An actively mode-locked Ho: YAG solid laser pumped by a Tm: YLF laser

A mode-locked pulse has the characteristics of high peak power and a short pulse. Ultrafast picosecond lasers emitting in the 2 µm spectral range have a wide range of application prospects and value in many fields, such as micromachining [1], laser spectrum technology [2], medicine [3], exciting high harmonics [4, 5] and nonlinear frequency conversion [6–8]. Therefore, researchers have much interest in 2 µm ultrafast lasers.

Both Ho³⁺ and Tm³⁺–Ho³⁺ co-doped crystals can be used to emit picosecond pulses in this range. Because the conversion is prone to loss between Tm³⁺ and Ho³⁺, the Tm³⁺–Ho³⁺ co-doped crystals must work at ultra-low temperatures to inhibit loss and improve the output efficiency [9, 10]. The Ho³⁺ crystals can be pumped by 1.9 µm lasers, called in-band pumped lasers, which have the advantage of producing minimal loss, low heat and high efficiency [11–13].

Mode-locked lasers can be divided into actively mode-locked (AML) or passively mode-locked (PML) lasers. Typically, actively mode-locked lasers can produce several tens or hundreds of picosecond width pulses and have higher energy in a single pulse as compared with passively mode-locked lasers. In recent years, there has been much research reported on actively mode-locked lasers emitting in the 2 µm spectral range. For example, in 1994 [14], Schepler et al obtained a mode-locked Tm, Ho: YLF based on the AOM pumped by a 795 nm laser diode (LD) with a pulse width of 370 ps, repetition frequency (RF) of 150 MHz and maximum output power of 300 mW. In 2003 [15], Galzerano et al achieved an AML Tm, Ho: BYF laser using the same method, pumped by a 780 nm LD with a pulse width of 97 ps, RF of 100 MHz and maximum output power of 20 mW. In 2013 [16], the Q-peak Company accomplished an AML Ho: YLF laser with a 290 ps pulse width, 81.36 MHz RF and 4 W output power pumped by a 1940 nm Tm³⁺ fiber laser.

To our knowledge, this is the first time a continuous wave mode-locked Ho: YAG laser based on AOM pumped by a 1.91 µm Tm: YLF laser has been reported on. In this experiment, with the dimensions of 1.5  ×  6 × 50 mm³ and the doped-concentration of 0.8 at.% Ho: YAG crystal, up to 1.04 W of CWML output lasing at 2097.25 nm was obtained when the pump power was 13.2 W. The pulse RF was 82.76 MHz and the mode-locked pulse width was 102 ps measured through a no-background second harmonic autocorrelation. Furthermore, the M² factor was calculated to be 1.146.

The geometry of an actively mode-locked Ho: YAG laser is shown in figure 1. The pumped laser was a Tm: YLF laser pumped by two fiber-coupled laser diodes to the 789 nm emission wavelength with maximal output power of 30.3 W for each. With an increase of the pumped power, the output wavelength of the Tm: YLF laser was in the range of 1908.1–1908.5 nm and the maximal output power of the Tm: YLF laser was 13.2 W. The M² factor of the Tm: YLF laser was equal to 2.43 at 11.7 W output power. The radius of the output beam was transformed through a lens with a focal length of 150 mm to match the resonator oscillation spot. The lens was located 300 mm after the output mirror of the Tm: YLF laser and the distance between the lens and the middle of the Ho: YAG crystal was 255 mm. The radius of the pump spot in the Ho: YAG crystal was measured to be about 0.156 mm by the 90/10 knife edge technique [17], which was well matched with the oscillation spot.

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The Ho: YAG crystal with a refraction index of 1.82 had a Ho-doped concentration of 0.8 at.%, a cross-section of 1.5 × 6 mm² and a length of 50 mm. Both end faces of the crystal were antireflection coated at both 1.9 and 2.1 μm. To effectively remove the heat generated for high power operation, the crystals were wrapped in indium foil and held in copper heat-sinks. The working temperature of the Ho: YAG and Tm: YLF crystals was kept at 17 and 15 °C, respectively. In addition, the double z-cavity resonator consisted of an output coupler M1 with a radius of curvature of 150 mm, four curved mirrors M2 (R = 400 mm), M3 (R = 300 mm), M6 (R = 600 mm) and M7 (R = 600 mm) as the reflect mirrors at 2.1 μm (R > 99%), three flat high reflect (R > 99%) mirrors at 2.1 μm M4, M5 and M8. Folding angles of M2, M3, M6, M7 were set to about 10° to compensate for the astigmatism. The lengths of L1, L2, L3 and L4 were 310, 350, 345 and 300 mm respectively.

