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Transparent laser ceramics have been demonstrated to offer tremendous processing and design advantages in the diode-pumped solid-state laser field. Successfully developed composite Nd:YAG/Cr:YAG ceramics realized a multi-megawatt three-beam output microchip laser for efficient engine ignition. After a progress review for Giant Micro-photonics, including their wavelength extension with micro-domain controlling, we’d like to discuss the next generation of high-brightness lasers based on anisotropic ceramics. The capability of transparent anisotropic ceramics, by using a new crystal orientation process based on large magnetic anisotropy induced by 4f electrons, offers extremely high-power laser materials such as RE:FAP and patterning process for multi-function integrated monolithic solid-state lasers.
©2011 Optical Society of America
1. Introduction
Development of solid-state sources, including laser and nonlinear optical frequency conversion, has contributed to broadening the new horizon in the quantum electronics field, owing to their high-brightness nature of giant pulses under Q switching and ultrashort pulses under mode locking. Moreover, their cutting edges are expected from the field of high-energy physics (i.e., laser fusion/laser ignition, laser accelerator, and vacuum decay) to precise measurement (i.e. optical comb, LIDAR), laser-based material processing (i.e., cutting, welding, drilling), and laser displays. On the other hand, its development has long been a materials-limited field. Advances in laser performance, such as increased optical powers, extended wavelength ranges, and longer lifetimes, are in general preceded by improvements in the materials on which the lasers are based. The past decade has witnessed a veritable revolution in the types and performance levels of solid-state lasers, largely due to development of micro-domain engineered new optical materials. Two such materials, micro-domain structured transparent laser ceramics and ferroelectrics for quasi-phase matched nonlinear optics, have emerged over the past decade as the basis for a field described as Micro Solid-State Photonics [1]. In both systems, the ability to manipulate a given material on a scale of the optical wavelength enables engineering of a large range of macroscopic behaviors.
In this paper, we’d like to review the recent progress of this field with regard to high-performance microchip lasers based on the micro-domain structure and boundary-controlled materials. The latest advance in laser ceramics is the orientation-controlled anisotropic gain medium by applying a magnetic field such as the electric field poling in the ferroelectric materials, as shown in Fig. 1 . The engineered, compact, solid-state optical materials are reliable, efficient, and multi-functional light sources. Moreover, their micro domain causes a new interaction for coherent radiation. This effect should be enhanced by a micro-cavity or periodic structure. These micro and/or microchip lasers by using advanced ceramics can provide extreme performances as a new generation of solid-state lasers.
2. High-brightness microchip lasers
2.1 Ceramic lasers as an early work of micro-domain controlling
In the late 1980s, the development of high-power (>50 W/cm), high-efficiency (>50%), high-temperature (>300 K) laser diodes and laser diode arrays have led to a renaissance-like revolution in solid-state lasers [2]. Rapid development in the Nd3+:Y3Al5O12 (Nd:YAG, cubic) laser has been established with using AlGaAs active quantum wells emitting at 808 nm. Compared with broadband arc or flash-lamps pump sources, a diode laser pump source possesses greatly increased spectral brightness, with an emission bandwidth of typically a few nanometers. The availability of such narrowband bright pump sources precipitated the drastic advances in downsizing, improved efficiency, improved stability, and long lifetime in diode pumped solid-state lasers (DPSSLs). Moreover, it made an efficient quasi-three- or quasi-four- level laser action at room temperature. Now, the Yb:YAG laser has been recognized as attractive for scaling both energy and average power in continuous wave (cw) and ultrafast lasers because of its suitable emission bandwidth and good thermo-optical properties with low quantum defect. Development of DPSSLs has been realized based on trivalent rear-earth (RE3+)-ion-doped YAG material [3–10]. In addition, the transparent laser ceramics should be making an impact in the field of solid-state lasers because of their numerous promises over melt growth methods, including (1) faster production times, (2) formation of solid solutions allowing the fabrication of multiphase transition materials that are highly homogeneous, and (3) the ability to engineer profiles and structures before sintering. In addition, (4) it is possible to increase its extraction power with their improved tensile strength as compared with single crystals. Much progress has been expected in improving the optical quality of ceramics as well as in exploring new laser materials [11–13]. However, the conventional polycrystalline ceramic materials have many light-scattering centers: a refractive index modulation around the grain boundary; index changes by inclusions or pores; segregations of the different phases; birefringence; and surface scattering by roughness, as shown in Fig. 2 . That is why ceramics were not suitable as laser media for a long time. We had to wait for the Nd:YAG ceramic laser until 1995. In addition, we made much effort to brush up those new materials: fundamental material characterization, single-axial mode oscillation, and its typical laser evaluations have been demonstrated. These pioneering works led to laser ceramics research in the 2000s [14–16].
