The present invention relates generally to semiconductor lasers and more specifically to a semiconductor laser structure which coaxially emits light at two or more different wavelengths, whose active regions are appropriately structured to provided gain augmentation at a selected output wavelength or wavelengths.
Semiconductor lasers, also referred to as solid state lasers or diode lasers, are well known in the art. These devices are based on the p-n junction from semiconductors, and quantum electronics from lasers. The devices generally consist of a layered semiconductor structure having one or more active layers sandwiched between cladding layers and bounded at their ends by cleaved facets which act as semitransparent mirrors. An optical resonator, or so-called Fabry-Perot cavity is thereby formed. An electrical potential is applied across the one or more active layers. The voltage drives either holes or electrons or both across the p-n junction (i.e., they are "injected"), and when these carriers recombine they emit light. Optical feedback caused by internal reflection from the cleaved facets allows "stimulation" of the recombination to provide coherent emission.
As a general rule, a semiconductor laser of this type emits coherent light at a single wavelength, which is a function primarily of the semiconductor material in the laser's light emitting region. This single wavelength emission is sufficient for many applications, such as communication systems, laser xerography, and other applications where the device's small size, low operating current, and other characteristics are beneficial. However, there are a number of applications where it is necessary or desirable to be able to select or tune the output wavelength of the laser to one of two or more possible output wavelengths during operation of the laser. Furthermore, in certain circumstances, some tuning of the output wavelength of the laser may be required to optimize its applicability. This is due in part to the fact that it is not possible to know precisely what the predominant emission wavelength will be in the wavelength gain spectrum of the laser.
A number of laser structures have been suggested which allow multiple wavelength emission. Of these there are basically two type--coaxial and separate source lasers. A coaxial laser is one capable of emitting, either separately or simultaneously, two different wavelengths from the same point in a single layer. Emanation is in the form of a beam which, for both wavelengths, is along the same longitudinal axis and which may be imaged to a single spot at any distance from the laser structure. A separate source laser is one which emits at two or more different wavelengths, either separately or simultaneously, each wavelength emanating from a different point in the structure (such as from separate layers such as disclosed in U.S. Pat. No. 5,048,040, which is incorporate by reference herein; see also copending U.S. patent application Ser. No. 07/579,218, filed Sep. 5, 1990 (commonly assigned), which is also incorporated by reference herein). The outputs of separate source lasers have also been combined external to the laser cavity to form a single beam in which the outputs of different wavelength are made approximately coaxial. See, Aiki et al., Frequency Multiplexing Light Source with Monolithically Integrated Distributed-Feedback Diode Lasers, Appl. Phys. Lett., vol. 29, no. 8, p. 506 (1976). As will become apparent, coaxial multiple wavelength lasers are of primary concern herein, although the present invention may find applicability in separate source lasers.
Furthermore, the present invention is most applicable to quantum-well heterostructure lasers. Typically, a quantum-well heterostructure laser consists of, inter alia, a substrate upon which is formed or deposited a cladding layer, an active layer, another cladding layer, and appropriate electrical contacts to the various layers. Waveguiding layers may also be incorporated into the structure where appropriate. Commonly, these structures are comprised of active layers of GaAs, cladding layers of Al.sub.x Ga.sub.1-x As, where 0.ltoreq.x.ltoreq.1, and where employed, waveguiding layers of Al.sub.y Ga.sub.1-y As, where 0.ltoreq.y.ltoreq.1 and y&lt;x. There are two primary types of quantum-well heterostructure lasers, those whose active layer is comprised of a single quantum-well (SQW) and those whose active layer is comprised of multiple quantum-wells (MQW). As will become clear from the following description, the latter, MQW laser, is most relevant to the present invention.
