378 378 378 378 378 378 378 378 378 378 378 378 378 0 0 378 378 Another approach enabling continuous mode hop free tuning of the EC-QCL system was achieved by simultaneously adjusting the grating angle and periodically varying the laser injection current to achieve a continuous shift of the Fabry-Prot comb of the gain chip (Pushkarsky et al., 2006b). Figure 7.7. In the grazing-incidence configuration (Fig. The MEMS- based EC-QCL system, inside a 2cc volume high heat load (HHL) laser package will offer a precise mode hop free tuning within approximately 8cm1 from its center wavelength. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 The most basic of tunable laser designs is that incorporating an output mirror coupler and a tuning grating in, Atomic Absorption, Methods and Instrumentation, Encyclopedia of Spectroscopy and Spectrometry, To ensure that only light of a wavelength specific to the analyte of interest is being measured, a line isolation device is required. 0 0 0 0 375 0 0 0 0 0 0 625 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 375 In a Littrow configuration, the radiation at the selected wavelength is retroreflected. 89 0 obj Figure 4. /Encoding /MacRomanEncoding These apertures (normally slits) are usually oversized in the direction perpendicular to the dispersion to allow the spectrum of the sky to be recorded directly adjacent to that of the target to permit accurate subtraction of the background. 0000036046 00000 n John F. Federici, Zoi-Heleni Michalopoulou, in Counterterrorist Detection Techniques of Explosives, 2007. General performance and wavelength tuning behavior of QC lasers in Littrow and Littman-Metcalf cavity configurations were studied in detail in Guipeng (2002), demonstrating similar wavelength tuning properties for both configurations. Fig. 0000057400 00000 n Although this is a very effective avenue to achieve fairly narrow linewidths, the issue of optical damage due to high intracavity power densities does introduce limitations. J. Allington-Smith, in Encyclopedia of Modern Optics, 2005. << /Font << /F8 119 0 R /F9 98 0 R /F10 90 0 R /F11 93 0 R /F12 91 0 R /T2 104 0 R >> For the values of R o investigated, the variation in the measured THz amplitude in a linear (solid squares) compared to curved geometry (open squares) is rather small. << /S 438 /L 645 /Filter /FlateDecode /Length 139 0 R >> Tuning is accomplished by slight rotation of the grating. /Info 85 0 R Geometry and location of detector array positions (solid squares) and source location (point O at an angle of) for near-field correction. 0 517 0 0 0 0 0 326 326 326 326 326 326 326 326 326 326 326 326 Using the usual complex-wave representation for probability amplitudes, apply Eq. 0000016381 00000 n endobj This linewidth performance was obtained at a pulse repetition rate (prf) of~8 kHz, laser peak powers of~1 kW, and laser conversion efficiencies of~5% (Duarte and Piper, 1984). 0000005039 00000 n endobj /Resources 89 0 R /N 9 Both cavity modes are separated by v 1/2nL, where L is the cavity length and n is the refractive index of the cavity medium. Wavelength tuning by rotation of the grating, in narrow-linewidth dispersive oscillators, imposes stringent constraints on the angular resolution of the grating kinematic mount. The two most commonly used configurations for grating-coupled EC systems which employ a QCL as a gain medium are the Littman-Metcalf (Phillips, 2007) and the Littrow configurations (Arnold, 1998). 7. Performance of Tunable Laser Oscillators Without Intracavity Beam Expansion, Steve J. Hill, Andy S. Fisher, in Encyclopedia of Spectroscopy and Spectrometry, 1999. /CropBox [ 0 0 612 792 ] Since |G| 2, the maximum attainable resolving power, according to eqn [14], is determined chiefly by the telescope aperture and angular slitwidth. Tunable laser oscillators without intracavity beam expansion are those laser resonators in which the intrinsic narrow beam waist at the gain region is not expanded using intracavity optics. << /Filter /FlateDecode /Length 8149 /Subtype /Type1C >> The relative reference phase for each detector' remains fixed because the infrared signals are delivered to the receiver through a fixed-length fiber pigtail. By altering the position of this dispersing element, light of only the desired wavelength passes through the exit slit to the detector. Further information on wavelength tuning can be found in Chapter 6, including details on birefringent filters. /Flags 34 7.2, yield very stable coherent output characterized by a time averaged laser linewidth of 1.4 GHz (or 0.0015 nm at a wavelength of 575 nm). In advance of the availability of multiple THz detectors for a complete N element detector array, we use a single homodyne detector at multiple positions to sample the phase and amplitude of the wavefront from a point-like THz source. /Type /Font %%EOF The experimental system that is used to demonstrate [72] the concept of interferometric imaging in the THz range is shown in Fig. /BaseFont /ANKGLH+CMBX12 b;QHL$;bJMWHp,Pa~zvG3hWQ | This HMPGI grating oscillator delivers a laser linewidth of 375 MHz, or 0.00042 nm at 580 nm (after Duarte, 1997). trailer /Flags 262178 The Daylight Solutions EC-QCL consists of a 25mm long optical cavity length, miniature grating tuning mechanism, and integrated current and temperature controls. If D = 4 m and d1 = 200 mm, the corresponding angle on the sky is 180 arc-sec. /PageLabels 76 0 R 0000020478 00000 n Owing to the relatively low-power requirements of the receiver, the optical power from a single set of lasers can be distributed using a fiber optic splitter to power each element of the imaging array. In a compensating configuration the multiple prism beam expander is designed to yield zero dispersion at a wavelength of choice (Duarte, 1985a). 0000013280 00000 n In this method the laser current value is selected to have one of the Fabry-Prot modes of the gain chip exactly coincide with the desired output frequency as the laser is tuned. Assuming tan = 2, the ratio in Eq. 378 378 378 0 378 378 378 378 378 378 378 0 378 378 378 ] 19(a)], the angles of incidence and diffraction are equal: i = 1. 7.2. One approach utilized in cavities incorporating gratings in a Littrow configuration, intracavity beam expansion, and an intracavity etalon, is to synchronize the motion of the grating and the etalon. (11.25). Due to the double diffraction from the grating the Littman-Metcalf configuration offers narrower linewidth than the Littrow configuration at the expense of both optical power and spectral tuning range due to decreased grating feedback strength. For smaller values of , the ratio LR is even more in favor of the Fabry-Perot. >> 378 378 378 378 378 378 378 378 378 378 378 378 378 378 378 378 /ItalicAngle 0 << The minimum resolvable linewidth or resulting laser linewidth obtainable from the etalon is given by, where is the effective finesse of the etalon. The grating equation becomes. /h/d/F/s/seven/i/W/a/L/E/t/g/M/u/A/one/k/fi/O/v/two/m/b) For a well-defined projected slit, assume the largest practical is about 1. The lasers are very sensitive to feedback. 0000025067 00000 n << If the disperser is replaced by a plane mirror (or if the disperser is removed in a transmissive system), the system acts as a versatile imager, with A = 1. The mode-hop free tuning ability and tuning range is significantly improved when an anti-reflection (AR) coating is implemented at the front facet of the Fabry-Perot QCL. /CapHeight 676 However, this configuration suffers from mode hopping due to the lack of control for the EC length when the laser was tuned. Diffraction gratings in external cavity lasers combine the functions of the filter and external mirror. 0000014782 00000 n 91 0 obj 0000017466 00000 n Grazing-incidence grating cavities. 862 738 707 0 0 419 0 0 676 1067 0 845 769 0 839 625 782 0 0 1162 9. Wavelength tuning of these systems is realized with a Littrow grating angle controlled by means of a stepper motor and an integrated absolute optical encoder with microprocessor-based closed loop controller (Pushkarsky et al., 2008; Caffey et al., 2010). 7.9a, or as a closed cavity, as shown in Fig. (13.5.10), we get. HlyPSIB"JxOCcoX!-Cn!@BG+2A%(PZ,3(8X Although the Fabry-Perot clearly has an advantage over an echelle system in terms of the LR product, there are other considerations as well. /T 238198 << For the Fabry-Perot we assume R=4E5, a resolving power appropriate for the observation of narrow spectral lines, for example. (11.18) to arrive at an equation for the probability of transmission in a MachZehnder interferometer. Copper-vapor laser-pumped HMPGI grating oscillators, with architecture similar to that depicted in Fig. This, together with the area coverage of either a scanning or imaging system, makes the Fabry-Perot especially suited to studies of individual spectral lines of extended, emission line sources such as gaseous nebulae. Further, frequency stability requirements demand the design of thermally stable resonators and hence the use of materials such as superinvar [23]. Figure 7.10. 94 0 obj 0000000016 00000 n 0 stream An important configuration in the tunable laser oscillators without intracavity beam expansion class that employs only the natural divergence of the intracavity beam for total illumination of the diffractive element is the grazing-incidence grating design (Shoshan et al., 1977; Littman and Metcalf, 1978). The radius of curvature of the curved path is Ro. Thus the echelle is well suited to the study of stars and near stellar sources. /Type /Catalog /FontBBox [ -34 -251 988 750 ] It is important to note that for = 500 nm and N = 40 the free spectral range of the Fabry-Perot for this resolving power is only 0.05 nm, compared to 5 or 10 nm for an echelle in this same wavelength range. and the minimum resolvable bandwidth provided by the etalon is given by the ratio FSR/F. (13.5.11) is about 100 for the same transmittances and beam diameters in each instrument. Note that , the angle subtended by the slit length at the collimator, is D/d1 times larger than the angular length projected on the sky. Also assume that the 600-lines/mm echelle grating is deployed in Littrow configuration for the shorter wavelength, at m = 3. As the photo-mixing process requires that the electric fields of the optical sources be collinear, polarization-maintaining fiber is used to deliver the optical power to the fiber pigtailed THz transmitter and receiver. 0000001515 00000 n The reflective finesse is given by (Born and Wolf, 1999). /ColorSpace << /Cs6 103 0 R >> In addition, the grazing-incidence configuration is typically used with a much higher angle of incidence, for example, i 85. 93 0 obj This implies maximizing cos, since = , which results in A > 1. >> endstream

