WAVELENGTH-STABILIZED NEAR-FIELD OPTOELECTRONIC DEVICE
An in-plane-emitting semiconductor diode laser employs a surface-trapped optical mode existing at a boundary between a distributed Bragg reflector and a homogeneous medium, dielectric or air. The device can operate in both TM-polarized and TE-polarized modes. The mode exhibits an oscillatory decay in the DBR away from the surface and an evanescent decay in the dielectric or in the air. The active region is preferably placed in the top part of the DBR close to the surface. The mode behavior strongly depends on the wavelength of light, upon increase of the wavelength the mode becomes more and more extended into the homogeneous medium, the optical confinement factor of the mode in the active region drops until the surface-trapped mode vanishes. Upon a decrease of the wavelength, the leakage loss of the mode into the substrate increases. Thus, there is an optimum wavelength, at which the laser threshold current density is minimum, and at which the lasing starts. This optimum wavelength is temperature-stabilized, and shifts upon temperature increase at a low rate less than 0.1 nm/K, indicating wavelength-stabilized operation of the device. The approach applies also to semiconductor optical amplifiers or semiconductor gain chips which are also wavelength-stabilized. Reflectivity of the surface-trapped mode from an uncoated facet of the device can be extremely low, also <1E-4 or even <1E-5 which is particularly advantageous for amplifiers or gain chips. For diode lasers, a specific intermediate reflective coating can be deposited on the facet to put its reflectivity into a range from 0.5% to 3%, which lies within targeted values for lasers. An optical integrated circuit can employ wavelength-stabilized amplifiers operating in a surface-trapped mode, wherein such devices amplify light propagating along a dielectric waveguide.
Latest VI SYSTEMS GmbH Patents:
- Wavelength-stabilized near-field optoelectronic device
- Optoelectronic device based on a surface-trapped optical mode
- Micropillar optoelectronic device
- Reliable high-speed oxide-confined vertical-cavity surface-emitting laser
- Optoelectronic device with enhanced lateral leakage of high order transverse optical modes into alloy-intermixed regions and method of making same
This is a continuation-in-part of U.S. patent application Ser. No. 16/364,180, filed Mar. 26, 2019, entitled: “Optoelectronic Device Based on a Surface-Trapped Optical Mode”. The aforementioned application is hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION Field of the InventionThe invention pertains to the field of optoelectronic devices. More particularly, the invention pertains to light emitting devices.
Description of Related ArtThere is a need in wavelength-stabilized semiconductor diode lasers for a wide range of applications, such as pumping of erbium- and ytterbium-doped fibers at about 975 nm, solid state laser pumping at 808 nm, frequency conversion at 1060 and 920 nm, and in telecommunications for wavelength division long haul transmission in multiplexing. In conventional edge-emitting lasers, the emission spectrum is broad and the wavelength shift of the lasing wavelength with temperature is governed by the thermal shift of the semiconductor energy band gap. For ˜1 μm range GaInAs/GaAs quantum well laser, the shift is about 0.4 nm/K. This makes their applications in telecommunication and longer reach datacommunication applications at high frequency impossible.
Conventional approaches to achieve wavelength-stabilized operation include rather expensive external cavity optical disk lasers or relatively low power single mode distributed feedback (DFB) lasers or volume fiber grating-coupled lasers.