The laser was actively mode-locked with an acousto-optic modulator (AOM) consisting of a 30 mm long Brewster-cut fused-quartz, with a refraction index of 1.456. The AOM had a high transmission (T > 99%) at 2.1 μm. The mode locker was placed near the high reflect mirror M8 and driven at a resonant frequency of 41.38 MHz. M8 was mounted on a translation stage, having an accuracy of 1 mm, which can be used to fine-tune the length of the cavity to match the derived frequency of the AOM by transferring the location of M8. The total physical length of the resonator cavity was nearly 1755.7 mm and the transmission of the output coupler was 10% at 2.1 μm. The drive power and the peak transmission modulation of the mode-locker was measured to be approximately 20 W and 27.5% at 2.1 μm, respectively. In addition, the oscillation spot in the acousto-optic modulator crystal was about 0.22 mm according to the ABCD propagation matrix.

When the AOM did not work, continuous wave (CW) laser output could be obtained. Figure 2 shows the average output power at different pump powers with CW laser output and the continuous wave mode-locked (CWML) pulse output. The straight lines are results of linear fits. For CW operation, the maximum output power was 3.25 W, pump power was 13.2 W, corresponding to a slope efficiency of 37.3% and an optical–optical conversion efficiency of 24.6%. For CWML operation, the pump threshold of the laser was 5.9 W with the output power of 79 mW. The maximum output average power was 1.04 W, corresponding to slope efficiency of 13.3%. Compared to each other, the maximal output power declined to about 1/3, which coincided with the AOM's transmission modulation of 27.5%. Under observation, the CWML Ho: YAG laser could be kept stable for more than 4 h. This proved that the laser had a good stability.

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Figure 3 shows output pulses of a CWML Ho: YAG laser within the timescales of 200 ns. The pulses were recorded with a 650 MHz Lecroy digital oscilloscope combined to a 1 GHz fiber-detector. As shown in figure 3, the repetition rate of pulses was 82.75 MHz, which could be constant with the cavity round-trip time of 12.08 ns. Figure 4 shows the radio frequency of the CWML Ho: YAG laser under an output of 1.04 W, which was recorded by the Agilent N9320A spectrum analyzer. As known from figure 4, the span of the RF spectrum was 1 MHz and the repetition of the center peak was 82.76 MHz, matching well with the AOM's driven frequency of 41.38 MHz. At this point, the maximum single pulse energy was 12.57 nJ. Moreover, because there were not any side peaks near the center peak, it proved that the laser was operating in CWML mode rather than the Q-switched mode-locking.

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In the experiment, we measured the width of the mode-locked pulse through a no-background second harmonic autocorrelation [18] with a 5  ×  5 × 4 mm³ KTP as the nonlinear crystal. Figure 5 shows the measured result at an output power of 1.04 W. As shown in figure 5, the black squares were the mean of the relative intensity of the second harmonic and the fit curve was Gaussian in shape. When the Gaussian curve was used to fit the autocorrelation trace, then the conversion factor between the autocorrelation trace width and the width of the mode-locked pulses was 1.414. As shown in figure 5, the full-width at half-maximum (FWHM) of the autocorrelation trace was 144.3 ps, corresponding to the width of a mode-locked pulse of 102 ps.

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Moreover, we measured beam radiuses under the maximum output power of 1.04 W at various distances from a lens with f = 150 mm placed about 293 mm after the output mirror, as shown in figure 6. In the experiment, the output beam quality factor was measured by 90/10 knife edge technology. As noted from figure 6, by fitting Gaussian beam standard expressions to these data, we calculated a beam quality M² of 1.146. Obviously, the output CWML laser had a good beam quality. Meanwhile, we recorded a 2D beam profile taken by a Spiricon Pyrocam I pyroelectric camera, as shown in the inset of figure 6.

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In addition, as shown in figure 7, we also measured the spectrum of the CW and CWML Ho: YAG lasers with a Bristol 721A IR Spectrum analyzer (precision of 0.424 nm). As we can see from figure 7, the output center wavelength for CW and CWML was 2090.49 and 2097.25 nm under a pump power of 13.2 W, respectively. The slightly red shift of the central wavelength results possibly from the combined effect of both the emission cross-section relation to the emitting wavelength and the change of intracavity losses.

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At last, we used a Glan prism to measure the polarization state of the CWML laser. The result showed that at the CWML operation the output laser was vertically polarized.

We have successfully completed a CWML Ho: YAG solid laser based on an AOM pumped by a 1.9 μm Tm: YLF laser. This is the first time a report on an active CWML Ho: YAG laser has been published. For CWML operation, the threshold of pump power was about 5.9 W. The maximum output power and single pulse energy of the Ho: YAG laser was respectively 1.04 W and 12.57 nJ with slope efficiency of 13.3% when the pump power was 13.2 W. The mode-locked pulse repetition frequency was 82.76 MHz matching with the length of the cavity. And measured by a second harmonic autocorrelation, the mode-locked pulse was 102 ps under the maximum output power of 1.04 W. Furthermore, the M² factor and output center wavelength were calculated to be 1.146 and 2097.25 nm at the maximum output power.

This work was supported by National Natural Science Foundation of China (No. 61308009, No. 61405047), China Postdoctoral Science Foundation funded project (No. 2013M540288), Fundamental Research funds for the Central Universities (Grant No. HIT.NSRIF.2014044), Grant No. HIT.NSRIF.2015042) and Science Fund for Outstanding Youths of Heilongjiang Province (JQ201310).