Semiconductor laser with integrated phototransistor for dynamic power stabilization

Abstract: A diode laser (82-90,92b,94b,96) of the buried heterostructure type with transverse injection and a lateral bipolar transistor structure (82-90,92a,94a) are monolithically and coaxially formed in the same set of semiconductor layers (72-80) to make an integral laser device that emits a constant level of optical power stabilized against unpredictable variations. The base region (86a) of the transistor structure is formed coaxially with the laser cavity (86b), so that laser optical power passes into or through the transistor base (86a). Electrical feedback is provided from the transistor to the laser via a resistor (110) between the collector (94a) of the transistor and the anode (96) of the laser, thereby controlling the current delivered by an external current source to the laser.

Description: The present invention relates generally to the field of semiconductor lasers, and more specifically to a semiconductor laser coaxially integrated with a phototransistor Semiconductor lasers generally consist of a planar layered semiconductor structure having one or more active layers bounded at their ends by cleaved surfaces that act as mirrors for the optical resonator. In one form of this structure a p-type optical waveguide for the lasing action can be formed by placing n-type regions on either side of the p-type active region, thereby forming an n-p-n laser structure. Similarly an n-type optical waveguide for the lasing action can be formed in an n-type active layer or layers, by placing p-type regions on either side of the n-type active region, thereby forming a p-n-p laser structure. Both of these structures are known as transverse junction lasers because the p-n junction lies across, i.e. transverse to, the active layers. The transverse n-p-n or p-n-p laser structure described above also functions as a bipolar transistor, as disclosed in detail in US-A-4,987,468 to Thornton. For example, FIG 1 shows the n-p-n structure fabricated by impurity-induced layer disordering (IID) on a semi-insulating substrate with the base, emitter, and collector contacts for the transistor all placed on the top surface of the semiconductor layers. Since the base region is necessarily kept very narrow, on the order of a few microns in width, the base contact is placed remote from the active base region with a conductive path allowed around and under one of the n-type regions. It is well known in the art that a transistor structure can be used as an optical detector with gain by introducing the incident light into the base region. In the phototransistor, excess electron-hole pairs are created in the base by absorption of the input light, the resulting current being proportional to the incident light power generated in the base is amplified by normal transistor action. Optical detectors monolithically integrated with diode lasers have been of considerable interest to those skilled in the art, as a way to monitor the level of power emitted by a diode laser, especially for monolithic arrays of independently addressed lasers. One approach, described by Merz, et al., in Integrated GaAs-Al x Ga 1-x As Injection Lasers and Detectors with Etched Reflectors , Applied Physics Letters vol. 30, No. 10, pp. 530-533, (1977), employs a detector region containing the same layer configuration as the laser, located outside the laser resonator but coaxial with the direction of lasing and coupled to the laser by an underlying passive waveguide layer. A second approach, described by Iga, et al., in GalnAsP/lnP Laser with Monolithically Integrated Monitoring Detector , Electronics Letters vol 16, pp. 342-343, (1980), employs a detector region containing the same layer configuration as the laser, separated from the laser by an etched groove. Both of these approaches employ in-line detectors which are outside the laser cavity and require at least one laser facet to be etched. One important application for a photodetector integrated with a diode laser is in a feedback loop that stabilizes the power emitted by the laser against unpredictable variations. Thermal fluctuations are especially deleterious to maintaining constant optical power output, especially during pulsed modulation. For example, heating of the laser chip unavoidably occurs when the applied laser current is abruptly increased at the beginning of a pulse. Since a laser's output power is temperature dependent, this time-dependent, or transient, heating normally causes the power output to decrease or "droop" during the pulse. Furthermore, transient heating during a sequence of pulses can have a cumulative effect on the temperature that depends on the number and frequency of the pulses. For example, if the time between successive pulses is large, the device will be given sufficient time to cool, so that the application of the driving current has a large temperature effect, i.e. droop, during the next pulse. The shorter the time between pulses, the less time the device has to