FIBER OPTIC LASERS
State of the art Erbium Doped Fiber Amplifiers (EDFA) are the modern
version of the travelling-wave solid state microwave lasers of the 1950's.
(France 1991).
The Erbium Doped Fiber Amplifier (EDFA) is the modern incarnation of
the dielectric single pass laser mentioned above, it is pumped by a diode laser.
Both share the following common properties :
- Operate on quantum transitions in metallic ions.
- A dielectric matrix is doped with a small concentration of ions
- Longitudinally pumped by highly monochromatic radiation.
- Very high single pass gain.
- Design attempts to minimize feedback.
- Somewhat low efficiency. (Power input versus power output)
- Broad bandwidth (below their Amplified Spontaneous Emission threshold).
- Very low noise amplifiers.
- Extensively used by the telecommunications industry.
To act like an amplifier radiation must not be allowed to reverse its path
or unstable oscillations may result (conventional laser resonators)
To minimize feedback the fiber ends can be :
- Antireflection coated.
- Cut at an angle.
- Immersed in a fluid of matching index of refraction.
The efficiency may not be very high, but fiber amplifiers provide incredibly
large gains in a single pass. However if the gain is too high, say greater than
10,000 times, even with complete suppression of feedback, the process of
amplified spontaneous emission (ASE or 'mirrorless lasing') will take
place with high efficiency and limit the achievable gain. Photons spontaneously
emitted at one end of the active medium are amplified on a single pass to an
intensity that significantly depletes the gain.
Since the spontaneous emission lifetime scales approximately as the inverse cube
of the transition frequency, this problem is negligible at microwave frequencies.
The problem becomes worse at the higher infrared frequencies, and amplified
spontaneous emission dominates the output at the highest frequencies such as in
optical or ultraviolet transitions, reducing the signal to noise of these
kind of amplifiers.
Like the ruby microwave laser, fiber amplifiers are used extensively by the
telecommunications industry as an all optical means of amplifying radiation.
(in fiber optic repeaters).
REFERENCES
- France, P.W.: 1991, editor Optical Fiber Lasers and Amplifiers
- ... ? : 1992, Fibre Lasers in Solid State Lasers,
eds. Inguscio,M., Wallentein,R. p.231
- SPIEconference on Doped Fiber Devices held in Denver, Co, Aug 4, 1996
- A Fiber Optic Chronology by Jeff Hecht
Laser History