1. Introduction

Integrated photonics sensors are contributing to many fields, including chemical sensing [1–4], temperature sensing [5, 6], and ultrasound detection [7, 8]. Recently, such sensors for detecting ultrasound based on ring resonators (RRs) fabricated in silicon-on-insulator (SOI) and polymer platforms have attracted attention [9–11], in view of their high sensitivity, small footprint, possibility of realizing a sensor array on chip, immunity to electromagnetic interference and mass producibility. Therefore, high quality interrogation of this type of sensors becomes crucial for their further development and applications.

RR sensors are often interrogated using an optical spectrum analyzer (OSA), for measuring the shift of the transmission spectrum [5, 12]. However, its resolution is limited to the pm range and the interrogation speed is low, which makes an OSA unsuitable for sensing narrow and fast periodic signals. An on-chip arrayed waveguide grating was developed for interrogating a RR gas sensor [13]. The arrayed waveguide grating is suitable for high speed, but its resolution is also too low for RR ultrasound sensors. Another method is the so-called modulation method, using a narrow linewidth tunable laser and a high speed photodetector [9–11]. By placing the laser wavelength at the linear flank of a RR resonance and measuring the resulting modulation of the transmitted power, the resonance-wavelength modulation can be extracted. However, the highest modulation amplitude measurable with this method is limited by the narrow width of a resonance, implying a short flank [9]. In [14] we demonstrate an interrogator for a RR ultrasound sensor based on a fiber Mach-Zehnder interferometer (MZI) with a 3 ×3 fiber coupler. Owing to the 3 ×3 coupler, at least two outputs of the MZI are non-zero for any operation wavelength, enabling detecting a resonance-wavelength modulation resulting from a 1.2 Pa ultrasound pressure [14]. However, due to the large footprint of the fiber circuit, the environmental phase drift of the MZI is hard to control, thus making this fiber interrogator unsuited for long-time measurements. In addition, it requires pre-measurement at high ultrasound pressure for a circle fitting procedure with the measured signals, adding complexity in using the fiber interrogator [14].

Here, as the next step in the development started in [14], we present a compact integrated photonics interrogator, consisting of a photonic integrated circuit (PIC) and a special light source. The PIC includes an integrated photonics MZI with a 3 ×3 multi-mode interferometer (MMI) and three photodetectors. With this miniaturization of the MZI, the temperature of the MZI can be well controlled during interrogation. Therefore, this new interrogator is very suitable for long-time measurements as well. Similar MZIs have been reported for interrogating fiber Bragg grating (FBG) sensors [15], but not for the RR sensors, which are challenging due to the multiple resonances, the limited control of the resonance wavelength, the narrow resonance linewidth and the low transmission of the fiber-to-RR coupling. We developed a novel light source to overcome these issues. The light source comprises a pump laser, an Er-doped fiber and a tunable FBG. It provides a spectrum with high power density, large tuning range and suitable bandwidth. These properties ensure a high S/N ratio of the output signals, a robust alignment of the source to the sensor, and a large measurement range. By taking into account instrumental offsets stably present in the acquired signals, we avoid the above-mentioned circle fitting and thus strongly simplify the interrogation procedure. We analytically present the interrogation procedure and experimentally demonstrate a low detection limit and a large measurement range of this interrogator. Together with the RR sensor, this integrated interrogator is very promising for medical applications, such as intravascular ultrasound imaging [16]. Therefore, we call this interrogator MediGator (medical interrogator).

2. Ring-resonator ultrasound sensor and MediGator design

2.1. Ring-resonator ultrasound sensor

Here we provide the background of the operation of the RR ultrasound sensor used in the interrogation experiments and present its main characteristics. The latter serve as input for developing the interrogation procedure and for matching of the MediGator and the RR sensor presented in Section 3.

