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Abstract
GaN-based semiconductor optical amplifier (SOA) and its integration with laser diode (LD) is an essential building block yet to be demonstrated for III-nitride photonic integrated circuits (PICs) at visible wavelength. This paper presents the InGaN/GaN quantum well (QW) based dual-section LD consisting of integrated amplifier and laser gain regions fabricated on a semipolar GaN substrate. The threshold current in the laser gain region was favorably reduced from 229mA to 135mA at SOA driving voltages, VSOA, of 0V and 6.25V, respectively. The amplification effect was measured based on a large gain of 5.7 dB at VSOA = 6.25V from the increased optical output power of 8.2 mW to 30.5 mW. Such integrated amplifier can be modulated to achieve Gbps data communication using on-off keying technique. The monolithically integrated amplifier-LD paves the way towards the III-nitride on-chip photonic system, providing a compact, low-cost, and multi-functional solution for applications such as smart lighting and visible light communications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The past decade witnessed the rise of III-nitride optoelectronic devices, such as violet-blue-green light-emitting diodes (LEDs), laser diodes (LDs), modulators and photodetectors (PDs), owing to its applications in white lighting, display, optical data storage, and sensing [1–4]. Recently, InGaN/GaN quantum well (QW) based LEDs and LDs were demonstrated as transmitters for high-speed visible light communications (VLC) and underwater wireless optical communications (UWOC) [5–8]. However, the functions of light generation, amplification, modulation, and detection at visible wavelength are currently enabled using discrete III-nitride components. Since the on-chip integration of various photonic devices offers the advantages of small footprint, low-cost and multi-functionality, it is of great interest to develop III-nitride photonic integrated circuits (PICs) at the visible wavelength [9]. Though GaAs- and InP-based PICs at the near-infrared wavelength have been studied [10], such photonic integration at the visible wavelength is still not available in III-nitride material systems. Towards such integration, GaN-based core-shell nanowire LEDs and PDs were placed and patterned to demonstrate optical coupling with SiN waveguides [11]. Besides, researchers fabricated the LED, PD and the suspended waveguide on the same GaN-on-silicon substrate [12] and such scheme enabled 30Mbps in-plane data communication link [13]. Compared to those based on LEDs, the photonics integration based on LDs are promising owing to its significant higher modulation bandwidth [14, 15]. Recently, a blue-emitting integrated waveguide modulator LD was demonstrated for white lighting and VLC [4]. GaN-based LDs with electroabsorption modulator and absorber were also reported for Q-switching [16, 17]. In addition, the integration of LD and PD was reported to achieve power monitoring and on-chip communication [18]. However, the III-nitride semiconductor optical amplifier (SOA) and its integration with LD are yet to be developed for the eventual realization of III-nitride PICs. III-nitride based optical amplifiers, which can be used as power boosters, in-line amplifiers and detector pre-amplifiers in optical data communication system, are important optical components for VLC links. In comparison with electrical amplifiers, optical amplifiers stand for a cheaper and compact solution. In particular, the millimeter-sized SOA can be monolithic integrated with laser chips, which enables the building of economical and high-performance devices for advanced optical networks.
In this paper, we demonstrated a violet-emitting dual-section LD with integrated semiconductor optical amplifier and laser gain regions (SOA-LD) sharing the same InGaN/GaN QW-based active region on semipolar GaN substrate. With increasing driving voltage at the SOA section (VSOA) beyond transparency, the amplification effect was measured based on the increase in optical output power of 8.2 mW (laser section injection current ILD = 250mA, VSOA = 0V) and 30.5 mW (ILD = 250 mA, VSOA = 6.25 V). A large effective gain of 5.7 dB at VSOA = 6.25 V was measured for the integrated SOA-LD. A proof-of-concept Gbps data communication link by modulating the integrated amplifier was demonstrated using on-off keying (OOK) modulation scheme, suggesting the modulated SOA-LD to be a promising device for VLC applications.
