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Pulsed Nd:YAP laser at 1432 nm pumped with high power laser diode
Optics Communications 283 (2010) 2881–2884
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Pulsed Nd:YAP laser at 1432 nm pumped with high power laser diode S. Wang a,⁎, X. Wang a, H. Rhee a, S. Meister a, H.J. Eichler a, J. Chen b a b
Institute of Optic and Atomic Physics, Technical University Berlin, Str. des 17, Juni 135, 10623 Berlin, Germany State Key Laboratory of Modern Optical Instrumentation, Zhejiang University Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 15 March 2010 Accepted 15 March 2010
a b s t r a c t LD end pumped Nd:YAP laser operation in the eye safe spectrum region at 1432 nm is reported. With pump energy of 256 mJ, maximum linearly polarized output of 26 mJ is obtained. The optical-to-optical overall efficiency is around 10%, and the slope efficiency is around 18%. The laser beam operates with spiking mode with a total emission period of less than 300 µs at 10 Hz. The stimulated emission cross section is estimated at around 0.85 × 10− 20 cm2. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The spectral range with wavelengths larger than 1.4 µm is called the eye safe region, because little energy is transmitted to the retina [1]. Water absorption increases very strongly as a function of wavelength and presents several peaks at 1.45, 1.95 and 2.9 µm [2]. Nd lasers working at 1.4 µm and the Er, Tm, Ho lasers working at around 1.54 µm and 2 µm can be used for surgery, because at these wavelengths water molecules present an optical absorption high enough to cut soft tissues without problems of high penetration, but low enough to prevent blood coagulation [2,3]. Also due to the large absorption coefficient in water, laser operation at these wavelengths can be used for remote sensing for detecting water vapor [4]. Er, Tm and Ho doped gain materials have a quasi-three-level nature and they normally show relatively high laser threshold and require efficient cooling [5,6]. Despite the normal laser lines around 1 µm and 1.3 µm, Nd doped crystals have also a laser line around 1.4 µm, which runs as a four level laser. Some papers already reported Nd:YAG laser operation at 1444 nm [4,7]. With flash lamp pumping, a maximum average output of 100 W was obtained at pump pulse energy of 5.5 J [4]. Compared with flash lamp pumping systems, laser diode pumping systems lead to comparatively higher conversion efficiency and are more compact. With continuous-wave (cw) LD pumping, output of 4.9 W was achieved with pump power 40 W [7]. To our knowledge, lasing with Nd:YAP crystals at 1432 nm is only reported in [7]. Output of 2.2 W with 27 W continuous-wave pump power and a slope efficiency of 8% were reported. Compared with Nd: YAG crystal, Nd:YAP crystal exhibits some advantages: it has a high value of thermal conductivity; it is birefringent and the laser emission is polarized. In this paper we investigate an LD end pumped Nd:YAP
⁎ Corresponding author. E-mail address: [email protected] (S. Wang). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.03.035
crystal at 1432 nm with high peak pump power up to around 1 kW in pulsed operation. 2. Experimental setup Fig. 1 shows the setup of a free running Nd:YAP laser at 1432 nm. The pump source (Laserline™) is working at 10 Hz with pulse duration of 300 µs. The pump laser diode provides a maximum peak power of around 1000 W at 803 nm wavelength, which corresponds to the absorption peak of the Nd:YAP laser crystal. The power is delivered by a fiber system (HIGHYAG™) with 1 mm core diameter and NA = 0.2. A telescope system with convex lenses L1 and L2 is used to focus the diode pump laser beam into the Nd:YAP crystal. The focal lengths of L1 and L2 are 100 mm and 150 mm respectively. The active medium is a 0.8% doped Nd:YAP crystal with 10 mm length, 9.5 mm diameter and with the b-axis in the laser beam direction. The crystal is AR coated at 1080 nm, 1341 nm and 1432 nm. Fig. 2 shows the energy level of Nd:YAP laser crystal. Transitions from energy level 4F3/2 to 4I13/2 of Nd:YAP lead to laser operation at 1341 nm and 1432 nm [8]. A spherical bi-convex quartz lens (≈50 mm diameter, 100 mm focal length) and a plane-convex cylindrical quartz lens (50 × 50 mm2, 100 mm focal length) are used to collimate the fluorescence output into the entrance slit of a monochromator. The fluorescence spectrum was measured with a spectrometric analyzer system on the base of a scanning grating monochromator in Czerny Turned arrangement (Mcpherson Model 270, dispersion of 6.8 /pixel with a gratin of 150 lines/mm) which was equipped with a Hamamatsu linear image sensor InGaAs-CCD (G9204-512D with 512 pixels). This provides good enough sensitivity in near IR range. Fig. 3 shows the fluorescence spectrum of Nd:YAP crystal from 1000 nm to 1500 nm. The measurement accuracy of this spectrum is 0.7 nm, limited by the grating of the spectrometer. From Fig. 3 it is clear that the emission cross section of Nd:YAP laser at 1432 nm is much smaller than at 1080 nm and 1341 nm. In
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S. Wang et al. / Optics Communications 283 (2010) 2881–2884
Fig. 1. Setup of a free running Nd:YAP laser at 1432 nm.
