† Corresponding author. E-mail:
An injection-seeded single-frequency Q-switched Nd:YAG laser is accomplished by using a phase modulated ramp-fire technique. A RbTiOPO4 (RTP) electro-optic crystal is selected for effective optical path length modulation of the slave self-filtering unstable resonator. This single-frequency laser is capable of producing 50 mJ pulse energy at 1 Hz repetition rate with a pulse width of 16 ns. The standard deviation of laser pulse intensity for consecutive 100 shots from the mean pulse intensity is less than 1.05%. A spectral linewidth of less than 0.5 pm with a frequency jitter of about 14 fm over 30 min is obtained.
All-solid-state Q-switched single-frequency lasers are widely used in many fields, such as nonlinear optics, high-resolution spectroscopy and Doppler lidar.[1–4] Currently, injection seeding is one of the most frequently used techniques to produce single-axial-mode output from a Q-switched laser. It is achieved by injecting a low power narrow linewidth single-frequency seed laser into a Q-switched slave oscillator. If the seed laser has sufficient power, in the absence of spatial hole burning, the mode competition in the slave cavity will establish single-axial-mode oscillation.[5] However, the slave cavity must be in resonance with the frequency of the seed when the Q-switch of the slave cavity is activated. Hence, an active feedback locking system is required to ensure the resonance.
Schmitt and Rahn described a stabilization feedback locking technique by minimizing the Q-switched pulse build-up time (MBUT).[6] However, the feedback signal was obtained only once after each laser shot. This led to difficulties in certain situations such as low repetition rate operation and high noise environments. Fry et al. reported a ramp-fire locking technique for maintaining a single-frequency operation in severe environments based on fast resonance detection.[7,8] In this approach, optimum axial mode locking was achieved for each laser shot. This became the most reliable approach to realizing the stable single frequency operation. However, these techniques generally rely on a piezoelectric transducer (PZT) mounted on a cavity mirror to modulate the length of the cavity. However, the low time response of PZT limits the response rate of ramp system and the repetition rate of laser operation. The moving of the cavity mirror also results in unstability of the resonator. Zhang et al. demonstrated single-frequency output using a LiNbO3(LN) crystal instead of PZT.[9] They obtained better spectrum performance by adopting LN compared with that by adopting the PZT.
In this work, we apply this intracavity phase modulation technique to a flashlamp-pumped Nd:YAG self-filtering unstable resonator (SFUR). There are several benefits in using the SFUR resonator. The SFUR resonator makes it easier for the laser to achieve seed injection mode matching and large mode volume. Finally, we obtain a high-energy, near-diffraction-limited TEM00 mode and stable narrow linewidth single-axial-mode pulse output.
The geometry of the single-frequency laser is shown in Fig.
The seed laser is a commercial 1064 nm single-frequency laser (Mephisto S200NE, Coherent) with a spectral linewidth on the order of 1 kHz and maximum output power of 200 mW. The output light is elliptically polarized, and is transformed into linear polarization state by a quarter-wave and a half-wave plate. A Faraday isolator (IO-5-1064-VHP, Thorlabs) with an isolation of 40 dB is inserted behind the seed laser to prevent feedback from damaging by the Q-switched pulse.
The Q-switched slave oscillator is a flashlamp-pumped self-filtering unstable resonator (SFUR) Nd:YAG laser.[10] The SFUR configuration is a confocal negative branch unstable resonator. A field-limiting pinhole is located on the common focal plane of the cavity mirrors. The aperture is chosen such that a plane wave incident on it is focused by mirror M1 on an Airy disk having the same diameter. Then this will result in the removal of the hot spot and the smoothing of the spatial profile. This yields
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A RbTiOPO4 (RTP) crystal is used for intracavity optical path length modulation. The Y-cut RTP crystal has dimensions of 3(Z)×3(X)×20(Y) mm3. The Z faces are plated with Ti electrodes onto which the modulating electric field is applied. The polarization of the input seed is parallel to the X-axis. In this case, the double pass effective optical path length change
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![](cpb_27_5_054212/cpb_27_5_054212_ieqn2.gif)
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Figure
![]() | Fig. 3. (color online) Plots of output energy versus pump energy in the cases of the laser seeded and unseeded injection. |
The beam spatial profile is shown in Fig.
The temporal shapes of the output pulse with seeded and unseeded injection are shown in Fig.
![]() | Fig. 5. (color online) Temporal profiles of the laser in seeded and unseeded operation, respectively. |
The pulse energy stability is measured by measuring the area of the temporal profile of the laser pulse with the built-in function of the oscilloscope. The results shown in Fig.
The frequency spectrum of the seeded injection laser is further analyzed with a commercial Fizeau wavemeter (Angstrom WS/7, HighFinesse) as shown in Fig.
However, the disadvantage of this laser is that the Q-switched firing time jumps randomly in one resonant cycle during the ramp, as shown in Fig.
We have demonstrated a flashlamp-pumped Q-switched self-filtering unstable Nd:YAG single-frequency laser by using a phase-modulated ramp-fire technique. An RTP crystal is used for optical path length modulation. The single-frequency laser is capable of producing over 50 mJ pulse energy with a near-diffraction-limited spatial beam profile. The pulse duration is about 16 ns with a jitter of 183 ps, and the pulse-to-pulse energy stability is approximately 1%. The laser linewidth is measured to be less than 0.5 pm and the frequency stability is 14 fm over 30 min. The developed single frequency laser is scalable and harmonic, and is being incorporated into an OPO system.
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