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We present what to our knowledge is the first demonstration of self-pumped phase conjugation in a laser-pumped solid-state Rhodamine-6G dye saturable amplifier. Phase-conjugate energy reflectivity of as much as 2100% at 557 nm has been obtained.
©2003 Optical Society of America
Self-pumped phase conjugation (SPPC) of an external input beam by use of a laser amplifier in self-intersecting loop geometry is particularly attractive because of its simplicity and efficiency [1–5]. Until now, SPPC has been demonstrated for several laser amplifiers, such as a flash-lamp-pumped Nd:YAG, a diode-pumped Nd:YVO4, and a laser-pumped Ti:sapphire.
Solid dye lasers [6–10], in which dye-doped polymer is used as a gain material, have received much attention, because they can offer compact, inexpensive, and tunable lasers in the visible region. However, their poor thermal properties including extremely low thermal conduction prevent power scaling and high-repetition operation. One solution for correcting the thermal effects is application of self-pumped phase-conjugate mirrors to the solid dye laser. In addition, the upper-level lifetime of dyes is much shorter (approximately nanoseconds) than that of Nd:YAG (~200 µs). The very short upper-level lifetime of dyes makes solid dye SPPC difficult to achieve.
In our previous reports [11,12] we demonstrated efficient phase conjugation by degenerate four-wave mixing in an injection-seeded solid dye laser. This system exhibits extraction of phase-conjugate energy of as much as ~140 times the total energy injected externally; however, it requires good spatial overlap between the injected beam and counterpropagating pump beams already existing in the cavity.
Here we present what we believe to be the first demonstration of a self-pumped phase-conjugate mirror with loop geometry in a solid dye laser amplifier. We obtained experimental phase-conjugate reflectivity of as much as 2100% at 557 nm.
The solid dye consisted of a matrix of poly(methyl methacrylate) with less than 0.1-wt.% Rhodamine-6G dye doping. It measured φ 50 mm × 8 mm. Population inversion was obtained by longitudinal laser pumping from both sides of the solid dye by use of a frequency-doubled Q-switched Nd:YAG laser with a pulse duration of 10 ns at 2.5 Hz. A commercial narrowband (<0.03 cm-1) Rhodamine-19 dye laser output was used as the injection beam for SPPC. The wavelength of the dye laser was 557 nm. The time delay between the frequency-doubled Nd:YAG laser and the dye laser pulse was controlled, thereby yielding the maximum amplification of the injected beam energy. Experimental values of single-pass energy gain for two pump levels of 400 and 200 mJ/cm2 were estimated to be 60 and 30, respectively. The saturation fluence Us (= ħω/σ) was 1.4 mJ/cm2.
A schematic diagram of the self-pumped phase-conjugate system with self-intersecting loop geometry is shown in Fig. 1. An injected pulse passes through the loop and encounters itself in the solid dye amplifier. If the injected pulse has sufficient coherence length, the self-intersection of the injected pulse forms an interference pattern that modulates the population inversion in the amplifier. The coherence length of the injected pulse was ~35 cm. Backward amplified spontaneous emission (ASE) is then diffracted from the gain grating to provide phase conjugation by a four-wave mixing process. The phase conjugation feeds back into the amplifier and emerges from the self-pumped phase-conjugate system. The self-intersecting loop is formed by three mirrors, M1, M2, and M3. M1 has high reflection for 557 nm and high transmission for 532 nm. M2 and M3 are total-reflection mirrors for 557 nm. The curvature of M3 is 1000 mm. A nonreciprocal transmission element (NRTE), consisting of a Faraday rotator (size, φ 2 mm × 20 mm) and a λ/2 plate placed between two Glan-air polarizers, provides high transmission loss in the clockwise direction but near-unity transmission in the counterclockwise direction.
Fig. 1. Experimental setup for self-pumped phase conjugation with double-pass geometry in a solid laser dye.
To make the loop as short as possible, the λ/2 plate was fixed to the Faraday rotator, and the loop length was thus ~27 cm. As a result, clockwise and counterclockwise transmission factors through NRTE were fixed to be 0.01 and 0.8 at 557 nm, respectively.
Figure 2 shows experimental phase-conjugate reflectivity and output energy as a function of input energy injected into the self-pumped phase-conjugate system at two levels of pumping energy. The injected input energy is normalized to the saturation energy Es = 44 µJ. A maximum phase-conjugate reflectivity of ~2100% was obtained for a single-pass gain of 60. Experimental peak reflectivity occurs near the threshold injected energy ~0.005 Es (0.2 µJ) for phase conjugation to occur. As the injected energy is increased, the phase-conjugate output energy increases because of the increase in diffraction efficiency by the gain grating and is limited eventually by the saturation of the amplifier itself. A maximum output energy of 71 µJ was obtained at an injected energy of ~E s (46 µJ). In the case of single-pass gain of 30, the maximum phase-conjugate reflectivity was ~53% for an injected energy of 1.6 µJ. A maximum output energy of 3 µJ for an injected energy of 16 µJ was extracted from the system.
Optimization of the clockwise and counterclockwise transmission factors through NRTE can allow the increase of phase-conjugate energy and the reduction of the injected beam energy required for obtaining the maximum phase-conjugate reflectivity.
Fig. 2. Experimental phase-conjugate reflectivity and output energy versus injected beam energy at (a) single-pass gain of 60 and (b) single-pass gain of 30.
To investigate the potential for phase correction in our self-pumped phase-conjugation system, we introduced a phase plate as a simulator of the aberration inside an optical pass for the injected beam. The spatial form of the phase conjugation is shown in Fig. 3. The phase conjugation was not at all affected by the aberration. The results show that the self-pumped phase-conjugation system exhibits good potential for phase correction. The measured frequency spectra of the injected beam and phase conjugation from the self-pumped phase-conjugation system are shown in Fig. 4. The spectral resolution of the employed fiber-coupled CCD spectrum analyzer was ~1–2 nm. Although the spectral resolution is not sufficient for quantitative discussion, it can be seen that the spectrum of phase conjugation is qualitatively consistent with that of the injected beam. A weaker peak at 563 nm in the spectrum of phase conjugation is due to ASE noise.
Moreover, temporal forms of the injected pulse and its phase conjugation are shown in Fig. 5. The injected energy was 0.5 µJ. The time taken for the phase-conjugate output to build up is ~5 ns. The phase conjugation exhibits a mode beating, which corresponds to the round-trip time of the self-intersecting loop of ~0.9 ns. These experiments show that the loop works as a ring resonator for phase conjugation that is oscillating [13].
Fig. 3. Experimental far-field patterns of (a) injected beam, (b) injected beam with the phase aberrator, and (c) phase conjugation.
Fig. 4. Frequency spectra of injected beam and phase-conjugate output from self-pumped phase-conjugate mirror.
In conclusion, we have demonstrated what to the best of our knowledge is self-pumped phase conjugation (SPPC) with a self-intersecting loop geometry in a Rhodamine-6G solid-laser-dye saturable amplifier pumped by a frequency-doubled Nd:YAG laser. A maximum phase-conjugate reflectivity of 2100% was obtained for a single-pass gain of 60. The maximum output energy was 71 µJ.
Acknowledgments
The authors acknowledge support from a scientific research grant-in-aid (10750027, 11555010) from the Ministry of Education, Science and Culture of Japan and from the Japan Society for the Promotion of Science.
References and links
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