DOI: 10.1007/s00340-003-1291-7 Appl. Phys. B 77, 505–508 (2003)
Lasers and Optics
Applied Physics B
y.f. chen1,u s.w. chen1 s.w. tsai2 y.p. lan2
Output optimization of a high-repetition-rate
diode-pumped Q-switched intracavity optical
parametric oscillator at 1
.57 µm
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
Received: 28 March 2003/Revised version: 6 June 2003 Published online: 30 September 2003 • © Springer-Verlag 2003 ABSTRACTThe output optimization of a high-repetition-rate diode-pumped Q-switched intracavity optical parametric oscil-lator at 1573 nm with a type-II non-critically phase-matched
x-cut KTP is experimentally and theoretically studied. The
op-timum output reflectivity is found to be 85%–90% for the maximum average power. The average conversion efficiency from diode-laser input power to OPO signal output power is up to 15% at a pulse repetition of 80 kHz. However, the opti-mum output reflectivity for the maxiopti-mum peak power is found to be 60%–70%; the overall peak power amounts to 3–4 kW at a pulse repetition of 80 kHz.
PACS42.60.Gd;42.65.Yj;42.55.Xi
1 Introduction
High-peak-power laser sources at wavelengths greater than 1.5 µm are vital for applications involving co-herent laser radar, remote sensing and active imaging [1]. In this spectral range, radiation is mostly absorbed in the ocu-lar fluid of the eye rather than in the retina. Eye safety is an important requirement for laser applications involving free-space propagation. This need has stimulated much interest in intracavity optical parametric oscillators (OPOs). In compar-ison with extracavity configurations, intracavity OPOs take advantage of the high power level within the oscillator to allow a low threshold and high efficiency. Even though in-tracavity OPOs have been proposed for over 30 years [2–4], only recently have their merits been noted by the availabil-ity of high-damage-threshold non-linear crystals and diode-pumped Nd-doped lasers [1, 5–7]. Several crystals belong-ing to the potassium titanyl phosphate (KTP) family, when pumped by Nd-doped laser pumps around 1050–1070 nm, generate signal wavelengths around 1.55 µm [8–11]. The conventional intracavity OPOs in which flash lamps or quasi-cw diodes are used as the pump sources typically restrict operations to low repetition rates, less than 1 kHz [12, 13]. Al-though a compact diode-pumped Q-switched intracavity OPO has been demonstrated at a frequency repetition rate from 1 to 20 kHz, the overall average power is less than 60 mW [14].
u Fax: +886-35/72-9134, E-mail: [email protected]
Recently, we demonstrated 1.33 W of 1573-nm output from an intracavity OPO based on a non-critically phase-matched KTP pumped by a diode-pumped acousto-optically (AO) Q-switched Nd: YVO4 laser operating at 80 kHz [15]. In this work, we experimentally study the output optimiza-tion of a high-repetioptimiza-tion-rate diode-pumped Q-switched intra-cavity optical parametric oscillator at 1.57 µm. Experimental results reveal that the output reflectivity for the maximum conversion efficiency is in the range 85%–90%, whereas the highest peak power can be obtained with the output reflec-tivity around 60%–70%. Numerical analysis is performed to confirm the experimental results.
