Appl. Phys. B 71, 827–830 (2000) / Digital Object Identifier (DOI) 10.1007/s003400000410
Applied Physics B
Lasers and Optics
High-power diode-end-pumped Nd:YVO
4
laser: thermally induced
fracture versus pump-wavelength sensitivity
Y.F. Chen1,∗, Y.P. Lan2, S.C. Wang2
1Department of Electrophysics, National Chiao Tung University Hsinchu, Taiwan, Republic of China (Fax: +886-35/729-134, E-mail: [email protected])
2Institute of Electro-Optical Engineering, National Chiao Tung University Hsinchu, Taiwan, Republic of China
Received: 19 February 2000/Revised version: 30 May 2000/Published online: 20 September 2000 –Springer-Verlag 2000 Abstract. We report two kinds of compact and efficient
diode-end-pumped TEM00lasers with output power> 25 W
at ≈ 50 W of incident pump power. One laser consists of a single 0.3 at. % Nd:YVO4 crystal in a V-type cavity, the
other laser includes two 0.5 at. % Nd:YVO4crystals in a
lin-ear cavity. Experimental results show that lowering Nd3+
concentration can be beneficial in extending the fracture-limited pump power but it also increases the sensitivity of the pump wavelength due to the overlapping efficiency.
PACS: 42.55.Rz; 42.60.Lh
Diode-end-pumped solid-state lasers with high beam quality and output power in the range of several tens of watts are rapidly becoming the preferred laser sources in numerous po-tential applications [1, 2]. Nd:YVO4 crystal has been often
used in diode-pumped lasers because of its large stimulated-emission cross section at lasing wavelength and its high ab-sorption over a wide pumping wavelength bandwidth. How-ever, power scaling with Nd:YVO4 crystal has been greatly
hindered by the thermally induced fracture [3, 4].
In a recent study [4], we found that the fracture-limited pump power for an end-pumped laser is inversely propor-tional to the absorption coefficient, i.e.,
Plim=
1
α
4πRT
ξ , (1)
where ξ is the fractional thermal loading, α is the absorp-tion coefficient at the pump wavelength and RTis the thermal
shock parameter which depends on the mechanical and ther-mal properties of the host material. The absorption coefficient of the laser crystal linearly increases with increasing doping concentration. Therefore, lowering Nd3+ concentration can
be beneficial in extending the fracture-limited pump power. In this letter, we demonstrate two kinds of compact and efficient diode-end-pumped Nd:YVO4lasers with TEM00output
pow-ers greater than 25 W at the pump power around 50 W. The
∗Corresponding author.
laser setups are a V-type cavity with a 0.3 at. % Nd:YVO4
crystal and a linear cavity with two 0.5 at. % Nd:YVO4
crys-tals, respectively. Experimental results show that even though the output efficiency is rather similar for both laser setups, the sensitivity of the output power to the diode temperature is quite different. The difference is attributed to the fact that the dependence of the overlapping efficiency on the pump wave-length is more critical for a YVO4 crystal with lower Nd3+
concentration.
1 Experiment
The pump power consists of two 30-W fiber-coupled diode-laser arrays (Coherent, FAP-81-30C-800-B) with the central wavelength of the lasers at 28◦C around 809 nm. The fibers were drawn into round bundles of 0.8-mm diameter and a nu-merical aperture of 0.2. The first cavity was a V-type res-onator formed by two reflection mirrors M1 and M2, an out-put coupler and a 0.3 at. % Nd:YVO4 crystal with 10-mm
length, as shown in Fig. 1a. The mirror M1 was a 100-cm-radius-of-curvature concave mirror with antireflection coat-ing at 809 nm on the entrance face and with high-reflection coating at 1064 nm and high-transmission coating at 809 nm on the second surface. The coating of the flat mirror M2 was the same as the mirror M1. The output coupler was a flat mir-ror with 85% reflection at 1064 nm. The total cavity length was around 80 mm. The second laser cavity was a linear cav-ity including an reflection mirror, an output coupler and two 0.5 at. % Nd:YVO4 crystal with 6-mm length, as shown in
Fig. 1b. The specifications of the mirror M3 and the output coupler are the same as those of M1 and the output coupler used in the first cavity, respectively. The total cavity length was also around 80 mm.
