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All-solid-state Singly and Doubly Q-switched, Laser-diode-pumped Intracavity Optical Parametric Oscillators

Author: WangJing
Tutor: ZhaoShengZhi
School: Shandong University
Course: Optical Engineering
Keywords: Optical parametrical oscillator EO switch doubly Q-switched KTA crystal rate equations Gaussian distribution conversion effeciency
CLC: TN248
Type: PhD thesis
Year: 2008
Downloads: 344
Quote: 2
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Abstract


Much interest in intracavity optical parametric oscillators (OPOs) has been stimulated due to the current interest in efficient, eye-safe coherent light sources for their applications such as telemetry, laser radar, communications and active imaging.It is efficient to predict the performance of Optical Parametric Oscillators (OPOs) by means of rate equations. Many researchers have been working on the rate-equation model since Oshman and Harris firstly derived the rate equations for a CW pumped Extracavity Optical Parametric Oscillator (EOPO) in 1968. The derived models can describe the operation of all possible OPOs, including the Q-switched pulse pumped Intracavity Optical Parametric Oscillators (IOPOs) which are preferred by the researchers because of its high efficiency. But all of these works based on the theory that the spatial distribution in the transverse section of the pump light, the fundamental light and the signal light is uniform or under the plane-wave approximation. For LD-pumped IOPOs, this assumption is not properly satisfied because the three beams are all Gaussian transversal distributions. For LD-pumped actively and passively Q-switched lasers, when the Gaussian distribution of the intracavity laser intensity is considered in rate equations, it has been shown that the numerical solutions of the equations are more close to the experimental results than those obtained under the plane-wave approximation. Therefore, for an LD-pumped IOPO, it is reasonable to take the spatial variations of the intracavity photon density, the population-inversion density of the gain medium into account.In order to make an OPO work more efficiently, the high peak intensity of the pumped fundamental laser is one of the most important factors. Therefore, it is very important to apply a high-quality Q-switch to obtaining the short pulses of the fundamental laser. An actively Q-switched laser (AO and EO switched) can obtain stable pulse train output, but the pulse width of it is usually wide. Passively Q-switched lasers with a saturable absorber can generate shorter pulse than actively Q-switched laser, but the pulse repetition rate is not very stable. When an actively and passively Q switches are simultaneously used in the cavity, it has been shown that this doubly Q-switched can generate shorter pulse with stable repetition rate and high peak power. So it is possible for LD-pumped doubly Q-switched IOPOs to obtain signal light with shorter pulse width and higher peak power.Among active Q-switching techniques, electro-optic (EO) Q-switching and acousto-optic (AO) Q-switching are often used, and the former is known to be advantageous over the latter in its faster switching and better hold-off ability. The traditional EO crystals, such as DKDP (KH2PO4) and LN (LiNO3), are often used in low repetition rate (<1 KHz) Q-switched lasers. Although the crystal BBO (β-BaB2O4) as an EO crystal is attractive because of its low damage threshold and low piezoelectric effect, it is still insufficient for an efficient OPO due to its several kHz limit in repetition rate. The excellent frequency-doubling crystal KTP (KTiOPO4) has large electro-optic coefficient and can be used as EO modulator at high-repetition-rate operation, but its application is limited due to its low resistivity.The new type of EO crystal RTP (RbTiOPO4) has high resistivity (1010-1012Ω·cm), large electro-optic coefficient, high damage threshold and low piezoelectric effect, so RTP crystal is considered to be promising for high repetition rate EO-Q-switching lasers. Though some reports on EO-switched IOPOs with several kHz repetition were presented, as far as we know, there is no the related report on a high repetition rate EO-switched IOPO.As far as the nonlinear crystal, compared with the similar and more common OPO crystal KTiOPO4 (KTP), the new crystal KTiOAsO4 (KTA) has large nonlinear optical coefficient and the advantage of enhanced transparency in the mid-infrared region (3-5μm), allowing high-power operation with minimal thermally induced refractive distortion in the OPO crystal. Because of the abovementioned advantages of KTA crystal, the KTA OPO can generate the higher signal output and the shorter pulse than the KTP OPO at some experimental condition.Based on the abovementioned thoughts, the main contents of this dissertation include:(Ⅰ) In the experiment, a laser-diode-pumped (LD pumped) actively Q-switched Nd:GdVO4/KTP IOPO with acousto-optic (AO) modulator is realized. The pulse width, the output power and the temporal profile are measured. The rate equations under Gaussian assumption are derived to fit the experimental data. It is found that the Gaussian rate equations agree with the experimental results better than the plane-wave equations. (ⅡI) By using RTP crystals as EO modulator, a flash-pumped high repetition rate (10 kHz-50 kHz) EO-Q-switched Nd3+:YAG/KTP IOPO is realized. The dependences of pulse width, the single-pulse energy and the peak power on flash-pump energy for the generated-signal light are obtained. By considering the gradual loss from the delayed switched time of the EO switch, the coupled rate equations of IOPO under the assumption of the Gaussian distributions of the intracavity photon densities are given and the numerical solution of the equations fits the experimental data well.