Patent Grant
9246312
This application claims priority from U.S. patent application Ser. No. 13/572,824, filed Aug. 13, 2012, which in turn claims priority from German Patent Application 10 2011 109 971.2, filed Aug. 11, 2011, both of which are incorporated herein by reference in their entireties.
The present invention is in the field of optics. In particular, the present invention relates to a tuneable VCSEL device. Such tuneable VCSEL devices are particularly useful for optical coherence tomography (OCT) applications.
Optical coherence tomography (OCT) is an optical signal acquisition and processing method. It captures micrometer resolution 3D-images within optically scattering media, such as bio-logical tissue. A distance from a scattering medium is measured by use of interferometric signals. In OCT, comparatively long wavelength light, typically in the near-infrared region is used. This has the advantage that the light may penetrate rather deeply into the scattering medium, allowing to obtain sub-surface images at near-microscopic resolution. OCT has proven particularly useful in the imaging of the human eye, allowing obtaining high resolution images of the retina and the anterior segment of the eye. A further very attractive application of OCT currently under development is intravascular imaging in cardiology.
As in any interferometric methods, it is possible to measure the interference signal in the time domain or in the frequency domain. Simply put, in the time domain the length of the interferometer reference arm is varied and the intensity of the interference signal is measured, without paying attention to the signal spectrum. Alternatively, the interference of the individual spectral components can be measured, which corresponds to the measurement in the frequency domain. There are also mixed variants of time and frequency domain OCT, of which the “time encoded frequency domain OCT”, which is also referred to as the “swept source OCT” has recently received particular attention. In the swept source OCT, each frequency scan allows to obtain exactly one depth scan of the OCT image. There is a common understanding that the swept source OCT is the most powerful and promising OCT variant that will be of increasing importance, in particular for ophthalmologic and cardiologic OCT imaging. Currently, there are three promising approaches to obtain tuneable laser sources allowing for higher sweep rates:
Accordingly, each of the three abovementioned light sources with a potential for higher frequency sweep rates than conventional tuneable lasers employs some sort of Fabry-Perot filter device, and the sweep frequency of the Fabry-Perot filter device turns out to be the limiting factor of the sweep frequency of the light source as a whole.
Between the two mirrors 12, an air gap is formed that defines an optical resonator cavity. By operation of the piezo-actuator 20, the optical path length between the two mirrors 12, or in other words, the length of the resonator cavity formed between the two mirrors 12 can be tuned. When the optical length of the round trip length of the cavity is an integer of a wavelength, then this wavelength together with a narrow band resonates inside the cavity and passes through the filter 10 with very low loss. Wavelengths not meeting this resonance condition, however, will not pass from one optical fiber 14 to the other, but will be blocked by the filter instead. This way, the FFP-TF 10 acts as tuneable narrow band pass filter.
The FFP-TF 10 design of
A further FFP-TF is commercially available from LambdaQuest, see Lambda Quest, L. “High Speed Optical Tunable Filter 2011”; Available from: www.lambdaquest.com/products.htm. The LambdaQuest FFP-TF has a U-shaped design, where the two arms of the U carry the ferrules with the fibers and where the arms are connected by a piezo-actuator. The general shape is shown in
At low operation frequencies of the piezo-actuator 24, i.e. at low sweep frequencies, the arms 22 will simply be moved to and fro according to the expansion and contraction of the piezo-actuator 24. At higher frequencies, the entire U-shaped structure will start to oscillate in a vibration mode that is indicated by the arrows 26 in
The vibration mode of the U-shaped structure can also be seen in the response function of the LambdaQuest filter recorded by the inventors which is shown in
Both FFP-TFs of
However, VCSELS based on MEMS technology also have considerable drawbacks in practice. The manufacturing requires a large number of etching steps that need to be carried out in a clean room. Also, since the material bridges suspending the mirror are very thin and hence difficult to manufacture, there is an inherent reproducibility problem. Also, since the top mirror is very thin, heat dissipation is rather difficult. Further, since the top mirror is actuated electrostatically, the necessary control voltages are rather high and the elongation depends highly non-linearly on the control voltage. And while in general the extremely small mass of the top mirror is an advantage when it comes to sweep frequency, the resonance wavelength tends to be unstable due to thermal noise (Brownian motion), see J. Dellunde, et al., “Trans-verse-mode selection and noise properties of external-cavity vertical-cavity surface-emitting lasers including multiple-reflection effects”, Journal of the Optical Society of America B-Optical Physics, vol. 16, pp. 2131-2139, November 1999.
