- Email: [email protected]
A high-power high-efficiency Cu-Ne-HBr (λ = 510.6, 578.2 nm) laser
OPTICS C O M MUNICATIONS
Optics Communications 94 (1992) 289-299 North-Holland
Full length article
A high-power high-efficiency Cu-Ne-HBr (2= 510.6, 578.2 nm) laser D . R . J o n e s , N . V . S a b o t i n o v I, A. M a i t l a n d a n d C.E. L i t t l e J.F. Allen Research Laboratories, Department of Physics and Astronomy, Universityof St Andrews, North Haugh, St Andrews, Fife KYI 6 9SS, UK Received 2 April 1992; revised manuscript received 22 June 1992
A high-power copper bromide laser, which utilizes the technique of in situ production of CuBr by reaction of copper and HBr, is reported. The 45 mm bore copper HyBrID (Hydrogen Bromide In Discharge) laser has produced a maximum average output power at 510.6 and 578.2 nm of 94 W at 21 kHz pulse recurrence frequency, with an efficiency (based on stored energy) of 1.5%. The corresponding specific output power was 52 mW cm-3. An efficiency of 2.7% has been achieved for an average output power of 40 W. Laser oscillation can be obtained less than 15 minutes after a cold start-up, with full power attained 5 min later.
1. Introduction The high operating t e m p e r a t u r e s ( 1400-1600 ° C ) o f c o n v e n t i o n a l c o p p e r - v a p o u r lasers ( C V L s ) are responsible for their long start-up times a n d complex tube construction. These disadvantages, however, can be c i r c u m v e n t e d by the use o f copper b r o m i d e as a " l o w " t e m p e r a t u r e ( ~ 5 0 0 ° C ) source o f the required a t o m i c c o p p e r density in the laser active region. Recently, we described a new type o f copper b r o m i d e laser ( C B L ) (13 m m bore, 30 cm active length) which p r o d u c e d a specific o u t p u t power o f 195 m W c m -3 [1 ]. This figure is one o f the highest reported (exceeded only by small, narrow-bore ( 3 4 m m ) , CVLs [2,3 ] ) for multikilohertz, longitudinally excited, c o p p e r - b a s e d lasers. The tube was simple to construct, materials were used well within their limits o f operation, a n d the t i m e to laser oscillation from a cold start could be shorter than 1 min. (cf. ~ 60 min for a small C V L ) . This c o p p e r b r o m i d e laser used a new technique, whereby the necessary copper halide was p r o d u c e d on d e m a n d in the laser tube by reaction o f copper metal with HBr, which was i n t r o d u c e d with the neon buffer gas. In previous Department of Metal Vapor Lasers, Institute of Solid State Physics, 72 Tzarigradsko Chaussee, Sofia 1784, Bulgaria.
types o f CBL, copper b r o m i d e was either placed in solid form along the floor o f the laser tube a n d vaporized by discharge (or external) heating [4], or originated as a v a p o u r from heated sidearms containing the b r o m i d e [5]. O u r use o f H B r reactant permits the simultaneous i n t r o d u c t i o n o f b r o m i n e a n d hydrogen in a fixed 1 : l ratio to the discharge; in small a m o u n t s ( ~< 1 t o r r ) , hydrogen is known [5 ] to significantly increase the output powers a n d efficiencies o f copper b r o m i d e lasers. F u r t h e r m o r e , by the use o f HBr, the copper concentration in the active region can within seconds be readjusted to that o p t i m u m for laser oscillation whenever the discharge conditions (e.g. neon pressure, voltage, pulse recurrence frequency ( P R F ) ) are altered. To distinguish this type o f laser from the two conventional classes o f CVL and CBL, o f which it is clearly a hybrid, we introduce the generic descriptor H y B r I D (hydrogen b r o m i d e in discharge). In some previous studies, gas (or v a p o u r ) additives have been observed to m o d i f y (for better or worse) the operating characteristics o f copper halide lasers. F o r example, HC1, C12 a n d HE were e x a m i n e d as additives to a double-pulse CuC1 laser by Vetter and N e r h e i m [6]. It was found that with HCI, the m i n i m u m a n d o p t i m u m delay times o f the excitation pulse c o m p a r e d with the dissociation pulse was
0030-4018/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
289
Volume 94, number 4
OPTICS COMMUNICATIONS
reduced, and the laser pulse energy increased by 15%. The effects were attributed to the (unidentified) influence of hydrogen and not of chlorine, since the addition of C12 was always detrimental to laser oscillation, and the effects of H2 were similar to those of HC1. Copper metal plated out onto the tube wall when H2 or C12 was used, but not when HC1 was used, indicating that the Cu-C1-CuC1 balance is markedly affected by the addition of those gases, as might be expected. In the case of the copper HyBrID laser, we find that the alumina wall remains free of copper indicating that the Cu-Br-CuBr balance lies towards the molecular recombination side. In another study [7], Kim et al. added alkali chlorides to a doublepulse CuC1 laser, and found that the temperature regime of operation was extended; the effect was attributed to a change in the balance of Cu-CuC1 in the laser gas mixture. Only in a very few instances has the addition of a reacting species been a necessary step in attaining laser oscillation. Aside from our own work on the use of C12 [ 8 ], Br2 [ 8 ], HC1 [ 9 ] and HBr to produce the copper halide in situ, we know of only one similar scheme. Saito and Taniguchi [ 10 ] began their studies using a laser tube with a heated sidearm containing metallic copper powder placed above AIBr3, A1C13 or BiC13 powders. A reaction of the vapours of the volatile metal halides and the metallic copper yielded volatile metal complexes such as CuA12C17 and CuAlzBr7 at heater temperatures of less than 100 ° C. In subsequent experiments [ 11 ], the metallic copper powder was removed from the sidearm to the floor of the laser tube. Volatile AIBr3 vapour was entrained in flowing He buffer gas, and carried to the laser active region where it reacted with copper powder on the tube floor (heated to 400-500°C). In the latter scheme the reaction to produce the copperbearing metal-vapour complex occurred in the laser active region. The laser power so obtained was reported to be at least as large as may be obtained using the same laser as a conventional copper chloride laser. However, the scheme is more complicated than the HyBrID laser we report, since in their case, the metal halides had to be preheated to produce the vapour. More seriously, the 1 : 7 ratio of copper atoms to halogen atoms in the metal-vapour complex gives the potential for problems of discharge stability as the concentration of atomic copper (and hence of 290
15 November 1992
the halogen) is increased. Furthermore, addition of the gas hydrogen, necessary for high powers and efficiencies, would be an additional complication for the metal-vapour complex system; no such complication exists for the HyBrID laser reported here. Given the high specific output powers we have demonstrated with small-bore copper HyBrID lasers, it is of interest to establish how this type of device may be scaled to higher output powers. To that end we have constructed and tested a large-bore (45 m m ) laser, the characteristics of which we report here.
