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A linearly polarized 1123 nm Nd:YAG laser using a Fabry–Perot filter as output mirror
Optics Communications 330 (2014) 143–146
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
A linearly polarized 1123 nm Nd:YAG laser using a Fabry–Perot filter as output mirror Zhongfa Wang, Xiaozhong Wang n, Mingchun Cai, Yikun Bu, Lujian Chen, Guoxiong Cai Department of Electronic Engineering, Xiamen University, Xiamen, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 April 2014 Received in revised form 18 May 2014 Accepted 19 May 2014 Available online 2 June 2014
We propose and demonstrate an 1123 nm Nd:YAG laser by exploiting a dielectric Fabry–Perot band-pass filter as laser output mirror. A fiber-pigtailed 808 nm laser diode array is used to pump an o111 4-cut Nd:YAG crystal with a plano-plane resonator cavity. The dielectric Fabry–Perot filter as output mirror is specially designed to suppress the strong emission around 1064 nm and facilitate the oscillation of 1123 nm weak emission. A linearly polarized 1122.5 nm laser is achieved. The maximum output power of the laser is 105 mW and the slope conversion efficiency is 4.58% with the threshold pump power of 2.9 W. Passively Q-switched 1123 nm laser is realized using a [100]-cut Cr:YAG crystal as Q-switch. FWHM pulse width of 121 ns and pulse repetition rate of 2.3 kHz is achieved at pump power of 5.3 W. The design principle of the Fabry–Perot filter used as output mirror is discussed and the advantages of the method are summarized. & 2014 Elsevier B.V. All rights reserved.
Keywords: Diode-pumped lasers Fabry–Perot filter polarization
1. Introduction Nd:YAG crystal is the most widely-used laser gain medium because of its excellent optical characteristics and mechanical properties. The output wavelengths of a Nd:YAG laser are usually focused at 1064, 1319 and 946 nm. Typical room temperature emission spectrum for the 4F3/2–4I11/2 transition of a Nd:YAG crystal shows that there are many emission bands, include 1052 nm, 1064 nm, 1073 nm, 1112 nm, 1116 nm and 1123 nm [1]. The output wavelengths of lasers based on the 4F3/2–4I11/2 transition of a Nd:YAG crystals are mostly around 1064 nm. Other wavelengths are usually suppressed by the stronger transitions near 1064 nm due to their lower stimulated emission intensity. In recent years, Nd:YAG lasers with output wavelengths 1123 nm have attracted the interests of many researchers. 1123 nm laser can be frequency-doubled to generate yellow laser for hyperfine spectra measurement of iodine molecule [2], used as laser source for differential absorption lidar to allow remote monitoring of atmospheric water vapor concentration [3]. 1123 nm laser can also be used as a pump source for thulium fiber laser to generate blue laser [4]. In order to suppress the 1064 nm emission oscillation and obtain the 1123 nm laser, specially coated mirrors are usually exploited [5–13]. Sometimes an etalon is used alone [14] or
n
Corresponding author. Fax: þ 86 592 2580141. E-mail addresses: [email protected], [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.optcom.2014.05.044 0030-4018/& 2014 Elsevier B.V. All rights reserved.
together with the specially coated mirrors [5,12] to obtain the 1123 nm laser. The etalon method makes the cavity complicated, while the specially coated mirrors have complex designs and demanding coating fabrication, since the wavelength of the suppressed bands is close to the desired band. In contrast, a Fabry–Perot band-pass filter is a traditional device and the separation of the pass bands can be as small as 6 pm [15]. The pass band of a Fabry–Perot filter (FPF) can be used to suppress the oscillations of stronger emissions in order to emphasize and obtain the oscillation of weak emissions in a laser, which may substitute for the special coated mirror or etalon method. In this paper, we use a Fabry–Perot filter as an output coupler for a Nd:YAG laser. The oscillation of the stronger emission near 1064 nm is suppressed and the desired 1123 nm laser output is obtained. The design of the FPF, the plan using the FPF as output mirror and the experimental setup are introduced in Section 2. The experimental results and analysis are presented in Section 3, followed by a brief conclusion in Section 4.
