IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 10, OCTOBER 1999 1241
Efficient High-Power Diode-End-Pumped
TEM
Nd:YVO Laser
Y. F. Chen, Member, T. M. Huang, C. C. Liao, Y. P. Lan, and S. C. Wang
Abstract—We demonstrate a compact and efficient diode-end-pumped TEM00 laser with output power of 25.2 W for 52 W of incident pump power by use of a single YVO4crystal with a Nd concentration of 0.3 at.%. InQ-switched operation 21-W of average power at a pulse repetition rate of 100 kHz and1.1-mJ pulse energy at a pulse repetition rate of 10 kHz were produced. At 10 kHz, the pulse width is around 10 ns and the peak power is higher than 100 kW.
Index Terms—Diode-pumped, Nd-doped laser,Q-switched.
D
IODE-PUMPED solid-state lasers with high beam qual-ity and output power in the range of several tens of watts are rapidly becoming the preferred laser sources in micromachining applications [1]. However, progress in power scaling of TEM operation has been limited by thermal fracture of the laser crystal [2]. Nowadays, the avoidance of thermally induced fracture plays a key role in laser design. The Nd:YVO crystal has been often used in diode-end-pumped lasers owing to its high absorption over a wide pumping wavelength bandwidth and large stimulated emission cross section at the lasing wavelength. Unfortunately, the thermal shock parameter of Nd:YVO is 3 times lower than that of Nd:YAG. Therefore, the maximum output power of Nd:YVO is usually several times lower than that of Nd:YAG. For a conventional 1.0 at% Nd:YVO crystal, the maximum output power limited by thermal fracture is approximately 6 W for one-end pumping [3].In our recent study [4], we found that the fracture-limited pump power, , for an end-pumped laser is inversely proportional to the absorption coefficient, i.e.,
(1) where is the fractional thermal loading, is the absorption coefficient at the pump wavelength and is the thermal shock parameter which depends on the mechanical and thermal properties of the host material. The absorption coefficient of the laser crystal linearly increases with increasing the dopant concentration. In other words, lower Nd concentrations can be beneficial in extending the fracture-limited pump power. Even though lowering dopant concentrations can extend the
Manuscript received April 29, 1999; revised June 17, 1999.
Y. F. Chen is with the Department of Electrophsyics, National Chiao Tung University, Hsinchu 30050, Taiwan, R.O.C.
T. M. Huang is with the Precision Instrument Development Center, National Science Council, Hsinchu 30050, Taiwan, R.O.C.
C. C. Liao, Y. P. Lan, and S. C. Wang are with the Institute of Electro-Optics, National Chiao Tung University, Hsinchu 30050, Taiwan, R.O.C.
Publisher Item Identifier S 1041-1135(99)07738-1.
Fig. 1. Scheme of the experimental setup.
fracture-limited pump power, the efficiency in the TEM mode may be reduced because of a poorer overlapping ef-ficiency. Therefore, a good laser design must integrate these factors to obtain the optimum concentration for scaling the output power at high beam quality. Our theoretical analysis [4] shows that the dopant concentration in Nd:YVO crystal must be larger than 0.25 at.% to obtain a slope efficiency in TEM mode higher than 45%. In this letter, we demonstrate an efficient diode-pumped Nd:YVO laser with a continuous-wave (CW) TEM output of 25 W by use of a Nd concentration of 0.3 at.%. The performance in -switched operation of this laser is also reported.
Fig. 1 is a schematic of the three-mirror laser cavity utilized in the experiment. The pump power consists of two 30-W fiber-coupled diode-laser arrays (FAP-81-30C-800-B) with the output wavelength of the lasers at 25 C ranging from 807 to 810 nm. The fibers were drawn into round bundles of 0.8-mm diameter and a numerical aperture of 0.2. The mirror was a 100-cm radius of curvature concave mirror with antireflection coating at 808 nm on the entrance face and with high-reflection coating at 1064 nm and high-transmission coating at 808 nm on the second surface. The coating of the flat mirror was the same as the mirror . The output coupler was a flat mirror with 70% reflection at 1064 nm. The total cavity length was around 90 mm. The Nd:YVO crystal, which has a 0.3 at.% Nd-ion concentration, was 12 mm long. The laser crystal was a-cut to obtain the high-gain transition and was wrapped with indium foil and mounted in a water-cooled copper block. The water temperature was maintained at 16 C. Both end surfaces of the Nd:YVO crystal were antireflection coated for 1064 nm The 20-mm-long switcher (Gooch and Housego) had antireflection coatings at 1064 nm on both faces and was driven at a 40.7-MHz center frequency with 3.0 W of radio-frequency (RF) power.
