Thi Nghiem Vu,1,2,* Andreas Klehr,1 Bernd Sumpf,1 Hans Wenzel,1 Götz Erbert,1 and Günther Tränkle1. 1Ferdinand-Braun-Institut, Leibniz-Institut für ...
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OPTICS LETTERS / Vol. 39, No. 17 / September 1, 2014
Tunable 975 nm nanosecond diode-laser-based master-oscillator power-amplifier system with 16.3 W peak power and narrow spectral linewidth below 10 pm Thi Nghiem Vu,1,2,* Andreas Klehr,1 Bernd Sumpf,1 Hans Wenzel,1 Götz Erbert,1 and Günther Tränkle1 1
2
Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet rd., Cau Giay, Hanoi, Vietnam *Corresponding author: Vu.Thi.Nghiem@fbh‑berlin.de Received June 3, 2014; accepted July 1, 2014; posted July 24, 2014 (Doc. ID 213295); published August 25, 2014 A spectrally tunable, narrow linewidth master oscillator power amplifier system emitting ns pulses with high peak power is presented. The master oscillator is a distributed feedback ridge waveguide (DFB-RW) laser, which is operated in continuous wave (CW) mode and emits at about 975 nm with a spectral line width below 10 pm. The oscillator can be tuned over a range of 0.9 nm by varying the injection current. The tapered amplifier (TA) consists of an RW section and a flared gain-guided section. The RW section of the amplifier acts as an optical gate and converts the CW input beam emitted by the DFB-RW laser into a train of short optical pulses, which are subsequently amplified by the tapered section. The width of the pulses is 8 ns at a repetition rate of 25 kHz. The peak power is 16.3 W. The TA preserves the spectral properties of the emission of the DBR-RW laser. The amplified spontaneous emission is suppressed by about 40 dB. © 2014 Optical Society of America OCIS codes: (140.3280) Laser amplifiers; (140.3490) Lasers, distributed-feedback; (140.3600) Lasers, tunable; (140.5960) Semiconductor lasers; (140.3538) Lasers, pulsed. http://dx.doi.org/10.1364/OL.39.005138
There is an increasing demand in continuously tunablewavelength narrow-spectral-linewidth high-peak-power light sources. In order to apply these sources to detect atmospheric gases, e.g., H2 O, using differential absorption light detection and ranging (DIAL), not only peak powers in the 10 W range and spectral widths adapted to the width of the absorption lines at atmospheric pressure (about 0.1 cm−1 , i.e., 10 pm at 975 nm) are required, but the wavelength must be continuously tunable. This is especially important for calibration, adjusting the working points, and selecting suitable absorption lines. In DIAL systems, the time between two laser pulses determines the maximal time interval between emission and detection without a temporal overlap of the emitted pulses. Together with the speed of light, the possible measuring range can be calculated [1]. A repetition rate of 25 kHz, i.e., a time between two pulses of 40 μs corresponds to a measuring range of 6000 m. The response time of the detection of the photons determines the spatial resolution of the LIDAR measurement, e.g., a response time of 8 ns corresponds to a resolution of 1.2 m. To distinguish between the resolution intervals, the emitted laser pulse should have a length at least not longer than the response time of the system. A promising concept to realize a tunable wavelength, high-power diode laser system emitting a spatial and spectral single mode is the master oscillator power amplifier (MOPA), where the spectral properties are defined by the master oscillator (MO) and a high peak power is provided by the power amplifier (PA). One possible approach is the use of an external cavity diode laser (ECDL) as the MO and one or two tapered amplifiers (TAs). Obland et al. [2] reported a 17 nm tuning range centered at about 832 nm, achieving an optical peak power of 0.25 W with a pulse width of 1 μs at a 0146-9592/14/175138-04$15.00/0