The sensor is very similar to the ones we used in our previous work [14]. It comprises a racetrack-shaped silicon RR located on a silicon dioxide membrane that has its vibrational mode in the MHz range. A difference is that the present sensor has a square membrane, while the previous ones had a circular membrane. Further, the racetrack is parallel to the <100> direction instead of the <110> direction. This gives a somewhat smaller resonance-wavelength shift in response to the applied strain [17]. A schematic of the sensor, a cross-section of the membrane region and a microscope image of the membrane are presented in Figs. 1(a)-1(c). RRs of the type shown in Fig. 1(c) were fabricated at IMEC through the ePIXfab MPW service [18] on a CMOS compatible SOI platform [220 nm Si layer, 2 µm buried oxide (BOX) layer]. The two directional couplers of the RR are identical. The realized waveguide width of the RR is 400 nm. Operational ultrasound sensors result from post-processing in the Kavli Nanolab Delft and subsequent packaging, using the same procedures as described in [7]. The truncated pyramidal hole under the membrane [Fig. 1(b)] is formed in a KOH etch, applied to locally remove the silicon substrate under the RR, using the BOX layer as etch stop. As can be seen in Fig. 1(c), the 84 µm ×84 µm membrane is well aligned to the RR.

 

Fig. 1 (a) Schematic of the RR ultrasound sensor: a RR on a square membrane (blue). (b) Cross-section of the membrane region, showing the various layers and their thickness. Membrane thickness is 2.65 µm. A glass platelet seals the air cavity under the membrane. (c) Microscope image of the membrane with the RR, taken from below. Membrane size is 84 µm ×84 µm.

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In the underwater sensing situation, the membrane’s vibrational mode is excited by acoustic waves of proper characteristics. The membrane vibrations induce time-periodic strain in the RR. This results in a modulation of the circumference L of the ring and the effective index n_e of the ring’s waveguide [17, 19]. These quantities determine the resonance condition n_e(λ_r)L = mλ_r (m is an integer), for which the ring resonates at resonance wavelength λ_r. Thus, the acoustic waves induce a modulation of the resonance wavelength of a magnitude proportional to the pressure of the waves. By interrogating the sensor, the resonance-wavelength modulation can be obtained. This yields information of the incident waves that is required for acoustic imaging applications.

The RR sensor is packaged for measuring the transmission at the drop port, implying that sensing uses a resonance peak instead of a dip. This strongly simplifies the analytical description of the output interrogation signals. The transmission to the drop port of our sensor with identical directional couplers can be expressed as [20]

Here ε and γ_r are defined by, respectively

The line shape of the transmission peak given by Eq. (2) is Lorentzian. ε is the maximum transmission value. γ_r is the full width at half maximum (FWHM) of the transmission peak. Obtained from characterization results, our RR sensors typically have γ_r ≈100 pm. The free spectral range of the RRs [FSR, with $\text{FSR}={\lambda }_r^2/\left(n_{g}L\right)$; n_g is the group index] is about 5 nm.

2.2. MediGator design

The MediGator comprises two main parts: the light source and the InP PIC. In interrogating the RR sensor, the light source is coupled to the input port of the RR, and the drop port of the RR is coupled to the input of the PIC. A schematic of the MediGator is shown in Fig. 2.

 

Fig. 2 MediGator (left) and experimental setup (right). The MediGator comprises the tunable light source of high brightness (upper part) and the photonic integrated circuit of MZI and PDs (lower part). Each PD is connected to a TIA. (TEC, thermoelectric cooling; WDM, wavelength division multiplexer; FBG, fiber Bragg grating; PIC, photonic integrated circuit; PD, photodetector; TIA, trans-impedance amplifier; AWG, arbitrary waveform generator; MMI, multi-mode interferometer)

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The light source is designed to have a high power density, a proper bandwidth and a tuning range exceeding the FSR of the RR. A high power density is needed for sufficient S/N ratio of the MediGator output signals, in view of the narrow RR resonance peak and transmission losses in the system, which is mainly due to fiber-chip connections. The light-source bandwidth should be large compared to γ_r and to the amplitude of the resonance-wavelength modulation resulting from acoustic waves. Further, it should be small compared to the FSR of the RR to enable selecting only a single resonance from the sensor transmission spectrum by tuning the position of the source spectrum. The requirement for the tuning range results from the limited control of the resonance wavelength due to the fabrication variations. A tuning range exceeding the FSR always allows centering the light-source spectrum at a resonance peak.

