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Current Status of the EAMs

How are current state-of-the-art EAMs doing?

Many researchers all over the world worked and are still working on the EAMs. As discussed in the EAM design, research focus was on the efficiency and the speed of the EAM as an optical modulator. Because the efficiency and the speed of the EAM are counter-acting each other, the optimization is always necessary for the given fiber-optic link requirement. With a decent modulator efficiency, a 3-dB bandwidth of 10 GHz is not difficult to realize. Further research is going on currently how to improve the bandwidth (another way of saying device speed) without too much compromising the modulator efficiency. A 3-dB bandwidth of ~40 GHz had been reported with the conventional lumped-element-electrode modulator structure with either a shorter device length or using a 50-ohm resistor parallel with a modulator (sacrificing 6 dB in RF gain).

One of the notable approach to overcome the efficiency-speed dilemma is the travelling wave modulator, which overcomes the conventional modulators' RC-time limit for the speed. In this approach, the microwave travels the microwave transmission line with the same or very similar speed of the optical beam, which is travelling in the optical waveguide placed parallel with the microwave transmission line. As this approach is not limited by the RC-time limit, the longer waveguide length (hence the bigger absorption, better efficiency) can be used. This enables to achieve a good efficiency modulator with a very large 3-dB bandwidth (>40 GHz, even 100 GHz). One important thing that should be noted here is that the length of the EAM is now limited by the propagation loss, not by the RC-time constant, as the propagation loss of the EAM is quite large (15 - 20 dB/mm).

Another important development occured in integrating the EAM with the light source, the laser diode. This shows one definite advantage of the EAM over Mach-Zehnder modulator, which cannot be integrated with the light source. The DFB lasers integrated with an EAM are already being produced by leading companies in 2.5 Gb/s or in 10 Gb/s version. The followings are examples:

  • Lucent's 10 Gb/s integrated EAM with a DFB laser: data sheet ([PDF Image] 703 kB)
  • Alcatel's 10 Gb/s EA modulated DFB laser: data sheet ([PDF Image] 342 kB), article ([PDF Image] 136 kB)
The problem of the EAM that still needs improvement is, primarily, the efficiency. The efficieny of the EAM is comparable to that of the Mach-Zehner modulator, but as the Mach-Zehnder modulator can take much higher optical power, the RF gain of the link using EAM is naturally smaller than that using a Mach-Zehnder modulator.

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