Optical temperature sensor with enhanced sensitivity by employing hybrid waveguides in a silicon Mach-Zehnder interferometer

: We report on a novel design of an on-chip optical temperature sensor based on a Mach-Zehnder interferometer configuration where the two arms consist of hybrid waveguides providing opposite temperature-dependent phase changes to enhance the temperature sensitivity of the sensor. The sensitivity of the fabricated sensor with silicon/polymer hybrid waveguides is measured to be 172 pm/°C, which is two times larger than a conventional all-silicon optical temperature sensor (~80 pm/°C). Moreover, a design with silicon/titanium dioxide hybrid waveguides is by calculation expected to have a sensitivity as high as 775 pm/°C. The proposed design is found to be design-flexible and robust to fabrication errors.

Diverse silicon photonic structures have been utilized to achieve temperature sensors, including ring resonators [11][12][13][14][15], Bragg reflectors [16,17], Fabry-Perrot (FP) cavities [18], and interferometers [19][20][21].However, the temperature sensitivity of above demonstrated sensors are typically limited to ~80 pm/°C.Recently, a silicon optical temperature sensor leveraging the Vernier effect of two cascaded ring resonators was demonstrated [23].While it achieved an enhanced temperature sensitivity of 293.9 pm/°C, the performances inherently depend on the fine tailoring of the free spectral regions (FSRs) of the two rings and, therefore, introduces restrictions to the fabrication tolerance.
In this paper, we propose and experimentally demonstrate a temperature sensor on silicon with an enhanced temperature sensitivity by employing an unbalanced MZI configuration with hybrid waveguides.The two arms of the MZI consist of non-identical optical waveguides providing opposite phase changes of the transmitted modes with respect to the temperature variation.While a silicon nanowire itself having silica or air cladding transmits a mode with a positive temperature-dependent phase change, a negative temperature-dependent phase change can be realized in a waveguide with negative-TOC material as the core.Materials with a negative TOC like polymers [24] and titanium dioxide (TiO 2 ) [25] have already been utilized as the cladding for a narrowed silicon waveguide to reduce the temperature dependence of the silicon photonic devices, but few reports have been published on utilizing these materials in temperature sensors.Recently, an MZI-based silicon temperature sensor with TiO 2 overlaying both arms have achieved a large temperature sensitivity of −340 pm/°C, since the TiO 2 used in the work has an unexplainably ultrahigh TOC of −5 to −7 × 10 −4 /°C [26].In our work, the negative-TOC material is used as the waveguide core in one arm of the proposed unbalanced MZI-based temperature sensor and, therefore, the temperature-dependent phase differential of the two arms is further intensified.The temperature sensitivity of the proposed device is measured to be 172 pm/°C utilizing silicon (Si) / polymer (SU-8) hybrid waveguides, and is calculated to be as high as 775 pm/°C if we employ Si/TiO 2 hybrid waveguides with the same large TOC of TiO 2 as in [26].The present design can be much more compact (arm lengths 30 μm) compared with the one proposed in [26], which is longer than 360 μm.Furthermore, our design is found to be quite design-flexible and fabrication-tolerant.