order to obtain 1432 nm laser operation, the cavity mirrors need special coatings to avoid laser running at 1080 nm and 1341 nm. In our experiments, the HR mirror has a transmission higher than 90% at 1080 nm and 803 nm, around 50% at 1341 nm, less than 1% transmission at 1432 nm. Two different output couplers (OC) are tested in the experiments. One mirror OC1 has a transmission of 4.8% at 1432 nm, more than 75% transmission at 1341 nm, more than 90% transmission at 1080 nm. The other mirror OC2 has a transmission of 9.6% at 1432 nm, more than 80% transmission at 1341 nm, around 70% transmission at 1080 nm. A HP 70950A optical spectrum analyzer (OSA), ranging from 600 to 1700 nm, is used to measure the wavelength of laser output. No laser lines other than 1432 nm are detected with output coupler OC1. With output coupler OC2, if the cavity length is longer than about 300 mm, laser operates at both 1080 nm and 1432 nm. This is because for LD end pumped solid state lasers, the laser output power (i.e. gain) is dependent on the ratio of laser beam to pump beam diameter. The optimum ratio is around 1 [9]. The laser beam diameter is determined by the cavity length and the thermal lens effect. When the cavity is longer and the thermal effect is stronger, the laser beam diameter is larger. Laser operation at higher wavelength suffers from a larger thermal effect [10]. The laser beam diameter is larger for a longer wavelength due to the thermal lens effect [11]. That is to say, the ratio of laser beam to pump beam diameter of a longer wavelength is larger. When the cavity length is small, the ratio at 1432 nm is closer to the optimum value, the net gain of the laser line at 1432 nm is larger than the net gain at 1080 nm and only 1432 nm is running. When the
Fig. 2. An energy level diagram of Nd:YAP laser crystal.
cavity length exceeds a certain value, the ratio at 1432 nm is further away from the optimum value, the net gain of the laser line at 1432 nm is comparable with the net gain at 1080 nm, two laser lines are running together. If we want to avoid the laser emission at 1080 nm with mirror OC2, the transmission at 1080 nm of this mirror needs to be increased. To characterize the laser emission, a wedged glass plate is used outside the cavity to split the output laser (Fig. 1). An InGaAs linear image sensor is taken to measure the laser beam diameter. An energy meter is employed to measure the output energy of the laser. 3. Laser output energy and thermal lens effect Fig. 4 shows the laser pulse energy versus energy of pump pulse at 10 Hz repetition rate. Maximum of 15 mJ output energy is obtained with output mirror OC1 with slope efficiency of 8%, and an optical-tooptical overall efficiency of 6%. Up to 26 mJ output energy is obtained with pump energy of 256 mJ with output mirror OC2. This results in a slope efficiency of 18%, and an optical-to-optical overall efficiency of 10%. The laser output was checked to be linearly polarized. The laser beam propagation factor M2 is measured at around 1.2. Fig. 5 shows the output energy versus the cavity length. In this experiment only OC1 is used in order to keep the laser running only at 1432 nm. The laser output energy firstly increases and then decreases with increase of the cavity length. This is caused by the thermal lens effect, resulting in an increasing cavity mode size with resonator length [11]. As a result, the overlap of laser and pump mode firstly increases and then decreases, so that the output energy also behaves similarly. From Fig. 5, if we assume a linear decrease of output energy dependent on cavity length, the output energy will go to zero when the resonant length is 1282 mm, i.e. x2 ≈ 1240 mm. Zero output means the cavity becomes unstable. We can roughly obtain the thermal lens coefficient by letting focal length of thermal lens
Fig. 3. Fluorescence spectrum of Nd:YAP crystal.
