1. Introduction

Thulium- (Tm³⁺), holmium- (Ho³⁺), and Tm³⁺, Ho³⁺-co-doped fibers have been shown to be efficient gain media for lasers operating near 2 µm [1–3] with several options available for diode pumping particularly at 0.8 µm [1] and 1.15 µm [4]. The pump wavelength of 1.56 µm from Er³⁺-doped fiber lasers, however, allows for core pumping with significantly reduced quantum defect [3]. The fiber lasers operating around 2 µm have become an important tool for many applications, especially those requiring narrow-line pulsed operation and high power.

Passively and actively Q-switched Tm³⁺-based fiber lasers have been recently demonstrated exhibiting pulse durations in the range from ~40 ns to 1 µs [3,5–7]. Among them, the most advanced 2 µm Q-switched fiber laser sources demonstrated to date rely on active switching techniques delivering, however, rather unstable pulses with fairly broad spectra [6,7]. Compared to active Q-switching, passive switching offers a more compact geometry but the pulse duration depends critically on the modulation depth of the saturable absorber and the cavity round-trip time [8,9]. Since the pulse width is directly proportional to the cavity round-trip time and inversely proportional to the modulation depth of the saturable absorber, the shortest pulse width achievable from fiber lasers is typically set by the cavity length, which can not be made as short as the value used in, for example, microchip lasers [10].

We have shown recently that under strong pumping conditions, the Q-switch pulses can be significantly compressed in lasers with a large volume of the gain medium due to strong dynamic gain deviations [11]. In this paper we demonstrate a compact passively Q-switched 2 µm fiber laser that delivers nearly Gaussian-shaped 20 ns pulses with narrow spectrum and a repetition rate of 57 kHz. The highly doped Tm³⁺, Ho³⁺-doped aluminosilicate fiber that was core-pumped with 1.56 µm erbium fiber laser enables the use of exceptionally short-length cavity and hence shorter pulse operation. Q-switched operation is initiated by a resonant antimonide-based semiconductor saturable absorber mirror with a modulation depth of 70% at 1970 nm. The large dynamic gain excursion provides an additional but important impact on Q-switched pulse characteristics.

2. Experiment

The all-fiber laser is shown schematically in Fig. 1. The oscillator comprised of a short piece of highly doped Tm³⁺(2.5 wt.%), Ho³⁺ (0.25 wt.%)-doped aluminosilicate fiber, a dichroic pump coupler, a narrow bandwidth fiber Bragg grating (FBG) and InGaSb-based semiconductor saturable absorber mirror (SAM). The fiber core diameter was 10 µm and the numerical aperture was 0.2. The splice loss with standard single mode fiber (SMF-28) was ~0.5 dB. The resonant SAM was manufactured using solid source molecular beam epitaxy and comprised 20 InGaSb quantum-wells placed within a GaSb cavity with a Fabry-Pérot resonance at 1970 nm, and 18 AlAsSb/GaSb pairs for the distributed Bragg reflector (DBR). To enhance the cavity induced effect and to increase the nonlinear response, the structure was capped with a 4-pair AlAsSb/GaSb DBR which increases the finesse of the Fabry-Pérot structure. The low intensity reflectivity of the SAM is shown in Fig. 2. At the resonant wavelength of 1970 nm, the modulation depth of the absorber approached ~70%. A ~300 µm-diameter collimated beam was directed onto the SAM; the fluence at the resonant wavelength was ~70 mJ/cm² which corresponded to fully bleached absorption. Continuous wave (CW) operation was studied by replacing the SAM with a highly reflective (HR) dielectric mirror. A 5.6 W, 1.56 µm single-mode Er³⁺-doped fiber laser was used as the pump source. Core-pumping combined with high Tm³⁺ doping allowed a short (22 cm) optimal cavity length. The operating wavelength of the Tm³⁺, Ho³⁺-doped fiber laser was set with a FBG output coupler with a reflectivity of ~65%. To study the effect of the absorber modulation depth on the Q-switching performance near the resonant wavelength of the SAM, four FBGs with center wavelengths of 1942 nm, 1948 nm, 1957 nm, and 1970 nm were manufactured and used.

 

Fig. 1. Schematic of the passively Q-switched Tm³⁺, Ho³⁺-doped aluminosilicate fiber laser.

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Figures 3(a) and (b) show the average output power for operation in both CW and Q-switched modes at 1948 nm (off-resonance) and 1970 nm (resonant). As expected, the nonsaturable losses at the resonant wavelength reduced the average power ratio for Q-switched and CW regimes from 95% to 78% as the modulation depth (ΔR_SAM) was increased from 17% to 70%, respectively. The overall cavity losses associated with the resonant SAM also increased the laser threshold from 900 mW to 1350 mW. The maximum average output power at 1970 nm was 800 mW.

 

Fig. 2. Low intensity reflectivity of the SAM. The arrows show the operation wavelengths of the laser. The resonant wavelength is 1970 nm.

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Figure 4(a) shows the effect of the modulation depth on the pulse width; at the highest absorbed pump power of 5 W, the pulse width decreased from 67 ns to 19.6 ns for ΔR_SAM= 10% and 70%, respectively. When the SAM was operated off-resonance at 1942 nm, 1948 nm or 1957 nm, the laser produced a pump dependent pulse width which approached, at high pump powers, values that are shorter than those expected from classical passive Q-switching theory [see dashed lines in Fig. 4(a)] [8,9]. This pulse shortening mechanism has been observed recently in another Tm³⁺, Ho³⁺-doped fiber laser and was found to originate from gain-induced pulse compression under strong pumping conditions [11]. The pulse width reduction in the current investigation was due to the large difference between the unsaturated

gain before the onset of the Q-switched pulse and the saturated gain from the significantly depleted population inversion after the Q-switched pulse. The passively Q-switched pulse width under strong pumping conditions has been shown to be inversely proportional to this gain difference [11]. We note that dynamical pulse compression resulting in the generation of pulses shorter than the cavity round-trip time has been reported for passively and self-Q-switched fiber lasers employing stimulated Brillouin scattering (SBS) [12–14]. In our study, the effects from SBS were not observed.

