Patent Application
20170033534
This invention relates to a grating reflector and VCSEL structures employing embodiments of the grating reflector.
Sub-wavelength high-index-contrast gratings (HCGs) have received lots of attention due to special properties such as broadband high reflection spectrum and ultra-high Q resonance effect. As a reflector, it can be approximately 50 times thinner than a conventional distributed Bragg reflector (DBR), but still offer high reflectivity over a much significantly broader spectral width, properties that make it useful in a wide application range, including lasers, photodetectors, filters, splitters, couplers, etc. They have been implemented in vertical cavity surface emitting lasers (VCSELs) and resonant-cavity-enhanced photodetectors (RCEPDs) in place of conventional DBRs. In addition several unique characteristics of HCGs in VCSEL structures, such as a strong single-transverse-mode operation, broad wavelength tunability, and light emission into an in-plane silicon photonics chip have been shown.
Using HCG as a high Q resonator, a very compact (small modal volume) with ultra-high quality factor lasing device has been demonstrated. Fully rigorous electromagnetic solutions known as RCWA exist for gratings, although they require heavy mathematical formalism. Different groups investigated the physics behind HCGs' properties. In all the literature on the HCG mirrors, the grating is surrounded by low index materials. Even if the device substrate is a high-index material, a layer with low-index material is said to be required to obtain the HCG properties.
U.S. Pat. No. 7,304,781 B2 is an example of patent prior art describing HCG mirrors. Again, the high-index regions are surrounded by material having a relatively low refractive index.
International patent application publication WO 2013/110004A1 discloses a “0-gap” HCG. The 0-gap HCG is defined by only three geometrical parameters, i.e., grating period, grating thickness, and grating duty cycle. The incident medium is high refractive index material 114. This 0-gap HCG does not provide a reflectivity higher than 99.5% according to
The structures in WO 2013/110004A1 therefore have some undesirable properties.
The present invention addresses some of these undesirable properties and provides an alternative VCSEL structure that allows for more design flexibility.
Despite the fact that in the literature HCGs consist of a grating sections having high-index material surrounded by a low-index material, the inventors of the present invention have realized that similar properties can be obtained even with a high-index material, a “cap layer”, abutting the grating structure. The resulting structure will be referred to as a grating reflector. Even with a relatively thick layer (several times the grating's thickness) of high-index material, this structure can have special and advantageous properties. The working mechanism of the grating reflector can be more complex and, more importantly, more flexible than the conventional HCG mirror. For practical purposes, the cap layer is somewhere between 300 nm to 1 micron at 1.5 micron wavelength, but it can also be thinner or thicker.
The addition of the cap layer provides several advantages over conventional HCGs. It can improve some of the reflection properties, e.g. broaden the bandwidth. From a fabrication standpoint, especially for devices with active material, it can ease the fabrication process due to the possibility of integrating active material inside the grating reflector. Furthermore, the invention can improve device performance, such as tuning rate, due to a smaller effective cavity length in RCEPDs.
Most of the materials that can be used in the present invention have chromatic dispersion, which is the phenomenon that the phase velocity of light travelling in the material varies with the wavelength of the light. In the present specification, particularly in the claims, “refractive index” or “index of refraction” of a material refers, unless otherwise specified, to generally accepted values of the refractive index for that material at a free-space wavelength of 1.5 um. Table 1 shows values for common high-index materials applicable in the context of the present invention. At high frequencies, the refractive indices for those materials change rapidly with decreasing wavelength, typically increasing at first, and then decreasing to values lower than 2.5. Table 1 also shows the refractive indices at a free-space wavelength of 250 nm to illustrate this.
This definition of refractive index used herein shall not be construed as limiting the scope of the invention. The definition is used because a number of materials that are advantageous in embodiments of aspects of the present invention have refractive indices within certain intervals at various wavelengths. Using the refractive index at a certain wavelength as a reference, the concept of refractive index or index of refraction as these entities pertain to the claims invention becomes unambiguous.