Schematic representation of interferometric imaging array receiver. /H [ 1515 604 ] The distance from the source to each detector position (xn) is given by Rn. /Root 87 0 R Current instruments on 8 m telescopes feature collimated beam diameters of D1 mm. A closed cavity means that the laser output is coupled from the output coupler mirror and not from the reflection losses of a dispersive or diffraction component. startxref 88 0 obj 0000016731 00000 n 7.10 (Duarte and Piper, 1981). That information is used to test the near-field imaging performance and techniques expected for an interferometric array. The predominant effect on the THz electric field is the additional change in phase from positioning the detectors on the linear compared with curved geometry. where R=(xo+xn-Rotan)2+Ro2 represents a constant offset phase. 0000032379 00000 n /Type /Page Assume m = 3. F.J. Duarte, in Lasers for Medical Applications, 2013. A laser beam fails to provide interference fringes when the distance from the beam splitter to the mirrors in a Michelson interferometer is 1 m. Estimate the linewidth of the laser. /Widths [ 375 313 0 0 563 563 563 563 563 563 563 0 0 0 0 0 0 0 0 0 850 0 813 Diffraction grating mountings. /FontDescriptor 92 0 R 90 0 obj The Tx emits radiation when a voltage bias is applied to the LT GaAs in the presence of the two infrared beams. The grating-to-collimator configuration may also be used at the penalty of lower R but does permit a smaller camera. Hb```f``} Ab,{8?/!Pl,~d*U iV 8B#f,B"i3W_]#p)0j^"+xemmjmqavXwH&-a_t5V6M,CJW\:}EuGcE~a'/:tr0Z+g&*!WVNRR~9eXL\u[i^nW235]suBhn`(10 F@ cglb># Pv`4s~7a&;;L\9010[00.``r`m&6c };@yILL] 0`JoucakcpM/MC:A2UC The value of assumed here is obviously appropriate for a large extended source.

