Group-III-Nitride Superluminescent Diodes for Solid-State Lighting and High-Speed Visible Light Communications

Group-III-nitride superluminescent diodes (SLDs) are emerging as light sources for white lighting and visible light communications (VLC) owing to their droop-free, low speckle noise, and large modulation bandwidth properties. In this paper, we discuss the development of GaN-based visible SLDs, and analyze their electro-optical properties by studying the optical power-bandwidth products and injection current densities. The significant progress in blue SLDs and their applications for white light VLC is highlighted. A blue SLD, with an optical power of >100 mW and large PBP of 536 mW·nm, is utilized to generate white light, resulting in a high color rendering index (CRI) of 88.2. In a modulation experiment designed for an SLD-based VLC system, an on–off keying scheme exhibits a 1.2 Gbps data rate, with a bit error rate of 1.8 × 10−3, which satisfies the forward error correction criteria. A high data rate of 3.4 Gbps is achieved using the same SLD transmitter, by applying the 16-quadrature-amplitude-modulation (16-QAM) discrete multitone modulation scheme for high-speed white light communication. The results reported here unequivocally point to the significant performance and versatility that GaN-based SLDs could offer for beyond-5G implementation, where white lighting and high spectral efficiency VLC systems can be simultaneously implemented.

region, highly doped GaN contact layer and AlGaN electron 119 blocking layer. The waveguide width is 15 μm. The fabrication 120 process has been discussed in detail elsewhere [43]. 121 For a single-pass SLD, the optical power can be defined as, 122 P ≈ P sp G s (j, L) , (1) where P sp is the spontaneous emission power, and G s is the (j) and cavity length (L), and is given by: where Γ is the confinement factor, g o is the gain coefficient, η 126 is the internal quantum efficiency, d is the active layer thickness 127 and α is the absorption coefficient [44]. 128 In a single-pass SLD, the reflectivity of the front and back 129 facets should be as small as possible, which can be achieved by 130 AR coating. However, in this case, the device requires a signifi-131 cantly large single-pass gain (G s ) to achieve high-power output, 132 which calls for a high injection current density (j), and a long 133 cavity length (L) [44]. A higher injection inadvertently leads to 134 joule heating, and a device with a long cavity length may not 135 be efficient. A preferred device design for achieving high-power 136 SLD is based on a double-pass configuration, in which a high-137 reflective (HR) mirror (R 2 > 0.9) is coated at the back facet 138 [39]. In a double-pass SLD, the output power is proportional to 139 the square of the gain, as For a given R 2 > 0.9, the front facet reflectivity (R 1 ) should 141 be kept as small as possible, to minimize the generation of spec-142 tral modulation, which is undesirable for SLDs due to its impact 143 to the coherence properties of SLD [44]. This challenge can be 144 solved by tilting the waveguide or facet beyond a certain angle. 145 A modal reflectivity below 10 −6 can be achieved in SLDs with 146 facet angles greater than 6° [45], [46]. Our SLD features a 12°147 tilted facet, in conjunction with an HR-coated back facet mirror, 148 as shown in Fig. 1.  153 shown. The device was measured using the Keithley 2520 laser 154 diode testing system with a calibrated Si photodetector. The de-155 vice has an output optical power of ∼105 mW at 1 A, as shown 156 in Fig. 2. A superlinear increase in the L-I curve is observed 157 following the onset of superluminescent characteristics, at ap-158 proximately ∼500 mA, with an increasing EQE observed in the 159 ASE regime. The SLD has a relatively low operating voltage of 160 4. 38 ∼ 4.78 V at a current of 500 ∼1000 mA, resulting in a low 161 (dV/dI) series resistance (≤1 Ω). 162 The electroluminescence (EL) spectra (Fig. 3) were measured 163 using a Yokogawa AQ6373B optical spectrum analyzer, exhibit-164 ing peak emissions at ∼443 nm. A blue-shift in the emission peak 165   Fig. 4a. 190 The results for which the SLD was driving under pulsed or CW condition are labelled differently. Of all the reported devices we 192 reviewed, the highest PBP is 931.7 mW·nm, calculated based 193 on an optical power of 121 mW and a peak FWHM of 7.7 nm at 194 9.5 kA/cm 2 , from a 446-nm-emitting SLD grown on a semipolar 195 GaN substrate with a passive absorber design operating in CW 196 mode reported in 2017 [54]. Other SLDs showing a PBP of > 197 650 mW·nm include a 447-nm-emitting semipolar SLD, with a 198 PBP of 775 mW·nm at 8 kA/cm 2 [25]; a 429-nm-emitting SLD, 199 with a PBP of 700 mW·nm at 15 kA/cm 2 [5]; and a 442-nm-200 emitting SLD, with a PBP of 682.5 mW·nm at 6.7 kA/cm 2 [7]. 201 A higher PBP is partially attributed to a higher optical power, 202 where an SLD with a higher material gain is favored. 203 Next, we consider the ratio of the PBP to the current density 204 ((mW·nm)/(kA/cm 2 )). This ratio provides a measure of how effi-205 ciently electrical charges are converted into optical power along 206 a determined optical bandwidth. Fig. 4b presents the PBP over 207 the current density vs. the emission wavelength for the reported 208 devices. The highest PBP to current density value reported is 209 101.87 (mW·nm)/(kA/cm 2 ), for a blue SLD using a tilted facet 210 configuration on a c-GaN substrate [7]. This is result of the high 211 quantum efficiency in this SLD originating from the high-quality 212 active region in the epitaxial layers. Our previously presented 213 semipolar SLDs also exhibit high values in terms of the PBP to 214 current density value, such as ∼98 (mW·nm)/(kA/cm 2 ) in [54] 215  TABLE I  ELECTRO-OPTICAL PROPERTIES OF III-NITRIDE SLDS P refers to the optical power from the SLD, FWHM refers to the optical bandwidth of the emission peak, PBP refers to the power-bandwidth product, J refers to the injection current density, and " * " refers to the results presented in this work. and 59 ∼ 97 (mW·nm)/(kA/cm 2 ) in [25], suggesting that such 216 devices are promising for high-brightness light emitters.  [40] and 20 mW in pulse mode 232 [55]. Intense efforts are required to further improve the optical 233 power of cyan-green SLDs.
234 Fig. 4. Plots of (a) PBP vs. current density, (b) PBP over current density vs. wavelength, and (c) optical power vs. wavelength in GaN-based SLDs reported in prior work. The numbers correspond to the references in which the data points were obtained. The results from SLDs operating in pulsed and CW mode have been labelled separately, and the performances of SLDs reported from KAUST Photonics Laboratory have been highlighted. " * " refers to the results presented in this work.

