1. Introduction

Superluminescent diodes (SLDs) have attracted extensive attention for many applications including optical coherence tomography (OCT) [1,2], optical fiber based sensors [3,4], external cavity tunable lasers [5–7], optoelectronic system [8], etc. A wide emission spectrum is required for these applications, which allows to the realization of improved resolution in the systems. It has been proposed that the characteristic of size inhomogeneity, naturally occurring in self-assembled QDs grown by Stranski-Krastanow (S-K) mode, is beneficial to broaden the spectral bandwidth of the device [9]. QDs have successfully been used as the active media in several broadband light-emitting devices, such as QD-SLDs [10–18], QD semiconductor optical amplifiers (QD-SOAs) [19,20] and QD broadband laser diodes [21,22]. Till now, for a typical QD-SLD with a single current-injection section inclining the waveguide at an angle with respect to the emission facet, can achieve a maximum spectrum bandwidth of about 100 nm based on the balance of QDs’ ground state (GS) emission and 1st excited state (ES) emission; but due to the gain saturation in the GS of QDs the output power is usually just about a few milliwatts.

Besides a wide emission spectrum, a high output power is also required in practical applications. As an example, in an OCT system, a high power is usually needed to enable great penetration depth and improve the imaging sensitivity [23]. For a regular QD-SLD device 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’ ES due to the low saturated gain of the QDs’ GS. Many efforts have been made towards high-power superluminescent devices. By using an intermixed p-doped QD structure as high-gain active region, the QD-SLD exhibits a high power of 190 mW with a 78-nm spectral bandwidth [18]. For geometrical designs of the high-power SLD device, A quantum-well SLD with a two-section structure which monolithically integrates an SLD with a tapered semiconductor optical ampliðer (SOA) has been reported [24], which exhibits an output power one or two orders of magnitude higher than the regular SLD devices. Numerical investigation [25] and experimental evidences [26,27] have shown that the emission spectrum and output power can be tuned independently in an SLD device with the multi-section structure. Recently, we have previously demonstrated high-performance QD-SLDs with the two-section structures [28, 29], which exhibits high output powers and simultaneous broad bandwidths. In [28], it exhibits a high-power QD-SLD device with two-section structure for the first time. High power (260 mW) and broadband spectrum (66 nm) is achieved at an optimum working point where the SOA current is 5 A and the SLD current is 0.1 A. But the investigation on working mechanisms of the two-section device is not presented, only the good performance of the superluminescent device is demonstrated. In [29], the high-power and broadband superluminescent device is achieved by monolithically integrating a conventional SLD with a tapered SOA. High output power is attributed to the single-pass amplification while the superluminescent light of the SLD section is propagating forward from the narrow end to the wide end of the tapered SOA. But for such a two-section SLD, the optimum working point is at high current-injection where the SOA current is about 8.5 A and the SLD current is about 0.2 A.

In this paper, a high performance QD-SLD device was achieved by using a two-section structure, which consists of a tapered titled ridge waveguide section and a straight titled ridge waveguide section. The working mechanisms of such a QD-SLD with the two-section structure is investigated. It is shown that double-pass gain is working in the two-section device. And wavelength-selective optical feedback to the emission of the QD’ GS and 1st ES can be achieved by tuning the current densities injected in the straight titled section. With the GS-dominant optical feedback from the rear facet under proper current-injection of the straight titled section, a high output power of 338 mW and a broad bandwidth of 65 nm is obtained simultaneously.

2. Experiments

The epitaxial structure of the QD-SLD devices in this study was grown by a Riber 32P solid-source molecular beam epitaxy machine on n-GaAs (001) substrate, which is the same as our previous study [28,29]. The active region consists of ten layers of self-assembled InAs QDs covered by 2-nm In_0.15Ga_0.85As and separated from each other by 35-nm GaAs spacer. Each QDs layer is formed by the deposition of 1.8-monolayer InAs at 480 °C. The areal density is about 4 × 10¹⁰ cm⁻² obtained by atomic force microscopy for an uncapped sample, which has the same growth parameters as the epitaxial structure of the device. The whole active region included the GaAs waveguide is sandwiched between 1.5-μm n- and p-type Al_0.5Ga_0.5As cladding layers grown at 620 °C. Finally, a p⁺-doped GaAs contact layer completes the structure.

