First experimental demonstration of self ... - OSA Publishing

First experimental demonstration of selfsynchronous phase locking of an optical array T. M. Shaya, Vincent Benhamb, J. T. Bakerc, Capt. Benjamin Warda, Anthony D. Sancheza, Mark A. Culpeppera, Sgt. D. Pilkingtona, Lt. Justin Springa, Lt. Douglas J. Nelsona, and Lt. Chunte A. Lua a

Air Force Research Laboratory, Directed Energy Directorate, 3550 Aberdeen Ave. SE, Kirtland AFB, NM 87117; b IIT Industries, 5901 Indian School Rd. NE, Albuquerque, NM 87110; c Boeing LTS Inc., P.O. Box 5670, Albuquerque, NM 87185 [email protected]

Abstract: A novel, highly accurate, all electronic technique for phase locking arrays of optical fibers is demonstrated. We report the first demonstration of the only electronic phase locking technique that doesn’t require a reference beam. The measured phase error is λ/20. Excellent phase locking has been demonstrated for fiber amplifier arrays. ©2006 Optical Society of America OCIS codes: (060.2320) Fiber Optics Amplifiers and oscillators; (140.3290) Laser Arrays

References and links 1. 2. 3. 4. 5.

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V. P. Gapontsev, “New Milestones in the development of super high power fiber lasers,” presented at Photonics West, OE/LASE 2006, San Jose, CA, Jan 21-26, 2006. P. K Cheo, A. Liu, and G. G. King, “A High Brightness Laser Beam from a Phase-Locked Multicore YbDoped Fiber Laser Array,” IEEE Photon. Technol. Lett. 13, 439-441, (2001). E. J. Bochove, P. K. Cheo, and G. G. King, “Self-organization in a multicore fiber laser array,” Opt. Lett. 28, 1200-1202, (2003). Hans Bruesselbach, D. C. Jones, M. S. Mangir, M. I. Minden, and J. L. Rogers,, “Self-organized coherence in fiber laser arrays,” Opt. Lett. 30, 1339-1341, (2003). R. J. Beach, M. D. Feit, S. C. Mitchell, K. P. Culter, J. W. Dawson, S. A. Payne, R. W. Mead, J. S. Hayden, D. Krashkevich, and D. A. Alunni, “Ribbon fiber with multiple phase-locked gain cores,” Proc. SPIE 4974, 7-16, 2003. R. A. Beach, M. D. Feit, R. H. Page, L. D. Brasure, R. Wilcox, and S. A. Payne, “Scalable antiguided ribbon laser,” J. Opt. Soc. Am. B 19, 1521-1534, (2002). C. J. Corcoran, “Experimental demonstration of a phase-locked laser array using a self-Fourier cavity,” Appl. Phys. Lett. 86, 201118-201121 (2005). B. W. Grimes, W. B. Roh, T. G. Alley, “Phasing of a two-channel continuous-wave master oscillator-power amplifier by use of a fiber phase conjugate mirror,” Opt. Lett. 30, 2415-2417, (2005). R. R. Rice et, J. A. Davis, J. S. Whitely, J. H. Hollister, and N. F. Ruggieri, “Coherent Fiber MOPA,” Presented at 14th Annual Solid State and Diode Laser Technology Review, Sean Ross, ed., Albuquerque, NM (2001). J. Abderegg, S. J. Brosnan, M. E. Weber, H. Komine, and M. G. Wickham, “8-watt coherently-phased 4element fiber array,” in Advances in Fiber Lasers, L. N. Durvasula, ed., Proc. SPIE 4974, 1-6 (2003). S.J. Augst, T. Y. Fan, and Antonio Sanchez, “Coherent Beam Combining and Phase Noise Measurements of Yt fiber Amplifiers,” Opt. Lett. 29, 474-476, (2004). Michael Wickham, “Coherently Coupled High Power Fiber Arrays,” in Fiber Lasers III: Technology, Systems, and Applications, Andrew J. W. Brown, Johan Nilsson, Donald J. Harter, and Andreas Tunnermann, eds., Proc. SPIE 6102, 61020U-1 to 61020U-5 (2006). “A Novel Technique for Phase Locking Optical Fiber Arrays,” T. M. Shay and Vincent Benham, in FreeSpace Laser Communications IV, Jennifer C. Ricklin and David G. Voelz, eds., Proc. SPIE 5550, 313-319 (2004). “First Experimental Demonstration of Fiber Array Phase Locking by RF Phase Modulation,” T. M. Shay and Vincent Benham, Proceedings of the 17th Solid State and Diode Laser Technology Review, Sean Ross, ed., pg. BEAM-7 (2004). “Self-synchronous Locking of Optical Coherence by Single-detector Electronic-frequency Tagging”, T. M. Shay, US Patent 7,058,098, June 2006.

