Copper vapor laser acoustic thermometry system

A copper vapor laser (CVL) acoustic thermometry system is disclosed. The invention couples an acoustic pulse a predetermined distance into a laser tube by means of a transducer and an alumina rod such that an echo pulse is returned along the alumina rod to the point of entry. The time differential between the point of entry of the acoustic pulse into the laser tube and the exit of the echo pulse is related to the temperature at the predetermined distance within the laser tube. This information is processed and can provide an accurate indication of the average temperature within the laser tube.

BACKGROUND OF THE INVENTION 
The present invention relates to a copper vapor laser (CVL) acoustic 
thermometry system. 
Metal vapor lasers, in particular copper vapor lasers (CVL), have been 
shown to be powerful, efficient sources of visible laser light. In order 
to optimize the performance of these devices and extend their usable 
life-time, it is desirable to operate them within a narrow (less than 
10.degree. C.) temperature range. The plasma tube temperature of a CVL 
could be held at an optimal value by varying the input power based on the 
current temperature. 
Conventional thermometry systems prove inadequate for CVL systems due to 
the high temperature involved (1450.degree. C.), resolution requirements, 
the intense electromagnetic fields within and around the plasma tube, and 
several mechanical constraints. It would, therefore, be desirable to 
provide an acoustic thermometry system for a CVL laser which provides an 
accurate indication of the average temperature within the CVL laser tube. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved copper 
vapor laser (CVL) acoustic thermometry system. 
It is a more particular object of the present invention to provide an 
improved CVL acoustic thermometry system which provides a very accurate 
indication of the average temperature within a CVL laser tube. 
The thermometry system includes alumina rod means for axially coupling an 
acoustic pulse a predetermined distance from a coupling point into the 
interior of a metal vapor laser such that an echo pulse corresponding to 
the acoustic pulse is reflected from the interior of the laser to the 
coupling point without affecting the operation of the laser. 
The system further includes means for measuring the period of time between 
the coupling of the acoustic pulse and the return of the echo pulse to the 
coupling point. The temperature at the predetermined point in the laser is 
proportional to the period of time indicated above, so that the system 
includes means for processing that measured time difference to provide a 
temperature indication at the predetermined interior point within the 
laser. 
Additional objects, advantages and novel features of the present invention 
will be set forth in part in the description which follows and in part 
become apparent to those skilled in the art upon examination of the 
following, or may be learned by practice of the invention. The objects and 
advantages of the present invention may be realized and attained by means 
of the instrumentalities and combinations which are pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE DRAWINGS 
Reference will now be made in detail to the preferred embodiment of the 
invention, an example of which is illustrated in the accompanying 
drawings. While the invention will be described in conjunction with that 
preferred embodiment, it will be understood that it is not intended to 
limit the invention to that embodiment. On the contrary, it is intended to 
cover alternatives, modifications and equivalents as may be included 
within the spirit and scope of the invention as defined by the appended 
claims. 
Referring now to FIG. 1, a control diagram employing an acoustic 
thermometry system to control a copper vapor laser (CVL) input power 
control voltage is depicted. 
The CVL system 10 of FIG. 1 is associated with an acoustic thermometry 
system 12 which provides an accurate indication of the operating 
temperature of CVL system 10. In FIG. 1, the temperature information from 
acoustic thermometry system 12 is provided to data interpretation means 
14, which generates the temperature information data to summing circuit 16 
via lead 18. Summing circuit 16 also receives a setpoint signal which is 
related to the desired operating temperature of the CVL. The difference or 
error signal on lead 20 is provided to the control algorithm means 22. The 
data interpretation means 14 and control algorithm means are desirably 
software implemented but could, if desired, be hardware implemented. The 
loop to CVL system 10 is thus completed via control algorithm means 22 
providing appropriate input power control voltage signals to CVL system 
10. 
This control loop is implemented by means of acoustic thermometry system 
12, the details of which will now be described in more detail. 
As previously described, conventional thermometry systems prove inadequate 
for CVL systems due to the high temperature involved (1450.degree. C.), 
resolution requirements, the intense electromagnetic fields within and 
around the plasma tube, and several mechanical constraints. An acoustic 
thermometry system according to the present invention has been designed 
and prototyped to meet the requirements of this application. The present 
invention far exceeds the resolution requirements and should work well in 
a CVL or other metal vapor laser environment. 
