A self-organizing laser based on a single diode-pumped Nd:YAG slab with reciprocal cavity completed by dynamic holographic gratings was created. The temporal dynamics, frequency spectrum and spatial characteristics of the generated beam were examined. A single-transverse-mode beam with up to 10 mJ pulse energy and 15% optical-to-optical efficiency was generated.
© 2004 Optical Society of America
Laser oscillators with a self-organizing cavity completed by holographic gratings induced in nonlinear media by generating beams themselves have attracted great interest in the recent years due to adaptability of their dynamic cavity [1–6]. This unique property of the self-organizing lasers is promising for generation of high average power beam with good quality. The main idea of the laser is self-consistent grating formation and increased intensity of the generated beam due to positive feedback provided by the dynamic gratings. Two main concepts of the holographic laser with optically induced population gratings in laser amplifiers are now under investigation: a cavity consisting of a nonreciprocal device [1–3], and a reciprocal scheme [4–6].
The first configuration of the self-organized laser system has incorporated a non-reciprocal device based on a Faraday rotating element to provide a differential transmission in the two loop directions (clockwise and anti-clockwise) that can optimize the grating writing process. The Faraday device introduces also a differential π - phase shift to achieve resonance by offsetting the anti-phase population inversion grating induced by the intensity interference pattern and this is essential in a self-starting oscillator for a pure gain grating [1–3]. The output from the laser system is via a partially reflective plane feedback mirror. Operation of the laser has been demonstrated in pulsed flash-lamp pumped Nd:YAG amplifiers [1,2], and continuous-wave diode-pumped Nd:YVO4 .
Another related kind of laser systems with self-intersecting loop geometry but without any nonreciprocal device has also been investigated. The powerful output from this system without non-reciprocal element is derived from the throughput of the self-intersecting loop section. The operation of the reciprocal cavity self-organizing laser can be explained by formation of a moving resonant refractive index grating that accompanies the population grating induced in the laser crystal [4–6]. This moving grating can complete the cavity providing energy transfer from a strong output beam to a weak intracavity beam. The self-starting generation in the reciprocal holographic laser was realized for a flash-lamp-pumped Nd:YAG crystal [4,5]. The self-organizing lamp-pumped Nd:YAG laser system has demonstrated the capability of pulse-repetitive generation of good-quality beams with an average power up to 300W . Also, similar schemes of the self-organizing laser with the reciprocal holographic cavity and a combination of flash-lamp-pumped and diode-pumped Nd:YAG laser crystals , or a continuous-wave diode-pumped Nd:YVO4 crystal , were recently examined. However, the mechanism and even the possibility of generation in the self-organizing laser based on a diode-pumped crystal with the reciprocal cavity remained disputable.
In this paper, we examine the generation possibility of the diode-pumped self-organizing laser based on a single Nd:YAG crystal with a reciprocal cavity completed by holographic population-accompanied gratings. Main emphasis was made on investigating spatial and temporal structures of the generated beams. The investigations of the near- and far-field of radiation demonstrated the possibility of highly efficient generation of a good-quality beam.
A Nd:YAG slab (with dimensions of 2×4×12 mm3 and Nd3+-ion concentration of 1 atomic %), produced by “ELS-96” Ent. (Moscow, Russia) was used for creation of the self-organizing laser. The Nd:YAG sample was side pumped through the 2-mm× 12-mm face (with AR at 808 nm) by a QCW-diode stackat 808 nm, produced by “JENOPTIK”, Germany (duration of pump pulses was 200-300 μs, their repetition rate was 10-500 Hz, and pulse power was varying up to 300 W). The fast-axis collimation of the diode output beam and an additional vertical lens formed a line focus on the crystal pump face with dimensions of approximately 100 μm × 10 mm. The slab width of 4 mm and an additional mirror at 808 nm behind the slab (see, Fig. 1) provided the good absorption of the pumping beam (the absorption coefficient in the Nd:YAG crystal was ~3 cm-1).
The Nd:YAG amplifier was operated in a bounce geometry where the optical beam approached at a grazing incidence angle to the pumped crystal face where it experiences total internal reflection. An internal bounce angle with respect to the pump face was varied to maximize generation output power. The geometry with total internal reflection from the pump face ensures beam access to the highest region of gain.
Several schemes of the cavity based on the Nd:YAG laser crystal pumped by the QCW diode stack were experimentally studied. The one-loop system composed of the diode-pumped Nd:YAG amplifier, three mirrors (M) at 1064 nm and two cylindrical lenses VCL1 and VCL2 was studied in the first experimental series (Fig. 1).
The beam intersection angle (θ) formed by the loop was varied from 0.01 rad to 0.1 rad (in air). For these angles good intersection of the interacting beams was achieved. The output from the throughput of the loop section was monitored for power, spatial quality, spectral composition and temporal form. A Faraday isolator (FI) was inserted at the output to minimize feedback into the laser from the diagnostic systems.
The laser oscillation was observed when the pumping power achieved a threshold level. Above the threshold, the output energy of the generated pulse increased almost linearly with an increase of pumping pulse energy (Fig. 2).
