Polarization-maintaining Research Articles

Theoretical and experimental investigations of dispersion-managed, polarization-maintaining 1-GHz mode-locked fiber lasers.

High-repetition-rate (up to GHz) femtosecond mode-locked lasers have attracted significant attention in many applications, such as broadband spectroscopy, high-speed optical sampling, and so on. In this paper, the characteristics of dispersion-managed, polarization-maintaining (PM) 1-GHz mode-locked fiber lasers were investigated both experimentally and numerically. Three compact and robust 1-GHz fiber lasers operating at anomalous, normal, and near-zero dispersion regimes were demonstrated, respectively. The net dispersion of the linear cavity is adjusted by changing types of PM erbium-doped fibers (EDFs) and semiconductor saturable absorber mirrors (SESAMs) in the cavity. Moreover, the long-term stability of the three mode-locked fiber lasers is proved without external control. In order to better understand the mode-locking dynamics of lasers, a numerical model was constructed for analysis of the 1-GHz fiber laser. Pulse evolution simulations have been carried out for soliton, dissipative-soliton, and stretched-pulse mode-locking regimes under different net dispersion conditions. Experimental results are basically in agreement with the numerical simulations.

Comments about birefringence dispersion, with group and phase birefringence measurements in polarization-maintaining fibers

A recent JEOS-RP publication proposed Comments about Dispersion of Light Waves, and we present here complementary comments for birefringence dispersion in polarization-maintaining (PM) fibers, and for its measurement techniques based on channeled spectrum analysis. We start by a study of early seminal papers, and we propose additional explanations to get a simpler understanding of the subject. A geometrical construction is described to relate phase birefringence to group birefringence, and it is applied to the measurement of several kinds of PM fibers using stress-induced photo-elasticity, or shape birefringence. These measurements confirm clearly that the difference between group birefringence and phase birefringence is limited to 15–20% in stress-induced PM fibers (bow-tie, panda, or tiger-eye), but that it can get up to a 3-fold factor with an elliptical-core (E-core) fiber. There are also surprising results with solid-core micro-structured PM fibers, that are based on shape birefringence, as E-core fibers.

Planar fiber-chip-coupling using angle-polished polarization maintaining fibers

We report on our latest developments of a planar fiber-chip-coupling scheme, using angle polished, polarization maintaining (PM) fibers. Most integrated photonic chip components are polarization sensitive and a suitable way to launch several wavelength channels with the same polarization to the chip is the use of PM fibers. Those impose several challenges at processing and handling to achieve a stable, permanent, and low-loss coupling. We present the processing of the fibers in detail and experimental results for our planar and compact fiber-chip-coupling technique.

Thulium-doped all-PM fiber chirped pulse amplifier delivering 314 W average power

Abstract A high-power all polarization-maintaining (PM) chirped pulse amplification (CPA) system operating in the 2.0 μm range is experimentally demonstrated. Large mode area (LMA) thulium-doped fiber (TDF) with a core/cladding diameter of 25/400 μm is employed to construct the main amplifier. Through dedicated coiling and cooling of the LMA-TDF to manage the loss of the higher order mode and thermal effect, a maximum average power of 314 W with a slope efficiency of 52% and polarization extinction ratio of 20 dB is realized. The pulse duration is compressed to 283 fs with a grating pair, corresponding to a calculated peak power of 10.8 MW, considering the compression efficiency of 88% and the estimated Strehl ratio of 89%. Moreover, through characterizing the noise properties of the laser, an integrated relative intensity noise of 0.11% at 100 Hz−1 MHz is obtained at the maximum output power, whereas the laser timing jitter is degraded by the final amplifier from 318 to 410 fs at an integration frequency of 5 kHz to 1 MHz, owing to the self-phase modulation effect-induced spectrum broadening. The root-mean-square of long-term power fluctuation is tested to be 0.6%, verifying the good stability of the laser operation. To the best of our knowledge, this is the highest average power of an ultrafast laser realized from an all-PM-fiber TDF-CPA system ever reported.

Environment-stable sub-100 fs Er: fiber laser with a 3 dB bandwidth of 78 nm.

