Laser optical feedback imaging insensitive to parasitic optical feedback Olivier Jacquin,* Eric Lacot, Corinne Felix, and Olivier Hugon Laboratoire de Spectrométrie Physique, UMR CNRS 5588, Université Joseph Fourier de Grenoble, 140 Avenue de la Physique, BP 87, Domaine Universitaire, 38402 Saint Martin d’Hères, France *Corresponding author:
[email protected] Received 11 April 2007; accepted 20 July 2007; posted 27 July 2007 (Doc. ID 81907); published 12 September 2007
We present an optical architecture for the laser optical feedback imaging (LOFI) technique that makes it possible to avoid the effect of the optical parasitic reflections introduced by the optical components located between the laser source and the studied object. These reflections damage phase and amplitude information contained in the images. This phenomenon is a leading problem that strongly limits the LOFI performance for weak feedback detection. Consequently, it is essential to be able to limit or avoid the effect of these parasitic reflections to reach the optimal LOFI performance. © 2007 Optical Society of America OCIS codes: 110.3175, 110.2970, 110.3080, 110.4280, 280.3420.
1. Laser Optical Feedback Imaging Technique
The laser optical feedback imaging (LOFI) technique is a sensitive imaging method combining optical heterodyne interferometry with the dynamic proprieties of class B lasers [1]. In this method, the interference takes place into the laser, between the intracavity light and the backscattered light by the studied target. The backscattered light is frequency shifted to create an intracavity optical beating. The laser output is thus modulated at the shift frequency ⍀. The amplitude and phase measurement of this modulation with a lock-in amplifier makes it possible to obtain amplitude (or reflectivity) images and phase (or profilometry) images [2] of a nocooperative target (a diffusing surface or volume, for example). To get the maximum sensitivity, the optical beating frequency must be resonant with the natural oscillation of the laser. That is why the shift frequency ⍀ must be equal to the laser relaxation frequency ⍀R. In this case, one has a great amplification of the optical beating. For a Nd:YAG microchip laser, this LOFI amplification is of the order of 106, which makes it possible 0003-6935/07/276779-04$15.00/0 © 2007 Optical Society of America 6779
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to be extremely sensitive to the light reinjected into the laser. Reflectivity as low as 10⫺13 is then easily detectable with a laser output power of a few milliwatts, with a bandwidth detection of 1 KHz [3]. In the LOFI technique the laser is the source and the detector (Fig. 1). The system is then self-aligned since the laser and the target are conjugated via the optics of the system, the backscattered photons come back into the laser cavity according to the reverse path principle. Consequently, the optical system needs no complex alignment. In the LOFI technique, an image is built up by a point-by-point scan. Figure 1 shows a schematic of the LOFI experiment. We used a Nd:YAG microchip laser emitting a few milliwatts at the wavelength of 1.064 m. The relaxation frequency ⍀R is approximately 1 MHz. A twoaxis galvanometric mirror scanner allows us to move the laser beam on the studied target. The frequency shift is obtained with two acousto-optic modulators operating respectively at 81.5 MHz (order ⫹1) and 81.5 MHz ⫺ ⍀兾2 (order ⫺1). The frequency shift is equal to ⍀兾2 when the light crosses the shifter. After a round trip, the total frequency shift of the reinjected light into the laser is thus equal to ⍀.
Fig. 1. Description of the classical LOFI experiment.
2. Parasitic Optical Feedback
Parasitic reflections are inherent in optical systems. They are generated by all the optical interfaces of the system, and they are more or less important according to the quality of the optical elements. These parasitic reflections are a leading problem in optical devices as, for example, in laser chains [4] or in optical telecommunication networks [5]. A significant parasitic reflection generated by an optical element located between the frequency shifter and the studied target limits the LOFI sensitivity for amplitude images. It affects amplitude and phase information contained in LOFI images. For a parasitic diffusing object with an effective reflectivity rP and for a target with an effective reflectivity rC, respectively located at distances dP and dC from the laser, the expressions of amplitude and phase extracted by the lock-in amplifier are [6]
冋
⫽ a tan
册
rC sin共c兲 ⫹ rP sin共p兲 , rC cos共c兲 ⫹ rP cos共p兲
R ⫽ GLOFI冑rC2 ⫹ rP2 ⫹ 2rCrP cos共C ⫺ p兲Pout,
(1)
(2)
with p ⫽ 共2兾兲2dP, c ⫽ 共2兾兲2dC, and Pout as the output laser power. If rP ⬍⬍ rC, the previous equations become ⬇ c and R ⬇ GLOFIrCPout. The phase is thus directly proportional to dC, the distance between the target and the laser, and the amplitude R is directly proportional to the effective reflectivity rC of the target. In the following, we will consider no parasitic reflection if rP ⬍⬍ rC, (i.e., if the parasitic reflections have a negligible effect on the measurements). On the other hand, if the inequality rP ⬍⬍ rC is not satisfied, the phase behavior is thus no longer linear according to the laser–target distance. As a consequence, the displacement measurements or profilometry images are degraded. One also notices that the R amplitude depends on rP, rC and the optical path difference between the parasitic signal and the backscattered signal from the studied target. In this situation, the least vibrations of the target generate fluctuations of R, which will disturb amplitude image
Fig. 2. R and are given in polar coordinates versus the distance dC, with parasitic reflection in the optical system.