The QD-SLD devices with index-guided ridge waveguide and two-section structure were fabricated. A schematic diagram of the geometrical design (not to scale) is shown in the inset of Fig. 1 . The device consists a straight titled section (S1) and a tapered titled section (S2). The straight section is 1-mm long and 10-μm wide. The tapered section is 2.3-mm long with a full flare angle of 6°, which expands linearly from 10-μm wide at the narrow end to 250-μm wide at the wide end. The ridge waveguide was fabricated using standard photolithography process and wet chemical etching. The etching profile entered the bottom GaAs waveguide layer. With a deep-etched ridge waveguide, optical feedback from the rear facet of the S1 section is expected. The center axis of ridge was aligned at 6° to the facet normal to suppress Fabry-Pérot cavity resonance. Ti/Au and AuGeNi/Au ohmic contacts were evaporated on the top and back of the wafer. A 20-μm-wide isolating stripe between the straight region and the tapered region generated by leaving the up Ti/Au and 0.5-μm semiconductor epilayers. After metallization, the device was cleaved and mounted p-side up on a copper heatsink using indium solder. The output facet of the S2 section were coated with a SiO₂ antireflection (AR) coating while the rear facet of the S1 section remained as-cleaved.

 

Fig. 1 P-I characteristic measured from the output facet of the S2 section with the S1 section un-pumped. The inset shows schematic design of the SLD device with two-section structure.

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The QD-SLD device was characterized by optical power-injection current (P-I) and electroluminescence (EL) measurements at room temperature under a pulsing (1 kHz repetition rate and 3% duty cycle) injection in the S2 section and a CW injection in the S1 section, respectively.

3. Results

The P-I characteristic measured from the output facet of the S2 section while the S1 section is un-pumped as a rear optical absorption region is shown in Fig. 1. The inset depicts the schematic diagram of the device structure. As shown in Fig. 1, a superluminescent characteristic is clearly observed by the superlinear increase in optical power with the current of the S2 section (I₂). Inspection of the emission spectra (as shown in Fig. 2 ) indicates that lasing is successfully suppressed for such a two-section QD-SLD, and that the output optical power is due to amplified spontaneous emission. For a given I₂ of 8 A, a maximum output power of 108 mW was obtained.

 

Fig. 2 (a) Normalized EL Emission spectra under different injection-currents of the S2 section. Some spectra are shifted vertically for clarity. (b) Spectral bandwidth and output power as a function of injection-current of the S2 section.

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Figure 2(a) shows the EL emission spectra under different injection-currents in the S2 section with the S1 section un-pumped. Figure 2(b) depicts the dependences of spectral bandwidth and output power by injection-currents of the S2 section. At a lower pumping level of 1 A, the emission spectrum exhibits a full width at half maximum (FWHM) of 52 nm with the center wavelength of 1.17 μm, which corresponds to the emission from the QDs’ GS. The relatively wide GS emission is attributed to the size inhomogeneity naturally occurring in self-assemble QDs. With the increase of I₂, the emission spectra are clearly broadened at the blue side, which should be attributed to the saturation of the QDs’ GS and sequential carrier filling of the higher-energy ES. For a given I₂ of 4 A, a wide spectrum with the maximum spectral bandwidth of 88 nm is achieved, which is attributed to the balance of QDs’ GS emission and 1st ES emission. However, due to the low saturated power of the GS emission, the output power is only 40 mW at I₂ = 4 A as shown in Fig. 2(b). It is shown that high output powers can be achieved at the high pumping levels of the S2 section, which is due to the contribution of the higher-energy ES. The 1st ES level could give out an optical gain with twice as many as the GS level due to the high angular momentum degeneracy [30]. But it is should be noticed, with an increase of injection levels from 4 A to 8 A, the device emits predominantly from the QDs’ ES and the spectral bandwidth becomes narrow gradually. For a given I₂ of 8 A, the emission spectrum exhibits a FWHM of 40 nm with the output power of 108 mW.

The characteristics of the two-section SLD device were measured when the S1 section is under proper current-injection (I₁) to tune optical feedback from the rear facet. The output-power characteristics versus I₂ under different I₁ are shown in Fig. 3 . Equal power curves in the range of 100 to 800 mW as function of the currents injected in the two sections of the device are shown in Fig. 4 . It can be seen that the output power of the two-section SLD device increases rapidly with the increasing current-injection in the S1 section. While the S1 section is un-pumped as a rear optical absorption region, the output power of the two-section SLD device is 108 mW at I₂ = 8 A. At I₂ = 8 A and I₁ = 200 mA, the output power of the device can reach above 1.2 W. Inspection of the emission spectra with various combinations of I₁ and I₂ shows that lasing appears when the output power is approximately 400 mW (referring to the solid circles in Fig. 4).

