Patent Application
20090052479
The invention relates to saturable absorber structures to be used for attaining passive mode locking of lasers in order to attain generation of ultra-short radiation pulses.
Saturable absorber mirrors are used inside laser resonators for generation of short and ultrashort optical pulses with pulse duration in the picosecond or sub-picosecond range. In particular, saturable absorber mirrors are very useful for the so-called self-starting passive mode locking of lasers, where a periodic train of optical pulses is generated spontaneously by the saturable absorber mirror element. The mechanism of mode locking relies on the non-linear, i.e. intensity dependent, reflectivity of the saturable absorber mirror. The mirror provides large absorption and low reflectivity for low intensity light incident upon it, while for large intensity light the absorption is reduced and reflectance increased; The intensity I where the absorption goes to half of its maximum value is called saturation intensity IS of the saturable absorber. Mathematically the intensity dependent absorption coefficient α(I) can be expressed as
α(I)=α0/(1+I/IS),
where α0 is the non-saturated—low intensity—absorption coefficient. Hence, the saturable absorber mirror favors light having high intensity by minimizing losses for such light, and the way the laser resonator achieves this is by locking the relative phases of different longitudinal modes of the laser resonator. Such phase locking exactly produces a periodic train of short optical pulses with high pulse intensity. There are other important requirements aside saturable absorption for the mirror to be useful for passive mode locking. One important property is the recovery time of absorption from the saturated low value back to the unsaturated high value. The recovery time should be sufficiently short for production of picosecond of sub-pico-second pulses. The absorption bandwidth of the saturable absorber should also be sufficiently large for the entire spectrum of the optical pulse to be efficiently absorbed. A third parameter is the non-saturable absorption of the mirror. This absorption the optical intensity cannot bleach, and eventually limits the maximum reflectance exhibited by the mirror or reflector. This part of absorption needs to be kept at minimum.
Most saturable absorber mirrors are based on the Semiconductor Saturable Absorber Mirror (=SESAM). A typical SESAM structure consists of an epitaxially grown Distributed Bragg Reflector (=DBR) on a single crystal semiconductor substrate and an active layer structure including the saturable absorber layers grown on top of the DBR. The DBR reflects the light at the lasing wavelength typically with better than 99% reflectance. The active structure includes single or multiple Quantum Well (=QW) layers that are positioned either at the antinodes or off-antinodes of the standing electromagnetic field within the SESAM structure. By changing the location of the QWs with respect to the standing wave the saturation intensity can be changed. As light is incident on the SESAM, it will experience varying reflectivity depending on its intensity due to saturable absorption in the QW layers. High optical quality QW layers are not suitable for mode-locking since the absorption recovery time is in the range of nanosecond instead of the required picosecond range. Hence, the quality of the QW material must be somehow degraded to facilitate reduction of recovery time, which is determined by the recombination time of the electrons and holes in the QW. Suggested methods to achieve this reduction rely on generation of dislocations, low temperature growth, doping the QW material, and ion implantation of the material. However, all these methods have their drawbacks. Dislocations and low temperature growth tend to make the material interfaces rough and also produce optical scattering, which generates non-saturable loss. Dislocations are not uniformly distributed and the reduction of carrier lifetimes must also be accompanied by relatively slow diffusion of carriers from dislocation free areas into the dislocations acting as recombination centers, whereupon passive mode-locking with picosecond or sub-picosecond pulses cannot be initiated at least in the case of fiber lasers. Furthermore, it is known in the art that the low-temperature growth itself produces e.g. arsenic precipitates reducing carrier lifetime, but unfortunately, said precipitates induce non-saturable losses and raise issues for device robustness and lifetime. Ion implantation degrades the structural quality of the saturable absorber layers and also the DBR layers in the mirror by causing intermixing of the layers. This degradation translates to reduction of optical quality of the structure and generates possible non-saturable loss. Furthermore, implantation is a rather expensive ex-situ process.
Due to the large number of semiconductor layers, the growth of a SESAM structure takes several hours, and a careful control on the layer thicknesses is required due to limited refractive index difference between the semiconductor DBR layers. Since the DBR mirror is grown first on the substrate, the substrate is usually left on the sample and it therefore induces large thermal impedance when the SESAM chip is mounted on a heat sink with the substrate facing the beat sink.