2. Filter design and experimental setup Fabry–Perot filter (FPF), a narrow band-pass filter, is usually an all-dielectric filter consisting of quarter-wave optical thickness layers for the mirrors and half-wave optical thickness or multiple half-wave optical thickness layers for the spacers [16]. In this paper, a single cavity Fabry–Perot band-pass filter with pass band centered at 1064 nm is designed and fabricated. The single-cavity
144
Z. Wang et al. / Optics Communications 330 (2014) 143–146
FPF is composed of quarter-wave isotropic layers of alternating high and low refractive indices, denoted as H and L, respectively. The stacked layer structure is substrate /(HL)\widehat5 2H (LH) \widehat5 /air and the phase thickness values of H, L layers are all π/2 at wavelength 1064 nm. The substrate is a 3-mm-thick parallel plane surfaces BK7 glass block. The high refractive index layers are composed of tantalum oxide (Ta2O5) while the low refractive index layers are composed of silica (SiO2). The two central high index layers form a half-wave spacer layer. The filter was deposited by a remote plasma magnetron sputtering process (System Control Technology S500). The measured transmission curve of the FPF is shown in Fig. 1, and shows that the peak transmission is 30.4% and is located at 1065.3 nm, due to a slight error in the deposition rate. The transmissions at 1064 nm, 1112 nm, 1116 nm and 1123 nm are 30%, 0.85%, 0.66% and 0.48%, respectively. There are totally 21 layers in the film. In contrast, the film in the specially coated mirrors should be about 50 layers to obtain the same transmission features. The experimental setup is shown schematically in Fig. 2. Light from a fiber-pigtailed 808 nm pump source (LIMO) is firstly collimated and depolarized, then focused on the o111 4-cut Nd:YAG by a plano-convex lens (f ¼15.29 mm). The diameter of the pump beam on the surface of Nd:YAG is about 190 mm. The 0.5% doped Nd:YAG (CASTECH INC.) is 3 mm 3 mm 8 mm, enclosed in indium foil and passively cooled in a copper block. The input surface of the Nd:YAG is anti-reflection-coated at 808 nm, 946 nm, 1319 nm and high-reflection coated near 1064 and 1123 nm, forming the front mirror (M1) of the laser cavity. The second surface of the Nd:YAG crystal is anti-reflection-coated around 1064 nm and 1123 nm. The FPF forms the output mirror
for the 63.5 mm-long laser cavity. The Fabry–Perot filter is installed on a 5-dimentional stage, which allows the FPF can be adjusted with the following 5 freedoms, X, Y, Z, tilt about X, tilt about Y with high precision. The 5-dimentional stage is fastened on a shockproof experimental table. Experiments showed that the FPF used as the output coupler can effectively suppress the oscillation near 1064 nm to obtain an 1123 nm laser output. The laser output is firstly filtered by a RG830 glass to absorb the residual 808 nm pump light, then measured by a power meter (COHERENT PM10) and an optical spectrum analyzer (HP 70951B).
3. Results and discussion Fig. 3 shows relationship between the output power and the pump power. The threshold pump power is 2.9 W. The slope efficiency is 4.58%. The maximum power obtained is 105 mW with pump power of 5.3 W. Thermal gradient with higher pump power may damage the crystal as the crystal is passively cooled. The output power could be higher with active cooling of the laser crystal. The low laser slope efficiency arises from the high reflection at 1123 nm of the Fabry–Perot filter using as the output mirror. The high refection is necessary to suppress the lasing of 1064 nm. If the transmission of the output mirror at 1064 nm is higher, then the transmission at 1123 nm could also be higher, which will improve the laser slope efficiency. Fig. 4 shows the spectrum of the laser. The center wavelength is 1122.5 nm. A Glan–Taylor polarizer is used to measure the
Fig. 3. Continuous output power properties of the Nd:YAG laser.
Fig. 1. Transmission curve of the Fabry–Perot filter (FPF). (a) Transmission outline of the manufactured FPF, (b) Detailed transmission curve around 1064 nm.
