Background of the Invention

The present invention relates to fiber optics as applied to free-space coherent and incoherent optical systems. The advantages of fiber optics in ground-based communication systems (whether telecommunications or radar, for example) is widely recognized. However, present space- borne optical systems (such as for communications and radar) typically rely on bulk optics, including mirrors and lenses rigidly mounted to an optical bench, to transfer light from optical sources to the exit aperture or from the entrance aperture to the optical receiver. Such designs are typically sensitive to thermal and mechanical perturbations, and may substantially impact upon the host payload and real estate utilization.

Summary of the Invention

One aspect of the present invention includes an optical communications and spatial tracking receiver system having an optical package coupled to an acquisition and tracking network, the optical package including an optical fiber and acting to receive optical signals and to transfer them into the fiber, the fiber transferring the signals to a receiver, the receiver processing the signals and providing an output to the network.

Various embodiments of this aspect may include any of the following features: wherein the optical package further includes at least one nutation device, the optical package transferring the optical signals into the fiber by nutation; wherein the nutation device is a beam steering mirror; wherein the nutation device is an active fiber

coupler; wherein the nutation device is a mechanical nutation device having a housing to which is affixed a flexure, the fiber guided by the flexure and terminating near an exposed end of the flexure, at least one actuator coupled between the flexure and the housing; wherein the actuator is electro¬ mechanical, piezo-electric or electrostatic; wherein the nutation device is driven at a mechanical resonance; wherein the nutation device further includes at leas _one.positional sensor coupled between the housing and the flexure; wherein the nutation device further includes at least two positional sensors, output signals of both sensors applied to a position sensor amplifier section of the optical package, where the sensor output signals represent positional location of the fiber and which information is used to maximize coupling of the optical signals into the core of the fiber; further including a local oscillator, the local oscillator providing a signal which is combined with the optical signal to perform coherent detection; wherein a local oscillator laser is a GaAlAs diode laser having an output power of 30 W operable at a wavelength of 0.86 micrometers; wherein the local oscillator and optical signals are combined in a fiber coupler; wherein the coupler is a 3 dB coupler; wherein the coupler is coupled to a double balanced receiver; wherein the position sensor output is fed back to a dither generator to compensate for changes in the response of the nutation device; and the system including a beam steering mirror, wherein the position sensor output is applied to a demodulator to demodulate the receiver output signal, the demodulator output being applied via a loop compensation network to control position of the beam steering mirror.

In another aspect of the invention, a fiber-based receiver unit for free-space optical communication, where a

focusing element focuses a received light beam at a desired focal point, includes an active fiber coupler, an optical fiber coupled to the active fiber coupler, the optical fiber having a terminus approximately near the focal point, the active fiber coupler disposed to radially translate the optical fiber terminus about the focal point, the fiber coupled to a receiver, the receiver coupled to a tracking network, the tracking network including actuation circuitry for assuring that the terminus of the optical fiber be maintained at or very near the focal point.

Various embodiments of this aspect may include any of the following features: wherein the received light beam is directed to the focal point via a beam steering mirror, the active fiber coupler is a nutation device and includes at least one nutation position sensor, and the tracking network comprises a loop compensation network for actively controlling the position of the mirror responsive to nutation position signals supplied by the position sensor; wherein the nutation device nutation is actively compensated by a position control network responsive to the nutation position signals; wherein the position control network includes a position sensor amplifier and a dither generator; the receiver unit further including a local oscillator, the output of which is combined with the received light beam in a fiber optic coupler, the output of the latter being coupled to the receiver, the receiver being double balanced; wherein the fiber optic coupler is a 3dB coupler; wherein the fiber and coupler are polarization preserving; wherein the output of the receiver and of the position sensor are coupled to a demodulator, the receiver so coupled via a bandpass filter and a detector for detecting the nutation signal imposed upon the received light beam.

In another aspect of the invention, a nutation device includes a housing to which is affixed a flexure, a fiber extending along the flexure and terminating about one end of the flexure, an actuator coupled between the flexure and the housing, the flexure coupled to a detector to cooperate with the actuator for driving the flexure at its mechanical resonance; the nutation device may include at least one position sensor and a dither generator, . hereby, changes. in the device may be actively corrected so that the nutator will be nutated at a desired dither frequency and amplitude.

Other advantages and features will become apparent from the following description of the preferred embodiment and from the claims.

Description of the Preferred Embodiment

First we briefly describe the drawings. FIG. 1 is a block diagram of a fiber-based coherent optical receiver system in practice of the present invention. FIG. 2a is a side view of one embodiment of an active fiber coupler.

FIG. 2b is a cross-sectional view of the coupler of FIG. 2a taken along line A-A in FIG. 2a.

FIG. 3 is a block diagram of a preferred fiber-based coherent optical receiver system in practice of the present invention.

FIG. 4 is a graphical representation of closed-loop and rejection transfer functions in practice of one embodiment of the present invention.

