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 pr...

<ol>
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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. 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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 ...

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11.2K vistas.  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.