Journal of Optics Applications July, 2013, Volume 2, Issue 3, PP.35-42
A Novel Laser Wavelength Measurement Method Based on Scanning Fabry-Perot Interferometer Xiao Xiao, Yuanfu Lu, Jianhua Chen, Xiaojing Gong, Guangzhi Feng, Wenlong Yu, Fengqi Yu, Jin Lei Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China Chinese University of Hong Kong, Hong Kong, China E-mail: xiao.xiao@siat.ac.cn
Abstract A novel laser wavelength measurement method based on scanning Fabry-Perot interferometer is proposed here. The two basic laser wavelength measurement instruments and the classification according to their operation principles are introduced at first, and then the Fabry-Perot interferometer-based wavelength measurement technique is discussed in details. The confocal cavity configuration based scanning Fabry-Perot interferometer SA200-12B as well as its novel application in the measurement of an external cavity diode laser output wavelength is articulated at last. Keywords: Laser Wavelength Measurement; Scanning Fabry-Perot Interferometer; Confocal Cavity Configuration
1 INTRODUCTION Until now, there are two basic instruments used for the measurement of the laser wavelength: one of which is Optical Spectrum Analyzer, and the other is Optical Wavelength Meter. According to the operation principle of Optical Spectrum Analyzer, it can be divided into three types: one type is based on Michelson interferometer; the other is based on Diffraction grating; the third is based on Fabry-Perot interferometer. For Optical Wavelength Meter, its operation principle is mainly based on Michelson interferometer. Michelson interferometer-based wavelength measurement technique belongs to dual-beam interference measurement technique, which can typically provide a measurement accuracy of better than 0.001 nm, would only be suitable to measure optical signals with discrete wavelength components, rather than to measure power spectral densities of optical noises. Diffraction grating-based wavelength measurement technique belongs to dispersion spectroscopic measurement technique, which is unable to easily provide a very fine spectral resolution because of the limitation by the grooveline densities of diffraction grating and the maximum optical beam diameter projected onto the grating, and has wide wavelength coverage, usually covers a wavelength range from 400 nm to 1700 nm [2]. Fabry-Perot Interferometer (FPI) was invented in 1879, as the most important precise measuring instrument since then, it has already obtained the massive use in each domain of scientific research. Charles Fabry developed the multi-beam interference theory of light during 1890~1892, and based on this theory with his colleague Alfred Perot in 1897 manufactured the first FPI composed of two parallel fixed plate glasses, both sides of which were coated with very thin metal silver film, and the index of reflection of its metal reflector was about 90% (the pilot model is shown in Figure 1) [3]. The biggest merit of FPI lies in its extremely high resolution, for modern FPI which has a use wavelength of about 500 nm, when the cavity thickness is 1 cm and the index of reflection of reflector is 95%, the winnable resolution is 1.2Ă—106. In special application situation, 2.5Ă—107 can be achieved, which is almost the 1~2 magnitudes of prism and grating spectrograph. At present, because FPI has the merit in the precise wavelength measurement that other optical testing instruments fail to compare with, it has been utilized in the analysis of atomic hyperfine structure, the calibration of standard meter according to the optical wavelength, laser cavity, and other - 35 www.joa-journal.org
special application situations.
FIG. 1 THE EARLIEST FABRY-PEROT INTERFEROMETER MODEL
2 FABRY-PEROT INTERFEROMETER-BASED WAVELENGTH MEASUREMENT TECHNIQUE Fabry-Perot interferometer-based wavelength measurement technique belongs to multi-beam interference measurement technique, can easily provide spectral resolution better than 10 MHz, which is approximately 0.08 pm in a 1550 nm wavelength window; however, it only has relatively narrow wavelength coverage compared with Michelson interferometer-based wavelength measurement technique [2]. In Fabry-Perot interferometer-based wavelength measurement technique, Fabry-Perot interferometer is usually employed as the optical measuring instrument. Optical spectrum analyzer based on scanning Fabry-Perot interferometer is a popular optical instrument for its superior spectral resolution. The basic configuration of a Fabry-Perot interferometer is shown in Figure 2, where two parallel mirrors, both having power reflectivity R, are separated by a distance d [1,6]. If a light beam is launched onto the mirrors at an incident angle α, a part of the light will penetrate through the left mirror and propagate to the right mirror at point A where part of the light will pass through the mirror and the other part will be reflected back to the left mirror at point B. This process will be repeated many times until the amplitude is significantly reduced due to the multiple reflection losses. The power transfer function of a Fabry-Perot interferometer is periodic in the frequency domain, and it can be expressed in equation (1) using the optical wavelength λ as the variable: (1 R)2 TFP ( ) , 2 dn cos (1) (1 R)2 4 R sin 2 ( ) where n is the refractive index of the material between the two mirrors.