In more detail, the source has similarities with the Er-doped fiber lasers studied in [21]. Here, however, we use light amplification based on a dual-pass configuration instead of a real cavity (see Fig. 2). The source is based on a 1480 nm Fabry-Perot pump laser (Anritsu, AF4B125EA75L) with an internal thermoelectric cooler (TEC). The pump laser is operated in continuous mode. Its light is coupled into an Er-doped fiber [Fibercore I-25 (980/125), length 3.5m] using a wavelength division multiplexer (WDM, Thorlabs, WD1450B).The 1480nm light excites the Er ions, which partly decay by spontaneous emission. The spontaneously emitted light in turn leads to stimulated emission by excited Er ions. Part of the fiber-guided spontaneously emitted spectrum reaches a tunable FBG (AtGrating, chirped type, 10 mm long). The FBG reflects the spectrum back into the fiber, where it induces further stimulated emission. As a result, a high brightness spectrum dominated by stimulated emission and of a bandwidth determined by the FBG reflection is coupled out from the fiber via the WDM. By stretching the FBG, the spectral position is tunable. The FBG is chosen to have a bandwidth large enough compared to γ_r of the transmission peak, to ensure that the peak remains within the source spectrum, even for high modulation amplitudes. After the WDM the light passes a polarization controller for setting the polarization in the optimum direction for the grating coupler of the RR, which is designed for TE polarization. For further stability of the source, the temperature of the pump laser and the FBG are also TEC regulated.

The InP PIC of the MediGator was fabricated at SMART Photonics [22]. It is designed as an MZI witha 2 ×2 MMI input, a 3 ×3 MMI output, and three photodetectors with a responsivity of 0.85 A/W for the light of wavelengths between 1520 and 1570 nm (see Fig. 2). In operating the MediGator, at least two of the three outputs of the 3 ×3 MMI are non-zero. This enables measuring very small phase changes when interrogating the sensor, as will become apparent in Section 5.2. Since the chip of the PIC is very small (4mm ×4.6 mm), its temperature can be properly stabilized using a TEC, on which the PIC is mounted. Thus, environmental phase drift of the MZI, an issue for the fiber interrogator [14], is virtually absent. Each photodetector output is connected to a trans-impedance amplifier (TIA, Analog Devices, ADA4899-1) with a gain of 2.2kV/A. Assuming there is no loss, the power transmission T_MZI,i (i = 1, 2, 3)to the i^th output of the 3 ×3 MMI is given by

In the ideal case p = q = 1, implying a fringe visibility q/p of unity as well. OPD is the optical path-length difference between the MZI arms. The phases φ_i, with φ_i = 0^° , 120^° and 240^°(i = 1, 2, 3), are the standard phases of the outputs of a 3 ×3 MMI. φ_e is a phase offset due to imperfections of the MZI and environmental influences, respectively. By Taylor expanding 1/λ around λ_r and only retaining the first order term, we arrive at the right side of Eq. (5). The FSR of the MZI equals ${\lambda }_r^2/\text{OPD}$. ψ_e is defined as −φ_e −4πOPD/λ_r.

