Optical amplifier

An optical amplifier includes a plurality of amplifiers which are disposed between a first reflective mirror and a second reflective mirror of an optical resonator arranged with the first reflecting mirror and the second reflecting mirror in face-to-face relationship and are adapted to amplify light through induced emission; and a beam combining and separating device which is adapted to combine the light made incident thereupon from the plurality of amplifiers and cause the light to emerge therefrom in the direction of the second reflecting mirror and is also adapted to separate the light from incident thereupon after being reflected by the second reflecting mirror and cause the light to emerge therefrom in the respective directions of the amplifiers. As the combination and separation of the light are repeatedly effected, a beam of light having high energy amplified by the amplifiers is obtained.

BACKGROUND OF THE INVENTION: 
1. Field of the Invention: 
The present invention relates to an optical amplifier, and more 
particularly to an optical amplifier which is capable of emitting a beam 
of light having high energy and is suitably used as a light source for an 
optical recording apparatus for recording information such as a character 
on a recording medium. 
2. Description of the Related Art: 
As an apparatus for recording information such as a character on a 
recording material by means of a beam of light, a laser computer output 
microfilmer (lasercom) which directly records information such as a 
character on a recording material including a microfilm by effecting 
scanning with a laser beam on the basis of information outputted by a 
computer is known (Japanese Patent Laid-Open No. 67722/1975). This 
lasercom has an argon laser for oscillating a laser beam, an optical 
modulator for optically modulating the laser beam in response to the 
character information, a rotating polygon mirror for polarizing in the 
horizontal scanning direction the laser beam modulated by the optical 
modulator, and a galvanometer provided with a polarizing mirror for 
polarizing in the vertical scanning direction the light reflected from the 
rotating polygon mirror. The arrangement is such that the laser beam 
outputted from the optical modulator is used to effect two dimensional 
scanning on a recording material via a scanning lens by means of the 
rotating polygon mirror and the galvanometer, thereby recording 
information such as a character on the recording material. 
With the above-described lasercom, however, since an argon laser which 
cannot be on-off controlled, an optical modulator and the like are 
necessary, a proposal has recently been made to use a semiconductor laser 
instead of the argon laser. As such semiconductor lasers, SDL-2410, 
SDL-2420 (brandnames of Spectra Diode Labs Inc.) Series and the like are 
available. With respect to laser beams emitted from laser oscillating 
regions of such a semiconductor laser, it is known that a phase difference 
exists, and that, when the phase difference between laser beams emitted 
from the laser beam oscillating regions is 180.degree., two lobes are 
formed in the direction along a surface of a p-n junction in a far field 
pattern. Accordingly, even if a semiconductor laser which forms these two 
lobes is used as a light source for recording on a recording material, the 
laser beam fails to focus into one spot, so that it is impossible to 
realize an optical system having a high degree of resolution. In 
particular, in cases where character information or the like is recorded 
with dots on a microfilm by using a laser beam, resolving power on the 
order of 3,360 dots/7.2 mm is required, so that it is necessary to effect 
recording with dots with a very high degree of accuracy, with the result 
that the aforementioned two lobes present a problem. 
For this reason, a proposal has hitherto been made to use one lobe by 
cutting the other (Appl. Phys. Lett. 41 (12), Dec. 15, 1982). In addition, 
Japanese Patent Laid. Open No. 98320/1987 discloses an arrangement in 
which the two lobes are separated from each other after a laser beam is 
converted into a parallel bundle of rays, and the two lobes are then 
combined into one by using a reflecting mirror, a half-wave plate, and a 
polarized beam splitter. 
However, with the aforementioned conventional optical system in which one 
lobe is cut, there is a problem in that since one half of the beam for 
forming a lobe is cut, the light intensity of the laser beam emitted in 
the direction along the surface of the p-n junction is reduced to one half 
or thereabouts, so that the efficiency is poor. Consequently, the 
application of this optical system is difficult with respect to a 
recording material which requires high energy in recording as in the case 
of a heat. mode recording material such as a laser direct recording film 
(LDF). 
If the above-disclosed optical system in which the lobes are combined is 
used, it is difficult in practice to obtain a completely parallel bundle 
of rays, and if the optical path lengths of the individual separated lobes 
are not equal, the position of a beam waist at the time when the beam is 
focused by a final lens becomes offset from an optical axis, thereby 
making it difficult to effect focusing at a high energy density. 
SUMMARY OF THE INVENTION: 
Accordingly, an object of the present invention is to provide an optical 
amplifier which is capable of emitting a light beam having a high energy 
density, thereby overcoming the above-described drawbacks of the 
conventional art. 
