Laurent de Schoulepnikoff,
Valentin Mitev,
Valentin Simeonov,
Bertrand Calpini,
and Hubert van den Bergh
L. Schoulepnikoff and V. Mitev are with the Observatory of Neuchâtel, Rue Observatoire 58, Neuchâtel CH 2000, Switzerland.
V. Simeonov, B. Calpini, and H. van den Bergh are with the Swiss Federal Institute of Technology, Laboratory for Air and Soil Pollution, Lausanne, Switzerland.
Laurent de Schoulepnikoff, Valentin Mitev, Valentin Simeonov, Bertrand Calpini, and Hubert van den Bergh, "Experimental investigation of high-power single-pass Raman shifters in the ultraviolet with Nd:YAG and KrF lasers," Appl. Opt. 36, 5026-5043 (1997)
Single-pass Raman cells pumped by either a quadrupled Nd:YAG (266-nm)
laser or a KrF excimer laser are studied. The Raman-active gases comprise
H2, D2, or
CH4, as well as a mixture of them, with the addition of
He, Ne, or Ar. A parametric study, in which the Stokes conversion efficiency
and the beam quality (M2) were
measured, was made. The first Stokes efficiency increases and all the Stokes
thresholds decrease with an increase in the lens focal length or the
M2 parameter of the pump beam.
The quality of the Stokes beams deteriorates when the active-gas pressure
increases but is improved by the addition of an inert gas. Laser-induced
breakdown is shown to be a factor that limits the conversion efficiency and
the quality of the Stokes beams. With a mixture of D2,
H2, and Ar, a 10–15-mJ pulse energy is obtained
(depending on the pump M2
parameter) in the first Stokes beam of D2 (289 nm) and
H2 (299 nm), with a full-angle divergence of 0.5 mrad
(at 86% power).
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Mixture of Active and Buffer Gases that Maximize the First Stokes Photon Conversion Efficiency (η̂)
Laser
Lens Focal Length (cm)
Active Gas
Active-Gas Pressure (atm)
Buffer Gas
Buffer-Gas Pressure (atm)
η̂ (%)
Su
25
H2
5
Ar
20
31
D2
10
20
35
50
H2
10
Ar
55
56
D2
10
30
47
75
H2
10
Ar
40
64
D2
10
40
47
Po
50
H2
10
Ar
25
41
D2
12
15
30
75
H2
1.5
He
10
35
3.0
Ne
8.0
32
10
Ar
33
61
D2
2.0
He
2.5
39
3.0
Ne
6.0
37
10
Ar
35
33
CH4
10
Ar
45
52
Table 5
Beam Quality Measured at the Output of the Raman Cella
Laser
Active Gas
Active-Gas Pressure (atm)
Buffer Gas
Buffer-Gas Pressure (atm)
x/y (Ex Laser)
Beam
M̃2 Parameter
Ex
H2
8.5
—
x
P
1.0 ± 0.1
S1
0.48 ± 0.04
S2
0.51 ± 0.1
y
P
1.6 ± 0.1
S1
0.66 ± 0.06
S2
0.48 ± 0.09
26
—
x
P
1.1 ± 0.1
S1
0.57 ± 0.09
S2
0.37 ± 0.04
y
P
1.6 ± 0.1
S1
0.94 ± 0.08
S2
0.57 ± 0.06
15
He
7.5
x
P
1.0 ± 0.1
S
1
0.41 ± 0.1
S2
0.23 ± 0.05
y
P
1.2 ± 0.1
S1
0.68 ± 0.08
S2
0.45 ± 0.07
Su
H2
5
Ar
25
S1
1.2 ± 0.1
Po
H2
2
—
S1
0.73 ± 0.06
10
—
P
3.3 ± 0.3
S1
3.4 ± 0.4
S2
12.5 ± 1.5
10
Ar
25
P
1.2 ± 0.1
S1
1.1 ± 0.2
10
He
10
S1
1.4 ± 0.2
D2
2.5
—
P
1.1 ± 0.1
S1
0.52 ± 0.04
The cell-lens focal length is 50 cm (Su
and Ex lasers) and 75 cm (Po laser). For the Ex laser, the knife-edge
measurement was performed along the biggest (x) and
the lowest (y) dimensions of the rectangular beam,
whereas for the Nd:YAG lasers the M2
parameters along two perpendicular axes were measured to be within
10% of each other and are presented as an average in the table. The
M2 parameter is normalized with
the M2 of the output pump beam
when the cell is filled with 25 atm of Ar, namely, 273
(x) and 27.6 (y) for the Ex laser,
16.1 for the Su laser, and 14.0 for the Po laser. The normalized
M2 is denoted as
M̃2. The uncertainties of
M2 are calculated from the
dispersion of the measurement points around the model curve,
Eqs. (6). P in the Beam
column refers to the residual output pump.
