Qswitched lasers are gaining more interest because of their ability for various applications in remote sensing, environmental monitoring, micro

Transcription

1 90 Qswitched lasers are gaining more interest because of their ability for various applications in remote sensing, environmental monitoring, micro machining, nonlinear frequency generation, laserinduced break down spectroscopy and so on (Zayhowski 2000). Nd:YAG passively Qswitched lasers are well studied using Cr 4+ :YAG as the saturable absorber. Cr 4+ doped crystals have become the promising materials for one micron passively Q switched lasers because of its compactness, high damage threshold, large absorption cross section and their inertness to the chemical and thermal environment. Other than 1.06 µm Qswitched operation, Qswitched lasers were also developed at 946 (Zayhowski 1996) and at 1123 nm (Chen and Lan 2004) using Nd:YAG crystal. In addition, diode pumped passively self Q switched lasers by Nd and Cr codoped YAG single crystals added much interest with a demonstration of 50 ns pulses with a slope efficiency of 20% (Dong et al 2000). After the demonstration of first successful laser oscillation by Ikesue et al (1995) on Nd:YAG ceramics, transparent ceramics are recently gaining much interest because of its large advantages over single crystals (Ikesue and Aung 2008). A combination of Nd:YAG transparent ceramics and Cr:YAG single crystal has been used to achieve diode pumped passively Q

2 91 switched pulses of 50 ns pulse duration (Wu et al 2007). Chromium doped YAG ceramics were also fabricated and an all ceramic self Qswitched Nd 3+ :YAG + Cr 4+ :YAG laser was demonstrated with a pulse duration of 20 ns and the possibility of fabrication of Nd 3+ and Cr 4+ codoped self Qswitched laser materials was also proposed (Takaichi et al 2002). Instead of a separate gain medium and a saturable absorber, Cr 4+ and Nd 3+ codoped YAG ceramics were prepared using solid state reaction and vacuum sintering and this acts both as the gain medium and the saturable absorber for self Qswitched solid state lasers (Li et al 2006a). Apart from the generation of Qswitched lasers, Nd:Cr:YAG has also been used for lamp and solar pumped solid state lasers. For the lasers that use solar power, cross pumping mechanism helps in increasing the pumping efficiency. Because of the efficient energy transfer that takes place between the 2E state of Cr 3+ ions to excited levels of Nd 3+ ions, the fluorescence efficiency of Nd 3+ ions has been increased by the presence of chromium ions for lamp pumped laser system (Kiss and Duncan 1964). In addition, Nd:Cr:YAG ceramics exhibit more advantages that include a strong absorption in the spectrum of sun light, high excited level life time and a large stimulated emission cross section (Saiki et al 2006). The ionic radius of Cr 3+ ion is larger and there are more chances of transformation of Cr 3+ to Cr 4+ ions. These make the growth of large size crystals of Cr 3+ doped YAG difficult to achieve. After the development of ceramic technology, it became possible to develop Cr 3+ doped Nd: YAG ceramics reducing Cr 4+ states in the matrix (Saiki et al 2007). A flash lamp pumped Nd 3+ and Cr 3+ codoped YAG ceramic was also efficiently demonstrated with the laser efficiency of two times higher than that of Nd: YAG ceramic rod (Yagi et al 2006). Recently the optical properties of Cr/Nd: YAG ceramics at two different concentration of Cr ions were investigated and it has been found that the gain is twice compared to Nd:YAG crystal (Endo 2010).

3 92 In this study, attempts were made to analyze the optical properties including transmission analysis, emission and the cross energy transfer mechanism between Cr to Nd ion at 0.1 and 3% Cr concentration. In addition, under laser diode pumping self Qswitched laser was demonstrated and it confirms the possibility of presence of two charge states of chromium ion. The transparent ceramic samples of Nd:Cr:YAG at 1% of Nd concentration and 0.1 and 3% concentration of chromium ions were prepared using nanocrystalline assisted modern ceramic techniques which were used before for the preparation of YAG ceramics (Yanagitani et al 1998, Yagi et al 2007a). The samples were prepared by the slip casting and vacuum sintering method. The aqueous solutions of aluminum, yttrium, neodymium and chromium chloride were initially mixed together. The mixed aqueous solution was added dropwise and mixed with aqueous solution of ammonium hydrogen carbonate. The obtained solution was filtered and washed with water for many times. The powder obtained was dried in an oven at 120 C. The obtained precursor was calcinated at around 1200 C to get YAG powder with particle dimensions of 200 nm. The YAG powder was then milled with a solvent, binder and a dispersion medium. The milled slurry was put into a gypsum mold and dried to obtain a desired form. The organic components were removed by calcination. The final materials were then sintered in vacuum at 1700 C for 20 h. After annealing, highly transparent Nd:Cr:YAG ceramics were obtained. The samples used for the measurements are shown in Figure 5.1. Transmittance spectra were recorded from nm using Hitachi spectrophotometer having a resolution of 2 nm.

4 93!"# $%&'$#%( )#$ )& *+,$-.. $%,/-.0. The samples obtained were transparent and had a green coloration. Transmission spectra recorded in the range of nm are shown in Figure 5.2. The absorption peaks in the lower wavelength region correspond to the chromium ions where as the absorption peaks in the higher wavelength region correspond to the neodymium ions. The broad absorption peaks observed at 587 nm and 430 nm correspond to transitions from 4 A 2 excited level to 4 T 2 and 4 T 1 levels of Cr 3+ ions respectively. A less intense absorption peak around 520 nm contributes to the green coloration of the samples. The increase in this absorption intensity causes the samples to appear more greenish as the Cr concentration was increased. Other absorption peaks at 530, 735, 748, 808 and 885 nm are attributed to Nd 3+ ions. When the concentration of chromium was increased from 0.1 to 3%, there are no

5 94 additional or absence of peaks but an increase in the absorption intensity corresponding to Cr 3+ was observed. This implies that the charge state of chromium remains the same. When observed carefully, the absorption peaks corresponding to the Nd ions are less strong as Cr concentration was increased. 0.8 Nd1Cr0.1YAG Nd1Cr3YAG Transmittance Wavelength(nm)!"# $%& (($%)#&'#)($*+($%&'$#%()#$ )& $($%0.*+)*%)#%($( *% 0 Photoluminescence spectra were recorded at two different pumping wavelengths. Laser diodes at 801 nm and at 407 nm (Nichia Ltd., Japan) of powers 1 W and 200 mw respectively were used as the excitation sources. Optical spectrum analyzer (OSA) ANDO AQ6315 A was used for collecting

6 95 the emitted photons from a direction perpendicular to the excitation sources. Photoluminescence spectra of both the ceramics under 801 nm laser diode pumping are shown in Figure 5.3. The spectra show the characteristic Nd 3+ emission peaks due to the transitions from 4 F 3/2 excited level to the various ground levels of Nd ions. The emission intensity was found to decrease with the increase in chromium concentration. As mentioned above, the reduction in absorption intensity corresponding to the neodymium ions may have led to the reduction in the emission intensity as well. This shows that Cr addition may have an adverse effect on the Nd ions leading to a reduction in its emission efficiency under 801 nm pumping. Figure 5.4 shows the luminescence spectra recorded at an excitation of 407 nm laser diode for the two samples. An intense emission peak around 706 nm was the characteristic emission peak of Cr 3+ ions because of the energy transfer from 2E excited level of the chromium ions to stable 4 A 2 ground level. Pump source excites the electrons to the broad 4 T 1 level which then decays nonradiatively to 2E level and the further transition from the 2E level gives this intense red emission band centered at 706 nm. The other dominant emission peaks above 800 nm are due to Nd 3+ emission. As can be seen from the spectra, the emission at 706 nm gets weakened as the concentration of chromium ions was increased to 3% where as the Nd 3+ emission has enhanced enormously. The intensity of the major emission peak at 1064 nm for the sample with 3% Cr concentration was more than twice the intensity for the sample with 1% Cr concentration. This is because of the efficient energy transfer mechanism between Cr 3+ to Nd 3+. As proposed earlier by Kiss and Duncan (1964), there was an efficient energy transfer between the 3d bands of Cr 3+ ions to 4f states of Nd 3+ ions in YAG matrix.

