High Thermo-optic Coefficient Research Articles

Microstructured optical fiber filled with glycerin for temperature measurement based on dispersive wave and soliton

A nonlinear temperature sensor was proposed drawing on dispersive wave (DW) and soliton in an in-house made silica microstructured optical fiber (MOF). Glycerin with high thermo-optical coefficient was filled into the MOF air holes as the thermal medium. Using a 1050 nm femtosecond laser as the pump source, DW and soliton were experimentally and theoretically explored for temperature sensing by detecting their central wavelength shift of 3-dB bandwidth. The results showed that the higher the average pump power, the higher the temperature sensitivity of DW and soliton. When the DW blue-shifted relative to the soliton, the temperature sensitivity of DW was higher than that of soliton, and the experimental maximum values were 0.928 nm/℃ (DW) and -0.923 nm/℃ (soliton). Our work explored the nonlinear phenomenon for optical fiber-based sensing technology and verified that both DW and soliton can be used to realize temperature sensors of high sensitivity and good mechanical strength. The sensing characteristic difference of DW and soliton revealed in this work might provide a new way for solving the cross-sensitivity of dual-parameter sensing in biological engineering, disease detection and environmental monitoring without introducing extra structural complexity.

Mode transition induced enhanced temperature sensitivity in long period waveguide gratings near dispersion turning point

We propose a highly sensitive temperature sensor based on long period grating inscribed high index coated ridge waveguide in which the unique feature of mode transition (MT) phenomenon combined with the high thermo optic coefficient (TOC) polymer ambient elevates the differential thermal dependence between the participating modes. Using the dual resonance behavior, the temperature sensitivity is found to be ∼100 nm/°C near the dispersion turning point (DTP) at around room temperature with good tolerances with respect to waveguide’s width and grating period. A judicious grating design has also been formulated to operate the proposed structure near the DTP for temperatures other than room temperature including human body temperature, making the device an excellent choice for different bio and chemical applications. A comparative study of temperature sensitivity near the DTP with a conventional ridge waveguide is used to show the importance of the MT in obtaining high sensitivity near the turning point.

Ultrahigh-sensitive and compact temperature sensor based on no-core fiber with PMMA coating.

A new mechanism between the temperature sensitivity and the length ratio of PMMA coating to no-core fiber (NCF) is reported to realize an optical fiber temperature sensor with ultra-high sensitivity and compact size by PMMA-coated no-core fiber. By both theory and experiment, it is found that the sensitivity has a linear response to the length ratio of PMMA coating to NCF rather than the conventional viewpoint that it depends on the length of PMMA. Based on this conclusion and the high thermo-optic coefficient of PMMA, the temperature sensitivity is significantly enhanced as high as -9.582 nm/℃ through a simple, compact, and inexpensive sensor with 5 mm NCF and 3 mm PMMA coating. Our work opens a new avenue of a significant increase in the detection sensitivity of miniaturized fiber temperature sensors.

Tapered multicore fiber interferometer for ultra-sensitive temperature sensing with thermo-optical materials.

An in-line interferometer based on tapered multicore embedded into a flexible thermo-optical material is proposed and investigated, theoretically and experimentally. The device consists of a tapered multicore fiber spliced between two single-mode fibers covered with PDMS, with high thermo-optic coefficient. The temperature sensitivity improvement obtained from PDMS applied on a tapered multicore fiber (TMCF) interferometer has been fundamentally and experimentally verified. The experimental results show the temperature sensitivity can be improved by reducing the tapered waist diameter of TMCF. The sensor exhibits the high sensitivity of 5-25 nm/°C within the decreasing temperature range from 50 °C down to 10 °C. A sequence of simulations and corresponding experiments are performed to clarify the evolution of the interference fading and consequently build the criteria for sensor design and reachable lower limit of temperature sensing. The proposed sensor can be employed as photonic thermometer with ultra-high sensitivity for biological and deep-sea applications, particularly based on the claimed quantitative criteria.

