Field of the Invention

The invention relates to a fibre-optic mode scrambler for homogenising laser illumination over a short timescale.

Background

Laser illumination is widely used in scientific and commercial applications, mainly because of its narrow spectral width and high radiance. However, the coherence properties of laser light also leads to unwanted interference fringes and speckle, making homogeneous illumination hard to achieve. Beam homogenisers may be used to overcome this drawback. These generally work by dynamically scrambling the wave front, resulting in fast moving speckle patterns that average out in time as well in space (e.g. in the case of a digital camera, over the exposure time and pixel area). Most beam homogenisers however, including commercial devices, operate in free space with mechanically moving parts, limiting the device to integration times of the order of milliseconds. Devices based on moving turbid fluids can achieve high speeds but suffer from high power losses as they scatter the incident power almost isotropically. Beam homogenisers based on fibre-optic mode scramblers, in contrast, reduce the radiance of the incoming laser only as much as given by the fibre core diameter and numerical aperture, can move at high frequency due to the small mass involved, and can facilitate in guiding the scrambled light to where it is needed. A problem with existing fibre-optic mode scramblers, however, is that the timescales required for homogenisation are generally too long for short exposure times. Homogenisation may require timescales of the order of milliseconds, whereas some applications require exposure times of the order of microseconds.

Summary of the Invention

In accordance with a first aspect of the invention there is provided a fibre-optic mode scrambler comprising: a series of fibre guides arranged to hold a portion of an optical fibre with a defined curvature between a pair of the fibre guides; an electroactive transducer positioned between the pair of fibre guides and arranged to contact the portion of the optical fibre along a contact length of the optical fibre; and a control module arranged to provide a drive signal to the transducer. In particular, the electroactive transducer may be arranged to contact the optical fibre only along the contact length. The contact between the electroactive transducer and the optical fibre may be a loose contact, i.e. the optical fibre may be able to move away from the electroactive transducer in response to an impact from the optical transducer. The position of the electroactive transducer is thereby adjustable to adjust a contact pressure between the transducer and the optical fibre.

Such a mode scrambler may provide high-speed, effective scrambling with the advantages of a fibre-optic based system (e.g. easy repositioning of the input source). In particular, the mode scrambler may provide scrambling at speeds far higher (e.g. sub-millisecond scrambling times) than the resonant frequency of the electroactive transducer.

In some embodiments, the contact length may be between 0.5cm and 2 cm, or between 0.8cm and l .2cm. Thus a whole length of the optical fibre may be actuated by the electroactive transducer, not just individual points. Contacting this optical fibre over such a length ensures that only a small pressure is exerted on the optical fibre by the electroactive transducer.

In some embodiments, the electroactive transducer may be a piezoelectric actuator.

In some embodiments, the electroactive transducer may be in the form of a membrane. Thus the electroactive transducer itself may sustain spatio-temporal dynamics, such as waves, as opposed to a rigid piezoelectric transducer.

In some embodiments, a resonant frequency of the electroactive transducer is between 20 and 50 kHz.

In some embodiments, each of the fibre guides may comprise a rod with a groove arranged to hold the optical fibre and allow vibrational movement in a direction parallel to a plane of the curvature of the optical fibre. The optical fibre is thus free to move away from the electroactive transducer in response to an impact from the electroactive transducer. In some embodiments, the fibre guides may be arranged to hold an optical fibre having a core diameter between lOOpm and 300pm, or between l50pm and 250pm. Scramblers using electroactive transducers with resonant frequencies in the range 20- 50kHz may be particularly effective when used with optical fibres having core diameters in these ranges.

In some embodiments, the resonant frequency of the electroactive transducer may be related to the core diameter of the optical fibre that the fibre guides are arranged to hold. For example, if a narrower core optical fibre is to be used, the fibre-optic mode scrambler may comprise an electroactive transducer with a higher resonant frequency. For example, if an optical fibre with a core diameter less than 150 pm is to be used, a resonant frequency higher than 40kHz may be necessary to achieve effective mode scrambling. Such narrow cores may desirable for certain applications, such as total internal reflection fluorescence (TIRF) microscopy.

In some embodiments, the pair of fibre optic guides may be arranged to hold a portion of the optical fibre with a radius of curvature of between 3cm and 9cm, or between 5cm and 7cm, or between 5.5cm and 6.5cm, or may be approximately 6cm.