Patent publication U.S. Pat. No. 4,860,296 discloses a controlled laser having an optical resonator, a laser gain medium placed inside the optical resonator, the laser gain medium being capable of emitting light and of lasing with the light, a multiple layer heterostructure placed inside the optical resonator, and means for varying an optical absorption of the multiple layer heterostructure for the light in order to control an optical gain of the optical resonator, and thereby control lasing of the laser gain medium. Passive mode locking is achieved by the light emitted by the gain medium controlling the optical absorption of the multiple layer heterostructure. Active mode locking and modulation are achieved by controlling the optical absorption of the multiple layer heterostructure by applying an electric field to the multiple layer heterostructure. Control of laser gain by an external light source is achieved by controlling the optical absorption of the multiple layer heterostructure by illuminating it with light from the external light source. An embodiment of the multiple layer heterostructure fabricated as a GaAs-AlGaAs multiple quantum well with a Type I superlattice band structure is a passive mode locker for a semiconductor diode laser. Especially there is defined a semiconductor device comprising a first layers of material having a nonlinear optical absorption at a predetermined optical frequency, whereupon a second layers of material alternated with said first layers, and the spacing of said first layers being an integer multiple of one-half the wavelength of light of said predetermined optical frequency. This means that the saturable absorber formed by multiple layer heterostructure is placed between the end mirrors of the optical resonator structure, but Fabry-Perot etalon is not discussed. The scheme presented to achieve passive mode locking relies on very tight focusing of the laser beam on the sample and diffusion of the photo-generated carriers.
According to patent publication U.S. Pat. No. 5,237,577 saturation intensity and loss of a saturable absorber are substantially independently regulated by positioning the saturable absorber element within a Fabry-Perot etalon defined by first and second reflective elements so that the saturable absorber element responds to light at optical wavelengths in the anti-resonant portion of the Fabry-Perot spectral response, that is, between optical wavelengths corresponding to resonance peaks. The resulting combination of elements is called a Fabry-Perot saturable absorber. Thickness of the saturable absorber element substantially sets the loss of the Fabry-Perot saturable absorber while changes in the reflectivity of the first reflective element onto which the light is incident substantially determines the saturation intensity (degree of nonlinearity) and assists in compensating loss of the saturable absorber element. In one exemplary embodiment, a high reflectivity first reflective element is positioned on the end of the saturable absorber element facing the incident optical radiation while a high reflectivity second reflective element is positioned on the opposite end of the saturable absorber. Dielectric material layers form the first reflective element whereas semiconductor layers form the second reflective element. A plurality of quantum well and barrier layers is employed to form the saturable absorber element. Especially the patent discloses an optical apparatus comprising first and second reflective elements being spaced apart to form a Fabry-Perot etalon therebetween, said Fabry-Perot etalon being characterized by a plurality of optical frequencies each frequency corresponding to a resonant condition, and semiconductor material having a nonlinear optical absorption substantially at a predetermined optical frequency and being positioned between said first and second reflective elements, said predetermined optical frequency being between any two adjacent optical frequencies in said plurality of optical frequencies so that said predetermined optical frequency occurs substantially at an optical frequency corresponding to an anti-resonant condition for said Fabry-Perot etalon.
Patent publication U.S. Pat. No. 5,701,327 suggests that low optical loss and simplified fabrication are achieved by a nonlinear reflector which incorporates one or more semi-conductor quantum wells within an n half-wavelengths strain relief layer (where n is an odd integer greater than zero) that is formed on a standard semiconductor quarter wave stack reflector. Growth of the half-wavelength layer is controlled so that dislocations are formed in sufficient concentration at the interface region to act effectively as non-radiative recombination sources. After saturation, these recombination sources remove carriers in the quantum well before the next round trip of the optical pulse arrives in the laser cavity. The nonlinear reflector is suitable for laser mode-locking at the high wavelengths associated with many currently contemplated telecommunications applications and provides, at such wavelengths, an intensity dependent response that permits it to be used for saturable absorption directly in a main oscillating cavity of a laser. Saturation intensity of the nonlinear reflector and thereby related laser mode-locking properties can be controlled by disposing the quantum well(s) at a particular position within the strain relief layer. Further, the patent describes that good mode-locking and good photoluminescence is obtained from quantum wells grown on such strain relaxed layers, while according to well known principles high photoluminescence intensity inevitably means that radiative carrier lifetime is of the same order as non-radiative lifetime.
