Optical bistable device

A method of operating and apparatus for an optical bistable device is disclosed in which an active laser medium is placed between two mirrors within an optical resonator and bistable switching is achieved by operating the active resonator across the transition borderline between the stable and unstable resonator regions. Several different embodiments are disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to an optical bistable device (OBC) for inherent 
potential applications such as optical switching, optical memory, bistable 
logic, differential amplification, optical transistor, discrimination, 
clipping, limiting, pulse shaping, and for performing a number of optical 
digital data processing functions. More specifically, the present 
invention relates to an optical bistable device in which an active laser 
medium is placed between two mirrors within an optical resonator whereby 
bistable switching is provided by operating the active resonator across 
the transition borderline between the stable-unstable resonator regions. 
In conventional optical bistable devices (OBC), nonlinear Fabry-Perot 
resonators containing saturable absorbers or nonlinear refractive-index 
material are utilized. A schematic diagram of such a device is shown in 
FIG. 1(a) wherein a laser beam is used for power input. Typical 
characteristics of such an optical bistable system are illustrated in FIG. 
1(b). 
This type of optical bistable device is referred to as "intrinsic system". 
U.S. Pat. No. 3,610,731, issued on Oct. 5, 1971 to H. Seidel and U.S. Pat. 
No. 3,813,605, issued on May 28, 1974 to A. Szoeke, both disclose an 
intrinsic system containing saturable absorbers. Nonlinear Fabry-Perot 
resonators containing nonlinear refractive index materials were 
demonstrated and described by Gibbs et al in 36 Phys. Rev. Lett. 1135 
(1976). The phenomenon appearing in one of such intrinsic systems is 
described in "Laser Focus", April 1982, page 79, which is incorporated 
herein by reference. 
Another type of optical bistable device involves a so-called hybrid system. 
In contrast to the intrinsic systems mentioned above, the microscopic 
nonlinearity in hybrids is synthesized using electro-optic feedback. The 
hybrid optical bistable containing a medium with a nonlinear refractive 
index and an electrical feedback loop was initially suggested and 
experimentally illustrated by Smith and Turner in 30 Appl. Phys. Lett. 280 
(1977). An example of an electro-optic hybrid analogue of a dispersive 
optical bistable device is shown in FIG. 2. The operating principle of 
such systems is described in "Laser Focus", April 1982, page 81, which is 
incorporated herein by reference. 
As mentioned above, various types of optical bistable devices which have so 
far been demonstrated in either hybrid or intrinsic systems require 
nonlinear refraction as the microscopic nonlinearity. For intrinsic 
optical bistable systems, an external laser is generally required for 
operation because of the need for high intensity and/or interference, 
while hybrid systems which could be driven by a broad band source require 
electro-optic feedback circuits. 
More specifically, in the nonlinear Fabry-Perot resonators containing 
saturable absorbers, which can be referred to as absorptive OB, 
comparatively large changes in absorption are required to give rise to 
absorptive bistability. Generally, it is difficult to produce absorptive 
OB because the absorber must saturate to a low level of residual 
absorption as described in "Laser Focus", April 1982, on page 81. 
The Fabry-Perot resonator containing nonlinear refractive index material, 
which can be referred to as dispersive OB, requires nonlinear media 
showing a large intrinsic nonlinear index of refraction which is difficult 
to find. In addition, those systems requiring multiple-beam interference 
need relatively coherent light and hence will generally require external 
lasers for their operation. 
For the hybrid optical bistable system, since the microscopic nonlinearity 
required in the cavity is synthesized by using electro-optic feedback, the 
switching speed of the device is generally limited. In addition, an 
external laser beam is required in most of the hybrid optical bistable 
systems. 
Now, in the study of input vs. output power characteristics of a 
flashlamp-pumped Nd:GGG rod geometry laser, the present inventor has 
observed that the output laser power increases with increasing flashlamp 
input power. As the flashlamp input power exceeds a certain power level, 
the laser output power is reduced to an insignificant level and laser 
action ceases if the input power is further increased. At that stage, the 
flashlamp input power is reduced and no laser action is observed until the 
input power decreases to a second power level which is smaller than the 
certain power level. As the input power is further reduced below the 
second power level, laser action resumes. A hysteresis effect is 
reproducible. 
This OB phenomenon has not been noticed so far in prior art laser 
resonators, which may be because of the fact that laser operation in the 
region of stable-unstable configuration transition has never been 
carefully studied. 
The present discovery led to the development of a more general concept for 
active optical bistable laser devices, as described herein. 
SUMMARY AND OBJECTS OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide an 
optical bistable device which overcomes the shortcomings of prior art 
optical bistable devices as mentioned above. 
