Fiber-optic Sagnac Interferometer Research Articles

A 1 kHz A-scan rate pump-probe laser-ultrasound system for robust inspection of composites.

We recently built a fiber-optic laser-ultrasound (LU) scanner for nondestructive evaluation (NDE) of aircraft composites and demonstrated its greatly improved sensitivity and stability compared with current noncontact systems. It is also very attractive in terms of cost, stability to environmental noise and surface roughness, simplicity in adjustment, footprint, and flexibility. A new type of a balanced fiber-optic Sagnac interferometer is a key component of this all-optical LU pump-probe system. Very high A-scan rates can be achieved because no reference arm or stabilization feedback are needed. Here, we demonstrate LU system performance at 1000 A-scans/s combined with a fast 2-D translator operating at a scanning speed of 100 mm/s with a peak acceleration of 10 m/s(2) in both lateral directions to produce parallel B-scans at high rates. The fast scanning strategy is described in detail. The sensitivity of this system, in terms of noise equivalent pressure, was further improved to be only 8.3 dB above the Nyquist thermal noise limit. To our knowledge, this is the best reported sensitivity for a noncontact ultrasonic detector of this dimension used to inspect aircraft composites.

사냑형 간섭계 광섬유 센서를 이용한 변압기유 내에서의 외부 음향 주파수 모니터링

The fiber optic Sagnac interferometer is well established as a sensor for detection of physical perturbations such as acoustic and vibration. In this paper acoustic signals generated in the cylindrical cavity submerged in transformer oil were measured by the fiber optic sensor array in one Sagnac loop. Two different external sound frequencies, and , were applied to the sensor array simultaneously by using piezoelectric with frequency range from 5 kHz to 90 kHz. Based on the experimental results, fiber optic sensor detected harmonic series of applied sound frequency such as , , , , , . Suggested fiber optic sensor array can be applied to monitor physical quantities such as internal sound pressure and vibration due to partial discharge in the real electric transformer system.

Sensitivity-enhanced temperature sensor with cascaded fiber optic Sagnac interferometers based on Vernier-effect

A novel fiber optic temperature sensor has been proposed and experimentally demonstrated with ~9 times sensitivity enhancement by using two cascaded Sagnac interferometers. These two Sagnac interferometers consist of the same type of polarization maintaining fibers with slightly different lengths. The working principle is analogous to a Vernier scale. One interferometer acts as filter, while the other is for temperature sensing. The envelope of the cascaded sensor shifts much more than single one with a certain enhancement factor, which related to the free space range difference between the filter and sensor interferometers. Experimental results show that the temperature sensitivity is enhanced from −1.46nm/°C based on single Sagnac configuration to −13.36nm/°C.

Optical fiber Sagnac interferometer for sensing scalar directional refraction: application to magnetochiral birefringence.

We present a setup dedicated to the measurement of the small scalar directional anisotropies associated to the magnetochiral interaction. The apparatus, based on a polarization-independent fiber Sagnac interferometer, is optimized to be insensitive to circular anisotropies and to residual absorption. It can thus characterize samples of biological interests, for which the two enantiomers are not available and/or which present poor transmission. The signal-to-noise ratio is shown to be limited only by the source intensity noise, leading to a detection limit of Δϕ = 500 nrad Hz(-1/2). It yields a limit on the magnetochiral index nMC < 4 × 10(-13) T(-1) at 1.55 μm for the organic molecules tested.

A new fiber-optic non-contact compact laser-ultrasound scanner for fast non-destructive testing and evaluation of aircraft composites.

Laser ultrasonic (LU) inspection represents an attractive, non-contact method to evaluate composite materials. Current non-contact systems, however, have relatively low sensitivity compared to contact piezoelectric detection. They are also difficult to adjust, very expensive, and strongly influenced by environmental noise. Here, we demonstrate that most of these drawbacks can be eliminated by combining a new generation of compact, inexpensive fiber lasers with new developments in fiber telecommunication optics and an optimally designed balanced probe scheme. In particular, a new type of a balanced fiber-optic Sagnac interferometer is presented as part of an all-optical LU pump-probe system for non-destructive testing and evaluation of aircraft composites. The performance of the LU system is demonstrated on a composite sample with known defects. Wide-band ultrasound probe signals are generated directly at the sample surface with a pulsed fiber laser delivering nanosecond laser pulses at a repetition rate up to 76 kHz rate with a pulse energy of 0.6 mJ. A balanced fiber-optic Sagnac interferometer is employed to detect pressure signals at the same point on the composite surface. A- and B-scans obtained with the Sagnac interferometer are compared to those made with a contact wide-band polyvinylidene fluoride transducer.

