The present disclosure relates to laser diodes characterized by multi-wavelength emission and, more particularly, to distributed Bragg reflector (DBR) quantum cascade (QCL) laser diodes. The present disclosure also relates to the use of such lasers as a mid-IR tunable source in the identification of molecular compositions in, for example, gas sensing and medical diagnostics, although the concepts of the present disclosure will enjoy broad applicability in a variety of fields.
The present disclosure is directed to multi-wavelength DBR QCL products that can be operated to generate several wavelengths sequentially in time. The resulting emission can be used, for example, to sample a broad absorption line. Particular embodiments of the present disclosure are limited to uni-polar QCLs, which use inter-sub-band transitions to produce photons, but it is also contemplated that the concepts of the present disclosure can be adapted for use with bi-polar lasers, which use inter-band transitions to produce photons.
In accordance with one embodiment of the present disclosure, a multi-wavelength distributed Bragg reflector (DBR) laser diode is provided comprising front and rear DBR sections and a plurality of dedicated tuning signal control nodes. The front DBR section comprises a plurality of front wavelength selective grating sections defining a plurality of distinct grating periodicities Λ1*, Λ2* . . . corresponding to distinct Bragg wavelengths λS1*, λS2* . . . . The rear DBR section comprises a plurality of rear wavelength selective grating sections defining a plurality of distinct grating periodicities Λ1, Λ2 . . . corresponding to distinct Bragg wavelengths λS1, λS2 . . . . The plurality of dedicated tuning signal control nodes are associated with individual ones of the front wavelength selective grating sections, individual ones of the rear wavelength selective grating sections, or both, and are constructed such that one or more tuning signals applied to one or more of the dedicated tuning signal control nodes spectrally aligns distinct Bragg wavelengths a selected one of the distinct Bragg wavelengths λS1*, λS2* . . . of the front DBR section with a selected one of the distinct Bragg wavelengths λS1, λS2 . . . of the rear DBR section.
The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
The general structure of a multi-wavelength DBR laser diode 10 according to the present invention is illustrated in
As will be appreciated by those familiar with DBR lasers, a DBR section of a DBR laser comprises Bragg gratings, i.e., a light-reflecting device based on Bragg reflection by a periodic structure. The periodic structure of the DBR section defines the Bragg wavelength λB of the laser. The front and rear DBR sections 20, 30 of the present disclosure do not rely upon periodic or aperiodic shifts in the grating phase Φ or chirped grating periodicities to generate multiple wavelength selection capabilities. Further, the respective reflectivity peaks of the front and rear DBR sections 20, 30 are spaced such that they do not overlap each other, although individual reflectivity peaks of the front DBR section 20 can be tuned to match a selected reflectivity peak of the rear DBR section 30, as will be explained in detail below.
The present disclosure is directed to the particulars of the front and rear DBR sections 20, 30. The respective structures of the waveguide core 45, the associated waveguide layers, the gain and phase sections 40, 50, and the anti-reflection coatings can be gleaned from readily available teachings in the art. As is illustrated in
As is illustrated schematically in
Although, in the embodiment illustrated in
As a further example, it is contemplated that one or more of the distinct Bragg wavelengths λS1*, λS2* . . . could be spectrally aligned with respect to the distinct Bragg wavelengths λS1, λS2 . . . in the “un-tuned” state. In which case, in a “tuned” state, one or more tuning signals could be applied to the dedicated front tuning signal control nodes 25 or rear tuning signal control nodes 30 to alter selected ones of the distinct Bragg wavelengths λS1*, λS2* . . . such that all but one of the distinct Bragg wavelengths λS1*, λS2* . . . are spectrally misaligned with respect to the distinct Bragg wavelengths λS1, λS2 . . . This configuration and procedure is illustrated in
As is further illustrated in
In cases where the front and rear tuning signal control nodes 25, 35 comprise thermal tuning nodes, e.g., micro-heater elements, it will typically be advantageous to ensure that each of the distinct Bragg wavelengths λS1*, λS2* . . . are shorter than the distinct Bragg wavelengths λS1, λS2 . . . so that a temperature increase initiated by one of the front thermal tuning nodes will increase the corresponding tuning wavelength to bring it into alignment with the target emission wavelength. It is also contemplated that the front and rear tuning signal control nodes 25, 35 may comprise electrical contacts for direct current injection to the front and rear wavelength selective grating sections. Finally, it is contemplated that individual ones of the tuning signal control nodes 25, 35 could be operated together, as a single control node, depending upon the operational demands of the particular application.
In cases where the gain section 40 of the laser diode 10 is characterized by a wavelength-dependent optical gain spectrum it will typically be preferable to arrange the front and rear wavelength selective grating sections of the front and rear DBR sections 20, 30 such that grating sections corresponding to reflectance peaks in relatively low gain portions of the optical gain spectrum are positioned relatively close to the gain section 40 of the laser diode 10, while grating sections corresponding to reflectance peaks in relatively high gain portions of the optical gain spectrum are positioned relatively far from the gain section 40 of the laser diode 10.
The waveguide core 45 of the laser diode 10 may comprise a stack of quantum cascade cores and each quantum cascade core may be configured to define a gain peak approximating one of the distinct Bragg wavelengths λS1, λS2 . . . of the rear wavelength selective grating sections. Alternatively, the waveguide core 45 of the laser diode 10 may comprise a single quantum cascade core with a gain spectrum that is broad enough to encompass the distinct Bragg wavelengths λS1, λS2 . . . of the rear wavelength selective grating sections. In many cases, the gain section 40 of the laser diode 10 will be characterized by a wavelength-dependent optical gain spectrum. To account for this, it is contemplated that quantum cascade cores with relatively low optical gains can be placed relatively close to the center of the optical mode of propagation of the laser diode 10, while quantum cascade cores with relatively high optical gains can be placed relatively far from the center of the optical mode of propagation of the laser diode 10. Alternatively, or additionally, cores with relatively low optical gain can be constructed with a greater number of stages or higher confinement factors, and cores with relatively high optical gain can be constructed with a fewer number of stages or lower confinement factors. As a further alternative, it is contemplated that shorter wavelength cores can be placed near the center of the waveguide core 45, with longer wavelength cores outside, because optical mode size at longer wavelengths is larger than at relatively short wavelengths.
Preferably, the waveguide core 45 of the laser diode 10 comprises a uni-polar QCL using inter-sub-band transitions to produce photons. However, it is also contemplated that the waveguide core 45 of the laser diode 10 may comprise a bi-polar laser using inter-band transitions to produce photons.
For example, and not by way of limitation, in one implementation of the concepts of the present disclosure, the distinct Bragg wavelengths λS1, λS2 . . . are selected to be the sampling wavelengths of a relatively broad absorption line, i.e., approximately 150 cm−1 spectral width. To this end,
It is noted that terms like “preferably,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure.
For the purposes of describing and defining the present invention it is noted that the terms “substantially” and “approximately” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terms “substantially” and “approximately” are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it is noted that the various details disclosed herein should not be taken to imply that these details relate to elements that are essential components of the various embodiments described herein, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Rather, the claims appended hereto should be taken as the sole representation of the breadth of the present disclosure and the corresponding scope of the various inventions described herein. Further, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present disclosure are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects.
It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.”
This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/556,434 filed on Nov. 7, 2011.
Number | Date | Country | |
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61556434 | Nov 2011 | US |