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>

<ol>
	<li>Hofer, M., Fermann, M. E., Haberl, F., Ober, M. H., Schmidt, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mode+locking+with+cross-phase+and+selfphase+modulation.">Mode locking with cross-phase and selfphase modulation.</a> Opt. Lett. 16 (7), 502-504 (1991).</li><li>Haus, H. A., Ippen, E. P., Tamura, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Additive-pulse+modelocking+in+fiber+lasers.">Additive-pulse modelocking in fiber lasers.</a> IEEE J. Quantum Electron. 30 (1), 200-208 (1994).</li><li>Hellwig, T., Walbaum, T., Gro&#223;, P., Fallnich, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Automated+characterization+and+alignment+of+passively+mode-locked+fiber+lasers+based+on+nonlinear+polarization+rotation.">Automated characterization and alignment of passively mode-locked fiber lasers based on nonlinear polarization rotation.</a> Appl. Phys. B. 101 (3), 565-570 (2010).</li><li>Radnatarov, D., Khripunov, S., Kobtsev, S., Ivanenko, A., Kukarin, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Automatic+electronic-controlled+mode+locking+self-start+in+fibre+lasers+with+non-linear+polarization+evolution.">Automatic electronic-controlled mode locking self-start in fibre lasers with non-linear polarization evolution.</a> Opt. Express. 21 (18), 20626-20631 (2013).</li><li>Shen, X., Li, W., Yan, M., Zeng, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electronic+control+of+nonlinear-polarization-rotation+mode+locking+in+Yb-doped+fiber+lasers.">Electronic control of nonlinear-polarization-rotation mode locking in Yb-doped fiber lasers.</a> Opt. Lett. 37 (16), 3426-3428 (2012).</li><li>Andral, U., Si Fodil, R., Amrani, F., Billard, F., Hertz, E., Grelu, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fiber+laser+mode+locked+through+an+evolutionary+algorithm.">Fiber laser mode locked through an evolutionary algorithm.</a> Optica. 2 (4), 275-278 (2015).</li><li>Olivier, M., Gagnon, M. D., Pich&#233;, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Automated+mode+locking+in+nonlinear+polarization+rotation+fiber+lasers+by+detection+of+a+discontinuous+jump+in+the+polarization+state.">Automated mode locking in nonlinear polarization rotation fiber lasers by detection of a discontinuous jump in the polarization state.</a> Opt. Express. 23 (5), 6738-6746 (2015).</li><li>Ulrich, R., Simon, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polarization+optics+of+twisted+single-mode+fibers.">Polarization optics of twisted single-mode fibers.</a> Appl. Opt. 18 (13), 2241-2251 (1979).</li><li>Chong, A., Logan, L. R., 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=Ultrafast+fiber+lasers+based+on+self-similar+pulse+evolution:+a+review+of+current+progress.">Ultrafast fiber lasers based on self-similar pulse evolution: a review of current progress.</a> Rep. Prog. Phys. 78 (11), 113901 (2015).</li><li>Komarov, A., Leblond, H., Sanchez, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Theoretical+analysis+of+the+operating+regime+of+a+passively-mode-locked+fiber+laser+through+nonlinear+polarization+rotation.">Theoretical analysis of the operating regime of a passively-mode-locked fiber laser through nonlinear polarization rotation.</a> Phys. Rev. A. 72, 063811 (2005).</li><li>Kobtsev, S., Smirnov, S., Kukarin, S., Turitsyn, S. <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+fiber+lasers+with+significant+variability+of+generation+regimes.">Mode-locked fiber lasers with significant variability of generation regimes.</a> Optical Fiber Technology. 20 (6), 615-620 (2014).</li><li>Kobtsev, S., Kukarin, S., Smirnov, S., Turitsyn, S., Latkin, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Generation+of+double-scale+femto/pico-second+optical+lumps+in+mode-locked+fiber+lasers.">Generation of double-scale femto/pico-second optical lumps in mode-locked fiber lasers.</a> Opt. Express. 17 (23), 20707-20713 (2009).</li><li>Churkin, D. V., Sugavanam, S., Tarasov, N., Khorev, S., Smirnov, S. V., Kobtsev, S. M., Turitsyn, S. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stochasticity+periodicity+and+localized+light+structures+in+partially+mode-locked+fibre+lasers.">Stochasticity periodicity and localized light structures in partially mode-locked fibre lasers.</a> Nat. Commun. 6, 7004 (2015).</li><li>Duling, I. N., Chen, C. J., Wai, P. K. A., Menyuk, C. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Operation+of+a+nonlinear+loop+mirror+in+a+laser+cavity.">Operation of a nonlinear loop mirror in a laser cavity.</a> IEEE J. Quantum Electron. 30 (1), 194-199 (1994).</li><li>Krempzek, K., Grzegorz, S., Kaczmarek, P., Abramski, K. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+sub-100+fs+stretched-pulse+205+MHz+repetition+rate+passively+mode-locked+Er+doped+all-fiber+laser.">A sub-100 fs stretched-pulse 205 MHz repetition rate passively mode-locked Er doped all-fiber laser.</a> Laser Phys. Lett. 10, 105103 (2013).</li><li>Shtyrina, O., Fedoruk, M., Turitsyn, S., Herda, R., Okhotnikov, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evolution+and+stability+of+pulse+regimes+in+SESAM-mode-locked+femtosecond+fiber+lasers.">Evolution and stability of pulse regimes in SESAM-mode-locked femtosecond fiber lasers.</a> J. Opt. Soc. Am. B. 26 (2), 346-352 (2009).</li></ol>