2. Experimental details
The layer structure of the dual-section integrated SOA-LD, as shown in Fig. 1(a), was grown using metal-organic chemical vapor deposition (MOCVD) technique. The epitaxial structure consists of four pairs of In0.1Ga0.9N/GaN QWs, 600 nm/350 nm p-GaN/n-GaN cladding layers, 60 nm/60 nm p-In0.025Ga0.975N/n-In0.025Ga0.975N separate confinement heterostructure waveguide layers, a 16 nm p-Al0.18Ga0.82N electron blocking layer, and highly doped p-GaN/n-GaN contact layers. The Pd/Au and Ti/Al/Ni/Au metallization stacks form the p- and n-electrodes, respectively, for the 300-µm long integrated SOA section and the 1190-µm long LD gain section [Fig. 1(b)]. The uncoated facets and 2-µm wide ridge waveguide were defined using UV photolithography and plasma etching. Both of the integrated SOA and LD gain sections have their Mg-doped p-GaN contact layer etched off to form a trench for electrical isolation at the p-contacts (> 100 kΩ). Thus, this allows independent electrical control of the two sections, while retaining the seamless optical coupling of the light from laser gain section to the integrated SOA section for light amplification.
Fig. 1 (a) Cross-sectional layered structure and (b) 3D illustration of the 405-nm emitting dual-section integrated SOA-LD on semipolar GaN substrate. The device involves 4 pairs of In0.1Ga0.9N/GaN multi-quantum-wells (MQWs) as active region and a pair of InGaN separate confinement heterostructure waveguide layers. The length of the integrated SOA section and LD gain section is 300 µm and 1190 µm, respectively. Inset: Photo of the fabricated device under optical microscope. (c) Photo of the device operating at room temperature. (d) The emission spectrum of the device at a laser gain section current of 250 mA and zero SOA section driving voltage, showing a peak emission at ~404.3 nm.
The fabricated, unpackaged dual-section SOA-LD was tested under continuous wave (CW) operation at room temperature, as shown in Fig. 1(c). The measurement setup consists of a thermoelectric cooler, a Keithley 2520 diode laser testing system with a calibrated Si PD and a Labsphere integrating sphere, a Keithley 2400 source meter to provide the driving voltage to the integrated SOA section, and an Ando AQ-6315A optical spectrum analyzer for spectra measurement. Initially, without biasing the integrated SOA section (VSOA = 0 V), the LD was lasing at a peak wavelength of 404.3 nm at a LD gain section current of 250 mA (ILD = 250 mA), as shown in Fig. 1(d).
In the data transmission experiments, the device was probed in the same setup, the laser gain section was probed by a set of DC probes and the integrated amplifier section was probed by a set of RF probes. A pseudorandom binary sequence (PRBS) 210-1 data stream was generated using the built-in pattern generator in the Agilent J-BERT N4903B bit error rate tester and the error detector in the same equipment was utilized to measure the bit-error rate (BER). The eye-diagram was measured using an Agilent DCA-86100C digital communication analyzer. The non-return-to-zero OOK (NRZ-OOK) data transmission setup also involves a Picosecond Pulse Labs 5543 bias tee, a Picosecond Pulse Labs 5865 broadband amplifier and Menlo Systems APD 210 high-speed Si avalanche photodetector (APD). The APD was placed approximately 10 cm from the light emitting facet of the device.
3. Results and discussions
Figure 2(a) presents the optical output power of the integrated SOA-LD vs. LD gain region injection current (L-ILD) relation at different driving voltages applied to the integrated waveguide SOA section (VSOA from 0 V to 6.25 V). The onset of lasing is identified by the rapid increase of optical output power collected from the front facet of the SOA-LD when ILD goes beyond threshold current Ith. To evaluate the effectiveness of the demonstrated dual-section integrated SOA-LD, we compared its outcomes with the optical power vs. current (L-I) relation of a single-section, 1200 µm long LD in Fig. 2(b). An Ith of 254 mA was observed in the 1200 µm LD. At VSOA = 0V, the SOA-LD exhibited a lower optical power due to losses in the unbiased SOA. A significant enhancement in optical emission was measured in the SOA-LD at VSOA of 5 V and 6V, outperforming the 1200 µm LD. An increase of the slope of L-ILD characteristics is observed with increasing VSOA beyond 4 V, suggesting the amplification of light emission owing to the integrated SOA section.
