低成本Mach-Zehnder干涉仪生物传感器的稳健测量技术

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本文主要探讨了一种创新且成本效益高的光子生物化学传感器测控技术，该技术应用在基于马赫-泽德勒干涉仪（Mach-Zehnder Interferometer, MZI）的集成光子传感器系统中。Mach-Zehnder干涉仪是一种常用的光学元件，它通过两个或多个光路的相位差来实现对微小物理量，如光强度或折射率变化的敏感测量。作者们提出了一种新型的测量方法，其核心是直接检测MZI臂间的相位变化。这个过程通过在干涉仪的一个臂上进行信号调制，并结合伪双频检测（pseudoheterodyne detection）来实现。与传统的依赖于可调光源或光谱分析器的方案不同，这种方法显著降低了设备成本，使得该技术更加经济实用。这种技术的独特之处在于，输出信号不受波长、功率变化以及全局温度波动的影响，从而展现出极高的鲁棒性。具体操作中，信号调制允许在固定光源下对干涉图案进行精细控制，而伪双频检测则能有效地分离出由被测量生物化学反应产生的相位变化信号，即使在复杂的环境条件下也能保持精确度。这使得该技术特别适合于对生物分子浓度、酶活性等微量化学物质进行实时、连续的监测，对于诸如医疗诊断、环境监控等领域具有巨大的应用潜力。此外，由于整个系统集成在一块芯片上，空间紧凑，易于大规模生产和集成，这进一步降低了系统的体积和复杂性，有利于实现小型化和便携式设备。这项研究不仅推动了光子传感器技术的发展，也为低成本、高灵敏度的生物化学检测提供了一种新的解决方案。

Robust and low-cost interrogation technique for

integrated photonic biochemical sensors based

on Mach–Zehnder interferometers

V. Toccafondo

* and C. J. Oton

Consorzio Nazionale Interuniversitario per le Telecomunicazioni, Via G. Moruzzi 1, 56124 Pisa, Italy

Scuola Superiore Sant’Anna, Via G. Moruzzi 1, 56124 Pisa, Italy

*Corresponding author: veronica.toccafondo@cnit.it

Received February 20, 2015; revised February 9, 2016; accepted February 16, 2016;

posted February 18, 2016 (Doc. ID 254169); published March 14, 2016

We describe and experimentally demonstrate a measuring technique for Mach–Zehnder interferometer (MZI)

based integrated photonic biochemical sensors. Our technique is based on the direct measurement of phase

changes between the arms of the MZI, achieved by signal modulation on one of the arms of the interferometer

together with pseudoheterodyne detection, and it allows us to avoid the use of costly equipment such as tunable

light sources or spectrum analyzers. The obtained output signal is intrinsically independent of wavelength, power

variations, and global thermal variations, making it extremely robust and adequate for use in real conditions. Using

a silicon-on-insulator MZI, we demonstrate the real-time monitoring of refractive index variations and achieve a

detection limit of 4.1 × 10

−6

OCIS codes: (130.6010) Sensors; (130.3120) Integrated optics devices; (060.2840) Heterodyne.

http://dx.doi.org/10.1364/PRJ.4.000057

1. INTRODUCTION

Silicon photonics [1] is becoming a very attractive technology

for the realization of photonic biosensors aimed at the detec-

tion of proteins, antibodies, DNA, contaminants, and so forth.

The main reason for this is their potential for mass production

because of being compatible with standard CMOS processes,

which would yield a very low cost per device while maintain-

ing a high sensing performance. However, at the moment they

are hardly employed outside academic research and are strug-

gling to find an appropriate market [2]. We believe that this is

due to several factors, among which is the very conservative

medical and biological environment, into which it is difficult

to introduce disruptive technology, or the fact that many of

those sensors rely on the use of expensive devices, such as

tunable lasers or optical spectrum analyzers, to perform the

readout of the sensor [2]. Moreover it is difficult to find inte-

grated photonic biosensors which are really robust to environ-

mental changes, such as the temperature of the surrounding

medium or the power and wavelength fluctuations of the light

source. Typically employed photonic structures include ring

resonators [3], photonic crystals [4], microdisks [5], or Mach–

Zehnder interferometers (MZIs) [6]. All these are based on

evanescent wave sensing and exhibit a change in their re-

sponse when the refractive index n

eff

of the surrounding

medium changes. The first three rely on the detection of a

change in the transmission spectrum of the device when n

eff

changes. In MZI-based sensors the phase difference induced

by n

eff

variations over one of the two interferometer arms

yields a modulation of the interferometric conditions at the

device output [2,6]. Typically this intensity variation is what

is measured when using MZI-based sensors, but it presents

some drawbacks, such as the change in sensitivity depending

on the interferometric condition (maximum around the quad-

rature points, but very low at the vertices), and the intrinsic

difficulty in distinguishing phase changes from intensity

variations. A measuring technique allowing us to avoid this

problem and in which the signal going through one of the MZI

arms is electrically modulated was already proposed in [7],

although the performance of the device with the modulation

elements was not demonstrated. In that paper, the interroga-

tion was based on a different technique in which a time delay

between a trigger and the quadrature point was monitored. In

[8] a technique was proposed for MZI interrogation based on

phase modulation of the laser source. However, that tech-

nique requires a very asymmetric MZI which produces a phase

response that is very sensitive to thermal fluctuations and

wavelength variations of the source.

In this paper we propose a technique based on pseudohe-

terodyne phase demodulation [9], capable of extracting the

phase variations in a robust way without having to tune the

laser source or use spectral analysis. In our experiment we

have used a dual-phase lock-in amplifier (LIA) for phase

demodulation, although it is expected that simpler phase-

demodulation components could be used too, which would

avoid the need of lock-in amplification. Our technique is in-

trinsically immune to power, temperature, and wavelength

fluctuations, and it allows the use of low-cost instruments

to perform the read-out, making it very promising for the

realization of low-cost point-of-care devices.

2. CONCEPT

The phase demodulation technique consists of the introduc-

tion of a periodic phase variation with a linear sawtooth with

a 2π amplitude, which produces an effect equivalent to a

V. Toccafondo and C. J. Oton Vol. 4, No. 2 / April 2016 / Photon. Res. 57

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