[1] Handbook of Optics, 3rd edition, Vol. 4. McGraw-Hill 2009
[2] D. E. Aspnes and A. A. Studna. Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV, Phys. Rev. B 27, 985-1009 (1983)
[3] http://www.filmetrics.com/refractive-index-database/Si3N4/Silicon-Nitride-SiN
[4] T. Bååk. Silicon oxynitride; a material for GRIN optics, Appl. Optics 21, 1069-1072 (1982)
[5] Gorachand Ghosh. Dispersion-equation coefficients for the refractive index and birefringence of calcite and quartz crystals, Opt. Commun. 163, 95-102 (1999)
[6] SOPRA N&K Database
[7] Philip E. Ciddor. Refractive index of air: new equations for the visible and near infrared, Appl. Optics 35, 1566-1573 (1996)
A first aspect of the invention provides a new type of VCSEL. This VCSEL comprises:
A first aspect of the invention provides a new type of a grating reflector. The grating reflector comprises:
The cap layer is a crucial part of the grating reflector. Generally, the grating reflector comprises:
A “side” of a layer refers to an in-plane oriented face of the layer that meets another in-plane oriented face of another layer, or meets a grating, or air. From the figures, it is clear that in a “grating layer” as referred to in the present invention, high-index sections and low-index sections alternate in an in-plane direction.
Compared to e.g. WO 2013/110004 A1, the first reflector and grating reflector form an optical cavity that can be significantly shorter than the cavity 60 in D1.
To obtain a stronger effect of the grating, it can be advantageous that the grating layer within the core grating region comprises at least 3 high-index sections.
The high-index regions of the grating region can be made of for instance Si or be InP-based or GaAs-based.
The second low-index layer may comprise or consist of for instance SiNx, SiO2, or AlOx, or an equivalent material fulfilling the conditions for a second low-index layer.
The cap layer comprises a first active region and may comprise at least two contacts positioned to allow a voltage to be applied across the first active region for either generating photons or changing an absorption of the first active region.
The cap layer may comprise a first cladding layer and a second cladding layer, with the first active region interposed therebetween.
In certain preferred embodiments, a thickness of the cap layer is at most 3 microns, such as at most 1.5 microns, such as at most 0.6 microns. Preferably, the thickness of the cap layer is in the interval 300 nm to 1.5 microns.
In some embodiments, the VCSEL structure further comprises a second active region arranged so the first low-index layer is situated between the first active region and the second active region.
In some embodiments, the first reflector is a distributed Bragg reflector. Alternatively, it is a grating reflector designed to have a high reflectivity, such as at least 99%, such as at least 99.5%, such as at least 99.8%, or even higher. Alternatively, it may be a high-contrast grating (HCG), a metallic reflector, or any other reflector having the abovementioned high reflectivity.
By adding at least two second active region contacts, application of a forward-bias voltage or reverse-bias voltage across the second active region is enabled. Then an optical output from the optical cavity can be modulated when a sufficient and time-varying reverse or forward bias voltage is applied across the second active region. Preferably, the first active region contacts are located on one side of the first low-index layer, and the second active region contacts are located on a side of the first low-index layer opposite the side of the first active region contacts. The contacts are typically not in direct contact with the first low-index layer, which the drawings will clearly show. The first low-index layer is typically non-conducting or at least has a high resistance, whereby the application of a voltage across the first active region is not affected by application of a voltage across the second active region.
In some embodiments, the VCSEL structure includes an output waveguide in the grating layer, the output waveguide comprising an external waveguide beginning from and extending beyond a projection of a first edge of the optical cavity onto the grating layer. This output waveguide enables coupling light from the optical cavity out in a lateral direction rather than in a vertical direction (such as through the first reflector). Preferably, this is combined with a first reflector and grating reflector having a combined normal-incidence reflectivities of at least 99.8%. Preferably, the normal-incidence reflectivity of the first reflector exceeds the normal-incidence reflectivity of the grating reflector. The result is that most of the optical power is coupled out via the output waveguide rather than in a direction normal to the first reflector.
Preferably, the external waveguide is at least 10 microns long.
In some embodiments, the VCSEL structure comprises a core section in which a normal-incidence reflectivity of the combined cap layer and grating layer is at least 99%, such as at least 99.8%.
In some embodiments, the core section extends to the beginning of the external waveguide.
In some embodiments, the VCSEL structure comprises a core section with grating parameters that result in a normal-incidence reflectivity of the combined cap layer and grating layer of at least 99%, such as at least 99.8%. The VCSEL structure furthermore comprises a coupling section between the core section and the external waveguide, and the coupling section has grating parameters that are different from the grating parameters of the core section.
In some embodiments, the grating layer in the coupling section comprises a narrow low-index section that has a width in the interval 35 to 65% of a smallest width of low-index sections within the core section, and the narrow low-index section abuts the external waveguide. In some embodiments, the grating layer within the coupling section consists of only the narrow low-index layer.
In some embodiments, the external waveguide is integral with a wide high-index section in the coupling section, the wide high-index region having a width exceeding a highest width of high-index sections within the core section.