According to a the invention, a tuneable VCSEL is provided, comprising:
Note that typically, VCSELs are MEMS devices, with the advantages but also the drawbacks referred to in the introductory portion. These drawbacks are particularly related to the rather complicated manufacturing, to the difficulties with regard to reproducibility and reliability, as well as to difficulties with the operating stability due to noise (Brownian motion) which affects the rather delicate moving mirrors of tuneable VCSELs based on MEMS.
According to the invention, a very robust tuneable VCSEL is provided that can be manufactured with much less demand for manufacturing equipment and that exhibits a good reliability and reproducibility.
Preferably, the resonator cavity between the two reflecting elements is free of any light guiding medium. This is different from tuneable fiber Fabry-Perot VCSELs as for example disclosed in U.S. Pat. No. 6,263,002, where the light is confined in a waveguide within the cavity. Dispensing with any light guiding medium in the resonator cavity allows for longer resonator length modulation amplitudes and higher sweep frequencies. At the same time, the total length of the resonator cavity without light guiding medium can generally be made shorter, hence allowing for a larger free spectral range (FSR).
According to the invention, light guiding materials in the resonator cavity can be dispensed with by providing a concave recess formed in one of the reflecting elements. The concave recess can be designed such as to provide a stable resonator for the wavelengths of interest. Simply put, a “stable resonator” is a resonator where the light will not escape even after plural reflections. For more details about stable resonators, reference is made to the textbook, “Lasers” of A. E. Siegman, University Science Books, 55D Gate Five Road, Sausalito, Calif. 94965, ISBN 0-935702-11-3.
In a preferred embodiment, the waveguide is in direct contact with one of the reflecting elements. Herein, the expression “direct contact” shall mean that no additional optical elements are provided between the waveguide and the reflecting element.
In a preferred embodiment, one of the reflecting elements comprises a polymer layer in which the concave recess is formed and that is coated with a reflecting layer, in particular a dielectric reflecting layer. Herein, the thickness of the polymer layer is preferably larger than 5 μm, more preferably larger than 8 μm and/or preferably smaller than 100 μm, more preferably smaller than 40 μm.
This embodiment has great advantages from a manufacturing point of view. The concave recess can be easily formed in the polymer layer such as by a mechanical indentation or by laser ablation. Further, the concave recess can be formed after the polymer layer has been applied to the end of the fiber and also the end face of a ferrule surrounding the fiber. Then, the concave recess can be integrally formed precisely at the desired location, avoiding any alignment of separate components that would make the manufacture more complicated and prone to errors. Direct laser machining of the concave structure would have the same advantages.
The first actuator preferably comprises at least one piezo-electric actuator, in particular, a lead-zirconate-titanate (PZT) actuator.
Preferably, the waveguide, typically an optical fiber, is accommodated in a ferrule. The ferrule can be made from glass, metal or zirconia and can be a standard telecommunications component that is manufactured with high precision but is available at fairly low prices.
In a preferred embodiment, one of the reflecting elements comprises a substrate, the substrate being provided with a reflecting layer. The substrate may be disposed on the end of a fiber ferrule, although no light is coupled out of the cavity through the substrate and the ferrule hence does not contain any fiber. However, using a fiber ferrule for mounting the substrate allows for a very precise positioning thereof employing inexpensive, “off-the-shelf” components. What is more, this allows mounting both reflecting elements—i.e. the reflecting element that is coupled to a fiber and the reflecting element that is not coupled to a fiber—in the same manner, preferably even using identical holding frames which permits a symmetric design and decreases manufacturing costs. For this purpose, the two ferrules preferably have at least the same outside diameter.
In a preferred embodiment, a gain medium is provided on top of one of the reflecting elements. The gain medium may be provided with an anti-reflective coating. Preferably, the thickness of the gain medium is a multiple of ¼ of the operating wavelength of the VCSEL.
