GB2236417A - Optical memory using optical fibre waveguides as second harmonic generator - Google Patents

Abstract

An optical memory has an array of phosphorus-germania doped silica optical fibre waveguides 2-5 capable of self- seeding by an optical signal from a laser 6, at a first frequency coupled a fibre by lens 8 to provide second harmonic (SH) generation. The efficiency of the SH generation is dependant on the intensity of the laser seeding signal which determines the memory state of the seeded fibre. The memory state of a fibre is read by introducing a probe signal from the laser 6 into the fibre and detecting the intensity of the second harmonic by means of a photodetector 12 after focusing by lens 10 and filtering out the remnant probe signal with a filter 14. <IMAGE>

Description

OPTICAL MEMORY This invention relates to optical memories. It relates, in particular, to the use of an optical waveguide capable of self-seeding a phase-matching periodicity by an optical signal at a first frequency to produce second harmonic on subsequent propagation of a signal at the first frequency along the waveguide and to an optical memory which utilises this effect.

According to a first aspect of the present invention a method of using such an optical waveguide as an optical memory having n memory states includes the steps coupling an optical signal at the first frequency at one of n distinct optical powers for a sufficient time to produce an associated, distinct, second harmonic conversion efficiency; and reading the optical waveguide by coupling an optical signal to the waveguide for a time insufficient to alter the second harmonic conversion efficiency of the optical waveguide and measuring the intensity of the second harmonic optical output, the measured intensity being indicative of the memory state of the optical waveguide.

The optical waveguide may be a O.9um core radius silica-germania optical fibre with a core-cladding difference of 0.015. This can provide a memory for which the first frequency is about 1064nm. With such a fibre a convenient range of input intensities is the range 800w to 1700w.

Recent investigations by the applicant indicate that a quicker memory response and greater efficiencies will be obtainable with phosphorus-germania doped silica optical fibres (11/20/0 and 150/0 by weight, respectively). It should be appreciated that other optical waveguides that exhibit such self-seeding of a phase-matching periodicity can be used as a component in the present invention. Similarly the use of materials other than those specifically mentioned in this application having improved response can be used advantageously according to the present invention.

According to a second aspect of the present invention an optical memory comprises at least one optical waveguide capable of being self-seeded by an optical signal at a first frequency to produce an optical signal at the second harmonic of the first signal to which has been coupled an optical signal at the first frequency at one of n distinct optical powers for a sufficient time to produce an associated, distinct, second harmonic conversion efficiency; and optical reading means for determining the second harmonic conversion efficiency of at least one of the waveguides whilst leaving the efficiency unchanged.

Such an optical memory may include an optical writing means for applying an optical signal at the first frequency at one of n distinct optical powers to at least one of the waveguides so that associated distinct second harmonic conversion efficiency may be established in the waveguide.

The optical memory may have a reading means which can determine the second harmonic conversion efficiency of any one of a selected one of the waveguides.

The optical memory may be provided with an optical writing means can selectively apply an optical signal to any one of the waveguides to put it into one of the n memory states.

Each optical waveguide may comprise a germania-doped silica optical fibre or preferably a phosphorus-germania doped silica fibre (11/20/0, 150/0 by weight respectively).

The principle of operation and an embodiment of the present invention will now be described with reference to the accompanying drawings of which Figure 1 is the mode dispersion diagram for an optical fibre; Figure 2 is a graph of the SHG growth of the fibre of Figure 1; Figure 3 is a graph showing the onset of guided mode SHG; Figure 4 is a graph showing the fluctuation is SHG growth; Figures 5 and 6 are graphs showing the decay of SHG power and rise of SHG power on charging the power of the input signal; Referring to Figure 1, fibres 2 to 5 are single mode at 1064 nm (pump wavelength) and 532nm having a core-cladding index difference of 0.015 and a core radius of O.9um. The mode dispersion diagram of the fibre is shown in Figure 2.It can be seen that modal phase-matching does not occur to any mode for a c2.gum. Phase-matching in this fibre is therefore only possible to Cerenkov radiation modes up to that core radius and when first exposed to 1064nm Q switched pulses from laser 6 the fibre 2 produces only Cerenkov radiation SHG.

The optical signal from the laser 6 is coupled via a lens 8 into a selected one of the fibres 2 to 5, here fibre 3, by moving the array of fibres 2 to 5 laterally (by means not shown). The output from the fibre 3 is focused by lens 10 onto a photodector 12 via a filter 14 which allows the second harmonic but not the fundamental, to reach the detector 12.

The laser 6 can be controlled by a controller 16 to emit an intensity sufficient to erase or set the memory in one of the n memory states in any selected one of the fibres 2-5.

The laser 6 can also be controlled to provide a read pulse of a fixed power in which case the intensity of the second harmonic detected by the photodetector 12 will be indicative of the memory state of the fibre.

Alternative arrangements of writing and reading are possible. For example the laser 6 may illuminate all the fibres 2-5 with a read signal, and means provided for allowing the output of only a selected one of the fibres 2-5 to reach the photodetector 12 thereby allowing selective reading of the fibres 2-5.

