Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-09-04
2001-07-10
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140AG
Reexamination Certificate
active
06259086
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for the detection of a light signal by means of a plurality of detection means which are spatially delimited from each other, by means of bringing together a sum signal, wherein narrow-band alignment signals are formed by means of multiplication or phase-sensitive rectification of differential signals formed from a plurality of detectors with the sum signal. The invention further relates to devices usable for executing the method.
BACKGROUND OF THE INVENTION
In the near future, optical free space communication between satellites as well as between a satellite and a ground station will become an important and, on board of the satellites a weight-saving, complement to the existing microwave technology. So-called optical terminals consist of one or several telescopes, which reduce the angular range of the field of vision of an optical receiver in the direction toward the counter-station and also see to a directional dissemination of the signals to be transmitted. Furthermore, several movable mirrors are provided, by means of which the alignment of the transmitting and receiving directions is performed. Besides the direct detection of the optical output of the transmitter of the counter-station as the transmission method, the coherent superimposition of the received light with the light of the same frequency of a local oscillator laser plays an important role, since in the process the indifference to interferences by radiation in the background is advantageous, besides a great sensitivity to the signal to be detected.
The output of the light signal to be detected is in general very low with the above mentioned systems, accordingly it is intended to use the largest possible portion of the light signal in a receiver for detecting the transmitted data. For example, because of the narrowband character of an error signal from the alignment device of the optical portion of the receiver, it would be conceivable to conduct a very small portion of the light entering the receiver to a field of detector means, which are delimited in respect to each other and are arranged in a plane, in order to obtain a directional error signal by means of the detection of the respectively illuminated detector means. However, such a method is only used for the rough alignment during the establishment of the connection. Since the light sources used for data transmission operate at a considerably lower optical output than special optical beacon transmitters used for establishing the connection, a coherent detection method is needed, which requires additional light output from a laser oscillator provided for the superimposition of the received light signal, as well as additional electronic means.
OBJECT AND SUMMARY OF THE INVENTION
It is accordingly the object of the invention hereinafter described to overcome the disadvantages of the prior art and to assure by means of a combined system the detection and demodulation of the information signal, along with the simultaneous derivation of a directional error signal, while making the best possible use of the output of the available light signal.
The present invention consists of detector means and several electronic modules arranged downstream thereof. Although only a single detection means is required for detecting of a, for example, phase-modulated light signal, into which both the light signal as well as an unmodulated light wave, which has the same mid-frequency or is relatively close to the mid-frequency of the light wave, are conducted, several detector means, which are delimited in respect to each other and are laterally offset, are used by the system in accordance with the invention. All detector means are illuminated in the same way by the locally generated, unmodulated light wave. The illumination by means of the light signal only takes place evenly as a function of the error in alignment of the optical receiving device in case of the disappearance, but in general is irregular. A mixing process takes place in each detector means by the squared conversion of the total amplitude of the light field into electrical current, from which a photo-flow with a mid-frequency arises, whose value corresponds to the difference between the optical mid-frequencies of the light signal and of the unmodulated light. A d.c. current is generated from this, which is proportionally added from the mean optical output of the light signals and of the unmodulated light. Since the optical output of the light signals falls below that of the unmodulated light by orders of magnitude, it is generally very difficult to generate a signal containing the alignment error from the d.c. current of the detector means. The signal current generated by multiplicative mixing is a proportional function of the output of the light signal impinging on the respective detection means, wherein the proportionality factor is correspondingly high because of the relatively high output of the unmodulated light.
The method in accordance with the invention contains the derivation of a signal corresponding to the alignment error from the signal flows of all detection means. By multiplying all signal flows with themselves, i.e. their squaring, it would be possible to generate a corresponding d.c. current from the individual signal flows. However, in this case there is no proportionality between the d.c. current and the optical output of the light signal in the individual detector means, and the sign of the error voltage would be lost. Furthermore, the error voltages would be a function of the strength of the incident light, i.e. of the transmission distance. It is now possible to remedy these two deficiencies by means of the invention, as will be described hereinafter.
The sum of all detector signals is formed, and their amplification is regulated by means of an amplifier, which can be adjusted in respect to an amplification factor in such a way that a signal level is created at the output, which is independent of the strength of all of the incident light.
This amplification regulation (AGC—automatic gain control) takes place, for example, on the basis of the comparison of the sum output signal with a reference variable. By the application of the same amplification factor, which is a function of the size of the sum signal, to the two difference signals x and y, the characteristic of the error signals (as a function of the amount of deviation of the light beam from the specified position) becomes independent of distance. It is therefore essential that the amplification regulation characteristics of the difference channels and the sum channel are matched to each other, which requires appropriate measures in accordance with switching technology. The actual formation of the narrow-band error signals now takes place by multiplying the AGC—regulated difference signals with the sum signal of a constant size. However, the so-called phase-sensitive rectification of the difference signals with the aid of the sum signal as the reference clock (so-called balanced modulation) offers itself as a more robust variant. Both alternatives basically realize the same function of the sign-correct demodulation of the error signal.
A large advantage results in that, besides the proportionality of the alignment error-dependent variable of the individual detector means obtained by this, its detection also is assured even with only very weak illumination of an individual detector means by the light signal. A further advantage resides in that because of the derivation of narrow-band error signals from the actual information signal, the highly sensitive coherent detection technique used in connection with the latter is also employed here, wherein furthermore no portion of the light signal is removed from conveying useful data. The multiple use of the electronic devices used for signal processing should be stressed, wherein the total dependability of the system is increased, in particular when used in satellite systems.
Further details, characteristics and advantages of the invention en
Deflorin Urs
Guggenbuehl Walter
Wandernoth Bernhard
Chambers Guy W.
Contraves Space AG
Lee John R.
Townsend and Townsend / and Crew LLP
LandOfFree
Method and device for receiving a phase-modulated light... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for receiving a phase-modulated light..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for receiving a phase-modulated light... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2452099