To this end, in accordance with the present invention, there is provided an 
optical amplifier in which, as shown in FIG. 3, between a first reflecting 
mirror A and a second reflecting mirror B of an optical resonator arranged 
with the first reflecting mirror A and the second reflecting mirror B 
having a smaller reflectivity than that of the first reflecting mirror A 
in face-to-face relationship, there are disposed a plurality of amplifying 
means C.sub.1, C.sub.2, . . . for amplifying light by means of induced 
emission and a beam combining and separating means D which is adapted to 
combine the light made incident thereupon from the respective amplifying 
means C.sub.1, C.sub.2, . . . and cause the light to emerge therefrom in 
the direction of the second reflecting mirror B and is also adapted to 
separate the light made incident thereupon after being reflected by the 
second reflecting mirror B and cause the light to emerge therefrom in the 
direction of the respective amplifying means C.sub.1, C.sub.2, . . . . 
In accordance with the present invention, light emitted from the plurality 
of amplifying means C.sub.1, C.sub.2. . . is made incident upon and 
combined by a beam combining and separating means D and is then made to 
emerge therefrom in the direction of the second reflecting mirror B so as 
to be reflected thereby. Since the path of the light is reversible, the 
light reflected by the second reflecting mirror B passes along a return 
path and, after being made incident upon the beam combining and separating 
means D and separated by a combined light emerging point thereof, the 
light is made emergent in the direction of the amplifying means C.sub.1, 
C.sub.2. . . . Then, after passing through the amplifying means, the light 
is reflected by the first reflecting mirror A. Accordingly, the light 
reciprocates between the first reflecting mirror A and the second 
reflecting mirror B, and if the period of this reciprocation is set at an 
integral multiple of the oscillating period of an optical wave, the phase 
of the oscillation when the light returns to its original position becomes 
identical, and standing waves are generated and the light is amplified by 
induced emission. During the initial period of oscillation of the light, 
light having various phases and planes of polarization disperse in all 
directions from various positions owing to natural emission. However, the 
greater the amplitude of the light, the greater the rate of induced 
emission, so that strong waves are amplified while weak waves are 
absorbed. Thus, the waves are gradually unified into those of one mode, 
and are ultimately emitted to the outside after being transmitted through 
the second reflecting mirror. 
As described above, in accordance with the present invention, it is 
possible to obtain the advantage that a light beam of high energy can be 
obtained since the light amplified by the plurality of amplifying means is 
combined and output. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description of the 
invention when read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS: 
Referring now to the accompanying drawings, a detailed description will be 
given of the preferred embodiments of the present invention. As shown in 
FIGS. 1 and 2, a biaxial crystal 10 having parallel planes 10A, 10B 
perpendicular to a Z-axis is disposed in such a manner that these parallel 
planes constitute planes of incidence and emergence. As such a biaxial 
crystal, it is possible to use tridimite (SiO.sub.2), mica (K.sub.2 O 
Al.sub.2 O.sub.3.6SiO.sub.2.2H.sub.2 O), chrysoberyl (BeAl.sub.2 O.sub.4), 
forsterite (MgSi.sub.2 O.sub.4), aragonite (CaO.CO.sub.2), fluorine type 
topaz (Al.sub.2 SiO.sub.4 (F).sub.2), gypsum (CaO.SO.sub.3.2H.sub.2 O), or 
the like. These biaxial crystals have a property in which if light is made 
incident upon its plane perpendicular to the Z-axis, the incident light is 
refracted conically. In addition, it is also possible to use ADP (NH.sub.4 
H.sub.2 PO.sub.4) which displays conical refractiveness in an electric 
field. 
Wave plates 12A, 12B 12C for correcting the phase of light, collimators 
14A, 14B, 14C, and semiconductor lasers 16A, 16B, 16C each coated in such 
a manner that one specular surface thereof has a reflectivity of 
approximately 90% and the other specular surface thereof is nonreflective, 
are disposed in order on the side of a plane 10A of the biaxial crystal 10 
corresponding to the bottom of a conical refraction, and are arranged in 
such a manner as to be aligned with the periphery of the bottom of the 
conical refraction. The angle of the plane of polarization of a laser beam 
emitted from each of these semiconductor lasers is determined in 
correspondence with the position of incidence of the laser beam upon the 
plane 10A of the biaxial crystal 10 such that the light combined by the 
biaxial crystal 10 becomes circularly polarized light. 
