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Ref. p. 245]

4.4 Phase conjugation

243

 

 

 

I (x )

x

(x )

E

x

E ( x )

x

/4

n ( x )

I ( x )

x

cc aaxiss

Coherent

beams

Interference

Space charge

distribution

Space charge field

Refractive index grating

Fig. 4.4.9. Formation of a photorefractive index grating due to a sinusoidal intensity pattern.

Fig. 4.4.10. Scheme of photorefractive total- internal-reflection phase conjugator (cat conjugator), left. The light propagation in the crystal can be seen due to scattering, right.

process may be highly e cient leading to large reflectivities well above 100 % in relation to the incoming power.

Self-pumped phase conjugators require only a single incident beam and because of their simplicity they are more advantageous for practical applications. The operation of photorefractive self-pumped phase conjugators is based on a non-linear optical process called beam fanning. When a single beam is incident on a photorefractive crystal, some light is scattered inside the crystal. This scattered light forms a set of gratings with the incident light and is amplified by two-wave mixing. This process was named fanning because a broad fan of scattered amplified light is generated emerging from the crystal.

Perhaps the most commonly used photorefractive self-pumped phase conjugator type is the socalled cat conjugator [82Fei]. In this case the first pump beam is generated from the incident beam by fanning, the second pump beam by backreflection on the crystal corner. Figure 4.4.10 shows a rhodium-doped barium titanate crystal which acts as a cat conjugator for an incident beam of 5 mW

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4.4.6 Photorefraction

[Ref. p. 245

 

 

 

 

 

 

 

 

 

 

 

Spatial

FP

 

 

Array

/2

 

 

=array810 nm

waveλ/2plate-

filter

etalon

 

 

λ=810nm

wave plate

 

 

 

 

BaTiO3: Rh

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Output : P = 230 mW

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 4.4.11. Coherent diode laser array coupled by a phase-conjugating BaTiO3:Rh crystal [98Lob].

optical power at 808 nm wavelength. The formed internal phase-conjugation loops can be observed in the lower right-hand corner of the crystal. Self-pumped phase-conjugate reflectivities as high as 60–80 % have been reported for visible and near-infrared wavelengths by numerous investigators using photorefractive crystals in various arrangements [85Gue, 86Pep, 95Mu, 94Wec, 97Huo].

The e cient operation of photorefractive phase conjugators at low and moderate power levels makes this type of device attractive especially for diode-laser applications. Free-running high-power diode laser arrays emit laser beams of poor spatial and spectral quality. Optical phase-conjugate feedback can increase both the spatial and the temporal coherence of the radiation. Figure 4.4.11 shows an external-cavity diode laser system comprising a photorefractive BaTiO3 crystal as phase conjugator, a Fabry-Perot etalon, and a spatial filter forcing the laser diode array to operate in a single spatial and a single longitudinal mode [98Lob]. The coherence length of the phase-conjugate laser system has been increased by a factor of 70 and the output has become almost di ractionlimited. The output power is reduced from 440 mW to 230 mW.

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References for 4.4

245

 

 

References for 4.4

72Kai

Kaiser, W., Maier M.: Stimulated Rayleigh, Brillouin and Raman Spectroscopy, Laser

 

Handbook, Arecchi, F.T., Schulz-DuBois, E.O. (eds.), Amsterdam: North-Holland Publ.

 

Co., 1972, pp. 1077–1150.

77Zel

Zel’dovich, B.Ya., Shkunov, V.V.: Wavefront reproduction in stimulated Raman scat-

 

tering; Sov. J. Quantum Electron. (English Transl.) 7 (5) (1977) 610–615.

82Fei

Feinberg, J.: Self-pumped, continuous-wave phase conjugator using internal reflection;

 

Opt. Lett. 7 (1982) 486.

82Pep

Pepper, D.M.: Nonlinear optical phase conjugation; Opt. Eng. 21 (1982) 156–183.

85Gue

G¨unter, P., Voit, E., Zha, M.Z., Albers, J.: Self-pulsation and optical chaos in self-

 

pumped photorefractive BaTiO3; Opt. Commun. 55 (1985) 210–214.

86Pep

Pepper, D.M.: Hybrid phase conjuagtor/modulators using self-pumped 0◦-cut and 45◦-

 

cut BaTiO3 crystals; Appl. Phys. Lett. 49 (16) (1986) 1001–1003.

88Gue

G¨unter, P., Huignard, J.-P.: Photorefractive materials and their applications I–II, Topics

 

in Applied Physics, Vol. 61–62, Berlin: Springer-Verlag, 1988.