Recently, all-epitaxial approaches to wavelength-stabilized semiconductor diode lasers have been developed. Tilted cavity laser (TCL) was disclosed in the U.S. Pat. No. 7,031,360, entitled “TILTED CAVITY SEMICONDUCTOR LASER (TCSL) AND METHOD OF MAKING SAME”, filed Feb. 12, 2002, issued Apr. 18, 2006, and in the U.S. patent application Ser. No. 11/194,181, entitled “TILTED CAVITY SEMICONDUCTOR DEVICE AND METHOD OF MAKING SAME”, filed Aug. 1, 2005, published online Dec. 15, 2005, publication US2005/0276296, both invented by the inventors of the present invention, whereas both are hereby incorporated herein in their entirety by reference. Tilted cavity laser is based on the resonant interaction between an antiwaveguiding cavity for the optical mode and a narrow-stopband multilayer interference reflector (MIR) supporting the wavelength-stabilized lasing at a certain tilt angle of the light in the MIR, and thus, at a certain wavelength of the cavity mode. The thermal shift of the wavelength can be controlled and completely temperature-insensitive lasing wavelength can be achieved. Implementation of the TCL concept is still challenging as it includes a need to grow epitaxially a complex, thick and difficult-to-calibrate MIR-structure to enable a narrow reflectivity peak of the MIR stopband required to provide a low leakage loss cavity mode only at a certain wavelength.
Tilted wave laser (TWL) was disclosed in the U.S. Pat. No. 7,421,001 entitled “EXTERNAL CAVITY OPTOELECTRONIC DEVICE”, filed Jun. 16, 2006, issued Sep. 2, 2008, and in the U.S. Pat. No. 7,583,712 entitled “OPTOELECTRONIC DEVICE AND METHOD OF MAKING SAME”, filed Jan. 3, 2007, issued Sep. 1, 2009, both invented by the inventors of the present invention, whereas both are hereby incorporated herein in their entirety by reference. TWL is based on the phase matching between the optical wave leaking from a thin active waveguide to a thick passive waveguide and returning back and the wave propagating along the active waveguide and thus provides wavelength-stabilized lasing. TWL emits laser light in two coherent vertical lobes allowing a quasi-Bessel-like self-healing beam. However, opposite to the TCL concept, the wavelength stabilization requires a weak coupling between the elements and the beam profile may become complex in case of wavelength-stabilized operation.
Recently, in the parent patent application U.S. Ser. No. 16/364,180, an optoelectronic semiconductor device was proposed based on a coupling between a resonant cavity mode of a vertical cavity surface-emitting laser (VCSEL) and a surface-trapped mode enabling in-plane-emitting semiconductor laser or semiconductor optical amplifier.
Since the resonant cavity mode of a VCSEL is wavelength-stabilized, modes coupled to it are also wavelength-stabilized. Therefore related in-plane-emitting devices will operate in a wavelength-stabilized mode. However, the structure is then excessively complex and can emit additional optical modes, unneeded for in-plane operation, such as the vertical cavity mode. To make the device emitting in an in-plane mode only is a challenging task.
Thus, there is a need in the art to employ a surface-trapped optical mode and to construct a significantly less complex wavelength-stabilized optoelectronic device.
SUMMARY OF THE INVENTIONAn in-plane-emitting semiconductor diode laser employs a surface-trapped optical mode existing at a boundary between a distributed Bragg reflector and a homogeneous medium, dielectric or air. The device can operate in both TM-polarized and TE-polarized modes. The mode exhibits an oscillatory decay in the DBR away from the surface and an evanescent decay in the dielectric or in the air. The active medium is preferably placed in the top part of the DBR close to the surface. The mode behavior strongly depends on the wavelength of light, upon increase of the wavelength the mode becomes more and more extended into the homogeneous medium, the optical confinement factor of the mode in the active medium drops until the surface-trapped mode vanishes. Upon a decrease of the wavelength, the leakage loss of the mode into the substrate increases. Thus, there is an optimum wavelength, at which the laser threshold current density is minimum, and at which the lasing starts. This optimum wavelength is temperature-stabilized, and shifts upon temperature increase at a low rate less than 0.1 nm/K, indicating wavelength-stabilized operation of the device. The approach applies also to semiconductor optical amplifiers or semiconductor gain chips which are also wavelength-stabilized. Reflectivity of the surface-trapped mode from an uncoated facet of the device can be extremely low, also <1E-4 or even <1E-5 which is particularly advantageous for amplifiers or gain chips. For diode lasers, a specific intermediate reflective coating can be deposited on the facet to put its reflectivity into a range from 0.5% to 3%, which lies within targeted values for lasers. An optical integrated circuit can employ wavelength-stabilized amplifiers operating in a surface-trapped mode, wherein such devices amplify light propagating along a dielectric waveguide.
