100-nm tunable femtosecond Cr:LiSAF laser mode
locked with a broadband saturable Bragg reflector
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Citation
Demirbas, Umit et al. "100-nm tunable femtosecond Cr:LiSAF laser
mode locked with a broadband saturable Bragg reflector." Applied
Optics 56, 13 (April 2017): 3812-3816 © 2017 Optical Society of
America
As Published
http://dx.doi.org/10.1364/AO.56.003812
Publisher
Optical Society of America
Version
Author's final manuscript
Citable link
https://hdl.handle.net/1721.1/121498
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Creative Commons Attribution-Noncommercial-Share Alike
100-nm Tunable Femtosecond Cr:LiSAF Laser
Mode-Locked with a Broadband Saturable Bragg-Reflector
U
MIT
D
EMIRBAS
,
1,2,3
J
ING
W
ANG
,
1
G
ALE
S.
P
ETRICH
,
1
S
HEILA
N
ABANJA
,
1
J
ONATHAN
R.
B
IRGE
,
1
L
ESLIE
A.
K
OLODZIEJSKI
,
1
F
RANZ
X.
K
ÄRTNER
1,3
AND
J
AMES
G.
F
UJIMOTO
1
1 Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
2 Laser Technology Laboratory, Department of Electrical and Electronics Engineering, Antalya International University, 07190 Antalya, Turkey 3Center for Free-Electron Laser Science, DESY and Department of Physics, The Hamburg Center of Ultrafast Imaging, University of Hamburg,
D-22607 Hamburg, Germany
*Corresponding author: umit79@alum.mit.edu and jgfuji@mit.edu
Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX
We report broad tunability of a femtosecond diode-pumped Cr:LiSAF laser mode-locked with a broadband saturable Bragg reflector (SBR). The SBR had seven pairs of AlxOy (n 1.5) and Al0.19Ga0.81As (n3.5) layers in its
Bragg stack, enabling 250 nm reflectivity bandwidth around 850 nm. A 6-nm-thick strained In0.14Ga0.86As quantum
well placed between Al0.19Ga0.81As cladding layers was used as a broadband saturable absorber in the 800-920 nm
wavelength range. The laser was pumped by 6 single mode diodes; four at 640 nn and two at 660 nm. Modelocking was self starting and femtosecond pulses could be continuously tuned from 800 nm to 905 nm by an intracvity birefringent filter with an out of plane optic axis. The pulse widths varied from 70-fs to 255-fs as the laser was tuned. The laser had an 85.5 MHz repetition rate and the output power varied from 80 mW to 180 mW with tuning. © 2017 Optical Society of America
OCIS codes: (140.3460) Lasers; (140.4050) Mode-locked lasers; (140.3600) Lasers, tunable; (140.3480) Lasers, diode pumped; (320.7090) Ultrafast lasers.
http://dx.doi.org/10.1364/AO.99.099999
1. INTRODUCTION
Cr3+:LiSAF is a versatile solid-state laser gain medium in the near
infrared that enables the construction of low-cost, efficient and compact continuous-wave (cw) and femtosecond (fs) laser systems [1-7]. Low-cost diodes in the red spectral region around 650 nm can be used as efficient pump sources [8, 9]. In cw operation, lasing thresholds as low as 2 mW, slope efficiencies above 50%, and tuning in the 780-1110 nm region have been demonstrated [10]. In mode-locked operation, pulses as short as 10-fs (2.3 mW average power)[4], and average powers as high as 580 mW (195-fs pulsewidth) have been realized [10]. Femtosecond tuning ranges of 809 nm to 910 nm [11], 807 nm to 920 nm [12], and 835 nm to 910 nm [13] have been obtained from Kerr-lens mode-locked (KLM) lasers. Alternatively, using single walled carbon nanotubes for mode-locking, fs tuning from 868 nm to 882 nm has been achieved [14]. A recent study has further demonstrated fs tuning in the 836 nm to 897 nm range using a graphene saturable absorber mode-locked Cr:LiSAF laser [15].