Fig. 2 Microstructure images of polycrystalline ceramics and light-scattering sources: (1) refractive index modulation around the grain boundary, (2) index changes by inclusions or pores, (3) segregations of the different phases, (4) birefringence, (5) surface scattering by roughness. The color of each crystal describes the direction of orientation.
Advancements in the design and development of polycrystalline ceramic laser media have balanced with drawback of scattering losses during the past decade and contributed to the enhancement of the power density and functionality of solid-state lasers [17–24]. Now, these devices have various scientific, medical, and industrial applications, such as communications, engine ignition, or inertia nuclear fusion, the last one being an important application of future laser technology. RE:YAGs are popular and promising materials for high-power applications because of their high quantum efficiency and superior thermo-mechanical properties. For example, cw power of a >100 kW JHPSSL Nd:YAG laser [24] and power density of a 0.19 MW/cm3 composite Yb:YAG microchip laser were demonstrated using ceramics gain media[22,23]. Like the above transparent polycrystalline ceramics and periodically poled ferro-electric devices, it is possible to enhance its optical effects for creation of new function by micro-domain structure and boundary controlling. Moreover, we can expect more effects of its optical function by using a micro-cavity structure. Thus, micro solid-state photonics is making an impact in the development of high-brightness microchip solid-state lasers.
2.2 Giant pulse microchip lasers
The high-power density chip laser will offer extremely high-power performance under scaling. The microchip laser, which is the smallest sized solid-state laser, maintains its short cavity of about few millimeters to 10 mm length. This short cavity has a benefit of enhancing the quantization effects to improve its laser coherence as single-axial mode oscillation. Furthermore, it is possible to reduce the Q-switched pulse duration to the subnanosecond region, because its cavity round-trip time τr is three to four orders shorter than conventional solid-state lasers. Namely, the Q-switched laser pulse width τp is given by
where τc is the cavity decay time and is given by the cavity round-trip time τr and the loss of cavity δ as τc = τr/δ = 2L/cδ, r is the initial inversion ratio, η is the energy extraction efficiency, L is the cavity length, and c is the speed of light [25]. Equation (1) indicates that the Q-switched laser pulse width could be shortened until cavity round-trip time. For example, it is possible to generate 12 ps pulse width directly from a 1 mm cavity length Nd:YAG microchip Q-switched laser. Consequently, the drastic brightness improvement should be expected only by cavity shortening due to high-peak-power single-frequency oscillation with a single transverse mode.In general, solid-state lasers have advantages in giant-pulse generation in Q-switching, for which the typical pulse width is a few hundred nanoseconds to a few nanoseconds, and ultrashort pulse generation in mode locking, in the few picoseconds to the few femtoseconds region. Unfortunately, for a long time, it has been difficult to cover the “pulse-gap” region between nanoseconds to picoseconds, even though we can expect a fruitful phenomenon for material interactions. In this situation, the promise of pulse duration from a few nanoseconds until the 10 ps region in the Q-switched microchip lasers is attractive. However, the disadvantage of microchip lasers is poor power and poor efficiency. Until recently, the peak powers of Q-switched microchip lasers were limited to a few kilowatts to the 100 kW level [26–29]. Laser ceramics has a potential to overcome this situation because of its high Nd3+-ion doping and highly homogeneous medium capability with engineered composite structure ability.
