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Transcript
Diode laser
The diode or semiconductor laser
Diode lasers are extremely compact (0.3×0.2×0.1 mm),
high efficiency (up to 40%), tuneable lasers, which
have low power consumption
Emission wavelengths range from 375 nm to 30 μm
Active medium is a p-n junction:
Energy levels are not discrete
Emission is from recombinant radiation occurring when electrons in the
conduction band recombine with holes in the valence band
Applying a bias voltage changes the energy levels to give a population inversion.
Photon energy related to band gap, Eg=EC – EV
Diode laser
Typical diode laser materials are GaAs,
InGaAs, InGaAsP, AlGaAs with
emission from 400 to 1600 nm.
Lead salt lasers have emission in
the mid-IR.
In general, the polished ends of the laser
are partially reflective and serve as
the cavity mirrors
Owing to the short cavity, the diode laser beam is more highly
diverging than other lasers
Diode laser
At low powers, multiple longitudinal modes are emitted
These modes are
separated by
Δν = c / 2nd
(n is the index of
refraction)
Tuning wavelength is accomplished by changing the current or
temperature.
“mode hops”: sudden changes in wavelength as the resonant
longitudinal mode changes in the cavity.
Diode laser
Mode hops are problematic if a significant tuning range is desired. A
larger tuning range can be achieved by increasing the length of
the cavity and including an wavelength selective element. Such a
system is known as an external cavity diode laser (ECDL).
ECDL configurations:
An advantage of current tuning is that the wavelength can be modified
at kHz to MHz rates – this high frequency modulation is useful for
improving the signal-to-noise ratio in many experiments.
For example, detecting gases based on their rotational vibrational
transitions