- Email: [email protected]
30 ps pulses source based on a fiber-coupled passively Q-switched microchip laser
Optics & Laser Technology 44 (2012) 145–147
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
30 ps pulses source based on a fiber-coupled passively Q-switched microchip laser Florent Doutre a,b, Dominique Pagnoux a,n, Vincent Couderc a, Alessandro Tonello a, Alain Jalocha b a b
Xlim Research Institute, UMR CNRS 6172, 123 Avenue A. Thomas, 87060 Limoges Cedex, France CILAS Company, 8 Avenue Buffon, BP 631, ZI La Source, 45063 Orle´ans Cedex, France
a r t i c l e i n f o
abstract
Article history: Received 10 November 2010 Received in revised form 19 May 2011 Accepted 20 June 2011 Available online 22 July 2011
We report the design of low-cost and compact short-pulse source based on a fiber-coupled Q-switched microchip laser. The combination of stimulated Raman scattering and nonlinear polarization rotation effects in the fiber associated with appropriate filtering makes it possible to tune pulse duration down to 32 ps with peak power above 3 kW. Pulse to pulse peak power fluctuation is below 4%. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Pulse shortening Non linear propagation Raman depletion
1. Introduction Powerful picosecond optical pulses are required for many applications in a number of domains such as nonlinear imaging, spectroscopy or continuum generation [1]. Among laser sources able to deliver such pulses, mode locked lasers exhibit attractive performances as they emit high peak power pulses in the femtosecond to picosecond regimes. However, they remain expensive devices, which require careful alignments of their components and are likely to suffer from a lack of robustness in operational environment. Actively or passively Q-switched microchip lasers are low cost compact sources capable of delivering pulses with duration below 200 ps. On one hand, active Q-switch operation involving lithium tantalate etalon allows emission of pulses as short as 115 ps with 80 kW peak power [2]; on the other hand, 37 ps/1.4 kW pulses from a microchip laser passively Q-switched by means of SESAM have been reported [3]. However, due to expensive electronics control for the former laser and low optical damage threshold for the latter one, these two configurations are not suitable for commercial production and the above performances are only obtained in laboratory. In addition, one can note that pulse duration is not tunable with such devices. Robust and reliable Q-switched lasers based on the use of a Cr:YAG saturable absorber have been demonstrated for a long time. The shortest reported pulses were 170 ps in duration with
n
Corresponding author. Tel.: þ33 5 55 45 72 47; fax: þ33 5 55 45 72 53. E-mail address: [email protected] (D. Pagnoux).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.06.008
17 kW peak power [2]. More generally, typical performances of commercially available microchip lasers are somewhat lower, i.e. pulse duration from 500 ps to 2 ns with a peak power of a few tens of kW. Laser pulse compression is a very efficient method for picosecond (or shorter) pulses shortening [4]. However compression of pulses longer than a few hundreds of picoseconds needs a very long compensating fiber. For example, Iwashita et al. used 104 km of negative dispersion fiber to compress 1.7 ns pulses down to 0.35 ns [5]. Compression of pulses from typical microchip lasers would need kilometers of fiber or long sized gratings, which prevent from compressing such pulses in a compact device. In our previous works, we showed that nonlinear polarization rotation (NPR) in a short piece (5 m) of single mode lowbirefringence fiber can be exploited to shorten 600 ps pulses by a factor 10, taking advantage of coherent coupling between polarizations [6]. More recently, we took advantage of stimulated Raman scattering (SRS) arising with peak power above 3 kW to improve and stabilize the shortening process [7]. As it will be reminded in Section 2, this improved shortening technique is based on a combination of the following two phenomena: (i) a deep depletion at the operating wavelength of the central part of the pulse to be shortened due to SRS, and (ii) exacerbated NPR, which allows to minimize low intensities by polarization filtering. The process is described in details in [7], supporting both numerical simulations and experimental validation. In this paper, we propose a simple efficient design of a short pulse source using the above mentioned technique. This source is based on a commercially available fiber-coupled Nd:YAG passively Q-switched microchip laser. We first describe the
146
F. Doutre et al. / Optics & Laser Technology 44 (2012) 145–147
experimental setup built to implement the method. Then, we report and discuss the temporal and spectral characteristics of the highly shortened pulses obtained with this device.
