Research Terms
Researchers at the University of Central Florida have created waveguides capable of simultaneously transmitting both short and long wavelength EM signals. Often used to transmit electromagnetic (EM) signals, optical fibers sometimes transmit signals of varying wavelengths, such as signals from opposite ends of the EM spectrum, from microwaves to infrared (IR). Due to the fibers' geometry, these transmissions typically require multiple fibers, where each fiber is configured to guide waves within a limited wavelength range. This large number of fibers increases the complexity of the communication systems utilizing them.
The UCF invention reduces this complexity. In some cases, the waveguides can transmit signals that have at least a two-fold difference in wavelength. This revolutionary method can be used in nonlinear optical interactions involving EM waves in different bands. It can also be used within a universal cable for EM wave transmission, which, for example, can reduce the bulk of cables in Navy vessels. The waveguides can also be used in nonlinear imaging (such as excitation in visible/near-infrared wavelengths, fluorescence in IR, and microwaves).
Technical Details
The UCF invention entails waveguides comprising composite photonic crystal fibers (PCF) with one or more unit cells, each capable of transmitting relatively short wavelength EM signals. The unit cells are arrayed in a hexagonal pattern around a primary core capable of transmitting relatively long wavelength EM signals. The fractal-like geometry of the fibers, where the unit cells also function as PCFs, gives the ability to simultaneously guide waves with very different wavelengths, including waves from opposite ends of the EM spectrum.
Researchers at the University of Central Florida have developed a process using nonlinear optical procedures to exactly reverse the propagation direction and phase variation of a beam of light. Until now, purely optical techniques have been used to realize optical phase conjugation (OPC), which is used for many applications, ranging from laser medicine to imaging to communications. However, the classical approach is limited by its inability to adapt the phase conjugation quickly and precisely. Now, using the UCF electronic optical phase modulation (E-OPC) process, optical transmission through complex media such as biological tissue, traditionally limited by multiple light scattering, can now be realized.
In laser medicine, this technique can fluorescently label a cell (such as a cancer cell) and use the advantages of OPC to selectively interact with these cells without effect to surrounding tissues. These actions can be thermal, mechanical or chemical, all mediated by the back propagated optical beam. This process can also be applied to defense applications where the light/electromagnetic waves come from objects of interest. Using this method, it is possible to use OPC to illuminate, interfere or destroy the object that was illuminated.
Technical Details
The UCF invention combines electronic and optical phase conjugation. Typically, there is an efficiency limitation due to phase conjugation relying on nonlinear optical processes. However, the invention resolves this issue by introducing electronic phase conjugation (E-OPC) as a complementary method. OPC defines a special relationship between two coherent optical beams propagating in opposite directions with reversed wavefronts and identical transverse amplitude distributions. The two-step process of E-OPC begins by measuring the wavefront of the signal wave (fluorescence) using a reference beam and charge-coupled device (CCD) arrays so that both quadratures of the wavefront are measured. The measured wavefront is then conjugated in the electrical domain and applied to a spatial modulator array that generates the phase modulated beam to back propagate to the sample. E-OPC removes the wavelength and efficiency limitation as CCDs respond over a broad spectrum and provides the unique ability to remove aberrations due to a turbid medium.
Researchers at the University of Central Florida have developed a new method of nonlinearity compensation that improves on current technologies by reducing the computational requirement and increasing computational efficiency by at least an order of magnitude. The method of nonlinearity compensation uses a dispersion-managed optical signal that is transmitted over a long-haul optical communication link.
Implemented using digital signal processing, the current technologies for fiber nonlinearity compensation are impractical for high-speed fiber links because of the large computational load. This includes nonlinearity precompensation, digital back propagation, and optical phase conjugation. The new UCF method enables a reduction in fiber nonlinearity and consequently an increase in spectral efficiency and transmission distance in fiber communication systems while reducing the computation needed. Reduced computational load also allows a reduction in ASIC chip size and power consumption by at least an order of magnitude. This technology is ideal for dispersion-managed fiber optic transmission systems, particularly long-distance (for example, transoceanic) systems.
