# 1. An optical cavity is formed by two plane mirrors ```EE421 sampleTEST 3
STAR DATE:
STARFLEET OFFICER:
______________________
CAPTAINS LOG:
1.
a) Discuss the three processes that affect energy level population in an operating
laser.
b) With a sketch and words explain why a 4-level laser is more efficient than a 3level laser.
2.
A ruby rod is 15 cm. in length and 2 cm. in diameter. It has a chromium ion
density of 1.6 x 1019 ions/cm3.
a) What is the minimum energy that must be pumped into this rod in order to make it
lase at the optical wavelength of 6943 angstroms?
b) Assume the overall efficiency of a laser using this ruby rod is 2%. What is the
value of the energy storage voltage if the capacitor value is 160uf and the rod is to be
pumped to its maximum output?
3.
A fiber cable is used to carry control signals for a federation shuttle craft. The
core index n1 = 1.602, cladding index n2 = 1.542, and outside index n0 = 1.000. The
signal wavelength  = 1 micron. The core diameter is 60 microns.
a)
How many modes can this fiber support?
b)
Find the maximum beam entrance cone angle 2m.
c)
A lens of focal length f is used to couple a laser with a 4 mm. diameter
beam into
the fiber. Find the focal length f that will allow the focused input
beam to match
the acceptance cone angle of the fiber calculated in a)
above.
d)
What is the minimum skip distance for this fiber?
4.
Given a ruby laser with a wavelength of .6943 microns and a ruby rod with the
following specs:
Length = 10 cm
Diameter = 2 cm Chromium ion density = 1.58 x 1019 ions/cm3
The slope efficiency is 1% and the (threshold lamp input)/(rod volume) is 350 J/cm3.
a)
Find the lamp input at threshold.
b)
The required lamp input for a 5 Joule output.
5.
Given the optical cavity below formed by one plano-convex lenses reflection coated on
the plane side and one plane mirror used as the output coupler. The focal lengths for the
feedback mirror is f1.
a)
Find the round-trip matrix starting on the output coupler mirror on the right.
b)
What are the conditions on L and f1 that will insure that this cavity is stable?
c)
If the cavity is used as a laser cavity where in the cavity does the smallest beam
diameter occur?
6. A laser is formed by an optical cavity that is composed of a concave mirror on the left with a
curvature of 1 km and a plane mirror used as an output coupler on the right. The mirrors are
separated by a distance of 2 meter. The FWHM of the laser material is 1500 MHz with a center
wavelength of 5000 angstroms.
a) Determine the stability of the cavity.
b) Find the number of axial mode frequencies in the output of the laser.
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