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Abstract
We report a high power quasi-continuous-wave (QCW) 620 nm laser from an external cavity diamond Raman laser utilizing intracavity frequency doubling in lithium triborate. Output power of 30 W for durations of 0.25 ms at 15% conversion efficiency was achieved with a beam quality factor M2 = 1.1 from a free-running Nd:YAG pump laser of M2 = 1.5. The critical design parameters that affect conversion efficiency and power were analysed with the aid of an analytical model. By adaptation to other pump technologies, the diamond approach provides a novel pathway towards high brightness CW beam generation in the visible and ultraviolet regions.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Solid state Raman lasers are a convenient technology for shifting the wavelength of conventional lasers to exotic wavelengths with enhanced brightness. A key advantage offered by the Raman interaction is the automatic phase-matching which makes the process insensitive to temperature and angle and forms the basis for Raman beam clean-up in which aberrated pumps can generate diffraction limited output. Cascading the process allows generation of higher-order Stokes wavelengths.
An important concern when increasing Raman laser power is the significant thermal loading arising from the decay of the Raman-generated optical phonon and impurity/defect absorption [1–6]. Raman fibers are promising laser media as they allow high pump intensities to be maintained over a longer distance due to optical confinement and possess thermal advantages by virtue of the distributed active medium. For example, CW output powers up to 3.9 kW and wavelengths spanning 1.1 – 1.5 microns have been demonstrated with Raman fiber lasers [7–10]. However, subsequent harmonic conversion is made difficult by linewidth broadening at elevated powers along with the complexities of mixing fiber and bulk crystal cavity elements.
Diamond has a narrow Raman linewidth (45 GHz compared to more than a THz for Raman fibers) and has recently emerged as an important high brightness continuous-wave (CW) Raman conversion material due chiefly to its high thermal conductivity, high Raman gain coefficient and wide transparency. The high thermal conductivity, accompanied by a low thermal expansion coefficient, enables efficient and thermally-unaffected performance over a wide power range. Furthermore, the high thermal conductivity enables CW conversion in an external cavity configuration which enables adaptation to a greater range of pumps [11]. To date, CW external cavity diamond Raman lasers (DRLs) have been demonstrated at Stokes powers up to 381 W and conversion efficiency (61%) and with good prospects for further power scaling [12, 13].
The wavelength diversity of Raman lasers increases by combining χ(2) nonlinear frequency mixing techniques such as second harmonic generation (SHG) and sum frequency mixing. The enhanced cavity field allows efficient generation of the second harmonic in Type I or II phase-matched crystals. In intracavity Raman lasers, the technique has been demonstrated in the yellow and orange spectral region with up to 10 W of CW power using a range of pump lasers, Raman crystals and harmonic crystals [14–25]. For external cavity Raman lasers, intracavity second harmonic generation has been reported only in the pulsed regime [26, 27]. Since diamond is well suited to continuous power conversion in an external cavity [11], it is a promising system for high power generation of visible and UV wavelengths via intracavity harmonic generation.
Here we report an intracavity frequency-doubled external cavity Raman laser operating in the quasi-CW regime. Using diamond, a Nd:YAG laser at 1064 nm was Raman shifted to the 1240 nm first Stokes wavelength and subsequently converted to 620 nm in a lithium triborate (LBO) harmonic crystal. The red output wavelength is in a region of interest for laser display applications, medicine and laser spectroscopy. A model for the laser is presented and used to identify the route to optimised efficiency at current and elevated power levels. In addition, the prospects for adapting this scheme to extended CW durations, higher power levels (eg., >100 W) and other wavelengths are discussed.
