1. INTRODUCTION

Stable pulsed lasers with high peak power and short pulse duration have attracted much interest in the fields of laser measurement, nonlinear optics, micromachining processes, and so on [1–4]. To date, the most common approach to produce ultrashort pulses is the passive continuous wave (CW) mode-locking method based on saturable absorption [5]. However, most of the reported CW mode-locking lasers have a high repetition rate of up to megahertz or even gigahertz, which is adjusted only by changing the cavity length. The high repetition rate could further lower the per-pulse energy and limit the application fields of CW mode-locking lasers. In fact, pulses with manageable low repetition rate and high pulse energy are needed in some applications, such as fluorescence lifetime measurement [1], chemical analysis based on laser ablation [3], and laser micromachining [4]. Thus, in order to generate short pulses with high peak power and optional low repetition rate, the dual-loss-modulation Q-switched and mode-locked (QML) technology is employed [6,7]. By simultaneously using an active modulator and a saturable absorber (SA) in a laser resonator, the dual-loss-modulated QML regime can apparently compress the duration of the $Q$-switched envelope. The repetition frequency of the mode-locking pulses underneath the $Q$-switched envelope is related to the cavity round-trip transit time. When the pulse width of the $Q$-switched envelope is compressed to be shorter than the cavity round-trip transmit time, only one subnanosecond mode-locked pulse in a $Q$-switched envelope exists [8]. In this case, the repetition rate of the single mode-locking pulse will equal that of the active modulator. In such way, a stable subnanosecond single mode-locking pulse with low repetition rate and high peak power can be realized.

In comparison to the active modulator, the role of an SA for pulse duration compression is more pronounced due to its fast loss modulation inside the cavity. Therefore, it is important to choose an excellent SA. In recent years, carbon nanotubes (CNTs), as an excellent SA, have been successfully employed in various lasers to generate ultrashort pulses [9–15]. In comparison with the conventional semiconductor SA (e.g., SESAM), CNT-based SAs offer excellent performance with the advantages of broad absorption spectra (0.8–2.1 μm), ultrafast recovery time, chemical stability, high optical damage threshold, and ease of manufacture [9,13,14]. In accordance with the wall number, CNTs can be divided into single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), and multi-walled CNTs (MWCNTs) [11,12,16]. To date, an MWCNT-based dual-loss-modulation subnanosecond single mode-locking $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.5}{\mathrm{Y}}_{0.5}{\mathrm{VO}}_4/\mathrm{KTP}$ green laser has been demonstrated [17]. However, subnanosecond single mode-locked lasers at 1.06 μm have not been reported until now, to our knowledge. Moreover, there is great interest in investigating the subnanosecond single mode-locking pulse characteristics versus CNTs with different wall structures.

In this paper, diode-pumped dual-loss-modulated QML $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ lasers at 1.06 μm with an electro-optic (EO) modulator and different CNT-SAs were presented. To get a higher pulse energy and shorter pulse duration, a mixed crystal $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ was used as the gain medium for its large energy storage capacity, broad gain spectrum, and small stimulated emission cross section [18]. An EO modulator was chosen as the active modulator owing to its fast switching and excellent hold-off ability, which was beneficial to obtaining a shorter pulse duration of the $Q$-switched envelope. By accurately designing the cavity parameters, a subnanosecond pulse with high stability, high peak power, and adjustable low repetition rate can be obtained. The dependence of subnanosecond pulse characteristics on pump power with different CNT structures was also investigated.

