Abstract:
A III-V nitride blue laser diode has an amplifier region and a modulator region. The amplifier region has a constant current to keep the region near the lasing threshold. The modulator region has a small varying forward current or reverse bias voltage which controls the light output of the laser. This two section blue laser diode requires much lower power consumption than directly modulated lasers which reduces transient heating and “drooping” of the light output.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a blue laser diode and, more particularly, to a two section blue laser diode with an amplifier region and a modulator region to reduce power output variations. 
     Solid state lasers, also referred to as semiconductor lasers or laser diodes, are well known in the art. These devices generally consist of a planar multi-layered semiconductor structure having one or more active semiconductor layers bounded at their ends by cleaved surfaces that act as mirrors. The semiconductor layers on one side of the active layer in the structure are doped with impurities so as to have an excess of mobile electrons. The semiconductor layers on the other side of the active layer in the structure are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. Layers with excess electrons are said to be n-type, i.e. negative, while layers with excess holes are said to be p-type, i.e. positive. 
     An electrical potential is applied through electrodes between the p-side and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers across the p-n junction so as to “inject” them into the active layers, where electrons recombine with holes to produce light. Optical feedback provided by the cleaved mirrors allows resonance of some of the emitted light to produce coherent “lasing” through the one mirrored edge of the semiconductor laser structure. 
     Semiconductor laser structures comprising group III-V nitride semiconductor layers grown on a sapphire substrate will emit light in the near ultra-violet to visible spectrum within a range including 360 nm to 650 nm. 
     The shorter wavelength of blue/violet laser diodes provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) laser diodes for laser printing operations and high density-optical storage. In addition, blue lasers can potentially be combined with existing red and green lasers to create projection displays and color film printers. 
     The III-V nitrides make possible diode lasers that operate at room temperature and emit shorter-wavelength visible light in the blue-violet range under continuous operation. The III-V nitrides comprise compounds formed from group III and V elements of the periodic table. The III-V nitrides can be binary compounds such as gallium nitride (GaN), aluminum nitride (AIN), or indium nitride (InN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium gallium nitride (InGaN), and quartemary alloys such as aluminum gallium indium nitride (AlGaInN). 
     These materials are highly promising for use in short-wavelength light emitting devices for several important reasons. Specifically, the AlGaInN system has a large bandgap covering the entire visible spectrum. III-V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under high electric current and intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown. 
     High speed and high resolution printing requires laser devices with little or no fluctuations of the output power. For example, the variation in the laser light output required for red and IR laser diodes for printing applications is smaller than 4% and those requirements would be similar for AlGaInN laser diodes. 
     Heat is generated through voltages drops across the metal electrode/semiconductor interfaces, which have a finite resistance, and through voltage drops across the resistive semiconductor layers. Energy is also introduced into the active region of the laser by injecting electrons into the conduction band and/or holes into the valence band. Electrons relax into the lowest energy state of the conduction band and holes relax into the lowest energy state of the valence band through non-light emitting processes and release their energy in the form of heat. 
     When a laser device is switched from the OFF to the ON state, transient heating, or heating that changes over time, can cause the light output of AlGaInN laser diodes to drop significantly. 
     As an illustrative example, an AlGaInN blue laser diode is forward biased with a constant current above the lasing threshold current. At the initial time t=0, with a constant current of 65 mA, the blue laser diode will have a first output power PI of 9.5 mW with a laser structure temperature of 20 degrees C., as shown in FIG.  1 . 
     However, as time increases with the blue laser diode above lasing threshold with the constant current, the temperature of the laser structure increases. This increased temperature results in a decreased output power for the AlGaInN laser diode. 
     At a subsequent time t=∞, still with a constant current of 65 mA, the blue laser will have a second output power P 2  of 6.2 mW with a laser structure temperature of 30 degrees C., as shown in FIG.  1 . The second output power P 2  is lower than the initial output power P 1 . Thus, the plot of output power versus time of FIG. 2 shows an initial output power of P 1  at turn-on, “drooping” to the second lower output power P 2  as the blue laser diode is operated. 
     Thermal fluctuations are especially deleterious to maintaining constant optical power output, especially during pulsed modulation. In virtually all of the applications of these lasers, it is necessary to modulate the output of the laser into a series of pulses. 
