AU614799B2

AU614799B2 - Optical heterodyne receiver

Info

Publication number: AU614799B2
Application number: AU26613/88A
Authority: AU
Inventors: Joachim Pietzsch
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1987-12-07
Filing date: 1988-12-06
Publication date: 1991-09-12
Anticipated expiration: 2008-12-06
Also published as: EP0319788B1; DE3883499D1; EP0319788A1; AU2661388A

Description

1% if 614799 S F Ref: 78721 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int Class Complete Specification Lodged: Accepted: Published: S Priority: Related Art: 4 Name and Address of Applicant: Address for Service: Siemens Aktiengesellschaft WittelsDacherplatz 2 8000 Muenchen FEDERAL REPUBLIC OF GERMANY Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Complete Specification for the invention entitled: Optical Heterodyne Receiver T',e following statement is a full description of this invention, including the best method of performing it known to me/us 5845/4 1 Abstract Optical heterodyne receiver for digital signals The frequency-shift-keying modulation (FSK) is advantageous for optical heterodyne reception since an additional modulator can be omitted in the transmitter; however, according to the prior art, frequency-shiftkeying modulated signals cannot be demodulated by means of homodyne receivers. To reduce the expenditure in the transmitter and, when higher bit rates are transmitted, also to reduce the expenditure of the receiver, an optical heterodyne receiver is proposed which, following an optoelectrical 900 hybrid on the input side, contains one or two demodulators which are connected in parallel on the input side and which in each case consist of two branches which contain an analog multiplier and a delay section and in which the signals of the branches are combined by means of an adder or a subtracting circuit.

Figure 1 0 0* o S S o 0 Siemens Aktiengesellschaft 87P1884DE Optical heterodyne receiver The invention relates to a heterodyne receiver for optical digital signals in accordance with the precharacterizing clause of Claim 1.

A heterodyne receiver of the type initially mentioned is known from Electronics Letters of 12th September 1985, Vol. 21, No. 19, Pages 867 and 868. The known receiver is used for demodulating optical DPSK-moduLated signals, it contains an optical 90° hybrid at the input which is connected to a helium-neon Laser as local Laser and to a transmission link for digital signals with a bit rate of 140 Mbit/s which is simulated in the experimental arrangement. Two PINFET diodes are separately connected directly to the outputs of the optical 900 hybrid so that, overall, a 900 hybrid with two optical inputs and two electrical o. outputs is obtained. The PINFET diodes are in each case connected to the input of one demodulator branch. Each of the demodulator branches contains an analog multiplier one input of which is connected directly and the other input of **which is connected via a delay section to the input of the branch and thus to one of the PINFET diodes. The outputs S of the analog multipliers are in each case separately •j connected via low-pass filters to the inputs of an analog adder at which the digital signal can be picked up after transmission.

The demodulator configuration known from Electronics Letters is also known in Journal of Lightwave Technology, Vol. LT-5, No. 4, April 1987, pages 561 to 572, particularly from Figure 11 of page 568. In this document, an FSK oemodulator is also described which is also constructed of two branches having one analog multiplier in each case, in which arrangement, however, one input of the analog multiplier receives the input signal of the respective branch directly and the other input receives the signal of the other branch in differentiated form. The output signal 2 of the two multipliers are combined via a subtracting circuit. For the phase diversity receiver described there is proposed a local laser the frequency of which deviates from that of the transmission signals by about 1/100 of the bit rate of the digital signals.

From the TUB5 lecture by D.W. Stowe at the Optical Fiber Conference, New Orleans 1983, it is already known to implement an optical hybrid by coupling together four 2 x 2 couplers, but the demands on the control of the optical path length between the couplers are extremely high.

From the papers by R.G. Priest, IEEE Journal of Quantum Electronics, Vol. QE (1982), pages 1601-1603, by K.P. Koo, A.B. Tveten and A. Dandridge in Appl, Phys. Lett. 41 of 1 october 1982, pages 616-618 and by A. Dandridge et al. in Internat. Conference on Optical Fibre Sensors, London, 1983, pages 48-52 on sensor technology and, particularly, on the fibre interferometer, 90* hybrids consisting of a 3 x 3 fibre coupler with connected photodiodes are known. A balanced 3 x 3 coupler with a 1200 phase shift in each case between the outputs is used S for implementing the 90° hybrid.

