LaserAnwendungsCentrum > Arbeitsgruppen > Angewandte Photonik (LAC) > Projekte > Fiber-coupled THz-source using difference-frequency generation in a DAST crystal

Fiber-coupled THz-source using difference-frequency generation in a DAST crystal

Konrad Hohmann, Wolfgang Schippers, Ulrike Willer, Christian Bohling, Wolfgang Schade, Tobias Schossig

Abstract: A passively Q-switched Cr4+:Nd3+:YAG microchiplaser is amplified in an Yb-fiber. The occurring sidemodes are used for THz-generation in a DAST crystal. These first investigations show quasi cw output powers in the order of 0.2 nW.

THz radiation is recently discussed extensively for various applications ranging from industrial process control, data communication to security applications. Major advantages of radiation in this spectral range are that it penetrates through e.g. plastics but is strongly reflected by metals and that molecules show distinct and distinguishable spectra so that a selective sensing of single species is possible. However, existing THz-sources are either very bulky and expensive, need cryogenic temperatures or emit only low power radiation. Furthermore the setup is often very complicated and sensitive so that field measurements are not possible.

Difference-frequency generation is a method widely used in the mid-infrared spectral range to set-up portable and rugged laser systems for environmental sensing and industrial process-control. For the THz-region, the organic crystal DAST (4-N, N-dimethylamino-4’-N’-methyl-stilbazolium tosylate) has been proposed as nonlinear material because of its high nonlinear coefficient [1, 2]. Its suitability for THz-generation has been shown for difference-frequency generation [3] and optical rectification [4, 5]. In this paper we use the nonlinear interaction of two adjacent modes of a fiber amplifier to generate THz radiation.
A passively Q-switched Cr4+:Nd3+:YAG laser is used to seed an Yb-fiber amplifier which is pumped by a P=50W laser diode emitting at a wavelength  of 976 nm. The amplified radiation is collimated and focused into the nonlinear crystal which is mounted in a rotation stage. The generated radiation is then aligned with two parabolic mirrors and focused onto a bolometer. As cut-off filter polyethylene plates are used.  At pump powers greater than P=14.5W side modes occur in the emission spectra of the fiber amplifier as shown in figure 1. Difference frequency generation in the DAST crystal provides the emission of THz-radiation.

 
Fig.1. Spectra of the emission of the fiber amplifier at a pump power (a)  P= 14.8W and (b) P=0W.

 

The DAST crystal is rotated to maximize the output power. The angle dependence of this signal confirms that the measured power is really due to THz-emission and not due to residual heat radiation not blocked by the filters since the background noise measured without pumping the amplifier does not show this dependence. Furthermore, the output power increases nonlinear with the pump-power of the fiber-amplifier while the emitted laser power increases linear. This is shown in figure 2. As can be seen, effective THz-emission sets on at P=14.5W, which coincides exactly with the beginning of the double mode behavior shown in figure 1.

These first measurements show output powers in the order of P=0.2nW. Further investigations will be performed to increase the output power. Direct attachment of the very thin nonlinear crystal onto the output-fiber of the amplifier could provide an effective and easy to handle fiber-coupled THz-source. 

 

Fig.2. THz outputpower and output power of the fiber amplifier as a function of pump power. 

 Financial support from the “Biophotonik Initiative des Landes Nieder­sachsen” is gratefully acknowledged.

References

  1. M. Walther, K. Jensby, S. R. Keiding, H. Takahashi, and H. Ito, “Far-infrared properties of DAST” Optics Letters, 25, 911-913 (2000).
  2. F. Pan, G. Knöpfle, Ch. Bosshard, S. Follonier, R. Spreiter, M.S. Wong, and P. Günter, “Electro-optic properties of the organic salt 4-N,N-dimethylamino-4’-N’-methyl-stilbazolium tosylate”, Appl. Phys. Lett. 69, 13-15 (1996).
  3. K. Kawase, M. Mizuno, S. Sohma, H. Takahashi, T. Taniuchi, Y. Urata, S. Wada, H. Tashiro, H. Ito, “Difference-frequency terahertz-wave generation from  4-N,N-dimethylamino-4’-N’-methyl-stilbazolium tosylate by use of an electronically tuned Ti:sapphire laser”, Optics Letters, 24, 1065-1067 (1999).
  4. X.-C. Zhang, X. F. Ma, Y. Jin, T.-M. Lu, E.P. Boden, P. D. Phelps, K. R. Stewart, and C. P. Yakymyshyn, “Terahertz optical rectification from a nonlinear organic crystal”, Appl. Phys. Lett. 61, 3080-3082 (1992).
  5. A. Schneider, I. Biaggio, P. Günter, “Optimized generation of THz pulses via optical rectification in the organic salt DAST”,Optics Comm. 224, 337-341 (2003).

 

 

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