Coherent light generators – Particular resonant cavity – Distributed feedback
Patent
1992-01-23
1993-05-18
Epps, Georgia Y.
Coherent light generators
Particular resonant cavity
Distributed feedback
372 45, 372102, H01S 308
Patent
active
052127125
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase-shifted distributed feedback laser device in which an effective refractive index of a laser active layer is discontinuous, and an improvement in a method of manufacturing the same.
2. Description of the Related Art
A DFB-LD (Distributed FeedBack-Laser Diode) plays an important role in long-distance large-capacity optical communication. A characteristic feature of this DFB-LD lies in a structure (i.e., a diffraction grating) in which a refractive index periodically changes in an optical waveguide. However, when cleavage of crystals is used in a reflecting mirror, it is difficult to oscillate the DFB-LD in a single longitudinal mode because the phase of a diffraction grating at the end of the DFB-LD has an adverse effect on oscillation in the single longitudinal mode.
Therefore, the following proposals have been made for the DFB-LD in terms of a device structure.
First, a .lambda./4-shifted DFB laser in which the phase of a diffraction grating is shifted by .pi. (.pi./2 as the phase of an oscillation frequency) in a central portion of a laser resonator has been developed (this laser is described in, e.g., Electronics Letters vol. 20, NO 24, 1984, pp 1,008 to 1,010). According to this proposal, laser light reliably oscillates in a single longitudinal mode. Second, an effective refractive index type .lambda./4-shifted DFB laser in which the width of a waveguide is changed in a central portion of a laser resonator to change the effective refractive index of an active medium has been developed. A characteristic feature of this proposal is that because the phase of waveguide light changes in a portion where the width of the waveguide changes, an effect similar to that obtained when a diffraction grating is shifted can be equivalently obtained (this laser is described in, e.g., IEEE JOURNAL OF QUANTUM ELECTRONICS. VOL. QE-23, NO 6JUNE 1987 pp 804 to 814).
In the .lambda./4-shifted DFB laser in which the phase of a diffraction grating is shifted, however, when a coupling coefficient .kappa. indicating a feedback quantity of a light wave is large, light reciprocated in a resonator is easily concentrated on a phase shift portion A of the diffraction grating, as shown in FIGS. 1A and 1B. Therefore, the performance of the laser is largely degraded, e.g., hole burning in the axial direction results, causing saturation of an optical output (this is described in, e.g., IEEE JOURNAL OF QUANTUN ELECTRONICS. VOL. 24, NO 11, November 1988). On the contrary, when the coupling coefficient .kappa. is small, light reciprocated in the resonator is easily concentrated on end faces B of the diffraction grating, as shown in FIGS. 2A and 2B. Therefore, it is very difficult to adjust the coupling coefficient .kappa., e.g., a threshold current rise or no satisfactory suppression ratio with an adjacent mode. Referring to FIGS. 1A and 2A, reference numeral 1 denotes an InP substrate; 2, a waveguide layer; 3, an active layer; 4, a cladding layer; 5, a contact layer; 6, electrodes; and 7, AR films.
In addition, in the DFB-LD in which the width of a waveguide is changed, concentration of light is decreased in a region where the phase is shifted. However, since even a waveguide having a uniform width is difficult to form, it is very difficult to control the width, the shape, or the like of a region where the phase is shifted. This proposal, therefore, is not useful unless a control apparatus capable of precisely controlling the width of a waveguide is developed. Furthermore, the far field pattern of output light undesirably large projections.
The following proposals have been made for the DFB-LD in terms of a manufacturing method.
First, a method of using both negative and positive resists in order to shift the phase of a diffraction grating has been developed (this method is described in, e.g., Electronics Letters vol. 20, NO 24, 1984, pp 1,008 to 1,010). Second, a method of using a phase shift film has been developed (this method is described in,
REFERENCES:
patent: 4775980 (1988-10-01), Chinone et al.
patent: 4796273 (1989-01-01), Yamaguchi
patent: 4847856 (1989-07-01), Sugimura et al.
K. Utaka et al., ".lambda./4-Shifted InGaAsP/InP DFB Lasers by Simultaneous Holographic Exposure of Positive and Negative Photoresists," Electronics Letters vol. 20, No. 24, pp. 1008-1010, Nov. 22, 1984.
H. Soda et al., "Stability on Single Longitudinal Mode Operation in GaInAsP/InP Phase-adjusted DFB Lasers," IEEE Journal of Quantum Electronics vol. QE-23, No. 6, pp. 804-814, Jun. 1987.
T. Kimura et al. , "Linewth Reduction by Coupled Phase-Shift Distributed-Feedback Lasers," Electronics Letters vol. 23, No. 19, pp. 1014-1015, Sep. 10, 1982.
Kinoshita et al., "Transient Chirping in Distributed Feedback Lasers: Effect of Spatial Hole-Burning Along the Laser Axis," IEEE Journal of Quantum Electronics vol. 24, No. 11, pp. 2160-2169, Nov. 1988.
Yamaguchi et al., "Phase shifted DFB-DC-PBH LD in 1.55 .mu.m wavelength range," Electronics, Information, and Communication Society Research Report OQE86-150, pp. 33-37.
Epps Georgia Y.
Kabushiki Kaisha Toshiba
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