Fig. 2 (a) Optical output power vs. LD gain region current (ILD) relation of integrated SOA-LD with varying SOA driving voltage (VSOA) at room temperature. (b) L-I relation of a 1200 µm long LD and its comparison with that of integrating SOA-LD at VSOA of 0 V, 5 V, and 6 V.
In particular, at a constant ILD of 250 mA in the LD gain section, the optical output power of the SOA-LD was enhanced from 8.2 mW at VSOA = 0 V to 9.0 mW, 17.5 mW, 28.0 mW, and 30.5 mW at VSOA of 4 V, 5 V, 6 V, and 6.25 V, respectively [Fig. 3(a)]. Moreover, a decrease of Ith from 229 mA at VSOA = 0 V to 135 mA at VSOA = 6.25 V was observed in the same figure. Figure 3(b) illustrates the voltage vs. current (V-I) relations of the integrated SOA section and the LD gain section. as well as the relation of the 1200 µm long LD for comparison purposes. When the SOA-LD operates at ILD = 250 mA and VSOA = 6 V (with SOA current ISOA = 67 mA), the device consumes a total electrical power of 2.14 W, corresponding to a wall plug efficiency (WPE) of 1.3%. On the other hand, the 1200 µm long LD operating at the same total system current of 317 mA, shows a WPE of 0.54% based on a total electrical power consumption of 2.27W. Thus, the amplification of light output in the dual-section integrated SOA-LD was verified.
Fig. 3 (a) Optical power vs. VSOA and Ith vs. VSOA at ILD = 250 mA. (b) Voltage vs. current (V-I) relations of the integrated SOA section and LD gain section in the SOA-LD, and the 1200 µm long LD.
The VSOA-dependent amplification effect originates from the switching of the gain in the SOA section from a low value to a high value when VSOA increases. Based on the same technology as a Fabry-Perot laser, the SOA amplify incident light through stimulated emission. The working principle of the integrated amplifier is similar to that of a GaAs-based SOAs working in the optical telecommunication wavelength regime [19], where the waveguide mode traveling through the active region of an SOA contributed to the optical transition of the electrically injected carriers in the conduction band. The effective gain in such semiconductor amplifier depends on the injection current. Those stimulated photons have the similar energy to the original optical signal, thus amplifying the optical signal. Since the integrated SOA and laser gain sections share the same epitaxial layers, the injection current density necessary to reach transparency can be obtained from the performance fitting of LDs with different cavity lengths on the same chip. A transparency current density (J0) of 900 A/cm2 was measured, suggesting a transparency current (I0) of 5.4 mA in the SOA section. This corresponds to a bias voltage (V0) of 3.5 V at transparency. Hence, the effective gain of the SOA-LD is defined as the ratio between the optical power at driving voltage VSOA and the optical power at transparency condition, expressed as,
Figure 4(a) shows the effective gain as a function of ILD at different VSOA. Gain saturation occurs at an LD current of approximately 250 mA. By increasing VSOA from 4 V to 6.25 V, an increasing gain was observed. Basically, a positive net gain at VSOA ≥ 4 V is expected when the InGaN/GaN QW active region is biased beyond transparency, which is consistent with the gain measurement. At ILD = 250 mA, the effective gain increases from 0.4 dB to 5.7 dB in response to increasing VSOA from 4 V to 6.25 V, with a slope of 2.36 dB/V, as shown in Fig. 4(b). The high gain observed in the device is partially attributed to the large electron-hole wavefunction overlap in InGaN/GaN QWs grown on semipolar GaN substrate, which exhibits reduced polarization field compared to that in conventional c-plane devices [20]. Apart from the high material gain in semipolar InGaN/GaN QWs, the device presented in current work also features a highly Mg-doped GaN contact layer to reduce the contact resistance and the InGaN-based, Al-free separate confinement heterostructure (SCH) layers to improve the reliability and manufacturability for high power operation [21].