S. Wang et al. / Optics Communications 283 (2010) 2881–2884
Fig. 4. Output energy at 1432 nm versus pump energy of laser diode at 803 nm wavelength with different output coupler transmissions.
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Fig. 6. Measurement of thermal lens coefficient K.
4. Stimulated emission cross section f = x2 ≈ 1240 mm. The pump energy is 256 mJ at repetition rate of 10 Hz, i.e. average pump power of 2.56 W. The thermal lens coefficient is calculated as K = 0.315 m− 1 W− 1. We also use the power maxima of asymmetric flat–flat resonators method [12] to determine the thermal lens coefficient K. With the increase of average pump power, the laser resonator becomes unstable because of the decrease of the focal length of thermal lens. At the point where the average output power decreases, the focal length of thermal lens is equal to the length of longer arm, that is x2 in our experiments. If the critical average pump power at this point is known, K = 1 / (x2Pavc). Here Pavc means the average pump power at the point when the output power decreases. In the experiments of thermal lens coefficient measurement, pump frequency of 30 Hz is used, which is because with higher pump frequency, the average pump power is higher when the pump energy is the same. Then a smaller x2 is needed to reach the unstable region of laser resonator, which results in easier alignment. According to [13], the thermal lens coefficient depends on the average pump power. So the thermal lens coefficient measured with pump repetition rate of 30 Hz should be nearly the same with thermal lens coefficient of 10 Hz. Fig. 6 shows the measurement of critical average pump power dependence on the cavity length x2. The thermal lens coefficient is obtained as K = 0.31 m− 1 W− 1 from the linear fitting of 1/x2 and Pavc, which is nearly the same compared with K = 0.315 m− 1 W− 1 obtained from Fig. 5.
Fig. 5. Nd:YAP laser output with variable cavity length, OC1 is used.
The value of stimulated emission cross section is one of the important parameters to determine the threshold and the efficiency for a given transition in a laser material. However we did not find much information on stimulated emission cross section at 1432 nm of Nd: YAP crystal, except 3.4 × 10− 20 cm2 reported in [3], and ≈10− 20 cm2 in [14]. Because of the large difference between these two values, we try to find out the simulated emission cross section of Nd:YAP crystal at 1432 nm. In this paper two methods are used to determine the emission cross section. First according to [15] the emission cross section σ can be calculated from 2
σ=
hνTπw0 2τs Pout
ð1Þ
where T is the transmission of output coupler, and w0 is the laser beam diameter in the laser rod. The stimulated transition lifetime τs is defined by τs = (Is/I)τ where τ is the fluorescence lifetime of the upper laser level, the saturation parameter Is is defined by Is = hυ/στ. The stimulated transition lifetime τs can be obtained from τs = 1 = f02 τ2c . The relaxation oscillation angular frequency f0 is determined by the second order derivation of the rate equations as the small oscillation rate during the steady state. The photo lifetime of the cavity is τc = ½x1 + x2 + nlrod = ½ðT = 2 + Li Þc, where n is refractive index of laser crystal and lrod is length of the laser crystal. The
Fig. 7. Temporal waveform of the output pulse.
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S. Wang et al. / Optics Communications 283 (2010) 2881–2884
Table 1 Measurement of stimulated emission cross section at 1432 nm, plane cavity, b-axis Nd: YAP rod, 10 mm in length, 9.5 mm in diameter, n = 1.9125 [17], x1 = 43 mm, x2 = 150 mm, T = 4.8%. Peak pump power [W]
412
500
589
677
Average output power during spiking train[W] Beam diameter in rod [mm] Relaxation oscillation circular frequency [KHz] Stimulated emission cross section [× 10− 20 cm2]
24.1
31.6
39.2
44.8
0.964 761.6
1.123 897.6
0.928 966.6
0.961 1047.2
0.8
1.1
0.7
0.8
5. Conclusion
single pass cavity loss Li, which includes the absorption, scattering and diffraction loss, is measured at around 3% by the Findlay–Clay method [16]. The laser beam diameter is measured by the InGaAs sensor, which is placed at a distance x2 away from output coupler. The beam diameter measured with the InGaAs sensor is as large as the beam diameter in the laser rod, for the output coupler is a plane mirror. The laser beam diameter w0 is defined as FWHM of the intensity. Fig. 7 shows the relaxation oscillation of the output pulse. At the end of the pump pulse, the output pulse tends to steady state with some turbulence. The relaxation oscillation angular frequency f0 is determined by the period of fluctuation. Table 1 gives the results of measurement. The measurement error of stimulated emission cross section comes mainly from the inexact measurement of laser beam diameter. From our measurements, the emission cross section of Nd: YAP crystal at 1432 nm is determined as (0.85 ± 0.25) × 10− 20 cm2. We also determined the stimulated emission cross section of Nd: YAP laser crystal at 1432 nm from the fluorescence spectrum. The emission cross section σ can be found by comparing the associated fluorescence intensity I1 to that of another line I2 whose cross section σ2 is known [8]. σ1 = σ2
I1 λ 1 3 : I2 λ 2
0.85× 10− 20 cm2 measured before. The main difference may come from the uncertainty of reflectivity difference between 1341 nm and 1432 nm of the grating in the monochromator.
