WO2015030569A1

WO2015030569A1 - All fibre based diaphragm-less optical microphone

Info

Publication number: WO2015030569A1
Application number: PCT/MY2014/000205
Authority: WO
Inventors: Chang Hong PUA
Original assignee: University Of Malaya
Priority date: 2013-08-30
Filing date: 2014-06-27
Publication date: 2015-03-05
Also published as: MY165534A

Abstract

An all fibre based optical microphone with dual wavelength fibre laser and fibre laser dynamics operating principles is fabricated according to the present invention to settle the difficulty of sound sensing using optical fibre without any intermediate or diaphragm. The optical microphone developed according to the present invention consists of a Single Mode Fibre (SMF), a 980 nm laser diode, a Wavelength Division Multiplexer (WDM), a single channel SMF output, an Erbium Doped Fibre (EDF), a 1 x 16 Arrayed Waveguide Gratings (AWG) with 16 channels (SMF) output (with jacket) and 2 channels SMF (without jacket) connected to the chosen channels as acoustic waves sensing element. The optical microphone can be used under harsh environment such as high electromagnetic interference especially for medical instruments like Magnetic Resonance Imaging (MRI) and able to bend into any sensor shape to fit in to any position. The optical microphone also can be used as hydrophone as it functions normally even under water.

Description

ALL FIBRE BASED DIAPHRAGM-LESS OPTICAL MICROPHONE

FIELD OF THE INVENTION

The present invention relates to an optical microphone. More specifically, the present invention relates to an all fibre based diaphragm-less optical microphone with dual wavelength fibre laser which is able to sense the acoustic waves without any intermediate or diaphragm.

BACKGROUND OF THE INVENTION

The fibre optical microphone is an entirely new microphone concept, first invented in Israel in 1984 by Drs. Alexander Paritsky and Alexander Kots. The fibre optical microphone has very specific advantages over conventional microphones. First, no electronic or metal components are used in the microphone head or the connecting fibres, so the microphone does not react to or influence any electrical, magnetic, electrostatic or radioactive fields (this is called EMI/RFI immunity). The fibre optical microphone is therefore ideal for use in areas where conventional microphones are ineffective or dangerous, such as inside industrial turbines or in magnetic resonance imaging (MRI) equipment environments. Another advantage is the physical nature of optical fibre light propagation. The distance between the microphone's light source and its photo detector may be up to several kilometers without need for any preamplifier and/or other electrical device. Finally, fibre optical microphones possess high dynamic and frequency range, similar to the best high fidelity conventional microphones. They are robust, resistant to environmental changes in heat and moisture and are excellent for noise-canceling applications.

In optical microphone, conversion of acoustical waves into electrical signals is achieved not by sensing changes in capacitance or magnetic fields (as with conventional microphones), but, instead by sensing changes in light intensity. During operation, light from a laser source travels through an optical fibre to illuminate the surface of a tiny, sound-sensitive reflective diaphragm Sound causes the diaphragm to vibrate, thereby, minutely changing the intensity of the light it reflects. The modulated light is then transmitted over a second optical fibre to a photo detector, which transforms the intensity-modulated light into electrical signals for audio transmission or recording.

However, the diaphragm inherently has mass which affects the ability of the microphone to accurately detect the original acoustic energy. As compare to electrical microphone, optical microphone becomes very important in medical field especially for application in strong Electro-Magnetic wave instrument such as MRI. Conventional electrical microphone will not work in such device as all the electrical signal will be distorted under such environment. While diaphragm-less optical microphone can do even more. Conventional diaphragm optical microphone requires stringent alignment to proper align the optical fibre together with the diaphragm. Besides that, the diaphragm is normally not designed for usage in wet, under water, or high temperature environment. A diaphragm-less optical microphone is different, due to the sensor head is fibre which the material is glass, it can operate in almost all conditions that a glass can stand as long as the structure and properties of the glass are unchanged. These abilities allow the diaphragm- less optical microphone to have wider applications that unable to be achieved by the existing microphones. Besides, the flexibility of the fibre allows the diaphragm- less optical microphone to shape into many different forms with minimum thickness of about 250 μπι only.

