F.-M. Kuo

100 GHz ultra-wideband (UWB) fiber-to-the-antenna (FTTA) system for in-building and in-home networks

Fiber-to-the-antenna (FTTA) system can be a cost-effective technique for distributing high frequency signals from the head-end office to a number of remote antenna units via passive optical splitter and propagating through low-loss and low-cost optical fibers. Here, we experimentally demonstrate an optical ultra-wideband (UWB) - impulse radio (IR) FTTA system for in-building and in-home applications. The optical UWB-IR wireless link is operated in the W-band (75 GHz - 110 GHz) using our developed near-ballistic unitraveling-carrier photodiode based photonic transmitter (PT) and a 10 GHz mode-locked laser. 2.5 Gb/s UWB-IR FTTA systems with 1,024 high split-ratio and transmission over 300 m optical fiber are demonstrated using direct PT modulation.

Extremely High Saturation Current-Bandwidth Product Performance of a Near-Ballistic Uni-Traveling-Carrier Photodiode With a Flip-Chip Bonding Structure

In this study, we demonstrate near-ballistic uni-traveling carrier photodiodes (NBUTC-PDs) with an optimized flip-chip bonding structure, wide 3-dB optical-to-electrical (O-E) bandwidth (> 110 GHz), and extremely high saturation current-bandwidth product performance (37 mA, > 110 GHz, > 4070 mAmiddot GHz). NBUTC-PDs with different active areas (28-144 mum2) are fabricated and flip-chip bonded with coplanar waveguides onto an AlN-based pedestal. The overshoot drift velocity of the electrons in the collector layer of the NBUTC-PD means that both the thicknesses of the collector layer and active areas of our device can be increased to reduce the density of the output photocurrent, compared to that of the traditional UTC-PD. This improves the high power performance without seriously sacrificing the speed performance. According to the measured O-E frequency responses, devices with even a large active area (144 mum2 ) can still have a flat O-E frequency response, from near dc to 110 GHz. A three-port equivalent circuit model for accurately extracting the 3-dB bandwidth of the devices is established. The extracted 3-dB O-E bandwidth of a device with a small active area (28 mum2) can be as high as 280 GHz under a load of 25 Omega . In addition, the saturation current measurement results indicate that after inserting a center bonding pad on the pedestal (located below the p-metal of the NBUTC-PD for good heat sinking), the saturation current performance of the device becomes much higher than that of the control device (without the center bonding pad), especially for the device with a small active area (28 mum2 ). The measurement and modeling results indicate that a device with a 144 mum2 active area and optimized flip-chip bonding pedestal can achieve an extremely high saturation current-bandwidth product (6660 mA-GHz, 37 mA, 180 GHz).

Remotely Up-Converted 20-Gbit/s Error-Free Wireless On–Off-Keying Data Transmission at W-Band Using an Ultra-Wideband Photonic Transmitter-Mixer

We demonstrate a remotely up-converted and distributed 20-Gbit/s wireless on-off-keying (OOK) data transmission link at the W-band that uses a near-ballistic uni-traveling-carrier photodiode (NBUTC-PD)-based photonic transmitter-mixer. This device consists of an active NBUTC-PD integrated with a planar passive circuit for feeding the intermediate-frequency (IF) modulation input and extracting the up-converted optical-to-electrical (O-E) output signals. An equivalent-circuit model is developed, which allows for the O-E and IF responses to be independently optimized. Accordingly, we can achieve both an ultra-wide O-E bandwidth (67-118 GHz) and IF modulation bandwidth (>;15 GHz) with a very-low coupling loss (<; 2 dB) from the NBUTC-PD to the WR-10 waveguide. We adopted a remotely distributed 1-ps optical pulse train source with a repetition rate at 93 GHz to serve as a high-performance photonic carrier, which is generated by a spectral line-by-line shaper utilizing the repetition-rate multiplication (RRM) technique. In contrast to lossy amplitude filtering, our RRM is based on applying periodic loss-less spectral phase filtering onto the 31-GHz comb lines. In comparison with the conventional 93-GHz sinusoidal carrier, the photogenerated millimeter-wave (MMW) power of this kind of carrier is 4 dB higher than that of PD under the same output photocurrent. In contrast to the traditional mode-locked laser, the fiber dispersion can be totally precompensated without additional dispersion compensation components. By use of such device and optical MMW source, we successfully demonstrate remotely distributed and up-converted 20-Gbit/s error-free OOK wireless data transmission link over a 25-km standard single-mode fiber.

