OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 8 — Apr. 13, 2009
  • pp: 6623–6628
« Show journal navigation

Nanosecond terahertz optical parametric oscillator with a novel quasi phase matching scheme in lithium niobate

D. Molter, M. Theuer, and R. Beigang  »View Author Affiliations


Optics Express, Vol. 17, Issue 8, pp. 6623-6628 (2009)
http://dx.doi.org/10.1364/OE.17.006623


View Full Text Article

Acrobat PDF (440 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We present an optical parametric oscillator pumped by a single mode Q-switched nanosecond Nd:YVO4 laser for terahertz generation in periodically poled lithium niobate with a new phase matching scheme. This new method leads to an emission of terahertz radiation close to the Cherenkov angle and to a parallel propagation of the pump and signal wave. The emission frequency of this novel source is chosen by the poling period to 1.5 THz. For spectral narrowing the signal wave of the OPO is injection seeded. In the optical spectrum also cascaded processes are observed demonstrating a powerful generation of terahertz waves.

© 2009 Optical Society of America

1. Introduction

2. Phase matching schemes

Fig. 1. Phase matching schemes. (a) Non-collinear phase matching also known as Cherenkov-phase matching. (b) “Conventional” quasi phase matching, grating vector parallel to pump wave propagation. (c) Slant-stripe periodic poling for quasi phase matching. (d) Novel quasi phase matching scheme with grating vector perpendicular to pump wave propagation.
θ=arccos[(nPλPnS(λP1λTHz1))(λTHznTHz)]
(1)

where λP, λS, λTHz, nP, nS and nTHz are the pump wavelength, the signal wavelength, the Terahertz wavelength, the refractive indices of the pump, signal and Terahertz wave, respectively. This angle is almost equivalent to the so called Cherenkov angle of

θCherenkov=arccos(nIRnTHz)
(2)

with the condition nPnS(= nIR) [6

6. M. Theuer, G. Torosyan, C. Rau, R. Beigang, K. Maki, C. Otani, and K. Kawase, “Efficient generation of Cherenkov-type terahertz radiation from a lithium niobate crystal with a silicon prism output coupler,” Appl. Phys. Lett. 88, 071122 (2006). [CrossRef]

]. Therefore outcoupling techniques for THz waves emitted in this specific direction are already well known (e.g. Cherenkov-cut or Si-prisms, see also [6

6. M. Theuer, G. Torosyan, C. Rau, R. Beigang, K. Maki, C. Otani, and K. Kawase, “Efficient generation of Cherenkov-type terahertz radiation from a lithium niobate crystal with a silicon prism output coupler,” Appl. Phys. Lett. 88, 071122 (2006). [CrossRef]

]).

The grating period as a function of the pump wavelength and the desired THz output frequency is given by

Λ=[(nTHzλTHz)2(nPλPnS(λP1λTHz1))2]12.
(3)

3. Pump enhancement of nanosecond pulses

In order to build a cavity which is pumped at 1064 nm and highly reflective at the signal wave (1070 nm for THz generation at 1.5 THz), no standard dichroic mirrors are available. Our solution to overcome this problem is the application of a pump enhancement cavity. The effect is, that a stabilization of the length of a highly reflective cavity on a multiple of the pump wavelength leads to an effective transmission of the pump field through the incoupling cavity mirror. Further, the pump intensity inside the cavity can exceed the incoming intensity. For the signal wave the cavity is still highly reflective. A requirement needed to obtain a pump enhancement of pulses is that the pulse length is larger than the roundtrip time inside the cavity (a further possibility is synchronous pumping, which is only applicable at high repetition rate systems, e.g. enhancement of Ti:Sapphire laser pulses). Then each pump pulse is enhanced by itself. Although the maximum enhancement factor achievable in this case is lowered in comparison to the maximum enhancement in the case of a continuous wave pump field, this scheme is still helpful to build a cavity which allows for a collinear overlap of the pump and signal wave. The stabilization of a cavity to a special wavelength is a well known and solved problem [7

7. T. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. 35, 441–444 (1980). [CrossRef]

, 8

8. R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, “Laser Phase and Frequency Stabilization Using an Optical Resonator,” Appl. Phys. B 31, 97–105 (1983). [CrossRef]

]. One convenient solution is the stabilization scheme by Hänsch-Couillaud which analyzes the ellipticity of the reflected pump wave. This reflection consists of two contributions. One is the direct reflection from the input coupler, the other one is the transmission through this mirror after the wave has taken one roundtrip inside the cavity. If the cavity is detuned (length is not a multiple of the wavelength) these two waves have a phase difference. The wave that has taken one roundtrip inside the cavity suffers a polarisation rotation caused by different nonlinear losses or conversions of the different polarization contributions. The overlap of the two waves outside the cavity results in an elliptically polarized wave, whose chirality is proportional to the length detuning of the cavity.

