Traveling-wave Photodetectors Research Articles

High-Power and High-Speed Ge/Si Traveling-Wave Photodetector Optimized by Genetic Algorithm

We experimentally demonstrate a 4-stage and an 8-stage Ge/Si distributed traveling-wave photodetectors (TWPDs) with high RF output powers and large bandwidths. The genetic algorithm is utilized to synergistically optimize multiple relevant design parameters, including the dimension of the coplanar waveguide electrode and the aperiodic arrangement of the PD array. After the genetic algorithm optimization, the 3dB bandwidth of the 4-stage TWPD is improved from 46 GHz to 54 GHz under the constraint that the bandwidth of the unterminated single PD element is 50 GHz. Meanwhile, the 3dB bandwidth of the 8-stage TWPD is improved from 33 GHz to 40 GHz. A good agreement between measured and calculated frequency responses is obtained. The measured maximum RF output powers of the aperiodic 4-stage and 8-stage TWPDs at 30 GHz are 2.5 dBm and 6.3 dBm, respectively. Furthermore, the effectiveness of our design methodology is verified by measurement results of devices fabricated at different silicon photonics foundries.

Silicon-based high-power traveling wave photodetector with inductive gain peaking.

We demonstrate Ge/Si high-power and high-speed distributed traveling wave photodetectors (TWPD) by using the inductive gain peaking technique. Input terminals of TW electrodes are open to enhance RF output efficiencies to output loads. Furthermore, optimized on-chip spiral inductors are incorporated at output terminals of TW electrodes to alleviate bandwidth degradations caused by the absences of matching impedances. A comprehensive equivalent circuit model is developed to calculate the frequency response of this scheme. It is used to optimize the design, and then is validated by measurement results. After inducing on-chip inductors, the bandwidths of 4-stage and 8-stage TWPDs are improved from 32 to 44 GHz and 16 to 24 GHz, respectively. Maximum RF output powers of 4-stage and 8-stage TWPDs with on-chip inductors are measured to be 5.7 dBm and 9.4 dBm at 20 GHz, respectively.

On-chip monitoring of far-field patterns using a planar diffractor in a silicon-based optical phased array.

We demonstrate the on-chip monitoring of far-field patterns in a silicon-based optical phased array (OPA) using a planar diffractor and traveling-wave photodetectors (PDs) integrated at the end of the radiator array. To reproduce the diffraction patterns within a silicon slab, the planar diffractor is designed with a diffraction region surrounded by an absorptive boundary and seven discrete outlet waveguides. Each outlet waveguide is linked to the photon-assisted tunneling PD which has a silicon p-n junction and is operated under a reverse bias to detect a sub-bandgap wavelength, 1.3 µm. With the 1×16 OPA and seven detectors, the positions of the main beams aligned to specific directions in the free space were clearly monitored.

High-power traveling-wave photodetector based on an aperiodically loaded open-circuit electrode.

We demonstrate a four-stage silicon distributed traveling-wave photodetector (TWPD) whose input terminal is open-circuit so as to enhance the radio-frequency (RF) output power. In order to mitigate the bandwidth drop caused by the RF reflection at the open input terminal, photodetector (PD) units are aperiodically loaded on the TW electrode. With the aid of an equivalent circuit model, we optimize spacings between PD units and then improve the 3dB bandwidth from 7 to 10GHz. A good agreement between measurement and modeling is obtained. Thanks to the cancellation of the input termination impedance, maximum RF output powers of the device are 10.2 and 6.5dBm at 5 and 10GHz, respectively. The corresponding power conversion efficiencies are 7.0% and 3.2%.

Silicon photonics traveling wave photodiode with integrated star coupler for high-linearity mm-wave applications.

Next-generation wireless communication will require increasingly faster data links. To achieve those higher data rates, the shift towards mmWave frequencies and smaller cell sizes will play a major role. Radio-over-Fiber has been proposed as a possible architecture to allow for this shift but is nowadays typically implemented digitally, as CPRI (Common Public Radio Interface). Centralization will be important to keep next-generation architectures cost-effective and therefore shared optical amplification at the central office could be preferable. Unfortunately, limited power handling capabilities of photodetectors still hinder the shift towards centralized optical amplification. Traveling wave photodetectors (TWPDs) have been devised to allow for high-linearity, high-speed opto-electronic conversion. In this paper, an architecture is discussed consisting of such a TWPD implemented on the iSiPP25G silicon photonics platform. A monolithically integrated star coupler is added in the design to provide compact power distribution while preserving the high linearity of the TWPD. The traveling wave structure (using 16 photodetectors) has a measured 3 dB bandwidth of 27.5 GHz and a fairly flat S21 up to 50 GHz (less than 1 dB extra loss). Furthermore, the output referred third-order intercept point at 28 GHz, is improved from -1.79 dBm for a single Ge photodiode to 20.98 dBm by adopting the traveling wave design.

