Patent Application
20230307888
Embodiments of the present disclosure relate to a distributed feedback laser, and in particular to a distributed feedback laser with complex coupling.
A distributed feedback laser (DFB) is a type of laser diode in which the active region of the device contains a periodically structured element or diffraction grating, which may include periodic changes in refractive index that cause reflection back into the laser cavity. The periodic change can be either in the real part of the refractive index, e.g. index modulated, or in the imaginary part, e.g. loss or absorption modulated. Conventional DFB lasers used in optical networks may exploit either loss coupling or index coupling mechanisms, respectively, involving a grating supporting a modulation of the material gain (loss) or a modulation of the material index of refraction.
In the case of complex-coupling, index-modulated and loss-modulated gratings may be combined together. Prior art solutions however show many drawbacks with respect to mechanical stability and process control.
An object of embodiments according to the present disclosure is to provide a more efficient distributed feedback laser with a complex-coupling grating structure.
Accordingly, a first apparatus includes a distributed feedback laser comprising:
In embodiments the first optical grating may have a substantially zero absorption coefficient; and the second optical grating may have a non-zero absorption coefficient.
The first optical grating may be configured to work at a first order of the laser wavelength; and the second optical grating may be configured to work at the first order of the laser wavelength.
In some embodiments the first optical grating may have a photoluminescence peak wavelength larger than the laser wavelength; and the second optical grating may have a photoluminescence peak wavelength less than the laser wavelength.
The first plurality of grating structures may be in a first epitaxial film or layer of a first group III-V semiconductor; and the second plurality of grating structures may be in a second epitaxial film or layer of a second group III-V semiconductor.
The distributed feedback laser may further comprises: a first electrode; a first electrical contact region between the first electrode and the first optical grating; a first spacer region between the optical waveguide gain medium and the first optical grating; a second electrode; a second electrical contact region between the second electrode and the second optical grating; and a second spacer region between the optical waveguide gain medium and the second optical grating.
In any of the aforementioned embodiments the first electrical contact region and the first spacer region may be comprised of one of an n-doped group III-V semiconductor or a p-doped group III-V semiconductor; and the second electrical contact region and the second spacer region may be comprised of the other of the n-doped group III-V semiconductor or the p-doped group III-V semiconductor.
In any of the aforementioned embodiments ηSpacer1 may be <ηIndexG which may be <ηwgm; wherein ηwgm is the refractive index of the optical waveguide gain medium, ηIndexG is a refractive index of the first optical grating, and ηSpacer1 is a refractive index of the first spacer region; and ηSpacer2 may be <ηwgm which may be <ηLossG; wherein ηSpacer2 is a refractive index of the second spacer region, ηwgm is the refractive index of the optical waveguide gain medium, and ηLossG is a refractive index of the second optical grating.
In any of the aforementioned embodiments the optical waveguide gain medium may be based on separate confinement heterostructure (SCH) and multiple quantum wells (MQW).
It is clear from the aforementioned embodiments that the distributed feedback laser is a vertical, buried structure. This presents many advantages in terms of mechanical stability, and heat evacuation when fabricated using semiconductor technologies.
Accordingly, a first method includes a method of manufacturing a distributed feedback laser comprising:
Step b) may further include forming the first plurality of grating structures in a first epitaxial film or layer of a first group III-V semiconductor, and step d) may further include forming the second plurality of grating structures in a second epitaxial film or layer of a second group III-V semiconductor.
In further embodiments step b) may also comprise:
In embodiments, the first electrical contact region and the first spacer region may be comprised of one of an n-doped group III-V semiconductor or a p-doped group III-V semiconductor, and the second electrical contact region and the second spacer region may be comprised of the other of the n-doped group III-V semiconductor or the p-doped group III-V semiconductor.
In embodiments, ηSpacer1 may be <ηIndexG which may <ηwgm; wherein ηSpacer1 is a refractive index of the first spacer region, ηwgm is the refractive index of the optical waveguide gain medium, and ηIndexG is a refractive index of the first optical grating; and ηSpacer2 may be <ηwgm which may be <ηLossG; wherein ηSpacer2 is a refractive index of the second spacer region, ηwgm is the refractive index of the optical waveguide gain medium, and ηLossG is a refractive index of the second optical grating.
