Compact, simple, economical and high peak power solid-state laser source with sub-nanosecond pulses is of practical importance in many fields such as range finding and 3D imaging in use of the time-of-flight methods, micro-processing and optical communication. Because the pulse energy of mode-locked lasers is limited by the short inter-pulse periods and the device configuration is complex in fabrication, a better way to obtain high peak power laser output is the passively Q-switched (PQS) microchip lasers discussed in section 1.4. Compared to the actively Q-switched microchip lasers, the PQS microchip lasers is simple, low cost and no need of any external high speed and high power consumption drivers and temperature control of
the electro- or acousto-optic Q-switches. But the choices of gain mediums and saturable absorbers are restricted to the suitable doped bulk crystals with matched emission and absorption spectrum. Even though the use of antiresonant Fabry-Perot saturable absorbers (A-FPSAs) or the so-called semiconductor saturable absorber mirrors (SESAMs) with adjustable wavelengths via bandgap engineering in PQS microchip lasers is demonstrated by Braun et al. in the spectral region of 1.06-μm, 1.34-μm and 1.5-μm [1-3], the operation wavelengths of these lasers are still mainly limited by the discrete energy levels of the doped ions of gain medium. Alternatively, the passively Q-switched lasers in combination with nonlinear frequency conversion techniques such as the harmonic generation, optical parametric oscillation and stimulated Raman scattering are developed to be the promising pulsed laser sources [4-6]. But the long active length of the nonlinear crystal hinders the application of intra-cavity pumping in which the cavity length should be short enough to afford sub-nanosecond operation.
Based on the contents of this thesis, if we combine simultaneous Q-switching and frequency-switching device and the heat spreader method discussed in Chapter 4 and section 3.3, a compact thin disk PQS microchip laser with intra-cavity wavelength conversion could be achieved via capillary bonding technique. The effectiveness of the use of diamond in contact with the Yb:YAG crystal with 1 mm thickness has been realized in our lab with continuous wave and PQS operation [7]. But the Cr4+:YAG crystal is separated from the gain chip and is just functioning as a nonlinear saturable absorber. Therefore, the emission wavelength is restricted to the Yb:YAG gain chip of 1030 nm and the cavity length is as long as 8.4 mm. If we take the AlGaInAs MQWs chip as shown in Chapter 4 instead of the Cr4+:YAG and construct a suppositional
Fiber-coupled LD@970nm
Focusing lens
Pump beam 1530nm laser output
Yb:YAG
φ4x1mm AlGaInAs MQWs
Diamond heat spreader HT@970nm
HR@1030nm
HR@1530nm HR@970nm
HR@1030nm PR@1530nm Heat sink
Fig. 5.2-1 Suppositional experimental setup of the simultaneously Q-switched and frequency-switched Yb:YAG/AlGaInAs MQWs microchip laser.
laser configuration as demonstrated in Fig. 5.2-1, we could obtain the 1530 nm pulsed output and the cavity length could be shortened well below 2.5 mm under the nearly equivalent experimental setup. In this framework, the two diamonds are bonded to the Yb:YAG crystal and AlGaInAs MQWs chip with thickness of 0.5 mm to enhance heat dissipation from the pump spot. The diamond wafer in contact with the gain chip is high-reflection (HR) coated at 1030 and 1530 nm and high-transmittance (HT) coated at 970 nm at the entrance side to serve as the front mirror. The other diamond wafer in contact with the AlGaInAs MQWs chip is HR coated at 970 and 1030 nm and partial reflection coated at 1530 nm at the output face to serve as the external mirror.
Compared to the PQS laser in combination with the nonlinear wavelength conversion, this laser setup offers several advantages. First, it is capable of intracavity pumping without significantly lengthening the cavity. Second, the fundamental laser emission is no need to be linear polarization. Third, the wavelength conversion efficiency is not strongly depending on the intracavity photon intensity. Four, there is no requirement of phase matching. Therefore, it is a simple, compact, monolithic and low cost potential laser configuration to produce sub-nanosecond near-infrared laser pulses and we are interested in realizing this laser module in the future.
Reference
[1] B. Braun, F. X. Kärtner, G. Zhang, M. Moser, and U. Keller, “56-ps passively Q-switched diode-pumped microchip laser,” Opt. Lett. 22, 381-383 (1997).