Patent publication U.S. Pat. No. 6,252,892 discloses an intracavity resonant Fabry-Perot saturable absorber (R-FPSA) induces mode locking in a laser such as a fiber laser. An optical limiter such as a two photon absorber (TPA) can be used in conjunction with the R-FPSA, so that Q-switching is inhibited, resulting in laser output that is CW mode-locked. By using both an R-FPSA and a TPA, the Q-switched mode-locked behavior of a fiber laser is observed to evolve into CW mode locking. Especially the patent claims e.g. a mode-locked laser, comprising: an optical cavity including a gain medium; a Fabry-Perot etalon within said cavity which is near resonance at the laser frequency; and a saturable absorber having nonlinear absorption characteristics and inducing mode-locked laser pulses, said absorber located within said Fabry- Perot etalon. Further the publication mentions ion implantation as the method of carrier lifetime reduction in the saturable absorber.
Patent publication U.S. Pat. No. 6,538,298 discloses a “low field enhancement” (LFR) semi-conductor saturable absorber device design in which the structure is changed such that it has a resonant condition. Consequently, the field strength is substantially higher in the spacer layer, resulting in smaller saturation fluence and in a higher modulation depth. However, the field in the spacer layer is still lower than the free space field or only moderately enhanced compared to the field in the free space. According to one embodiment, the absorber device is a Semiconductor Saturable Absorber Mirror (SESAM) device. In contrast with SESAMs according to the state of the art, a structure including the absorber and being placed on top of a Bragg reflector is provided, which essentially fulfills a resonance condition whereby a standing electromagnetic wave is present in the structure. In other words, the design is such that the field intensity reaches a local maximum in the vicinity of the device surface. According to the example calculations given in the patent reflectivity modulation 1.4% at most can be achieved in such kind of structures. The publication does not suggest any specific means for carrier lifetime reduction.
According to patent publication U.S. Pat. No. 6,551,850 the non-linear optical material characteristics of a semiconductor material grown at low temperatures can be significantly improved by the following measures: Doping with foreign atoms and/or additional thermal annealing. As an example is said that when GaAs grown at 300° C. is doped with Be to a concentration of 3·1019 cm−3, then the response time is reduced from 480 femtoseconds to 110 femtoseconds, without the absorption modulation being reduced by this or the non-saturable absorption losses being increased. Semiconductor materials, during the production of which at least one of the above measures was implemented, manifest influenceable, in particular short response times as well as simultaneously high absorption modulations and low non-saturable absorption losses. The publication suggests that these materials are eminently suitable for non-linear optical applications, such as optical information processing, optical communication or ultrashort laser pulse physics.
It is an object of the invention to attain an improved saturable absorber mirror, which addresses many of those problems associated with conventional saturable absorber mirrors of the prior art, and which is usable in lasers having high gain active media. For passive mode-locking of lasers having a high gain active medium, such as fiber lasers, it is needed a saturable absorber mirror that can produce large contrast between high intensity reflectivity and low intensity reflectivity, i.e. large modulation of reflectivity for passive mode-locking. Preferably said difference between the high and low intensity reflectivities should be at least a few percent, because the cavity losses in high gain lasers are generally quite high. This is the reason why saturable absorber mirrors designed to be used with low gain laser media may not work properly with high gain laser media. A good structural integrity of the saturable absorber mirror structure shall be also achieved thus minimizing nonsaturable losses. Furthermore, an efficient heat sinking of the devices shall be possible, because effective cooling is important for long-lived components, especially mode-locked laser oscillators with high average output powers. Further, it is an object of the invention to attain a method, which enable producing said improved saturable absorber mirror.
