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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 25 — Dec. 5, 2011
  • pp: 25623–25631
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Miniature wavelength-selectable Raman laser: new insights for optimizing performance

Xiaoli Li, Helen M. Pask, Andrew J. Lee, Yujing Huo, James A. Piper, and David J. Spence  »View Author Affiliations


Optics Express, Vol. 19, Issue 25, pp. 25623-25631 (2011)
http://dx.doi.org/10.1364/OE.19.025623


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Abstract

We report a miniature, wavelength-selectable crystalline Raman laser operating either in the yellow (588 nm) or lime (559 nm) selected simply by changing the temperature of an intracavity LBO crystal. Continuous-wave (CW) output powers are 320 mW and 660 mW respectively, corresponding to record diode-visible optical conversion efficiencies of 8.4% and 17% for such miniature devices. The complex laser behavior arising from interplay between nonlinear processes is studied experimentally and theoretically. We show that the interplay can lead to complete suppression of the first-Stokes field and that the phase matching conditions for maximum visible powers differ markedly for different length LBO crystals. By using threshold measurements, we calculate the round-trip resonator losses and show that crystal bulk losses dominate over other losses. As a consequence, Raman lasers utilizing shorter LBO crystals for intracavity frequency mixing can produce higher visible output power. These are new considerations for the optimum design of CW intracavity Raman lasers with visible output.

© 2011 OSA

1. Introduction

Stimulated Raman Scattering (SRS) is an inelastic process arising from the third-order nonlinearity of Raman active materials. It has been widely used for frequency conversion to extend the spectral coverage of solid-state lasers, not only in the infrared region but also in the visible through sum-frequency generation (SFG) of fundamental and Stokes output or through second harmonic generation (SHG) of either of these [1

1. H. M. Pask, P. Dekker, R. P. Mildren, D. J. Spence, and J. A. Piper, “Wavelength-versatile visible and UV sources based on crystalline Raman lasers,” Prog. Quantum Electron. 32(3–4), 121–158 (2008). [CrossRef]

, 2

2. P. Cerný, H. Jelínková, P. G. Zverev, and T. T. Basiev, “Solid state lasers with Raman frequency conversion,” Prog. Quantum Electron. 28(2), 113–143 (2004). [CrossRef]

]. Compact, efficient, all-solid-state lasers generating visible emission spanning the green-to-yellow are attractive for applications in ophthalmology, biomedicine and display technologies. Motivated by these applications, significant effort has been directed at optimizing such laser sources. The most efficient and practical solid-state CW Raman lasers reported to date have been mainly based on a variety of tungstate crystals including BaWO4, SrWO4, KGW, and vanadate crystals including Nd:YVO4, Nd:GdVO4, and Nd:LuVO4 [3

3. L. Fan, Y. Fan, Y. Duan, Q. Wang, H. Wang, G. Jia, and C. Tu, “Continuous-wave intracavity Raman laser at 1179.5 nm with SrWO4 Raman crystal in diode-end-pumped Nd:YVO4 laser,” Appl. Phys. B 94(4), 553–557 (2009). [CrossRef]

6

6. Y. Lü, X. Zhang, S. Li, J. Xia, W. Cheng, and Z. Xiong, “All-solid-state cw sodium D2 resonance radiation based on intracavity frequency-doubled self-Raman laser operation in double-end diffusion-bonded Nd3+:LuVO4 crystal,” Opt. Lett. 35(17), 2964–2966 (2010). [CrossRef] [PubMed]

].

In this paper, we present the performance of an efficient miniature self-Raman laser that generates either lime or yellow output by tuning the intracavity LBO crystal temperatures for phase-matching of either SFG or SHG. Despite the very modest diode pump power of 3.8 W, output powers of 320 mW at 588 nm and 660 mW at 559 nm were achieved with high diode to visible conversion efficiencies of 8.4% and 17% respectively, comparable to the best conversion efficiencies previously reported for order-of-magnitude higher pump powers. Contrary to usual expectations, the best laser performance is achieved with a short LBO crystal, a consequence of the interplay between SRS and frequency mixing, as well as crystal losses. We compare laser operation using two LBO crystals of different length, and make estimates of the round trip losses by laser threshold measurements. By measuring the residual fundamental and Stokes emission through the output coupler, along with the visible emission, we show that complex interplay between the intracavity fields occurs in these lasers, originating from competition between the non-linear effects. We use rate equation analysis to explain this complex behavior leading to new understanding of factors limiting power and efficiency.

