1. Field of the Invention
The present invention relates to an optical pulse compression device for generating ultra-short pulses necessary to realize ultra-fast optical communication technology.
2. Background Art
A conventional optical pulse compression device comprises, for example, a light source for generating an optical pulse; a rare-earth doped fiber amplifier for increasing the peak power of a pulse generated by the light source; an optical pulse compression fiber for compressing the width of the output pulse outputted from the optical fiber amplifier. This optical pulse compression fiber has a feature in that the group velocity dispersion within the optical fiber is anomalous and the absolute value of the dispersion decreases from the input end towards the output end in the direction of the light propagation.
Before explaining the operation of the optical pulse compression device, it is necessary to explain what an optical soliton is which performs an important role in producing compression of the pulse width.
An optical soliton is a stable optical pulse generated as a result of a balance between the pulse width widening caused by an anomalous dispersion effect in the optical fiber on the one hand, and pulse width compression caused by a self-phase modulation effect, on the other hand. The characteristic feature is that the soliton pulse is propagated in the optical fiber without altering its waveform. The peak power required to generate a fundamental soliton of the first order, N=1, is given by the following formula: EQU P.sub.N=1 =0.776(.lambda..sup.3 /(.pi..sup.2 cn.sub.2)).multidot.(.vertline.D.vertline./.tau..sup.2).multidot.p.omega.. sup.2 ( 1)
In Equation (1), D stands for a dispersion in the optical fiber at wavelength .lambda., c is the speed of light, n.sub.2 is a non-linear refractive index of the optical fiber, .tau. is a pulse width, and .omega. is a spot size of the optical fiber.
The group velocity dispersion refers to a frequency-dependent amount of change in the group velocity, and to produce an optical soliton, the sign of the group velocity dispersion must be anomalous.
The operation of the optical pulse compression device will be explained next.
An optical pulse generated by a light source propagates through the rare-earth doped fiber amplifier (shortened to fiber amplifier hereinbelow) and is launched into the optical pulse compression fiber (shortened to pulse compression fiber herein below).
while propagating through the fiber amplifier, the energy of the optical pulse is increased within the optical fiber, which leads to an increase in the peak power of the optical pulse. When the peak power of the optical pulse which passed through the fiber amplifier satisfies Equation (1) at the input end of the pulse compression fiber, a fundamental soliton of N=1 is produced.
As described above, the absolute value of the group velocity dispersion in the pulse compression fiber becomes gradually smaller from the input end to the output end in the direction of the pulse propagation. While propagating in such a pulse compression fiber, the optical soliton maintains a constant energy by decreasing the pulse width. In other words, in accordance with Equation (1), the energy E (=.tau..multidot.P.sub.N=1) of an optical soliton is proportional to .vertline.D.vertline./.tau., therefore, if the energy does not change, the pulse width inevitably becomes narrower when the value of .vertline.D.vertline. becomes smaller.
It follows that as the optical soliton propagates in the pulse compression fiber, the pulse width gradually becomes narrow. Further, because the group velocity dispersion at the output end of the pulse compression fiber is anomalous and if its value is close to 0, the pulse width of the optical soliton at the output end is significantly narrower than that of the optical soliton at the input end (termed "input soliton" hereinbelow).
Therefore, by using a pulse compression fiber having an anomalous group velocity dispersion in which the absolute value of dispersion becomes gradually smaller from the input end to the output end in the direction of the light propagation, it is possible to compress the pulse width of the input soliton in keeping with the nature of the optical soliton.
Further, as can be seen from Equation (1), a product of energy and pulse width (E.multidot..tau.) is a constant in the fundamental soliton (N=1). Therefore, if the fundamental soliton is adiabatically amplified in the propagation direction, its energy increases in proportion to the degree of amplification (optical gain) while maintaining the above relationship, thus causing the pulse width to decrease inversely with the optical gain. This phenomenon is called the adiabatic narrowing by optical soliton amplification.
There are methods, other than the use of a pulse compression an fiber presented above, for compressing optical soliton. An example is a method based on adiabatic narrowing of an optical soliton using rare-earth doped optical fiber having a constant group velocity dispersion. In this case, the amount of the adiabatic narrowing is dependent on the power of the pumping light source; thus the pulse width of the compressed pulses can be controlled by adjusting the pump power.
However, in the conventional devices for optical pulse compression presented above, there is a limit to the narrowing of the pulse width achievable by these compression techniques. That is, in the method utilizing the reduction in the group velocity dispersion in the optical fiber, although the pulse width does become narrower as the optical soliton propagates in a pulse compression fiber, it is insufficient to generate ultra-short pulses in the femtosecond-range (10.sup.-15 second:fs).
One reason is that a propagation loss is suffered by the optical soliton while it is propagating through the optical fiber. When the optical intensity is attenuated due to the propagation loss, the pulse waveform broadens and interferes with the pulse compression. Thus, one of the problems in the conventional optical pulse compression devices is the broadening of the optical pulses due to the propagation loss.
Similarly, although the adiabatic narrowing of the optical soliton is able to reduce the pulse width, because the conventional device is based on a rare-earth doped optical fiber having a constant dispersion, compression of the optical soliton is mediocre, and it is difficult to generate ultra-short pulses in the femtosecond-range.