Download Bistability and Optical Control of a Distributed Bragg Reflector Laser

610
OPTICS LETTERS / Vol. 18, No. 8 / April 15, 1993
Bistability and optical control of a distributed-Bragg-reflector
laser
M. Margalit, R. Nagar, N. Tessler, G. Eisenstein, and M. Orenstein
Advanced OptoelectronicsCenter, Department of Electrical Engineering Technion - Israel Institute of Technology, Haifa 32000, Israel
U. Koren and C. A. Burrus
AT&T Bell Laboratories,Holmdel, New Jersey 07733
Received November 3, 1992
We demonstrate bistable operation and optical control of a specially designed distributed-Bragg-reflector laser.
Optically controlled turn-on and turn-off with 1-pJ pulses at 1.5 ,m and 2-pJ pulses at 1.3 Am, respectively, is
demonstrated
as well as all-optical flip-flop operation.
Bistable operation of diode lasers is thought to be
an important feature of advanced optical signal processing. It is also an attractive means for studying
basic nonlinear phenomena in diode lasers. Over
the years there have been demonstrations of many
bistable diode lasers.1 - 6 These studies differ by the
specific underlying nonlinear mechanism that governs the bistable operation, by the type of control used
(electrical, optical, or a combination of the two), and
by the required switching energies.
In this Letter we describe a novel bistable diode
laser that is based on a specially designed distributedBragg-reflector (DBR) structure. The laser exhibits
both power and wavelength bistability and can be
operated as an all-optical flip-flop. The mechanism
governing the operation of the present laser results
from an interplay between the wavelength of a lasing mode and the wavelength-dependent reflectivity
function of the Bragg section. This interplay, coupled with conventional laser nonlinearities, gain saturation, and carrier- and thermal-induced frequencyshifting effects,7 results in bistable characteristics.
The operation of the device relies on two properties of the specially designed two-section 1.5-pm
DBR structure, depicted in Fig. 1(a): First, a 250,tm-long Bragg reflector consisting of a low-loss
shallow grating that yields a reflection function
bistable operation of the laser is the adjustment
of the spectral overlap such that the single cavity
mode is placed on the short-wavelength side (the blue
side) of the Bragg-reflection function, at low injection
levels. This particular type of mode-DBR overlap,
together with conventional gain nonlinearities and
carrier effects,7 causes a self-adjusting mechanism
of the phase condition that leads to bistable
characteristics.
Exploitation of the index-of-refraction nonlinearity
in a laser cavity for achieving bistability was reported
before,'0 -' 2 where either a multimode laser and a
dispersive external cavity or a forward-biased twosection laser'1 " 2 was used.
Optical
signals
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2 - section
(a)
with a relatively narrow bandwidth of = 225 GHz
FVHM. Second, a short (130 /tum) strained-layer
quantum-well gain medium8 that defines a large
modal spacing of =300 GHz. The optical coupling
between the two sections is essentially lossless and
reflectionless.9 This laser operates such that the
Bragg reflector overlaps spectrally one cavity mode
at most [Fig. 1(b)], thus it can be turned on and
off by controlling the phase condition of the cavity.
The latter can be accomplished since the cavity can
be adjusted so that no frequency, where there is
sufficient gain, satisfies the N x 2 fr phase condition
necessary for lasing. The laser can be turned into
and out of this condition by an electrical drive or by
optical signal injection. The key point in obtaining
1B
DBR laser
A 225GHz
Bragg
'300GHz
Cavity
(b)
1.
1 550nm
Fig. 1. (a) Schematic diagram of the bistable laser.
(b) Conceptual description of mode spacing. The inset
shows the mode-DBR overlap on the blue side of the
Bragg-reflection function.
© 1993 Optical Society of America
April 15, 1993 / Vol. 18, No. 8 / OPTICS LETTERS
branch of the LI bistable curve. For operation on
the upper branch, a current increase results in the
usual thermal red shift, whereas a current decrease
causes a gain decrease and a mode blue shift. The
latter is a combination of a carrier-number-induced
red shift and the usual thermally induced blue shift,
which dominates. At any current level on the upper
trace of the hysteresis loop (Fig. 2), the mode is red
shifted compared with the mode at the same current
on the lower trace. The losses, which are determined
by the mode position relative to the Bragg-reflection
function, are reduced along the upper trace, and
therefore the laser turns off at a current lower than
the turn-on current. This spectral behavior along
the hysteresis loop is shown in Fig. 3, where a wavelength domain hysteresis loop is shown. This loop,
together with the fixed position (in wavelength) of the
Bragg-reflection function, is responsible for the power
_ 3?1-
0 2-
0.
1
17
15
21
19
23
25
bistability
(mA)
Current
Gain
Fig. 2. Measured bistable LI curve.
4.2
-
0LO
4.1
LO
E
a
4.0
3.9
C
3.8
3.7
15
21
18
Gain
24
Current
27
30
(mA)
Fig. 3. Wavelength bistability curve.
Figure 2 shows the measured bistable light output
power versus the injected current (LI curve) for a
given current swing to the gain section and zero
current to the Bragg section. The explanation of
the bistable operation, with reference to the inset of
Fig. 2, is as follows.
Below threshold,
611
an increase
in current causes a material gain increase and a
simultaneous cavity loss increase owing to a small
blue shift of the mode frequency such that its overlap with the Bragg-reflection bandwidth decreases.