2. Laser design

The cavity layout for our compact lime–yellow wavelength selectable laser is shown in Fig. 1
Fig. 1 Cavity layout for self-Raman experimental laser.
. The laser was designed for low-threshold and high-efficiency operation, and to achieve these we used small spot sizes and kept cavity losses as low as possible. The fundamental (1064 nm) and Stokes (1176 nm) wavelengths were resonated in the high Q cavity, and we used temperature tuning of an LBO crystal to select the visible output wavelength of the laser: the output was 559 nm (lime) when the LBO was phase-matched for SFG of the fundamental and Stokes fields, and 588 nm (yellow) when the LBO was phase-matched for SHG of the Stokes field.

The pump source used in this work was a high-brightness free-space diode laser (Unique Mode: UM4200-M20-CB-TEC), providing horizontally-polarized pump light with a maximum output power of 3.8 W at 808 nm. We used a telescope arrangement for expanding and collimating the diode output and then focusing it into the Nd:YVO4 crystal through a 50 mm focal length lens. The focal spot radius was estimated to be 80 µm. Over 98% of the pump was absorbed in the laser crystal.

We chose an a-cut, 1 at.% Nd:YVO4 crystal with dimensions of 4 mm × 4 mm × 3 mm long as the combined laser/Raman medium, taking account of the high Raman gain (4.5 cm/GW [7

7. A. A. Kaminskii, K.-i. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4 - new efficient χ(3)-materials for Raman lasers,” Opt. Commun. 194(1–3), 201–206 (2001). [CrossRef]

]) and high emission cross-section (14.1×10−19 cm2 [8

8. Y. Sato and T. Taira, “Comparative study on the spectroscopic properties of Nd:GdVO4 and Nd:YVO4 with hybrid process,” IEEE J. Sel. Top. Quantum Electron. 11(3), 613–620 (2005). [CrossRef]

]) of this material. The Nd:YVO4 crystal was oriented so that the pump light was polarized along its ‘c’ axis, to take advantage of the high 808 nm absorption (48.4 cm−1), a factor of four times higher than for light polarized along the ‘a’ axis [8

8. Y. Sato and T. Taira, “Comparative study on the spectroscopic properties of Nd:GdVO4 and Nd:YVO4 with hybrid process,” IEEE J. Sel. Top. Quantum Electron. 11(3), 613–620 (2005). [CrossRef]

]. In order to reduce the number of intracavity surfaces and minimize resonator losses, a high-reflectivity (HR) coating (R>99.994% @ 1064 /1176 nm) was deposited directly onto the pumped face of the Nd:YVO4 crystal. Here and below the quoted reflectivities are as specified by the suppliers. On the other side, the crystal was anti-reflection (AR) coated (R = 0.026% @ 1064nm, R = 0.045% @ 1176 nm). The crystal was water cooled to 20°C.

Two lithium triborate (LBO) crystals (Castech Inc.) with dimensions of 4 mm × 4 mm (cross section) and either 10 mm or 5 mm long were evaluated for intracavity frequency-mixing. They were cut for noncritical phase matching (NCPM) (θ=90°, φ=0°), AR coated (R = 0.42%-0.47% @1064 nm, R=0.095%-0.098%@1176 nm). A copper block incorporating a resistive heater was used to house the LBO crystal and enable heating of the crystal from ambient up to 160°C, and the temperature of this mount was monitored using a PT100 sensor.

The resonator was formed by the combination of the HR coating on the pumped face of the Nd:YVO4 crystal, and an output coupler with radius of curvature (ROC) 50 mm. The total cavity lengths were ~17 mm and ~15 mm when the 10 mm and 5 mm long LBO crystals were used respectively (the minimum cavity length was set by the component mounts). The output coupler was coated for high transmission (T~80%) at 559 nm and 588 nm (T~95%), and high reflectivity (R>99.994%) at 1064 and 1176 nm. The spectral properties of the output were monitored using a fibre-coupled spectrometer having 0.05 nm resolution.

3. Experimental results using LBO crystals with different lengths

3.1 Laser output powers at 559 nm and 588 nm

Figure 2(a)
Fig. 2 Visible output power as function of incident pump power (a) using 5 mm long LBO (b) using 10 mm long LBO.
shows results obtained using the 5 mm long LBO crystal. We observed the maximum visible output powers (at 3.8 W maximum pump power) were 660 mW and 320 mW, at 559 nm and 588 nm respectively, for which the LBO mount temperature was 98°C and 45°C. The corresponding conversion efficiencies were 17% and 8.4%. Figure 2(b) shows the experimental results for the 10 mm long LBO crystal. The maximum power at 559 nm was 420 mW, and at 588 nm 195 mW, obtained for the same 3.8 W incident pump power and LBO mount temperatures set at 88°C and 50°C respectively. The temperatures for the two LBO crystal lengths were noticeably different. Threshold pump powers were identical for the two visible wavelengths, 0.34 W for the 5 mm LBO case and 0.6 W for the 10 mm LBO case. Note that the thresholds for both visible lines are the same as threshold for SRS. The output beam quality parameter (M2) of the outputs were 2.5 and 1.5 at 559 nm and 588 nm respectively, at maximum output power.