2. Experimental arrangement
The experimental arrangement is shown in Fig. 1. The Raman laser cavity was a 107.5 mm long near-concentric resonator consisting of an input coupler and output coupler of 50 mm radii of curvature. A focusing lens (50 mm focal length) was used to focus the pump beam into the middle of a 4 × 1 × 8 mm long diamond that was anti-reflection (AR) coated for the first Stokes wavelength. The diamond (ultra-low birefringence, Element Six Ltd) had a manufacture-specified nitrogen impurity level of approximately 20–40 ppb and a birefringence of less than 10−5 in the propagation direction (as measured using Metripol). The LBO crystal of dimensions 4 × 4 × 10 mm and cut at θ = 85.8° and ϕ = 0° (and temperature tuned to 310.8 K) was placed on a translation stage to enable adjustment of the Stokes beam size in the LBO and thus the strength of the nonlinear interaction. The input coupler was highly transmissive for the pump (T >97%), highly reflective for the 1240 nm first Stokes (R >99.9%) and partially transmissive for the 620 nm second harmonic output (T =66%). The output coupler was highly reflective for the pump and provided approximately 0.5% and 30% transmission for the first Stokes and second harmonic output respectively. Note that these mirrors, although were not optimal for efficiently exiting single-ended 620 nm output, were selected with the primary aim of providing robust (high damage threshold) operation suitable for a first demonstration and parametric study of dynamics. The generated SHG power was measured from the final input turning mirror (as indicated in Fig. 1) and calibrated using the transmissivities of the output coupler, input coupler, focussing lens (FL), half-wave plate (HWP 2) and the turning mirror at 620 nm.
Fig. 1 Experimental arrangement of the external cavity DRL with intracavity frequency doubling: HWP-half-wave plate, FL-focussing lens, TM-turning mirror, IC-input coupler, and OC-output coupler. The inset diagram specifies the diamond and LBO crystal orientations with respect to the propagation axis.
The pump source was a free-running Nd:YAG laser at 1064 nm with a pulse duration of 0.25 ms operating at 40 Hz repetition rate (1% duty cycle) and a beam quality factor M2 = 1.5. Since threshold is attained within a microsecond and thermal gradients establish in the diamond on the timescale of a few microseconds [28], its behaviour during the majority of the pulse is representative of steady-state continuous operation. This assumption is supported by tests which show that the DRL efficiency is independent of repetition rate and on-time duration within the range provided by the pump laser (rates 20–80 Hz and durations up to 0.1–0.5 ms). At high powers, heating of the LBO is also anticipated to become an important consideration due to impurity absorption. Thermal gradients are established much more slowly in LBO (time constant is approximately 20 ms) due to its 103 times lower thermal conductivity. As a result, steady-state thermal conditions are not achieved within the pump duration and the current investigation is representative of CW operation only under the assumption that thermal effects in the LBO are negligible. This issue is discussed in detail in Section 6. A half-wave plate (HWP 1) and a polarizing cube combination were used to attenuate the pump beam and a Faraday rotator provided isolation from the back-reflected pump. A half-wave plate (HWP 2 in Fig. 1) was used to align the pump polarization with respect to the diamond axes.
3. Experimental results - Laser performance
The laser performance was investigated as a function of LBO position. Figure 2 shows the Stokes and second harmonic output plotted as a function of the Stokes beam radius at the midpoint of the LBO (wLBO), which was calculated from the cavity parameters using Gaussian beam propagating software. The Stokes leakage (top figure) from the cavity increases with wLBO consistent with the lower nonlinear output coupling and hence greater build-up of intracavity Stokes power. Over the same range, the SHG power (bottom figure) increases initially up to the 30 W maximum at a diamond-LBO separation of 12 mm (corresponding to wLBO = 155 µm) and decreases thereafter. For separations less than optimum, over-coupling occurs in which the high rate of Stokes-SHG conversion suppresses the Stokes field that in turn reduces the transfer of power from the pump and hence the maximum obtainable overall conversion efficiency to the harmonic. Conversely, for larger than optimum separations, under-coupling occurs in which the Stokes-to-SHG conversion becomes a smaller fraction of the overall losses of the intracavity Stokes field. These effects are borne out in the numerical model detailed below (Section 4).
Fig. 2 Measured and calculated Stokes power leakage (top) and SHG power (bottom) as a function of wLBO at 200 W pump power. The model parameters correspond to the optimum condition given in Tab. 1.
After optimizing the LBO position, the laser behaviour was characterized as a function of injected pump power. Figure 3(a) shows the generated SHG power, the Stokes power leakage and residual pump power. At a pump power of 97 W, the onset of 1240 nm first Stokes output and 620 nm second harmonic output was observed simultaneously. For higher pump powers, the SHG power increased monotonically up to the 30 W maximum (as limited by the available pump power of 204 W) achieving a maximum conversion efficiency of 14.9%. The M2 of the beam at the highest output power (30 W) was measured to be 1.1.