Another object of the present invention is to provide a new class of 
optical bistable devices which requires neither a saturable absorber nor 
nonlinear refractive-index material in the resonator. 
Yet another object of the present invention is to provide an innovative 
type of optical bistable device which requires no electro-optic feedback. 
Still another object of the present invention is to demonstrate optical 
bistable effects with an incoherent, broad band source or even with 
non-optical means. 
Another object of the present invention is to provide a new type of active 
bistable optical device which is composed simply of a Fabry-Perot 
resonator and an active laser medium with an excitation source. 
Briefly described, the present optical bistable device comprises an optical 
resonator comprising a pair of mirrors and an active laser medium 
exhibiting lens effects between the mirrors, and a light source, wherein 
laser-action-induced focusing or defocusing in the active laser medium 
provides the feedback necessary for optical bistability and hysteresis 
effects even with the use of noncoherent light sources. 
With these and other objects, advantages and features of the invention that 
must become hereinafter apparent, the nature of the invention must be more 
clearly understood by reference to the following detailed description of 
the invention, the appended claims and to the several drawings attached 
herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As mentioned previously, in the study of input vs output power 
characteristics of a flashlamp-pumped Nd:GGG rod geometry laser, the 
present inventor has observed that the output laser power increases with 
increasing flashlamp input power. As the flashlamp input power exceeded a 
certain level denoted by P.sub.in (down) as shown in FIG. 3, the laser 
output power was reduced to an insignificant level and laser action ceased 
if the input power was further increased. At that stage, the flashlamp 
input power was reduced and no laser action was observed until the input 
power decreased to a level denoted by P.sub.in (up) which was smaller than 
P.sub.in (down). As the input power was further reduced below P.sub.in 
(up), laser action resumed. A hysteresis effect was clear and 
reproducible. That discovery has led to the present invention, some 
embodiments of which are described hereinafter. 
Surprisingly, the active optical bistable device of the present invention 
does not need coherent light, for it exhibits optical bistable 
characteristics in response to even incoherent input light. In addition, 
the laser medium of the present active optical bistable device per se 
exhibits the optical bistable characteristics, so that the present device 
does not need any electro-optic feedback. 
The laser medium may be a gas (e.g. CO.sub.2, N.sub.2, H.sub.3, etc.), a 
liquid (e.g. organic dye) or a solid (e.g. activated laser crystals such 
as YAG, GGG, Ruby, GSGG, etc.), or semiconducting materials (such as GaAs, 
etc.). 
Referring now in detail to the drawings, wherein like parts are designated 
by like reference numerals throughout, there is illustrated in FIG. 4 a 
homogeneous active medium 10 in thin rectangular slab with pump sources 11 
and 12 on the opposite sides thereof for excitation of the active medium 
10. In one embodiment, the medium may comprise neodymium doped YAG 
(yttrium aluminum garnet), GGG (gadolinium gallium garnet), or GSGG 
(gadolinium scandium gallium garnet), or other activated laser crystals, 
while the pump sources 11 and 12 may comprise flashlamps or an LED (light 
emitting diode) array. 
Two optically plane parallel end faces 13 and 14 at each longitudinal end 
of slab 10 of active medium are coated with antireflection (AR) thin film 
at the lasing wavelength of the active medium 10. Mirrors 15 and 16 form 
an optical cavity which provides feedback for laser action in the medium 
10. The surface toward the resonator cavity of mirror 16 has a high 
reflectivity (HR) coating while that of mirror 15 has a partially 
transmissive coating. The radii of curvature of mirrors 15 and 16 are 
properly selected depending upon the operating condition of the bistable 
switching. Extending parallel to the longitudinal axis 17 of the body are 
two planes 18 and 19 to produce a channel for excitation of the active 
medium 10. The width of the active medium 10 is much larger than its 
thickness. 
The thickness of the active medium 10 is properly chosen such that the 
resonator can only allow the lowest order (spatial) mode to oscillate. 
In order to explain the basic principles of the invention, the theory 
necessary to analyze resonators that contain optical elements other than 
the end mirrors will first be explained. That theory will then be applied 
to the case of a resonator containing a laser medium. Under excitation, 
the laser medium acts like a lens of an effective focal length, f.sub.eff, 
which may be either positive or negative depending upon the laser medium 
and excitation processes. 
The pertinent parameters of a resonator equivalent to one with an internal 
thin lens are: 
EQU g.sub.1 =1-L.sub.2 /f-L.sub.0 /R.sub.1 g.sub.2 =1-L.sub.1 f-L.sub.0 
/R.sub.2 (1) 
where L.sub.0 =L.sub.1 +L2-(L.sub.1 L.sub.2 /f) and f is the focal length 
of the internal lens; L.sub.1 and L.sub.2 are the spacings between mirrors 
M.sub.1, M.sub.2 and the lens; and R.sub.1 and R.sub.2 are the radii of 
curvature of mirrors M.sub.1, M.sub.2. 
