OTDM switch on a single chip blazes at 20 Gbits/sec

OTDM switch on a single chip blazes at 20 Gbits/sec

ACHIM STRASS

Researchers at the Heinrich Hert¥Institute in Berlin report that they have fabricated an optical time-division multiplexing (OTDM) switch as a photonic integrated circuit on a single chip for the first time. Additionally, the researchers have verified functionality of this chip as an all-optical add/drop multiplexer in optical communications subsystem experiments.

The Heinrich Hert¥Institute is recognized as a leading research institution in communications technology. The OTDM research was supported by the Federal Ministry for Research and Technology of Germany and by the Federal State of Berlin.

An add/drop multiplexer enables the addition and dropping out of channels in optical networks, thereby allowing a flexible, multinode architecture. The researchers say they have achieved both the adding and dropping of 5-gigabit-per-second channels to a 20-Gbit/sec channel. Chip size was only 4.0 ¥ 1.5 mm, with the potential for further size reduction. The demultiplexing operation occurs at speeds as high as 40 Gbits/sec.

Monolithic integration of such components is necessary to achieve compact, stable, high-performance devices at low cost. Practical applications could come in the next few years in long-haul transmission systems, high-speed local area networks or packet-switched networks.

Many applications, such as add/drop multiplexing, fast multiplexing, and demultiplexing for optical networks with transmission over several hundred kilometers, are possible with high-speed OTDM point-to-point links. Monolithic integration is practical for cost-effective, high-volume fabrication of electronic and optoelectronic devices.

OTDM switching is deployed in high-speed optical-fiber transmission systems. Research on fast optical switches is ongoing throughout the worldwide scientific community to overcome the speed limitations of electronic switching systems.

The Institute`s objective is to strengthen Germany`s telecommunications industry. It focuses on electronic imaging and global photonic networks, which comprise mostly optical transmission and information technologies.

Niraj Agrawal, project leader, points out, "Optically controlled switching devices are expected to play an important role in future high-capacity communications networks based on both OTDM and wavelength-division multiplexing [WDM] techniques."

Time-division multiplexing means that many data channels can be sent over a single fiber by using different time slots for different channels. The same is true for WDM, where many channels are transmitted over different wavelengths. However, the number of WDM channels is limited, and the technique requires complex schemes for wavelength stabilization, as well as expensive multiwavelength laser sources and stable filters. By using an OTDM switch, large network capacity can be obtained cost-effectively.

Experiments using this chip demonstrated transmission at 40 Gbits/sec over a distance of 100 km, followed by demultiplexing combined with all-optical clock-recovery, thus overcoming the speed bottlenecks associated with electronic switching devices.

Add/drop operation

As shown in the figure, the switch is structured as a Mach-Zehnder interferometer (MZI), where two semiconductor laser amplifiers (SLAs) provide the required nonlinearity. The principle of add/drop operation is as follows: The MZI consists of two phase sections, with SLAs inserted between two cascaded, 3-decibel couplers. The input coupler distributes the input light simultaneously to both branches of the interferometer. The two lightwaves interfere at the second coupler in such a way that light comes out at port 3. By shifting the phase of light in one of the branches with respect to the other, the light can be fully switched to port 4. This phase shift can be obtained in principle by using the electro-optic effects due to externally applied voltages or gain saturation of an SLA obtained by optical control pulses.

Says Agrawal, "The time-dependent electro-optically induced phase shift windows are generally unsuitable for switching of individual bits in case of high-speed multigigabit signals. Therefore, we have integrated a semiconductor laser amplifier in both branches of the MZI switch."

By injecting an optical control pulse from a 5-gigahert¥pulse source into the MZI switch, the refractive index changes due to the gain saturation of the laser amplifiers, leading to the phase shift of the lightwaves in its two branches. A symmetric interferometer would not produce any switching effect.

However, if the two SLAs are slightly shifted (in this device, 300 microns correspond to 3 picoseconds), the counter-propagating control and the desired data pulses coexist in only one of the amplifiers. Thus, phase shift is achieved between the two branches only for this bit, thereby switching only this bit to port 4. That means signal X was added, and signal C was dropped.

The wavelength of all pulses was about 1550 nm. For input, 5-Gbit/sec optical data signals were multiplexed by a tunable-fiber delay-line multiplexer to a 10-, 20- or 40-Gbit/sec data signal and then coupled into the interferometer. At port 3, the pulses were observed with a streak camera. The researchers found that the extinction ratio of the bits that were dropped out was 10 dB. The control pulses were generated by a mode-locked laser giving pulses with a full-width-half maximum of 1.8 psec and a repetition rate of 5 GHz. Synchronization of the data and control pulses was done manually.

The design and fabrication of the integrated MZI are sophisticated. Bulk indium gallium arsenide phosphide quaternary layers buried within indium phosphide (InP) cladding layers with bandgap wavelengths of 1.55 and 1.25 microns were used as cores of the active and passive waveguides, respectively. A butt coupling scheme was employed for active/passive transitions. An etched mesa buried heterostructure was used for the active region.

For the 3-dB couplers, multimode interference devices were used because of their compact size and low polarization sensitivity. Semi-insulating iron-doped InP was grown by metal organic chemical vapor phase epitaxy as blocking layers. Here, a silicon nitride mask was used for patterning.

Compact circular bends with radii of 500 microns were used for connecting the interferometer branches to the input/output waveguides, which were separated by 1 millimeter. Moreover, these waveguides were inclined at 7° to cleaved facets to reduce reflectivity. No antireflection coating was employed. q

Achim Strass writes from Munich, Germany.