The optical resonance, acoustic frequency response, noise equivalent pressure, and acoustic angular response of the etalon must be characterized to understand device performance and determine its suitability as a high-frequency ultrasound array detector. The basic experimental setup is shown in . A polarized and collimated continuous-wave (CW) laser beam with power of 4 mW and tunable wavelength from 1440 nm to 1590 nm (Agilent/HP 8168F, Agilent Technologies, Santa Clara, California) travels through a polarized beamsplitter and a quarter wave plate. It is then focused onto a 20-μm spot on the surface of the etalon mounted at the bottom of a water tank. The 20-μm laser focal spot defines a 20-μm ultrasound detection element size. The reflected beam’s polarization after traveling through the quarter wave plate is perpendicular to that of the incident beam; therefore, it is reflected off the polarized beamsplitter and is then focused into an amplified InGaAs photodetector connected to the computer for data capture. A piezoelectric transducer is generally placed above the etalon as the source of ultrasound waves.
Block diagram of the experimental setup to characterize the etalon.
The reflected optical intensity depends on the optical wavelength,50
and displays a sharp drop when the optical path of two-way travel in the etalon bulk equals an integer multiple of the wavelength. This is called the resonance condition
and creates the mechanism for ultrasound detection. The optical resonance can be measured by recording the reflected optical intensity when the wavelength of the probe CW laser is tuned. As is shown in , the resonance wavelength is 1538 nm with FWHM of 6.3 nm.
Optical resonance of the etalon structure.
The quality factor of the etalon can be estimated to be:50
=1.57 is the refractive index of SU-8; d
m is the thickness of the SU-8 layer; λ
=1538 nm is the resonance wavelength; and R
=0.85 is the estimated optical reflection coefficient of the two gold layers. The theoretical FWHM of the resonance can be determined using Δλ1/2
≈6.6 nm, in good agreement with experimental results. When this device is used for ultrasound detection, the wavelength of the probe laser is tuned to 1536.5 nm, the point of largest slope yielding the largest optical modulation when the resonance condition is changed by the acoustic pressure of the incident ultrasound waves.
The frequency response of the etalon can be calculated using the method developed by Beard and his group.37
Following this approach, the frequency response of the etalon is:
m is the thickness of the etalon bulk layer, d
m is the overall thickness of the etalon, including the protection layer, c
=2500 m/s is the acoustic velocity in SU-8, and f
is the acoustic frequency. R0
is the acoustic reflection coefficient between SU-8 and water, and R1
is the coefficient between SU-8 and glass. They can be estimated as the following:
=1.5 MRayl, Zglass
=14.7 MRayl, and ZSU8
=2.9 MRayl are acoustic impedances of water, glass, and SU-8, respectively. The calculated curve is shown in as the dashed line.
Fig. 4 (a) Spectra of the transducer pulse-echo signal and etalon detection signals from 11-μm and 5.9-μm etalons. (b) Experimental and theoretical acoustic frequency responses of an 11-μm etalon. (c) Experimental and theoretical acoustic (more ...)
Experimentally, the frequency response of the etalon was characterized with a 50-MHz piezoelectric transducer with aperture diameter of 2.5 mm and focal length of 4 mm (LiNbO3, Resource Center for Medical Ultrasonic Transducer Technology, University of Southern California). First, a pulse-echo signal reflected from a glass substrate is recorded by the piezoelectric transducer. Then the transducer is placed a focal length away above the etalon, as shown in the setup in , and the signal from the piezoelectric transducer is recorded from the etalon. An 11-μm etalon is also used for comparison purposes.
The frequency response of the etalon is derived by dividing the spectrum of the etalon signal by the square root of the spectrum of the transducer pulse echo. The spectrum of the transducer pulse-echo signal and spectra from both etalons are shown in . The derived frequency response of the 11-μm-thick etalon, together with the theoretical curve, is shown in . The derived frequency response of the 5.9-μm-thick etalon, and the corresponding theoretical curve, are shown in . Clearly, a thinner etalon has a higher frequency response and a broader bandwidth than a thicker one, as illustrated in . These results suggest that the 5.9-μm-thick etalon is suitable for ultrasound detection above 50 MHz.