A robust all polarization-maintaining (PM) passively mode-locked Er-doped fiber laser is demonstrated based on the biased nonlinear amplifier loop mirror (NALM). With a π/2 nonreciprocal free-space phase shifter, stable single pulse mode locking can be obtained at the central wavelength of 1565.7 nm with a 3 dB spectral bandwidth of 24.6 nm in the soliton regime. The repetition rate of the pulse train is 98.13 MHz. The direct output pulse duration is 109 fs, which is nearly transform-limited. After the intracavity dispersion management, the robust self-started mode-locking in the stretched-pulse regime is realized at 1564 nm, and the 3 dB spectral bandwidth reaches up to 78 nm. The repetition rate of the pulse train is 199.6 MHz. In particular, the direct output pulse width is only 77 fs with a low integrated relative intensity noise (RIN) of only 0.0044% (integrated from 1 Hz to 1 MHz). To the best of our knowledge, this is the shortest pulse width directly from the all-PM NALM laser oscillator.

Polarized Supercontinuum Generation in CS2-Core All-Normal Dispersion Photonic Crystal Fiber

We numerically analyze a polarization maintaining (PM) supercontinuum generation (SCG) in all-normal dispersion liquid (CS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )-core photonic crystal fiber (LC-PCF). The proposed LC-PCF affords a high birefringence (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> ) from 1.0 μm to 2.2 μm wavelength with fundamental mode behavior. The linear polarization and high coherence spectral width for X and Y polarized axes covers 1.32 μm∼2.28 and 1.28 μm∼2.24 μm with pump wavelength of 1.55 μm at pump power of 2 kW, respectively. Furthermore, we found that the ability of maintaining linear polarization state of SC source for X-axes is better than that of Y-axes as the pulse duration varies from 0.05 ps to 0.6 ps at the same pump wavelength. In addition, the proposed LC-PCF-based SC source is a good candidate for applications such as biomedical imaging, fluorescence lifetime imaging fields, and frequency comb sources.

Simultaneous Strain and Temperature Discrimination in 18650 Li-ion Batteries Using Polarization-Maintaining Fiber Bragg Gratings

In this work, a fiber Bragg grating (FBG) sensor network inscribed in a polarization-maintaining (PM) fiber is proposed to proceed with a multipoint simultaneous temperature and strain discrimination in different locations (positive and negative terminals, and middle) on a cylindrical Li-ion battery. The birefringence property of the PM fibers, together with FBG sensors, allowed such an application using only one fiber line fixed to the edges of the battery. The battery was subjected to two different charge/discharge cycles, one with nominal charging and discharging conditions (1.00 C and 1.13 C, respectively) and another with abusive conditions (1.88 C for charge and 2.39 C for discharge). The PM-FBG sensors registered maximum temperature and strain variations at the end of the abusive discharge process of the battery; the positive terminal achieved a 28.7 ± 0.3 °C temperature variation, while the center achieved 221 ± 10 με strain variation. The results indicate a different strain variation behavior in the middle location when compared to the negative and positive terminals, as well as a higher temperature variation in both terminals when compared to the middle location. The use of PM-FBG sensors successfully demonstrates their feasibility in locally tracking and discriminating strain and temperature shifts in a battery surface. To our knowledge, this is the first study using the application of PM-FBG sensors to monitor and discriminate critical safety parameters in Li-ion batteries.

Consecutive 1015-1105-nm wavelength tunable "figure-of-9" mode-locked Yb:fiber oscillator.

A widely wavelength tunable mode-locked Yb-doped fiber oscillator based on nonlinear amplifier loop mirror (NALM) is reported, in which only a piece of short (∼0.5 m) single-mode polarization-maintaining (PM) Yb-doped fiber is employed, instead of the frequently used long (a few meters) double cladding (DC) fiber in previous papers. Experimentally, the center wavelength can be consecutively tuned from 1015 to 1105 nm by tilting the silver mirror, corresponding to a tuning range of 90 nm. To the best of our knowledge, this is the broadest consecutive tuning range in Yb:fiber mode-locked fiber oscillator. In addition, the mechanism of wavelength tuning is tentatively analyzed and attributed to the combined action of the spatial dispersion induced by a tilting silver mirror and the limited aperture in the system. Specific to the wavelength of 1045 nm, the output pulses with 13-nm spectral bandwidth can be compressed to 154 fs.

