GB2235284A

GB2235284A - Optical spectrum analyser

Info

Publication number: GB2235284A
Application number: GB8906702A
Authority: GB
Inventors: Jeremy Kenneth Arthur Everard
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-03-23
Filing date: 1989-03-23
Publication date: 1991-02-27
Also published as: GB8906702D0

Abstract

An optical spectrum analyser for an optical input (1) uses the fact that the Fourier transform of the autocorrelation function produces the power spectrum. The analyser consists of a Michelson interferometer where one or both paths (4, 5) contain a variable delay. This delay may be implemented using an optical fibre wrapped around a size varying cylinder for example a PZT (6, 7). By arranging for the total delay to be very large then even small changes in diameter produce a large differential delay. The optical signals from the two paths are applied to a photo-detector (10). The electrical output of the photodetector is sampled and digitised (11) and processed to produce the Fourier transform (12) and hence the power spectrum. A modified embodiment dispenses with optical fibres and produces the optical delay by using a rotating glass block in one of the paths. The spectrum analyzer may be used to detect backscattered light from an optical fibre, for use in temperature and/or strain measurement. Signals from a reference laser (2) may be used to trigger the samples and digitizer (11). <IMAGE>

Description

OPTICAL SPECTRUM ANALYSER SPECIFICATION The techniques that are applied to radio communications systems are now being applied to optics and it is therefore essential to be able to offer similar performance instruments with performance scaled to the parameters at RF frequencies. These instruments would be used both in the research and development laboratories and in the field. In this invention an optical spectrum analyser which uses Fourier transform spectroscopy which is based on the fact that the autocorrelation function of a time domain waveform and the power spectral density form a Fourier Transform pair is described. As The auto-correlation function is easy to measure and FFTs (Fast Fourier Transforms) are easy to implement digitally a high performance spectrum analyser with resolution bandwidths of Ghz can be built.Infra-red Fourier Spectroscopy is a fairly standard technique, however in this invention a very simple way to implement the autocorrelation function is described allowing a compact and cheap instrument operating in the communications bands from approximately 0.5cm to 211 to be made.

In this invention an optical spectrum analyser is invented which uses the fact that the Fourier transform of the auto-correlation function produces the power spectrum. The analyser consists of a Michelson Interferometer where one or both paths contain a variable delay. This delay is implemented using an optical fibre wrapped around a size varying cylinder for example a piezo-electric cylinder (PZT) (or other shape) or electro-mechanical cylinder or vibrating mechanical cylinder. By arranging for the total delay to be very large then even small changes in diameter produce a large differential delay.

The system is shown in Figure 1. The analyser consists of a fibre Michelson interferometer made up of optical fibres (1 and 2) wrapped around two size varying cylinders (3 and 4) for example piezoelectric transducers hereafter called PZrs which are interconnected to two fibre or bulk optics couplers (5 and 6). The input optical signal which is defined as the "wanted optical signal" is split into two paths using an optical fibre directional coupler or bulk optics coupler (5). Half the signal passes down one path (1) and the other half passes down the other path (2). The optical signals are recombined via an output coupler (6).

The voltage on each PZT is varied in anti-phase causing the delay in one path to be increased and the other path decreased and vice versa. Other types of size varying cylinders can be used including vibrating cylinders which are caused to vibrate by any means.

Light is launched into the input coupler (5) and the output from the output coupler (6) is incident on a photodetector (7) for example a photodiode or photoconductor or avalanche photodiode or photomultiplier.

The amplitude of the output waveform from the photodetector which is called the "wanted electrical signal" is electrically sampled at many different delays and digitised in the sampler and digitiser (8). The output on the photodiode as the differential delay is varied is the autocorrelation function of the optical signal which when processed by taking the Fourier transform is the spectrum of the optical signal. The output from the sampler and digitiser (8) is processed to obtain the Fourier transform and hence spectrum in a hardware processor or computer (9).