In some embodiments, the pair of fibre guides may be arranged to hold a plurality of portions of optical fibre. The electroactive transducer may be arranged to contact each portion of the plurality of portions of optical fibre held by the pair of fibre guides. For example, the plurality of portions may comprise portions of separate optical fibres, and/or may comprise a plurality of portions of the same optical fibre. For example, an optical fibre may be wound in multiple loops, a portion of each loop being held by the pair of fibre guides. Contacting multiple loops of the same optical fibre with the electroactive transducer may increase the scrambling speed.

In some embodiments, the series of fibre guides may comprise a plurality of pairs of fibre guides, each pair of fibre guides arranged to hold a portion of the optical fibre with a defined curvature. The fibre-optic mode scrambler may comprise a plurality of electroactive transducers; each transducer positioned between a respective pair of fibre guides and arranged to contact along a contact length the portion of the optical fibre held by the respective pair of fibre guides. Using a cascade of multiple transducers may increase the scrambling speed. In some embodiments, the fibre-optic mode scrambler may further comprise a static mode scrambler arranged to contact the optical fibre. For example, the static mode scrambler may be a micro-bend mode scrambler, such as that described in GB2405488(B), which is hereby incorporated by reference. The static mode scrambler may strongly couple the fibre modes, helping to equalise the mode population. The static mode scrambler may this increase the efficiency of the fibre-optic mode scrambler, and help to make full use of the available numerical aperture. In particular embodiments, the static mode scrambler may be arranged to contact the optical fibre before the electroactive transducer. It has been found that this arrangement may lead to smoother output beam. Alternatively, the static mode scrambler may be arranged to contact the optical fibre after the electroactive transducer, which may lead to better initial averaging of the speckle pattern of the beam passing through the fibre-optic mode scrambler.

In some embodiments, the fibre-optic mode scrambler may further comprise a tuning arrangement operable to adjust a contact pressure between the electroactive transducer and the optical fibre. The tuning arrangement may be operable to adjust the contact pressure to optimise the scrambling provided by the fibre-optic mode scrambler.

In some embodiments, the tuning arrangement may comprise an audio transducer arranged to detect sound from a vibrating contact between the transducer and the optical fibre. The control module may be arranged to receive an output from the audio transducer, and may be configured to adjust the contact pressure to maximise the output from the audio transducer. For example, the control module may be configured to adjust a position of the transducer to adjust the contact pressure.

Surprisingly, it has been found that at a critical contact pressure, an audible white noise is created by the vibrating contact between the electroactive transducer and the optical fibre. The onset of this white noise coincides with a drastic improvement in the speckle averaging produced by the fibre-optic mode scrambler. Thus the effectiveness of the mode scrambler can be optimised by listening to the sound generated by the vibrating contact. This allows the fibre-optic mode scrambler to be easily tuned by a simple arrangement of an audio transducer connected to the control module. In some embodiment, the fibre-optic mode scrambler may further comprise an optical fibre held by the series of fibre guides. In accordance with a second aspect of the invention there is provided a method of constructing a fibre-optic mode scrambler, the method comprising: holding an optical fibre with a series of fibre guides such that a portion of the optical fibre is held with a defined curvature between a pair of the fibre guides; and positioning an electroactive transducer between the pair of fibre guides to contact the optical fibre along a contact length, the electroactive transducer controlled by a drive signal from a control module.

The method may further comprise adjusting a contact pressure between the electroactive transducer and the optical fibre. In particular, the contact pressure may be adjusted based on a sound from a vibrating contact between the transducer and the optical fibre. The contact pressure may be adjusted so as to maximise the sound from the vibrating contact. The contact pressure may be adjusted manually by an operator listening to the sound generated by the vibrating contact between the electroactive transducer and the optical fibre; or automatically using an audio transducer as described above in relation to the first aspect of the invention.

Detailed Description

The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which:

figure 1 is a schematic representation of an example fibre-optic mode scrambler;

figure 2 shows results of speckle contrast for various types of optical fibre; figures 3a and 3b are schematic representations of an alternative fibre-optic mode scrambler;

figure 4 shows speckle patterns generated by the fibre-optic mode scrambler of figures 3a and 3b, taken at various exposure times, and for polarised (top row) and unpolarised (bottom row) light;

figure 5 shows the measured speckle contrast and shot noise versus exposure time; and

figure 6 illustrates a method of constructing a fibre-optic mode scrambler. Figure 1 shows an example fibre-optic mode scrambler 100, comprising an electroactive ultrasonic transducer 101 in loose contact with an optical fibre 102. A portion l02a of the optical fibre 102 is held between a pair of fibre guides l03a, l03b. The pair of fibre guides l03a, l03b holds the portion l02a such that it has a defined curvature, i.e. the fibre guides l03a, l03b hold the portion l02a with a slight bend. Bending the fibre 102 changes the optical path lengths within the fibre 102. The fibre guides l03a, l03b in the illustrated example are separated by a distance of approximately 5cm.