2. Experimental setup The experimental setup is shown in Fig. 1. The light source is a commercially available fiber-coupled Nd:YAG microchip laser from Horus Laser company. Passive Q-switching is performed by a Cr:YAG crystal and emitted linearly polarized pulses at l ¼1064 nm, which are 500 ps long. The average emitted power is controlled through the current of the CW 808 nm diode pumping the Nd:YAG crystal. This control allows managing the laser repetition rate, the peak power of the generated pulses remaining quasi-constant for repetition rates from few kilohertz to 20 kHz. The nominal repetition rate is set to 16 kHz. The laser output is directly connected to a single mode low birefringence fiber (HI 980 fiber from Corning, cutoff wavelength¼980 nm, beat length 1 m, zero dispersion wavelength¼ 1.3 mm). As we plan to use a shortening method taking advantage of polarization effects, an in-line polarization controller (General Photonics squeeze and twist PLC-900 device) is set a few centimeters far from the input end of the fiber in order to tune the input polarization to any state. The measured mean power at fiber output is 85 mW, corresponding to pulse peak power of 10.5 kW. In order to generate only the first order Stokes line, the fiber length is progressively reduced down to 80 cm. The spectrum at the fiber output is presented in Fig. 2. The output end of the fiber is connected to an optical microbench in which the output beam is collimated prior to spectral and polarization filtering. A bandpass filter centered at 1064 nm, with 3 dB attenuation at this wavelength is set at the fiber output in order to block the Stokes wavelength of SRS
Polarisation controller
Microchip laser
(E1118 nm), which corresponds to the powerful part of the pulse above the Raman threshold. The central part of the pulse is then completely depleted at 1064 nm, and the transmitted lateral parts constitute a set of two pulses, which exhibit different amplitudes and asymmetric shapes. Because of these differences of level, both pulses at 1064 nm have experienced different NPR in the fiber. Therefore, filtering by a properly oriented polarizer (extinction ratio ¼30 dB, attenuation of the transmitted polarization o0.1 dB) makes it possible to select only one pulse with very short duration. Due to the reduced number of elements in the setup and due to their small size, the overall source volume is less than 1 l, making it suitable for portable applications.
3. Experimental results and discussion The resulting pulse is simultaneously analyzed in the temporal and spectral domains. Pulses temporal profile is obtained using a high-speed optical sampling module (Tektronix 80C04) in a sequential 20 GHz oscilloscope (Tektronix CSA8000). Spectrum is acquired using an optical spectrum analyzer (OSA Advantest Q8384 resolution 10 pm). The input polarization controller and the output analyzer are tuned in order to obtain single short pulses at the output with
Bandpass filter 1064 nm +/- 3nm
Nonlinear Fiber L= 80 cm
Output Glan polariser
Fig. 1. Schematics of the experimental setup.
Fig. 2. Spectrum at the output end of a 80 cm long fiber.
Fig. 3. Examples of shortened pulses (a) 84 ps duration (b) 32 ps duration. Initial pulse profile at fiber output is shown in dashed line. Inset: spectrum of the 32 ps pulse.
F. Doutre et al. / Optics & Laser Technology 44 (2012) 145–147
147
peak to peak fluctuations remain below 4%. Let us note that, as the high-speed detector used for the measurements is very sensitive, the fraction of the output pulse sent to this detector must be very small. In these conditions, the signal to noise ratio is evaluated to be smaller than 15 dB making the above value of the fluctuations somewhat overestimated.
4. Conclusion
Fig. 4. Upper and lower envelops of the pulse over a 1 h acquisition time. Inset: zoom on the peak power envelops.
