Chrysoberyl solid state lasers

A chrysoberyl solid state laser comprising: PA1 (a) a rod-like laser medium composed of a chrysoberyl single crystal containing trivalent titanium ions as luminescent ions, the direction of c-axis of said crystal being made approximately the same as the longitudinal direction of the rod. PA1 (b) a means for generating excitation light for exciting said luminescent ions to emit light therefrom. PA1 (c) a means for focussing excitation light, generated by said means for generating excitation light, in said laser medium, and PA1 (d) a light resonator for generating oscillated laser light by resonating the light emitted from said luminescent ions with said focussed excitation light.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to chrysoberyl solid state lasers and more 
particularly it relates to such lasers which are provided with a laser 
medium consisting of chrysoberyl (BeAl.sub.2 O.sub.4) single crystal 
containing trivalent titanium ions as luminescent ions, an excitation 
light emission means, an excitation light focussing means and a light 
resonator and in which the directions of crystal axes of the chrysoberyl 
single crystal constituting said laser medium are specified. 
2. Description of the Prior Art 
Solid state lasers are being increasingly used in various industrial fields 
because of their small size, high output power, easy maintenance and 
excellent stability. 
Among the solid state lasers, those having trivalent titanium ions as 
luminescent ions are expected to be used in various fields since they can 
be tuned so that they oscillate to produce a wavelength over a continuous 
and extremely wide range. Among others, chrysoberyl single crystals doped 
with trivalent titanium ions have been proposed for use as very promising 
solid state lasers media as described in U.S. Pat. No. 4,765,925 which 
corresponds to Japanese Pat. Appln. Laid-Open Gazette No. Sho.62-216286 
assigned to the same assignee as the present application. This single 
crystal has a wide absorption band, the center of wavelengths of which is 
500 nm, it emits light in a wide region of from 600 nm to over 950 nm, and 
wavelength-variable solid state lasers having wavelength tunability in the 
above wide region are expected to be realized. 
Further, the present inventors have also proposed a solid state laser in 
which excitation light emitted from a source of excitation light is 
focussed on the laser medium with a focussing lens and then luminescent 
ions are excited in the laser medium thereby to emit oscillated laser 
light. This solid state lasers, however, is not satisfactory in size, 
output etc. 
This invention was made in view of the above circumstances. 
The primary object of this invention is to provide chrysoberyl solid state 
lasers which are further improved in energy efficiency over the above 
conventional lasers. 
SUMMARY OF THE INVENTION 
The above object of this invention is achieved by the provision of 
chrysoberyl solid state lasers which are provided with a laser medium 
consisting of chrysoberyl single crystals containing trivalent titanium 
ions as luminescent ions, a means for generating excitation light, and a 
means for focussing excitation light and a light resonator and in which 
the directions of crystal axes of the chrysoberyl single crystal 
constituting the laser medium are specified. 
More specifically, the crux of this invention resides in a chrysoberyl 
solid state lasers which is provided with: 
(a) a rod-like laser medium composed of a chrysoberyl single crystal 
containing trivalent titanium ions as luminescent ions, the direction of 
c-axis of said crystal being made approximately the same as the 
longitudinal direction of the rod, 
(b) a means for generating excitation light for exciting said luminescent 
ions to emit light therefrom, 
(c) a means for focussing excitation light, generated by said means for 
generating excitation light, in said laser medium, and 
(d) a light resonator for generating oscillated laser light by resonating 
the light emitted from said luminescent ions with said focussed excitation 
light, and in which the laser medium is disposed so that the direction of 
b-axis of the chrysoberyl single crystal constituting the laser medium is 
approximately perpendicular to the plane including both the longitudinal 
direction of the laser medium and that of the means for generating 
excitation light. 
The trivalent titanium ion-containing chrysoberyl single crystal which is 
used as a laser medium in this invention belongs to the orthorhombic 
system and is represented by space group D.sub.2h.sup.16 -Pmnb, and the 
lattice constant thereof is a=0.5476 nm, b=0.9404 nm and c=0.4425 nm. 
Further, in such a chrysoberyl single crystal, the a-axis, b-axis and 
c-axis are perpendicular to one another in their respective axial 
directions. 
The chrysoberyl single crystal used in this invention is prepared by being 
grown by a Czochralski technique or a floating zone melting technique, and 
the former is suitably used in order to prepare a chrysoberyl single 
crystal of large size and high quality. In one example of the Czochralski 
technique, starting materials to be described later are introduced into an 
iridium-made crucible which is then placed in a high-frequency induction 
heating type Czochralski furnace or the like to completely melt the 
starting materials, after which seed crystals are contacted with the 
surface of the melted materials while rotating the seed crystal and the 
seed crystal so contacted is slowly pulled up from the crucible to grow 
chrysoberyl single crystal, this technique being a generally-used one. In 
this case, there can satisfactorily be obtained a trivalent titanium 
ion-containing chrysoberyl single crystal which is free of inclusions and 
segregated matters by selecting the conditions for growth. 