Principle and design
In a temperature sensor based on an unbalanced MZI with the two arms denoted as arm 1 and arm 2, the temperature sensitivity S, i.e., the interference wavelength shift with respect to the temperature T, is given by [27] where λ is the interference wavelength, L 1 (L 2 ) is the length of arm 1 (arm 2), n g1 (n g2 ) and n eff1 (n eff2 ) correspond to the group index and the effective index of the mode propagating in arm 1 (arm 2), respectively.To simplify the design, we consider the situation where the two arms are of equal lengths, i.e., L = L 1 = L 2 .In this way, expression (1) can be written as , and, therefore, is independent on the arm lengths.In order to increase S, one way is to increase the differential of the phase change of the two arms with respect to the temperature, i.e., maximize dn eff1 /dT-dn eff2 /dT.This can be done more easily by employing materials with positive and negative TOCs in the waveguide cores in arm 1 and arm 2, respectively.Following the principle above, we schematically present our design of the MZI-based temperature sensor on silicon in Fig. 1.Light is split into two parts by a power splitter based on a directional coupler and the two parts recombine in a second directional coupler after having propagated along arm 1 and arm 2. For arm 1, the waveguide is a conventional singlemode silicon waveguide with the same width (w 1 ) as the directional couplers.In arm 2, there is a narrow (w 2 <200nm) silicon nanowire in the center with two tapers connecting to the single-mode silicon waveguides with widths w 1 .An SU-8 polymer waveguide is covering the narrow nanowire part and the tapers.Here, SU-8 is used because it has a negative TOC (−1.21 × 10 −4 /°C) [28], and is easily fabricated and biocompatible [29], which suggests a potential for the proposed temperature sensor to be applied in biosensing applications.The lengths of the narrow nanowire and the tapers are denoted as L and L t , respectively.Inset (a) in Fig. 1 shows the cross-section of arm 1 and the power distribution of the transversemagnetic (TM) mode.The height of the silicon waveguide is fixed at 250 nm and the upper cladding is air.From the power distribution, one can see that the mode is tightly confined in the silicon and, therefore, has a positive temperature-dependent phase change.Inset (b) in Fig. 1 shows the cross section of the waveguide in the narrow nanowire part of arm 2 and the power distribution of the TM mode is also given for a nanowire width of 40 nm.We use the configuration of having a narrow silicon nanowire overlaid with a SU-8 waveguide cladding rather than having a pure SU-8 waveguide to avoid the losses generated from the abrupt terminals of the tapers.From the power distribution in inset (b), one can see that almost all the light of the mode is leaked to the SU-8 and, hence, will have a negative temperaturedependent phase change.Figures 2(a) and 2(b) show the calculated group index (n g ) and the effective index variation with the temperature (dn eff /dT) of the waveguides in arm 1 and arm 2 when the width of the silicon part changes.Here, the wavelength is 1550 nm and the silicon height, the SU-8 height (h SU-8 ) and the SU-8 width (w SU-8 ) are fixed at 250 nm, 2930 nm and 3850 nm, respectively.For SU-8, the refractive index and the TOC are 1.574 and −1.21 × 10 −4 /°C, respectively [28].It is reasonable that both the group index and the variation of the effective index with the changing temperature decrease for a smaller silicon width for both arms, since there will be less light confined in the silicon.Particularly, the effective index variation with changing temperature starts to be more and more negative as the silicon width decreases below ~230 nm and reaches the minimum (−1.2 × 10 −4 /°C), close to the TOC of SU-8 material.Figures 2(c From Fig. 2(d), one can find that the temperature sensitivity generally increases when w 2 increases from ~120 nm and w 1 decreases.Although a larger w 2 and a smaller w 1 will give a higher temperature sensitivity, it will also give a larger FSR as shown by Fig. 2(c), since the differential between n g1 and n g2 is smaller.Knowing that the spectral output of an MZI has a trigonometric shape, a large FSR indicates a tiny power change per unit wavelength change and will impose difficulties on the optical spectral analyzer (OSA) to determine the interference wavelength center during the spectral interrogation.When w 2 <120 nm, the temperature sensitivity more or less keeps constant, since almost all the light is localized in the SU-8.In our design, we choose w 1 = 460 nm and w 2 = 40 nm, which gives an FSR of 4.2 nm and a temperature sensitivity S of 158 nm/°C.For the proposed MZI structure, we also tune the coupling length of the input directional coupler in order to achieve a 30% / 70% splitting of power to arm 1 and arm 2, respectively, to account for the 2.6 dB excess loss in the two tapers of arm 2 [30].