2. Experimental details The laser discharge region was confined within an alumina tube of length 120 cm and internal diameter 45 mm. With an interelectrode spacing of 114 cm, the active volume was 1815 cm 3. The alumina tube was sleeved within a fused silica cylinder of internal diameter 56 ram. The ends of the silica tube were fixed via Viton O-rings to stainless steel end-flanges. Cylindrical electrodes of molybdenum were press fitted into the end-flanges, and projected 30 m m into the alumina cylinder at each end. Fused silica windows were fixed by rubber O-rings to the ends of the end-flanges, and set at 5 ° to the normal of the tube axis. Regions of 40 cm and 60 cm length separated the electrode areas from the cathode and anode end windows, respectively. Each end-flange was watercooled and had a gas inlet or outlet. The laser cavity comprised a fiat high reflector at the cathode end, and a flat uncoated quartz flat at the anode end. The tube was normally draped with a 5 m m layer of Saffil insulation, although this was removed at the higher input powers. Three aluminium strips along the length of the tube formed a current return of low inductance, whilst still enabling the discharge tube to be cooled by free convection. Storage and peaking capacitors were mounted directly on the laser head. A single EEV type CX1535 thyratron was used in a standard resonant-charging circuit to deliver average powers of up to 7 kW to the laser head at PRFs of up to 27 kHz. The ratio of storage capacitance to peaking capacitance was always maintained at 2: 1. Flat pieces of copper ( 1 5 × 1 2 × 0 . 5 m m 3) were placed at intervals of 10 cm along the floor of the
15 November 1992
OPTICS COMMUNICATIONS
Volume 94, number 4
alumina tube. The neon (99.99% purity) and HBr (99.8% purity) were admitted via separate fine metering valves to the laser at the anode end-flange, and were removed at the cathode end-flange via a valve and pressure gauge to a rotary vacuum pump. Gas flow rates in the range 0.5 to 1 litreatmhr - t were employed.
3. Laser characteristics
3.1. Effect of varying HBr pressure In fig. 1, the laser output power is plotted as a function of the HBr partial pressure. The neon pressure was 17 torr, the PRF was 20 kHz, the storage capacitance was 1.67 nF (hot) and the charging voltage was 14.4 kV. Laser power is seen to increase linearly with HBr pressure up to around 1 tort, and thereafter to fall slowly. This general form of the dependence is typical of that obtained over the entire range of laser operating conditions investigated, although the optimum pressure of HBr for lasing varied with neon pressure so as to maintain the ratio of neon to HBr approximately constant (15:1 ). This implies that at higher neon pressures, larger copper densities can be tolerated. Increasing the HBr con-
centration to beyond that which corresponds to maximum laser output power led to the onset of slight discharge instabilities, as was the case with the smallbore copper HyBrID laser described in ref. [ 1 ]. These instabilities are believed to be related to an increasing concentration of atomic bromine (which is a strong electron attacher) in the active volume of the discharge. Laser oscillation begins as a thin green annulus at the lowest HBr concentrations. As the HBr pressure is increased, the annulus fills in towards the centre of the beam, until, at maximum output power, the beam has a radial intensity profile which resembles a gaussian distribution. The ratio of the green to yellow peak intensities was noted as the HBr pressure was altered. There was little effect on the ratio with variation in Ne pressure. However, it was found that upon addition of HBr to the laser tube, oscillation always began on the green line first. The green:yellow ratio reduced with increasing HBr pressure, and under maximum average power conditions the ratio was approximately 5:4. When the HBr pressure was increased further, the intensity of the yellow line began to exceed the intensity of the green, and slight discharge instabilities and contraction appeared.
100
80 O
0~.
N
6o
/O
o
~
4o /o
I~, t
/
o/O 17 torr Ne pressure 20 kHz PRF 14.4 kV charging voltage 1.67 nF storage capacitance
°/O/
20 / 0
i
0
i
i
i
I
i
,
i
1
HBr pressure (tort) Fig. 1. Laser output poweras a functionof HBr partial pressure. 291
V o l u m e 94, n u m b e r 4
15 N o v e m b e r 1992
OPTICS COMMUNICATIONS
3.2. Neon pressure
Clearly, the fact that the output powers have not reached a plateau indicates that higher output powers than the observed 94 W (at 1.5% efficiency) are possible at higher input powers (voltages), albeit at efficiencies less than 1.5%. M a x i m u m efficiency o f 2.25%, for the matching conditions o f this experiment, c o r r e s p o n d e d to an average output power o f 62.5 W. The tube wall temperature (judged from the barely visible red glow o f the laser tube when switched straight off in a d a r k e n e d r o o m ) was m a i n t a i n e d in the range 5 5 0 - 6 0 0 ° C by adjusting the thermal insulation a r o u n d the tube as input power (voltage) was altered. At the highest powers ( >_-5 k W ) , however, the tube t e m p e r a t u r e could not be prevented from rising to 600-700 ° C. Fortunately, laser output power p r o v e d to be insensitive to tube wall temperature in the range 5 0 0 - 8 0 0 ° C , so that the d a t a in fig. 3 retain their general applicability despite the variation in tube wall t e m p e r a t u r e during the measurements (see §3.5 below).
Figure 2 shows the d e p e n d e n c e o f the average laser output power and efficiency (based on stored energy) as functions o f neon gas pressure. The charging voltage was 14.4 kV, the P R F was 20 k H z a n d the storage capacitance was 1.67 nF. M a x i m u m power was achieved in the range 15 to 30 torr neon pressure. At 22 torr, an o u t p u t power o f 78 W was obtained with an efficiency o f 2.1%. M a x i m u m o u t p u t power occurs at lower neon pressures than those in the small-bore laser described in ref. [ 1 ]. This is consistent with the c o m m e n s u r a t e l y lower electric field at b r e a k d o w n o f the present laser, i.e. the E / p ratio is similar ( 5 - 6 . 5 V / c m t o r r for m a x i m u m p o w e r ) in the two lasers.
3.3. Charging voltage The dependences o f laser o u t p u t power a n d efficiency on charging voltage a n d input power are typified in fig. 3. Here, the P R F was 21 k H z and the neon pressure was 16 torr; the H B r pressure was set for m a x i m u m o u t p u t p o w e r each time the voltage was altered. The storage capacitance fell from 2.01 n F to 1.61 n F as the input power was raised from 0.53 to 6.23 kW, a n d this has been taken into account for calculation o f the efficiencies. The p o w e r supply limited the available charging voltage to a r o u n d 19 kV.
3.4. Pulse recurrence frequency O u t p u t power versus P R F is shown in fig. 4 for charging voltages o f 10.6, 12.5, 14.4 and 16.3 kV. The neon pressure was 16 torr. A value o f P R F close to that giving m a x i m u m output power (for which the storage capacitance was ~ 1.95 n F ) was chosen as a
100
80
~0~-0~0~
<
O e~
N
2 60
,-,1 ~D
O
~
e
4o
>
20 kHz PRF 14.4 kV chargingvoltage HBr pressureoptimizedat each point 1.67 nF storagecapacitance
20 0
I
0
l0
i
I
I
I
I
20
30
40
50
0
60
Neon pressure (torr)
Fig. 2. The dependences of average laser output power and efficiency (based on stored energy ) on neon buffer-gas pressure.
292
15 November 1992
OPTICS COMMUNICATIONS
Volume 94, number 4
Input power (kW)
0
80
~
4
,
,
• ""
,
,
/
/
1 /
] "-" "°"°"-..o.
•
]2
.~
>
°/
1
/o/°
20
0
6
, ,/3 - - - / 0 / 0 ~] ,
o
4o
5
,
/ /
6o
• 1,1
•
/0/0/0 /l~O~ ~O
o ~
3
,
21 kHz PRF 16 torr Ne pressure HBr pressureoptimizedat each point
o N
,
u
i-
,~
2
1
100
J/°/, 5
0
, , 10 Charging voltage (kV)
i 15
,
0 20
Fig. 3. Laser output power and efficiencyas functions of chargingvoltage and electrical input power.
100
u
16 torr Ne pressure 1.95 nF storage capacitance
80
/~unn
@
i/
"~ 6o
U"
40
_u~o
~,0 ~ L
0 ~O
16.3 kV INAI~II-IN-N.II~. / .tO-~m /e.l.
