Semiconductor laser diode with advanced window structure

The invention relates to a semiconductor laser diode structure with increased catastrophic optical damage (COD) power limit, featuring three sections, sometimes called windows, at the output facet of the diode. These include an optically transparent section, a current blocking section and a partially current blocking section.

TECHNICAL FIELD

The present invention relates to semiconductor lasers, in particular high power lasers where catastrophic optical damage to the facets is one of the main factors limiting the maximum optical output power.

BACKGROUND OF THE INVENTION

One limiting factor for the maximum power of a semiconductor laser diode design is the catastrophic optical damage (COD) limit at the output front facet of the laser diode. Essentially the very high optical power density, current density and carrier density interact with defects, non-radiative recombination centers, optical absorption areas and the semiconductor/coating/air interface to cause excess heating and eventually destructive failure. Various methods have been employed to increase the COD limit which will be discussed along with the new inventions.

Note that the COD generally occurs at the output front facet due to the higher optical power density relative to the rear facet. This invention applies primarily to the front facet window, however the same considerations as disclosed in this invention can also be applied to the rear facet. The front facet typically has a low reflectivity coating or coating stack deposited after an optional surface passivation. The rear facet is typically coated with a high reflectivity coating or coating stack. Also note that this window design applies to any high power semiconductor laser at any lasing wavelength including, but not limited to, single-mode lasers, multi-mode lasers, fiber-coupled lasers, distributed Bragg reflector (DBR) lasers and distributed feedback (DFB) lasers.

One of the first structures to improve COD is the use of a transparent window area3as shown inFIG. 1, which shows an output end10of a semiconductor laser in longitudinal cross-section. Basically a quantum well active region2is isolated from semiconductor/facet coating6/air interface at front facet5. This reduces the local optical absorption and heating near the front facet5. Also typically the transparent window area3blocks most of injected current1, which is reduced to a leakage current11considerably smaller than the injected current1. The transparent window area3can be formed using a variety of methods including, but not limited to, local etch and regrowth, ion implantation, or diffusion intermixing.

Welch et al. (U.S. Pat. No. 4,845,725) disclose a preferred structure, which employs impurity induced disordering to smear the interfaces between active region and cladding layers producing a waveguide layer with increased bandgap and thus a transparent window region at the laser facet and a graded transverse refractive index profile. Window regions having transparent waveguide layers can be produced by impurity induced disordering, i.e. the diffusion of silicon, zinc, tin or other impurity through the semiconductor layers to form the window region.

The fabrication of the structure by Welch et al. entails several diffusion or implantation steps as well as crystallographic disordering, all of which are notorious for introducing non-radiative recombination centers. Since the introduced non-radiative recombination centers extend into the active layer of the laser, which is electrically pumped, they represent a serious laser degradation and reliability risk.

There are two problems with this design. There can be leakage current through the transparent window area3or, even worse, along the facet5, which has non-radiative recombination centers, even with facet coating passivation6. Also the interface16between the quantum well and transparent window typically will have defects and current flowing near those defects will recombine non-radiatively.

Another structure for improving COD uses a current blocking area, or unpumped structure4, as shown inFIG. 2, which is a longitudinal cross-sectional view of an output end20of a semiconductor laser. In this case one or more blocking materials or layers4prevent current flow near the front facet5and therefore reduce non-radiative recombination near the front facet5. The current blocking area4can be formed using a variety of methods including, but not limited to, patterned contact metal, a blocking insulator (such as silicon nitride, silicon dioxide, aluminum oxide, titanium oxide, etc.), a current blocking implant or diffusion, or current blocking semiconductor layers (such as etching off the contact layer or doing an etch and blocking re-growth).

Yu et al. (U.S. Pat. No. 6,373,875) disclose such a semiconductor laser structure incorporating a current blocking area4, specifically to prevent current leakage near the facet5, as illustrated inFIG. 2.

There are two problems with this design. There will be optical absorption in the quantum well active region2along the section25where it is not pumped to transparency. Also the semiconductor/facet coating6/air interface at the front facet5will typically be a location of defects causing further optical absorption.

Kamejima (U.S. Pat. No. 4,759,025) discloses a semiconductor laser structure which attempts to resolve COD problems by using an intermixing technique. The laser structure is grown with thin multiple layers located at the intended active layer. The area to be pumped electrically by an electric current is thermally interdiffused by laser irradiation to form a mixed crystal exciting region having a band gap narrower than that of the surrounding layer, which is thus transparent to emitted light. In particular the non-interdiffused area near the laser facets becomes transparent to the laser emission. Also, the pumping current flows preferentially through the lower bandgap interdiffused area, thereby reducing exposure of the laser facets to the pumping current.

