Search

The signal-to-noise ratio and bandwidth of photonic time-stretch (PTS) systems have previously been limited due to the nonlinear distortion caused by modulator response and dispersive propagation. In this paper, we present a novel method for the reconstruction of the input signal from two phase-diverse intensity measurements. The method consists of optical phase retrieval at the measurement point followed by simulated optical backpropagation to the modulation point. By numerical simulation of a PTS system with realistic parameters, we analyze the proposed method and compare it with the previously suggested maximum ratio combining (MRC) method. We show that the proposed optical backpropagation method, unlike the MRC method that treats the system as being linear from electrical input to output, can reconstruct the signal even at a large modulation depth and that the method is insensitive to biasing error or drift and not overly sensitive to misestimation of system parameters. Furthermore, to reduce the computational effort associated with the simulation of PTS systems, we present a numerical propagation method whereby the required number of sampling points is reduced by several orders of magnitude.

A novel interferometric scheme for photonic analog-to-digital conversion is for the first time experimentally demonstrated at a real-time sample rate of 40 gigasamples/s. The scheme includes sampling as well as binary encoding, and the input signal in the experiment was a 1.25-GHz sinusoidal tone that was successfully digitized with a nominal resolution of 21 digital levels. Single-sample measurements yielded an effective number of bits (ENOB) of 2.6, which was limited by thermal detection noise while multisample averaged measurements resulted in an ENOB of 3.6 bits, mainly limited by phase drift. Apart from the experimental results, this paper covers an extensive theoretical analysis of the system, including calculations on the fundamental maximum bandwidth, the required optical power, the generated binary code, and its error robustness, as well as the impact of detection noise on the signal-to-noise ratio of the digitized signal. The major benefits of this interferometric scheme are that only one standard phase modulator is required and that the phase swing does not have to be larger than plusmnpi to reach the full digital value space

Phase modulating spatial light modulators (SLMs) can be used to alter the shape of a laser wavefront to achieve a deflection or change in the shape of a laser beam. This paper reports the results of characterization, simulation and optimization of a one-dimensional liquid crystal (LC) SLM. The device has a large ratio between LC layer thickness and pixel pitch that results in a fringing field between pixels. In effect, the applied phase patterns will be lowpass filtered and the loss of high frequency components limits, for instance, the usable steering range. A method is presented where intensity measurements in the far field are used to determine how the phase modulation at the SLM is distorted. The inhomogeneous optical anisotropy of the device was determined by modelling the liquid crystal director distribution within the electrode-pixel structure. Finite-difference time-domain (FDTD) simulations were used to calculate the light propagation through the LC. The simulated phase distortion was co...

Tunable signal generation in the gigahertz to terahertz range was demonstrated by photomixing of a chirped optical pulse and its time-delayed replica. The chirped pulses were created by chromatically dispersing femtosecond pulses from a mode-locked fiber ring laser, and frequency tuning was achieved by adjusting the time delay and/or the amount of group velocity dispersion. It is shown that the signal phase is locked to the femtosecond-laser pulses and that an extinction ratio close to 100% can be obtained. Analytical calculations on the signal chirp introduced by higher order dispersion are also presented and compared with the experimental data

A novel interferometric analog-to-digital converter (ADC) scheme for real-time conversion at multiple-gigahertz bandwidth is presented and theoretically analyzed. The input analog signal is applied as an optical phase modulation in one of the arms of a Mach-Zehnder interferometer and the digital output is obtained in encoded form as a set of optical intensities in the interference pattern. Particular benefits of the proposed device are its simple design which facilitates integration, the extremely high bandwidth, and the inherent 2/spl pi/-periodicity that in comparison with conventional flash ADCs relaxes the demands on the comparators and allows unwrapping of the output for higher resolution.

Results from our experiments on inertial navigation in crash testing of vehicles are presented. A custom designed inertial measurement unit (IMU) has been developed, and several tests in dummy calibration rigs including full-scale Euro NCAP collisions have been performed at our partners' crash test laboratories. For reference, the IMU data is compared to camera data and traditional single-axis inertial sensors.

Building on the optimal-rotation-angle method, an algorithm for the design of inter-element coherent arrays of diffractive optical elements (DOEs) was developed. The algorithm is intended for fan-out DOE arrays where the individual elements fan-out to a sub-set of points chosen from a common set of points. By iteratively optimizing the array of elements as a whole the proposed algorithm ensures that the light from neighbouring elements is in-phase in all fan-out points that are common to neighbouring DOEs. This is important in applications where a laser beam scans the DOE array and the fan-out intensities constitute a read-out of information since the in-phase condition ensures a smooth transition in the read-out as the beam moves from one DOE to the next. Simulations show that the inter-element in-phase condition can be imposed at virtually no expense in terms of optical performance, as compared to independently designed DOEs.

