Anomalous noise spikes beating from a fine-splitting mode of surface-emitting laser with imperfect true-roundness aspect

Abstract

High-frequency discrete and distinct mode-beating noise (MBN) spectra are an anomalous phenomenon observed in the related intensity noise spectrum from the vertical-cavity surface-emitting laser (VCSEL), which reveals strong correlation with fine-splitting transverse modes caused by non-circular emission aspect of the VCSEL with imperfect true-roundness. Such a phenomenon is demonstrated for the first time, which seriously degrades the transmission performance of high-speed and broad-band encoding data carried by the multi-mode (MM) VCSEL biased under a large DC bias. In this work, the 4-ary quadrature-amplitude modulation (4-QAM) orthogonal frequency-division multiplexing (OFDM) and the non-return-to-zero on–off keying (NRZ-OOK) transmissions with the direct modulation of different 850-nm MM VCSELs, as suffered from the MBN-induced notches of the signal-to-noise-ratio (SNR) spectra, are characterized under their high-level lasing conditions with multi-mode numbers. The significant SNR degradation on the receiving and decoding performances caused by the cross-correlation between MBN and data spectra is also examined by tuning the lensed fiber along the transverse (X/Y)-axis of the VCSEL (perpendicular to the surface normal) when collimating the output beam, which finds the disorderly shifted MBN spectra, as attributed to the collection of light emitted from the overlapped gain region of different transverse modes. In contrast, the MBN reveals invariant spectral features while the lensed fiber is either back-and-forth tuning along the vertical (Z)-axis (parallel with the surface normal) or tilting its angles from the surface normal under the same horizontal position and distance concerning the MM VCSEL. Such MM VCSEL exhibits several MBN spikes to decay the SNRs of NRZ-OOK and QAM OFDM data encoded around 21 and 34 GHz. For other MM VCSELs with reducing numbers of their transverse lasing modes and deforming roundness of their circular emitting aspects, the corresponding MBN spectral peaks can also be respectively observed at 14/17/24 GHz and 4.5/10.5/13 GHz to degrade the SNRs of the encoded data at such frequencies. Even though pre-emphasized encoding technology is employed, such an MBN effect is strongly correlated with the asymmetric circular aspect of the VCSEL with a degraded roundness ratio, which still affects the data modulation performance of the MM VCSEL for data center communication.