The atmosphere in which the growth is carried out may generally be a 
hydrogen gas, nitrogen gas or argon gas atmosphere, or an atmosphere of a 
mixture of these gases, and it is preferable that the partial pressure of 
oxygen in the system for growth be in the range of from 10.sup.-9 to 
10.sup.-17 atm. 
The starting materials previously described are beryllium oxide (BeO), 
aluminum oxide (Al.sub.2 O.sub.3) and, in addition, titanium (III) oxide 
(Ti.sub.2 O.sub.3) as a luminescent ion. It is preferred that trivalent 
titanium ions be contained in an amount by weight of 0.01-1.0% in the 
single crystal. If the content of trivalent titanium ions is less than 
0.01 wt. % then the luminescence will be weakened, and, on the other hand, 
if the content thereof is more than 1.0 wt. % then the residual absorption 
coefficient, in an oscillation wavelength region, of the resulting matrix 
composed of chrysoberyl single crystal will undesirably increase. 
The thus obtained chrysoberyl single crystal is worked to obtain a rod 
which is cut at both the ends and further optically polished at both the 
ends to obtain a laser medium. When the chrysoberyl single crystal is to 
be worked to obtain the rod, it is worked so that its c-axis direction 
becomes the longitudinal direction of the resulting rod-like laser medium. 
There are no particular restriction in the cutting of both the ends of the 
rod and, however, the cutting may be effected perpendicularly to the 
longitudinal direction of the rod or may be effected to make a Brewster's 
angle with respect to the axial direction thereof. In addition, both the 
ends so polished may be subjected to AR (antireflection) coating as 
required. 
It is necessary that the solid state laser of this invention have a means 
for generating excitation light to excite the luminescent ions in the 
chrysoberyl single crystal. The excitation light generating means is 
required to have light emission spectra corresponding to the absorption 
bands of the Ti.sup.3+ and may be a straight-tube type flashlamp, arc lamp 
or like lamp in which xenon (Xe), krypton (Kr) or like gas is used. 
It is further necessary that the solid state laser of this invention have a 
means for focussing excitation light in the chrysoberyl single crystal. 
The excitation light focussing means may be such a one as to enable the 
excitation light emitted from the excitation light generating means to be 
focussed in the laser medium, and it is exemplified by a single elliptic 
cylinder-shaped or double elliptic cylinder-shaped reflection focussing 
mirror. The reflection mirror surface is required to have satisfactory 
reflection properties in a wavelength region corresponding to the 
absorption bands of Ti.sup.3+ and may be such a one that is plated with 
gold, silver, aluminum or the like. 
It is still further necessary that the solid state laser of this invention 
have a light resonator for generating oscillated laser light by resonating 
light emitted from the luminescent ions, and the light resonator may be of 
any optional construction and may have the same construction as a 
normally-used conventional light resonator. 
The chrysoberyl solid state laser of this invention is provided with the 
above-mentioned laser medium, means for generating excitation light, means 
for focussing excitation light and light resonator, and it is more 
preferable that said chrysoberyl solid state laser be such that the 
direction of b-axis of the chrysoberyl single crystal constituting the 
laser medium is approximately perpendicular to the plane including both 
the longitudinal direction of the laser medium and that of the excitation 
light-generating means. 
The present chrysoberyl solid state laser is not particularly restricted in 
the other constructional parts thereof, and it may have the same other 
constructional parts, such as a prism, Q switch or other oscillation 
control elements placed in the light path of the light resonator, as the 
conventional ordinary solid state lasers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention will be better understood by the following Examples. 
EXAMPLE 1 
The starting materials, which consisted of a 99.99% pure beryllium oxide, a 
99.999% pure aluminum oxide and a 99.9% pure titanium (III) oxide in 
respective amounts of 19.5 wt. %, 79.4 wt. % and 1.1 wt. %, were 
introduced into an iridium-made crucible and melted in a high-frequency 
induction heating type Czocharlski furnace where a gas mixture prepared 
from nitrogen, hydrogen and a very small amount of water vapor (H.sub.2 O) 
so as to obtain a partial oxygen pressure of 10.sup.-11 atm. in the 
crystal-growing atmosphere had been introduced. After the starting 
materials were completely melted, a seed crystal was contacted with the 
surface of the melt while slowly rotating them and then pulled up at a 
velocity of 0.5 mm/hr from the crucible thereby to obtain chrysoberyl 
single crystals containing trivalent titanium ions. Further, since the 
melting point of the resulting chrysoberyl single crystal was 
approximately 1870.degree. C., the growth of the chrysoberyl single 
crystal was carried out in the neighborhood of this temperature. 
