High energy, high brightness picosecond master oscillator power amplifier with output power 65.5 W

Xuesheng Liu; Xuesheng Liu; Xuesheng Liu; Wenzeng Jia; Wenzeng Jia; Wenzeng Jia; Yiheng Song; Yiheng Song; Yiheng Song; Song Yang; Song Yang; Song Yang; Shu Liu; Shu Liu; Shu Liu; Youqiang Liu; Youqiang Liu; Youqiang Liu; Anru Yan; Anru Yan; Anru Yan; Zhiyong Wang; Zhiyong Wang; Zhiyong Wang

doi:10.1364/OE.383724

1. Introduction

Picosecond (ps) pulses with high average power, high beam quality and high brightness have been in great demand in different fields, especially in micromachining process [1–3], pulse laser ranging [4,5], optical communication [6], plasma generation [7], nonlinear frequency conversion [8,9] and other investigations in basic science. All these applications benefit from the excellent characteristics of picosecond pulses with narrow pulse width, narrow spectrum, high beam quality, high pulse repetition rate (PRR) and so on. The laser system of semiconductor saturable absorber mirrors (SESAM) passively mode-locked seed generator in combination with the master oscillator power amplifiers (MOPA) is the most promising approach to achieve the features above-mentioned.

In order to obtain the desired ps pulses, priority should be given to stable picosecond seed sequences with high beam quality and low PRR. The direct generation of high-beam-quality ps pulses has been extensively reported from a microchip laser [10], fiber laser [11,12] and Nd:YVO₄ rob laser [13–20], etc. However, for the sake of stability, all-solid-state lasers can efficiently reduce the requirements for the design of subsequent amplifiers. At the same time, the Z-type [19] and multi-reflection folded cavity structures [13–18] have been more and more applied to passively mode-locked ps laser because of the limitation of optical length and PRR on linear cavity [20]. Typically, in 2010, X. Wushouer [13] reported a LD-end-pumped SESAM mode-locked Nd:YVO₄ oscillator with a folded cavity, which had a power of 1.8 W, a PRR of 86 MHz and a beam quality factor of M²<1.14. And then, an Yb-doped photonic crystal fiber amplifier and a dual-end-pumped composite YVO₄-Nd:YVO₄-YVO₄ amplifier were adopted to scale up the power to 53 W with M²<1.15 and optical-optical (o-o) efficiency of 42.4%. Their research provided reference for high-power, high-beam-quality picosecond pulse generation from a low-power seed. On the other hand, regenerative amplifiers [16,17] are a common means of reducing PRR for the seed, but it also makes laser system more complex and bulkier.

Compared with the undesired nonlinear effect and inevitable laser induced damage caused by high peak power in fiber amplifiers, all-solid-state Nd³⁺-doped amplifiers with high emission cross section have been widely applied in 1064-nm picosecond pulse generation. In the process of power scaling, the accumulation of thermal effect and the mismatch of mode field will lead to the deterioration of output characteristics. In view of this, the multi-pass multi-stage amplifiers with end pump have a better performance so far. In 2015, Aleksandrov [21] demonstrated a LD-end-pumped Nd:YVO₄ laser using χ²-lens mode-locking in PPMgSLT SHG crystal. By means of negative intra-cavity self-phase modulation, 6.1 ps self-started mode-locked pulses with 6.1 W average power and 600 MHz PRR were generated. In 2016, Ying Chen [16] reported a 1064-nm, low-PRR, high-peak-power ps MOPA system comprised a ps mode-locked oscillator, a regeneration amplifier and three-stage Nd:YAG slab amplifiers, which enable to achieve a single pulse energy of 8.2 mJ and a peak power of 324 MW under a PRR of 5 kHz and a pulse duration of 25.3 ps. The beam quality factors were measured to be 2.8 and 2.2 along the x and the y direction, respectively, corresponding to an output beam brightness of 5.88 × 10⁸ W·cm⁻²·Sr. Recently in 2018, Zhang Hongmei [17] developed an industrial-grade high-energy LD-end-pumped picosecond MOPA laser. The output power of 200 mW with pulse width 15.8 ps and PRR 93.85 MHz was realized from the SESAM mode-locked Nd:YVO₄ oscillator, which was used as the seed. After regenerative amplifier and traveling-wave amplifier, a pulse energy of 12 mJ was achieved finally with a tunable PRR in the range of 1∼20 Hz. In the same year, Bin Liu and Yong Wang [12] presented a ps MOPA system based on hybrid end-pumped Nd:YVO₄ amplifiers and side-pumped Nd:YAG amplifiers with high peak power and high beam quality. The mode-locked fiber seed laser with pulse duration 4.8 ps and PRR 30 kHz was amplified to over 31.8 W with a pulse duration of 10.5 ps, corresponding to a peak power over 100 MW for the first time. The beam quality was optimized to be M²=1.26 by wave front aberration self-compensation. At the basics of previous studies, we need to scale the power for further while maintaining the beam quality.

