1. Introduction

High-repetition-rate and short-pulse eye-safe 1.5 $\mathrm{\mu}\textrm{m}$ lasers with high pulse energy are attractive for many applications, such as quantum information, communication, and laser rangefinders [1,2]. For rangefinders application, laser pulses should have high repetition rates to attain high-speed scanning, short pulse duration to ensure high accuracy, and high pulse energy to achieve long-distance detection [1].

There are many active media used to generate high-repetition-rate and short-duration pulses in 1.5 $\mathrm{\mu}\textrm{m}$ solid-state lasers. Er:Yb:glass had gradually become the most favorite commercial 1.5 $ \mathrm{\mu}\textrm{m}$ laser medium since the Er:Yb:glass pulse laser was firstly reported in 1965 [3]. However, the poor thermal conductivity (0.85 W/m⁻¹K⁻¹) of the glass would lead to a serious thermal effect and degrade the performance of glass lasers at the high continuous-wave pump power. With the development of InGaAs diode lasers, numerous Er³⁺,Yb³⁺ codoped crystals with high thermal conductivity have been investigated to search for the promising 1.5 $ \mathrm{\mu}\textrm{m}$ laser gain media, such as Er:Yb:RAl₃(BO₃)₄ (R = Y, Lu and Gd) [4–10], Er:Yb:YVO₄ [11], Er:Yb:GdCa₄O(BO₃)₃ [12,13], and Er:Yb:Y₃Al₅O₁₂ [14]. Due to their higher thermal conductivity, higher energy transfer efficiency (larger than 88%), and weaker up-conversion loss, the Er:Yb:RAl₃(BO₃)₄ crystals have been demonstrated to be efficient 1.5 $ \mathrm{\mu}\textrm{m}$ laser media in the past few years.

Passive Q-switching is a simple method to generate the high-repetition-rate and short-pulse lasers. In 2012, a diode-pumped passively Q-switched Er:Yb:YAl₃(BO₃)₄ (Er:Yb:YAB) laser with a pulse duration of 5 ns and a repetition rate of 60 kHz was realized for the first time when a Co²⁺:MgAl₂O₄ crystal was used as a saturable absorber [15]. By applying a diffusion-bonded sapphire/Er:Yb:YAB/sapphire composite crystal instead of a single Er:Yb:YAB crystal as the gain medium, an efficient 1.52 $\mathrm{\mu}\textrm{m}$ pulse laser with a repetition rate of 105 kHz was established. But the corresponding pulse duration was up to 315 ns [16]. To minimize the thermal effect in the Er:Yb:YAB, two sapphire crystals were directly attached to two surfaces of the Er:Yb:YAB crystal. Then an efficient 1.52 $ \mathrm{\mu}\textrm{m}$ pulse laser with a repetition rate of 77 kHz and a pulse duration of 7 ns was obtained [17]. At the same time, the passively Q-switched monolithic laser based on an optical contact bonded Co²⁺:MgAl₂O₄/Er:Yb:GdAl₃(BO₃)₄ composite crystal was also demonstrated. A 1.52 $ \mathrm{\mu}\textrm{m}$ pulse laser with a repetition rate of 21 kHz and a pulse duration of 4 ns was achieved under the continuous-wave pumping [18]. The main drawbacks of passively Q-switched lasers are low pulse energy and significant pulse jitter [19]. The large jitter is not conducive to precise control through the external trigger, limiting the application of passively Q-switched lasers in some fields.

In order to obtain high pulse energy, acousto-optic Q-switched Er:Yb:YAB lasers have been investigated. In 2011, the firstly acousto-optic Q-switched Er:Yb:YAB pulse laser with the energy of 91 $ \mathrm{\mu}\textrm{J}$ and a pulse duration of 135 ns was realized at a repetition rate of 5 kHz [20]. Then, a 1.52 $\mathrm{\mu}\textrm{m}$ Er:Yb:YAB pulse laser with the energy of 210 $ \mathrm{\mu}\textrm{J}$, a duration of 45 ns, and a repetition rate of 1 kHz was also demonstrated in 2013 [21]. The pulse duration of acousto-optic Q-switched lasers is limited by the velocity of sound in the acousto-optic modulator. The typical pulse duration of acousto-optic Q-switched lasers is limited in tens of nanoseconds. It is difficult to generate the nanosecond pulses in lasers with low stimulated emission cross sections by the acousto-optic Q-switching [19].

The cavity dumping, whose pulse duration depends on the round-trip transit time in the cavity rather than on the gain of the active medium, is an elegant method to obtain high-repetition-rate and short-duration pulses in lasers based on crystals with low stimulated emission cross sections, such as Tm:YAP [22], Nd:YAG (emitting at 946 nm) [23], and Er:YAG [24].

To the best of our knowledge, the actively cavity-dumped Q-switched Er:Yb:YAB laser has not been reported. In this work, an electro-optically cavity-dumped Q-switched Er:Yb:YAB pulse laser was firstly demonstrated and the pulse characteristics of the laser were investigated in detail.

