Study of a QCW Light-emitting-diode (LED)-pumped Solid-state Laser

Transcription

1 Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011, pp Study of a QCW Light-emitting-diode (LED)-pumped Solid-state Laser Kangin Lee, Sangyoon Bae, Jin Seog Gwag, Jin Hyuk Kwon and Jonghoon Yi Department of Physics, Yeungnam University, Gyeungsan , Korea (Received 31 December 2010, in final form 8 September 2011) The lasing of solid-state lasers pumped by light emitting diodes (LEDs) was studied to replace the quasi-continuous-wave (QCW) laser diode in pulse laser pumping. The investigated solid-state gain media included Nd-doped solid-state materials (Nd:YAG, Nd:glass, Nd/Cr:YAG), Ti:sapphire, and solid dye. The gain medium was surrounded by arrays of LEDs very closely. The distribution of the LED radiation absorbed in the gain medium was calculated by using non-sequential ray tracing software. The calculated data transferred to the cavity analysis software and the lasing characteristics were simulated. The calculated results for the absorbed LED distribution and the absorption efficiency in the Nd:YAG rod were compared to experimentally measured fluorescence profile and the absorption efficiency and were found to be accurate within an error of 11%. Among the investigated gain media, Nd/Cr:YAG showed the lowest lasing threshold. We also found that the use of reflector in the pumping chamber could lower the lasing threshold of Nd:YAG to half the lasing threshold without the reflector. PACS numbers: Jb, Rz, Hj Keywords: Light-emitting-diode, LED, Solid-state laser DOI: /jkps I. INTRODUCTION Solid-state lasers have been intensively developed over the past several decades for their wide applications in industry and the military [1 3]. Solid-state lasers pumped by arc lamps are rapidly being replaced by diode-pumped solid-state lasers (DPSSLs). Ti:sapphire lasers and solid dye lasers have also recently been pumped by DPSS green lasers, instead of Ar + ion lasers. Still, most solidstate lasers are pumped by CW diode lasers because quasi-continuous-wave (QCW) laser diodes are very expensive as replacement for flash lamps. Thus, flash-lamppumped solid-state lasers account for a large portion of the high-energy, low-repetition-rate pulse laser market even though they generate a large amount of heat and have short lifetimes. About 40 years ago, the light-emitting diode (LED) was suggested as a pump source for solid-state lasers [4 10]. During early development, the LED had a poor electricity-to-light conversion efficiency, and the output power was very low. Reinberg et al. cooled the LED and the gain material to an extremely low temperature of 77 K to improve the conversion efficiency and the lifetime of the LED [4]. To overcome low absorption efficiency, a Nd:YAG single crystal fiber was used as a gain medium [6,7]. Farmer and Kiang used a gold reflector to concentrate highly diverging LED light to the Nd:YAG rod jhyi@yu.ac.kr [8]. Due to the low output power of the LED, there have been rare reports on LED-pumped lasers since several early developments. The concept of pumping using a semi-conductor-based light source with DPSSL has succeeded. Recently, LEDs, which have low cost and high output power, have developed rapidly for display and illumination applications, and their applications in laser pumping are gaining attention [11 16]. Yang et al. pumped a polymer waveguide by using an InGaN blue LED [13]. They used 10 times higher peak current compared with the normal CW driving current of a LED to get enough pump intensity for lasing. The high cost of diode lasers hinders wide application of DPSSLs. Further, diode lasers are easily damaged by humidity, static electricity, and dust. Pumping using LEDs has merits not only in cost but also in many practical aspects. LEDs are designed to resist static discharge. The emitter is encapsulated by a molded lens, isolating dust and humidity. A broad range of LED spectra allows direct pumping of a tunable gain material such as Ti:sapphire, Alexandrite, or solid dye. As CW laser diodes have different emitter designs, compared with QCW diode lasers, to manage generated heat, they can be operated only in the CW mode. In the case of LED pumping, a laser can be operated in CW, as well as QCW, modes with the same LED source. Even with traditional pump sources such as flash-lamps or arclamps, operation in both modes with the same lamp is impractical. In this work, we investigate the performance of a LED

