High average power (105†W) Fe:ZnSe laser pumped by radiation of laser diode side-pumped Er:YAG lasers.

A high average power re-frequency operation Fe:ZnSe laser using laser diode side-pumped free-running Er:YAG lasers as activating sources is presented. Two pieces of subsurface layer doped Fe:ZnSe polycrystal are adoptive in a reflective resonator configuration and face-cooled by liquid nitrogen. A maximal Fe:ZnSe laser power of 105 W at a wavelength of 4.1 µm is achieved upon pumping by ten home-made Er:YAG lasers with fiber coupled output working at a frequency of 250 Hz and a pulse duration of ∼420 µs. Corresponding to the maximum Fe:ZnSe laser power, the optical-optical efficiency and slope efficiency with respect to the absorbed pump power are 43% and 44% respectively. The beam quality factor M2 is measured to be 3.4. To the best of our knowledge, it is the highest output average power of an Fe:ZnSe laser reported.

Performance of the mid-infrared Q-switched LD side-pumped Er:YSGG MOPA laser.

We report on a laser-diode (LD)-pumped master-oscillator power amplifier (MOPA) mid-infrared laser system based on an LD side-pumped Er:YSGG seed laser that can operate in both free-running and Q-switched regimes. In the free-running mode of the seed laser, the maximum amplified single-pulse energy was 83.4 mJ. In Q-switched mode of the seed laser, a maximum single-pulse energy of 7.8 mJ was achieved at 100 Hz repetition rate with the pulse width of 90 ns, corresponding to the peak power of 86.7 kW and the single-pass amplification factor of 1.66. The results indicate that the LD side-pumped MOPA structure is an effective way to realize a nanosecond ∼3µm mid-infrared laser with high repetition rate and high pulse energy.

1.14†kW peak power mid-infrared Er:YAG planar waveguide MOPA laser.

We report on a quasi-continuous Er:YAG planar waveguide laser operated at 2.94 µm based on the major oscillator power amplification configuration. With the total pump peak power of 32.01 kW, a maximum output peak power of 1.14 kW was obtained at the seed injection peak power of 184.4 W operated at 400µs, 40 Hz. Furthermore, the numerical simulation results indicate that better performance of the laser could be obtained with the higher injected seed laser power. To the best of our knowledge, this is the first experimental demonstration of 2.94 µm planar waveguide laser with an Er doped host material.

148-W single-frequency Nd:YAG Innoslab µs-amplifier at 1064†nm with high efficiency.

In this paper, a single-frequency quasi-continuous-wave partially pumped slab (Innoslab) amplifier at 1064 nm is reported. The 4.4-W single-frequency seed laser was amplified to 148.1 W with optical-optical efficiency of 30.4%. The output pulse duration was 141.4 µs at the repetition of 500 Hz. The beam quality factors of M2 were 1.41 and 1.37 in the horizontal and vertical direction respectively. The experimental results match well with the numerical simulation. To the best of our knowledge, this is the first report on single-frequency Nd:YAG Innoslab amplifier with such a high output power and efficiency.

300 W two-stage Nd:YAG Innoslab microsecond amplifier.

In this paper, a two-stage partially pumped slab (Innoslab) microsecond amplifier at 1064 nm is reported. The 4.4-W single-frequency seed laser is amplified to 303.6 W, with an overall optical-optical efficiency of 25.7%. The overlapping efficiency of the first- and second- amplifier stage is 67% and 55.6%, respectively. The pulse width is 145.0 µs, at a repetition rate of 500 Hz, and the beam quality factor of M2 is 1.84 and 1.71 in the horizontal and vertical directions, respectively. With higher overlap between the pump volume and the seed laser mode, the output power and optical-optical efficiency can be further improved.

208 W all-solid-state sodium guide star laser operated at modulated-longitudinal mode.

A 208 W all-solid-state modulated-longitudinal-mode quasi-continuous-wave sodium guide star (SGS) laser was developed by sum-frequency of a 1064 nm laser and a 1319 nm laser. The laser contained two spectral lines separated by 1.72 GHz for re-pumping the sodium atoms. To suppress absorption saturation effect of the sodium atoms induced by the high light intensity, we used a white noise generator to modulate the 1064 nm single frequency seed laser in the frequency domain. The line width of the modulated-longitudinal-mode 589 nm laser was maximally broadened to 0.74 GHz compared to the initial line width of ~0.30 GHz. A bright SGS with photon return flux of 56800 photons/s/cm2 during the pulse length was obtained.

