Accuracy of optical angle estimator operating through the turbulent atmosphere

Performance limitations of optical detector arrays (ODA) are evaluated taking into account background radiation, quantum noise, and internal noises of the photodetector as well as the impact of the atmosphere on the optical wave propagation. It is shown that the accuracy of the ODA-based angle estimator rapidly decreases when the range exceeds a certain limit. Thus, it is possible to define the maximum range of the ODA. Behavior of the maximum range as a function of wavelength and of target radiation energy is investigated. Large maximum ranges are obtained at important wavelengths of 0.63, 3.83, and 10.6 microm. It is shown that the ODA performance is limited by the quantum noise for the small-target radiation energy and by the atmospheric turbulence and attenuation for the large-target radiation energy.     

Heterodyne detection through rain, snow, and turbid media: effective receiver size at optical through millimeter wavelengths

Both scattering and turbulence can effect the spatial coherence of short wavelength signals propagating through the open atmosphere. In this paper, the influence of forward scattering on heterodyne receiver performance is investigated, taking into account turbulence. It is shown that the effect of forward scattering is to reduce the effective heterodyne receiver area through spatial coherence degradation. A common approach to scattering as an attenuation phenomenon is not always valid. Generally, this approach underestimates the SNR. The accuracy of the attenuation approach depends on the ratio R of the actual receiver diameter to the scattering particle diameter. If R >100, scattering is essentially large angle and the typical treatment of scattering as an attenuation effect is indeed justified. However, for small R, forward scattering is primarily small angle, field coherence is noticeably affected by forward scattering, and the attenuation approach is not valid. Further, it is shown that the SNR is improved when the ratio of the scattering particulate size to turbulence coherence diameter decreases. From the practical point of view, the most important result of this study is that small receivers use their area more effectively than large receivers. Thus, an array of several small receivers may perform better than one large receiver with the same total area. The treatment here is particularly relevant for coherent detection through clouds, fog, precipitation, and turbid media in general, including liquid media.     

Nonlinear crosstalk and two countermeasures in SCM-WDM opticalcommunication systems

We investigate, theoretically and experimentally, crosstalk between wavelengths in subcarrier-multiplexed (SCM) wavelength-division multiplexed (WDM) optical communication systems. Crosstalk arises mainly from interactions between subcarriers on one wavelength and the optical carrier of another wavelength. In a dispersive fiber, crosstalk can be attributed to stimulated Raman scattering (SRS) and cross-phase modulation (XPM) combined with group velocity dispersion (GVD). We investigate the phase relationship between SRS-induced and XPM-induced crosstalks. Crosstalks induced by SRS and XPM add in the electrical domain and can interfere constructively or destructively. Experimental results show that the combined crosstalk level can be as high as 40 dBc after 25 km of SMF with two wavelengths and 18 dBm per wavelength of transmitted power. We propose two crosstalk countermeasures. The first countermeasure uses parallel fiber transmission. We show theoretically that both SRS-induced and XPM-induced crosstalks can be cancelled to the first order. We present an experimental demonstration of concept which has achieved 15 dB of crosstalk cancellation over 200 MHz. The second countermeasure uses optical carrier suppression. We show, theoretically and experimentally, that by suppressing the optical carrier, we can significantly reduce crosstalk while maintaining the same link budget and carrier-to-noise ratio (CNR) at the receiver, 20 dB of crosstalk reduction over 2 GHz has been demonstrated experimentally     

Experimental demonstration of an access point for HORNET-Apacket-over-WDM multiple-access MAN

Hybrid opto-electronic ring network (HORNET) is a novel packet-over-WDM multiple-access network designed by the Stanford Optical Communications Research Laboratory (OCRL) to provide efficient bandwidth sharing among a large number of access points (APs) in a metropolitan area. The HORNET network eliminates the cost and complexity of SONET equipment by transmitting IP/ATM packets directly over the wavelength division multiplexing (WDM) layer. To improve performance above that of a conventional ring network, HORNET employs a multiple-access architecture using fast tunable transmitters and a novel carrier-sense multiple-access with collision avoidance (CSMA/CA) media access control (MAC) protocol. The OCRL has constructed a testbed to demonstrate the ability of a HORNET AP to transmit packets using a fast-tunable transmitter and a novel MAC protocol and to asynchronously receive packets in the packet-over-WDM architecture. The experimental results confirm that HORNET successfully achieves the following functions: 1) fast (low overhead) wavelength tuning using a fast-tunable transmitter; 2) collision-free packet transmission over a multiple-access network via the CSMA/CA MAC protocol; and 3) fast clock and data recovery using the embedded clock tone (ECT) technique     

Cross-phase modulation in dispersive fibers: theoretical andexperimental investigation of the impact of modulation frequency