2 Experimental set-up
A schematic of the actively Q-switched intracav-ity OPO laser is shown in Fig. 1. The active medium was an a-cut 0.3 − at%. Nd3+, 7-mm-long Nd: YVO4 crystal. A Nd: YVO4crystal of low doping concentration was used to avoid thermally induced fracture [16]. Both sides of the laser crystal were coated for antireflection at 1064 nm (R< 0.2%). The pump source was a 15-W 809-nm fiber-coupled laser diode with a core diameter of 0.8-mm and a numerical aper-ture of 0.16. A focusing lens with 12.5-mm focal length and 85%coupling efficiency was used to re-image the pump beam into the laser crystal. The waist radius of the pump beam was around 0.36 mm. The input mirror, M1, was a 1-m radius-of-curvature concave mirror with antireflection coating at the diode wavelength on the entrance face (R< 0.2%), high-reflection coating at the lasing wavelength (R> 99.8%) and high-transmission coating at the diode wavelength on the other surface (T> 95%). The output coupler had a dichroic coating that was highly reflective at 1064 nm (R> 99.8%) and partially reflective at 1573 nm. Several output couplers with different reflectivities (50%≤ Rs≤ 95%) at 1573 nm were used in the experiment to study the output optimization. The 10-mm-long AO Q-switcher (NEOS) was antireflection coated at 1064 nm on both faces and was driven at a 80 MHz center frequency with 3.0 W of rf power. To avoid damage to the intracavity optical components, the Q-switcher was oper-ated above 10 kHz. The OPO cavity was formed by a cooper-ated KTP crystal and an output coupler. The 20-mm-long KTP crystal was used in type-II non-critical phase-matching con-figuration along the x-axis (θ = 90◦andϕ = 0◦) to have both
506 Applied Physics B – Lasers and Optics
FIGURE 1 Schematic of the actively Q-switched intracavity OPO laser
a maximum effective non-linear coefficient and no walk-off between the pump, signal and idler beams. One surface of the KTP crystal was coated to have high reflectivity at the signal wavelength of 1573 nm (R> 99.8%) and high transmission at the pump wavelength of 1064 nm (T> 95%). The other face of the KTP crystal was antireflection coated at 1573 nm and 1064 nm. Both the Nd: YVO4 and KTP crystals were
wrapped with indium foil and mounted in a water-cooled cop-per block. The water temcop-perature was maintained at 25◦C. The overall Nd: YVO4 laser cavity length was 55 mm and
the OPO cavity length was 23 mm. The pulse temporal be-havior at 1573 nm was recorded by a LeCroy 9362 digital oscilloscope (500 MHz bandwidth) with a fast germanium photodiode.
3 Results and discussion
Figure 2 shows the average output power at 1573 nmas a function of the repetition for different output couplers at an absorbed pump power of 12.6 W. The ra-dius of the OPO beam was estimated to be around 0.3 mm. Over the entire frequency range, the beam quality M2
fac-FIGURE 2 The average output power at 1573 nm as a function of the repe-tition for different output couplers at an absorbed pump power of 12.6 W
tor was found to be less than 1.3. Basically, increasing the pulse repetition rate can efficiently increase the average out-put power at 1573 nm, except that the OPO reflectivity is too low (Rs= 50%). It can be also seen that a higher output
coupler reflectivity (85%≤ Rs≤ 90%), on average, leads to
higher conversion efficiency. However, if the OPO output re-flectivity is too high, the stored energy is not fully extracted in a single output pulse. Since the remaining energy is sufficient to evolve the pump field, the OPO threshold can be reached again and a second signal pulse is produced. As shown in Fig. 3a and b, a train of OPO pulses is usually produced for higher values of Rs, whereas a single pulse can be generated
with a lower output reflectivity. In addition, the pulse width was found to be generally less than 5 ns. The short pulse width
FIGURE 3 Oscilloscope traces showing a train of pump and signal pulses when Rs= 90% (a), and a single pulse for pump and signal fields when Rs=
60% (b). Both results were measured at a pulse repetition rate of 80 kHz and an absorbed pump power of 12.6 W
CHENet al. Output optimization of a diode-pumped Q-switched intracavity optical parametric oscillator 507
comes from the effective cavity dump of the intracavity OPO. Therefore, if high peak power is preferred, the OPO output re-flectivity needs to be chosen to maximize the conversion in a single pulse.
