Elsevier

Optics Communications

Volume 313, 15 February 2014, Pages 186-194
Optics Communications

Silicon-on-sapphire waveguides design for mid-IR evanescent field absorption gas sensors

https://doi.org/10.1016/j.optcom.2013.10.022Get rights and content

Abstract

Major trace gases have absorption lines in mid-IR. We propose silicon-on-sapphire waveguides at mid-IR for gas sensing based on evanescent field absorption. This can provide a general platform for multipurpose sensing of different types of gases in a reusable fashion. Three types of waveguides (strip, rib and slot) are investigated on their geometrical dependence of evanescent-field ratio (EFR) and propagation loss to serve as the proposed gas sensor. Slot waveguide provides the highest EFR (>25%) in mid-IR with moderate dimension, but its fabrication can be more challenging and its high loss (~10 dB/cm) impairs the sensing resolution and necessitates higher input power in longer waveguides. Strip and rib waveguides can achieve similar EFR with smaller dimensions. We analyze the detection of CO2 in atmosphere based on its mid-IR absorption peak at ~4.23 µm as a case study. Numerical analysis based on up-to-date commercial mid-IR detector parameters shows that a resolution of 2 ppm, 5 ppm and 50 ppm can be achieved in cooled InSb, room-temperature HgCdTe and room-temperature PbSe detectors respectively by using 1 cm waveguides. Effect of waveguide loss also has been investigated.

Introduction

Gas sensors have found wide applications and high demands in chemistry, biology, medicine, manufacturing and environmental industry [1]. Major types of gas sensors include catalytic, electrochemical, thermal, ultrasonic, semiconductor and infrared sensors. Their detection mechanisms can rely on the thermal, chemical, mechanical, electrical or optical property of the functional material [2]. However, most of these sensors are bulky and some even require a thermal management system [2]. Their integrability and portability are limited and they are not applicable in scenarios where limited space is available. It is attractive to have miniature sensors that could be fitted into small chip areas. In this way, they could be integrated with on-chip control and processing circuits to build a chip-scale sensing system, which can be deployed in distributed sensing networks. Chip-level gas sensors have been demonstrated using micro-ring resonators and Mach–Zehnder interferometers in near-IR spectrum [3], [4]. They both utilize the refractive index variation due to different types and concentrations of the gas immersing the evanescent field. However, the refractive index change by the gas itself alone in these sensors is too small to be detected. A film is often required to be coated on device surfaces to enhance the refractive index variation and hence to increase the sensitivity. However, a particular film is often reactive to only one type of gas and different materials have to be deposited for different gas species. Hence, a general-purpose sensor is not applicable. Moreover, the response time is also limited by the chemical or physical reaction rate and its lifetime is usually short. To probe further, besides refractive index change, gas type and concentration also affect the absorption experienced by the evanescent field. This property in planar waveguides can be also utilized to achieve gas sensing as has been shown in fiber optics [5], [6], [7]. While fiber optics and waveguide technology are well-developed and become mature in the famous near-IR telecommunication band, there are very few types of industrially or environmentally important gases with absorption peaks in the near-IR wavelengths. In contrast, the mid-IR spectrum window contains absorption peaks for a wide variety of trace gases, including H2O, CO, CO2, NO, N2O, NO2 and CH4 [5], [8]. If viable waveguide-based sensors for these gases are available at affordable price, they can be integrated with electrical circuitry to support system-on-chip applications for chemical, biological, industrial and environmental monitoring. Unfortunately, the SOI and fiber technology cannot be extended easily to mid-IR due to the strong loss associated with the oxide layer. Free-standing silicon waveguides with air substrate are proposed to circumvent this limitation in mid-IR [8], [9], [10]. However, this type of waveguide is unstable mechanically or optically in the long term especially under harsh natural conditions, where the sensors usually are. Another way to circumvent the substrate loss is to use a non-lossy substrate material. Among many proposed candidates, sapphire has gained popularity for its superior mechanical and thermal stability and a transparency window up to ~6 µm. Silicon-on-sapphire (SOS) waveguides are proposed and proven to be a good mid-IR alternative to SOI in the near-IR [11], [12], [13], [14], [15], [16]. With the abundance of gas absorption peaks in the mid-IR spectra, SOS platform can support the detection of all the gas species mentioned earlier.

