Johan Nilsson

• Teknologie doktor
• Professor at University of Southampton

This study examines the influence of quenching dynamics on the efficiency of erbium-doped fiber amplifiers (EDFAs) with high erbium-ion (E³⁺-ions) doping concentrations, comparing pulsed and continuous wave core-pumping methods. Our findings indicate that quenching, driven by energy transfer upconversion, substantially impacts signal gain in these...

A thulium-doped fiber laser operating quasi-continuous-wave generated 198 W of instantaneous output power in 0.2-ms pulses at 50 Hz repetition rate. The duty cycle becomes 1% and the average output power 2.0 W. This was cladding-pumped with 408 W from 0.79-um diode lasers (average pump power 4.1 W). The pump switch-on time was ~10 us at full power,...

A two-channel ytterbium-doped fiber amplifier system with active phase-locking reaches a differential phase noise of only 40 mrad (X/160) at 200-W channel power. Frequencies above 30 Hz did not require noise suppression, thus simplifying advanced beam-shaping through coherent beam combination.

A high-power 1.9-µm Tm³⁺-doped silica fiber laser generates 7.3-dB of gain at 2.3 µm when cladding- pumped with 207 W of power at 793 nm. We attribute the 2.3-µm gain to the transition ³H4 → ³H5 of Tm³⁺.

Parallel two channel Yb-fiber amplifiers with low differential phase noise above 100 Hz, up to 200 W, were phase-locked for coherent beam combination using neural networks and principal component analysis, with lambda/120 stability at 900 cycles/s control.

We report here the first implementation of chemically specific imaging in the exhaust plume of a gas turbine typical of those used for propulsion in commercial aircraft. The method used is chemical species tomography (CST) and the target species is ${{\rm CO}_2}$ , absorbing in the near-infrared at 1999.4 nm. A total of 126 beams propagate transver...

This Joint Issue of Optics Express and Optical Materials Express features 36 state-of-the art articles written by authors who participated in the international conference Advanced Solid State Lasers held online from October 3-7, 2021. This review provides a summary of these articles covering a wide spectrum of topics around solid-state lasers from...

This joint issue of Optics Express and Optical Materials Express features 36 state-of-the art articles written by authors who participated in the international conference advanced solid state lasers held online from October 3-7, 2021. This review provides a summary of these articles covering a wide spectrum of topics around solid-state lasers from...

Practical application of coherent beam combination of multiple fibres necessitates phase identification and optimisation in real-time. Here, we solve this mathematical challenge via deep learning, and hence demonstrate the potential for real-time bespoke beam shaping.

Coherent beam combination of multiple fibres can be used to overcome limitations such as the power handling capability of single fibre configurations. In such a scheme, the focal intensity profile is critically dependent upon the relative phase of each fibre and so precise control over the phase of each fibre channel is essential. Determining the r...

We demonstrate experimentally that random phase modulation of an erbium-doped fiber ring-laser by an intra-cavity electro-optic phase modulator did not inhibit ultrashort-pulse operation. Stable and self-starting ultrashort-pulse operation with a single pulse circulating in the cavity was achieved even when the phase modulator was driven with rando...

A Cr ²⁺ :ZnSe crystal with quasi-continuous-wave single-mode pumping at 1.9 μm generates over 10 dB of single-pass small-signal gain at 2.3 μm. Simulations suggest 30 dB is possible. We attribute the difference to quenching and focal mismatch.

We theoretically study stimulated Brillouin scattering (SBS) in a standard single-mode fiber (SMF), taking Brillouin-enhanced four-wave-mixing (BEFWM) effects into account. In particular, we investigate the case when there is non-negligible back-reflection of the forward-pump field at the rear fiber end although such reflection is typically weak an...

Coherent beam combination of multiple fibres can be used to overcome limitations such as the power handling capability of single fibre configurations. In such a scheme, the focal intensity profile is critically dependent upon the relative phase of each fibre and so precise control over the phase of each fibre channel is essential. Determining the r...