According to the first aspect of the invention it is provided a saturable absorber structure with multiple-layer epitaxial heterostructure absorbers, comprising: at least a first absorber layer of a quantum well semiconductor QW-material with two opposite surfaces, said QW-material having a nonlinearly on radiation intensity dependent optical absorption at a predetermined optical frequency range of an electromagnetic radiation fed into said absorber structure in direction normal to said opposite surfaces; at least one first contacting layer of a first optically transparent semiconductor material against a surface or surfaces of said first absorber layer(s), said first contacting layer(s) having a lattice fit or a pseudomorphism with said first absorber layer(s); and a first Bragg-reflector with a plurality of quarter wavelength layers. Said at least one absorber layer of the QW-material has a thickness of at maximum 60 nm; and said first optically transparent semiconductor material of the contacting layer(s) is a reactive R-material, which semiconductor material contains two or more main components, at least one dopant, and at least one metallic alloying element substituting one of said main components and enhancing the incorporation of said dopant(s), said metallic alloying element having a concentration at least 50 atomic-% of that main component it substitutes; whereupon the charge carriers originating in said QW-material of the first absorber layer(s) has a first recombination time at maximum 100 picoseconds determined by recombination of the charge carriers at sites of said dopant(s), thus forming a fast saturable absorber. By this way short charge carrier lifetimes in the saturable absorber layers can be achieved without the need to introduce dislocations or perform ion implantation, both of which would introduce non-saturable losses and broaden the spectral features of the absorber material. Strong confinement of the photo-generated carriers in the absorber layers is achieved due to the generally large band-gap of the surrounding contacting layers of R-material. This minimizes carrier leakage out of the absorber layers and improves the time dynamics of the processes that result in the saturation of absorption.
One may also choose not to place the above-mentioned contacting layers around all absorber layers. In such structures two different kinds absorber layers exist, one with fast recovery time (fast saturable absorber) and the other with slow recovery time (slow saturable absorber). For this purpose it is provided at least a second absorber layer of a quantum well semiconductor QW-material with two opposite surfaces, said QW-material having a nonlinearly on radiation intensity dependent optical absorption at a predetermined optical frequency range of an electromagnetic radiation fed into said absorber structure in direction normal to said opposite surfaces; and at least one second contacting layer of an optically transparent semiconductor material against a surface or surfaces of said second absorber layer(s), said second contacting layer(s) having a lattice fit or a pseudomorphism with said second absorber layer(s), whereupon said optically transparent semiconductor material of the second contacting layer(s) is a neutral N-material, which has a lower concentration of said at least one metallic alloying element and/or a lower concentration of said at least one dopant than said R-material of the first contacting layer(s); whereupon the charge carriers originating in said QW-material of the second absorber layer(s) has a second recombination time longer than 100 picoseconds, thus forming a slow saturable absorber. The unique ability to combine slow and fast saturable absorbers in the same structure enables the optimization of initiation and stabilization of mode locking. Further advantage is that both slow and fast saturable absorber layers can be grown into the same structure by placement of the contacting layers.
The lattice fit or said pseudomorphism between the first contacting layer(s) and the first absorber layer(s) is arranged to be so good that said QW-material of the first absorber layer(s) has a dislocation density at maximum 200×104/cm2, or smaller than 10×104/cm2, or smaller than 5×103/cm2: This way the point defects at the inter-faces between the contacting layers and the saturable absorber layers are distributed uniformly, and diffusion distances of carriers to these point defects are therefore minimized. Furthermore, all structural damages are kept negligible, and the heat generated in the non-radiative recombination of carriers is also uniformly distributed, which enhances device lifetime.
These similar or different absorber layers ca be positioned in different ways. The absorber structure comprises at least two absorber layers of the QW-material with contacting layers of the R-material or the optional N-material on one side or on both sides of each of said absorber layers, forming two or more absorber units. In one alternative the absorber units are positioned each at or in the proximity of at least one or each antinodes of the standing wave of said radiation, as shown in
The quantum well semiconductor QW-material according to the invention can e.g. be of the type GaX1In1−X1As, or GaX1In1−X1AsY1P1−Y1, or GaX2In1−X1AsY1N1−Y1, in which materials the mole fraction X1 is smaller than 0.5. Alternatively the quantum well semiconductor QW-material can e.g. be of the type (AlX1Ga1−X1)Y1In1−Y1As, in which materials the mole fraction X1 is smaller than 0.5.
In the reactive R-material according to the invention the two or more main components of the R-material are selected from among Gallium, Indium, Arsenic, and Phosphorus. Typical examples for the R-material are e.g. (M1RGa1−R)In1−X2As, (M11−R)GaX2In1−X2P, (M1RGa1−R)In1−X2AsY2N1−Y2, and M1RAs1−R, in which the mole fraction R is higher than 0.6, or higher than 0.7, or higher than 0.8. Especially in the R-material of the first contacting layer(s): the metallic alloying element is a metal of group III other than Gallium and Indium, and the dopant is an element of group VI and/or group VIII. Preferably the metal of group III is aluminum, and the dopant element of group VI or VIII is oxygen and/or-iron and/or chromium and/or nickel.
