Part III Optical Communications

Transcription

1 Part III Optical Communications Gong-Ru Lin and Yin-Chieh Lai Introduction The earliest history of optical communication technologies can be dated back to ancient times when smoke and beacon fires were used for signaling attacks from the enemy. One historical example is the ancient defensive system of the Great Wall in China, where beacon towers are built around every 5 km to observe enemies and to send/transmit messages. It is commonly believed that the oldest beacon towers actually appeared earlier than the Great Wall and can be dated back to the West Zhou Dynasty (Eleventh Century BC 771 BC). When enemy attacks were observed, alarm messages were sent by releasing smoke in daytime or by lighting fires at night. The amount of smoke or the number of fires indicated the size of the enemy. In modern terminology, this ancient system is an optical wireless communication system which utilized an incoherent light source (sun or firelights) with the intensity modulation scheme aiming for propagation distance around 5 km. The equivalent data rate was of course very low. Other visual techniques like hydraulic telegraphs, ship flags, and semaphore lines were also the earliest forms of optical communication. The modern break-through in optical communication occurred when A.G. Bell invented and demonstrated the first photophone in It was still an optical wireless system which utilized an incoherent light source (initially sunlight and later arc lamps). The modulation scheme is by using a flexible mirror attached to a speaking tube. The mirror oscillates between convex and concave when there are voices, encoding the audio vibration onto the optical beam divergence. The receiver was a parabolic mirror with a photoconductor (the selenium cells) placed at its focal point. The optical beam divergence modulation signal is converted into the electrical amplitude modulation signal due to the altering amount of light incident on the photoconductor through the optical setup. The equivalent data rate was now the audio rate and the achieved propagation distance is about 200 m initially and eventually up to 14 km by With the concept of low loss optical fibers proposed in 1966 by C.K. Kao and practically demonstrated in 1970 by Corning, the early commercial optical fiber telephone

2 102 Part III: Optical Communications systems were successfully developed around The system was now an optical wire system which utilized a coherent light source (GaAs semiconductor laser around the wavelength of 850nm). The graded-index multimode fiber is used as the transmission medium and the employed modulation scheme should be the fundamental digital intensity modulation scheme (on-off keying). The data rate was of the order of 45 Mb/s and the optical propagation distance (or the repeater spacing) was up to 10 km. In the 1980s, the light source for fiber communication was switched to InGaAsP semiconductor lasers around the wavelength of 1300 nm to reduce fiber loss and single-mode fiber was used to eliminate modal dispersion for achieving longer propagation distance. The bit rates were now of the order of Gb/s with repeater spacing up to 50 km. In the 1990s, with the availability of Erdoped fiber amplifiers and wavelength-division multiplexing (WDM) technologies, the optical wavelength for long-distance fiber transmission was shifted to wavelength around 1550 nm. The dispersion-shifted fiber was developed to reduce fiber dispersion at this wavelength. By properly engineering the dispersion map of the fiber link for reducing the impacts of fiber nonlinearity, the maximum optical propagation distance was extended long enough to across the biggest ocean with use of Er-doped fiber amplifiers to compensate for fiber loss at every 10 km. The maximum system capacity was multiplied using the WDM and other multiplexing technologies up to the Tb/s level with reasonable long propagation distance. Continual development in the 2000s has further increased the possible system capacity to the tens Tb/s level with the use of advanced optical modulation formats like differential phase shift keying (DPSK), quadrature amplitude modulation (QAM), and orthogonal frequency division multiplexing (OFDM) for better performance and higher spectral efficiency. In particular, the coherent optical OFDM modulation scheme allows the direct/adaptive compensation of various channel linear/nonlinear responses through digital signal processing techniques to achieve greater flexibility and better performance. In 2010, with the recent space division multiplexing (SDM) techniques that utilize multi-core or multimode fibers for another multiplexing degree of freedom, net transmission rates in the hundreds Tb/s level per fiber have been made feasible. These heroic optical transmission achievements clearly indicate that we have already developed systematic methods and efficient technological solutions to utilize the wide bandwidth offered by optical fibers. Besides the goal of achieving the highest data rates at longer propagation distance, which mainly aims at core network applications, optical communication technologies are also driven by another important goal of achieving cost-effective high data rates at shorter propagation distance, which mainly aims toward access network applications. The passive optical network (PON) structure has been well accepted as the solution for fiber access networking systems. To achieve higher data rates, coarse wavelength-division multiplexing (CWDM) or even dense wavelengthdivision multiplexing (DWDM) technologies will also be needed in access networks. Since the cost issues become more dominant when the technologies are brought closer to end users, cost-effective optical communication solutions have become the R&D focus for access network applications. Among the various