Technical Details
The UCF method of nonlinearity compensation begins with a dispersion-managed optical signal transmitted over an optical communication link and virtually divides the communication link into multiple steps. Next, the method includes performing lumped dispersion compensation on a received optical signal to obtain a waveform. Then digital back propagation is applied to the waveform by performing dispersion compensation and nonlinearity compensation for each of the multiple steps derived from the communication link. The process generates an estimate of the transmitted signal base.
Researchers at the University of Central Florida have developed a technology to increase capacity for optical communication systems using a coupled multi-core fiber (CMCF) structure. The new structural innovation increases the effective area of fiber modes to decrease the limitation of Kerr nonlinearity. The invention takes advantage of the crosstalk between the cores of a conventional multi-core fiber. Instead of avoiding crosstalk, the CMCF can use it for enhanced data transmission by shortening the core-to-core distance compared to conventional multi-core fiber. Optical energy that extends beyond the boundaries, evanescent fields, can easily couple into adjacent fiber cores. As the set of cores is coupled, it acts as a larger core and allows data transfer over increased bandwidth.
Offering higher mode density and larger mode effective area than conventional multi-core fiber, the new fiber design can also prevent the mode coupling of supermodes, with design freedom including core-to-pitch ratio and core arrangement. Simulation results have shown lower modal dependent loss, mode coupling, and differential modal group delay compared to few-mode fiber, making the new CMCF design a candidate for spatial division multiplexing and single-mode operation.
Technical Details
According to Xia (2011), for single-mode operation, CMCFs can attain larger effective index difference and effective area than FMFs. As a result, CMCFs tend to have less mode coupling and nonlinearity, which is important for efficient long-haul transmission. This invention is a passive, coupled multi-core fiber wherein the cores each support a spatial mode and are positioned close enough to cause coupling between their modes, generating supermodes capable of transmitting data.
Researchers at the University of Central Florida have developed a new imaging amplification technique that allows space multiplexing to increase fiber capacity in practical application. Increasing the number of cores allows an increase in total input power for the signal beams, increasing the optical power conversion efficiency for the imaging amplifier within a space-multiplexed optical transmission system. Compared to ED-FAs, an imaging amplifier is simpler, since it can be used to amplify signals from many cores, with each core supporting one or several spatial modes. When multi-core fibers are used, involving higher total input power, optical power conversion efficiency as high as 50 percent has been achieved.
To keep up with higher and higher bandwidth demand, multiplexing techniques offer promising ways to increase the capacity of current fiber infrastructure, including space-multiplexed optical transmission. While multimode and multicore fibers can multiply fiber capacity, space-multiplexed optical transmission remains limited to several tens of kilometers without a practical amplification technique. Previous amplification techniques have been unsatisfactory, or, at best, leave room for improvement; commercial erbium-doped fiber amplifiers (ED-FAs) can't be used in space-multiplexed transmission.
Technical Details
The UCF imaging amplification technique exploits the parallelism in bulk optics to provide the additional degrees of freedom needed to amplify signals from multi-core and multimode fibers. The facet of an input multimode or multi-core fiber is mapped or imaged to the facet of an output fiber after passing through an amplifying region. An image amplifying system can comprise an input fiber, a first lens, a bulk amplifier that comprises a gain medium, a second lens, and an output fiber. The input and output fibers should be substantially identical, the same multimode fibers or the same multi-core fibers. The lenses can be individual lenses or groups of lenses. The bulk amplifier comprises a single mass of material, such as silica or phosphate glass, that is doped with an appropriate amplifying medium, such as erbium, erbium/ytterbium, or any other element or elements that provide gain at the signal wavelength. The image amplifying system can also incorporate components including a side or longitudinal pump and a reflector that reflects unabsorbed pump energy back to the bulk amplifier.
Researchers at the University of Central Florida have developed a new photonic signal processing technique for space-multiplexing optical signals. The processing method is comparatively simple, uses only one pair of single lenses, and doesn't require complicated alignment or calibration.
Optical communication systems using multi-core fibers enable the high-bandwidth advantages of space-multiplexed systems. However, the lack of a practical photonic signal processing technique has limited its use. Most current optical signal processing components designed for an on-axis incoming beam can't be applied to a multi-core fiber because the beams from a multi-core fiber are both on-axis and off-axis. Alternatively, previous methods required that system components be multiplied by the number of signals transmitted.
