Semiconductor laser diodes have unparalleled broad range of application, including optical data transmission and storage , metrology , spectroscopy , material processing , and various kinds of medical treatments . They are made of semiconductor material that can be integrated into electrical circuit, and they have very low cost suitable for mass production.
However, their output power cannot compare to gas lasers, their beam quality is not as good as solid state lasers , and they don’t have the wide emission range like mode-locked lasers. What makes them so attractive is that, unlike any other lasers, all the output parameters of diode lasers can be modified, including its intensity, wavelength, linewidth, phase, polarization, etc. In another word, they offer the user more control over the outcome than any other laser sources.
Among the aforementioned parameters of laser diodes the most important ones are of course intensity (amplitude or optical power) and wavelength (optical frequency). However, due to the existence of carrier induced effect , these two factors are always tangled together. In this post, we will explore how to independently modulate these two factors by using a secondary control over the diode laser.
Arbitrary frequency intensity generation
In our post, the objective is to develop an arbitrary power and frequency waveform generator using DFB laser. Given certain range of intensity variation and frequency variation, we want to continuously and coherent scan the entire area defined by the area. Ideally this method should be independent of modulation frequency, which means any point within the range can be
reached statically as well.
In order to achieve this goal, first we need to modulate the frequency and intensity of DFB laser separately. If we can realize pure frequency modulation and pure amplitude modulation, then by combining these two, arbitrary can be accomplished.
The problem is that in semiconductor laser, pure AM or FM modulation cannot be achieved through simple current modulation. Because of Krammer-Kronig relation, the refractive index and optical gain are related by this complex integral. As a result, changing the density of electrically injection carriers always change both the frequency and amplitude of laser output.
Therefore in I – v map, the laser moves in diagonal direction. The question is if we can find a secondary modulation scheme that moves along the orthogonal direction to the current modulation. Then by changing the phase between two modulations, both pure AM and FM can be realized.
Assume two modulation schemes A and B, both result in some frequency variation and intensity variation, so that
Eq. can be rearranged to cancel their AM part of FM part. For example, if pure FM is desired, then
The parameter cannot be zero. Later in this work it can be shown that is proportional to the chirp parameter of modulation scheme A, while is proportional to the chirp parameter of modulation scheme B. Therefore two modulation schemes cannot have the same chirp parameter.