The sampling digitising and Fourier transform techniques can be performed using similar techniques to those described in "Fourier Transform Infared Spectrometry" ISBN 0 471-09902-3 by Wiley Interscience.

A reference electrical signal can be obtained by launching a reference precalibrated laser down the same fibre via the input or output coupler and taking the output at the output or input coupler respectively. An example of where the reference laser hereafter called the "reference optical signal" is launched into the input coupler is shown in Figure 2.

The wanted optical signal is launched into one arm (1) of the optical fibre or bulk optics coupler (3 ) and the reference signal is launched into the other arm of the coupler (2) where the coupler splits the wanted optical signal and the reference optical signals into the two arms (4) and (5) which are wrapped around the size varying cylinders (6) and (7).The optical signals are then recombined in a fibre coupler (8) and because the wanted optical signal and the reference optical signal are at different frequencies they are separated by a frequency selective beam splitter or diplexer (9) and the wanted optical signal is incident on the photodetector (10) the electrical output of which is sent into a sampler and digitiser (11) and Fourier transformed and analysed in a computer and or hardware processor (12) where the reference optical signal is incident on another photodetector (13) which is used to generate the trigger pulses for the sampler and digitiser (11).

The reference laser can for example be a narrow linewidth He Ne laser or a single mode semiconductor laser. These laser beams once passed down both arms of the fibre are added in a bulk optics or fibre coupler and then incident on a photodetector to produce a reference sinewave which is the autocorrelation function of the reference laser. The zero crossings and or peaks of this reference optical signal are detected on a separate photodetector and used to trigger the sampling circuitry which samples the "wanted electrical signal" which is the photodetected "wanted optical signal".

Optical filters and beam splitters are used to separate the wanted optical signal from the reference optical signal to produce the wanted electrical signal and the reference electrical signal respectively. The input optical wanted signal is the signal whose spectrum is being measured. A separate photodetector is used to detect the optical reference signal to covert this to the reference electrical signal. The reference electrical signal is therefore used to provide the sampling points for the wanted electrical signal and is also used to provide electronic feedback to control the electrical waveform applied to the PZT to produce a near linear sweep of delay with time.

The wanted electrical signal is therefore sampled at many different delays (for example evertime the reference electrical signal goes through a peak or zero crossing) and then digitised. The Fourier transform and hence spectrum is then calculated and displayed. The spectrum can be calculated by taking the Fourier transform. The reference signal can be doubled in frequency to enable sampling just at the zero crossing positions of the doubled reference signal.

The delay variation that can be achieved using 50 metres of fibre wrapped around a single 7cm by 6cm PZT drum is of the order of 5 mm if the PZT is driven both positive and negative. This can be increased by passing the signal down the fibre more than once as shown in Figure 3. The optical signal is launched into one arm (6) of the input coupler (5) which splits the wanted optical signals into fibres (3) and (4) which are wrapped around size varying cylinders (PZT's) (7) and (8). Mirrors (1) and (2) which reflect each signal back along each fibre are placed on the end of each fibre (3) and (4) and the wanted optical signal output is taken from the other arm (4) of the input coupler (5). This signal is then detected on a photo-detector (9) and then sampled and digitised in a sampler and digitoser (10) and Fourier transformed in a hardware processor or computer (11).

With this arrangement a total of 10 mm in glass per fibre hence 20 mm total in the two fibres which is 30mm in air can be obtained. Resolution bandwidths of approximately 10 Ghz can therefore be achieved. This resolution bandwidth can be further reduced by increasing the number of turns of fibre on the drum or by changing the diameter and or length of the size varying drum. The optical signal detection and processing is as described in the system shown in Figure 1.

