Laser Tuning With Diffraction Gratings

The grating equation mλ=d(sinα + sinβ) can be differentiated to give the angular spread (dispersion) of the spectrum:

dβ/dλ = (sinα + sinβ) / λcosβ

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:

dβ/dλ = 2tanβ / λ

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.

Lasers that employ certain organic dyes as their gain media have the ability to lase over a broad range of frequencies. The combination of a diffraction grating and one of these dyes in a laser allows the production of laser light that has a narrow line width and which is tunable through the gain profile of the dye.

Dye laser wavelength tuning, in the visible region of the spectrum, is done in two different modes. The classical one is with a grating in the autocollimating (Littrow) mount where the beam requires expansion to fill the grating in order to obtain adequate resolution. Telescope or prism optics fulfill this need. The alternative approach is to use the grating in a fixed grazing incidence mode together with a rotating reflecting tuning element in the form of either a mirror or a second grating.

Littrow tuning is done either with fine pitch, first order gratings (typically 1800 or 2400 g/mm frequency, either ruled or holographic) or a coarser grating used in higher orders. For the latter, a 600 g/mm, 54° blaze angle grating is particularly useful because it covers the visible spectrum in orders 3 to 7 with free spectral ranges that match the dyes and prevent overlap.

Grazing incidence tuning is done in first order only and 1800 g/mm, 2000 g/mm, and 2400 g/mm holographic gratings are preferred. The gratings have their ruled width filled by incidence angles of 80° to 88°. Steep angle usage leads to special grating dimensions such as 16.5 x 58 x 10 mm.

The gratings below are suitable for use in dye laser tuning in orders 3 to 8. These gratings can be used either in Littrow or grazing incidence mode. They are available on standard substrates or on special rectangular blanks. The maximum ruled area is groove length x ruled width. Click on a Master Grating Code (last 4 digits of a grating's part number) below to view master grating efficiency curves. Use the request a quote to get a quote based on your requirements.

Molecular lasers have a wide variety of uses, ranging from materials processing to medical applications. These high-power lasers generally produce gain over a wide spectral range. Gratings are used in these lasers to control and narrow the lasing wavelength.

Molecular lasers, operating both pulsed and continuous-wave (cw) in the infrared, typically have their output wavelength tuned by Littrow-mounted gratings. High efficiency is obtained by operating in the first order at diffraction angles >20°. This corresponds to l/d ratios from 0.67 to 1.8 (where d is the groove spacing), which ensures that only the zero and first orders can diffract. The output will be polarized in the S-plane (i.e., with the electric vector perpendicular to the grooves) because the efficiency will be several times greater than in the P-plane (electric vector parallel to the grooves).

Dispersion is a function of the tangent of the diffraction angle b and is chosen from medium (b ≈ 20°) to very high (b > 50°) as required. Note from the master grating options below, which summarizes gratings most suitable for molecular laser tuning, that high efficiency corresponds to diffraction angles that can be significantly greater than the groove or blaze angles. This is a consequence of the electromagnetic nature of diffraction from deep groove gratings. For maximum efficiency, any of these gratings can be supplied in the form of gold replicas.

Some molecular lasers operate at high power, capable of destroying gratings. In the case of pulsed lasers, extra thick replica films may be of help. In the case of cw lasers, replicas on metal substrates are superior to glass because of greater thermal conductivity; in some cases it is advisable to use water cooled substrates. In all cases, close attention to groove geometry maximizes reflection, minimizes absorption, and leads to improved grating performance. The informati0on below serves as a guide to the typical power levels a grating can be expected to survive.

• Pulsed lasers at 1.06 μm
  • Standard gold replica gratings can withstand 300 mJ/cm² pulses of 100 ps duration.
• CW lasers at 10 μm
  • Standard gold replica on copper 100 W/cm²
  • Water-cooled gold replica on copper 200 W/cm²

There are a number of masters available which are used to produce replicas with high S-plane efficiency for use with CO₂, CO, HF, or DF lasers (see below). For this type of application, we suggest you advise us of the following specifications:

• Spectral region of interest
• Peak power
• Pulse duration
• Beam size

The gratings below are suitable for use in the specific molecular lasers indicated. Gratings in this table are listed by laser type. The maximum ruled area is groove length x ruled width. Click on a Master Grating Code (last 4 digits of a grating's part number) below to view master grating efficiency curves. Use the request a quote to get a quote based on your requirements.