DOCSLIB.ORG

Practical Tips for Two-Photon Microscopy

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Practical Tips for Two-Photon Microscopy Appendix 1 Practical Tips for Two-Photon Microscopy Mark B. Cannell, Angus McMorland, and Christian Soeller INTRODUCTION blue and green diode lasers. To provide an alignment beam to which the external laser can be aligned, light from this reference As is clear from a number of the chapters in this volume, 2-photon laser needs to be bounced back through the microscope optical microscopy offers many advantages, especially for living-cell train and out through the external coupling port: studies of thick specimens such as brain slices and embryos. CAUTION: Before you switch on the reference laser in this However, these advantages must be balanced against the fact that configuration make sure that all PMTs are protected and/or commercial multiphoton instrumentation is much more costly than turned off. the equipment used for confocal or widefield/deconvolution. Given Place a front-surface mirror on the stage of the microscope and these two facts, it is not surprising that, to an extent much greater focus onto the reflective surface using an air objective for conve- than is true of confocal, many researchers have decided to add a nience (at sharp focus, you should be able to see scratches or other femtosecond (fs) pulsed near-IR laser to a scanner and a micro- mirror defects through the eyepieces). The idea of this method is scope to make their own system (Soeller and Cannell, 1996; Tsai to cause the reference laser beam to bounce back through the et al., 2002; Potter, 2005). Even those who purchase a commercial optical train and emerge from the other laser port. To do this, select multiphoton system find that it helps to understand a bit more about filter settings that will allow some of the light from the internal how to optimize the performance of the fs laser system.1 This laser to exit the chosen coupling port. In order to bring two laser Appendix has been added to the Handbook to provide the basic beams to co-linearity, a beam-steering device is essential. A single- alignment and operating information that such people need. mirror beam steerer provides angular control while changing the First, the safety announcement... separation between the mirrors of a 2-mirror steerer provides beam translation (Fig. A1.1). It is also possible to achieve beam translation with a second LASER SAFETY angular control mirror. After adjusting the incoming near-IR beam to an intensity where it can be viewed without totally overwhelm- 2 Light sources for multiphoton microscopy are almost without ing the reference beam, adjust one mirror to make both laser spots exception very powerful pulsed lasers (laser class IV). It is vital merge at the surface of the other (angle-adjustable) mirror. Then that any personnel who perform alignment or other operations that that mirror is adjusted to bring the beams to co-linearity. We find carry a risk of beam exposure are familiar with and follow laser it useful to use a piece of light-blue paper as this shows the dimmed safety regulations. During routine operation one MUST ensure infrared beam well. If the laser has been tuned to the far part of that accidental exposure to the pulsed laser beam is prevented by the spectrum, you may have to use an IR viewer or viewer card to providing proper shielding and interlocks. visualize the beam. During alignment, protective eyewear is not an option — it is essential! See http://www.osha.gov/SLTC/laserhazards/ for US TESTING ALIGNMENT AND guidelines. SYSTEM PERFORMANCE On a regular basis and particularly subsequent to laser alignment, LASER ALIGNMENT the performance of the multiphoton microscope should be tested. The prime indicator of proper alignment of an imaging system is Just as in any other type of microscopy, correct optical alignment its point-spread function, as measured by using a sample contain- is crucial for achieving optimal, diffraction-limited performance in ing sub-resolution fluorescent beads. A test slide can be prepared 2-photon microscopy. The alignment of external lasers such as the by letting a drop of diluted beads dry onto a coverslip. The beads Ti:S or similar 2-photon sources into a laser scanning microscope are then embedded in a drop of Sylgard elastomer (Dow Corning, can be simplified if a well-aligned “internal” or reference laser is USA) with a microscope slide placed on top. We usually use available. In commercial confocal microscopes, typical candidate 0.2mm beads from Molecular Probes (Eugene, OR). These are lasers include Argon-ion or green HeNe lasers or, more recently, available in a range of colors suitable for 2-photon microscopy. It 1 The Multiphoton Users Group e-mail network at ·mplsm-users@ 2 As is explained below, this can be achieved by over-closing the slit and/or yahoogroups.comÒ, operated by