Chapter 4 Passively Q-switched and mode-locked Yb:YAG lasers
4.3 Dual-wavelength mode-locked operation
Ultrashort pulse lasers with terahertz (THz) beat frequency have become more and more important to wide applications such as medical imaging [61], plasma physics [62], quantum communication [63], optical sampling [64], and astrophysics [65].
Dual-wavelength synchronously mode-locked lasers [66-68] have been extensively used to generate the ultrashort optical pulse train with THz beat frequency. The demonstrations for dual-wavelength synchronous mode locking include Ti:sapphire lasers [68-75], semiconductor lasers [66], and rare-earth doped solid state lasers such
as Nd-doped disordered crystal lasers [67, 76-79] and Yb:LYSO laser [80].
Among rare-earth doped crystals, the Yb:YAG crystal has been identified to be a promising material for generating compact efficient ultrashort laser pulses [81-86]
owing mainly to the small quantum defect, broad absorption and fluorescence spectra, and high quantum efficiency. A dual-wavelength mode-locked Yb:YAG ceramic laser at 1033.6 nm and 1047.6 nm was recently demonstrated in single cavity [85].
However, since the optical paths of the dual-wavelength modes were spatially separate in the laser cavity, there was no observation of ultrashort pulse train with THz beat frequency. Besides, the low optical-to-optical conversion efficiency leads to the average output power to be only 8 mW. Therefore, it is highly desirable and practically useful to develop a high-power dual-wavelength synchronously
mode-locked Yb:YAG laser for generating ultrashort pulses with THz beat frequency.
The spontaneous emission spectrum of the Yb:YAG crystal reveals that there are two main peaks under the optical excitation at 940 nm [87]: the primary and secondary peaks are located around 1032 nm and 1049 nm, respectively. One of the key issues for achieving dual-wavelength operation in a Yb:YAG laser is to preciously control the gain-to-losses ratios at 1032 nm and 1049 nm [88-89]. On the other hand, the synchronization of dual-wavelength mode-locked beams is indispensable for generating an ultrashort pulse train with THz beat frequency. It has been shown that the cross saturation of the saturable absorber is beneficial to the synchronization of dual-wavelength mode-locked beams [67, 76]. In this work, we design a semiconductor saturable absorber mirror (SESAM) not only to assist the synchronous
mode locking but also to balance the output coupling for achieving dual-wavelength operation at 1032 nm and 1049 nm. With the fabricated SESAM, we successfully develop a high-power dual-wavelength harmonically mode-locked Yb:YAG laser to generate a pulse train with a pulse duration of 1.54 ps at a repetition rate of 80.3 GHz.
A diamond heat spreader is employed to enhance the heat removal efficiency of the Yb:YAG medium for scaling up the output power [83, 86]. The maximum average output power can be up to 1.1 W under the absorbed pump power of 5.18 W, corresponding to the optical-to-optical conversion efficiency of 21.2% and slope efficiency of 29.4%. The autocorrelation traces display a profound modulation with the period corresponding to the beat frequency of 4.92 THz. Since the overall modulation depth is greater than 80%, the effective pulse duration within the mode-locked pulse is as short as 83 fs.
(A)Experimental setup
Figure 4.12 presents the schematic of the experimental setup. The gain medium was an 11 at.% doped Yb:YAGcrystal cut along the [111] direction with 1.03 mm in length and 4 mm in diameter. One of the end facet of the crystal was coated for high reflection (HR, R>99.8%) from 1030 nm to 1100 nm and high transmission (HT, T>95%) at 940 nm served as the front mirror. The rear facet was coated for high reflection (HR, R>99%) at 940 nm to increase the absorption efficiency of the pump power and with high-transmission (HT, T≈95%) from 1030 nm to 1100 nm. It has been confirmed [83] that the partial reflection (R≈5%) on the rear facet for the lasing spectral range can introduce a significant etalon effect to achieve the harmonic mode locking. The uncoated, single crystal synthetic diamond of 4.5 mm square and 0.5 mm thickness was used as a heat spreader and capillary bounded to the front mirror side of the gain medium as described in Ref. [83, 86]. The transmittance of the diamond heat spreader was about 70% at 940 nm. The front facet of the diamond was in contact with a copper heat sink which is cooled by a thermal-electric cooler (TEC) and maintained at a temperature of 14°C. The rear facet of the gain medium was tightly attached to a copper plate with a hole of 2 mm in diameter, where an indium foil was used to improve the thermal contact. The pumping source was a 940-nm fiber-coupled laser diode with a core diameter of 400 µm and a numerical aperture of 0.2. The focusing lens with 25 mm focal length and 90% coupling efficiency was used to
Ch4 Passively Q-switched and mode-locked Yb:YAG lasers
Fig. 4.12 The schematic diagram of the mode-locked Yb:YAG laser experimental setup. (S1: HT at 940 nm, HR at 1030~1100 nm; S2: HT at 1030~1100 nm, HR at 940
nm; HT: high transmission; HR: high reflection).
