Electronic Design
Sensitive Transimpedance Amps Make 40G And 100G Optical Transport A Reality

Sensitive Transimpedance Amps Make 40G And 100G Optical Transport A Reality

The growth of the Internet continues with the explosion of video, expanding mobile wireless applications, and the subsequent evolution of cloud computing. As a result, carriers and data centers are having to scale their core networks to the next level—that is, from 10G to 40G networks and ultimately to 100G. With 40G adoptions doubling annually and 100G expected to grow to more than 20% by 2013, carriers are looking to special modulation methods to achieve the speed and reliability expected of core backbone networks.

To make 40G and 100G equipment available, a whole new batch of special components is necessary to meet those data speed requirements. One critical part is the transimpedance amplifier (TIA) (see the figure) that accepts the input from the indium-phosphide (InP) photo detector diodes on the fiber. Now, Inphi Corp. has solved that problem with the Inphi 2850TA, the industry’s first OIF-compliant (Optical Internetworking Forum) 100G coherent TIA. Inphi is also extending its line of 40G products with the introduction of its 2251TA, the highest-sensitivity linear TIA for 40G differential quadrature phase-shift keying (DQPSK) applications.

The main problems with 40G transport on single-mode fiber are chromatic dispersion (CD) and polarization mode dispersion (PMD). Older on-off keying (OOK), optical duo-binary, and differential phase-shift keying (DPSK) no longer can the job. As a result, DQPSK is solving the problem. It keeps the laser on continuously and splits the signal into classical I/Q signals where the optical baud rate is only half the electronic bit rate.

To accommodate such signals, special TIAs with greater bandwidth and linearity are needed. Inphi’s 2251TA is a differential input linear TIA and variable gain amplifier (VGA) that supports data rates to 22.5 Gbits/s. Typical bandwidth is 20 GHz. Designed for 40G DQPSK receivers, it has the highest sensitivity and linearity needed to support DQPSK. Customers now can extend their reach by addressing the challenges of PMD.

The 2251TA delivers the highest sensitivity over a wide range of optical signal-to-noise ratios (OSNRs), which improves link margins. Also, it offers the greatest linearity (<5% total harmonic distortion, or THD), which makes possible the use of electrical dispersion compensation (EDC) technology to combat distortion due to PMD. It provides the lowest power dissipation available. And when it’s combined with Inphi’s recently introduced 2811DZ differential driver in surface-mount technology, customers will have a cost-effective, complete transmit and receive solution for next-generation 40G DQPSK systems.

100G represents a challenging tenfold leap in data rates over current systems, putting severe demands on the underlying technologies in the transceivers and line cards of next-generation networks. Inphi’s 2850TA differential input linear TIA/VGA offers an alternative. To achieve the 100G mark, the industry has developed a dual polarization and coherent detection scheme.

The 2850TA is the first 100G coherent TIA/VGA (variable-gain amplifier) available that complies with the OIF’s rigorous standards for this new transmission mode. This new mode uses dual polarization with QPSK to quadruple the amount of data that can be transmitted over a single wavelength of light. Coherent detection recovers the polarization and phase information. It is a breakthrough approach to achieving 100G serial bit-rate transmission over existing metro and backbone networks. The availability of the 2850TA receiver lets equipment designers build 100G coherent receivers for long-haul and metro regional networks.

The dual-differential linear 2850TA TIA/VGA offers the highest bandwidth (greater than 25 GHz) available, giving customers more margin in 100G system design. It also boasts superior linearity for coherent receivers, at less than 5% THD, exceeding OIF requirements. And, it takes advantage of its low power dissipation (250 mW/channel) to conserve energy.

One of the first applications of the 2850TA is in the 100G coherent receiver offered by u2t Photonics AG of Germany. Its CPRV1010A uses the Inphi devices to achieve the demodulation of the dual-polarization QPSK.

Engineering samples of the 2850TA dual-differential linear TIA/VGA for 100G coherent applications and the 2251TA high-sensitivity linear TIA/VGA for 40G DQPSK applications are available now.

Inphi Corp.

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