in CCD sensors and
semiconductor elements in CMOS sensors. They are known to reflect a small fraction of incident light, but could
also act as a diffraction grating, resulting in the optical response that could be utilized to identify the presence of
such a sensor. A laser-based detection system is proposed that accounts for the elements in the optical train of the
camera, as well as the eye-safety of the people who could be exposed to optical beam radiation. This paper presents
preliminary experimental data, as well as the proof-of-concept simulation results.
Lasers play an ever-increasing role in aerospace communication systems by providing the most logical connectivity channels. They drive the advancements in modern optoelectronics; however, successful implementation of this technology hinges on having an equally advanced beam-steering system for tracking the communication counterpart in the presence of complex maneuvers and the resident vibration of the airframe. The work presented in this paper concentrates on the development of agile acousto-optic beam-steering systems for laser communication terminals, which use constant-gain controllers augmented with an adaptive Kalman filter. Experimental results are presented to demonstrate communication performance as a function of the sampling rate in the tracking loop.
Laser beam propagating through the atmosphere encounters dynamic turbulence, which creates spatial and temporal fields of the refractive index. The resulting wavefront distortions lead to severe performance degradation in the form of reduced signal power and increased BER, even for short-range links. To alleviate this problem, an electrically addressed liquid crystal spatial light modulator (SLM) can be used to correct the wavefront by dynamically changing the optical path delays. Application of Zernike Formalism reduces the complexity of calculation of the SLM control signals by approximating the required phase profile. A real-time wavefront correction procedure utilizing Simplex optimization by Nelder and Mead was previously demonstrated. The performance of such procedure could be improved by proper re-initialization to avoid sub-optimum solutions. Interference-based phase estimation is proposed for this task and its potential was demonstrated in a proof-of-concept theoretical study. This paper presents the modification in the previously developed system and the corresponding experimental results, which show dynamic correction of the phase distortions.
Dynamically changing turbulence in the atmosphere distorts the wavefront of the laser beam propagating
through it. The resulting spatial and temporal fields of the refractive index lead to performance degradation
in the form of reduced signal power and increased BER, even for short link ranges. An electrically
addressed liquid crystal spatial light modulator (SLM) can be used to correct the optical path difference
(OPD) pattern resulting from the atmospheric distortions. Approximating the phase profile of the distorted
beam using well-known Zernike formalism reduces the complexity of controlling each pixel of the SLM.
Real time correction of the wavefront can be achieved using the Simplex optimization procedure by Nelder
and Mead. Previously, some modifications have been proposed to overcome the local minima problems as
well as the faster convergence. Yet the better and faster performance could be achieved by more accurate
prediction of the simplex initialization along with the modifications in the simplex procedure. This paper
presents the experimental results of such modifications to the earlier proposed system.
Laser beam propagating through the dynamically changing atmosphere is subjected to severe wavefront distortions caused by the optical turbulence. The resulting spatial and temporal fields of the refractive index lead to performance degradation in the form of reduced signal power and increased BER, even for short link ranges. An electrically addressed liquid crystal spatial light modulator (SLM) is proposed to perform correction of the optical path difference (OPD) pattern resulting from the atmospheric distortions. Controlling every individual pixel of the SLM is a rigorous and time-consuming task that calls for a stable and simple procedure that could be performed in real-time. This could be addressed by approximating the phase profile of the distorted beam using Zernike formalism, which provides efficient mapping between large number of SLM pixels and smaller number of coefficients of Zernike polynomials. A possible solution to the dynamic correction problem is the application of Simplex optimization by Nelder and Mead, which is well known for fast improvement of an optimization metric. As has been shown before, this approach presents a problem of locking up in local minima while correcting dynamic changes. This paper presents experimental results of different approaches to resolve this problem by modifying simplex procedure as well as modification in a previously presented experimental setup.
Laser beam propagating through the atmosphere is subjected to severe wavefront distortions due to the optical turbulence. This leads to reduction in the received power, ultimately resulting in the BER degradation, even for short ranges. Optical properties of the atmospheric channel change over time; hence, maintaining a reliable link requires dynamic wavefront control to mitigate the effects of the atmospheric turbulence. An electrically addressed programmable nematic liquid crystal spatial light modulator (SLM) is proposed to perform this task. Wavefront correction is achieved by computing a phase shift for each pixel of the SLM, which could be a rigorous and time-consuming procedure. Hence, the goal is to obtain a stable and relatively simple approach to dynamically control the modulator elements. The phase profile of the distorted beam can be approximated using Zernike formalism or another type of wavefront polynomial, which provides efficient mapping between a large number of SLM pixels and a much smaller number of approximation coefficients. Furthermore, wavefront correction needs to be performed in real-time; hence the Simplex method by Nelder and Mead, known for fast improvement of an optimization metric, is used to adjust the approximation coefficients. The phase profile obtained from the optimization procedure is imposed on the received beam by the SLM. This facilitates the reduction of the optical path difference (OPD) present in the distorted wavefront by applying an inverse OPD, and mitigating the effects of the optical turbulence. This paper presents a basic algorithm as well as the experimental results.