Technical Details
The UCF technique provides the additional degrees of freedom needed for spatial multiplexing by exploiting the parallelism in bulk optics. In this technique, the facet of an input fiber is mapped or imaged to the facet of an output fiber after passing through a region where light associated with all signals travels in pre-designed directions. Off-axis beams from an input fiber can be tilted by an angled facet on the output end of the input fiber; a central facet, perpendicular to an optical axis of the system and an outer facet that extends either forward or backward from the central facet; a wedge prism; or a tapered input fiber with a frustoconical end. Tilting the beams as such and crossing them from the focal point of a single lens can make them parallel to the optical axis, enabling multi-core fiber optical system processing.
An optical system including this new technique can operate as a band pass filter, a polarization sensitive or polarization insensitive optical isolator, an optical switch, or an optical cross-connect. The processing technique can be applied to multi-core broadcast and distribution optical networks and multi-core, multi-access networks.
University of Central Florida researchers have invented a system that requires less physical space and is less expensive than other laser brightness enhancement systems. By uniquely integrating few-mode fiber and photonic lantern technologies, the new system enables the ability to collect, combine, and enhance the output brightness of multiple lasers that emit different spatial modes.
Traditional multi-mode optical fiber combiner systems are designed to conserve brightness. As a result, they never exceed the sum of the input brightness of their laser sources. The UCF system overcomes this limitation by using unique photonic lanterns with the fundamental properties of waveguides (fibers) designed into them. The design maximizes the energy density of laser sources to the smallest possible product of area and divergence of a waveguide. Thus, it increases the brightness of combined laser output without resorting to energy exchange processes (such as those used in nonlinear optics).
The example figure depicts an application of the system for a semiconductor laser. In the example, light from multiple laser sources (1, 2, 3) is collected by fast axis collimating (FAC) and slow axis collimating (SAC) lenses. A focusing lens (5) then directs the light into few-mode fibers (6) that are connected to a photonic lantern (7). The PL separates the output of the FMFs into single modes and then recombines them into another PL (8), which connects to another FMF (9). The second FMF contains a super mode (10) that maximizes the brightness of all of the individual laser sources. Each PL is designed to operate over a large optical bandwidth, which is essential to match the optical bandwidth emitted from visible semiconductor lasers.
The research team is looking for partners to further develop the technology for commercialization.
University of Central Florida researchers have developed an innovation in remote sensing technology that may help speed the large-scale deployment of LIDAR (light detection and ranging) in autonomous vehicles. With its novel few-mode (FM) preamplified receiver architecture, the new UCF LIDAR system has demonstrated a signal-to-noise ratio (SNR) that is almost an order of magnitude higher than that of other systems. Besides providing much improved sensitivity and signal quality, the new eye-safe LIDAR system also allows for lower transmitter (laser) power, and thus, reduced costs.
The invention is a LIDAR system that attaches to the roof or front of a motor vehicle. The system includes a transmitter with amodulated infrared light source that emits an optical signal toward an object. It also includes a receiver that collects light reflected from the object. The receiver consists of a few-mode pre-amplifier, such as an Erbium-doped fiber amplifier (EDFA) or a semiconductor optical amplifier (SOA), which supports two or more spatial modes. The system outputs the amplified signal to a photodetector with a p-intrinsic-n (PIN) photodiode and a processing unit.
Improving the Sensitivity of LiDARs Using Few-Mode Pre-amplified Receivers, Frontiers in Optics / Laser Science OSA Technical Digest , Optical Society of America, 2018, conference paper FW7A.2
Researchers at the University of Central Florida have designed a single device that integrates mode demultiplexing, local oscillator power splitting, and optical 90-degree phase deconstruction using a multi-plane light converter (MPLC). As a result, the invention enables more reliable and robust coherent optical signals for communications systems.
Today's current optical front ends use hybrids that require a large footprint and complicated receiver structures. In contrast, the UCF Mode-Demultiplexing Hybrid (MDH) can reduce the number of devices needed in a space-division multiplexing (SDM) coherent receiver while reducing power consumption. Besides simplifying current coherent optical front ends for mode-division multiplexing (MDM) receivers, the MDH can also improve multicore fiber systems, where the mode demultiplexer is replaced by a fan-out device.
