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Abstract
We report a second Stokes diamond Raman laser at 1.49 μm capable of high power and large-scale-factor brightness enhancement. Using a quasi-continuous 1.06 μm pump of power 823 W (0.85% duty cycle) and M2 up to 6.4, a maximum output power of 302 W was obtained with an M2 = 1.1 providing an overall brightness enhancement factor of 6.0. The pulse length of ~210 μs was selected to ensure operation was representative of steady-state continuous lasing conditions in the diamond bulk. Accompanying theoretical calculations indicate that even more strongly aberrated pumps may be used to efficiently generate high beam quality output and with higher brightness enhancement factors. This diamond-based beam conversion technique addresses needs for high brightness and efficient eye-safe sources using low-brightness 1 μm pumps and reveals a widely-applicable route to practical high brightness lasers of increased wavelength range.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
There is intense interest to develop higher powered laser in eye-safe wavelength range (often taken as λ > 1.4 μm) for applications in remote sensing, ranging, laser radar and optical countermeasures. Er-doped crystalline and fiber lasers have emerged as important approaches [1–4]. At present, the highest reported powers for single-mode Er-doped lasers are 297 W and 264 W for Yb-sensitized and in-band pumped fiber lasers, and multi-mode is 405 W for large-core fiber laser, respectively [2,3,6]; 420 W highly multi-mode output for a cryogenically cooled Er:YAG slab laser [1]; and 75 W single-mode output for an in-band pumped Er:YAG planar waveguide laser [4]. Increasing the average power of Er lasers is challenging, however, more so than Yb and Tm lasers, due to problems associated with upconversion, dopant ion clustering, narrow pump absorption bandwidth, and poorer overlap of the absorption spectrum with mature diode pumps [5,6].
Raman lasers offer an alternative route to the eye-safe region via cascaded Stokes conversion from mature 1 μm lasers. Recently, cascaded Raman shifting in silica fibers has generated up to 301 W of output power at 1.5 μm with high beam quality [7]. However, for increased powers, management of spectral broadening in Raman fiber lasers may complicate design and may be a disadvantage in some applications [7–10]. Transverse-mode instability also represents a possible limitation for beam brightness [11]. Bulk diamond is an attractive Raman medium for high power generation [12,13] and features a much narrower Raman linewidth (45 GHz) and larger Raman frequency shift (40 THz) than Raman fibers. To date, diffraction-limited output has been demonstrated at the first Stokes wavelength (1.23 μm) with power of 154 W and 750 W in the continuous-wave (CW) and quasi-CW regimes respectively [13,14], and 100 W for quasi-CW operation at the second Stokes (1.49 μm) [15]. The capacity for the Raman interaction to provide high brightness output from low-coherence pumps [16–20] also is available from diamond [21,22]. Recently, a 12.7 times increase in brightness has been obtained for first-order laser for an external-cavity diamond Raman laser (DRL) with CW pumping [21]. A 1.7 times increase has also been reported for a second-Stokes pulsed nanosecond laser with an average power of 16.2 W [22]. To date, cascaded DRLs in the CW regime with brightness enhancement have not been reported.
Here we report a three-fold increase in power of a quasi-CW second-order Stokes DRL using low beam quality pump. With a pump laser of beam quality factor M2 = 6.4, 302 W of output power at 1.49 μm was obtained, with an M2 = 1.1 under all pump conditions, yielding maximum brightness enhancement factor (BEF) of 6.0. The output power is the highest single-mode power reported for Er-doped and Raman fiber lasers (~300 W) [2,3]. The measurements are in good agreement with model calculations, which we use to optimize and predict performance over wider range of power and input beam quality. The results highlight a novel pathway to high brightness eye-safe lasers based on relatively incoherent 1.0–1.1 μm pumps.
2. Experimental setup
Figure 1 depicts the experimental setup used to demonstrate brightness enhancement in an external cavity DRL cascaded to second Stokes wavelength of 1.49 μm. A quasi-CW diode-pumped Nd:YAG laser was utilized as a variable-beam-quality pump source for investigation of DRL characteristic performance as a function of pump brightness. The maximum pump laser average power obtained was 7 W at 1.06 μm with pulse duration of 210 μs and repetition rate of 40 Hz. Based on the oscilloscope trace (inset of Fig. 1) the calibrated on-time pump output power was up to 823 W. Our earlier thermal analysis of diamond demonstrated that tens of microseconds pulse durations are sufficient for establishment of steady-state thermal gradients [23]. Therefore, the DRL performance during the pump pulses is equivalent to the case of CW operation with one (or more) crystal faces held constant at ambient temperature. The pump beam quality factors were in the range M2 = 4.3 to 6.4 where the M2 value is determined from the horizontal and vertical beam quality factors using .
