The authors would like to thank Christian Olivier and Philippe Chr&#233;tien for valuable help concerning electronics, &#201;ric Girard at GiGa Concept Inc. for support with the motorized polarization controller, professor R&#233;al Vall&#233;e for the loan of the commercial polarimeter and professor Michel Pich&#233; for many fruitful discussions.
This work was supported by the Fonds de recherche du Qu&#233;bec - Nature et technologies (FRQNT), the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Summer Jobs.

1. Setting Up a Fiber ML Fiber Laser Including a Motorized PSC
<ol>
	<li>Gather the following components: a single-mode erbium-doped fiber, a 980/1,550 nm wavelength division multiplexer (WDM), a 980/1,550 nm WDM-1,550 nm isolator hybrid component, a 50/50 fiber coupler, a fiber polarizer, a motorized PSC, two 980 nm laser pump diodes, a 99/1 fiber coupler and a manual inline PSC.</li>
	<li>Cut the erbium-doped fiber and all the other fiber-pigtailed components to fit with the desired cavity design. 
	NOTE: The presented automation procedure is suitable for fiber lasers based on nonlinear polarization rotation. It should work for different operating regimes such as the soliton laser, the stretched-pulse laser, the dissipative soliton laser and the similariton laser. The latter regime is used in this experiment.</li>
	<li>To build the laser cavity, use a fiber fusion splicer to join the cavity components in the order shown in the diagram (Figure 3). Before performing each fusion splice, clean the fibers ends with isopropyl alcohol and cleave them with a fiber cleaver. 
	NOTE: The internal components of the laser are, in clockwise order in the ring cavity, a motorized PSC, a 980/1,550 nm WDM, an erbium-doped fiber, a 980/1,550 nm WDM isolator hybrid component, a 50/50 output coupler and a fiber polarizer. The external components are a 99/1 fiber coupler and a manual inline PSC (as discussed in steps 1.7 and 1.8). 
	NOTE: A fiber segment of approximately 30 cm must be inserted in the motorized PSC before the splices are performed with the other components of the cavity. Although a standard single-mode fiber will work, the use of polyimide-coated fiber is recommended for this segment because it is more resistant to the pressure exerted by the screws of the controller and will thus last longer.</li>
	<li>Join the pump laser diodes to the WDMs using the fusion splicer. Again, clean the fibers ends with isopropyl alcohol and cleave them with a fiber cleaver before performing each fusion splice.</li>
	<li>Connect the laser diodes to their respective temperature controllers and current drivers.</li>
	<li>Connect the intra-cavity motorized Yao-type fiber-squeezer PSC (Figure 4) to its driving module and then connect the driving module to the USB port of a computer. 
	NOTE: This port is identified by number &#34;COM4&#34; as shown in the &#34;Device Manager&#34; of the computer.</li>
	<li>At the output of the laser, i.e. the 50/50 coupler&#39;s port not spliced yet, splice a 99/1 coupler. 
	NOTE: The 99% port is the usable output. The 1% port is used to monitor the polarization state in the automation procedure.</li>
	<li>Insert a manual PSC along the fiber of the 1% port. To do so, remove the screws and open the PSC. Insert the fiber in the appropriate slot and then put the screws back into their holes and screw them in.</li>
	<li>Splice an angle-polished fiber connector (APC) at the end of the 1% port fiber (after the manual PSC). Clean and cleave the fibers ends before performing the fusion splice.</li>
	<li>Connect the 99% output to an optical spectrum analyzer (OSA) using a bare-fiber adaptor. 
	NOTE: As discussed later, the optical spectrum seen on the OSA will provide an alternate way of checking if the laser is ML.</li>
	<li>Secure all the fibers and the components in the cavity properly with polyimide film tape. 
	NOTE: The fibers and components must be prevented from moving under any conditions such as when the table vibrates or fans blow air. The polyimide film tape is used in order to avoid damaging the fibers.</li>
	<li>Tighten the pressure screws of the intra-cavity PSC until the fiber begins to be slightly squeezed.</li>
	<li>Turn on the pump lasers diodes and adjust their currents to their maximum values as specified by the laser diode manufacturer.</li>
	<li>Start the instrument communication interface. In the &#34;Peripherals and Interface&#34; column on the left, choose &#34;COM4&#34;. Click on &#34;Open VISA test panel&#34;. Click on &#34;Input/Output&#34;. Then, in &#34;Select or enter command&#34; type &#34;SM,500,3000\n&#34; and click on the &#34;Query&#34; button. This commands the PSC to rotate by 3,000 steps of 0.1125&#176; in the clockwise direction. While doing so, the PSC reaches a mechanical stop.</li>