In another embodiment of the present invention no dielectric is attached to the top DBR layer, and the DBR is bounded from the top by air.
In yet another embodiment of the present invention the optical gain is generated in the active medium due to photoexcitation.
In a further embodiment of the present invention the optical gain is generated in the active medium due to electron beam excitation.
To analyze optical modes in the laser structure (300) it is useful to specify the reference frame and the polarization of the optical mode.
The active region contains three quantum wells, each having a thickness of 8 nm, with 9 nm-thick spacer. The boundary between two layers is shifted with respect to the ideally periodic structure in order to place the quantum wells into a local maximum of the in-plane component of the electric field. The refractive indices of the quantum wells are taken 3.65 which refers to compressively strained Ga(1-x)In(x)As quantum wells having a refractive index higher than that of GaAs.
Magnetic field profile in the TM-polarized optical mode is found from the wave equation
Once magnetic field is found, the non-vanishing components of the electric field are found from the Maxwell's equations Ex(z)=−ik0−1ε−1(z)[dHy(z)/dz] and Ez(z)=−neffε−1(z)Hy(z), where k0=2π/λ. We note that |Ex(z)| has local maxima where Hy(z) and Ez(z) have nodes. The active region is placed at a local maximum of |Ex(z)|. We note that, according to Maxwell's equations, the z-component of the electric displacement field Dz(z)=ε(z)Ez(z) is continuous at the interfaces, and the electric field component Ez(z) exhibits discontinuities. In particular, |Ez(z)| in the air close to the surface increases versus its value in the DBR by a factor of ε1=n12=12.25.
It should be noted that the structure presented in
where m is an integer number, and a resonant cavity in a tilted cavity laser disclosed in U.S. Pat. No. 7,031,360 and U.S. Ser. No. 11/194,181, would have a thickness
No resonant cavity of this kind is present in the structure disclosed in the present patent application.
The strength of interaction of the optical mode with the active region is characterized by the optical confinement factor. The formula for the optical confinement factor for the TM-polarized optical mode was derived in the papers by Y.-Zh. Huang, Zet al. “Analysis of the optical confinement factor in semiconductor lasers,” Journal of Applied Physics, volume 79, issue 3, pages 3827-3830 (1996), and T. D. Visser, et al., “Confinement factors and gain in optical amplifiers,” IEEE Journal of Quantum Electronics, volume 33, issue 10, pages 1763-1766 (1997), whereas both papers are hereby incorporated herein in their entirety by reference. The optical confinement factor of the TM-polarized mode equals
where the integration in the denominator is taken over the DBR structure and the air. It follows from Eq. (4) that a significant enhancement of the electric field in the air results in a decrease of ΓTM.
The latter has a strong implication on the facet reflectivity. The facet reflectivity, calculated in the Fresnel approximation equals
A one skilled in the art will agree, that the Fresnel approximation gives with a good accuracy the facet reflectivity for a conventional edge-emitting laser, in which the optical mode is entirely located within the semiconductor structure, and the effective mode index lies within the interval of the indices of the semiconductor materials, e. g., between 3.0 and 3.5. For the structure considered in
Extremely low facet reflectivity, say, below 10−3, is advantageous for a semiconductor optical amplifier or for a semiconductor gain chip used in an external cavity system, where it is necessary to suppress lasing from an alone standing semiconductor chip. For using a structure as an edge-emitting laser, the front facet reflectivity should be preferably in a range of a few percent, e. g., between 0.5% and 3%.