.J The heterodyne receiver known from Electronics Letters 1985 is used 20 for demodulating DPSK signals, but it is more advantageous for the optical heterodyne reception to transmit frequency-shift-keying-modulated (FSK) signals since no additional modulator is used for generating these at the transmitter side since the semiconductor laser diode can be directly modulated via the laser current in frequency shift keying modulation. Because of the demands on quality which are less in comparison with PSK modulated signals, that is to say the width of the emitted spectral band of the semiconductor diode provided as transmitting laser, the CPDFSK (continuous phase differential frequency shift keying) modulation is also of advantage, part from FSK.

Thus, the present invention has the object of developing an optical receiver of the type initially mentioned in such a manner that it is also suitable for receiving FSK or CPDFSK modulated signals.

In accordance with the present invention there is disclosed a heterodyne receiver for digital optical signals, comprising on an input

T

3 side a 90° hybrid having optical inputs for received light and the light of a local laser the frequency of which differs from that of the received light by substantially less than the bit repetition rate of the digital signal to be transmitted from two electrical outputs at which two signals shifted by 90° with respect to one another appear, with a first and a second branch of an ensuing demodulator, wherein each of the branches comprises an input connected through a photocurrent amplifier to one of the two electrical outputs of the 900 hybrid, said receiver further comprising a first branch of a (first) demodulator (DEM1) having a first delay section (VGL1) and a first analog multiplier (Ml) and a second branch of the (first) demodulator (DEM1) having a second delay section (VGL2) and a second analog multiplier w!herein a first input of the first analog multiplier (MI) is connected with the input of the first branch of the (first) demodulator via the first delay section (VGLI), a first input of the second analog multiplier (M2) is connected with the input of the second branch of the (first) demodulator (DEM1) via the ~second delay section (VGL2), a second input of the first analog multiplier (Ml) is directly connected with the input of the second branch of the (first) demodulator (DEM1), a second input of the second analog 2W multiplier (M2) is directly connected with the input of the first branch of the (first) demodulator (DEMI), the output of the first analog multiplier (Ml) is connected with a positive input of a first subtracting circuit (DUB1) and the output of the second analog multiplier (M2) Is connected with a negative input of the first subtracting circuit (SUBI), and a first data output (DAD) is connected with the output of the first subtracting circuit (SUB1).

In the text which follows, the invention will be explained In greater detail with reference to illustrative embodiments shown in the drawing, in which Figure 1 shows a first optical heterodyne receiver according to the Invention for FSK-modulated signals, and IAD/1251o T I ~I II 3A Figure 2 shows an optical heterodyne receiver according to the invention which, compared with the embodiment of Figure 1, allows an improved frequency control of the local laser and can be used quite universally.

The optical heterodyne receiver according to Figure 1 contains at the input side an optoelectrical 90* hybrid H which exhibits a first optical connection E for the received signal and a second optical connection U for the signal of the local laser LL and contains a 3 x 3 fibre coupler coupled to three photodlodes PD1, PD2, PD3. In this arrangement, the cathode of the first and the anode of the second photodiode PD1, PD2 are connected to one another and to the input of a first photoamplifier V1, the anode of the third photodiode is directly connected to the input of a second photoamplifler V2. The photodiodes are also connected to bias voltage sources, not shown in the drawing.

15 The photoamplifiers V1, V2 are constructed as amplifying current/voltage converters.

The outputs of the two photoampliflers V1, V2 are in each separately connected to the inputs of a first and of a second branch of a first demodulator DEMI. In 0SSO@* S S bOs 0 t S IAD/1251o -1 -4 this arrangement, the input of the first demodulator branch is connected via a first delay section VGL1 to the first input of a first analog multiplier and directly to the second input of a second analog multiplier. Correspondingly, the input of the second demodulator branch is connected directly to a second input of the first analog multiplier M1 and via a second delay section VGL2 to a first input of the second analog multiplier M2. The output of the first analog multiplier M1 is connected to the positive and the output of the second analog multiplier M2 is connected to the negative input of a first subtracting circuit SUB1. The output of the first subtracting circuit represents the first data output DA1. The first and the second delay section VGL1, VGL2 in this arrangement are dimensioned in such a manner that their delay times are T being the total 2 fH fH being the total frequency deviation of the optical transmission signal.

The optical transmission receiver according to the invention is already operable in this form, the control input of the Local laser LL will then be connected via a low-pass filter 0 to the first data output DA1 to regulate the local Laser.

