Optimization of the nonlinear crystal length for high-power single-frequency intracavity frequency-doubling lasers.

We report a method of optimizing the nonlinear crystal length of the intracavity frequency-doubling laser, which is achieved by maximizing the output power of the frequency-doubling laser in the case of ensuring the single longitudinal mode (SLM) operation of the laser. The optimal length of the nonlinear crystal for an SLM oscillation of the intracavity frequency-doubling laser is firstly theoretically predicted by comparing the losses introduced by the nonlinear crystal with different lengths with that of ensuring the SLM operation of the laser. Then three nonlinear LiB3O5 (LBO) crystals with the length of 18, 20, and 22 mm are adopted to be the frequency-doubling components in the experiment. By recording the output power and monitoring the longitudinal mode structure of the laser, it is found that the nonlinear LBO crystal with the length of 20 mm is the best candidate, since the output power is higher than that of the LBO crystal with the length of 18 mm, and the SLM operation of the laser is readily achieved compared to that of the LBO crystal with the length of 22 mm. The experimental results well agree with the theoretical predictions. The current method can pave a good way to attain a single-frequency continuous-wave intracavity frequency-doubling laser.

How important is community participation to eco-environmental conservation in protected areas? From the perspective of predicting locals' pro-environmental behaviours.

Community participation is considered an effective measure to protect the eco-environment and to improve people's livelihoods in protected areas. However, it has not received enough attention at the practical level in most developing countries, and it is unclear how important it is in stimulating locals' pro-environmental behaviours to achieve eco-environmental protection goals. This study focuses on the relationship among community participation, perception changes in livelihood capitals and place attachment, which are related to residents' production, livelihoods, and pro-environmental behaviours. The study uses a convenience sampling method in the Nanling National Nature Reserve, China. Regression analysis results show that community participation is the most powerful predictor of pro-environmental behaviours. Furthermore, community participation moderates the relationship between place attachment and pro-environmental behaviours. In addition, perception changes in livelihood capitals positively affect pro-environmental behaviours in the high-level community participation group while having negative or positive results in the low-level community participation group. The findings, which emphasize the importance of community participation in conservation, provide a better understanding of the differences in pro-environmental behaviours between high and low community participation groups and will aid future development and conservation planning of these initiatives.

Influence of the pump scheme on the output power and the intensity noise of a single-frequency continuous-wave laser.

The influence of the pump scheme on the intensity noise of the single-frequency continuous-wave (CW) laser is investigated in this paper, which is implemented in a single-frequency CW Nd:YVO4 1064 nm laser by comparing the traditional 808 nm pumping scheme (TPS) to the direct 888 nm pumping scheme (DPS). Under the conditions that the lasers with TPS and DPS have the same cavity structure and the cavity mirrors, as well as the same operation state including the thermal lens of the laser crystals and the mode-matching between the pump laser mode and the laser cavity mode at the laser crystals, the output power of the laser with DPS is up-to 32.0 W, which is far higher than that of 21.1 W for the laser with TPS. However, the intensity noise of the DPS laser including resonant relaxation oscillation (RRO) frequency of 809 kHz, RRO peak amplitude of 31.6 dB/Hz above the shot noise level (SNL) and the SNL cutoff frequency of 4.2 MHz, respectively, is also higher than that of 606 kHz, 20.4 dB/Hz and 2.4 MHz for the TPS laser. After further analyses, we find that the laser crystal with high doping concentration and long optical length is employed for DPS laser in order to improve the pump laser absorption efficiency, which can simultaneously increase the dipole coupling between the active atoms and the laser cavity, and then results in a high RRO frequency with a large amplitude peak as well as a high SNL cutoff frequency of the laser.

Intensity noise suppression of a high-power single-frequency CW laser by controlling the stimulated emission rate.

The intensity noise of a high-power single-frequency continuous-wave (CW) laser is very harmful for applications in precise measurements and quantum communication. By simply lengthening the length of a laser resonator to decrease the stimulated emission rate of the laser, the coupling strength of all noise sources in the resonant laser field will be reduced, and thus the intensity noise of the output laser will be prominently suppressed. Based on theoretical analyses of the laser noise spectra, we experimentally implement a low-noise high-power single-frequency laser with a 1050 mm long resonator. With the assistance of an intracavity imaging system and nonlinear second-harmonic generation, the amplitude of the resonant relaxation oscillation peak and the shot-noise level (SNL) cutoff frequency are successfully reduced to 8.6 dB/Hz above SNL and 1.0 MHz, respectively, under the output power of 16 W. The work provides an effective way to develop a high-quality laser with high output power and low intensity noise.

Realization of a 101 W single-frequency continuous wave all-solid-state 1064 nm laser by means of mode self-reproduction.

We realized a 101 W single-frequency continuous wave (CW) all-solid-state 1064 nm laser by means of mode self-reproduction in this Letter. Two identical laser crystals were placed into a resonator to relax the thermal lens of the laser crystals, and an imaging system was employed to realize cavity mode self-reproduced at the places of the laser crystals. Single-frequency operation of the resonator was realized by employing a new kind of high extinction ratio optical diode based on the terbium scandium aluminum garnet crystal to realize a stable unidirectional operation of the laser, together with introducing a large enough nonlinear loss to the resonator to effectively suppress the multi-mode oscillation and mode hopping of the laser. As a result, a 101 W single-frequency 1064 nm laser in a single-ring resonator was achieved with 42.3% optical efficiency. The measured power stability for 8 h and the beam quality were better than ±0.73% and 1.2, respectively.

Investigation about the influence of longitudinal-mode structure of the laser on the relative intensity noise properties.

The influence of the longitudinal-mode structure (LMS) of the laser on the relative intensity noise (RIN) properties was investigated in this paper after an all-solid-state continuous-wave (CW) single-frequency 1064 nm laser with output power of 50.3 W was achieved. The LMS of the laser was manipulated by controlling the temperature of the nonlinear lithium triborate (LBO) crystal deliberately introduced to the resonator. When the laser worked with single-longitudinal-mode (SLM) operation, the stable RIN spectrum was observed and measured. Moreover, with the decrease of the nonlinear conversion efficiency (NCE), the frequency and amplitude of the resonant-relaxation oscillation (RRO) peak regularly shifted to the higher frequencies and increased, respectively. However, with further decrease of the NCE, the laser began to work with the multi-longitudinal-mode (MLM) or mode-hopping operation and the unstable RIN spectra of the laser were both observed not only at low frequencies but also at high frequencies. Once the NCE was moved away, the MLM or mode-hopping can only enhance the fluctuation of the laser RIN spectrum at the low frequencies (lower than the frequency of RRO). The experimental results directly revealed the relationship between the LMS of the laser and the RIN spectra, and confirmed definitely that the key to achieve a stable high power laser with low intensity noise was to realize single-frequency operation of the laser with free MLM and mode-hopping.

Intensity noise manipulation of a single-frequency laser with high output power by intracavity nonlinear loss.

The influences of intracavity nonlinear losses on the intensity noise of output lasers are theoretically and experimentally investigated with an all-solid-state single-frequency laser with high gain. By means of tuning the temperature of a nonlinear crystal deliberately placed inside the laser resonator, the intracavity nonlinear loss of the laser is controlled. The dependence of the frequency and amplitude of the resonant relaxation oscillation peak on the nonlinear loss is studied in detail for both fundamental-wave (FW) and second-harmonic-wave (SHW). We find that, by controlling the temperature of the nonlinear crystal, the intensity noises of the laser can be transferred between FW and SHW. The theoretical predictions are in good agreement with the experimental measurements. The obtained results can be applied to manipulate and suppress the laser noises.