Patent Description:
Short and high pressure transients in fluids are produced in many industrial and medical applications, for example laser induced breakdown spectroscopy, shock wave lithotripsy and laser vitreolysis. Measuring pressure waves in liquids is usually performed using a hydrophone. Several types of hydrophones already exist, such as those based on a piezoelectric sensor, PDVF membrane or needle hydrophone. The main disadvantages of such hydrophones is a relatively slow response (low bandwidth) and low damage threshold. For measuring shockwaves such as those produced by laser induced breakdown in liquids, a fast (bandwidth above <NUM>) hydrophone capable of withstanding positive and negative pressures in the range of kbar is required and a fiber-optic hydrophone meets those requirements.

There are two types of fiber-optic hydrophones. The first type uses the principle of interference, where the oncoming pressure waves change the optical path difference between two light beams, producing an interferometric response which can be measured and the amplitude of the pressure waves can then be determined.

The present invention, however, is based on another type of a fiber-optic hydrophone, i.e. a reflective fiber-optic hydrophone. In this case a source of probe light, usually from a laser diode is coupled into an optical fiber. The probe light then passes through a fiber-optic component, capable of coupling the probe light into the fiber-optic probe, while coupling the reflected light to another optical fiber, which is guided to a photodetector. The oncoming pressure waves modulate the optical reflectivity of the fiber-optic probe and by measuring the reflected light power with a photodetector, the pressure can be determined. In the simplest form disclosed in <CIT> a flat and perpendicularly-cleaved (to the light propagation direction) fiber tip is used as a fiber-optic probe and changes of the refraction index of the fluid with the oncoming pressure waves cause the modulation of reflectivity at the fiber-fluid interface.

Different structures can be added to the fiber tip or a shaped (i.e. non-flat) fiber tip can be used, usually to increase sensitivity but possibly lowering the damage threshold and making the probe increasingly more difficult to service in case the probe is damaged by the shockwave.

In order to achieve a fast response of a hydrophone, the probe needs to be as small as possible. In fiber-optic hydrophones, the probe volume is the light beam area times the wavelength of the light used. Usually multimode fibers are used, where sufficient optical power (several watts) can easily be coupled into the fiber, providing sufficient sensitivity of the hydrophone. Such fiber-optic hydrophones have a beam diameter around <NUM> and wavelength around <NUM>, already making the probe size significantly smaller when compared to other hydrophone types.

To further increase the bandwidth of a fiber-optic hydrophone and also reduce the lateral position and tilt sensitivity of the probe, a single-mode fiber can be used, with a beam diameter usually around <NUM>. This, however, results in a high probe light intensity which can have detrimental effects for example by locally heating the fluid around the fiber tip due to the absorption of light in the fluid, possibly leading to boiling and thermally induced damage to the surrounding fluid, which will disrupt the measured signal, while thermally-induced damage is also especially problematic in biological and medical applications of the fiber-optic probe.

Consequently, lower probe-light power needs to be used when using a single-mode fiber, usually in the range of <NUM> - <NUM> mW. Using a low power light source and considering the already low reflectivity at the fiber-fluid interface (usually below <NUM>%) and low changes of reflectivity with changes of pressure <MAT>, where R is the fiber-fluid interface reflection and p is the pressure, poses a challenge for measuring such low-power signals while maintaining a high signal to noise ratio. The technical problem is thus the design of a reflective fiber optic hydrophone that addresses these issues. It is the aim of the invention to provide a fiber-optic hydrophone with a high bandwidth and lowering the lateral position and tilt sensitivity with incident light power < <NUM> mW to avoid disruption of surrounding fluid.

A basic fiber-optic hydrophone is described in <CIT>, consisting of a fiber-probe with a beam diameter < <NUM>. The reflected signal is measured with a photodiode with subsequent electronic amplification. Disadvantage of such amplification is lower bandwidth. A possibility to sample a part of the probe-light before reflection is also described to subtract the amplitude fluctuations of the light source from the output signal.

An optical hydrophone with a higher shockwave damage resistance is described in <CIT>. Instead of using the tip of the fiber as a probe, a bulk optically transparent material, being more resistant to shockwave pressure, is used. To maintain the small probe size, which is determined by the light beam size, a set of relay optics is used to focus the light beam onto the bulk-fluid interface. Although this indeed can make a sufficiently small probe area, the bulk material with a much larger size when compared to an optical fiber can disrupt the shockwave propagation in fluid in turn producing measurement that does not reflect the actual pressure in unobstructed shockwave propagation. This solution differs from the present invention in the design of the hydrophone.

A single-mode fiber-optic hydrophone is described in <CIT>. Due to the low probe light power of <NUM> mW, an avalanche photodiode is used to amplify the measured signal. This differs from the present invention. The drawback of using an avalanche photodiode to measure shockwave dynamics is the limited bandwidth of such photodiodes, typically up to around <NUM>.

Another possible geometry of an optic-fiber hydrophone is described in <CIT>. Here, a laser resonator is built with the fiber-probe tip serving as one of the reflectors forming a resonator. Due to the nature of laser resonators, the output power is highly susceptible to the changes of reflection at the resonator reflector. This provides a very high pressure-sensitivity and such a device is useful as a microphone (i.e. measuring very low pressure waves) in air and using this technique as a hydrophone for measuring pressure in fluids is mentioned as well. However, such a device is limited in response time (bandwidth) by the resonator round-trip time (usually above <NUM> ns), leading to a bandwidth below <NUM> and is therefore not suitable as a hydrophone for measuring the shockwave dynamics in fluids, where a bandwidth above <NUM> is required.

The technical problem is solved as defined in the independent claims, wherein preferred embodiments of the invention are defined in the dependent claims. The main difference between the present invention and known reflective fiber-optic hydrophone is the use of an optical amplifier, which has never been proposed or used before in reflective hydrophones. The main advantage of using fiber-optic amplifiers compared to electronic amplifiers is larger bandwidth, i.e. higher than <NUM>. This is a consequence of operation of fiber-optic amplifiers, as they amplify the optical signal/power, wherein electronic amplifiers such as avalanche photodiodes amplify the electronic signal created after conversion of light (photons) into electrical current (charge).

The fast and highly sensitive fiber-optic hydrophone according to present invention thus comprises at least:.

The source of the probe light can operate either in continuous-wave (CW) or pulsed-regime, wherein pulse durations are in the range from <NUM> ns to <NUM>. The wavelength of the source light is chosen such that the light power can be further amplified in the selected single-mode optical fiber amplifier. Such wavelengths are known to a person skilled in the art of fiber amplifiers. Preferably, the wavelength is chosen around <NUM> for further amplification in ytterbium-doped fibers or around <NUM> for further amplification in erbium-doped fibers. Other wavelengths can be chosen and the essence of the invention is not limited by the exact wavelength of the probe light. Single mode optic fiber is used with the aim of keeping the sensor as small as possible, so that the beam diameter is approximately <NUM>, compared to <NUM> or larger in multi-mode fibers. Smaller beam diameter enables higher bandwidth.

Said first fiber-optic element is preferably a fiber-optic circulator, but other elements known to the person skilled in the art of fiber optics can be chosen, such as a fiber-optic coupler.

The amplifier amplifies the probe light power through a mechanism of stimulated light emission and its output is guided to the photodetector that converts the incoming optical power into an electrical signal, which can be measured with for example an oscilloscope.

Additionally, a fiber-optic wavelength filter can be used in order to prevent other wavelengths (i.e. other than the wavelength of the probe light) to enter the fiber optic amplifier and disrupt the measurement. Using the fiber-optic wavelength filter is preferred especially when measuring shockwaves produced by laser-induced breakdown in a fluid as a part of the laser light used to produce the shockwave or light originating from the induced plasma might get coupled into the fiber-optic probe and disrupt the fiber-optic amplifier, producing oscillations and spikes in the measured signal.

Furthermore, a fiber-optic coupler can be used to couple a part of the incoming probe light into another single-mode fiber and using the said part of the probe light to subtract any probe light intensity instabilities (i.e. noise) from the measured signal. This can be achieved for example by guiding both the amplified reflected probe light and the said part of the incoming probe light into a balanced photodiode detector that internally subtracts both signals. In this case the lengths of both inputs to a balanced photodiode detector should be matched such, that the time delay between both signals is significantly less than <NUM> ns to allow for instabilities suppression in the measured bandwidth (above <NUM>), preferably the lengths of both fibers should be matched within <NUM>.

