Ding, Y.

Eichler, H.J.

Pogany, P.

Roentgen, P.

Sprenger, M.

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August 1, 1997 / Vol. 22, No. 15 / OPTICS LETTERS 1147Photorefractive power transfer from parallel laser
diode amplifiers into a single output beam
M. Sprenger, Y. Ding, and P. Pogany
Optical Institute, Technical University of Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany
P. Roentgen
IBM Research Division, Zurich Research Laboratory, Saeumerstrasse 4, 8803 Rueschlikon, Switzerland
H.-J. Eichler
Optical Institute, Technical University of Berlin, Strasse des 17. Juni 135, 10623 Berlin, GermanyReceived May 1, 1997
Photorefractive BaTiO3:Ce was used to combine coherently a signal beam and two amplified pump beams
at l ­ 678 nm. The master laser beam was split and then amplified in two semiconductor laser amplifiers
with gain of as much as 6. The amplifiers were made from quantum-well laser chips by use of antireflection
coating. 40% of the power could be transferred to the signal beam without transferring the phase distortions
of the pump beams.  1997 Optical Society of AmericaThe photorefractive effect has been studied
intensively for many applications.1 In this Letter
multiwave mixing in BaTiO3 is used to operate semi-
conductor laser amplifiers in parallel. So far, with
quantum-well lasers based on one chip, power of only
,100 mW has been reached.2 The reason for this is
the high optical power densities of these lasers, which
thermally destroy the sensitive nanostructures of the
laser diode, especially the facets. Therefore modular
systems are needed for more power. With photore-
fractive multibeam coupling we can operate amplif iers
in parallel and preserve the beam quality of the signal
beam. Another more complicated way of combining
beams is collinear interferometric coupling.3
In multibeam coupling several coherent beams in-
terfere in a photorefractive crystal. The interference
pattern causes a refractive index grating. In BaTiO3
without an applied electric field this index grating is
phase shifted by py2 relative to the intensity grat-
ing.4 This phase shift leads to a power transfer from
the pump beams into the signal beam without phase
cross talk. The beam quality of the signal is preserved
after this amplif ication process.5
For beam combination we use a multibeam coupling
arrangement in which two pump beams overlap with
each other as well as with the signal beam. In Ref. 6
an alternative scheme was presented in which mul-
tiple pump beams interacted with the signal beam se-
quentially. In the case of sequential coupling each
pump beam does not interact with the signal over
the whole length of the crystal. Therefore the power
transfer is smaller. In the experiments a single-mode
quantum-well laser diode with an emission wavelength
of l ­ 678 nm was used (Fig. 1). The radiation was
collimated to a 4.3-mW beam at each side with an el-
liptical shape of 1 mm 3 5 mm. The focal length of
the lenses was f ­ 5 mm, and the numerical aperture
was 0.5. The beams were p polarized to the incident
plane of the crystal with a polarization ratio of PyS ­0146-9592/97/151147-03$10.00/0400:1. After passing an optical isolator the pump
beam was split and each beam was directed to an am-
plifier. To focus the beams into the active layer also,
we used 5-mm lenses. For combination of the ampli-
fied pump beams and the signal beam, photorefractive
BaTiO3:Ce, provided by the Institute of Physics, Chi-
nese Academy of Sciences, Beijing, was used. Doping
with Ce made the crystal more sensitive to red light.
Parallel to the c axis the dimension was 7.1 mm, and
in the transverse direction, 4.7 mm 3 4.65 mm. At
l ­ 678 nm the refractive indices were n0 ­ 2.40 and
ne ­ 2.35.7 The index grating was erased between two
measurements by an argon-ion laser beam with 10-mW
of power.
In preliminary measurements the optimum coupling
parameters for three beams were determined. The
half-angle between the outer pump beam and the
signal beam was u ­ 25–. The angle between the c
axis and the grating vector (KG ) was b ­ 30–. The
Fig. 1. Experimental setup for parallel operation of laser
diode amplifiers. On the pump-beam side of the master
laser the beam was split and then amplif ied in two laser
diodes coated with an antiref lection layer. Two optical
isolators were used to reduce the feedback. The energy
was transferred to the signal beam in photorefractive