The stability condition of the active resonator can be then expressed by: 
EQU 0&lt;g.sub.1 g.sub.2 &lt;1 (2) 
All cavity configurations are unstable unless they correspond to points 
lying in the area, as illustrated by FIG. 12, enclosed by a branch of the 
hyperbola g.sub.1 g.sub.2 =1 and the coordinate axes g.sub.1 g.sub.2 =0. 
The research in the prior art is substantially directed to the stable 
region. 
The general theory of optical bistability of this invention is as follows. 
There is a laser medium whose effective focal length, f.sub.eff. depends 
upon both the pump (excitation) input power, P.sub.in, and the laser power 
in the resonator P.sub.ls. It is presumed that the condition in the 
precise function form of f.sub.eff against P.sub.in and P.sub.ls is 
written in its most general form: 
EQU 1/f.sub.eff =1/f(P.sub.in)+1/f(P.sub.ls); (3) 
where f(P.sub.in) and f(P.sub.ls) are the focal lengths due to pump- and 
laser-induced focusing (or defocusing) of the resonator, respectively. 
Note that both f(P.sub.in) and f(P.sub.ls) may be either positive or 
negative. 
The pump-induced lens effect may arise from nonuniform gain distribution 
due to nonuniform pumping, nonuniform temperature distribution and 
thermally-induced distortion. On the other hand, the laser-induced lens 
effect may be a result of a self-focusing (or self-defocusing), transverse 
spatial hole burning, heating of laser medium due to an absorption or 
laser energy, and optical distortion of the resonator mirrors due to local 
heating by laser radiation. 
Equation (3) is the key expression for optical bistability in the novel 
features of this invention. The effective focal length of the laser medium 
can presumably be expressed by equation (3) due to pump- and 
laser-action-induced lensing effects as mentioned above. As a result, the 
cavity g parameters of the active resonator as shown in equation (1) 
depend upon both the pump power and the laser power. In other words, under 
the same pumping conditions, the cavity g parameters with laser action 
taking place have different values from those without laser oscillation in 
action. 
The cavity g parameters have subsequent influence on the laser operation of 
the active resonator as shown in equation (2). The active resonator can 
generate coherent light output if the stability condition of the g 
parameters as shown in equation (2) is met. Under the circumstances when 
either g.sub.1 g.sub.2 &gt;1 or g.sub.1 g.sub.2 &lt;0 is satisfied, the cavity 
configuration becomes unstable and laser action ceases. The condition 
specified by g.sub.1 g.sub.2 =0 or g.sub.1 g.sub.2 =1 sets the borderline 
between the regions of stable and unstable configurations of the 
resonator. 
As the operating condition of the active resonator, which changes with both 
input power and laser power, crosses the borderline, the active resonator 
undergoes a phase transition, i.e. stable-unstable configuration 
transition or lasing-nonlasing state transition, where optical switching 
occurs. Note that the functions of the pump sources are two-fold. First, 
they produce population inversion (i.e. gain) in the laser medium, and 
secondly, they create a lensing effect in the laser medium, as shown by 
the first term on the right hand side of equation (3). In addition, the 
pump sources may take several different forms. For example, they may be in 
the form of a noncoherent light, e.g. a flash-lamp in a solid-state (YAG, 
ruby, or GGG) laser or in the form of an electrical current, e.g. the 
injection current in a semiconductor laser. 
The conditions for bistability (hysteretic behavior) of this invention 
depend upon the signs (positive or negative) of f(P.sub.in) and 
f(P.sub.ls). Using a graphing method (see FIG. 8), the value of 
1/f.sub.eff is plotted against P.sub.in with laser action as a parameter. 
The borderline condition for stable-unstable resonator transition (i.e., 
g.sub.1 g.sub.2 =1 or g.sub.1 g.sub.2 =0) can be obtained from Eq. (1), 
which is indicated by the dashed line in FIG. 8. Note that both 
f(P.sub.in) and f(P.sub.ls) are positive in this case. 
The regenerative action underlying the switching is as follows. Increasing 
the pump input power, P.sub.in, increases the 1/f.sub.eff value which is 
equal to 1/f(P.sub.in). Since the resonator is unstable, there is no laser 
action for P.sub.in below point A. At point C, the resonator condition 
changes from a stable to an unstable state and laser action ceases, which 
is indicated by the transition from point C to point D. The bistable 
switching is clearly seen. 