Technical Details
The UCF invention comprises an MDH apparatus and methods for designing the MDH. In one example, the apparatus can consist of an entrance plane adapted to receive inputs, including a signal with a specified phase/amplitude profile and a reference. The signal and reference inputs are spatially separated, occupy non-overlapped areas, and can coherently interfere. The MDH contains a reflective cavity MPLC with input optically coupled to the signal and reference inputs. Lastly, an exit plane is optically coupled to an output of the MPLC, which has spatially overlapping signal and reference spots. Each spot has a different respective phase shift imposed by the MPLC so that all the output spots are mutually orthogonal. Thus, a single MDH enables mode demultiplexing and optical 90-degree mixing of the inputs. In numerical simulations, the research team demonstrated the use of a three-mode MDH using four phase plates—one more than required by an MPLC mode demultiplexer. The performance is comparable to that of commercial single-mode 90-degree hybrids.
Partnering Opportunity
The research team is looking for partners to develop the technology further for commercialization.
Researchers at the University of Central Florida have invented a technique for capturing and reconstructing images of phase objects with greater accuracy and quality than conventional optical diffraction tomography (ODT) inversion methods. Example objects include transparent samples such as biological cells, tissues and optical fibers with high contrast, complicated structure, or sizeable optical path difference (OPD).
With such targets, most non-iterative tomographic reconstruction methods adopt the weakly-scattering assumption, which degrades the imaging quality. In contrast, the UCF Iterative Optical Diffraction Tomography (iODT) method, with its algorithmic process, reconstructs such objects and those with complex permittivity with better accuracy, fast convergence, and sub-wavelength resolution.
Technical Details: The UCF iODT approach improves the reconstruction quality of multiply scattering two-dimensional and three-dimensional phase objects by iteratively reducing the error between the fields diffracted by the reconstructed object and the true fields measured experimentally or obtained through simulations of phantoms for all illumination angles.
In one example application, the first iteration of iODT provides an estimate of the unknown refractive index (RI) profile using the standard linearized ODT inversion algorithm. Subsequent iterations improve the estimate by applying a perturbative correction based on differences between the fields diffracted by the imperfectly reconstructed object and the measured fields diffracted by the true object. The process includes translating this error into an error in the associated complex phase and then computing a correction to the reconstructed object function. The method uses the Rytov approximation in every iteration as it is more applicable to the perturbative function, as opposed to the original function. Since the magnitude (distribution) of the perturbative function becomes smaller (smoother) at higher iterations, the Rytov approximation improves. Further, as expected, the number of phase vortices in the perturbative complex phase is gradually reduced in a self-healing process as the iterations converge. In essence, the embodied iterative algorithm is a nonlinear reconstruction based on perturbative expansion, much like a higher-order Born or Rytov expansion for forward propagation.
Partnering Opportunity: The research team is looking for partners to develop the technology further for commercialization.
Stage of Development: Prototype available.
Optical Fiber Refractive Index Profiling by Iterative Optical Diffraction Tomography, Journal of Lightwave Technology, Vol. 36, No. 24, December 15, 2018
The University of Central Florida invention is a device that incorporates a conventional laser monolithically integrated with a totally undoped semiconductor optical amplifier (SOA) that can produce more than 20dB of optical gain in an overall length of less than 100 µm. A 2-D array of such devices can easily be flip-chip bonded onto a silicon photonic integrated circuit (PIC) for applications such as high power free-space communications.
The University of Central Florida invention enables a photonic iterative solver (PIS) for many large-scale linear inverse and optimization science and engineering problems. Example applications include matrix inversion, image reconstruction, high-dimensional inverse problems, numerical eigen problems, and ab initio ray-tracing engines.
Conventional iterative solvers with floating-point computing units are slow and inefficient at handling large-scale or high-dimensional data. Though systems using a fixed-point data format have the potential to train large-scale neural networks, fixed-point is not suitable for iterative solvers that require precision on the output. The limited precision can stagnate the iterative solver before reaching the optimal solution. Also, existing fixed-point iterative solvers in analog computing hardware have no error management mechanisms to resolve the error stagnation problem.
The UCF solution offers a hybrid analog and digital computational system that uses a dynamic fixed-point format to reach the same precision level as a conventional floating-point iterative solver. Compatible with the latest hardware advancements in high-performance computing, the invention achieves high speed and energy efficiency with fixed-point, highly parallelized architecture.