BaTiO3. 1997 Optical Society of America
1148 OPTICS LETTERS / Vol. 22, No. 15 / August 1, 1997angle between the two pump beams (d) was set to
2.5–. Relative to the crystal normal we had incidence
angles of
a1 ­ 255–, a2 ­ 252.5–, aS ­ 25– (1)
outside the crystal.
Coherence is an important condition for the coupling
process in photorefractive crystal. With a Michelson
interferometer a coherence length of lc ø 5 cm was de-
termined. The optical beam path lengths of the sig-
nal and the pump beams (passing the optical isolator)
were equalized. Strained GaInP quantum-well lasers
with two 10-nm wells and a 4-nm barrier were used as
master oscillators. Stable single-mode operation was
obtained with a 5-mm surface ridge structure and a
resonator length of L ­ 500 mm.8 The lasers emitted
as much as 10 mW of power on each side in cw opera-
tion at a wavelength of l ­ 678 nm. The threshold
current was roughly Ith ­ 45 mA at room temperature.
All laser diodes and amplifiers [semiconductor laser
amplifiers (SLA’s)] were stabilized to room tempera-
ture (T ­ 22 –C) to avoid a change of output power and
mode hopping.
The amplifiers were of the same type as the master
laser but were coated with an antiref lection (AR) layer.
The chips, with a refractive index of nld ø 3.5, were
coated with HfO2 (nHfO2 ­ 1.89) at T ­ 40
–C in vacuum
to reduce the longitudinal mode oscillations. We cal-
culated a remaining ref lectivity of RAR ­ 1.6%.9 With
the coating a shift in the maximum wavelength of
the mode spectrum up to 3 nm to shorter wavelengths
was observed. A broadening of the mode spectrum
was also detected. With higher current the modu-
lation depth increased, so that without temperature
stabilization the amplif ication was sometimes lower,
depending on the movement of the mode spectrum with
changing temperature.
The coupling of the master laser beam into the active
layer of the amplifiers was done with a collimator and
an xyz micrometer stage. Tests without an optical
isolator resulted in a loss of coherence. An isolator
with two Faraday rotators and polarizers reduced the
feedback so strongly that multibeam coupling was
possible. With this setup a reduction of feedback by
a factor of 1024 was obtained.
All components of the optical isolator were uncoated
and caused losses of ,40%. After the beam splitter
only 30% of the master laser power, ,1 mW, could
be injected into each amplifier. The output power of
the outer amplif ier, 1, was ,7 mW, and that of the
inner amplif ier, 2, was only 3 mW, because of the
worse quality of this chip. The signal was attenuated
by neutral-density f ilters to ,2 mW. The maximum
amplification depended strongly on the adjustment of
the focused beam. Figure 2 shows the amplification
of the SLA relative to current of 0–120 mA. The mas-
ter laser supported constant power of 1.28 mW in front
of the amplifier. The modulation of the output power
was caused by the shift of the SLA spectrum through
current variation, whereas the master laser wave-
length stayed constant. Similar ref lectivity of the AR-
coated facets would have reduced this modulation.With beam intensity ratios of r1S ­ IP1yIS ­ 3.5
and r2S ­ IP 2yIS ­ 1 we measured the time-dependent
photorefractive amplif ication g, where IS is the inten-
sity of the signal beam and IP1 and IP2 are the pump
beam intensities. Figure 3 shows signal-beam ampli-
fication to g ­ IS,amplf yIS,no amplf ­ 3.8 and pump-beam
depletion of 60% measured behind the crystal. With a
slightly modif ied adjustment of the beam position in
the crystal g ­ 4.5 was reached. The output power
behind the crystal was 5.4 mW. As Fresnel losses de-
crease the signal-beam power while the beam passes
the crystal, we found a real amplif ication of g0 ­ 2.5,
which is given by the ratio of signal intensities behind
and in front of the crystal. In this arrangement we did
not observe any significant beam fanning.
The power transfer from the pump beams to the sig-
nal beam was calculated with Fresnel losses included
(hR , the real case) and without these losses (hnoR ), be-
cause the surface of the photorefractive crystal could
also be AR coated:
hR ­
PS sleff d 2 PS s0d
PP 1s0d 1 PP 2s0d
, (2)
hnoR ­
PS sleffd 2 PS s0d s1 2 RSd2
PP 1s0d s1 2 RP1d2 1 PP 2s0d s1 2 RP2d2
, (3)
Fig. 2. Amplification gSLA versus the relative injection
current of amplifier 1. Amplification of as much as a
factor of 7 were obtained, whereas the master power stayed
constant. The modulation was caused by the spectrum
shift with increasing current.
Fig. 3. Time-dependent photorefractive multibeam cou-
pling with three beams. The two pump beams were am-
plified in SLA’s.
August 1, 1997 / Vol. 22, No. 15 / OPTICS LETTERS 1149Fig. 4. Cross section of the signal beam behind the
crystal. The shape of the beam is preserved. The pump
beam had two intensity peaks after passing a phase
disturbation.
where PS sleffd is the power of the signal beam after the
crystal, leff is the effective interaction length (in this
case leff ø lcrystal ­ 4.65 mm), Pis0d is the beam power
in front of the crystal, and Ri is the ref lection losses for
p polarization.
With these definitions we obtained power-transfer
efficiencies of hR ­ 32% and hnoR ­ 41%. Without
amplifiers at a beam ratio of r1S ­ r2S ­ 10 we
measured slightly higher values: hR,no ampl ­ 39%
and hnoR,no ampl ­ 46%, where the pump beams were
depleted as much as 90%.
These values include , 13% absorption losses.
Although some absorption is clearly needed for a
photorefractive grating, too much absorption should
decrease the eff iciency of the process by converting
otherwise useful output light into heat. Theoreti-
cal estimates indicate that a 10-mm-long crystal
with the same absorption and coupling coefficients
should provide transfer efficiencies of as much as
hnoR ­ 62%. The transfer eff iciency also depends
on the beam ratio. With an increasing beam power
ratio the power transfer decreased. Hence with beam
ratios of riS ­ 1 and no amplifiers we obtained a power
transfer of hR,no ampl ­ 45% and hnoR,no ampl ­ 71%.
Also, the angle between the two pump beams should
be small for the best coupling with the signal beam
and minimal coupling between the pump beams. The
small coupling efficiency with operating amplifiers is,
on the one hand, caused by the depolarization of theamplifiers. On the other hand, the amplifiers have a
worse transverse quality, caused by some facet defect,
than the master laser.
The possible phase distortions of the amplif iers were
not transferred to the signal. For demonstration, a
visibly inhomogeneous plastic f ilm was put into the
pump-beam path. The resulting pump beam is shown
in the inset of Fig. 4. Despite the severely distorted
pump beam, the amplified signal-intensity distribu-
tion corresponds closely to the signal-intensity distri-
bution without amplif ication. The amplif ied signal
beam was 1.5 times (without amplification 1.45 times)
diffraction limited owing to optical inhomogeneities of
the BaTiO3 crystal.
In summary, we have amplif ied a weak signal beam
by multibeam coupling. The two pump beams have
been amplified more than seven times in semiconductor
laser amplifiers. The amplified beams have been
coupled in a BaTiO3:Ce crystal. With this setup we
obtained power transfers of the magnitude of hR ­
32% and hnoR ­ 41%. Despite the bad quality of the
pump beams, the signal-beam profile was maintained,
and only the power was transferred. With high-power
laser diode arrays as amplif iers, a high-power, near-
diffraction-limited beam should be achievable. An
AR-coated photorefractive crystal of better quality
would provide higher power transfer of ,50%. For
example, with two 200-mW arrays a beam of high
quality and 200 mW of power would be expected.
References
1. P. Gu¨nter and J. P. Huignard, Photorefractive Materials
and Their Applications (Springer-Verlag, Berlin, 1988).
2. W. W. Chow, S. W. Koch, and M. Sargent, Semi-
conductor-Laser Physics (Springer-Verlag, Berlin, 1994).
3. G. L. Schuster and J. R. Andrews, Opt. Lett. 18, 619
(1993).
4. J. Feinberg, J. Appl. Phys. 51, 1297 (1980).
5. A. E. T. Chiou and P. Yeh, Opt. Lett. 10, 621 (1985).
6. S. MacCormack and R. W. Eason, J. Appl. Phys. 67, 7160
(1990).
7. J. Harada and K.-H. Hellwege, Ferroelectrics: Oxides,
Vol. III 16a of Landolt-Bo¨rnstein Numerical Data and
Functional Relationships in Science and Technology
(Springer-Verlag, Berlin, 1981).
8. P. Unger, G.-L. Bona, R. Germann, P. Roentgen, and
D. J. Webb, IEEE J. Quantum Electron. 29, 1883 (1993).
9. I. P. Kaminow, IEEE J. Quantum Electron. QE-19, 493
(1983).


Photoreactive power transfer from parallel laser diode amplifiers into a single output beam

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Photoreactive power transfer from parallel laser diode amplifiers into a single output beam

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