Thermo-Optic Effects in Congruent-LiTaO 3 Based Continuous-Wave Optical Parametric Oscillator

We present an experimental study on single-frequency, singly-resonant optical parametric oscillator (SRO) comprised of 50 mm long congruently-grown, 8-mol%-doped, periodically-poled LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (MgO:cPPLT) crystal. The SRO is pumped by a single-frequency, fiber laser pumped green laser. Our observations reveal that the SRO output (idler) scales linearly with the pump power ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ) near the SRO threshold ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p(th)</sub> ). However, the SRO idler power ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> ) exhibits strong saturation at pump power ≈ 1.5P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p(th)</sub> which eventually switches-off the SRO operation on further pump-power enhancement. By coarsely tuning the MgO:cPPLT crystal temperature from 50 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C to 170 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C, the SRO idler wavelength is varied from 1810-1920 nm wavelength band. A maximum idler power P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> =583 mW was obtained when the input pump power was ≈ 3.1 W. The idler power saturation is accompanied by discernible distortion in idler beam profile. An early onset of saturation in MgO:cPPLT is essentially a consequence of strong linear absorption exhibited by MgO:cPPLT at pump wavelength, its high thermo-optic coefficient as well as low thermal conductivity. The results are theoretically analyzed by solving the heat conduction equation which provides a deeper insight about the impact of pump-induced thermo-optic effects on a three-wave interaction process.

Ultrasensitive temperature sensor and mode converter based on a modal interferometer in a two-mode fiber.

This paper presents an ultrasensitive temperature sensor and tunable mode converter based on an isopropanol-sealed modal interferometer in a two-mode fiber. The modal interferometer consists of a tapered two-mode fiber (TTMF) sandwiched between two single-mode fibers. The sensor provides high-sensitivity temperature sensing by taking advantages of TTMF, isopropanol and the Vernier-like effect. The TTMF provides a uniform modal interferometer with LP01 and LP11 modes as well as strong evanescent field on its surface. The temperature sensitivity of the sensor can be improved due to the high thermo-optic coefficient of isopropanol. The Vernier-like effect based on the overlap of two interference spectra is applied to magnify the sensing capabilities with a sensitivity magnification factor of 58.5. The temperature sensor is implemented by inserting the modal interferometer into an isopropanol-sealed capillary. The experimental and calculated results show the transmission spectrum exhibit blue shift with increasing ambient temperature. Experimental results show that the isopropanol-sealed modal interferometer provides a temperature sensitivity up to -140.5 nm/°C. The interference spectrum has multiple dips at which the input LP01 mode is converted to the LP11 mode. This modal interferometer acts as a tunable multi-channel mode converter. The mode converter that can be tuned by varying temperature and mode switch is realized.

Ultra-Broadband Polymer 1 × 2 Thermo-Optic Mode Switch

Optical mode switch is a key device for reconfigurable mode-division-multiplexing (MDM) technique. Here, an ultra-broadband 1×2 mode switch based on cascaded Mach-Zehnder interferometer and asymmetric Y-branch is proposed. The input E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> or E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode will be converted to E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> mode and switched between two output ports by changing the working state of electrode. When E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> mode is launched, the measured insertion loss of fabricated switch is 3.3 dB (off-state)/3.4 dB (on-state), and corresponding crosstalk is -18.8 dB (off-state)/-16.1 dB (on-state) at 1550 nm, respectively. Due to the adoption of polymer with high thermo-optic coefficient as the waveguide material, the measured switching power is 10.3 mW. The captured far-field output mode patterns prove the wavelength independence of proposed mode switch within the range from 1500 to 1630 nm. This broadband operation mode switch has potentials in wavelength-division-multiplexing & MDM complex systems, as well as cost-effective low-end switching applications.

Demonstration of an Ultra-Sensitive Temperature Sensor Using an Asymmetric Mach-Zehnder Interferometer

We present the design, fabrication and characterization of an on-chip, ultra-high sensitivity temperature sensor using asymmetric Mach-Zehnder interferometer (MZI) on silicon-on-insulator (SOI) platform. Both the group index and the sensitivity differences between two arms are considered for simultaneous sensitivity and linear range enhancement. The fabrication of the sensor only needs single lithography without introducing any high thermo-optic coefficient (TOC) materials. The experimental results demonstrate a sensitivity of 445 pm/°C from 27 °C to 67 °C, which is five times larger than a conventional all-silicon photonic temperature sensor (~80 pm/°C).

Highly-sensitive and wide-range temperature sensor based on polymer-filled micro-cavity in fibre Bragg grating by temperature segmentation

A highly-sensitive and wide-range temperature sensor based on a polymer-filled Mach-Zehnder (MZ) micro-cavity in commercial fibre Bragg grating (FBG) was proposed. The micro-cavity was fabricated rightly across half of the fibre core at the FBG position by using femtosecond laser micromachining, and then it was filled with the polymer with high thermo-optical coefficient. A temperature segmentation demodulation method was presented by monitoring the MZ peak and FBG peak in the transmission spectrum synchronously, which expanded the temperature measurement range to more than 80 °C with ultra-high sensitivity (9.33 nm/°C) and perfect linearity.

Cascaded fiber MZIs for simultaneous measurement of pressure and temperature

A hybrid cascade structure composed of two Mach–Zehnder interferometers (MZI) is proposed and demonstrated to measure gas pressure and temperature, simultaneously. The key sensing components of the two MZIs are all based on a tapered single mode fiber (TSMF) spliced between two multimode optical fibers. The difference is that one of the TSMFs is covered with PDMS material, while the other one is exposed to air. Because of the large changes in the refractive index versus air pressure and the high thermo-optic coefficient of PDMS, the MZI covered with PDMS has a high sensitivity to gas pressure and temperature. While the MZI exposed to air has only high gas pressure sensitivity because the air is directly involved in the sensing as a signal of interferometer and the effective refractive index of air varies with the pressure. Experimental results show that the gas pressure and temperature sensitivities of the MZI filled with PDMS can be as high as 78.553 nm/MPa and 8.455 nm/°C, respectively, and another MZI can obtain −8.405 nm/MPa and 0.038 nm/°C, respectively. Therefore, the simultaneous measurement can be achieved by utilizing two MZIs with different gas pressure and temperature sensitivities.

High sensitivity and ultra compact fiber-optic microtip SPR thermometer coated with Ag/PDMS bilayer film

In this paper, we report an ultra-compact reflective SPR microtip temperature probe, which can achieve 2-D spatial resolution of several microns. The pencil-shaped all-fiber microtip proposed by us is a SO2 cone-cylinder with a length of 40 μm and a gradient diameter of 2 μm-10 μm, which can be used to develop multi-type tiny functionalized sensors. Due to the strong evanescent field of the ultra-fine microtip, the surface plasmon resonance (SPR) can be effectively stimulated by depositing nano-silver (Ag) film on its surface. Owing to the high refractive index (RI) sensitivity of SPR effect and the high thermo-optic coefficient (TOC) of polydimethylsiloxane (PDMS), the temperature sensitivity of microtip SPR probe coated with PDMS micron-film can reach to 1.985 nm/°C. Teflon 1600 nano-film on the surface of the SPR thermometer can effectively enhance the non-stickiness, moisture-proof and wear resistance. The test results show that the probe is hardly disturbed by humidity. This new sensing platform can be demonstrated by assembling high-performance sensing materials and ultra-compact fiber-optic structures.

Ultra-high sensitivity SPR temperature sensor based on a helical-core fiber.

A novel compact ultra-high sensitivity optical fiber temperature sensor based on surface plasmon resonance (SPR) is proposed and demonstrated. The sensor is fabricated by employing a helical-core fiber (HCF), which is polished as a D-type fiber on the helical-core region and coated with a layer of Au-film and polydimethylsiloxane (PDMS). The theoretical and experimental results show that the resonant wavelength and sensitivity of the proposed sensor can be effectively adjusted by changing the twisting pitch of HCF. Due to the high refractive index sensitivity of the sensor and the high thermo-optic coefficient of PDMS, the maximum sensitivity can reach -19.56 nm/°C at room temperature when the twist pitch of HCF is 2.1 mm. It is worth noting that the sensitivity can be further improved by using a shorter pitch of HCF. The proposed SPR temperature sensor has adjustable sensitivity, is easy to realize distributed sensing, and has potential application prospects in biomedical, healthcare, and other fields.

Improving the sensitivity of WGM pressure sensors with oxyfluoride glass microspheres

Oxyfluoride glass microspheres doped Nd3+ were synthesized and calibrated to function as optical pressure sensors. When the microspheres were placed in a vacuum chamber and excited at 532 nm, Nd3+ typical emission spectrum was recorded. Due to the specific geometry of the microspheres, that allow them to act as an optical resonator, it was also possible to record Whispering Gallery Modes (WGM) superimposed to the Nd3+ typical emission spectrum. Furthermore, when decreasing pressure, changes in the Fluorescence Intensity Ratio (FIR) of the 2H9/2,4F5/2 → 4I9/2 (820 nm) and 4F3/2 → 4I9/2 (890 nm) Nd3+ transitions as well as a red-shift of the WGM was observed. Both changes were calibrated as a function of pressure. Relative sensitivity and limit of detection were estimated for both techniques as a way of comparing their performance, also taking into account the previous results found in the literature. Due to the high thermal expansion and thermo-optical coefficient of oxyfluoride glasses and the high energy gap of Nd3+ thermally coupled levels, the sensor presented groundbreaking levels of relative sensitivity and limit of detection.

Infiltrated hollow core fiber interferometer for high sensitive temperature and refractive index sensing

A simple fiber sensor based on liquid infiltrated modal interferometer created in hollow core fiber is proposed and demonstrated for temperature and refractive index (RI) sensing with high sensitivities. The fiber sensor is constructed by splicing a section of hollow core fiber filled with high index liquids between single mode fibers. The infiltration of the liquids changes the transmission property of the hollow core fiber, and modal interference is introduced in such a section of fiber. Due to the high thermo-optic coefficient of the liquid, the sensor is highly sensitive to temperature change. The sensor exhibits different temperature sensitivities in different temperature ranges, and the highest sensitivity (−1.4 nm/°C) is obtained in the temperature range from 22 °C to 36 °C. The RI response of the sensor is also investigated, and the sensor exhibits a spectrum shift towards longer wavelength as the ambient liquid RI increases, and a highest sensitivity of 884 nm/RIU is achieved. The proposed sensor is simple and cost effective, and is promising to be developed for high accuracy temperature measurements in the fields of biomedical and chemical applications.

High-sensitivity and large-range fiber optic temperature sensor based on PDMS-coated Mach-Zehnder interferometer combined with FBG.

Although numerous efforts have been dedicated towards developing fiber sensors with high performances, challenges still remain in achieving high-quality temperature sensors with high sensitivity, large measurement range and high stability. This study proposes a compact fiber optic temperature sensor based on PDMS-coated Mach-Zehnder interferometer (MZI) combined with FBG, and it can realize both high-sensitivity and large-range temperature measurement. The MZI is based on Thin No-Core Fiber (TNCF) with lateral-offset. Owing to the high refractive index sensitivity of MZI and the high thermo-optic coefficient of PDMS, the sensor can achieve a high temperature sensitivity (>10 nm/°C). Besides, by optimizing the TNCF length, the cascaded FBG can be used to locate different temperature intervals in units of approximately 10 °C, and therefore the detectable temperature range is largely extended. The experimental test demonstrates that the average sensitivities of 11.19 nm/°C, 8.53 nm/°C, 7.76 nm/°C, 7.27 nm/°C are achieved at the temperature around 30 °C, 40 °C, 50 °C and 60 °C, and it shows excellent consistency and repeatability during the thermal cycle tests.

A Fiber Ring Laser Sensor with a Side Polished Evanescent Enhanced Fiber for Highly Sensitive Temperature Measurement.

We demonstrate a highly sensitive and practical fiber-based temperature sensor system. The sensor is constructed based on a fiber ring laser (FRL) as well as a side-polished fiber filled with isopropanol. The laser cavity of the sensing part fiber is polished by the wheel fiber polishing system with residual thickness (RT) is selected to detect the temperature in the FRL. Thanks to the high thermo-optic coefficient of isopropanol, the sensitivity of the proposed temperature sensor could be effectively improved by filling isopropanol in the cost-less side polished single mode fiber. Refractive index (RI) of isopropanol changes with the surrounding temperature variation allowing high-sensitivity temperature sensing. Experimental results demonstrate that the side polished fiber can efficiently excite high-order cladding modes which enhance the modular interference increase the interaction between the evanescent wave and the isopropanol. Besides, the results show that the sensitivity can be as high as 2 nm/°C in the temperature range of 25–35 °C.

Lossy Mode Resonances Generated in Planar Configuration for Two-Parameter Sensing

This work shows a new sensor structure for simultaneous measurement of two parameters, temperature and refractive index. The optical configuration consists of incidence of light on the edge of a soda-lime coverslip fully coated with a CuO thin film and partially coated with a PDMS thick layer. This planar configuration permitted to generate two separated lossy mode resonances (LMRs): one centered at 600 nm and the other at 1000 nm. The second resonance is induced by the PDMS layer and it can be used to measure the temperature due to its high thermo-optic coefficient (the sensitivity is −1.75 nm/°C in the temperature range from 20 to 40 °C), whereas the first resonance is used for sensing refractive index with sensitivity of 1460 nm/RIU in the refractive index range from 1.3328 and 1.37. Finally, a calibration test was carried out using a calibrated oil series with refractive index ranging from 1.33 to 1.36. This work demonstrates the possibility of generating multiples resonances in a single structure as simple as a coverslip, which can be used as a multi-parameter interchangeable sensor, especially suitable for biological applications or the detection of heavy metals in water.

Electrically controlled 1 × 2 tunable switch using a phase change material embedded silicon microring.

Phase change material Ge2Sb2Te5 (GST) has recently emerged as a highly promising candidate for photonic device applications owing to its high optical contrast, self-holding bi-stability, and fast material response. Here, we propose and analyze a 1×2 tunable switch using a GST embedded silicon microring resonator exploiting high optical contrast during GST phase change and a high thermo-optic coefficient of amorphous phase GST. Our device exhibits high extinction ratios of 25.57 dB and 18.75 dB at through and drop ports, respectively, with just a 1 µm long GST layer. The two states of the switch are realizable by electrically inducing phase change in GST. For post phase change from amorphous to crystalline and vice versa, the fall time down the 80% of phase transition temperature is ∼66ns and ∼45ns, respectively. The resonance wavelength shift per unit active length is 0.661 nm/µm, and the tuning efficiency is 1.16 nm/mW. The large wavelength tunability (4.63 nm) of the proposed switch makes it an attractive option for reconfigurable photonic integrated circuits.

A Fiber-Attached Coupler for Transmission Bandpass Whispering Gallery Mode Resonator

A fiber attached coupler for transmission bandpass whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The coupler fabricated by femtosecond laser ablation consists of a pair of curved waveguides with a curvature radius of 50 μm, by which the WGM field is excited and collected through evanescent field coupling, respectively. As a proof of concept, a polymer microsphere functioning as the WGM cavity is embedded into a femtosecond laser ablated hole between the two curved waveguides. A passband with a full width of 0.15 nm at half maximum is observed when the microsphere is surrounded by air, verifying the feasibility of the proposed coupler for constructing a WGM cavity-based transmission narrow bandpass optical fiber filter. Benefitting from the relatively high thermo-optical coefficient, the passband of a packaged transmission bandpass WGM resonator can be thermo-optically tuned with a sensitivity of 446.6 pm/°C.

A highly-sensitive temperature sensor based on GeO2–SiO2 long period fiber grating

A highly-sensitive long period grating temperature sensor has been introduced here, which is composed of GeO2 in the core and SiO2 in the cladding. Due to high thermo-optic coefficient contrast of the core and the cladding, the proposed sensor provides high temperature sensitivity of S = Δλ/ΔT = −2.3871 nm/°C in the temperature range of 71 °C to 406 °C. The grating period and the length of the sensor are 445.4 nm and ∼25 mm, respectively and the operating wavelength range is λ∼1.48–2.28 μm. The proposed sensor has been investigated considering material dispersion for the core and the cladding at different temperatures. Also, the conditions required for coupling of the fundamental core mode to the higher order cladding modes with respect to lattice constant have been regarded. The phase matching curves has been considered in the design of the proposed sensor and the structure has been optimized to present the maximum sensitivity, achievable in the regarded temperature range. The presented optical temperature sensor is feasible with low cost which benefits the high sensitivity, large detection range, and large operating wavelength range and it can find potential applications in the measurement of temperature in toxic, unreachable, and far distant environments such as in temperature control of lasers, and in oil or gas industries.