Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1998-04-28
2001-03-06
Arroyo, Teresa M. (Department: 2881)
Optical waveguides
With optical coupler
Input/output coupler
C372S045013, C372S102000
Reexamination Certificate
active
06198863
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical filters, and to optical sources incorporating such filters. In particular it relates to tunable filters and sources suitable for us, for example, in optical networks employing wavelength division multiplexing (WDM).
2. Related Art
The use of WDM and wavelength routing in optical networks has been proposed to provide the increased bandwidth required, for example, for the provision of multimedia services. Tunable lasers and filters are essential components for the implementation of such systems and desirably should provide tuning ranges of around 40 nm to allow complete coverage of the erbium amplifier transmission window around 1550 nm, for example.
It has previously been proposed to use a distributed Bragg reflector laser (DBR) as a tunable source [Oeberg M et al., “Complete single mode wavelength coverage over 40 nm with a super structure grating DBR laser” 14th Semi. Laser Conf. Proceedings, 1994, pp32-33]. While a tuning range as broad as 40 nm has been claimed for such a device, it has the disadvantages of being complicated to fabricate and to control. Achieving the broad tuning range requires adjustment of control currents in both the superstructure grating (SSG) and in a phase control section of the laser.
Another approach is the use of a laser with a grating-assisted vertically-coupled filter (GAVCF). The paper by Kim I., et al., “Broadly tunable vertical-coupler filter tensile-strained InGaAs/InGaAsP multiple quantum well laser”, Appl. Phys. Lett., 1994, 64, (21), pp2764-2766, discloses one such device. Although a tuning range of 70 nm is claimed, and only one control current is required, the device is not able to cover all wavelengths but hops between longitudinal modes of the laser cavity.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided an optical filter characterised by at least two gratings located in a waveguide region of a semiconductor optical device, each grating having a multiple peak optical passband and the gratings being spaced apart in the waveguide region and forming an optical cavity having a transmission comb-filter characteristic.
The first aspect of the present invention provides a filter particularly suitable for use in a semiconductor laser, but also capable of other uses, for example as a wavelength selective channel filter. The filter, termed by the inventors as “in-line Fabry Perot” (ILFP) adopts a novel structure in which a pair of gratings are spaced apart within a waveguide region of an optical semiconductor device. As described in further detail below this structure functions as a Fabry Perot filter and has a response comprising a comb of equally spaced wavelengths which can be tuned over a relatively narrow range of less than 1 nm. By contrast with conventional Fabry Perot filters, the filter of the present invention does not require cleaved mirror facets and so is much easier to integrate monolithically with other devices. In particular, the in-line filter may be combined with a GAVCF to provide a device having an overall response which is widely tunable over specified wavelengths, which requires only a single control current, and which remains single-moded over substantially all of its range.
Preferably the gratings are superstructure gratings (SSG's).
Although other filter types, for example sampled gratings or computer generated gratings, may be used, the use of SSG's is preferred as providing optimism efficiency. An SSG operates in a similar manner to a sampled grating, but uses a grating with a linearly chirped period to obtain a number of high peaks. In prior art systems, the peaks produced by SSG or sampled grating structures are reflection peaks. Systems embodying the present invention, by contrast, give transmission peaks. A linearly chirped grating has a period which changes in a number of equal steps from a start period &Lgr;
a
to an end period &Lgr;
b
, where the start period may be greater than or less than the end period. Alternatively, since it is difficult to fabricate a grating with a linearly chirped period, this may be approximated using a fixed period grating with a number of discrete phase shifts. The term SSG as used herein encompasses a filter made in this manner, as well as a filter with a truly linearly chirped period.
Preferably the semiconductor optical device includes an optical gain element and the gratings are spaced apart within a single continuous active region of the optical gain element.
The in-line filter of the present invention may with advantage be integrated within an active device such as a semiconductor laser to provide a compact and tunable optical source.
According to a second aspect of the present invention, there is provided a filter system comprising a plurality of filters in accordance with the first aspect of the invention coupled in series along the waveguide region with a phase discontinuity at the interface between the filters.
Here, as elsewhere in the present specification, “phase” or “period” denotes the spatial phase or period of a grating. Although an in-line filter in accordance with the first aspect of the invention can be formed from just a single pair of gratings, the present inventors have found that the characteristics of the in-line filter can be further enhanced by concatenating a number of such filters, with a phase shift at the interface between them.
Preferably the plurality of filters are arranged symmetrically about a central region of the waveguide and there is a phase shift of pi at the interface between the waveguides at the said central region. In this arrangement, when using chirped gratings, the direction of increasing frequency is the same for all the gratings throughout the system.
According to a third aspect of the present invention, there is provided a grating-assistance vertically-coupled filter (GAVCF) comprising a first waveguide region, a second waveguide region aligned with the first waveguide region and spaced apart from the first waveguide region in a first direction, and a grating associated with the second waveguide region, characterised in that the said grating is provided by a periodic variation in the width of the second waveguide in the plane normal to the said first direction.
This aspect of the present invention provides a novel structure for a GAVCF device. Conventionally, the grating in a GAVCF has been etched above, below or into the stronger waveguide. The device of the present invention, by contrast, uses a grating which is formed laterally about the waveguide. This produces a structure which is easier to fabricate and provides enhanced yields.
As noted in the introduction above, lasers incorporating a GAVCF structure can be used to provide a tunable source suitable for WDM applications. The GAVCF of the third aspect of the invention may be used in such a laser, preferably, although not necessarily, in combination with the in-line filter of the first and second aspects.
The present invention also encompasses a method of manufacturing a GAVCF in accordance with the third aspect of the present invention.
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Lealman Ian F
Okai Makoto
Robertson Michael J
Arroyo Teresa M.
British Telecommunications public limited company
Inzirillo Gioacchino
Nixon & Vanderhye P.C.
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