Cheng-Ting Tsai

450-nm GaN laser diode enables high-speed visible light communication with 9-Gbps QAM-OFDM

A TO-38-can packaged Gallium nitride (GaN) blue laser diode (LD) based free-space visible light communication (VLC) with 64-quadrature amplitude modulation (QAM) and 32-subcarrier orthogonal frequency division multiplexing (OFDM) transmission at 9 Gbps is preliminarily demonstrated over a 5-m free-space link. The 3-dB analog modulation bandwidth of the TO-38-can packaged GaN blue LD biased at 65 mA and controlled at 25°C is only 900 MHz, which can be extended to 1.5 GHz for OFDM encoding after throughput intensity optimization. When delivering the 4-Gbps 16-QAM OFDM data within 1-GHz bandwidth, the error vector magnitude (EVM), signal-to-noise ratio (SNR) and bit-error-rate (BER) of the received data are observed as 8.4%, 22.4 dB and 3.5 × 10(-8), respectively. By increasing the encoded bandwidth to 1.5 GHz, the TO-38-can packaged GaN blue LD enlarges its transmission capacity to 6 Gbps but degrades its transmitted BER to 1.7 × 10(-3). The same transmission capacity of 6 Gbps can also be achieved with a BER of 1 × 10(-6) by encoding 64-QAM OFDM data within 1-GHz bandwidth. Using the 1.5-GHz full bandwidth of the TO-38-can packaged GaN blue LD provides the 64-QAM OFDM transmission up to 9 Gbps, which successfully delivers data with an EVM of 5.1%, an SNR of 22 dB and a BER of 3.6 × 10(-3) passed the forward error correction (FEC) criterion.

Going beyond 4 Gbps data rate by employing RGB laser diodes for visible light communication

With increasing interest in visible light communication, the laser diode (LD) provides an attractive alternative, with higher efficiency, shorter linewidth and larger bandwidth for high-speed visible light communication (VLC). Previously, more than 3 Gbps data rate was demonstrated using LED. By using LDs and spectral-efficient orthogonal frequency division multiplexing encoding scheme, significantly higher data rates has been achieved in this work. Using 16-QAM modulation scheme, in conjunction with red, blue and green LDs, data rates of 4.4 Gbps, 4 Gbps and 4 Gbps, with the corresponding BER/SNR/EVM of 3.3 × 10⁻³/15.3/17.9, 1.4 × 10⁻³/16.3/15.4 and 2.8 × 10⁻³/15.5/16.7were obtained over transmission distance of ~20 cm. We also simultaneously demonstrated white light emission using red, blue and green LDs, after passing through a commercially available diffuser element. Our work highlighted that a tradeoff exists in operating the blue LDs at optimum bias condition while maintaining good color temperature. The best results were obtained when encoding red LDs which gave both the strongest received signal amplitude and white light with CCT value of 5835K.

Tricolor R/G/B Laser Diode Based Eye-Safe White Lighting Communication Beyond 8 Gbit/s

White light generation by mixing red, green, and blue laser diodes (RGB LDs) was demonstrated with Commission International de l’Eclairage coordinates of (0.2928, 0.2981), a correlated color temperature of 8382 K, and a color rendering index of 54.4 to provide a maximal illuminance of 7540 lux. All the white lights generated using RGB LDs were set within the risk group-1 criterion to avoid the blue-light hazard to human eyes. In addition, the RGB-LD mixed white light was diffused using a frosted glass to avoid optical aberration and to improve the performance of the lighting source. In addition, visible light communication (VLC) by using RGB-LD mixed white-light carriers and a point-to-point scheme over 1 m was performed in the directly modulated 16-QAM OFDM data format. In back-to-back transmission, the maximal allowable data rate at 10.8, 10.4, and 8 Gbps was determined for R, G, and B LDs, respectively. Moreover, the RGB-LD mixed white light-based indoor wavelength-division multiplexing (WDM)-VLC system yielded a total allowable transmission data rate of 8.8 Gbps over 0.5 m in free space. Such a high-speed RGB-LD mixed WDM-VLC system without any channel interference can be used to simultaneously provide data transmission and white lighting in an indoor environment.