Femtosecond pulses can also be generated using saturable Bragg reflectors (SBRs) [16], also known as semiconductor saturable
absorber mirrors (SESAMs) [17]. Advantages of using SBRs for mode-locking include self-starting mode-locked operation, immunity to environmental fluctuations and reduced cavity alignment requirements. As a result, higher average powers are usually obtained from SBR mode-locked lasers. However, in standard SBRs, absorbers are integrated onto AlAs/AlGaAs Bragg mirrors, and the low-index contrast (n0.5-0.6) between the layers limits the reflectivity bandwidth (60 nm, R>%99.5) [18]. This limits the available tuning range from SBR mode-locked Cr:LiSAF lasers. Tuning over 50 nm around 850 nm with pulse durations from 45 fs to 200 fs was demonstrated using Cr:LiSAF with intracavity prisms [19] and a 45 nm tuning range (828-873 nm) with 120 fs to 220 fs pulses were demonstrated using double chirped mirrors (DCMs) and a birefringent tuning plate [18]. As an alternative to standard SBRs, oxidized SBRs increase the index contrast in the Bragg stack by using low-index AlxOy
(n1.6) layers instead of AlAs (n3) [20-23]. This large index contrast of around 1.9 in the Bragg stack enables high reflectors with bandwidths larger than 250 nm around a central wavelength of 800 nm [20-23].
In this manuscript, we describe a femtosecond diode-pumped Cr:LiSAF laser with >100 nm tuning range. A broadband SBR with a Bragg stack consisting of seven pairs of AlxOy and Al0.19Ga0.81As layers
was used to initiate and sustain mode-locking. The AlxOy layer is
created by the wet thermal oxidization of the AlAs layers [21]. A 6-nm-thick strained In0.14Ga0.86As quantum well was used in the SBR to
provide a broadband saturable absorber in the 800-920 nm wavelength range. Modelocking was self starting and femtosecond pulses could be continuously tuned from 800 nm to 905 nm with an out of plane optic axis intracavity birefringent filter [24]. The pulses were nearly transform limited with durations ranging from 70-fs to ~255-fs as the laser was tuned. To the best of our knowledge, this is the broadest tuning range ever reported for any SBR mode-locked femtosecond solid state laser.
2. EXPERIMENTAL
Figure 1 shows a schematic of the single-mode diode-pumped Cr:LiSAF laser. The gain medium was pumped by four 640 nm (Hitachi Inc., HL6385DG) and two 660 nm (Hitachi Inc., HL6545MG) linearly-polarized, single-mode diodes (D1-D6) with circular outputs. Each 640-nm laser diode produces about 200 mW of output and was driven at 350 mA (above the typical rating of 150 mW, 280-350 mA, at 25C), while each 660-nm laser diode produces about 150 mW output and was driven at 220 mA (above the typical rating of 130 mW, 180-220 mA). The 640 nm diodes were TEC-cooled to 15C, whereas no active cooling was applied to the 660 nm diodes. The diode outputs collimated by aspheric lenses and combined using polarizing beam splitting (PBS) cubes and dichroic filters. Two 65-mm focal length achromatic lenses focused the pump beams into the Cr:LiSAF crystal.
PBS Cr:LiSAF D1, TM 640 nm M1 DCM 75 mm M2 DCM 75 mm f=65 mm M5 DCM 150 mm 3% OC D2, TE 660 nm D 3 , TE 6 4 0 n m Dichroic filter f=65 mm PBS D4, TM 640 nm D 6 , TE 6 4 0 n m D5, TE 660 nm M3 DCM M4 DCM HR (cw) Oxidized/Broadband SBR BRF Dichroic filter
Fig. 1. Schematic of the single-mode diode-pumped Cr:LiSAF laser for tunable mode-locking with the broadband/oxidized SBR (saturable Bragg reflector). PBS: polarizing beam splitter cube, BRF: birefringent tuning plate, DCM: double chirped mirror, D1-D6: diodes 1 to 6.