The first Bragg-reflector and the optional second Bragg-reflector can be made of di-electric materials. This kind of reflectors is easier to manufacture than a semiconductor DBR due to larger refractive index differences achieved with dielectric materials. Typically only a few quarter layer pairs are required to achieve more than 99% reflectivity, especially when an enhancing metal layer, such as gold or silver, is deposited on top of the last dielectric layer. Thus, very large reflectance bandwidth can be achieved.
In a preferred embodiment of the invention the absorber structure, i.e. the Hybrid Saturable Absorber Mirror (=HSAM) chip, is bonded onto the heat sink from the first Bragg-reflector, so that the absorber layers of the quantum well semiconductor QW-material with the contacting layers of the first and/or second optically transparent semiconductor material extend away from said heat sink and said first Bragg-reflector. This way the distance from the absorber layers to the heat sink is of the order of a micrometer, since no thick substrate exists between the heat sink and absorber layers. Hence, the chip has low thermal impedance and can operate with high output powers from the mode-locked laser.
According to the second aspect of the invention it is provided a method for producing a saturable absorber structure with multiple-layer epitaxial heterostructure absorbers, comprising: taking a substrate of a semiconductor material; depositing a Bragg-reflector with a plurality of quarter wavelength layers; epitaxially growing one or more first absorber layers of a quantum well semiconductor QW-material, said QW-material being of a type that has a nonlinearly on radiation intensity dependent optical absorption at a predetermined optical frequency range of an electromagnetic radiation; epitaxially growing one or more first contacting layers of a first optically transparent semiconductor material prior to and/or after said growing of the first absorber layer(s) so that said first contacting layer(s) has a lattice fit or a pseudomorphism with said first absorber layer(s). Further in said method: said epitaxial growing of the first absorber layer(s) of the QW-material is finished when a predetermined thickness at maximum 60 nm is reached; and in said epitaxial growing of the first contacting layer(s) one or several main components are supplied, at least one dopant is supplied, and at least one metallic alloying element is supplied, which element either is an additional component or substitutes one of the several main components, and results in a concentration at least 50% of the substituted atomic fraction, so that a reactive R-material is formed for providing a first recombination time at maximum 100 picoseconds for the charge carriers originating in said QW-material of the first absorber layer(s).
According to the aimed product the method can further comprise epitaxially growing one or more second absorber layers of a quantum well semiconductor QW-material composition, said QW-material being of a type that has a nonlinearly on radiation intensity dependent optical absorption at a predetermined optical frequency range of an electromagnetic radiation; and epitaxially growing one or more second contacting layers of a second optically transparent semiconductor material composition prior to and/or after said growing of the first absorber layer(s) so that said second contacting layer(s).has a lattice fit or a pseudomorphism with said second absorber layer(s), and in said epitaxial growing of the second contacting layer(s) is supplied a lower concentration of said at least one metallic alloying element and/or a lower concentration of said at least one dopant than in said first contacting layer(s); whereupon a second recombination time longer than 100 picoseconds is provided for the charge carriers originating in said QW-material of the second absorber layer(s).
Spacer layer or layers prior to or after epitaxial growth of each first and second absorber layer(s), or prior to or after epitaxial growth of each first and second contacting layers are also grown as needed. The first Bragg-reflector with a plurality of quarter wavelength layers is deposited after said epitaxial growing of the first and second absorber layer(s) and the first and second contacting layer(s) and the spacer layer(s) preferably using one or more appropriate dielectric materials.
According to the product in question the saturable absorber structure can be adhered to a heat sink at an end surface of the structure, and especially on top side of said first Bragg-reflector, as shown in
Especially for attaining said reactive R-material of the first contacting layer(s) the method further comprises: feeding additional gas or gases towards the latest epitaxially grown layer so that component(s) thereof is/are transferred as said at least one additional metallic alloying element and/or as said at least one dopant into said layer, thereby forming said reactive R-material; and/or allowing component(s) of gas or gases present against the latest epitaxially grown first contacting layer(s) of said R-material to be transferred as said at least one dopant onto said layer(s).