2. Experimental arrangement

The electro-optically cavity-dumped Q-switched laser is schematically depicted in Fig. 1. The pumping source was a 976 nm fiber-coupled diode laser with a core diameter of 105 µm and a numerical aperture of 0.22. The pumping wavelength coincides with the peak absorption wavelength of the Er:Yb:YAB crystal. The gain medium, an a-cut Er(1.5 at.%):Yb(12 at.%):YAB crystal with dimensions of 3 × 3 × 1.5 mm³, was placed in a V-shaped cavity. Two sapphire crystals with dimensions of 3 × 3 × 1 mm³ were tightly attached to the surfaces of the Er:Yb:YAB crystal to mitigate the thermal load. The Er:Yb:YAB and sapphire crystals were mounted in a Cu holder by screws. The temperature of the holder was kept around 286 K by the circulating water. The incident surface of the Er:Yb:YAB crystal coated for high reflection in 1500-1600 nm and high transmission at the pump wavelength was used as the pump mirror (PM). The output surface of the Er:Yb:YAB crystal was coated for high transmission in 1500-1600 nm. Both surfaces of the sapphire crystals were coated for high transmission in 1500-1600 nm. The pump beam was collimated and focused into the crystal by a pair of plano-convex lenses with the same focal length of 50 mm. A plano-concave mirror with a curvature radius of 100 mm used as the folding mirror M1 was placed at a position about 50 mm away from the PM so that the beam diameter between M1 and M2 remained approximately constant. The constant beam diameter minimized the variation of beam diameter through two EO-switch crystals, enhancing the EO-switch efficiency. Two thin-film polarizers (TFP) were inserted in the cavity at Brewster’s angle to provide high reflection for the s-polarized beam and high transmission for the p-polarized beam. A quarter-wave plate (QWP) and an RTP EO-switch were located between the TFP2 and the high reflection mirror M2. The axes of the QWP were oriented at 45° with respect to p and s polarizations, rotating the s-polarization 90° in double pass. When no HV was applied to the EO-switch, the s-polarized radiation selected by the TFPs would be rotated to p polarization. The radiation was transmitted through TFP2, thus preventing the optical feedback. A low Q was established in the resonator, and the inversion population in the crystal was accumulating. When the quarter-wave retardation voltage (1.0 kV here) was applied to the EO-switch, the s-polarized radiation would not be changed through the QWP and EO-switch in double pass. The resonator was switched to a high Q, and the stored energy in the crystal was converted to the optical radiation in the cavity. Once the retardation voltage was moved, the stored energy in the cavity was dumped through the TFP2 with the transmission of 100%. The period of the HV applied to the EO-switch was called the high-Q phase, and conversely, the period of the HV turned off was called the low-Q phase. The QWP and EO-switch were coated for high transmission in 1500-1600 nm. The EO-switch was driven by a HV switch consisting of a HV driver and a delay generator. The HV driver was capable of regulating the HV from 0.6 kV to 1.2 kV. The delay generator provided the HV pulse width ranging from 100 ns to 650 ns with a rise and fall time of 1 ns. The round-trip time corresponding to the cavity length was up to 2.8 ns.

 

Fig. 1. Schematic diagram of the electro-optically cavity-dumped Q-switched Er:Yb:YAB laser

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3. Results and discussion

The pump laser with pulse mode (2.5 ms pulse duration, 200 Hz repetition rates, and 27 W peak power) was used in this experiment. Pulse profiles of the intracavity and the dumped-output were monitored through the M2 and the TFP2, respectively, by two 12.5 GHz InGaAs photodetectors (ET-5000, Electro-optic Technology) connected to an oscilloscope with a bandwidth of 500 MHz (DSO-X 3052A, Agilent).

The performance of the cavity-dumped laser significantly depended on the length of HV time applied to the EO-switch. The average output power and pulse energy of cavity-dumped laser versus HV time at the repetition rate of 100 kHz are shown in Fig. 2(a). When the HV time was less than 412 ns, pulse energies of the intracavity and the dumped-output both maintained at a single value. The dumped-out pulse energy increased linearly with the increment of the HV time. As the HV time was increased from 412 ns to 618 ns, pulse energies of the intracavity and the dumped-out were bifurcated. Meanwhile, the average output power continued to rise and attained the maximum value of 890 mW at the time of 574 ns. The maximum stable pulse energy of 10 µJ with a duration of 3.1 ns and a repetition rate of 100 kHz was obtained at the HV time of 412 ns, corresponding to the average output power of 521 mW. The pulse laser wavelength measured by a wavelength meter (821, Bristol) was centered at 1531.1 nm, as shown in Fig. 2(b). The traces of pulse train and single pulse at the HV time of 412 ns are presented in Fig. 3. The amplitude variation between various pulses was generally kept within 3%. The trailing edges in the evolutions of two single pulses were attributed to the depolarization effects caused by the thermally induced birefringence in the EO-switch and the QWP. The depolarization effects rotated the polarization direction of the beam in the cavity so that the transmission of the TFP2 could not reach up to 100% during the dumping phase.

