Adjustable VCSEL with Distortion-Compensated Semiconductor DBR

Abstract

Tunable VCSELs (TVCSELs) employ expanded material systems with expanded mechanical and optical design space for semiconductor DBR mirrors on GaAs substrates. One is the InGaAs / AlGaAsP material system, which adds indium (In) for a higher refractive index and contrast with the higher DBR layer's refractive index, reducing the bandgap of the InGaAs H layer. Adding phosphorus (P) provides independent control of the bandgap and strain of the AlGaAsP low-index L layer. The tensile strain in the AlGaAsP L layer compensates for the compressive strain in the InGaAs H layer, lowering the cumulative strain of the multilayer DBR structure. Another option is the InGaAsN(Sb) / AlGaAsP material system, where both types of layers can be lattice-matched to GaAs.

Description

Background Art

[0001] (Related Application) This application claims the benefit of U.S. Provisional Application No. 63 / 091,412, filed Oct. 14, 2020, under 35 U.S.C. § 119(e), the entire disclosure of which is hereby incorporated by reference in its entirety.

[0002] (Background of the Invention) Adjustable vertical-cavity surface-emitting lasers (VCSELs) with micro-electromechanical (MEMS) movable mirrors are finding applications in telecommunications. Matsui, Y., Vakhshoori, D., Wang, P., Chen, P., Lu, C.-C., Jiang, M., Knopp, K., Burroughs, S., and Tayebati, P. "Complete polarization mode control of long-wavelength tunable vertical-cavity surface-emitting lasers over 65-nm tuning, up to 14-mw output power" IEEE J. Knopp, K., Vakhshoori, D., Wang, P., Azimi, M., Jiang, M., Chen, P., Matsui, in Quantum Electronics 39, 1037-1048 (2003) and [2001 Digest of LEOS Summer Topical Meetings] TuA1.3, 31-32 (2001). Y., McCallion, K., Baliga, See A., Sakhitab, F., Letsch, M., Johnson, B., Huang, R., Jean, A., DeLargy, B., Pinzone, C., Fan, F., Liu, J., Lu, C., Zhou, J., Zhu, H., Gurjar, R., Tayebati, P., MacDaniel, D., Baorui, R., Waterson, R., and Van der Rhodes, G., "High power MEMs-tunable vertical-cavity surface-emitting lasers." They have also found applications in optical coherence tomography (OCT).For example, see B. Johnson, W. Atia, S. Woo, C. Melendez, M. Kuznetsov, T. Ford, N. Kemp, J. Jabbour, E. Mallon, and P. Whitney, "Tunable 1060nm VCSEL with pump, and SOA for OCT and LiDAR," SPIE Photonics West BiOS Proceedings, 10867 (2019), and Flanders, DC, Kuznetsov, ME, Atia, WA, and Johnson, BC, "OCT system with bonded MEMS tunable mirror VCSEL swept source." See also U.S. Patent Application Publication US2016 / 0329682 A1 (Patent Document 1) (November 10, 2016).

[0003] To date, most sweep source OCT applications have occurred within the 1,310 nanometer (nm) and 1,060 nm bands. Furthermore, regarding OCT, since depth resolution in OCT is inversely proportional to the tuning range, achieving the widest tuning range in optical frequency is extremely important. See J. Fujimoto and W. Drexler, "Introduction to Optical Coherence Tomography," in [Optical Coherence Tomography: Technology and Applications, First Edition], Drexler, W. and Fujimoto, JG, eds., ch. 1, 1-40, Springer (2008).