The 6-mm long (path length) Cr:LiSAF crystal (from II-VI inc.) has a Cr concentration of 1.5%. The crystal absorbed >99% of the TM-polarized pump at 660 nm, ~90% of the TE-TM-polarized pump at 660 nm due to reflectively losses and ~77% (0.9 x 85%) of the TE-polarized pump at 660 nm. In total, the Cr:LiSAF crystal absorbed ~900 mW of the ~1 W of incident pump power. An astigmatically-compensated, x-folded laser cavity, with 75 mm radius of curvature (ROC) pump mirrors (M1-M2) was used. The cavity was first optimized for CW operation, generating 400 mW of output power with 1 W of incident pump power using a 3% output coupler. To initiate and sustain mode-locked operation the broadband SBR was inserted into the cavity at a second focus produced by a curved mirror M5 (ROC = 150 mm). Dispersion compensation was performed using custom-designed double chirped mirrors (DCMs) with a GDD of -80 fs2 per bounce and
bandwidth from 800 nm to 940 nm were used for all mirrors in the cavity [25, 26] . Two flat mirrors (M3 and M4) were used to provide 6
bounces per pass (12 per round trip) to compensate dispersion and the total estimated cavity dispersion was -700 fs2.
The broadband SBR contained 7 pairs of Al0.19Ga0.81As/AlxOy as the
Bragg stack, and a 6–nm-thick strained In0.14Ga0.86As quantum well
fabricated between Al0.19Ga0.81As barriers as the absorber section (Fig.
2). The SBR was originally grown by molecular beam epitaxy with AlAs layers and Al0.19Ga0.81As layers within the Bragg stack. Mesas with 500
µm diameter were etched and the AlAs was transformed into AlxOy by
wet thermal oxidation [21]. Fig. 3 shows a microscopy image of an etched mesa from the oxidized SBR sample. Currently, the center region of the mesas is not fully oxidized; however, for a typical spot size (ω30 m 1/e intensity radius) on the SBR, the nonuniformly oxidized sample worked within the laser cavity. In order to obtain fully oxidized mesas, the fabrication process is being modified [23]. The use of low-index AlxOy material(n1.6) in place of AlAs (n3) in the Bragg mirror
stack increased the index contrast from 0.5 to 1.9 and enabled a reflectivity bandwidth of 250 nm centered at 850 nm (e.g. Fig. 8) [21]. Two considerations were included in the absorber design to enable broadband tuning. First, the position of the quantum well inside the standing wave electric field pattern was chosen to obtain a constant absorption. Secondly, the quantum well thickness was chosen to remove any discontinuities in the density of states in the tuning range of the SBR. Also, operating the laser in the soliton regime relaxed the dependence on SBR recovery time [19].
Layer Material Physical thickness (nm) Refractive index Purpose 0 GaAs - - Substrate
1 AlAsAlxOy 134.27 1.60 Bragg layer
2 Al0.17Ga0.83As 61.36 3.50 Bragg layer
… … … … Bragg layers
11 AlAsAlxOy 134.27 1.60 Bragg layer
12 Al0.17Ga0.83As 61.36 3.50 Bragg layer
13 AlAsAlxOy 134.27 1.60 Bragg layer
14 Al0.17Ga0.83As 20 3.50 Barrier
15 In0.15Ga0.85As 6 3.67 Absorber
16 Al0.17Ga0.83As 15 3.50 Barrier
17 Al0.1Ga0.9As 5 3.56 Cap
Fig. 2. (Top) Refractive index and physical thickness of each layer in the broadband oxidized SBR design. (Bottom) Detailed structure of the design with thickness and material information. The Bragg stack consists of 7 pairs of Al0.19Ga0.81As/AlxOy designed for a central
wavelength of 860 nm. The 6-nm thick strained In0.14Ga0.86As quantum
well is sandwiched between Al0.19Ga0.81As barriers. The substrate is
GaAs, and a 5 nm thick Al0.1Ga0.9As cap layer is used for protection
Fig. 3. Microscope images of etched mesas of the broadband/oxidized SBR. The center regions of the mesas are not fully oxidized.
