AU606965B2

AU606965B2 - An fsk homodyne receiver

Info

Publication number: AU606965B2
Application number: AU18840/88A
Authority: AU
Inventors: Robert Schneider
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1987-07-10
Filing date: 1988-07-08
Publication date: 1991-02-21
Anticipated expiration: 2008-07-08
Also published as: EP0298484B1; DE3868447T; AU1884088A; EP0298484B2; DE3868447D1; EP0298484A1

Description

S F Ref: 60438 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATI

(ORIGINAL)

FOR OFFICE USE: Class Int Class t Complete Specification Lodged: Accepted: Pub lished: Priority: Related Art: ij.Ti;T1 Li~; 1 I I .4 of Applicant: Address for Service,.

Siemens Aktiengesellschaft Witteisbacher Platz 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: An FSK Homodyne Receiver The following statement is a. full description of this invention, inclwdinu, the best m~ethod of performing It known to mie/us 5845iS i 4& 0

ABSTRACT:

AN FSK HOMODYNE RECEIVER: In coherent optical communications transmission the frequency-shift modulation method is frequently used as, on the one hand, the transmitting laser can be modulated directly and, on the other hand, the demands on the line-width of the laser diodes which 0000 0o 0 are used are not great. Previously it has only been possible to o000 0 receive frequency-shift modulated signals using heterodyne receivers 0 0 0 0 as, in the case of homodyne reception, the two frequency positions of i 0 0 the transmission signal led to the same LF-level. In accordance with the invention, in an optical homodyne receiver, following a 900 hybrid junction the signal path is sub-divided into two C 00 it arms (A1,A2), and a delay element (TG) is included in one arm, at least, After the multiplication of the signals of the two arms, and filtering out any additional frequency components, the useful signal (DQ) is obtained.

.3 Declaration was/w ee the first application s- made in a Convention country in respect of the invention the subject of the application.

Declared at Sydney this 6th day of July 1988 SFP4 To: The Commissioner of Patents Signature of Declarant(s) 1/81

I

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ll I I 1 lfl AN FSK HOMODYNE RECEIVER: The invention relates to an optical homodyne receiver in a coherent optical communications receiver in a laser communication system.

In coherent optical communications transmission, the use of frequency-shift modulation (FSK) is highly desirable as, on the one ooo o hand, the transmitting laser can be modulated directly in a manner .00 known per se and as, on the other hand, the demands on the 0oo 0 o o 000 line-widths of the laser diodes which are used are not as great as in 0 00 0 0 transmission, reception in accordance with the homodyne principle, in 0o which the transmitting laser and the local laser at the receiving end 0 00 00°°0 have the same frequency, is highly desirable. Thus this principle 0 00 requires amplifiers which have only half the bandwidth in comparison 00 oo 0 0DO to heterodyne reception with the same bit rate and modulation.

In the past, this superheterodyne principle has been used 0 00 00oo exclusively for the reception of frequency-shift modulated light signals. The reason for this is that, in the case of binary frequency-shift modulation, only the two discrete frequencies, fT fH and fT fH, are generated at the transmitting end, where fT is the carrier frequency, and fH is half the frequency j swing used at the transmitting end. Following the superimposition at M the receiving end of these two discrete frequencies with the carrier frequency fT, in both cases a frequency corresponding to half the frequency swing fH is obtained so that, at the receiving end, it is not possible to differentiate between the two discrete frequencies generated at the transmitting end.

I f.

2 One object of the present invention is to further develop an optical ,homodyne receiver of the type referred to in the introduction in such manner that it is suitable for the reception of frequency-shift modulated signals with a frequency range which, in particular, is high in relation to the data rate.

In accordance with the invention, there is provided an optical homodyne receiver comprising a 90° hybrid junction having first and second inputs, said first input being optically coupled to a signal input of said receiver, and two outputs each being connected via low-pass filters and amplifiers to the two inputs of a multiplier having an output connected via a loop filter to a control input of a local laser, said second input being optically connected to an output terminal of the local laser, wherein, for the reception of frequency-shift modulated signals, a first delay element is Included into the link between an output terminal of the 900 hybrid J5 junction and the multiplier, and the output of the multiplier is connected .o via a third low-pass filter to the signal output of the optical homodyne S receiver.

0o" A particular advantage of the solution proposed by the invention 0 consists in the surprisingly low additional outlay for the reception of )p frequency-shift modulated signals in optical homodyne receivers which are provided for the reception of frequency-shift modulated signals. Thus it is also easily possible to construct an optical homodyne receiver which contains a disconnectible delay element, and thus is suitable both for the reception of frequency-shift modulated signals and phase-shift modulated .25 signals.

°os o0 o 0 Sc 0 0 a 0 0 0 0 o 00 0 0 0o 11(I 0 STA/I100o I 3 Sn the following the invention will be explained in detail in the form of an exemplary embodiment which is illustrated in the drawing, in which:- The drawing illustrates an optical homodyne receiver for frequency-shift modulated signals provided with an optical input DE for the transmission signal which is connected to an input of an optical-electronic 900 hybrid junction H. This 900 hybrid junction is a coamercially available triple-fibre coupler with three symmetrical inputs and three symmetrical outputs.

