The financial support by DFG (German research foundation, project CZ 55/30-1) for parts of this work is gratefully acknowledged.

1. Assembling the Experimental Setup
<ol>
	<li>Preparing the proximal side
	<ol>
		<li>Place and fix the laser providing a collimated light beam &#8211; or use a fiber-coupled laser with collimation optics at the exit facet of the fiber.</li>
		<li>Put the polarizing beam splitter (PBS) to split the laser beam into reference and object beam. Turn the orientation of the half wave plates (HWP) by rotating the HWP in its rotation mount until the power of reference beam and object beam (at the distal side) is roughly the same. Check this by putting a screen into both reference and object beam. Choose the orientation of the PBS so that the polarization of the reference beam fits the polarization-sensitive spatial light modulator (SLM).</li>
		<li>Put a beam splitter (BS) into the reference beam to split the reference beam into two beams. Place the optical modulators (OM) such that these two beams coming from BS1 can pass OM1 and OM2, respectively.</li>
		<li>Combine the two beams passing OM1 and OM2 at BS2 employing two mirrors. Adjust the beam splitters and mirrors so that both beams are spatially separated.</li>
		<li>Carefully align BS5 to ensure that the direction of incidence of both beams is perpendicular to the pixel plane of the SLM, ignoring BS3 and BS4 at first. At first, nothing is displayed on the SLM, i.e., it acts like a mirror until the end of the calibration (throughout step 2).</li>
		<li>Adjust the position and the distance between the two lenses (L) constituting a Keplerian telescope in order to get a sharp image of the SLM plane on the complementary metal-oxide semiconductor (CMOS) camera. Watch the correct orientation of L1 and L2 (flat sides are facing each other) to minimize aberrations.</li>
	</ol>
	</li>
	<li>Preparing the distal side
	<ol>
		<li>Use BS7 to split the object beam into two beams and combine them at BS8 employing two mirrors. Again, adjust the beam splitters and mirrors so that both beams are spatially separated.</li>
		<li>Deflect both beams using BS9 to aim them to the microscope objective (OBJ). Focus OBJ2 on the distal end of the multimode fiber (MMF). Check the focus by observing the back reflection from the MMF employing L3 and a charge-coupled device (CCD) camera.</li>
	</ol>
	</li>
	<li>Connecting proximal and distal side
	<ol>
		<li>Collimate the light from the object beam exiting the MMF employing OBJ1.</li>
		<li>Split the object beam using BS6, ignore the linear polarizer (LP) at first. Combine both object beams with both reference beams at BS3 and BS4 employing a mirror. Adjust the beam splitters and mirrors so that each pair of reference and object beam overlap at the SLM, intersecting with a small angle (less than 1&#176;).</li>
		<li>Ensure that the power of the reference and the object beam are approximately equal by turning the orientation of the HWP, according to step 1.1.2.</li>
		<li>Check the interference pattern (off-axis hologram) at the CMOS camera and adjust the intersection angle accordingly. Increase the angle, until the interference fringe spacing roughly equals the size of two pixels on the CMOS camera.</li>
		<li>Adjust the orientation of the LP to match the polarization of object and reference beam in order to get a maximum contrast of the interference pattern in the CMOS camera image, so that the camera image shows distinct fringes.</li>
	</ol>
	</li>
</ol>
2. Calibrating the System
<ol>
	<li>Calibrating the pixel relation between SLM and CMOS
	<ol>
		<li>Illuminate the whole SLM using only one of the reference beams and block the other reference and objects beams.</li>
		<li>Capture an image of the SLM with the CMOS camera.</li>
		<li>Get the coordinates of the upper left corner of the SLM in the CMOS camera image, e.g. using graphics software and the mouse cursor at the PC. Use these pixel coordinates as the point of origin regarding the SLM.</li>
		<li>Remove all beam blocks.</li>
	</ol>
	</li>
	<li>Calibrating the signal paths
	<ol>
		<li>Block both reference beam 2 and object beam 2.</li>
		<li>Capture an image of the hologram with the CMOS camera. Evaluate the phase in the recorded hologram using angular spectrum method19. Calculate the inverted phase in the corresponding region of beam 1.</li>
		<li>Remove the former beam blocks and now block both reference beam 1 and object beam 1.</li>
		<li>Capture an image of the hologram with the CMOS camera. Measure the phase in the recorded hologram using angular spectrum method again. Calculate the inverted phase at the corresponding region of beam 2.</li>
		<li>Remove all beam blocks.</li>
	</ol>
	</li>
</ol>
3. Transmitting the Signals
<ol>
	<li>Block the object beam.</li>
	<li>Stitch the calculated inverted phase images at the corresponding regions of beam 1 and 2 together and display the entire image on the SLM, typically using the computer graphics port.</li>
	<li>Start the modulation of the input signals 1 and 2 by activating OM1 and OM2.</li>
	<li>Observe the output signals 1 and 2 on the CCD camera.</li>
</ol>

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The authors have nothing to disclose.

The assembling of the experimental setup (step 1 in the protocol) requires a thorough alignment of the optical components with respect to each other. The most important aspect is the rectangular incidence of the reference beams onto the SLM in order to ensure a high PBR.
In order to enhance the setup to more than two transmitted signals, additional beam splitters could be used. As an alternative, a fiber-based implementation would be more compact and robust allowing the system to be portable for in situ investigations in biophotonics. If a single-side access is possible only, model-based calibration solutions20 need to be accomplished as a future step. The more signals are transmitted, the more modes will be required so more pixels on both the SLM and the CMOS camera will have to be involved for achieving a PBR. Moreover, the number of pixels should be larger than or equal to the number of modes. In addition, the pixel size of the SLM should be twice the size of the smallest speckle diameter at the proximal side. It is further recommended that the SLM has a bit depth of at least four bit. The pixel number of the camera denoted with CMOS should exceed the number of the SLM pixels. However, instead of the CMOS camera any other detector type may be employed, e.g. CCD. The same holds for the camera denoted with CCD.
One limitation of the proposed method is that the light source requires a large coherence length (low spectral bandwidth) to assure interference in the hologram needed for the phase measurement. In addition, the system must be stable, i.e. no changes of the fiber or the optical setup between the calibration and the transmission are tolerable that are faster than the duration of the calibration, which is currently below 1 s. For long fibers and high signal frequencies, the group velocity dispersion of the different fiber modes has to be taken into account and may deteriorate the signal. To compensate for that, gradient-index fibers or the correction of spatiotemporal distortions21 may be used.
In contrast to previous phase conjugation approaches, our proposed SDM method can be used in applications, where independent light signals have to be transmitted. Phase conjugation methods are advantageous regarding time performance, compared to iterative approaches or complete matrix determination.
One further potential application field may be endoscopic light delivery, for instance at optical traps or in optogenetics. For optogenetics, our method is advantageous regarding the selective illumination of single neurons in order to analyze the behavior of the brain and better understand neurodegenerative diseases.