We demonstrate the mitigation of stimulated Brillouin scattering (SBS) in a double-clad single mode Yb-doped optical fiber amplifier through external phase modulation of narrow linewidth laser radiation using optimized periodic waveforms from an arbitrary waveform generator. Such optimized phase modulation waveforms are obtained through a multi-obj...

We use a model to investigate both the temporal and spectral characteristics of a signal lightwave which has been spectrally broadened through phase modulation with a maximal-length sequence (MLS), which is a common type of pseudo-random bit sequence. The enhancement of the stimulated Brillouin scattering (SBS) threshold of the modulated lightwave...

This Joint Issue of Optics Express and Optical Materials Express features 15 articles written by authors who participated in the international online conference Advanced Solid State Lasers held 13–16 October, 2020. This review provides a summary of the conference and these articles from the conference which sample the spectrum of solid state laser...

This Joint Issue of Optics Express and Optical Materials Express features 15 articles written by authors who participated in the international online conference Advanced Solid State Lasers held 13–16 October, 2020. This review provides a summary of the conference and these articles from the conference which sample the spectrum of solid state laser...

This Joint Issue of Optics Express and Optical Materials Express features 15 articles written by authors who participated in the international online conference Advanced Solid State Lasers held 13–16 October, 2020. This review provides a summary of the conference and these articles from the conference which sample the spectrum of solid state laser...

We show that optical fibers doped with as little as 7 mole% P2O5 have 27% lower temperature sensitivity of accumulated phase (relevant in interferometry) as compared to standard optical fibers. We believe full temperature insensitivity can be reached with higher levels of doping.

We experimentally demonstrate Raman amplification of signal pulses in a high-order Bessel mode ( ${{\rm{LP}}_{06}}$ ) at a wavelength of 1121 nm in a 335-m step-index fiber with a 70-µm diameter, 0.227-NA pure-silica core. This was pumped by 5-ns multimode pulses at 1065 nm from a Yb-doped fiber master oscillation power amplifier. The mode purity o...

We demonstrate experimentally that random phase modulation of an erbium-doped fiber ring-laser with an intra-cavity electro-optic phase modulator did not inhibit ultra-short pulse operation (so-called mode-locking). Stable mode-locking was achieved even when the phase modulator was driven with random sequences sufficiently fast and strong to render...

We demonstrate efficient pulse-energy extraction from a partly quenched erbium-doped aluminosilicate fiber amplifier. This has a high erbium concentration that allows for short devices with reduced nonlinear distortions but also results in partial quenching and thus significant unsaturable absorption, even though the fiber is still able to amplify....

Conventional models of Er/Yb co-doped fibers assume all ytterbium ions are equally involved in the energy transfer with erbium ions, governed by a singular transfer rate. This would predict output power clamping once ytterbium parasitic lasing starts, contrary to the observations that the output continued to grow albeit at a slower rate. One study...

This joint issue of Optics Express and Optical Materials Express features 17 state-of-the art articles written by authors who participated in the international conference Advanced Solid-State Lasers held in Vienna, Austria, from September 29 to October 3, 2019. This introduction provides a summary of these articles that cover numerous areas of soli...

This joint issue of Optics Express and Optical Materials Express features 17 state-of-the art articles written by authors who participated in the international conference Advanced Solid-State Lasers held in Vienna, Austria, from September 29 to October 3, 2019. This introduction provides a summary of these articles that cover numerous areas of soli...

We report 9.2-dB Raman amplification of pulses at 1121 nm in an orbital angular momentum mode with charge 15 in 30 m of 50-μm-diameter step-index-core fiber. The amplified signal mode-purity is measured to 78.4%.

We demonstrate efficient pulse-energy extraction from a partly-quenched erbium-doped aluminosilicate fiber amplifier. This has a high erbium-concentration, which allows for short devices with reduced nonlinear distortions, but which also results in partial quenching and thus significant unsaturable absorption, even though the fiber is still able to...

38 μI pulse energy with peak power of 1.3 kW and 0.3 nm spectral width is obtained at a short-wavelength of 1851 nm by actively Q-switching a thulium fiber laser cladding-pumped at 792 nm.

