Ultrafast laser processing has diverse scientific, industrial and medical applications. Until recently, this process has been idealized as a one-way interaction — the laser beam modifies the material, which would be the end of the story. The idea of the material modifying the laser beam, in return, and that this could open new doors appears to have been overlooked. In many cases, such two-way interactions either did not occur, or were unnoticed, if present, and actively prevented, if noticed.
Our approach is to explicitly design for and exploit such interactions, and this approach has already led to several striking advances. Here, we invoke nonlinearities in the form of positive feedback between laser beam-induced changes in the material and material change-induced effects back on the laser beam. We first showed that we could create laser-induced spatial nanostructures on various material surfaces with unprecedented uniformity (Ilday et al., Nature Photon., 2013), which we later extended to creation of self-organized 3D structures inside silicon (Ilday et al., Nature Photon., 2017). This perspective also led to the extremely efficient regime of ablation-cooled laser-material ablation (Ilday et al., Nature, 2016), self-assembly of colloidal nanoparticles (Ilday et al., Nature Commun., 2017), and intracavity optical trapping, where the trap is placed inside the cavity of a laser, giving rise to nonlinear feedback forces (arXiv:1808.07831).These demonstrations will be discussed, compared and contrasted.
Optical solitons and their interaction with other solitons or with dispersive wave shed by solitons under modulation instabilities or perturbation constitute a versatile experimental and theoretical platform for studying the nature of complex dynamics occurring in laser cavities [1-3] in addition to common physical principles in terms with a range of other nonlinear, non-equilibrium, coupled systems outside of optics. <p> </p>A soliton is energy localization of dissipative structures of electric field which evolves from noise in laser cavities. It is stationary solution of nonlinear Schrödinger equation that balances the effects of chromatic dispersion with nonlinearity during propagation in a medium. Strong pumping in soliton regime drives a laser system in to a multi pulsing self-organized system. Such a system in fiber medium is ubiquitous and always attracts research interest. <p> </p>Multisoliton pulses or soliton bunches generated from different systems through a short and long range interaction due to acoustic waves generated from electrostriction and its perturbation induced refractive index change of the medium by a propagating pulse on the next pulse in the neighborhood . A short range interaction can occur as a result of pulses overlapping, acoustoptic interaction or it can occur when dispersive waves at the tail of pulses interact with a back ground field or with solitons near to its [1, 4, 5].
Spectral beam combining of Tm-doped fiber lasers can increase the laser output power while simultaneously maintaining the single mode beam quality. We report on a spectral beam combining technique based on highly efficient in-housemade WDM cascade. We demonstrate continuous wave power combining employing a WDM cascade consisting of four fiber laser sources with emission wavelengths of 1920, 1949, 1996 and 2030 nm. A combined power of up to 38 W resulted in a combining efficiency of 69%.
We propose a novel approach for trapping micron-sized particles and living cells based on optical feedback. This approach can be implemented at low numerical aperture (NA=0.5, 20X) and long working distance. In this configuration, an optical tweezers is constructed inside a ring cavity fiber laser and the optical feedback in the ring cavity is controlled by the light scattered from a trapped particle. In particular, once the particle is trapped, the laser operation, optical feedback and intracavity power are affected by the particle motion. We demonstrate that using this configuration is possible to stably hold micron-sized particles and single living cells in the focal spot of the laser beam. The calibration of the optical forces is achieved by tracking the Brownian motion of a trapped particle or cell and analysing its position distribution.
Photoacoustic microscopy, as an imaging modality, has shown promising results in imaging angiogenesis and
cutaneous malignancies like melanoma, revealing systemic diseases including diabetes, hypertension, tracing drug
efficiency and assessment of therapy, monitoring healing processes such as wound cicatrization, brain imaging and
mapping. Clinically, photoacoustic microscopy is emerging as a capable diagnostic tool. Parameters of lasers used
in photoacoustic microscopy, particularly, pulse duration, energy, pulse repetition frequency, and pulse-to-pulse
stability affect signal amplitude and quality, data acquisition speed and indirectly, spatial resolution. Lasers used
in photoacoustic microscopy are typically Q-switched lasers, low-power laser diodes, and recently, fiber lasers.
Significantly, the key parameters cannot be adjusted independently of each other, whereas microvasculature and
cellular imaging, e.g., have different requirements. Here, we report an integrated fiber laser system producing
nanosecond pulses, covering the spectrum from 600 nm to 1100 nm, developed specifically for photoacoustic
excitation. The system comprises of Yb-doped fiber oscillator and amplifier, an acousto-optic modulator and a
photonic-crystal fiber to generate supercontinuum. Complete control over the pulse train, including generation
of non-uniform pulse trains, is achieved via the AOM through custom-developed field-programmable gate-array
electronics. The system is unique in that all the important parameters are adjustable: pulse duration in the range
of 1-3 ns, pulse energy up to 10 μJ, repetition rate from 50 kHz to 3 MHz. Different photocoustic imaging probes
can be excited with the ultrabroad spectrum. The entire system is fiber-integrated; guided-beam-propagation
rendersit misalignment free and largely immune to mechanical perturbations. The laser is robust, low-cost and
built using readily available components.
One of the key challenges for the next-generation light sources such as X-FELs is to implement a timing
stabilization and distribution system to enable ~ 10 fs synchronization of the different RF and laser sources
distributed in such facilities with distances up to a few kilometers. These requirements appear to be beyond the
capability of traditional RF distribution systems based on temperature-stabilized coaxial cables. A promising
alternative is to use an optical transmission system: A train of pulses generated from a laser with low timing
jitter is distributed over length-stabilized fiber links to remote locations. The repetition frequency of the pulse
train and its higher harmonics contain the synchronization information. At the remote locations, RF signals
are extracted simply by using a photodiode and a suitable bandpass filter to pick the desired harmonic of the
laser repetition rate. Passively mode-locked Er-doped fiber lasers provide excellent long-term stability. The
laser must have extremely low timing jitter, particularly at high frequencies (>1 kHz). Ultimately, the timing
jitter is limited by quantum fluctuations in the number of photons making up the pulse and the incoherent
photons added in the cavity due to spontaneous emission. The amplitude and phase noise of a home-built
laser, generating 100-fs, 1-nJ pulses, was characterized. The measured phase noise (timing jitter) is sub-10 fs,
from 1 kHz to Nyquist frequency. In addition to synchronization of accelerators, the ultra-low timing jitter
pulse source can find applications in next-generation telecommunication systems.
Recent advances in femtosecond fiber lasers are described. Self-similar evolution of parabolic pulses in a modelocked laser can be exploited to substantially increase the pulse energy and peak power that can be achieved without wave-breaking. Experimentally, pulse energies as high as 10 nJ and peak powers as high as 80 kW are obtained from Yb fiber lasers operating in the wave-breaking-free regime.
In recent years a considerable effort has been aimed at developing and characterizing glasses with large and fast optical nonlinearities, motivated largely by the potential application of these materials in all-optical switching devices. We will review our work in this area, with a focus on femtosecond measurements of new materials at 1.3 and 1.55 microns. These provide all possible information about the nonlinear response of the materials. Recent studies of As-S-Se, Ge-As-Se, and Ge-As-S-Se glasses reveal several materials with nonlinear refractive indices 1000 times larger than that of fused silica, as well as materials with figures of merit that are adequate for effective optical switching. These studies also provide information about the dispersion of the nonlinearities, which in turn sheds light on the mechanisms responsible for the nonlinear response.