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Abstract
We demonstrate an 806 nm laser diode end-pumped continuous-wave (CW) and actively Q-switched (AQS) orthogonally-polarized dual-wavelength Nd:YLF laser operating at 1314 and 1321 nm. Benefitting from the small difference of the stimulated emission cross sections at 1314 and 1321 nm in Nd:YLF crystal, the power equalized emissions at both wavelengths were achieved by simply titling the output coupler. A maximum CW output power of 9.2 W was obtained with the incident pump power of 32.5 W, giving an optical-to-optical conversion efficiency of approximately 28% and a slope efficiency of approximately 33%. Furthermore, active Q-switching was realized by inserting a Brewster-cut acousto optic modulator. For an incident pump power of 30 W, this oscillator delivered an average power of 6.5 W at a pulse repetition frequency (PRF) of 20 kHz, and a pulse energy of 2.6 mJ with a peak power of approximately 72 kW at a PRF of 1 kHz.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Orthogonally-polarized dual-wavelength lasers attract intensive attentions because of their significant applications in self-sensing metrology [1], medical instruments [2], holography [3], laser interference [4], and terahertz generation by difference-frequency mixing [5,6]. In addition, the efficient powerful continuous-wave (CW) orthogonally-polarized dual-wavelength laser at 1.31 and 1.32 µm would be of great use for a silver atom optical clock [7,8]. Specifically, the second harmonic of the 1322.6 nm laser can be utilized to interrogate the silver atom clock transition [9,10], and the fourth harmonic of the 1312.0 nm laser can be useful for sub-Doppler cooling of the silver atom on the D2 resonance line [8,11]. In addition, the 1.3 µm orthogonally-polarized dual-wavelength laser can be Raman-shifted to the 1.5 µm region, which has the applications such as eye-safe differential lidar [12] and long-range free-space optical communication [13].
Traditionally, the 4F3/2 → 4F13/2 transition of Nd3+ is the most common approach for generating the 1.3 µm dual-wavelength laser radiation. For instance, L. Guo et al. demonstrate a dual-wavelength Nd:YAG ceramic laser at 1319 and 1338 nm with a continuous wave (CW) output power of 5.9 W and a passively Q-switched (PQS) average output power of 226 mW [14]. The quasi continuous wave (QCW) dual-wavelength Nd:YAG laser operating at 1319 and 1356 nm was reported by M. Chen et al. to deliver a 9.4 W average output power [15]. However, those dual-wavelength laser lines were either the natural polarizations or having the same polarizations [16], which makes it difficult to separate the beams at the two wavelengths. Compared with the oxide crystals, fluoride crystals have strong emission spectral anisotropy, which is beneficial for generating the orthogonally-polarized dual-wavelength laser. In addition, fluoride crystal has been widely used for generating the high energy pulse laser due to their long fluorescence lifetimes [17] and weak thermal lens effect [18]. In the past decade, several orthogonally-polarized dual-wavelength lasers operating at 1.31 and 1.32 µm have been demonstrated based on the fluoride crystals, such as Nd:LLF [19,20] and Nd:YLF [21]. For example, H. Li et al. reported a dual-wavelength Nd:LLF laser working at 1314 and 1321 nm with a maximum output power of 6.08 W for CW operation, and a maximum energy per pulse of 108.7 µJ with a peak-to-peak intensity fluctuation of < 8% for PQS operation. However, due to the large difference of the stimulated emission cross sections for both polarizations in Nd:LLF crystal (2.2 × 10−20 cm2 for 1314 nm on σ-polarization and 5.1 × 10−20 cm2 for 1321 nm on π-polarization), it is hard to realize the power-equalized emissions at two wavelengths without the aid of additional birefringent element, and sometimes there is even one wavelength oscillation. As for the dual-wavelength laser, a similar number of photons for two wavelengths are of great importance for practical applications. Another well-known fluoride crystal Nd:YLF has very close stimulated emission cross sections for both polarizations (2.0 × 10−20 cm2 for 1314 nm on σ-polarization and 2.3 × 10−20 cm2 for 1321 nm on π-polarization), which could be helpful for generating the power-equalized dual-wavelength laser emission via modulating the loss induced from the Fresnel reflection on the output coupler [22]. So far, the orthogonally-polarized dual-wavelength Nd:YLF laser at 1313 and 1321 nm has been obtained by inserting an uncoated glass plate. However, due to the large insertion loss resulting from the additional optical element, the maximum CW output power generated from the output coupler was only 1.8 W with an optical-to-optical conversion efficiency of 9.8%. As far as we know, no work has been reported on a 1.3 µm orthogonally-polarized dual-wavelength Nd:YLF laser without the help of any additional optical element. In addition, it is noteworthy that thermal effects are especially problematic under 1.3 µm operation (compared with 1.0 µm operation) due to the larger quantum defect, which will increase the risk of thermal fracture in Nd:YLF crystal. To overcome this drawback, a lower Nd doping concentration and a larger pump beam volume are usually utilized [23,24]. Even so, under a high pump level, the Nd:YLF laser usually oscillated at 1314 nm on σ-polarization because of the strong negative thermal lensing effect associated with the π-polarization [25,26]. Hence, it is more challenging to generate an efficient high-power 1.3 µm orthogonally-polarized dual-wavelength Nd:YLF laser source.
