Laser oscillator

Laser apparatus is described in which the optical cavity of a laser oscillator is specially provided with a polarizing device which has on its outer face a partially reflecting coating (such as a thin partially reflecting layer of low-loss dielectric material). The polarizing device serves both as an output coupler for narrow linewidth laser emission from the optical cavity and also as a means for substantially suppressing amplified spontaneous emission (ASE). A multiple-prism Littrow-mounted grating (MPL) dye laser apparatus embodying the invention achieves a laser linewidth of less than about 0.1 GHz (.DELTA..nu.), a ratio of intensity I of the ASE to the intensity I.sub..lambda. of the desired laser emission of about 10.sup.-10, an efficiency of somewhat more than 10%, and a short optical cavity length of less than 10 cm. The apparatus is also more rugged, more stable in frequency with changes in temperature, and less costly than similar previous lasers.

FIELD OF THE INVENTION 
The present invention relates to an improved tunable laser oscillator, and, 
more particularly, to a dye laser oscillator having low amplified 
spontaneous emission (ASE), narrow linewidth, good efficiency, and which 
is rugged and stable. 
BACKGROUND OF THE INVENTION 
In the inventor's U.S. Pat. No. 4,891,817, which has an assignee in common 
with the present patent application, there is described a multiple-prism, 
Littrow-mounted grating (MPL) dye laser oscillator. In addition to 
efficient narrow linewidth lasing (e.g., .DELTA..nu..ltoreq.1 GHz), this 
type of oscillator has the ability to yield a relatively low level of 
amplified spontaneous emission (ASE). A low ASE level is desirable for 
many applications including spectroscopy, isotope laser separation, and 
lidar and laser radar. 
Amplified spontaneous emission in a dye laser has a number of basic 
characteristics which distinguish it from laser emission. First, most ASE 
occurs in the early stages of the emission process prior to narrow 
linewidth lasing. This stochastic ASE radiation is much lower in intensity 
than the main laser radiation and exhibits very much higher divergence. 
The ASE radiation is very broadband and hence all parasitic broadband 
reflections or unwanted modes in the resonator (laser oscillator) cavity 
should be minimized. A more comprehensive discussion of ASE radiation is 
given in an article entitled "Flashlamp pumped narrow-linewidth dispersive 
dye laser oscillators: very low amplified spontaneous emission levels and 
reduction of linewidth instabilities", by F.J. Duarte, et. al., Applied 
Optics, Vol. 29, No. 21, Jul. 20, 1990, pps. 3176 to 3179. 
The free spectral range (FSR) of an optical cavity is defined as c/2L, 
where "c" is the velocity of light in free space (2.997925.times.10.sup.8 
m./sec.) and "L" is the optical length of the cavity. For a cavity length 
L of 10 cm., the FSR is 1.5 GHz. As will be explained more fully 
hereinafter, it is desirable that the FSR be large (larger than the 
dispersive linewidth). This in turn means that the optical length of the 
cavity should be made short (for example, less than 10 cm). 
For increased efficiency in isotope separation (for example, the separation 
of .sup.235 U from .sup.238 U) it is highly desirable that the linewidth 
of the laser emission be as narrow as possible, and tunable to an exact 
frequency. On the other hand, in medical treatment of internal tumors it 
is desirable to utilize a thin optical fiber in transmitting a laser beam 
to the point of application. This in turn means that the "quality" of the 
beam should be high in order for the beam to propagate efficiently through 
the optical fiber. And the "quality" of the beam is related to narrow 
linewidth. 
It is desirable to have an optical cavity in a dye laser apparatus which is 
shorter than that with previous arrangements, is simpler and less 
expensive, and has lower ASE along with narrow-linewidth laser emission 
and good efficiency. In addition, it is desirable to have an increase in 
ruggedness of the physical apparatus and greater temperature stability 
compared with prior arrangements. 
SUMMARY OF THE INVENTION 
In accordance with the present invention in one embodiment thereof, there 
is provided an MPL dye laser in which the partially reflecting mirror 
customarily used at the output of such a laser is replaced with a 
specially provided polarizer device, the outer face of which has a 
partially reflecting dielectric coating. This partially reflecting 
polarizer device has its plane of polarization aligned parallel to the 
plane of polarization of the laser emission from the optical cavity of the 
laser. Thus the desired narrow-linewidth laser emission is transmitted 
through the polarizer device, with a predetermined small percentage (e.g., 
5% to 20%) of the polarized laser light being reflected by the outer 
partially reflecting face back into the cavity. The reflected laser light 
energy provides positive feedback primarily only of the desired narrow 
linewidth laser emission from the cavity. The ASE randomly generated in 
the optical cavity, being unpolarized, is highly discriminated against by 
passage through and then back from the output polarizer device with its 
reflecting outer face. This arrangement thus substantially reduces the ASE 
within the cavity and provides a very low ratio of intensity I of ASE to 
the intensity I.sub..lambda. of the narrow linewidth laser emission 
(I.sub.ASE /I.sub..lambda.). Ratios of the order of 10.sup.-10 for 
I.sub.ASE /I.sub..lambda. are obtained. A further advantage of a short 
length optical cavity is that this increases the number of intracavity 
passes "R" of the laser light, and the increasing of R helps decrease the 
dispersive linewidth ".DELTA..lambda.". A full discussion of this effect 
is given in Chapter 4 of the book "Dye Laser Principles", edited by Duarte 
and Hillman, and published by Academic Press, 1990, (See particularly 
Equation 4.55 on page 161, and pages 173 et. seq.). In addition, because 
the effective length L of the optical cavity is shortened by the 
simplified arrangement of cavity elements provided by the invention, the 
free spectral range (FSR) is made greater than the dispersive linewidth. 
This in turn prevents unwanted multiple modes of lasing within the cavity. 
And the suppression of unwanted modes further contributes to the obtaining 
of narrow linewidth laser emission. The elimination of a separate 
non-polarizing output mirror not only reduces the number of physical 
elements employed in this new arrangement, but also increases the output 
efficiency of the laser. Because the length of the optical cavity has been 
shortened by the specially provided partially reflecting polarizer device, 
the effects of thermal expansion and contraction within the optical cavity 
on the output frequency of the laser are reduced. The invention is 
described in conjunction with an MPL dye laser oscillator. However the 
invention is also well suited to other laser oscillators such as a hybrid 
multiple-prism, grazing-incident (HMPGI) oscillator as shown and described 
in detail in the above-identified book by the inventor entitled "Dye Laser 
Principles". 
In accordance with a specific aspect of the invention there is provided a 
special output device for the optical cavity of a laser oscillator. The 
output device comprises a multi-prism polarizer the outer face of which is 
coated with a thin partially reflecting layer of a low-loss dielectric 
material. This results in ASE levels being substantially reduced, the 
length of the optical cavity made shorter, and the free spectral range 
made substantially greater than the dispersive linewidth of the laser 
compared to previous oscillators. 
By virtue of the present invention, it is possible to obtain in a given 
laser apparatus in combination the desirable features of low ASE, 
narrow-linewidth laser output, short length of optical cavity, good 
efficiency, together with the physical, thermal and cost advantages of a 
simplified arrangement of optical elements. 
A better understanding of the invention, together with its important 
advantages will best be gained from a study of the following description 
given in conjunction with the accompanying drawings and claims.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is shown (not to scale and partially broken 
away) a graphical schematic diagram of a relationship of free spectral 
range (FSR) with modes of lasing in an optical cavity. As mentioned 
previously, FSR is defined as c/2L, where "L" is the effective length of 
the optical cavity and "c" is the velocity of light in free space. By way 
of example, a first lasing mode 1 is indicated by a vertical dashed line 
12, and a second lasing mode 2 is indicated by a vertical dashed line 14. 
Mode 1 occurs at a frequency .nu..sub.1 and mode 2 occurs at a frequency 
.nu..sub.2. For an optical cavity length as short as 10 cm., the FSR is as 
wide as 1.5 GHz. Thus lasing frequency .nu..sub.1 is separated from lasing 
frequency .nu..sub.2 by 1.5 GHz. Centered around the frequency .nu..sub.1, 
and shown by way of example, is a desired narrow linewidth laser emission 
indicated at 15 and bounded by the solid, near-vertical lines 16 and 18. 
The amplitude or intensity I.sub..lambda. of this laser emission 15 is as 
indicated. The linewidth of this emission 15 is defined as the range of 
frequency (.DELTA..nu.) between the vertical line 16 and the vertical line 
18. It is highly desirable for the laser linewidth or .DELTA..nu., to be 
as narrow as possible As indicated here the laser linewidth .DELTA..nu. is 
less than 100 MHz. By way of example, the dispersive emission of the 
optical cavity of the laser apparatus 20 is schematically shown here by 
the near-Gaussian bell-shaped curve 19 (indicated by the dot-dashed line). 
The dispersive linewidth ".DELTA..lambda." (expressed as a range of 
wavelength .lambda. rather than frequency .nu.) is given by the equation 
4.55 on p. 161 of the above-identified book by the inventor entitled "Dye 
Laser Principles". Where multiple modes of lasing within the optical 
cavity are possible, the linewidth of the desired laser linewidth 15 is 
undesirably broadened into the linewidth .DELTA..lambda. of the dispersive 
linewidth 19. The dispersive linewidth .DELTA..lambda. is, for example, 
about 500 MHz. In the present configuration, the dispersive linewidth 19 
is narrower or smaller in frequency range than the FSR. With this the 
case, lasing mode 2 (dashed line 14) is not excited within the optical 
cavity. Therefore the laser emission 15 will not be undesirably broadened 
in its linewidth (.DELTA..nu.) by lasing in the optical cavity at more 
than mode 1. The laser emission 15 has a much narrower linewidth than that 
of the dispersive linewidth 19. 
A second advantage of a shorter length L for the optical cavity is an 
improvement in the stability of frequency versus temperature change 
experienced by the laser oscillator. By virtue of the present invention, 
the effective length of the optical cavity of a laser oscillator can be 
made shorter than 10 cm. By way of example, for the oscillators described 
in the above-identified article "Flashlamp pumped narrow-linewidth 
dispersive dye laser oscillators: very low amplified spontaneous emission 
levels and reduction of linewidth instabilities", the effective optical 
cavity length L was about 40 cm. And the linewidth of the laser emission 
was that of the dispersive linewidth .DELTA..lambda., and could not be 
made narrower. On the other hand, a pure "grazing incidence" type of laser 
oscillator (such as described on page 142 of the above-identified book by 
the inventor entitled "Dye Laser Principles") may have a short cavity, but 
the efficiency is far less than that of the laser oscillator provided in 
accordance with the present invention. Moreover, in a pure "grazing 
incidence" oscillator the laser output is unpolarized, and this makes it 
unsuitable for use in conjunction with the present invention. 
Referring now to FIG. 2, there is shown schematically a dye laser apparatus 
20 in accordance with the present invention. The apparatus 20 produces a 
narrow linewidth laser output beam indicated at 22 by parallel dashed 
lines. The diameter of this beam 22 is indicated at W and is typically a 
fraction of a millimeter. As shown here a dye cell 24 (which can be like 
the one described in the above-identified U.S. Pat. No. 4,891,817) is 
"pumped" or excited by a beam 26 from a source such as a copper laser. 
This phenomenon is well known in the art. The pulse repetition frequency 
(prf) of such a source is in the range of 5 kHz to 20 kHz. Forming part of 
an optical cavity of the laser apparatus 20 is a first prism 30 which 
receives laser emission from the dye cell 24 at an incident angle 
indicated at .phi..sub.1,1. Laser light (indicated by the shaded area) 
from the prism 30 is directed at an angle .phi..sub.1,2 onto a second 
prism 32 and thence is refracted at an angle .psi..sub.1,2 in an expanded 
beam (shaded area) onto a Littrow-mounted grating 34. The angle of light 
incident on and diffracted from the grating 34 is indicated by an angle 
.THETA.. The relationships of these angles to the laser beams within the 
multiple-prism Littrow-mounted grating (MPL) portions of the optical 
cavity are given in detail in the above-identified book by the inventor 
entitled "Dye Laser Principles." 
After being diffracted back from the Littrow-mounted grating 34, through 
the prism 32 and the prism 30, the laser light is highly polarized and 
frequency narrowed. The plane of polarization here lies parallel to the 
plane of FIG. 2. This polarized light passes to the left back through the 
dye cell 24 for further amplification and becomes the narrow linewidth 
laser beam 38 having the diameter W. As this polarized beam 38 continues 
to the left from the dye cell 24, it encounters a specially provided, 
partially reflecting polarizer device 40. The outer or left-most face of 
this polarizer device 40 is made partially reflecting by a suitable 
coating 42, such as a very thin layer of low-loss dielectric material, 
which gives about 5% to 20% reflection of the laser beam 38. The remaining 
80% to 95% of the beam 38 passes through the reflecting coating 42 and 
becomes the laser output beam 22. This laser beam 22 is polarized in a 
plane parallel to the plane of FIG. 2. The laser beam 22, by way of 
example, has a narrow linewidth (such as illustrated by .DELTA..nu. in 
FIG. 1) of less than 100 MHz at a wavelength (which is tuneable) in the 
visible light spectrum (for example, 575 nm). Output efficiency of the MPL 
laser apparatus 20 is somewhat greater than 10%. The ratio of I.sub.ASE 
/I.sub..lambda. is about 10.sup.-10. The FSR is about 1.5 GHz and the 
effective length of the optical cavity is less than 10 cm. 
Referring now to FIG. 3, there is shown a broken away perspective view of 
the partially reflecting polarizer device 40. The outer surface (left most 
face) of the device 40 is coated with a partially reflecting layer 42 of a 
low-loss dielectric material such as produced by the CVI Laser Corp., of 
Albuquerque, N. Mex. The plane of polarization of the beam 38 (see FIG. 2) 
is here indicated by the vertical double-headed arrow 44 (which in FIG. 2 
lies in the plane of the drawing). The device 40 is angularly oriented so 
that its plane of polarization (indicated by the double headed arrow 48) 
is parallel with the arrow 44. As a consequence, the polarized beam 38 is 
transmitted through the device 40 virtually without loss. A small 
percentage (5% to 20%) of the polarized beam 38 is fed back into the 
optical cavity by the reflecting layer 42, as was previously explained. 
The remainder of the beam 38 (80% to 95%) passes through the device 40 and 
emerges as the narrow linewidth laser output beam 22. The device 40 serves 
as an output coupler for the laser beam 22 and as part of the optical 
cavity of the laser apparatus 20. This positive feedback of a portion of 
the polarized beam 38 back into the optical cavity and the high degree of 
discrimination against unpolarized ASE by the device 40 substantially 
reduces unwanted ASE levels in the output beam 22, as was previously 
explained. The device 40 is advantageously fabricated from a multi-prism 
polarizer commercially available, for example, as a Glan-Laser prism 
polarizer, Part No. MGLB-10 sold by the Karl Lambrecht Corp. of Chicago, 
Ill. The outer face of the Glan-Laser prism polarizer is specially coated 
with a partially reflecting layer 42, as described above, in order to 
obtain the device 40 as used in the laser apparatus 20 of FIG. 2 in 
accordance with the present invention. 
It is to be understood that the embodiment of the invention described 
herein is illustrative of the general principles of the invention. 
Modifications may readily be devised by those skilled in the art without 
departing from the spirit and scope of the invention. For example, lasers 
other than the MPL dye laser described may be used, and the polarizer 
device 40 is not limited to the Glan-Laser multiprism unit described or to 
a particular part number. Also the multiple-prism beam expander can be 
composed of more than two prisms. Still further, the invention is not 
restricted to a particular wavelength of laser operation, or to a given 
length of optical cavity, or to a particular active laser medium.