1. Introduction

Recent progress of Giant Micro-Photonics toward ubiquitous power laser provides the extreme giant power with fruitful applications in the fields of nonlinear frequency conversion, Terahertz wave generation, laser ignition, Master Oscillator Power Amplifier (MOPA), and laser material process such as laser peening, phase transition, new material creation under intense shock wave etc [1–6]. The development of high-power and high-field solid-state laser is limited by the generated heating in the gain media during laser operation. The resulted high temperature gradients in transverse and longitudinal directions of the crystal create the so-called thermal lens effect with spherical aberrations requiring additional compensation for the system. In addition, a depolarization loss will reduce the energy extraction from the amplifier.

In a conventional microlaser based amplifier system, a side-pumped non-bonding Nd³⁺:YAG rod was used for sub-GW peak power at low repetition rate 10 Hz due to the thermal effects at higher repetition rate [6]. Several efforts have been made to remove heat from gain media. Hanson et al. reported a one-disk structure of YAG/Nd³⁺:YAG/YAG by diffusion bonding technology with heat removing from two-faces of Nd³⁺:YAG [7]. Mason et al. reported kilowatt average power of 105 J, 10 ns at 10 Hz from Yb³⁺:YAG ceramics based multi-slab by using large-sized cryogenic gas cooling in the amplifier system [8]. Since laser pump diodes are a major cost for solid-state lasers, it is especially important to consider a candidate gain media that allowing fewer pumping diodes with long storage lifetimes or excellent heat-removing capability. The Mercury laser system demonstrated over 60 J at repetition rate 10 Hz around 1047 nm from a diode-pumped Yb³⁺:S-FAP solid-state laser [9]. Yb³⁺:S-FAP has more fortuitous spectral features than Yb³⁺:YAG that enable it to operate efficiently even at room temperature [10].

To develop a compact sub-nanosecond giant-pulse tiny integrated laser for high-repetition-rate and high-field laser system, a new architecture of Distributed Face Cooling (DFC) is proposed in this article through fabricating monolithic multi-disk with distributed transparent heat sink (THS, e.g. sapphire) among thin gain medium (TGM, e.g. thin Nd³⁺:YAG) by Surface Activated Bonding (SAB) technology. A saturable absorber (SA, e.g. [110]-cut Cr⁴⁺:YAG) is used for giant pulse laser generation. As it was indicated in our previous work, high-quality output beam could be successfully achieved from above mentioned DFC chip but a strong thermal degradation could appear from Nd³⁺:YAG single chip during high-power pumping process [11]. DFC laser with monolithic multi-disk structure combines both the advantages of conventional rod laser and thin-disk laser. Firstly, DFC structure offers a flexible way of obtaining the same high gain cross section as required in rod laser. Secondly, distributed transparent heat sink among the multi-disk allowing for a direct diode end-pump operation leading to a compact and simplified laser system, which is different from the parabolic mirror based recycled pump scheme thin-disk laser structure. Thirdly, DFC structure could be successfully realized through room temperature SAB technology without thermal effects arising from post-annealing-process that could happen in diffusion bonding technology. Hence, the simplified monolithic multi-disk DFC chip could offer a flexible and smart way of removing heat from the gain medium effectively. The monolithic multi-disk DFC chip is both prospective for oscillator cavity and amplifier system by designing the thickness of gain medium and heat sink substrate.

In this article, DFC tiny integrated laser will be illustrated in three aspects. (1) The round-trip cavity loss will be estimated in [100]-cut Nd³⁺:YAG and a-cut sapphire based 9-disk DFC chip bonded by SAB technology. (2) A DFC tiny integrated laser from [111]-cut Nd³⁺:YAG and c-cut sapphire based 8-disk DFC chip will be experimentally presented with the reduction of thermal-induced birefringence at repetition rate 1 kHz as compared with those measured from [111]-cut YAG/Nd³⁺:YAG chip. The depolarization ratio (D_pol) from DFC chip at different repetition rate will also be evaluated. (3) A preliminary giant pulse tiny integrated laser from 8-disk bonded DFC chip will be presented together with 3-pulse burst modes.

2. Experiment setup

Figure 1 gives the principle of room temperature Surface Activated Bonding (SAB) technique in fabricating a DFC chip. Figure 1(a) shows the initial state of polished sapphire and Nd³⁺:YAG disk. The typical ZYGO-measured roughness value Sa, showing difference in height of each point compared to the arithmetical mean of the surface, was less than 500 nm in Nd³⁺:YAG and less than 50 nm in sapphire. After cleaning with acetone solution, the surface is activated through a pre-treatment by Fast Atom Beam (FAB) as shown in Fig. 1(b). The dangling bond is then formed on the surfaces as illustrated in Fig. 1(c) before starting the bonding process. After loading a certain value of pressure for some period, the bonding interface of DFC chip is firmly formed between two high-quality surfaces as shown in Fig. 1(d). The fabricated DFC chip undergoes no post-annealing process when compared with those prepared by thermal diffusion bonding technology. Room temperature SAB technology could also be used in high-power semiconductor lasers with improved heat dissipation [12].

 

Fig. 1 Principle of room temperature Surface Activated Bonding (SAB). (a) Initial state of disk; (b) Fast Atom Beam (FAB) irradiation; (c) Dangling bond formation; (d) Bonding formation.

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Figure 2 gives an example of SAB fabricated DFC chip. The thin gain medium (TGM) Nd³⁺:YAG crystal used in the experiments was with aperture size of 8 × 8 mm² and thickness of 1 mm. The transparent heat sink (THS) sapphire crystal was with aperture size of 10 × 10 mm² and thickness of 1 mm. The first sapphire disk was coated with anti-reflection (AR) at 808 nm and 1064 nm (coating A) on the side exposing to the air. The other side of sapphire crystal facing the Nd³⁺:YAG was coated with AR at 808 nm & high reflection (HR) at 1064 nm (coating B). The end disk was coated with HR at 808 nm & high transmission (HT) at 1064 nm (coating C) allowing for two-path absorption in the cavity to enhance the absorption efficiency. In our first trials of bonding between coated interfaces, the hand-shaking intensity was not strong. Only after improving the coating design, we succeeded in bonding coated sapphire and Nd³⁺:YAG by SAB technology. We are working on more data to clarify the mechanism. Instead, we used optical-contact method to place the coated sample in the cavity. Optical-contact method is to contact two high-optical-quality surfaces through mechanical pressure at room temperature. When removing the mechanical pressure, the contact surface will be detached. Hence, the optical-contact method is different from the room temperature SAB technology. The latter one could allow for strong bonding at atomic level between two materials through an ionic bonding or Van der Waals force regardless of the difference of thermal expansion or refractive index.

 

Fig. 2 Example of Distributed Face Cooling (DFC) chip fabricated by Surface Activated Bonding (SAB) technology.

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To determine the cavity loss from DFC chip, 9-disk DFC chip including 4 Nd³⁺:YAG and 5 sapphire was used. The output coupler (OC) mirrors with 4.46% and 3.10% transmission at 1064 nm were used to measure the slope efficiency under continuous wave pump power. Fiber coupled CW diode laser (LIMO) with maximum incident pump power of 86 W and fiber diameter of 400 µm was used. A collimated pump beam diameter 920 µm was used. The front sapphire plate with input mirror coating and the first Nd³⁺:YAG disk was contacted through optical-contact method by a certain mechanical pressure. The other 7 interfaces were bonded from the in-house SAB device. A power meter (FL300A-V1, Ophir) with maximum average power of 300 W was used.

The thermal-induced-birefringence was evaluated by fiber coupled quasi-continuous wave (qCW) diode laser (DILAS) allowing for a maximum pump peak power 600 W and maximum pulse duration 300 μs. 8-disk DFC chip combining [111]-cut 1.0 at.% Nd³⁺:YAG crystal and c-cut sapphire crystal was used. The front sapphire crystal with input mirror coating was placed in the cavity by optical contact method. The other 6 interfaces were bonded by SAB technology. The thermal-induced-birefringence, in the form of depolarization (D_pol), was then characterized in two different passively Q-switched lasers. The first case of passively Q-switched laser was done by using a [110]-cut Cr⁴⁺:YAG crystal with initial transmission T₀ = 20% and a flat output coupler with 60% reflection at 1064 nm. The second case of passively Q-switched laser was done by using a [110]-cut Cr⁴⁺:YAG crystal with initial transmission T₀ = 30%, while the output surface of Cr⁴⁺:YAG crystal was coated with 50% reflection at 1064 nm. The depolarization ratio of output laser beam was then characterized after half-wave plate at 1064 nm and a laser line cube polarizer with high damage threshold (TYPE: PBSO-1064-050). The split laser beam was detected by energy & power sensor (PE 25-C, PE 10-C), which was connected to a handheld laser power & energy meter (Nova II).

