Efficient, frequency-stable laser-diode-pumped Nd:YAG laser

Analysis and Dynamic Monitoring of Indoor Air Quality Based on Laser-Induced Breakdown Spectroscopy and Machine Learning

The air quality of the living area influences human health to a certain extent. Therefore, it is particularly important to detect the quality of indoor air. However, traditional detection methods mainly depend on chemical analysis, which has long been criticized for its high time cost. In this research, a rapid air detection method for the indoor environment using laser-induced breakdown spectroscopy (LIBS) and machine learning was proposed. Four common scenes were simulated, including burning carbon, burning incense, spraying perfume and hot shower which often led to indoor air quality changes. Two steps of spectral measurements and algorithm analysis were used in the experiment. Moreover, the proposed method was found to be effective in distinguishing different kinds of aerosols and presenting sensitivity to the air compositions. In this paper, the signal was isolated by the forest, so the singular values were filtered out. Meanwhile, the spectra of different scenarios were analyzed via the principal component analysis (PCA), and the air environment was classified by K-Nearest Neighbor (KNN) algorithm with an accuracy of 99.2%. Moreover, based on the establishment of a high-precision quantitative detection model, a back propagation (BP) neural network was introduced to improve the robustness and accuracy of indoor environment. The results show that by taking this method, the dynamic prediction of elements concentration can be realized, and its recognition accuracy is 96.5%.

Unidirectional single-frequency operation of a continuous-wave Alexandrite ring laser with wavelength tunability

High resolution spectroscopy, metrology and quantum technologies (e.g. trapping and cooling) require precision laser sources with narrow linewidth and wavelength tunability. The widespread use of these lasers will be promoted if they are cost-effective, compact and efficient. Alexandrite lasers with a broad tuning band pumped efficiently by low-cost red diodes are a potential candidate, but full performance as a precision light source has not been fully achieved. We present in this work the first continuous-wave (CW) and single-frequency operation of a unidirectional diode-end-pumped Alexandrite ring laser with wavelength tunability. An ultra-compact bow-tie ring cavity is developed with astigmatic compensation and a novel 'displaced mode' design producing CW output power > 1 W in excellent TEM₀₀ mode (M² < 1.2) when using a low brightness pump (M² ≥ 30). Wavelength tuning from 727 - 792 nm is demonstrated using a birefringent filter plate. This successful operation opens the prospects of precision light source applications.

Single frequency MOPA based on Nd:YAG single crystal fiber and rods

We demonstrate a single frequency 1064 nm master oscillator power amplifier (MOPA) system operating in macro-micro pulse scheme. The repetition rate for the macro pulses was 300 Hz with pulse duration of 300 μs. Micro pulses operated at 25 kHz. The master laser was a single-longitudinal-mode electro-optically Q-switched Nd:YAG laser with an output power of 250 mW and pulse duration of 33 ns. Three stages of power amplifiers based on Nd:YAG single crystal fiber and rods were designed. The final output power reached 31.3 W with pulse duration of 30 ns and linewidth of less than 130 MHz. Micro pulse energy of 13.9 mJ was obtained with a peak power of up to 464 kW. The beam quality factors (M²) were measured to be 1.56 and 1.76 in horizontal and vertical directions, respectively.

Experimental study of the generation of a blue laser by intracavity frequency doubling of a cw Nd:GdVO4 laser with lithium borate

Efficient cw intracavity frequency doubling of a diode end-pumped Nd:GdVO4 laser that operates in the 4F(3/2) --> 4I(9/2) transition at 912 nm is demonstrated. A 15 mm long lithium borate crystal, cut for critical type I phase matching at room temperature, was used for second harmonic generation of the fundamental laser. A maximum output power of 14.8 W in the deep blue spectral range at 456 nm was achieved at an incident pump power of 40 W. In 2 h we achieved an optical-to-optical conversion efficiency of 37% with a better than 2.57% power stability.