ð2Þ
If we take the emission cross section at 1341 nm as 1.22 × 10− 19 cm2 from [2], the emission cross section at 1432 nm is determined as 1.33 × 10− 20 cm2 from Eq. (2) according to the fluorescence spectrum measured in Fig. 3. This value is in the same order compared with
LD end pumped Nd:YAP laser at 1432 nm is studied. The emission cross section is estimated at 0.85 × 10− 20 cm2 at 1432 nm, which is around 30 times smaller than at 1080 nm and 15 times smaller than at 1340 nm. Mirrors with high transmission coating at 1080 nm and 1340 nm are necessary to avoid lasing at these two wavelengths. Compared with Nd:YAG crystal which has an emission cross section of 4.5 × 10− 20 cm2 at 1444 nm [4], it is expected that Nd:YAP is less efficient in lasing. Due to its birefringent nature, Nd:YAP is still a promising material at 1.4 µm range because it is polarization maintaining. References [1] Russell L. McCally, C. Brent Bargeron, Jennifer A. Bonney-Ray, W. Richard Green, Johns Hopkins, APL Technical Digest. 26 (2005) 46–55. [2] R. Moncorgé, B. Chambon, J.Y. Rivoire, N. Garnier, E. Descroix, P. Laporte, H. Guillet, S. Roy, J. Mareschal, D. Pelenc, J. Doury, P. Farge, Opt. mater. 8 (1997) 109–119. [3] J.J. Romero, E. Montoya, L.E. Bausá, F. Agulló-Rueda, M.R.B. Andreeta, A.C. Hernandes, Opt. mater. 24 (2004) 643–650. [4] N. Hodgson, W.L. Nighan, D.J. Golding, D. Eisel, Opt. Lett. 19 (1994) 1328–1330. [5] T.Y. Fan, G. Huber, R.L. Byer, P. Mitzscherlich, Opt. Lett. 12 (1987) 678–680. [6] G. Huber, E.W. Duczynski, K. Petermann, IEEE J. Quantum Electron. 24 (1988) 920–923. [7] H.M. Kretschmann, F. Heine, V.G. Ostroumov, G. Huber, Opt. Lett. 22 (1997) 466–468. [8] M.J. Weber, T.E. Varitimos, J. Appl. Phys 42 (1971) 4996–5004. [9] W.P. Risk, J. Opt. Soc. Am. B 5 (1988) 1412–1423. [10] S. Wang, H.J. Eichler, X. Wang, F. Kallmeyer, J. Ge, T. Riesbeck, J. Chen, Appl. Phys. B: Lasers Optics 95 (2009) 721–730. [11] S. Wang, X. Wang, F. Kallmeyer, J. Chen, H.J. Eichler, Appl. Phys. B: Lasers Optics 92 (2008) 43–48. [12] N. Hodgson, H. Weber, Laser resonators and beam propagation, second ed., Springer, USA, 2005, pp. 704–708. [13] H.J. Eichler, A. Haase, R. Menzel, A. Siemoneit, J. Phys. D: Appl. Phys. 26 (1993) 1884–1891. [14] Lasers and Electro-optcs 2001, 169–170 (2001) (The names of the authors were not written on the paper.). [15] H. Shen, T. Lian, R. Zheng, Y. Zhou, G. Yu, C. Huang, H. Liao, Z. Zheng, IEEE J. Quantum Electron. 25 (1989) 144–146. [16] D. Findlay, R.A. Clay, Phys. Lett. 20 (1966) 277–278. [17] Z. Zeng, H. Shen, M. Huang, H. Xu, R. Zeng, Y. Zhou, G. Yu, C. Huang, Appl. Opt. 29 (1990) 1281–1286.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Pulsed Nd:YAP laser at 1432 nm pumped with high power laser diode S. Wang a,⁎, X. Wang a, H. Rhee a, S. Meister a, H.J. Eichler a, J. Chen b a b
Institute of Optic and Atomic Physics, Technical University Berlin, Str. des 17, Juni 135, 10623 Berlin, Germany State Key Laboratory of Modern Optical Instrumentation, Zhejiang University Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 15 March 2010 Accepted 15 March 2010
a b s t r a c t LD end pumped Nd:YAP laser operation in the eye safe spectrum region at 1432 nm is reported. With pump energy of 256 mJ, maximum linearly polarized output of 26 mJ is obtained. The optical-to-optical overall efficiency is around 10%, and the slope efficiency is around 18%. The laser beam operates with spiking mode with a total emission period of less than 300 µs at 10 Hz. The stimulated emission cross section is estimated at around 0.85 × 10− 20 cm2. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The spectral range with wavelengths larger than 1.4 µm is called the eye safe region, because little energy is transmitted to the retina [1]. Water absorption increases very strongly as a function of wavelength and presents several peaks at 1.45, 1.95 and 2.9 µm [2]. Nd lasers working at 1.4 µm and the Er, Tm, Ho lasers working at around 1.54 µm and 2 µm can be used for surgery, because at these wavelengths water molecules present an optical absorption high enough to cut soft tissues without problems of high penetration, but low enough to prevent blood coagulation [2,3]. Also due to the large absorption coefficient in water, laser operation at these wavelengths can be used for remote sensing for detecting water vapor [4]. Er, Tm and Ho doped gain materials have a quasi-three-level nature and they normally show relatively high laser threshold and require efficient cooling [5,6]. Despite the normal laser lines around 1 µm and 1.3 µm, Nd doped crystals have also a laser line around 1.4 µm, which runs as a four level laser. Some papers already reported Nd:YAG laser operation at 1444 nm [4,7]. With flash lamp pumping, a maximum average output of 100 W was obtained at pump pulse energy of 5.5 J [4]. Compared with flash lamp pumping systems, laser diode pumping systems lead to comparatively higher conversion efficiency and are more compact. With continuous-wave (cw) LD pumping, output of 4.9 W was achieved with pump power 40 W [7]. To our knowledge, lasing with Nd:YAP crystals at 1432 nm is only reported in [7]. Output of 2.2 W with 27 W continuous-wave pump power and a slope efficiency of 8% were reported. Compared with Nd: YAG crystal, Nd:YAP crystal exhibits some advantages: it has a high value of thermal conductivity; it is birefringent and the laser emission is polarized. In this paper we investigate an LD end pumped Nd:YAP
⁎ Corresponding author. E-mail address: [email protected] (S. Wang). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.03.035
crystal at 1432 nm with high peak pump power up to around 1 kW in pulsed operation. 2. Experimental setup Fig. 1 shows the setup of a free running Nd:YAP laser at 1432 nm. The pump source (Laserline™) is working at 10 Hz with pulse duration of 300 µs. The pump laser diode provides a maximum peak power of around 1000 W at 803 nm wavelength, which corresponds to the absorption peak of the Nd:YAP laser crystal. The power is delivered by a fiber system (HIGHYAG™) with 1 mm core diameter and NA = 0.2. A telescope system with convex lenses L1 and L2 is used to focus the diode pump laser beam into the Nd:YAP crystal. The focal lengths of L1 and L2 are 100 mm and 150 mm respectively. The active medium is a 0.8% doped Nd:YAP crystal with 10 mm length, 9.5 mm diameter and with the b-axis in the laser beam direction. The crystal is AR coated at 1080 nm, 1341 nm and 1432 nm. Fig. 2 shows the energy level of Nd:YAP laser crystal. Transitions from energy level 4F3/2 to 4I13/2 of Nd:YAP lead to laser operation at 1341 nm and 1432 nm [8]. A spherical bi-convex quartz lens (≈50 mm diameter, 100 mm focal length) and a plane-convex cylindrical quartz lens (50 × 50 mm2, 100 mm focal length) are used to collimate the fluorescence output into the entrance slit of a monochromator. The fluorescence spectrum was measured with a spectrometric analyzer system on the base of a scanning grating monochromator in Czerny Turned arrangement (Mcpherson Model 270, dispersion of 6.8 /pixel with a gratin of 150 lines/mm) which was equipped with a Hamamatsu linear image sensor InGaAs-CCD (G9204-512D with 512 pixels). This provides good enough sensitivity in near IR range. Fig. 3 shows the fluorescence spectrum of Nd:YAP crystal from 1000 nm to 1500 nm. The measurement accuracy of this spectrum is 0.7 nm, limited by the grating of the spectrometer. From Fig. 3 it is clear that the emission cross section of Nd:YAP laser at 1432 nm is much smaller than at 1080 nm and 1341 nm. In
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Fig. 1. Setup of a free running Nd:YAP laser at 1432 nm.
