A comprehensive comparison on the data transmission performance of photonic crystal structured micro light-emitting diodes (PC-µLEDs) with different aperture sizes is realized for high-speed visible light communication application. The 120×120-µm2 large PC-µLED exhibits the largest optical power of 580 µW and the highest external quantum efficiency of 2.5%; however, it also demonstrates the lowest analog modulation bandwidth of only 72 MHz. By contrast, the smallest PC-µLED with 20×20-µm2 aperture emits the lowest optical power of 37 µW but provides the highest 3-dB bandwidth of 162 MHz. After optimizing the operating parameters for data transmission, the trade-off between output power and encoding bandwidth is observed to improve the transmission performance. The PC-µLED with mesa length of 60–80 µm can transmit on-off keying (OOK) data format at 500 Mbit/s under error-free BER criterion. In particular, the device with a mesa area of 80×80-µm2 successfully carries the 300-MBaud 4-level pulse amplitude modulation (PAM-4) data with corresponding data rate of 600 Mbit/s under KP4 forward error correction (FEC) required BER. Furthermore, the quadrature-amplitude-modulation (QAM) orthogonal frequency division multiplexing (OFDM) data transmission is also performed, and the highest data rate of 2 Gbit/s under FEC criterion is allowable by using the PC-µLED with a mesa aperture of 80×80-µm2.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Lighting-emitting-diodes (LEDs) with different colors and structures have been proposed to enable the indoor optical and structure communication (OWC) with rapidly increased transmission capacity for smart-house and home-entertainment applications in recent years [1–4]. Chen et al. operated the Si p-n junction LED at avalanche breakdown region to perform the electro-optical modulation at 11 GHz . Up to now, the blue LED is the most prospective candidate for the next-generation lighting fidelity (LiFi) and OWC network due to its long-term stability, compact size, harmless substances, and low maintenance cost, etc [6,7]. In previous works, Vucic et al. have employed the phosphorescent white LEDs encoded by on-off-keying (OOK) data format to demonstrate the OWC link at 230 Mbit/s . However, general LEDs with larger emission aperture of ∼200-1000 µm2 usually exhibits lower modulation bandwidth and data rate. The improvement on analog bandwidth of LED relies strictly on reducing its emission aperture size, as which suppresses the RC constant to enhance the modulation bandwidth. More recently, versatile small-size LEDs including resonant cavity LEDs (RCLEDs), micro LEDs array, photonic crystal LEDs (PCLEDs), and surface-roughened LEDs were successively developed to meet this demand [9–12]. In particular, Fujii et al. roughened the n-type layer surface of GaN LED to enhance its light extraction efficiency . Among these devices with new structures, the photonic crystal structured micro LED (PC-µLED) not only benefits from the reduced emission aperture but also boosts up the light extraction efficiency, which shortens the photon lifetime to enhance both the external quantum efficiency (EQE) and the radiative recombination rate [13–15].