High-power laser performance of a-cut and c-cut Yb:LuVO4 crystals

We report high-power laser operation that was achieved with a-cut and c-cut Yb:LuVO(4) crystals. Up to 8.3 W of pi-polarized output power was generated from an a-cut, 2 mm uncoated crystal with an optical conversion efficiency of 59%. The highest slope efficiency was 80%. Laser oscillation obtained with c-cut crystal was also linearly polarized, with two orthogonal polarization components along [110] and [(-)110] directions. The highest output power achieved was 4.5 W, with an optical conversion efficiency of 33% and a slope efficiency of 40%.

End-pumped 1.5 microm monoblock laser for broad temperature operation

We describe a next-generation monoblock laser capable of a greater than 10 mJ, 1.5 microm output at 10 pulses/s (pps) over broad ambient temperature extremes with no active temperature control. The transmitter design is based on a Nd:YAG laser with a Cr4+ passive Q switch and intracavity potassium titanyl phosphate optical parametric oscillator. To achieve the repetition rate and efficiency goals of this effort, but still have wide temperature capability, the Nd:YAG slab is end pumped with a 12-bar stack of 100 W (each) diode bars. Different techniques for focusing the pump radiation into the 4.25 mmx4.25 mm end of the slab are compared, including a lensed design, a reflective concentrator, and a lens duct. A wide temperature operation (-20 degrees C to 50 degrees C) for each end-pumped configuration is demonstrated.

Efficient, reliable, long-lifetime, diode-pumped Nd:YAG laser for space-based vegetation topographical altimetry

A highly efficient, diode-pumped, Nd:YAG laser is described. The oscillator utilizes an unstable resonator design with a Gaussian reflectivity output coupler and a side-pumped zigzag slab gain medium. The laser produces 18-mJ, 10-ns pulses at a repetition rate of 242 Hz in a near-TEM00 mode with an optical efficiency of up to 14%. An extended performance test was recently concluded in which the transmitter operated at reduced output for more than 4.8 x 10(9) shots with no optical damage. Design criteria, beam quality, and lifetime data are presented.

Design of efficient lens ducts

Lens ducts have the potential to couple the output from a laser diode array efficiently into the gain medium of a solid-state laser in an end-pumped configuration. Using a ray-tracing method we investigate different design approaches of lens ducts and demonstrate the possibility to obtain an output beam with a symmetric profile that is insensitive to the small displacement from the output surface of a lens duct.

High-power monolithic unstable-resonator solid-state laser

We report the operation of a diode-pumped monolithic Q-switched unstable-resonator solid-state laser that generates 2.15-mJ, 2-ns pulses in a single transverse mode and a single longitudinal mode. We show that the unstable resonator is effective in suppressing the spatial and the temporal instability of the laser beam in a disk-shaped laser whose transverse dimension is comparable with or larger than its longitudinal dimension.

Tunable optical microwave source using spatially resolved laser eigenstates

A two-propagation-axis solid-state laser is shown to provide a widely tunable optical microwave source. The spatial separation of the laser eigenstates is shown to enable an étalon to act as a coarse tuner, forcing oscillation in any nonadjacent cavity modes. The frequency difference between opposite helicoidal eigenstates operating in nonadjacent cavity modes can then be tuned continuously. The beat note from such a solid-state laser is shown to vary from dc to 26 GHz, i.e., 30 times the laser free-spectral range, and is limited only by the free-spectral range of the étalon.

Theory and optimization of lens ducts

Lens ducts are simple optical devices that have found application in the coupling of pump radiation from extended two-dimensional semiconductor laser diode arrays into solid-state laser gain media. The operation of these devices relies on the combined effects of lensing at their curved input surface and channeling by total internal reflection off their canted planar sides, to contain and couple semiconductor diode laser light efficiently to the input face of a solid-state laser crystal or glass. The lens duct provides a robust method for amplifying the irradiance of laser diode array pump sources and has made possible a scalable diode end-pumping architecture that offers the opportunity to expand significantly the number of ions and transitions that can be practically engaged in diode-pumped solid-state laser systems. An analytic model that describes the transfer efficiency of lens ducts and aids in the optimization of their design is presented.

