Optical waveguides – With optical coupler – Particular coupling function
Patent
1995-03-29
1996-10-08
Healy, Brian
Optical waveguides
With optical coupler
Particular coupling function
385 15, 385115, 385116, 385129, 372 94, 372108, G02B 626, H01S 3083
Patent
active
055639682
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a multimode imaging component for dividing or coupling radiation, comprising a waveguide having an end which is provided with at least one window for transmitting radiation into or out of the waveguide. The invention also relates to a ring laser provided with a multimode imaging component.
2. Discussion of the Related Art
A multimode imaging component is also known as multimode interference component and multimode waveguide. The term "multimode imaging" will hereinafter be abbreviated to MMI.
An MMI component is known, inter alia from German Patent Specification DE 25 06 272. This component is very suitable for use as a radiation divider or coupler in integrated optical systems due to its satisfactory suitability for integration with other components, wide manufacturing tolerances, insensitivity to polarization and radiation wavelength, low radiation losses and versatility.
An application of an MMI component is disclosed in the article "Design and fabrication of integrated InGaAsP ring lasers with MMI outcouplers" by M. J. N. van Stralen et al on pp. 2-24 and 2-25 of the conference report of the ECIO '93 in Neuchatel, Switzerland. The MMI component is used in this application for coupling radiation from a ring laser to two output waveguides. The MMI component has two ends each provided with two windows and is arranged in the ring laser in such a way that the MMI component is part of the laser ring in which the radiation is generated. The radiation of the laser ring is passed to the output waveguides via two windows, one at each end. The ring laser, comprising the laser ring, the MMI component and the waveguide, is integrated on a single substrate.
A drawback of this known construction is that the spectrum of the radiation in the output waveguides does not comprise only the desired laser mode of the ring laser under all circumstances, but also a multitude of other, unwanted spectral components.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia an MMI component which does not have the above mentioned problem.
This object is achieved by an MMI component as described in the opening paragraph, which MMI component according to the invention is characterized in that the part of the end located outside the window is provided with means for reducing reflection of radiation in the direction of the longitudinal axis of the waveguide.
The invention is based on the newly gained recognition that the problems of the known ring laser are caused by reflection of radiation within the MMI component on those parts of the two ends which are located outside the windows. The relevant parts reflect the incident radiation in the direction of the longitudinal axis of the MMI waveguide so that a standing wave may be produced between the two ends. For said application in the ring laser, the internal space of the MMI waveguide is filled with a laser-active medium, with the two ends constituting the mirrors of a resonant cavity. The radiation generated in this resonant cavity leads to the unwanted spectral components in the output waveguides of the ring laser. By reducing the reflections at the ends, there will be less laser action in the resonant cavity and the unwanted spectral components are smaller.
Also in applications of the MMI component in which the waveguide does not comprise any laser-active medium, the measure according to the invention can be used to great advantage, for example, for reducing internal reflections which lead to an unwanted distribution of radiation across input and output waveguides.
The means for reducing reflection preferably comprise an anti-reflection coating, a radiation-absorbing coating, a diffuse scattering face or an oblique face which guides the reflected radiation into a direction where it does not disturb the desired radiation distribution.
It is to be noted that FIG. 16 of said German Patent Specification shows MMI components having oblique faces at the ends. However, these faces are
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patent: 4871228 (1989-10-01), Roos
patent: 5469460 (1995-11-01), Van Roijen et al.
"Design and Fabrication of Integrated InGaAsP Ring Lasers with MMI-outcouplers", M. J. N. Van Stralen, R. Van Roijen, E. C. M. Pennings, et al. pp. 2-24 and 2-25.
Pennings Engelbertus C. M.
Van Roijen Raymond
Healy Brian
Marion Michael E.
U.S. Philips Corporation
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