We thank Drs. E. Cronin-Furman and M. Weitzman (Olympus Corporation of the Americas Scientific Solutions Group) for assistance in acquiring images. This work was supported by National Institutes of Health Grant K22CA181611 (to B.Q.S.) and the Richard and Susan Smith Family Foundation (Newton, M.A.) Smith Family Award for Excellence in Biomedical Research (to B.Q.S.).

<ol>
	<li>Savage, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+parametric+oscillators.">Optical parametric oscillators.</a> Nature Photonics. 4, 124 (2010).</li><li>Xu, C., Wise, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+advances+in+fibre+lasers+for+nonlinear+microscopy.">Recent advances in fibre lasers for nonlinear microscopy.</a> Nature Photonics. 7, 875 (2013).</li><li>Kieu, K., Wise, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-fiber+normal-dispersion+femtosecond+laser.">All-fiber normal-dispersion femtosecond laser.</a> Optics Express. 16, 11453-11458 (2008).</li><li>Fekete, J., Cserteg, A., Szip&#337;ocs, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-fiber+all-normal+dispersion+ytterbium+ring+oscillator.">All-fiber all-normal dispersion ytterbium ring oscillator.</a> Laser Physics Letters. 6, 49-53 (2009).</li><li>Krolopp, &. #. 1. 9. 3. ;., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Handheld+nonlinear+microscope+system+comprising+a+2+MHz+repetition+rate,+mode-locked+Yb-fiber+laser+for+in+vivo+biomedical+imaging.">Handheld nonlinear microscope system comprising a 2 MHz repetition rate, mode-locked Yb-fiber laser for in vivo biomedical imaging.</a> Biomedical Optics Express. 7, 3531-3542 (2016).</li><li>Fermann, M. E., Hartl, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrafast+fibre+lasers.">Ultrafast fibre lasers.</a> Nature Photonics. 7, 868-874 (2013).</li><li>Szczepanek, J., Karda&#347;, T. M., Michalska, M., Radzewicz, C., Stepanenko, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Simple+all-PM-fiber+laser+mode-locked+with+a+nonlinear+loop+mirror.">Simple all-PM-fiber laser mode-locked with a nonlinear loop mirror.</a> Optics Letters. 40, 3500-3503 (2015).</li><li>Bowen, P., Singh, H., Runge, A., Provo, R., Broderick, N. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mode-locked+femtosecond+all-normal+all-PM+Yb-doped+fiber+laser+at+1060+nm.">Mode-locked femtosecond all-normal all-PM Yb-doped fiber laser at 1060 nm.</a> Optics Communications. 364, 181-184 (2016).</li><li>Chong, A., Buckley, J., Renninger, W., Wise, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-normal-dispersion+femtosecond+fiber+laser.">All-normal-dispersion femtosecond fiber laser.</a> Optics Express. 14, 10095-10100 (2006).</li><li>Kieu, K., Renninger, W., Chong, A., Wise, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sub-100+fs+pulses+at+watt-level+powers+from+a+dissipative-soliton+fiber+laser.">Sub-100 fs pulses at watt-level powers from a dissipative-soliton fiber laser.</a> Optics Letters. 34, 593-595 (2009).</li><li>Wise, F. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Femtosecond+Fiber+Lasers+Based+on+Dissipative+Processes+for+Nonlinear+Microscopy.">Femtosecond Fiber Lasers Based on Dissipative Processes for Nonlinear Microscopy.</a> IEEE Journal of Selected Topics in Quantum Electronics. 18, 1412-1421 (2012).</li><li>Nielsen, C. K., Keiding, S. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-fiber+mode-locked+fiber+laser.">All-fiber mode-locked fiber laser.</a> Optics Letters. 32, 1474 (2007).</li><li>Liu, Z., Ziegler, Z. M., Wright, L. G., Wise, F. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Megawatt+peak+power+from+a+Mamyshev+oscillator.">Megawatt peak power from a Mamyshev oscillator.</a> Optica. 4, 649-654 (2017).</li><li>Sidorenko, P., Fu, W., Wright, L. G., Olivier, M., Wise, F. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Self-seeded,+multi-megawatt,+Mamyshev+oscillator.">Self-seeded, multi-megawatt, Mamyshev oscillator.</a> Optics Letters. 43, 2672-2675 (2018).</li><li>Li, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-power+ultrafast+Yb:fiber+laser+frequency+combs+using+commercially+available+components+and+basic+fiber+tools.">High-power ultrafast Yb:fiber laser frequency combs using commercially available components and basic fiber tools.</a> Review of Scientific Instruments. 87, 093114 (2016).</li><li>Bale, B., Kieu, K., Kutz, J., Wise, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transition+dynamics+for+multi-pulsing+in+mode-locked+lasers.">Transition dynamics for multi-pulsing in mode-locked lasers.</a> Optics Express. 17, 23137-23146 (2009).</li><li>Davoudzadeh, N., Ducourthial, G., Spring, B. Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Custom+fabrication+and+mode-locked+operation+of+a+femtosecond+fiber+laser+for+multiphoton+microscopy.">Custom fabrication and mode-locked operation of a femtosecond fiber laser for multiphoton microscopy.</a> Scientific Reports. 9, 4233 (2019).</li><li>Renninger, W., Chong, A., Wise, F. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Area+theorem+and+energy+quantization+for+dissipative+optical+solitons.">Area theorem and energy quantization for dissipative optical solitons.</a> Journal of the Optical Society of America. 27, 1978-1982 (2010).</li></ol>

