Single fundamental mode photonic crystal vertical cavity surface emitting laser with 9 GHz bandwidth - Electronics Letters

Introduction: Stable single fundamental mode operation of a laser has drawn attention for a variety of applications in short and mid-range high quality optical networks, such as local area networks (LAN) and storage area networks (SAN). For these purposes, vertical cavity surface-emitting lasers (VCSELs) have been considered as promising optical sources over distributed feedback lasers not only because of their easy array scaling, on-wafer characterisation, low power consumption, and easy packaging, but also because of high volume and low cost manufacturability. Despite these promises, the development of a VCSEL which operates in a single fundamental mode during modulation has been limited due to device parasitics such as large intrinsic resistance and capacitance [1–7]. The 3 dB small signal modulation bandwidth for multimode VCSELs has been reported as high as 14.5 GHz for a proton-implanted VCSEL [2, 3] and 17 GHz for an oxide-confined VCSEL [4]. However, the highest reported singlemode small signal modulation is 6 GHz for an oxideconfined VCSEL [5] and 8.5 GHz for a proton-implanted VCSEL [6]. For oxide-confined VCSELs, constraining the thin oxide apertures to less than 3 mm in diameter which is necessary for single fundamental mode operation, reduces the rollover current and limits the output power, as well as creating a significant challenge for mass production. The decrease in rollover current and high capacitance caused by a thin oxide layer limits the modulation bandwidth of oxide-confined VCSELs [1, 4–6], whereas proton-implanted VCSELs suffer from thermal lensing which limits their modulation bandwidth [2]. It has been shown that singlemode operation of a VCSEL under continuous wave (CW) operation might be changed into multimode operation when the current is modulated. For example, a singlemode VCSEL with a 30 dB side mode suppression ratio (SMSR) exhibited multimode operation with an SMSR of 15 dB under 2.5 Gbit=s large signal modulation [7]. A VCSEL with micromachined surface relief demonstrated consistent singlemode operation both under CW and small signal modulation up to 5 GHz, but it operated in a single higher order transverse mode [8]. Another approach to attain a stable single fundamental mode operation is the incorporation of a photonic crystal pattern in the VCSEL (PC-VCSEL) [9–11], but the modulation behaviour of this device has not been studied. We report a PC-VCSEL which operates in a single fundamental mode with a record 9 GHz small signal bandwidth. Emission spectra under CW and small signal modulation conditions show that the PC-VCSEL remains in single fundamental mode operation.