/
/010
-'e\
/O
14.4 kV
0 -o-O-O~O / 0
ii/ / /NN//O/O ~
o
k
u
"~" 12.5 kV
~
D/n 0 o.O-Ot ' " O-O-O'O" _ o..O -o"
O. 0 /
106kV
20
0
I
0
n
10
F
a
i
a
20
30
Pulse recurrence frequency (kHz) Fig. 4. Laser output power as a function of PRF for various chargingvoltages. standard, from which the P R F was quickly adjusted a n d returned to for each m e a s u r e m e n t taken. This procedure m i n i m i z e d variations in tube wall temperature a n d storage capacitance during the experiment. F r o m the figure it can be seen that the P R F which corresponded to m a x i m u m output power decreased as the charging voltage increased. However, as charging voltage increased, m a x i m u m output
power occurred for higher i n p u t powers (ranging from 2.52 kW for 10.6 kV to 4.66 kW for 16.3 kV). That observation, together with the fact that the laser output had its highest intensity on the tube axis at each of the voltages is evidence that the m a x i m u m P R F associated with a given charging voltage is not dictated by the rate of removal of heat from the laser gas mixture to the tube wall, but rather by volumet293
V o l u m e 94, n u m b e r 4
OPTICS COMMUNICATIONS
ric kinetic processes in the interpulse period. In order to establish the influence of impedance matching on laser output power, the output power was measured as a function of PRF for the range of storage/peaking capacitors 0.74 to 3.81 nF (operating values). The charging voltage was maintained at 14.4 kV, and the neon pressure was set to 16 torr; the HBr pressure was set for that giving maximum output power, and PRF quickly varied as described above. The results are shown in fig. 5. The optimum combination of circuit-to-tube impedance matching, input pulse energy and discharge current density, when the charging voltage was 14.4 kV and the neon pressure was 16 torr occurred for the storage capacitance of 1.95 nF with a PRF of 20-21 kHz. The general trend in fig. 5 is for the peak in laser power to occur at lower PRFs as the storage capacitance (input pulse energy, current density) is raised. In fig. 6, laser efficiency is plotted as a function of PRF for various storage (indicated in the figure) and peaking capacitances. The experimental conditions are the same as those applicable to fig. 5. A clear trend towards higher operating efficiencies at lower capacitances is seen. Maximum efficiency is shown as a function of storage capacitance in fig. 7. The highest efficiency of 2.7% (corresponding to an output power of 40 W) was obtained with a storage capacitance of 0.74 nF. The efficiency corresponding to maximum laser output power also rose as capaci-
15 N o v e m b e r 1992
tance (input energy) was reduced (up to 2.5% efficiency with an output power of 45 W) - see fig. 8. The trends in fig. 7 and fig. 8 are possibly due to the effects on the laser level populations of changes in the rates of ionization of copper from excited or ground states implied by the changes in stored energy delivered to the tube. However, other factors need to be considered also. For example, as capacitance is reduced, the risetime of the current pulse becomes shorter; matching of the circuit to the discharge changes as capacitance is altered, etc. We see from fig. 5 that for larger charging voltages, maximum output power occurs at lower PRFs, and from fig. 6 that the use of larger capacitances also dictates lower PRFs. These two observations are consistent with optimum PRF being lowered as a result of increasing prepulse densities of electrons and/ or metastable copper atoms. An increase in preionization density would lower the breakdown voltage of the laser tube. Hence, for a given charging voltage and neon pressure, the tube voltage at breakdown will decrease significantly as PRF is raised if the effects of increasing preionization are marked. Indeed, when the neon pressure was 16 torr and the charging voltage on the storage capacitance of 1.98 nF was 14.4 kV, an increase in PRF from 9 kHz to 20 kHz caused the tube voltage to fall from 17 to 13 kV (fig. 9 below). The observations of a large fall in tube breakdown
100 14.4 kV charging voltage 16 ton" Ne pressure '-
1.95 nF 2.51 nF
80
~.~..~=~"-a""'...~a,,.
60
/
0 J n U.AA/-"P~I"~"O/ - - A ,--
D,6/..7.n 40
3.81 nF.
"~
/
20 u
0
0
- O ~
o~O---o-o. . . .
I
10
1.00 nF o..-
/ O _/ram "hI
/O
,.°A;" . / ~-'m/" .o'2--'" A'/ / o~'/~-
0
0
~,,~
o/
1.39 nF
e
0.74 nF
I
20
e
30
Pulse reeurrenee frequeney (kHz) Fig. 5. The dependenceof laser output power on PRF for a range of storage capacitances. In each case, the peaking capacitancewas half the storage capacitance. 294
15 N o v e m b e r 1992
OPTICS COMMUNICATIONS
V o l u m e 94, n u m b e r 4
3
i
i /
& i
2
~ o ~ l - ~ l ' ~ l
J m / I
a~
A_A~--/-~...~_a / n
i:l~:l
,
0.74
nF
\ u~
2.51 nF
e~ ~
x
1.39 nF
1.95 nF
3.81 nF
14.4 kV charging voltage 16 torr Ne pressure I
0
"'a...a_
n
~.
0
~'~
O.
1
0
6
•
"1-1~
u
l''l'~'l
O~
e
f
10
e
30
20
Pulse recurrence frequency (kHz) Fig. 6. Laser efficiencyas a functionof PRF for various storagecapacitances. 3
,
,
e'~'--e.e
<
,
100
e
80
/o/°-°~o~e~-
/o
>
14.4 kV charging voltage 16 torr Ne pressure PRF optimized at each point 0
i
0
I
i
1
I
i
2
I
3
i
6o
~.
40
--
2o
~ .<
0
4
Storage capacitance (nF) Fig. 7. M a x i m u m laser efficiency a n d the c o r r e s p o n d i n g laser o u t p u t p o w e r as functions of storage capacitance.
voltage with increasing PRF indicate that the preionization density plays a major role in determining the roll-over of output power around 20 kHz, although the present experiments cannot conclusively rule out increased copper metastable densities, which are understood to limit the output power at high PRFs in elemental CVLs [ ! 2 ]. 3.5. R e m a r k s on tube wall temperature
As a first step in gauging the influence of tube temperature on the laser's operation, we have operated
the laser with insulation adjusted to give tube wall temperatures of just below 500°C (no red glow of the tube in a darkened room) and then about 800°C (bright red glow). The PRF was varied in the same manner as described in section 3.4. It was found that, in each case, the dependence of laser power on PRF was the same, and that the maximum output powers were within 5% of each other. Also, with general operation, we find that laser powers are reproducible from day to day to better than 5%, even though the outside tube insulation may differ slightly in thickness and distribution between experiments. The con295
Volume 94, number 4
OPTICS C O M M U N I C A T I O N S
15 November 1992
100 \ ~: e~
O
\
80
t.
2 60
<
/'/
40
~ o
xl =
~9
>
14.4 kV charging voltage 16 tort Ne pressure PRF optimized at each point
20
o
0
1
2
3
4
Storage capacitance (nF) Fig. 8. M a x i m u m laser output power and the corresponding efficiency as functions of storage capacitance.
.~
600
4oo
j.,
~
~
i
200
=
'
~
~.
600
4
400
2
200
o
.=.
o
(a)
"-ii
\~--
~
T i m e (50 n s / d i v . )
~
o
{b)
20
20
20
2o
~
I~/o~vo,To
]
-20
(50
ns/div.)
1
1
1
1
-20
Time
(50 n s / d i v . )
Time
-20
I
Time
-20
(50
ns/div.)
Fig. 9. Oscillograms of laser intensity (green and yellow combined), current pulse, and charging and tube voltages (a) for 9 kHz PRF, corresponding to m a x i m u m efficiency of laser generation in figure 6, and (b) for 20 kHz PRF, corresponding to m a x i m u m average output power in figure 5. The storage capacitance was 1.95 nF, the charging voltage was 14.4 kV and the Ne pressure was 16 torr.