In addition, for both the fusion driver and the internal combustion engine, laser ignition has been investigated for a long time because of its high efficiency and high-energy firing capability. From the point of view of protecting the global environment and preserving fossil resources, the improvement of the efficiency of conventional internal combustion gasoline engines and the reductions of CO2 and harmful pollutant emissions have become more important. Laser ignition can change the concept of ignition innovatively and has many advantages over conventional electric spark plug ignition. The laser ignition plasma can be located anywhere within the combustion chamber. Its optimal positioning away from the cold cylinder wall allows the combustion flame front to expand rapidly and uniformly in the chamber and thus improves the efficiency. In addition, laser ignition has advantage for simultaneous, spatial multipoint ignition within a chamber. This shortens combustion time dramatically and improves the output and efficiency of engines effectively. Furthermore, a laser can ignite leaner or high-pressure mixtures that are difficult to be ignited by a conventional electric spark plug. However, the major difficulty of laser ignition for practical applications, especially for automobiles, is the dimension of the high-power lasers. For breakdown in fuel mixtures, light intensities in the order of 100 GW/cm2 are necessary at the focal point of ignition. Unfortunately, the commercially available giant pulse laser heads are table-top sized owing to the complexities of the laser cavity and the cooling system.
Progress of micro solid-state photonics gives us the solution for a compact giant pulse laser. The high-quality Nd:YAG/Cr:YAG composite monolithic chips are available by ceramic lasers. Up to 6 MW peak output and 500 ps pulse width at 1064 nm was obtained with passively a Q-switched Nd:YAG/Cr:YAG microchip laser [30,31]. Figure 3(a) shows the combustion process events and laser pulse durations. The microchip-based Q-switched laser pulses between the conventional Q-switched lasers and mode-locked lasers to cover the conventional pulse-gap region, which is the key time region for the combustion process, as shown in Figs. 3(a) and 3(b) [32]. The brightness of the subnanosecond and diffraction limit pulse microchip laser was calculated to be >0.3 PW/sr-cm2, and the brightness temperature was estimated to be 0.23–0.46 ZK (1021 K) (cf. the temperature of sun surface: ~6000 K). By the sophisticated laser operation of a five-pulse train, a 100% ignition was successfully demonstrated in a lean mixture of an A/F 17.2, where spark plug ignition failed. Finally, ignition tests for a real vehicle engine were performed. This kind of ceramic microchip laser has been realized within a spark plug size. In these experiments, we have used separated single-crystal Nd:YAG and Cr:YAG chips at first. After parameter optimization, we have designed a Nd:YAG/Cr:YAG monolithic chip by the sintering method, as shown in Fig. 4 (1.1 at.% Nd:YAG ceramics with 7.0 mm length and T0 = 25% Cr:YAG with 2.5 mm length) [33]. The advantage is just its uniformity for multiple-beam system. A photo of the all-ceramics, composite Nd:YAG/Cr4+:YAG monolithic laser with three-beam output is presented in Fig. 5 , where air breakdown is illustrated. An automobile electrical spark plug is shown for comparison. In this system, the beam energy variation is small enough to 5% tuning.
Fig. 5 Photo of the composite, all-ceramics passively Q-switched Nd:YAG/Cr4+:YAG monolithic laser with three-beam output.
In addition, it is possible to drive it by the automobile battery. These are surprising results for megawatt Q-switched lasers, and it is possible to apply them for nonlinear wavelength conversion from the ultraviolet to the terahertz region [34].
