Jhih-Min Wun

Oxide-Relief and Zn-Diffusion 850-nm Vertical-Cavity Surface-Emitting Lasers With Extremely Low Energy-to-Data-Rate Ratios for 40 Gbit/s Operations

We demonstrate novel structures of a vertical-cavity surface-emitting laser (VCSEL) for high-speed (~40 Gbit/s) operation with ultralow power consumption performance. Downscaling the size of oxide aperture of VCSELs is one of the most effective ways to reduce the power consumption during high-speed operation. However, such miniaturized oxide apertures (~2 μm diameter) in VCSELs will result in a large differential resistance, optical single-mode output, and a small maximum output power (<; 1 mW). These characteristics seriously limit the maximum electrical-to-optical (E-O) bandwidth and device reliability. By the use of the oxide-relief and Zn-diffusion techniques in our demonstrated 850-nm VCSELs, we can not only release the burden imposed on downscaling the current-confined aperture for high speed with low-power consumption performance, but can also manipulate the number of optical modes inside the cavity to maximize the E-O bandwidth and product of bit-rate transmission distance in an OM4 fiber. State-of-the-art dynamic performances at both room temperature and 85 °C operations can be achieved by the use of our device. These include extremely high D-factors (~13.5 GHz/mA1/2), as well as record-low energy-to-data ratios (EDR: 140 fJ/bit) at 34 Gbit/s operation, and error-free transmission over a 0.8-km OM4 multimode fiber with a record-low energy-to-data distance ratio (EDDR: 175.5 fJ/bit.km) at 25 Gbit/s.

Single-Mode, High-Speed, and High-Power Vertical-Cavity Surface-Emitting Lasers at 850 nm for Short to Medium Reach (2 km) Optical Interconnects

The vertical-cavity surface-emitting lasers (VCSELs) with high single-mode (narrow linewidth) output power are essential to minimize chromatic dispersion and to further improve the bit-rate distance product in a multimode fiber, which has a significant propagation loss (~3.5 dB/km) at 850 nm wavelength. Here, we demonstrate the detailed design considerations and fabrication of a single-mode, high-power, and high-speed VCSELs at the 850 nm wavelength with oxide-relief and Zn-diffusion apertures for the application of short (0.3 km) to medium reach (2 km) optical interconnects. By optimizing the relative geometric sizes between two such apertures in our demonstrated 850-nm VCSELs, we can not only attain high single-mode output power (~6.5 mW), but also with a reasonable threshold current (<; 2.0 mA). Furthermore, the spatial hole burning effect induced low-frequency roll off can also be minimized in our optimized structure to obtain a maximum data rate up to 26 Gbit/s. The record-high bit rate-distance products for OM4 MMF transmission under ON-OFF keying (14 Gbit/s × 2.0 km) modulation formats have been successfully demonstrated by the use of our VCSEL.

Photonic chirped radio-frequency generator with ultra-fast sweeping rate and ultra-wide sweeping range

A high-performance photonic sweeping-frequency (chirped) radio-frequency (RF) generator has been demonstrated. By use of a novel wavelength sweeping distributed-feedback (DFB) laser, which is operated based on the linewidth enhancement effect, a fixed wavelength narrow-linewidth DFB laser, and a wideband (dc to 50 GHz) photodiode module for the hetero-dyne beating RF signal generation, a very clear chirped RF waveform can be captured by a fast real-time scope. A very-high frequency sweeping rate (10.3 GHz/μs) with an ultra-wide RF frequency sweeping range (~40 GHz) have been demonstrated. The high-repeatability (~97%) in sweeping frequency has been verified by analyzing tens of repetitive chirped waveforms.

Design and demonstration of ultra-fast W-band photonic transmitter-mixer and detectors for 25 Gbits/sec error-free wireless linking.

A 25 Gbits/s error-free on-off-keying (OOK) wireless link between an ultra high-speed W-band photonic transmitter-mixer (PTM) and a fast W-band envelope detector is demonstrated. At the transmission end, the high-speed PTM is developed with an active near-ballistic uni-traveling carrier photodiode (NBUTC-PD) integrated with broadband front-end circuitry via the flip-chip bonding technique. Compared to our previous work, the wireless data rate is significantly increased through the improvement on the bandwidth of the front-end circuitry together with the reduction of the intermediate-frequency (IF) driving voltage of the active NBUTC-PD. The demonstrated PTM has a record-wide IF modulation (DC-25 GHz) and optical-to-electrical fractional bandwidths (68-128 GHz, ~67%). At the receiver end, the demodulation is realized with an ultra-fast W-band envelope detector built with a zero-bias Schottky barrier diode with a record wide video bandwidth (37 GHz) and excellent sensitivity. The demonstrated PTM is expected to find applications in multi-gigabit short-range wireless communication.

High-depth-resolution 3-dimensional radar-imaging system based on a few-cycle W-band photonic millimeter-wave pulse generator

We demonstrate that a near-single-cycle photonic millimeter-wave short-pulse generator at W-band is capable to provide high spatial resolution three-dimensional (3-D) radar imaging. A preliminary study indicates that 3-D radar images with a state-of-the-art ranging resolution of around 1.2 cm at the W-band can be achieved.

