Prism folded laser cavity with controlled intractivity beam polarization

A folded Q-switched laser cavity uses total internal reflections of prisms to fold the intracavity beam. The index of refraction and the orientation of the prisms are selected to induce a predetermined phase delay between the two orthogonal polarization components of the linearly polarized intracavity beam. The predetermined phase delay rotates the intracavity beam polarization to an orthogonal direction such that an intracavity polarizer rejects the beam and the laser is held off from lasing. An electro-optic Q-switch cell is intermittently turned "on" whereby a second predetermined phase delay is induced onto the intracavity beam. The combination of the two predetermined phase delays results in a beam polarization orientation that is not rejected by the polarizer and the laser is not held off from lasing.

FIELD OF THE INVENTION 
The present invention relates to laser systems, and in particular to a 
folded cavity Q-switched laser system. 
BACKGROUND OF THE INVENTION 
A Q-switched laser produces pulses of light at high peak powers. An 
intra-cavity shutter is closed, preventing laser action and allowing the 
energy in the laser gain medium to build up. The shutter is then opened 
quickly, causing the rapid build-up of the pulse of laser light. Such a 
shutter is called a Q-switch. 
A conventional Q-switched laser system 2 is illustrated in FIG. 1. The 
laser system 2 includes a resonant cavity formed by two optics: a high 
reflecting mirror (HR) 4 and an output coupling mirror (OC) 6. A gain 
medium 8 is placed inside the cavity. When a flashlamp means 9 for 
exciting the gain medium 8 is activated without a Q-switch device 
activated, an intracavity laser beam 10 is produced that circulates 
between the HR 4 and the OC 6. The laser beam 10 is amplified twice during 
each round trip through the cavity as it traverses the excited gain medium 
8. The OC 6 partially transmits a predetermined amount of the intracavity 
beam 10 to create the output beam 12. 
The polarizer 14 linearly polarizes the intracavity beam 10 by offering low 
optical loss for one (preferred) polarization component and high optical 
loss for the orthogonal one (non-preferred) polarization component. Under 
these conditions, beam 10 is forced to build up in the preferred 
polarization direction only. 
Pulsed output is created by intermittently "holding off" the laser. When 
the losses in the cavity exceed the gain produced by the gain medium 8, 
lasing in the cavity ceases, and the laser is "held off". When gain 
exceeds losses, lasing begins again. 
To hold off a high gain laser, an electro-optic Q-switch device (EO cell) 
16 is placed in the cavity. When high voltage is applied to the EO cell 16 
by driver 17 (the cell is on), it acts as a wave plate, thereby having a 
slow polarization axis and a fast polarization axis. Light polarized in 
the direction of the slow polarization axis travels slower through the EO 
cell 16 and becomes phase delayed relative to light polarized in the 
direction of the fast polarization axis, which travels faster through the 
EO cell 16. When no voltage is applied to the EO cell 16 (the cell is 
off), no phase delay occurs. The phase delay induced by an EO cell 16 is 
proportional to the voltage applied to the EO cell 16. 
FIG. 2 illustrates the gradual polarization changes caused by various phase 
delays induced upon linearly polarized light having a polarization 
direction 45.degree. to the fast and slow axes of an EO cell 16. As the 
phase delay is increased from 0.degree., elliptically polarized light is 
created. The elliptical polarization becomes circularly polarize light 
when the phase delay is 90.degree.. As the phase delay is increased from 
90.degree., elliptically polarized light is created whereby the elongated 
axis is orthogonal to the elongated axis of the elliptical polarized light 
created by phase delays of less than 90.degree.. At 180.degree. of phase 
delay, linearly polarized light is created which is orthogonal to the 
linearly polarized light at 0.degree. phase delay. Accordingly, a phase 
delay of 180.degree. results in a 90.degree. rotation of the linear 
polarization direction. 
The phase delay effect of the EO cell 16 is used to rotate the linear 
polarization direction of the intracavity beam 10 by 90.degree. such that 
the polarizer 14 rejects the beam. The loss produced by the polarizer 14 
is sufficient to hold off the laser. When the EO cell 16 is in its "off" 
state, there is no polarization rotation, and the laser is no longer held 
off. 
While the laser is held off and the gain medium 8 is being excited 
(pumped), a large amount of energy is stored in the gain medium 8, since 
there is no intracavity laser beam to extract such energy. When the 
Q-switch state is changed quickly, losses drop and lasing begins. The 
intracavity beam 10 quickly builds and extracts the stored energy thereby 
producing a very high power light pulse. 
