光電式次兆赫波發射器及其應用之研究

Abstract

在本論文中，我們研究可利用短脈衝雷射激發之次兆赫波(0.1~1THz)發射器，包括結合高速光電二極體和印刷天線之次兆赫波發射器以及光導天線兩種型式。不同型式的元件可產生不同波段之脈衝訊號，我們探討此頻段相關量測系統，元件特性，以及探討其在寬頻傳輸之應用。 首先我們以波長為800nm短脈衝鈦藍寶石雷射激發，以產生出高功率次兆赫波輻射，並予以調制；本研究中首先探討分別由改良利用低溫成長砷化鎵(LTG-GaAs) 為基材的分離式傳輸複合光二極體 (STR-PD)以及以砷化鎵/砷化鋁鎵(GaAs/AlGaAs)為基材的單載子傳輸光二極體(UTC-PD)，並結合溝槽天線以及單極圓碟微波天線之次兆赫波激發器之輻射效率以及其操作行為，以符合寬頻通訊實驗所需。研究中成功分別驗證了: (1)中心頻率100GHz(約75GHz~120GHz)以及(2)中心頻率500GHz的高功率輻射元件之可行性。我們並詳細的探討此兩種不同的高速二極體結合相同結構之寬頻天線對於兆赫波發射之效果，研究顯示此兩種設計存在截然不同電流響應機制，但皆具有高功率之輻射輸出功率。 此外，短脈衝雷射亦可激發光導天線以產生兆赫輻射。我們嘗試利用製作程序較為簡單之多重氧離子佈值之砷化鎵(GaAs:O) 光導天線取代傳統低溫砷化鎵(LT-GaAs)做為激發器，並探討其性質差異。我們量測多重氧離子佈植砷化鎵 ( ions/cm2(500 & 800 ), ions/cm2 (1200 ) 薄膜材料和低溫成長之砷化鎵製程之光導天線，驗證其材料替代之可行性。本研究除探討此兩種光導天線在高偏壓下操作之飽和現象外，亦驗證其產生連續兆赫波之可行性。 對於兆赫波與次兆赫波之量測，除了可利用800nm 之鈦藍寶石鎖模脈衝雷射以及1550nm光纖雷射做為激發源外，本研究中也架設了利用800nm雷射二極體拍頻連續波激發系統，可做為連續波兆赫波激發之用。在量測此高頻微波的功率方面，除了分別利用熱輻射儀量測並比較各元件之絕對功率外，也架設基於低溫砷化鎵光導天線之時域光譜儀，以量測元件輻射出來之電場以及傅式轉換後之頻譜。為了進一步展示次兆赫波寬頻通訊系統，也架設了一利用高速號角天線(W頻段,75~110 GHz)做為接受器之寬頻通訊系統，以提高系統之資料傳輸率。 最後在次兆赫波發射器在寬頻通訊應用部分，我們分別做了以下展示: (1) 以Manchester編碼用改善通訊系統之傳輸品質以及(2)以光纖鎖模脈衝雷射(波長為1550nm)為系統光源以產生寬頻載波(W頻段,75~110GHz)，並量測此系統之最高資料傳輸率。在第一部份我們在利用鈦藍寶石鎖模雷射做為激發光源，並以低溫砷化鎵光導天線做為系統之傳輸接收器，成功的將誤碼率由10-8降低到10-12。在第二部分，我們利用了光纖鎖模雷射之高脈衝重覆率的優點，成功的展示了在W頻段下2.5Gbit/s的高資料傳輸率。

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.