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Abstract
In this letter, a sub-pm linewidth, high pulse energy and high beam quality microsecond-pulse 766.699 nm Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser is demonstrated. At an incident pump energy of 824 mJ, the maximum output energy of 132.5 mJ at 766.699 nm with linewidth of 0.66 pm and a pulse width of 100 µs is achieved at a repetition rate of 5 Hz. To the best of our knowledge, this is the highest pulse energy at 766.699 nm with pulse width of hundred micro-seconds for a Ti:sapphire laser. The beam quality factor M2 is measured to be 1.21. It could be precisely tuned from 766.623 to 766.755 nm with a tuning resolution of 0.8 pm. The wavelength stability is measured to be less than ±0.7 pm over 30 min. The sub-pm linewidth, high pulse energy and high beam quality Ti:sapphire laser at 766.699 nm can be used to create a polychromatic laser guide star together with a home-made 589 nm laser in the mesospheric sodium and potassium layer for the tip-tilt correction resulting in the near-diffraction limited imagery on a large telescope.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Adaptive optics (AO) at large ground-based telescopes aims at providing diffraction-limited images by correcting the incoming wavefronts for atmospheric phase disturbances [1,2]. A limitation to AO is the low probability of finding a reference star both bright enough to feed the wavefront sensor and close enough to the program object [3,4]. To overcome this limitation, the concept of an artificial laser guide star (LGS) has been proposed for extending the observation area and correcting atmospheric distortion of the image [5].
Nevertheless a monochromatic LGS suffers a major drawback: the wavefront tip-tilt cannot be measured because the tip-tilt induced by the atmosphere on the way back from the focused laser spot had already been induced in reverse on the way to the spot from the telescope [4]; this principle prevents us from using only a monochromatic LGS for high-resolution long-exposure imaging and full sky coverage [1,6].
The most promising way to correct the tip-tilt is to use a polychromatic laser guide star (PLGS) [4]. PLGS aim is to ensure 100% sky coverage for near-diffraction limited observations. Its principle is to induce a radiative cascade of mesospheric sodium by laser excitation. The wavefront tilt is determined from the measurable difference of the tilts induced by the atmospheric turbulence at two of the returned wavelengths. The key point of the feasibility is given by the precision requested on the measurement of the differential tip–tilt. The latter is directly linked to the return flux available, i.e., to the intensity of the PLGS, the transition probability (in s-1) of the corresponding energy level of sodium atom or potassium atom, and the atmospheric transmittance of the laser wavelength. The French project ELPOA (Polychromatic Laser Guide Star for Adaptive Optics) is originally on the double resonant excitation of the 3S1/2-3P3/2 transition at 589 nm (D2 line) followed by the 3P3/2–4D5/2 transition at 569 nm [6]. Later, another excitation scheme has been proposed: the direct excitation of the mesospheric sodium at 330 nm (3S1/2–4P3/2) [7,8]. Our group proposed a new approach based on the double resonant excitation of the 3S1/2-3P3/2 transition at 589 nm (D2 line) followed by the 3P3/2-3D5/2 transition at 819.7 nm for generating BPRF of sodium atoms [9].
Here, we firstly propose a new PLGS approach based on the two-photon excitation of the mesospheric sodium (Na) and potassium (K) by means of two lasers, respectively at 589 (3S1/2–3P3/2) and 766.7 nm (4S1/2–4P3/2). Compared to the aforementioned two schemes, the 766.7 nm laser could lead to a more return flux. Due to the relatively higher transition probability of 4P3/2 (K) level for 766.7 nm (3.8×107 s-1) than that of 4D5/2 (Na) level for 569 nm (1.2×107 s-1) and 4P3/2 (Na) level of sodium atom for 330 nm (2.75×106 s-1) [10], which corresponds to a higher saturation power density to be able to generate more return flux. Furthermore, the atmospheric transmittance of K LGS at 766.7 nm is higher than that of Na LGS at 569 nm and 330 nm.
