Journal of the Korean Physical Society, Vol. 51, No. 1, July 2007, pp. 322∼326

The Lasing Characteristics of a Passively Q-Switched Nd:YVO4 Laser Using a Cr:YAG Saturable Absorber

Jonghoon Yi∗ and Jin Hyuk Kwon Department of Physics, Yeungnam University, Kyungsan 712-749

(Received 31 January 2007)

A diode-pumped, passively Q-switched Nd:YVO4 laser was fabricated. A composite crystal, 0.6 % Nd doped YVO4 crystal block bonded with an undoped YVO4 crystal cap, was used. The end cap reduced thermal lensing as well as thermal stress, when the crystal was end pumped by using a fiber coupled diode laser. Cr:YAG crystals with initial transmission of 80 % or 90 % were used as saturable absorbers. For each Cr:YAG crystal, several output couplers with different reflectivity were tested. The variations in the pulsewidth, the pulse repetition rate, and the average output power of the passively Q-switched laser were measured while varying the diode pump power for each value of the output coupler reflectivity and the Cr:YAG initial transmission. The dependences of the lasing characteristics on the output coupler reflectivity and the Cr:YAG initial transmission were explained.

PACS numbers: 42.55.Xi, 42.60.Gd, 42.60.Jf Keywords: Cr:YAG laser, Nd:YVO4 laser, Passive Q-switching, Diode-pumped laser, Solid state laser

I. INTRODUCTION medium is simple in structure, and small scale lasers can be easily fabricated. Although the thermal problem remains, bulk crystals with both ends diffusion bonded The trend of current laser development targets ro- with undoped crystal can reduce the thermal problem bust, maintenance-free, small, and efficient laser sources. much [7]. The crystal dissipates the thermal energy not Diode-pumped solid-state lasers are found to be suitable only through the laser crystal’s side surfaces but also for this, and they have been extensively developed over through the crystal’s end surfaces in contact with the the last decade. At the initial stage of the development, undoped crystal. Thus, reasonably high power can be the pumped gain media of most lasers were bulk solid obtained by using a small-size composite crystal. state crystals such as Nd:YAG or Nd:YVO4. Recently, To achieve high peak power, Q-switching or mode lock- gain media shaped as thin disks or long hair-like fibers ing is generally used to narrow the pulse duration [8]. Ac- have been extensively studied in an effort to overcome tive Q-switching produces very stable and regular pulses. the thermal problem [1–4]. However, it is difficult to fabricate a compact, actively When the pump power is increased, the heat origi- Q-switched laser because electro optic or acousto optic nating from quantum defects and unwanted absorption Q-switching devices are bulky and expensive. Passive results in thermal lensing and stress. Diode pumping Q-switching gives many advantages in cost and size. For greatly reduced the unwanted absorption and the large these reasons, passive Q-switching has attracted interest thermal load related with flashlamp pumping. However, for long time. Recently, Cr:YAG has been extensively there are residual thermal problem still, especially for investigated for use as a saturable absorber (SA) for a high-power lasers. A thin disk laser dissipates the ther- laser with a wavelength near 1 µm [9,10]. The optical mal energy through its wide back surface, which is di- and the mechanical properties of the Cr:YAG crystal are rectly contacted with the water-cooled surface [1,2]. A well summarized by Kalisky [11]. Cr:YAG is also used fiber laser efficiently dissipates thermal energy as it has as a gain medium for a laser in the wavelength range a large surface-to-volume ratio [3,4]. These lasers show of 1300 ∼ 1500 nm. The pulse duration from passively enhanced performances compared with diode-pumped Q-switched laser is in the tens of ns range [9]. When lasers using bulk solid laser crystals or ceramics [5,6]. Cr:YAG was applied to a microchip laser, it produced For thin disk and fiber lasers, however, a complicated pulses as short as 300 ps [10]. multi-pass pumping geometry and a large capital cost for We developed a passively Q-switched, diode-pumped development are problems. Pumping a small-size bulk Nd:YVO4 laser. A thin anti-reflection coated Cr:YAG crystal was used as a SA. The crystal is very stable com- ∗E-mail: [email protected] pared with an organic SA [12]. The dependence of the -322- The Lasing Characteristics of a Passively Q-Switched··· – Jonghoon Yi and Jin Hyuk Kwon -323-

