Optical fibre distributed feedback laser

Details Optical fibre distributed feedback laser Optical fibre distributed feedback laser

1997-03-20

1998-06-23

Healy, Brian

Coherent light generators

Optical fiber laser

372 96, 372 98, 385 37, 385123, 385141, H01S 330, G02B 634

Patent

active

057712514

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION

This invention relates to optical fibre lasers.


BACKGROUND OF THE INVENTION

Many light sources for optical fibre communications are electrically pumped Fabry-Perot (F-P) type semiconductor lasers, where the optical feedback is provided by reflections from the laser cavity end-surfaces (mirrors).
A lasing mode in a F-P type laser has to meet two conditions:
The threshold gain is the same for all modes satisfying the phase condition. Since the gain difference between the adjacent longitudinal modes is generally relatively small and there are many modes which satisfy the phase condition, these lasers may exhibit multi-mode operation.
In applications such as high speed optical communication where dynamic single-mode operation under modulation is required, it is common to use semiconductor distributed Bragg reflector (DBR) lasers.
A DBR laser is a F-P-type laser where the end-reflectors are Bragg gratings which have a wavelength dependent reflection owing to their periodic spatial variation of the refractive index.
The reflection from a Bragg grating is strongest at the Bragg wavelength .lambda..sub.B which fits the grating period .LAMBDA. (.lambda..sub.B =2n.LAMBDA., where n is the average refractive index of the medium). The lasing conditions for a DBR laser are the same as for a F-P laser with mirrors, but since the gratings only reflect within a narrow spectral band around the Bragg wavelength the mode closest to the Bragg wavelength will experience stronger feedback than the other longitudinal modes. If the difference in reflectivity (feedback) between adjacent longitudinal modes is sufficiently large, only a single longitudinal mode will tend to oscillate under modulation conditions.
Semiconductor distributed feedback lasers have been proposed. In these devices a grating extends across the lasing region and causes distributed feedback of the laser radiation.
DBR lasers in which the gratings and the gain medium are formed from optical fibre have been reported. These devices have the advantage that they are compatible with optical fibre communication links, they are simple in construction, and can offer low intensity noise and narrow linewidths.
However, in order to achieve single frequency operation from a DBR fibre laser, two apparently conflicting requirements must be addressed.
Firstly, the length of the fibre cavity (the fibre between the two end-reflectors) must be made as short as possible. The reason for this is that the spectral spacing between resonant cavity modes increases with a decreasing cavity length. When the resonant cavity modes are more widely separated, the differences in the round-trip cavity loss between adjacent modes caused by the wavelength dependent reflection of the gratings are more exaggerated, so it is more likely that a single oscillation mode will dominate.
However, a second requirement results from the fact that fibre lasers are longitudinally pumped optically, in contrast to the electrical pumping of semiconductor lasers. This feature means that the fibre cavity should be made long enough to allow sufficient energy at the pump wavelength to be absorbed.
The first requirement listed above leads to a desired cavity length of about a few centimetres or less. However, in erbium (Er.sup.3+) doped fibres commonly used as the gain medium, the pump absorption in such short laser lengths is normally only a few percent, and hence the so-called slope efficiency of these lasers is very low (less than 1%) even with high Er.sup.3+ -concentration. If the pump absorption is increased by doping the fibre more heavily, so-called concentration quenching occurs leading to a loss of slope efficiency and the lasers are also then prone to strong self-pulsation. Although a "MOPA" (Master Oscillator Power Amplifier) configuration has been proposed, where the residual pump power is used to pump a fibre amplifier, the output from a MOPA is more noisy than the direct output from a laser, and it usually requires an optical isolator between the laser and the amplifi

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