Established techniques for measuring the transmission matrix (TM) of a multimode fiber (MMF) allow for spot scanning at the distal end of the fiber through phase control at the proximal end, enabling ultrathin medical endoscopes and other applications that benefit from controllable light fields in MMF. Adding this capability to fibers utilized for other applications allows imaging to be performed within these areas. One outstanding limitation of this technology is the need to re-calibrate the fiber upon bending or other environmental perturbation. Here, we demonstrate a modified shape-sensor fiber that allows both shape sensing and imaging within the same fiber. In addition to permitting an image at the end of a shape sensor probe, the unification of these two technologies opens up the possibility of using the reconstructed fiber shape to mathematically update the calibration of the imaging waveguide in a dynamic environment, as has been proposed in the prior literature. Creating a robust method for maintaining knowledge of the fiber’s TM as the fiber is manipulated is critical for clinical deployment of this technology.
We demonstrate record high energy of 2 mJ, with four nanosecond pulses a peak power of ⪆420 kW and average power of 660 W, in a fiber amplifier using a novel 26 μm mode-field diameter Yb-doped gain fiber. The TMI threshold for this fiber was measured to be 1kW. This is achieved at a diffraction limited beam quality of M2=1.14.
Multimode fibers (MMFs) have a very large number of propagating modes per unit area and therefore allow for imaging with a very large number of pixels relative to their diameter. This makes MMFs perfect candidates for ultrathin endoscopes in applications such as deep brain imaging. However, the accuracy of the input-output relation that is needed, e.g., for distal spot scanning without moving parts, requires a new calibration after the fiber position or temperature has been significantly altered.
While neural networks have been used before to attempt to solve these challenges, we present an MMF-based imaging method that tolerates and classifies different fiber positions, using two single-layer fully-connected neural networks that only require the optical intensity without measuring the optical phase. One network learns the nonlinear relation between the input and output intensities and allows for image reconstruction in the presence of position changes, while the other network classifies that position change for different images. We show that our method is superior to memory-effect-based position sensing, both for small position changes where the relation between position change and output specklegram rotation angle is linear, as well as for larger position changes where this linearity and uniqueness break down. We also show that the position classification results are robust to temperature and polarization perturbations, and that our position classifier is able to effectively generalize. Likewise, we show that our imaging network also is robust to 30°C perturbations in temperature and 10° in polarization.
Transmission matrix measurements relating the electric field at the ends of standard step-index and graded-index multimode fibers promise to enable next generation miniaturized endoscopes. Relatively few measurements of specialty fibers and components have been demonstrated. Here, we present transmission matrix measurements and distal control through a variety of specialty fibers, including fibers for harsh environments, a polarization maintaining fiber, coreless fibers, a rectangular core fiber, multicore fibers, and a pump signal combiner. The calibration of these fibers and structures enables their dual-use for imaging and their original design application and allows control of the spatial profile of the light used in sensing, power delivery, and amplification.
A compact, robust, and inexpensive fiber-based source for coherent Raman imaging would benefit both re-searchers and the clinical application of these imaging techniques. However, the relative intensity noise of fiber sources has precluded their use for stimulated Raman scattering microscopy without the use of electronic noise cancellation. A recently demonstrated fiber optical parametric oscillator was used to achieve high-quality images using coherent anti-Stokes Raman scattering microscopy, and demonstrated that the self-consistent nature of the oscillator aided low-noise frequency conversion. Thus, reducing the intensity noise on the fiber laser used to pump this device will be a critical step in creating a fiber-based source for stimulated Raman scattering microscopy. We will report the design and construction of high-energy dissipative soliton fiber lasers as a potential source of quiet picosecond pulses at 1 μm, along with application to pumping the optical parametric oscillator.
A compact, alignment-free, and inexpensive fiber source for coherent Raman spectroscopy would benefit the field considerably. We present a fiber optical parametric oscillator offering the best performance from a fiber-source to date. Pumping the oscillator with amplified pulses from a 1 μm fiber laser, we achieve widely spaced, narrowband pulses suitable for coherent anti-Stokes Raman scattering microscopy. The nearly transform limited, 2 ps signal pulses are generated through the use of normal dispersion four wave mixing in photonic crystal fiber, and can be tuned from 779-808 nm, limited by the tuning range of the seed laser. The average signal power can reach 180 mW (pulse energies up to 4 nJ). The long-wavelength idler field is resonant in the oscillator, and the use of a narrow bandpass filter in the feedback loop is critical for stable operation, as seen in both simulation and experiment. Due to the self-consistent nature of the oscillator, this source provides lower relative intensity noise on its output pulses than parametric amplifiers based on the same frequency conversion process. We present high quality images of mouse tissues taken with this source that exhibit an outstanding signal to noise ratio at top imaging speeds.
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