For higher requirements for the accuracy of the frequency control of the local laser, a second demodulator DEM2 is 4 00 connected in parallel with the first demodulator DEM1 at the input side.

In these cases, the system is controlled for an intermediate frequency of zero. If the intermediate frequency differs from zero, the input of a frequency dis- :00* criminator tuned to the intermediate frequency is connected to the input of one of the two demodulator branches wh~ilst the output of the frequency discriminator is connected via the low-pass filter TPF to the control input of the local laser LL.

The second demodulator DEM2, like the first demodulator DEM1, consists of two branches the inputs of which are in each case separately connected to the outputs of the first and second photocurrent amplifier V1, V2. The first branch of the second demodulator DEM2 contains a third analog multiplier M3, the first input of which is connected via a third delay section VGL3 to the input of the first branch and the second input of which is connected directly to the input of the second branch. The second branch of the second demodulator DEM2 contains a fourth analog muLtiplier M4, the first input of which is connected via a fourth delay section VGL4 to the input of the second branch and the second input of which is connected directly to the input of the first branch. The output of the third analog multiplier M3 is connected to the positive input and the output of the fourth analog multiplier M4 is connected to the negative input of a second subtracting circuit SUB2 the output of which is connected to a second data output DA2.

This second data output DA2 is connected via a low-pass filter TPF to the control input of the local laser LL.

Whilst the delay time Ti of the first and of the second delay section VGL1, VGL2 is not greater than half a bit period so that it is sufficiently sure that there is no bit change within the delay time, the delay time T2 of the 0 third and of the fourth delay section VGL3, VGL4 is typically only a fraction of the delay time T1 and can be freely selected within certain ranges.

The optical heterodyne receiver shown in Figure 2, like the heterodyne receiver according to Figure 1, contains an optoelectrical 900 hybrid H with the two subsequent photoamplifiers V1 and V2 at the input side. As in the case of the heterodyne receiver according to Figure 1, the outputs of these photoamplifiers are connected to a first demodulator DE1 the output of which is connected to the first data output DA1.

The input of the first demodulator DEM1 is connected in parallel with a third demodulator DEM3. For this purpose, the input of a first branch of the third demodulator is connected in parallel with the input of the first branch of the first demodulator and, correspondingly, the input of the second branch of the first demodulator is connected in parallel with the input of the second branch of the third demodulator. The first branch of the third demodulator contains a fifth analog multiplier M5, the I -6 first input of which is connected via a fifth delay section and the second input of which is connected directly to the input of the first branch of the third domodulator.

Correspondingly, the second branch contains a sixth analog multiplier M6, the first input of which is connected via a sixth delay section VGL6 and the second input of which is connected directly to the input of the second branch of the third demodulator DEM3. The outputs of the fifth and of the sixth analog multiplier M5, M6 are in each case separately connected to inputs of an adder ADD the output of which is connected to a third data output DA3. In this form, the optical transmission receiver according to Figure 2 already represents a universal structure which can be modified in accordance with the code format of thr respective trans- S mission signals. Thus, the delay time of the first and of the second delay section VGL1, VGL2 must be set to be shorter than or equal to and that of the fifth and of the sixth delay section VGL5, VGL6 must be set to be approximately equal to the bit period of the digital signals for S* the reception of CPDFSK signals. In this case, the third data output DA3 must be connected to the useful signal output and the first data output DA1 must be connected to S the input of the low-pass filter TPF. To obtain a good high-frequency characteristic and high stability, the delay sections are advantageously constructed as delay lines as also in all other embodiments.

In a further application for FSK or CPFSK signals, 0 4 the first data output DA1 is the useful signal output and the third data output DA3 is connected via the low-pass filter TPF to the control input of the local laser LL.

The third demodulator DEM3 here operates as a frequency discriminator for the intermediate frequency. In this connection, the delay times of the fifth and sixth delay section VGL5 and VGL6 are approximately set to the value 1 T 4 ZF where ZF is the intermediate frequency. The delay times of the delay sections VGL1, VGL2 are about 1 2f

H

In the illustrative embodiment, the first and the i

J

7 third data output DA1, DA3 are combined by a seventh analog multiplier M7, the output of the analog multiplier M7 is connected to a fourth data output DA4. Such an arrangement can be utilized in various ways.