The transmission of multiple light signals through a multimode fiber (MMF) is evident in communications engineering1 and biophotonics2. In communications engineering, space-division multiplexing (SDM) is believed to be a viable solution in order to enhance the transmission capacity of optical fibers for future data transfer applications benefiting from a higher utilization of the limited space, compared to multiple single-mode fibers3. In biophotonics, biological samples are manipulated by light transmitting through an MMF endoscope4. For example, the independent optical control of individual neurons using MMF endoscopes is of interest for optogenetics in order to study neuronal networks in the brain5. However, the light projected onto the MMF input facet is subject to distortion due to mode mixing and dispersion during propagation to the output facet of the MMF. As a result, the light propagation is altered, which makes signal transmission challenging.
Wavefront shaping methods6,7 are applied in scattering media using spatial light modulators (SLM) and enable the compensation for the distortion due to scattering during light propagation8. There are iterative approaches that optimize the output using an optical feedback9. These approaches are rather time consuming because of the necessity for numerous iterations and the high degree of freedom, corresponding to a large number of modulator elements. Another approach is to completely determine the distortion within the MMF described by its transmission matrix10. If the number of modes to be transmitted is large, this will be time consuming as well. In contrast, digital optical phase conjugation (DOPC) is considered to be fast and advantageous here, since only few focal spots have to be generated at the output facet of the MMF. Phase conjugation approaches have also been demonstrated for focusing or imaging through biological tissue12,13,14.
So far, DOPC was employed for a single time signal only15,16, and was applied for the transmission of light through an MMF17. A DOPC approach for multiple independent signals has not been accomplished. We have developed an enhanced DOPC method providing the independent transmission of multiple light signals using individual wavefront shaping for each signal employing a single phase-only SLM18. To this aim, the SLM is segmented into regions, one for each signal to be transmitted. The proposed experimental setup is depicted in Figure 1, where a calibration is performed in a) before the actual transmission happens in b).
<img alt="Figure 1" src="/files/ftp_upload/55407/55407fig1.jpg" /> 
Figure 1: Experimental setup. BS = beam splitter, CCD = charge-coupled device, OM = optical modulator, CMOS = complementary metal-oxide semiconductor, HWP = half wave plate, L = lens, LP = linear polarizer, MMF = multimode fiber, OBJ = microscope objective, PBS = polarizing beam splitter, SLM = spatial light modulator (phase only) &#8212; only relevant beams for&#160;(a) the calibration and&#160;(b) the transmission are depicted <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55407/55407fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table><tbody><tr><td>spatial light modulator</td><td>Holoeye</td><td>PLUTO-VIS-016</td><td></td></tr><tr><td>CMOS camera</td><td>Mikrotron</td><td>MC4082</td><td></td></tr><tr><td>diode-pumped solid state laser</td><td>Laser Quantum</td><td>torus 532</td><td></td></tr><tr><td>CCD camera</td><td>IDS</td><td>U3-3482LE-M CMOS</td><td>camera; suitable as well</td></tr><tr><td>lens 1</td><td>Qioptiq</td><td>G063204000</td><td></td></tr><tr><td>lens 2</td><td>Qioptiq</td><td>G063203000</td><td></td></tr><tr><td>lens 3</td><td>Thorlabs</td><td>AC508-180-A-ML</td><td></td></tr><tr><td>multimode fiber</td><td>Thorlabs</td><td>M14L02</td><td></td></tr><tr><td>beam splitters</td><td>Thorlabs</td><td>BS013</td><td>9x</td></tr><tr><td>polarizing beam splitters</td><td>Thorlabs</td><td>PBS251</td><td></td></tr><tr><td>mirrors</td><td>Thorlabs</td><td>PF10-03-P01</td><td>5x</td></tr><tr><td>microscope objectives</td><td>Thorlabs</td><td>RMS20X</td><td>2x</td></tr><tr><td>half wave plates</td><td>Thorlabs</td><td>WPH10M-532</td><td>2x</td></tr><tr><td>linear polarizer</td><td>Thorlabs</td><td>LPVISB050-MP2</td><td></td></tr><tr><td>optical modulators</td><td>Thorlabs</td><td>MC2000B-EC</td><td>2x</td></tr><tr><td>linear and rotation stage for CMOS camera</td><td>Thorlabs</td><td>XYR1/M</td><td></td></tr><tr><td>fiber connector</td><td>Thorlabs</td><td>S120-SMA</td><td>2x</td></tr><tr><td>reducing ring for microscope objectives</td><td>Qioptiq</td><td>G061621000</td><td>2x</td></tr><tr><td>xy adjustment for objective adapters</td><td>Qioptiq</td><td>G061025000</td><td>2x</td></tr><tr><td>z translation mount for fiber adapter</td><td>Thorlabs</td><td>SM1Z</td><td>2x</td></tr><tr><td>rods for fiber alignment to objectives</td><td>Qioptiq</td><td>G061210000</td><td>8x</td></tr><tr><td>mounts for lenses 1 and 2 plus two phantom mounts</td><td>Qioptiq</td><td>G061047000</td><td>4x</td></tr><tr><td>rail carriers for objective and lens mounts</td><td>Qioptiq</td><td>G061372000</td><td>6x</td></tr><tr><td>rail for rail carriers</td><td>Qioptiq</td><td>G061359000</td><td>2x</td></tr><tr><td>adapter for CCD camera to 1 post</td><td></td><td></td><td>in-house</td></tr><tr><td>adapter for laser to 4 posts</td><td></td><td></td><td>in-house</td></tr><tr><td>mount for lens 3</td><td>Thorlabs</td><td>LMR2/M</td><td></td></tr><tr><td>mounts for half wave plates</td><td>Thorlabs</td><td>RSP1D/M</td><td>2</td></tr><tr><td>mounts for mirrors</td><td>Thorlabs</td><td>KM100</td><td>5x</td></tr><tr><td>mount for linear polarizer</td><td>Thorlabs</td><td>RSP05/M</td><td></td></tr><tr><td>mounts for beam splitters and SLM</td><td>Thorlabs</td><td>KM100PM/M</td><td>11x</td></tr><tr><td>clamping arms for beam splitters and SLM</td><td>Thorlabs</td><td>PM4/M</td><td>11x</td></tr><tr><td>posts for mounts, rail carriers and adapters</td><td>Thorlabs</td><td>TR75/M</td><td>29x</td></tr><tr><td>holders for posts</td><td>Thorlabs</td><td>PH50/M</td><td>29x</td></tr><tr><td>pedestals for holders</td><td>Thorlabs</td><td>BE1/M</td><td>29x</td></tr><tr><td>clamping forks for pedestals</td><td>Thorlabs</td><td>CF125</td><td>29x</td></tr></tbody></table>