All laser crystals were a-cut to obtain the high-gain π transition and were wrapped with indium foil and mounted in water-cooled copper blocks. The water temperature was maintained at 20◦C. Both end surfaces of the Nd:YVO4
crystal were antireflection coated for 1064 nm (R< 0.2%). A focusing lens with 25-mm focal length and 85% coup-ling efficiency was used to re-image the pump beam into
828
Fig. 1a,b. Schematic of diode-end-pumped experimental setup: a V-type
cavity with a single 0.3 at. % Nd:YVO4crystal and b linear cavity with two 0.5 at. % Nd:YVO4crystals
the laser crystal. The average pump-spot radius was around 320µm. Considering the thermal effect, the TEM00 radii at
the Nd:YVO4crystal are found to be about 200–250µm. In
other words, the present mode-to-pump size ratio was around 0.6–0.8. 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 TEM00mode [5].
Figure 2 shows the input–output characteristics for the present laser systems. It can be seen that the laser setup with two 0.5 at. % Nd:YVO4crystals has a lower threshold pump
power and a slightly higher slope efficiency. With the linear cavity shown in Fig. 1b, the highest output power of 25.5 W was achieved at the total pump power of 48 W. However, ex-perimental result shows that, once the incident pump power in any end exceeded around 25 W, the output power with two 0.5 at. % Nd:YVO4crystals immediately dropped, and output
characteristics were not reproducible when the pump power
Fig. 2. Plot of the input–output characteristics for the present laser setups
Fig. 3. Plot of dependence of the relative output power on the temperature
of the laser diode
was decreased. On the other hand, the highest output power in the cavity shown in Fig. 1a was 25.2 W at the maximum pump power of 52 W. No immediate damage of the laser crystal was observed in the experiment shown in Fig. 1a be-cause the fracture-limited pump power of 0.3 at. % Nd:YVO4
crystal exceeded the available pump power. The output beam quality was nearly the same for both laser cavities and the
M2 parameter has been estimated to be< 1.3 over the com-plete output power range with the algorithms of knife-edge technique [6]. According to the present result, it is possible to scale the output power to 50 W with two 0.3 at. % Nd:YVO4
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 un-doped 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.
Since the emission wavelength of the laser diode varies with its junction temperature at a rate:∆λ/∆T ∼= 0.3 nm/◦C, a few degrees change can scan the absorption band of the present laser systems. We have measured the output power as a function of the diode temperature in a range of about 30◦C, i.e., about 9 nm around the optimum wavelength. The output powers relative to the optimum result at 27◦C are displayed in Fig. 3. It is seen that the sensitivity of the output power is quite different for both laser setups. As analyzed later, we believe that this difference arises from the dependence of the overlapping efficiency on the pump wavelength.
2 Analysis
Even though lowering the absorption coefficient can ex-tend the fracture-limited pump power in end-pumped lasers, the efficiency in the TEM00mode may be reduced because
space-829 dependent rate equation analysis, the slope efficiency Se are
given by [7] Se= T T+ LηQ λ p λl ηo, (2)
where T is the power transmission of the output coupler,ηQ
is the quantum efficiency, and L denotes the round-trip cav-ity excess losses which include thermally induced diffraction losses, nondiffraction round-trip losses such as scattering at interfaces, imperfect reflection, and excited-state absorption. If concentration quenching is not significant, the quantum efficiencyηQ is almost equal to unity. The overlapping
effi-ciencyηoare given by [7]
ηo=( sl(x, y, z)rp(x, y, z)dν)2 s2 l(x, y, z)rp(x, y, z)dν , (3)
where rp(x, y, z) is the normalized pump intensity and sl(x, y, z) is the normalized cavity mode intensity
distri-bution. The beam profile of a fiber-coupled laser diode,
rp(x, y, z), can be approximately described as a top-hat
dis-tribution [5]: rp(x, y, z) = αe −αz πω2 p(z)[1 − e−αl] Θ(ω2 p(z) − x2− y2) , (4)
whereωp(z) is the pump size in the active medium, and Θ( )
is the Heaviside step function. Using the usual M2 propaga-tion law, the pump size is given by
