1. Introduction

High power pulse Yb-doped fiber lasers sources operated on single transverse mode around 976 nm are highly demanded for silica-based detection as well as building coherent green-blue light or deep ultraviolet coherent sources. By nonlinear wavelength conversion to 488 nm or 326 nm, these sources can play a significant role in underwater communication, photolithography, high density data storage and so on. All the practical applications mentioned above require linearly polarized, high peak power and high pulse energy laser outputs.

The Yb ions in a silica host have interesting properties around 976 nm [1]. In fact, it present absorption around 915 nm and 976 nm, and have a strong emission cross-section around 976 nm with a true three-level behavior and a lower one around 1030 nm with the quasi-four-level behavior. In contrast to emission around 1030~1080 nm, efficient laser operation at 976 nm is difficult to obtain in Yb-doped fiber. It lies in the high threshold which requires sufficient pump intensity to get a population inversion of almost 50% for the three-level nature. Besides, the reabsorption at 976 nm becomes significantly larger in a long active fiber. It causes spurious oscillations that restrict the fiber length and pump absorption, leading to low slope efficiency.

The point to overcome these problems is to increase gain around 976 nm [2–5]. For example, the ring doping [2] reduces the overlap of the lasing field and thus the gain at all wavelengths, increasing the gain around 976 nm relative to other wavelengths because of the larger emission cross-section. In 2003, R. Selvas et al. reported a specific designed Yb-doped jacketed-air clad (JAC) fiber [3]. With smaller clad to core area ratio compared to conventional fiber geometry and a higher clad numerical aperture (NA), it ensured sufficient high pump intensity in the Yb-doped core, leading to significant laser generation at 977 nm. Using this type of fiber, a Q-switched 980 nm Yb-doped fiber laser [4] was presented with 250 mW of average output power for repetition rates between 0.2 and 0.65 MHz, 60 W of maximum peak power. Suppressing the unwanted emission at longer wavelengths is another method [6–8]. Just like solid-core photonic band-gap fibers [6] reported in 2008, was introduced band-gap losses in the wavelength range above 1000 nm by adjusting the opto-geometrical parameters of fiber. Other techniques, including the incorporation of long period gratings [7] and the distributed feedback laser configuration [8] also had many reports. But in such cases, both the limited small pumping area of the fiber and the power scalability of the pump source restrict the maximum obtained average power of the oscillators only to be W-class, and peak power far below kW-class.

With the emergence of ultra-large core Yb-doped rod-type PCF, two groups develop 100 W-class diffraction-limited lasers at 977 nm in the CW regime [9, 10] in 2008 at the same time. The large pump absorption and short length of the PCFs are very effective for the suppression of any undesired nonlinear effects, which is crucial for the generation of high-peak-power pulses. From then on, more three-level Yb-doped laser systems operated in the pulse regime have been proposed [11, 12]. But the linearly-polarized, short pulse, high peak power oscillator operated on the three-level system is rarely reported by far.

In this letter, by introducing linear polarization component and optimizing cavity parameters [13, 14], we demonstrate a linearly-polarized short-pulse oscillator on the three-level system. The laser is based on a 36 cm short-length non-polarization-maintaining (non-PM) rod-type PCF in the acousto-optic modulator (AOM) Q-switched regime, adopting backward pump to promote the three-level gain against spurious oscillations at the quasi-four-level. In such a short length and strong pump condition of the fiber, laser gain around 978 dominates the dynamics of the laser. Although without any additional wavelength filter, the wavelength of output laser is centered in 978 nm. The laser can easily generate average output power over 2 W with PER as high as 43 dB. For variable repetition rate between 10 kHz and 200 kHz, laser presents stable pulse trains and pulse duration below 50 ns. At the repetition rate of 10 kHz, pulse of sub-10 ns duration, ~130 kW peak power is achieved. With this performance, this source can be potentially used in many applications, opening up a high-efficiency method to achieve 488 nm blue-green light or even shorter wavelengths through frequency conversion.

