Patent Description:
An example for a known optical element comprising a periodic grating is disclosed by the document <CIT>.

High power laser amplifiers have many applications, including industrial, commercial, military, etc. Designers of laser amplifiers are continuously investigating ways to increase the power of the laser amplifier for these applications. One known type of laser amplifier is a fiber laser amplifier that employs a doped fiber and a pump beam to generate the laser beam, where the fiber has an active core diameter of about <NUM>-<NUM> or larger.

Improvements in fiber laser amplifier designs have increased the output power of the fiber to approach its theoretical power and beam quality limit. To further increase the output power of a fiber amplifier some fiber laser systems employ multiple fiber lasers that amplify and combine the seed beams in some fashion to generate higher powers. A design challenge for fiber laser amplifier systems of this type is to combine the beams from a plurality of fibers in a coherent manner so that the beams provide a single beam output having a uniform phase over the beam diameter such that the beam can be focused to a small focal spot. Focusing the combined beam to a small spot at a long distance (far-field) defines the beam quality of the beam, where the more coherent the individual seed beams the more uniform the combined phase and the better the beam quality.

In one known multiple fiber amplifier design, a master oscillator (MO) generates a seed beam that is split into a plurality of split seed beams each having a common wavelength where each seed beam is amplified. The amplified beams are then collimated and directed to a diffractive optical element (DOE) that combines the coherent amplified beams into a single output beam. The DOE has a periodic structure formed into the element so that when the individual amplified beams each having a slightly different angular direction are redirected by the periodic structure all of the beams diffract from the DOE in the same direction. Each seed beam is provided to a phase modulator that controls the phase of the beam so that the phase of all the seed beams is maintained coherent. However, limitations on bandwidth and phasing errors limits the number of amplified beams that can be coherently combined, thus limiting the output power of the laser.

To overcome these limitations and further increase the laser power, multiple master oscillators are provided to generate seed beams at different wavelengths, where each of the individual wavelength seed beams is split into a number of split seed beams and where the seed beams within each group of seed beams have the same wavelength and are mutually coherent. The seed beams within each group of coherent seed beams at a respective wavelength are first coherently combined by a DOE, and then the coherently combined beams are directed to a spectral beam combination (SBC) grating at slightly different angles that diffracts the beams in the same direction as a single combined beam of multiple wavelengths. The SBC grating also includes a periodic structure for combining the beams at the different wavelengths.

An example for a hybrid fiber laser amplifier system of this type is disclosed in by the document <CIT>. This document discloses various embodiments for a hybrid fiber laser amplifier system, where each embodiment includes a DOE for providing coherent beam combining and an SBC grating for providing spectral beam combining, as discussed above. In one particular embodiment, the document <CIT> discloses a combination of the DOE and SBC grating into a single optical element, where the periodic structure for the DOE and the SBC grating are orthogonal to each other.

The document <CIT> discloses an optical device that includes an integrated DOE and SBC grating that is suitable to be employed in the fiber laser amplifier system disclosed in the document <CIT>. The optical device is fabricated by forming a periodic pattern into a top surface of an optically flat substrate in one direction that defines a periodic structure for the DOE. A multi-layer dielectric high-reflection (HR) coating is deposited on the substrate so that it conforms to the periodic pattern and is accurately reproduced therein. A top dielectric layer is deposited on the HR coating that also conforms to the periodic pattern and that is etched to form periodic grooves for the SBC grating in a second direction orthogonal to the first direction. Although the optical device of the document <CIT> has been shown to be effective as a combined DOE and SBC grating, improvements in optical performance and fabrication complexity can be realized. For example, the fabrication process of the optical device according to the document <CIT> requires separate fabrication steps for the DOE and the SBC grating.

The drawings and the following description disclose and discuss certain features of the invention but do not define the invention.

<FIG> is a schematic diagram of a known fiber laser amplifier system <NUM> including a plurality of N master oscillators (MO) <NUM> each generating a seed beam of an appropriate line-width on an optical fiber <NUM> for separate beam channels or wavelength groups <NUM> having a slightly different wavelength λ<NUM>, λ<NUM>,. The MO <NUM> may also incorporate a phase modulator to broaden its line-width to suppress nonlinearities in subsequent amplification. The seed beam on each of the fibers <NUM> is sent to a beam splitter <NUM> that splits the seed beam into a plurality of M split seed beams where each split beam is provided to a separate phase modulator <NUM>. The splitter <NUM> and the plurality of phase modulators <NUM> in each wavelength group <NUM> are separate devices, but are shown here as a single device because they can be implemented on a single chip. The phase modulators <NUM> correct the phase of each split seed beam so that all of the beams are coherent and in phase with each other as will be discussed below. In this example, each wavelength group <NUM> includes five seed beams, however, the number of seed beams in each wavelength group <NUM> can be any number suitable for a particular application, and will be represented herein as M seed beams. The M split seed beams from the phase modulators <NUM> are each sent to a fiber amplifier <NUM>, where the amplifiers <NUM> represent the doped amplifying portion of a fiber <NUM> that receives an optical pump beam (not shown).

