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The output wavelength of the dye laser can be tuned by turning the Littrow mounted diffraction grating end reflector about an axis parallel to the grooves. The equation relating wavelength to angle of incidence on the grating is nl = 2d sin i. Using this relationship, one can calibrate the mirror mount micrometer to i and, using a grating as one of the reflectors in a dye laser, cause the spectral line width of the output to be reduced to a narrow region around the Littrow wavelength. Maximum reduction is attained by increasing dispersion to the practical limit. As can be seen from the equation for angular dispersion,
this limit is approached for the high blaze angle characteristic of echelles and finely spaced echellettes. Even greater selectivity can be attained by adding a Fabry-Perot etalon to the grating-mirror cavity. The efficiency of the spectral condensation in organic dye lasers is quite high, with figures of 70% being typical.
Gratings are normally used in one of two configurations: Littrow or grazing incidence (see figure below).
GRAZING INCIDENCE GRATINGS
Grazing incidence is a simple and inexpensive optical configuration that can tune and increase the resolution of a dye laser. A holographic grating, functioning as an end reflector in a dye laser cavity, is positioned so that laser radiation strikes the grating almost perpendicular to the grating normal. As the angle of incidence approaches 89 degrees, a relatively large area of the grating is illuminated by the laser beam, increasing angular dispersion and resolving power significantly. The sizes of the grazing and mirror (12.7 x 50.8 mm) are optimized for grazing incidence. The grating is fixed and tuning is achieved by rotation of the mirror. The laser beam is diffracted twice in grating incidence, resulting in a two fold increase in resolution. Low grating efficiency is characteristic of the grazing incidence configuration but is compensated for by the high gain of the dyes used.
| LITTROW SERIES GRATINGS |
| SPECIFICATIONS |
 Dimensional Tolerances |
- |
±0.5mm |
| Thickness Tolerances |
- |
±0.5mm |
| EFFICIENCIES |
| Roled |
- |
60 - 80% at blaze |
| Holographic |
- |
45 - 65% at peak |
| Clear Apertore |
- |
90% |
| Groove parallelism to edge |
- |
±0.5° |
|
GROOVES
PER MM
|
BLAZE λ
(nm)
|
BLAZE
ANGLE
|
DISPERSION
(nm/mr)
{@ 500 nm}
|
12.5 x 25 x 6 CAT. NO |
25 x 25 x 9.5 CAT. NO. |
30 x 30 x 9.5 CAT. NO. |
50 x 50 x 9.5 CAT. NO. |
| RULED |
| 1200 |
300
|
10° 2' |
0.82 |
3-4131 |
3-2131 |
3-3131 |
3-5131 |
| 1200 |
500
|
17° 26 |
0.79 |
3-4151 |
3-2151 |
3-3151 |
3-5151 |
| 1200 |
750
|
26° 45 |
0.74 |
3-4171 |
3-2171 |
3-3171 |
3-5171 |
| 1200 |
1000 |
36° 52 |
0.67 |
3-4111 |
3-2111 |
3-3111 |
3-5111 |
| 1800 |
500
|
26° 45 |
0.50 |
3-4851 |
3-2851 |
3-3851 |
3-5851 |
|
Price($)*
|
75.00 |
99.00 |
105.00 |
159.00 |
* Prices are in USD and are list prices only.
| GROOVES PER MM |
BLAZE ANGLE |
BLAZE λ (nm) |
DISPERSION (nm/mr) @ 500 nm |
| ECHELLES |
| 31.6 |
Echelle
|
63 |
14.37 |
| 316 |
Echelle |
63 |
1.44 |
| 79 |
Echelle |
75 |
3.28 |
| 79 |
Echelle |
63 |
5.75 |
|
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|
|