KEYWORDS: Telecommunications, Interference (communication), Acousto-optics, Signal to noise ratio, Laser communications, Bragg cells, Sensors, Electronics, Laser systems engineering, Signal detection
Laser communication systems hold great promise for broadband applications. This technology uses much higher-than-RF region of the spectrum and allows concentration of the signal within a very small spatial angle, thus offering unsurpassed throughput, information security, reduced weight and size of the components and power savings. Unfortunately, these intrinsic advantages do not come without a price: small beam divergence requires precise positioning, which becomes very critical at high bit rates. Complex motion patterns of the communicating platforms, resident vibrations, and atmospheric effects are known to cause significant signal losses through the mechanisms of the pointing errors, beam wander and other higher-order effects. Mitigation of those effects is achieved through the multiple means of fast tracking and wavefront control. In this paper we focus on the application of a beam steering technology and its effect on the communication performance of the system. We present the results of an experimental study of a laser communication link subjected to pointing distortions. These distortions are generated by a special disturbance element in the optical setup, which recreates specific operation environments with particular spectral characteristics. The acousto-optic technology is used to build an agile tracking system to assure the maximum signal reception in spite of the harsh operational conditions. The received communication signal is recorded and statistically analyzed to calculate the bit-error-rates. This paper presents the synthesis of a tracking system and the experimental results characterizing the communication performance under uncompensated pointing disturbance and with tracking.
Implementation of long-range laser communication systems holds great promise for high-bandwidth applications. They are viewed as a technology that in the nearest future will handle most of the "last mile" communication traffic for the individual subscribers, corporate offices, military, and possibly deep space probes. Indeed, lasers allow for concentration of energy within tightly focused beams and narrow spectral interval, thus offering high throughput, information security, weight and size of components and power requirements that could not be matched by RF systems. However, the advantages of optical communication systems come in the same package with several major challenges. In particular, high data rates should be complemented by high-precision wide-bandwidth position control of a laser beam. In many applications the ability to maintain a link is affected by the complex maneuvers performed by mobile communication platforms, resident vibrations, and atmospheric effects. The search for the most effective and reliable way to shape and steer the laser beam is an on-going effort. This paper is focused on the application of acousto-optic technology as an alternative to electro-mechanical devices. With realization that an acousto-optic Bragg cell is only a component of the entire communication system, which should perform complex tasks of acquisition, pointing, and tracking of the remote terminal, we present an attempt to consider this problem from the "systems" point of view.
Laser technology plays an ever-increasing role in aerospace and communication systems and is often viewed as a technology that has the potential for providing the material base for high-bandwidth applications. Laser provides the most logical connectivity channel for mobile systems requiring high data rates, low power consumption, covert operation, and high resistance to jamming. While advancements in modern opto-electronics have resulted in small size, reliable and power efficient lasers and modulators, successful operation of any communication technology hinges upon the ability to develop an equally advanced beam steering/positioning system. In many aerospace applications, when the transmitting optical platform is placed on board of an airplane, the ability to track the target is affected by the complex high-speed maneuvers performed by the aircraft and the resident vibration of the airframe. The tracking system must assure that in spite of the relative motion of both the transmitting and receiving stations and adverse environments, such as vibration, mutual alignment of two systems will be maintained to minimize communication errors. The work presented in this paper concentrates on the development of agile beam steering systems for laser communication terminals. Acousto-optic Bragg cells are used as deflectors while feedback information is generated by a quadrant detector. The control system is synthesized using a relatively simple constant-gain controller augmented with an adaptive Kalman filter to mitigate the effects of measurement noise in the tracking system. Laboratory experiments are conducted to investigate communication performance as a function of the sampling rate in the beam position feedback.
Performance of laser communication links between ground terminals, both fixed and mobile, and satellites is generally limited by several factors. Continuous movement of the communicating platforms, complemented by mechanical vibrations, is the main cause of pointing errors. In addition, atmospheric turbulence causes changes of the refractive index along the propagation path, thus creating wavefront distortions of the optical beam resulting in spatio-temporal redistribution of the received energy. The total effect of these phenomena leads to an increased bit-error probability under adverse operation conditions. This paper presents a combined approach to the analysis of a laser link in the presence of pointing errors and turbulence effects, and their contribution to the increased bit-error rates (BER). Analysis of both uplink and downlink communication is performed in the simulation environment. Two distinct approaches to wavefront distortion modeling are used for these scenarios. In uplink propagation the beam is distorted in the initial transition through the atmosphere, and then it travels over a long distance in free space, where even more self-interference occurs. In downlink communication the effects of distortion are only observed during the final transition through the atmosphere, and; therefore, are less severe. Communication performance under different conditions is assessed in terms of the bit-error rate as a function of the pointing error variance and the scintillation index.
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