     Transient heating during a sequence of pulses can have a cumulative effect on the temperature depending on the number and frequency of the pulses. For example, if the time between successive pulses is large, the laser diode will be given sufficient time to cool, so that the application of the driving current has a large temperature effect (i.e., a large droop in output power will occur at turn-on of the next pulse). The shorter the time between pulses, the less time the laser diode 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 the output pulse obtained with a constant level of input current. 
     Another related consequence of transient heating of a laser is wavelength variation during a pulse and over long streams of pulses. Essentially, the operating wavelength of a laser diode is dependent on the temperature of the laser diode. If the temperature varies, the wavelength of operation will vary. The effect of this variation of wavelength, for example in the laser xerography application, is to vary the energy that can be written onto the photoreceptor. This can also translate directly into variations in the spot size and pattern on the photoreceptor. 
     Digital printing requires accurate control of the optical energy delivered in each pulse. 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. 
     Due to the poor thermal conductivity of the sapphire substrate and the relatively high electric power consumption of III-nitride baser laser devices, transient heating is an issue for AlGaInN devices. For example, AlGaInN laser devices have threshold currents in the order of 50 mA and operating voltages of 5 V (compared to about 15 mA and 2.5 V for red lasers). 
     It is an object of the present invention to provide a blue laser with reduced power output variations due to transient heating. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an III-V nitride blue laser diode has an amplifier region and a modulator region. The amplifier region has a constant current to keep the region near the lasing threshold. The modulator region has a small varying forward current or reverse bias voltage which controls the light output of the laser. This two section blue laser diode requires much lower power consumption than directly modulated lasers which reduces transient heating and “drooping” of the light output. 
     The light output of the laser diode is controlled by absorption changes in the modulator region. Absorption changes can be induced using field effects (e.g. QCSE) or carrier effects (e.g. band filling or field screening) which require much lower power consumption than directly modulated lasers. 
     Since only a small section of the laser diode is used to control the output power, the resulting lower capacitance should also be beneficial for achieving higher modulation speeds. 
     Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained and understood by referring to the following detailed description and the accompanying drawings in which like reference numerals denote like elements as between the various drawings. The drawings, briefly described below, are not to scale. 
     FIG. 1 is a plot of power output versus input current for a blue laser diode showing the effect of transient heating of the laser diode. 
     FIG. 2 is a plot of power output versus time for the blue laser diode of FIG. 1 showing power output “droop”. 
     FIG. 3 is a cross-sectional side view of the blue laser diode of the present invention after III-V nitride film growth. 
     FIG. 4 a  is a cross-sectional front view of the two section ridge-waveguide blue laser diode with gain region and modulator region of the present invention. 
     FIG. 4 b  is a cross-sectional side view of the two section blue laser diode with gain region and modulator region of the present invention separated by an etched trench. 
     FIG. 4 c  is a cross-sectional side view of the two section blue laser diode with gain region and modulator region of the present invention separated by an electrically insulating ion implanted region. 
     FIG. 5 is a top view of the two-section blue laser diode of FIG.  4 . 
     FIG. 6 a  shows the measured light output vs. current characteristic for a blue two-section laser diode for different modulator section reverse bias voltages. 
     FIG. 6 b  shows the measured light output vs. reverse bias voltage characteristic for a blue two-section laser diode at a constant gain section current. 
     FIG. 7 a  shows the measured two-section laser diode emission spectra at a modulator section voltage of 0 V (OFF state). 
     FIG. 7 b  shows the measured two-section laser diode emission spectra at a modulator section voltage of 6 V (ON state). 
    
    
     DESCRIPTION OF THE INVENTION 
     In the following detailed description, numeric ranges are provided for various aspects of the embodiments described. These recited ranges are to be treated as examples only, and are not intended to limit the scope of the claims hereof. In addition, a number of materials are identified as suitable for various facets of the embodiments. These recited materials are to be treated as exemplary, and are not intended to limit the scope of the claims hereof. 