Technical Details: The UCF invention comprises a system, computer program product, and method of creating a hybrid analog and digital computational system. It incorporates a residual iterative algorithm to solve the set of solution values for equations. The residual iterative algorithm includes an outer update loop computed using a digital computing device with residue values initially set to a first initial value and a set of solution update values set to a second initial value.
The residual iterative algorithm also includes an inner residual loop that is iteratively computed using an analog accelerator until one or more inner residual loop stopping criteria are met and return a set of solution update values. Next, the system uses the new values to update the set of residue values and a range of the next set of solution update values, thereby adjusting the computational precision of the inner residual loop.
Partnering Opportunity: The research team is looking for partners to develop the technology further for commercialization.
Stage of Development: Prototype available.
The University of Central Florida invention offers an amplifier array with the efficiency/high-power characteristics of edge-emitting devices and the large area/thermal management advantages of surface-emitting devices. Coherent array emitters have been considered a viable approach to scaling laser output power. There are two schools of thought in implementing this on a chip. One uses edge-emitting amplifiers/lasers, and the other uses surface-emitting devices. The UCF invention combines the best of both worlds.
Technical Details: Existing chip-scale CBC techniques suffer from low power, poor scalability, and/or high optical loss. As a solution to these issues, the UCF technology enables a laser light source using an array of semiconductor optical amplifiers (SOAs) on a substrate. Each of the two or more SOAs receive seed light from a common seed source, and the plurality of SOAs provide an array of SOA output beams associated with amplifying the seed light. The UCF light source invention also enables the use of a series of phase masks to coherently combine the array of SOA output beams into a single output beam.
Partnering Opportunity: The research team is seeking partners for licensing, research collaboration, or both.
The University of Central Florida invention comprises methods for mitigating atmospheric turbulence in free-space coherent optical communications. Such turbulence distorts the wavefront of a signal, resulting in amplitude and phase error at the detector. To mitigate this problem, the UCF invention uses an adaptive optics photonic integrated circuit (PIC) in conjunction with wavefront sensors and feedback controls for wavefront correction. The technology also includes arrayed incoherent receivers to eliminate costly and slow adaptive optics for FSO communications.
Technical Details: The UCF technology incorporates the following:
Partnering Opportunity: The research team is seeking partners for licensing, research collaboration, or both.
Researchers at the University of Central Florida have developed a transmission system for optical communications, featuring a modulation format designed to intensify data transfer with high spectral efficiency, while reducing the need for additional equipment. The proliferation of bandwidth-intense services and cloud computing drives the demand for higher data transfer rates. In turn, high-capacity optical transmission systems require increased spectral efficiency due to finite bandwidth. This system offers doubled spectral efficiency with polarization-division multiplexing, transmitting two independent channels simultaneously at the same wavelength. This high efficiency is achieved without polarization control, improving dispersion tolerance and reducing system cost. Ideal for commercial optical communication system applications, the novel modulation system can be used for bandwidth-intense and distance-sensitive applications.
Technical Details
The UCF innovation's core concept, a modulation format named differential polarization-phase-shift keying (DPolPSK), provides a new constant intensity encoding of lightwave phase and polarization without the need to recover the state of polarization (SOP) at the receiver. Because the demodulation technique features a process essentially not affected by the slow polarization change during transmission, complex and costly dynamic polarization control at the receiver is eliminated. The system consists of an electrical encoder, an optical encoder, optical demodulators, and balanced optical signal detectors. The optical demodulator offers the flexibility of effective performance at 1-bit or 2-bit delay. Additionally, the system can include a multiplexer/demultiplexer, further maximizing finite bandwidth.
Researchers at the University of Central Florida have developed the first all-optical carrier phase and polarization recovery scheme for Phase-Shift Key (PSK) signals. The technique also provides a means to carry out reliable optical carrier synchronization using a phase sensitive oscillator. Coherent optical communication is an active area in modern optics research.
Most existing strategies use interference between an incoming optical signal and a local oscillator signal for phase recovery. However, phase recovery schemes such as phase-locked loops and injection locking, are only effective if optical carrier information is available. Hence, efficient carrier phase and polarization recovery is important. The absence of reliable carrier recovery technologies thus poses a challenge and hindrance to the frequent use of coherent optical communication. In addition, all-optical signal processing techniques for coherent optical modulated signals such as regeneration of PSK, require a polarization and phase locked local oscillator.
