Download optical cavity

7. Optical cavity( optical resonator)
An optical cavity or optical resonator is an arrangement of mirrors
that forms a standing wave cavity resonator for light waves. Optical
cavities are a major component of lasers, surrounding the gain medium
and providing feedback of the laser light
7.1 Resonator types
Different resonator types are distinguished by the focal lengths of the
two mirrors and the distance between them. (Flat mirrors are not often
used because of the difficulty of aligning them to the needed precision.)
The geometry (resonator type) must be chosen so that the beam
remains stable (that the size of the beam does not continually grow with
multiple reflections, as shown below.
Resonators types
7.2 Resonator Stability
Stability diagram for a two-mirror cavity.
Only certain ranges of values for R1, R2, and L produce stable
resonators in which periodic refocussing of the intracavity beam is
produced. If the cavity is unstable, the beam size will grow without limit,
eventually growing larger than the size of the cavity mirrors and being
lost. By using methods such as ray transfer matrix analysis, it is possible
to calculate a stability criterion:
0≤
≤ 1.
Values which satisfy the inequality correspond to stable resonators.
The stability can be shown graphically by defining a stability parameter,
g for each mirror:
,
and plotting g1 against g2 as shown. Areas bounded by the line g1 g2 = 1
and the axes are stable. Cavities at points exactly on the line are
marginally stable; small variations in cavity length can cause the
resonator to become unstable, and so lasers using these cavities are in
practice often operated just inside the stability line.
A simple geometric statement describes the regions of stability: A cavity
is stable if the line segments between the mirrors and their centers of
curvature overlap, but one does not lie entirely within the other
9. Laser Modes‫أنماط الليزر‬
‫وذلك‬mirrors ‫ بواسطة المرايا‬feedback ‫للحصول على الليزر نستخدم التغذية العكسية‬
active
‫ خالل مروره بالوسط الفعال‬light amplification‫لتكبير الشعاع الضوئي‬
‫ لهذه المرايا دور في التأثير على االشعاع الكهرومغناطيسي داخل المكبر‬،‫ لليزر‬material
: ‫حيث ينتج نوعين من االنماط تعرف كالتي‬
longitudinal modes‫ االنماط الطولية‬
transverse modes.‫النماط المستعرضة‬

 Longitudinal modes are only specific frequencies are possible
inside the optical cavity of a laser, according to standing wave
condition. The same l,m but different n.
 Transverse modes are created in cross section of the beam,
perpendicular to the optical axis of the laser. Different l or m but
the same n.