High-birefringence Fiber Loop Mirror Research Articles

Wavelength continuously tunable and wavelength spacing variable comb filter based on a High-Birefringence fiber loop mirror

We demonstrated an all-fiber comb filter that can continuously tune the wavelength for four switchable wavelength spacings. The comb filter consists of a fiber loop mirror (FLM) that is inserted with three polarization maintaining fibers (PMFs) and three polarization controllers (PCs). The transmission characteristics of the comb filter was theoretically analyzed. The output characteristics of the comb filters shows a dependence of the orientation angles of the PCs. With the angles being optimally set, the comb filter with four different wavelength spacings can be achieved. The wavelength continuously tunable and wavelength spacing variable mechanisms are elaborated. The three PCs are employed to enable the tunable and switchable functions. The two PCs between the PMFs serve as a polarization rotator to align the birefringent axes of the PMFs for varying the wavelength spacing while the PC adjacent to the optical coupler can be used for wavelength tuning function. When the two quarter-waveplates (QWPs) of the PC are optimally set at π/4 and - π/4, the half-waveplate (HWP) acts as a wavelength tuning element, and when the QWPs are set at zero degrees, the HWP serves as an amplitude tuning element. The center wavelength of the comb filter could be linearly tuned with the rotation angle of the HWP, and a π/2 change in the rotation angle leads to a wavelength change in one free-spectral-range (FSR). A π/4 change in the rotation angle could bring the normalized peak transmission from its maximum at 1 to minimum at 0. The proposed tunable technique has been verified by our experimental results. We experimentally demonstrated the comb filters can switch among FSRs of 0.75 nm, 1.01 nm, 1.51 nm, and 3.1 nm with tunable functions.

Continuously Tunable Comb Filter Based on a High-Birefringence Fiber Loop Mirror With a Polarization Controller

A spectrum tunable method is presented for a high-birefringence fiber loop mirror (HiBi-FLM) comb filter with a polarization controller (PC) in the loop, where not only the wavelength of the comb filter can be continuously tuned with peak transmission remaining at unity, but the peak amplitude can be also continuously tuned without wavelength shift. By using the Jones matrix, we theoretically analyze the comb filter. The analytical expressions of the reflectivity and transmissivity are derived in terms of the orientation angles of three waveplates of the PC. The optimum conditions for wavelength and peak amplitude tunable operations are obtained. Our theoretical results show that when the two quarter-waveplate (QWP) angles are optimally set, the PC could act as a phase shifter or a polarization rotator, which empowers the comb filter to be wavelength continuously tunable or to be peak amplitude variable from 0 to 1 by only rotating the half-waveplate (HWP) of the PC. Moreover, the wavelength shift has a linear relationship with the HWP angle. When the two QWP angles are not optimized, PC has a combined effect of a phase shifter and a polarization rotator, leading to a wavelength shift accompanied by a change of the peak amplitude when tuning the HWP. Our theoretical prediction has been verified by experimental results.

Sensitivity amplification of high birefringence fiber loop mirror temperature sensor with Vernier effect

A fiber optic temperature sensor consisting of two cascaded high birefringence fiber loop mirrors (Hi-Bi FLMs) was proposed and experimentally demonstrated to enhance the temperature sensitivity and resolution by Vernier effect. The FSRs for the two Hi-Bi FLMs are designed to be slightly different to generate the Vernier effect. The working principle for the proposed sensor is analogous to Vernier-scale, one Hi-Bi FLM acts as a reference, while the other is for temperature sensing. The temperature sensitivity and resolution for the cascaded configuration are much higher than those for single Hi-Bi FLM due to the Vernier effect. A Gauss fitting algorithm is introduced to fit the Vernier spectral envelope to accurately trace the wavelength shift of the envelope peak. Experimental result shows that the temperature sensitivity and resolution can be improved from − 2.24 nm/°C and ± 0.0223 °C (single Hi-Bi FLM) to − 23.68 nm/°C and ± 0.0021 °C (cascaded configuration), respectively. The enhancement factors for temperature sensitivity and resolution are both 10.6, which are in good agreement with the theoretical prediction (10.7). The proposed sensor with high sensitivity, accuracy, easy fabrication and low cost is highly desirable for some applications that needs precise temperature control, such as micro-nano satellites, lasers, some chemical reaction process and enzyme reaction process.