transmission of multiple signals through an optical fiber using wavefront shaping

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the light distortion during the propagation within the fiber. Our methodology is based on digital optical phase conjugation employing only a single spatial light modulator, where the optical wavefront is individually modulated at different regions of the modulator, one region per light signal. Digital optical phase conjugation approaches are considered to be faster than other wavefront shaping approaches, where (for example) a complete determination of the wave propagation behavior of the fiber is performed. In contrast, the presented approach is time-efficient since it only requires one calibration per light signal. The proposed method is potentially appropriate for spatial division multiplexing in communications engineering. Further application fields are endoscopic light delivery in biophotonics, especially in optogenetics, where single cells in biological tissue have to be selectively illuminated with high spatial and temporal resolution.

Typical output signals at the distal side of the 2 m long fiber are depicted in Figure 2. Note that the desired focal spot (peak) is accompanied by an undesired speckle pattern (background), which is due to imperfection of the DOPC as a matter of principle. The corresponding peak-to-background ratio (PBR) amounts to 53 (solely signal 1 is &#39;on&#39;), 36 (solely signal 2 is &#39;on&#39;) and 20 (both signals 1 and 2 are &#39;on&#39;) here, respectively. The PBR can be increased when a fiber that supports a larger number of modes (currently: 1710) is used.
Due to the finite PBR, a crosstalk results between the output signals, which is visualized in Figure 3. The crosstalk between to periodic signals with the frequencies f1 and f2 amounts to -24 dB (from signal 2 to signal 1) and -29 dB (from signal 1 to signal 2).
<img alt="Figure 2" src="/files/ftp_upload/55407/55407fig2.jpg" /> 
Figure 2: Image of distal fiber end, transmission of output signal 1 (left), signal 2 (center) and both signal 1 and signal 2 (right). Intensity [a. u.] <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55407/55407fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/55407/55407fig3.jpg" /> 
Figure 3: Temporal frequency spectrum of the transmitted output signal 1 (left) and 2 (right). Amplitude [a. u.] <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55407/55407fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping at JoVE.com

Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping

We demonstrate the transmission of multiple independent signals through a multimode fiber using wavefront shaping employing a single spatial light modulator. By modulating the wavefront for each signal individually, spatially separated foci are transmitted. Potential applications are multiplexed data transfer in communications engineering and endoscopic light delivery in biophotonics.