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Received 11 October 2006; revised 20 November 2006; accepted 20 November 2006

11 December 2006 / Vol. 14, No. 25 / OPTICS EXPRESS 12015

1. Introduction To achieve the high brightness required for many laser applications it is necessary to phase lock multiple element optical arrays. Recently, IPG Photonics has reported 2.5-kW of power out of a single mode fiber with a near diffraction limited optical beam [1]. The intensity and hence the power available from a single-mode optical fiber is limited either by optical surface damage or nonlinear optical effects. These limitations can be overcome by coherent beam combining of the power from multiple optical fibers. We have demonstrated a novel coherent beam combining system that offers not only highly accurate and robust phase locking, but in addition, is readily scalable to more than 100 elements. Furthermore, this is the first phased array locking system that doesn’t require an external reference beam. The results of the first experimental demonstration for two new electronic coherent beam combining techniques, the self-referenced LOCSET and the self-synchronous LOCSET techniques are presented. Accurate control of the optical phase is required for any phase locked multi-fiber approach. In a master oscillator power amplifier configuration, the optical paths of each of the fibers must be locked to within a fraction of the wavelength in order to coherently combine the individual outputs into a single, high-power beam. As a result of time varying thermal loads and other disturbances, active feedback is required in order to provide for stable coherent addition. There have been a number of experimental and theoretical research efforts addressing the need the for very high brightness fiber laser sources. The technical approaches that have been attempted include the optical self-organized approaches [2-8] and RF phase locking methods [9-11]. Electronic phase locking has demonstrated high fringe visibility for both passive [9-14] and amplified systems [10-12] and powers of 470 watts [12] have been phase locked using these methods. In the previous electronic phase locked fiber arrays, the reference beam was phase modulated at an RF frequency [9-12] and all of the previous systems required an external reference beam in their systems [9-14]. The light emerging from each element was then interfered with the light from a reference beam at the photodetector or an array of photodetectors. The light from each element must be sent to a spatially isolated photodetector, because the RF phase modulation was impressed solely upon the reference beam. Good fringe visibilities of > 94% and hence very low phase errors have been consistently achieved using electronic phase locking methods. Previously, we presented [13,14] the first reports of a novel coherent beam combination system called Locking of Optical Coherence by Single-Detector Electronic-Frequency Tagging or LOCSET. The LOCSET technique preserved the strengths and simplicity of previous electronic phase locking while providing scaling to very large numbers of elements. In the LOCSET technique, each element of the amplifier array is phase modulated with a unique RF frequency, thus the phase shift for each element is tagged by that elements’ unique RF frequency. In the self-referenced and self-synchronous LOCSET techniques [15] the array elements are phase modulated at unique RF frequencies exactly as was demonstrated in the first LOCSET technique [13,14]. The optical phase shift between the optical wave in the unmodulated element (for self-referenced LOCSET) or the array mean phase (for self-synchronous LOCSET) are measured separately in the electronic domain and the phase error signal is fed back to the corresponding elements LiNbO3 phase modulator to minimize the phase error for that element. The phase error signal for an individual phase modulated element originates from the RF beat note generated by the interference between the overlapping fields of the individual array element with the fields from the other array elements. Therefore, like our previous LOCSET technique [13,14] the fields of all of the array elements must overlap on the photodetector to obtain the error signal. The theoretical model for both the self-referenced and self-synchronous LOCSET are summarized in the next section. 2. Summary of the Self-Synchronous and Self-Referenced LOCSET theory In self-synchronous LOCSET all of the array elements are phase modulated, while in the selfreferenced LOCSET configuration one array element is unmodulated while all of the

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remaining array elements are phase modulated. The results of the theoretical model for selfreferenced LOCSET and self-synchronous LOCSET methods are summarized in the succeeding paragraphs. Assuming that the unmodulated and phase modulated fields are plane waves and are identically polarized, then the unmodulated element optical field, Eu(t) and the ith array element optical fields, Ei(t) are, Eu ( t ) = Eu 0 ⋅ Cos ( ω L ⋅ t + φu ) and (1) Ei ( t ) = Ei 0 ⋅ Cos ( ω L ⋅ t + φi + β i ⋅ Sin ( ωi ⋅ t ) ) , (2) where Eu0 and Ei0 represent the field amplitudes for the unmodulated element and ith phase modulated element, respectively. ωL represents the laser frequency. φu and φi represent the optical phases of the unmodulated and the ith array elements, respectively. βi represents the phase modulation amplitude for the ith array element. ωi represents the RF modulation frequency for the ith array element. The optical fields from the unmodulated array element and all of the phase modulated array elements are superimposed on the photodetector so that the total field is, ET ( t )

=

∑ E (t ) N

Eu ( t )

+

,

j

(3)

j =1

where N is the number of phase modulated elements in the optical array. The photodetector current is, iPD ( t ) = RPD ⋅ A ⋅

εo ⎧ 2 ⎛ ⋅ ⎨ Eu ( t ) + ⎜ μo ⎩ ⎝

∑ E ( t ) ⎞⎟⎠ ⎜⎛⎝ ∑ E ( t ) ⎟⎞⎠ + 2 ⋅ E ( t ) ⋅ ∑ E ( t )⎬⎫⎭ , N

N

l

N

j

l =1

u

j =1

j

(4)

j =1

where l and j represent the summation indices for the phase modulated elements, μo and εo represent the magnetic and electric permeabilities of free space, RPD represents the responsivity of the photodetector, and A represents the photodetector area. The phase control signal is extracted from the photocurrent using coherent demodulation in the RF domain. The photodetector current is multiplied by sin(ωi t) and integrated over a time, τ, where ωi represents the phase modulation frequency of one of the phase modulated elements. The integration time, τ, is selected long enough to isolate the individual phase control signals of the phase modulated elements and short enough so that the phase control loop can effectively cancel the phase disturbances of the system. The phase control signals for the ith array element of the self-referenced and the self-synchronous LOCSET systems are given by, τ 1 S Si = ⋅ ∫ iPD ( t ) ⋅ Sin ( ωi ⋅ t ) ⋅dt , (5) τ 0 where SSi represents the phase error control signal. In the self-referenced LOCSET configuration one array element is not phase modulated whereas, in the self-synchronous LOCSET configuration all of the array elements are phase modulated. The phase error signal for a self-synchronous LOCSET system is obtained by evaluating Eq. (5), under the following conditions; ωi is equal one of the array phase modulation frequencies and the integration time, τ >> 2 π/|(ωi-ωj)| for all i and j when j≠i. Under those conditions, Eq. (5), the phase error signal, for the self-synchronous LOCSET system is given to an excellent approximation by, S SSi = RPD ⋅

Pi ⋅ J1 ( β i ) ⋅

⎡ ⎢ ⎣

∑ J (β ) ⋅ N

0

j =1

j

Pj ⋅ Sin (φ j − φi )

⎤ ⎥ ⎦

,

(6)

where SSSi represents the phase error control signal for the self-synchronous configuration of the system, J0 represents a Bessel function of the first kind of zero order, J1 represents a Bessel function of the first kind of order one, φi and φj represent the optical phases of the ith #75875 - $15.00 USD