Enhanced high-slope-efficiency and high-power LD side-pumped Er:YSGG laser.

We designed a high-slope-efficiency and high-power laser diode (LD) side-pump Er:YSGG laser based on the analysis for the effect of the crystal dimension and the LD distribution on the temperature and energy distributions in laser crystal. A maximum output power of 34.9 W for a 2.8 µm mid-infrared laser was achieved at 200A, 120 Hz, and 500 µs pulse width, corresponding to the slope efficiency of 13.7% and optical-optical efficiency of 12.7%. Moreover, the beam quality $M_x^2/M_y^2$Mx2/My2 factors of the laser were measured to be 5.15/5.19 and the far-field divergences ${\Theta _x}/{\Theta _y}$Θx/Θy were 9.52/9.75 mrad.

Narrow-linewidth, quasi-continuous-wave ASE source based on a multiple-pass Nd:YAG zigzag slab amplifier configuration.

We present investigations into a narrow-linewidth, quasi-continuous-wave pulsed all-solid-state amplified spontaneous emission (ASE) source by use of a novel multiple-pass zigzag slab amplifier. The SE fluorescence emitted from a Nd:YAG slab active medium acts as the seed and is amplified back and forth 8 times through the same slab. Thanks to the angular multiplexing nature of the zigzag slab, high-intensity 1064-nm ASE output can be produced without unwanted self-lasing in this configuration. Experimentally, the output energy, optical conversion efficiency, pulse dynamics, spectral property, and beam quality of the ASE source are studied when the Nd:YAG slab end-pumped by two high-brightness laser diode arrays. The maximum single pulse energy of 347 mJ is generated with an optical efficiency of ~5.9% and a beam quality of 3.5/17 in the thickness/width direction of the slab. As expected, smooth pulses without relaxing spikes and continuous spectra are achieved. Moreover, the spectral width of the ASE source narrows versus the pump energy, getting a 3-dB linewidth of as narrow as 20 pm (i.e. 5.3 GHz). Via the sum frequency generation, high-intensity, smooth-pulse, and narrow-linewidth ASE sources are preferred for solving the major problem of saturation of the mesospheric sodium atoms and can create a much brighter sodium guide star to meet the needs of adaptive imaging applications in astronomy.

Relaxation oscillation suppressed, narrow linewidth, high beam quality, 1319 nm long-pulsed duration laser.

We first report a 1319 nm long-pulsed duration laser with relaxation oscillation suppressed by placing lithium triborate crystals into the laser cavity. Two more etalons were added into the laser cavity to further increase the difference in cavity loss between oscillating and non-oscillating modes. With very little reduction in average output laser power, the smooth temporal profile laser pulse was obtained with a pulse duration of 150 µs. The average output power was 5.45 W, and the beam qualities Mx2 and My2 were 1.12 and 1.20.

Surface enhanced nonlinear Cherenkov radiation in one-dimensional nonlinear photonic crystal.

We study the configuration of efficient nonlinear Cherenkov radiation generated at the inner surface of a one-dimensional nonlinear photonic crystal, which utilizes the combination of both quasi-phase matching and total internal reflection by the crystal surface. Multi-directional nonlinear Cherenkov radiation assisted by different orders of reciprocal vectors is demonstrated experimentally. At specific angles, by associating with quasi-phase matching, the incident fundamental wave and total internal reflection wave format completely the phase-matching scheme, leading to great enhancement of harmonic wave intensity.

Nonlinear Cherenkov radiation at the interface of two different nonlinear media.

We discuss the nonlinear response due to the spatial modulation of the second-order susceptibility at the interface between two nonlinear media, and experimentally demonstrate that the nonlinear Cherenkov radiation is enhanced by the interface of two nonlinear crystals with a large disparity in χ(2). In our experiment, the intensity of the nonlinear Cherenkov radiation generated at the nonlinear interface was approximately 4 to 10 times that at the crystal boundary. This result suggests potential applications to efficient frequency conversion.

Sum-frequency nonlinear Cherenkov radiation generated on the boundary of bulk medium crystal.