In this paper, we demonstrate an efficient high-power orthogonally-polarized dual-wavelength Nd:YLF laser at 1314 and 1321 nm for both CW and actively Q-switched (AQS) modes without any additional optical elements, for the first time. The resonator structure was designed carefully to make two orthogonally-polarized laser beams both working in the thermally stable region. A maximum CW output power of 9.2 W was obtained with a slope efficiency of about 33%. Meanwhile, by Q-switching the laser cavity with an acousto-optic modulator, we attained a maximum average output power of 6.5 W at 20 kHz and the largest pulse energy of 2.6 mJ at 1 kHz. To our knowledge, these are the highest output powers of 1.3 µm orthogonally-polarized dual-wavelength laser sources in both CW and Q-switched modes.
2. Experimental setup and resonator design
A schematic diagram of experimental setup is illustrated in Fig. 1. The a-cut Nd:YLF crystal with the doping concentration of 0.55 at. % and dimension of 4 × 4 × 20 mm3 was employed as the gain medium. It was coated for high-transmission (HT) at 806 nm, 1047-1053 nm and 1314–1321 nm on the front surface, high reflection (HR) at 806 nm and HT at 1047–1053 nm and 1314-1321 nm on the rear surface. Here, the HR coated at 806 nm was used to improve the longitudinally temperature uniformity, thereby ensuring a high absorption efficiency of approximately 95%. The gain medium was wrapped with an indium foil and mounted into a micro-channel water-cooled copper block, and the water temperature was maintained at 15°C to efficiently cool the crystal. A 50 W, 806 nm, CW fiber-coupled laser diode (LD) with a fiber core diameter of 400 µm and a numerical aperture of 0.22 (Focuslight Technologies Inc., FL-S-50-808-400) was employed as the pump source. Two plane-convex coupling lenses (1:2 magnification) coated anti-reflection (AR) at 806 nm was used to re-image the pump beam with a waist radius of ∼400 µm in the center of the gain medium. M1 was a plane-concave mirror coated for HT at 806 nm and 1047-1053 nm, HR at 1314-1321 nm. The HT coated at 1047-1053 nm was applied to avoid the parasitic oscillation at 1 µm. Two plane mirrors (M2) coated for part-reflectivity (PR) at 1314 and 1321 nm (TOC = 5% and 10%) were available. The output couplers were placed into a high-stability optical mount, thus we could accurately adjusted their inclination relative to the cavity axis. A 46-mm-long acousto-optic Q-switcher (Gooch & Housego, I-QS027-4S4H-B5) was inserted in the cavity between the Nd:YLF crystal and output coupler. The acousto-optic Q-switcher driven at a 27.12 MHz center frequency with a radio-frequency power of 100 W was AR coated at 1314-1321 nm on both facets. The laser spectra were recorded with an optical spectrum analyzer (Zolix, Omni-λ300) with a resolution of 0.1 nm. The pulse temporal behaviors were recorded by a fast photodiode (Thorlabs, DET08CL/M) connected to a digital oscilloscope with a bandwidth of 6 GHz.
Fig. 1. Experimental layout of 1.3 µm orthogonally-polarized dual-wavelength laser system. LD, laser diode; AOS, acousto-optic Q-switcher; PBS, polarizing beam splitter.
