The era of solid-state laser science and technology was opened by Maiman’s demonstration of “the flash-lamp-pumped ruby laser” in 1960, which is indeed the very first laser of all kinds [1]. Since then, we have seen remarkable, continuous advances in lasers, encompassing solid-state lasers and all other types of lasers. The progress includes a series of new scientific findings and technological developments in terms of materials, sources, and applications. As one premier example, the advent of semiconductor diode lasers combined with novel materials, such as crystals, ceramics, and fibers, has crowned solid-state lasers with unprecedented efficiency and flexibility. Over the last five decades, solid-state lasers have continuously been developed and deployed into numerous fields of our daily life, from a simple laser pointer to laser machinery for heavy industry or laser particle accelerators for high-energy physics. In fact, the 27 years of the former Advanced Solid-State Photonics (ASSP) meeting, together with Advances in Optical Materials (AIOM) and Fiber Laser Applications (FILAS) have served as an impetus to the mainstream of such advances, which were combined together in 2013 [2].

The Advanced Solid-State Lasers (ASSL) Congress 2013 was the inaugural meeting as one integrated congress, combining the three topical meetings formally known as ASSP, AIOM, and FILAS [2]. This historical, exciting new congress was held in Paris, France, from October 27 to November 1, 2013, offering researchers from all over the world great opportunities for presenting and discussing at one place their latest significant achievements in areas of materials, sources, and applications. Notwithstanding the radical change in the conference format, the ASSL Congress successfully managed to keep the style of topical meetings, running single-track technical sessions, which was greatly appreciated by attendees because no one needed to rush off to pick up presentations of interest. The single-track format indeed offered great stimulus for close interactions and networking among attendees and colleagues. The congress also provided selections of short courses and a series of special events, which also allowed for great encouragement and refreshment to attendees. The co-located topical meetings, including Application of Lasers for Sensing and Free-Space Communication (LS&C) and Mid-Infrared Coherent Sources (MICS), were another opportunity for looking around the recent advances in the associated research areas. The conference banquet at the historic Maison de la Chimine near the Pantheon and the Sorbonne, sponsored by IPG Photonics, was greatly enjoyed and appreciated by attendees. Finally, the Postdeadline Session (jointly with MICS) highlighted the utmost recent advances in materials, sources, and applications. It is worth noting that during the congress, 25 invited, 87 contributed oral, and 157 contributed poster papers were presented, together with 10 postdeadline papers.

In general, the ASSL Congress includes topics categorized as follows [3]:

Materials:

This ASSL 2013 focus issue, jointly prepared by Optics Express and Optical Materials Express, includes 21 contributed papers (18 for Optics Express and 3 for Optical Materials Express) selected from the voluntary submissions from attendees who presented at the ASSL Congress 2013 and have extended their work into complete research articles. While they do not cover the whole range of research topics presented and discussed at the congress, readers may be able to configure part of the foci of the presentations and discussions given at the congress, particularly in the following areas:

Topics for Optics Express:

It is worth noting that the number given in parentheses denotes the number of contributed papers in the corresponding category. While it is not easy to divide all the contributed papers into separate categories with no conflict, the editors classify them, considering the primary focus of the work.

Here we give brief introductions of the contributed papers as follows:

Topics for Optics Express

Ion doped crystal lasers: Solid-state lasers based on ion doped crystals having novel features are invariably of interest at all times. Two ion doped crystal lasers based on Nd:YAG and Er:YAG are discussed here [4,5].

Yoon et al. present the first multi-watt demonstration of a diode pumped cryogenically cooled Nd:YAG laser operating at 947 nm [4]. At the liquid nitrogen temperature (77 K), the authors obtained 3.8 W output power for 12.8 W of absorbed pump power with a slope efficiency of 47% under continuous wave (CW) operation. Another interesting point with this paper is the authors’ extensive experimental study on the temperature-dependent spectroscopic properties of Nd:YAG. They found that an increase of ~2.5 times for both the absorption and emission cross-sections was obtainable when the gain medium was cooled down from 300 to 77 K.

