Abstract

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 W_m were varied from W_ml=2A.O to W_m40=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 America