JPS6030458B2 - optical transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical transmitter with less distortion in optical power intensity.

Such a device is particularly suitable for analog transmission in the field of optical fiber communications. The development of practical fiber systems is just beginning. Such systems use modulated light sources, primarily semiconductor-type light emitting diodes (LEDs).
is used to guide this optical output to the receiving end via an optical fiber link. The energy of the optical power exiting the optical fiber impinges on a photodetector, typically a reverse biased P-11N semiconductor diode or the like, generating charge carriers within the photodetector; This is amplified to produce a useful output signal. Such a system requires three important elements: an optical power source or transmitter, an optical fiber link, and a photodetector or receiver.

If the transmission mode is analog, its operation is affected to such an extent that it is rendered useless as a practical system due to nonlinear distortions. The problem of nonlinear distortion is, of course, nothing new for optical fiber systems. However, in the system we are considering here, nonlinear distortion is an even greater obstacle than in other systems. This is because the intensity of the light output produced by a light source such as an LDE does not vary with sufficient linearity with respect to the electrical input signal. For this reason, second- and third-order nonlinearities, for example, cause distortions in the transmitted analog video signal, which limits the quality of the transmission. Therefore, transmission over long distances is not possible. Of course, photodetectors are not perfectly linear either, but their effect on signal distortion has been found to be much smaller compared to the distortion caused by LEDs. The present invention is an optical transmitter that uses an LED as an electric-to-optical converter and a photodetector as an optical-to-electrical converter, and has better linearity of optical output with respect to an electric input signal than conventional ones. Provides optical transmitters.

Simply put, the final electrical-to-optical converter in this optical transmitter is modulated or excited by a modified electrical signal rather than by the original electrical input signal. As will be appreciated by those skilled in the art, very small amounts of strain can be utilized.
The electrical-to-optical transfer characteristics of the individual electrical-to-optical converters depend on the similarity. Therefore, an optical transmitter having two parallel signal paths may be used to couple a first component of the electrical input signal to a first of the signal paths and a second component of the input signal to one of the signal paths. a first electrical-to-optical converter optically coupled to an optical-to-electrical converter, the second signal path having a signal splitting means for coupling to a second path of the electrical delay of the first and second
An optical transmitter, characterized in that it has means for generating an instantaneous deviation of the signal at the end of the signal path, and a second electro-optical converter for converting the instantaneous deviation into the form of an optical output. provided. The optical transmitter described above linearizes the optical power of a converter that is not very linear, but it is clear that the more linear the converter used, the more linear the final output will be. be.

Such linearization complicates the system and degrades the signal/noise ratio. However, the latter problem can be ignored if the initial input signal/noise ratio is reasonable. The signal/noise ratio degradation should not exceed much more than 5 dB. Next, embodiments of the present invention will be described with reference to the accompanying drawings.

Referring to FIG. 1, an optical transmitter according to the invention includes a power divider 10 that divides an input signal Sin into two signal components S and S2.

The signal S, is supplied to an optical driver 11 which drives (ie biases and modulates) a light emitting diode LED12. The output light of the light emitting diode 12 is transmitted to the optical receiver 1
A photodiode 13, such as a P-1-N photodiode, is electrically coupled to a photodiode 13, such as a P-1-N photodiode. The output of the optical receiver 14 is sent to the subtraction side port of the adder 15.
The other addition side port of adder 15 receives signal S2 delayed by time 7 by delay circuit 16. This delay time is chosen to be equal to the delay that the signal S, undergoes between the input of the optical driver 11 and the output of the optical receiver 14. The reason for this is that, in a practical sense, the signals S and S2 should arrive at the adder 15 with a phase difference of 0 as much as possible. Typically, the delay time 7 is on the order of a few billionths of a second. The output of the adder 15 is sent to the same optical driver 7 as the optical driver 11. Optical driver 7 is also LED
The same LDE 18 as 12 is driven. To explain the operation of the circuit in an easy-to-understand manner, we will introduce the first-order distortion cancellation effect (firs
torder distortion cancellation
tion effects) will be explained first.

