The beauty of tunability

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Tunable lasers

The economics of new optical networks requires a shift from current fixed dense wavelength division multiplexing to flexible DWDM networks. These, in turn, require high-performance tunable optical devices and modules that can be rapidly provisioned through software.

By Edward Haroff, GEID Press, Geneva

Tunable laser components have long been a "holy grail" of the industry and have spawned numerous technical approaches by a dozen start-up companies competing with more established suppliers. With no panacea in sight to serve the entire market, several tuning techniques promise different solutions.

Next-generation DWDM systems continue to squeeze more wavelength channels, at higher bit-rate rates, into the C-band transmission window (1530-1565nm). The number of channels (up to 160) in current DWDM deployments requires that each wavelength has a fixed wavelength laser (distributed feedback laser). The availability of tunable transmitter cards makes life a lot easier as an adequate back-up for all channels in the DWDM system.

Not just cheaper, as one laser can deliver any wavelength, these tunable cards are considerably simpler from an inventory and logistics perspective.

In the near term, cheaper DWDM systems should permeate smaller communication lengths, such as metro networks. Tunable lasers promise to deliver high performance at lower cost. Market growth would permit dramatic increases in manufacturing volumes, resulting in tunable laser alternatives costing around 20% more than a fixed DFB device.

Finally, tunable lasers open the door to a realistic approach for an all-optical networking future. If the wavelengths at each point can be dynamically varied, the bandwidth capacity of each link can be adjusted to reflect customer usage.

With this type of DWDM architecture, network nodes become passive, which in turn allows service providers to reduce complexity of cross-connects across the network. As point-to-point fibre links give way to mesh networks in which signals can take more complex routes - crossing a number of nodes between their sources to destination - dynamically changing the routing becomes critical to functionality.

Name that tunable
The term "tunable laser" has come to describe an increasingly broad range of technologies, from monolithic semiconductor lasers to micro-electro-mechanical system (MEMS)-based lasers and fibre lasers.

These can generally be classed into six technology groups:
Monolithic widely tunable diodes are relatively well known, having been pursued by universities and research institutions for many years. Though these multi-contact lasers do not suffer from mechanical issues, the device characteristics are extremely complex and change as the device ages. Ingenious control circuits and algorithms are required to stabilise these devices, and integrated amplifiers are needed to boost the power to required levels.

Narrowly tuneable devices, such as distributed Bragg reflector lasers (DBR) are generally preferred by the more experienced manufacturing teams from the larger corporations, who are aware of the pitfalls of the common paths to widely tunable devices.

Sampled grating DBR lasers (SG-DBR) use repeated sections of short grating and unmodified waveguides to create a comb-like reflection spectrum, with the spacing of the peaks determined by the spacing of the gratings.

To make the device widely tunable, the grating coupler with rear sampled reflector laser (GCSR) replaces the DBR with a filter that has a larger tuning range. Adding a suitable sampled grating and a phase-control section makes tuning speeds and ranges comparable with SG-DBR.

Others have chosen external cavity approaches to avoid the intricacies of laser diode manufacture. The advantage of modifying or shrinking conventional external cavity tunable lasers with moving parts is that only simple gain elements are required. Still, mechanical tolerances and stability remain as challenges. External-cavity lasers (ECL) use micro-electro-mechanical systems (MEMS) to achieve a higher power level and wider tuning range.

Tunable vertical-cavity surface-emitting lasers (VCSEL) generally use a movable element to adjust the cavity length and tune the laser, but either requires an external pump or cannot deliver appropriate output power. An array of lasers of different wavelengths combined with a star coupler into a single output is also difficult to scale.

To tune or not to tune
Alternatives to tunability also use an array of lasers, but instead of using the conventional star coupler to combine the beams, a much more efficient external combiner is employed, resulting in minimal excess loss. In a new product from Santur, a California-based start-up, DFB devices have much faster tuning (<10ms) by changing only the temperature of the lasing stripe instead of the entire optical subassembly.

Unlike other array-based approaches, where only a small amount of the power can be coupled to the fibre, this type of fibre-based distributed-feedback laser (DFB) combiner delivers nearly the entire laser output power. The resulting device performs like a DFB, and can be manufactured at low cost using a proprietary automated packaging approach.

