Chang, CC

Selviah, DR

Tang, LC

English

UCL Discovery

FRIDAY AFTERNOON / CLEO/PACIFIC RIM / 207 - h : 0  -10 c  -20 ~  -30 . tZ  -60  -  0.2  0.4  0.6  0.8  1  1.2 Wavelength  (pm ) FM2  Fig. 1.  2 n(dn/dT)  versus wavelength for LBO crystal: curves are the computed values; solid circles are the experimental data'  at 20°C. terials even for limited  datae3  The opti- mum fitted constants G and H are shown in Table 1 along with the other physical parameters  at  20°C.  The  experimental points and the fitted curves are shown in Fig. 1. All  the  quoted  values  including precision are used. It is interesting to note that the fit gap E, is close to the XPS va- lence band peak  ositions 'D',  as denoted by French et al.  These optical constants are  capable  to  interpret  the  existing NLODs, particularly  the  recently  intro- duced  femtosecond  traveling-wave  par- ametric generations. 1.  G. Banfi, et al., Opt. Lett. 18, 1633- 1635 (1993). 2.  S.  Velsko, et al., IEEE J. QE 27, 3.  G. Ghosh, Opt. Lett. 19, 1391-1393 (1994). 4.  G. Ghosh, IEEE Phot. Tech. Lett. 6, 5.  R. French, et al., Phys. Rev. B 44, P 2182-2192 (1991). 431-433 (1994). 8496-8502  (1991). FM3  1400 Efficient phase conjugation using four-wave mixing in a broad-area laser diode Peter Kurz, Ron Nagar,* Takaaki Mukai, NTT  Basic Research Laboratories, 3-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa  243-01, Japan There is now a strong interest in devel- oping fast, efficient phase conjugators for a variety of  applications. Here, we focus on  a  phase  conjugation  scheme  where two external  fields  are  injected  into  a broad-area  laser  diode using a spatially nondegenerate mixing geometry (Fig. 1). Our approach has the advantage of  pro- viding  a  phase  conjugate  mirror  with nanosecond response times restricted only by the carrier lifetime inside the laser di- ode. This  compares favorably with more conventional schemes for phase conjuga- tion using photorefractive materials. The scheme presented in Fig. 1  has al- ready been shown to work in principle.',' However, the observed phase  conjugate reflectivities (defined as the ratio of  the phase conjugate signal power  to the in- put probe power) were  rather  low, less than 3%. We  achieve higher reflectivities by operating  the laser  diode  above  the free-running laser threshold. For  a  phase  conjugator  we  use  a broad-area laser diode with a 50 pm wide emission stripe and reflectivities of  15% and 95% at the front and rear facet, re- Broad-area  laser diode  7 FM3  Fig. 1.  Two extemal fields (dashed lines) are injected into the broad-area laser diode. A carrier density grating is generated by pump and probe beams traveling to the left inside the broad-area laser diode. The pump beam reflected from the rear facet is scattered by this grating to produce a conjugate beam counterpropagating to the incident probe beam. In the far field we observe the amplified pump and probe beams, and the phase conjugate signal. 120  130  140  150 Bias current [mA] FM3  Fig. 2.  Dependence of  the phase conjugate reflectivity on the bias current of  the broad-area diodelaser. At input powers of  390 pW for the pump and 280 pW for the probe and laser diode can be operated at a current of  10% above threshold (133 mA) in an injection-locked regime. spectively. A pump beam is injected col- linearly  to  the  laser  axis  and  a  probe beam is injected at an angle of  5.2" to the axis. The pump frequency w1 and probe frequency  w2 are tuned  200  MHz apart. The  phase  conjugate  reflectivities  are measured by spectrally resolving the far field of  the light emitted by the laser di- ode. This is done by scanning the far field with an optical fiber whose output is an- alyzed with a Fabry-Perot etalon. At the frequency 2wl -  w2 we observe the phase conjugate signal propagating counter to the input probe beam. We  increase  the  bias  current  of  the broad-area laser above threshold operat- ing it in the injection-locked regime. The phase  conjugate  reflectivity is  strongly dependent on the bias current as shown in Fig. 2. The lasing threshold is 133 mA. We operate between 124 mA and 145 mA where  the  conjugate  reflectivity ranges from 4% to 23%. For bias currents above 145  mA,  injection-locking  deteriorates and free running modes of  the laser di- ode are observed, reducing  the stability and  efficiency  of  the  phase  conjugate process. The observed phase conjugate reflec- tivity of  23%  is considerably more than results previously reported. This suggests that the broad-area  laser diode is an in- teresting candidate for phase conjugation applications where a fast time response is needed. An example might be the line- width  reduction  of  laser  diodes  using phase conjugate feedba~k.