2. Experiment setup

The architecture of the short-pulse linear-polarization AOM Q-switched rod-type PCF laser operated at 978 nm is schematically shown on Fig. 1. The Q-switched fiber oscillator is built on a 36 cm long non-PM large-mode-area Yb-doped double-clad rod-type PCF (NKT Corp, DC-200/85-Yb-ROD). Its core diameter is 85 μm (NA_core = 0.02) and inner cladding is 200 μm. Both fiber ends are in great care by splicing a perpendicularly cleaved end-cap with large anti-reflective coating in a certain spectral range to avoid Fresnel reflections and parasitic lasing. To backward pump the three-level laser system, a fiber-coupled laser diode of 200 µm diameter (NA = 0.22) emitting around 915 nm with 60 W maximum output power is applied. The light sources are respectively directed by 20°dichroic mirror M₁ (R>99% at 915 nm), M₂ (T>90% at 915 nm, R>95% from 976 nm to 1030 nm), and M₃ (the same parameters as M₂). The laser cavity is formed by the high reflection mirror HR (R>99%, centered at 976 nm, FWHM of 100 nm) and the output coupler OC (R = 30% at 976 nm). Operated at its first-order diffraction position to achieve Q-switched operation, an AOM with diffraction efficiency (90%, perpendicular to acoustic propagation; 60%, parallel to acoustic propagation) sensitive to polarization state is integrated into the cavity in front of the mirror HR. A Glan-Taylor prism (GTP, less than 0.45 dB insertion loss for wavelength from 976 nm to 1030 nm) with polarizing direction matching with the direction of the maximum diffraction efficiency of AOM to promote linearly polarized light is inserted into the cavity near the mirror OC. Coated with high transmittance around 978 nm, the GTP is perpendicular to the laser propagation direction. The total optical cavity length is ~1.2 m. A 600 MHz digital oscilloscope (Lecroy, WR62XR) and a fast photometer (THORLABS, DET02AFC) are used to measure the pulse profile.

 

Fig. 1 Experiment set-up of the linearly-polarized short-pulse Q-switched 978 nm laser oscillator.

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3. Results

The performances of AOM Q-switched rod-type PCF lasers operated on the three-level system are investigated in the linearly-polarized mode and the random-polarized mode (corresponding to with and without the introduction of GTP). In Fig. 2(a) plots the output average laser power and pulse duration as a function of the absorbed pump power at the repetition rate of 25 kHz in two modes. The linearly-polarized mode is represented by the red curve with square dot, while the random-polarized mode is represented by the black curve with square dot. It can be observed that the curves of average output laser power versus absorbed pump power rise smoothly in both modes. No roll-off is observed. The lasing threshold of the three-level system is reached up to an absorbed pump power of 12.5 W at 915 nm, which is considered by the high population inversion of three-level nature. The power in the random-polarized mode grows a little faster than the one in the linearly-polarized mode. The reason lies in that the insertion of GTP brings an extra loss at one polarization direction. The pulse duration decreases quickly with the increasing of the absorbed pump power in both modes while it in the random-polarized mode always presents slightly shorter than that in the linearly-polarized. But pulse widths become identical with the growing of the absorbed pump power. Finally, the shortest pulse durations are both reached 13 ns at 25 kHz repetition rate when the maximum average power measured is 2.6 W in the random-polarized mode and 2.1 W in the linearly-polarized mode. Figure 2(b) exhibits the steady pulse train at maximum output power in the linearly-polarized mode. The stability of pulse-to-pulse is estimated to above 90%. The slope efficiency with respect to the absorbed pump power limited to 13~17% can be a consequence of the short length of gain fiber, dichroic mirrors, GTP and AOM induced intra-cavity losses, leaving the unabsorbed pump power up to 26 W at maximum incident pump power of 57 W. If the double pass configuration coupled back with a highly reflective mirror at 915 nm is used like [9, 10], the slope efficiency can be increased. In the meantime, the measured PERs of the output lasers are 3 dB and 43 dB, respectively. The huge polarization property difference between two modes is in consequence of the coactions of the strong polarization selectivity of GTP and different diffraction efficiency of two orthogonal directions of the AOM. These make laser output linearly polarized light in the direction of the maximum diffraction efficiency of AOM.

 

Fig. 2 (a) Output laser average power and pulse duration as a function of the absorbed pump power at 25 kHz (b) The trace of the output pulse train at maximum output power of 2.1 W in the linearly-polarized mode at 25 kHz.

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By controlling the modulation frequency and optimizing the gate time width of the AOM, stable Q-switched operation can be acquired at different repetition rate. The pulses duration and peak power influenced by the repetition rate in the linearly-polarized mode is depicted in Fig. 3 at maximum incident pump power. To eliminate the uncertainly of interpulse ASE in low repetition rate, the pulse energy is measured by energy detector (OPHIR, PE9-C) when repetition rate is lower than 25 kHz. For higher repetition rate discussed in the experiment, the ratio of the interpulse ASE power to the total measured output power is far below 10%, such that the effect of interpulse ASE is ignored. The peak power is calculated from the measured single pulse energy and corrected by the factor of the pulse shape. In principle, as the repetition of AOM increases from 10 kHz to 200 kHz, pulse duration increases almost linearly. Up to the repetition rate of 200 kHz, the maximum pulse duration is less than 50 ns. With the increase of repetition rate, the pulse peak power exhibits a sharp decline. For repetition rates higher than 80 kHz, the pulse peak power is below 1 kW. The shortest pulse of 9 ns is obtained at the repetition rate of 10 kHz, which is pretty close to the cavity round-trip time τ (τ = 2L/c, where c is the light velocity in vacuum, L is the optical cavity length). Insert of Fig. 3 shows the record pulse temporal profile of the 9 ns with pulse energy of 120 μJ, giving birth to a maximum pulse peak power of ~130 kW in this experiment.