2. EXPERIMENTAL SETUP

The schematic configuration of the dual-loss-modulated QML $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ laser with EO modulator and CNT-SA is shown in Fig. 1. According to the ABCD matrix theory, a four-mirror Z-type resonator was designed with three cavity arm lengths of 57, 74, and 8.6 cm, respectively. The pump source was a commercial fiber coupled laser diode (FAP-I system, Coherent Inc., USA) emitting at 808 nm. The diameter of the coupled fiber core was 400 μm. The pump beam was collimated and focused into the laser gain medium with a spot radius of 200 μm by an optics system. An a-cut mixed crystal $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ with dimensions $3\mathrm{mm}\times 3\mathrm{mm}\times 10\mathrm{mm}$ and Nd-doping concentration of 0.38 at. % was employed as the gain medium. Both surfaces of the laser crystal were anti-reflection (AR) coated at 808 and 1064 nm. To efficiently dissipate heat deposition, the laser crystal was wrapped with indium foil and mounted in a copper block water-cooled to 17°C. Flat mirror M1, AR coated at 808 nm on the two surfaces and high-reflection (HR) coated at 1064 nm on the pump surface, was adopted as the input mirror. Spherical concave mirrors M2 and M3 were both HR coated at 1064 nm with radii of curvature of 500 and 150 mm, respectively. Flat mirror M4 with 10% transmission at 1064 nm was employed as an output mirror. With a polarizer and a $\lambda /4$ plate, an EO modulator (BBO crystal) was employed as the active $Q$-switcher, while three kinds of CNTs (SWCNT, DWCNT, and MWCNT) without AR coating were used as the passive SAs. The substrates of the CNT-SAs were three identical hydrophilic quartz wafers with thickness of about 0.5 mm, and the SWCNT, DWCNT, and MWCNT thin films have thicknesses of about $\sim 10$, $\sim 15$, and $\sim 25\mathrm{\mu m}$, respectively. For $Q$-switching with simultaneous mode locking using an SA, the buildup time for the mode-locked pulses has to be sufficiently short due to the limited round-trip time of the intensity fluctuation. In our experiments, the CNT SA was placed at a tight focusing position (i.e., beam waist near M4) to minimize the mode-locking buildup time. In this position, laser intensity on CNT-SA is higher than its saturated fluence and lower than its damage threshold for preventing performance degradation. According to the ABCD matrix, the average beam waist radius at the position of the absorber is about 115 μm. The pulse characteristics were recorded by a 16 GHz digital oscilloscope (Agilent DSO-X91604A, 80 G samples/s sampling rate, Tektronix Inc., USA) and a fast pin photodiode detector (New Focus 1414) with a rise time of 14 ps. The output powers were measured by a PM100D energy and power meter (Thorlabs Inc., USA).

 

Fig. 1. Schematic configuration of the dual-loss-modulated QML $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ laser.

Download Full Size | PDF

3. RESULTS AND DISCUSSION

The SWCNTs, DWCNTs, and MWCNTs used in our experiment were fabricated by the vertical evaporation technique. As we know, saturable absorption arises mainly from the energy bandgaps in semiconducting CNTs. Since the bandgap energy is determined by the tube diameter of the CNTs, the diameter value and distribution is a critical factor. The mean diameters of SWCNTs, DWCNTs, and MWCNTs are 1.5, 5, and 30 nm, respectively, which are controlled within a narrow distribution range to guarantee excellent optical properties. The fabrication procedures of CNT-SAs are similar to what is described in Ref. [16]. To further investigate the saturable absorption properties of all three CNT samples, the nonlinear transmission of CNT-SAs was measured by the pump–probe setup, as shown in Fig. 2. The laser source is a homemade acousto-optic (AO) $Q$-switched $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser with pulse duration of 22 ns at 1.06 μm. The MWCNT-SA have the largest optical loss in both the linear and nonlinear regimes. The highest nonsaturable loss was caused mainly by the scattering and nonuniformity of the MWCNTs. By fitting the data, we estimated modulation depths of 2.1%, 3.8%, and 4.9% for the SWCNT, DWCNT and MWCNT absorbers, respectively.

 

Fig. 2. Nonlinear transmission of CNT-SAs used in the experiment.

Download Full Size | PDF

In the experiment, the threshold pump powers of the dual-loss-modulated QML lasers with SWCNT-SA, DWCNT-SA, and MWCNT-SA are 1.35, 1.4, and 1.6 W, respectively. At the QML laser stage, once the pump power exceeds the threshold power, QML pulses with high stability and high peak-to-background contrast can be obtained. By constantly increasing the incident pump power, more and more inversion population density can be generated in the gain medium in unit time, and the pulse width of the $Q$-switched envelope will be gradually compressed. Because the time interval between two adjacent mode-locking pulses underneath one $Q$-switching envelope is the cavity round-trip transmit time, when the $Q$-switched envelope duration is shorter than the cavity round-trip transmit time, the laser will operate on single mode-locking stage with low repetition rate controlled by the EO modulator. The main influencing factors for the realization of single mode-locking are the active modulation frequency, the saturable absorption of the CNT-SAs, and the incident pump power. To investigate the dependence of the pulse characteristics CNT-SAs with different wall structures more intuitively, the modulated frequency of EO modulator was fixed at 1 kHz.

According to the fluctuation mechanism [19,20], the short pulse build-up time and small absorption recovery time are of benefit to the generation of QML pulses. To our knowledge, a low initial transmittance of SA can cause a short build-up time of the $Q$-switched pulse. Based on the data of Fig. 2, one can conclude that the pulse build-up times with DWCNT-SA and MWCNT-SA are shorter than that obtained with SWCNT-SA. In addition, the absorption recovery times of DWCNT ($\sim 220\mathrm{ps}$) and MWCNT ($\sim 330\mathrm{ps}$) are much smaller than that of SWCNT ($\sim 600\mathrm{ps}$) [11,12,16]. Therefore, lasers based on DWCNTs and MWCNTs can obtain QML pulses with high stability and short pulse width more easily. Figure 3 shows the number of mode-locked pulses underneath a $Q$-switched envelope versus the pump power for three kinds of CNT-SAs. It can be seen that the number of the mode-locking pulses underneath a $Q$-switched envelope decreases monotonically with increasing pump power. At pump power of 5.36 W, seven, five, and three mode-locking pulses underneath a $Q$-switched envelope were obtained from the dual-loss-modulated QML laser with SWCNT-SA, DWCNT-SA, and MWCNT-SA, respectively. When the pump power reached 9.84 W, the single mode-locking pulse underneath a $Q$-switched envelope can be obtained with the three kinds of CNT-SAs. However, there was another very small mode-locking pulse in the $Q$-switched envelope with the SWCNT-SA. Thus, compared with the SWCNTs, we can see that the DWCNTs and MWCNTs have better ability to compress the pulse duration of the $Q$-switched envelope. This is due to the large insertion loss of DWCNTs and MWCNTs. According to $Q$-switching theory, a large optical loss and nonsaturable loss of an SA can generate a large inversion population density in the gain medium, which can further generate a narrow $Q$-switched envelope. At pump power of 10.72 W, single mode-locking operation of the dual-loss-modulated QML lasers was realized with all three kinds of CNT-SAs. The temporal profiles of the pulse trains are illustrated in the last column of Fig. 3, and the corresponding data are recorded for calculating pulse stability.

 

Fig. 3. Oscilloscope traces of pulses from the dual-loss-modulated QML laser with different CNT-SAs under different pump power: (a) SWCNT-SA, (b) DWCNT-SA, and (c) MWCNT-SA.

Download Full Size | PDF

The pulse-to-pulse amplitude fluctuations (the ratio between the largest deviation and the mean pulse amplitude) of the lasers with different CNT-SAs are all less than 4%, which demonstrates the high stability of CNT-based dual-loss-modulated QML sub-nanosecond single mode-locking lasers. Obviously, the pulse stability of the laser with MWCNT-SA is better than the other two. This results mainly from the structure of the MWCNTs, which takes the form of a stack of concentrically rolled graphene sheets. The outer walls can protect the inner walls from damage or oxidation, and the thermal stability and laser damage threshold of MWCNTs are higher than those of the other two, which is beneficial for improving the pulse stability [21,22].

When the EO modulator was switched off, the lasers operated in singly passively QML state with CNT-SAs. The experimental results show that the pulse width and pulse repetition rate of the $Q$-switched envelope are much larger than those of the dual-loss-modulated QML laser. Under pump power of 10.72 W, the envelope pulse width and the envelope repetition rate of the singly passively QML laser with MWCNT-SA can reach 158 ns and 97 kHz, respectively, corresponding to a pulse-to-pulse amplitude fluctuation of about 16%, which is shown in Fig. 4. Thus, it can be concluded that the EO modulator plays a significant role in compressing the pulse width of the $Q$-switched envelope and enhancing the stability of the QML laser.

 

Fig. 4. Temporal shape of the pulse train and the $Q$-switched envelope in the singly passively QML laser with MWCNT-SA at the pump power of 10.72 W.

