| 摘要: | 隨著光纖通訊的進步，與人們對傳輸頻寬需求的日與劇增，射頻光纖系統已成為未來最可行的系統(radio-over-fiber system)之ㄧ，將寬頻的微波信號經由光纖傳遞，可以擁有低的傳輸損耗與高傳輸頻寬而沒有相位延遲的問題，在最後用戶端前的一公里再利用光子轉換發射器(photonic transmitter)將信號輻射至空氣中，以提供用戶端寬頻無線傳輸的服務，然而在較低的傳輸頻段已無法負荷如此寬頻的信號傳輸，因此在頻帶上的選擇必須往更高頻的W頻段設計，然而卻使此系統中關鍵的元件光電混波器 (optoelectronic mixer) 與光子轉換發射器在設計上更為困難，換言之需要極佳性能的光檢測器才能達到系統的需求，日前，NTT研究團隊所提出一種單載子傳輸光檢測器(Uni-Traveling-Carrier photodiode, UTC-PD)，此結構成功解決早期傳統PIN結構因較慢電洞累積於空乏層而造成之飽和效應，由於此結構將原本由電洞所主導的元件飽和效應改由較快的電子所主導，因而大大的提升元件之速度與飽和功率，在此篇論文中我們提出一種創新的結構近彈道傳輸單載子光檢測器(Near-Ballistic-Transport Uni-Traveling-Carrier photodiode, NBUTC-PD)，改善先前UTC-PD在偏壓與載子傳輸速上的限制，此結構已被證實有極高的頻寬與飽和電流乘積，在本論文中做了一系列完整的研究與探討。 首先，將介紹側照式NBUTC-PD之設計原理與量測結果，借由在原本的UTC結構中的N掺雜集極層(N-collector layer)中插入了一個P種類電場控制層(P+-Charge layer)，及一層未摻雜之電場承受層(Undoped Electric-field-suffered layer)，大部分的電場皆落在Electric-field-suffered layer，使電子可以在大部分的區域中都以峰值速度(Peak velocity)傳輸，只有在極薄的Electric-field-suffered layer中以飽和速度傳輸(Saturation velocity)，將此結構與漸耦合式光波導結合，不但可以得到高的飽和電流頻寬乘積(> 1280 mA-GHz, @ 40 GHz)也可以擁有高的光響應度(1.14A/W)。因根據前面量測結果發現，所展示的元件具有奇特的頻寬增加現象當光電流上升時，為了近一步釐清其中的物理意義，我們根據所量測到的頻寬響應與S參數建立了一個二端的等校電路，可以將此現象歸因於載子傳輸的峰值速度對電場的非線性與交流電容的減少，此模型的建立不但對元件特性有深一層的了解，更提供了將此元件與其他物件整合時所需的元件模型。接下來，爲進ㄧ步將所研發的元件應用在W頻段的射頻光纖系統中，對元件磊晶結構與尺寸上必須做更進ㄧ步的設計與微縮，因此提出背射型的NBUTC-PD，在磊晶結構上因載子傳輸速度的提升，使空乏區的最佳厚度可以增加進而降低元件電容，在元件背後整合微透鏡不僅可以增加元件響應度還可以增加對準誤差，根據量測的結果此元件可以在同等的元件尺寸下有較高的元件速度與飽和功率(120GHz, 24.6mA, 2952mA-GHz)。然後，我們進一步將元件與被動的帶通濾波器結合做為光電混波器，利用元件特有的偏壓相關的非線性現象，成功的將低頻的資料信號升頻至60GHz與100GHz，並將升頻轉換損耗分別的對光信號與電信號做分析研究，我們發現因低通的濾波器可以在元件產生駐波，進而增加元件非線性與降低轉換損耗，根據量測結果此元件可達到接近1dB的轉換增益，與大於15GHz的調制頻寬。 最後，將元件與側面發射的Yagi天線結合做為光子發射器，利用元件特有的偏壓相關的非線性現象，可以將資料信號直接藉由調變元件偏壓後直接升頻到W頻段，此元件與先前的研究做比較，在合理的接收功率下不但省去昂貴的光調制器與繁複的Si透鏡製作，並且同時將發射器與混波器用ㄧ個元件同時實現，根據量測結果此元件做為光電混波器使用時可達到接近-2.4dB的轉換損耗，而做爲光子發射器時可達大於-14.1dBm的偵測功率於30mA光電流下。在最後利用此元件成功實現1.25Gbit/s BPSK射頻光纖無線傳輸之系統，根據所量測之眼圖所推算之錯誤向量強度可2低於29 當接收之毫微米波功率為-20Bm而光電劉惟15mA時。 To keep pace with progress in fiber communications and the rapid increase in demand for large data capacity it is hoped that radio-over-fiber communication systems will become feasible systems in the future. To do this it is necessary to translate broadband data signals in optical fiber that have low propagation loss and high data capacity without phase delay problems. The final signal is radiated in free space over the last mile to the end user via a photonic transmitter. The system can provide broadband wireless communication services for the user. However, the low frequency bands are too crowded to translate such broadband signals. Thus, it is necessary to select a higher frequency band such as the W band. This however increases the difficulty of designing the optoelectronic mixer and photonic transmitter whose core component is a photodiode. In other words, we need a photodiode with extremely high performance to match the system requirements. Recently a research group at Nippon Telegraph and Telephone Corpration (NTT) has demonstarted a uni-traveling-carrier photodiode (UTC-PD), which minimizes the saturation effect of the traditional pin photodiode, induced by the acumulation of slow photo-generated holes in the absorption region. In this device, the dominant carrier of the saturation phenomenon has been changed from a slow hole carrier to a fast electron carrier, thus greatly improving its speed and saturation power performance. In this study, we demonstrate a new near-ballistic-transport uni-traveling-carrier photodiode (NBUTC-PD) structure, in which the trade-off between the carrier drift velocity and the bias voltage that occurs in traditional UTC-PDs is relaxed. The results indicate that this new structure has superior high bandwidth saturation than