296
Volume 94, number 4
OPTICS COMMUNICATIONS
clusion to be drawn from these observations is that the output characteristics of the laser, for temperatures up to about 800°C, are largely insensitive to the tube wall temperature. This is attributed to the efficient reaction of HBr and its discharge products with the surfaces of the copper pieces in the temperature range 500-800°C, compared with the reverse reactions which would return copper to the surfaces. The wide temperature range of operation of HyBrID lasers makes them easier to design than CVLs. In contrast to a HyBrID laser, the atomic copper concentration in a CVL is set by the tube wall temperature. Due to the nature of the thermal insulation within the vacuum envelope of a CVL, the laser can operate optimally for a very narrow range of input powers, only. The engineering and operation of a HyBrID laser is considerably simpler than a CVL because the tube wall temperature and atomic copper concentration are parameters which are essentially decoupled.
3.6. Laser pulse, current and voltage waveforms The oscillograms of laser intensity, current pulse, tube voltage and charging voltage are shown in fig. 9 for a storage capacitance 1.95 nF, a charging voltage of 14.4 kV and a Ne pressure of 16 torr, for the PRFs corresponding to maximum efficiency (9 kHz) and maximum output power (20 kHz). The laser intensity is an average over the cross-section of the output beam. It is clear that under the conditions of maximum efficiency, lasing begins close to the beginning of the current-pulse rise and terminates near the point of zero measured tube voltage (just after the current pulse reaches its peak). Thus, laser oscillation occurs during the time that the bulk of the input energy is delivered to the tube. For the conditions of maximum output power, there is a delay in the onset oflasing associated with a marked shoulder at the beginning of the current pulse, when the tube voltage is high, so that the temporal overlap of lasing with input energy deposition is reduced compared with the maximum efficiency conditions. Thus, better voltage hold-off would eliminate the slower initial increase in current at these higher input powers, and larger efficiencies and output powers would be expected as a result. The lower tube voltage hold-
15 November 1992
offs at the higher charging voltages (currents) are consistent with a larger prepulse density of electrons in the laser tube. During both the maximum efficiency and maximum power conditions the total duration of the laser pulses (green and yellow combined) reached 75 ns with the largest storage capacitances (e.g. 75 ns current-pulse risetime with 3.06 nF). The green pulse generally terminated 10 ns before the yellow pulse. Shorter laser pulses were obtained with the use of smaller storage capacitances, due to the shorter current-pulses which resulted (45 ns long laser pulse with 0.74 nF storage capacitance; 50 ns current-pulse risetime).
3. 7. Laser start-up characteristics Figure 10 illustrates the start-up characteristics of the laser. An input power of 2.5 kW was delivered to the cold tube by applying a charging voltage of 9.6 kV at a PRF of 20 kHz, with a neon pressure of 20 torr. Due to heating of the storage capacitors, this input power quickly fell to around 1.7 kW. Two minutes into the warm-up, the voltage was increased to 14.4 kV, raising the input power to approximately 3.8 kW. By 5 min the input power had equilibrated to 3.5 kW. At this time, a partial pressure of 0.5 torr of HBr was admitted to the laser tube. Laser oscillation began 13 min after initiating the discharge. At 15 min, the HBr pressure was increased to 1 torr. Twenty minutes after switching on, a maximum laser output power of 66 W was reached. The 20 minute time to full power should be compared with that of high-power CVLs, which normally take over 60 min to reach laser threshold, and even longer to attain full power.
3.8. Remarks on power scaling by tube bore If the upper limit to PRF is determined by volumetric processes such as interpulse electron-ion recombination, then HyBrID lasers should be scalable in tube bore without the need to significantly reduce PRF for maximum laser output power. This is indeed found to be the case experimentally. For the present 45 mm bore laser, the maximum output power occurred at a PRF of 21 kHz. We found earlier [ 1 ] that a HyBrID laser of 13 mm bore produced maximum output power in the similar PRF 297
Volume 94, number 4
OPTICS COMMUNICATIONS
80
I
~
60
I
15 November 1992
I
I
-~o--O-O--O-O-O-O--O
O
O--
I
t
o I ol
=
o
I
40
o
I
L
O
I
o
/
20
o
I
o o 0
i
0
i
20
I
40
a
I
I
60
80
100
Time after cold switch-on (minutes) Fig. 10. Behaviour of output power upon start-up of the laser tube from cold.
range of 21-22 kHz. We thus find that increasing the active volume of HyBrID lasers by increasing the tube bore does not carry the penalty of having to reduce the PRF. For elemental CVLs on the other hand, the PRF for maximum output power must be reduced as the tube bore is increased [ 12 ], since strong radial gradients in gas temperature cause increasing thermal prepulse densities of the metastable lower laser levels, with the largest densities produced along the tube axis. The small-bore laser was characterized by higher input power loadings at maximum output power (44 W c m -3 versus the 3.9 W c m -3 of the large-bore laser). Part of this difference is attributed to the fact that the ratio of surface area (inside) to active volume was larger (by a factor of 3) in the case of the small-bore laser. The ratio of specific output powers for the small- and large-bore lasers was 4:1 (195 m W c m - 3 of. 52 mW c m - 3 ) . Although the specific output power is lower in the case of the large-bore laser, the operating efficiency is higher. This is possibly the result of the lower peak current densities (27 A c m -2 cf. 320 A c m -2) for the 45 m m bore laser, which will alter the laser level populations via changes in the rates of ionization, as remarked earlier. (Note, however, that it is believed that the efficiency of the small-bore laser may be improved by 298
careful optimization of circuit-to-discharge matching. )
4. Conclusions We have demonstrated that the copper HyBrID laser can be operated with large bores and active volumes, to yield average output powers approaching 100 W at an efficiency of 1.5%. This type of laser is clearly superior to high-power CVLs, in terms of efficiency, start-up time, simplicity of construction and output power per unit volume. There seems to be no fundamental limitation to obtaining output powers considerably in excess of 100 W from copper HyBrID lasers by increasing both bore and length of the active region further.
Acknowledgements We are indebted to British Nuclear Fuels Ltd for supporting our work. We also express gratitude to EEV Ltd for technical advice during the experiments, and to the British Council, Bulgaria, for making possible the visit to St Andrews of Dr Sabotinov from the Institute of Solid State Physics, Sofia. In addition we wish to thank Frits Akerboom for prep-
Volume 94, number 4
OPTICS COMMUNICATIONS
a r a t i o n o f t h e silica l a s e r t u b e s , a n d C h r i s M a r t i n f o r calibration of the laser power meter.
References [ 1 ] E.S. Livingstone, D.R. Jones, A. Maitland and C.E. Little, Opt. Quantum Electron. 24 (1992) 73. [2]A.A. Isaev, M.A. Kazaryan and G.G. Petrash, Kratk. Soobshch. Fiz. No. 2 (1973) 27. [ 3 ] V.B. Vorobev, S.V. Kalinin, I.I. Klimovskii, I. Kostadinov, V.A. Krestov, V.N. Kubasov and O. Marazov, Soy. J. Quantum Electron. 21 ( 1991 ) 1067. [4] O.S. Akirtava, V.L. Dzhikiya and Yu.M. Oleinik, Sov. J. Quantum Electron. 5 (1976) 1001.
15 November 1992
[ 5 ] D.N. Astadjov, N.K. Vuchkov and N.V. Sabotinov, IEEE J. Quantum Electron. QE-24 (1988) 1927. [ 6 ] A.A. Vetter and N.M. Nerheim, Appl. Phys. Lett. 30 ( 1977 ) 405. [7] S.W. Kim, R.J. Niefer, J.B. Atkinson and L. Krause, Optics Comm. 63 (1987) 269. [8] E.S. Livingstone and A. Maitland, J. Phys. E 22 (1989) 63. [9]D.R. Jones, A. Maitland and C.E. Little, unpublished experiments. [ 10 ] H. Saito and H. Taniguchi, IEEE J. Quantum Electron. QE21 (1985) 1308. [ 11 ] T. Kano, H. Taniguchi and H. Saito, Trans. IEICE E70 (1987) 312. [ 12 ] R.R. Lewis, Opt. Quantum Electron. 23 ( 1991 ) $493.