     Reference is now made to FIG. 3 wherein is described the basic two section III-V nitride based semiconductor alloy diode laser  100  of the present invention. The semiconductor laser structure  100  has a C-face ( 0001 ) or A-face ( 1120 ) oriented sapphire (Al 2 O 3 ) substrate  102  on which a succession of semiconductor layers is epitaxially deposited. The laser structure  100  includes a thin buffer layer  103 , also known as a nucleation layer, formed on the sapphire substrate  102 . The buffer layer  103  acts primarily as a wetting layer, to provide smooth, uniform coverage of the top surface of the sapphire substrate  102 . The buffer layer  103  can comprise any suitable material. Typically, the buffer layer  103  is formed of a binary or ternary III-V nitride material, such as, for example, GaN, AIN, InGaN or AlGaN. The buffer layer  103  typically has a thickness of from about 10 nm to about 30 nm. The buffer layer  103  is typically undoped. 
     A second III-V nitride layer  104  is formed on the buffer layer  103 . The second III-V nitride layer  104  is an n-type GaN or AlGaN layer. The second III-V nitride layer  104  acts as a lateral n-contact and current spreading layer. The second III-V nitride layer  104  typically has a thickness of from about 1 μm to about 10 μm. The second III-V nitride layer  104  is typically n-type GaN:Si or AlGaN:Si. 
     A third III-V nitride layer  105  is formed over the second III-V nitride layer  104 . The third III-V nitride layer  105  is a defect reducing layer. The third III-V nitride layer  105  typically has a thickness of from about 25 nm to about 200 nm. The third III-V nitride layer  105  is typically n-type InGaN:Si with an In content smaller than the InGaN quantum well(s) in the active region  108 . 
     A fourth III-V nitride layer  106  is formed over the third III-V nitride layer  105 . The fourth III-V nitride layer  106  is an n-type cladding layer. The fourth III-V nitride layer  106  typically has a thickness of from about 0.2 μm to about 2 μm. The fourth III-V nitride layer  106  is typically n-type AlGaN:Si with an Al content larger than the third or the second III-V nitride layer. 
     A fifth III-V nitride layer  107 , which is a waveguide layer, is formed over the fourth III-V nitride layer  106 . The fifth III-V nitride layer  107  is typically n-type In GaN:Si, GaN:Si, InGaN:un or GaN:un with an In content smaller than the InGaN quantum well(s) in the active region  108 . The overall thickness of the fifth III-V nitride layer  107  is typically from about 0.05 μm to about 0.2 μm. 
     On top of the fifth III-V nitride layer  107 , the InGaN quantum well active region  108  is formed, comprised of at least one InGaN quantum well. For multiple-quantum well active regions, the individual quantum wells typically have a thickness of from about 10 Å to about 100 A and are separated by InGaN or GaN barrier layers which have typically a thickness of from about 10 Å to about 200 Å The InGaN quantum wells and the InGaN or GaN barrier layers are typically undoped or can be Si-doped. 
     A sixth III-V nitride layer  109 , which is a carrier confinement layer, is formed over the InGaN (multiple) quantum well active region  108 . The sixth III-V nitride layer  109  has a higher band gap than the quantum well active region. The sixth III-V nitride layer  109  is typically p-type Al x Ga l−x N:Mg with an Al content in the range from x=0.05 to x=0.4. The overall thickness of the sixth III-V nitride layer  109  is typically from about 5 nm to about 100 nm. 
     A seventh III-V nitride layer  110 , which is a waveguide layer, is formed over the sixth III-V nitride layer  109 . The seventh III-V nitride layer  110  is typically p-type InGaN: Mg or GaN:Mg with an In content smaller than the InGaN multi-quantum well(s) in the active region. The overall thickness of the seventh III-V nitride layer  110  is typically from about 50 nm to about 200 nm. 
     A eighth III-V nitride layer  111  is formed over the seventh III-V nitride layer  110 . The eighth III-V nitride layer  111  serves as a p-type cladding layer. The eighth III-V nitride layer  111  typically has a thickness of from about 0.2 μm to about 1 μm. The eighth III-V nitride layer  111  is typically AlGaN:Mg with an Al content larger than the seventh III-V nitride layer. 
     A ninth III-V nitride layer  112  is formed over the eighth III-V nitride layer  111 . The ninth III-V nitride layer  112  forms a p-contact layer for the minimum-resistance metal electrode, to contact the p-side of the heterostructure  100 . The ninth III-V nitride layer  112  typically has a thickness of from about 10 nm to 200 nm. 