808nm pump source
T@808nm:95% M1: R@1123nm:99.75% T@1064nm:0.23% Fiber
FPF T@1064nm:30% T@1123nm: 0.48%
Depolarizer
Collimator
Focus lens Nd:YAG
Fig. 2. Experimental setup.
RG830 Fig. 4. Optical spectrum and polarization of the laser.
Z. Wang et al. / Optics Communications 330 (2014) 143–146
polarization state of the output laser. The variation of power with the transmission axis orientation angle of the polarizer is shown in the inset of Fig. 4, which shows that the laser output is linearly polarized. As the laser crystal and the cavity is isotropic, the FPF may account for the polarized laser output. Measuring results show that the polarizing angle of the laser output changes with the orientation of the FPF. The polarization feature of the FPF needs to be further investigated. Furthermore, the passively Q-switched characteristics of the laser are investigated. A [100]-cut Cr:YAG crystal with initial transmission of 99% is inserted between the laser crystal and the FPF. The Cr:YAG crystal is installed on a rotation stage and rotated to align the polarization direction of the 1123 nm laser with the [010]- or [001]-axis of the Cr:YAG crystal to maximize the output power [17]. The results are shown in Fig. 5 and Fig. 6. With the increase of the pump power, the pulse repetition rate is increased and the pulse width is decreased, which is in accordance with the typical results of passively Q-switched lasers [17]. The pulse shape at pump power of 5.3 W is shown in Fig. 6. The FWHM pulse width is about 121 ns and the pulse repetition rate is 2.3 kHz. The threshold condition for a diode-end-pumped solid-state laser can be written as [18], ln R1i þ Li hvp 1 P th;i ¼ 2lηi f i si τi ∭ si ðr; zÞr p ðr; zÞdv i ¼ 0; 1 ð1Þ
Fig. 5. Passively Q-switched characteristics of the laser.
Fig. 6. Pulse shape with pump power of 5.3 W. Upper right inset: pulse repetition rate features.
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where subscript i refers for the i transition wavelength, Ri is the reflectivity of the output mirror, Li is the round-trip cavity loss, l is the length of the laser crystal, ηi is the quantum efficiency, f i is the fraction of the 4F3/2 population that resides in the Stark component used as the upper laser level, hνp is the pump photon energy, si is the emission cross-section, τi is the fluorescence lifetime at upper level, si ðr; zÞ is the normalized cavity mode intensity distribution, and r p ðr; zÞ is the normalized pump intensity distribution in the active medium, i ¼ 0 is for the 1064 nm, and i ¼ 1 is for the 1123 nm. The two wavelengths have the same upper 4F3/2 laser level with close wavelengths in the same cavity, so s0 ðr; zÞ ¼ s1 ðr; zÞ, r 0 ðr; zÞ ¼ r 1 ðr; zÞ, L0 ¼ L1 , and τ0 ¼ τ1 . Therefore, the threshold can be expressed as: ln R1i þ Li P th;i ¼ C ηi si f i hvp 1 C¼ 2lτi ∭ si ðr; zÞr p ðr; zÞdv i ¼ 0; 1 ð2Þ where C is a equal parameter for the two wavelengths. In order to suppress the oscillating around 1064 nm, the threshold for 1123 nm laser should be lower, P th;1 o P th;0 , which means that ln R11 þ L1 ln R10 þ L0 oC ¼ P th;0 ð3Þ P th;1 ¼ C η 1 s1 f 1 η0 s0 f 0 For the o1 1 1 4-cut Nd:YAG crystals, η0 ¼ 808 nm=1064 nm ¼ 75:9%, η1 ¼ 808 nm=1123 nm ¼ 72%, s0 ¼ 4:6 10 19 cm2 , s1 ¼ 0:3 10 19 cm2 , f 0 ¼ 40%, f 1 ¼ 60%, and L0 is assumed to 0.01. In our experiment, R0 ¼ 70%, R1 ¼ 99:52%. Taking the above-known data into Eq. (3), we get P th;1 o P th;0 . Therefore, the 1064 nm is effectively suppressed. Following the same procedure, we could show that the 1112 nm and 1116 nm can also be suppressed. The measured peak transmission wavelength of the FPF is slightly shifted from the wavelength anticipated by the theory, because the layer thickness was controlled by deposition time. A more precise method to control the layer thickness in the filter deposition would result in smaller shift of the