AU592476B2 - Method and apparatus for spectral measurement - Google Patents

Description

592476 COMMONWEALTH OF AUSTRALIA FORM PATENTS ACT 1952 COMPLETE SPECIFICAT ION FOR OFFICE USE: Class Int.Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: f Tkla doK,,qjn.-nt WnWIM ILK 4 19 I Nlfir4Ct Wr P)CLbSttjo h vd 3C *'.Priority: 'Related Art: Name of Applicant: 'Address of Applicant: SActual Inventor: PLESSEY OVERSEAS LIMITED VICARAGE LANE, ILFORD, ESSEX IG1 4AQ,

ENGLAND

JOHN EDWARD CARROLL PETER THOMAS JOHNSON Address for Service: SHELSTON WATERS, 55 Clarence Street, Sydney Complete Specification for the Invention entitled: "METHOD AND APPARATUS FOR SPECTRAL MEASUREMENT" The following statement is a full description of this invention, including the best method of performing it known to me/us:- 1 r- 1 -14- METHOD APPARATUS FOR SPECTRAL MEASUREMENT TECHNICAL FIELD The present invention concerns a method and apparatus for spectral measurement, and, in particular, a technique for measuring the complex correlation coefficients for an electromagnetic radiation signal.

a .o Spectral lineshape and linewidth measurements of semiconductor lasers are important for determining the viability of such sources for coherent applications. The spectral lineshape also provides information about the noise processes within the laser itself. Both amplitude and frequency stability are of interest.

D I I BACKGROuND ART I r.

Existing measurement techniques are of limited frequency resolution.

Hitherto, linewidths of coherent sources have been A measured using commercially available spectrum analysers.

For optical sources it has been usual to split the optical signal into two parts. One part is delayed with respect to the other by a sufficient amount so that there is no mutual coherence between the two signals. On homodyning

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-2- *n *00 4* 44 4 0rn 4* 49. a 0 44 aI 4 these two signals, the difference frequency is nominally zero and this is spectrally analysed. Thb measured linewidth of this baseband signal is then twice that of the source. Only the spectral amplitude is measured and phase infcrmation is not given by this measurement.

It is known to use a 4 X 4 multiport junction to S generate correlation coefficients and by way of background the interested re der is referred to the following article by N.G.Walker et al, entitled "Simultaneous Phase 010 Amplitude Measurements on optical signals using a multiport juncion" Electronic Letters Vol 20 No 23 pages 981-983 (Nov 1984).

DISCLOSURE OF THE INVENTION The invention considered herein is intended to provide a method and apparatus for spectral measurement, the same being capable of affording improved frequency resolution.

According to the invention thus there is provided a method for spectral measurement, a method of the type wherein an input signal and a reference signal are (;ompared and quadrate complex amplitude product terms derived therefrom, characterised in that said signals are offset in frequency by a variable frequency shift; and, r.rur

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:t I ttt I I C i *-^a>a«stSwa^ y -3each said term is integrated for a suitable period of time

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a ltl lt1 I I t( I ct and sampled, samples being taken for different values of the frequency shift by which said signals are offset.

In the method aforesaid the input signal and the reference signal may be derived from different sources.

In this case the samples taken correspond to complex cross-correlation coefficients for each value of frequency shift. Alternatively, the input signal and the reference signal may be derived from a common source, the samples then corresponding to the complex auto-correlation coefficients of that source.

In order to obtain satisfactory averaging, the period of integration is chosen to correspond to many periods of the frequency shift. The term "suitable period" as used hereinbefore, shall be construed accordingly.

Apparatus for performing the method aforesaid may comprise:amplitude and phase comparator means, this having first and second input ports and being responsive then to input signal and reference signal respectively, this means providing quadrature complex amplitude product terms as output in response to said signals; frequency shift means inserted at one of said input ports, to pply each of a set of different frequency offsets; and, i- -4integration and sampling means, co-operative with said amplitude and phase comparator means, reponsive thus to said product terms.

