Optical components

GigaTera Sends Pulses Racing

What could turn out to be breakthrough technology for high-speed transmitters is being developed by GigaTera AG, a Zurich-based startup.

The technology, in the form of pulse-generating lasers, eliminates requirements for a couple of components that are normally required in 40-Gbit/s transmitters. As a result, module makers can achieve significant reductions in cost, size, and power requirements, according to Andros Payne, GigaTera’s CEO.

The startup was spun out of the Swiss Federal Institute of Technology (ETH) in Zürich (see Swiss On a Roll). It got its first round of funding in February 2001 and now has 20 on staff. These include Dr. Ursula Keller, who is well known for her research into ultra-fast lasers.

The best way of understanding what GigaTera is up to is to compare these two figures, supplied by the company, showing the building blocks for high-speed transmitters:

First, a bit of background. The design of 40-Gbit/s transmitters is more complicated than that of normal transmitters, because they employ a different modulation scheme. Called "return to zero" (RZ) modulation, it consists of a string of pulses, with pulses "removed" to indicate a zero. (Most transmitters today use non-return to zero (NRZ) modulation -- see Optical Modulation). The RZ format is chosen because it has a higher resistance to impairments such as chromatic dispersion, which smudge the optical signal as it travels down the fiber.

The top diagram, labeled “Current RZ Transmission Approach” shows that high-speed transmitter modules need six buildings blocks -- three optical ones (colored orange) and three electronic ones (colored gray). The modulation is done in two stages in order to produce pulses of the necessary quality.

On the optics side, light is generated by the CW (continuous wave) laser in the top left and then enters an RZ dual-drive pulse modulator. This interrupts the light to create a stream of regular pulses, which is fed into a second modulator (labelled NRZ), one that blocks some of the pulses so that the pattern that’s left represents the data to be carried.

On the electronic side, a multiplexer is used to convert the incoming parallel streams of data into two outputs -- one creating a clock signal to generate the regular string of light pulses, the other to create the data signal used to wipe out the appropriate pulses using the second modulator. Each modulator is driven by a modulator driver.

Making a pulse modulator driver that can operate at 40 GHz is challenging, according to Payne, who says Germany’s SHF Communication Technologies AG is one of only a few companies that make them today. Translation: They’re expensive.

In the GigaTera, approach shown in the bottom diagram, the pulse modulator and its tough-to-make driver are eliminated by the use of GigaTera’s pulse-generating laser. This does more than reduce space requirements and cost. It also reduces power requirements, because the pulse modulator would typically have an insertion loss of around 5 decibels, according to Payne. Eliminating it means that the laser needn’t be as powerful.

GigaTera’s pulse-generating laser “is really an optical clock. You could keep time with it,” says Payne. It’s made using special “saturable absorber” semiconductor material that becomes more transparent as the light gets more intense. GigaTera has managed to use this material in a laser cavity to create regular pulses rather than a continuous stream of light. “Until August of last year, no one thought this was possible,” says Payne, although he won't say how the company solved the problem of laser instability that has prevented other researchers from commercializing the same idea.

Creating a stable pulse train was only part of the challenge, he adds. The pulses must also have the correct repetition frequency to comply with Sonet timing requirements. GigaTera has managed to achieve this, though again it won't say how. At the Optical Fiber Communication Conference and Exhibit (OFC) last month, it showed that the laser meets Sonet jitter specifications, in a demonstration at the Agilent Technologies Inc. (NYSE: A) booth (see GigaTera Demos 40 GHz Pulse Source).

Payne adds that the technology could be used for more than just 10-Gbit/s and 40-Gbit/s transmitter modules. Different streams of pulses can be combined to boost bandwidths even further, using optical time-division multiplexing (OTDM). It might also be used for research into optical signal processing, which generally requires pulsed laser sources. GigaTera has developed two versions of its laser, one targeting Sonet and the other for research into newer ideas like OTDM. It expects to start sampling both types in mid 2002.

GigaTera’s $7 million first funding round came from JK&B Capital and Broadband Capital AG (see Swiss Startup Scores $7 Million). Then, last November, Silicon Light Machines and Cypress Semiconductor Corp. (NYSE: CY) invested $2 million in the company (see GigaTera Gets Cypress Cash). Here's the link: Dr. Keller and Kurt Weingarten, her husband and GigaTera’s CTO, were both students of David Bloom, a founder of Silicon Light Machines, now part of Cypress.

— Peter Heywood, Founding Editor, and Pauline Rigby, Senior Editor, Light Reading
spookydoodle 12/4/2012 | 10:30:59 PM
re: GigaTera Sends Pulses Racing Nothing new with this approach. The Pulse generator is a Passively Mode Locked laser whoes pulse train is stablized by Data Clock using electrical injection to the Saturable absorber region. The Data clock must be closer to the repetition rate of Laser or at its subharmonic frequency. However a large amount of electrical power is required to reduce timing jitter of generated optical pulses.
coffeebean 12/4/2012 | 10:30:58 PM
re: GigaTera Sends Pulses Racing
This approach was proposed by Bell Labs many years ago. I beleve that Prof Keller participated in work there for a while, seems unlikely to be of interest now.
redface 12/4/2012 | 10:30:56 PM
re: GigaTera Sends Pulses Racing How does Giga-Tera keep the ITU laser wavelengths precise? Is the saturable absorbers inside a monolithic DBR laser cavity, or is this an external cavity laser? I suppose in this pulsed laser, there is a DBR section, a saturable absorber section, and a free-space external cavity section to adjust for time delay so that the pulse frequency matches laser cavity length.

This technology seems to be viable only for 40 Gb/s applications.
meaty_urologist 12/4/2012 | 10:30:49 PM
re: GigaTera Sends Pulses Racing Hmmm... with all those sections it seems this may not be simpler at all. Each section needs some control signal. Their new picture seems to show that they have eliminated the TEC. Fat chance. I suspect the TEC control must be far more precise for the cavity resonant frequncy to be well controlled.
Besides hasn't Lucent published work on a device that combines laser, RZ modulator, and data modulator all into one chip for precisely this application? Why does this "new breakthrough" now require multiple chips and multiple packages to do the same thing? Any customers?!?
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