Si Optical Modulator Research Articles

High-extinction-ratio Si optical modulator loaded with integrated polarizer

We propose a carrier-depletion type Si optical modulator for transverse electric (TE) polarization loaded with an integrated polarizer. The integrated TE-pass polarizer with a bent waveguide has a compact and simple structure and demonstrate a polarization extinction ratio of 41.3 dB and insertion loss of 0.67 dB. This Si optical modulator loaded with the integrated polarizer achieved a maximum extinction ratio of 54.5 dB. It is expected to have a wide range of applications in digital coherent optical communication and optical sensing fields.

A Si Optical Modulator Based on Fano-Like Resonance

In this letter, we present an experimental demonstration of a Si optical modulator based on Fano-like resonance, which is the first of its kind in the literature. The Fano-like resonance is obtained through weak coupling between two optical resonators in the prototype device and a PN junction is designed for carrier-depletion type electro-refractive modulation. 20 Gb/s data rate on-off keying (OOK) is obtained experimentally on the prototype device. Compared with a ring resonator modulator with a similar optical resonance linewidth, a Fano-like resonance based modulator can potentially have a higher extinction ratio (ER) because the Fano-like resonance has an asymmetric spectrum line shape and the line shape on one side can be sharper than the Lorentz resonance line shape of a ring resonator.

High-efficiency and high-speed narrow-width MOS capacitor-type Si optical modulator with TM mode excitation.

We developed a high-speed and high-efficiency narrow-width metal-oxide-semiconductor (MOS) capacitor-type Si optical modulator (Si-MOD) by applying TM optical mode excitation. We designed and fabricated an optical-mode-converter structure from TE to TM mode. Even in the case of a 200-nm width, the Si MOS-MOD showed high-modulation efficiency in TM mode (about 0.18 Vcm), and the electrical capacitance decreased as the MOS junction width decreased. We also demonstrated high-speed operation at 32 Gbps and 40 Gbps for the 30-µm-long Si MOS-MOD in TM mode.

(Invited) High-Performance Si Optical Modulator and Ge Photodetector and Their Application to Silicon Photonics Integrated Circuit

We designed the optimum Si optical modulator (Si-MOD) structure with applying p-type-doped strained SiGe. By applying the strained Si70Ge30 layer, about twice larger optical modulation efficiency in VπL can be obtained in the simulation. In case of a relaxed Si70Ge30 layer, enhancement factor in VπL decrease to about 50%, but optical loss is comparable to that without a SiGe layer. We demonstrated a high modulation efficiency of 0.67 Vcm, which is about 50% more efficient than that of Si-MODs with just a lateral pn junction. From the Raman spectra and SIMS analyses, Ge composition of SiGe was estimated to be around 35%. Crystalline strain of SiGe was estimated to be 1.7% on the 0.04 mm2 square pattern, which is comparable to the theoretical value. On the other hand, it was estimated to be 1.1% for the selective growth of SiGe on the Si WG with a lateral pn junction. Therefore, biaxial strain of SiGe was a little relaxed, because the selective growth of SiGe pattern has a large aspect ratio with 400 nm width and 800 μm length. The Si-MOD showed 27.3 GHz bandwidth at -3 Vdc with 200μm-long phase-shifter. We demonstrated eye diagrams of NRZ (non-return-to-zero) 40 Gbps of 231-1 PRBS (pseudorandom binary sequence) and PAM (pulse amplitude modulation)-4 40Gbps at 1.3 μm wavelength under the condition of differential RF drive with 1.6-2.0 Vpp at -2Vdc. The optical loss of the Si-MOD is estimated to be 1.0-1.5 dB/mm, which is comparable to that of simulation. With increase in temperature up to 80℃, optical loss increased by about 0.3-0.5 dB, which would originate from band-gap shrinkage of a strained-SiGe layer. We obtained the clear eye diagram at 25 Gbps with 27-1 PRBS from the optical transceiver chip when the CMOS-driver output voltage was 0.9 Vpp and Vdc was -0.3V. ER (extinction ratio) of about 2 dB was obtained in case of 800μm-long phase shifter with 4-divided electrodes. We also studied a high-speed and high-efficiency of surface-illumination type Ge photodetector (Ge-PD) with a 1800 nm thick Ge layer. By optimizing the anti-reflection coating stack structure, high responsivity of 0.8-0.9 A/W was uniformly obtained within the wafer. At 3 Vdc, about 15 GHz bandwidth was obtained. In case of the Ge-PD with thick Ge-layer, photo-carrier transit time mainly limits the frequency bandwidth. Therefore, we optimized the layered structure of Ge thickness on SOI to satisfy the high-responsivity and high-bandwidth. We demonstrated an output waveform from integrated CMOS-TIA (trans-impedance amplifier) at 25 Gbps with 27-1 PRBS at 1.3 μm wavelength at 3.3 Vdc. Clear eye opening was obtained, which would contribute to the efficient optical interconnect.

(Invited) High-Performance Si Optical Modulator and Ge Photodetector and Their Application to Silicon Photonics Integrated Circuit

We developed a high speed and high efficiency of MOS (metal-oxide-semiconductor) capacitor type and depletion type Si optical modulators (Si-MODs) with a pn junction by applying a p-type-doped strained SiGe layer which was stacked on the lateral pn junction type Si-MOD. We designed the optimum Si-MOD structures and demonstrated a high modulation efficiency of 0.16Vcm for MOS capacitor type Si-MOD. We also demonstrated high modulation efficiency of 0.67 Vcm for depletion type Si optical modulator with a p-type-doped strained SiGe layer, which is about 50% more efficient than that of Si-MOD with a lateral pn junction. We studied a high speed and high efficiency of vertically-illuminated p-i-n Ge photodetector (Ge-PD) with Ge thickness of 1800 nm. We developed a vertical p-i-n structure of Ge-PD and demonstrated high-responsivity of 0.8-0.9 A/W and high-speed of 25 Gbps at 1.3 μm wavelength.

High-speed and highly efficient Si optical modulator with strained SiGe layer

We developed a high-speed and highly efficient depletion-type Si optical modulator (Si-MOD) with a pn junction by applying a p-type-doped strained SiGe layer, which was stacked on a lateral pn junction-type Si-MOD. We designed an optimum Si-MOD, which is one of the most efficient Si-MODs with a pn junction, and demonstrated highly efficient modulations of 0.67 and 0.81 V·cm for VπL at dc reverse bias voltages of −0.5 and −2 Vdc, respectively. We also demonstrated a high-speed operation of 25 Gbps for the Si-MOD at a wavelength of around 1.3 µm.

Forward bias operation of silicon photonic Mach Zehnder modulators for RF applications.

In this paper, we demonstrate that forward bias (+0.9V) of a high-speed silicon (Si) optical Mach-Zehnder modulator (MZM) increases the radio-frequency (RF) link gain by 30 dB when compared to reverse bias operation (-8V). RF applications require tunable, narrowband electro-optic conversion with high gain to mitigate noise of the optical receiver and realize high RF spur-free dynamic range. Compared to reverse bias, the forward bias gain rolls off more rapidly but offers higher RF link gain improvement of more than 13.2 dB at 20 GHz. Furthermore, forward bias is shown to result in comparable spurious-free dynamic range (SFDR: 104.5 dB.Hz2/3). We demonstrate through an analytical dc transfer curve the existence of simultaneous high gain and OIP3 and verify the theoretical results with measurement under forward bias at a bias point of around +0.9 V.