In this work, a high pulse energy, high beam quality and high brightness SESAM mode-locked MOPA laser system was demonstrated. A folded-cavity structure was considered in master oscillator to increase the length of cavity under the premise of saving space. A compact RTP electro-optic switch as the pulse picker was adopted to reduce the PRR. Meanwhile, a combination of small-signal multi-pass amplifiers and large-signal single-pass amplifiers was studied so as to avoid the problem of power and beam quality degradation caused by thermal effect and mode mismatching. Ultimately, an average power of 65.5 W with pulse duration 16.9 ps, PRR 496.85 kHz was obtained. And beam quality factors of final output were measured to be M_x²=1.36 and M_y²=1.32, respectively. The corresponding output beam brightness was as high as 3.22 × 10⁹ W·cm⁻²·Sr, which was the highest for ps pulses at 1064 nm so far.

2. Theoretical analysis

The necessary theoretical analysis is not only the guidance for design of experimental scheme, but also the preliminary verification of its feasibility. We have used mature theories to simulate the mechanism of master oscillator and power amplifiers, respectively, and some improvement approaches were adopted to our experiment accordingly.

There are many factors affecting the output characteristics of high-power all-solid-state CW oscillator, including crystal doping concentration, crystal size, intracavity loss and pump characteristics. According to the empirical formula built up by Li [22], the output power of Nd:YVO₄ laser under strong pumping conditions is given by the Eq. (1):

(1)$$\begin{aligned}{P_{out}} &= {Z_p} \times {Z_T} \times \frac{{{\lambda _p}}}{\lambda } \times \frac{T}{W} \times {P_{in}}\\ &= {Z_p} \times [{1 - \exp ( - C \ast {d^b} \ast l)} ]\times \frac{{{\lambda _p}}}{\lambda } \times \frac{T}{{{C_1} \ast d \ast l + {C_2} \ast {P_{in}} + {W_0} + T}} \times {P_{in}} \end{aligned}$$

where ${P_{out}}$ and ${P_{in}}$ are the output power and the pump power of the oscillator, ${Z_p}$ is the quantum efficiency, ${Z_T}$ is the absorption efficiency of pump laser, ${\lambda _p}$ and $\lambda $ are the wavelengths of the pump beam and the oscillator beam, W is the intra-cavity loss. Generally, the intra-cavity loss W is composed of the loss T as the transmittance of the output coupling mirror, the loss ${W_0}$ as intra-cavity diffraction and incomplete mirror reflection and the loss ${W_1}$ as the absorption of the intra-cavity components. Among them, ${W_0}$ and T are determined by the structure parameters of the resonator, while ${W_1}$ is related to the crystal doping concentration d, the optical length l and the pump power ${P_{in}}$. Therefore, the absorption efficiency of the pump laser ${Z_T}$ and the intra-cavity loss W could be represented by Eqs. (2) and (3).

(2)$${Z_T} = 1 - \exp ( - {T_p} \ast l) = 1 - \exp ( - C \ast {d^b} \ast l)$$

(3)$$W = {W_1} + {W_0} + T = {C_1} \ast d \ast l + {C_2} \ast {P_{in}} + {W_0} + T$$

In which ${T_P}$ represents the absorption coefficient of the pump laser and C, b, ${C_1}$, ${C_2}$ are all experimental constants.

According to the empirical formula mentioned above, a simulated curve via MATLAB of the output power of the oscillator we designed as a function of the pump power was obtained as shown in Fig. 1.

Fig. 1. Simulated curve of output power of oscillator as a function of the pump power.

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With the increase of the pump power, the output power of oscillator grows almost linearly and a theoretical output power of 5.67 W is obtained at a pump power of 8.92 W. Meanwhile, when the pump power continues to increase, the heat loss will increase, resulting in slight decline in conversion efficiency, and the curve slope tends to decrease. And apparently the deterioration of thermal effect will also lead to poor beam quality. Therefore, the contradiction between high output power and excellent beam quality should be considered fully when choosing the pump power of seed.

The extracted energy during amplifying depends on the energy stored in the amplifier gain medium prior to the signal input. The amplifier gain expression of laser pulse is the Eq. (4) according to the Franz-Nodvik theory [23].

(4)$$G = \frac{{{E_s}}}{{{E_{in}}}}\ln \left\{ {1 + \left[ {\exp (\frac{{{E_{in}}}}{{{E_s}}}) - 1} \right] - {G_0}} \right\}$$

where ${E_s}$ and ${E_{in}}$ are the saturation energy density and the input energy density, ${G_0}$ is the small-signal gain coefficient, which relates to the pump energy.

(5)$${G_0} = \exp ({g_0}l) = \exp (\beta {E_s}l)$$

where ${g_0}$ is the gain coefficient per unit length and l is the length of gain medium. Due to Nd:YVO₄ is a four-level system, ${E_s}$ could be expressed as the Eq. (6). In which h is the Planck constant, $\nu $ is the pulse frequency and $\sigma $ is the simulated emission cross-section.

(6)$${E_s} = \frac{{h\nu }}{\sigma }$$

After substituting Eqs. (5) and (6) into the Eq. (4), the output power of the laser amplifier could be expressed as the Eq. (7). Where n is the number that the laser passing through the gain medium, ${g_0}l$ is the small-signal gain factor of single path.

(7)$${E_{out}} = (\frac{{{E_s}}}{n})\ln \left\{ {1 + \left[ {\exp (\frac{{n{E_{in}}}}{{{E_s}}}) - 1} \right]\exp (n\sigma {N_0})} \right\}$$

(8)$${N_0} = \frac{{{g_0}l}}{\sigma }$$

The empirical formula Eq. (7) was substituted into MATLAB for simulation. Based on the parameters of single-pass amplifier stage, an output energy calculated by theory of 82.07 µJ and 193 µJ were obtained for the 3^rd-stage and the 4^th-stage amplifier, respectively.

3. Experimental setup

A high-power all-solid-state master oscillator power amplifier (MOPA) system was presented to obtain high pulse energy output at 1064nm, which consisted of a mode-locked picosecond oscillator, a four-pass and a dual-pass traveling-wave amplifier stages and two single-pass Nd:YVO₄ amplifier stages.

Figure 2 shows the schematic diagram of the picosecond master oscillator, in which a semiconductor saturable absorption mirror (SESAM) was used for mode-locking. The developed master oscillator mainly comprised a SESAM with a saturable relaxation time of 1 fs, a modulation depth and a unsaturated loss of 0.5% and 0.5% respectively, a Nd:YVO₄ crystal rod with a size of 3 × 3×7 mm³ and a Nd³⁺ doped concentration of 0.5 at.%, a dichroic reflector M0 with a 1064 nm high-reflection (HR) coating and an 808 nm anti-reflection (AR) coating, a output coupler (OC) with a transmittance of 8% at 1064 nm, a set of coupling lenses (Coupler 1) with a magnification of 1:2 and a 30 W pump laser diode (LD) with emission wavelength of 808 nm and coupling fiber core diameter of 200 µm.