4. Experimental setup

As pump source a Q-switched single mode Nd:YVO4 laser (Xiton Photonics GmbH) is used. It emits pulses with a length of about 33 ns at a repetition rate of 10 kHz. This high repetition rate is the most significant difference to THz-OPOs reported by other groups. So far mostly 10 Hz and once 350 Hz repetition rate systems have been reported [1

1. K. Kawase, J.-i. Shikata, and H. Ito, “Terahertz wave parametric source,” J. Phys. D: Appl. Phys. 35, R1–R14 (2002). [CrossRef]

, 2

2. T. J. Edwards, D. Walsh, M. B. Spurr, C. F. Rae, and M. H. Dunn, “Compact source of continuously and widely-tunable terahertz radiation,” Opt. Express 14, 1582–1589 (2006). [CrossRef] [PubMed]

]. The average output power of the laser is up to 7 W, thus a pulse energy of 0.7 mJ is available at the laser output. Single mode operation of the laser is achieved by injection seeding with a MISER leading to a bandwidth below 60 MHz. The OPO itself is also seeded by a grating stabilized diode laser tunable from 1064 nm to at least 1076 nm. Therefore this seed laser is in principle useful to build OPOs for THz frequencies up to 3 THz when pumped at 1064 nm. For the purpose of cavity length stabilization we apply the Hänsch-Couillaud stabilization scheme and a commercially available locking system. The OPO cavity itself is built by two curved mirrors with a radius of curvature of 200 mm each. The transmittance of the mirrors is 5 % for both optical wavelengths. The dimensions of the used 5% MgO-doped congruent LiNbO3 crystal are 50×3×1 mm3. The crystal is poled as shown in Fig. 1(d) with a periodicity Λ of 43.7 μm. With the refractive index of lithium niobate at the pump wavelength of 2.15 the cavity roundtrip time is 0.85 ns, which is significantly less than the pulse duration. Output coupling of the THz radiation was achieved by use of five high resistivity Si prisms. The base of each prism is 10 mm, the height is 7 mm and the angles 40 and 50 degrees. So the direction of emission of the THz radiation is perpendicular to one surface of the Si prisms.

Fig. 2. Experimental setup of the OPO including the pump laser (Nd:YVO4), the grating stabilized seed laser and the Hänsch-Couillaud stabilization scheme.

5. Considerable losses

The extraction of the generated THz out of the nonlinear crystal is a nontrivial problem. Several restrictions and sources of loss lead to a limitation of the extractable THz power.

The phase matching scheme and the fact that a linear cavity in combination with asymmetric Si prisms is used lead to a reduction of the extractable THz power by a factor of four: A factor of two is contributed by the fact, that the poling is symmetric and leads to emission of THz into the direction of both y-facets of the crystal. The linear setup of the cavity results in a further factor of two. In our setup with the used asymmetric Si prisms only one direction of emission can be accessed.

One of the major problems of THz generation inside LiNbO3 crystals is the high absorption coefficient in the THz region [9

9. L. Palfalvi, J. Hebling, J. Kuhl, A. Peter, and K. Polgar, “Temperature dependence of the absorption and refraction of MgO:doped congruent and stoichiometric LiNbO3 in the THz range,” J. Appl. Phys. 97, 123505 (2005). [CrossRef]

]. At the frequency of 1.5 THz the absorption is about 45 cm-1. Assuming the generation position inside the crystal being 500 μm from the exit facet this results in an absorption of 91 %.

With our outcoupling scheme two interfaces on the way from the inside of the crystal to the outside have to be overcome which lead to inevitable Fresnel losses. The interface LiNbO3-Si leads to a loss of about 7 %, the interface Si-air to a loss of about 30 %.

The generation of THz radiation in a nonlinear crystal suffers from considerable diffraction, when the beam waist inside the crystal is small. The divergency angle of the emitted beam is enhanced by refraction at the interface LiNbO3-Si as well as at the interface Si-air. The limited collection angle of the following mirror leads to a decrease of the fraction of collectable THz power.

