1. Introduction

Broadband tunable laser sources play an essential role in a wide range of applications including precision measurements [1], spectroscopic analysis [2], biomedical treatments [3] and environmental monitoring [4]. Furthermore, semiconductor lasers emitting around the 1.55 μm wavelength, which corresponds to the silica fiber C band transparency window, experience minimal power attenuation, and so are of great interest for fiber optical communication applications, such as dense wavelength-division multiplexing (WDM) [5]. Among the reported tunable laser source designs, external-cavity (EC) configurations are the most popular because of their simplicity, low cost, and superior performance [6]. Grating coupled EC lasers based on semiconductor quantum wells (QWs) enabled tuning across a 240 nm wavelength band centered on 1.5 μm [7]. However, due to the high saturated gain of the QW ground state, an extremely large current density of 33 kA/cm² was required to realize such a large tuning range.

In contrast, EC lasers based on quantum dot (QD) gain media are well suited for broadband tuning at low injection current operation. The large inhomogeneous broadening of self-assembled QDs broadens the gain spectra of QD devices [8, 9]. The relatively low ground-state saturation gain makes QD excited states easier to fill at fairly low current densities [10]. GaAs-based InAs QD-EC lasers have achieved many high performance results thanks to the high quality epitaxial material growth [11–13]. For example, a broad tuning band of 201 nm was realized by such lasers at a maximum bias of 2.87 kA/cm² [12]. Very recently, Haggett et al. demonstrated a tapered tunable QD-EC laser with a tuning range of 100 nm and output power of up to 0.62 W [13]. However, difficulty in reaching the 1.55 μm optical communication window with InAs/GaAs QD materials resulted in research attention shifting to InP-based InAs QD materials. So far, only a few results have reported based on tunable InAs/InP QD-EC lasers. Ortner et al. first demonstrated a 1.55 μm InAs/InP QD-EC laser with a broad tuning range of 166 nm, but the EC laser had an output power of only a few microwatts [14]. Chen et al. reported InAs/InP QD-EC lasers grown by gas source molecular beam epitaxy [15], which had a wide tuning range of 98 nm with a maximum output power of 7 mW. Compared with InAs/GaAs QD-EC lasers, high-performance InP-based QD-EC lasers are still challenging to fabricate due to the difficulty of QD growth in a small lattice mismatch material system and the complex strain distribution across the InAs and InP growth interfaces [16, 17].

In this paper, we demonstrated an improved InAs/InP QD material structure grown by metal-organic vapor phase epitaxy (MOVPE) for use in broadband tunable external-cavity lasers emitting in a 1.55 μm band. It is found that introducing a high AsH₃ flow during the interruption after the growth of the InAs QDs can promote QD ripening and thus increases the number of large QDs emitting at longer wavelengths, greatly improving the optical gain of the QD active medium in lower energy states. For the as-cleaved QD external-cavity laser, a tuning range of 78.5 nm was achieved with an output power of 33 mW at an injection current of 1000 mA. Moreover, by adding anti-reflection (AR)/high-reflection (HR) facet coatings, a broadly tunable QD-EC laser was obtained, with a tuning range of 140.4 nm (1436.6 nm-1577 nm) and a maximum output power of 6 mW.

2. Experiment details

The InAs/InP QD laser samples used in this study were grown by a commercial AIXTRON 3 × 2 FT MOVPE system on exactly (100) oriented n-type InP substrates. The undoped active region of the QD sample consisted of seven stacked InAs QDs alternated with InGaAsP barrier layers, with a room temperature (RT) bandgap wavelength of 1.1 μm (1.1 Q). The QD layers were formed by depositing 3.3 monolayers of InAs, then interrupting growth for 5 seconds while flowing AsH₃ to encourage adatom diffusion and material redistribution between the dots and wetting layer. The AsH₃ flow was varied from 3.9 × 10⁻⁶ mol/min to 2.0 × 10⁻⁵ mol/min to control the size of the dots, and hence their lasing wavelength. Afterward, a double cap process was used to eliminate giant and defective islands, thus narrowing the full width at half maximum (FWHM) of the gain spectrum. The density of the stacked QDs was approximately 3 × 10¹¹ cm⁻². The active layers were embedded in a 150 nm thick 1.1 Q waveguide core, providing both carrier and optical confinement. More details of the material growth can be found elsewhere [18]. For device fabrication, 6 μm wide ridge waveguides were fabricated using standard photolithography and wet etching techniques. The chips were cleaved to form Fabry-Perot (FP) lasers with cavity lengths varying from 1 mm to 3 mm.

