In this paper, we demonstrate the efficient 1.3 um dual-wavelength operation of LD end-pumped Nd:YAG ceramic laser. With a plano-concave cavity, a maximum continuous-wave dual-wavelength output power of 5.92 W is obtained under an incident pump power of 20.5 W, giving a slope efficiency of 30.3% and an optical-optical conversion efficiency of 29.0%. With Co2+:LaMgAl11O19 crystal as the saturable absorber, the passively Q-switched dual-wavelength operation is achieved for the first time to our knowledge. The maximum passively Q-switched average output power is 226 mW, the minimum pulse width is 15 ns, and the highest pulse repetition rate is 133 kHz.
© 2010 OSA
In recent years, ceramic laser materials have been developed rapidly because of their favorable properties . Comparing with Nd:YAG crystal, Nd:YAG transparent ceramic has many advantages in terms of fabrication, cost, neodymium concentration, size, as well as structures . These advantages make Nd:YAG laser ceramic become an alternative to its single crystal. As an important emission band besides 1.06 µm and 0.94 µm in Nd3+ ion, laser emission at 1.3 μm should also be studied in detail due to its potential applications in the fields of medical treatment, optical fiber communication, efficient production of red radiation by frequency doubling, and yellow radiation by sum-frequency (using radiation of 1.06 μm). In the past, the maximum output power was obtained to be 36 W at the wavelength of 1319 nm .
Recently, simultaneous multiple wavelengths lasing has been of great interest for many applications such as medical instrumentation, spectral analysis, optical frequency up-conversion, and THz frequency generation, etc. By using of Nd:YAG ceramic, we have achieved the promising dual-wavelength laser at 1052 and 1064 nm . As is known, there are numerous laser radiations near 1.3 um due to the stark splitting of Nd3+ ion such as 1319, 1320, 1334, 1335, 1338, 1341 and 1356 nm, corresponding to 4F3/2→4I13/2 transition. Among the radiations above, the most efficient are 1319 nm and 1338 nm in Nd:YAG ceramic . Laser-diode (LD) side-pumped 1.3 um Nd:YAG ceramic lasers have been reported previously, with emphasis on the single wavelength of 1319 nm or 1338 nm, respectively [3,6]. The dual-wavelength laser is potential for the generation of ultrahigh repetition rate pulse by optical beating, new-wavelength laser by sum-frequency and coherent terahertz (THz) radiation by difference frequency generation etc.. Considering the comparable emission cross-sections at 1319 nm and 1338 nm in Nd:YAG, it is possible to achieve the dual-wavelength Nd:YAG ceramic laser at this wavelength band. In this paper, we report, for the first time to our knowledge, the efficient 1319 nm and 1338 nm dual-wavelength operation of LD end-pumped Nd:YAG ceramic laser, including continuous-wave (CW) output and passively Q-switched output. With a plano-concave cavity, a maximum CW output power of 5.92 W is obtained under an incident pump power of 20.5 W, giving a slope efficiency of 30.3% and an optical-optical conversion efficiency of 29.0%. With Co2+:LaMgAl11O19 (Co:LMA) as the saturable absorber, we get the maximum dual-wavelength passively Q-switched average output power of 226 mW, the minimum pulse width of 15 ns, and the highest pulse repetition rate of 133 kHz. We proposed that, with this dual-wavelength laser, a novel sum-frequency wavelength of 664 nm or a radiation source of 3.2 THz can be realized.
2. Experimental setup
The experimental setup shown in Fig. 1 is based on a simple plano-concave resonator. The pump source employed in the experiment is a fiber-coupled laser-diode (LD) with a central wavelength around 808 nm. Through a focusing optics (N.A.= 0.22), the output of the pump source is focused into the laser medium with a spot radius of 0.256 mm. The input mirror M1 is concave with radius of curvature of 200 mm, antireflection (AR) coated at 808 nm on the flat facet, and high-transmission (HT) coated at 808 nm, high-reflection (HR) coated at 1319 nm and 1338 nm on the concave facet. The output-coupler (OC) M2 is a flat mirror with different transmissions of 1.4%, 4.1%, 12.5% at 1319 nm and 1.3%, 3.9%, 12.0% at 1338 nm, respectively. All the mirrors used in the experiment are HT coated at 1.06 um to suppress its oscillation in the resonator. Although the two radiations of 1319 nm and 1338 nm share the same upper level 4F3/2 of Nd3+ ion which will induce the competition of the inversion populations, the similar stimulated emission cross sections between 1319 nm (0.8×10−19cm2) and 1338 nm (0.9×10−19cm2) are favorable for the dual-wavelength operation [3,6]. By appropriately choosing the transmission values of the two wavelengths through the output coupler, the two wavelengths (which have almost the same thresholds) are obtained whether in CW or Q-switched operation. The Nd:YAG ceramic (uncoated) with Nd concentration of 2 at.% is cut with dimensions of 3×3×4.8 mm3 (4.8 mm corresponding to the light-passing direction). The Co:LMA (uncoated) crystal with the Co2+ doping concentration of 0.5 at. % and dimensions of 3×3×0.2 mm3, corresponding to the initial transmission of 89.6% at 1319 nm and 89.7% at 1338 nm, is used as the saturable absorber.
