Ultrawide-band/high-frequency photodetectors

Abstract

The two main trends in the progress of ultrawideband/high-frequency photodetectors (PD's), improving the bandwidth-efficiency product and obtaining a high saturation current, are reviewed. With respect to achieving large bandwidthefficiency, the limiting factors and potentials of edge-coupled (waveguide, waveguide-fed, traveling-wave, periodic-travelingwave), resonant-cavity, and refracting-facet photodiodes, as well as the avalanche photodiode are discussed. Regarding high-saturation current, the author estimated how much the space-charge effect limits the saturation current and two ways to reduce the space-charge effect are outlined. One way is to distribute the photocarriers along the edge-coupled PD's and the other is to increase the carrier velocity using a uni-traveling carrier structure. The waveguide-photodiode-based technologies that we have developed are also presented; namely the design and fabrication of a 100-GHz waveguide photodiode (WGPD), uni-traveling carrier WGPD, 60-GHz packaging, and a 20-GHz large-core WGPD for the planar lightwave circuit integration. A 50-Gb/s receiver opto-electronic integrated circuit technology based on the WGPD is also presented.

Millimeter-wave photonic wireless links for very high data rate communication

“…47 As layer instead of in the intrinsic absorption layer as in the traditional p-i-n photodiode. In the p-type absorption layer, photogenerated holes are the majority carrier and should relax directly to the anode of the UTC photodiode without transport [28]. Photogenerated electrons, on the other hand, will diffuse through the p-type absorption layer and drift to the InP depletion and cathode layers.…”

“…Compared with using the MMW electrical amplifier to further amplify a weak electrical signal from the high-speed photodiode, a combination of an erbium-doped fiber amplifier and high-power, high-speed photodiode can convert the intense optical signal to a strong MMW signal directly. This not only releases the burden imposed on the MMW amplifier but also provides superior noise performance [28,29]. With the integration of a high-directivity antenna, it should be possible to achieve high gain (directivity) in the broad frequency regime and radiate the wideband digital data carried on the MMW wave efficiently [30][31][32].…”

“…It can thus be understood that electrons are the only active carriers in the UTC photodiode structure. This leads to a significant improvement in the effective carrier drift velocity of the photodiode during operation and excellent SCBP performance [28,70,71]. The major difference between the UTC photodiode and the NBUTC photodiode is the insertion of an additional p-type charge layer in the collector layer to control the distribution of the internal electrical field, which results in an over-shoot of the electron drift velocity even under high reverse bias voltage (-3 V) [56][57][58][71][72][73][74].…”

“…Compared with the approach of using several MMW amplifiers to further amplify the small photocurrent from a high-speed photodiode, the combination of highspeed, high-power photodiodes with a high-power erbium-doped fiber amplifier is an attractive alternative to costly, narrow bandwidth, noisy electrical MMW amplifiers with limited gain and saturation power performance [29]. The (peak) saturation-current-bandwidth product (SCBP) is a key parameter for evaluating the performance of these high-power, high-speed photodiodes [28,29,[56][57][58][59]. By increasing the saturation current of the photodiode, it is possible to boost the injected optical power and further increase the maximum available MMW power produced therefrom.…”

“…where J max is the maximum output current density from the photodiode, V bias is the reverse bias voltage, Φ is the built-in potential of the photodiode, ε is the dielectric constant in the depletion region, V carrier is the effective drift velocity of the photogenerated carriers (including the drift velocity of electrons and holes [28]) and D is the depletion layer thickness. As can be seen, a thinner depletion layer, higher effective carrier drift velocity, or higher reverse bias voltage should improve the maximum output current density of the photodiode.…”