Nothing is more basic to cable TV than tuning. Video channel tuning up and down the TV dial is a common experience for millions of cable subscribers who blissfully surf each night through wave after wave of cable TV programming.

But in the optical fiber portions of cable operators' hybrid fiber/coax networks, a different type of tuning on a different type of wave is emerging.

Alcatel's tunable laser
Alcatel’s tunable laser and design support kit. Included software helps familiarize customers
with the tunable laser, demonstrating
how wavelengths are changed.
Tunable lasers are devices that improve upon today's fixed-wavelength lasers through their ability to tune to more than one ITU (International Telecommunications Union) wavelength. Used in conjunction with dense wavelength division multiplexing (DWDM) platforms, tunable lasers offer both short-term and long-term advantages to network operators.

"We're actually very interested in tunable lasers," says Don Loheide, vice president of engineering technology for Charter Communications Inc. The reasons why address both cost issues in the near term, and network efficiency down the road.

In today's DWDM systems, multiple wavelengths of light are able to travel on a single fiber. But to produce each of those wavelengths, separate lasers are required for each wavelength.

Sixteen-channel DWDM systems, for example, require 16 laser assemblies or circuit boards. This creates a significant inventory problem for operators, who must stock spares for all wavelengths used, in the event of a failure or outage.

Tunable lasers solve the inventory issues associated with stocking spares by using sophisticated optical technologies to tune light to multiple channels. Today's tunable lasers coming to market can switch from between four and 100 channels each. With one laser that can tune to more than one wavelength, operators have to stock far fewer spare lasers, which, for long-haul applications, can cost upwards of $20,000 to $40,000 each.

In addition, the costs associated with trading out spares in the field, done in conjunction with the central office serving the laser, are also significant, notes Arlon Martin, vice president of marketing for tuner laser developer Agility Communications Inc.

But tunable lasers can serve a much more significant role than merely acting as spare parts. Instead, tunable lasers can enable the remote provisioning or re-provisioning of wavelengths. This would allow a technician at a remote terminal to re-set the laser to another wavelength or channel to accommodate dedicated services or shifting traffic patterns on the network.

The first step for cable: Tuning the return path

While the main drivers of tunable lasers have typically fallen outside of the cable industry, Harmonic Inc. has introduced tunable laser technology for cable network applications. "We've been tinkering around in the lab for awhile" with tunable lasers, says Patrick Harshman, president of broadband access networks for Harmonic. Specifically, Harmonic has been applying tunable laser technology for its digital return path equipment.

In November, Harmonic introduced a DWDM-based, tunable return path transmitter, the NDT 3059A. The new tunable digital transmitter return product line, consisting of nine temperature-hardened models, can be hosted in Harmonic's PWRBlazer node platforms. Each NDT 3059A uses a wavelength-adjustable, cooled DFB (distributed feedback) laser that can be tuned in blocks of four adjacent wavelengths. The nine models collectively support up to 36 wavelengths on the ITU grid, which defines standard telecommunications wavelengths, typically within the C band, or from 1530 nanometers to 1565 nm.

The NDT 3059A digitally transmits two streams (5 MHz to 48 MHz) from any PWRBlazer optical node to a headend or hub location.

The evolution to tunable lasers in the return path is a progression from Harmonic's DWDM return path transmitters, which first rolled out in 1999. Previous digital return transmitters, such as Harmonic's NDT 3049A, operate at fixed wavelengths on a 200 gigahertz, ITU C channel. Because the NDT 3059A is able to tune 36 wavelengths at 100 GHz spacings, the new system can now support twice as many wavelengths in the same spectrum.

Near-term, the primary application of the new tunable laser is for sparing, as an operator can keep nine tunable lasers in inventory to cover all 36 wavelengths. By remote activation or by engaging a DIP switch, a technician can tune each laser to any one of four wavelengths. This capability reduces the number of spares a tech must keep on hand for node maintenance.