235
White light can be generated using violet-blue light emit-236 ters exciting one or multiple phosphors as color converters 237 [58]. A comparison of the spectra, chromatic properties, and 238 emission patterns in white light generated using LEDs, SLDs, 239   [41]. For a high-power blue SLD on a c-GaN 291 substrate, a modulation bandwidth of ∼400 MHz has recently 292 been reported [7]. In this section, we study the performance of 293 VLC systems based on blue SLD transmitters, and investigate 294 the system performance using various modulation techniques, 295 i.e., OOK and DMT.

297
Non-return-to-zero OOK (NRZ-OOK) modulation has been 298 widely employed in VLC systems, owing to its low complexity. 299 To study the data transmission capability of an SLD-based VLC 300 system, we employ a pseudorandom binary sequence (PRBS) 301 2 10 -1 data stream generated by the Agilent N4903B J-BERT pat-302 tern generator. A data transmission experiment is conducted by 303 measuring the bit error rates (BERs) and eye diagrams at differ-304 ent data rates (Fig. 6). The setup also involves a PSPL5866 linear 305 amplifier, PSPL5580 bias tee, and 1-GHz APD210 Si avalanche 306 photodetector as the receiver. The eye diagrams are collected 307 using a DCA-86100C digital communication analyzer and are 308 depicted in Figs. 6b and 6c. A data rate of 1.2 Gbps was achieved 309 with a BER of 1.8 × 10 −3 , which is below the forward error 310 correction (FEC) limit of 3.8 × 10 −3 . The corresponding eye 311 diagram (Fig. 6c) shows open eyes at 1.2 Gbps, demonstrating 312 that GaN-based SLDs can be utilized as high-speed transmitters 313 in VLC systems.
where S r (k) and S i (k) are the real and the imaginary parts of 337 S(k), respectively. Then (4) can be written as: In the majority of cases, N is an even number. Thus, s i (n) can 339 be written as: From (7), we can conclude that if we want to obtain a real-341 valued time domain signal for the IDFT, then the complex signal 342 of the frequency domain in a specific period N should satisfy the 343 following relationship, where Q(k) is the k th complex symbol of 16-QAM, and 345 Q (k) is the conjugated complex data of Q(k). In addition, 346 S(0) = S( N 2 ) = 0. In practice, the 0 to (N/2-1)th subcarriers 347 are placed on the positive frequency carriers, while the (N/2) 348 to (N-1)th subcarriers are placed on the corresponding negative 349 frequency carriers. If the QAM symbols satisfy the relationship 350 depicted in (8), then we can obtain a real-valued time domain 351 signal by the IDFT, which can be directly modulated on the blue 352 SLD and received by the Si APD.  Fig. 7 illustrates the system structure. At the transmitting 354 side, a decimal random data stream from 0-15 is mapped into 355 16-QAM symbols. The five lowest frequency subcarriers will 356 be filled with zeros, owing to the low SNRs presented in these 357 subcarriers. To transform the complex data into real-valued time 358 domain data, we will conjugate the origin complex symbols and 359 assemble them. Up-sampling can suppress the frequency spec-360 trum and broaden the time domain signal, which improves the 361 system performance. Then, after applying the IDFT the real-362 valued time domain signal is generated. A cycle prefix (CP) 363 is employed to mitigate the multi-path effect. A Tektronix 364 AWG7122C arbitrary waveform generator (AWG) was utilized 365 in this experiment.  estimated using a DMT signal, where all subcarriers were modu-393 lated by binary phase-shift keying (BPSK) with an equal power. 394 We illustrate the achievable QAM order against the frequency in 395 Fig. 9. The first five subcarriers at the low-frequency end were 396 intentionally set to zero. It should be noted that a number of 397 subcarriers are capable of supporting 32-QAM DMT, with few 398 of them supporting 64-QAM. Therefore, it is possible to achieve 399 a higher system performance using SLD as the transmitter with 400 bit-and power-loading DMT modulation techniques. , respectively. Although 411 many reports show relatively large modulation bandwidth, their 412 low optical powers, which are way below 10 mW, limit the prac-413 tical application in white-lighting and long-range VLC systems 414 [75]. Although most of the devices mentioned above are unpack-415 aged devices, it may need further researches in improving the 416 light extraction efficiency and utilizing arrayed micro-LEDs to 417 enhance the output power for VLC applications.