Strain and temperature discrimination using a fiber Bragg grating concatenated with PANDA polarization-maintaining fiber in a fiber loop mirror

A highly sensitive strain and temperature sensor which is formed by concatenating a fiber Bragg grating (FBG) with a section of PANDA polarization-maintaining fiber (PMF) in a fiber loop mirror (FLM) has been proposed and experimentally demonstrated to perform temperature and strain measurement. The sensing principle of high birefringence fiber loop mirror (HiBi-FLM) and FBG has been analyzed with formula derivation. Due to the different temperature and strain sensitivities of HiBi-FLM resonant dip and FBG resonant peak, the changes in applied strain and ambient temperature can be simultaneously determined by measuring the two resonance wavelengths shifts. High temperature sensitivity of −1.93 nm/℃ and strain sensitivity of 37.5 pm/με have been achieved respectively over the range of 20∼55 ℃ and 0∼789.6 με. The discrimination errors for temperature and strain are measured to be ± 0.5 ℃ and ± 18 με, respectively. The proposed sensor enjoys simple structure, good linearity, high sensitivity and low cost, and thus shows a good application prospects in various fiber-optic sensing applications.

Highly sensitive temperature sensor based on cascaded HiBi-FLMs with the Vernier effect

We propose and experimentally demonstrate a highly sensitive temperature sensor based on cascaded high birefringence fiber loop mirrors (HiBi-FLMs) with the Vernier effect. The two HiBi-FLMs have almost the same free spectrum range (FSR), which can be regarded as two scales of different periods of an optical Vernier-scale, and act as the fixed part and the sliding part, respectively. A Gauss fitting algorithm is introduced to fit the Vernier spectrum envelope to accurately trace the spectral shift of the envelope. The temperature sensitivity and temperature resolution of the cascaded configuration are much larger than those of the individual HiBi-FLMs due to the Vernier effect. The experimental result shows that the temperature sensitivity of the proposed sensor can be improved from − 1.723 n m / ∘ C (single HiBi-FLM) to − 43 n m / ∘ C (cascaded configuration) by employing the Vernier effect, and the temperature resolution is enhanced from ± 0.029 ∘ C to ± 0.001162 ∘ C . The amplification factors for temperature sensitivity and temperature resolution are both 24.96, which shows good agreement with the theoretical predictions. The proposed sensor with huge temperature sensitivity, high resolution, and simple configuration may be a promising candidate for some applications that need precise temperature control.

Laser Wavelength Estimation Method Based on a High-Birefringence Fiber Loop Mirror

A simple method for the estimation of the wavelength of a fiber laser system is proposed. The method is based on the use of a high-birefringence-fiber loop mirror (HBFLM). The HBFLM exhibits a periodic transmission/reflection spectrum whose spectral characteristics are determined by the length and temperature of the high-birefringence fiber (HBF). Then, by the previous characterization of the HBFLM spectral transmission response, the central wavelength of the generated laser line can be estimated. By using a photodetector, the wavelength of the laser line is estimated during an HBF temperature scanning by measuring the temperature at which the maximum transmitted power of the HBFLM is reached. The proposed method is demonstrated in a linear cavity tunable Er/Yb fiber laser. This method is a reliable and low-cost alternative for laser wavelength determination in short wavelength ranges without the use of specialized and expensive equipment.