The foregoing summary, and the following detailed description of the preferred embodiments of the present invention, will be better understood when read in conjunction with the accompanying drawings, in which:
With reference now to the drawings,
The sacrificial layer 12 used during production enables one to detach the actual saturable absorber structure 10 from the substrate 11 at a later processing stage of the saturable absorber structure. For instance, the substrate can be removed by chemically etching it with a first etchant. The composition of the sacrificial layer 12 is chosen such that it is not significantly etched by said first etchant, and therefore the etching will stop when reaching the sacrificial layer 12. Next the sample is subjected to a second etchant, this time selectively etching the sacrificial layer 12, but not significantly etching the rest of the layers 13, 1415 or any of the subsequent dielectric materials. Alternatively, the sacrificial layer 12 may left as an integral part of the remaining structure. These kinds of etchants and materials for the sacrificial layer are generally known, and are not described in detail. The saturable absorber structure 10 is bonded, if heat sink is used, to a heat sink 21 at the time of etching to provide support for the thin layer structure as shown in
Preferably the first absorber layers 13, 13a, 13b etc. are quantum wells, i.e. quantum well semiconductor QW-material. The absorber layers have two opposite surfaces 3a, 3b, and a thickness S of at maximum 60 nm, and preferably at maximum 50 nm, or between 1 nm and 40 nm, i.e. from one to few tens of nanometer. The composition of the absorber layers 13 is chosen such that they are absorbent to the laser radiation B, and that the saturation of absorption can be achieved with reasonable saturation intensities. For example, for a 1.06 μm laser radiation the absorber material may be GaXIn1−XAs with X≈0.25. QW-materials with nonlinearly on radiation intensity dependent optical absorption, and applicable as materials for absorber layers are generally known, and for the first absorber layers any of the known or possible new materials ca be used. As examples of these applicable QW-materials can be mentioned GaX1In1−X1AsY1, P1−y1, Gax1In1−x1AsY1N1−Y1 and GapX1In1−X1As, in which the mole fraction X1 is smaller than 0.5. QW-materials like (AlX1Ga1−X1)Y1In1−Y1As, in which the mole fraction X1 is smaller than 0.5, can be alternatively used. Some more detailed examples are listed in the table “Material Examples” below.
The first contacting layers 14 are of a first optically transparent semiconductor material, which more specifically according to the invention contains two or more main components, at least one dopant M2, and at least one metallic alloying element M1 substituting one of said main components and enhancing the incorporation of said dopant(s), said metallic alloying element having a concentration at least 50 atomic-% of that main component it substitutes, which kind of semiconductor material is called a reactive R-material. The mentioned main components of the R-material are nowadays typically Gallium and/or Indium and/or Arsenic and/or Phosphorus, i.e. without the metallic alloying element M1 and without the dopant M2 the composition “R-material” may resemble QW-materials like those mentioned above. But to attain the actual reactive R-material at least one of these main components is replaced partly or totally by a metallic element M1. The first contacting layers 14, 14a, 14b, 14c etc. are grown adjacent to each of the first saturable absorber layers 13, 13a, 13b etc. either at both sides of them—i.e. against both opposite surfaces 3a and 3b—as shown in
According to the invention the reactive R-material contains, not only metallic alloying element M1, but also at least one dopant M2. Definition “dopant” means here, as traditionally, an alloying element, which is present in a concentration at maximum 10−4 mole fraction. The dopant M2, which is an element or elements of group VI of the Periodic Table, or possibly group VIII of the Periodic Table, can be introduced in the first contacting layer(s) by the same way as other alloying elements in the epitaxial structure, which is the generally known alternative, and which is not described in detail. The element used as the dopant M2 can oxygen and/or iron and/or chromium, but preferably oxygen. Alternatively the growth of the layer(s) may be interrupted at or close to the heterointerfaces between the first contacting layers 14 and the first absorber layers 13 to further enhance the adsorption of impurity atoms, such as oxygen, during the interrupt. The dopant atoms may originate from the gas or gases