cool between one pulse and the next, leading to a sustained increase in the temperature of the laser. This sustained temperature increase results in a further decrease in amplitude of the output pulse obtained with a constant level of input current. There is presently a need in the art for apparatus and methods which provide accurate and reproducible control of the optical energy contained in each pulse of an intensity-modulated light beam. In systems currently known to those skilled in the art, a predetermined amount of energy is delivered in each pulse by turning on the optical beam to a desired power level for a fixed time interval. This approach requires that the laser output power be reproducible from pulse to pulse and constant during a pulse, in order that the optical energy delivered in each pulse be accurately controlled. Accurate control is especially important in printing with different grey levels formed by varying the number of exposed spots or when exposing very closely spaced spots in order to control the formation of an edge. Power fluctuations can arise from many sources, including for example ambient thermal fluctuations, laser self-heating, laser degradation, fluctuations in the drive current, and/or the pattern of modulation. Since these fluctuations occur within each pulse and are very difficult to predict, control, or eliminate, they are commonly not compensated for. Thus there is presently a need in the art for apparatus and methods which provide accurate and reproducible control of the level of pulsed optical power emitted by an intensity-modulated diode laser. Each of the integrated detectors of the prior art discussed above is formed by a simple p-n junction diode. A p-n junction diode of this type provides an electrical signal proportional to the laser power but does not provide amplification of the photo-generated current. Consequently such a detector requires an additional stage of amplification to produce feedback control sufficient to maintain constant laser output in the presence of unpredictable variations in laser temperature, drive current, or modulation pattern. Additional amplification can be obtained from an off chip circuit or by monolithically incorporating a remote transistor on the chip, e.g. as described by Katz, et al., in a Monolithic Integration of GaAs/GaAlAs Bipolar Transistor and Heterostructure Laser , Applied Physics Letters vol. 37, pp. 211-213 (1980). However, instantaneous stabilization of the laser's output power requires high-speed electrical coupling from the detector to the transistor to the laser. Consequently, external circuitry is adequate to compensate slow variations of power but not for instantaneous stabilization during short pulse modulation such as encountered in laser printing systems. Similarly, with on-chip amplification, the required response times dictate that the transistor amplifier be physically located close to the laser/detector structure. Consequently, remote amplifying transistors are not suitable for use with more than two closely spaced lasers on a chip because the transistors must be placed too far from the laser/detector structure to allow adequately fast response. Accordingly, there is presently a need in the art for a compact monolithic apparatus emitting constant optical power either continuously or from pulse to pulse. Patent Abstracts of Japan, vol. 11, No. 187 (E-516), 16 June 1987, discloses a compound semiconductor device in which a semiconductor laser and a light receiving transistor are formed integrally with each other, so that the light emitting and receiving elements are monolithically formed in one unit and an electric signal received by the light-receiving element is amplified. JP-A-62086775 discloses a circuit for stabilising the optical output of a light emitting diode connected in parallel to a photo-transistor, whose collector is connected via a resistor to the anode of the diode. The light emitting diode and the photo-transistor are powered by a common voltage source. The present invention provides a semiconductor device comprising: a semi-insulating substrate having monolithically formed thereon with a common semiconductor layer structure a transverse junction laser within which is defined an optical cavity, and a heterojunction bipolar transistor, the optical cavity of the laser being axially aligned with a base region of the transistor such that optical output from the optical cavity impinges upon said base region, and means providing electrical connection between the anode of said laser and the collector of said transistor, whereby the optical output from said laser incident upon said base region causes a stabilizing feedback electrical current to be generated by said transistor and which is communicated to said laser which is powered by a separate current source, to thereby stabilize the optical output of said laser. The collector of the transistor is preferably connected via a resistor to the anode of the laser in order to form a feedback path for the transistor current to