To remove the heat generating from Nd:YAG ceramic and Co:LMA crystal under high pump power levels, the ceramic is wrapped with indium foil and mounted in a water-cooled copper block, the Co:LMA crystal is attached on a copper block without cooling water. The temperature of cooling water is controlled to be 15 °C. The laser output power is measured by a power meter (EPM 2000, Molectron Inc.) and temporal behavior of the Q-switched laser is recorded by a 500-MHz digital oscilloscope (TDS 3052, Tektronix Inc.) and fast photodiode detector (D400FC, Thorlabs Inc., bandwidth 1GHz). The output wavelengths are detected by a spectrum analyzer (MS9710C, Anritsu Inc.).
3. Experimental results and discussions
The thermal lens effect of diode end-pumped Nd3+ laser at 1.3 um is stronger than that at 1.06 um due to the larger quantum defect as well as strong excited state absorption for the 1.3 um transition . Consequently, when the pump power increases to a critical value, the strong thermal lens effect will cause the laser cavity to become unstable, which will result in a rapid decrease of the output power. In order to reduce the influence of thermal lens effect and reduce the optics dissipation, the length of the cavity is adjusted as short as possible, which is about 15 mm. Removing Co:LMA from the cavity, we get the CW dual-wavelength operation with different output coupler transmissions (T) of 1.4%, 4.1%, 12.5% at 1319 nm and 1.3%, 3.9%, 12.0% at 1338 nm respectively, of which the total output power curves versus the incident pump power are shown in Fig. 2 .
Because of the similar wavelengths between 1319 nm and 1338 nm, it is difficult for us to achieve a split mirror which is high transmission at 1319 nm but high reflection at 1338 nm (or the contrary). During our experiment, we obtain the individual output power of 1319 nm and 1338 nm by using the equations as following:Fig. 1; P2 is the measured residual part of P1 that transmits through M3 (M3 is slightly declining in order to avoid the interference between forward and backward laser beams).
As is shown in Fig. 2, laser thresholds increase from 0.74, 0.88 to 1.94 W with the increasing of the output coupler transmissions and the laser output powers increase almost linearly with the pump powers. Using the output coupler with transmissions of 4.1%, 3.9% (at 1319 nm, 1338 nm), we get the highest dual-wavelength total output power of 5.92 W (corresponding to optical-optical conversion efficiency and slope efficiency of 29.0% and 30.3%), in which the individual power at 1319 nm and 1338 nm are 2.56 W and 3.36 W. This is more efficient than the previous result obtained with Nd:YAG ceramic at this wavelength band (optical-optical conversion efficiency of 12.5% ). From this figure, it can be found that the output powers are not saturated, which mean that much higher output powers could be obtained if we increase the pump powers continuously. At the same time the spectrum was monitored showing no other lasing wavelengths besides 1319nm and 1338nm.
To meet the requirement of simultaneous dual-wavelength oscillation, output couplers with similar transmissions at 1319 nm and 1338 nm have been adopted to control the gain competition between the dual-wavelength lines. Additionally, if we reasonably assume the approximately equal mode size parameters for the two wavelengths, and the same pump geometry, threshold balancing can be approximately expressed as following [8,9]:
The CW total and respective output powers with output coupler transmissions of 4.1%, 3.9% (at 1319 nm, 1338 nm) are shown particularly in Fig. 3 . Although the two modes have almost the same thresholds, the output power at 1338 nm is larger than that at 1319 nm. This may derive from the fact that there is a difference in the stimulated emission cross sections at 1338 nm (0.9×10−19cm2) and 1319 nm (0.8×10−19cm2). The whole process can be divided into three sections: In 1 ~4.5 W pump power, both of the output for 1319 nm and 1338 nm increase rapidly; In 4.5 ~13.5 W pump power, the output power for 1338 nm increases obviously but it is unchanged approximately for 1319 nm; In 13.5 ~20.5 W pump power, the situation is reversed. Such behavior can be attributed to the same up-level (4F3/2) of 1319 nm and 1338 nm transitions, and the output ratio is a result of the intense competition. Figure 4 presents the measured threshold pump power as a function of ln(1/R), where R is the reflectivity of the output couplers. Based on the Findlay-Clay method, the intracavity loss mainly generated by the ceramic is estimated to be 5.3%.
In order to obtain stable single wavelength output, we insert another mirror (slightly tilted) with transmissions of 23%, 72% (at 1319 nm, 1338 nm) into the cavity. Due to the bigger inserting loss of 1319 nm than that of 1338 nm, only the single wavelength of 1338 nm is achieved. Compared with previous oscillation of 1338 nm in simultaneous dual-wavelength operation, the single mode output curve shown in Fig. 5 is smoother because of the disappearance of gain competition, and the maximum single wavelength output power of 1338 nm is 1.07 W.
3.2 Passive Q-switching
For laser operations at 1.3 um, several saturable absorbers such as semiconductor (SESAMs) [10,11] and V3+-doped crystals [12,13] have been employed as the passive Q-switchers. Recently, tetrahedral Co2+-doped crystals whose broad near-infrared absorption band is located at 1030-1660 nm have also been shown to be useful Q-switchers for a number of Nd3+ lasers operating at 1.3 um [14–16].
To generate efficient passive Q-switching, it is necessary that the saturation in the absorber should occur earlier than that in the gain medium. Generally, it can be achieved by augmenting the ratio of the effective mode area in the gain medium to that in the saturable absorber for the passively Q-switched Nd3+ lasers, which is called the second threshold condition and can be expressed as following :
By inserting Co:LMA crystal into the cavity, 1.3 μm passively Q-switched output is obtained. Figure 6 shows the average output powers with different output coupler transmissions. It can be seen that the corresponding thresholds are 3.24 W, 3.79 W, and the maximum average output powers are 115mW, 226 mW. At the same time, we check the spectrum at different incident pump power and do not find any other wavelength beside 1319 nm and 1338 nm. The Q-switched dual-wavelength operation with output coupler transmissions of 12.5%, 12.0% (at 1319 nm, 1338 nm) is shown particularly in Fig. 7 . At high pump levels, the power of 1319 nm increases rapidly and surpasses 1338 nm finally.
The dual-wavelength pulse profile is observed by a digital oscilloscope. During the experiment, we get stable pulses without time delay between 1319 nm and 1338 nm. From Fig. 8 , it can be seen that the pulse width decreases with the increasing of pump power. In addition, the transmissions of the output coupler also have influence on the output pulse width. Under the same pump power, a narrower pulse width is obtained when the output coupler with higher transmissions is used. The minimum pulse width of 15 ns is obtained under pump power of 19 W. This value is much shorter than that obtained by Huang et.al (42 ns) , Li et.al (45 ns)  and Qi et. al (44.8 ns) , and is the shortest pulse width with Co:LMA as saturable absorber to our knowledge. The corresponding pulse profile is shown in Fig. 9 .
The pulse repetition rate versus the incident pump power is demonstrated in Fig. 10 . When the pump power is 19 W, the highest pulse repetition rates are 133 kHz and 91 kHz, respectively. Based on the measured average output power and pulse repetition rate, the pulse energy can be calculated. The maximum pulse energy is 2.5 μJ under the pump power of 19 W. According to the measured pulse width, the highest peak power is calculated to be 167 W.
In conclusion, we have demonstrated CW and passively Q-switched 1.3 um dual-wavelength operation of LD end-pumped Nd:YAG ceramic lasers. With the optimum output coupler, we get the maximum CW dual-wavelength output power of 5.92 W. Using Co:LMA as saturable absorber, the maximum passively Q-switched average output power, the minimum pulse width and the highest pulse repetition rate are 226 mW, 15 ns and 133 kHz, respectively. Generally, our experimental results show that the two modes at 1319 nm and 1338 nm have the similar radiative property in Nd:YAG ceramic, with which one can get a novel sum-frequency wavelength of 664 nm or a radiation source of 3.2 THz.
This work is supported by the National Natural Science Foundation of China (60978027, 50925205 and 50990303), Natural Science Foundation of Shandong Province, China (ZR2009FM015), Innovation Fund for the Post-Doctoral Program of Shandong Province (200802029), and China Postdoctoral Science Foundation funded project (200904501184).
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