The thus obtained chrysoberyl single crystal was cut to obtain a rod having 
a dimension of 5 mm.phi..times.57 mml. In this case, the rod was prepared 
so that the direction of c-axis of the chrysoberyl single crystal agrees 
with the longitudinal direction of the rod, and was then cut at both the 
ends perpendicularly to the longitudinal direction. Furthermore, the cut 
surfaces of the rod ends were optically polished and then coated with a 
single-layered AR coat of MgF.sub.2 whose center wavelength was 830 nm, 
thereby to obtain a laser medium. The thus obtained laser medium was 
attached to a holder which was 360.degree. rotatable around, as the axis, 
the imaginary line connecting the centers of the end surfaces of the laser 
medium to each other. The laser medium so attached, together with 
excitation light sources and the like, form a solid state laser. FIG. 1 
shows the construction of an experimental apparatus using the said solid 
state laser. 
In FIG. 1, numerals 1 and 2 show a reflecting mirror and a reflection 
focussing mirror, respectively, while numerals 3 and 4 show excitation 
light sources, respectively. Numerals 5, 6 and 7 in the same figure, show 
a laser medium, a reflecting plane mirror and a path of the oscillated 
laser light, respectively. 
The reflecting mirror 1 is one which has a 2000 mm radius of curvature and 
which reflects 100% of light having a wavelength in the range of 830 
nm.+-.50 nm. The reflection focussing mirror 2 is a double elliptic 
reflection focussing mirror which is plated with silver at the inside and 
which has a common-focus elliptic shape in cross section. The excitation 
light sources 3 and 4 are xenon flashlamps with a pulse time of 10.sup.-5 
s (=10 .mu.s). The laser medium 5 is a rod of chrysoberyl single crystal. 
The reflecting plane mirror 6 is one which reflects 95% of light having a 
wavelength in the range of 830 nm.+-.50 nm. 
FIG. 2 is a view showing the positional relationship between the excitation 
light source and the laser medium, viewed in the axial direction of these 
two components. The two excitation light sources (flashlamps) and the 
laser medium (chrysoberyl crystal rod) are located so that the central 
axes of said three components constitute a common plane and are parallel 
to each other. In FIG. 2 (a), the a-axis of chrysoberyl is included in the 
said common plane, while in FIG. (b), not the a-axis but the b-axis is 
included. 
As shown in FIG. 1, the laser medium 5 is located so that the path 7 of 
light resonated in a light resonator composed of the reflecting mirror 1 
and the reflecting plane mirror 6, and the longitudinal direction of the 
laser medium 5 approximately agree with each other, and is also located on 
the common focus of the reflection focussing mirror 2 as shown in FIG. 2. 
The excitation light sources 3 and 4 are located on the other two focuses. 
The reflecting mirror 1 and the reflecting plane mirror 6 are located on 
the light path 7 of the laser light emitted from the laser medium 5 as 
explained before. 
Excitation lights emitted from the excitation light sources 3 and 4 are 
focussed on the laser medium 5 which is located at the common focus. The 
titanium ions in the laser medium 5 are then excited by the excitation 
lights to emit light. After this emitted light is resonated by oscillating 
between the reflecting mirror 1 and the reflecting plane mirror 6 (the 
path of resonated light being designated at numeral 7), the resonated 
light is outputted as oscillated laser light (wavelength 750-950 nm) from 
the light resonator. 
With this experimental apparatus, the rod-shaped laser medium 5 was rotated 
around the longitudinal axis thereof thereby to arrange the b-axis 
direction of the chrysoberyl single crystal perpendicularly to the plane 
including the c-axis direction of said single crystal and the longitudinal 
direction of the excitation light-generating means (FIG. 2 (a)), thereby 
measuring the intensity (mJ) of oscillated laser light obtained. The 
results obtained are shown by open circles " " in FIG. 3. 
EXAMPLE 2 
The procedure of Example 1 was followed except that the a-axis direction of 
the chrysoberyl single crystal was arranged perpendicularly to the plane 
including the longitudinal direction of the laser medium and that of the 
excitation light emission means (FIG. 2 (b)), thereby to measure the 
intensity of oscillated laser light 7 obtained. The results obtained are 
shown by closed circles " " in FIG. 3. 
These results of Examples 1 and 2 clearly show that using the laser of this 
invention, oscillated laser lights can be otained efficiently and, in 
addition, energy efficiency is further improved by arranging the b-axis 
direction of the chrysoberyl single crystal perpendicularly to the plane 
including the longitudinal direction of the laser medium and that of the 
excitation light emission means. 
EXAMPLE 3 (Variation) 
The reflection focussing mirror for excitation light has a co-focus 
elliptic shape in cross section and, however, the said mirror is not 
necessarily required to have such a shape in cross section and may have a 
single elliptic shape as shown in FIG. 4. In this case, energy efficiency 
can be further improved by arranging the b-axis direction of the 
chrysoberyl single crystal perpendicularly to the plane including the 
longitudinal direction of the laser medium and that of the excitation 
light emission means. 
Further, the laser medium is rod-like in shape and, however, it may be 
slab-like (plate-like) in shape for use as desired. 
EFFECT OF THE INVENTION 
As mentioned above, chrysoberyl solid state lasers have been improved in 
energy efficiency by the present invention.