Jonathan F. Holzman

Error rate performance comparison of coherent and subcarrier intensity modulated optical wireless communications

A detailed analysis and comparison is carried out for optical wireless communications (OWCs) with coherent and subcarrier-intensity-modulation-based systems, which are the two major implementations for detection-threshold-free operation without irreducible error floors. Error rate performance is studied for communications with binary phase-shift keying, differential phase-shift keying, and noncoherent frequency-shift keying over weak-to-strong (gamma-gamma distributed) turbulence conditions. Series-form error rate expressions are also derived for diversity reception schemes, including maximum ratio combining, equal gain combining, and selection combining. Based on our analysis, it is found that coherent OWC systems typically outperform subcarrier intensity modulation systems, with 24-30 dB improvements in sensitivity, mainly due to their elimination of thermal and background noise effects. The performance improvements of coherent systems are confirmed through numerical studies. The findings can offer significant benefits for future OWC systems that are subject to transmitted power limitations.

Error Rate Analysis of M-ary Coherent Free-Space Optical Communication Systems with K-Distributed Turbulence

Coherent free-space optical (FSO) communications is analyzed for long-range, high turbulence FSO systems operating under K-distributed turbulence conditions. Exact error rates are presented for M-ary phase-shift keying (MPSK) and M-ary quadrature amplitude modulation (MQAM) using a closed-form moment generating function. Maximum ratio combining (MRC) reception is employed in the system with MPSK and MQAM to mitigate the turbulence effects, and asymptotic error rate analysis is presented for this MRC reception. The error rate performance degradation caused by phase compensation error is also studied, and it is found that the impact of phase compensation error on the error rate performance can be small when the standard deviation of the phase compensation error is kept below twenty degrees.

Two-photon photoconductive terahertz generation in ZnSe

We report on the generation of free-space terahertz (THz) radiation using polycrystalline ZnSe as the photoconductive (PC) substrate. It is found that the ZnSe emitter can be operated at peak fields up to 125 kV/cm without dielectric breakdown, and can be photoexcited (through two-photon absorption) at pump energy fluences up to 28 mJ/cm2 without saturating. The relationship between the THz field strength and the excitation conditions of the PC gap are interpreted through a photocarrier transport model.

Performance analysis of coherent wireless optical communications with atmospheric turbulence

Coherent wireless optical communication systems with heterodyne detection are analyzed for binary phase-shift keying (BPSK), differential PSK (DPSK), and M-ary PSK over Gamma-Gamma turbulence channels. Closed-form error rate expressions are derived using a series expansion approach. It is shown that, in the special case of K-distributed turbulence channel, the DPSK incurs a 3 dB signal-to-noise ratio (SNR) penalty compared to BPSK in the large SNR regime. The outage probability is also obtained, and a detailed outage truncation error analysis is presented and used to assess the accuracy in system performance estimation. It is shown that our series error rate expressions are simple to use and highly accurate for practical system performance estimation.

Exact error rate analysis of equal gain and selection diversity for coherent free-space optical systems on strong turbulence channels

Exact error rate performances are studied for coherent free-space optical communication systems under strong turbulence with diversity reception. Equal gain and selection diversity are considered as practical schemes to mitigate turbulence. The exact bit-error rate for binary phase-shift keying and outage probability are developed for equal gain diversity. Analytical expressions are obtained for the bit-error rate of differential phase-shift keying and asynchronous frequency-shift keying, as well as for outage probability using selection diversity. Furthermore, we provide the closed-form expressions of diversity order and coding gain with both diversity receptions. The analytical results are verified by computer simulations and are suitable for rapid error rates calculation.

Ultrafast photoconductive self-switching of subpicosecond electrical pulses

A novel photoconductive switch is proposed. The geometry of this ultrafast switch allows the rising edge of an ultrashort optical pulse to both turn on and turn off a terahertz electrical transient, making the device independent of the substrate material and charge carrier lifetime. A lumped-element model is used to analyze the operation of the switch. The model employed describes the photoexcitation of both a microstrip photoconductive switch layout and a coplanar photoconductive switch layout. It is found that both of the layouts are capable of achieving subpicosecond switching, with the coplanar layout offering greater ease of fabrication and device tunability.

Ultrafast carrier dynamics in InP photonic crystals

Ultrafast time-resolved reflectivity investigations are performed on InP-based photonic crystals with a wide range of structural parameters. It is found that the structure plays a critical role in determining the recombination dynamics of the photo-injected charge carriers. For sufficiently large etched sidewall area densities the carrier lifetime is decreased to a level below 100 ps.

Coherent Wireless Optical Communications With Predetection and Postdetection EGC Over Gamma–Gamma Atmospheric Turbulence Channels

Wireless optical communication systems with coherent detection are analyzed for Gamma-Gamma distributed turbulence channels. In addition to the shot noise, we consider the impacts of both turbulence amplitude fluctuations and phase fluctuations on the error performance. Error rate analyses of predetection and postdetection equal gain combining (EGC) are carried out. We derive the exact error rate expressions for predetection and postdetection EGC using a characteristic function method. In the case of predetection EGC, we also study the impact of phase noise compensation error on the error rate performance. It is shown that the error rate performance of predetection EGC is sensitive to phase noise compensation errors for both weak and strong turbulence conditions. In order to alleviate the impact of phase noise, postdetection EGC with differential phase-shift keying is introduced and analyzed. In addition, postdetection EGC is compared with predetection EGC in the presence of phase noise compensation errors, and it is found to be an effective alternative to predetection EGC with low complexity implementation.

Two-dimensional flow dynamics in digital microfluidic systems

A two-dimensional model for the fluid flow in a digital microfluidic system is introduced, and the results are compared to experimental data. Resistive flow effects based upon contact line forces, filler effects and shear forces are applied in the model. It is found that the induced vertical velocity components are critical to the overall motion, as velocity and pressure gradients, together with microdroplet surface deformations, can limit the desired horizontal velocity. These effects are particularly important for digital microfluidic systems characterized by higher Reynolds numbers.

Free-space detection of terahertz radiation using crystalline and polycrystalline ZnSe electro-optic sensors

We present an experimental investigation demonstrating the use of single-crystal and polycrystalline ZnSe for the detection of freely propagating terahertz (THz) radiation. It is found that polycrystalline ZnSe electro-optic sensors exhibit preferred crystallographic orientations and can provide high bandwidth detection capabilities. The polycrystalline grain size and orientation are shown to be important in detecting distortion-free THz wave forms. By proper choice of the polycrystalline sensor thickness, a THz detection bandwidth comparable to that obtained with a single-crystal sensor is demonstrated. The application of polycrystalline material for free-space electro-optic sampling (FS–EOS) permits the possibility of utilizing nonlattice-matched thin-film integrated THz FS–EOS sensors and generators.