The authors declare no competing interests.

The protocols outlined here synthesize know-how and expertise that have been common practice in the laser physics laboratory for decades, but which is frequently unfamiliar to many biomedical researchers. This work attempts to make this ultrafast fiber laser technology more accessible to the broader community. The ANDi fiber laser design is well-established, as first developed in seminal works by Wise and colleagues3. However, implementations of this technology by other groups have sometimes resul...

Solid state femtosecond (fs) pulsed lasers are widely used for microscopy and biological research. One typical example is the usage of multiphoton excitation (MPE) fluorescence microscopy, where high peak power and low average power are desired to facilitate the MPE process while minimizing photodamage mechanisms. Many high-performance solid-state lasers are commercially available, and when combined with an optical parametric oscillator (OPO), the laser wavelength can be tuned over a wide range1. For example, commercial oscillator-OPO systems generate &#60;120 fs pulse durations (typically with an 80 MHz pulse repetition rate) and &#62;1 W average power from 680 to 1,300 nm. However, the cost of these commercial tunable fs laser systems is significant (&#62;$200,000), and solid-state systems generally require water cooling and are not portable for clinical applications.
Ultrashort pulsed fiber laser technology has matured in the past few years. The cost of a commercial fs pulsed fiber laser is typically significantly lower than solid-state lasers, albeit without the capability of broad wavelength tuning afforded by the solid-state systems mentioned above. Note that fiber lasers can be paired with OPOs when desired (i.e., hybrid fiber-solid-state systems). The large surface-to-volume ratio of fiber laser systems enables efficient air cooling2. Hence, fiber lasers are more portable than solid-state systems due to their relatively small size and simplified cooling system. Further, fusion splicing of the fiber components reduces system complexity and mechanical drift in contrast to the free-space alignment of the optical components making up solid-state devices. All of these features make fiber lasers ideal for clinical applications. In fact, all-fiber lasers have been developed for low-maintenance operation3,4,5, and all-polarization-maintaining (PM)-fiber lasers are stable to environmental factors including changes in temperature and humidity as well as mechanical vibrations2,6,7,8.
Here, a method is presented to build a cost-efficient fs pulsed ANDi fiber laser with commercially available parts and standard fiber splicing techniques. Methods to characterize pulse repetition rate, duration, and coherence (full mode-lock) are also presented. The resulting fiber laser generates mode-locked pulses that can be compressed to 70 fs with a repetition rate of 31 MHz and a wavelength centered at 1,060 to 1,070 nm. The maximum power output from the laser cavity is approximately 1 W. The pulse physics of ANDi fiber lasers elegantly utilizes nonlinear polarization evolution intrinsic to optical fiber as a key component of the saturable absorber2,3,9,10,11. However, this means that the ANDi design is not easily implemented using PM fiber (although an all-PM fiber implementation of ANDi mode-locking has been reported, albeit with low power and ps pulse duration12). Thus, environmental stability requires significant engineering. Next generation fiber laser designs, such as the Mamyshev oscillator, have the potential to offer complete environmental stability as all-PM-fiber devices capable of an order-of-magnitude increase in intracavity pulse energy as well as offering significant decreases in pulse duration to enable applications that rely on broad pulse spectra13,14. Custom fabrication of these innovative new fs fiber laser designs requires know-how and fiber splicing experience.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Adapters, mirrors, posts, mounts, and translational stage (optomechanics)</td>