To operate the giant pulse DFC tiny integrated laser at 1 kHz repetition rate, the same pump source and DFC chip was used as those in above-mentioned thermal-induced-birefringence measurement. The Intense and Fast Pulse Pump (IFPP) was used, in which one first pulse pumps up the upper-level population, and then dumps it rapidly by Q-switching that could come close to complete pumping efficiency leading to reduced thermal problems [2]. A [110]-cut Cr⁴⁺:YAG crystal with initial transmission T₀ = 20% was used as saturable absorber. The output coupler (OC) mirror was coated with partial reflection (PR) 60% at 1064 nm. The beam quality value M² was measured by a CCD camera (SP620U) and BeamGage software (Ophir). The pulse duration was measured by oscilloscope (DPO71604C, 16 GHz, Tektronix). Finally, the burst modes from kHz DFC laser were characterized.

3. Results and discussions

3.1 Round-trip cavity loss from DFC chip

Figure 3 gives the schematic diagram of measuring round-trip cavity loss and the output power from 9-disk DFC chip under continuous wave (CW) laser pump. The input mirror was coated on the sapphire crystal facing Nd³⁺:YAG surface in order to shorten the cavity. The cavity length was 15 mm. The round-trip cavity loss L_i could be estimated from the relation between the incident pump power and the laser output power following the equation $L_i=\frac{(T_2/T_1)\cdot T_1-({\eta }_{s2}/{\eta }_{s1})\cdot T_2}{{\eta }_{s2}/{\eta }_{s1}-T_2/T_1}$, where η_s1 and η_s2 represents the slope efficiency at different output coupler (OC). The chiller temperature for diode laser (TD) was set at 12 °C for cavity loss measurement to realize the optimum absorption wavelength for Nd3+:YAG pumping under full diode laser power. The 9-disk DFC chip gave an output power of 47.7 W under pump power of 86 W and no roll-off curve was observed. Figure 3 also shows that the slope efficiency η_s was 64.6% when the transmission of output coupler (T_OC) was 4.46% at 1064 nm, while that was 61.8% when T_OC was 3.10%. The round-trip cavity loss L_i was then calculated as 0.51% in 9-disk DFC chip. The estimated value of round-trip cavity loss is at the same level as that was reported in 0.9% Nd³⁺:YAG single chip [13]. It indicates that such DFC chip is suitable for high-gain and low-loss required oscillator cavity or amplifier system.

 

Fig. 3 Round-trip cavity loss and the output power from 9-disk Sapphire/Nd^3+:YAG DFC chip under continuous wave (CW) laser pump. T_D stands for chiller temperature for diode laser.

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The Fresnel reflection could be deduced from the equation. The reflection index is n=1.78 for sapphire plate and n=1.82 for Nd³⁺:YAG. Thus, the Fresnel reflection loss occurring at one Sapphire/Nd³⁺:YAG interface is 0.012%. Accordingly, the total Fresnel reflection loss from the two-path designed 8 Sapphire/Nd³⁺:YAG interfaces is then calculated as 0.2%. The similar refractive index of sapphire and Nd³⁺:YAG allows a low Fresnel reflection loss in the bonded interfaces. Moreover, the front and end sapphire surfaces facing air (refractive index n=1) in the 9-disk DFC chip were coated with dielectric thin-film layers to reduce Fresnel reflection. From the estimation of laser output by using different transmission of output coupler as shown in Fig. 3, the measured cavity loss of 9-disk DFC chip was 0.51% under CW pump. It could be concluded that DFC chip is a suitable gain media for constructing low-loss cavity even the refractive index difference between two materials is 2.2%. New heat sink with higher thermal conductivity could be considered for fabricating DFC chip even the refractive index is different from the gain media. It points out a new research direction of optical shaping and patterning materials for optics.

