Dec 10, 20244 min read

Integrated SiGe Electro-Optical Modulator for Mid-Infrared Spectroscopy

Introduction

The mid-infrared (mid-IR) spectral range, spanning wavelengths from approximately 3 to 20 μm, is of great interest for various applications such as defense, medical diagnosis, and environmental monitoring. This is because many molecules exhibit strong absorption lines in this wavelength range, enabling their detection and identification through spectroscopic techniques. To develop integrated mid-IR spectroscopic sensing systems, one of the key active components is an efficient electro-optical modulator operating in the long-wave spectral range. This tutorial article presents the first experimental demonstration of a PIN electro-optical modulator embedded in a SiGe waveguide, operating across a wide mid-IR wavelength range from 5.5 μm to 10 μm.

Device Design

The integrated SiGe electro-optical modulator (EOM) exploits the free carrier plasma dispersion effect in SiGe waveguides to achieve efficient modulation. The device structure, shown in Figure 1, features a PIN diode embedded within the SiGe waveguide. The epitaxial growth involves several layers, starting with a 3-μm-thick Si0.6Ge0.4 layer deposited on a non-doped Si substrate, followed by a 3-μm-thick graded SiGe layer (Ge fraction increasing linearly from 40% to 100%), and finally a 2-μm-thick Si0.3Ge0.7 layer on top to ensure effective optical confinement in the waveguide and away from the upper metallic contact.

To exploit the free carrier plasma dispersion effect, a PIN diode is embedded in the waveguide. The first 1-μm-thick Si0.6Ge0.4 layer is heavily N-doped with a concentration of 1 × 10^18 cm^-3, while the upper 300-nm-thick layer of Si0.3Ge0.7 is P-type doped with a concentration of 5 × 10^18 cm^-3. The region between the N and P regions is unintentionally doped. The device fabrication process involves etching to a depth of 7.5 μm to define the SiGe waveguide and create access to the bottom contact on the N-doped region.

Figures 2a and 2b show scanning electron microscope (SEM) images of the fully integrated electro-absorption modulator, clearly depicting the signal and ground line contacts positioned on top of the waveguide and within the etched region of the device, respectively. Figure 2c illustrates the optical mode profile for the TE polarization at a wavelength of 7.8 μm, demonstrating the effective confinement of the optical mode within the SiGe waveguide.

Fig. 2. Device fabrication. a-b) SEM images of the fabricated device from the top view and lateral view, respectively. c) Optical profile for TE polarization at the wavelength of 7.8 µm.

Electro-Optical Characterization

To evaluate the modulation performance of the EOM, static DC measurements were first carried out. An external cavity-based mid-infrared quantum cascade laser (QCL) was used as the tunable light source, covering wavelengths from 5.2 μm to 11.2 μm. The light was coupled in and out of the photonic circuit using a pair of ZnSe lenses.

Figure 3 reports the extinction ratio (ER) for a 5.9-mm-long modulator in both depletion and injection regimes. In the depletion regime (Figure 3a), the relative optical transmission increases with a higher reverse bias voltage, attributed to the expansion of the depletion width within the active region, leading to a reduction in free-carrier absorption. Conversely, in the injection regime (Figure 3b), the concentration of electrons and holes injected into the active region increases, resulting in an increase in free-carrier absorption and a decrease in the relative optical transmission as more current is injected.

Fig. 3. Static DC characterization of EOM. a) Depletion regime. b) Injection regime.

Notably, at a wavelength of 10 μm, an extinction ratio of 3 dB was achieved in the depletion regime at a reverse voltage of -15V, while in the injection regime, the extinction ratio reached 10 dB for a 115 mA current injected into the device.

Dynamic operation was characterized using a continuous-wave mid-IR QCL at a wavelength of 7.8 μm. An RF signal with an amplitude of 8V peak-to-peak was applied to the device through a bias-tee (-4VDC in depletion and 4VDC for injection). The modulated optical output was collected and converted into an electrical signal using a fast mid-IR photodetector with a -3 dB bandwidth of 700 MHz. This modulated signal was then analyzed using an electrical spectrum analyzer (ESA).

Figure 4 illustrates the measured beat notes corresponding to different modulated RF frequencies in the depletion and injection regimes. The results show that effective modulation can take place up to 1.5 GHz, with a signal-to-noise ratio of 10 dB.

Fig. 4. High frequency operation. The peak amplitude of the beat notes observed in ESA at various RF frequencies at the wavelength of 7.8 µm for injection and depletion regimes. Background noise is obtained as laser is off.

Conclusion

This work reports the first broadband electro-absorption modulator operating from 5.5 μm to 10 μm wavelength, based on a PIN diode embedded in a SiGe waveguide. High extinction ratios were observed, with more than 3 dB in the depletion regime and 10 dB in the injection regime at a wavelength of 10 μm. Furthermore, high-speed operation was achieved, with modulation frequencies up to 1.5 GHz. These results pave the way for the development of mid-infrared on-chip spectroscopic sensing systems and open up new possibilities for integrated mid-IR photonics.

Reference

[2] T. H. N. Nguyen, V. Turpaud, N. Koompai, J. Peltier, S. Calcaterra, G. Isella, J.-R. Coudevylle, C. Alonso-Ramos, L. Vivien, J. Frigerio, D. Marris-Morini, "Integrated SiGe electro-optical modulator based on PIN diode in the 5.5-10 µm wavelength range," Centre de Nanosciences et de Nanotechnologies, Université Paris-Saclay, CNRS, 91120 Palaiseau, France; L-NESS, Dipartimento di Fisica, Politecnico di Milano, Polo di Como, 22100 Como, Italy, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.