89Agr

Agrawal, G.P.: Nonlinear fiber optics, Boston: Academic Press, 1989.

91Cro

Crofts, G.J., Damzen, M.J., Lamb, R.A.: Experimental and theoretical investigation of

 

two-cell stimulated-Brillouin-scattering systems; J. Opt. Soc. Am. B 8 (1991) 2282–2288.

93Yeh

Yeh, P.: Introduction to photorefractive nonlinear optics, New York: John Wiley & Sons,

 

Inc 1993.

94Wec

Wechsler, B.A., Klein, M.B., Nelson, C.C., Schwartz, R.N.: Spectroscopic and pho-

 

torefractive properties of infrared-sensitive rhodium-doped barium titanate; Opt. Lett.

 

19 (8) (1994) 536–538.

95Jac

Jackel, S., et al.: Low threshold, high fidelity, phase conjugate mirrors based on CS2

 

filled hollow waveguide structures; JNOPM 11 (1995) 89–97.

95Mu

Mu, X., Shao, Z., Yue, X., Chen, J., Guan, Q., Wang, J.: High reflectivity self-pumped

 

phase conjugation in an unusually cut Fe-doped KTa1−xNbxO3 crystal; Appl. Phys.

 

Lett. 66 (1995) 1047.

95Nol

Nolte, D.D.: Photorefractive e ects and materials, Norwell: Kluwer Academic Publish-

 

ers, 1995.

96Sol

Solymar, L., Webb, D.J., Grunnet-Jepsen, A.: The physics and applications of photore-

 

fractive materials, Oxford: Clarendon Press, 1996.

97Eic

Eichler, H.J., Haase, A., Kunde, J., Liu, B., Mehl, O.: Fiber phase-conjugator as re-

 

flecting mirror in a MOPA-arrangement, Solid State Lasers VI, San Jos´e (California);

 

Proc. SPIE (Int. Soc. Opt. Eng.) 2986 (1997) 46–54.

97Eic

Eichler, H.J., Kunde, J., Liu, B.: Quartz fibre phase conjugators with high fidelity and

 

reflectivity; Opt. Commun. 139 (1997) 327–334.

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References for 4.4

 

 

97Huo

Huot, N., Jonathan, J.M.C., Rytz, D., Roosen, G.: Self-pumped phase conjugation in a

 

ring cavity at 1.06 µm in cw and nanosecond regimes using photorefractive BaTiO3:Rh;

 

Opt. Commun. 140 (1997) 296.

97Yos

Yoshida et al.: SBS phase conjugation in a bulk fused-silica glass at high energy opera-

 

tion, CLEO 1997; OSA Tech. Dig. Ser. 11 (1997) 117–118.

98Eic

Eichler, H.J., Dehn, A., Haase, A., Liu, B., Mehl, O., R¨ucknagel, S.: High repetition rate

 

continuously pumped solid state lasers with phase conjugation, Solid State Lasers VII,

 

San Jos´e (California); Proc. SPIE (Int. Soc. Opt. Eng.) 3265 (1998) 200–210.

98Lob

Lobel, M., Petersen, P.M., Johansen, P.M.: Single-mode operation of a laser-diode array

 

with frequency-selective phase-conjugate feedback; Opt. Lett. 23 (1998) 825.

99Eic

Eichler, H.J., Mehl, O.: Multi amplifier arrangements with phase conjugation for power-

 

scaling of high-beam quality solid state lasers, Solid State Lasers VIII, San Jos´e (Cali-

 

fornia): Proc. SPIE (Int. Soc. Opt. Eng.) 3613 (1999) 483–492.

02Eic

Eichler, H.J., Mocofanescu, A., Riesbeck, T., Risse, E., Bedau, D.: Stimulated Brillouin

 

scattering in multimode fibers for optical phase conjugation; Opt. Commun. 209 (2002)

 

391–395.

03Rie

Riesbeck, T., Risse, E., Eichler, H.J.: Pulsed solid-state laser systems with high bright-

 

ness by fiber phase conjugation; Proc. SPIE (Int. Soc. Opt. Eng.) 5120 (2003) 494–499.

04Rie

Riesbeck, T., Risse, E., Mehl, O., Eichler, H.J.: Multi-kilohertz pulsed laser systems

 

with high beam quality by phase conjugation in liquids and fibers; in: Brignon, A.,

 

Huignard, J.-P. (eds.): Phase conjugate laser optics, Hoboken, New Jersey: John Wiley

 

& Sons, 2004.

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