Studies On Mid-infrared Tunable Lasers

Tunable mid-infrared laser sources operating at the two transparency windows of 3 ~5μm and 8 ~12μm in the atmosphere are more and more important with applications in laser remote sensing, laser target detecting, spectra analysis, atmospheric environment monitoring, space communicating and some applications in military. Study in this thesis for doctorate is focused on the lasers which can deliver the wavelength in the two subranges: tunable CO2 laser is investigated in the window of 8 ~12μm; solid-state erbium laser, second harmonic generation (SHG) of CO2 laser with nonlinear optical crystals and optical parametric oscillator are demonstrated in the window of 3 ~5μm. The study consists of three parts as following.I: Tunable mechanical Q-switched CO2 laser with longitudinal DC dischargeA fast chopper driven by high-speed motor together with a grating in order to select different lines is used to set up the mechanical Q-switching of tunable CW CO2 laser with longitudinal DC discharge. In order to achieve higher pulse peak power of CO2 laser, the apertures of the chopper should be narrower and the rotation speed of it should be much faster. A specific scheme is used for simple mechanical Q switch in our experiment as shown in Fig.1. Employed with two concave Au mirror for beam focus, a fast chopper for the control of Q switch at the focal point, Au grating for selecting different lines, Brewster angle scheme for polarizer, a simple tunable mechanical Q-switched CO2 laser is demonstrated with polarizing output. Fig.1 Schematic setup of mechanical Q-switched CO2 laser with longitudinal DC discharge: G, the grating; M1, M2, focusing mirrors; CP, the chopper; ZS, ZnSe window; LT, laser tube; OPM, the output mirror.The laser parameters of the simple mechanical Q-switched CO2 laser with the gas mixture (Xe: CO2: N2: He=1: 2.5: 2.5: 17.5) at an average total pressure of 20 Torr are: pulse repetition and peak power are up to 740 Hz and 2 kW, respectively; typical pulse width is 200~300 ns. When the discharge is 10 mA, the laser pulse is shown in Fig.2. The peak power is 1745 W, with pulse duration of 192 ns and a repetition rate of 619 Hz. We can see the obvious tailing of the pulse. Other dynamic parameters of the laser are investigated as shown in Fig.3. The stability of Q switch is directly influenced by the motor of the chopper. The relations between Q-switched pulse repetition and control voltage of motor, operating time are shown in Fig.3a and Fig.3b, respectively. Considering the gas discharge and the stability of motor, the Q-switched laser can work well after half hour of its operation. Q-switched pulse average power and energy are shown in Fig.3c and Fig.3d, respectively. By adjusting the grating, all different lines of 9 .2~12.9μmcan be generated.There are two theoretical models to analyze the dynamics of the simple mechanical Q-switched CO2 laser: One is vibro-rotational level model based on the rate equations, which is so-called four-level model (4LM); the other is six-temperature model based on the dynamics of CO2 laser, which is so-called 6TM. The schematic diagrams of the two models are shown in Fig. 4. 4LM 6TM Fig.4 Schematic diagrams of the two models: (left) Levels 1 and 2 are the lasing levels. J denotes the number of rotational levels involved in each manifold which related with laser levels,γR and JγR denote the relaxation rates in or out of the lasing levels; (right) Continuous, arrowed lines denote V-V transitions; dotted, arrowed lines denote V-T energy transfer processes; dashed, arrowed lines denote electron excitation processes. The dynamical process of the mechanical Q-switched CO2 laser can be described by two stages: One stage is Q-switching close, which corresponds to the duration when the chopper cuts down the light beam. There exists a big loss in the cavity and almost no oscillation happens. The laser intensity is nearly zero during the time. But due to discharge, the population inversions increase linearly within the time before reaching the maximum value. The other stage is Q-switching open, which corresponds to the light beam passing through one