A closer look at the sample-grating distributed Bragg reflector tunable laser and its applications

By KEVIN AFFOLTER
Agility Communications

The optical networking industry experienced exponential growth from 1996 to 2001, during which time a record amount of inventory was created by organizations embracing DWDM technology. As the number of wavelengths supported on an optical fiber grew from 1 to 200 in just 5 years, the supply chain had to ramp up to support 200 times greater complexity. DWDM technology also created difficulties in forecasting, and supplying lasers with the correct wavelength, which meant missed revenue opportunities up and down the supply chain. As such, tunable lasers, which promise to support dynamic network architectures, thereby reducing inventory and sparing costs, are generating heated discussion.

One approach to developing tunable lasers is based on sampled-grating distributed Bragg reflector (SG-DBR) technology. The SG-DBR widely tunable lasers can serve multiple market segments while addressing the concerns of reliability and cost evolution through a solid-state, monolithically integrated architecture and automated assembly process.

SG-DBR technology

The SG-DBR laser is a solid-state device based on industry standard indium phosphide chip technology and standard packaging processes. It is widely tunable; the laser can be monolithically integrated with a semiconductor optical amplifier (SOA) for high power, long-haul applications, or with an electro-absorption modulator (EAM) for metro-core applications. A highly automated packaging process is available because SG-DBR technology is based on a single opto-electronic chip, which is a key factor in enabling faster time to volume production.

The SG-DBR comprises four sections: front mirror, back mirror, gain and phase. Sampled gratings are used to form the mirror sections. A sampled grating is similar to the continuous gratings found in distributed-feedback lasers except here the grating is sampled, or non-continuous. By sampling the grating, multiple reflectivity peaks are formed. These peaks are spaced apart in wavelength at a period inversely proportional to the period of the sampling. The front and back mirrors of the laser are sampled at different periods such that only one of their multiple reflection peaks can coincide at a time, this is known as the Vernier effect. In this way the desired ITU channel can be selected by tuning the two mirrors such that the closest reflection peak of each mirror is aligned at the desired channel and lasing occurs. It is this tuning mechanism, which enables the C-band to be covered by a single laser.

In addition to the wide tunability, the SG-DBR platform offers many other advantages. Monolithic integration of an SOA provides the ability to deliver high output power into the optical fiber as well as realizing a variable attenuator function to allow output power to be varied. The SOA section also provides a beam blanking function thereby preventing spurious emissions during wavelength switching. It is also possible to integrate an EAM, allowing extended reach tunable operation in a very compact and low cost footprint.

Short-term applications

Applications for tunable lasers range from the immediate to those up to 5 years away. Short-term applications are expected to achieve wide deployment in the next 2 years. Longer-term applications will be widely deployed in 2 years or more.

As new DWDM systems are deployed, tunable lasers will replace fixed wavelength devices, initially for one-time provisioning applications. This is typical of the approach pursued today by many optical networking vendors. The drivers for one-time provisioning are ease of forecasting and planning, inventory reduction, and streamlined manufacturing processes. Introducing a widely tunable laser into the equation means that lasers can be forecast by volume alone rather than by wavelength.

One-time provisioning also reduces inventory requirements. Inventory stocking is used as a tactic to accommodate fluctuating demands. However, the recent industry downturn left vendors and carriers with billions of dollars of idle inventory. Tunable lasers remove the need to hold inventory at such massive levels. Implementing widely tunable lasers is likely to save businesses hundreds of millions of dollars.

Finally, in the event of a failure, a widely tunable spare can replace a fixed-wavelength laser. This drastically reduces the quantity of spares needed and their associated costs.

Longer-term applications

Deployment of optical add/drop multiplexing has been hampered by the lack of flexibility in today's networks due to the cost premium required to implement the equipment required to create dynamic architectures. A reconfigurable optical add/drop multiplexer (R-OADM) would allow network operators to change the dropped or added capacity at a remote site without an expensive bandwidth manager. This results in major savings for the network provider.

R-OADMs can be implemented in several ways -- from a tunable filter approach to a back-to-back multiplexer with a switching fabric in between. Both approaches require tunable lasers. Tunable lasers support network flexibility, without requiring a fixed transponder for every wavelength.

Another application area for tunable lasers is photonic crossconnects. Of great interest for some time, photonic crossconnect technology is just starting to come to fruition. A coarse bandwidth management function can be implemented using a photonic switch surrounded by fixed transponders to overcome wavelength blocking and provide SONET-like performance monitoring. Implementing the same function using tunable transponders can result in a 50% reduction in transponder cards. In this application the tunable laser enables new functionality, which drastically decreases the capital, and operating costs of the network.

Wavelength switching at the packet level is another application enabled by tunable lasers. A key feature of a solid-state tunable laser is that it can be switched from one wavelength to another in a few nanoseconds. This enables wavelength switching at the packet level, which can then be used to route data traffic based on color through a passive router such as an N x N arrayed waveguide grating. In this application, changing the wavelength at any input port will cause the data to emerge on a different output port.