ω2 p(z) = ω2po 1+ λpMp2 nπω2 po (z − zo) 2 , (5)
whereωpo is the radius at the waist, λp is the pump
wave-length, Mp2 is the pump beam quality factor, and zo is focal
plane of the pump beam in the active medium. For a single transverse mode TEM00, sl(x, y, z) can be given by
sl(x, y, z) = 2 πω2 l(z)l exp −2x2+ y2 ω2 l(z) , (6) and ωl(z) = ωo 1+ [(z − zl)λ1/πω2o]2≈ ωo. (7)
Hereωois the beam waist of the laser mode, zlis the position
of the beam waist, and the point z= 0 is taken to be the in-cident surface of the active medium. Substituting (4) and (6) into (3), the overlapping efficiencyηocan be expressed as
ηo= F(α, ωo)2 F(α, ωo/ √ 2), (8) and F(α, ωo) = 1 2 α 1− e−αl l o ω o ωp(z) 2 × 1− exp −2ωp(z) ωo 2 e−αzdz. (9)
To investigate the concentration dependence of the over-lapping efficiency, we have measured the absorption coef-ficient for a-cut 0.3 and 0.5 at. % Nd:YVO4 crystals with
Lambda 900 spectrometer. The lengths used in the meas-urement are 10 and 6 mm for 0.3 and 0.5 at. % Nd:YVO4
crystals. Since the light source in the spectrometer was un-polarized, the measured absorption spectra were the com-bination of absorption spectra in the π and σ components of the Nd:YVO4 crystal. The present results are practically
useful because the pump source from a fiber-coupled laser diode is unpolarized. With the measured spectra, the absorp-tion coefficients were deduced from the exponential law of absorption and shown in Fig. 4. It is seen that the amplitude is proportional to the doping concentration, and the observed structure and peak positions are insensitive to the doping concentration.
Even though a longer crystal can be used to increase the absorption efficiency for a lower doping concentration, the overlapping efficiency may become poorer because of the di-vergence of the pump beam. Applying the measured absorp-tion coefficients to (8) and (9), we calculated the dependence of the overlapping efficiency on the diode temperature for the present cavities. The cavity mode size was determined by the ABCD matrix approach with the thermal lensing effect. For a fiber-coupled laser diode, the thermal lens is given by [1, 8]
fth−1= l 0 ξPabs 4πKc αe−αz 1− e−αl [dn/dT + (n − 1)αT] ω2 p(z) dz, (10)
where ξ is the fractional thermal loading, Kc is the
ther-mal conductivity, Pabs is the absorbed pump power, dn/dT
is the thermal-optic coefficients of n, αT is the thermal
ex-pansion coefficient along the a axis. The parameters used in the calculation are as follows: ξ = 0.24, ωpo= 0.32 mm,
M2
p≈ 310, Kc= 0.0523 W/K cm, dn/dT = 3.0 × 10−6/K,
αT= 4.43 × 10−6/K, and n = 2.165. The value of zo was
Fig. 4. The absorption coefficient for unpolarized light as a function of the
830
Fig. 5. Calculation result for the influence of thermal lens on the cavity
mode diameter
found to be 2.5 and 2 mm for 0.3 and 0.5 at. % Nd:YVO4
crystals under the condition of the optimum result. Fig-ure 5 shows the calculation result for the influence of ther-mal lens on the cavity mode diameter. It can be seen that the mode diameter in resonator shown in Fig. 1a is slightly larger than that in resonator shown Fig. 1b due to the cavity structure and the thermal lensing effect. Even so, the mode size is weakly sensitive to the thermal lens for most of the pump-power range studied here. Figure 6 shows the depen-dence of the overlapping efficiency on the diode tempera-ture for 0.3 and 0.5 at. % Nd:YVO4 crystals. It can be found
that the dependence is in agreement with the result shown in Fig. 3.
3 Conclusion
We have demonstrated two types of compact and efficient TEM00 Nd:YVO4 lasers with output power greater than
25 W. We utilized the YVO4 crystals with low doping
con-centrations to avoid the thermally induced fracture. One laser was to use a single 0.3 at. % Nd:YVO4 crystal in a V-type
cavity and the other laser was to use two 0.5 at. % Nd:YVO4
Fig. 6. The theoretical analysis for the dependence of the overlapping
effi-ciency on the temperature of the laser diode
crystals in a linear cavity. From the experimental results it can be concluded that even though there is substantial scope for further power scaling of end-pumped Nd:YVO4 lasers
with low Nd concentrations, the temperature regulation of the laser diode becomes severe because of the overlapping efficiency.
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