55 65 75 85 95 105 700 750 800 850 900 950 1000 1050 1100 Wavelength (nm) T ra n s m is s io n (% ) 21 19 18 17 15 15 21 19 18 17
Fig. 4. Calculated transmission characteristics of the Cr:LiSAF laser cavity, as a function of wavelength for different birefringent plate rotation angles (rotation angles in the range from 15 to 21, see differently colored graphs). The calculation has been performed for a 3-mm-thick crystal quartz birefringent plate with an optical axis tilted 45 with respect to the surface of the plate.
A 3-mm-thick crystal quartz birefringent filter (BRF) with an optical axis 45 to the surface of the plate was used for the tuning of the femtosecond pulses. The BRF was inserted at Brewsters angle into the cavity near the output coupler mirror. The TE polarized intracavity beam had Fresnel reflection losses from the laser crystal and the BRF plate surfaces, which provided a modulation depth of around 45% in one round trip. Figure 4 shows the transmission characteristics of the BRF containing linear laser cavity around the wavelength region of interest for one round trip calculated using Jones matrices [24, 27-32]. The transmission is calculated for several different rotation angles of the birefringent plate in the range from 15 to 21 (rotating the plate about an axis normal to the surface). For this specific BRF, in this rotation angle range, the free spectral range and the modulation depth of the plate is maximized [31]. Note from Fig. 4 that, the filter had a full width half maximum of around 20 nm, that could potentially enable tuning with sub-50-fs long pulses. However, for low gain laser materials like Cr:LiSAF, assuming a tolerable loss level of 2.5 %, the bandwidth reduces to around 7 nm, which corresponds to tuning with 100 fs level pulses. The free spectral range of the plate is greater than 300 nm, preventing wavelength jumps and instabilities. Lastly, the
plate has a very large tuning rate versus plate rotation value (40 nm/degree), due to its out of plane optical axis. As Lovolt et al. pointed out, this is important since the BRF then enables tuning in a very narrow rotation angle range, where the modulation depth of the filter does not vary appreciably [30].
3. RESULTS AND DISCUSSION
Figure 5 shows the measured laser efficiency curve for the free running laser operation without the birefringent tuning plate, with 12 bounces on mirrors M3 / M4 and an estimated total cavity dispersion of -700 fs2. Different regimes of operation as a function of pump power
(or by varying energy fluence on the SBR) were observed. For absorbed pump powers greater than 650 mW (720 mW incident), stable cw locked operation was obtained, where the mode-locking was self-starting and robust against environmental fluctuations. When mode-locked at full pump power (1 W incident, 900 mW absorbed), the free running laser operated at ~850 nm and had a ~100 fs pulse duration with approximately 185 mW of average power (2.16 nJ pulse energy). The repetition rate of the laser was 85.5 MHz and RF signal peak measured with a photodiode and spectrum analyzer was >70 dB above the background noise level. The average power is slightly lower compared to a standard AlGaAs/AlAs-based SBR mode-locked Cr:LiSAF laser. At 1 W of pump power, 250 mW of average power (100-fs, 2.5 nJ pulses at 100 MHz) was obtained at 850 nm from a Cr:LiSAF laser using a standard SBR. The reduced output is attributed to the slightly higher passive (non-saturable) losses of the broadband SBR. The origin of the losses is under investigation. 0 40 80 120 160 200 0 150 300 450 600 750 900
Absorbed pump power (mW)
A ve ra g e o u tp u t p o w e r (m W ) Q-switched mode-locking CW mode-locking CW
Fig. 5. Measured efficiency curve of the mode-locked Cr:LiSAF laser, showing different regimes of operation.
For femtosecond tuning experiments, the birefrigent tuning plate was inserted into the cavity at Brewster angle with almost no power penalty. The mode-locked output could be continuously tuned from 800 nm to 905 nm. Figure 6 shows the measured pulse width and pulse energy as a function of wavelength. The pulse duration varied from ~70 fs to ~254 fs and the average output power varied from 80 mW to 180 mW across the tuning range. As an example, Fig. 7 shows the measured autocorrelation trace for the shortest pulses (70-fs), where the laser central wavelength was at 880 nm. Tuning was performed by adjusting only the face normal rotation of the birefringent plate, without adjusting the pump power or any other components. If the position of the SBR is also adjusted (changing the spot size on the SBR) the tuning range is increased slightly from 800-905 nm to 795-910 nm. Figure 8 shows optical spectra of the pulses at different wavelengths in the tuning range, along with the calculated
reflectivity for the broadband SBR including the quantum well absorption and with the absorption set to zero to model saturation. The periodic variation in the pulse duration as a function of tuning range is probably due to the imperfect alignment of the gain crystal creating an undesired birefringence.