A quarter wave plate 18, a polarized beam splitter 20 acting as a mode 
selector, and a second reflecting mirror 22 having a reflectivity of 
approximately 2-5% are arranged in order on the side of the plane 10B of 
the biaxial crystal 10 corresponding to an apex of the conical refraction. 
A junction plane of this polarized beam splitter 20 forms an angle of 
45.degree. with respect to an optical axis and allows only the laser beam 
polarized in a specific direction to pass therethrough in the direction of 
the second reflecting mirror 22. It should be noted that the specular 
surface of each of the semiconductor lasers having a reflectivity of 
approximately 90% serves as a first reflecting mirror, and this specular 
surface and the second reflecting mirror composes an optical resonator. 
The operation of this embodiment will be described hereinunder. Laser beams 
emitted from the semiconductor lasers 16A, 16B, 16C are made incident upon 
the biaxial crystal 10 via the collimators 14A, 14B, 14C and the wave 
plates 12A, 12B, 12C. The laser beams made incident upon the biaxial 
crystal 10 are refracted conically and are then combined and made emergent 
from the plane 10B as circularly polarized light. This circularly 
polarized light passes through the quarter wave plate 18 so as to be 
converted to linearly polarized light, is then transmitted through the 
polarized beam splitter 20, and is made incident upon the second 
reflecting mirror 22. At this time, part of the laser beam is reflected by 
the junction plane of the polarized beam splitter 20, and only the laser 
beam polarized in a specific direction is transmitted through the 
polarized beam splitter 20. The laser beam thus transmitted through the 
polarized beam splitter 20 is reflected by the second reflecting mirror 
22, passes through the same optical path as the forward path, and after 
being separated into linearly polarized light by the biaxial crystal 10 it 
is made incident upon the semiconductor lasers and is reflected by their 
specular surfaces. In this manner, the laser beam reciprocates between the 
specular surfaces of the semiconductor lasers and the second reflecting 
mirror, is amplified by the semiconductor lasers by means of induced 
emission, and is unified into waves of one mode by the polarized beam 
splitter. When the intensity of the laser beam reaches a predetermined 
level, the laser beam is transmitted through the second reflecting mirror 
and is thereby emitted to the outside. 
Referring now to FIG. 4, a description will be given of another embodiment 
of the present invention. In this embodiment, light amplifiers in which 
both specular surfaces of semiconductor lasers are coated so as to be 
nonreflective are used instead of the semiconductor lasers. As shown in 
FIGS. 5A and 5B, each of these light amplifiers is arranged as follows: An 
active region 28 is clamped by an n-type semiconductor region 24 and a 
p-type semiconductor region 26, and an anode 30 having a stripe width T is 
provided in the n-type semiconductor region 24, while a cathode 32 is 
provided in the p-type semiconductor region 26. Specular surfaces 34, 36 
are coated so as to be nonreflective. 
Light amplifiers 40A, 40B, 40C are arranged in the same positions as those 
of the semiconductor lasers 16A, 16B, 16C shown in FIGS. 1 and 2, and a 
first reflecting mirror 42 having a reflectivity of approximately 90% is 
disposed in such a manner as to oppose specular surfaces of the light 
amplifiers 40A, 40B, 40C. It should be noted that since in this embodiment 
the parts other than the first reflecting mirror and the light amplifiers 
are identical to those of the above-described embodiment, corresponding 
parts are denoted by the same reference numerals, and a description 
thereof will be omitted. In this embodiment as well, the laser beam is 
emitted in the same way as the above-described embodiment. 
Also, since the wave plates 12A, 12B, 12C in these embodiments are used for 
correcting the phase, they can be omitted in cases where the correction of 
the phase is not necessary. In addition, the number of the semiconductor 
lasers is not restricted to the one mentioned above, and more than two or 
three semiconductor lasers can be provided, in which case a laser beam 
having energy corresponding to the number of the semiconductor lasers can 
be obtained. Furthermore, although a description has been given of an 
example in which a biaxial crystal is used as the beam combining and 
separating means, it is possible to use a Wollaston prism, a Rochon prism, 
a Senarmont prism or the like in which two rectangular prisms of a 
uniaxial crystal are joined together and the incident light is separated 
into normal rays and abnormal rays. Moreover, although a description has 
been given of an example in which a polarized beam splitter is used as the 
beam selector, it is also possible to use an etalon or the like 
alternatively, or this beam selector may be omitted. Also, the present 
invention can be applied to a light source section for such as an optical 
disk, a thermal printer, or a laser printer.