To date, there are far fewer lidar observations of the K layer than of the Na layer, this is because the average distribution of sporadic K was lower than that of sporadic Na in the mesosphere and lower thermosphere region (80-105 km). However, K and Na showed different seasonal variations despite both being alkali metals [11]. From over 2 years of lidar data, the results of the field experiment revealed that the Na occurrence had a maximum (19.3%) in May-June months and a minimum (1.6%) in January-February months, while the K occurrence had a maximum (4.9%) in January-February months and a minimum (1.0%) in September-October months at Beijing (40.6 °N, 116.2°E). That is in the January–February months the K occurrence rate (4.9%) was much higher than that of Na (1.6%) [12]. Therefore, the proposed approach can greatly enhance the ability to correct the tip–tilt during a certain period time.
Usually, 766.7 nm laser is generated from Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser. However, most attention has been concerned on continuous wave [13] or short pulse [14,15]. In addition, optical parametric oscillators (OPO) also give an alternative route to access 766.7 nm. Similarly, most effort has been made on continuous wave [16] and short pulse [17,18]. Moreover, very complicated adjustments are unavoidable, which limit its practical use. Recently, the rapid improvement in diode laser based on gallium nitride (GaN) materials opens the way for direct diode-laser pumping of Ti:sapphire laser to produce 766.7 nm [19]. Unfortunately, this approach is difficult to obtain high pulse energy due to lack of high performance diode lasers in the required blue-green spectral region [20].
Moreover, the microsecond pulse laser demonstrate a excellent performance in artificial LGS, because it can provide a gateable pulse format to eliminate the interference of atmospheric Rayleigh scattering compared with the continuous wave (CW) beacon laser and it can be easier to obtain narrow linewidth compared with the short pulse laser. Thus, it can remarkably improve the signal noise ratio of the AO system [21].
In this letter, we report a sub-pm linewidth, high pulse energy and high beam quality microsecond-pulse 766.699 nm Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser at 532 nm for generating PLGS. A birefringent filter and an etalon are used to obtain the required wavelength and narrow linewidth. The wavelength of the narrow linewidth Ti:sapphire laser could be tuned from 766.623 to 766.755 nm with a tuning resolution of 0.8 pm by adjusting the temperature of the etalon. The wavelength stability is better than ±0.7 pm over 30 min. The maximum output energy of 132.5 mJ at 766.699 nm is achieved with linewidth of 0.66 pm, a pulse width of 100 µs and beam quality factor of M2=1.21. This is, to the best of our knowledge, the highest pulse energy at 766.699 nm with pulse width of hundred micro-seconds for an all solid state laser.
2. Experimental details
The experimental configuration is schematically depicted in Fig. 1. The Ti:sapphire laser was pumped by a frequency-doubled Nd:YAG laser at 532 nm. Frequency doubling of the laser was accomplished by a home-made p-polarized 1064 nm Nd:YAG master oscillator power amplifier (MOPA) laser system in Lithium triborate (LBO) nonlinear crystal. The master oscillator had a stand wave cavity with two flashlamp-pumped modules based on Nd:YAG rods (Φ8 mm × 145 mm). Similarly, the power amplifier consisted of two single-pass flashlamp-pumped amplifier modules with Nd:YAG rods (Φ8 mm × 145 mm). After amplification, the MOPA provided 2.75-J of pulse energy and 130-µs of pulse width at a pulse repetition frequency (PRF) of 5 Hz. The LBO crystal with dimensions of 4 mm × 4 mm × 60 mm was cut at θ= 90° and φ= 0° for type I noncritical phase matching operation with no walk-off effect at 148.2 °C. Both ends of the LBO were antireflection coated at 1064 nm and 532 nm. An aperture in front of the LBO was employed to clean the spatial profile of pump beam at 1064 nm. Thus it was beneficial for improving the green beam quality. As a result, the 532 nm laser provided a pulse energy of 824 mJ (5 Hz, 102-µs). The energy conversion efficiency from 1064 nm to 532 nm was about 30%. The dichroic flat mirrors M1 and M2 with high reflection (HR) coating at 532 nm and antireflection (AR) at 1064 nm were employed to separate residual 1064 nm and reflect 532 nm.