     The laser structure  100  can be fabricated by a technique such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy as is well known in the art. MOCVD growth is typically performed on 2-inch or 3-inch diameter sapphire substrate wafer. The substrate  102  can be a C-face ( 0001 ) or A-face ( 1120 ) oriented sapphire (Al 2 O 3 ) substrate. The sapphire substrate wafers are of standard specifications including an epitaxial polish on one side and a typical thickness of 10-mil to 17-mil. Other examples of substrates include, but are not limited to 4H—SiC, 6H—SiC, AIN or GaN. In case of growth on a GaN substrate, the second III-V nitride layer  104  can be directly formed on top of the substrate  102  without the deposition of a nucleation layer  103 . The substrate temperatures during growth are typically 550 degrees C. for the GaN nucleation layer, 1000 degrees C. to 1100 degrees C. for the GaN and AlGaN layers and 700 degrees C. to 800 degrees C. for the InGaN layers. In addition, the reactor pressure may be controlled between 50 Torr and 740 Torr. As organometallic precursors for the MOCVD growth TMGa (trimethylgallium), TMAl (trimethylalurninum), TMIn (trimethylindium) and TEGa (triethylgallium) are used for the group III elements and NH 3  (ammonia) is used as the nitrogen source. Hydrogen and/or nitrogen are used as carrier gas for the metalorganic sources. For the n-doping, 100 ppm SiH 4  diluted in H 2  is used, and for the p-doping, Cp 2 Mg (cyclopentadienylmagnesium) is used. Other examples of p-type dopants include, but are not limited to, Mg, Ca, C and Be. Examples of n-type dopants include, but are not limited to, Si, O, Se, Te and N-vacancies. 
     After MOCVD growth, the Mg p-doping in (Al)GaN:Mg layers is activated by RTA (rapid thermal annealing) at 850 degrees Celsius for 5 minutes in N 2  ambient. A ridge waveguide structure is formed by dry-etching into the p-GaN waveguide layer  110  with CAIBE (chemical assisted ion beam etching) or RIE (reactive ion beam etching) in an Ar(argon)/Cl 2 (chlorine)/BCl 3 (borontrichloride) gas mixture as shown in FIG. 4 a.    
     In order to form the gain and the modulator section of the device, an isolation trench  114  is etched into the ridge-waveguide as shown in FIG. 4 b . The isolation trench  114  can be formed in the same step as the ridge-wave-guide etch or can be formed in a separate step in order to obtain a different etch depth. An electrically insulating layer (e.g. silicon-oxy-nitride or SiO 2  or Si 3 N 4 ) (not shown in the Figure) can be deposited, e.g. by plasma-enhanced chemical vapor deposition (PE-CVD), into the trench in order to reduce the disturbance of the optical mode and serve as passivation layer for the exposed surfaces. 
     Dry-etching using CAIBE (chemical assisted ion beam etching) or RIE (reactive ion beam etching) in an Ar/Cl 2 /BCl 3  gas mixture is used to access the GaN:Si layer  104  for n-contact formation. An n-contact  120  is formed over the second III-V nitride layer  104 , which is functioning as a lateral contact layer. 
     The n-electrode  120  of FIG. 4 a  is common to both the amplifier region  116  and the modulator region  118 . The n-contact metal can be deposited by thermal evaporation, electron-beam evaporation or sputtering. Typically Ti/Al, Ti/Au or Ti/Al/Au are used as n-metal contacts. The n-contacts are annealed in N 2  ambient at 500 degrees Celsius in order to reduce the contact resistance. A dielectric isolation layer  113  (e.g. silicon-oxy-nitride or SiO 2  or Si 3 N 4 ) is deposited by plasma-enhanced chemical vapor deposition (PE-CVD) on top of the ridge-waveguide. Alternatively polyimide can also used for isolation. Windows for p-contact formation are etched into the dielectric isolation layer using RF plasma etching in a CF 4 /O 2  ambient. 
