Hybrid coherent-optical, RF signal channelizer

[0001] This application is related to commonly owned co-pending patent application, Ser. No. 08/759,901, filed on Dec. 4, 1996, entitled “All Optical RF Signal Channelizer,” by Wickham, et al.

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical channelizer system and more particularly to an optical channelizer system adapted to provide RF signal channelization at frequencies in the order of 100 GHZ or more which instantaneously channelizes the entire signal spectrum into channels having a few megahertz.

[0004] 2. Description of the Prior Art

[0005] Various factors are driving communications systems toward higher frequency and larger bandwidth. For example, available communications bandwidth is disappearing as the number of users and the bandwidth per user escalates forcing communication links to higher frequencies. In addition, newer communication systems, such as military communications are increasingly being driven toward higher frequencies and larger bandwidth user requirements. For example, modem missile seekers and imaging radar systems are utilizing frequencies around 100 GHz to achieve antenna directivity and higher resolution for small aperture systems. In order to accommodate relatively high bandwidth and high frequencies, RF communications systems are known to employ optical carrier transmission of the RF signal. Examples of such systems are disclosed in U.S. Pat. Nos. 4,468,766; 4,695,790; 5,005,946; 5,347,535; 5,377,035 and U.S. Statutory Invention Registration H1059.

[0006] Optical channelizers are used to perform spectral analysis of the newer high frequency communication signals. Such optical channelizers are known to process relatively wider bandwidth than comparable RF channelizers for real time identification of complex signals. Such optical channelizers are used in particular to channelize or divide up a relatively wide bandwidth signal into several subbands or channels. Although such optical channelizers are known to process relatively wider bandwidth than known RF channelizers, many optical channelizers utilize acousto-optic technology which limits the bandwidth to a few gigahertz. Examples of optical communications systems utilizing acousto-optic technology are disclosed in U.S. Pat. Nos. 4,448,494; 4,644,267; 5,005,946; 5,105,380; 5,327,142 and 5,363,221.

[0007] In order to resolve this problem, optical channelizers have been developed which do not depend on the acousto-optic technology as disclosed in commonly owned co-pending U.S. patent application Ser. No. 08/759,901 filed on Dec. 4, 1996. In that application, as illustrated in FIGS. 1 and 2, an optical channelizer, generally identified with the reference numeral 20 includes a diffraction grating. The RF signal spectrum, generally identified with the reference numeral 22, is spatially mapped to positions across a 1×n photo-detector array 24 by the diffraction grating. A mode locked laser 26 provides a comb of optical local oscillator (LO) frequencies 28 (FIG. 2), separated by the desired channel spacing. The LO signals are optically odyned with the signal spectrum at the photo-detector array 24. In this way, each detector generates the same band of optical frequencies so that each channel may use the same post detection electronics. The band is centered on an intermediate frequency (IF) that is determined by adjusting the offset (Fig. 1) in the incident angles at which the signal and the local oscillator lasers illuminate the grating. A complex signal spectrum (phase and amplitude) is extracted by measuring the in-phase and quadrature components of the heterodyne products. Each detector is followed by a IF filter whose bandwidth is generally set to the channel spacing in order to achieve a hundred percent frequency coverage. Unfortunately, due primarily to size constraints, the optical channelizer, disclosed in the ‘901 patent application, is limited to minimal channel bandwidth of about 1 GHz. Although it is possible to improve the channelizer sensitivity, dynamic range and resolution by reducing the bandwidth of the post detection IF filter, such an approach compromises the frequency coverage. For example, a 1 GHz channelizer with 40 MHz IF only detect signals within the IF filters pass band, thus being blind to 960 MHz out of every 1 GHz of bandwidth or 96% of all possible signals. Thus, there is a need for an optical channelizer for use with signals up to 100 GHz with improved resolution.

[0008] The present invention relates to an optical channelizer system that is adapted to provide instantaneous RF signal channelization at frequencies on the order of 100 GHz or more into channels having bandwidth as small as a few megahertz. In order to improve the channel resolution the optical channelizer system is formed from a plurality of parallel channelizers, for example, twenty-five parallel sub channelizers which can provide 1 GHz channel spacing and 40 MHz signal resolution.