Steve Potter at Georgia Tech, enrolled its reducing pump power, because mode-locking is not required. We typically 500th member in 2003. use <20mw @ 800nm and <10mW at 720nm. Mark B. Cannell, Angus McMorland, and Christian Soeller • Department of Physiology, FMHS, University of Auckland, New Zealand 900 Handbook of Biological Confocal Microscopy, Third Edition, edited by James B. Pawley, Springer Science+Business Media, LLC, New York, 2006. Practical Tips for Two-Photon Microscopy • Appendix 1 901 AB C FIGURE A1.1. (A) 2D simplification of the beam alignment process using a conventional beam-steerer. A vertical translation of a tilted mirror is used to bring the two beams to a common point on a second, tiltable mirror. (B) Rotation of the second mirror at the point of the common spot makes the two beams co-linear. (C) The co-linear beams after alignment. takes only about 30 minutes to prepare 10 slides in this way. Once ticularly if the system is to be used for 2-photon flash photolysis the elastomer has set, these slides will last for months if kept in a or combined confocal and multiphoton co-localization studies. dark drawer. As a result, they provide a good standard to check the In our laboratory we perform a basic system test with a pre- microscope sensitivity and resolution provided you have recorded pared bead sample on a daily basis. This check (usually conducted microscope and laser settings (including center wavelength, laser following system startup) is well worth the ~5 minutes it takes, power and bandwidth/pulse length) with each reference image. especially if it helps avoid debugging signal problems later when With proper alignment, the beads should blur approximately a precious biological sample is on the stage. evenly as you focus above and below them. Asymmetric blurring above-and-below best focus indicates spherical aberration while motion of the centroid of intensity means that the objective LASER SETTINGS AND OPERATION aperture is filled asymmetrically. The spatial resolution (without a pinhole) should be similar to confocal performance, values Historically, the mode-locked lasers used for 2-photon imaging between 0.2–0.4mm in plane, full-width at half maximum could be quite temperamental and ensuring that proper laser oper- (FWHM) and 0.5–0.8mm out of plane (in the z direction) should ation was a large part of the challenge of running a multiphoton be attainable when using a high-numerical-aperture (NA ~1.3) microscope. With the advent of fully computer-controlled turn-key objective. laser systems, this has become less of an issue. In any case, as the A very weak and noisy signal can have a number of causes. If most versatile source for 2-photon imaging is still the tunable there is no problem with the detectors or emission filters (most of Ti:S laser in the femtosecond configuration, we will focus on it which would also be apparent when operating the microscope with here. Regardless of whether you are using a fully automated or a conventional [1-photon] laser excitation), check that the laser manually adjusted Ti:S system, it is important to monitor and beam fills the objective rear aperture fully and evenly by rotating optimize the laser output before imaging. the objective turret to an empty position, placing a lens tissue over The choice of center wavelength is generally determined by the opening and inspecting the pattern of illumination (using an IR the fluorochromes to be excited. As a general rule of thumb you viewer if necessary). The beam should be accurately centered in should try to use the longest wavelength compatible with the dyes the empty socket and should form a uniform circle of light that in your sample as this will help minimize photodamage and also will cover the rear aperture (~8–10mm wide) of a typical objec- reduce scattering of the excitation light. Data on excitation spectra tive lens. If the light intensity at the rear aperture is low (<10mW) is now available from many sources in the literature and, if in make sure that no IR-opaque optical items are obstructing the illu- doubt, there are mailing lists where one can ask other researchers mination path.3 It is also possible that the beam is so badly mis- for advice (see http://groups.yahoo.com/group/mplsm-users/ and aligned that only scattered light is being observed. You can check http://listserv.acsu.buffalo.edu/archives/confocal.html). for this by ensuring that adjustments of the alignment mirrors have the expected effects on the spot in the BFP. If the microscope is a combined confocal/multiphoton system, MONITORING LASER PERFORMANCE the bead slide is also a useful tool to disclose alignment offsets between the 2-photon laser system and any other lasers. In partic- During tuning and imaging, laser operation can be very conve- ular you should check for any axial offsets (i.e., focus shifts), par- niently monitored using a spectrum analyzer.
