JP2002518857A - High power semiconductor light source - Google Patents

Abstract

The semiconductor light source comprises super luminescent diodes (SLDs) 110 disposed on a faceted substrate 58, each of which is separated by a channel 120 and comprises a mode expander region. 160. Each SLD has a diamond-shaped active region in which the front and rear ends of each superluminescent diode are tapered. The mode expander region 160 is located on at least one side of the taper of the SLD and tapers into a waveguide extending to the facet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD 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 having 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 diodes are powerful, compact and efficient light sources, with power density capacities on the order of 10 ⁷ W / cm ² generated in active layers 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 this active layer, and current can flow therethrough. It is formed by epitaxial growth on a substrate. One common method of forming an active waveguide is to form a channel in the structure by etching to form a ridge. Light is guided through the ridge by total internal reflection because the refractive indices of the channel and cladding regions are slightly lower than the effective refractive index of the active region below the ridge. Typically, the walls of the ridge are parallel and light is transmitted in a zig-zag path through the ridge at a number of discrete angles defined by the propagation modes in the structure. For each mode there is an effective index of refraction, which is determined by the zigzag angle. The lowest mode has the smallest zigzag angle.

FIG. 1a is a perspective view of a typical ridge waveguide laser or Fabry-Perot laser. FIG. 1B is a top view showing the propagation characteristics. In a Fabry-Perot laser, feedback for laser oscillation is provided by reflection from a facet. One of the facets reflects only partially, and thus a portion of the light is emitted. The spectrum of light is very narrow, as shown in FIG. 1c, and is often less than 1 Angstrom. When the width of the ridge is within the range specified by Expression 1 described below, 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 with a spatial distribution as shown in FIG. 1d. Most applications require that the laser operate in single mode.

In the laser of FIG. 1a, the active waveguide is perpendicular to the facet. The same structure is achieved by treating the active waveguide at an angle with respect to the facet.
Can be configured as 2a and 2b show the same laser structure configuration 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. Figure 2 shows the waveguide ridge angle.
Can be selected so that as little light as possible is reflected and combined as shown in FIG. For example, if the angle of the waveguide is 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. 2d.

[0005] SLD is a light source that has several important functions. First, it is an optical amplifier capable of amplifying optical signals of 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 coherence. Third, it includes a tunable laser without unwanted laser modes caused by facet 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.

Low power (several mW) semiconductor lasers having an active layer length less than about 500 μm and a width of about 1-5 μm are widely used in CDs, CD-ROMs, optical data storage, and optical communications. I have. However, in a single "guide" mode that can be easily coupled to a single mode optical fiber, about 10 to 10
New applications requiring powers on the order of 0 W (watts) are emerging. Examples of such applications are light sources in projectors or laser printers. In view of their high saturation power density, such power can be achieved by semiconductor laser materials by forming a sufficiently wide active waveguide layer.
For example, an active waveguide having a width of 1 mm and a thickness of 1 μm has an area of 10 ⁻⁵ cm ² and has an area of 10 ⁻⁵ cm ^2.
Will produce output power up to 0W. However, such power can only be obtained in multi-mode and not in single mode.

As described above, waves propagate in multiple modes, which are characterized by the discrete zigzag angles 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. For a waveguide with a core refractive index of n ₁ , a cladding refractive index of n ₂ , and a wavelength of λ, the width for a single mode is determined by Equation 1 below. (1) for a typical laser having a d ≦ λ / [2 (n 1 2 -n 2 2) 1/2] n 1 = 3.2 and n ₂ = 3.195, d is a wavelength of 980nm Is less than 3 μm and the area for a thickness of 1 μm is less than 3 × 10 ⁻⁸ cm ² , and therefore the maximum output is about 0.3 W. 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 strip widths of about 100 μm have been fabricated for output powers of a few watts, and arrays with higher powers have been obtained, but they are multimode and have limited use.

[0008]

[Problems to be solved by the invention]

It is an object of the present invention to generate a high power laser (about 10-100 W) with a single mode output to manage the power available in a wide strip width active area.

[0009]

[Means for Solving the Problems]

The present invention provides a semiconductor light source, the semiconductor light source comprising a plurality of superluminescent diodes (SLD) disposed on a first conductive type substrate having facets.
Each of the SLDs has a diamond-shaped active region such that the front and rear ends of each SLD are tapered, and further separates each superluminescent diode. A plurality of channels and a mode expansion region disposed on at least one side of the taper of the SLD, the mode expansion region being tapered into a waveguide extending to the facet. It is characterized by. Mode expansion is desired for high power,
It 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 with a mechanism for optical feedback in that mode, is coupled to the facet.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be more fully understood from the following detailed description of the preferred embodiments which refers to the accompanying drawings. Preferred embodiments of the system and method of the present invention that meet the above objectives and provide other features are described below with reference to FIGS. Those skilled in the art will readily recognize that the following description with reference to these drawings does not limit the technical scope of the present invention, but is merely illustrative.

