Method for making a high power semiconductor laser diode
Semiconductor laser diodes, particularly high power ridge waveguide laser diodes, are often used in opto-electronics as so-called pump laser diodes for fiber amplifiers in optical communication lines. To provide the desired high power output and stability of such a laser diode and avoid degradation during use, the present invention concerns an improved design of such a device, the improvement concerns a method of suppressing the undesired first and higher order modes of the laser which consume energy and do not contribute to the optical output of the laser, thus reducing it's efficiency. This novel effect is provided by a structure comprising CIG—for Complex Index Guiding—elements on top of the laser diode, said CIG being established by fabricating CIG elements consisting of one or a plurality of layers and containing at least one layer which provides the optical absorption of undesired modes of the lasing wavelength. This CIG preferably contains an insulating layer as a first contact layer to the semiconductor. The CIG elements are manufactured by a selected sequence of processing steps, in particular several masking steps, and are specifically shaped, both in thickness and coverage of the lasers semiconductor body, to provide desired suppression characteristics. Further, the CIG elements may be combined with the contact layer usually providing the electrical input power to the laser diode.
This application is a continuation-in-part of U.S. application Ser. No. 10/245,199, filed Sep. 17, 2002. The entire disclosure of this application is hereby incorporated herein by reference.
DESCRIPTION1. Field of the invention
This invention relates to semiconductor laser diodes, in particular to ridge waveguide (RWG) diodes, and a method for making such diodes. RWG laser diodes are especially used as pump lasers in fiber optic networks and similar applications since they provide the desired narrow-bandwidth optical radiation with a stable light output power in a given frequency band. Naturally, output power and stability of such laser diodes are of crucial interest. The present invention relates to an improved method for making such a laser diode, i.e. an improved manufacturing process, the improvement in particular concerning the structure and design of the laser diode; it also relates to laser diodes manufactured by such an improved process.
2. Background of the Invention
Coupling light of a semiconductor laser diode into an optical fiber is a central problem within the field of optical networks, in particular when high power transmission/coupling is desired. Due to increasing channel density in DWDM (Dense Wavelength Division Multiplexing) long haul networks, and the power requirements at elevated temperatures in metro networks, maximizing the laser diode's operating light output power is a primary design criterion. The useful operating power is mainly limited by a “kink” in the L-I curves, i.e. the light output over current curves, indicating a beam steering in lateral direction. The occurrence of such a kink is influenced by the real refractive index step, the gain profile as well as spatial hole burning and local heating in the laser diode. Depending on the device structure, the laser diode suffers at a certain power level from the resonance between the fundamental mode and higher order modes in lateral direction. This has been shown by J. Guthrie et al in “Beam instability in 980 nm power lasers: Experiment and Analysis” in IEEE Pot. Tech. Lett. 6(12), 1994, pp. 1409-1411. Generation of higher order modes is highly undesirable since efficient laser to fiber coupling is only possible with the fundamental mode.
Since weakly guided semiconductor devices like ridge waveguide (RWG) laser diodes are preferred for high power applications, as shown by B. E. Schmidt et al in “Pump laser diodes”, Optical Filter Telecommunications IVA, Editors: Kaminov and Li, Academic Press, 2002, ISBN 0-12-395172-0, pp. 563-586, an improvement in RWG designs appears highly desirable.
Bowler U.S. Pat. No. 6,141,365 describes a semiconductor laser with a kink suppression layer. Reportedly, the latter limits the establishment of higher order lateral modes and thus increases the kink power of the device. Bowler also discloses disposing an optical layer along the optical axis of an RWG laser on both sides of the laser's ridge. However, shape and size of this kink suppression layer is essentially determined by the photoresist mask used to form the ridge. Bowler does not address utilizing the kink suppression layer's shape, thickness, and/or material for any particular purpose apart from general kink suppression. Also, the lasers described by Bowler have output powers of no more than 200 to 300 mW which is insufficient for many of today's technical applications.
Thus, it is a general object of this invention to devise a reliable design for a high power RWG laser diode which in particular provides a stable light output under all operating conditions and a sufficiently long life of such laser diodes. Hereinbelow, the term “high power” is used for an optical output power approximating 1 W. Laser diodes with 918 mW linear kink-free power have been realized with a design according to the present invention.