However, the net effect is an increase in gain so
that eventually the laser reaches threshold. This
operation of the laser at threshold is unstable since
any excess carriers above the clamping level red shift
the mode, thereby reducing the cavity losses and
lowering the clamping level, which in turn causes
generation of more excess carriers until the laser
stabilizes. This process explains the discontinuity
of the LI curve and the generation of the upper
of Fig. 2.
The effect of light injection on the hysteresis loop
is depicted in Fig. 4. The experiment uses a diode
laser emitting near 1.5 ,um as a source for the turnon signal and a second diode laser emitting near
1.3 Am for the turn-off signal. Both optical signals
are coupled to the gain section as seen in Fig. 1(a).
Figure 4(a) shows the effect of the 1.5-/Lmsignal. It
is clear that it affects only the turn-on threshold of
the hysteresis loop. The 1.5-,umsignal saturates the
gain, which causes a modal red shift. This increases
the mode-DBR overlap, thereby reducing the losses
and turning the laser on at a lower level of injected
current.
In contrast, the 1.3-/,.m signal affects only the turn
of threshold. This signal is absorbed in the cavity,
which causes a carrier density increase with no significant thermal effects. The resultant blue mode
shift increases the mode-DBR mismatch, enhancing
the cavity losses and causing the laser to turn off
at a current higher than the corresponding current
without light injection.
The results shown in Fig. 4 suggest the possibility
of optically controlled turn-on and turn-off by using
the 1.5- and 1.3-pumsignals, respectively. This enables one to operate the laser in a flip-flop mode.
(b)
(a)
4-
4
D 3-
I3
S
3:
0 2-
o2
0.
0.
1-
fi
15 17
19 21 23 25
Gain Current (mA)
16 17 19 21 23 25
Gain Current (mA)
Fig. 4. Effect of optical control signals on the hysteresis
loop (the dashed lines represent the boundaries of the
hysteresis loop with no optical injection). (a) Effect of
the 1.5-,m signal, (b) effect of the 1.3-,um signal.
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OPTICS LETTERS / Vol.18, No. 8 / April 15, 1993
also demonstrated and is depicted in Fig. 5. The 1.5um pulse turns the laser on, and the 1.3-/um pulse
turns the laser off. The switching energies were 1
and 2 pJ for turn-on and turn-off, respectively.
Finally, for comparison we examine the characteristics of a second laser that differs from the first
one only in that its cavity mode overlaps the Braggreflection function on the long-wavelength side (red
side), as in the inset of Fig. 6(a).
0
ON
OFF-
DBR
Laser
0.27 0.30 0.32 0.35 0.37 0.40
T ("s)
Fig. 5. Dynamic behavior of the optical flip-flop.
(a)
Figure 6(a) shows
the measured LI curve of that laser which exhibits no
bistability. Below threshold, a current increase reduces losses (owing to the mode blue shift) while also
increasing the gain until the laser reaches threshold,
which in this case is a stable one. Above threshold,
a current decrease retraces the LI curve so that no
hysteresis loop is obtained. Optical control of this
device by injection of a 1.5-/Lm optical signal into
the gain section was demonstrated. This causes a
red shift (owing to gain saturation), which further
increases the losses and increases the threshold. A
measurement of the effect is shown in Fig. 6(b). The
1.5-,um optical signal can therefore turn the laser off
(without latching to a second stable state), so it can
serve as a wavelength-converting device.
(b)
References
S
'S
01
30
01
1. I. H. White and J. E. Carrol, Electron. Lett. 19, 558
(1983).
4,
0o
2. T. Odagawa and S. Yamakoshi, Electron. Lett. 25,
1429 (1989).
0.
3. H. Shoji, Y. Arakawa, and Y. Fuji, J. Lightwave Technol. 8, 1630 (1990).
15
19 23 27
31 35
Gain Current (mA)
4. M. Okada, H. Kikuchi, K. Takizawa, and H. Fujikake,
IEEE J. Quantum Electron. 27, 2003 (1991).
5. Y. Ozeki and C. L. Lang, IEEE J. Quantum Electron.
15
19
23 27 31
35
Gain Current (mA)
Fig. 6. (a) Measured LI curve with no bistability. The
inset shows the mode-DBR overlap on the red side of the
Bragg-reflection function. (b) Effect of a 1.5-gm optical
control signal.
Such an optical flip-flopwas tested both statically and
dynamically. For the static operation, the device
was dc biased below threshold
(18.5 mA), and we
observed laser turn-on when the optical signal at
1.5 Iumwas applied. Subsequent removal of the 1.5,um signal left the laser turned on. Next we applied
the 1.3-Aumoptical signal and observed laser turnoff. Removal of the 1.3-,gm signal left the laser in
its off state. Dynamic operation of the flip-flop was
27, 1160 (1990).
6. M. J. Adams, Opt. Quantum Electron. 21, 15 (1989).
7. K. Peterman, Laser Diode Modulation and Noise
(Kluwer Academic, Dordrecht, The Netherlands,
1988).
8. P. J. A. Thijs, in Digest of the IEEE Thirteenth InternationalSemiconductorLaser Conference(Institute of
Electrical and Electronics Engineers, New York, 1992),
paper A-1.
9. T. L. Koch and U. Koren, J. Lightwave
274 (1990).
Technol. 8,
10. P. Glas and R. Muller, Opt. Quantum Electron. 14,
375 (1982).
11. F. S. Felber and J. H. Marburger, Appl. Phys. Lett.
28, 731 (1976).
12. A. N. Olsson, W. T. Tsang, and R. A. Logan, Appl.
Phys. Lett. 44, 375 (1984).