Load more

Recommended publications
  • Resolving Power
    The 5 I’s of Culturing Microbes • Inoculation • Isolation • Incubation • Inspection • Identification 8/18/12 MDufilho 1 Table 4.1 Metric Units of Length 8/18/12 MDufilho 2 Microscopy • General Principles of Microscopy – Wavelength of radiation – Magnification – Resolution – Contrast 8/18/12 MDufilho 3 Figure 4.1 The electromagnetic spectrum 400 nm 700 nm Visible light X UV Infra- Micro- Radio waves and Gamma rays rays light red wave Television Increasing wavelength 10–12m 10–8m 10–4m 100m 103m Crest One wavelength Trough Increasing resolving power 8/18/12 MDufilho 4 Figure 4.2 Light refraction and image magnification by a convex glass lens-overview Light Air Glass Focal point Specimen Convex Inverted, lens reversed, and enlarged 8/18/12 MDufilho image 5 Principles of Light Microscopy • Magnification occurs in two phases – – The objective lens forms the magnified real image – The real image is projected to the ocular where it is magnified again to form the virtual image • Total magnification of the final image is a product of the separate magnifying powers of the two lenses power of objective x power of ocular = total magnification 8/18/12 MDufilho 6 8/18/12 MDufilho 7 Resolution Resolution defines the capacity to distinguish or separate two adjacent objects – resolving power – Function of wavelength of light that forms the image along with characteristics of objectives • Visible light wavelength is 400 nm–750 nm • Numerical aperture of lens ranges from 0.1 to 1.25 • Oil immersion lens requires the use of oil to prevent refractive loss
    [Show full text]
  • Pockels Cells (EO Q-Switches)
    Pockels Cells (EO Q-switches) The Electro-Optic Effect The linear electro-optic effect, also known as the Pockels effect, describes the variation of the refractive index of an optical medium under the influence of an external electrical field. In this case certain crystals become birefringent in the direction of the optical axis which is isotropic without an applied voltage. When linearly polarized light propagates along the direction of the optical axis of the crystal, its state of polarization remains unchanged as long as no voltage is applied. When a voltage is applied, the light exits the crystal in a state of polarization which is in general elliptical. In this way phase plates can be realized in analogy to conventional polarization optics. Phase plates introduce a phase shift between the ordinary and the extraordinary beam. Unlike conventional optics, the magnitude of the phase shift can be adjusted with an externally applied voltage and a λ/4 or λ/2 retardation can be achieved at a given wavelength. This presupposes that the plane of polarization of the incident light bisects the right angle between the axes which have been electrically induced. In the longitudinal Pockels effect the direction of the light beam is parallel to the direction of the electric field. In the transverse Pockels cell they are perpendicular to each other. The most common application of the Pockels cell is the switching of the quality factor of a laser cavity. Q-Switching Laser activity begins when the threshold condition is met: the optical amplification for one round trip in the laser resonator is greater than the losses (output coupling, diffraction, absorption, scattering).
    [Show full text]
  • Laser Modulation Solutions for Two-Photon Microscopy
    A Coherent White Paper Laser Modulation Solutions for Two-Photon Microscopy Since the seminal work in two-photon laser scanning fluorescence microscopy published in 1990 (Denk, et al., 1990), the technique has benefitted from step changes in laser technology. These improvements have furthered the penetration of the technique initially from a physics laboratory, to cell biology, disease studies and advanced neuroscience imaging. One box, tunable Ti:Sapphire lasers started this trend around 2001. Some years later, automatic dispersion control was added to the lasers to optimize the pulse duration at the sample plane of the microscope. As probes excitable at wavelengths longer than the upper limit of Ti:Sapphire lasers became more mature and efficient, after 2010 laser companies turned to Optical Parametric Oscillators to address this need for an augmented color palette, deeper imaging and less photodamage. In this article, we discuss the next phase in this evolution; the integration of fast power modulation in the laser system, and how this enables faster setup time, highest performance and low cost of ownership. Requirements for laser power control in two-photon microscopy. In its simplest form, continuous control of laser power can be achieved by addition of a phase retarding waveplate and polarizing analyser. By rotation of the waveplate, the transmission of the laser power through the analyser can typically be changed from 0.2% transmission to ~99%. By motorizing the waveplate this process can be automated to alter the power at the imaging plane in the microscope to equalize focused fluences at different depth frames, for example. Most modern laser scanning two-photon microscopes, however, require a faster modulation speed.