Technical Details
The UCF invention comprises a system and methods for optical carrier phase and polarization recovery. In one embodiment, an optical transmitter transmits an optical signal to an optical carrier synchronizer, which includes a phase-sensitive oscillator. Use of the phase-sensitive oscillator results in a recovered optical carrier being output from the optical carrier synchronizer. In the example, the optical carrier synchronizer consists of a pump generator that provides a pump signal, or "pump," to the phase-sensitive oscillator via an optical coupler. The phase-sensitive oscillator can include a phase-sensitive amplifier with a nonlinear optical fiber loop mirror (NOLM), a polarization controller. It can also include a reflector in the form of a fiber Bragg grating (FBG ).Researchers at the University of Central Florida have developed a novel process for directly converting silica photonic crystal fiber (PCF) into pure silicon PCF. In photonic crystal devices, there exists an ability to confine light to a very small cross-section. This results in a new style of guiding which has lead researchers around the world to envision novel applications that exploit this highly confined and guided light.
One such application, the photonic crystal fiber (PCF), is a threaded version of the photonic crystal and is applicable in communications, lasers, nonlinear conversion, and more. Research into silica (SiO2) PCF devices has been intensive and productive; proving itself a valuable technology. Unfortunately, the transition to pure silicon (Si) devices, featuring low loss in the mid-IR, high optical damage threshold, and wide availability, is not yet commercially feasible.
Technical Details
The UCF technology defines a process that uses magnesiothermic reduction. During the process, a magnesium gas pulls the oxygen away from the silica device. This is the very first method which has shown the ability to fabricate pure silicon PCF.
The fabrication method preserves the nanostructure of the silica in the raw material during the conversion to silicon. In some embodiments, the hollow-core silica fiber used as a raw material has a honeycomb-shaped photonic crystal structure around a hollow core. The use of silicon in the converted fiber provides low absorption loss in the mid-IR range, an extremely high optical damage threshold, and excellent thermal conductivity. The use of nanostructured silicon (ns-Si) extends the low absorption loss into near-IR and improves third-order non-linearity (because of quantum confinement).
Researchers at the University of Central Florida have developed a digital signal processing (DSP) method of post-compensation for impairments. Optical signal processing hinges on the system's ability to compensate for the error accumulated by optical pulses as they travel through air or a waveguide apparatus, such as a fiber optic cable. The optical signal unavoidably succumbs to a variety of impairments, including absorption, dispersion, nonlinearities, and amplifier noise. In the past, optical techniques have attempted to physically compensate for these errors but have led to only marginal success as physical systems vary greatly in detail. Therefore, the trend has shifted toward electronic error correction.
Technical Details
First applied to artificial neural networks, backward propagation of errors (back propagation) is a method for teaching a data correcting element the response of a nonlinear system when it is not possible to use standard impulse response methods. With the UCF technology, this method is applied to an algorithm that learns the amount of each optical error present in the system using a known teaching signal. A received and impaired signal is sent backward through a virtual version of the system to gauge error weights. The system then uses that knowledge to compensate for normal operational signal recognition.
When an optical signal travels through a fiber channel, the fiber introduces various types of distortion. Researchers at the University of Central Florida have developed a method to compensate for this optical distortion, using backward propagation in the electrical domain to restore optical signal quality/integrity. The received signal, distorted by imperfections in the physical channel, is processed by modeling channel parameter values that are opposite to those of the distortion-causing physical channel. By modifying the physical channel distorted signal with the same effect a converse physical channel would have, this development can restore signal as if it had been transmitted through a perfect physical channel.
Examples of impairments include fiber dispersion, self-phase modulation or SPM (an intra-channel impairment), cross-phase modulation or XPM (an inter-channel impairment), and four-wave mixing or FWM (another inter-channel impairment).
Technical Details
In this new method, the distorted optical signal is demultiplexed by a frequency demultiplexer and provided to one or more optical detectors, which convert the distorted optical signal to a signal in the electrical domain. The distorted electrical signal is processed in the electrical (digital) domain by impairment compensation logic to remove distortion produced in the optical (physical) domain. Carried within the demultiplexed and compensated electrical signal is data that is a replica or near replica of the originally transmitted data.