To effectively overcome the encodable bandwidth limitation for traditional LED, the novel data formats with high spectral usage efficiency are utilized to elevate data rate. Khalid et al. employed a discrete multi-tone (DMT) format to modulate a phosphor-coated LED with allowable data rate as high as 1 Gbit/s . In 2015, Manousiadis et al. preliminarily demonstrated the blue µLED for carrying the orthogonal frequency division multiplexing (OFDM) data at 1.05 Gbit/s with frequency-domain pre-equalization . Afterwards, the pre-emphasis (or called pre-equalization) technique is often used to analyze the frequency response of the whole system before data transmission, which precisely detunes the frequency spectrum of the data at same average energy level to fit the channel response. Hence, such a pre-emphasis can effectively compensate the distorted frequency response to further enhance the transmission rate [18,19]. The throughput compensation by data pre-emphasis can effectively improve the clarity of eye diagram when executing 4-level pulse amplitude modulation (PAM-4) and OOK data transmission [20,21]. In addition, Argyris et al. employed the monolithic photonic integrated circuits to generate the chaotic optical signal, which further elevated the transmission data rate to 2.5 Gbit/s . Based on the advantage for the pre-emphasis technology, Tsonev et al. developed the pre-equalization technique to optimize the transmission rate up to 3 Gbit/s for the µLED carrying with 64-quadrature amplitude modulation (QAM) OFDM data . When considering the optical fiber wireline network at visible wavelength (especially for blue light), the plastic optical fiber is one of candidates to efficiently collect the optical power due to its low loss, large bending tolerance, cost-effectiveness, easy connection and robustness when comparing with the commercial single- or multi-mode glass fibers. When the PC-µLED delivers the high-spectral-usage data, Geng et al. have ever adopted a step index plastic optical fiber to promote the collectable optical power . Although employing the new data format with high spectral efficiency harvests the insufficient bandwidth of LED and performing the digital signal processing increases the raw data rate for direct encoding, the ultimate way to further promote the encodable bandwidth relies on sacrificing the emission mesa size of LED, which can effectively overcome bottle-neck of the narrow analog modulation bandwidth at the cost of power degradation. The small-size LED with fast frequency response also contributes to the weak output with decreasing signal-to-noise ratio (SNR). Nevertheless, the compromised power and frequency response of the µLED and its relationship with the mesa size need to be explored for obtaining the optimized communication transmission.
In this work, the PC-µLED chips designed and fabricated with different mesa areas are demonstrated and compared for pre-emphasized OOK/PAM-4 and pre-leveled 16-QAM OFDM data transmissions in both free-space and plastic fiber links. The basic characteristics including the luminance-to-current-voltage curve, differential resistance and frequency responses for the PC-µLED chips and their dependence with mesa size are analyzed and compared. For the OOK and data transmission, the pre-emphasis technology is utilized to optimize the transmission eye-diagram and bit-error performances. In addition, the key parameters including error vector magnitude (EVM), SNR and bit error rate (BER) for the 16-QAM OFDM data transmission are also optimized by changing the bias current, amplitude of signal and pre-leveling slope. Finally, the PC-µLED with optimized mesa area to compromise the trade-off between throughput power and encodable bandwidth is obtained via the comparison of aforementioned parameters.
2. Principle and experimental setup
2.1 Device design and fabrication
At first, the typical GaN LED structure was grown on a c-plane sapphire substrate via standard procedure. To characterize the mesa size dependent performance, the square mesa PC-µLEDs were fabricated with length (L) ranging from 20 µm to 120 µm at an increment of 20 µm. Figure 1(a) shows the top-view optical microscope (OM) images of the PC-µLEDs with different mesa areas. The lengths of mesa area for different PC-µLEDs were designed as 20, 40, 60, 80 and 120 µm, respectively, as shown from left to right part of Fig. 1(a). In these work, these mesa lengths of 20, 40, 60, 80, and 120 µm are common sizes for commercial micro- and mini-LEDs, which can generate sufficient output power to perform the VLC transmission. The PhC structure is used to improve the modulation bandwidth of LED. In detail, the 3-D device structures of the PC-µLED chip are illustrated in Fig. 1(b). The thicknesses of the n- and p-type GaN layers are controlled as 2 µm and 250 nm, respectively. The layer number of multiple quantum wells is fixed as 10. Then, the mask patterns of PC array was designed to hexagonally align with interval and radius of 500 nm and 200 nm, respectively. Moreover, the depth of PC nanohole is also set as 700 nm. After forming the metallic mask as etch-stop pattern via electron-beam lithography, the inductively coupled plasma reactive-ion etching (ICP-RIE) was used to etch the p-GaN and multiple quantum wells (MQW) layers, which construct the PC nanohole array within the epitaxial LED structure. The p-type Ni/Au (5 nm/5 nm) contact layer was deposited so as to avoid the short circuit in active layer owing to the constitution of photonic crystal nanohole. The thickness of n-type Ti/Al/Ni/Au is individually controlled as 15 nm/100 nm/15 nm/150 nm. Finally, the n- and p-type probe pads are fabricated with Ti/Au contact thickness of 20 nm/200 nm.