3. Interrogation procedure and matching of the MediGator and the ring-resonator sensor

3.1. Interrogation procedure

Using Eqs. (2) and (5) for the transmission of the RR and MZI, we calculate the MediGator outputs. For incident sinusoidal ultrasound waves of frequency f₀, the resonance-wavelength modulation is written as $\widetilde{\delta {\lambda }_r}\left(t\right)={\delta }_0\sin \left(2\pi f_{0}t\right)$, where δ₀ is the resonance-wavelength modulation amplitude. We rewrite Eq. (2) as

Following the flow of the signals in Fig. 2 and using the integrating property of the photodetectors, the output of the three TIAs can be written as

Here α represents the wavelength independent overall transmission coefficient of components other than the RR and the MZI between the light source and photodetector. P_LS is the power density of the light source. Because γ_r is more than one order smaller than the FBG bandwidth and δ₀ in practice is below 50 pm, we approximate the light-source spectrum as a constant. R_ph is the responsivity of the photodetector and G is the gain of the TIAs.

We calculate the integral in Eq. (7) by contour integration in the complex plane, using the residue theorem, to arrive at V_i given by:

The procedure to arrive at Eq. (8) is similar to the one in [14], but simpler, as a result of using the drop-port transmission and the wide FBG bandwidth. The cosine in Eq. (8) has 2πδ₀ sin (2π f₀t)/FSR in its argument, leading to Bessel function expansions and harmonics of frequency f₀ [23]. The final result is that the cosine equals a linear combination of an expansion in even harmonics and an expansion in odd harmonics. The prefactors of the expansions are cos (2πλ_r/FSR + φ_i + ψ_e) and sin (2πλ_r/FSR + φ_i + ψ_e), respectively. In addition, a DC term is part of the expansion in even harmonics, which as a result of the above prefactor depends on φ_i. The DC term and the term p in Eq. (8) add up to the total DC signal contribution to $V_i\left({\widetilde{\delta \lambda }}_r\left(t\right)\right)$.

Using the values of φ_i, two mutual orthogonal voltages V_x and V_y can be calculated from the V_i :

The phase modulation Φ(t)induced by the acoustic waves can be retrieved from V_x and V_y using the arctangent function:

Here Φ₀ = 2πδ₀/FSR is the phase-modulation amplitude and ΔΦ = 2πλ_r/FSR + ψ_e is a phase offset.

Eqs. (8)-(11) suggest the following interrogation procedure. First, for a known ultrasound pressure applied to the sensor, time traces of the signals V_i (t)are measured. Then, from these the orthogonal voltages V_x (t) and V_y (t) are constructed, leading to the phase modulation Φ(t). Applying fast Fourier transform (FFT) to Φ(t) leads to Φ₀ = 2πδ₀/FSR, from which finally δ₀ is obtained.

3.2. Matching of the MediGator and the ring-resonator sensor

The MediGator should be sensitive enough to detect a resonance-wavelength modulation below 1 pm. Given Φ₀ = 2πδ₀/FSR, a smaller FSR of the MZI leads to a higher phase modulation, i.e., a higher sensitivity. However, due to the exponential prefactor of the cosine in Eq. (8), the oscillatory part of V_i strongly decreases with decreasing FSR. To answer the question of the optimum matching of the MediGator and the RR sensor or equivalently the combination of FSR and γ_r giving the maximum signal, we consider the FSR dependence of the sensitivity of V_i to small changes of λ_r, i.e., the function

To find the optimum FSR, we apply the condition ∂S/∂(FSR) = 0. In this, we treat the sine function in Eq. (12) as a constant. This makes sense, since considering the FSR dependence of |∂V_i/∂λ_r | [effectively done when considering ∂S/∂(FSR)] should not be affected by a changing argument of the sine. Thus, the phase offset ΔΦ = 2πλ_r/FSR + ψ_e is constant or λ_r keeps pace with a change of FSR, such that λ_r remains at the same phase position of T_MZI,i. The condition ∂S/∂(FSR)= 0 then readily leads to the matching condition FSR = π ×γ_r. For γ_r ≈100 pm (typical value of our RRs), this implies an optimum FSR of about 314 pm.