Additional fibre guides l04a, l04b, cause further bends in the portion l02a to produce the defined curvature. Portion l02a passes between the fibre guides l03a and l04a; and between the fibre guides l03b and l04b. This results in a defined curvature of the optical fibre 102 in a way which allows deformation of the plane of curvature.

The electroactive transducer 101 loosely contacts a portion l02a between the pair of fibre guides l03a, l03b. This is achieved by gently pushing the electroactive transducer into contact with the fibre 102 such that an emitting surface of the transducer 101 contacts the portion l02a along a contact length l _c . The contact length is typically around lcm.

In use, the transducer 101 is driven at a resonant frequency by a drive signal from a control module 111. For example, the drive signal may comprise a square wave, and may have an amplitude of 20Vpp, equivalent to 9Vrms in the fundamental wave.

The contact pressure between the electroactive transducer 101 and the optical fibre 102 may be adjusted to optimise the scrambling effect. Surprisingly, it has been found that at a critical weak contact pressure point, an audible white noise is emitted from the vibrating contact between the electroactive transducer 101 and optical fibre 102, and the speckle averaging of the scrambler 100 is dramatically improved. The contact pressure can thus be optimised by adjusting the pressure to maximise the sound produced. This may be performed manually, by an operator listening for the sound and adjusting the pressure. Alternatively, an audio transducer, such as a microphone, may be positioned to detect any sound produced by the vibrating contact between the electroactive transducer 101 and the optical fibre 102. The audio transducer may communicate the received signal to the control module 111, which may adjust the contact pressure until the amplitude of the detected sound is maximised. The control module 111 may adjust the contact pressure by adjusting the position of the electroactive transducer 101.

In an alternative method of optimising the contact pressure between the electroactive transducer 101 and the optical fibre 102, the intensity fluctuations of light exiting the mode scrambler 100 may be observed using a spectrum analyser. Optimum alignment of the electroactive transducer 101 relative to the optical fibre 102 is achieved when the spectral power in the high frequency tail of the observed spectrum is maximised.

To further increase scrambling efficiency, and to make use of the full numerical aperture of the fibre, the scrambler 100 may further comprise one or more static mode scramblers The microbends introduced by the static mode scramblers may strongly couple the fibre modes and help to equalise their population, leading to smaller speckle compared to the overall beam size, as well as a more homogenous beam profile. For example, the static mode scrambler may comprise a static mode scrambler as described in GB2405488 (B), which is incorporated herein by reference.

The effectiveness of the scrambler 100 may be demonstrated by analysing the rms speckle contrast of speckle patterns of beams transmitted through the optical fibre 102. Figure 2 shows the speckle contrast results for a fibre-optic mode scrambler such as scrambler 100, for different types and core diameters of optical fibre 102. Unpolarised light of 532nm was used, and the exposure time was controlled by the camera recording the speckle patterns. Alternative positions of a static microbend scrambler were used - either before or after the electroactive transducer

In figure 2, the open symbols represent the speckle contrast with the microbend after the transducer, and the filled symbols represent the speckle contrast with the microbend before the transducer. Black symbols represent an exposure time of 0.2ms, red symbols represent an exposure time of 20ms.

The three polymer clad fibres represented on the right of figure 2 (BLF37-200, FT200- EMT, BFL48-200) are high-numerical aperture (NA) polymer clad fibres with a core diameter of 200pm; the silica clad fibres represented on the right of figure 2 (AFS50- 125Y, AFS 105-125Y, AFS200-220Y) are silica clad with a numerical aperture of 0:22 and core diameters of 50pm, l05pm and 200pm respectively.

