Ultrafast Laser Fabrication of Novel Fiber Mach-Zehnder Interferometer Sensors and Its Cost-Effective Alternative Manufacturing Methods (Invited Paper)

The recent progresses of fiber sensor fabrication in our group are reviewed.Novel inline fiber Mach-Zehnder interferometer(MZI) sensors with various structures are proposed and manufactured by femtosecond laser fabrication and fusion splicing for high-quality sensing of refractivity-sensitive parameters such as temperature,concentration,humidity,pressure,stress and strain: a) for an MZI sensor with a trench on a single-mode fiber,the refractive index(RI) sensitivity of acetone vapor is about 104 nm/RIU(refractive index unit) and the temperature sensitivity is 51.5 pm/℃ from 200 to 875 ℃;b) For an MZI consisting of two micro-air-cavities,the sensitivity is 501.5 nm/RIU and the detection limit is 1.994×10-6 RIU at the refractive index of 1.4;c) to reduce the fabrication cost,a new fusion-splicing based method is proposed to fabricate MZI sensors;the sensitivity is 664.57 nm/RIU with a detection limit of 1.5×10-6 RIU and its cost is tens of times cheaper than those of commercialized long period fiber Gratings;Also,5×10-5 acetone vapors are successfully detected by the MZI sensors coated with zeolite thin films.

Electro-Optical Effect Measurement of Thin-Film Material Using PM Fiber Mach-Zehnder Interferometer

A polarization-maintaining （PM） fiber Mach-Zehnder （MZ） interferometer has been established to measure the EO effect of very thin film materials with optical anisotropy. Unlike a common MZ interferometer, all the components are connected via polarization-maintaining fibers. At the same time, a polarized DFB laser with a maximum power output of 10mW is adopted as the light source to induce a large extinction ratio. Here,we take it to determine the electro-optical coefficients of a very thin superlattice structure with GaAs, KTP, and GaN as comparative samples. The measured EO coefficients show good comparability with the others.

Simulation of distributed optical fiber disturbance detection system based on Sagnac/Mach-Zehnder interferometer and cross-correlation location

A distributed optical fiber disturbance detection system consisted of a Sagnac interferometer and a Mach-Zehnder interferometer is demonstrated. Two interferometers outputs are connected to an electric band-pass filter via a detector respectively. The central frequencies of the two filters are selected adaptively according to the disturbance frequency. The disturbance frequency is obtained by either frequency spectrum of the two interferometers outputs. An alarm is given out only when the Sagnac interferometer output is changed. A disturbance position is determined by calculating a time difference with a cross-correlation method between the filter output connected to the Sagnac interferometer and derivative of the filter output connected to the Mach-Zehnder interferometer. The frequency spectrum, derivative and cross-correlation are obtained by a signal processing system. Theory analysis and simulation results are presented. They show that the system structure and location method are effective, accurate, and immune to environmental variations.

A Novel Mach–Zehnder Interferometer Based on Hybrid Liquid Crystal–Photonic Crystal Fiber

We propose a novel all fiber Mach-Zehnder interferometer（MZI） based on photonic crystal fiber（PCF） filled with liquid crystal（LC）. The interference between the core mode and the cladding modes of a PCF is utilized.To excite the cladding modes, a region is formed using fiber fusion splicer. Due to the fact that varying effective index difference between the core region and the LC-filled cladding region can cause different transmission spectra,we mainly study the MZIs with different LC-filled structures and different lengths of LC filling. The measured results demonstrate that quite clear interference spectra can be obtained. Through analysis spatial frequency spectrum and temperature spectrum of two MZIs with different LC-filled structures, we can obtain that the MZI with adjacent two LC-filled holes has clearer interference spectrum and higher temperature sensitivity. Thus we choose this MZI to measure the temperature sensitivity with different lengths of LC filling. When the length of LC filling is 2 cm, the temperature sensitivities can be enlarged to 1.59 nm/C. The interferometer shows a good temperature tunability and sensitivity, which can be a good candidate for a highly tunable optical filtering and temperature sensing applications.

