Broadband Ultrafast Signal Generation and Dissemination via Dynamic Fiber Optic Links

Abstract

Mode-locked fiber lasers, with rare-earth-doped fibers offering numerous advantages over alternative gain media, including excellent beam quality, simple thermal management, and a compact and robust design, are in high demand for multiple applications in the scientific, technological, medical, and industrial arenas. Broadband femtosecond supercontinuum sources find applications in fields such as Optical Coherence Tomography, fluorescence lifetime imaging, and frequency metrology. Mode-locked fiber lasers offer an excellent candidate for supercontinuum generation owing to high peak power, low average power, and all fiber operation, given the availability of highly nonlinear fibers with appropriate zero dispersion wavelength; photonic crystal fibers (PCF) at 1um wavelength and highly nonlinear fiber (HNLF) at 1.5um. Supercontinuum generation involves nonlinear effects like self-phase modulation (SPM) and four-wave mixing (FWM) that cause spectral broadening. However, conventional spectral broadening often suffers from supercontinua with degraded spectral flatness. The profile of the broadened spectrum depends on the properties of the medium, as well as the power and temporal profile of the input pulse. To improve the supercontinuum spectrum, we can shape the pulse before broadening. However, the envelope is sensitive to the pulse spectral phase, which can vary over time, leading to sub-optimal performance. We address this issue in the first portion of this thesis by adaptively optimizing the input pulse using an automated closed-control loop that perturbs the spectral phase. A programmable Fourier pulse shaper modifies the C-band sub-picosecond pulses from a mode-locked fiber laser before spectral broadening in HNLF. An evolutionary strategy algorithm processes the measured spectrum and adaptively optimizes the spectral phase to realize a smooth supercontinuum with a broad Gaussian spectrum iteratively. We achieved a 4X bandwidth enhancement of the input pulse with a high degree of agreement between the generated supercontinuum and the Gaussian target, with spectral fluctuations less than 3dB across the bandwidth. Robust delivery of ultrashort pulses to multiple satellite locations over a fiber-optic network dynamically from a central location reduces complexity and overhead. Accurate characterization of the pulses remotely without specialized equipment is essential for this to work well. In the second part of this thesis, we demonstrate a simple measurement module for remote characterization using a second-harmonic crystal and power detectors at the fundamental and second-harmonic wavelengths. A centrally located pulse shaper-based interferometer creates pulse pairs with varying time delays during the characterization phase. Together with the remote detectors, this provides the field and intensity autocorrelations, which describe the spectral and temporal domain of the pulse. We demonstrate our technique by transmitting dispersion compensated sub-400fs pulses over a 50 meter and 100 meter length of optical fiber. The pulses are shaped adaptively prior to transmission to compensate for the distortions. The pulse intensity and power spectrum measured remotely agree with those made at the source. This provides an easy distribution method for femtosecond lasers from central to satellite locations via standard optical fiber links and their remote characterization. This method, however, proved inadequate for detecting optical non linearities as the spectral broadening seen by a pulse pair with varying delay differs from that of a pair of pulses undergoing nonlinear broadening separately. To overcome this drawback, we propose to launch the variable-delay pulse pair with no temporal overlap to avoid combined nonlinear distortions and measure the auto correlation at the output by adding a fixed delay interferometer to our detector module. The in-house fabricated fixed delay element consisted of a quartz plate with its surfaces coated by partially reflecting Bragg mirrors. This enables us to detect the spectral changes to the sub~400fs pulses in the presence of nonlinearities in the delivery links. To summarize, this thesis is a work towards a concise ultrashort signal generation and dissemination system that can generate signals at required wavelengths using nonlinear optical methods and distribute them to multiple satellite locations. The pulse-width and spectra are characterized remotely and can be adjusted according to user requirements.