1. General information

Since 1985 the Advanced Solid State Lasers (ASSL) Conference has been the world’s premier forum for presenting the most recent advances in solid-state laser science and technology. In 2016 ASSL continued to feature prestigious technical programs and events including celebration of the 100th anniversary of The Optical Society (OSA) [1]. It is therefore our pleasure to introduce this joint focus issue of Optics Express and Optical Materials Express addressing topics of the ASSL Conference 2016 held at The Westin Boston Waterfront in Boston, USA, on October 30 − November 3, 2016. In fact this focus issue becomes the fourth of its kind since ASSL 2013 [2–4]. The ASSL Conference 2016 was co-located with the Application of Lasers for Sensing & Free Space Communication (LS&C) and the Laser Applications Conference (LAC) as part of the 2016 OSA Laser Congress and Exhibition. The ASSL 2016 technical program featured 2 plenary and 10 invited speakers, 77 contributed oral presentations, 106 posters and 5 postdeadline papers reporting the latest advances and innovations in the fields of Materials and Laser Sources [1]. In addition, the Congress featured 2 short courses and a series of special events, including 3 networking receptions and the conference banquet sponsored by IPG Photonics, which offered great opportunities to attendees for invaluable encouragement, refreshment, and networking. The Exhibition featured more than 50 companies, representing diverse aspects of solid-state laser systems and their implementations. Overall, the ASSL 2016 technical program and other events were greatly enjoyed and appreciated by attendees.

2. Topical areas

2.1 ASSL 2016 conference topics

The topics of ASSL Conference 2016 were divided into Materials and Laser Sources [1]:

Materials included laser crystals and ceramics, laser fibers and thin-films as well as nonlinear crystals for ultraviolet (UV), visible, and mid-infrared (IR) spectral ranges, ion-doped crystals and nano-materials for ultrafast laser devices.

Laser Sources covered both free-space and fiber sources, including nonlinear sources, narrow-line and semi-conductor lasers, near-IR and mid-IR sources, as well as high-average power, ultra-short pulse lasers and beam combining.(For other general topics of the ASSL Conference 2016, visit the conference website [5].)

2.2 ASSL 2016 focus issue topics

This ASSL 2016 focus issue, jointly prepared by Optics Express and Optical Materials Express, includes 20 contributed papers (14 for Optics Express and 6 for Optical Materials Express) selected from the voluntary submissions from attendees who presented at the ASSL Conference 2016 and have extended their work into complete research articles.

While the articles in this focus issue by no means can reflect the whole range of research topics and contributions to the conference, they still can give a taste of the breadth and depth of the presentations and discussions given at the conference, particularly in areas categorized as follows:Optics Express

The numbers given in parentheses in the above lists denote the number of contributed papers in the corresponding category. It is worth noting that each paper was classified into a single category, depending on its primary keyword. The next sections will provide brief introductions to the individual Optics Express and Optical Materials Express contributions.

3. Optics Express contributions

3.1 Ultrafast lasers

Takada, Chiba, Yoshitomi, Torizuka, and Misawa [6] report ultrafast green pulse generation from an ytterbium (Yb)-doped fiber laser system based on a chirped-pulse amplification scheme. The system consists of a mode-locked Yb-doped fiber oscillator, a pulse stretcher, a pulse picker, a Yb-doped fiber preamplifier, a gain-narrowing compensator, a dispersion compensator, a Yb-doped fiber power amplifier, a pulse compressor, and finally a second-harmonic generator based on a lithium triborate (LBO) crystal. Based on the system, they generate green optical pulses at 515–538 nm with 35 nJ pulse energy, 41 fs pulse duration, and 3 MHz repetition rate.

Canbaz, Kakenov, Kocabas, Demirbas, and Sennaroglu [7] demonstrate the first graphene mode-locked laser with sub-20-fs pulse duration. A diode-pumped Cr³⁺:LiSrAlF₆ (Cr:LiSAF) laser delivers pulses as short as 19 fs at a ~100 MHz repetition rate. At a pump power of few 100 mW the average output power amounts to 8.5 mW at the center wavelength, which at somewhat longer pulse durations could be tuned between 836 and 897 nm.