Kamejima's structure is unsuitable for high power lasers for a fundamental reason, however. To achieve high output power levels and high efficiency, a single or multiple quantum well (MQW) active layer consisting of one or more very thin semiconductor layers is generally preferred. The intermixing process on which Kamejima's structure depends destroys the quantum well structure.

An object of the present invention is to improve the laser structure for use at high optical power output levels by mitigating the adverse thermal effects in the vicinity of the laser facets such as recombination diverting some of the pumping current to non-radiative recombination centers and optical absorption.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a semiconductor laser structure pumped with an injected electrical current for high optical power output. The key features include an optically transparent section, and a current blocking section at an output facet of the diode.

The optically transparent section is protected from the injected pumping current by the overlapping current blocking section.

Another aspect of the present invention relates to an additional partially current blocking section. To further shield the output facet from the pumping current, the partially current blocking section produces a reduced profile in the injected pumping current density. Sufficient pumping current is provided however to maintain optical transparency this section.

DETAILED DESCRIPTION

A first embodiment, shown in longitudinal cross-section inFIG. 3, comprises an output facet end30of a semiconductor laser incorporating a quantum well active region2, a contact region for injecting pumping current1into a main section38of the quantum well active region2, an optical reflector generally known as a laser facet5coated with a facet coating6for passivation and reflectivity control, a current blocking area34, and a transparent window33. The quantum well active region2may comprise a single quantum well, or a multiple quantum well structure. Semiconductor materials suitable for the quantum well active region2include, but are not limited to, AlGaAs, GaAs, GaAsP, InP, InGaAs, and InGaP. The output optical field profile is represented by curve37. The transparent window33limits the optical absorption under the current blocking area34, which substantially removes undesired electrical injection current leakage through the transparent window33, or along the front facet5. It also substantially removes electrical injection current near the interface36between the quantum well active region2and transparent window33. For this reason length, L2, of the current blocking area34would typically be longer than length, L1, of the transparent window33. However, there may be material combinations and structures, in which L2is shorter than L1. Furthermore these lengths need to be optimized for best laser diode performance, high COD power limit and highest reliability under operating conditions.

FIG. 4is a top view of the first embodiment showing the output facet end40of a semiconductor laser with a current injection area48which extends to within a distance L2from the facet5coated with facet coating6. The transparent window43extends from the facet5for a distance L1. The output optical field profile is represented by curve47.

A final problem to be addressed is excess optical loss under the current blocking area34that is not within the transparent window area33as shown inFIG. 3.

Accordingly a second embodiment of the invention, shown in longitudinal cross-section inFIG. 5, comprises an output facet end50of a semiconductor laser incorporating a quantum well active region2, a contact region1for injecting pumping current into a main section58of the quantum well active region2, an optical reflector generally known as a laser facet5coated with a facet coating6for passivation and reflectivity control, a total current blocking section54of length L2A, a partially blocking section55of length L2B−L2Athrough which pumping current from a contact region51is injected into a partially pumped section52of the quantum well active region2, and a transparent window section53of length L1. The output optical field profile is represented by curve57.

A section the quantum well active region, which does not receive electrical pumping will absorb lasing light emitted by a pumped section58of the quantum well active region2, thus reducing lasing efficiency and causing localized heating. The transparent window section53is introduced to offset such potential problems by reducing optical losses by absorption and consequent heating.

The total current blocking section54removes undesired electrical injection current leakage through the transparent window section53or along the front facet5and removes electrical injection current near the interface56between the quantum well active region2and transparent window section53. The partially blocking section55provides a means for reducing electrical current density in the partially pumped section52of the quantum well active region2produced by pumping current from the contact area51at an end portion(s) of the quantum well active region2. Typically the current density in the partially pumped section52is a fraction of the current density resulting from injecting pumping current from the contact region1into the main section58of the quantum well active region2.

For the reasons discussed in relation to the first embodiment, a typical design has L1<L2A. For the partially blocking section55to have finite length for obtaining a profiled transition in current density between the total current blocking section54and the main section58pumped by pumping current from contact region1, the lengths must also fulfill the relationship, L2B−L2A>0. Furthermore these lengths need to be optimized for best laser diode performance, high COD power limit and highest reliability under operating conditions.

Beneath the partially blocking section55the current density in the partially pumped section52will be lower than in the main lasing section58but just enough to reduce optical loss and improve the overall COD power limit.

As shown inFIGS. 6a-d, the contact areas68aa,68bb,68ccand68dd(corresponding to51inFIG. 5and disposed generally above the partially blocking section55) can be formed using a variety of methods including, but not limited to, patterned contact metal, multiple contacts, a patterned blocking insulator (such as silicon nitride, silicon dioxide, aluminum oxide, titanium oxide, etc.), a patterned current blocking implant or diffusion, or patterned current blocking semiconductor layers (such as etching off the contact layer or etching and blocking re-growth).

InFIG. 6aa top view of the third embodiment shows the output facet end of a semiconductor laser60awith current injection area68a. An extra contact68aaadjacent to the total current blocking section64and separated by a gap69from the main contact area68acan be used to inject a reduced current density in the quantum well active region below it.

InFIG. 6ba top view of the fourth embodiment shows the output facet end of a semiconductor laser60bwith a plurality of contact stripes68bbdisposed between the total current blocking section64and the main contact area68bfor injecting a reduced current density in the quantum well active region below it (e.g. the partially pumped section52inFIG. 5).

InFIG. 6ca top view of the fifth embodiment is shown comprising the output facet end of a semiconductor laser60cwith current injection area68ccomprising a plurality of contact fingers68ccextending to the total current blocking section64for injecting a reduced current density in the quantum well active region below it (e.g. the partially pumped section52inFIG. 5).

The sixth embodiment shown inFIG. 6dis similar to the fifth embodiment, the difference being in the tapering of the extended contact fingers68ddof the current injection area68dfor controlling the spatial profile of the injected current density.

Various experiments were performed to show that adding a partial pumping region improves the COD limit and device reliability.FIG. 7shows results for applying various current levels to a small separate contact (such as68aainFIG. 6a) near the front facet5. In the case of no current in the extra contact68aanear the front facet5(corresponding to the embodiment ofFIG. 3) the COD level ranges from 3.2 W to 4.0 W. In the case of uniform current injection (corresponding to the design in prior artFIG. 1with transparent window only) the COD ranges from about 3.2 W to 3.8 W. At approximately one-half baseline current density the COD range narrows and increases to 3.6 W to 4.1 W. This indicates that a patterned pumping scheme designed to inject about one-half the current density can improve the COD.

FIG. 8shows the result of using a patterned partial pumping section (such as inFIG. 5andFIG. 6) designed for injecting about one-half the current density. Going from unpumped to 50 μm partially pumped to 100 μm partially pumped improves COD level significantly. This particular set of samples illustrates the greatest improvement but in no set of samples tested case does the COD level become worse.

FIG. 9shows the yield results of an experiment for laser diodes with a baseline combined transparent and unpumped current blocking section compared to laser diodes with an additional partially pumped/partially blocked section. The yield through a standard burn-in increases significantly from 58% to 95%, while the yield to a destructive COD test after burn-in increases even further from 78% to 100%. Again, this particular set of samples shows the most improvement but in no set of samples does the COD level become worse when using a partially pumped section.

The above experiments were conducted while injecting one half of the current density; however, alternative amounts of current injection are within the scope of this invention, e.g. preferred current density about 30% to 70%, more preferred current density about 35% to 60%, most preferred current density about 40% to 50%.

Preferred lengths for a laser cavity, LC, are greater than about 0.5 mm, more preferred values range approximately from 0.5 mm to 10 mm, the most preferred range being about 0.7 mm to 5 mm.

Accordingly, the current blocking area has a preferred length L2Afrom the output facet of up to about 5% of the total laser cavity length LC, more preferably between about 1 μm and 50 μm, and most preferably from about 2 μm to 30 μm.

The length L1of the transparent window area is equal to or less than the length L2Aof the current blocking area. Preferably L1should be greater than about 0.5 μm and less than about 1% of LC.

The partially pumped (or partially current blocking) section has a length L2Bfrom the output facet of up to about 10% of LC, more preferably between about 10 μm and about 5% of LC, and most preferably between about 30 μm and about 3% of the LC, wherein LC>L2B>L2A>=L1. The partial current blocking area extends from the current blocking area to the desired distance L2Bfrom the front facet5.