Arbitrary Spectral Synthesis and Waveform Generation With HiBi Fiber Loop Mirrors

An arbitrary spectral synthesis scheme with parallel-connecting high-birefringence fiber loop mirrors (HiBi-FLMs) based on Fourier synthesis theory has been proposed and demonstrated. Three typical spectra of triangle, rectangle, and sawtooth shape have been synthesized by implementing only four HiBi-FLMs. The experimental results are in good agreement with theoretical simulations with goodness of fit of 0.9565. Furthermore, higher precise optical spectrum with narrower bandwidth can be obtained by adopting longer polarization-maintaining fiber and more sections of HiBi-FLMs. Besides, a typical application of arbitrary waveform generation has been implemented. By incorporating with frequency-to-time mapping, triangle- and sawtooth-shaped electrical pulses with repetition rate of 1 GHz and pulse width of ~860 ps have been generated.

Simultaneous Measurement of Hydrogen Concentration and Temperature Based on Fiber Loop Mirror Combined With PCF

Simultaneous measurement of hydrogen concentration and temperature with high sensitivity and high precision was realized by integrating a photonic crystal fiber (PCF) modal interferometer and a high-birefringence fiber loop mirror (HBFLM). The PCF was coated with sol-gel Pd/WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> coating, whose refractive index and volume would be changed along with the variation of surrounding hydrogen concentration or temperature, and then the resonant dip of PCF modal interferometer would shift accordingly. Meanwhile, the resonant dip of the HBFLM has a high sensitivity to surrounding temperature. By integrating PCF modal interferometer and HBFLM together, the two independent resonant dips that can be simultaneously monitored at the output spectrum of the integrated interferometers would all shift with the change of external hydrogen concentration or temperature. Finally, combined with the dual-wavelength matrix method, the hydrogen concentration and temperature could be simultaneously measured. Experimental results showed that the resonant dip of the PCF modal interferometer moved in the short wavelength direction with the increase of hydrogen concentration, and the sensitivity of the hydrogen concentration was -1.12 nm/% within the concentration range from 0% to 1%. In the process of temperature change, the HBFLM realized a higher temperature sensitivity of -1.84 nm/°C. The sensor had high sensitivity, low cost, and simple structure.

Monitoring the Etching Process of Hi-Bi Fiber Through a Fiber Loop Mirror

A method for monitoring the etching process of Hi-Bi fiber is proposed and demonstrated. The birefringence of the etching fiber is accurately calculated by tracking the shift of the interference spectrum in a high-birefringence fiber loop mirror interferometer. Then, the degree of etching can be inferred by the relationship between the etching-fiber birefringence and the fiber structure. A section of PANDA fiber is used as an example to experimentally demonstrate the proposed method. The results show that the calculated birefringence is consistent with the theoretical simulation. Furthermore, four different structural states, which reflected in four different rate of change, are presented in the whole etching process of PANDA fiber. This method has the advantages of easy realization, high precision, and repeatability, and it can be used in the preparation of Hi-Bi microfiber photonic devices.

Hydrogen sensor based on high-birefringence fiber loop mirror with sol-gel Pd/WO3 coating

A hydrogen sensor based on high-birefringence fiber loop mirror (HBFLM) with sol-gel Pd/WO3 coating was demonstrated. The sensing structure was formed by inserting a section of polarization maintaining fiber (PMF) with Pd/WO3 coating in fiber loop mirror. The Pd/WO3 coating was prepared by sol-gel method and coated on PMF by dip-coating method, which is simple fabrication, low cost, and has good binding force with fiber. When the hydrogen concentration around Pd/WO3 coating was changed, it would induce the strain change of polarization maintaining fiber, and then shift the interference spectrum of HBFLM. Therefore, the hydrogen concentration can be measured by monitoring the wavelength shift of the interference spectrum. Experimental results showed that the resonance wavelength had a blue shift with the increase of hydrogen concentration and the total shift of the resonance wavelength was ∼2.18nm within the concentration range of 0–1%. The sensor had simple structure, low cost, high sensitivity, good repeatability, and well stability.