Figure 3 also shows the low pump power limit laser performance comparison of Nd³⁺:YAG single chip with that of 9-disk DFC chip by using 3.1% output coupler with diode chiller temperature of TD = 24 °C to maintain the optimum pump wavelength for low diode laser power region. A curve roll-off from Nd³⁺:YAG single chip appeared after the incident pump power reached 41.7 W. To clearly illustrate the roll-off of output power in single chip, the chiller temperature for diode laser (T_D) was set at 24 °C. Accordingly, the diode laser gave higher output power at lower chiller temperature together with shifted wavelength resulting in the shifted laser threshold as shown in Fig. 3. The same experimental condition was also applied to DFC chip. A slope efficiency of 62.2% without roll-off was obtained in DFC chip after pump power reached around 50W. It could be noted from Fig. 3 that Nd³⁺:YAG single chip suffered crystal crack at pump power around 54 W partly due to the high pump power induced thermal lens effects, while the 9-disk DFC chip underwent no crack damage at pump power of 86 W indicating a higher capability of thermal reduction. The testimony of high-power induced thermal lens in Nd³⁺:YAG single chip is a solid proof that the design of DFC structure could offer an ideal solution to relieve the heating in high-power and high-repetition-rate laser system.

3.2 Thermally-induced-birefringence effects

The estimation of depolarization ratio in [111]-cut YAG/Nd³⁺:YAG chip was previously reported [14]. A fiber-coupled 120 W, 808 nm diode laser was used as pump source. The [111]-cut 4 mm-thick Nd³⁺:YAG single chip was end-cap bonded to a 1 mm-thick pure YAG by using adhesive-free-bonding technique. A [110]-cut Cr⁴⁺:YAG crystal with initial transmission T₀ = 40% was used for passively Q-switched laser. A flat mirror with a transmission of 50% at 1064 nm was used as output coupler. The reported depolarization ratio from [111]-cut YAG/Nd³⁺:YAG chip was increased from 0.1% at 100 Hz to 12% at 1 kHz.

Figure 4 shows the schematic diagram of thermal-induced-birefringence measurement and the depolarization ratio (D_pol) of passively Q-switched 1064 nm laser from Sapphire/[111]-cut Nd³⁺:YAG based DFC chip and YAG/Nd³⁺:YAG chip at different repetition rate. The [111]-cut Nd³⁺:YAG was used for parallel comparison. The depolarization (D_pol) could be described as the ratio of the depolarized energy to the total energy following the equation $D_{pol}=E_{\perp }/\left(E_{\parallel }+E_{\perp }\right)$, where $E_{\perp }$and$E_{\parallel }$ is the measured energy after a half-wavelength plate and polarized beam splitter.

 

Fig. 4 Thermal-induced-birefringence from Sapphire/Nd³⁺:YAG based DFC chip and YAG/Nd³⁺:YAG chip.

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The thermal-induced-birefringence from Sapphire/[111]-cut Nd³⁺:YAG based DFC chip was evaluated in two conditions under pump peak power of 600 W and pump pulse duration of 44 µs. The total thickness Nd³⁺:YAG in the DFC chip was the same as that used in [111]-cut YAG/Nd³⁺:YAG chip. In the first case using [110]-cut Cr⁴⁺:YAG crystal with initial transmission T₀ = 20% and a flat output coupler with 60% reflection at 1064 nm, the polarized output energy 1.12 mJ and 13.6 μJ was obtained under pump energy 26.4 mJ at repetition rate 1 kHz along the vertical and horizontal direction of the beam propagation direction, separately. Accordingly, the depolarization ratio was 1.2% at 1 kHz from Sapphire/[111]-cut Nd³⁺:YAG based DFC chip, which was around 10 times reduction as compared with that from [111]-cut YAG/Nd³⁺:YAG chip. In the second case from a [110]-cut Cr⁴⁺:YAG crystal with T₀ = 30% having output surface coated with 50% reflection at 1064 nm, the output energy 1.03 mJ and 3.12 μJ was obtained along the vertical propagation direction of the Q-switched laser beam, separately. Accordingly, the depolarization ratio was 0.3% at 1 kHz from Sapphire/[111]-cut Nd³⁺:YAG based DFC tiny integrated laser, which was around 40 times reduction compared with that from [111]-cut YAG/Nd³⁺:YAG microlaser.