We demonstrated experimentally a method to generate the sum-frequency Nonlinear Cherenkov radiation (NCR) on the boundary of bulk medium by using two synchronized laser beam with wavelength of 1300 nm and 800 nm. It is also an evidence that the polarization wave is always confined to the boundary. Critical conditions of surface sum-frequency NCR under normal and anomalous dispersion condition is discussed.

Large acceptance of non-collinear phase-matching second harmonic generation on the surface of an anomalous-like bulk dispersion medium.

We investigate several bandwidths of non-collinear phase-matching second harmonic generation, which is generated by sum-frequency of the incident and reflected wave on the inner surface of a z-cut 5%/mol MgO : LiNbO3 crystal. The bandwidths of angle, temperature and wavelength in this configuration are measured to be about 0.51°, 4.1°C and 6 nm, respectively. The large acceptance of non-collinear phase-matching second harmonic generated on the surface shows attractive potential in the application of wavelength conversion.

Cherenkov second-harmonic Talbot effect in one-dimension nonlinear photonic crystal.

We demonstrate a new method to generate second-harmonic Talbot effect through degenerate Cherenkov radiation in one-dimension anomalous-dispersion-like nonlinear photonic crystals. In anomalous-dispersion-like medium, the degenerated nonlinear Cherenkov radiation can be achieved and is parallel to domain walls, of which the intensity is adjusted by the second-order nonlinear coefficient. In this system the one-dimension nonlinear photonic crystal can be regarded as a nonlinear grating, which is necessary for nonlinear Talbot effect. This is a new method to generate enhanced nonlinear Talbot effect in addition to the quasi-phase-matching technique reported previously.

Enhanced nonlinear Cherenkov radiation on the crystal boundary.

We demonstrate a method to generate enhanced nonlinear Cherenkov radiation (NCR) in the bulk medium by utilizing the total reflection on the physical boundaries inside the crystal. This is the experimental demonstration of enhanced NCR by a sharp χ(2) modulation from 0 to 1, and a new way for generating enhanced NCR in addition to using the waveguide structures and the ferroelectric domain walls, which also possesses a better beam quality for applications.

Nonlinear Cherenkov radiation in an anomalous dispersive medium.

By using ultrathin nonlinear media, the phase velocity of nonlinear polarization waves can be continuously adjusted. We realized nonlinear Cherenkov radiation (NCR) in an anomalous dispersion medium experimentally, which breaks the minimum speed limit of NCR. This modified nonlinear Cherenkov radiation has two new special states with respect to its prior, i.e., the degenerate state and extreme state. Also, periodically arranged ultrathin nonlinear media can enhance NCR and realize the nonlinear Smith-Purcell effect.

Enhancement of UV Second-Harmonic Radiation at Nonlinear Interfaces with Discontinuous Second-order Susceptibilities.

We investigate the generation of ultraviolet (UV) second-harmonic radiation at the boundary of a UV transparent crystal, which is derived from the automatic partial phase matching of the incident wave and the total internal reflection. By adhering to another UV non-transparent crystal with a larger second-order nonlinear coefficient χ(2), a nonlinear interface with large disparity in χ(2) is formed and the enhancement of UV second-harmonic radiation is observed experimentally. The intensity of enhanced second harmonic wave generated at the nonlinear interface is up to 11.6 times that at the crystal boundary. As a tunable phase-matching method, it may suggest potential applications in the UV, and even vacuum-UV region.

Time-reversed wave mixing in nonlinear optics.

Time-reversal symmetry is important to optics. Optical processes can run in a forward or backward direction through time when such symmetry is preserved. In linear optics, a time-reversed process of laser emission can enable total absorption of coherent light fields inside an optical cavity of loss by time-reversing the original gain medium. Nonlinearity, however, can often destroy such symmetry in nonlinear optics, making it difficult to study time-reversal symmetry with nonlinear optical wave mixings. Here we demonstrate time-reversed wave mixings for optical second harmonic generation (SHG) and optical parametric amplification (OPA) by exploring this well-known but underappreciated symmetry in nonlinear optics. This allows us to observe the annihilation of coherent beams. Our study offers new avenues for flexible control in nonlinear optics and has potential applications in efficient wavelength conversion, all-optical computing.