Download Full Size | PDF

Figure 5(a) shows the pulse width of the $Q$-switched envelope versus the pump power for CNT-SAs with different wall structures at the QML stage. Demonstratively, the pulse width of the $Q$-switched envelope decreases monotonically with increasing pump power. With MWCNTs as the SA, the pulse width in the QML stage is much smaller than that with the other two. For the MWCNT-SA, DWCNT-SA, and SWCNT-SA, the threshold pump powers for obtaining subnanosecond single mode-locking pulses are 6.28, 8.08, and 9.84 W, respectively. For the laser with the SWCNT-SA, when the pump power increased to 9.84 W, $Q$-switched envelopes containing a large mode-locking pulse and one or two very small peaks can be obtained. Moreover, these small pulse peaks cannot be eliminated completely just by increasing the pump power. This imperfectly single pulse operation in the case of SWCNTs results mainly from the small nonsaturable loss and modulation depth of the SWCNT-SA, which makes it relatively easy to reach saturation. As we know, when the SA is close to saturation, it has no ability to further compress the pulse width. On the other hand, because the open time of the EO switch is longer than the pulse duration of the large single mode-locking pulse, there is still gain enough in the laser crystal to support the small pulse oscillation in the cavity after the SWCNTs recover from the saturated state. Since more than 85% of the energy of the $Q$-switched envelope was concentrated into the main pulse, single mode-locking operation was considered to have been achieved.

 

Fig. 5. Dependence of the pulse width on the pump power for different CNT-SAs: (a) the pulse width of the $Q$-switched envelope, and (b) the pulse width of the single mode-locking pulse.

Download Full Size | PDF

Figure 5(b) gives the pulse width of the single mode-locking pulse versus the pump power for CNT-SAs with different wall structures. The mode-locking pulse duration further decreases with increasing pump power. To our knowledge, the mode-locking pulse width is determined mainly by two factors: one is the absorption recovery time of the SA and the other is the gain bandwidth of the laser. In our experiment, the DWCNTs and MWCNTs have similar recovery times that are smaller than that of SWCNTs. Thus, the difference of the single mode-locking pulse duration obtained with DWCNTs and MWCNTs is very small. When the pump power reached 10.72 W, the shortest pulse durations were 748, 525, and 490 ps with the SWCNT-SA, DWCNT-SA, and MWCNT-SA, respectively. The results we obtained are on the same level with those observed in similar configurations in previous work [23,24]. Then, the pulse width of the laser without an SA is investigated as a comparison in our experiment. At pump power of 10.72 W, the shortest pulse duration of 220 ns was obtained from the actively QML laser only with EO. The results show that the pulse duration of the $Q$-switched envelope is significantly compressed by the dual-loss-modulation mechanism.

The average output powers of the dual-loss-modulated QML lasers versus the pump power for the three kinds of CNT-SAs are demonstrated in Fig. 6. Benefiting from the high transmission of SWCNTs, at the same pump power, the average output power obtained from the dual-loss-modulated QML laser with the SWCNT-SA is higher than that with the other two. At pump power of 10.72 W, the highest average output powers of dual-loss-modulated QML $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ lasers with a SWCNT-SA, DWCNT-SA, and MWCNT-SA were 827, 689, and 474 mW, respectively. Considering the degradation of optical parameters of CNT-based SAs, in order to check the robustness of CNT-SAs, the dual-loss modulated subnanosecond $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ lasers with the three kinds of CNT-SAs were kept operating at the maximum radiation power for at least 4 h every day during one week. The fluctuation of the average output power $\mathrm{\Delta }P$ is measured at pump power of 10.72 W. For the lasers with an SWCNT-SA, DWCNT-SA, and MWCNT-SA, $\mathrm{\Delta }P$ is less than 3.1%, 2.9%, and 2.6%, respectively. No damage and optical degradation of the CNT-SAs were observed in the experiment. According to the average output power and the pulse repetition rate, the per-pulse energy of the $Q$-switched envelopes were obtained and are also shown in Fig. 6.

 

Fig. 6. Average output power and pulse energy of $Q$-switched envelope versus the pump power for different CNT-SAs.