tunable durations. As the technique requires the use of a low birefringence fiber neutral axis orientation is difficult to evaluate, and therefore polarization orientation angle is meaningless since it cannot reefer to a fiber axis. However, we verified that the given orientations of the polarizers provide repeatable results, i.e. the temporal shape of the emitted pulse is always the same for a given set of orientations. Examples of experimental shortened pulses emitted by the device are plotted in Fig. 3. The shortest pulse duration, which has been obtained is 32 ps FWHM (depicted in Fig. 3b solid black line), corresponding to a reduction factor higher than 15. To our best knowledge, this is the shortest pulse ever obtained by means of a source based on a Q-switched laser. We must notice that some energy remains in the pulse pedestals. However, more than 64% of the overall detected energy is contained in the 32 ps pulse. The mean power measured at the setup output is 2.7 mW, which corresponds to a loss of about 15 dB in the filtering process. This somewhat high loss is due to the shortening method, which plays a role in drastically carving the initial pulse. Taking into account the emitted pulse duration, the repetition rate and the part of the energy located in the pedestals, the output pulse peak power is about 3.3 kW. The decrease of the peak power due to the shaping device is thus only 5 dB, including the 3 dB contribution of the 1064 nm interferential filter. The spectrum of the 32 ps shortened pulse is shown in the inset of Fig. 3. It exhibits 340 pm FWHM, giving a time bandwidth product DtDn 2.9, far higher than the value obtained for transform limited pulses. The spectrum profile with two maxima is very similar to that of pulses which undergo self phase modulation. This shape suggests that the pulse suffers from a significant chirp. Therefore, temporal compression could be operated in order to reach additional duration reduction. Shortened pulses temporal shape stability over a long time has also been studied. The upper and lower envelops of the pulses, recorded over 1 h are displayed in Fig. 4. This figure shows that
We realized a compact picosecond laser source using a microchip oscillator fibered with a standard Corning HI 980 optical fiber. The 500 ps pulses emitted from a bulk Nd:YAG laser are shortened down to 32 ps by means of an interplay between SRS and NPR processes occurring in normal dispersion regime (l ¼1064 nm). The output pulse has a peak power of 3.3 kW with peak to peak fluctuations lower than 4%. The output pulses duration is tunable, offering additional potentiality to this pulse source. The overall volume of the source is less than 1 l, making it attractive for portable applications. Improvements of the device are under study. First, the timebandwidth product of 32 ps pulse being high (close to 2.9), we plan to perform pulse compression and pulses as short as 11 ps are expected. Thus, the source should be able to cover the picosecond range with kilohertz repetition rate. In addition, because of the moderate repetition rate, the amplification of output pulses with reasonable mean power can also be realized. We performed preliminary experiments suggesting that MW peak power is achievable. This system could advantageously replace mode-locked laser source coupled with regenerative amplifier producing megawatt picoseconds pulses.
Acknowledgements The authors thank the French ANRT (National Association for Technical Research) for its financial support to Florent Doutre via a convention with the CILAS Company. References [1] Okuno M, Kano H, Leproux P, Couderc V, Hamaguchi H. Ultrabroadband multiplex CARS microspectroscopy and imaging using a subnanosecond supercontinuum light source in the deep near infrared. Opt Lett 2008;33: 923–5. [2] Zayhowski JJ. Microchip lasers. Opt Mater 1999;11:255–67. ¨ [3] Spuhler GJ, Paschotta R, Fluck R, Braun B, Moser M, Zhang G, Gini E, Keller U. Experimentally confirmed design guidelines for passively Q-switched microchip lasers using semiconductor saturable absorbers. J Opt Soc Am B 1999;16: 376–88. [4] Alfano RR. The supercontinuum laser source. 2nd ed. Springer; 2006. [Chapter 10]. [5] Iwashita K, Nakagawa K, Nakano Y, Suzuki Y. Chirp pulse transmission through a singlemode fiber. Electron Lett 1982;18:873–4. [6] Doutre F, Pagnoux D, Couderc V, Tonello A, Vergne B, Jalocha A. Large temporal narrowing of subnanosecond pulses in a low-birefringence optical fiber. Opt Lett 2008;33:1789–91. [7] Doutre F, Pagnoux D, Couderc V, Tonello A, Jalocha A. Shortening pulses from subnanosecond to picosecond by means of ultrafast temporal filtering in an optical fiber. Opt Lett 2009;34:2087–9.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