In a preferred embodiment, one of the reflecting elements is formed by a plate, said plate having a free portion that is free to carry out plate vibrations and a supporting portion where the plate is supported. Herein, the plate may, but need not, carry the gain medium.
The free portion is preferably a central portion and the supporting portion is a circumferential portion at least partly surrounding the central portion of the plate, wherein the supporting portion is directly or indirectly supported by the first actuator means. Preferably, the first actuator means is ring-shaped or is comprised of a plurality of actuators arranged along the circumferential portion of said plate. Such plate may resonate with high amplitude at very high frequencies, thereby modulating the resonator cavity length with a high frequency and in effect allowing for a high sweep frequency.
In a preferred embodiment, the VCSEL comprises a second actuator means, in particular a second piezo-electric actuator. The second piezo-electric actuator may be devised for low frequency actuation such as to compensate drifts in the cavity length and the like, while the first actuator means allows for the frequency sweeps in operation.
In one embodiment, the radius of the resonator cavity is larger than 40 μm, preferably larger than 80 μm and most preferably larger than 150 μm. In addition or alternatively, the radius of the resonator cavity may be smaller than 5 mm, preferably smaller than 1 mm and most preferably smaller than 800 μm.
In a preferred embodiment, the free spectral range of the VCSEL is larger than 1 nm, preferably larger than 30 nm and most preferably larger than 80 nm.
The center wavelength of the tuneable VCSEL is preferably chosen among one of the following ranges: 750-870 nm, 1000-1100 nm, 1250-1350 nm and 1480-1600 nm. In a preferred embodiment, the VCSEL is operated in one of the 5th to 15th order.
As mentioned before, the tuneable VCSEL of the invention is of high practical value even if its sweep frequency is not particularly high. However, preferably the sweep frequency of the frequency tuning is at least 1 kHz, more preferably at least 50 kHz. Much higher frequencies can, nevertheless, be obtained by using one or more of the features described below.
Preferably, the tuneable VCSEL is pumped using a pump diode having a wavelength between 950 and 1010 nm. In a preferred embodiment, the optical fiber is a polarization conserving fiber, i.e. a fiber in which no or little optical power is transferred from one polarization mode to another. In the alternative, the optical fiber can also be a polarizing fiber.
In a preferred embodiment, at least one of the gain medium, the reflecting elements or the substrate is structured such as to provide a polarizing effect. This can for example be achieved with a so-called “wire grid” polarizer structure.
The tuneable VCSEL may be operated in an inert gas atmosphere or in a vacuum.
When the gain material is optically pumped, the pumping light may be coupled to the gain medium from the back side, i.e. from the side facing away from the resonator cavity. Herein, the pump light may be coupled to the gain medium by means of a non-guided beam or via a waveguide.
In a preferred embodiment, the pump light is generated by a multi-mode laser diode. However, in alternative embodiments, the gain medium may also be electrically pumped.
In a preferred embodiment, the volume of the gain medium that is optically or electrically pumped is at least twice as large as the volume covered by the optical resonator mode, preferably at least five times as large and most preferably at least 20 times as large.
In order to efficiently dissipate heat from the device, the tuneable VCSEL device is preferably supported by a thermically conductive material. However, the tuneable VCSEL may also comprise means for actively stabilizing the temperature, such as a Peltier element. In addition, the VCSEL device may be sealed by moisture-barrier forming moulding material which may also be a heat conductive material.
While in the preferred embodiment the concave recess is formed in a polymer layer that is directly attached to an optical waveguide or fiber end, in some embodiments the concave recess can also be provided directly in the fiber end, for example by means of laser ablation, etching or polishing.
In yet a further embodiment, the first or second reflecting elements can be formed by a mirror surface that is directly applied to a piezo-electric element forming the first actuator means.
In one embodiment, the VCSEL is pumped with a quantum dot laser.
In one embodiment, the piezo-actuator forming the first or second actuator means has a mass larger than 10 μg, preferably larger than 100 μg and most preferably larger than 1 mg. Also, the minimum dimension of the first or second piezo-actuators in operating direction may be 0.3 mm or more, preferably 1.0 mm or more and most preferably 1.9 mm or more.