3 Part III: Optical Communications 103 challenges that can be investigated, cost-effective light sources for CWDM/DWDM PON systems are certainly much desired. Distributed feedback (DFB) semiconductor lasers have been the mature light sources for optical communication. The possibility of using other lower cost light sources for fiber communication systems has been continually investigated in many research studies. Some notable examples include the various proposals of utilizing light emitting diodes (LED), amplified spontaneous emission (ASE) broadband sources, semiconductor optical amplifiers (SOA), as well as injection-locked Fabry-Perot (FP) semiconductor lasers for transmitting optical data with the propagation distance of the km order. On the other hand, optical communication technologies also become more important for applications that require high data rates at even shorter propagation distances. Optical fiber cables for ethernet and high speed USB links have been commercially available. Parallel optical fiber transmitter/receiver modules for computer boardto-to-board connection have also been practically used. For these applications, the propagation distance is of the order of meters and the data rate is at the level of 1 10 Gb/s per fiber. Vertical cavity surface emitting lases (VCSEL) around 800nm are typically employed with the use of multi-mode fibers for reducing the packaging cost. In these applications, the cost-effective VCSEL and other surface emitting type laser sources also play a central role for related technology development. In recent years, optical wireless communication for in-building applications emerges as another R&D focus. The propagation distance is of the 1 10 m order with the free space as transmission medium. This development trend is mainly motivated by the success of radio wireless communication for networking applications. Among the possible approaches, visual light communication (VLC) has attracted a lot of research interest mainly because the possibility of using lighting LEDs for transmitting data potentially offers a new cost-effective light source for new optical communication applications. The light source is now an incoherent one and the channel bandwidth is severely limited by the LED modulation bandwith. The challenges are thus how to cost-effectively achieve high enough data rates under severe modulation bandwith constraints. Coding and signal processing techniques are the main tools that can be explored to boost the performance, besides the improved design of the LED modules. It is also possible to explore the use of cheap visible lasers for transmitting optical wireless data. The modulation bandwith constraint can be greatly relaxed in this way, although the possible combination with lighting applications has been sacrificed. These proposals may eventually lead to whole new applications of optical communication in our daily life. In view of the above technology trends, in this chapter we present some of our recent research results in the following three focus area: (1) Visual light communication based on lighting LEDs (2) Visual light communication based on laser pointer lasers (3) Colorless laser diode sources for DWDM PON applications The emphases are on the development of innovative approaches including new optical sources and new transmission schemes for solving emergent optical communication problems. The main contents are summarized below:

4 104 Part III: Optical Communications (1) Visual light communication based on lighting LEDs LED-based VLC offers many transmission advantages, making it a promising technology for future short-range communication applications. A common advantage offered by VLC is that the visible electromagnetic spectrum is not regulated (license-free), which makes it relatively easy to develop new products for VLC. Most importantly, LED-based VLC can also be further integrated with lighting/signaling systems to develop innovative optical communication solutions. We first discuss the VLC R&D activities that are happening in the world. Then we discuss important technological aspects of VLC, including the main challenges and possible solutions. Topics to be covered include enhancing the transmission data rate of VLC, mitigation of optical background noise, achieving bidirectional transmission, and finally, using AC-LED for VLC. (2) Visual light communication based on laser pointer lasers We provide an overview of modern VLC systems based on laser pointer lasers (LPL) that can offer higher transmission rate and longer free-space link than those based on high-brightness LED (HB-LED), red-green-blue (RGB) LED, and phosphor-based LED. The methodologies used to improve the LPL freespace transmission are theoretically discussed. With the simultaneous assistance of the preamplifier and adaptive filter, the amplitude and phase errors can be nicely compensated. The signal-to-noise ratio (SNR) and bit error rate (BER) of the systems can also be further improved. In addition to the discussion of VLC communication systems, the feasibility of integrating various bidirectional passive optical fiber networks with the LPL free-space VLC transmission schemes are also explained. These integrated transport systems are shown to be distinguished not only because of the simplicity in hybrid integration of PON and VLC applications, but also because of the convenience for installation. (3) Colorless laser diode sources for DWDM PON applications The weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) injectionlocked by three master sources with different degrees of coherence, and directly modulated by either the on-off-keying (OOK) at up to 10 Gbit/s or the 16 quadrature-amplitude-modulation (QAM) orthogonal frequency division multiplexing (OFDM) at 12.5 Gbit/s are demonstrated. The injection master coherence shows impacts on noise, bandwidth, and bit-error-rate (BER) performances of the slave WRC-FPLD transmitter. By using the highly coherent master, the injection-locked WRC-FPLD reduces its relative intensity noise peak at 5 GHz by 18 db and increases its throughput frequency response by 5 db. This enhances the signal-to-noise ratio from 10.5 db to 18.9 db and the on/off extinction ratio from 10.4 db to 11.4 db, enabling the error-free OOK transmission at 10 Gbit/s with a requested receiving power sensitivity of 15 dbm at BER of Simulated eye diagrams from modified rate equations are also performed to show such improvements. With highly coherent injection-locking, the back-to-back transmitted BER of the WRC-FPLD carried 16-QAM OFDM data with a modulation bandwidth of 3.1 GHz can be minimized to

5 Part III: Optical Communications 105 In comparison, the WRC-FPLD injection-locked WRC-FPLD pair with partial coherence can also provide an 8-Gbit/s OOK with a receiving power sensitivity of 19 dbm at BER of Such a cost-effective colorless source also supports a 12.5-Gbit/s 16-QAM OFDM transmission with a BER of to serve multi-channel passive optical network in next generation.