Due to the opposite thermal lens effect of Nd:YLF crystal in two orthogonal polarizations, the structure of resonator should be designed systematically to realize efficient 1.3 µm dual-wavelength laser emission under a high pump power. According to [18], the thermal lens focal length can be described by
Fig. 2. (a) Thermal lens focal lengths versus the incident pump power for 1314 nm on σ-polarization and 1321 nm on π-polarization. (b) Calculated laser beam sizes of the 1314 and 1321 nm TEM00 modes inside the gain medium as a function of the cavity length for the case of R = 150 mm, where the dashed vertical line indicates the constraint of Lcav ≈ 0.88R.
3. Results and discussion
First of all, we investigated the CW orthogonally-polarized dual-wavelength laser at 1314 and 1321 nm with an optimal cavity length of approximately 130 mm. Throughout the experiment, by slightly inclining the output coupler to introduce the Fresnel loss difference, the power ratio of the two polarizations was maintained to be 1:1. The evolution of the total dual-wavelength output power with the incident pump power for two different output couplers is shown in Fig. 3. The incident pump power thresholds were 4.7 and 6.2 W for the 5% and 10% couplers, respectively. It can be seen that the dual-wavelength output power increases linearly with the incident pump power, and the 5% output coupler provides better results than that with the 10% output coupler. By adopting the 5% output coupler, the maximum dual-wavelength output power was up to (9.2 < ± 0.1) W under the incident pump power of 32.5 W, resulting in an optical-to-optical conversion efficiency of 28.3% ± 0.3% and a slope efficiency of 32.6% ± 0.4%. Compared with the previous result by Nd:LLF crystal [19], the CW dual-wavelength output power at 1314 and 1321 nm has been improved by about 1.5 times. In addition, owing to the relatively high pump absorption efficiency, the conversion efficiency from the incident pump power to the dual-wavelength output power is about 1.4 times higher than that previously reported (∼20.5%). Increasing the output coupling transmittance to 10%, the maximum output power and slope efficiency decreased to (6.7 < ± 0.1) W and 27.5% ± 0.3%, respectively. To avoid the thermal fracture of Nd:YLF crystal, the maximum incident pump power was limited to 32.5 W. With the 5% output coupler, the spectral property of the CW dual-wavelength laser at the full output power was registered, as illustrated in Fig. 4. The central wavelengths of the dual-wavelength laser were measured to be 1314.0 and 1321.0 nm with the same spectral width (FWHM) of 0.7 nm. We can see that the two wavelengths have approximately the same emission intensity. With the aid of a polarizing beam splitter (PBS), we found that both laser emissions were linearly polarized. The emission at 1314 nm had the polarization direction along the b axis of Nd:YLF corresponding to the σ-polarization, while the polarization direction of the 1321 nm laser was along c axis (π-polarization). In addition, the beam quality factors of the 1314 and 1321 nm lasers were measured by using a scanning-knife-edge method, respectively. With the 5% output coupler, the M2 factor of 1321 nm laser on π-polarization maintained less than 1.5 over the full range of pump powers. On the contrary, the M2 factor of 1314 nm laser on σ-polarization was 1.3 near pump power threshold, and then increased to be 2.8 at the full pump power of 32.5 W. This phenomenon has been reported by several groups [24,27,28], and the beam quality deterioration of σ-polarized component was mainly attributed to the gain competition with π-polarized component.
Fig. 3. CW output power with respect to the incident pump power with different output couplers for the 1.3 µm orthogonally-polarized dual-wavelength laser.
Fig. 4. Optical spectrum of the 1.3 µm orthogonally-polarized dual-wavelength laser at the highest CW output power of 9.2 W.
To achieve high peak power orthogonally-polarized dual-wavelength laser operation, the acousto-optic Q-switcher mentioned above was inserted into the laser cavity between the Nd:YLF crystal and output coupler. Similar to CW operation, the power ratio of the 1314 nm laser and 1321 nm laser was also maintained at 1:1 by appropriately titling the output coupler. With the 5% and 10% output couplers, the output performances of the Q-swithed 1.3 µm orthogonally-polarized dual-wavelength laser under the incident pump power of 30 W were investigated by increasing the pulse repetition frequency (PRF) from 1 to 20 kHz, as visualized in Fig. 5. With the output coupler of 5%, the dual-wavelength average output power increased from 2.6 W at 1 kHz to 6.5 W at 20 kHz, respectively, resulting in the pulse energy decreasing from 2.6 to 0.33 mJ. The corresponding relative uncertainties of the average output power (or pulse energy) were determined to be 3.3% and 1.5%, respectively. After being separated with a PBS, the power uncertainties of the 1314 and 1321 nm lasers were less than 5.2% and 7.2% over one hour, respectively. Furthermore, the 10% output coupler produced a maximum dual-wavelength average output power of 5.8 W at a PRF of 20 kHz and a maximum pulse energy of 2.4 mJ at a PRF of 1 kHz, respectively.