FIG. 2 ILLUSTRATION OF A FABRY-PEROT INTERFEROMETER WITH TWO PARTIALLY REFLECTING MIRRORS SEPARATED BY A DISTANCE D [2] - 36 www.joa-journal.org
For a fixed optical wavelength λ, equation (1) is a periodic transfer function of the incidence angle α. If it is assumed that the incidence angle is α=0, this Fabry-Perot interferometer configuration is known as a collinear configuration. With α=0, equation (1) can be simplified to equation (2): TFP ( )
(1 R)2 , (1 R)2 4 R sin 2 (2 nd / )
(2)
In this simple case, the power transmission is a periodic function of the signal optical frequency. Figure 3 shows two examples of power transfer functions in a collinear Fabry-Perot interferometer configuration where the mirror separation is d = 5 mm, the media refractive index is n = 1, and the mirror reflectivity is R = 0.95 for Figure 3(a) and R = 0.8 for Figure 3(b). With a higher mirror reflectivity, the transmission peaks become narrower and the transmission minima become lower. Therefore the Fabry-Perot interferometer has better frequency selectivity with high mirror reflectivity. In Figure 3(b), free spectral range (FSR) is the frequency separation ∆f between adjacent transmission peaks of an FPI, which is inversely proportional to the cavity optical length nd; and the FSR can be found from equation (1) as: FSR f
c , 2nd cos
(3)
FIG. 3 TRANSMISSION VERSUS THE SIGNAL WAVELENGTH OF A COLLINEAR FABRY-PEROT INTERFEROMETER WITH MIRROR SEPARATION D = 5 MM, MEDIA REFRACTIVE INDEX N = 1, AND MIRROR REFLECTIVITY R = 0.95 (A) AND R = 0.8 (B) [2]
HPBW, the width of each transmission peak of the FPI power transfer function, which indicates the frequency selectivity of the FPI, can be expressed as: HPBW
(1 R)c 2 nd R cos
,
(4)
The ratio between the FSR and HPBW is defined as Finesse: Finesse
FSR R , HPBW 1 R
Finesse is a quality measure of FPI that depends only on the effective mirror reflectivity R. - 37 www.joa-journal.org
(5)
Contrast is the ratio between the transmission maximum and the transmission minimum of the FPI power transfer function. It specifies the ability of wavelength discrimination if the FPI is used as an optical filter. From the power transfer function equation (1), the highest transmission is Tmax = 1 and the minimum transmission is Tmin = (1-R) 2/ [(1-R) 2+4R]. Therefore the contrast of the FPI is: T 4R 2 Finesse 2 C max 1 1 ( ) , T (1 R)2 min
(6)
From equation (2), the wavelength λm corresponding to the mth transmission peak can be found as:
m
2nd , m
(7)
By changing the length of the cavity, this peak transmission wavelength will move. When the cavity length is scanned by an amount of (8) d m , 2n The mth transmission peak frequency will be scanned over one entire FSR. To measure the signal spectral density over a wavelength band corresponding to an FSR, it is only needed to sweep the cavity length for approximately half the wavelength. This is the basic mechanism to make an OSA using Fabry-Perot interferometer. The mechanical scanning can usually be accomplished using a voltage-controlled piezo-electric transducer (PZT); therefore the mirror displacement (or the change in cavity length d) is linearly proportional to the applied voltage [2].
