Quadrature processed LIDAR system

#This disclosure relates to quadrature signal processing of local oscillator and Doppler frequency-shifted signals in a lidar or other coherent optical systems.

#A primary obstacle of fiber lidar is assumed to be the birefringent depolarization of the local oscillator (LO) signal from the transmitted carrier after splitting from the lidar output path. The effect can destroy the heterodyne efficiency at the detector and hence lidar operation unless polarization preserving fiber is utilized in the system past the split point in homodyne systems. This effect is assumed worse in heterodyne systems utilizing different LO and transmitter sources. The only form of the optical fiber lidar “immune” from this effect utilizes a local oscillator signal taken from the Fresnel reflection at the end of the transmit fiber immediately preceeding the output telescope. However, this latter mode of operation is not required as conventionally assumed. Laboratory tests have shown that phonon modulation of the birefringence in the local oscillator path gives rise to AM modulation of the detected signals within the dynamic range required of the lidar to perform its basic task. This provides a statistically detectable signal.

#Furthermore, in conventional lidar systems, a frequency offset between a local oscillator signal and a transmitted beam has been traditionally required. This has traditionally been achieved in homodyne operation via a frequency shifting device such as an expensive acousto-optic (A/O) modulator, or in heterodyne operation by maintaining a fixed offset between the frequencies of the two coherent sources. It is desirable to perform such heterodyning or homodyning without the use of such acousto-optic modulators.

#FIG. 1 is a schematic representation of an optical fiber lidar using an acousto-optical modulator;

#FIG. 2 is a schematic representation of a quadrature signal mixing assembly for bi-directional Doppler signal processing;

#FIG. 3 is a schematic representation of a quadrature processed optical fiber lidar;

#Applications for coherent Doppler lidars include velocity sensing applications (platforms and objects), volumetric/fluidic flow sensing, vibration monitoring, range to target and other related standoff sensing applications. The lidar detects the Doppler frequency shift imposed on coherent light scattered from a moving target by mixing the scattered, frequency shifted light with a reference beam of light (local oscillator) which is not shifted in frequency on the detector. A difference frequency results from this mixing process which is proportional to the velocity of the scattering medium. It is the Doppler frequency shift imposed on the light scattered from the target that provides the mechanism used for velocity detection. The reference beam can be either derived from the transmit beam (homodyne operation) or derived from another stable coherent source (heterodyne operation). By measuring the Doppler shift from three (or more) spatially separated lidar beams a complete vector velocity can be computed along with statistical velocity information.

#In general, fiber lidar systems utilize the same optical functions to perform the lidar mission, except the optical elements are created by guided-wave optics (i.e. optical fiber devices). The laser source is generally a combination of a suitable solid state, DFB laser diode and one or more cascaded optical fiber amplifiers of the appropriate wavelength, although fiber or free-space lasers could be used as the source elements. For the most part, the amplifier of choice is the erbium-doped fiber amplifier (EDFA) operating at a wavelength of 1.54 μm. In one embodiment of an offset homodyne fiber lidar 100 shown in FIG. 1, the output 134 of the laser amplifier/source combination 102 is fed through a duplex element 110 to the end of a fiber 112 located at the focal point of an appropriate lens 114. In FIG. 1, the local oscillator (LO) signal 346, is split off by a tap coupler 106 prior to the duplex element 110 to be offset shifted in frequency by the A/O modulator 118, 120, 122. The frequency shifted LO signal 148 is then recombined with the returning Doppler signal 146 in a combining coupler 128 as shown in FIG. 1. The main beam 140 is transmitted to the target (not shown), such as atmospheric scatterers, through the lens 114 which also couples the backscattered light 142 into the return fiber path 144 through the duplex element 110. The two signals 146, 148 then mix due to the superposition of the electric field vectors on the detector 128 to generate a signal 150 at the Doppler difference frequency according to the square of the electric field intensity. Electronic processing of the signal 150 is then used to produce a Doppler velocity spectrum 152. The offset frequency must be greater than the highest Doppler velocity component. System electronic bandwidth must be twice this frequency to accept both positive and negative Doppler velocity.

#If the optical fiber quadrature processing assembly 200 shown in FIG. 2 is substituted for the combiner 128 shown in FIG. 1 and the system diagram modified as shown in FIG. 3, the A/O modulator 118, 120, 122 may be omitted and the system electronic bandwidth cut in half due to the effective use of the phase information in the optical carrier 134, 138, 140. The signals when processed according to the equations below result in a Fourier power spectrum centered around zero frequency instead of being centered around the offset frequency of the A/O modulator as in the case of FIG. 1. Such a network may also be used in coherent optical fiber systems (e.g., communications, sensors) operating over a wide range of wavelengths or may be used with free-space lidars with the appropriate optical coupling elements. In, FIG. 2, the fixed −90 degree phase shift of the couplers 202, 204, 206, 208, 210 is inherent in the coupled mode equations that describe the physics of the devices. These couplers 202, 204, 206, 208, 210 may then be used in mixing polarized or non-polarized optical sources at the optical detectors 214, 216 to generate the quadrature Doppler components. Those in-quadrature signal components may then be processed as the analytic function for the Fourier transform (sin(±ωd)t−j cos(±ωd)t) to develop a signed velocity spectrum. While the equations below are used in RF spectrum analysis and are standard in communications textbooks for illustrating Fourier transform theory, heretofore it has not been connected to optical lidar signal processing using the phase characteristics of the coupled—mode equation.