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Technical Insight

Essient Photonics brings new thinking to optical modulation technology

One of the new companies making its debut at OFC was Essient Photonics, a spin-out from Glasgow University in Scotland that specializes in optical modulators and photodetectors. These devices take advantage of the properties of resonant tunneling diodes (RTDs). The company has announced $7 million in series A funding from Pond Ventures, a venture-capital firm that specializes in early-stage investment in European technology companies.

Essient s novel RTD-based technology for making 10 and 40 Gbit/s optical modulators and photodetectors will significantly lower the drive voltage and footprint compared with current devices. At OFC Essient announced that its modulators can operate with a drive of 0.1 V, significantly lower than the several volts needed for lithium niobate and InP- or GaAs-based devices. The new modulators are only about 5% of the size of a typical lithium niobate modulator.

The company is headquartered in Glasgow and is following a fabless business model. The funding will be used to take existing prototype devices through to the stage where they are suitable for sampling to commercial customers. "We are about six months away from getting devices to customers," said Jeremy Chappell, Essient s VP of marketing. Essient is using Compound Semiconductor Technologies in Glasgow to fabricate the devices in the current development phase, and other potential fab partners are also being assessed. "One thing that came across very strongly when speaking to potential customers was that they need reliability of supply, and for that reason we are looking at a couple of foundry partners," said Chappell.

How does it work?

Essient s approach makes use of a technology developed and patented by the company s founder and chief scientist Charles Ironside. This involves using the properties of RTDs in a novel way to modulate light.

RTDs use a quantum well between two thin barrier layers to create an electron resonator. Electrons incident on the double barrier can tunnel through the first barrier into the well and then reflect between the barriers, creating standing waves. Electrons of a specific energy will constructively interfere, and so the standing waves created represent discrete energy states. Regions of negative differential resistance arise because of electron confinement and coupling in the well. The I-V curve of an RTD is characterized by peaks and valleys in the current as the bias voltage is varied. The peaks represent the bias condition at which resonance occurs, with the valleys being a bias point that prevents electrons passing through the energy states in the standing wave, effectively switching the device off.

RTDs have attracted much interest because of their ability to operate at high frequencies and provide a very large increase in current for a small applied bias voltage. An AC signal below 1 V can induce high-speed switching of the RTD and therefore the electric field it generates. Ironside has applied the RTD principle to waveguides, using the high-frequency change in electric field to shift the absorption band edge of a material, thus making an electroabsorption modulator.

Ironside s patent describing an RTD-based modulator represents the initial development of the idea, while more recent improvements are now subject to the patent application process. Though most RTD research to date has concentrated on GaAs/AlAs structures, Essient s RTD uses an InGaAs well with AlAs barrier layers. This enables the device to operate at telecom wavelengths and also gives a bigger difference between the peak and valley currents in the device s I-V curve.

The patent describes an RTD structure consisting of a 6 nm InGaAs QW between two 2 nm AlAs barrier layers (figure 1). The RTD is sandwiched between two InGaAlAs spacers that form the waveguide core. The InP substrate and a top InAlAs layer provide waveguide cladding, and an InGaAs contact layer is grown on the top cladding layer. Ridge waveguides are then etched and ohmic contacts deposited.

Early performance data

The patent describes typical devices with a 2 x 100 µm2 active area. These have peak current densities of 20 kA/cm2 with a peak-to-valley current ratio of 4. The difference between peak and valley voltages is 0.8 V. The modulator bandwidth and extinction ratio are related to the RTD s structure and electrical characteristics. The extinction ratio is directly related to the peak-to-valley current ratio in the RTD I-V curve. For a device with a 4 x 200 µm2 active area the extinction ratio peaked at 28 dB for 1565 nm light (figure 2). Essient says that at present the devices can operate over a 50 nm range of wavelengths, an important feature for devices that may be used in WDM systems.

A key feature of Essient s technology is that in principle the device can operate in reverse as a light detector. The RTD will then act as an integrated amplifier for the photocurrent generated by the incident light.

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