Linearisation of a cascaded system of analogue optical links

We describe a linearisation technique suitable for compensation of nonlinearities in a cascaded system of analogue optical links. Experimentally, we have observed an improvement of more than 30 dB in 2nd-order distortion, reducing distortion to ?53 dB in a cascaded system of four optical transmitter/receiver modules.     

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,     

Multigigabit multichannel optical interconnection module forbroadband switching system

This paper describes optical transmitter and receiver modules for package-to-package interconnection in broadband switching networks such as an asynchronous transfer mode switch fabric. These modules, which include the multiplexer and demultiplexer, can reduce the number of connections and the problem of skew between links. Five-channel optical transmitter and receiver modules were fabricated and demonstrated at 2.8 Gbit/s with a power dissipation of 4.5 W per channel. Moreover, temperature-insensitive optical interconnection was successfully demonstrated by driving a laser with a constant bias current over the threshold and by deducting the optical signal offset. The output power of the transmitter module was -4.2 dBm. Nonuniformity of the transmitter output powers across the range of optical channels was <2.1 dB. Receiver sensitivity for a bit error rate of 10^-11 was -9.3 dBm. Nonuniformity of the receiver sensitivities was <1.5 dB. The power penalty of the receiver sensitivity due to crosstalk was 1 dB. The connection distance was >250 m     

Experimental characterization of integrated optical wireless components

Line of sight optical links can provide extremely high bandwidth communications between terminals, but in order to maintain alignment between transmitter and receiver, tracking is required. In this letter, we report results from a "solid-state" tracking transmitter and receiver. The transmitter consists of a custom complementary metal-oxide-semiconductor (CMOS) integrated circuit that is flip-chip bonded to a seven-element resonant cavity light-emitting diode. The receiver uses a custom seven-element InGaAs detector array that is flip-chip bonded to a CMOS integrated circuit. Results from an initial link demonstration show overall system operation at 100 Mb/s/channel, for Manchester coded data.     

Principles and design of bidirectional optical isl using GaAlAs laser at λ = O·85 μm and direct detection PPM

This paper gives a general introduction to bidirectional optical intersatellite links. The design, principles and link budget are presented, and the four subsystems—transmitter, receiver, optical channel and acquisition/tracking system—are described. GaAlAs semiconductor laser and a silicon avalanche photodiode are used as the optical transmitter and receiver. Pulse polarization position modulation is used to obtain a higher sensitivity. The acquisition/tracking signal is extracted from the top-modulation data stream. The effect of background noise is analysed, and direct detection and heterodyne detection are compared.     

New type of optical fibre analogue SSB data communication system using edge‐emitting LED coupled to single‐mode optical fibre

This paper describes a single‐mode(SM) optical fibre SSB data communication system for use in place of power‐line data transmission. In the optical transmitter, SSB data is fed through the squarewave frequency (SWFM) modulator to the edge‐emitting LED (ELED) module. The light beam of the ELED module is fed to the SM optical fibre having a length of 12km. In the optical receiver, the light beam received through the SM optical fibre is detected by the PIN photodiode module, and fed to the SWFM detector to obtain the double‐pulse frequency modulation (DPFM) signal voltage. The DPFM signal voltage at the output of the SWFM detector is fed to the low‐pass filter (LPF) to obtain analogue SSB data. When the transmission was experimentally carried out, the spurious component was as low as −50–−70 dB and the SNR of SSB data as high as more than 65–70 dB. The optical transmitter and receiver were attached to the conventional analogue SSB data communication system to constitute a new optical fibre analogue SSB data communication system. During the field test utilizing the new system, the crosstalk and SNR were 60–70 dB and 60–70 dB on each channel, respectively. Copyright © 1999 John Wiley & Sons, Ltd.     

Monolithic millimeter wave optical receivers

A single stage monolithic millimeter wave optical receiver circuit was designed and fabricated using a metal-semiconductor-metal (MSM) photodetector and a pSeudomorphic Modulation Doped Field Effect Transistors (SMODFET) on a GaAs substrate for possible applications in chip-to-chip and free space communications. The MSM photodetector and the SMODFET epitaxial material were grown by molecular beam epitaxy (MBE). Device isolation was achieved using an epitaxially grown buffer between the MSM detector layers and SMODFET. The photodetector was designed for maximum absorption at optical wavelength of 770 nm light and the SMODFET impedance matching network was optimized for 44 GHz. The monolithic millimeter wave optical receiver circuit achieved 3 dB gain over a photodetector at 39 GHz, which was the limit of the measurement system. TOUCHSTONE model of the circuit indicated 6.6 dB gain over the photodetector and 5.7 dB total gain including the insertion loss of the photodetector at 44 GHz     

