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development of a diode-pumped passively mode-locked Nd:YVO4 laser operated at pulse repetition rate lower than 10 Mhz



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development of a diode-pumped passively mode-locked Nd:YVO4 laser operated at pulse repetition rate lower than 10 Mhz




Do Quoc Khanh*, Nguyen Dinh Hoang, Nguyen Trong Nghia, Phung Viet Tiep, Nguyen Van Hao, Dao Duy Thang, Trinh Dinh Huy, Pham Hong Minh


Institute of Physics, Vietnam Academy of Science and Technology
10 Dao Tan, Ba Dinh, Hanoi, Vietnam
Abstract

The research and development of a passively mode-locked diode-pumped Nd:YVO4 laser operating at 1064 nm and at pulse repetition rates as low as 8.8 MHz were presented. The mode-locking of the laser was achieved with a low-modulation semiconductor saturable absorber mirror (SESAM – 2 %) and a long resonator based on the multi-pass cavity configuration and folded in two with two plan mirrors. As a result, the laser is compact and well set –up on an optical table (120 x 40 cm) without the use of special optical component. Using a CW diode pumping power of 1.8 W at 808 nm, the laser provided a stable train of 12 ps pulses (FWHM) with a peak power of 4 kW, corresponding to an average laser power of 410 mW (without amplification). Some applications to photonics research are realized with this laser.



Key words: laser resonator, diode-pumped laser, mode-locking

  1. INTRODUCTION

Fluorescence is widely used in biomedicine and photonic organic materials to track specific fluorophores and to study characteristics of molecular energy transfer. Fluorescence lifetime measurements add dynamic information but they require pumping pulsed laser sources emitting visible radiation. Because the fluorescence lifetime of numerous organic molecules is in the nanosecond to hundreds of picoseconds range, short laser pulses of several tenths of picoseconds are ideally needed from reliable pumping laser sources. The pulse repetition rate of the pumping laser source used is also an important parameter in experiments: High repetition rates ensure fast acquisition of the fluorescence decay signal and allow the dynamic processes to be studied. However, if the repetition rate exceeds several megahertz, problems appear in the complex signal-processing devices during the acquisition of the data. Therefore, a pulse repetition rate less than 10 MHz seems to be a good requirement for this kind of application. Some ways have been proposed to develop short pulsed lasers operating at such low repetition rates. A classical approach to generating short visible pulses is the use of cavity dumping [1]. The second one is the development of a Q-switched microchip laser operating at a fixed wavelength in the IR region (1064 nm) and the use of nonlinear processes to reach the visible range [2]. However, in such systems the use of amplification stages is needed. The third way is based on mode-locked laser oscillators [3, 4, 6, 7]. However, standard mode-locked solid state lasers usually produce pulses at repetition rate of 50-100 MHz that determined entirely by their resonator length. To obtain a pulse repetition rate lower than 10 MHz, the resonator length should be larger than 15 m. To achieve such a long resonator, one introduced a multiple-pass cavity (MPC) based on the classical Herriott-style MPC with two concave mirrors [5]. This resonator configuration was recently used successfully to decrease the frequency of a Ti:Al2O3 mode-locked laser to 4 MHz [6, 7].

In this presentation, we report the operation of a passively mode-locked diode-pumped Nd:YVO4 oscillator at a repetition rate lower than 10 MHz, emitting pulses of 12 ps duration with 410 mW of average power. It is a simple, cheap and picosecond laser source developed with commercially available component which still offers stability and high enough average and peak power as well as a long-term stable mode-locking operation at low-repetition rate without cavity dumping.



