| Summary: | 博士 === 國立交通大學 === 光電工程系所 === 97 === This study investigates of the key components of optical electrical devices operating in the THz and Sub-THz range (0.1~1THz), including photoconductive (PC) antennas and photonic transmitters (PTs). PTs are integrated high speed photo diodes with printed planar antennas designed based on the required radiation frequency range. This study also examines related high frequency measurement systems and broadband communication applications.
The feasibility of several novel photonic transmitters is demonstrated first, which are designed for high peak power generation and wireless ultra-wideband (UWB) communication. Initially, the feasibility of a PT composed of a low-temperature-grown GaAs (LTG-GaAs) based separated-transport-recombination photodiode (STR-PD) and a micromachined slots antenna is demonstrated. Under femto-second (fs) optical pulse illumination, this device radiates strong electrical pulses (300 mW peak power) at a designed frequency of 500GHz. A traditional LTG-GaAs based PT under high, externally applied electrical fields (>50kV/cm) is then eliminated using our STR-PD based PTs (STR-PTs). Monolithic integration of a GaAs/AlGaAs based uni-traveling-carrier photodiode (UTC-PD) with a broadband micromachined antenna creates UTC-PD based PTs (UTC-PTs) that can also radiate strong sub-THz pulses (20mW peak-power) with a narrow pulse-width (<2ps) and wide bandwidth (100~250GHz). The bias dependent peak output-power of both PTs (UTC- and STR-PD based) makes them highly promising for use as a data modulator/emitter for a photonic UWB system.
This study also describes in detail the characterization of two high power PTs based on two high power photodiodes, UTC-PD and STR-PD. Both PDs have the same depletion layer thickness, i.e. the same theoretical RC-limited bandwidth, and are monolithically integrated with the same broadband micro-machined circular disk monopole antennas. However, the STR-PD based transmitter exhibits a significantly different dynamic and static performance from that of the UTC-PD based transmitter due to a low temperature grown GaAs (LTG-GaAs) based recombination center inside the active region, as well as a much thinner thickness of an effective depletion layer. Under optical pulse excitation (~480pJ/pulse), the STR-PD based transmitter exhibits a markedly lower maximum average output photocurrent (1.2mA vs. 0.3mA) than that of the UTC-PD transmitter. This is despite the fact that the radiated electrical pulse width and maximum peak power, which are determined by the same THz time domain spectroscopic (TDS) system, of both devices are comparable.
Next, high power THz generation by using PC antennas is studied by comparing the emission properties of LT-GaAs PC antennas with GaAs:O PC antennas in the pulse and CW mode. GaAs:O PC antennas can generate a higher THz power than LT-GaAs based both in the pulsed and CW modes. The bandwidths of GaAs:O PC antennas and LT-GaAs PC antennas are measured at approximately 1THz both under pulse (TDS) and CW (photomixing) pumping. However, the THz power of LT-GaAs PC antenna becomes saturated in CW mode, while GaAs:O does not. This finding suggests that GaAs:O PC antenna is a more reliable THz emitter than LT-GaAs, which is difficult to reproduce.
To excite THz and Sub-THz radiation, not only are a Ti:Sappire laser (��=800nm) and fiber mode locked laser (��=1550nm) used, but a CW excitation system is also established, which consists of two laser diodes (��=800nm) . The radiated powers of all devices are compared using a Helium-cooled bolometer. Additionally, radiated electrical fields are measured by a TDS system, which is based on LTG-PC antennas. The power spectrum of devices can be determined following fast Fourier transformation (FFT). A wideband communication system is also adopted by using a high speed horn (W band, 75~110GHz) antenna as a receiver to demonstrate the effectiveness of sub-THz wideband communication, which displays an improved data transmission rate.
Finally, this study demonstrates the feasibility of wideband communication applications by using our sub-THz emitters as follows: (1) Communication quality of the LTG-GaAs PC antennas based TDS system is improved by using Manchester coding; and (2) A wideband carrier (W band, 75~110GHz) is generated by using a fiber mode-locked laser as system optical source and determining its maximum data transmission rate. In (1), the Ti: Sapphire laser is adopted as the excitation source and we demonstrate that the bit error rate (BER) improved from 10-8 to10-12 by using Manchester coding. In (2), data transmission of 2.5Gbit/s at W band is successfully demonstrated by utilizing the advantage of a high repetition rate of fiber mode-locked laser.
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