Stack-type diode laser device

Two thin-clad laser diodes are disposed to form a stack-type diode laser device. The diodes emit two beams that are substantially parallel and in proximity such that they share many fiberoptic systems designed for a single beam. The diodes are coupled by leaky waves through top surfaces. The leaky waves are generated by a thin metal contact layer or diffractive gratings. The stack-type device is employed for single-mode power enhancement and tunable lasers.

FEDERALLY SPONSORED RESEARCH

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SEQUENCE LISTING OR PROGRAM

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BACKGROUND

1. Field of Invention

This invention relates to semiconductor lasers, and particularly to stack-type semiconductor laser devices.

2. Description of Prior Art

As used here, the term “semiconductor laser” or “diode laser” means edge-emitting diode lasers. Edge-emitting diode lasers have a horizontal cavity and emit light beams in a direction parallel to the wafer's plane or an active region of the wafer. Most semiconductor lasers in use are edge-emitting and are used for laser printers, fiberoptic telecommunication systems, and optical data storage devices.

A diode emits an output light beam with a spatial power distribution. The output of a single transverse mode diode, or a single-mode diode, has a single lobe in its spatial power distribution, while a multimode output has multiple lobes. Single-mode diode lasers are desirable in many fields, especially in fiberoptic telecommunications and optical data storage. However one of the weaknesses of single-mode diode lasers is that they have relatively low output power.

The importance of tunable wavelength diode lasers has grown dramatically in fiberoptic telecommunications in recent years. Tunable lasers have three important specifications, i.e., output power, wavelength tuning range, and tuning speed. A typical high output power is about 20 milliwatts. A typical wide tuning range is about 40 nanometers, e.g., the diode can emit light from 1525 to 1565 nm. A fast tuning speed is in a range of several nanoseconds. Among various tunable lasers, distributed feedback (DFB) lasers offer high power, but suffer a narrow tuning range and a slow tuning speed. Three-section distributed Bragg reflector (DBR) lasers have a fast tuning speed and moderate power, but a narrow tuning range. Sampled grating DBR (SG-DBR) lasers and superstructure grating DBR (SSG-DBR) lasers have a wide tuning range and a fast tuning speed, but low output power. Conventional tunable external cavity diode lasers benefit from high power and a wide tuning range, but suffer a slow tuning speed. Thus current tunable lasers can't satisfy the three specifications: high power, wide tuning range, and fast tuning speed.

One approach to overcome limitations on the single-mode output power and tunable laser involves stacking two thin-clad laser diodes to form a stack-type diode laser device and coupling the diodes, as is disclosed in my above regular U.S. patent application. The thin-clad diodes emit two beams which are substantially parallel and proximate such that they can simultaneously feed a fiberoptic system designed for a single beam. When the diodes are coupled in phase, their outputs can be combined to increase single-mode power, or novel tunable lasers can be created to meet the three specifications. However, the disclosed coupling mechanisms rely on external feedbacks, which require additional optics. The resulting stack-type device is complicated and bulky.

OBJECTS AND ADVANTAGES

Accordingly, several main objects and advantages of the present invention are:

(a). to provide an improved stack-type diode laser device;

(b). to provide such a device which has a simple and compact structure;

(c). to provide such a device which has a simple and compact structure and generates higher output power than that obtainable from a single laser diode;

(d). to provide a tunable laser which has a simple and compact structure and has high power, a wide tuning range, and a fast tuning speed.

Further objects and advantages will become apparent from a consideration of the drawings and ensuing description.

SUMMARY

In accordance with the present invention, two thin-clad diodes are stacked together with the top surfaces opposite each other. The diodes are coupled by leaky waves through their top surfaces. The leaky waves can be generated by a thin metal contact layer or diffractive gratings. The resulting device is simple, compact, and can be used to combine output power of the diodes or create a tunable laser which has high power, a wide tuning range, and a fast tuning speed.

ABBREVIATIONS

DBR Distributed Bragg reflector

DFB Distributed feedback

REFERENCE NUMERALS IN DRAWINGS

DETAILED DESCRIPTION

FIGS. 1-Aand1-B show schematic cross-sectional and perspective views of a typical prior-art ridge-guide laser diode. The diode is fabricated on an n-type substrate20, which usually is a thin square or rectangular semiconductor chip with a thickness around one hundred microns. Deposited on substrate20are an n-type cladding layer14, an active region10of the diode, where light is generated, a p-type cladding layer12, a p-type contact layer16, two separate insulator regions15, and a metal contact layer18as a top electrode. The diode emits light beams from an elliptical region22on the edge or side of the diode.

As shown in the figures, a protrusion, ridge, or boss is formed by etching layer16and part of layer12in the center portion of the chip. A top surface comprises the upward surface area of the protruded part of layer18. The layers between active region10and the top surface have a total layer thickness of about two microns for a regular thick-clad diode. In the case of a thin-clad design, the total layer thickness can be reduced to several tenths of one micron.

FIG. 1-Cillustrates schematically a prior-art stack-type diode laser device. The arrangement comprises a lens system48, a single-mode fiber50, and thin-clad diodes40and42which generate respective output beams44and46. The beams are coupled into fiber50by lens system48. The diodes are opposite each other, or in other words, their top surfaces face each other. Due to the nature of thin-clad diode, when diodes44and46are disposed such that they are in close distance, beams44and46can be arranged to be substantially parallel and proximate. For example, the separation between the two beams can be smaller than one micron. In such a case, two beams at 1.55-micron wavelength can share lens system48and fiber50simultaneously with a relatively small additional coupling loss, which is about 0.2 dB comparing to a case where a single diode is coupled to a single-mode fiber through a coupling lens.

However, the diodes inFIG. 1-Cdon't interact with each other directly. In the prior-art, the diodes are coupled by feedbacks which coupling optics generates. The coupling optics makes the stack-type device complicated and bulky.

FIG.2-A—Stack-Type Device with a Thin Metal Contact Layer

FIG. 2-Ashows schematically a cross-sectional view of a preferred embodiment of a stack-type device according to the invention (Supporting and bonding structures are not shown). The cross section is cut along a direction of light propagation. Thin-clad ridge-guide diodes37and38are opposite and each have a similar layered structure to that ofFIG. 1-A. Layers18and19are thin metal contact layers. Active regions24and25lie between regions26and28and regions17and21, respectively. Regions26and28contain cladding and contact layers. Regions17and21contain cladding and substrate layers. The diodes are disposed such that their top surfaces are proximate and their output beams (not shown) are parallel to a certain degree.

Because of the thin-clad design, light waves propagating inside the diode interact with the metal contact layer in a greater degree than a thick-clad diode. Parts of the light waves are reflected back by the metal layer, and parts of them leak out through it. Since the diodes' top surfaces are opposite each other, a portion of the leaking waves of one diode enters the other diode and vice versa. Thus each diode has a portion of its light wave coupled in the other diode. When the coupling intensity is strong enough, the diodes influence each other in terms of phase and mode selection of their propagating light waves. The light waves inside the diodes can be locked in phase.

The coupling efficiency of the two diodes depends upon the intensity of the leaking waves. The leakage intensity in turn depends upon property of the metal contact layer, the metal layer thickness, and the diode structure. The thinner the metal layer, the larger the leakage. But the metal contact layer can't be too thin. First, leakage causes power loss; second, the metal contact layer is responsible for electrical contact so it must have an adequate thickness.

FIG.2-B—Stack-Type Device Coupled to a Single-Mode Fiber

When two diodes of a stack-type device are coupled in phase directly, the device can have a simple and compact structure to provide power enhancement and a wavelength tuning mechanism.FIG. 2-Bshows schematically a cross-sectional view of an embodiment of a stack-type device according to the invention. Thin-clad diodes37and38are phase locked by leaking waves through their top surfaces. Their output beams are coupled into a single-mode fiber54by lens system48. Fiber54has an anti-reflection coating52on its angled end to reduce unwanted feedback, which can cause instability of output power and wavelength.

When light waves in diodes37and38are in phase, constructive interference occurs between their output beams and their output power is combined. The diodes can be of same or different types that have a thin-clad design and a thin metal contact layer, e.g., Fabry-Perot, DFB, DBR, or a semiconductor amplifier, which is sometimes called a gain chip, as along as their output spectra partially overlap. When a DBF diode having a narrow spectral width around 1550 nm and a broadband diode having a spectral range from 1535 to 1565 nm are coupled, the DFB diode dominates output wavelength of the broadband diode, and the diodes' power are combined at the DFB's wavelength. The stack-type device inFIG. 2-Bis simple and compact and achieves larger single-mode output power than a single diode.

It is well known to those skilled in the art that phase locking between two diodes in such stack-type device is affected by diode characteristics and the spacing between the two top surfaces besides coupling efficiency. Therefore diode structure and dimensions and the spacing must be optimized. Moreover, drive current and temperature of the diodes can be used to fine tune phase relation between the diodes. In a passive phase locking mechanism, values of the drive current and temperature are chosen and fixed. In an active case, the values are adjusted in a feedback loop according to output power received by a power monitor.

The embodiment ofFIG. 2-Bcan also be used to create schemes to tune the output wavelength. The tuning mechanism is quite similar to that of a SG-DBR or a SSG-DBR laser. SG-DBR and SSG-DBR lasers have a phase section, which fine tunes the phase, a gain section, which amplifies the light, and two Bragg reflector sections, which reflect light and feature a comb-like reflective spectrum. The output wavelength is selected by matching one peak of one comb to another peak of the other comb. To tune the wavelength, at least one spectrum is moved so that the two peaks coincide at another wavelength. Since a Bragg reflector causes considerable power loss and is difficult to fabricate, SG-DBR and SSG-DBR lasers, each having two of such reflector, suffer from low power and yield concerns.

To employ the embodiment ofFIG. 2-Bas a tunable laser similar to SG-DBR and SSG-DBR lasers, diodes37and38are designed to have a phase section, a gain section, and a Bragg reflector section which is located close to the diode's rear facet. The Bragg reflector gives the diodes a comb-like output spectrum. Since the diodes are coupled, their output wavelength can be generated by matching two peaks of the comb-like spectra. The resulting tunable laser has a similar fast tuning speed and a similar wide tuning range to SG-DBR and SSG-DBR lasers. But since each diode of the stack-type device has only one Bragg reflector, it has larger output power and a better yield. In addition, the total output power is further increased by constructive interference of the two diodes. Therefore the stack-type diode laser device produces high output power, fast tuning speed, and wide tuning range in a simple and compact structure.

FIGS.3and4—Thin-Clad Diodes with Diffraction Gratings

Besides a thin metal contact layer, there are other ways for light waves to leak out through the top surface of a stack-type diode device.FIG. 3shows schematically a cross-sectional view of a thin-clad diode having a diffraction grating on its top surface according to the invention. Like diode38inFIG. 2-A, the diode has region28containing cladding and contact layers, active region25, and region21containing cladding and substrate layers. As inFIG. 3, an x-axis lies in the light propagation direction and a z-axis in a direction perpendicular to the top surface. A grating area usually consists of repetitive arrays of grating elements. InFIG. 3, grating elements30and32are arranged along x-axis. Grating element30represents a metal row, which is made by selectively etching or depositing metal contact layer. Element32represents a row where metal layer is etched. The rows are parallel to the y-axis (Not shown). A grating period L is the length addition of elements30and32, as illustrated inFIG. 3. All the metal rows are connected electrically.

When interacting with an impinging light wave, a diffractive grating may generate waves in different directions. Each direction corresponds to a diffraction order which is represented by an integer, e.g.0,1, −1,2, −2, etc. Elements30and32are constructed to generate a second-order diffraction. When the light waves propagate along the x-axis inside the diode, a second-order diffraction causes a portion of the light to radiate out through the top surface along the z-axis. Although the grating functions as a distributed Bragg reflector (DBR), it is often called grating coupler since the grating is frequently used to couple light waves traveling inside an edge-emitting laser diode into an optical fiber in a direction perpendicular to the diode's surface.

DFB or DBR diodes in a stack-type structure may employ a second-order grating and be coupled by second-order diffraction assuming the diffraction sections are aligned. When the diodes are the same, the spacing between their top surfaces must be optimized so that the light waves in the diodes are in phase. When different diodes are used, their structure and dimensions must be optimized, too.

FIG. 4shows schematically a cross-sectional view of another thin-clad diode having a diffraction grating on its top surface according to the invention. The cross section is cut along a direction of light propagation in the diode. The diode ofFIG. 4is similar to the diode ofFIG. 3but has a different grating structure. InFIG. 4, a grating element36represents a row without a metal layer, and a grating element34a metal row; the latter is made by etching part of the contact layer, then depositing a metal contact layer in the etched place. As inFIG. 3, all the metal rows are connected electrically. Since element34is closer to the active region than element32inFIG. 3, it encounters stronger impinging light waves which propagate inside the diode. As a result, elements34and36generate stronger second-order diffraction and leaky waves.

CONCLUSION, RAMIFICATIONS, AND SCOPE

Accordingly, the reader will see that two thin-clad diodes of a stack-type device can be coupled by leaky waves through their top surfaces. The resulting stack-type diode laser device has a simple and compact structure. It produces a larger single-mode output than a single diode. In addition, it provides a tunable laser that has a wide tuning range, a fast tuning speed, and high output power.

Although the above description contains many specificities, these should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the presently preferred embodiments. Numerous modifications, alternations, and variations will be obvious to those skilled in the art.

For example, the diodes of a stack-type device can have different leaking structures, as long as adequate coupling efficiency is achieved.

Furthermore, other leaking structures besides a thin metal contact layer or a second-order grating which cause part of the light waves to radiate out through the top surface, can be used to couple stacked diodes.

Therefore the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.