299
Optics Communications 94 (1992) 289-299 North-Holland
Full length article
A high-power high-efficiency Cu-Ne-HBr (2= 510.6, 578.2 nm) laser D . R . J o n e s , N . V . S a b o t i n o v I, A. M a i t l a n d a n d C.E. L i t t l e J.F. Allen Research Laboratories, Department of Physics and Astronomy, Universityof St Andrews, North Haugh, St Andrews, Fife KYI 6 9SS, UK Received 2 April 1992; revised manuscript received 22 June 1992
A high-power copper bromide laser, which utilizes the technique of in situ production of CuBr by reaction of copper and HBr, is reported. The 45 mm bore copper HyBrID (Hydrogen Bromide In Discharge) laser has produced a maximum average output power at 510.6 and 578.2 nm of 94 W at 21 kHz pulse recurrence frequency, with an efficiency (based on stored energy) of 1.5%. The corresponding specific output power was 52 mW cm-3. An efficiency of 2.7% has been achieved for an average output power of 40 W. Laser oscillation can be obtained less than 15 minutes after a cold start-up, with full power attained 5 min later.
1. Introduction The high operating t e m p e r a t u r e s ( 1400-1600 ° C ) o f c o n v e n t i o n a l c o p p e r - v a p o u r lasers ( C V L s ) are responsible for their long start-up times a n d complex tube construction. These disadvantages, however, can be c i r c u m v e n t e d by the use o f copper b r o m i d e as a " l o w " t e m p e r a t u r e ( ~ 5 0 0 ° C ) source o f the required a t o m i c c o p p e r density in the laser active region. Recently, we described a new type o f copper b r o m i d e laser ( C B L ) (13 m m bore, 30 cm active length) which p r o d u c e d a specific o u t p u t power o f 195 m W c m -3 [1 ]. This figure is one o f the highest reported (exceeded only by small, narrow-bore ( 3 4 m m ) , CVLs [2,3 ] ) for multikilohertz, longitudinally excited, c o p p e r - b a s e d lasers. The tube was simple to construct, materials were used well within their limits o f operation, a n d the t i m e to laser oscillation from a cold start could be shorter than 1 min. (cf. ~ 60 min for a small C V L ) . This c o p p e r b r o m i d e laser used a new technique, whereby the necessary copper halide was p r o d u c e d on d e m a n d in the laser tube by reaction o f copper metal with HBr, which was i n t r o d u c e d with the neon buffer gas. In previous Department of Metal Vapor Lasers, Institute of Solid State Physics, 72 Tzarigradsko Chaussee, Sofia 1784, Bulgaria.
types o f CBL, copper b r o m i d e was either placed in solid form along the floor o f the laser tube a n d vaporized by discharge (or external) heating [4], or originated as a v a p o u r from heated sidearms containing the b r o m i d e [5]. O u r use o f H B r reactant permits the simultaneous i n t r o d u c t i o n o f b r o m i n e a n d hydrogen in a fixed 1 : l ratio to the discharge; in small a m o u n t s ( ~< 1 t o r r ) , hydrogen is known [5 ] to significantly increase the output powers a n d efficiencies o f copper b r o m i d e lasers. F u r t h e r m o r e , by the use o f HBr, the copper concentration in the active region can within seconds be readjusted to that o p t i m u m for laser oscillation whenever the discharge conditions (e.g. neon pressure, voltage, pulse recurrence frequency ( P R F ) ) are altered. To distinguish this type o f laser from the two conventional classes o f CVL and CBL, o f which it is clearly a hybrid, we introduce the generic descriptor H y B r I D (hydrogen b r o m i d e in discharge). In some previous studies, gas (or v a p o u r ) additives have been observed to m o d i f y (for better or worse) the operating characteristics o f copper halide lasers. F o r example, HC1, C12 a n d HE were e x a m i n e d as additives to a double-pulse CuC1 laser by Vetter and N e r h e i m [6]. It was found that with HCI, the m i n i m u m a n d o p t i m u m delay times o f the excitation pulse c o m p a r e d with the dissociation pulse was
0030-4018/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
289
Volume 94, number 4
OPTICS COMMUNICATIONS
reduced, and the laser pulse energy increased by 15%. The effects were attributed to the (unidentified) influence of hydrogen and not of chlorine, since the addition of C12 was always detrimental to laser oscillation, and the effects of H2 were similar to those of HC1. Copper metal plated out onto the tube wall when H2 or C12 was used, but not when HC1 was used, indicating that the Cu-C1-CuC1 balance is markedly affected by the addition of those gases, as might be expected. In the case of the copper HyBrID laser, we find that the alumina wall remains free of copper indicating that the Cu-Br-CuBr balance lies towards the molecular recombination side. In another study [7], Kim et al. added alkali chlorides to a doublepulse CuC1 laser, and found that the temperature regime of operation was extended; the effect was attributed to a change in the balance of Cu-CuC1 in the laser gas mixture. Only in a very few instances has the addition of a reacting species been a necessary step in attaining laser oscillation. Aside from our own work on the use of C12 [ 8 ], Br2 [ 8 ], HC1 [ 9 ] and HBr to produce the copper halide in situ, we know of only one similar scheme. Saito and Taniguchi [ 10 ] began their studies using a laser tube with a heated sidearm containing metallic copper powder placed above AIBr3, A1C13 or BiC13 powders. A reaction of the vapours of the volatile metal halides and the metallic copper yielded volatile metal complexes such as CuA12C17 and CuAlzBr7 at heater temperatures of less than 100 ° C. In subsequent experiments [ 11 ], the metallic copper powder was removed from the sidearm to the floor of the laser tube. Volatile AIBr3 vapour was entrained in flowing He buffer gas, and carried to the laser active region where it reacted with copper powder on the tube floor (heated to 400-500°C). In the latter scheme the reaction to produce the copperbearing metal-vapour complex occurred in the laser active region. The laser power so obtained was reported to be at least as large as may be obtained using the same laser as a conventional copper chloride laser. However, the scheme is more complicated than the HyBrID laser we report, since in their case, the metal halides had to be preheated to produce the vapour. More seriously, the 1 : 7 ratio of copper atoms to halogen atoms in the metal-vapour complex gives the potential for problems of discharge stability as the concentration of atomic copper (and hence of 290
15 November 1992
the halogen) is increased. Furthermore, addition of the gas hydrogen, necessary for high powers and efficiencies, would be an additional complication for the metal-vapour complex system; no such complication exists for the HyBrID laser reported here. Given the high specific output powers we have demonstrated with small-bore copper HyBrID lasers, it is of interest to establish how this type of device may be scaled to higher output powers. To that end we have constructed and tested a large-bore (45 m m ) laser, the characteristics of which we report here.
2. Experimental details The laser discharge region was confined within an alumina tube of length 120 cm and internal diameter 45 mm. With an interelectrode spacing of 114 cm, the active volume was 1815 cm 3. The alumina tube was sleeved within a fused silica cylinder of internal diameter 56 ram. The ends of the silica tube were fixed via Viton O-rings to stainless steel end-flanges. Cylindrical electrodes of molybdenum were press fitted into the end-flanges, and projected 30 m m into the alumina cylinder at each end. Fused silica windows were fixed by rubber O-rings to the ends of the end-flanges, and set at 5 ° to the normal of the tube axis. Regions of 40 cm and 60 cm length separated the electrode areas from the cathode and anode end windows, respectively. Each end-flange was watercooled and had a gas inlet or outlet. The laser cavity comprised a fiat high reflector at the cathode end, and a flat uncoated quartz flat at the anode end. The tube was normally draped with a 5 m m layer of Saffil insulation, although this was removed at the higher input powers. Three aluminium strips along the length of the tube formed a current return of low inductance, whilst still enabling the discharge tube to be cooled by free convection. Storage and peaking capacitors were mounted directly on the laser head. A single EEV type CX1535 thyratron was used in a standard resonant-charging circuit to deliver average powers of up to 7 kW to the laser head at PRFs of up to 27 kHz. The ratio of storage capacitance to peaking capacitance was always maintained at 2: 1. Flat pieces of copper ( 1 5 × 1 2 × 0 . 5 m m 3) were placed at intervals of 10 cm along the floor of the
15 November 1992
OPTICS COMMUNICATIONS
Volume 94, number 4
alumina tube. The neon (99.99% purity) and HBr (99.8% purity) were admitted via separate fine metering valves to the laser at the anode end-flange, and were removed at the cathode end-flange via a valve and pressure gauge to a rotary vacuum pump. Gas flow rates in the range 0.5 to 1 litreatmhr - t were employed.