Given that each LBO crystal had identical coatings, the substantially lower threshold and higher maximum output powers for the lasers utilizing the shorter LBO crystal indicate that bulk losses in these crystals are significant. This is discussed further in Section 4.

The diode to visible conversion efficiencies achieved here are very high, approaching those obtained in [5

5. A. J. Lee, D. J. Spence, J. A. Piper, and H. M. Pask, “A wavelength-versatile, continuous-wave, self-Raman solid-state laser operating in the visible,” Opt. Express 18(19), 20013–20018 (2010). [CrossRef] [PubMed]

] where pumping with a much higher power of 29 W delivered conversion efficiencies of 12.1% and 16.9% in the yellow and lime respectively. The powers reported here are also substantially higher than the yellow power of 220 mW reported in [9

9. X. Li, A. J. Lee, H. M. Pask, J. A. Piper, and Y. Huo, “Efficient, miniature, cw yellow source based on an intracavity frequency-doubled Nd:YVO4 self-Raman laser,” Opt. Lett. 36(8), 1428–1430 (2011). [CrossRef] [PubMed]

] using the same 3.8 W pump diode (operation at 559 nm was not reported in [9

9. X. Li, A. J. Lee, H. M. Pask, J. A. Piper, and Y. Huo, “Efficient, miniature, cw yellow source based on an intracavity frequency-doubled Nd:YVO4 self-Raman laser,” Opt. Lett. 36(8), 1428–1430 (2011). [CrossRef] [PubMed]

]). Note that, for simplicity, only the visible emission propagating through the output coupler was measured in this work. Even higher powers and efficiencies may be possible by using an intracavity mirror as in [5

5. A. J. Lee, D. J. Spence, J. A. Piper, and H. M. Pask, “A wavelength-versatile, continuous-wave, self-Raman solid-state laser operating in the visible,” Opt. Express 18(19), 20013–20018 (2010). [CrossRef] [PubMed]

] and [9

9. X. Li, A. J. Lee, H. M. Pask, J. A. Piper, and Y. Huo, “Efficient, miniature, cw yellow source based on an intracavity frequency-doubled Nd:YVO4 self-Raman laser,” Opt. Lett. 36(8), 1428–1430 (2011). [CrossRef] [PubMed]

] to collect the back-propagating visible emission. The improvements in this work are due to many factors including the directly-deposited HR coating on the Nd:YVO4 crystal, a shorter resonator with consequently smaller cavity mode and higher intensities, and most particularly a shorter LBO crystal for reasons associated with laser dynamics and losses that are developed in the following sections.

3.2 Optical dynamics: dependence on phase matching temperature

To further investigate the behavior of the laser operating with 5 mm or 10 mm long LBO crystals, we monitored the visible output power along with the residual fundamental and Stokes fields leaking through the output coupler, as functions of LBO temperature (affecting the phase matching within the resonator). Normally highest yellow and lime outputs would be expected for LBO temperatures which correspond to optimum calculated phase matching conditions for those wavelengths, with considerably lower output away from these temperatures. However, we see from Fig. 3(a)
Fig. 3 Experimental measurements of visible, Stokes and fundamental field intensities as a function of LBO temperature tuning for (a) 5 mm LBO crystal, and (b) 10 mm LBO crystal along with theoretical plots, (c) and (d), of the non-linear coupling strengths for the associated LBO crystals.
and 3(b), the observed behavior is not the case, particularly for the longer crystal.

For comparison with the experimental results, we also plotted in Figs. 3(c) and 3(d), the relative strength of the SHG and SFG non-linearity η, which is proportional to crystal length lD2 and sinc2 term. This takes the form [10

10. W. Koechner, Solid State Laser Engineering, 4th ed. (Springer-Verlag, 1996), Chap. 10.1.

]
ηlD2sinc2(π(TTM)lD/ΔT)
(1)
where ΔT is the temperature acceptance range in units of K.cm. For SHG to 588 nm, the phase-matching temperatureTM=41°CandΔT=7.1K.cm, and for SFG to 559 nm, TM=89°CandΔT=6.2K.cm [11

11. A. V. Smith, “SNLO nonlinear optics code,” AS-Photonics, Albuquerque, NM, http://www.as-photonics.com/SNLO.html

, 12

12. K. Kato, “Temperature-tuned 90° phase-matching properties of LiB3O5,” IEEE J. Quantum Electron. 30(12), 2950–2952 (1994). [CrossRef]

].