System-level radiation hardening of fiber-optic sensor for applications in the radiant condition

In order to improve the radiation hardness of fiber-optic sensor for applications in the radiant condition, especially in the condition that the commercial optical components are used, the system-level radiation hardening technique is proposed by taking into account fiber-optic Sagnac interferometer as an example, in this paper. A model for describing the relationship between the random walk coefficient (RWC) of the closed-loop Sagnac interferometer and the optical parameters in optical loop, the radiation-induced attenuation (RIA) in fiber coil is presented. Then, the numerical simulation for the influence of optical parameters on the degradation sensitivity of RWC to the increase of RIA in fiber is implemented. It is found that the tolerance of fiber-optic Sagnac interferometer on the RIA in fiber coil can be improved by using the light source with high output power, making the diameter of fiber coil as large as possible, and utilizing the over-modulation technique. Furthermore, increasing the output power of light source is the most efficient and feasible method. The increase of output power from 0.2mW to 3mW can reduce the degradation of RWC by more than one order of magnitude.

Phenomenon in an alcohol not full-filled temperature sensor based on an optical fiber Sagnac interferometer

An alcohol not full-filled high-birefringence photonic crystal fiber (HiBi-PCF) temperature sensor based on an optical fiber Sagnac interferometer (OFSI) is demonstrated and investigated in detail. A new phenomenon that the resonant dip wavelengths of the temperature sensor blueshift with temperature increasing is observed, which is contrary to that of the previously reported alcohol filled HiBi-PCF OFSI temperature sensor. By considering the influences of the group birefringence and the thermo expansion of alcohol, this phenomenon is explained very well. The temperature sensitivity of the proposed sensor is about -1.17 nm/°C and is only one-sixth of that of the alcohol full-filled HiBi-PCF OSFI.

Temperature Sensor by Using Selectively Filled Photonic Crystal Fiber Sagnac Interferometer

In this paper, we report an optical fiber Sagnac interferometer (OFSI)-based temperature sensor constructed by a selectively filled polarization-maintaining photonic crystal fiber (PM-PCF). The transmission spectrum of the OFSI is in sinusoidal form and is sensitive to temperature. Simulation results predicted a higher sensitivity by selective filling than nonselective filling. In experiments, we used an extremely low-cost process to realize the selective filling. A sensitivity of 2.58 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{nm}/^{\circ}\hbox{C}$</tex> </formula> was achieved with an 11.7-cm-long PM-PCF. The sensitivity dependence on the infiltration length ratio was also investigated.

Physical quantities measurement by using the fiber optic sensor in the pendulum ball collision

It is interesting to measure the impact force when the moving pendulum ball collides to the fixed body. In this paper physical quantities were measured using by fiber optic sensor when a pendulum ball collides to the fixed ball on the wall. Both steel ball dimensions are 1 inch in diameter. The fiber optic Sagnac interferometer is well established as a sensor for acoustic and vibration. It is made a 1mm penetrated hole and optical fiber in the Saganac loop passed through the hole. The ball was welded on the wall as a fixed ball. When the external force applied to the fixed ball the optical fiber in the Sagnac loop detects the impact force. The output signal is proportional to the output voltage in the oscilloscope. Based on the result suggested fiber optic sensor can measure the impact intensity and this technique can be expanded to the moving bodies.

Robust and environmental insensitive fiber optic Sagnac interferometer for microwave photonic applications

A technique that allows a fiber optic Sagnac interferometer based microwave photonic device to be implemented using non-polarization maintaining components inside the Sagnac loop while still obtaining an output that is insensitive to changes in environmental conditions is presented. It is based on inserting the non-polarization maintaining components in between a polarization beam combiner and a Faraday rotator mirror inside the loop. The technique also introduces a phase bias to the light propagating inside the loop. Experimental results demonstrate that the discretely and continuously tunable Sagnac loop based signal processors implemented using non-polarization maintaining components have an environmentally insensitive frequency response.

The Application Technique of Signal detecting for Optic Fiber Temperature Sensor

A temperature sensor based on polarization non–reciprocity(PNR)in fiber-optic Sagnac interferometer(PSI)can measure the temperature of one point in high precision. The phasic subtraction of two light beams was shifted while temperature field around the probe changed. Analyzing the temperature measuring theory of Sagnac fiber optic temperature sensor, a linear output model Was established. The all digital signal detection was based onsquare–wave modulation and demodulation. An all digital signal detecting system using C8051 F060 as its COre Was completed. The experimental data achieved by temperature experiments agree with theory well. The input and output of system achieve a linearity of 10°C. Discussion shows that this kind of temperature sensor can reach high precision.

Magneto-optic fiber Sagnac modulator based on magnetic fluids

A magneto-optic modulator with a magnetic fluid film inserted into an optical fiber Sagnac interferometer is proposed. The magnetic fluid exhibits variable birefringence and Faraday effect under external magnetic field that will lead to a phase difference and polarization state rotation in the Sagnac interferometer. As a result, the intensity of the output light is modulated under the external magnetic field. Moreover, the modulator has a high extinction ratio and can easily be integrated in a single-mode fiber system. The performance of the modulator is not affected by ambient temperature variation from room temperature to 40 °C.

Magnetic pulse generation for high-speed magneto-optic switching

In this article, the magnetic pulse characteristics needed to achieve high-speed magneto-optic (MO) switching are investigated. A fiber-based, MO, low-voltage optical switch capable of 200 ns switching is presented, along with the special circuit characteristics for magnetic field generation for high-speed switching. The switch consists of the optical system, the MO material (bismuth substituted iron garnet [(Bi1.1Tb1.9)(Fe4.25Ga0.75)O12]), and a high-speed magnetic field driving circuit. A Faraday rotator is placed within the interferometric loop of a fiber-optic Sagnac interferometer, and interference at the output ports is controlled by the applied field. The fast switching speed is accomplished via the special design of the magnetic pulse generation circuitry. The applied magnetic field overshoots the field necessary to achieve the desired Faraday rotation and then settles to a steady state field. If the duration of the overshoot is less than the time it takes the material to saturate, a fast optical switching time can be achieved without saturating the material. The effects of the overshoot amplitude and duration and steady-state amplitude on optical rise time (determined by domain wall velocity) are studied and experimental results are presented.

Harmonic and Intermodulation Performance of Sagnac Fiber-Optic Interferometer

In this article, closed-form expressions are obtained for the amplitudes of the harmonic and intermodulation products generated when driving the fiber-optic Sagnac interferometer by a DC bias voltage plus an equal-amplitude two-tone signal. The results clearly show the strong dependence of the harmonic and intermodulation performance on the DC bias voltage and/or the phase bias of the fiber-optic Sagnac interferometer. The results obtained suggest that the DC bias voltage and/or the phase bias play an important role in deciding the linearity of the fiber-optic Sagnac interferometer and, hence, its suitability for analog applications.

STUDY OF LINEAL AND NON-LINEAL TRANSMISSION OF AN OPTICAL FIBER SAGNAC INTERFEROMETER AS A BIDIRECTIONAL DEVICE

The optical fiber Sagnac interferometer is a versatile system that has been investigated for a variety of applications such as optical switchers, filters, demultiplexers and passive mode-locked laser. In many cases, this arrangement is designed using a symmetrical coupler with two of their ports connected making a loop and generally the analysis have been focused in the transmission of the signal propagated in only one direction. Therefore in the present work a complementary study of the system considering the analysis for the lineal and non-lineal transmission as a bidirectional device has been performed. The experimental setup consists of different optical fiber lengths inside the cavity loop (between 100 and 500 m) with highly twisted singlemode fiber, a quarter wave retarder placed asymmetrically in one arm and a 50/50 coupler. The results have shown that for low optical powers, it is possible to adjust the system transmission in both propagation directions with the rotation of the retarder wave. On the other hand, in high optical power levels, this arrangement showed that the transmission increases slowly for the case when both the input and the output beams have the same polarization. This behavior can be used for pedestal suppression in a light pulse. Furthermore, for the case when the output signal polarization is orthogonal respect to the input one, the transmission changes quite fast. This effect can be used for applications such as the passive mode-locking.

