電控吸收外調式調變器之線性化技術及其在HFC網路之寬頻服務應用

Abstract

在本論文中, 我們成功地實作出1550 nm 分波長多工電控吸收外調式調變傳輸器, 並應用在HFC網路之寬頻服務. 我們實作出一套可調式三階及五階非線性補償電路, CTB可消除 22 dB達到 –65 dBc. 在廣播有線電視47.8公里傳輸上, 可以維持低於 –60dBc的傳輸要求. 在窄播的傳輸方面, 也成功地完成 70 公里的傳輸. 我們發現電控吸收外調式調變器具有 low-chirp 的特性 可避免光纖色散所造成的失真問題, 限制窄播接收功率的動態範圍與傳輸距離的主要原因在於電控吸收外調式調變傳輸器本身RIN太高, 造成 CNR 的衰退.不過此傳輸器仍成功地完成 HFC網路之寬頻傳輸實驗, 具有 3dB的窄播接收功率動態範圍.

A predistortion circuit with adjustable amount of third-and fifth-order predistortions is studied both theoretically and experimentally. The circuit is used to cancel the third- and fifth-order intermodulations distortion of a 1.55μm integrated electroabsorption modulator/DFB laser. The CSO obtained is -61 dBc and the CTB is reduced by 22 dB to -65 dBc. This performance is maintained after fiber amplification and propagation through 47 km of standard fiber due to the modulator’s low chirp. Dithering of the DFB laser’s injection current is employed to increase the stimulated Brillouin scattering (SBS) threshold to + 12 dBm. Also, an experiment with a AM-VSB broadcast channel and one QAM-format narrowcast channel is performed to confirm the performance of 1550-nm narrowcast transmission systems using EML (Electroabsorption Modulator integrated Laser ) over 70km of standard fiber, through two cascaded Erbium-Doped Fiber Amplifiers with sufficiently low distortion performance and 3 dB dynamic range of received power. We find that dispersion-induced distortion of the MQW–EA modulator is smaller than that of the direct-modulated laser. Carrier-to-noise ratios (CNR’s) of 35 dB for channel 116 (the worst channel) are measured after 70 km transmission. It is found that the deterioration of the broadcast CNR is mainly caused by the cascaded erbium doped fiber amplifiers (EDFA’s), RIN noise and NLD from EML; the narrowcast CNR deterioration is mainly due to the EDFA intensity noise from high received power of broadcast channel. It shows that the output power of the modulator should be higher to improve CNR.