Study of a QCW Light-Emitting-Diode (LED)-Pumped Solid-State Laser

Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011, pp. 3239∼3245

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 712-749, 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: 85.60.Jb, 42.55.Rz, 42.70.Hj Keywords: Light-emitting-diode, LED, Solid-state laser DOI: 10.3938/jkps.59.3239

I. INTRODUCTION [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 us- Solid-state lasers have been intensively developed over ing a semi-conductor-based light source with DPSSL has the past several decades for their wide applications in in- succeeded. Recently, LEDs, which have low cost and dustry and the military [1–3]. Solid-state lasers pumped high output power, have developed rapidly for display by arc lamps are rapidly being replaced by diode-pumped and illumination applications, and their applications in solid-state lasers (DPSSLs). Ti:sapphire lasers and solid laser pumping are gaining attention [11–16]. Yang et al. dye lasers have also recently been pumped by DPSS pumped a polymer waveguide by using an InGaN blue green lasers, instead of Ar+ ion lasers. Still, most solid- LED [13]. They used 10 times higher peak current com- state lasers are pumped by CW diode lasers because pared with the normal CW driving current of a LED to quasi-continuous-wave (QCW) laser diodes are very ex- get enough pump intensity for lasing. pensive as replacement for flash lamps. Thus, flash-lamp- The high cost of diode lasers hinders wide application pumped solid-state lasers account for a large portion of of DPSSLs. Further, diode lasers are easily damaged the high-energy, low-repetition-rate pulse laser market by humidity, static electricity, and dust. Pumping us- even though they generate a large amount of heat and ing LEDs has merits not only in cost but also in many have short lifetimes. practical aspects. LEDs are designed to resist static dis- About 40 years ago, the light-emitting diode (LED) charge. The emitter is encapsulated by a molded lens, was suggested as a pump source for solid-state lasers [4– isolating dust and humidity. A broad range of LED 10]. During early development, the LED had a poor spectra allows direct pumping of a tunable gain mate- electricity-to-light conversion efficiency, and the output rial such as Ti:sapphire, Alexandrite, or solid dye. As power was very low. Reinberg et al. cooled the LED and CW laser diodes have different emitter designs, com- the gain material to an extremely low temperature of 77 pared with QCW diode lasers, to manage generated heat, K to improve the conversion efficiency and the lifetime they can be operated only in the CW mode. In the case of the LED [4]. To overcome low absorption efficiency, a of LED pumping, a laser can be operated in CW, as Nd:YAG single crystal fiber was used as a gain medium well as QCW, modes with the same LED source. Even [6,7]. Farmer and Kiang used a gold reflector to con- with traditional pump sources such as flash-lamps or arc- centrate highly diverging LED light to the Nd:YAG rod lamps, operation in both modes with the same lamp is impractical. ∗E-mail: [email protected] In this work, we investigate the performance of a LED- -3239- -3240- Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011

Fig. 1. (Color online) Structure of the investigated LED- pumped 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 afford- able. Calculation by ray tracing software enabled us to calculate the LED energy absorbed in the gain media. Fig. 2. Spectra of (a) the white LED and (b) the blue LED The calculated result was transferred to cavity analy- used in the experiment and the calculation. sis software for the simulation of LED-pumped laser- output characteristics [17,18]. The investigated gain media in the calculation included Nd:YAG, Nd:glass, from LED reached the gain medium’s surface directly. Nd/Cr:YAG, solid dye, and Ti:sapphire. From the cal- To estimate the accuracy of the calculation, we mea- culation, we could estimate the minimum requirement sured fluorescence profile from the rod cross-section and for the LED pump power to get lasing and the slope compared it with the calculated distribution. To reflect efficiency for each gain medium. To test the accuracy the actual experimental conditions in the simulation, we of the simulation, we calculated the distribution of the measured the output power and the spectrum of the light absorbed energy over a cross-section of the rod and com- emitted from each LED by using an integrating sphere pared the result with the experimentally measured fluo- and spectrometer (SMS-500, Sphere optics). The mea- rescence profile of a LED-pumped Nd:YAG laser. sured 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 II. DESIGN AND SIMULATION used here, higher efficiencies up to 50% are expected in the near future. The measured spectra of the LEDs are The gain material was pumped by emission from the shown in Figs. 2(a) and (b). For the white LED, part LED directly sent to the gain media, as illustrated in of the blue light with a wavelength of 461 nm was con- Fig. 1. Commercially available high-power white LEDs verted to yellow light by the phosphor. Unconverted blue (S42180, Seoul Semiconductor) and blue LEDs (B42180, light was mixed with yellow light, giving white light. The Seoul Semiconductor) were used as pumping sources, and spectrum of the white LED had a full width at half max- they were mounted very close to the gain media. A set imum (FWHM) of 207.1 nm. For the blue LED, the of 10 LEDs was mounted linearly on a copper square spectrum had a FWHM of 25.9 nm, which was still very bar. Four assembled bars surrounded a cylindrical, rod- broad compared with the widths of most absorption lines shaped gain medium symmetrically. The gap between of Nd:YAG as shown in Fig. 3(a). The intensity of the the surface of the LED and the laser rod was 1 mm. The beam, I(z), after propagating a distance z in the gain gap distance was decided by considering the divergence medium is given as [19] angle of the LED. The size of each gain medium was 4 mm in diameter and 100 mm in length. From the calcu- Z λ2 I(z) = f (λ) exp(−α(λ)z)dλ, (1) lation, we found that about 79.7% of the beam emitted e λ1 Study of a QCW Light-emitting-diode (LED)-pumped Solid-state Laser – Kangin Lee et al. -3241-