6. Results

A typical optical spectrum of the OPO is shown in Fig. 3. Peak height or area is not to be taken as a measure of intensity. Besides the pump wavelength at 1064 nm and the signal wavelength of 1070 nm further generated wavelengths are observed. The additional peak at 1076 nm is caused by a cascaded process, where a photon at 1070 nm decays in a further THz photon and a 1076 nm photon. This is a very useful process to overcome the Manley-Rowe limit, because, in principle, one pump photon can decay in more than one THz photon by passing through this cascaded process (see also [10

10. K. L. Vodopyanov, “Optical THz-wave generation with periodically-inverted GaAs,” Laser & Photon. Rev. 2, 11–25 (2008). [CrossRef]

]). The highest cascaded process observed so far is of the third order. An unintended process which occurs is the sum frequency generation (SFG) of the pump and the THz wave leading to a peak at 1058 nm. This is a further indication of the high efficiency of this source, but unfortunately it is also a source of loss of THz photons. However, the power of the SFG wave observed in our experiment is negligible so far. The lowest A1-symmetry polariton mode of the LiNbO3 crystal leads to a Raman peak at 7.6 THz and is therefore the reason for the Raman lasing process at 1093.5 nm. This peak is only observed when the OPO is not seeded and can be seen in the shown spectrum because of the chopped seed laser (so the spectrum is the superposition of the seeded OPO and the Raman laser).

Fig. 3. Left: Typical optical spectrum including cascaded, SFG and Raman processes. Right: Threshold behavior and correlation between signal power and THz (Golay cell-) signal.

The THz output was detected with a Golay cell and the lock-in technique. The power of the generated signal wave was measured after the separation from the pump wave using a grating behind the OPO. On the right side of Fig. 3 the threshold behavior and the good correlation between the signal and the THz power is obvious. The measured intracavity signal power of about 1.5 W corresponds to a total THz power inside the crystal of about 8 mW. It is evident that only a fraction of this power can be extracted out of the crystal for reasons already discussed.

Fig. 4. Left: Fabry Perot scan of the THz output. Right: Tunability of the OPO measured by the peak wavelength of the signal wave and the Golay cell signal.

A FPI-scan of the THz-wave performed with a FPI consisting of two silicon wafers is shown on the left side of Fig. 4. The scan range was limited to about 15 mm, thus, only an upper bandwidth limit of about 3 GHz can be estimated, but is expected to be much less as the two input waves (pump and seed) are single mode. Further experiments to determine the THz linewidth with more sophisticated measurement equipment will be carried out in the near future.

By measuring the signal output wavelength with a high resolution optical spectrometer, the tuning characteristics of the OPO were measured as can be seen on the right graph of Fig. 4. The seed laser was tuned by tilting the grating and the Golay cell signal was taken to measure the relative power of the THz output. The peak wavelength of the signal wave was used to calculate the THz frequency. The OPO tuning bandwidth was found to be about 100 GHz, limited by the finite bandwidth of the QPM-scheme.

7. Conclusion

We have presented the generation of monochromatic THz radiation in an OPO by applying a novel QPM-scheme in LiNbO3. This new scheme leads to a parallel propagation of the optical waves (pump and signal) and therefore to a better interaction of these two. The pump and signal resonant cavity was achieved by applying a pump enhancement with a Hänsch-Couillaud stabilization scheme. A tunability of about 100 GHz and an upper bandwidth limit of 3 GHz was shown. Further, cascaded processes of higher orders were observed which is an important result for future experiments to overcome the Manley-Rowe limit.

Acknowledgments

We acknowledge the support of the Bundesministerium für Bildung und Forschung (BMBF, FKZ 13N9297).

References and links

1.

K. Kawase, J.-i. Shikata, and H. Ito, “Terahertz wave parametric source,” J. Phys. D: Appl. Phys. 35, R1–R14 (2002). [CrossRef]

2.

T. J. Edwards, D. Walsh, M. B. Spurr, C. F. Rae, and M. H. Dunn, “Compact source of continuously and widely-tunable terahertz radiation,” Opt. Express 14, 1582–1589 (2006). [CrossRef] [PubMed]

3.

J. A. L’huillier, G. Torosyan, M. Theuer, C. Rau, Y. Avetisyan, and R. Beigang, “Generation of THz radiation using bulk, periodically and aperiodically poled lithium niobate Part 2: Experiments,” Appl. Phys. B 86, 197–208 (2006). [CrossRef]

4.

C. Weiss, G. Torosyan, J.-P. Meyn, R. Wallenstein, R. Beigang, and Y. Avetisyan, “Tuning characteristics of narrowband THz radiation generated via optical rectification in periodically poled lithium niobate,” Opt. Express 8, 497–502 (2001). [CrossRef] [PubMed]

5.

