Insertion loss of the modulator has been investigated in the monolithically integrated electro-absorption modulator/DFB laser light source, and low-threshold current and high-efficiency operation has been attained in the monolithic device by AR-HR coating on the facets. Monolithic light source in which an electro-absorption modulator and a DFB laser are integrated into one chip has been demonstrated to operate successfully at lOGb/s[l],[2]. However, the problem in the integrated device is that the insertion loss of the electro-absorption modulator lowers the slope efficiency and limits the light output power. Here, we have investigated the loss budget of the insertion of the modulator, and demonstrated that the small slope efficiency due to the loss can be recovered by introducing HR coating on the rear facet of the device. Fig.1 shows a schematic diagram of the integrated light source. The photoluminescence wavelength of the absorption layer was selected to be 1.40p.m for the lasing wavelength of 1.55p.m. The length of the modulator region is 200p.m and the laser length is 300p.m. On the front facet we have performed the AR coating using SiN. The reflectivity of the coating has been estimated to be 0.5 %. To evaluate the insertion loss of' the modulator, we have fabricated the integrated devices and solitary DFB lasers from the same wafer, and evaluated the slope efficiencies. Fig. 2 compares the slope efficiencies of the integrated devices and the solitary DFB lasers whcn no voltage is applied to the modulator. On the average, the slope efficiency of the integrated device was one half of the solitary DFB laser. This means that the absorption loss plus the coupling loss at the butt joint is about 3dB. The coupling loss has been calculated theoretically for the butt joint dimensions of the actual device. The loss is predominantly due to the coupling to the radiation mode caused by the thickness difference at the butt joint. The loss was 0.45dB. Further improvement of the butt joint will reduce the total loss to 2.5dB. The 3dB of the insertion loss of the present device limits the slope efficiency and the output power. We have performed the HR coating on the facet of' the DFB laser side using quarter wavelength thickness Si02 layer and the amorphous Si layer. Two pairs of coating give the reflectivity of about 90%. Fig.3 compares the slope efficiencies between the conventional AR-cleave type integrated device and the AR-HR type device. The slope efficiency has increased about a factor of 2 and the average value was 0.2W/A, which is almost comparable to the solitary DFB lasers. Fig.4 shows the distribution of the threshold current of the AR-HR device. The threshold current was irom 8 to 22 mA which was 10-20% lower than that of the AR-cleave type device. The AR-HR coated devices exhibited extinction ratio of about 13 dB for operating voltage of -5V. The modulation characteristics at 10 Gb/s was almost the same as that of the previously reported AR-cleave type device. In summary, the insertion loss of the modulator has becn investigated in the integrated device, and the AR-HR facet coating has becn dcmonstratcd to be effective to recover the loss of the modulator. Very low threshold currents of 822mA and high efficiencies of around 0.2W/A comparable to the solitary DFB lasers have becn obtaincd for the first timc in thc monolithically integrated device. References