2.2 Data transmission setup
The experimental setup for synthesizing the OOK/PAM-4/QAM-OFDM data streams to encode the PC-µLED vacuum chucked on the probe station system for plastic optical fiber assisted visible light communication (VLC) is shown in Fig. 1(c). The PC-µLED chip was placed on a thermal-stabilized heat sink with circulated water-pipe cooler to precisely control the device temperature at 22°C (room temperature). At beginning, the electrical OOK and PAM-4 data streams were exported from an arbitrary waveform generator (AWG, Keysight 8195A). For QAM-OFDM data stream generation, the homemade MATLAB program generated a serial pseudo-random bit sequence (PRBS) data stream with a length of 215-1 and mapped them with made serial-to-parallel distribution into 16-QAM symbols before uploading onto the OFDM subcarriers in frequency domain. Then, the computer generated 16-QAM OFDM data stream was also loaded into the AWG with a sampling rate of 65 GS/s for exporting data waveform. Subsequently, the data stream output from AWG was pre-amplified by a 40-dB amplifier (Mini Circuits ZKL-1R5). The amplified data stream was then combined with a DC bias current from a current source (ILX, LDC-3724B) via a bias-tee (Picosecond, 5540A) to drive the PC-µLED chip via a ground-signal-ground (GSG) probe (GGB, 40A-GSG-100-DP). Afterwards, the encoded PC-µLED output was built-coupling into the plastic optical fiber segment, and the delivered PC-µLED optical data was converted back to electrical signal by using a photodetector (PD, New Focus 1601). The same amplifier (Mini Circuits ZKL-1R5) with high power gain of 40 dB and low noise figure of 2.8 dB was used for post-amplification to enhance the SNR of received OOK, PAM-4, an OFDM data streams. The eye diagrams of the post-amplified OOK and PAM-4 data waveforms were obtained by employing a digital serial analyzer (DSA, Tektronix 8300) with a receiving bandwidth of 30 GHz. On the other hand, the amplified 16-QAM OFDM waveform in time domain was sent into a real-time DSA (Tektronix DPO77002SX) with a sampling rate of 100 GS/s. Finally, a built-in commercial software program (Tektronix, 80SJNB) was utilized to analyze the related bathtub curve and to evaluate the received BER from the converted OOK or PAM-4 data waveforms. For decoding the received 16-QAM OFDM data stream, its waveform in time domain was decoded by an off-line homemade MATLAB program to obtain the constellation plots for calculating EVM, SNR and BER.
2.3 Data waveform synthesis
To enhance the LED modulation depth, the preamplifier is required to provide the PhC-µLED with enlarging amplitude after output from the AWG under inherent limitation on peak-to-peak voltage. Accordingly, the amplified electrical data waveforms of OOK, PAM-4 and OFDM data formats at 0.5 GBaud are shown in Fig. 2(a). The error-free (<10−24) electrical OOK data stream reveals clear eye diagram with a peak-to-peak voltage of 3.8 V, a rise/fall time of 245/250 ps, a root-mean-square jitter of 12.4 ps, and a Q factor of 12.2 dB. The error-free (<10−24) electrical PAM-4 data exhibits a peak-to-peak voltage of 3.7 V, and a jitter tolerance of 17.6/18.7/17.3 ps for top/middle/bottom eye diagram. The 16-QAM OFDM shows distinct constellation plot with EVM of 1.7%, SNR of 35.2 dB and BER of 2.39×10−140.
To compensate the finite channel response with data pre-emphasis for improving the allowable data rate, the detail and procedure have been described in previous works [21,23,24]. Figure 2(b) displays the pre-emphasis processes for the OOK and PAM-4 data stream. The frequency response of the transmission channel including magnitude and phase parts is measured with inserting a broadband signal. Next, either the modified OOK or PAM-4 data streams are reproduced by an AWG under the feedback of the induced distortion amplitude and phase variation for compensating the throughput degradation caused by the whole transmission channel.
