High-power, quasi-continuous wave blue laser output is obtained by intracavity frequency-tripling of a 1.319 µm Nd:YAG laser with two LiB3O5 crystals. The conversion efficiency and the stability of the blue laser output power at 439.7 nm were both improved through the suppression of 1.338 µm operation by means of a thin YAG etalon. 7.6 W average power at 439.7 nm was demonstrated at 5 kHz with pulse width of 170±10 ns (FWHM). The beam quality factor M2 is 12±2 in both horizontal and vertical directions. The long-term stability of the blue light is better than 1% at an output of 6 W for 200 operation hours.
© 2006 Optical Society of America
High power blue light source is desired for many applications, such as optical data storage, submarine communications, spectroscopy, biotechnology, large image projection and medical applications. For these applications, the high power blue laser source must have a high electrical to optical efficiency, compact package, long and reliable life time. A diode pumped solid state blue laser is a promising method to such a blue light source.
Up to now, there are several ways to get a high power diode-pumped solid-state laser system in the blue spectral region (power of 1 W and beyond). The first one is the frequency-doubling of the diode-pumped quasi-three-level Nd3+ Laser operated on the 4F3/2→4I9/2 transition [1–4]. The second one is the tunable output produced directly by optical parametric oscillators (OPOs) or by frequency-doubling of a tunable laser system in the near-infrared spectral region, such as OPOs pumped by the all-solid-state laser [5–9]. The first way is the efficient one to get watt level blue laser output, however the output of laser oscillated on the 4F3/2→4I9/2 transition is limited because of the considerable reabsorption loss due to thermal population at the lower level caused by the quasi-three-level operation. The highest output power reported of a blue light was 2.8 W by first method . The second method potentially represents the tunable and flexible solution, average powers of 10.1 W in the blue at 450nm was obtained . However it is often complicated for routine operation.
Frequency-tripling of diode-pumped solid-state laser operated at 1.3 µm is a possible way to obtain high power blue radiation. Ref. 10 and Ref. 11 show that the blue light can be obtained efficiently in this way with their experiments using picosecond lasers at 1.3 µm and the periodically poled nonlinear crystals [10–11]. However the highest output power published in the blue by this way so far was limited to 1 watt. Our group reported the generationg of 4.3 W QCW blue light at 440 nm  using V shape resonator two year ago.
However, the maximum optical-to-optical efficiency is about 1.1%. The main cause of low efficiency is that two wavelengths can be obtained around 1.3 µm at the same time from one Nd:YAG laser. Besides the R2 to X1 transition (1319 nm) there is a probable competition from the R2 to X3 transition (1338 nm). The coexistence of the two wavelengths is a problem for the generation of high efficiency frequency conversion to get efficient output of blue light.
In this paper, we reported a significant improvement in the generation of the high power blue light by frequency tripling inside a compact and simple diode-pumped solid-state Nd:YAG laser operated at 1.3 µm. 7.6 W blue light at 439.7 nm with high beam quality (M2=12±1) was obtained, the conversion efficiency and the stability of the blue output power were both improved through the suppression of 1.338 µm operation by means of a thin YAG etalon. The long-term stability of the blue light is better than 1% at an output of 5.4 W. To the best of our knowledge, the blue laser reported in this paper has the highest output power among the all-solid-state QCW blue lasers ever reported.
2. Experimental setup
The experimental arrangement of the system is illustrated in Fig. 1. The laser system in our experiment contains two identical diode-pumped Nd:YAG laser modules in a folded-mirror resonator designed to oscillate at 1.3 µm. The resonator is a four-mirror design. This cavity provides two tight intracavity focuses, which is valuable for frequency conversion.
The flat mirror M1 was coated for high reflection at 1319 nm and high transmission at 659.5nm (R1319nm≈99.8%, T659nm≈98%). The concave mirror M2 was coated for high reflection at 1319 nm and 659.5nm, while high transmission at 439.7 nm (R1319nm≈99.8%, R659.5nm≈99.8% T439.7nm≈97%), which is the output coupler for 439.7 nm laser. The concave mirror M3 and M4 were both coated for high reflection at 1319 nm, 659.5 nm and 439.7 nm (R1319nm≈99.8%, R659nm≈99.8%, R439.7nm ≈99.7%). The coatings of the four mirrors have the transmission greater than 70% at 1064 nm. The high transmission at 1064 nm for the components is necessary since the gain of the oscillation line at 1064 nm is much higher than that at 1319 nm. The highly wavelength-selective dielectric coatings suppress the laser oscillation from the strongest transition at 1064 nm and provide optimum operation conditions at 1319 nm. Each face of the elements in the cavity was coated for high transmission at 1319 nm, 659.5 nm and 439.7 nm to minimize insertion losses. In order to compensate for the thermal birefringence, and thus to improve the output power and beam quality, a quartz 90° polarization rotator at 1319 nm was inserted between the two identical laser modules . With this technique, the radial and tangential components of the polarizations are exchanged between two identical laser rods to achieve equal phase retardation in the cross-section along the entire Nd:YAG rods. An acousto-optic modulator with high diffraction loss at 1319 nm was used for Q switching to generate the repetition rate of 5 kHz. Pulsed operation was necessary for efficient operation of the intracavity frequency conversion. Because Nd:YAG is an isotropic laser medium and emits unpolarized radiation, it was also necessary to polarize the laser beam with a Brewster plate in order to maximize frequency conversion in our experiment.