It is an object of the present invention to prevent higher order modes such as "diamond" structures and "adiabatic" power transfer means for mode expanding waveguides having relatively large cross sections. This is achieved by creating a basic structure for generating power in a large area active waveguide having means for generating the same. Means are also provided for monolithically combining the basic structure with the desired large passive structure, which is adiabatically tapered to a single mode waveguide. In the case of a laser, further means are provided to ensure that the feedback takes place only in the desired vibration mode.

As mentioned in the background, the output power of a single mode laser is determined by the width required for single mode operation due to the zigzag nature of the propagation, so that the volume occupied by the active waveguide is small. Therefore, it is limited to a fraction of 1W. Widening the waveguide section increases the number of propagation modes and generates mode output power, but wastes extra power. 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.

A detailed description of a diamond-shaped SLD with a double taper structure that achieves the purpose of blocking the propagation of higher-order modes is given in US patent application Ser. No. 08/857929. This patent application is incorporated herein by reference. This diamond shape is substantially called a rhombus, a rhombus rotator, or a parallelogram when viewed in two dimensions.

FIG. 3 shows an exemplary diamond SLD designed to operate at 830 nm
It is a top view of one Embodiment of. 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 facets 15 and 20 at the ends. The taper angle α of both halves of the diamond shape is selected to be less than the lowest mode zigzag angle to ensure that the zigzag pattern is blocked. To prevent radiation loss, a taper angle of less than about 3 degrees is selected with respect to the longitudinal centerline of the diamond as shown in FIG. The entire diamond itself is arranged at an angle θ with respect to the facet. Is preferably less than the critical angle at the facet of about 15 degrees. Preferably, the strip angle θ ranges from about 5 to 10 degrees. The taper angle α is the angle between the side wall of the diamond and the central axis of the strip, and the angle θ of the strip is the angle of the longitudinal axis of the strip to the normal of the facet.

The way in which zigzag propagation is prevented is shown in FIG. In FIG. 4, dashed line A shows the waveguide with parallel side walls and the zigzag propagation of light rays inside. Line B shows the portion of the diamond near the left facet, and solid line C shows that the light reflected from one side wall 17 diverges from the opposing side wall 18, indicating that zigzag propagation is blocked. In fact, tapers reduce the angle of reflection and collimate light efficiently.

The width at the facet 15 is about 10 μm (and about 10 ⁻⁷ cm ^{2 in} cross section),
The SLD of FIG. 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 taper to the desired final dimension. Additional power is extracted.

One object of the present invention combines the structure of a mode expander (modex) with the concept of the diamond double taper structure of FIG. 3 to extract all available power at the output of a Gaussian distribution shape. Achieved by: Mode expanders reduce the power density for driving high power. A complete description of an SLD with a mode expander is described in US patent application Ser. No. 08 / 946,180. The combination of a mode expander and a diamond structure to capture power in a thick passive waveguide is shown in FIG. 5a.

FIG. 5 a shows a truncated diamond SLD structure 50 having a mode expander region 60 in a semiconductor chip 40. FIG. 5a shows a diamond ridge structure 50, which instead of extending to facets 52, 54, modex in semiconductor structure 40.
3 except that the diamond terminates somewhere in region 60.
Is similar to

The refractive index of the region 60 outside the diamond is smaller than the refractive index of the diamond portion 50. As the light field generated in the diamond SLD structure 50 consequently approaches the diamond tip, it expands from the diamond to a lower index of refraction region 60 where it propagates away from the diamond 50. This structural part is lossless and passive.

FIG. 5 b shows a side view of the structure of FIG. 5 a, showing 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 typical 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. According to the present invention, an n-type cladding layer is disposed on a substrate 58, which has one thick layer (thick guide) 70 having a slightly lower refractive index than diamond as described above.
And a different layer (outer n-type cladding) 73 having the same n-type but a different composition giving 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.