Y. Sasaki, Y. Avetisyan, K. Kawase, and H. Ito, “Terahertz-wave surface-emitted difference frequency generation in slant-stripe-type periodically poled LiNbO3 crystal,” Appl. Phys. Lett. 81, 3323 (2002). [CrossRef]

6.

M. Theuer, G. Torosyan, C. Rau, R. Beigang, K. Maki, C. Otani, and K. Kawase, “Efficient generation of Cherenkov-type terahertz radiation from a lithium niobate crystal with a silicon prism output coupler,” Appl. Phys. Lett. 88, 071122 (2006). [CrossRef]

7.

T. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. 35, 441–444 (1980). [CrossRef]

8.

R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, “Laser Phase and Frequency Stabilization Using an Optical Resonator,” Appl. Phys. B 31, 97–105 (1983). [CrossRef]

9.

L. Palfalvi, J. Hebling, J. Kuhl, A. Peter, and K. Polgar, “Temperature dependence of the absorption and refraction of MgO:doped congruent and stoichiometric LiNbO3 in the THz range,” J. Appl. Phys. 97, 123505 (2005). [CrossRef]

10.

K. L. Vodopyanov, “Optical THz-wave generation with periodically-inverted GaAs,” Laser & Photon. Rev. 2, 11–25 (2008). [CrossRef]

OCIS Codes
(190.4970) Nonlinear optics : Parametric oscillators and amplifiers
(230.6080) Optical devices : Sources

ToC Category:
Nonlinear Optics

History
Original Manuscript: February 10, 2009
Revised Manuscript: March 31, 2009
Manuscript Accepted: April 4, 2009
Published: April 7, 2009

Citation
D. Molter, M. Theuer, and R. Beigang, "Nanosecond terahertz optical parametric oscillator with a novel quasi phase matching scheme in lithium niobate," Opt. Express 17, 6623-6628 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-8-6623


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. K. Kawase, J.-i. Shikata, and H. Ito, "Terahertz wave parametric source," J. Phys. D: Appl. Phys. 35, R1-R14 (2002). [CrossRef]
  2. T. J. Edwards, D. Walsh, M. B. Spurr, C. F. Rae, and M. H. Dunn, "Compact source of continuously and widelytunable terahertz radiation," Opt. Express 14, 1582-1589 (2006). [CrossRef] [PubMed]
  3. J. A. L’huillier, G. Torosyan, M. Theuer, C. Rau, Y. Avetisyan, and R. Beigang, "Generation of THz radiation using bulk, periodically and aperiodically poled lithium niobate Part 2: Experiments," Appl. Phys. B 86, 197- 208 (2006). [CrossRef]
  4. C. Weiss, G. Torosyan, J.-P. Meyn, R. Wallenstein, R. Beigang, and Y. Avetisyan, "Tuning characteristics of narrowband THz radiation generated via optical rectification in periodically poled lithium niobate," Opt. Express 8, 497-502 (2001). [CrossRef] [PubMed]
  5. Y. Sasaki, Y. Avetisyan, K. Kawase, H. Ito, "Terahertz-wave surface-emitted difference frequency generation in slant-stripe-type periodically poled LiNbO3 crystal," Appl. Phys. Lett. 81, 3323 (2002). [CrossRef]
  6. M. Theuer, G. Torosyan, C. Rau, R. Beigang, K. Maki, C. Otani, and K. Kawase, "Efficient generation of Cherenkov-type terahertz radiation from a lithium niobate crystal with a silicon prism output coupler," Appl. Phys. Lett. 88, 071122 (2006). [CrossRef]
  7. T. W. H¨ansch, and B. Couillaud, "Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity," Opt. Commun. 35, 441-444 (1980). [CrossRef]
  8. R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, "Laser Phase and Frequency Stabilization using an Optical Resonator," Appl. Phys. B 31, 97-105 (1983). [CrossRef]
  9. L. Palfalvi, J. Hebling, J. Kuhl, A. Peter, and K. Polgar, "Temperature dependence of the absorption and refraction of MgO:doped congruent and stoichiometric LiNbO3 in the THz range," J. Appl. Phys. 97, 123505 (2005). [CrossRef]
  10. K. L. Vodopyanov, "Optical THz-wave generation with periodically-inverted GaAs," Laser & Photon. Rev. 2, 11-25 (2008). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1. Fig. 2. Fig. 3.
 
Fig. 4.
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited