Innovation and cost reduction, as always, were headline themes at the annual Optical Fiber Communication (OFC) conference and exhibition, held in Los Angeles at the end of February. Out on the show floor, where a mood of cautious optimism prevailed, it was clear that fibre-optics vendors are gearing up to take advantage of the industry's new-found emphasis on areas like fibre-to-the-premises (FTTP), broadband services delivery and CWDM.
Encouragingly, over in the more rarefied atmosphere of the conference halls, there was also plenty of evidence suggesting that the industry's brightest and best have readjusted their development roadmaps to match the post-slump demands of the network operators. Optical amplification is a case in point. In a conference session devoted to alternative amplifiers, Gerlas van den Hoven of Genexis in the Netherlands emphasized how component and subsystem vendors have come up with a myriad of optical-amplifier designs to help telcos ramp performance out at the edge of their networks.
The costs of erbium-doped fibre amplifiers (EDFAs), the workhorses of long-haul and metro core WDM networks, have come down enough to permit deployment in metro edge and access systems. The trouble is, the roll-out of CWDM in such networks calls for amplifiers that work over a wavelength range of 1270-1610 nm (EDFAs only operate in the C- and L-bands between 1530 and 1610 nm). "Nature matched erbium to the low-loss window of optical fibre," said Van den Hoven. "Everything else will take a lot of engineering."
The good news is that metro and access networks generally have less stringent requirements on amplifier performance than long-haul systems, so researchers can turn to an alternative solution - the semiconductor optical amplifier (SOA). SOAs, which typically use III-V semiconductor compounds to generate optical gain, currently provide less amplification at 1550 nm than EDFAs. Their big plus, however, is that they can be engineered to amplify between 1200 and 1700 nm.
In a paper presented at the OFC post-deadline sessions, a Japanese collaboration from Fujitsu, the Optoelectronic Industry and Technology Development Association (OITDA) and the University of Tokyo detailed an SOA with what they say is a record bandwidth of 120 nm (1410-1530 nm). By using InP quantum dots as the active material, the researchers increased the bandwidth and ramped the usable output power to 23 dBm, claimed to be the highest reported value for any SOA.
Quantum dots also offer an ultrafast gain response, enabling the SOA to follow the intensity changes of 40 Gbit/s signals, as well as to suppress gain fluctuation. Other figures of merit include a gain of above 20 dB, a noise figure below 7 dB and a 3 dB saturation output power exceeding 19 dBm.
Other ways to shift the amplifiable bandwidth regions away from the C- and L-bands are to fabricate rare-earth-doped fibre amplifiers with dopants other than erbium (including thulium, praseodymium, neodymium and ytterbium) or to use non-silica fibre (such as fluoride, phosphate or tellurite glasses).
Combining both approaches, NTT Photonics Laboratories of Japan described a wideband rare-earth-doped fibre amplifier designed for eight-channel CWDM applications. NTT's amplifier comprises a hybrid EDFA-TDFA (thulium-doped fibre amplifier) arranged in parallel with an erbium-doped tellurite fibre amplifier (EDTFA).
The device amplifies eight channels, with four wavelengths (1470, 1490, 1510 and 1530 nm) amplified by the EDFA-TDFA and the remainder (1550, 1570, 1590 and 1610 nm) by the EDTFA section, before recombination via a WDM coupler. NTT tested the amplifier by transmitting eight signals through 2 x 100 km spans of singlemode fibre and two in-line amplifiers. The set-up achieved an average gain of 22.5 dB with a gain variation of 2 dB between 1463-1537 and 1543-1617 nm.
Technology drivers
In his overview presentation, Van den Hoven explained how optical amplification also acts as an enabler for the development of integrated components, turning high-insertion-loss devices into lossless ones. He cited single-chip amplifiers such as SOAs and erbium-doped waveguide amplifiers as suitable for integration, although he said that issues such as noise and power consumption may limit widespread application at present.
Van den Hoven concluded by looking at the emergence of low-cost devices such as linear optical amplifiers and gain-clamped SOAs, which are currently being developed to meet the cost and space constraint of metro and access networks. "There are many amplifier technologies, and each has its merits. The trick is to match each technology to the application," he said.
CWDM also featured prominently in a conference session covering planar lightwave circuits (PLCs). Shin Kamei from NTT Photonics Laboratories presented details of an eight-channel CWDM filter based on an arrayed waveguide grating (AWG) with multimode waveguides.
Kamei explained that CWDM systems require inexpensive and compact mux/demux filters with low loss and a wide passband. PLC-based filters such as AWGs are both mass-producible and compact, but their performance can be inferior to other designs. "With conventional AWGs there is a trade-off between loss and bandwidth," he said.
Kamei elaborated: "The spectral response of an AWG is determined by the coupling between the focusing field on the imaging plane and the eigenmodes of the output waveguide. To widen the AWG passband, the coupling efficiency must be kept almost constant when the position of the focusing field is within a certain range of the plane." But in a standard AWG, keeping the coupling efficiency constant requires the focusing field to be distorted, which causes field mismatching and intrinsic losses.
NTT addressed this issue by replacing the singlemode output waveguides in conventional AWGs with multimode waveguides. In the multimode version, the total coupling efficiency is 100% provided the position of the focusing field is within the range of the multimode waveguide width. "The multimode-waveguide AWG realizes a wide passband with no intrinsic loss," said Kamei.
The researchers fabricated a 20 nm-spaced eight-channel AWG filter for unidirectional CWDM systems. The AWG acts as a demultiplexer with the multimode output waveguides connected to receivers via multimode fibre. High-index-contrast (Δn = 1.5%) waveguides minimized the chip size to 8 x 40 mm. The device achieved a flat and wide passband for every channel with insertion losses of less than 1.7 dB, a 1 dB bandwidth of above 14 nm and a crosstalk below -29 dB for adjacent channels.
The researchers also fabricated a version for use as a mux/demux at either end of a bidirectional CWDM system. This AWG has a 2 x 9 port configuration in which the odd output ports are singlemode waveguides for multiplexing and the even output ports are multimode waveguides for demultiplexing.
This bidirectional device exhibited insertion losses of below 1.6 or 4.3 dB, and a 1 dB bandwidth of more than 13.5 or 5 nm, for the multimode and singlemode ports respectively. The crosstalk between adjacent ports was less than -35 dB (multimode) and below -40 dB (singlemode). "We have demonstrated that multimode-waveguide AWGs are suitable and attractive filters for application to general CWDM systems," said Kamei.
Other presentations in the PLC session saw the technology applied to devices for use in FTTP networks, particularly passive-optical networks (PONs) that deploy passive splitters to share optical bandwidth between multiple users. The compactness and ease of mass production of PLC-based components make them ideal for use in broadband PONs.
Researchers working in NTT's Photonics Laboratories and Access Network Service Systems Laboratories described low-loss silica-waveguide WDM filters for video-distribution services. In line with the ITU-T specification G.983.3 (which sets the wavelength bands for video-distribution services and ATM–PON transport services as 1550-1560 and 1260-1500 nm, respectively) the NTT team fabricated 1260-1500/1550-1560 nm WDM filters, both as single-channel and eight-channel arrays.
The WDM filters comprise silica-on-silicon (Δn = 0.3%) cross-waveguides, with a 30 μm thick dielectric multilayered thin-film filter inserted into a groove formed at the waveguides' intersection. When connected with fibres and packaged, the single-channel device was 35 mm long with a 5 mm diameter, and the eight-channel version measured 60 x 8 x 6 mm.
The optical performance of both WDM filters meets the system performance that is described in G.983.3. The eight-channel device, for example, exhibits insertion losses of below 0.82 dB in the 1260-1500 nm range and below 0.66 dB between 1550 and 1560 nm, as well as high isolation of more than 42 dB at 1550-1560 nm.
The researchers claim that for the first time, the optical performance of a PLC-type filter is comparable to that of bulk-type filters. And according to the research paper, "these compact and high-performance filters will enable us to provide cost-effective video distribution services in FTTP systems".
Counting the costs
Given all the talk about FTTP deployments in North America at the show, it was no surprise to find that PONs were also a key theme in two conference sessions dedicated to access networks, with several speakers examining ways to implement these set-ups at lower costs. For starters, the network research team at Samsung Electronics in Korea described the use of Fabry-Perot laser diodes to transmit high-definition (HD) TV and Ethernet data over a WDM-based PON.
Uncooled Fabry-Perot laser diodes have been touted as a low-cost option for WDM-PON sources, but previously had issues with thermal reliability and high injection power requirements. Samsung developed an amplified spontaneous emission (ASE)-injected Fabry-Perot laser diode that it says delivers good bit-error-rate performance over a wide temperature range with low injection power.
To maximize performance, the Samsung researchers modified the laser diode to exhibit narrow mode spacing, low reflectivity at the front facet and high reflectivity at the rear facet. In a WDM-PON testbed serving two subscribers, the device successfully demonstrated video-on-demand delivery with two HDTV channels (2 x 19.39 Mbit/s) and a fast Ethernet connection (100 Mbit/s) over a 20 km uplink and downlink.
At -17 dB injection power, the laser diode exhibited error-free transmission over 25 km at 155 Mbit/s, across a temperature range of 70 °C. This low injection power requirement means that low-cost ASE sources such as SOAs could be employed.
Elsewhere, a team from the COBRA Institute at the Eindhoven University of Technology in the Netherlands presented details of a high-capacity multiservice in-house network. COBRA employed a technique called mode-group diversity multiplexing to integrate multiple services (such as cable TV, voice and high-speed data) onto one multimode fibre network. The authors say that this method offers the same functionality as wavelength multiplexing but at a lower cost.
In theory, using all of a multimode fibre's modes to transport information could greatly increase its capacity. But, in practice, modal dispersion severely limits the link bandwidth. To address this issue the COBRA researchers used selective-mode launching, which excites less than half of the fibre modes and enables mode-group diversity multiplexing to be used without the problems of mode mixing.
For low-cost implementation, COBRA proposed using an integrated array of vertical-cavity surface-emitting lasers to launch each data signal into a different group of modes. At the receiver end, an array of integrated photodetectors coupled to the fibre's large core performs spatially resolved detection of the mode groups. The researchers reckon that several classes of services can be carried over a single in-house infrastructure, including Gigabit Ethernet user terminals and broadband wireless LAN terminals.
• This article originally appeared in FibreSystems Europe in association with LIGHTWAVE Europe April 2004 p19.