Photonic Generation and Wireless Transmission of Linearly/Nonlinearly Continuously Tunable Chirped Millimeter-Wave Waveforms With High Time-Bandwidth Product at W-Band

We demonstrate a novel scheme for photonic generation of chirped millimeter-wave (MMW) pulse with ultrahigh time-bandwidth product (TBP). By using a fast wavelength-sweeping laser with a narrow instantaneous linewidth, wideband/high-power photonic transmitter-mixers, and heterodyne-beating technique, continuously tunable chirped MMW waveforms at the W-band are generated and detected through wireless transmission. Compared with the reported optical grating-based wavelength-to-time mapping techniques for chirped pulse generation, our approach eliminates the problem in limited frequency resolution of grating, which seriously limits the continuity, tunability, and TBP of the generated waveform. Furthermore, by changing the alternating current (AC) waveform of the driving signal to the sweeping laser, linearly or nonlinearly continuously chirped MMW pulse can be easily generated and switched. Using our scheme, linearly and nonlinearly chirped pulses with record-high TBPs (89-103 GHz/ 50 μs/7 × 105) are experimentally achieved.

Spectral Power Enhancement in a 100 GHz Photonic Millimeter-Wave Generator Enabled by Spectral Line-by-Line Pulse Shaping

We report generation of high-modulation-depth photonic millimeter-wave (MMW) waveforms by applying line-by-line pulse shaping on a phase-modulated continuous-wave frequency comb. The optimized 20 and 100 GHz optical waveforms are then converted into electrical MMW signals using a near-ballistic uni-traveling-carrier photodiode (NBUTC-PD). A 7.4 dB MMW power enhancement is experimentally achieved by using 2.6 ps optimized pulses at a 100 GHz repetition rate, as compared with excitation by a conventional sinusoidal signal for the NBUTC-PD operated at the same photocurrent. This is in qualitative agreement with a theoretical analysis of spectral power enhancement by optical short pulses comprised of equi-amplitude frequency lines over sinusoidal excitation.

Linear-Cascade Near-Ballistic Unitraveling-Carrier Photodiodes With an Extremely High Saturation Current–Bandwidth Product

Saturation current-bandwidth product (SCBP), the key of figure of merit in high-speed and high-power photodiodes (PDs), is mainly limited by the tradeoff between carrier drift time in depletion layer and RC-limited bandwidth of conventional PDs. Here, we present a revolutionary photodiode structure: linear-cascade photodiodes (LCPDs), designed to further improve the SCBP performance. Our demonstrated LCPD structure can greatly increase the SCBP without using a complex distributed structure of the traveling-wave PD or reducing the load resistance (output RF power). Two flip-chip bonding packaged near-ballistic unitraveling-carrier photodiode (NBUTC-PD) units are employed in our LCPD structure. It exhibits a great improvement in SCBP compared to that of the control device with a single NBUTC-PD. A two-port equivalent-circuit model is established for the LCPDs and the modeling results clearly indicate that the increase in SCBP can be attributed to the significant reduction in its total capacitance due to the serial connection. Furthermore, we find that only when each PD unit in the LCPD structure has the same amount of injected optical power and modulated frequency of optical signal, the whole structure exhibits a carrier transit time as short as that of a single PD. Under the proper optical excitation, we can achieve a record high SCBP (7500 mA·GHz and 100 GHz) for two-element LCPDs under a 50 Ω load.

