In this paper, recent progress in millimeter-wave (MMW) photonic gigabit wireless communication is reviewed. This technique is attractive partly because the MMW signal can be easily distributed from central to base stations through the use of a low-loss optical fiber. This radio-over-fiber approach facilitates the transmission of MMW signals. An MMW photonic transmitter, comprised of high-power photodiodes with integrated antennas for MMW signal broadcasting, is needed for signal generation only over the last mile. The development of several different low-noise optical MMW sources and high-power photonic transmitters and photodiodes for optical MMW wireless links is summarized. The performance of photonic wireless links with extremely high data rates (>10 Gbit s−1) developed based on these key components and using different modulation schemes is also reviewed. Finally, some advanced commercially available products and the prospects of a future gigabit wireless communication era are discussed.

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Millimeter-wave photonic wireless links for very high data rate communication

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.

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 near-ballistic uni-traveling-carrier photodiode (NBUTC-PD) with a scaled-down epi-layer structure designed for high-power performance in the sub-THz frequency regime. Compared with UTC-PDs, NBUTC-PDs offer a higher optimum bias voltage for the near-ballistic transport of electrons, which leads to improvement in the output power performance. Furthermore, a small load resistance ( <; 50nΩ), which would sacrifice the output power for minimizing the output ac voltage swing on dc bias point, is not necessary. By scaling down the collector layer thickness and active area of the NBUTC-PDs, we achieve a large optical-to-electrical bandwidth (250 GHz) and a high saturation current (17 mA), which is close to the theoretical maximum, under a 50-Ω load and -2-V bias.

Design and Analysis of Ultra-High-Speed Near-Ballistic Uni-Traveling-Carrier Photodiodes Under a 50- $\Omega$ Load for High-Power Performance

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.

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 the feasibility of monolithic integration of evanescently coupled Uni-Traveling Carrier Photodiodes (UTC-PDs) having a bandwidth exceeding 100 GHz with Multimode Interference (MMI) couplers. This platform is suitable for active-passive, butt-joint monolithic integration with various Multiple Quantum Well (MQW) devices for narrow linewidth millimeter-wave photomixing sources. The fabricated devices achieved a high 3-dB bandwidth of up to 110 GHz and a generated output power of more than 0 dBm (1 mW) at 120 GHz with a flat frequency response over the microwave F-band (90-140 GHz).

High-speed photodiodes for InP-based photonic integrated circuits.

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).

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

F.-M. Kuo

Papers

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