acquisition. One has the same fluctuations of R for a vibration of the parasitic object or for atmospheric optical distortion between the parasitic object and the target. Intuitively, we can also understand that it will be difficult to detect target reflectivity lower than those of the parasitic object, which then limits the sensitivity of the LOFI technique. To illustrate the effect of parasitic reflection on the R and parameters, we have introduced a great parasitic echo 共rP ⬇ rC兲 using a microscope slide. This parasitic echo is similar to that caused by optics with no antireflective coating. We have measured at the frequency ⍀, the reflectivity and the phase for a single point on the target (no scanning), with a movement of the target along the optical axis. This movement is obtained from a piezoelectric translation. We measure thus the following parameters: R共dC兲 and 共dC兲 for a dC variation between 0 and a few . These measurements are given in polar coordinates in Fig. 2. Figure 2 shows a circle with radius rC, corresponding to a constant reflectivity of the target and a longitudinal displacement greater than . However, the circle is not centered because the measured amplitude R is not constant. Figure 2 shows also that the measured phase is not proportional to dC. Indeed, if you imagine a circle located in the top right frame, then the phase measured by the lock-in amplifier is included between 0 and 兾2, whereas it actually varies between 0 and 2. In the absence of important parasitic reflection, i.e., if rP ⬍⬍ rC, the circle is perfectly centered with a constant amplitude R and a phase proportional to dC. The use of antireflective-coated optics [7] at the working wavelength in the optical device makes it possible to avoid parasitic reflections to verify the inequality rP ⬍⬍ rC. However, it is difficult and兾or expensive to verify this inequality for rC reflectivity as low as 10⫺13. For example, if the LOFI technique is coupled with a microscope to realize biological images [8], it is difficult to find objectives with 20 September 2007 兾 Vol. 46, No. 27 兾 APPLIED OPTICS
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antireflective coating at the wavelength of 1.064 m and expensive or not reasonable to use an antireflective-coated microscope slide. The surface of the sample also causes an important echo that limits the possibilities of investigations under this surface. These examples highlight the importance of being able to avoid the adverse effects caused by parasitic reflections. 3. Antireflection LOFI Device
The aim of this work is to present a LOFI device that allows the measurement of a backscattered signal by the target without the adverse effects caused by parasitic reflections. For that purpose, the studied target is lighted by two laser beams instead of one single laser beam. The principle of the experiment is shown in Fig. 3. The laser beam is split by a 50兾50 beam splitter located before the frequency shifter. The LOFI system is thus composed of two parallel optical arms. The optical frequency is not the same for both arms. For the top arm in Fig. 3, there is no frequency shift because the light does not cross the frequency shifter, whereas there is a frequency shift for the bottom arm. Both laser beams are finally focused in a single point of the target by a lens. The scanning is still performed by two galvanometric mirrors. The backscattered light then has two possibilities to be reinjected into the laser. Indeed, it can come back by the bottom arm or by the top arm (Fig. 3). In each case, the frequency shift is different. If the laser–target path and the target–laser path are different, then the frequency shift is ⍀兾2, because the reinjected light has crossed the frequency shifter only once. If the laser–target path and the target–laser path are the same, then the frequency shift is ⍀ or zero because the reinjected light has crossed the frequency shifter twice or never. If there is a parasitic reflection, it takes place before the target, outside the overlapping point of both beams, which means that the laser–target path and the target–laser path are necessarily the same. Hence, the frequency shift for parasitic reflection is ⍀ or zero. Thus, an amplitude and phase measurement at frequency ⍀兾2 with the device presented in Fig. 3 makes it possible to avoid the adverse effects caused by parasitic reflections.
Fig. 3. LOFI device insensitive to optical parasitic reflection. 6781
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Fig. 4. R and given in polar coordinates measured at ⍀兾2 frequency with antireflection system.