Compact, repetition rate locked all-PM fiber femtosecond laser system based on low noise figure-9 Er:fiber laser

We demonstrate a compact femtosecond fiber laser system based on all polarization-maintaining (PM) fiber and fiber components integrated structure. The figure-9 oscillator which incorporated a nonlinear amplifying loop mirror in the cavity features a 103.4-MHz high repetition rate with up to 93.1 dB signal-to-noise ratio of the radio frequency spectrum, 0.0056% [1 Hz, 1 MHz] integrated root-mean-square amplitude noise at the fundamental repetition rate and 63.7-fs timing jitter [100 Hz, 1 MHz]. Meanwhile, the fundamental repetition frequency was also locked to a stable radio frequency reference by using a self-designed frequency actuator and a relative frequency stability of 2.1 × 10−12 at 1-s gate time was obtained. Moreover, benefitting from the large positive group-velocity dispersion and negative third-order dispersion at 1.5-μm wavelength band, we also achieved 48.2 fs compressed pulse duration as well as an amplified average power of 199 mW via one-stage all-PM fiber amplifier and compressor. At last, as a performance proof, by directly splicing 38-cm long PM highly nonlinear fiber to the pulse compressor, a broadband coherent supercontinuum spanning from 950 nm to 2150 nm was generated. Our all-PM fiber laser system is suitable for the further buildup of a low noise PM fiber optical frequency comb.

All-Fiber Wavelength-Tunable Narrow-Linewidth Polarization-Maintaining Tm-Doped Fiber MOPA System

An all-fiber wavelength-tunable narrow-linewidth polarization-maintaining (PM) Tm-doped fiber master oscillator power amplifier (MOPA) system is presented. We demonstrate an all-fiber ring cavity Tm-doped fiber master-oscillator (MO) with a tuning range of 110 nm (from 1925 to 2034 nm). The maximum output power of 459 mW is obtained at 1992.9 nm for 16 W of launched pump power at 793 nm, corresponding to a slope efficiency of 5.6% concerning launched pump power. By using a one-stage Tm-doped fiber amplifier combined with a high gain of &gt;10 dB, the maximum slope efficiency is 12.6% and the output power is 5.65 W at 1993 nm. The 3 dB linewidth is less than 0.5 nm, M2 ≈ 1.25, and the polarization extinction ratio (PER) reaches 21.4 dB. The influence of different active fiber lengths on laser amplification is also studied.

Low-Loss Flexible Polarization-Maintaining Hollow Waveguide for Linearly Polarized 100 GHz Radiation Transmission and Subwavelength Imaging

Terahertz imaging has been developed rapidly in recent years. However, conventional terahertz imaging systems are often bulky due to the free space propagation. Various subwavelength sized probes have been developed to realize high resolution. However the tiny probes often suffer from high loss. A flexible metallic polarization-maintaining (PM) hollow waveguides with elliptical cross section is investigated and fabricated. The waveguide is used for transmitting linearly polarized terahertz (THz) radiation and flexible imaging. Bore size of the waveguide is on the order of wavelength. Transmission loss is as low as 0.92 dB/m. High birefringence of 0.08 and measured polarization ratio of 97.2% are achieved by the waveguide at 100 GHz. Moreover, a scanning 100 GHz imaging system is established by using the waveguide as a flexible probe. The PM capability of the fabricated waveguide brings benefits compared to conventional non-PM circular waveguide. The linearly polarized fundamental modes are easily excited and can be propagated through the fabricated waveguides to one meter away with polarization maintenance, which results in compressing of output beam and realizes a high resolution. A resolution better than λ/3 is achieved. It is further experimentally verified that the waveguide is robust to bending and twisting deformation, and maintained the imaging quality during flexible imaging. A subwavelength bore-sized single-polarization single-mode waveguide is also fabricated to simultaneously achieve high horizontal and vertical resolutions. The discussed waveguide is easy to fabricate and may find important applications in THz microscopy and flexible polarization imaging.

Generation of 99.8 fs, 25 kW Peak-Power, Dispersion-Managed Pulses Directly from an Yb-Doped Figure-of-9 Fiber Laser.

We reported on the generation of 99.8 fs, 25 kW peak-power, dispersion-managed pulses directly from a passively mode-locked Yb-fiber laser oscillator with a figure-of-9 configuration. The introduction of strongly injected pump power and optical components with a high damage threshold enables high-power operation, while the polarization-maintaining (PM) fiber supports environmentally stable self-started mode-locking. Mode-locking in the soliton-like and negative-dispersion regime is characterized by the dispersion management via tuning the separation distances between a pair of gratings inside the cavity. The oscillator generates stable pulses with up to 40.10 mW average power at a 16.03 MHz repetition rate, corresponding to a pulse energy of 2.5 nJ. To the best of our knowledge, it is the highest peak-power directly obtained by a laser oscillator with a figure-of-9 configuration.

