Amplification of interleaved optical signals

ES 2 397 673 T3

DESCRIPTION

Amplification of interleaved optical signals

Background

Free space optical communication systems are capable of transmitting high data rate data over long distances. Acquisition schemes and precise beam tracking and targeting capabilities are required for communication between mobile platforms (eg, air, space, and ground vehicles). Particularly with aerial platforms, where aircraft movement can be rapid and unpredictable, it is critical that the point and track scheme provide adequate guidance for directing the data laser beams.

Consider a scenario in which two optical communication terminals, whose relative positions may change, are coupled in two-way communication (eg, either one or both terminals are mobile). At each terminal, one of the options for determining the angular direction of the other terminal (i.e., the far end terminal) is the division of a portion of the data signal (e.g., a laser beam) received from the terminal. far end and determine the angle of arrival of the split data signal. This approach has a number of disadvantages. The power of the received signal must be divided between two detectors, one to detect the aiming angle and the other to receive the data. By using a part of the received data signal for angular position detection, only the remaining part of the received data signal is available for data reception, thereby reducing the signal strength in the receiver and reducing the maximum operating range of the system. On the other hand, it is desirable to minimize the beamwidth of the data signal in order to maximize the signal strength and operating range. Given the limited angular range of the data signal, initial acquisition of a remote terminal is difficult with the data signal. Similarly, once a communication link has been established between the terminals, it can be difficult for the terminals to continuously track each other using narrow data signal lasers, as each terminal can get out of the beam quite quickly. when the relative angular direction of the terminals is changing rapidly.

Another option for determining the angular direction of a remote end terminal is to separately generate and transmit both the data signal and a beacon signal. The beacon signal may have a wider beam width, which is more suitable for acquisition and tracking. However, if two separate signals are created, the size, weight, and power of the system typically double. Additionally, the two signals must be combined together with high power, which is typically done in free space using expensive optics that require careful alignment.

Patent document EP 0 653 852 A1 describes a free space optical communication system in which a pilot signal and a main signal can be superimposed one on top of the other by wavelength multiplexing, time division multiplexing or wavelength multiplexing. code.

Consequently, there remains a need for a system capable of generating two optical signals, such as data and beacon signals in an optical communication system, without significantly increasing the size, weight and power of the system relative to a system of individual signals and without diminishing the signal strength.

Summary

One technique for generating a data signal and a beacon signal for free space optical communications involves generating a data signal having a first optical wavelength and a beacon signal having a second optical wavelength. The data signal is encoded with the data by modulation with a first modulation rate. The beacon signal is an inverted version of the data signal and can be further modulated with a second modulation rate that is lower than the first modulation rate. The data and beacon signals are optically combined to produce a combined signal in which the power attributable to the beacon signal is interleaved and substantially does not overlap in time with the power attributable to the data signal. The combined signal is amplified through a fiber amplifier, and the combined signal is supplied to the transmitter optics to transmit the data signal and the beacon signal to free space.

The interleaving technique of the invention allows both the data signal and the beacon signal to be amplified using a single amplifier, such as an erbium-doped fiber amplifier, while allowing both signals to be amplified to the full range of the amplification available from the amplifier. Due to the generation of a clear beacon signal for acquisition and tracking, it is not necessary to use a part of the data signal at the far end receiver for this purpose, so there is no decrease in signal strength. available for data signal detection at a far end receiver. However, the beacon signal is generated without significantly increasing the size, weight, power, and cost requirements of an optical transmitter system. Modulating the beacon signal with a lower modulation rate by, for example, on-off keying simplifies the detection of the beacon signal at the far-end receiver.