repetition rate of 20 kHz. Nehrir and Repasky [3,4] used an ECDL as the MO and either a two-stage amplification with a continuous wave (CW) tapered preamplifier and a 1 μs pulse width driven second TA [3] or a 1 μs pulse width for the TA [4]. At a wavelength of 824 nm, a spectral width of 0.001 pm (≤0.00001 cm−1 ) and a side-mode suppression ratio (SMSR) of 45 dB were obtained at peak powers of 1 or 7 W [3], respectively. The generated pulses at repetition rates of 20 kHz [2,3] or 10 kHz [4] were applied for water vapor profiling, corresponding to measurement ranges of 7.5 and 15 km, respectively, at a spatial resolution of about 150 m. It should be noted that in [2–4] the applied pulses were limited to τ 1 μs duration by the electronics used. Distributed feedback ridge waveguide (DFB-RW) lasers can operate mode hop free with narrow linewidth and excellent SMSR. The wavelength can be finely tuned by changing the injection current or the temperature. They are also well known as reliable and mechanically stable light sources. A DFB-RW laser emitting at 975 nm with a tuning range of about 2 nm by adjusting the injection current suitable for the detection of water vapor was reported by Wenzel et al. [5]. Sumpf et al. [6] used a DFB laser at 940 nm with a tuning range of about 4.7 nm by adjusting the injection current and a tuning of 7.1 nm by changing the injection current and temperature for the measurements of H2 O lines and the determination of line-broadening coefficients. However, in order to obtain a higher peak power, thus conserving the desired spectral properties, our group recently reported a MOPA system emitting at 1060 nm, which consisted of a DFB-RW laser used as the MO and a TA with an integrated optical gate [7]. An optical peak power of 16 W with a pulse width of τ 2 ns and a repetition rate of f 800 MHz was obtained. A spectral © 2014 Optical Society of America
September 1, 2014 / Vol. 39, No. 17 / OPTICS LETTERS
Fig. 1.
Scheme of the MOPA setup.
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Fig. 3. Color scale mapping of the optical spectrum of the DFB laser in dependence of the current.
reflectivities of R ≤ 5 × 10−4 in the spectral range from 950 to 980 nm. The device is mounted p side up on a C mount, allowing an electrically separated excitation of the two sections. The PA is operated at a heat sink temperature of 30°C, which is selected to avoid self-lasing. If, in the TS, a current pulse with a length of τTS 15 ns, a repetition rate of f 25 kHz, and an amplitude of I TS 18.4 A is injected, an average output power of 1.4 mW is measured (Fig. 4). The insert shows the spectrum of the amplified spontaneous emission (ASE). The peak wavelength of the ASE spectrum is about λ 969 nm, and the 3 dB full
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linewidth below 10 pm with an SMSR of better than 46 dB was reported. In this work, the concept of an all semiconductor MOPA system, as presented in [7], is transferred to the spectral range of 975 nm, which is selected according to the strength of the H2 O lines in this region, to allow a path length of about 6000 m in DIAL experiments [8,9,10]. Particular attention is paid to obtain a tunable emission wavelength with a spectral linewidth below 10 pm and a peak power of more than 10 W in order to meet the requirements of the application. The pulse width and repetition rate are reasonable for very high spatial resolution in a short length range. A scheme of the MOPA setup is shown in Fig. 1. A DFB laser, as described in [5,11], is used as the MO and operated in CW mode. The emitted light is collimated with an antireflection-coated aspheric lens having a focal length of 3.1 mm and a numerical aperture NA 0.68 (Thorlabs, 352330-B). The light passes through an optical isolator (Thorlabs, IO-980-5-HP) and is divided into two parts by a beam splitter 50∶50 (Thorlabs, BS014). One part is focused into the RW section (P in ) of the TA by a second aspheric lens. Another part is sent to a power meter or an optical spectrum analyzer (OSA) for monitoring the power and spectrum, respectively. The DFB laser has a cavity length of L 3 mm and a ridge width of W RW 3 μm, providing single lateral mode operation. The laser is mounted p side down on a standard C mount. At T 35°C, the threshold current is I th 35 mA. The slope efficiency S, determined slightly above threshold current, is S 0.62 W∕A. At an injection current of 600 mA, the output power is P 340 mW (see Fig. 2). The optical spectra are measured with an OSA with a resolution of 10 pm and a dynamic range of 60 dB (Advantest Q8384). The two spectra inserted in Fig. 2 are measured at 100 mA (40 mW) and 600 mA (340 mW). The peak wavelengths are 974.50 and 975.40 nm, respectively. The DFB laser operates in single mode with a spectral linewidth smaller than 10 pm. The dependence of the optical spectra on the injection current is shown in Fig. 3 as a color scale mapping. A tuning range of 0.9 nm is obtained by varying the current from 100 to 600 mA. The two-section PA is based on an InGaAs double quantum well embedded in an asymmetric AlGaAs super large optical cavity, resulting in a narrow vertical far-field divergence of 14° FWHM. As indicated schematically in Fig. 1, the amplifier with a total length of 6 mm consists of a 2 mm long index-guided RW section with a 4 μm wide ridge and a 4 mm long gain-guided tapered section (TS) with a full taper angle of 6°. The front and the rear facets are passivated and antireflection coated with
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Fig. 4. Power-current characteristic for the tapered section (TS) of the amplifier at T 30°C under pulsed operation with τTS 15 ns, f 25 kHz and inserted spectrum of the ASE at I TS 18.4 A, τTS 15 ns, f 25 kHz.
OPTICS LETTERS / Vol. 39, No. 17 / September 1, 2014
width of the spectrum is 18 nm. No lasing modes are observed. The RW section of the PA (Fig. 1) serves as an optical gate (denoted OG). It absorbs the coupled CW beam of the DFB laser without current injection. However, if a current pulse of defined width τOG and amplitude I OG is injected, the OG becomes transparent, and the optical beam passes the OG. In the experiments described in this work, a current pulse amplitude of I OG 100 mA is used. The generated optical pulse is then subsequently amplified in the amplifier section TS where a current with an amplitude of I TS ≤ 18.4 A is injected. Due to the targeted 8 ns optical pulse length within our experiments, the pulse width τTS has to be larger than τOG , but not too long in order to prevent the generation of the ASE after emission of the optical pulse. The time needed to accumulate enough excess carriers in the active region amounts to several times the carrier lifetime, which is of the order of 1 ns. Hence a delay between the pulse through the TS and the OG of about 3.5 ns is chosen, as discussed in [7]. After the OG pulse, the same delay time for the depopulation can be assumed. Therefore, a pulse width of τTS 15 ns is selected. The repetition rate f of the current pulses injected into both sections (OG and TS) is 25 kHz. Current pulses for the OG and the TS are applied by driver circuits based on GaN transistors, as described in [12]. They provide nanosecond-range current pulses with a free choice of the repetition rate and a peak current up to 20 A. The transient behavior of the optical output pulse and the optical spectrum of the MOPA, depending on input power or current of the DFB laser, are simultaneously recorded using a beam splitter 50∶50 (Thorlabs, BS014). To detect the optical peak power, a high-speed InGaAs PIN photodiode (C30617BH EXCELITAS) is combined with an oscilloscope (Tektronix TDS754D). The optical peak power is then calculated by [13]
optical peak power PP / W
Pt 16 a) 12
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0 5 10 15 0 5 10 15 0 5 10 15 0 5 10 15 20 time τ /ns Fig. 5. Temporal pulse shapes emitted by the MOPA when (a) the MO is turned off and for different input powers [(b) P in 5.3 mW, (c) 8.9 mW, (d) 11.8 mW, (e) 15 mW, (f) 20 mW, (g) 35 mW, and (h) 50 mW] for I TS 18.4 A, τDelay 3.5 ns, τPulse 8 ns, I PP 100 mA, τTS 15 ns, f 25 kHz.
