Distributed feedback semiconductor laser

Details Distributed feedback semiconductor laser Distributed feedback semiconductor laser

1992-06-17

1994-02-22

Epps, Georgia Y.

Coherent light generators

Particular resonant cavity

Distributed feedback

372 45, 372 46, H01S 308

Patent

active

052894940

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to a distributed feedback semiconductor laser which is used as an electro-optic converter. This invention relates, more particularly, to a gain-coupled distributed feedback laser diode (GC-DFB-LD).
This invention is highly suitable as the light source for a long-distance and large-capacity optical communication system, an optical information processing system, an optical memory system, an optical measuring instrument and other opto-electronic devices.


BACKGROUND TECHNOLOGY

The distributed feedback semiconductor laser generates stimulated emission of light by distributed optical feedback to an active layer with a diffraction grating provided near the active layer. The device can produce stimulated emission of excellent lasing spectral characteristics by a relatively simple construction, Therefore, it has been the target of various R & D efforts and is expected to be used as a light source suitable for a long-distance and large-capacity optical communication system, an optical information processing system, an optical memory system and an optical measuring instrument.
Such distributed feedback semiconductor laser has an optical waveguide structure, wherein an active layer is surrounded with transparent hetero-junction semiconductor layers, for efficient stimulated emission. R & D efforts are recently directed toward distributed feedback of light by periodically changing the refractive index in a transparent optical waveguide layer which is placed very close to the active layer. In that case, a diffraction grating, having triangular cross section for instance, is formed on the interface of the optical waveguide layer on the side farther from the active layer.
In the light distributed feedback by such index coupling, however, feedback to phase cannot be matched for the light in Bragg wavelength which is reflected correspondingly to the period of the thickness change in the optical waveguide layer. Because of this phase match condition, stable lasing cannot be obtained and two longitudinal lasing mode whose wavelengths are separated symmetrically in the vertical direction across the Bragg wavelength may possibly be generated at once. Even if only one such longitudinal-mode-lasing takes place, it is difficult to select previously which of the two wavelengths would be lasing. So, the precision in setting lasing wavelength is seriously deteriorated.
In sum, the light distributed feedback using index coupling which is based on the periodical perturbation of the refractive index in the optical waveguide layer has an inherent problem of degeneracy by longitudinal lasing mode of two wavelengths, which is difficult to avoid.
There have been proposed various solutions for the problem. One of them proposed a structure to shift the phase by 1/4 wavelength substantially at the center of the diffraction grating. Those proposals, however, are not quite effective as they tend to make the construction of a laser more complicated, require additional manufacturing steps only for the solution of the degeneracy and need formation of anti-reflection coating on the facets of the laser.
Kogelnik et al. proposed a theory in their paper titled "Coupled-Wave Theory of Distributed Feedback Lasers", Journal of Applied Physics, 1972, Vol. 43, pp. 2327-2335, whereas a stop band is produced around the Bragg frequency when distributed feedback of the light is conducted by index coupling, if distributed feedback is conducted by gain coupling based on the periodical perturbation of gain factors, such stop band would not be produced and longitudinal mode lasing of exclusively single wavelength would be obtained. They did not mention in their paper the specific construction to realize the theory, rather they merely discussed on the result of their theoretical studies.
Some of the present inventors have invented novel semiconductor lasers applying the basic theory of Kogelnik et al., and filed patent applications as follows: (Publication No. JP-A 3-34489) 1989) (Publication No. JP-A 3-49283 to 3-49287)
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H. Kogelnik et al, "Coupled-Wave Theory of Distributed Feedeback Lasers", Journal of Applied Physics, vol. 43, pp. 2327-2335 (1972).
Y. Luo et al, "Purely Gain-Coupled Distributed Feedback Semiconductor Lasers", Applied Physics Letters, vol. 56, pp. 1620-1622 (1990).
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