ES 2 397 673 T3

The invention proposes a method for generating a data signal and a beacon signal for optical communications in free space, the method comprises generating a data signal having a first optical wavelength, the data signal is modulated with a first modulation rate to encode the data; generating a beacon signal having a second optical wavelength, the beacon signal comprises an inverted version of the data signal; optically combining the data and beacon signals to produce a combined signal in which the power attributable to the beacon signal is interleaved and substantially does not overlap in time with the power attributable to the data signal; amplify the combined signal; and transmitting the data signal and the beacon signal to free space. Advantageous features are mentioned below. The beacon signal can be further modulated with a second modulation rate that is lower than the first modulation rate. The first modulation can be on-off switch modulation (OOK) with a rate of at least one megahertz, and where the second modulation is OOK modulation with a rate less than one megahertz. The second modulation can have a duty cycle of approximately 50%. Additionally, the combined signal can be amplified by an erbium-doped fiber amplifier (EDFA). In a preferred step the beacon signal comprises: supplying the data signal in electrical form to a beacon signal path; inverting the data signal on the beacon signal path; modulating the data signal in the beacon signal path with a second modulation rate that is less than the first modulation rate; and converting the data signal in the beacon signal path to an optical signal with the second optical wavelength, wherein inversion, modulation and conversion of the data signal along the beacon signal path produces the signal beacon with the second optical wavelength. The conversion of the data signal to an optical signal can be performed downstream of at least one of the inversion of the data signal and the modulation of the data signal in the beacon signal path. The conversion of the data signal to an optical signal can be performed upstream of at least one of the inversion of the data signal and the modulation of the data signal in the beacon signal path. The inversion of the data signal can be done downstream of the conversion of the data signal to the optical signal. In a preferred step the data signal comprises: supplying the data signal in electrical form to a data signal path; and converting the data signal on the data signal path into the data signal with the first optical wavelength. An alternative method of generating first and second optical signals comprising: supplying a common signal to the first and second signal paths, the common signal being modulated with a first modulation rate to encode the information; generating, from the common signal in the first signal path, a first optical signal having a first optical wavelength; generating, from the common signal in the second signal path, a second optical signal having a second optical wavelength, the second optical signal comprises an inverted version of the common signal and is further modulated with a second rate of modulation that is less than the first modulation rate; optically combining the first and second optical signals to produce a combined signal in which the power attributable to the first optical signal is interleaved and substantially does not overlap in time with the power attributable to the second optical signal; amplify the combined signal; and supplying the combined signal to the transmitting optics for transmission of the first and second optical signals.

The invention proposes an apparatus for generating a data signal and a beacon signal for optical communications in free space, comprising; a data signal generator configured to generate a data signal having a first optical wavelength, the data signal being modulated with a first modulation rate to encode the data; a beacon signal generator configured to generate a beacon signal having a second optical wavelength, the beacon signal comprising an inverted version of the data signal; an optical combiner configured to optically combine the data and beacon signals to produce a combined signal in which the power attributable to the beacon signal is interleaved and substantially does not overlap in time with the power attributable to the data signal; a fiber amplifier configured to amplify the combined signal; and transmitter optics for transmitting the data signal and the beacon signal to free space. Preferably the beacon signal generator may be configured to further modulate the beacon signal with a second modulation rate that is lower than the first modulation rate. The first modulation can be on-off switching (OOK) modulation with a rate of at least one megahertz, and where the second modulation is OOK modulation with a rate less than one megahertz. The beacon signal generator preferably comprises: an inverter, a modulator, and a laser module arranged along a beacon signal path configured to receive the data signal in electrical form at an input end and supply the signal signal. beacon to the optical combiner at one end of the output, wherein: the inverter is configured to invert the data signal over the beacon signal path; the modulator is configured to modulate the data signal on the beacon signal path with the second modulation rate; and the laser module is configured to convert the data signal on the beacon signal path to an optical signal with the second optical wavelength. Furthermore the modulator may comprise a switch configured to apply on-off modulation to the data signal. The laser module can be arranged downstream of at least one of the inverter and the modulator along the beacon signal path. The laser module can be arranged upstream of at least one of the inverter and the modulator along the beacon signal path. The inverter can be arranged downstream of the laser module. Furthermore, the data signal generator comprises a laser module configured to convert the data signal in electrical form into the data signal with the first optical wavelength. The laser module can be a tunable laser seed module. The fiber amplifier may comprise an erbium doped fiber amplifier. An alternative apparatus for generating first and second optical signals comprises: means for