The Nd:YAG rod is surrounded by a flow tube and three circular reflectors. Pump light from the diodes is coupled into the Nd:YAG rod directly. The Nd:YAG rod in each module, which is doped with 0.6 at.% Nd3+, has a diameter of 3 mm and 80 mm in length. To reach the maximal overlap of the pump energy volume with the volume occupied by a low-order resonator mode, the pump energy distribution within the gain medium has been optimized through the geometric parameters of our pumping configuration according to the design of our four-mirror folded cavity. The pump energy distribution matched maximally with the oscillation beam profile helps to increase the gain extraction in the medium and the beam quality of the output laser . Two LBO crystals are used as the nonlinear crystals for intracavity frequency conversion in the experiment. One is for the second harmonic generation (SHG), which was cut into 4×4×20 mm3 with type-I phase matching SHG (θ=84°, φ=0°); The other is the third harmonic generation (THG), which was cut into 4×4×40 mm3 for type-II noncritical phase matching (θ=0°,φ=0°). The LBO was chosen for its high optical damage threshold, low absorption at both the fundamental, second harmonic and third harmonic outputs, moderate nonlinear coefficients and sufficient birefringence to provide phase matching needed in our experiment. LBO was cut for type-II noncritical phase matching for frequency tripling in the experiment, which is very valuable to generate high power blue laser. The crystals are placed in their respective ovens, whose temperatures were all maintained by temperature controllers to a precision of ±0.1 K. In order to reach optimum second harmonic generation and frequency tripling simultaneously, a special Z-type resonator, which has two focuses, was designed. One focus is located near the middle of the folded arm M3 and M4, another focus is located near the middle of the folded arm M2 and M3, the SHG LBO and the THG LBO crystal is set close to these focuses, respectively. Having two focuses is valuable to take the advantage of the strongest power density of the beam in the cavity. In the first LBO crystal (named as SHG-LBO in Fig. 1), some fraction of the fundamental radiation at 1319 nm is converted to the second harmonic radiation at 659.5 nm. In the other LBO crystal (named as THG-LBO in Fig. 1), unconverted fundamental radiation is mixed with the second harmonic to produce the third harmonic radiation at 439.7 nm.
In order to avoid the simultaneous existence of two waves and to select a wanted wavelength, a solid etalon is placed in the laser cavity. The etalon is made of undoped solid YAG and is not coated, the thickness and finesse of the etalon are 0.6mm and 1.27, respectively. YAG material, not silica, is selected because of high refractive-index and perfect laser performance. It can improve the transmission difference between 1.319 and 1.338 µm from 12% to 28% than silica etalon. The insertion loss for the etalon at 1.319µm has been calculated to be less than 0.1%.
3. Results and discussion
First, when the etalon is absent from the laser cavity, two fundamental wavelengths was obtained around 1.3 µm in the output of a Nd:YAG laser, one is the R2→X1 transition at 1.319 µm and the other is the R2→X3 transition at 1.338 µm. Figure 2 shows the coexistence of two transitions and their relative laser performance under high pumping power level, which was measured by Anritsu optical spectrum analyzer MS9710B from the leak of the mirror M1. It consists with the same effective stimulated emission cross sections data for both transitions as A.A.Kaminskii reported : 0.95×10-19 cm-2 and 1.0×10-19 cm-2 respectively .
Second, we tuned the temperature of the THG-LBO out of the phase-match value, the maximum power of red light obtained is about 9.35 W, which is measured by Ophir laser power meter F300A. The spectrum of the red beam is analyzed and it is displayed in Fig. 3. The radiation at 659.5 nm and 669 nm were generated by frequency doubling of 1.319 µm and 1.338 µm, while the output at 664.2 nm can be attributed to the sum-frequency between 1.319 µm and 1.338 µm. It indicates that the Nd:YAG laser oscillated on the two transitions simultaneously under high pump power.
Third, we tuned the temperature of the THG-LBO around the phase-match value, the maximum power of blue light obtained is about 2 W, the stability of the blue laser output is very poor, the peak to peak value is up to 1 W. The spectrum of blue laser output is analyzed by monochromatic grating spectrometer of 1 nm resolution power, and four separate blue lines are observed. Multiple blue lines are the result of cross frequency tripling between the fundamental wavelengths and their frequency doubling, which is depicted in table 1.
One LBO crystal is used as the frequency doubler of 1.3 µm radiation of Nd:YAG, which was cut into 4×4×20 mm3 with type-I phase matching SHG (θ=84°,φ=0°). The mixture accept bandwidth is up to 1300 cm-1·cm, which is enough to obtain high conversion efficiency of both frequency doubling and sum-frequency of two lines of 1.3 µm radiation of Nd:YAG. Two fundamental radiations and three red lines coexist simultaneously in resonator. Six types of frequency tripling processes are possible, and four blue lines could be obtained. The 439.7 nm blue is the self frequency tripling of the 1319 nm radiation, the 446nm blue line is the self frequency tripling of the 1338 nm radiation, the 441.8 nm and 443.9 nm blue lines are the cross frequency tripling of 1319 nm an 1338 nm.
Obviously, the coexistence of the two fundamental wavelengths is a big problem for the generation of high efficiency frequency tripling to get blue light. First, the existence of 1.338 µm radiation, consuming the energy stored in the Nd:YAG material, reduces the peak power density of 1.319 µm radiation in the cavity; Second, the sum-frequency and cross frequency tripling between 1.319 µm and 1.338 µm consumes a larger portion of the energy oscillated at 1.319 µm in the cavity. Therefore, the coexistence of the two transitions holds back the efficient increase of total blue output power at the high pump power level; Third, the mode competition between two transitions around 1.3 µm in the cavity makes the output unstable dramatically.
Finally, in order to avoid the simultaneous existence of two fundamental waves and to select a wanted wavelength, we designed a solid etalon to be placed in the laser cavity. The etalon is made of undoped thin YAG without coating. YAG material, not silica, is selected because of high refractive-index and perfect laser performance. It can improve the transmission difference between 1.319 µm and 1.338 µm from 12% to 28% than silica etalon. The insertion loss for the etalon at 1.319 µm has been calculated to be less than 0.1%. We measured the spectrum distribution of the blue laser with Avantes mini fiber spectrummete AvaSpec2048 of 0.5 nm resolution power. Figure 4. shows that only single blue spectrum at 439.7 nm of 1.319 µm Nd:YAG laser was obtained at 480 W pumping power.
Experimentally, we measured the 439.7 nm blue light output power of with Ophir laser power meter F300A as a function of the pump power of the diodes. Figure 5 shows the blue output power versus the pump power at 808 nm under the repetition rate of 5 kHz. The blue light output started at a pumping power around 170 W, the start point of power out is about 1 W. Because the resonator is designed to oscillate at high pumping power level, it is in an unstable region under 170 W pumping power. It increases rapidly as the pumping power of the diode laser increases and is not saturated at 480 W, the corresponding maximum blue output power is 7.6 W, with pulse width 170±10 ns. The slope efficiency is about 2.1%. To the best of our knowledge, this experimental result is the highest power reported for an all-solid-state nanosecond blue laser.
In order to integrate a large screen laser projector, one high power blue laser device of 6.0 W was manufactured. It has been operating for 200 hours up to now, the time trace of the output power at the output power level of 6 W is shown in Fig. 6, the fluctuation of the red beam output power was better than 1.0%.
Beam quality is important for many applications. The beam quality factor M2 becomes poorer as the pump power increases. At maximum output of 7.6 W, the M2 value is 12±2 in both horizontal and vertical directions measured by Spiricon beam analyzer. The beam profile of the far-field was show in Fig. 7. At a lower output, the beam quality is much better. For example, at an output of 3 W, the value of M2 is reduced to about 8±1 in both directions. The poorer quality at higher output is mostly due to that the laser tends to oscillate in higher-order transverse modes under high pump power. But, it is enough for laser projecting application.
In conclusion, high power blue laser by intracavity frequency tripling of a diode-pumped 1.3 µm has been demonstrated by utilizing two side-pumped laser modules and intracavity frequency-tripling in LBO crystals. The conversion efficiency and the stability of the blue laser output power at 439.7 nm were both improved through the suppression of 1.338 µm operation by means of a thin YAG etalon. 7.6 W average power at 439.7 nm was obtained at 5 kHz with pulse width of 170±10 ns (FWHM). The beam quality factor M2 is 12±2 in both horizontal and vertical directions. The long-term stability of the blue light is better than 1% at an output of 6 W for 200 operation hours.
This work was supported by the Knowledge Innovation Programme of the Chinese Academy of Sciences, the National High Technology Research and Development Programme of China under Grant No 2002AA311120 and 2005AA31120, the National Key Basic Research and Development Programme of China under Grant No G1998061413, and the council for Science and Technology of Beijing under Grant No H020420060060110.
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