JP4649614B2 - Phase conjugate mirror - Google Patents

Description

  The present invention relates to a phase conjugate mirror, and more particularly to a phase conjugate mirror that is preferably used for controlling an ultrashort pulse laser.

  The phase conjugate wave is light showing spatial reversibility, and when a phase conjugate mirror that generates a phase conjugate wave is used, various phase distortions that the light undergoes during propagation can be automatically compensated. For example, when this is applied to a high-power laser device, any thermal aberration generated in the laser device is automatically compensated, and a high-power laser can be generated while maintaining high beam quality.

  As a material for realizing this phase conjugate mirror, disordered crystals (vanadate crystals) have recently attracted attention. By utilizing the third-order nonlinear optical effect called saturation amplification effect exhibited by this laser crystal, it is considered that a highly functional phase conjugate mirror that simultaneously satisfies high reflectivity and high-speed response can be constructed.

  However, the line width of the laser transition of the vanadate crystal is as narrow as about 1 nm, and a broad-band laser such as an ultrashort pulse laser of sub-picosecond or less causes an extreme decrease in reflectance and pulse broadening due to spectral narrowing. At present, there is no practical phase conjugate mirror for ultrashort pulse lasers.

It is assumed that a Rh: BaTiO ₃ crystal is used as a phase conjugate mirror for a pulse laser, and this Rh concentration, crystal orientation, and refractive index lattice spacing are optimized to achieve a phase conjugate wave reflectivity exceeding 50% in the picosecond region. There is a report.

However, even when the phase conjugate mirror is used, the wavelength bandwidth and the response speed are in a trade-off relationship, and the refractive index wavelength dispersion of the BaTiO ₃ crystal is large. There remains a problem that time response and reflectance are low with respect to femtosecond pulses from second to second, and further, a light damage threshold is low due to a nonlinear phenomenon involving light absorption. In fact, a high-power laser using a photorefractive phase conjugate mirror has a limit of an average output of 50 W and a pulse energy of 1 mJ even in the picosecond region. Furthermore, it is very difficult to apply to ultrashort pulse lasers of sub-picosecond or less.

  Therefore, an object of the present invention is to provide a phase conjugate mirror that solves the above-described problems and exhibits higher reflectivity, wider bandwidth, faster response, and higher optical damage threshold.

  The present inventors diligently studied the above problem, and provided a phase conjugate mirror that exhibits higher reflectivity, broadband, faster response, and higher optical damage threshold by utilizing the gain saturation effect, which is one of the nonlinear optical effects. In view of this, in view of the fact that a phase conjugate mirror satisfying the above performance can be provided by using a plurality of vanadate crystals to form a mixed crystal, the present invention has been completed.

  That is, the present invention is characterized in that a vanadate mixed crystal is used in the phase conjugate mirror.

In this case, the vanadate mixed crystal preferably contains at least any two of Nd: YVO ₄ , Nd: GdVO ₄ , Nd: LuVO ₄ and Nd: LaVO ₄ , and further vanadate. The crystal is Nd: Gd _x Y _1-x VO ₄ , Nd: Lux _x Gd _1-x VO ₄ (0 <x <1), or Nd: La _x Gd _1-x VO ₄ (0 <x <1 ) Is also desirable.

  As described above, the present invention can solve the above-described problems and provide a phase conjugate mirror that exhibits higher reflectivity, wide band, high speed response, and high optical damage threshold.

  Hereinafter, the phase conjugate mirror according to the present embodiment will be described with reference to the drawings. It should be noted that the phase conjugate mirror of the present invention can be implemented in various modes, and of course is not limited to the embodiments described below.

First, FIG. 1 shows an outline of a phase conjugate mirror 1 according to this embodiment and an optical system using the same. The phase conjugate mirror 1 according to the present embodiment is provided to reflect the laser light amplified by the laser amplifier 2. Here, the phase conjugate mirror 1 is characterized by using a vanadate mixed crystal. The vanadate mixed crystal is a mixture of a plurality of crystals made of a complex oxide of rare earth elements and vanadium. Examples of this vanadate mixed crystal are not particularly limited, but for example, at least one of vanadate crystal groups represented by Nd: YVO ₄ , Nd: GdVO ₄ , Nd: LuVO ₄ and Nd: LaVO ₄ Those containing two are applicable (in this case, “Nd:” indicates that Nd ions are doped). Of course, it is possible to add other necessary configurations to these optical systems as long as necessary.

  FIG. 2 is a schematic perspective view of an aspect of the phase conjugate mirror 1. The phase conjugate mirror 1 shown in FIG. 2 includes a laser element 11 and a diode 12 that is an example of a light supply unit that introduces light from the lower surface of the laser element. Laser light is incident on the laser element, and a phase conjugate mirror is realized by reflecting these.