of the slits on chopper. The population inversion begins to decrease, at the same time the laser intensity begins to increase. When the gain equals the loss in the cavity, a laser pulse with high peak power is emitted. Fig.5 shows the time evolutions of the laser intensity and population inversion using the two models. Both the two models give good results to simulate the process. As we know, the vibrational and rotational levels play a very important role in the laser dynamical processes. The time evolutions of the laser intensity as a function of the rotational levels J for the two models are shown in Fig.6. From the figures, it is obvious that both pulse shapes from the two models can not be used for describing the Q-switched pulse when J equals zero. When J ranges from 15 to 25, the main variation for the 4LM is on the peak intensity, while for the 6TM is on the delay time. This is consistent with the results reported in the literatures for the P 20 line.Vibro-rotational level theory based on rate equations takes emphasis on the relaxation process of rotational level. 6TM takes into account the dissociation of CO2 molecules, the effects of electrical excitation on different energy levels, as well as the rotational relaxation of rotational levels, which can describe the energy transfer processes of different gases such as N2 and He. Fig. 7a and Fig.7b show the variations of the laser intensity for different non-dissociation coefficients and electron density (which corresponds to discharge current) in the cavity, respectively. It is consistent with our experimental observation that the laser intensity in the cavity will be reduced with the increase of the dissociation coefficients and the decrease of the discharge current. So we added few Xenon in mixture gas. It can lead to a decrease in the dissociation of CO2 molecules and increase the output power.As shown in Fig.8, the two models are used to simulate the output pulse shape. The calculated results of laser pulse waveform for the first big peak are perfectly consistent with the experimental one. But for the second little peak, 6TM is suitable only at the time scale and a little higher tailing is gaven. Much delay time is given for 4LM for the second one. The tailings of the pulses are both obvious in numerical solutions for the two models. II: Active and passive Q-switched solid-state erbium lasersQ-switched Er:Cr:YSGG lasers (λ= 2.79μm) are demonstrated by three sets of shutters. Using ultra-thin single-crystal InAs epilayers grown on GaAs substrates as absorber, a passive Q-switching is investigated. We demonstrate two types of electrooptically Q-switched Er:Cr:YSGG laser. Q-switching and polarizing for the one are realized with only one LiNbO3 crystal with both ends cut at Brewster's angle and for the other they are achieved by a cubic LiNbO3 crystal and a Glan-Taylor prism, respectively. In order to compare and analyze, a plane-parallel dielectric mirror M1 with output coupling coefficient of 0.28 for 2.79μm is used as output mirror, high reflectivity mirror M2 is a mirror with Au coated and a curvature of 2 m. The lasing crystal is 110 mm in length and 4 mm in diameter with AR coated on both ends. The dopant concentrations of Er3+ and Cr3+ are 4×1021cm ?3 and 5×1019cm ?3, respectively. It is pumped in an elliptical silvered monolithic enclosure cavity by a xenon flashlamp with the pulse duration around 150μs (FWHM) at 1~20 Hz. The experimental results are recorded under the repetition of 1 Hz. The same cavity lengths of 50 cm are considered. The three schematic experimental setups of Q-switched Er:Cr:YSGG laser are shown in Fig.9. The typical Q-switched pulse shapes of three schemes are shown in Fig.10.The results of lasing experiments are concluded as follow:1) Two layers InAs with different thickness 0.2μm and 0.5μm are used as passive shutters, which are placed at the Brewster angle (72~75 degree) in the laser cavity. Using 0.2μm InAs as absorber, Q-switched pump threshold is 21 J and the narrowest pulse width is 300 ns. A single-pulse stable maximum energy of 20.3 mJ is recorded. The passive Q-switching is a simple method to obtain short pulses. The thicker the layer, the higher the absorption and the shorter the pulse widths. The disadvantage is its limited damage threshold. For 0.5μm InAs, the oscillation threshold nearly equals its damage threshold.2) Lasing experimental results of electro-optical Q-switching with LiNbO3 crystal are listed in table 1. 