where ϕt is the temporal shape of the optical pulse, P avg is average power, and T is period (T 1∕f ). In Fig. 5, the temporal shapes of the output of the TA are shown in dependence of the input power. The temporal shape of the optical pulse without the input (ASE) has a pronounced plateau with a pulse width of about 11 ns and a power amplitude of 3.8 W. The rise and fall times determined from 10% and 90% of the pulse are 2.2 and 2 ns, respectively [Fig. 5(a)]. Figures 5(b)–5(h) show the optical output pulses at different input powers varied between 5.3 mW [Fig. 5(b)] and 50 mW [Fig. 5(h)]. It can be seen that the optical peak power increases from 10 to 15 W when the input power is increased from 5.3 to 15 mW [Figs. 5(b)–5(e)] and reaches a constant output power of 16.3 W for larger input power [Figs. 5(f)–5(h)]. The pulse width as well as rise and fall times are almost unchanged. A small ripple observed at the top of the optical pulse may be caused by unwanted feedback effects. At the beginning and the end of the amplifier pulse, the pedestals indicate contributions due to ASE. The dependence of the optical output power on the input power is shown in Fig. 6 (solid squares). For highinput power P in , the signal power saturates, and the ASE is suppressed. To determine the saturation power P sat , the formula [14] P P −P in −P in sig sat P sat 1 − e e P out P ASE max max
(2)
is used. As P ASE max , the experimental result shown in Fig. 5(a) (P ASE 3.8 W), and for P sig max , the saturated power of max 16.3 W is used. This leads to a saturation power of P sat 5.3 mW. The resulting plot is given as a solid line in Fig. 6. Figure 7 shows the optical spectrum of MOPA at a peak power of about 16.3 W (at P in 20 mW, I TS 18.4 A, τDelay 3.5 ns, τPulse 8 ns, I PP 100 mA, and f 25 kHz) in the wavelength range from 940 to 990 nm. The peak wavelength of the ASE spectrum of 969 nm is slightly shorter than the emission wavelength of the MO. The peak value of the intensity of the ASE is optical output peak power PP / W
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Fig. 6. Optical peak power versus input power for I TS 18.4 A, τDelay 3.5 ns, τPulse 8 ns, I PP 100 mA, τTS 15 ns, f 25 kHz (black solid squares). Equation (2) was used for curve fitting (red curve).
September 1, 2014 / Vol. 39, No. 17 / OPTICS LETTERS Optical peak power PP / W
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Fig. 7. Optical spectrum of the MOPA at 16 W peak power (at P in 20 mW, I TS 18.4 A, τDelay 3.5 ns, τPulse 8 ns, I PP 100 mA, and f 25 kHz).
suppressed by 40 dB (SMSR) compared with the intensity of the lasing peak (974.82 nm). In Fig. 8, optical spectra of the MOPA at different wavelengths obtained by adjusting the current injected into the DFB laser are shown. The peak wavelength is tuned from 974.50 to 975.40 nm when varying the current I DFB from 100 to 600 mA. A 3 dB full width of 10 pm is measured, limited by the resolution of the optical spectrum analyzer. A SMSR of about 40 dB over the whole wavelength range is obtained. The dependence of the optical peak power on the wavelength shift (see Fig. 8) is given in Fig. 9. In the wavelength range from 974.50 to 974.66 nm, the peak power rises with increasing wavelength due to the low input power. In the wavelength range from 974.66 to 975.40 nm, the output power remains constant with P P 16.3 W. Nanosecond optical pulses at a wavelength of about 975 nm were generated using an all semiconductor MOPA system. The DFB-RW laser used a MO and was operated in CW mode. The RW section of the amplifier worked as an optical gate and converted the CW input beam emitted by the MO into a train of short optical pulses, which were subsequently amplified by the TS. An optical peak power of 16.3 W with a pulse width of 8 ns is obtained. The emission wavelength could be tuned over 0.9 nm when varying the current of the DFB laser. The spectral width is smaller than 10 pm, and the SMSR is 40 dB. These parameters meet the demands of a light source suitable for the calibration of water vapor absorp-20
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Fig. 8. Optical spectra of the MOPA for different injection currents to the DFB laser for τDelay 3.5 ns, τPulse 8 ns, I PP 100 mA, I TS 18.4 A, τTS 15 ns, f 25 kHz.