Fig. 5. Average output power and pulse energy of the AQS 1.3 µm orthogonally-polarized dual-wavelength laser as a function of the pulse repetition frequency under an incident pump power of 30 W for two different output couplers.
Figure 6 depicts the dependence of pulse widths and peak powers at 1314 and 1321 nm on the incident pump power with TOC = 5% and 10%. We can see that the pulse durations of the 1314 and 1321 nm lasers are both increased with the PRF, and the pulse duration of the 1314 nm laser is slightly narrower than that of the 1321 nm laser. It can be seen from Fig. 6(a) that the pulse durations of the 1314 and 1321 nm lasers with the 5% output coupler decreased from 135 and 150 ns at 20 kHz to 31 and 43 ns at 1 kHz, respectively. While the 10% output coupler was adopted [Fig. 6(b)], the pulse durations of the 1314 and 1321 nm lasers decreased from 300 and 340 ns at 20 kHz to 30 and 39 ns at 1 kHz, respectively. Meanwhile, the maximum peak powers at 1314 and 1321 nm were calculated to be 41.9 and 30.2 kW for the 5% output coupler as well as 40 and 30.8 kW for the 10% output coupler, respectively. Compared with previously reported with other PQS 1.3 µm orthogonally-polarized dual-wavelength Nd:LLF lasers [19,20], the resulting peak power has been increased by two orders of magnitude.
Fig. 6. Pulse width and peak power at 1321 and 1314 nm as a function of pulse repetition frequency at full pump power of 30 W for an output coupler transmission TOC = (a) 5% and (b) 10%.
As for the 5% output coupler, the pulse trains and single pulse profiles at both wavelengths were monitored simultaneously with two fast photodiodes after being separated with a PBS. As illustrated in Fig. 7(a), under the PRF of 1 kHz and full output power of 2.6 W, the peak-to-peak intensity fluctuations at 1314 and 1321 nm were measured to be about 9.3% and 10.1%, respectively. As depicted in Fig. 7(b), the two laser pulses with orthogonal polarizations are overlapped in the time domain. In addition, we monitored the total temporal profile of the dual-wavelength laser, and a stable single pulse was observed on the oscilloscope, indicating that there was no time delay between the two wavelengths.
Fig. 7. (a) Q-switched laser pulse trains at PRF of 1 kHz and (b) the corresponding temporally expanded Q-switched pulse profiles of the 1314 and 1321 nm lasers under the full output power with the 5% output coupler.
4. Conclusion
In this work, based on a carefully designed linear resonator, the high performance CW and AQS orthogonally-polarized dual-wavelength Nd:YLF laser at 1314 and 1321 nm has been achieved by modulating the output coupler for the first time. Maximum CW dual-wavelength laser power of 9.2 W was obtained with an optical-to-optical efficiency of about 28% and a slope efficiency of about 33%. For AQS regime, this laser setup produced a maximum average output power of 6.5 W at 20 kHz, and the largest pulse energy of 2.6 mJ at 1 kHz resulting in a combined peak power > 70 kW. These results represent a significant improvement in terms of the average power and peak power over previous reports, and also set a new records for the CW and Q-switched 1.3 µm orthogonally-polarized dual-wavelength lasers. The single-longitudinal mode dual-wavelength Nd:YLF laser at 1314 and 1321 nm could be further developed by using a ring cavity configuration, which has important applications in silver atom optical clock and microwave signal generation.
Funding
Key Project of Natural Science Foundation of China (61935010); National Key Research and Development Program of China (2017YFB1104500); National Natural Science Foundation of China (61735005); Guangdong Project of Science and Technology Grants (2016B090917002, 2018B030323017); Guangzhou Science and Technology Project (201903010042).
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