ES 2 397 673 T3 supplying a common signal to the first and second signal paths, the common signal is modulated with a first modulation rate to encode the information; means for generating, from the common signal in the first signal path, a first optical signal having a first optical wavelength; means for generating, from the common signal in the second signal path, a second optical signal having a second optical wavelength, the second optical signal comprises an inverted version of the common signal and is further modulated with a second modulation rate that is lower than the first modulation rate; means for optically combining the first and second optical signals to produce a combined signal in which the power attributable to the first optical signal is interleaved and substantially does not overlap in time with the power attributable to the second optical signal; means for amplifying the combined signal; and means for transmitting the first and second optical signals.

The foregoing and even additional features and advantages of the present invention will become more apparent upon consideration of the following definitions, descriptions, and descriptive figures of specific embodiments thereof, wherein similar reference numerals throughout the various figures are used to designate similar components. While these descriptions go into specific details of the invention, it should be understood that variations may exist and will be apparent to those skilled in the art based on the descriptions herein.

Brief description of the drawings

Fig. 1 is a top-level block diagram of an example transmitter system illustrating the concepts of the invention.

Fig. 2 is a block diagram illustrating an implementation of the transmitter system shown in Fig. 1.

Fig. 3 is a signal time sequence diagram showing segments of the data and beacon signals generated by a transmitter system in accordance with one embodiment of the invention.

Fig. 4 is a diagram conceptually illustrating the combination of the data and beacon signals in an interleaved manner without overlapping.

Fig. 5 is a functional flow diagram illustrating the operations that are performed to generate the data and beacon signals in accordance with an embodiment of the invention.

Fig. 6 is a block diagram illustrating an implementation of the transmitter system shown in Fig. 1.

Fig. 7 is a block diagram illustrating an implementation of the transmitter system shown in Fig. 1.

Detailed description

This specification describes a beacon and data signal creation technique for free space optical communication in which only one amplifier, such as an erbium-doped fiber amplifier, is needed to amplify both signals prior to transmission. By generating the beacon signal using the data signal shape, the beacon signal power is interleaved and does not overlap in time with the data signal power when the data and beacon signals are optically combined . The two signals can be combined with low power and then amplified together with a common amplifier. In this way, the total maximum power of the amplifier in the data signal is obtained, and the beacon signal is able to use the amplifier during the periods when it is not needed for the data signal to create a maximum full power but a beacon signal of half half power. As a result, a beacon signal can be transmitted without reducing the power available for the data signal and without significantly increasing the size, weight, and cost of the transmitter system.

FIG. 1 is a top-level block diagram of an example transmitter system 100 illustrating the concepts of the invention. A data signal is supplied by a data source 110, such as a modem. The data signal may be an electrical signal encoded with data to be transmitted to a remote end terminal. For example, data source 110 may encode data up to the data signal by on-off switching (OOK) with a first modulation rate. With OOK modulation, the data signal alternates sequentially between a first power level and a second power level which is preferably a very low or no power level, resulting in full power intervals and substantially no power intervals. A logical 0 can be represented by the absence of power in an interval, and a logical 1 can be represented by the presence of energy in an interval, or vice versa. Optionally, a coding scheme can be employed that ensures an on-off duty cycle of approximately 50% (ie, the signal is at full power approximately half the time and no power approximately half the time). To transmit information quickly, the first modulation rate can be at least one megahertz (MHz) and can be many orders of magnitude higher, possibly greater than one or many gigahertz (GHz).

The data signal can be used to transmit virtually any type of information or data, including, but not limited to, the following: sensor data, navigation signals, voice / audio signals,

ES 2 397 673 T3 image, video signals, data relating to an application running on a processor, control signals and communication protocol or header signals (for example, in relation to the communication protocol, handshaking , routing, equipment configuration, etc.). In particular, sensors that collect intelligence, surveillance and reconnaissance information generate a considerable amount of data and can benefit from the high data rates used in optical communications for the transmission of information in a reasonable period of time.