Si-rich SiNx based Kerr switch enables optical data conversion up to 12 Gbit/s

Silicon photonic interconnection on chip is the emerging issue for next-generation integrated circuits. With the Si-rich SiNx micro-ring based optical Kerr switch, we demonstrate for the first time the wavelength and format conversion of optical on-off-keying data with a bit-rate of 12 Gbit/s. The field-resonant nonlinear Kerr effect enhances the transient refractive index change when coupling the optical data-stream into the micro-ring through the bus waveguide. This effectively red-shifts the notched dip wavelength to cause the format preserved or inversed conversion of data carried by the on-resonant or off-resonant probe, respectively. The Si quantum dots doped Si-rich SiNx strengthens its nonlinear Kerr coefficient by two-orders of magnitude higher than that of bulk Si or Si3N4. The wavelength-converted and cross-amplitude-modulated probe data-stream at up to 12-Gbit/s through the Si-rich SiNx micro-ring with penalty of −7 dB on transmission has shown very promising applicability to all-optical communication networks.

Blue Laser Diode Enables Underwater Communication at 12.4 Gbps

To enable high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances, a 450-nm blue GaN laser diode (LD) directly modulated by pre-leveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity of up to 10 Gbps. The proposed UWOC in tap water provided a maximal allowable communication bit rate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m, exhibiting a bit rate/distance decaying slope of −0.847 Gbps/m. When conducting the same type of UWOC in seawater, light scattering induced by impurities attenuated the blue laser power, thereby degrading the transmission with a slightly higher decay ratio of 0.941 Gbps/m. The blue LD based UWOC enables a 16-QAM OFDM bit rate of up to 7.2 Gbps for transmission in seawater more than 6.8 m.

Suppressing the relaxation oscillation noise of injection-locked WRC-FPLD for directly modulated OFDM transmission

By up-shifting the relaxation oscillation peak and suppressing its relative intensity noise in a weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) under intense injection-locking, the directly modulated transmission of optical 16 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data-stream is demonstrated. The total bit rate of up to 20 Gbit/s within 5-GHz bandwidth is achieved by using the OFDM subcarrier pre-leveling technique. With increasing the injection-locking power from -12 to -3 dBm, the effective reduction on threshold current of the WRC-FPLD significantly shifts its relaxation oscillation frequency from 5 to 7.5 GHz. This concurrently induces an up-shift of the peak relative intensity noise (RIN) of the WRC-FPLD, and effectively suppresses the background RIN level to -104 dBc/Hz within the OFDM band between 3 and 6 GHz. The enhanced signal-to-noise ratio from 16 to 20 dB leads to a significant reduction of bit-error-rate (BER) of the back-to-back transmitted 16-QAM-OFDM data from 1.3 × 10(-3) to 5 × 10(-5), which slightly degrades to 1.1 × 10(-4) after 25-km single-mode fiber (SMF) transmission. However, the enlarged injection-locking power from -12 to -3 dBm inevitably declines the modulation throughput and increases its negative throughput slope from -0.8 to -1.9 dBm/GHz. After pre-leveling the peak amplitude of the OFDM subcarriers to compensate the throughput degradation of the directly modulated WRC-FPLD, the BER under 25-km SMF transmission can be further improved to 3 × 10(-5) under a receiving power of -3 dBm.

4-Gbit/s visible light communication link based on 16-QAM OFDM transmission over remote phosphor-film converted white light by using blue laser diode

Visible Light Communication (VLC) as a new technology for ultrahigh-speed communication is still limited when using slow modulation light-emitting diode (LED). Alternatively, we present a 4-Gbit/s VLC system using coherent blue-laser diode (LD) via 16-quadrature amplitude modulation orthogonal frequency division multiplexing. By changing the composition and the optical-configuration of a remote phosphor-film the generated white light is tuned from cool day to neutral, and the bit error rate is optimized from 1.9 × 10(-2) to 2.8 × 10(-5) in a blue filter-free link due to enhanced blue light transmission in forward direction. Briefly, blue-LD is an alternative to LED for generating white light and boosting the data rate of VLC.

60-GHz Millimeter-wave Over Fiber with Directly Modulated Dual-mode Laser Diode.