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 border="0" cellpadding="0" cellspacing="0" width="903">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Bare-Fiber adaptor</td>
			<td style="width:163px;">Bullet</td>
			<td style="width:183px;">NGB-14</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:391px;">Drop-in polarization controller</td>
			<td style="width:163px;">General Photonics Corp.</td>
			<td style="width:183px;">Polarite PLC-006</td>
			<td>Manual polarization controller.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:391px;">DSP In-line polarimeter</td>
			<td style="width:163px;">General Photonics Corp.</td>
			<td style="width:183px;">POD-101D PolaDetect</td>
			<td>Polarimeter with USB/serial computer connectivity.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Fiber Cleaver</td>
			<td style="width:163px;">Fitel</td>
			<td style="width:183px;">S323</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">FiberPort</td>
			<td style="width:163px;">Thorlabs Inc.</td>
			<td style="width:183px;">PAF-X-2-C</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Fixed Fiber-to-Fiber Coupler Bench</td>
			<td style="width:163px;">Thorlabs Inc.</td>
			<td style="width:183px;">FBC-1550-APC</td>
			<td>Any optical bench could be used. A 3-way bench would even be better.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Fusion Splicer</td>
			<td style="width:163px;">Fujikura</td>
			<td style="width:183px;">FSM-40PM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">High resolution all fiber polarization controller</td>
			<td style="width:163px;">Giga Concept Inc.</td>
			<td style="width:183px;">GIG-2201-1300</td>
			<td>All-fiber motorized polarization controller with USB computer connectivity.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">InGaAs PIN PD module</td>
			<td style="width:163px;">Optoway</td>
			<td style="width:183px;">PD-1310</td>
			<td>Pigtailed photodiode.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Instrument communication interface</td>
			<td style="width:163px;">National Instruments</td>
			<td style="width:183px;">NI MAX</td>
			<td>It comes packaged with National Instruments drivers (NI-VISA, NI-DAQmx, etc.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Operational amplifier</td>
			<td style="width:163px;">Texas Instruments</td>
			<td style="width:183px;">TLO81ACP</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Optical Powermeter</td>
			<td style="width:163px;">Newport</td>
			<td style="width:183px;">818-IS-1 with 1835-C</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Optical spectrum analyzer</td>
			<td style="width:163px;">Anritsu</td>
			<td style="width:183px;">MS9710C</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Oscilloscope</td>
			<td style="width:163px;">Tektronix</td>
			<td style="width:183px;">TDS2022</td>
			<td>Oscilloscope with GPIB computer connectivity.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Polarizing beamsplitter module</td>
			<td style="width:163px;">Thorlabs Inc.</td>
			<td style="width:183px;">PSCLB-VL-1550</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Polyimide Film Tape</td>
			<td style="width:163px;">3M</td>
			<td style="width:183px;">5413</td>
			<td>Tape to fix the components on the table without damaging the fibers.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:391px;">Graphical programming language interface (GPLI)</td>
			<td style="width:163px;">National Instruments</td>
			<td style="width:183px;">LabVIEW</td>
			<td>Interface to program in G Programming Language and communicate with laboratory instruments.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:391px;">Polarimeter controlling software</td>
			<td style="width:163px;">General Photonics Corp.</td>
			<td style="width:183px;">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...

