T.K. Fong

CORD: contention resolution by delay lines

The implementation of optical packet-switched networks requires that the problems of resource contention, signalling and local and global synchronization be resolved. A possible optical solution to resource contention is based on the use of switching matrices suitably connected with optical delay lines. Signalling could be dealt with using subcarrier multiplexing of packet headers. Synchronization could take advantage of clock tone multiplexing techniques, digital processing for ultra-fast clock recovery, and new distributed techniques for global packet-slot alignment. To explore the practical feasibility and effectiveness of these key techniques, a consortium was formed among the University of Massachusetts, Stanford University, and GTE Laboratories. The consortium, funded by ARPA, has three main goals: investigating networking issues involved in optical contention resolution (University of Massachusetts), constructing an experimental contention-resolution optical (CRO) device (GTE Laboratories), and building a packet-switched optical network prototype employing a CRO and novel signaling/synchronization techniques (Stanford University). This paper describes the details of the project and provides an overview of the main results obtained so far.

Improving the dynamic range of a coherent AM analog optical link using a cascaded linearized modulator

An AM coherent analog optical link is experimentally demonstrated to achieve the highest spurious-free dynamic range reported to date for a coherent system by employing a linearized cascaded electro-optic modulator. The link gives a spurious-free dynamic range of 115 dB/spl middot/Hz/sup 2/3/, with a 34-dB reduction in third-order intermodulation distortion over conventional Mach-Zehnder modulators. With the linearized modulator, our experimental results are just 3 to 6 dB away from the fundamental limit of this link; the remaining penalty is mainly due to RF components.<<ETX>>

CORD-a WDM optical network: control mechanism using subcarrier multiplexing and novel synchronization solutions

Stanford University is implementing and investigating CORD, a 2.488 Gb/s//spl lambda/ WDM ATM packet-switched network experiment. CORD performs optical contention resolution at the receiver using optical switches and delay lines. CORD uses the multichannel subcarrier multiplexing (MSCM) technique to transmit the control channel and the payload data on the same wavelength and an ultra-fast clock recovery technique to synchronize the received control packets in every slot. We have experimentally shown that CORD incurs an MSCM penalty of 2.2 dB. In addition, we have demonstrated that the clock recovery scheme performs clock acquisition within 4 preamble bits with a penalty of 1 dB.

Linewidth-insensitive coherent AM analog optical links using semiconductor lasers

Coherent AM analog optical links are analyzed taking into account laser phase noise. With the optimum selection of the IF bandwidth, coherent AM links can be made essentially independent of the laser linewidth. Under shot-noise-limited conditions, the fundamental limit of the dynamic range (FLDR) for coherent links is 148 dB-Hz per 1 mW of the received optical power versus 151 dB-Hz/mW for conventional direct detection links. However, when the received optical power is less than 1 mW, coherent links can improve the dynamic range by some 7 dB as compared to direct detection links.<<ETX>>

Optical local area network technologies

To utilize the large bandwidth of optical fiber, optical LANs must employ architectures that fundamentally differ from current single-channel LAN architectures. With computer processor speeds continuing to grow exponentially and multimedia applications growing even faster, there is a strong need for higher-speed local area networks (LANs) that can handle the traffic generated by tomorrows LAN users. Optical fiber is well suited for high-speed traffic transport, but the busty nature of computer traffic and large number of users makes it difficult to utilize the fibers capacity in LANs. The incorporation of multiple payload channels in future LANs is seen as a necessity; WDM is a good candidate for achieving this. The rapidly improving optical component technologies allow more flexible WDM architecture designs for various emerging applications.<<ETX>>

Linewidth-insensitive coherent AM optical links: design, performance, and potential applications

The use of coherent detection in analog optical links offers several advantages over direct detection: improved receiver sensitivity, inherent frequency translation, and the ability to utilize angle modulation and separate wavelength division multiplexed (WDM) signals. In this paper, we investigate an externally modulated coherent AM optical link. We study the dynamic range of the coherent AM link, considering receiver noise, laser phase noise, laser relative intensity noise (RIN), and system nonlinearities. With proper selection of the receivers IF bandwidth, the coherent AM link can be made insensitive to the laser linewidth. For optical powers less than 5 mW, RIN of less than /spl minus/160 dB/Hz reduces the spurious-free dynamic range (SFDR) by less than 3 db with the use of a balanced receiver. The external modulator nonlinearity is the dominant nonideal effect; it reduces the SFDR by 5-19 dB from the theoretical limit for 100% modulation index. We compare the performance of the coherent AM link with that of a conventional direct detection link for two applications: point-to-point links and distribution networks. When the received optical power is less than 1 mW, the coherent link can provide higher SFDR than the direct detection link. Thus, coherent links are well-suited for long distance point-to-point links and FM video distribution systems. >

Linewidth-insensitive coherent optical analog links

Coherent optical analog links offer several important advantages over conventional direct detection links including the ability to separate wavelength-division multiplexed signals, frequency translation, and utilization of angle modulation formats. However, the wide linewidth of semiconductor lasers can cause substantial performance degradation of these links. This paper analyzes the signal-to-noise ratio and the dynamic range of amplitude modulated (AM) and frequency modulated (FM) coherent links, and compares them to direct detection links. It is concluded that a properly designed AM system is insensitive to laser linewidth. For optical powers less than 1mW, the performance of coherent AM links is better than that of direct detection links; for optical powers greater than 1mW, the performance of the two links is nearly the same. Coherent FM links have the potential to increase the SNR and the dynamic range by more than 10 dB as compared to direct detection and coherent AM links. However, FM links are potentially sensitive to laser linewidth, and require elaborate phase noise cancellation techniques when semiconductor lasers are used.

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 /spl mu/W. 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 /spl mu/W, 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/spl middot/Hz/sup 2/3/, link gain=-25 dB and noise figure=78 dB; this performance has been obtained with a received optical signal power of 85 /spl mu/W, and a local oscillator power at the photodetector of 228 /spl mu/W. 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,. >

Novel distributed slot synchronization technique for optical WDM packet networks

Recently, a number of projects aimed at the implementation of optical packet-switched network prototypes have been announced. A common and critical element of these networks is the need to achieve time-slot synchronization over the whole network. Stanford University is currently implementing CORD, a two-node optical wavelength-division multiplexing (WDM) packet-switched network testbed with a star topology, featuring all-optical receiver contention resolution by means of optical switches and delay lines. This paper describes a flexible, distributed digital slot synchronization technique for CORD which is robust and scalable.

CORD-A WDM optical network: design and experiment of fast data synchronization by pilot-tone transport

CORD is a 2.5-Gb/s//spl lambda/ wavelength division multiplexing (WDM) packet switched network experiment. We have experimentally demonstrated a pilot-tone transport for the high-speed data synchronization in CORD. The optical power penalty is minimized to about 1 dB by equalizing the frequency response of the payload data channel and optimizing the pilot-tone modulation depth. The main advantages of the pilot-tone transport are: 1) a simple transceiver structure; and 2) a fast clock recovery time (within 40 b, 16 ns).