control the current delivered by a separate current source to the laser. The bias conditions and circuit parameters are chosen such that the laser emits the desired light power when pulsed on with a current pulse of appropriate magnitude. If the power output of the laser changes during the pulse, feedback from the transistor adjusts the current delivered to the laser to maintain the light output constant. For example, if the power output decreases from its set value, the collector current in the transistor decreases, thereby allowing the laser current to increase and return the output power to its set value. On the other hand, if the power output increases from its set value, the collector current in the transistor increases, thereby allowing the laser current to decrease and return the output power to its set value. The base region of the transistor may be internal to the optical cavity of the laser with the light being coupled from the lasing region into the transistor base (1) across a narrow region of inactive semiconductor material, or (2) by an integrally formed, passive two dimensional optical waveguide. Alternatively, the base region of the transistor is external to the optical cavity of the laser with the light being coupled from the laser cavity into the transistor base (1) through a distributed grating reflector that functions as one mirror of the laser cavity, or (2) across a narrow groove etched in the semiconductor layers. The present invention furthr provides an array of integrated laser/transistor structures according to claim 6 of the appended claims, the structure being formed in a single set of semiconductor layers on a single substrate. The output power from each integrated structure in this monolithic array is separately controlled by its independent data signal and simultaneously stabilized against unpredictable variations by the integrated feedback control provided by the phototransistor. Such integrated control is advantageous because it requires no high speed connections to the chip and because it coaxially integrates the transistor with the laser, thereby allowing the light emitting regions to be closely spaced. The present invention further provides a rotary output scanning (ROS) apparatus according to claim 7 of the appended claims. In operation, a light beam is generated by the light source which is modulated in response to an image data signal. The light beam is scanned across at least a portion of a surface of the image plane means in a fast scan direction, as well as scanned across at least a portion of a surface of the image plane means in a slow scan direction which is normal to the fast scan direction. During the scanning, the image data signal modulates the light beam on and off to form distinct or overlapping spots of exposure on the surface of the image plane. The energy used to form each spot can be accurately controlled by the duration of the optical pulse because the pulse amplitude is stabilized against unpredictable variations. The pulse amplitude may be held fixed from spot to spot so that spots of constant size are formed on the surface of the image plane or may be changed from spot to spot so that spot size on the surface of the image plane varies in a desired way. The control of optical energy provided by the present invention makes the areal size and/or illumination level of the exposure on the image plane independent of unknown or unpredictable variations in the amplitude of the optical power emitted by the optical source. Such independence is especially important in multiple beam ROS employing a monolithic optical source composed of closely spaced emitters, where the modulation of one emitter induces an unpredictable variation of power emitted by the other source. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 shows a side or elevation view of a lateral heterojunction bipolar transistor of the prior art; FIG. 2 shows a side or elevation view of a one embodiment of the integrated transistor/laser structure of this invention with light coupled through an inactive region of semiconductor layers; FIG 3 shows the electrical circuit for the embodiment of this invention shown in FIG 2; FIG 4 shows a side or elevation view of another embodiment of the integrated transistor/laser structure of this invention with light coupled through a passive two-dimensional optical waveguide; FIG. 5 shows a side or elevation view of yet another embodiment of the integrated transistor/laser structure of this invention where the transistor is separate from the laser by a distributed grating reflector; FIG 6 shows a side or elevation view of still another embodiment of the integrated transistor/laser structure of this invention where the laser has formed therein a distributed grating reflector; and FIG. 7 shows a side or elevation view of yet another embodiment of the integrated transistor/laser structure of this invention where the transistor is separated from the optical cavity of the laser by a narrow etched groove. In general, like reference numerals