2 Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011 Fig. 1. (Color online) Structure of the investigated LEDpumped solid-state laser. pumped solid-state laser that uses a very simple pumping chamber structure similar to that of the laser side pumped by diode lasers. When a QCW current is applied to the LED, the peak output power from LED can be increased to several times higher than the CW output power even with the same LED. In previous studies [4 10], special LED chips with spectra matched with the absorption bands of the gain media were especially fabricated for pumping. In this work, we use commercial LEDs with dome lenses on chips that have been developed for illumination applications, and are easily affordable. Calculation by ray tracing software enabled us to calculate the LED energy absorbed in the gain media. The calculated result was transferred to cavity analysis software for the simulation of LED-pumped laseroutput characteristics [17,18]. The investigated gain media in the calculation included Nd:YAG, Nd:glass, Nd/Cr:YAG, solid dye, and Ti:sapphire. From the calculation, we could estimate the minimum requirement for the LED pump power to get lasing and the slope efficiency for each gain medium. To test the accuracy of the simulation, we calculated the distribution of the absorbed energy over a cross-section of the rod and compared the result with the experimentally measured fluorescence profile of a LED-pumped Nd:YAG laser. II. DESIGN AND SIMULATION The gain material was pumped by emission from the LED directly sent to the gain media, as illustrated in Fig. 1. Commercially available high-power white LEDs (S42180, Seoul Semiconductor) and blue LEDs (B42180, Seoul Semiconductor) were used as pumping sources, and they were mounted very close to the gain media. A set of 10 LEDs was mounted linearly on a copper square bar. Four assembled bars surrounded a cylindrical, rodshaped gain medium symmetrically. The gap between the surface of the LED and the laser rod was 1 mm. The gap distance was decided by considering the divergence angle of the LED. The size of each gain medium was 4 mm in diameter and 100 mm in length. From the calculation, we found that about 79.7% of the beam emitted Fig. 2. Spectra of (a) the white LED and (b) the blue LED used in the experiment and the calculation. from LED reached the gain medium s surface directly. To estimate the accuracy of the calculation, we measured fluorescence profile from the rod cross-section and compared it with the calculated distribution. To reflect the actual experimental conditions in the simulation, we measured the output power and the spectrum of the light emitted from each LED by using an integrating sphere and spectrometer (SMS-500, Sphere optics). The measured output power for each white LED was 0.31 W, and that for blue LED was 0.46 W. Although the measured electricity-to-light conversion efficiencies were 7.6% and 11.4%, respectively, for the white and the blue LEDs used here, higher efficiencies up to 50% are expected in the near future. The measured spectra of the LEDs are shown in Figs. 2(a) and (b). For the white LED, part of the blue light with a wavelength of 461 nm was converted to yellow light by the phosphor. Unconverted blue light was mixed with yellow light, giving white light. The spectrum of the white LED had a full width at half maximum (FWHM) of nm. For the blue LED, the spectrum had a FWHM of 25.9 nm, which was still very broad compared with the widths of most absorption lines of Nd:YAG as shown in Fig. 3(a). The intensity of the beam, I(z), after propagating a distance z in the gain medium is given as [19] I(z) = λ2 λ 1 f e (λ) exp( α(λ)z)dλ, (1)