  1. EXPERIMENTALS AND DISCUSSION

The configuration of the MPC used in our laser system is in fact a version of the Herriott-style MPC folded in two with the help of two plane mirrors (Fig. 1). The distance of both concave mirrors (r = 2 m) from the two plane mirrors is fixed at approximately the focal length of the concave mirrors. The beam is periodically focused and defocused after each reflection on the concave and plane mirrors, respectively. The analysis about this laser resonator configuration was presented in detail [2]. The main advantage of this MPC configuration is that simply changing the alignment of the plane or the concave mirrors allows almost continual control of the number of passes through the MPC. The remarkable freedom of the easy control of the beam path in the MPC is the result of the use of the two plane mirrors. Each reflection on them, when the proper alignment is achieved, actually resets the beam into the MPC in such a way that it is forced to stay in the MPC for an arbitrarily large number of round trips. Furthermore, the use of other four plane mirrors could supply better and easier control of the number of reflections, compressing of course the final setup of the laser resonator on the optical table (L120 x W40 cm2). Additionally, in our case it was not necessary to drill or cut the concave mirrors for the entrance and exit of the beam.

Group 99

Fig. 1: Schematic of the diode-pumped passively mode-locked Nd:YVO4 laser resonator with the MPC consisting of two pairs of plan-concave mirrors
In this system, a 3 x 3 x 3 mm3, 0.1% doped Nd:YVO4 crystal (Casix) is pumped by a diode laser at 808 nm of 2-W maximum power (ATC-Semiconductor Devices). Passive mode-locking is achieved with the use of a semiconductor saturable absorber mirror – SESAM (Batop) [8-12] with a 2 % modulation depth. All mirrors of the laser resonator were provided by Casix. The mode-locked laser operation of such a system was presented in detail [12]. Reducing the pulse repetition rate to several MHz while maintaining the same average laser power provides higher peak powers. To avoid malfunction of the SESAM, therefore, a concave mirror of its curvature radius of 50 cm was used to focus the laser beam on the SESAM.

Fig. 2: a) Intensity auto-correlation trace of 12ps pulse duration (assuming a Gaussian profile), b) Average powers of the mode-locked Nd:YVO4 laser for different output mirrors.

Group 83

Fig. 3: Oscilloscope traces of a fast photodiode showing the mode-locked laser pulse train with a 115 ns separation.

During the alignment of the MPC inside the laser resonator, various lengths of the laser resonator could be used, resulting each time in the operation of the mode-locked laser system on a number of different repetition rates. With the use of an output mirror of 20 % transmission at 1064 nm, the laser provides the pulse duration of 12 ps (Fig. 2a) and an average laser power of 410 mW, corresponding to about 4 kW peak power. Fig. 3 presents a stable operation at a repetition rate of 8.8 MHz with a 115 ns separation. Specially, by adjusting the MPC alignment, we could achieve the laser operation at 5 MHz with the pulse duration of 14 ps and a peak power of about 7.0 KW. The second and third harmonic generations at 532 nm and 355 nm are straightforward.



  1. cONCLUSION.

The research and development of a passively mode-locked diode-pumped Nd:YVO4 laser operating at 1064 nm and 532 nm at pulse repetition rates lower than 10 MHz have been achieved. The mode-locking of the laser was obtained with a low-modulation semiconductor saturable absorber mirror (2%) and a multiple-pass resonator configuration folded with two concave mirrors (r = 2 m). As a result, the laser system was compact and well arranged on an optical table (120 x 40 cm2). Such a picosecond laser source is very demanded and attractive for applications in biology and photonics.

IV. aCKNOWLEDGEMENT

The authors would like to thank for the financial support from the Vietnam NAFOSTED (Physics, Project N0. 103.06.89.09).



References

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  2. S. Forget, F. Balembois, G. Lucas-Leclin et al, Opt. Commun. 220, 187 (2003).

  3. N. H. Schiller, X. M. Zhao, X. C. Liang, L. M. Wang et al, Appl. Opt. 28, 946 (1989)

  4. J. B. Deaton, Jr., A. D. W. Mckie, J. B. Spicer, and J. W. Wagner, Appl, Phys. Lett. 56, 2390 (1990); J. B. Deaton, Jr., and J. W. Wagner, Appl. Opt. 33, 1051 (1994).