A V-shaped cavity was used in the experiment where L1, L2, and L4 were 275 mm, 244 mm, and 65 mm, respectively. M1 was a concave mirror with a radius of curvature (ROC) of 550 mm and coated for high reflection (HR, R>99%) from 1030 nm to 1100 nm. A pair of SF10 prisms with a tip-to-tip distance (L3) of 480 mm was employed to compensate the cavity dispersion. A SESAM was designed to assist the synchronous mode locking and to balance the output coupling for achieving dual-wavelength operation. The modulation depth of the SESAM was found to be approximately 1.2% at 1040 nm. The SESAM device was monolithically grown on an undoped 350 µm thick GaAs substrate by metalorganic chemical vapor deposition (MOCVD) to comprise single strained In0.27Ga0.73As/GaAs quantum well (QW) grown on the Bragg mirror. The QW has a thickness of 8 nm. The Bragg mirror consists of ten-pair AlAs/GaAs quarter-wavelength layers. Figure 4.13 shows the
transmittance spectrum for the SESAM. It can be seen that the transmittances are approximately 5.8% and 3.8% at 1032 nm and 1049 nm. The back side of the GaAs substrate was coated for antireflection at 1040 nm (R<1%).
900 930 960 990 1020 1050 1080 1110 1140 1170 1200 0
20 40 60 80 100
Transmittance (%)
Wavelength (nm) SESAM
Fig. 4.13 Transmittance spectra of the SESAM.
(B) Experimental results and discussion
Figure 4.14 depicts the average output power versus the absorbed pump power in the dual-wavelength harmonically mode-locked operation. The maximum average output power is approximately 1.1 W under the maximum absorbed pump power of 5.18 W, corresponding to the optical-to-optical conversion efficiency of 21.2% and slope efficiency of 29.4%. The optical-to-optical conversion efficiency is considerably higher than the earlier results of dual-wavelength mode-locked lasers which range from 0.03% to 10% by utilizing rare-earth doped crystals such as Nd-doped disordered crystals [67, 76-79], Yb: LYSO crystal [80], and Yb:YAG ceramics [85].
With the thermal management of a diamond heat spreader, not only the output efficiency is remarkably increased but also the beam distortion is significantly
Ch4 Passively Q-switched and mode-locked Yb:YAG lasers improved [15].
0 1 2 3 4 5 6
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Average output power (W)
Absorbed pump power (W)
Mode-locked with SESAM
Fig. 4.14 Dependence of the average output power on the absorbed pump power in the dual-wavelength harmonically mode-locked operation.
The optical spectrum of laser output was measured with a Fourier-Michelson optical interferometer (Advantest, Q8347) with a resolution of 0.003 nm. Figure 4.15 depicts the experimental result for the lasing spectrum at the maximum absorbed pump power of 5.18 W. It can be seen that there are dual lasing bands with central wavelengths at 1031.67 nm and 1049.42 nm. As a result, the difference between the central frequencies is 4.92 THz. The values for the full width at half maximum (FWHM) of the spectral bands at 1031.67 nm and 1049.42 nm are 1.08 nm and 0.89 nm, respectively. The spectral intensity ratio of the two bands was numerically calculated to be 1:0.8. The values of the mode spacing within each spectral band are found to be approximately 80.3 GHz. This mode spacing precisely corresponds to the free spectral range of the etalon effect caused by the Yb:YAG crystal with an optical length of about 1.87 mm. It has been demonstrated [83] that the partial reflection on the surface of the gain medium could introduce a significant mode selection for
effectively generating high-order harmonic mode locking.
1025 1030 1035 1040 1045 1050 1055
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Intensity (a.u.)
Wavelength (nm)
17.747 nm
∆ν =4.92 THz
Fig. 4.15 Optical lasing spectrum obtained at the maximum absorbed pump power of 5.18 W.
