Providers continue to look for new ways to diminish installation costs for these high-bandwidth fibre systems.
Major telecom carriers in Korea, Japan, Europe and the U.S. are installing high-bandwidth fibre connections to the home and network nodes (FTTx) based around the IEEE’s Gigabit Ethernet passive optical network (GEPON), the ITU’s broadband passive optical network (BPON), and the ITU’s gigabit passive optical network (GPON) standards.
The deployments offer high revenue opportunities for optical module vendors, system vendors, and national carriers investing in strategic IPTV and triple-play services for voice, video, and data. With widescale deployments estimated to grow to more than 50 million ports shipped by 2011, the market is highly competitive.
Significant challenges for the service providers of FTTx systems are installation costs and the amount of time for return on investment. Highend IPTV services must also compete with low-cost cable broadcast systems. A solution is to deliver entrylevel RF video broadcast over the 1550nm wavelength. This produces a competitive offering for broadcast video subscriptions over the fixed-line telecom network and a migration path onto higher-end IPTV services.
The network infrastructure consists of a central-office optical line terminal (OLT) that transmits and receives data together with broadcast RF video to an optical network terminal (ONT) located at the customer premises. Typically, 16-32 optical splits are achieved between the OLT and ONT, increasing the economy of scale and lowering the cost-per-customer.
THE FTTX NETWORK
The FTTx standards specify a transmit wavelength of 1310nm (upstream data), and a receive wavelength of 1490nm (downstream data) (Fig. 1). For GPON and BPON, an optional downstream RF video-streaming wavelength of 1550nm can be used. The three optical signals are converted into electrical signals by a combined laser and optical receiver called a triplexor. The triplexor interfaces with a transceiver laser driver and limiting amplifier to terminate the fibre connection.
Let’s look at the design challenges and solutions in producing modules with respect to data paths, video stage, module control, and diagnostics monitoring systems. It’s based on the Phyworks Triple-Play project.
MODULE DESIGN CHALLENGES
GPON standards compliance means meeting optical specifications for receiver sensitivity, low crosstalk, transmit mask margin, transmit burst timing, and more. These must be met over an industrial temperature range of -40 to +85°C. Modules also have to meet digital specifications, particularly the extended Digital Diagnostics Monitoring (DDM) requirement. This is mandated by the system suppliers and network operators.
To keep costs competitive in this high-volume market, ease of manufacturing is essential. Moreover, low power consumption becomes a rather crucial issue.
Phyworks has worked with FTTh module suppliers, equipment suppliers, and carriers in the development of integrated devices that address their design challenges. Recently, it produced a demonstration module to prove the effectiveness of the company’s approach.
At the heart of the system lies a burst-mode transceiver, the PHY2078, which combines a laser driver and limiting amplifier in a single chip (Fig. 2). The device uses a mixed-signal design.
The PHY2078’s low crosstalk is a factor in ensuring compatibility between RF video and digital data inside the triple-play module. Transmitter modulation control is based on a lookup table. This technique enables well-controlled extinction ratio setting over the temperature range of -40 to +85°C. The digital automatic-power control loop maintains average power independent of temperature, burst length, or laser aging. Also, a patented startup algorithm enables the APC (Automatic Power Control) loop to find the correct operating point in under 1µs.
On the receiver side, the company offers complementary transimpedance amplifiers (TIAs) for GEPON and GPON with high sensitivity: -33 and -29dBm, respectively, when implemented with PIN photodiodes. These TIAs can be combined with avalanche photodiodes to further increase optical receiver sensitivity.
System sensitivity can be compromised by crosstalk from transmitter to receiver. Care is taken to minimise crosstalk in both the PHY2078 chip implementation and demonstration board layout. The video-path receiver is implemented using a third-party amplifier. By optimising power and ground layout, minimal crosstalk from the digital transmit path is achieved.
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Digital Diagnostic Monitoring specifies that module supply voltage, temperature, laser bias current, transmit power, and receive power are monitored. For GPON triple-play module applications, video-receive power, video-output RF power, and videostage supply voltage are usually added to monitoring requirements.
To implement these monitors, the Phyworks optical module demonstrator includes an MCU that uses ADC and DAC information from the PHY2078, plus measurements taken by its own ADC. It then reports these values over an I2C interface.
The triplexor on the demonstrator module is based on the Phyworks PHY1097 transimpedance TIA.
CUTTING THE COSTS
The integrated nature of the PHY2078 enables a triple-play optical module to be implemented on a single-sided board, reducing both design and assembly costs.
During module production, parameters usually must be adjusted to ensure the correct optical performance of the unit. In first-generation laser-driver solutions, this meant physically changing resistors.
Using the PHY2078, all parameters are set using digital registers, which are then stored in nonvolatile memory. This implementation facilitates a fully automated test setup, as required for high-throughput, lowcost manufacturing.
MINIMISING POWER CONSUMPTION
Low power consumption in the optical module is important from several perspectives. First, in GPON systems, backup power must be provided to enable emergency calls in case of power outage. Backup power is expensive and lower power consumption reduces the amount of backup power that’s needed.
Second, power consumption translates into heat, which can have a negative effect on the lifetime of the system. Reducing the power consumption will extend operating life, and thus lower the cost of ownership.
Third, most GPON systems are placed in a “hardened environment,” since they’re going to be located outside. The addition of internally generated heat can mean that the laser will have to operate at much higher temperatures, which will severely impact efficiency. Lowering the power consumption will drop the temperature. Furthermore, it makes it possible for the laser to operate at a more efficient point.