Watch this Scientific Journal Video about Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements at JoVE.com

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 ...

automation-mode-locking-nonlinear-polarization-rotation-fiber-laser

Research

Education

JoVE Core

JoVE Journal

Electrical Engineering

Engineering

Unit 1

Diagrams and Signal Flow Graphs

SCIENTISTS IN ACTION

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

Methods for Measuring the Orientation and Rotation Rate of 3D-printed Particles in Turbulence

Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing technology is used to fabricate particles with slender arms connected at a common center. Shapes explored are crosses (two perpendicular rods), jacks (three perpendicular rods), triads (three rods in triangular planar symmetry), and tetrads (four arms in tetrahedral symmetry). Methods for producing on the order of 10,000 fluorescently dyed particles are described. Time-resolved measurements of their orientation and solid-body rotation rate are obtained from four synchronized videos of their motion in a turbulent flow between oscillating grids with R&#955; = 91. In this relatively low-Reynolds number flow, the advected particles are small enough that they approximate ellipsoidal tracer particles. We present results of time-resolved 3D trajectories of position and orientation of the particles as well as measurements of their rotation rates.

We use 3D printing to fabricate anisotropic particles in the shapes of jacks, crosses, tetrads, and triads, whose alignments and rotations in turbulent fluid flow can be measured from multiple simultaneous video images.

Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing ...

Preparation and High-temperature Anti-adhesion Behavior of a Slippery Surface on Stainless Steel

Anti-adhesion surfaces with high-temperature resistance have a wide application potential in electrosurgical instruments, engines, and pipelines. A typical anti-wetting superhydrophobic surface easily fails when exposed to a high-temperature liquid. Recently, Nepenthes-inspired slippery surfaces demonstrated a new way to solve the adhesion problem. A lubricant layer on the slippery surface can act as a barrier between the repelled materials and the surface structure. However, the slippery surfaces in previous studies rarely showed high-temperature resistance. Here, we describe a protocol for the preparation of slippery surfaces with high-temperature resistance. A photolithography-assisted method was used to fabricate pillar structures on stainless steel. By functionalizing the surface with saline, a slippery surface was prepared by adding silicone oil. The prepared slippery surface maintained the anti-wetting property for water, even when the surface was heated to 300 &#176;C. Also, the slippery surface exhibited great anti-adhesion effects on soft tissues at high temperatures. This type of slippery surface on stainless steel has applications in medical devices, mechanical equipment, etc.

Slippery surfaces provide a new way to solve the adhesion problem. This protocol describes how to fabricate slippery surfaces at high temperatures. The results demonstrate that the slippery surfaces showed anti-wetting for liquids and a remarkable anti-adhesion effect on soft tissues at high temperatures.

Anti-adhesion surfaces with high-temperature resistance have a wide application potential in electrosurgical instruments, engines, and pipelines. A ...

Measurements of Soil Carbon by Neutron-Gamma Analysis in Static and Scanning Modes

The herein described application of the inelastic neutron scattering (INS) method for soil carbon analysis is based on the registration and analysis of gamma rays created when neutrons interact with soil elements. The main parts of the INS system are a pulsed neutron generator, NaI(Tl) gamma detectors, split electronics to separate gamma spectra due to INS and thermo-neutron capture (TNC) processes, and software for gamma spectra acquisition and data processing. This method has several advantages over other methods in that it is a non-destructive in situ method that measures the average carbon content in large soil volumes, is negligibly impacted by local sharp changes in soil carbon, and can be used in stationary or scanning modes. The result of the INS method is the carbon content from a site with a footprint of ~2.5 - 3 m2 in the stationary regime, or the average carbon content of the traversed area in the scanning regime. The measurement range of the current INS system is &#62;1.5 carbon weight % (standard deviation &#177; 0.3 w%) in the upper 10 cm soil layer for a 1 hmeasurement.