F2 was a focus lens with the focal length of 500 mm, which was used to enhance the density of 532 nm pump power and make a good mode matching between the pump beam and the Ti:sapphire oscillating beam. The diameter of pump beam in the Ti:sapphire crystal was about 1.5 mm. The beam waist diameter of pump beam in the Ti:sapphire crystal was about 1.5 mm. Ti:sapphire resonator was consisted of M3, M4, M5, prism, and Ti:sapphire crystal. The dichroic mirror (M3) was high-reflection coated in the range from 720 nm to 860 nm (R>99.8%) at 45 degree and high-transmission coated for the range from 520 nm to 540 nm (T>95%) at 45 degree. The flat mirror (M4) was high-reflection coated in the range from 720 nm to 860 nm (R>99.8%). The output coupler M5 was a flat mirror with a transmission of 10% around the center wavelength of 766.699 nm. The Ti:sapphire crystal with dimensions of 4 mm × 4 mm × 30 mm, was cut at Brewster angle at both end facets with respect to the direction of c axis for the highest absorption in the pumping light (>80%), highest emission in the infrared, and the lowest residual absorption at the laser wavelengths. To alleviate thermal effects in the gain medium, the Ti:sapphire crystal was wrapped with indium foil and tightly mounted in a water-cooled copper sink, and the water temperature were kept at 20 °C. A fused-silica prism was placed behind the output facet of the Ti:sapphire crystal, which was used as the dispersion element to tune the wavelength of output. The incident and emergent angles correspond the Brewster's angle to reduce the insertion loss, the apex angle of the prism was about 69° at center wavelength of 766.699 nm. The birefringent filter (BF) at the incident angle of 57° was used to obtain single wavelength around 766.7 nm and suppress other wavelengths of Ti:sapphire such as 800 nm. To get the tunable output wavelength with narrow linewidth around 766.699 nm, the etalon made of suprasil 3002 (Heraeus) were adopted with the free spectral range (FSR) of 148 GHz and the finesse of 3.31.
The total resonator length was 310 mm. The arm lengths L1, L2, L3 and L4, were 105 mm, 35 mm, 35 mm, and 105 mm, respectively, for an optimum cavity design. To obtain higher power output and better beam quality, the mode size of the Ti:sapphire beam must be well matched with that of 532 nm pump beam in the center of the Ti:sapphire crystal. To generate stable power output, the Ti:sapphire laser must operate at the stable power point which is influenced by pump power and pump beam waist. Considering the thermal effect of the Ti:sapphire crystal, the fundamental mode size of the optical resonator operated in the stable region was calculated based on ABCD ray transfer matrix as shown in Fig. 2. The inset in Fig. 2 shows the thermal focal of the Ti:sapphire crystal for our resonator configuration in the range from 9000 mm to 11000 mm for pump power from 3W to 4.5W. While the pump power was 4.12 W and the waist diameter of the 532 nm pump beam in the center of the Ti:sapphire crystal was 1.5 mm, the thermal focal length of the Ti:sapphire crystal was estimated to be around 10600 mm.
Fig. 2. Calculated fundamental mode size of the optical resonator operated in the stable region as a function of thermal focal of the Ti: sapphire. Inset: the estimated operation range for the thermal focal of the Ti:sapphire crystal in our resonator configuration.
3. Theoretical analysis
The output performance of Ti:sapphire laser resonator can be modeled with laser rate equations given by Koechner [22]:
Table 1. Parameters for Ti:sapphire and the laser cavity
4. Results and discussion
The output energy of Ti:sapphire laser at 766.699 nm was measured by an energy meter (Ophir PE-80). Fig. 3 shows the output energy as a function of the incident pump pulse energy. The black dots represent the measured pulse energy. As seen from Fig. 3, the threshold pump energy is 200 mJ per pulse, and the laser output energy grows monotonically with the increasing input energy. The maximum output pulse energy was achieved to be 132.5 mJ at the pump pulse energy of 824 mJ, corresponding to an optical-to-optical conversion efficiency of 16.1%, and a slope efficiency of 21.6%. The red line was the calculated result. It can be seen that the theoretical simulation is reasonable agreement with the measured data. Moreover, the output energy does not show any roll-over effect up to the maximum pump energy, indicating that higher output can be achieved with higher pump energy.
