BACKGROUND

Field

[0001] This invention relates generally to a pulsed fiber laser amplifier system and, more particularly, to a pulsed fiber laser amplifier system that employs a plurality of optical sources providing seed pulse beams at different wavelengths and a single fiber amplifier chain that amplifies all of the seed pulse beams at different time intervals, where the spaced apart amplified seed pulse beams are combined into a single pulse output beam using a spectral-temporal beam combiner.

Discussion

[0002] 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 10-20 μιτι or larger.

[0003] 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 combine the fiber 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 the more uniform the combined phase front the better the beam quality.

[0004] In one known multiple fiber amplifier design, a master oscillator (MO) generates a signal beam that is split into a plurality of fiber beams each having a common wavelength where each fiber beam is amplified. The amplified fiber beams are then collimated and directed to a diffractive optical element (DOE) that combines the coherent fiber beams into a signal output beam. The DOE has a periodic structure formed into the element so that when the individual fiber 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 fiber beam is provided to a phase modulator that controls the phase of the beam so that the phase of all the fiber beams is maintained coherent. However, limitations on bandwidth and phasing errors limits the number of fiber beams that can be coherently combined, thus limiting the output power of the laser.

[0005] To overcome these limitations and further increase the laser power, multiple master oscillators are provided to generate signal beams at different wavelengths, where each of the individual wavelength signal beams are split into a number of fiber beams and where each group of fiber beams has the same wavelength and are mutually coherent. Each group of the coherent fiber beams at a respective wavelength are first coherently combined by a DOE, and then each group of 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.

[0006] One specific application for fiber laser amplifiers is for 3-D

Ladar range finding of objects that may be at a considerable distance. Laser beam pulses emitted by the fiber laser amplifier are reflected off of the object being targeted, and the reflected pulses from the object are collected by a receiver that includes, for example, an avalanche diode charge coupled device (CCD) array that provides both temporal and spatial imaging of the object to provide range information. By employing pulse widths of about 1 ns, images of the object can be achieved with a spatial resolution of inches at a range of 100 kilometers or more, thus providing a usable system for both terrestrial and space applications.

[0007] For certain fiber laser range finder systems, the fiber amplifiers operate in the < 200 kHz range, where the energy storage capability and hence the average power capability of the fiber is limited because of peak power limitations. Typical large core fibers, i.e., 40 μιτι, often employed for these applications can store energies of about 2.5 mJ/pulse, but are limited to about 250 μϋ/pulse at 1 ns pulse width by nonlinearities, such as self-phase modulation, in the fiber, which broadens the beam spectrum beyond what is usable for many applications. Further, these applications often require electrically efficient compact laser sources in the 200 W average power range. Such amplifier systems require pulse energies of 5 mJ/pulse, 0.5-1 ns pulse widths, and are limited to repetition rates of less than 40 kHz by a single photon counter detector readout electronic rate and require laser spectral bandwidths less than 1 nm so that the receiver can discriminate against solar background effects.

[0008] The pulse energies referred to above for laser range finding applications cannot be achieved using a single fiber amplifier chain because of the material limitations of the fiber. In order to obtain the desired power for these applications a number of fiber amplifier chains operating at different wavelengths are typically employed, where the fiber beam from each fiber amplifier chain is combined by, for example, a spectral beam combiner to provide a single pulse overlapped output beam at the desired power level. However, providing many fiber chains to provide the desired power significantly increases the complexity of the system, the cost of the system and the ability to package the system in a reasonable manner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a schematic illustration of a known fiber laser amplifier system including a plurality of fiber laser amplifying chains;

[0010] Figure 2 is a schematic illustration of a fiber laser amplifier system including a plurality of optical seed sources, a single fiber amplifying chain and a spectral-temporal beam combiner;

[0011] Figure 3 is a graph with time on the horizontal axis and amplitude on the vertical axis showing a combined pulse envelope for multiple beam input pulses combined by a passive beam combiner;

[0012] Figure 4 is a graph with time on the horizontal axis and amplitude on the vertical axis showing a combined pulse envelope for multiple beam input pulses combined by an electro-optic beam combiner; [0013] Figure 5 is an isometric view of a wavelength dependent temporal delay (WDTD) optics employed in the spectral-temporal beam combiner in the amplifier system shown in figure 2;

[0014] Figure 6 is an illustration of a beam delay path for one of the amplified pulse beams in the WDTD optics;

[0015] Figure 7 is an illustration of another beam delay path for one of the amplified pulse beams in the WDTD optics; and

[0016] Figure 8 is an illustration of another beam delay path for one of the amplified pulse beams in the WDTD optics.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] The following discussion of the embodiments of the invention directed to a pulsed fiber laser amplifier system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.