Table 6
Experimental Partial Pressures in a Mixture of H2, D2, and Ar that Yield Equal Pulse Energies in the first Stokes beams of D2 and H2a
Laser
H2 Pressure (atm)
D2 Pressure (atm)
Ar Pressure (atm)
Input Pump Pulse Energy (mJ)
S1 Pulse Energy (mJ)
Su
5.0
20
1.0
50
10
7.0
19
33
50
14
10
15
>20
50
–
Po
3.5
9.0
5.5
70
7.0
4.0
12
20
70
9.0
5.0
10
>20
70
–
5.5
15
12
70
9.0
7.0
20
17
55
7.0
A lower bound in the Ar pressure column
and correspondingly an – in the S1
energy column means that, in the indicated pressure range, equal energy in
both first Stokes beams could not be reached. A 75-cm focal-length lens was
used.
Table 7
Spectrum at Output of a Raman Cell Filled with a Mixture of H2, D2, and Ar, as Dispersed by a Gratinga
Wavelength (nm)
Identification
239.5
AS1(H2, 266.0)
246.4
AS1(D2, 266.0)
258.0
AS1(H2, 289.0)
266.0
Pump
274.4
AS1(D2, 299.0)
279.6
AS1(H2, 316.3)
283.6
AS2(D2, 341.5)
289.0
S1(D2, 266.0)
299.0
S1(H2, 299.0)
309.8
AS1(D2, 341.5)
316.3
S2(D2, 266.0)
328.4
S1(D2, 299.0)
S1(H2, 289.0)
341.5
S2(H2, 266.0)
349.3
S3(D2, 266.0)
364.2
S2(D2, 299.0)
S1(H2, 316.3)
380.3
S2(H2, 289.0)
S1(D2, 341.5)
390.2
S4(D2, 266.0)
397.9
S3(H2, 266.0)
408.7
S3(D2, 299.0)
S1(H2, 349.3)
The lines are identified with the
following notation: For example,
S1(D2,
299.0) indicates the first Stokes beam in D2 of the
299.0 nm beam, which is the first Stokes beam in H2
of the 266.0-nm input pump. The parameters are 70 mJ at cell input, Po laser,
5.5-atm H2, 15-atm D2, 12.5-atm
Ar, and 75-cm focal-length lens.