One should note that a conventional semiconductor edge-emitting laser typically generates an optical mode having an effective mode index within the interval of the refractive indices of semiconductor materials, say, between 3.0 and 3.5. The reflectivity of an as-cleaved, uncoated facet equals approximately ˜0.3. Then a high reflective coating is applied to the rear facet to increase its reflectivity to a level >95%, or, more preferably, to a level >98%. An anti-reflective coating is applied to a front facet to decrease its reflectivity to a level of several percent, e. g., between 0.5% and 3%. Intermediate reflective coating is specifically needed if a laser is configured to operate in a surface-trapped mode.
The modeled coating has the thickness of the first layer D1diel=260 nm and the thickness of the second layer D2diel=31 nm.
It follows from
It should be noted that if the DBR is bounded from the top not by the air, but by a dielectric, an as-cleaved uncovered facet also exhibits strong anti-reflective properties. For example, if the top dielectric material is SiO2 with the refractive index 1.45, Eq. (5) gives 3.4% of the facet reflectivity.
To determine the wavelength and current density at which the lasing starts we consider the criterion of the lasing threshold
where gth is the threshold material gain in the QWs, L is the chip length, and R1 and R2 are the reflectivities of the front and the rear facets, respectively. The leakage loss to the substrate is related to the outgoing wave in the substrate,
The modal absorption loss is connected with the absorption coefficient in the material α(z) via an integral similar to (5),
where z=0 refers to the substrate level and z=z0 is the top surface of the semiconductor structure. The absorption coefficient in the semiconductor materials is assumed α(z)=1 cm−1.
To evaluate external loss of the last term on r. h. s. of Eq. (4) we consider a chip length L=4 mm, a high reflective coating on the rear facet R2≈1 and an intermediate reflective coating on the front facet R1≈0.02. To address the threshold current density we apply a logarithmic dependence of the gain in QWs versus current density
Parameters of Eq. (9) for the quantum wells emitting at wavelengths ˜1 μm are extracted from experimental data given in the paper by P. Crump, et al., “975 nm high power diode laser with high efficiency and narrow vertical far field enabled by low index quantum barriers,” Applied Physics Letters, volume 96, issue 13, paper 131110 (2010), whereas the paper is hereby incorporated herein in its entirety by reference. Extracted values are g0=1190 cm−1 and J0=203 A/cm2.
The leakage loss in
The threshold material gain, according to Eq. (6) is governed by the wavelength dependencies of the optical confinement factor and of the total loss. As the wavelength increases, the main loss is the external loss, practically independent of the wavelength, the optical confinement factor ΓTM drops, and the threshold material gain rapidly increases. As the wavelength decreases, approaching the shortwavelength edge of the stopband, ΓTM depends only weakly on λ, while the leakage loss to the substrate increases drastically, leading to a rapid increase of the threshold material gain.
To characterize the dependence of the threshold gain and threshold current density on the wavelength (
Since material gain spectrum of the QWs is rather broad, the lasing wavelength at the threshold is governed by the minimum in the threshold current density in
A change in the top layer thickness leads to a shift of the wavelength, at which the threshold material gain and the threshold current density is minimum, i. e. a shift of the wavelength, at which lasing starts.
A change in the top layer thickness, from ˜0.8 d1 to d1 is about 14 nm, which indicates that it is certainly possible with the present epitaxial technology, to meet this interval of thicknesses and, furthermore, a targeted thickness within this interval. Additional possibility of adjustment includes depositing a thin layer of a dielectric. For example, SixNy with the index n*˜2 can be used. To keep the same optical thickness of the deposited layer, the dielectric layer used for adjustment will be √{square root over ((n12−1)/(n*2−1))}≈1.9 times thicker than the corresponding semiconductor layer, which makes adjustment even easier. Furthermore, by varying the thickness of the deposited dielectric layers across the semiconductor wafer one can realize multiwavelength devices on the same wafer.