Fig. 3 Measured SHG and Stokes power (top) and residual pump power (bottom) as a function of incident pump power for Stokes beam radius in the LBO of (a) 155 µm and (b) 550 µm. Model determined results (dashed lines) are included for comparison (for experimental conditions given in Tab.1).
It was noted that the residual pump power above threshold remains approximately constant up to 180 W of injected pump due to conversion to the first Stokes. However, above 180 W the residual pump was observed to increase. Although an increase in residual pump power may signal the onset of cascaded second Stokes generation (1485 nm) [29], the second Stokes threshold was not attained in this case owing to the high transmission of 1485 nm for both the input and output couplers (98% and 84% respectively). Since there is no commensurate saturation in Stokes and SHG output, the origin for this increase is unclear.
The detailed output characteristics for the under-coupled regime are shown in Fig. 3(b). When compared to the optimum condition (Fig. 3(a)), this regime is characterized by a much higher intracavity Stokes field and consequently much higher depletion of the pump. (Note that operation in the over-coupled regime was not investigated in detail due to a higher risk of LBO coating damage under extended operation.)
The output polarization of the Stokes and SHG beams were measured as a function of input polarization. For all pump polarizations, the Stokes and SHG polarization were horizontal and vertical respectively (ie., along the 〈100〉 and crystal directions). It has been reported previously that the output polarization properties of high-Q Raman lasers are perturbed from the direction of highest gain and becomes fixed to the local birefringence axes [30]. As the dominant birefringence element in the cavity is the LBO, and its optic axis is aligned close to the horizontal axis (see Fig. 1), the measured Stokes polarization is consistent with the expected polarization behaviour. The vertical SHG polarization is consistent with the type I phase matching condition.
4. Laser model
4.1. Model description
A model has been developed to provide guidance for improving efficiency and predicting the performance at elevated power levels. The model was adapted from the model equations for a CW external cavity DRL [31] to include a nonlinear output coupling loss term for conversion from the intracavity Stokes field to the SHG output. The spatial distributions for the pump, Stokes and SHG beams are assumed to be Gaussian and thermal effects are neglected. The z-dependence of the intracavity Stokes intensity inside the cavity is taken to be constant owing to the low round-trip gain and loss of the high–Q cavity.
Under these conditions, the Stokes power generated for double pass pumping is [31]
where η = λP/λS is the quantum defect of the inelastic Raman scattering process, PP is the input pump power, is the intracavity Stokes power and G is the Raman gain in the focussed geometry which takes into account the crystal length, confocal parameters of pump and Stokes, gain reduction factor for the pump and Stokes wavelength and beams overlap mismatch between the pump and Stokes as explained in [31].The nonlinear output due to SHG is
where , V/A is the plane-wave impedance, ωS is the frequency of the Stokes beam, deff is the effective nonlinear coefficient of the crystal and is the area of the Stokes beam in the LBO.Losses for the Stokes field also include crystal losses and leakage through the cavity mirrors. The total crystal loss is
where αd and are distributed loss terms for absorption and scattering loss for the diamond and LBO respectively, and Ld and are the crystal lengths, and κ is the total reflection loss. The Stokes power leakage through the output coupler with transmission T for the Stokes isFor the steady-state condition of equal Stokes generation and loss, a solution for the required pump power for a given intracavity Stokes power (and thus the generated SHG) is obtained
The first term in the brackets takes into account the Stokes leakage through the output coupler and parasitic losses in the cavity. The second term which depends on the square of the intracavity Stokes power corresponds to the SHG. Also from [31], the residual pump power is calculated using Equations (5) and (6) are used to predict the Stokes, SHG, and residual pump power. Table 1 summarizes the parameters used for fitting the model results with the experimental results. The output coupling, diamond and LBO lengths, M2 values of the pump and Stokes, and the second-order nonlinear coefficient deff were taken as fixed parameters. The values for gS, wP, wS, and parasitic losses (αd, , and κ) were varied within a nominal range to fit the experiment data. The gS value of 8 cm/GW is within the range of the reported values in the literature (8–15 cm/GW at 1064 nm) [32, 33]. The parameter αd is taken to be 0.37%/cm which is within the range expected for diamond samples with 20–40 ppb nitrogen content [34, 35]. The value for the LBO of 0.37%/cm is slightly higher than the absorption coefficient value of about 0.1%/cm at 1064 nm given by the manufacturer (Castech Inc.). The κ value of 0.9 % corresponds to approximately 0.1% reflection loss per diamond surface and approximately 0.13% per LBO surface (from the manufacturer’s data sheet [36]). The pump and Stokes beam-waist radii in the diamond (wP and wS) are calculated using LASCAD laser design software from the known values of incident pump beam radius, the focal length of the input lens, radii of curvature of the input and output couplers and position of diamond in the cavity.
4.2. Comparison with experiment
Figure 4 shows examples of model output corresponding to the over-, optimally- and under-coupled regimes for a representative set of parameters. The figures reveal the reduction of SHG power and the corresponding increases in Stokes and residual pump fields that are characteristic to the under- and over-coupled regimes.
Fig. 4 SHG, Stokes and residual pump power as a function of pump power for wLBO = 450 µm (top), 155 µm (middle) and 70 µm (bottom) featured by under-, optimum and over-coupling regimes respectively. The parameters used for these plots are T = 0.5%, κ = 0.9%, αd = 0.37%/cm, , Ld = 8 mm, , ωP = 42 µm, ωS = 47 µm, , and .
The model results are included in experimental plots of Figs. 3(a) and 3(b) for optimum- and under-coupling regimes. In both the regimes, there is broad agreement with the experimental data; the most notable discrepancy occurs for the residual pump powers at higher injected pump powers (see in particular Fig. 3(b)). This is a known issue for this type of model, and is attributed to the absence of diffraction effects in the one-dimensional treatment[31]. The over-evaluation of the modelled residual pump power values compared to experiment indicates that the model derived loss and/or gain coefficients are likely to be over-estimates.
Figure 2 also includes model-calculated Stokes leakage and SHG power as a function of wLBO at 200 W of pump power. Again good agreement is obtained with the data, except for consistently lower measured values of SHG power for wLBO greater than the optimum value. This discrepancy is attributed to the progressive misalignment of the cavity as the LBO was moved from the diamond towards the output coupler.
5. Thermal considerations
The thermal effects in diamond and LBO need to be considered when increasing the pump power and on-time duration. In diamond, the major heating mechanisms are the decay of Raman generation optical phonons and the optical absorption by impurities and defects. Although the nature of heat deposition and transfer in diamond is not completely understood, an upper bound for the steady-state thermal lens strength may be calculated by assuming a uniform heat deposition along the crystal length [12, 32, 37]
where Pdep is the total heat load deposited, is the thermo-optic coefficient, n is the refractive index, ν is the Poisson’s ratio, Cr,ϕ are the photoelastic coefficients in cylindrical coordinates, κ is the thermal conductivity (2200 W/mK), and αT is the thermal expansion coefficient. Diamond has a moderately high and unusually small αT and hence the major contributor to lensing in Eq. (7) is due to the thermo-optic term [32]. For the conditions of 30 W of output, the heat loads due to phonon decay and impurity absorption are calculated to be 6.7 W and 3.2 W respectively yielding . ABCD analysis indicates that a thermal lens of focal length as short as 2.6 cm may be accommodated by the cavity with a small length adjustment, hence it is deduced that is at least an order of magnitude weaker than the limit for cavity stability.The thermal lens strength in LBO, which occurs primarily by impurity absorption of the intracavity Stokes field, was evaluated using for steady-state conditions the expression from ref [38]
where here κ and correspond to material parameters for LBO, α is the coefficient for impurity absorption and wLBO is the average beam size in the LBO. Note that this expression neglects anisotropic and thermal expansion effects, but nevertheless is used to gain a first pass estimate of the lens strength. Given the Stokes absorption coefficient in LBO is α = 0.1%/cm [36], κ = 3.03 W/mK [36] and taking dnz/dT = −8.9 × 10−6 /°C [36], heat deposited in the LBO due to Stokes absorption is 4 the W yielding a value of . ABCD analysis indicates that the current cavity accommodates LBO thermal lenses of up to −0.6 cm which corresponds to a pump power 320 W and 95 W of SHG output.6. Discussion
The intracavity frequency-doubled DRLs described here shows the potential to generate high power and high beam quality output in the red region of the spectrum. The power demonstrated compares favourably with other competing technologies such as frequency doubling of rare-earth doped lasers [39–42], semiconductor disk lasers [43, 44], VECSELs [45, 46] and laser diodes [47]. Also, the combined effect of Raman beam clean-up and the lack of thermally-induced saturation in the diamond provide important advantages for generating high brightness output from high power pumps of poor beam quality (see [48]).