[2] R. Fluck, B. Braun, E. Gini, H. Melchior, and U. Keller, “Passively Q-switched 1.34-μm Nd:YVO4 microchip laser with semiconductor saturable-absorber mirrors,” Opt. Lett. 22, 991-993 (1997).
[3] R. Fluck, R. Häring, R. Paschotta, E. Gini, H. Melchior, and U. Keller, “Eyesafe pulsed microchip laser using semiconductor saturable absorber mirrors,” Appl.
Phys. Lett. 72, 3273-3275 (1998).
[4] R. Bhandari and T. Taira, “> 6MW peak power at 532 nm from passively
Q-switched Nd:YAG/Cr4+:YAG microchip laser,” Opt. Express 19, 19135- 19141 (2011).
[5] H. Zhu, G. Zhang, H. Chen, C. Huang, Y. Wei, Y. Duan, Y. Huang, H. Wang, and G. Qiu, “High-efficiency intracavity Nd:YVO4\KTA optical parametric oscillator with 3.6 W output power at 1.53 μm,” Opt. Express 17, 20669-20647 (2009).
[6] Y. T. Chang, K. W. Su, H. L. Chang, and Y. F. Chen, “Compact efficient Q-switched eye-safe laser at 1525 nm with a double-end diffusion-bonded Nd:YVO4 crystal as a self-Raman medium,” Opt. Express 17, 4330-4335 (2009).
[7] W. Z. Zhang, Yi-Fan Chen, K. W. Su, K. W. Huang, and Y. F. Chen,
“Performance enhancement of sub-nanosecond diode-pumped passively Q-switched Yb:YAG microchip laser with diamond surface cooling,” Opt.
Express 20, 22602-22608 (2012).
Curriculum Vitae
Personal Data
Name: Yi-Fan Chen Birthday: Jan. 7, 1986 Nationality: Taiwan (R.O.C.) Birthplace: Taipei Telephone (M): +886-911-202-823 Sex: Male
E-mail: [email protected]
Education
2007~2012 Ph.D. in Dep. of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2004~2007 B.S. in Dep. of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2001~2004 Taipei ChengGong Senior High School, Taipei
Work Experience
2007~2012 T.A. of Fundamental Physics
Speciality
Laser Physics and Optically-Pumped Semiconductor Lasers
Publication List
Journal paper:
[1] Y.-F. Chen, K. W. Su, W. L. Chen, K. F. Huang, and Y. F. Chen, “High- peak-power optically pumped AlGaInAs eye-safe laser at 500-kHz repetition rate with an intracavity diamond heat spreader,” Appl. Phys. B 108, 319-323 (2012).
IF=2.189 => 點數=2.5x100%=2.5
[2] Y.-F. Chen, Y. C. Lee, S. C. Huang, K. W. Huang, and Y. F. Chen, “AlGaInAs multiple-quantum-well 1.2-μm semiconductor laser in-well pumped by an Yb-doped pulsed fiber amplifier,” Appl. Phys. B 106, 57-62 (2012).
IF=2.189 => 點數=2.5x100%=2.5
[3] W. Z. Zhuang, Yi-Fan Chen, K. W. Su, K. F. Huang, and Y. F. Chen,
“Performance enhancement of sub-nanosecond diode-pumped passively Q-switched Yb:YAG microchip laser with diamond surface cooling,” Opt.
Express 20, 22602-22608 (2012).
IF=3.587 => 點數=3x70%=2.1
[4] H. L. Chang, S. C. Huang, Yi-Fan Chen, K. W. Su, Y. F. Chen, and K. F. Huang,
“Efficient high-peak-power AlGaInAs eye-safe wavelength disk laser with optical in-well pumping,” Opt. Express 17, 11409-11414 (2009).
IF=3.587 => 點數=3x50%=1.5
[5] S. C. Huang, H. L. Chang, Yi-Fan Chen, K. W. Su, Y. F. Chen, and K. F. Huang,
“Diode-pumped passively mode-locked 1342 nm Nd:YVO4 laser with an AlGaInAs quantum-well saturable absorber,” Opt. Lett. 34, 2348-2350 (2009).
IF=3.399 => 點數=3x50%=1.5
Conference paper:
[1] K. W. Su, Yi-Fan Chen, S. C. Huang, A. Li, S. C. Liu, Y. F. Chen, and K. F.