As described below, nothing else compares with the unique aspects of the present invention. US Patent No. 7561277B2 discloses a method to fabricate a new Fabry- Perot diaphragm-fibre optic microphone by using MEMS technology in processing and packaging. US Patent No. 6281976B1 provides fibre optic Fabry-Perot interferometer diaphragm sensor and method of measurement. US Patent No. 6014239 A disclosed an diaphragm-less optical microphone which includes a laser and beam splitter cooperating therewith for splitting a laser beam into reference beam and a signal beam. However, there are several problems that remain unsolved. The first issue is the random fluctuation of the optical output background noise of the sensor. The second issue is the transient generation during the modulation, which will distort the original acoustic wave pattern, although detection is still possible. In view of the foregoing, the present invention has developed a unique all fibre based diaphragm-less optical microphone with dual wavelength fibre laser which is able to show the optical response to acoustic waves at 23dB below the triggering threshold of laser dynamics.

SUMMARY

An object of the present invention is to provide an all fibre based diaphragm-less optical microphone which is able to sense the acoustic waves without using any intermediate or diaphragm The optical microphone can be used under harsh environment such as high electromagnetic interference especially for medical instruments like Magnetic Resonance Imaging (MRI) and able to bend into any sensor shape to fit in to any position. The optical microphone also can be used as hydrophone as it functions normally even under water. In one embodiments of the present invention, the optical microphone is constructed with a dual-wavelength fibre laser and based on the dynamic operation principles to sense the acoustic waves or sound using all fibre based material without the need for an intermediate medium or a diaphragm. This will eliminate all the disadvantages of a conventional diaphragm optical microphone.

The optical microphone developed according to the present invention consists of a 980 nm laser diode, a Single Mode Fibre (SMF), a 980/1550 nm Wavelength Division Multiplexer (WDM), a flat-cleaved SMF output, an Erbium Doped Fibre (EDF), a 1 x 16 Arrayed Waveguide Gratings (AWG) with 16 channels SMF (with jacket). Among the 16 channel outputs, 2 selected channel outputs are connected to 2 different SMF fibers (without jacket) as acoustic waves sensing element.

A further object, features and advantages of the present invention will be readily apparent from the following description. BRIEF DESCRIPTION OF THE DRAWING/FIGURES

The accompanying drawings, which are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and together with the description serve to explain the principles of the present invention.

Figure 1 shows the schematic diagram of the dual wavelength fibre laser setup constructed using 1 x 16 channels AWG; Figure 2 shows the optical spectrum of dual wavelength fibre laser pumped at 23.2 mW;

Figure 3 shows the optical output from (a) output of (4); and (b) output of (11) and output of (12), with FFT frequency distributions that have been measured and calculated using the built-in MATH function of the digital oscilloscope;

Figure 4 shows the detected optical level from both outputs (11) and (12), with calculated FFT frequency distributions for (a) output (11) and (b) output (12); Figure 5 shows the random spiking due to the transient effect from (a) outputs of (11) and (12) and (b) summation of outputs (11) and (12); and

Figure 6 shows the optical outputs from (11) and (12) for modulation by 500-Hz acoustic waves at amplitudes of (a) 65.0 dBA and (b) 88.3 dBA.

DETAILED DESCRTPTION OF THE PRESENT INVENTION

In the following detailed description, reference is made to various specific embodiments in which the present invention may be practiced. These embodiments are described with sufficient details to enable those methods in the present invention to be practiced and it is to be understood that other embodiments may be employed and that structural and logical changes may be made without departing from the spirit or scope of the present invention. In general, the present invention represents an all fibre based optical microphone which is able to use under harsh environment such as wet, under water or high temperature environment which is unable to be achieved by existing microphones. In a specify embodiment, an optical microphone with dual wavelength fibre laser and fibre laser dynamics operating principles is fabricated according to the present invention to settle the difficulty of sound sensing using optical fibre without any intermediate or diaphragm. Wavelengths competition or interaction in the dual wavelength fibre laser is highly sensitive to the fibre condition which is dependant to the environment stability making it sensitive to the sound wave although the interaction between sound wave and fibre is still very low. The sensitivity of the diaphragm-less optical microphone can go as low as 65 dBA.