[0004] Two of the most important factors affecting the tuning range of a VCSEL are (1) the laser resonator mirror bandwidth and (2) the length of the laser resonator. Much effort has been spent on broadband mirror technology, including high refractive index contrast dielectric mirrors and GaAs / AlAs oxide mirrors. See Jayaraman, V., Jiang, J., Potsaid, B., Robertson, M., Heim, PJS, Burgner, C., John, D., Cole, GD, Grulkowski, I., Fujimoto, JG, Davis, AM, and Cable, AE, "VCSEL swept light sources" in [Optical Coherence Tomography: Technology and Applications, Second Edition], Drexler, W. and Fujimoto, JG, eds., ch. 22, 659-686, Springer (2015). The GaAs / AlAs material system inherently possesses a fairly high achievable exponential contrast, i.e., a high-refractive-to-low-refractive-index ratio of the selected material, enabling epitaxially grown distributed Bragg reflector (DBR) mirrors to support an adjustment range of approximately 100 nm. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] U.S. Patent Application Publication No. 2016 / 0329682 [Overview of the project] [Means for solving the problem]

[0006] (Summary of the invention) Nevertheless, adjustment ranges exceeding 100 nm require improved epitaxially grown mirrors. Generally, there is a need for semiconductor DBRs with higher reflectivity, wider spectral width, and fewer layers.

[0007] Improvements can be achieved by a DBR mirror layer with higher refractive index contrast. Semiconductor materials with high bandgap energy have a low refractive index, while materials with a low bandgap have a high refractive index. One way to obtain higher DBR refractive index contrast is to decrease the bandgap of the DBR high refractive index H layer and increase the bandgap of the DBR low refractive index L layer. This can be achieved by extending the material space for semiconductor DBR mirrors on a GaAs substrate beyond AlGaAs.

[0008] Furthermore, both the bandgap and strain of the DBR mirror layer must be considered and controlled. This approach can include complete control over the strain of the DBR structure by either balancing the compressive and tensile strains of the alternating mirror layers or by using mirror layer materials that are lattice-matched to the substrate.

[0009] Specific examples of DBR mirrors on GaAs substrates include (strain-balanced) InGaAs / GaAlAsP or (lattice-matched) InGaAsN / GaAlAsP.

[0010] From a general perspective, this approach can be applied not only to InP and GaAs such as GaSb, but also to other semiconductor systems.

[0011] Furthermore, from a general perspective, this approach can also be applied to optically and electrically excited VCSELs, tunable and non-tunable devices, and passive and active devices.

[0012] In summary, adjustable VCSELs (TVCSELs) employing extended material systems for semiconductor DBR mirrors on GaAs substrates can achieve improved performance. Standard systems based on GaAs / AlGaAs generally have limited H / L DBR refractive index contrast and built-in cumulative distortion across multiple layers, as well as limited DBR mirror spectral reflection bandwidth and TVCSEL tuning range.

[0013] One option for GaAs substrates is the InGaAs / AlGaAsP material system. This involves adding indium (In) to the GaAs layer for a higher refractive index and higher DBR refractive index contrast, reducing the band gap of the InGaAs H layer. Adding phosphorus (P) to the AlGaAs layer controls the strain of the AlGaAsP L layer. The tensile strain of the AlGaAsP L layer compensates for the compressive strain of the InGaAs H layer, reducing the cumulative strain of the multilayer DBR structure.

[0014] This provides an increase in the DBR mirror spectral reflection bandwidth and TVCSEL tuning range. For example, in one specific design, the DBR mirror bandwidth was increased from 101.5 nm to 104.5 nm (laser tuning range 103.7 nm) with respect to a standard DBR. Here, the mirror bandwidth is measured at a reflectivity level of 99.5%.

[0015] Another option for GaAs is the InGaAsN / AlGaAsP material system. It uses indium (In) and nitrogen (N) to achieve independent control of the strain and bandgap of the dilute InGaAsN H layer, with a lower bandgap and higher refractive index than the starting GaAs. This allows for the creation of a lattice-matched H layer on GaAs. Often, some amount of antimony (Sb) is also added to the dilute nitride InGaAsN. Such a pentagonal alloy InGaAsNSb allows for the epitaxial growth of lower-defect and higher-quality heterostructures on GaAs, with a bandgap of up to 0.8 eV. The use of phosphorus (P) in AlGaAsP allows for the creation of a lattice-matched L layer on GaAs. This approach resulted in increased DBR refractive index contrast, DBR mirror spectral reflection bandwidth, and TVCSEL tuning range. For example, the DBR mirror bandwidth increased from 101.5 nm to 109.6 nm with respect to a standard DBR (laser tuning range 108.9 nm).