     In FIGS. 4 a  and  4   b , a p-electrode  200  is deposited on top of the amplifier contact layer  201  for the amplifier region  116 . A p-electrode  300  is deposited on top of the modulator contact layer  301  for the modulator region  118 . The two p-electrodes  200  and  300  are separate and distinct and allow for independent addressability of the amplifier region  116  and the modulator region  118 . Ni/Au, NiO/Au, Pd/Au, Pd/Au/Ti/Au, Pd/Ti/Au, Pd/Ni/Au, Pt/Au or Pd/Pt/Au can be deposited as p-contact metal by thermal evaporation, electron-beam evaporation or sputtering. 
     The laser facets can be formed either by cleaving or dry-etching (e.g. CAIBE). A SiO 2 /TiO 2 , SiO 2 /Ta 2 O 5  or SiO 2 /HfO 2  high reflective coating can be deposited on the backside of laser diode facets  126  by e-beam evaporation in order to enhance the mirror reflectivity. A SiO or SiO 2  anti-reflective coating can be deposited on the front side of the laser diode facet  124  using e-beam evaporation. 
     As seen in FIGS. 4 b  and  5 , the trench  114  separates the laser structure  100  into an amplifier region  116  and a modulator region  118 . The amplifier region  116  has a metal p-contact  200 , a p-contact layer  201 , an upper cladding layer  202 , an upper waveguide layer  203  and an upper confinement layer  204  The modulator region  118  has a metal p-contact  300 , a p-contact layer  301 , an upper cladding layer  302 , an upper waveguide layer  303  and an upper confinement layer  304 . 
     The amplifier p-contact layer  201  is separate and distinct from the modulator p-contact layer  301  but both are formed from the p-contact layer  112  before the groove  114  is etched. The amplifier upper cladding and waveguide layers  202  and  203  are separate and distinct from the modulator upper cladding and waveguide layers  302  and  303  but both are formed from the upper confinement and waveguide layer  110  and  111  before the groove  114  is etched. The amplifier upper confinement layer  204  is separate and distinct from the modulator upper confinement layer  304  but both are formed from the p-upper confinement layer  109  before the groove  114  is etched. 
     The active quantum well layer  108 , the lower waveguide layer  107 , the lower cladding layer  106 , the defect reducing layer  105 , the thick (Al)GaN current spreading layer  104 , the buffer layer  103  and the substrate  102  are common to both the amplifier region  116  and the modulator region  118 . 
     The optimum depth of the isolation trench  114  will depend on whether complete electric isolation is desired or minimum disturbance of the optical mode. Complete electric isolation is obtained if the trench  114  is etched through all the p-type layers and may even reach into or beyond the MQW active region  108 . This might, however, disturb the optical mode traveling between the modulator and the gain section of the device and lead to scattering losses. If the trench is only partially etched into the p-type layers some electric connection between the gain and modulator section remains. The cross-talk between these two section, however, is expected to be quite small, because of the high lateral resistance of the p-type GaN and AlGaN layers. For example, if the trench  114  is only etched into the p-AlGaN cladding layer  111  (as shown in FIG. 4 b ), the remaining GaN:Mg waveguide layer  110  and p-AlGaN layer  109  would yield a series resistance of about 3 MΩ between the modulator and the gain section (assuming a waveguide thickness of 100 nm, a trench width of 10 μm and a ridge-waveguide width of 2 μm, a hole concentration of 10 18  cm −3  and a mobility of 1 cm 2 /Vs). The trench can also be refilled with an electrically insulating dielectric layer (e.g. silicon-oxy-nitride or Si 3 N 4 ) (not shown in the Figure), which can be deposited by plasma-enhanced chemical vapor deposition (PE-CVD)) in order to improve the optical coupling between the modulator and the gain section. 
     Alternatively, electric isolation between the modulator and gain section can be obtained by ion implantation and without the etching of a trench. Ion implantation of the area  115  between the modulator and the gain section for example with protons (H + ) or oxygen ions would make this area electrically insulating and have the additional benefit that the optical waveguide would not be disturbed. An example of such a structure is shown in FIG. 4 c . The ion implanted region could reach into or beyond the InGaN MQW active region in order to prevent and carriers from the gain section leaking into the modulator section of the device. 