The data signal is supplied to an optical data signal generator 120 on a data signal path and to a beacon optical signal generator 130 on a beacon signal path that is in parallel with the data signal path. The optical data signal generator 120 converts the data signal into an optical signal with a first optical wavelength i, which is supplied as an output to an optical fiber. The optical output data signal retains the data modulation with the first modulation rate contained in the original data signal.

The beacon optical signal generator 130 converts the data signal into a beacon optical signal with a second optical wavelength λ 2 that is different from the first optical wavelength. The beacon signal is an inverted modified version of the data signal. In particular, the beacon signal preserves the data modulation of the data signal, but the beacon signal has power during certain time intervals when the data signal has no power, and the beacon signal has no power during the time intervals in which the data signal has power. In addition, the beacon optical signal generator 130 further modulates the beacon signal with the second modulation rate that is less than the first modulation rate (eg, at least an order of magnitude lower and perhaps several orders of magnitude lower). of smaller magnitude), as described in more detail below. The beacon optical signal generator 130 supplies the beacon optical signal as an output to an optical fiber. Therefore, the resulting beacon signal is modulated with the first (data) modulation rate, albeit inverted, and with the second (beacon) modulation rate that is less than the data modulation rate.

An optical combiner 140 receives the optical fibers from the optical data signal generator 120 and the beacon optical signal generator 130, which respectively contain the data and beacon signals with optical wavelengths λ 1 and λ2, and combines the signals data and beacon in a combined signal on a common output fiber. Due to the inversion of the beacon signal relative to the data signal, within the combined signal, the power attributable to the beacon signal is interleaved and substantially does not overlap in time with the power attributable to the data signal. The combined signal is supplied to a fiber amplifier 150, which amplifies the combined signal. In this way, a common amplifier amplifies the data signal and the beacon signal without sacrificing full amplification of either signal. The amplified combined signal is then supplied to optical transmitter 160, which transmits the data and beacon signals into free space. The transmitter optics 160 can discriminate between the data and beacon signal based on their different wavelengths in order to process the signals separately. For example, the data and beacon signals can be directed to different paths, and a greater beam divergence can be applied to the beacon signal in order to produce a greater beamwidth.

By way of example, the optical wavelengths used for transmission and reception of data and beacon lasers may be in the eye-safe range of the spectrum (i.e. wavelengths greater than 1.4 microns) , such as the wavelengths of the telecommunications C and L bands or between about 1530 nm and 1600 nm. These wavelengths allow commercially available optical components to be used in the laser transceiver. However, the invention is not limited to a certain range of optical wavelengths. Therefore, as used herein and in the claims, the term "optical" refers generally to the wavelength range of electromagnetic signals within which "optical" equipment (eg, equipment, transmitters, receivers, etc. of optical communications), including the visible spectrum, infrared wavelengths and ultraviolet wavelengths.

Fig. 2 is a block diagram illustrating an implementation of the transmitter system 100 shown in Fig. 1. In this example, the optical data signal generator comprises a laser module 205 such as a tunable laser seed module. , which may be a commercially available Small Form Factor Pluggable (SFP) laser module that provides an interface between a device supplying data (eg Ethernet traffic) and an optical fiber. In this example, the laser module 205 converts the data signal with electrical form into an optical signal with the first wavelength λ ₁.

The beacon optical signal generator includes an inverter 210, a modulator 220, and a laser module 230 arranged along the beacon signal path. The inverter 210 receives the data signal electrically and generates an electrical output signal that is the logical negation of the data signal (that is, the output signal is a logical 1 when the data signal is a logical 0, and the output signal is a logical 0 when the data signal is a logical 1.

Modulator 220 may be a switch that chops the beacon signal by modulating the beacon signal by turning on and off at a relatively low frequency to produce a square wave signal whose modulation frequency can be detected at the remote end, such as is described in more detail at

ES 2 397 673 T3 continued. In effect, the switch applies a particular OOK modulation to the beacon signal, but with a modulation rate significantly less than the data modulation rate (for example, at least an order of magnitude lower and perhaps several orders of magnitude lower ). By way of example, the on-off modulation rate (ie, the aforementioned second or beacon modulation rate) of the beacon signal may be in the range of 4 to 15 kHz and may be programmable. A 50% on-off duty cycle can be used to produce a balanced square wave. In general, the invention is not limited to any particular chopping frequency or duty cycle; thus, higher or lower frequencies than the interval examples can also be used. However, the data modulation rate must be higher enough than the beacon modulation rate so that the data modulation that remains encoded in the beacon signal is not detectable or seen by the beacon detector at the end. remote. In the context of this example, the beacon modulation rate should be below 1 MHz which is clearly distinguishable from the data modulation rate.