3. Giant Micro-photonics
3.1 Quasi-phase-matched device based high-performance nonlinear wavelength conversion
The interest in quasi-phase-matched (QPM) devices has increased because of their advantages over the birefringent-phase-matching (BPM) method [35]. Using various electric-field poling techniques, periodically poled LiNbO3 (PPLN) for high-efficiency nonlinear wavelength conversion has been demonstrated: owing to its large electro-optical, piezoelectric, and nonlinear-optical effects (~25.2 pm/V @ 1.064 μm [36]), the applications include signal processing for optical communications and ultrafast pulse generation and shaping. Lately, attention has been directed toward the periodically poled structured stoichiometric LiTaO3 (PPsLT) [37] and MgO-doped congruent LiNbO3 (PPMgLN) [38], owing to its higher resistance to photorefractive damage and much lower coercive fields compared with LN. However, the nature of charge concentration during the poling process hinders patterning of the periodically poled QPM structure. To overcome this limitation, a multi-pulse field poling technique for large cross-section MgLN was developed [39]. Also, the coercive field necessary to invert the crystal polarization was drastically reduced at an elevated temperature of 250°C: the coercive field was 1.2 kV/mm, which is about 1/4 compared with that for MgLN at room temperature (RT), and about 1/17 of that for LN at RT. Finally, a structure for quasi-phase matching of a 5 mm × 5 mm large aperture using 5 mol.% MgO-doped periodically poled LiNbO3 (LA-PPMgLN) for high-power wavelength conversion was realized [40–42]. A free-running degenerate LA-PPMgLN OPO pumped by a Q-switched 1.064 μm Nd:YAG laser exhibits maximum output pulse energy of 110 mJ with high slope efficiency of 75% around room temperature. Broad spectral bandwidth (Δλ~100 nm) around the degeneracy wavelength was suppressed by using a VBG as an output coupler. With this LA-PPMgLN, a tunable, narrow-bandwidth (<2 cm−1) MIR optical parametric system has been developed: a continuously tunable tuning range from 4.6 to 11.2 μm, producing a maximum output energy of 2.0 mJ at 5.1 μm [43]. Practical use of the MIR source is demonstrated by MIR-UV double-resonance spectroscopy of jet-cooled acetanilides [44]. On the other hand, the generation of 2-cycle (15.7-fs), 740 μJ pulse at the carrier wavelength of 2.1 μm and the repetition rate of 1 kHz, from a broadband OPCPA system based on the LA-PPMgLN, has been demonstrated. A novel method of seed spectral shaping is used to solve the nontrivial problem of determining the true super fluorescence level and the amplified signal energy from the total pulse energy of 920 μJ [45]. These results indicate that the micro-domain structure or grain boundary engineered QPM devices, open new doors, so to speak, for Giant Micro-photonics, such as that shown in Fig. 6 .
3.2 Micro-domain-controlled laser ceramics as the next-generation laser ceramics
The history of the diode-pumped solid-state laser is summarized in Fig. 7 . Figures 7(a) and 7(b) correspond to the RE:YAG single crystal and ceramics. A revolution in power scalability is expected from the Yb:YAG ceramic laser for extremely high-power operation. However, this material must be used under cryogenic conditions [46,47] because of its relatively small stimulated-emission cross section at room temperature. On the other hand, Yb3+-doped hexagonal fluorapatite materials have large stimulated emission and absorption cross sections, as well as long fluorescence lifetimes [48]. Furthermore, they show excellent optical characteristics and therefore are suitable for high-power laser operation at room temperature. An output energy of 70 J has been achieved with a ytterbium-doped strontium fluorapatite Yb:Sr10(PO4)6F2 (Yb:SFAP) single crystal, as shown in Fig. 7(c) [49]. In general, laser-grade anisotropic single crystals, such as apatite and YVO4 (tetragonal), are grown by melt techniques [50–52]. However, it is difficult to obtain large-volume or composite media whose power scalability and functionality are comparable to conventional ceramic lasers. Therefore,ceramics from non-cubic materials requires precise crystal orientation control, so that optical polycrystalline anisotropic materials realized by a ceramics process are expected to contribute significantly to the field of laser technology. Various techniques have been proposed to control the crystal orientation in polycrystalline ceramics. However, it was too difficult to obtain uniaxial and homogeneously oriented laser-grade ceramics. Just recently, it has been demonstrated that the crystal orientation of diamagnetic and paramagnetic materials, as well as that of magnetic materials, can be controlled by imposing a high magnetic field of 10 T. Figure 8 presents the new method proposed for orientation control of the micro-domain structure in RE3+-doped anisotropic laser ceramics. The principle of ceramic particle handling process by magnetic field can be explained from the perspective of magnetization energy. When a nonmagnetic material is submerged in a magnetic field, the imparted magnetization energy U is approximately given by
where μ0 is the magnetic permeability in vacuum, V is the volume of the material, χ⊥ is themagnetic susceptibility of the hard axis, Δχ is the difference in the magnetic susceptibility between the magnetic easy axis and the magnetic hard axis, H is the intensity of the imposed magnetic field, and θ is the angle between the magnetic field direction and the magnetic easy axis,Fig. 8 Schematic of the RE-assisted magnetic orientation method for fabrication of RE-doped anisotropic laser ceramics.
