JP3093338B2 - Fiber optic link - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to improvements in fiber optic links.

[0002]

BACKGROUND OF THE INVENTION In very short-range communications, conventional microwave signal transmission has been achieved via coaxial or waveguide transmission media. For longer distance communications, fiber optic links have replaced these transmission media because of their low loss, small size, and the ability to avoid electromagnetic interference at microwave frequencies. However, conventional fiber optic links are limited to the bandwidth of optical devices (eg, lasers and photoelectric detectors).

[0003] A typical fiber optic link configuration consists of a series of components: an optical transmitter, an optical fiber, and an optical receiver. For transmitting microwave signals, the optical transmitter uses either an injection current modulated laser or an external modulator that modulates the steady state light output of the laser using electro-optic effects. size,
It is preferable to use an injection current modulated laser in terms of weight and effect. An optical fiber is a transmission medium that guides an optical signal from an optical transmitter to an optical receiver. The optical receiver includes a pin photodiode, an avalanche photodiode, or an optical FET, which is a non-linear element, as a photoelectric detector. Photoelectric detectors typically convert an optical signal into an electrical signal by a process called direct luminance detection.

[0004] Modulated signals often consist of digital or analog data signals with a bandwidth of up to 500 MHz and analog carrier signals, usually microwaves in the quasi-millimeter wave frequency range. The data signal is usually frequency-converted by a carrier, and the up-converted data signal is applied to the laser directly or via an external modulator. Another method is to transmit a data signal and a carrier signal via two fiber optic links. After the two optical signals have been detected by the respective photoelectric detectors, the resulting two electrical signals are amplified and then the data signal is up / down converted to a microwave or quasi-millimeter wave frequency.

[0005]

Microwave synthesis of a data signal and a carrier signal as electric signals before modulating the electric signal into an optical signal reduces performance characteristics of the entire circuit such as gain and dynamic range. Has been previously recognized. For this purpose, it has already been proposed to separate and transmit these signals via two fiber optic links and then to frequency-convert the signals detected by the photoelectric detectors in a frequency converter. However, this system is not only expensive, but also requires the addition of microwave components to achieve the same end result.

It has already been proposed to extend the bandwidth of microwave optical fiber links by laser harmonic generation and photodiode synthesis. Methods using laser harmonic generation degrade system performance due to laser non-linearities. Further, a method using photodiode combining, although feasible, requires either a local oscillator or an additional reference carrier fiber optic link in the optical receiver. In this way, the prior art cannot provide the required high bandwidth fiber optic link without increasing cost or increasing the size of the fiber optic link.

Each of the above fiber optic links is based on a transmit / receive (T / R) module. These T / R modules have been a key component of many communication systems, including satellite communication platforms, antenna remote systems, and distribution networks for personal communications.

[0008]

SUMMARY OF THE INVENTION In an optical fiber link according to the present invention, a transmitted carrier signal is detected and amplified, and the amplified signal is re-injected into a photoelectric detector. The photoelectric detector combines the re-injected carrier and the data signal to be detected by the frequency conversion action of the nonlinear circuit element to perform up / down conversion of the data signal. Thus, only one photodetector detects both the data signal and the carrier signal and performs signal up / down conversion. Further, the photoelectric detector can be used as a fundamental or subharmonic frequency converter by installing an appropriate filter in the optical receiver circuit.

[0009]

