Abstract:
One embodiment is an optical image preamplifier having an input through which a laser signal is received and amplified, said laser signal emanating from a target illuminated by a laser transmitter or generated by multiple lasercom transmitters in the field of view; the optical image preamplifier also having an output; and a focal plane array having an input operatively coupled to the output of the optical preamplifier. Embodiments of the present method and apparatus may be utilized to overcome photodetector and post-detection electronic noise to permit near quantum-limited receiver sensitivity with simple focal plane technologies. These embodiments enable ladar, wavefront sensor and multiple access lasercom systems that provide high sensitivity with the wide bandwidth and wavelength flexibility of semiconductor laser media.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to waveguide amplifiers and, more particularly, to optical preamplification of an entire image prior to delivery to a focal plane array of a photopumped semiconductor image amplifier. 
       BACKGROUND 
       [0002]    Ladar (Laser Detection and Ranging) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The sensitivity of focal plane arrays (FPAs) used for ladar and multiple-access lasercom is not sufficient in many instances, so gain is frequently employed to increase the signal level from the FPA. Internal gain, for example as occurs in avalanche photo detectors (APDs), improves sensitivity, but complicates FPA design and signal readout. The use of an external optical image preamplifier to raise the signal above the noise level of the detector array and post-detection electronics can significantly improve system performance; however, image amplifier technology has not generally been adequate for this application. The use of a photopumped semiconductor waveguide amplifier gain medium overcomes many issues with prior art image amplifiers. 
       SUMMARY 
       [0003]    One embodiment of the present method and apparatus encompasses an apparatus. The apparatus may comprise: an optical image preamplifier having an input through which a back scatter signal from a target is received and amplified, the optical image preamplifier also having an output, and the optical image preamplifier being photopumped; and a focal plane array having an input operatively coupled to the output of the optical image preamplifier. 
         [0004]    Another embodiment of the present method and apparatus encompasses an apparatus. The apparatus may comprise: a target illuminated with laser illumination that produces a weak back scatter signal of the target; an optical preamplifier having an input through which a back scatter signal is received and amplified, the optical image preamplifier being a photopumped semiconductor image amplifier having an output; a focal plane array having an input operatively coupled to the output of the optical image preamplifier; relay optics and a narrow band filter disposed between the optical image preamplifier and the focal plane array; wherein an entire image is optically preamplified prior to delivery to the focal plane array. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0005]    The features of the embodiments of the present method and apparatus are set forth with particularity in the appended claims. These embodiments may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: 
           [0006]      FIG. 1  depicts an embodiment according to the present method and apparatus. 
           [0007]      FIG. 2  depicts an embodiment according to the present method and apparatus of an optical image preamplifier. 
           [0008]      FIG. 3  depicts a semiconductor waveguide fabrication process according to the present method and apparatus. 
           [0009]      FIG. 4  is a graph depicting lattice constant versus band gap energy. 
           [0010]      FIG. 5  shows an example of one embodiment of a multimode semiconductor waveguide. 
           [0011]      FIG. 6  depicts a semiconductor waveguide  1000  having a heat sink  1002  with partially transmitting coatings  1004  to provide optical feedback. 
           [0012]      FIG. 7  is a graph showing gain v. optical frequency for an amplifier with partially transmitting coatings. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In general the embodiments of the present method and apparatus may be referred to as an optical image preamplifier, an optical preamplifier, an image preamplifier or a photopumed semiconductor preamplifier. A photopumped semiconductor image preamplifier may be considered a particular version of an image preamplifier, which is in turn a member of the class of optical preamplifiers. The function of an image preamplifier is to boost the power level of the image to overcome noise in the detector array and post-detection electronics; this is what the use of heterodyne mixing is intended to accomplish, but heterodyne mixing is very difficult for amplifying an image because the wavefronts must be precisely matched at the array pixels. A preamplifier is far simpler since no wavefront alignment is required. 
         [0014]    Optical preamplification is an attractive method of increasing ladar return signals, but the added optical noise minimizes the benefit obtained from the gain of the optical amplifier. By combining an optical amplifier with a narrowband optical bandpass filter, spontaneous emission noise added by the amplifier is partially rejected, allowing the gain from the amplifier to be realized as an increase in sensitivity. Spontaneously emission noise cannot be totally rejected, hence leading to a theoretical noise figure of at least 3 db. 
         [0015]    A purpose of the optical image preamplifier is to enable comparable sensitivity without the use of a heterodyne mixing. Heterodyne mixing can also overcome noise in the detector and post-detection electronics, but requires near perfect wavefront matching (i.e., is exquisitely sensitive to phase variations across the input wavefront) and just does not work with an image, the wavefront of which contains drastic phase variations by nature. Additionally, an optical preamplifier can compensate for low quantum efficiency (QE) of the photodetector array to avoid degradation of the signal-to-noise ratio (SNR), a benefit that is not accomplished by other methods cited earlier such as internal avalanche gain or heterodyne detection. 
         [0016]      FIG. 1  depicts an embodiment according to the present method and apparatus. In this embodiment a target  102  is illuminated by laser illumination  104  and a weak back scattered signal  106  is directed to a receive telescope  108 . Following the receive telescope  108 , is an image preamplifier  110 , relay optics  112 , narrow band filter  114 , focal plane array/receiver optical integrated circuit (ROIC)  116  and signal processing electronics  118 . 
         [0017]    Embodiments of the present method and apparatus preamplify images in a photopumped waveguide to enhance sensitivity of optical receivers. As a result gains of 30 dB and NF of 3 dB are feasible. There is good image quality with high MTF. Either continuous waves or short pulses may be used for pumping, the gain following pump intensity. The wavelength of operation supports ladar systems using, for example Nd:YAG, Yb fibers, and Er fibers, and the optical gain may be temporal waveform controlled by pump to enable range gating. 
         [0018]    Embodiments of the present method and apparatus reduce ladar transmitter power and aperture, and enable a wider choice of FPA (focal plane array) technologies. By raising the signal above the receiver noise, embodiments of the present method and apparatus may reduce SWP and cost of ladar systems, enable multiple access lasercom receivers, and enhance wavefront sensors for AO. There are many benefits of the embodiments of the present method and apparatus. Prior art optical preamplifiers using fibers did not provide high gain and low noise with excellent imaging properties, but photopumping a multimode semiconductor waveguide amplifier enables efficient image amplification without serious image degradation nor excessive additive noise. Some of the benefits are: high gain (˜30 dB); low noise figure (NF ˜3 dB); face pumping by simple low power diode bars; large gain bandwidth (30-50 nm); supports CW, wideband data, short pulses, chirped or other coherent waveforms; wide operating wavelength range (750-2000 nm) can cover important ladar and lasercom wavelengths; fabricated by established epitaxial growth and wafer processing; pulsed pumping permits range gated operation; and avoids problems inherent in electrical pumping. 
         [0019]    Embodiments of the present method and apparatus permit high sensitivity receiver operation using simple and low cost detector arrays. For example, embodiments may include: PIN photodiodes rather than GM APDs; resolvable spots and MTF determined by waveguide dimensions (large numerical aperture); and CW or pulsed operation (gain follows pump intensity). Embodiments of the present method and apparatus may extend to MWIR, which may be feasible using appropriate semiconductor materials and cryogenic cooling. 
         [0020]      FIG. 2  depicts an embodiment according to the present method and apparatus of an image preamplifier  200 . In this embodiment a photopumped self imaging waveguide  202  is located on a heat sink  204 . Located above the photopumped self imaging waveguide  202  are diode bar pumps  206 ,  208 . 
         [0021]      FIG. 3  depicts a semiconductor waveguide fabrication process according to the present method and apparatus. As an example, fabrication and processing may be performed with a GaAs wafer  301 . Epitaxial layers  302  may be grown on large, for example, 3 or 4 inch substrates  304  to form a semiconductor multimode waveguide  306  with quantum wells using quaternary alloys to control band gap and lattice constant. Wafers may be lapped to desired thickness from the substrate side, AR  307  coated and bonded to heat sinks to produce a waveguide  308  with a heat sink  310 . 
         [0022]      FIG. 4  is a graph depicting lattice constant versus band gap energy. Quantum wells may have both band gap and lattice constant mutually controlled. Quaternary III-V alloys (e.g. GaInAsSb, etc.) may be used. The quantum well thickness also modifies wavelength.  FIG. 4  is a three dimensional representation of the thermal resistivity for In 1-x , Ga x , As y , P 1-y  quaternary alloy over the entire range of compositions. Thus, the quantum well amplifier may operate with a highly uniform temperature distribution. 
         [0023]      FIG. 5  shows an example of one embodiment of a multimode semiconductor waveguide  500 . For an amplifier of thickness d with N quantum wells of thickness t W , an overlap factor T may be calculated as T=Nt W /d. The net gain (G=exp(gTL) depends on the gain coefficient g for each well and the length L of the amplifier. Quantum well gain may depend on carrier density in a complex way, but can be large (e.g., 100-1000 cm −1 ). Carrier density may also depend on pump intensity and amplifier power in a complex way. Features may be a large area semiconductor waveguide, high index contrast, and high order multimode design. Waveguide modes (ignoring quantum wells) may be: 
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         [0024]    Signal along length L of amplifier may be calculated as function of pumping and input field distribution to determine image signal gain. The heat flow within the waveguide may be determined to obtain the operating temperature distribution, which is anticipated to be small and constant in time. Amplifier may be designed for Talbot self-imaging length (i.e., L=4nd2/λ) for the operating wavelength. 
         [0025]      FIG. 6  depicts a semiconductor waveguide  1000  having a heat sink  1002  with partially transmitting coatings  1004 , and  FIG. 7  is a graph showing gain v. optical frequency. By using a partially transmitting coating in place of an antireflection coating, the self-imaging waveguide amplifier exhibits resonant behavior. The gain becomes periodic (Δv=c/2nL) with signal wavelength, and the peak gain may be increased by finesse (Q) of resonator. Also, the required pump power is reduced for a given gain, and ASE and spectral background are suppressed. However, gain may not be increased arbitrarily or parasitic oscillation could occur. 
         [0026]    The present method and apparatus are not limited to the particular details of the depicted embodiments and other modifications and applications are contemplated. Certain other changes may be made in the above-described embodiments without departing from the true spirit and scope of the present method and apparatus herein involved. It is intended, therefore, that the subject matter in the above depiction shall be interpreted as illustrative and not in a limiting sense.