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 sourc....

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 suc....

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

List of materials used in this article

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.