Patent Application
20110063701
The present invention generally relates to lasers and specifically to a method for combining numerous laser beams with preserved or improved beam quality for power and brightness scaling. The invention also relates to an optical planar system based on planar holography and to a method of improving brightness of light beams with the use of the digital optical planar holography system.
Many attempts have been made to combine laser beams by means of various devices and methods. For example, U.S. Pat. No. 4,246,548, granted to E. M. Rutz in 1981, describes a coherent laser array with an external resonator comprising spherical lenses, plane mirrors, and a spatial filter for transverse mode selection.
J. R. Leger, et al, in U.S. Pat. Nos. 4,813,762 issued in 1989 and 5,033,060 issued in 1991 proposed a coherent beam combining with an array of phase correctors and Talbot imaging. All components are placed into an external resonator.
Edwards, et al, in U.S. Pat. No. 5,029,964 issued in 1991 disclosed a laser-beam combiner for a plurality of high-power lasers, including a holder arrangement to receive the ends of the optical fibers, three collimating lenses, and a single focusing lens. The ends of the three optical fibers are located substantially on a common plane and are circularly spaced at 120-degree intervals. The three collimating lenses are also located substantially on a common plane, and their axes are circularly spaced at 120-degree intervals. Each of the collimating lenses is aligned with one of the optical fibers. The laser beams are directed by the collimating lenses, as collimated beams, onto the focusing lens with the axes of the individual beams parallel to each other.
Horikawa, et al, in U.S. Pat. No. 5,216,544 issued in 1993 proposed a beam-combining device comprising an airtight housing and a laser-beam source unit housed in the housing. The laser-beam source unit comprises laser-beam sources and collimator optical systems respectively positioned in optical paths of laser beams, which are radiated from the laser beam sources in order to collimate the laser beams. Optical-path adjusting elements are respectively positioned in the optical paths of the laser beams in order to radiate the laser beams along optical paths that are parallel and close to one another. The laser-beam sources, the collimator optical systems, and the optical-path adjusting elements are supported on a single support.
Veldkamp, et al, in U.S. Pat. No. 4,649,351 issued in 1987 disclosed an apparatus and method for coherently adding laser beams, including a diffraction grating illuminated by a plurality of lasers. The laser beams are combined coherently by phase locking and diffracting the beams into a single beam. The diffraction grating is configured to generate, upon illumination, substantially equal intensities of diffraction orders corresponding to the number of lasers while suppressing higher unwanted orders.
L. B. Glebov in his publication, “High brightness laser design based on volume Bragg gratings” in Proc. SPIE Vol. 6216 (2006), developed the idea of combining laser beams with thick (volume) Bragg gratings fabricated on photo-thermal-refractive glass plates by exposing them to ultraviolet laser radiation.
Rothenberg, et al, in U.S. Patent Application 20080084605 published in 2008 describes a hybrid beam-combining system and method for combining a plurality of coherent and incoherent light beams into a composite high-power diffraction-limited beam. Each N oscillator transmits light at one of N different wavelengths, and each wavelength is split into M constituent beams. M beams in each of the N groups are phase-locked by a phase modulator using phase-correction signals. The phase-locked beams are amplified and coupled into an M×N fiber array. Beams emerging from the array are collimated and incident on a diffractive optical element operating as a beam combiner to combine M outputs at each N wavelength into a single beam. The N single beams are incident and spectrally combined on a grating that transmits a composite beam at a nominal 100% fill factor. A low-power sample beam, taken from the N beams emerging from the diffractive optical element, is measured for phase deviations from which the phase-correction signals are derived and fed back to the phase modulators. The diffractive optical element may include a weak periodic grating for diffracting the low-power sample. The diffractive optical element may also be combined with the spectral combining grating into a single optical element.
Rothenberg, et al, in U.S. Patent Application 20080084598 published in 2008 discloses a method and system for coherent beam combining using an integrated diffractive beam combiner and sampler, which combines plural low-power light beams into a coherent high-power light beam by means of a diffractive optical element operating as both a beam combiner and beam sampler. An oscillation source transmits a master signal that is split into plural beams propagating at a common wavelength. Each beam is phase-locked by a corresponding phase modulator according to a phase-correction signal. The beams are directed through a fiber array to the diffractive optical element to allow efficient coherent combination of the beams at a desired diffraction order. The diffractive optical element includes a periodic-sampling grating for diffracting a low-power sample beam representative of the combined beam. A phase-detection stage detects phases of constituent beams in the sample beam from which the phase-correction signals are derived and fed back to the phase modulators. The diffractive optical element may be further modified to collimate beams diverging from the fiber array and to focus the sample beam onto a phase detector.