 

Fig. 2. (a) Average output power and pulse energy versus HV time at the repetition rate of 100 kHz. (b) Spectrum of the cavity-dumped laser at the HV time of 412 ns.

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Fig. 3. (a) Trains of the intracavity pulse (top) and the dumped-output pulse (bottom) at the HV time of 412 ns. (b) Evolutions of single pulse in the intracavity pulse (top) and the dumped-output pulse (bottom) at the HV time of 412 ns.

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Bifurcations in the HV time of 412-618 ns were similar to the period-doubling frequency bifurcations in regenerative amplifiers [25,26], which may lead to the instability of the laser. When the HV time exceeded the optimal storage time of the energy in the cavity, the gain remained at the end of the high-Q phase was quite small. During the upcoming low-Q phase, the small gain could not be restored to the value at the beginning of the previous high-Q phase. The pulse energy dumped out at the end of the following high-Q phase was relatively smaller than that of the previous pulse. At the HV time of 450 ns, the intensity of the dumped-out high-energy pulse (HEP) was higher than that of the dumped-out low-energy pulse (LEP), as shown in Fig. 4(a). As the HV time was further increased to 546 ns, the bifurcation became more severe, as shown in Fig. 5(a). Interestingly, the intensity of the dumped-out HEP was much lower than that of the dumped-out LEP. Owing to the higher average intracavity power at the HV time of 546 ns, the depolarization effects in the EO-switch and QWP were more severe than those of 450 ns. The transmission of TFP2 in the high-Q phase was no longer 0. In this case, the cavity-dumped laser during the high-Q phase could be regarded as a typical electro-optically Q-switched laser. Before being dumped, part of the intracavity laser had leaked through the TFP2, as shown in Fig. 5(b). Energy in the cavity used for dumping was so small that the peak intensity of the dumped-out HEP was significantly lower than that of the LEP. The duration of the HEP was no longer maintained at 3.1 ns and was broadened to 62 ns.

 

Fig. 4. (a) Trains of the intracavity pulse (top) and the dumped-output pulse (bottom) at the HV time of 450 ns. (b) Evolutions of contiguous single pulse in the intracavity pulse (top) and the dumped-output pulse (bottom) at the HV time of 450 ns.

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Fig. 5. (a) Trains of the intracavity pulse (top) and the dumped-output pulse (bottom) at the HV time of 546 ns. (b) Evolutions of contiguous single pulse in the intracavity pulse (top) and the dumped-output pulse (bottom) at the HV time of 546 ns.

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The pulse parameters of some important Q-switched lasers based on Er,Yb codoped materials are listed in Table 1. Although the pulse energies of acousto-optic lasers were greatly larger than that of other lasers, their pulse durations and repetition rates were longer and smaller, respectively. According to Ref. [21], the pulse energy and duration varied significantly with the repetition rate. With the rise of the repetition rate from 1 kHz to 30 kHz, the pulse energy decreased to 25 $\mathrm{\mu}\textrm{J}$, and the pulse duration increased to 94 ns. It is difficult for Er:Yb:YAB crystal to generate the high-repetition-rate and short-duration pulses by using the acousto-optic Q-switching. Compared with the pulse duration of acousto-optic lasers, pulse durations in passive Q-switched and cavity-dumping lasers vary slightly with the change of repetition rates. Although the pulse energy was up to 10 $\mathrm{\mu}\textrm{J}$, the amplitude between various pulses was changed significantly due to thermal effects [17]. The stable pulse energy with a 7% amplitude variation was obtained by decreasing the pump power, and the repetition rate dropped from 77 kHz to 67 kHz. The amplitude variation of the pulse (10 $\mathrm{\mu}\textrm{J}$, 100 kHz, 3.1 ns, 3% amplitude variation) obtained in this work is significantly lower than that of passive lasers mentioned above. It can be seen that the cavity-dumping is an elegant method for Er:Yb:YAB crystal to produce high-repetition-rate, short-duration, and low jitter pulses.

Table 1. Pulse parameters of some important Q-switched lasers based on Er,Yb codoped materials.

View Table

4. Conclusion

An electro-optically cavity-dumped Q-switched Er:Yb:YAB pulse laser was realized for the first time. The maximum stable pulse energy of 10 µJ with pulse duration of 3.1 ns was obtained at the repetition rate of 100 kHz. The length of HV time and the depolarization in the EO-switch and the QWP impacted the pulse characteristics significantly. The improper length of HV time would lead to the bifurcation of pulse energy. The depolarization in EO-switch and QWP turned the cavity-dumped laser into a typical electro-optically Q-switched laser; in this case, the HEP duration was no longer only determined by the cavity round-trip time and was broadened to tens of nanoseconds. Setting the width of HV time properly and reducing the depolarization effects are essential to obtaining a short-pulse and stable cavity-dumped laser.