To gain further insight in the signals V_i (t) given by Eq. (8) and their FSR dependence, we performed calculations using MATLAB [24]. We first calculate time traces V_i (t), using the input parameters p = q = 1, λ_r = 1550 nm, γ_r = 100 pm, δ₀ = 35 pm, f₀ = 1 MHz and FSR = π ×γ_r (optimum matching). Fig. 3(a) shows traces V_i (t). To obtain these V_i(t) traces, we choose ψ_e in the argument of the cosine of Eq. (8) such that a minimum of T_MZI,1(λ) coincides with λ_r, or equivalently that sin (2πλ_r/FSR + φ₁ + ψ_e)= 0. This is close to the real case in the interrogation experiments reported below. V₂ and V₃ in this situation are the strongest signals of equal strength, both for the DC and AC component. The latter component has a period of 1 µs, corresponding to the ultrasound frequency f₀. V₁ is dominated by frequency 2 f₀, in agreement with the Bessel function expansions of the cosine in Eq. (8) and the resonance coinciding with a minimum of T_MZI,1(λ).

 

Fig. 3 Calculated output signals of the MediGator, for the interrogation situation specified in the text. (a) Output signals V_i in the time domain. Signal V₁ is dominated by the 2nd harmonic of the ultrasound frequency f0 due to the alignment of the RR resonance wavelength to a minimum of the MZI transmission at output 1. (b) Amplitude of the signals V_i as a function of the dimensionless ratio FSR/(πγ_r). The signal amplitude cannot be directly compared with the signal magnitude in (a) in view of the much weaker modulation amplitude.

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The FSR dependence of the oscillatory component of V_i is obtained from traces as in Fig. 3(a), by calculating from these the amplitude as a function of FSR, in this case for δ₀ = 1 pm, the required detection limit. In the sweep of FSR, we again maintain a constant phase offset ΔΦ = 2πλ_r/FSR+ ψ_e. The other parameters are kept as above. We calculate the FSR dependence for 30 random values of ΔΦ in the range [0, 2π] using MATLAB, to check whether the general behavior of the FSR dependence depends on ΔΦ. This turns out to not be the case. A typical result is presented in Fig. 3(b), for ΔΦ = 100.26^°. The maximum of the three curves occurs for FSR/(π ×γ_r)= 1, in agreement with the above analytical result for optimum matching. It follows that in practice a deviation from optimum matching should be weak in view of the strong FSR dependence of the amplitude of V_i, in particular for FSR values below the one for optimum matching.

4. Characterization of the ring-resonator sensor, the light source and the Mach-Zehnder interferometer

Using the characteristics of the fabricated RR sensor, we arrived at the final design of the MZI and the light source and then fabricated these MediGator components, of which we present the characterization.

The sensor is characterized from its optical transmission without applied ultrasound, by sweeping the wavelength of a tunable laser (Santec, TSL-550, output power 0.5 mW, step size 5pm) coupled to the sensor input and by measuring the output power at the sensor drop port with a photodetector (Newport, 1811-FC-AC). Between the laser and the sensor a polarization controller (Thorlabs, FPC562) is used to set the optimum polarization for the grating coupler. In Fig. 4(a) the resulting normalized transmission of the peak centered at 1550.755 nm is plotted (this peak is used in the interrogation experiments below), along with a fit of Eq. (2) to the data points. The fit describes the measured transmission very well, leading to γ_r = 108.17 pm. The FSR of the RR sensor is 5.02 nm.

 

Fig. 4 (a) Transmission peak of the RR sensor. The red curve is a fit of Eq. (2) to the data points. (b) Emission spectra of the MediGator light source. Straining of the FBG results in the observed red shift of the spectra, i.e., the source is tunable. The spectra have a -3 dB bandwidth of 1.5 nm and a maximum power density of 9 dBm/nm. The tuning range shown is 5.7 nm. (c) MZI transmission spectra of the three outputs V_i. The continuous curves are fits of Eq. (5) to the experimental data, giving φ_i =0^°, 121.1^° and 242.7^° (i = 1, 2, 3).