A Silicon Aperiodically Distributed Traveling-Wave Photodetector With Enhanced RF Output Power

We propose an aperiodic loading technique to enhance the RF output power of a silicon distributed traveling-wave photodetector (TWPD), which can be used to handle high optical powers in analog optical links. This technique contains a traveling wave electrode whose input terminal is open circuit so as to avoid shunting any photocurrent from the photodetector (PD) array. As a result, the RF output to the load impedance that locates at the other terminal is enhanced. To mitigate the bandwidth reduction caused by the strong RF reflection at the open input terminal, the spacing between adjacent PD units gradually increases from the input to the load terminals. By breaking the balanced distribution of the photocurrent from each PD among the forward and the backward propagation directions, this arrangement weakens the strength of the interference between the forward and the reflected backward RF waves, and hence improves the bandwidth. In order to validate this operation principle, an analytical model is developed to calculate the frequency response of the silicon distributed TWPD. The model then is used to optimize both periodically and aperiodically distributed TWPDs on silicon. After the optimization, the 3-dB bandwidth of a four-stage aperiodic TWPD is 25.7 GHz, which is only 0.8 GHz lower than that of a four-stage periodic TWPD. However, since the aperiodic TWPD eliminates the matching impedance at its input terminal, its RF output power on the load is four times as strong as that of the periodic TWPD from 1 to 24.7 GHz.

High-power dual-fed traveling wave photodetector circuits in silicon photonics

We introduce the concept of dual-illuminated photodetectors for high-power applications. Illuminating the photodetector on both sides doubles the number of optical channels, boosting DC and RF power handling capability. This concept is demonstrated utilizing multiple-stage dual-illuminated traveling wave photodetector circuits in silicon photonics, showing a maximum DC photocurrent of 112 mA and a 3-dB bandwidth of 40 GHz at 0.3 mA. Peak continuous-wave RF power is generated up to 12.3 dBm at 2 GHz and 5.3 dBm at 40 GHz, at a DC photocurrent of 55 mA. High speed broadband data signals are detected with eye amplitudes of 2.2 V and 1.3 V at 10 Gb/s and 40 Gb/s, respectively. A theoretical analysis is presented illustrating design tradeoffs for the multiple-stage photodetector circuits based on the bandwidth and power requirements.

Silicon-based traveling-wave photodetector array (Si-TWPDA) with parallel optical feeding.

We demonstrate silicon-based traveling-wave photodetector arrays (Si-TWPDAs) with parallel optical feeding by integrating multiple Germanium photodetectors. Such Si-TWPDAs feature the merit of high optical saturation power with remaining the large operation bandwidth. The impedance-matched traveling-wave electrode design takes into account the individual Ge photodetector loading effect. Optical waveguide delay lines are designed in order to balance the electrical phase delay of the traveling-wave electrode. The maximum linear photocurrent at -4V biased voltage are respectively 16 mA, 38 mA, and 65 mA with integrating 1, 2, and 4 photodetectors, upon the saturation power of 40 mW, 100 mW, and 160 mW. This corresponds to a normalized photocurrent generation of >0.32 mA/µm3 and a normalized saturation power of 0.8 mW/µm3. The extracted fiber access responsivity is ~0.42 A/W and the intrinsic responsivity of ~0.82 A/W. The measured 3-dB bandwidth for 4-channel TWPDA is ~20 GHz.

Strong Light Absorption of Traveling Wave Photodetectors in Superconducting Plasmon Optical Fiber

By using an analytical model and a finite element method, we investigate a new, very sensitive, superconducting traveling wave photodetector made by an optical fiber, which includes a gold core and another thin gold layer in between two active superconducting (yttrium barium copper oxide, YBCO) layers. It is possible to tune the geometrical parameters of the fiber such as only two guided modes (fundamental degenerate HE11 and plasmon nondegenerate TM0) are highly confined in the interior YBCO layer. For a particular fiber structure, a single degenerate HE11 mode can be obtained if the gold core layer is replaced with a LaAlO3 layer. One advantage of our optical fiber with a gold core is that the power fraction of the HE11 and TM0 modes is mainly absorbed (above 94 %) in the interior superconducting layer. Our optical fiber can be used in miniaturizing plasmonic superconducting photodetectors due to sub-wavelength dimensions and very efficient absorption.

Efficient Sub-wavelength Plasmonic Superconducting Photodetectors in Ultra-Thin Planar Waveguides and in Optical Fibers

By using a finite-element method we investigate the efficiency of sub-wavelength plasmonic superconducting photodetectors in ultra-thin planar waveguides (the incident wave is P-polarized) and in optical fibers (a traveling wave). The unit cell of a periodic sub-wavelength planar waveguide is made by a gold strip that is encapsulated in an YBCO layer and further enclosed above by an air layer and beyond by a thick gold plate substrate. For a planar structure with a single unit cell, the power absorption in YBCO layer is very large (between 0.929 and 0.975) for a fixed wavelength λ=1.55 μm when the distance between the strips and gold substrate is varied between 17 nm and 32 nm. When the wavelength is varied between 1.1 μm and 1.9 μm, the power absorption in YBCO layer is between 0.944 and 0.965 with a maximum of 0.976 at λ=1.40 μm. The superconducting traveling wave photodetector is made by an optical fiber, which includes a gold core surrounded by a number of smaller gold cylinders at a small distance from the central gold region and that are encapsulated in an active YBCO layer and further enclosed by an air layer. In our structure of the fiber with 18 smaller gold cylinders, the imaginary parts of the effective index of plasmon modes (two nondegenerate modes and eight twofold degenerate modes) and the power absorption (between 0.904 and 0.976) in active YBCO layer are very large. The first nine modes are highly confined in the YBCO layer in the space between the neighboring gold cylinders and the other nine modes are highly confined in the YBCO layer but in the space between the small gold cylinders and the gold core. Although in our structure, the thickness of the YBCO layer is 10 times smaller in comparison with that of a recently published model with 32 small gold cylinders, the losses in active YBCO layer are also very large.

Very High Absorption of Traveling Wave Photodetectors in Superconducting Fiber Plasmon–Polariton Optical Waveguides

By using a finite element method, we investigate a new, very sensitive, superconducting traveling wave photodetector made by an optical fiber, which includes a high index layer (LaAlO3) with small losses, a number of metallic (gold) cylinders, and a single active superconducting layer (YBCO). Although in our structure of the fiber with 32 smaller gold cylinders, the number of the modes is large (3 nondegenerate modes and 16 twofold degenerate modes), the imaginary parts of the HE11, TM01, HE12, and higher order plasmon modes and also the power absorption (0.93 for HE11,0.91 for HE12,0.95 for TM01 and larger than 0.93 for all higher order plasmon modes) in the active superconducting layer are larger in comparison with that of a fundamental mode from a LaAlO3–YBCO–Au planar waveguide or with that of the HE11, TM01, HE12 modes from a model previously published where the gold cylinders are replaced with a single gold layer. The nondegenerate HE11 mode and the highest nondegenerate plasmon mode are highly confined in the YBCO layer in the space between the neighboring gold cylinders. Also, the plasmon mode TM01 is highly confined in the YBCO layer but on the exterior surface of the gold cylinders. The HE12 and the higher plasmon modes are highly confined in the YBCO layer in the space between the neighboring gold cylinders or very close to their exterior surface. The confinement regimes of the light and the power absorption efficiency in the superconducting layer can be optimized by only acting on the fiber geometry.

High-Speed Traveling-Wave Photodetector with a 3-dB Bandwidth of 410 GHz

A high-speed traveling-wave photodetector (TWPD) with an InGaAs absorber is designed and realized. The bandwidth of the TWPD is measured using electro-optic sampling techniques. The bandwidth is 410 GHz, which shows that the RC limitation is overcome. While the TWPD shows a low responsivity of 0.06 A/W at 1,550 nm, this value can be improved through further optimization of the structure without a sacrifice in bandwidth.

Absorption Efficiency of Traveling Wave Photodetectors in Superconducting Fiber Plasmon–Polariton Optical Waveguides

By using an analytical model and a finite element method, we investigate a new, very sensitive, superconducting traveling wave photodetector made by an optical fiber, which includes a high index layer (LaAlO3) with small losses, a metallic (gold) layer, and two active superconducting layers (YBCO). While the fundamental guided degenerate mode HE11 is highly confined in the interior YBCO layer, the plasmon non-degenerate mode TM01 and plasmon degenerate mode HE12 are tightly confined in the exterior superconducting layer. In our structure of the fiber with five layers (without a buffer layer), the imaginary parts of the HE11, TM01, HE12 modes and the power absorption efficiency in superconducting layers are larger in comparison with that of a fundamental mode from a LaAlO3–YBCO–Au planar waveguide. The confinement regimes of the light and the power absorption efficiency in superconducting layers can be optimized by only acting on the fiber geometry.