Donoghue, et al, in U.S. Patent Application 20020181035 published in 2002 proposed a method and system for combining multiple low-power laser sources to achieve high-efficiency, high-power output using holographic transmission methodologies. A holographic beam combiner (HBC) is used to combine output from many lasers into a single-aperture, diffraction-limited beam. The HBC is based on the storage of multiple holographic gratings in the same spatial location. By using a photopolymer material such as quinone-doped polymethylmethacrylate (PMMA) that uses the novel principle of “polymer with diffusion amplification” (PDA), it is possible to combine a large number (N) of diode lasers with an output intensity and brightness 0.9 N times as much as those of combined outputs of individual N lasers. The HBC is a small, inexpensive-to-manufacture, and lightweight optical element. The basic idea of the HBC is to construct multiple holograms onto a recording material, with each hologram using a reference-beam incident at a different angle but keeping the object beam at a fixed position. When illuminated by a single read beam at an angle matching one of the reference beams, a diffracted beam is produced in the fixed direction of the object beam. When multiple read beams that match multiple reference beams are used simultaneously, all beams can be made to diffract in the same direction and under certain conditions that depend on the degree of mutual coherence between input beams.
Miller in U.S. Pat. No. 7,265,896 published in 2007 described laser-beam combining for nonlinear frequency conversion with nonlinear feedback to the sources being combined. Such nonlinear feedback can advantageously reduce output-power sensitivity from the wavelength conversion device to variations in the nonlinear coefficients of the conversion device. The reason for this reduced sensitivity is that in preferred embodiments, feedback power increases if a nonlinear coefficient decreases and tends to increase the power supplied to the conversion device, thus mitigating the effect of the reduced nonlinear coefficient.
Miller in U.S. Pat. No. 7,423,802 issued in 2008 disclosed a multicolor illumination module providing light in a plurality of colors, including Red-Green-Blue (RGB) light and/or white light. In one embodiment the illumination module includes a laser configured to produce an optical beam at a first wavelength, a planar lightwave circuit coupled to the laser and configured to guide the optical beam, and a waveguide optical-frequency converter coupled to the planar lightwave circuit and configured to receive the optical beam at the first wavelength, to convert the optical beam at the first wavelength into an output optical beam at a second wavelength, and possibly to provide optically coupled feedback, which is nonlinearly dependent on the power of the optical beam at the laser first wavelength.
In U.S. Pat. No. 7,394,953 issued in 2008, Nagarajan, et al, proposed configurable optical combiners and decombiners to be implemented as arrayed waveguide grating-based multiplexors and demultiplexors.
Welch, et al, in U.S. Pat. No. 7,340,122, issued in 2008 described a similar monolithic integrated circuit in which the arrayed waveguide grating multiplexor is implemented on the same chip as laser sources and their modulators.
Peters, et al, in U.S. Pat. No. 7,444,048 issued in 2008 described arrayed waveguide gratings that are deployed for beam combining as well. In addition, in one embodiment, elliptical Bragg gratings are proposed for beam combining.
Although arrayed waveguide gratings solve the problem of combining laser beams inside integrated optics without using free-space optical components, they still are large and inherently complex structures that typically cover several square centimeters in area and comprise multiple waveguides. Arrayed waveguide gratings are also very sensitive to any phase error, particularly at the end of the arrayed waveguides; therefore very careful and rigorous design is critical for correct operation. Prototyping is prohibitively expensive, both in terms of cost and time; hence, modeling tools are required to provide detailed analysis of the arrayed waveguide grating operation. However, due to component size, only approximate semi-analytical methods that do not fully characterize structure and performance are available. Also, arrayed waveguide gratings do not support fabrication defects or structure degradation; any defect in one of the array arms, for example, will result in dropping the corresponding laser beam from the combined output.
Bragg gratings, proposed in U.S. Pat. No. 7,444,048 by Peters, et al, are more robust than arrayed waveguide gratings because local defects are not fatal and do not drastically degrade general performance and because the Bragg grating works as a digital hologram, wherein even a fraction of total structure performs the same functions with lower throughput. However, in this patent, N Bragg gratings, required for combining N laser beams, are placed sequentially on a planar with grating #1 being the closest to a laser diode array and grating #N being the farthest. This configuration is not optimal for phase synchronization, system miniaturization, loss minimization, and scaling in the number of combined laser beams and can be significantly improved by overlaying the gratings, as explained below.
Therefore, there is a need for a miniature beam combiner to be implemented as a component of integrated optic circuits.
It is an object of the present invention to provide a miniature optical monolithic or hybrid beam combiner, without free-space components, that can be fabricated on a wafer by standard mass-production methods and that comprises a compact chip-level optical device.
Another object of the present invention is to provide a compact integrated optical device for the combining of laser beams for improved laser brightness.
A further object is to provide a compact optical chip that, based on digital planar holography (DPH), improves the brightness of an output beam by enhancing coherence and fusion of light beams emitted from laser diodes arranged into an array.