The 100 GHz spacing is key for cable operators because, as Loheide points out, with eight wavelengths on a fiber, "we don't want to spare all eight wavelengths, but if you have to spare four (at 200 GHz spacing) and it costs you twice as much for a tunable, you haven't gained anything."

Typically, Charter uses seven wavelengths, reserving the eighth wavelength as a spare. In case of a failure, the eighth wavelength could be used, but that entails a re-configuration of the optical multiplexer which receives wavelengths–a process that can add a half-hour to Mean Time to Repair, "which we hate to see," says Loheide.

What would be more attractive, he adds, is a tunable laser that covers the entire eight-channel band that would be, for instance, twice the cost of a single-wavelength fixed laser. "That would make a heck of lot of sense to us," he notes.

After 12 months of using baseband digital return methods, Loheide says there's been a vast improvement in tunable laser development. "It's coming along," he says.

NDT 3059A
Harmonic's NDT 3059A digital return path transmitter.
Harmonic builds its own tunable laser modules, which allows the company to design devices for HFC node environments rather than relying on lasers made for other telecom uses. "One of the key aspects to making this play in the field is the ability to do things in a very compact, temperature-hardened way," says Harshman. "The biggest technical challenge we faced was dealing with the size and temperature constraints." In fact, tackling these issues involved the development of closely held intellectual property by Harmonic. Therefore, company officials are loath to disclose details of its tunable technology at this time.

Beyond return path applications, Harshman says that Harmonic is exploring ways to incorporate "tunability" into its Gigabit Ethernet transport platform, including interconnects between headends and hubs. With video-on-demand becoming more prevalent, and more traffic moving from content server farms to the network edge, "the traffic patterns and relative demand loads are still emerging," says Harshman. "Innovative wavelength switching may prove to be very helpful in these kinds of networks."

In the forward path, Loheide, noting Charter's passive hub architecture for VOD channels, says that as fewer and fewer homes are served by a Data-Over-Cable Service Interface Specification (DOCSIS) channel, an ITU grid laser will likely be dedicated to a few nodes, and in this case, a tunable laser could play a role.

Looking further ahead, Loheide says that if cable is successful in addressing connectivity (such as Gigabit Ethernet) and wavelength services for business, tunable lasers could also come into play helping to provision nodes dynamically where bandwidth is needed.

Today, specific wavelengths are dedicated to hub sites for VOD. Often, they're underutilized, says Loheide. Those wavelengths could be re-routed to a different hub site for a different application. For example, at 3 p.m. on a Thursday afternoon, when VOD services are less requested, a wavelength could be delivered to a business, and re-routed at night to carry traffic during peak VOD times.

While these applications are clearly a few years away, tunable laser vendors, from startups to entrenched giants, are moving quickly to address costs, faster switching times and wider tuning ranges.

Single-chip tunable laser

Pushing integration and single-chip designs in the tunable laser sector is Agility Communications Inc., a Santa Barbara, Calif.-based company that last September announced it had received a third round of funding, led by General Motors Investment Management Corp., worth $83 million. Other strategic investors include Ciena Corp. and Tellabs Inc.

Agility's 3105 CW
Agility’s 3105 CW Widely Tunable Laser Assembly.
Last month (March), the company announced two new tunable laser products, both of which push the envelope for integration and size.

Targeted for long-distance applications, the 3105 CW Widely Tunable Laser Assembly (TLA) is a 10 milliwatt (mW) laser that tunes to 100 C-band channels. The laser, based on Agility's Sampled Grating Distributed Bragg Reflector (SG-DBR), is integrated with a semiconductor optical amplifier (SOA).

A single-chip, integrated design is meant to drive the cost of tunable lasers down into the realm of fixed-wavelength lasers, says Martin. The 10 mW power metric means the laser can get as bright as single lasers tuned to one channel.