 

Fig. 3 Pulses duration and peak power influenced by the repetition rate in the linearly-polarized mode at maximum incident pump power. Inset: Pulse temporal profile of the 9 ns pulse at 10 kHz.

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Although the measured output power is up to 2.2 W at 10 kHz repetition rate, the contribution of interpulse ASE reaches 1 W, resulting in the effective power ratio that pulse power to measured output power to be only 54%. This significant interpulse ASE build-up is mainly due to the strong pump that the three-level operation needs to overcome transparency and actually store a great amount of energy for pulsed extraction the gain at 976 nm. What calls for special attention is that, in the random-polarized mode, at the same pump condition and repetition rate, the measured output power is 2.9 W, however, the effective power ratio is 40%. It means that the interpulse ASE contributes a larger part for the total output power in the random-polarized mode. The rise of effective power ratio in the linearly-polarized mode primarily attributes to the insert of GTP, which prevents the laser oscillation in the polarization direction of lowest diffraction efficiency of AOM. In general, the lower diffraction efficiency of AOM, the severer interpulse ASE is built. When the most serious interpulse ASE of one polarization direction is completely suppressed, the entire effective power ratio gets enormous improved. In the meantime, the suppressed effect of interpulse ASE by GTP becomes more obvious while repetition rate gets higher.

In previous reports [9–12], extra dichroic mirrors used to promote the laser generation at 976 nm and avoid increasing of 1030 nm must be set in the experiment configuration. In our previous work [13, 14], we found that the highest peak of emission spectrum of this short-length rod-type PCF is at 977 nm with 1030 nm get suppressed. But in forward pump condition [14], the gain at 1030 nm still has advantage over that of 978 nm due to the strong reabsorption of the 978 nm in the fiber. In this experiment, to improve the efficiency of the laser [15], backward pump is implemented. It changes the population inversion distribution of the fiber and boosts the gain of 978 nm to dominate the dynamics of this laser. Even if the wavelength-selective mirror in [13] used to introduce extra ~30 dB of intra-cavity losses for 1030 nm is discarded, the output spectrum measured at 25 kHz, 2.1 W output average power in the linearly-polarized mode, shown in the Fig. 4, implies that the oscillator still works in the three-level system.

 

Fig. 4 Spectrum of the 2.1 W output power at 25 kHz in the linearly-polarized mode. Insert, far field image of the beam.

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The spectrum presents a ~2.5 nm FWHM (3 dB) centered in 977.9 nm (resolution 0.05 nm). The relative broad bandwidth results from no narrow band-pass elements included in the entire oscillator cavity. Even so, no nonlinear effect (such as SRS or SBS) is detected. It must be noticed that there is still a suppression of ASE around 1030 nm of more than 40 dB below the laser peak signal at 978 nm. Actually, even though we replace the high reflection mirror HR with a boardband reflection mirror with high reflection from 950 nm to 1100 nm, the output spectrum of the laser keeps at 978 nm, too. But the suppression of ASE around 1030 nm exhibits a little less than 35 dB. The far field image of the output beam from the three-level rod-type PCF laser, measured at full output power, is also shown in the insert of Fig. 4. Even with a core diameter of 85 µm, the output beam has a M² of 1.21 close to the diffraction limit, which attribute to the especial feature of the ultra-large mode area Yb-doped rod-type PCF.

4. Conclusion

In conclusion, we have demonstrated a short pulse, linearly-polarized AOM Q-switched pulse oscillator at 978 nm without any additional wavelength filter. With the introduction of GTP, the laser can easily work in the linearly-polarized mode of 43 dB PER with average power of over 2 W. Meanwhile, without the introduction of GTP, the laser is in the random-polarized mode with nearly 3 W average output power. For variable repetition rate between 10 kHz and 200 kHz under the control of the active Q-switched driver, laser presents stable pulse trains and pulse duration below 50 ns. Based on a short-length and high absorption rod-type PCF, the laser produces pulse duration as short as 9 ns, pulse energy of 120 μJ and peak power of 130 kW at the repetition rate of 10 kHz in the linearly-polarized mode. Acquiring such attractive features, this linearly polarized source would be particularly attractive for further power scaling, open up a high-efficiency method to demonstrate blue-green laser or ultraviolet source through nonlinear wavelength conversion.

Acknowledgment