| dc.description.abstract | 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. | en_US |