The seed beams within each wavelength group <NUM> of M fibers <NUM> are combined into a one-dimensional array of seed beams by a suitable optical device <NUM> so that all of the amplified beams from all of the fibers <NUM> are combined to define a two-dimensional fiber array <NUM> of amplified beams <NUM>, where each column of the beams <NUM> in the array <NUM> includes the beams from the fibers <NUM> within one of the wavelength groups <NUM>, which have the same wavelength λi, where i = <NUM>, <NUM>, <NUM>,. , N for each of the columns. It will be understood that the array <NUM> has been rotated <NUM>° from the propagation path of the beams <NUM> so that the orientation of the beams <NUM> can be visualized. It is noted that the configuration of the beams <NUM> in the array <NUM> is shown in an orientation where the spacing between the beams <NUM> of the longer wavelength is greater. Particularly, the wavelength λ<NUM> of the beams <NUM> in the left most column are closer together because the wavelength λ<NUM> is for exemplary purposes assumed to be the shortest and the wavelength λN of the beams <NUM> in the right most column of the array <NUM> are spaced farther apart because the wavelength λN is assumed the longest.

The MxN amplified beams <NUM> from the array <NUM> are collimated by collimating optics <NUM>, where the beams <NUM> have slightly different angles of propagation as a result of their position in the array <NUM>. The collimated amplified beams <NUM> are directed to a DOE <NUM> that is positioned in the back focal plane of the collimating optics <NUM> to ensure an optimal overlap of all of the beams <NUM> on the DOE <NUM> at the same location. The DOE <NUM> is an optical element having a periodic structure that directs the beams <NUM> in respective columns that are coherent into respective single beams for each of N columns in array <NUM> so that N number of coherently combined beams <NUM>, each at respective wavelengths λi, where i = <NUM>, <NUM>, <NUM>,. , N, propagating in slightly different directions are reflected from the DOE <NUM>. The DOE <NUM> will also diffract a number of spurious order beams <NUM> as a result of DOE inefficiencies. Proper phasing results in an efficient combination of M beams from each group <NUM> at wavelength λi.

The N combined beams <NUM> diffracted by the DOE <NUM> are sampled by a splitter <NUM> so that N sample beams of low power, one for each wavelength λi, are generated, where each sample beam has a slightly different angular displacement. A lens <NUM> focuses the N sample beams to spatially separated phase detectors <NUM>, such as photodetectors, where each detector <NUM> detects the phase of the M constituent beams at one of N specific wavelengths, which have been combined by the DOE <NUM>. The phase detectors <NUM> measure the phase of the combined beam at the particular wavelength λi and provide an electrical measurement signal to a synchronous phase processor <NUM>, where a separate processor <NUM> is provided for each of the detectors <NUM>.

The phase of the constituent beams in each N combined beam can be distinguished in a single output from the phase detector <NUM> by uniquely dithering or coding the constituent beams in phase or amplitude, such as by using distinct frequencies for frequency modulation (FM) or amplitude modulation (AM), distinct codes for code division multiple access (CDMA) or time division multiple access (TDMA), etc., so that a synchronous detector scheme can distinguish the constituent phase signals for each seed beam in the combined beam. Such a technique is disclosed, for example, in the document <CIT>. Each synchronous phase processor <NUM> decodes the distinct constituent phases in the measurement signal from the phase detector <NUM>, and generates phase error correction signals for each seed beam that are sent to the corresponding phase modulator <NUM> so that adjustments to the phase of the individual seed beams in the fiber amplifiers <NUM> causes all of the constituent seed beams to be locked in phase. Because the array of amplified beams fully overlap and are combined into a single beam, no gaps are present between beams on the DOE and side lobes owning to reduced fill factor are eliminated, and the output beam can be focused to a nearly diffraction limited spot to nearly reach the theoretical limit of brightness provided by the total combined power of the beams.

The N angularly displaced combined beams <NUM> that pass through the beam sampler <NUM> are relayed by relay optics <NUM> and are imaged onto an SBC grating <NUM> to provide spectral beam combination of all of the N combined beams <NUM> of varied wavelengths while preserving the required angles of incidence. The wavelength λi of each of the N combined beams <NUM> is selected in accordance with the angular dispersion of the SBC grating <NUM> to precisely compensate for angular deviation. The SBC grating <NUM> includes a periodic grating structure to diffract the N combined beams <NUM> having different angles into a common direction. Thus, a single diffraction limited output beam <NUM> is provided at the output of the fiber amplifier system <NUM> that combines all of the M×N beams with high efficiency and with low power in the spurious diffracted orders.