Referring now to FIGS. 2 and 3, the acoustic thermometry system according 
to the present invention operates in the following manner. In FIG. 2, a 
narrow electrical pulse is created and sent to a piezoelectric crystal 
transducer 26. The resulting acoustic energy in the form of a 
compressional wave is then coupled into a small diameter (0.125") rod 28, 
which has the majority of its length (36") within the laser tube 24. The 
wave then propagates down the length of rod 28. As predicted by wave 
theory, a nearly total reflection occurs at the end of rod 28 and the 
reflected wave travels back toward the transducer 26. Since both the 
length of rod 28 and the velocity of sound in rod 28 are temperature 
dependent quantities, the time difference between the excitation pulse and 
the returning echo is a function of temperature. This time difference is 
measured and the rod temperature is calculated from this data. 
FIG. 3 depicts a block diagram of a CVL acoustic thermometry system 
according to the present invention in more detail. In FIG. 3, a pulse 
generator circuit 30 provides a control pulse to pulser circuit 32, which 
generates the excitation pulse. In turn, a switching network 34 is 
connected to receive the excitation pulse from pulser 32 and, in addition, 
the control pulse from pulse generator 30. Switching network 34 switches 
the excitation pulse for connection to transducer 36, which couples the 
excitation pulse to alumina rod 38 in the form of an acoustic sound wave. 
The sound wave travels along alumina rod 38 in a manner as described in 
conjunction with FIG. 1. The returning echo pulse along alumina rod 38 is 
coupled from transducer 36 to switching network 34, which in turn switches 
the returning echo to echo detection circuitry 40. Echo detection 
circuitry 40 determines when the main echo has been detected and couples 
this event to the time to digital converter (TDC) circuit 42. 
TDC 42 also receives the excitation pulse from pulser 32 in order to 
determine the time difference between the excitation pulse and the 
returning echo. 
This information is connected to microcomputer 42 which processes the 
information and provides to display 46 the necessary information for 
graphically illustrating the desired temperature information. 
To exist within the environment of the CVL plasma tube with the strong 
electromagnetic fields and high temperatures, the probe of the acoustic 
thermometry system must be composed of a dielectric with a very high 
melting temperature. In a preferred embodiment, Alumina (Al.sub.2 O.sub.3) 
was chosen as the rod material and lithium niobate (LiNbO.sub.3) was 
chosen as the transducer material. 
Referring now to FIG. 4, a prototype CVL acoustic thermometry probe is 
illustrated wherein transducer 60 and alumina rod 62 are shown with their 
actual dimensions, in a preferred embodiment. Although transducer 60 is 
outside of the CVL tube, it will experience relatively high temperatures 
due to conduction through rod 62. Lithium niobate was an ideal candidate 
for this application due to its high Curie temperature (1150.degree. C.). 
Alumina was also a desired choice due to the fact that the laser tubes 
themselves are composed of this material and is readily available. Very 
thin (&lt;1.0 mm) sapphire fibers may offer some advantages over alumina 
rods. 
Referring to FIG. 3, a standard laboratory pulser 32 can be used as the 
source of the excitation pulse. A solid state switching network 34 routes 
the excitation pulse and the returning echoes into the appropriate 
subsystems. The echoes are fed into a threshold based detection circuit 
40. Its output and the excitation pulse are then fed into a commercially 
available Time-to-Digital-Converter (TDC) 42. The accuracy and resolution 
of this thermometry system are dependent on the resolution of TDC 42. In 
the prototype, a one nanosecond TDC was employed which, in theory, should 
yield a temperature resolution of less than 0.5.degree. C. Upon 
investigation, however, a resolution of approximately 1.0.degree. C. was 
seen. This far exceeds the requirements of the desired application. 