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Output spectra of the light source for a drive current of the pump laser of 600 mA (interrogation conditions), measured with an OSA (Anritsu, MS9710C), are presented in Fig. 4(b) as a function of the strain applied to the FBG. The measured spectra have a rather flat top, decay steeply and show side lobes characteristic of the FBG reflection spectrum. The power density and the spectral shape are virtually strain independent. The maximum power density is approximately 9 dBm/nm, about one order of magnitude higher than for a high end superluminescent diode emitting around 1550 nm [25]. The −3 dB bandwidth of the spectra is 1.5 nm, large compared to γ_r = 108.17 pm and to changes of resonance positions of a silicon RR due to typical environmental temperature variations occurring at a rate of about 80 pm/K [5]. This ensures robust alignment of the source spectrum to the resonance and a wide range of applicable resonance-wavelength modulations. In addition, the bandwidth is smaller than the FSR of the sensor. In Fig. 4(b) we demonstrate a tuning range of the source spectrum of 5.7nm. This exceeds the FSR of the sensor. The characteristics of the light source make it very suitable for actuating the sensor in the interrogation situation.

The MZI is characterized from its transmission as well, also using the tunable laser and external polarization controller to set the polarization for the PIC, which supposes TE polarization. The three output voltages V_i of the MediGator are sampled using a data acquisition system (based on National Instruments NI 5734, max. sampling rate 120 MSa/s). We find that the acquisition system has stable DC offsets and the MediGator generates constant DC voltages (offsets). These would be added to the DC components of the real signal. We calibrate the DC offsets and subtract these from the measured transmission signals. This leads to the transmission functions in Fig. 4(c), which at first glance show an approximately 120^° phase difference. This is confirmed by the phases φ_i =0^°, 121.1^° and 242.7^° (i = 1, 2, 3) extracted from the fits of Eq. (5) to the measured transmission data, which are plotted in Fig. 4(c) as well. The three outputs each have a fringe visibility of 0.99.

From the transmission data we find that FSR=391.2 pm, to be compared with the design value π ×γ_r = 340 pm. We attribute the difference to fabrication variations. The ratio FSR/(π ×γ_r) is 1.15, implying only a slight deviation from the optimum in Fig. 3(b).

From transmission measurements of the MZI we find that its stability, judged from wavelength drift of the transmission curves, amounts to 5 pm in a one hour time period in the stable lab environment. This high stability results from temperature control with the TEC. The temperature control also yields negligible drift of the three DC signals resulting from the MZI transmission drift. These properties enable long time measurements with the MediGator.

5. Experiments with the MediGator

5.1. Measurement of the frequency response of the sensor

For interrogation experiments, the frequency response of the sensor should be known. We measure the frequency response using time-delay spectrometry [26]. We use the interrogator setup of Fig. 2, but replace the data acquisition system and arbitrary waveform generator (AWG) by a frequency spectrum analyzer (FSA, Rigol, DSA815-TG) and its tracking generator, respectively. The sensor and a transducer (Olympus, V314-SU) are coaxially mounted 240 mm apart on the opposite sides of a U-shaped frame in a water tank (see Fig. 2), giving a normal incidence of ultrasound onto the sensor. Using the OSA we align the light-source spectrum to the resonance peak by tuning the FBG and then couple the sensor drop port to the MZI via a polarization controller.