    [Show full text]
  • Measurement of Quadratic Terahertz Optical Nonlinearities Using Second
    Measurement of quadratic Terahertz optical nonlinearities using second harmonic lock-in detection Shuai Lin,1 Shukai Yu,1 and Diyar Talbayev1, ∗ 1Department of Physics and Engineering Physics, Tulane University, 6400 Freret St., New Orleans, LA 70118, USA (Dated: October 16, 2018) Abstract We present a method to measure quadratic Terahertz optical nonlinearities in Terahertz time- domain spectroscopy. We use a rotating linear polarizer (a polarizing chopper) to modulate the amplitude of the incident THz pulse train. We use a phase-sensitive lock-in detection at the fundamental and the second harmonic of the modulation frequency to separate the materials’ responses that are linear and quadratic in Terahertz electric field. We demonstrate this method by measuring the quadratic Terahertz Kerr effect in the presence of the much stronger linear electro-optic effect in the (110) GaP crystal. We propose that the method can be used to detect Terahertz second harmonic generation in noncentrosymmetric media in time-domain spectroscopy, with broad potential applications in nonlinear Terahertz photonics and related technology. arXiv:1810.06427v1 [physics.app-ph] 26 Sep 2018 1 I. INTRODUCTION Nonlinear Terahertz (THz) optics has blossomed into an exciting and active area of research with the advent of table-top high-field THz sources1–5. Among a multitude of non- linear phenomena, we focus here on the nonlinearities that are quadratic in THz electric field ET . Such quadratic effects can be induced in the second and third orders via the non- linear polarizabilities χ(2)(ET )2 and χ(3)(ET )2Eω, where Eω is an optical field. The second order nonlinearity χ(2) can lead to second harmonic generation (or sum frequency mixing) in noncentrosymmetric crystals.
    [Show full text]
  • Nonlinear Optical Characterization of 2D Materials
    nanomaterials Review Nonlinear Optical Characterization of 2D Materials Linlin Zhou y, Huange Fu y, Ting Lv y, Chengbo Wang y, Hui Gao, Daqian Li, Leimin Deng and Wei Xiong * Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] (L.Z.); [email protected] (H.F.); [email protected] (T.L.); [email protected] (C.W.); [email protected] (H.G.); [email protected] (D.L.); [email protected] (L.D.) * Correspondence: [email protected] These authors contributed equally to this work. y Received: 7 October 2020; Accepted: 30 October 2020; Published: 16 November 2020 Abstract: Characterizing the physical and chemical properties of two-dimensional (2D) materials is of great significance for performance analysis and functional device applications. As a powerful characterization method, nonlinear optics (NLO) spectroscopy has been widely used in the characterization of 2D materials. Here, we summarize the research progress of NLO in 2D materials characterization. First, we introduce the principles of NLO and common detection methods. Second, we introduce the recent research progress on the NLO characterization of several important properties of 2D materials, including the number of layers, crystal orientation, crystal phase, defects, chemical specificity, strain, chemical dynamics, and ultrafast dynamics of excitons and phonons, aiming to provide a comprehensive review on laser-based characterization for exploring 2D material properties. Finally, the future development trends, challenges of advanced equipment construction, and issues of signal modulation are discussed. In particular, we also discuss the machine learning and stimulated Raman scattering (SRS) technologies which are expected to provide promising opportunities for 2D material characterization.