In one embodiment, the first and/or second reflecting element, including its carrier substrate may have a thickness of 50 μm or more, preferably of 100 μm or more and most preferably of 1 mm or more. Further, the first and/or second reflecting element may have a diameter of 200 μm or more, preferably of 500 μm or more and most preferably of 1.9 mm or more.
In a preferred embodiment, said first actuator means is
Herein, the first actuator means is configured to modulate the optical path length between said first and second reflecting elements by a modulation amplitude to thereby sweep the optical resonator cavity through a band of optical resonance frequencies with a sweep frequency of 70 kHz or more, preferably 100 kHz or more and more preferably 200 kHz or more. Herein, the tuneable VCSEL device is further characterized by one or both of the following features:
Accordingly, this embodiment avoids an MEMS structure but relies on an ordinary mechanical design instead, including a first actuator means that is directly or indirectly mechanically coupled with said first reflecting element. Herein, the first actuator means may for example be a piezo-actuator.
However, the VCSEL device of this embodiment differs from the prior art FFP-TFs of
Further, according feature (b), the first actuator means is directly coupled with or indirectly coupled with said first reflecting element via said supporting member such as to substantially drive the first reflecting element only, but not the second reflective elements. This is different from both prior art filter designs as shown in
Note that the expression “substantially drive the first reflecting element only” accounts for the fact that since the first actuator means and the second reflecting means are part of the same structure, they will necessarily be somehow coupled, too, and that strictly speaking in view of the finite masses involved it cannot be excluded that the second reflective element is slightly driven by the first actuator means as well. However, the mass of the first reflecting element and—if present—the supporting member is considerably smaller than that of the remainder of the Fabry-Perot tuneable filter device that for all practical purposes it can be said that the first reflecting element is substantially driven only.
As will be explained below in more detail, each of features (a) and (b) will allow to push the sweep frequency limit considerably higher than in the prior art filters of
Any of these features is characteristic for elements acting as a system of coupled oscillating elements.
In some cases, independently oscillating elements can be identified by the existence of pronounced nodes with almost zero amplitude within the structure, as will be shown below with reference to
Note that the “resonance” is the resonance in the response function of the tuneable VCSEL device as a whole, i.e. the response function of the modulation amplitude of the optical path length as a function of driving frequency of the first actuator means. Although this optical path length modulation amplitude does not depend on the oscillation amplitude of the individual element alone, it is seen that in some embodiments of the invention, the resonance in the response function is nevertheless attributable to the oscillation of an individual element as a part of a system of coupled oscillating elements.
Preferably, the resonance that is attributed to one of the first actuator means, the reflecting element and the supporting member has a Q-factor of 3 or more, preferably of 8 or more.
In a preferred embodiment, the amplitude of the oscillation of the first reflective element is larger than that of the first actuator when operated at said sweep frequency of 70 kHz or more. This way, a considerable modulation amplitude of the optical path length can be obtained even at smaller oscillation amplitudes of the first actuator.
In a preferred embodiment, a phase shift between two quantities chosen from a control signal, the elongation of the first actuator means in operation direction, the elongation of the supporting member, and the elongation of the first reflecting element is between 45° and 135°, preferably between 70° and 110°. Herein, no phase shift (i.e. 0 phase) would correspond to an elongation state of two elements that would lead to an identical effect on the optical path length between the first and second reflecting elements. For example, an elongation of the first actuator means that leads to a decrease in the optical path length and an elongation of the supporting member that would lead to a yet further decrease of the optical path length would be considered as being in phase, i.e. having a phase shift of 0°. A phase shift as defined above is indicative of a system of coupled oscillating elements. In particular, a phase shift close to 90° is indicative of an energy transfer between the two coupled oscillating elements that allows for large modulation amplitudes.
Preferably, the resonance frequencies of at least two elements out of the first actuator means, the supporting member and the first reflecting element differ by less than 60%, preferably by less than 30% and most preferably by less than 10%. This allows for an efficient energy transfer between the at least two elements.