3. Results and discussion
3.1 Basic characteristic of PC-µLED
The optical power-current (P-I) curves are shown in Fig. 3(a). For the PC-µLED with a mesa length above 60 µm, the biased current is limited no longer than 100 mA to avoid the current source from the over bias of compliance DC voltage. In addition, the biased currents for the PC-µLED devices with mesa lengths of 20 µm and 40 µm are confined below 40 and 25 mA, respectively, because their smaller size could cause carrier overflow and junction heating problems under high current injection. The output power of these PC-µLEDs is respectively enhanced from 0.05 mW to 0.6 mW as the mesa length increases from 20 µm to 120 µm. In addition, the dPout/dIbias slope obtained from P-I curve can be used to evaluate differential quantum efficiency (ηdiff = (q/hv)(dPout/dIbias)) of 0.8%, 1%, 1.8%, 2.1% and 2.5% for the PC-µLEDs with mesa lengths of 20, 40, 60, 80 and 120 µm, respectively. From the current-voltage (I-V) curves of PC-µLED shown in Fig. 3(b), the lowest differential resistance is decreased to 8 Ω with enlarging the mesa length to 120 µm. The PC-µLED with a smaller mesa size possesses higher differential resistance to match the impedance of 50 Ω at larger bias, whereas the larger PC-µLED has to be operated at lower bias to prevent from impedance mismatch and signal reflection. Under different biased conditions, the relationship between capacitance and mesa size of devices should be discussed to realize the effect of their RC time constant on the analog modulation bandwidth. Figure 3(c) shows the capacitance of PC-µLED with the applied voltage ranging from -2 to + 7 V.
The capacitance of PC-µLED with mesa lengths of 20, 40, 60, 80 and 120 µm at bias voltage of 6 V are -10, -24, -52, -77 and -93 pF, respectively. The absolute value of capacitance for PC-µLED is proportional to the mesa area of device under the same biased voltage; however, the capacitance is reduced to negative value with further increasing forward bias. In general, according to previous work, the extra sub-band gap states caused from the sidewall defects in PC-µLED may induce the negative capacitance effect . The capacitance depletion of device remains positive under reverse-biased voltage for all the devices. When the carrier/dopant concentration increases or the forward bias enlarges, the depletion width is suppressed to decrease the depletion capacitance . Furthermore, the equivalent electrical circuit of the PC-µLED mainly limits its modulation bandwidth by the RC time constants. The cut-off frequency of electrical circuit for PC-µLED is calculated from the dV/dI and C-V curve, as shown Fig. 3(d). Operating at a bias voltage of 4 V provides the allowable bandwidths of 0.4, 0.2, 0.7, 0.3 and 0.68 GHz for PC-µLEDs with mesa lengths of 20, 40, 60, 80 and 120 µm, respectively. The smaller PC-µLEDs possess the larger differential resistance to cause the smaller cut-off frequency. In addition, the capacitance of small-size PC-µLED is significantly decreased to up-shift the cut-off frequency for extending its modulation bandwidth. By increasing the bias voltage to 6 V, the cut-off frequency of PC-µLEDs with mesa lengths of 20, 40, 60, 80 and 120 µm can greatly improve to 1.56, 1.52, 0.9, 0.42 and 0.41 GHz.