Here, we present the protocol for in situ measurement of soil carbon using the neutron-gamma technique for single point measurements (static mode) or field averages (scanning mode). We also describe system construction and elaborate data treatment procedures.

The herein described application of the inelastic neutron scattering (INS) method for soil carbon analysis is based on the registration and analysis of ...

Ultrasonic Fatigue Testing in the Tension-Compression Mode

Ultrasonic fatigue testing is one of a few methods which allow investigating fatigue properties in the ultra-high cycle region. The method is based on exposing the specimen to longitudinal vibrations on its resonance frequency close to 20 kHz. With use of this method, it is possible to significantly decrease the time required for the test, when compared to conventional testing devices usually working at frequencies under 200 Hz. It is also used to simulate loading of material during operation in high speed conditions, such as those experienced by components of jet engines or car turbo pumps. It is necessary to operate only in the high and ultra-high cycle region, due to the possibility of extremely high deformation rates, which can have a significant influence on the test results. Specimen shape and dimensions have to be carefully selected and calculated to fulfill the resonance condition of the ultrasonic system; thus, it is not possible to test the full components or specimens of arbitrary shape. Before each test, it is necessary to harmonize the specimen with the frequency of the ultrasonic system to compensate for deviations of the real shape from the ideal one. It is not possible to run a test until a total fracture of the specimen, since the test is automatically terminated after initiation and propagation of the crack to a certain length, when the stiffness of the system changes enough to shift the system out of the resonance frequency. This manuscript describes the process of evaluation of materials' fatigue properties at high-frequency ultrasonic fatigue loading with use of mechanical resonance at a frequency close to 20 kHz. The protocol includes a detailed description of all steps required for a correct test, including specimen design, stress calculation, harmonizing with the resonance frequency, performing the test, and final static fracture.

A protocol for ultrasonic fatigue testing in the high and ultra-high cycle region in axial tension-compression loading mode.

Ultrasonic fatigue testing is one of a few methods which allow investigating fatigue properties in the ultra-high cycle region. The method is based on ...

Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser (laser-SARPES), and demonstrate a power of this technique for studying solid state physics. Laser-SARPES achieves two great capabilities. Firstly, by examining orbital selection rule of linearly polarized lasers, orbital selective excitation can be carried out in SAPRES experiment. Secondly, the technique can show full information of a variation of the spin quantum axis as a function of the light polarization. To demonstrate the power of the collaboration of these capabilities in laser-SARPES, we apply this technique for the investigations of spin-orbit coupled surface states of Bi2Se3. This technique affords to decompose spin and orbital components from the spin-orbit coupled wavefunctions. Moreover, as a representative advantage of using the direct spin detection collaborated with the polarization-variable laser, the technique unambiguously visualizes the light polarization dependence of the spin quantum axis in three-dimension. Laser-SARPES dramatically increases a capability of photoemission technique.

Here, we combine polarization-variable 7-eV laser with spin- and angle-resolved photoemission technique to visualize the spin-orbital coupling effect in solid states.

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser ...

Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

Materials showing coupling phenomena between magnetism and (ferro)electricity, i.e., magnetoelectric effects, have attracted a great deal of attention due to their potential applications for future device technologies such as sensors and storage. However, conventional approaches, which usually utilize materials containing magnetic metal ions (or radicals), have a major problem: only a few materials have been found to show the coupling phenomena at room temperature. Recently, we proposed a new approach to achieve room-temperature magnetoelectrics. In contrast to conventional approaches, our alternative proposal focuses on a completely different material, a &#34;liquid crystal&#34;, free from magnetic metal ions. In such liquid crystals, a magnetic field can be utilized to control the orientational state of constituent molecules and the corresponding electric polarization through magnetic anisotropy of the molecules; it is an unprecedented mechanism of the magnetoelectric effect. In this context, this paper provides a protocol to measure ferroelectric properties induced by a magnetic field, that is, the direct magnetoelectric effect, in a liquid crystal. With the method detailed here, we successfully detected magnetically-tuned electric polarization in the chiral smectic C phase of a liquid crystal at room temperature. Taken together with the flexibility of constituent molecules, which directly affects the magnetoelectric responses, the introduced method will serve to allow liquid crystal cells to acquire more functions as room-temperature magnetoelectrics and associated optical materials.