     The light beam  128  emitted by the laser structure  100  is emitted from the mirror  124  adjacent to the modulator region  118  in order to minimize any spontaneous emission from the amplifier region  116  in the OFF state. 
     For this invention, the active layer  108  under the amplifier region  116  is the amplifier active layer  130  and the active layer  108  under the modulator region  118  is the modulator active layer  132 . 
     In the two region blue laser structure  100 , the amplifier region  116  is strongly pumped with current to serve as a light emitting region, and the modulator region  118  is pumped with a lower current level than the amplifier region to allow high frequency modulation. Alternatively the modulator region can be also reverse biased, which also enables high frequency operation. 
     The amplifier region  116  is of much greater length than the modulator region  118 . Accordingly, the active layer  130  of the amplifier region  116  provides essentially all of the gain required to produce the desired output intensity. The active layer  132  of the modulator region  118  controls the output of laser  100  by switching the internal loss from a high value to a low value. 
     The amplifier region  116  will be forward biased by an input current applied through the p-electrode  200  and the n-electrode  120 . The current will cause electrons to flow from the n-doped layers of the current spreading layer  104 , the defect reducing layer  105 , the lower cladding layer  106  and lower waveguide layer  107  into the amplifier active layer  130 . The current also causes holes to flow from the p-doped layers of the amplifier contact layer  201 , the amplifier upper cladding layer  202 , the amplifier upper waveguide layer  203  and the amplifier upper current confinement layer  204  into the amplifier active layer  130 . Recombination of the electrons and holes in the amplifier active layer  130  at a sufficient current will cause stimulated emission of light  128 . 
     The current applied to the amplifier region  116  is adjusted so that enough gain is generated in the amplifier active layer  130  to overcome the total optical loss including the mirror loss and the optical loss from of the entire active layer  108  in the two section laser structure  100 , when the modulator section  118  is in the ON state (ON state=the modulator active layer  132  is in the low-loss-state) but is not exceeding the total optical loss including the mirror loss and the loss of the entire active layer  108  if modulator section  118  is in the OFF state (OFF state=the modulator active layer  132  is in the high-loss-state). The amplifier current is kept constant. The two-section laser structure  100  modulates the laser emission and laser output power by varying the optical loss in the active layer  132  of the modulator section of the laser device. If the modulator section is its high-loss-state (OFF state), the gain produced in the amplifier section is not large enough to overcome the total optical loss and therefore lasing is prohibited. If the modulator is switched to its low-loss-state (ON state) the optical gain produced in the amplifier section will be large enough to overcome the total optical loss and lasing will be allowed. The optical loss in the modulator section can be either varied by applying a forward current or by applying a reverse bias voltage to the modulator region  118 . 
     There are two basic modes of modulation for the two-section blue laser structure  100  of the present invention. 
     One mode, called the “forward current modulation mode”, is one in which the amplifier region  116  is sufficiently forward biased to cause stimulated light emission and a negligible minimal forward bias current is applied to the modulator region. The modulator region  118  can be forward biased by an input current applied through the p-electrode  300  and the n-electrode  120 . The current will cause electrons to flow from the n-doped layers of the current spreading layer  104 , the defect reducing layer  105 , the lower cladding layer  106  and lower waveguide layer  107  into the modulator active layer  132 . The current also causes holes to flow from the p-doped layers of the modulator contact layer  301 , the modulator upper cladding layer  302  and the modulator upper waveguide layer  303 , and the modulator current confinement layer  304  into the modulator active layer  132 . If no current is applied, the modulator section  118  is in the high-loss-state and lasing is prohibited (OFF state). Injection of the electrons and holes in the modulator active layer  132  at a sufficient current will reduce the loss in the modulator active layer  132 . As the modulator section  118  is in its low-loss-state, the gain from the amplifier section  116  will be sufficiently high to overcome the total loss and lasing is permitted (ON State). 
     Another mode, called the “reverse bias modulation mode”, also has the amplifier region sufficiently forward biased to cause stimulated emission, but has a reverse bias voltage applied to the modulator region. 