will be used to denote like elements as between each of the aforementioned figures. Reference is now made to FIG. 2, wherein there is shown a first embodiment of this invention comprising an optically integrated transistor/laser structure 70. Device 70 comprises a semiconductor support 72 which is a semi-insulating substrate or support layer that blocks electrical connection between conducting regions of the integrated structure. Epitaxially deposited on support 72 are at least three semiconductor layers 74, 76, 78 wherein layers 74 and 78 are of wider bandgap material than semiconductor layer 76. Also semiconductor layers 74, 76, 78 are formed of the same conductivity type, shown here for example as p-type. As an example support 72 may be a substrate of semi-insulating GaAs, layer 74 may be p-Ga 1-x Al x As, layer 76 may be p-GaAs or p-Ga 1-y Al y As and laye7 38 may be p-Ga 1-z Al z As where x≃z>y. The structure is completed with cap or ohmic contact layer 80 which may be p-GaAs. The integrated structure has charge carrier regions with wide bandgap formed by impurity enhanced layer interdiffusion, most commonly known as impurity induced disordering (IID), as disclosed, for example in US-A-4,639,275. As shown in FIG. 2, the selectively disordered regions 82 and 84 cause at least a partial disordering or interdiffusion of the compound semiconductor materials comprising layers 74-80 within those regions. In particular, layer 76 in regions 82 and 84 is converted into a wide bandgap alloy of materials comprising material from layers 74, 76, and 78, e.g. Ga 1-A Al A As where A≃z>y. Also if the disordering is accomplished with an n-type impurity, such as Si, regions 82 and 84 will be of n-type conductivity. The undisordered portion 86 of layer 76 between disordered regions 82 and 84 comprises a narrow bandgap layer or channel, e.g. of p-GaAs, bounded by wider bandgap undisordered portions of layers 74 and 78. The disordered alloy regions 82 and 84 also form an optical waveguide with region 86 as its core, suitable for use as the active region of a diode laser. Further, p-n heterojunctions 88 and 90 are formed, respectively, between n-type region 82 and p-type region 86 and between n-type region 84 and p-type region 86. As an example, layer 76 and consequently region 86 after completion of the disordering process, may be a narrow bandgap layer of 100 nm thickness, or may be sufficiently thin to form a quantum well layer and exhibit quantum size effects, e.g. 6 to 10 nm thick, or may be a multiple quantum well or superlattice structure of alternating thin layers of GaAs and AlAs or GaAs and GaAlAs. Contact to regions 82, 84, and 86 is accomplished by metallization electrodes 92a, 92b, 94a, 94b, and 96. A distinguishing feature of this invention is that the contacts to regions 82 and 84 are each divided into two electrically isolated sections, respectively, 92a/92b and 94a/94b. Electrodes 92a and 94a , respectively, contact one section 86a of regions 82 and 84, which function respectively as the emitter and collector regions of an n-p-n transistor. In one embodiment of this invention, the base region of the transistor has no contact since the transistor is operated open base. Electrodes 92b and 94b, respectively, contact the other section 86b of regions 82 and 84, and are operated in parallel to form the cathode of the laser. The sections 86a and 86b form the unitary, integrated region 86. In order to eliminate the effect of potentially competing heterojunctions in layer 76 and 80, proton implants are performed into regions 102, 104, and 106, to damage the crystal lattice thereby rendering these regions of high resistance. Proton implant regions 104 and 106 are also formed deep enough to isolate different devices on the same chip. A shallow proton implant in region 100 deactivates the parasitic base layer formed by the GaAs capping layer 80 between disordered regions 82 and 84. The anode of the laser is provided by electrode 96 which contacts a path for current down through the p-layers into layer 74 and under the proton bombarded region 102. The ohmic contact to the base under electrode 96 may be facilitated with a Zn diffusion into cap layer 80 (not shown in Fig. 2). Another distinguishing feature of this invention is that the optical waveguide 86 formed by disordered regions 82 and 84 comprises the amplifying region 86b of the laser structure and the base region 86a of the transistor. The amplifying region 86b of the laser is formed between the portion of region 82 contacted by electrode 92b and the portion of region 84 contacted by electrode 94b. The base region 86a of the transistor is formed between the portion of region 82 contacted by electrode 92a and the portion of region 84 contacted by electrode 94b. Thus the base region 86a of the transistor is an integral part of the laser resonator, such that optical power generated in the amplifying region 86b is optically coupled to and guided by the transistor base 86a. In the embodiment of Fig. 2, coupling between the amplifying region 