30 ps pulses source based on a fiber-coupled passively Q-switched microchip laser Florent Doutre a,b, Dominique Pagnoux a,n, Vincent Couderc a, Alessandro Tonello a, Alain Jalocha b a b
Xlim Research Institute, UMR CNRS 6172, 123 Avenue A. Thomas, 87060 Limoges Cedex, France CILAS Company, 8 Avenue Buffon, BP 631, ZI La Source, 45063 Orle´ans Cedex, France
a r t i c l e i n f o
abstract
Article history: Received 10 November 2010 Received in revised form 19 May 2011 Accepted 20 June 2011 Available online 22 July 2011
We report the design of low-cost and compact short-pulse source based on a fiber-coupled Q-switched microchip laser. The combination of stimulated Raman scattering and nonlinear polarization rotation effects in the fiber associated with appropriate filtering makes it possible to tune pulse duration down to 32 ps with peak power above 3 kW. Pulse to pulse peak power fluctuation is below 4%. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Pulse shortening Non linear propagation Raman depletion
1. Introduction Powerful picosecond optical pulses are required for many applications in a number of domains such as nonlinear imaging, spectroscopy or continuum generation [1]. Among laser sources able to deliver such pulses, mode locked lasers exhibit attractive performances as they emit high peak power pulses in the femtosecond to picosecond regimes. However, they remain expensive devices, which require careful alignments of their components and are likely to suffer from a lack of robustness in operational environment. Actively or passively Q-switched microchip lasers are low cost compact sources capable of delivering pulses with duration below 200 ps. On one hand, active Q-switch operation involving lithium tantalate etalon allows emission of pulses as short as 115 ps with 80 kW peak power [2]; on the other hand, 37 ps/1.4 kW pulses from a microchip laser passively Q-switched by means of SESAM have been reported [3]. However, due to expensive electronics control for the former laser and low optical damage threshold for the latter one, these two configurations are not suitable for commercial production and the above performances are only obtained in laboratory. In addition, one can note that pulse duration is not tunable with such devices. Robust and reliable Q-switched lasers based on the use of a Cr:YAG saturable absorber have been demonstrated for a long time. The shortest reported pulses were 170 ps in duration with
n
Corresponding author. Tel.: þ33 5 55 45 72 47; fax: þ33 5 55 45 72 53. E-mail address: [email protected] (D. Pagnoux).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.06.008
17 kW peak power [2]. More generally, typical performances of commercially available microchip lasers are somewhat lower, i.e. pulse duration from 500 ps to 2 ns with a peak power of a few tens of kW. Laser pulse compression is a very efficient method for picosecond (or shorter) pulses shortening [4]. However compression of pulses longer than a few hundreds of picoseconds needs a very long compensating fiber. For example, Iwashita et al. used 104 km of negative dispersion fiber to compress 1.7 ns pulses down to 0.35 ns [5]. Compression of pulses from typical microchip lasers would need kilometers of fiber or long sized gratings, which prevent from compressing such pulses in a compact device. In our previous works, we showed that nonlinear polarization rotation (NPR) in a short piece (5 m) of single mode lowbirefringence fiber can be exploited to shorten 600 ps pulses by a factor 10, taking advantage of coherent coupling between polarizations [6]. More recently, we took advantage of stimulated Raman scattering (SRS) arising with peak power above 3 kW to improve and stabilize the shortening process [7]. As it will be reminded in Section 2, this improved shortening technique is based on a combination of the following two phenomena: (i) a deep depletion at the operating wavelength of the central part of the pulse to be shortened due to SRS, and (ii) exacerbated NPR, which allows to minimize low intensities by polarization filtering. The process is described in details in [7], supporting both numerical simulations and experimental validation. In this paper, we propose a simple efficient design of a short pulse source using the above mentioned technique. This source is based on a commercially available fiber-coupled Nd:YAG passively Q-switched microchip laser. We first describe the
146
F. Doutre et al. / Optics & Laser Technology 44 (2012) 145–147
experimental setup built to implement the method. Then, we report and discuss the temporal and spectral characteristics of the highly shortened pulses obtained with this device.