    [Show full text]
  • Part 2: Laser in Pulsed Operation
    Part 2: Laser in pulsed operation 1 Theoretical principles 1.1 Generation of short laser pulses The output power of existing continuous wave laser systems is between a few milliwatts (He-Ne lasers) and a few hundred watts (Nd or CO2 lasers). However, there is a possibility to increase the output power of the laser for a small period of time by pulsed laser operation. Solid-state lasers are particularly suitable for this purpose, as they can achieve pulse peak output powers of up to 1012 W. This value corresponds approximately to the average electrical energy generation of the entire world. The difference, however, lies in the period in which this performance is achieved. While all the power plants together reach this value continuously, a single laser produces this high output power only for a duration of 10−13 s. In this case, the extremely short pulse duration appears to be disadvantageous, but there are also applications which exactly require this. One example is laser ablation, which is a method in material processing. Here, a small volume of material at the surface of a work piece can be evaporated if it is heated high enough in a very short amount of time. On the other hand, supplying the energy gradually would allow the heat to be absorbed into the bulk of the piece, never attaining a sufficiently high temperature above the evaporation point of the material. Other applications rely on the very high peak pulse power to obtain strong non-linear optical effects, like it is necessary for efficient second-harmonic generation or for optical parametric oscillators (OPO) which converts an input laser wave into two output waves of lower frequencies.
    [Show full text]
  • Second Harmonic Generation Microscopy for Quantitative Analysis
    PROTOCOL Second harmonic generation microscopy for quantitative analysis of collagen fibrillar structure Xiyi Chen1, Oleg Nadiarynkh1,2, Sergey Plotnikov1,2 & Paul J Campagnola1 1Department of Biomedica l Engineering, University of Wisconsin-Madison, Madison, Wisconsin, USA. 2Present addresses: Department of Physics, University of Utrecht, Utrecht, The Netherlands (O.N.); US National Institutes of Health, Heart, Lung and Blood Institute, Bethesda, Maryland, USA (S.P.). Correspondence should be addressed to P.J.C. ([email protected]). Published online 8 March 2012; doi:10.1038/nprot.2012.009 Second-harmonic generation (SHG) microscopy has emerged as a powerful modality for imaging fibrillar collagen in a diverse range of tissues. Because of its underlying physical origin, it is highly sensitive to the collagen fibril/fiber structure, and, importantly, to changes that occur in diseases such as cancer, fibrosis and connective tissue disorders. We discuss how SHG can be used to obtain more structural information on the assembly of collagen in tissues than is possible by other microscopy techniques. We first provide an overview of the state of the art and the physical background of SHG microscopy, and then describe the optical modifications that need to be made to a laser-scanning microscope to enable the measurements. Crucial aspects for biomedical applications are the capabilities and limitations of the different experimental configurations. We estimate that the setup and calibration of the SHG instrument from its component parts will require 2–4 weeks, depending on the level of the user′s experience. INTRODUCTION Over the past decade, the nonlinear optical method of SHG micros­ on the harmonophores and their assembly that can be imaged, as copy has emerged as a powerful tool for visualizing the supra­ the environment must be noncentrosymmetric on the size scale molecular assembly of collagen in tissues at an unprecedented level of of λSHG; otherwise, the signal will vanish.