order to obtain 1432 nm laser operation, the cavity mirrors need special coatings to avoid laser running at 1080 nm and 1341 nm. In our experiments, the HR mirror has a transmission higher than 90% at 1080 nm and 803 nm, around 50% at 1341 nm, less than 1% transmission at 1432 nm. Two different output couplers (OC) are tested in the experiments. One mirror OC1 has a transmission of 4.8% at 1432 nm, more than 75% transmission at 1341 nm, more than 90% transmission at 1080 nm. The other mirror OC2 has a transmission of 9.6% at 1432 nm, more than 80% transmission at 1341 nm, around 70% transmission at 1080 nm. A HP 70950A optical spectrum analyzer (OSA), ranging from 600 to 1700 nm, is used to measure the wavelength of laser output. No laser lines other than 1432 nm are detected with output coupler OC1. With output coupler OC2, if the cavity length is longer than about 300 mm, laser operates at both 1080 nm and 1432 nm. This is because for LD end pumped solid state lasers, the laser output power (i.e. gain) is dependent on the ratio of laser beam to pump beam diameter. The optimum ratio is around 1 [9]. The laser beam diameter is determined by the cavity length and the thermal lens effect. When the cavity is longer and the thermal effect is stronger, the laser beam diameter is larger. Laser operation at higher wavelength suffers from a larger thermal effect [10]. The laser beam diameter is larger for a longer wavelength due to the thermal lens effect [11]. That is to say, the ratio of laser beam to pump beam diameter of a longer wavelength is larger. When the cavity length is small, the ratio at 1432 nm is closer to the optimum value, the net gain of the laser line at 1432 nm is larger than the net gain at 1080 nm and only 1432 nm is running. When the
Fig. 2. An energy level diagram of Nd:YAP laser crystal.
cavity length exceeds a certain value, the ratio at 1432 nm is further away from the optimum value, the net gain of the laser line at 1432 nm is comparable with the net gain at 1080 nm, two laser lines are running together. If we want to avoid the laser emission at 1080 nm with mirror OC2, the transmission at 1080 nm of this mirror needs to be increased. To characterize the laser emission, a wedged glass plate is used outside the cavity to split the output laser (Fig. 1). An InGaAs linear image sensor is taken to measure the laser beam diameter. An energy meter is employed to measure the output energy of the laser. 3. Laser output energy and thermal lens effect Fig. 4 shows the laser pulse energy versus energy of pump pulse at 10 Hz repetition rate. Maximum of 15 mJ output energy is obtained with output mirror OC1 with slope efficiency of 8%, and an optical-tooptical overall efficiency of 6%. Up to 26 mJ output energy is obtained with pump energy of 256 mJ with output mirror OC2. This results in a slope efficiency of 18%, and an optical-to-optical overall efficiency of 10%. The laser output was checked to be linearly polarized. The laser beam propagation factor M2 is measured at around 1.2. Fig. 5 shows the output energy versus the cavity length. In this experiment only OC1 is used in order to keep the laser running only at 1432 nm. The laser output energy firstly increases and then decreases with increase of the cavity length. This is caused by the thermal lens effect, resulting in an increasing cavity mode size with resonator length [11]. As a result, the overlap of laser and pump mode firstly increases and then decreases, so that the output energy also behaves similarly. From Fig. 5, if we assume a linear decrease of output energy dependent on cavity length, the output energy will go to zero when the resonant length is 1282 mm, i.e. x2 ≈ 1240 mm. Zero output means the cavity becomes unstable. We can roughly obtain the thermal lens coefficient by letting focal length of thermal lens
Fig. 3. Fluorescence spectrum of Nd:YAP crystal.