We investigated, theoretically and experimentally, the impact of modulation frequency on cross-phase modulation (XPM) in dispersive fibers. A simple expression for the XPM index has been derived and verified by experiment. The XPM index is found to depend on fiber length, fiber chromatic dispersion, wavelength separation between the signal and the pump, and the intensity modulation frequency. At high modulation frequencies, the XPM index is inversely proportional to the product of the modulation frequency and wavelength separation     

Experimental linewidth-insensitive coherent analog optical link

We constructed an experimental linewidth-insensitive coherent analog optical link. The transmitter utilizes an external electro-optic amplitude modulator and a semiconductor laser. The receiver consists of a heterodyne front-end, a wideband filter, square law detector and narrowband lowpass filter. We performed experimental measurements and theoretical analyses of the spurious-free dynamic range (SFDR), link gain and noise figure for both the coherent AM and the direct detection links; we investigated the dependencies of the foregoing parameters on the received optical signal power, laser linewidth, IF bandwidth, and the laser relative intensity noise (RIN). By selecting a wide enough bandpass filter, we made the coherent AM link insensitive to laser linewidth. The coherent AM link exhibits a higher SFDR than the corresponding direct detection link when the received optical signal power is less than 85 μW. The noise figure for the coherent link is greater than that for the direct detection link under all conditions investigated. For received optical signal powers greater than 4 μW, the link gain for the direct detection link is greater than that for the coherent AM link. The following are the link parameters that have been achieved for the coherent AM link investigated: SFDR=88 dB·Hz^2/3, link gain=-25 dB and noise figure=78 dB; this performance has been obtained with a received optical signal power of 85 μW, and a local oscillator power at the photodetector of 228 μW. The link performance can be further improved by auxiliary subsystems such as a balanced receiver and impedance matched transmitter and receiver ends; and/or by using better optical and electrical devices like higher power lasers, linearized optical modulators, low-noise and high gain RF amplifiers, and optical amplifiers,     

A summary of the HORNET project: a next-generation metropolitan area network

Metropolitan area networks are currently undergoing an evolution aimed at more efficiently transport of data-oriented traffic. However, the incoming generation of metro networks is based on conventional technology, which prevents them scaling cost-effectively to ultrahigh capacities. We have developed a new architecture and set of protocols for the next generation of metro networks. The architecture, named HORNET (hybrid optoelectronic ring network), is a packet-over-wavelength-division multiplexing ring network that utilizes fast-tunable packet transmitters and wavelength routing to enable it to scale cost-effectively to ultrahigh capacities. A control-channel-based media access control (MAC) protocol enables the network nodes to share the bandwidth of the network while preventing collisions. The MAC protocol is designed to transport variable-sized packets and to provide fairness control to all network end users. The efficiency and the fairness of the MAC protocol is demonstrated with custom-designed simulations. The implementation of the MAC protocol and the survivability of the network have been demonstrated in a laboratory experimental testbed. The article summarizes the accomplishments of the HORNET project, including the design, analysis, and demonstration of a metro architecture and a set of protocols. The HORNET architecture is an excellent candidate for next-generation high-capacity metro networks.     

Polarization-independent one-pump fiber-optical parametricamplifier

One-pump fiber optical parametric amplifiers (OPAs) can be rendered polarization independent by using a polarization-diversity technique. We have experimentally demonstrated a fiber OPA with peak signal gain of 9 ± 0.2 dB when the signal polarization angle was varied from 0° to 90°. Power penalty of less than 1 dB was measured in a 10-Gb/s transmission system     

SUCCESS-HPON: A next-generation optical access architecture for smooth migration from TDM-PON to WDM-PON

Optical access networks are considered to be a definite solution to the problem of upgrading current congested access networks to ones capable of delivering future broadband integrated services. However, the high deployment and maintenance cost of traditional point-to-point architectures is a major economic barrier. Current TDM-PON architectures are economically feasible, but bandwidth-limited. In this article we first discuss the possible role of WDM in access networks and investigate the associated issues. We then present the Stanford University Access Hybrid WDM/TDM Passive Optical Network (SUCCESS-HPON), a next-generation hybrid WDM/TDM optical access architecture that focuses on providing a smooth migration path from current TDM-PONs to future WDM-PONs. The first testbed for this architecture is described, along with the experimental results obtained, including feasibility of bidirectional transmission on the same wavelength on the same fiber for access networks and ONU modulation of upstream data on continuous waves provided by the OLT, eliminating the need for tunable components at the ONUs. The development of a second testbed and the issues it will address, including the implementability of the SUCCESS-HPON MAC protocol and scheduling algorithms, are also described.     

Sequential estimation of the position of an optical beam

The estimation of optical beam position by means of detector arrays is investigated. Both unsequential and sequential approaches are considered. Performance analysis is accomplished for both cases. It is shown that the sequential algorithm greatly reduces estimation error with the same average expenditure of the signal beam energy. The reduction is, however, obtained only in the case of shot-noise-limited estimation.