| 摘要: | 850nm波段高速垂直共振腔面射型雷射(Vertical-Cavity-Suface-Emitting-Laser)，作為多模光纖通訊通道的光源已是多種應用的主流，像是超短距離數據通訊、超級電腦、HDMI2.1或DisplayPort2.0纜線等等。為了提升上述系統的數據速度，脈衝幅度調製(pulse amplitude modulation)、正交頻率多工(orthogonal frequency division multiplexing)、前饋均衡(feed forward equalization)等複雜調變技術，可以解決850nmVCSEL速度上的限制。然而，使用上述複雜調變技術讓VCSEL中的相對強度噪音(RIN)更為重要。因此，同時擁有高速調製與低相對強度噪音的VCSEL設計，對於下一代光互聯系統而言非常重要。
低相對強度噪音的VCSEL，可以透過增加頂部P型分布式布拉格反射鏡來實現，但這會讓腔體內部的光子生命時間(p)增加，導致頻寬(E-O)和輸出功率劣化。控制VCSEL的極化是另一種有效率降低相對強度噪音的方法，在許多發表單極化VCSEL的技術中，像是將磊晶長在方向偏移的砷化鎵基板上、在DBR層做橢圓表面蝕刻，其中最有效的方法是在頂部DBR反射鏡做線性光柵，此技術可以得到大且穩定的正交極化抑制比(OPSR>20 dB)。儘管如此，線性光柵會誘發額外腔內損耗導致p下降，影響相對強度噪音(RIN)的表現，而我們發現了另一種方法:電鍍銅基板，可以在不減少p的情況下改善RIN。
參考了這麼多控制極化的方法，最初我們設計了圓形凸台結構(磊晶長在方向偏移的砷化鎵基板上)，得到微小極化差異(~6dB)且應證晶片在<011->方向有比較強的極化。為了取得更大的極化差異，我們也做了矩形凸台結構(磊晶長在方向偏移的砷化鎵基板上)，並觀察到了比較大的極化差異(10dB)。我們進一步在它背面電鍍銅基板利用誘發應力提升各項表現，成效包括了在不犧牲臨限電流與輸出功率情況下使光譜線寬變窄、RIN降低6dB、更大的極化差異(15dB)、更好品質的眼圖(25Gbit/sec)。
由於頻譜不理想，我們使用鋅擴散(圓與橢圓)來改善我們的頻譜，結果橢圓鋅擴散因長邊較長較少損耗無法達成單模，而圓形鋅擴散不僅達到了單模單極化(正交極化斥拒比: > 20 dB)且有著世界紀錄高的單模功率(10 mW; 單模斥拒比: >15 dB)和亮度(5.89MW(cm-2sr-1))。並且能夠在25 Gbit/sec 操作。
;Using high-speed vertical-cavity surface-emitting lasers (VCSELs) at around 850 nm wavelength regime as the light source in multi-mode fiber (MMF) based communication channel has become the main stream for several applications, such as very short-reach data communication, high-performance-computing (HPC) system, and HDMI 2.1 or DisplayPort 2.0 cables. In order to boost up the data rate per channel in the above-mentioned systems,complex modulation/de-modulation techniques, such as pulse-amplitude modulation(PAM),orthogonal frequency-division multiplexing (OFDM) , and feed forward equalization (FFE) ,have been demonstrated to alleviate the speed bottleneck for an 850 nm VCSEL under direct modulation. Nevertheless, the use of the afore-mentioned techniques to boost the data rate makes the relative intensity noise (RIN) in the VCSEL even more critical. The designing of VCSELs which simultaneously offer high modulation-speeds and very low RIN is thus very important to meet the requirements of the next-generation optical interconnect system. The low RIN performance in VCSEL can be realized by increasing the pair numbers of top p-type distributed Bragg reflector (DBR) mirror, which leads to the increase of photon lifetime (p) inside cavity. However, the larger p usually decreases both the net E-O bandwidths and slope efficiency of VCSEL . By properly controlling the polarization states of VCSEL output is one of the other effective ways to reduce RIN. Among all the reported single-polarized VCSEL techniques, such as epi-layers grown on misoriented GaAs substrate and elliptical surface etching on DBR layers, implementing the linear grating on the top DBR mirror is one of the most effective way to have a large and stable orthogonal polarization suppression ratio (>20 dB) over the whole range of bias current . Nevertheless, the grating induced additional intra-cavity loss may reduce p and degrades its RIN performance. In this work, we demonstrate a new technology :electroplating copper substrate to improve RIN and attain single polarized output without reducing p.
In this work, firstly we implemented the VCESLs with circular mesa, which was grown on the misoriented GaAs substrate. We achieved a small polarization difference between 011- and 011 orientation as 6 dB.. In order to obtain a larger Orthogonal polarization suppression ratio (OPSR) , we demonstrated rectangular mesa grown on the same misoriented GaAs substrate. Using this approach, we observed a larger PSR at nearly 10dB. In addition, by electroplating with the copper substrate to induce strain in our device, the further improvements in OPSR without sacrificing their threshold currents and slope efficiencycan be achieved.With this approach, we observe linewidth narrowing in the output optical spectra and the significant reduction of RIN(~6dB) simultaneously over a wide frequency range. Moreover, flat electrical-to-optical (E-O) frequency response and improved 25Gbits/sec transmission performance with low jitter than those of references without electroplatingcan be observed . Whereas, the far-field pattern (FFP) obtained using our rectangular mesa structure shows highly multimode. In order to further improve the FFP , we use Zn diffusion (circular and elliptical shape) technique in the circular mesa structure. The elliptical shaped Zn diffused circular mesa has less loss in the longer side as a result of which no significant improvement in the modes were observed. Nevertheless,using circular shaped Zn diffused circular mesa structure, we can simultaneously achieve highly single mode, single polarize (OPSR>20dB) with a record high single-mode power (10mW, SMSR>15dB), high brightness over 5.89mW(cm-2sr-1), and capable of 25Gbit/sec operation. |