It is a further primary object of this invention to provide an advantageous and economical manufacturing method for a novel high power RWG laser diode, allowing reliable mass production of such laser diodes.
It is a more specific object of this invention to provide a RWG laser diode design optimally suited for realizing laser diodes with kink-free output powers in the 1 W region, and an increase of about 25% in median linear power (taken over about 700 devices) compared to a standard design.
DISCLOSURE OF THE INVENTIONThe principal design idea of the invention is to develop a structure of a high power RWG laser diode which controllably introduces additional optical losses for first and higher order modes, whereas the fundamental (or 0th order) mode experiences only minor influences.
It is known that high order lateral modes, e.g. the first order mode, exhibit a broader extension of the optical field in lateral direction than the fundamental mode. In other words, the lateral extension of the desired fundamental mode is smaller than that of the undesirable first order and higher order modes. These undesired modes can be suppressed by introducing optically absorbing regions parallel to the ridge waveguide.
Hence, depending on the location, an absorbing layer can function as a suppression layer for the first and higher order modes, without introducing significant absorption of the fundamental mode.
Due to the increased loss in the first order mode, resonant coupling occurs at much higher power levels and hence the linear power, i.e. the kink-free power, of the laser diode is significantly increased. Since attenuation of first and higher order modes is stronger than the same for the fundamental mode, this layer acts as a mode-discrimination element.
The absorption layer can be made of any material in which the imaginary part of the complex index of refraction is not zero for the wavelength in question, i.e. the lasing wavelength. The element that discriminates first and higher order modes can be a single layer or contain multiple layers, where at least one layer must have the desired absorption properties. Number and location of these mode-discrimination elements (or Complex Index Guiding, CIG, elements) within the laser diode structure as well as shape and number of layers contained within the element depend on the laser design and have to be individually optimized.
The improvement achieved by adding CIG elements to a standard RWG structure can be demonstrated. The linear power for a laser diode with CIG elements as described is significantly higher than for a similar standard laser diode. In one trial embodiment of a laser diode according to the invention, about 900 mW kink-free light output power was reached at an operating current of around 1.1 A. The median linear, i.e. kink-free, power taken over about 700 laser diodes increased by about 25% for laser diode structures containing CIG elements compared to standard diodes.
In a first series of experiments, the photoresist etching mask already used for ridge etching was employed as mask for RIE etching the insulating layer, similar to the method described by Bowler in U.S. Pat. No. 6,141,365, cited above. The insulating layer at both sides of the ridge was etched down to the semiconductor. Subsequently, the p-contact metallisation (Ti/Pt/Au) was deposited. The Ti layer of the metallisation functioned as the optically absorbing layer, i.e. the CIG element, in this case. Depending on the laser design, the linear power was increased anywhere from 10% to 20%. At the same time, the efficiency decreased by 10% to 20%, indicating significant absorption of the fundamental mode.
In further experiments, the design was improved by laterally varying the distance of the CIG elements relative to ridge and herewith the extension of the modes. The purpose of this variation is to optimize absorption of higher order modes relative to the fundamental mode and thus optimize linear power and minimize efficiency losses. Furthermore, a thin insulating layer was added to the CIG element. This layer is electrically insulating and does not absorb light of the lasing wavelength. It is located between the semiconductor body and the absorption layer. The overall absorption now not only depends on the material of the absorption layer and the location of the CIG element, but also on the thickness of this insulating layer, i.e. the vertical distance of the modes from the absorption layer. Additionally, the insulator electrically separates the absorption layer, which is a conductor in the present case, from the semiconductor and thus eliminates the possibility of leaking currents.
These variations rendered very interesting results and thus form an essential part of this invention. They will be described in detail later. In three variations, the CIG elements were located at 0, 300, and 600 nm distance relative to both sides of the ridge, i.e. measured from ridge etching mask. The thin insulating layer, here Si3N4, was part of the CIG elements for all experiments and had a thickness of about 25 nm. On average, the linear power of these laser diodes increased by about 25% relative to laser diodes without CIG elements. Relative to standard laser diodes, the average efficiency was reduced by about 10% for lasers, where the CIG elements were located right next to the ridge, i.e. at 0 nm from ridge etching mask. For the two designs where the CIG elements were taken further from the ridge, i.e. 300 nm and 600 nm relative to the ridge etching mask, the efficiency was reduced by only about 5%.