A 335-m fiber with a 70-μm diameter, 0.227-NA pure-silica step-index core reaches 15-dB average-power Raman gain for 1112-nm pulses in a high-purity, high-order Bessel-mode (LP07). The 812-W pump peak power is compatible with continuous-wave pumping.

A multimode silica-core fiber Raman laser was pumped directly by five wavelength-combined multimode diode lasers. The pump wavelengths were spread over a bandwidth of 700 cm-1, namely, at 976, 969, 950, 940, and 915 nm. This is larger than the Raman peak shift of 490 cm-1. The output power reached 31.5 W with an overall slope efficiency of 42% at t...

With the help of a robust model for stimulated Brillouin scattering (SBS) in Yb-doped fiber amplifiers, we Pareto-optimize phase modulation formats for suppressing SBS. We achieved 1.6 times enhancement in SBS threshold for 100MHz linewidth.

We demonstrate active inter-pulse amplitude control and intra-pulse instantaneous-phase equalization in bursts of 50 1-ns pulses in a 1.15-mJ, B~16 rad narrow-linewidth linearly-polarized divided-pulse Yb-fiber amplification system and theoretically compute 90% coherent pulse-stacking efficiency. © 2019 The Author(s) OCIS codes: (120.5050) Phase me...

We suppress stimulated Brillouin scattering in an erbium-doped optical fiber amplifier for 50-ns-long transform-limited signal pulses by counter-directional pumping with a pulse burst. The pump pulse burst is codirectional with the parasitic Brillouin Stokes wave, which therefore undergoes cross-phase modulation and thus spectral broadening due to...

A passively mode-locked erbium-doped fiber laser incorporates an intra-cavity electroopticphase modulator. Stable soliton mode-locking was achieved even when the phase modulatorwas driven with noise to render the cavity modeless.

We numerically simulate and optimize a high-power fiber Raman amplifier cladding pumped by spectrally combined diode lasers at wavelengths from ~0.9 to ~1µm in the continuous-wave regime. This amplified a signal at the first-Stokes wavelength of 1024 nm. We found that it was possible to add pumps over an increasingly wide wavelength span up to ~90...

We use numerical simulations with the beam propagation method (BPM) and rate equations to investigate the pump absorption and amplification characteristics in double-clad ytterbium-doped fibers with small cladding-to-core area ratios, in the range 1–3. The presence of modes with low overlap with the doped region (or alternatively, skew rays) hamper...

This joint issue of Optics Express and Optical Materials Express features 28 state-of-the-art articles written by authors who participated in the international “Advanced Solid State Lasers” conference, held in Boston November 4–8, 2018. This review provides a summary of these articles that cover the spectrum of solid state lasers from materials res...

This joint issue of Optics Express and Optical Materials Express features 28 state-of-the-art articles written by authors who participated in the international “Advanced Solid State Lasers” conference, held in Boston November 4–8, 2018. This review provides a summary of these articles that cover the spectrum of solid state lasers from materials res...

Narrow-linewidth bursts of 50 1-ns pulses from a Yb-doped fiber amplifier system reach 2.5-mJ burst energy and 250 W average power at 100 kHz burst frequency with 19-dB polarization extinction ratio at 1064 nm.

Photodarkening (PD) can degrade the output power and increase the thermal load in Yb-doped fiber lasers and amplifiers by increasing the propagation loss in the core [1,2]. PD can be attributed to color centers [3] and is reported to grow when the fiber is in use, over times from a few minutes to many hours (e.g., [2-4]). Typically, this is charact...

Thulium-doped fiber lasers (TDFLs) offer a broad ³F₄ → ³H emission band ~1600-2200 nm. Thanks to "two-for-one" cross-relaxation process in TDFLs [1], that enables to achieve output power > 1 kW at ~2045 nm with 0.79-µm cladding-pumping. Efficient cross-relaxation requires high Tm-concentration (> 3wt%) [1, 2, refere...

