Wavelength division multiplexing (WDM) is such an effective transmission scheme that the transmission capacity has already risen to more than 1 Tb/s.111 The multiplexing wavelength range for WDM transmission has been limited to 1.53-1.56 pm by the Er-doped fiber amplifier (EDFA) gain band. Recently the possibility to expand the wavelength limit of the EDFA gain band to around 1.59 pm was reported.121 This implies the potential for extremely wide wavelength range and large-capacity WDM transmission. On the other hand, the preparation of such multichanneled light sources, whose lasing wavelengths are tuned to standardized wavelength grids for WDM is very expensive because a lot of epitaxial wafers are needed to cover the wide wavelength range. Simultaneous fabrication of LDs with different wavelengths on the same wafer is a very promising way to reduce the cost for different wavelength LD preparation.1 ’1 However, uniform lasing characteristics "are difficult to obtain over such a wide EDFA gain wavelength range (1.53-1.59 pm) due to the gain-peak separation from the lasing wavelength within a wafer. As the detuning (deviation of the Bragg-wavelengths from the gain-peak wavelength) becomes larger, lasing characteristics deterioration occurs, such as threshold current increase and Fabry-Perot mode oscillation.141 In this paper, we report different-wavelength DFB-LDs fabricated on a single wafer that can operate over a wide wavelength range of 1.52-1.59 pm by overcoming the detuning problem. Gain peak adjustment of the active layer to the Bragg-wavelength was accomplished successfully by the narrow-stripe selective MOVPE technique, which controlled the bandgap i energy of the active layer.[5J Fabrication Figure 1 is a schematic drawing of the simultaneous fabrication of different-wavelength DFB- LDs on the same wafer. Forty different pitch gratings of Ai=235.98 to A4o=247.68 nm in 0.3nm steps were formed on an n-InP substrate usipg EB lithography and wet-chemical etching Subnanometer pitch control in the grating was attained by varying the EB-scanning field size.[4J We introduced X/4-shifts in the grating every 300 pm. Fig. 1. Detuning-adjusted DFB-LDs of different wavelength fabricated on a wafer In order to adjust the gain-peak wavelength to the Bragg wavelength of the grating for each channel, we applied the bandgap-energy-control technique using selective MOVPE. Si02 mask stripes were formed on the grating substrate in such a way as to provide a 1.5-pm-wide stripe window for the selective growth of a MQW active layer. The Si02 mask widths Wm were varied from Wml=2A.O to Wm40=35.1 pin in 0.3-pm steps to correspond to the grating pitch. The InGaAsP/InGaAsP compressive-strained MQW active layer (Nw=6), grown at 635t: under a pressure of 150 torrj5^ showed the photoluminescence (PL) peak wavelength change from 1511 nm for channel 1 to 1575 nm for channel 40. The active stripes were then embedded with a p-InP clad in a second selective MOVPE growth. Finally, the DFB-LDs were cleaved to a 300-pm-long cavity with the X/4-shift located at the cavity center, and the end facets were AR-coated with SiON.
© 1997 Optical Society of AmericaPDF Article