High power ultrafast laser sources based on multimode fiber (Conference Presentation)

Single transverse mode fiber lasers have recently achieved output powers at the kilowatt level. These breakthroughs can be attributed to the maturation of high power diode pump technology at 980 nm and the use of large mode area (LMA) fibers. In the continous wave (cw) operation regime, LMA fibers, through their reduction of device length and increase of the effective area, have been effective in the reduction of deleterious nonlinear optical effects such as stimulated Raman scattering (SRS). The use of LMA fiber has been less effective in the suppression of stimulated Brillouin scattering (SBS), for which the threshold can be several orders of magnitude lower than for SRS. In this work we use refractive index profiles and index modifying dopant distributions for the mitigation of SBS. Our theoretical and modeling efforts led to an experimentally demonstrated increase in SBS threshold of 2.5 dB for single mode fiber and 6 dB for LMA amplifier fiber. We discuss the use of this SBS-suppressive fiber in the demonstration of a high-power, narrow linewidth fiber amplifier.

Regenerated fiber Bragg gratings (RFBGs) have been created in large mode area (LMA) fibers, and their key properties for their application as high-temperature sensors were experimentally characterized. UV written Type-I seed gratings in standard single-mode fibers (SMF-28) and in low numerical aperture LMA fibers with 125 μm and 250 μm cladding diameters were regenerated, and their wavelength drift characteristics at high temperatures, their temperature sensitivities, and their mechanical strengths were tested and compared with each other. We found that low dopant concentrations in the fiber cores led to lower wavelength drift rates during and after regeneration. A temperature calibration from room temperature to 800 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C showed that LMA fibers have generally similar but slightly lower temperature sensitivities than SMF-28 fibers. The tensile strength tests showed that LMA fibers with 250 μm cladding diameter have similar tensile fracture stresses as SMF-28 fibers but four times larger tensile force tolerances because of their twofold fiber diameter. Our research shows that using LMA fibers for RFBGs offers significant advantages for high-temperature sensing than their counterparts in standard fibers, which are also applicable to other silica fiber-based high-temperature sensors.

Femtosecond inscription of fiber gratings into polarization maintaining (PM) fibers and large mode area (LMA) fibers is studied. Transmission spectra for a Bragg grating written in a panda PM fiber is obtained where the polarization orientation with respect to the index modification of the grating is of particular interest. Transmission spectrum of a grating written in a LMA fiber with large core and cladding diameters is also obtained.

We demonstrate a robust, compact, low-cost, pulsed, linearly polarized, 1064 nm, Yb:fiber laser system capable of generating ~100 kW peak power pulses and >17 W average power at repetition rates of 80 – 285 kHz. The system employs a configurable microchip seed laser that provides nanosecond (~1.0 – 1.5 ns) pulse durations. The seed pulses are amplified in an all-fiber, polarization maintaining, large mode area (LMA) fiber amplifier optimized for high peak power operation. The LMA Yb:fiber amplifier enables near diffraction limited beam quality at 100 kW peak power. The seed laser, fiber amplifier, and beam delivery optics are packaged into an air-cooled laser head of 152×330×87 mm3 with pump power provided from a separate air-cooled laser controller. Due to the high peak power, high beam quality, spectral purity, and linearly polarized nature of the output beam, the laser is readily frequency doubled to 532 nm. Average 532 nm powers up to 7 W and peak powers exceeding 40 kW have been demonstrated. Potential for scaling to higher peak and average powers in both the green and infrared (IR) will be discussed. This laser system has been field tested and demonstrated in numerous materials processing applications in both the IR and green, including scribing and marking. We discuss recent results that demonstrate success in processing a diverse array of representative industrial samples.