Commercially available pulsed fibre lasers at ~1.5 µm have many uses in imaging, defense, communications and light radar (LIDAR) [1]. For 3D scanning LIDAR, higher signal-to-noise ratio requires lasers with high average power and high pulse repetition rate (ideally several MHz) for faster scanning rate, whereas to improve distance resolution requir...

We propose and implement the use of an intensity modulated pump, counter-directional to a forward-propagating signal for suppressing unwanted stimulated Brillouin scattering (SBS) of spectrally narrow signals in optical fiber Raman amplifiers. The modulated Raman pump cross-phase-modulates and thus spectrally broadens the parasitic Brillouin Stokes...

We experimentally demonstrate 15 dB of Raman amplification of 1115 nm pulses in an orbital angular momentum mode (OAMM) with charge , in 5 m of step-index 25 μm-diameter-core fiber. The total output reaches 4.5 kW of peak power and 68.5 μJ of energy in pulses at 4 kHz repetition rate. An Yb-doped fiber source pumps the Raman amplifier at 1060 nm wi...

We demonstrate efficient pulse-energy extraction from partly-quenched highly erbium-doped aluminosilicate fiber amplifiers. Although the average-power efficiency degrades, the highest achievable pulse energy is largely unaffected, and our results point to an increasingly promising outcome for short pulses. In one case, using 0.2-μs, 23-μJ pulses to...

An Er-nanoparticle-doped fiber laser cladding-pumped at 975 nm generates 110 W at 1605 nm with 40.6% slope efficiency and M 2 ~7. High Er-concentration (4×10 25 ions/m 3) and pump brightness allows for adequate pump absorption even without Yb-codoping. There is continued strong interest and progress in the power scaling of fiber lasers operating at...

We demonstrate femtosecond dissipative solitons generated from an all-PM Ytterbium-doped fiber laser. The simplified fiber ring cavity has been shown to reliably self-start and provides a route to a robust platform for future development.

We suppress stimulated Brillouin scattering (SBS) of narrow-line signal pulses through pulse-burst counter-pumping of an Erbium fiber amplifier. Cross-phase-modulation broadens the Stokes wave, which reduces parasitic SBS power transfer without needing signal-broadening.

A fiber Raman laser pumped by five multimode diode lasers at 976, 969, 950, 940, and 915 nm generates 31.5 W of output power at a single wavelength of 1018 nm with 42% slope-efficiency.

We demonstrate active inter-pulse amplitude control and intra-pulse instantaneous-phase equalization in bursts of 50 1-ns pulses in a 1.15-mJ, B~16 rad narrow-linewidth linearly-polarized divided-pulse Yb-fiber amplification system and theoretically compute 90% coherent pulse-stacking efficiency.

We experimentally investigate the dynamics of backscattered light due to stimulated Brillouin scattering (SBS) in a narrow-linewidth continuous wave (CW) fiber amplifier. We observe the onset of sharp intensity variations in the backscattered radiation as we increase the pump power, which when analyzed using Karl-Pearson’s correlation coefficient r...

We use numerical simulations to investigate the effects of photodarkening (PD) on pulse compression and coherent combination following chirped-pulse amplification in Yb-doped fiber amplifiers. Even though we increase the pump power to keep the pulse energy constant, PD degrades the pulses due to phase distortions resulting from heating acting alrea...

This publisher’s note amends the spelling of the second author’s name in Appl. Opt. 57, 6388 (2018)APOPAI0003-693510.1364/AO.57.006388.

We report the first demonstration of Raman amplification in a fiber of a single Bessel-like higher order mode using a multimode pump source. We amplify the LP08-mode with a 559-µm² effective mode area at a signal wavelength of 1115 nm in a pure-silica-core step-index fiber. A maximum of 18 dB average power gain is achieved in a 9-m long gain fiber,...

We demonstrate a fiber Raman laser directly pumped by two spectrally combined multimode diode lasers at ~950 nm and ~976 nm. The emission wavelength becomes 1020 nm, which corresponds to the 1st Stokes of 976 nm. The pump separation of 270 cm-1 is both much smaller than the Raman peak shift of 440 cm-1 and much larger than the Raman linewidth of ar...