3 Study of a QCW Light-emitting-diode (LED)-pumped Solid-state Laser Kangin Lee et al Table 1. Properties of the laser gain media used in the calculations. Gain Medium Nd:YAG Solid dye Nd:glass Ti:sapphire Nd/Cr:YAG Dopant 1.0 at.% 0.8 milli-mol/l ion/cm 3 ND 1.0 mol% 0.1 wt.% concentration Cr 3.0 mol% Emission cross-section (cm 2 ) (1064 nm) (580 nm) (1064 nm) (795 nm) (1064 nm) Peak absorption coefficient (cm 1 ) (589 nm) (490 nm) (590 nm) (480 nm) (460 nm) Ref , , 26 Table 2. Calculated ratio of the LED beam absorbed by the gain media to the emitted LED beam for the blue and the white LED pump beams. Gain Medium Nd:YAG Solid dye Nd:glass Ti:sapphire Nd/Cr:YAG Absorption efficieny for white LED pumping (%) Absorption efficiency for blue LED pumping (%) Fig. 3. Measured absorption coefficients of (a) Nd:YAG as a function of wavelength. (b) LED spectrum before (thick solid line) and after (thin solid line) transmitting through Nd:YAG. where λ 1 and λ 2 are the lower and the upper limits, respectively, of the emission spectrum f e (λ) of the LED; α(λ) is wavelength-dependent absorption coefficient of the gain medium. The calculation by ray tracing (ZEMAX) considered the measured emission spectra of LEDs by importing the relative strengths of the emitted light intensities at 24 equally spaced, different wavelengths within the emission bandwidth in the input data field [17]. In a similar way, wavelength-dependent absorption coefficient data were imported in the calculation reflecting the absorption spectrum of each gain media. The radiant intensity of the LED showed a Gaussian profile with a FWHM of 125, giving the maximum output in the normal direction. The angular distribution of radiation from the LED was also considered in the calculation. The gain media considered in the calculation are as follows: 1.0-at.% Nd-doped YAG, 0.8 milli-mol/l Rh- 6G-doped PMMA solid dye, glass substrates doped with Nd 3+ ions with a density of ion/cm 3, YAG with a Nd 3+ density of 1.0 mol% and a Cr 3+ density of 3.0 mol% (Nd/Cr:YAG), and a 0.1-wt.% Ti 3+ -ion-doped sapphire crystal. The optical and the physical properties of solid dye in Refs. 20 to 22 were used in the calculation. For the absorption coefficients of Nd:glass and Ti:sapphire, the data reported in Refs. 23 and 24 were used. For Nd/Cr:YAG, data reported in Refs. 25 and 26 were used. The stimulated emission cross-sections at each lasing wavelength and related data for calculations are summarized in Table 1. After the wavelength-dependent absorption coefficient α(λ) and the emission spectra, f e (λ), of the LEDs had been obtained, the absorbed LED pump power was calculated. Table 2 shows the ratio of LED power absorbed by the gain media to the emitted LED power obtained from the calculation. For Nd:YAG, solid dye, Nd:glass, Ti:sapphire, and Nd/Cr:YAG, the cal-

4 Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011 Fig. 4. Laser output energies as functions of the output coupler reflectivity for (a) the white LED pumping case and (b) the blue LED pumping case. The maximum output energy from the LEDs is 192 mj. culated results showed absorption efficiencies of 6.2%, 24.3%, 8.4%, 4.2%, and 43.4%, respectively, in case of white LED pumping. For the case of blue LED pumping, the absorption efficiencies were to 4.5%, 53.8%, 4.7%, 7.6%, and 52.7% respectively. Solid dye showed the highest absorption efficiency of 53.8% as the emission band of the blue LED was located close to the peak absorption wavelength of the solid dye. If green LEDs were used, Ti:sapphire also had a higher efficiency. The result shows that a reflector is required in the pumping chamber when a gain medium with a low absorption efficiency is used, to enhance the absorption efficiency through repeated transmission through the gain medium. Figure 3(a) shows the measured absorption coefficients versus wavelength for a 1.0-at.% Nd-doped YAG crystal (Casix) obtained using a spectrometer (Varian Inc., Cary 500), and Fig. 3(b) shows the measured spectra of white LED before and after transmitting through the Nd:YAG crystal. From the measured spectra, we found that 7.4% of the power emitted from the white LED was absorbed by the Nd:YAG rod. This result was in good agreement with the calculated value of 6.2% with an error of 11%. In the calculation, the LEDs were assumed to have a maximum total pump energy of 192 mj (4.8 mj 40 ea.). The temporal duration of the pump pulse was assumed to be 200 µs considering the fluorescence lifetime of Nd:YAG. As the fluorescence lifetimes of Ti:sapphire (3.2 µs) and solid dye ( 10 ns) are much shorter than the pump duration, the QCW pumpings of Ti:sapphire and solid dye are similar to CW pumping with a LED of 960-W power for 200 µs. In the case of Nd/Cr:YAG, the fluorescence lifetime is near 600 µs, much longer than Nd:YAG. In this case, a pump pulse with a 200-µs temporal duration was applied for comparison of the lasing performance. The calculated data for the distribution of the LED energy absorbed by the gain media were imported to LASCAD [18]. From a calculation