peak wavelength. The design of Fabry–Perot filter is important to obtain the desired results. One main designing principle of the FPF is that the FWHM of the FPF (30 nm in this paper) should be smaller than the spectral separation between the suppressed wavelength and the desired wavelength (59 nm in this paper). This principle can guarantee that the suppressed wavelength is located at the center of the pass band and the desired wavelength lies outside of the pass band, thus brings maximum loss for the former and enough gain for the later. The measured transmission at 1064 nm is 30% in our filter, which is enough to suppress the oscillation of the 1064 nm emission for the present pump power level. The transmission at 1064 nm should be increased with the increase of the pump power to effectively suppress its oscillation. The relationship between the transmission at the suppressed wavelength and the working power level need to be further studied. Compare with the etalon or specially coated mirror methods, the Fabry–Perot filter method presented in this paper has several advantages. Firstly, the selection of the oscillation wavelength is easy and precise. All you need is using a Fabry–Perot filter with its pass band located near the suppressed wavelength and with the desired wavelength outside or on the edge of the pass band. Secondly, the wavelength separation between the two wavelengths is widely variable, which can be as small as 6 pm [15] and as big as possible decided by the laser material. Thirdly, the Fabry–Perot filter is easy to design and fabricate. The coating
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composed of 21 layers. In comparison, the coating of the specially coated mirrors is usually around 50 layers to obtain the same effect. 4. Conclusion In summary, a Fabry–Perot band-pass filter has been used as the output coupler of a solid-state laser. A single cavity Fabry– Perot band-pass filter with pass band centered at 1064 nm was designed and manufactured. The FPF was used as an output coupler of an o111 4-cut Nd:YAG laser to suppress the strong emission near 1064 nm. A linearly polarized 1122.5 nm laser with output power of 105 mW and slope efficiency of 4.58% was thus obtained. The passive-Q-switch characteristics of the laser are investigated. With the increase of the pump power, the pulse repetition rate increases while the pulse width decreases. The FWHM pulse width is about 121 ns and the pulse repetition rate is 2.3 kHz at pump power of 5.3 W. The design principle of the Fabry–Perot filter used as output mirror is discussed and the advantages of the method are summarized. Acknowledgments This work is financially supported by the China Scholarship Council (CSC) (2010631516) and the Natural Science Foundation of Fujian Province(2013J01254).
References [1] W. Koechner, Solid-State Laser Engineering, Springer, New York, 1992. [2] T. Yang, F. Meng, Y. Zhao, Y. Peng, Y. Li, J. Cao, C. Gao, Z. Fang, E. Zang, Appl. Phys. B 106 (2012) 613. [3] S. Lehmann, J. Bosenberg, in: A. Ansmann, R. Neuber, P. Rairoux, U. Wandinger (Eds.), Advances in Atmospheric Remote Sensing with Lidar, Selected Papers of the 18th International Laser Radar Conference, Springer-Verlag, 1996, pp. 309–312. [4] R. Paschotta, N. Moore, W.A. Clarkson, A.C. Tropper, D.C. Hanna, G. Maze, IEEE J. Quantum Electron. 3 (1997) 1100. [5] N. Moore, W.A. Clarkson, D.C. Hanna, S. Lehmann, J. Bö senberg, Appl. Opt. 38 (1999) 5761. [6] Y.F. Chen, Y.P. Lan, S.W. Tsai, Opt. Commun. 234 (2004) 309. [7] X.P. Guo, M. Chen, G. Li, B.Y. Zhang, J.D. Yang, Z.G. Zhang, Y.G. Wang, Chin. Opt. Lett. 2 (2004) 402. [8] E.J. Zang, J.P. Cao, Y. Li, T. Yang, D.M. Hong, Opt. Lett. 32 (2007) 250. [9] S.S. Zhang, Q.P. Wang, X.Y. Zhang, Z.J. Liu, W.J. Sun, S.W. Wang, Laser Phys. 19 (2009) 2159. [10] C.Y. Li, Y. Bo, F. Yang, Z.C. Wang, Y.T. Xu, Y.B. Wang, H.W. Gao, Q.J. Peng, D.F. Cui, Z.Y. Xu, Opt. Express 18 (2010) 7923. [11] C.Y. Li, Y. Bo, Y.T. Xu, F. Yang, Z.C. Wang, B.S. Wang, J.L. Xu, H.W. Gao, Q.J. Peng, D.F. Cui, Z.Y. Xu, Opt. Commun. 