The measurements in figure 2 illustrate that core diameter of the fibre is an important factor in performance of the scrambler, as both silica and polymer clad fibres with 200pm core diameters work approximately equally well. In contrast, the smaller core diameter fibres show reduced speckle averaging, i.e. their speckle patterns are almost static. This is consistent with coupled non-linear oscillators (transducer and fibre) being the cause of efficient scrambling. For some applications, such as total internal refection fluorescence (TIRF) microscopy, an effective fibre mode scrambler with a smaller core diameter may be achieved using an ultrasonic transducer operating at a higher frequency.

The silica clad fibre AFS200-220Y was chosen for the final design because its speckle pattern appears to average out to a somewhat more uniform beam profile at long exposure times. However, for some applications this disadvantage may be outweighed by the smaller speckle-to-beam size ratio achievable with higher numerical aperture.

The data in figure 2 also demonstrate that placing the static microbend mode scrambler before the ultrasonic transducers leads to a smoother end result, even though the reverse order may lead to slightly better averaging initially.

Figures 3a and 3b show a top-down view and side view respectively of an alternative example fibre-optic mode scrambler 300. Scrambler 300 comprises a cascade of multiple electroactive transducers 301, each contacting portions 302a of an optical fibre 302 held between respective pairs of fibre guides 303a, 303b, 304.

The optical fibre 302 is wound around two supports 305, 306 and through the fibre guides 303a, 303b, 304 to create a stadium shaped coil. As can be seen most clearly in figure 3b, multiple loops of the optical fibre 302 are wound around the supports 305, 306 and through the fibre guides 303a, 303b, 304, so that each electroactive transducer 301 contacts a plurality of portions 302a of the optical fibre 302. In the illustrated examples 15 loops of the optical fibre 302 are used, with four electroactive transducers 301, yielding 60 points of contact between the fibre and an electroactive transducer 301. In other embodiments, any other number of optical fibre loops and transducers 301 (with respective numbers of pairs of fibre guides 303a, 303b) may be used. For example, between 10 and 20 loops may be used, with between 2 and 6 transducers 301. Using multiple loops of optical fibre wound through a cascade of multiple electroactive transducers 301 increase the speed of scrambling.

The illustrated scrambler 300 further comprises optical connections 309, 310, connected to respective first and second ends of the optical fibre 302. The optical connections 309, 310 allow light to be coupled into the optical fibre 302, and transmitted from the optical fibre 302 after being scrambled. The connections 309, 310 may allow further optical fibres to be coupled to optical fibre 302, or may be configured to receive an incident light beam which is not confined to an optical fibre. In alternative examples, first and second ends of the optical fibre 302 may extend out of the coil to enable light to be coupled into the optical fibre 302. In such examples optical connections 309, 310 may be omitted.

The scrambler 300 may be constructed by tightly winding the fibre 302 around the half-cylinder supports 305, 306, separated by more than their diameter, to create a stadium-shaped coil. The coil is then clamped and the half supports 305, 306 brought closer together to produce some slack in the fibre 302. In the illustrated example, the supports 305, 306 are able to slide towards each other when clamping bolts 307, 308 are loosened. Fibre guides 303a, 303b, 304 (e.g. nylon M6 bolts) are then inserted one by one to shape the straight fibre sections into a series of bends, while keeping the individual loops evenly separated. On every other bend a transducer 301 is brought into loose contact with the fibre, with the position of each transducer 301 being optimised individually for maximum audible noise as described above. A control module 311 provides a drive signal to each transducer 301. The control module 311 may also be configured adjust the contact pressure for each transducer 301 by individually adjusting the position of each transducer 301.

Figure 4 shows speckle patterns for a scrambler similar to scrambler 300, recorded using a Thorlabs (RTM) beam profiler with the CCD chip surface positioned l24mm from the output facet of the multimode fibre 302. The pixel size of the CCD was 6.45pm x 6.45pm. With these parameters, the average speckle size was around 60 pixels, and spatial averaging could be neglected. In order to exclude any effects of laser linewidth, a single-frequency Titanium Sapphire laser at approximately 780nm was used for illumination. For measurements of polarised light, an additional polarising beam splitter was inserted between the fibre end (i.e. optical connection 310) and camera. The illuminating intensity was adjusted for each exposure time to give a CCD saturation around 0:4...0:7. The CCD chip used in the beam profiler had an electronic shutter, and the exposure time was varied from 20ps to 20ms using the beam profiler software. However, because of finite attenuation of the electronic shutter, it was necessary to also switch the illuminating laser with an acousto-optic modulator (AOM). To accommodate any jitter in signals or delay times the AOM was switched 5ps before after the camera-defined exposure window.