The amplitude and phase comparator means may comprise, for example a 4 X 4 multiport junction formed of a network of beam splitters or power dividers. For application to signals at optical frequency, the power dividers may each consist of a fibre coupler. In the latter case, the optical outputs from the 4 X 4 junction may be detected by diodes and the diode currents combined in subtractive pairs to provide the requisite product terms aforesaid.

For optical application, the frequency-shift means mal have the form of a coiled optical fibre bonded about the periphery of a piezoelectric crystal former which latter is driven by a periodic voltage drive signal. For convenience, the drive signal provided may be of sinusoid waveform.

The integration timing and frequency shift adjustment may be controlled by suitable electronic and/or computer means eg. by a microprocessor. Measured data may be stored in and processed by the same electronic and/or computer means or, in preference to this, by another dedicated computer or processor.

I iL-- 1. i r BRIEF INTRODUCTION OF THE DRAWINGS In the drawings accompanying this specification:- Figure 1 is a block schematic diagram showing apparatus provided in accord with this invention.

Figure 2 is a block schematic diagram showing the optical part of the apparatus including a 4 X 4 multiport junction comprised of fibre couplers; Figure 3 is a block diagram of the electrical processing part of apparatus and which may be used in conjunction with the optical apparatus shown in the preceding figure; Figure 4 shows, in side and front elevation, a frequency shift device for optical application and for use in the apparatus of figure 2 preceding; Figure 5 illustrates the waveform of a drive signal, a signal that may be used to operate the device of the preceding figure.

DESCRIPTION OF A PREFERRED EMBODIMENT So that this invention may be better understood, an embodiment thereof will now be described and reference will be made to the accompanying drawings. The i -v description that follows is given by way of example only.

The technique that is considered here is general and is applicable to any frequency. For the purpose of description, however, consideration will now be restricted to optical aDplication and in particular to an embodiment suited to signals at eg. 1.55 microns wavelength and thereabouts. With a single source, spectral Sauto-correlation function information about noise in D semiconductor lasers can be obtained, and it is this 10 measurement that will be considered below. Alternative to -this, different sources can be applied to the comparator inputs and cross-correlation measurement obtained.

As shown in figure 1 the signal from a laser source 1 is divided by means of a beam splitter (or power divider) 3 to provide an input signal A and a reference signal B.

One of these signals, the reference signal B, is applied directly to a first input port I, of an amplitude and phase comparator 5. The other signal, the meaured input j signal A, is applied to a second input pont 1, via a frequency-shift device 7, which device serves to offset the frequency f( -6/27f' of the input signal and by a prescribed and variable amount The comparator 5 serves to provide complex amplitude product tirms differing by quadrature phase and to this end provides at its output ports signals Pl, to P 4 which I I -7are phase diverse and comprised of the mixed input signals

A,B.

A practical arrangement for the optical part of the comparator 5, a 4 X 4 multiport fibre-coupler junction, is shown in figure 2. This junction comprises a network of four 3dB fused fibre couplers 9,11,13 and 15, all arranged symmetrically. In two of the branches of this S network, phase-shift components 17 and 19 are inserted.

J These latter components 17 and 19 are adjusted to produce S"i0 the requisite phase diversity. Polarisation controllers 21 and 23 are inserted in the remaining two branches of *the network and are adjusted for optimum polarisation alignment.

S, The four optical output signals PI, to P 4 15 provided by the multiport junction, are directed onto a set of corresponding photodiodes 25, 27, 29, and 31 and detected. The diode currents, after individual pre-amplification by amplifiers 33, 35, 37,and 39, are then referred in pairs (Plr P2) and (P 3 P4) to differential amplifiers 41, and 43. The resulting currents correspond to differences of power measurements as given below:- Pl P2 2A(t)B(t) cos(Sl2t-); 1M P 3 4~ 2A t) B t) s where: P 1 to P 4 are the power measurements obtained, A(t) and B(t) are the amplitudes of the incut and reference signals respectively, J-Lis the inserted ancular frequency offset, and, 0, is an ar-bitrary phase dependent upon the optical path lengths through the network. It will be noted that these- two terms correspond to the complex amplitude products of the input and reference signals and differ by quadrature phase.