One of key important implications of the wavelength-stabilized operation of a laser is a significantly reduced thermal shift of the lasing wavelength as compared to that in a conventional edge-emitter. To address the effect of temperature on the surface-trapped mode, we assume that the gain spectrum of the active medium is much broader than the spectral widths in
The features
A one skilled in the art will appreciate that various types of the active medium can be employed in the device. Apart of multiple quantum wells as in the embodiment of
Once a particular embodiment of a semiconductor laser is discussed in connection to the embodiment of
It was noted in the parent patent application Ser. No. 16/364,180, that the DBR having a perfect periodicity and thus, the top layer thickness equal dtop=d1 does not localize at the surface a TE-polarized optical mode. It was however shown, that a significant increase in the top layer thickness, and using a layer with a thickness dtop=λ0/(2n1) allows to localize a TE-polarized surface-trapped mode.
To model TE-polarized modes, which obey the wave equation for the single non-vanishing component of the electric field Ey(z)
a laser multilayer structure similar to that of
leakage loss of the TE-polarized mode equals
and absorption loss equals
The dependence of the optical confinement factors of the TE modes versus wavelength presented in
Leakage loss of the TE modes in
Correspondingly, the threshold material gain in
A one skilled in the art will appreciate that the similar structures localizing surface-trapped TM- or TE-polarized optical mode can be used as semiconductor optical amplifiers. The transparence material gain obeys equation similar to (6),
ΓTM/TEgtransparency=αleakageαabsmodal, (14)
and the transparency current density can be found from
Then the same effect of the interplay of an optical confinement factor decreasing upon increasing wavelength and a leakage loss increasing upon decreasing wavelength results in a stabilization of the wavelength, the transparency current at which is minimum. This wavelength also shifts upon temperature increase at the same low rate ˜0.07 nm/K.
Once a semiconductor optical amplifier operates at a fixed current density Joperation, the dimensionless gain of the amplifier
where L is the chip length, also has a maximum at a certain wavelength. This wavelength also shifts upon temperature increase at the same low rate ˜0.07 nm/K. Thus, a further embodiment of the present invention is a wavelength-stabilized semiconductor optical amplifier.
A lower fraction of the optical power accumulated in the air for the TE-polarized mode (
A one skilled in the art will appreciate, that, instead of a DBR, a different type of a multilayer interference reflector (MIR) can be used, in which the surface-trapped mode will exhibit and oscillatory decay away from the surface.
All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.
Although the invention has been illustrated and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiments set out above but to include all possible embodiments which can be embodied within a scope encompassed and equivalents thereof with respect to the feature set out in the appended claims.
Claims
1. An in-plane emitting optoelectronic device comprising
- a) a multilayer interference reflector bounded from the top by a homogeneous medium having a refractive index lower than the refractive index of the topmost layer of said first multilayer reflector;
- b) an active medium located within said multilayer interference reflector;
- c) a substrate contiguous to said multilayer interference reflector at a side opposite to said homogeneous medium; wherein said multilayer interference reflector localizes a surface-trapped optical mode; wherein said surface-trapped optical mode i) is localized at a boundary between said multilayer interference reflector and said homogeneous medium; and ii) exhibits an evanescent decay in said homogeneous medium away from said boundary; and iii) exhibits an oscillatory decay in said multilayer interference reflector away from said boundary; and iv) the optical confinement factor of said surface-trapped optical mode decreases upon increase in wavelength of light; and v) the leakage loss of said surface-trapped optical mode into said substrate increases upon decrease in wavelength of light; and wherein said optoelectronic device operates in a wavelength-stabilized optical mode.
2. The optoelectronic device of claim 1,
- wherein said optoelectronic device contains no resonant cavity.
3. The optoelectronic device of claim 1,
- wherein said homogeneous medium is selected from the group consisting of: a) a dielectric material; and b) air.
4. The optoelectronic device of claim 1,
- wherein said surface-trapped optical mode is selected from the group consisting of: i) a TM-polarized optical mode; and ii) a TE-polarized optical mode.
5. The optoelectronic device of claim 1,
- wherein a fraction of the optical power of said surface-trapped optical mode accumulated in said homogeneous medium is above ten percent.