We investigate mode-area-scaling and bending performances of a Yb-doped large-mode-area fiber with an elongated non-circular core. Such fiber can be bent in the plane of its short axis to suppress bending effects, such as mode area reduction and mode profile distortion. Meanwhile, the other orthogonal axis can be stretched for mode area scaling. Ca...

We propose and demonstrate a phase retrieval method using a novel variant of the dispersion scan (‘-scan’) technique via both simulations and experimental measurements on a femtosecond fiber laser. The method combines a map of group-delay-dispersion-scanned (d-scan) second-harmonic generation (SHG) spectra together with the fundamental spectrum as...

We demonstrate temporal-based single-shot ns pulse characterization on a nanosecond 1064-nm Yb-doped fiber amplifier system. Both the amplitude and phase stability of bursts containing 35 1-ns pulses are measured and analyzed.

A spatially gain-tailored fiber Raman laser with Ge-doped core and P-doped inner cladding generates 46 W of instantaneous power at 1084 nm with 64% slope efficiency when cladding-pumped by a 1030-nm quasi-cw multimode disk laser.

The Advanced Solid State Lasers 2017 Conference (ASSL) was held from October 1 to 5, 2017. It was an extraordinary conference at the Nagoya Congress Center in Nagoya, Japan. ASSL 2017 again suggested an impressive platform where miscellaneous perceptions with a variety of approaches to optics, photonics, sensing, laser technology, laser systems, an...

The Advanced Solid State Lasers 2017 Conference (ASSL) was held from October 1 to 5, 2017. It was an extraordinary conference at the Nagoya Congress Center in Nagoya, Japan. ASSL 2017 again suggested an impressive platform where miscellaneous perceptions with a variety of approaches to optics, photonics, sensing, laser technology, laser systems, an...

A fiber Raman laser pumped by two wavelength-combined multimode diode lasers at 950 nm and 976 nm generates up to 23 W of output power at a single wavelength (1020 nm) with 51% slope efficiency.

We use numerical simulations to investigate the effects of photodarkening (PD) on single-channel and coherently combined Yb-doped Fiber CPA Systems. The system design optimizing for the lossy fiber caused by PD at ending of life shows improvements on both pulse compression and combining efficiency.

An Er-nanoparticle-doped fiber laser cladding-pumped at 975 nm generates 110 W at 1605 nm with 40.6% slope efficiency and M2~7. High Er-concentration (4×1025 ions/m3) and pump brightness allows for adequate pump absorption even without Yb-codoping.

We analyze the Brillouin Stokes in optical fibers, taking four-wave-mixing effects into account, and report that the phase-mismatch in the four-wave-mixing process may have a significant impact on the stimulation process in short optical fibers.

A low-birefringence 5-m-long fiber amplifier with a highly Yb-doped 40-μm, 0.028-NA phosphosilicate core produces linearly polarized (7.2-dB PER) 20-ns, 0.8-mJ pulses with 69-GHz linewidth, M2 = 1.2, and 120 W of average output power at 1083 nm.

p>We experimentally demonstrate 15-dB Raman amplification of 1115-nm, 20-ns pulses of charge l 2 orbital angular momentum mode in a 5-m multimode-pumped step-index fiber with measured mode purity of 83.2%.</p

We experimentally demonstrate 6.5 dB amplification of an orbital angular momentum (OAM) beam through a co-pumped fiber Raman amplifier based on a commercial step-index few mode fiber. Preliminary estimate of mode purity upon amplification is 85%.

We theoretically investigate nonlinear optimization of periodic phase modulation for suppression of stimulated Brillouin scattering (SBS) in single-mode optical fibers. We use nonlinear multi-parameter Pareto optimization to find modulations that represent the best trade-off between SBS and optical linewidth, as measured by its RMS value. The optim...

We demonstrate simultaneous spatial and arbitrary temporal pulse-shaping with a picosecond Yb-doped fiber laser system generating an output power > 10 W. By using a self-developed feed-back control algorithm on a single polarization optical pulse shaper, we obtained arbitrary complex pulse structures in the time domain with high accuracy and at hig...