The signal S, is converted into an equivalent optical signal (a, S, +Δ) via the optical driver 1 and the LED 12. Here a
．． is the electrical-to-optical conversion coefficient, and Δ is the amount of distortion caused by the LED 12. The optical signal (a, S, 10△) is P-
It is directly connected to a 1-L photodiode 13, where it is converted into an electrical signal and amplified by an optical receiver 14.
In the embodiment of FIG. 1, the gain of the optical receiver 14 is set such that the initial electrical input signal S, becomes distorted.

Of course, this is based on the assumption that no distortion is added by the P-1-N photodiode 13 and the optical receiver 14 following it. Therefore, the signal S,
The overall gain that is experienced from the input end of the optical driver 11 to the output end of the optical receiver 14 is unity. If power divider 10 power $2 = steal. If the signal is divided so that the output signal of the adder 15 becomes S2-(S.sufficient)=ru.-S.-total=S.-all.

The output light power of the LED 18 is SOut=a・(
S・-kai) 10△=a. S. becomes. Of course, S is much smaller than S, and the distortion caused by this term can be ignored as a first approximation. In other words, the electrical signal S, is LE
This means that the light is converted into the optical output Sout without D distortion. A more detailed analysis reveals that not all distortions are completely removed.

Let us assume the transfer function of the LED as follows. SLED=
a, S+a2S20a3S30a4S40...Here, SLoo is the optical output power of the LED, and S is the LED
is the current signal flowing through the

a, is of course the conversion efficiency of the LED, and a2, a3,
. . . are quadratic, tertiary, . . . distortion coefficients, and it is often sufficient to consider terms up to a3. When the circuit of FIG. 1 is analyzed under the condition that S2=cross, it can be seen that theoretically there is no quadratic distortion term. The third-order distortion term becomes small if there is a condition such that <<'a31.

Those skilled in the art know that there are now cases in which this condition applies. Higher-order distortions can be ignored and do not need to be considered. Next, while referring to Figure 2,
Let's discuss a more general case than the one shown in the figure.
Amplifier 1 with a raw loss coefficient on the path female side of signal S2
9 have been added.

Furthermore, the overall gain (or loss) factor of the path of the signal S, is assumed to be G in FIG. Assuming that G=1, the case of K=1 corresponds to the case of quadratic distortion cancellation shown in FIG. In order to cancel the third-order distortion, K must be set to a value other than 1.

That is, K must satisfy the following cubic equation. K3-Hinamaki-1:.

This cubic equation always has a solid line, and this real root is the desired value of K.

In practice, an intermediate optimum setting is achieved by adjusting the gain of amplifier 19. A point in between can be chosen that completely cancels out either the quadratic or cubic distortion terms.

A more general case occurs when G takes a value other than 1.

In this case, for quadratic distortion cancellation, the value of K is K=20
-1.

This means that the gain factor of the amplifier 19 is equal to G'. Also, by varying the value of K, third-order distortion cancellation is achieved instead of second-order distortion cancellation. It can be seen that employing either quadratic or cubic strain cancellation results in an overall reduction of all strain terms.

It is obvious that the higher the order of the distortion term, the less noticeable the improvement effect. Before discussing other minor details, it should be noted that measurements taken along the lines of the example in FIG.
I would like to mention that Mother B has decreased.

These improvements were achieved for fundamental frequencies varying from a few KH2 to a few MH2. The elements used were off-the-shelf elements.
For example, as the adder 15, 180 as shown in FIG.
0 hybrid was used. Since input signals E and E2 (S and S2 in FIG. 1) are in phase, the 1800 Hybrid outputs the difference between them. As such a hybrid, Amac's part number H for 0.2-39MH2
There are HI08 etc. An alternative to the apparatus shown in FIG. 3 is the apparatus shown in FIG.