Fig. 2. Calculated thermal stress for the diode pumped Nd:YVO4 crystal (a) with an undoped YVO4 end cap and (b) without the end cap. Fig. 1. Schematic diagram of the experimental setup and laser layout.

and the copper housing was installed on a 5-axis mount, lasing characteristics on output coupler reflectivity, as which adjusted the height and the tilting angle of the well as on initial transmission of Cr:YAG, is investigated. crystal. Several flat output couplers with reflectivities The developed compact, passively Q-switched laser can (R) of 77 %, 81 %, 88 %, and 94 % were used to test be used for laser spectroscopy, micro-machining, and the dependence of lasing characteristics on the cavity sensing applications. loss. Optics mounting blocks were installed on a linear rail for easy adjustment of the relative distance between optics. The total cavity length was 40 mm. For high efficiency, the pump beam radius is adjusted to match II. EXPERIMENTS AND RESULTS the resonator mode waist [13]. The focused beam size was 520 µm and was about 10 % larger than calculated Figure 1 shows the geometry of a fabricated diode- TEM00 mode size of 470 µm at the crystal. The pump pumped Nd:YVO4 laser and experimental setup. A beam size was measured by scanning a knife edge across fiber-coupled diode laser (Thomson CSF) sends 10-W the beam at focus. pump power (808 nm) to one end of the Nd:YVO4 crystal The mode size was calculated using LASCAD soft- surface through an input coupler. The peak wavelength ware, which takes into account the effect of thermal lens- and spectral width of the diode laser depend on the driv- ing in the crystal [14]. The calculated mode size was ing current. The absorption coefficient of the Nd:YVO4 rather large because the cavity was very short and be- crystal sensitively depends on the diode laser wavelength cause the mirror and output coupler had flat surfaces. and bandwidth. When the diode power was 10 W, the The beam was focused at a point 0.5 mm inside the en- diode wavelength was 808.5 nm, and the spectral width tering surface of the doped Nd:YVO4 crystal. The LAS- was 2.1 nm. The wavelength was 805.7 nm and the band- CAD also calculates thermal stress of the diode-pumped width was 1.7 nm when the diode power was 5.1 W. The crystal, and the calculation results are shown in Fig. input coupler has a transmission of 90 % at diode wave- 2. The composite Nd:YVO4 crystal showed 33 % lower length of 808 nm and a reflectivity of 99 % at the lasing thermal stress compared with a crystal without end cap wavelength of 1064 nm. The core diameter of the fiber (bulk crystal). From the calculation, it is found that the was 600 µm. The diode laser emitted from the fiber end thermal stress is the most severe at the side surfaces of was collimated and focused by using 5-cm-focal-length the composite crystal, which are the closest positions to lenses. The entrance surface of the Nd:YVO4 crystal is the heat source (pump focus). For a bulk crystal with- located at 10 mm from the surface of the input coupler. out bonding, the maximum stress occurs at the edge of A 0.6 at.% doped Nd:YVO4 crystal was used as a gain the pump beam’s entrance surface of the crystal. Ther- medium, and its size was 4 × 4 × 6 mm3 (Onyx product). mal stress becomes large where the thermal gradient is Both ends were bonded with undoped YVO4 crystals of steep, which is at the cooled side surfaces of the crys- 4 × 4 × 3 mm3 in size. The total length of the compos- tal. Large thermal gradients induce thermal stress on ite crystal was 12 mm. The crystal was wrapped with the crystal’s side surfaces, and when it becomes larger indium foil, then, it was tightly enclosed in water cooled than the mechanical fracture limit, the crystal cracks copper block. The cooling water temperature was set at [15]. The pump beam’s input surface of the compos- 20 ◦C, and the same flowing water cooled another copper ite crystal is effectively conduction cooled through the block enclosing the Cr:YAG disk. Cr:YAG crystals with bonded crystal whereas the bulk crystal surface is air initial transmissions (T ) of 80 % or 90 % were used as cooled, which is less efficient. Fast cooling lowers the SAs. The Cr:YAG crystal disk had a 1-mm thickness, surface temperature of the crystal and reduces the ther- -324- Journal of the Korean Physical Society, Vol. 51, No. 1, July 2007