The method for measuring with the hydrophone according to invention comprises the following steps:
Firstly, the optic probe is placed in the liquid, in which pressure waves should be measured or determined,.

Secondly, the source of probe light emits light, which is transmitted to the first single mode fiber and to the fiber optic element such as a fiber optic circulator or coupler and finally through the tip of the single-mode fiber optic probe into the surrounding liquid. A part of the incoming probe light is reflected at the fiber-liquid interface and the amount of reflected power depends on the pressure at the fiber-liquid interface due to the dependence of refractive index of the liquid on the pressure. This dependence can be written as: <MAT>.

Where nL is the refractive index of liquid and p is the pressure at the fiber-liquid interface. The value of A is around <NUM> · <NUM>-<NUM> MPa-<NUM> for water. The reflectivity of the fiber-fluid interface is then calculated from Fresnel equations for reflection at the perpendicularly cleaved fiber (i.e. angle of incidence = <NUM>) as: <MAT> Where R is the reflectivity of the fiber-liquid interface and nF and nL are the refractive indices of fiber (around <NUM> for fused silica glass) and liquid (around <NUM>) respectively. From these two equations the changes of reflectivity <MAT> can be obtained. In most cases the changes of fiber refractive index can be neglected due to low compressibility of glass used in typical optical fibers.

Said reflected probe light is guided to the second fiber-optic element, where it is combined with the pump light. This is followed with amplification of the combined reflected probe light in the single-mode fiber-optic amplifier, and measurement of the signal with the photodetector that converts the incoming optical power into an electrical signal, which can be measured with for example an oscilloscope. Measured current/voltage is then conversed into pressure. As the dependency is linear, measured values are multiplied with a known factor dependent on the fluid and refractive index of the glass.

The hydrophone according to the invention may be used in many industrial and medical applications, for example in laser induced breakdown spectroscopy, shock wave lithotripsy and laser vitreolysis.

The fast and highly sensitive reflective fiber-optic hydrophone according to the invention will be described in further detail on the basis of exemplary embodiments and figures, which show:.

In the scope of the invention as described here and defined in the claims other embodiments of the hydrophone clear to the skilled person in the art of fiber-optic technology are possible, the scope of the invention being solely defined by the appended claims. <FIG> shows a scheme of the preferred embodiment of the fast and highly sensitive fiber-optic hydrophone and comprises a source of probe light <NUM> which is coupled into a single-mode fiber 1a. The probe light is guided into a first fiber-optic element <NUM> that transmits a part of the incoming probe light 2a into a second single-mode fiber 2b and a part of the light reflected from a fiber-optic probe <NUM> into a third single-mode fiber 2c. The said fiber-optic element <NUM> can be a fiber coupled circulator or a fiber coupler or any other fiber-optic component meeting the above-mentioned requirements (i.e., transmitting part of the incoming light into the second fiber and transmitting a part of the reflected light into the third fiber), known to a person skilled in fiber-optic technology. The output of the said element 2b is optionally guided into a fiber-optic wavelength filter <NUM> and finally to a fiber-optic probe <NUM>, which is placed into a fluid and positioned such that the pressure waves to be measured cause sufficient changes to the reflectivity of the probe. The reflected light from the probe coupled into a third fiber 2c is then guided into a second fiber-optic element <NUM> that combines both the reflected probe light 2c and the pump light from a pump source <NUM> into a common, fifth fiber 6a. The said element can be either a wavelength-division-multiplexer (WDM) or a multimode pump combiner according to the type of the pump light source <NUM>, which may be a single-mode or a multimode laser diode connected to the second fiber-optic element <NUM> with a fourth single or multimode optic fiber 5a. The said common fiber guiding both the probe light and the pump light is coupled into a doped fiber amplifier <NUM>, which through a mechanism of stimulated light emission amplifies the probe light power and the output of the doped fiber is guided to a photodetector <NUM> that converts the incoming optical power into an electrical signal that can be measured with for example an oscilloscope. A typical electrical signal as produced by the photodetector <NUM> is schematically shown in <FIG>.

<FIG> shows a second embodiment of the improved fast and highly sensitive fiber-optic hydrophone, which comprises a source of probe light <NUM>' which is coupled into a single-mode fiber 1a'. The probe light is guided into a fiber-optic element <NUM>' that transmits a part of the incoming probe light 2a' into a second single-mode fiber 2b' and a part of the light reflected from fiber-optic probe <NUM>' into a third single-mode fiber 2c'. The said fiber-optic element <NUM>' can be a fiber coupled circulator or a fiber coupler or any other fiber-optic component meeting the above-mentioned requirements, known to a person skilled in fiber-optic technology. The output of the said element 2b' is optionally guided into a fiber-optic wavelength filter <NUM>' and into a fiber-optic coupler <NUM>', which couples a part of the probe light into a fiber-optic probe <NUM>' and another part into a separate single-mode fiber 4b' that is guided into the photodetector to be used to subtract the intensity fluctuations of the probe light source from the measured signal. The reflected light from the probe coupled into a third fiber 2c' is then guided into another fiber-optic element <NUM>' that combines both the reflected probe light 2c' and the pump light from a pump source <NUM>' into a common fiber. The said element can be either a wavelength-division-multiplexer (WDM) or a multimode pump combiner according to the type of the pump light source <NUM>' (single-mode or a multimode laser diode). The said common fiber for guiding both the probe light and the pump light is coupled into a doped fiber amplifier <NUM>', which through a mechanism of stimulated light emission amplifies the probe light power and the output of the doped fiber is guided to a balanced photodetector <NUM>' that subtracts the part of the probe light 4b', containing the probe light fluctuations, from the amplified probe light from the amplifier <NUM>' and converts the subtracted optical power into an electrical signal that can be measured with for example an oscilloscope. A typical electrical signal as produced by the photodetector <NUM>' is schematically shown in <FIG>.

In <FIG> a schematic electrical signal from the photodetector of the first embodiment of the hydrophone is shown. The electrical signal contains the probe light intensity instabilities A and is offset in amplitude B. The offset of the signal results from a non-zero static reflectivity of the fiber-optic probe, when no shockwave is present. A positive pressure peak C and a negative pressure peak D arising from shockwave propagation are shown.

Claim 1:
A fast and highly sensitive reflective fiber-optic hydrophone for measuring pressure waves in liquids, wherein it comprises at least:
- a source of probe light (<NUM>, <NUM>') coupled into a first single-mode fiber (1a, 1a'),
- the first single-mode fiber (1a, 1a') coupled into a first fiber-optic element (<NUM>, <NUM>') for transmitting at least a part of the incoming probe-light (2a, 2a') propagating in the forward direction from the first single-mode fiber (1a, 1a') into a second single-mode fiber (2b, 2b') and for transmitting at least a part of a reflected light from a probe (<NUM>, <NUM>') propagating in the backward direction into a third single-mode fiber (2c, 2c'),
characterized in that the hydrophone further comprises
- the single-mode fiber-optic probe (<NUM>, <NUM>') having a perpendicularly-cleaved or polished single-mode fiber tip, so that the surface of the fiber tip is perpendicular to the probe-light propagation direction, wherein the probe (<NUM>, <NUM>') is connected to the second single-mode fiber (2b, 2b'),
- a source (<NUM>, <NUM>') of pump light coupled into a fourth optical fiber (5a) with a wavelength chosen such that it allows for pumping an optical amplifier,
- a second fiber-optic element (<NUM>, <NUM>') for combining the reflected probe light from the third single-mode fiber (2c, 2c') and the pump light into a fifth optical fiber (6a),
- a single-mode fiber-optic amplifier (<NUM>, <NUM>') for amplifying optical signal from the fifth optical fiber (6a), said amplifier (<NUM>, <NUM>') comprising a doped single-mode fiber with length between <NUM> and <NUM> to allow for sufficient absorption of the pump light and sufficient amplification of the probe light, wherein the dopant is chosen according to the chosen probe light wavelength, preferably ytterbium-doped or erbium-doped fiber is used, and
- a photodetector (<NUM>, <NUM>') with sensitivity above <NUM> A/W and bandwidth above <NUM> to measure the previously amplified probe light.