[0016] Another alternative to GaAs is AlGaAs / AlGaAsP, in which case the high-aluminum-containing, low-refractive-index AlGaAsP layers incorporate phosphorus P to reduce their compressive strain or to further lattice-match these layers grown on GaAs.

[0017] For the purposes of the present invention, it will generally be understood that materials comprising ternary elements such as (In)GaAs, quaternary elements such as GaAlAs(P), and pentary elements such as InGaAsN(Sb) include compositions in which one or more of the constituent chemical elements are present at zero fractional concentration.

[0018] In general, according to one aspect, the present invention is characterized by a laser including a semiconductor distributed Bragg reflection mirror, the mirror comprising a layer of InGaAs and a layer of AlAsP or GaAlAsP.

[0019] Preferably, the layer has alternating compressive / tensile strain on the semiconductor substrate. These alternating strain layers can create a very low cumulative composite strain on the semiconductor substrate.

[0020] In some embodiments, in addition, quantum well layers are present to provide gain. These quantum well layers can be interspersed with (In)GaAs layers and AlAsP or GaAlAsP layers.

[0021] The laser is often a vertical cavity surface emitting laser such as an adjustable vertical cavity surface emitting laser, and potentially involves a MEMS membrane including a reflective coating that defines the ends of the laser resonator.

[0022] Generally, according to another aspect, the present invention features a laser including a semiconductor distributed Bragg reflector mirror, the mirror comprising a layer of InGaAsN(Sb) and a layer of AlAsP or GaAlAsP.

[0023] The quantum well layers can be interspersed with InGaAsN(Sb) layers and AlAsP or GaAlAsP layers.

[0024] Generally, according to another aspect, the present invention features a semiconductor distributed Bragg reflector mirror, the mirror comprising a layer of InGaAs and a layer of AlAsP or GaAlAsP.

[0025] Generally, according to another aspect, the present invention features a semiconductor distributed Bragg reflector mirror, the mirror comprising a layer of InGaAsN(Sb) and a layer of AlAsP or GaAlAsP.

[0026] Generally, according to another aspect, the present invention features a laser including a semiconductor distributed Bragg reflector mirror, the mirror comprising a layer of InGaAs or InGaAsN(Sb) and a layer of AlAsP or GaAlAsP.

[0027] The above and other features of the present invention, including various novel details of the component configurations and combinations, as well as other advantages, are herein described more specifically with reference to the accompanying drawings and pointed out in the claims. It will be understood that the specific methods and devices embodying the present invention are shown by way of illustration and not as limitations of the present invention. The principles and features of the present invention may be employed in various and numerous embodiments without departing from the scope of the present invention. The present invention provides, for example, the following: (Item 1) A laser including a semiconductor distributed Bragg reflection mirror, wherein the mirror is InGaAs layer, A layer of AlAsP or GaAlAsP A laser equipped with a laser. (Item 2) The laser according to item 1, wherein the layer has alternating compressive / tensile strains on a semiconductor substrate. (Item 3) The alternating strain layers generate very low cumulative composite strain on the semiconductor substrate, as described in item 2 of the laser. (Item 4) The laser described in item 1, further comprising a quantum well layer for providing gain. (Item 5) The laser according to item 4, wherein the quantum well layer is scattered together with the (In)GaAs layer and the AlAsP or GaAlAsP layer. (Item 6) The laser is a vertical-cavity surface-emitting laser, as described in item 1. (Item 7) The laser is a tunable vertical cavity surface-emitting laser, as described in item 6. (Item 8) The laser according to item 7, wherein the laser includes a MEMS film having a reflective coating that defines the edges of the laser resonator. (Item 9) A laser including a semiconductor distributed Bragg reflection mirror, wherein the mirror is InGaAsN(Sb) layer, A layer of AlAsP or GaAlAsP A laser equipped with a laser. (Item 10) The laser according to item 9, wherein the layer has alternating compressive / tensile strains on a semiconductor substrate. [[ID=�5]] (Item 11) ] The alternating strain layers generate very low cumulative composite strain on the semiconductor substrate, as described in item 9 of the laser. (Item 12) The laser according to item 9, wherein the layer is substantially lattice-matched to the semiconductor substrate. (Item 13) The laser described in item 9, further comprising a quantum well layer for providing gain. (Item 14) The laser according to item 13, wherein the quantum well layer is scattered together with the InGaAsN(Sb) layer and the AlAsP or GaAlAsP layer. (Item 15) The laser is a vertical-cavity surface-emitting laser, as described in item 9. (Item 16) The laser is a tunable vertical cavity surface-emitting laser, as described in item 15. (Item 17) The laser according to item 16, wherein the laser includes a MEMS film having a reflective coating that defines the edges of the laser resonator. (Item 18) A semiconductor distributed Bragg reflective mirror, wherein the mirror is InGaAs layer, A layer of AlAsP or GaAlAsP A semiconductor distributed Bragg reflective mirror equipped with the following features. (Item 19) The aforementioned layer is a mirror according to item 18, having alternating compressive / tensile strains on a semiconductor substrate. (Item 20) The alternating strain layers generate very low cumulative composite strain on the semiconductor substrate, as described in item 19. (Item 21) A semiconductor distributed Bragg reflective mirror, wherein the mirror is InGaAsN(Sb) layer, A layer of AlAsP or GaAlAsP A semiconductor distributed Bragg reflective mirror equipped with the following features. (Item 22) The aforementioned layer is a mirror as described in item 21, which is substantially lattice-matched to the semiconductor substrate. (Item 23) The aforementioned layer is a mirror according to item 21, having alternating compressive / tensile strains on a semiconductor substrate. (Item 24) The alternating strain layers generate very low cumulative composite strain on the semiconductor substrate, as described in item 23. (Item 25) A laser including a semiconductor distributed Bragg reflection mirror, wherein the mirror is A layer of InGaAs or InGaAsN(Sb), A layer of AlAsP or GaAlAsP A laser equipped with a laser. [Brief explanation of the drawing]

[0028] In the accompanying drawings, reference letters refer to the same parts throughout the different drawings. The drawings are not necessarily to scale, and instead, the focus is on illustrating the principles of the invention.

[0029] [Figure 1] Figure 1 is a schematic cross-sectional view of a tunable vertical-cavity surface-emitting laser (TVCSEL).

[0030] [Figure 2] Figures 2A and 2B are plots of reflectance as a function of wavelength in nanometer units, showing the improved reflectance of a strain-equipped DBR design using AlAsP / InGaAs (AlAs(0.872)P(0.128) / In(0.05)Ga(0.95)As) compared to a reference standard DBR design using AlAs / GaAs.

[0031] [Figure 3] Figure 3 shows a design for a layered structure of a half-VCSEL employing combined gain and DBR mirrors using AlAsP / InGaAs mirrors.

[0032] [Figure 4] Figures 4A and 4B are plots of reflectance as a function of wavelength in nanometer units, showing the improved reflectance of the dilute nitride DBR design, AlAsP / InGaAsN, i.e., AlAs(0.9595)P(0.0405) / In(0.018)Ga(0.982)As(0.994)N(0.006), compared to a reference standard DBR design using AlAs / GaAs.

[0033] [Figure 5] Figure 5 shows a design for a layered structure of a half-VCSEL employing combined gain and DBR mirrors, using InGaAsN and (L)AlAsP-based mirrors. [Modes for carrying out the invention]

[0034] (Detailed description of preferred embodiments) Herein, the present invention will be described in more detail with reference to the accompanying drawings illustrating illustrative embodiments of the invention. However, the present invention can be embodied in many different forms and should not be construed as being limited to the embodiments described herein. Rather, these embodiments are provided so as to ensure that this disclosure is thorough and complete and to fully convey the scope of the invention to those skilled in the art.

[0035] Where used herein, the term “and / or” includes any combination of one or more of the enumerated items relating to it. Furthermore, singular forms and the articles “a,” “an,” and “the” are intended to include plural forms as well, unless otherwise explicitly stated. Furthermore, when used herein, the terms “includes,” “comprises,” “including,” and / or “comprising” specify the existence of a described feature, integer, step, action, element, and / or component, but do not exclude the existence or addition of one or more other features, integers, steps, actions, elements, components, and / or groups thereof. Furthermore, when an element containing a component or subsystem is referred to and / or indicated as being connected to or combined with another element, it will be understood that there may be elements that directly connect to or combine with, or intervene in, the other element.

[0036] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as they are generally understood by those skilled in the art to the extent of this invention. Furthermore, terms such as those defined in commonly used dictionaries should be interpreted as having the meaning consistent with their meanings in the context of the relevant art, and it will be understood that they should not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0037] Figure 1 is a schematic cross-sectional view of a tunable vertical cavity surface-emitting laser (TVCSEL) 100.

[0038] In principle, these devices often include a movable MEMS film 100 fabricated from silicon or a silicon compound. A high-reflectivity (HR) coating 112 is deposited on this film 110, and this coating forms a mirror at one end of the laser resonator. Often, the HR film coating 112 has a reflectivity of 99.0% to 99.9%. An anti-reflective (AR) coating 115 is typically deposited on the film 110 opposite the HR coating. In common embodiments, the movable film is electrostatically deflected. Thermally actuated film movement is also used for VCSEL laser tuning.

[0039] Importantly, the half-VCSEL device 120 typically includes a semiconductor gain region 122 containing a quantum well for amplifying light within the resonator. A semiconductor distributed Bragg reflector (DBR) 126 forms the other mirror of the laser resonator. Light within the VCSEL laser resonator propagates between the two mirrors in a direction perpendicular to the semiconductor surface. Typically, a semiconductor window 124 is formed across the gain region 122 for functions such as preventing excited carriers from reaching the semiconductor surface as light travels between the half-VCSEL 120 and the gap 114 separating the proximal end of the half-VCSEL 120 and the inner surface of the HR coating 112, surface passivation, and anti-reflective (AR) properties. The DBR 126 is formed on the substrate 128.

[0040] More specific details of the design and fabrication of one exemplary TVCSEL can be found in U.S. Patent No. 10,109,979, issued October 23, 2018, by Flanders, Kuznetsov, Atia, and Johnson, which are incorporated herein by reference. Additional innovations are described in U.S. Patent Publication No. US2019 / 0348815, published November 14, 2019, by Johnson, Malonson, Atia, Kuznetsov, Getz, and Whitney, and U.S. Patent Publication No. US2019 / 0386461, published December 19, 2019, also incorporated herein by reference.

[0041] The semiconductor DBR126 is formed by alternating high-refractive-index semiconductor layers (H) and low-refractive-index semiconductor layers (L). The tunable VCSEL uses a semiconductor DBR mirror structure with alternating GaAs / AlGaAs, i.e., high / low H / L refractive index layers, formed on a GaAs substrate 128. A large number of H / L pairs are required to achieve the high reflectivity magnitude required for these low-gain vertical cavity laser structures, and to achieve the spectrally broadband reflectivity required for tuning broadband wavelengths.

[0042] Nevertheless, such DBR structures have two properties that limit the performance of TVCSEL devices: namely, material strain that affects the DBR mirror spectral reflection bandwidth and refractive index contrast of the H / L layer.

[0043] High cumulative strain in semiconductor DBR structures causes strong epitaxial wafer warping, limiting device reliability and DBR mirror reflectivity bandwidth. High strain in the DBR can affect the distorted quantum well gain region above the DBR, and excessive strain can be relieved through the generation of dark line defects, leading to device degradation. High cumulative strain in the DBR mirrors limits the number of H / L layer pairs that can be used within the mirror without generating defects. This limits the DBR mirror reflectivity bandwidth, which increases with a larger number of pairs. Reducing DBR strain makes the device more reliable and can increase the mirror bandwidth by increasing the number of H / L pairs that can be reliably used.

[0044] The limited refractive index contrast between the H-index semiconductor layer and the L-index semiconductor layer limits the DBR mirror spectral bandwidth. The DBR mirror spectral bandwidth limits the tuning range of the tunable VCSEL when sufficient gain bandwidth is available.

[0045] However, H / L semiconductor materials with higher refractive index contrast can improve mirror bandwidth, and therefore the TVCSEL laser tuning range can be increased. Thus, there is a need for materials with a higher H refractive index and a lower L refractive index. In addition, the extended range of semiconductor DBR materials also requires strain control of these materials when they are epitaxially grown on GaAs.

[0046] Regarding semiconductor materials, a smaller energy band gap results in a larger refractive index value, and a larger energy band gap results in a smaller refractive index value. The dispersion of refractive index, i.e., the variation in refractive index with wavelength, needs to be considered for broadband semiconductor DBR mirrors. Furthermore, to increase the H / L refractive index contrast, H materials with lower band gaps and L materials with higher band gaps are required.

[0047] In the GaAs / AlGaAs material system for DBR, the highest refractive index contrast is achieved with respect to the GaAs / AlAs binary composition, and the resulting mirror reflection bandwidth is limited to around 100 nanometers (nm). In the AlGaAs material system, GaAs has the lowest energy band gap, and AlAs has the highest energy band gap. For 40(50) mirror pairs, the reflection bandwidth is 98.5(101.5) nm at a 99.5% DBR reflectivity level. In this material system, grown on a GaAs substrate 128, AlAs also exhibits the highest strain.

[0048] Based on this understanding, strain-compensated semiconductor DBR structures can be used to improve the performance of TVCSELs.

[0049] In the initial innovation, In is added to the H layer to shift from GaAs to InAs, forming an In(x)Ga(1-x)As layer. This modification can provide compressive strain, i.e., a lower band gap. H layers with a reduced band gap increase their refractive index. For L layers, P is added to shift from AlAs to AlP, or from AlGaAs to AlGaAsP. Thus, these layers are characterized by tensile strain and a higher band gap. L layers with a slightly increased band gap slightly decrease their refractive index.

[0050] As a result, the InGaAs / AlAsP (or InGaAs / GaAlAsP) mirror increases the DBR refractive index contrast due to a larger reflection bandwidth and a smaller effective penetration depth, and therefore a shorter optical resonator when paired with a second mirror. The shorter optical resonator produces a larger longitudinal mode spacing or free spectral range FSR of the laser optical resonator, which is important for tunable lasers, in which case a larger FSR potentially allows for a larger laser tuning range. In addition, the H and L layer compositions are tuned to achieve the resulting DBR strain equilibrium, for reduced wafer warpage and improved reliability of the semiconductor structure.

[0051] Generally, in many embodiments, the mirrors for TVCSELs incorporate 45 pairs, 50 pairs, or more pairs of equal, or 40 pairs or more H / L layers.

[0052] Figures 2A and 2B show the improved reflectivity achieved by a strain-balanced DBR design using AlAsP / InGaAs, where AlAs(0.872)P(0.128) / In(0.05)Ga(0.95)As. This is compared to a reference standard DBR design using AlAs / GaAs.

[0053] The mirror has 50 pairs. Strain compensation is H = InGaAs = compression and L = AlAsP = tension.

[0054] As demonstrated by the plot, the strain-equilibrium DBR bandwidth increases to 104.5 nm compared to the 101.5 nm bandwidth of a standard GaAs / AlAs DBR mirror on GaAs.

[0055] In addition, these principles can be extended to the structure of combined gain and DBR mirrors, as disclosed in U.S. Patent Application No. 16 / 993,953 (now U.S. Patent Application No. 2021 / 0050712), filed on 14 August 2020, by Johnson, Kuznetsov, and Whitney, which is incorporated herein by reference.

[0056] Figure 3 shows a design for a layered structure of a half-VCSEL employing combined gain and DBR mirrors using GaAlAsP / InGaAs mirrors.

[0057] This combined mirror includes two layers 20-21 to form a window 124. The final epitaxial window layer 21 has an InGaP compositional lattice matched to GaAs. Such a material has a large band gap to prevent excited electron-hole carriers from reaching the surface. Chemically, the InGaP layer does not oxidize in contact with air and is therefore preferred over the AlGaAs window layer, which may suffer such harmful oxidation.

[0058] Layers 1-19 form a passive DBR mirror 126, i.e., a mirror formed by combining layers 1-2 and an active semiconductor gain mirror region 122, i.e., layers 3-19. Repeated layer pairs 1-2 include GaAlAsP (low index) and InGaAs (high index) layers that form a passive DBR mirror without gain. Layers 3-19 form a gain-embedded DBR (GEDBR) active mirror region with reflection gain, which is formed on top of the passive DBR mirror. Here, InGaAs gain quantum well layers (5, 9, 13, 17) are inserted inside the GaAs layers of the GaAlAsP (low index) / GaAs (high index) mirror layer pair of this active mirror with the gain GEDBR region. Other compositions of the GEDBR H layer, such as AlGaAs, are also possible. The use of such an active-gain GEDBR mirror structure allows for shorter TVCSEL laser resonators, resulting in larger FSR longitudinal mode spacings and a wider tolerance for tuning.

[0059] Further innovations to increase DBR bandwidth involve extending the material system to eliminate strain in high-index layers.

[0060] DBR mirrors composed of GaAs / (Ga)AlAs(H / L) have a large cumulative internal distortion.

[0061] On the other hand, InGaAs / (Ga)AlAsP DBR mirrors can have zero-equilibrium cumulative distortion. Furthermore, the reduction in the band gap of InGaAs compared to GaAs, and the increase in the band gap of (Ga)AlAsP compared to (Ga)AlAs, provide greater refractive index contrast and greater spectral bandwidth than GaAs / (Ga)AlAs-based DBR mirrors.

[0062] The InGaAs / (Ga)AlAsP layer pair has zero-equilibrium strain, where the compressive strain of the InGaAs H layer is compensated by the tensile strain of the (Ga)AlAsP L layer. However, the individual InGaAs layers are still strained, and the product of the required maximum strain thicknesses of these layers limits the amount of bandgap reduction that can be achieved, and therefore limits the increase in DBR mirror bandwidth.

[0063] To overcome this limitation, the material system is further extended to include dilute InGaAsN nitride by adding nitrogen N to the InGaAs H layer. Such dilute nitride material can be grown lattice-matched with GaAs, thus enabling a reduction in the maximum bandgap and an increase in the DBR spectral bandwidth, which is limited only by the requirement of a non-absorbing mirror layer at the laser wavelength.

[0064] Furthermore, adding a certain amount of antimony (Sb) to dilute nitride InGaAsN forms a pentagonal alloy InGaAsNSb. Such pentagonal alloy InGaAsNSb materials enable the epitaxial growth of low-defect, high-quality heterostructures on GaAs, with a correspondingly large refractive index as the band gap decreases to 0.8 eV. Pentagonal alloy InGaAsNSb materials can also be used as high-index materials for high-quality, high-index contrast DBR mirrors. A reference for dilute nitrides is Mircea Guina et al., "Molecular Beam Epitaxy of Dilute Nitride Optoelectronic Devices," in "Molecular Beam Epitaxy," 2nd edition, Mohamed Henini ed., Elsevier Inc, 2018.

[0065] InGaAsN(Sb) / (Ga)AlAsP DBR mirrors can be grown with both H / L layers that are substantially lattice-matched to GaAs. This eliminates both the strain of the individual layers and the requirement to compensate for structural strain by alternating relatively large compressive / tensile strains in the individual layers. Alternatively, strain-compensated structures with low individual layer strains can also be grown in this material system. The DBR spectral bandwidth in this material system can be greater than that of GaAs / (Ga)AlAs(P)DBR mirrors.

[0066] Figures 4A and 4B show the improved reflectivity achieved by the dilute nitride DBR design AlAsP / InGaAsN, i.e., AlAs(0.9595)P(0.0405) / In(0.018)Ga(0.982)As(0.994)N(0.006). This is compared to the reference standard DBR design using AlAs / GaAs.

[0067] The mirror has 50 pairs. Both (H)InGaAsN and (L)AlAsP are lattice-matched to GaAs. As shown, the dilute nitride DBR bandwidth can be increased to 109.6 nm compared to the 101.5 nm bandwidth of a standard GaAs / AlAs DBR mirror on GaAs.

[0068] Figure 5 shows a design for a layered structure of a half-VCSEL employing combined gain and DBR mirrors, using InGaAsN and (L)GaAlAsP-based mirrors.