The results using the 5 mm long LBO, show that the yellow output was maximized around 45°C, and the lime at around 98°C, with fairly symmetric behavior above and below these temperatures. The temperatures that we recorded experimentally, corresponding with the maximum visible emission at these two wavelengths, are notably higher than the theoretical phase matching temperature (with a larger discrepancy for the higher phase matching temperature) consistent with the measured LBO mount temperature being higher than the temperature on the axis of the LBO crystal. The measured widths (span of temperatures) of the emission peaks are consistent with the expected sinc2 dependence on temperature, with the total width of central peak of the sinc2 function being 28.6°C for the yellow output, and 24.8°C for the lime output for the 5 mm crystal. What is surprising is that substantial visible output power was still generated at LBO temperatures far from the optimum value, in the wings of the sinc2 curve. This result indicates that even for non-linear strengths of only a few percent of the peak value, there was still sufficient coupling to generate substantial visible laser output.

The results for the 10 mm long crystal are significantly different from those for the shorter crystal. We expect the peak non-linear coupling to be four times higher compared to the 5 mm case due to the lD2 dependence, and the temperature acceptance bandwidths to be half as wide. In fact, around where maximum lime emission was generated using the 5 mm long LBO crystal (~98 o C), no lime at all was generated when using the 10 mm long LBO, and the 1176 nm field completely stopped lasing over a significant temperature range (92°C-100°C). The cessation of the 1176 nm field over this temperature range is even more surprising given that there was strong intracavity fundamental (1064 nm) intensity, evidenced by the direct measurement of the residual fundamental field through the output coupler. The maximum lime emission was observed on either side of the temperature range seeing cessation of 1176 nm field, and the maximum output power was lower compared to the 5 mm case. Yellow emission was observed at similar temperatures to what was observed in the 5 mm case, but the output power appeared largely independent of temperature over a 30°C temperature range. Again, the weak non-linear coupling in the wings of the sinc2 function was strong enough even to provide near-maximum output, suggesting that there was far more non-linearity than required at the phase matched temperature,

There are two aspects of these experimental results that require further explanation and confirmation. Firstly, why does the 5 mm long LBO crystal give much more output power than the 10 mm LBO? Secondly, why does the lime and Stokes output completely and suddenly cease near the peak of the temperature tuning of the 10 mm LBO crystal?

4. Estimation of round trip losses

At Raman threshold, the Stokes field is, by definition, extremely small and so the effective output coupling of the LBO crystal can be neglected. The total cavity losses at threshold are then comprised of the unwanted losses due to mirror leakage, surface reflections, and bulk losses in the crystals. The threshold for Stokes oscillation can be written [13

13. D. J. Spence, P. Dekker, and H. M. Pask, “Modelling of continuous wave intracavity Raman lasers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 756–763 (2007). [CrossRef]

]. Note the original work in [13

13. D. J. Spence, P. Dekker, and H. M. Pask, “Modelling of continuous wave intracavity Raman lasers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 756–763 (2007). [CrossRef]

] did not consider backwards SRS. It has been included here, giving rise to an additional factor of 2 in the denominator.
PP=πr2λF(TS+LS)(TF+LF)4gRlRλP
(2)
where the fundamental wavelengthλF = 1064 nm, pump wavelengthλP = 808 nm, Raman gain gR =4.5 cm/GW [7

7. A. A. Kaminskii, K.-i. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4 - new efficient χ(3)-materials for Raman lasers,” Opt. Commun. 194(1–3), 201–206 (2001). [CrossRef]

], Raman crystal length lR = 3 mm, ris the radius of cavity mode in the Raman crystal, and TF,TSare the transmission of the output coupler at fundamental and Stokes wavelengths, respectively. LF,LSare the losses for fundamental and Stokes fields, respectively. To first approximation these are the same for the two wavelengths, thus we assumeLF=LS=L. We can then calculate the round trip losses L using the measured value of PP and other measured or known laser parameters.

As an aside, we note that more complex methods using multiple measurements, such as Caird analysis [14

14. J. A. Caird, S. A. Payne, P. R. Staber, A. J. Ramponi, L. L. Chase, and W. F. Krupke, “Quantum electronic properties of the Na3Ga2Li 3F12:Cr3+ laser,” IEEE J. Quantum Electron. 24(6), 1077–1099 (1988). [CrossRef]

] and Findlay-Clay method [15

15. D. Findlay and R. A. Clay, “The measurement of internal losses in 4-level lasers,” Phys. Lett. 20(3), 277–278 (1966). [CrossRef]

] are normally required to measure the losses of conventional lasers. Such methods are not viable here because the losses are not dominated by a fixed output coupler transmission in our Raman lasers. The output couplers have very low transmission, and for the yellow and lime lasers, the effective output coupling due to frequency mixing within LBO crystal actually goes to zero at Raman threshold. In this case, the crystal losses are the dominant contributor to resonator losses, and therefore can be reasonably estimated by a single measurement.

Table 1

Table 1. Round Trip Losses Calculation

table-icon
View This Table
shows the measured thresholds, experimental parameters, and calculated round trip losses for the yellow lasers constructed with the 5 mm and 10 mm long LBO crystals, as well as for a self-Raman laser with no LBO crystal in the cavity. For all the three cases, we used the same output coupler with TF=0.0064%, TS=0.0042% and adjusted the physical length of the cavity to achieve the same cavity mode size of 96 µm inside the Nd:YVO4 crystal, determined using LASCAD (software package for Laser Cavity Analysis and Design). The M2 beam quality was measured to be 1.2 near threshold for each case.

From the measured thresholds, we calculated the round trip loss for each laser: 0.145% for the self-Raman laser; 0.214% for the 5 mm LBO yellow laser, and 0.286% for the 10 mm LBO yellow laser. We estimated the accuracy of the above values to be approximately 10%, primarily due to the uncertainties in the radius of the Raman cavity mode and the observed laser threshold. Uncertainty in the Raman gain coefficient would also impact on the loss measurements; no error is estimated in [7

7. A. A. Kaminskii, K.-i. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4 - new efficient χ(3)-materials for Raman lasers,” Opt. Commun. 194(1–3), 201–206 (2001). [CrossRef]

], however the gain coefficient is said to be “no less than 4.5 cm/GW”, leading to (if anything) our underestimating the round trip loss. From these cavity losses, we inferred the additional insertion loss caused by 5 mm and 10 mm LBO crystals to be 0.07% and 0.14%, respectively. This 1:2 ratio of losses indicates that the insertion losses caused by LBO crystals are dominated by bulk losses rather than the surface losses that are identical in both cases. It is interesting then that in this laser, the AR coatings on the LBO (R=0.42% - 0.47% @1064 nm, R=0.095% - 0.098%@1176 nm) do not appear to contribute to the intracavity loss. We believe that laser runs on longitudinal modes that experience no coating losses due to etalon effects, a contention which is supported by our observation of spectral mode selection effects associated with the LBO. We also note that experiments performed using a different 10 mm crystal with much better AR coatings (0.004% at 1064 nm, 0.064% at 1173 nm) showed the same Raman threshold. This gives further confirmation that the AR coatings do not contribute significantly to the round trip loss in this laser. Here, we present only the results for the 10 mm and 5 mm long LBO crystals coated by Castech (as detailed in the method section) to allow immediate comparison and conclusion regarding the effect of crystal length.

Likewise, we attribute the losses in the self-Raman laser to bulk scattering losses in the 3 mm long Nd:YVO4 crystal, noting the excellent coatings on the intracavity faces of the Nd:YVO4 crystal. The round trip scattering loss 0.145% determined for the Nd:YVO4 self-Raman resonator corresponds to a per centimeter loss of 0.24% for the highly doped Nd:YVO4 crystal used in this work.

5. Theoretical analysis

We now move on to analyze theoretically the complex behavior of the yellow and lime output for our two lasers, with a view to understanding the complete suppression of the Stokes and lime fields for the laser with 10 mm long LBO at temperatures where we would usually expect the most output.

For the yellow laser, we can think of the SHG process as a non-linear output coupler for the Stokes field, with the effective output coupling being zero at Raman threshold, increasing as the Stokes field grows with increasing pump power. To get optimum output power at our maximum pump power, we require this effective output coupling to take its optimum value for that Stokes field strength, just as the fixed output coupler for a conventional laser needs to be optimized. If the SHG strength is too low or too high, the yellow output will be less than its potential maximum, quite similar to the case of frequency doubling a conventional laser [16

16. R. Smith, “Theory of intracavity optical second-harmonic generation,” IEEE J. Quantum Electron. 6(4), 215–223 (1970). [CrossRef]

]. For the 5 mm LBO, the yellow output peaked symmetrically as the temperature was tuned, indicating that the coupling strength at the phase matching temperature was at or below its optimum value. For the 10 mm LBO, the yellow output power was flat or even slightly dipped around the same phase matching temperature, suggesting that the peak coupling strength may have been too high, i.e. the Stokes field was over-output-coupled.

