Self-starting femtosecond Cr4+:YAG laser mode locked
with a GaInNAs saturable Bragg reflector
C G Leburn1, A D McRobbie1, S Calvez2, A A Lagatsky1, D Burns2,
H D Sun2, J A Gupta3, G C Aers3, C T A Brown1, M D Dawson2 and
W Sibbett1
1 J.F.
Allen Physics Research Laboratories, School of Physics and
Astronomy, University of St Andrews, KY16 9SS, UK
2 Institute of Photonics, University of Strathclyde, 106 Rottenrow,
Glasgow. G4 0NW, UK
3 Institute for Microstructural Sciences, National Research Council of
Canada, Ottawa,K1A 0R6, Canada
Corresponding author: cgl@st-andrews.ac.uk
Abstract. We report the first demonstration of a self-starting passively mode-locked
femtosecond Cr4+:YAG laser that incorporates a GaInNAs saturable Bragg reflector
(SBR) to initiate the mode-locking mechanism around 1500 nm. This low-loss SBR
mediates the generation of femtosecond pulses with durations as short as 132 fs and
output powers >100 mW.
1. Introduction
Progress in the field of solid state lasers has demonstrated their ability to produce ultrashort
pulses in a variety of configurations, utilising various gain media. One of the most promising
of this type of materials is Cr4+:YAG, possessing a large gain bandwidth between 1300 and
1500 nm and an absorption band, coinciding with the emission wavelength of several
commercially available pump sources. Over the years Cr4+:YAG lasers have usually
generated pulses through use of the Kerr-lens mode-locking mechanism[1-3], however, this
mechanism is rarely self-starting and relies on critical cavity alignment.
The other viable route to generating femtosecond pulses involves using saturable Bragg
reflectors (SBRs) and semiconductor saturable absorbing mirrors (SESAMs) to initiate the
mode-locking process[4-6]. Previously reported saturable absorbers for operation in this
wavelength region have been based on InGaAs quantum wells on GaAs based Bragg
mirrors or InGaAsP quantum wells on InP based Bragg mirrors, both of which have
recognized shortcomings[7]. The indium concentration required for longer wavelength
operation (>1m) of an InGaAs device results in a lattice constant differential that is beyond
the critical level for a coherently strained InGaAs:AlAs/GaAs Bragg mirror structure. Strain
relaxation occurs with the inevitability of high non-saturable losses and low damage
thresholds, which makes the devices unusable in near-infrared solid state lasers such as
Cr4+:YAG. The active YAG medium has relatively low gain so its lasing efficiency suffers
when lossy intracavity elements are inserted into the system. This means that Cr 4+:YAG
lasers that incorporate InGaAs on GaAs SBRs[7-9] have inferior performance characteristics
compared to their continuous wave (cw) or Kerr-lens mode locked counterparts.The
alternative scheme of growing an InGaAsP absorber on an InP-based mirror structure offers
narrow bandwidth Bragg mirror reflectivity, due to the small refractive index contrast
achievable with materials that are lattice matched to InP, but this restricts significantly the
pulse durations that can be generated due to the lack of laser bandwidth that can be
accessed.
Recently, however, a significant new route in the design and development of more
suitable SBR devices for ultrashort pulse lasers has been demonstrated[10-13]. Sun et
al.[10] gave the first demonstration of passive mode locking using a GaInNAs SBR,
incorporated within a high power picosecond Nd-based laser operating near 1300nm, thus
demonstrating the high damage threshold of GaInNAs based saturable devices. The key has
been the use of GaAs-based SBRs that incorporate GaInNAs quantum wells for saturable
absorption. Inclusion of small percentages of nitrogen not only reduces the strain, but also
shifts the absorption edge very effectively into the 1200-1600nm region. The low-loss nature
of these GaInNAs SBRs is crucial for the development of efficient and reliable Cr 4+-based
lasers. In fact, McWilliam et al.[13] have been successful in generating 130fs pulses in the
1300nm spectral region from a Cr4+:forsterite solid state laser that incorporates a GaInNAs
SBR. For Cr4+:YAG lasers the inclusion of a suitable SBR eliminates the need for critical
cavity alignment and makes these systems more reliable and robust that their Kerr-lens
mode-locked counterparts.
2. Experimental setup
The GaInNAs SBR that was incorporated into our laser system was provided by the
Institute of Photonics at the University of Strathclyde. The GaInNAs SBR consisted of a 30layer-pair GaAs (113.4nm)/AlAs (132.1nm) Bragg stack incorporating a nominally 7nm-thick
Ga0.6In0.4N0.027As0.973 single quantum-well embedded between two 20nm-thick GaN0.045As0.955
barriers having a nominal transition wavelength of 1560nm. The MBE growth was performed
at 600°C - lowered to 410°C for the saturable absorber. No subsequent annealing was
performed. Reflectivity characterisation showed a >171nm-stop-band Bragg stack. The low
temperature growth and high nitrogen content of the quantum well prevented any
photoluminescence to be observed even at low temperature.