3) As in many other solid state lasers, the input pump energy also influences the Q-switched pulse width of three schemes for Er:Cr:YSGG laser。The Q-switched pulse width as a function of pumping energy is shown in Fig.11 (left). It is obvious for three schemes that the more the pumping energy, the less the pulse width. The shortest pulse width of eletro-optical Q-switching is 40~50 ns. It is obvious that Q-switched pulse width of InAs absorber is longer than that of the electro-optical. 4) Comparison of lasing energy property for three schemes is shown in Fig.11 (right). In contrast to the Brewster angle scheme, the pumping energy not only for Q-switched operation but also for the region of free running is lower for the Glan-Taylor prism scheme. Pumping energy for InAs passive Q-switching is the lowest one among the three schemes. But only at the point of Q-switched pulse energy, the absorber scheme and Brewster angle scheme are better than Glan-Taylor prism scheme. Employing the traveling amplifier of Er:Cr:YSGG in our lab, the maximum pulse energy is up to 30~40 mJ, which can be effectively used in nonlinear optics such as pump source of mid-infrared OPO and medical applications. Second harmonic generations (SHG) of the tunable CO2 laser are demonstrated with crystals of AgGaGexS2(1+x),x=0,1,HgGa2S4,ZnGeP2. Phase-matched properties are investigated. Schematic experimental setup of SHG is shown in Fig.12.1) Employing AgGaS2 crystal with dimension of 10×7×20mm3, measured phase-matched angles are 56.963o, 57.515o and 39.6°for pump wavelengths 9.51, 9.61 and 2.79μm, respectively. Temperature variation of SHG phase-matched angleΔΘpm in AgGaS2 isΔΘpm/dT=16.6″°C-1 atλ=9.55μm. SHG phase-matched angles in XZ plane of AgGaGeS4 crystal with dimension of 10×15×2.1mm3 are 29.7 and 27.6°for pump wavelengths 9.48 and 9.6μm, respectively. Measured experimental data for SHG two crystals are well within the different diagrams. The differences among different diagrams can be caused mainly by crystal cut and inaccuracies approximation, and by different crystal composition and/or physical properties due to different growth technologies.2) In our experiment a mechanically polished single crystal HgGa2S4 plate with dimensions 10×12×3.1 mm3 and color variations from light green to orange over the big plane area was used. Chemical composition has been determined by electron probe microanalysis (EPMA) of LEO-1430 device with scanning area 0.1×0.1 mm2, 5 nm deep, for several positions at the 10×12 mm2 sample surface. The actual composition is Hg1.02±0.08Ga1.90±0.04S4 without any reasonable relation to local area color. Optical transparency at short-wavelength end in visible range has been measured with Shimazdu UV 3101PC (0.3~3.2μm ) as shown in Fig.13. Different color crystals are located at different positions as shown in Fig.13 inset: 1 is a local part of most light-green colour, 2 is light-green, 3 and 4 are yellow; 5, 6 and 7 are orange, 8 is deepest orange part. At 10% level the short-wavelength end of transparency range found at 528 nm for deepest orange part of the crystal is shifted to longer wavelength in reference to 520 nm for yellow and 512 nm for light-green part of the crystal. Long-wavelength transparency end is studied with Specord 80 IR (2.5-25μm) and the same result 14.3μm for different colors is record.To confirm correlation between the colour variation and phase matching (PM) conditions we have carefully measured phase-matched angle (PMA) for SHG under CO2 laser pump at the points 1, 4 and 7 on the crystal surface shown in Fig.13a inset. The relation between different color crystals and angular mismatch under the pump wavelength 9.512μm is shown in Fig.14a. To improve the accuracy the changes of SHG output powers versus angular mismatch were also developed and approximated. In such a way an influence of