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Abstract
We report the relative intensity noise (RIN) characteristics of an InAs quantum dot (Qdot) laser epitaxially grown on the Ge substrate. It is found that the minimum RIN of the Ge-based Qdot laser is around −120 dB/Hz, which is 15 dB higher than that of a native GaAs-based Qdot laser with the same layer structure. The higher RIN in the Ge-based laser can be attributed to the high-density epitaxial defects of threading dislocations and antiphase domain boundaries.
© 2017 Optical Society of America
1. Introduction
High-speed photonic integrated circuits on Si provide a cost-effective communication solution for data centers, fiber-optic networks, and high performance computers [1]. Various photonic devices including semiconductor lasers, optical modulators, photodiodes, and passive waveguides have been demonstrated on the Si platform [2–5]. Semiconductor lasers are usually integrated on Si through the flip-chip bonding and the wafer bonding schemes [6–8]. An alternative promising approach is to monolithically integrate III-V semiconductor lasers on Si using the direct epitaxial growth approach, which has advantages of low cost, high yield, and large-scale integration capability [9,10]. However, at present this technique is very challenging due to the lattice mismatch between III-V compounds and Si, which leads to high-density defects of threading dislocations [11,12]. In addition, antiphase domain boundaries are formed arising from the growth of a polar material on a nonpolar substrate [13,14]. The defects act as non-radiative centers and hence restrain the radiative recombination of semiconductor lasers [15]. This problem is circumvented through employing quantum dots (Qdots) instead of quantum wells (Qwells) in the laser's active region, thanks to the high tolerance of Qdots to the epitaxial defects [16,17]. Indeed, InAs Qdot lasers epitaxially grown on Ge, Ge-on-Si and Si substrates have been successfully achieved in recent years [9,10,18–23].
In comparison with Qwell lasers, Qdot lasers exhibit superior static performances like lower threshold current density and higher temperature stability [24,25]. In addition, Qdot lasers also show enhanced dynamic characteristics, such as larger modulation rate [26], reduced sensitivity to residual optical feedback [27], lower phase noise as well as lower relative intensity noise (RIN) [28–31]. Particularly, the RIN in semiconductor lasers increases the bit-error rate of optical signals and hence limits the data transmission rate in fiber-optic communication networks [32]. The RIN characteristics of Qdot lasers have been extensively investigated, and the lowest RIN of InAs/GaAs Qdot lasers was as small as −160 dB/Hz with a highly damped resonance peak [20,31].
Ge- and Si-based InAs Qdot lasers have been demonstrated to show superior static characteristics over Qwell lasers as well [9,20,22]. In comparison with native GaAs-based InAs Qdot lasers, however, the lasing threshold becomes larger while the differential quantum efficiency becomes lower due to the high-density epitaxial defects [9,23,33]. The defect density of Ge- and Si-based Qdot lasers is typically on the order of 106–108 cm−2, which is at least two orders of magnitude higher than GaAs-based lasers [2,20,22]. However, there are few studies discussing the defect impacts on the lasers' dynamic performances. Very recently, A. Y. Liu et al. reported that the optical feedback sensitivity of InAs Qdot lasers monolithically integrated on Si was about 20 dB lower than Qwell lasers heterogeneously integrated on Si, which was obtained through the RIN measurements [34]. The RIN of the Si-based Qdot lasers was measured to be in the range of −140 dB/Hz – −150 dB/Hz. In this work, we report the RIN characteristics of a Ge-based InAs Qdot laser epitaxially grown by the gas-source molecular beam epitaxy (GSMBE) technique. It is found that the minimum RIN of the Ge-based Qdot laser is about −120 dB/Hz, which is 15 dB higher than that of a GaAs-based Qdot laser with the same layer structure.
2. Laser device and experimental setup
The laser samples were grown on a 4-inch Ge(001) wafer with an off-cut of 6° towards the [111] plane by the GSMBE [19]. As shown in Fig. 1, the epitaxy starts with a 600-nm thick GaAs buffer layer, followed by a 1 μm n-type doped GaAs contact layer. A 1.5 μm n-type Al0.3Ga0.7As cladding layer is grown above the contact layer, followed by 20 periods of superlattice (SL) consisting of AlGaAs/GaAs (2 nm/2 nm). The active region has 5 stacked InAs Qdot layers separated by GaAs spacer layers. The InAs Qdots were grown at 500 °C and its deposition thickness was fixed at 2.2 monolayers (ML). Above the active region is another SL structure, a 1.5 μm p-type Al0.3Ga0.7As cladding layer, and finally a 300 nm GaAs p-contact layer. Figure 1(b) shows the atomic force microscope (AFM) image of the uncapped InAs Qdots grown on the Ge substrate, and the dot density is estimated to be in the range of 1010 cm−2. The epitaxial defect density of the laser samples is evaluated by calculating the surface defect density in the GaAs buffer layer based on its AFM image, which is estimated to be in the range of 105–106 cm−2. The above laser structure was fabricated into ridged lasers using standard lithography and wet etching techniques. The ridge was etched down to right above the active region, while the n-type contact window was etched down to the bottom contact layer. Ti/Pt/Au metal layers were deposited on the p-GaAs contact layer and Ge/Au/Ni/Au layers were on the n-GaAs contact layer. Finally, the laser facets were cleaved without any coating. The cavity length and width of the Ge-based InAs Qdot laser device under study are 4.4 mm and 6 μm, respectively. The photoluminescence spectra of the Ge-based laser samples were reported in [19].