Summary form only given. Many applications require high power good beam quality lasers with fiber delivered output. Very often, the necessary powers are above the damage threshold and nonlinear effects threshold of single mode (SM) fibers. This may be overcome by two approaches. A first approach is to use either large mode area fibers or photonic crystal fibers (PCFs), which have a very large and nearly SM cores. Nevertheless, this approach is limited by the achievable nearly SM core size, which is 40 μm for a flexible PCF. A second possible approach is to impose a Gaussian mode in a highly multimode (MM) step-index fiber with very large core diameter. In this approach, it was found that in passive highly MM fibers the fundamental mode can be maintained over short distances (~20 cm) in the case of straight fibers [1], or over longer distances in the case of coiled fibers [2]. Additionally, off-centered coupling of the fundamental mode into highly MM (100-400 μm core diameter), 2 m long, passive fibers was studied [4], along with its effect on the output beam quality [3]. Finally, SM excitation in active MM fibers, as means for achieving high quality output in an amplifier configuration, was also reported [2].Additionally, offcentered coupling of the fundamental mode into highly MM (100-400 m core diameter), 2 m long, passive fibers was studied [4], along with its effect on the output beam quality [3]. Finally, SM excitation in active MM fibers, as means for achieving high quality output in an amplifier configuration, was also reported [2]. Here we experimentally investigate propagation of the fundamental mode in highly MM passive and active fibers. We resorted to well controlled, free-space coupling of a Gaussian beam to the MM fiber, instead of tapering or other complex ways of mode coupling. The experimental setup in the case of a passive fiber consists of a 1 μm CW Gaussian seed source, coupling optics, and a commercially available highly MM passive fiber with 50 μm core diameter and 0.15 numerical aperture (NA). This fiber supports ~200 spatial modes and can handle almost 1 MW of peak power. The measured effect of the fiber length and coiling radius on the beam quality (M2) is shown in Fig 1 (left). As evident, the beam quality improves with larger coiling radius and with shorter fiber length. In addition, a clear minimum of the M2 parameter (<;1.2) is observed at a coiling radius of 7.5 cm for all fiber lengths. This demonstrates fundamental mode propagation in highly passive MM fiber for more than 15 m without significant degradation. In the case of an active double-clad fiber, the experimental setup is similar but includes a 2 m long fiber and a pump source with suitable coupling optics in a counter propagating pump configuration. In these experiments our active fiber is extremely MM (supports ~340 spatial modes), with 140 μm core diameter and 0.07 NA, which can handle up to 7.5 GW peak power. To the best of our knowledge, this is the largest active core diameter that was used for preservation and amplification of the fundamental mode. Our experimental results, which are presented in Fig. 1 (right), show successful preservation of the fundamental mode in this fiber with M2<;2. Additionally, the amplification results show some degradation in beam quality with increasing pump power that corresponds to the increase in amplified spontaneous emission (ASE). Since the fundamental mode field diameter is significantly smaller than the active core diameter, highly multimode ASE is generated, degrading the beam quality. A more detailed discussion as well as additional experimental results will be presented.

Transverse mode competitions are compared between the large mode area (LMA) fiber lasers with and without flattened fundamental modes (FMs). Numerical simulations show that the flattened-FM fiber lasers can support single-mode (SM) operation without employing other mode selection methods. This property is useful for designing LMA fiber lasers with SM operation. For the flattened-FM fiber amplifiers, the photonic crystal fiber (PCF) with a flattened FM is proposed to shape the Gaussian beam into a flat-top one, which is coupled into the input end of the active fiber as a seed. The SM operation based on this approach can be obtained.

Highly nonlinear effects are observed in graded-index multimode optical fibres. Multimode fibres are of interest for next-generation telecommunications systems and the construction of high-energy fibre lasers. However, relatively little work has explored nonlinear pulse propagation in multimode fibres. Here, we consider highly nonlinear ultrashort pulse propagation in the anomalous-dispersion regime of a graded-index multimode fibre. Low modal dispersion and strong nonlinear coupling between the fibre's many spatial modes result in interesting behaviour. We observe spatiotemporal effects reminiscent of nonlinear optics in bulk media—self-focusing and multiple filamentation1,2—at a fraction of the usual power. By adjusting the spatial initial conditions, we generate on-demand, megawatt, ultrashort pulses tunable between 1,550 and 2,200 nm; dispersive waves over one octave; intense combs of visible light; and a multi-octave-spanning supercontinuum. Our results indicate that multimode fibres present unique opportunities for observing new spatiotemporal dynamics and phenomena. They also enable the realization of a new type of tunable, broadband fibre source that could be useful for many applications.