MZI/FPI Fiber Optic Dual-parameter Sensor Based on a Double Cone and Air Cavity Structure(Invited)

This paper proposes a Mach Zehnder/Fabry Perot Interferometer(MZI/FPI)fiber sensor based on Single-mode Fiber(SMF)and Hollow-core Fiber(HCF),which has high sensitivity to temperature and lateral loads.The proposed device consists of two single-mode fiber cones formed by manually controlling the fusion splicer and an air cavity formed by fusing a section of hollow-core fiber.The structure of the sensor is a double cone cascaded air cavity.At the beginning of the design,we compared the basic transmission spectra of single cone structure and double cone structure experimentally,and therefore chose to use double cone structure and air cavity cascade.Light undergoes its first reflection at the first interface between the single-mode fiber and the air cavity structure,and its second reflection at the second interface between the air cavity structure and the single-mode fiber.The two reflected light waves produced by the two reflections form FP interference,which can be used to measure lateral loads.The transmitted light is excited through the first cone,and a portion of the core mode light is excited to the cladding,while another portion of the core mode light continues to propagate in the core.The light couples at the second cone,and the cladding mode light couples back into the core,forming MZ interference with the core mode light,which can be used to measure temperature.The use of hollow-core fiber to form an air cavity has little effect on transmitted light,while avoiding the problem of crosstalk in dual parameter measurements.By designing temperature and lateral load experiments,this article verifies the sensitivity characteristics of this sensor to temperature and lateral loads.A significant redshift phenomenon was observed in the temperature experiment.A significant redshift phenomenon also occurred in the lateral load experiment.Through wavelength demodulation,the experimental results show that the wavelength sensitivity of the sensor to temperature is 56.29 pm/℃in the range of 30℃to 80℃.The wavelength sensitivity of the sensor to lateral loads is 1.123 nm/N in the range of 0~5 N.In addition,we have prepared multiple sets of fiber optic sensors with this structure and conducted repeated experiments to verify that the sensing performance of this structure of fiber optic sensors for temperature and lateral load is relatively stable.Also,the different waist diameters of cones will have a certain impact on the transmission spectrum of MZ,while the length of the air cavity will also have a certain impact on the reflection spectrum of FP.This article lists some fiber optic sensors for dual parameter measurement of temperature and lateral load.Compared with the listed sensors,the fiber optic sensor proposed in this article has better sensitivity to temperature and lateral load.And the fiber optic sensor proposed in this article has a simple manufacturing process,low production cost,and good performance,which has certain prospects in scientific research and industrial production.

High Sensitive Refractive Index Sensor Based on Cladding Etched Photonic Crystal Fiber Mach-Zehnder Interferometer

A high sensitive refractive index sensor based on the cladding etched photonic crystal fiber (PCF) Mach-Zehnder interferometer (MZI) is proposed, which is spliced a section of photonic crystal fiber between two single modes fibers (SMFs).The interference fringe of the MZI shifts with the variation of the ambient refractive index (RI). It is found that the RI sensitivity slightly decrease with an increase in the interference length. The sensitivities of MZI with 35inm PCF, 40mm PCF, and 45mm PCF are 106.19nm/RIU, 93.33nm/RIU, and 73.64nm/RIU, respectively, in the range of 13.33 to 1.381. After etched, the RI sensitivity of the MZI could be improved obviously. The RI sensitivities of the MZI with 35 mm PCF are up to 211.53nm/RIU and 359.37nm/RIU when the cladding diameter decreases to 112(im and 91 pm, respectively. Moreover, the sensor is insensitive to temperature, and the measured sensitivity is only 9.21 pm/℃ with the range from 20℃ to 500℃. In addition, the sensor has advantage of simple fabrication, low cost, and high RI sensitivity.

Mach-Zehnder Interferometer for High Temperature(1000℃)Sensing Based on a Few-Mode Fiber

A Mach-Zehnder interferometer(MZI)for high temperature(1000°C)sensing based on few mode fiber(FMF)was proposed and experimentally demonstrated.The sensor was fabricated by fusing a section of FMF between two single-mode fibers(SMFs).The structure was proven to be an excellent high temperature sensor with good stability,repeatability,and high temperature sensitivity(48.2 pm/C)after annealing process at a high temperature lasting some hours,and a wide working temperature range(from room temperature to 1000 C).In addition,the simple fabrication process and the low cost offered a great potential for sensing in high temperature environments.

Strain and Temperature Discrimination Based on a Mach-Zehnder Interferometer With Cascaded Single Mode Fibers

An in-fiber Mach-Zehnder interferometer is proposed for the discrimination of strain and temperature.The sensor is based on two cascaded standard single mode fibers using three peanut tapers fabricated by simple splicing.The cascaded structure excites more frequency components,which induce four sets of interference dips in the transmission spectrum.One set of the spectrum dips have different sensitivities to temperature and strain from those of the other three.The sensor can discriminate strain and temperature by monitoring the wavelength shifts of two spectrum dips.Repeated experiments are taken both for strain and temperature increasing and decreasing scenarios.Experimental results show that Dip 1 has an average strain sensitivity of-0.911 pm/με and an average temperature sensitivity of 49.98pm/℃.The strain sensitivity for Dip 2 is negligible and its average temperature sensitivity is 60.52pm/℃.The strain and temperature resolutions are±3.82με and±0.33℃.