The model described in Sec. 4 provides a basis for optimizing design. The model predicts substantial increases in SHG power for the same basic design by paying attention to cavity losses, increasing the diamond length and by using tighter pump focussing. For example, for T < 0.1%, αd < 0.2%/cm (for 20 ppb CVD diamond [34, 35]), (approximate absorption and scattering losses for low-loss LBO [49]), Ld = 15 mm, and wP = 30 µm (which is within the range for the current cavity), the SHG power potentially increases to more than 90 W with approximately 45% conversion efficiency at the current pump power level of 204 W. Furthermore, it should be noted that the Stokes was horizontally polarized and thus the Raman gain in the current configuration is 75% of the maximum achievable value [32, 33]. Lower threshold and higher power is thus expected by orienting the LBO phase-matching plane, and hence the Stokes polarization, parallel to the maximum 〈111〉 gain direction for diamond (which is 35.3° from horizontal in the present arrangement).
The concept described here offers the potential to further explore more wavelengths in the visible and UV spectral regions by making use of pumps at other wavelengths. High-power Yb-doped fiber lasers provide a potentially important pump source due to the highly practical nature of the technology, wide tunability (1010–1070 nm for example [50]) and the capacity for high power. The corresponding first Stokes wavelength range 1167–1248 nm enables harmonic in the range 584–624 nm. Further diversification of output wavelengths may be obtained through harmonic conversion of higher-order Stokes fields by adapting concepts from pulsed lasers [26, 27].
In the present work, the output linewidth was the order of 20 GHz (a convolution of the pump and Raman linewidths) which may be too broad for some applications such as those based on interferometry and line spectroscopy. Recently it was shown that CW DRLs are well suited to generation of single longitudinal mode output, even in a standing wave resonator by paying attention to mode overlap and thermally-induced cavity length changes [51, 52]. These concepts may be adapted to the present scheme to provide a route towards high power single-longitudinal mode output, and to develop novel spectrally and spatially bright lasers to address demand for brighter sources in applications such as holography, guide stars and atom cooling.
A potentially important extension of our concept is to use visible pump sources to generate wavelengths in the deep UV where there is demand in spectroscopy, atom cooling and trapping, photolithography, and material processing. The main routes to CW deep-UV output have been through external–cavity resonant SHG and SFG conversion [53–58], intracavity frequency doubling of gas lasers [59] and Ce doped lasers [60]. Progress has been hampered by the paucity of visible laser gain media suitable for intracavity harmonic generation. With the exception of 266 nm (the Nd fourth harmonic), the maximum power achievable obtained is generally limited to few hundred milliwatts and at higher power it is difficult to maintain the beam quality. Adaptation to mature 532 nm pump technology, for example, is anticipated to provide a highly practical route to 287 nm via the second harmonic with outstanding potential for multi–watt power and with high beam quality.
7. Conclusion
We have demonstrated frequency extension of a high power DRL to the red spectral region by intracavity frequency doubling. A Type I phase–matched LBO was used in an external cavity DRL to generate quasi-CW 30 W output power at 620 nm with 15% conversion efficiency and excellent beam quality (M2 = 1.1) for 204 W of available pump power. The SHG power as functions of pump power and LBO position were found to agree well with an analytical model developed for the system.
Extension to higher power and to continuous operation was considered by examining the thermal effects that are expected to come into play in the diamond and LBO in steady-state CW operation. Despite the passive role of the LBO in the cavity, its thermal lens strength by impurity absorption is calculated to be more than an order of magnitude higher than in diamond due to its much lower thermal conductivity and is the predicted mechanism for limiting power. For the current basic design, CW powers approaching 100 W are predicted before LBO thermal effects impact severely upon the performance.
These results reveal a generic approach to generating high brightness visible and UV output through the combination of a diamond Raman element and intracavity harmonic conversion.
Funding
Australian Research Council (ARC) (DP150102054) and U.S. Air Force Research Laboratory (FA2386-15-1-4075). R.J.W. acknowledges the support of a Macquarie University Research Fellowship.
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