[0015] The present invention relates to an optical channelizer system that is adapted to provide RF signal channelization at frequencies on the order of 100 GHz or more which simultaneously channelizes the signal spectrum into channels having bandwidths as small as a few MHz. The optical channelizer system in accordance with the present invention is adapted to provide improved resolution while at the same time maintaining a suitable physical size so that it is deployable on various terrestrial and airborne platforms. In particular, as illustrated in FIGS. 3-5, the optical channelizer system in accordance with the present invention is formed from a plurality of subchannelizers, arranged to operate in parallel.

[0016] Although, the system is described and illustrated for channelizing signals having a bandwidth of a 100 GHz or more, it will be understood by those of ordinary skill in the art, that the principles of the present invention are broadly applicable to signals having a bandwidth more or less than a 100 GHz. An important aspect of the present invention is that the problems associated with the channelizer illustrated in FIGS. 1 and 2 and discussed above are solved by dividing the signal spectrum and the local oscillator comb frequencies prior to applying those signals to an optical channelizer, such as, the optical channelizer 20, discussed above and illustrated in FIGS. 1 and 2. However, it should be understood that the principles of the present invention can be utilizing various types of channelizers, such as channelizers based on integrated optical arrayed wave guide gratings, for example, U.S. Pat. No. 5,943,452.

[0017] Referring to FIG. 3, the optical channelizer system in accordance with the present invention is generally identified with the reference numeral 32. The optical channelizer system 32 includes an optical channelizer 20, for example, the optical channelizer 20 illustrated in FIGS. 1 and 2 and discussed above, a m ×n dimensional photo detector array 34 as opposed to the 1×n detector array used with the channelizer 20, illustrated in FIG. 2, and a source of a local oscillator frequency comb, with the frequencies, separated, for example, by 1 GHz. In order to generate multiple signal beams, the signal spectrum is passed through a Bragg cell 34. Such Bragg cells are generally known in the art. An example of a suitable Bragg cell 34 is disclosed in I. C. Chang, “Acousto-optic Devices and Applications,” IEEE Trans-Sonics and Ultrasonics, Vol. 50-23, P. 2 (1976). The Bragg cell 26 can be driven to replicate the signal spectrum to generate, for example, 25 signal beams, with each version of the signal spectrum being shifted, for example, by 40 MHz with respect to its predecessor. As shown in FIG. 3, in order to provide such separation, a periodic chirp signal, for example 1 GHz, generally identified with the reference numeral 38, is used to drive the Bragg cell 36. In order to replicate the signal spectrum 25 with each version of the signal spectrum frequency shifted by, for example, 40 MHz with respect to its predecessor, the period of each chirp signal is selected to be {fraction (1/40)} MHz or 25 ns. The 25 signal beams are applied to the optical channelizer 20 and translated unto the m×n detector array 34. As a result, the image of each signal spectrum is imaged onto a different row of the m×n detector array 34 and translated horizontally. An optical amplifier (not shown) may be used to amplify the signal before it has been replicated 25 times by the Bragg cell 36.

[0018] FIG. 4 illustrates a system for spatially dividing the local oscillator input beam, for example 25 times. As shown in FIG. 5, a monolithic, optical splitter/amplifier integrated circuit (MOSAIC) may be used to spatially divide the local oscillator signal. The output of the MOSAIC 30, in turn, is applied to the optical channelizer 20. In particular, the resulting images of the local oscillator spectrum are projected onto horizontal rows of the photo detector array 34. With such an arrangement, each photo detector 34 processes a different 40 MHz segment of the signal with identical post detection electronics. The signal frequencies detected by adjacent elements in a particular row differ by 1 GHz. Neighboring rows operate on segments that are displaced by 40 MHz and so on for each role until the entire 1 GHz is spanned. With such an arrangement a number of subchannelizers are formed and operated in parallel.

[0019] Alternatively, the signal spectrum can be replicated identically, for example, by being passed through the MOSAIC 40, while the local oscillator beam is passed through the Bragg cell 36 to generate frequency shifted versions of the LO spectrum. Since it is desirable to maintain the LO spots centered on their respective detectors, the rows of detectors may have to be skewed.

[0020] Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, the invention can be implemented utilizing virtually any type of optical channelizers, such as one based on integrated optical arrayed waveguide gratings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.