A first possibility for utilizing the combination of the first and of the third data output DAl, DA3 via the seventh analog multiplier M7 consists in the fact that the delay times of the first, second, fifth and sixth delay section have been selected to be 1- where fH is the frequency deviations used on the transmitting side which is equal to or greater than the bit repetition rate of the digital signals. In this case, the first data output DA1 is connected to the useful signal output and the low-pass filter TPF is connected to the fourth data output DA4 as 49 9.

is shown in Figure 2.

Figure 2 can be modified by selecting the delay times S, of the first, second, fifth and sixth delay section VGLI, VGL2, VGL5, VGL6 to be In this case, the useful

H

signal output is connected to the fourth data output DA4 whilst the low-pass filter TPF is connected to the first data output DA1. This second case offers the advantage to provide the possibility of optimum modulation of the receive

F*

signal even with a very small frequency deviation which can then be about half the bit repetition rate of the digital Ssignals. In this arrangement, the intermediate frequency is controlled for zero.

In the case of control for an intermediate frequency which is different from zero, either an additional frequency discriminator can be connected in parallel with the input of one of the demodulators, in which arrangement the output of the frequency discriminator controls the local laser LL via a low-pass filter, or a fourth demodulator DEM4, which is constructed like the third demodulator DEM3, can be connected in parallel with the demodulators DEM1 and DEM3. In this arrangement, the demodulator DEM4, the delay sections of which have delay times T 1/(4 ZF), operates as a frequency discriminator for the intermediate frequency ZF and controls with its output the local laser LL via a -8- Low-pass fiLter~ 8 Patent ctainis 2 Figures *O 45 a a 5 0 eat...

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Claims

1. A heterodyne receiver for digital optical signals, comprising on an input side a 90° hybrid having optical inputs for received light and the light of a local laser the frequency of which differs from that of the received light by substantially less than the bit repetition rate of the digital signal to be transmitted from two electrical outputs at which two signals shifted by 900 with respect to one another appear, with a first and a second branch of an ensuing demodulator, wherein each of the branches comprises an input connected through a photocurrent amplifier to one of the two electrical outputs of the 90° hybrid, said receiver further comprising a first branch of a (first) demodulator having a first delay section and a first analog multiplier and a second branch of the (first) demodulator having a second delay section and a second analog multiplier, wherein a first input of the first analog I* re multiplier is connected with the input of the first branch of the (first) demodulator via the first delay section, a first input of the second 9 analog multiplier is connected with the input of the second branch of the (first) demodulator via the second delay section, a second input of the first analog multiplier is directly connected with the input of the 20 second branch of the (first) demodulator, a second input of the second analog multiplier is directly connected with the input of the first S" branch of the (first) demodulator, the output of the first analog multiplier is connected with a positive input of a first subtracting j} .circuit and the output of the second analog multiplier is connected with a negative input of the first subtracting circuit, and a first data S: output is connected with the output of the first subtracting circuit. 1i

2. A heterodyne receiver according to claim 1, wherein the first and the second delay sections are dimensioned in such a manner that their respective delay times are 1 T1 2f H IAD/1251o 1~ ~s 10 where fH is the total frequency deviation of the optical transmission signal.

3. A heterodyne receiver according to claim 1, or 2, wherein the first data output is connected via a low-pass filter to the control input of the local laser.

4. A heterodyne receiver according to claim 1 or 2, wherein the input of one of the two demodulator branches is connected to the input of a frequency discriminator tuned to an intermediate frbquency and having an output connected via a low-pass filter to the control input of the local laser. 9

5. A heterodyne receiver according to claim 1 or 2, wherein the input of the first branch of the first demodulator is connected in parallel with the. input of a first branch of a second demodulator, and the input of the second branch of the first demodulator is connected in parallel with the input of a second branch of the second demodulator, that the input of the first branch of the second demodulator is connected via a third delay section to the first input of a third analog multiplier and to the second input of a fourth analog multiplier and to the second 2 input of the third analog multiplier, that the output of this third analog multiplier is connected to the positive input of a second analog subtracting circuit and the output of the fourth analog multiplier is *q ,connected to the negative input of the second analog subtracting circuit, that the output of the second analog subtracting circuit is connected to a second data output and via a low-pass filter to the control input of the local laser.

6. A heterodyne received according to claim 5 when dependent on claim 2, wherein the third and the fourth delay section exhibit a delay time which is a fractibn of the delay time T1.