3. Laser characteristics
3.1. Effect of varying HBr pressure In fig. 1, the laser output power is plotted as a function of the HBr partial pressure. The neon pressure was 17 torr, the PRF was 20 kHz, the storage capacitance was 1.67 nF (hot) and the charging voltage was 14.4 kV. Laser power is seen to increase linearly with HBr pressure up to around 1 tort, and thereafter to fall slowly. This general form of the dependence is typical of that obtained over the entire range of laser operating conditions investigated, although the optimum pressure of HBr for lasing varied with neon pressure so as to maintain the ratio of neon to HBr approximately constant (15:1 ). This implies that at higher neon pressures, larger copper densities can be tolerated. Increasing the HBr con-
centration to beyond that which corresponds to maximum laser output power led to the onset of slight discharge instabilities, as was the case with the smallbore copper HyBrID laser described in ref. [ 1 ]. These instabilities are believed to be related to an increasing concentration of atomic bromine (which is a strong electron attacher) in the active volume of the discharge. Laser oscillation begins as a thin green annulus at the lowest HBr concentrations. As the HBr pressure is increased, the annulus fills in towards the centre of the beam, until, at maximum output power, the beam has a radial intensity profile which resembles a gaussian distribution. The ratio of the green to yellow peak intensities was noted as the HBr pressure was altered. There was little effect on the ratio with variation in Ne pressure. However, it was found that upon addition of HBr to the laser tube, oscillation always began on the green line first. The green:yellow ratio reduced with increasing HBr pressure, and under maximum average power conditions the ratio was approximately 5:4. When the HBr pressure was increased further, the intensity of the yellow line began to exceed the intensity of the green, and slight discharge instabilities and contraction appeared.
100
80 O
0~.
N
6o
/O
o
~
4o /o
I~, t
/
o/O 17 torr Ne pressure 20 kHz PRF 14.4 kV charging voltage 1.67 nF storage capacitance
°/O/
20 / 0
i
0
i
i
i
I
i
,
i
1
HBr pressure (tort) Fig. 1. Laser output poweras a functionof HBr partial pressure. 291
V o l u m e 94, n u m b e r 4
15 N o v e m b e r 1992
OPTICS COMMUNICATIONS
3.2. Neon pressure
Clearly, the fact that the output powers have not reached a plateau indicates that higher output powers than the observed 94 W (at 1.5% efficiency) are possible at higher input powers (voltages), albeit at efficiencies less than 1.5%. M a x i m u m efficiency o f 2.25%, for the matching conditions o f this experiment, c o r r e s p o n d e d to an average output power o f 62.5 W. The tube wall temperature (judged from the barely visible red glow o f the laser tube when switched straight off in a d a r k e n e d r o o m ) was m a i n t a i n e d in the range 5 5 0 - 6 0 0 ° C by adjusting the thermal insulation a r o u n d the tube as input power (voltage) was altered. At the highest powers ( >_-5 k W ) , however, the tube t e m p e r a t u r e could not be prevented from rising to 600-700 ° C. Fortunately, laser output power p r o v e d to be insensitive to tube wall temperature in the range 5 0 0 - 8 0 0 ° C , so that the d a t a in fig. 3 retain their general applicability despite the variation in tube wall t e m p e r a t u r e during the measurements (see §3.5 below).
Figure 2 shows the d e p e n d e n c e o f the average laser output power and efficiency (based on stored energy) as functions o f neon gas pressure. The charging voltage was 14.4 kV, the P R F was 20 k H z a n d the storage capacitance was 1.67 nF. M a x i m u m power was achieved in the range 15 to 30 torr neon pressure. At 22 torr, an o u t p u t power o f 78 W was obtained with an efficiency o f 2.1%. M a x i m u m o u t p u t power occurs at lower neon pressures than those in the small-bore laser described in ref. [ 1 ]. This is consistent with the c o m m e n s u r a t e l y lower electric field at b r e a k d o w n o f the present laser, i.e. the E / p ratio is similar ( 5 - 6 . 5 V / c m t o r r for m a x i m u m p o w e r ) in the two lasers.
3.3. Charging voltage The dependences o f laser o u t p u t power a n d efficiency on charging voltage a n d input power are typified in fig. 3. Here, the P R F was 21 k H z and the neon pressure was 16 torr; the H B r pressure was set for m a x i m u m o u t p u t p o w e r each time the voltage was altered. The storage capacitance fell from 2.01 n F to 1.61 n F as the input power was raised from 0.53 to 6.23 kW, a n d this has been taken into account for calculation o f the efficiencies. The p o w e r supply limited the available charging voltage to a r o u n d 19 kV.
3.4. Pulse recurrence frequency O u t p u t power versus P R F is shown in fig. 4 for charging voltages o f 10.6, 12.5, 14.4 and 16.3 kV. The neon pressure was 16 torr. A value o f P R F close to that giving m a x i m u m output power (for which the storage capacitance was ~ 1.95 n F ) was chosen as a
100
80
~0~-0~0~
<
O e~
N
2 60
,-,1 ~D
O
~
e
4o
>
20 kHz PRF 14.4 kV chargingvoltage HBr pressureoptimizedat each point 1.67 nF storagecapacitance
20 0
I
0
l0
i
I
I
I
I
20
30
40
50
0
60
Neon pressure (torr)
Fig. 2. The dependences of average laser output power and efficiency (based on stored energy ) on neon buffer-gas pressure.
292
15 November 1992
OPTICS COMMUNICATIONS
Volume 94, number 4
Input power (kW)
0
80
~
4
,
,
• ""
,
,
/
/
1 /
] "-" "°"°"-..o.
•
]2
.~
>
°/
1
/o/°
20
0
6
, ,/3 - - - / 0 / 0 ~] ,
o
4o
5
,
/ /
6o
• 1,1
•
/0/0/0 /l~O~ ~O
o ~
3
,
21 kHz PRF 16 torr Ne pressure HBr pressureoptimizedat each point
o N
,
u
i-
,~
2
1
100
J/°/, 5
0
, , 10 Charging voltage (kV)
i 15
,
0 20
Fig. 3. Laser output power and efficiencyas functions of chargingvoltage and electrical input power.
100
u
16 torr Ne pressure 1.95 nF storage capacitance
80
/~unn
@
i/
"~ 6o
U"
40
_u~o
~,0 ~ L
0 ~O
16.3 kV INAI~II-IN-N.II~. / .tO-~m /e.l.
