Page 1 .... obtained with: (a) CW light injection off, (b) CW light injection on. (injected power ... Seo et al. reported [4] that CW light injection reduced the.
Gainswitched DFB laser diode pulse source using continuous wave light injection for jitter suppression and an electroabsorption modulator for pedestal suppression P. G u n n i n g , J.K. Lucek, D.G. M o o d i e , I(.Smith, R.P. Davey, S.V. Chernikov, M.J. G u y , J.R. T a y l o r and A.S. Siddiqui
Indexing terms: Semiconductor junction lasers, Distributedfeedback lasem, Jitter 4ps pulses with low temporal jitter ( 0 . 6 ~and ~ ) reduced pedestal (> 25dB extinction) have been generated from a 1548nm gainswitched distributed feedback laser diode (DFB) pulse source at 2.5GH.z using continuous wave (CW) light injection for jitter suppression and an electroabsorption modulator (EAM) for
pedestal suppression.
Introduction: gainswitched DFB-SLDs are more susceptible to (uncorrelated) timing jitter than gainswitched Fabry-Perot SLDs [I]. Techniques for addressing this problem include self-seeding. which involves reflecting a portion of a pulse back into the laser cavity to reduce timing jitter 121. However, this method depends on appropriate adjustment of the external cavity length to the pulse repetition rate. More recently, continuous wave (CW) light injection was used to suppress the pulse timing jitter [3, 41. Electroabsorption modulators (EAM) can be used to generate optical pulse trains from a CW optical source, where the strongly nonlinear variation of device optical absorption characteristics with applied (reverse) bias voltage enable the formation of picosecond duration pulses from a simple sinusoidal electrical drive that can be readily synchronised to a network clock [5]. The synthesis of the two methods, gain-switching and electroabsorption modulation, where the chirped pulses from a gainswitched DFB laser diode were temporally filtered with an EAM to reduce pulsewidth variation, has been demonstrated 161. We report an extension of this method where CW light injection is used to reduce uncorrelated timing jitter and an EAM is used to suppress the pulse pedestal. The pulses that resulted had an uncorrelated root-meansquare timing jitter (URTJ) of 0.6ps and a pedestal extinction of >25dB.
cavity CW optical source injected light through port 2 into the gainswitched DFB-SLD cavity having first passed through an optical isolator and a 50/50 coupler. A set of controllers was used to alter the polarisation state of the injected light before it entered the cavity of the DFB-SLD. Port 4 was used to monitor the injected CW light. The gainswitched pulses which exited port 3 were filtered by a l . l n m bandpass filter to remove spectral extremities and reduce the nonlinear chirp before injection into an erbium doped fibre amplifier (EDFA) that boosted the power to the EAM to +4dBm. The EAM employed an InGaAsPAnGaAsP multiple quantum well absorber layer. The low capacitance buried ridge structure was a modification of that previously described [5]. It comprised a 0.8,pn wide active mesa encased in a 5 p n thick Fedoped InP blocking structure. The modulator was 3 7 0 p long and was fully packaged in a high speed connectorised fibre-pigtailed module. At 1550nm the fibre to fibre insertion loss of the module was 7.3dB, its modulation depth was 30.4dB and its 3dB electrical bandwidth was 14GHz. The EAM was driven by a 12.5GHz electrical sinewave generated by a separate frequency synthesiser that was locked to the 2.5GHz synthesiser and amplified to 11V (peak to peak.) An adjustable electrical delay line was used to allow temporal adjustment of the switching window created by the EAM with respect to the gainswitched pulses. The pulses that emerged from the EAM were then compressed using a negatively dispersive optical fibre (D= 13ps/nm.) l o t '
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Fig. 2 High .speed sampling oscilloscope tYace.s a CW light injection on; b CW light injection off
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Fig. 1 Experrmvntal setup
Experiment: The 2.5 GHz electrical sinewave generated by a frequency synthesiser was amplified and combined with an adjustable D C bias current (via a bias-tee) to enable gain switching of a DFB contained within a high speed package. The DFB used was a ridge waveguide device with a centre wavelength of 1546.51~11and threshold current of 39mA at 15°C. The DFB temperature and DC bias current were maintained at 15°C and 60m.4, respectively, throughout the measurements. The electrical signal to the DFB package had a peak to peak voltage of -lOV measured across a 50Q load. The gainswitched optical pulse stream that resulted had a mean optical power of -3dBm and was injected into the top arm of a 50/5O coupler, denoted 'port 1' in Fig. 1. A tunable external
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Results: Fig. 2a shows the sampling oscilloscope traces that demonstrate the beneficial effect of CW light injection in suppressing the temporal jitter of the gainswitched optical pulses. In contrast, Fig. 20 was recorded without CW light injection and portrays the reduced pulse definition indicative of timing jitter. Fig. 2 also shows that CW light injection advanced the turn-on of the gainswitched pulses by -15-2Ops. Fig. 3 shows the RF spectra obtained with: (a)CW light injection off, (b) CW light injection on (injected power = -8.4dBm, wavelength 1547.6nmJ and (e) background noise-floor (no power incident to spectrum analyser.) The reduction of the phase noise background due to uncorrelated timing jitter with CW light injection is evident in Fig. 3a and b. Using an extension of the method outlined by Leep et al. [7], we calculated a URTJ of 3.6ps without CW light injection (from the dataset used to generate Fig. 3a and e). Similarly, we calculated an URTJ of 0.6ps with CW light injection (from the dataset used to generate Fig. 3b and c). The criterion: LV
