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IMP ROVING PULSES FROM 2-CONTACT S ELF~PU LSATING DFB SEMICONDUCTOR LASERS

A.J. Lowery, P.C.R Gurney Photonics Research Laboratory, Department ofElectrka1 and Electronic Engineering

The University of Melbourfie, Parhille, VIC 3052, Australia

Abstract: We propose that short pulses from a self-pulsating DFB laser will be shortened and stabilised by optical filtering and optical feedback. Simulations predict 60 ps pulses at 2.5 GHz with a contrast ratio >10 dB. Introduction: Mohrle et al. [ 11 have measured multi-mode self-pulsations in 2-contact 1535- nm DFB lasers when the contact current densities were unequal. The pulsations were at around 3 GHz, making them useful for clock pulses in Gbit/s optical transmission systems. Similar pulsation have been observed by Phelan et al. [2] in three-contact lasers. Bandelow et al. [3] have modelled self-pulsating in 2-contact lasers using a single-mode model. In this paper, we show that the pulsations are a result of mode-hopping between an unstable asymmetric mode [4] and a non-dominant symmetrical mode [4] using a detailed multi-mode laser model [5]. We have found that the rate of pulsation can be increased by increasing the current inhomogeneity, which is an advantage over single-contact devices. Furthermore, the strongest output pulsations will occur at the facet with the largest current density, unlike single-contact devices where strong pulsations appear at one or other facet randomly. We show that thee improvements can be made to the device as a clock source. 'These are: (a) to antireflection (AR) coat the laser facets to achieve short pulse generation over a wide operating range of currents; (b) to filter the output pulsations in order to achieve shorter output pulses with an increased contrast ratio; (c) to feed back a small proportion of the filtered output to the laser to reduce timing jitter. Results: We simulated a 1550-nm uniform grating AR-coated DFB laser with a bulk active region and a Bragg coupling of 4.0 using the transmission-line laser model (TLLM) [5]. The front contact was driven at 60 mA and the rear contact at 45 mA. Strong pulsations (Figla) appeared at the front facet, and weak pulsations at the rear facet (Figlb). The repetition frequency was 2.5 GHz and could be increased by increasing the contact current difference. The method of pulse generation is similar to that in single-contact devices driven at high currents [5]. Analysis [4] shows that a single-contact device can have an unstable asymmetric mode and a stable but higher-threshold symmetrical mode, when biassed at high currents. Thus neither mode is indefinitely stable, leading to self-pulsation [5]. In the 2-contact device the asymmetric mode is favoured at lower total currents because of the unequal current injection densities. Our simulations show that a high-power pulse will rapidly reduce any asymmetry in the carrier density, and so is quickly quenched. The device will then lase in the symmetrical mode until an asymmetric mode regrows due to the current density asymmetry. Our simulations predict pulsations for a wide range of current combinations, unlike in the uncoated device in [l]. However, if 1% facet reflectivities were used, the number of curnmt combinations causing self-pulsation decreased considerably. This could explain why the currents for self-pulsation were so critical in [ 11, and shows that AR coating is desirable. The optical spectrum from the front facet (Fig.2) shows a heavily chirped lower-frequency mode and a narrower upper-frequency mode, as seen in [2]. The large chirping suggests that the lower frequency mode is a train of very short pulses. Thus, the short pulses can be recovered by optical band-pass filtering of the lower mode. The front facet output after a 25-GHz FWHM filter (Fig.3) shows an improved contrast ratio, and pulse shortening to approx. 60 ps from 130 ps. Because the low-frequency (asymmetric) mode grows from a low power, its timing will be influenced by noise. Thus, injection locking should improve the timing jitter. Fig.4 shows the RF spectra of the laser without feedback, and with feedback from the filtered output (0.01% with

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266 ps delay). The width of the spectral peaks is reduced by the feedback, suggesting a reduced amplitude andor timing jitter. Surprisingly, the repetition frequency is also reduced. Conclusions: We show that 2-contact DFB lasers can be used as stable pulse sources and that their characteristics can be improved by AR coating, filtering the output, and feeding back a small proportion of the filtered output to the laser. We acknowledge the support of the Australian Photonics Cooperative Research Centre. [ 11 [2] [3] 141 [SI

MOHRLE, et al.: Photon. Technol. Lett., 1993,4, pp.976-978 PHELAN,P., et al.: ZEE Proc. J: Optoelectron., 1994,30 BANDELOW,U.,et hl.: Photon. Technol. Lett., 1993, 4, pp.1176-1179 TROMBORG,B., et al.: Photon. Technol. Lett., 1992, 28, pp.985-988 LOWERY, A.J., Electron. Lett., 1993,29, pp.1852-1853

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Fig. la: Optical output fromji-ont facet. Fig.2: Optical spectrum from front facet.

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Fig. 1 b: Optic& output fiom rear facet. Fig.4a: RF spectrum without feedback

Fig.3: Filtered front facet output - Fig.4b: RF spectrum with feedback.

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