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the ...

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9.8K Views.  TU Dresden. We demonstrate the transmission of multiple independent signals through a multimode fiber using wavefront shaping employing a single spatial light modulator. By modulating the wavefront for each signal individually, spatially separated foci are transmitted. Potential applications are multiplexed data transfer in communications engineering and endoscopic light delivery in biophotonics.

Video: Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping

Quasi-light Storage for Optical Data Packets

Today's telecommunication is based on optical packets which transmit the information in optical fiber networks around the world. Currently, the processing of the signals is done in the electrical domain. Direct storage in the optical domain would avoid the transfer of the packets to the electrical and back to the optical domain in every network node and, therefore, increase the speed and possibly reduce the energy consumption of telecommunications. However, light consists of photons which propagate with the speed of light in vacuum. Thus, the storage of light is a big challenge. There exist some methods to slow down the speed of the light, or to store it in excitations of a medium. However, these methods cannot be used for the storage of optical data packets used in telecommunications networks. Here we show how the time-frequency-coherence, which holds for every signal and therefore for optical packets as well, can be exploited to build an optical memory. We will review the background and show in detail and through examples, how a frequency comb can be used for the copying of an optical packet which enters the memory. One of these time domain copies is then extracted from the memory by a time domain switch. We will show this method for intensity as well as for phase modulated signals.

The article describes a procedure to store optical data packets with an arbitrary modulation, wavelength, and data rate. These packets are the basis of modern telecommunications.

Today's telecommunication is based on optical packets which transmit the information in optical fiber networks around the world. Currently, the processing ...

Engineering

Fabrication and Characterization of Disordered Polymer Optical Fibers for Transverse Anderson Localization of Light

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed polymer optical fiber is composed of 80,000 strands of poly (methyl methacrylate) (PMMA) and polystyrene (PS) that are randomly mixed and drawn into a square cross section optical fiber with a side width of 250 &mu;m. Initially, each strand is 200 &mu;m in diameter and 8-inches long. During the mixing process of the original fiber strands, the fibers cross over each other; however, a large draw ratio guarantees that the refractive index profile is invariant along the length of the fiber for several tens of centimeters. The large refractive index difference of 0.1 between the disordered sites results in a small localized beam radius that is comparable to the beam radius of conventional optical fibers. The input light is launched from a standard single mode optical fiber using the butt-coupling method and the near-field output beam from the disordered fiber is imaged using a 40X objective and a CCD camera. The output beam diameter agrees well with the expected results from the numerical simulations. The disordered optical fiber presented in this work is the first device-level implementation of 2D Anderson localization, and can potentially be used for image transport and short-haul optical communication systems.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. This microstructured fiber can transport a small localized beam with a radius that is comparable to the beam radius of conventional optical fibers.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed ...

Writing Bragg Gratings in Multicore Fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...

Generation and Coherent Control of Pulsed Quantum Frequency Combs

We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional states on-chip in a practical way have remained elusive due to the increasing complexity of the quantum circuitry needed to prepare and process such states. Here, we outline how high-dimensional, frequency-bin entangled, two-photon states can be generated at a stable, high generation rate by using a nested-cavity, actively mode-locked excitation of a nonlinear micro-cavity. This technique is used to produce pulsed quantum frequency combs. Moreover, we present how the quantum states can be coherently manipulated using standard telecommunications components such as programmable filters and electro-optic modulators. In particular, we show in detail how to accomplish state characterization measurements such as density matrix reconstruction, coincidence detection, and single photon spectrum determination. The presented methods form an accessible, reconfigurable, and scalable foundation for complex high-dimensional state preparation and manipulation protocols in the frequency domain.

A protocol is presented for the practical generation and coherent manipulation of high-dimensional frequency-bin entangled photon states using integrated micro-cavities and standard telecommunications components, respectively.

We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional ...

High-Throughput Total Internal Reflection Fluorescence and Direct Stochastic Optical Reconstruction Microscopy Using a Photonic Chip

Total internal reflection fluorescence (TIRF) is commonly used in single molecule localization based super-resolution microscopy as it gives enhanced contrast due to optical sectioning. The conventional approach is to use high numerical aperture microscope TIRF objectives for both excitation and collection, severely limiting the field of view and throughput. We present a novel approach to generating TIRF excitation for imaging with optical waveguides, called chip-based nanoscopy. The aim of this protocol is to demonstrate how chip-based imaging is performed in an already built setup. The main advantage of chip-based nanoscopy is that the excitation and collection pathways are decoupled. Imaging can then be done with a low magnification lens, resulting in large field of view TIRF images, at the price of a small reduction in resolution. Liver sinusoidal endothelial cells (LSECs) were imaged using direct stochastic optical reconstruction microscopy (dSTORM), showing a resolution comparable to traditional super-resolution microscopes. In addition, we demonstrate the high-throughput capabilities by imaging a 500 &#181;m x 500 &#181;m region with a low magnification lens, providing a resolution of 76 nm. Through its compact character, chip-based imaging can be retrofitted into most common microscopes and can be combined with other on-chip optical techniques, such as on-chip sensing, spectroscopy, optical trapping, etc. The technique is thus ideally suited for high throughput 2D super-resolution imaging, but also offers great opportunities for multi-modal analysis.

Chip-based super-resolution optical microscopy is a novel approach to fluorescence microscopy and offers advantages in cost effectiveness and throughput. Here, the protocols for chip preparation and imaging are shown for TIRF microscopy and localization-based super-resolution microscopy.

Total internal reflection fluorescence (TIRF) is commonly used in single molecule localization based super-resolution microscopy as it gives enhanced ...

Biochemistry

Shaping the Amplitude and Phase of Laser Beams by Using a Phase-only Spatial Light Modulator

The aim of this article is to visually demonstrate the utilization of an interferometric method for encoding complex fields associated with coherent laser radiation. The method is based on the coherent sum of two uniform waves, previously encoded into a phase-only spatial light modulator (SLM) by spatial multiplexing of their phases. Here, the interference process is carried out by spatial filtering of light frequencies at the Fourier plane of certain imaging system. The correct implementation of this method allows arbitrary phase and amplitude information to be retrieved at the output of the optical system.
It is an on-axis, rather than off-axis encoding technique, with a direct processing algorithm (not an iterative loop), and free from coherent noise (speckle). The complex field can be exactly retrieved at the output of the optical system, except for some loss of resolution due to the frequency filtering process. The main limitation of the method might come from the inability to operate at frequency rates higher than the refresh rate of the SLM. Applications include, but are not limited to, linear and non-linear microscopy, beam shaping, or laser micro-processing of material surfaces.

We show how to encode the complex field of laser beams by using a single phase element. A common-path interferometer is employed to mix the phase information displayed into a phase-only spatial light modulator to finally retrieve the desired complex field pattern at the output of an optical imaging system.

The aim of this article is to visually demonstrate the utilization of an interferometric method for encoding complex fields associated with coherent laser ...

Low-cost Custom Fabrication and Mode-locked Operation of an All-normal-dispersion Femtosecond Fiber Laser for Multiphoton Microscopy

A protocol is presented to build a custom low-cost yet high-performance femtosecond (fs) fiber laser. This all-normal-dispersion (ANDi) ytterbium-doped fiber laser is built completely using commercially available parts, including $8,000 in fiber optic and pump laser components, plus $4,800 in standard optical components and extra-cavity accessories. Researchers new to fiber optic device fabrication may also consider investing in basic fiber splicing and laser pulse characterization equipment (~$63,000). Important for optimal laser operation, methods to verify true versus apparent (partial or noise-like) mode-locked performance are presented. This system achieves 70 fs pulse duration with a center wavelength of approximately 1,070 nm and a pulse repetition rate of 31 MHz. This fiber laser exhibits the peak performance that may be obtained for an easily assembled fiber laser system, which makes this design ideal for research laboratories aiming to develop compact and portable fs laser technologies that enable new implementations of clinical multiphoton microscopy and fs surgery.

