Boris Levit

Temporal differentiation of optical signals using a phase-shifted fiber Bragg grating

We propose and experimentally demonstrate an all-optical (all-fiber) temporal differentiator based on a simple pi-phase-shifted fiber Bragg grating operated in reflection. The proposed device can calculate the first time derivative of the complex field of an arbitrary narrowband optical waveform with a very high accuracy and efficiency. Specifically, the experimental fiber grating differentiator reported here offers an operation bandwidth of approximately 12 GHz. We demonstrate the high performance of this device by processing gigahertz-bandwidth phase and intensity optical temporal variations.

Reconfigurable generation of high-repetition-rate optical pulse sequences based on time-domain phase-only filtering.

We propose and demonstrate a fiber-based phase-only filtering technique for programmable optical pulse shaping, in which the filtering operation is implemented in the time domain by means of an electro-optical (EO) phase modulator. The technique has been applied for generating customized ultrahigh-repetition-rate optical pulse sequences (>40 GHz) from single input pulses by driving the EO phase modulator with a periodic electronic waveform (RF tone). The generated output pulses are replicas of the input pulse and both the repetition rate and the envelope profile of the generated sequences can be controlled and tuned electronically using this approach.

Spectro-temporal imaging of optical pulses with a single time lens

Spectro-temporal imaging (time-to-frequency conversion) constitutes a simple and direct (single-shot) technique for the high-resolution measurement of fast optical temporal waveforms. Here, we experimentally demonstrate that spectro-temporal imaging of an optical pulse can be achieved with a single time lens (quadratic phase modulator) operating under the appropriate conditions. As compared with the conventional solution, our proposal avoids the use of an input dispersive device preceding the time lens, thus, representing a much simpler and more practical alternative for implementing spectro-temporal imaging.

Broadband arbitrary waveform generation based on microwave frequency upshifting in optical fibers

An interesting method for broadband arbitrary waveform generation is based on the frequency upshifting of a narrowband microwave signal. In this technique, the original microwave signal is imaged into a temporally compressed replica using a simple and practical fiber-based system. Recently, it has been shown that the conventional limitations of this approach (e.g., bandwidth limitations) can be overcome by exploiting a temporal self-imaging (Talbot) effect in fiber. This effect can be used whenever the signal to be imaged is a quasi-periodic waveform (e.g., microwave tones or any arbitrary periodic waveform). This paper provides a comprehensive study of the microwave frequency upshifting technique with special focus on the Talbot-based approach. Following a theoretical analysis of the design constraints of the conventional approach, the Talbot-based solution is theoretically investigated in detail. In particular, the design specifications of a Talbot-based microwave upshifting system are derived, and the practical capabilities and constraints of these systems (e.g., in terms of achievable bandwidth) are stated and examined. The theoretical findings are confirmed by means of numerical simulations. Moreover, a numerical study of the influence of higher-order (second-order) dispersion terms on system performance is presented, and some additional design rules to minimize the associated detrimental effects are given. The results show that microwave frequencies up to a few hundreds of gigahertz over nanosecond temporal windows can be easily obtained with the described technique using input optical bandwidths in the terahertz range. This has been experimentally confirmed.

Repetition-rate multiplication of optical pulses using uniform fiber Bragg gratings

A simple method for repetition-rate multiplication of optical pulses using uniform Bragg gratings is demonstrated. The grating formation system for this application requires positioning accuracy of only 1 μm. A simple method of control for each of the gratings in the writing process is proposed. Compensation of fiber dispersion using rate multiplication of pulses is also demonstrated.

Complete characterization of optical pulses by real-time spectral interferometry

We demonstrate a simple method for complete characterization (of amplitudes and phases) of short optical pulses, using only a dispersive delay line and an oscilloscope. The technique is based on using a dispersive delay line to stretch the pulses and recording the temporal interference of two delayed replicas of the pulse train. Then, by transforming the time domain interference measurements to spectral interferometry, the spectral intensity and phase of the input pulses are reconstructed, using a Fourier-transform algorithm. In the experimental demonstration, mode-locked fiber laser pulses with durations of approximately 1 ps were characterized with a conventional fast photodetector and an oscilloscope.

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.

Simplified temporal imaging systems for optical waveforms

We demonstrate that temporal imaging (TI) of optical pulses (distortionless compression or expansion of the optical temporal waveform) can be achieved with a system comprising a quadratic phase modulator (time lens) followed by a dispersive device. As compared with the conventional solution, the proposed configuration does not require the use of an input dispersive device preceding the time lens, thus resulting in a much simpler and more practical alternative for implementing TI of optical signals.

Reshaping periodic light pulses using cascaded uniform fiber Bragg gratings

The authors demonstrate the use of cascaded uniform fiber Bragg gratings (FBGs) for the generation of periodic optical pulses with arbitrary waveform. It is a significantly simplified structure compared to complex FBG shapes. The pulse shaping is based on splitting of the input pulses by low-reflecting FBGs into a number of replicas and their superposition with proper amplitude, time delay, and phase shift that depend on the FBG parameters. The reflection amplitude and phase of each grating are unambiguously determined by the needed pulse shape. This method was experimentally verified for converting sinusoidally phase-modulated radiation of continuous-wave laser diode into a Gaussian pulse train with a pulsewidth of 30 ps. A method for controlling the parameters of FBGs during their fabrication process is also presented. It is done by measuring the spectral interference between the reflections from the FBGs and the fiber end by an optical spectrum analyzer and performing a fast Fourier transform. The method allows correction of the FBGs until the needed parameters are obtained during the writing process, as well as at any time after that.

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.