  5. Herriott, H. Kogelnik, and R. Kompfner, Appl. Opt. 3, 523 (1994).

  6. S. H. Cho, F. X. Kärtner, U. Morgner, E. P. Ippen, J. G. Fujimoto et al. Opt. Lett. 26, 560 (2000).

  7. S. H. Cho, B. E. Bouma, E. P. Ippen and J. G. Fujimoto, Opt, Lett. 24, 417 (1999).

  8. U. Keller, D. A. B. Miller, G. D. Boyd, T. H. Chiu, J. F. Ferguson et al. Opt. Lett. 17, 505 (1992).

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  10. C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser et al. J. Opt. Soc. Am. B 16, 46 (1999)

  11. G. J. Spühler, T. Südmeyer, R. Paschotta, F. Moser et al. Appl. Phys. B 71, 19 (2000).

  12. N. T. Nghia, Do Q. Khanh et al. ASEAN J. Scien.& Tech. for Develop. 24, 1-2, 139 (2007).



FABRICATION OF POROUS LAYER ON THE NONDOPED AMORPHOUS SIC THIN FILM BY ANODIC ETCHING METHOD IN THE HF/H2O/H2O2 SOLUTION




Luong Truc Quynh Ngan1*, Cao Tuan Anh2, Nguyen Thi Thu Ha1, Pham Thi Lien2, Pham Thi Mai Hoa1,3 and Dao Tran Cao1


1 Institute of Materials Science, 18 Hoang Quoc Viet, Cau Giay Distr., Hanoi, Vietnam

2 Institute of Physics, 10 Dao Tan Street, Ba Dinh Distr., Hanoi, Vietnam

3Delft University of Technology, Mekelweg 4, 2628 CD Delft, the Netherlands
Abstract

Due to the inertness of the nondoped amorphous SiC (i-aSiC) material to the chemical impact, for making it porous by the anodic etching method people often have to add surfactants, e.g. Triton-X100, to the etching solution. However it has been found that the presence of Triton in the etching solution will extinguish the photoluminescence from the porous i-aSiC layer. This is the reason why we are seeking for other etching solutions. In this report we present another etching solution, in which H2O2 is used instead of Triton-X100. Results showed that with the appropriate ratio of components in the HF/H2O/H2O2 solution, in the i-aSiC thin film we can manufacture a porous layer with the porosity similar to the porosity of the porous layer obtained by etching in the HF/H2O/Triton-X100 solution with optimal composition.



Keywords: porous amorphous SiC, thin film, electrochemical etching.

  1. INTRODUCTION

The discovery of strong photoluminescence (PL) from porous silicon (PSi) of Canham [1] in 1990 has inspired researches on the porous SiC (PSiC), because people hoped that like PSi, PSiC will also have a strong photoluminescence. In practice, this hope has became a reality. Furthermore, the interest in PSiC has grown up because unlike the red luminescence from PSi, which is extinguished very quickly in air due to oxidation, it has been found that the blue luminescence from PSiC degrades very little in air at room temperature.

Up to now, most of research groups in the world are focused on fabrication and studying of porous crystal SiC, porous amorphous SiC (aSiC) is not sufficient studied. In literature we could find only few works concerning porous aSiC [2-4], so we have decided to focus our research on this material. Today, the main method for making SiC porous is the electrochemical etching (anodization) method. Due to the high resistance, the nondoped amorphous SiC (i-aSiC) thin films are especially difficult to etch by anodization. In previous works [5, 6], we showed that we have found a simple way to etch anodically this kind of films. Our method is consisted of adding Triton X100 surfactant to the HF/H2O electrolyte in order to make the etching process easier to occur. Unfortunately, the PL measurements have shown that the addition of Triton X100 to the electrolyte weakens significantly the PL from porous i-aSiC, especially the PL in the blue region [5]. Therefore if we would like to have non-degraded PL, we must find out another substance for replacing Triton X100. Among the papers about PSiC in the literature we have seen that in the reports [7, 8] the authors have mentioned the use of the electrolyte containing H2O2 for etching the crystal SiC (n-type 3C-SiC). Particularly, the authors of [7] showed that the aqueous solution containing 2.5 % HF and 0.5% H2O2 had the highest etching rate. Following these papers, we have tried to replace Triton X100 by H2O2 in the electrolyte. By this replacement we could have the high porosity of the etched layer together with the high PL intensity from this layer. The results of this study are presented in this report.