Fabrication and measurement
The designed temperature sensor was fabricated in a silicon-on-insulator material having 250 nm of silicon on top of a 3 μm buried-oxide (BOX) layer.Inverse tapers with SU-8 waveguide cladding [30] were used to couple light between tapered and lensed fibers and the silicon waveguides.First-step electron-beam lithography followed by an inductive plasma etching was utilized to pattern the silicon including the MZI and the inverse taper couplers.A 2930-nm thick SU-8 (SU-8 2005, MicroChem) was spin-coated on the chip and the SU-8 structures were selectively defined on the silicon by another electron-beam lithography step.The fabricated sensors were characterized using a tunable laser (AQ4321D, Ando) and an OSA (AQ6317B, Ando) to record the transmission spectra for the sensors.A polarization controller (PC) was used before the sample to select the TM mode.The temperature of the chip was controlled by a temperature feedback system consisting of a semiconductor cooler beneath the sample holder, an integrated precision temperature sensor (LM335) inside the sample holder and a temperature controller (LDC 502, SRS).Figures 4(a) and 4(b) show the measured and normalized spectra of the temperature sensor with Si/SU-8 hybrid waveguides and the all-Si reference temperature sensor at different temperatures, respectively.Here, a silicon straight waveguide with the same fiber-waveguide couplers is utilized for the normalization.As expected, one can see that both the spectra of the proposed and the reference temperature sensors have a red shift with increasing temperature.From Fig. 4(a), one can find the FSR of the proposed temperature sensor to be 4.22 nm, which agrees well with the calculations in Fig. 2(c).The insertion loss for the sensor can also be extracted to be ~1.6 dB.From Fig. 4(b), the FSR of the reference all-Si temperature sensor is found to be ~11 nm, which agrees nicely with a MZI with an arm length difference of 50 μm.
Figure 4(c) gives the interference wavelength shift with respect to the temperature for the proposed (red circle) and the reference (blue square) temperature sensors.Here, the reference wavelengths for the proposed and the reference sensors are 1543.7 nm and 1544.6 at room temperature (24.1 °C), respectively.Linear fittings are also shown to obtain the temperature sensitivities, which are extracted to be 172 pm/°C and 70 pm/°C for the proposed and the reference temperature sensors, respectively.The measured sensitivity of the all-silicon reference temperature sensor agrees well with previous reports [16][17][18].Considering the effect of the two tapers with L t = 20 μm, the calculated temperature sensitivity is 163 pm/°C, which agrees well with the measured sensitivity.For such a measured temperature sensitivity, the proposed sensor has a potential to measure temperature changes as small as 0.006 °C considering that the current setup has a wavelength resolution of 1 pm.The sensing range defined as FSR/S can be extracted as ~25 °C.Here, the length L is increased to 290 μm.From Fig. 5(a), one can find that the temperature sensor has an extinction ratio, an insertion loss, and a temperature sensitivity (171 pm/°C) comparable to the temperature sensor having L = 190 μm in Fig. 4(a).Only the FSR is changed to 2.9 nm due to a different arm length.Therefore, for the present design, it is easy to obtain different sensing ranges, set by the FSR, without degrading the performance by simply tuning the length L. For example, one is expected to obtain a sensing range of ~100 °C by shortening L to 20 μm.Moreover, the undegraded performances shown in Fig. 5(b) for the sensor with w 2 = 90 nm also demonstrates a good fabrication tolerance of the proposed temperature sensor.Full CMOS-compatibility is usually desired for mass-productive and low-cost integrated sensors.It is well known that TiO 2 is CMOS-compatible.Moreover, TiO 2 has a negative TOC and, therefore, can be exploited to further increase the sensitivity of our hybrid temperature sensor.TiO 2 deposited by different methods and at different temperatures can provide different refractive indices and TOCs as summarized by Lee in [31].In the following calculations, we use the same refractive index (2.13) and TOC (−7 × 10 −4 /°C) of the TiO 2 as in [26].Replacing the SU-8 in Fig. 1 with TiO 2 , we can obtain a full CMOS compatible temperature sensor with Si/TiO 2 hybrid waveguides.Figure 6 presents the calculated FSR and the temperature sensitivity S at the wavelength 1550 nm of the proposed sensor for different TiO 2 waveguide widths (w TiO2 ) and different TiO 2 waveguide heights (h TiO2 ).Here, w 2 = 40 nm and in order to get a large FSR (sensing range), we choose a short arm length of 30 μm.It is noteworthy that a longer arm length will not degrade the temperature sensitivity.Also noteworthy, here we use the transverse-electric (TE) mode since for this polarization the TiO 2 does not need to be very thick to confine light in the TiO 2 and, hence, will make the fabrication easier.From Fig. 6, one can find that the temperature sensitivity can be as high as 775 pm/°C when w TiO2 = 700 nm and h TiO2 = 600 nm.Meanwhile, the corresponding FSR is 43.5 nm and the sensing range is ~56 °C.For the fabrication of the TiO 2 waveguides, one can use dry etching employing a chromium mask as demonstrated in [32].

Conclusion
In conclusion, we have proposed and experimentally demonstrated a Mach-Zehnder interferometer-based temperature sensor with hybrid waveguides on silicon.In the present design, one arm is silicon waveguide and the other is a hybrid waveguide with a negative thermo-optic coefficient enhancing the temperature sensitivity.A hybrid silicon/SU-8 waveguide is employed to experimentally verify the principle and a temperature sensitivity is measured to be 172 pm/°C, which is two times larger than the reference all-silicon Mach-Zehnder interferometer temperature sensor (70 pm/°C) and the conventional all-silicon temperature sensors with other configurations (~80 pm/°C).Our hybrid design has a good flexibility where one can simply change the arm lengths to obtain different sensing ranges while simultaneously keeping the performances.Temperature sensors with different arm lengths and widths have been fabricated and reveals no obvious performance degradation, which suggests a good fabrication tolerance.Furthermore, the proposed design is calculated to give a temperature sensitivity as high as 775 pm/°C by employing a hybrid silicon/TiO 2 waveguide.This paves the way for full CMOS compatible optical temperature sensors with high sensitivities for use in e.g.bio sensing or chemical analysis.