FIG. 9 shows bistable switching under the conditions where f(P.sub.in) is 
positive and f(P.sub.ls) is negative. FIG. 10 describes the hysteretic 
behavior for the situation where both f(P.sub.in) is negative and 
f(P.sub.ls) is positive. 
In order to clearly explain the present invention over the prior art 
intrinsic optical bistable system, some features are compared as follows. 
The optical resonator used in the intrinsic (passive) OB system is designed 
to provide a cavity for interference to take place such that the nonlinear 
Fabry-Perot interferometer can produce a transmission spectrum for the 
external laser beam where maximum transmission occurs under the condition 
that the external laser beam is on-resonance with the passive cavity while 
minimum transmission takes place under the condition that the external 
laser beam is off-resonance with the passive cavity, whereas the optical 
resonator used in the present invention is designed to provide an optical 
feedback in the active cavity for laser action to occur. 
In the prior art laser resonators, most of the efforts which have been made 
so far are directed to maintain the cavity g parameters at the same values 
throughout the operating region of the laser in order to retain constant 
laser beam parameters, e.g., spatial modes and beam divergence. Whereas, 
in the present invention, the operation of the laser resonator is designed 
in such a way that its cavity g parameters change with the operating 
conditions and deliberately make the g parameters cross the borderline for 
the stable-unstable configuration of the resonator as the operating 
condition varies to achieve the ON-to-OFF and OFF-to-ON transitions of 
laser action. 
Additional embodiments of the present optical bistable devices are 
described as follows, showing modifications of the present invention. 
The features of the embodiment shown in FIG. 5 are similar to those 
described in FIG. 4 except that the cavity mirrors are not used and the 
end faces 23 and 24 of the active medium 10 are constructed as cavity 
mirrors for laser oscillation. The end face 24 has a high reflectivity 
(HR) coating on it whereas the end face 23 is coated with partially 
transmissive film for output. 
Referring now to FIG. 6, another embodiment of the present invention 
comprises a rectangular semiconducting crystal (e.g. GaAs). The active 
laser region 30 is in the p-n junction between the p-type region 31 and 
the n-type region 32. Two plane faces 33 and 34 are metallized for making 
contact with bias leads 35 and 36 through which the injection current 37 
provides gain for the active medium in the p-n juction region 30. Two 
optical end faces 38 and 39 form a resonant cavity for laser oscillation 
with the p-n junction region 30. The radius of curvature of face 39 is 
properly selected so that the laser resonant cavity defined thereby is 
unstable when no current is injected. The end face 39 is coated with high 
reflectivity thin film at the laser wavelength. The end face 38 is either 
cleaved or polished for laser output. 
The laser described herein provides a bistable output controlled by the 
injection current. At low injection current, the resonant cavity is 
unstable. Therefore, no laser action is taking place. As the injection 
current reaches a certain level, the resonant cavity becomes stable due to 
pump-induced lensing in the active region and laser action occurs. That 
process completes the switching ON of the device. At that stage, if the 
injection current is reduced, the resonant cavity will become unstable at 
a lower injection current when laser action ceases. Such process concludes 
the OFF switching. 
FIG. 7(a) shows still another alternate embodiment of the present 
invention. A homogeneous active medium 40 which exhibits very weak or even 
no laser-induced lens effect is placed in an optical cavity formed by 
mirrors 41 and 42. A passive medium 43 showing strong laser-power (or 
intensity)-dependent lens effect is disposed in the cavity defined by 
mirrors 41 and 42. Two focusing lenses 44 and 45 are shown in FIG. (7a) 
are placed with the passive medium 43 sandwiched therebetween to increase 
laser power density in the passive medium 43 such that laser-induced lens 
effect in the passive medium 43 is enhanced. 
In FIG. 7(b), which also shows another embodiment of the present invention 
similar in manner to that shown in FIG. 7(a), the lensing enhancement 
described above is achieved by the curved surfaces 41 and 46. 
In the embodiments shown in FIGS. 7(a) and 7(b), the pump-induced lens 
effect required for bistable operation is provided by the active medium 
40, while the laser-induced lens effect is produced in the passive medium 
43. 
The active medium may be made of a gas (e.g., CO.sub.2, He and N.sub.2 and 
their mixtures), liquid (e.g. dye solution) or solid (e.g. Ti.sup.3+ - and 
Cr.sup.3+ -doped fluoride crystals). The passive medium 43 may likewise be 
made of a gas (e.g. CS.sub.2, liquid (e.g. Nitrobenzene), or solid (e.g. 
LiNbO.sub.3). 
While preferred embodiments of this invention have been shown and 
described, it will be appreciated that other embodiments will become 
apparent to those skilled in the art upon reading this disclosure, and, 
therefore, the invention is not to be limited by the disclosed 
embodiments.