Huang, “Low-temperature study of lasing characteristics for 1.3-μm AlGaInAs quantum-well laser pumped by an actively Q-switched Nd:YAG laser,” Proc.
SPIE 7578, 75780Z 1-7 (2010).
總點數=2.5+2.5+2.1+1.5+1.5=10.1
Appl Phys B (2012) 108:319–323 DOI 10.1007/s00340-012-4954-4
High-peak-power optically pumped AlGaInAs eye-safe laser at 500-kHz repetition rate with an intracavity diamond heat spreader
Y.-F. Chen· K.W. Su · W.L. Chen · K.F. Huang · Y.F. Chen
Received: 23 November 2011 / Revised version: 1 February 2012 / Published online: 21 March 2012
© Springer-Verlag 2012
Abstract We report on a compact efficient high-repetition-rate (>100 kHz) optically pumped AlGaInAs nanosecond eye-safe laser at 1525 nm. A diamond heat spreader bonded to the gain chip is employed to improve the heat removal.
At a pump power of 13.3 W, the average output power at a repetition rate 200 kHz is up to 3.12 W, corresponding to a peak output power of 560 W. At a repetition rate 500 kHz, the maximum average power and peak power are found to be 2.32 W and 170 W, respectively.
1 Introduction
High-peak-power high-repetition-rate laser sources have been in demand for the applications in the eye-safe wave-length regime near 1.55-µm such as free-space communica-tion, gas sensing, spectroscopy, and medical treatment. The eye-safe laser sources can be realized in several ways in-cluding stimulated Raman scattering (SRS) or optical para-metric oscillation (OPO) pumped by the high-peak-power Nd-doped lasers [1–7] and the solid state lasers directly use the Er3+-doped or Cr4+-doped gain media [8–11].
Optically-pumped multiple-quantum-wells (MQWs) semiconductor disk lasers have been developed to pro-vide low-divergence, circular, and high quality nearly-diffraction-limited output beams with flexible choices of emission wavelengths via bandgap engineering [12,13]. The quaternary alloys lattice matched to InP including InGaAsP
Y.-F. Chen· K.W. Su · W.L. Chen · K.F. Huang · Y.F. Chen ()
Department of Electrophysics, National Chiao Tung University, 1001 TA Hsueh Road, Hsinchu 30050, Taiwan
e-mail:[email protected]
and AlGaInAs are developed in the quantum confined struc-ture of the semiconductor lasers for generating the radiation at the NIR region [14–18]. Although the InGaAsP systems were commonly employed in the semiconductor lasers of the NIR region in the early stage [19,20], the AlGaInAs systems have been verified to have higher conduction band offsets and better carrier confinements. So far, the maximum average output power ever reported for the eye-safe laser based on the AlGaInAs material was found to be 2.6 W at the continuous-wave operation [21].
Recently, the AlGaInAs eye-safe pulsed laser was real-ized with an actively Q-switched 1.06-µm laser as a pump source [22,23]. However, the average output power and the pulse repetition rate were restricted to 0.52 W and 20 kHz, respectively, due to the poor heat dissipation from the pump area. Here we report, for the first time to our knowledge, on a high-repetition-rate (100–500 kHz) high-power optically pumped AlGaInAs nanosecond eye-safe laser at 1525 nm with an intracavity diamond heat spreader to enhance the heat removal. We employ an Yb-doped pulsed fiber ampli-fier to be a pump source for providing various pulse repeti-tion rates. With a pump width of 28-ns at a repetirepeti-tion rate of 200 kHz, the average output power and peak output power under an average pump power of 13.3 W are found to be up to 3.12 W and 560 W, respectively. The maximum average power and peak power at a repetition rate 500 kHz are found to be 2.32 W and 170 W, respectively. The overall slope ef-ficiency is maintained as high as 27.3 % at a pulse repetition rate between 100 and 500 kHz.
2 Experimental setup
Figure 1 shows the experimental configuration of the Al-GaInAs MQWs laser at the eye-safe region driven by a
320 Y.-F. Chen et al.
1.06 µm Yb-doped pulsed fiber amplifier (SPI redEN-ERGY G3). The pump source could be operated to provide consecutive pulses with the pulse duration in the range of 9–200 ns and the repetition rate ranging from 10–500 kHz.