In a free-space optical system, optical fibers can enable remotely locating the transmitter laser, local oscillator laser (in the case of coherent systems), and the receiver spaced apart from the front-end optics. This yields flexibility in the mechanical, thermal, and electrical design which can enable reduction of size, weight and stability requirements of the optical module and can lead to easier integration and better utilization of the host platform. A key element of a fiber optic optical module is the receiver subsection. This receiver must not only perform the communication or radar signature function, but must also serve as a spatial tracking sensor to the line-of-sight deviations induced by the host platform.

FIG. 1 shows a block diagram of a fiber-based coherent optical receiver 10 in practice of the present invention which can perform both the communication function and tracking function. (Note that for non-coherent systems the same block diagram applies with the exception of the local oscillator 25 laser and optical coupler 24.) The information-carrying light beam (or field) 12 is collected using a telescope and course pointing device, such as a gimballed optical telescope 14, so as to apply light beam 12, via mirror 17 of steering mirror assembly 18, to an optical fiber 22. More particularly, telescope 14 images light beam 12 via relay optics 16 onto the surface of steering mirror 17, which in turn directs light beam 12 via a focusing lens 19 to a focal point 13 (which point is desired to coincide with the exposed end of core 20 of optical fiber 22) .

It is preferable that the free-space coupling end of optical fiber 22 be carried in an active fiber coupler 34 to serve as the tracking error sensor. It is possible that such device could incorporate piezo electric or electrostatic

technology. However, an electro-magnetic actuator is preferred.

For a coherent system, as shown in FIG. 1, light beam 12 is combined with light from a local oscillator laser

5 25 by means of a fiber optic coupler 24, the combined signals are then passed via a receiver (such as a dual balanced receiver) 26 as an electronic signal to a communications package 28 (which conventionally processes the. communication data supplied by signal light beam 12) and also to an

10 acquisition and tracking network 29. Preferably, network 29 is electrically coupled to both the active fiber coupler 34, and to an actuator assembly 30 (which drives beam steering mirror 17) . In such embodiment, the acquisition and tracking network 29 acquires or angularly tracks the received signal

1.5 beam 12 by driving mirror 17 and coupler 34 so as to position light beam 12 upon core 20 of optical fiber 22 at focal point

13 to maximize coupling of the light beam from free-space into the fiber.

As seen in FIGS. 2a, 2b, a preferred mechanically

20 nutated active fiber coupler 34 includes a housing 36 to which are coupled magnetic poles 38, 40. These poles are provided with actuator coils 42, 44, respectively. (It will be understood, however, that the present invention recognizes that nutation can be provided in many forms, such as via

25 moving coils and fixed magnets or moving magnets and fixed coils.) Optical fiber 22 is mounted within a flexure element

50 and preferably terminates at a first end 52 of flexure 50

(which is the location at which light beam 12 is applied from free-space to fiber core 20) . (For coherent systems, the

30 fiber and fiber optical coupler each are preferably single mode and polarization preserving. For non-coherent systems, no such coupler is required and the fiber need not be

polarization preserving.)

Coupler 34 thus may form a part of a tracking error sensor by employing standard nutation techniques. A dither generator imposes a small circular scan on coupler 34 by actuation of flexure 50. (A larger dither amplitude increases tracking performance but decreases communication performance. ) Tracking error signals are extracted by synchronously detecting the amplitude of the nutation signal. The resulting error signals are fed back to correct for tracking errors. Flexure 50 is flexibly coupled to housing 36 by means of flexing joint 51 (although we have recently fabricated flexure 50 as a spring element without a joint 51, with good results) .. Whether with or without joint 51, flexure 50 is preferably configured to develop a desired mechanical response (amplitude and phase) at the desired nutation frequency. For example, the nutator will nutate at a frequency of approximately 10 times the desired beam steering mirror bandwidth. In order to use small, low power mechanical actuators, the mechanical resonance of the flexure can be designed to provide a mechanical amplification at the desired nutation frequency. This mechanical resonance feature is optional, although favorably incorporated into the preferred embodiment described below.

FIG. 2a shows nutation device 34 having two opposed positional sensors 66, 68. These sensors are optional, although they are favorably incorporated in the preferred embodiment described below.

Turning now to FIG. 3, a preferred embodiment 55 of a fiber-based free-space optical system includes telescope 14 and relay optics 16, which apply light beam 12 to mirror 17 of beam steering mirror 18, which in turn focuses the light beam at focal point 13 near the end of core 20 of optical

fiber 22 via focusing lens 19. The optical fiber is presented to the light beam by means of nutation device 34.