High-speed and high-efficiency Si optical modulator with MOS junction, using solid-phase crystallization of polycrystalline silicon

We developed a high-speed and high-efficiency MOS-capacitor-type Si optical modulator (Si-MOD) by applying a low optical loss and a low resistivity of a polycrystalline silicon (poly-Si) gate with large grains. To achieve a low resistivity of a poly-Si film, a P-doped poly-Si film based on Si2H6 solid-phase crystallization (SPC) was developed, which showed a comparable resistivity to that of P-doped single-crystal Si. In addition, high-temperature annealing (HTA) after SPC was effective for realizing low optical loss. We designed the optimum Si-MOD structure and demonstrated a very high modulation efficiency of 0.3 V cm, which is very efficient among the Si-MODs developed thus far. High-speed (15 Gbps) operation was achieved with a small footprint of the 200-µm-long phase shifter and a low drive voltage of 1.5 Vpp at a low optical insertion loss of −2.2 dB and 1.55 µm wavelength.

High-performance MOS-capacitor-type Si optical modulator and surface-illumination-type Ge photodetector for optical interconnection

We developed a high-speed and high efficiency MOS-capacitor-type Si optical modulator (Si-MOD). We designed the optimum structure and demonstrated a very high modulation efficiency (VπL) of 0.28–0.30 V cm at 1.3 µm wavelength. We also demonstrated a high speed of 25 Gbps for the Si-MOD integrated with a Si-waveguide-coupled Ge photodetector (Ge-PD), and also high-speed operation of 15 Gbps with a CMOS driver. We further demonstrated a high modulation efficiency of 0.16 V cm with a low optical loss (α) of 3.5 dB/mm, using a carrier accumulation mode. In this case, the figure of merit (FOM) of αVπL is less than 7 dBV. We demonstrated a high-speed operation of 25 Gbps for the Si-MOD with a short phase shifter of 60 µm length. We also demonstrated a surface-illumination-type pin Ge-PD, which shows both a high bandwidth of 24 GHz and a high-efficiency of 0.8–0.9 A/W in the case of 30 µm Ge diameter.

Numerical Analysis of Carrier-Depletion Strained SiGe Optical Modulators With Vertical p-n Junction

The modulation characteristics of carrier-depletion strained SiGe optical modulators with a vertical p-n junction are numerically analyzed by technology computer-aided design simulation and finite-difference optical mode analysis. In addition to the strong optical confinement in the vertical direction for the fundamental transverse electric field mode, the vertical p-n junction effectively depletes the strained SiGe layer in which the plasma dispersion effect is enhanced owing to the mass modulation of holes by strain. We predict that a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> optical modulator exhibits V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> L at 1.55 μm of as small as 0.31 V-cm at a bias voltage of -2 V, which is ~1.8 times smaller than that of a Si optical modulator. The product of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> L and the phase-shifter loss (αV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> L) is also expected to be as low as 18.3 V-dB at -2 V, enabling optical modulation with a 5-dB extinction ratio in a symmetric Mach-Zehnder modulator.

A 40 Gbit/s optical link on a 300-mm silicon platform

We demonstrated 40 Gbit/s optical link by coupling a silicon (Si) optical modulator to a germanium (Ge) photo-detector from two separate photonic chips. The optical modulator was based on carrier depletion in a pn diode integrated in a 950-µm long Mach-Zehnder interferometer. The Ge photo-detector was a lateral pin diode butt coupled to a silicon waveguide. The overall loss, which is mainly due to coupling (3 grating couplers times ~4 dB) was estimated to be lower than 18 dB. That also included modulator loss (4.9-dB) and propagation loss (<1 dB/cm). Both optoelectronic devices have been fabricated on a 300-mm CMOS platform to address high volume production markets.

Customized drive electronics to extend silicon optical modulators to 4 gb/s

The data transmission bandwidth of a metal oxide semiconductor (MOS) capacitor Si optical modulator is extended from 1 to 4 Gb/s through the introduction of custom-designed low-impedance drive circuitry. Two distinct drive circuits were produced and tested-the first targeting 2.5 Gb/s data rate and 3 dB extinction ratio (ER), and the second having reduced voltage swing (1.3 V single-ended swing) while achieving an open eye at 4 Gb/s. The speed, power, and ER data collected are used to build a quantitative discussion of the challenges in achieving a power-efficient free-carrier modulator at bit rates above 1 Gb/s.