Fig. 3. CW Nd:YVO4 laser output power as a function of Fig. 4. Q-switched Nd:YVO4 laser output power as a func- the diode pump power for several values of the output coupler tion of the diode pump power for several values of the output reflectivity. coupler reflectivity (R). Two Cr:YAG crystals with 80 % or 90 % initial transmissions (T ) were used for each set of output couplers. mal stress. When we tried a bulk Nd:YVO4 crystal with a 1-mm obtained. thickness, the crystal cracked when the pump power ex- A Cr:YAG crystal with 80 % or 90 % initial transmis- ceeded 7 W. The cracked position is close to the position sions was used as a SA. After the SA had been inserted predicted by the calculation. From the LASCAD cal- inside the cavity, the CW lasing ceased, and pulsed out- culation, we could estimate that the bonded crystal in put was observed. Various characteristics of the output this experiment could be used even at the pump power pulses were measured as a function of the diode pump of 30 W. For the case when both sides are pumped, the power. The measurements were repeated for each set of maximum pump power can be increased to 50 W. How- output coupler and SA. The passively Q-switched laser ever, in actual experiments, misalignment of the focusing output power was decreased to about 12 ∼ 42 % of the point from the center of the crystal should be consid- CW laser maximum power. The maximum Q-switched ered. When the pump beam is displaced from the center output power was obtained when the output coupler position to the side wall by 1 mm accidentally during reflectivity was 81 %. The slope efficiency was 12 % the initial alignment procedure, the thermal stress at the in this case. Compared to the CW lasing, the output crystal’s side surface increases by more than 30 %, and power was affected more by the reflectivity change of the increased stress can crack the crystal. the output coupler. Thus, finding the optimal reflectiv- The undoped end cap also reduces thermal lensing of ity was important for efficient operation of the passively the gain medium. From the LASCAD calculation results, Q-switched laser. The slope efficiency increased to 19 % the focal length of the thermal lens in the crystal is about when we used a Cr:YAG crystal with an initial transmis- 30 cm at a 10-W pump power for the case of a composite sion of 90 %. The laser output also increased compared crystal. However, the calculated focal length of the ther- with the results when an R = 80 % SA was used. The mal lens reduces to about 21 cm for the case of the bulk increased slope efficiency is caused by the lower intra- crystal, which is close to the experimental results [16]. cavity loss due to absorption in the SA. The laser ceased The output characteristics of a laser with a shorter ther- passive Q-switching and began CW lasing when the re- mal lens are affected more by pump power variations. flectivity of the output coupler was 94 %, and the pump Also, a laser with a shorter thermal-lens focal length be- power was larger than 6 W. comes unstable at lower pump powers compared with a The output from the passively Q-switched laser laser with a longer thermal-lens focal length. showed a series of pulses with very large fluctuations To see the maximum laser output power and its de- of the amplitude, pulse-to-pulse temporal intervals, and pendence on the reflectivity of the output coupler, we pulse durations. After averaging more than 32 pulses, measured the output of a CW (continuous wave) laser. we estimated the pulse duration. Fig. 5 shows the mea- Fig. 3 shows the results. When the reflectivity was 77 sured results. As the pump power increases, the pulse %, the laser showed the highest slope efficiency of 40 duration decreases when T = 80 % Cr:YAG is used. The %. At a maximum pump power of 9.7 W, the conver- increased gain due to the increased pump power amplifies sion efficiency was 34 %, and a 3.3-W CW output was the pulse rapidly and this results in a shorter pulse du- The Lasing Characteristics of a Passively Q-Switched··· – Jonghoon Yi and Jin Hyuk Kwon -325-