283 (2010) 2885. [12] Z. Wang, Y. Bo, S. Xie, C. Li, Y. Xu, F. Yang, J. Xu, Q. Peng, J. Zhang, D. Cui, Z. Xu, Appl. Phys. B 104 (2011) 45. [13] Y. Gao, T.Z. Zhao, C.Y. Li, W.Q. Ge, Q. Wu, Z.H. Shi, G. Niu, J. Yu, Z.W. Fan, Y. G. Wang, Opt. Commun. 286 (2013) 261. [14] J. Marling, IEEE J. Quantum Electron. 14 (1978) 56. [15] Z. Al-Qazwini, H. Kim, J. Lightwave Technol. 30 (2012) 3157. [16] H.A. Macleod, Thin-Film Optical Filters, 3rd ed., Institute of Physics Publishing, Bristol and Philadelphia, 2001. [17] Y.F. Ma, X. Yu, F.K. Tittel, R.P. Yan, X.D. Li, C. Wang, J.H. Yu, Appl. Phys. B 107 (2012) 339. [18] T.Y. Fan, R.L. Byer, IEEE J. Quantum Electron. 24 (1988) 895.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
A linearly polarized 1123 nm Nd:YAG laser using a Fabry–Perot filter as output mirror Zhongfa Wang, Xiaozhong Wang n, Mingchun Cai, Yikun Bu, Lujian Chen, Guoxiong Cai Department of Electronic Engineering, Xiamen University, Xiamen, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 April 2014 Received in revised form 18 May 2014 Accepted 19 May 2014 Available online 2 June 2014
We propose and demonstrate an 1123 nm Nd:YAG laser by exploiting a dielectric Fabry–Perot band-pass filter as laser output mirror. A fiber-pigtailed 808 nm laser diode array is used to pump an o111 4-cut Nd:YAG crystal with a plano-plane resonator cavity. The dielectric Fabry–Perot filter as output mirror is specially designed to suppress the strong emission around 1064 nm and facilitate the oscillation of 1123 nm weak emission. A linearly polarized 1122.5 nm laser is achieved. The maximum output power of the laser is 105 mW and the slope conversion efficiency is 4.58% with the threshold pump power of 2.9 W. Passively Q-switched 1123 nm laser is realized using a [100]-cut Cr:YAG crystal as Q-switch. FWHM pulse width of 121 ns and pulse repetition rate of 2.3 kHz is achieved at pump power of 5.3 W. The design principle of the Fabry–Perot filter used as output mirror is discussed and the advantages of the method are summarized. & 2014 Elsevier B.V. All rights reserved.
Keywords: Diode-pumped lasers Fabry–Perot filter polarization
1. Introduction Nd:YAG crystal is the most widely-used laser gain medium because of its excellent optical characteristics and mechanical properties. The output wavelengths of a Nd:YAG laser are usually focused at 1064, 1319 and 946 nm. Typical room temperature emission spectrum for the 4F3/2–4I11/2 transition of a Nd:YAG crystal shows that there are many emission bands, include 1052 nm, 1064 nm, 1073 nm, 1112 nm, 1116 nm and 1123 nm [1]. The output wavelengths of lasers based on the 4F3/2–4I11/2 transition of a Nd:YAG crystals are mostly around 1064 nm. Other wavelengths are usually suppressed by the stronger transitions near 1064 nm due to their lower stimulated emission intensity. In recent years, Nd:YAG lasers with output wavelengths 1123 nm have attracted the interests of many researchers. 1123 nm laser can be frequency-doubled to generate yellow laser for hyperfine spectra measurement of iodine molecule [2], used as laser source for differential absorption lidar to allow remote monitoring of atmospheric water vapor concentration [3]. 1123 nm laser can also be used as a pump source for thulium fiber laser to generate blue laser [4]. In order to suppress the 1064 nm emission oscillation and obtain the 1123 nm laser, specially coated mirrors are usually exploited [5–13]. Sometimes an etalon is used alone [14] or
n
Corresponding author. Fax: þ 86 592 2580141. E-mail addresses: [email protected], [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.optcom.2014.05.044 0030-4018/& 2014 Elsevier B.V. All rights reserved.