The second input LE of the 900 hybrid junction is connected to the optical output of a local laser LL, the light emitted from which has the same frequency, fT, as that of the o 0 t o a transmitted light. The three symmetrical outputs of the triple-fibre 0 00 coupler are optically coupled to three photo-diodes, PD!,PD2 and PD3 01 which are commercially available PIN photo-diodes. The symmetrica.

fibre coupler brings about a uniform power distribution of the Sa'. mutually combined light signals with a respective coupling degree of approximately 33% and, with an at least approximately loss-free coupler, due to the delay conditions, the photo-currents generated by the three photo-diodes, PD1,PD2 and PD3 are each displaced in phase by 1200 relative to one another. The first and second photo-diodes, PD1 and PD2, are electrically connected in push-pull so that a difference current can be obtained from the central tapping of the two photo-diodes. The first arm Al of the optical homodyne receiver is connected to this central tapping, and the second arm A2 o 0 00*00 o~ 00 000040 0 00 0 0 0 0 00 00 0 00 0 000 is connected to 'the 'third photo-diode. For the elimination of" frequencies outside the signal frequency band, each of the two arm, is provided at its input end with a low-pass filter, TPl and TP2 respectively, which are each followed by a respective first or second amplifier, V.1 and V2. The output of the first amplifier 111, which is located in the link between the f irst output terminal of the 900 hybrid junction and a multiplier BM, is followed by a first delay element TG. The output of the second amplifier V2 is directly connected to a second input of the multiplier BIM, which is a commercially available balanced mixer. The output of the multiplier B24 is connected via a third low-pass filter, which eliminates the snm frequencies produced by the multiplication, to the signal output DQ of the optical homodyne receiver and illustrated in broken lines to a loop filter FS. Having been amplified in a third amplifier V3, a low-frequency component of the output signal is fed by the loop filter FS to the control input of the local laser LL, and effects the frequency regulation thereof.

The mode of operation of the optical homodyne receiver corresponding to the invention is based on the recognition that, due to the delay of the signal in the first arm Al by a specific time interval T and subsequent multiplication by the signal of the second arm A2, In addition to the swing frequency 2 fH, depending upon the modulation state, one of the two levels ±UnaxQ x sin (2ntfH x T) occur. However, these level.s themselves represent the demodulated signal.

0 0 0 The maximum amplitude is obtained for Tmax 1/(4EH).

However, it should be ensured that the delay T is selected to be sufficiently small that the amplitude of the preceding bit of the passing signal is negligible at the sampling time of the following bit and thus has no repercussions. This is ensured for T Tmax if the frequency swing to fH is at least equal to the bit rate of the digital signals and the sampling takes place in the bit centre.

The LO-regulation is based on the fact that the middle output level both represents a guage of the centre-of-gravity frequency of the base-band signal, which is to be zero, and supplies the sign thereof. For this purpose the data signal must contain no frequencies below the cut-off frequency of the regulating loop.

However, the LO-regulation can function only when the delay time T is distinctly shorter than Tmax as, for T Tmax, the middle output level is always zero. TIt latter requires a second mixer BMS and a second delay element TGS, where TGs TG for the LO-regulation.

The second mixer BMS can likewise consist of a balanced mixer, and the second delay element TGS is designed to be such that its delay time corresponds at the maximum to half the delay time of the first delay element TG. Here the second delay element TGS is connected at its input to the first delay element TG, and the output of the second delay element TGS is connected to the first input of the second mixer BMS. The other input of the second mixer BMS is connected to that input of the first mixer BM which is not connected to the first delay element TG. The broken 1ine connection from the output of the third low-pass filter TP3 to the loop filter FS is then interrupted, and the loop filter FS is connected to the output of the additional mixer.

;i 6 The proposed receiver can form a vital link in a coherent optical communications transmission system with advantageous characterist ics.

Claims

2. An optical homodyne receiver as claimed in Claim 1, wherein a first delay element is selected, with a time delay which is sufficiently smnall that the amplitude of the preceding bit of the passing signal is negligible at the time of occurrence of the following bit.
3. An optical homodyne receiver as claimed in Claim 1 or Claim 2, wherein a first delay element is selected, with a time delay corresponding to half the period of the frequency swing used at the transmitting end for the generation of the transmission signal. STA/I00o I'
4. An optical homodyne receiver as claimed in Claim 1, wherein a commercially available balanced mixer is provided as the multiplier. An optical homodyne receiver as claimed in Claims 1 to 3, wherein the input of the first delay element is connected in parallel with the input of a second delay element, whose output is connected to the first input of a second mixer, whose other input is connected to that input of the first mixer which is not connected to the output Sof the first delay element, and that the connection from the output of the third low-pass filter to the input of the loop filter iz Interrupted, and this input is connected to the out;ut of the second mixer. a 6. An optical homodyne receiver as claimed in Clai 5, wherein the delay time of the second delay element corresponds at the maximun to half the delay time of the first delay element.
7. An optical homodyne receiver substantially as described with reference to the drawing.
8. A coherent optical communications transmission system incorporating a receiver as claimed in Claim 7. DATED this SIXTH day of JULY 1988 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON