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

Summary

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.

Abstract

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.

Introduction

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 signal^1,2. 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.

Protocol

1. Setting Up a Fiber ML Fiber Laser Including a Motorized PSC

1. 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.
2. 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.
3. 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.
4. 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.
5. Connect the laser diodes to their respective temperature controllers and current drivers.
6. 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 "COM4" as shown in the "Device Manager" of the computer.
7. At the output of the laser, i.e. the 50/50 coupler'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.
8. 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.
9. 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.
10. 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.
11. 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.
12. Tighten the pressure screws of the intra-cavity PSC until the fiber begins to be slightly squeezed.
13. Turn on the pump lasers diodes and adjust their currents to their maximum values as specified by the laser diode manufacturer.
14. Start the instrument communication interface. In the "Peripherals and Interface" column on the left, choose "COM4". Click on "Open VISA test panel". Click on "Input/Output". Then, in "Select or enter command" type "SM,500,3000\n" and click on the "Query" button. This commands the PSC to rotate by 3,000 steps of 0.1125° in the clockwise direction. While doing so, the PSC reaches a mechanical stop.
15. In the "Select or enter command" of the "COM4" test panel, type "SM,500,-10\n" and click on the "Query" button. The PSC then rotates approximately 1° 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.
16. If ML is not reached, repeat 1.15 until either ML or the maximum angle attainable with the PSC is reached.
17. 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.
18. 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.

2. Analyzing the Polarization of the Output Signal

1. Link the 1% tap to a commercial polarimeter.
2. Connect the polarimeter to the computer using a USB port.
3. In the "Select or enter command" of the "COM4" test panel, type "SM,500,3000\n" and click on the "Query" button.
4. Run the commercial polarimeter controlling software and start the polarization measurement by clicking on the "Start" button.
5. In the "Select or enter command" of the "COM4" test panel, type "SM,500,-10\n" and click on the "Query" button. Observe the polarization state on the polarimeter.
6. 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.
7. 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 "Measurement-→Oscilloscope" 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.

3. Setting Up a Feedback Loop to Automate the Alignment of the PSC Using the Commercial Polarimeter Measurements

1. Turn off the computer.
2. Connect the serial port of the commercial polarimeter to the serial port "COM1" of the computer. Restart the computer and the polarimeter.
3. Start the graphical programming language interface (GPLI) that will allow the reading of the polarimeter via "COM1" and the control of the motorized PSC via "COM4".
4. In the GPLI, click on "Blank VI". Then, select "Window→Tile Left and Right".
   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.
5. 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 "COM4" are the same as the ones presented in steps 2.3 and 2.5. The command to read S₁ on the commercial polarimeter via "COM1" is "*SOP?\n".
6. Save the script by clicking on "File→Save" and then run it by clicking on the "→" button. The PSC is brought back to its mechanical stop, then it rotates by steps of approximately 1° until ML is reached, showing the value of S₁ as it evolves.

4. Building a Rudimentary Homemade Polarization Analyzer

1. Connect an oscilloscope to the computer using the GPIB interface.
2. Put a polarizing beam splitter cube (PBS) on an optics bench.
3. 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.
4. Connect a fiber-pigtailed InGaAS PIN photodiode to the first output.
5. Connect the photodiode to a trans-impedance circuit (Figure 9).
6. Connect the electrical output of the circuit to channel 1 of the oscilloscope.
7. Turn on the trans-impedance circuit.
8. In the GPLI, read the average value of the voltage on channel 1 of the oscilloscope via the GPIB connection by using the commands "MEASU:IMM:SOU ch1;" to select channel 1 of the oscilloscope, "MEASU:IMM:TYPE mean;" to define the measurement to be an average voltage, "MEASU:IMM:VAL?" to get the value and finally "MEASU:IMM:UNI?" to obtain the units of the measurement. Save the script by clicking on "File→Save" and then run it by clicking on the "→" button.
9. 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.
10. 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.
11. 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 P_x from the measured voltage.
12. Connect a second fiber-pigtailed InGaAS PIN photodiode to the second output of the PBS.
13. Connect the photodiode to a second trans-impedance circuit.
14. Connect the electrical output of the circuit to channel 2 of the oscilloscope.
15. Turn on the trans-impedance circuit.
16. In the GPLI, read the average value of the voltage on channel 2 of the oscilloscope via the GPIB connection by using the commands "MEASU:IMM:SOU ch2;" to select channel 2 of the oscilloscope, "MEASU:IMM:TYPE mean;" to define the measurement to be an average voltage, "MEASU:IMM:VAL?" to get the value and finally "MEASU:IMM:UNI?" to obtain the units of the measurement. Save the script by clicking on "File→Save" and then run it by clicking on the "→" button.
17. Turn the laser on at an arbitrary pump power.
18. 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.
19. 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 P_y from the measured voltage.
20. After setting up the second detector to measure P_y, use the GPLI to compute the first Stokes parameter S₁ defined as S₁ = (P_x-P_y)/(P_x+P_y). The homemade rudimentary polarization analyzer is now ready to use.

5. Replacing the Commercial Polarimeter by the Homemade Polarization Analyzer in the Automation Process

1. Connect the 1% output of the laser to the homemade polarization analyzer input (as was done in step 4.9).
2. 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° (using the command "SM,500,-10\n" sent to "COM4") and reads out the value of S₁ from the homemade polarization analyzer at each step.
3. Modify the script developed in 3.5 so that, instead of using the value given by the commercial polarimeter, it gets P_x and P_y from the homemade polarization analyzer and then computes S₁ = (P_x-P_y)/ (P_x+P_y) .
4. Use the new script based on the homemade polarization analyzer to ML the laser automatically in a way similar to step 3.6.

Results

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...

Discussion

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...

Materials

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