It is noted that although the DOE <NUM> and the SBC grating <NUM> are shown as reflective structures that reflect the optical beams that impinge thereon, other fiber laser amplifier system designs may employ transmissive elements where the optical beams that impinge the DOE or SBC grating propagate through the optical element.

<FIG> is a schematic diagram of a known fiber laser amplifier system <NUM> similar to the fiber laser amplifier <NUM>, where like elements are identified by the same reference numeral. The amplifier system <NUM> includes an integrated DOE and SBC grating optical element <NUM> instead of the separate DOE <NUM> and the SBC grating <NUM> discussed above that provides a single optical element that combines both the coherent beams and the beams of differing wavelengths. This provides the advantage of fewer optical elements and a more compact size for the amplifier system. The low power sample beam directed towards the lens <NUM> is the <NUM>th order reflection from the SBC grating in the optical element <NUM>. The <NUM>st or higher order diffracted beams from the SBC grating is part of the output beam <NUM>. In order to fabricate the optical element <NUM>, a DOE design is implemented for a combination of a specific number of M beams, and thus a phase function ϕ(x).

The document <CIT> referred to above discloses an integrated DOE and SBC grating that is suitable to be used as the optical element <NUM>. <FIG> is a recreation of <FIG> from the document <CIT> , and shows an optical element <NUM> including a substrate <NUM>, a multi-layer HR coating <NUM>, and a top dielectric layer <NUM>. A DOE periodic structure <NUM> is formed into the height h(x) of the substrate <NUM> in an x-direction, and the coating <NUM> and the layer <NUM> follow that pattern when they are deposited. This defines a smooth height function h(x) = λ[ϕ(x)/4π], which is typically of a magnitude on the order of the wavelength λ, and is a periodic function of position along one axis of the optical surface with period d. For each of the N columns in the fiber array <NUM>, the wavelength λ determines the ratio λi/d, which gives the required angular separation of the M beams within that column. The top layer <NUM> is etched to form SBC grating channels <NUM> in a y-direction, such as by depositing and patterning a photoresist layer and exposing the photoresist layer using known holographic or lithographic techniques, so that the element <NUM> includes both DOE and SBC gratings.

As will be discussed below, the present disclosure proposes an optical element having an integrated DOE and SBC grating that is also suitable to be used as the optical element <NUM> and that provides certain advantages over the element <NUM>, such as a reduction in the number of fabrication steps and a more precise and efficient DOE.

<FIG> is a profile view of a fabrication step for an integrated DOE and SBC grating optical element <NUM> that can be used for the optical element <NUM>. <FIG> is a top view and <FIG> is an isometric view of the element <NUM> after a final fabrication step. The element <NUM> includes a substrate <NUM>, such as glass or silicon, and being a few millimeters to centimeters thick, having an optical flat surface, and a multi-layer dielectric high-reflection (HR) coating <NUM> deposited on the substrate <NUM>. The multi-layer coating <NUM> includes an alternating sequence of a high index of refraction dielectric layer <NUM> and a low index of refraction dielectric layer <NUM>, many of which are known in the art. In this example, each of the layers <NUM> and <NUM> is about one-quarter of a wavelength λ in optical thickness, and the total thickness of all of the layers <NUM> and <NUM> could be on the order of <NUM>-<NUM>. A top dielectric layer <NUM>, such as silica or another suitable oxide, that is a few µm or less in thickness is deposited on the HR coating <NUM>.

A photoresist coating <NUM> is deposited on the layer <NUM> and is patterned for an etching process that defines periodic structures for the DOE and an SBC grating. Thus, instead of forming the periodic pattern for the DOE in the substrate <NUM> and causing the coating <NUM> to conform to the pattern as was done in the document <CIT>, both the periodic patterns for the DOE and the SBC grating are formed at the same time by one etching process. More specifically, the layer <NUM> is etched into its top surface to simultaneously produce a DOE periodic structure <NUM> having groove positions shifted in a y-direction by a distance that is described by a function Δy(x) that is periodic in an x-direction and appropriately shaped channels or grooves <NUM> that are periodic in the y-direction that provide the periodic structure for the SBC grating. In this example, the grooves <NUM> have a rectangular cross-sectional shape for a particular application. However, as will be appreciated by those skilled in the art, other shapes for other applications may be equally applicable, such as trapezoidal, sawtooth, triangular, etc. Although the periodic structure <NUM> for the DOE is formed in the x-direction, it is noted that the periodic structure for the DOE in the document <CIT> is formed into and out of the substrate, which leads to a height variation in the top layer <NUM>, whereas in this disclosure, the periodic structure <NUM> for the DOE is formed by varying groove positions across a flat plane defining the layer <NUM>.