The generation threshold depends on the intersection angle of optical waves in the Nd:YAG crystals (Fig. 3). The threshold was the lowest at smallest intersection angles. As the angle increased, we first observed a nonlinear increase of the threshold, and then there was an area where the threshold stabilized and even slightly decreased. With a further increase of the angle, the threshold increased again. The similar threshold dependence was recently observed in the scheme with an additional amplifier inside the linear arm of the cavity . This behavior of generation threshold at small intersection angles can be explained by the predominating contribution of the thermal component of the refractive index gratings completing the cavity, and the predomination of the electronic gratings at the larger intersection angles. Indeed, the decay time (τT) of the thermal grating is proportional to the square of the grating period (Λ), τT=Λ2/(4π2D) (where D is the thermal conductivity); and the period is inverse proportional to the intersection angle, Λ≈λ/θ (τT was 2…100 μs for the experimental angles). In turn, the diffraction efficiency of the dynamic holographic grating increases with increasing grating decay time (proportionally to the time in the stationary regime). On the other hand, the decay time (τe) of the electronic grating (caused by the index changes due to different polarizability of the excited and unexcited Nd3+ ions ) is determined only by the decay time of population inversion, τe=T1=250 μs. Besides, the angle dependence of the threshold indicates a leading role of the index grating in the cavity formation (in comparison with the gain grating accompanying the population grating whose decay time is independent on the period).
3. Temporal dynamics and spatial structure of the generated beams
The temporal dynamics and frequency spectra of the generating beams were studied (Fig. 4). The frequency spectra were registered both by a Fabry-Perot interferometer (free spectrum range ∆v = 5 GHz) and a spectrophotometer (with resolving capacity of about 0.01 Å).
The generation of a single pulse with duration of several hundreds of nanoseconds was observed near the oscillation threshold. In this case the singe-mode generation was registered both by the Fabry-Perot interferometer and the spectrophotometer (Fig. 4(a)). A weak wavelength jitter of the generating mode (from pulse to pulse) was observed.
The number of both the generated pulses and the spectral components above the oscillation threshold increased with an increase of the pumping power (Figs. 4(b) and 4(c)). Eventually the observable range of the generation frequency amounted to ∆v~1.23 cm-1 (which is comparable with half-width of luminescence line of the Nd:YAG laser transition at 1.064 μm, ∆v~3 cm-1). The frequencies of the majority of generated modes were essentially shifted in respect to the center of the luminescence line. The multi-frequency generation can be explained by formation of longitudinal modes of the dynamic cavity (when the different waves diffract on the same holographic grating) .
The spectrum of generated waves in the self-starting diode-pumped laser was compared with the measured generation spectrum of a reference flash-lamp-pumped Nd:YAG laser with linear cavity created by ourselves. A weak Stokes shift of the line of the self-organizing laser with respect to the line center of the linear-cavity laser was detected, but it can be explained by the difference in the temperature of the Nd:YAG crystals under flash-lamp and diode pumping. As an argument for this explanation, the generated frequency of the self-organizing laser increased (by a frequency jump of ~0.8-1.0 cm-1) with a decrease in controllable temperature of the diode-pumped Nd:YAG crystal (at 15-20 degrees).
The transverse profile of the generated beam was measured. The spatial quality of the generating beam was good for both the near and far field, with M2<1.2 in the vertical and horizontal plane (Fig. 5).
4. Double-loop self-starting laser
The self-starting oscillations were obtained and investigated also in a double-loop scheme where the optical beam passed 6 times through the diode-pumped slab (Fig. 6). The output beam passing through a Faraday isolator (similarly as in the one-loop scheme) was monitored for power, spatial quality, frequency spectra and temporal dynamics.
In this scheme the threshold of generation was lower than in the laser with one-loop cavity, the output energy increased more than two times in comparison with the one-loop scheme. Due to the additional passage through the crystal, the absorbed pump energy was used more efficiently; as a result, energy of the generated beam was increased (Fig. 2), and optical-to-optical efficiency of the generation reached more than 15 %. The generating beam has good quality even at the biggest output pulse energy.
The temporal dynamics and frequency spectra of the generating beams were the same as in the one-loop cavity laser: one mode generation near the threshold and multimode oscillations above the threshold (Fig. 4).
In conclusion, the experiments demonstrated the self-starting generation in a self-organizing laser with reciprocal holographic cavity completed by dynamic gratings formed inside the diode-pumped Nd:YAG crystal by generated beams themselves. The strong dependence of generation threshold on the intersection angle of the optical waves in the Nd:YAG crystal was measured. The single longitudinal mode generation can be obtained near self-oscillation threshold. Adding one more pass through the loop to the reciprocal dynamic cavity can increase the output energy and optical-to-optical efficiency. Pulse energy of the generated good-quality beam up to 10 mJ and optical-to-optical efficiency up to 15% were obtained by QCW diode pumping with a repetition rate up to 500 Hz. Similar schemes can be used for creation of high-average-power diode-pumped lasers with good beam quality.
This research was supported in part by the NATO Science For Peace foundation (through grant 974143), the International Association for the promotion of co-operation with scientists from the New Independent State of the Soviet Union (through grant 03-51-4893), the Russian Foundation for Basic Research (through grants 02-02-81042 Bel. and NSh-1625.2003.2) and the program Nonlinear Optics of Unique Laser System of the Russian Academy of Science.
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