The TLA is targeted for use in DWDM systems spanning the full C-band, with tunability for 100 ITU channels with 50 GHz spacing. Other components of the TLA include a wavelength locker, beam blanking to eliminate spurious emissions, and microprocessor control and monitoring. The wavelength locker is a component that captures a small output of light that checks its color to ensure the wavelength is "on channel."

The second product, aimed at metro network applications, is an electro-absorption modulation laser (EML). Agility's 4245 EML, also based on an SG-DBR platform, contains an integrated SOA, wavelength locker, modulator driver, control electronics and firmware. It "runs 200 kilometers before it runs out of usable light," says Martin.

According to Agility, the EML works at speeds up to 2.7 gigabits per second and supports forward error correction across the full C-band with 50 GHz spacing.

The integrated modulator in the EML, Martin explains, serves much like a shutter in a camera. It is essentially a pattern generator that enables the on/off light pulses to travel to a receiver. This contrasts with the continuous wave 3105, which uses an external modulator to add data to the light stream.

The Agility SG-DBR implementation consists of four primary sections, horizontally aligned: two mirrors front and back, a gain section that controls power, and a phase section which fine tunes wavelengths. Each of the mirrors, says Kevin Affolter, director of marketing for Agility, has a grating, which magnified, appears like teeth in a comb or fingers on a hand. The gap between the teeth or fingers is different on the front and back mirrors, meaning that the two mirrors, at any given point in time, can only line up to create one wavelength.

By lining up two peaks in the grating on the front and back mirror, a coarse wavelength is produced. Based on look-up tables embedded in the device, it's determined what values of current are needed to create and change wavelengths. The current changes the refractive index of the mirror, thus changing the comb or finger pattern of the grating. The phase section of the laser trims the wavelength to line up with a desired, specific wavelength.

Heavy telecom hitters

To date, most of the development for tunable lasers has been focused on telecom applications, and heavyweights Nortel Networks, ADC and Alcatel Optronics, among others, continue to develop tunable laser products.

Nortel has several tunable lasers in its portfolio, including a narrowly tuned DFB laser that tunes via temperature difference to eight channels with 50 GHz spacing. Another laser, Nortel's ML-20, is based upon vertical-cavity surface-emitting laser (VCSEL) technology, with mirrors on the top and bottom of a semiconductor. The mirrors are based on micro-electro mechanical systems (MEMS) technology. The ML-20 "is designed for full bandwidth tuning," says Tom Dudley, director of marketing for Nortel's optical components division, and can tune based on any channel spacing, as low as 25 GHz. "You're not confined to a specific channel spacing with our laser," says Dudley.

A lower-cost version of the ML-20 tunes to the equivalent of 20-plus nanometers of the C-band. Both lasers include a bandwidth locker, control system electronics and a full transmitter.

Just introduced from Nortel is a tunable laser that includes a Gallium Arsenide modulator in the package.

With enough power at up to 20 mW to handle long-haul applications, Alcatel Optronics' A1935TLI can tune to eight wavelengths at 50 GHz spacing over a range of 30 channels, according to Marco Albasio, technical support director for Alcatel. Future applications of the device, says Albasio, are designed to accommodate virtual private networking, or wavelength-on-demand services and require fast switching times, which on the A1935TLI, run at about 10 ms, says Albasio.

Alcatel is using DBR (Distributed Bragg Reflector) lasers which rely on the injection of an electrical current that changes the index of refraction of a mirror in the assembly to change wavelengths.

Further down the road, Alcatel will be developing a tunable laser to facilitate Internet Protocol (IP) optical routing, requiring a complex component set, enabling arrays of lasers to be switched in nanosecond time frames.

Already hitting less than 50 nanosecond switch times is ADC's "grating-assisted codirectional coupler with rear sampled reflector" (GCSR) tunable laser, according to Rick Masloski, vice president and general manager of ADC's photonics division. The Anywave NYW-50 covers the entire C-band (80 channels) at 50 GHz spacing. In addition to sparing and fixed laser replacement, the Anywave NYW-50 is also targeted for add/drop multiplexer applications.