Fig. 5. Q-switched Nd:YVO4 laser pulse duration as a Fig. 6. Variation of p for Cr:YAG initial transmissions of function of the diode pump power for several values of the T = 80 % and T = 90 % for output coupler reflectivity of 77 output coupler reflectivity (R). Two Cr:YAG crystals with %, 81 %, 88 % and 94 %. 80 % or 90 % initial transmissions (T ) were used for each set of output couplers. the SA initial transmission. On the contrary, Pavel et al. [7] used the following parameter p to decide on the pulse ration. The figure also shows that the pulse duration de- energy, the pulsewidth, and the peak power: creases with increasing output coupler reflectivity, which also increases the gain in the cavity. However, the pulse − ln T 2 p = . (4) duration is almost constant when a T = 90 % Cr:YAG − ln R + L − ln T 2 is used, but the pulse duration is longer than it is for the T = 80 % Cr:YAG case. The denominator, − ln R + L − ln T 2, in p represent the The dynamics of the population inversion density in a fractional power loss per round trip. Thus, p means the gain medium, n, the photon density inside the resonator, contribution of SA to the fractional round-trip power φ, and the population density of absorbing state in SA, loss. Pavel et al. used the parameter to explain the ns, were well analyzed by Degnan [17]. However, the dependence of the lasing characteristics on T for the excited-state absorption cross-section σes in SA was ig- Nd:YAG laser. The experimental results in this work, nored in this work, and the effect of σes was considered the dependence of lasing characteristics on reflectivity, by other groups later [7]. The modified rate equations can also be explained with help of the calculations done are given as follows [7,9]: by Pavel et al. [7]. According to their calculation, when p increases, the peak power and the pulse energy increase. dn n = W − − γσcφn, (1) On the contrary, the ratio of the final population inver- dt τ sion before Q-switching to the initial population inver- dn n − n s = si s − γ σ cφn , (2) sion before Q-switching decreases with increasing p. dt τ s s s When the value of T goes from 0 to 1, p becomes dφ φ small, which means that the SA becomes less effective in = [2σnl − 2σsnsls dt tr controlling the photon density, thus leading to slow Q- switching. In this case, other parameters, such as R and −2σes(nsi − ns)ls − (L − ln R)]. (3) L, affect the lasing characteristics more. As p becomes In the equations, σ and σs are the stimulated emission smaller, the output characteristics approach those of a cross section of the gain medium and the absorption cross CW laser with high cavity loss. The pulse repetition section of the SA, respectively. The lengths of the gain rate increases more with decreasing p, and eventually Q- medium and the SA are represented by l and ls. The switching ceases as shown in Fig. 5. When T goes from thermal population reduction factors are represented by 1 to 0, p becomes closer to 1, and the SA becomes more γ and γs. Also, τ and τs denote the excited-state relax- effective. Increasing p leads to fast Q-switching because ation times for the gain medium and the SA, respectively, the population inversion ratio, before the Q-switching c is the speed of light, tr is the cavity round-trip time, to after the Q-switching, changes more. In Fig. 5, the nsi is the initial population of SA, R is the reflectivity pulsewidth in the T = 80 % case is narrower than that of output coupler, L is the residual loss, and W denotes in the T = 90 % case due to increased gain in the pulse the pumping rate. Degnan used the ratio of small signal buildup process. The p in the T = 80 % case is higher gain to cavity loss as the main parameter in the calcula- than the p in the T = 90 % case, as can be seen in Fig. tion to find the optimum output coupler reflectivity and 6, where p is calculated for various R and T . -326- Journal of the Korean Physical Society, Vol. 51, No. 1, July 2007

between T = 80 % and T = 90 % for various output cou- pler reflectivity were well explained by considering the ratio of the fractional loss due to Cr:YAG to the cavity round-trip loss.

ACKNOWLEDGMENTS

This work was supported by the Regional Innovation Center Program of the Ministry of Commerce, Industry, and Energy of Korea.

REFERENCES

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