86b and transistor base 86a is provided by the undisturbed waveguide 86 formed between regions 82 and 84 Optical power is absorbed in base region 86a to create electron-hole pairs. The electrons diffuse to and are collected by the reverse-biased collector junction 90, leaving excess holes in the p-type base that lower the forward-biased potential on the emitter-base junction 88, causing additional electrons to be injected and consequently an increased collector current. The transistor then operates as a phototransistor with a base current that is directly proportional to the instantaneous value of the laser's internal optical power and amplified by normal transistor action. The collector electrode of the transistor is connected to the anode of the laser via resistor 110 in order to form an automatic feedback control for the optical power output. For example, FIG. 3 shows electrical circuit 120 for providing automatic feedback control of the laser power output during pulse modulation. Pulsed current is supplied to laser 124 by laser driver 122, thereby turning on the laser to emit the desired optical power. The total current supplied by driver 122 to integrated structure 70 comprises current 128 supplied to laser 124 plus current 130 supplied to transistor 126. Laser 124 is optically coupled to transistor 126 by optical power 132. Consequently, if the power output of the laser changes with time, for example due to an increase in the ambient temperature or self-heating from the current pulse itself, electrical feedback from the transistor adjusts the portion of the driver current delivered to the laser, thereby maintaining a constant level of output power. For example, if the power output decreases from its set value, the collector current 130 in the transistor decreases, thereby allowing the laser current to increase and return the output power to its set value. Alternatively, if the power output increases from its set value, the collector current 130 in the transistor increases, thereby decreasing the laser portion 128 of the driver current to return the output povver to its set value. Other feedback arrangements are also possible, as will be appreciated by one skilled in the art. FIG. 4 shows a second embodiment 140 of this invention, wherein the phototransistor is optically coupled to the lasing region by a narrow, low loss waveguide 142 formed by impurity-induced disordering as disclosed in US-A-4,802,182. Other parts of integrated structure 140 are substantially identical to the parts in integrated structure 70 of FIG. 2 and, therefore carry identical numerical identification. Utilization of the low loss waveguide is advantageous because it allows the separation between the lasing section 86b of the structure, excited between electrodes 92b and 94b, and the transistor section of the structure, activated between electrodes 92a and 94a, to be increased compared to integrated structure 70, thereby increasing the electrical isolation between the phototransistor and the laser sections. For example, proton implants can be made between electrodes 92a and 92b and between 94a and 94b, thereby rendering these regions of hign resistance. FIG. 5 shows a third embodiment 160 of this invention, wherein the phototransistor is optically coupled to the laser structure but is formed outside of the laser resonator 86b. Integrated structure 160 is substantially identical to integrated structure 140 of FIG. 4 and, therefore like parts carry identical numerical identification. The distinguishing feature ot structure 160 is that the transistor is separated from the laser by a short region of low loss waveguide 142 containing a distributed Bragg reflector 162. The optical waveguide can be formed, for example, by IID, as disclosed in US-A-4,802,182. The distributed Bragg reflector 162 can be formed by providing a diffraction grating etched in the top cladding of the optical waveguide. The reflector provides enough reflectivity to establish one mirror of the laser cavity 86b while coupling a portion of the light into the base 86a of the phototransistor. FIG. 6 shows a variation of the embodiment shown in FIG. 5 in which a distributed Bragg reflector 164 is positioned not in a waveguide region, but in the active region 86b of the laser nearest the transistor. FIG. 7 shows a fourth embodiment of this invention, wherein the phototransistor is optically coupled to the laser structure but is formed outside of the laser resonator 86b. Integrated structure 180 is substantially identical to integrated structure 70 of FIG. 2 and, therefore like parts carry identical numerical identification. The distinguishing feature of structure 180 is that the transistor is separated from the laser by a narrow channel 182 etched between the laser electrodes 92b, 94b and the transistor electrodes 92a, 92b. As described by Bona, et al., In Beam Properties of AlGaAs Power Lasers with High-Quality Etched Mirrors, IEEE Photonics Technology Letters, vol. 3, pp. 412-413, (1991) chemically assisted ion-beam etching can be used