Subsequently, the power-to-frequency throughput responses of all PC-µLED chips under small-signal analog modulation at different biased currents are illustrated Fig. 4. The maximal 3-dB bandwidths of PC-µLEDs with mesa lengths of 20, 40, 60, 80 and 120 µm are measured as 162, 136, 112, 96, and 72 MHz, respectively. Among them, the PC-µLED with a mesa length of 20 µm exhibits the highest 3-dB bandwidth because the RC time constant is suppressed by the limited diffusion capacitance and differential resistance to broaden the modulation bandwidth. It is also observed that the 3-dB bandwidth of the PC-µLED can be extended by increasing the DC bias current. Typically, the 3-dB bandwidth of PC-µLED can be theoretically described as [27–30]
3.2 OOK transmission performance of PC-µLED
In addition, the received data stream for the PC-µLED with a mesa length of 20 µm is not clear because its relatively weak optical output power is below the receiving criterion set by the photodetector. Therefore, the weak OOK signal for the device with a mesa length of 20 µm is merged into the noise background. Figure 5 shows the eye diagrams of the non-return-to-zero OOK (NRZ-OOK) data streams without and with pre-emphasis for the PC-µLEDs with mesa lengths of 40, 60, 80, and 120 µm, providing the root mean square (RMS) jitters and quality factor (Q-factor) of 480 ps/4.8 dB, 391 ps/4.9 dB, 435 ps/4.5 dB, and 2930 ps/4.1 dB, respectively.
After performing the pre-emphasis procedure, the RMS jitter/ Q-factor of received data after optimization can be respectively improved to 308 ps/5.8 dB, 146 ps/7.5 dB, 135 ps/8.2 dB, and 177 ps/7.7 dB. Obviously, employing data waveform pre-emphasis can significantly improve the eye-diagram with reduced RMS jitter, and enhance Q-factor after transmitting the OOK data stream. To further inspect the size dependent transmission capacity of PC-µLEDs, the pre-emphasized OOK data streams at different bandwidths are transmitted and Fig. 6(a) demonstrates their corresponding BERs. With pre-emphasis, the PC-µLEDs with mesa lengths of 60, 80, and 120 µm can support the error-free (BER < 10−12) OOK transmission capacity up to 0.5, 0.5, 0.4 Gbit/s, respectively. The eye diagrams obtained at the highest OOK transmission data rate for PC-µLEDs with different mesa areas are shown in Fig. 6(b).
Particularly, the weak optical power and large jitter degrade the transmission performance for the device with a mesa length of 40 µm or smaller, which provides a BER of 5×10−8 at 0.2 Gbit/s failing to pass the error free criterion due to the small amplitude and the large RMS jitter of < 0.59 V and >343 ps, respectively. For larger PC-µLED, the smaller RMS noise reduces to 50 mV for the devices with mesa length of 60-80 µm. Nevertheless, the 80×80-µm2 large PC-µLED possesses larger optical power with higher SNR than others, and the 120×120-µm2 PC-µLED reveals better transmission performance at 300 Mbit/s. Beyond 300 Mbit/s, the superiority of the 120×120-µm2 large PC-µLED no longer maintains due to its degraded SNR or data at high-frequency region.
3.3 PAM-4 transmission performance of PC-µLED
On the other hand, the pre-emphasized PAM-4 data format is also selected to increase the spectral usage efficiency for overcoming the limited bandwidth of the PC-µLED. The forward error correction (FEC) is inserted into the transmitted data as a parity code, which enables to assess the data errors suffering from the noise of the transmission channel. In this work, the Reed-Solomon (544, 514, t = 15, m = 10) encoder based KP4-FEC is adopted for the algorithm, which is standardized by international Telecommunication Union-Telecommunication Standardization Sector (ITU-T) G.975  for low-cost ameliorating the digital optical communication networks. Figure 7(a) shows the received BER of the PAM-4 data carried by different PC-µLED chips at different Baud rates. The maximal allowable Baud rate of PAM-4 data stream for 80×80-µm2 large PC-µLED is 300-MBaud or 600 Mbit/s with its BER of 8.6×10−5 to pass through KP4-FEC criterion.
