Orthogonally Polarized Lasers

Orthogonally Polarized Lasers and their Applications Shulian Zhang and Thierry osch Precision metrologists have traditionally thought of lasers as mere light sources. Now, orthogonally polarized lasers are being applied as sensors themselves due to their unique intrinsic properties. They re being developed and used to measure displacement, wave plates retardation, force and weight. 38 OPN May 007 www.osa-opn.org

Orthogonally polarized lasers are advanced coherent light sources that can be used as the carrier for self-sensing instruments in metrology. ecause the traditional Zeeman laser (Z-laser) outputs a frequency difference that is generally smaller than MHz, it has limited applications. An orthogonally polarized laser, on the other hand, is capable of outputting two frequencies, o-light and e-light, with a 90 angle between their polarization directions. The difference between the two frequencies may range from 1 MHz to 1 GHz for a HeNe laser, and as much as several GHz for an Nd:YAG laser. The origin of this dual-frequency output lies in the induced birefringence within the resonant laser cavity. Moreover, it is easy to adjust the frequency difference and detect the intensities of the two frequencies separately. Thus, one will observe a number of unusual characteristics in cavity tuning and optical feedback, in addition to detecting optical correlative patterns associated with mode competition. These properties can be translated into the working principles of self-sensing metrology instruments. Indeed, several such self-sensing lasers are currently under development for example, nanometer displacement rulers based on laser cavity tuning or feedback effects, retardation measures of wave plates based on frequency splitting or feedback effects, and force (weight) measurement sensors. Limitations of Z-lasers in interferometers For years, HeNe Z-lasers served as light sources in dual frequency laser interferometers, which are used in equipment for integrated circuit manu- Several self-sensing lasers are currently under development for example, nanometer displacement rulers based on laser cavity tuning or feedback effects, retardation measures of wave plates based on frequency splitting or feedback effects, and force (weight) measurement sensors. [ An orthogonally polarized laser ] M 1 Q M A quartz crystal plate in the laser cavity is used to produce an orthogonally polarized beam. (Q=quartz plate; M 1 and M are mirrors.) Frequency splitting process of a HeNe laser [ observed with a confocal scanning interferometer ] 1.. 3. 4. When the birefringence is increased from zero, only one frequency oscillates at the beginning. When the birefringence reaches a certain value, a new frequency will appear near the original frequency and the power of the original frequency is evenly divided by these two frequencies. As the birefringence becomes larger, so too does the frequency difference. facturing, machinery processing, machine tool appraisal, block gauge appraisal, 3D nano-machining, etc. However, the frequency differences of longitudinal and transverse Z-lasers are typically less than 1.5 MHz and 1 MHz, respectively. If the frequency difference is enlarged to more than MHz by increasing the strength of the magnetic field, the output power of the laser will be too weak to use, and the laser s longevity will decrease, too. These conclusions have been proved theoretically and experimentally. Such small frequency differences restrict the measurement speed of interferometers to less than 700 mm/s. Hence, manufacturers must use costly and complicated acoustic-optical modulators to modify the frequency of a portion of the laser beam and then combine the modified portion with the other portion to form a beat and achieve a large frequency difference. Frequency splitting technology by intercavity birefringence To obtain a laser with a large frequency difference, Zhang and colleagues placed a birefringent element (e.g., a quartz crystal or a glass plate with force applied thereon) in the cavity of a HeNe laser, changing it into an orthogonally polarized laser (or birefringence laser, -laser) with a frequency difference in proportion to the magnitude of the birefringence. The direct ratio between the frequency difference and the birefringence exists only when the frequency difference is larger than some critical frequency, which is typically about 40 MHz. In a -laser with a frequency difference larger than 40 MHz, two frequencies may oscillate together and stably. However, in a -laser with a frequency OPN May 007 39 1047-6938/07/05/0038/6-$15.00 OSA

