GB2245715A - Optical sampler - Google Patents

Abstract

An optical sampler comprises an all-fibre Sagnac loop (6) having one port (12) coupled to a source of pulses to be sampled (22) via an optical coupler (18). A pulsed laser source (2) is also coupled to the port (12) by the coupler (18). The loop (6) exhibits a non-linearity which is sufficient for the optical sampling pulses from the laser (20) to cause a relative phase shift in part of the signals from the laser (22) as they propagate round the optical fibre loop (6) whereby that part of the optical signal is switched to a second port (16). The signal output from the port (16) is sampled via a detector (26) and displayed on an oscilloscope (30). By making the repetition rate of the optical sampling pulses different from that of the signal to be sampled, the sampling pulse will switch successive parts of the signal to be sampled to the output port (16) with each period. <IMAGE>

Description

OPTICAL SAMPLER This invention relates to an optical sampler and in particular, but not exclusively, to its use as part of an oscilloscope for displaying the shape of repetitive optical signals.

Currently oscilloscopes for displaying the optical shape of pulses include an optical detector onto which the optical pulses are directed. The optical detector converts the received optical signal to an electrical signal which is displayed on a conventional oscilloscope. The resolution of the displayed pulse shape is limited by the speed of response of the optical detector. By using photodiodes a resolution of 50ps is attainable. If higher resolutions are required it is necessary to use other methods of analysis. Streak cameras can provide resolutions in the order of 2ps for example but only at great capital expense.

According to the present invention an optical sampler comprises: an optical waveguide coupler having a first and a second port, constituting a first pair of ports, and a second pair of ports in which substantially equal first signal portions of an optical signal at a first wavelength received at a port of one pair are coupled to the two ports of the other pair of ports; an optical waveguide coupling together the second pair of ports and including an interaction section which includes a material having a non-linear refractive index; a source of optical sampling pulses at a second wavelength; control means for controlling the repetition rate of the optical sampling pulses; a first coupling means for coupling the optical sampling pulses to the interaction section so the optical sampling pulses propagate along it in substantially one direction;; a second optical coupling means for coupling an optical signal at the first wavelength to the first port; the magnitude of non-linearity of the non-linear material being sufficient for the optical sampling pulses to cause a relative phase shift in part of the first signal portions as they propagate round the optical fibre whereby that part of the optical signal at the first wavelength is switched to the second port.

In this specification by "non-linear" we mean that the refractive index of a material varies with the intensity of the transmitted signal. Typically the refractive index n is given by the formula n + nO + n2/E/2 where nO is the linear refractive index, n2 is the Kerr coefficient and /E/2 the intensity of the transmitted signal.

By "optical" is meant that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.

For a zero intensity optical sampling pulse the first optical waveguide coupler and the optical waveguide, which form a Sagnac antiresonant interferometer, act as a loop mirror to an optical signal coupled to the first port of the first pair of ports (the "input port") in that the signal entering the coupler at the first port will be reflected, i.e. it will exit, from the same port. This is because the two counter propagating portions are of substantially equal intensity and so maintain the same relative phase as they travel around the loop. When an optical sampling pulse propagates along the interaction section of the waveguide it induces, via its interaction with the non-linearity of the interaction section, a phase shift in that part of the first portion which co-propagates with it.The condition for reflection is therefore broken and some, and preferably substantially all, of the repetitive optical signal to be sampled exits the second port of the first pair of ports (the "output port").

If the repetition rate of the optical sampling pulses is equal to the repetition rate of the input signal they will stay synchronised with it in which case the parts of the repetitive signal switched to the second port, i.e. the sampler output, form a series of pulses of constant amplitude. If the repetition rate of the optical sampling pulses is different from that of the signal to be sampled the sampling pulse will switch successive parts of the signal to be sampled to the second output port with each period. The output in this case is a train of pulses modulated in the same way as the input signal being sampled but with a longer period.If the frequency of the optical sampling pulses, f2, is given by f2 = (n + 6). fl where n is an integer, s < 1, and fl is the repetition rate of the signal to be sampled, then the period output of the output signal from the sampler is given by output /6 output input where tinput is the repetition period of the input signal.

By making a 1 the period can be made sufficiently large for the output signal to be detected and the sampled signal waveform to be resolved using a photodiode and amplifier combination having a response time much slower than the repetition rate of the sampled, input signal and displayed on a standard oscilloscope.

The resolution of the oscilloscope display is limited by the width of the optical sampling pulses which equal the width of the switched part of the input signal. The present invention can therefore readily achieve resolutions of less than a picosecond.