We experimentally demonstrate the performance of the recently proposed "OBP" algorithm based on optical phase retrieval by numerical back-propagation. Using this technique, distortion due to the modulator transfer function as well as the group velocity dispersion in a photonic time-stretch ADC (TS-ADC) can be suppressed, and frequency fading eliminated thanks to the use of a dual output "phase diverse" modulator. The algorithm was tested with a swept input signal frequency from 10 to 50 GHz. as well as with a two-tone signal, and is shown to recover the signal even in the presence of fading with a distortion suppression of up to 26 dB. These results also constitute the first experimental demonstration of channel combining in a phase diverse TS-ADC.

The performance of wavelength-based photonic analog-to-digital converters (ADCs) is theoretically analyzed in terms of resolution and bandwidth as well as of noise tolerance. The analysis applies to any photonic ADC in which the analog input signal is converted into the wavelength of an optical carrier, but special emphasis is put on the spectrometerlike setup in which the wavelength is mapped to a spatial spot position. The binary output signals are then retrieved by an array of fan-out diffractive optical elements that redirect the beam onto the correct detectors. In particular, the case when the input signal controls the wavelength directly such that it will chirp in frequency during each sampling pulse or interval is studied. This chirping obviously broadens the spot on the diffractive optical element array; the effect of this broadening on noise tolerance and comparator accuracy is analytically analyzed, and accurate numerical calculations of the probability of error are presented.

A novel interferometric scheme for photonic analog-to-digital conversion (ADC) is experimentally demonstrated at a real-time sample rate of 40 gigasamples/s. The input signal in the experiment was a 1.25 GHz sinusoidal tone and the resolution 21 digital levels with an effective number of bits (ENOB) of 3.6. The major benefits of this scheme are that only one standard phase modulator is required and that the phase swing does not have to be larger than ±π to reach the full digital value space.

Using a tunable semiconductor laser diode and an array of diffractive optical elements (DOEs), a time-continuous, free-space optical analog to digital converter (ADC) with five bits of resolution was experimentally evaluated. The signal to be A/D-converted was fed to the tuning sections of a grating coupled twin-guide sampled reflector (GCSR) laser diode, giving a quasi-continuous tuning range of 10 nm that spanned 40 longitudinal modes. The 32 central modes were mapped to specific digital output values by first converting wavelength to deflection angle using a diffraction grating and then focusing on an array of beam splitting DOEs. Each DOE element generated a five-spot digital code word in the detector plane. Using Gray code, only one code bit changed value at a time. Thus, the beam could straddle two adjacent DOE elements without large read out errors. Furthermore, the grating components of the elements in the DOE array were all in-phase to keep the spot focused when such straddling occurs. The SNR of a converted 10 MHz sine signal covering 23 modes was 21 dB, mainly limited by tuning hysteresis. This SNR corresponds to 3.2 effective bits. The laser's analog tuning bandwidth was found to be 45 MHz, probably limited by the carrier lifetime in the passive tuning sections, but we also measured the ADC performance at 100 MHz. As the studied ADC system is time-continuous, the sampling was done in the digital domain.

A commercial linear one-dimensional, 1x4096 pixels, zero-twist nematic liquid crystal spatial light modulator (SLM), giving more than 2π phase modulation at λ = 850 nm, was evaluated for beam steering applications. The large ratio (7:1) between the liquid crystal layer thickness and pixel width gives rise to voltage leakage and fringing fields between pixels. Due to the fringing fields the ideal calculated phase patterns cannot be perfectly realized by the device. Losses in high frequency components in the phase patterns were found to limit the maximum deflection angle. The inhomogeneous optical anisotropy of the SLM was determined by modelling of the liquid crystal director distribution within the electrode-pixel structure. The effects of the fringing fields on the amplitude and phase modulation were studied by full vector finite-difference time-domain simulations. It was found that the fringing fields also resulted in coupling into an unwanted polarization mode. Measurements of how this mode coupling affects the beam steering quality were carried out and the results compared with calculated results. A method to compensate for the fringing field effects is discussed and it is shown how the usable steering range of the SLM can be extended to ± 2 degrees.

A linear one-dimensional, 1x4096 pixel, zero-twist nematic liquid crystal spatial light modulator (SLM) was evaluated for laser beam steering and tracking applications. The commercially obtained SLM is designed to operate at, λ = 850 nm, allowing more than 2 π phase modulation. Due to voltage leakage the phase modulation experienced by the wave front differed from the ideal calculated phase patterns. This cross talk between pixels reduces the diffraction efficiency. Different methods developed to compensate for this effect are presented. The usable steering range of the SLM was extended to ± 2 degrees using improved phase patterns. A simple model was developed to simulate the optical effects of the voltage leakage. Preliminary tracking experiments were carried out in a laboratory set-up using a moving corner cube retro reflector. The beam steering SLM was implemented in a transceiver for free-space optical communication. Initial results using the transceiver up to 180 m range are presented.