Efficient operation near the quantum limit in external cavity diamond Raman laser

Stimulated Raman scattering (SRS) is an efficient and important nonlinear frequency conversion process to develop lasers in new wavelength regions [1–4]. Automatic phase-matching and beam quality improvement through Raman beam clean-up are fascinating features. With the emergence of excellent Raman crystals, such as YVO₄ [5, 6], Ba (NO₃)₂ [7], BaWO₄ [8–10], KGd(WO₄)₂ [11–14], diamond [15–37], etc, all-solid-state Raman lasers have been attracting more and more attention because of their good mechanical and thermal properties, compactness and high conversion efficiency. Chemical vapor deposition (CVD) diamonds are an attractive Raman gain medium due to their excellent optical and thermal properties including the highest Raman shift (1332.3 cm⁻¹), a very wide transparency range (from 225 nm to 2.5 μm and longer than 6 μm), a high Raman gain coefficient (10–12 cm GW⁻¹ @1064 nm), and exceptionally high thermal conductivity (2000 W m^-1 K^-1). The diamond provides the first Stokes at ∼1240 nm for a ∼1064 nm pump laser, which is important for a range of medical and defence applications, and the second Stokes at ∼1485 nm in the 'eye-safe' region [38]. The external cavity configuration has the advantage of being easily adapted to mature pump lasers. Thus, external cavity diamond Raman lasers (DRLs) are promising and efficient wavelength converters in the IR spectrum. High-quality CVD single diamonds have led to the rapid development of external cavity DRLs operating in the deep UV [15], visible [16, 17], near-IR [18–35] and mid-IR spectrum [36, 37] in CW [18–24], QCW [19, 25–28] and pulsed regimes [29–35], to name just a few. In particular, through removing spatial hole burning, stable single-longitudinal-mode (SLM) operation from DRLs has been achieved [39–41], which makes it possible to generate high-spectral-density emission from simple resonators.

Recently, external cavity DRLs have shown highly efficient Raman conversion [17, 29, 31]. In 2009, an external cavity DRL generated 1.2 W at 573 nm with a slope efficiency of 75% at a pulse repetition of 5 kHz [17]. In 2010, an external cavity DRL generating 2.0 W at 1240 nm was investigated. The slope efficiency of 84% was very close to the 85.8% first Stokes quantum efficiency [29]. In 2011, an efficient second Stokes 1.485 µm external cavity DRL was reported. The average output power was 1.63 W at 5 kHz. The slope efficiency was 56% [31].

External cavity DRLs with a pulsed output of ten to tens of nanoseconds are also developing rapidly [4, 29, 33–35], and may find applications in tracking and ranging. In 2013, an efficient pulsed DRL at 1240 nm and 1485 nm with an output power up to 14.5 W was reported. The conversion efficiency and pulse energy of the second Stokes were 43% and 0.36 mJ [33]. In 2016, an output energy of 1.2 mJ at 1240 nm with a slope efficiency of 54% was achieved, while the pulse energy of 0.7 mJ was obtained in the eye-safe spectral region at 1485 nm [35]. In 2018 [4], it was mentioned that an output energy of 9.7 mJ with a slope efficiency of 46% was produced in a pulse 8 ns in duration with a peak power of 1.2 MW.

SRS is a cascading nonlinear frequency conversion process. When the first Stokes reaches enough power density, it will be converted to the second Stokes as a pump source. Cascading to higher Stokes orders has been reported in DRLs [21, 22, 24, 27, 31, 33–35]. Therefore, we can realize dual-/multi-wavelength DRLs, which will have a wide range of applications in division multiplexing communication, optical signal processing, optical sensing, precision spectroscopy, etc. In 2013, an efficient pulsed DRL which operated simultaneously at two Stokes (1240 nm and 1485 nm) with 14.5 W total output power (corresponding to 0.41 mJ total output pulse energy) and 65% slope efficiency was reported [33]. In 2016, an external-cavity DRL generating simultaneously at two Stokes (1240 nm and 1485 nm) with ∼0.7 mJ enhanced the total output pulse energy and a 24% slope efficiency was realized [35]. As shown in the above experiments, the DRLs predominantly generated the second Stokes at the maximum pump level, which led to a very small output pulse energy of the first Stokes (∼0.04 mJ).