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and jth array elements, respectively and finally, Pi and Pj represent the optical power incident upon the photodetector from the ith and jth elements, respectively. For these experiments, the ith phase control loop operating point, φi-φj, is set to zero for all of the other array elements. Experimentally, this condition is achieved by adjusting the variable RF phase adjustors, which are shown in Fig. 2, to optimize the optical power on the photodetector. It is only necessary to make this adjustment once for any system set up. When a self-synchronous LOCSET system is adjusted to optimize the optical power on the photodetector, the control loop for each of the N elements strives to zero φi-φj, for all of the other array elements, thus locking the phases of the array elements even in the presence of disturbances. If ωi is one of the array phase modulation frequencies and if the integration time, τ >> 2 π/(ωi-ωj) for all i and j when j≠i then from Eq. (5), the phase error signal, for the selfreferenced LOCSET system is to an excellent approximation, S SRi = RPD ⋅

Pi ⋅ J1 ( β i ) ⋅

⎡ P ⋅ Sin (φu − φi ) + ⎢ u ⎣

∑ J (β )⋅ N

0

j =1

j

⎤ ⎥ ⎦

Pj ⋅ Sin ( φ j − φi ) ,

(7)

where SSRi represents the phase error control signal for the self-referenced LOCSET configuration, φu presents the optical phase of the unmodulated array element and Pu represents the optical power incident upon the photodetector from the unmodulated array element. In Eq. (7) the first term in the bracket has the same form as the phase error signals used by previous phase locked arrays and the second term in the bracket is the selfsynchronous term given in Equation (6). For these experiments, the ith phase control loop operating point, φi-φu, is set to zero for all of the array elements. Experimentally, this condition is achieved by adjusting the variable RF phase adjustors, which are shown in Fig. 2, to optimize the optical power on the photodetector. When a self-referenced LOCSET system is adjusted to optimize the optical power on the photodetector, the control loops adjust the phases of the phase modulated array elements to track the phase of the unmodulated element, thus the phases of the array elements are locked in phase even in the presence of disturbances. Assuming that the loop integrator time constant, τ >> 1/|ωi- ωj| for any pair of phase modulation frequencies and that a portion of the central lobe of the far-field pattern of the array be imaged on the photodetector active area, then the phase error signal for the selfreferenced LOCSET and self-synchronous LOCSET configurations are presented in Eqs. (6) and (7), respectively. 3. Experimental Systems for Self-Referenced and Self-Synchronous LOCSET A block diagram of the experimental system is shown in Fig. 1. The master oscillator is a Lightwave Electronics series 122 Nonlinear Planar Ring Oscillator. The optical power from the master oscillator is coupled into the single mode polarization maintaining input fiber for the EOSpace 1x8 power splitter that has a separate phase modulator in each of the 8 legs. The outputs of the EOSpace power splitter/phase modulator are coupled into 8 single mode polarization maintaining optical fibers. The optical signals from the EOSpace power splitter/phase modulator legs are then directed either directly into the collimating optics or coupled through fiber amplifiers and then into the collimating optics. The collimated output beams from the array are sampled by a beam splitter and a small fraction of the output power from the array is sent to a focusing lens that images the central lobe of the far field onto a single photodetector. A small amplitude RF phase modulation is applied to each of the phase modulators at a unique RF frequency for each array element. Therefore, the photocurrent contains the RF phase modulation frequencies of each array element and the amplitudes of those RF frequency components contain the optical phase error signals for the array elements, as is shown in Eq. (5). Next, the RF phase modulation frequencies, corresponding to each array element, are isolated in the electronic domain and the optical phase error signals corresponding to each array element are separately extracted and fed back to the corresponding phase modulator for that array element and the phase of that array element is locked to the same phase as the other array elements. Each array element has a functionally #75875 - $15.00 USD

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Fiber Amps Master Oscillator 100 mW

Beam sampler

PM

9 beams

PM PM

1 X 8 PM, Splitter