Resonator interferometry of pulsed submillimeter-wave lasers

Steffen, Hans; Kneubühl, Fritz Kurt

doi:10.1109/jqe.1968.1075016

Cited by 76 publications

(4 citation statements)

References 85 publications

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“…Figure 9 illustrates a portion of a typical laser resonator interferogram, with the output power plotted as a function of decreasing far-infrared laser cavity length [42][43][44][45] .…”

Section: Discussionmentioning

confidence: 99%

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

Jackson

Zink

2015

JoVE

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The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and Terahertz imaging. For about 45 years, the generation of coherent, far-infrared radiation has been accomplished using the optically pumped molecular laser. Once far-infrared laser radiation is detected, the frequencies of these laser emissions are measured using a three-laser heterodyne technique. With this technique, the unknown frequency from the optically pumped molecular laser is mixed with the difference frequency between two stabilized, infrared reference frequencies. These reference frequencies are generated by independent carbon dioxide lasers, each stabilized using the fluorescence signal from an external, low pressure reference cell. The resulting beat between the known and unknown laser frequencies is monitored by a metal-insulator-metal point contact diode detector whose output is observed on a spectrum analyzer. The beat frequency between these laser emissions is subsequently measured and combined with the known reference frequencies to extrapolate the unknown far-infrared laser frequency. The resulting one-sigma fractional uncertainty for laser frequencies measured with this technique is ± 5 parts in 10 7. Accurately determining the frequency of far-infrared laser emissions is critical as they are often used as a reference for other measurements, as in the high-resolution spectroscopic investigations of free radicals using laser magnetic resonance. As part of this investigation, difluoromethane, CH 2 F 2 , was used as the far-infrared laser medium. In all, eight far-infrared laser frequencies were measured for the first time with frequencies ranging from 0.359 to 1.273 THz. Three of these laser emissions were discovered during this investigation and are reported with their optimal operating pressure, polarization with respect to the CO 2 pump laser, and strength. Video LinkThe video component of this article can be found at

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“…Figure 9 illustrates a portion of a typical laser resonator interferogram, with the output power plotted as a function of decreasing far-infrared laser cavity length [42][43][44][45] .…”

Section: Discussionmentioning

confidence: 99%

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

Jackson

Zink

2015

JoVE

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“…For far-infrared lasers, e.g. hydrogen, deuterium, and iodide cyanide lasers [137] or THz quantum-cascade lasers [138], the thermal contribution also has to be considered. For a laser at λ ¼ 10 μm, e.g.…”

Section: Schawlow-townes Approximationmentioning

confidence: 99%

Spectral coherence, Part I: Passive-resonator linewidth, fundamental laser linewidth, and Schawlow-Townes approximation

Pollnau

Eichhorn

2020

Progress in Quantum Electronics

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The degree of spectral coherence characterizes the spectral purity of light. It can be equivalently expressed in the time domain by the decay time τ or the quality factor Q of the light-emitting oscillator, the coherence time τ coh or length ℓ coh of emitted light or, via Fourier transformation to the frequency domain, the linewidth Δν of emitted light. We quantify these parameters for the reference situation of a passive Fabry-P erot resonator. We investigate its spectral line shapes, mode profiles, and Airy distributions and verify that the sum of all mode profiles generates the corresponding Airy distribution. The Fabry-P erot resonator is described, as an oscillator, by its Lorentzian linewidth and finesse and, as a scanning spectrometer, by its Airy linewidth and finesse. Furthermore, stimulated and spontaneous emission are analyzed semi-classically by employing Maxwell 0 s equations and the law of energy conservation. Investigation of emission by atoms inside a Fabry-P erot resonator, the Lorentz oscillator model, the Kramers-Kronig relations, the amplitude-phase diagram, and the summation of quantized electric fields consistently suggests that stimulated and spontaneous emission of light occur with a phase 90 in lead of the incident field. These findings question the quantum-optical picture, which proposed, firstly, that stimulated emission occurred in phase, whereas spontaneous emission occurred at an arbitrary phase angle with respect to the incident field and, secondly, that the laser linewidth were due to amplitude and phase fluctuations induced by spontaneous emission. We emphasize that the first derivation of the Schawlow-Townes laser linewidth was entirely semi-classical but included the four approximations that (i) it is a truly continuous-wave (cw) laser, (ii) it is an ideal four-level laser, (iii) its resonator exhibits no intrinsic losses, and (iv) one photon is coupled spontaneously into the lasing mode per photon-decay time τ c of the resonator, independent of the pump rate. After discussing the inconsistencies of existing semi-classical and quantum-optical descriptions of the laser linewidth, we introduce the spectral-coherence factor, which quantifies spectral coherence in an active compared to its underlying passive mode, and derive semi-classically, based on the principle that the gain elongates the photon-decay time and narrows the linewidth, the fundamental linewidth of a single lasing mode. This linewidth is valid for lasers with an arbitrary energy-level system, operating below, at, or above threshold and in a cw or a transient lasing regime, with the gain being smaller, equal, or larger compared to the losses. By applying approximations (i)-(iv) we reproduce the original Schawlow-Townes equation. It provides the hitherto missing link between the description of the laser as an amplifier of spontaneous emission and the Schawlow-

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“…In any case, most THz lasers can be treated with homogeneously broadening due to the small Doppler contribution to the widths of their emission lines [16,17]. We take into account a THz laser with the output coupler at one end and assume the transmittance of the output coupler is T, whereas the loss B and the absorption A from all mirrors are assumed in the other laser mirror.…”

Section: Output Power Influenced By Slight Shift Of Transmittancementioning

confidence: 99%

A theoretical investigation of wedged capacitive strip-grating output couplers for optically pumped terahertz lasers

Zuo

Meng

et al. 2010

Optics Communications

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Resonator interferometry of pulsed submillimeter-wave lasers

Cited by 76 publications

References 85 publications

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

Spectral coherence, Part I: Passive-resonator linewidth, fundamental laser linewidth, and Schawlow-Townes approximation

A theoretical investigation of wedged capacitive strip-grating output couplers for optically pumped terahertz lasers

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