Fig. 1 (a) Epitaxial layer structure of the Ge-based Qdot laser; (b) Atomic force microscope image of a 5 × 5 μm2 region of InAs Qdots on the Ge substrate.
Figure 2 shows the experimental setup for the measurement of the Ge-based Qdot laser's RIN. The tested laser is biased by a DC current source, and the temperature is kept constant at 20 °C using a thermo-electric cooler. The laser output is coupled into a single-mode lensed fiber, and the optical spectrum is record by a high-resolution (0.02 nm) optical spectrum analyzer (OSA). In addition, the optical signal is converted into the electrical domain through a low-noise photodiode (New Focus 1414) with a bandwidth of 25 GHz. Passing through a bias-tee, the DC voltage is measured by an oscilloscope, while the AC signal is amplified by a broadband amplifier (Keysight N4895A-S30) with a typical small-signal gain of 30 dB. The gain of the amplifier is characterized using a vector network analyzer. The amplified noise spectrum is measured on an electrical spectrum analyzer (ESA).
3. Results and discussion
The Ge-based Qdot laser was operated in the continuous-wave lasing mode, and Fig. 3(a) shows the fiber-coupled optical power as a function of the pump current. The laser exhibits a threshold current of Ith = 300 mA, and the laser power goes up to 3.8 mW at 1.5 × Ith. Figure 3(b) shows that the laser spectrum centers around 1046 nm at 1.1 × Ith, and the two dominate lasing peaks are due to the mode competition effects associated with the inhomogeneous broadening effects [32]. A higher pump current (1.3 × Ith) excites the stimulated emission of more Qdots of different sizes and hence broadens the lasing spectrum. The total noise NT measured on the ESA consists of the intrinsic laser noise NL, the thermal noise Nth, and the shot noise of the photodiode Nq. Nth is independent on the optical power, and is measured when the laser is turned off. Nq is a white noise determined by Nq = 2qIDCRL, with q being the elementary charge, IDC the DC current and RL the load resistance of the ESA [35]. The laser noise NL is calculated by subtracting Nth and Nq from the total noise NT. Finally, the RIN of the tested laser is obtained as
where PDC is the electrical DC power, RBW is the resolution bandwidth of the ESA (200 kHz), and G is the gain of the amplifier. Figure 4 illustrates the power spectral densities of the different noise sources, where the amplifier's gain influence is already removed. The total noise (solid line) is dominated by the intrinsic laser noise (dash-dot line) and the thermal noise (dashed line), while the shot noise of the photodiode (dotted line) is more than 30 dB smaller and hence is negligible. The spectral density of the laser noise exhibits a clear resonance peak at 0.8 GHz. In addition, another broad peak appears at 9.0 GHz, which is not common in the noise spectra of semiconductor lasers. We believe this peak is due to the impurity of longitudinal modes arising from the Qdot size dispersion, where two sets of carrier populations with an energy separation of about 9.0 GHz are excited [36].Fig. 3 (a) Coupled laser power versus the pump current. (b) Optical spectra of the laser pumped at 330 mA and 390 mA.
Figure 5(a) shows the measured RINs of the tested Ge-based InAs Qdot laser (6-μm ridge), which are extracted using Eq. (1). The RIN at low frequencies (<4.0 GHz) is relatively high, and it reduces with the increasing frequency. This high RIN is attributed to the low-frequency driven current noise, thermal noise, and mode partition noise [32]. The RIN exhibits a strong resonance peak around 1.0 GHz due to a small damping factor, which suggests that the Ge-based Qdot laser is underdamped. In contrast, The RINs of native GaAs-based Qdot lasers are usually overdamped [30,37]. The damping difference between the two type lasers can be attributed to the different Qdot sizes and residual strains [38–40], which alter the carrier scattering rates into the lasing ground state of Qdots [41,42]. In addition, the resonance frequency of the Ge-based laser increases with the pump current as expected. For frequencies higher than 4.0 GHz, the RIN reduces with the pump current, and its minimum level saturates around −122 dB/Hz (dashed line). The second peak around 9.0 GHz is due to the dot size dispersion as discussed in Fig. 4. Meanwhile, we fabricated another Ge-based InAs Qdot laser with a ridge width of 4 μm (the cavity length remains 4.4 mm). The lasing threshold reduces to 220 mA and the optical spectrum peaks around 1045 nm similar to that in Fig. 3(b). Figure 5(b) shows that the RIN characteristics of the 4-μm ridge laser are similar to those of the 6-μm ridge one, whereas the minimum RIN level is slightly lower (−125 dB/Hz). This is because a narrower ridge reduces the number of lateral optical modes [30].
Fig. 5 RINs of (a) 6-μm ridge and (b) 4-μm ridge Ge-based InAs Qdot lasers. The dashed lines indicate the minimum RIN level.
In order to unveil the influences of growth defects in the Ge-based lasers on the RIN characteristics, we study the RIN of a native GaAs-based InAs Qdot laser with the same layer structure for comparison. The GaAs-based laser sample was grown on a n-type GaAs(100) substrate by the GSMBE technique. The active region consists of five InAs Qdot layers with a dot density of about 2.0 × 1010 /cm2. The cavity length of the laser is 1.5 mm, the ridge width is 6 μm, and both cavity facets are as-cleaved. The lasing threshold of the GaAs-based laser is 120 mA. As shown in Fig. 6(a), its optical spectrum centers around 1076 nm, which is about 30-nm longer than that of the Ge-based laser in Fig. 3(b). Similarly, Ge-on-Si- and Si-based Qdot lasers have been observed to show blue-shifted optical spectra as well [17, 33], which are caused by the smaller dot sizes and the larger residual strains in comparison with those in GaAs-based lasers [38–40,43]. In addition, the lasing spectrum of the GaAs-based laser is broader owing to its higher carrier injection efficiency with fewer epitaxial defects [23]. Figure 6(b) shows that the resonance peak of the RIN in the GaAs-based laser around 1.8 GHz is less pronounced than that in the Ge-based laser (Fig. 5), which indicates a lager damping factor [44]. In addition, the minimum level of the GaAs-based laser's RIN is about −137 dB/Hz, which is 15-dB lower than that of the 6-μm ridge Ge-based laser (Fig. 5(a)). The degradation of the RIN performance in the Ge-based InAs Qdot laser can be attributed to the high-density epitaxial defects including threading dislocations and antiphase domain boundaries. The epitaxial defects in Qdot lasers accelerate the Shockley-Read-Hall non-radiative recombination rate [32], which increases carrier populations in the wetting layer and in the excited states. The large carrier populations induce stronger photon variations under the perturbation of spontaneous emission noises and carrier noises, and thereby leads to higher RINs in the Ge-based lasers. A detailed theoretical study of the epitaxial defect effects on the RIN of Qdot lasers will be reported elsewhere.
Fig. 6 GaAs-based InAs Qdot laser. (a) The optical spectra at 130 mA and at 160 mA. The lasing threshold is Ith = 120 mA. (b) The laser's RIN at various pump currents. The dashed line indicates the minimum RIN level.
4. Conclusion
In conclusion, we present the RIN characteristics of InAs Qdot lasers monolithically grown on Ge by the GSMBE technique. The RIN spectra of the lasers exhibit a more pronounced resonance peak than that of the native GaAs-based InAs Qdot laser with the same layer structure. In addition, it is found that the minimum RIN of the Ge-based laser is about 15 dB higher than that of the GaAs-based one, which is due to the high-density epitaxial defects arising from the mismatch between GaAs and Ge. This work is of prime importance for improving dynamic performances of Qdot lasers on the Ge or Si platform, and for designing photonic integrated circuits in optical communication systems.
Lastly, it is commented that the Ge-based laser studied in this work has a longer cavity than the GaAs-based one, which results in different active region temperatures although the heatsink temperatures are kept the same. According to Ref [45], a longer laser cavity slightly reduces the RIN of semiconductor lasers. Therefore, the RIN difference of Ge- and GaAs-based Qdot lasers with identical cavity length can be slightly larger than 15 dB. Future work will compare the RIN and phase noise performances of Ge- and GaAs-based InAs Qdot distributed feedback lasers with the same epitaxial layers and cavity structures.
Funding
Shanghai Pujiang Program (17PJ1406500); National Natural Science Foundation of China (NSFC) (61675128).
Acknowledgments
The authors would like to thank Prof. Frédéric Grillot at Télécom ParisTech, France for fruitful discussions.
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