In the rena in ing electr ical proces s ing part of th'e apparatus, as shown in figure 3, these latter signal terms are integrated and sampled to p.'rovide a se't of data f Pl P 2 )dt 7=(P3 P 4 )dt It can be shown that the complex term Z defined by:z X+jy JP P2dt y P (3 -P 4 )dt) is equivalent to Z (a)=exp (j4)fA B* dc./- Fo tL 7N-/-L x and The time averaged~ signal outpu-ts thus provide a measure of the correlation coefficients For practical measuremnent, the time average is taken over a -9f inite interva2-C an interval that is long compared, with the period of the frequency shift ).2n-Z The integration then approximates infinite limits, and is consistent with the theory.

The quadrature terms thus each pass through an integrator 45, 47 and are sampled after the period of integration L by respective sampl e- and--hold -units 49 and t P I P51. The samples are then dig'Jt'ised by respective analogue- to-d ig ital convertors 53 and 55 and the data 17,7 stored in the memory of a computer 57 for'subseuen- data processing.

The computer 57, acts as both a controller and a data-logger. The computer 57 controls the frequency of a signal generator 590, which in turn controls the frecuency of fset -Q inserted by the optical frequency shift unit 7.

Whils, the frequency shift has a fixed value for the integration tjmeT, th.o frequency shift. is valid at a value-. A pulse, the "valid" pulse, is then sent to the integrators 45, 47 and the sample-and-hold circuits 49, 20 51, The integrators 45, 47 detect the rising edge of this pulse and are reset. Whilst the frequency shift-72- is valid, the power differences of the inultiports (ie the quadrature terms) are integrated. When the frequency shift-f2. becomnes invrlld, je, at the falling edge of the 25 pulse, the sample and hold circuits 49, 51 sample the 0 44 04 4 4444 4444 4 4444 4 04 44~44 4 4 4444 4 0 44 4 4 44 04 4 4 44 4044 40 44 4 44 44 0 4 44 044$ O 4 44,4 4 44444* 0 0 4, 0 4 44 4 44 0 Ott 4<4 4 4 outouts of the integrators 45, 47 and analogue to digital conversion commences. When the digital data x,y is ready, the computer 57 is interrupted and the data is logged.

When the computer 57 has sufficient data (xlr yPr) (x2, 172r); (31, at that particular frequency shiftS, it increases :he frequency of the signal gerrator 59, thus increasing the optical frequency shift, and the procedure above can be reoeated, Provision is made to allow for different integration 10 times a-ppropriate to different frequency shifts. Rocug hyI spealking the integration time must be many periods of the frectuenc~v shift in order to obtain satisfactory averaging and reject-ion of unwanted frequency components so that the inftegration approxima-'.,4s to one over infinite limi"ts.

1$ With just, the one computer 57, the control of the procedure will1 eifher bi done as interrupts to the processing program, or control and dat%-a-logg~ing will preceed the processing. Therefore, two computers or microprocessors would be preferable, one dedicated to 20 control, the other dedicated to data processing.

Choice of frecluency shift unit 7 is not critical.

Any' unit capable of providing a constant frequency shift over the requisite range of integration time values and over the ranvgo of shifts derived then would suffice.

The frequency shifter 7 shown in figure 4 is based -11upon change of signal phase, which change is proortional to time over a per,-od of time -C The shifter 7 comprises a length of optical fibre 61 which has been wound around and bonded to the periphery of a piezoelectric crystal- 63 which, in the embodiment shown, is tubular in shape.