The diaphragm-less optical microphone is built based on the principle of fibre laser dynamics. A conventional Single Mode Fibre (SMF) and an Arrayed Waveguide Gratings (AWG's) are used to form a dual wavelength linear cavity fibre laser cavity. The cavity contains an Erbium Doped Fibre (EDF) at length of 5 meter. The EDF is acting as the gain medium for the fibre laser to lase. The energy source of the fibre laser is come from a 980 nm laser diode which pumped into the EDF through the guidance of the Wavelength Division Multiplexer (WDM). The setup is shown in figure 1.

Figure 1 shows the schematic diagram of the optical microphone of the present invention. As indicated in figure 1, the optical microphone consists of a 980 nm laser diode, indicated by the reference numeral (1); a Single Mode Fibre (SMF), indicated by the reference numeral (2); a 980/1550 nm Wavelength Division Multiplexer (WDM), indicated by the reference numeral (3); a flat-cleaved SMF output, indicated by the reference numeral (4); an Erbium Doped Fibre (EDF), indicated by the reference numeral (5); a 1 x 16 Arrayed Waveguide Gratings (AWG), indicated by the reference numeral (6) with 16 channels SMF (with jacket), indicated by the reference numeral (7). Among the 16 channel outputs (7), 2 selected channel outputs are connected to 2 different SMF fibers (without jacket) as acoustic waves sensing element, indicated by the reference numeral (11) and (12) respectively, have a 90° flat-cleaved output to produce ~ 4% back-reflection as feedback to the system. While the remaining 14 channels are organized to have angle-cleaved outputs, indicated by the reference numeral (13) to avoid significant back-reflection. As referring to figure 1, the optical microphone fabricated according to the present invention is constructed by the conventional SMF. The input of the SMF (2) is connected to the 980 nm laser diode (2) for generating optical radiation into the SMF

(2) and another end of the SMF (2) is connected with the 980 nm port of the WDM

(3) . The 1550 nm port of the WDM is left unconnected and cleaved at 90° flat-cleaved to provide ~ 4% back reflection (4). The third port of the WDM (3) is connected with the 5 meter long EDF (5). The other end of the EDF (5) is connected to the 1 x 16 Arrayed AWG (6) followed by the 16 channels SMF (with jacket) output (7).

When the 980 nm laser is strong enough to pump the Erbium ions in EDF (5) to population inversion condition, the linear cavity of the fibre laser will start to lase. When the fibre starts to lase, it will go through almost similar lasing process as a conventional laser such as laser dynamics. During the operation of the fibre laser, the laser is very sensitive to external turbulence including acoustic waves that will disturb the balance of the lasing condition. When there are external turbulence interrupt, it will create a temporary grating that will change the feedback of the laser back to the gain medium and leads to the changing of lasing power. Observing the power change of the fibre laser, it is able to observe the acoustic activities happen around the fibre setup including sound. For better understanding and to further illustrate the present invention in greater details and not by way of limitation, the following exemplary embodiments will be given.