difference of less than 40 MHz, one of the frequencies will always be extinguished. The frequency splitting process of a HeNe laser can be easily observed by using a confocal scanning interferometer. Increasing the birefringence of the intercavity quartz crystal or the glass plate from zero, we observe that only one frequency oscillates at the beginning, and it displays a peak on the screen of the confocal scanning interferometer. When the birefringence reaches a certain value, a new frequency whose intensity is half of the original will appear near the original frequency, which displays two peaks. In other words, the power of the original frequency is evenly divided by these two frequencies. The critical frequency is found to vary between 30 and 60 MHz; 40 MHz was a statistical average for the smallest frequency difference. We named the range of 0-40 MHz the strong competition section. As the birefringence becomes larger, the frequency difference becomes larger. In order to obtain the frequency difference in the range of 0 to 40 MHz, Jin et al. applied a transverse magnetic field of about 10 T in a -laser, weakening the competition between the frequencies to a great extent and causing the frequency that would otherwise be extinguished to oscillate. Such a laser is called a Zeemanbirefringence laser (Z-laser), which may output frequencies with the difference ranging from less than 1 MHz to approximately 1,000 MHz. Solid-state pumped lasers such as the Nd:YAG microchip -laser are capable of outputting frequencies with a difference of several gigahertz. An Nd: YAG microchip -laser typically uses a laser diode for pumping the laser crystal with diameters of 3 to 5 mm and thicknesses between 1 and mm. Observing mode competition in tuning a -laser cavity In addition to orthogonally polarized outputs, another notable Intensities [mv] A transverse magnetic field () is used to weaken the competition between the two polarization modes. F is the force applied to M to cause frequency splitting. PZT 9 8 7 6 5 4 3 1 0 [ A Zeeman-birefringence (Z) laser ] M 1 F M M [ Tuning the laser cavity ] [ Cavity tuning intensity curve ] 0 4 6 8 10 1 14 16 18 0 4 6 8 Time [s] As PZT tunes the laser cavity, there are three polarization combinations: o-light only, o-light and e-light oscillating together, and e-light only. M1 Q W D PS Lead zirconium titanate, or PZT, tunes the laser cavity. Two detectors (D 1 and D ) detect the intensities of o-light and e-light. W is the glass window with anti-reflective layers, and PS is the polarizing beam splitter. property of a -laser is the competition between two frequencies as the laser cavity is tuned, which can also be observed by a confocal scanning interferometer. The so-called cavity tuning is accomplished by moving the laser s cavity mirror along its normal so that the two frequencies are tuned. y doing this, the researchers found that the laser exhibited three distinct forms of polarization behavior: an oscillating o-light with a non-oscillating e-light; an o-light and an e-light oscillating simultaneously; and an oscillating e-light with a non-oscillating o-light. As the laser switches from one form to another, the transfer in their intensities occurs between the o-light and e-light; an increase for one is accompanied by a decrease in the other. This kind of mode competition can be seen in a simple D 1 experimental configuration. The two lights are separated by a polarizing beam splitter and result in two spots that can be brought onto a screen. The two spots alternate between dark and bright as the laser cavity is tuned. This is called mode competition resulting from laser cavity tuning. The net gain is big enough to keep the two frequencies oscillating only in the vicinity of the central frequency of the lasing medium, but not so large as to keep the two frequencies oscillating at the edge of the gain region of the lasing medium. Equipment that demonstrates the laser frequency split effect and mode competition phenomena have been used as experimental and teaching systems in many universities. Feedback effects in orthogonally polarized lasers Indeed, researchers have learned a good deal about laser physics by studying feedback in semiconductor lasers. The feedback system looks simple, but the physics are complicated. The system requires a reflecting surface (M 3 ) placed out of the laser that reflects the output beams back into the laser s cavity. Then, one can 40 OPN May 007 www.osa-opn.org

observe feedback phenomena, and the characteristics of the laser change greatly when the reflectivity and/or position of the M 3 change. In early research on laser feedback, scientists paid little attention to the action between the adjacent modes of the lasers, perhaps due to the lack of appropriate methods to detect their intensities separately. Now, with orthogonally polarized lasers, the adjacent modes of orthogonally polarized light can be easily spatially separated by using a polarized beam splitter such as a Wollaston prism. One can detect or compare the variation of their intensities (or powers) as the feedback mirror moves. When feedback is considered without regard to orthogonal polarizations, it is referred to as one-end input and one-end output. In other words, all output frequencies of the laser are reflected and fed back to the interior of the laser by a mirror, and the variation in the total intensity is detected by a single detector. In such systems, the intensity variation of each single frequency and the mutual influence between two adjacent frequencies are hitherto unknown. Feedback in which orthogonal polarizations are taken into account is called feedback with two-end input and two-end output, wherein either one or both of the two polarizations of a laser can be fed back into the laser via reflection of a mirror, and two detectors are used to measure the intensity variation of the two polarizations. The orthogonally polarized optical feedback presented here can be separated into two types. The first is the empty (isotropy) external cavity feedback of the orthogonal polarized laser. The second is the