JP4514951B2 - High power semiconductor light source - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser light source, and more particularly to a semiconductor laser using a diamond-shaped active structure of a superluminescent diode (hereinafter referred to as SLD) structure as a gain medium that provides a sufficient volume for high output power.
[0002]
[Prior art]
Semiconductor lasers and superluminescent diode is a powerful, a good source of efficient compact, 10 ⁷ W / cm ² A moderate power density capacity is generated in the active layer with a thickness of 1 μm or less. A semiconductor laser diode has a waveguide structure in which an active layer is sandwiched between an n-doped cladding layer and a p-doped cladding layer having a slightly lower refractive index than that of the active layer, and current can flow therethrough. It is formed by epitaxial growth on a substrate. In one common method of forming an active waveguide, a channel is formed in the structure by etching to form a ridge. Since the refractive index of the channel and cladding regions is slightly lower than the effective refractive index of the active region under the ridge, light is guided through the ridge by total reflection. Typically, the ridge walls are parallel and light is transmitted in a zigzag path through the ridge at a number of discrete angles defined by the propagation modes in the structure. An effective refractive index is associated with each mode, which is determined by the zigzag angle. The lowest mode is the one with the smallest zigzag angle.
[0003]
FIG. 1a is a perspective view of a typical ridge waveguide laser or Fabry-Perot laser. Further, b in FIG. 1 is a top view showing propagation characteristics. In the Fabry-Perot laser, feedback for laser oscillation is performed by reflection from a facet. One of the facets reflects only partly and therefore part of the light is emitted. The spectrum of light is very narrow as shown in FIG. 1c, often less than 1 angstrom. When the width of the ridge is within the range specified by Equation 1 described later, only the lowest order mode is allowed to propagate. The laser is then called single mode and its output is a diffraction limited beam with a Gaussian-like far field pattern having a spatial distribution as shown in FIG. In most applications, the laser is required to operate in a single mode.
[0004]
In the laser of FIG. 1a, the active waveguide is perpendicular to the facet. The same structure can be configured as an SDL by processing the active waveguide at an angle with respect to the facet. 2a and 2b show the form of the same laser structure of FIG. 1a as SDL. When the optical waveguide is not perpendicular to the facet, the reflection from the facet leaves the waveguide at an angle and only a very small portion of the reflected light can be coupled to the waveguide. The angle of the waveguide ridge can be selected so that as little light as possible is reflected and coupled, as shown in FIG. 2c. For example, if the waveguide has an angle of about 6 degrees, the reflected wave is about 10 ⁻⁶ . Without reflection, there is no feedback and the output is a broad spectrum of naturally occurring radiation as shown in FIG.
[0005]
The SLD is a light source having several important functions. First, it is an optical amplifier that can amplify optical signals that are several orders of magnitude. Second, it is a broadband light source used in some applications such as fiber optic gyroscopes and light sources for images with low coherency. Third, it includes a tunable laser that is free of unwanted laser modes caused by faceted reflections, allowing the laser to be inserted into an external cavity (the space between the two mirrors). These three functions can be used in the present invention.
[0006]
Low power (several mW) semiconductor lasers having an active layer length of less than about 500 μm and a width of about 1 to 5 μm are widely used in CD, CD-ROM, optical data storage, and optical communications. However, new applications have arisen that require power on the order of about 10-100 W (watts) in a single “guide” mode that can be easily coupled to a single mode optical fiber. Examples of such applications are projector or laser printer light sources. In view of their high saturation power density, such power can be achieved with semiconductor laser materials by forming a sufficiently large width active waveguide layer. For example, an active waveguide with a width of 1 mm and a thickness of 1 μm is 10 ⁻⁵ cm ^2. Will produce output power up to 100W. However, such power can only be obtained in multimode and not in single mode.
[0007]
As described above, the wave propagates in multiple modes, which are characterized by the discrete zigzag angle of the waveguide, with the lowest mode being the smallest zigzag angle. At any given wavelength, the number of modes is determined by the refractive index of the waveguide core and the surrounding cladding. In order to operate in its lowest mode, the waveguide width d cannot exceed a certain minimum value. The refractive index of the core is n ₁ And the refractive index of the cladding is n ₂ For a waveguide with a wavelength of λ, the width for a single mode is determined by Equation 1 below.