7. A heterodyne receiver according to claim 1, wherein the input of a rirst branch of a third demodulatoris connected in parallel with the input of the first branch of the first demodulatorand the input of a sec.ind branch of the third demodulator is connected in parallel with the input of the second branch of the first demodulator; the first branch of the third demodulator comprises a fifth delay section and a fifth analog multiplier and the second branch of the third demodulator comprises a IAD/'1251o A P j 11 sixth delay section and a sixth analog multiplier; a first input of the fifth analog multiplier Is connected through the fifth delay section, and a second input of the fifth analog multiplier is directly connected with the input of the first branch of the third demodulator; a first input of the sixth analog multiplier is connected through the sixth delay section, and a second input of the sixth analog multiplier is directly connected with the input of the second branch of the third demodulator; and the outputs of the fifth and the sixth analog multiplier are connected with separate inputs of an adder with the output of which a third data output is connected.

S8. A heterodyne receiver according to claim 7, wherein the first data output provides a signal output of said receiver and the third data I" output is connected to the local laser via a low-pass filter for the reception of PSK-modulated transmission signals. 15

9. A heterodyne receiver according to claim 8, wherein the delay times of the fifth and of the sixth delay section substantially correspond to 1/4fZF where fZF is the intermediate frequency.

A heterodyne receiver according to claim 7, wherein the delay times of the first and of the second delay section are less than or equal to a bit period associated with the bit repetition rate, and the delay times of the fifth and of the sixth delay section are substantially equal to the bit period, that the first data output is connected to the local S"c laser via low pass filter for the reception of CPDFSK signals and the third data ou put provides a signal output of said receiver.

11. A heterodyne receiver according to claim 7, wherein the first data output and the third data output are connected to the inputs of a seventh multiplier and the output of the latter Is connected to a fourth data output which is connected to Ihe local laser via a low-pass filter, the first data output providing a signal output of said receiver and the delay times of the first, second, fifth, sixth delay section are approximately 1 2fH where fH is the frequency deviation used at the output(s) of said I receiver, said deviation being equal to or greater than the bit repetition rate of the digital signals. f IAD/1251o i -s I 12 S. S S 0S S S OS 0 *55* S S S* 0 S

12. A heterodyne receiver according to claim 7, wherein the first and the third data outputs are connected to the Inputs of a seventh analog multiplier the output of which is connected to a fourth data output which provides a signal output of said receiver, the local laser being connected via a low-pass filter to the first acta output and that the delay times of the first, second, fifth and sixth delay section are equal to 1 4fH where fH is the frequency deviation used at the output(s) of said receiver, said deviation being equal to or greater than the bit repetition rate of the digital signals.

13. A heterodyne receiver according to claim 7, wherein the first and the third data outputs are connected to the inputs of a seventh 15 analog multiplier the output of which is connected to a fourth data output which provides a signal output of said receiver, the local laser being connected via a low-pass filter to the output of a frequency discriminator which is tuned to an Intermediate frequency and the input of which is connected to the input of one of the demodulators and that the delay times of the first, second, fifth and sixth delay section are equal to 1 4fH where fH Is the frequency deviation used at the output(s) of said 25 receiver, said deviation being equal to or greater than the bit repetition rate of the digital signals.

14. A heterodyne receiver according to claim 7, wherein the first and the third data output are connected to the inputs of a seventh analog multiplier the output of which Is connected to a fourth data output which provides a signal output of said receiver, that the first and second branch, respectively, of the third demodulator are connected In parallel with the first and second branch, respectively, of a fourth demodulator at the input side, that the fourth demodulator Is constructed like the third demodulator, that the delay times of the first, second, fifth and sixth delay section are equal to IAD/1251o il-li 13 where fH Is the frequency deviation used at the output(s) of said receiver, said deviation being equal to or greater than the bit repetition rate of the digital signals and the delay times of the delay sections contained in the fourth demodulator are equal to 1 /4fZF where fzf is frequency of the intermediate frequency signal of the receiver.

A heterodyne receiver as claimed in any one of the preceding claims, wherein the 900 hybrid is preferably constituted by a 3x3 fibre coupler with photodiodes connected to the output and a first input terminal of the 3x3 fibre coupler is connected with the input for the received light and a second input terminal is connected with the input for the light of the local laser, wherein the third input terminal remains unconnected.

16. A heterodyne receiver for digital optical signals substantially as described herein with reference to Fig. 1 or Fig. 2 of the drawings. 9 0** DATED this TWENTY-EIGHTH day of JUNE 1991 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON S.. S 0I SI 0l SI( SI at IAD/1251o