Fig. 2. Experimental setup of SESAM mode-locked picosecond oscillator.

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The pump beam centered in 808 nm, which delivered from the LD (Nlight, P2-030-0808-3-A-R01-S0119), was coupled into the Nd:YVO₄ crystal via the Coupler 1. The Nd:YVO₄ crystal rod adopted in the experiment was cut along the a-axis, in which the end faces were AR coated at both 1064 nm and 808 nm. The self-starting passively mode-locking was initiated by the SESAM (Batop, SAM-1064-1-1ps-4.0-25.4s), and the picosecond signal laser oscillated in the multi-reflection folded cavity. The folded cavity structure was designed to make our laser system more compact, the overall length and the folding angle of the folded cavity were 5.124 m and 8 degree respectively. M1, M2, M3 and M4 were concave mirrors, each was HR coated at 1064 nm.

The master oscillator as a seed source would work with subsequent four-stage travelling-wave amplifiers to form the MOPA system. The schematic diagram of mode-locked picosecond MOPA system is presented in Fig. 3. Stable picosecond pulse sequence from the seed source with a PRR of 30 MHz was delivered into the pulse picker after a convex lens L1 with 200-mm focal length and a faraday isolator. The pulse picker used in the experiment was a RTP (RbTiOPO₄) electro-optic switch with the size of 5 × 5×25 mm³. By adjusting the switching time and operating frequency of pulse picker to allow one out of every 60 pulse trains to pass, the PRR of the seed could be reduced to 500 kHz subsequently. The purpose of the isolator was to protect the seed oscillator from the damage of backward power. Due to the insertion loss of the pulse picker, the output power after that was reduced to only 30 mW. The picked seed pulse, which was converted to horizontal polarization by a polarizer at Brewster’s angle, was subsequently amplified up to be 16.19 W by the 1^st-stage four-pass amplifier and the 2^nd-stage dual-pass amplifier.

Fig. 3. Schematic diagram of SESAM mode-locked picosecond MOPA system.

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Two single-pass travelling-wave amplifier stages of nearly identical construction were adopted for further power scaling. The active medium of each single-pass amplifier was a a-cut Nd:YVO₄ crystal rod with a size of 4 × 4×32 mm³ and a Nd³⁺ concentration of 0.6 at.%. The end face of Nd:YVO₄ crystal rod was cut at 2 degree to suppress the generation of ASE, and both ends were AR coated at 1064 nm and 808 nm. The active medium was longitudinally pumped by 808 nm laser diode with the power of 130 W, (Nlight, 1067385IOIPN) coupled into a 400 µm optical fiber (NA = 0.22). The pump beam was focused by a 1:2 reimaging unit (Coupler 2 and 3) and injected into Nd:YVO₄ crystal through the mirror M8 and M10, which were both AR coated at 808 nm and HR coated at 1064 nm. The signal beam coupled into the 3^rd-stage and the 4^th-stage single-pass amplifiers via convex lens L2 and L3 converted the energy stored in the active medium into signal power. Plane mirrors M1∼M7 and M9 in Fig. 3 were all HR coated at 1064 nm.

Moreover, the Nd:YVO₄ crystal rods and the pump sources were wrapped by indium foil and placed in copper heat sinks, which were cooled by the water chiller with a temperature set of 19 ℃. Water-cooling manner and good contact between heat sinks and indium foil might ensure excellent heat dissipation effect, thus compensating the thermal effect of the active medium and improving laser beam output characteristics under the condition of high-power operation.

4. Experimental results and discussion

During the experiments, we characterized and evaluated the output indexes of seed oscillator and MOPA amplifiers respectively.