In a preferred embodiment, the first reflecting element is formed by a plate that has a free portion that is free to carry out plate vibrations, and a supporting portion where the plate is supported. Herein, the “plate vibrations” refer to vibrations normal to the rest plane of the plate. A freely vibrating plate is one example of a first reflecting element that is loosely coupled with the rest of the filter device. Obviously, the frequencies of the plate vibrations can be much higher than e.g. the resonance of the entire U-shaped structure of the prior art FFP-TF of
In a preferred embodiment, the free portion is a central portion and the supporting portion is a circumferential portion at least partially surrounding said central portion of said plate. As will be shown in a specific embodiment below, this arrangement allows for high frequency and high amplitude plate vibrations permitting a very fast frequency sweep over a large frequency range.
In a preferred embodiment, the supporting portion is directly or indirectly supported by said first actuator means. Herein, the expression “directly supported” could for example mean that the plate is glued directly to said first actuator means. However, the supporting portion may also be indirectly supported by the first actuator means via some intermediate element provided therebetween.
In a preferred embodiment, the first actuator means is ring-shaped or comprised of a plurality of actuators arranged along the circumferential portion of said plate. This means that the central free portion of the plate will be driven from the circumferential portion surrounding said central free portion. The inventors could confirm that this way very high oscillations of the free portion can be achieved. Graphically speaking, driving the free portion of the plate from the circumferential portion focuses the energy from the ring-shaped first actuator means or circumferentially arranged plurality of actuators to the center of the free portion, where it leads to very large oscillation amplitudes, which in turn lead to a large modulation amplitude of the optical path length.
In a preferred embodiment, the plate is made from one of glass, quartz, sapphire and diamond. Further, the thickness of the free portion of the plate is preferably 0.02 mm or higher, more preferably 0.05 mm or higher and most preferably 0.1 mm or higher. Alternatively or in addition, the thickness of the free portion of the plate is preferably 2.0 mm or lower, preferably 0.5 mm or lower and most preferably 0.2 mm or lower.
Preferably, the plate has a front side facing the resonator cavity and a back side facing away from the resonator cavity, wherein a fiber focusing means is provided opposite to the back side of the plate such as to focus light exiting from the resonator cavity through said plate into an optical fiber. Note that this design is different from the typical fiber FP-TF as shown in
Preferably, the fiber focusing means comprises at least one gradient-index (GRIN) lens. A GRIN lens is a lens having a gradual variation of the refractive index of its material. Further, the back side of the plate facing away from the resonator cavity is preferably coated with an anti-reflective coating.
Preferably, the supporting member has a tapered shape, having a larger cross section at its back end and tapering towards the front end thereof. This tapered shape allows for a comparatively large back end surface in relation to the total mass of the supporting member and is hence preferable as compared with, for example, a cylindrical supporting member design. Namely, since the back end surfaces is where the supporting member is attached to the first actuator means, this allows combining a comparatively light weight supporting structure with a first actuator means of a given size and hence mass. In fact, in practice this will allow to provide a supporting member having a mass that is considerably smaller than that of the first actuator means. The weight of the supporting member can be further reduced by manufacturing it from light metals such as aluminum or magnesium. Again, the reduced mass of the supporting member allows for higher sweep frequencies of the tuneable filter device.
The geometry of the supporting member can be further devised such as to display a resonance at an eigenfrequency that is higher than the desired sweep frequency of 70 kHz or more, preferably 100 kHz or more and most preferably 200 kHz or more. For example, the supporting member and the first actuator means may act as a system of coupled oscillators and the oscillation frequency of the supporting member may then lead to a resonance in the response function of the optical path length modulation of the filter device that is attributable to the supporting member oscillations. The desired shape of the supporting member can be established in an electrical discharge machining process, in particular by spark machining, spark erosion or wire erosion. This way, the stiffness and oscillating frequency of the supporting member can be adjusted as desired.