Dynamic Analysis of High-Efficiency InP-Based Photodiode for 40 Gbit/s Optical Interconnect Across a Wide Optical Window (0.85 to 1.55 μm)

The detailed dynamic analysis of novel high-speed InP-based photodiodes (PDs) has been performed. Such device can sustain an invariable high external efficiency (~74%; no antireflection coating) across a wide optical operation window (0.85 to 1.55 μm). Furthermore, compared with the traditional GaAs-based high-speed PD for optical interconnect applications, our proposed device structure can offer an enlarged device active diameter and eliminate the degradation in responsivity performance when the desired speed performance is increased. This is because the strong photoabsorption process and the elimination of slow hole drift in the In0.53Ga0.47As based collector layer at 0.85-μm wavelength operation. By measuring the dynamic performance of PDs with different active diameters across such wide optical window, we can accurately extract the electron drift-velocity under different wavelengths excitations in the In0.53Ga0.47As collector layer. This result indicates that at short-wavelength (0.85 μm) operation, the photogenerated electron in the In0.53Ga0.47As collector suffers from significant intervalley scattering effect due to its high excess energy. By using such device with diameter of optical window as large as 40 μm, 40 Gbit/s error-free transmissions have been successfully demonstrated through 5-km single-mode (SMF-28) and 0.1-km multimode (OM4) fibers at long- and short-wavelengths operations with reasonable sensitivity, respectively.

Photonic High-Power 160-GHz Signal Generation by Using Ultrafast Photodiode and a High-Repetition-Rate Femtosecond Optical Pulse Train Generator

We demonstrate photonic high-power MMW generation at subTHz (160 GHz) frequencies by using ultrafast near-ballistic unitraveling carrier photodiodes (NBUTC-PD), which have a miniaturized active area (24 μm2 ) and flip-chip bonding package for good heat-sinking. Under optical sinusoidal signal excitation with a ~85% modulation depth, 165-GHz optical-to-electrical 3-dB bandwidth, 18-mA saturation current, +5.11-dBm maximum output power at 160-GHz operating frequency has been demonstrated. In order to further mitigate device-heating, we developed a high-power pulsed optical signal source with increased optical modulation depth: a femtosecond optical short-pulse generator with extremely high repetition rate (160 GHz) and pulsewidth as short as 285 fs. With this novel source, we generated high MMW power (+7.8 dBm) with an effective 120% optical modulation depth at 160 GHz directly from the NBUTC-PD.

Strong Wavelength Detuning of 850 nm Vertical-Cavity Surface-Emitting Lasers for High-Speed (>40 Gbit/s) and Low-Energy Consumption Operation

The strong (>20 nm) wavelength detuning technique has been demonstrated to enhance the modulation speed and high-temperature characteristics (at 85 °C), as well as lower the required driving current density performance of oxide-relief 850-nm vertical-cavity surface-emitting lasers (VCSELs) for >40 Gbit/s operation. By increasing the wavelength detuning from 15 to 20 nm, a significant improvement in the electrical-to-optical (E-O) bandwidth (20 to 27 GHz) of the VCSEL can be observed. This detuning design (~20 nm) is incorporated along with a Zn-diffusion structure into our oxide-relief VCSEL with a miniaturized oxide-relief aperture (~3 μm). Highly single-mode, high-speed (26 GHz) operation, and moderate differential resistance (100 Ω) values can be simultaneously achieved. In addition, it is found that devices with a further larger detuning wavelength (>20 nm) and enlarged oxide-relief apertures (~8 μm) can sustain the same maximum E-O bandwidth (26 GHz) as that of a miniaturized (~3 μm) VCSEL, resulting in the lower driving current density (8 versus 18.8 kA/cm2) required for high-speed performance. Excellent transmission performance, which includes an extremely low energy-to-data rate ratio (EDR: 228 fJ/bit; over 100 m OM4 fiber) and record-low driving-current density (8 kA/cm2; 3.5 mA) has been successfully achieved for 41 Gbit/s error-free transmission for these VCSELs.

Type-II GaAs0.5Sb0.5/InP Uni-Traveling Carrier Photodiodes With Sub-Terahertz Bandwidth and High-Power Performance Under Zero-Bias Operation

We successfully demonstrate ultrafast uni-traveling carrier photodiodes (PD) with sub-terahertz bandwidth (∼170 GHz) and high-power performance under zero bias and at 1.55-μm optical wavelength operation. By using a type-II (GaAs_0.5Sb_0.5/InP) absorption-collector interface and inserting an n-type (1 × 10¹⁸ cm⁻³⁾ charge layer in the collector, the current blocking (Kirk) effect can be greatly minimized. A stack of undoped Al<italic>_x</italic>In_0.52Ga_0.48−xAs layers with different Aluminum mole fractions (<italic>x</italic>: 0.2 to 0.08) and bandgaps is adopted as the collector layer. This graded-bandgap design can provide a built-in electric field and further shorten the internal collector transit time. The demonstrated PD structure achieves a 3-dB optical-to-electrical bandwidth of 170 GHz and subterahertz output power −11.3 dBm at 170 GHz, a record among all the reported zero-bias PDs.

High-power THz-wave generation by using ultra-fast (315 GHz) uni-traveling carrier photodiode with novel collector design and photonic femtosecond pulse generator

UTC-PDs with novel collector design, flip-chip package, and ultra-wide bandwidth (315GHz) are successfully demonstrated. Under optical femtosecond pulse train illumination with THz repetition rate, it achieves record-high maximum CW output power (1.04 mW) at 280GHz operation.