In this paper, we realize efficient operation of the first Stokes (1240.3 nm) and the second Stokes (1485.8 nm) in external cavity DRLs. Firstly, an output pulse energy of 0.26 mJ at the first Stokes is generated with a slope efficiency of 84.3%, which is very close to the quantum limit of 85.8%. Secondly, the output pulse energy of 0.26 mJ at the first Stokes and 0.18 mJ at the second Stokes are generated simultaneously. The total Raman output pulse energy is 0.44 mJ. As far as we know, a slope efficiency of 73.0% is the highest slope efficiency in dual-wavelength external cavity DRLs. Through efficient cavity coupling management, the energy ratio of the second Stokes to the first Stokes at the maximum pump level can be tuned from 0.69 to 1.92.

In the experiment, the Raman gain medium was a 7 mm (L) $\times$ 2 mm (W) $\times$ 2 mm (H), low-nitrogen, low birefringence, CVD-grown single crystal diamond (Element Six Ltd., UK). The diamond was cut for pump propagation along the <110> direction and anti-reflection coated at ∼1064 nm, ∼1240 nm and ∼1485 nm. The experimental setup of the external cavity DRL, associated with the pump laser and the launch optics, is shown in figure 1. A home-built Q-switched Nd:YVO₄ laser was adopted as the pumping source of the external cavity DRL, which delivered a maximum pulse energy of 1 mJ with a pulse duration of ∼16 ns and a repetition rate of 20 Hz at 1064.4 nm. The power of the pump laser was varied by using the first half-wave plate (HWP 1) and a home-built high-power optical isolator to ensure constant beam and pulse characteristics. The second half-wave plate (HWP 2) was used to align the polarization axis of the pump laser to the preferred <111> axis of the diamond to access the maximum Raman gain [29]. The pump laser was focused into the diamond through a plano-convex focus lens (FL) with a focal length of 125 mm to achieve a waist radius of ∼65 μm. The external cavity diamond Raman resonator consisted of an input mirror (IM) with a 100 mm curvature radius and a planar output coupler (OC). The overall resonator length was ∼8.0 cm, which gave calculated cold-cavity fundamental waists of ∼138 μm at ∼1240 nm and ∼151 µm at ∼1485 nm. Different IMs and OCs were used in the experiment. They had different transmissions at the pump laser, the first and second Stokes (detailed in table 1). The OCs were high reflection (HR) coated (R = 99.9%) at ∼1064 nm, which allowed for a second pass of the pump laser through the diamond to raise the conversion efficiency. Two long-pass filters (LFs) were used. The first one (LF 1, Thorlabs, FELH1150) was able to separate the first and second Stokes from the residual pump laser. The second one (LF 2, Thorlabs, FELH1350) separated the second Stokes from the first.

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The laser output power was measured with a power meter (Coherent, FIELDMAXII-TO(ROHS), PS19Q). The laser spectrum was recorded with an optical spectrum analyzer (Yokogawa, AQ6370D, 600–1700 nm). An oscilloscope (Rohde & Schwarz, RTO2044, 4 GHz, 20 GSa s⁻¹) with an InGaAs amplified detector (Thorlabs, PDA10CF-EC, 700–1800 nm) was used to characterize the laser pulse.

The spectra of the pump laser and the first Stokes were measured at the maximum pump level with IM 1 and OC 1, as shown in figure 2. The central wavelength of the pump laser and the first Stokes are 1064.4 nm and 1240.3 nm, respectively. The frequency shift between the pump laser and the first Stokes is 1332.3 cm⁻¹. Due to the high transmission of IM 1 at ∼1485 nm, the second Stokes was not generated. In figure 3, the pulse widths of the pump laser and the first Stokes are 16.1 ns and 10.2 ns, respectively. It can be seen that SRS frequency conversion leads to pulse compression of the Stokes components.