littrow Figure 7.8. The diffracted light from the second pass is a retroflection of the incident light from the first pass. An elegant approach to long-range wavelength scanning in single-longitudinal-mode oscillators is the use of synchronous scanning methods. 0000038724 00000 n >> /L 240036 This tuning range is sufficient to perform high-resolution gas-phase spectroscopy of molecules with narrow spectral features. In order to achieve continuous, fine mode-hop free tuning, the cavity length and grating angle must track each other. /BaseFont /ANKGMH+CMR12 This linewidth performance corresponds to single-longitudinal-mode emission and was obtained at a prf of~8 kHz, laser peak powers of~800 W, and laser conversion efficiencies of~4% (Duarte and Piper, 1984). The THz modules are LT grown GaAs bowtie-type photo-mixers. /StemV 109 grating littrow measurement nanostructures /CharSet (/C/o/c/y/n/l/e/P/f/T/I/period/r/h/J/d/s/i/a/t/g/u/k/O) /ItalicAngle 0 Longitudinally-laser pumped hybrid multiple-prism pre-expanded near grazing incidence (HMPGI) grating solid-state dye laser oscillator. 0000004174 00000 n F.K. A schematic diagram of such an instrument is shown in Figure 5. 0 0 0 0 762 666 0 0 0 707 0 0 0 0 0 0 0 0 0 0 0 0 490 0 435 544 0000026991 00000 n 7.2. /LastChar 180 /Pages 77 0 R For ELTs, the implied sizes are 0.52 m, requiring huge optics, accurately co-aligned mosaics of dispersing elements, and enormous support structures to provide the required stability. A topic of considerable interest in grating tuned cavities is long-range wavelength tuning. 139 0 obj