			<td>Thorlabs</td>
			<td>TR6-P5 (3x), AD12NT (2x), PFSQ20-03-M01, PFSQ05-03-M01, KMS, KM100C, KM100CL, KM200S, LT1, LT101, UPH2-P5, UPH3-P5 (2x)</td>
			<td>Standard optical components</td>
		</tr>
		<tr>
			<td>Advanced optical fiber cleaver</td>
			<td>AFL</td>
			<td>CT-100</td>
			<td></td>
		</tr>
		<tr>
			<td>Autocorrelator</td>
			<td>Femtochrome</td>
			<td>FR-103XL/IR/FA/CDA</td>
			<td></td>
		</tr>
		<tr>
			<td>Beamsplitter mount</td>
			<td>Thorlabs</td>
			<td>BSH1/M</td>
			<td></td>
		</tr>
		<tr>
			<td>Factory fusion splicer</td>
			<td>AFL</td>
			<td>FSM-100P</td>
			<td></td>
		</tr>
		<tr>
			<td>Fiber collimators</td>
			<td>OZ Optics (Canada)</td>
			<td>LPC-08-1064-6/125-S-1.6-7.5AS-60-X-1-2-HPC</td>
			<td>3x</td>
		</tr>
		<tr>
			<td>Fiber-coupled,high-speed photodiode detector</td>
			<td>Thorlabs</td>
			<td>DET08CFC</td>
			<td></td>
		</tr>
		<tr>
			<td>Free-space isolator</td>
			<td>Thorlabs</td>
			<td>IO-5-1050-HP</td>
			<td></td>
		</tr>
		<tr>
			<td>Free-space isolator</td>
			<td>Thorlabs</td>
			<td>IO-3D-1050-VLP</td>
			<td></td>
		</tr>
		<tr>
			<td>Half waveplate</td>
			<td>Union Optics (China)</td>
			<td>WPZ2312</td>
			<td>2x</td>
		</tr>
		<tr>
			<td>High power multimode fiber pump module</td>
			<td>Gauss Lasers (China)</td>
			<td>Pump-MM-976-10</td>
			<td></td>
		</tr>
		<tr>
			<td>High power pump and signal combiner</td>
			<td>ITF Technology (Canada)</td>
			<td>MMC02112DF1</td>
			<td></td>
		</tr>
		<tr>
			<td>Index matching gel</td>
			<td>Thorlabs</td>
			<td>G608N3</td>
			<td></td>
		</tr>
		<tr>
			<td>Optical spectrum analyzer</td>
			<td>Keysight</td>
			<td>Agilent 70951B</td>
			<td></td>
		</tr>
		<tr>
			<td>Oscilloscope</td>
			<td>Keysight</td>
			<td>Agilent 54845A</td>
			<td></td>
		</tr>
		<tr>
			<td>Passive double clad fiber(5/130 &#956;m)</td>
			<td>ITF Technology (Canada)</td>
			<td>MMC02112DF1</td>
			<td>3m, Included with combiner</td>
		</tr>
		<tr>
			<td>Polarizing beamsplitter</td>
			<td>Thorlabs</td>
			<td>PBS253</td>
			<td></td>
		</tr>
		<tr>
			<td>Quarter waveplates</td>
			<td>Union Optics (China)</td>
			<td>WPZ4312</td>
			<td>2x</td>
		</tr>
		<tr>
			<td>Quartz birefringent filter plate</td>
			<td>Newlight (Canada)</td>
			<td>BIR1060</td>
			<td></td>
		</tr>
		<tr>
			<td>RF spectrum analyzer</td>
			<td>Tektronix</td>
			<td>RSA306B</td>
			<td></td>
		</tr>
		<tr>
			<td>Single mode fiber (6/125 &#956;m)</td>
			<td>OZ Optics (Canada)</td>
			<td>LPC-08-1064-6/125-S-1.6-7.5AS-60-X-1-2-HPC</td>
			<td>1m, Included with collimators</td>
		</tr>
		<tr>
			<td>Single mode fiber coupler</td>
			<td>AFW (Australia)</td>
			<td>FOSC-2-64-30-L-1-H64-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Transmission diffraction grating 1</td>
			<td>LightSmyth</td>
			<td>T-1000-1040-3212-94</td>
			<td>For compressor</td>
		</tr>
		<tr>
			<td>Transmission diffraction grating 2</td>
			<td>LightSmyth</td>
			<td>T-1000-1040-60x12.3-94</td>
			<td>For compressor</td>
		</tr>
		<tr>
			<td>Waveplate rotation mount</td>
			<td>Thorlabs</td>
			<td>RSP1/M</td>
			<td>4x</td>
		</tr>
		<tr>
			<td>Ytterbium-doped single mode double clad fiber</td>
			<td>Thorlabs</td>
			<td>YB1200-6/125DC</td>
			<td>3m</td>
		</tr>
	</tbody>
</table>