The pump spot diameter was controlled to be approximately 700± 20 µm to have a good spatial overlap with the las-ing mode. The laser resonator was designed to be a linear plane-plane cavity which was stabilized by the thermally induced lens of the gain material. The flat mirror at the pump side was coated with antireflection coating at 1.06 µm (R < 0.2 %) at the entrance face and with high-reflection coating (R > 99.8 %) at 1.53 µm and high-transmission (T > 95 %) at 1.06 µm at the other face. The reflectivity of the flat output coupler was 70 % at 1.53 µm. The overall cavity length is approximately 15 mm.
The gain structure is composed of 30 groups of triple QWs spaced at half-wavelength intervals by AlGaInAs bar-rier layers as shown in the inset of Fig. 1. The thickness of the quantum wells are designed to be 8 nm. The Al-GaInAs barriers are only used to separate the QWs not used as strain compensation layers. The resonant-periodic-gain (RPG) structure was designed to locate the QWs at the antin-odes of the lasing field standing wave [24,25]. The periodic AlGaInAs QW/barrier layers were grown on a Fe-doped InP transparent substrate by metalorganic chemical-vapor depo-sition. The Fe-doped InP transparent substrate with high transmission at the pump and lasing wavelength is used to solve the problem of the lack of good distributed Bragg re-flectors (DBRs) for the InP-based systems. The function of conventional DBRs was replaced with an external reflective mirror. A window layer of InP was deposited on the gain structure to prevent surface recombination and oxidation.
In contrast to the conventional barrier pumping scheme, the present gain medium was designed to be suitable for in-well pumping to enhance the quantum efficiency [26]. It has been
confirmed that the slope efficiency with the in-well pump-ing scheme was significantly higher than that with the bar-rier pumping scheme [23]. In the experiment, the single-pass absorption of this gain chip is 81–84 % for repetition rate ranged from 30–500 kHz under 28 ns pump pulse width.
A 4.5-mm square, 0.5-mm thick piece of uncoated sin-gle crystal diamond heat spreader was bonded to the MQWs side of the cleaved 2.5-mm square piece of the gain chip to improve the heat removal. Although the heat spreader ap-proach has been used in a variety of high power optically pumped semiconductor lasers [27–29], to the best of our knowledge, the diamond heat spreader is for the first time to be applied to the transparent semiconductor gain medium.
The other side of the diamond was in contact with a cop-per heat sink which was cooled by a thermal-electric cooler (TEC), where the temperature was maintained at 15◦C. The substrate side of the gain chip was attached tightly to a cop-per plate with a hole of 2-mm diameter, where an indium foil was employed to be the contact interface. The contact uniformity was further confirmed by inspecting the interfer-ence fringe coming from the minute gap between the gain chip and the diamond heat spreader. The package configura-tion of the gain medium can be seen in Fig.1.
3 Experimental results and discussion
We fixed the duration of pump pulses to be 28 ns for making a detailed comparison at different pulse repetition rates. The spectral information was monitored by an optical spectrum analyzer (Advantest Q8381A) with a diffraction monochro-mator which could be used for the high-speed measurement of pulsed light with a resolution of 0.1 nm. Figure2shows the room temperature spontaneous-emission spectrum of AlGaInAs MQWs pumped with an average absorbed power
High-peak-power optically pumped AlGaInAs eye-safe laser at 500-kHz repetition rate with an intracavity 321
Fig. 2 Room temperature surface emitting spontaneous emission spectrum under a 100-kHz pump repetition rate at an average absorbed power of 0.8 W
Fig. 3 Output performances of the eye-safe laser without and with the diamond heat spreader at a 30-kHz repetition rate
of 0.8 W at a pulse repetition rate of 100 kHz. It can be seen that the photoluminescence peak at a low pump power was approximately at 1500 nm.
