High peak power Nd:YAG laser pumped by 600-W diode laser stack

The Q-switched laser with triangle slab made of Nd:YAG crystal side pumped by 600-W quasi-cw diode laser stack has been designed. The multimode (M²≈2.6) output energy of about 42 mJ was demonstrated in free running mode for110-mJ pump energy. In Q-switch experiments, the KDDP Pockels cell was placed between the slab and rear mirror in plane-plane cavity with output coupler of 84% transmission. The energy of 8 mJ in 2.1-ns pulse duration was obtained for near TEM₀₀ output beam. For passive Q-switching by means of Cr:YAG crystal of 12.6% unsaturated transmission, the energy of 5.1 mJ in 2.5-ns pulse duration was obtained for output beam close to TEM₀₀ mode.     

Narrow-band UV radiation (250–260 nm) from intracavity doubling a single-mode ring dye laser

Generation of continuous-wave, tunable UV radiation (250–260 nm) by intracavity doubling a coumarin-515 ring dye laser is described. A cooled (200–280 K) ADP crystal with end faces cut at Brester's angle is placed inside the laser ring cavity which has been compensated for astigmatism and coma. UV output powers at 254 nm of 120 μW and 60 μW are achieved with the laser operating multimode (bandwidth ? 20 GHz) and single-mode (bandwidth ? 50 MHz), respectively. Continuous single-mode scans over the 253.7 nm mercury profile demonstrate sub-Doppler resolution of the Hg 6s6p ³P^O₁ - 6s²¹S₀ transition.     

Influence of the modal composition of the radiation on the oscillation dynamics of a CO2-laser with a saturable absorber

The influence of the modal composition (transverse modes of a single longitudinal mode) of the laser radiation on the oscillation dynamics of a CO₂-laser with a saturable absorber in the cavity is investigated. Passive Q-switching in initiation of the fundamental mode TEM₀₀ and the modes of the first family TEM₀₁ and TEM₁₀ is considered. It is established that in the case of multimode oscillations sequences of single pulses with an irregular change in their amplitude can be observed. The multimode oscillations obtained are compared with similar single-mode oscillations, and their special features and differences are discussed. Reported at the Second International Scientific and Technical Conference on Quantum Electronics, Minsk, November 23–25, 1998. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 681–687, September–October, 1999.     

Dynamically reconfigurable characteristics of a double phase conjugate mirror using Sn2P2S6 crystals and their application to optical inter-satellite communication

A double phase conjugate mirror (DPCM), created by two mutually incoherent beams entering photorefractive nonlinear materials, can generate a phase conjugate beam whose reflectivity may be greater than 100%. Even though the conditions of the incident beams are changed, the DPCM can be dynamically reconfigured by using a Sn₂P₂S₆ crystal with a high response speed. These features of the DPCM are advantageous, particularly in an optical inter-satellite communication system. In particular, use of the phase conjugate beam from the DPCM offers wavefront compensation and amplification in satellite communication. In addition, the dynamically reconfigurable DPCM using a Sn₂P₂S₆ crystal relaxes the acquisition accuracy of the signal beam in the system. In this study, the temporal and spatial operating characteristics of the DPCM using a Sn2P2S6 crystal were first clarified. Next, an inter-satellite system based on the DPCM was proposed, and it was demonstrated that our system significantly improves the tolerance of the acquisition accuracy and tracking time.     

High-power neodymium-doped mixed vanadate bounce geometry laser, mode locked with nonlinear mirror

The highest power neodymium-doped mixed vanadate laser oscillator is presented. Using a crystal of Nd:Gd_0.6Y_0.4VO₄ in the bounce geometry, average output powers of 27.5?W in multimode and 23?W in TEM₀₀ operation were achieved. The first nonlinear mirror mode-locked operation of a mixed vanadate laser is also presented, with 16.8?W output power??the highest power mode-locked mixed vanadate oscillator, to the best of our knowledge. Self-starting continuous-wave mode locking was observed at a repetition rate of 100?MHz, pulse duration of 12.7?ps and central wavelength of 1063.8?nm, in TEM₀₀ mode.     