Since the refractive index of the p-type cladding layer 65 and the outer cladding layer 73 is lower than that of the thick guide layer 70, the light field propagates in the thick guide 70 by the p-type cladding layer 65 and the outer cladding layer 73. And propagates through the thick guide 70 in the direction of the facets 80, 82. While the active layer of the SLD structure 50 has an overall thickness of less than 1 micron, the thickness of the thick guide 70 can be several microns, typically 3-4 microns. A high power 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. In the active region, the power density is higher due to mode expansion. It is a lossless region of density. If the waist portion of the diamond is about 20 μm and the length is about 2 mm, the generated power is about 3 to 4 W, and as shown in FIG. (Having a length of 0.1 mm to about 0.2 mm).

Without lateral guidance, the field expands laterally to the facet and is emitted at an angular width determined by the taper angle of the diamond. The lateral guide extends by extending the channel region 53 in FIG.
Alternatively, it may be achieved by adiabatically tapering to the desired output beam size near the facet.

Referring to FIG. 5b, the device is typically a binary III-type device such as GaAs or InP.
It is composed of group V compounds. In a GaAs system, structure 58 is typically about 10
The first main surface is made of n-type GaAs having a thickness of 0 μm and is parallel to the (100) crystal plane or slightly different in direction. The cladding layers 65, 70, 73 are typically composed of Al _x Ga _{1 -x} As, where 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, typically Al _x Ga _1-x As, where x is generally about 0 and 0.1, depending on the emitted wavelength and I have. Cap layer 75 is typically about 0.5 μm thick and is typically composed of p-type GaAs. 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.

According to the present invention, a high power single mode structure is formed from this geometry, and the output is formed to satisfy conditions for efficient coupling to an external device such as a single mode optical fiber. Have been. Additional power is generated from the diamond structure 50 by increasing its waist. However, the transition from the diamond structure 50 to the mode expander 60 has a diamond width of about 5 μm.
It does not occur until it is less than m. 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 when it escapes the active area. However, the problem of filamentation leads to large area structures that can significantly reduce the life of the device. At very high power densities, some impurities, some refractive index changes are induced by the drive current, which results in local changes in power density. These changes result in hot spots or filaments that degrade the material and cause high absorption. The degraded area spreads slowly over time, producing heating that can be a catastrophic failure. To maintain 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 a strip burns out, the defect is localized, so that other strips do not affect it.

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 is provided in parallel, separated by a narrow channel 120. Diamond structure 11
The lengths of 0 need not be the same. Diamond width at waist is 20-30μ
m is preferable. Channel 120 may be as narrow as about 5 μm. The width must be sufficient to prevent overlapping of the individual diamond structure 110 fields, but the thick waveguide mode expander region beyond the individual waveguide transitions
At 160 it is possible to overlap. The mode expander area 160
A relatively long (approximately 1-2 mm) tapered waveguide is formed to allow the combination of power from all individual diamonds, with an additional taper 17.
It is generated with a thick waveguide passive region to produce 0 and achieves the desired mode size in facets 180,182.

The structure shown in FIG. 6 is sufficient to generate a laser. It is necessary to provide a suitable facet coating to provide feedback. However,
It may not be enough to ensure that substantially all available power is present in a single transverse mode. To achieve single mode, a single mode guide section is provided to filter out higher order single modes and feedback is provided only for the desired lowest mode guide. The length of such a guide is preferably on the order of a few cm, 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, which can cause vibrations to occur in some unwanted modes. The array 110 of individual diamond structures is therefore configured as an SLD,
Process it at an angle (about 5-10 degrees) with respect to the centerline of 0. FIG. 7 shows this preferred embodiment.

FIG. 7 shows a preferred structure 100 of the SLD structure. Two facets 80, 1
A single mode optical fiber 190, 192 is coupled to the output at 82. The single mode optical fibers 190 and 192 function as mode filters as described above. The output dimension of the thick tapered passive waveguide 170 is about 3 μm × 6 μm low, which is sufficient to couple about 80% or more of the light available in optical fibers 190,192. Passive waveguide 170 has a much higher power density capacity than active diamond material due to its low loss. If each element provides about 3-4 W, only about 20-30 elements take it and produce about 100 W of output light. Small overall dimensions (about 2 mm x 4 mm including guard space and channel)
), A 3 inch diameter wafer produces over 400 chips at a time,
Therefore, a low-cost device is obtained.