The 980 nm laser diode (1) driven at a current of 79.2 mA is used to pump the 5 m long EDF (5) at a power of 30.0 mW and to create a population inversion in the fiber. At this pump power, an Amplified Spontaneous Emission (ASE) spectra with an average power of 4.72 mW is generated. The ASE that propagates toward the AWG (6) will be sliced into 16 different wavelength outputs (7), with one wavelength for each physical channel. Among the 16 channel outputs (7), 2 selected channel outputs, indicated by the reference numeral (11) and (12) respectively, have a 90° flat-cleaved output to produce ~ 4% back-reflection as feedback to the system - while the remaining 14 channels are organized to have angle-cleaved outputs, indicated by the reference numeral (13) to avoid significant back-reflection. The two selected wavelengths are partly reflected back into the cavity and experience stimulated emission when passing through the EDF (5). The amplified dual wavelength signal then experiences another 4% back reflection at the other end of the laser cavity, which is indicated by the reference numeral (4) and the gain is sufficient to produce laser action at the two selected wavelengths. This setup forms a dual-wavelength laser cavity approximately 12 m long.

Figure 2 shows the optical spectrum of the dual-wavelength fibre laser obtained from output (4), with lasing wavelengths of 1532.10 nm and 1532.90 nm. While for output (11) and output (12), only single wavelength will be observed due to the demultiplexing effect of AWGs (5), Output (11) will only carry the wavelength at 1532.10 nm and output (12) will carry the wavelength at 1532.90 nm. Figure 3 presents the photodetected output power from output (4) and both outputs (11) and (12), measured by a digital oscilloscope. Figure 3(a) shows output (4), which is the total optical output of the fibre laser in the backward direction, at a photodetected voltage level of 2.18 V and approximates an optical power of 1.15 mW. This output is stable at low frequencies, but exhibits fluctuations of the power level at frequencies in the range from 19.7 kHz to 22.0 kHz - with the peak fluctuation level at 20.8 kHz being measured using the built-in Fast Fourier Transform (FFT) capability of the MATH function in the digital oscilloscope.

The FFT spectrum displayed on the digital oscilloscope panel is shown in Figure 4(a). The fluctuation noise and its frequency spectrum is directly related to the fibre laser dynamics, depending on parameters such as the pump power, laser cavity length, erbium concentration, cavity reflectivity and cavity loss. Figure 4(b) shows the optical power from output of (11) and output of (12) measured simultaneously using both input channels on the digital oscilloscope, at wavelengths of 1532.10 nm and 1532.90 nm respectively, at independent maximum levels of 260 mV. It is clearly demonstrated that the fluctuations of the optical output power at the two wavelengths are totally inverted with respect to each other, i.e. that the levels are substantially complementary.

Increasing the optical power level at one wavelength causes the optical power of the second wavelength to decrease by the same amount. To identify the inverted nature of the two outputs, a summation of both outputs was performed using the built-in MATH function of the digital oscilloscope. The result is to produce a steady output level similar to that of output (4), but at a much lower power level (on average, a photo- detected level of 532 mV) as shown in Figure 3(b). There are several reasons for the difference in the output power. One is the difference between the output power levels from the forward and backward directions of the fibre laser. Due to the fact that the fibre laser is not pumped to the population inversion condition throughout the length of the EDF (5) (at the low pump power levels being used), re-absorption will happen towards the end of the EDF (5) where there is less inversion population. A second reason could be the losses caused by the AWG (6), due to insertion and coupling losses. A further reason is the loss caused by the other 14 channels (13) that are in the range from 70 μ up to 150 μW; however these losses at the ASE level are less significant. Analysis of the optical power fluctuation in the frequency domain has been performed by studying the FFT of the optical power fluctuation. Figure 4(a) and 4(b) show the same optical output power level from output (11) and (12), measured using both channels of the digital oscilloscope simultaneously for a sampling period of 5 s. In the case of figure 4(a), the FFT spectrum taken is from the output of (11), with the output of (12), shown to indicate the opposite response of the output of (11). Similarly, for figure 4(b), the FFT spectrum is shown for the output of (12), with the output of (11) trace as the comparison. From the two FFT spectra, it can be observed that there are three distinct peaks, at frequencies of 3.50 kHz, 5.45 kHz and 20.8 kHz. The 20.8 kHz peak is contributed by the resonant frequency of the fibre laser system, as mentioned previously and is observed on all of the optical outputs (4), (11) and (12). The other two peaks, at 3.50 kHz and 5.45 kHz, are contributed by the resonant frequency of each mode that oscillates at different wavelengths and can only be observed in outputs

(11) and (12). The existence of both resonant frequency peaks at both outputs indicates that they are correlated with each other. The FFT frequency distribution from outputs (11) and (12) are almost identical with only slight differences in their amplitude that are not significant enough to merit further analysis. These frequencies of 3.5 kHz and 5.45 kHz can be understood as the mode competition frequency or how frequent of the dominant laser shifts from one wavelength to another. This phenomenon is more obvious when the laser is under a 'chaotic' state as in figure 5.