already present in the atmosphere around the epitaxially grown first contacting layer(s) of said R-material and be transferred as said at least one dopant M2 onto said layer(s). The impurity atoms, i.e. dopant(s), create deep energy levels into the energy band structure and act as effective sink of carriers generated in the first absorber layers 13 forming the saturable absorber. Hence, the carrier lifetime will be drastically reduced thus facilitating the mode-locking generation in the saturable absorber structure 10. The first contacting layers 14 have a further benefit that they are usually of a much wider band-gap than the first absorber layers 13. Therefore, carriers generated within the first absorber layers 13 are effectively confined by the heterobarriers, and are not able to thermally leak out of the absorber layers 13. To further enhance the effect of the first contacting layers 14, gas or gases containing suitable impurity atoms, such as O2, may be injected or fed through a leak valve during the growth or growth interrupts towards the latest epitaxially grown layer so that component(s) thereof is/are transferred as the dopant(s) M2 into the layer. This ensures that sufficient number of impurity atoms get adsorbed and embedded into the first contacting layer 14 to reach sufficient shortening of the carrier lifetimes, i.e. the first recombination time, in the first absorber layers 13 down to at maximum 100 picoseconds, or shorter than 100 picoseconds. After introducing both the metallic element M1 and the dopant M2 with or without stopping the epitaxial growth for this purpose the first contacting layer(s) of the reactive R-material is formed. The thickness of the contacting layer(s) 14, 14a, 14b . . . can vary considerably, contacting layer(s) may be as thin as a few atoms, or hundreds of nanometers, or even more. It is important that there is either a good lattice fit—especially when the contacting layers are not extremely thin—between each of the absorber layers and the respective contacting layer or layers, or a pseudomorphism—when the contacting layers are very thin—between each of the absorber layers and the respective contacting layer or layers, so that excessive linear and two- and three-dimensional lattice defects are avoided. Accordingly, lattice fit or pseudomorphism respectively between the first contacting layer(s) and the first absorber layer(s) is so good that said QW-material of the first absorber layer(s) has a dislocation density at maximum 200×104/cm2, or smaller than 10×104/cm2, or smaller than 5×103/cm2.
In the following table is listed some possible QW-materials with nonlinearly on radiation intensity dependent optical absorption, and some R-materials—the dopant is not marked—that can be used together with the mentioned QW-materials. Further, the table tells some optional N-materials, which are discussed later, and the wave-length at which the saturable absorber in question works.
In addition to the fast saturable absorbers according to the invention, as disclosed above, and comprising first absorber layer(s) 13, 13a, 13b of a QW-material together with first contacting layers 14, 14a, 14b, 14c of an R-material, the saturable absorber structure 10 according to the invention can comprise further absorber layer(s), i.e. second absorber layer(s) 13, 13c, and second contacting layer(s) 14, 14d, 14e to attain longer recombination times. This is because slow saturable absorbers are preferred for initiation of mode locking, while fast saturable absorbers are preferred for supporting stable short pulse mode-locked operation of the laser. For this purpose there is at least a second absorber layer 13, 13c of a quantum well semiconductor QW-material of the described type with two opposite surfaces 3c, 3d. There is also at least one second contacting layer 14, 14d, 14e of an optically transparent semiconductor material against a surface or surfaces 3c and/or 3d of said second absorber layer(s). Deviating from what is already explained in this text, in this case the optically transparent semiconductor material of the second contacting layer(s) is a neutral N-material. N-material has a lower concentration of the metallic alloying element M1 and/or a lower concentration of the dopant M2 than said R-material of the first contacting layer(s) 14, 14a, 14b, 14c. This way the charge carriers originating in the QW-material of the second absorber layer(s) 13, 13c has a second recombination time longer than 100 picoseconds, thus forming a slow saturable absorber. Hence, the unique capability to combine both slow and fast saturable absorbers in the same structure allows one to optimize the device for both initiation and supporting the mode locking.