Equation (3) shows that the magnetic torque is proportional to H2 and Δχ. We found that the spin–orbit interaction of the 4f electrons of an RE3+ ion strongly enhances the net magnetic anisotropy Δχ, and thus a large magnetic torque per unit volume can be generated. The magnetic anisotropy Δχ is enhanced by RE doping [Δχ = 10−4~10−6 (RE doped), Δχ = 10−6~10−7 (undoped)]. Therefore, it is possible to realize highly oriented anisotropic ceramics with a 1.4T magnetic field toward extreme lasers of Giant Micro-photonics in Fig. 7(d). Uniaxial oriented laser-grade Nd:FAP and Yb:FAP ceramics were successfully fabricated by newly developed spin-orbital orientation controlling process [53,54]. The photograph of 2 at.% Nd:FAP transparent anisotropic laser-grade ceramics (0.5 mm in thickness and 3.5 mm in width) have been fabricated with the above process, as shown in Fig. 9 . Lately, we have demonstrated the diode pumped laser oscillation in these anisotropic Nd:FAP ceramics and new micro-domain orientation controlling and patterning method for composite structured anisotropic laser ceramics by imposition of the gradient magnetic field during slip casting [55].
Here, we’d like to propose the new patterning method by using a gradual magnetic field. The translational magnetic force acting on a substance can be obtained as shown in Eq. (4),
Therefore, magnetic force FM is in proportion to (H⋅∇)H and magnetic susceptibility difference between substances. The schematic of the multi-functional micro-domain controlling process under magnetic field is shown in Fig. 8. Simultaneous micro-domain controlling of crystal orientation and rare-earth distribution is available during slip casting process, and it is possible to obtain arbitrary shapes of rare-earth patterns by designing the magnetic field gradient. This RE-assisted magnetic patterning process for laser ceramics can be applied to other RE-doped transparent laser media such as RE: FAP, YAG, and YVO4.
4. Conclusion
In conclusion, we have successfully demonstrated micro solid-state photonics–based high-brightness lasers and wavelength extension. Up to a 190 kW/cm3 power density 1 μm wavelength microchip laser was obtained in Yb:YAG/YAG composite ceramics. Moreover, a multi-megawatt three-beam output giant pulse laser has been demonstrated using Nd:YAG/Cr:YAG ceramic monolithic microchip in an automobile spark plug compatible size. In addition, periodically poled Mg-doped LiNbO3 devices extend the wavelength of 1 μm wavelength lasers to the broadband spectrum, their wavelengths ranging from 5 nm (soft X-ray) to 300 μm (THz wavelength), as shown in Fig. 10 . Now, micro-domain-controlled new materials/devices allow us the fruit of full photonics world.
Eventually, we have established a way to align the laser ceramics orientation. The spin–orbit interaction of 4f electrons of an RE3+ ion strongly enhances the net magnetic anisotropy, and thus a large magnetic torque can be generated. We improved the fabrication process of transparent anisotropic ceramics toward Giant Micro-photonics and realized the laser action in anisotropic ceramics with RE:FAP. Moreover, RE-assisted magnetic patterning process for laser ceramics can be applied to multi-function integrated monolithic solid-state lasers. This innovation in laser ceramics with controlled microstructures and grain boundaries is expected to contribute significantly to the field of laser science and technology.
Acknowledgments
The author is grateful to Dr. M. Tsunekane, Dr. H. Ishizuki, and Dr. J. Akiyama of IMS for their support. This work was partially supported by MEXT, JSPS, JST and Genesis Research Institute, Inc.
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53. J. Akiyama, Y. Sato, and T. Taira, “Laser ceramics with rare-earth-doped anisotropic materials,” Opt. Lett. 35(21), 3598–3600 (2010). [CrossRef] [PubMed]
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