(1) d ≦ λ / [2 (n ₁ ² -N ₂ ^{2 1/2} ]
n ₁ = 3.2 and n ₂ For a typical laser with = 3.195, d is less than 3 μm at a wavelength of 980 nm and the area for a thickness of 1 μm is 3 × 10 ⁻⁸ cm ^2. And therefore the maximum power is about 0.3W. Despite the high power capacity of semiconductor laser materials, its output as a single mode laser is only a fraction of 1W. Large area lasers with a strip width of about 100 μm were fabricated for an output power of several watts, and the arrays yielded higher power, but they were multimode and limited in use.
[0008]
[Problems to be solved by the invention]
It is an object of the present invention to produce a high power laser (about 10-100 W) with a single mode output to manage the power available in the wide strip width active region.
[0009]
[Means for Solving the Problems]
The present invention provides a semiconductor light source, which comprises a plurality of superluminescent diodes (SLDs) disposed on a first conductivity type substrate having facets, each SLD of which It has a diamond-shaped active region such that the front end portion and the rear end portion of the end portion are tapered, and further, a plurality of channels respectively separating each superluminescent diode, and the taper of the SLD. A mode expansion region disposed on at least one side, the mode expansion region being tapered in a waveguide extending to the facet. Mode expansion is desired for higher powers, but can also be used for intermediate powers less than about 3-5W. According to one aspect of the invention, a mode filter, preferably a single mode optical fiber having a mechanism for optical feedback in that mode, is coupled to the facet.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The invention will be more fully understood from the following detailed description of the preferred embodiments with reference to the accompanying drawings, in which:
A preferred embodiment of the system and method of the present invention that meets the above objectives and provides other features will now be described with reference to FIGS. Those skilled in the art will readily recognize that the following description with reference to the drawings is merely illustrative, not limiting the technical scope of the present invention.
[0011]
The object of the present invention is to prevent the structure from generating higher order modes such as "diamond" structures and "adiabatic" power transfer means for mode expanding waveguides having a relatively large cross section. This is accomplished by creating a basic structure for generating power in a large area active waveguide having: Also, means are provided for monolithically combining the basic structure with the desired large passive structure, and the large passive structure is adiabatically tapered to the single mode waveguide. In the case of a laser, means are further provided to ensure that feedback is only performed in the desired vibration mode.
[0012]
As described above in the background art, the output power of a single mode laser is determined by the required width for single mode operation due to the zigzag nature of the propagation, so the volume occupied by the active waveguide is small and 1 W Is limited to a fraction of If the waveguide portion is widened, the number of propagation modes increases and mode output power is generated, but extra power is wasted. This is because it cannot be coupled to an external structure like a single mode optical fiber. It is preferable to prevent zigzag propagation in order to prevent the generation of higher order modes.
[0013]
A detailed description of a diamond-shaped SLD, a double taper structure that achieves the purpose of preventing higher order mode propagation, is described in US patent application Ser. No. 08/857929. This patent application is hereby incorporated by reference. This diamond shape is substantially called a rhombus, a rhombus rotating body, or a parallelogram when viewed in two dimensions.
[0014]
FIG. 3 is a top view of one embodiment of an exemplary diamond SLD designed to operate at 830 nm. The SLD 10 has a length of about 1.0 mm, a width of about 57 μm at the central waist, and a width of about 10 μm at the end facets 15 and 20. The taper angle α of both halves of the diamond shape is selected to be smaller than the lowest mode zigzag angle to ensure that the zigzag pattern is blocked. In order to prevent radiation loss, a taper angle of less than about 3 degrees with respect to the longitudinal centerline of the diamond is selected as shown in FIG. The entire diamond itself is placed at an angle θ with respect to the facet. The strip angle θ is preferably smaller than the critical angle at a facet of about 15 degrees. Preferably, the strip angle θ is in the range of about 5 to 10 degrees. The taper angle α is the angle between the diamond sidewall and the central axis of the strip, and the strip angle θ is the angle of the longitudinal axis of the strip relative to the facet normal.
[0015]
The manner in which zigzag propagation is blocked is illustrated in FIG. In FIG. 4, a broken line A indicates a waveguide having parallel side walls and zigzag propagation of internal rays. Line B shows the diamond portion near the left facet, and solid line C shows a state in which the light reflected from one side wall 17 diverges from the opposite side wall 18 and indicates that zigzag propagation is blocked. In fact, the taper reduces the reflection angle and collimates the light efficiently.
[0016]
The width at facet 15 is about 10 μm (and the cross-sectional area is about 10 ⁻⁷ cm ²⁾ 3) operates at an output power of about 175 mW or more, and when a reflection mirror is provided to operate as a laser, generates an output power of a profile having a Gaussian distribution shape of about 1.8 W. If the end of the active taper is cut off at a point where the cross-section is still large, the field can be expanded to a lossless passive waveguide that is thicker than the active layer and can be tapered between the desired final dimensions. Additional power is extracted.