The noise equivalent pressure of the current system can be measured by replacing the high-frequency transducer in the setup of with a calibrated 10-MHz transducer (V312, Panametrics NDT, Waltham, Massachusetts). It has a diameter of 6.35 mm and focal length of 19 mm and generates a negative peak pressure of about -2.6 MPa at focus when driven by a commercial pulser/receiver (5077PR, Panametrics NDT, Waltham, Massachusetts). The etalon was put at the focal plane of the transducer. The system outputs a root mean squared noise of 6.4 mV over 25 to 75 MHz with a 32.5-dB gain amplifier and a peak signal of 101 mV without the amplifier. Therefore, the noise-equivalent pressure (NEP) within the specified 50 MHz bandwidth is estimated to be:
For an optoacoustic detection element of 20 μ
m, NEP of 3.9 kPa over a 50-MHz bandwidth is at least as good as, if not significantly better than, polyvinylidene fluoride (PVDF) hydrophones of equivalent size.38
For example, a sensitivity of 6 nV/Pa over a bandwidth of 40 MHz was reported for a 40-μ
m-diam PVDF needle hydrophone (HP 0.04 mm Interchangeable Probe, HPM04/1, Precision Acoustics Ltd, Dorchester, UK).51
The output noise level of the preamplifier is 60 μ
V, which yields an NEP of 10 kPa for this 40-μ
m PVDF hydrophone. Taking into account the difference in the effective element size and assuming that NEP is inversely linear with area, the NEP of a 20-μ
m PVDF hydrophone should be 40 kPa, so the etalon is actually much more sensitive than a typical PVDF hydrophone of similar size.
The NEP of the etalon is proportional to the minimum detectable optical power of the photodetector and the Young’s modulus of the etalon bulk material and is inversely proportional to the intensity of the etalon probe beam and the quality factor of the etalon (i.e., etalon thickness).37
Increasing the etalon thickness is the easiest way to reduce the NEP. For example, NEP of 2 kPa was measured with 11-μ
m etalons using the method described earlier. Other straightforward methods to reduce the NEP are to increase the probe beam intensity and increase photodetector sensitivity. Also, the quality factor can be increased by higher optical reflectivity of the two gold layers, which will also contribute to a lower NEP. This has significant implications on photoacoustic imaging applications because extensive averaging can be avoided with better detection sensitivity, thus ensuring real-time imaging.
The same experimental setup as shown in can be used to measure the angular response of the etalon. A signal generator (8647A, Agilent Technologies, Santa Clara, California) outputs a CW signal at different frequencies (30 to 80 MHz with a 10-MHz step). This signal was gated with a 400-ns pulse (at a pulse repetition rate of 1 kHz) from a wave form generator (33250A, Agilent Technologies) using a frequency mixer (ZFM-4, Mini-Circuits, Brooklyn, New York). The gated CW burst was amplified by a home-made power amplifier (37-dB gain) and then drove a 50-MHz piezoelectric ultrasound transducer (LiNbO3, Resource Center for Medical Ultrasonic Transducer Technology, University of Southern California). The transducer was mounted on a mechanical stage capable of rotation and 3-D translation.
After setting an angle (between the transducer axis and the normal direction of the etalon), the transducer was moved to focus on the active element defined by the optical spot on the etalon surface, and signals corresponding to different (CW) frequencies detected by the etalon were then recorded by an oscilloscope (WaveSurfer 432, LeCroy, Chestnut Ridge, New York). The signal magnitudes at different frequencies and angles from 0 to 45 deg are shown in . Note that at frequencies up to 50 MHz, the relative attenuation at 45 deg compared with 0 deg is only 3 dB, nearly negligible. Only at frequencies above 80 MHz does the attenuation at 45 deg exceed 10 dB. Therefore, the etalon is suitable for high-frequency ultrasound detection even at large angles, making it ideal for photoacoustic imaging applications.
Angular response from 0 to 45 deg at acoustic frequencies of (a) 30 MHz, (b) 40 MHz, (c) 50 MHz, (d) 60 MHz, (e) 70 MHz, and (f) 80 MHz.