In embodiments, step b) may further comprise:
The step d) may further comprise:
All mentioned embodiments of the distributed feedback laser can be integrated in many other devices to be become a simple optical function. For example it can be integrated in devices such as Electro absorption-Modulated Lasers (EML), Integrated Mach-Zehnder Modulator Lasers, Master Oscillator Power Amplifiers Laser (MOPA Laser). Moreover all described embodiments can be easily integrated on different platforms such as “native” semiconductors or Silicon on isolator (SOI) as the technological process for the realization of the lasers is the same for III-V semiconductors and II-VI semiconductors.
Some example embodiments will be described in greater detail with reference to the accompanying drawings, wherein:
While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art.
Applying a complex grating structure to a DFB laser takes advantage of two features that improve the single frequency behavior of the DFB laser through the simultaneous presence of index and gain/loss modulation. In the case of complex-coupling, index-modulated and loss-modulated (parity-time) gratings are combined together. A feature of the asymmetric coupling may be obtained in gratings with elements of parity-time symmetry, by using an appropriate π/2 phase shift, i.e. for a Λ/4, between the loss modulation and the index modulation.
With reference to
The DFB laser 10 may include an upper electrical contact region 21 over the optical waveguide gain medium 13, and a lower electrical contact region 22 under the optical waveguide gain medium 13. The upper electrical contact region 21 may be comprised of one of an n-doped or a p-doped material, such as an n-doped or p-doped group III-V semiconductor, such as n-InP or p-InP, and the lower electrical contact region 22 may be comprised the other of the n-doped or p-doped material, such as the n-doped or p-doped group III-V semiconductor, such as doped n-InP or p-InP. The first optical grating 11 may be positioned in one of the upper electrical contact region 21 or the lower electrical contact region 22, and the second optical grating 12 may be positioned in the other of the upper electrical contact region 21 or the lower electrical contact region 22. A substrate 25, adjacent to the lower electrical contact region 22, may support the various layers of the DFB laser 10. The substrate 25 may be comprised of a group III-IV semiconductor material or a group III-V semiconductor material, e.g. InP, but the use of any suitable material is possible. An upper, e.g. metal, electrode 27 may be provided at a top end of the upper electrical contact region 21, and a lower, e.g. metal, electrode 28 may be provided at a bottom end of the lower electrical contact region 22, configured to applying electrical energy, e.g. voltage or current, to the DFB laser 10.
The first (index-modulated) optical grating 11 may be defined as a transparent DFB grating, because an optical index ηIndexG+ikIndexG of the first optical grating 11 may be approximated only by the real number ηIndexG, whereby the absorption coefficient is equal to kIndexG=0. An optical Bragg grating may be a transparent device with a periodic variation of the refractive index, so that a large reflectance (reflectivity) may be reached in some wavelength range (bandwidth) around a certain wavelength λB which fulfills the Bragg condition:
λB=2η0Λ
where λB is the Bragg wavelength of light, η0 the effective refractive index of the optical mode, and Λ the grating period. If this condition is met, the wavenumber of the grating matches the difference of the wavenumbers of the incident and reflected optical waves. The effective index η0 of the optical mode is the mean of the optical indexes of materials seen by optical mode 40 based on the confinement in each of the materials. The effective index η0 is between the lower index of the encapsulating materials of the waveguide, e.g. cladding region 35, the upper electrical contact region 21, the lower electrical contact region 22, the first optical grating 11 and the second optical grating 12, and the higher index of the waveguide, e.g. the core of the optical wave guide gain medium 13. The effective index η0 may be close, but not the same as, the index of the core of the optical waveguide gain medium 13, because the main part of the optical mode 40 travels therethrough.
The second optical grating 12 may be defined as the loss DFB optical grating, because an optical index nLossG+ikLossG has a non-zero absorption coefficient kLossG≠0.