Optical fiber mode filters 190, 192 to fabricate a laser from the structure of FIG.
One tip is coated with a high refractive index mirror or configured as a highly reflective grating 195, and the other tip is provided with a partially reflecting mirror, or grating 197. This means that feedback is provided only in modes that propagate in the optical fiber, regardless of the number of modes that can propagate in the semiconductor structure 100. The result is a high power single mode laser, the output power of which 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. Slope efficiency is 1 W at a supply voltage of about 2 volts
/ Ampere low. The single mode optical fibers 190, 192 can be replaced by a suitably long multimode optical fiber to provide a specific mode number of multimode outputs to the pump. Thus, the pump is fed back to set a particular mode pattern where the optical fiber is an SLD with sections and the light source is a laser.

There are many applications for semiconductor lasers from 10 to 00 W, without feedback, the device has an equivalent laser output power of about 1/4 for the same drive current, acts as an SLD, Applicable to coherent medical imaging, smart structure and time domain reflectometer. Applications as lasers include light sources for plate printing, surgery, and electronic projection systems, and the like.

In view of the above, the basic concept of a semiconductor laser according to the invention is to use a superluminescent active structure of the SLD structure as a gain medium to provide sufficient volume for high output power. is there. The SLD structure blocks feedback from the facets and otherwise causes the laser to oscillate in an undesirable mode.
Mode feedback is provided by an optical fiber structure outside the semiconductor chip.

Although illustrated and described herein with reference to certain embodiments, the present invention is not limited to the details shown, but rather various modifications may be made within the scope of the present invention. Can be performed without departing from Thus, semiconductor materials other than those illustrated and other compositions of the selected semiconductor material may be used.

Also, the conductivity types may be reversed (similarly) substituted. Variations, differing from the manufacturing method shown here, are also possible by those skilled in the art, and other techniques,
For example, another method for forming a semiconductor layer can be effectively used.

[Brief description of the drawings]

FIG. 1 is a perspective view and a top view of a conventional 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 superluminescent diode of the prior art, and FIG.
FIG. 3 is a characteristic diagram of the reflection versus the angle of the LD, and its output spectrum characteristic diagram.

FIG. 3 is a top view of a diamond SLD according to one embodiment of the present invention.

FIG. 4 is an explanatory diagram of a collimating effect of a waveguide having a taper for preventing zigzag propagation according to the present invention.

FIG. 5 is a structural diagram of a combination of a mode expander for capturing power in a thick passive waveguide according to a preferred embodiment of the present invention and a diamond structure.

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.

FIG. 7 shows an SL with a single mode optical fiber coupled to the output with two facets
FIG. 7 is a schematic diagram of a preferred embodiment of the present invention showing the pattern of FIG. 6 in a D structure.

Claims (10)

[Claims] A superluminescent diode disposed on a substrate of a first conductivity type having facets, each superluminescent diode having a front end and a rear end of each superluminescent diode. Having a tapered diamond-shaped active region, and further comprising a plurality of channels respectively separating each superluminescent diode, and disposed on at least one side of the taper of the superluminescent diode. And a mode expansion region, wherein the mode expansion region is tapered in a waveguide extending to the facet. 2. The semiconductor light source according to claim 1, wherein said superluminescent diodes are arranged in a monolithic array. 3. In each of the superluminescent diodes, a first cladding layer of a first conductivity type, an active layer, and a second cladding layer of a second conductivity type are sequentially arranged on the substrate. The semiconductor light source according to claim 1, further comprising a structure for guiding a light field emitted from the active layer to the mode expansion layer. 4. The first cladding layer comprises a first layer and a second layer,
The first layer has a lower refractive index than the second layer, and the second layer has a lower refractive index than the first layer.
4. The semiconductor light source according to claim 3, wherein said semiconductor light source is disposed so as to cover said layer. 5. The invention of claim 1 wherein said channel is allowed to overlap in said enlarged area while having a width sufficient to prevent overlap of the light field of said superluminescent diode. Semiconductor light source. 6. A plurality of superluminescent diodes are arranged on a semiconductor substrate having facets, each of the superluminescent diodes being a diamond-shaped diode having a tapered front and rear end of each superluminescent diode. An active region, forming a plurality of channels respectively separating each superluminescent diode, forming a mode expanding region on at least one side of the taper of the superluminescent diode, Is a taper in a waveguide extending to the facet. 7. The method of claim 6, including the step of placing said superluminescent diodes in a monolithic array. 8. 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. 7. The method of claim 6, wherein the substrate is of a first conductivity type, and wherein the layer is arranged to direct a light field emitted from the active layer to the mode expansion layer. 9. The method of claim 1, wherein forming the first cladding layer comprises depositing a second layer over the first layer, wherein the first layer has a lower refractive index than the second layer. 9. The method of claim 8, wherein said method has a ratio. 10. The method of claim 6, including the step of depositing a facet coating on said facet.