Observation of the mode competition behaviour for the fibre laser under chaotic conditions has been made by introducing a small impact on the bench where the fibre laser is placed and obtained by knocking using a metal ruler. The perturbation generated will cause the laser to undergo chaotic operating conditions and produce random spiking effects that may be described as transients - and have been well explained in terms of the laser dynamics (Khanin, 2006; Erneux and Glorieux, 2010). Figure 5(a) shows the spiking generated at both output of (11) and output of (12), due to laser dynamic transient effects. For better visualization, outputs (11) and (12) were slightly offset, at a different amplitude level from the base level, in order to separate the two outputs clearly on the oscilloscope panel. It is clear that, under chaotic operating conditions, lasing at 1532.90 nm from output of (11) dominates by producing major high energy spikes, as compared to output of

(12) . When the captured images of the two outputs are placed together, it can be observed that most of the spikes generated in the output of (11) fill up the empty gap between the spikes generated in the output of (12). In other words, at most of the time, under chaotic conditions, only one mode at a time survives. The total output level from output of (8) and output of (9) together can be obtained by adding up the two outputs, as presented in figure 5(b). The domination of the pulsing effect is probably related to the stability and laser peak power. The domination of the lasing effect will switch alternatively between the two lasing wavelengths, causing the low frequency distribution in the FFT spectrum as given in figure 4. To observe the effect of these small perturbations on the oscillation of the two wavelengths, low amplitude acoustic waves at 500 Hz were generated near the fibre laser using a multimedia speaker connected to a personal computer. The amplitude of the acoustic waves was increased slowly until outputs of (11) and (12) responded to the acoustic wave. Figure 6 shows the optical outputs from the outputs of (11) and (12) under modulation by 500 Hz acoustic waves at sound levels of: (a) 65.0 dBA SPL (sound pressure level) (or equivalent to 68.5 dB SPL) and (b) 88.3 dBA SPL (or equivalent to 91.5 dB SPL) which are equivalent to 0.0514 N/m² and 0.752 N/m² respectively. In a more general form, 65.0 dBA SPL is close to the sound made by a vacuum cleaner operating at a distance of 1 m, while 88.3 dBA SPL is similar to sound of a diesel truck from 10 m away (SengpieL, 2011). The larger the level of the sound pressure, the larger the pressure exerted onto the fibre, which in turn causes a large change in the refractive index of the fibre which in turn leads changes in the loss and gain of the fibre laser. Both optical outputs have a sinusoidal form at 500 Hz. The larger modulation amplitude shown in figure 6(b) is due to the larger acoustic wave amplitude. Another difference between figures 6(a) and 6(b) that can be observed is that, in figure 6(a), the outputs of (11) and (12) are "out of-phase" by 180°, while they are "in-phase" in figure 6(b). We believe that the outputs from (11) and (12) are "out-of-phase" not because of delay or changes of path length that cause a relative phase shift between outputs (11) and (12). Interaction of the acoustic waves with the fibre causes modulation of the modes oscillating in the fibre laser. However, the acoustic modulation at this amplitude (65.0 dBA SPL) is far too weak to modulate the total gain and loss of the system. Hence, when one of the modes experiences changes in gain or loss, the other mode will experience an opposite effect to compensate for the changes in the system and to maintain a stable lasing condition. Thus, when the outputs from (11) and (12) in figure 6(a) are summed or the total output power from A is measured, the modulation effect on the laser is small. As the acoustic modulation increases, the sinusoidal amplitude of output (11) and (12) also increase. This will continue until at a certain stage whereby the output of (11) and (12) become "in- phase" as shown in figure 6(b). The transition from "out-of-phase" to "in-phase" happens when the acoustic waves is approximately 87 dBA. After the transition state, there is no observation of modulation amplitude suppression in both of the outputs of (11) and (12). Above 88.0 dBA the acoustic amplitude is large enough to modulate the refractive index in the core along the fibre, causing fluctuations in the total gain and loss of the fibre laser and the outputs from (11) and (12) show "in-phase" modulation. Furthermore, it is obviously possible to observe the modulation pattern on the total output - either directly from output (4) or from the summation of outputs (11) and (12), as shown in figure 6(a).