Fig. 1 .
Fig. 1.Schematic of the proposed MZI-based temperature sensor with Si/SU-8 hybrid waveguides.Insets (a) and (b) show the cross section and the mode (power) distribution of the waveguides in arm 1 and in the narrow nanowire part of arm 2, respectively.
Figures 2(a) and 2(b) show the calculated group index (n g ) and the effective index variation with the temperature (dn eff /dT) of the waveguides in arm 1 and arm 2 when the width of the silicon part changes.Here, the wavelength is 1550 nm and the silicon height, the SU-8 height (h SU-8 ) and the SU-8 width (w SU-8 ) are fixed at 250 nm, 2930 nm and 3850 nm, respectively.For SU-8, the refractive index and the TOC are 1.574 and −1.21 × 10 −4 /°C, respectively[28].It is reasonable that both the group index and the variation of the effective index with the changing temperature decrease for a smaller silicon width for both arms, since there will be less light confined in the silicon.Particularly, the effective index variation with changing temperature starts to be more and more negative as the silicon width decreases below ~230 nm and reaches the minimum (−1.2 × 10 −4 /°C), close to the TOC of SU-8 material.Figures2(c) and 2(d) show the calculated FSR and the temperature sensitivity S, respectively, of the proposed temperature sensor as a function of the width of the silicon nanowire in arm 2, when the width of arm 1 is fixed at different widths.Here, the FSR is calculated by

Fig. 2 .
Fig.2.The group index (a) and the effective index variation with the temperature changes (b) as a function of the waveguide width in arm 1 or arm 2. The FSR (c) and the temperature sensitivity (d) of the proposed temperature sensor as a function of the width of the silicon nanowire in arm 2, when the width of arm 1 is 300 nm (blue diamond), 360nm (red square) and 460 nm (green circle).Here, the wavelength is 1550 nm.

Fig. 3 (
a) shows a scanning electron microscopy (SEM) image of the fabricated temperature sensor with Si/SU-8 hybrid waveguides.The inset in the green frame in Fig.3(a)shows the close-up view of the interface part of the Si waveguide and the SU-8 waveguide and one can find that the two waveguides are well-aligned.Since the SU-8 waveguide is quite wider (3850 nm) than the silicon waveguide (460 nm), ultra-precise alignment is not necessary and, therefore, the present design is relatively fabrication tolerant.As a reference for the performances of the proposed temperature sensor with hybrid waveguides, an all-silicon MZI-based temperature sensor with an arm length difference of 50 μm was fabricated and is also shown on the SEM image in Fig.3(b).

Fig. 3 .
Fig. 3. SEM image of the proposed temperature sensor with (a) Si/SU-8 hybrid waveguides and (b) the all-Si temperature sensor for reference.The inserted SEM image in the green frame in (a) shows the close-up view of the interface part of the Si waveguide and the Si/SU-8 waveguide.

Fig. 4 .
Fig. 4. Measured and normalized spectra of the proposed temperature sensor with (a) Si/SU-8 hybrid waveguides and (b) the reference all-Si temperature sensors recorded at different temperatures.(c) The shift of the interference wavelength as a function of the temperature for the proposed (red circle) and the reference (blue square) sensors.Here, the wavelengths for the proposed and the reference sensors are 1543.7 nm and 1544.6 at 24.1 °C, respectively.

Figure 5
Figure5show the measured and normalized spectra of the proposed temperature sensor with Si/SU-8 hybrid waveguides at different temperatures when the silicon narrow nanowire in arm 2 is (a) 40 nm and (b) 90 nm.Here, the length L is increased to 290 μm.From Fig.5(a), one can find that the temperature sensor has an extinction ratio, an insertion loss, and a temperature sensitivity (171 pm/°C) comparable to the temperature sensor having L = 190 μm in Fig.4(a).Only the FSR is changed to 2.9 nm due to a different arm length.Therefore, for the present design, it is easy to obtain different sensing ranges, set by the FSR, without degrading the performance by simply tuning the length L. For example, one is expected to obtain a sensing range of ~100 °C by shortening L to 20 μm.Moreover, the undegraded performances shown in Fig.5(b) for the sensor with w 2 = 90 nm also demonstrates a good fabrication tolerance of the proposed temperature sensor.

Fig. 5 .
Fig. 5. Measured and normalized spectra of the proposed temperature sensor with Si/SU-8 hybrid waveguides at different temperatures for the width of the narrow nanowire of arm 2 is 40 nm (a) and 90 nm (b), respectively.Here, the length of the narrow nanowire L is 290 μm.

Fig. 6 .
Fig. 6.Calculated FSR and temperature sensitivity of the proposed temperature sensor with Si/TiO 2 hybrid waveguides as a function of the TiO 2 waveguide width at different TiO 2 waveguide heights.Here, the arm lengths are 30 μm and w 2 = 40 nm.