In the embodiment shown in FIG. 1, the EO cell 16 is turned off to generate 
the pulse, and turned on to hold off the laser. There are drawbacks to 
this type of design. First, an EO cell driver 17 is required that can 
rapidly switch the high voltage off to turn off the EO cell 16 fast enough 
to generate the shortest light pulse. It is easier to design a driver 17 
that turns the high voltage on quickly, rather than one that turns the 
high voltage off quickly. Secondly, a high voltage must be applied to the 
EO cell 16 during the relatively long period of time while the optical 
gain medium is being pumped. Since EO cells tend to degrade when high 
voltages are excessively applied, it is not beneficial to have the 
Q-switch turned on to hold off the laser. 
The above mentioned problems are solved by inserting a quarter-wave plate 
18 into the cavity, as shown in FIG. 3, where the laser is held off when 
the EO cell 16 is off, and the light pulse is generated when the EO cell 
16 is turned on. During a round trip, the quarter-wave plate 18 rotates 
the beam polarization by 90.degree., which holds off the laser. When the 
EO cell is turned on, it either adds an additional 90.degree. polarization 
rotation, or it adds a polarization rotation equal and opposite to the 
quarter-wave plate induced rotation, either of which switches the laser 
on. Therefore, a less complicated driver can be used, and degradation of 
the EO cell can be avoided (since the EO cell voltage is applied only when 
the pulse is being generated). The drawback to this cavity configuration 
is the requirement of an additional intracavity element. 
There is a need for a Q-switched laser that does not use an intracavity 
passive quarter-wave plate whereby the pulse is generated when the EO cell 
16 is turned on, and the laser is held off when the EO cell 16 is off. 
A drawback to both of the above described embodiments is that the cavity is 
too long for many applications. Simply shortening the cavity is not a 
viable solution because certain intracavity dimensions are required to 
obtain the desired output beam 12. For example, shortening the cavity can 
increase the divergence of the output beam 12. 
A prior art solution to reduce the dimensions of the cavity is a folded 
cavity configuration, as illustrated in FIG. 4. Two turning mirrors 20 are 
used to create a "U" shaped cavity, thereby reducing the overall length of 
the laser head. The use of turning mirrors 20 in the cavity, however, 
presents several problems. First, intracavity mirrors 20 coated to reflect 
at 45.degree. can damage easily. The high intracavity powers, especially 
with Q-switched lasers, can degrade such optical coatings. Further, and 
more importantly, 45.degree. turning mirrors 20 alter the polarization 
state of the reflected intracavity beam in a way that is difficult to 
control. The effect on the polarization state can vary from one coating to 
the next for the same optical coating design. Therefore, laser cavities 
sensitive to the polarization of the intracavity beam, such as EO cell 
Q-switched lasers, cannot use folding mirrors without degradation to laser 
performance and reliability. 
There is a need for a folded laser cavity in which the turning optics do 
not alter the polarization state of the intracavity beam in an 
uncontrollable way, and are more reliable than 45.degree. surface coated 
optics. 
SUMMARY OF THE INVENTION 
The present invention solves the aforementioned problems by using prisms to 
turn the intracavity laser beam in a folded Q-switched laser cavity. 
The Q-switched laser cavity of the present invention includes a folded 
resonant cavity having a gain medium, a polarizer, an EO cell, and turning 
prisms. The turning prisms reflect the intracavity beam by total internal 
reflection (TIR). Each TIR phase shifts the beam by a known, controllable 
amount, depending upon angle of incidence and refractive index of the 
prisms. The angle of incidence and refractive index are selected to induce 
a predetermined phase delay of 180.degree. in the linearly polarized 
intracavity beam during each round trip. The resulting beam after the 
phase delay is therefore polarized orthogonally to its original 
polarization direction, which is subsequently rejected by the polarizer to 
hold off the laser. The EO cell is then turned on to induce an additional 
180.degree. phase delay, or subtract out the 180.degree. prism induced 
phase delay, during each round trip. The resulting beam has a polarization 
orientation that is not rejected by the polarizer. 
The EO cell is turned on to create the laser beam pulse, and turned off to 
hold off the laser. Therefore, a passive quarter-wave plate is not 
required inside the cavity to utilize a simplified EO cell driver. 
An alternate embodiment of the present invention includes a polarization 
sensitive gain medium, wherein a separate intracavity polarizer element is 
not needed in the cavity. Such a gain medium amplifies two orthogonal 
polarization components, but one much more than the other. The result is 
that the lesser amplified polarization component never oscillates in the 
cavity because the gain for that component never reaches the cavity 
losses. Therefore, the gain medium causes linearly polarized light to 
oscillate in the cavity without the use of linear polarizer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiment of the present invention is a folded cavity, 
Q-switched laser system 2, as shown in FIG. 5. 
The resonant cavity is formed by a high reflecting mirror (HR) 4, two 
turning prisms 30, and an output coupling mirror (OC) 6. Inside the cavity 
there is a gain medium 8, a polarizer 14, and a Q-switch electro-optic 
cell (EO cell) 16. 