Absolute frequency stabilization of an injection-seeded optical parametric oscillator

A method is described that provides absolute frequency stabilization and calibration of the signal and idler waves generated by an injection-seeded optical parametric oscillator (OPO). The method makes use of a He-Ne stabilized transfer cavity (TC) to control the frequencies of the cw sources used to seed both the pump laser and OPO cavity. The TC serves as a stable calibration source for the signal and idler waves by providing marker fringes as the seed laser is scanned. Additionally, an acoustic-optic modulator (AOM) is used to shift the OPO seed laser's frequency before locking it onto the TC. The sidebands of the AOM are tunable over more than one free spectral range of the TC, thereby permitting stabilization of the signal and idler waves at any frequency. A ±25-MHz residual error in the absolute frequency stabilities of the pump, signal, and idler waves is experimentally demonstrated, which is roughly 30% of the 160-MHz near-transform-limited linewidths of the signal and idler pulses.

Diode-pumped single-frequency Nd:YVO(4) laser with a set of coupled resonators

350 mW of single-frequency power from a diode-pumped solid-state Nd:YVO(4) laser has been obtained from a coupled resonator design without any intracavity elements. Single-frequency operation was obtained by use of a very short laser rod and a coupled resonator design. The two coupled resonators were formed by the two faces of a very short Nd:YVO(4) laser crystal and an output coupling mirror. The interaction of the two coupled cavities caused a modification of the eigenmodes supporting laser action in a single longitudinal mode. This design, which is extremely simple, represents a cost-eff icient way of obtaining single-frequency output.

Effect of spatial hole burning in a mode-locked diode end-pumped Nd:YAG laser

We have controlled the amount of spatial hole burning in a diode end-pumped Nd:YAG laser to quantitatively analyze its role in active mode locking. We demonstrate inhomogeneous broadening of the lasing bandwidth to >60 GHz, accompanied by a pulse-width reduction from 40 to 15 ps and an ~30% increase in the time-bandwidth product. Both homogeneous and inhomogeneous broadening have been included in a successful calculation of the steady-state pulse width.

High-sensitivity two-dimensional thermal- and mechanical-stress-induced birefringence measurements in a Nd:YAG rod

A novel polarimeter for measuring the two-dimensional (2D) thermal- and mechanical-stress-induced birefringence in solid-state laser materials such as Nd:YAG is proposed. Using this device, we could sensitively measure the direction of the principal birefringence axis as well as the phase shift δ with sign when δ < π/4. The 2D thermal- and mechanical-stress-induced birefringence in a laser-diode-pumped Nd:YAG rod was successfully measured with the proposed polarimeter. We also found an active quarter-wave Nd:YAG phase retarder.

Noise characteristics of a Nd:YVO(4) laser pumped by laser diode modulated at high frequency

We report, for the first time to our knowledge, on the noise characteristics of a Nd:YVO(4) laser pumped by a laser diode modulated at high frequency. We have investigated noise characteristics of a Nd:YVO(4) laser pumped by a laser diode that is oscillating in a stable multilongitudinal mode because of modulation by a high-frequency (several hundred megahertz) current. As a result, low-noise operations of -130 dB/Hz above frequencies of 1 MHz have been achieved. This noise level is comparable to the level obtained when pumping a Nd:YVO(4) laser with a laser diode that is oscillating in a single longitudinal mode. However, a noise peak corresponding to relaxation oscillations of the Nd:YVO(4) laser has appeared around a frequency of several hundred kilohertz.

High-efficiency 1.06-µm output in a monolithic Nd:YVO(4) laser

A miniature 1.5-mm-long monolithic Nd:YVO(4) laser was end pumped with a Ti-sapphire laser. The Nd-doping density was 3 at. %. The maximum output power obtained with 885 mW of absorbed pump power was 495 mW at 1.064 µm. The maximum slope efficiency was 57%, and the output power was only slightly dependent on pump polarization. Laser diode pumping was also demonstrated. Thermal effects were observed to reduce the output power and required active cooling of the laser crystal.