measurement instabilities resulted in scattering of experimental points near the tops of diagrams on the determination of PMA is significantly reduced. From Fig.14a one can see that maximal difference in PMA for different colour local points is ~1.5°. Our data for light green local point 1 are close to Takaoka's data [CLEO'98, CWP39, P.253-254] for his green colored or something yellowish HgGa2S4 crystal. Our experimental data for yellow and orange points 4 and 7 are shifted in the direction to Petro's diagrams (Opt. Commun., 2004, 235, 219-226) for yellow and orange colour crystals. Temperature tuning of SHG PM pumped by different wavelength 9.48,9.51,9.53μm is shown in Fig.14b. With the increase of the crystal temperature, phase-matched angles increase slightly. Temperature dispersion of dθ/dT= 0.36′/1°C at pump wavelengthsλ=9.48-9.53μm is recorded.Based on the analysis of the available sellmeier equations and experimental data, dispersion equations for different colour crystals are proposed as where y is a weighting coefficient. At y=1 and 0, the dispersion of refractive indices is governed by Sellmeier equation from Takaoka's and Petro's. As shown in Fig.15a, under the relation, the value y=0 well describes our experimental data for local position 7. The values y=0.55 and 0.32 are related to positions 4 and 1, respectively. When we change the weighting coefficient y, phase-matched diagrams simulated are well agreed with available experimental data. With the help of short-wavelength end of HgGa2S4 crystal transparency at 10% level shown in Fig.13a and the new dispersion relations, short-wavelength end of different color HgGa2S4 crystal transparency at 10% level can be concluded and shown in Fig.15b. The dispersion relations open the possibility for accounting real refractive index variation synchronous with crystal colour variation and are applicable for PM conditions determination in wide spectral range, which are helpful for the development of different colour HgGa2S4 crystals in device applications. 3) Employing tunable Q-switched CO2 laser, temperature property of SHG phase-matched angles in ZnGeP2 crystal is investigated. The dimension of ZnGeP2 crystal is 6. 9×10.3×4.3mm3, which is cut at 70 degree. As shown in Fig16a, our experiment data are well consisted with the theoretical predictions at the 9μm waveband of CO2 laser for the different temperatures. But there exists a big difference between the experiment and simulation at the 10μm waveband except at the temperature of 500 K.Based on the point of almost same wavelength of SHG (4793.11369nm) of CO2 laser at P(24) line and CO fundamental band P(14) (4793.1242nm), CO gas detection is investigated. The experimental data and simulation curve are shown in Fig.16b. The detectable concentration of the detecting system is about 100ppm (0.01%). (2) 3 ~5μmoptical parametric oscillatorAmong the mid-infrared nonlinear crystals, AgGaS2 (AGS) crystal is one of the few crystals which can be pumped by commercially available 1.064μm Nd:YAG laser to achieve phase-matched down-conversion into theλ> 5μm region. AgGaS2 type-I singly resonant optical parametric oscillator (SRO OPO) pumped by a ns 1.064μm Nd:YAG laser is demonstrated experimentally. Two OPO cavity schemes are investigated: two identical plane-parallel dielectric mirrors are used as OPO cavity mirrors for the usual one; a plane-parallel dielectric mirror together with a prism is used as OPO cavity devices for the other. Schematic experimental setups for two schemes are shown in Fig.17. AgGaS2 crystal used with 10×7 mm2 in cross section, 20 mm in length,θ=47°and ?=45°cut for type-I phase-matching is supplied by MolTech GMBH, Germany. In order to reduce the loss so as to the oscillation threshold, both cross sections are well antireflection AR coated: high transparent HT1.06μm >99% at pump wavelength 1.06μm, and also at signal and idler wavelengths HT1.3-1.7μm ~97.5%—99%, HT3-5μm ~97%—98.5%, respectively. The two identical flat mirrors M1, M2, which are used as the cavity mirrors with coatings of HT1.06μm>95%, HR1.3-1.7μm>99-99.4% and HT3-5μm>88-98%, are designed for SRO OPO. A homemade electro-optically Q-switched Nd:YAG laser and amplifier pumped by flashlamp with pulse width 10-20 ns depending on the input energy is used as OPO pump source. A diaphragm is inserted in the cavity of pump laser to restrain the higher-order mode oscillation. The laser can operate at the frequency of 20 Hz. The wavelength tuning ranges are from 2.6 to 5.3μm and 2. 