The inverted and modulated (chopped) data signal is then supplied along the beacon signal path to laser module 230 which may be similar to laser module 205 in the data signal path (eg, tunable laser seed module or SFP), which converts the input electrical signal into an optical signal with the second optical wavelength Á2 to produce the beacon optical signal.

Fig. 3 is a time diagram showing representative parts of the data and beacon signals one on top of the other for comparison. The data signal is modulated with a first (data) modulation rate and comprises a logical sequence of ones and zeros that results in a signal that alternates between a first state in which power is present and a second state in which the power is present. that there is practically no power depending on the data values ​​being transmitted. As shown in the left part of Fig. 3, the parts of the beacon signal contain the same data modulation as the data signal, except that the beacon signal comprises inverted data whose logic state is the opposite of that of the data signal, so that the beacon signal contains power during the intervals when the data signal contains no power, and the beacon signal contains no power during the intervals when the data signal contains power. The on-off modulation of the beacon signal results in portions of the beacon signal remaining in a logic zero (no power) state. These parts of the beacon signal do not contain inverted data modulation. It should be noted that the rate of this on-off modulation of the beacon signal is a significantly lower rate than that of data modulation.

Returning to Fig. 2, the optical combiner can be implemented with a fiber combiner 240 that combines the data and beacon signals on the fiber. Fig. 4 illustrates the effect of combining the data and beacon signals. In the combined signal, the parts of the data signal that contain the signal power (shown with oblique crosshatches from left to right upward in Fig. 4) are interspersed with the parts of the beacon signal that contain signal power (shown with oblique crosshatches from left to right down in Fig. 4) in such a way that the power of the two signals does not substantially overlap in the time. It should be noted that the two signals are still distinguishable by virtue of their different wavelengths. On the other hand, during the periods when the beacon signal is off due to on-off modulation, only the data signal is present in the combined signal.

As shown in Fig. 2, the fiber amplifier can be implemented with a single-mode erbium-doped fiber amplifier (EDFA) 250 whose output can be supplied to a collimator 260 that receives the signal. combined at the end of the fiber and supplies a collimated beam of free space to the transmitter optics. The wavelengths of the data and beacon signals A (1, Á2) can be selected to be within the amplification band of the EDFA 250. If, for example, the EDFA 250 has a maximum power of 5 watts, a sequence typical data, with an equal number of logical zeros and ones, will have an average data power of 2.5 watts. By using strips of logical zeroes for transmission of the beacon signal, the beacon signal will not use any of the power of the data signal. Assuming a 50% beacon modulation duty cycle with a kHz level rate, the beacon signal will be produced with an average power of 1.25 watts using the 2.5 watts of "left" power available from the EDFA 250 for half of the periods when the data signal is in a logic zero state. It should be noted that each of the two signals in the combined signal is the output of the EDFA 250 at the maximum power setting of the EDFA 250.

A separate beacon wavelength allows full use of the incoming beacon signal at the far end receiver rather than splitting a fraction of the received data signal and tracking either the DC data signal or a modulation small amplitude and lower frequency at the top of the data signal.

The chopped beacon signal provides a number of advantages. A unique beacon hash frequency (second modulation rate) can be used for each terminal in an optical communication system, such that detection of the hash frequency identifies which terminal is sending the beacon signal. For example, two terminals can establish optical communication with each other and keep track, where one terminal uses a beacon hash frequency of 6,000 Hz and the other terminal uses a beacon hash frequency of 9,000 Hz.