Simple single-frequency He-Ne laser

For a (0.5–0.6)m long He–Ne laser tube, single longitudinal mode 632.8 nm generation is obtained, using a higher gas-mixture pressure and application of an axial magnetic field. An efficient and very simple mode selection is obtained by gas-mixture pressure increase only. A multimode power to single-mode power conversion coefficientk=0.75 is obtained by increasing the tube pressure. A further increase tok=0.85 is possible with optimum magnetic field and gas pressure.     

High efficiency TEM00 mode, diode-pumped, conduction-cooled, Nd:YAG zig-zag slab laser

We report a compact, conduction-cooled, highly efficient, continuous wave (CW) Nd:YAG slab laser in diode-side-pumped geometry. To achieve high efficiency, a novel laser head for Nd:YAG slab has been developed. For an absorbed pump power of 27.6 W, maximum output power of 10.4 W in multimode and 8.2 W in near-diffraction-limited beam quality has been obtained. Slope and optical-to-optical conversion efficiencies are 45.3% and 37.7% in multimode with beam quality factors (M²) in x and y directions equal to 32 and 8, respectively. TEM₀₀ mode operation was achieved in a hybrid resonator with slope and optical-to-optical conversion efficiencies of 43.2% and 29.7%, respectively. Beam quality factors in x and y directions are ?1.5 and ?1.6 for the whole output power range. The laser radiation was linearly polarized and polarization contrast ratios are >1200:1 in the multimode and 1800:1 in the TEM₀₀ mode operation. In passive Q-switching with Cr⁴⁺:YAG crystal of 68% initial transmission, 18 ns pulsewidth has been achieved with an average power of 2 W at a repetition rate of 16 kHz.     

多模气体激光纵模线型函数及其频宽的实验观测

本文根据多模激光时间相干性ｇ（１）（τ）的准周期性特点，提出了一种观测多模激光纵模线型函数及其频宽的新方法－程差２ｋＬ法。文章介绍了测量原理与方法，并以双模Ｈｅ－Ｎｅ激光器为例，给出了相应的实验结果，研究表明，多模气体激光的纵模线型函数为洛仑兹（Ｌｏｒｅｎｔｚ）线型函数，相应的纵模频宽约为１０＾７Ｈｚ。     

6.3 GHz Linearly Tuning Single-Frequency Nd:YVO<Subscript>4</Subscript> Laser

We propose a linearly frequency-tuning single-frequency Nd:YVO₄ laser. We achieve a single-frequency tuning operation with a tuning range of 6.3 GHz and a maximum power of 0.5 W at 1,064 nm by tilting a thin coated etalon and changing the voltage applied to an RTP crystal in a synchronous way. The laser is linearly tuned with a standard frequency variation of ～190 MHz. We estimate the average tuning speed to be 0.81 GHz/s. The tuning range obtained is more than three times the longitudinal mode spacing of the laser resonator.     

Response of a BaTiO3 phase conjugate mirror to broadband and narrowband radiation

The bandwidth of a BaTiO₃ self-pumped phase conjugate mirror is measured in three different ways. 1) The crystal is exposed to infrared light with a phase change that is sinusoidal in time and the degree of phase modulation on the reflected wave is measured. As the modulation frequency is increased from 0.1 to 3 GHz, the modulation transfer function of the phase conjugate mirror decreases by a factor of 2. 2) The crystal is exposed to visible light from a laser operated both in a single longitudinal mode and in multiple longitudinal modes. When the laser bandwidth is increased from 20 MHz to 2 GHz the reflectivity of the phase conjugate mirror increases by up to a factor of 7. 3) A laser cavity is formed with the crystal as one end mirror and the lasing bandwidth is measured. Depending on the exact conditions, lasing bandwidths range from 2 to 240 GHz.     