Optical backplane system using waveguide-embedded PCBs and optical slots

As discussed in this paper, a practical optical backplane system was demonstrated, using a waveguide-embedded optical backplane board, processing boards, and optical slots for board-to-board interconnection. A metal optical bench was used as a packaging die for the optical devices and the integrated circuit chips in both the transmitter and the receiver processing boards. The polymer waveguide was produced by means of a hot-embossing technique and was then embedded following a conventional lamination processes. The average propagation loss of these waveguides was approximately 0.1 dB/cm at 850 nm. The dimension and optical properties of the waveguide in an optical backplane board were unchanged after lamination. As connection components between transmitter/receiver processing boards and an optical backplane board, optical slots were used for easy and repeatable insertion and extraction of the boards with a micrometer-scale precision. A 1/spl times/4 850-nm vertical-cavity surface-emitting laser array was used with 2 dBm of output power for the transmitter and a p-i-n photodiode array for the receiver. This paper successfully demonstrates 8 Gb/s of data transmission between the transmitter processing board and the optical backplane board.     

Line strip spot-diffusing transmitter configuration for optical wireless systems influenced by background noise and multipath dispersion

In this letter, we propose and evaluate a novel optical wireless configuration that employs a multibeam transmitter in conjunction with a narrow field-of-view direction diversity receiver. Such a configuration overcomes the drawbacks and combines the advantages of both types of optical wireless links, including line-of-sight and diffuse transmission. A multibeam transmitter placed on the communication floor was adopted to produce multiple diffusing spots on the middle of the ceiling in the form of a line strip. The design goal is to reduce the effect of intersymbol interference and to improve the signal-to-noise ratio (SNR) when the system operates under the constraints of background noise and multipath dispersion. Simulation results show that our line strip multibeam transmitter (LSMT) with only three branches diversity gives about 23 dB SNR improvement over the conventional system. The results also show that the multipath dispersion, which induces pulse spread, is significantly reduced when the LSMT with diversity detection is used.     

8.4--A 3.39-micron infrared optical heterodyne communication system

An optical heterodyne communication system is described which employs a separate stable laser local oscillator at the receiver. The theoretical advantage of quantum-limited reception has been realized, demonstrating an improvement in receiver sensitivity of more than 40 dB over that of a conventional photodetector receiver. The fundamental sources of noise in the system are identified as laser oscillator frequency noise, atmospheric phase noise, atmospheric amplitude noise, and quantum noise. The quantitative characteristics of these noise sources are analyzed as they influence the operation of AM and FM laser communications.     

ASK heterodyne receiver sensitivity measurements with two in-line 1.5 ?m optical amplifiers

A heterodyne amplitude shift keying configuration was used to evaluate the performance of resonant-type optical amplifiers in a coherent lightwave communication system. With two in-line amplifiers, each with a gain of 25 dB, the total loss between the transmitter and the receiver was 95 dB at 10-9 bit error rate. At a 100 Mbit/s data rate, the receiver sensitivity without amplifiers was -49.3 dBm. With two amplifiers the receiver sensitivity could be maintained at -49.0 dBm.     

Frequency division multiplexed optical networks using heterodynedetection

The optical heterodyne process is described. Because the best attainable system gain (transmitter power divided by receiver sensitivity) is only about 50 dB for lightwave systems, as compared with values approaching 100 dB at radio frequencies, it is extremely important in building an optical network (such as a local area network, or LAN) to minimize excess tap losses. It is shown that a star coupler provides a nearly ideal means for interconnecting a multiterminal network. Three areas in which problems unique to optical systems have been discovered are discussed. Theses are transmitters for coherent optical systems, optical frequency determination and control, and polarization control and optical receivers. Experimental progress is briefly discussed     

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 receiver's 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 -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     

Adaptive detector arrays for optical communications receivers

An optimal adaptive array receiver for use in groundbased optical communications is investigated. Kolmogorov phase screen simulations are used to generate realistic focal-plane distributions of the received optical fields in the presence of turbulence. The array detection concept reduces interference from background radiation by effectively assigning higher confidence levels at each instant of time to those detector elements that contain significant signal energy and suppressing those that do not. A simpler suboptimum structure that replaces the continuous weighting of the optimal receiver by a hard decision over each detector element is also described. It is shown that, for photon counting receivers observing Poisson distributed signals, performance improvements of up to 5 dB can be obtained over conventional single-detector photon counting receivers when observing turbulent optical fields in high background environments.     