Fig. 4 (a) Effective gain vs. ILD relation of the SOA-LD at different VSOA. (b) Effective gain vs. VSOA relation at ILD = 250 mA.
The amplification effect of the integrated SOA section in the dual-section LD can be further studied from spectral characteristics. At ILD = 200 mA, the emission spectra of the integrated SOA-LD at VSOA of 0 V and 6.25 V are shown in Fig. 5. The amplification ratio, which is defined as the intensity ratio between the two emission spectra, is also shown in the figure. Compared to the emission spectra at zero driving voltage in the integrated SOA section (VSOA = 0 V), the emission peak after amplification (with VSOA = 6.25 V) exhibits a significant increase in intensity as well as a narrowing of the peak full width at half maximum (FWHM) from 3 nm to 0.6 nm. Therefore, with amplification originating from the integrated SOA, the device was switched from spontaneous emission (at VSOA = 0 V) to stimulated emission regime (at VSOA = 6.25 V), which is consistent with the L-ILD relations shown in Fig. 2. Finally, a peak amplification ratio of 18.4 is observed at the lasing wavelength of 404 nm. These spectral characteristics of the dual-section SOA-LD further confirm the amplification effect obtained from the integrated SOA section.
Fig. 5 Emission spectra of the SOA-LD at ILD = 200 mA with VSOA = 0 V and 6.25 V. The amplification ratio (Ramp) is also shown with a peak Ramp of 18.4 at 404 nm.
For a proof-of-concept demonstration of high-speed data communication using the proposed SOA-LD scheme, we performed a modulation measurement by applying the data signal to the integrated amplifier while pumping the gain region with a constant driving current of 250 mA, as illustrated in Fig. 6(a). Instead of directly modulate the laser gain section, the modulated SOA scheme was measured. At a data rate of 1 Gbit/s, a clear open eye was observed [Fig. 6(b)]. A corresponding BER of 3.4 x 10−4 is measured, which is well passing the FEC limit of 3.8 x 10−3. This result supports that the demonstrated SOA-LD is a promising transmitter for future VLC systems.
Fig. 6 (a) Schematic of the NRZ-OOK data transmission measurement using the SOA-LD as transmitter. BERT stands for bit error rate tester and DCA stands for digital communication analyzer. (b) Eye diagram of 1 Gbit/s data rate showing with a clear open eye.
4. Conclusions
In conclusion, we demonstrated the InGaN/GaN QW-based dual-section laser diode consisting of a laser gain section and an integrated SOA section fabricated on a semipolar GaN substrate. The high-gain (5.7 dB) integrated SOA can effectively enhance light output and leading to a reduced power consumption and increased efficiency in the SOA-LD. Gbps data communication link was enabled using the integrated amplifier. The presented III-nitride SOA-LD will constitute an important building block for the realization of III-nitride PICs at visible wavelength for a plethora of emerging applications, such as smart lighting, optical communications, and optical switching based on visible light.
Funding
King Abdulaziz City for Science and Technology (KACST), Grant No. KACST TIC R2-FP-008 and KACST-KAUST- UCSB Solid-State Lighting Program. This work was partially supported by the King Abdullah University of Science and Technology (KAUST) baseline funding, BAS/1/1614-01-01, KAUST funding KCR/1/2081-01-01, and GEN/1/6607-01-01, as well as KAUST-KFUPM Special Initiative (KKI) Program, REP/1/2878-01-01.
Acknowledgments
Portions of this work were presented at the OSA Light, Energy and the Environment Congress in 2017, SW3C.2.
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