The optical waveguide coupler may be a dichroic coupler coupling substantially all of an optical signal at the second wavelength received at one port of one pair of ports to one port of the other pairs of ports. In this case the first coupling means may conveniently comprises a dichroic coupler for coupling both the optical signal at the first wavelength and the optical sampling pulses at the second wavelength to the first port.

This permits coupling of both the optical signal to be sampled and the optical sampling pulses to the first port. The dichroic optical waveguide coupler then provides that the input optical signal is split into two equal intensity counterpropagating portions whilst the optical sampling pulses propagate in one direction only round the waveguide loop and hence the interaction section.

Alternatively, the optical sampling pulses can be coupled to the interaction section by means of a pair of dichroic couplers, one located at a respective end of the interaction section.

Other schemes for selectively coupling the optical sampling signals for one-way propagation through the interaction may be devised for use with the present invention.

The waveguide loop coupling the second pair of ports together may constitute the interaction region. Alternatively a non-linear material may be introduced into loop. For example a semiconductor doped optical fibre may be spliced into the loop.

This may be advantageous if lower power optical sampling pulses are to be used to obtain a sufficiently high non-linear interaction to effect satisfactory switching to the output port.

The optical sampler may include means for separating signals at the first and second wavelengths which exit the second port.

This may comprise a dichroic coupler, filter or other demultiplexing means. This is to separate the sampled parts of the input signal from the sampling pulses if both exit the second port if necessary.

The waveguides may comprise single mode optical fibres as they provide a particularly simple medium for constructing the optical sampler using well established technology for the production of the couplers and waveguide loops. Other waveguides may be used to carry out the present invention, for example planar waveguides on a lithium niobate substrate. The present invention is not restricted to any specific type of waveguide; clearly any waveguide technologies or combinations of- them, having the appropriate properties may be used to carry out the present invention.

According to a second aspect of the present invention an optical sampling oscilloscope comprises an optical sampler according to the first aspect of the invention including; an optical detector optically coupled to the second port and an electrical oscilloscope for displaying the electrical output from the optical detector.

Embodiments of the present invention will now be described with reference to the accompanying drawings in which Figure 1 is a schematic diagram of an optical oscilloscope according to the present invention; Figure 2 is a graph of the calculated non-linear phase imposed on a cw input signal by an optical sampling pulse; and Figure 3 is a oscillogram of a sampled optical signal.

The optical sampling oscilloscope of Figure 1 comprises a Sagnac antiresonant interferometer 2 defined by a single silica telecommunications optical fibre 4 formed into an optical fibre loop 6 with portions of the optical fibre 6 being formed into a dichroic fused optical fibre coupled 8 having a first pair of ports 10,12 and a second pair of ports 14, 16. In this embodiment the fibre of the loop 2 itself constitutes the interaction section as it exhibits a non-linear refractive index. The fibre loop 6 was 100m long.

A modelocked Nd:YAG laser 20 provides a pulsed optical sampling signal with a pulse width of about 100ps and repetition rate of 75.66mhz at l.3um which is coupled into the first port 10 by means of a dichroic coupler 18.

Polarisation controllers 27 in the loop 27 are used to obtain matching of the counter-propagating portions of the input signal for complete reflection and transmission.

A gain-switched DFB laser 22 provides a repetitive optical signal at 1.53rum, constituting the input signal, which is also coupled to the port 10 of the coupler 8 by means of the optical coupler 18.

The coupler 8 is manufactured in well known manner so as to couple equal portions of the input signal coupled to port 10 to the ports 14 and 16 to produce two counterpropagating equal intensity portions in the loop 6 and to couple substantially all of the pulsed optical sampling signal into port 14 (an extinction ratio of 37dB at 1.3;m) so the sampling signal propagates in only one direction in the loop 6.

The operation of the oscilloscope of Figure 1 will now be explained in terms of a cw optical input signal at 1.53;m and a pulsed optical signal at l.3m are propagating round the loop 6.

Under these conditions the portion of the cw signal co-propagating in the same direction and with the pulsed optical signal can be described by the following pair of coupled equations in normalised units.

In equations 1, A is the pulsed signal and B is the input signal which is propagating in the same direction as A. The group delay is given by ss'A and ss'B for the appropriate waves. Since B is small we can neglect terms of order B2. In addition, if we transform into the frame moving with the group velocity of the B wave then equation (1) becomes

where ss ss A ss B ( ) is the difference in group delays of the two waves. Note, we have also neglected the SPM of the pulse signal since the nonlinear response is unaffected by this term.

The solution for A is simply a travelling wave given by

The equation for B can now be integrated to give

where L is the length of the loop 6. Equation (5) is exact even when SPM is included in the pump. The expression in brackets in equation (5) represents the phase change f of the input signal (B) caused by the pulsed optical sampling signal Pa(t) The reflected B signal from the loop mirror can be simply expressed in terms of this phase as Bref = B (1 + cos ())Bin/ (6) (see N.J. Doran and D. Wood, nonlinear Optical Loop Mirror" Optics Lett 13 56-58 (1988)). This expression shows that the low power B signal is modulated by the high power A signal pulse.In addition, the difference in group delay between the two signals leads to a broadening of the reflected pulse because the phase depends upon the integral of the pump pulse PA(t). As an example, if the A signal is given by A(t) = U sech(t) (7) then the nonlinear phase change is given by +(t) = u2 (tanh(t) - tanh(t-Ass'L)/Ass' (8) This will reduce the resolution of the optical sampler, however it is possible to eliminate this problem by ensuring that the zero dispersion wavelength is midway between the two wavelengths or by using dispersion flattened fibre.

Referring briefly to Figure 2 there is shown the nonlinear phase for a loop length which is small compared to the pulse walk off length l/(Ass' ). The non-linear phase charge is converted by the interference of the counterpropagating portions at the coupler to a modulation in the probe signal.

The pulse repetition rate of the laser 20 is controlled by a control 24 in a known manner to be slightly different from an integer multiple that of the input signal to be displayed, provided in this example by the laser 22 which was modulated at 0.45GHz. Those parts of the input signal switched to the output port 16 of the coupler 8 are selectively routed to an optical detector 26 via a dichroic coupler 28. The electrical output of the optical detector 30 is displayed by an oscilloscope 30.

Figure 3 shows an oscillogram of the optical signal reaching the detector 26 when 6 = 1 x 10 5 displayed on a millisecond time scale on the oscilloscope 30.

In this particular example a 10W peak power sampling pulse is required to completely switch a part of the input signal from the laser 22 to the port 16 of the coupler 8 and the detector 26.

The clock 24 can be used to generate the drive frequency via frequency generators (not shown) for both the laser 20 providing the input signal to be sampled -and the laser 22 generating the optical sampling pulses so as to provide a stable sample of the input signal.

Alternatively, the control means 24 may have a single frequency generator locked to a pulsed synchronisation signal derived from the laser 22 whose output is to be sampled.

Claims (11)

1. An optical sampler comprising an optical waveguide coupler having a first and a second port, constituting a first pair of ports, and a second pair of ports in which substantially equal first signal portions of an optical signal at a first wavelength received at a port of one pair are coupled to the two ports of the other pair of ports; an optical waveguide coupling together the second pair of ports and including an interaction section which includes a material having a non-linear refractive index; a source of optical sampling pulses at a second wavelength; control means for controlling the repetition rate of the optical sampling pulses; a first coupling means for coupling the optical sampling pulses to the interaction section so the optical sampling pulses propagate along it in substantially one direction;; a second optical coupling means for coupling an optical signal at the first wavelength to the first port; the magnitude of non-linearity of the non-linear material being sufficient for the optical sampling pulses to cause a relative phase shift in part of the first signal portions as they propagate round the optical fibre whereby that part of the optical signal at the first wavelength is switched to the second port.

2. An optical sampler as claimed in claim 1 in which the optical waveguide coupler is a dichroic coupler coupling substantially all of an optical signal at the second wavelength received at one port of one pair of ports to one port of the other pairs of ports.

3. An optical sampler as claimed in claim 2 in which the first coupling means comprises a dichroic coupler for coupling both the optical signal at the first wavelength and the optical sampling pulses at the second wavelength to the first port.

4. An optical sampler as claimed in claim 1 in which the coupling means comprises a pair of dichroic couplers one located at a respective end of the interaction section.

5. An optical sampler as claimed in any preceding claim in which the waveguide constitutes the interaction section.

6. An optical sampler as claimed in any one of claims 1 to 4 in which the waveguide interaction section comprises a doped waveguide.

7. An optical sampler as claimed in any preceding claim including means for separating signals at the first and second wavelengths which exit the second port.

8. An optical sampler as claimed in any one of the preceding claims in which the optical waveguides comprise single mode optical fibres.

9. An optical oscilloscope comprising an optical sampler as claimed in any preceding claim including an optical detector optically coupled to the second port and an electrical oscilloscope for displaying the electrical output from the optical detector.

10. An optical sampler as hereinbefore described.

11. An optical oscilloscope as hereinbefore described.