We present 18 dB peak Raman amplification of 60 ns, 1115 nm, LP08 mode pulse in a 9 m long fiber with 555 µm2 mode at ~36.7% depletion of the 1060 nm multimode pump pulse.

Continuous-Wave 3.1 – 3.6 μm Difference-Frequency Generation of Dual Wavelength-Tunable Fiber Sources in PPMgLN-Based Rapid-Tuning Design Junqing Zhao, Fuqiang Jia, Yutong Feng, and Johan Nilsson Abstract—We report on a single-frequency continuous wave (CW) difference-frequency generation (DFG) source based on single-frequency wavelength-tunable po...

Laser linewidth broadening through phase modulation is an effective way of suppressing stimulated Brillouin scattering (SBS) [1]. However, several applications require laser linewidth to be as narrow as possible. These contradicting requirements call for phase modulation formats that allow for suppression of SBS while maintaining the laser power wi...

We investigate errors in small-signal absorption spectra that result from re-emission in single-mode fibers with overlapping absorption and emission spectra. Experiments on Er-doped fibers and simulations of Er and Yb-doped fibers show that the re-emission can severely distort the spectrum, especially the peak, under common measurement conditions,...

Here, we theoretically and numerically analyze the abnormal SBS combined with BEFWM in short optical fibers, and report that EIT-like Brillouin-induced transparency and slow-light effects can occur to the anti-Stokes in short optical fibers.

An Yb-free Er-doped fiber laser with a 146-μm diameter core produces a record-breaking output power of 405 W at 1.6 μm with a slope efficiency of 37% when cladding-pumped at 977 nm.

We observe that the pump power required to reach a given Brillouin gain in short optical fibers when excited by narrow line-width continuous wave light, is less than expected. The lowering of the pump power is correlated with reduced group velocity of the Stokes wave.

In this paper we report on the first tomographic reconstructions of CO2 concentration distributions created by gaseous phantoms. The tomography system has been developed as part of project FLITES which aims to provide images of various target chemical species within the exhaust plume of turbine engines.We provide a brief overview of the instrumenta...

We demonstrate beyond 10W simultaneous temporal and spatial pulse-shaping on a picosecond fiber laser system. Our proposed technique can substantially enhance the capability and efficiency of the existing ultrashort fiber laser systems for high precision material processing.

A method to produce spatially resolved images of the distribution of absorbing particles in the exhaust plume of a modified helicopter gas turbine engine is presented. Over a small region of the plume, in situ sensing of soot particles by laserinduced incandescence (LII) is demonstrated using fiber lasers with higher power (∼10 W), longer pulse dur...

We report on the installation and commissioning of two systems for the measurement of cross-sectional distributions of pollutant species in jet exhaust, within the engine ground test facility at INTA, Madrid. These systems use optical tomography techniques to estimate the cross-sectional distributions of CO2 and soot immediately behind the engine....

We present simulations for an all-fiber tandem pumping configuration and demonstrate kW-level output through a careful management of the in-band pump power level. Experimental results are also presented for a booster amplifier at 1030 nm.

We describe novel fiber Raman lasers pumped directly by spectrally combined high power multimode laser diodes at ∼975 nm and emitting at ∼1019 nm. With a commercial multimode graded-index fiber, we reached 20 W of laser output power with a record slope efficiency of 80%. With an in-house double-clad fiber, the beam quality improved to M 2 = 1.9, al...

We study the phase distortion induced in optical pulses due to thermal effects in Sm doped silica fibers. We experimentally measure the phase change accurately using an interferometric technique.

We show that errors in the absorption measured in rare-earth-doped fibers can exceed 50% and severely distort the spectral shape. This is a result of re-emission in fibers with overlapping absorption and emission spectra.

We theoretically investigate mode area scalability in a bent rectangular-core fiber. We discuss designs that mitigate bending effects and promote robust FM operation. We show that fiber fabrication is possible through conventional routes.