0000017140 00000 n Light of all wavelengths enters the monochromator through an entrance slit and is then split into specific wavelengths using either a prism, or more commonly, a diffraction grating. ScienceDirect is a registered trademark of Elsevier B.V. ScienceDirect is a registered trademark of Elsevier B.V. Tunable External-Cavity Semiconductor Lasers, The dispersion of the grazing-incidence configuration is therefore twice that of the, INSTRUMENTATION | Astronomical Instrumentation, For a 600-lines/mm echelle grating deployed in, Narrow-Linewidth Laser Oscillators and Intracavity Dispersion, Gratings, prisms, and etalons are widely used as tuning elements in dispersive cavities. /ProcSet [ /PDF /Text /ImageC ] This is necessary to suppress mode hopping. An interesting scheme based on a miniature EC system wavelength tuning has been implemented by Daylight Solutions (www.daylightsolutions.com). >> The line-width of each laser is ~2 MHz with an output power of ~100 mW. <<

with the echelle in the Littrow configuration. 0000025045 00000 n @}tq}+*5V]X)sWWn0l8]xlg;},Q. /ID[<40ed4c60c74d98cb3027a8ff5a65eb47><40ed4c60c74d98cb3027a8ff5a65eb47>] 2c). 8. /Widths [ 742 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 613 0 0 0 558 0 0 0 302 0 0 0 The second stage translates the Rx laterally at different locations corresponding to different detectors' in the interferometric imaging array. Multiple-etalon systems are described in detail in the literature (Maeda et al., 1975; Pacala et al., 1984). [:6>r^im~kbYz[Xm:bz 3=n[P"E-v_h}K1_| Here, it should be mentioned that pure grazing-incidence configurations, although compact, provide either too low efficiency in their closed cavity version or higher amplified spontaneous emission (ASE) in their open cavity alternative (Duarte, 1990a). /Linearized 1

In this case, the effect of the optical cavity formed by the laser itself is eliminated and stronger feedback from the diffraction grating to the laser chip is achieved. Another approach to achieve single frequency operation is to integrate a Fabry-Prot QCL with broadband gain medium (Faist et al., 2001; Faist et al., 2002) into an EC configuration (Maulini et al., 2004; Maulini et al., 2005; Wysocki et al., 2005; Maulini et al., 2006).

(13.4.11) and division into Eq.

The value of x o represents the distance from the xn = 0 detector to the origin. endobj For trace gas detection, Littrow configured external cavity quantum-cascade laser (EC-QCL) systems (Lewicki et al., 2007b; Wysocki et al., 2008; Karpf and Rao 2009; Scherer et al., 2009; Tsai and Wysocki 2010; Spagnolo et al., 2010) are more often employed than Littman-Metcalf systems (Phillips et al., 2007). /Contents [ 111 0 R 113 0 R 115 0 R 117 0 R 124 0 R 126 0 R 128 0 R 130 0 R ] /Ascent 693 A typical angle of incidence for the Littrow configuration is i 50. 375 375 0 0 375 0 0 0 0 0 375 0 375 375 0 0 0 0 0 0 0 0 0 0 0 0 A conceptual diagram of the receiving array is shown in Fig. To ensure that only light of a wavelength specific to the analyte of interest is being measured, a line isolation device is required. @l j For a pure grazing-incidence cavity, or an HMPGI oscillator incorporating a dispersionless multiple-prism expander, the basic grating equation applies: where is the angle of incidence and is the angle of diffraction (see Fig. The diffracted beam is subsequently reflected back toward the grating by the tuning mirror. The electric field correlation can then be calculated for each pair' of detector positions, thereby mimicking the performance of an N element detector array. 0000017088 00000 n 0000026069 00000 n An EC-QCL system consists of three main optical elements: a QCL gain chip, an aspherical lens to collimate laser light, and a diffraction grating which acts as a wavelength-selective filter. Copyright 2022 Elsevier B.V. or its licensors or contributors. The expression for the FRS has its origin in v = c/x. The THz radiation is emitted within 30 [102] using a hyper-hemispheric silicon lens that is attached to the THz module. :5:mfvVmmN37tK>-l6. 431EJ1f(C\`+k640@_2?.=5-7#!.>nj;G'-v;^Tf9*4IHNgsMJ!&ee3x1Nc2iA[60if/i4d3.M4s.7gh;P04oL`u! >> McNicholl and Metcalf [67] provide a scalar diffraction analysis for Littrow and grazing-incidence cavities. The finesse of the etalon is a function of the flatness of the surfaces (often in the range of /100 /50), the dimensions of the aperture, and the reflectivity of the surfaces.

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