Spectral properties of a Tm:Ho:YAG laser in active mirror configuration

3+ :Ho³⁺:YAG laser is operated in active mirror configuration. This laser is characterised by a rather thin crystal (3 mm thickness in our experiment) and a very short resonator (about 3.2 mm). Therefore this laser is well suited for operation in a single longitudinal mode. Single-frequency operation is demonstrated with and without an intracavity etalon. It is further shown that single-frequency emission is reached with a stable cw emission. With a slight detuning of the outcoupling mirror, spiking can be achieved, resulting in an emission on different lines. The assignment of the observed laser lines to the known transitions between Stark sublevels of the ⁵I₇ and the ⁵I₈ level is discussed and compared with predictions in the literature. Received: 2 December 1997     

Multimode fiber laser beam cleanup based on stochastic parallel gradient descent algorithm

We present experimental research on multimode fiber laser beam cleanup based on a stochastic parallel gradient descent (SPGD) algorithm. The multimode laser is obtained by injecting a 1064 nm central wavelength single mode fiber laser into a multimode fiber and the system is setup by using phase only liquid crystal spatial light modulators (LC-SLM). The quality evaluation function is increased by a factor of 10.5 and 65% of the laser energy is encircled in the central lobe when the system evolves from open-loop into close-loop state. Experimental results indicate the feasibility of the multimode fiber laser beam cleanup by adaptive optics (AO).     

Noise characteristic of sum-frequency mixing orange solid-state laser

A laser-diode-pumped Nd:YVO₄ orange solid-state laser is described in this paper. The fundamental wavelengths at 1064 and 1342 nm oscillate simultaneously and generate the 593.5 nm wavelength by intracavity sum-frequency mixing in a KTP crystal using type II phase matching. The noise characteristic of laser output has been measured and analyzed at different pumping powers. The relationship between the amplitude noise of sum-frequency mixing output and the longitudinal modes of fundamental frequency has been investigated. The results show that the sum-frequency mixing output has low noise if one of the fundamental wavelengths is single longitudinal mode. The experiment shows that amplitude noise of the intracavity sum-frequency mixing laser is lower than that of intracavity frequency doubled laser with the same structure.     

Intracavity Raman laser generating a third stokes component at 1.5 μm

The lasing properties of an intracavity Nd:KGW Raman laser which converts the multimode radiation of an Nd:KGW laser operating on the ⁴ F _3/2–⁴ I _11/2 transition into the third Stokes component at a wavelength of 1.5 μm are studied. The energy in the third Stokes component is found to increase essentially linearly with the electrical energy delivered to the flashlamp. Lasing at the third Stokes component begins in the central portion of the Nd:KGW crystal and then propagates to its boundaries. Reducing the geometric aperture of the multimode pump beam in the Raman crystal lowers the divergence of the Stokes emission. For a source pump energy of 6 J, the intracavity Raman laser emits 14.7 mJ pulses of duration 3–4 nsec which are safe to the eyes. The divergence of the Raman laser beam at a level of 86% of the total energy is ≈ 9 mrad.     

Second-harmonic power of KTiOPO4 with double refraction

Second harmonic (SH) power of the type-II phase matching in KTiOPO₄ (KTP) is calculated for the fundamental beams of TEM₀₀ mode within the near field approximation (parallel Gaussian beam) taking the beam walk-off effect among the two fundamental and the SH beams into account. The slit intensity and the total SH power are calculated as functions of the beam radius and the crystal length. Two approximations are made for the SH power calculation at tight beam focusing condition. The measurements of the SH power in KTP are carried out for three crystals of 6, 3, and 1.5 mm thickness. The effective nonlinear coefficient for Nd: YAG incident radiation is estimated to be 4.3 pm/V with 10% uncertainty, taking the enhancement factor due to the multimode structure of the laser beam into account.     