In one embodiment, the lateral and vertical far-field show stable single mode outputs above 900 mW and no lateral beam steering was observed in the whole power range.
The three experimentally evaluated locations of the CIG elements show clearly that optimization reduces the detrimental effect on the fundamental mode and thus increases the efficiency and kink power even further.
The laser diodes with the improved CIG design were tested under accelerated conditions for stability, failures and degradation. The CIG-improved lasers showed stable performance, indicating highly reliable operation. No distinctive features were observed compared to standard laser diodes. The operating conditions were 900 mA constant current at 85° C. heat sink temperature, 3000 hrs.
To summarize, the invention concerns a process for making a novel high power ridge waveguide semiconductor laser design containing one or more CIG elements (Complex Index Guiding elements). These CIG elements consist of at least one layer that absorbs light of the lasing wavelength, but may contain a plurality of absorbing and non-absorbing layers. The novel laser exhibits high stability with increased kink power. The CIG elements are preferably located to both sides of the ridge along the optical axis. Precise location and shape of the CIG element as well as number and location of layers in the CIG element depend on the laser design and are chosen to achieve maximum efficiency and/or maximum kink power.
The novel manufacturing process according to the invention allows control of the distance relative to the extension of fundamental and first order modes and hence optimization of increased kink power vs. optical losses. Experimental results show an increased kink power of about 25% (median) and very good life-time results.
As already addressed, the position of the absorbing layer relative to the fundamental mode is rather critical. This is due to the fact that absorption of the first order mode is desired, but absorption of the fundamental mode is undesirable since it results in reduced efficiency. The described novel manufacturing method allows control of the distance of the absorbing layer relative to the ridge by a self-aligning process. This optimizes the kink power increase by absorption of the first order mode without significantly loosing efficiency by absorption of the fundamental mode. Since the location of the CIG elements can be defined independently of the ridge and its etching mask, any epitaxial design and any ridge design can be used.
The fabrication method according to the invention has the further advantage that it does not put limitations on the CIG elements in terms of position, thickness, material and deposition method. Also, the novel method facilitates the introduction of a thin insulating layer underneath the absorption layer to electrically separate the semiconductor from the metal and thus avoid leaking currents and to modify the overall absorption.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTSIn the following, various embodiments of the invention, including some basic considerations and both the laser structure and the manufacturing process, shall be described by reference to the drawings, in which:
The lower part of
Initially, a manufacturing method of RWG laser diodes according to the invention shall be described since many details will become clear from the preferred manufacturing process. Different stages and variations of this method are illustrated in
Please note that the figures showing the RWG laser diode are not to scale, in particular are the thicknesses of the various layers greatly exaggerated to make them visible. Please note also that the manufacturing process is only explained with regard to the present invention and is insofar incomplete as those steps and measures known to the person skilled in the art are not mentioned or described.
In the next step, shown in
Whereas the steps themselves above are more or less state of the art, they form the basis for subsequent steps focusing on the invention.
The steps illustrated in
In the step shown in
To provide the masking necessary for the fabrication the CIG element(s), the photoresist is etched to a desired shape, here specifically a variable width or distance, measured from the ridge center. A preferred method for this shaping step is RIE, i.e. Reactive Ion Etching. This results in the shaping masks 5 illustrated in
More precisely,
In a subsequent step, shown in
After the etching process described in
As shown in
-
- It provides the contact layer for the usual P-contact metallisation on top of the ridge.
- It provides the absorption necessary for suppressing the undesired first and higher order modes of the laser by forming absorption layers (or CIG elements) 8a and 8b at both sides of the ridge. As stated earlier, the location where absorption takes place, i.e. where the CIG element is effective, is confined to those areas left and right of the ridge where the semiconductor body 2 is not covered by the thick insulator strips 6a and 6b. If desired, the absorption layer may extend over only part of the semiconductor body's length. A person skilled in the art will know how to achieve this.
Consequently, this absorption layer must have two important material properties:
-
- It must be a material in which the imaginary part of the complex refractive index is non-zero for the wavelength in question, i.e. the lasing wavelength.