Fiber up-taper assisted Mach-Zehnder interferometer for high sensitive temperature sensing

A new in-line Mach-Zehnder interferometer （MZI） sensor consisting of a stub of multi-mode fiber and an up-taper was proposed and demonstrated. Temperature measurement can be carried out by detecting wavelength shift. Dependency of sensitivity on interferometer length and dip wavelength was discussed. Experimental results showed a maximum temperature sensitivity of 113.6pm/℃ can be achieved, which is superior to most fiber temperature sensors based on in-line MZIs within the range from 20℃ to 80℃, also a good mechanical strength can be obtained. The proposed sensor is a good candidate for temperature measurement, due to the advantages of simple structure, easy fabrication, cost- effective and high sensitivity.

A novel time-to-live countdown scheme based on asymmetric Mach-Zehnder interferometer and Fabry-Perot semiconductor optical amplifier

We propose a novel optical time-to-live （TTL） processing scheme using asymmetric Mech-Zehnder inter-ferometer （AMZI） and Fabry-Perot semiconductor optical amplifier （FP-SOA）. AMZI transfers M TTL pulses into M-1 pulses and two residual pulses with 6-dB power difference. FP-SOA enhances the power difference between the M - 1 pulses to the residual pulses to more than 10 dB. A numerical model is established for verifying the feasibility of this scheme.

Highly sensitive salinity sensor based on Mach-Zehnder interferometer with double-C fiber

This paper proposes a highly sensitive,compact,and low-cost optical fiber salinity sensor based on the Mach-Zehnder interferometer.The sensor is constructed using a single mode fiber(SMF)-no-core fiber-double-C fiber(DCF)-NCF-SMF structure,with the DCF prepared by etching the dual side-hole fiber with HF acid.The DCF’s large-size exposed microfluidic channels solve the previous microstructured optical fiber’s challenging liquid filling and replacement problems.Theoretical simulations and experiments demonstrate that the sensor is suitable for high-sensitivity salinity measurement.The sensor exhibits a high salinity sensitivity of-2.26 nm/‰ in the salinity range of 10‰-50‰,as demonstrated by the experimental results.Additionally,the sensor exhibits some fascinating characteristics,including high repeatability,hysteresis,reversibility,and stability.

Temperature Insensitive Bending Sensor Based on In-Line Mach-Zehnder Interferometer

A simple and compact fiber bending sensor based on the Maeh-Zehnder interferometer was proposed. A photonic crystal fiber （PCF） with a length of 10mm was spliced by collapsing air holes with two conventional single mode fibers to consist of an all fiber bending sensor. The sensitivity of 0.53nm/m-1 was obtained at 1586nm for the curvature range from 0 to 8.514 m-1. The temperature sensitivity was very low. The measurement error due to the temperature effect was about 8.68x10-3 m-1/℃, and the temperature effect in the curvature measurement could be ignored. This device can avoid the cross sensitivity of the temperature in the curvature measurement.

Analysis of the Interference Signal of the Distributed Optical Fiber Sensing Based on DSP

In this paper, the author analyzes characteristics and extracting method of interference signal of the distributed optical fiber sensing. In the distributed optical fiber sensing, realizing alarm and positioning function only through the cross-correlation operation will increase the load of the system, can make misinformation rate of the system be improved greatly. Therefore, before the localization algorithm, adding a interference signal feature recognition is very necessary, can reduce unnecessary operation loss and reduce the load of the system, also reducing the number of the false positives.

Cost-Effective Fiber Multiplexing System Based on Low Coherence Interferometers and Application to Temperature Measurement

Based on the low-coherence interferometric principles, a cost-effective all-fiber Mach-Zehnder multiplexing system is proposed and demonstrated. The system consists of two interferometers： sensing interferometer and demodulation interferometer. By scanning an optical tunable delay line back and forth constantly with a stable speed, sensing fibers with different optical paths can be temporal interrogated. The system is experimentally proved to have a high performance with a good stability and low system noises. The multiplexing capacity of the system is also investigated. An experiment of measuring the surrounding temperature is carried out. A sensitivity of 12 gm/℃ is achieved within the range of 20 ℃ to 80 ℃. This low cost fiber multiplexing system has a potential application in the remote monitoring of temperature and strain in building structures, such as bridges and towers.