to form a smooth, vertical surface on the laser side of the channel, thereby providing the mirror for the laser resonator 86b. On the transistor side of the channel, the surface is formed at an angle to the axis of the waveguide 86 in order to reduce reflection of light back into the laser. The third and fourth embodiments of this invention are advantageous in that the phototransistor absorbs optical power emitted from the laser cavity 86b, whereas in the first and second embodiments the phototransistor absorbs optical power within the laser cavity 86b. Consequently, a phototransistor placed within the laser cavity increases the internal loss of the laser, thereby increasing the laser's threshold. Each of the above embodiments can be utilized to form an array of integrated laser/transistor structures in a single set of semiconductor layers formed on a single substrate. The output power from each integrated structure in this monolithic array can be separately controlled by its independent data signal. Simultaneously with this modulation, the optical power emitted by each integrated laser structure is stabilized against unpredictable variations by the integrated feedback control provided by the phototransistor. This integrated control is advantageous because it requires no high speed connections to the chip and because it coaxially integrates the transistor with the laser, thereby allowing the light emitting regions to be closely spaced. Finally, the present invention may be embodied in a raster output scanning (ROS) apparatus which includes, inter alia, an output-stabilized solid state laser source (of single or multiple addressable spot output) for emitting at least one light beam, means for pulse modulating the light beam in accordance with a data signal, means for scanning the light beam in a raster fashion, image plane means, such as a photoreceptive element, for receiving the scanned light beam, and means for focusing the light onto the image plane means. The present invention finds particular advantage in such a ROS apparatus, as previously described. In general, to those skilled in the art, many variants of the above-described embodiments will be apparent. For example, while the above described structure is formed to have twin lateral p-n junctions (e.g., as shown by 88 and 90 of FIG. 2) between the anode and cathode of the laser, the laser may alternatively be formed to include only a single lateral p-n junction. Further, although IID techniques are described above with regard to formation of the laser/transistor structure, any other suitable technique maybe employed which results in the described structure or a structure equivalent thereto.

Claims: A semiconductor device comprising: a semi-insulating substrate (72) having monolithically formed thereon with a common semiconductor layer structure a transverse junction laser (82-90,92b,94b,96) within which is defined an optical cavity (86b), and a heterojunction bipolar transistor (82-90,92a,94a), the optical cavity (86b) of the laser being axially aligned with a base region (86a) of the transistor such that optical output from the optical cavity (86b) impinges upon said base region, and means (110) providing electrical connection between the anode (96) of said laser and the collector (94a) of said transistor, whereby the optical output from said laser (82-90,92b,94b,96) incident upon said base region (86a) causes a stabilizing feedback electrical current to be generated by said transistor (82-90,92a,94a) and which is communicated to said laser which is powered by a separate current source, to thereby stabilize the optical output of said laser. The semiconductor device of claim 1, wherein said optical cavity includes a distributed Bragg reflector (164) located proximate said transistor. The semiconductor device of claim 1, further comprising an optical waveguide (142) disposed between and in optical communication with said optical cavity (86b) of said laser and said base region (86a) of said transistor. The semiconductor device of claim 3, wherein said optical waveguide (142) includes a distributed Bragg reflector (162). The semiconductor device of claim 1, wherein said optical cavity (86b) of said laser and said base region (86a) of said transistor are a unitary, integrated structure (86). An array of independently addressable semiconductor laser light sources, each light source comprising a semiconductor device according to any of claims 1 to 5. A raster output scanner apparatus of the type having an improved light source for emitting at least one light beam, means for pulse modulating the light beam in accordance with a data signal, means for scanning the light beam in a raster fashion, image plane means for receiving the scanned light beam, and means for focusing the light onto the image plane means, the improved light source comprising a semiconductor device according to any of claims 1 to 5.

Inventors: PAOLI THOMAS L, PAOLI, THOMAS L.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: H01S3
IPC codes: H01S5, H01L31