The spacer layers 15 separate the at least two absorber layers or the multiple absorber layers from each other, and provide the correct spacing L1 between the different absorber layers 13, 13a, 13b, 13c . . . with their respective contacting layers 14, 14a, 14b, 14c, 14d, 14e . . . placed at or close to different antinodes A of the standing wave pattern, as shown in
Since at least one mirror or reflector, i.e. the first Bragg-reflector 23 is deposited on the absorber structure, and the saturable absorber structure 10 is typically in connection either with a separate mirror, not shown in the Figures, or with a second Bragg-reflector 24 deposited on absorber structure, so that there is a distance between the first Bragg-reflector 23 and the second Bragg-reflector 24 or the separate mirror, Fabry-Perot etalon is formed. Then a standing wave will exist within the saturable absorber structure when an electromagnetic radiation B with a wavelength λ is fed into the structure 10 in a direction perpendicular to the planes of the layers 13, 14, 15 and reflector(s). For those familiar with the art it is clear that nodes i.e. amplitude maxima, and antinodes A i.e. amplitude minima, form in the standing wave pattern at a given laser wavelength λ. The absorber layers 13 are preferably placed at or close to the antinodes A when minimizing the saturation intensity is preferred, as already explained. One or multiple absorber layers 13, 13a, 13b, 13c . . . can be grouped around each antinode, and one or multiple of such groups may be placed into the structure, one at each antinode or other preferred position close to the antinode. The number and position of the absorber layers 13, 13a, 13b, 13c . . . are determined by the required saturation intensity. It is preferred alternative to have the first and the second Bragg-reflectors 23, 24, which both are formed of a multitude of quarter wavelength layers 19 with two different refractive indices alternating, which are optically transparent semiconductor material, or—as the alternative preferred—optically transparent dielectric material. Bragg-reflectors are generally known, are not described in detail. The first Bragg-reflector 23 may also contain on top of the dielectric stack a metallic high reflectivity layer, which in combination with the di-electric stack enhances the reflectivity and reduces the number of pairs required in the dielectric stack for the target reflectivity for the said reflector. The metallic layer may also serve to facilitate better bonding adhesion via the bonding material 22 to the heat sink 21, as described below.
Further, the saturable absorber structure 10 can comprise a heat sink 21, which is positioned against the first Bragg-reflector 23. In this arrangement the absorber layers 13, 13a, 13b, 13c of the quantum well semiconductor QW-material with said contacting layers 14, 14a, 14b, 14c, 14d, 14e of the first and/or second optically transparent semiconductor material extend away from said heat sink and said first Bragg-reflector. The heat sink 21 comprises a-high thermal conductive material, for example diamond or copper-diamond composite. The saturable absorber structure is attached to the heat sink 21 with a thermally conductive material 22, such as metallic solder.
The saturable absorber structure 10 of
Part of a more complete saturable absorber structure 10 is shown in
Another more complete saturable absorber structure 10 is shown in
During the manufacturing of the saturable absorber structure 10 according to the invention, including the sacrificial layer 12—if utilized—is first epitaxially grown on the single crystal substrate 11. Next the absorber layer(s) 13 together with the contacting layer(s), i.e. at least the first absorber and contacting layers; but also the second absorber and contacting layers, as well as the possible spacing layer(s) are epitaxially grown onto the sacrificial layer 12, or onto the substrate. The steps of epitaxial growing are repeated as many times as is needed to attain the predetermined number of different layers. The structure after these process steps corresponds to that shown in
As a short summary. The present invention describes a saturable absorber structure 10, which can also be called as a Hybrid Saturable Absorber Mirror (HSAM). One embodiment of the HSAM consists of a dielectric mirror on top of which the semiconductor saturable absorber structure is located. The fast recovery time of the absorber layers is achieved in situ by the epitaxial growth process using special contacting layers of a reactive R-material against one side or both sides of the absorber layers of a QW-material. Typically the saturable absorber structure comprises at least two absorber layers 13, 13a, 13b of the QW-material and at least two contacting layers 14, 14a, 14b, 14c of the R-material so that at least one R-material is in contact with each of said absorber layers of the QW-material, whereupon the layers 13, 14 of the QW-material and the R-material form a multi-quantum-well structure. On top of the saturable absorber a second dielectric reflector may be placed for controlling the saturation intensity and the group delay properties of the absorber structure, whereupon reduction of the carrier lifetimes to picosecond or sub-picosecond regime is possible without incurring non-saturable losses like scattering and non-saturable saturable absorption, at the same time keeping structural damages negligible, and which would allow the use of large spot sizes on the saturable absorber sample. The dependence of the standing wave intensity on the reflectivity of the first reflector 23 and the second reflector 24 can be calculated by using e.g. the well-known transfer matrix method. For those familiar with the art, it is clear that also the optical thickness of the saturable absorber structure 10 at the laser wavelength is affecting the standing wave intensity at the said wavelength. There are different alternatives for connections between a saturable absorber and a laser. The saturable absorber structure 10 according to invention can be connected using any of the known or new devices, arrangements, and methods applicable to the intended use. Connections between laser and saturable absorber are, accordingly, not described in this text.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI2005/000188 | 4/21/2005 | WO | 00 | 2/13/2008 |