Experimental Investigation of Ultrafast Yb:fiber Oscillators Based on Several Dominant Mode Locking Methods

To find out which kind of mode-locked fiber oscillator is superior, six kinds of typical mode-locked Yb:fiber oscillators, including nonlinear polarization evolution (NPE)-based oscillators (i.e. conventional NPE-based oscillator, polarization maintaining (PM) NPE-based oscillator), nonlinear amplifying loop mirror (NALM)-based oscillators (i.e. figure-8 oscillator, all-fiber figure-9 oscillator, free-space figure-9 oscillator), and semiconductor saturable absorption mirror (SESAM)-based oscillator, were presented and crosswise compared. Except for the conventional NPE-based one, all the others were constructed with all-PM fiber components. Experimentally, detailed crosswise comparisons were carried out quantitatively and qualitatively for different oscillators in terms of spectral bandwidth, spectral shape, pulse repetition rate, signal-to-noise ratio (SNR), mode-locking threshold, self-starting mode-locking pump power, robustness, component life, construction simplicity, and so on. In addition, subjective sensations on each oscillator were also provided, which could be helpful for decision-making when one wants to make or purchase an oscillator. Finally, the all-fiber figure-9 oscillator was suggested to be the best choice even it needs higher pumping power, which could be an ideal seed for high power ultrafast laser amplifiers.

Polarization-maintaining all-fiber tunable mode-locked laser based on a thermally controlled Lyot filter.

We propose and demonstrate for the first time, to the best of our knowledge, a thermally controlled all polarization-maintaining (PM) fiber Lyot filter. This filter is implemented in an all-PM mode-locked fiber laser to achieve wavelength tunability. When operating in the single-wavelength tunable mode, the center wavelength can be tuned across a range from 1546 nm to 1571 nm. Furthermore, the laser can also operate in a dual-wavelength mode with center wavelengths at 1545 nm and 1571 nm. The temperature sensitivity achieved in our all-PM fiber Lyot filter is 0.602 nm/°C, which is over 46 times higher than other fiber-based filters such as a fiber Bragg grating filter (0.013 nm/°C). This highly stable and versatile wavelength-tunable all-PM fiber mode-locked laser is a promising source for various applications requiring wavelength tunability and/or dual-wavelength output, such as coherent Raman microscopy and dual-comb spectroscopy.

Highly stable Q-switched and mode-locked pulse generation from an all-PM figure-9 fiber laser.

A highly stable figure-9 Yb-doped fiber laser with all polarization-maintaining (PM) double-cladding fiber is demonstrated. Through leveraging the saturable absorption effect of a nonlinear amplifying loop mirror, both the Q-switched and mode-locked operation are realized by adjusting the pump power. With increasing the pump power from the threshold to the maxima, the repetition rate of the Q-switched pulses is linearly increased from 14.9 kHz to 138.0 kHz with the pulse duration accordingly reduced from 3.9 µs to 970 ns. The corresponding maximum average power and pulse energy are respectively 2.34 W and 17 µJ, which are more than ten times larger than the common material-based Q-switched all-fiber lasers. In addition, in the process of increasing and decreasing the pump power, an optical bistability that manifested as a significant power jumping effect is observed, while its effect on the pulse repetition rate and duration is trivial. Whereas for the single pulse mode-locked operation, a maximum output power of 56.3 mW with a fundamental repetition rate of 12.5 MHz is realized, corresponding to a pulse energy of 4.5 nJ. To the best of our knowledge, it is much higher than the most of previous works concerning figure-9 all-PM-fiber lasers of which the emitted pulse energy is generally less than 1 nJ. After being compressed by a pair of diffraction grating, a minimum pulse width of 378 fs and a maximum peak power of 9.76 kW are respectively obtained. In addition, through characterizing the spectral and temporal properties of the laser source, the excellent stability of both the Q-switched and mode-locked operations is verified.