[0017]
One object of the present invention is achieved by combining the structure of a mode expander with the diamond double taper concept of FIG. 3 to extract all of the available power with a Gaussian distributed output. Is done. Mode expanders reduce the power density to drive high power. A complete description of an SLD with a mode expander is described in US patent application Ser. No. 08 / 946,180. A combination of a mode expander and a diamond structure for trapping power in a thick passive waveguide is shown in FIG.
[0018]
FIG. 5a shows a truncated diamond SLD structure 50 having a mode expander region 60 in the semiconductor chip 40. FIG. In FIG. 5a, a diamond ridge structure 50 is shown, which instead of extending to facets 52, 54, the SLD of FIG. 3 except that the diamond terminates somewhere in the modex region 60 in the semiconductor structure 40. Similar to structure 10.
[0019]
The refractive index of the outer region 60 of the diamond is smaller than the refractive index of the diamond portion 50. As a result, the light field generated in the diamond SLD structure 50 approaches the diamond tip so that it expands from the diamond to the low index index region 60 where it propagates away from the diamond 50. This structural part is lossless and passive.
[0020]
FIG. 5 b shows a side view of the structure of FIG. 5 a and shows the conversion to the mode expander region 60. FIG. 5 b shows the layer structure of the device and the properties in the expander region 60. A normal laser or SLD includes one cladding layer on the p-side or n-side of the active layer, and the active layer includes a quantum structure that forms the SLD. In accordance with the present invention, an n-type cladding layer is disposed on the substrate 58, which is the same n-type as one thick layer (thick guide) 70 having a refractive index slightly lower than diamond as described above. It consists of two parts, another layer (outer n-type cladding) 73 with another composition that gives a lower refractive index than the thick guide layer 70. A typically low refractive index p-type cladding layer 65 is then deposited over the active layer of the diamond SLD structure 50.
[0021]
Since the refractive index of the p-type cladding layer 65 and the outer cladding layer 73 is lower than the refractive index of the thick guide layer 70, the optical field is propagated in the thick guide 70 by the p-type cladding layer 65 and the outer cladding layer 73, and is thick. Propagates through the guide 70 in the direction of facets 80 and 82. The active layer of the SLD structure 50 has a total thickness of less than 1 micron, but the thickness of the thick guide 70 can be several microns, typically 3-4 microns. High power, which is essentially close to the saturation power density, is generated in the diamond SLD structure 50, and the field configuration and power equivalent to the waveguide single mode are extracted from the active layer region. It becomes a lossless region of density. If the diamond waist is about 20 μm and the length is about 2 mm, the generated power is about 3 to 4 W, and most of it uses a taper as shown in FIG. Captured by a mode expander 60 (having a length of 0.1 mm to about 0.2 mm).
[0022]
Without lateral guidance, the field expands laterally to facets and radiates with an angular width determined by the diamond taper angle. Lateral guidance is achieved by extending the channel region 53 in FIG. 5a toward the facet or by adiabatically tapering to the desired output beam size near the facet.
[0023]
Referring to FIG. 5b, the device is typically comprised of a binary III-V compound such as GaAs or InP. In a GaAs system, the structure 58 is typically composed of n-type GaAs with a thickness of about 100 μm and has a first major surface that is parallel to or slightly different from the (100) crystal plane. The cladding layers 65, 70, 73 are typically Al _x Ga _1-x It is composed of As, and the value of x is about 0.3 to 0.4, typically about 0.4. The active layer is typically about 0.08 μm thick and is typically Al _x Ga _1-x As and x is generally about 0 and 0.1 and is radiated and wavelength dependent. The cap layer 75 is typically about 0.5 μm thick and is typically composed of p-type GaAs. In the InP structure, the layer is In _x Ga _1-x As _y P _1-y X and y are appropriately selected for the desired operating wavelength.
[0024]
In accordance with the present invention, a high power single mode structure is formed from this geometric configuration, and the output is formed to satisfy conditions for efficient coupling to an external device such as a single mode optical fiber. . Additional power is generated from the diamond structure 50 by enlarging its waist. However, the transition from the diamond structure 50 to the mode expander 60 does not occur until the diamond width is about 5 μm or less. The power available at this 5 μm point exceeds saturation. The problem is overcome by forming a multi-tooth shaped diamond tip, so that the light field has a wide width only after it escapes from the active region. However, the filamentation problem results in a large area structure that can significantly reduce the lifetime of the device. At very high power density, some impurities, some refractive index changes are induced by the drive current, which causes local changes in power density. These changes result in hot spots or filaments that degrade the material and cause high absorption. The degraded area slowly spreads in time, resulting in heating that becomes a catastrophic failure. In order to gain high power and maintain reliability, it is preferable to use several relatively narrow active diamond strips and combine them in parallel. In this way, even if the strip burns out, the defect is localized, so that it does not affect other strips.
