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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 23 — Nov. 5, 2012
  • pp: 25654–25661

Measurement of the intrinsic linewidth of terahertz quantum cascade lasers using a near-infrared frequency comb

M. Ravaro, S. Barbieri, G. Santarelli, V. Jagtap, C. Manquest, C. Sirtori, S. P. Khanna, and E. H. Linfield  »View Author Affiliations


Optics Express, Vol. 20, Issue 23, pp. 25654-25661 (2012)
http://dx.doi.org/10.1364/OE.20.025654


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Abstract

We report the measurement of the frequency noise power spectral density of a quantum cascade laser emitting at 2.5THz. The technique is based on heterodyning the laser emission frequency with a harmonic of the repetition rate of a near-infrared laser comb. This generates a beatnote in the radio frequency range that is demodulated using a tracking oscillator allowing measurement of the frequency noise. We find that the latter is strongly affected by the level of optical feedback, and obtain an intrinsic linewidth of ~230Hz, for an output power of 2mW.

© 2012 OSA

OCIS Codes
(030.1640) Coherence and statistical optics : Coherence
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: August 21, 2012
Revised Manuscript: September 28, 2012
Manuscript Accepted: October 3, 2012
Published: October 29, 2012

Citation
M. Ravaro, S. Barbieri, G. Santarelli, V. Jagtap, C. Manquest, C. Sirtori, S. P. Khanna, and E. H. Linfield, "Measurement of the intrinsic linewidth of terahertz quantum cascade lasers using a near-infrared frequency comb," Opt. Express 20, 25654-25661 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-23-25654


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References

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  18. The ratio 2.6x10−3/0.08 = 0.032 between the ε coefficients at minimum and maximum isolation corresponds to the field amplitude isolation, i.e. to a power isolation of 1x10−3, or 30dB. By directly measuring the performance of our isolator using a power detector we found instead an isolation of 16dB. There are several possible explanations for this large difference. The most likely is related to the thickness of the quartz wave plate (3.1+/−0.005mm) being much larger that the QCL wavelength. Therefore, given the QCL large free spectral range of 16GHz, the amount of isolation is strongly dependent on the Fabry-Perot lasing mode number, which can change depending on the feedback conditions. For technical reasons the direct measurement was performed with the QCL operating in pulsed mode, thus lasing on several longitudinal modes, which underestimates the isolation. Instead the QCL was lasing in continuous wave on a single-mode when we measured the frequency pulling.
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