The temporal behavior of the laser output was analyzed by exploiting the schemes of first- and second-order autocorrelations. The first-order autocorrelation trace was measured with a Michelson interferometer. The second-order autocorrelation trace was performed with a commercial autocorrelator (APE GmbH, PulseCheck). First of all, we measured the autocorrelation traces in a delay-time span of 50 ps to display the pulse repetition rate of the experimental mode-locked pulse train. Under this time span, the resolutions of the first- and second-order autocorrelations are 67 fs and 200 fs, respectively. Figures 4.16(a) and 4.16(b) show the measured results at the maximum absorbed pump power. It can be seen that the laser output exhibits a state of tenth-order harmonic mode locking (relative to the fundamental mode locking pulse repetition rate of 80.3 MHz) with the pulse repetition rate of 80.3 GHz. The tenth-order harmonic mode locking originates from the etalon effect that is caused by the partial reflection on the surface of the gain medium. The
Ch4 Passively Q-switched and mode-locked Yb:YAG lasers
Fig. 4.16 Experimental traces of the temporal behavior of (a) first- and (b) second-order autocorrelations in a delay-time span of 50 ps. Resolution: 67 fs and 200
fs for the first- and second-order autocorrelations, respectively.
To identify the modulation of the beat frequency, we measured the autocorrelation traces with higher resolutions in a delay-time span of 8 ps. Under this time span, the resolutions of the first- and second-order autocorrelations are 8 fs and 20 fs, respectively. Figures 4.17(a) and 4.17(b) show the measured results at the maximum absorbed pump power. It can be seen that both the first- and second-order autocorrelations display interference patterns with modulation depths higher than 80%
in the mode-locked pulse. Assuming the temporal intensity of the second-order autocorrelation trace to be a sech2 profile, the duration of the mode-locked pulse can be deduced to be 1.54 ps, as shown in Fig. 4.17(b). The periodic modulation within the autocorrelation traces clearly corresponds to the 4.92-THz beat frequency of the dual-wavelength laser. The deep modulations in autocorrelation traces indicate that the dual-wavelength mode-locked pulses are synchronous to a certain extent. Figure 4.18 shows the second-order autocorrelation trace in a delay-time span of 180 fs to evaluate the pulse duration arising from the beating. In terms of a cosine-like shape, the effective pulse duration of the beating exactly corresponds to the FWHM of the
measured autocorrelation trace. As a consequence, the effective pulse duration of the dual-wavelength mode-locked laser is approximately 83 fs.
-4 -3 -2 -1 0 1 2 3 4
Fig. 4.17 Experimental traces of the temporal behavior of (a) first- and (b) second-order autocorrelations in a delay-time span of 8 ps. Resolution: 8 fs and 20 fs
for the first- and second-order autocorrelations, respectively.
-90 -60 -30 0 30 60 90
Fig. 4.18 Experimental trace of the temporal behavior of second-order autocorrelation of the single beat pulse.
Ch4 Passively Q-switched and mode-locked Yb:YAG lasers
(C) Conclusions
I have experimentally demonstrated a high-power harmonically mode-locked Yb:YAG laser with a pulse duration of 1.54 ps at a repetition rate of 80.3 GHz. The power scale-up was improved by employing a diamond heat spreader to enhance the heat removal efficiency of the Yb:YAG crystal. At the absorbed pump power of 5.18 W, the maximum average output power was up to 1.1 W, corresponding to the optical-to-optical conversion efficiency of 21.2% and slope efficiency of 29.4%. An appropriate SESAM has been developed to balance the output coupling and to achieve the dual-wavelength synchronous mode-locked operation at 1032 nm and 1049 nm.
The autocorrelation traces revealed that the modulation depth of the mode-locked pulse in the beat frequency of 4.92 THz could be generally higher than 80% and the effective pulse duration was as short as 83 fs.
4.4 Conclusions
In this chapter, I demonstrate efficient, high peak power PQS Yb:YAG microchip laser by using Cr4+:YAG crystal as the saturable absorber. Energy and output power scaling together with pulse stability improvement were achieved via employing a diamond window as the heat spreader. Besides, high repetition rate, self-mode-locked operation was also attained using the same gain chip and heat spreader. Various order of harmonic mode locking was obtained by means of tunning the cavity length to match a commensurate ratio of the gain chip length. Furthermore, by replaceing a semiconductor saturable absorber mirror as the output coupler, dual-wavelength mode-locked Yb:YAG laser was demonstrated. Synchronous, dual-wavelength mode locking was speculated to come from the balance of dispersion compensation and the cross-saturation of the saturable absorber. As a result of the optical beating of the dual spectral bands, THz modulation frequency was generated with an ultrashort pulse duration.
Ch4 Passively Q-switched and mode-locked Yb:YAG lasers
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