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 ND 1.0 mol% 1.0 at.% 0.8 milli-mol/l 1019 ion/cm3 0.1 wt.% concentration Cr 3.0 mol% Emission 2.8 × 10−19 3.0 × 10−16 2.8 × 10−20 2.8 × 10−19 6.5 × 10−19 cross-section (cm2) (1064 nm) (580 nm) (1064 nm) (795 nm) (1064 nm) Peak absorption 2.5 29.9 3.4 0.6 30 coeﬃcient (cm−1) (589 nm) (490 nm) (590 nm) (480 nm) (460 nm) Ref. 24 20 – 22 23, 24 24 25, 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 6.2 24.3 8.4 4.2 43.4 white LED pumping (%) Absorption eﬃciency for 4.5 53.8 4.6 7.6 52.7 blue LED pumping (%)

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 Nd3+ ions with a density of 1019 ion/cm3, YAG with a Nd3+ density of 1.0 mol% and a Cr3+ density of 3.0 mol% (Nd/Cr:YAG), and a 0.1-wt.% Ti3+-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 sum- Fig. 3. Measured absorption coefficients of (a) Nd:YAG marized in Table 1. After the wavelength-dependent ab- as a function of wavelength. (b) LED spectrum before (thick sorption coefficient α(λ) and the emission spectra, fe(λ), solid line) and after (thin solid line) transmitting through of the LEDs had been obtained, the absorbed LED pump Nd:YAG. 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 where λ1 and λ2 are the lower and the upper lim- dye, Nd:glass, Ti:sapphire, and Nd/Cr:YAG, the cal- its, respectively, of the emission spectrum fe(λ) of the -3242- Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011

sumed 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 Fig. 4. Laser output energies as functions of the output output energy when blue LEDs were used were 96% for coupler reflectivity for (a) the white LED pumping case and Nd:YAG, 97% for solid dye, and 93% for Nd/Cr:YAG. (b) the blue LED pumping case. The maximum output en- However, lasing was not observed for Ti:sapphire and ergy from the LEDs is 192 mJ. Nd:glass at this pump energy. Figure 5 shows the calculated output energies of white LED pumped lasers and blue LED pumped lasers as culated results showed absorption efficiencies of 6.2%, functions of the LED pump energy for each gain medium. 24.3%, 8.4%, 4.2%, and 43.4%, respectively, in case of The output energies of the white LED required to the white LED pumping. For the case of blue LED pumping, reach lasing thresholds of Nd:YAG, solid dye, Nd:glass the absorption efficiencies were to 4.5%, 53.8%, 4.7%, and Nd/Cr:YAG were 45.3 mJ, 26.0 mJ, 181 mJ, and 5.1 7.6%, and 52.7% respectively. Solid dye showed the high- mJ, respectively. On the other hand, the lasing thresh- est absorption efficiency of 53.8% as the emission band olds of Nd:YAG, solid dye and Nd/Cr:YAG were 54.2 of the blue LED was located close to the peak absorp- mJ, 18.3 mJ, and 4.8 mJ when a blue LED was used for tion wavelength of the solid dye. If green LEDs were pumping. used, Ti:sapphire also had a higher efficiency. The result Although Nd:glass has an absorption peak near 600 shows that a reflector is required in the pumping cham- nm and its absorption spectrum is broad, the lasing ber when a gain medium with a low absorption efficiency threshold is higher than that of Nd:YAG because the is used, to enhance the absorption efficiency through re- emission cross-section is 10 times smaller than that of peated transmission through the gain medium. Figure Nd:YAG [23,24]. Ti:sapphire showed a lower absorption 3(a) shows the measured absorption coefficients versus efficiency of 4.2% because the LED spectrum is located wavelength for a 1.0-at.% Nd-doped YAG crystal (Casix) on the wing of the wide absorption band. Due to the obtained using a spectrometer (Varian Inc., Cary 500), low absorption coefficient, a much higher pump LED and Fig. 3(b) shows the measured spectra of white LED power is required compared with the case of Nd:YAG before and after transmitting through the Nd:YAG crys- laser. Even though blue LED pumping is more favorable tal. From the measured spectra, we found that 7.4% of for the Ti:sapphire laser, the absorption efficiency is at the power emitted from the white LED was absorbed 7.6%, and the required pump energy for lasing threshold by the Nd:YAG rod. This result was in good agreement is 383 mJ. with the calculated value of 6.2% with an error of 11%. The slope efficiencies of the white LED pumped In the calculation, the LEDs were assumed to have a Nd:YAG and blue LED pumped Nd:YAG were 1.5% and maximum total pump energy of 192 mJ (4.8 mJ × 40 1.1%, respectively. Considering 40 LEDs participate in ea.). The temporal duration of the pump pulse was as- pumping, each white LED should have an output en- Study of a QCW Light-emitting-diode (LED)-pumped Solid-state Laser – Kangin Lee et al. -3243-

Fig. 7. (Color online) Photo of a blue-LED-pumped Nd:YAG laser.

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. Fig. 5. Laser output energies as functions of (a) the white LED pump energy and (b) the blue LED pump energy.

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. Fig. 6. Laser output energy as a function of the white LED To see the accuracy of the calculated results for the pump energy when a reflector is enclosed in the gain medium. 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 ergy of 1.1 mJ, also, each blue LED should have an out- the fabricated pumping chamber. Each blue LED had put energy of 1.4 mJ to reach the lasing threshold for a maximum output energy of 0.1 mJ. The LEDs were Nd:YAG. For the case of a LED-pumped solid dye laser, mounted on four copper bars. The surface of each cop- each white LED should have an output energy of 0.65 per bar was coated with an electrical insulator to isolate mJ, and each blue LED should have an output energy it from the parallel metal wires connecting the electrodes of 0.46 mJ to reach the lasing thresholds. The pump of the LEDs. Both ends of the copper bar were tightly energies per LED to reach the lasing threshold were the fixed to a housing made of brass. The base plate of the lowest for Nd/Cr:YAG, and they were 0.13 mJ for the housing was cooled by water at a temperature of 20 ◦C. white LED and 0.12 mJ for the blue LED. The blue LED Both ends of the Nd:YAG rod were wrapped with in- pumped solid dye showed the highest slope efficiency of dium foil and then secured at the hole of the housing for -3244- 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

Several solid-state lasers pumped by QCW LEDs were investigated. The accuracy of the calculation was con- firmed by comparing the measured absorption efficiency and absorbed power distribution over the laser rod cross- section 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 las- Fig. 9. (Color online) (a) Blue LED output for various the ing threshold of 4.8 mJ/pulse, and solid dye showed the peak driving currents and (b) the total emitted LED energy highest slope efficiency of 23.6%. To get lasing with the as a function of the peak current for several different QCW test setup for the fluorescence measurement, we had to current pulses. 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 reflec- conduction cooling. Fluorescence from the Nd:YAG rod tor could reduce the blue LED pump energy for lasing of was observed while the blue LEDs were turned on. Fig- Nd:YAG to 22.3 mJ/pulse from 54.1 mJ/pulse. The re- ure 8(a) shows a fluorescence image of the rod taken by sults demonstrate that lasing of LED-pumped Nd:YAG using a CCD camera and image capture program. Fig- is accessible with a simple direct pumping configuration ure 8(b) is cross-sectional view of calculated distribution when a pumping chamber with reflector is used. Further, of LED beam absorbed by the Nd:YAG rod, and both when Cr/Nd:YAG is used, an optical-to-optical efficiency results are quite similar, implying a high accuracy for of 13% can be obtained by using the tested blue LEDs the calculation. even without a reflector. 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 ACKNOWLEDGMENTS as a function of the peak current for a repetition rates of 25 Hz and a current duration of 200 µs. The peak This research was supported by Yeungnam University current of the QCW pulse was increased to 10 times the Research Grants in 2009. 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 REFERENCES peak current. The result shows that the pump pulse energy from a single LED was 0.27 mJ, 2.9 times higher [1] A. Minassian, B. Thompson and M. J. Damzen, Appl. than the energy from a cw LED during 200 µs. Although Phys. B 76, 341 (2003). the tested blue LED showed less energy than the LED [2] X. Ya, Q. Liu, M. Gong, X. Fu and D. Wang, Appl. Phys. in Ref. 13, the results demonstrated that QCW pumping B 95, 323 (2009). could be an effective means of increasing the peak pump [3] Y. Sun, H. Zhang, Q. Liu, L. Huang, Y. Wang and M. power. The pump energy of the tested blue LED was Gong, Laser Phys. Lett. 7, 722 (2010). Study of a QCW Light-emitting-diode (LED)-pumped Solid-state Laser – Kangin Lee et al. -3245-

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