together with the specially coated mirrors [5,12] to obtain the 1123 nm laser. The etalon method makes the cavity complicated, while the specially coated mirrors have complex designs and demanding coating fabrication, since the wavelength of the suppressed bands is close to the desired band. In contrast, a Fabry–Perot band-pass filter is a traditional device and the separation of the pass bands can be as small as 6 pm [15]. The pass band of a Fabry–Perot filter (FPF) can be used to suppress the oscillations of stronger emissions in order to emphasize and obtain the oscillation of weak emissions in a laser, which may substitute for the special coated mirror or etalon method. In this paper, we use a Fabry–Perot filter as an output coupler for a Nd:YAG laser. The oscillation of the stronger emission near 1064 nm is suppressed and the desired 1123 nm laser output is obtained. The design of the FPF, the plan using the FPF as output mirror and the experimental setup are introduced in Section 2. The experimental results and analysis are presented in Section 3, followed by a brief conclusion in Section 4.
2. Filter design and experimental setup Fabry–Perot filter (FPF), a narrow band-pass filter, is usually an all-dielectric filter consisting of quarter-wave optical thickness layers for the mirrors and half-wave optical thickness or multiple half-wave optical thickness layers for the spacers [16]. In this paper, a single cavity Fabry–Perot band-pass filter with pass band centered at 1064 nm is designed and fabricated. The single-cavity
144
Z. Wang et al. / Optics Communications 330 (2014) 143–146
FPF is composed of quarter-wave isotropic layers of alternating high and low refractive indices, denoted as H and L, respectively. The stacked layer structure is substrate /(HL)\widehat5 2H (LH) \widehat5 /air and the phase thickness values of H, L layers are all π/2 at wavelength 1064 nm. The substrate is a 3-mm-thick parallel plane surfaces BK7 glass block. The high refractive index layers are composed of tantalum oxide (Ta2O5) while the low refractive index layers are composed of silica (SiO2). The two central high index layers form a half-wave spacer layer. The filter was deposited by a remote plasma magnetron sputtering process (System Control Technology S500). The measured transmission curve of the FPF is shown in Fig. 1, and shows that the peak transmission is 30.4% and is located at 1065.3 nm, due to a slight error in the deposition rate. The transmissions at 1064 nm, 1112 nm, 1116 nm and 1123 nm are 30%, 0.85%, 0.66% and 0.48%, respectively. There are totally 21 layers in the film. In contrast, the film in the specially coated mirrors should be about 50 layers to obtain the same transmission features. The experimental setup is shown schematically in Fig. 2. Light from a fiber-pigtailed 808 nm pump source (LIMO) is firstly collimated and depolarized, then focused on the o111 4-cut Nd:YAG by a plano-convex lens (f ¼15.29 mm). The diameter of the pump beam on the surface of Nd:YAG is about 190 mm. The 0.5% doped Nd:YAG (CASTECH INC.) is 3 mm 3 mm 8 mm, enclosed in indium foil and passively cooled in a copper block. The input surface of the Nd:YAG is anti-reflection-coated at 808 nm, 946 nm, 1319 nm and high-reflection coated near 1064 and 1123 nm, forming the front mirror (M1) of the laser cavity. The second surface of the Nd:YAG crystal is anti-reflection-coated around 1064 nm and 1123 nm. The FPF forms the output mirror
for the 63.5 mm-long laser cavity. The Fabry–Perot filter is installed on a 5-dimentional stage, which allows the FPF can be adjusted with the following 5 freedoms, X, Y, Z, tilt about X, tilt about Y with high precision. The 5-dimentional stage is fastened on a shockproof experimental table. Experiments showed that the FPF used as the output coupler can effectively suppress the oscillation near 1064 nm to obtain an 1123 nm laser output. The laser output is firstly filtered by a RG830 glass to absorb the residual 808 nm pump light, then measured by a power meter (COHERENT PM10) and an optical spectrum analyzer (HP 70951B).