7 96!"# 0 && *% &'#)($ *+ )#$ )& $( $% #1) ($( *% 2$3#4#%!(5*+6% 1064 Nd1Cr0.1YAG Nd1Cr3YAG Intensity (a.u) Wavelength (nm)!"# 7 && *% &'#)($ *+ )#$ )& $( $% #1) ($( *% 2$3#4#%!(5*+78%

8 97 The energy transfer process has been represented in Figure 5.5 as per the energy levels of Cr 3+ and Nd 3+ ions. This enables an efficient cross pumped laser system under lamp pumping. As the chromium concentration was increased, the lifetime of its 2E band gets reduced, leading to the fast energy transfer to the neodymium ions and this leads to the emission enhancement in the near IR region.!"# %#!9($%&+##)5$% &/#(2##% 0: $% 0: *%& % ($%&'$#%()#$ )&

9 98 7 A fibercoupled laser diode with an emission wavelength of 807 nm (Unique Model No. UM 7800_100_20) was used as a pumping source for an end pumped laser experiment. The laser was focused on the sample using a coupling optics consisting of two lenses with a pump spot of about 200 µm diameter and the sample temperature was maintained at 20 C by a water cooler. The laser cavity of 50 mm was made between a flat mirror which has antireflection coated at the pumping wavelength and a concave mirror which has high reflection coating for the pumping wavelength at one side and partial transmission for the wavelength range of nm on the other side. Output coupler with T = 1% for lasing wavelength and having 250 mm radius of curvature was used in the experiment. The LD output power and the transmitted laser power through the samples were measured. Thus the absorbed pump power of the sample was calculated and the output power dependency on the absorbed power was plotted as shown in Figure 5.6. The slope efficiencies at T = 1% output coupler at 0.1% and 3% of chromium concentrations are 21% and 19.5% respectively. The maximum output power at an absorbed pump power of 2.5 W was 490 and 410 mw for 0.1% and 3% Cr doped samples respectively. The lasing threshold was slightly higher at 3% of chromium concentration. The lasing threshold values were found to be 264 mw and 877 mw of incident pump power for 0.1% and 3% of Cr doped Nd:YAG samples respectively.

10 Nd: Cr 0.1%: YAG Average Output Power (mw) Nd: Cr 3%: YAG Absorbed Pump Power (W)!"# ; #'#%#%)# *+ *"('"( '*2# *% $/&*/# '"' '*2# "%#4$& %!*'#$( *% %)#$ )& 7 7 #4+2 ()5#$&##%#$( *%"& %! $%&'$#%(#$ )& 2 ()5 %! Under continuous wave operation, the population inversion was fixed at its threshold value when oscillation starts. When the lasing operation was in pulsed operating conditions, the population inversion was seen to exceed the threshold value only by a relatively small amount, due to the onset of stimulated emission. If the shutter is introduced into the laser cavity, and when the shutter was closed, laser action was prevented. Under this condition, the value of the population inversion may far exceed the threshold population

11 100 holding when the shutter was absent. If the shutter was now opened suddenly, the laser will exhibit a gain that exceed losses; stored energy may then be released in the form of a short and intense light pulses (Hellwarth 1961). Since this operation involved switching the cavity Qfactor from a low to a high value, the technique is usually called Qswitching. This technique allows one to generate laser pulses with a duration comparable to the photon decay time and high peak power. Several methods have been developed to achieve switching of the cavity Q. Most commonly used methods are (1) Electrooptical shutters, (2) Rotating prisms, (3) Acousto optical switches, (4) Saturable absorbers. These devices are generally grouped into two categories, active and passive Qswitches. In an active Qswitching device, some external active operation such as a change of voltage must be applied to this device to produce Qswitching. In a passive Qswitch, the switching operation is automatically produced by the optical nonlinearity of the element used, such as a saturable absorber (Svelto 1998). Passive Q switch was commonly used method for pulsed operation. A saturable absorber consists of a material that absorbs at the laser wavelength and has a low value of saturation intensity. The material becomes more transparent as the fluence increases. At high fluence levels, the material saturates resulting in a high transmission. This bleaching or saturation process in a saturable absorber was based on saturation of a spectral transition. If such a material with high absorption at the laser wavelength was placed inside the laser resonator, it will initially prevent laser oscillation. As the gain increases during a pump pulse and exceed the roundtrip losses, the intracavity power density increases dramatically causing the passive Qswitch to saturate. Under this condition, the losses are low and a Qswitch pulse builds up. Even