Download Full Size | PDF

When the dual-loss-modulated QML lasers operate on the subnanosecond single mode-locking pulse stage, the pulse energy of the $Q$-switched envelope can be regarded as the pulse energy of the single mode-locking pulse. Then, by dividing the per-pulse energy with the pulse duration of the single mode-locking pulse, the peak powers of the dual-loss-modulated QML laser at the single mode-locking stage can be obtained, and they are shown in Fig. 7. For the three kinds of CNT-SAs, the peak powers increase with the rise of pump power. At the same pump power, the peak power generated from the dual-loss-modulated QML laser with the DWCNT-SA is higher than that with the other two. This is mainly because the QML laser with the DWCNT-SA can generate a relatively high pulse energy and small pulse duration. The highest peak power obtained with the SWCNT-SA, DWCNT-SA, and MWCNT-SA are 1.105, 1.312, and 0.967 MW, respectively. The pulse energies and peak powers of doubly QML pulses obtained in this paper are of the same orders as those obtained in Ref. [23], and much larger than those obtained in Ref. [24]. Through analyzing the results, one can see that the dual-loss-modulation single mode-locking technology with EO modulator and CNT-SAs is a simple and effective method for generating subnanosecond pulses with high pulse energy and peak power.

 

Fig. 7. Peak powers of single mode-locking pulses as a function of the pump power.

Download Full Size | PDF

By using a Wavescan Laser spectrometer (resolution 0.4 nm, APE GmbH, Germany), the corresponding spectra of the pulsed lasers with different kinds of CNT-SAs are measured, and they are shown in Fig. 8. From the figure we can see that the laser emission wavelength was around 1064 nm without significant change due to the similar saturated absorption mechanism of the three kinds of CNT-SAs.

 

Fig. 8. Spectrum of the subnanosecond $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ lasers.

Download Full Size | PDF

From the experimental results, one can see that the DWCNT type is the most suitable SA for generating high peak power subnanosecond pulses in the dual-loss-modulated QML lasers. With the MWCNT-SA, the pulse energy and the peak power of the dual-loss-modulated QML laser are lower than that with the SWCNT-SA and the DWCNT-SA, but the large nonsaturable loss and modulation depth makes it much easier to achieve subnanosecond single mode-locking pulse status. In addition, considering its high thermal and laser damage threshold, the MWCNT-SA is also an appropriate candidate for the generation of subnanosecond single mode-locking pulses.

In general, by employing different EO/CNT groups, low repetition rate subnanosecond $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ lasers with high peak power are realized. The introduction of the EO switch means the sub-nanosecond pulse laser has nearly no timing jitter or pulse-to-pulse amplitude fluctuation. In addition, owing to its lower pulse repetition rate, this sub-nanosecond pulse laser has a potential for generating large per-pulse energy and high peak power, which can satisfy the requirements for high stability, specific repetition rate, and high peak power in some applications. To our best knowledge, the pulse durations previously achieved at 1 μm in passively QML lasers with conventional SAs were of the order of tens of nanoseconds [25–27]. Since the dual-loss-modulation method was employed, the pulse duration of the QML laser was greatly compressed, while the peak power was improved significantly [24,28]. The maximum pulse energy of 1.15 mJ was obtained from a dual-loss-modulated QML subnanosecond laser with EO modulator and ${\mathrm{Cr}}^{4+}\text{:}\mathrm{YAG}$, corresponding to a peak power of 3.15 MW [23].

In comparison to most of the results obtained with conventional SAs in similar configurations [8,24,28], our experimental results demonstrates better performances of SWCNTs, DWCNTs, and MWCNTs in compressing the duration of the $Q$-switched envelope at 1.06 μm, providing new potential efficacy for CNTs in high power ultrafast laser systems.

4. CONCLUSION

By employing the mixed crystal $\mathrm{Nd}\text{:}{\mathrm{Lu}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{VO}}_4$ as the laser medium, doubly QML lasers with EO modulator and CNT-SAs at 1.06 μm are realized and subnanosecond pulses with high peak power and low repetition rate can be obtained. The related laser characteristics are studied. The experimental results show that the MWCNT-SA needs the smallest pump power to generate subnanosecond pulses among the three CNT-SAs we used. The shortest pulse durations are 490, 525, and 748 ps for the MWCNT-SA, DWCNT-SA, and SWCNT-SA, respectively. The maximum peak power of 1.312 MW is generated from the QML laser with the DWCNT-SA.