Kwon, Vazquez-Zuniga, Lee, Kim, and Jeong [8] theoretically investigate quasi-mode-locked (QML) multi-pulse dynamics in a fiber ring laser cavity in the anomalous dispersion regime. They show that the laser cavity can operate in five constitutively different QML regimes, accounting for their coherence and multi-pulse formation features, which include the incoherent noise-like-pulse, partially-coherent noise-like-pulse, symbiotic, partially-coherent multi-soliton, and coherent multi-soliton regimes. In particular, they, for the first time, numerically clarify and confirm the symbiotic regime, in which noise-like pulses and multi-solitons coexist stably in the cavity that has recently been observed experimentally. They verify that the operation regimes based upon their shot-to-shot coherence characteristics and multi-pulse dynamics are determined, critically depending on the nonlinear-phase shift per roundtrip and the net dispersion of the laser cavity. They finally quantify and visualize all those characteristics onto simple contour maps, which will be very useful and helpful in discussing and clarifying the complex QML dynamics.

3.2 Parametric oscillators and amplifiers

Murray, Runcorn, Guha, and Taylor [9] present results of high average power mid-IR generation via parametric conversion in periodically poled lithium niobate, employing synchronized nanosecond pulsed ytterbium and erbium fiber amplifier systems. They could generate greater than 6 W of mid-IR radiation tunable in the spectral range of 3.31–3.48 μm, at power conversion efficiencies exceeding 75%, with near diffraction limited beam quality (M² = 1.4). They highlight that the use of fiber-based, synchronized pump and signal sources was the key to achieving high output beam quality and very high conversion efficiency. They also present numerical studies of the developed source, demonstrating the validity of the numerical approach in the heavily depleted pump regime.

Demas, Prabhakar, He, and Ramachandran [10] achieved four-wave mixing of higher-order modes of step-index multi-mode fibers to address wavelength ranges typically not addressed by fiber lasers. At 10-kW-class peak power and ~300-ps pulse duration, output wavelengths of 880, 974, 1173, and 1347 nm are obtained at ~20 kHz repetition rate. Frequency doubling of these sources will allow for blue, green, orange or red emission.

Peterson, Zuegel, and Bromage [11] present a demonstration of a high-energy femtosecond optical parametric oscillator using BiBO crystal, which is synchronously pumped by 1 ps pulses at 1030 nm from a Yb:YAG thin-disk pump laser. This parametric oscillator produces signal wavelength tunable from 1.99 to 2.2 µm, when operating near degeneracy in a non-collinear geometry. Obtained signal power exceeds 2 W of average power with 455 fs pulses at 7.08 MHz repetition rate, with corresponding energy of up to 359 nJ per pulse. This demonstration represents a power and energy scalable ultrashort pulse source, which is tunable in the infrared.

3.3 Q-switched lasers

Lan, Mateos, Wang, Serres, Loiko, Li, Pan, Griebner, and Petrov [12] report a holmium microchip laser passively Q-switched by a semiconductor saturable absorber. It is based on Ho:YAG ceramic, which is in-band pumped by a thulium fiber laser at 1.91 µm wavelength. Q-switching was achieved using several different commercially available transmission-type saturable absorbers, which enabled generation of stable 89 ns duration and 3.2 µJ energy pulses at 141 kHz repetition rate, producing 450 mW at 2.089 µm with 37% slope efficiency.

Kausas, Loiseau, Aka, Zheng, Zheng, and Taira [13] present a detailed comparison between the properties of YCa₄O(BO₃)₃ (YCOB) crystals grown by different methods for second-harmonic generation of a Q-switched Nd³⁺:YAG microchip laser and reveal that under proper conditions similar or even better performance (conversion efficiency of 70.2%) can be obtained compared to LiB₃O₅ (LBO). Moreover, the wide temperature acceptance of YCOB has been confirmed

3.4 Fiber lasers

Aubrecht, Peterka, Koška, Podrazký, Todorov, Honzátko, and Kašík [14] present the self-sweeping of laser wavelength in a holmium fiber emitting at ~2100 nm. They demonstrate the so-called spontaneous laser line sweeping (SLLS) within an ~4 nm interval with a sweep rate of ~0.7 nm/s from longer towards shorter wavelengths, and discuss the physical origins of the reverse direction of the SLLS regime. They, however, point out that there is a stability issue, because it can only last for a couple of minutes, and believe that further research efforts on deeper understanding of heating effects in the active medium, optimization of the cavity of the holmium fiber lasers, and investigation of the effect of refractive index grating to the sweeping may resolve the issue.