419
Emerging VLC and simultaneous lighting applications re-420 quire high-brightness and high-speed group-III-nitride light 421 emitters. GaN-based superluminescent diodes offer consider-422 able competitive advantages over LEDs and LDs. In this study, 423 we have discussed the design and electro-optical properties of 424 GaN-based SLDs for the abovementioned purpose. A blue SLD 425 with >100 mW optical power operating in CW has been pre-426 sented, exhibiting a large PBP of 536 mW·nm. The demon-427 strated device offers a benchmark towards the development of 428 Watt-level violet-blue SLDs for matching the optical power re-429 ported in high-power LDs [76]. By combining the blue SLD 430 with phosphor, white-light with a CRI of 88.2 has been produced. The SLD-based VLC system offers a high data rate of          process has been discussed in detail elsewhere [43].

121
For a single-pass SLD, the optical power can be defined as, where P sp is the spontaneous emission power, and G s is the 123 single-pass optical gain. G s is a function of the current density 124 Fig. 1. Schematic of the fabricated double-pass SLD with an HR-coated back facet and 12°tilted front facet.
(j) and cavity length (L), and is given by: where Γ is the confinement factor, g o is the gain coefficient, η 126 is the internal quantum efficiency, d is the active layer thickness 127 and α is the absorption coefficient [44].

128
In a single-pass SLD, the reflectivity of the front and back 129 facets should be as small as possible, which can be achieved by 130 AR coating. However, in this case, the device requires a signifi-131 cantly large single-pass gain (G s ) to achieve high-power output, 132 which calls for a high injection current density (j), and a long 133 cavity length (L) [44]. A higher injection inadvertently leads to 134 joule heating, and a device with a long cavity length may not 135 be efficient. A preferred device design for achieving high-power 136 SLD is based on a double-pass configuration, in which a high-137 reflective (HR) mirror (R 2 > 0.9) is coated at the back facet 138 [39]. In a double-pass SLD, the output power is proportional to 139 the square of the gain, as For a given R 2 > 0.9, the front facet reflectivity (R 1 ) should 141 be kept as small as possible, to minimize the generation of spec-142 tral modulation, which is undesirable for SLDs due to its impact 143 to the coherence properties of SLD [44]. This challenge can be 144 solved by tilting the waveguide or facet beyond a certain angle. 145 A modal reflectivity below 10 −6 can be achieved in SLDs with 146 facet angles greater than 6° [45], [46]. Our SLD features a 12°147 tilted facet, in conjunction with an HR-coated back facet mirror, 148 as shown in Fig. 1. shown. The device was measured using the Keithley 2520 laser 154 diode testing system with a calibrated Si photodetector. The de-155 vice has an output optical power of ∼105 mW at 1 A, as shown 156 in Fig. 2. A superlinear increase in the L-I curve is observed 157 following the onset of superluminescent characteristics, at ap-158 proximately ∼500 mA, with an increasing EQE observed in the 159 ASE regime. The SLD has a relatively low operating voltage of 160 4.38 ∼ 4.78 V at a current of 500 ∼1000 mA, resulting in a low 161 (dV/dI) series resistance (≤1 Ω).