Irrespectively of the detailed design of the VCSEL device, the inventors have noticed that in preferable embodiments, the masses of the first actuator means, the supporting member (if present) and the reflecting elements may fulfill one or more of the following criteria:
Note that when the individual components first actuator means, supporting member and first reflecting element act as a system of coupled oscillators, it is not always the total mass of the respective component that governs the resonance frequency, but rather the mass that is actually moved during oscillation. For example, with reference to the oscillating plate design, the resonance frequency will depend on the mass of the free portion of the plate rather than on the total mass of the plate, which may have sections that are not involved in the oscillation at all. Accordingly, in a preferred embodiment the amplitude weighted mass of the reflecting element defined as the integral of the product of the local peak-to-peak displacement A(x,y,z) times the mass element dm, with A normalized such that the volume integral over all x,y,z of the function A(x,y,z) is 1, is less than 20 mg, preferably less than 5 mg and most preferably less than 1 mg.
As mentioned before, in preferred embodiments, the first actuator means is a piezo-actuator and in particular, a lead-zirconate-titanate actuator. The first piezo-actuator is preferably ring-shaped with a hole having a diameter of 0.2 to 8.0 mm, preferably of 0.6 to 3.5 mm and most preferably of 0.7 to 2.1 mm. In one embodiment, the first piezo-actuator is operated at said sweep frequency with 0.1 to 50% of its maximum elongation.
In a preferred embodiment, the maximum possible modulation amplitude provided by the first piezo-actuator at said sweep frequency is at least 100 nm, preferably at least 500 nm and most preferably at least 1 μm. In a preferred embodiment, the first piezo-actuator comprises a single ceramic layer.
In a preferred embodiment, the tuneable VCSEL device comprises a second actuator means for providing an optical path length modulation at low frequencies. Herein, the expression “low frequency” means a frequency that is below the aforementioned sweep frequency, for example at a frequency as low as 200 Hz. Such low frequency operation is also referred to as “DC operation” herein. The second actuator means is preferably a second piezo-actuator, in particular a lead-zirconate-titanate piezo-electric element.
The second actuator means can be used for calibrating the cavity length to a desired mean value, while the first actuator means is employed to modulate the cavity length with regard to this mean value. This way, drifts in the cavity length can be compensated by the second actuator means, while the entire stroke available from the first actuator means can be used for the modulation.
Preferably, the modulation amplitude provided by the second actuator means at low frequencies is at least 100 nm, preferably at least 500 nm and most preferably at least 1 μm.
When the second actuator means is formed by a second piezo-actuator, its length in operation direction is preferably more than 0.5 mm, preferably more than 1.0 mm and most preferably more than 1.9 mm. The second piezo-actuator is preferably a multi-layer piezo electric actuator.
Preferably, the first and second actuator means are operated by means of two separate electrical circuits.
When the first and/or second actuator means are formed by a piezo-actuator, the Curie temperature is preferably more than 180° C. This will permit soldering connections with the piezo-actuator. Also, this will prevent that the operation is compromised when the piezo-actuators heat up during operation.
In a preferred embodiment, the tuneable VCSEL device comprises driving means for driving the first actuator means. Herein, the driving means may comprise a microcontroller, hard-wired circuitry or both.
In a preferred embodiment, the driving means is adapted to drive the first actuating means at a resonance frequency of the response function of the optical path length modulation of the FP-TF device. As will be shown below, this resonance may be attributable to one of the first actuator means, the supporting member or the first reflecting element. So in some cases it can also be said that the driving means is adapted to drive the first actuator means at the resonance of the first actuator means, the support member or the reflecting element, respectively.
In one embodiment, the driving means is adapted to drive the first actuator means with a harmonic, i.e. sinusoidal signal corresponding to a ground resonance of the tuneable VCSEL device. This will lead to large modulation amplitudes of the optical path.
In a preferred embodiment, the driving signal of the driving means may contain components of the higher harmonics of the ground resonance, in particular the third harmonics. By a suitable combination of the ground resonance and its higher harmonics, the wavelength sweep can be to some extent linearized in that the optical path length modulation is more similar to a triangle waveform rather than a purely sinusoidal waveform.
In one embodiment, the first actuator means is a piezo-actuator, and the driving means comprises an inductivity that is provided in parallel to the piezo-actuator. Since the piezo-actuator has a considerable capacitance, the inductivity provided in parallel to the piezo-actuator forms an LC-parallel-oscillator circuit that can be adjusted to the desired driving frequency.