the current product. We report and discuss this research in the following manuscript. First, we introduce the design principles and measurement results of the edge-illuminated NBUTC-PD. By inserting an additional p delta-doped layer and an undoped electric-field-buffer layer (E) to the n-doped collector of a traditional UTC-PD, most of the electrical field will cross the E layer. Thus, the electron carrier will drift across the collector layer with an overshoot velocity; only in the thin E layer will it have a saturation drift velocity. By incorporating the structure and the evanescently coupled waveguide, we can not only achieve a high saturation current-bandwidth (>1280mA-GHz, @40GHz), but also high responsivity (1.14A/W). In order to investigate this observed particular bandwidth enhancement phenomenon based on the measurement results, we establish a two port equivalent circuit model which functions according to the measured S parameters and the frequency response. The modeling results suggest that we can attribute the phenomenon to the overshoot drift velocity and the AC reduction capacitance. The established model can not only help us understand the physical characteristics but also provide useful information for the design of other associated components. Then, we demonstrate a back-illuminated NBUTC-PD produced by improving the epi-layer structure and downscaling the active area in order to make it applicable to the W-band ROF system. Because of the overshoot drift velocity, we can further increase the depletion region to reduce the junction capacitance and increase the device speed. By incorporating a microlense, we can improve the device responsivity and alignment tolerance. According to the measurement results, our device can achieve higher speed and saturation current (120GHz, 24.6mA, 2952mA-GHz) than has been reported previously. Then, we discuss how to integrate the device with a band pass filter to serve as an optoelectronic mixer. By taking advantage of the special bias dependence nonlinearity, we can successfully up-convert the intermedium-frequency (IF) signal to 60GHz and 100GHz. The up-conversion loss under different IF and optical local-oscillator (LO) signal powers is also investigated. We found that the filter can reflect the IF power. It forms a standing-wave pattern with a voltage-maximum-peak located around the active device. This phenomenon increases the device nonlinearity and reduces the up-conversion loss. High up-conversion gain (1dB) and large modulation bandwidth (>15GHz) have been observed in our results. Then, we discuss the integration of the device with the planar Yagi antenna to serve as a photonic transmitter. By taking advantage of the special bias dependence nonlinearity, we can up-convert the intermedium-frequency (IF) signal to the W-band through direct modulation. In comparison with that reported previously, not only can we remove the costly optical modulator, and the complex fabrication of the Si lens, but we can also realize the optoelectronic mixer and photonic transmitter in a single devcie. When serving as a photonic mixer under bias modulation operation, the demonstrated device can achieve a -2.4dB internal-conversion-gain at 106GHz while serving as a photonic transmitter we have a maximum detected power at 106GHz of around -14.1dBm with an output photocurrent of 30mA output photocurrent. Finally, we realize 1.25Gbit/s wireless BPSK data transmission bases on this novel device. The measured eye diagram and error vextor magnitude indicates the suitability of the NBTU-PD for application to W-band ROF communication systems. |