0 0.5 1 1.5 2 2.5 780 800 820 840 860 880 900 920 Wavelength (nm) P u ls e E n e rg y ( n J ) 0 60 120 180 240 300 P u ls e W id th ( fs ) Pulse energy Pulse width
Fig. 6. Summary of fs tuning results with Cr:LiSAF laser. The graph shows the variation of laser pulsewidth and pulse energy as the laser central wavelength is tuned.
0.0 0.3 0.5 0.8 1.0 -300 -200 -100 0 100 200 300 Delay (fs) S H G I n te n si ty (a u )
Fig. 7. Sample autocorrelation trace measured for the pulses with a central wavelength of 880 nm. The autocorrelation FWHM is 108 fs, corresponding to a 70-fs pulse duration (assuming a sech2 pulse). The
average power is 103 mW, with 1.2 nJ pulse energy at 85-MHz repetition rate. The optical spectrum bandwidth is 12 nm (FWHM) at 880 nm with a 0.33 time-bandwidth product.
Pulse durations as short as 30-fs centered at 880 nm with an output power of 150 mW could be generated by decreasing the number of bounces on the DCMs to operate at an estimated total cavity dispersion of -250 fs2. However, broad continuous tuning could not be
obtained with short pulses. Moreover, for shorter pulses, parasitic two-photon absorption in the SBR occurs at lower energy levels, causing multiple pulsing instabilities. Two photon absorption might be reduced using a different SBR design, which might ultimately facilitate the generation of sub-10-fs long pulses.
We believe that the femtosecond tuning range was limited by the gain bandwidth of the Cr:LiSAF crystal on the short wavelength side (795 nm) and by the absorber band edge of the SBR on the long wavelength side (920 nm). It should be possible to design an absorber with a band edge at longer wavelengths which might enable tuning above 920 nm. However, the gain of Cr:LiSAF decreases above
900 nm. In addition, there are water absorption lines in the 930-960 nm region, so purging of the laser cavity might be necessary.
99 99.25 99.5 99.75 100 750 775 800 825 850 875 900 925 950 975 Wavelength (nm) S B R r e fl e ct io n ( % ) -800 -700 -600 -500 -400 T o ta l ca vi ty G V D ( fs^ 2 ) _ GVD Small signal ref. Sat. ref.
Fig. 8. Example spectra from the Cr3+:LiSAF laser, showing tunability
of the central wavelength of the laser from 800 nm to 905 nm. Calculated small signal and saturated reflectivity of the SBR are also shown.
4. CONCLUSION
In summary, we have presented what is, to our knowledge, a record femtosecond tuning range (795-910 nm: 115 nm) from a SBR mode-locked solid-state laser. This work shows that it is possible to achieve self-starting mode-locked operation and broadband tuning of femtosecond pulses with specially designed SBRs based on broadband Bragg-mirrors. The oxidized SBR concept for broadband fs tuning might in principle be applied to other gain media such as Ti:Sapphire, Cr:Forsterite, Cr:YAG and Cr:ZnSe as well. The specific results shown here suggest that broadly tunable femtosecond Cr:LiSAF lasers may be useful in areas such as multiphoton microscopy, where tunability is of key importance.
Funding Information. National Science Foundation (ECS-0900901);
Air Force Office of Scientific Research (FA9550-12-1-0499 and FA9550-15-1-0473); National Institutes of Health (2R01-CA075289-19, 4R01-CA178636-04 and 5R01-NS057476-02); The Scientific and Technological Research Council of Turkey, (TUBITAK, 114F191), European Union Marie Curie Career Integration Grant (PCIG11-GA-2012-321787) and Thorlabs Inc.
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