birefringence (anisotropy) external cavity feedback for single-mode lasers with a potential orthogonal mode. The laser output intensity of the orthogonal-polarized optical feedback differs greatly from [ Feedback scheme for orthogonally polarized lasers ] PZT Laser intensity 1,00 1,000 M 3 Q M M D 1 PS e-light 800 600 400 00 With orthogonally polarized lasers, the adjacent modes of orthogonally polarized light can be easily spatially separated by using a polarized beam splitter such as a Wollaston prism. One can detect or compare the variation of their intensities (or powers) as the feedback mirror moves. (e+o) light [ Feedback curves of a -laser ] o-light 0 0 0 40 60 80 100 10 140 160 180 00 the conventional one-end input and oneend output optical feedback. ased on this concept, Zhang and his colleagues made a series of findings, mainly in HeNe and microchip Nd:YAG lasers since 000. The major results include three aspects: the competition patterns between o-light and e-light; sub-fringes, l/4, l/80 in one integral conventional fringes of l/ width; and the polarization flipping at any moving direction variation of feedback mirror M 3. Applications in self-sensing metrology In precision metrology, a laser is merely regarded as a conventional light source. However, orthogonally polarized lasers are being applied as sensors themselves due to their unique intrinsic properties. c Laser nanometer ruler A little movement of one mirror of a laser can cause a huge change in the laser s frequency. This movement can be measured by detecting the frequency variation. Zhang and his colleagues created such a laser nanometer displacement ruler, also called PZT voltage The feedback curves of orthogonally polarized lasers with a reflectivity of 50 percent. The blue dots indicate o-light and the green circles indicate e-light. From Opt. Express 13, 6558-63 (005). D 1 a displacement self-sensing laser, using a 10-mm-long HeNe laser with a plate of quartz crystal in the cavity. y continuously moving one laser cavity mirror, the researchers formed four states of polarization in the output of the laser a single o-light; both an o-light and an e-light; a single e-light; and no light. The output state shifts once as the mirror moves by a length of 1/8 wavelength, and completes a cycle after the mirror moves by a length of 4 3 1/8 wavelengths, or 1/ wavelength. Using this information, one can construct an apparatus for measuring displacement. Without needing to be frequency stabilized, the laser can be deemed as operating at its central frequency and is traceable to the optical wavelength OPN May 007 41

due to its self-calibration. Working at this central frequency of the laser is advantageous, because there is only a single pass by the central frequency for each movement of 1/ wavelength. Currently, the apparatus has a measurement range of 15 mm, a resolving power of 79 nm, a standard deviation of 0. mm, a linearity of 0.005 percent and a zero drift of 0.16 mm/hour. The laser nanometer displacement ruler offers good immunity disturbances in airflow, temperature variation and better stability than some interferometers. It is thus appropriate to use under normal operating conditions. c Frequency-splitting wave plate measurement system Current methods for measuring phase retardations of wave plates are complex to operate, especially considering that they must be traced to natural standards, and cannot measure with high accuracy. Most methods need the aid of standard quarter wave plates or a high-accuracy goniometer. ased on the principle of -lasers that the frequency difference output is proportional to the magnitude of the Current methods for measuring phase retardations of wave plates are complex to operate, especially considering that they must be traced to natural standards, and cannot measure with high accuracy. birefringence Zhang and his colleagues developed an apparatus for measuring retardations of wave plates using laser frequency splitting. This method is capable of obtaining the retardation of a wave plate by simply putting it in a laser and measuring the frequency difference output from the laser. This technique is advantageous for its high precision and traceability to a natural reference the optical wavelength. It may also function as a benchmark for the measurement of wave plates. c Laser feedback wave plate measurement system Fei et al. developed an apparatus for measuring wave plates via optical feedback into a birefringent laser. Their work is based on the finding that, under laser feedback, the duty cycle of two orthogonal polarizations within a cycle of intensity modulation is linear with respect to the retardation of a wave plate under investigation. The apparatus is capable of performing fast online measurement of wave plate retardation with accuracy up to 0.1 degree, and has accomplished [ Laser frequency splitting wave-plate measurement system ] (a) Laser intensity [00 mv/div] 5 4 3 1 Time [.5 ms/div] Laser intensity PZT voltage PZT voltage [100 V/div] (b) Laser intensity [00 mv/div] 5 4 3 1 Time [.5 ms/div] Laser intensity PZT voltage PZT voltage [100 V/div] In feedback with a phase plate in the external cavity, the ratio between the width of orthogonally polarized beams is in direct proportional to the phase retardation of the phase plate. See Opt. Comm. 46, 505-10 (005). The polarization flipping position changes with the variation of external cavity birefringence. 4 OPN May 007 www.osa-opn.org