Larat et al. present an Er:YAG laser system comprising one oscillator and two single-pass amplifiers end-pumped by a fiber-coupled 1.47 μm diode [5]. The laser system could operate at 1.65 μm, capable of generating 120 mJ pulses with a beam quality factor (M²) of 3.1~3.7 at a repetition rate of 30 Hz. Although further pulse-energy scaling was limited by the damage of the pump dichroic mirrors, the authors think that there should still be room for improvement if a fiber-coupled diode laser system with higher power with the same brightness becomes available.

Planar waveguide lasers: Waveguide lasers offer a great opportunity for generating coherent light in a very small dimension, which eventually allows for facilitating a variety of integrated optics applications. A number of waveguide fabrication techniques include ultrafast laser inscription and direct femtosecond-laser writing techniques [6,7].

Macdonald et al. present a compact mid-infrared channel waveguide based on Cr:ZnS, which was fabricated by ultrafast laser inscription technique and could operate at 2333 nm with a maximum power of 101 mW of CW output that was only limited by available pump power [6]. In particular, this wavelength is very useful for LIDAR, atmospheric sensing, and laser surgery.

Salamu et al. report Nd:YAG ceramic waveguide lasers fabricated by direct femtosecond-laser writing technique [7]. Various waveguide geometries were investigated for CW and pulse operations at 1.06 and 1.3 μm wavelengths. For example, the authors could generate laser pulses of 2.8 mJ at 1.06 μm and of 1.2 mJ at 1.3 μm.

Short pulse sources: High-energy short pulse sources are invariably of great importance because they facilitate numerous scientific and industrial applications. Possibly, this category is one of the most vivid research areas in solid-state laser science and technology. In general, to scale up the energy of the pulses in the ultrafast regime, very typical amplification techniques are utilized, such as chirped pulse amplification (CPA) or regenerative amplification [8–10].

Peng et al. present a monolithic fiber CPA system that could generate sub-500 fs pulses with 913 μJ pulse energy and 4.4 W average power at 1.55 μm wavelength, the estimated peak power of which approached 1.9 GW [8]. The authors highlight that the outstanding performance of this system was attributed to the straight and short length of the booster amplifier as well as to adaptive pulse shaping and spectral broadening techniques introduced to mitigate the overall nonlinear phase accumulation. They expect that additional system improvements, such as further increases in the effective mode area of the amplifier fiber, increasing Er-doping concentration, or longer pulse stretch factors, would lead to pulse energy well in excess of 1 mJ.

João et al. present a CPA laser system generating 1.24 mJ, 390 fs pulses at 1035 nm, which comprised a 2.8 mJ Yb:CaF₂ regenerative amplifier and a matched stretcher-compressor device based on a single-chirped volume Bragg grating and a compact, low-dispersion Treacy compressor [9]. The authors highlight that the use of a Treacy grating pair at the stretcher substantially improved the output pulse length to a value of ~390 fs, which had otherwise been limited to 1.7 ps.

Pouysegur et al. report the generation of 100-fs-level pulses obtained from an Yb:KYW-based regenerative amplifier [10]. The proposed innovative and simple femtosecond amplifier architecture was featured by the tailored control of the linear and nonlinear phase via the inclusion of a negative pre-chirper just after the seed laser. This allows for laser operations in 100-fs-level pulse duration, which have been accessible mainly with relatively more complicated Ti:sapphire-based ultrafast lasers. This pulse source eventually generated 32 μJ, 145 fs pulses centered at 1026 nm.

Modelocked oscillators: This category forms another vivid research area, providing seed lasers for short pulse source systems as well as being stand-alone sources for many other applications, such as optical combs, time-resolved measurements, imaging, etc.

Ishibashi et al. report the first demonstration of a modelocked Cr⁴⁺:YAG single-crystal fiber laser operating at 1520 nm with 120 fs pulse duration and 23 mW output power in a single-pulse-per-round-trip configuration, which was obtained in a Z-fold cavity with a semiconductor saturable absorber mirror (SESAM) [11]. The authors emphasize that although the Cr⁴⁺:YAG single-crystal fiber could support multiple waveguide modes, a single-transverse-mode operation was obtained with the aid of the proper design of the external cavity.