In recent few years, femtosecond fiber lasers have been developed with ever increasing performances in terms of pulse energy. The energy produced in a mode-locked fibre oscillator has been increased from some nanojoules to several hundreds of nanojoules. This breakthrough results from the emergence of low-nonlinearity Yb-doped large mode area (LMA) fibers and progress in development of high contrast semiconductor saturable absorber mirrors (SESAM). As the first reported laser based on this concept, an Yb-doped fiber laser generating 0.9 W of average output power in 500-fs soliton pulses was demonstrated [1]. More recently, a significant energy scaling (up to more than 0.25 µJ) in femtosecond pulses has been demonstrated thanks to the use of these new fiber designs in purely-normal dispersion laser configurations [2,3]. Further energy scaling should be possible by incorporation of one or more segments possessing large positive dispersion to achieve the so-called chirped-pulse oscillator configuration [4]. In this contribution, we report on generation of high-energy femtosecond pulses from a mode-locked chirped-pulse oscillator featuring large-mode-area fibers. The laser is based on a 1.3 m long LMA microstructure fiber featuring a 40 µm Yb-doped core and a fast saturable absorber mirror for passive mode-locking. The total cavity dispersion is controlled by insertion of a passive LMA microstructure fiber with a core diameter of 25 µm. Preliminary results are obtained using a 3 m long LMA passive fiber leading to a total cavity dispersion of 0.09 ps2. In this configuration, we obtain self-starting modelocking for an average output power of 500 mW at a repetition rate of 33 MHz. The single-pulse operation is maintained up to an average output power as high as 3 W, which results in an energy per pulse of 91 nJ.

Based on a depressed-clad index profile design, a PM Yb-doped large mode area (LMA) fiber with an effective mode area of 450 μm 2 was designed and fabricated. The fiber was used to amplify 10-ns pulses at 1064 nm with a repetition rate of 100 kHz, and an output energy higher than 200 μJ was obtained, within a bandwidth of 0.5 nm. The fiber was coiled on a 12-cm diameter mandrel to obtain a single-mode output having a measured M 2 value of 1.04. The output polarization extinction ratio was higher than 20 dB. The photodarkening excess loss of the Ytterbium//Phosphorus Aluminum co-doped fiber was measured to be a factor 5 lower than that of a reference Ytterbium/Aluminum co-doped fiber. It is shown how a depressed-clad index profile design can improve higher-order mode filtering while keeping the coiling diameter practical for compact fiber amplifier packaging.

Summary form only given. Yb-doped femtosecond fiber chirped pulse amplifiers (FCPAs) are extremely attractive to replace their solid-state counterparts in when it comes to robustness and ease of operation. However, in most Yb-doped FCPAs delivering high-energy output pulses the advantages of robustness and ease of operation are forfeited since free-space optical components are used to stretch and compress the pulses. The main bottleneck to obtain fiber based stretching and compression is to match the dispersion of the stretcher and compressor. Recent advances have yielded good results with fiber-based stretchers and free space compressors [1-3]. However, the free space compressors require rather large distances between the compressor elements, which reduces the robustness of the system. On the other hand, while anomalous dispersion solid-core fibers have been available, e.g. photonic crystal fibers [4] or higher-order mode fibers [5], such fibers are not suitable as compressor fibers for pulse energies exceeding several nJ, because of the high nonlinearities experienced in the solid core. The use of a hollow-core (HC) photonic bandgap fiber (PBF) can alleviate this problem, an approach that was demonstrated in Er-doped fiber lasers [6]. Short lengths of HC PBF have been used to compress pulses from Yb-fiber oscillators [7], but not yet in Yb-FCPA. Here, we present a μJ-level femtosecond Yb-FCPA that uses a dispersion-matched dispersion compensating fiber (DCF) stretcher [8] and a HC PBF compressor.The Yb-fiber laser system consists of an all normal dispersion Yb-fiber oscillator operating at 40 MHz repetition rate, two PM single mode fiber (SMF) preamplifiers and a PM large mode area (LMA) fiber amplifier. The pulses from the oscillator are stretched in a hybrid fiber-pulse stretcher that consists of 8 m of ClearLite 980 fiber followed by 5 m of DCF. After amplification, the pulses are compressed in 25 m of HC PBF. The measured group delay dispersion of the compressor at 1060 nm is about -2 ps2 resulting in a stretched pulse duration of ~30 ps. Because the effective area of the DCF is rather small (~6.7 μm2), the seed energy was limited to 0.2 nJ and the pulses were pre-stretched in the ClearLite 980 fiber in order to minimize the nonlinearities in the DCF stretcher. After the first amplification stage a fiber pigtailed acousto-optic modulator (AOM) was used to reduce the repetition rate to 2 MHz. In preliminary test experiments, we used a large mode area (LMA) power amplifier as a final amplification stage. The LMA has a core diameter of 30 μm, corresponding to a mode field area of ~625 μm2. Free space coupling and mode conversion was used to couple the amplified pulses into the HC PBF. The output pulses from the compressor were characterized by second harmonic frequency resolved optical gating (SH FROG). The shortest compressed pulse duration was achieved by optimizing the fiber length of the pre-stretcher. Figure 1 shows the retrieved pulse with a duration of 220 fs and its spectral phase for a pulse energy of 250 nJ. At the moment experiments using a power amplification stage utilizing a single mode fiber with 10 μm core diameter are underway. This will allow us to directly couple (by fusion-splicing) the light from the power amplifier to the HC PBF. The results from these experiments will be presented at the conference. We thank K. Jespersen and L. Gruner-Nielsen from OFS Denmark for their support, providing the fiber stretcher and measuring the stretcher and compressor dispersion.