/
/010
-'e\
/O
14.4 kV
0 -o-O-O~O / 0
ii/ / /NN//O/O ~
o
k
u
"~" 12.5 kV
~
D/n 0 o.O-Ot ' " O-O-O'O" _ o..O -o"
O. 0 /
106kV
20
0
I
0
n
10
F
a
i
a
20
30
Pulse recurrence frequency (kHz) Fig. 4. Laser output power as a function of PRF for various chargingvoltages. standard, from which the P R F was quickly adjusted a n d returned to for each m e a s u r e m e n t taken. This procedure m i n i m i z e d variations in tube wall temperature a n d storage capacitance during the experiment. F r o m the figure it can be seen that the P R F which corresponded to m a x i m u m output power decreased as the charging voltage increased. However, as charging voltage increased, m a x i m u m output
power occurred for higher i n p u t powers (ranging from 2.52 kW for 10.6 kV to 4.66 kW for 16.3 kV). That observation, together with the fact that the laser output had its highest intensity on the tube axis at each of the voltages is evidence that the m a x i m u m P R F associated with a given charging voltage is not dictated by the rate of removal of heat from the laser gas mixture to the tube wall, but rather by volumet293
V o l u m e 94, n u m b e r 4
OPTICS COMMUNICATIONS
ric kinetic processes in the interpulse period. In order to establish the influence of impedance matching on laser output power, the output power was measured as a function of PRF for the range of storage/peaking capacitors 0.74 to 3.81 nF (operating values). The charging voltage was maintained at 14.4 kV, and the neon pressure was set to 16 torr; the HBr pressure was set for that giving maximum output power, and PRF quickly varied as described above. The results are shown in fig. 5. The optimum combination of circuit-to-tube impedance matching, input pulse energy and discharge current density, when the charging voltage was 14.4 kV and the neon pressure was 16 torr occurred for the storage capacitance of 1.95 nF with a PRF of 20-21 kHz. The general trend in fig. 5 is for the peak in laser power to occur at lower PRFs as the storage capacitance (input pulse energy, current density) is raised. In fig. 6, laser efficiency is plotted as a function of PRF for various storage (indicated in the figure) and peaking capacitances. The experimental conditions are the same as those applicable to fig. 5. A clear trend towards higher operating efficiencies at lower capacitances is seen. Maximum efficiency is shown as a function of storage capacitance in fig. 7. The highest efficiency of 2.7% (corresponding to an output power of 40 W) was obtained with a storage capacitance of 0.74 nF. The efficiency corresponding to maximum laser output power also rose as capaci-
15 N o v e m b e r 1992
tance (input energy) was reduced (up to 2.5% efficiency with an output power of 45 W) - see fig. 8. The trends in fig. 7 and fig. 8 are possibly due to the effects on the laser level populations of changes in the rates of ionization of copper from excited or ground states implied by the changes in stored energy delivered to the tube. However, other factors need to be considered also. For example, as capacitance is reduced, the risetime of the current pulse becomes shorter; matching of the circuit to the discharge changes as capacitance is altered, etc. We see from fig. 5 that for larger charging voltages, maximum output power occurs at lower PRFs, and from fig. 6 that the use of larger capacitances also dictates lower PRFs. These two observations are consistent with optimum PRF being lowered as a result of increasing prepulse densities of electrons and/ or metastable copper atoms. An increase in preionization density would lower the breakdown voltage of the laser tube. Hence, for a given charging voltage and neon pressure, the tube voltage at breakdown will decrease significantly as PRF is raised if the effects of increasing preionization are marked. Indeed, when the neon pressure was 16 torr and the charging voltage on the storage capacitance of 1.98 nF was 14.4 kV, an increase in PRF from 9 kHz to 20 kHz caused the tube voltage to fall from 17 to 13 kV (fig. 9 below). The observations of a large fall in tube breakdown
100 14.4 kV charging voltage 16 ton" Ne pressure '-
1.95 nF 2.51 nF
80
~.~..~=~"-a""'...~a,,.
60
/
0 J n U.AA/-"P~I"~"O/ - - A ,--
D,6/..7.n 40
3.81 nF.
"~
/
20 u
0
0
- O ~
o~O---o-o. . . .
I
10
1.00 nF o..-
/ O _/ram "hI
/O
,.°A;" . / ~-'m/" .o'2--'" A'/ / o~'/~-
0
0
~,,~
o/
1.39 nF
e
0.74 nF
I
20
e
30
Pulse reeurrenee frequeney (kHz) Fig. 5. The dependenceof laser output power on PRF for a range of storage capacitances. In each case, the peaking capacitancewas half the storage capacitance. 294
15 N o v e m b e r 1992
OPTICS COMMUNICATIONS
V o l u m e 94, n u m b e r 4
3
i
i /
& i
2
~ o ~ l - ~ l ' ~ l
J m / I
a~
A_A~--/-~...~_a / n
i:l~:l
,
0.74
nF
\ u~
2.51 nF
e~ ~
x
1.39 nF
1.95 nF
3.81 nF
14.4 kV charging voltage 16 torr Ne pressure I
0
"'a...a_
n
~.
0
~'~
O.
1
0
6
•
"1-1~
u
l''l'~'l
O~
e
f
10
e
30
20
Pulse recurrence frequency (kHz) Fig. 6. Laser efficiencyas a functionof PRF for various storagecapacitances. 3
,
,
e'~'--e.e
<
,
100
e
80
/o/°-°~o~e~-
/o
>
14.4 kV charging voltage 16 torr Ne pressure PRF optimized at each point 0
i
0
I
i
1
I
i
2
I
3
i
6o
~.
40
--
2o
~ .<
0
4
Storage capacitance (nF) Fig. 7. M a x i m u m laser efficiency a n d the c o r r e s p o n d i n g laser o u t p u t p o w e r as functions of storage capacitance.
voltage with increasing PRF indicate that the preionization density plays a major role in determining the roll-over of output power around 20 kHz, although the present experiments cannot conclusively rule out increased copper metastable densities, which are understood to limit the output power at high PRFs in elemental CVLs [ ! 2 ]. 3.5. R e m a r k s on tube wall temperature
As a first step in gauging the influence of tube temperature on the laser's operation, we have operated
the laser with insulation adjusted to give tube wall temperatures of just below 500°C (no red glow of the tube in a darkened room) and then about 800°C (bright red glow). The PRF was varied in the same manner as described in section 3.4. It was found that, in each case, the dependence of laser power on PRF was the same, and that the maximum output powers were within 5% of each other. Also, with general operation, we find that laser powers are reproducible from day to day to better than 5%, even though the outside tube insulation may differ slightly in thickness and distribution between experiments. The con295
Volume 94, number 4
OPTICS C O M M U N I C A T I O N S
15 November 1992
100 \ ~: e~
O
\
80
t.
2 60
<
/'/
40
~ o
xl =
~9
>
14.4 kV charging voltage 16 tort Ne pressure PRF optimized at each point
20
o
0
1
2
3
4
Storage capacitance (nF) Fig. 8. M a x i m u m laser output power and the corresponding efficiency as functions of storage capacitance.
.~
600
4oo
j.,
~
~
i
200
=
'
~
~.
600
4
400
2
200
o
.=.
o
(a)
"-ii
\~--
~
T i m e (50 n s / d i v . )
~
o
{b)
20
20
20
2o
~
I~/o~vo,To
]
-20
(50
ns/div.)
1
1
1
1
-20
Time
(50 n s / d i v . )
Time
-20
I
Time
-20
(50
ns/div.)
Fig. 9. Oscillograms of laser intensity (green and yellow combined), current pulse, and charging and tube voltages (a) for 9 kHz PRF, corresponding to m a x i m u m efficiency of laser generation in figure 6, and (b) for 20 kHz PRF, corresponding to m a x i m u m average output power in figure 5. The storage capacitance was 1.95 nF, the charging voltage was 14.4 kV and the Ne pressure was 16 torr.
