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Optical amplifiers are also key components used for processing optical and terahertz signals. Amplifiers are realized with a number of different technologies, including as laser, semiconductor optical, Raman, and optical parametric amplifiers. Two major categories of optical amplifiers include fiber-based and planar optical waveguide amplifiers.13

For photonic based transmitters, optical-electric (O/E) converters are key devices. Unitraveling-carrier photodiodes (UTC-PDs) and photodiodedevices based on positive-intrinsic-negative (PIN) photodiodes (PIN PDs) serve as efficient O/E converters. A combination of these two approaches can provide conversion with reasonable output power through 380 GHz, with photocurrent of 10 mA and bias voltage of 1.1 V yielding output power of 110 μW. This output power can be increased to 400 μW using photocurrent of 20 mA. Figure 3 depicts output powers for three O/E converter technologies: UTC-PD, PIN-PD, and low-temperature (LT) GaAs photomixers.2

3. This plot compares the high-frequency performance of three O/E converter technologies: UTC-PD, PIN-PD and low-temperature (LT) GaAs photomixers.

As the use of different electronic devices has shown over the past few years, the performance of terahertz systems can be steadily improved, including the noise and phase performance. Figure 4 shows the conceptual design of a transceiver for terahertz applications, to demonstrate how different electronic devices, such as sources, modulators, transistors, and amplifiers, can be applied to improve the performance of terahertz transceiver systems.

4. This simple block diagram shows an electronic terrhertz transceiver.

A variety of electronic sources for terahertz applications have been developed in recent years. Some of the signal sources are integrated-circuit (IC) oscillators based on transistors, resonant tunable diodes,14 Bloch oscillators,15 or plasmaticoscillators.16 Hot-electron-bolometer (HEB) superconductor mixers have also been developed over the past decade, providing some of the highest sensitivity levels for heterodyne receivers at frequencies above 1 THz. Low-noise amplifiers (LNAs) based on indium-phosphide (InP) high-electron-mobility-transistor (HEMT) devices are also capable of operation at these frequencies.17

At the conjunction of these two technologies, fourth or quad pixel technology limits noise levels for high-speed spectroscopy and imaging applications. The operation and processing speed of these pixel-based systems can be significantly improved by means of integrated HEB circuits and intermediate-frequency (IF) amplifiers integrated in multipixel focal plane arrays (FPAs).18 The low power consumption of LO sources and the low operational noise of HEB mixers make them suitable for this kind of integration, especially in monolithic-microwave-integrated-circuit (MMIC) IF amplifiers. Proper use of input matching networks provides the necessary frequency tuning and opportunities for using MMIC devices with different sources and at different operating frequencies. Chip devices based on single-pixel HEB array technology have been fabricated with close to 30-dB gain across bandwidths as wide as 10 GHz at terahertz frequencies.

Figure 5 shows the base structure of a single-pixel HEB array.17 The active elements consist of a photon-cooled HEB, constructed from NbN films deposited on a silicon substrate. The three composite focal plane elements, which consist of HEB and MMIC IF amplifiers, have demonstrated operation at frequencies to 1.6 THz. In theory, the integrated quasioptical pixels can be operated at even higher frequencies. Research has indicated the possibility for developing FPAs with potentially hundreds of pixels. High-frequency HEB FPAs can be useful in applications such as astronomy, long-distance detection, and medical diagnostic systems.

5. These are the essential components of a single-pixel HEB.

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