[0069] Layers 1-19 form a combined mirror having a passive DBR mirror 126, i.e., layers 1-2, and an active semiconductor gain mirror region 122, i.e., layers 3-19. Repeated layers 1-2 include GaAlAs(P) (low index) and InGaAsN(Sb) (high index) layers that form a passive DBR mirror without gain. Layers 3-19 form a gain-embedded DBR (GEDBR) active mirror region with reflection gain, which is formed on top of the passive DBR mirror. Here, InGaAs gain quantum well layers (5, 9, 13, 17) are inserted inside the GaAs layers of the GaAlAs(P) (low index) / GaAs (high index) mirror layer pair of this active mirror with the gain GEDBR region.

[0070] Other compositions of the GEDBR H layer, such as AlGaAs, are also possible. Such GaAlAs(P) / InGaAsN(Sb) composition mirrors allow for independent adjustment of the distortion and bandgap / refractive index of the mirror layer. This provides a much wider design space for the mechanical and optical properties of the DBR mirror, including individual layers as well as cumulative mechanical strain and strain compensation, optical reflection bandwidth, etc. The gain-embedded DBR region also benefits from the use of materials with a wider design space. The use of such active-gain GEDBR mirror structures enables shorter TVCSEL laser resonators, resulting in larger FSR longitudinal mode spacings and a wider allowable adjustment range.

[0071] While the present invention is shown and described, in particular with reference to its preferred embodiments, it will be understood by those skilled in the art that various modifications in form and detail can be made within the scope of the invention as encompassed by the accompanying claims.

Claims

1. A laser including a semiconductor distributed Bragg reflection mirror, wherein the mirror is GaAs substrate and A plurality of layers disposed on the GaAs substrate, wherein the plurality of layers include a plurality of layers of InGaAs having compressive strain, alternating with a plurality of layers of AlAsP or GaAlAsP having tensile strain and a plurality of layers of InGaAs having compressive strain. Equipped with, A laser comprising multiple layers of InGaAs having compressive strain, alternating with multiple layers of AlAsP or GaAlAsP having tensile strain, thereby generating substantially zero-equilibrium cumulative strain on the GaAs substrate.

2. The laser according to claim 1, further comprising a plurality of quantum well layers for providing gain.

3. The laser according to claim 2, wherein the plurality of quantum well layers are inserted inside the plurality of InGaAs layers and the plurality of AlAsP or GaAlAsP layers.

4. The laser according to claim 1, wherein the laser is a vertical cavity surface-emitting laser.

5. The laser according to claim 4, wherein the laser is a tunable vertical cavity surface-emitting laser having broadband wavelength tuning characteristics.

6. The laser according to claim 5, wherein the laser includes a MEMS film that includes a reflective coating defining the edges of the laser resonator.

7. A semiconductor distributed Bragg reflection type mirror, wherein the mirror is GaAs substrate and A plurality of layers disposed on the GaAs substrate, wherein the plurality of layers include a plurality of layers of InGaAs having compressive strain, alternating with a plurality of layers of AlAsP or GaAlAsP having tensile strain and a plurality of layers of InGaAs having compressive strain. Equipped with, Multiple layers of InGaAs having compressive strain, alternating with multiple layers of AlAsP or GaAlAsP having tensile strain, mirror the GaAs substrate, generating substantially zero-equilibrium cumulative strain.

8. A laser including a semiconductor distributed Bragg reflection mirror, wherein the mirror is GaAs substrate and A plurality of layers disposed on the GaAs substrate, wherein the plurality of layers include a plurality of layers of InGaAs or InGaAsN(Sb) having compressive strain, alternating with a plurality of layers of AlAsP or GaAlAsP having tensile strain and a plurality of layers of InGaAs or InGaAsN(Sb) having compressive strain. Equipped with, A laser comprising multiple layers of InGaAs or InGaAsN(Sb) having compressive strain, alternating with multiple layers of AlAsP or GaAlAsP having tensile strain, thereby generating substantially zero-equilibrium cumulative strain on the GaAs substrate.

Images