Transmitting 40G signals over long distances presents significant challenges. At 10G, the modulation format of choice is the conventional on/off keying (OOK) due to its simplicity and low cost. As data rates increase to 40G and beyond, however, chromatic dispersion and, even more important, polarization-mode dispersion (PMD) limit the distance over which the data can be transmitted. In fact, 40G transmission using conventional OOK modulation format is restricted to a distance of 2 km maximum. For longer distances, advanced modulation formats are required.

Differential quadrature phase shift keying (DQPSK) is an important modulation format for next-generation 40G optical transport systems. It has excellent tolerance against chromatic dispersion, PMD, optical add-drop multiplexing (OADM) filtering, and optical noise. In addition, 40G DQPSK is spectrally efficient and can be deployed side by side with current 10G line cards in standard 50 GHz wavelength spacing, which is a big benefit for network operators. This article will discuss key enabling technologies for 40G DQPSK systems and offer a roadmap for future development.

Current Status of 40G DQPSK Systems
The advantages of 40G DQPSK have been demonstrated in laboratories and field trials for many years. However, due to the lack of availability of key enabling components, commercial deployment of 40G DQPSK systems has not been possible until now. Figure 1 shows a typical block diagram of a 40G DQPSK line card using commercially available components, which includes modulators, modulator drivers, MUX/DEMUX, and demodulators/receivers. These are the key enabling components for 40G DQPSK systems.

Modulators
Modulators convert electrical data into optical bit stream. DQPSK is a phase modulation format that converts two bits of data in the electrical domain into a single bit of data in the optical domain. The bit in the optical domain can have four possible values, "0," " π/2," "π." and "3 π/2" versus two possible values "0" and "1" in the electrical domain. Thus, it is possible to encode the data at a rate of 2 bits/symbol transmitted (a symbol is an optical bit) and reduce the transmission rate by a factor of 2. Currently, the technology of choice is Lithium Niobate (LN) modulator in metal package with GPPO connectors. Fujitsu and Sumitomo Osaka Cement Company (SOCC) modulators are the two popular choices for 40G DQPSK. Both require approximately 8Vpp differential RF drive.

Drivers
It is important to match the performance of the modulator and driver for optimum performance. Impedance mismatch between the modulator and driver results in excessive jitter due to multiple reflections. Drive voltage is optimized based upon modulator size, bandwidth, insertion loss, and material properties. Early 40G DQPSK modulators required a drive voltage as high as 12Vpp, which put severe demands on the driver in terms of reliability and power consumption. The current modulators from Fujitsu and SOCC require approximately 8Vpp differential, which can be met with current driver technology.

Figure 3 shows a photo of a 40G DQPSK modulator driver. This driver delivers an output voltage of 8Vpp differential and is well matched with both the Fujitsu and SOCC modulators. Its high gain allows the 2514DZ to accept a single-ended input as low as 500 mVpp from the MUX while delivering a large output voltage of 8Vpp differential.

MUX/DEMUX
The MUX function is to multiplex 16 lanes of electrical bit streams into two lanes of 20G electrical bit streams. The MUX also includes a pre-coder, on-chip VCOs, clock multipliers, and de-skew functions. On the receive side, the DEMUX de-serializes two lanes of 20G electrical bit streams back to 16 lanes of lower speed data. Key challenges for the MUX/DEMUX are jitter performance, power consumption, and packaging. Figure 4 shows a photo of a Sierra-Monolithics 40G DQPSK MUX in a BGA package with GPPO connectors. GPPO connectors are currently required for the high speed connection between the MUX, modulator drivers, and modulators.

Demodulators and Receivers
The 40G DQPSK signal contains information regarding the phase difference of two successive bits. A 40G DQPSK demodulator function is to convert the phase difference information back to intensity. Figure 5 shows a demodulator by Optoplex.

Because the information is contained by the difference of the two optical bits, balanced receivers are used after the. A balanced receiver amplifies the differential input into the receiver. Thus if the two optical bits are in phase, the phase difference between them is small and the balanced receiver will convert the phase information into a "0." If the two optical bits are out of phase by "π/2," the phase difference between them is large, and the balanced receiver will convert the phase difference into a "1." As shown in the block diagram in Figure 1, two balanced receivers are needed per system. U2t Photonics, Picometrix, and Yokogawa are the current top 40G receiver vendors. Figure 6 shows a photo of a balanced receiver from U2t Photonics and Figure 7 shows a photo of a balanced receiver from Picometrix.

Future development and roadmap
Unlike previous 40G modulation formats, 40G DQPSK is based entirely upon a 20G transmission bit rate. Future development will take advantage of this unique feature to drive down the size, cost, and power of 40G DQPSK components and transponders. Some of the key enabling technologies are discussed below.

Modulators: Currently, modulators are Lithium Niobate (LN) Mach-Zhender modulators with GPPO connectors. In the near term, expect to see lower voltage drive requirements, which will lead to lower power consumption for drivers. However, due to its inherent size limitation, GPPO will remain the connector technology of choice.

InP-based Mach Zhender modulators: LN modulators will face strong competition from more compact InP-based Mach Zhender modulators, which are gaining market share at 10G. We expect to see competitive offerings of this type of modulator for 40G DQPSK. Bookham, for example, has just announced successful demonstration of its 40G DQPSK tunable transmitter assembly that includes a tunable laser and a compact (10-mm) InP-based Mach Zhender modulator.

Demodulators and balanced receivers: Currently, 40G DQPSK demodulators are based upon free-space optics or fiber solutions, which require large size packages. In the near future, demodulators and balanced receivers will be integrated into the same package. Photonics integrated circuit (PIC) technology is a good candidate for this functional block, but other hybrid approaches based upon "silicon bench" also look promising.

Drivers and MUX/DEMUX: Driver and MUX/DEMUX technology will likely to migrate from metal package with GPPO connectors to a surface mount ceramic package, which will lead to significant reductions in size and packaging cost. We expect this type of innovation will enable the deployment of 40G DQPSK in high volume.

Summary
40G DQPSK technology is emerging from the research environment to become a commercial reality. The current generation of 40G DQPSK components, based upon the 40G GPPO interconnect technology, are large in size and expensive. Future generations of 40G DQPSK components will be based upon surface mount technology, which will drastically reduce their size and cost. Further integration of optical components will be needed to make 40G DQPSK transponders more compact and cost competitive. This technology is a fertile ground for research and development activities worldwide and significant progress is sure to be made in the next few years.

About the Author
As one of Inphi's founders, Dr. Loi Nguyen has over 20 years of experience in the development of high-speed devices and integrated circuits. Dr. Nguyen holds seven U.S. patents and is an author of more than 50 scientific publications. He has served on technical committees of the IEEE International Electron Devices Meetings, the IEEE Device Research Conference, and the IEEE International Solid State Devices Meetings. Dr. Nguyen holds B.S. and Ph.D. degrees in electrical engineering from Cornell University and an MBA from the Anderson School of Management at UCLA. To reach Dr. Nguyen, email: lnguyen@inphi-corp.com.