In this paper, we propose an evanescent field absorption gas sensor based on SOS waveguide in mid-IR wavelength. This provides a general solution to the sensing of different trace gases, which could result in inexpensive and robust miniature gas sensors with fast response and large reusability. Another advantage is that the platform can potentially be used to detect different types of gases at the same time with broadband mid-IR sources (e.g. thermal radiators). The mid-IR optical signal can be detected indirectly using silicon waveguide and near-IR detectors [16], [17]. Meanwhile, mid-IR sources are getting available also through the parametric process in silicon waveguides [18], [19]. Also integrated mid-IR detectors on a silicon substrate have been also demonstrated [20], [21]. With further advancements in mid-IR photonics, an on-chip gas sensing system would become possible in the foreseeable future. The paper is organized mainly in three parts. We first present the sensing mechanism of the proposed SOS waveguide based absorption evanescent sensor. The SOS waveguide is then designed for high detection resolution of the target gas concentration. Evanescent field ratio (EFR) is defined as the ratio of evanescent field power in the cladding to the total power of the waveguide mode. The geometrical dependence of EPR, as well as propagation loss, provides a guideline for the design of SOS waveguide for mid-IR absorption sensing application. In particular, three types of common waveguides (strip, rib and slot waveguides) are included in the design. Although all three types are capable of delivering a platform to sense the target gas with high resolution, slot waveguide is shown to possess the highest EFR values with moderate dimension and hence higher sensing performance if other conditions are the same. However, it is more fabrication intensive and prone to having higher propagation loss, which makes a higher input power necessary to compensate the loss and to maintain acceptable sensing resolution for longer waveguides. On the other hand, rib waveguide offers the lowest propagation loss among the three types, favoring its application as an evanescent gas sensor. In the end, we apply this sensing mechanism to detect CO2 concentration fluctuation at the atmospheric base level. Our analysis based on the up-to-date mid-IR detector parameters shows that a resolution as low as 2 ppm can be readily achieved with these waveguides at atmospheric environment.

Section snippets

Evanescent absorption sensing

As light travels through the silicon waveguide, most of the energy is confined within the silicon core. However, as shown in Fig. 1, there will be also evanescent field extending to the substrate region and cladding region formed by the surrounding media. In the cladding, the wave interacts with the gas surrounding the waveguide and goes through power attenuation if the light wavelength is properly aligned with the absorption lines of the gas. The absorption is dependent on the concentration of

Waveguide design

According to Eq. (3), in order to increase the detection sensitivity, the waveguides need to be designed for maximum gas–field interaction to achieve a large η value. A waveguide with small electrical dimension is generally desired to support strong evanescent field [25]. In this paper, we investigate three types of SOS waveguides, including strip, rib and slot waveguides, for absorption based gas sensing in mid-IR as shown in Fig. 2. W and H are the width and height of waveguide, respectively.

CO2 sensing

Providing moderate EFR values, these waveguides can be incorporated to form evanescent field absorption gas sensors. As an example, we calculate the sensing resolution of CO2 fluctuation in the atmosphere using parameters from commercial detectors and laser. The incident power of the input light source is set at 10 mW and its RIN is −139 dBc/Hz [33]. The base concentration c of CO2 is chosen to be the latest atmospheric level of 396.8 ppm [34]. The absorption coefficient of CO2 is 1214 L cm−1/mol

Summary

In summary, strip, rib and slot waveguides are designed for high evanescent field ratio to increase sensing resolution of the proposed evanescent field absorption gas sensor. Slot waveguide can provide the highest EFR (>25%) in mid-IR with moderate waveguide dimension, but its fabrication is more challenging especially when slot width is small for high EFR. And its propagation loss is also much higher than those of other two types, which makes higher input power necessary to achieve acceptable

Acknowledgment

This research was supported by DARPA Young Faculty Award, #66001-10-1-4036 and EU PIRG07-GA-2010-268370 grant. The authors want to thank Sam Crivello from Daylight Solution for valuable discussion. The authors would also like to thank the anonymous reviewers for their valuable comments and suggestions to improve the quality of the paper.

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