Passively assembled optical interconnection system based on an optical printed-circuit board

We propose a passively assembled chip-to-chip optical interconnection system using fiber-optic technology. To demonstrate the system, three components were prepared: a fiber-embedded optical printed-circuit board (OPCB), optical transmitter/receiver modules, and 90/spl deg/-bent fiber connectors. All components were assembled using precise guide pins and holes so that complete passive alignment was achieved in the OPCB. An optical link of 5-Gb/s/ch signals with a total link loss of -1.5 dB has been successfully demonstrated from the assembled system.     

Gigabit receiver ICs for optical communications

The authors discuss gigabit receiver ICs for optical communications, focusing on their circuit and package design, the performance of receivers that were fabricated, and their application to a 1.6 Gb/s optical receiver. The key technologies for the receivers are discussed, and a design based on these key technologies is proposed. The proposed design is used to fabricate six receiver ICs (eight chips) using an ultra-high-speed bipolar process with transistors having a unity gain bandwidth of 6-8 GHz. The receivers are suitable for long-haul optical transmission at bit rates up to 1.6 Gb/s. Experimental results show that the 1.6 Gb/s receiver has an optical dynamic range of more than 23 dB without any adjustment, and the received average optical power required to maintain a 10^-11 error rate is less the -31 dBm     

Multiple spot diffusing geometries for indoor optical wireless communication systems

In order to improve the performance of indoor optical wireless communication links, two multispot diffusing geometries based on diamond and line strip spot distribution geometries are proposed, analysed and compared to the known uniform spot distribution. Such geometries combine the advantages of the diffuse and the line‐of‐sight systems, giving great robustness and ease of use. The novel line strip multibeam transmitter geometry has resulted in a receiver signal‐to‐noise ratio (SNR) improvement of about 4 dB compared to the conventional diffuse system as well as a significant reduction in the pulse spread. Simulation and comparison results for both the conventional diffuse system and the three multispot diffusing geometries are presented. Further, pulse responses, SNR, and delay spread results at various locations are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Sampled-grating DBR laser-based analog optical transmitters

Sampled-grating distributed Bragg grating (SGDBR) laser-based widely tunable optical transmitters are investigated for application in high-performance analog links. More than 45 nm tuning range, 40 dB sidemode suppression ratio, and peak relative intensity noise below -153 dB/Hz is measured. SGDBR lasers integrated with semiconductor optical amplifiers and electroabsorption modulators (EAMs) are characterized with spurious free dynamic range of 125-127 dB/spl middot/Hz/sup 4/5/ over the wavelength tuning range. It is also shown how the modulation response of the EAM is affected by the optical power to limit the performance of the analog transmitter.     

Analysis of diffuse optical wireless channels employing spot-diffusing techniques, diversity receivers, and combining schemes

In this letter, the performance of an indoor optical wireless spot-diffusing system using various multibeam transmitter configurations, in association with direction diversity and combining techniques, is assessed and compared under the impact of multipath dispersion and ambient light noise through theoretical analysis and computer simulation. Computer simulation for three different multibeam transmitter configurations and a conventional diffuse transmitter is carried out. Diversity receiver and wide field-of-view (FOV) receiver configurations are evaluated in conjunction with the proposed configurations. For the diversity-detection case, a receiver comprising an array of narrow FOV detectors (three and seven segments) oriented in different directions is used to maximize the collected signals and minimize noise. A novel line-strip multibeam system (LSMS) is investigated with single and diversity receiver configurations, and is compared with other spot-diffusing methods. Combining schemes, including selection combining, maximum ratio combining, and equal gain combining are employed for the presented configurations. Our results indicate that the performance improvement obtained through the use of LSMS with a three-direction diversity receiver is about 20 dB signal-to-noise ratio enhancement over the conventional diffuse system, and 26 dB when combining techniques are used. Root mean square delay-spread performance for the proposed configurations, at different positions on the communication floor, are also evaluated and compared.     

A low-cost multichannel optical transmission system for videosignals

A novel low-cost multichannel optical transmission system for video signals is described. The system uses time-division multiplex pulse-position modulation (TDPPM). The transmitter and receiver are realized in a standard bipolar process while an external low-cost laser and PIN photodiode are used. A maximum of 16 channels can be multiplexed, while achieving a weighted signal-to-noise ratio (SNR) of 54 dB, a differential gain of 1% and a differential phase of 0.5°. For a launched power of 0 dBm (pulse amplitude), the optical budget for a 45 dB weighted SNR exceeds 20 dB. Besides video signals the system is also capable of handling other signals such as digital coded audio signals or computer data. The feasibility of a four-channel system has been confirmed by measurements