- For the process described, it must also be suitable as a first contact layer for the p-contact metallisation. Conductors such as Ti and Cr are suitable in this case.
Any other steps in the manufacturing process to complete the RWG laser diode remain essentially standard and are well known to a person skilled in the art. These steps thus need not be described here.
Depending on the laser design (e.g. ridge shape, epitaxial design) the lateral extension of the modes within the laser diode varies. Accordingly, changes must be made with regard to the optimal location of the CIG elements to achieve the desired maximum absorption of first and higher order modes and minimum absorption of the fundamental mode. It is therefore important to have a process that allows variable placement and shape of the CIG elements independent of, but adapted to, the laser's ridge shape and design. The present invention provides this flexibility and adaptability.
Some alternatives for the deposition and the arrangement of the absorption layer(s) or complex index guiding (CIG) element(s) will be addressed in the following.
In a next step a thin insulating layer, again preferably 25 nm, is deposited covering the entire semiconductor body 2, thus forming the first layer of the CIG elements 7a and 7b as shown in
Since this thin insulator covers the entire surface of the semiconductor body, it also covers the contact area on top of the ridge. In this latter area, the thin insulator must be removed to provide electrical contact of the semiconductor with the p-metal. This can be done by any common method with photoresist masks and subsequent etching, preferably RIE etching. A person skilled in the art will know how to realize this. The result is shown in
Finally, the p-metal layer 9, which also provides and functions as the absorption layer of the CIG elements 8a and 8b, is deposited resulting in a structure shown in
The third alternative process is similar to the previous one, but allows the utilization of different materials for the CIG element independent of the thick insulating layer(s) and the p-metal layer.
In a next step, an absorption layer is deposited, also covering the entire body and forming the necessary absorption layers for the CIG elements. This is shown in
A fourth alternative is described in
Any of the above described embodiments my be applied to a laser diode of the so-called “straight-flared-straight” structure as disclosed in Pawlik et al. U.S. Pat. No. 6,798,815, assigned to the assignee of the present invention and incorporated herein by reference.
Further modifications will readily occur to a person skilled in the art and the invention is therefore not limited to the specific embodiments, details, and steps shown and described hereinbefore. Modifications may be made without departing from the spirit and scope of the general inventive concepts as defined in the appended claims.
Claims
1. A method for making a high power laser diode with a semiconductor body and a ridge waveguide as active region,
- comprising the following steps
- (a) providing said semiconductor body with said ridge waveguide by a first mask, in particular a photoresist mask,
- (b) depositing an insulator layer over at least part of said semiconductor body including said first mask,
- (c) depositing a photoresist on said insulator layer,
- (d) removing part of said photoresist in a controlled way to provide a second mask,
- (e) removing or thinning at least part of said insulator layer where it is uncovered by said second mask,
- (f) removing both said first and said second masks, and
- (g) depositing an absorption layer, in particular as part of a complex index guiding (CIG) element.
2. The method according to claim 1, wherein
- the second mask has a predetermined size wider than the ridge—waveguide.
3. The method according to claim 1, wherein
- the absorption layer serves as part of a complex index guiding (CIG) element and as contact layer.
4. The method according to claim 1, wherein
- the insulator is thinned to a predetermined thickness where it is uncovered by the second mask to provide insulator areas of a first thickness under said second mask and of a second thickness outside said second mask.
5. The method according to claim 1, wherein
- a third mask is applied as one of a plurality of process steps in generating multiple CIG elements to both sides of the optical axis of the waveguide.
6. The method according to claim 5, wherein
- the third mask provides for two or more longitudinally contiguous CIG sections, each said section having a different lateral extension, in particular at least one of said sections extending laterally to the border of the semiconductor body.
7. The method according to claim 1, including
- (f1) removing together with the first and the second masks at least part of a first insulator layer,
- (f2) depositing a second, preferably thin, insulator layer over at least part of the semiconductor body including the ridge waveguide, and
- (f3) before depositing the absorption layer, removing said second insulator layer at least partly in a contact region of said ridge waveguide,
- (h) depositing a contact layer, in particular a P-contact layer.
8. The method according to claim 7, including the steps of
- (f2′) after deposition of the second insulator layer, depositing an absorption layer on said second insulator layer over at least part of the semiconductor body including the ridge waveguide, and
- (f3′) at least partly removing said absorption layer and said second insulator layer in a contact region of said ridge.