S. Wang et al. / Optics Communications 283 (2010) 2881–2884
Fig. 4. Output energy at 1432 nm versus pump energy of laser diode at 803 nm wavelength with different output coupler transmissions.
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Fig. 6. Measurement of thermal lens coefficient K.
4. Stimulated emission cross section f = x2 ≈ 1240 mm. The pump energy is 256 mJ at repetition rate of 10 Hz, i.e. average pump power of 2.56 W. The thermal lens coefficient is calculated as K = 0.315 m− 1 W− 1. We also use the power maxima of asymmetric flat–flat resonators method [12] to determine the thermal lens coefficient K. With the increase of average pump power, the laser resonator becomes unstable because of the decrease of the focal length of thermal lens. At the point where the average output power decreases, the focal length of thermal lens is equal to the length of longer arm, that is x2 in our experiments. If the critical average pump power at this point is known, K = 1 / (x2Pavc). Here Pavc means the average pump power at the point when the output power decreases. In the experiments of thermal lens coefficient measurement, pump frequency of 30 Hz is used, which is because with higher pump frequency, the average pump power is higher when the pump energy is the same. Then a smaller x2 is needed to reach the unstable region of laser resonator, which results in easier alignment. According to [13], the thermal lens coefficient depends on the average pump power. So the thermal lens coefficient measured with pump repetition rate of 30 Hz should be nearly the same with thermal lens coefficient of 10 Hz. Fig. 6 shows the measurement of critical average pump power dependence on the cavity length x2. The thermal lens coefficient is obtained as K = 0.31 m− 1 W− 1 from the linear fitting of 1/x2 and Pavc, which is nearly the same compared with K = 0.315 m− 1 W− 1 obtained from Fig. 5.
Fig. 5. Nd:YAP laser output with variable cavity length, OC1 is used.
The value of stimulated emission cross section is one of the important parameters to determine the threshold and the efficiency for a given transition in a laser material. However we did not find much information on stimulated emission cross section at 1432 nm of Nd: YAP crystal, except 3.4 × 10− 20 cm2 reported in [3], and ≈10− 20 cm2 in [14]. Because of the large difference between these two values, we try to find out the simulated emission cross section of Nd:YAP crystal at 1432 nm. In this paper two methods are used to determine the emission cross section. First according to [15] the emission cross section σ can be calculated from 2
σ=
hνTπw0 2τs Pout
ð1Þ
where T is the transmission of output coupler, and w0 is the laser beam diameter in the laser rod. The stimulated transition lifetime τs is defined by τs = (Is/I)τ where τ is the fluorescence lifetime of the upper laser level, the saturation parameter Is is defined by Is = hυ/στ. The stimulated transition lifetime τs can be obtained from τs = 1 = f02 τ2c . The relaxation oscillation angular frequency f0 is determined by the second order derivation of the rate equations as the small oscillation rate during the steady state. The photo lifetime of the cavity is τc = ½x1 + x2 + nlrod = ½ðT = 2 + Li Þc, where n is refractive index of laser crystal and lrod is length of the laser crystal. The
Fig. 7. Temporal waveform of the output pulse.
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Table 1 Measurement of stimulated emission cross section at 1432 nm, plane cavity, b-axis Nd: YAP rod, 10 mm in length, 9.5 mm in diameter, n = 1.9125 [17], x1 = 43 mm, x2 = 150 mm, T = 4.8%. Peak pump power [W]
412
500
589
677
Average output power during spiking train[W] Beam diameter in rod [mm] Relaxation oscillation circular frequency [KHz] Stimulated emission cross section [× 10− 20 cm2]
24.1
31.6
39.2
44.8
0.964 761.6
1.123 897.6
0.928 966.6
0.961 1047.2
0.8
1.1
0.7
0.8
5. Conclusion
single pass cavity loss Li, which includes the absorption, scattering and diffraction loss, is measured at around 3% by the Findlay–Clay method [16]. The laser beam diameter is measured by the InGaAs sensor, which is placed at a distance x2 away from output coupler. The beam diameter measured with the InGaAs sensor is as large as the beam diameter in the laser rod, for the output coupler is a plane mirror. The laser beam diameter w0 is defined as FWHM of the intensity. Fig. 7 shows the relaxation oscillation of the output pulse. At the end of the pump pulse, the output pulse tends to steady state with some turbulence. The relaxation oscillation angular frequency f0 is determined by the period of fluctuation. Table 1 gives the results of measurement. The measurement error of stimulated emission cross section comes mainly from the inexact measurement of laser beam diameter. From our measurements, the emission cross section of Nd: YAP crystal at 1432 nm is determined as (0.85 ± 0.25) × 10− 20 cm2. We also determined the stimulated emission cross section of Nd: YAP laser crystal at 1432 nm from the fluorescence spectrum. The emission cross section σ can be found by comparing the associated fluorescence intensity I1 to that of another line I2 whose cross section σ2 is known [8]. σ1 = σ2
I1 λ 1 3 : I2 λ 2
0.85× 10− 20 cm2 measured before. The main difference may come from the uncertainty of reflectivity difference between 1341 nm and 1432 nm of the grating in the monochromator.
