MECHANICALLY Q-SWITCHED CODOPED Er-Yb GLASS LASER
UNDER Ti:SAPPHIRE AND LASER DIODE PUMPING
E. Tanguy, JP. Pocholle, G. Feugnet, C. Larat, M. Schwarz
Laboratoire Central de Recherches, Thomson-C.S.F.,
Domaine de Corbeville, 91404 Orsay Cedex, France.
A. Brun, P. Georges
Institut d'Optique Théorique et Appliquée, CNRS, B.P.
147, 91403 Orsay Cedex, France.
Indexing terms: Q-Switch, Lasers
A simple Q-switched TEM00 Er3+,Yb3+:glass laser end pumped by a Ti:Al2O3
laser or by a broad-area high-power laser diode is demonstrated. In both case, the FWHM
pulse duration is about 50 ns and the peak power is more than 100 W.
A compact laser emitting in the 1.5 µm eye-safe wavelength range find very interesting applications in the fields of telemetry or optical communications [1]. Codoped Er3+,Yb3+ phosphate glass [2], and laser diode pumped leads to a low-cost, compact micro-laser emitting at this wavelength [3].
In this letter an Er3+:Yb3+ microlaser pumped by a Ti:Al2O3 laser and a broad area laser-diode are presented. Continous and Q-Switch operations were performed.
The laser cavity shown in fig 1, is made of a 2 mm thick KIGRE QX/ER Er3+
: Yb3+ : phosphate glass disc and a plane output mirror. One face of the disc
is high reflectivity coated at 1540 nm (R>99.9%) and the transmission at 980 nm is
about 95%. The other disc face is antireflection coated at 1540 nm (R<0.03%). The
output mirror reflection is estimated to be around 99% at 1535 nm. The overall cavity
length is 2.5 to 3 mm.
A 200 mm lens focuses the Ti:Al2O3 laser operating at
980 nm leading to a measured spot radius within the disc about 100 µm. The Rayleigh range
of the pump beam is about 2 cm. Thus the spot size can be considered constant into the
active material. The output power from the Er3+,Yb3+:glass laser is
shown in figure 2 as a function of the pump power.
The power was measured by a thermopile power meter. A semiconductor filter rejects the unabsorbed pump power. The maximum output power obtain is 100 mW at a 540 mW pump power level. The optical-optical efficiency is 18.5%, the laser threshold is 90 mW and the slope efficiency is 22%. The measured output beam was almost diffraction limited (M2 = 1.3) with a 7 mrad angular divergence leading to an intra-cavity beam radius around 90 µm. As expected in plano-plano cavity, the intra-cavity and the pump beam have slightly the same dimension.
A one slit ( 1 mm width) mechanical chopper is inserted into the cavity between the active material and the output mirror (cavity length 8 mm). With this cavity length, the laser is less efficient. The CW output power is about 30 mW for a 360 mW pump level used. Q-switch operation was observed at 140 Hz repetition rate. The FWHM pulse duration is 48 ns as shown in fig 3 and the pulse energy is 8.6 µJ (measured with a pyroelectric joulemeter). Thus the peak power was estimated to be 180 W. The Q-switch mode average power is 1.2 mW. The average power should be the same order of magnitude that in CW mode operation. A possible explanation is that the Q-switch rise time is not fast enough ( 5 µs) to release completely the stocked energy.
Figure 2 : Outpout power versus pump power
The same cavity laser configuration was pumped by a broad-area high-power fiber pigtailed laser diode emitting around 980 nm ( = 200 µm). The pump beam angular divergence at the output of the fiber FWHM is 15°. The optical fiber was in contact with the active material. An optical-optical efficiency of 8% is obtained (see fig 2). The laser threshold is 190 mW and the slope efficiency is 11%. The beam profile are almost gaussian but a M2 measurement was not performed.
In laser-diode pumped configuration, the threshold power is twice higher and the slope efficiency is two times lower than in Titanium-Sapphire pumped configuration. A possible explanation is that the laser diode beam is divergent whereas the Titanium-Sapphire is almost collimated in the active material.
In Q-switched regime, 52 ns FWHM pulses are obtained. In that case, the chopper disc presents three 0.5 mm width slits. The measured energy per pulse is 6.2 µJ. Thus the peak power is 120 W for a 650 mW pump power level.
As shown in figure 2, the experimental data are well fitted in both cases by a rate equation analysis [4]. This one differs from the model published by P. Laporta et al [5] because it takes into account the non-constant energy transfer between Er3+ and Yb3+. As no shortering of the erbium 4I13/2 level lifetime was observed, the " up-conversion " phenomena is not considered in this model.
The thresholds are high because the large intra-cavity spot size. The plano-plano cavity is only stabilized by the gain guiding and/or the thermal lens. A plano-concave cavity with an appropriate radius of curvature might have smaller spot size and lead to a smaller threshold [6]. But this cavity configuration was not studied because a goal of this work is to make a microchip and it is more difficult to make it with a plano-concave cavity configuration.
Fig 3 : Temporal pulse shape.
In conclusion, we have demonstrated a simple efficient and compact Q-switch
TEM00 Er3+ : Yb3+ laser end pumped by a Ti:Al2O3
laser and by a broad-area high-power laser diode. In both case, the FWHM pulse duration
was about 50 ns and the peak power is more than 100 W.
E. Tanguy, JP. Pocholle, G. Feugnet, C. Larat, M. Schwarz (Laboratoire
Central de Recherches, Thomson-C.S.F., Domaine de Corbeville, 91404 Orsay Cedex, France)
References
[1] V.P. Gapontsev, S.M. Matitsin, A.A. Isineer, V.B. Kravchenko,
" Erbium glass and their applications ", Optics and Laser Technology,
14. pp. 189-196, 1982.
[2] Shibin Jiang et al., " Laser and thermal performance of a new
erbium doped phosphate laser glass ", KIGRE INC. technical paper.
[3] P. Laporta, S. Taccheo, S. Longhi and O. Svelto, " Diode-pumped
microchip Er-Yb:glass laser ", Opt. Lett, 1993, 18, pp 1232-1234.
[4] Unpublished works
[5] P. Laporta, S. Longhi, S. Taccheo and O. Svelto, " Analysis and
modeling of the erbium-ytterbium laser ", Opt. Commun., 100 (1993) 311-321.
[6] P. Laporta, S. Taccheo and O. Svelto, " High power and high
efficiency diode-pumped Er : Yb : glass laser ", Electron. Lett., 1992, 28, pp
490-492.