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Under various incident pump levels, the Raman output pulse energy with OC 1 and OC 2 were measured, as shown in figures 4(a) and (b), respectively. In figure 4(a), the output pulse energy of the first Stokes increases linearly with the incident pump energy above the threshold (∼0.5 mJ). The maximum output pulse energy is 0.26 mJ and the corresponding peak power is 25.5 kW. The conversion efficiency at the maximum pump level is 32.2%. The slope efficiency is 84.3%, which is comparable to that in [29] and closely approaches the quantum limit of 85.8%. Much more output pulse energy can be generated by higher power pump lasers and by increasing the beam waist radius to ensure that the peak power density of the incident pump laser is under the threshold of coating damage and parasitic nonlinear effects such as self-focusing. It should be noted that the output power exhibited substantial amplitude fluctuations when operated near the threshold and became more stable at higher pump level. These changes may be caused by mode instabilities and mechanical vibrations. In figure 4(b), lower output coupling (6.9%) leads to a lower threshold (∼0.45 mJ) but also lower slope efficiency (69.0%). Thus the output coupling should be optimized for highly efficient operation of DRLs. With OC 2, the maximum output pulse energy is 0.24 mJ and the corresponding conversion efficiency is 29.5%.

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As shown in [42], the Raman laser threshold ${P_{pTh.1s}}$ for the first Stokes generation with second-pass pumping is given by

$$\begin{equation}{P_{pTh.1s}} = \frac{{{T_{1s}} + 2{\alpha _{1s}}L}}{{4{G_{1s}}}}\frac{{{\lambda _{1s}}}}{{{\lambda _p}}}\end{equation} \tag{ 1 }$$

where ${T_{1s}}$ is the transmission of the output coupler at the first Stokes. ${\alpha _{1s}}$ is the distributed loss coefficient for the first Stokes which includes all major parasitic loss. $L$ is the length of the diamond. ${\lambda _p}$ and ${\lambda _{1s}}$ are the wavelength of the pump and the first Stokes. ${G_{1s}}$ is the Raman power gain in the focused geometry and contains a reduction factor for overlap mismatch between the pump and the Stokes beams. By using a model similar to [42], we have simulated how beam matching influences Stokes output, threshold and conversion efficiency, as shown in figure 5. In simulation, the beam waist radius of the pump is constant and equal to that in the experiment (65 μm). Increasing the first Stokes waist radius, the Stokes output pulse energy and conversion efficiency decrease and the threshold increases. When the first Stokes waist radius is above ∼185 μm, the threshold exceeds the incident pump energy and the first Stokes cannot be generated. If the waist radius of the first Stokes is the same as that of the pump (65 μm), conversion efficiency will increase to ∼65%. So good beam matching can contribute to high conversion efficiency.

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In the next experiment, IM 2 with a low transmission at the second Stokes (∼20.0%) was used instead of IM 1. The overall resonator length was shortened to ∼3.7 cm. The calculated cold-cavity fundamental waists were ∼143 μm at ∼1240 nm and ∼156 µm at ∼1485 nm. The spectra (figure 6) and pulse shapes (figure 7) were measured at the maximum pump level with IM 2 and OC 1. The first Stokes (1240.3 nm) and the second Stokes (1485.8 nm) were generated simultaneously, thus achieving dual-wavelength external cavity DRLs with large spectral spacing (245.5 nm). No other higher order Stokes was detected due to the limitation of the wavelength range of the optical spectrum analyzer (600–1700 nm). The pulse widths of the pump laser and the second Stokes are 16.1 ns and 7.8 ns, respectively. Obviously, by a cascaded SRS process, the pulse width (7.8 ns) is shorter than that (10.2 ns) in figure 3.