The tracking generator drives the transducer with a sine wave that is swept in frequency and the MediGator signals V_i are sequentially measured by the FSA. The bandwidth filter of the FSA (width 3 kHz) is swept at the same rate as the drive voltage, taking into account the time delay of the acoustic waves between the transducer and the sensor. The frequency is swept from 0.5 MHz to 1.5 MHz. For each V_i we average 50 individual frequency sweeps to obtain the final results, which still include the transducer’s frequency characteristic. From these we choose the result for output 3, which gives the highest signal. Among the three outputs, output 3 is closest to the maximum sensitivity according to Eq. (12). The maximum resonance-modulation amplitude is 2.5 pm. By using such small modulation, the second harmonic in V₃ is negligible, which ensures an accurate measurement. We measure the transducer’s frequency characteristic using a hydrophone (Precision Acoustics, 1mm) and correct for it, to obtain the normalized frequency response of the sensor presented in Fig. 5. The frequency response has its maximum at 0.995 MHz, the frequency of the membrane’s lowest vibrational mode. The −6 dB bandwidth, defined as the FWHM of the frequency response divided by the membrane fundamental resonance frequency, is 10.0%.

 

Fig. 5 The normalized frequency response of the RR sensor, giving the resonance frequency of 0.995 MHz.

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5.2. Interrogation experiments

For the interrogation experiments, we use the complete setup in Fig. 2. First, the pressure of acoustic waves emitted by the transducer, driven by the AWG (Rigol, DG1022), is calibrated using the hydrophone. As in Section 5.1, the light-source spectrum is aligned to the sensor resonance. Then, acoustic waves of frequency f₀ = 0.995 MHz, the resonance frequency of the membrane, are sent to the sensor. In this situation traces V_i(t) are measured, using a sampling frequency and measurement time of 30 MHz and 50 ms, respectively. The traces are post-processed using the offset subtraction and noise reduction. For noise reduction we use Gaussian bandpass filters of 100 Hz FWHM and centered at frequency f₀ = 0.995 MHz and its harmonics. Examples of time traces V_i (t)are presented in Fig. 6(a), for an ultrasound pressure of 442.4Pa. The measured traces V_i(t)are close to the calculated traces in Fig. 3(a). Compared to the case of Fig. 3(a), the RR resonance is located slightly to the right of the minimum of T_MZI,1, which gives V₃ a larger amplitude but a lower DC component compared to V₂. The high degree of resemblance of the measured and calculated traces indicates that the interrogator and the interrogation procedure work as predicted. From the V_i (t)we obtain V_x and V_y plotted in Fig. 6(b). In V_x the second harmonic is also clearly visible. From V_x and V_y and using Eq. (11), we extract δ₀ = 34.2 pm for this pressure.

In Fig. 6(c) the Fourier transform spectra of V_i (t) are presented for a pressure of 1.47 Pa, the lowest pressure used. Even at this pressure, signal spikes at 0.995 MHz about two times higher than the noise floor are clearly discernible in the transforms of V₂ and V₃, which is enough to extract the resonance-modulation amplitude. The signal in the transform of V₁ is below the noise level. The smallness of the latter signal is explained from the resonance wavelength being close to a minimum of T_MZI,1, as indicated before. In that case, according to Eq. (12) the signal amplitude of V1 is very close to zero. The V_i(t)for 1.47 Pa yield δ₀ = 126 fm. This value and the S/N ratio of about two indicate that the MediGator is capable of detecting a modulation amplitude down to 100 fm.

 

Fig. 6 (a) Measured output signals V_i when interrogating the RR sensor. The applied pressure amplitude is 442.4 Pa. A strong 2nd harmonic is observed in V₁. (b) V_x and V_y as a function of time, calculated from V_i in (a). (c) Fourier transform of the three V_i for a pressure amplitude of 1.47 Pa. For V₂ and V₃, spikes at the ultrasound frequency 0.995 MHz clearly stand out of the noise floor at this low pressure.

