Hybrid laser power supply

The present invention provides three embodiments of circuitry, responsive to the application of an energizing pulse to the high pressure tube of a hybrid laser, for de-energizing the normally continuously energized low-pressure tube of the hybrid laser for only a given time interval which is longer than the duration of the high-energy pulse of coherent wave energy generated by the high pressure tube when energized. This solves a prior-art problem of sporadic spike emissions by the low-pressure tube during the time interval in which echoes from objects of interest are being received by a LIDAR employing a hybrid laser transmitter portion.

BACKGROUND OF THE INVENTION 
I. Field Of The Invention 
This invention relates to a power supply for use with a hybrid laser and, 
more particularly, to such a power supply for energizing a hybrid laser 
employed as the transmitting portion of a LIDAR (Laser Infra-red Detection 
And Ranging) system. 
II. Description Of The Prior Art 
As known in the prior art, hybrid lasers are employed to provide high 
energy pulses of coherent wave energy in a single longitudinal mode (and 
preferably a single transverse mode). Reference is made to U.S. Pat. No. 
4,554,666, which issued Nov. 19, 1985. This patent discusses the state of 
the hybrid laser art in some detail. 
Briefly, a hybrid laser is comprised of an optical resonant cavity in which 
is situated a narrow-bandwidth, low-energy active lasing medium (such as a 
continuously-excited low pressure CO.sub.2 laser tube) and a 
wide-bandwidth, high-energy active lasing medium (such as a pulse-excited 
high-pressure TEA (i.e., transverse-excited atmospheric) CO.sub.2 laser 
tube). The two laser tubes are arranged in serial relationship with one 
another within the optical resonant cavity, so that coherent light 
reflected from a first reflector at one end of the optical resonant cavity 
must travel through both laser tubes before reaching a second reflector at 
the other end of the optical resonant cavity. Consequently, the bandwidth 
of the high-energy coherent wave energy pulses (generated by the 
high-pressure laser tube) is constrained by the narrow-bandwidth of the 
low-energy, low-pressure laser tube. 
LIDAR, which is similar to radar, employs coherent wave energy at infra-red 
wavelengths generated by a laser, rather than microwave coherent wave 
energy at radio wavelengths, to measure the distance to objects, the 
velocity of such objects by Doppler techniques, etc. As is known, LIDAR is 
often used for the purpose of making atmospheric measurments of various 
types. 
A CO.sub.2 hybrid laser is particularly suitable for use as the transmitter 
portion of a LIDAR. This is true because of the ability of a CO.sub.2 
hybrid laser to generate single longitudinal mode, high peak power short 
pulses of infra-red wave energy. The high peak power extends the ranging 
distance of a LIDAR. The small length of each pulse increases the 
resolution in range with which objects at slightly different distances may 
be distinguished, and the single longitudinal mode of the pulse coherent 
wave energy makes it possible to measure relatively small object 
velocities with high precision by Doppler techniques. By way of example, 
it would be desirable to employ a LIDAR comprised of a hybrid CO.sub.2 
laser transmitter in a polar-orbit weather satellite for the purpose of 
making accurate measurements of wind velocity at closely spaced points 
(every few kilometers) over the face of the earth. Such measured wind 
velocity data could then be used, along with other weather data, to 
provide (by computer analysis of the measured data) much more accurate 
long-term, world-wide weather forecasting than is currently achievable. 
An essential requirement of both radar and LIDAR is that no power be 
radiated by the transmitter during the entire time interval following the 
transmission of each exploratory pulse during which reflected echoes from 
objects of interest may be received by the radar or LIDAR. Otherwise, the 
relatively low-power echo wave energy would be swamped by such undesired 
transmitter power leaking into the receiver system. 
In the past, the low pressure laser tube of the CO.sub.2 hybrid laser 
transmitting portion of a LIDAR was continuously excited. Therefore, the 
hybrid laser would normally emit a certain amount of low-power infra-red 
light, deriving from the low pressure tube laser during the period between 
successive high-power pulses of infra-red light deriving from the high 
pressure laser tube. However, in practice, the creation of each high power 
laser pulse (generated in response to an applied excitation pulse to the 
high pressure laser tube) results immediately thereafter in the Q of the 
cavity being spoiled due to optical and thermal effects. This spoiling of 
the Q of the cavity squelches lasing of both the low and high pressure 
laser tubes, although the low pressure laser tube continues to be 
energized and excited (i.e., the plasma current continues to flow 
therein). It is fortunate that the lasing of the low pressure tube of the 
hybrid laser of the transmitting portion of a LIDAR takes place, since the 
production of laser light from the low pressure tube would inevitably be 
detected in the receiver optics and possibly swamp any received signal. 
The problem in the prior art is that after a time interval of roughly 100 
microseconds (.mu.s)--highly dependent on laser design--the low pressure 
tube of the hybrid laser begins to sporadically emit low-power spikes of 
infra-red light for a certain time before the spoiled Q of the cavity 
recovers and the low pressure tube resumes continuous lasing. While the 
total time of the spoiled Q of the cavity to recover is sufficient for 
object echoes of interest to be received by the receiving portion of the 
LIDAR, the time interval of only 100 .mu.s is substantially shorter than 
that which is required to receive the more distant object echoes of 
interest. Therefore, the problem with the prior art is that the receiving 
portion of the LIDAR is disrupted by the detection of the sporadic 
infra-red spikes. The present invention is directed to a solution to this 
problem. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the continuous excitation of the 
low pressure laser tube of the hybrid laser is automatically disrupted in 
synchronization with the firing of the high pressure laser tube for a 
given predetermined time from the instant of the main discharge of the 
high pressure laser tube. 
More specifically, the present invention is directed to an improvement in 
the power supply used for energizing the respective low-pressure and 
high-pressure laser tubes of a hybrid laser. As in the prior art, such a 
power supply includes (1) first means for intermittently applying an 
energizing pulse to the high-pressure laser tube thereby permitting the 
high-pressure laser tube to generate a high-energy pulse of coherent wave 
energy of a certain duration at the beginning of such an applied 
energizing pulse, and (2) second means for normally continuously 
energizing the low-pressure laser tube, thereby permitting the 
low-pressure laser tube to normally continuously generate low-energy 
coherent wave energy. However, in accordance with the improvement of the 
present invention, the second means includes third means coupled to the 
first means and responsive to the application of an energizing pulse to 
the high pressure laser tube for de-energizing the low-pressure laser tube 
for only a given time interval from the beginning of the energizing pulse, 
the given time interval being longer than the certain duration of the high 
energy pulse. The effect is that the low-pressure laser tube generates no 
coherent wave energy during the given time interval.

PREFERRED EMBODIMENTS OF THE INVENTION 
Referring to FIG. 1, the prior art power supply includes pulsed 
high-pressure (H.P.) laser excitation section 100, for energizing the 
high-pressure (H.P.) TEA laser tube 102 (which preferably employs CO.sub.2 
as its active lasing medium), and continuous low-pressure (L.P.) laser 
excitation section 104, for energizing L.P. laser tube 106 (which also 
preferably includes CO.sub.2 as its active lasing medium). 
FIG. 1 shows a single voltage supply 108 having respective "common," 
E.sub.H, and E.sub.L terminals. The "common" terminal may be grounded, as 
indicated in FIG. 1. 
Voltage supply 108 is a relatively high voltage supply that provides 
respective first and second D.C. voltages on terminals E.sub.H and E.sub.L 
of given polarity with respect to the grounded common terminal of voltage 
supply 108. The first voltage on terminal E.sub.H, which is used to 
energize pulsed H.P. laser excitation section 100, has a magnitude that is 
substantially higher than the magnitude of the second voltage on terminal 
E.sub.L, which is used to energize the continuous L.P. laser excitation 
section 104. By way of example, the magnitude of the first voltage on 
terminal E.sub.H may be of a magnitude of about 40 kilovolts, while the 
second voltage on terminal E.sub.L may have a magnitude of about 9 
kilovolts. 
Pulsed H.P. laser excitation section 100 is comprised of capacitance C, a 
charging circuit for capacitance C and a discharging circuit for 
capacitance C. The charging circuit for capacitance C includes resistance 
R.sub.1 connecting one end of capacitance C to terminal E.sub.H of voltage 
supply 108 and resistance R.sub.2 connecting the other end of capacitance 
C to the grounded common terminal of voltage supply 108. 
Capacitance C, which must store sufficient energy to provide a high power 
excitation pulse to H.P. laser tube 102 on discharge, has a relatively 
high value, such as 30 to 40 nanofarads. The time constant for the 
charging circuit for capacitance C may be as short as a few milliseconds 
or as long as one-half second. A short time constant permits a high pulse 
repetition for H.P. laser tube 102, but requires a higher charging current 
from voltage supply 108. Depending on the time constant, R.sub.1 has a 
value that ranges from somewhat below 0.1 megohm to somewhat above 10 
megohms. R.sub.2 has a much lower value in the order of tens of ohms. 