 

Fig. 4. (a) The effect of the pump power and operation wavelength (hence ΔR_SAM) on the pulse width. Dashed lines show the pulse width calculated from theory without taking into account the gain induced pulse compression. (b) The effect of the modulation depth on pulse compression at different power levels exceeding the threshold power. Solid lines are the fittings according to egn. (8) in [11].

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Fig. 3. Average output power in CW operation with HR mirror and Q-switched operation with SAM at the wavelengths of (a) 1948 nm corresponding to the absorber modulation depth of ΔR_SAM=17%, and (b) 1970 nm corresponding to ΔR_SAM=70%.

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As can be seen in Fig. 4(a), for ΔR_SAM=70 % at 1970 nm, the modulation depth of the SAM played a dominant role in setting up the width of the Q-switched pulse producing pulses nearly independent of the pump power with compression from 21.3 ns to 19.6 ns. At a low modulation, however, the pulse compression effect contributed significantly to the shaping of the pulse; see results for ΔR_SAM=10%. Figure 4(b) illustrates further the effect of the modulation depth on pulse compression which we define as (τ_P,threshold-τP)/τ_P,threshold, where τ_P,threshold is the pulse width at the lasing threshold and τ_P is the pulse width at a given pump power. The experimental results for pulse compression, shown in Fig. 4(a), are also plotted against SAM modulation depth in Fig. 4(b) for two values of the pump power. As can be seen, the pulse compression at a pump power that is 3.6 times the laser threshold (black scatter) decreased from 43% for ΔR_SAM=10% to 8% for ΔR_SAM=70%. The solid lines represent the numerical results on Eqn. (8) in [11] and are in a good agreement with the experimental data.

Although the repetition rate of the laser, as seen from Fig. 5, increased linearly with pump power at high modulation depth, saturation effects appeared with off-resonance operation. As expected from classical theory, the repetition rate decreased with ΔR_SAM as f_repetition ~ (ΔR_SAM)^{- 1}. The pulse energy at off-resonance was pump dependent as observed in Fig. 5 for ΔR_SAM=10% at λ=1942 nm. For resonant operation at 1970 nm with ΔR_SAM=70%, the pulse energy was somewhat independent on the pump power with a mean value of ~15 µJ. The solid lines in Fig. 5 represent numerical simulations based on E_released=E_sat,gΔg, where E_sat,g is the gain saturation energy, and Δg the gain variation which takes into account pulse compression for operation well above threshold by replacing the value for the gain variation Δg=2q0 used in the near-threshold analysis [9] (dashed lines in Fig. 5) by Eqn. (7) from Ref. [11]. The pulse energy is now pump dependent at low modulation depth but differs from the value expected from classical theory in agreement with experimental observations. With large modulation depth, the pulse energy is effectively constant and close to the calculated near-threshold value. The difference in the laser characteristics (i.e. the pulse width, repetition rate, and pulse energy) between the off-resonant and resonant operation of the absorber is therefore due to the pulse compression effect that is strong for small modulation depth but is minimal at resonant operation, where ΔRSAM attains large values.

 

Fig. 5. The effect of the pump power and operation wavelength (hence ΔR_SAM) on the Q-switched pulse energy and repetition rate. Solid lines are the numerical fittings, and the dashed lines the near-threshold limit values for the pulse energy.

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The corresponding peak power at the resonant wavelength was 0.7 kW and the laser slope efficiency and optical conversion efficiency were 23% and 16%, respectively. A typical oscilloscope trace reveals a nearly Gaussian-shaped pulse without any substructure, as shown in Fig. 6. The small asymmetry in the pulse shape is due to the large initial inversion leading to a fast rise time and a slightly slower decay time of the pulse [15]. Optical spectra at the four operating wavelengths are shown in the inset of Fig. 6. The spectral width was <0.6 nm which was limited by instrument resolution. The pulse jitter ranged from ~20 µs to 5 µs as the repetition rate increased, see inset to Fig. 6. The large jitter, which is typical for passively Q-switched lasers, is caused by amplified spontaneous emission, fluctuations in the pump power, loss and temperature effects [16]. The pulse stability in amplitude and time (pulse duration) were better than 20% and ~5%, respectively.

 

Fig. 6. Oscilloscope trace of a 19.6 ns Q-switched pulse observed at the resonant wavelength of the SAM. The insets show the narrow optical spectra at the four operating wavelengths (right) and the jitter vs. repetition rate of the laser (left).

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3. Conclusion

We have studied a passively Q-switched ~2-µm Tm³⁺, Ho³⁺-doped fiber laser using a high contrast InGaSb resonant semiconductor saturable absorber mirror. The compact core-pumped all-fiber laser incorporating an optimized absorber mirror and pumping far above the threshold delivered 20 ns pulses with a narrow spectrum, an average output power of 800 mW, and pulse energy of 15 µJ. The absorber modulation depth was shown to have a significant influence on the contribution of dynamic gain induced pulse compression on the Q-switched pulse characteristics.