The distributed feedback laser 10 based on the first and second buried semiconductor gratings 11 and 12 with index-loss (parity-time) symmetry provides a strong asymmetry of the output for the optical power working at the first order, whereby a majority up to substantially all the optical power of the output light may be output from the front face 15. Accordingly, an optical treatment may not be necessary on the rear face 16 of the DFB laser 10, which provides an interesting industrial simplification and consequently a cost reduction. Moreover, the threshold current intensity is smaller than usual DFB laser due to a better quantum efficiency.
The first, i.e. index-modulated, optical grating 11, which may be positioned over the optical waveguide gain medium 13, may have a photoluminescence peak wavelength below the laser wavelength λDFB. The greater the difference between the laser wavelength λDFB and the photoluminescence peak wavelength of the semiconductor corresponding to the first optical grating 11, the more efficient is the first optical grating 11. Indeed the optical index of the first optical grating 11 ηIndexG+ikIndexG may be approximated by the real number ηIndexG because the absorption coefficient kIndexG is substantially equal to 0. For a laser wavelength λDFB of 1.55 μm, the first optical grating 11 may comprise a first plurality of grating structures comprising a group III-V semiconductor, such as aluminum gallium indium arsenide (AlGaInAs) or gallium indium arsenide phosphide (GaInAsP), which have a photoluminescence peak wavelength of 1.1 μm to 1.2 μm. The first optical grating 11 may comprise a first plurality of grating structures in a first epitaxial film or layer of the aforementioned group III-V semiconductor.
The second, i.e. loss-modulated, optical grating 12, which may be positioned under the optical waveguide gain medium 13, may have a photoluminescence peak wavelength greater than the laser wavelength λDFB. The greater the difference between the laser wavelength λDFB and the photoluminescence peak wavelength of the semiconductor corresponding to the second grating 12, the more efficient is the second, i.e. loss, optical grating 12. Indeed the optical index of the second optical grating 12 ηLossG+ikLossG has a non-zero absorption coefficient kLossG≠0. For a laser wavelength km of 1.55 μm, the second optical grating 11 may comprise a second plurality of grating structures comprising a group III-V semiconductor, such as indium gallium arsenide (InGaAs), which has a photoluminescence peak wavelength of about 1.65 μm. The second optical grating 12 may comprise a second plurality of grating structures in a second epitaxial film or layer of the aforementioned group III-V semiconductor.
The optical waveguide gain medium 13 may be spaced apart from the first optical grating 11 by a first spacer region 31, which may be comprised of the same material as the upper electrical contact region 21, e.g. n-InP or p-InP. Optical indexes of the different layers may respect the inequation, ηSpacer1<ηIndexG<ηwgm, wherein ηwgm is the refractive index of the optical waveguide gain medium 13, ηIndexG is a refractive index of the first optical grating 11, and ηSpacer1 is a refractive index of the first spacer region 31. The optical waveguide gain medium 13 may be spaced apart from the second grating 12 by a second spacer region 32, which may be comprised of the same material as the second electrical contact region 22, e.g. n-InP or p-InP. Optical indexes of the different layers may respect the inequation, ηSpacer2<ηwgm<ηLossG, wherein ηwgm is the refractive index of the optical waveguide gain medium 13, ηLossG is a refractive index of the second optical grating 12, and ηSpacer2 is a refractive index of the second spacer region 32.
In the illustrated example of
The first and second gratings 11 and 12 are configured to reflect light of the first order of the desired optical wavelength λDFB of the DFB laser 10. The period Λ of the first, e.g. index-modulated or transparent, grating 11 is defined by the wavelength λDFB and is the same as the period Λ of the second, i.e. loss-modulated, optical grating 12. The period Λ follows from the equation λDFB=2η0Λ, where η0 is the effective refractive index of the optical mode, as defined hereinabove. However, there may be an offset corresponding to about Λ/4±Λ/8 between the grating structures of the first and second gratings 11 and 12. If the layers of the second, i.e. loss, grating 12 and the first, i.e. transparent, grating 11 were permuted, the offset corresponding to Λ/4 will be also permuted.