Low-splicing-loss polarization-maintaining photonic-crystal-fiber-based Sagnac interferometer and its cladding-mode effect

A low-splicing-loss polarization-maintaining photonic-crystal-fiber (PMPCF)-based optical fiber Sagnac interferometer (OFSI) and its cladding-mode effect are demonstrated experimentally and analytically. An OFSI with minimum splicing loss of about 2.6 dB is achieved. The weak cladding modes induced by residual mode field diameter mismatch at the fusion splice cause a filtering effect on the transmission spectra of the OFSIs. This is especially noticeable for short length PMPCF-based OFSIs. Experimental results show that the relatively maximum ripple fluctuation induced by the cladding mode can reach 5.7%. The cladding-mode effect can be eliminated by slightly bending the PMPCF, but this introduces additional insertion loss.

Switchable and tuneable multi-wavelength Er-doped fibre ring laser using Sagnac filters

We demonstrate experimentally a simple configuration to perform wavelength-tuning and multi-wavelength operation in a fibre ring laser that is composed of an Erbium-doped fibre and includes a fibre-optic Sagnac interferometer as a spectral filter. We consider three different Sagnac filters including a 3 dB coupler, two wave retarders (WRs) and respectively 7 cm, 1 and 7 m of high birefringence fibre. The transmission spectrum of each filter is sinusoidal with a 70, 6, and 0.85 nm period, and its maxima can be shifted over one period by adjusting the angle of the retarders in the loop. By adjusting the WRs included in the filter and in the laser ring, tuneable single wavelength and several two- or three-wavelength lasing regimes were observed. In particular, fine adjustments of the WRs allow observing two- and three- wavelength operation with wavelength separations well below the homogeneous bandwidth of the gain medium. The stability of these modes of operation however strongly depends on environmental conditions at room temperature.

Application of fibre-optic rotational seismometer in investigation of seismic rotational waves

AbstractApplication of a fibre-optic Sagnac interferometer as a rotational seismometer is presented in this paper. It is a new device which parameters (sensitivity equal to 4.3×10−8 rad/s for 2σ) are comparable with the parameters of typical mechanical rotational seismometers. However, a direct measurement of rotation without influence of linear motions for fibre-optic rotational seismometer designed it for a direct measurement of a ground rotation component. Experimental data obtained during simultaneous application of the above two types of sensors are also presented. Research of near-field seismic events, the amplitude of which has been identified in the range of 1.5×10−6 rad/s to 2×10−7 rad/s, shows directly that, so-called, seismic rotational waves exist independently of typical seismic waves generated during earthquakes.

An experimental investigation of the optical switching characteristics using an optical Sagnac interferometer incorporating one and two ring resonators

We present the use of a fiber optic Sagnac interferometer incorporating one and two fiber optic ring resonators to characterize experimentally the optical switching behaviors. These prototypes were formed by the Sagnac interferometer and serially connected to the ring resonators. The input optical signal was launched via one end of the interferometer, where the polarization control was employed to obtain the maximum optical transmission power. Results obtained have shown that the increasing in switching time (i.e. narrower spectral width) and power can be achieved.

Application of the fibre-optic Sagnac interferometer in the investigation of seismic rotational waves

This paper presents new results connected with the fibre-optic Sagnac interferometer application in the investigation of seismic rotational waves. In general, it is a fibre-optic rotational seismometer representing sensitivity equal to 4.27 × 10−8 rad s−1 which can detect seismic rotational waves in a direct way. This paper presents the experimental results obtained simultaneously by a fibre-optic and a standard rotational seismometer installed in the Ojcow seismic observatory. The main advantage of such an approach is the possibility of recognizing the velocity of seismic rotational waves, which is different from the velocities of other seismic waves. The experimental data presented in this paper are confirmed by the theoretical description of the nature of seismic rotational waves.