Mangold et al. present their recent study on a modelocked integrated external-cavity surface emitting laser (MIXSEL) [12]. In fact, this device facilitates stable and self-starting fundamental passive modelocking in a simple straight cavity, combining a vertical-external-cavity surface-emitting laser with a SESAM in a single semiconductor layer stack, with which the average power scaling is readily obtainable such as semiconductor disk lasers. The authors showed that a MIXSEL could support pulse repetition rate scaling from ~5 to >100 GHz with excellent beam quality and high average power. They could obtain average power of >1 W and pulse durations of <4 ps up to a pulse repetition rate of 15 GHz. In particular, they could demonstrate 101.2 GHz at 127 mW average output power and 570 fs pulse duration, which is the highest pulse repetition rate from any fundamentally modelocked semiconductor disk laser.

Tolstik et al. report a series of developments of graphene modelocked Cr:ZnS lasers [13,14]. The first laser configuration demonstrated mid-infrared pulses of only 5.1 optical cycles, i.e., 41 fs, operating at 2.4 μm with 190 nm spectral bandwidth, which is the shortest pulse width ever reported [13]. The pulse energy and average output power were scaled to 2.3 nJ and 250 mW at 108 MHz repletion rate, respectively. The second laser configuration demonstrated high-energy graphene modelocked mid-infrared pulses at 2.4 μm with 42 nm spectral bandwidth operating in the positive dispersion regime [14]. It is highlighted that this is the first demonstration of a solid-state dissipative soliton laser modelocked by graphene ever reported. The output pulses of 0.87 ps pulse duration with 15.5 nJ pulse energy could be compressed down to 189 fs.

Fiber Lasers: In general, fiber lasers allow for a unique ability to combine high power, high gain, high efficiency, and broad gain bandwidth. In particular, the performances of Yb-doped fiber lasers have been outstanding since a kilowatt single-mode operation of a large-core Yb-doped fiber was first demonstrated [15]. The low cost of ownership enables practical and cost-effective industrial laser applications, so that they are now competing with and replacing their bulk-type counterparts in many areas [16]. We believe that this research area will continue to expand, particularly in industrial and medical application areas.

Piccoli et al. propose and demonstrate for the first time a simple optical bleaching scheme to mitigate the photodarkening-induced losses of Yb-doped alumino-silicate fibers with high inversion and doping concentration above 1 wt%, which is based on a co-injection of a few mW of light at ~550 nm wavelength into the active fiber [17]. The authors could demonstrate operation at above 90% of the pristine output power level in several lasers with up to 30% Yb ions in the excited state. They also provide a comprehensive analysis of main parameters closely linked with the photobleaching performance via quantitative measurements of the excited state absorption cross-section in the visible range.

Agrež et al. present a gain-switched laser based on microstructured Yb-doped fiber, which could generate output pulses of 2.3 kW peak power with duration of <60 ns [18]. The laser output also exhibited a high polarization extinction ratio of >21 dB and a narrow spectral width of <0.13 nm. These laser properties make the laser setup suitable for use as a compact source for second harmonic generation.

Laser combining: Laser combining techniques open up the possibilities for ultimate power scaling of lasers [19], which include both coherent and incoherent combining techniques. A passive coherent beam combining technique is discussed here.

Rosenstein et al. demonstrate a Q-switched, intracavity combined fiber laser system based on two Yb-doped, rod-type photonic crystal fibers [20]. The output from the two fibers are passively phase-locked into a single output beam in a power oscillator configuration, generating a record peak power of ~0.7 MW with pulse duration of ~10 ns at 1 kHz repetition rate. The peak power can further be scaled up with increasing the number of phase-locked fibers or employing higher pump powers. The authors believe that the high peak power realized in the relatively simple configuration will offer numerous applications such as material processing, light detection and ranging, and medicine.

Nonlinear sources: Wavelength conversion based on nonlinear scatterings in fibers is one of the areas having dramatically expanded over the last decade since various novel types of dispersion-tailored fibers, such as photonic crystals fibers [21], became available together with the advent of high-peak power laser sources.

Demircan et al. present a novel scheme for generating a multi-octave supercontinuum based on the strong pulse reshaping at an optical event horizon [22]. This scheme actually exploits the authors’ own recent work on the two-pulse collision at a group velocity horizon between a dispersive wave and a soliton. They provide detailed theoretical and computational studies on the interaction between dispersive waves and a fundamental soliton in fiber for the generation of a supercontinuum encompassing the whole transparency region of fused silica, ranging from 300 to 2300 nm.

Nonlinear effects in fibers: While nonlinear scatterings, such as Raman scattering and Brillouin scattering, are sometimes actively utilized to generate specific Stokes signals, they can also severely limit the power scaling of optical fiber sources, so that the mitigation of such nonlinear scatterings has been a long-lasting issue over the last decade [23,24].

Park et al. challenge to analyze the stimulated Brillouin scattering issue with high-power optical fibers in an intuitive and much simplified manner [25]. For this, the authors introduce a novel quasi-mode interpretation methodology, which can represent acoustic radiation modes in acoustically antiguiding optical fibers in terms of discrete, virtual acoustic modes, so called, acoustic quasi-modes. The distinctive feature of this method compared to other previous methods is that the acoustic modal properties, including mode center frequencies and Brillouin linewidths for acoustic normal modes, can be determined in a straightforward manner without going through solving complicated, complex-valued transcendental characteristic equations. They present the detailed theory on it and discuss its effectiveness in comparison with a well-known experimental result previously reported in [24].

Optical technologies: Well-controlled manipulation of light has always been sought in optical systems comprising lasers as light sources. Two novel optical technologies, including beam manipulation based on the birefringence of biaxial crystals and the polarization control via a cavity locking technique, are discussed here.

Grant et al. present the evolution of conically diffracted Gaussian beam in free space, using various combinations of four biaxial crystals of the monoclinic double tungstate family [KGd(WO₄)₂] [26]. The authors also provide a theoretical model on it to predict the conical diffraction patterns, which turn out to be in good agreement with the experimental results. The novel feature of the conical diffraction may be useful for bio-medical applications, such as imaging, optical tweezers, etc. The authors also hope that their work will lead to a tangible expansion of the application space.

Kosuge et al. demonstrate a four-mirror ring cavity together with a three-mirror image invertor to produce a small spot inside the cavity which is eligible for generating γ-rays via laser Compton scattering (LCS) [27]. The polarization control via this cavity configuration is crucial because the angular distribution of nuclear resonance fluorescence γ-rays via multipole transitions is dependent on the polarization of LCS γ-rays which is basically identical to that of the laser. The authors expect that this cavity locking technique can generate polarization selectable LCS γ-rays for the purpose of nondestructive detection of isotopes in the spent nuclear fuel by using nuclear resonance fluorescence.

Topics for Optical Materials Express

Crystalline materials: Temperature coefficients of refractive index and thermal expansion coefficients of laser materials are important parameters for solid-state laser design. Sato et al. measured these parameters of YAG, YVO₄ and GdVO₄ precisely, using a highly accurate interferometric technique [28].

Laser materials: Compact and efficient Alexandrite lasers are attractive for several applications. Yorulmaz et al. demonstrate a low-threshold and efficient Alexandrite laser that is pumped by only one state-of-the-art single-spatial-mode diode [29]. While pure CW operation was obtained in most circumstances, self-Q-switching was also observed in slightly misaligned laser cavities.

Waveguide and optoelectronic devices: Karakuzu et al. present a practical approach to the numerical optimization of the guiding properties of buried microstructured waveguides in a z-cut lithium niobate crystal by using direct femtosecond laser inscription [30]. This low-loss waveguide can be used in the mid-infrared region beyond 3 μm wavelength.

In conclusion, we witnessed the success of the ASSL Congress 2013 as the only international, integrated event regarding solid-state lasers running single-track technical sessions for presenting and discussing materials, sources, and applications, all together. We believe that joining in this exciting event would offer great opportunities for seeing the utmost recent advances in solid-state laser science and technology and also for foreseeing their future. We hope this focus issue offers a good snapshot of a variety of topical discussions held at the ASSL Congress 2013 and will contribute to the further expansion of the associated research areas. Finally, we all invite you to join the upcoming ASSL Congress 2014 that is to be held in Shanghai on November 16–21, 2014.