Referring to Figure 3, after about an hour's exposure, SHG in the LPol mode is visible in the core. This guided SHG grows exponentially for several hours, then saturates at a level 107 times higher than the initial radiation SHG. The maximum conversion efficiency observed at saturation is about 1/20/e (5W peak SH power). It is believed that it is the radiation SHG that acts as the seed for the writing of the X(2) grating that is required to obtain conversion from the IR to the second harmonic guided mode.

Figure 4 shows the initial stage of the preparation process.

The radiation SHG grows by a factor of 10 (to 2xlO 7 W peak) before the onset of guided mode SHG possibly because the radiation SHG must reach a critical intensity necessary to seed the fibre. Experiments on the same fibre type at 1053 nm show similar conversion efficiencies and preparation rates.

It has been noticed by the applicant that sometimes during the latter stages of preparation, the second harmonic power fluctuates as it grows with a period of 10 s and a modulation depth of 100/0. (This is shown in Figure 5). Furthermore, we have observed transient effects in fully prepared fibres. For example, referring to Figure 6, the power launched into a fibre fully prepared at a power of 1.1 kW peak is raised above 1.7kW there is a corresponding transient increase in the second harmonic power followed by a rapid decay (1 min.), reaching lower steady value of about 0.8 W peak.

Referring to Figure 7, if the input power is then suddenly reduced back to 0.8 kW, the second harmonic power falls but then increases slowly until it reaches the original conversion efficiency, taking about fifteen minutes. The SHG efficiency can be cycled between values as shown in Figure 8. We believe these results show competition between writing and erasure with only IR launched into the fibre. These observations suggest that the preparation process involves competition between the writing and erasure of the x#2# erasure of the X(2)grating.Ifthesecond harmonic power is greater than 0.8 W peak, then erasure dominates until the power falls to this maximum equilibrium power.

The reversible self erasure process illustrated in Figure 8 forms the basis of the optical memory according to the present invention. When an optical signal at first wavelength is propagated down the fibre at, in the illustrated example, 1800W for a length of time, the second harmonic power generated by the fibre will rise to point b after which it will decay to point c.

The optical signal can be removed after which the optical fibre will have a SHG conversion efficiency given by the curve O-d,c.

If a short pulse of light at the first frequency is subsequently coupled to the fibre a second harmonic signal will be generated with an efficiency given by the curve O,d,c. In particular interrogation by an 800 W signal would produce an 8K signal indicated by point d. This therefore indicates a first state of fibre.

Passing light at a second intensity for a length of time, in the illustrated example 800W, the SH signal will first have an output power indicated by the point d (because the fibre is in state o-d-c) but it will rise to point a, that is the fibre now has a SH conversion efficiency indicated by the curve o-a-b which again is retained- on turning off the optical signal.

Interrogation by the same 800W signal will now produce SHG output as indicated by point a, this therefore indicates a second state of the fibre.

Further states can be defined - for example exposure at 1000w would set the fibre to point e, a further distinct SHG efficiency state defined by an SH efficiency curve o-e.

This mechanism is applicable to any optical medium capable of self seeding in which the SH conversion efficiency is dependant on the intensity of the optical signal causing the self-seeding.

A fibre can be prepared by external seeding in known manner before using the optical fibre as a memory as described above.

Claims (10)

1. A method of using an optical waveguide capable of self seeding by an optical signal at a first frequency to produce an optical signal at the second harmonic of the first signal as an optical memory having n states including the steps of: coupling an optical signal at the first frequency at one of n distinct optical powers for a sufficient time to produce an associated, distinct, second harmonic conversion efficiency; and reading the optical waveguide by coupling an optical signal for a time insufficient to alter the second harmonic conversion efficiency of the optical waveguide and measuring the intensity of the second harmonic optical output, the measured intensity being indicative of the memory state of the optical waveguide.

2. A method as claimed in claim 1 in which the optical waveguide is a phosphorus-germania doped silica optical fibre, about 11/20/0 and about 15 /o by weight, respectively.

3. A method as claimed in claim 2 in which the first frequency is about 1064nm.

4. A method as claimed in claim 3 in which the n distinct optical powers lie in the range of about 800W to about 1700W.

5. An optical memory comprising at least one optical waveguide capable of self seeding by an optical signal at a first frequency to produce an optical signal at the second harmonic of the first signal to which has been coupled an optical signal at the first frequency at one of n distinct optical powers for a sufficient time to produce an associated, distinct, second harmonic conversion efficiency; and optical reading means for determining the second harmonic conversion efficiency of at least one of the waveguide whilst leaving the efficiency unchanged.

6. An optical memory as claimed in claim 5 including optical writing means for applying an optical signal at the first frequency at one of n distinct optical powers to at least one of the waveguides so that associated distinct second harmonic conversion efficiency may be established in the waveguide.

7. An optical memory as claimed in one of claims 5 and 6 in which the interrogation means can selectively determined the second harmonic conversion efficiency of any one of the waveguides.

8. An optical memory as claimed in claim 6 in which the optical writing means can selectively apply an optical signal to any one of the waveguides.

9. An optical memory as claimed in any one of claims 6 to 8 in which each optical waveguides comprises a phosphorus germania-doped silica optical fibre, about 11/20/0, and about 50/0 by weight, respectively.

10. An optical memory as hereinbefore described.