3. Results and discussion Fig. 3 shows relationship between the output power and the pump power. The threshold pump power is 2.9 W. The slope efficiency is 4.58%. The maximum power obtained is 105 mW with pump power of 5.3 W. Thermal gradient with higher pump power may damage the crystal as the crystal is passively cooled. The output power could be higher with active cooling of the laser crystal. The low laser slope efficiency arises from the high reflection at 1123 nm of the Fabry–Perot filter using as the output mirror. The high refection is necessary to suppress the lasing of 1064 nm. If the transmission of the output mirror at 1064 nm is higher, then the transmission at 1123 nm could also be higher, which will improve the laser slope efficiency. Fig. 4 shows the spectrum of the laser. The center wavelength is 1122.5 nm. A Glan–Taylor polarizer is used to measure the
Fig. 3. Continuous output power properties of the Nd:YAG laser.
Fig. 1. Transmission curve of the Fabry–Perot filter (FPF). (a) Transmission outline of the manufactured FPF, (b) Detailed transmission curve around 1064 nm.
808nm pump source
T@808nm:95% M1: R@1123nm:99.75% T@1064nm:0.23% Fiber
FPF T@1064nm:30% T@1123nm: 0.48%
Depolarizer
Collimator
Focus lens Nd:YAG
Fig. 2. Experimental setup.
RG830 Fig. 4. Optical spectrum and polarization of the laser.
Z. Wang et al. / Optics Communications 330 (2014) 143–146
polarization state of the output laser. The variation of power with the transmission axis orientation angle of the polarizer is shown in the inset of Fig. 4, which shows that the laser output is linearly polarized. As the laser crystal and the cavity is isotropic, the FPF may account for the polarized laser output. Measuring results show that the polarizing angle of the laser output changes with the orientation of the FPF. The polarization feature of the FPF needs to be further investigated. Furthermore, the passively Q-switched characteristics of the laser are investigated. A [100]-cut Cr:YAG crystal with initial transmission of 99% is inserted between the laser crystal and the FPF. The Cr:YAG crystal is installed on a rotation stage and rotated to align the polarization direction of the 1123 nm laser with the [010]- or [001]-axis of the Cr:YAG crystal to maximize the output power [17]. The results are shown in Fig. 5 and Fig. 6. With the increase of the pump power, the pulse repetition rate is increased and the pulse width is decreased, which is in accordance with the typical results of passively Q-switched lasers [17]. The pulse shape at pump power of 5.3 W is shown in Fig. 6. The FWHM pulse width is about 121 ns and the pulse repetition rate is 2.3 kHz. The threshold condition for a diode-end-pumped solid-state laser can be written as [18], ln R1i þ Li hvp 1 P th;i ¼ 2lηi f i si τi ∭ si ðr; zÞr p ðr; zÞdv i ¼ 0; 1 ð1Þ
Fig. 5. Passively Q-switched characteristics of the laser.
Fig. 6. Pulse shape with pump power of 5.3 W. Upper right inset: pulse repetition rate features.