In this report, we present a protocol to examine direct magnetoelectric effects, i.e., induction of ferroelectric polarization by applying magnetic fields, in liquid crystals. This protocol provides a unique approach, supported by the softness of liquid crystals, to achieve room-temperature magnetoelectrics.

Materials showing coupling phenomena between magnetism and (ferro)electricity, i.e., magnetoelectric effects, have attracted a great deal of attention due ...

Carrier Lifetime Measurements in Semiconductors through the Microwave Photoconductivity Decay Method

This work presents a protocol employing the microwave photoconductivity decay (&#956;-PCD) for measurement of the carrier lifetime in semiconductor materials, especially SiC. In principle, excess carriers in the semiconductor generated via excitation recombine with time and, subsequently, return to the equilibrium state. The time constant of this recombination is known as the carrier lifetime, an important parameter in semiconductor materials and devices that requires a noncontact and nondestructive measurement ideally achieved by the &#956;-PCD. During irradiation of a sample, a part of the microwave is reflected by the semiconductor sample. Microwave reflectance depends on the sample conductivity, which is attributed to the carriers. Therefore, the time decay of excess carriers can be observed through detection of the reflected microwave intensity, whose decay curve can be analyzed for estimation of the carrier lifetime. Results confirm the suitability of the &#956;-PCD protocol in measuring the carrier lifetime in semiconductor materials and devices.

As one of the important physical parameters in semiconductors, carrier lifetime is measured herein via a protocol employing the microwave photoconductivity decay method.

This work presents a protocol employing the microwave photoconductivity decay (&#956;-PCD) for measurement of the carrier lifetime in semiconductor ...

Implementation of a Nonlinear Microscope Based on Stimulated Raman Scattering

Stimulated Raman scattering (SRS) microscopy uses near-infrared excitation light; therefore, it shares many multi-photon microscopic imaging properties. SRS imaging modality can be obtained using commercial laser-scanning microscopes by equipping with a non-descanned forward detector with proper bandpass filters and lock-in amplifier (LIA) detection scheme. A schematic layout of a typical SRS microscope includes the following: two pulsed laser beams, (i.e., the pump and probe directed in a scanning microscope), which must be overlapped in both space and time at the image plane, then focused by a microscope objective into the sample through two scanning mirrors (SMs), which raster the focal spot across an x-y plane. After interaction with the sample, transmitted output pulses are collected by an upper objective and measured by a forward detection system inserted in an inverted microscope. Pump pulses are removed by a stack of optical filters, whereas the probe pulses that are the result of the SRS process occurring in the focal volume of the specimen are measured by a photodiode (PD). The readout of the PD is demodulated by the LIA to extract the modulation depth. A two-dimensional (2D) image is obtained by synchronizing the forward detection unit with the microscope scanning unit. In this paper, the implementation of an SRS microscope is described and successfully demonstrated, as well as the reporting of label-free images of polystyrene beads with diameters of 3 &#181;m. It is worth noting that SRS microscopes are not commercially available, so in order to take advantage of these characteristics, the homemade construction is the only option. Since SRS microscopy is becoming popular in many fields, it is believed that this careful description of the SRS microscope implementation can be very useful for the scientific community.

In this manuscript, the implementation of a stimulated Raman scattering (SRS) microscope, obtained by the integration of an SRS experimental set-up with a laser scanning microscope, is described. The SRS microscope is based on two femtosecond (fs) laser sources, a Ti-Sapphire (Ti:Sa) and synchronized optical parametric oscillator (OPO).

Stimulated Raman scattering (SRS) microscopy uses near-infrared excitation light; therefore, it shares many multi-photon microscopic imaging properties. ...

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 ...