4~5.3μm as shown in Fig.18 for usual type cavity and prism type scheme, respectively. They are pumped by a 1.064μm Nd:YAG laser with pulse width 15 ns and spot diameter 1.5 mm. To our knowledge, it is the biggest range for nanosecond AgGaS2 type-I SRO OPO. In order to lower the oscillation threshold, the double-pump scheme is considered. As shown in Fig.17, the pump laser is not fed back into the OPO cavity when pump laser is not vertical to 1.064μm reflective filter, F, but the pump laser can be fed back into the cavity for a double pump pass by adjusting the filter, F vertical to pump laser. Fig.19a shows the pump threshold energy density as a function of various idle wavelength outputs for single and double pump pass AGS SRO OPO. 1.4 times higher oscillation threshold of SSRO than DSRO at 4μm is concluded, which is well consistent with 1.6 times of the theoretical prediction. The predicted variabilities of threshold on the idler are confirmed by two schemes. The longer the output wavelength is, the higher the oscillation threshold is, which can be interpreted in section two.Output energy of the idler is our main interest. Firstly, the output energy of idle light under single pass pump is monitored. The pump pulse width (FWHM) is 15 ns and its beam diameter is 1.3 mm with frequency of 1 Hz. The relation between pump energy and output energy is shown in Fig.19b. Square points are at fixed output wavelength 4μm, 2.8 cm in cavity length. 270μJ is recorded with the maximum laser-to-idler conversion efficiency 3.5% without any damage appears. The optical damage of cavity mirrors inside occurs at the input pump power density up to 34 MW/cm2, while no damage appears on the crystal surface in OPO cavity, which is due to the good growth, polishing and coating technique on the crystal. In fact, we also observe the backward energy output after mirror m5 as shown in Fig.17. In order to improve the output energy, a telescope with two lens of f=5 cm, f=10 cm is inserted between Glan prism G and mirror m5 to enlarge the pump spot size. The output energy with double pump pass is shown in Fig.19b by the circular points. The maximum energy 560μJ is recorded for the usual cavity scheme, which is over 2 times higher than that reported for the same type OPO in reference. Considering the filter's loss at the idler, the maximum one of 620μJ has been generated. Enlarging the pump spot size, 580μJ is recorded at fixed output wavelength 4μm for the prism cavity scheme.In conclusion, the contributions of the thesis can be summarized as:A simple tunable Q-switched CO2 laser with longitudinal DC discharge is setup. There are two theoretical models used to analyze the dynamics of the simple mechanical Q-switched CO2 laser: One is vibro-rotational level model based on the rate equations, which is so-called four-level model (4LM); the other is six-temperature model based on the dynamics of CO2 laser, which is so-called 6TM. Optimized Q-switched CO2 laser can be applied on quantum optics and nonlinear optics.Active and passive Q-switched Er:Cr:YSGG lasers (λ= 2.79μm) are set up in three schemes. There is no available report on Q-switched Er:Cr:YSGG lasers in China. The master oscillation together with traveling amplifier can be effectively used in nonlinear optics such as pump source of mid-infrared OPO and medical applications.The properties of AgGaGexS2(1+x),x=0,1,HgGa2S4,ZnGeP2 crystals are investigated using SHG of tunable CO2 laser, which are helpful for the crystals studied in device applications. An effective method of CO gas detection with CO2 SHG technique is demonstrated.A wide-tunable, high-energy output optical parametric oscillator is demonstrated with AgGaS2, which can completely overlap the main transmission window 3-5μm of the atmosphere and provide an effective source for applications.