28 GHz optical injection-locked
1.55 lm VCSELs
free-running
detuning = -0.122 nm
detuning = -0.06 nm
detuning = 0 nm
0
A record resonance frequency of 28 GHz and an intrinsic laser
3 dB bandwidth of 34 GHz is reported for a directly modulated
injection-locked 1.55 mm VCSEL. The small-signal modulation
response is experimentally investigated using polarisation-maintaining
components.
Introduction: Recently, optical injection locking has been shown to
be an effective technique to enhance laser resonance frequency [1, 2].
Using a vertical cavity surface emitting laser (VCSEL) as the follower
and a distributed feedback (DFB) laser as the master, we have shown
that such enhancement can be obtained over a large range of injection
power and wavelength detuning ( ¼ lDFB lVCSEL), demonstrating
the robustness of the technique. In this Letter, small-signal modulation results show that the VCSEL resonance frequency is enhanced
from 6.6 to 28 GHz, which, to the best of our knowledge, is the
highest VCSEL resonance frequency.
33 GHz
9.4528 GHz
-80
Free-running
-0.122 nm
-100
-0.06 nm
0.0 nm
-120
0
The system and device parasitic are found by curve-fitting the
difference of two S21 curves. The intrinsic laser modulation response,
free from all parasitic, is obtained.
The injection ratio was approximated by comparing the reflected
master light power when the VCSEL is turned off, against the freerunning VCSEL light output power. This assumes that the measured
reflected light actually interacts with the VCSEL. In reality, some of the
collected light could be due to a reflection from the surface; hence the
actual injection-coefficient may be lower than quoted.
Experimental results: Fig. 1 shows the S21 of the VCSEL biased at
1.0 mA for an approximate injection power of 1 dBm, for various
wavelength detuning. All parasitics have been removed from the data.
The highest injection-locked relaxation oscillation frequency ( fr)
obtained is 28 GHz (4.3X improvement from 6.6 GHz free-running).
The lowest damping (sharpest resonance peaks) occur for the lowest
detuning values (0.122 nm detuning in this case). Since the S21
measurements were instrumentation limited, the 3 dB bandwidth of
the locked laser is estimated using the curve-fit frequency response,
and is found to be as high as 34.3 GHz. In Fig. 2, the VCSEL is now
biased at 2.0 mA with an injection power of 2.5 dBm, and the
resonance frequency is enhanced from 9 to 27 GHz (3X). For a
fixed VCSEL bias at 1 mA, the resonance frequency is plotted over
the locking range in Fig. 3, showing highest fr for highest injection
power.
ELECTRONICS LETTERS 15th April 2004
fr
g (GHz)
-
6.6
28
27
26
26
1
7
15
7
7
7
10
20
frequency, GHz
free-running
detuning = 0.085 nm
detuning = 0.125 nm
detuning = 0.263 nm
0
30
40
relative response, dB
injection-locked
27.2 GHz
26.6 GHz
25.5 GHz
free-running
9.3 GHz
-40
33.8 GHz
32.3 GHz
-60
12.3 GHz
-80
-100
Free-running
0.085 nm
-120
0.125 nm
0.263 nm
0
fz
fp
fr
g (GHz)
25
20
23
8
8
8
9.3
27
27
26
53.1
1
3.5
16
10
20
frequency, GHz
33.8 GHz
30
40
Fig. 2 Experimentally measured small-signal response of same
injection-locked VCSEL, biased at 2 mA
Solid lines are curve-fitted
Inset: Fitting parameters
0.6
ð1Þ
fp
30
22
19
Solid lines are curve-fitted with (1). VCSEL biased at 1 mA
Inset: Fitting parameters
1 þ ð f =fz Þ2
1
2 2
1 þ f =fp
1 ð f =fr Þ2 þðg=2pfr Þ2 ð f =fr Þ2
þ const
fz
33.8 GHz
Fig. 1 Experimentally measured small-signal response (S21) of
injection-locked VCSEL
: stable locking measurement
: unstable locking measurement
20
0.5
wavelength detuning, nm
H 2ð f Þ ¼ A 34.3 GHz
-60
-20
Experimental setup: Experiments were performed using a five-QW
InGaAlAs=InP 1.55 mm VCSEL [3]. For this device, the threshold is
0.4 mA and its resonance frequency is 6.6 GHz at 1 mA, 9.3 GHz at
2 mA. The setup is similar to that used in [4], except that the VCSEL is
injection locked using a polarisation-maintaining (PM) circulator and
an Ortel=Emcore DFB laser (relative intensity noise <165 dB=Hz).