It is known that the total energy E.sub.T of a charge carrier (e.g., an electron) in the quantum well is composed of its energy in the three orthogonal directions x, y, and z, and can therefore be written as EQU E.sub.T =E.sub.x +E.sub.y +E.sub.z ( 1)
where E.sub.i is the carrier energy in the ith direction relative to the energy at the bottom of the well. For the direction normal to the plane of the quantum well layer, denoted herein by z, the thickness of the semiconductor layer in which the carrier is confined is on the order of the carrier de Broglie wavelength (.lambda.=h/p, where h is Planck's constant and p is carrier momentum). For layers of this thickness one-dimensional quantization occurs thereby restricting the carrier's energy E.sub.z to discrete values E.sub.n with n=1, 2, 3, . . . . These levels (referred to herein as "quantum levels") are typically illustrated by a bound-state diagram such as that shown in FIG. 1 for a GaAs/Al.sub.x Ga.sub.1-x As quantum well structure. See, e.g., Holonyak et al., Quantum-Well Heterostructure Lasers, IEEE J. Quant. Elec., vol. QE-16, no. 2, p. 170, (1980). The active layer having quantized states is essentially an energy "well" whose depth is a function of the bandgap of the active layer material and whose width is equal to the thickness of the active layer. FIG. 1 diagrams a single quantum well structure. However, by forming the active layer to have a number of bandgap changes, for example, multiple quantum wells may be formed.
In many applications, the motion of the individual carriers in the x and y directions is allowed over distances much larger than the de Broglie wavelength and therefore is not quantized. In these nonquantized semiconductor layers, the energy components for the x and y directions are given by EQU E.sub.x =p.sub.x.sup.2 /2m.sub.e ( 2) EQU and EQU E.sub.y =p.sub.y.sup.2 /2m.sub.e ( 3)
where m.sub.e is the effective mass of the electron and p.sub.i is the component of the electron momentum in the ith direction. In certain applications, such as so-called quantum wire devices, motion in one or both of the x and y directions may be restricted such that quantum size effects occur. For a discussion of this type of device see U.S. Pat. No. 5,138,625 and incorporated by reference herein.
Carriers (e.g., electrons) are injected into the active layer with sufficiently high energy that they enter the conduction band with excess energy. It is known as a general feature of semiconductors that carriers injected into a layer at an initial energy level rapidly scatter downward in energy (thermalize) to ultimately occupy the unfilled energy state with lowest energy. Thus, electrons injected into the quantum well rapidly settle to the lowest energy states in the conduction band, i.e., states whose total energy, given by EQU E.sub.T =p.sub.x.sup.2 /2m.sub.e +p.sub.y.sup.2 /2m.sub.e +E.sub.1( 4)
is a minimum, where E.sub.1 is the lowest energy state in the z direction (i.e., the first quantum level). Note that the total energy in the quantum well can not be less than E.sub.1 due to the energy quantization. As the number of carriers in the quantum well is increased, electron states at increasingly higher energies are occupied as p.sub.x.sup.2 /2m.sub.e +p.sub.y.sup.2 /2m.sub.e increases. If the density of the injected carriers increases sufficiently, the total energy E.sub.T given by equation (4) becomes equal to E.sub.2, allowing carriers to occupy states at the next highest quantum level. This process of progressively filling unoccupied states of increasing energy is called "bandfilling" because the conduction band is filled with the increasing number of electrons and the valence band is filled with the increasing number of holes. Increasing the number of carriers further will eventually fill states with energy greater than E.sub.2 and thereby allow carriers to occupy states at the third quantum level E.sub.3 and so on. Similar bandfilling occurs with holes in the valence band.
When an electron in the conduction band and a hole in the valence band recombine they radiate their energy in the form of light. In a typical solid state laser, part of this light energy is emitted, while part is absorbed in the active layer. Part of the absorbed energy increases the carrier concentrations in the conduction and valence bands by photogeneration of electrons and holes. When more electrons and holes recombine than are created by absorbed light, the quantum well is said to have optical gain. When the gain of the quantum well is equal to the loss of the resonator, a point referred to as "threshold," the device begins to lase.
The wavelength of the light emitted in recombination is a function of the energy given up in recombination, as given by ##EQU1## When the level of carrier injection is relatively low (but above threshold) laser oscillation of wavelength .lambda..sub.1 corresponding to the recombination of carriers in quantum level E.sub.1 will occur in the resonator. If lasing in quantum level E.sub.1 is prevented while simultaneously increasing the injected current, the quantum well layer will bandfill, allowing carriers to occupy the second quantum level E.sub.2. When the injected current is increased sufficiently, laser oscillation of wavelength .lambda..sub.2 occurs.