Stable femtosecond pulse generation relying on a simple NALM-based all PM Tm-doped fiber laser

Herein, an ultrafast mode-locked Tm-doped fiber laser with all polarization-maintaining (PM) fiber relying on nonlinear amplifying loop mirror was firstly presented. The simple all-PM mode-locked oscillator can deliver output power of ∼1.20 mW and pulse repetition of ∼7.267 MHz at central wavelength of ∼1950.2 nm. The pulse duration was measured after boosting the power to ∼20 mW by a fiber amplifier, which shows the full width at half maximum (FWHM) of ∼357 fs. Moreover, the all-PM mode-locked oscillator has exhibited a good power stability with fluctuation of ∼1.08 % during 36 h monitoring. This stable fiber laser might be served as a potential ultrashort pulse seed source for the applications of medical treatment, precision processing and mid-infrared light source.

2.3 W, Linearly-Polarized Superfluorescent Generation From a Polarization-Maintaining Er3+-Doped Fluoride Fiber Amplifier Around 2.8 μm

We present a high-power, polarization-maintaining (PM) Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fluoride fiber amplifier, which enables watt- level, linearly-polarized (LP) superfluorescent outputs around 2.8 μm by using a non-PM Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fluoride fiber superfluorescent source (FSS) after polarization controlling as the seed. Based on an optimized scheme, the gained superfluorescent signal with a 20 dB bandwidth of 95 nm (2710∼2805 nm) can yield a maximum power of 2.32 W and a polarization extinction ratio (PER) of up to ∼23.6 dB. To our knowledge, this is the first demonstration of a mid-infrared (mid-IR) PM fiber amplifier, and the power also represents, until now, the highest level from a FSS in the mid-IR. Further power scaling is only limited by the seed power, and can be anticipated by boosting seed power or/and adopting multi-stage amplification. This work provides an alternative way for high-power, LP, mid-IR FSS, enabling a range of real-world applications.

Dynamic polarization response of polarization-maintaining fibers by periodic thermal cycling method.

We report a periodic thermal cycling method to investigate the dynamic response of the polarization of a laser propagating through polarization-maintaining (PM) optical fiber, driven by periodic weak temperature modulation. Consequently, temperature modulation on the surface of the coating material of the PM fiber was found to cause a continuous periodic change in the polarization state of the output laser without approaching the steady state of the resulting dynamic polarization response. Additionally, the response was found to depend on the temperature-modulation frequency and amplitude. These experimental results are qualitatively in good agreement with that of the simple theoretical model. Our research would be applied not only to the method of measuring properties of a PM optical fiber by utilizing the continuous modulation of the differential refractive index with a wide modulation-frequency range but also to various applications of the dynamic control of the periodic refractive index in materials.

3.96 kW All-Fiberized Linearly Polarized and Narrow Linewidth Fiber Laser with Near-Diffraction-Limited Beam Quality.

In this paper, we realize a 3.96 kW all-fiberized and polarization-maintained (PM) amplifier with narrow linewidth and near-diffraction-limited beam quality. Based on a master oscillator power amplifier (MOPA) configuration seeded with phase-modulated single-frequency laser, a 3.96 kW signal laser is achieved with a 3 dB linewidth of 0.62 nm at the pump power of 5.02 kW. At the maximum output power, the polarization extinction ratio (PER) is ~13.9 dB, and the beam quality (M2 factor) is M2x = 1.31, M2y = 1.41. As far as we know, this is the maximum output power of PM narrow linewidth fiber laser with near-diffraction-limited beam quality and all-fiber format.

Influence of injected signal polarization on SBS, SRS, spectral broadening, and self-pulsing properties in high-power fiber amplifier

In this paper, we construct a narrow-linewidth amplifier based on polarization-maintaining (PM) fiber, which can be injected by a fiber oscillator seed or white noise signal phase-modulated seed. The stimulated Brillouin scattering (SBS), SRS, spectral broadening, self-pulsing properties of the fiber amplifier when its’ injected signal is 0° or 45° relative to the slow axes of the PM fiber are studied. It is indicated that when the injected signal is launched at 45° relative to the slow axes, the nonlinear effects are significantly suppressed compared with the case of the injected signal is launched parallel to the slow axis. The spectral broadening rate is reduced from 0.14 pm W−1 to 0.0747 pm W−1, the ratio of Raman to laser peak intensity is increased from 20 dB to 30 dB, the SBS threshold is increased by >24%, and the self-pulsing threshold is increased by 28.7%. It is worth noting that the nonlinear properties corresponding to the injected polarization state in our experiment are quite different from some earlier works.