	<li>In the &#34;Select or enter command&#34; of the &#34;COM4&#34; test panel, type &#34;SM,500,-10\n&#34; and click on the &#34;Query&#34; button. The PSC then rotates approximately 1&#176; counterclockwise. Check if ML is reached by looking at the optical spectrum on the OSA. ML is reached when the full-width at half maximum of the optical spectrum is of the order of a few tens of nanometers (Figure 5). If ML is reached, keep the birefringence and the angle fixed and go to step 1.18.</li>
	<li>If ML is not reached, repeat 1.15 until either ML or the maximum angle attainable with the PSC is reached.</li>
	<li>If the maximum angle of the PSC is reached before ML occurs, increase the birefringence of the PSC by tightening the pressure screws slightly and repeat steps 1.14, 1.15 and 1.16 as many times as required to get ML.</li>
	<li>Once ML is reached, decrease the pump powers to their minimum value allowing ML to self-start. To do so, reduce the pump powers until ML is lost. Then, bring them back slowly toward the smallest value that will make the ML reappear. Turn the pumps off and back on again and check if the laser mode locks by itself. Increase the pump powers slightly more to ensure the ML is stable and will self-start each time the laser is turned on.</li>
</ol>
2. Analyzing the Polarization of the Output Signal
<ol>
	<li>Link the 1% tap to a commercial polarimeter.</li>
	<li>Connect the polarimeter to the computer using a USB port.</li>
	<li>In the &#34;Select or enter command&#34; of the &#34;COM4&#34; test panel, type &#34;SM,500,3000\n&#34; and click on the &#34;Query&#34; button.</li>
	<li>Run the commercial polarimeter controlling software and start the polarization measurement by clicking on the &#34;Start&#34; button.</li>
	<li>In the &#34;Select or enter command&#34; of the &#34;COM4&#34; test panel, type &#34;SM,500,-10\n&#34; and click on the &#34;Query&#34; button. Observe the polarization state on the polarimeter.</li>
	<li>Repeat step 2.5 as many times as required to cover the whole range of angles allowed by the intra-cavity PSC. Observe that the polarization state varies very smoothly with the angle except at the specific angles where ML is reached as can be seen by watching simultaneously the width of the optical spectrum on the OSA.</li>
	<li>Repeat steps 2.3 to 2.6 but this time, instead of just watching the polarization state, record the values of the Stokes parameters S1, S2 and S3 as functions of the angle of the PSC (Figure 6). To see these values clearly, choose &#34;Measurement-&#8594;Oscilloscope&#34; in the menu of the software and look for the mean values of S1, S2 and S3. Simultaneously watch the optical spectrum and record the angles for which the laser is ML.</li>
</ol>
3. Setting Up a Feedback Loop to Automate the Alignment of the PSC Using the Commercial Polarimeter Measurements
<ol>
	<li>Turn off the computer.</li>
	<li>Connect the serial port of the commercial polarimeter to the serial port &#34;COM1&#34; of the computer. Restart the computer and the polarimeter.</li>
	<li>Start the graphical programming language interface (GPLI) that will allow the reading of the polarimeter via &#34;COM1&#34; and the control of the motorized PSC via &#34;COM4&#34;.</li>
	<li>In the GPLI, click on &#34;Blank VI&#34;. Then, select &#34;Window&#8594;Tile Left and Right&#34;. 
	NOTE: The screen will then be divided in two parts. The block diagram is displayed on the right. It is used to create the script using different functions associated with different icons. The front panel is displayed on the left. It is used to display the commands and the measurements when the script is running.</li>
	<li>In the block diagram window of the GPLI, develop a ML automation script to be used with the commercial polarimeter (see Figure 7). 
	NOTE: This script reads S1 from the polarimeter and uses its value to provide feedback and reach the proper alignment of the PSC angle leading to ML. The detection of ML is achieved by searching for a discontinuity in the variation of S1 as the angle is varied. 
	NOTE: The commands used to control the PSC via &#34;COM4&#34; are the same as the ones presented in steps 2.3 and 2.5. The command to read S1 on the commercial polarimeter via &#34;COM1&#34; is &#34;*SOP?\n&#34;.</li>
	<li>Save the script by clicking on &#34;File&#8594;Save&#34; and then run it by clicking on the &#34;&#8594;&#34; button. The PSC is brought back to its mechanical stop, then it rotates by steps of approximately 1&#176; until ML is reached, showing the value of S1 as it evolves.</li>
</ol>
