The device characteristics of GaSb-based mid-infrared (MIR) photonic-crystal (PC) surface-emitting lasers (SELs) were investigated with respect to depths of etched PC holes. Measurement of below-threshold emission spectra identifies the bandgap as well as band-edge modes. The bandgap separation, which is a function of feedback coupling, increases with increasing depth. From within, the Bragg frequencies and their detuning from lasing frequencies can be determined. Moreover, with increasing depth, the threshold pumping density decreases exponentially to a saturation level, which is assigned to minimum device modal gain of certain value. The relative threshold gain is then plotted as a function of normalized frequency detuning. The gain-detuning relationship of PCSELs is similar to that of one-dimensional (1D) distributed feedback (DFB) lasers.
Monolithic passively mode-locked lasers are investigated based on chirped multilayer InAs/InGaAs quantum-dot (QD) structure. The forward and backward tracings of light–current characteristics show two kinks and two hysteresis loops. The optical spectra reveal two lasing wavelengths around 1273 and 1230 nm, which are identified as two ground-state emissions of two differently chirped QD layers. The corresponding radio-frequency (RF) spectra of high-speed detector reveal two RF peaks at 16.21 and 16.03 GHz, which are attributed to fundamental mode-locking of two respective wavelengths. The laser pulses are confirmed by optical autocorrelator to exhibit dual-wavelength mode-locking. The pulsed characteristics of two lasing wavelengths are also discussed in terms of operating conditions.
Monolithic passively mode-locked lasers are investigated based on chirped multilayer InAs/InGaAs QDs. Three chirped wavelengths, with stacking numbers of 2, 3 and 5 layers, are designed with capped InGaAs thickness of 4, 3 and 1 nm, respectively. The ridge-waveguide devices of 5-μm width and 3-mm length are fabricated to have absorber-to-gain length ratio of 1:9. A curve tracer is used to analyze the hysteresis on the light-current curve. Two kinks in the L-I curve are observed at threshold current near 50 mA and at higher current of about 150 mA. The lasing wavelength just above threshold is centered at 1268 nm and the RF spectrum of mode-locking is peaked at 13.32 GHz. At well above threshold of 200 mA, another RF peak at 13.21 GHz occurs that corresponds to shorter lasing wavelength around 1233 nm. The two lasing wavelengths are originated from ground-state transitions of two groups of InAs/InGaAs QDs. Simultaneous dual-wavelength mode-locking is therefore achieved at rather low forward current and low reverse bias by incorporating this novel design of QD structure.
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