Bên cạnh đó. các đường cong điều chỉnh nói chung tương tự, ngoại trừ cho sự chỉ đạo của độ cong. Như vậy, lựa chọn của tinh thể phi tuyến tốt nhất có thể được dựa trên những cân nhắc khác hơn so với giai đoạn phù hợp với đường cong. | 384 Paul Zorabedian Camera Lenses There are at least three published reports on the use of camera lenses as collimators in ECLs. Heckscher and Rossi 57 reported the use of a TV camera lens for intracavity collimation of a Littrow grating cavity but gave no indication of the feedback strength obtained. Sommers 58 evaluated several camera lenses from 25-mm focal length to 50-mm focal length . The lenses gave only about 1 feedback when used with a grating and it was concluded that spherical aberration was responsible for the poor performance since the lenses were not used in their intended geometry. Fleming and Mooradian successfully employed camera lenses in an ECL 38 . They used 50-mm focal length seven-element lenses. All air-glass surfaces were AR coated. Ball Lenses Glass spheres can be used to couple the gain medium to waveguide or fiberpigtailed external filters. However the spherical aberrations are too great to be useful for collimation in bulk optic cavities. Lensed Fiber Lensed optical fiber 59 can be used to couple the gain medium to fiberpigtailed external cavities. However this method requires the fiber to be in very close proximity to the facet which gives rise to the danger of facet damage. There is also a very high sensitivity of the coupling loss to lateral misalignment. Optics for Beam Expansion and Shaping Cylindrical Lenses A cylindrical lens can be used in an ECL 60 to form a line illumination on a diffraction grating. This implements a degenerate resonator in one dimension and provides a high degree of angular misalignment tolerance while maintaining high spectral selectivity. Critical to the success of this technique is the fact that the cylinder axis can be inclined with respect to the optical axis at a large angle to match the grating angle of incidence without introducing a large amount of spherical aberration. This is because the cylinder lens has no power in this plane and appears to be a