A discharging circuit for C includes normally-open switching element 110, 
which, when closed, connects the aforesaid one end of capacitance C 
directly to the grounded common terminal of voltage supply 108, and the 
H.P. laser tube 102 connecting the aforesaid other end of capacitance C to 
the grounded common terminal of voltage supply 108. Switching element 110 
may be comprised of a normally-open controlled switch, such as a thyratron 
or a triggered spark gap, which is closed only when a control trigger 
signal is applied to a control electrode thereof. 
As long as normally-open switching element 110 remains open, no energizing 
current can flow through H.P. laser tube 102. This permits capacitance C 
to be charged relatively slowly through resistances R.sub.1 and R.sub.2 
toward the high voltage (e.g., 40 kilovolts) on terminal E.sub.H. After a 
time interval that is at least sufficient for capacitance C to become 
fully charged, a control trigger signal is applied to the control 
electrode of normally open switching element 110, thereby closing 
switching element 110. This provides pulsed H.P. laser tube 102 with an 
energizing pulse, as capacitance C discharges very quickly through closed 
switching element 110 and H.P. laser tube 102. Further, as soon as 
capacitance C is discharged, switching element 110 reopens, and charging 
of capacitance C begins all over again. This process continues 
periodically (or at least intermittently) to provide a series of spaced 
pulses of coherent wave energy from laser tube 102. 
As shown in FIG. 1, the continuous L.P. laser excitation section 104 is 
comprised of resistance R.sub.3, having one end thereof connected to 
terminal E.sub.L of voltage supply 108 and the other end thereof connected 
L.P. laser tube 106. Laser tube 106 is serially connected between the 
other end of resistance R.sub.3 and the grounded common terminal of 
voltage supply 108. Resistance R.sub.3, which operates as a 
current-limiting resistance, may have a value of nearly 0.9 megohms to 
permit L.P. laser tube 106 to be continuously energized by a current of 
about 10 milliamperes produced by a voltage of about 9 kilovolts at the 
E.sub.L terminal of voltage supply 108. 
It is to be noted that in FIG. 1, the energization of pulsed H.P. laser 
excitation section 100 is completely independent from the energization of 
continuous L.P. laser excitation section 104. Therefore, the single 
voltage supply 108 shown in FIG. 1 may be (and in practice often is) 
replaced by two separate voltage supplies--one of which supplies the 
voltage at the E.sub.H terminal of voltage supply 108 to pulsed H.P. laser 
excitation section 100 and the other of which supplies the voltage at the 
E.sub.L terminal of voltage supply 108 of the continuous L.P. laser 
excitation section 104. In this latter case each of the two voltage 
supplies would have its own individual common terminal (which may or may 
not be grounded). 
FIGS. 2, 3 and 4, each of which illustrates a different embodiment of the 
present invention, all provide means responsive to the application of an 
energizing pulse to H.P. TEA laser tube 102 for de-energizing L.P. laser 
tube 106 for a time interval more than sufficient for all LIDAR echoes of 
interest to be received. However, the implementation shown in the FIG. 2 
embodiment of the present invention is significantly less desirable than 
the respective implementations shown in the FIGS. 3 and 4 embodiments of 
the present invention. 
Referring to FIG. 2, voltage supply 208 includes only an E.sub.H terminal 
and a grounded common terminal. The relatively high magnitude voltage 
(e.g. 40 kilovolts) at terminal E.sub.H of voltage supply 208 with respect 
to ground is used to energize both pulsed H.P. laser excitation section 
100 and continuous L.P. laser excitation section 204, as shown in FIG. 2. 
Specifically, current-limiting resistance R.sub.4 of L.P. laser excitation 
section 204 has one end thereof connected to the junction of resistance 
R.sub.1 and capacitance C of H.P. laser excitation section 100, with the 
other end of current-limiting resistance of R.sub.4 being connected to one 
end of L.P. laser tube 106. This differs from the prior art shown in FIG. 
1, wherein the current-limiting resistance R.sub.3 is connected between 
terminal E.sub.L of voltage supply 108 and one end of L.P. laser tube 106. 
In other respects, the structure of the FIG. 2 embodiment of the present 
invention is similar to that of the prior art shown in FIG. 1. 
If the current flow through L.P. laser tube 106, of the FIG. 2 embodiment 
(after capacitance C is fully charged) is about 10 milliamperes and the 
magnitude of the voltage at terminal E.sub.H of the voltage supply 208 is 
40 kilovolts, the sum of the resistance values of R.sub.1 and R.sub.4 has 
to be nearly 4 megohms. However, the maximum voltage charge across 
capacitance C at the time that switching element 110 is triggered is equal 
to the difference between the 40 kilovolts at terminal E.sub.H of voltage 
supply 208 and the voltage drop across resistance R.sub.1 caused by the 10 
milliampere current for L.P. laser tube 106 flowing through resistance 
R.sub.1. In order not to unduly lower the voltage charge of capacitance C 
(which would lower the power of the excitation pulse of H.P. laser tube 
102 and the power of the coherent wave pulse generated thereby), the 
resistance value of R.sub.1 must be relatively small (e.g., 0.1 megohms). 
One result of this is that the charging circuit for C in the FIG. 2 
embodiment is always quite short (a few milliseconds), regardless of 
whether the pulse repetition rate of the LIDAR is low or high. A more 
important result of the relatively small resistance value for R.sub.1 in 
the FIG. 2 embodiment is that the resistance value of current-limiting 
resistance R.sub.4 is relatively large (e.g., nearly 3.9 megohms). The 10 
milliampere current flowing through a current-limiting resistance R.sub.4 
value of nearly 3.9 megohms dissipates nearly 390 watts. This wasted power 
of nearly 390 watts of the FIG. 2 embodiment of the present invention is 
much greater than the wasted power of only nearly 90 watts of the FIG. 1 
prior art (i.e., the power dissipation resulting from ten milliamperes 
flowing through the nearly 0.9 megohm current-limiting resistance 
R.sub.3). This large amount of wasted power, which requires that power 
supply 208 be much larger than power supply 108, is a big disadvantage of 
the implementation of the FIG. 2 embodiment of the present invention. 
Nevertheless, the FIG. 2 embodiment of the present invention does serve to 
de-energize continuous L.P. laser tube 106 for a time interval at least 
sufficient for all LIDAR echoes of interest to be received (and thereby 
solve the above-discussed problem with the prior art, wherein the 
receiving portion of the LIDAR is disrupted by the detection of sporadic 
infra-red spikes). Specifically, the closing of normally-controlled 
switching element 110 in the FIG. 2 embodiment (as in the prior art shown 
in FIG. 1) results in capacitance C quickly discharging through H.P. laser 
tube 102,--thereby energizing H.P. laser tube 102 with an energizing 
pulse--after which, switching element 110 reopens and capacitance C 
recharges. However, in the FIG. 2 embodiment, the closing of normally-open 
and controlled switching element 110 also results in shorting out L.P. 
laser excitation section 204, thereby de-energizing and extinguishing the 
plasma in L.P. laser tube 106. Even after control switching element 110 
reopens, L.P. laser tube 106 of the FIG. 2 embodiment remains de-energized 
until capacitance C recharges to the relatively high striking voltage of 
L.P. laser tube 106. Furthermore, even after L.P. laser tube 106 has been 
struck, the current therethrough will not reach its operating level of 10 
milliamperes until capacitance C is substantially fully charged (i.e., a 
time interval of a few milliseconds following the generation of the 
immediately preceding excitation pulse). 
The respective embodiments of the present invention shown in FIGS. 3 and 4 
employ all the structure employed by the prior art shown in FIG. 1, as 
well as additional structure not employed by the prior art. This structure 
common to the embodiments of the present invention shown in FIGS. 3 and 4 
and the prior art shown in FIG. 1 functions in the same manner as that 
described above in connection with FIG. 1. 
Specifically, LP laser excitation section 304 of the FIG. 3 embodiment of 
the present invention further includes additional structure, not employed 
by the prior art shown in FIG. 1, for de-energizing L.P. laser tube 106 
for a time interval sufficiently long for all LIDAR echoes of interest to 
be received following the energization of H.P. laser tube 102 by an 
excitation pulse, before L.P. laser tube 106 is re-energized. This 
additional structure is comprised of capacitance C.sub.2 having one end 
thereof connected to the junction of resistance R.sub.1 and capacitance C 
and the other end thereof connected to the junction of current-limiting 
resistance R.sub.3 and one end of L.P. laser tube 106. This additional 
structure further includes a diode D.sub.1, shunting L.P. laser tube 106, 
poled as shown in FIG. 3 for a positive supply or reverse poled for a 
negative supply. 
In FIG. 3, it is assumed that the respective voltages of both the E.sub.H 
and E.sub.L terminals of voltage supply 108 have a positive polarity with 
respect to the grounded common terminal. First assume that capacitance C 
has been fully charged, normally-open controlled switching element 110 
remains open and that LP laser tube 106 is being continuously energized by 
about 10 milliamperes of current flowing from terminal E.sub.L through 
current-limiting R.sub.3. In this case, the top plate of capacitance 
C.sub.2 will be charged to the same positive potential as capacitance C 
(40 kilovolts), while the bottom plate of capacitance C.sub.2 will be at a 
positive potential of only a few hundred volts (caused by the voltage drop 
across L.P. laser tube 106 when it is conducting) because diode D.sub.1, 
poled as shown, is reversed bias. Now assume that switching element 110 is 
closed, resulting in the discharge of capacitance C and the application of 
an excitation pulse to H.P. laser tube 102. The effect of this on L.P. 
laser excitation section 304 is to ground the top side of capacitance 
C.sub.2, thereby tending to drop the bottom plate of capacitance C.sub.2 
to a negative potential of nearly 40 kilovolts. However, diode D.sub.1, 
poled as shown in FIG. 3, becomes forward biased and clamps the bottom 
plate of capacitance C.sub.2 to ground. The result is that there is no 
voltage across L.P. laser tube 106, thereby de-energizing L.P. laser tube 
106. 
After switching element 110 reopens, L.P. laser tube 106 remains 
de-energized until the bottom plate of capacitance C.sub.2 reaches a 
positive potential sufficient to strike L.P. laser tube 106 into 
conduction (the length of time being dependent on both the charging time 
constant of capacitance C through resistance R.sub.1 and also, to a 
certain extent, on such factors as (1) the time constant of capacitance 
C.sub.2 through resistances R.sub.1, R.sub.3 and voltage supply 108, and 
(2) the magnitudes of the voltages at terminals E.sub.H and E.sub.L of 
voltage supply 108. In any case, the time interval between the closing of 
switching element 110 and the striking of the L.P. laser tube 106 is at 
least a few milliseconds (i.e., more than enough time for all LIDAR echoes 
of interest to be received before energization and lasing of L.P. laser 
tube 106 resumes). 
In the FIG. 4 embodiment, the only additional structure of L.P. laser 
excitation section 404 of the FIG. 4 embodiment, that is not also employed 
in the prior art shown in FIG. 1, is diode D.sub.2, poled as shown, (again 
for a positive supply), which is connected between the junction of 
resistance R.sub.1 and capacitance C and the junction of current-limiting 
resistance R.sub.3 and L.P. laser tube 106. 
It is assumed in FIG. 4 that the respective voltages on terminals E.sub.H 
and E.sub.L of voltage supply 108 have a positive polarity with respect to 
the grounded common terminal. In this case, the diode D.sub.2, poled as 
shown, will be reversed biased by the positive voltage charge on 
capacitance C prior to the closing of switching element 110, while L.P. 
laser tube 106 remains energized and continuously lasing. However, the 
closing of switching element 110 results in the grounding of the top plate 
of capacitance C, thereby forward biasing diode D.sub.2 and clamping the 
junction of current-limiting resistance R.sub.3 and L.P. laser tube 106 to 
ground. Therefore, L.P. laser tube 106 becomes de-energized. Even after 
switching element 110 reopens, L.P. laser tube 106 will remain 
de-energized until diode D is again reverse biased by capacitance C 
recharging to a positive potential having a magnitude equal to the 
striking voltage of L.P. laser tube 106. After the resumption of 
conduction by L.P. laser tube 106, in response to the striking thereof, 
there is a large voltage drop across current-limiting resistance R.sub.3, 
which results in the diode D.sub.2 again being reversed biased. However, 
in all cases, the time constant for charging C is sufficiently long so 
that L.P. laser tube 106 is not re-energized and does not resume lasing 
until a time interval has elapsed following the generation of an 
excitation pulse applied to H.P. laser tube 102 that is sufficiently long 
for all LIDAR echoes of interest to be received (e.g., at least a few 
milliseconds). 
It has been assumed in describing the FIGS. 3 and 4 embodiments of the 
present invention that voltage supply 108 provides positive polarity 
voltages at terminals E.sub.H and E.sub.L with respect to the grounded 
common terminal. However, this is not a requirement of the present 
invention. Should voltage supply 108 provide negative polarity voltages at 
terminals E.sub.H and E.sub.L thereof, the respective FIGS. 3 and 4 
embodiments will operate in a similar manner to that described above if 
the poling of the diode D.sub.1 in FIG. 3 is reversed and the poling of 
the diode D.sub.2 in FIG. 4 is reversed. 
It has been verified, in practice, that the low pressure laser tube of a 
hybrid laser can be switched off for periods of many milliseconds, 
reliably and in synchronization with the discharge of the high pressure 
TEA laser tube. Thus, in spaceborne applications, as a result of the long 
time of flight of the laser pulse, the techniques disclosed herein should 
prove especially useful for any type of hybrid gas laser remote sensing 
systems.