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.

It is critical to verify mode-locked operation upon completion of the fiber laser fabrication procedures. Signatures of optimal fs pulse generation and laser stability are as follows: First, the output pulse may be sufficiently characterized by the instrumentation outlined in step 6. The pulse spectrum output from the laser oscillator should be centered near 1,070 nm with the characteristic cat-ear or Batman shape that indicates mode-locking as predicted by numerical simulation of ANDi pu...

Watch this Scientific Journal Video about Low-cost Custom Fabrication and Mode-locked Operation of an All-normal-dispersion Femtosecond Fiber Laser for Multiphoton Microscopy at JoVE.com

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

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 ...

low-cost-custom-fabrication-mode-locked-operation-an-all-normal

Research

JoVE Journal

Engineering

7.4K Views.  Northeastern University. 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.

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

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

Fabrication, Operation and Flow Visualization in Surface-acoustic-wave-driven Acoustic-counterflow Microfluidics

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we present the fabrication protocol for a multilayered SAW acoustic counterflow device. The device is fabricated starting from a lithium niobate (LN) substrate onto which two interdigital transducers (IDTs) and appropriate markers are patterned. A polydimethylsiloxane (PDMS) channel cast on an SU8 master mold is finally bonded on the patterned substrate. Following the fabrication procedure, we show the techniques that allow the characterization and operation of the acoustic counterflow device in order to pump fluids through the PDMS channel grid. We finally present the procedure to visualize liquid flow in the channels. The protocol is used to show on-chip fluid pumping under different flow regimes such as laminar flow and more complicated dynamics characterized by vortices and particle accumulation domains.

In this video we first describe fabrication and operation procedures of a surface acoustic wave (SAW) acoustic counterflow device. We then demonstrate an experimental setup that allows for both qualitative flow visualization and quantitative analysis of complex flows within the SAW pumping device.

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we ...

Fabrication and Operation of a Nano-Optical Conveyor Belt

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and&#160;limits the&#160;resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological ...

Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a new and inexpensive procedure to force mode locking in a pre-adjusted nonlinear polarization rotation fiber laser. This procedure is based on the detection of a sudden change in the output polarization state when mode locking occurs. This change is used to command the alignment of the intra-cavity polarization controller in order to find mode-locking conditions. More specifically, the value of the first Stokes parameter varies when the angle of the polarization controller is swept and, moreover, it undergoes an abrupt variation when the laser enters the mode-locked state. Monitoring this abrupt variation provides a practical easy-to-detect signal that can be used to command the alignment of the polarization controller and drive the laser towards mode locking. This monitoring is achieved by feeding a small portion of the signal to a polarization analyzer measuring the first Stokes parameter. A sudden change in the read out of this parameter from the analyzer will occur when the laser enters the mode-locked state. At this moment, the required angle of the polarization controller is kept fixed. The alignment is completed. This procedure provides an alternate way to existing automating procedures that use equipment such as an optical spectrum analyzer, an RF spectrum analyzer, a photodiode connected to an electronic pulse-counter or a nonlinear detecting scheme based on two-photon absorption or second harmonic generation. It is suitable for lasers mode locked by nonlinear polarization rotation. It is relatively easy to implement, it requires inexpensive means, especially at a wavelength of 1550 nm, and it lowers the production and operation costs incurred in comparison to the above-mentioned techniques.

A protocol to detect and automate mode locking in a pre-adjusted nonlinear polarization rotation fiber laser is presented. The detection of a sudden change in the output polarization state when mode locking occurs is used to command the alignment of an intra-cavity polarization controller in order to find mode-locking conditions.

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a ...

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Acoustofluidic devices use ultrasonic waves within microfluidic channels to manipulate, concentrate and isolate suspended micro and nanoscopic entities. This protocol describes the fabrication and operation of such a device supporting bulk acoustic standing waves to focus particles in a central streamline without the aid of sheath fluids.

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

Design and Fabrication of an Optical Fiber Made of Water

In this report, an optical fiber of which the core is made solely of water, while the cladding is air, is designed and manufactured. In contrast with solid-cladding devices, capillary oscillations are not restricted, allowing the fiber walls to move and vibrate. The fiber is constructed by a high direct current (DC) voltage of several thousand volts (kV) between two water reservoirs that creates a floating water thread, known as a water bridge. Through the choice of micropipettes, it is possible to control the maximal diameter and length of the fiber. Optical fiber couplers, at both sides of the bridge, activate it as an optical waveguide, allowing researchers to monitor the water fiber capillary body waves through transmission modulation and, therefore, deducing changes in surface tension.
Co-confining two important wave types, capillary and electromagnetic, opens a new path of research in the interactions between light and liquid-wall devices. Water-walled microdevices are a million times softer than their solid counterparts, accordingly improving the response to minute forces.

This protocol describes the design and manufacture of a water bridge and its activation as a water fiber. The experiment demonstrates that capillary resonances of the water fiber modulate its optical transmission.

In this report, an optical fiber of which the core is made solely of water, while the cladding is air, is designed and manufactured. In contrast with ...

Femtosecond Laser Filaments for Use in Sub-Diffraction-Limited Imaging and Remote Sensing

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode scanners. In classical optics, the intensity that can be brought to bear on a remote target is limited by the spot size of the laser at the distance of the target. This spot size has a lower bound determined by the diffraction limit of classical optics. However, amplified femtosecond laser pulses generate intensity sufficient to modify the refractive index of the ambient air and undergo self-focusing. This self-focusing effect leads to the generation of highly intense laser filaments which maintain their intensity and small sub-millimeter diameter size at distances well beyond the classical Rayleigh length. Such intensity provides the capability of remote scanning, imaging, sensing, and spectroscopy with enhanced spatial resolution. We describe a technique for generating filaments with a femtosecond regenerative chirped-pulse amplifier, and for using the resulting filament to conduct imaging and spectroscopic measurements at remote distances of at least several meters.

High-intensity femtosecond pulses of laser light can undergo cycles of Kerr self-focusing and plasma defocusing, propagating an intense sub-millimeter-diameter beam over long distances. We describe a technique for generating and using these filaments to perform remote imaging and sensing beyond the classical diffraction limits of linear optics.

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode ...