A high-performance quantum dot superluminescent diode with a two-section structure
Based on InAs/GaAs quantum dots [QDs], a high-power and broadband superluminescent diode [SLD] is achieved by monolithically integrating a conventional SLD with a semiconductor optical amplifier. The two-section QD-SLD device exhibits a high output power above 500 mW with a broad emission spectrum of 86 nm. By properly controlling the current injection in the two sections of the QD-SLD device, the output power of the SLD can be tuned over a wide range from 200 to 500 mW while preserving a broad emission spectrum based on the balance between the ground state emission and the first excited state emission of QDs. The gain process of the two-section QD-SLD with different pumping levels in the two sections is investigated.
Keywordsquantum dot superluminescent diode two-section structure optical amplification
Superluminescent diodes [SLDs] have attracted extensive attention for a wide range of applications, such as optical coherence tomography [OCT] [1, 2], optical fiber-based sensors [3, 4, 5], external cavity tunable lasers [6, 7, 8], optoelectronic systems , etc. A wide emission spectrum corresponding to a low degree of coherence is required for these applications of SLD, which allows the realization of sensors with improved resolution. It has been proposed that self-assembled quantum dots [QDs] [10, 11, 12] and quantum well grown on a high-index surface are beneficial to broaden the spectral bandwidth of the device . Till now, QDs have successfully been used as the active media in several broadband light-emitting devices, such as QD-SLDs [14, 15, 16, 17, 18, 19, 20], QD semiconductor optical amplifiers [SOAs] [21, 22, 23], and QD broadband laser diodes [24, 25, 26]. For QD-SLD devices, a high power of 200 mW  and a wide spectral bandwidth of more than 140 nm [27, 28] have been achieved. Most recently, an intermixed QD-SLD exhibits a power of 190 mW with a 78-nm spectral bandwidth .
For a typical SLD device structure with a single current-injection section, the high output power can only be obtained at a high pumping level, where the device demonstrates a narrow spectrum emitted predominantly from the QDs' excited state [ES] due to the low saturated gain of the QD ground state [GS]. It is difficult to achieve high-power and broad-emitting spectrum simultaneity. However, a high-power SLD that is broadband emitting is required in some fields. As an example, in an OCT system, a high power is usually needed to enable greater penetration depth and improve the imaging sensitivity . Numerical investigation  and experimental evidence [32, 33] have shown that this limitation can be overcome by using a multi-section structure in an SLD device, which allows the emission spectrum and output power to be tuned independently. A quantum-well SLD with a two-section structure which integrates monolithically an SLD with an SOA has been reported, which exhibits an output power that is one or two orders of magnitude higher than that in conventional SLD devices .
In this paper, a QD-SLD device, which has a two-section structure monolithically integrating an SLD with an SOA, is fabricated. A high power (500 mW) with a broad emission of 86 nm is obtained. By properly controlling the current injection in the two sections of the QD-SLD device, the power tunability over a wide range from 200 to 500 mW is achieved, with the preservation of a nearly constant spectral width.
The epitaxial structure of the QD-SLD device in this study was grown by a Riber 32P solid-source molecular beam epitaxy machine on n-GaAs(001) substrate. The epitaxial structure consists of ten InAs-QD layers separated from each other by a GaAs spacer; each of them is formed by depositing a 1.8-monolayer InAs at 480°C and covered by a 2-nm In0.15Ga0.85As. Ten QD layers plus the GaAs waveguide layers form the whole active region which is sandwiched between 1.5-μm n- and p-type Al0.5Ga0.5As cladding layers. Finally, a p+-doped GaAs contact layer completes the structure.
Results and discussion
It can be seen from the above results that the output power and spectrum bandwidth can be tuned by properly controlling the current densities injected in the two regions of the QD-SLD. Figure 4 shows equal power curves as function of the currents injected in the two sections. Data points (solid squares) at which the GS and ES1 have nearly identical emission intensities, corresponding to the maximum bandwidth of the emission spectrum, are also shown in Figure 4. It can be found that the output power can be tuned over a wide range of 200 to 500 mW while preserving a broad emission spectrum. The high output power and wide power tunability is due to the two-section structure which integrates a tapered SOA section. Current combinations at which the device begins lasing are also shown in Figure 4 (solid circles). Working points of the QD-SLD device can be set in the lower left region of the borderline. An optimum working point is found in the figure that the SOA current is in the 8- to approximately 8.5-A range and the SLD current is 0.2 to approximately 0.25 A, at which a 500-mW output power and an 86-nm bandwidth are achieved simultaneously.
In conclusion, a high-power QD SLD with a broad bandwidth in the emission spectra is achieved by the two-section structure which monolithically integrates an SLD with a tapered SOA. Properly controlling the current densities injected in the two sections, the QD-SLD device exhibits a maximum output power above 500 mW and a simultaneously broad bandwidth of 86 nm. Also, the output power can be tuned over a wide range from 200 to 500 mW while preserving a nearly constant spectral width.
This work was supported by the National Basic Research Program of China (no. 2006CB604904) and the National Natural Science Foundation of China (nos. 60976057, 60876086, and 60776037).
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