The polarizer 14 produces an intracavity beam 10 linearly polarized in a 
preferred direction. Since the laser will naturally oscillate with light 
polarized in the orientation having the highest intracavity gain, the 
polarizer 14 effectively rejects the polarization component orthogonal to 
the preferred polarization direction by introducing loss to that 
orthogonal component. The polarizer 14 transmits the preferred 
polarization component with virtually no loss. 
A commonly used polarizer is a Brewster plate polarizer 32, as shown in 
FIG. 6. A Brewster plate polarizer 32 is a transparent plate, such as 
glass or fused silica, that is tipped at "Brewster's Angle" relative to 
the beam 10 in one axis only. The Brewster plate polarizer defines a plane 
of incidence, which is the plane defined by the input beam 10 and the 
normal vector 35 (to the plate surface 34). The incident light has two 
orthogonal polarization components, S and P. The S component is 
perpendicular to the plane of incidence, and the P component is parallel 
(or in) the plane of incidence The S component of the beam 10 is partially 
reflected at both plate surfaces 34. The P component (preferred direction) 
of the beam 10 is transmitted through the plate 32 with virtually no loss. 
There are four reflected beams illustrated by FIG. 6 because the beam 10 
is travelling in both directions. 
The loss caused by the S component reflections at the plate 32 is usually 
enough to prevent the S component of light from circulating in the cavity. 
If greater losses are required, such as in higher gain lasers, one or both 
of the plate surfaces 34 can be coated to increase the reflectivity for 
the S polarization component of the light. 
As discussed below, it is desirous that the linearly polarized intracavity 
beam 10 have equal vertical and horizontal components. Therefore, the axis 
in which the plate 32 is tipped at Brewster's angle, which is in the plane 
of the plate 32, is 45.degree. from vertical. 
The EO cell 16 is placed between the Brewster plate polarizer 32 and the 
prisms 30. Materials such as KD*P or lithium niobate serve as excellent 
Q-switch cells. Driver 17 applies high voltage to the EO cell 16, causing 
the EO cell 16 to have a fast axis and an orthogonal slow axis. The EO 
cell 16 is rotated about the beam so that the fast and slow axes are 
oriented 45.degree. to the direction of the P polarization component 
passed by the Brewster plate polarizer 32. 
Turning prisms 30 are used to fold the cavity by reflecting the intracavity 
beam 10 by total internal reflection (TIR). The prisms 30 are each 
oriented to turn the beam 90.degree. in the horizontal plane. The prisms 
30 are made of glass having a refractive index of 1.56. 
A total internal reflection phase shifts the two polarization components of 
the incident light in a controllable way, as discussed in "Principles of 
Optics" by Max Born and Emil Wolf, Pergamon Press, sixth edition, 1980, p. 
50. The amount of relative phase shift is expressed by: 
##EQU1## 
where .delta. is the optical phase shift between the S and P components 
(at the TIR face), n is the refractive index of the optical material, and 
.theta. is the angle of incidence on the TIR surface. Therefore, as can be 
seen by the above formula, if the prisms are formed with material having a 
refractive index of 1.56, and the angle of incidence at the TIR surface is 
45.degree., then the phase shift per TIR is 45.degree.. 
When no voltage is on the EO cell 16, the EO cell 16 has no effect on the 
polarization of the beam 10. The beam 10 reflects through both prisms 30, 
the HR 4, and back through both prisms 30 in each round trip through the 
cavity, thereby encountering four TIR's that result in a total phase shift 
of 180.degree.. Since the beam 10 is polarized 45.degree. from the 
vertical, the beam 10 has equal S and P components. Therefore, the 
180.degree. phase shift corresponds to a 90.degree. rotation of the linear 
polarization direction of the beam 10 (See FIG. 2). The rotated beam 10 is 
rejected by the Brewster plate polarizer 32 which holds off the laser. 
When the "quarter-wave" voltage is applied by driver 17 to the EO cell 16, 
the EO cell 16 shifts the phase between the polarization components of 
beam 10 by 90.degree. per pass (180.degree. during each round trip). 
Therefore, with the EO cell 16 on, the prisms 30 and the EO cell 16 
combine to provide a total phase shift of 360.degree. during each round 
trip, which results in a 180.degree. rotation of the linear polarization 
direction. The 180.degree. rotated beam is linearly polarized light in the 
same direction as the original beam. Therefore, the 180.degree. rotated 
beam is transmitted by the Brewster plate polarizer 32 without loss. Thus, 
the laser is switched on when the EO cell 16 is turned on. 
To decrease intracavity losses, the prisms 30 are coated with an 
antireflection coating on surfaces 36 where the intracavity beam 10 enters 
and exits the prisms 30. As the angle of reflection changes, so too should 
the prism shape such that surfaces 36 are normal to the intracavity beam 
10 to minimize intracavity loss. 