Conclusion, the power fluctuation of each lasing wavelength in the dual wavelength diaphragm- less optical microphone fabricated according to the present invention has been investigated. By investigating the power level for each individual lasing wavelength, greater sensitivity to acoustic wave modulation, in comparison with the sensitivity of the total output power has been observed, since the diaphragm-less optical microphone will always tend to reduce the induced system fluctuation by compensating the loss or gain changes for other wavelengths or modes in the diaphragm-less optical microphone. With this investigation, the sensing of the diaphragm-less optical microphone to acoustic waves has been reduced by more than 23 dB - with possible further improvement being obtainable by optimization of the diaphragm-less optical microphone.

References

Y. I. Khanin, Fundamentals of Laser Dynamics. Cambridge, U.K.: Cambridge International Science Publication, 2006. T. Erneux and P. Glorieux, Laser Dynamics. New York: Cambridge Univ. Press, 2010. E. Sengpiel. (2011, Jul. 4). Decibel Table— SPL— Loudness Comparison Chart [Online]. Available:http://wvm.sengpiekudio.com/TableOfSoundPressureLevels.htm

Claims

I CLAIM:

1. An all fibre based diaphragm- less optical microphone for sensing the acoustic waves consists of:

i) a 980 nm laser diode;

ii) a Single Mode Fibre (SMF);

iii) a 980/1550 nm Wavelength Division Multiplexer (WDM);

iv) a flat-cleaved SMF output;

v) an Erbium Doped Fibre (EDF);

vi) a 1 x 16 Arrayed Waveguide Gratings (AWG) with 16 channels SMF output; and

vii) 2 channels SMF (without jacket) output as acoustic waves sensing element.

2. The optical microphone according to claim 1, wherein the 16 channels SMF output further consists of two 90° flat-cleaved output channels to produce 4% back-reflection as feedback to the optical microphone and fourteen angle- cleaved outputs channels to avoid significant back-reflection to the optical microphone.

3. The optical microphone according to claim 1, wherein the 2 channels SMF output is constructed and arranged without the fibre outer jacket to facilitate acoustic waves sensing activity.

4. The optical microphone according to claim 1, wherein the flat-cleaved SMF output is a 90° flat-cleaved SMF to provide 4% back reflection to the optical microphone.

5. The optical microphone according to claim 1, wherein the preferably length of the EDF is 5 meter.

6. The optical microphone according to claim 1, wherein the input of the SMF is connected to the 980 nm laser diode for generating optical radiation into the SMF and another end of the SMF is connected with the 980 nm port of the WDM; wherein the 1550 nm port of the WDM is left unconnected and cleaved at 90° flat-cleaved to provide 4% back reflection to the optical microphone; wherein the third port of the WDM is connected with the 5 meter long EDF; wherein the other end of the EDF (5) is connected to the 1 x 16 Arrayed AWG and followed by the 16 channels SMF output.

7. The optical microphone according to claim 1, wherein the optical microphone is able to perform under harsh environment such as high electromagnetic interference and able to bend into any sensor shape to fit in to any position.

8. The optical microphone according to claim 1, wherein the optical microphone can be used as hydrophone as it functions normally under water.