Here we report for the first time a passive mode-locking of single section Fabry-Perot (FP) lasers based on InAs quantum dots(QDs) grown on (113)B InP substrate. Devices under study are a 1 and 2 mm long laser diodes emitting around 1.58 µm. Self-starting pulses with repetition rates around 23 and 39 GHz and pulse widths down to 1.5 ps are observed after propagation through a suitable length of single-mode fiber for intracavity dispersion compensation. A RF spectral width as low as 20 kHz has been obtained leading to a low timing jitter RMS.
© 2013 Optical Society of America
Mode-locked laser (MLL) diodes have been regarded for the last two decades as the centre of interest for a large range of photonics and optical telecommunication applications. According to their capability of generating short pulses at high repetition rates, these sources are well suited for ultra-high bit rate optical telecommunications. In addition, MLLs may serve as building blocks for the development of all optical systems for clock distribution, clock recovery, radio over fiber signal generation and optical sampling so as to overcome the electronic bandwidth limitation at a reasonable cost . For all these applications, low timing jitter is necessary to fulfill bit rate error. QDs are ideal nanostructures for the MLL because of their fast carrier dynamics and their large effective gain bandwidth resulting from inhomogeneous dispersion in volume of dots and leading to an ultra-short pulse generation . The small coupling of the amplified spontaneous emission to the optical mode resulting from the low confinement factor in the dots leads to a low phase noise. It’s known that the timing jitter is related to the phase noise . For applications in the 1.3 µm telecommunication window, QD-lasers have already outperformed the quantum well (QW) lasers . On the other hand for operation in the C and L telecommunication band, much attention is devoted to InAs nanostructures grown on InP substrates. Stransky-Krastanov growth of InAs nanostructures on InP, when grown by Gas source Molecular Beam Epitaxy (GSMBE), leads to the formation of Quantum Dashes (QDashes) on nominal (001) InP substrate  or QDs on the (113)B InP substrate . QDashes obtained by GSMBE growth on (001)InP have already demonstrated good results for mode-locking in single or two section devices [7,8]. In contrast, despite the high density (1011/cm2) of small QDs achieved on (113)B InP substrate using GSMBE and the good laser results with low threshold current density , no mode-locking results have been reported on this substrate so far. In fact, InAs/InP QD based MLL have been demonstrated on InP nominal substrate using chemical beam epitaxy (CBE)  or on misoriented InP substrate using GSMBE . In this work, we report, for the first time to our knowledge, the fabrication and characterization of a mono-section InAs/InP QD MLL on (113)B substrate emitting around 1.58 µm.
2. Fabrication and static characterizations
The active region (AR) consists of 9 layers of QDs embedded in a lattice matched GaInAsP barrier with a corresponding gap of 1.18 µm (Fig. 1(a)) grown by GSMBE on an n doped (113)B InP substrate using the double cap technique . Figure 1(b) shows an atomic force microscopy (AFM) of an uncapped layer of InAs QDs grown on InP (113)B, showing a very high density (1011 /cm2) of small QDs (diameter of 30 nm). As shown in the Fig. 2(a), these QDs exhibit a photoluminescence spectrum centred at 1.52 µm, with a full width at half maximum of 177 nm determined by the inhomogeneous size dispersion of InAs QDs within the 9 stacked QD layers sample. Measurements on broad area lasers processed from such epitaxial structure show an internal efficiency of about 46%, and a modal gain and internal losses of about 25 cm−1 (Fig. 2(b)) and 6 cm−1, respectively. Single section FP ridge lasers have been fabricated by photolithography and inductively coupled plasma (ICP) reactive ion etching. Benzocyclobutene (BCB) is used for laser passivation and planarization. A ridge width of 2 µm has been chosen to ensure single transverse mode operation of the lasers. FP Lasers with cleaved facets cavity lengths of 1 and 2 mm, exhibit threshold currents of about 60 and 80 mA respectively.
