Investigation of Laser Diode Properties 

Introduction


A laser diode is a solid state device that emits photons of light when electrons travel across the forward bias of the p-n junction. These lasers were first demonstrated in 1962 but have only recently gained enough popularity to be considered "common". The growth in diode laser use can be attributed to techonological advancements in laser production and the increasing number of applications for these devices. These applications are very diverse and it would be impossible to list them here, but the following are some of the most common uses for laser diodes.

To understand the operation of a laser diode, it is necessary to review the characteristics of semiconductors and ordinary diodes. A semiconductor is exactly what the name implies-- a material that is sometimes an insulator and sometimes a conductor. Conduction in any material is a function of the energy gap between the valence band of electrons and the conduction band. In the valence band, electrons are closely tied to an atom and cannot move around freely. In the conduction band, however, the electrons are free to move about and act as electricity. For an insulator, the energy gap between these bands (known as the "band gap") is large and electrons are rarely promoted into the conduction band. Oppositely, a conductor is a material that has almost no band gap energy and many of the electrons can be promoted from the valence band into the conduction band. A semiconductor is a material that lies in between these two extremes. Depending on the conditions, namely temperature and applied voltage, a semiconductor can act as either a conductor or an insulator. The following diagram helps to illustrate this point.

A diode is a special type of doped semiconductor. Doping is a process whereby semiconductors are grown with impurities that give either an excess or a derth of electrons to the material. Semiconductors grown with an excess of electrons are known as n-type semiconductors and those grown with a derth of electrons are known as p-type semiconductors. In a diode, a piece of p-type semiconductor is placed next to a piece of n-type semiconductor. The region where these two materials meet is known as the p-n junction. If the conditions in the semiconductor are conducive to conduction, the extra electrons in the n-type material are able to move to the p-type material and recombine into the valence band.

In a diode laser, the thermal energy of the electrons and the current applied to the device move the excess electrons from the valence band of the n-type material into the conduction band, thus performing a population inversion. Here, they are able to move into the p-type material where they fall back into the valence band. This action releases energy in the form of a photon. These photons, released in a direction parallel to the p-n junction, are either re-absorbed by other electrons or propogate in such a manner that they induce additional transitions from the conduction to the valence band, thereby releasing more photons. If the probability of such "stimulated emission" is greater than the probability of absorption, lasing is acheived in the diode. If half the wavelength emitted is an integer division of the cavity length, the photons will most likely be reflected at the ends of the diode due to the large change in the index of refraction when moving from the diode to the surrounding medium. In some diodes, however, the index in refraction may not differ enough from that of the surrounding medium, and the diode must be treated with a partially reflective coating. Photons emitted whose wavelengths are not integer divisions of the cavity length will not be amplified inside the diode. Those that do meet this qualification are amplified and occasionally escape the diode, producing the diode laser beam.

One of the most useful characteristics of a diode laser is its ability to tune its wavelength. Output wavelength in a laser diode is a function of the diode temperature and applied current. Even more convenient is the relatively linear behavior of the wavelength vs. current and wavelength vs. temperature curves. The linearity of the diode laser tuning is only interrupted by a series discontinuities, or mode hops, in the graph. Mode hopping is a phenomenon that occurs when the wavelength or laser cavity size has changed such that an additional half wavelength of light can fit within the diode's laser cavity. By measuring the size of these mode hops, and knowing that each single mode hop originates from an additional half wavelength of light being amplified in the laser cavity, the size of the laser cavity can be calculated from the following equation:

where lc is the cavity length, xo is the index of refraction of the diode material, and delta lambda is the size of the mode hop.

Tunable diode lasers have many applications in physics because of their abilitity to zero in on certain atomic transitions in atoms. This application was demonstrated in our experiment by using the laser to observe the transition of electrons in rubidium between the 5s and 5p1/2 states.

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