Sensitivity of Traveling Wave Photodetectors in Superconducting Fiber Plasmon–Polariton Optical Waveguides

By using an analytical model and a finite element method, we investigate a new, very sensitive, superconducting traveling wave photodetector made by a fiber waveguide, which includes a high index layer, a metallic layer, and an active superconducting layer. A comparison with the corresponding superconducting box shaped waveguide shows that a larger number of modes (HE11, TM01 and HE12) are obtained in optical fiber due to the surface plasmon–polariton modes at the interfaces between gold and air layers or between gold and YBCO layers. The radial component of the electric field is perpendicular to the metal surface and has sign changes at the gold boundaries as in the simple case of surface plasmon polaritons on metal cylinder with dielectric core. In a structure of the fiber with six layers, the imaginary parts of the TM01, HE12 modes, and the power absorption efficiency in superconducting layer are larger in comparison with that of the fiber with five layers. The confinement regimes of the light and the power absorption efficiency in superconducting layer can be optimized by only acting on the fiber geometry.

Power Absorption Efficiency in Superconducting Fiber Optical Waveguides

By using an analytical model and a finite element method, we investigate a new, very sensitive, superconducting traveling wave photodetector made by a fiber waveguide, which includes two high index layers and an active superconducting layer. A comparison with the corresponding superconducting box shaped waveguide shows that a larger width of the superconducting layer can be used to obtain single degenerate hybrid mode HE11. The real part of the propagation constants in the fiber is smaller in comparison with that of the box-shaped waveguide with similar dimensions but the imaginary part of the propagation constant and the power absorption efficiency in superconducting layer are comparable with those of the corresponding box-shaped waveguide. The confinement regimes of the light and the power absorption efficiency in superconducting layer can be optimized by only acting on the fiber geometry.

Efficiency of Traveling-Wave Photodetectors in Superconducting Box-Shaped Metamaterial Optical Waveguides

Using finite element simulations we investigate a new, very sensitive, superconducting traveling-wave photodetector made from a modified box-shaped waveguide, which includes a high-index layer, a metamaterial layer and an active superconducting layer. The maximum of the electric field is shifted from the middle of the waveguide towards an interface which includes the active YBCO layer for an optimum imaginary part of the refractive index of the metamaterial. For some values of the refractive index of metamaterial and an optimized geometry of the waveguide, the power absorption efficiency in superconducting layer is comparable with or better than that of the plasmon–polariton optical waveguide.

Sensitivity of Traveling Wave Photodetectors in Superconducting Box-Shaped Plasmon-Polariton Optical Waveguides

A new, very sensitive, superconducting traveling-wave photodetector made by a modified box-shaped waveguide, which includes a high-index layer, a metallic layer, and an active superconducting layer, is studied. The light confinement regimes and the power absorption efficiency in superconducting layer can be maximized by optimizing only the waveguide geometry.

Power Absorption Efficiency in Superconducting Box-Shaped Optical Waveguides

A very sensitive superconducting traveling wave photodetector made of a modified box-shaped waveguide, which includes two high index layers and an active superconducting layer, is studied. The optical propagation constants and the power absorption efficiency for guided modes are determined using the finite element method; the results show that by acting only on the waveguide geometry, different confinement regimes of the light in the absorption superconducting layer can be achieved and optimized.

<I>A Special Issue on</I> Nanoscale Modeling and Simulation

This special issue of Journal of Computational and Theoretical Nanoscience consists of 2 review and 12 research original papers. The increasing power of today’s computers forms the basis for an increasing significance of computer simulations in all fields, from quantum mechanics to biology, processes, devices, and applications in nanoscience. This special issue was intended to share very important results in advanced simulations techniques, multiscale modeling, as well as properties and applications to various materials and device problems from the leading research groups in Republic of Korea. A large number of manuscripts were received. After going through a very strict reviewing process, 2 review and 12 research papers were selected for publication in this special issue. One review paper on nanotube oscillators presents important progresses for their properties and applications in outstanding works by means of molecular dynamics simulations. Another review paper on traveling wave photodetectors presents recent advanced model using finite-difference time-domain (FDTD) method. The contributed papers

Demonstration of balanced coherent detection using polymer optical waveguide integrated distributed traveling-wave photodetectors

We have demonstrated balanced coherent detection using distributed balanced traveling-wave photodetectors integrated with single mode polymer optical waveguides. Balanced distributed traveling-wave photodetectors having 3 dB bandwidth of 20 GHz exhibited 20 dB signal to noise ratio improvement measured at 15 GHz modulation frequency in a balanced coherent detection demonstration.