     The reverse bias input voltage to the modulator region  118  can be applied through the p-electrode  300  and the n-electrode  120 . By applying a reverse bias to the modulator region p-n junction formed by the lower waveguide layer  107 , the InGaN MQW active region  1132  and the upper current confinement layer  304 , the electric field in the modulator section p-n junction can be changed. If no voltage is applied, the modulator section  118  is in the high-loss-state and lasing is prohibited (OFF state). As the modulator section  118  is in its low-loss-state, by applying an external reverse bias, the gain from the amplifier section  116  will be sufficiently high to overcome the total loss and lasing is permitted (ON state). 
     Examples of the measured laser diode characteristics of devices operating in the “reverse bias modulation mode” are shown in FIGS. 6 and 7. The amplifier region in this example has a length of 700 μm, the modulator region has a length of 100 μm and the ridge-waveguide has a width of 3 μm. Amplifier and modulator region are separated by a 20 μm wide trench, which was etched into the upper GaN waveguide layer  110 . FIG. 6 a  shows the light output vs. current characteristic for such a two-section laser device with different reverse bias voltages applied to the modulator section. With no voltage applied to the modulator section, the loss in the modulator section is high resulting in a high threshold current (I th ˜225 mA). If a reverse bias voltage is applied to the modulator section the loss in the modulator section is reduced leading to a reduction of the threshold current. For example at a modulator voltage of U mod =6 V the threshold current is reduced to I th ˜190 mA). In the “reverse bias modulation mode”, the laser is operated with the current in the gain (amplifier) section set to a constant current of I gain =225 mA (indicated by the dotted curve in FIG. 6 a ). The light output vs. modulator voltage characteristic of the same two-section laser diode is shown in FIG. 6 b . The current in the gain (amplifier) section was set to a constant current value of I gain =225 mA. As the reverse bias voltage applied to the modulator section increases, the loss in the modulator section is reduced and the light output increases from 0.5 mW (at zero bias) to ˜3 mW (at U mod =7.2 V). The corresponding laser diode emission spectra are shown in FIGS. 7 a  and  7   b . At a modulator section voltage U mod =0 V the loss in the modulator section is high, thus prohibiting lasing (as shown in FIG. 7 a ). If the modulator section reverse bias U mod  is raised to 6 V, the loss the modulator section is reduced and consequently lasing is permitted, as can be seen in the spectra of FIG. 7 b.    
     The varying modulator region current and the varying dissipated electric power in the modulator region is significantly smaller in comparison to the constant current and the dissipated electric power in the amplifier region. For example, if the two-section laser diode is operated in the “forward current modulation mode”, the current density necessary to modulate the absorption in the modulator section (typically 100 A/cm 2  to (1000 A/cm 2 ) is only a fraction of the current density necessary to produce sufficient gain in the amplifier section (typically 2000 A/cm 2  to 5000 A/cm 2 ). Furthermore the length of the modulator section is much smaller (typically {fraction (1/10)} to ⅕ of the length of the amplifier section) and therefore the current to switch the modulator section is even smaller. In the case when the two-section laser diode is operated in the “reverse bias modulation mode”, the dissipated electric power in the modulator region will be even smaller. Since the modulator section is operated in reverse bias, no current is injected in the modulator section active region. The only current flowing in the modulator section is the photocurrent, which is induced by the absorbed light from the amplifier region. Accordingly, the laser structure  100  operates at an elevated but constant temperature due to the constant amplifier region current. The varying modulator region current will have minimal temperature effect to the laser structure  100 . Modulating the laser from the non-light emitting, or OFF state, to the light emitting, or ON state, does -only result in a small increase in the operating temperature of the laser. 
     Since only the smaller modulator region  118  is used to control the output power of the emitted laser beam, the resulting lower capacitance will help achieve higher modulation speeds for the blue laser diode  100 . 
     The laser diode structure according to the invention described above can be applied to any device requiring compact laser structures, including high resolution laser printing devices, digital printers, display devices, projection displays, high density optical storage devices, including magneto-optical storage devices, including CD-ROM and DVD&#39;s whereby data is stored on a magneto-optical disk, fiber-optic communications devices, including for fiber optic emitters and repeaters and undersea communications devices (sea water is most transparent in the blue-green spectrum). The LED structure according to the invention can also be applied to any device requiring compact LED structures, including illumination devices and full color displays, including monolithically integrated pixels for full color displays. 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.