For the lime laser, the SFG process also acts as a non-linear output coupler, in this case presenting a loss to both the fundamental and Stokes fields. In contrast to SHG, if the SFG strength is too high, the Stokes field is extinguished – there is a critical coupling strength over which the Stokes field suddenly does not lase. We can gain further insight into the dynamics of this effect by considering the form of the differential rate Eq. (3) for the power in the Stokes field:
dPRdt=PRτR+c1gRlRAPRPFc2deffSFG2κSFGlD2APRPFc3deffSHG2κSHGlD2APR2
(3)
wherePF,PRare fundamental and Stokes intracavity powers respectively,τRis the intracavity photon lifetime,gRis the stimulated Raman gain coefficient,lR, lD are Raman and nonlinear crystal lengths, respectively,deffis the effective nonlinearity of the doubling crystal, and Ais the mode area in the crystals. c1,c2,c3 are collected constants and laser parameters that do not need to be considered further here. κSFGandκSHG are the sinc2 factors between 0 and 1 conveying the quality of the phase matching as a function of temperature. The first term on the right hand side represents resonator losses, the second represents Raman gain, the third is active for SFG mixing of the fundamental and Stokes fields (lime output), and the fourth is for SHG of the Stokes field (yellow output).

When the LBO temperature is in the region for lime emission by sum frequency generation (SFG), we can approximate that κSHG0 and so the last term in Eq. (3) can be neglected. We also note that the terms for Raman gain and SFG have the same dependence on the powers of the intracavity field and thus we can combine them for the case of SFG to get the simplified Eq. (4)
dPRdt=PRτR+(αβ)PRPF
(4)
where α and βare collected coefficients for the Raman and SFG processes respectively. From this equation, it is clear that the SFG can be thought of as reducing the effective Raman gain. When the coefficient βis too large, which means SFG is too strong, the Stokes field can be completely suppressed as it will see no net gain at all and will cease to oscillate. While one would expect the Stokes field to be somewhat diminished for too strong SFG coupling (as has been reported for an external cavity Raman laser with intracavity SFG [17

17. K. Koch and G. T. Moore, “Raman oscillation with intracavity sum-frequency generation,” IEEE J. Quantum Electron. 35(1), 72–78 (1999). [CrossRef]

]), here we are describing the sudden and complete suppression of the Stokes field by the presence of the SFG interaction, a regime for which the Stokes field will not lase for any incident pump power. The expression for β in Eq. (3) shows that the value of β can be varied with different combinations of κSFG and lD. For the 10 mm LBO crystal, clearly at the peak of the phase matching the value of β is sufficient to turn off the Stokes field, but we can reduce the coefficient βby detuning the nonlinear crystal temperature from the phase matched value, upon which the Stokes field and lime output is restarted, and local optima are found.

6. Conclusions

Our comprehensive investigation of lime and yellow wavelength-selectable Raman lasers using 5 mm and 10 mm LBO crystals has led to two important observations that have enabled us to demonstrate an efficient wavelength-versatile Raman laser pumped by only 3.8 W of diode power.

First, in the high-Q resonators that are typical of efficient intracavity CW Raman lasers, bulk crystal losses (e.g. due to scattering and absorption) become increasingly important. For the lasers presented here, the intracavity losses are dominated by the bulk losses of the crystals themselves, rather than losses at the AR-coated crystal surfaces or unwanted transmission through cavity mirrors. This unusual situation leads to new conclusions regarding the optimum length of the intracavity crystals, notably that shorter Raman and SHG/SFG crystals are favored to reduce losses. This is why in this work, lasers with 10 mm LBO generate lower output powers than those with 5 mm LBO, and is one reason why miniature Raman lasers with just 3 mm of Raman active material in the cavity have relatively similar efficiencies to more standard designs with up to 20 mm of Raman active material [5

5. A. J. Lee, D. J. Spence, J. A. Piper, and H. M. Pask, “A wavelength-versatile, continuous-wave, self-Raman solid-state laser operating in the visible,” Opt. Express 18(19), 20013–20018 (2010). [CrossRef] [PubMed]

]. The dominance of crystal losses is also a significant factor favoring self-Raman laser designs over those using separate laser and Raman crystals, for which the total length of material in the cavity can be substantially reduced.

Second, we have observed experimentally a complete suppression of the first-Stokes optical field in a Raman laser using intracavity SFG, and explained this phenomenon by considering the rate equation for the first-Stokes optical field in the presence of SFG. Understanding these dynamics is absolutely critical to the design of efficient Raman lasers incorporating intracavity SHG.