FIG. 1 shows a system configuration of a basic optical fiber link including an optical receiver according to the present invention. The left part of the optical fiber 3 is each component of the optical transmitter T, and the right part is each component of the optical receiver R (hereinafter, FIGS. 2 to 5).
The same). The data signal 6 and the reference carrier signal 7 are first combined in the microwave power combiner 11 to become an electric combined signal. The electric composite signal is subjected to electric-optical conversion by the optical modulator 1 to become an optical signal. The generated optical signal is transmitted, propagates through the optical fiber 3, is detected by the photoelectric detector 2 in the optical receiver, and is demodulated into an electric composite signal. After that, the demodulated electric composite signal is reflected by the band-pass filter 5 whose pass band is set according to the frequency of the reference carrier, so that only the reference carrier signal is filtered. Thereafter, the reference carrier signal passes through a microwave circulator 8 and is amplified through a block 9. Here, the block 9 may be a microwave amplifier or an injection lock oscillator. Both of these devices output their own carrier frequency at high power levels. If an injection-locked oscillator is used, block 9 is operated in either the fundamental or subharmonic mode. From the above, this block 9 can be called an amplifier. Thereafter, the amplified reference carrier signal passes through the microwave circulator 8 again and is re-injected into the photoelectric detector 2. In the photoelectric detector 2, the photoelectric detector functions as a frequency converter due to the non-linear response of the photoelectric detector at the high power level of the re-injected amplified reference carrier signal, and the detected data signal is the amplified reference carrier signal. Is up-converted in the photoelectric detector (see FIG. 5). The up-converted data signal is transmitted to the output port 10 through the band-pass filter 4 that allows only the up-converted data signal to pass. FIG. 5 shows how the data signal, the reference carrier, the amplified reference carrier, and the upconverted data signal propagate in the photoelectric converter. As will be appreciated, the bandpass filter 4 must filter only the data signal upconverted from the detected data signal, the detected reference carrier signal, and the upconverted data signal. Furthermore, since the bandpass filter 4 can be designed to pass either the fundamental wave or the harmonic, the photoelectric detector can be used in both the fundamental mode and the subharmonic mode. This sub-harmonic mode can extend the bandwidth of the fiber optic link. The process of down-conversion can also be performed on the fiber optic link of FIG. The data signal 6 is transmitted to the microwave power combiner 1
The rest of the circuit functions as described above, except that the upconverted data signal is applied to input port 6 instead of supplying to 1.

The embodiment shown in FIG.
1 to the output port 10 are the same as in FIG. The basic difference from FIG. 1 is that it comprises two independent optical modulators 1 and 1 ′, to each of which a data signal 6 and a reference carrier signal 7 are applied. These two optical signals are combined in the optical power combiner 12. Thus, the embodiment of FIG. 2 combines the two microwave signals and reduces the effects of internal modulation involving one optical modulator of the fiber optic link of FIG. 1 transmitted to one optical modulator 1. be able to. The down conversion process is also performed on the optical fiber link of FIG. The data signal 6 is transmitted to the first optical modulator 1
Instead of supplying the upconverted data signal to this input port 6, the rest of the circuit functions as before.

The embodiment shown in FIG. 3 is the same as the optical fiber link of FIG. 1 except that an optical amplifier 13 which is a fiber amplifier or a semiconductor amplifier is inserted between the optical fiber 3 and the photoelectric detector 2. . As a modification of this embodiment, the optical amplifier 13 may be inserted at a point A between the optical modulator 1 and the optical fiber 3. In any case, the total amount of gain required for the microwave amplifier 9 that performs low noise amplification in the optical region can be reduced. These embodiments have advantageous performance such as higher system gain, higher frequency conversion gain, and lower system noise values. The processing in the down-conversion is also shown in the fiber optic link of FIG. The rest of the circuit functions as described above, except that instead of supplying the data signal 6 to the microwave power combiner 11, the up-converted data signal is applied to the input port 6.

The embodiment shown in FIG. 4 is the same as the optical fiber link of FIG. 2 except that an optical amplifier 13 is inserted between the optical fiber 3 and the photoelectric detector 2 in the optical receiver R. As a modification of this embodiment, an optical amplifier 13 is inserted at points B and B 'between the two optical modulators 1 and 1' and the optical power combiner 12, and as another modification, an optical amplifier is 13 may be inserted at a point C between the optical power combiner 12 and the optical fiber 3. In either case, low noise amplification is generated in the optical domain, which can reduce the gain required of the microwave amplifier 9.
Furthermore, the integrity of the two signals is ensured by using two optical modulators. These embodiments provide the best performance of all of the above fiber optic links.
The processing in the down-conversion is also executed in the optical fiber link of FIG. The rest of the circuit functions as described above, except that instead of supplying the data signal 6 to the first optical modulator 1, the up-converted data signal is applied to the input port 6.

[0013]

As is apparent from the above description, the optical fiber link according to the present invention can extend the bandwidth of the optical fiber link as compared with the conventional optical fiber link configuration, and can use only a single optical fiber. And the number of optical and electrical devices can be reduced, and the simple configuration can improve the performance of the fiber optic link, resulting in an economical transmission /
Receiver equipment can be designed.

[Brief description of the drawings]

FIG. 1 is an embodiment of an optical fiber link including an optical transmitter that combines a data signal and a reference carrier signal in a microwave power combiner and performs optical modulation in an optical modulator, and an optical receiver.

FIG. 2 illustrates an optical fiber link including an optical transmitter that optically modulates a data signal and a reference carrier signal using independent optical modulators, and then combines the optical signals with an optical power combiner, and an optical receiver. This is an example.

FIG. 3 is an embodiment of the embodiment of FIG. 1 including amplifying the optical signal by an optical amplifier before photoelectric detection for receiving the optical signal.

FIG. 4 is an embodiment of the embodiment of FIG. 2 including amplifying the optical signal by an optical amplifier before photoelectric detection for receiving the optical signal.

FIG. 5 is a diagram showing a propagation state of each signal in a photoelectric detector.

[Explanation of symbols]

T: each component of the optical transmitter R: each component of the optical receiver 1, 1 '... an optical modulator for transmitting an electric signal as an optical signal 2 .... a photoelectric detector 3 .... an optical fiber 4 .... Band-pass filter (center frequency is n * reference carrier frequency + data signal frequency or n * reference carrier frequency-data signal frequency. [N is an integer] .5 ... Single frequency band-pass filter (center frequency is
Reference carrier frequency) 6 Data signal (low frequency for up-conversion) 7 Reference carrier signal (microwave frequency) 8 Microwave circulator 9 Microwave amplifier 10 Output of up / down-converted data signal 11 ... Microwave power combiner 12 ... Optical power combiner 13 ... Optical amplifier

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-2-226921 (JP, A) JP-A-5-22228 (JP, A) JP-A-49-65116 (JP, A) JP-A-2- 53306 (JP, A) JP-A-62-122344 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 H01L 31/00-31/10

Claims (5)

(57) [Claims] 1. An optical transmitter for transmitting a single optical signal by optically modulating a data signal and a carrier signal separately or after combining them, and an optical fiber for transmitting an optical signal from the optical transmitter. In an optical fiber link including an optical receiver that receives the optical signal transmitted by the optical fiber and demodulates the optical signal into an electric signal, the optical receiver includes a photoelectric detector that demodulates the transmitted optical signal into an electric signal. Filter, means for filtering the carrier signal from the demodulated electrical signal, means for amplifying the carrier signal filtered by the filtering means, and photodetection of the carrier signal amplified by the amplifying means. Means for re-injecting the data signal with the re-injected amplified carrier signal, wherein the photoelectric detector has a non-linear response. An optical fiber link characterized by being inverted. 2. An optical transmitter for optically modulating and transmitting a single optical signal after individually or combining a data signal and a carrier signal, an optical fiber for transmitting an optical signal from the optical transmitter, An optical fiber link comprising an optical receiver that receives the optical signal transmitted by an optical fiber and demodulates the signal into an electric signal, wherein the optical transmitter electrically combines a data signal and a carrier signal; and Means for modulating the combined data signal and carrier signal into an optical signal, the optical receiver comprising: a photoelectric detector for demodulating the transmitted optical signal into an electric signal; and a carrier detector for demodulating the carrier signal from the demodulated electric signal. Means for filtering the signal, means for amplifying the carrier signal filtered by the filtering means, and reinjection of the carrier signal amplified by the amplifying means into the photoelectric detector. The optical detector has nonlinear response, and up / down-converts the data signal with the re-injected amplified carrier signal. . 3. The optical fiber link of claim 2, wherein said optical transmitter further comprises means for optically amplifying said optical signal before transmitting said optical signal modulated by said modulating means. Alternatively, the optical fiber link comprising means for optically amplifying the optical signal before the optical receiver demodulates the optical signal by the photoelectric detector. 4. An optical transmitter for separately or combining a data signal and a carrier signal and then optically modulating and transmitting as a single optical signal; an optical fiber for transmitting an optical signal from the optical transmitter; An optical fiber link comprising an optical receiver for receiving the optical signal transmitted by an optical fiber and demodulating the optical signal into an electric signal, wherein the optical transmitter independently modulates a data signal and a carrier signal into an optical signal. Means for optically combining respective optical signals modulated by the modulating means, wherein the optical receiver comprises: a photoelectric detector for demodulating a transmitted optical signal into an electric signal; and Means for filtering the carrier signal from the electric signal, means for amplifying the carrier signal filtered by the filtering means, and carrier amplified by the amplifying means Means for reinjecting a signal into said photodetector, said photodetector having a non-linear response and up / down converting said data signal with said reinjected carrier signal. An optical fiber link characterized in that: 5. The optical fiber link according to claim 4, wherein said optical transmitter further comprises means for optically amplifying said optical signal between said independent modulating means and said combining means. Before the optical signal synthesized by the synthesizing means is transmitted, the optical signal includes means for optically amplifying the optical signal, or the optical transmitter demodulates the optical signal by the photoelectric detector. A fiber optic link comprising means for optically amplifying said optical signal.