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Technical Note 1
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 , |
(1) |
(see Figure 1) can be differentiated to give the angular spread (dispersion) of the spectrum:
 . |
(2) |
When the grating is operated in the Littrow configuration (in which the light is retro-diffracted; see Figure 2), the equation for the dispersion simplifies to
 . |
(3) |
Figure 1 – The Grating Equation. Here a is the incidence angle, b is the diffraction angle, m is the (integral) diffraction order, l is the wavelength of the light and d is the spacing between adjacent grooves.
TECHNIQUES
There are two main methods for selecting a narrow spectral band of light, Littrow and Littman. These methods are shown in Figures 2 and 3. Littman tuning offers higher angular spread and hence narrower spectral feedback; as the angle of incidence is increased toward 90°, however, the efficiency of the grating drops significantly.
Figure 2 – Laser tuning using a grating in Littrow mode as the feedback element.
Figure 3 – Littman tuning using a grating near grazing incidence.
Figures 4 and 5 show typical efficiency curves for two popular plane holographic gratings, with groove frequencies of 1800 g/mm and 2400 g/mm. The curves are measured near Littrow.
Figure 6 gives some insight into the decrease in S-plane efficiency as the incidence angle a increases.
Figure 4 – Efficiency curve: 1800 g/mm. Red dashed curve: P-plane; solid black curve: S-plane.
Figure 5 – Efficiency curve: 2400 g/mm. Red dashed curve: P-plane; solid black curve: S-plane.
Figure 6 – Efficiency curve: 1800 g/mm, for several incidence angles a.
ORDERING INFORMATION
Popular grating sizes are shown below. Different sizes can be accommodated: please contact us with your specifications, and we will provide pricing.
| grating size | grooves | catalog |
| (mm) | per mm | number |
| 28.5 x 58 x 10 | 1800 | 35-23-04-330H |
| 16.5 x 58 x 10 | 1800 | 35-23-08-330H |
| 50 x 100 x 16 | 1800 | 35-23-14-330H |
| 12 x 110 x 16 | 1800 | 35-23-16-330H |
| 12.7 x 50 x 10 | 1800 | 35-23-21-330H |
| 110 x 110 x 16 | 1800 | 35-53-21-330H |
| 28.5 x 58 x 10 | 2400 | 35-23-04-430H |
| 16.5 x 58 x 10 | 2400 | 35-23-08-430H |
| 12.7 x 50 x 10 | 2400 | 35-23-21-430H |
| 110 x 110 x 16 | 2400 | 35-53-15-430H |
FOR FURTHER INFORMATION
For additional information, please contact us.
SOME TECHNICAL REFERENCES
- M. Littman, "Single-mode operation of grazing incidence pulsed dye laser," Optics Letters 3, 138 (1978).
- M. de Labachelerie and G. Passedat, "Mode-hop suppression of Littrow grating-tuned lasers," Applied Optics 32, 269–274 (1993).
- T. Day, "Continuously Tunable Diode Lasers," Lasers & Optronics, 15–17 (June 1993).
- P. Zorabedian et al., "Multiple-prism grating designs tune diode lasers," Laser Focus World, 103 (February 1993)
- H. Lotem, "Littrow-mounted diffraction grating cavity," Applied Optics 33, 930–934 (1994).
Newport Corporation
705 St. Paul Street, Rochester, New York 14605 USA
Telephone: (585) 248-4100, Fax: (585) 248-4111
E-mail: gratings@newport.com,
Web Site: http://gratings.newport.com
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Founded in 1969, Newport is a pioneering single-source solutions provider of laser and photonics components to the leaders in scientific research, life and health sciences, microelectronics, industrial manufacturing, and homeland security markets. | ||||
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