6. The optoelectronic device of claim 5,
- wherein a fraction of the optical power of said surface-trapped optical mode accumulated in said homogeneous medium is above thirty percent.
7. The optoelectronic device of claim 6,
- wherein a fraction of the optical power of said surface-trapped optical mode accumulated in said homogeneous medium is above fifty percent.
8. The optoelectronic device of claim 1,
- wherein the wavelength of said wavelength-stabilized surface-trapped optical mode shifts upon temperature increase at a rate below zero point one nanometer per Kelvin.
9. The optoelectronic device of claim 1,
- d) further comprising a front facet;
- wherein said front facet is an uncoated front facet, and
- wherein said front facet has a reflectivity of said uncoated front facet below four percent.
10. The optoelectronic device of claim 9,
- wherein the facet reflectivity of said uncoated front facet is below one times ten to the power minus three.
11. The optoelectronic device of claim 10,
- wherein the facet reflectivity of said uncoated front facet is below one times ten to the power minus four.
12. The optoelectronic device of claim 11,
- wherein the facet reflectivity of said uncoated front facet is below one times ten to the power minus five.
13. The optoelectronic device of claim 10,
- e) further comprising an intermediate reflective coating deposited on said front facet.
14. The optoelectronic device of claim 1,
- wherein said active medium is placed in said multilayer interference reflector at a distance from said boundary not exceeding one micrometer.
15. The optoelectronic device of claim 14,
- wherein said active medium is placed in said multilayer interference reflector at a distance from said boundary not exceeding zero point five micrometer.
16. The optoelectronic device of claim 1, further comprising
- a means of generating non-equilibrium carriers in said active medium.
17. The optoelectronic device of claim 16,
- wherein said means of generating of non-equilibrium carriers is selected from the group consisting of: (i) current injection; (ii) photoexcitation; and (iii) electron beam excitation.
18. The optoelectronic device of claim 1, further comprising a p-n junction;
- wherein said active medium is located within said p-n junction.
19. The optoelectronic device of claim 1,
- wherein said active medium is selected from a group consisting of:
- i) a bulk material;
- ii) single or multiple quantum well;
- iii) single or multiple sheet of quantum wires;
- iv) single of multiple sheet of quantum dots; and
- v) any combination of i) through iv).
20. The optoelectronic device of claim 1,
- wherein said multilayer interference reflector and said active medium are formed of semiconductor materials selected from the group consisting of: (i) III-V materials; (ii) III-N materials; (iii) II-VI materials; (iv) group IV materials; and (v) any combination of (i) through (iv).
21. The optoelectronic device of claim 1,
- wherein said optoelectronic device is selected from the group consisting of: (i) edge-emitting laser; (ii) semiconductor optical amplifier; and (iii) semiconductor gain chip.
22. An optical integrated circuit, comprising
- (i) least one optoelectronic device according to claim 1; and
- (ii) a planar optical waveguide.
23. The optical integrated circuit of claim 22,
- wherein said at least one optoelectronic device according to claim 1 is a semiconductor optical amplifier.
24. The optical integrated circuit of claim 23,
- wherein said planar optical waveguide (2270) is formed of a first dielectric material;
- wherein a second dielectric material (2480) is attached in the lateral plane to at least one facet of said semiconductor optical amplifier;
- wherein the absolute value of a difference in refractive indices between the dielectric in the dielectric waveguide (2270) and the dielectric (2480) in the spacers is below zero point zero two;
- wherein the facet reflectivity of said at least one facet of said semiconductor optical amplifier is below one times ten to the power minus three.
Type: Application
Filed: Oct 17, 2019
Publication Date: Oct 1, 2020
Applicant: VI SYSTEMS GmbH (Berlin)
Inventors: Vitaly SHCHUKIN (Berlin), Nikolay LEDENTSOV (Berlin)
Application Number: 16/656,323