Recent attempts to achieve giant pulse microlaser by combining [100]-cut Nd³⁺:YAG gain crystal with [110]-cut Cr⁴⁺:YAG saturable absorber succeeded in demonstrating intrinsically reduced depolarization ratio. The anisotropy of the saturable absorption in Cr⁴⁺:YAG even with the isometric crystal structure produced the modulation of the single polarized giant pulse without other polarization elements in the micro laser cavity. Sakai et al. and Bhandari et al. reported the stabilized linear polarization by using [110]-cut Cr⁴⁺:YAG crystal [15–17]. The difference in time constant of the absorption saturation in single crystalline Cr⁴⁺:YAG for E // [110] are two times faster than that for E // [111], which enables to control the polarization of the oscillation mode by means of using [110]-cut Cr⁴⁺:YAG saturable absorbers. Sato et al. described that the mechanism of anisotropy of saturable absorption in Cr⁴⁺:YAG cubic garnet systems comes from the nonlinearity which is caused by the two second-ordered tensors included cascade process between the absorption and the excited state absorption [18]. Bhandari et al. reported a reduction of depolarization ratio to 20% at 1 kHz by replacing [111]-cut YAG/Nd³⁺:YAG by [100]-cut YAG/Nd³⁺:YAG chip [14]. [100]-cut Nd³⁺:YAG crystal has 5 times reduction from that of [111]-cut YAG/Nd³⁺:YAG crystal. Accordingly, we could expect a 200 times reduction from Sapphire/[100]-cut Nd^3+:YAG based DFC module as compared with that from [111]-cut YAG/Nd³⁺:YAG crystal.

3.3 Burst modes from giant pulse DFC tiny integrated laser

A preliminary performance of kHz giant pulse tiny integrated laser was implemented in Sapphire/[111]-cut Nd³⁺:YAG based DFC chip. The sapphire crystal with input mirror coating was placed in the cavity by optical contact method. In the single-pulse laser operation with output energy 1.12 mJ and pulse width 767 ps, the pump energy range was between 23.4 mJ and 36 mJ. The M² value was both 4.0 along the vertical and horizontal direction of the beam propagation direction. The divergence was 15.2 mrad. A shoulder profile was observed in the 200% magnification times of beam profile figure, which might be originated in the optical contact surface between the heat sink sapphire plate and gain media Nd³⁺:YAG disk. In other words, the optical-contact interface would degrade the beam quality. A fully SAB-fabricated DFC chip is under preparation to avoid such shoulder profile and improve the beam quality.

The experimental results of 3-pulse burst modes at 1 kHz were achieved. A total energy 3.1 mJ was obtained at pump pulse duration 120 μs. The output energy did not follow strictly to the stair-shape rule that keeping the constant pulse energy until the following pulse was generated. The interval of pulse generation was around 21 μs, 30 μs and 27 μs during the 1st pulse, the 2nd pulse and the 3rd pulse. This is probably due to the thermal effects and pump wavelength control in the current kHz setup. Further DFC chip design including [100]-cut Nd³⁺:YAG and heat sink with higher thermal conductivity such as diamond would be considered to improve the heat removing capability.

4. Summary

Shape design of Sapphire/[111]-cut Nd³⁺:YAG based Distributed Face Cooling (DFC) giant-pulse tiny integrated laser is reported with the realization of thermal reduction. The round-trip cavity loss was estimated as 0.51% in 9-disk DFC chip. The depolarization ratio of Q-switched laser from DFC chip got 40 times drastically reduction compared with that from conventional YAG/Nd³⁺:YAG chip. The DFC chip underwent no crack damage at pump power of 86 W indicating a higher capability of thermal reduction while the Nd³⁺:YAG single chip at pump power around 54 W suffered crystal crack. The testimony of thermal lens relief showed that DFC structure is promising for high-power and high-repetition-rate laser system.