Tables (7)
Table 1
Parameters Used in the Calculation of the Raman Gain at 295 K [Eq. (1)]a
Mixture of Active and Buffer Gases that Maximize the First Stokes Photon Conversion Efficiency (η̂)
Laser
Lens Focal Length (cm)
Active Gas
Active-Gas Pressure (atm)
Buffer Gas
Buffer-Gas Pressure (atm)
η̂ (%)
Su
25
H2
5
Ar
20
31
D2
10
20
35
50
H2
10
Ar
55
56
D2
10
30
47
75
H2
10
Ar
40
64
D2
10
40
47
Po
50
H2
10
Ar
25
41
D2
12
15
30
75
H2
1.5
He
10
35
3.0
Ne
8.0
32
10
Ar
33
61
D2
2.0
He
2.5
39
3.0
Ne
6.0
37
10
Ar
35
33
CH4
10
Ar
45
52
Table 5
Beam Quality Measured at the Output of the Raman Cella
Laser
Active Gas
Active-Gas Pressure (atm)
Buffer Gas
Buffer-Gas Pressure (atm)
x/y (Ex Laser)
Beam
M̃2 Parameter
Ex
H2
8.5
—
x
P
1.0 ± 0.1
S1
0.48 ± 0.04
S2
0.51 ± 0.1
y
P
1.6 ± 0.1
S1
0.66 ± 0.06
S2
0.48 ± 0.09
26
—
x
P
1.1 ± 0.1
S1
0.57 ± 0.09
S2
0.37 ± 0.04
y
P
1.6 ± 0.1
S1
0.94 ± 0.08
S2
0.57 ± 0.06
15
He
7.5
x
P
1.0 ± 0.1
S
1
0.41 ± 0.1
S2
0.23 ± 0.05
y
P
1.2 ± 0.1
S1
0.68 ± 0.08
S2
0.45 ± 0.07
Su
H2
5
Ar
25
S1
1.2 ± 0.1
Po
H2
2
—
S1
0.73 ± 0.06
10
—
P
3.3 ± 0.3
S1
3.4 ± 0.4
S2
12.5 ± 1.5
10
Ar
25
P
1.2 ± 0.1
S1
1.1 ± 0.2
10
He
10
S1
1.4 ± 0.2
D2
2.5
—
P
1.1 ± 0.1
S1
0.52 ± 0.04
The cell-lens focal length is 50 cm (Su
and Ex lasers) and 75 cm (Po laser). For the Ex laser, the knife-edge
measurement was performed along the biggest (x) and
the lowest (y) dimensions of the rectangular beam,
whereas for the Nd:YAG lasers the M2
parameters along two perpendicular axes were measured to be within
10% of each other and are presented as an average in the table. The
M2 parameter is normalized with
the M2 of the output pump beam
when the cell is filled with 25 atm of Ar, namely, 273
(x) and 27.6 (y) for the Ex laser,
16.1 for the Su laser, and 14.0 for the Po laser. The normalized
M2 is denoted as
M̃2. The uncertainties of
M2 are calculated from the
dispersion of the measurement points around the model curve,
Eqs. (6). P in the Beam
column refers to the residual output pump.
Table 6
Experimental Partial Pressures in a Mixture of H2, D2, and Ar that Yield Equal Pulse Energies in the first Stokes beams of D2 and H2a
Laser
H2 Pressure (atm)
D2 Pressure (atm)
Ar Pressure (atm)
Input Pump Pulse Energy (mJ)
S1 Pulse Energy (mJ)
Su
5.0
20
1.0
50
10
7.0
19
33
50
14
10
15
>20
50
–
Po
3.5
9.0
5.5
70
7.0
4.0
12
20
70
9.0
5.0
10
>20
70
–
5.5
15
12
70
9.0
7.0
20
17
55
7.0
A lower bound in the Ar pressure column
and correspondingly an – in the S1
energy column means that, in the indicated pressure range, equal energy in
both first Stokes beams could not be reached. A 75-cm focal-length lens was
used.
Table 7
Spectrum at Output of a Raman Cell Filled with a Mixture of H2, D2, and Ar, as Dispersed by a Gratinga
Wavelength (nm)
Identification
239.5
AS1(H2, 266.0)
246.4
AS1(D2, 266.0)
258.0
AS1(H2, 289.0)
266.0
Pump
274.4
AS1(D2, 299.0)
279.6
AS1(H2, 316.3)
283.6
AS2(D2, 341.5)
289.0
S1(D2, 266.0)
299.0
S1(H2, 299.0)
309.8
AS1(D2, 341.5)
316.3
S2(D2, 266.0)
328.4
S1(D2, 299.0)
S1(H2, 289.0)
341.5
S2(H2, 266.0)
349.3
S3(D2, 266.0)
364.2
S2(D2, 299.0)
S1(H2, 316.3)
380.3
S2(H2, 289.0)
S1(D2, 341.5)
390.2
S4(D2, 266.0)
397.9
S3(H2, 266.0)
408.7
S3(D2, 299.0)
S1(H2, 349.3)
The lines are identified with the
following notation: For example,
S1(D2,
299.0) indicates the first Stokes beam in D2 of the
299.0 nm beam, which is the first Stokes beam in H2
of the 266.0-nm input pump. The parameters are 70 mJ at cell input, Po laser,
5.5-atm H2, 15-atm D2, 12.5-atm
Ar, and 75-cm focal-length lens.