A number of efforts have been undertaken to use bandfilling in SQW and MQW devices to obtain coaxial multiple wavelength emission. For example, Epler et al., in Broadband Tuning (.DELTA.E.about.100 meV) of Al.sub.1-x Ga.sub.1-x As Quantum Well Heterostructure Lasers With An External Grating, Appl. Phys. Lett., vol. 43, no. 8, p. 740 (1983) describe a bandfilled SQW diode laser tuned by a grating between wavelengths corresponding to the n=1 and n=2 transitions. Tokuda et al., in Emission Spectra of Single Quantum Well Lasers With Inhomogeneous Current Injection, J. Appl. Phys., vol. 64, no. 3, p. 1022 (1988), describe a split contact SQW laser which, by way of bandfilling, can provide gain over a large spectral region. Also, Tokuda et al., in Lasing Wavelength of An Asymmetric Double Quantum Well Laser Diode, Appl. Phys. Lett., vol. 51, no. 4, p. 209 (1987), describe bandfilling of a tailored double quantum well structure allowing the selection of the wavelength output from six possible allowed transitions. (N.B., lasing at each and every transition originates in both wells.) Ikeda et al., in Asymmetric Dual Quantum Well Laser-Wavelength Switching Controlled By Injection Current, Appl. Phys. Lett., vol. 55, no. 12, p. 1155 (1989) described use of the lowest energy levels in two wells of different composition to provide emission at two different wavelengths.
Efforts have also been undertaken to control, or tune, the output between one of two possible wavelengths. For example, Fang et al., in Longitudinal Mode Behavior and Tunability of Separately Pumped (GaAl)As Lasers, Appl. Phys. Lett., vol. 44, no. 1, p. 13 (1984) describe a laser structure having a separately contacted modulator region along the axis of the laser cavity allowing changing the carrier concentration, and hence the output wavelength over a limited range. Iwaoka et al., in U.S. Pat. No. 4,893,353, dated Jan. 9, 1990 and U.S. Pat. No. 4,912,526, dated Mar. 27, 1990 discuss varying the operating temperature of the laser in order to switch between various output wavelengths.
All of the above approaches suffer from a common disadvantage in that, as the laser is switched or shifted toward a shorter wavelength of operation the output intensity decreases. In order to compensate for this intensity decrease, the laser is often operated at an increased current. While this does allow an increase in output intensity, it also puts demands on the current source, increases heat generation resulting in degraded efficiency of operation, increases strain on the structure leading to shortened lifespan (similar also to the drawbacks of the thermal modulation discussed above), the large difference between the thresholds of the long and short wavelengths introduces large thermal transients when the wavelength is switched, etc.
In order to increase the output intensity of single frequency lasers it has been suggested that all of the wells of a multiple quantum-well laser be structured identically so that the quantum levels of all of the wells are aligned at the same energy bandgap. See Y. Arakawa and A. Yariv, Theory of Gain, Modulation Response, and Spectral Linewidth in AlGaAs Quantum Well Lasers, IEEE J. Quant. Elec., vol. QE-21, no. 10, p. 1666 (1985). In this way, the output of each of the wells acts in conjunction at the same wavelength to thereby increase the gain of the laser. No disclosure or suggestion is made as to compensation for the intensity drop in multiple wavelength lasers.
Additional disadvantages of various of the above-described methods and apparatus include: temperature cycling resulting from or required to vary the output wavelength accelerates the degradation of the performance of the laser, often to the point where the lifespan of the laser is significantly shortened; a number of the methods and apparatus are not adaptable to wavelength tuning during operation of the device (that is, once the diffraction grating is placed in the laser beam path it is not possible to practically and accurately adjust or change the grating or filter to alter the wavelength fed back to the laser); it is very difficult if not impossible to accurately heat only one laser of an array of lasers without affecting the neighboring lasers (this is also an issue when operating the laser in close proximity to other devices, such as transistors, when operating the laser in a small-scale integrated system); and, a number of the methods and apparatus require additional apparatus and control, for example to vary the laser's temperature.
There is a present need in the art for a coaxially emitting multiple wavelength laser capable of increased output intensity, especially at the higher output frequencies, which at the same time minimizes any increase in threshold current for the higher output frequencies. There is also an alternative need to be able to operate the multiple wavelength laser with a minimal but nonzero difference between the thresholds of the longest and shortest wavelengths of operation. Also needed in the art are lasers able to operate simultaneously on two widely separated wavelengths. Finally, it is desirable to be able to provide a multiple wavelength laser with improved matching of the internal optical gain to the fundamental (gaussian) laser mode TE.sub.00. As will be described below, various aspects of the present invention address these needs.