The wavelength detuning and injection power was adjusted by tuning
the DFB temperature, current, and coupling loss. The polarisation
of the DFB signal is adjusted to match that of the VCSEL by rotating
the fibre circulator port-2. For non-matching polarisation, no locking
was possible. The VCSEL was directly modulated and characterised
using a 40 GHz network analyser and a lightwave analyser. The
experimental small-signal frequency response (S21) data is fitted to
the theoretical injection-locked laser frequency response by using the
equation as [1]:
28.3 GHz
27.3 GHz
26.3 GHz
free-running
6.6 GHz
-40
injection-locked
18
0.4
0.3
16
record fr
28 GHz
0.2
0.1
14
12
0
10
-0.1
-0.2
-15
stable locking range
hysteresis locking region
-10
-5
approximate injection power, dBm
8
0
injection-locked resonance frequency, GHz
X. Zhao, M. Moewe, L. Chrostowski, C.-H. Chang,
R. Shau, M. Ortsiefer, M.-C. Amann and
C.J. Chang-Hasnain
relative response, dB
-20
Fig. 3 Contour plot of resonance frequency against detuning and injection
power with VCSEL at 1 mA
Stably locked region between solid lines
Discussion: It has been analytically predicted that the resonance
frequency is expected to vary as the square root of injection power [5]:
o2r ¼
vg ðdg=dnÞS
Sinj
þ þ kc2
S
tp
ð2Þ
This relationship is confirmed by our experiments, by plotting the
resonance frequency against the square root of the injection power, in
Fig. 4. The results were obtained from the S21 measurements. The line
of best fit is also shown for reference, where the slope of the line is the
product of the injection coupling parameter kc and the coupling
efficiency.
Vol. 40 No. 8
resonance frequency, GHz
25
Acknowledgments: The authors wish to thank P. C. Chen at Emcore
Inc. for the donation of the DFB laser, as well as DARPA grant
22549-23802.
20
injection-locked
15
# IEE 2004
Electronics Letters online no: 20040349
doi: 10.1049/el:20040349
10
free-running
X. Zhao, M. Moewe, L. Chrostowski, C.-H. Chang and C.J. ChangHasnain (Department of Electrical Engineering and Computer
Science, University of California, Berkeley, CA 94720, USA)
5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Sqrt, injected power, mW
14 February 2004
0.7
0.8
Fig. 4 Experimental resonance frequency (measured by S21) against
square root of estimated injection power
For higher injection powers, the resonance peak was beyond
measurement, though the S21 behaved as though a higher frequency
resonance peak was present. A natural question to ask is if there is a
limit to the resonance frequency enhancement for higher injection
power, and if so, to what frequency. We have also observed that
the optical spectra exhibit optical sidemodes at precisely the resonance
frequency measured in the S21, most easily observed at the negative
detuning edge of locking where the damping is small. Based on
these observations, we believe that resonance frequencies >40 GHz
are possible.
Conclusion: A record resonance frequency of 28 GHz and an intrinsic 3 dB bandwidth of 34 GHz are achieved with polarisation-matched
injection-locking of a 1.55 mm VCSEL. In agreement with our theory,
the resonance frequency enhancement is not believed to be limited,
and will continue increasing with even higher injection powers.
E-mail: [email protected]
R. Shau and M. Ortsiefer (VERTILAS GmbH, Lichtenbergstr. 8,
D-85748 Garching, Germany)
M.-C. Amann (Walter Schottky Institut, Technische Universität
München, Am Coulombwall 3, D-85748 Garching, Germany)
References
1
2
3
4
5
Chang, C.H., Chrostowski, L., and Chang-Hasnain, C.J.: ‘Injection
locking of VCSELs’, accepted for publication in IEEE J. Sel. Top.
Quantum Electron., 2004
Xue Jun, M., Tai, C., and Wu, M.C.: ‘Experimental demonstration of
modulation bandwidth enhancement in distributed feedback lasers with
external light injection’, Electron. Lett., 1998, 34, pp. 2031–2032
Ortsiefer, M., Shau, R., Mederer, F., Michalzik, R., Rosskopf, J.,
Bohm, G., Kohler, F., Lauer, C., Maute, M., and Amann, M.C.: ‘Highspeed modulation up to 10 Gbit=s with 1.55 mm wavelength InGaAlAs
VCSELs’, Electron. Lett., 2002, 38, pp. 1180–1181
Chrostowski, L., Chang, C.H., and Chang-Hasnain, C.J.: ‘Enhancement
of dynamic range in 1.55 mm VCSELs using injection locking’, IEEE
Photonics Technol. Lett., 2003, 15, pp. 498–500
Li, L.: ‘Static and dynamic properties of injection-locked semiconductor
lasers’, IEEE J. Quantum Electron., 1994, 30, pp. 1701–1708
ELECTRONICS LETTERS 15th April 2004
Vol. 40 No. 8