(Ⅲ) In the experiment, a laser-diode-pumped doubly switched Nd:GdVO4/KTP IOPO with AO and a Cr4+: YAG saturable absorber is realized. In the rate equations, the intracavity photon densities and the initial population-inversion density are assumed to be Gaussian spatial distributions. By numerically solving the introduced rate equations based on Gaussian assumption, the dependences of pulse width, single-pulse energy and peak power on pump power for the generated-signal light are obtained. For a comparison, the experimental results of the AO singly Q-switched Nd:GdVO4/KTP IOPO, involving the pulse width, the peak power and the pulse energy, are measured. It can be found that when the other conditions are the same, the doubly Q-switched IOPO can generate the shorter pulse width and higher peak power than that of the singly AO-switched one. The experimental results agree with the theoretical values well.(Ⅸ) By simultaneously using both an EO modulator and a Cr4+:YAG saturable absorber in the cavity, a xenon flash-lamp-pumped doubly Q-switched Nd:YAG/KTP/IOPO is presented. The dependences of the single-pulse energy, the pulse width, and the peak power on flash-pumping energy for the generated-signal light are obtained. The output characteristics of the doubly Q-switched IOPO with an EO switch and a Cr4+:YAG crystal are compared with those of the singly Q-switched IOPO with an EO switch. The coupled rate equations of doubly Q-switched IOPO under the assumption of the Gaussian distributions of the intracavity photon densities are given and the numerical solution of the equations fits the experimental data well.(Ⅴ) A laser-diode-pumped doubly switched Nd:GdVO4/KTA IOPO with an AO and a Cr4+: YAG saturable absorber is realized. For a comparison, the experimental results of the AO singly Q-switched Nd:GdVO4/KTA IOPO, involving the average power, the pulse width, the peak power and the pulse energy of the signal light, are measured. Gaussian equations are used to fit the experimental data and the numerical solutions of the coupled rate equations agree with the experimental results well.(Ⅵ) The wavelength of the output signal light from the LD pumped KTP / KTA IOPO is measured. The signal wavelengths with different crystal temperature for a LD pumped KTA IOPO are obtained. It is found that, in the experimental temperature range (10-29℃) the value of wavelength is the lest at TKTA = 23℃.The main innovations of this dissertation are as follows:(Ⅰ) When the intracavity photon densities and the initial population-inversion density are assumed o be Gaussian spatial distributions, the Gaussian rate equations describing the performance of IOPO are firstly derived. In the equations, the influence of the pump rate, the thermal effect in the gain medium and the diffraction loss of the actively Q-switch are considered. Also the different photon densities at different positions in the laser cavity are taken into account. Finally, the average radius of the pump light, the average population-inversion density of the gain medium and the population densities of the excited-state of the saturable absorber are considered to be Gaussian spatial distributions. The calculated values are compared with that from the plane wave assumption. It is found that the Gaussian theory fits the experimental data better, especially when the pump power is high in our experiment.(Ⅱ) A high repetition rate electro-optical-Q-switched intracavity optical parametric oscillator (IOPO) is realized by using two RbTiOPO4 (RTP) crystals as electro-optic (EO) modulator. By considering the gradual loss from the delayed switched time of the EO switch, the coupled rate equations of IOPO under the assumption of the Gaussian distributions of the intracavity photon densities are given.(Ⅲ) A laser-diode-pumped doubly switched Nd:GdVO4/KTP IOPO with AO and a Cr4+: YAG saturable absorber is firstly reported in experiment. The experimental results are compared with those of singly actively Q-switched IOPO. The doubly Q-switched IOPO can generate the shorter pulse width and higher peak power than that of the singly switched one.(Ⅳ) A xenon flash-lamp-pumped doubly Q-switched Nd:YAG/KTP/IOPO is presented in experiment. The experimental results are compared with those of singly actively Q-switched IOPO. The doubly Q-switched IOPO can generate the shorter pulse width and higher peak power than that of the singly switched one and have high conversion efficiency. (Ⅴ) By using the new crystal KTA as the nonlinear material, a Q-switched KTA IOPO is realized. The output characteristic of it is compared with that of a KTP IOPO. The KTA IOPO can generate shorter signal pulse and higher peak power.

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