[0018] Figure 1 is a schematic illustration of a known pulsed fiber array laser amplifier system 10 including a plurality of fiber amplifier chains 12 each including a pulsed laser seed beam source 14, for example, a pulsed DFB diode, a laser, a solid-state laser, etc. producing a seed beam on a fiber 16 at different wavelengths λι, λ ₂ , ..., λ _Ν . The wavelength spacing between adjacent chains 12 can be any suitable wavelength difference for spectral beam combining as discussed herein. In this non-limiting example, the system 10 includes ten of the amplifier chains 12 as representative of a typical laser range finding system. A controller 18 controls each of the seed beam sources 14 so that they generate their pulses in synchronization, i.e., at the same time.

[0019] Each of the chains 12 also includes a plurality of strategically positioned Faraday isolators 20 that allow light to propagate only in the desired direction so as to prevent optical beams from entering components and potentially causing damage thereto, and a plurality of strategically positioned tap couplers 22 that allow a small portion of the seed beam at various stages of amplification in the chain 12 to be separated from the main seed beam for diagnostic purposes. Each of the fiber chains 12 also includes a plurality of amplification stages 24 each including an emitter pump diode 26 that generates a pump beam provided to a pump coupler 28 that amplifies the seed beam in a fiber amplifier 30 in a known manner. In an alternate embodiment, other types of optical amplifiers may be applicable. In one non-limiting example, the fiber amplifiers 30 are a polarization maintaining (PM), single mode (SM), ytterbium (Yb) doped length of fiber having a 10 μιτι core. A final fiber amplification stage 32 may, for example, provide more light amplification through multiple emitter pump diode arrays 34 and a 25/40 μιτι, SM, PM, Yb doped fiber amplifier 36 and counter-pumping. An amplified pulse beam 38 is then output from each of the amplifier chains 12 at the same time. The amplification of the pulsed beam at the various amplification stages 24 in each chain 12 is carefully controlled so that the peak power limitation of the fiber 16 is not exceeded.

[0020] Each amplified pulse beam 38 at the particular wavelength λ from the chains 12 is directed to a spectral beam combiner (SBC) 40 including a first grating 42 that directs each of the pulse beams 38 impinging the grating 42 from a different direction to be directed to a second grating 44 at the same location so all of the pulse beams 38 are combined as a single output beam pulse 46 at the desired power level and propagating in the same direction.

[0021] The present invention proposes reducing the number of components in a pulsed fiber laser amplification system of the type discussed above by employing a single fiber amplifier chain, where pulses at the different wavelengths from a plurality of different seed beam sources are amplified by the fiber chain at different points in time so that the power limitations of the fiber is not exceeded.

[0022] Figure 2 is a schematic illustration of a pulsed fiber amplifier system 50 of the type referred to above including a single fiber amplifier chain 52, where like elements to the system 10 are identified by the same reference number. In this non-limiting embodiment, the beam power output of the laser system 50 is the same as the beam power output of the laser system 10 by including the same number of the seed sources 14 each producing the same wavelength seed beam at the same pulsewidth and the same number of the amplification stages 24 in the fiber chain 52. Instead of each of the seed sources 14 being synchronized to produce its pulses at the same time as in the system 10, the seed sources 14 in the system 50 are controlled by a controller 54 so that only a single one of the seed sources 14 is producing its pulsed seed beam at a particular point of time. The timing of the seed pulse beam is selected so that the temporal overlap of the seed pulse beams propagating through fiber chain 52 does not exceed the peak power limitations of the fiber 58, but where the pulses are close enough in time so that they can be temporally combined as will be discussed below.

[0023] All of the pulsed seed beams from the seed sources 14 are provided to a 1 x N coupler 56 that directs each seed pulse beam onto the fiber 58 for amplification in the same manner as each of the fiber chains 12 discussed above. In one embodiment, the coupler 56 is a passive combiner that passes the Gaussian distributed pulses unaffected. Figure 3 is a graph with time on the horizontal axis and amplitude on the vertical axis showing a seed beam output from the coupler 56, where each of the seed pulse beams from the seed sources 14 are shown as Gaussian pulses 60, and where the combined pulses 60 define a combined pulse envelope 62. In an alternate embodiment, the coupler 56 is an electro-optic 1 x N combiner that allows the pulse-shapes to be programmable. This embodiment is illustrated in figure 4 showing a graph with time on the horizontal axis and amplitude on the vertical axis, where the seed pulse beams from each of the seed beam sources 14 is shown as a square pulse 66 having a combined pulse envelope 68.

[0024] The combined pulse envelope put on the fiber 58 from the coupler 56 is amplified in the fiber chain 52 in the same manner as the separate pulses amplified in the fiber chains 12 to produce an amplified pulse envelope beam 70. The amplified pulse envelope beam 70 from the fiber chain 52 is then sent to a spectral-temporal beam combiner (STBC) 72 that causes the pulses in the envelope beam 70 to overlap both spectrally and temporally to produce an output pulse beam 74 having the power of all of the pulse beams combined. The STBC 72 includes a first SBC grating 76 that receives the beam 70 and directs each of the pulses in the beam 70 having the different wavelengths propagating in a slightly different direction to be received by a second SBC grating 78. The grating 78 redirects the separate pulse beams to a wavelength dependent temporal delay (WDTD) optics 80 that delays each of the separate pulse beams of the different wavelengths for a different period of time depending on when the pulse was generated so that all of the pulse beams are output from the WDTD optics 80 at the same time and in separate directions. The pulse beams from the optics 80 are directed to a third SBC grating 82 that redirects the beams so that they impinge a fourth SBC grating 84 at the same location to be output as the single pulse beam 74 having the desired power. Thus, the system 50 emits amplified laser beam pulses at high power and at predetermined intervals in the manner discussed herein.

[0025] The WDTD optics 80 can be any suitable optical system that provides the desired optical delay of each of the separate pulse beams and can employ any combination of one or more of mirrors, dispersive materials of different lengths, lenses, etc.

[0026] Figure 5 is an isometric view of a WDTD optics 90 providing a general illustration of the type of optics that can be employed for the optics 80 in the system 50. The optics 90 includes a mirror array 92 at one end of the optics 90, a mirror array 94 at an opposite end of the optics 90, and a lens array 96 including lenses 98 positioned therebetween. Each of the mirror arrays 92 and 94 would include a configuration of many mirror elements (not shown) oriented and configured together at different angles to reflect the pulse beams consistent with the discussion herein. Each of the pulse beams enter the optics 90 through the top or bottom of the mirror array 92 at a particular angular orientation depending on how the beam is reflected from the grating 78. That particular pulse beam will be focused by a particular one of the lenses 98 to be directed onto a particular mirror element in the mirror array 94 to be reflected back through another one of the lenses 98 and be reflected by a mirror element on the mirror array 92 to again be sent back through another one of the lenses 98 to the mirror array 94. The mirror elements and the lenses 98 are strategically positioned so that the number of reflections between the mirror arrays 92 and 94 for a particular pulse beam depends on its angular orientation and will provide the desired delay for that beam within the optics 90 so that when it is directed out of the optics 90 through the top or bottom of the mirror array 94 it is at the same time as all of the other pulsed beams entering the optics 90 at different times for the particular output pulse beam being generated. In one non-limiting embodiment, the mirror and lens arrays 92, 94 and 96 can be made as monolithic sub-blocks using chemically assisted precision optical bonding techniques, well known to those skilled in the art.

[0027] Figures 6-8 illustrate three optical paths within the WDTD optics 90 to show the beam delay as just described. Particularly, figure 6 shows a delay optics 1 10 for a pulse beam 1 12 having one wavelength that is reflected off of two mirror elements 1 18 and 120 and is focused by two lenses 122 and 124 to provide three passes through the optics 90 and provide the shortest delay.

[0028] Figure 7 shows a delay optics 126 for a pulse beam 128 having another wavelength that is reflected off of four mirror elements 130 and is focused by three lenses 132, as shown, to provide five passes through the optics 90 and give a longer delay than the delay optics 1 10.

[0029] Figure 8 shows a delay optics 134 for a pulse beam 136 having another wavelength that is reflected off of six mirror elements 138 and is focused by four lenses 140, as shown, to provide seven passes through the optics 90 and provide a longer delay than the delay optics 126.

[0030] The foregoing discussion disclosed and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.