As the PAM-4 data format usually requires rigorous SNR level to achieve the same BER with the OOK format, the blurred eye diagram of the received PAM-4 data for 40×40-µm2 large PC-µLED contributes to the unqualified BER after software decoding because of its insufficient encoding throughput, as shown in the left part of Fig. 7(b). The increase of mesa to 60×60-µm2 successively achieves the better transmission performance of OOK data format, which becomes inferior for comparing with the 120×120-µm2 device when transmitting the PAM-4 data stream. The main reason is attributed to its weaker data amplitude and lower SNR in spite of its broader frequency response. From the bathtub BERs of pre-emphasized PAM-4 data stream at 200 MBaud shown in Fig. 6(c), the PC-µLEDs with mesa lengths of 60, 80 and 120 µm permit the jitter tolerance for the top/middle/bottom eye widths as large as 3.21/2.69/1.85 ns (0.37/0.64/0.54 UI), 3.23/2.46/1.88 ns (0.38/0.49/0.65 UI) and 3.08/2.14/1.58 ns (0.62/0.43/0.32 UI), respectively. For PAM-4 data transmission, the weak output for small-size PC-µLED fails to deliver the PAM-4 data with sufficient throughput and SNR, which especially contributes to the decayed top eye of the received PAM-4 data so as to degrade the whole BER.
3.4 OFDM transmission performance of PC-µLED
Regarding the QAM OFDM analysis, the FEC criterion is based on the Bose-Chaudhuri-Hocquengham (1020, 988) encoder , which defines the error-free BER of obtained data below 3.8×10−3. To increase maximal transmission capacity, both the bandwidth and SNR are enhanced by increasing the biased current, which is shown in Fig. 8. For instance, the bias current of 60×60-µm2 large PC-µLED adjusts from 30 mA to 50 mA such that the EVM/SNR/BER can be improved from 15.8%/16 dB/1.8×10−3 to 13.7%/17.3 dB/4.07×10−4. However, the overdriven bias up to 70 mA conversely deteriorates the SNR of low-frequency OFDM subcarriers due to the P-I curve saturation. The optimized DC bias at 25 80 90 mA for devices with mesa lengths of 40, 80 and 120 µm respectively allows the maximal transmission bandwidths at 0.2, 0.3 and 0.2 GHz for 16-QAM OFDM data stream, and the average SNR of 15.1/18.2/19.3 dB and BER of 3.9×10−3/1×10−4/1.4×10−5 at raw data rates of 0.8, 1.2 and 0.8 Gbit/s are also obtained.
Appropriately detuning the output voltage from AWG is an alternative way to further improve the transmission performance as the modulation depth enhances for different-size PC-µLEDs individually. Figure 9 shows the receiving spectra, peak-to-average power ratio (PAPR) and BER of the 16-QAM OFDM data with different amplitudes when delivering by PC-µLED chips with different mesa areas. In comparison with the PC-µLEDs with mesa lengths of 40 and 120 µm, the 120×120-µm2 device carrying with 0.8-Gbit/s 16-QAM OFDM data shows significantly degraded throughput, whereas 40×40-µm2 device exhibits relatively flat throughput within allowable bandwidth. For detailed comparison, the PAPR as the power ratio of instantaneous peak to the average level of the OFDM waveform in time-domain is introduced and defined as 
3.5 Comparison of µ-LED with different mesa size at same data bandwidth
With setting the modulation bandwidth at 0.2 GHz for device performance comparison, Fig. 10 displays the time-domain waveforms, RF spectra, SNR responses, constellation plots and PAPRs of the received QAM-OFDM data for different-size PC-µLEDs. To obtain the best performance, the biased current and AWG amplitude are respectively optimized to 25/50/80/90 mA and 700/700/600/600 mV for PC-µLEDs with mesa lengths of 40/60/80/120 µm. The time-domain waveforms and PAPRs in Figs. 10(a) and 10(b) show the variation on waveform amplitude of received 16-QAM OFDM data at 0.8 Gbit/s. The PAPR is suppressed by 1 dB when decreasing the mesa length from 120 µm