For the presented master oscillator configuration, the variations of output power and o-o efficiency as a function of pump power are displayed in Fig. 4. An average output power of 3.39 W could be obtained with a pump power of 8.92 W at the pump current 2 A with an o-o efficiency of 38%, superior to other reports with the same type [14–16]. And the mode-locked pulse train was recorded by a digital oscilloscope (Tektronix, MDO4104C), the PRR of 29.27 MHz is easy to read from Fig. 5, corresponding to a single pulse energy of 0.116 µJ. The power fluctuation of the seed laser was also measured for twice with the instability less than 2% (1.28% and 1.56% respectively), as plotted in the lower-right inset of Fig. 4.

Fig. 4. Seed output power and o-o efficiency versus the pump power for master oscillator. Insert figure shows the power fluctuation measured in 2 hours.

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Fig. 5. Mode-locked pulse train of the master oscillator.

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At the average output power of 3.39 W, the beam quality factor M² of seed laser shown in Fig. 6 was measured to be 1.12 and 1.07 along the x, y axis direction, respectively. The difference in M² between two different directions was related to the asymmetry of crystal cooling structure. Seed laser with high beam quality and conversion efficiency is one of the most important factors that determined the performance of high power MOPA laser with excellent output characteristics. The optimized design of the master oscillator has met the requirements of MOPA system in terms of seed laser pulse width and beam quality.

Fig. 6. Measured beam quality M² of the seed (Insert: near-filed beam intensity distribution).

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After passing through the pulse picker, the power of seed laser was attenuated to 30 mW at a PRR of 500 kHz. In order to maximize the ultimate output power, we set the temperature of cooling water for active medium to be 19 ℃ and adjusted all parts coaxial. The amplified average output power and corresponding o-o efficiency of four-stage multi-pass and single-pass amplifiers versus separate pump power are displayed in Fig. 7 (green dots represent the working points in the experiment). Over all, the output power increased linearly with the pump power. It is fully demonstrated that the optimized design of small-signal multi-pass amplifier and large-signal single-pass amplifier could restrain the thermal lensing effect well within the pump range. As shown in Figs. 7(a) and 7(b), the average output power was amplified to 3.84 W and 16.19 W with the o-o efficiency of 20.42% and 38.96% after the 1^st-stage four-pass amplifier and the 2^nd-stage dual-pass amplifier respectively. The following two single-pass amplifier stages had exactly the same structural design. Under the pump power of 100.6 W at the pump current of 7 A, an average output power of 35.7 W with an o-o efficiency of 19.39% was obtained from the 3^rd-stage single-pass amplifier, corresponding to a single pulse energy of 71.4 µJ. Similarly, the 4^th-stage single-pass amplifier produced a maximum average output power of 65.5 W of 1064 nm at the pump current of 8 A and the pump power of 116.2 W, corresponding to an o-o efficiency of 25.6% for the 4^th-stage single-pass amplifier. The equivalent o-o efficiency of four-stage travelling-wave amplifiers was:

(9)$${e_{o - o}} = \frac{{{P_{out}} - {P_{in}}}}{{{P_{pump}}}} = \frac{{6\textrm{5}.5W - 0.03W}}{{2\textrm{67}\textrm{.16}W}} = 24.\textrm{5}\%$$

It can be seen from Figs. 7(a)–7(d) that there are slight discontinuities of the signal power before and after entering the next amplifier stage under the threshold condition of pump lasers. This is mainly because of the absorption loss due to mirrors and crystal rods as well as the diffraction loss owing to aperture limitation. At the same time, the gain of amplifiers is not large enough that resulting in a relatively lower o-o efficiency under low pump power, as shown in Figs. 7(c) and 7(d). In addition, a repetition rate for the final output pulse of 496.85 kHz was measured accurately by a 200 MHz bandwidth photodetector (CONQUER, PR-200M3150), and the corresponding single pulse energy was calculated to be 131.83 µJ. However, this measured result is lower than the theoretical simulation in section 2, mainly because the influence of thermal effect was not considered in the previous simulation model.