Pursuant to the present invention, the preferred embodiment described above 
can be altered in terms of the turning angle(s), number of prisms, 
Brewster plate polarizer orientation, and prism refractive index so long 
as the combination can intermittently hold off and turn on the laser by 
operating the EO cell 16. For example, a single prism having the 
appropriate refractive index and angle of incidence can be used to fold 
the beam to cause a 180.degree. phase shift. Alternately, 3 or more prisms 
with the appropriate refractive index (or different indices) and angles of 
incidence can be used to fold the beam to cause the 180.degree. phase 
shift. Lastly, the EO cell 16 could alternate between two different 
voltages that result in different phase shifts. 
The preferred embodiment can also be altered to generate the light pulse 
when the EO cell 16 is off, and hold off the laser when the EO cell 16 is 
on. For example, adding a quarter-wave plate, and/or making appropriate 
changes regarding turning angle(s), number of prisms, Brewster plate 
polarizer orientation, and prism refractive index such that there is a 
total phase delay of 360.degree., 720.degree., etc. in each round trip 
when the EO cell 16 is off. 
In a second embodiment of the present invention, the gain medium 8 negates 
the need for the polarizer, as illustrated in FIG. 7. Certain materials 
used as the gain medium 8, such as Alexandrite, have a preferred crystal 
axis such that light polarized along this axis experiences higher gain 
than light polarized orthogonally. 
The Alexandrite gain medium (FIG. 7) is oriented in the cavity to 
preferentially amplify one of the two orthogonal polarization components 
that are oriented 45.degree. to the vertical. With the EO cell 16 turned 
on in the Alexandrite laser, both of the orthogonal polarization 
components experience either a 180.degree. polarization rotation, or no 
net polarization rotation, in each round trip. Therefore, the light 
component having its polarization direction aligned with the Alexandrite 
for amplification is preferentially amplified on every round trip. The 
result is that the laser is turned on. When the Q-switch cell is turned 
off, both orthogonal polarization components of the light beam 10 
experience a polarization rotation of 90.degree. on each round trip, so 
they exchange places each round-trip. The average round-trip gain for 
multiple passes is now lower, and the average gain is insufficient to 
overcome the intracavity losses (including high output coupling mirror 
transmission out of the cavity). 
The above described embodiments illustrate how the EO cell 16 induces an 
additional phase shift to the phase shift caused by the prisms to yield a 
total phase shift of 360.degree.. By swapping the slow and fast axis of 
the EO cell (by rotating the EO cell 16 by 90.degree. or switching the 
voltage leads on the cell), the EO cell 16 will induce a negative phase 
shift (relative to the prism induced phase shift), which in effect 
subtracts out the phase shift induced by the prisms on the beam 10. In 
that case, there is a 0.degree. net phase shift in each round trip when 
the EO cell 16 is on, yielding the same effect as a 360.degree. phase 
shift in each round trip. 
In practice, it may not be possible to use prisms with a refractive index 
that induces a total phase shift of 180.degree., either because of 
availability or because glass with that exact index incurs optical damage 
in Q-switched lasers. However, prisms having a refractive index that 
induce a total phase shift of somewhere near 180.degree. can sometimes 
still hold off the laser. In such a case, to turn "on" the laser, the 
voltage applied to the EO cell and the EO cell orientation in the cavity 
must be chosen to either induce a phase shift equal and opposite to that 
induced by the prisms to result in a net induced phase shift of 0.degree. 
in each round trip, or to induce a phase shift which, when combined with 
the prism induced phase shift, results in a net induced phase shift of 
360.degree. in each round trip. 
The Applicant has produced an Alexandrite laser system as shown in FIG. 7 
that outputs 0.75 J pulses at 10 Hz. The Q-switch cell 16 is made of KD*P 
with an applied voltage of .about.2000 volts. The EO cell 16 turns on in 
about 30 ns, and is kept on for about 1 microsecond. The output coupling 
mirror 6 is coated to reflect 35% and transmit 65% of the intracavity beam 
10. The Alexandrite rod is 4.times.90 mm, and is excited by a 1.2 Kw 
flashlamp. The prisms 30 are of BK-7 from Schott Optical Glass Inc., 
having a refractive index of 1.51, and were oriented to turn the beam 
90.degree. per reflection (45.degree. angle of incidence). The 1.51 
refractive index and 45.degree. angle of incidence result in a total prism 
induced phase shift of 154.degree., which is sufficient to hold off the 
laser. The applied voltage to the EO cell and the EO cell orientation were 
selected to result in subtracting the 154.degree. phase shift from the 
cavity when the EO cell is turned on. 
It is to be understood that the present invention is not limited to the 
embodiments described above and illustrated herein, but encompasses any 
and all variations falling within the scope of the appended claims.