[0025]
FIG. 6 shows a preferred structure for generating high power in which a monolithic array of a plurality of individual thin active ridge diamond structures 110 having a taper are provided in parallel, separated by narrow channels 120. The lengths of the diamond structures 110 need not be the same. The diamond width at the waist is preferably 20-30 μm. The width of the channel 120 may be as narrow as about 5 μm. The width must be sufficient to prevent field overlap of the individual diamond structures 110, but allow for thick waveguide mode expander regions 160 beyond the individual waveguide transitions to overlap. To do. The mode expander region 160 is formed in a relatively long (about 1 to 2 mm) tapered waveguide to allow the combination of power from all individual diamonds, creating an additional taper 170. To achieve the desired mode size at facets 180, 182.
[0026]
The structure shown in FIG. 6 is sufficient to generate a laser. It is necessary to provide an appropriate facet coating to provide feedback. However, it may not be sufficient to ensure that substantially all available power is present in a single transverse mode. To achieve a single mode, a single mode guide section is provided to filter out higher order single modes and feedback is only provided for the desired lowest mode guide. The length of such a guide is preferably about several centimeters, but any length is possible. One way to provide a single mode filter is to couple a single mode optical fiber to the structure 100. However, it is important to avoid reflections from facets 80, 182 that can cause vibrations that occur in some undesirable modes. Thus, the individual diamond array 110 is configured as an SLD and processes it at an angle (about 5-10 degrees) with respect to the centerline of the waveguide 170. FIG. 7 illustrates this preferred embodiment.
[0027]
FIG. 7 shows a preferred structure 100 of the SLD structure. It has single mode optical fibers 190, 192 coupled to the output at two facets 80, 182. The single mode optical fibers 190 and 192 function as a mode filter as described above. The output dimension of the thick passive waveguide 170 with taper is about 3 μm × 6 μm low, which is sufficient to couple about 80% or more of the light available in the optical fibers 190, 192. The passive waveguide 170 has a much higher power density capacity than the active diamond material because of its low loss. If each element gives about 3-4W, only about 20-30 elements take it and produce about 100W of output light. Because of the small overall dimensions (about 2 mm × 4 mm including guard space and channels), a 3 inch diameter wafer produces more than 400 chips at a time, thus resulting in a low cost device.
[0028]
To make a laser from the structure of FIG. 7, one tip of the fiber optic mode filter 190, 192 is coated with a high index mirror or configured as a high reflective grating 195, while the other tip is partially A reflecting mirror or grating 197 is provided. Regardless of the number of modes that can propagate through the semiconductor structure 100, feedback is provided only in the modes that propagate through the optical fiber. The result is a high power single mode laser whose output power is determined by the number of elements in the structure. The maximum beam divergence, which is half the full width of the output light, is determined by the numerical aperture of the optical fiber and is about 15 degrees. The slope efficiency is as low as 1 W / ampere at a supply voltage of about 2 volts. Single mode optical fibers 190, 192 can be replaced by appropriate lengths of multimode optical fibers to provide a pump with a specific mode number of multimode outputs. Thus, the pump is an SLD with an optical fiber section and is fed back to set a specific mode pattern in which the light source is a laser.
[0029]
There are many applications for 10 to 00W semiconductor lasers, without feedback, the device has an equivalent laser output power of about 1/4 for the same drive current, functions as an SLD, and low coherent medical Applicable to video, smart structure and time domain reflectometer. Laser applications include light sources for plate printing, surgery, and electronic projection systems.
[0030]
In view of the above, the basic concept of the semiconductor laser according to the present invention is to use a superluminescent active structure of SLD structure as a gain medium in order to provide sufficient volume for high output power. The SLD structure prevents feedback from facets, otherwise it oscillates the laser in an undesired mode. Mode frequent feedback is provided by means such as an optical fiber structure external to the semiconductor chip.
[0031]
Although illustrated and described herein with reference to specific embodiments, the invention is not limited to the detailed structure shown, but rather various modifications depart from the scope of the invention. Can be done without. Accordingly, semiconductor materials other than those exemplified and other compositions of selected semiconductor materials can be used.