using LAS- CAD, the thermal effects in the gain media, as well as laser power, could be obtained. In the calculation, the cavity length was 104 mm, and cavity mirrors had flat surfaces except for the case of solid dye. The thermal lens of most gain media was very long, ranging from 200 m to 600 m. Due to strong absorption at the rod surface, the solid dye rod showed a thermal lens with a negative focal length of 600 m. When the solid dye was pumped by white LEDs or blue LEDs, the radius of curvature of the end mirror for the stable resonator condition was 100 m. Figures 4(a) and (b) show the calculated output energies for both cases of white LED pumping and blue LED pumping as functions of the output coupler reflectivity. The cavity loss was assumed to be 0.02 in the calculation. The output coupler reflectivities for maximum output energy when blue LEDs were used were 96% for Nd:YAG, 97% for solid dye, and 93% for Nd/Cr:YAG. However, lasing was not observed for Ti:sapphire and Nd:glass at this pump energy. Figure 5 shows the calculated output energies of white LED pumped lasers and blue LED pumped lasers as functions of the LED pump energy for each gain medium. The output energies of the white LED required to the reach lasing thresholds of Nd:YAG, solid dye, Nd:glass and Nd/Cr:YAG were 45.3 mj, 26.0 mj, 181 mj, and 5.1 mj, respectively. On the other hand, the lasing thresholds of Nd:YAG, solid dye and Nd/Cr:YAG were 54.2 mj, 18.3 mj, and 4.8 mj when a blue LED was used for pumping. Although Nd:glass has an absorption peak near 600 nm and its absorption spectrum is broad, the lasing threshold is higher than that of Nd:YAG because the emission cross-section is 10 times smaller than that of Nd:YAG [23,24]. Ti:sapphire showed a lower absorption efficiency of 4.2% because the LED spectrum is located on the wing of the wide absorption band. Due to the low absorption coefficient, a much higher pump LED power is required compared with the case of Nd:YAG laser. Even though blue LED pumping is more favorable for the Ti:sapphire laser, the absorption efficiency is at 7.6%, and the required pump energy for lasing threshold is 383 mj. The slope efficiencies of the white LED pumped Nd:YAG and blue LED pumped Nd:YAG were 1.5% and 1.1%, respectively. Considering 40 LEDs participate in pumping, each white LED should have an output en-

5 Study of a QCW Light-emitting-diode (LED)-pumped Solid-state Laser Kangin Lee et al Fig. 7. (Color online) Photo of a blue-led-pumped Nd:YAG laser. Fig. 5. Laser output energies as functions of (a) the white LED pump energy and (b) the blue LED pump energy. Fig. 6. Laser output energy as a function of the white LED pump energy when a reflector is enclosed in the gain medium. ergy of 1.1 mj, also, each blue LED should have an output energy of 1.4 mj to reach the lasing threshold for Nd:YAG. For the case of a LED-pumped solid dye laser, each white LED should have an output energy of 0.65 mj, and each blue LED should have an output energy of 0.46 mj to reach the lasing thresholds. The pump energies per LED to reach the lasing threshold were the lowest for Nd/Cr:YAG, and they were 0.13 mj for the white LED and 0.12 mj for the blue LED. The blue LED pumped solid dye showed the highest slope efficiency of Fig. 8. (Color online) Distributions of the absorbed blue LED beam over a cross-section of the Nd:YAG rod obtained by (a) measurement and (b) calculation. Arrows indicate the direction of the pump LED beam. 23.6% and an optical-to-optical energy conversion efficiency of 16.8%. The lower optical-to-optical efficiency for Nd:YAG is caused by its low absorption efficiency. A reflector enclosing the gain medium can enhance the absorption efficiency greatly. According to the calculation, assuming a 99% reflector of 8 mm in diameter is placed with its symmetry axis overlapping the Nd:YAG, the absorption efficiency increases to 13.5% from 6.2% for the while LED pumping case. Also, the lasing threshold was lowered to 26.7 mj, and the slope efficiency was increased to 3.2%, as shown in Fig. 6. Compared with the results shown in Fig. 5(a), the slope efficiencies of gain media with lower absorption efficiencies are notably enhanced while Nd/Cr:YAG shows a slight enhancement. To see the accuracy of the calculated results for the performance of LED-pumped solid-state lasers, we fabricated a pumping chamber with a geometry similar to the one shown in Fig. 1. Figure 7 shows a photo of the fabricated pumping chamber. Each blue LED had a maximum output energy of 0.1 mj. The LEDs were mounted on four copper bars. The surface of each copper bar was coated with an electrical insulator to isolate it from the parallel metal wires connecting the electrodes of the LEDs. Both ends of the copper bar were tightly fixed to a housing made of brass. The base plate of the housing was cooled by water at a temperature of 20 C. Both ends of the Nd:YAG rod were wrapped with indium foil and then secured at the hole of the housing for