12 101 though, saturable dye contained in a cell can be mostly used, solid state and gaseous saturable absorbers are also used. The most common material employed as a passive Qswitch is Cr 4+ : YAG. The Cr 4+ ions provide the high absorption cross section of the laser wavelength and the YAG crystal provides the desirable chemical, thermal, and mechanical properties required for long life. In the Cr and Nd codoped YAG ceramic, self Qswitched laser operation has been achieved in this present investigation. The chromium ions present in the same matrix, acts as the saturable absorber where as the Nd ions present in the ceramic causes the lasing generation. The combined process generates the pulsed lasers. Initially, ceramic techniques were used to prepare highly Cr 3+ doped and Cr 4+ reduced Nd:YAG ceramics. It was assumed that the developed ceramics did not contain Cr 4+ ions (Saiki et al 2007). Recently, Endo (2010) pointed out that Cr 4+ ions are not completely suppressed in Nd:Cr:YAG ceramics. The presence of Cr 4+ ions are responsible for the Q switch operation in this Nd:Cr:YAG ceramics. The pulsed laser operation was recorded using InGaAs PIN photodiode connected in line with an oscilloscope. The lasing experiment was carried out using a 5 W laser diode with an emission wavelength of 807 nm. The experimental setup used for the pulsed lasing operation is shown in Figure 5.7.

13 !"# 81'# #%($4&#("'"&#+*&#4+&2 ()5#4$&#*'#$( *% %)#$ )& 10

14 103 The lasing operation is observed to be pulsed operation and this is the first demonstration of self Qswitched pulsed operation by an Nd and Cr codoped ceramic sample under laser diode pumping. The train of pulses under pulsed operation at the pumping power of 2 W incident power for Nd:Cr0.1%:YAG at T = 1% output coupler is shown in Figure 5.8. %) #%('"''*2# <... Intensity (a.u) Time (s)!"# 6 $ % *+ '"4&#& */&#3# " %! (5# &#4+ &2 ()5# *'#$( *% %..)#$ )& The pulse duration which is the full width at half maxima of the pulses was found to be 420 ns as shown in Figure 5.9. This pulse duration is very long compared to the values obtained before using single crystals of Nd:Cr:YAG and the combination of single crystals and ceramics. The oscilloscope trace is shown in Figure 5.10.

15 104 %) #%('"''*2# <... Intensity (a.u) (420ns) Time (s)!"# = "4&# "$( *% #$&"# % $ & %!4# '"4&# */&#3# +*..)#$ )&!"# &) 44*&)*'#($)#*+(5##*%&($( *%*+'"4&#4$&#*'#$( *%

16 105 It is well known that the pulse duration is dependent on the thickness of the gain media as well. Basically, the longer gain medium results in a shorter pulses and higher threshold (Zhou et al 1993). The thickness of the samples that were used in the present measurement was about 1mm thick and this small thickness may be one of the reasons for longer pulse duration. Pulse duration remains constant when the pumping power was increased where as the repetition rate increases with the increase in pumping power. The pulses became unstable with an increase in pumping power above 3W. The pulse duration for 3% Cr concentration was much higher of the order of ms and the instability also was much higher leading to an erratic behavior of the pulses at higher pumping power. Pulses of less duration may be possible if the thickness of the samples can be improved and the concentration of chromium ions are at an optimum level. Transparent ceramics of Nd:Cr:YAG were prepared at an Nd concentration of 1% whereas the concentration of chromium ions were varied between 0.1 and 3%. The samples showed very good transparency with an appearance of greenish coloration as the Cr concentration was increased. The samples experienced almost saturable absorption at a higher concentration of Cr ions. The emission studies were performed separately under two different wavelengths for exciting Nd 3+ and Cr 3+ ions in two different cases. Under Cr 3+ excitation scheme, it was found that there is an efficient energy transfer from Cr 3+ to Nd 3+ ions and the energy level transitions have been discussed. Lasing oscillation experiments were performed using a 807 nm laser diode in an end pumped pumping scheme. Slope efficiencies for Nd1%:Cr0.1%:YAG and Nd1%:Cr3%:YAG were 21 and 19.5% respectively. Combined optical results imply that an increase in Cr ion concentration has a detrimental effect on the performance of Nd 3+ ions whereas Cr 3+ ions have an increased performance

17 106 with an increase in Cr ion concentration. Self Qswitched operation has been demonstrated under the same lasing experimental conditions and the pulses were detected using an InGaAs photodiode. The presence of traces of Cr 4+ ions in addition to Cr 3+ ions in Nd:Cr:YAG ceramics acted as saturable absorber and led to the generation of laser pulses. Pulse duration of 420 ns has been observed for Nd1%:Cr0.1%:YAG ceramics. For the sample with 3% of Cr ion concentration, pulses became unstable even at low pumping power.