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The relationship between the output pulse energy and the incident pump energy is shown in figure 8. In figure 8(a), the maximum total Raman output pulse energy is 0.44 mJ, yielding a slope efficiency of 73.0%. As far as we know, this is the highest ever achieved slope efficiency in dual-wavelength external cavity DRLs [33, 35]. The conversion efficiency of the total Raman output at the maximum pump level is 51.8%. The output pulse energy of the first (second) Stokes is 0.26 (0.18) mJ, yielding a slope efficiency of 27.1% (45.9%). The corresponding peak power of the second Stokes is 23.1 kW. At the maximum pump level, the energy ratio of the second Stokes to the first is 0.69. At various pump levels, the output pulse energy of the second Stokes is less than that of the first. The threshold of the first (second) Stokes is ∼0.33 (∼0.47) mJ.

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In order to increase the energy ratio of the second Stokes to the first Stokes at the maximum pump level, an OC 2 with a lower transmission for the first Stokes was used instead of OC 1. In figure 8(b), the maximum total Raman output pulse energy is 0.38 mJ, yielding a slope efficiency of 65.0% and a conversion efficiency of 44.7%. The output pulse energy of the first (second) Stokes is 0.13 (0.25) mJ, yielding a slope efficiency of 13.3% (51.7%). The slope efficiency of the second Stokes is comparable to that in [31]. At the maximum pump level, the energy ratio of the second Stokes to the first increases to 1.92. Under ∼0.57 mJ incident pump energy, the first and second Stokes have the same output pulse energy (∼0.10 mJ). The lower transmission of OC 2 at ∼1240 nm (∼1485 nm) contributes to the lower threshold of the first (second) Stokes, which is ∼0.31 (∼0.38) mJ. Combined with the experimental data in the previous paragraph, the energy ratio of the second Stokes to the first at the maximum pump level can be tuned from 0.69 to 1.92 by managing the cavity coupling, thus providing an efficient way to realize a dual-wavelength laser with flexible energy ratios from an external cavity DRL system.

In figure 9, we show that the second Stokes output coupling can influence the energy ratio through a model similar to [22, 42]. Because a high output coupling at the second Stokes is required for efficient conversion, the second Stokes reflectivity changes from 5% to 55%. The first Stokes output coupled power ${P_{1sOut}}$ is given by [22]

$$\begin{equation}{P_{1sOut}} = \frac{{ - ln{R_{2s}} + 2{\alpha _{2s}}L}}{{4L{g_{2s}}}}{A_2}{T_{1s}}\end{equation} \tag{ 2 }$$

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where ${R_{2s}}$ is the reflectivity at the second Stokes. ${\alpha _{2s}}$ is the distributed loss coefficient for the second Stokes, which accounts for absorption and scattering in the Raman crystal. ${A_2}$ is the effective area of the interaction between the first and second Stokes beams in the diamond. ${g_{2s}}$ is the Raman gain coefficient for the second Stokes. Thus, the output pulse energy of the first Stokes is inversely proportional to ${R_{2s}}$ (shown in the upper part of figure 9). The second Stokes output coupled power ${P_{2sOut}}$ is given by

$$\begin{equation}{P_{2sOut}} = \left( {{P_p} - {P_{pTh.2s}}} \right)\left( {1 - R_{2s}^{\frac{{{\lambda _{2s}}{A_2}}}{{{\lambda _p}{A_1}}}}{e^{\frac{{ - 2{\alpha _{2s}}L{\lambda _{2s}}{A_2}}}{{{\lambda _p}{A_1}}}}}} \right)\frac{{{\lambda _p}}}{{{\lambda _{2s}}}}{\eta _{2sOut}}\end{equation} \tag{ 3 }$$

