Laser diode pumped solid state laser with miniaturized quick disconnect laser head

A compact laser head for a solid state laser has a miniaturized laser rod and output coupling mirror which form a miniaturized laser cavity. A miniaturized frequency doubler crystal placed in the cavity provides frequency doubled output. The laser head is connected by an optical fiber to a separate power suply which contains a laser diode pumping source. A quick disconnect connector enables the fiber optic to be easily connected to the laser head. Pumping radiation is transmitted through the optical fiber to longitudinally end pump the laser rod using fiber coupling imagery. The fiber is aligned with the rod by the connector and the pumping radiation is imaged into the rod by a focussing sphere. The pumping volume is matched to the lasing volume which is determined by the cavity geometry. The quick disconnect laser head allows interchange of different heads with different output characteristics while using a single power supply.

BACKGROUND OF THE INVENTION 
The invention relates generally to solid state lasers such as Nd:YAG lasers 
and more particularly to compact packaging of solid state lasers. 
U.S. patent application Ser. No. 730,002, filed May 1, 1985, and CIP 
application Ser. No. 811,546, filed Dec. 19, 1985, described solid state 
lasers which included a laser rod end pumped by a laser diode. The pumping 
volume of the laser diode was matched to the laser rod to optimize pumping 
efficiency and the laser cavity was configured to provide a beam waist 
within the cavity at which a frequency doubler crystal could be placed. 
Tne laser diode was packaged in the same assembly. Each laser was designed 
to produce a particular output frequency determined by the material of the 
laser rod and the presence or absence of a doubler crystal. However, for 
the widest variety of applications and for the greatest ease of use, it is 
desirable to have a laser with the most compact packaging possible and a 
laser with interchangeable components so that a number of different output 
characteristics would be available from the same laser system. Since the 
output characteristics are largely determined by the design and components 
of the laser cavity, it is desirable to have a compact laser head which is 
a separate unit from the rest of the laser system and which can be readily 
coupled and decoupled to the rest of the system. Thus laser heads 
producing different output characteristics can be readily substituted. It 
is also desirable to end pump the laser rod. 
U.S. Pat. Nos. 4,387,297 issued June 7, 1983 to Swartz et al. and 4,409,470 
issued Oct. 11, 1983 to Shepard et al. disclose a hand held typically 
gun-shaped laser-tube based laser scanning head. The head may also be 
streamlined or box-shaped. The head typically has a volume of 50-100 cubic 
inches and weights 1-2 pounds. The use of a semiconductor laser diode in 
place of a He-Ne laser tube allows the lower sizes of the indicated ranges 
to be achieved. Power supplies, scanning motors and mirrors, and other 
circuitry are all included in the scanner head. The head is coupled to 
other components such as computer and data storage circuitry through an 
electrical cable. 
U.S. Pat. No. 4,383,318 issued May 10, 1983 to Barry et al. shows a laser 
pumping system in which optic fibers in a fan-in arrangement concentrate 
energy from an array of LED's or diode lasers to points along the length 
of a laser rod. 
U.S. Pat. No. 4,035,742 issued July 12, 1977 to Schiffner shows a device 
for optically pumping solid state lasers having a waveguide between the 
pumping source and laser rod disposed at an angle to the surface of the 
rod determined by the index of refraction of the waveguide. 
U.S. Pat. No. 3,982,201 issued Sept. 21, 1976 to Rosenkrantz et al. shows 
an end pumped solid state laser in which a diode laser array is pulsed at 
a rate and duty cycle to produce Cw operation. 
SUMMARY OF THE INVENTION 
Accordingly it is an object of the invention to provide a solid state laser 
having a miniaturized laser head. 
It is also an object of the invention to provide a compact solid state 
laser head which may be easily connected or disconnected from a laser 
diode pumping source. 
It is a further object to provide a compact solid state laser head which is 
longitudinally end pumped. 
It is another object of the invention to provide a solid state laser system 
with readily interchangeable laser heads. 
The invention is a laser diode pumped solid state laser having compact 
packaging with a miniaturized laser head coupled through a fiber optic to 
a power supply which includes a laser diode. The laser head contains a 
laser rod mounted in a housing with optical components to define a laser 
cavity and provide output coupling. A quick disconnect coupling for the 
fiber optic to the laser head is provided and the laser head contains an 
imaging lens to image the output of the fiber optic into the laser rod to 
longitudinally end pump the laser rod. The fiber optic allows the laser 
rod to be end pumped oy the laser diode in a separate power supply by a 
pumping scheme using fiber coupling imagery. The laser head housing is 
made as small as possible and all components therein are miniaturized. The 
use of particular mounting means for the components, in particular ball 
and tube mounts, allows the use of very small components and the least 
amount of space. The components are positioned to match the pumping volume 
of the laser diode pumping source which is transmitted to the laser head 
through an optic fiber and imaged into the laser rod to the lasing volume 
of the rod. By position and geometry of the optical elements defining the 
laser cavity a desired beam profile in the cavity can be produced, which 
according to one aspect of the invention, are used to provide TEM.sub.OO 
output A frequency doubler crystal can also be mounted in the laser head 
in the optical cavity, preferably at a beam waist, to provide frequency 
doubled output. In accordance with the invention, various quick disconnect 
heads are readily interchangeable and operable with a single power supply 
which includes a laser diode pumping source. Each head can be designed to 
provide particular output characteristics. Thus a very versatile system is 
provided in which only the laser heads are interchanged. The small size of 
the laser head and the ability to move the laser head a distance from the 
power supply are highly advantageous for a variety of applications. 
Furthermore, the laser diode can be replaced when necessary without any 
adjustment or realignment of the laser head components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A quick disconnect compact solid state laser head 10 in accordance with the 
invention is shown in FIG. 1 The laser head 10 has a hollow housing 12 
which is preferably substantially cylindrical or tubular and typically 
made of stainless steel. At one end of housing 12 is end cap 14, typically 
made of plastic, e.g. teflon impregnated Delrin, which screws onto or is 
otherwise attached to housing 12. A mirror 16 is mounted at the end of 
housing 12 and inside end cap 14. Mirror 16 preferably has a concave inner 
surface and substantially flat outer surface. Mirror 16 forms a part of 
the laser optical cavity and is the output coupler for the laser cavity. 
Mirror 16 is held in a ball mount 18 which is rotatably mounted in end cap 
14 between beveled edge 20 of housing 12 and beveled edge 22 of end cap 
14. Ball mount 18 has a hollow tube 24 extending therefrom into the 
interior of housing 12. Set screws 26 extend through housing 12 and 
contact tube 24 so that the angular position of ball mount 18 can be 
adjusted; there are typically three or four set screws 26 spaced around 
the circumference of the housing. 
Near the opposite end of housing 12 is mounted a solid state laser rod 28 
which is held in a holder or mount 30 which fits within the housing 12; 
the rod 28 may be held in place by a set screw (not shown) which also 
stresses the rod to polarize the output. Alternatively, laser rod 28 can 
be mounted in a ball mount if desirable to adjust its angular orientation. 
Mount 30 also holds a spherical lens or focussing sphere 32 in a spaced 
relationship to laser rod 28; tne lens 32 may be epoxied in place. 
Spherical lens 32 is mounted against beveled edge 34 in end portion 36 of 
mount 30; end portion 36 is wider than the portion of mount 30 which holds 
laser rod 28. An end cap 38 is placed at the end of housing 12 and 
contains the end portion 36 of mount 30. End cap 38 is typically made of 
teflon impregnated Delrin. End cap 38 also contains coupling means 40 
which allow an optical fiber 42 to be connected to laser head 10. Coupling 
means 40 is preferably a standard fiber optic connector, either bayonet 
type or SMA (screw-on) type, e.g. Amphenol 905 and 906 series connectors 
from Allied Corp., or any other coupling means which provides fiber 
alignment and quick connect/disconnect. Coupling means 40 holds optical 
fiber 42 so that its end 44 is in close proximity to spherical lens 32. 
Laser rod 28, spherical lens 32 and the end 44 of optical fiber 42 are 
positioned so that the output of optical fiber 42 is imaged into laser rod 
28 to provide efficient longitudinal end pumping of laser rod 28. Coupling 
means 40 provides proper alignment of fiber 42 whicn is reliable each time 
the fiber is connected to the laser head. 
A frequency doubler crystal 46 may also be mounted in the housing 12 in 
order to produce a frequency doubled output. Doubler crystal 46 is mounted 
in a ball mount 48 which is held against beveled edge 50 on the interior 
of housing 12 by ball retainer ring 52 which is spring loaded by spring 54 
which is held by spring retainer 56 which is mounted in housing 12. Ball 
mount 48 has a hollow tube 58 extending therefrom longitudinally in 
housing 12. Set screws 60 extend through housing 12 and contact tube 58 so 
that the angular position of ball mount 48 can be adjusted; typically 
three or four set screws 60 are used. 
In accordance with the principles described in U.S. patent application Ser. 
No. 730,002, filed May 1, 1985, and CIP application Ser. No. 811,546, 
filed Dec. 19, 1985, which are herein incorporated by reference, and the 
packaging techniques of the present invention a very short optical cavity 
is produced. The optical cavity is defined by surface 62 of mirror 16 and 
surface 64 of laser rod 28. Surface 64 is transmissive to pumping 
radiation but reflective to the lasing output of laser rod 28 and the 
frequency doubled radiation in cases where the doubler crystal 48 is used. 
By proper selection of the curvature of the optical surfaces and the 
distances between the optical surfaces, the beam profile within the cavity 
is controlled. In particular a beam waist is formed within the cavity 
which provides the optimal position for placement of the doubler crystal 
46. Also by mode matching the beam profile to the cavity dimensions single 
transverse mode operation, e.g., TEM.sub.OO mode, can be achieved. 
The optical elements 16, 28, 46 are provided in housing 12 at the 
appropriate positions according to a particular cavity design. The 
elements are centered along the bore of housing 12. To perform the initial 
alignment of optical elements 16 and 46, ball mounts 18 and 48, 
respectively, are rotated. The angular adjustment of doubler crystal 46 in 
ball mount 48 is illustrated in FIG. 2. The crystal 46 is mounted in a 
channel through ball mount 48. Ball mount 48, typically made of teflon 
impregnated aluminum, is rotatably held between beveled edge 50 of housing 
12 and retaining ring 52. Tube 58 projects from ball mount 48 into the 
bore of housing 12. A plurality of set screws 60, typically three or four, 
extend through the housing 12 and contact tube 58. By adjustment of set 
screws 60, the tube 58 can be oriented in different positions, as 
illustrated, thereby rotating attached ball mount 48 and cnanging the 
orientation of crystal 46. These ball mounts provide a very compact 
configuration and ease of alignment; a ball mount could be used for the 
laser rod. 
A significant feature of the invention is the longitudinal pumping scheme 
using fiber coupling imagery. A view of the overall laser system 66 is 
shown in FIG. 3 in which laser head 10 is coupled by optical fiber 42 to a 
power supply 68. Power supply 68 contains a laser diode pumping source 
which is suitable for pumping the solid state laser rod in laser head 10. 
The pumping radiation is transmitted from power supply 68 to laser head 10 
by optical fiber 42. As shown in FIG. 1, the pumping radiation transmitted 
through optical fiber 42 is imaged by spherical lens 34 onto the end face 
64 of laser rod 28. In accordance with the invention the image size from 
the fiber is matched to the mode size in the laser rod. The image size 
from the fiber is determined by the fiber diameter and divergence of the 
light from the fiber. The distances from the spherical lens to the fiber 
and to the laser rod determine the imaging ratio. A spherical lens 
(focussing sphere) is preferred for its ease of centration in housing 12 
and for its lack of alignment problems. The lasing volume in the rod is 
determined by the cavity configuration, i.e., the length of the cavity and 
the curvature of the output coupler mirror and front surface of the laser 
rod. Thus for any desired cavity configuration, the pumping radiation from 
the fiber optic can be imaged into the desired lasing volume of the rod 
for the most efficient operation. The use of the fiber optic coupling 
allows the laser head to be very compact and contain only the optical 
elements while all the electronic and other elements, including the 
pumping source, can be placed in a separate, stationary power supply. 
Since the optical fiber can be quite long, this system configuration 
provides great flexibility in the use of the laser, making the laser head 
highly portable. Also because of the quick disconnect feature, different 
laser head can be readily interchanged. Thus a variety of different laser 
heads which have different output characteristics can be used, essentially 
giving the user the benefit of several different systems but without the 
expense and redundancy of entire separate systems since only a new laser 
head is required with the same power supply to have an entire new system. 
Since the laser head contains only the optical components, the 
availability of different outputs becomes relatively economic. Also down 
time in the case of a laser head failure is minimized since a replacement 
head can easily be substituted. A further advantage to the use of fiber 
optic coupling imagery for pumping the rod is that in the event that the 
pumping source must be replaced, the laser diodes can be easily replaced 
and matched into the fibers without need for realignment of the laser head 
since the imaging of the fiber into the rod is not affected. 
As an illustrative embodiment of the invention, a preferred laser head 
configuration is about 8.4 cm long and about 1.0 cm in diameter. The laser 
rod is a Nd:YAG crystal which is about 5 mm long and 3 mm in diameter. The 
spherical lens is 5 mm diameter; there is a space of about 1.8 mm between 
between the end of the fiber and the spherical lens and a space of about 3 
mm from the lens to the end of the laser rod. The doubler crystal is a KTP 
crystal about 5 mm by 3 mm by 3 mm; the doubler crystal is 2.2 cm from the 
laser rod and 3.1 cm from the output coupler mirror. A number of different 
optical fibers can be used, the smaller the fiber, the higher the 
brightness, but the greater difficulty in alignment. A 200 micron diameter 
fiber, e.g., NRC FC-PC, a 125 micron diameter fiber, e.g. Corning 1504, 
and a 100 micron diameter fiber, e.g., NRC FC-MLD, all available from 
Newport Research Corporation, Fountain Valley, Calif., can be used. In a 
particular embodiment, a 200 micron fiber is used with 1:1 imaging to 
produce a 200 micron diameter mode volume in a 3 mm diameter Nd:YAG laser 
rod. By mode matching mode size is then 200 micron, and only TEM.sub.OO 
output is obtained. The principles of the invention can be applied to form 
even smaller laser heads, as small as 4 cm length and 7 mm diameter. Laser 
rods with lengths of about 1 mm and doubler crystals with lengths of about 
2 mm can be used. 
Changes and modifications in the specifically described embodiments can be 
carried out without departing from tne scope of tne invention which is 
intended to be limited only by the scope of the appended claims.