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Another advantage of the hash beacon signal is that each terminal can transmit and receive beacon signals at the same wavelength, since the beacon hash frequency can be used to differentiate one beacon signal from the other. By assigning a different beacon modulation frequency to each of the two transceivers on the link, beacon modulation provides a means to reduce the possibility of self-interference due to reflections from the outgoing transmitting beacon. Furthermore, this greatly simplifies the overall beacon design and system architecture, as neither the beacon laser module nor the receiver need to be tunable or adjusted (the beacon signal can always be detected without the need for tuning) and beacon hardware can be identical on all terminals. For example, in the implementation of Fig. 2, laser module 230 would not have to be tunable. It should be noted that it is much easier from a design and operational point of view to adjust the hash frequency of the beacon signal to provide differentiation between beacon signals than to use different wavelengths as a differentiator. Optionally, of course, both the different hash frequencies and the different optical wavelengths can be used to differentiate the source terminals if necessary.

Another advantage of chopping the beacon signal is that the on-off modulation creates an alternating current square wave signal whose frequency is that of the chopping frequency. Consequently, the laser chopped beacon signal can be AC ​​coupled to a position sensor detector at the remote end terminal. By AC coupling the position sensor detector, continuous signals that create a DC bias do not affect the detection process. Thus, for example, unwanted signals such as stray background light, solar radiation including direct and indirect sunlight, and fluctuating reflection, which tend to be constantly transmitting light, simply cause signal polarization of DC when detected together with the chopped beacon signal and are automatically filtered by AC coupling. Beacon modulation also facilitates accurate measurements under a wide range of environmental conditions as AC coupling within the high-gain signal chain is immune to various DC offsets and offset drifts from op amps and leakage. of the detector. Band-pass filtering of the received beacon signal can also attenuate the effects of electromagnetic interference from power lines that induce 50-400 Hz hum or other noise sources in the environment. Therefore, the chopped beacon laser beam provides additional immunity to these types of interference, and enables optical communication terminals to keep track of far-end terminals when such interference is present.

Additionally, AC coupled position sensing detectors for determining the angle of arrival of laser beams have frequency bandwidths that extend down to a few megahertz. In optical communications, data can be modulated on the laser beam using modulation in the order of gigahertz up to hundreds of gigahertz. This modulation makes the data-bearing lasers virtually invisible to AC coupled position sensing detectors, which currently do not have the bandwidth for gigahertz modulation frequencies. Consequently, the modulation of data remaining in the beacon signal (at the rate of MHz or greater) is too high in frequency to be detected by the beacon detector at the remote end of this scheme. Instead, the far-end beacon receiver electronics only see frequency modulation in kHz.

The example described above involves modulation of the beacon signal with a second modulation rate that is lower than the data modulation rate to provide these advantages over beacon signal detection at the rover. However, the invention is not limited to schemes that require additional modulation of the beacon signal, and the beacon signal can be transmitted without additional modulation (apart from data modulation), and other detection schemes can be used. on the rover to detect the beacon signal. The transmitter optics can apply greater beam divergence to the beacon signal than to the data signal to aid acquisition and tracking using the beacon signal. Fig. 5 is a functional flow chart summarizing the operations performed to generate optical data and beacon signals, as described above in connection with Fig. 1. At step 510, a data signal is generated having a first optical wavelength, where the data signal is modulated with a first (data) modulation rate to encode the data. In step 520, a beacon signal is generated having a second optical wave. The beacon signal is an inverted version of the data signal and is optionally further modulated with a second (beacon) modulation rate that is lower than the first modulation rate. The data and beacon signals are optically combined in step 530 to produce a combined signal in which the power attributable to the beacon signal is interleaved and substantially does not overlap in time with the power attributable to the data signal. The combined signal is then amplified (step 540) and the data signal and beacon signal are transmitted to free space (step 550).