3.5 Visible lasers

Metz, Marzahl, Huber, and Kränkel [15] realized continuous wavelength tuning of Tb³⁺-doped fluoride lasers around 540 nm at 100 mW-level output power. The prospects for further improvement of the performance are shown by efficient operation of a stoichiometric, i.e., 100%-doped, TbF₃ laser. This achievement reveals that significantly higher doping concentrations and thus higher absorption efficiencies are feasible also in other host materials.

3.6 Laser amplifiers

Nagymihaly, Cao, Papp, Hajas, Kalashnikov, Osvay, and Chvykov [16] report on the prospects of multi-100-W-class average power Ti:sapphire power amplifiers. Detailed simulations on the temperature increase under different operating conditions are performed. They reveal that 100 TW class Ti:sapphire power amplifiers can be operated in thin-disk geometry at 300 W average power and 100 kHz repetition rate. Assuming water-cooling of both optical surfaces of the Ti:sapphire disk, the temperature increase is calculated to be as low as ~30 K.

3.7 Laser materials

Diebold, Jia, Graumann, Yin, Emaury, Saraceno, Tao, and Keller [17] demonstrate a thin-disk laser based on the gain material Yb:GGG. They highlight that the Yb:GGG material can be a promising contender to the well-established Yb:YAG laser crystal for high power applications in that it has many desirable properties for the thin disk geometry, including a high thermal conductivity that is nearly independent of the doping concentration, a low quantum defect, low temperature growth, and a broadband absorption spectrum. They demonstrate continuous-wave laser output power above 50 W with the Yb:GGG thin disk, which is an order of magnitude higher than previously achieved with this material in the bulk geometry. They emphasize that its maximum slope efficiency reaches 67% and its output characteristics are comparable with a state-of-the-art Yb:YAG disk under identical pumping conditions. They also point out via finite-element-method simulations the advantageous heat-removal capabilities of Yb:GGG in comparison with Yb:YAG, which can eventually lead to >50% lower temperature rise and thereby significantly less thermal lensing for Yb:GGG disks in comparison with Yb:YAG disks.

3.8 Laser-induced breakdown

In order to bridge a known gap in the reported measurements of the laser induced breakdown in gases, Lim and Taira [18] demonstrated a continuously duration-tunable MW peak power pulsed laser with pulse durations tunable from 0.5 to 9 ns. They then investigated laser induced breakdown in laboratory air as a function of pulse duration in the sub-nanosecond region. The laser was based on Nd:YAG/Cr:YAG ceramic, and operated at 100 Hz with peak powers from 0.5 MW up to 6 MW. Air breakdown threshold measurements confirmed that the breakdown threshold intensity I_th is almost constant at pulse durations τ longer than the pulse duration limit for cascade ionization τ_CI, and scales as τ⁻² for τ < τ_CI. It was also found that τ_CI is not constant and depends on focusing conditions. The advantages of sub-nanosecond giant-pulse based on ceramic micro-lasers for breakdown will allow for more efficient laser ignition.

3.9 Fiber characterization

Bock, Plötner, De Vries, Nold, Haarlammert, Schreiber, Eberhardt, and Tünnermann [19] present modal content measurements (S²) of two different negative curvature hollow-core photonic crystal fibers: a kagome fiber and an ice cream cone fiber. They analyze the sensitivities of the fibers towards mode matching, bending, and polarization, and discuss their higher-order mode (HOM) suppression characteristics, in particular. They show that the kagome fiber reaches a HOM suppression of 17 dB for optimized parameters of bending, coupling and polarization, and that the ice cream cone fiber exhibits stable few-mode operation for all measured polarization parameters with a large HOM suppression of 23 or 42 dB for the worst-case or optimal conditions, respectively. They stress that for both fibers a proper choice of the incident polarization state is crucial for obtaining good HOM suppression.