Focusing lens with 12.5-mm focal length and 87% coupling efficiency was used to re-image the pump beam into the laser crystal. The pump-spot radius was around 350 m. Considering the thermal lensing effect, the TEM radii at the Nd:YVO crystal are calculated as 260–300 m. In other
1242 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 10, OCTOBER 1999
Fig. 2. A plot of the dependence of the on the pump power for several resonator lengths.
words, the present mode-to-pump size ratio was around 0.8. Use of a mode-to-pump size ratio less than unity is based on the fact that a central portion of the highly aberrated thermal lens presents less loss to the TEM mode [5]. For a fiber-coupled laser diode, the thermal lens power can be given by [6]
(2) where is the thermal conductivity, is the absorbed pump power, is the refractive indexes along the axis of the Nd:YVO crystal, is the thermooptic coefficients of is the thermal expansion coefficient along the axis, is the crystal length, and is the pump size in the active medium. Using the usual propagation law, the pump beam is given by
(3) where is the radius at the waist, is the pump wave-length, is the pump beam quality factor, and is focal plane of the pump beam in the active medium. The refractive powers were calculated by using the following parameters: W/K cm, /K,
mm, mm,
and /K. The refractive power is around 8.4 diopters at 52 W of absorbed pump power. With the calculated refractive power, the stability of the resonator can be confirmed from the value of the factor . Fig. 2 shows the dependence of the on the pump power for several resonator lengths. It can be seen that the resonator with mm is in the stable region for the pump power up to 60 W. On the other hand, the resonator with -mm steps into the unstable region for the pump power higher than 46 W. The theoretical predictions agree very well with the experimental results.
Fig. 3. A plot of the average output power as a function of the incident pump power. Circles: CW mode operation. Squares:Q-switched mode at a repetition rate of 100 kHz.
Fig. 3 shows the average output power in CW mode and -switched mode at a repetition rate of 100 kHz as a function of the incident pump power. The output power in CW mode was measured before insertion of the -switch into the resonator. The highest output power of 25.2 W was achieved at the maximum pump power of 52 W. The average slope efficiency was 50% with respect to the incident pump power. The parameter has been measured to be 1.3 over the complete output power range. According to the present result, it is possible to scale the output power to 50 W with two Nd:YVO crystals pumped from both ends with total pump power of 100 W. Recently, a composite crystal structure [3], which is fabricated by diffusion-bonding a doped crystal to an undoped piece of the same cross section, was used to reduce the thermally induced stress. We believe that higher output power with better beam quality can be achieved with a composite crystal structure. With the -switch in the cavity, stable -switched mode operation at a pulse repetition rate up to 100-kHz was accomplished. As shown in Fig. 3, the highest average output power obtained at a 100-kHz pulse repetition rate was 21 W at pump power of 52 W.
Fig. 4 shows the average output power and pulse width at a pump power of 52 W as a function of the pulse repetition rate. It can be seen that at low pulse repetition rates the pulse width is short and the energy per pulse is high, whereas at higher pulse repetition rates the energy per pulse is low and the pulsewidth is long but the average power is high. The highest -switched pulse energy of 1.1 mJ was achieved at 10 kHz. Below 10 kHz, no further increase of the pulse energy was observable due to the 90 100 s lifetime of the upper laser level of Nd:YVO crystal. The major advantage of Nd:YVO crystal is its ability to retain short pulse width even at very high pulsed repetition rates. At full pump power, the pulse width varies from 10 ns at 10 kHz to 18 ns at 100 kHz. With a Nd:YAG crystal in the present cavity, the pulsewidth increases from 20 ns at 10-kHz to 50 ns at 100 kHz. The pulse width in Nd:YVO crystal is about two to three times shorter than
CHEN et al.: EFFICIENT HIGH-POWER DIODE-END-PUMPED TEM Nd:YVO LASER 1243
Fig. 4. A plot of average output power and pulse width as a function of the
Q-switched pulse repetition frequency.
that in Nd:YAG due to a shorter lifetime in Nd:YVO crystal. Although the average output power of Nd:YVO crystal at 10-kHz drops by 50% compared with operation at 100 kHz, the short pulse width leads to peak power greater than 100 kW. This compares with the peak power in diode-pumped Nd:YAG and Nd:YLF lasers at low pulse repetition rates [7], [8]. For high pulse repetition rates ( 30 kHz), the output performance of the Nd:YVO system is generally better than that of the Nd:YAG or Nd:YLF system due to higher optical-to-optical conversion efficiency.
We have demonstrated a highly efficient, high-power Nd:YVO laser in CW and -switched modes. To scale up
output power, a YVO crystal of low Nd concentration (0.3 at.%) was used to avoid thermally induced fracture. 25.2 W of TEM CW mode output power with good beam quality was obtained at a 52-W pump power. This result indicates that there is substantial scope for further power scaling of end-pumped Nd:YVO lasers with a low Nd concentration. In switched operation, we demonstrate the potential of the YVO crystal to generate a pulse energy in the millijoulle range with high pulse repetition rates. Such a laser source will be interesting for micromaterials processing applications.
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