While for the last few years the output and efficiency of visible Raman lasers have been substantially boosted by improvements to mirror and crystal coatings, these improvements appear to be approaching a limit. The present results show that future advances might now be driven by improvements of crystal quality both of the laser and nonlinear materials. We have also demonstrated the potential for making practical miniature visible laser sources based on mm-scale crystal Raman lasers, giving ~100 mW CW powers using low-power (1-2W) diode-pumping.

References and links

1.

H. M. Pask, P. Dekker, R. P. Mildren, D. J. Spence, and J. A. Piper, “Wavelength-versatile visible and UV sources based on crystalline Raman lasers,” Prog. Quantum Electron. 32(3–4), 121–158 (2008). [CrossRef]

2.

P. Cerný, H. Jelínková, P. G. Zverev, and T. T. Basiev, “Solid state lasers with Raman frequency conversion,” Prog. Quantum Electron. 28(2), 113–143 (2004). [CrossRef]

3.

L. Fan, Y. Fan, Y. Duan, Q. Wang, H. Wang, G. Jia, and C. Tu, “Continuous-wave intracavity Raman laser at 1179.5 nm with SrWO4 Raman crystal in diode-end-pumped Nd:YVO4 laser,” Appl. Phys. B 94(4), 553–557 (2009). [CrossRef]

4.

L. Fan, Y.-X. Fan, Y.-Q. Li, H. Zhang, Q. Wang, J. Wang, and H.-T. Wang, “High-efficiency continuous-wave Raman conversion with a BaWO(4) Raman crystal,” Opt. Lett. 34(11), 1687–1689 (2009). [CrossRef] [PubMed]

5.

A. J. Lee, D. J. Spence, J. A. Piper, and H. M. Pask, “A wavelength-versatile, continuous-wave, self-Raman solid-state laser operating in the visible,” Opt. Express 18(19), 20013–20018 (2010). [CrossRef] [PubMed]

6.

Y. Lü, X. Zhang, S. Li, J. Xia, W. Cheng, and Z. Xiong, “All-solid-state cw sodium D2 resonance radiation based on intracavity frequency-doubled self-Raman laser operation in double-end diffusion-bonded Nd3+:LuVO4 crystal,” Opt. Lett. 35(17), 2964–2966 (2010). [CrossRef] [PubMed]

7.

A. A. Kaminskii, K.-i. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4 - new efficient χ(3)-materials for Raman lasers,” Opt. Commun. 194(1–3), 201–206 (2001). [CrossRef]

8.

Y. Sato and T. Taira, “Comparative study on the spectroscopic properties of Nd:GdVO4 and Nd:YVO4 with hybrid process,” IEEE J. Sel. Top. Quantum Electron. 11(3), 613–620 (2005). [CrossRef]

9.

X. Li, A. J. Lee, H. M. Pask, J. A. Piper, and Y. Huo, “Efficient, miniature, cw yellow source based on an intracavity frequency-doubled Nd:YVO4 self-Raman laser,” Opt. Lett. 36(8), 1428–1430 (2011). [CrossRef] [PubMed]

10.

W. Koechner, Solid State Laser Engineering, 4th ed. (Springer-Verlag, 1996), Chap. 10.1.

11.

A. V. Smith, “SNLO nonlinear optics code,” AS-Photonics, Albuquerque, NM, http://www.as-photonics.com/SNLO.html

12.

K. Kato, “Temperature-tuned 90° phase-matching properties of LiB3O5,” IEEE J. Quantum Electron. 30(12), 2950–2952 (1994). [CrossRef]

13.

D. J. Spence, P. Dekker, and H. M. Pask, “Modelling of continuous wave intracavity Raman lasers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 756–763 (2007). [CrossRef]

14.

J. A. Caird, S. A. Payne, P. R. Staber, A. J. Ramponi, L. L. Chase, and W. F. Krupke, “Quantum electronic properties of the Na3Ga2Li 3F12:Cr3+ laser,” IEEE J. Quantum Electron. 24(6), 1077–1099 (1988). [CrossRef]

15.

D. Findlay and R. A. Clay, “The measurement of internal losses in 4-level lasers,” Phys. Lett. 20(3), 277–278 (1966). [CrossRef]

16.

R. Smith, “Theory of intracavity optical second-harmonic generation,” IEEE J. Quantum Electron. 6(4), 215–223 (1970). [CrossRef]

17.

