Recently, fiber lasers and amplifiers develop rapidly due to the demands of high energy and high peak power pulses in industries[
High Power Laser Science and Engineering, Volume. 4, Issue 3, 03000e31(2016)
Review of fiber superluminescent pulse amplifications
High coherence of the laser is indispensable light sources in modern long or short-distance imaging systems, because the high coherence leads to coherent artifacts such as speckle that corrupt image formation. To deliver low coherence pulses in fiber amplifiers, we utilize the superluminescent pulsed light with broad bandwidth, nonlongitudinal mode structure and chaotic mode phase as the seed source of the cascaded fiber amplifiers. The influence of fiber superluminescent pulseamplification (SPA) on the limitations of the performance is analyzed. A review of our research results for SPA in the fibers are present, including the nonlinear theories of this low coherent light sources, i.e., self-focusing (SF), stimulated Raman scattering (SRS) and self-phase modulation (SPM) effects, and the experiment results of the nanosecond pulses with peak power as high as 4.8 MW and pulse energy as much as 55 mJ. To improve the brightness of SPA light in the future work, we introduce our novel evaluation term and a more reasonable criterion, which is denoted by a new parameter of brightness factor for active large mode area fiber designs. A core-doped active large pitch fiber with a core diameter of 190 mm and a mode-field diameter of 180 mm is designed by this method. The designed fiber allows neardiffracted limited beam quality operation, and it can achieve 100 mJ pulse energy and 540 Waverage power by analyzing the mode coupling effects induced by heat.
1 Introduction
Recently, fiber lasers and amplifiers develop rapidly due to the demands of high energy and high peak power pulses in industries[
Pulses with 26 mJ energy, 50 ns width and near diffraction limited beam quality have been reported with large pitch fiber in Q-switched fiber laser whose core diameter is
To improve the peak power further, we developed a new technology, i.e., utilizing broadband superluminescent pulsed light, such as the superluminescent diode light, super fluorescence or spontaneous emission light, as the seed source of the cascaded fiber amplifiers in as early as 2009 under the support of National High Technology Research and Development Program of China. We termed it as a superluminescent pulse amplification (SPA) technology. Broad bandwidth of wavelength helps to increase the SBS threshold in the VLMA fiber further, which makes higher peak power in fiber more achievable. Moreover, superluminescent light with nonlongitudinal mode and chaotic mode phase will suppress the modal interference, and make the light field intensity in the fiber more uniform, which mitigates the frequency modulation to amplitude modulation (FM-to-AM) effects, and scale up the peak power in the fiber.
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In this paper, the limiting factors in achieving nanosecond pulses with high pulse energy and high peak power in fiber amplifiers are reviewed first. Then the influence of superluminescent pulses on the limitations of fiber amplifiers’ performance is analyzed. Moreover, a review of our experimental results for superluminescent pulses amplified in the fibers is made. Nanosecond pulses with peak power as high as 8 MW and pulse energy as much as 100 mJ are demonstrated. To improve the brightness of SPA light in the future work, we introduce our novel evaluation term and a more reasonable criterion which is denoted by a new parameter of brightness factor for active large mode area fiber designs. A core-doped active large pitch fiber with a core diameter of
2 Theoretical analysis for the limitations
2.1 Classical limitations for fiber amplifiers
Besides the extractable energy, the performance of fiber amplifiers is mainly limited to nonlinear effects and fiber damages.
Nonlinear effects
Even though most of the advantages of fiber amplifiers and lasers benefit from the fiber geometry, this geometry is also the source of its main limitations. Specifically, the long path lengths of the tightly confined light in the fiber core make the system more susceptible to nonlinear effects, even at modest powers. Various nonlinear effects can occur in optical fibers, but those that most degrade the performance of nanosecond fiber lasers are SBS (which usually dominates with narrowband signals), SRS (which usually dominates with broadband signals), self-phase modulation (SPM, which is important for short and ultrashort pulses) and SF (which sets the ultimate power limit).
The thresholds of SRS and SBS can be calculated as below[
Different from the SBS and SRS effects which have sharp power thresholds, the SPM effect which results in spectral broadening has no threshold, and occurs in fiber amplifiers ubiquitously. The output spectrum induced by SPM can be calculated by solving the nonlinear Schrodinger (NLS) equations for coherent and incoherent pulses, respectively.
The ultimate limit for the fiber lasers is always SF[
Damage limits
Bulk damage and surface damage of fibers also limit the performance of fiber lasers and amplifiers. The study of Smith and Do[
As for surface damage, in general, facets are more fragile and irregularities, roughness or dirt on the facet dramatically reduce the damage threshold. However, in the study of Smith and Do, it was shown that by proper polishing, the facet damage threshold can be made equal to the bulk damage. Moreover, the end cap spliced at the output of fibers can eliminate the surface damage for most fiber systems.
2.2 Theory analysis for the thresholds of superluminescent pulses amplified in fiber amplifiers
SF effects
With rapid development of high power pulsed fiber lasers and amplifiers, the pulse peak powers are getting increased. Megawatt level peak powers have been reported. With such high peak power, SF effect becomes the most important factor that limits the fiber amplifier output power. SF effect has been theoretically predicted and experimentally verified for over 50 years. Theory of SF effect gives the widely accepted power threshold for SF
For
SRS effects
SRS is an important limiting factor for achieving high peak power intensity in fiber amplifier systems. It was proposed to use partially coherent light to increase the SRS threshold significantly. In our study[
Nevertheless, both the SRS experiment and theory results of incoherent light demonstrate that SRS has no relation to the phase status of the light. Therefore, partially coherent light does not help to break through the SRS limit. To understand the physical details, our theoretical model for SRS of partially coherent light in fibers is developed in Ref. [
SPM effects
For incoherent pulses, according to the Khinchin theorem, the output spectrum due to SPM is the Fourier transform of the first-order electric-field correlation function
However, in fiber amplifiers where high peak power is generated, we notice that the predicted results with Equation (
3 Experimental results for superluminescent pulse amplification in fiber amplifiers
We have reported the generation of nanosecond pulses with 55 mJ energy and 4.8 MW peak power in a multistage fiber amplifier system[
And at the same pump condition (250 mJ, 1 ms), without injection of seed pulses, the forward amplified spontaneous emission (ASE) energy is measured (0.56 mJ). With the seed pulses, the ASE energy must be lower than this value. So, the ASE ratio must be lower than 1% in the largest pulse energy we obtained.
The feasibility of 50 mJ pulse energy and 5 MW peak power delivered by the multistage fiber amplifier system is analyzed in Figure
It is noticeable that because of the multimode fibers used in the multistage fiber amplifier system, the beam quality is not good. The measured beam quality beta factor which refers to the value that the actual beam’s far-field divergence divided by the reference, i.e., the diffraction-limit beam’s far-field divergence is 11.
To explore the performance of superluminescent pulses in single-mode operation, we also report the construction of a cascaded fiber amplifier where a
4 very large mode fiber design
The fiber output of high power, high energy and high beam quality at the same time is the future developing direction. To satisfy the developing requirement, the first step is designing and manufacturing the very large mode fiber. The fiber designs are usually guided in terms of transversal mode discrimination, i.e., different propagation losses, gains between modes. However, no standard of how much the discrimination should be is unified. The brightness of the fiber laser is a relative synthetical and representative performance parameter. To evaluate the brightness of superluminescent light, we present a novel evaluation term and a more reasonable criterion which is described by a new parameter of brightness factor for active large mode area fiber design[
With both desirable factors including high brightness and high nonlinear effect resistance taken into consideration, we propose a new parameter of brightness factor
A core-doped active large pitch fiber with a core diameter of
For the large pitch fiber with a core diameter of
5 Conclusions
In this paper, to achieve nanosecond pulses with high pulse energy and high peak power in fiber amplifiers, we develop a new technology called SPA technology, i.e., utilizing broadband superluminescent pulsed light to seed the cascaded fiber amplifiers. The limiting factors in achieving nanosecond pulses with high pulse energy and high peak power in fiber amplifiers and the influence of superluminescent pulses on the limitations of fiber amplifiers’ performance is reviewed. Moreover, a review of our experimental results for superluminescent pulses amplified in the fibers is made. In multimode operation, for nanosecond pulses with peak power as high as 4.8 MW and pulse energy as much as 55 mJ are demonstrated. In single-mode operation, linearly polarized, 7.5 ns pulses with 1.5 mJ energy, 123 kW peak power are achieved. Moreover, we developed a new evaluation of the brightness of superluminescent light, and based on this criterion, a novel fiber with a core diameter of
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Haitao Zhang, Xinglai Shen, He Hao, Qinghua Li, Mali Gong. Review of fiber superluminescent pulse amplifications[J]. High Power Laser Science and Engineering, 2016, 4(3): 03000e31
Special Issue: HIGH ENERGY DENSITY PHYSICS AND HIGH POWER LASER
Received: Mar. 25, 2016
Accepted: Jul. 4, 2016
Published Online: Nov. 7, 2016
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