6 Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011 about 1/5 the required energy for the lasing threshold of Nd:YAG. If a pumping chamber with reflector is used, lasing can be achieved when the energy of the tested LED is increased to only two times higher. Even without the reflector, when the Nd:YAG is replaced by Nd/Cr:YAG, an output energy of 1.4 mj/pulse is expected when a pump energy of 10.8 mj/pulse (0.27 mj 40) is emitted from the 40 blue LEDs tested in this work. III. CONCLUSION Fig. 9. (Color online) (a) Blue LED output for various the peak driving currents and (b) the total emitted LED energy as a function of the peak current for several different QCW current pulses. conduction cooling. Fluorescence from the Nd:YAG rod was observed while the blue LEDs were turned on. Figure 8(a) shows a fluorescence image of the rod taken by using a CCD camera and image capture program. Figure 8(b) is cross-sectional view of calculated distribution of LED beam absorbed by the Nd:YAG rod, and both results are quite similar, implying a high accuracy for the calculation. When QCW current pulses were sent to the LEDs, the peak output power from the LEDs could be increased to several times higher than the peak power from the CW LEDs. Figure 9(a) shows the emitted blue LED output as a function of the peak current for a repetition rates of 25 Hz and a current duration of 200 µs. The peak current of the QCW pulse was increased to 10 times the normal CW current for the LEDs. When the applied peak current was increased to larger than 10 A, the LEDs were damaged. Figure 9(b) shows the measured pump pulse energy from the LED as a function of the applied peak current. The result shows that the pump pulse energy from a single LED was 0.27 mj, 2.9 times higher than the energy from a cw LED during 200 µs. Although the tested blue LED showed less energy than the LED in Ref. 13, the results demonstrated that QCW pumping could be an effective means of increasing the peak pump power. The pump energy of the tested blue LED was Several solid-state lasers pumped by QCW LEDs were investigated. The accuracy of the calculation was confirmed by comparing the measured absorption efficiency and absorbed power distribution over the laser rod crosssection with the calculated results. In the calculation, several gain media (4-mm diameter and 100-mm length), such as Nd:YAG, Nd:glass, solid dye, Ti:sapphire, and Nd/Cr:YAG, were investigated for white LED and blue LED pumping. From the calculation, not only the absorption efficiency of pump LED beam in each gain medium but also the lasing threshold and the slope efficiency of each laser could be estimated. Among the tested gain media, Nd/Cr:YAG showed the lowest lasing threshold of 4.8 mj/pulse, and solid dye showed the highest slope efficiency of 23.6%. To get lasing with the test setup for the fluorescence measurement, we had to use Nd/Cr:YAG. When Nd:YAG with a 1.0-at% Nd concentration was used, a 5 times higher power blue LED was needed for lasing. A pumping chamber with a reflector could reduce the blue LED pump energy for lasing of Nd:YAG to 22.3 mj/pulse from 54.1 mj/pulse. The results demonstrate that lasing of LED-pumped Nd:YAG is accessible with a simple direct pumping configuration when a pumping chamber with reflector is used. Further, when Cr/Nd:YAG is used, an optical-to-optical efficiency of 13% can be obtained by using the tested blue LEDs even without a reflector. ACKNOWLEDGMENTS This research was supported by Yeungnam University Research Grants in REFERENCES [1] A. Minassian, B. Thompson and M. J. Damzen, Appl. Phys. B 76, 341 (2003). [2] X. Ya, Q. Liu, M. Gong, X. Fu and D. Wang, Appl. Phys. B 95, 323 (2009). [3] Y. Sun, H. Zhang, Q. Liu, L. Huang, Y. Wang and M. Gong, Laser Phys. Lett. 7, 722 (2010).

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