162
The electroluminescence (EL) spectra (Fig. 3) were measured 163 using a Yokogawa AQ6373B optical spectrum analyzer, exhibit-164 ing peak emissions at ∼443 nm. A blue-shift in the emission peak 165  where an SLD with a higher material gain is favored. 203 Next, we consider the ratio of the PBP to the current density 204 ((mW·nm)/(kA/cm 2 )). This ratio provides a measure of how effi-205 ciently electrical charges are converted into optical power along 206 a determined optical bandwidth. Fig. 4b presents the PBP over 207 the current density vs. the emission wavelength for the reported 208 devices. The highest PBP to current density value reported is 209 101.87 (mW·nm)/(kA/cm 2 ), for a blue SLD using a tilted facet 210 configuration on a c-GaN substrate [7]. This is result of the high 211 quantum efficiency in this SLD originating from the high-quality 212 active region in the epitaxial layers. Our previously presented 213 semipolar SLDs also exhibit high values in terms of the PBP to 214 current density value, such as ∼98 (mW·nm)/(kA/cm 2 ) in [54] 215 TABLE I ELECTRO-OPTICAL PROPERTIES OF III-NITRIDE SLDS P refers to the optical power from the SLD, FWHM refers to the optical bandwidth of the emission peak, PBP refers to the power-bandwidth product, J refers to the injection current density, and " * " refers to the results presented in this work. and 59 ∼ 97 (mW·nm)/(kA/cm 2 ) in [25], suggesting that such 216 devices are promising for high-brightness light emitters.    Fig. 5. These results 269 confirm that GaN-based SLDs can be integrated into indoor 270 lighting systems, offering a desirable CRI. A higher CRI can 271 be achieved by further engineering the phosphor mixture, such 272 as adding more color conversion elements in the green, cyan and 273 red color regime.

274
IV. SLD AS TRANSMITTER FOR VLC 275 While LEDs have been well-studied as transmitters in VLC 276 systems, owing to their wide availability, their relatively small 277 modulation bandwidth limits the data transmission rate in LED-278 based VLC systems [26]. Hence, many researchers have pro-279 posed different schemes to address this challenge. For example, 280 a hardware equalizer that suppresses the low-frequency domain 281 SNR to compensate for the high-frequency response can broaden 282 the LED bandwidth, thus enabling an LED-based VLC system 283 to achieve higher data rate [61], [62].

284
In our previous work, we demonstrated a GaN-based SLD 285 exhibiting a significantly higher 3-dB modulation bandwidth, 286 thus making it attractive for use as a transmitter in VLC systems. 287 A blue-emitting SLD on a semipolar GaN substrate exhibits 288 a modulation bandwidth of ∼500 MHz [54]. In a subsequent 289 report, a 405-nm emitting semipolar SLD exhibited a bandwidth 290 of up to 807 MHz [41]. For a high-power blue SLD on a c-GaN 291 substrate, a modulation bandwidth of ∼400 MHz has recently 292 been reported [7]. In this section, we study the performance of 293 VLC systems based on blue SLD transmitters, and investigate 294 the system performance using various modulation techniques, 295 i.e., OOK and DMT.