In the system of FIG. 3, the output of TDC 42, the time difference between 
the excitation pulse and first echo, is fed into a microcomputer 44. The 
computer then takes this time value and converts it into a temperature 
through the use of a polynomial evaluation. The polynomial can be 
generated by a curve fitting program using several previously determined 
data points. Look-up-tables could also be employed to perform this 
conversion. A digital filtering algorithm is then employed to smooth the 
resulting temperature data. The filtered data is then displayed on a 
standard video terminal 46. The temperature data can be sent to the 
control algorithm 22 to control the input power of the CVL. 
Referring now to FIGS. 5A and 5B, schematic diagrams of a suitable 
switching network 34 are depicted. In one embodiment, a typical switching 
network could be Analog Devices Model No. AD759001, which is a well known 
analog switch. In FIG. 5A, the excitation pulse of FIG. 3 is input to 
switches 70 which, with appropriate control pulses from the Pulse 
Generator 30 of FIG. 3, operate to close switches 70 for switching the 
excitation pulse to the transducer 36 of FIG. 3. 
Similarly, the echo pulse from transducer 36 of FIG. 3 is switched via the 
analog switch of FIG. 5B for input to the echo detection circuitry 40 of 
FIG. 3. The return acoustic pulse from the transducer 36 is switched via a 
suitable control pulse from Pulse Generator 30 of FIG. 3 for switching to 
the echo detection circuitry 40 of FIG. 3. 
Referring now to FIGS. 6 and 7, the operation of the echo detection 
circuitry 40 of FIG. 3 is described in more detail. 
FIG. 7 depicts a timing diagram illustrating the amplified echo signal from 
switching network 34. In a preferred embodiment, the present invention 
detects the first zero crossing of the amplified echo signal after the 
amplified echo signal reaches the predetermined value in terms of 
amplitude. This point (the first zero crossing after the amplified echo 
reaches the predetermined value) is indicated at t.sub.4 of FIG. 7. In 
FIG. 6, the amplified echo signal is input to a threshold comparator 80, 
the output of which is input as a clock to a threshold flipflop 84, which 
in turn is connected as a "D" input to a zero crossing flipflop 86. Both 
flipflops 84, 86 are cleared by a delay signal 78, which keeps the 
flipflops 84, 86 in a low state until a time slightly before the echo 
signal is expected. The echo signal in FIG. 6 is also input to a zero 
crossing comparator 82, the output of which is input as a clock to zero 
crossing flipflop 86. 
At time t.sub.1, the output of zero crossing comparator 82 changes state, 
as the amplified echo signal is passing a zero crossing point. At time 
t.sub.2, the outputs of threshold comparator 80 and threshold flipflop 84 
change states, and at time t.sub.3, the output of threshold comparator 80 
changes its state back to its original condition, which indicates that the 
amplified echo signal is passing through the predetermined reference value 
92 toward the zero crossing point, as indicated in FIG. 7. 
As indicated above, at time t.sub.4, the amplified echo signal passes 
through the zero crossing, which is indicated by the output of zero 
crossing comparator 82 changing state together with the output of zero 
crossing flipflop 86 also changing state. In FIG. 7, the output signal 
from gate 90 is then input to the time to digital converter 42 of FIG. 3. 
The processing of the output signal from gate 90 can then proceed, as 
described previously. 
The amplified echo signal continues to oscillate, as indicated in FIG. 7, 
so that the output of zero crossing comparator 82 changes state at time 
t.sub.5 and the output of threshold comparator 80 changes state at time 
t.sub.6. However, it is at time t.sub.4 that the logic of FIG. 6 provides 
an output signal which enables the present invention to determine the 
temperature within the laser by suitable processing, as described 
previously. 
Although the device is a very useful tool as it stands, many future 
extensions are possible for this technology. Improvements that would 
increase system resolution, accuracy, cost effectiveness, and reliability 
are straight-forward and hold a great deal of promise. The application can 
also be expanded upon in order to get a temperature profile of the laser 
rather than an average temperature. It is also possible to envision other 
applications of this thermometry system to gas discharge lasers or other 
types of lasers. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. The embodiment was chosen and described in 
order to best explain the principles of the invention and its practical 
application to thereby enable others skilled in the art to best utilize 
the invention in various embodiments and with various modifications as are 
best suited to the particular use contemplated. It is intended that the 
scope of the invention be defined by the claims appended hereto.