A schematic of the laser cavity and pumping geometry used for the Cr4+:YAG laser is
shown in Figure 1.
Figure 1 Schematic diagram of Cr4+:YAG laser system incorporating a low-loss GaInNAs SBR.
The Yb:fibre pump laser (IPG) produced up to 10W of linearly polarised, near-diffraction
limited light at 1064nm. This pump beam passed through a 1.5x expanding telescope before
being tightly focused into the Cr4+:YAG crystal by a 80mm focal length convex lens (pump
spot size, wp ~ 40m). The Brewster-cut Cr4+:YAG rod had a small-signal pump absorption
coefficient of 1.2cm-1, was 20mm in length and mounted in a water-cooled copper jacket that
was maintained at a temperature of 12 ºC. This rod was placed in a 4-mirror asymmetric,
astigmatically compensated Z-fold cavity. The two curved folding mirrors had radii of
curvature of -100mm and -75mm for the long and short arms respectively, had highreflectivity (HR) coatings for wavelengths in the 1460-1600nm range and high transmission
at the pump laser wavelength (1064nm). A wedged 0.5% output coupler (O/C) was located at
the end of the long arm of the cavity and an HR mirror or the SBR structure terminated the
short arm of the cavity. The choice of an asymmetric cavity allowed the mode size on the
SBR to be easily varied by adjusting the length of the short arm of the resonator. For
femtosecond operation, two fused silica prisms were incorporated in the long arm of the laser
resonator with a tip-to-tip separation of 185mm.
3. Results and discussion
Figure 3 illustrates the power transfer characteristics for the cw laser, with the 0.5% O/C in
place. With a HR mirror in the short arm of the cavity cw operation took place when the
incident pump power was greater than 0.8W. A similar threshold was reached when the HR
mirror was replaced with the SBR, indicating that the SBR had negligible non-saturable
losses.
160
HR
GaInNAs SBR
140
Output power (mW)
120
100
80
60
40
20
0
0
1
2
3
4
5
6
Incident pump power (W)
Figure 2 Power transfer characteristics of Cr4+:YAG laser system.
Mode locking was self-starting when the incident pump power was increased to >1.6W. Initial
results yielded highly unstable mode locking. To combat this instability an intracavity slit was
placed at the end of the long arm. The inclusion of this intracavity slit greatly enhanced the
stability of the system and allowed the production of the results that are presented below.
1.0
1.0
(b)
(a)
FWHM18.2nm
Normalised intensity
Normalised intensity
0.8
0.6
0.4
0.32
0.6
0.4
0.2
0.2
0.0
1460
FWHM132fs
0.8
1470
1480
1490
Wavelength (nm)
1500
1510
1520
0.0
-400
-200
0
200
400
Time (fs)
Figure 3 Measured optical spectrum (a) and corresponding internsity autocorrelation (b) of the
mode-locked Cr4+:YAG laser.
A representative intensity autocorrelation and spectral profile (centre wavelength of 1490nm)
are included as Figure 3. By assuming a sech2 intensity profile, the pulse duration was
determined to be 132fs. With the corresponding spectral width of 18.2nm, the implied timebandwidth product was 0.32, indicating near transform limited pulses. The pulse repetition
rate of the laser in this configuration was 159MHz. The mode-locking instability in the
absence of an intra-cavity slit and the low threshold mode locking indicated that the
modulation depth of the SBR was ~0.2%. This parameter value is thought to be the main
limitation of the current mode-locking performance. These successful initial results will be
thus furthered by investigations of the laser characteristics with new SBRs whose a
modulation depth will be increased to 1-2%, a more typical value for these low gain
systems[14]. Due to the success of these initial results we are now investigating the
possibility of having a more suitable device grown.
In conclusion, this is the first time a GaInNAs SBR structure has been incorporated into a
4+
Cr :YAG laser cavity to demonstrate mode locking in the femtosecond domain (at a centre
wavelength of 1490nm). This confirms the potential of low-loss GaInNAs-based SBRs as
mode-locking elements in femtosecond solid-state lasers operating around the
telecommunications window of 1550nm. It is to be expected that GaInNAs-based SBRs will
greatly enhance the practicality of ultrashort-pulse lasers in the technologically relevant
1200-1600nm spectral region.
The authors would like to thank R. Wang and M. Bresee for technical support, D. Dalacu,
D. Poitras and Z.R. Wasilewski for helpful discussions and acknowledge the EPSRC through
the UPC programme for its continued support.
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