This phase conjugate mirror 1 uses the vanadate mixed crystal to make the most of the gain saturation effect and achieves the ultimate performance for near-infrared ultrashort pulse light ranging from subpicosecond to femtosecond (high reflectivity, ultra-wideband, Ultrafast response, high light damage threshold). The gain saturation effect is a nonlinear optical effect that originates from laser gain saturation that occurs via stimulated emission in a laser element. As long as the phase conjugate mirror has a sufficiently wide gain bandwidth, the phase conjugate mirror formed by the gain saturation effect is potentially orders of magnitude faster (less than 1 picosecond) and is subject to laser amplification. The phase conjugate reflectance becomes extremely large (greater than 100%). Furthermore, it shows performance far surpassing conventional phase conjugate mirrors, such as a high damage threshold (about GW / cm ² ) because it hardly involves absorption. Therefore, if this phase conjugate mirror is applied to a high-power laser, a high-quality ultrashort pulse laser exceeding an average output of 100 W and a pulse energy of 10 mJ becomes possible.

In consideration of the above, it is more preferable that the Banadata mixed crystal is a mixed crystal of Nd: GdVO ₄ crystal and Nd: YVO ₄ crystal (Nd: Gd _x Y _1-x VO ₄ ). This is because the center wavelengths of these fluorescence spectra are slightly different from 1063 nm and 1064 nm, and the mixed crystal can obtain a wide gain band. Moreover, it has a high thermal conductivity and is suitable for high average output. Further, a mixed crystal of Nd: LuVO ₄ crystal and Nd: GdVO ₄ crystal (Nd: Lux _x Gd _1-x VO ₄ ), a mixed crystal of Nd: LaVO ₄ crystal and Nd: GdVO ₄ crystal (Nd: La _x Gd _1-x VO ₄ ) is also preferable. In these mixed crystals, since the center wavelength of the fluorescence spectrum is shifted to the long wavelength side of 1066 nm, a wide gain low bandwidth of about 10 nm can be obtained. Table 1 below shows the stimulated emission cross section and the oscillation wavelength in each vanadate crystal.

In addition, this vanadate mixed crystal can appropriately adjust the stimulated emission cross section, the fluorescence spectrum, and the thermal conductivity by appropriately adjusting the combination of the above listed polycrystals and the mixed crystal ratio. A phase conjugate mirror that matches the performance of the pulsed light can be realized as appropriate. For example, in the case of Nd: Gd _x Y _1-x VO ₄ , Nd: La _x Gd _1-x VO ₄ and Nd: La _x Gd _1-x VO ₄ listed above, x is larger than 0 and smaller than 1, but these It is possible to optimize by appropriately adjusting.

  The vanadate mixed crystal can employ various manufacturing methods, and is not particularly limited. However, the physical constant of the vanadate mixed crystal greatly depends on parameters such as the concentration of ions to be added and the mixed crystal ratio. For this reason, it is preferable to manufacture by the floating zone method (FZ method) with high controllability. A well-known method can be adopted as the FZ method, and a schematic diagram of this method is shown in FIG.

  As described above, according to the present phase resonance mirror, it is possible to provide a phase conjugate mirror that exhibits higher reflectivity, wide bandwidth, high speed response, and high optical damage threshold.

The gain bandwidth was actually measured using vanadate mixed crystals. The vanadate mixed crystals that were specifically measured are two types, Nd: Gd _0.6 Y _0.4 VO ₄ and Nd: Gd _0.4 Y _0.6 VO ₄ . The fluorescence spectrum of these mixed crystals is shown in FIG. According to this result, the measured fluorescence spectra are all larger than 4 nm, and are several times wider than those of the Nd: YVO ₄ and Nd: GdVO ₄ crystals, respectively, and are converted to Fourier transform limit pulses, which are below 500 fs. I was able to confirm.

  Further, laser oscillation was actually performed in the CW and Q switch operations. FIG. 5 shows the results of the output Q-switch operation using the vanadate mixed crystal. According to this result, an output of 17 W was observed at the time of Q switching with respect to the excitation light power of 41 W. This is a high efficiency approaching 50% in terms of light-light conversion efficiency, and it was confirmed that it could be used as a laser crystal. In addition, the maximum repetition frequency at the time of Q switch is 700 kHz, and the possibility of practical use was confirmed. The same operation was performed in the CW operation, and an output of 19 W with respect to the excitation light power of 41 W could be observed during CW, and the same effect was confirmed.

  As described above, according to the present phase resonator mirror, it was confirmed that a phase conjugate mirror exhibiting higher reflectivity, broadband, high speed response, and high optical damage threshold can be provided.

The figure which shows the outline of the phase conjugate mirror 1 and an optical system using the same. The perspective schematic diagram of a phase conjugate mirror. The figure which shows the outline | summary of FZ method. The figure which shows the fluorescence spectrum of a vanadate mixed crystal. The figure which shows Q switch operation | movement using a vanadate mixed crystal.

Claims (1)

Nd: Gd _x Y _1-x VO ₄ (0.4 ≦ x ≦ 0.6) , Nd: Lu _x Gd _1-x VO ₄ ( 0.4 ≦ x ≦ 0.6 ) or Nd: La _x Gd _{1 A} phase conjugate mirror using a vanadate mixed crystal containing _−x VO ₄ ( 0.4 ≦ x ≦ 0.6).