Fig. 7. Average signal output power and o-o efficiency of four-stage travelling-wave amplifiers versus pump power: (a) the 1^st-stage four-pass amplifier, (b) the 2^nd-stage dual-pass amplifier, (c) the 3^rd-stage single-pass amplifier, and (d) the 4^th-stage single-pass amplifier.

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The autocorrelation trace of the amplified pulse was directly measured by an interferometric autocorrelator (APE, Pulse check USB SM 600). As shown in Fig. 8, the pulse duration was measured to be 16.9 ps, which marked a maximum peak power of 7.8 MW we achieved.

Fig. 8. Measured autocorrelation trace (Gaussian fitting) for the amplified pulse.

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Moreover, the beam quality factor M² at the output energy of 131.83 µJ was monitored with the knife-edge method. Hyperbolic fitting was performed for the data of variation of beam radii at different positions and the M_x² and M_y² we calculated according to the fitting parameters were 1.36 and 1.32, respectively, as is shown in Fig. 9. And we note a slight degradation in M² compared to the injected seed laser. As can be seen from the beam profiles illustrated in Figs. 6 and 9, it is the subtle change in Gaussian intensity distribution after multi-stage amplifiers that leads to the deterioration in beam quality. Even so, the beam quality factors of 1.36 and 1.32 are good enough to meet the requirements of practical applications. From the perspective of practical applications, especially in frequency conversion field, brightness B is the parameter we concern more, which is defined as [24]:

(10)$$B = \frac{{C \ast P}}{{{\lambda ^2} \ast M_x^2 \ast M_y^2}}$$

where P represents the average output power and C is a constant depending on the beam profile (C = 1 for a Gaussion-type beam). The brightness of final output pulse after four-stage multi-pass amplifiers was calculated to be 3.22 × 10⁹ W·cm⁻²·Sr, which is 1.8, 5.5 and 6.9 times respectively higher than other works in [12] (31.8 W, M²=1.26, B = 1.77 × 10⁹ W·cm⁻²·Sr), [16] (41 W, M_x²=2.8, M_y²=2.2, B = 5.88 × 10⁸ W·cm⁻²·Sr) and [10] (10.2 W, M_x²=1.37, M_y²=1.40, B = 4.70 × 10⁸ W·cm⁻²·Sr).

Fig. 9. Measured beam quality factor M² of the amplified pulse (Insert: near-filed beam intensity distribution).

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5. Conclusion

In summary, A SESAM mode-locked, multi-stage and multi-pass amplifying picosecond master oscillator power amplifier system was demonstrated with high pulse energy and high beam quality ps pulse. After pulse picker and four-stage traveling-wave amplifiers, the final average output power of 65.5 W was obtained. The maximum single pulse energy of 131.83 µJ was achieved with a repetition rate of 496.85 kHz, and the maximum peak power reached 7.8 MW with a pulse duration of 16.9 ps. The equivalent o-o efficiency of four-stage multi-pass amplifiers was 24.5%. Meanwhile, the beam quality factors M_x² and M_y² were measured to be 1.36 and 1.32, respectively, corresponding to the brightness of 3.22 × 10⁹ W·cm⁻²·Sr. As far as we all know, this is the highest brightness for a picosecond pulsed Nd:YVO₄ MOPA laser at 1064 nm. We will further optimize the thermal cavity model to provide reference for the design of the same type of lasers. Furthermore, this high energy, high brightness picosecond laser will have an enormous application potential in laser processing, especially in hard and brittle materials.

Funding

National Key Research and Development Program of China (2017YFB0305800); 19 Science and Technology Innovation Service Capacity Building - Basic Scientific Research Business Expenses (Research Category) (101000546319528).

Disclosures

The authors declare no conflicts of interest.

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High energy, high brightness picosecond master oscillator power amplifier with output power 65.5 W

Abstract

1. Introduction

2. Theoretical analysis

3. Experimental setup

4. Experimental results and discussion

5. Conclusion

Funding

Disclosures

References

Cited By

Figures (9)

Equations (10)

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