  1. EXPERIMENTAL

SiC original samples are the nondoped amorphous SiC (i-aSiC) thin film samples with the thickness of 1 m fabricated by plasma-enhanced CVD deposition at 400 oC on the standard Si substrate. These samples have been fabricated at DIMES Institute, Delft University of Technology, Netherlands.

We have made i-aSiC films porous by anodization. Normally, the SiC sample have been anodized using a current constant source, but in this work we have anodized i-aSiC films using a voltage constant source. The reason is that the voltage variation region of normal current constant sources are only several tens of Volt, while if we use a current constant source to anodize the nondoped amorphous SiC thin films with very high resistance, the required voltage variation region of the current constant source is hundreds of Volt. Therefore, all our samples have been anodized using a voltage constant source (type 051 TYP, a product of Hungary), which output voltage (we used 150V for all samples) is applied to the load consisting of a sample connected in series with a 90 kΩ resistor. All of the anodization processes were carried out in 30 minutes. The HF electrolyte was kept in a teflon tank. The i-aSiC/Si sample serves as the anode (positive voltage was imposed on Al layer which was evaporated on the backside of the n-aSiC/Si sample), and a platinum plate is used as the cathode. During the etching, the voltage and current are monitored and recorded. As etching proceeds, normally we have observed the reduction of voltage drop on the n-aSiC sample (which corresponds to the increasing of the anodizing current). In general, the initial voltage drop on the SiC sample was 130 V (which corresponds to 0.22 mA current), at the end of the etching process, the voltage drop was only about 20 V (which corresponds to the 1.44 mA current). The used electrolytes will be presented in the next section. After the etching process, the samples were washed in the deionization water and dried in air.



For testing of the morphology and PL properties of the obtained porous layers on the i-aSiC samples, all etched samples have been cut into two parts. One part was used for the morphology measurements by the S-4800 Field Emission Scanning Electron Microscope (Hitachi), while the other part was used for the PL measurements (with 325 nm light excitation from a He-Cd laser).

  1. RESULTS AND DISCUSSION

    1. Anodic etching of the i-aSiC thin film in HF aqueous solution containing H2O2

First of all, we have tried to fabricate the porous i-aSiC layer in the electrolyte comprising H2O, HF and H2O2 with the ratio of 205:5:1 (2,5 % concentrated (48 %) HF and 0,5 % H2O2 in H2O) which has been proposed by Shor et al [7]. However, the SEM images have showed that the porosity of the etched porous SiC layer was very low. Therefore, we have decided to change the HF and H2O2 concentrations in the etching solution with the purpose to find out the electrolyte which will give the best porosity. To perform this work, based on the previous obtained results that the sample which has been etched in the solution with 1% HF has the highest porosity, firstly we keep the HF concentration at 1% while changing the H2O2 concentration in the etching solution. We have varied the H2O2 concentration between 1 and 20 %. We have found that the electrolyte containing 10 % H2O2 was the best etching solution. Then, for finding out the best HF concentration in the electrolyte, we keep the H2O2 concentration at 10 % while changing the HF concentration in the electrolyte. At last, we have found that the electrolyte comprising 5% HF and 10% H2O2 was the best etching solution. These results have been shown in fig. 1. From this figure we can see that the porosity (including both the pore size and the pore density) of the porous layer formed on the original a-SiC film after being etched in the best electrolyte is much higher than the porosity of the same film after being etched in other electrolytes. The pore diameter of the sample with the best porosity is varied from 10 to 40 nm, and we would like to note that they are still somewhat smaller than that of the sample which has been etched in the HF aqueous solution containing Triton X100.