A directly modulated dual-mode laser diode (DMLD) with third-order intermodulation distortion (IMD3) suppression is proposed for a 60-GHz millimeter-wave over fiber (MMWoF) architecture, enabling new fiber-wireless communication access to cover 4-km single-mode-fiber (SMF) and 3-m wireless 16-QAM OFDM transmissions. By dual-mode injection-locking, the throughput degradation of the DMLD is mitigated with saturation effect to reduce its threshold, IMD3 power and relative intensity noise to 7.7 mA, −85 dBm and −110.4 dBc/Hz, respectively, providing huge spurious-free dynamic range of 85.8 dB/Hz2/3. This operation suppresses the noise floor of the DMLD carried QPSK-OFDM spectrum by 5 dB. The optical receiving power is optimized to restrict the power fading effect for improving the bit error rate to 1.9 × 10−3 and the receiving power penalty to 1.1 dB. Such DMLD based hybrid architecture for 60-GHz MMW fiber-wireless access can directly cover the current optical and wireless networks for next-generation indoor and short-reach mobile communications.

Direct QAM-OFDM Encoding of an L-band Master-to-Slave Injection-Locked WRC-FPLD Pair for 28 × 20 Gb/s DWDM-PON Transmission

The 560 Gb/s (20 Gb/s × 28 channels) DWDM-PON with 16 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) transmission over 25-km SMF by using the master-to-slave injection-locked weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) pair is demonstrated. After master-to-slave injection-locking, the BER of the 16-QAM OFDM data stream under back-to-back and 25-km transmissions can be improved from 3.3 × 10-3 to 2.1 × 10-5 and from 1.4 × 10-1 to 1.2 × 10-3, respectively. With OFDM subcarrier pre-leveling, the BER of 16-QAM OFDM data transmitted by the master-to-slave injection-locked WRC-FPLD over 25-km transmission is further improved from 1.2 × 10-3 to 2.1 × 10-4, concurrently enabling the 28 channel transmissions at 20 Gb/s with BER below FEC-limit of 3.8 × 10-3. The back-to-back transmission of pre-leveled 64-QAM OFDM data at 30 Gb/s under is also demonstrated to achieve FEC limited BER at receiving power of -6 dBm by using the main of the master-to-slave injection-locked slave WRC-FPLD. The partially coherent injection-locking of maser-to-slave WRC-FPLD pair is comparable with the coherent DFB injection-locked WRC-FPLD, which is capable of serving as the universal transmitters with competitive performance of a power penalty as low as 0.5 dB between WRC-FPLD-to-WRC-FPLD and DFB-to-WRC-FPLD.

Power fading mitigation of 40-Gbit/s 256-QAM OFDM carried by colorless laser diode under injection-locking

The pre-compensation on power fading effect of a colorless laser diode (CLD) carried 40-Gbit/s 256-QAM OFDM transmission during 25-km is demonstrated. By offsetting the DC bias to thrice the threshold (I(th)) and increasing the injection to 0 dBm, the CLD not only enhances its coherence but also suppresses modulation throughput declination and reduces the relative intensity related noise floor to -50 dBm. Modeling the receiving power of the delivered 256-QAM OFDM subcarriers is established, indicating that raising the bias to 3I(th) down-shifts the power fading induced notch to 8.8 GHz. This further degrades the OFDM subcarrier peak power by -2.9 dB after 25-km transmission, and the corresponded signal-to-noise ratio (SNR), error vector magnitude (EVM) and bit-error-rate (BER) are 26.1 dB, 4.9% and 6.5 × 10(-3), respectively. Pre-leveling the OFDM subcarrier as well as the modulation throughput effectively compromises the over-bias enlarged power fading to promote transmission. With a pre-leveled power slope of 1.5 dB/GHz for 256-QAM OFDM data, the modulation throughput declination of the high biased CLD significantly mitigates under BtB transmission, enabling the receiving sensitivity at -7.2 dBm with SNR, EVM and BER of 29.9 dB, 3.1% and 1.5 × 10(-4), respectively. Increasing the pre-leveling slope to 3.2 dB/GHz minimizes the fiber dispersion induced power fading, which improves the receiving SNR, EVM and BER to 27.4 dB, 4.2% and 2.6 × 10(-3), respectively, with receiving sensitivity of -3 dBm and power penalty of 4.2 dB after 25-km SMF transmission.