 

Fig. 3 Output power versus current of the S2 section under different injection-current of the S1 section.

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Fig. 4 Equal power curves (solid lines) as function of the currents injected in the two sections of the device. The solid circles show the combinations at which the device begins lasing.

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When the S1 section is pumped, part of the increasing output power of the two-section SLD device is attribute to the effect of the emission coming from the S1 section. Another reason is the double-pass amplification of the emission from the S2 section by the optical feedback from the rear facet of the S1 section. The primary increase of the output power should be attributed to the double-pass amplification by the optical feedback rather than the effect of the weak emission coming from the S1 section. The evidence as shown in the inset of Fig. 3 is that a much slighter effect on the output power by the increasing injection-current in the S1 section while both facets of the two-section SLD device are with AR coating. In addition, the high output power benefits the design of the tapered S2 section for high optical gain. With a full flare angle of 6°, the beam will expend freely to fill the full tapered region owing to diffraction [31]. The optical density will be reduced, which increases the saturated power.

Figures 5(a) and 5(c) show the EL emission spectra under different I₁, for a given I₂ of 4 A and 6 A respectively. The dependences of spectral bandwidth and output power by injection-currents of the S1 section exhibits in Figs. 5(b) and 5(d). As we have expected, for the QD-SLD device with the two-section structure, it can be found that the spectrum shape and emission bandwidth can be tuned by properly controlling the current densities injected in the S1 section.

 

Fig. 5 Normalized EL emission spectra under different I₁, for a given I₂ of 4 A (a) and 6 A (c) respectively. Some spectra are shifted vertically for clarity. The dependences of spectral bandwidth and output power by injection-currents of the S1 section, for a given I₂ of 4 A (b) and 6 A (d) respectively. The inset of Fig. 5(c) is a segment of the high-resolution spectrum at I₂ = 6 A and I₁ = 100 mA.

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As shown in Figs. 5(a) and 5(b), under a given I₂ of 4 A, the EL emission spectrum exhibits a balance of the QDs’ GS emission and 1st ES emission while the S1 section is un-pumped. When the S1 section is pumped, by tuning the current densities injected in the S1 section, the wavelength-selective optical feedback to the emission of the QDs’ GS and 1st ES can be achieved. At I₂ = 4 A and I₁ = 50 mA, the GS emission provides the main contribution to the spectrum due to GS-dominant optical feedback under low pumping level of the S1 section. At I₂ = 4 A and I₁ = 150 mA, the balance of the QDs’ GS emission and 1st ES emission appears again, which is due to the increasing optical feedback of the QDs’ 1st ES. A broad emission bandwidth of 70 nm and a high output power of 260 mW is achieved simultaneously at I₂ = 4 A and I₁ = 150 mA.

For a given I₂ of 6 A, the EL emission spectra under different I₁ are shown in Fig. 5(c). When the S1 section is without pumped, a narrower spectrum with the FWHM of 51 nm is obtained by the main contribution of the emission from the QDs’ 1st ES. At I₂ = 6 A and I₁ = 40 mA, it reachs a balance between the QDs’ GS emission and 1st ES emission due to GS-dominant optical feedback from the rear facet of the S1 section. A broad emission spectrum of 86 nm with the output power of 186 mW is obtained. At I₂ = 6 A and I₁ = 75 mA, a broad emission spectrum of 65 nm and a high output power of 338 mW is achieved simultaneously. The optical spectrum ripple of ~0.07 dB is observed by a high-resolution spectral measurement at I₂ = 6 A and I₁ = 100 mA as shown in the inset of Fig. 5(c).

4. Conclusion

We have demonstrated a high-power InAs/GaAs QD-SLD device with broad bandwidth in the emission spectra by using a two-section structure that consists a straight titled section and a tapered titled section. The tapered section supplies for a high optical gain and the straight titled section is used to tune the optical feedback from the rear facet. It is shown that wavelength-selective optical feedback to the emission of the QDs’ GS and 1st ES can be achieved by tuning the current densities injected in the straight titled section. Under proper pumping level of the straight titled section, a high output power of 338 mW and a broad emission spectrum of 65 nm is obtained simultaneously due to the GS-dominant optical feedback from the rear facet of the two-section QD-SLD device.