Synthetic aperture ladar with chirped modelocked waveform
The present invention uses an intra-cavity modulation approach to modulate a laser transmitter at bandwidths greater than a few gigahertz (GHz) in a non-ambiguous waveform by chirping the laser and simultaneously mode locking. Accordingly, the inventive system includes a source of a beam of electromagnetic energy; a mechanism for mode locking the beam; and an arrangement for chirping the beam. In the illustrative embodiment, the source is a laser. The mode locking mechanism may be an active element or a passive element. The beam is chirped with a translation mirror. The translation mirror may be driven with a piezo-electric drive coupled. In the best mode, the carrier is chirped with an electro-optical crystal disposed in the cavity of the laser. The laser is chirped to the free-spectral range limit, which is typically a few hundred megahertz, by scanning the optical length of the laser resonator. After simultaneously mode locking, the waveform created is a train of mode locked pulses that have an optical carrier frequency chirped in time.
1. A ladar system comprising:
first means for generating a chirped mode-locked laser beam;
second means for recording phase and frequency information pertaining to said laser beam and subsequently transmitting said laser beam from said system in response thereto;
third means for receiving a reflected version of said laser beam and providing a received signal in response thereto, said third means including an In-phase (I) and Quadrature (Q) receiver for implementing I and Q detection and outputting said received signal having I and Q electrical signal components in response thereto; and
fourth means for correcting said received signal based on said phase and frequency information recorded by said second means.
2. A ladar method comprising the steps of:
generating a chirped mode-locked laser beam;
recording phase and frequency information pertaining to said laser beam and subsequently transmitting said laser beam from said system in response thereto;
receiving a reflected version of said laser beam and providing a received signal in response thereto, said third means including an In-phase (I) and Quadrature (Q) receiver for implementing I and Q detection and outputting said received signal having I and Q electrical signal components in response thereto; and
correcting said received signal based on said recorded phase and frequency information.