Shay Keren

Interrogation of fiber gratings by use of low-coherence spectral interferometry of noiselike pulses

We demonstrate an innovative method for a real-time interrogation of fiber Bragg gratings based on low-coherence spectral interferometry of noiselike pulses. By analyzing the spectral interference at the output of a Michelson interferometer we obtained the impulse response of the grating with a time resolution of ~350 fs . Using the Gabor transformation, we could directly detect nonuniform regions inside the grating and could measure the spatial dependence of the resonance wavelength along the grating.

Measuring the structure of highly reflecting fiber Bragg gratings

We demonstrate a new technique that enables us to measure the structure of highly reflecting fiber Bragg gratings. The impulse response function is measured from both sides of the grating using a low-coherence spectral interferometry technique. An inverse scattering algorithm is used to extract the refractive-index profiles from the measured impulse responses. The reconstruction of the grating is performed by combining the refractive-index profiles, measured from both sides of the grating. The transfer function of the optical spectrum analyzer is measured and used to correct the measured results. The interrogation of an apodized grating with a reflectivity of 99.91% is demonstrated.

Measuring temperature profiles in high-power optical fiber components

We demonstrate a new method for measuring changes in temperature distribution caused by coupling a high-power laser beam into an optical fiber and by splicing two fibers. The measurement technique is based on interrogating a fiber Bragg grating by using low-coherence spectral interferometry. A large temperature change is found owing to coupling of a high-power laser into a multimode fiber and to splicing of two multimode fibers. Measurement of the temperature profile rather than the average temperature along the grating allows study of the cause of fiber heating. The new measurement technique enables us to monitor in real time the temperature profile in a fiber without the affecting system operation, and it might be important for developing and improving the reliability of high-power fiber components.

Distributed three-dimensional fiber Bragg grating refractometer for biochemical sensing

We demonstrate a three-dimensional (3-D) distributed refractometer that measures refractive index in a small volume. The sensor is based on an evanescent-wave fiber Bragg grating that is interrogated by low-coherence spectral interferometry. The measurement can be performed on a short time scale without the need for a mechanical scan. The new sensor was used to simultaneously measure glucose concentration in several droplets along a single sensor and to interrogate the time-dependent evaporation process of a water droplet. The new measurement technique might enable novel 3-D distributed sensors to be developed and multiple discrete measurements to be made in a small volume.

Data storage in optical fibers and reconstruction by use of low-coherence spectral interferometry.

We demonstrate optical data storage in optical fibers and reconstruction by use of low-coherence spectral interferometry. The information was stored by means of writing fiber Bragg gratings with different central wavelengths at different locations of the fiber. We need only a single short pulse is needed to read all the stored data. The maximum theoretical reconstruction rate that can be obtained with our technique is 10 Tbits/s. Our storage technique can be useful for identifying users in optical communication networks.

Modeling the saturation induced by broad-band pulses amplified in an erbium-doped fiber amplifier

We theoretically study the saturation of a homogeneously broadened optical amplifier with a slow response time. This model approximates well the behavior of the erbium-doped fiber amplifier (EDFA). When a broad-band pulse propagates inside such amplifier the saturation is determined by the overlap between the amplifier gain profile and the pulse spectrum rather than by the energy of the pulse. This effect may significantly increase the output power of an EDFA that amplifies broad-band pulses.

Interrogation of fiber gratings using spectral interferometry of a low-coherence light source

Summary form only given. Fiber Bragg gratings are important for various applications in optical communication systems and in optical metrology. Several methods for analyzing the performance of complex fiber gratings have been recently demonstrated. However, those methods do not give direct information on the spatial distribution of the grating. By measuring the spatial distribution of the grating one can improve the quality of gratings and increase the spatial resolution of fiber sensors. We present an innovative yet simple method for interrogating fiber Bragg gratings. The method is based on analyzing the interference between a low-coherence beam that is reflected from a grating and a reference beam. Using the Fourier and the Gabor transformations we can find the impulse response of the grating and its response to short pulses with different central frequencies. Because fiber gratings are distributed elements and because the propagation time of photons depends on the location of the region that scattered the photons, the impulse response contains information on the spatial distribution of the grating. We describe two methods for obtaining the spatial distribution and the chirp of the grating from its impulse response.