We have employed the rate equation model developed by Debuisschert et al. [17] to confirm the experimental results. Since the present IOPO is resonant only on the signal, the evo-lution equation of the idler field can be suppressed. With this adiabatic elimination, the rate equations for the four-level Q-switched laser with IOPO are given by:
dn dt = −cσϕPn, (1) dϕp dt = lcr lca cσnϕp+ ∆ϕp −lnl lcaσ opoϕsϕp −ϕp trp ln 1 R + L , (2) dϕs dt = cσopoϕp(ϕs+ ∆ϕs) − ϕs trs ln 1 Rs + Ls , (3) where n is the inversion population density of the gain medium, c is the speed of light,ϕpis the pump photon
dens-ity,ϕsis the signal photon density, lcais the optical length of
the laser cavity,σ is the stimulated emission cross section of the gain medium, trp is the round-trip time in the laser
cav-ity, lcr is the length of the gain medium, lnl is the length of
the non-linear crystal,∆ϕpis the spontaneous emission
inten-sity, L is the round-trip pump wave intensity loss in the laser cavity, R is the global reflectivity of the laser cavity mirrors,
trsis the round-trip time in the OPO resonator,σopois the
ef-fective OPO conversion cross section,∆ϕsis the noise signal
intensity, Lsis the round-trip signal wave intensity loss in the
OPO cavity and Rsis the output reflectivity of the OPO mirror.
Equations 1–3 are essentially identical to those used by De-buisschert et al. [17], except that the effective OPO cross sec-tion is used to describe the conversion rate. The effective OPO cross section,σopo, can be derived from the parametric gain
coefficient for small gains of the single resonator oscillator: 2σopolnl= 8ω1ω2d2efflnl2 n1n2n3εoc2 w2 3 w2 2+ w23 , (4)
whereω1andω2are the idler and signal frequencies,
respec-tively; n1, n2 and n3 are the refractive indices at the idler,
signal and pump wavelengths, respectively; deffis the
effect-ive non-linear coefficient;ε0 is the vacuum permittivity; and
w2andw3are the mode sizes for the signal and laser waves,
respectively.
Figure 4a and b depicts the calculated temporal profiles corresponding to the experimental results shown in Fig. 3a and b, respectively. The good agreement between experimen-tal and numerical results indicates that the theoretical model is successful in predicting the production of several signal pulses. Figure 5 shows the experimental and theoretical re-sults for the average output power versus the OPO output reflectivity at a repetition rate of 80-kHz and an absorbed pump power of 12.6-W. It can be seen that the theoretical calculations agree quite well with the experimental data. The optimum output coupler for the maximum average power is
FIGURE 4 The calculated temporal profiles corresponding to the experi-mental results shown in Fig. 3a and b, respectively
FIGURE 5 Experimental and theoretical results for the average output power versus the OPO output reflectivity at a repetition rate of 80 kHz and an absorbed pump power of 12.6 W
508 Applied Physics B – Lasers and Optics
FIGURE 6 Experimental and theoretical results for the output peak power versus the OPO output reflectivity at a repetition rate of 80 kHz and an ab-sorbed pump power of 12.6 W
found to be approximately Rs= 85%. With the optimum
out-put coupler, the conversion efficiency from the diode laser input power to OPO signal output power is up to 15%.
Although the highest conversion efficiency was obtained with an output coupler of Rs= 85%, the peak power was
a maximum with an output coupler of Rs= 60%–70%, as
shown in Fig. 6. It can be seen that with an output coupler of Rs= 60%–70%, the peak power can amount to 3–4 kW at
a pulse repetition rate of 80 kHz and an absorbed pump power of 12.6 W. From the experimental results shown in Figs. 5 and 6, an output coupler of Rs= 80% is found to be a
bet-ter choice for the tradeoff between the average power and the peak power.
Finally, it is worthwhile to mention that the pulse repeti-tion rates in the present experiment (> 20 kHz) are consid-erably higher than those in conventional Q-switched lasers
(< 1 kHz). On the whole, the optimum output reflectivity in-creases with increasing pulse repetition rate; this is the reason why the optimum output reflectivity is significantly higher than that for conventional output couplers in Q-switched lasers.
4 Conclusions
We have studied the output optimization of a high-repetition-rate diode-pumped Q-switched intracavity optical parametric oscillator at 1573 nm with a type-II non-critically phase-matched x-cut KTP crystal. Numerical calculations have also been performed to confirm the experimental results. It was found that the maximum conversion efficiency can be obtained with an output coupler of 85%–90% at the cost of peak power. If high peak power is desired, the output reflectiv-ity needs to be around 60%–70%.
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