Ultrashort pulse sources are ubiquitous in scientific research as well as industrial applications. Delivering ultrashort pulses with high fidelity over a fiber-optic network to multiple target locations on a time-sharing basis can potentially overcome their complexity in operation and reduce overhead. We previously demonstrated a mechanism to deliver dispersion-compensated sub-400fs pulses in the C-band to different satellite locations using standard telecom-fiber links, as well as characterize them using a compact detector module at the delivery location assisted by a pulse shaper at the source, both controlled remotely via the cloud. The measurement procedure relied on creating a pulse pair with varying delays before launching them into the delivery fiber and measuring first and second-order autocorrelations at the remote location. However, this method proved inadequate to detect the optical nonlinearities as the spectral broadening seen by a pulse pair with varying delays differs from that of a pair of pulses undergoing nonlinear broadening separately since the degree of overlap between the pulses varies with the delay. 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 autocorrelation 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, which provided a delay ≈ 11ps between the reflections from the front and back surfaces. Both surfaces were coated by custom-engineered partially reflecting semiconductor coatings to give ≈ 40% power in both reflections. The addition of the fixed delay element enables us to detect the spectral changes to the sub 400 fs pulses in the presence of nonlinearities in the delivery links using a compact detector module with no movable parts.
Ultrashort optical pulses find uses in many areas such as multiphoton microscopy, spectroscopy, and material processing. These pulse sources are complex systems that are resource-intensive. This necessitates methods for the robust delivery of pulses to time-varying satellite locations. Characterization of the power spectrum and the temporal profile of the delivered pulses without the need for specialized equipment at the satellite location is highly desirable. Here, we demonstrate a simple method using a compact measurement apparatus at satellite locations with power detectors at the fundamental and second harmonic wavelengths (Germanium and Silicon detectors, respectively). The module also includes a thin β-Barium Borate crystal for second harmonic generation and a communication link to the source using standard data protocols. A pulse shaper at the source emulates an interferometer by creating pulse pairs with varying time delays. At satellite locations, fundamental and SHG power measurements of the pulse pair provide the field autocorrelation function (Fourier transform of the power spectrum) and the intensity autocorrelation function, respectively. Transform-limited pulses can be delivered by compensating the measured dispersion dynamically using the pulse shaper. We have delivered sub-picosecond pulses from a C-band mode-locked fiber laser with a bandwidth of 20nm over 50 and 100m using existing telecom fiber links. The pulse widths and spectra obtained using the remote measurement matched with those made directly at the satellite location. This provides easy distribution and remote characterization for femtosecond lasers from a central location to various satellite locations.
Broadband femtosecond supercontinuum sources find applications in fields such as Optical Coherence Tomography, fluorescence lifetime imaging, and frequency metrology. A mechanism to achieve the required spectral bandwidth is to broaden the output of a femtosecond laser source in nonlinear media such as highly nonlinear fibers (HNLF) utilizing a combination of nonlinear effects such as self-phase modulation (SPM) and four-wave mixing (FWM). 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 the temporal profile of the input pulse. The pulse can be shaped before broadening to improve the supercontinuum spectrum. However, the envelope is highly sensitive to the pulse spectral phase, potentially time-varying, resulting in a sub-optimal performance with any single pass optimization approach. Here, we overcome this by adaptively optimizing the input pulse by perturbing the spectral phase in an automated closed-control loop. A 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 is used to process the measured spectrum and adaptively optimize the spectral phase to realize a smooth supercontinuum with a broad Gaussian spectrum iteratively. We allowed the spectral phase to evolve with multiple variables across the pulse. We achieved a 4X bandwidth enhancement of the input pulse with high fidelity between the supercontinuum spectra and the target Gaussian shape. Spectral fluctuations were <3dB across the bandwidth of the generated supercontinuum.
Cascaded Raman fiber lasers are agile and scalable offering high optical powers at various wavelength bands inaccessible with rare-earth doped fiber lasers. Although several architectures for building cascaded Raman lasers exist, only the use of cascaded Raman resonators (CRRs) provide a high degree of power-independent wavelength conversion. A cascaded Raman resonator comprises of nested cavities built with two sets of high reflectivity fiber Bragg gratings at fixed Stokes wavelengths and thus can be used only for a fixed input wavelength; thereby restricting its use to a specific Ytterbium-doped fiber laser. The need for fabricating separate grating sets for each input wavelength compromises the simplicity and cost-effectiveness of this technique. Here, we demonstrate through experiment and simulations that the simple inclusion of a distributed broadband reflector at the first-order Stokes component along with the grating sets makes the CRR module very flexible to the input wavelengths, with remarkable improvement in efficiency over a widerange of inputs. In our experiment, a 17W Ytterbium-doped fiber laser tunable from 1055nm to 1080nm is used to pump a CRR module designed for an input wavelength of 1117nm and output wavelength of 1480nm. In conventional operation, for a non-resonant pump input into the CRR, nearly all the output was still unconverted pump. However, with the addition of the broadband distributed feedback reflector for the first-order Stokes component we achieved the 6thorder Stokes at 1480nm over the entire tuning range with a significant improvement in conversion ranging from ~33% to 86% of output at 1480nm.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.