This content has been downloaded from IOPscience. Please scroll down to see the full text.
Download details:
IP Address: 140.113.38.11
This content was downloaded on 28/04/2014 at 10:27
Please note that terms and conditions apply.
Elimination of Dynamic Instabilities in the +c-face Incident Photorefractive BaTiO3 Mutually
Pumped Phase Conjugator
View the table of contents for this issue, or go to the journal homepage for more 1999 Jpn. J. Appl. Phys. 38 L567
(http://iopscience.iop.org/1347-4065/38/5B/L567)
Part 2, No. 5B, 15 May 1999 c
°1999 Publication Board, Japanese Journal of Applied Physics
Elimination of Dynamic Instabilities in the +c-face Incident Photorefractive
BaTiO
3Mutually Pumped Phase Conjugator
Chi Ching CHANG, Tzu Chiang CHEN1, Li Chuan TANG2and Hon Fai YAU1
Department of Applied Physics, Chung Cheng Institute of Technology, Tahsi, Taoyuan 33509, Taiwan, R.O.C. 1Institute of Optical Sciences, National Central University, Chungli, Taoyuan 32054, Taiwan, R.O.C. 2Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, 30050, Taiwan, R.O.C. (Received December 25, 1998; accepted for publication March 24, 1999)
Experimental observations of the dynamic instabilities in the+c-face incident mutually pumped phase conjugator (MPPC) of a BaTiO3crystal are reported for the first time. Experiments show that it is possible to stabilize the phase-conjugate output
by choosing a proper geometry formed from the crystal and two incident beams. By choosing the proper geometry the essential configuration of the MPPC attained is a “kite” rather than a “fish head”. Data also indicate that the phase-conjugate output can reach a very steady state with a high value (∼32%) and is insensitive to angular and positional variations.
KEYWORDS: phase conjugation, mutually pumped phase conjugator, photorefractive, BaTiO3, injection locking
L567 The mutually pumped phase conjugator (MPPC) has be-come an attractive photorefractive device in which two mutu-ally incoherent beams interact indirectly and emerge as the phase conjugates of one another. In the past decade, sev-eral interaction geometries1–7) of the MPPC have been pro-posed. These geometries differ from one another according to the entrance face on which the incident beams (to be phase conjugated) impinge, the number of total internal reflections and the number of interaction regions. Recently, additional geometries8–10) for the MPPC, with distinct configurations,
such as the “fish head”,8) the “plate-form”9) and the
“rain-bow”,10) were discovered for effective coupling of two
mu-tually incoherent laser sources. In these configurations, the two beams were incident to the+c face of the crystal. Based on the idea of how the stimulated photorefractive backscat-tering self-pumped phase conjugation (SPB-SPPC)11)occurs, the difficulty in these configurations is that the SPB inter-actions cannot support the generation of self-pumped phase conjugation (SPPC)12)due to this special beam/crystal geom-etry. With the “+c-face incident” geometry, one can improve the performance of the mutually pumped phase conjugation without special doping or crystal orientation cutting.
The injection locking of incoherent laser sources, achieved by the class of MPPCs,13–17) can be regarded as a
photore-fractive holographic coupling between two mutually inco-herent laser sources.18) However, MPPC stability during the
coupling will determine the performance of the injection locking process. MPPC dynamic instabilities were previ-ously observed in several configurations.19, 20) In the
“mod-ified bridge” configuration, various instabilities (regular and irregular pulsations, periodic oscillations, and optical chaos) were observed by changing the incident geometric parame-ters. In the “bird-wing” configuration phase conjugate, dy-namic instability outputs, including regular and irregular os-cillations, were detected by precisely choosing the experi-mental geometry. Therefore, how to effectively stabilize the phase-conjugate output of the MPPC is still an open question. In this study, we demonstrate that a better+c-face incident MPPC for a nominally undoped BaTiO3 crystal is possible
if the proper geometry formed from the crystal and incident beams is chosen. The results will demonstrate that the phase-conjugate output can reach a very steady state with a higher value when the optical path inside the crystal has the “kite” configuration (as shown in inset (a) of Fig. 1) rather than the
full use of the crystal’s coupling strength.21)Two beam split-ters, BS1and BS2, were used to couple the mutually pumped
phase-conjugate outputs into the photodetectors (which were connected to an x -t chart recorder) for detection. A cam-era set was utilized to capture the top view of the MPPC optical paths inside the crystal. The two incidence angles,
θAandθB (as measured outside the crystal), were set to be
unequal or equal to one another but at Brewster’s angle of the BaTiO3crystal to avoid direct reflection from the crystal
face,+c. Two lenses, L1 and L2, whose focal lengths were 50 mm, were used to diverge the two incident beams before they entered the crystal to provide sufficiently large beam di-ameters to achieve significant beam fanning and subsequent beam coupling inside the crystal. The two input beams in this geometry were made to be mutually incoherent, so that very little competing photorefractive grating (such as the reflection grating) was formed. This mutual incoherence was achieved by simply removing the etalon from the cavity of the Ar+ laser and by making the optical path different around 200 cm between the two incident beams, which was larger than the Ar+laser coherence length Lc(∼3 cm). A white light source
with a fiber bundle was used to illuminate the crystal for about two min between consecutive measurements to erase any in-dex gratings formed within the crystal in the previous mea-surement.
In the first set of experiments, we conducted the following fish-head configuration8) (inset (b) of Fig. 1). The kite con-figuration also accounts for the angular and lateral positional acceptance of the incident beams from the operation of the MPPC.
A schematic of the experimental arrangement for the +c-face incident MPPCs is shown in Fig. 1. A single do-main, 0◦-cut, nominally undoped BaTiO3 crystal (a× b ×
c = 5.16 mm×4.74 mm×5.00 mm with the c-axis along
the 5.16 mm edge) was employed for producing mutually pumped phase-conjugate waves. The crystal was mounted onto a translation/rotation stage so that the angular (θ) and positional (z) dependence of the MPPC effect could be in-vestigated. Using a variable beam splitter (VBS), an argon ion laser beam (λ = 488 nm) was split into two beams, IA
and IBand directed onto the+c face of the BaTiO3crystal to
form the kite or fish-head configurations, respectively, inside the crystal. Both beams were extraordinarily polarized with respect to the crystal by rotating a half-wave plate to make
tests to verify the observation of the dynamic instabilities of
the+c-face incident MPPC, especially in the fish-head
con-figuration (or fish-head MPPC: FHMPPC). Two unexpanded Gaussian beams, each having a power of IA = 17 mW and IB= 20 mW with an area of ∼0.95 mm2, were incident at the distance d = zA− zB = 2 mm onto the crystal’s +c face.
The lateral positions (zAand zB) of the two beams were
mea-sured from the crystal corner (z= 0) to the center of the area of each beam incident onto the+c face. When both of the two incident beams impinged upon the crystal, the two phase-conjugate light beams could be detected simultaneously while the fish-head configuration was forming inside the crystal. Both MPPC phase-conjugate outputs with the fish-head con-figuration were stable when the incidence angles of those two mutually incoherent beams were smaller than 55◦.8)
How-ever, when we symmetrically increased the external angleφ (= θA+θB) between the two input beams to greater than 110◦
(i.e.,θA= θB> 55◦), the phase-conjugate outputs of the
fish-head conjugation became unstable. Figure 2(a) shows the temporal response of both established phase-conjugate out-puts, which varied irregularly and markedly at the angle be-tween the two incident beams, 134◦, i.e., the Brewster’s angle (∼67◦for BaTiO3at=488 nm) of each beam. It was found
that the fluctuation in the outputs was greater than 75% with respect to the mean value of at least five min. With the same symmetrically incident conditions mentioned above, we ro-tated the crystal along the axis vertical to the plane of the two input beams with angles1θ and d kept constant at 2 mm. In Fig. 3(a), both phase-conjugate outputs also reveal a dynamic instability with about 80% variation as the crystal was rotated clockwise by an angle of1θ = −10◦. To study the lateral positional response of the FHMPPC, we shifted the crystal along the z-axis. We found that both phase-conjugate
out-Fig. 1. Experimental arrangement for demonstrating and investigating the+c face incident type of MPPCs.
L568 Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 2, No. 5B C. C. CHANGet al.
Fig. 2. Temporal evolution of the MPPC’s phase-conjugate output of (a) the fish-head and (b) the kite configurations. Photographs in the figures show the optical path formed inside the crystal when the phase conjugation process is established.
and lateral positional acceptance of the KMPPC. To mea-sure the angular response, we rotated the crystal clockwise and/or counter clockwise along the axis vertical to the plane of the input beams. The phase-conjugate output power var-ied slightly with the rotated angle in an asymmetrical path (as shown in Fig. 4(a)). As indicated in Fig. 4(b), the out-puts maintain a stable and fast response when the crystal has been rotated clockwise by an angle of 10◦. The photograph in the upper-left corner of Fig. 4(b) shows the kite configuration with a slight deformation. To measure the lateral positional response, we shifted the crystal along the z-axis back and forth. Figure 5(a) shows that the phase-conjugate outputs var-ied symmetrically with the lateral positioning of the intersec-tion and increased on both sides (zA= zB) at z= 1.5 mm &
3.5 mm. As indicated in Fig. 5(b), both phase-conjugate out-puts were highly stable, and the fluctuation of the outout-puts was within 3%. Unlike the FHMPPC, the KMPPC can generate phase conjugation easily regardless of the lateral movement of the intersection of the input beams on either side of the+c face. Neither counter nor counter clockwise movement along the axis vertical to the plane of the two input beams affected KMPPC phase conjugation generation.
In conclusion, we demonstrated another geometry, the kite configuration, of the +c-face incident MPPC of a BaTiO3
crystal. The phase-conjugate output of the kite geometry was not only more stable but also generated higher reflectivity. As with the existing MPPCs, the performance of the KMPPC, MPPC phase-conjugate outputs as also observed (Fig. 3(b))
while a deformed fish-head configuration, as shown in the upper-left corner of Fig. 3(b), was forming inside the crystal. In the next set of experiments, we proposed another config-uration, the kite, for the MPPC (or kite MPPC: KMPPC) with
+c face incident geometry to overcome the drawback
men-tioned above. Once the phase conjugation process was estab-lished, the MPPC with the kite configuration showed a greater stability and higher phase-conjugate output than the fish-head configuration, while the mutually incoherent beam(s) were in-cident at a large angle with respect to the normal direction of the+c face. Figure 2(b) illustrates the temporal response of the phase conjugation in the kite configuration with large inci-dence angles (θA= θB= 67◦). The photograph in the
upper-left corner of Fig. 2(b) shows the optical beam path formed inside a BaTiO3crystal when the KMPPC is well established.
Both phase-conjugate waves were generated less than five s apart. As indicated in Fig. 2(b), the output powers of the phase conjugation were strikingly stable, and the fluctuation of the outputs was within about 5% within five min. Com-pared to the fish-head conditions, the phase-conjugate output of the kite geometry was not only more stable but also gener-ated higher reflectivity.
In the following experiments, we scrutinized the angular puts of the FHMPPC were sensitive to positional variations at greater incidence angles with respect to the normal direction of the crystal+c face as well. The dynamic instability of the Fig. 3. Temporal evolution of the MPPC’s phase-conjugate output of the fish-head configurations for (a) the crystal rotated by an angle1θ = −10◦ and (b) the crystal shifted along the z-axis 1 mm away from the crystal corner (z= 0).
Fig. 4. (a) Plot of the KMPPC’s phase-conjugate reflectivity as a function of the rotated angle1θ. (b) Temporal evolution of the KMPPC’s output as the crystal is rotated by angle1θ = −10◦.
L570 Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 2, No. 5B C. C. CHANGet al.
Fig. 5. (a) Plot of KMPPC’s phase-conjugate reflectivity as a function of the lateral position of the intersection point z. (b) Temporal evolution of the KMPPC’s phase-conjugate output when the crystal is shifted along the z-axis and the intersection point z is set to 1.5 mm away from the crystal corner (z= 0).
especially the positional and angular acceptances, makes this MPPC very promising for practical applications such as in-jection locking lasers and optical free space communications.
Acknowledgements
The authors would like to thank the National Science Coun-cil, Taiwan ROC for supporting this project under the contract number NSC 88-2215-E-014-001.
1) S. Weiss, S. Sternklar and B. Fisher: Opt. Lett. 12 (1987) 144. 2) A. M. C. Smount and R. W. Eason: Opt. Lett. 12 (1987) 498. 3) M. D. Ewbank: Opt. Lett. 13 (1988) 47.
4) E. J. Sharp, W. W. Clark III, M. J. Miller, G. L. Wood, B. D. Monson, G. J. Salamo and R. R. Neurgaonkar: Appl. Opt. 29 (1990) 743. 5) M. D. Ewbank, R. A. Vazquez, R. R. Neurgaonkar and J. Feinberg: J.
Opt. Soc. Am. B 7 (1990) 2306.
6) D. Wang, Z. Zhang, Y. Zhu, S. Zhang and P. Ye: Opt. Commun. 73 (1989) 495.
7) L. Zhang, J. Lu, J. Zhang, Z. Shao, J. Chen, X. Mu, H. Chen, M. Jiang, W. Zhang and G. Zhang: Jpn. J. Appl. Phys. 36 (1997) 2661. 8) C. Chang and D. R. Selviah: Opt. Lett. 20 (1995) 677.
9) J. Zhang, S. X. Dou, H. Gao, Y. Zhu and P. Ye: Opt. Lett. 20 (1995) 985.
10) H. Wang, N. Yoshikawa, S. Yoshikado and T. Aruga: Opt. Lett. 21 (1996) 561.
11) T. Y. Chang and R. W. Hellwarth: Opt. Lett. 10 (1985) 408. 12) J. Feinberg: Opt. Lett. 8 (1982) 480.
13) S. Sternklar, S. Weiss, M. Segev and B. Fischer: Opt. Lett. 11 (1986) 528.
14) M. Segev, S. Weiss and B. Fischer: Appl. Phys. Lett. 50 (1987) 1397. 15) T. Shimura, M. Tamura and K. Kuroda: Opt. Lett. 18 (1993) 1645. 16) M. W. Wright and J. G. McInerney: Opt. Commun. 110 (1994) 689. 17) A. A. Kamshilin, T. Jaaskelainen, V. V. Spirin, L. Yu. Khriachtchev, R.
Onodera and Y. Ishii: J. Opt. Soc. Am. B 14 (1997) 2331. 18) P. Yeh: IEEE J. Quantum Electron. 25 (1989) 844.
19) D. Wang, Z. Zhang, X. Wu and P. Ye: J. Opt. Soc. Am. B 7 (1990) 2289. 20) G. Hussain, S. W. James and R. W. Eason: J. Opt. Soc. Am. B 7 (1990)
2294.
21) P. Yeh: IEEE J. Quantum Electron. 25 (1989) 484.
22) M. Cronin-Golomb, B. Fisher, J. O. White and A. Yariv: IEEE J. Quan-tum Electron. QE-20 (1984) 12.