In Fig. 2, the optical beacon signal generator is implemented with an inverter 210, a modulator (switch) 220, and a laser module 230 arranged in sequence along the beacon signal path. It will be appreciated, however, that the invention is not limited to this particular configuration or arrangement. The block diagrams shown in Figs. 6 and 7 illustrate other possible implementations of the transmitter system. For example, in Fig. 6, a laser module 610 is arranged upstream of an inverter 620 and a modulator 630. In this case, the inverter 620 and modulator 630 operate on the optical data signal at a wavelength A 2 instead of on a signal as in the case of the arrangement of Fig. 2. In Fig. 7, an inverter 710 is arranged upstream of a laser module 720, which in turn is upstream of a modulator 710. Here, the inverter 730

ES 2 397 673 T3 operates on an electrical signal, while modulator 730 operates on the optical signal produced by laser module 720.

On the other hand, although the systems shown in Figs. 2, 6 and 7 involve optical signals that are transmitted, combined and amplified through optical fibers and a fiber amplifier, the principles of the invention can be employed in the context of any of a wide variety of mechanisms to transmit, combine and amplify signs. For example, data and beacon signals can be combined and amplified in free space rather than fiber.

Data and beacon signals can be generated by a wide variety of devices, and the invention is not limited to these examples. One or more of the inversion, modulation and optical conversion operations carried out to create the beacon signal can be carried out by a single device or they can be carried out by devices contained in a common integrated circuit. For example, beacon reversal and modulation can be performed by a single chip or device, or the laser module can contain a switching mechanism that performs on-off modulation. Furthermore, the modulator switch and / or the inverter can be implemented in hardware or firmware such as FPGA logic. Regardless of the specific mechanisms used, the creation of the data and beacon signals requires that the signals can be combined in an interleaved manner without the power attributable to the two signals substantially overlapping in time so that the signals can be fully amplified by a common amplifier. This is accomplished in this example because the beacon signal includes the same data modulation pattern as the data signal but with an inverted shape.

In the examples described above, the beacon signal is modulated primarily for the purpose of allowing beacon signal detection (eg, by detecting a square wave at a particular frequency in the kilohertz range). However, it is also possible to modulate the beacon signal with the second modulation rate to further encode the data in the beacon signal. For example, the control information regarding beacon tracking could be encoded into the signal using OOK modulation with the second modulation rate rather than simply turning the beacon signal on and off with a constant frequency.

The transmitter system for generating optical data and beacon signals described herein can be used in an optical communication terminal (eg, laser) designed to operate in a laser communication system with mobile platforms, where the relative positions of the terminals change over time. The system may include, for example, terminals mounted on aerial platforms, satellites, ships, boats or land vehicles, as well as stationary terminals that communicate with terminals mounted on mobile platforms (for example, combinations of air-to-air and air-to-ground links ).

Although the invention has been described in the context of free space optical communications where a beacon signal is used for acquisition and tracking and a separate data signal is used to transmit the data, more generally the concepts of the The invention can be used in an optical system where two optical signals are to be transmitted and one of the signals requires a relatively higher modulation rate. For example, OOK modulation with a first modulation rate can be applied to a common signal to encode data, and the common signal can be supplied to first and second signal paths in the manner described above. A first optical signal having a first optical wavelength can be generated from the signal supplied to the first signal path. A second optical signal having a second optical wavelength can be generated from the common signal in the second signal path, wherein the second optical signal comprises an inverted version of the common signal and is further modulated with a second rate. modulation that is less than the first modulation rate to encode additional data. In other words, the second optical signal does not have to be a beacon signal for acquisition and tracking, but could be another data signal carrying data modulated through the second lower frequency modulation. The two signals can then be optically combined to produce a combined signal in which the power attributable to the first optical signal is interleaved and substantially does not overlap in time with the power attributable to the second optical signal, as described above. The combined signal can then be amplified, thereby amplifying both the first and the second signal. The two signals can then be transmitted. It should be noted that this mechanism is not limited to transmission through free space in a free space optical communication system and can be used in an optical system using any transmission medium.

Having described preferred embodiments of a new and improved technique for amplifying interspersed optical signals, it is believed that other modifications, variations, and changes will be suggested to those skilled in the art in light of the teachings set forth herein. Therefore, it should be understood that all such variations, modifications, and changes are believed to be within the scope of the present invention as defined in the appended claims. While specific terms are used herein, they are used in a generic and descriptive sense and not for limitation purposes only.