The TL1010-C is the first in a line of widely tunable laser products that will debut this year using Santur's patented device and packaging technologies. These DFB products have a powerful combination of high-power achieved without the use of a semiconductor optical amplifier (SOA), wide tunability and stability in harsh environments.

In combination with an external modulator, the TL1010-C suits a variety of DWDM systems offered by network providers. The wide tunability and lower cost structure enable flexibility in system design, lower inventory costs, and streamlined logistics for network providers of metro, long-haul, and submarine systems. All the desired features of a standard, high-performance, fixed-wavelength DFB laser are provided, with the additional features of 36nm wide tunability and the functionality of variable optical attenuation in a standard package.

The simple design of the patented DFB array and low-loss coupling scheme enables pure DFB performance with an intrinsically low cost structure approaching that of a fixed wavelength device. Since the optical characteristics are identical to fixed wavelength DFBs, no system redesigns or compromises are necessary to make use of the tuneable laser. The Telcordia-ready package of the TL1010-C also integrates a wavelength locker for ultra-stable 50GHz ITU grid operation and built-in control electronics for ease of use (further details at www.santurcorp.com).


Optical DWDM network trends
When asked to predict major technology trends in optical components, market-research firm RHK said it sees new optical components supporting three key trends in optical DWDM networks evolution:

(a) Capacity growth in core networks (increases in both the number of channels and bit-rate per channel)
(b) Optical edge function in the metro network
(c) Reconfiguration in optical network

To put sales numbers on the total transmitter market, RHK and another market forecaster, ElectroniCast, both indicate growth approaching USD5bn in the next few years.


Wide-band tunable will "blow a hole" in the fixed wavelength market
Bookham Technology has developed a new DS-DBR (Digital Supermode Distributed Bragg Reflector) laser using a novel tuning approach to maximise output power. The company views the monolithic device as a breakthrough: when combined with its gallium arsenide-based integrated modulator platform the DS-DBR becomes the powerhouse for 10Gbit/s and 40Gbit/s transmitter solutions.

Key features include: excellent power uniformity with tuning; a simplified set up, calibration and control; and scalability in manufacture through use of monolithic, planar device structures.

The widely tunable laser is intended for dynamic wavelength routing and active system reconfiguration in addition to sparing and inventory management solutions.

Andy Carter, Bookham Technology's VP for Active Component R&D, said: "This device combines the key features required by systems manufacturers today. It has excellent side mode suppression ratio (SMSR). The device is monolithic, providing scalability in manufacture using industry-standard technologies.

"We will also be supplying these devices in our integrated transmitter platform with support electronics, giving customers all the advantages of tunability with none of the problems of fibre splicing. This really has the potential to blow a major hole in the fixed wavelength laser market."
www.bookham.com

CLose-Grid calibrates widely tunable lasers
Manufacturers of widely tunable lasers face a significant characterisation problem. Although laser chips are diced from similar wafers, no two chips are exactly the same in their tuning characteristics. Furthermore, the device has to be fully built and packaged before its characterisation can be "mapped" and proper currents for different lasing wavelengths are fully identified.

CLose-Grid is a new suite of products from Tsunami Photonics, Dun Laoghaire, Co Dublin, Ireland designed to support the test, calibration and control of both widely and narrowly tunable lasers. It combines hardware and software that provides the test and frequency-coverage validation for both chip-on-carrier lasers and fully packaged laser modules.

Screening at chip-on-carrier is achieved within 30s, and a full ITU-T look-up table is generated in less than 2min with frequency-setting precision of 1GHz. Better accuracy (500MHz and 250MHz) will be achieved soon, the company says.

The key feature of CLose-Grid is the manner in which it establishes the unique combination of currents associated with each individual wavelength and then stores this information in the look-up table. At the dicing stage - or chip-on-carrier stage - it only takes 30s to generate a 2-D mode map of the laser and to determine if the chip is capable of generating the wavelengths needed to cover the entire C- and L-bands.

After the quality laser chip has been packaged, characterisation takes less than 2min to fully characterise the laser module, generate the look-up table and evaluate stability of selected ITU channels.

www.tsunamiphotonics.com

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