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

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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 sev- eral 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 pump- ing chamber structure similar to that of the laser side pumped by diode lasers. When a QCW current is ap- plied to the LED, the peak output power from LED can be increased to several times higher than the CW out- put 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 fab- ricated for pumping. In this work, we use commercial LEDs with dome lenses on chips that have been devel- oped 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.
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