High-Performance Zn-Diffusion 850-nm Vertical-Cavity Surface-Emitting Lasers With Strained InAlGaAs Multiple Quantum Wells

We demonstrate a high-performance Zn-diffusion 850-nm vertical-cavity surface-emitting laser (VCSEL). By the use of strained InAlGaAs/AlGaAs multiple quantum wells for the active region, our structure can have a much higher maximum output power, higher differential quantum efficiency (DQE), and larger modulation current efficiency (D-factor) than those of non-strained control GaAs/AlGaAs VCSELs. Two different Zn-diffusion depths were adopted in our devices with the same single-oxide current-confined aperture (~6 μm) to further optimize the static and dynamic performance, respectively. The device with a deep Zn-diffusion depth (~1.2 μm) shows an optimized static performance, which includes a low threshold current (0.8 mA), high DQE (90% at ~1.2 mA), and a maximum output power as high as 9.7 mW. On the other hand, the device with a shallow Zn-diffusion depth (<; 0.6 μm) demonstrates good dynamic performance and exhibits a large D-factor (9.5 GHz/mA1/2), high maximum data rate (32 Gbit/s error-free) performance, and very-high data-rate/power-dissipation ratio (5.25 Gbit/s/mW) under an extremely small driving voltage (Vpp: 0.25 V).

High-Speed

A high-speed W -band integrated photonic transmitter is demonstrated. The presented integrated photonic transmitter is essentially developed with a near-ballistic uni-traveling-carrier photodiode integrated with a broadband front end through the flip-chip assembling technique. Technically, compared to our previous design, a W-band bandpass filter is exploited to significantly increase the transmitter IF modulation bandwidth. The demonstrated integrated photonic transmitter has a flat broad IF modulation response, as well as a broad optical-to-electrical (O-E) bandwidth. Specifically, the variation of the normalized IF modulation response, ranging from dc to around 13 GHz, is within 3 dB, and the normalized 3-dB O-E bandwidth is about 24 GHz. On the other hand, an up to 20-Gb/s high data-rate wireless transmission realized with the presented transmitter is demonstrated. The integrated photonic transmitter is expected to find applications in high-speed radio-over-fiber communications.

W

For the first time, the internal carrier dynamic inside GaN-based green light-emitting diodes (LEDs) during operation has been directly observed using the demonstrated electrical-optical pump-probe technique. Short electrical pulses (~100 ps) were pumped into high-speed cascade green LEDs, and the output optical pulses were probed using high-speed photoreceiver circuits. Using such a method, the recombination time constant of the carriers can be directly measured without any assumption about the recombination process. A high-speed cascade LED structure was adopted in the experiments to eliminate the influence of the RC delay time on the measured responses. Our measurement results indicate that both single- and three-LED cascade structures have the same internal response time due to current continuity. Furthermore, based on responses measured under different temperatures (from 25°C to 200°C), the origin of the efficiency droop in GaN-based green LEDs under a high bias current density may be attributed to the strong nonradiative Auger effect rather than device heating or carrier overflow. The demonstrated measurement scheme and high-speed cascade device structure offer a novel and simple way to straightforwardly investigate the internal carrier dynamic inside the active layers of the LED during forward-bias operation.

-Band Integrated Photonic Transmitter for Radio-Over-Fiber Applications

A W-band photonic transmitter-mixer, constructed by integrating a planar quasi-yagi radiator for feeding the WR-10 waveguide-based horn antenna and a near-ballistic uni-traveling-carrier photodiode, is used with a mode-locked fiber laser to obtain 2.5-Gb/s impulse-radio (IR) wireless data transmission at around a center frequency of 100 GHz. The bias-modulation technique provides less jitter and a longer maximum transmission distance compared with the technique of modulating the optical pulse train using an electrooptics modulator. Using the bias-modulation technique, we achieve a 2.5-Gb/s IR wireless data transmission.