WO2005011076A1

WO2005011076A1 - Weakly guiding ridge waveguides with vertical gratings

Info

Publication number: WO2005011076A1
Application number: PCT/IB2004/002438
Authority: WO
Inventors: Benoit Reid
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2003-07-31
Filing date: 2004-07-30
Publication date: 2005-02-03
Anticipated expiration: 2006-01-31

Abstract

The present invention provides a design and method of fabrication of a ridge waveguide structure with vertical gratings enabling weak guiding of light. In weakly guiding waveguide structures only a small portion of light from an optical mode is guided by the waveguide, with majority of the optical mode being present in a material outside of the waveguide structure. The present invention further provides an integrated optical device comprising a weakly guiding ridge waveguide formed on a substrate, said weakly guiding ridge waveguide including a ridge with a vertical grating structure, said weakly guiding ridge waveguide guiding an optical mode substantially beneath the ridge. The waveguide structure with vertical gratings according to the present invention uses materials of appropriate refractive index to provide weak guiding of light. The waveguide structure according to the present invention further enables lateral coupling of light from the waveguide structure to other optical structures that may be placed in a lateral plane that is parallel to the surface of the waveguide structure.

Description

WEAKLY GUIDING RIDGE WAVEGUIDES WITH VERTICAL GRATINGS

FIELD OF THE INVENTION

The present invention pertains to the field of ridge waveguides. More specifically, the present invention relates to weakly guiding ridge waveguides with vertical gratings.

BACKGROUND

Since their initial invention in 1962, semiconductor lasers have become a significant part of the electronics infrastructure. These important devices provide integral elements of communication and computer systems, and are key components in a host of widely used technological products, including compact disk players and laser printers. Designs have evolved from basic light emitting diodes to multi-layer heterostructure lasers with wideband performance.

Waveguide structures are commonly used in lasers to restrict the three dimensional propagation of electromagnetic waves to a single dimension. For example, the edge- emitting laser is usually based on a waveguide structure, formed from different semiconductor materials to provide waveguiding in one direction. The use of mirrors constructed from crystallographic "cleaves", enables the required optical reflections. A development of the basic laser constructed from a series of semiconductor layers, is the construction of a "ridge" in the top cladding layer of the slab guide. An example of this design has been shown by Kamp et al, in "High Performance Laterally Gain Coupled InGaAs/AlGaAs DFB Lasers", IPRM 1998. Kamp et al. and have shown that lateral gratings can be successfully fabricated using electron-beam lithography. The cross- section of such a laser is shown in Figure 1, wherein the laser design comprises lateral Cr grating, such that the evanescent field of the laser mode couples to the lateral Cr grating. In this configuration, the ridge improves the lateral, namely horizontal, waveguiding of the optical mode. Other ridge options have been developed allowing alternate methods of construction and providing improved concentration of the optical signal in the ridge area. In general the semiconductor layers of the laser guide the light in the "vertical" direction and the ridge guides the light in the "horizontal" direction approximately between the ridge sidewalls. The cleaved mirrors and gratings, if present, provide feedback by reflections in the "longitudinal" direction.

Gratings may be formed directly adjacent to the active layer, or as part of the active layer itself. Additionally the gratings can be placed on either the horizontal or vertical surfaces of a ridge laser configuration. The vertical placement of these gratings can be called sidewall gratings.

For applications, such as telecommunications, where the laser wavelength must be more closely controlled, it is common to use a periodic grating close to the active region. Distributed Bragg Reflector (DBR) lasers and Distributed Feedback (DFB) laser structures both rely on the use of periodic grating structures. In a DBR laser the active region, which provides gain, and the gratings, which provide wavelength selectivity, are separated longitudinally, h a DFB laser the two functions are combined.

A key part of the progress in the design of semiconductor lasers has been the development of the associated manufacturing methods. There has been a continuous effort to find ways to improve alternate techniques for building these lasers and methods of improving the accuracy of the final constructed laser. The performance of a laser is highly dependent on the final physical accuracy of the layers, gratings and electrical contacts as well as the accuracy of the composition of the materials used. The manufacturing process requires multiple steps of deposition and etching of layers of materials, and requires the materials to be relatively pure. The availability of metal organic chemical vapour deposition (MOCVD), molecular beam epitaxy (MBE), advanced deposition techniques for dielectric and metal films, stepper photolithography and electron-beam lithography has allowed significant steps for the improvement in production capability. Furthermore, semiconductor device fabrication is an expensive process and the design must take into account the required quality and cost of the units when made in large quantities. The yield and cost of semiconductor lasers are highly dependent on the number of steps required during the manufacturing process. Therefore, continued effort is needed to both simplify the method of fabrication, and reduce the number and types of different processes used. The standard method of fabricating lasers uses photolithography. In this process geometrical shapes on a mask are transferred to the semiconductor layers. This process is well known, proven and ideally suited to manufacturing. However due to diffraction, the photolithography process introduces a distortion of the required image, resulting in a blurring of the original image and a limitation in its resolution, typically achieving resolution levels in the range of 0.3 microns. Resolution defines the ability to discriminate between two points. A resolution of 0.3 microns means that it is not possible to determine any feature smaller than 0.3 microns. For a free space wavelength of 1.55 microns, the guide wavelength in a semiconductor laser will be approximately 0.48 microns, giving a first order grating spacing of 0.24 microns, and as such gratings of this order may not be fabricated using photolithography.

The gratings may also be fabricated using a holographic method whereby two laser beams, which are split from a single laser, interfere with each other. The interference pattern comprises fringes made of bright and dark lines. A photoresist is illuminated with this pattern, which creates the grating. The pitch of the grating, which is usually first order, but could be of any order, is determined by the angle between the beams. Excellent accuracy can be achieved with this method and the holographic method is very suitable when the grating is identical over the whole surface of the wafer. However, difficulties occur when the gratings are more complicated, and not uniform across the wafer, for example, patches of grating or variable gratings over the length of the wafer. For this scenario electron-beam lithography would typically be used.

Electron-beam lithography uses a scanning beam of electrons to define the desired pattern in the photoresist film. This technique is capable of very high resolution, which can be better than 0.03 microns. The process is a very flexible technique that can work with a variety of materials and patterns. However it is a slow process, being orders of magnitude slower than photolithography and requires more complicated and expensive manufacturing equipment.

Figure 2 illustrates a schematic diagram of a DFB waveguide laser structure proposed by Miller et al. in "A Distributed Ridge Waveguide Quantum Well Heterostructure Laser", IEEE Photon. Tech. Lett., vol.3, 1991. ΔQ is the period required to satisfy the Bragg condition, w is the laser stripe width and L_G is the length of the etched region of the grating. Miller et al. have demonstrated the ability to create third and fifth order DFB gratings using electron-beam lithography.

Lammert et al, in "InGaAsP-InP ridge- waveguide DBR lasers with first order surface gratings fabricated using CAIBE", IEEE 1997, disclose an InGaAsP-inP Ridge- Waveguide DBR Laser with a First Order Grating as illustrated in Figure 3. Lammert et al. show the feasibility of creating a ridge laser with first order gratings fabricated by a combination of electron-beam lithography and chemically assisted ion beam etching.

As illustrated in Figure 4, Y. Watanabe et al, in "Laterally Coupled Strained MQW Ridge Waveguide Distributed-Feedback Laser Diode Fabricated by Wet-Dry Hybrid Etching Process", IEEE Photon. Techn. Lett. Vol. 10, 1998, show a ridge waveguide with both lateral and vertical gratings. Watanabe et al. demonstrate the successful application of gratings with a ridge waveguide, using first order gratings fabricated with electron-beam lithography.

In certain requirements deeply etched gratings are needed as illustrated in Figure 5, by J. Wiedmann et al. in "Gain As/TnP Distributed Reflector Lasers Consisting of Deeply Etched Vertical Gratings", Jpn. L. Appl. Phys., Vol. 40, 2001. J. Wiedmann et al. disclose a DBR laser comprising a cavity with a vertical grating and a deeply etched DBR facet and demonstrate the possibility of deeply etching vertical gratings using electron-beam lithography.

Vertical grating waveguide structures typically use first order gratings, that is the grating spacing or pitch, is equal to half the optical wavelength in the waveguide, since essentially no light is scattered perpendicular to the grating with such waveguides. Higher order gratings have a pitch that is a multiple of half the optical wavelength, that is, a second order grating would have twice the spacing and a third order grating would have three times the spacing. In the case of higher order gratings light can be scattered from the waveguide in a direction perpendicular to the grating, unlike with first order gratings. This effect has been used to couple light in and out of planar waveguides using surface, or horizontal gratings. Furthermore, ridge waveguides used in semiconductor laser structures are typically strongly guiding waveguides. In strongly guiding waveguides, the optical mode is almost entirely located in the ridge since the surrounding material is of a lower refractive index as illustrated in Figure 6. For example, WO 03102646 discloses the use of side gratings on both tightly and strongly guided waveguides using first order gratings, to form a DFB laser. These are illustrated in Figure 7 and 8, respectively. US Patent No. 5,659,640 further discloses tightly guiding buried waveguides using first order gratings to produce a filter that may be optically connected to a laser, with the waveguide structure fabricated using a selective growth technique. In addition, US Patent No. 5,930,437 discloses a strongly guiding buried geometry waveguide that is buried in a lower refractive index material.

Strongly guiding waveguides are typically used since these waveguides will have a good overlap with the gratings due to the presence of the optical mode in the ridge. However, when light is coupled out of a strongly guiding waveguide, the light travels in air next to the ridge. Therefore, in applications where metal connections to the waveguide are required, for example, such connections can interfere with the light from strongly guiding waveguides. This can be a problem when integrating various optical circuits and devices.

Therefore, there is a need for a new design and method of fabrication of weakly guiding ridge waveguides with vertical gratings to overcome problems with integration of optical circuits and devices. In addition, there is a need to minimize the cost and complexity of fabrication of weakly guiding ridge waveguides with vertical gratings.

This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention. SUMMARY OF THE INVENTION

An object of the present invention is to provide weakly guiding ridge waveguides with vertical gratings. In accordance with an aspect of the present invention, there is provided an integrated optical device comprising a weakly guiding ridge waveguide formed on a substrate, said weakly guiding ridge waveguide including a ridge with a vertical grating structure, said weakly guiding ridge waveguide guiding an optical mode substantially beneath the ridge.

In accordance with another aspect of the invention, there is provided a method of forming an optical waveguide comprising the steps of: depositing one or more layers of an optical waveguide material on a substrate material, a layer of said substrate material having a first index of refraction and a layer of said optical waveguide material having a second index of refraction, wherein said first index of refraction is higher than the second index of refraction; creating a mask on said optical waveguide material, said mask having a pattern, said pattern defining a ridge waveguide region with vertical gratings; and etching away a portion of the optical waveguide material determined by the mask, thereby forming a ridge waveguide with vertical gratings.

In accordance with another aspect of the invention, there is provided a method of forming an optical waveguide comprising the steps of: depositing one or more layers of an optical waveguide material on a substrate material, a layer of said substrate material having a first index of refraction and a layer of said optical waveguide material having a second index of refraction, wherein said first index of refraction is higher than the second index of refraction, said optical waveguide material being photon, electron, ion or neutral atom sensitive; exposing the optical waveguide material to a predetermined radiation or particle in a pattern defining a ridge waveguide region with vertical gratings, whereby the optical waveguide material exposed to the predetermined radiation is altered, thereby defining a ridge waveguide with vertical gratings.

In accordance with another aspect of the present invention, there is provided a method of forming an optical waveguide comprising the step of selectively depositing one or more layers of an optical waveguide material defining a ridge waveguide region with vertical gratings, said optical waveguide material being selectively deposited on a substrate material, a layer of said substrate material having a first index of refraction and a layer of said optical waveguide material having a second index of refraction, wherein said first index of refraction is higher than the second index of refraction, said step of selectively depositing, thereby forming a ridge waveguide with vertical gratings.

In accordance with another aspect of the present invention there is provided an integrated optical device comprising: a substrate having one or more layers of material thereon, one of said layers having a first index of refraction; and a ridge waveguide formed on said substrate, said ridge waveguide including a vertical grating structure, said ridge waveguide having one or more layers, one of said layers having a second index of refraction; wherein said first index of refraction is higher than the second index of refraction thereby forming a weakly guiding waveguide.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a schematic layout of a ridge waveguide laser, and shows the evanescent field of the laser mode coupling to the lateral Cr grating as shown by Kamp et al.

Figure 2 is a schematic diagram of a distributed feedback ridge waveguide laser, defining the grating period requirement needed to satisfy the Bragg condition as shown by Miller et al.

Figure 3 is a schematic diagram of an InGaAsP-InP ridge-waveguide DBR laser with a first order grating as shown by Lammert et al.

Figure 4 shows a laterally coupled strained MQW ridge waveguide distributed-feedback laser diode fabricated by wet-dry hybrid etching process as shown by Y. Watanabe et al.

Figure 5 shows a schematic of a DBR laser comprising a cavity with a vertical grating and a deeply etched DBR facet as shown by from J. Wiedmann et al.

Figure 6 shows a strongly guiding corrugated ridge waveguide. Figure 7 shows a schematic of a tightly guiding ridge waveguide according to Hastings et al.

Figure 8 shows a schematic of a strongly guiding ridge waveguide according to Hastings et al.

Figure 9 shows a weakly guiding corrugated ridge waveguide.

Figure 10 shows one embodiment of the present invention wherein gratings on a semiconductor laser allow transfer of optical energy to another device.

Figure 11 shows a basic semiconductor laser structure before the formation of a ridge.

Figures 12A to 12D demonstrate the key_. steps of the fabrication process for a laser employing one embodiment of the present invention.

Figure 13 A illustrates power-current characteristics of a Fabry-Perot laser and a corrugated-ridge DFB laser employing one embodiment of the present invention.

Figure 13B shows typical spectra of the corrugated-ridge laser of Figure 13 A measured at various drive currents.

Figure 13C shows the measured heterodyne signal for the structure of Figure 13 A.

Figure 14A illustrates one embodiment of the present invention wherein the grating pitch is different on either side of the ridge.

Figure 14B illustrates one embodiment of the present invention wherein gratings are designed in sets.

Figure 14C illustrates one embodiment of the present invention wherein sets of gratings have varying separation therebetween. Figure 14D illustrates one embodiment of the present invention wherein the amplitude of the gratings is different at two ends of the ridge.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The term "light" is used to define radiation in any region of the electromagnetic spectrum.

The term "optical" is used to define a relation to visible light as well as radiation in any other regions of the electromagnetic spectrum.

The term "photolithography" is used to define a lithography process in which photons from any region of the electromagnetic spectrum may be used to produce a particular pattern on a radiation sensitive material. For example, such a lithography process may comprise the use of G-line (436 nm), H-line (405 nm) and/or I-line (365 nm) wavelengths of radiation. Other examples of photolithography processes include deep ultraviolet (UN) lithography and x-ray lithography. The term "photolithography" is further used to define various techniques of lithography, for example, stepper lithography, mask lithography, holography techniques, and any other lithography techniques as would be readily understood by a worker skilled in the art.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

The present invention provides a design and method of fabrication of a ridge waveguide structure with vertical gratings enabling weak guiding of light. In weakly guiding waveguide structures only a small portion of light from an optical mode is guided by the waveguide, with majority of the optical mode being present in a material outside of the waveguide structure. For example, assume an optical mode 93 is generated by a device such as a laser or other optical device as would be readily understood by a worker skilled in the art, in a substrate 92 and a waveguide structure with vertical gratings 91 is in contact with the material in which the optical mode is present, as illustrated in Figure 9. When the refractive index of the substrate material 92 is higher than that of the ridge waveguide 91 then the optical mode will remain primarily in the substrate 92 with only a small percentage of light entering the waveguide 91, thus resulting in weak guiding of light from the optical mode. In one embodiment of the present invention, the waveguide structure with vertical gratings uses materials of appropriate refractive index to provide weak guiding of light. For example, in one embodiment, the optical mode may be confined within a high refractive index layer. In a further embodiment, the optical mode may be centered within the high refractive index material.

In another embodiment, the weakly guiding waveguide is designed such that the size and dimensions of the ridge waveguide limit the optical mode from substantially entering the ridge waveguide, thereby resulting in weak guiding of light by the waveguide structure. Other methods, as would be readily understood by a worker skilled in the art, may also be used in designing weakly guiding waveguide structures. In other embodiments, the optical mode of the device may not be generated directly below the waveguide structure of the present invention but may be guided to other material that is in contact with the waveguide structure.

An advantage of weakly guiding waveguide structures is that essentially no light travels in the air next to the ridge waveguide. Therefore, metal connections can be made to the weakly guiding waveguide structure, which enables integration of various optical circuits that may comprise weakly guiding waveguide structures.

Embodiments of the present invention further enable the use of a photolithographic process, such as stepper lithography, for definition of the vertical grating geometries, which is a less complex and cheaper lithography process when compared to the electron-beam lithography process, which is typically used for current designs of vertical gratings. The present invention also allows for a variety of vertical grating structures to be defined and fabricated in the ridge for weakly guiding waveguide structures.

The waveguide structure according to the present invention further enables lateral coupling of light from the waveguide structure to other optical structures that may be placed in a lateral plane that is parallel to the surface of the waveguide structure. This can be useful when multiple optical circuits or devices are integrated onto a single chip, for example. In one embodiment, the lateral light output from a ridge waveguide structure with vertical gratings is incident upon a detector, which can detect the intensity as well as wavelength of the light.

In addition, the present invention may be used to achieve spatial modulation of the loss of light along the waveguide, which results from a variation in the position of the optical mode along the length of the waveguide structure. In addition, the design and fabrication of the waveguide structure according to the present invention may simplify the overall fabrication process of structures that incorporate the waveguides. For example, the number of fabrication steps may be reduced for certain devices incorporating the waveguide structure of the present invention.

The waveguide structure of the present invention may be used in various devices such as semiconductor lasers, filters, tunable filters, and other devices as would be readily understood by a worker skilled in the art. For example, appropriate embodiments of the present invention may be used with Distributed Feedback (DFB) lasers or Distributed Bragg Reflector (DBR) lasers. Both types of laser designs rely on the use of periodic grating structures for providing high reflectivity of light resulting from spacing the gratings an integer number of half wavelengths apart, for example λ/2, 3λ/2, 5λ/2, and so on, thus enabling the reflections to be in phase with each other. One embodiment of the present invention can additionally be used to produce vertical gratings containing phase shifts, for example a quarter wavelength phase shift, wherein this alteration can cause the structure to act as a resonator that stores energy. Furthermore, one embodiment of the present invention may be used in tunable semiconductor lasers, for example, DBR lasers that rely on tuning of the injected current.

Embodiments of the present invention may also be used with various optical integrated circuits. The grating associated with the ridge may be passive or active, and will be active if a contact is placed thereon. In one embodiment an electrode is formed on the top surface of the waveguide structure. Furthermore, in another embodiment, the waveguide may be used as a tunable filter, for example, wherein the wavelength of light passing through a waveguide may depend on the voltage or cunent applied to an electrode on the waveguide. In addition, the scattering of light may further be dependent upon the applied current or voltage. In a further embodiment, the vertical grating symmetry, pitch and/or depth are variable thus providing various filtering effects and a plurality of design potentials for filters designed according to embodiments of the present invention.

As described earlier, first order gratings are typically used in ridge waveguides with vertical gratings, as essentially no light is scattered perpendicular to the grating plane with this grating configuration. However, by using higher order gratings in the present invention, the vertical grating patterning process can be faster, less complex and more economical by enabling the use of photolithography in place of the typically used electron-beam lithography for grating fabrication. In addition the light scattered from the vertical grating of higher order grating designs according to the present invention can be advantageously used to couple light laterally from the waveguide structure to other optical circuits and structures within a particular proximity.

Use of Photolithography

The use of current state-of-the-art photolithography techniques for the generation of a first order vertical grating structure can be problematic due to the manufacturing resolution of the photolithographic process and the desired level of accuracy of the gratings for this form of grating structure. This present invention may use higher order vertical gratings, which have larger spacing dimensions when compared to first order gratings, thus potentially enabling the manufacture of these gratings using a photolithographic process.

Since higher order gratings have larger spacing dimensions when compared to first order gratings, it can be possible to use fabrication techniques which have a lower resolution compared to fabrication techniques required for first order gratings. Therefore, it can be possible to use photolithography to create the vertical grating structure on the sidewalls of a ridge waveguide structure having higher order gratings, for example second, third or higher order. For example, for a free space wavelength of 1.55 microns, the waveguide wavelength in a semiconductor laser is approximately 0.48 microns, giving a first order spacing of 0.24 microns, a second order spacing of 0.48 microns, a third order spacing of 0.72 microns, and so on. As previously stated, the most commonly available photolithography systems have a resolution in the order of approximately 0.3 microns. Therefore using a third order grating with a pitch of 0.72 microns, the smallest feature namely the gap or tooth of the grating would be approximately 0.36 microns, assuming that the grating has a 50/50 mark-to-space ratio. Such a grating size can thus be fabricated using a photolithographic process, however as the smallest feature size is close to the resolution limit of the photolithographic method, the vertical grating may have some level of erosion.

There are other types of photolithographic systems that use electromagnetic photons in the deep ultraviolet and x-ray regions that may physically achieve a higher resolution than the commonly available photolithography systems. The smallest feature that can be reproducible is physically limited by the diffraction of the light used for exposure of the photoresist film and in ideal conditions, light can be typically focused to a point smaller than approximately half its wavelength. As technology evolves, it may be possible to use a radiation source having a wavelength of approximately 0.12 microns, which would enable the fabrication of vertical first order gratings using a photolithographic process.

Photolithography is a widely used lithographic process in semiconductor fabrication and is more economical, partially as a result of well established systems and techniques, compared to other types of lithography such as electron-beam lithography. It is also a less complex method of patterning compared to electron-beam lithography, which as described earlier is the typical method used in vertical first order grating waveguide fabrication.

As described earlier, holographic patterning techniques are commonly used to pattern gratings. An advantage of using a mask or stepper photolithography technique as opposed to a holographic technique for the fabrication of the vertical gratings is that the design can encompass variations of grating structure and is not limited to an interference pattern. For example different orders of vertical gratings may be combined on a particular ridge thereby extending the design capability for a device having this type of waveguide structure. Therefore a very broad range of options in vertical grating combinations and designs are available using a mask or stepper photolithography fabrication technique for ridge waveguides having vertical ridge gratings. In addition, the photolithographic process for manufacturing such waveguides can be highly repeatable and can meet the needs of an efficient manufacturing process. Furthermore, photolithography does not have the complexity of programming a complicated track for etching, as is needed for electron-beam lithography.

Coupling of Light

In one embodiment of the present invention, the vertical gratings allow coupling of light to occur between the waveguide structure and other devices. These devices may be other waveguides, photodetectors or other optical systems. Because the gratings in the present invention are vertical, the coupling of the light occurs laterally, that is, in a plane that is parallel to the top surface of the ridge waveguide of the present invention. It is well known that a grating can couple light in and out of a waveguide, however most gratings are horizontal therefore this coupling occurs in the plane perpendicular to the surface of the wafer. According to the present invention, the coupling occurs horizontally, thereby allowing the possibility of integrating many devices onto the same chip, with the transfer of light therebetween.

Furthermore, in the present invention, coupling of light can occur laterally from the optical mode below the ridge material. As described above, in weakly guiding waveguides the optical mode is primarily present below the ridge waveguide as illustrated in Figure 9. Light 94 from the optical mode can therefore also be coupled out laterally within the substrate material 92. Conversely, in strongly guiding waveguides as illustrated in Figure 6, since the optical mode is primarily located in the ridge waveguide, light 64 is primarily coupled laterally from the waveguide 61 rather than within the substrate 63.

It is also well known that with higher order gratings light can be scattered in a plane perpendicular to the grating, however this is not readily achievable with first order gratings. For even order gratings, such as second order, essentially 100% of the light may be coupled out of the grating when the grating pitch is exactly one wavelength long. When this condition is not satisfied there is less light coupled out of the grating. For odd order gratings, there can be some light that is scattered out wherein the amount of coupled light depends on the grating pitch. An advantage of having light scattered in a direction perpendicular to the grating as in the case of higher order gratings is that the light can be coupled to other optical circuits and structures that are in a lateral plane parallel to the plane of the waveguide structure. As is readily understood by a worker skilled in the art, it is possible to couple light from a vertical first order grating by using blazing techniques which entail controlling the cross-sectional shape of the vertical grooves or teeth to concentrate the light in a desired region. In addition, in some embodiments of the present invention an antireflective (AR) coating may be applied to the emitting surface of the vertical grating and in other embodiments this may not be necessary.

Figure 10 shows the top view of a weakly guiding ridge waveguide with vertical gratings 101 according to one embodiment of the present invention, whereby the optical signal 102 is transmitted from the waveguide to another adjacent device 103. The ability of the present invention to couple light laterally in a plane parallel to the top surface of the waveguide structure can be an important advantage. For example, there are certain applications that require data to be transmitted on the light using a directly- modulated laser or using a laser with an external modulator, for example a Mach- Zehnder modulator. The laser and the modulator are typically fabricated on two separate chips and need to be aligned in the package with other optics and electronics. For manufacturing ease and cost reduction, there is a tendency to integrate more functions on a single chip. It is therefore advantageous to laterally couple the light between such devices wherein this coupling is enabled by the use of a weakly guiding ridge waveguide with vertical gratings according to the present invention.

In one embodiment the light from the waveguide structure of the present invention may be laterally coupled to a detector. The detector may be used to monitor the power of the light or the wavelength of the light, for example. In one embodiment, the detector may be a reverse-biased p-n junction such that any light entering the detector produces a proportional current that can be measured, thereby providing a measure of the power of the light coupled from the waveguide. Depending on the grating and detector design, some or all the light coupled from the waveguide can be detected. Therefore, the power of the light propagating within the waveguide may also be determined. In the case of wavelength monitoring, the detected light can provide an indication of the wavelength of the light coupled from the waveguide structure. This wavelength monitoring can be possible since light that couples out of a vertical grating, typically a blazed or higher order grating, does so parallel to the plane of the substrate, such that the angle the propagating light makes with the direction of the waveguide is a function of the wavelength of the light and of the pitch of the grating. Therefore, the light that contacts a detector or one of an array of detectors is a function of the wavelength. Therefore, the angle at which the light is detected is indicative of its wavelength. An array of detectors may thus be used to detect various wavelengths of light that may be coupled out of the waveguide structure. In one embodiment, the light coupled out of the waveguide structure may be small such that the detector or detectors act as an in-line wavelength monitor, thereby enabling most of the light to continue to propagate along the waveguide with only a small amount of the light used to determine the wavelength thereof. As would be readily understood, any desired amount of light may be used for such detection. In another embodiment of the present invention, the light coupled from the waveguide may be incident upon another waveguide structure designed to couple the light therein.

The weakly guiding ridge waveguide with vertical gratings according to the present invention may further be used to introduce spatially modulated variations in the loss of light along the waveguide structure, that is, modulated loss coupling can be introduced in the waveguide structure. This may be an approximately periodic variation in the optical loss from the waveguide as the light propagates along the waveguide structure. As the ridge width varies along the vertical grating, the optical mode comes closer to the surface where the ridge is narrower as compared to where the ridge width is wider. Therefore, placing a lossy material, for example a metal, on the surface of the teeth of the vertical grating can result in a greater loss of light from the narrower regions of the waveguide compared to the wider regions, which can result in the optical loss associated with the device varying or being modulated, along the length of the waveguide structure. Spatial modulation of loss may also be introduced along the waveguide structure by placing materials with other properties that affect the loss characteristics of light from the waveguide. For example, the material may have a non-linear effect on the loss or may suppress loss. Furthermore, in other embodiments, the material may be placed only on particular regions of the grating surface in order to introduce desired loss characteristics to desired regions of the waveguide.

For a ridge waveguide structure with vertical gratings used in a DFB semiconductor laser, the active region is remote from the ridge, and as a result, the cunent is not modulated, subsequently resulting in no gain coupling. Using this modulated loss coupling technique described above, there may be the ability to introduce loss coupling. As described earlier, in higher order gratings, there will be some scattered light that is modulated along the cavity to give loss modulation. According to one embodiment of the present invention, there is a capability to modify the design to provide such a modulation.

In one embodiment of the present invention the coupling can be small since the optical mode is below the surface and because higher order gratings are used. With DFB semiconductor laser designs, a typical design goal is for kL ~ 1, where k is the coupling coefficient and L is the length of the laser. A higher value of kL is required for a directly modulated laser. If k is small, then L needs to be longer. A usual DFB laser would be around 200-400 microns in length. A DFB laser using the weakly guided waveguide structure with vertical gratings according to one embodiment of the present invention would be between 1000-2000 microns in length, which has the advantage of providing improved power dissipation and hence higher optical power capability.

Fabrication Process

A description of one example of the implementation of the invention will now be made with reference to the accompanying drawings. In this example, a weakly guiding ridge waveguide with vertical gratings of second order or higher is used in a semiconductor laser structure. The design is not limited by substrate material, for example InP, or GaAs or GaN or whether the substrate is a p-substrate or n-substrate. In addition, a worker skilled in the art would readily understand the types of layers, materials and thicknesses, that would be necessary in order to manufacture a ridge semiconductor laser with a weakly guiding waveguide structure with vertical gratings according to the present invention, wherein these materials would be designed to be compatible with the substrate being used. In this example a vertically-coupled DFB ridge type semiconductor laser of GaAsP/InP type is manufactured by means of metal organic chemical vapor deposition (MOCND) using an organometallic compound gas, together with standard deposition techniques.

As shown in Figure 11, a wafer of an n-InP crystalline substrate 60 is prepared having a predetermined plane orientation. Chemical etching is performed to clean the surface of the wafer. Then a layer 50 of InGaAsP is formed on the cleaned surface of the wafer 60 in order to fabricate an active layer region for the semiconductor structure. The process used for forming this layer can involve methods such as epitaxial growth, liquid phase epitaxial growth, metal organic chemical vapor deposition, molecular beam epitaxial growth for example, or alternate techniques as would be readily understood by a worker skilled in the art of semiconductor manufacture. The active layer 50 could be a bulk layer, or a single quantum well layer, or multiple quantum well layers that are mainly composed of InGaAsP. Subsequently, a cladding layer 40 made of a material such as p- InP is deposited on the active layer 50. An etch termination 30 layer is then deposited on the cladding layer 40. A second cladding layer 20 made of a material for a ridge stripe such as p-InP is then deposited on the etch termination layer 30. Finally, a contact layer 10 is deposited on top of the cladding layer 20. The composition of an appropriate contact layer for this material system would be readily understood by a worker skilled in the art.

A sequence of steps for the subsequent fabrication of a ridge waveguide with vertical gratings according to one embodiment of the present invention is illustrated in Figure 12A to Figure 12D. Figure 12A illustrates the completion of the structure as defined above in relation to Figure 11 with a dielectric layer 5 deposited on top. Contact layer 10 is not shown for simplicity, however is present on top of the ridge material 20. A photoresist layer (not shown) is deposited on this structure and a photolithography process is used to transfer the desired grating pattern to the photoresist, thus defining the pattern for the vertical gratings. A stepper lithography process, for example, may be used in the photolithography process. The pattern is then transferred from the patterned photoresist layer to the dielectric layer 5 by etching thus forming a hard mask 51, followed by removal of the photoresist layer. The resulting structure is illustrated in Figure 12B. The etching process for the gratings can use a combination of processes including plasma and liquid base chemical reactions to transfer the pattern defined in the hard mask 51 into the contact layer 10 (not shown in Figure 12) and ridge material 20 thus forming structure 201 as illustrated in Figure 12C. The hard mask 51 is then removed as illustrated in Figure 12D and the basic ridge waveguide 201 having the desired order vertical gratings is ready for the remaining standard steps of fabrication. In this example, as illustrated in Figures 12C and 12D, the optical mode 1 is positioned substantially beneath the ridge thereby resulting in a weakly guiding waveguide.

As would be readily understood by a worker skilled in the art variations of the above fabrication processes may be used to fabricate the weakly guiding ridge waveguides with vertical gratings according to the present invention.

A dielectric (not shown), for example, silicon dioxide or silicon nitride, may then be deposited over the entire region embedding the ridge and the gratings. A via can be formed in the dielectric layer on the top of the ridge thus exposing the contact layer 10. Finally ohmic contacts can be fabricated on the top side and the back side of the laser. Embodiments of the present invention may similarly be implemented in various other laser structures.

Embodiments of the present invention may be used with various laser structures. For example, the laser may comprise a substrate of a first conductivity type, for example n- InP; a cladding layer of the same conductivity type, for example n-InP, wherein this cladding layer can include a plurality of layers of the same material with different doping levels; an active region, which can be doped with either conductivity type, or undoped and can be fabricated from alloys, for example alloys of InGaAsP, wherein the active region can include a plurality of layers and/or single or multiple quantum wells; a cladding layer of the opposite conductivity type, for example p-InP, wherein this cladding layer can comprise a plurality of layers of the same material with different doping levels and the cladding layer can comprise an etch-stop layer for the fabrication; a third-order grating (or any other order) substantially etched vertically on the sidewalls in the p-cladding layer forming the ridge. The structure can have a contact layer and additionally electrodes on both sides. As is well known to one skilled in the art, there are opportunities for a variety of reasons to incorporate additional layers, cleaning steps and other processes usual in the fabrication of such a device. The terms "on" and "on top of refer to the sequence of construction, and would allow the normal additional minor processes necessary for such construction.

The above example describes the use of a mask, which is used to transfer the required waveguide geometry to the waveguide material, followed by an etching process, however, a worker skilled in the art would readily understand that other fabrication processes may be used to fabricate the weakly guiding waveguide with vertical gratings according to the present invention. For example, in one embodiment, the material may be selectively deposited or grown to form the waveguide structure. In another embodiment, a material that is sensitive to photons, electrons, ions or neutral atoms, is deposited and patterned using lithographic techniques, wherein the material properties change upon exposure. These changes may include variations in refractive index or solubility of the material in certain chemicals, for example. Subsequent to exposure, either the altered or unaltered material may be etched away thereby forming the ridge waveguide with vertical gratings.

A typical DFB ridge waveguide laser requires a two-growth sequence, with a grating fabrication step in between for the construction of the vertical gratings. With the present invention the creation of the vertical grating during the ridge step allows the laser to be fabricated with two fewer steps in its overall construction, for example. This reduction in the number of steps in the manufacturing process can provide a significant reduction in the overall cost to manufacture the laser and can provide a greater yield in the overall production of these lasers. These benefits can arise from the ability to fabricate the gratings after the full structure is grown.

Considering the integration of lasers and Mach-Zehnder chips as described above, each of these chips are typically fabricated using different processes. By assembling the laser and the Mach-Zehnder on the same chip, the number of fabrication steps can increase to produce a single chip, with a possible consequent decrease in the yield and increase in cost of this chip. Since embodiments of the invention may decrease the number of steps to fabricate the laser, there can be a corresponding decrease in the overall number of steps to fabricate the combined laser and Mach-Zehnder modulator on a single chip. This is one example of many system designs where embodiments of the invention can simplify and improve the overall performance, allowing increased yield and reduced cost.

Device Performance

Figure 13 A to Figure 13C illustrate the performance of a DFB laser structure employing one embodiment of the present invention. This device was fabricated using stepper lithography for the definition of the vertical grating waveguide structure.

Figure 13 A shows the power output for the above laser and a Fabry-Perot laser structure as a function of injection current. Curve 131 illustrates the power output for the Fabry- Perot laser and curve 132 illustrates the curve for the DFB laser structure fabricated with an embodiment of the present invention.

Figure 13B illustrates typical spectra of the DFB laser at various drive currents. Curve 133 represents a 100 mA drive current, curve 134 represents a 300 mA drive current, and curve 135 represents a 700 mA drive current. The curves have been displaced vertically and horizontally for clarity. A high SMSR (side-mode suppression ratio) is demonstrated in Figure 13B.

Figure 13C illustrates the measured heterodyne signal 137 used to extract the optical linewidth, and demonstrates the nanow bandwidth for the DFB laser which is less than 1 MHz at - 3 dB from the peak.

Further Embodiments

The present invention further allows the fabrication of weakly guiding ridge waveguides with vertical gratings having a wide range of grating depths, widths and spacing, thus enabling many designs to be produced, for example chirp grating designs. Figure 14A to Figure 14D illustrate vertical grating designs according to various embodiments of the present invention. These embodiments are provided as examples and do not limit the types of designs achievable. Figure 14A illustrates an embodiment in which the vertical grating structures are different on either side of the ridge 141. It can be seen that vertical gratings 142 have a different pitch compared to gratings 143. Figure 14B illustrates vertical gratings in sets 145, or sampled gratings, along the ridge 144, where each set 145 is separated by a particular distance 146. Lastly, Figure 14C illustrates chirped sampled vertical gratings 148, where the spacing 149 between the sets varies. Figure 14D illustrates an embodiment in which the amplitude of the vertical gratings varies at two ends of the ridge 150. Gratings 151 have a smaller amplitude compared to gratings 152. The embodiment illustrated in Figure 14D may be used in an anti-reflective/high-reflective (AR/HR) DBR laser, for example. In further embodiments the gratings may have other variations in pitch and/or amplitude. In yet further embodiments the gratings may be angled/blazed gratings.