Fig. 1 Schematic diagram of the DRL setup for brightness enhancement of second-order Stokes. Inset shows the oscilloscope trace of pump (red line) and second Stokes (yellow line).
A half-wave plate in combination with a polarizer was used to attenuate the pump power without altering the pump beam quality. Another half-wave plate present in front of the DRL served to align the pump linear polarization along 〈111〉 axis of diamond to access the maximum of Raman gain [24]. The pump beam was focused into the diamond using a 100-mm-focal-length lens to achieve a waist radius in the range of 50–75 μm, determined by the pump M2 and beam size on the lens. The diamond used was a single-crystal 8 mm long with 4 × 1.2 mm2 cross-section and end facets antireflection coated at the first Stokes wavelength (λ = 1.24 μm). The input coupling mirror had 100 mm radius of curvature with high transmission at 1.06 μm, and high reflectivity at 1.24 μm and 1.49 μm. The output coupling mirror had 100 mm radius of curvature with 55% transmission at 1.49 μm, 99% reflectivity at 1.06 μm and 99.9% reflectivity at 1.24 μm. The output-coupling was slightly below the optimal values of 80–90% predicted using the models of [15,25]. The total cavity length was 202 mm, providing a TEM00 mode radius of 67 μm and 74 μm for 1.24 μm and 1.49 μm wavelengths respectively. A long pass filter was used to eliminate the residual pump and first Stokes light from the output.
3. Results and discussion
The high cavity finesse at 1.24 μm enabled a low threshold for the first Stokes and buildup of a high intracavity power sufficient for reaching second Stokes threshold with moderate pump power. For pump beam quality M2 = 4.3, 5.3, and 6.4, the DRL had first Stokes lasing thresholds of 27 W, 30 W and 36 W, respectively; and second Stokes lasing thresholds of 47 W, 54 W and 64 W, respectively [see Fig. 2(a)]. Using the available pump power and the most degraded pump beam quality (832 W, M2 = 6.4), the maximum second Stokes output power obtained was 302 W, with 37% conversion efficiency. The slope efficiency was 41 ± 2% for all three investigated M2 values.
Fig. 2 (a) Output second Stokes power versus pump power for input beams with M2 = 4.3 (black squares), M2 = 5.3 (red circles), and M2 = 6.4 (blue up triangles). (b) Beam quality measurements and near-field profiles of the pump with M2 = 6.4 (left), and second Stokes with M2 = 1.1 (right).
The output beam quality was measured at the maximum output power and found to be M2 = 1.1 ± 0.04 in all three pump conditions. Figure 2(b) compares the M2 factors on both axes of the pump and output beams for the lowest pump beam quality case (M2 = 6.4), in which the inset shows the near-field profiles. There was no evidence of output beam quality degradation or crystal damage even upon application of pump power more than 10 times above lasing threshold. Since the first-Stokes intensity is clamped at the second-Stokes threshold, and the second-Stokes output coupling ratio is high, the output power scales very efficiently without reaching the high intra-cavity powers likely to cause damage. Therefore, higher power and brightness scaling are expected in the present configuration by increasing pump power.
In order to quantify the brightness increase, we use the BEF defined as
where B is beam brightness, ηe is the power conversion efficiency from the pump to the second Stokes output, and λ is wavelength. Figure 3 shows the brightness of the pump, second Stokes beam, and the corresponding BEF as a function of pump power under three different pump conditions. Higher BEFs were obtained for degraded pump beams due to the constant high beam quality of the Stokes output (M2 = ~1.1), despite the increase in threshold. Here BEFs up to 2.7, 4.0 and 6.0 were obtained at the maximum second Stokes output power in each pump condition.Fig. 3 Brightness of the pump (black squares), second Stokes (red circles) and BEFs (green triangles) as a function of pump power with (a) M2 = 4.3, (b) M2 = 5.3, and (c) M2 = 6.4.