Low-noise operation (-1 40 dB/Hz) in close-coupled Nd:YVO(4) second-harmonic lasers pumped by single-mode laser diodes

Very-low-noise operation (as low as -140 dB/Hz) of close-coupled Nd:YVO(4) lasers is reported. This low noise level has been attained by controlling the spectra of the pumping laser diodes into the single longitudinal mode by suppressing mode hopping. The small-packaged second-harmonic laser (Φ12 × 16 mm) showed highly stabilized 530 nm/6 mW operations with a relative intensity noise of -140 dB/Hz.

Single-frequency diode-pumped Nd:YAG ring laser with no intracavity elements

We report on a technique for unidirectional operation of a ring laser with no intracavity elements other than the gain material. The Faraday effect in the gain medium combined with an out-of-plane Brewster-angled rod design produces an optical diode. Fabrication of the gain material as a prism permits a simple two-mirror ring-cavity arrangement. Single-frequency operation of a diode-pumped Nd:YAG laser based on this design is demonstrated. Using a 1-W diode pump, we achieved a single-frequency output of 190 mW at 1.064 microm.

Pumping configuration without focusing lenses for a small-sized diode-pumped Nd:YAG slab laser

We propose an end-pumping method for a small-sized diode-pumped Nd:YAG laser. The miniature Nd:YAG slab is directly coupled to the pump diode without focusing lenses. With the use of total reflection on the slab surfaces, the pump light is confined to the laser mode volume. With a diode-laser power of 890 mW, a TEM(00) output power of 210 mW is obtained.

Doppler lidar atmospheric wind sensor: reevaluation of a 355-nm incoherent Doppler lidar

We reevaluate the performance of an incoherent Doppler lidar system operating at 354.7 nm, based on recent but well-proven Nd:YAG laser technology and currently available optical sensors. For measurements in the lower troposphere, up to ~5 km altitude, and also in the Junge-layer of the lower stratosphere, a wind component accuracy of +/- 2 m/s and a vertical resolution of 1 km should be obtained with a single pulse from a 1-J laser, operating at Polar Platform altitudes (700-850 km) and high scan angles (55 degrees ). For wind measurements in the upper troposphere (above ~5 km altitude) and stratosphere (above and below the Junge layer) the concentration of scatterers is much lower and higher energies would be required to maintain +/-2m/s accuracy and 1 km vertical resolution, using single laser pulses. Except for the region in the vicinity of the tropopause (10 km altitude), a 5-J pulse would be appropriate to make measurements in these regions. The worst case is encountered near 10 km altitude, where we calculate that a 15-J pulse would be required. To reduce this energy requirement, we would propose to degrade the altitude resolution from 1 km to 2-3 km, and also to consider averaging multiple pulses. Degrading the vertical and horizontal resolution could provide an acceptable method of obtaining the required wind accuracy without the penalty of using a laser of higher output power. We believe that a Doppler lidar system, employing a near ultraviolet laser with a pulse energy of 5 J, could achieve the performance objectives required by the major potential users of a global space-borne wind observing system.

Single-mode operation of a standing wave miniature Nd laser pumped by laser diodes

Single-longitudinal-mode operation is reported for miniature Nd:YAG and Nd:BEL standing wave resonators. The best performance obtained was 18 mW with laser diode pumping and 31 mW with dye laser pumping. The miniature lasers were also operated multimode at power levels as high as 470 mW. A completely integrated, miniature Nd:BEL laser was constructed and is described. Results for several different resonator configurations are compared and it is concluded that the threshold pump power for multimode oscillation in each is consistent with an earlier observation that energy diffusion increases with absorbed pump power density. This allows single frequency oscillation to be obtained at high excitation rates with axially extended cavities.

Temperature tunable 1341-nm monolithic Nd(3+):YAIO(3) laser

A short cavity monolithicN d(3+):YA10(3) laser pumped by a forty-stripe diode array has produced 7 mW of TEM(00)power in only three longitudinal modes. The central mode is smoothly temperature tunable from 1341.0 to 1341.6 nm at the rate of 15.0 +/- 0.5 pm/ degrees C.