K. Koch and G. T. Moore, “Raman oscillation with intracavity sum-frequency generation,” IEEE J. Quantum Electron. 35(1), 72–78 (1999). [CrossRef]

OCIS Codes
(140.3480) Lasers and laser optics : Lasers, diode-pumped
(140.3550) Lasers and laser optics : Lasers, Raman
(190.2620) Nonlinear optics : Harmonic generation and mixing
(190.5650) Nonlinear optics : Raman effect

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: October 18, 2011
Revised Manuscript: November 10, 2011
Manuscript Accepted: November 21, 2011
Published: November 30, 2011

Citation
Xiaoli Li, Helen M. Pask, Andrew J. Lee, Yujing Huo, James A. Piper, and David J. Spence, "Miniature wavelength-selectable Raman laser: new insights for optimizing performance," Opt. Express 19, 25623-25631 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-25-25623


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References

  1. H. M. Pask, P. Dekker, R. P. Mildren, D. J. Spence, and J. A. Piper, “Wavelength-versatile visible and UV sources based on crystalline Raman lasers,” Prog. Quantum Electron.32(3–4), 121–158 (2008). [CrossRef]
  2. P. Cerný, H. Jelínková, P. G. Zverev, and T. T. Basiev, “Solid state lasers with Raman frequency conversion,” Prog. Quantum Electron.28(2), 113–143 (2004). [CrossRef]
  3. L. Fan, Y. Fan, Y. Duan, Q. Wang, H. Wang, G. Jia, and C. Tu, “Continuous-wave intracavity Raman laser at 1179.5 nm with SrWO4 Raman crystal in diode-end-pumped Nd:YVO4 laser,” Appl. Phys. B94(4), 553–557 (2009). [CrossRef]
  4. L. Fan, Y.-X. Fan, Y.-Q. Li, H. Zhang, Q. Wang, J. Wang, and H.-T. Wang, “High-efficiency continuous-wave Raman conversion with a BaWO(4) Raman crystal,” Opt. Lett.34(11), 1687–1689 (2009). [CrossRef] [PubMed]
  5. A. J. Lee, D. J. Spence, J. A. Piper, and H. M. Pask, “A wavelength-versatile, continuous-wave, self-Raman solid-state laser operating in the visible,” Opt. Express18(19), 20013–20018 (2010). [CrossRef] [PubMed]
  6. Y. Lü, X. Zhang, S. Li, J. Xia, W. Cheng, and Z. Xiong, “All-solid-state cw sodium D2 resonance radiation based on intracavity frequency-doubled self-Raman laser operation in double-end diffusion-bonded Nd3+:LuVO4 crystal,” Opt. Lett.35(17), 2964–2966 (2010). [CrossRef] [PubMed]
  7. A. A. Kaminskii, K.-i. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4 - new efficient χ(3)-materials for Raman lasers,” Opt. Commun.194(1–3), 201–206 (2001). [CrossRef]
  8. Y. Sato and T. Taira, “Comparative study on the spectroscopic properties of Nd:GdVO4 and Nd:YVO4 with hybrid process,” IEEE J. Sel. Top. Quantum Electron.11(3), 613–620 (2005). [CrossRef]
  9. X. Li, A. J. Lee, H. M. Pask, J. A. Piper, and Y. Huo, “Efficient, miniature, cw yellow source based on an intracavity frequency-doubled Nd:YVO4 self-Raman laser,” Opt. Lett.36(8), 1428–1430 (2011). [CrossRef] [PubMed]
  10. W. Koechner, Solid State Laser Engineering, 4th ed. (Springer-Verlag, 1996), Chap. 10.1.
  11. A. V. Smith, “SNLO nonlinear optics code,” AS-Photonics, Albuquerque, NM, http://www.as-photonics.com/SNLO.html
  12. K. Kato, “Temperature-tuned 90° phase-matching properties of LiB3O5,” IEEE J. Quantum Electron.30(12), 2950–2952 (1994). [CrossRef]
  13. D. J. Spence, P. Dekker, and H. M. Pask, “Modelling of continuous wave intracavity Raman lasers,” IEEE J. Sel. Top. Quantum Electron.13(3), 756–763 (2007). [CrossRef]
  14. J. A. Caird, S. A. Payne, P. R. Staber, A. J. Ramponi, L. L. Chase, and W. F. Krupke, “Quantum electronic properties of the Na3Ga2Li 3F12:Cr3+ laser,” IEEE J. Quantum Electron.24(6), 1077–1099 (1988). [CrossRef]
  15. D. Findlay and R. A. Clay, “The measurement of internal losses in 4-level lasers,” Phys. Lett.20(3), 277–278 (1966). [CrossRef]
  16. R. Smith, “Theory of intracavity optical second-harmonic generation,” IEEE J. Quantum Electron.6(4), 215–223 (1970). [CrossRef]
  17. K. Koch and G. T. Moore, “Raman oscillation with intracavity sum-frequency generation,” IEEE J. Quantum Electron.35(1), 72–78 (1999). [CrossRef]

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