SILICON-PHOTONICS-BASED SEMICONDUCTOR OPTICAL AMPLIFIER WITH N-DOPED ACTIVE LAYER

Abstract

A semiconductor optical amplifier for high-power operation includes a gain medium having a multilayer structure sequentially laid with a P-layer, an active layer, a N-layer from an upper portion to a lower portion in cross-section thereof. The gain medium is extendedly laid with a length L from a front facet to a back facet. The active layer includes multiple well layers formed by undoped semiconductor material and multiple barrier layers formed by n-doped semiconductor materials. Each well layer is sandwiched by a pair of barrier layers. The front facet is characterized by a first reflectance Rf and the back facet is characterized by a second reflectance Rb. The gain medium has a mirror loss α.sub.m about 40-200 cm.sup.−1 given by: α.sub.m=(½L)ln{1/(Rf×Rb)}.

Claims

1-20. (canceled)

21. A semiconductor optical amplifier for high-power operation of a tunable laser, the semiconductor optical amplifier comprising: a gain medium comprising a multilayer structure with a P-layer and an N-layer; a front facet having a first reflectance; a back facet having a second reflectance, the first reflectance and the second reflectance configured such that the gain medium at least one of generates, reflects, and amplifies laser light; and an N-doped active layer having undoped well layers alternating with N-doped barrier layers formed between the P-layer and the N-layer.

22. The semiconductor optical amplifier of claim 21, wherein: the gain medium has a length L from the front facet to the back facet; and the gain medium has a mirror loss m between 40 and 200 cm-1 in accordance with m=(½)ln{1/(Rf×Rb)}, where Rf is the first reflectance and Rb is the second reflectance.

23. The semiconductor optical amplifier of claim 21, wherein the N-doped active layer is configured to provide saturation power of at least 15 dBm with an operation temperature up to 50° C. for the gain medium.

24. The semiconductor optical amplifier of claim 21, wherein the N-doped barrier layers have a width between 5 and 15 nm and a bandgap between 0.9 and 1.1 eV.

25. The semiconductor optical amplifier of claim 24, wherein each of the N-doped barrier layers comprises a modulation doping area with an N-type dopant concentration range of 1.0×10.sup.18 cm.sup.−3 to 3.0×10.sup.18 cm.sup.−3 in a width range of 7 nm to 10 nm based on a barrier layer width of 10 nm.

26. The semiconductor optical amplifier of claim 21, wherein the undoped well layers have a width between 4 and 8 nm and a bandgap of 0.8 eV.

27. The semiconductor optical amplifier of claim 21, wherein the gain medium has a mirror loss greater than 45-100 cm.sup.−1 for a length of 1 mm, the front facet is provided with an anti-reflective coating, the first reflectance is less than or equal to 0.005%, the back facet is provided with a reflective coating, and the second reflectance is greater than 90%.

28. The semiconductor optical amplifier of claim 27, wherein the N-doped active layer is configured as a linear waveguide through an optical cavity of the gain medium, and wherein the linear waveguide forms a non-perpendicular angle relative to the front facet and a substantially perpendicular angle relative to the back facet.

29. The semiconductor optical amplifier of claim 21, wherein the gain medium has a mirror loss greater than 90-200 cm.sup.−1 for a length of 1 mm, the front facet is provided with an anti-reflective coating, the first reflectance is less than or equal to 0.005%, the back facet is provided with an anti-reflective coating, and the second reflectance is less than or equal to 0.005% to form a symmetric semiconductor optical amplifier.

30. The semiconductor optical amplifier of claim 29, wherein the gain medium is configured as an optical cavity for amplifying the laser light in the N-doped active layer, and wherein the laser light is input through the back facet once and emitted through the front facet.

31. The semiconductor optical amplifier of claim 30, wherein the N-doped active layer is configured as a linear waveguide through an optical cavity of the gain medium, and wherein the linear waveguide forms a non-perpendicular angle relative to the front facet and a non-perpendicular angle relative to the back facet.

32. The semiconductor optical amplifier of claim 21, wherein the P-layer comprises a P-type confinement layer attached to an upper portion of the N-doped active layer and a P-type clad layer attached to a side of the P-type confinement layer more distal to the N-doped active layer than the P-type confinement layer.

