Tunable VCSELs

Tunable Vertical Cavity Surface Emitting Lasers (VCSELs) are poised to meet the varied protocol, dynamic traffic pattern and cost target requirements of metro optical networks. Metro networking requirements are changing due to the enormous capacity deployed in long-haul WDM (wavelength division multiplexing) transport systems and the introduction of that capacity with mesh-capable optical and photonic cross-connects between metro and long-haul connection points.

Additionally, the continued growth of end user demands for various high-bandwidth services such as DSL, GbE and Fiber Channel are fueling the metro fire. As a result, there is a growing need for more flexible and higher capacity metro architectures that have more optical user connections at the network edge than the traditional single wavelength synchronous metro networks originally designed for aggregated voice traffic. Tunable VCSELs will play a critical role in meeting traffic patterns and protocol diversity with detailed performance and cost requirements unique to metro environments.

Traditional approaches to metro networking

SONET/SDH architectures

The most widely deployed optical systems in metro are SONET/SDH systems, which are usually deployed in a ring topology for protection capabilities. Due to synchronous network timing, SONET/SDH is designed for efficient single wavelength sharing at many nodes. Client connections that are typically below the optical transmission rate are multiplexed for efficient bandwidth utilization and minimal de-multiplexing at each node along the ring.

This SONET/SDH approach is reaching its limit in many of today’s networks as client’s synchronous and asynchronous bandwidth requirements near the transmission line rate become more common. This approach has also complicated the provisioning of client services between interconnected rings and across cascaded rings. The result has been unacceptable provisioning complexities and “mean time to connect” metrics for many carriers.

SONET/SDH with electrical enhancements

Recent enhancements to single wavelength SONET/SDH systems have addressed more efficient single wavelength sharing through adapting various protocols to a single SONET frame (“multiservice”). Asynchronous and synchronous signals are adapted to a single protocol and provisioned on shared wavelengths. The system capacity per wavelength is increased and the approach is extendable to multiple wavelengths with WDM. Although protocol specific signaling controls and interfaces enhance functionality, they commonly introduce transparency issues in asynchronous protocols.

SONET/SDH with WDM enhancements

WDM has been integrated into the same platform to extend SONET/SDH-based metro system capacity. These types of systems have multiple wavelength capacity and flexible SONET/SDH protection schemes. Optical gain elements and design rules used by optical mux/de-mux filters to access the wavelengths (the “wavelength plan”) become important capacity planning issues to maintain transmission reach or data rate. A common approach is wavebanding, which bands wavelengths into groups to minimize pass-thru losses from optical filters for wavelengths dropped at other network nodes. This extends the total system reach and allows certain wavelength bands to be dropped into the client-facing muxes/switches at one node while other wavelengths may be accessed at others. (See Figure 1 for a two-wavelength example.)

Wavelength banded architectures are well suited for static networking environments where limited capacity reconfiguration is typical. As the optical capacity gets closer to the end users, however, churn of services and customers on and off the network are the rule, not the exception.

Electrical switching with WDM

Another approach to Metro Networking is to integrate switching into the WDM nodes, which creates an Optical Cross Connect (OCC). Typically, this is implemented without any wavebanding, as shown in Figure 2. The switch matrix, which is more capable than SONET/SDH node mux/switch matrices, enables more flexible mesh architectures that can be restored by extending existing data-centric routing protocols.

This complexity may not be required at all points in the network and is typically used today between long-haul and metro networks only.

Tunable metro network architectures for protocol transparent remote provisioning

Recent tunable metro architecture investigations, which have proven technically and economically feasible with the advent of tunable VCSELs at 1.5 mm, have moved to the forefront of system designs. Systems designed to leverage directly modulated tunable VCSELs, optical gain, optical filters and optical switch elements can be combined to provide dynamic remote provisioning of protocol transparent services that scale in capacity and economy.

Tunable laser/fixed filter architectures

A tunable source in a banded wavelength plan (see Figure 1) that is capable of tuning across one band of wavelengths can be dynamically and remotely configured to meet client traffic demands. As the laser frequency is tuned within the drop band of the band filter, the wavelength is dropped to a channel filter, which then drops it to the receiver or rejects it into the add multiplexer to continue along the span. This wave continues until reaching a node with the channel filter matched to that frequency.

Fixed laser/tunable filter architectures

Similar system functionality with fixed wavelength lasers and tunable filters capable of “hitless” tuning is also possible. One frequency may be dropped to the receiver at a node and when the filter is tuned to another frequency, that wavelength is dropped to the receiver.

