A key component of any optical fiber network is the laser that transmits information down the fiber. Unlike the silicon-based electronic circuits that control the data flow through the network, these lasers are made from semiconductor materials other than silicon, which is a poor light-emitter. This makes integrating lasers with silicon electronic circuits cumbersome and expensive, and so reducing the number of lasers in the network could substantially lower the cost of connecting users to the internet.
Optics Express - 10Gbps monolithic silicon FTTH transceiver without laser diode for a new PON configuration
A new passive optical network (PON) configuration and a novel silicon photonic transceiver architecture for optical network unit (ONU) are proposed, eliminating the need for an internal laser source in ONU. The Si transceiver is fully monolithic, includes integrated wavelength division multiplexing (WDM) filters, modulators (MOD) and photo-detectors (PD), and demonstrates low-cost high volume manufacturability.
7 page paper
One widely adopted scheme for reducing the number of expensive lasers in the network is to transmit data to multiple homes at once using a single laser, with a transmission protocol ensuring that the correct data packet is sent to the correct user. Yet although this configuration reduces the number of lasers considerably, each connected household still needs a laser to send data back the other way.
The network architecture proposed by Zhang and his co-workers eliminates the laser at the consumer end. Instead, they propose using two strands of optical fiber: one to transmit data to the consumer as usual and another to send a continuous laser beam to all linked consumers. An integrated silicon chip at the consumer end picks up the incoming continuous laser beam, encodes it with the signal intended for back transmission, and then redirects this laser beam back to the internet provider. “Fiber is cheaper than lasers, particularly as it can be used for more than 20 years once it is installed,” says Zhang.
In their experiment, the researchers also demonstrated the practical viability of this scheme for the operation of commercial fiber-optic networks. They fabricated an integrated silicon circuit for this task and have already achieved successful operation at speeds of up to 10 gigabits per second. “Given the cost benefits, these transceiver devices may significantly accelerate the deployment of optical fiber networks,” says Zhang. “Our work has attracted serious commercial interest for collaboration on the development of silicon photonic transceivers.”
GPON physical layer accepts single fiber and dual fiber based network structures. In the single fiber network, transceivers with bidirectional optical sub-assembly (BOSA) are used, while transceivers with separate transmitter optical sub-assembly (TOSA) and receiver optical sub-assembly (ROSA) are used in dual fiber network. Single fiber network with BOSA based transceivers is preferred by industry. In the proposed network topology, ONUs share one centralized light source from the OLT unit, the transceiver’s function in each ONU can be monolithically integrated on one silicon chip. By adopting this configuration, the cost structure of the transceiver will be fundamentally changed and the cost of each ONU will be significantly decreased. Although the fiber cost will be doubled in this topology, the fiber cost is a one-time investment in the network, and fibers last for tens of years. Compared to the fiber, the ONUs change or upgrade more frequently. Therefore, much lower cost at ONUs will be definitely more attractive than less fibers.
A new PON configuration is proposed. In this configuration, the internal light source is eliminated. Instead, ONUs share a continuous wave light source which is centralized at the OLT. A novel silicon monolithic transceiver without internal laser diode for ONU in PON is fabricated. It includes a WDM filter, a Si modulator and a Ge photodetector. The monolithic transceiver chip size is only 2mm by 4mm. The crosstalk between the Tx and Rx is less than −20dB. The transceiver chip is integrated on a SFP + transceiver board. Both Tx and Rx showed data rate capabilities of up to 10Gbps. The silicon transceiver chip can be further integrated with CMOS modulator drivers and receiver amplifiers so that the more functions can be monolithically integrated on silicon chip. By implementing this scheme, the ONU transceiver size can be significantly reduced and the assembly processes will be greatly simplified. The results demonstrate the feasibility of mass manufacturing monolithic silicon ONU transceivers using low cost CMOS processes, taking silicon photonics a step closer to practical network deployment.
Here is a white paper from 2006 that analysed Fiber to the home costs for Verizon (FIBER TO THE HOME: Capital Costs and the Viability of Verizon’S FIOS)
(i) say $2000 for fiber,
(ii) $700 for drop and ONU,
(iii) $200 common electronics,
(iv) $300 converters.
That is a total of $3,200.
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