    [Show full text]
  • Lecture 11: Introduction to Nonlinear Optics I
    Lecture 11: Introduction to nonlinear optics I. Petr Kužel Formulation of the nonlinear optics: nonlinear polarization Classification of the nonlinear phenomena • Propagation of weak optic signals in strong quasi-static fields (description using renormalized linear parameters) ! Linear electro-optic (Pockels) effect ! Quadratic electro-optic (Kerr) effect ! Linear magneto-optic (Faraday) effect ! Quadratic magneto-optic (Cotton-Mouton) effect • Propagation of strong optic signals (proper nonlinear effects) — next lecture Nonlinear optics Experimental effects like • Wavelength transformation • Induced birefringence in strong fields • Dependence of the refractive index on the field intensity etc. lead to the concept of the nonlinear optics The principle of superposition is no more valid The spectral components of the electromagnetic field interact with each other through the nonlinear interaction with the matter Nonlinear polarization Taylor expansion of the polarization in strong fields: = ε χ + χ(2) + χ(3) + Pi 0 ij E j ijk E j Ek ijkl E j Ek El ! ()= ε χ~ (− ′ ) (′ ) ′ + Pi t 0 ∫ ij t t E j t dt + χ(2) ()()()− ′ − ′′ ′ ′′ ′ ′′ + ∫∫ ijk t t ,t t E j t Ek t dt dt + χ(3) ()()()()− ′ − ′′ − ′′′ ′ ′′ ′′′ ′ ′′ + ∫∫∫ ijkl t t ,t t ,t t E j t Ek t El t dt dt + ! ()ω = ε χ ()ω ()ω + ω χ(2) (ω ω ω ) (ω ) (ω )+ Pi 0 ij E j ∫ d 1 ijk ; 1, 2 E j 1 Ek 2 %"$"""ω"=ω +"#ω """" 1 2 + ω ω χ(3) ()()()()ω ω ω ω ω ω ω + ∫∫d 1d 2 ijkl ; 1, 2 , 3 E j 1 Ek 2 El 3 ! %"$""""ω"="ω +ω"#+ω"""""" 1 2 3 Linear electro-optic effect (Pockels effect) Strong low-frequency
    [Show full text]
  • State-Of-The-Art Fiber Optics for Short Distance Frequency Reference Distribution
    N89-27878 January-March 1989 TDA Progress Report 42-97 State-of-the-Art Fiber Optics for Short Distance Frequency Reference Distribution G. Lutes and L. Primas Communications Systems Research Section A number of recently developed fiber-optic components that hem the promise of unprecedented stability for passively stabilized frequency distribution links are character- ized. These components include a fiber-optic transmitter, an optical isolator, and a new type of fiber-optic cable. A novel laser transmitter exhibits extremely low sensitivity to intensity and polarization changes of reflected light due to cable flexure. This virtually eliminates one of the shortcomings in previous laser transmitters. A high-isolation, low- loss optical isolator has been developed which also virtually eliminates laser sensitivity to changes in intensity and polarization of reflected light. A newly developed fiber has been tested. This fiber has a thermal coefficient of delay of less than 0.5 parts per million per °C, nearly 20 times lower than the best coaxial hardline cable and 10 times lower than any previous fiber-optic cable. These components are highly suitable for distribution systems with short extent, such as within a Deep Space Communications Complex. In this article these new components are described and the test results presented. I. Introduction mary causes of degradation. These effects are caused by distri- bution system noise, which reduces the SNR, and variations in The transmitter exciter, local oscillator, and receiver delay the environmental temperature, which cause delay changes. calibration system in a Deep Space Station (DSS) require The degree of delay change is dependent on the Thermal Coef- stable frequency references.
    [Show full text]
  • Improved Laser Based Photoluminescence on Single-Walled Carbon Nanotubes
    Improved laser based photoluminescence on single-walled carbon nanotubes S. Kollarics,1 J. Palot´as,1 A. Bojtor,1 B. G. M´arkus,1 P. Rohringer,2 T. Pichler,2 and F. Simon1 1Department of Physics, Budapest University of Technology and Economics and MTA-BME Lend¨uletSpintronics Research Group (PROSPIN), P.O. Box 91, H-1521 Budapest, Hungary 2Faculty of Physics, University of Vienna, Strudlhofgasse 4., Vienna A-1090, Austria Photoluminescence (PL) has become a common tool to characterize various properties of single- walled carbon nanotube (SWCNT) chirality distribution and the level of tube individualization in SWCNT samples. Most PL studies employ conventional lamp light sources whose spectral dis- tribution is filtered with a monochromator but this results in a still impure spectrum with a low spectral intensity. Tunable dye lasers offer a tunable light source which cover the desired excitation wavelength range with a high spectral intensity, but their operation is often cumbersome. Here, we present the design and properties of an improved dye-laser system which is based on a Q-switch pump laser. The high peak power of the pump provides an essentially threshold-free lasing of the dye laser which substantially improves the operability. It allows operation with laser dyes such as Rhodamin 110 and Pyridin 1, which are otherwise on the border of operation of our laser. Our sys- tem allows to cover the 540-730 nm wavelength range with 4 dyes. In addition, the dye laser output pulses closely follow the properties of the pump this it directly provides a time resolved and tunable laser source.
    [Show full text]
  • A Polarization-Insensitive Recirculating Delayed Self-Heterodyne Method for Sub-Kilohertz Laser Linewidth Measurement
    hv photonics Communication A Polarization-Insensitive Recirculating Delayed Self-Heterodyne Method for Sub-Kilohertz Laser Linewidth Measurement Jing Gao 1,2,3 , Dongdong Jiao 1,3, Xue Deng 1,3, Jie Liu 1,3, Linbo Zhang 1,2,3 , Qi Zang 1,2,3, Xiang Zhang 1,2,3, Tao Liu 1,3,* and Shougang Zhang 1,3 1 National Time Service Center, Chinese Academy of Sciences, Xi’an 710600, China; [email protected] (J.G.); [email protected] (D.J.); [email protected] (X.D.); [email protected] (J.L.); [email protected] (L.Z.); [email protected] (Q.Z.); [email protected] (X.Z.); [email protected] (S.Z.) 2 University of Chinese Academy of Sciences, Beijing 100039, China 3 Key Laboratory of Time and Frequency Standards, Chinese Academy of Sciences, Xi’an 710600, China * Correspondence: [email protected]; Tel.: +86-29-8389-0519 Abstract: A polarization-insensitive recirculating delayed self-heterodyne method (PI-RDSHM) is proposed and demonstrated for the precise measurement of sub-kilohertz laser linewidths. By a unique combination of Faraday rotator mirrors (FRMs) in an interferometer, the polarization-induced fading is effectively reduced without any active polarization control. This passive polarization- insensitive operation is theoretically analyzed and experimentally verified. Benefited from the recirculating mechanism, a series of stable beat spectra with different delay times can be measured simultaneously without changing the length of delay fiber. Based on Voigt profile fitting of high- order beat spectra, the average Lorentzian linewidth of the laser is obtained. The PI-RDSHM has advantages of polarization insensitivity, high resolution, and less statistical error, providing an Citation: Gao, J.; Jiao, D.; Deng, X.; effective tool for accurate measurement of sub-kilohertz laser linewidth.
    [Show full text]
  • Saturated Absorption Spectroscopy
    Ph 76 ADVANCED PHYSICS LABORATORY —ATOMICANDOPTICALPHYSICS— Saturated Absorption Spectroscopy I. BACKGROUND One of the most important scientificapplicationsoflasersisintheareaofprecisionatomicandmolecular spectroscopy. Spectroscopy is used not only to better understand the structure of atoms and molecules, but also to define standards in metrology. For example, the second is defined from atomic clocks using the 9192631770 Hz (exact, by definition) hyperfine transition frequency in atomic cesium, and the meter is (indirectly) defined from the wavelength of lasers locked to atomic reference lines. Furthermore, precision spectroscopy of atomic hydrogen and positronium is currently being pursued as a means of more accurately testing quantum electrodynamics (QED), which so far is in agreement with fundamental measurements to ahighlevelofprecision(theoryandexperimentagreetobetterthanapartin108). An excellent article describing precision spectroscopy of atomic hydrogen, the simplest atom, is attached (Hänsch et al.1979). Although it is a bit old, the article contains many ideas and techniques in precision spectroscopy that continue to be used and refined to this day. Figure 1. The basic saturated absorption spectroscopy set-up. Qualitative Picture of Saturated Absorption Spectroscopy — 2-Level Atoms. Saturated absorp- tion spectroscopy is one simple and frequently-used technique for measuring narrow-line atomic spectral features, limited only by the natural linewidth Γ of the transition (for the rubidium D lines Γ 6 MHz), ≈ from an atomic vapor with large Doppler broadening of ∆νDopp 1 GHz. To see how saturated absorp- ∼ tion spectroscopy works, consider the experimental set-up shown in Figure 1. Two lasers are sent through an atomic vapor cell from opposite directions; one, the “probe” beam, is very weak, while the other, the “pump” beam, is strong.
    [Show full text]