Abstract:Abstract:We report efficient acousto-optic control of speckle contrast at the output of multimode fibers (MMFs) using a cylindrical piezoelectric transducer (PZT) vibrating in the radial direction. With appropriate packaging of an MMF around the PZT, periodic stretching and subsequent intensity modulation were achieved over the wound fiber to result in timeaveraged smoothing of the output within a short time. It was experimentally confirmed that light passed through the vibrating PZT-fiber assembly maintains t… Show more
“…It is interesting to notice that while in modal noise experiments, the aim is to reduce the contrast to 0 and destroy the speckle structure to avoid errors [20], in our case we look to maximize this contrast. …”
Section: Speckle Intensity Contrast and Enhancementmentioning
Multimode fibers can guide thousands of modes capable of delivering spatial information. Unfortunately, mode dispersion and coupling have so far prevented their use in endoscopic applications. To address this long-lasting challenge, we present a robust scanning fluorescence endoscope. A spatial light modulator shapes the input excitation wavefront to focus light on the distal tip of the fiber and to rapidly scan the focus over the region of interest. A detector array collects the fluorescence emission propagated back from the sample to the proximal tip of the fiber. We demonstrate that proper selection of the multimode fiber is critical for a robust calibration and for high signal-to-background ratio performance. We compare different types of multimode fibers and experimentally show that a focus created through a graded-index fiber can withstand a few millimeters of fiber distal tip translation. The resulting scanning endoscopic microscope images fluorescent samples over a field of view of 80µm with a resolution of 2µm.
“…It is interesting to notice that while in modal noise experiments, the aim is to reduce the contrast to 0 and destroy the speckle structure to avoid errors [20], in our case we look to maximize this contrast. …”
Section: Speckle Intensity Contrast and Enhancementmentioning
Multimode fibers can guide thousands of modes capable of delivering spatial information. Unfortunately, mode dispersion and coupling have so far prevented their use in endoscopic applications. To address this long-lasting challenge, we present a robust scanning fluorescence endoscope. A spatial light modulator shapes the input excitation wavefront to focus light on the distal tip of the fiber and to rapidly scan the focus over the region of interest. A detector array collects the fluorescence emission propagated back from the sample to the proximal tip of the fiber. We demonstrate that proper selection of the multimode fiber is critical for a robust calibration and for high signal-to-background ratio performance. We compare different types of multimode fibers and experimentally show that a focus created through a graded-index fiber can withstand a few millimeters of fiber distal tip translation. The resulting scanning endoscopic microscope images fluorescent samples over a field of view of 80µm with a resolution of 2µm.
“…The 2 nd harmonic, 10 Hz component, can be seen in the spectra. This effect was explained by a descriptive model 9, 30 previously. However, our numerical result further demonstrates that the 3 rd harmonic may be present, as shown in (b2) and (c2).Figure 1Numerical simulation results of the angular power flow distribution of a large core MMF under external perturbations.…”
Large core optical multimode fiber provides benefits such as a large light-coupling tolerance, easy handling, and delivery of higher light power without undesirable nonlinear effects. In this research, we exploit the effects of external perturbation on the power flow within the large core fiber and present two relevant applications, namely a perturbation sensor and a doughnut beam tuner. Since conventional multimode fiber power flow model does not take into consideration the perturbation effect, we modify the power flow model so that the influence of time varying perturbation can be theoretically analyzed. Based on our theory, we further conduct the numerical simulation and experiments on these two applications. For the fiber vibration sensor, the proposed numerical model shows that the sensor sensitivity depends on the intensity profile of the launched beam and also the higher-order harmonics that were not reported previously can become interferences to affect the signal. For the beam tuner application, we prove both theoretically and experimentally that the doughnut intensity profile at the fiber output can be tuned in real-time by applying external perturbations to the fiber. We expect that the results can be useful to further exploit the external perturbation on large core fiber in various applications.
“…Since many modes propagate along the fiber with various propagating constants, there is a phase shift between different waveguide modes. Since the phase shift is very sensitive to the ambient environment of the MMF [12], the vibration caused by the VCM changes the phase shifts rapidly [13]. When the VCM is vibrating at a high frequency, the phase shifts between different modes vary on a time scale faster than the integration time of the camera.…”
Section: A Multimode Fibermentioning
confidence: 99%
“…The mode coupling between various waveguide modes changes when the fiber is bent and vibrated [13]. If the fiber deformation is large and fast enough to create fast-changing random mode coupling in a large range, the mode power distribution also changes with time randomly.…”
Speckle, due to the interference of coherent light having random phases, can seriously degrade image quality in a laser based projection system. The speckle is even worse when the diode pumped solid state (DPSS) lasers are used as light sources since the laser linewidth from the DPSS lasers is usually very narrow. In this paper, a de-speckle system consisting of a multimode fiber (MMF), a dielectric elastomer actuator (DEA) and a lightpipe is proposed and demonstrated, which can reduce speckle significantly. The speckle reduction components used in the system are optimized, taking into account speckle reduction ability and optical efficiency. The proposed de-speckle system is low in cost and easy to set up. It features a low optical power penalty and is compatible with large venue laser projection for applications such as digital cinema. In the proposed de-speckle system, test images captured by a camera with a setup following the recently proposed speckle measurement standard can achieve a speckle contrast ratio as low as 4.7%. By adding a voice coil motor (VCM) to vibrate the MMF to generate temporal diversity, the speckle contrast ratio can be further reduced to 3.7%.
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