In some embodiments, the VCSEL structure furthermore comprises a confinement section having grating parameters different from grating parameters of the core section. In some embodiments, a vertical resonance wavelength in the confinement section differs from a vertical resonance wavelength in the core section. Then there is no mode in the confinement section matching the vertical resonance wavelength mode in the core section, and thus light from the core section cannot propagate into the confinement section.
In some embodiments, the grating layer in the confinement section is a Bragg reflector or equivalent structure having a stop band around a vertical cavity resonance wavelength of the core section. In some embodiments, it comprises alternately high-index and low-index sections each of which has an optical width of one quarter of the vertical resonance wavelength of the core section. Generally, the widths may also fall within +/−25% of the one quarter of the vertical resonance wavelength of the core section.
In some embodiments, the external waveguide tapers from a first width at the beginning of the external waveguide, to a narrower width.
In some embodiments, a normal-incidence reflectivity of the first reflector equals or exceeds a normal-incidence reflectivity of the combined cap layer and grating layer in the core section. This ensures a higher coupling of optical power from the optical cavity into the external waveguide.
In some embodiments, the VCSEL structure comprises:
In some embodiments, the first active region contacts are located in a north position and a south position relative to the core section and the external waveguide is located in an east position relative to the core section, when seen in a direction normal to the first reflector.
The invention will now be exemplified with reference to the accompanying drawings. Reference signs in this specification, including in the claims, are not to be construed as limiting the scope of the invention. The drawings are not necessarily drawn to scale.
A one-dimensional grating used in a grating layer of a grating reflector could for instance be made of parallel bars of a first material, spaced evenly and being separated by a second material, such as SiO2 or by air or other gaseous substance. The first material could for instance be Si or InP or GaAs or other high-index material. In some embodiments, the grating is non-periodic, such as apodized or chirped or almost-periodic or quasi-periodic or consisting of several sections with different grating periods and/or grating width. Such options are well-known by the person skilled in the art and are applicable as gratings in embodiments of the present invention. The selection of grating depends on the desired properties.
It is important to note that the cap layer in accordance with the invention need only to consist of high-index material in a limited region, namely within the core grating region projection defined by the core grating region described previously.
In
In many embodiments, however, the cap layer 30 can be active, and typically a composite layer as shown in
In many embodiments, the high-index sections 21 of the grating layer is group-IV based, typically made of Si. At the same time, the cap layer is typically group-III-V based. In that case, the grating reflector is referred to as a hybrid grating reflector, indicating this hybridised nature.
The refractive indices of both the grating 21 and the cap layer 30 might in some embodiments range from 2.9 to 3.7. As discussed, the refractive indices of surrounding media 10 and 50 and the grating gaps 22 are low, e.g., between 1.0 and 1.8 or 2. The thickness of the grating layer 20 can be, but is not limited to, around 1-1.2 times of the wavelength of interest divided by the refractive index of the grating sections 21. For example, this could be 500 nm for a Si grating when the wavelength of interest is 1550 nm; here, the wavelength of interest can be a central wavelength of the wavelength range where the grating reflector 40 has high reflectivity values. The thickness of the cap layer 30 could range from 0.02 to 2.2 times of the wavelength of interest divided by the refractive index of the cap layer 30, but this is a matter of design and not to be considered a limitation. For example, this can be 10 nm to 1 μm if the cap layer is made of InP and the wavelength of interest is 1550 nm. The refractive indices and thicknesses given above are example values; other values may be used depending on designs, as also discussed
As shown in
The present invention builds in part on the fact that the grating reflector can be designed to have a significantly higher bandwidth than conventional HCGs. In some embodiments of the grating reflector, the normal-incidence reflectivity is at least 99%, such as at least 99.8%. In these embodiments, the grating reflector is novel and inventive, as the prior art has not disclosed the surprisingly effective combination of an HCG and a cap layer that makes it possible to have a high reflectivity across a broad range. In the prior art, a high-index layer is not designed for the obtaining a high reflectivity across a broad range. The presence of the low-index layer on the second side of the cap layer gives the surprising effect. Preferably, a thickness of the cap layer is at most 3 microns. It is even possible to achieve a broad bandwidth around 1550 nm with a cap layer smaller than 1.5 microns. This may advantageously be combined with a grating layer having a thickness of between 200 nm and 700 nm.
