1. Introduction

Diode-pumped passively mode-locked lasers with high average output powers are used in multiple fields such as micromachining and nonlinear optics. Development of high power mode-locked oscillators with high efficiency, however, is not straightforward mainly due to the thermal effects that inevitably arise and degrade laser performance of such systems. Among Nd-doped laser crystals, Nd:vanadate (Nd:YVO₄) is one of the most efficient and exhibits very high emission cross-section, fairly broad absorption profile suitable for diode-pumping, and strong birefringence which is useful for further frequency conversion processes [1–3].

On the other hand Nd:YVO₄ suffers from its inferior thermal properties in comparison to Nd:YAG. For this reason it has been extensively studied in order to determine the induced thermal effects which are the main factor preventing it from being used in very high power applications [4–9].

As opposed to changing the cavity design parameters or the geometry of the laser crystal (gain medium) in order to partially compensate for the thermal effects, recent increase in availability and feasibility of the laser diodes at longer wavelengths allowed Nd:YVO₄ crystal to utilize other absorption lines further away from the strongest and standard one at 808 nm.

This change in the pump wavelength results in lower quantum defect (QD) which is defined as the difference in energy between the absorbed pump and the emitted laser photons. Lower QD affects the laser system in two ways. First, it lowers the heat generation per stimulated photon which ultimately results in lower thermal load inside the gain medium. The reduction of thermal effects due to lower QD has been reported for 880, 888, and 914 nm pump wavelengths [10–14]. Moreover, the lower absorption at 880, 888, and 914 nm, allows for the absorbed pump power to be more optimally spread across a larger volume, thus further lowering the thermal lensing effects. This contributes to a better stability of the system and opens up a way for laser power scaling.

Secondly, it elevates the intrinsic limitation in efficiency of the laser system imposed by the QD. According to the relationship between the slope efficiency η and the internal cavity loss L_i (T is the transmission of the output coupler), η = (T/(T + L_i)) × (λ_pump / λ_laser), the slope efficiency can be described only by the ratio of the pump and laser wavelengths when L_i = 0 [15]. Therefore, the theoretical upper limit of slope efficiency of the laser system (operating at 1064 nm) can increase from 75.9% in 808 nm pumping case to 85.9% in the case of 914 nm pumping. Fortunately, high-brightness laser diodes around this wavelength are widely used for pumping of Yb-doped fiber lasers. Therefore, in-band pumping at 914 nm offers an attractive possibility to simultaneously achieve both high output power and high slope efficiency. Indeed, Nd:YVO₄ lasers in continuous wave regime achieved slope efficiency of up to 81% [13]. At the same time, Nd:YVO₄ lasers in mode-locked (ML) regime under 808, 880, and 888 nm pumping have been also reported [16–18]. The highest reported slope efficiency to date in ML regime was 71% in [17]. However, there was only one report by Liang et al. [19] on mode locking performance of a Nd-doped gain medium with low quantum defect pumping at the 0.9 µm absorption lines of the ⁴F_3/2 → ⁴I_9/2 transition. A diode-pumped and mode-locked Nd:Gd_0.6Y_0.4VO₄ laser demonstrated optical-to-optical efficiency of 27% and slope efficiency of only 44% which was caused by the non-optimal mode-matching condition for TEM₀₀ mode [19].

In this work we report the operation of a highly efficient Nd:YVO₄ laser in ML regime with in-band pumping at 914 nm. The laser produced a maximum average output power of 6.7 W and pulses with duration of 16 ps. The achieved 77.1% slope efficiency, to the best of our knowledge, is the highest reported to date for Nd-doped lasers in the ML regime.

2. Experimental setup

A 20-mm- long a-cut Nd:YVO₄ crystal (Castech) with doping level of 1.5 at.% was antireflection (AR) coated for both pump and laser wavelengths in order to reduce internal cavity loss. The crystal was pumped by a fiber-coupled laser diode (IPG Photonics) with 110 μm core diameter and numerical aperture of 0.12 at 914 nm. The laser diode was water cooled at 16 °C to control the wavelength shift due to temperature change and was capable of delivering 16.8 W of pump power at 914 nm. At this wavelength its spectrum was ~3.8 nm wide at FWHM (full width at half maximum). The pump was focused into a 275 μm spot radius at the center of the crystal through a set of coupling lenses with 1:5 magnification. The Nd:YVO₄ crystal was wrapped in indium foil and constantly water cooled on the top and bottom surfaces in a metal holder at 16 °C in order to control the thermal effects.

A 5-mirror cavity was used with a reflective semiconductor saturable absorber mirror (SESAM) as one of the end mirrors as shown in Fig. 1. The dichroic pump mirror M₂ served as a folding mirror and was AR coated for 914 nm and highly reflected (>99.9%) laser radiation at 1064 nm. The radii of curvature of M₁ and M₃ were 400 mm and 500 mm, respectively. The distances L1, L2, L3, and L4 were 33, 47.5, 48.1, and 45.0 cm, respectively. At the highest absorbed pump power (and with thermal lensing taken into account) the spot size of the laser mode at the center of the crystal was 270 μm and ensured constant and efficient mode-matching with the pump beam while the spot size on SESAM was 175 μm. The laser cavity was designed to have a proper spot size on SESAM in order to control its fluence and to achieve mode-locking with the highest output power [20]. This enabled us to avoid multi-pulse mode-locking and damage to the SESAM.

 

Fig. 1 Experimental setup for mode- locked operation of a Nd:YVO4 laser diode-pumped at 914 nm. LD, laser diode.

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The SESAM (BATOP GmbH) with 1.2% modulation depth, 0.8% non-saturable loss, 90 μJ/cm² saturation fluence, and relaxation time constant of ~10 ps was used. The SESAM was soldered onto a gold plated Cu-cylinder.