The amplitude of the output waveform from the photodetector is sampled at many different delays for example evertime the reference electrical signal goes through a peak or zero crossing and then digitised. The Fourier transform and hence spectrum is then displayed. The spectrum can be calculated by taking the Fourier transform. The reference signal can be doubled in frequency to enable sampling just at the zero crossing positions. The sampling digitising and Fourier transform techniques can be performed similar techniques to those described in "Fourier Transform Infared Spectrometry" ISBN 0 471-09902-3 by Wiley Interscience. The wanted electrical signal may only be sampled at the zero crossings of the reference electrical signal.

This system shown in Figure 2 where the signal is reflected back from the far end of the fibre such that it propagates down the fibre twice therefore gives twice the variation. Further reflections may be possible by launching the light into different birefringent axes each time a reflection occurs.

In fact this can be further improved by sending the light down a high birefringence fibre which has two birefringence axis say A and B where the light is launched into axis A and it propagates down the fibre in this axis A and at the end of the fibre the polarisation is is then rotated by 90 degrees and the light reflected back down the fibre now in axis B and then the light is reflected one more time so that the light passes down the fibre again in axis B thus propagating down the fibre three times. This does however rely on high isolation between the two axes, and accurate non-frequency- dependent polarisation rotators.In fact the polarisation variation can be reduced by putting electrically driven wire coils around the cylinder walls to compensate for polarisation rotation by using the Faraday effect where the polarisation of the optical signal is monitored and the monitored signal is used to provide negative electrical feedback to reduce the variation of polarisation.

Another system is shown in Figure 4. The optical input signal is split in a fibre or bulk coupler (1). Now only one of the optical fibre paths (2) is arranged to have a variable delay by varying the size of the cylinder (3) and the other path is a fixed delay consisting of a fibre wrapped on a drum (4). The two optical signals are added using fibre couplers or bulk optics (5) and then applied to a photo-detector (6). The electrical output from the photo-detector (6) is sampled and digitised in a sampler and digitiser (7) and Fourier transformed and displayed in a hardware processor and/or computer (8). The optical signal detection and processing is as described in the system shown in Figure 1.

This system can also be improved by adding mirrors to double the differential delay variation as shown in the system described with Figure 3.

The techniques described below can be applied to all the svstems Great care must be taken in the fibre wrapping process as chaotic behaviour can occur due to fibre slippage. A reverse bias will be applied to the size varying drum eg PZT while wrapping.

It is essential that the variation of polarisation with delay is kept to a minimum and therefore high birefringence fibre should be used.

Measurement of the input signal with high speed using analogue techniques can be achieved. The electrical correlation waveform produced by the reference signal can be varied in frequency by passing the reference optical signal down a section of the total delay or more than the total delay used for the wanted optical signal. The optical reference signal can then be multiplied with the input signal in the detector. This can also be used for measuring multiple sidebands.

This instrument can be interfaced to a personal computer with a mouse, light pen and touch screen display monitoring. These spectrum analysers can also be used as optical frequency counters.

If larger changes in diametre can be produced by approaching the breaking strain of the fibre which is of the order of 0.3 to 0.5% change in length then even if only 0.1% increase is used then the change in delay is ten centimetres in glass (1Scms in air). This variation in delay will allow a frequency resolution in the final instrument of a few Ghz or less than a GHz.

The extension of the fibre can be further improved by using different types of fibre that can be stretched more for example plastic fibres and polymers as long as they can be made single mode. It may be possible to use some forms of multimode fibre.

To obtain the correct sampling points for the digitisor a reference laser is passed down the same fibre. The autocorrelation measured with this laser produces a sine wave and the zero crossings and peaks are used to trigger the sampling circuitry optical filters and beam splitters will be used to separate these signals.

Note these circuits can be used to filter and measure the input signals with high speed in a number of ways. For example the correlation waveform produced by the reference signal can be varied in frequency either by mixing with an electrical offset frequency or by passing the reference signal down a section of the total delay used for the input signal or more than the delay seen by the input signal. The optical reference signal can then be multiplied with the input signal and the amplitude of the signal measured at the beat frequ ?nay.