A method is presented to build a custom low-cost, mode-locked femtosecond fiber laser for potential applications in multiphoton microscopy, endoscopy, and photomedicine. This laser is built using commercially available parts and basic splicing techniques.

A protocol is presented to build a custom low-cost yet high-performance femtosecond (fs) fiber laser. This all-normal-dispersion (ANDi) ytterbium-doped ...

Characterization of SiN Integrated Optical Phased Arrays on a Wafer-Scale Test Station

Optical phased arrays (OPAs) can produce low-divergence laser beams and can be used to control the emission angle electronically without the need for moving mechanical parts. This technology is particularly useful for beam steering applications. Here, we focus on OPAs integrated into SiN photonic circuits for a wavelength in the near infrared. A characterization method of such circuits is presented, which allows the output beam of integrated OPAs to be shaped and steered. Furthermore, using a wafer-scale characterization setup, several devices can easily be tested across multiple dies on a wafer. In this way, fabrication variations can be studied, and high-performance devices identified. Typical images of OPA beams are shown, including beams emitted from OPAs with and without a uniform waveguide length, and with varying numbers of channels. In addition, the evolution of output beams during the phase optimization process and beam steering in two dimensions is presented. Finally, a study of the variation in the beam divergence of identical devices is performed with respect to their position on the wafer.

Here, we describe the operation of a SiN integrated photonic circuit containing optical phased arrays. The circuits are used to emit low divergence laser beams in the near infrared and steer them in two dimensions.

Optical phased arrays (OPAs) can produce low-divergence laser beams and can be used to control the emission angle electronically without the need for ...

Multicolor Fluorescence Detection for Droplet Microfluidics Using Optical Fibers

Fluorescence assays are the most common readouts used in droplet microfluidics due to their bright signals and fast time response. Applications such as multiplex assays, enzyme evolution, and molecular biology enhanced cell sorting require the detection of two or more colors of fluorescence. Standard multicolor detection systems that couple free space lasers to epifluorescence microscopes are bulky, expensive, and difficult to maintain. In this paper, we describe a scheme to perform multicolor detection by exciting discrete regions of a microfluidic channel with lasers coupled to optical fibers. Emitted light is collected by an optical fiber coupled to a single photodetector. Because the excitation occurs at different spatial locations, the identity of emitted light can be encoded as a temporal shift, eliminating the need for more complicated light filtering schemes. The system has been used to detect droplet populations containing four unique combinations of dyes and to detect sub-nanomolar concentrations of fluorescein.

Multicolor fluorescence detection in droplet microfluidics typically involves bulky and complex epifluorescence microscope-based detection systems. Here we describe a compact and modular multicolor detection scheme that utilizes an array of optical fibers to temporally encode multicolor data collected by a single photodetector.

Fluorescence assays are the most common readouts used in droplet microfluidics due to their bright signals and fast time response. Applications such as ...

Optimizing Brillouin Optical Time-Domain Analyzers Based on Gain Spectrum Engineering

Demonstrated is a unique method for sensing performance enhancement in Brillouin optical time-domain analyzers (BOTDA). A Brillouin gain spectrum (BGS) is superimposed with two symmetric Brillouin loss spectra (BLS). This leads to a complex engineered spectrum shape that is more resistant to the sensing system noise. Instead of only one pump and probe interaction as in the conventional BOTDA setup, three optical probe waves are exploited, with one probe located in the BGS and the other two symmetrically in the BLS. Due to the resistance and insensitivity of the engineered spectrum shape to the noise, the sensing performance is enhanced by 60% and the measurand resolution is doubled.

A protocol for Brillouin optical time-domain analyzers based on gain spectrum engineering is presented. Enhancements in the sensing performance, including sensing range and measurand resolution are achieved and the excess Brillouin intensity noise is studied. The protocol introduces a new way to enhance distributed Brillouin sensing performance.

Demonstrated is a unique method for sensing performance enhancement in Brillouin optical time-domain analyzers (BOTDA). A Brillouin gain spectrum (BGS) is ...