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Fig. 9. Optical peak power of the MOPA in dependence on the input wavelength where the current injected into the DFB laser is varied.
tion lines under atmospheric pressure in a DIAL experiment. In the future, this system will be used in experiments for absorption spectroscopy of water vapor. Thi Nghiem Vu gratefully acknowledges the financial support by the Ministry of Education and Training of Socialist Republic of Vietnam (322_Project) and the DAAD (code no. A/10/76664). We acknowledge the continuous technical support by Jacqueline Hopp. References 1. L. Fiorani, J. Optoelectron. Adv. Mater. 1, 1 (1999). 2. M. D. Obland, K. S. Repasky, A. R. Nehrir, L. Carlsten, and J. A. Shaw, J. Appl. Remote Sens. 4, 43515 (2010). 3. A. R. Nehrir and K. S. Repasky, J. Atmos. Ocean. Technol. 28, 131 (2011). 4. A. R. Nehrir, K. S. Repasky, and J. L. Carlsten, Opt. Express 20, 25137 (2012). 5. H. Wenzel, A. Klehr, M. Braun, F. Bugge, G. Erbert, J. Fricke, A. Knauer, P. Ressel, B. Sumpf, M. Weyers, and G. Tränkle, Proc. SPIE 5595, 110 (2004). 6. B. Sumpf, A. Klehr, G. Erbert, and G. Tränkle, Appl. Phys. B 106, 357 (2012). 7. T. N. Vu, A. Klehr, B. Sumpf, H. Wenzel, G. Erbert, and G. Tränkle, Semicond. Sci. Technol. 29, 035012 (2014). 8. B. Datt, Australian Journal of botany 47, 909 (1999). 9. J. Penuelas, J. Pinol, R. Ogaya, and I. Filella, Int. J. Remote Sens. 18, 2869 (1997). 10. L. S. Rothman, I. E. Gordon, Y. Babikov, A. Barbe, D. Chris Benner, P. F. Bernath, M. Birk, L. Bizzocchi, V. Boudon, L. R. Brown, A. Campargue, K. Chance, E. A. Cohen, L. H. Coudert, V. M. Devi, B. J. Drouin, A. Fayt, J.-M. Flaud, R. R. Gamache, J. J. Harrison, J.-M. Hartmann, C. Hill, J. T. Hodges, D. Jacquemart, A. Jolly, J. Lamouroux, R. J. Le Roy, G. Li, D. A. Long, O. M. Lyulin, C. J. Mackie, S. T. Massie, S. Mikhailenko, H. S. P. Müller, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. Perevalov, A. Perrin, E. R. Polovtseva, C. Richard, M. A. H. Smith, E. Starikova, K. Sung, S. Tashkun, J. Tennyson, G. C. Toon, Vl, G. Tyuterev, and G. Wagner, J. Quantum Spectro. Rad. Trans. 130, 4 (2013). 11. H. Wenzel, J. Fricke, A. Klehr, A. Knauer, and G. Erbert, IEEE Photon. Technol. Lett. 18, 737 (2006). 12. A. Liero, A. Klehr, S. Schwertfeger, T. Hoffmann, and W. Heinrich, in IEEE MTT-S International Microwave Symposium Digest, Anaheim, California, May 25–27, 2010. 13. S. Schwertfeger, A. Klehr, T. Hoffmann, A. Liero, H. Wenzel, and G. Erbert, Appl. Phys. B 103, 603 (2011). 14. T. Ulm, F. Harth, H. Fuchs, J. A. L’huillier, and R. Wallenstein, Appl. Phys. B 92, 481 (2008).