2,5%HF, 0,5%H2O2



1%HF, 10%H2O2



5%HF, 10%H2O2



10%HF, 10%H2O2


Fig. 1. SEM images of i-aSiC samples after anodization (the darker points present the etched pores). The anodization conditions are the same for all samples except the HF and H2O2 concentration of the electrolytes

      1. Photoluminescence from the porous layer on the i-aSiC sample



Fig. 2. PL spectra of i- aSiC samples after anodization in the electrolytes with different H2O2 concentration.

The results of PL measurements on the porous n-aSiC layers using the 325 nm light excitation from a He-Cd laser are shown in fig. 2 and 3. In fig. 2 we show the PL spectra of 3 i-aSiC samples with the same 1 m thickness after etching in the same anodization conditions but in the different electrolytes: 1 % HF, 1 % HF + 1 % Triton and 1 % HF+ 10 % H2O2. We can see clearly that the sample which was etched in the 1 % HF + 10 % H2O2 electrolyte has the strongest PL intensity and the sample which was etched in the 1 % HF+ 1 % Triton electrolyte has the lowest PL intensity. It means that if the PL intensity of the sample etched in the 1% HF electrolyte was taken as a standard, than the 10 % H2O2 addition to the electrolyte will enhance the PL intensity of the porous layer, while the 1 % Triton addition to the electrolyte will weaken the PL intensity of the porous layer. Thus, the Triton replacement by H2O2 has the advantage of ensuring the higher porosity of the porous i-aSiC layer together with enhancement of the PL intensity from this layer at the same time.



Fig. 3. PL spectra with 325 nm excitation of i- aSiC samples after anodization in the electrolytes with 10 % H2O2 and different HF concentration.

From the Fig. 3 we can say that the sample with the best porosity (which has been etched in the 5% HF + 10% H2O2 electrolyte) has the PL intensity much more lower than the sample with the much lower porosity, for example the sample etched in the 1% HF + 10% H2O2 electrolyte. In general for the i-aSiC samples etched electrochemically in the electrolyte containing H2O2 the rule is not the higher porosity will give the stronger PL intensity but the more dilute HF etchant will give the stronger PL intensity. The physical reason of this phenomenon is still unclear and requires further researches.



      1. CONCLUSION

The HF/H2O/H2O2­ solution can be used as an electrolyte in the electrochemical etching process for making i-aSiC porous, in which the solution with 5% HF and 10% H2O2 in water will give the highest porosity with the pore size varied between 10 and 40 nm. Moreover, H2O2, even when it was used with large concentration as indicated above, does not reduce the PL of the porous i-aSiC layer. This is a very big advantage of H2O2 in comparison with Triton X100. Another result is that the PL from the porous layer in the i-aSiC sample which has been etched in the electrolyte containing H2O2 is not directly proportional to porosity of the porous layer but inversely proportional to the HF concentration in the electrolyte.

      1. ACKNOWLEDGMENTS

The authors would like to thank MSc. Do Hung Manh for and MSc. Nguyen Thi Thanh Ngan for the SEM and photoluminescence measurements. This work was supported by the National Foundation for Science and Technology Development.

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  5. Dao Tran Cao, Cao Tuan Anh, Nguyen Thi Thu Ha, Huynh Thi Ha, Bui Huy, Pham Thi Mai Hoa, Pham Hong Duong, Nguyen Thi Thanh Ngan and Ngo Xuan Dai, Journal of Physics: Conference Series 187, 012023 (2009).

  6. Cao Tuan Anh, Dao Tran Cao, Nguyen Thi Thu Ha, Huynh Thi Ha, Pham Thi Mai Hoa, Pham Hong Duong and Nguyen Thi Thanh Ngan, Advances in Optics Photonics Spectroscopy and Applications V, Nha Trang, Viet Nam, September, 2008, p. 542.

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