The QD lasers were investigated by a grating-coupled EC arrangement in the Littrow configuration with a cavity length of around 12 cm [19]. The radiation emitted from the front facet was collimated with an AR-coated aspheric lens (0.5 numerical aperture). The 600 lines/mm groove density and 1.6 μm blazed wavelength grating provided optical feedback in the first diffraction order. Wavelength tuning was achieved by changing the incidence angle of the grating. The emission from the back facet of the QD gain device was collected via an optical fiber bundle into a grating monochromator to measure the emission spectrum.

3. Experimental results and discussion

3.1 QD active medium

Growth interruption after InAs QDs deposition is usually used to encourage material redistribution and adatom diffusion, which allows QD ensembles to develop comparatively stable configurations [20, 21]. We investigated the influence of growth interruption with different AsH₃ flows on the lasing characteristics of the QD gain medium. Figure 1(a) shows the RT photoluminescence (PL) spectrum from the QD sample grown with a 5 s growth interruption accompanied by a 3.9 × 10⁻⁶ mol/min AsH₃ flow (referred to as structure A, hereinafter) and the RT lasing spectrum of the 3 mm QD laser fabricated from the same sample under an injection current of 1.1 times the threshold current I_th. The device lased at a much shorter wavelength than the PL spectral peak, with a blue-shift of 58 nm. A similar blue shift of the lasing wavelength has also been reported by Kim et al. [22]. This shift is mainly attributed to the large size distribution of the Stranski-Krastanov growth mode and the finite number of carriers accommodated by each QD energy state, the optical gain of larger QDs with lower lying energy states cannot compensate for the total loss. As the injection current increases, smaller dots with higher energy levels or the excited states of larger QDs with higher degeneracy and gain are progressively filled, which provide sufficient gain to sustain lasing and results in the observed blue-shift of the lasing wavelength [23]. Enhancing the optical gain of the lower lying energy states would therefore reduce the blue-shift effect. In our system, higher AsH₃ flow during growth interruption was introduced to enhance the ripening of QDs. Figure 1(b) shows the RT PL spectrum for a QD sample grown with a 5 s growth interruption accompanied by a 2.0 × 10⁻⁵ mol/min AsH₃ flow (referred to as structure B, hereinafter). Structure B exhibited about twice the peak intensity and a narrower line width relative to structure A. The corresponding lasing spectrum for a 3 mm long device at an injection current of 1.1 × I_th is also shown in Fig. 1(b), the lasing wavelength blue-shift is reduced to only 25 nm.

 

Fig. 1 Effect of AsH₃ flow on QD structure PL and lasing spectra. (a) RT PL (dash-dot line) and lasing at 1.1 × I_th (solid line) spectra for structure A, (b) RT PL (dotted line) and lasing at 1.1 × I_th (solid line) spectra for structure B.

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To further explore gain saturation and state filling effects, the net modal gain spectra were calculated from the amplified spontaneous emission spectra of 1 mm long QD lasers by the series expansion method [24]. Figure 2(a) shows the net modal gain spectra for a structure A QD laser at different continuous-wave (CW) injection currents. A double-peaked structure of ground states is observed, which originates from two subsets of the QD ensemble. This dual-mode distribution corresponds to two ground state gain values: the ground state gain of the large QDs, which have a gain peak located at 1.57 μm at 100 mA, and the ground state gain of the small QDs, which have a gain peak located at 1.51 μm at 100 mA. As the injection current increases, the ground states of the larger QDs quickly saturate and the maximum net modal gain cannot compensate for the cavity loss. Nevertheless, the ground states of smaller QDs become more densely populated until lasing eventually develops. The larger number of small QDs in the ensemble results in higher gain. Figure 2(b) shows the net modal gain spectra for a structure B QD laser. Compared with the structure A QD laser spectra, the structure B QD laser spectra exhibit single-mode distributions and increased optical gain of the ground state due to the increased AsH₃ flow during the growth interruption. The high AsH₃ flow leads to increased adatom mobility and island clustering, which promotes QD ripening. As a result, the smaller QDs become larger, and the number of large QDs increases, resulting in a single-mode distribution of QDs with an increased optical gain at the ground state [20]. Therefore, the observed deviation of the lasing wavelength from the PL peak is greatly reduced, as shown in Fig. 1(b).

 

Fig. 2 Net gain spectra at various CW injection currents for 1 mm long QD lasers with (a) structure A and (b) structure B. The dashed arrow indicates the direction of gain change with increasing injection current.

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3.2 External cavity laser

Tuning characteristics were investigated for 2 mm long QD lasers fabricated with structures A and B in an EC configuration and driven by a pulsed current with a 3% duty cycle at 1 kHz. The structure A laser achieved a tuning bandwidth of 39 nm, covering wavelengths from 1498.5 nm to 1537.5 nm, as shown in Fig. 3(a). The tuning range only covered a small part of the net gain spectrum at the ground states of the smaller QDs. At shorter wavelengths, the tuning range is limited by the finite injection current and competition from FP resonance. At longer wavelengths, the EC tuning is limited as the net gain is not sufficient to compensate for the total cavity loss, even with the high reflectivity cavity provided by the external grating [14]. The structure B laser achieved increased optical gain from the lower lying energy states due to the increased AsH₃ flow during the growth interruption, resulting in a tuning range that extends much more into long wavelengths, relative to that of the structure A laser, as shown in Fig. 3(b). This is consistent with the enhanced ground state gain observed in Fig. 2(b). Because a cleaved cavity facet with about 30% reflectivity for InP based material can create a coupled-cavity effect in the EC tuning experiment, small FP resonance peaks are visible in Fig. 3(a) and (b). Lasing at lower energy states can also lead to a lower threshold current. For structure B, due to the greater gain contributed by the lower energy states, lasing can occur before the gains of the lower energy states saturate. Therefore, the threshold current of the structure B laser is lower than that of the structure A laser, which requires population of the higher energy states. The light-current (L-I) curves for free-running QD lasers and EC lasers tuned to 1524 nm are presented in Fig. 4, in consideration of the high output powers and weak FP resonances at the tuning wavelength of 1524 nm. Threshold current values of 450 mA and 355 mA were observed for the free running QD lasers fabricated with structures A and B, respectively. The threshold current is decreased by nearly 100 mA by adopting structure B instead of A. In addition, the EC lasers have lower threshold currents and higher output powers than the corresponding free-running QD devices due to the high feedback effect from the grating. The EC QD laser fabricated with structure B attained an output power as high as 33 mW at a pulsed injection current of 1000 mA (1 kHz repetition rate and 3% duty cycle).

 

Fig. 3 Tuning spectra of EC lasers based on 2 mm long QD gain devices with (a) structure A and (b) structure B, driven by a pulsed injection current of 800 mA (1 kHz repetition rate and 3% duty cycle).

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Fig. 4 L-I curves of the free-running QD lasers (blue dotted line and red dashed line) and the EC lasers tuned to 1524 nm (blue solid line and red dash-dot line) for structures A and B, respectively.