Electrodes 65, 67 are provided on the inside and outside surfaces of the tubular crystal 63 and. a periodic drive *voltage applied, The crystqil 63 will thus expand and contract periodically, and will vary the Zength of the f ib re 6. The optical, path length and thus signal phase will be varied Pet-iodic4lyv accordingly* For practlc J.

purposes it is suIfficient, to apply a swn -r voltage drive signal (fig As can be speer 2r(= the graphical representationt the voltage change With tinea, is dV/dt,, approx.J'ma..co~ a linear ohanqe betwan 0-nlt,j#g 0 00 t2. and therefore wij. pz'oVlde 4 l-na pa change over the period t2 -tj he t 1 ,tz, will di.fEor acoordinq to drive si~inai.

**efrequency and or amp~itde, Th@ r4Agtri (5 41 log i be Us~ed to provide positive and neqtive frc,-ency shiIftl rospect.ively. .oho 4egt4ety shift .cart tlius be elhnled by changing eq the frequtency oe the driv hgn4 Corrpnding qhn~ 'a iteqatlon, tilitn would h :14 effected by the COriputer Or i r~e~~

Claims (10)

1. A method for spectral measurement, a method of the type wherein an input signal and a reference signal are compared and quadrature complex amplitude product terms derived therefrom, characterised in that said signals are offset in frequency by a variable frequency shift; and, a pair of such terms is integrated for a suitable period of time and sampled, samples being taken for difference values of the frequency shift by which said signals are offset.

2. Apparatus for performing the method of claim 1, this apparatus comprising:- amplitude and phase comparatot means this having first and second input ports (Il, 12) and being responsive thus to input signal and reference signal respectively, this means providing quadrature complex amplitude product terms as output in response to said signals frequency shift means inserted at one of said input S'0 ports (12) to offset the frequency of the input signal and, integration and sampling means 45, 47, 49 and 51, co-operative with said amplitude and phase comparator means responsive thus to said product terms. oo -13-

3. Apparatus, as claimed in claim 2, wherein the amplitude and phase comparator means comprises a 4 X 4 multiport optical fibre coupler junction 11, 13, and a set of four photodiodes (25, 27, 29 and 31) ie each responsive to a respective output port of the junction; and, a pair of differential amplifiers (41 and 43) connected to the photodiodes in pairwise manner (25 and 27, 29 and 31).

4. Apparatus, as claimed in either claims 2 or 3, wherein the frequency shift means is a device of the type capable of inserting a change of phase and wherein this phase change is variable in linear proportion to time.

5. Apparatus, as claimed in claim 4, wherein the frequency shift mears cormprises a length of optical fibre (61); and, S phase control means (63) co-operative with said fibre (61) to change the length thereof periodically.

6. Apparatus, as claimed in claim 5 wherein the phase control means (63) comprises:- a piezoelectric crystal former and, a voltage generator this being connected to electrodes (65, 67) adjacent to the former (63) to apply a periodic voltage thereto. i ft ft tw If If S 9*1 I I If@ I *1 t 9 II -14-

7. Apparatus, as claimed in claim 6, wherein the voltage generator (59) is adapted to provide a drive voltage of sinusoid waveform.

8. Apparatus, as claimed in any one of preceding claims 2 to 7, wherein the value of each frequency offset, provided by said frequency shift means is controlled by computer control means (57).

9. Apparatus, as claimed in claim 6, wherein the frequency of the periodic voltage is controlled by computer nontrol 10 means (57). Apparatus, as claimed in any one of preceding claims 2 to 9, wherein timing control for. said integration and sampling means (45, 47; 49and 57) is provided by computer control means (57).

11. Apparatus for performing the method of claim I, this apparatus being constructed, adapted and arranged to perform substantially as described hereinbefore with reference to and as shown in the accompanying drawings. DATED this 26th day of October, 1987 PLESSEY OVERSEAS LIMITED Attorney: PETER HEATHCOTE Fellow institute of Patent Attorneys of Australia of SHELSTON WATERS