3. Mode-locking results and discussions
Mode-locking performances of the devices described above are evaluated in this section. The lasers are operated in CW regime and at room temperature of 20°C controlled by a Peltier cooler. The laser signal is collected by an antireflection coated lensed fiber followed by an optical isolator to prevent feedback from reflections into the laser cavity. Then, the collected signal is analysed using an optical spectrum analyser, an RF electrical spectrum analyser associated with a 50 GHz bandwidth InGaAs photodiode detection and an autocorrelator are also used for RF and pulses measurements. Measurements were performed at 192 and 361 mA respectively for the 1 and 2 mm devices. These operating points were selected by choosing the optimal RF spectra in term of power and width. Figure 3 shows the variation of the RF peak width (Δf) versus the injection current density (J) for the 1 mm (a) and 2 mm (b) cavity length devices. Insets of the figure show the RF spectra for both devices.
The RF peak width decreases with increasing the injection current. The repetition rates are about 39 and 23 GHz and the optimal RF peak width at −3 dB is around 20 and 83 kHz respectively for the two devices which is comparable with the RF peak width of a single section QDashes MLLs . From the RF peak width, we can extract the integrated RMS timing jitter. In fact, in passively MLL the RF frequency noise is induced principally by the relatively broadband spontaneous emission and is consequently essentially white. This leads to a lorentzian-shaped of the power spectral density [13,14]. In consequence, the RF width Δf determines completely the RMS of the integrated timing jitter using the following expression [15,16]:8] and monosection QDashes MLLs (166 fs in the range [16 MHz-320 MHz]) .
The optical spectrum width increases with injection current and reaches a maximum value of 4.0 nm and 4.8 nm respectively (Fig. 4). Self-starting pulses were observed by autocorrelation after propagation through 220 and 230 m of single mode fiber (SMF) respectively to fully compensate the group delay dispersion of the laser. Figure 5 exhibits the optical pulse width Δτ versus the injection current density of the 1 mm (a) and 2 mm (b) cavity length devices. The autocorrelation traces are presented in the inset of the figure. Similarly, the pulse width decreases with the current and reach its minimum value for the optimal current presenting the minimum value of the RF peak width. Assuming a Gaussian fit, we deduce a pulse width of about 1.5 ps for the 1 mm long device and 1.8 ps for the 2 mm long one, after propagation in 220 and 230 m of SMF respectively (in order to compensate the group delay dispersion). We can calculate the time bandwidth product (TBP) to be 0.73 and 1.03 respectively for the 1 mm and 2 mm cavity length devices. These values are above the 0.44 Gaussian Fourier limit indicating some residual frequency chirp being present in the pulses. Recently, a shorter pulse value of 295 fs has been reported on a single section QD MLL using CBE . Here we report for the first time a mode-locking regime on an InAs QD laser elaborated on InP (113)B substrate using GSMBE. This kind of substrate allows obtaining very high density of QD by layer. Higher QD densities lead to an increase of the maximal modal gain, a decrease of the threshold current for the ML, and may enable MLL on shorter cavity and thus higher frequency. Also, real QD like behavior, in association with an important gain, may enhance non-linear effects, which are supposed to drive self-pulsating lasers and to improve their performances. Thus QD monosection MLL devices are expected to demonstrate superior performances in comparison with 1D or 2D like active layers. Furthermore, we can notice on Fig. 5 that the optical pulse width decreases with increasing the gain current. This process has already been observed for the monosection device MLL   and has been attributed to a compensation of the dispersion by nonlinear effects enhanced by higher carrier density and larger intracavity laser fields . Thanks to this feature, the monosection MLLs devices can deliver more power comparing to the multi-section devices.
Mode-locking without the presence of saturable absorber is demonstrated for the first time for single-section QD lasers based on (113)B InP substrates. Lasers with repetition frequencies of 39 and 23 GHz exhibit self-starting pulses with pulse widths down to 1.5 ps after intracavity dispersion compensation by light propagation through suitable lengths of SMF. A low integrated timing jitter RMS has been demonstrated leading to a good mode-locking regime with low noise.
This work was supported by the French National Research Agency through the project TELDOT.
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