33. The semiconductor optical amplifier of claim 32, wherein the P-type confinement layer comprises (i) a first confinement sublayer having a thickness between 5 and 15 nm with a bandgap between 0.9 and 1.1 eV attached to a barrier layer with a bandgap between 0.9 and 1.1 eV next to the upper portion of the active layer and (ii) a second confinement sublayer having a thickness between 15 and 25 nm and a bandgap between 1.1 and 1.2 eV attached to a side of the first confinement sublayer more distal to the N-doped active layer than the first confinement sublayer.

34. The semiconductor optical amplifier of claim 21, wherein the N-layer comprises an N-type confinement layer attached to a lower portion of the N-doped active layer and an N-type clad layer attached to a side of the N-type confinement layer more distal to the N-doped active layer than the N-type confinement layer.

35. The semiconductor optical amplifier of claim 34, wherein the N-type confinement layer comprises (i) a first confinement sublayer having a thickness between 5 and 15 nm with a bandgap between 0.9 and 1.1 eV attached to a barrier layer with a bandgap between 0.9 and 1.1 eV next to the lower portion of the N-doped active layer and (ii) a second confinement sublayer having a thickness between 15 and 80 nm and a bandgap between 1.1 and 1.2 eV attached to a side of the first confinement sublayer more distal to the N-doped active layer than the first confinement sublayer.

36. A tunable laser, comprising: a reflective semiconductor optical amplifier (RSOA) configured to reflect and emit laser light; and a semiconductor optical amplifier (SOA) configured to receive and amplify the laser light emitted from the RSOA, at least one of the RSOA and the SOA comprising a gain medium comprising a multilayer structure with a P-layer and an N-layer, a front facet having a first reflectance, a back facet having a second reflectance, the first reflectance and the second reflectance configured such that the gain medium is configured for transmission of laser light, and an N-doped active layer formed between the P-layer and the N-layer.

37. The tunable laser of claim 36, the N-doped active layer having undoped well layers alternating with N-doped barrier layers.

38. The tunable laser of claim 36, further comprising: a wavelength tuner configured to receive, via a first waveguide, the laser light emitted from the RSOA and tune a wavelength of the laser light.

39. The tunable laser of claim 38, further comprising: a wavelength locker configured to lock the wavelength of the laser light as tuned by the wavelength tuner and provide, via a second waveguide, the laser light to the SOA.

40. The tunable laser of claim 39, further comprising a silicon photonics substrate, the RSOA, the SOA, the wavelength tuner, and the wavelength locker being formed on the silicon photonics substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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.

[0028] FIG. 1 is a schematic diagram of a tunable laser based on a reflective semiconductor optical amplifier (RSOA) combined with a semiconductor optical amplifier (SOA) according to an embodiment of the present invention.

[0029] FIG. 2 is a schematic diagram of a RSOA with a gain medium in both top and side views according to an embodiment of the present invention.

[0030] FIG. 3 is a schematic diagram of a SOA with a gain medium in both top and side views according to an embodiment of the present invention.

[0031] FIG. 4 is a simplified bandgap diagram of a modulated n-doped active layer in the gain medium of RSOA/SOA according to an embodiment of the present invention.

[0032] FIG. 5 is a graph showing the relation between the reciprocal of internal efficiency and cavity length of a gain medium with/without n-doped active layer in a nominal optical cavity according to an embodiment of the present invention.

[0033] FIG. 6 is a graph showing the relation between mirror loss and transparent current density of a gain medium with/without n-doped active layer in a nominal cavity according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention relates to optical telecommunication techniques. One aspect of the present invention provides a reflective semiconductor optical amplifier (RSOA) and/or a semiconductor optical amplifier (SOA) with high-power gain medium for elevated-temperature operation. More particularly, the present invention provides RSOA/SOA with n doped active layer for making a wide-band wavelength-tunable laser in high-speed data communication application, though other applications are possible.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Please note, if used, the labels inner, outer, left, right, front, back, top, bottom, end, forward, reverse, clockwise and counter clockwise 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.

[0040] In an aspect, the present disclosure provides a reflective semiconductor optical amplifier (RSOA) and/or a semiconductor optical amplifier (SOA) with high-power operability at elevated temperature. In an example, the RSOA/SOA with high-power operability is applied to form a wide-band wavelength tunable laser in silicon photonics platform. FIG. 1 is a schematic diagram of a tunable laser based on a reflective semiconductor optical amplifier (RSOA) combined with a semiconductor optical amplifier (SOA) according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, the tunable laser module 100 includes a RSOA with a first gain medium 110 on a silicon photonics substrate 10 to generate a stimulated emission, a wavelength tuner 170 formed in the silicon photonics substrate 10 and coupled to the RSOA to receive a reflected light in a wide band tuned based on the stimulated emission, a wavelength locker 180 formed in the silicon photonics substrate 10 and coupled to the wavelength tuner 170 to lock wavelengths of the light, and a SOA with a second gain medium 160 in an output path coupled to the wavelength locker to pass the light with a single amplification in power.

[0041] Optionally, the first gain medium 110 of RSOA is a laser chip flip-bonded on the silicon photonics substrate 10. The laser chip includes an active region 115 capped in a clad region configured in a waveguide form along its length throughout an optical cavity having a 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 relative to the front facet 102 to reduce direct back reflection of the light thereby but with a substantially perpendicular angle relative to back facet 101 to maximize the reflection. The laser light is reflected by the back facet 101 and emitted out through the front facet 102 into a first waveguide 191 via a coupler 130. Through the first waveguide 191 the laser light is delivered to the wavelength tuner 170. In an embodiment for the tunable laser in high-power operation, the first gain medium 110 of RSOA is configured in a laser-diode chip to yield a high saturation power P.sub.sat 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 shorter 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.

[0042] Referring to FIG. 1, the laser light, after passing through the wavelength tuner 170 and wavelength locker 180, with its wavelength locked in a specific value in a wide-band spectrum (such as C-band), is inputted via an input coupler 150 in the SOA where it is further amplified in power before being outputted via an output coupler 140. Optionally, the second gain medium 160 of SOA is another laser chip flip-bonded on the silicon photonics substrate 10. The gain medium 160 includes a gain region 165 capped in a clad region configured in a waveguide form extended in a length L through an amplifying cavity from a back facet 142 coupled to the input coupler 150 and a front facet 141 coupled to the output coupler 140. In the embodiment, the front facet 141 is characterized by a first reflectance Rf and the back facet 142 is characterized by a second reflectance Rb. Optionally, both the back facet 142 and the front facet 141 are coated by anti-reflective coating to give a very low value, e.g., 0.005%, of the first reflectance Rf and the second reflectance Rb for forming a symmetric semiconductor optical amplifier to allow the light to pass through once and be amplified in power. Optionally, the active region 165 in waveguide form is configured to be in a curved shape with a non-perpendicular angle relative to either the back facet 142 or the front facet 141 to reduce direct back reflection of the light thereby. This light received via the back facet 142 from the input coupler 150 is simply passed through the length L of the cavity and amplified therein before outputting via the front facet 141. In the embodiment, for making the tunable laser in high-power operation, the second gain medium 160 of SOA can be also configured in a chip to yield a high saturation output power P.sub.sat>15 dBm by design, meaning to have a high amplified maximum output power at elevated temperature of about 50° C.

[0043] In theory, the saturation output power P.sub.sat of a gain medium or a laser-diode chip can be expressed as following:

[00001]$\begin{array}{cc}{P}_{\mathrm{sat}}^c=\left(\frac{dw}{\Gamma }\right)\left(h\nu \frac{1}{a}\frac{1}{{\tau }_s}\right).& \left(1\right)\end{array}$

In the expression (1), d is a thickness of an active layer of the gain medium, w is a width of the active layer, Γ is optical confinement factor, a is a differential gain, and τ.sub.s is carrier lifetime. Conventional methods have been used to improve P.sub.sat by 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 wider emitter is the gain is also reduced. In order to obtain higher gain for the device, longer cavity length and higher operating current are normally required to achieve high power. But high operating current mostly is not recommended for device in elevated temperature operation.

[0044] The present disclosure provides an improved SOA (or RSOA) with high saturation power P.sub.sat for elevated temperature operation with reasonably high gain but reduced carrier lifetime τ.sub.s and differential gain a by providing a gain medium with n-doped active layer in the gain chip for the SOA or RSOA. FIG. 2 is a schematic diagram of a RSOA with a gain medium in both top and side views according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in part A), a top view of the gain medium 110 of the RSOA reveals a linear active region 115 laid in an optical cavity between a front facet with anti-reflective (AR) coating and a back facet with high-reflective (HR) coating. In part B) of FIG. 2, a cross-section side view of the main medium 110 reveals a multi-layer structure with the active region 115 is shown as an active layer in a central portion of the multi-layer structure sandwiched between a P-layer at upper portion and a N-layer at lower portion which forms a semiconductor diode with a p-type electrode (p-metal) on the P-layer at the upper portion and a n-type electrode (n-metal) beneath the N-layer at the lower portion. Note, the terms of upper or lower merely are referred to the figure illustration and do not limit actual device in just one setting in orientation. Optionally, the P-layer includes a p-type optical confinement layer and a p-type clad layer. Optionally, there is a contact layer between the P-layer and the p-metal. Optionally, the N-layer includes a n-type optical confinement layer and a n-type clad layer. Optionally, the N-layer is formed on a substrate and the n-metal is formed at a back side of the substrate. Optionally, the semiconductor diode is configured as a laser diode with the optical cavity between the front facet and the back facet for RSOA.

[0045] When the laser diode is driven by a current source across the p-type electrode and the n-type electrode, a carrier-stimulated emission in the active layer 115 is generated between the front facet and the back facet before lasing out the front facet. Optionally, the active layer 115 is provided as a n-doped layer to increase carrier density and reduces the carrier lifetime significantly. Optionally, the active layer 115 itself is also a multilayer structure configured to form a multi-quantum-well structure. Depending on working wavelength spectrum, different semiconductor materials including one or more compound semiconductors or a combination of InAsP, GaInNAs, GaInAsP, GaInAs, and AlGaInAs may be employed for forming the multi-layers in the active layer with multi-quantum-well structure. Optionally, the n-type dopant for the active layer 115 can be Si or Se. Optionally, the doping concentration can be in a range of 1.0×10.sup.18 cm.sup.−3 to 3.0×10.sup.18 cm.sup.−3.

[0046] Although P-doped active layer may be employed to reduce the carrier lifetime but it also causes high differential gain. Thus, p-doped active layer is not a good solution for improving Psat of RSOA by reducing differential gain.

[0047] FIG. 3 is a schematic diagram of a SOA with a gain medium in both top and side views according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. In an embodiment, the gain medium for SOA can be substantially same as the gain medium for RSOA described above in FIG. 2. Similarly, in Part A), a top view of the gain medium 160 of the SOA reveals a linear active region 165 laid in a cavity between a back facet and a front facet both with anti-reflective coating. In part B) of FIG. 3, a cross-section side view of the main medium 160 reveals a multi-layer structure with the active region 165 is shown as an active layer in a central portion of the multi-layer structure sandwiched between a P-layer at upper portion and a N-layer at lower portion which forms a semiconductor diode with a p-type electrode (p-metal) on the P-layer at the upper portion and a n-type electrode (n-metal) beneath the N-layer at the lower portion. Note, the terms of upper or lower merely are referred to the figure illustration and do not limit actual device in just one setting in orientation. Optionally, the P-layer includes a p-type optical confinement layer and a p-type clad layer. Optionally, there is a contact layer between the P-layer and the p-metal. Optionally, the N-layer includes a n-type optical confinement layer and a n-type clad layer. Optionally, the N-layer is formed on a substrate and the n-metal is formed at a back side of the substrate. Optionally, the semiconductor diode is configured as a laser diode with the optical cavity between the back facet and the front facet for SOA.

[0048] Optionally, the active layer 165 itself is a multilayer structure configured to form a multi-quantum-well structure or a hetero-junction structure. Depending on working wavelength spectrum, different semiconductor materials including one or more compound semiconductors or a combination of InAsP, GaInNAs, GaInAsP, GaInAs, and AlGaInAs may be employed for forming the multi-layers in the active layer with multi-quantum-well structure. Optionally, the n-type dopant for the active layer 165 can be Si or Se. Optionally, the doping concentration can be in a range of 1.0×10.sup.18 cm.sup.−3 to 3.0×10.sup.18 cm.sup.−3. Optionally, the active layer 165 is substantially the same as the active layer 115 in a multi-layer structure and in composition of respective layer within the multi-layer structure. Optionally, both the active layer 165 and the active layer 115 are respective parts of two gain chips diced from a same die in a same wafer fabricated under a same semiconductor manufacture process.

[0049] In an embodiment, the active layer 115 of RSOA or the active layer 165 of SOA is a modulated n-doped multi-quantum-well structure. The multi-quantum-well structure of the active layer provides a hetero-junction structure including multiple well layers separated by respective barrier layers to achieve high-power light wave oscillation to yield high saturation lasing power for RSOA or output power for SOA comparing to a bulk active layer. FIG. 4 is a simplified bandgap diagram of a modulated n-doped active layer in the gain medium of RSOA/SOA according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, the active layer provides a physical stack of multilayers including multiple well layers 11 separated by respective barrier layers 12. The active layer stack is located between an upper optical confinement stack and a lower optical confinement stack. The upper optical confinement stack is further attached with an upper clad layer and the lower optical confinement stack is further attached with a lower clad layer. Optionally, the upper optical confinement stack plus the upper clad is provided as a P-layer with p-type doping characteristics as shown in FIG. 2 and FIG. 3. Optionally, the lower optical confinement stack plus the lower clad layer is provided as a N-layer with n-type doping characteristics as shown in FIG. 2 and FIG. 3. Optionally, the clad layer is selected from InP-based semiconductor material with different doping characteristics. Referring to an example shown in FIG. 4, the upper optical confinement stack includes a barrier layer 10a, two confinement layers 21a and 22a with different thicknesses and different bandgaps. The lower optical confinement stack in the example also includes a barrier layer 10b and two confinement layers 21b and 22b with different thicknesses and different bandgaps.

[0050] In an aspect, the present invention provides a gain medium with modulated n-doped active layer stack for generating high saturation power in a semiconductor amplifier such as RSOA and SOA. The specific material choices for the well layer and barrier layer, and parameters like thickness and doping characteristics are varied to yield different bandgaps and oscillation characteristics for the emitted light within the active layer stack in order to provide a specific light spectrum range that is fit, for example, for the application of the RSOA or SOA in forming a C-band or O-band wavelength tunable laser.

[0051] Referring to an embodiment shown in FIG. 4, the active layer stack is composed by three well layers 11 with a thickness t1 and a low energy bandgap of about 0.8 eV which is in a corresponding amplified wavelength range. For example, the SOA or RSOA is for amplifying wavelength in C-band. Each pair of well layers is separated by a barrier layer 12 with a thickness t2 and an energy bandgap of about 0.9-1.1 eV. Optionally, there can be two well layers. Optionally, there can be four well layers. Optionally, the well thickness is about 4-8 nm. Optionally, the barrier thickness is about 5-15 nm. Optionally, the well layer 11 is selected from an InGaP-based composite semiconductor material. Optionally, the barrier layer 12 is selected from a GaInAs-based composite semiconductor material. Optionally, the active layer stack is modulated n-doped such that the well layer 11 is undoped and the barrier layer 12 is doped by n-type impurity. Optionally, the n-type doping area 15 in the barrier layer 12 includes a width no greater than that of the barrier layer thickness t2. Optionally, the n-type doping area 15 contains Si (or Se) with a concentration in a range of 1.0×10.sup.18 cm.sup.−3 to 3.0×10.sup.18 cm.sup.−3 and a width in a range of 7 nm to 10 nm if the barrier layer is 10 nm.

[0052] Referring to FIG. 4, optionally, a barrier layer 10a (10b) is provided between the confinement layer stack and the active layer stack. Optionally, this barrier layer 10a (10b) also contains a n-doping area width, a doping concentration, and an energy bandgap all are comparable to or substantially the same as those characteristics in the barrier layer 12. Optionally, the upper confinement stack includes a first p-type confinement sublayer 21a of a thickness of about 5-15 nm same as that of the barrier layer 10a, and a bandgap of about 0.9-1.1 eV slightly higher than that of the barrier layer 10a (same as the active layer stack at about 0.9-1.1 eV) to contain the charged carrier within the active layer stack. the upper confinement stack includes a second p-type confinement sublayer 22a of a thickness of about 15-25 nm and a bandgap of about 1.1-1.2 eV higher than that of the first p-type confinement sublayer 21a to more effectively contain the charged carriers within the active layer stack for generating stimulated emission. Similarly, the lower confinement stack includes a first n-type confinement sublayer 21b of a thickness of about 5-15 nm same as that of the barrier layer 10b, and a bandgap of about 0.9-1.1 eV slightly higher than that of the barrier layer 10b (same as the active layer stack at about 1 eV). Further, the lower confinement stack also includes a second p-type confinement sublayer 22b of a thickness of about 38 nm and a bandgap of about 1.1-1.2 eV. Referring to FIG. 4, the upper clad has an even higher bandgap of about 1.3-1.4 eV. Optionally, the upper clad is InP doped with p-type dopant. The lower clad also has a bandgap of about 1.3-1.4 eV and made by InP doped with n-type dopant.