Another object of the invention is to provide an optical chip that incorporates a beam combiner based on the principle of DPH.
The invention provides a method and an optical device for combining laser beams on the principle of DPH. The method and apparatus of the invention improve brightness of an output beam by enhancing coherence and fusion of light beams emitted from laser diodes arranged in an array. Improvement is achieved due to correlating phases of individual light beams. The invention applies to optical chips of a hybrid or monolithic configuration; the former can be realized by fabricating a DPH beam combiner on a separate chip, which is optically coupled to a laser diode array, while the latter presumes embedding the beam combiner into the same planar optical waveguide wherein the laser array is located. DPH is a flexible technology that is suitable for planar waveguides made of various materials and for this reason applies to virtually any diode laser array. DPH optical components, including beam combiners, are digital holograms generated in a computer and manufactured by standard mass-production methods, such as binary microlithography or nanoimprinting.
Every DPH component has a complicated hierarchical structure, which can be considered as a supergrating that comprises multiple overlaid subgratings, each of which consists of standard binary features (for example, etched grooves of a rectangular configuration) formed in a planar waveguide in order to modulate its effective refractive index. Specific dimensions that are shorter than predetermined wavelengths emitted by lasers define each binary feature.
A light beam that is confined inside the planar waveguide is forced to propagate through and interacts with the DPH structure. The DPH structure is made so that it correlates phases and fusion of beams emitted from a plurality of laser diodes of an array, which results in improved brightness of the output beam emitted from the optical chip. Light propagation along the DPH structure allows the length of interaction to be as long as needed so that multiple functions can be implemented in a single hologram.
Each subgrating is a group of DPH features that constitutes a part of the supergrating and interacts with a selected laser diode, or laser diodes, to accomplish a predetermined function. In other words, each subgrating functionally acts as coherent connector of individual lasers, is part of the supergrating, and interacts with a selected laser diode, or laser diodes, to accomplish a predetermined function.
It has been shown in the description of the prior art that there are two types of combiners:
The invention relates only to a coherent DPH beam combiner. The coherent DPH beam combiner of the invention is placed in a common resonator for all diode lasers. Therefore, the coherent beam combiner participates in light generation and provides beam coupling, phase correlation, and beam fusion for all lasers by partial reflection of each beam into all diode lasers of the array. This leads to light generation in all laser diodes at the same wavelength and with the same output phase so that the coherently combined beam has the same quality as that of any laser diode in the array, while brightness of the beam is scaled proportionally to the number of lasers in the array.
In the beam-combining system 120a, components that are similar those in the system shown in
The beam-combining system 120a also incorporates a partially reflecting mirror 130a. In the structure of the optical waveguide 122a, the DPH beam combiner 129a and the mirror 130a form an optical resonator 132a. In the resonator 132a, the light applied from the laser-active medium 126a bounces back and forth between the DPH beam combiner 129a and the partially transparent mirror 130a, thereby enhancing stimulated emission. Thus, it has been shown that the DPH beam combiner 129a accomplishes the function of a full-reflection mirror.
It should be noted that in addition to the above, the DPH beam combiner 129a participates in light generation and provides phase correlation and fusion of all laser beams by partial reflection of each beam into all diode lasers of the array. In other words, the resulting radiation of the output beam acquires coherency. Thus, this leads to light generation in all laser diodes at the same wavelength and with the substantially same phase downward to the output of the system so that the coherently combined beam has the same quality as that of any laser diode in the array while brightness of the beam is scaled proportionally to the number of lasers in the array.
Alternatively, the coherent-beam system of the invention can be embodied as a beam-combining system 120b (shown in
The coherent output beams B2 and B3 are collimated or focused by respective output optics 124a and 124b.
Similar to the system of the previous modification, the DPH beam combiner 131b participates in light generation and provides phase correlation and fusion of all beams emitted from all lasers by partial reflection of each beam into all diode lasers of the array 126b. In other words, the resulting radiation of the output beam acquires coherency. This leads to light generation in all laser diodes at the same wavelength and with the same phase at the output so that the coherently combined output beam B3 has the same quality as that of any laser diode in the array while brightness of the beam is scaled proportionally to the number of lasers in the array.
In both systems 120a and 120b, every DPH beam combiner 129a and 131b, respectively, has a complicated hierarchical structure, which in approximation can be considered substantially as a supergrating consisting of standard binary features (for example, etched grooves of a rectangular shape) formed in a planar waveguide in order to modulate its effective refractive index. Each binary feature is defined by three dimensions (width, length, and depth) that are shorter than the predetermined wavelength emitted by the laser that interacts with the aforementioned grooves.