Researchers at the University of Central Florida have developed a method for backward propagation to undo the nonlinear impairments experienced by polarization-division multiplexed (PMD) WDM channels, using the Manakov equation (ME). The method accounts for PMD induced polarization scattering by monitoring the Jones matrix of the fiber for each WDM channel along the fiber length, for example, at each span, to be used in the backward propagation.
Channel impairments in transmission systems result in signal degradation and thus limit the carrying capacity of these systems. As complex electrical signal propagates along each span of an optical channel and is distorted by fiber impairments, polarization-dependent nonlinearity impairment compensation logic can undo signal degradation.
While backward propagation can be used to compensate for nonlinear impairments in wavelength-division multiplexed (WDM) systems, based on solving the nonlinear Schrodinger equation (NLSE), advanced transmission schemes such as polarization-division multiplexing or polarization-interleaving require a vectorial form of NLSE.
Technical Details
Polarization-dependent nonlinearity impairment compensation logic solves the vectorial NLSE to model for compensation of various impairments, including non-linear impairments with their polarization dependence. The polarization-specific component of each compensated signal is then demultiplexed to estimate the originally transmitted data.
Researchers at the University of Central Florida have developed systems and methods of compensating for transmission impairment over an optical transmission channel using wavelet-based FIR filters. Transmitting a signal over long haul optical fiber links leads to undesirable distortions, which in order for the signal to be effectively received, these distortions must be compensated for and the original signal recovered. Signal distortion results from linear processes, such as absorption and fiber chromatic dispersion, and non-linear processes, such as cross-phase modulation, four-wave mixing, and amplifier noise. Previously, physical means such as specialty fibers were used to correct these signals after transmission. Now, digital methods make use of inverse Fourier-Transform calculations to compensate for both linear and non-linear distortion.
Compared to previous electronic correction techniques using inverse Fourier-Transform methods, wavelet-based filtering offers a significant computational advantage by reducing filter length by more than four-fold. By using wavelet filters, you can now compensate for linear and non-linear impairments in an optical communication link in the electronic domain. Electronic filtering provides a simplicity that physical domain compensation systems cannot offer, while offering the capability to retrofit to existing networks.
Technical Details
To compensate for the distortion, the UCF technology first generates a model of the fiber by calculating the propagation of a wave through the fiber and creates a virtual fiber. This model is then used to real-time correct the transmitted signal by propagating the distorted signal backwards through the system in an electronic virtual environment. By reversing the sources of error, this electronic approach is more versatile and adaptable than physical means, as those are limited to material and physical characteristics. This technology is extendable to multi-wavelength schemes and can even be retrofitted to existing transmission systems.
Researchers at the University of Central Florida have created a method involving post-compensation that can modify a received signal, compensate for the impairment, and can be implemented in either the optical domain or in the electrical/electronic domain. With the use of coherent detection and digital signal processing (DSP), post-compensation offers great flexibility when adaptive compensation is used within this scheme. It has also been proven to be very effective in chromatic dispersion compensation as well as intra-channel nonlinearity compensation.
In recent years, electrical dispersion compensation (EDC) and electrical nonlinearity compensation (ENLC) have received significant research activity as well as commercial interest. Specifically, EDC has the potential to decrease optical signal distortion in the electrical domain after detection in a photoreceiver. Optical channel impairments, caused by semiconductor optical amplifiers (SOA), can also result in signal degradation and limited carrying capacity.
Technical Details
The UCF post-compensation method comprises receiving an optical signal distorted in the physical domain by an SOA and propagating the distorted optical signal backward in the electronic domain in a corresponding virtual SOA. The techniques of this post-compensation method involve the use of digital backward propagation in the electrical domain to convert a received optical signal into an estimated transmitted signal that then compensates for nonlinear impairments introduced by SOAs that reside along the transmission link(s).
With a simulated DSP speed of 25 GHz, the impairment compensation of a 1200-kilometer link requires 22.3M of multiply-accumulate (MAC) units. The computational efficiency is 464 kMAC/bit with latency of 10.2 microseconds, considerably smaller than the transmission latency of 6 milliseconds in the fiber. Based on simulation results, for a 12 x 100 Gb/s 16-QAM/WDM (quadrature amplitude modulation/wavelength-division-multiplexing) system using nonzero dispersion-shifted fiber (NZ-DSF), the computational load is halved by the complementary filter pair design. The transmission distance is increased from 500 to 1200 kilometers by ENLC while preserving the same Q-value.
