Patent Application
20220190560
The present invention relates to optical communication techniques. More particularly, the present invention provides a gain medium structure for semiconductor optical amplifier for high-power elevated temperature operation.
Over the last few decades, the use of communication networks exploded. In the early days Internet, popular applications were limited to emails, bulletin board, and mostly informational and text-based web page surfing, and the amount of data transferred was usually relatively small. Today, Internet and mobile applications demand a huge amount of bandwidth for transferring photo, video, music, and other multimedia files. For example, a social network like Facebook processes more than 500 TB of data daily. With such high demands on data and data transfer, existing data communication systems need to be improved to address these needs.
40-Gbit/s and then 100-Gbit/s data rates wide-band DWDM (Dense Wavelength Division Multiplexed) optical transmission over existing single-mode fiber is a target for the next generation of fiber-optic communication networks. More recently, optical components are being integrated on silicon substrates for fabricating large-scale photonic integrated circuits that co-exist with micro-electronic chips. a whole range of photonic components, including filters, (de)multiplexers, splitters, modulators, and photodetectors, have been demonstrated, mostly in the silicon-photonics platform. The silicon-photonics platform on silicon-on-insulator substrate is especially suited for standard WDM communication bands at 1300 nm and 1550 nm, as silicon (n=3.48) and its oxide SiO2 (n=1.44) are both transparent, and form high-index contrast, high-confinement waveguides ideally suited for medium to high-integration silicon photonics integrated circuits (SPICs).
Semiconductor optical amplifier in silicon photonics platform have been implemented for many applications of optical communication. For example, wavelength tunable lasers based on the semiconductor optical amplifier (SOA) and reflective semiconductor optical amplifier (RSOA) are provided as key elements in SPICs for wide-band optical communication with increasing spectral efficiency. However, technical challenges exist for developing gain chip for high-power SOA/RSOA for operation at elevated temperature in wide-band high-speed data communication application. Therefore, improved techniques are desired.
The present invention relates to optical telecommunication techniques. One aspect of the present invention provides a gain medium structure for a semiconductor optical amplifier (SOA) and/or a reflective semiconductor optical amplifier (RSOA) for tunable lasers in high-power elevated temperature application. More particularly, the present invention provides a gain medium having wider n side separated confinement heterostructure, narrower quantum well, and optimized active layer confinement factor used in SOA/RSOA with high saturation power at elevated temperature for a wide-band wavelength tunable laser in high-speed data communication application, though other applications are possible.
In an embodiment, the present invention provides a gain medium for semiconductor optical amplifier in high-power operation. The gain medium includes a substrate with n-type doping and a lower clad layer formed overlying the substrate. The gain medium further includes a lower optical confinement stack overlying the lower clad layer. Additionally, the gain medium includes an active layer comprising a multi-quantum-well heterostructure with multiple well layers characterized by about 0.8% to 1.2% compressive strain respectively separated by multiple barrier layers characterized by about −0.1% to −0.5% tensile strain. The active layer overlays the lower optical confinement stack. The gain medium further includes an upper optical confinement stack overlying the active layer, the upper optical confinement stack being set thinner than the lower optical confinement stack. Furthermore, the gain medium includes an upper clad layer overlying the upper optical confinement stack. Moreover, the gain medium includes a p-type contact layer overlying the upper clad layer.
In an alternative embodiment, the present invention provides a method of forming a gain medium for semiconductor optical amplifier in high-power operation. The method includes a step of providing a substrate with n-type doping. The method further includes a step of forming lower clad layer formed overlying the substrate. Additionally, the method includes a step of forming a lower optical confinement stack overlying the lower clad layer. The method further includes a step of forming an active layer comprising a multi-quantum-well heterostructure with multiple well layers characterized by about 0.8% to 1.2% compressive strain respectively separated by multiple barrier layers characterized by about −0.1% to −0.5% tensile strain. The active layer is configured to overlay the lower optical confinement stack. Furthermore, the method includes a step of forming an upper optical confinement stack overlying the active layer, the upper optical confinement stack being set thinner than the lower optical confinement stack. The method further includes a step of forming an upper clad layer overlying the upper optical confinement stack. Moreover, the method includes a step of forming a p-type contact layer overlying the upper clad layer. In the embodiment, the lower optical confinement stack is made with a larger thickness than the upper optical confinement stack. In the embodiment, the active layer is configured with an optimized confinement factor ratio of 1.27+/−0.15 (1/μm)
The present invention achieves these benefits and others in the context of known technology of semiconductor optical amplifier for tunable laser. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings.
The following diagrams are merely examples, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many other variations, modifications, and alternatives. It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this process and scope of the appended claims.
The present invention relates to optical telecommunication techniques. One aspect of the present invention provides a gain medium structure for a semiconductor optical amplifier (SOA) and/or a reflective semiconductor optical amplifier (RSOA) for tunable lasers in high-power elevated temperature application. More particularly, the present invention provides a gain medium having wider n side separated confinement heterostructure, narrower quantum well, and optimized active layer confinement factor used in SOA/RSOA with high saturation power at elevated temperature for a wide-band wavelength tunable laser in high-speed data communication application, though other applications are possible.
The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments. Thus, the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
In the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without necessarily being limited to these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.
The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All the features disclosed in this specification, (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
Furthermore, any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. Section 112, Paragraph 6. In particular, the use of “step of” or “act of” in the Claims herein is not intended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.
Please note, if used, the labels inner, outer, left, right, front, back, top, bottom, end, forward, reverse, clockwise and counterclockwise have been used for convenience purposes only and are not intended to imply any particular fixed direction. Instead, they are used to reflect relative locations and/or directions between various portions of an object.
In an aspect, the present disclosure provides a gain medium of a reflective semiconductor optical amplifier (RSOA) and/or a semiconductor optical amplifier (SOA) for tunable laser application with high-power operability at elevated temperature.
When the gain medium 110 is driven by a bias current a carrier-stimulated emission in the active region 115 is generated between the front facet and the back facet before lasing out the front facet. The stimulated light emission is transmitted along with the active region 115 which acts as a light waveguide. Specifically in this top view diagram, the linear active region 115 appears like a linear waveguide which forms an angled (a) intersection with the front facet with AR coating so that the stimulated light emission mostly is not reflected to itself. While, the linear active region 115 is substantially perpendicular to the back facet with HR coating allowing a substantially total reflection of the stimulated light emission from the back facet. Eventually, the stimulated light emission generated in the active region 115 exits the front facet through the AR coating.
In part B) of
In part B) of
In a specific embodiment, the gain medium with the active layer in multi-quantum-well structure for both the RSOA and SOA is configured to yield high saturation output power for high-power tunable laser operated at elevated temperature. In theory, the saturation output power Psat of a gain medium with the active layer as shown in
In the expression (1), d is a thickness of the active layer of the gain medium, w is a width of the active layer, Γ is optical confinement factor, α is a differential gain, and τs is carrier lifetime. In order to enhance the saturation output power Psat, conventional approaches included increasing the width w of the active layer or reducing optical confinement factor Γ (or increasing ratio of d/Γ). Downside of reduced confinement factor or increased width of the active layer is the gain of the gain medium is also reduced. In order to obtain higher gain for the (SOA/RSOA) device, longer cavity length and higher operating current are normally required to achieve higher saturation output power. But high operating current mostly is not recommended for devices in elevated temperature operation.
In this disclosure, an improved gain medium used for SOA (or RSOA) with high saturation power Psat with reasonably high gain is provided with an active layer in modulated n-type doping and an optimized optical confinement factor Γ to reduce carrier lifetime τs and differential gain α. This makes it a very suitable gain medium for the SOA or RSOA in high-power tunable laser application.
In the embodiment, the gain medium is a multilayer heterostructure including an active layer configured as a multi-quantum-well structure. Optionally, the active layer includes modulated doping with n-type electronic impurity. Optionally, the active layer is undoped. The multi-quantum-well structure of the active layer is provided as a hetero-junction structure including multiple well layers separated by respective barrier layers. In the embodiment, specific material choices for these well layers and barrier layers, and physical parameters like thickness and doping characteristics are designed to yield different bandgaps and oscillation characteristics for the stimulated light emission within multi-well structure of the active layer in order to provide a certain light spectrum range that is fit, for example, for the applications of the RSOA or SOA for tunable laser operating in wide-band (C-band or L-band) wavelengths.