[0032]
Also, the conductivity type may be replaced with the opposite (similar). Variations different from the manufacturing methods shown here are also possible by those skilled in the art, and other techniques such as another method for forming a semiconductor layer can be used effectively.
[Brief description of the drawings]
FIG. 1 is a perspective view and a top view of a prior art Fabry-Perot semiconductor laser, an output spectrum of the semiconductor laser, and a characteristic diagram showing a spatial distribution of the semiconductor laser.
FIG. 2 is a perspective view and a top view of a prior art superluminescent diode, its reflection versus angle characteristic diagram of its SLD, and its output spectrum characteristic diagram.
FIG. 3 is a top view of a diamond SLD according to an embodiment of the present invention.
FIG. 4 is an explanatory diagram of a collimating effect of a waveguide having a taper that prevents zigzag propagation according to the present invention.
FIG. 5 is a structural diagram in which a mode expander for capturing power and a diamond structure are combined in a thick passive waveguide according to a preferred embodiment of the present invention.
FIG. 6 is a schematic diagram of a preferred embodiment of the present invention for generating high power with a monolithic array of parallel diamond ridges separated by narrow channels.
7 is a schematic diagram of a preferred embodiment of the present invention showing the pattern of FIG. 6 of an SLD structure with a single mode optical fiber coupled to the output at two facets.

Claims (14)

A plurality of superluminescent diodes arranged in parallel on a substrate of a first conductivity type having facets, each of the superluminescent diodes having a tapered shape at the front end and the rear end of each superluminescent diode has an active region of a diamond shape that is configured as a part, are separated by each of superluminescent diode channel,
Further comprising the super luminescent one mode expander region being formed adjacent to the distal end portion of the portion of the tapered St. diode,
This mode expander region and extend to the facets, semiconductor, characterized in that it is configured in a tapered shape to couple to waveguide receives light field emitted by each superluminescent diode light source.   The semiconductor light source of claim 1, wherein the superluminescent diodes are arranged in a monolithic array. Each of the superluminescent diodes has a structure in which a first clad layer of a first conductivity type, an active layer, and a second clad layer of a second conductivity type are sequentially disposed on the substrate,
2. The semiconductor light source of claim 1, wherein the structure is arranged to guide a light field emitted from the active layer to the mode expander layer.   The first cladding layer is composed of a first layer and a second layer, the first layer has a lower refractive index than the second layer, and the second layer is the first layer. 4. The semiconductor light source according to claim 3, which is disposed so as to cover the layer. 2. The semiconductor light source according to claim 1, wherein each of the channels has a width sufficient to prevent overlap of light fields of the superluminescent diodes, but allows the channels to overlap in the expander region. 4. The semiconductor according to claim 3, wherein the structure including the first cladding layer, the active layer, and the second cladding layer is configured to guide an optical field emitted from the active layer to the mode expander region. light source. The semiconductor light source of claim 1, wherein the mode expander region is configured to allow vertical and lateral expansion of a light field emitted by each superluminescent diode. A plurality of superluminescent diodes formed in parallel on a semiconductor substrate having a facet, their respective superluminescent diodes of diamond shaped front and rear portions Ru Oh part of the tapered shape of each superluminescent diode Having an active region, forming a plurality of channels separating each superluminescent diode between each superluminescent diode ;
Forming a mode expander region adjacent to one tip portion of the tapered portion of the superluminescent diode;
This mode expander region and extend to the facets, the semiconductor light source, characterized in that the Ru configured in a tapered shape to couple to waveguide receives light field emitted by each super luminescent diode Manufacturing method. The method of claim 8 including placing the superluminescent diodes in a monolithic array. Each of the superluminescent diodes includes a step of sequentially forming a first cladding layer of a first conductivity type, an active layer, and a second cladding layer of a second conductivity type on a substrate,
9. The method of claim 8, wherein the substrate is of a first conductivity type, and the layer is arranged to direct a light field emitted from the active layer to the mode expander layer. Forming the first cladding layer includes depositing a second layer over the first layer, the first layer having a lower refractive index than the second layer. The method according to claim 10 . The method of claim 8 including depositing a facet coating on the facet. The method of claim 10, wherein the first cladding layer, the active layer, and the second cladding layer are configured to direct an optical field emitted from the active layer to the mode expander region. The mode expander region is the super luminescent diode.
9. The method of claim 8, wherein the method is configured to allow vertical and lateral expansion of the light field emitted by the card.

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