The groove position function Δy(x) for the periodic structure <NUM> is a smoothly varying function typically having a magnitude on the order of the wavelength of interest that is periodic along the x-direction and having a period d. For each of the N columns in the fiber array <NUM>, the wavelength λi determines the ratio λi/d, which gives the required angular separation of the M beams within that column. The period of the periodic structure <NUM> is typically about <NUM> times the wavelength λ or larger of the beam of interest so that maximum angles of the groove edges with respect to the x-direction are typically small, such as on the order of tens of mrad. It is noted that the typical period of the periodic structure <NUM> for the DOE is about <NUM> to combine the coherent beams and the period of the grooves <NUM> for the SBC grating is typically about <NUM> - <NUM> to combine the beams of varying wavelength.

The same optical functions for a combined DOE and SBC grating discussed above can be provided for a transmissive optical element. For a transmissive optical element, the HR coating <NUM> would be eliminated, and either the substrate <NUM> is patterned and etched directly or a cap layer is applied and etched directly on the substrate <NUM>, or on top of an anti-reflection (AR) multi-layer dielectric coating. It is also noted that if the integrated DOE and SBC grating device is a transmissive device, either of the coating <NUM> and the dielectric layer <NUM> may or may not need to be included, and if the integrated DOE and SBC grating device is a reflective device, both the coating <NUM> and the dielectric layer <NUM> would be required.

A holographic fabrication process can be employed where an interference pattern between impinging beams generates light and dark fringes that expose the photoresist coating <NUM> to generate the periodic patterns necessary to define the DOE and the SBC grating. To achieve both orthogonal periodic structures, the coating <NUM> is exposed not by straight interference fringes, but where the fringe positions, and thus the grooves <NUM>, vary along the x-direction. If the basic grating period is p, then a shift of the grooves <NUM> from their normal periodic position along the y-direction by Δy(x) results in a phase shift of the diffracted beam by ϕ(x) = 2πΔy(x)/p. In the document <CIT>, the varying phase shift along the x-direction was achieved by etching the substrate to vary the surface height h(x) along the x-direction so that the phase shift is ϕ(x) = 4π h(x)/λ. Thus, the SBC grating writing process itself is leveraged to provide the necessary x-direction dependent phase variation ϕ(x) via shifted grating groove locations Δy(x) to achieve the DOE functionality.

Any suitable holographic patterning technique for achieving the required x-direction dependence of fringe and groove positions can be used, such as scanning beam interference lithography (SBIL). Through SBIL, a focused exposure beam is scanned across the photoresist coating <NUM> while varying the relative phase between two constituent beams that form fringes in the illuminated focal spot. A raster scan can be employed where the x-direction is scanned while varying the relative phase ϕ(x) between the beams, thus varying the fringe y-direction positions Δy(x) as a function of the position along the x-direction. The raster scan would then progress across the optical element <NUM> at successive y-direction positions reproducing the identical x-direction variation in the position of the exposure fringes and corresponding periodically placed grooves, and thereby providing the required diffractive phase variation ϕ(x). Since the phase function ϕ(x) is controlled electronically in SBIL, the groove position function Δy(x) can be very precise, more so than the height etching process needed in the document <CIT>, and therefore the current described method can create a more accurate and efficient DOE functionality. As a result, a wide range of DOE phase designs can be incorporated into the fringe and grating groove positions, where the limiting resolution of the focused scanning beam in the x-direction can be one to several microns. Thus, the SBIL technique enables µm-scale variations in groove position along the x-direction. Typically, the grating period p is less than <NUM>, such as ~ <NUM>, whereas the scale of variation for the DOE along the x-direction is much slower, such as greater than <NUM>, such as ~ <NUM>. The resulting diffractive optic thus has a slowly varying groove position along the x-direction, which can be tailored to provide the DOE coherent beam combination function, as well as the conventional grating function along the y-direction to spectrally combine a number of beams. In an alternate example, raster scanning along the y-direction can be employed to provide the periodic varied groove position pattern Δy(x) for the DOE, where the phases between the SBIL beams are adjusted between scans to shift the grooves.

Claim 1:
An integrated optical device (<NUM>) that includes a diffractive optical element for combining coherent beams having different angular displacements and a spectral beam combiner for combining incoherent beams having different wavelengths and angular displacements, said device comprising:
a substrate (<NUM>) having a top planar surface; and
a periodic structure (<NUM>) formed into the substrate in a first direction that defines the diffractive optical element and a periodic grating having channels formed into the substrate in a second direction substantially orthogonal to the first direction that defines the spectral beam combiner, wherein the periodic structure includes periodic modulations along the length of the channels that are orthogonal to a channel-to-channel periodicity of the periodic grating.