3 THE CONFOCAL CAVITY CONFIGURATION BASED SCANNING FABRY-PEROT INTERFEROMETER AS WELL AS ITS APPLICATION IN THE MEASUREMENT OF AN EXTERNAL CAVITY DIODE LASER OUTPUT WAVELENGTH There are various types of commercially available scanning FPIs, such as plano-mirror design, confocal-mirror design, and all-fiber design. The confocal-mirror design specially mentioned here uses a pair of concave mirrors whose radii of curvature are equal to their separation, resulting in a common focus point in the middle of the cavity, as is shown in Figure 4. An FPI using this confocal configuration usually has much higher finesse compared with plano-mirror configuration. The reason is that the focus of the incident beam reduces possible finesse degradation due to mirror surface imperfections. This configuration also has better tolerance to the quality of the incident beam. However, the cavity length of the FPI is determined by the curvature of the mirror, which cannot be adjusted. Therefore, FPI using confocal-mirror design has a fixed free spectral range and is not as flexible as that using planomirror design.
FIG. 4 CONFIGURATION OF A SCANNING FABRY-PEROT INTERFEROMETER USING CONFOCAL-MIRROR DESIGN
The scanning Fabry-Perot interferometer based on confocal cavity configuration uses a piezoelectric transducer to vary the separation between its two concave mirrors, and then the cavity will act as a very narrow band-pass filter. The SA200-12B, a high finesse confocal-mirror based scanning FPI manufactured by THORLABS Corporation, is used to examine the fine structures of the spectral characteristics of continuous wave (CW) lasers, as is shown in Figure 5. It has a finesse of 209, a free spectral range of 1.5 GHz, a resolution of 7.5 MHz, and a mirror separation of - 38 www.joa-journal.org
50 mm [4]. With SA200-12B, the laser wavelength of an irradiation from an external cavity diode laser (ECDL) in the infrared band can be measured.
FIG. 5 THE SCHEMATIC DIAGRAM OF A HIGH FINESSE CONFOCAL-MIRROR BASED SCANNING FABRYPEROT INTERFEROMETER SA200-12B
FIG. 6 SCHEMATIC DIAGRAM OF AN ECDL OPTICAL SPECTRUM MEASUREMENT USING SA200-12B
Generally, an additional fixed laser wavelength reference is needed if the absolute ECDL output wavelength is calibrated and measured using a scanning Fabry-Perot interferometer such as SA200-12B [8]. Without the fixed laser wavelength reference, just the ECDL output line-width and modulation sidebands can be measured using SA20012B. However, Figure 6 shows a schematic diagram of an ECDL optical spectrum (including wavelength) measurement using SA200-12B only. This scanning FPI is driven by a saw-tooth voltage waveform, and therefore its mirror displacement is linearly scanned. As a result, the peak transmission frequency of the SA200-12B transfer function is also linearly scanned. A photodiode is used at the SA200-12B output to convert the optical signal into electrical waveform, which is then displayed on an oscilloscope. To synchronize with the mirror scanning, the oscilloscope is triggered by the saw-tooth waveform [5]. In this spectrum measurement system, the spectral resolution is determined by the width of the SA200-12B transmission peak and the frequency coverage is limited by its FSR. Because of the periodic nature of the SA20012B transfer function, the spectral content of the ECDL optical signal to be measured has to be limited within one SA200-12B FSR. All the spectral components outside one SA200-12B FSR will be folded together, thus introducing measurement errors. In Figure 6, the optical spectrum of the ECDL in infrared band has an FSR of 666.7 MHz less than half the SA200-12B FSR, and two ECDL FSRs are thus contained in one SA200-12B FSR; however, due to the special design of the ECDL, its output optical spectral content will be limited within its two FSRs [7]. Therefore, all the ECDL output optical spectral contents will be inside one SA200-12B FSR, which prevents the spectral folding as well as the introduction of measurement errors. The optical path difference of two adjacent transmitted light in SA200-12B is four times its cavity length due to the confocal-mirror design of SA200-12B, as is shown in Figure 4; hence for an ECDL output wavelength 位, the SA200-12B output optical signal will change from an interference maxima to its adjacent interference maxima when the SA200-12B cavity length is scanned by 位/4 from a position where the constructive interference for 位 occurs. - 39 www.joa-journal.org
FIG. 7 A VIRTUAL SCANNING WAVEFORM OF THE ECDL OUTPUT OPTICAL SPECTRAL CONTENTS WITHIN TWO SA200-12B FSRS. THE SLOPING STRAIGHT LINE ON TOP REPRESENTS PZT VOLTAGE VERSUS TIME CURVE; THE CURVES ON BOTTOM REPRESENT THE ECDL OUTPUT LONGITUDINAL MODES; M AND M+1 ARE THE INTERFERENCE ORDER NUMBER OF THE ECDL OUTPUT LONGITUDINAL MODES
Figure 7 gives a virtual scanning waveform of the ECDL output optical spectral contents within two SA200-12B FSRs. It can be seen from it that the actual ECDL output optical spectral content only contains three longitudinal modes, of which the wavelengths are λ1, λ2, and λ3, respectively. The ECDL output optical spectral content in the second SA200-12B FSR with the interference order number of m+1 are as the same as that in the first SA200-12B FSR with the interference order number of m. According to the discussion above, following relationships are found between the variables λ1, λ2, and λ3, so as to obtain their values.
