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Effect of threshold carrier density on spectral linewidth in DFB lasers with high output power has been studied. A narrow spectral-linewidth of 0.3 MHz and the record output power of 180 mW are achieved in 1.55 pm DFB lasers with reduced threshold carrier density.Recently, high output power 1.55 pm distributed feedback (DFB) lasers have become key devices as CW sources for external modulators in a variety of analog or digital fiber communication systems with erbium-doped fiber amplifiers (EDFA). These lasers are also strongly required to have narrow spectral-linewidth and low noise characteristics [I]. However, linewidth rebroadening at high output power has limited operating power. Previously, we have proposed high power 1.55 pm DFB laser with Mass-Transport Grating (MTG) which is composed of InAsP buried with InP [2]. In this study, we have investigated the influence of threshold carrier density on spectral linewidth. As a result, very narrow spectral-linewidth with the highest output power ever reported in 1.55 pm DFB lasers has been demonstrated.The fabricated laser has an active region which contains 5 pairs of 6-nm-thick compressively strained InGaAsP (Aa/a=O.9 %) well layer and 10-nm-thick InGaAsP (A, =1.05 pm) barrier layer.The volume of total active layers pad) has been varied to investigate the influence of threshold carrier density on spectral linewidth by changing the cavity length and the width of active region (Va,=21, 28, and 32 pm3). For three lasers, the Bragg wavelength of the DFB laser (ADw) and the photoluminescence peak wavelength (ApL) are set to 1555 nm and 1570 nm, respectively. Figure 1 shows the measured spectral linewidth characteristics for the three types. For the laser with the smallest Vact of 21 pm3, spectral linewidth increases due to the rebroadening at the output power of more than 60 mW. However, with the largest V,,, spectral linewidth of less than 0.5 MHz is realized at the output power of more than 20 mW. Figure 2 shows the measured spectrum at the injection current of Ith+0.5 mA at the heat temperature of 25 "C. With increasing Vact from 21 to 32 pm3, it can be seen that the shift of gain peak (ApL -kain) to the side of short wavelength is reduced from 23 to 5 nm due to the reduction on band filling effect. Observed reduction in linewidth is considered to be due to the suppression of carrier over flow to barrierheparate confinement heterostructure layers, which is caused by the increase of threshold carrier density [3]. In Fig. 3, the product of linewidth and output power (Av-Po) characteristics as a function of detuning (hDpB-kab) is studied. The AvP0 product decreases from 20 to 10 MHz-mW with changing the detuning from 8 to -10 nm. Also, these measured results are well fitted to the calculation considering the decrease in a-parameter by negative detuning [4]. From these results, it is confirmed that it is effective to decrease threshold carrier density for decreasing linewidth power product and suppressing the rebroadening at the high output power operation in DFB lasers...

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High power VCSEL (Vertical-Cavity Surface-Emitting Lasers) as light sources for free space optical communications have been developed. These VCSEL devices have 4, 9, or 16 simultaneously driven spots-similar to a single spot VCSEL and have a single cone-shaped Far Field Pattern (FFP) under high optical output power operations at 10, 20 and 40mW. Since the multi spots are driven simultaneously, these devices have a high optical output power even if the optical output power from each spot is small. As a result, the Mean Time To Failure (MTTF) obtained from a 4 spot multi-mode 850nm VCSEL array is well over 1,000,000 hrs at 70°C under a 10mW operation.The VCSEL arrays also show a wide-open 2.5Gbps eye diagram at standard driving conditions-more than sufficient for optical free space communications such as optical wireless HDTV transmission systems. The simultaneously driven 9 and 16 spot multi-mode 850nm VCSEL arrays also show similar wide-open 2.5Gbps eye diagrams under standard driving conditions.Simultaneously driven multi spot VCSEL arrays are useful as light sources for Free Space Optics (FSO) because of their high speed modulation and high reliability under high optical output power operation.