$$\begin{equation}{P_{pTh.2s}} = \frac{{{T_{1s}} + 2{\alpha _{1s}}L}}{{{T_{1s}}}}\frac{{{\lambda _{1s}}}}{{{\lambda _p}}}{P_{1sOut}}{\left[ {1 - exp\left( { - \frac{{4{G_{1s}}}}{{{T_{1s}}}}{P_{1sOut}}} \right)} \right]^{ - 1}}\end{equation} \tag{ 4 }$$

where ${P_p}$ is the pump power. ${P_{pTh.2s}}$ is the pump power at the threshold for the second Stokes generation. ${\lambda _{2s}}$ is the second Stokes wavelength. ${A_1}$ is the effective area of the interaction between the pump and the first Stokes beams in the diamond. ${\eta _{2sOut}}$ is the ratio of the output coupling to the total cavity round-trip loss. In (3), the pump power injected beyond the threshold of the second Stokes (the first bracketed term) is directly proportional to ${R_{2s}}$, and the fraction of the pump power converted in the Raman process (the second bracketed term) is inversely proportional to ${R_{2s}}$. Thus as shown in the upper part of figure 9, when ${R_{2s}}$ is less than ∼30%, the greater amount of pump power injected contributes to an increase of the second Stokes output pulse energy. When ${R_{2s}}$ is more than ∼30%, less conversion from the pump to the Raman process leads to a decrease of the second Stokes output pulse energy. According to the output pulse energy of the first and second Stokes, we calculate the energy ratio of the second Stokes to the first (shown in the lower part of figure 9). As ${R_{2s}}$ increases from 5% to 55%, the energy ratio increases from ∼ 0.5 to ∼ 2.5. Thus to manage the energy ratio, the output coupling of the second should be carefully considered.

For practical applications, it is always necessary to realize highly efficient operations at the selected Stokes. By comparing figures 4(a) to 8(a), it is obvious that to realize the high slope efficiency of the first Stokes, especially close to the quantum limit, the second Stokes must be suppressed. Maximizing the transmission of IM and OC at the second Stokes is a simple and effective method. Alternatively, the laser cavity can be designed for maximizing the second Stokes pulse energy so that a DRL can operate efficiently in the 'eye-safe' region. By comparing figures 8(a) to (b), low output couplings of the first Stokes and high output couplings of the second Stokes are needed to maximize the slope efficiency of the second Stokes. For dual-wavelength external cavity DRLs, we can further increase the energy ratio (the second Stokes to the first Stokes) by increasing the pump power (figure 8) and improving the reflectivity of the second Stokes appropriately in the high output coupling region (figure 9).

We have demonstrated highly efficient external cavity DRLs based on a high-quality CVD single diamond and analyzed how to realize efficient operation. Firstly, an output pulse energy of 0.26 mJ at the first Stokes is generated and the corresponding peak power is 25.5 kW. The slope efficiency 84.3% closely approaches the quantum limit of 85.8%. Then we generate the first Stokes (1240.3 nm) and the second Stokes (1485.8 nm) simultaneously. The maximum output pulse energy of the first Stokes and the second Stokes are 0.26 mJ and 0.18 mJ, yielding the highest total slope efficiency of 73.0% in dual-wavelength external cavity DRLs. Through efficient cavity coupling management, the energy ratio of the second Stokes to the first at the maximum pump level can be tuned from 0.69 to 1.92. The achieved external cavity DRL with dual-wavelength and flexible energy ratios will have potential applications in various areas, such as wavelength division multiplexing operation, due to the fact that it can output two different wavelengths simultaneously. Furthermore, high-efficiency external cavity DRLs have great potential for practical applications, such as remote sensing, scanning lidar and laser range finding.

The authors wish to thank Yu Pang, Zhimeng Huang, Huanian Zhang and Yulong Tang for their expert advice during the project. This work is supported by National Natural Science Foundation of China (NSFC) (61605191), Research Foundation of CAEP (TCGH1001-01) and Presidential Foundation of CAEP (YZJJLX2016011). The authors declare no conflicts of interest.