Conventional high energy laser beam pointing and control systems are "narrow" field of view (FOV) systems that require a substantial amount of telescope steering so as to redirect the laser beam from a first point to a second point. As can be appreciated, the greater the distance over which the beam must be steered, the greater is the amount of time required to reposition the beam. When used in, for example, an optical communication system, these characteristics may provide less than optimum system performance. A wide FOV pointing system is desirable at least for the reason that more rapid pointing operations may be performed.
However, the provision of a wide FOV (WFOV) laser beam directing system presents a problem in the operation of an Outgoing Wavefront Sensor (OWS) that is a component of the beam directing system. The OWS, such as a Hartmann-type wavefront sensor, is employed to measure an aberration of a sampled portion of the outgoing wavefront so that aberration correction can be applied. A deformable mirror (adaptive optic) is one conventional technique to correct for an aberration of the outgoing wavefront.
In the Hartmann wavefront sensor, a radiation detector array is employed at a focal plane of a transfer lens to measure a plurality of spot image positions of focussed samples of the outgoing wavefront. If the spot positions deviate from positions associated with a non-aberrated wavefront, then an aberration may be present in the outgoing wavefront. The measured spot image deviations are employed to determine an amount of aberration correction to be applied by the deformable mirror. A plurality of Holographic Optical Element (HOE) diffraction gratings can be employed to sample the outgoing wavefront at a number of locations and to direct the samples to the transfer lens of the Hartmann wavefront sensor.
The aforementioned problem arises in the operation of the wavefront sensor over the WFOV. The desired wavefront shape leaving the beam directing system is that of a plane. The wavefront sensor must measure deviations from this condition. If, while operating over the FOV, the optical path from the out-coupled HOE sample beams to a transfer lens of a detector assembly (i.e., the wavefront sensor relay, or transfer, optical subsystem) itself introduces aberrations to the HOE sample beams, the aberrations being equivalent to a departure from a planar condition, then measurement of the true aberration of the outgoing wavefront is made difficult.
However, due to pupil distortion as a function of field angle, the spots positions pattern also changes, in addition to the change resulting from wavefront aberrations. This problem becomes more pronounced if the FOV of the system is widened, as the pupil distortion is a cubic function of the FOV.
Many prior systems attempt to direct the HOE sample beams through a hole in a secondary mirror. However, this results in extreme difficulties in removing shifts of the entire HOE spot pattern, due to slewing over the FOV. One conventional wide FOV design approach requires a large camera, in conjunction with a transfer lens using an extremely fast F-number of approximately F/1. Another approach employs a set of steering mirrors to reduce the overall spot shifts. In either case, these conventional approaches add significant complexity and/or require a large mass to be positioned behind the secondary mirror.
It is thus an object of this invention to provide a low intrinsic distortion wavefront sensor that provides for accurate wavefront aberration detection over a wide field of view.
It is another object of this invention is to provide a wide FOV laser pointing system having a wavefront sensor that accurately measures an aberration of an outgoing wavefront.