Spatial beam self-cleaning is one of the most intriguing nonlinear effects in multimode optical fibers: it involves the transformation of a speckled intensity pattern into a smooth beam with a bell-shaped profile at the fiber output. This effect was initially observed above some threshold value of peak power for sub-nanosecond pulses in passive multimode (MM) fibers with graded-index (GRIN) profile; a similar behavior is also observed with other types of fibers or with femtosecond pulse durations. For example, it was reported that a highly-multimode pulsed beam propagating in active (Yb-doped) MM GRIN fiber tapers experiences self-cleaning after amplification to peak powers of &#8275;20 kW, with a beam quality parameter improved from &#8275;10 to &#8275;4. Here we study the amplification of a continuous-wave (CW) single-frequency radiation in a similar MM GRIN fiber taper. Surprisingly, the beam cleaning is observed at much lower output powers. The beam quality is improved to near-Gaussian (&lt;1.5) after amplification to &#8275;20 W of CW power. We study the dynamics of the output transverse modes by a real-time mode decomposition technique. This reveals a relatively slow (1 Hz range) and unusual evolution in the first few seconds, followed by a relaxation into a stationary state, consisting of a bright spot on a low-power speckled pedestal.

Mode instability (MI) has become one of the main restrictions for further power scaling of large mode area (LMA) fiber systems. A periodical large-small-core (PLSC) filter in a strongly pumped LMA fiber amplifier based on multimode interference and self-imaging effect is proposed to suppress high order modes and improve the beam quality. An investigation on the modeling of light propagation within this filter structure is presented. Transverse mode competition and individual transverse mode power distribution in a strongly pumped fiber amplifier are theoretically analyzed. By employing this PLSC mode filter in the 30/400 strongly pumped LMA fiber amplifier, the percentage of the fundamental mode rises from 27.8% (without filter) to 95.6%, and the percentage of the high order modes correspondingly reduces from 72.2% (without filter) to 4.4%. By the modal power decomposition, the optical field pattern in free space is established and the M2 parameter of beam quality decreases dramatically from 2.24 to 1.12 (0 relative phase) and from 3.01 to 1.28 (π/2 relative phase). This study provides a new method to achieve high beam quality in LMA fiber amplifiers and this filter would be extended to larger mode area fiber amplifiers.

New progress for novel design for achieving large effective mode area fibers while maintaining single mode operation for fiber lasers and fibre amplifiers is addressed in the presentation.

We analyze numerically the dependence of segmented cladding fibers (SCF) characters, including the effective V-number, the effective fundamental mode area and the permissible bending radius of fundamental mode, on the structure parameters of the fiber and the signal wavelength with the methods of radial effective index and transmission matrix. The results show that a single mode and large mode area fiber with an acceptable permissible bending radius can be obtained when we design SCF with optimized structure parameters. The effective fundamental area is as large as 1948.93μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with proper fiber parameters. It will benefit the fabrication of the SCF for applications in high power fiber lasers.

We observe a nonlinear spatial self-cleaning process for femtosecond pulses in graded-index (GRIN) multimode fiber (MMF). Pulses with ∼80 fs duration at 1030 nm are launched into GRIN MMF with 62.5 μm core. The near-field beam profile at the output end of the fiber evolves from a speckled pattern to a centered, bell-shaped transverse structure with increasing pulse energy. The experimental observations agree well with numerical simulations, which show that the Kerr nonlinearity underlies the process. This self-cleaning process may find applications in ultrafast pulse generation and beam-combining.