Coherent optical array receivers for the mitigation of atmospheric turbulence and speckle effects

Dual sideband receiver for radio-over-fiber

A dual sideband reception scheme for radio-over fiber (RoF) links is introduced. It is shown that the new receiver can increase the performance of noise-limited systems by up to 3 dB (2.97 dB in a lab back-to-back experiment). The receiver scheme exploits the fact that current RoF links do not realize their full potential. This is because in typical RoF receivers, the radio-frequency (RF) signals are mapped back to the optical domain by means of electro-optical modulator. In this process energy typically is lost as only one of the two generated sidebands is subsequently used. The suggested receiver exploits the signal of both sidebands. The receiver scheme was subsequently tested in a full optical-RF-optical transmission link at RF carrier frequencies of 228 GHz over a free-space channel spanning distances of 1400 m for symbol rates of up to 48 Gbaud 4 QAM. Here, we could achieve SNR improvements of up to 2.6 dB.

Performance enhancement of free-space optical communications under atmospheric turbulence using modes diversity coherent receipt

This paper numerically and experimentally investigates the performance of free-space optical receiver using modes diversity coherent receipt under moderate-to-strong turbulence. By utilizing a three-mode photonic lantern with digital maximum ratio combining, a 40 Gbps QPSK optical signal is received. The turbulence strength is measured by the ratio of beam diameter to atmospheric coherence length, D/r0. The larger the D/r0, the stronger the turbulence is, and vice versa. Compared with conventional single mode fiber based receipt, the required transmitted power can reduce by 4.6 dB and 4 dB at 10% interruption probability under turbulence strength D/r0 = 8 and 16. The required transmitted power at bit error ratio of 2.2 × 10^-2 can relax by 4.2 dB and 5 dB under turbulence strength D/r0 = 8 and 16.

Multi-aperture digital coherent combining for free-space optical communication receivers

Space-to-ground optical communication systems can benefit from reducing the size, weight, and power profiles of space terminals. One way of reducing the required power-aperture product on a space platform is to implement effective, but costly, single-aperture ground terminals with large collection areas. In contrast, we present a ground terminal receiver architecture in which many small less-expensive apertures are efficiently combined to create a large effective aperture while maintaining excellent receiver sensitivity. This is accomplished via coherent detection behind each aperture followed by digitization. The digitized signals are then combined in a digital signal processing chain. Experimental results demonstrate lossless coherent combining of four lasercom signals, at power levels below 0.1 photons/bit/aperture.

Photonic-lantern-based coherent LIDAR system

In this work, a photonic-lantern-based coherent LIDAR system is experimentally demonstrated and the voltage signal-to-noise ratio improvement is analyzed. A voltage signal-to-noise ratio (SNR(V)) improvement of 2.8 is demonstrated experimentally for photonic-lantern-based coherent receivers relative to single-mode coherent receivers. The voltage signal-to-noise ratio improvement is obtained when other parameters are kept constant. We have also analyzed the effect of random optical power distribution among the single-mode fibers. We found that the distribution does not significantly impact the SNR(V) improvement. The mean value of voltage signal-to-noise ratio improvement is found to be ~ 2.4.

Free-space to single-mode collection efficiency enhancement using photonic lanterns

We demonstrate single-mode collection efficiency enhancement for free space optical systems using a photonic lantern to collect scattered infrared light from diffuse objects at far- and near-field distances. A single-mode collection efficiency improvement of ∼8  dB is demonstrated in the near-field region relative to standard single-mode fiber. The insertion loss properties of the photonic lantern are also analyzed, and an additional insertion loss penalty is observed for near-field distances when the transmitted beam is collimated. The photonic lantern can be used for coherent detection systems such as light detection and ranging and free-space optical communication with improved collection efficiency and nearly perfect mode matching.

Ultrawideband coherent noise lidar range-Doppler imaging and signal processing by use of spatial-spectral holography in inhomogeneously broadened absorbers

We introduce a new approach to coherent lidar range-Doppler sensing by utilizing random-noise illuminating waveforms and a quantum-optical, parallel sensor based on spatial-spectral holography (SSH) in a cryogenically cooled inhomogeneously broadened absorber (IBA) crystal. Interference between a reference signal and the lidar return in the spectrally selective absorption band of the IBA is used to sense the lidar returns and perform the front-end range-correlation signal processing. Modulating the reference by an array of Doppler compensating frequency shifts enables multichannel Doppler filtering. This SSH sensor performs much of the postdetection signal processing, increases the lidar system sensitivity through range-correlation gain before detection, and is capable of not only Doppler processing but also parallel multibeam reception using the high-spatial resolution of the IBA crystals. This approach permits the use of ultrawideband, high-power, random-noise, cw lasers as ranging waveforms in lidar systems instead of highly stabilized, injection-seeded, and amplified pulsed or modulated laser sources as required by most conventional coherent lidar systems. The capabilities of the IBA media for many tens of gigahertz bandwidth and resolution in the 30-300 kHz regime, while using either a pseudo-noise-coded waveform or just a high-power, noisy laser with a broad linewidth (e.g., a truly random noise lidar) may enable a new generation of improved lidar sensors and processors. Preliminary experimental demonstrations of lidar ranging and simulation on range-Doppler processing are presented.