[0053] In the embodiment to implement the modulated n-doped active layer stack in the gain medium for RSOA or SOA to reduce carrier lifetime without increasing differential gain for achieving higher saturation power [see expression (1)]. At the same time, the gain 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 smaller cavity length so that the device can be operated at elevated temperature of 50° C. or higher.

[0054] FIG. 5 is a graph showing the relation between the reciprocal of internal efficiency and cavity length of a gain medium with/without n-doped active layer in a nominal optical cavity according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, the reciprocal of external differential quantum efficiency and is plotted against cavity length of a gain medium for either a conventional un-doped active layer stack (Δ) or a modulated n-doped active layer stack (O). These data are collected from cavity length varied from 0.05 cm to 0.2 cm. The solid line and the dashed line are respective linear fit lines for the two sets of data. The internal loss is calculated from the gradient of each straight line (solid or dashed). Apparently, the modulated n-doped active layer yields a lower loss (see Table 1 below) compared to the case with the un-doped active layer. In consequence, the internal losses of the gain medium without and with the n-doped active layer stack were estimated at 8.96 cm.sup.−1 and 6.77 cm.sup.−1, respectively. This indicates that the internal efficiency can be improved for achieving higher output power by modulation doping with the n-type dopant in the active layer stack of the gain medium.

TABLE-US-00001 TABLE I T Loss η.sub.i G.sub.0 J.sub.tr Wafer (° C.) cm.sup.−1 (%) cm.sup.−1 mA/cm Std 25 8.96 0.89 14.42 41.73 n-doped 25 6.77 0.84 11.12 27.21

[0055] FIG. 6 is a graph showing the relation between mirror loss and transparent current density of a gain medium in a nominal optical cavity according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. In additional to the internal loss, the gain medium, as it is applied in a nominal optical cavity, also has a mirror loss depending on reflectance values of two facets thereof. As shown, the mirror loss of the gain medium in a nominal optical cavity (with about 27% reflectivity) containing an undoped active layer stack (Δ) or a modulated n-doped active layer stack (O) is plotted against the logarithm of a threshold current density. The solid line and the dashed line are respective linear fit lines for the two sets of data. The threshold gain G.sub.0 of the gain medium is calculated from the gradient of each straight line (solid or dashed), showing that G.sub.0 is slightly lowered from 14.42 (cm.sup.−1) to 11.12 (cm.sup.−1) (see Table I above). A lower threshold gain value of the gain medium leads to a lower differential gain. Based on expression (1), lower differential gain results in higher saturation power of the gain medium can produce, which translates to a higher lasing power as the gain medium is in a RSOA device and a higher maximum output power as the gain medium is in a SOA device, and combined, a higher output power for the tunable laser based on the RSOA and SOA. Based on the graph of FIG. 6, a transparent current density J.sub.tr can be obtained as the current density at cavity length of infinity, or the mirror loss equals zero. The results of Table I indicate that the transparent current density J.sub.tr is lowered a lot at about 27 mA/cm.sup.2 for the gain medium with n-doped active layer stack compared to about 42 mA/cm.sup.2 for the gain medium with undoped active layer stack. Lower transparent current density means lower carrier lifetime, which yields higher saturation power as shown in the expression (1).

[0056] Yet, for a slightly lower G.sub.0 around 11 cm.sup.−1, the chip gain for either a RSOA or SOA device with the gain medium as described in a typical 1 mm cavity length can be still at least 20 dB (at room temperature) with a nominal operating current of about 300 mA. This ensures a high saturation power of at least 15 dBm when the ROSA or SOA device is operated at elevated temperature of about 50° C. There is no need to extend the cavity length or hike the injection current in order to achieve high enough gain especially at the elevated temperature environment. Additionally, the gain medium with the modulated n-doped active layer stack being applied in the RSOA device or SOA device has much higher mirror loss than typical laser device due to the facet reflectance setting of the gain medium cavity given a similar cavity length. For a RSOA device with a gain medium in a 1 mm long cavity having one HR facet with >90% in reflectance and one AR facet with about 0.005% in reflectance, a mirror loss is about 49.5 cm.sup.−1. For a SOA device with a gain medium in a 1 mm long cavity having both AR facets, the mirror loss is about 90 cm.sup.−1 or greater.

[0057] 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.