To implement similar functionality, tunable filters that are not capable of hitless tuning -- in other words, they impact the performance of other system wavelengths while tuning -- require optical isolation schemes during tuning to not affect other wavelengths in operation. Small port count optical switching is today’s best choice, making the tunable laser/fixed filter a cost winner for wavebanded dynamic systems.

All-optical wavelength reconfiguration metro architecture

The introduction of optical switch technologies has allowed systems designers to consider wavelengths as the unit of granularity for services. Open wavelength, rather than wavebanded, use plans are typical and support mesh network architectures. The nodes in these systems are effectively optical routers (Figure 3) that can leverage existing routing protocols for optical capacity rearrangement due to provisioning requests or protection events.

Quantifying the “routing power” of nodes shows that reconfigurable optical add/drop multiplexers (OADMs) that affect the degree of connectivity and amount of logical trunking in a network are key performance metrics for protection and restoration. In all-optical reconfiguration systems that use the same amount of optical switch fabric, nodes implemented with tunable lasers have increased routing power and provided comparable service protection and restoration at lower cost points.

Wavelength conversion-based architecture

Network elements designed to minimize optical switch fabric may leverage cross-phase modulation effects in gain stages to enable wavelength conversion and increase routing power further. Tunable sources are used as an external input to a gain stage that converts one input wavelength to another without an optical-to-electrical step or optical switching. This increases the rearrangement capabilities of the nodes while maintaining optical transparency for the services transported on the working wavelengths.

Optical switching and WDM-enabled with tunable components

A metro deployment constructed from the previously discussed system architectures is depicted in Figure 4. The multiservice platforms and cross-connect elements enhanced with tunable lasers (multi-colored background symbols) are used at different points in the network.

Tunable network architectures can be leveraged for remote transparent service provisioning, optical protection and potentially for packet switching. The time for optical layer reconfiguration will set limits on the application. Today’s applications are geared toward provisioning (~1 sec) and protecting the optical layer (“hot standbys” -- ~10mS). There is continued research into leveraging nS tuning speeds in optical packet networks where information must be stored and read faster than under existing techniques.

Optical amplification is utilized in tunable network architectures as needed to increase bit rate, distance and number of nodes traversed before regeneration is required, and the Optical Signal to Noise Ratio (OSNR) sets limits in typical metro systems design. An approximation for a 100km transmission span, 4 multiplexing nodes and a native tunable VCSEL power of –10dBm results in an OSNR ~ 9 to 10dB at the receiver, which should exceed today’s strict Bit-Error Rate SONET metrics.

Economic Benefits of Tunable Metro Network Architectures

The traditional fixed wavelength approaches previously discussed are quite useful. But as system capacities grow via WDM, costs associated with hot standbys for protection, sparing fixed wavelength sources to meet “mean time to restore” metrics and inventory management for both manufacturers and service providers do not satisfy cost targets required for metro networks. Enhancing metro network architectures with tunability is the key to delivering bandwidth efficiently and profitably.

Inventory management

System manufacturers must maintain and manage an inventory of spare line cards to assure that network build schedules are met. Optical line cards, however, typically cost between $10,000 and $20,000, and this inventory becomes a significant expense to total system costs. A 5-year cost-savings case study is depicted in Figure 5.

Hot standbys Metro systems typically implement protection schemes at the optical layer to provide resiliency equal to or better than SONET/SDH. This normally requires one-for-one optical source standbys – one for the work path and one for the protect path. With tunable lasers the ratio of work-to-protect line cards can be changed (e.g., 1:2, 1:4, 1:8, etc.), depending on the wavelength use plan and switching capability built into the system. As shown in Figure 6a, the number of required hot standby spares is dramatically reduced by using tunable transmitters in their place. Figure 6b shows the cumulative savings over five years in hot standby line card costs.

Sparing

Network operators must also maintain an inventory of spare line cards near its deployed equipment to meet typical “mean time to repair” metrics of two hours for their customers. The ability to tune the optical transmitter dramatically reduces the number of spare transmitters. Stocking requirements are reduced depending on the tunable laser’s tuning range and the system’s total wavelength count.

Remote provisioning and reconfiguration benefits

The incremental revenue from using remote reconfiguration benefits of tunable-based systems can be defined and compared to traditional 24x7 services. (For example, a service available for ¾ of the day would be priced at 20% above ¾ of the 24x7 rate if it were offered at peak periods.)

Conclusion

Tunable VCSELs are a key ingredient for enabling metro network architectures to meet the unique needs of metro network applications. Tunable metro networks allow carriers to cost-effectively maximize transmission capacity and offer a new wave of service flexibility and higher-value services that address the varied protocols and changing demands of metro networks.

Charles Duvall is Applications Consultant for Bandwidth9.

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