technology photodiodes
Figure 1. A simplified diagram of analogue optic link
These analogue links can be viewed as replacements for conventional electrical cables or waveguides, which are often impractical, due to their high loss and limited bandwidth. One reason why analogue optical links can realise very high speeds is that they are able to overcome the limitations of analogue-to-digital and digital-to-analogue conversion, which are found in digital transmission. With analogue optical links it is possible to transmit signals over vast distances by using a substantial amount of RF power between a centre station and remote locations, such as antenna feeds. This reduces the complexity of the instruments held at remote locations, and cuts their maintenance requirements.
The performance of these links depends on the capability of the photodiode in providing optical-to- electrical conversion. A high gain, large bandwidth link demands a diode with high-power-handling capacity and high-speed operation. High linearity of the photodiode is valued too, because this minimises the signal distortion in the link and enables it to maintain a large, spurious-free dynamic range.
Figure 2. Schematic diagram of an OEO based on a transposed gain approach
High-performance photodiodes can also be deployed in oscillators, an indispensable component in virtually all modern electronic systems. Most of these oscillators are electronic, and they tend to rely on ‘high-Q’ quartz crystal resonators to achieve high spectral purity. But as frequency increases their performance drops off, due to either a weakening of the resonance or an introduction of phase noise, which results from frequency multiplication.
It is possible to address these weaknesses with an optoelectronic oscillator (OEO) (see Figure 2 for an example of this approach). This class of oscillator produces an ultra-stable microwave signal at
frequencies of up to tens of gigahertz by exploiting the low-loss properties of optical resonators. Approaches can involve long fibre delay lines, Fabry–Pérot cavities and whispering gallery mode resonators. Many OEOs are based on a transposed gain oscillator, while others use a dual-mode laser or an optical frequency division technique. One of the strengths of these OEOs is that they generate microwave signals in both the electrical and optical domains, a trait that makes them specifically suitable for integration with other photonic systems.
Ideally, high-RF-power-output photodiodes are used in OEOs, because this increases the signal-to-noise ratio and lowers the system phase noise. Further enhancements in performance are possible when the RF power output from the photodiode is large enough to eliminate the electronic amplifier and its corresponding noise from the loop. The introduction of extra phase noise during the photo-detection process can be addressed with low distortion photodiodes.
Device design
PIN photodiodes are widely used to meet the requirements for high optoelectronic conversion efficiency and large bandwidth. These devices contain an intrinsic absorber sandwiched between heavily doped n-type and p-type layers that give rise to a space charge region. When photons hit the device, they spawn electron-hole pairs, which are pulled apart by an applied bias voltage acting in partnership with an internal electric field established by ionized dopants. Electrons and holes flow in opposite directions, creating a photocurrent in the external circuit.
Many PINs operate at 1.55 µm, the wavelength for long- distance optical communication. Producing this class of device involves epitaxial growth of a lattice-matched InGaAs layer on an InP substrate. A key decision is to select the optimal thickness for the depleted, ternary absorbing layer: Get it too thick and the diode is too slow, due to excessive transit times; but get it too thin and the device does not absorb enough incident light, leading to a signal that is too weak.
Another key characteristic for the photodiode is its power handling capability. This is strongly influenced by the space-charge effect, and can be traced back to the spatial distribution of photo-generated carriers as they pass through the depletion layer. In the photodiode, the electric field generated by the free carriers – the space- charge field – opposes that established by ionized dopants and the applied bias voltage. This leads to a total electric field in the depletion region that drops to almost zero at high current densities. When this happens, carrier transit time increases and RF power output falls, due to a combination of compression and saturation. Making matters worse, the voltage drop across the load resistor reduces the effective bias voltage, pushing the diode toward saturation (see Figure 3 for a summary of the major factors limiting
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www.compoundsemiconductor.net April / May 2012
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