As a light beam is confined inside the planar waveguide, it is forced to propagate through and interact with the DPH structure, which results in phase correlation and fusion of the beams. Light propagation along the DPH component allows the length of interaction to be as long as needed so that multiple functions can be implemented in a single hologram.
Alternatively, the beam-combining system may be incorporated into system 120c (shown in
As mentioned above, all optical components in systems 120a, 120b, and 120c are implemented as integrated devices in the form of a planar optical chip. Optionally, there can be one planar chip for the laser active media 126a, 126b, 126c and DPH beam combiners 129a, 131b, or 123c together, or two planar chips, i.e., one for the laser media and another for the DPH beam combiners. In the second case, both chips are optically coupled with each other.
According to the present invention, each DPH beam combiner is implemented as a combination of nanogrooves embedded into a planar waveguide for periodical modulation of its refractive index. The modulating function is calculated based on specific optical-transfer functions, desirable in said combiner, and implemented by standard mass-production methods such as nanolithography or nanoimprinting. Numerous nanofeatures (e.g., in an amount of 105-106) can be aggregated into multiple subgratings, each of which is responsible for a specific optical-transfer function.
In fact, each subgrating need not comprise a periodic structure according to the general definition of a periodic structure. Instead, each subgrating is a group of DPH features specifically selected to accomplish a predetermined function from multiple functions of the DPH beam combiner. All subgratings are superimposed on the same planar area, forming a supergrating that performs all desired functions.
Each supergrating is generated as a mathematical superposition of elliptic, parabolic, or hyperbolic subgratings with a spatial period of an approximate one-half wavelength according to the following method. The first to be created is a two-dimensional analog-generating function A(x,y) representing a superposition of modulation profiles of the refractive index. Each modulation function corresponds to the equivalent of a subgrating. Determined in this step is a two-dimensional generating function A(x,y) that resembles an interference pattern of wavelengths emitted from multiple sources at different wavelengths. The generating function A(x,y) is a mathematical linear superposition of elliptic subgrating integration.
The next step is binarization of a two-dimensional analog-generating function A(x,y), which was produced in the previous step. Binarization is achieved by applying a threshold value and assigning 1 to all areas above the predetermined threshold and 0 to the remaining areas in order to obtain a digital two-dimensional generating function B(x,y).
Next, the complex shape islands in B(x,y) with the value of 1 are simplified for presentation as a combination of standard microlithographic or nanolithographic features (short straight lines and dashes). This is accompanied by conversion to function C(x,y).
The last step is lithographic fabrication of the calculated standard microlithographic or nanolithographic features by etching all binary features as function C(x,y) to a calculated depth on a planar waveguide.
An example of the structure of a beam-combining system according to the invention is shown in
Light beams such as beams 224, 226, 228, and 230 shown in
For example, in a structure that contains three laser diodes 220a, 220b, and 220c, as shown in
All subgratings are mathematically superimposed on the same planar area (not shown), forming a supergrating, where each feature works toward the best synergetic performance of all desired functions. In general, for the structure shown in
Therefore, the total number of subgratings m is the sum of the arithmetic progression:
m=N+(N−1)+(N−2)+ . . . +1, (1)
i.e.,
m=0.5N(N+1) (2)
As mentioned, a DPH beam combiner can be used as a full-reflection resonator mirror or as a partial-reflection output mirror. Such modifications can be provided by varying the DPH-combiner length: short supergratings reflect only partially, reflection coefficient grows with structure length and after becoming saturated does not depend on additional increase in length, i.e., full-reflection component. These options can be better understood by considering
Shown in
In the modification of a beam-combining system 400 shown in
All configurations shown in
Strictly speaking, in all modifications shown in
All conditions and considerations described above for a plurality of single-mode lasers apply in this invention, resulting in synchronizing all component beams and generating respective single-mode output beams 612a, 612b, and 612c of increased brightness and decreased divergence.
Thus, it has been shown that this invention provides a method and an optical device for combining laser beams on the principle of digital planar holography. The method and apparatus of the invention improve brightness of an output beam by correlating phases and improving fusion of coherent beams emitted from a plurality of laser diodes by means of digital planar holography (DPH). The invention applies to optical chips of a hybrid or monolithic configuration; the former can be realized by fabricating a DPH beam combiner on a separate chip, which is optically coupled to a laser diode array, while the latter presumes embedding the beam combiner into the same planar optical waveguide wherein the laser array is located.
Although the invention has been shown and described with reference to specific embodiments, these embodiments should not be construed as limiting the areas of application of the invention and that any changes and modifications are possible provided that these changes and modifications do not depart from the scope of the attached patent claims. For example, the system may not contain mirrors at all but instead may contain two DPH beam combiners functioning as the respective mirrors. Replacement of all mirrors with the DPH beam combiners of this invention applies to all combinations of the system components described above.