Long Period Fiber Grating Based Refractive Index Sensor With Enhanced Sensitivity Using Michelson Interferometric Arrangement

The long period fiber grating （LPFG） is widely used as a sensor due to its high sensitivity and resolution. However, the broad bandwidth of the attenuation bands formed by the mode coupling between the fundamental core mode and the cladding modes constitutes a difficulty when the device is used as a conventional sensor. To overcome this limitation, a Michelson interferometer-type sensor configuration has been developed, using an LPFG grating pair formed by coating a mirror at the distal end of the LPFG. This sensor configuration is more convenient to use and is able to overcome the limitations of the single LPFG based sensor as the shifts in the attenuation bands being more easily detectable due to the formation of the sharp spectral fringe pattern in the LPFG based Michelson interferometer. In this work, I studied the LPFG based Michelson interferometer as the refractive index sensor and discussed the sensitivity enhancement of the LPFG based Michelson interferometer as a refractive index sensor by employing higher order cladding modes and by reducing the cladding radius. The results demonstrated the HE17 mode with a cladding radius of 62.5 μm, in the range of surrounding refractive index （SRI） of 1 - 1.45, and its resonant peak showed a wavelength shift of 26.99nm/RIU. When the cladding region was further reduced to 24μm, the resonant peak showed a wavelength shift of 569.88 nm/RIU, resulting in a sensitivity enhancement of nearly 21 times. However, as the cladding region was etched further, then the HE17 order cladding mode and higher mode would be cut off. Therefore, the implementation of high sensitivity for SRI sensing with the reduced cladding in the LPFG based Michelson interferometer is dependent on the proper combination of the cladding radius and cladding mode order.

MZI wavelength interleaving filter based on double microring structure

To improve the output characteristics of all-fiber Mach-Zehnder interferometer(MZI)-interleaver,a dual microring-assisted MZI interleaver is designed.According to its structure,the output expression of the device is derived from the signal flow diagram.After simulation analysis,the optimal structural parameters during the transmission process are obtained.In addition,the coupling coefficient and transmission loss of the coupler are analyzed.The results show that the improved interleaver output line wave is closer to the square wave,and its 25 dB cutoff band bandwidth and 0.5 dB passband bandwidth are significantly improved,with the values of 41.2 GHz and 18.9 GHz,respectively.The device has a certain resistantability to deviation,and the transmission loss has less influence on the extinction characteristics of the filter.

Low power all optical switches

In this paper, we propose a new design of all fiber optical switches by using a high finesse ring resonator (RR) side coupled Mach-Zehnder interferometer. We will show that by compensating the total loss in the RR the switching power can be decreased greatly and by loss, compensating the bistability effect in RR can be cancelled and the switching performance can be improved. In addition, we will show that by using Erbium doped fiber for fabricating the RR we can obtain switching power threshold in mW range.

Application of MZI Symmetrical Structure With Fiber Balls and Seven-Core Fiber in Microdisplacement Measurement

An optical fiber microdisplacement sensor based on symmetric Mach-Zehnder interferometer (MZI) with a seven-core fiber and two single-mode fiber balls is proposed. The rationality and manufacturing process of the MZI sensing structure are analyzed. The fabrication mechanism of the Mach-Zehnder sensor by CO2 laser is described in detail. Experimental results show that temperature sensitivities of the two dips are 98.65 pm/℃ and 89.72pm/℃, respectively. The microdisplacement sensitivities are 2017.71 pm/mm and 2457.92 pm/mm, respectively. The simultaneous measurement of temperature and microdisplacement is demonstrated based on the sensitive matrix. The proposed Mach-Zehnder interference sensor exhibits the advantages of compact structure, simple manufacturing process, and high reliability.

Mid-infrared all-optical modulators based on an acetylene-filled hollow-core fiber

We report all-optical mid-infrared phase and intensity modulators based on the photo-thermal effect in an acetylene-filled anti-resonant hollow-core fiber.Optical absorption of the control beam promotes the gas molecules to a higher energy level,which induces localized heating through non-radiative relaxation and modulates the refractive index of the gas material and hence the accumulated phase of the signal beam propagating through the hollow-core fiber.By modulating the intensity of the control beam,the phase of the signal beam is modulated accordingly.By use of a 1.53μm near-infrared control beam,all-optical phase modulation up to 2.2πrad is experimentally demonstrated at the signal wavelength of 3.35μm.With the phase modulator placed in one arm of a Mach-Zehnder interferometer,intensity modulation with on-off ratio of 25 dB is achieved.The gas-filled hollow-core-fiber modulators could operate over an ultra-broad wavelength band from near-to mid-infrared and have promising application in mid-infrared photonic systems.