4. Experimental Results
To validate the principle of the optical device presented in Fig. 3, we have realized the same experiment as the one described in Section 2. The parasitic optical feedback is generated by a microscope slide placed on both arms just after the frequency shifter. We have measured at the frequency ⍀兾2 the reflectivity and the phase for a single point on the target (no scanning), with a movement of the target along the optical axis. We measure thus the following parameters: R共dC兲 and 共dC兲 for dC variations between 0 and a few . These measurements are plotted in polar coordinates in Fig. 4. It shows a centered circle that means measurements of R and parameters insensitive to the parasitic reflection contribution. The R amplitude is constant, corresponding to the point of investigation on the target. The measured amplitude with the lock-in amplifier is thus directly proportional to rC, the effective reflectivity of the target at this point. The measured phase is also correct. Indeed, it is directly proportional to dC, the distance between the laser and the target. These results validate the possibility of avoiding the adverse effects caused by parasitic reflections with the optical system proposed in Fig. 3. With the optical device presented in Fig. 3, we have also realized an image with a controlled parasitic reflection. Images have been realized at the frequencies ⍀ and ⍀兾2. The imaged object is a piece of a metallic ruler. Figure 5 shows images obtained for different investigated configurations. Figures 5(a) and 5(b) give images realized without parasitic reflection. For measurement at the frequency ⍀, the top arm of the optical system is shut to approach as much as possible a classical LOFI set up. Figure 5 shows that images (a) and (b) realized respectively at the frequencies ⍀ and ⍀兾2 without significant parasitic reflection 共rP ⬍⬍ rC兲 are of the same quality. Then we have realized the same images with significant parasitic reflection 共rP ⬇ rC兲 caused by a microscope slide in the optical system (Fig. 3). We note that the image
realized at the frequency ⍀ is completely drowned in the noise generated by the parasitic reflection, the image obtained is a continuous amplitude corresponding to the amplitude of light reinjected into the laser by the parasitic reflection. On the other hand, we note that the measurement at the frequency ⍀兾2 yields an image of the same quality with or without parasitic reflection. We can conclude from these results that the proposed device allows us to realize a LOFI image without the adverse affect caused by parasitic reflections in the optical system.
5. Conclusion
In this paper, we have shown that parasitic reflections damage the amplitude and phase information contained in LOFI images. To avoid these adverse effect, we have presented a LOFI device insensitive to optical parasitic reflection. The experimental results show that the proposed device allows us to avoid the adverse effect caused by a parasite in the optical system on amplitude and phase LOFI images. The next step is to propose an equivalent system selfaligned, in which it is not necessary to overlap both laser beams as in the proposed device in this paper. For that, we could, for example, use birefringent optics to split the laser beam and to select the roundtrip path.
References
Fig. 5. LOFI images realized respectively at the frequencies ⍀ and ⍀兾2, with and without parasitic reflection (i.e., with and without the microscope slide placed before the metallic ruler) to validate the principle of the antireflection scheme for the LOFI technique. (a) Detection frequency ⍀, without parasitic reflection; (b) detection frequency ⍀兾2, without parasitic reflection; (c) detection frequency ⍀, with parasitic reflection; (d) detection frequency ⍀兾2, with parasitic reflection.
1. E. Lacot, R. Day, and F. Stoeckel, “Laser optical feedback tomography,” Opt. Lett. 24, 744 –746 (1999). 2. E. Lacot and O. Hugon, “Phase sensitive laser detection by frequency-shifted optical feedback,” Phys. Rev. A 70, 053824 (2004). 3. E. Lacot, R. Day, and F. Stoeckel, “Coherent laser detection by frequency-shifted optical feedback,” Phys. Rev. A 64, 043815 (2001). 4. M. E. Storm, “Controlled retroreflection: a technique for understanding and eliminating parasitic lasing,” J. Opt. Soc. Am. B 9, 1299 –1304 (1992). 5. P. Megret, L. Wuilmart, J. C. Froidure, M. Blondel, “Bit-errorrate in optical fiber links with optical reflections,” in Proceedings of IEEE Conference on Lasers and Electro-Optics Society (IEEE, 1997), Vol. 2, pp. 87– 89. 6. R. Day, “Une nouvelle technique d’imagerie laser basée sur la reinjection décalée en fréquence. Laser optical feedback imaging (LOFI),” Ph.D. thesis (University J. Fourier, France, 2000), pp. 51–55, http://www-lsp.ujf-grenoble.fr/pdf/theses/dyrd.pdf. 7. J.-M. Mackowski, “Coatings principles,” in Proceedings of the NATO Advanced Study Institute on Optics in Astrophysics (Springer, 2005), Vol. 198, pp. 327–342. 8. O. Hugon is preparing a manuscript to be called “Cell imaging by coherent backscattering microscopy using frequency-shifted optical feedback in a microchip laser” and to be submitted to Ultramicroscopy.
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