4. Optical Materials Express contributions

4.1 Laser materials

Loiko, Serres, Mateos, Tacchini, Tonelli, Veronesi, Parisi, Di Lieto, Yumashev, Griebner, and Petrov [20] report on a detailed comparative study of the spectroscopic and thermo-optic properties of tetragonal Tm:LiLnF₄ (Ln = Y, Gd, and Lu) crystals indicating their suitability for highly-efficient microchip lasers diode-pumped at ~791 nm and operating at ~1.91 μm. They demonstrated that an a-cut 8 at.% Tm:LiYF₄ micro-laser could generate 3.1 W of linearly polarized output at 1904 nm with a slope efficiency of 72% and a laser threshold of only 0.24 W. They also demonstrated that for 8 at.% Tm:LiGdF₄ and 12 at.% Tm:LiLuF₄ lasers, the output power reached ~2 W with slope efficiencies of 65% and 52%, respectively. They emphasize that thermal lensing in all Tm:LiLnF₄ crystals is weak, positive and low-astigmatic, and that the Tm:LiLnF₄ lasers have great potential for operation beyond ~2 μm via vibronic coupling as well as for passively Q-switched microchip lasers.

Beecher, Grant-Jacob, Hua, Prentice, Eason, Shepherd, and Mackenzie [21] describe the laser performance of three Yb-doped crystal waveguides prepared by pulsed laser deposition. The Yb:YAG waveguide exhibits optical quality similar to that of Czochralski-grown Yb:YAG, which was demonstrated with 70% slope efficiency and >16 W of output power.

4.2 Nonlinear optical materials

Nakano, Akiyama, and Shoji [22] measured the electro-optic (EO) coefficients of undoped and 1.7-mol% MgO-doped stoichiometric LiNbO₃ (SLN) using high-quality crystals and a reliable AC-field applying method. The EO coefficients at the wavelength of 633 nm were determined to be r₃₃ = 30.2 ± 0.6 pm/V and r₁₃ = 9.1 ± 0.2 pm/V for undoped SLN, and r₃₃ = 29.8 ± 0.6 pm/V and r₁₃ = 9.1 ± 0.2 pm/V for MgO-doped SLN. The obtained values are found to be nearly the same with those (r₃₃ = 30.3 ± 0.6 pm/V and r₁₃ = 9.5 ± 0.2 pm/V) for congruent undoped LiNbO₃. Measurements near the short wavelength transmission cutoff are needed and planned.

4.3 Nonlinear optics, devices

Kubota, Atarashi, and Shoji [23] successfully fabricated a quasi-phase matching (QPM) stack of thirty 106 µm thick and 5.5 × 5.0 mm aperture GaAs plates for second-harmonic generation of CO₂ lasers at 10.6 µm by using the room-temperature-bonding technique. They are in the process of fabricating 100-plate or more stacked GaAs-QPM structures for realizing highly efficient wavelength conversion.

4.4 Laser damage

Bach, Mero, Pasiskevicius, Zukauskas, and Petrov [24] obtained surface damage threshold intensity and fluence values for uncoated blank Rb:KTiOPO₄ in temporal regimes (pulse durations of ~0.3 and ~1.0 ps and a repetition rate of 100 kHz) where such data were previously unavailable. They conclude that quasi-phase matched frequency conversion in Rb:KTiOPO₄ is potentially interesting for the first two stages of high-average power few-cycle, chirped-pulse optical parametric amplifier systems. Feasibility for application in further stages with higher average powers has yet to be evaluated by damage and thermal behavior studies with larger beam sizes.

4.5 Spectroscopy, saturation

Sato and Taira [25] investigated the polarization dependence of saturable absorption in Cr⁴⁺:YAG. They theoretically proposed that its general analytical formula can be expressed in terms of the intensity and the polarization angle of the pump beam. They also derived the transmission formulas for specific incident surfaces of (100), (110), and (111) planes. In order to prove their model, they examined the polarization dependence in the transmittance of (110)-cut Cr⁴⁺:YAG. Considering pump depletion in Cr⁴⁺:YAG, they could evaluate its polarization-dependent transmittance, which is consistent with past spectroscopic parameters of Cr⁴⁺:YAG.

5. Conclusion

We witnessed the ASSL Conference 2016 continue to succeed as the world’s premier forum regarding solid-state lasers, featuring single-track technical sessions for presenting and discussing Materials and Laser Sources. We hope this focus issue provides a taste of the breadth and depth of the variety of topical discussions held at the conference in 2016, contributing to further advances in the associated research areas.

Finally, we invite you to join the upcoming ASSL Conference 2017 that is to be held in Nagoya, Japan, on October 1−5, 2017, believing that joining in this exciting conference shall continue to offer privileged opportunities for seeing the utmost recent advances in solid-state laser science and technology.