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Facet coating of the laser cavity has effectively increased EC tuning ranges [14, 19]. Since AR facet coating can effectively suppress FP resonance of the laser chip, it allows the EC laser to operate under relatively high current injection conditions and be tuned towards short wavelengths. At the same time, HR facet coating used in the other laser cavity facet can reduce the total optical loss and thus increase the tuning range towards long wavelengths. Overall, in order to expand the wavelength tuning range, AR (R≈1%) and HR (R≈86%) dielectric stacks were separately deposited on the two cavity facets of a 2 mm long QD laser chip fabricated with structure B. The tuning spectra of the EC laser were evaluated at a pulsed injection of 1500 mA (1 kHz repetition rate and 3% duty cycle) at 18 °C, as shown in Fig. 5. A wide tuning range of 140.4 nm was achieved, with the wavelengths varying from 1436.6 nm to 1577 nm.

 

Fig. 5 Tuning spectra of 2 mm long EC InAs/InP QD gain device using structure B and AR/HR coating driven by a pulsed injection current of 1500 mA (1 kHz repetition rate and 3% duty cycle).

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Figure 6(a) shows the threshold current density of the structure B InAs/InP QD EC laser with AR/HR coating as a function of tuning wavelength. The minimum threshold current density occurs around 1557 nm, where the as-cleaved gain device exhibits a peak in the ground state gain spectra. Another local threshold current density minimum occurs around 1484 nm, which corresponds to the excited state peak gain. The threshold current density slightly increases in the wavelength range between the ground and first excited states due to the reduced available optical gain, which is easily observed in the net modal gain spectra in Fig. 2(b). At extremely long wavelengths, the lower density of states of the QD ensemble causes the decrease of the material gain, resulting in increasing threshold current density. At short wavelengths, the threshold current density gradually increases with the tuning towards shorter wavelengths. This can be attributed to more carriers being needed to populate the corresponding higher energy states to sustain device lasing.

 

Fig. 6 (a) Threshold current density versus tuning wavelength Dependence of threshold current density on wavelength and (b) L-I curves at the wavelengths of 1452 nm (black solid line), 1500 nm (blue dashed line) and 1545 nm (red dotted line) for the EC laser fabricated with structure B and AR/HR coating. The inset shows the output power as a function of lasing wavelength at a pulsed injection current of 1500 mA.

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The output powers from the HR coated back facet of the structure B QD laser at different EC tuning wavelengths were also measured under pulsed operation mode at 18 °C. The L-I curves of the EC laser at tuning wavelengths of 1545 nm, 1500 nm, and 1452 nm are plotted in Fig. 6(b), which correspond to the locations of the ground state, the gap between the ground and excited states, and the excited state, respectively, as shown in Fig. 2(b). The slope efficiency significantly decreases as the laser is tuned towards longer wavelengths. Moreover, the output power level gradually saturates with increasing injection current when the EC QD laser operates at the longer wavelengths of 1500 nm and 1545 nm. This is possibly due to the corresponding lower energy states more easily saturating than the higher energy states and additional carriers preferentially filling the higher energy states under the high pump level. Such effects have also been observed in other QD lasers [11]. The increased slope efficiency at the shorter wavelength of 1452 nm can be attributed to the higher degeneracy of the QD excited states, which results in higher material gain [14]. The inset in Fig. 6(b) gives the output power versus tuning wavelength for a fixed driving current of 1500 mA. The resulting maximum output power is about 6 mW, measured from the HR coated back facet.

4. Conclusions

In conclusion, we have demonstrated a broadband tunable InAs/InP QD laser with external-cavity arrangement in Littrow configuration, based on MOVPE epitaxial growth. It is found that high AsH₃ flow during the interruption after the growth of QDs can effectively increase the optical gain of lower energy state, which broadens the EC laser tuning range and reduces threshold current. Based on the improved QD material structure, the as-cleaved QD external-cavity laser demonstrated an improved tuning range from 39 to 78.5 nm, with a maximum output power of 33 mW at an injection current of 1000 mA. Moreover, by adding AR/HR facet coatings to the QD gain device, a broad tuning range of 140.4 nm (1436.6 nm-1577 nm) and a maximum output power of 6 mW were achieved.