how often does the letter p occur in:
active coherent beam combining of laser oscillators is an attractive way to achieve high output power in a diffraction limited beam. here we describe an active beam combining system used to coherently combine 21 semiconductor laser elements with an 81% beam combining efficiency in an external cavity configuration compared with an upper limit of 90% efficiency in the particular configuration of the experiment. our beam combining system utilizes a stochastic parallel gradient descent (spgd) algorithm for active phase control. this work demonstrates that active beam combining is not subject to the scaling limits imposed on passive-phasing systems.
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fig. 1. coherent beam combining experimental configuration. twenty-one element array is incident on a doe. a diffraction grating provides feedback and wavelength selectivity.
fig. 2. single gain element output power in external cavity at 960 nm. output power is the combined total from all ports.
fig. 3. stochastic parallel gradient descent (spgd) detector signal. spgd detector measures the zero-order power incident from the doe. the convergence time is typically 4 ms. the spgd loop is turned on at t=0.
fig. 4. passive phasing: far-field image of doe. the power in the zero-order is ∼5% of the total power (η∼5%). the current per element is set at 400 ma.
fig. 5. active phasing: far-field image of the doe capturing all the diffracted orders when spgd is turned on. 81% of the power is captured in the zero-order beam when spgd is activated. the remaining 19% of the power is diffracted into higher doe orders and is scattered. the average current per element is 400 ma.
fig. 6. near-field spectrometer. each element in the array is imaged onto a line. an etalon is used in a telescope configuration to map frequencies onto circular rings.
fig. 7. near-field spectrometer output. array position is on the horizontal axis and frequency is on the vertical axis. array elements with equal frequency are mapped onto circular rings (similar to interference pattern between a plane and spherical wave).
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The passage contains one hundred and twelve "p"s.
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