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where subscript i refers for the i transition wavelength, Ri is the reflectivity of the output mirror, Li is the round-trip cavity loss, l is the length of the laser crystal, ηi is the quantum efficiency, f i is the fraction of the 4F3/2 population that resides in the Stark component used as the upper laser level, hνp is the pump photon energy, si is the emission cross-section, τi is the fluorescence lifetime at upper level, si ðr; zÞ is the normalized cavity mode intensity distribution, and r p ðr; zÞ is the normalized pump intensity distribution in the active medium, i ¼ 0 is for the 1064 nm, and i ¼ 1 is for the 1123 nm. The two wavelengths have the same upper 4F3/2 laser level with close wavelengths in the same cavity, so s0 ðr; zÞ ¼ s1 ðr; zÞ, r 0 ðr; zÞ ¼ r 1 ðr; zÞ, L0 ¼ L1 , and τ0 ¼ τ1 . Therefore, the threshold can be expressed as: ln R1i þ Li P th;i ¼ C ηi si f i hvp 1 C¼ 2lτi ∭ si ðr; zÞr p ðr; zÞdv i ¼ 0; 1 ð2Þ where C is a equal parameter for the two wavelengths. In order to suppress the oscillating around 1064 nm, the threshold for 1123 nm laser should be lower, P th;1 o P th;0 , which means that ln R11 þ L1 ln R10 þ L0 oC ¼ P th;0 ð3Þ P th;1 ¼ C η 1 s1 f 1 η0 s0 f 0 For the o1 1 1 4-cut Nd:YAG crystals, η0 ¼ 808 nm=1064 nm ¼ 75:9%, η1 ¼ 808 nm=1123 nm ¼ 72%, s0 ¼ 4:6 10 19 cm2 , s1 ¼ 0:3 10 19 cm2 , f 0 ¼ 40%, f 1 ¼ 60%, and L0 is assumed to 0.01. In our experiment, R0 ¼ 70%, R1 ¼ 99:52%. Taking the above-known data into Eq. (3), we get P th;1 o P th;0 . Therefore, the 1064 nm is effectively suppressed. Following the same procedure, we could show that the 1112 nm and 1116 nm can also be suppressed. The measured peak transmission wavelength of the FPF is slightly shifted from the wavelength anticipated by the theory, because the layer thickness was controlled by deposition time. A more precise method to control the layer thickness in the filter deposition would result in smaller shift of the peak wavelength. The design of Fabry–Perot filter is important to obtain the desired results. One main designing principle of the FPF is that the FWHM of the FPF (30 nm in this paper) should be smaller than the spectral separation between the suppressed wavelength and the desired wavelength (59 nm in this paper). This principle can guarantee that the suppressed wavelength is located at the center of the pass band and the desired wavelength lies outside of the pass band, thus brings maximum loss for the former and enough gain for the later. The measured transmission at 1064 nm is 30% in our filter, which is enough to suppress the oscillation of the 1064 nm emission for the present pump power level. The transmission at 1064 nm should be increased with the increase of the pump power to effectively suppress its oscillation. The relationship between the transmission at the suppressed wavelength and the working power level need to be further studied. Compare with the etalon or specially coated mirror methods, the Fabry–Perot filter method presented in this paper has several advantages. Firstly, the selection of the oscillation wavelength is easy and precise. All you need is using a Fabry–Perot filter with its pass band located near the suppressed wavelength and with the desired wavelength outside or on the edge of the pass band. Secondly, the wavelength separation between the two wavelengths is widely variable, which can be as small as 6 pm [15] and as big as possible decided by the laser material. Thirdly, the Fabry–Perot filter is easy to design and fabricate. The coating
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composed of 21 layers. In comparison, the coating of the specially coated mirrors is usually around 50 layers to obtain the same effect. 4. Conclusion In summary, a Fabry–Perot band-pass filter has been used as the output coupler of a solid-state laser. A single cavity Fabry– Perot band-pass filter with pass band centered at 1064 nm was designed and manufactured. The FPF was used as an output coupler of an o111 4-cut Nd:YAG laser to suppress the strong emission near 1064 nm. A linearly polarized 1122.5 nm laser with output power of 105 mW and slope efficiency of 4.58% was thus obtained. The passive-Q-switch characteristics of the laser are investigated. With the increase of the pump power, the pulse repetition rate increases while the pulse width decreases. The FWHM pulse width is about 121 ns and the pulse repetition rate is 2.3 kHz at pump power of 5.3 W. The design principle of the Fabry–Perot filter used as output mirror is discussed and the advantages of the method are summarized. Acknowledgments This work is financially supported by the China Scholarship Council (CSC) (2010631516) and the Natural Science Foundation of Fujian Province(2013J01254).
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