Download I. Introduction - Politecnico di Torino

Mode locking and bandwidth enhancement in single section
ridge laser with two spatial modes.
A.Enarda, P.Resneaua, M.Calligaroa, O.Parillauda, M. Krakowskia*,
M.Valloneb, P.Bardellab, I.Montrossetb
a
Alcatel Thales III-V Lab, RD 128, 91767 Palaiseau France
b
Politecnico di Torino, Torino, Italy
e mail: *[email protected]
Abstract: With a single section ridge multi-quantum well
laser diode having two spatial modes, we demonstrate mode
locking and modulation bandwidth enhancement at
1580nm.
Index Terms — Mode locking, modulation bandwidth .
0.14W/A respectively. Above 160mA there is a kink in
the light current characteristics at 20°C corresponding to
the lasing of a second spatial mode as shown by the slow
axis far-field (Fig.2).
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I. INTRODUCTION
50°C
9
20°C
Semiconductor mode locked laser diodes are very
compact and efficient sources for generation of short
optical pulses at high repetition rate. They are mainly
used in optical telecommunications [1] (high bit rate
transmissions, all optical clock recovery, millimeter wave
generation). They can also be used in signal processing
(optical sampling) and in medical applications.
Usually, mode locked laser structures comprise a gain
section and a saturable absorber. However some active
materials or laser geometries allow mode locking without
two sections geometry. This is for instance the case with
QDashes material, which have shown mode locking with
a very low RF linewidth, with only one section [1].
In this paper we present a new way of getting mode
locking, by using only one section ridge multi-quantum
well (MQW) laser structure.
Optical power/facet (mW)
8
70°C
7
6
5
4
90°C
3
2
1
0
0
50
100
150
200
250
Current (mA)
Fig.1
Light-current characteristics at various
temperatures.
II. LASER STRUCTURE
The MQW laser structure was grown on an InP
substrate using Metal Organic Chemical Vapor
Deposition (MOCVD). The active region consists of 8
identical compressively strained GaInAsP quantum wells
separated by partially tensile strained GaInAsP barriers
for laser emission at 1580nm.
A 2.2µm wide single section ridge was defined. With
the chosen index difference, the waveguide supports two
modes, with effective indices computed at 3.208 and
3.180. For high bandwidth modulation, a low capacitance
structure with thick polymer isolation and localized p
type contact was realized. The cavity length is 300µm.
The as cleaved laser is mounted on AlN submount for
measuring dynamic properties.
Fig. 2 a Slow (top) and fast (bottom) far-fields at
140mA, 20°C
III. EXPERIMENTAL RESULTS
The light-current characteristics of the ridge laser have
been characterized at different temperatures (Fig.1).
Threshold current and efficiency at 20°C are 25mA and
Fig. 2 b Slow (top) and fast (bottom) far-fields at
180mA, 20°C
The optical spectra under continuous operation show a
doubling of each Fabry-Perot mode above 160mA,
corresponding to the two spatial mode.
There is a 12dB gain between the two applied currents
of 150mA and 159mA. The pedestal is at -38dB of the
peak, which is representative of a low jitter [2]. The
5kHz RF linewidth is that of the spectrum analyzer.
With a 50GHz vectorial network analyzer we have
measured the frequency response (S21) under small
signal modulation at different currents. The results
obtained at 20°C are shown on fig.5.
21
I=60mA
18
I=80mA
15
I=100mA
S21 (dB)
12
Fig. 3 a Optical spectrum at 160mA, 20°C
I=120mA
9
I=140mA
6
I=160mA
I=180mA
3
0
-3
-6
-9
-12
0
5
10
15
Frequency (Ghz)
20
25
Fig. 5
Fig. 3 b Optical spectrum at 180mA, 20°C
The separation between the main peak and the lateral
ones is 0.09nm at 1577nm, corresponding to a frequency
of 10.5GHz.
We have measured the RF spectra with a 50GHz
Rhode & Schwartz spectrum analyzer and a U2t 50GHz
detector. We have obtained, without modulating the
laser, a peak at 10.5GHz with a linewidth of 18MHz, for
a CW current of 150mA at 27°C, where the laser has also
two spatial modes. This corresponds to the beating of the
two spatial modes, which can be considered as a
particular regime of passive mode locking. For active
mode locking, we have applied a modulation of -7dBm
with an Anritsu synthesizer. This resulted in RF spectra
depicted in fig.4.
-20
-25
159mA / 27°C / Iphd=0.76mA
12dB gain in power
at 10.6Ghz
-35
-45
P (dBm)
We have demonstrated mode locking at 10.6GHz with
a single section 300µm long MQW Ridge laser, emitting
at 1577nm, due to the beating of two spatial modes which
are both lasing above 150mA at room temperature. We
have also demonstrated an enhancement of the resonance
peak and modulation bandwidth linked to this regime of
operation.
The authors gratefully acknowledge the support of the European
Commission through the EU FP7 grant agreement N°224366 (ICT
DELIGHT.project) The authors would like to thank J.P.Le Goec,
Y.Robert and E.Vinet for excellent technical assistance, A.Shen and
F.V.Dijk for fruitful discussions.
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pedestal at -38dB
of the peak
-50
IV. CONCLUSIONS
ACKNOWLEDGEMENTS
150mA / 27°C / Iphd=0.72mA
-30
Frequency responses (S21) at different
currents, T=20°C
From 60mA up to 140mA, the resonance frequency
and the damping increase. At 140mA the -3dB bandwidth
is 17.3GHz. At 150mA, the second spatial mode is
lasing. At 180mA, this enhances both the amplitude of
the resonance peak (+19.5dB) and the -3dB bandwidth
(22.6GHz). Same behavior can be seen at 25°C, but it
disappears above 30°C, together with the apparition of
the second spatial mode lasing.
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REFERENCES
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[1] G.H.Duan et al, "High Performance InP-based Quantum Dash
Semiconductor
Mode-Locked
Lasers
for
Optical
Communications", Bell Labs Technical Journal 14(3),63-84 (2009)
[2] D.von der Linde, “Characterization of the Noise in
Continuously Operating Mode-Locked Lasers”, Applied Physics B
vol. B 39 (4),201-218, April 1986.
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-90
-250
-200
-150
-100
-50
0
50
Frequency (kHz)
100
150
200
250
Fig. 4 RF spectra at 150mA and 157mA, 27°C
with a -7dBm modulation at 10.6GHz