Passively mode-locked quantum-well laser with a saturable absorber having gradually varied bandgap Junjie Xu, Song Liang*, Songtao Liu, Lijun Qiao, Siwei Sun, Qiufang Deng, Yongguang Huang and Hongliang Zhu (
[email protected]) Abstract A novel passively mode-locked quantum-well laser, which saturable absorber (SA) has gradually varied bandgap, is fabricated. Light pulses are obtained at a repetition frequency of 226 GHz with a minimum pulse width of 605 fs under an appropriate mono current bias. Introduction High frequency optical pulse generation has many potential applications including ultrahigh bit rate optical communication systems or terahertz generation. Among the methods to obtain optical short pulses, monolithic semiconductor mode-locked lasers (SMLLs), which have a gain section and a saturable absorber (SA) as basic parts, have attracted great interest due to their compact size, mechanical stability, low cost and high reliability. However, there is only a limited bias parameters region in which saturable absorption can touch and thus stable mode-locking occurs, which limits their commercial applications. A larger tolerance to variations in operating conditions is needed in order to promote widespread practical applications of SMLLs. The narrow operation range of the SMLLs can be attributed to the fact that the properties of the absorbing section are strongly dependent on the bandgap detuning between the gain and SA region [1-3]. SA with wavelength independent absorption characteristics was suggested to be a better choice to obtain wide parameter range of stable operation of SMLLs [3,4]. In this paper, we present a novel SMLL design which has a MQW SA with a gradually changed bandgap structure along the ridge direction, realized by selective area growth (SAG) technique, which renders a degree of wavelength independent absorption characteristics in the SA. Device structure and fabrication The structure of the fabricated device is shown schematically in Fig. 1(a). The device has an asymmetric colliding pulse mode-locked (ACPM) structure [5]. With this ACPM structure, the repetition rates are ‘L/l’ times of the roundtrip frequency, as shown in Fig. 1(a). The device is fabricated by a two-step metal organic chemical vapor deposition (MOCVD) process. The first MOCVD step is a SAG process. A PL wavelength difference is thus fabricated of about 90 nm between the MQWs in the SAG region and the region where there is no mask, whose PL wavelengths are 1550 nm and 1460 nm, respectively, as shown in Fig. 1(b). The effect of the SAG masks on the MQW wavelength decreases with the distance from the SAG masks, leading to the continuously varied bandgap between the MQWs from the gain to SA sections. The long wavelength and the short wavelength MQWs are used as the gain materials and the passive waveguide materials, respectively. The 50μm wide region between the gain region and the passive region is used as an absorber, in which the absorption edge varies gradually from 1550 nm to 1460 nm. Thus, the MQWs for all the three parts of the lasers are obtained simultaneously in one epitaxy step. A p cladding is then grown and a 2.5μm wide ridge waveguide is fabricated. Device Properties The threshold current of the laser is 30 mA, corresponding to a 2.069 kA/cm2. The slope efficiency of the light emission is about 0.124 mW/mA and over 21 mW light power can be obtained when the gain current is larger than 250 mA. Repetition rates and pulse durations are measured by an auto-correlator based on second harmonic generation. The measured autocorrelation pulse train including five pulses obtained at 120 mA gain current is shown in Fig. 2. The average repetition rate of the pulse is 226 GHz. For our device, the total length of the cavity L = 580
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μm and the absorber is placed at about L/m=190 μm from one of the facet. The 226 GHz repetition rate corresponds roughly to the value of m = 3 times the round trip frequency (75.3 GHz) of the cavity, indicating clearly a third harmonic asymmetric colliding-pulse mode-locking happened in the device [5]. The shortest pulse width is observed from the laser at a gain current of 120 mA. The experimental data of an isolated pulse fitted with a hyperbolic secant type profile is shown in Fig. 3. The autocorrelation width is 0.937 ps, deconvolving to a pulse width of 605 fs, which is the shortest one obtained with no reverse bias voltage applied on the SA for InGaAsP MQWs SMLLs. The full width at half maximum of the spectrum is 4.59 nm, corresponding to a time bandwidth product (TBP) of 0.347, which is close to the limit of a hyperbolic secant profile (0.315). Acknowledgment The work was supported by the National Science Foundation of China (NSFC) (61635010, 61474112, 61574137, 61321063, 61320106013, 61274071), the National 863 Program of China (2013AA014502), and the National Key Research and Development Program of China (2016YFB0402301); The 863 Program (2013AA014402)
References [1] Vincenzo Pusino et al., “Passive mode-locking in semiconductor lasers with saturable absorbers bandgap shifted through quantum well intermixing”, Photon. Res. Vol.2, No. 6 (2014) [2] D. Kunimatsu et al., “Passively Mode-Locked Laser Diodes with Bandgap-Wavelength Detuned Saturable Absorbers”, IEEE Photonics Technology Letters, Vol.11, No.11, (1999) [3] Julien Javaloyes et al., “Detuning and Thermal Effects on the Dynamics of Passively Mode-Locked Quantum-Well LasersJournal of Quantum Electronics, Vol.48, No.12, (2012) [4] David Massoubre et al., “Analysis of thermal limitations in high-speed microcavity saturable absorber all-optical switching gates”, Journal of Lightwave Technology, Vol.24, No.9 (2006) [5] Takanori Shimizu et al., “Asymmetric Colliding-Pulse Mode-Locking in InGaAsP Semiconductor Lasers”, Semiconductor Optical Review, Vol.2, No.6 (1995)
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