Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-06-18
2004-04-27
Eckert, George (Department: 2815)
Coherent light generators
Particular active media
Semiconductor
C372S017000, C372S046012
Reexamination Certificate
active
06728282
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to intersubband (ISB) optical devices and, more particularly, to engineering the gain/loss profile of ISB devices in order to realize an optical device having a predetermined function or characteristic.
BACKGROUND OF THE INVENTION
The term ISB optical device embraces light emitters (e.g., lasers, spontaneous emitters) as well as light absorbers (e.g., photodetectors). Thus, an ISB light emitter may include a single stage, non-cascaded device of the type described in a paper by C. Gmachl et al.,
Appl. Phys. Lett
., Vol. 73, No. 26, pp. 3830-3832 (December 1998), which is incorporated herein by reference. The term may also include a multiple stage, cascaded device of the type described in the F. Capasso et al.,
Solid State Communications
, Vol. 102, No. 2-3, pp. 231-236 (1997) and J. Faist et al.,
Science
, Vol. 264, pp. 553-556 (1994), which are also incorporated herein by reference; i.e., a quantum cascade (QC) device that includes a multiplicity of essentially identical repeat units (or active regions). Each active region, in turn, includes a multiplicity of essentially identical radiative transition (RT) regions and a multiplicity of essentially identical injection/relaxation (I/R) regions interleaved with the RT regions. The RT regions, which include quantum well regions interleaved with barrier regions, as well as the I/R regions each comprise a multiplicity of semiconductor layers. At least some of the layers of each I/R region are doped, but in any case the I/R regions as well as the RT regions are unipolar. In addition, the term ISB active region is intended to embrace both diagonal radiative (i.e., optical) transitions as well as vertical optical transitions. A diagonal transition involves optical transitions between upper and lower energy levels or states where the wave functions (the moduli squared) corresponding to the levels are substantially localized in different quantum wells of the same RT region for all repeat units. See, also F. Capasso et al., U.S. Pat. No. 5,457,709 issued on Oct. 10, 1995, which is incorporated herein by reference. On the other hand, in the case of a vertical transition the excited and lower energy states are both substantially localized in the same quantum well of a single RT region for all repeat units. See, F. Capasso et al., U.S. Pat. No. 5,509,025 issued on Apr. 16, 1996, which is also incorporated herein by reference. Both types of transitions are also described in the article by F. Capasso et al., supra. This article, as well as the '025 patent, point out that the I/R regions of a vertical transition QC laser may include minibands and a minigap between the minibands to form an effective Bragg reflector for electrons in the excited state and to ensure swift electron escape from the lower states.
In addition, the source may be designed to operate at a single center wavelength, as in the papers discussed above, or it may operate in multiple wavelengths as described, for example, by A. Tredicucci et al.,
Nature
, Vol. 396, pp. 350-353 (November 1998) and by F. Capasso et al., U.S. Pat. No. 6,148,012 issued on Nov. 14, 2000, both of which are incorporated herein by reference.
Yet another form of ISB optical device is known as a superlattice (SL) laser in which the wavefunctions of the laser levels are spread over a multiplicity of quantum wells within each RT region. Laser action is achieved through unipolar injection by inter miniband tunneling. See, G. Scamarcio et al.,
Science
, Vol. 276, pp. 773-776 (May 1997), which is incorporated herein by reference. The SLs may be pre-biased as described by A. Tredicucci et al.,
Appl. Phys. Lett
., Vol. 73, No. 15, pp. 3101-3103 (October 1998) and F. Capasso et al., U.S. Pat. No. 6,055,254 issued on Apr. 25, 2000, both of which are also incorporated herein by reference.
One characteristic of the ISB devices described above is the cascading scheme in which electrons traverse a stack of many (e.g., 30-100) essentially identical active regions. As noted above, in the prior art all stages of the cascade (i.e., all of the repeat units) are essentially identical to one another. We refer to such devices as having a homogeneous cascade. Although laser based on homogeneous cascades exhibit low threshold currents, high average power outputs and large slope efficiency, to date they have been limited to applications in which they function as narrow-band light emitters, typically mid-infrared lasers.
Thus, a need remains in the art for an approach that more fully exploits the capabilities of ISB devices, and in particular enables them to be employed to generate essentially arbitrary gain/loss profiles to satisfy a broad range of applications.
SUMMARY OF THE INVENTION
In accordance with one aspect of our invention, an optical device includes a stack of at least two different ISB optical sub-devices in which the gain/loss profiles of the individual ISB sub-devices are mutually adapted, or engineered, so as to generate a predetermined overall function for the combination. We define this combination device as being heterogeneous since not all of the individual ISB sub-devices are identical to one another and refer to it hereinafter as a HISB device. Illustratively, the parameters of each individual ISB sub-device that might be subject to this engineering process include: the peak energy of the ISB optical transitions (emission or absorption) associated with each RT region, the position of each sub-device in the stack; the oscillator strengths of these ISB transitions; the energy bandwidth of each transition; and the total length of the RT and I/R regions of each ISB sub-device. In one embodiment, our approach may be used to engineer a gain profile that has peaks at a multiplicity of different wavelengths, thus realizing a multi-wavelength HISB optical source in which the applied electric field self-proportions itself so that each individual ISB sub-device experiences the appropriate field strength for its particular design. Alternatively, the gain profile may be engineered to be relatively flat over a predetermined wavelength range. In another embodiment, our approach may be used to generate a function that compensates for a characteristic of another device. For example, our HISB device may be engineered to have a gain profile that compensates for the loss profile of another device. Alternatively, the gain/loss profile may be engineered to produce a nonlinear refractive index profile in our device that compensates for that of another device (e.g., an optical fiber).
REFERENCES:
patent: 5126803 (1992-06-01), Hager et al.
patent: 5457709 (1995-10-01), Capasso et al.
patent: 5509025 (1996-04-01), Capasso et al.
patent: 6055254 (2000-04-01), Capasso et al.
patent: 6148012 (2000-11-01), Capasso et al.
patent: 6324199 (2001-11-01), Capasso et al.
patent: 6463088 (2002-10-01), Baillargeon et al.
A. Tredicucci et al.,A multiwavelength. . . , Nature, vol. 396, pp.350-353 (Nov. 1998).
C. Gmachl et al.,Dependence of the Device. . . , IEEE J. of Selected Topics in QE, vol. 5, No. 3, pp. 808-816 (May/Jun. 1999).
C. Sirtori et al.,Quantum cascade. . . , Appl. Phys. Lett., vol. 66, No. 24, pp. 3242-3244 (Jun. 1995) (May 1998).
J. Faist et al.,High power mid-infrared. . . , Appl. Phys. Lett., vol. 68, No. 26, pp. 3680-3682 (Jun. 1996) (May 1998).
G. Scamarcio et al.,High-Power Infrared. . . , Science, vol. 276, pp. 773-776 (May 1997).
Tredicucci et al.,High performance interminiband. . . , Appl. Phys. Lett., vol. 73, No. 15, pp. 2101-2103 (Oct. 1998).
C. Gmachl et al.,Noncascaded intersubband. . . , Appl. Phys. Lett., vol. 73, No. 26, pp. 3830-3832 (Dec. 1998).
F. Capasso et al.,Infrared(4-11&mgr;m) . . . , Solid State Comm., vol. 102, No. 2-3, pp. 231-236 (1997).
Capasso Federico
Cho Alfred Yi
Colombelli Rafaelle
Gmachl Claire F.
Ng Hock Min
Eckert George
Lucent Technologies - Inc.
Nguyen Joseph
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