Comparing the spectra of
It is a key aspect that the grating reflector includes the “cap layer” described above. The hybrid reflector is defined by four geometrical parameters, i.e., grating period, grating thickness, grating duty cycle, and cap layer thickness. In the previously discussed WO2013/110004 A1, there are only three parameters available. The high-index layer in WO 2013/110004A1 situated at a location similar to the cap layer in the grating reflector does not play the same role. An important reason for this is that the incident medium in the present invention is a low refractive index material 50. In the prior art, the cavity 60 is made of high-index material, while in the present invention, the cavity is made of low-index material, which leads to fundamentally different optical modes.
VCSEL with a Hybrid Grating Reflector Operating as an Integrated Modulator
The active region 102 can be made with materials and a configuration as described above in relation to layer 32 of the grating reflector, that is: it can be bulk material or contain one or more quantum wells, one or more quantum dot layers, one or more quantum wire layers, one or more quantum dash layers, a buried heterostructure (BH) and so on, or a combination of such materials and material structures. The layer 101 and/or 103 may include an optical confinement structure and/or an electrical confinement structure. This is a matter of design and affects device efficiency and optical and electrical properties. The illustrated design is simple and efficient.
Electric current is supplied to the active material 102 through cladding layers 101 and 103 for light generation. Metal contacts 105 and 106 are used for supplying electric current. Structures for lateral confinement of electronic current such as an oxide aperture and a tunnel junction can be included in either cladding layers 101 or 103, or in both of them. The low-index layer 50 is SiO2, or AlO or BCB, for example.
The reflector 210 can for instance a distributed Bragg reflector, a conventional high-index-contrast grating, or another grating reflector or hybrid grating reflector, or other type of suitable mirror.
By supplying reverse bias or forward bias to the active material 32, the refractive index and absorption coefficient of the active material 32 can be changed. Metal contacts 35 and 36, or metal contacts 35 and 105 are used for this supply. In
VCSEL with Light Generation from a Hybrid Grating Reflector
In
In
For lateral confinement of current, a structure such as an oxide aperture or a tunnel junction can be included for instance in the cladding 31 or 33. Alternatively, the active material 32 can be included within a buried heterostructure.
When light is incident on the hybrid grating reflector 40, it excites several modes in the grating layer 20 and the cap layer 30. These excited modes collectively form a standing wave intensity pattern within the hybrid grating reflector. The light intensity at one of the anti-node positions of the standing wave pattern within the cap layer 30 is comparable to or even higher than that at one of the anti-node positions within the optical cavity. Thus, equivalently efficient stimulated emission as in conventional VCSEL structures can be obtained. This is fundamentally different from the principles of the prior art.
An apparent structural difference from the conventional VCSEL structures is that the active material for light generation is located not in the optical cavity per se but in a reflector. As a result, the volume, V, of the optical mode formed by the optical cavity is considerably smaller in the VCSEL structure shown in
Another consequence is that an equivalent series resistance, R, and capacitance, C, are considerably smaller, easing the RC time constant limit, the extrinsic modulation speed to some extent scaling as (RC)−1/2.
The structure in
After forming the grating pattern in the grating layer 20, the cap layer 30 is wafer-bonded onto the grating layer 20.
The low-index layer 50 can be made of SiNx, SiO2, or AlOx. SiNx and SiO2 should be deposited, AlOx can be epitaxially grown and oxidized, and air can be formed by sacrificial etching.
The reflector 210 can be a deposited dielectric DBR, an epitaxially grown DBR, a high-index-contrast grating, or another grating reflector or hybrid grating reflector.
Fabrication of the embodiment in
RCEPD with Light Absorption in a Hybrid Grating Reflector
The structures in
The amount of light absorption can be enhanced if a light-absorbing material is positioned at one of light intensity anti-nodes of the standing wave pattern in the optical cavity. In the structures in
Lateral Emission into an In-Plane Waveguide
In some embodiments, the confinement section 626 may have different grating parameters from the core section 625, leading to a different resonance wavelength in the confinement section 626. This provides transverse confinement.
In some embodiments, the grating parameters of the grating layer of the confinement section 626 can be chosen to form a Bragg reflector. This provides transverse confinement.
In some embodiments, the cap layer of the confinement section 626 has a Bragg reflector pattern. This provides transverse confinement.
The external waveguide could alternatively extend in another direction relative to the grating structure in the grating layer, such as parallel to the bars in case of a grating such as that in
| Number | Date | Country | Kind |
|---|---|---|---|
| 14163768.6 | Apr 2014 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/057522 | 4/7/2015 | WO | 00 |