In the preferred embodiment, for a coherent system, a single-mode, polarization-preserving fiber 22 is used to carry light beam 12 to a single-mode polarization-preserving coupler 70. The other input to coupler 70 (preferably at 3 dB coupler) contains light from a local oscillator laser 72, which is provided via a single-mode, polarization-preserving fiber optic cable 23. For non-coherent systems, no coupler or local oscillator is required and the fiber need not be polarization preserving. Optionally, a Faraday isolator 74 (and collimating and focusing lenses 73, 75) may be used to minimize feedback to the local oscillator laser, since isolation is critical to system performance. Laser 72 may be, for example, a GaAlAs diode laser, having an output power of 30 mW, operated at a wavelength of 0.86 micrometers.

The local oscillator field and the applied light field are combined in coupler 70. The combined signal is carried by means of fiber outputs 76, 78 to a receiver (preferably a double balanced receiver) 80. This receiver, in turn, is electrically coupled to a communications package 81 and acquisition and tracking network 82.

The output signal 84 of receiver 80 is applied to demodulator 90 of network 82 through a noise -limiting bandpass filter 86 and detector 88. The electrical signal output from detector 88 contains information relating to the dither signal imposed (via nutation device 34) onto the received optical power in fiber 22 and is applied to demodulator 90. Also, position sensor amplifier 92 couples information from nutation position sensors 66, 68 to demodulator 90.

Nutation driver circuit 100 includes a dither generator 102 and a power amplifier 104, which, in a

simplified embodiment of the present invention, can apply a constant amplitude signal (such as a sine wave) to the nutation driver circuit (magnets 38,40 and respective coils 42,44). A constant sinusoidal signal causes the nutator to oscillate in a circular manner at a desired frequency (such as 10 kHz) , without any direct error compensation. In the preferred embodiment, the nutation driver circuit 100 compensates for any drift in the characteristics of coupler 34 to ensure that fiber core 20 oscillates in a circular manner at the desired frequency (which in the case of the resonant fiber nutator is at or near the mechanical resonance) . Preferably, information from position sensor amplifier 92 is applied to dither generator 102. Dither generator 102 in turn responsively corrects for drift by varying its output in order to drive fiber core 20 in a circular scan at the desired nutation frequency as best as possible dead-center at focal point 13, while at the same time the position sensor amplifier 92 output applied to demodulator 90 will apply tracking errors via loop compensator 94 and power amp 96 to actuator 30 of beam steering mirror assembly 18 to reposition mirror 17, so as to apply light beam 12 as best as possible dead-center to core 20 at focal point 13.

In one embodiment, demodulator 90 continually correlates the output from detector 88 with the position of fiber core 20. If the output of detector 88 is unchanged over one or more nutation cycles, then no repositioning signal need be applied to the steering mirror actuator 30 to reposition mirror 17. However, if the output of detector 88 is changing over one or more nutation cycles, then the demodulated tracking error signal is applied to drive actuator 30 by means of loop compensation and power amplifier circuits 94, 96. That is, demodulator 90 can favorably correlate the position

of fiber core 20 with received power to extract tracking information, so as to provide error compensation to steering mirror assembly 18 to better position focal point 13 at the exposed end of core 20. (While FIG. 3 is one dimensional, it will be appreciated that the tracking and correlation process can include both azimuth and elevation.)

While actuation of the steering mirror assembly can be eliminated by feeding back position signals directly to the active fiber coupler 34 only, there are two reasons for including such actuation. First, its use can increase dynamic range. Second, the point-ahead requirement for free-space communication systems is more easily implemented by combining the transmit and receive beams (with the appropriate angular offset) after the steering mirror has tracked out platform disturbances. In order to support the tracking loop bandwidths necessary for free-space optical communication systems (such as open-loop cross-over frequencies of 100 Hz to 1 kHz) dithering at high frequency (such as at approximately 1 kHz to 10 kHz) is required. FIG. 4 shows the measured closed-loop and rejection transfer functions that were obtained in one embodiment by feeding back the error signal output of demodulator 90 to a typical, two-dimensional fast steering mirror assembly 18.

(These measurements apply for both azimuth and elevation axes.) This design included a 9 kHz dither frequency, 500 Hz open-loop cross-over frequency, and 43* of open-loop phase margin. The resulting -3 dB closed-loop bandwidth is approximately 1.1 kHz.

It will now therefore be understood that the present invention makes it possible to accurately position the focal point 13 of the received field 12 at the exposed end of the fiber core 20 of fiber 22, so as to optimally transfer light

beam 12 from free-space into the fiber. This is achieved by means of, in one embodiment, steering the light beam by actuation of the beam steering mirror assembly 18 and generating an error signal (such as by dithering the beam steering mirror or by means of separate optical detectors, e.g., quadrant detectors). In another embodiment, improved coupling may be achieved by use of an active fiber coupler (such as a dithered nutation device) 34, which can be responsively driven to assure accurate coupling of beam 12 into fiber 22 at focal point 13. In a preferred embodiment, nutation device 34 and steering mirror assembly 18 are both responsively controlled to assure accurate coupling of beam 12 into fiber 22 at focal point 13, using a dithering technique for generation of error signals. Other embodiments are within the following claims.