~ *Department of  Electrical Engineering, Technion-Israel, Haifa 32000, Israel 1.  M. Lucente, G. M. Carter, J. G. Fuji- moto, Appl. Phys. Lett. 53, 467 (1988). M. Lucente, G. M. Carter, J. G. Fuji- moto, Appl. Phys. Lett. 53, 1897 (1988). L. Petersen, U. Giese, T.  PJ.  Nielsen, IEEE J. Quantum Electron. 30, 25 (1994). 2. 3. FM4  1415 Arch double phase conjugation in photorefractive BaTiO,  crystal Chi Ching Chang, Li Chuan Tang, David R. Selviah,'  Department of  Applied Physics, Chung Cheng Institute of  Technology, Tahsi, Taoyuan 33509, Taiwan, ROC The  double  phase  conjugate  mirror (DPCM) couples  two incoherent  beams and has been demonstrated  im  a variety of  materials and ge~metries.'.~  It has ap- plications  in  optical  processing,  laser phase  locking  and  optical  neural  net- works. Two phase conjugate outputs are produced  simultaneously in the interac- tion of  the two mutually incoherent laser beams within the photorefractive  crystal by  four-wave  mixing  (FWM).  The DPCMs reported to date can be catego- rised  by  the number  of  internal  reflec- tions the beams experience before emerg- ing; zero (double  hase conjugation and modified bridge)','one  (bird-wing):  two (mutually  incoherent  beami  coupling; MIPC)4  and three (frog-legs)." In this paper a novel DPCM geometry, the "arch'  (shown in Fig. I),  in photore- fractive BaTiOg crystals is demonstrated. Both incident beams make an acute angle with the negative,  -C,  crystal axis. This leads to an effective mutual interaction as the  two fans emanating frorn the  input beams have a large region of  overlap and do not suffer from Fresnel losses at inter- nal reflections. The new DPCM exhibits phase conjugation with zero internal re- flections. Qualitatively, it appears to ex- \  f '/  I, BaTiO, FM4  Fig. 1.  Schematic of  interaction geometry for the Arch-DPCM. 208 1 CLEO/PAClFlC RIM 1 FRIDAY AFTERNOON perimentally  verify  recent  theore tical predictions.b The  geometry  of  the Arch-DPCM is shown in  Fig. 1. When  only  one beam was incident on the crystal (Figs. 2(a) & (b)), apart from the directly propagating beam, a divergent fan of  beams was es- tablished. When two beams were simul- taneously  incident  on  the  crystal  (Fig. 2(c)) we  observed  that  they  began  to branch out and bend toward the +C axis, and  an  arch  shape  began  to  form.  A steady arch was observed to be complete after 1.5 seconds (Fig. 2(d)). The depen- dence of the phase conjugate reflectivities of  each beam as a function of  the input beam power ratio is shown in Fig. 3. The  two  incident  beams  enter  the crystal  (Figs. 2(a) &  (b)), and produce strong photorefractive  beam  fanning to- wards the +C axis. The two input beams have  the same wavelength but are mu- tually incoherent. Each incident beam in- terferes with its own scattered light from scattering centres within the crystal and writes a set of  fan gratings. A set of  iden- tical gratings with similar phase, period and position, occurring in both fans add constructively. The fanned light resulting from one beam Bragg diffracts from the fan gratings formed by the other beam to FM4  Fig. 2.  A temporal sequence of photographs illustrating the formation process of  the Arch DPCM, taken from above the BaTi03 crystal for 60" angle of incidence for both beams. (a) only the left beam incident, (b) only the right beam incident, (c) immediately after both beams are incident, (d) 1.5 seconds after both beams are incident showing the arch. Incident Pouci huo  (lbg(l,ll,)) FM4  Fig. 3.  Plot of  the phase conjugate powers, IT and I:,  as a function of the beam power ratio, I1/I2, between two beams. generate  the  phase  conjugate beam  by FWM. As a result,  the fans couple and bend into each other, to form the arch. The  authors  wish  to  thank  the  Na- tional Science Council, Taiwan, ROC and Chung  Cheng  Institute  of  Technology, Taiwan, ROC for supporting tlus project. "Department of  Electronic and  Electrical Engineering, University College London, Torrington Place, London  WCIE,  7]E, UK 1. 2. B. Fischer, S.  Stemklar, Appl. Phys. Lett. 51, 74 (1987). D. Wang, Z. Zhang, Y.  Zhu, S. Zhang, P. Ye,  Opt. Commun. 73,  495 (1989). M. D. Ewbank, Opt. Lett. 13, 47 (1988). M. C. Smount, R. W. Eason, Opt. Lett. 12, 498 (1987). M. D. Ewbank, R. A. Vazquez, R. R. Neurgaonkar, J. Feinberg, J. Opt. Soc. Am. B 7, 306 (1990). A. A. Zozulya, M. Saffman, D. Z. Anderson, Phys. Rev. Lett. 73, 818 (1994). 3. 4. 5. 6. FM5  1430 Studies of a new mechanism of self-pumped phase conjugation in photorefractive crystals Peixian Ye,  S.  X. DOLI,  lnstitufe  ofphysics, Chinese Academy of  Sciences, P.O. Box  603, Beijing  100080, China Traditionally,  two typical mechanisms for intemal  self-pumped phase  conjugation (SPPC)  generation  in  photorefractive crystals have been idenhfied. One is stim- ulated  photorefractive  backscattering (SPB).' The  other  is  total-intemal-reflec- tion  (TIR).'  SPPC  generation  with  the former depends on SPB interaction. With the  later, SPPC  generation  depends on four-wave mixing (FWM) and total inter- nal reflections. In  this case, optical path loop(s) exists in the crystal [Fig. l(a)]. In this paper, we mainly present our studies on a new SPPC generation mech- anism in photorefractive crystals, stimu- lated  photorefractive backscattering and four-wave  mixing  (SPB-FWM).  This mechanism was first observed in 0.4 wt% Fe-doped  KT&,,Nb,,O,  (KTN:Fe)  at 514.5  nm? Where  SPPC  is  also  estab- lished with FWM interactions. However, one of  the pump beams for FWM inter- actions, i.e., beam that propagates back- ward  to  the WM  regions, is produced through SPB amplification of  backscatter- ing light by the fanning beam. Thus the SPPC establishment does not require the (a)  (b) FM5  Fig.  1.  Schematic optical beam patterns that can be observed in an intemal self-pumped phase conjugator when the SPPC mechanism is (a) total- intemal-reflection and (b) stimulated photorefractive backscattering and four- wave mixing. intemal reflections by  the  crystal  faces. Typical beam pattems are schematically shown in  Fig.  l(b). This  mechanism  is also  found  to  dominate  in various  Ce- doped BaTi03 phase  conjugators in al- most  the  whole  visible  wavelength  re- gion. SPPC reflectivity as high as 80% has been obtained in these phase conjugators. In  both KTN:Fe and BaTi03:Ce crys- tals, it has been observed that, in a fixed beam/crystal  geometry, a  transition  be- tween SPB-FWM and TIR happens with varying  ~avelength.4,~  At  short  wave- lengths,  SPB-FWM  dominates  in  the SPPC production. At long wavelengths, TIR will be responsible for the SPPC pro- duction. At medium wavelengths,  vari- ous beam patterns different from that as shown in Fig.  1 can  be  observed.  The phenomenon  of  mechanism  transition with wavelength is found to result from the  fact that  the contra-directional  two- wave  mixing  gain  coefficient  yZK  de- creases with increasing wavelength. And thus it should be quite general in photo- refractive crystals. This phenomenon can be explained in theory. Phase conjugators working with SPB- FWM  mechanism has two  main advan- tages over that working with TIR mech- anism: (a) only the incidence face needs to  be  polished;  (b) dimensions  of  the phase  conjugator  could  be  small.  The transition  property  of  SPPC mechanism make  it  possible  for  us  to  choose  the mechanism of  a phase conjugator by var- ying the wavelength. It should be noted that SPB interaction in  photorefractive  crystals  can  be strengthened by doping. As doped crys- tals become more easily available, it is ex- pected that SPB-FWM mechanism should become more common and important in SPPC  generation  with  photorefractive crystals. 1. 2. 3. T.  Y.  Chang, R. W.  Hellwarth, Opt. Lett. 10, 408 (1985). J. Feinberg, Opt. Lett. 7, 486 (1982). Y.  Lian, H. Gao, P.  X. Ye,  Q. Guan, J. Wang, Appl. Phys. Lett. 63, 1745 (1993). 4.  Y.  Lian, S. X. Dou, H. Gao, Y. Zhu, X. Wu, C. Yang, I?  X. Ye,  Opt. Lett. 19, 610 (1994). Y.  Lian, S.  X. Dou, J. Zhang, H. Gao, Y.  Zhu, X. Wu, C. Yang, P.  X. Ye, Opt. Comm. 110, 192 (1994). 5. FM6  1445 Two possibilities of self-pumped phase conjugation of the laser beams in an inverted laser crystal with a feedback loop 0.  L. Antipov, S. I. Belyaev, A. S. Kuzhelev, Applied Physics Institute of  the Russian Academy of  Science 603600, 46 Uljanou Str., Nizhny Novgorod, Russia The effect of  self-pumped  phase  conju- gation  (SPPC) was observed recently in the inverted Nd:YAG-rod with  the loop schemes.'2 Two possibilities of  this effect realization were discussed: first is the for- mation of  PC-wave in the resonator with the  holographic  mirror  (RHM-scheme) induced in an inverted laser crystal by in- terference field  of  the  pumping waves; second  is  the  simultaneous  stimulated resonant  scattering  (SE) of  two  inter- secting (in the general case noncoherent 

Arch double phase conjugation in photorefractive BaTiO3 crystal

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