We have employed a steady-state model to obtain the second Stokes threshold and hence investigating the brightness enhancement dependence on pump M2 and output coupling. The pump power required for second Stokes threshold Pth is [15,26]
whereis the threshold condition for second Stokes lasing [see Eq. (5) in [15]], T1S = 0.1% is the round-trip out-coupling loss for the first Stokes, T2S is the output coupler transmissivity for the second Stokes, η1S,2S is the quantum conversion efficiency of the first- (η1S = λp/λ1S) and second-Stokes (η2S = λ1S/λ2S), α = 0.37%.cm−1 is the absorption loss in the diamond, L = 8 mm is the length of diamond, and G1S,2S is the Raman gain in the focused geometry [see Eq. (7) in [26]]. The model assumes the beam profiles are Gaussian-shaped with the beam overlap integral calculated as a function of pump M2 for TEM00 Stokes beams. The assumed high beam quality of Stokes beams is justified by observations in [21] and herein. We set the pump waist radius equal to the first-Stokes TEM00 mode radius i.e. 67 μm. The pump waist size is kept constant for all M2 values, which is achieved in practice by increasing the beam size on the focusing lens. The thresholds as a function of pump M2 with different T2S, shown in Fig. 4(a), indicate that increasing the M2 up to 20 leads only to a small increase (< 20%) in threshold. For comparison of our experimental results with the model, we calibrated the pump power required in a 67 μm waist radius for equivalent pump intensity. The experimental thresholds (black squares) for the three pumping conditions are in good agreement with the model (T2S = 55%, red line).Fig. 4 (a) Threshold of second Stokes laser as a function of pump M2 in the range 1–20 with output coupler transmissivity T2s = 95%, 90%, 70%, 55%, 50% and 30%, respectively; black squares show the calibrated experimental data of thresholds for the three cases (55.8 W, 58.5 W and 60.4 W for M2 = 4.7, 5.3 and 6.4 respectively). (b) Slope efficiency as a function of pump output coupler transmissivity with pump M2 = 1, 5, 10, 20 and 50, respectively.
The slope efficiency for the second Stokes beam is calculated using,
where η1Sη2S ( = 71.6%) is the quantum conversion efficiency from the pump to the second Stokes, ηout is the ratio of the output coupling to the cavity loss, ηd is the fraction of power depleted from the pump [15] given byThe resulting slope efficiency as a function of T2S, plotted in Fig. 4(b) for several values of pump M2, shows that slope efficiency increases with T2S as expected [15,25]. Meanwhile, the slope efficiency only decreases by a few % for increasing pump M2 up to 10 and by about 10–15% at M2 = 20. A more significant reduction in slope, up to 30%, is shown for M2 = 50. As G1S,2S is proportional to [26], the increase of pump M2 reduces the Raman gain for the first Stokes G1S, whereas the Raman gain for the second Stokes G2S is unaffected. Thus the ratio G1S/G2S decreases with M2, lowering the rate of pump depletion and consequently also the slope efficiency. In our experiment for M2 = 4.3–6.4 and T2S = 55%, the slope efficiency observed to be 41 ± 2% compared to theoretical estimate of 48 ± 2% [see Fig. 4(b)]. This variation can be attributed to small alignment errors of the Raman cavity.The BEF was calculated using Eqs. (1) and (4) and given , for the typical operating condition of 10 times above threshold. As shown in Fig. 5, the experimental BEFs (black squares) closely match the relevant model curve (T2S = 55%; see also inset). The result indicates that higher BEFs can be achieved with higher output coupler transmission and higher pump M2. For example, when using a pump M2 = 20, 90% output coupling and pump power of 1000 W, up to 548 W of second Stokes power and a BEF of 93 is predicted.
Fig. 5 BEF as a function of pump M2, with pump power equal to 10 × Pth; black squares show the experimental data of BEFs for the three cases (2.7, 4.0 and 6.0 for M2 = 4.3, 5.3, and 6.4) at maximum pump power.
4. Conclusions
We have demonstrated an efficient eye-safe DRL with large brightness enhancement of the second Stokes beam compared to the pump. A quasi-CW output power of 302 W with M2 = 1.1 is achieved, which matches the state of the art for Er-doped fiber lasers (including Er/Yb co-doped fibers) and Raman fiber lasers. We have extended our second-Stokes analytical model to account for pump beams of arbitrary M2, predicting a minor decrease in slope efficiency for pump M2 up to 20. Our concept provides an innovative approach to high-power eye-safe lasers that is applicable to kW-level pumps including those with relatively poor beam quality. Being a bulk crystal device, it avoids the problem of transverse mode instability that currently acts to limit brightness in fiber lasers. At the current power levels, thermal lensing in the diamond is not found to be significant. Much higher thermally-unaffected power is anticipated by using high pump powers and potentially also by using diamond that is cryogenically cooled and with enhanced isotopic purity to enhance heat transport from the active region [13,27]. The concept may be adapted and extended to other wavelength regions by using other pumps or via higher-order (3+) cascading. For example, to generate high brightness red output near 0.62 – 0.67 μm by using second harmonic pumps near 0.53 μm. A large number of wavelength options are conceivable as a result of the wide transparency of diamond (0.23–3.8 µm, and > 6 µm).
Funding
Australian Research Council (ARC) (DP150102054); Air Force Research Laboratory (FA2386-15-1-4075).
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