Fiber-coupling efficiency for free-space optical communication through atmospheric turbulence

High-speed free-space optical communication systems have recently used fiber-optic components. The received laser beam in such a system must be coupled into a single-mode fiber at the input of the receiver module. However, propagation through atmospheric turbulence degrades the spatial coherence of a laser beam and limits the fiber-coupling efficiency. We numerically evaluate the fiber-coupling efficiency for laser light distorted by atmospheric turbulence. We also investigate the use of a coherent fiber array as a receiver structure and find that a coherent fiber array that consists of seven subapertures would significantly increase the fiber-coupling efficiency.

Effectiveness of simultaneous independent realizations at low carrier-to-noise ratio to improve heterodyne Doppler lidar performance. I. Theory and numerical simulations

It is shown that the performance of heterodyne Doppler lidar (HDL) can be improved by (i) at least one good realization for every single shot or (ii) several simultaneous good realizations for accumulation. Until now, several simultaneous independent realizations at high carrier-to-noise ratio (CNR) have been considered. At low CNR, noise may have a detrimental effect on the accumulation techniques. We determine the chances of getting "heavy" speckles in HDL signals from many receiver-detector units on a single-shot basis and several good realizations on a single-shot basis, which is required for an effective accumulation. The use of multiple receiver-detector units at low CNR is worthwhile in contexts such as space lidar, where optimized treatment is at a premium. We conclude on the effectiveness of many receiver-detector units in parallel in order to achieve simultaneous independent realizations at low CNR to improve the performance of HDL on a statistical basis.

Bit-error rate for free-space adaptive optics laser communications

An analysis of adaptive optics compensation for atmospheric-turbulence-induced scintillation is presented with the figure of merit being the laser communications bit-error rate. The formulation covers weak, moderate, and strong turbulence; on-off keying; and amplitude-shift keying, over horizontal propagation paths or on a ground-to-space uplink or downlink. The theory shows that under some circumstances the bit-error rate can be improved by a few orders of magnitude with the addition of adaptive optics to compensate for the scintillation. Low-order compensation (less than 40 Zernike modes) appears to be feasible as well as beneficial for reducing the bit-error rate and increasing the throughput of the communication link.

Four-Element Receiver for Pulsed 10-mum Heterodyne Doppler Lidar

A four-element photomixer receiver has been tested in a 10-mum heterodyne Doppler lidar. It addresses a reduction of the variance of the power scattered off distributed aerosols targets at ranges as long as 8 km. An improvement in performance is expected when the four independent signals recorded on every single shot are combined. Two summation techniques of the four signals have been implemented: a coherent summation of signal amplitude and an incoherent summation of intensities. A phasing technique for the four signals is proposed. It is based on a more suitable correlation time with discernible self-consistent packets (SCP's). The SCP technique has been successfully tested, and the results obtained with a coherent summation of the four signals, i.e., variance reduction, carrier-to-noise ratio improvement, and velocity accuracy improvement, are in agreement with theory.

Autonomous beam alignment for coherent Doppler lidar with multielement detectors

Autonomous beam alignment for coherent Doppler lidar requires accurate information about optical misalignment and optical aberrations. A multielement heterodyne detector provides the required information without a loss in overall system performance. The effects of statistical variations from the random backscattered field (speckle field) are determined with computer simulations for both ground-based operation with a fixed calibration target and for space-based operation with random target backscatter.

Experimental verification and theory for an eight-element multiple-aperture equal-gain coherent laser receiver for laser communications

The detection and processing of laser communication signals are affected by the fading induced onto these signals by atmospheric turbulence. One method of reducing this fading is to use an array of detectors in which each of the detector outputs are added together coherently. We present experimental verification and theory of a 1.06 mum eight-element coherent receiver used to mitigate the effects of fading over a 1-km outdoor range. The carrier-to-noise ratio (CNR) was measured on a single channel and was then compared with the CNR obtained from the coherent sum of the eight channels. The increase of the mean CNR for the coherent sum as compared with a single aperture was observed proportional to the number of the apertures under different conditions of atmospheric turbulence. The measured mean CNR gain fitted the theoretical prediction well when the laser intensity fluctuations followed the gamma distribution.

Aperture dependence of the mixing efficiency, the signal-to-noise ratio, and the speckle number in coherent lidar receivers

With the aid of the van Cittert-Zernike theorem we develop an analytical expression for the ensemble-averaged heterodyne mixing efficiency in coherent lidar receivers that are looking at a diffuse target that is in the receiver's far field. Our extremely simple and straightforward analysis shows that the dependence of the mixing efficiency on the receive aperture size dR first follows a parabolic decrease and later approaches a (dR)(-2) function. As a consequence, the signal-to-noise ratio does not increase proportionally to the aperture area but saturates. For the system model chosen, the heterodyne mixing efficiency exhibits the same functional dependence on the lidar geometry as the reciprocal of the number of speckle cells within the receive aperture.