297
Non-return-to-zero OOK (NRZ-OOK) modulation has been 298 widely employed in VLC systems, owing to its low complexity. 299 To study the data transmission capability of an SLD-based VLC 300 system, we employ a pseudorandom binary sequence (PRBS) 301 2 10 -1 data stream generated by the Agilent N4903B J-BERT pat-302 tern generator. A data transmission experiment is conducted by 303 measuring the bit error rates (BERs) and eye diagrams at differ-304 ent data rates (Fig. 6). The setup also involves a PSPL5866 linear 305 amplifier, PSPL5580 bias tee, and 1-GHz APD210 Si avalanche 306 photodetector as the receiver. The eye diagrams are collected 307 using a DCA-86100C digital communication analyzer and are 308 depicted in Figs. 6b and 6c. A data rate of 1.2 Gbps was achieved 309 with a BER of 1.8 × 10 −3 , which is below the forward error 310 correction (FEC) limit of 3.8 × 10 −3 . The corresponding eye 311 diagram (Fig. 6c) shows open eyes at 1.2 Gbps, demonstrating 312 that GaN-based SLDs can be utilized as high-speed transmitters 313 in VLC systems.
where S r (k) and S i (k) are the real and the imaginary parts of 337 S(k), respectively. Then (4) can be written as: In the majority of cases, N is an even number. Thus, s i (n) can 339 be written as: From (7), we can conclude that if we want to obtain a real- where Q(k) is the k th complex symbol of 16-QAM, and 345 Q (k) is the conjugated complex data of Q(k). In addition, 346 S(0) = S( N 2 ) = 0. In practice, the 0 to (N/2-1)th subcarriers 347 are placed on the positive frequency carriers, while the (N/2) 348 to (N-1)th subcarriers are placed on the corresponding negative 349 frequency carriers. If the QAM symbols satisfy the relationship 350 depicted in (8), then we can obtain a real-valued time domain 351 signal by the IDFT, which can be directly modulated on the blue 352 SLD and received by the Si APD. 353 Fig. 7 illustrates the system structure. At the transmitting 354 side, a decimal random data stream from 0-15 is mapped into 355 16-QAM symbols. The five lowest frequency subcarriers will 356 be filled with zeros, owing to the low SNRs presented in these 357 subcarriers. To transform the complex data into real-valued time 358 domain data, we will conjugate the origin complex symbols and 359 assemble them. Up-sampling can suppress the frequency spec-360 trum and broaden the time domain signal, which improves the 361 system performance. Then, after applying the IDFT the real-362 valued time domain signal is generated. A cycle prefix (CP) 363 is employed to mitigate the multi-path effect. A Tektronix 364 AWG7122C arbitrary waveform generator (AWG) was utilized 365 in this experiment.  BERs and corresponding constellation diagrams at 3 Gbps and 374 3.4 Gbps at different data rates are presented in Fig. 8. 375 At 3.4 Gbps, the SLD-based VLC system has a BER of 376 3.7 × 10 −3 , which satisfies the FEC criteria of 3.8 × 10 −3 . To the 377 best of our knowledge, this is the first reported SLD-based VLC 378 system with a data rate higher than 3 Gbps using the DMT modu-  Fig. 9. Estimated QAM orders for each subcarrier for bit-loading scheme. estimated using a DMT signal, where all subcarriers were modu-393 lated by binary phase-shift keying (BPSK) with an equal power. 394 We illustrate the achievable QAM order against the frequency in 395 Fig. 9. The first five subcarriers at the low-frequency end were 396 intentionally set to zero. It should be noted that a number of 397 subcarriers are capable of supporting 32-QAM DMT, with few 398 of them supporting 64-QAM. Therefore, it is possible to achieve 399 a higher system performance using SLD as the transmitter with 400 bit-and power-loading DMT modulation techniques.

401
It is also worth mentioning that new types of group-III-nitride 402 light emitters have also been developed recently showing poten-403 tial in high-speed modulation, such as mini-LEDs, micro-LEDs 404 and semipolar/nonpolar light emitters. For example, micro-405 LEDs show a maximum 3-dB bandwidth and optical powers 406 of ∼500 MHz and ∼2 mW in [71], ∼800 MHz and < 3 mW in 407 [72], and ∼1 GHz and ∼1.7 mW in [73], respectively. Reported 408 semipolar/nonpolar light emitters show a bandwidth and power 409 of ∼1 GHz and ∼1.5 mW in [25], ∼1.5 GHz and ∼1.3 mW in 410 [74], and ∼2.5 GHz and ∼2 mW in [52], respectively. Although 411 many reports show relatively large modulation bandwidth, their 412 low optical powers, which are way below 10 mW, limit the prac-413 tical application in white-lighting and long-range VLC systems 414 [75]. Although most of the devices mentioned above are unpack-415 aged devices, it may need further researches in improving the 416 light extraction efficiency and utilizing arrayed micro-LEDs to 417 enhance the output power for VLC applications.

419
Emerging VLC and simultaneous lighting applications re-420 quire high-brightness and high-speed group-III-nitride light 421 emitters. GaN-based superluminescent diodes offer consider-422 able competitive advantages over LEDs and LDs. In this study, 423 we have discussed the design and electro-optical properties of 424 GaN-based SLDs for the abovementioned purpose. A blue SLD 425 with >100 mW optical power operating in CW has been pre-426 sented, exhibiting a large PBP of 536 mW·nm. The demon-427 strated device offers a benchmark towards the development of 428 Watt-level violet-blue SLDs for matching the optical power re-429 ported in high-power LDs [76]. By combining the blue SLD 430 with phosphor, white-light with a CRI of 88.2 has been produced. The SLD-based VLC system offers a high data rate of