A stable, self-starting, continuous wave mode-locked output with the highest output power was achieved with a 15% output coupler (OC). The mode-locked pulse train was recorded by a 6 GHz digital oscilloscope (Tektronix, TDS6604) with a fast InGaAs photodetector (Thorlabs, DET08CFC/M) giving an overall temporal resolution of ~100 ps. An autocorrelator with a delay range of 200 ps (Femtochrome Research, FR-103XL) was used to measure the pulse duration and monitor multi-pulse instabilities.

3. Results and discussion

The pump absorption at 914 nm displayed nonlinearity due to the wavelength shift of the laser diode. This was taken into account for further analysis of the performance of the developed laser oscillator. Pump absorption was 66% at the highest level of pump power.

The laser output power versus the absorbed pump power is presented in Fig. 2(a). After 0.94 W of output power, Q-switched mode-locked (QS-ML) operation was observed as shown in the pulse train in the middle inset. Above 4.1 W of output power, the laser started operating in continuous wave mode-locked regime. At 11.0 W of absorbed pump power, the maximum output power of 6.7 W was obtained with 60.7% of optical-to-optical efficiency and 77.1% of slope efficiency in the ML regime. This slope efficiency is much higher than the reported values of 55.2% for the commonly used pumping scheme at 808 nm [21–26] and higher than 71% previously reported at 880 nm [17]. Assuming a reasonable internal cavity loss of ~0.9% that was introduced by different cavity elements (i.e. crystal and mirrors which were all specified to have <0.1% loss per surface/reflection) and 0.8% from the non-saturable loss introduced by the SESAM, the maximum possible slope efficiency can be calculated as ~77.2%. This is in good agreement with the experimentally achieved slope efficiency and shows that it is mainly limited by the intracavity loss. Therefore by carefully managing of the intracavity loss either by using a SESAM with lower non-saturable loss or by lowering the number of optical elements inside the cavity, it is possible to increase the slope efficiency even further.

 

Fig. 2 (a) Average output power of the Nd:YVO₄ laser versus the absorbed pump power. Insets: intensity profile at maximum output power, QS-ML pulse train and ML pulse train over 100 μs and 100 ns, respectively; (b) Optical spectrum of the laser output in continuous wave and ML operation regimes.

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The obtained optical-to-optical efficiency with respect to incident pump power (40%) can be also increased by using a longer gain medium with higher doping concentration to maximize pump absorption. Since the beam quality of the laser diodes available at 914 nm is much better than for the 808 nm diodes, maintaining good mode-matching even in a long crystal is not an issue.

The slope efficiency shows no sign of decreasing at the highest output power which suggests that the output power was not limited by the thermal lensing effects but by the available pump power. Due to the lower absorption coefficient of Nd:YVO₄ at 914 nm as compared to the other absorption lines, a longer crystal or multiple pass pumping scheme can be used to further increase the output power.

The beam quality factor M² was measured by a CCD camera beam profiler to be less than 1.4. Throughout the experiment, the output beam exhibited TEM₀₀ mode beam profile as shown in the inset of Fig. 2(a). This is due to the fact that the cavity was designed with preliminary measurement of thermal lensing effect [14,27] which at the highest pump power level was found to be ~160 mm. The insets in Fig. 2(a) also show the pulse trains in the QS-ML and ML regimes within the time spans of 100 μs and 60 ns, respectively.

Figure 2(b) shows the broadening of the optical spectrum when the laser operation changes from the continuous wave to the continuous wave mode-locked regime. The FWHM of the spectra are 0.14 and 0.07 nm, respectively, and was limited by the resolution (0.07 nm) of the available optical spectrum analyzer (Anritsu, MS9710B).

At maximum output power a pulse width of 16 ps was measured by an intensity autocorrelator corresponding to the time bandwidth product of 0.593 which indicates a chirped pulse. According to the recorded spectrum, the generation of pulses shorter than 10 ps should be possible. The autocorrelation trace with its corresponding fitting assuming sech²-shaped pulses is presented in Fig. 3(a). The radio frequency (RF) spectrum in Fig. 3(b) displays stable mode locking at ~87 MHz repetition rate with noise suppressed to better than 51 dB. In addition, the inset in Fig. 3(b) shows an RF spectrum spanning 0-500 MHz also indicating clean, stable mode locking without multi-pulse instabilities.

 

Fig. 3 (a) Autocorrelation trace (dots) and sech² fit (line) of the output pulses in ML operation. (b) Radio frequency (RF) spectrum of the mode-locked laser. Inset: RF signal in the 0-500 MHz range.

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The measured pulse duration and repetition rate corresponded to the peak power and maximum pulse energy of 4.8 kW and 77.2 nJ, respectively.

4. Conclusions

In summary, we have demonstrated a diode-pumped passively mode-locked Nd:YVO₄ laser with optical-to-optical efficiency up to 60.7% and slope efficiency of 77.1% under the new pumping wavelength of 914 nm. This is the highest slope efficiency for the ML Nd:YVO₄ lasers reported to date and for the ML Nd-doped lasers in general. This result for the first time confirms the advantages of in-band pumping at 914 nm over the standard pumping at 808 nm in ML regime. The measured slope efficiency was limited by the intracavity losses and therefore can be further improved by careful intracavity loss management. The maximum average output power of 6.7 W was achieved with a pulse duration of 16 ps and was limited only by the available pump power. Further power scaling can be done both by the increase of pump power or increase of absorbed pump power using a longer gain medium and higher doping concentration. We believe that the same concept can be extended and to other Nd-ion based gain media to produce highly efficient mode-locked laser systems.