The shape of the optical mode 40 in the optical waveguide gain medium 13 and surrounding cladding may be adjusted, e.g. in order to make the optical mode as circular as possible, by tuning various different dimensions of the laser elements, such as the width WTransG of the first grating 11, the width WLossG of the second grating 12, the thickness TTransG of the first grating 11, the thickness TLossG of the second grating 12, the thickness TSpacer1 of the first spacer region 31 and the thickness TSpacer2 of the second spacer region 32, width of the optical waveguide gain medium 13, and type of cladding around the optical waveguide gain medium 13.
In
All of the upper electrical contact regions 21, 321 and 421 may comprise the same material, and may perform the same functions. All of the optical waveguides gain mediums 13, 413 may comprise the same material, and may perform the same functions. The ridges 39, 339, 439, 539, 639 and 739 may have a width 1.5 μm to 10 μm wide. The positions of the first (index-modulated) optical grating 11 and the second, i.e. loss-modulated, optical grating 12 relative to the substrate 25 may be interchangeable, whereby the first optical grating 11 may be between the substrate 25 and the optical waveguide gain medium 13.
An exemplary method of manufacture is illustrated in
As hereinbefore discussed, ideally the second grating layer 912, corresponding to the second e.g. loss-modulated, optical grating 12, comprises a semiconductor material with a photoluminescence peak greater than the laser wavelength λDFB. For example, when the lower contact region 22 comprises n-InP for the n-doped III-V semiconductor layer and the laser wavelength λDFB is about 1.55 μm, the semiconductor material corresponding to the second, e.g. loss, optical grating 12 should have a photoluminescence peak wavelength greater than 1.55 μm, e.g. InGaAs, which has a photoluminsecence wavelength around 1.65 μm. The greater the difference between the laser wavelength λDFB and the photoluminsecence peak wavelength of the semiconductor material corresponding to the second grating 12, the more efficient is the second grating 12.
A mask layer 950, e.g. a dielectric layer such as SiO2 or Si3N4, may then be deposited over the spacer layer 932, and the second grating 12 may be defined by lithography, e.g. the dielectric layer is patterned like the second optical grating 12, and becomes an etching mask. With reference to
With reference to
With reference to
The first spacer region 31, which may comprise a p-doped III-V semiconductor layer, may then be provided, e.g. deposited or epitaxy, over the optical waveguide layer 913. A first grating layer 911, from which the first optical grating 11 may be formed, may then be provided, e.g. deposited or epitaxy, over the first spacer region 31. The upper electrical contact region 21, which may comprise a p-doped III-V semiconductor layer, e.g. p-InP, may then be provided, e.g. deposited or using an epitaxy process, over the first grating layer 911.
As an example, in the case of a laser wavelength λDFB around 1.55 μm, the first spacer region 31 and the upper electrical contact region 21 may comprise a p-doped III-V semiconductor layer, such as p-doped Indium Phosphide (p-InP). Ideally, the material forming the first, e.g. index-modulated, optical grating 11, may have a photoluminescence peak wavelength less than 1.55 μm, e.g. aluminum gallium indium arsenide (AlGaInAs) or gallium indium arsenide phosphide (GaIsAsP) quaternary semiconductors with a photoluminescence peak wavelength from 1.1 μm to 1.2 μm. The greater the difference between the laser wavelength DFB and the photoluminsecence peak wavelength of the material, e.g. semiconductor material, corresponding to the first, e.g. transparent, optical grating 11, the more efficient is the first optical grating 11.
A second mask layer 960, e.g. a dielectric layer such as SiO2 or Si3N4, may then be deposited on the upper electrical contact region 21 to define the first, e.g. loss, optical grating 11 by lithography. The second mask layer 960 may be patterned like the first optical grating 11 to become an etching mask. The first spacer region 31, the first grating layer 911, and the upper contact region 21 may then be etched, e.g. by reactive ion etching (RIE) or wet chemical solution, to form the first optical grating 11 and the first spacer region 31. (
With reference to
With reference to
The ridge, e.g. ridge 39, 339, 439, 539, 639 or 739, may then be etched (
With reference to
With reference to
The foregoing description of one or more example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the disclosure be limited not by this detailed description.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22305342.2 | Mar 2022 | EP | regional |