Frequency splitting phenomenon of dual transverse modes in a Nd:YAG laser

We observed frequency splitting phenomenon of dual transverse modes (TEM_00q and TEM_01q) in a Nd:YAG microchip standing wave laser utilizing intracavity stress birefringence effects. Four resonance frequencies (ν_00qe, ν_00qo, ν_01qe, and ν_01qo, respectively) were produced and tuned by changing the diametral compression force applied on the laser crystal. The transverse mode frequency spacing for the same longitudinal mode number (Δν_trans) was 1.16 GHz, and the magnitude of frequency splitting (Δν) ranged from 0 MHz to 1.16 GHz. Based on this phenomenon, a four-mode differential standing wave laser, whose signal characteristics were a little like those of a four-mode differential travelling wave laser gyro however with a much larger frequency splitting range, was produced. The theoretical analysis is in good agreement with the experimental results. This phenomenon not only can be used to make lasers with two or more frequency differences, but also can be used to make high-resolution self-sensing laser sensors (e.g. laser force sensors and laser accelerometers).     

Multi-wavelength ytterbium doped fiber laser based on longitudinal mode interference

Multi-wavelength Ytterbium-doped fiber laser (YDFL) is demonstrated using a longitudinal mode interference assisted by a four-wave mixing (FWM) effect in a ring laser cavity. The gain medium is a 16 m long of the fabricated (Ytterbium-doped fiber) YDF, which has a core composition of 0.8 wt % of Yb₂O₃, 1.8 wt % of Al₂O₃ and 23 wt % of GeO₂, Ytterbium ion fluorescence lifetime of 1.1 ms and absorption of 9.0 dB/m at the pump wavelength of 976 nm. 20 m long photonic crystal fiber (PCF) is used to provide FWM effect so that the energy of different oscillating lines can be redistributed to improve multi-wavelength operation. The proposed laser generates 12 lines of optical comb with a line spacing of approximately 0.59 nm at 1035 nm region.     

Effect of pump spectra and axial mode separation on the single longitudinal mode performance in end pumped solid-state lasers with semi-monolithic gain medium

We present a theoretical model to study the effect of the pump spectrum and axial mode separation on the single longitudinal mode operation of a laser with a homogeneously broadened semi-monolithic gain medium of short absorption depth. The characterizing parameter is the ratio of the pump power up to which single longitudinal mode is possible to the pump power at the lasing threshold and is denoted by r_max. A numerical study using a Gaussian-shaped pump spectrum reveals that, for a small value of the axial mode separation and for a crystal with short absorption depth at the pump wavelength, the value of r_max reduces significantly with increase in the FWHM of the pump spectrum and thereby results in degradation of the single longitudinal mode performance of the system. However, for large value of the axial mode separation, the SLM performance was found to be nearly independent of the variations in the spectral bandwidth of the pump beam, location of its peak emission wavelength and its polarization.     

Injection seeding for single-mode operation in an optically pumped high-power D2O laser

Spectrally narrow, pulsed outputs consisting of almost a single mode have been obtained from an optically-pumped high-power (200kW) D₂O laser by adopting the injection seeding method, where single-mode radiation (seed pulse) from a low-power, compact D₂O laser has been injected into the main D₂O laser. Spectrally narrow outputs with high power having spectral widths as narrow as 5 MHz have been obtained, when the seed pulses with frequency tuned to one longitudinal mode of the main D₂O laser have been injected at a time sufficiently before the lasing of the main laser took place. The experimental results have been compared with those of numerical simulation modified to include the injection field with varying injection times.     

Mathematical modeling of tunable TEA CO2 lasers

A mathematical model, based on the Landau–Teller equations of six-temperature model for the CO₂–N₂–He–CO system, to describe the process of dynamic emission in tunable TEA CO₂ lasers is introduced. In this model, the Landau–Teller equations are rewritten with regard to fine longitudinal mode frequencies in the laser resonator. These revised equations can be utilized to estimate the laser output spectra as well as other laser output pulse parameters. Examples are given to show the modeling results of non-tunable, grating tuned or injection-locking TEA CO₂ lasers.