Also, the driving means may comprise an LC-series-oscillator circuit connected in series with the first actuator means, where the LC-series circuit is adjusted to the desired driving frequency. If it is intended to drive the first actuator means with a superposition of different frequencies, then a number of corresponding LC-series circuits can be provided in parallel to each other, but each in series with the first driving means.
Also, in order to add a constant bias to the driving signal without disturbing the other components, the driving means may comprise a bias-T circuit with an AC-input and a DC-input.
Each of the features and constructive details can be employed in all possible combinations.
a) is a perspective view of an FP-TF device
b) is a side view of the FP-TF device of
a) is a sectional view of the FP-TF device of
b) shows a detailed view of the portion of
a) is a contour plot showing the elongation or excursion of the glass plate acting as the first reflecting element in the FP-TF of
b) is a 3D-plot showing the same data as
c) is another 3D-plot showing the same data as
a), (b) show similar data as
a) is a perspective view of a second example of a FP-TF
b) is a sectional view of the second example of the FP-TF according to
a) is a perspective view of a tuneable VCSEL,
b) is a sectional close-up view of the resonator cavity of the tuneable VCSEL of
a) is a perspective view of a further tuneable VCSEL allowing for high sweep frequencies,
b) is a side view of the tuneable VCSEL of
c) is a sectional view of a part of the tuneable VCSEL of
d) is a sectional enlarged view showing the resonator cavity of the tuneable VCSEL of
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiments illustrated in the drawings and specific language will be used to describe the same. It will, nevertheless, be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates.
As is shown in
While specific examples will be described below, various embodiments generally allow for higher frequencies of the tuneable filter by employing one or both of two features that can already be explained with reference to
According to the second feature (b), the actuator means is directly coupled or indirectly coupled via the supporting member with one reflecting element, referred to as the “first reflecting element” herein, such as to substantially drive this first reflecting element only. This is different from conventional designs where the mechanical actuator is symmetrically coupled to both reflecting elements forming the resonator cavity.
A first example of an FP-TF device 28 will be described with reference to
As is best seen in the sectional view of
As is further seen best in
In the shown example, the glass plate 48 has a thickness of about 0.2 mm. Instead of glass, other materials could also be used, such as quartz, sapphire or diamond. The first piezo-actuator 50 is attached to a first holding frame 52 that may also be made from aluminum. The first holding frame 52 supports a fiber focusing means 54 that receives light transmitted from the resonator cavity 56, i.e. the air gap formed between the second reflective surface 40 and the glass plate 48 (see
The back side of the plate 48, i.e. the side facing away from the resonator cavity 56 is coated with an anti-reflective coating. Further, the fiber focusing means 54 preferably comprises a gradient index lens 59 for focussing the light into the first optical fiber 58.
In
Further, it is seen that the first piezo-actuator 50 is coupled with the first reflective element, i.e. the plate 48 only, such that the plate 48 is substantially driven by the first piezo-actuator 50 only. Herein, the term “substantially” shall again indicate that of course the second ferrule 36 is also remotely coupled with the first piezo-electric actuator 50, but that due to the much higher moment of inertia of the structure inbetween, this will not lead to any significant movement of the second ferrule 36.
In particular, the response function shows a very sharp peak at about 397 kHz, which corresponds to the base mode of the oscillating plate 48. This has been confirmed by the inventors in an interferometric measurement of a space resolved detection of the mechanical oscillations, the results of which being summarized in
c) shows the same data as
Turning back to
Finally, the response function of
So in summary, it can be seen from the response function of
The performance can be further improved if the resonance frequencies of the first actuator means and the first reflecting element or the supporting member are adjusted to each other, so that the energy transfer from one element to the other is enhanced. Preferably, the ground mode resonances of at least two elements among the first actuator means, the supporting member (if present) and the first reflecting elements differ by 60% or less, preferably by 30% or less and most preferably by 10% or less.