The University of Central Florida invention is an adaptive system that enables adjustable ranges for parameters and intermediary variables while greatly reducing computer power consumption. The invention determines the adjustable ranges based on the statistics of the parameters/variables during the adaptation process, which reduces errors caused by clipping or low precision and offers improved performance in range-limited adaptive systems.
Unlike other adaptive systems that rely on floating-point numbers for all or a portion of their parameters, the UCF invention uses fixed-point data types and does not use floating-point numbers. In one example application, the UCF adaptive system can effectively train artificial neural networks (ANNs). A class of adaptive systems, ANNs transform inputs (such as text, audio, images, and videos) to outputs of desired formats to achieve various tasks.
Technical Details: The UCF invention comprises an adaptive system (such as an adaptive control system, an adaptive proportional-integral-derivative (PID) controller, or an ANN) and a novel method for adjusting a parameter in the system. For instance, a parameter (fixed-point or analog) with a finite range is adjusted based on a difference between the output signal and a target output signal. Typically, the parameter is an analog electrical signal, a digital electrical signal, an analog optical signal, a digital optical signal, or a digital-and-analog hybrid signal.
Finite ranges of individual parameters are adjusted based on the statistics of the parameter values. For an individual parameter, the statistical distribution can be estimated from the historical values of the parameters during the adaptation process. As an example, if the adaptive system employs discrete, iterative feedback steps, such as those in a reinforcement learning system, the distribution can be fitted from the parameter values from the previous iterations. In another example, if the adaptive system employs continuous feedback, such as a real-time motion system, the distribution can be fitted from a portion of or the whole time series of the parameter in the past.
Partnering Opportunity: The research team is seeking partners for licensing, research collaboration, or both.
Stage of Development: Prototype available.
The University of Central Florida invention is a photonic matrix accelerator that works with floating-point numbers. Other photonic accelerators only work with fixed-point numbers, significantly limiting the dynamic range of analog neural networks (ANNs). With the UCF floating-point photonic accelerator (FPA), multiplications are performed by coherent mixing and accumulations are performed in the spatial-mode or wavelength domain. The UCF power-efficient floating-point analog tensor accelerator provides a foundational and vertical technology applicable across the spectrum of applications in harnessing artificial intelligence for both commercial and defense applications.
Technical Details
The UCF approach effectively enhances the dynamic range of analog computation. However, repeated updates of the ANN weight matrix are required in training processes, and this configuration always needs high-speed modulation, since accumulations are implemented using time-division multiplexing (TDM), resulting in excessive energy expenditure. This construction has a direct implication to the invention’s scalability. For example, assume that the integration time for accumulation is 200 picoseconds, corresponding to a 5 gigahertz (GHz) clock rate. Using a maximum modulation speed of 500 GHz, the number of weights per column is only 100. Thus, the approach can be scaled to much larger sizes and encode a greater number of exponent levels.
Researchers at UCF have developed a new solution that replaces the slow and costly deformable mirror loop with a detector array and digital signal processing (DSP) chip. Free-space optical communication has become an alternative to fiber-based technologies and is implemented when physical connectivity is impractical (for example, satellite communication).
Unfortunately, transmission of light through the atmosphere induces devastating wavefront distortions that ruin the signal's content. Therefore, the inclusion of signal corrective technologies is required. Currently, adaptive optics techniques are used to perform this correction, and consist of feedback loops with deformable mirrors that sample and iteratively correct the signal. While this technology has allowed free-space communication to become successful, physical correction of the optical signal is both costly and slow.
Technical Details
The invention samples an incoming signal (whether for communication or imaging) through a detector array that references against a local oscillator source generating an interference pattern from which the atmospheric wavefront distortion is calculated. These readings are subtracted from the incoming signal as a correction. Performance of this technique approaches a system free of wavefront distortion and could be used to greatly increase the capabilities of any system that suffers from random media wavefront distortion.
Electronic wavefront correction for PSK free-space optical communications, Electronics Letters, Vol. 43, Issue 20, (27 Sept. 2007): 1108-1109.