Referring to
Optionally, the active layer is undoped. Optionally, the active layer, particularly, the well layers and barrier layers are modulated doped with n-type electronic impurity at about 1.0×1018 cm−3 to 3.0×1018 cm−3 to reduce carrier lifetime τs and differential gain a for enhancing saturation output power of the light emission generated therein. Additionally, the design with a narrower quantum well in the active layer is to have a lower confinement factor Γ so that the saturation output power can be reduced (see expression (1)). Optionally, the number of the well layer can be more than three. The confinement factor ratio Γ/d needs to be adequate for the gain medium being applied for ROSA or SOA. If it is too small, the gain becomes low ire SOA and threshold current becomes high for RSOA. If it is too large, the saturation power would be low. The adequate value of the confinement factor ratio Γ/d is depended on cavity length used in an application. For an application with 1 mm cavity length for either a RSOA or SOA device, an optimized value for confinement factor ratio of Γ/d is 1.27+/−0.15 (1/μm).
In a specific embodiment, the lower optical confinement stack, including layers 2 and 3, is made by InGaAsP-based material. Layer 2 is designed with a thickness of about 60+/−10 nm and layer 3 with a thickness of about 8+/−2 nm. Referring to
Beneath the layer 2 is the lower clad layer 1 which is made by InP-based material with a thickness of about 1000 nm formed on the substrate 0 which is optionally also an InP-wafer with n-type doping level >1e18 cm−3. Optionally, the lower clad layer 1 is doped to n-type at a level of about 1e18 cm3. Optionally, a n-metal may be formed beneath the lower clad layer 1 on the substrate 0 or formed beneath the substrate 0 to form an electrode contact. Above the layer 12 is the upper clad layer 13 which is made also by InP-based material with a thickness of about 2300 nm. The upper clad layer 13 is doped with p-type impurity at a level of 1.0×1018 cm−3. Optionally, an upper electric contact layer 14 based on InGaAs is formed on the upper clad layer 13. Optionally, the upper electric contact layer 14 is designed with a thickness of about 300 nm and with p-type doping level >1.0×1019 cm−3.
In another aspect of the present disclosure, the gain medium described hereabove is configured to form a reflective semiconductor optical amplifier (RSOA) with an AR front facet and a HR back facet or a semiconductor optical amplifier with AR coating on both front facet and back facet for a tunable laser application.
Optionally, the first gain medium 110 of RSOA is a flip chip bonded on the silicon photonics substrate 1000. The chip includes an active region 115 capped by optical confinement heterostructure in a clad region configured as a waveguide throughout an optical cavity length L from a back facet 101 to a front facet 102, within which a stimulated emission or laser light is generated. The front facet 102 is characterized by a first reflectance Rf and the back facet 101 is characterized by a second reflectance Rb. Optionally, the first reflectance Rf is very low in value, e.g., 0.005%, provided by an anti-reflective coating on the front facet 102. Optionally, the second reflectance Rb is very high in value, e.g., >90%, provided by a high-reflective coating on the back facet 101. Optionally, the active region 115 in waveguide form is configured to be in a curved shape with a non-perpendicular angle α relative to the front facet 102 (as seen in
Referring to
In an embodiment of RSOA/SOA for tunable laser in high-power operation, the first gain medium 110 of RSOA is configured as a laser-diode chip to yield a high saturation power Psat at elevated temperature by design. The high saturation power means that the RSOA is configured to produce a high stable laser power. Yet, the higher saturation power can be achieved by using a low driving current with a short (˜1 mm) cavity to keep the gain high enough in order for it to operate at elevated temperature of ˜50° C. with a saturation power higher than 15 dBm. In the embodiment, the second gain medium 160 of SOA can be also configured in a chip to yield a high saturation output power Psat>15 dBm by design, meaning to have a high amplified maximum output power at elevated temperature of about 50° C.
In the embodiment, a modulated n-doped active layer stack is provided with a narrow (5+/−2 nm) well multi-quantum-well structure in the gain medium for making chips for RSOA or SOA with smaller confinement factor and reduced carrier lifetime without increasing differential gain to achieve higher saturation power [see expression (1)]. The barrier GaInAsP material in the active layer also is tuned to produce photoluminescence at 1250 nm˜1350 nm to allow carrier distribution more uniformly in the active layer and have shorter carrier lifetime. At the same time, the gain parameter of the gain medium containing the modulated n-doped active layer stack is kept high enough to have the RSOA or SOA device to operate at lower injected current with a small (˜1 mm) cavity length. The optimized active layer confinement factor (Γ/d) is limited in 1.27+/−0.15 (1/μm) so that the device can be operated at elevated temperature of 50° C. or higher.
While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.