Fig. 3. Output energy of the Ti: sapphire laser at 766.699 nm versus pump energy at 532 nm. Black dots represent the experimental data; the results of simulation are shown by the red line.
The pulse temporal characteristics of the 766.699 nm Ti:sapphire laser were recorded by a high-speed Si-detector (Thorlabs DET 10A/M, 1 ns rise time) connected to a digital oscilloscope (Tektronix DPO 4104 B-L, 1 GHz) at the full output pulse energy. A typical pulse trains of Ti:sapphire laser is shown in Fig. 4. It depicts the pulse trains at a PRF of 5 Hz and the fluctuation of the pulse intensity is about ±0.9%. The inset of Fig. 4 expresses an expanded single pulse profile. It can be seen from the inset that the pulse width of the 766.699 nm laser is about 100 µs, which is approximately the same as the pump pulse width of 102 μs at 532 nm.
Wavelength tuning was realized by tuning the temperature of the etalon. Fig. 5 shows the wavelength as a function of the temperature of the etalon. As shown in Fig. 5, when the temperature was changed from 38.6°C to 55.5°C, the wavelength was continuously tuned from 766.623 to 766.755 nm. The slope of the tuning curve was 8 pm/°C, and the step-length of temperature during the temperature tuning was 0.1°C, corresponding to a tuning resolution of 0.8 pm.
The laser wavelength and linewidth are measured by the wavelength meter (WS-7, High Finesse GmbH). As shown in Fig. 6, the central wavelength of the laser is located to potassium 4S1/2–4P3/2 transition at 766.699 nm. The wavelength stability over 30 min is measured to be better than ±0.7 pm and the linewidth is 0.66 pm. The wavelength stability is about to be upgraded with methods such as locking the output wavelength to a wavelength meter.
Furthermore, the repetition rate of the laser was too low that it was hard to measure the beam quality by general beam-propagation analyzer. A home-made CCD camera-based system was employed to measure beam quality on the low repetition rate (<10 Hz) laser. To ensure an accurate measurement of beam quality, the samples of the transverse beam profile must be taken near the beam waist and several Rayleigh ranges away from the waist [23]. As shown in Fig. 7, the measured beam quality factor M2 is about 1.21, fitted from several measured beam diameter by the hyperbolic equation [24,25]. The inset in Fig. 7 exhibits far-field two-dimensional (2D) beam intensity profile, which shows that the laser operates in a near Gaussian mode.
5. Conclusion
In summary, a sub-pm linewidth, a high pulse energy, high beam quality microsecond-pulse Ti:sapphire laser at 766.699 nm, pumped by a frequency-doubled Nd:YAG laser at a repetition rate of 5 Hz is demonstrated. At an incident pump energy of 824 mJ, the output energy is up to 132.5 mJ with linewidth of 0.66 pm and a pulse width of 100 µs, corresponding to a slope efficiency of 13.6% from the 532 nm pump laser to the Ti:sapphire laser. With an optimum cavity design, the good beam quality factor M2=1.21 is achieved. The wavelength stability is measured to be better than ±0.7 pm over 30 min. It could be tuned from 766.623 to 766.755 nm with a tuning resolution of 0.8 pm. The sub-pm linewidth, high pulse energy and high beam quality Ti:sapphire laser at 766.699 nm with pulse width of hundred micro-seconds can be used to efficiently generate a PLGS together with a 589 nm laser [21] for the tip-tilt correction of ground-based telescopes to lead to a 100% sky coverage.
Funding
Beijing Science and Technology Planning Project (Z221100006722005).
Acknowledgments
The authors would like to thank L. Feng for fruitful discussions. Funding from the Beijing Science and Technology Planning Project and China National Nuclear Corporation Youth Talent Project are gratefully acknowledged.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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