calibration similar to that obtained with the SC-VIS optical phase compensator made by Thorlabs and a wave plate made by Fujian CASTECH Crystals Inc. Moreover, an apparatus is currently under development for in situ measurement of wave plates. c Laser feedback ruler Researchers working on displacement measurement with feedback effects often use software to process displacement signals. In order to distinguish direction and to subdivide half-wavelengths, it is important to use appropriate optical structures and hardware circuits. Mao and colleagues have developed a displacement meter using strong optical feedback from a highly reflective mirror into Z-lasers which is structured by a Z-laser and a highly reflective mirror for feedback. The state of polarization weaves of the laser in feedback (which may be only o-light; both o-light and e-light; only e-light; and then no light) shifts sequentially as the object being measured or the feedback mirror moves along the axis of the laser, and the time interval of the polarization shift corresponds to a displacement of the feedback mirror by 1/8 wavelength. Unlike the laser nanometer ruler, in which the laser cavity varies in length, this apparatus has a fixed cavity length and hence a larger measurable range up to 50 mm or more. oth types of rulers have specific advantages: The laser feedback ruler has a large measurable range, while the laser nanometer ruler has higher accuracy with the auto-traceability to the optical wavelength. c Microchip Nd:YAG laser force/weight measurement system Holzapfel and colleagues conducted a series of studies using LD-pumped Nd:YAG microchip lasers at 1.06 µm for measuring both static and dynamic force and weight. The force being measured is applied along a diameter of a round microchip Nd:YAG pumped by a semiconductor laser with tens of milliwatts power and 808-nm wavelength, causing one frequency of the laser to split into two. The frequency difference of the microchip Nd:YAG laser is proportional to the force/weight applied to it. This laser sensor can measure forces ranging from less than 10-7 N to more than 10 N, with the sensitivity of 6.6 MHz/N. The test of dynamic force to an Nd:YAG laser is produced by a piezo-translator with a sine voltage of adjustable frequency. The modulation frequency ranges from DC up to 100 KHz. In this frequency range, there is good agreement between theory and experimental results. The laser feedback ruler has a large measurable range, while the laser nanometer ruler has higher accuracy with the auto-traceability to the optical wavelength. Future perspectives Several research groups are studying high-speed interferometers that incorporate -lasers and Z-lasers. Laser nanometer rulers and laser feedback rulers could be used in various kinds of displacement measurement instruments. They offer lower cost than current interferometers and higher accuracy (linearity, stability and repeatability) than commercially available inductance transducers. The frequency splitting wave plate measurement system is awaiting authorization as a benchmark with the function of traceability to a natural reference (i.e., the optical wavelength). The laser wave plate measurement system will be tested on a line of wave plate products. The microchip Nd:YAG laser force/weight measurement system is also promising, but should be modified to become more user friendly. Although much work has been done on orthogonally polarized lasers and their applications, further research is needed to exploit the full potential of these devices. t [ Shulian Zhang (zsl-dpi@mail.tshinghua.edu.cn) is with the State Key Laboratory of Precision Measurement Technology and Instruments, Tsinghua University, eijing, China. Thierry osch is with the engineering school ENSEEIHT and director of the electronics laboratory (LEN7) in Toulouse, France. ] [ References and Resources ] >> S. Yang and S.L. Zhang. The frequency split phenomenon in a HeNe laser with a rotation quartz crystal plate in its cavity, Opt. Comm. 68, 55-7 (1988). >> W. Holzapfel and W. Seffgast. Precise force measurement over 6 decades applying the resonator-internal photoelastic effect, Appl. Phys., 49(), 69-7 (1989). >> J. Yuye and S.L. Zhang. Zeeman-birefringence HeNe dual frequency lasers, Chinese Phys. Lett.18(4), 533-6 (001). >> L. Cui and S.L. Zhang, Semi-Classical theory model for feedback effect of orthogonally polarized dual frequency He-Ne laser, Opt. Express 13, 6558-6563 (005). >> W. Du et al. Using a cat s eye cavity to improve displacement selfsensing laser, Sensors & Actuators A, Physical, 1, 76-8 (005). >> L.G. Fei et al. Polarization flipping and intensity transfer in laser with optical feedback from an external birefringence cavity, Opt. Comm. 46, 505-10 (005). >> X. Zong et al. Intensity tuning characters of dual-isotope quasi-isotropic lasers, Chinese Phys. Lett. (8), 1906-8 (005). >> Z. Xiaobin and S.L. Zhang. Measurement of retardations of arbitrary wave plates by laser frequency splitting technology, Opt. Eng. 45(3) (006). >> S.L. Zhang. Orthogonal Polarization Lasers, Press of Tsinghua University, 005, second ed., June 006. >> L.G. Fei and S.L. Zhang. The discovery of nanometer fringes in laser self-mixing interference, Opt. Comm. 73, 6-30 (007). OPN May 007 43