Frequency-modulated Nd:YAG microchip lasers

Tunable, single-frequency, Nd:YAG microchip lasers have been piezoelectrically frequency modulated over several hundred megahertz at rates from dc to 25 MHz.

Efficient diode-array-pumped Nd:YAG and Nd:Lu:YAG lasers

Nd:YAG (YAG) and Nd:Lu:YAG (LYAG) are investigated as high-efficiency laser materials utilizing a diode-array side-pump geometry. We demonstrate higher laser efficiencies than previously reported for transversely pumped YAG. A YAG rod yielded an optical slope efficiency of 47.7%, while a LYAG rod yielded an optical slope efficiency of 50.3%. Increased performance of these materials is attributed to higher pump absorption and lower losses.

Single-frequency microchip Nd lasers

Optically pumped, single-frequency, Nd-doped, solid-state lasers have been constructed using flat-flat cavities, which were diced from large dielectrically coated wafers of various crystals. For example, a Nd:YAG laser with a cavity length of 730 microm has operated at room temperature in a single longitudinal mode from a threshold of less than 1 mW to greater than 40 times the threshold. Theslope efficiency was greater than 30%. Heterodyne measurements showed an instrument-limited linewidth of 5 kHz. The microchip lasers demonstrate ways to reduce greatly the cost and complexity offabricating small lasers and electro-optic devices.

Diode-pumped 1.34-microm Nd(3+):YAlO(3) laser

We have pumped Nd(3+):YAlO(3) lasers with a 40-stripe, 500-mW, 807-nm diode laser. Maximum output powers at 1.34 microm of 26 mW multimode and 20 mW in a single transverse mode were achieved. The several longitudinal modes had a total spectral width of ~0.4 nm.

Wavelength tunable alexandrite regenerative amplifier

We describe a wavelength tunable alexandrite regenerative amplifier which is used to amplify nanosecond slices from a single-frequency cw dye laser or 50-ps pulses emitted by a diode laser to energies in the 10-mJ range. The amplified 5-ns slices generated by the cw-pumped line narrowed dye laser are Fourier transform limited. The 50-ps pulses emitted by a gain-switched diode laser are amplified by more than 10 orders of magnitude in a single stage.

Stable intracavity doubling of orthogonal linearly polarized modes in diode-pumped Nd:YAG lasers

The stability of the output light from a diode-pumped intracavity frequency-doubled Nd:YAG laser was studied. An intracavity nonlinear crystal, such as Type II phase-matched potassium titanyl phosphate, was used for frequency doubling. The incident beam consisted of two orthogonal linearly polarized modes. When the polarization eigenvectors were parallel to the E and O axes of the crystal, a large amplitude fluctuation was observed; however, when the azimuthal angle between the polarization eigenvectors and the axis was 45 degrees , the light output was stabilized. The experimental results are explained by analyzing the coupling of the two orthogonal linearly polarized modes through a sum-frequency-generation process.

Single-frequency Q-switched operation of a diode-laser-pumped Nd:YAG laser

We describe the performance of a single-frequency Q-switched Nd:YAG laser operating at 1.06 microm and optically pumped by a 100-mW diode laser. This system provides pulses of 6-microJ energy with peak powers greater than 125W at 40-nsec duration with less than 16-MHz linewidth.

Numerical simulation and experimental studies of longitudinally excited miniature solid-state lasers

We present a numerical model for the transient response of a longitudinally pumped miniature solid-state laser. The model is suitable for both regenerative amplifiers and oscillators, provided the latter run in a single mode. The results of our calculations compare well with measurements of the peak output powers and pulse widths for a Nd:YAG rod pumped by a ten-stripe diode laser array. Our model predicts saturation at peak powers of approximately twice the 850 mW reported here due to filling of the lower laser level. To overcome this power limitation due to saturation, we also explore the use of miniature Nd:glass laser amplifiers to boost the single-frequency Nd:YAG pulses to powers exceeding 200 W.