As illustrated in the Figures, the sizes of layers or regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of the present invention. Once again, as stated previously, various aspects of the present invention are described with reference to a layer or structure being formed on a substrate or other layer or structure. As will be appreciated by those of skill in the art, references to a layer being formed "on" another layer or substrate contemplates that additional layers may intervene. Furthermore, relative terms such as beneath may be used herein to describe one layer or regions relationship to another layer or region as illustrated in the

Figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in the Figures is turned over, layers or regions described as "beneath" other layers or regions would now be oriented "above" these other layers or regions. The term "beneath" is intended to encompass both above and beneath in this situation.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

I CLAIM:

1. An integrated optical device comprising a weakly guiding ridge waveguide formed on a substrate, said weakly guiding ridge waveguide including a ridge with a vertical grating structure, said weakly guiding ridge waveguide guiding an optical mode substantially beneath the ridge.

2. The integrated optical device according to claim 1, wherein the vertical grating structure is a higher order vertical grating structure.

3. The integrated optical device according to claim 2, wherein the higher order vertical grating structure provides a means for coupling light from the integrated optical device to at least one alternate optical device.

4. The integrated optical device according to claim 1, wherein the vertical grating structure is a first order vertical grating structure.

5. The integrated optical device according to claim 4, wherein the vertical grating structure comprises vertical grooves having a cross-sectional shape and pitch to concentrate the light in a desired region.

6. The integrated optical device according to claim 5, wherein the vertical grating structure provides a means for coupling light from the integrated optical device to at least one alternate optical device.

7. The integrated optical device according to claim 3 or 6, wherein the alternate optical device is an optical circuit.

8. The integrated optical device according to claim 1, wherein the vertical grating structure is symmetric on opposite sides of the ridge.

9. The integrated optical device according to claim 1, wherein the vertical grating structure is asymmetric on opposite sides of the ridge.

10. The integrated optical device according to claim 1, wherein the ridge has a length, and wherein the vertical grating structure has a variable pitch along the length of the ridge.

11. The integrated optical device according to claim 1, wherein the ridge has a length and wherein the vertical grating structure has a variable amplitude along the length of the ridge.

12. The integrated optical device according to claim 1, further comprising a material deposited on the ridge waveguide thereby providing a modulation of loss of light from the ridge.

13. The integrated device according to claim 12, wherein the material is a metal.

14. The integrated device according to claim 12, wherein the material is a polymer.

15. The integrated device according to claim 12, wherein the material is deposited on a portion of the ridge.

16. The integrated optical device according to claim 1, wherein the vertical grating structure is designed to provide a phase shift.

17. The integrated optical device according to claim 3 or 6, wherein the alternate optical device is a detector.

18. The integrated optical device according to claim 3 or 6, wherein light from the integrated optical device is coupled to multiple detectors, each detector being positioned to receive light of a particular wavelength or a particular range of wavelengths.

19. The integrated optical device according to claim 1, wherein the integrated optical device is a waveguide.

20. The integrated optical device according to claim 1, wherein the integrated optical device is a laser.

21. The integrated optical device according to claim 20, wherein the laser is a DBR laser or a DFB laser.

22. The integrated optical device according to claim 1, wherein the integrated optical device is a filter or a tunable filter.

23. An integrated optical device comprising: a) a substrate having one or more layers of material thereon, one of said layers having a first index of refraction; and b) a ridge waveguide formed on said substrate, said ridge waveguide including a vertical grating structure, said ridge waveguide having one or more layers, one of said layers having a second index of refraction; wherein said first index of refraction is higher than the second index of refraction thereby forming a weakly guiding waveguide.

24. A method of forming an optical waveguide comprising the steps of: a) depositing one or more layers of an optical waveguide material on a substrate material, a layer of said substrate material having a first index of refraction and a layer of said optical waveguide material having a second index of refraction, wherein said first index of refraction is higher than the second index of refraction; b) creating a mask on said optical waveguide material, said mask having a pattern, said pattern defining a ridge waveguide region with vertical gratings; and c) etching away a portion of the optical waveguide material determined by the mask, thereby forming a ridge waveguide with vertical gratings.

25. The method of forming an optical waveguide according to claim 24 wherein the vertical gratings are higher order vertical gratings and wherein patterning of the mask is performed using photolithography.

26. The method of forming an optical waveguide according to claim 25, wherein photolithography is selected from the group comprising stepper lithography, holography, deep ultraviolet lithography, mask lithography and x-ray lithography.

27. A method of forming an optical waveguide comprising the steps of: a) depositing one or more layers of an optical waveguide material on a substrate material, a layer of said substrate material having a first index of refraction and a layer of said optical waveguide material having a second index of refraction, wherein said first index of refraction is higher than the second index of refraction, said optical waveguide material being photon, electron, ion or neutral atom sensitive; b) exposing the optical waveguide material to a predetermined radiation or particle in a pattern defining a ridge waveguide region with vertical gratings, whereby the optical waveguide material exposed to the predetermined radiation is altered, thereby defining a ridge waveguide with vertical gratings.

28. The method of forming an optical waveguide according to claim 27 further comprising the step of etching away the optical waveguide material that is altered.

29. The method of forming an optical waveguide according to claim 27 further comprising the step of etching away the optical waveguide material that is unaltered.

30. A method of forming an optical waveguide comprising the step of selectively depositing one or more layers of an optical waveguide material defining a ridge waveguide region with vertical gratings, said optical waveguide material being selectively deposited on a substrate material, a layer of said substrate material having a first index of refraction and a layer of said optical waveguide material having a second index of refraction, wherein said first index of refraction is higher than the second index of refraction, said step of selectively depositing, thereby forming a ridge waveguide with vertical gratings.