A new all-optical signal-processing device for ultra-high-speed networks

A variety of factors are currently driving demand for real-time, network-based transmission of high-resolution video content in applications such as immersive TV conferencing, video streaming, and remote diagnosis. High-definition TV (HDTV) needs a bit rate of 1.5Gbps, and digital cinema requires 6Gbps for real-time streaming. NHK, the Japanese public broadcasting company, is developing ultra-high-definition TV (UHDTV) (i.e., 7680×4320 pixels) at 72Gbps.¹ However, existing Internet Protocol (IP) networks are inadequate to transmit this kind of super-high-capacity video content in real time. Consequently, an entirely new communication infrastructure is called for. One of the more promising schemes for a next-generation network is optical time division multiplexing (OTDM) employing ultrafast all-optical signal-processing devices.

To date, various optical signal-processing devices operating above 160Gbps have been used in OTDM transmission experiments. These include fiber-based applications such as nonlinear optical loop mirrors (NOLMs),² semiconductor optical amplifiers (SOAs),^3,4 and semiconductor electroabsorption modulators (EAMs).⁵ But NOLMs have the disadvantage that they are large. SOAs can be complicated to implement because of their slow time-response component. And, EAMs suffer from large insertion loss and limited response speed. What is needed is a low-power-consuming, simple, all-optical signal-processing device operating at, for example, 160Gbps, which corresponds to about 100 channels of HDTV or two channels of UHDTV.

If we could control the phase of light, we could construct a switching device using lightwave interference. Recently, we discovered a new ultrafast all-optical cross-phase modulation (XPM) effect associated with the so-called intersubband transition (ISBT) in an ultrathin indium gallium arsenide/aluminum arsenide antimonide quantum well.⁶ All-optical phase modulation can be carried out on transverse electric mode (TE)-polarized light using a transverse magnetic mode (TM)-polarized gate pulse. An attractive feature of this approach is that phase modulation takes place in the lossless TE mode, and hence we can build low-insertion-loss, ultrafast signal-processing devices. XPM is the result of refractive index dispersion associated with the band-to-band transition, which is influenced by the ISBT.⁷ The response speed is governed by the electron relaxation time in the conduction band, around 1ps.

Figure 1. Mach-Zehnder-type gate module structure with an intersubband transition (ISBT) quantum well. PBS: Polarizing beam splitter. PZT: Piezoelectric transducer.

We developed a Mach-Zehnder interferometer-type optical gate module using space optics.⁸ Figure 1 shows the configuration in schematic form. The polarizing beam splitter was used to send the 160Gbps TE signal along the two arms of the interferometer. Next, TM gating signals were injected into the ISBT device from both sides to cause a phase change in the signal light. This space-optic module measures 137×100mm. The module enabled us to achieve error-free demultiplexing (DEMUX) of a 160Gbps signal to a 40Gbps one using 2pJ/facet×2 gate pulse energy. This corresponds to 160mW. Figure 2 shows the ‘eye pattern’ of the 160Gbps signal and the demultiplexed 40Gbps signal. We also evaluated the bit-error rate to verify error-free performance.

Figure 2. Eye patterns produced by the 160Gbps and demultiplexed 40Gbps signals.

To conclude, error-free DEMUX operation of a 160Gbps optical signal was accomplished using a novel XPM effect in ISBT. This method has the advantage that ISBT possesses an intrinsically fast response of around 1ps, and the device is lossless for TE mode. In addition, electric power is not required, as in SOAs. The gate pulse power at present is still high (160mW). However, we can reduce it by improving the quantum-well structure. Our final objective is to develop a very small Mach-Zehnder interferometer-type gate module of 1–2mm by hybrid integration with a silicon-wire waveguide. Such a low-power device can be expected to play a key role in future ultrafast OTDM networks.