9. The method according to claim 7, including the steps of
- (f2″) after deposition of the second insulator layer, depositing a stack of absorbing layers and insulator layers over at least part of the semiconductor body including the ridge waveguide, and
- (f3″) removing said stack and said second insulator layer in a contact region of said ridge waveguide, essentially leaving said stack as CIG elements at both sides of said ridge waveguide.
10. The method according to claim 7, wherein
- the contact layer serves as part of the CIG element.
11. The method according to claim 1, wherein
- the insulator layer, especially Si3N4, is deposited over essentially the whole surface of the semiconductor body,
- the photoresist is deposited over at least the center part of said semiconductor body,
- said photoresist is removed, especially etched, to a desired distance from said ridge waveguide, thus providing the second mask,
- said insulator layer is thinned or removed, especially etched, in particular etched down to the semiconductor surface so that only insulator areas covered by said second mask remains,
- said masks are removed by lifting off,
- at least one conductive layer is deposited as absorption layer of a complex index guiding (CIG) element, said conductive layer including at least one of Ti, Cr, Pt, Au, Si, or Ge.
12. A method for making a laser diode with a semiconductor body having an active region, a lower cladding layer, an upper cladding layer with a ridge waveguide, and a top metallization for current injection, said laser diode further including an optically absorbing element for suppressing first and higher order modes of said laser diode, said absorbing element being part of one or more complex index guiding (CIG) elements, the method comprising: fabricating an insulation layer and an absorption layer, the insulation layer being provided on at least part of said upper cladding layer, separating at least part of said absorption layer from said laser semiconductor body, whereby said insulation layer is fabricated with a predetermined thickness having a maximum close to said ridge waveguide and a minimum, including zero, distant from said ridge waveguide.
13. The method according to claim 12, wherein
- at least one CIG element is fabricated to comprise or consist of two or more sections located along the optical axis of the waveguide, each said section having a predetermined extension along the optical axis of said waveguide
14. The method according to claim 12, wherein
- two sections each of substantially constant thickness, a first, greater thickness close to the ridge waveguide and a second, smaller thickness distant from said ridge waveguide are fabricated, said two sections forming the insulation layer separating the absorption layer from the semiconductor body.
15. The method according to claim 12, wherein
- the CIG element is fabricated as a plurality, or stack of, insulating and absorption layers.
16. The method according to claim 12, wherein
- at least two CIG elements are fabricated as layered structures, preferably located on both sides of the ridge waveguide and extending along part of or the full length of the semiconductor body.
17. The method according to claim 12, wherein
- the CIG element is shaped for maximizing the ratio of the suppression of first and higher order modes to the suppression of the fundamental mode.
18. The method according to claim 12, wherein
- the semiconductor body is made of a first material, including GaAs or InP based materials, and the complex index guiding element is made of a second material or a stack of second materials, in particular either a conductor or a semiconductor, including at least one of Ti, Cr, Pt, Au, Si, Ge, or an insulator, in particular at least one of TiO2, Si3N4, AlN, SiO2.
19. The laser diode according to claim 12, wherein
- the insulation layer is fabricated to separate the absorption layer from the laser semiconductor body in the vicinity of the ridge waveguide only, preferably covering at least part of the sides of said ridge and/or part of said semiconductor body.
20. The method according to claim 12, wherein
- the first greater thickness of the insulator close to the ridge waveguide is chosen to minimize absorption of the fundamental mode by the absorption layer, preferably to zero, and the second smaller thickness distant from the ridge waveguide is chosen to maximize absorption of the first and higher order mode, while keeping absorption of the fundamental mode at a minimum.
21. The method according to claim 15, wherein
- materials and/or thickness for at least one CIG element are selected to maximize the ratio of the suppression of first and higher order modes to the suppression of the fundamental mode, in particular maximizing suppression of first and higher order modes while minimizing the absorption of the fundamental mode.
22. The method according to claim 12, wherein
- the two sections of the absorption layer are fabricated to abut against a common shoulder which is self-aligned with the ridge waveguide.
23. A method for making a high power diode with a semiconductor body and a ridge waveguide laser as active region,
- comprising the following steps
- (a) providing a first mask over said ridge waveguide,
- (b) depositing a first insulator layer over at least part of said semiconductor body,
- (c) depositing a photoresist over at least the center part of said semiconductor body,
- (d) thinning or removing, especially etching, said photoresist to a desired distance from said ridge waveguide, thus providing a second mask, exposing part of said semiconductor body,
- (e) thinning or removing at least part of said insulator layer where it is uncovered by said second mask,
- (f) depositing at least one absorption layer as part of a complex index guiding (CIG) structure over at least part of said semiconductor body, and
- (g) lifting off both said masks, thus exposing said ridge waveguide and said semiconductor body at least partly, whereby parts of said first insulator layer and of said absorption layer remain on said semiconductor body, and
- (h) depositing a further layer as contact layer, especially a P-contact layer.
24. The method according to claim 23, wherein
- step (f) is replaced by step (f):
- depositing, over at least part of the semiconductor body distant from the waveguide ridge, an insulating layer and an absorption layer or a stack of alternating insulating and absorption layers as part of a complex index guiding (CIG) structure.
25. A method for making a high power diode with a semiconductor body and a ridge waveguide laser as active region, comprising the following steps
- (a) providing a first mask over said ridge waveguide,
- (b) depositing a photoresist over at least part of said semiconductor body,
- (c) removing, especially etching, said photoresist to a desired, variable distance from said ridge waveguide, thus providing a second mask, exposing part of said semiconductor body,
- (d) depositing an absorption layer as part of a complex index guiding (CIG) structure over at least part of said semiconductor body, said first mask and said second mask,
- (e) lifting off both said first and said second masks, thus exposing at least part of said ridge waveguide and of said semiconductor body, whereby parts of said absorption layer remain on said semiconductor body,
- (f) depositing an insulator layer over at least part of said semiconductor body,
- (g) opening a contact area, especially on top of said ridge waveguide, and
- (h) depositing a further layer as contact layer, especially a P-contact layer.
26. The method according to claim 25, wherein
- the second mask has a predetermined size wider than said ridge waveguide.
27. The method according to claim 25, wherein
- step (d) is replaced by step (d′):
- depositing an absorption layer and an insulator layer or a stack of absorption and insulator layers over at least part of the semiconductor body as part of a complex index guiding (CIG) structure.
28. The method according to claim 25, wherein
- a third mask is applied as one of a plurality of process steps in generating multiple CIG elements to both sides of the optical axis of the waveguide.
29. A high power laser diode fabricated according to a method defined in any of the claims 1, 4, 7, 10, 11, 22, or 24, said laser diode comprising a semiconductor body, a ridge waveguide, an active region, and a structure of optically absorbing elements for suppressing selected modes of said laser diode, said structure constituting part of one or more complex index guiding (CIG) elements,
- said laser diode preferably having front and back facets and, extending between said facets, said ridge waveguide having: a center segment with a substantially constant first cross section, preferably having a length of 40-70% of the diode length, two tapered segments extending and widening from the center segment towards said facets in opposite direction, and two end segments between said tapered segments and said facets, each said end segment having a substantially constant cross section larger than said first cross section, in particular a first one of said tapered segments having a length of about 30-60% of the diode length and a second one of said tapered segments having a length of up to 10% of the diode length.
30. A high power laser diode fabricated according to a method defined in claim 25, said laser diode comprising:
- a semiconductor body, an active region, a lower cladding layer, an upper cladding layer with a ridge waveguide, a top metallization for current injection, said laser diode further including a structure of optically absorbing elements constituting part of one or more complex index guiding (CIG) elements for suppressing selected, especially first and higher, order modes of said laser diode, said structure extending along the length of said semiconductor body with a predetermined width across said semiconductor body, preferably having a variable width or having sections with at least two widths, preferably one wider width extending across a first part of the semiconductor body and one narrower width extending across only a fraction or a second part of said semiconductor body.
Type: Application
Filed: Jan 21, 2005
Publication Date: Sep 15, 2005
Inventors: Silke Traut (Niederlenz/Zurich), Berthold Schmidt (Erlenbach/Zurich), Boris Sverdlov (Adliswil/Zurich), Achim Thies (Zurich)
Application Number: 11/040,246