ð2Þ
If we take the emission cross section at 1341 nm as 1.22 × 10− 19 cm2 from [2], the emission cross section at 1432 nm is determined as 1.33 × 10− 20 cm2 from Eq. (2) according to the fluorescence spectrum measured in Fig. 3. This value is in the same order compared with
LD end pumped Nd:YAP laser at 1432 nm is studied. The emission cross section is estimated at 0.85 × 10− 20 cm2 at 1432 nm, which is around 30 times smaller than at 1080 nm and 15 times smaller than at 1340 nm. Mirrors with high transmission coating at 1080 nm and 1340 nm are necessary to avoid lasing at these two wavelengths. Compared with Nd:YAG crystal which has an emission cross section of 4.5 × 10− 20 cm2 at 1444 nm [4], it is expected that Nd:YAP is less efficient in lasing. Due to its birefringent nature, Nd:YAP is still a promising material at 1.4 µm range because it is polarization maintaining. References [1] Russell L. McCally, C. Brent Bargeron, Jennifer A. Bonney-Ray, W. Richard Green, Johns Hopkins, APL Technical Digest. 26 (2005) 46–55. [2] R. Moncorgé, B. Chambon, J.Y. Rivoire, N. Garnier, E. Descroix, P. Laporte, H. Guillet, S. Roy, J. Mareschal, D. Pelenc, J. Doury, P. Farge, Opt. mater. 8 (1997) 109–119. [3] J.J. Romero, E. Montoya, L.E. Bausá, F. Agulló-Rueda, M.R.B. Andreeta, A.C. Hernandes, Opt. mater. 24 (2004) 643–650. [4] N. Hodgson, W.L. Nighan, D.J. Golding, D. Eisel, Opt. Lett. 19 (1994) 1328–1330. [5] T.Y. Fan, G. Huber, R.L. Byer, P. Mitzscherlich, Opt. Lett. 12 (1987) 678–680. [6] G. Huber, E.W. Duczynski, K. Petermann, IEEE J. Quantum Electron. 24 (1988) 920–923. [7] H.M. Kretschmann, F. Heine, V.G. Ostroumov, G. Huber, Opt. Lett. 22 (1997) 466–468. [8] M.J. Weber, T.E. Varitimos, J. Appl. Phys 42 (1971) 4996–5004. [9] W.P. Risk, J. Opt. Soc. Am. B 5 (1988) 1412–1423. [10] S. Wang, H.J. Eichler, X. Wang, F. Kallmeyer, J. Ge, T. Riesbeck, J. Chen, Appl. Phys. B: Lasers Optics 95 (2009) 721–730. [11] S. Wang, X. Wang, F. Kallmeyer, J. Chen, H.J. Eichler, Appl. Phys. B: Lasers Optics 92 (2008) 43–48. [12] N. Hodgson, H. Weber, Laser resonators and beam propagation, second ed., Springer, USA, 2005, pp. 704–708. [13] H.J. Eichler, A. Haase, R. Menzel, A. Siemoneit, J. Phys. D: Appl. Phys. 26 (1993) 1884–1891. [14] Lasers and Electro-optcs 2001, 169–170 (2001) (The names of the authors were not written on the paper.). [15] H. Shen, T. Lian, R. Zheng, Y. Zhou, G. Yu, C. Huang, H. Liao, Z. Zheng, IEEE J. Quantum Electron. 25 (1989) 144–146. [16] D. Findlay, R.A. Clay, Phys. Lett. 20 (1966) 277–278. [17] Z. Zeng, H. Shen, M. Huang, H. Xu, R. Zeng, Y. Zhou, G. Yu, C. Huang, Appl. Opt. 29 (1990) 1281–1286.