The pump source is a standard commercial product and its maximum output power is dependent on the pulse repeti-tion rate. The maximum output powers of the pump source are approximately 5 W and 20 W for the repetition rates of 30 kHz and 100–500 kHz, respectively. Consequently, we present the experimental results for the laser performance in different figures for the conditions of 30 kHz and 100–
500 kHz, respectively. To investigate the losses introduced by the intracavity diamond heat spreader, we make a com-parison between the performance of the AlGaInAs eye-safe laser without and with the diamond at the repetition rate of 30 kHz, as shown in Fig. 3. Note that the thermal
ef-fect at the repetition rate of 30 kHz is not significant for the absorbed pump power less than 2 W. It can be seen that the output power without the heat spreader displays a thermally induced roll-over effect for the average absorbed power higher than 2.3 W. In contrast, the slope efficiency ob-tained with the heat spreader can remain nearly constant for the absorbed pump power up to the maximum pump power of 3.3 W, where the maximum pump power is just limited by the pump source at the repetition rate of 30 kHz. This result confirms the improvement of the power scalability by use of the diamond heat spreader. On the other hand, the slope effi-ciencies obtained without and with the heat spreader can be found to be 33 % and 28 %, respectively. With these slope efficiencies and the output reflectivity of 70 %, the losses in-troduced by the heat spreader can be estimated to be 7.5 %.
Even though there is a room for improving the introduced losses, the diamond heat spreader can extend the operation frequency up to 500 kHz, as shown in the following results.
Figures4(a) and (b) show the output performances with-out and with the diamond heat spreader, respectively, for the repetition rate in the range of 100–500 kHz. The maximum average output powers without the heat spreader can be seen to decrease from 0.45 W down to 0.11 W for the repeti-tion rate increasing from 100 kHz to 500 kHz. On the other hand, the average output powers with the heat spreader can be almost maintained linear for the absorbed pump power reaching the maximum value of 13.3 W at the repetition rate within the range of 200–500 kHz. Since the diamond can effectively reduce the thermal effects, the overall beam quality M2 was found to be better than 1.3 for all the pump powers. The maximum average output powers can be found to be up to 3.12 W and 2.32 W for the repetition rates of 200 kHz and 500 kHz, respectively. The roll-over phe-nomenon observed in Fig.4(b) for the case of 100 kHz was attributed to the pump-saturation effect. With an absorbed pump power of 10 W and a pump diameter of 700 µm, the pump intensity for the pump duration of 28 ns at 100 kHz could be calculated to be 0.93 MW/cm2. Since the satura-tion intensity of the MQW absorpsatura-tion was measured to be approximately within 0.8–1.0 MW/cm2, the power roll-over phenomenon at 100 kHz was considered to come from the pump-saturation effect.
Figure 5(a) depicts the lasing spectrum with the heat spreader under an average absorbed power of 2.5 W at a repetition rate of 100 kHz. The lasing spectrum can be seen to comprise dense longitudinal modes with the bandwidth to be approximately 10 nm and the center wavelength to be located at 1515 nm. With increasing the average absorbed power, the center wavelength has significant redshifts due to the pump power induced the local heating on the gain medium. Figure5(b) shows the dependence of the red-shift on the absorbed pump power for the laser operation without and with the heat spreader at a repetition rate of 100 kHz. It
322 Y.-F. Chen et al.
Fig. 4 Output performances without (a) and with (b) the diamond heat spreader for repetition rates in the range of 100–500 kHz
Fig. 5 (a) Lasing spectrum with the heat spreader under an average absorbed power of 2.5 W at a repetition rate of 100 kHz. (b) Dependence of the red-shift on the absorbed pump power for the laser operation without and with the heat spreader at a repetition rate of 100 kHz
can be seen that the redshift measured for the laser without using the heat spreader is considerably larger than the result with the heat spreader. This substantial difference also con-firms the local heating to be considerably improved by use of the diamond heat spreader.
The temporal behavior of the laser output was recorded with a LeCrory digital oscilloscope (Wave pro 7100, 10 G samples/s, 1 GHz bandwidth). Figure6shows the input and output pulse trains as well as the extended pulse shape of the single pulse for the result obtained with an average ab-sorbed power of 2.5 W at a repetition rate of 200 kHz. It can be seen that the time delay of the output pulse with respect to
The temporal behavior of the laser output was recorded with a LeCrory digital oscilloscope (Wave pro 7100, 10 G samples/s, 1 GHz bandwidth). Figure6shows the input and output pulse trains as well as the extended pulse shape of the single pulse for the result obtained with an average ab-sorbed power of 2.5 W at a repetition rate of 200 kHz. It can be seen that the time delay of the output pulse with respect to