A Fibre Bragg Grating Interrogation Technique Based on High Birefringence Fibre Loop Mirror and WDM

In this paper, a fibre Bragg grating (FBG) interrogation technique based on high Birefringence fibre loop mirror (Hi-Bi FLM) and wavelength division multiplexer (WDM) is proposed and demonstrated. The approximate linear edge of the transmittance of the Hi-Bi FLM, which is a sinusoidal function of wavelength, is used to interrogate the sensing FBGs and WDMs is used to realize wavelength discrimination .Suitable for both static and dynamic sensing, this interrogation method has the advantages of all fibre design and high stability.

Temperature and refractive index sensor using a high-birefringence fiber loop mirror and single mode-coreless-single mode fiber structure

ABSTRACTA novel fiber optic sensor for the simultaneous measurement of refractive index and temperature is reported. The sensor consists of a high-birefringence fiber loop mirror and a section of single mode-coreless-single mode fiber structure. The single mode-coreless-single mode fiber structure served as a refractometer while the high-birefringence fiber loop mirror was used to measure temperature. The multimode interference valley of the single mode-coreless-single mode fiber structure was sensitive to the surrounding refractive index of liquids (96.42 nm/refractive index unit) and had almost no response to temperature fluctuations. The high-birefringence fiber loop mirror was highly sensitive to temperature (1.98 nm/°C) but was insensitive to changes in refractive index. The theoretical and experimental results demonstrated simultaneous measurement of temperature and refractive index. The optimum resolution was 2.07 × 10−4 refractive index units and 0.01°C.

High resolution polarization-independent high-birefringence fiber loop mirror sensor.

In this work, two all polarization-maintaining (PM) high-birefringence (Hi-Bi) fiber loop mirrors (FLM) which are immune to external polarization perturbations are validated both theoretically and experimentally. Simplified and stable versions of classical FLMs were attained using a PM-coupler and by fusing the different Hi-Bi fiber sections with an adequate rotation angle between them. Since the polarization states are fixed along the whole fiber loop, no polarization controllers are needed. This simplifies the operation and increases the stability of the systems, which were also validated as ultra-high resolution sensors, experimentally obtaining a resolution of 6.2∙10-4 °C without averaging.

A novel temperature and strain sensing method based on high-birefringence fiber loop mirror

A novel temperature and strain sensor based on a fiber loop mirror (FLM) consisting of high-birefringence polarization-maintaining fibers is proposed. The output spectrum was theoretically deduced based on Jones matrix. Two equal-length high-birefringence fibers (HBFs) with different birefringence indices were inserted into the FLM. The temperature and strain sensing system was built. The sensitivity coefficients of temperature and strain were calibrated respectively. The double parameters measurement of temperature and strain was realized. High resolution of 2.38μɛ has been achieved for strain sensing. Meanwhile, the temperature resolution is 0.016°C.

Pattern effect reduction scheme for high-speed all-optical amplification system

A method for high-bit-rate optical pulse amplification without a pattern effect (PE) phenomenon is numerically analyzed and presented. In the proposed new scheme, the input signals are applied to a series of 1×2 optical switches and semiconductor optical amplifiers (SOAs). Based on the input signal bit rate and the desired PE reduction rate, it is shown that the number of these devices can be easily optimized. For reducing the SOA nonlinearities on the output signal, a high-birefringent fiber loop mirror is used as an optical Gaussian filter. The achieved results depict that symmetry and the time-bandwidth product of the output signal obtained by this filter are significantly improved. The simulations are performed for high bit-rate signals (>50 Gbps); therefore, in the SOA model, all relevant nonlinear effects that occur in the subpicosecond regimes are taken into account.