In an optical fibre the V number can vary from about 0.5 to 2.5 for the fibre to remain single mode. The fibre can therefore be arranged to cover a large frequency range. If it is necessary to cover many bands a number of different size single mode optical fibres can be used.

Optical fibres can be used to measure wavelengths in the range of 0.5 microns to 2 microns and this range will increase as new fibres become available.

Compensation for the change in attenuation with wavelength can be incorporated in the computer controller.

A single fibre may be used as both the variable delayed arm and the reference arm. This could be a birefringent fibre where the fibre is wound such that the strain field is along one of the axes or at an angle to the axes such that the strain increases or reduces the birefringence such that the delay between the signals passed down the fibre is varied. The same signal is launched in both axes by lauching the wanted optical signal at 45 degrees to the birefringence axes such that the delay between them at the other end is varied by the strain from the PZT and hence the applied voltage.

The fibre could be a low birefringence fibre where variable birefringence is introduced using the PZT.

The fibre could be circular or rectangular or any shape that will propagate light.

The PZT could either be a piezo-electric ceramic or any material demonstrating piezo-electric properties or any electromechanical or size changing system.

The output optics would need to recombine the two beams with the same polarisation onto a photo-detector.

The correlation can be obtained digitally using A to D convertors sampling and fourier transforming including fast fourier transforms.

The spectrum of the wanted optical signal can also be obtained by measuring the spectrum of the wanted electrical signal by arranging for the delay variation with time to be very nearly linear. The spectrum could also be measured using analogue techniques such as lockin amplifier techniques where the wanted electrical output signal is multiplied by quadrature signals of known frequencies in a multipler where the output of the multipler is correlated when the quadrature signals and the wanted electrical signals are the same frequency. The amplitude of the output of the multipler would be proportional to the amplitude of the wanted optical signal at the relevant frequency. The intensity phase and frequency of the wanted optical signal could therefore be measured.The difference frequency between two wanted optical signals could also be obtained by measuring the beat frequency of the wanted electrical signal.

The PZT can be driven sinusoidally or with sawtoothes or ramps and flyback or any waveform to produce the auto-correlation function. A reference laser beam can also be passed down the fibre such that the fringes can be used to trigger the sample and hold before the analgue to Digital convertor (A to D) and also to provide feedback to the electrical drive signal to produce a linear change in delay with applied voltage.

Where PZT is referred to in the text this can be replaced by size varying cylinder anywhere in the text.

The reference signal could be used to produce a nearly continuous output without excessive phase discontinuities by changing the direction of delay change of the PZT at a zero crossing point on the electrical reference signal where the reference waveform may be arranged to be the same frequency by only passing the reference optical signal down a section of the delayed fibre or more than the delayed fibre. If the reference electrical signal and electrical output of the wanted optical signal are the same frequency then their correlation can be obtained by multiplication.

Photo-conductors can also be used to detect and decorrelate the signal to obtain time resolved spatial information of the spectrum where the light signal is incident on the phtotconductor and a spread spectrum code is applied to one electrical terminal on the photoconductor and the electrical ouput taken from the other terminal as described in Patent application 8816250.

Polarisation rotators may be used to correct for unwanted polarisation change at any point in the system.

Optical sampling systems may also be used to sample the optical output of the couplers prior to electronic detection.

Electrical coils could be wrapped around the walls of the size varying cylinders such that the polarisation of the optical wave can be varied by Faraday rotation by passing a current through the coils. Detection and feedback techniques could be used to maintain a given polarisation state.

These fibre analysers can be used in distributed temperature sensors as described in patent application GB2190186A (date of filing 9th May 1986) to measure the complete spectrum of the Raman and Brillouin lines by arranging for the spectrum of the autocorrelation function and the pulsed signal not to overlap, and by using processing techniques in the detector.

In the sensor a laser or LED is modulated with a pseudo-random code for example maximal length or Golay codes and the beam launched down the fibre. The backscattered light is then passed through the fibre spectrum analyser. The photodetector, which could be an avalanche photo-diode or a photoconductor is used to detect and correlate the signal to obtain spatial information by switching the bias on the APD or photo conductor (as described in patent application 8816250 "Low noise opto-electronic correlators and mixers" ) with a delayed version of the spread spectrum code where the delay is equal to the round trip time in the fibre between the optical source and the wanted portion of the fibre. As the delay is varied the spectrum of the backscatter from different points can be obtained.Similarly a DC bias can be applied to the photodetector and a multiplier placed after the photodetector in which the correlation occurs where one input to the multipler is from the photodetector and the other input is the delayed version of the spread spectrum code. By arranging for the spectrum of the autocorrelation function and the spectrum of the spread spectrum code not to overlap, the spatial variation of spectrum can be obtained.

The light signal is incident on the photoconductor and a spread spectrum code is applied to one electrical terminal on the photoconductor and the electrical ouput taken from the other terminal as described in Patent Application 8816250.

Measurement of the input signal at high speed using analogue techniques can be achieved. The correlation waveform produced by the reference signal can be varied in frequency by passing the reference signal down a section of the total delay used for the input signal. The optical reference signal can then be multiplied with the input signal in the detector. This can also be used for measuring multiple sidebands. Coherent gain of optical signals may also be possible by adding this optical signal.

It may also be possible to measure strain by monitoring the frequency of the Brillouin and Raman lines which change with strain.

A combined temperature and strain sensor may therefore be built by monitoring the amplitude and frequency of the Raman and Brillouin lines allowing a combined temperature and strain sensor. The ratio of Stokes to Antistokes and also to Rayleigh lines would remove the effect of the strain on the temperature sensor.

Another System Another system is shown in Figure 5. A Michelson Interferometer containing a beam splitter (1) and two mirrors (2) and (3) is set up, however one path contains a rotating glass block (4). As light travels more slowly in glass (c/n) where c is the free space speed of light and n is the refractive index of glass) the delay varies with angle of rotation.

By varying the delay in one path and keeping the delay constant in the other path the output of the interferometer when applied to a photo-diode (square law device) is the auto-correlation function of the input waveform. This waveform is sampled and digitised in a sampler and digitiser (6). The Fourier transform and hence spectrum is calculated and then displayed in a hardware processor and or computer (7). This instrument can be interfaced to a personal computer with a mouse, light pen and touch screen display monitoring. These spectrum analysers can also be used as optical frequency counters. These analysers can be used to measure the coherence length of a semiconductor laser.

To calibrate the system a reference laser for example a He-Ne or semiconductor laser of known frequency is passed down the delayed path and used to produce an autocorrelation function consisting essentially of a sine wave. This sine wave is then used to trigger the sample and hold and digital to analogue convertor at the zero crossings and at the peaks.

Claims

1. An optical fibre spectrum analyser comprising two high birefringence optical fibres, means for coupling signal to be measured into both fibres, means for electromechanically stretching the fibres to produce a scanned delay between input and output optical signals, means for combining output signals on a photodetector, means for sampling output from photodetector, means for plotting spectrum by performing Fourier Transform, and means for measuring the spatial distribution of the incident light by launching a modulated laser signal into a medium and measuring the backscatter using the said spectrum analyser.

2. A spectrum analyser as described in claim 1 where the spectrum of light scattered from an optical fibre is measured to obtain the spatial variation of the spectrum of the backscattered Ram an or Brillouin light and hence the spatial distribution of temperature along the fibre.

3. A spectrum analyser as described in claim 1 where only one of the fibres is stretched.

4. A spectrum analyser as described in claim 1 where only a single high birefringence fibre is used and the birefringence is changed by electromechanical stretching to cause a scanned delay between the two linear polarisation states where the output light is then sampled as in claim 1.

5. An optical fibre spectrum analyser substantially as described in the drawings.