In figure 4, the top row of images show the results of the polarised measurements, and the bottom row of images show the results of the unpolarised measurements. As can be seen, as exposure time increases, so does the uniformity of the speckle pattern. The uniformity is very high for exposure times above 2ms. However, the uniformity is still very good even at 0.2ms, illustrating that a fibre-optic mode scrambler according to the present invention is capable of sub-ms scrambling times.

Images were analysed by choosing a region of interest / _ί;· in the centre of the beam, where the variation of the average beam profile was negligible, and normalising to obtain only the fluctuations in relative intensity A _tj , where

with

where the sum is over all pixels {i=l,...,N, JH) within the region of interest.

Of the resulting normalised sub-image, the autocorrelation function was calculated and normalised to give the mean square at zero displacement. Shot and electronic read-out noise was uncorrelated across pixels and appeared as a single-pixel peak in the autocorrelation; in contrast the speckle-related peak was much broader. Consequently, the two contributions can be separated, and the mean square speckle contrast can be determined from the averaged nearest neighbours of the zero displacement peak, while the remainder to the actual peak height is attributed to shot and readout noise.

The measured static speckle diameter on the CCD chip was 432pm, as determined from the FWHM of the autocorrelation function with the transducer switched off. The speckle diameter d corresponding to the equivalent speckle area in the case of a uniformly illuminated circular random scatterer is given by d = 4 z/nD, where z is the distance from the scatterer and D the diameter of the illuminated spot. Applying this formula to the fibre core with D = 200pm yields an expected speckle diameter d = 620pm, which compares reasonably well to our measured value, despite the two being based on differing definitions (FWHM vs. equivalent area) and physical models (multi -mode fibre core vs. uniformly scattering disc).

Fully developed, single mode speckle has a contrast (D/ ² )/(/) ² = 1, where the average is an ensemble average. For static speckle patterns, the average can be taken over space; for dynamically scrambled speckle, assuming ergodicity, the average can be taken over time at one position in space, or over space in an instantaneous snapshot. As speckle are assumed uncorrelated, the power spectral density of fluctuations, in space or in time, should be at and band-limited to a frequency f _n , referred to here as the noise bandwidth, which corresponds to the speckle size or the correlation time, respectively. The power spectral density can then be estimated as i _n = l/f _n below f _n and above zero. A finite exposure time T acts as a moving-average low pass filter with a single-sided noise bandwidth of 1/2T. The filtered power will then be « 1/2 Tf _n and the rms contrast: rms

In unpolarised speckle, the local instantaneous intensity is the sum of two statistically independent speckle patterns; consequently, the contrast is expected to be reduced by a factor of 1/V2 compared to polarised speckle.

Figure 5 shows the speckle contrast and shot noise (including readout noise) versus exposure time T. Speckle contrast scales as T ^m for short exposure times (see equation (2)). For long exposure times, the scaling is obscured by the appearance of static inhomogeneities. Shot noise dominates the image contrast for exposure times beyond 2ms.

The mode scrambler was tested with an input of up to around 500mW; the theoretical power handling capability of the fibre should be much higher. However the transmission efficiency is only approximately 60% at 780nm. Of this, around 8% can be attributed to Fresnel reflections at the uncoated input and output facets of the fibre; attenuation in the fibre itself should account for less than 10%. The remaining substantial loss may be the result of the many bends involved in the mode scrambler - 15 full turns close to the minimum specified bend radius 50mm, plus two microbends in the static mode scrambler.

Figure 6 illustrates a method 600 of constructing a fibre-optic mode scrambler, such as a scrambler 100 or scrambler 300.

In a first step 601 of the method, an optical fibre is held with a series of fibre guides such that a portion of the optical fibre is held with a defined curvature between a pair of the fibre guides

In a second step 602, an electroactive transducer is positioned between the pair of fibre guides to contact the portion of optical fibre along a contact length. The electroactive transducer is controlled, in use, by a drive signal from a control module, as described above in relation to scrambler 100.

The contact pressure between the electroactive transducer and the optical fibre may be adjusted to optimise the speckle averaging of the mode scrambler. For example, the sound generated by the vibrating contact between the electroactive transducer and the optical fibre may be maximised, as described above in relation to scrambler 100.

Other embodiments are intentionally within the scope of the invention as defined by the appended claims.