/ 4 1 1 , V2 2 / 4 2 V1
(9)
/ 4 2 2 , V3 3 / 4 3 c c 1.5 T1, 1 2 7 V2
(10) (11)
Among the three equations above, ∆V1, ∆V2, ∆V3, and ∆T1 can be read from the oscilloscope, whose units are mV and ms respectively; c is the propagation velocity of light in vacuum and its unit is m/s; λ1, λ2, and λ3 are the unknown wavelength variables, whose units are nm; 1.5 represents the free spectral range of SA200-12B, and its unit is GHz; 7 represents the free spectral range of SA200-12B in time domain, and its unit is ms. Then, the values of λ1, λ2, and λ3 can be acquired as follows: 7c(V2 V1) (12) 1 , 1.5 V2 T1
2
3
7c(V2 V1) 1.5 * V1 * T1
(13)
,
7cV3 (V2 V1) 1.5 * V1 * V2 * T1
.
(14)
4 CONCLUSIONS From the three formulae above, not only the value of λ1, λ2, and λ3 can be obtained, but also the measurement accuracy of λ1, λ2, and λ3 determined by the measurement and reading accuracy of ∆V1, ∆V2, ∆V3, and ∆T1 is known. - 40 www.joa-journal.org
With a high performance digital storage oscilloscope, the measurement and reading accuracy of ∆V1, ∆V2, ∆V3, and ∆T1 can be guaranteed to a high level; therefore, so does the measurement accuracy of λ1, λ2, and λ3. With this measurement method, the ECDL output longitudinal mode wavelength, line-width, and modulation sidebands can be measured just using SA200-12B.
ACKNOWLEDGMENT This work is supported by 2008 Shenzhen Basic Research Project Fund (grant SY200806300217A), 2009 Shenzhen Technology Research and Development Fund / STRDF (grant O702011001), 2010 Guangdong―Chinese Academy of Sciences Comprehensive Strategic Cooperation Project (grant 2010A090100014), and 2011 TIPC, Key Laboratory of Functional Crystals and Laser Technology, Chinese Academy of Sciences (grant JTJG201109).
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Xiao Xiao and Yu Fengqi, “Investigation of a Linearly Tunable External Cavity Diode Laser in a Configuration with Single Cavity All-Dielectric Thin-Film Fabry-Perot Filter,” SPIE Optical Engineering. 50, 034201, 2011.
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AUTHORS Xiao Xiao earned his BS and MS degrees
mainly engages in the research of Terahertz biomedical imaging,
in science and technology of optical
non-linear optical microscope.
information at Huazhong University of
Chen Jianhua is a research assistant at
Science and Technology. He is currently a
Shenzhen
doctoral student in the department of integrated Institutes
electronics of
Advanced
at
Institutes
Technology,
Shenzhen
Chinese
of
Advanced
Academy
of
Science. He received his Master's degree
Technology,
in Electronic Engineering from Tsinghua
Chinese Academy of Sciences.