to 40 µm because of amplitude reduction and waveform clipping. Both the PAPR and the SNR of received OFDM data at 200-MHz for the 120×120-µm2 large device are the largest among all devices, which lead to the clearest constellation plot with smallest EVM of 10.4%, as compared to those of 17.1%/11.8%/10.5% for the devices with mesa lengths of 40/60/80 µm. In more detail, each subcarrier SNRs of the received 16-QAM OFDM data at 0.2-GHz bandwidth for all PC-µLED chips are compared in Fig. 10(e). The SNR of received 16-QAM OFDM data at 11-MHz can be enhanced from 18.8 to 20.7 dB with increasing the mesa length from 40 to 120µm. However, all PC-µLEDs show SNR of OFDM subcarriers under 100 MHz slightly lower than prospective results due to the low-frequency noise from ambient light and from the baseline wander effects . In addition, the SNR of OFDM carriers beyond 0.15-GHz bandwidth for the 120×120-µm2 PC-µLED reveals steepened degradation due to its lower modulation bandwidth than others. In contrast, the average BER beyond 450-MHz bandwidth exhibits different trend. The 60-µm large PC-µLED provides a BER of 3.9×10−3 which is better than those of 7.9×10−3 for 80-120 µm large devices, as the large-size PC-µLEDs also possess high decaying throughput slope (dB/Hz) in frequency response.
To effectively enlarge the average SNR of received data, the pre-leveled 16-QAM-OFDM format slightly sacrificing the low-frequency SNR to compensate the degraded high-frequency SNR is realized [4, 35, 36]. Figure 11 shows the SNR responses for all OFDM subcarriers, the received BERs and the constellation plots of 40/60/80/120 µm large PC-µLEDs delivering 1/2/1.8/1.8-Gbit/s 16-QAM OFDM data without and with OFDM subcarrier pre-leveling. No matter the pre-leveling or not, the 1/1.8-Gbit/s OFDM data carried by the 40/120-µm-large PC-µLEDs cannot pass FEC criterion. Because the 40-µm large device exhibits lower output power and SNR in low-frequency region, whereas the 120-µm large device fails by its inherent bandwidth limitation.
After performing the OFDM pre-leveling with optimizing power-to-frequency slopes to 0.4 and 0.3, the modulation bandwidths for the PC-µLEDs with mesa length of 60 and 80 µm are significantly improved to 0.5 and 0.45 GHz, for 16-QAM OFDM data rates of 2 and 1.8 Gbit/s, respectively. The constellation plot exhibits more centralized points with corresponding EVM of 17.2%/17.4% and FEC qualified BER of 3.5×10−3/3.8×10−3. Nonetheless, the OFDM data over pre-leveled with slopes of 0.6 and 0.5 dB/GHz excessively sacrifices the data delivered by OFDM subcarriers at low-frequency region, which deteriorates the respective average SNR of 14.5/14.3 dB and BER of 6.5×10−3/7.8×10−3 by over pre-distortion. With a proper pre-leveling slope, the 60×60-µm2 large PC-µLED allows the highest data rate of 2-Gbit/s within a bandwidth of 0.5 GHz.
In summary, the related parameters such as EQE, differential resistance (R), capacitance, optical power, frequency response, OOK and PAM-4 data rate, and OFDM without and with pre-emphasis or pre-leveling at optimized bias current and modulation amplitude are listed in Table 1 for comparison. Among all devices, the small-size PC-µLED reveals the lowest capacitance in effective electronic circuit to access the highest modulation frequency response of 162 MHz. The optical power of PC-µLED with mesa length of 20 µm is 4.2/8.7/11/12.9 dB less than others with mesa lengths of 40/60/80/120 µm, which inevitably leads to the significantly deteriorated SNR at low-frequency region. Consequently, the PC-µLEDs with mesa lengths of 60/80 µm provide an optimized trade-off between modulation throughput and bandwidth to implement the 500-Mbit/s OOK data, 600-Mbit/s pre-emphasized PAM-4 data and 2-Gbit/s pre-leveled OFDM data streams for visible light communication.
In 2006, Lee et al. used the patterned sapphire substrates to generate PhC structure in LED with mesa size of 350×350 µm2, which obtains the maximal power of 9 mW and the external quantum efficiency of 16.4% when biased at 20 mA . In 2013, Yin et al. observed the PhC LED with negative refraction in emission region  and suppressed radiative recombination lifetime of 2.1 ns . This PhC-µLED was also utilized to perform the VLC transmission. For comparison, the transmission performances among previous works are listed in Table 2. By defining the spectral usage efficiency (SUE) as the ratio of allowable transmission rate to the analog modulation bandwidth of transmitters. In 2018, Lin et al. also employed the PhC LED to demonstrate QAM OFDM transmission at 2 Gbit/s with an EVM of 17.07%, a SNR of 15.35 dB, a BER of 3.3×10−3, and a SUE of 11.2 bit/s/Hz . Chi et al. demonstrated the QAM-OFDM transmission at 9 Gbit/s by using the 450 GaN LD . In addition, Wang also increased the QAM level to 64 for the 405-nm LD with enabling data rate of 25.8 Gbit/s . In contrast, the PhC-µLED used in this work only exhibits analog bandwidth of 96 MHz for QAM-OFDM transmission, which achieves total data rate of 1.8 Gbit/s to obtain a SUE of 18.8 bit/s/Hz comparable with that for LD transmitter.
The PC-µLED chips with different mesa areas are compared for pre-emphasized OOK/PAM-4 and pre-leveled 16-QAM OFDM data transmissions in both free-space and plastic fiber links. Notably, the PC-µLED chip with mesa area of 120×120-µm2 exhibits the highest optical power of 580 µW with a differential quantum efficiency of 2.5%, but also reveals the high capacitance of 133 pF to increase the RC time delay and reduce the encodable data bandwidth down to 72 MHz. Decreasing the PC-µLED mesa area to 20×20-µm2 provides the lowest capacitance of 10 pF to improve the modulation bandwidth up to 162 MHz. The smaller PC-µLED not only causes the larger differential resistance but also induces optical power saturation and Auger scattering. The maximal BtB transmission capacity of 500 and 400 Mbit/s for PC-µLED with mesa sizes of 60-80 µm can be guaranteed with error-free receiving. The superior throughput performance enables the 80×80-µm2 PC-µLED achieving the highest 600-Mbit/s or 300-MBaud PAM-4 transmission with KP4-FEC qualified BER of 8.6×10−5, whereas the weaker output from the smaller PC-µLEDs fails to deliver the PAM-4 data with sufficient throughput and SNR especially for the decayed top eye. After pre-leveling with power-to-frequency slopes of 0.4 and 0.3 dB/GHz for 16-QAM-OFDM transmission, the modulation bandwidths for the PC-µLEDs with mesa length of 60 and 80 µm are significantly improved to 0.5 and 0.45 GHz for 16-QAM OFDM at 2 and 1.8 Gbit/s, respectively. The decoded constellation plot declares that the PC-µLEDs with mesa lengths of 60/80 µm can provide an optimized trade-off between modulation throughput and bandwidth to implement the 500-Mbit/s OOK data, 600-Mbit/s pre-emphasized PAM-4 data and 2-Gbit/s pre-leveled OFDM data streams for visible light communication. Among all candidates, the PC-µLED with a mesa length of 80 µm reveals the best performance for PAM-4 transmission, whereas the 60-µm large PC-µLED reveals the highest transmission capacities for both OOK and 16-QAM OFDM data streams.
Ministry of Science and Technology, Taiwan (MOST 106-2221-E-002-152-MY3, MOST 107-2218-E-992-304-, MOST 107-2221-E-002-158-MY3, MOST 107-2221-E-002-159-MY3, MOST 108-2218-E-992-302-).
The authors declare no conflicts of interest.
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