Bookham targets integration sweet spot for metro network

Optoelectronic integration represents the future for InP, says Andy Carter, Bookham s vice-president of research and development. He believes, however, that some technologists have become carried away with producing very large, highly integrated chips, and that they are overlooking issues of cost and yield.

"[Bookham has] the right balance between high yields and the performance that the market demands," said Carter. According to him, Bookham s unique combination of a tunable laser and an integrated broadband modulator provides the ideal small-form-factor solution for 10 Gb/s metro networks.

Bookham s method is to select the best materials technology for each device, as opposed to manufacturing overly complex devices that "force functions" onto a particular material system. The company believes that its linear integration approach, which uses a sequence of components for a signal channel rather than parallel integration of multiple channels, is driven by market demand.

Bookham has packaged a digital supermode distributed Bragg reflector (DS-DBR) laser with a spot-size converter Mach-Zehnder (SSC-MZ) modulator to produce a broadband transmitter that targets the 10 Gb/s metro overlay network. This module, which builds upon the company s fixed-wavelength design, uses an industry-standard butterfly box, with the laser and modulator mounted on a common thermo-electric cooler to improve stability.

Carter points out that Bookham s customers are not demanding higher levels of integration in which the tunable laser and broadband modulator are combined on the same chip. However, the company s laser and modulator could be scaled monolithically to meet future network requirements.

Although Carter believes that monolithic transmitters have issues associated with cost and yield, fixed-wavelength monolithic designs are available on the market. For example, Apogee Photonics, which was formed by the merger of T-Networks and ASIP (see "T-Networks and ASIP merge to form Apogee" Compound Semiconductor September 2005 p15), manufactures a 10 Gb/s electro-absorption modulator and a single-frequency laser on a single chip; and Infinera Corporation has deployed highly integrated photonic circuits in its own digital network system (see "Infinera moves in for the long haul as fiber recovers" Compound Semiconductor September 2005 p29).

Bookham s DS-DBR lasers, which were first launched at OFC 2004, are said to offer several advantages over fixed-wavelength sources and to make ideal drop-in replacements in metro networks.

Today s metro networks typically operate at 2.5 Gb/s across the C-band (1530-1565 nm) using fixed-wavelength lasers. However, Carter says that the move from circuit-based services to packet services has increased the deployment of 10 Gb/s Ethernet networks. Bookham s tunable laser, alongside its broadband modulator, can be adjusted to emit at any wavelength within the band and to support the increased data rates. Tunable lasers benefit manufacturers because they can reduce the number of components that are produced, and customers can carry a vastly reduced inventory, says Carter.

One advantage offered by DS-DBR lasers that is highly valued by Bookham s customers is what Carter describes as "wavelength protection". Since tunable laser emission can be set remotely, if one of these lasers is fitted into a network it can be used to replace any fixed-wavelength laser that fails. And looking further ahead, customers may be able to improve data transmission by adjusting the wavelengths of their tunable lasers, noted Carter. The laser also features "dark tuning", which prevents network interference by switching out transmission when the laser is retuned.

Bookham s 1.7  mm-long DS-DBR design features four main sections: a rear phase grating, which produces a comb of reflection lines; a phase control section; a multiquantum well gain section; a multicontact grating at the front of the device, which is used for coarse wavelength selection (figure 1). A semiconductor optical amplifier is situated beyond the front grating to provide 3-4 dB of gain.

The DS-DBR laser is capable of broadband gain. Its emission wavelength is coarsely tuned by adjusting the front and rear gratings, before the phase current (Iphase) is used to fine-tune the wavelength. The front grating has a broad reflection profile, but the rear grating contains seven sharp reflection peaks between 1520 nm and 1560 nm spaced 6.8 nm apart. These sharp peaks control the lasing wavelength and prevent modal competition by selecting just one of the available longitudinal modes.

The tunable lasers are manufactured in a similar way to the fixed-wavelength sources that Bookham has produced for many years. The process starts with two separate MOCVD growth steps on 3 inch InP wafers. This is followed by grating fabrication, further growth, and additional steps that involve dielectric and metal depositions and photolithography. The wafer is then thinned, cleaved, coated and finally packaged. The resulting lasers have a 42.5 nm tuning range that covers 214 different telecommunication channels, and an output power of 13.5 ± 0.1 dBm.

Bookham has developed broadband MZ modulators that can be integrated with the company s lasers to form fullband tunable transmitters for metro networks. Until recently, Carter believed that the linearity of these modulators might be insufficient for broadband operation, and he was surprised that only small adjustments enabled this device to cover the entire C-band.

MZ modulators are sophisticated optical switches that contain an interferometer. Laser light entering the device is split into one of two paths, and the phase difference between the two recombining signals dictates the device s output. If no phase difference exists, recombination is constructive (the "on" state), but if the two signals are exactly out of phase, destructive interference occurs corresponding to no light output from the device (the "off" state).

The relative phase is adjusted by biasing the two waveguides. Changes in the applied electric field alter the refractive index of the waveguide, adjusting its refractive index and its relative phase shift. In fact, Bookham has designed these modulators to split the incoming light into two signals of differing magnitude. By using differential biasing, the device can impart a negative chirp to the emitting light. This increases the distance that the light can propagate before dispersion compensation is needed.

Bookham is now producing its third generation of MZ modulators, although these latest devices are the company s first to be really suited to broadband operation. "The InP modulator has its genesis within Nortel, who were pioneers of this component from the mid-1990s onwards," said Carter.

Bookham has built on Nortel s earlier development of low-voltage modulators by improving features such as chirp and extinction ratio (the difference in light intensity between the on and off states). Carter cites the introduction of SSCs as one example of a technology that has been used to meet the changing demands of customers. Earlier modulators had a very small spot size, which made them difficult to package with a low loss. However, the SSC delivers efficient, low-cost coupling into the MZ waveguide.

The company s latest SSC-MZ modulator, which is used in products such as the LMC10 series of transmitters, has an overall chip length of 3 mm. Incorporated into the device is an input SSC, an MZ modulator, a multistage electro-absorption variable optical attenuator (VOA), a tap/detector that can be used with an external circuit to adjust the bias on the modulator and optimize performance, and an output SSC.

A key advantage of the SSC-MZ modulator is that the performance of the VOA and detector, as well as the RF bandwidth, which reveals the maximum switching speed of the device, can be predicted and controlled without screening every manufactured component. Instead of full-scale testing, the RF performance of a small proportion of the modulators is determined by chip-on-carrier measurements. Insertion losses, switching voltages, DC extinction ratios, and absorption as a function of voltage are also measured for various wavelengths.

Carter says that after recording data from more than 20,000 devices fabricated on more than 100 wafers, Bookham has demonstrated the high yield of the process. This is because the chip s location on the wafer produces only small variations in performance, provided that the wafer has been grown and processed to specification. The performance of Bookham s MZ modulators is at least the equal of their larger legacy LiNbO3 rivals, which are already deployed in metro networks, says Carter.

In trials, Bookham s integrated laser/modulator package has transmitted 10.7 Gb/s across the whole C-band, through dispersion of 1600 ps/nm over standard industry fiber with penalties below 1 dB. The company will start sampling the transmitter to customers later this year. The response could shape the type of products seen in tomorrow s network, and the level of integration seen in optoelectronic components.