4. Building a Rudimentary Homemade Polarization Analyzer
<ol>
	<li>Connect an oscilloscope to the computer using the GPIB interface.</li>
	<li>Put a polarizing beam splitter cube (PBS) on an optics bench.</li>
	<li>Set up three FC/APC fiber optics port collimators with the PBS (Figure 8). 
	NOTE: One of the ports is the input. The other two are the outputs for the x- and y- polarization components of the signal.</li>
	<li>Connect a fiber-pigtailed InGaAS PIN photodiode to the first output.</li>
	<li>Connect the photodiode to a trans-impedance circuit (Figure 9).</li>
	<li>Connect the electrical output of the circuit to channel 1 of the oscilloscope.</li>
	<li>Turn on the trans-impedance circuit.</li>
	<li>In the GPLI, read the average value of the voltage on channel 1 of the oscilloscope via the GPIB connection by using the commands &#34;MEASU:IMM:SOU ch1;&#34; to select channel 1 of the oscilloscope, &#34;MEASU:IMM:TYPE mean;&#34; to define the measurement to be an average voltage, &#34;MEASU:IMM:VAL?&#34; to get the value and finally &#34;MEASU:IMM:UNI?&#34; to obtain the units of the measurement. Save the script by clicking on &#34;File&#8594;Save&#34; and then run it by clicking on the &#34;&#8594;&#34; button.</li>
	<li>Connect the 1% output of the laser at the input port of the PBS and turn the laser on at an arbitrary pump power. This sends a 1,550 nm optical signal to the input.</li>
	<li>Measure the average voltage at the first output. Then, disconnect the fiber-pigtailed photodiode and replace it by a commercial power-meter. Measure the optical power at this output.</li>
	<li>Repeat step 4.10 while varying the power of the input optical signal. The voltage should vary linearly with the optical power. Find the coefficients of this linear relation. 
	NOTE: This relation will be used in step 4.20 to obtain Px from the measured voltage.</li>
	<li>Connect a second fiber-pigtailed InGaAS PIN photodiode to the second output of the PBS.</li>
	<li>Connect the photodiode to a second trans-impedance circuit.</li>
	<li>Connect the electrical output of the circuit to channel 2 of the oscilloscope.</li>
	<li>Turn on the trans-impedance circuit.</li>
	<li>In the GPLI, read the average value of the voltage on channel 2 of the oscilloscope via the GPIB connection by using the commands &#34;MEASU:IMM:SOU ch2;&#34; to select channel 2 of the oscilloscope, &#34;MEASU:IMM:TYPE mean;&#34; to define the measurement to be an average voltage, &#34;MEASU:IMM:VAL?&#34; to get the value and finally &#34;MEASU:IMM:UNI?&#34; to obtain the units of the measurement. Save the script by clicking on &#34;File&#8594;Save&#34; and then run it by clicking on the &#34;&#8594;&#34; button.</li>
	<li>Turn the laser on at an arbitrary pump power.</li>
	<li>Measure the average voltage at the second output. Then, disconnect the fiber-pigtailed photodiode and replace it by a commercial power-meter. Measure the optical power at this output.</li>
	<li>Repeat step 4.18 while varying the power of the input optical signal. Ensure that the voltage varies linearly with the optical power. 
	NOTE: Find the coefficients of this linear relation. This relation will be used in step 4.20 to obtain Py from the measured voltage.</li>
	<li>After setting up the second detector to measure Py, use the GPLI to compute the first Stokes parameter S1 defined as S1 = (Px-Py)/(Px+Py). The homemade rudimentary polarization analyzer is now ready to use.</li>
</ol>
5. Replacing the Commercial Polarimeter by the Homemade Polarization Analyzer in the Automation Process
<ol>
	<li>Connect the 1% output of the laser to the homemade polarization analyzer input (as was done in step 4.9).</li>
	<li>Measure the first Stokes parameter S1 as a function of the angle of the PSC (Figure 10) by repeating step 2.7 using the homemade polarization analyzer (instead of the commercial polarimeter). Observe the S1 graph automatically updating at each step. Observe a discontinuous jump in the value of S1 when ML occurs (this is the case while using the commercial polarimeter). 
	NOTE: Use a GPLI script to perform this task automatically. This script is based on a loop that varies the angle of the PSC by steps of 1&#176; (using the command &#34;SM,500,-10\n&#34; sent to &#34;COM4&#34;) and reads out the value of S1 from the homemade polarization analyzer at each step.</li>
	<li>Modify the script developed in 3.5 so that, instead of using the value given by the commercial polarimeter, it gets Px and Py from the homemade polarization analyzer and then computes S1 = (Px-Py)/ (Px+Py) .</li>
	<li>Use the new script based on the homemade polarization analyzer to ML the laser automatically in a way similar to step 3.6.</li>
</ol>