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Finally, we determine the sensitivity of the sensor, defined as ∂δ₀/∂P, where P is the pressure of the incident ultrasound. The sensitivity is obtained by interrogating the sensor for 24 pressures in the range 1.47 −442.4 Pa and determining the related δ₀ values. The results are plotted in Fig. 7, along with a linear fit to the data points. The plot shows excellent linearity of the relation between the pressure and the modulation amplitude across the whole pressure range. The slope of the fitted line is 77.2 fm/Pa, which is the sensitivity. For comparison, we also determine the sensitivity using the modulation method explained in Section 1, for which the results are shown in Fig. 7 as well. Very good agreement of the two methods is found for pressures below 150 Pa, leading to the same sensitivity value for both methods. Above 150 Pa, however, the modulation-method points lie increasingly below the MediGator points. This limitation of the modulation method arises from the short linear flank of the transmission peak. We emphasize that the MediGator does not have this limitation, leading to high quality interrogation up to the highest pressure used here, which is not limited by the MediGator itself.

 

Fig. 7 Amplitude of the resonance-wavelength modulation of the sensor as a function of the ultrasound pressure, measured with the MediGator. The inset zooms in on the low pressure range. The line is a linear fit to the MediGator data points. Its slope is the sensitivity, which amounts to 77.2 fm/Pa. For reference, results obtained with the modulation method are presented as well. The modulation method only agrees with the MediGator approach below 150 Pa. Above this value the modulation method increasingly underestimates the resonance-wavelength modulation.

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6. Conclusion and outlook

We designed, fabricated and demonstrated an integrated photonics interrogator, called MediGator, for interrogating a ring-resonator (RR) ultrasound sensor. The MediGator comprises i) a novel light source with proper bandwidth around 1550 nm, high power density and large tuning range and ii) an InP Mach-Zehnder interferometer (MZI) with a free spectral range matched to the sensor characteristic. Using its tunable FBG, the light-source spectrum can always be aligned to a single resonance peak of the RR spectrum, the feature for ultrasound sensing. The sensor in the present interrogation demonstration has a 84 µm ×84 µm square membrane, on which the RR is located, and works in the MHz range. Our mathematical description of the signal flow in the MediGator and sensor, apart from yielding matching of the MediGator to the sensor for maximum MZI outputs, leads to the interrogation procedure applied in the experiments. Thanks to the small size of the integrated photonics MZI with the 3 ×3 MMI output and photodetectors, its temperature can be accurately regulated during long-time measurements for avoiding environmental phase drift, an issue of our previous fiber MZI based interrogator.

Using the MediGator, we obtain the sensor’s acoustic frequency response, giving a resonance frequency of 0.995 MHz and a −6 dB bandwidth of 10.0%. Interrogation experiments show high MediGator performance, reflected in ease of aligning the light source to the RR resonance and high linearity of the plot of applied acoustic pressure versus resonance-wavelength modulation. This results in a sensitivity of 77.2 fm/Pa of the sensor. The minimum ultrasound pressure detected is 1.47 Pa, resulting after FFT of the measured signals. Compared to the modulation method, the MediGator exhibits a three times larger measurement range, which in the experiments is limited by the highest applied pressure and not by the MediGator itself. In spite of one MediGator output signal accidentally being rather close to zero as a result of the wavelength position of resonance peak, the MediGator operates very well, underlining the strength of using a 3 ×3 MMI at the MZI output. Summarizing, the primary merits of the MediGator are ease of operation, temperature robustness, low detection limit, large measurement range, compactness and cost effective, making the instrument very suitable for interrogating the RR ultrasound sensor.

Further improving the S/N ratio and enabling even lower detection limit are possible through several methods: improving the fiber-chip coupling, adding second-stage amplifiers after the TIAs, and further shielding of the MediGator for noise from the environment. We envision that real-time interrogation of the RR ultrasound sensor can be realized by implementing real-time signal filtering and processing to obtain the resonance-wavelength modulation. In addition, the architecture of the MZI chip can be adapted for interrogating an array of RR sensors by adding arrayed waveguide gratings or cascaded MZIs for multiplexing and demultiplexing [27, 28]. Finally, we note that owing to the temperature stabilization of the crucial parts of the MediGator, it can also interrogate RR sensors (not limited silicon ones) for sensing of signals slower than induced by ultrasound waves. This makes the MediGator highly versatile.