A differential amplifier 20 is used here. The use of differential amplifiers is common in signal processing technology. An optical driver 21 is directly connected to the differential amplifier 20 . For an excellent optical driver suitable for such applications, see mEE TransactionS. n1
Beat tmmetati. n andMeasuremen
James in ts magazine (1975 pp230-232)
C. The paper “120MHz Bandwidth Linear Signal Transmission System Using Optical Fiber (AI2)” by Mr. Blackbmn
skin day 2 eurondwidm L ship ar Si starvation
Tra ship mission S tem UsingFi
4 (p. 231) of ``Ber Optics''. Of course, a less sophisticated amplifier may suffice in some cases. Such an optical driver directly biases and modulates the LED 18. Regarding the optical receiver, the arrangement as shown in FIG. 5 is most appropriate.

In FIG. 5, a reverse bias resistor R is utilized. Its resistance value is typically chosen to optimize the noise behavior of the subsequent amplifier 22. A simple and suitable amplifier is Texasin.
It is XL152 from Sulluments. Also available from RCA is the photodiode/amplifier integrated circuit C30818/819, which is particularly suitable for analog applications up to several MHz. FIG. 6 shows another arrangement to obtain the same strain reduction effect.

In this arrangement, the path of signal S2 is divided into two paths carrying equal amounts of signal power. The added path is delayed by a delay circuit 27 so that both signals arriving at the final adder 26 arrive in phase. The main path leaving the delay circuit 16 is through an amplifier/attenuator 23 in order to ensure that all signals reaching the first adder 24 have the same magnitude. The output of summer 24 passes through amplifier 25 to summer 26, whose output drives the final optical driver. When K = 1, the quadratic strain term cancels out as in the case of Fig. 1, but when the value of K is a3 K = thin sweat box, the cubic strain term cancels out, and if string l<l
a21, the quadratic strain term becomes small.

This condition generally always holds. In this case as well, by setting the value of K to an intermediate value (preferably determined by experiment), it is possible to obtain an optimal value between the quadratic distortion cancellation and the cubic distortion cancellation.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an optical transmitter according to the present invention. FIG. 2 shows a modification of FIG. FIG. 3 illustrates the use of an 1800 hybrid converter to generate a deviation of two in-phase signals for use in the transmitter of FIG. 1 or FIG. 2. FIG. 4 shows a differential amplifier followed by an optical driver and a light emitting diode for creating a deviation between two signals, as used in the transmitter of FIG. 1 or FIG. 2. Figure 5 shows the P-1-N used in the transmitter of Figure 1 or Figure 2.
It shows how to bias a photodiode and connect it to an optical receiver. FIG. 6 is a modification of a portion of the transmitter of FIG. 10...Power divider, 11,1
7 and 21... optical driver, 12 and 18
...Light emitting diode, 13...Photodiode, 14...Optical receiver, 15, 24 and 26...
...Adder, 16 and 27...Delay circuit,
19, 22, 23 and 25...Amplifier, 20
...Differential amplifier. F'G. 3 shores NS” 柤rG. 4''G. 5

Claims (1)

[Claims] 1. An optical transmitter having two parallel signal paths,
A first component S_1 of the electrical input signal Sin is coupled to a first of the signal paths, and a second component S_2 of the input signal is coupled to a first one of the signal paths.
a first electrical-to-optical converter 12, the first signal path being optically coupled to an optical-to-electrical converter 13; means 15 for generating a momentary deviation of the signal at the terminations of the first and second signal paths, the second signal path producing a predetermined electrical delay; a second electro-optical converter 18 for converting into an output form. 2. The first and second electrical-to-optical converters are similar electrical
2. An optical transmitter according to claim 1, having optical transmission characteristics such that the delay caused by the second signal path is substantially equal to the overall delay of the first signal path. . 3. The optical transmitter of claim 2, wherein the first and second signal-to-optical converters are semiconductor light emitting diodes, and the optical-to-electrical converter is a semiconductor photodiode. 4. The optical transmitter of claim 3, wherein said second signal path includes means for adjusting the signal power therethrough. 5. Claim 1, wherein the first component of the input signal contains half the power contained in the second component.
The optical transmitter according to item 2 or 3. 6. The ratio of the signal power contained by the signal at the ends of the first and second signal paths is (K+1) to G, where G is the quotient of the power at the end of the first signal path divided by the power at its starting point. 4. The optical transmitter according to claim 1, 2 or 3, wherein K is a predetermined real number.