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The authors have nothing to disclose.

It has been shown that it is possible to automate the ML of NPR fiber ring lasers by using a feedback loop based on output polarization measurements. To realize this task it is crucial to insert an adjustable PSC in the cavity. The output coupler of the cavity must be located just before the polarizer in order to see a difference between the polarization state of a CW signal and a pulse signal (Figure 2). The birefringence of the PSC must be pre-adjusted so that ML can be found and the pump power must be...

The purpose of this article is to present an automation alignment procedure to get mode locking (ML) in nonlinear polarization rotation fiber lasers. This procedure is based on two essential steps: detecting the ML regime by measuring the polarization of the output signal of the laser and then setting-up a self-start control system to get to ML.
Fiber lasers have become an important tool in optics nowadays. They are an efficient source of coherent near-infrared light and they are now extending into the mid-infrared portion of the electromagnetic spectrum. Their low cost and ease of use have made them an attractive alternative to other sources of coherent light such as solid-state lasers. Fiber lasers can also provide ultrashort pulses (100 fsec or less) when a ML mechanism is inserted in the fiber cavity. There are many ways to design this ML mechanism such as nonlinear loop mirrors and saturable absorbers. One of these, widely used for its simplicity, is based on nonlinear polarization rotation (NPR) of the signal1,2. It uses the fact that the polarization ellipse of the signal undergoes a rotation proportional to its intensity as it propagates in the fibers of the laser cavity. By inserting a polarizer in the cavity, this NPR leads to intensity-dependent losses during a roundtrip of the signal.
The laser can then be forced to ML by controlling the polarization state. Effectively, the high-power portions of the signal will be subjected to lower losses (Figure 1) and this will eventually lead to the formation of ultrashort pulses of light when the laser is turned on and starts from a low-power noisy signal. However, the drawback of this method is that the polarization state controller (PSC) must be properly aligned to get ML. Usually, an operator finds the ML manually by varying the position of the PSC and analyzing the output signal of the laser with a fast photodiode, an optical spectrum analyzer or a nonlinear optical auto-correlator. As soon as the emission of pulses is detected, the operator stops varying the position of the PSC since the laser is ML. Obviously getting the laser to self-start automatically leads to an important gain in efficiency. This is especially true when the laser is subject to perturbations changing the alignment or the cavity configuration since the operator has to go through the alignment procedure again and again. In the last decade, different methods have been proposed to achieve this automation. Hellwig et al.3 used piezo-electric squeezers to control polarization in combination with a full analysis of the polarization state of the signal with an all-fiber division-of-amplitude polarimeter to detect ML. Radnarotov et al.4 used liquid-crystal plate PSCs with an analysis based on the RF spectrum to detect ML. Shen et al.5 used piezo-electric squeezers to control polarization and a photodiode/high-speed counter system to detect ML. More recently, a strategy based on an evolutionary algorithm was presented in which the detection is provided by a high-bandwidth photodiode in combination with an intensimetric second-order autocorrelator and an optical spectrum analyzer. The control is then performed with two electronically driven PSCs inside the cavity6.
This article describes an innovative way of detecting ML and its application to an automation technique forcing the fiber laser to ML. The detection of ML of the laser is achieved by analyzing how the output polarization state of the signal varies as the angle of the PSC is swept. As will be shown, the transition to ML is associated with a sudden change in the polarization state detectable by measuring one of the Stokes parameters of the output signal. The fact that a pulse is more intense than a CW signal and will undergo a more important NPR explains this change. Since the output of the laser is immediately located before the polarizer in the cavity, the polarization state of a pulse at this location is different from the polarization state of a CW signal (Figure 2) and will be used to discriminate the ML state. Theoretical aspects of this procedure and its first experimental implementation were presented in Olivier et al.7. In this article, the emphasis will be on the technical aspects of the procedure, its limitations and its advantages.
This technique is relatively simple to implement and does not require sophisticated measuring instruments to detect the ML state and automate the alignment of the laser to get ML. A PSC adjustable externally through a programmable interface is required. Different PSCs could be used in principle: piezo-electric squeezers, liquid crystal, wave-plates rotated by a motor, magneto-optic crystals or a motorized all-fiber PSC based on squeezing and twisting the fiber8. In this article, the latter is used, an all-fiber motorized Yao-type PSC. To detect the polarization state an expensive commercial polarimeter can be used. However, since only the value of the first Stokes parameter is required, a polarizing beam splitter in combination with two photodiodes will be sufficient as shown in this article.
All these components are inexpensive for the widely used erbium-doped fiber lasers. A feedback loop based on this procedure can find ML in a few minutes. This response time is suitable for most applications of fiber lasers and is comparable to the other existing techniques. In fact, the response time is limited by the electronics used to analyze the polarization of the signal. Finally, although the procedure is applied here to a similariton9 erbium-doped fiber laser, it could be used for any NPR based fiber laser as soon as the above mentioned equipment or its equivalent becomes available at the wavelength of interest.