296
Volume 94, number 4
OPTICS COMMUNICATIONS
clusion to be drawn from these observations is that the output characteristics of the laser, for temperatures up to about 800°C, are largely insensitive to the tube wall temperature. This is attributed to the efficient reaction of HBr and its discharge products with the surfaces of the copper pieces in the temperature range 500-800°C, compared with the reverse reactions which would return copper to the surfaces. The wide temperature range of operation of HyBrID lasers makes them easier to design than CVLs. In contrast to a HyBrID laser, the atomic copper concentration in a CVL is set by the tube wall temperature. Due to the nature of the thermal insulation within the vacuum envelope of a CVL, the laser can operate optimally for a very narrow range of input powers, only. The engineering and operation of a HyBrID laser is considerably simpler than a CVL because the tube wall temperature and atomic copper concentration are parameters which are essentially decoupled.
3.6. Laser pulse, current and voltage waveforms The oscillograms of laser intensity, current pulse, tube voltage and charging voltage are shown in fig. 9 for a storage capacitance 1.95 nF, a charging voltage of 14.4 kV and a Ne pressure of 16 torr, for the PRFs corresponding to maximum efficiency (9 kHz) and maximum output power (20 kHz). The laser intensity is an average over the cross-section of the output beam. It is clear that under the conditions of maximum efficiency, lasing begins close to the beginning of the current-pulse rise and terminates near the point of zero measured tube voltage (just after the current pulse reaches its peak). Thus, laser oscillation occurs during the time that the bulk of the input energy is delivered to the tube. For the conditions of maximum output power, there is a delay in the onset oflasing associated with a marked shoulder at the beginning of the current pulse, when the tube voltage is high, so that the temporal overlap of lasing with input energy deposition is reduced compared with the maximum efficiency conditions. Thus, better voltage hold-off would eliminate the slower initial increase in current at these higher input powers, and larger efficiencies and output powers would be expected as a result. The lower tube voltage hold-
15 November 1992
offs at the higher charging voltages (currents) are consistent with a larger prepulse density of electrons in the laser tube. During both the maximum efficiency and maximum power conditions the total duration of the laser pulses (green and yellow combined) reached 75 ns with the largest storage capacitances (e.g. 75 ns current-pulse risetime with 3.06 nF). The green pulse generally terminated 10 ns before the yellow pulse. Shorter laser pulses were obtained with the use of smaller storage capacitances, due to the shorter current-pulses which resulted (45 ns long laser pulse with 0.74 nF storage capacitance; 50 ns current-pulse risetime).
3. 7. Laser start-up characteristics Figure 10 illustrates the start-up characteristics of the laser. An input power of 2.5 kW was delivered to the cold tube by applying a charging voltage of 9.6 kV at a PRF of 20 kHz, with a neon pressure of 20 torr. Due to heating of the storage capacitors, this input power quickly fell to around 1.7 kW. Two minutes into the warm-up, the voltage was increased to 14.4 kV, raising the input power to approximately 3.8 kW. By 5 min the input power had equilibrated to 3.5 kW. At this time, a partial pressure of 0.5 torr of HBr was admitted to the laser tube. Laser oscillation began 13 min after initiating the discharge. At 15 min, the HBr pressure was increased to 1 torr. Twenty minutes after switching on, a maximum laser output power of 66 W was reached. The 20 minute time to full power should be compared with that of high-power CVLs, which normally take over 60 min to reach laser threshold, and even longer to attain full power.
3.8. Remarks on power scaling by tube bore If the upper limit to PRF is determined by volumetric processes such as interpulse electron-ion recombination, then HyBrID lasers should be scalable in tube bore without the need to significantly reduce PRF for maximum laser output power. This is indeed found to be the case experimentally. For the present 45 mm bore laser, the maximum output power occurred at a PRF of 21 kHz. We found earlier [ 1 ] that a HyBrID laser of 13 mm bore produced maximum output power in the similar PRF 297
Volume 94, number 4
OPTICS COMMUNICATIONS
80
I
~
60
I
15 November 1992
I
I
-~o--O-O--O-O-O-O--O
O
O--
I
t
o I ol
=
o
I
40
o
I
L
O
I
o
/
20
o
I
o o 0
i
0
i
20
I
40
a
I
I
60
80
100
Time after cold switch-on (minutes) Fig. 10. Behaviour of output power upon start-up of the laser tube from cold.
range of 21-22 kHz. We thus find that increasing the active volume of HyBrID lasers by increasing the tube bore does not carry the penalty of having to reduce the PRF. For elemental CVLs on the other hand, the PRF for maximum output power must be reduced as the tube bore is increased [ 12 ], since strong radial gradients in gas temperature cause increasing thermal prepulse densities of the metastable lower laser levels, with the largest densities produced along the tube axis. The small-bore laser was characterized by higher input power loadings at maximum output power (44 W c m -3 versus the 3.9 W c m -3 of the large-bore laser). Part of this difference is attributed to the fact that the ratio of surface area (inside) to active volume was larger (by a factor of 3) in the case of the small-bore laser. The ratio of specific output powers for the small- and large-bore lasers was 4:1 (195 m W c m - 3 of. 52 mW c m - 3 ) . Although the specific output power is lower in the case of the large-bore laser, the operating efficiency is higher. This is possibly the result of the lower peak current densities (27 A c m -2 cf. 320 A c m -2) for the 45 m m bore laser, which will alter the laser level populations via changes in the rates of ionization, as remarked earlier. (Note, however, that it is believed that the efficiency of the small-bore laser may be improved by 298
careful optimization of circuit-to-discharge matching. )
4. Conclusions We have demonstrated that the copper HyBrID laser can be operated with large bores and active volumes, to yield average output powers approaching 100 W at an efficiency of 1.5%. This type of laser is clearly superior to high-power CVLs, in terms of efficiency, start-up time, simplicity of construction and output power per unit volume. There seems to be no fundamental limitation to obtaining output powers considerably in excess of 100 W from copper HyBrID lasers by increasing both bore and length of the active region further.
Acknowledgements We are indebted to British Nuclear Fuels Ltd for supporting our work. We also express gratitude to EEV Ltd for technical advice during the experiments, and to the British Council, Bulgaria, for making possible the visit to St Andrews of Dr Sabotinov from the Institute of Solid State Physics, Sofia. In addition we wish to thank Frits Akerboom for prep-
Volume 94, number 4
OPTICS COMMUNICATIONS
a r a t i o n o f t h e silica l a s e r t u b e s , a n d C h r i s M a r t i n f o r calibration of the laser power meter.
References [ 1 ] E.S. Livingstone, D.R. Jones, A. Maitland and C.E. Little, Opt. Quantum Electron. 24 (1992) 73. [2]A.A. Isaev, M.A. Kazaryan and G.G. Petrash, Kratk. Soobshch. Fiz. No. 2 (1973) 27. [ 3 ] V.B. Vorobev, S.V. Kalinin, I.I. Klimovskii, I. Kostadinov, V.A. Krestov, V.N. Kubasov and O. Marazov, Soy. J. Quantum Electron. 21 ( 1991 ) 1067. [4] O.S. Akirtava, V.L. Dzhikiya and Yu.M. Oleinik, Sov. J. Quantum Electron. 5 (1976) 1001.
15 November 1992
[ 5 ] D.N. Astadjov, N.K. Vuchkov and N.V. Sabotinov, IEEE J. Quantum Electron. QE-24 (1988) 1927. [ 6 ] A.A. Vetter and N.M. Nerheim, Appl. Phys. Lett. 30 ( 1977 ) 405. [7] S.W. Kim, R.J. Niefer, J.B. Atkinson and L. Krause, Optics Comm. 63 (1987) 269. [8] E.S. Livingstone and A. Maitland, J. Phys. E 22 (1989) 63. [9]D.R. Jones, A. Maitland and C.E. Little, unpublished experiments. [ 10 ] H. Saito and H. Taniguchi, IEEE J. Quantum Electron. QE21 (1985) 1308. [ 11 ] T. Kano, H. Taniguchi and H. Saito, Trans. IEICE E70 (1987) 312. [ 12 ] R.R. Lewis, Opt. Quantum Electron. 23 ( 1991 ) $493.
299