Diode Laser—Pumped Solid-State Lasers

Diode laser-pumped solid-state lasers are efficient, compact, all solid-state sources of coherent optical radiation. Major advances in solid-state laser technology have historically been preceded by advances in pumping technology. The helical flash lamps used to pump early ruby lasers were superseded by the linear flash lamp and arc lamp now used to pump neodymium-doped yttrium-aluminum-garnet lasers. The latest advance in pumping technology is the diode laser. Diode laser-pumped neodymium lasers have operated at greater than 10 percent electrical to optical efficiency in a single spatial mode and with linewidths of less than 10 kilohertz. The high spectral power brightness of these lasers has allowed frequency extension by harmonic generation in nonlinear crystals, which has led to green and blue sources of coherent radiation. Diode laser pumping has also been used with ions other than neodymium to produce wavelengths from 946 to 2010 nanometers. In addition, Q-switched operation with kilowatt peak powers and mode-locked operation with 10-picosecond pulse widths have been demonstrated. Progress in diode lasers and diode laser arrays promises all solid-state lasers in which the flash lamp is replaced by diode lasers for average power levels in excess of tens of watts and at a price that is competitive with flash lamp-pumped laser systems. Power levels exceeding 1 kilowatt appear possible within the next 5 years. Potential applications of diode laser-pumped solid-state lasers include coherent radar, global sensing from satellites, medical uses, micromachining, and miniature visible sources for digital optical storage.

Second-harmonic generation of a continuous-wave diode-pumped Nd:YAG laser using an externally resonant cavity

We report 13% second-harmonic conversion efficiency of a 15-mW, cw, diode-laser-pumped Nd:YAG oscillator. 2 mW of single-axial-mode 532-mm radiation was generated by externally resonant second-harmonic generation in a monolithic MgO:LiNbO(3) nonlinear crystal cavity. The measured finesse of 450 for the monolithic external cavity indicated that absorption and scatter losses in the doubler were less than 0.8%.

Stress-induced tuning of a diode-laser-excited monolithic Nd:YAG laser

We report the application of a stress-induced change in the cavity of a monolithic Nd:YAG laser as a means for precisely tuning the laser's emission frequency. By using a piezoelectric transducer to vary the applied stress, we can scan the laser emission frequency rapidly and monotonically over as much as 76.5 GHz. As a consequence of the stress-induced birefringence, the device can also be operated simultaneously in two orthogonally polarized modes that exhibit different tuning rates with applied stress.

Coherent laser radar at 1.06 microm using Nd:YAG lasers

A coherent laser radar system operating at the 1.06-microm Nd:YAG laser wavelength has been built and operated. A laser-diode-pumped monolithic ring laser served as the master oscillator. A single flash-lamp-pumped zigzag slab amplified the oscillator output to a power of 2.3 kW. Single-mode optical fiber was used to collect and mix the return signal with the local-oscillator output. Signals from clouds at a range of 2.7 km and from atmospheric aerosols at a range of 600 m were detected.

Frequency stability and offset locking of a laser-diode-pumped Nd:YAG monolithic nonplanar ring oscillator

The frequency stability of laser-diode-pumped, monolithic Nd:YAG solid-state unidirectional nonplanar ring oscillators was studied by heterodyne measurements. We obtained cw single-axial- and transverse-mode power of 25 mW at 1064 nm at a slope efficiency of 19%. Two independent oscillators were offset locked at 17 MHz with frequency fluctuations of less than +/-40 kHz for periods of 8 min.

Diode-pumped continuous-wave Nd:glass laser

We report on diode-laser pumping of monolithic Nd:glass laser oscillators. End pumping with a single-stripe diode laser, a threshold of 2.2 mW, and a slope efficiency of 42% were observed on a 2-mm-long oscillator with a mode radius of 35 microm. The oscillator generated 2.5 mW of single-ended output power in many (>20) axial modes.

Monolithic Nd:YAG fiber laser

A single-crystal Nd:YAG fiber with polished and coated endfaces has been operated as a monolithic, guided-wave laser oscillator. The 47-microm-diameter, 7-mm-length fiber oscillator operated with a threshold of 3.7 mW and a slope efficiency of 10.5%. Seventy-five percent of the laser power was in the fundamental spatial mode.