<table><tbody><tr><td>Bare-Fiber adaptor</td><td>Bullet</td><td>NGB-14</td><td></td></tr><tr><td>Drop-in polarization controller</td><td>General Photonics Corp.</td><td>Polarite PLC-006</td><td>Manual polarization controller.</td></tr><tr><td>DSP In-line polarimeter</td><td>General Photonics Corp.</td><td>POD-101D PolaDetect</td><td>Polarimeter with USB/serial computer connectivity.</td></tr><tr><td>Fiber Cleaver</td><td>Fitel</td><td>S323</td><td></td></tr><tr><td>FiberPort</td><td>Thorlabs Inc.</td><td>PAF-X-2-C</td><td></td></tr><tr><td>Fixed Fiber-to-Fiber Coupler Bench</td><td>Thorlabs Inc.</td><td>FBC-1550-APC</td><td>Any optical bench could be used. A 3-way bench would even be better.</td></tr><tr><td>Fusion Splicer</td><td>Fujikura</td><td>FSM-40PM</td><td></td></tr><tr><td>High resolution all fiber polarization controller</td><td>Giga Concept Inc.</td><td>GIG-2201-1300</td><td>All-fiber motorized polarization controller with USB computer connectivity.</td></tr><tr><td>InGaAs PIN PD module</td><td>Optoway</td><td>PD-1310</td><td>Pigtailed photodiode.</td></tr><tr><td>Instrument communication interface</td><td>National Instruments</td><td>NI MAX</td><td>It comes packaged with National Instruments drivers (NI-VISA, NI-DAQmx, &lt;em&gt;etc.&lt;/em&gt;)</td></tr><tr><td>Operational amplifier</td><td>Texas Instruments</td><td>TLO81ACP</td><td></td></tr><tr><td>Optical Powermeter</td><td>Newport</td><td>818-IS-1 with 1835-C</td><td></td></tr><tr><td>Optical spectrum analyzer</td><td>Anritsu</td><td>MS9710C</td><td></td></tr><tr><td>Oscilloscope</td><td>Tektronix</td><td>TDS2022</td><td>Oscilloscope with GPIB computer connectivity.</td></tr><tr><td>Polarizing beamsplitter module</td><td>Thorlabs Inc.</td><td>PSCLB-VL-1550</td><td></td></tr><tr><td>Polyimide Film Tape</td><td>3M</td><td>5413</td><td>Tape to fix the components on the table without damaging the fibers.</td></tr><tr><td>Graphical programming language interface (GPLI)</td><td>National Instruments</td><td>LabVIEW</td><td>Interface to program in G Programming Language and communicate with laboratory instruments.</td></tr><tr><td>Polarimeter controlling software</td><td>General Photonics Corp.</td><td>PolaView</td><td>Comes with the polarimeter General Photonics POD-101D.</td></tr></tbody></table>

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.

NPR mode-locked fiber lasers are known to provide a large variety of pulsing regimes such as Q-switched pulses10, coherent ML pulses, noise-like pulses, bound states of ML pulses, harmonic ML and complex structures of interacting ML pulses11. In the laser described here, after the birefringence of the PSC was fixed to be able to get ML, the pump power was adjusted to be relatively near the threshold of single-pulse ML. In doing so, the number of competing regimes was...

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

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 ...

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11.3K Views.  Université Laval. 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.

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

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 ...

Characterization of Anisotropic Leaky Mode Modulators for Holovideo

Holovideo displays are based on light-bending spatial light modulators. One such spatial light modulator is the anisotropic leaky mode modulator. This modulator is particularly well suited for holographic video experimentation as it is relatively simple and inexpensive to fabricate1-3. Some additional advantages of leaky mode devices include: large aggregate bandwidth, polarization separation of signal light from noise, large angular deflection and frequency control of color1. In order to realize these advantages, it is necessary to be able to adequately characterize these devices as their operation is strongly dependent on waveguide and transducer parameters4. To characterize the modulators, the authors use a commercial prism coupler as well as a custom characterization apparatus to identify guided modes, calculate waveguide thickness and finally to map the device's frequency input and angular output of leaky mode modulators. This work gives a detailed description of the measurement and characterization of leaky mode modulators suitable for full-color holographic video.

This work describes fabrication and characterization of anisotropic leaky mode modulators for holographic video.

Holovideo displays are based on light-bending spatial light modulators.  One such spatial light modulator is the anisotropic leaky mode modulator.  This ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Chemistry

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

We present a cooling method for a cold Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap (ODT). Our method employs the anharmonicity of the ODT, in which the hotter atoms at the edge of the trap feel the anharmonic components of the trapping potential, while the colder atoms in the center of the trap feel the harmonic one. By modulating the trap depth with frequencies that are resonant with the anharmonic components, we selectively excite the hotter atoms out of the trap while keeping the colder atoms in the trap, generating parametric cooling. This experimental protocol starts with a magneto-optical trap (MOT) that is loaded by a Zeeman slower. The precooled atoms in the MOT are then transferred to an ODT, and a bias magnetic field is applied to create an interacting Fermi gas. We then lower the trapping potential to prepare a cold Fermi gas near the degenerate temperature. After that, we sweep the magnetic field to the noninteracting regime of the Fermi gas, in which the parametric cooling can be manifested by modulating the intensity of the optical trapping beams. We find that the parametric cooling effect strongly depends on the modulation frequencies and amplitudes. With the optimized frequency and amplitude, we measure the dependence of the cloud energy on the modulation time. We observe that the cloud energy is changed in an anisotropic way, where the energy of the axial direction is significantly reduced by parametric driving. The cooling effect is limited to the axial direction because the dominant anharmonicity of the crossed-beam ODT is along the axial direction. Finally, we propose to extend this protocol for the trapping potentials of large anharmonicity in all directions, which provides a promising scheme for cooling quantum gases using external driving.

We present a parametric driving method to cool an ultracold Fermi gas in a crossed-beam optical dipole trap. This method selectively removes high-energy atoms from the trap by periodically modulating the trap depth with frequencies that are resonant with the anharmonic components of the trapping potential.

We present a cooling method for a cold Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap (ODT). Our method employs ...

Fabrication Procedures and Birefringence Measurements for Designing Magnetically Responsive Lanthanide Ion Chelating Phospholipid Assemblies

Bicelles are tunable disk-like polymolecular assemblies formed from a large variety of lipid mixtures. Applications range from membrane protein structural studies by nuclear magnetic resonance (NMR) to nanotechnological developments including the formation of optically active and magnetically switchable gels. Such technologies require high control of the assembly size, magnetic response and thermal resistance. Mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and its lanthanide ion (Ln3+) chelating phospholipid conjugate, 1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine-diethylene triaminepentaacetate (DMPE-DTPA), assemble into highly magnetically responsive assemblies such as DMPC/DMPE-DTPA/Ln3+ (molar ratio 4:1:1) bicelles. Introduction of cholesterol (Chol-OH) and steroid derivatives in the bilayer results in another set of assemblies offering unique physico-chemical properties. For a given lipid composition, the magnetic alignability is proportional to the bicelle size. The complexation of Ln3+ results in unprecedented magnetic responses in terms of both magnitude and alignment direction. The thermo-reversible collapse of the disk-like structures into vesicles upon heating allows tailoring of the assemblies&#39; dimensions by extrusion through membrane filters with defined pore sizes. The magnetically alignable bicelles are regenerated by cooling to 5 &#176;C, resulting in assembly dimensions defined by the vesicle precursors. Herein, this fabrication procedure is explained and the magnetic alignability of the assemblies is quantified by birefringence measurements under a 5.5 T magnetic field. The birefringence signal, originating from the phospholipid bilayer, further enables monitoring of polymolecular changes occurring in the bilayer. This simple technique is complementary to NMR experiments that are commonly employed to characterize bicelles.

Fabrication procedures for highly magnetically responsive lanthanide ion chelating polymolecular assemblies are presented. The magnetic response is dictated by the assembly size, which is tailored by extrusion through nanopore membranes. The assemblies&#39; magnetic alignability and temperature-induced structural changes are monitored by birefringence measurements, a complimentary technique to nuclear magnetic resonance and small angle neutron scattering.

Bicelles are tunable disk-like polymolecular assemblies formed from a large variety of lipid mixtures. Applications range from membrane protein structural ...

Label-free Single Molecule Detection Using Microtoroid Optical Resonators

Detecting small concentrations of molecules down to the single molecule limit has impact on areas such as early detection of disease, and fundamental studies on the behavior of molecules. Single molecule detection techniques commonly utilize labels such as fluorescent tags or quantum dots, however, labels are not always available, increase cost and complexity, and can perturb the events being studied. Optical resonators have emerged as a promising means to detect single molecules without the use of labels. Currently the smallest particle detected by a non-plasmonically-enhanced bare optical resonator system in solution is a 25 nm polystyrene sphere1. We have developed a technique known as Frequency Locking Optical Whispering Evanescent Resonator (FLOWER) that can surpass this limit and achieve label-free single molecule detection in aqueous solution2. As signal strength scales with particle volume, our work represents a &#62; 100x improvement in the signal to noise ratio (SNR) over the current state of the art. Here the procedures behind FLOWER are presented in an effort to increase its usage in the field.

We have developed a label-free biosensing system based on optical resonator technology known as Frequency Locking Optical Whispering Evanescent Resonator (FLOWER) that is capable of detecting single molecules in solution. Here the procedures behind this work are described and presented.

Detecting small concentrations of molecules down to the single molecule limit has impact on areas such as early detection of disease, and fundamental ...

Bioengineering

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 ...

A Photonic System for Generating Unconditional Polarization-Entangled Photons Based on Multiple Quantum Interference

We present a high-performance source of unconditional polarization-entangled photons that have a high-emission rate, a broadband distribution, are degenerated and postselection free. The property of the source is based on the multiple quantum interference effect with a round-trip configuration of a Sagnac interferometer. The quantum interference effects make it possible to use the high generation efficiency of the polarization-entangled photons to process parametric down-conversion, and separate degenerated photon pairs into different optical modes without a postselection requirement. The principle of the optical system was described and experimentally used to measure the fidelity and Bell parameters, and also to characterize the generated polarization-entangled photons from a minimum of six combinations of polarization correlated data. The experimentally obtained fidelity and Bell parameters exceeded the classical local correlation limit and are clear evidence of the generation of unconditional polarization-entangled photons.

We describe an optical system for the generation of unconditional polarization-entangled photons based on multiple quantum interference effects with a detection scheme to estimate the experimental fidelity of generated entangled photons.

We present a high-performance source of unconditional polarization-entangled photons that have a high-emission rate, a broadband distribution, are ...

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 ...

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

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