Next, a second example of the FP-TF device will be explained with reference to
An important constructional difference between the fiber FP-TF device 66 of
Further, both the first piezo-actuator 74 and the conical supporting member 76 are ring-shaped, allowing the first optical fiber to be fed therethrough. This allows for a more compact design as compared to the design of
Also, when comparing the fiber FP-TF device 66 of
In some examples, the FP-TF filter may comprise a sensor for wavelength calibration, i.e. a sensor that allows measuring the optical path length in absence of any modulation due to the first actuator means. In one example, the optical path length can be estimated by a capacitance measurement employing a capacitive sensor. For this, at least one metal component is provided at a portion of the first optical element such as to form part of a capacitor employed in the capacitance sensor. The capacitance obtained should be in a range of 0.1 to 100 pF, preferably in a range from 1.0 to 40 pF, and most preferably between 10 and 30 pF. This can be e.g. achieved by depositing a conductive film on part of the first reflecting element, such as a metal coating or an indium tin oxide (ITO) coating. The advantage of an ITO coating is that it is and can hence be provided on the entire surface of the first reflecting element. Preferably, the distance between the capacitor plates is less than 0.1 mm, more preferably less than 0.05 mm and most preferably less than 0.03 mm. The area of the capacitor plate is preferably larger than 0.1 mm2, preferably larger than 1 mm2 and most preferably larger than 5 mm2.
Further provided is an LC-series oscillator circuit 96 that is connected in series with the first piezo actuator 50, 74. This LC-series circuit is as well adjusted to the desired driving frequency. If it is intended to drive the first piezo-actuator 50, 74 at different frequencies, for example higher harmonics of a ground resonance, then several of the LC-series circuits 96 can be provided in parallel.
Finally, a bias-T circuit 98 is provided, allowing to bias the AC-driving signal with a DC-input without disturbing the other components.
a) is a perspective view of a tuneable VCSEL 100 according to an embodiment of the invention.
With reference to
With reference to
A substrate 122 is provided on the end face of the other ferrule 106, i.e. the lower ferrule in the schematic drawing of
As is seen from
Moreover, as is seen particularly in
By operating the first piezo-actuators 104 (see
A particular advantage of the VCSEL 100 of
Further, both reflecting elements are formed on the end faces of ferrules 106. The manufacturing precision of the ferrules directly carries over to the manufacturing precision of the tuneable VCSEL 100. In fact, the main remaining task is to provide a very precisely drilled hole in the holding members 102 for receiving the ferrules 106.
So in summary, the whole tuneable VCSEL 100 can be manufactured comparatively easily with very good mechanical precision. This compares favorably to ordinary MEMS-based VCSELs that need to be manufactured in clean room conditions involving a large number of etching and lithography steps, the complexity of which makes small series production at reasonable cost difficult.
Further, it is emphasized that the light in the resonator cavity 110 propagates freely, i.e. it is not confined by any waveguiding material. At the same time, the light is confined in the resonator 10 due to the concave recess 118 which, at the operating wavelength of the tuneable VCSEL 100 provides for a stable resonator 110. Dispensing with any waveguiding material in the resonator cavity 110 makes the construction more simple and also allows for large resonator length modulation, i.e. a large tuning range.
While in the embodiment shown in
A further tuneable VCSEL 128 is shown in
The left half of the tuneable VCSEL 128 as shown in
A further holding member 132 is provided, which in this case, however, is different in shape from the holding member 130. The holding members 130, 132 are connected via two piezo-actuators 134, which, unlike the piezo-actuators 104 of
As is particularly seen in
A substrate 122 with a reflecting layer 124 and a gain medium layer 126 provided thereon is attached to the first piezo-actuator 138. As is seen in
By operating the first piezo-actuator 138, the substrate 122 with the reflective layer 124 and the gain medium 126 on top can be caused to oscillate in a similar way as was described with reference to
Finally,
Although preferred exemplary embodiments are shown and specified in detail in the drawings and the preceding specification, this should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiments are shown and specified and all variations or modifications should be protected that presently or in the future lie within the scope of protection of the invention.
| Number | Date | Country | Kind |
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| 10 2011 109 971 | Aug 2011 | DE | national |
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| 2113804 | Nov 2009 | EP |
| WO 9912235 | Mar 1999 | WO |
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| Number | Date | Country | |
|---|---|---|---|
| 20140369374 A1 | Dec 2014 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13572824 | Aug 2012 | US |
| Child | 14476838 | US |