University on the topic of ultra-high
Lu Yuanfu earned his PhD degree in
speed optical fiber transmission and
optics at photophysical Key Laboratory,
signal processing. His current research focuses on developing
the Institute of Physics, Chinese Academy
data acquisition and imaging schemes for AR/OR-Photo-acoustic
of Sciences. In 2009, he joined Shenzhen
medical devices.
Institutes of Advanced Technology (SIAT)
Gong Xiaojing is an assistant professor at
as an assistant researcher. During doctoral
Shenzhen
period, he mainly engaged in the research
Instruments and Mechanics from the
number of research projects of the Natural Science Foundation
University of Science and Technology of
of China and major national research equipment independent
indexed by SCI, and has applied for a patent. He currently
Advanced
He received his PhD degree in Fine
meanwhile, he had participated in the national "863 Program", a
papers in important domestic and international academic journals
of
Technology, Chinese Academy of Science.
of solid-state lasers and nonlinear frequency conversion;
innovation projects. In addition, he has published more than 20
Institutes
China in 2007. He joined the IBHE of SIAT in 2007. He focused in the research of Bio-photonics, including Optical Coherence Tomography/ Microscopy, Photoacoustic
- 41 www.joa-journal.org
Tomography/Microscopy,
Terahertz
biomedical
imaging, etc. He has authored and co-authored more than 10
Technology (SIAT) as a full professor and director of the
journal and conference articles, with 3 Chinese patents and one
integrated electronics department. Before joining SIAT, he
U. S. patent pending.
worked at Rockwell Science Center (USA), Intel (USA), Feng Guangzhi earned his master degree in
Condensed
Matter
Physics
at
Changchun Institute of Optics and Fine Mechanics and Physics. In 2008, he
Teradyne (USA), Valence Semiconductor (USA), and Suzhou CAS IC Design Center (China). His R&D interests include CMOS RF integrated circuit design, CMOS sensor design, wireless sensor networks, RFID, and wireless communications.
joined Shenzhen Institutes of Advanced
Jin Lei earned his PhD degree at Tsinghua
Technology (SIAT) as an engineer.
University in 1997. After graduation, he
During master period, he mainly engaged
served as an associate professor at
in the research of high power semiconductor laser beam shaping
Tsinghua University. He had many visits
and coupling, and he has published one paper and applied for
to the United Kingdom, Belgium, Hong
two patents. After graduation, he served as an optical engineer in
Kong, responsible for or involved in
the famous Han's Laser Technology Co., Ltd. He participated the
setting up a number of large advanced
development of laser processing equipment in the UV, IR band
instrument platform, such as a large sputtered atoms the laser
as a project backbone, and applied for one patent. In 2009, he
single atom detection spectrometer; China's first multi-photon
joined Shenzhen Institutes of Advanced Technology (SIAT) bio-
excitation biological imaging platform; nonlinear Biomedical
photonics center, and his current main research target is terahertz
Photonics of the Chinese University of Hong Kong microscope;
fast imaging system.
Shenzhen advanced school of the Chinese Academy of Sciences Yu Wenlong earned his master degree in
THz imaging systems, etc. He is one of the first batches of
software engineering at Harbin Institute of
researchers in the field of bio-photonics research; responsible for
Technology. In 2008, he joined Shenzhen
or participated in a number of national research projects,
Institutes of Advanced Technology (SIAT)
including 973 projects, 863 projects and research programs. The
as a research assistant. During master
establishment of Shenzhen Chinese Sleep and Health Study Key
period, he participated in the National
Laboratory of THz Medical Imaging Laboratory, classic life
Natural Science Foundation project: 3D
sciences joint laboratories and so on. Major research direction is
point cloud data feature segmentation-
the use of advanced scientific imaging instrument exploration of
based algorithm.
TCM scientific explanation, based on the basis of TCM clinical
Yu Fengqi earned his PhD degree in Integrated Circuits and Systems Lab (ICSL) at UCLA. In 2006, he joined Shenzhen
Institutes
of
Advanced
research, and he tries to develop medical devices with Chinese characteristics, the final application to the universal health care system. He is now a Professor of the Biomedical Engineering TCM scientific and applied technology Research Office.
42 www.joa-journal.org