High-birefringence fiber loop mirror sensor using a WDM fused fiber coupler

An intensity-based highly birefringent (Hi-Bi) fiber loop mirror (FLM) sensor is proposed which uses a wavelength-division multiplexing (WDM) fiber coupler. The effect of integrating the WDM coupler in a FLM configuration is first studied. A section of Hi-Bi (bow-tie) fiber of length 0.26 m is then placed in the fiber loop, making the spectral response of the device simultaneously dependent on the Hi-Bi fiber section and WDM coupler characteristics. When strain is applied to the sensing head, the spectral signal is modulated in amplitude, in contrast with the conventional Hi-Bi FLM sensors in which there are wavelength shifts. The sensor was characterized in strain and a sensitivity of (-2.2±0.4)×10(-3) με(-1) for a range of 300 με was attained. The self-referenced character of the sensor is noted.

A Selectable Multiband Bandpass Microwave Photonic Filter

A multiband bandpass microwave photonic filter (MPF) whose passband number and position can be selected is theoretically analyzed and experimentally demonstrated. The proposed MPF is based on a wide-band optical source (WBOS) and a two-order high-birefringence fiber loop mirror (HB-FLM), which serves as a slicing filter. Two segments of high-birefringence fiber (HBF) with the lengths of 3 m and 6 m are used in the HB-FLM, and three typical spectral periods or their combinations can be independently achieved by simply adjusting the polarization controllers (PCs) in the HB-FLM. Subsequently, the light source is sliced with uniform or mixing wavelength spacing. A coil of single-mode fiber (SMF) is then used to act as a dispersive medium to introduce time delay between taps. Thus, a single or multiband bandpass response is obtained at the output of a high-speed photodetector (PD). In addition, the passband centered at dc frequency is removed due to the use of phase modulation. All of the radio frequency (RF) characteristics of the proposed MPF show good agreement with the theoretical prediction. It has the merits of good flexibility, high spectrum efficiency, and great potential of extension.

Raman comb lasing in a ring cavity with high-birefringence fiber loop mirror

We demonstrate a 400-wavelength Raman comb lasing based on the nonlinear polarization rotation (NPR) in a ring cavity with high-birefringence fiber loop mirror (HBFLM). The comb-line intensities are equalized by the NPR mechanism. As the length of polarization-maintaining fiber (PMF) in the HBFLM is changed from 5.4 to 120m, the line spacing varies from 1.17 to 0.0533nm and the 10dB spectral bandwidth is kept at the similar level. In case of a 120m long PMF, more than 400 wavelengths with the signal-noise ratio higher than 10dB are obtained in the 10dB bandwidth of 22.4nm.

Influence of monitoring point wavelength on axial strain sensitivity of high-birefringence fiber loop mirror

The influence of the monitoring point wavelength on the axial strain sensitivity of high-birefringence fiber loop mirror is investigated. The theoretical expression for the axial strain sensitivity of high-birefringence fiber loop mirror is developed. The results show that the sensitivity increases with the wavelength of the monitoring point increasing when the high-birefringence fiber material is certain, that the sensitivity is constant and the wavelength shift is linear versus strain for the certain monitoring point. The axial strain sensitivities of the different wave peaks are monitored in experiment. The experimental results of data fitting are in good agreement with the theoretical ones. The research results help to improve the strain sensitivity, the temperature sensitivity, etc. of high-birefringence fiber loop mirror.

Simultaneous measurement of refractive index and temperature using multimode interference inside a high birefringence fiber loop mirror

Abstract A fiber optic sensor for simultaneous measurement of refractive index and temperature is presented. The sensing probe is realized by introducing a multimode interference device inside a high birefringence fiber loop mirror resulting in a configuration capable of refractive index and temperature discrimination. The multimode interference peak is sensitive to the surrounding refractive index (90 nm/RIU) and slightly responsive to the temperature (0.01 nm/°C). On the other hand, the birefringent fiber loop mirror is highly sensitive to temperature (2.36 nm/°C) and it has almost no response to refractive index. Using a power ratiometric peak detection scheme, a temperature independent refractive index measurement can be achieved with a resolution of ±2.25 × 10−5 RIU.