2. Experimental setup The experimental setup is shown in Fig. 1. The light source is a commercially available fiber-coupled Nd:YAG microchip laser from Horus Laser company. Passive Q-switching is performed by a Cr:YAG crystal and emitted linearly polarized pulses at l ¼1064 nm, which are 500 ps long. The average emitted power is controlled through the current of the CW 808 nm diode pumping the Nd:YAG crystal. This control allows managing the laser repetition rate, the peak power of the generated pulses remaining quasi-constant for repetition rates from few kilohertz to 20 kHz. The nominal repetition rate is set to 16 kHz. The laser output is directly connected to a single mode low birefringence fiber (HI 980 fiber from Corning, cutoff wavelength¼980 nm, beat length 1 m, zero dispersion wavelength¼ 1.3 mm). As we plan to use a shortening method taking advantage of polarization effects, an in-line polarization controller (General Photonics squeeze and twist PLC-900 device) is set a few centimeters far from the input end of the fiber in order to tune the input polarization to any state. The measured mean power at fiber output is 85 mW, corresponding to pulse peak power of 10.5 kW. In order to generate only the first order Stokes line, the fiber length is progressively reduced down to 80 cm. The spectrum at the fiber output is presented in Fig. 2. The output end of the fiber is connected to an optical microbench in which the output beam is collimated prior to spectral and polarization filtering. A bandpass filter centered at 1064 nm, with 3 dB attenuation at this wavelength is set at the fiber output in order to block the Stokes wavelength of SRS
Polarisation controller
Microchip laser
(E1118 nm), which corresponds to the powerful part of the pulse above the Raman threshold. The central part of the pulse is then completely depleted at 1064 nm, and the transmitted lateral parts constitute a set of two pulses, which exhibit different amplitudes and asymmetric shapes. Because of these differences of level, both pulses at 1064 nm have experienced different NPR in the fiber. Therefore, filtering by a properly oriented polarizer (extinction ratio ¼30 dB, attenuation of the transmitted polarization o0.1 dB) makes it possible to select only one pulse with very short duration. Due to the reduced number of elements in the setup and due to their small size, the overall source volume is less than 1 l, making it suitable for portable applications.
3. Experimental results and discussion The resulting pulse is simultaneously analyzed in the temporal and spectral domains. Pulses temporal profile is obtained using a high-speed optical sampling module (Tektronix 80C04) in a sequential 20 GHz oscilloscope (Tektronix CSA8000). Spectrum is acquired using an optical spectrum analyzer (OSA Advantest Q8384 resolution 10 pm). The input polarization controller and the output analyzer are tuned in order to obtain single short pulses at the output with
Bandpass filter 1064 nm +/- 3nm
Nonlinear Fiber L= 80 cm
Output Glan polariser
Fig. 1. Schematics of the experimental setup.
Fig. 2. Spectrum at the output end of a 80 cm long fiber.
Fig. 3. Examples of shortened pulses (a) 84 ps duration (b) 32 ps duration. Initial pulse profile at fiber output is shown in dashed line. Inset: spectrum of the 32 ps pulse.
F. Doutre et al. / Optics & Laser Technology 44 (2012) 145–147
147
peak to peak fluctuations remain below 4%. Let us note that, as the high-speed detector used for the measurements is very sensitive, the fraction of the output pulse sent to this detector must be very small. In these conditions, the signal to noise ratio is evaluated to be smaller than 15 dB making the above value of the fluctuations somewhat overestimated.
4. Conclusion
Fig. 4. Upper and lower envelops of the pulse over a 1 h acquisition time. Inset: zoom on the peak power envelops.
tunable durations. As the technique requires the use of a low birefringence fiber neutral axis orientation is difficult to evaluate, and therefore polarization orientation angle is meaningless since it cannot reefer to a fiber axis. However, we verified that the given orientations of the polarizers provide repeatable results, i.e. the temporal shape of the emitted pulse is always the same for a given set of orientations. Examples of experimental shortened pulses emitted by the device are plotted in Fig. 3. The shortest pulse duration, which has been obtained is 32 ps FWHM (depicted in Fig. 3b solid black line), corresponding to a reduction factor higher than 15. To our best knowledge, this is the shortest pulse ever obtained by means of a source based on a Q-switched laser. We must notice that some energy remains in the pulse pedestals. However, more than 64% of the overall detected energy is contained in the 32 ps pulse. The mean power measured at the setup output is 2.7 mW, which corresponds to a loss of about 15 dB in the filtering process. This somewhat high loss is due to the shortening method, which plays a role in drastically carving the initial pulse. Taking into account the emitted pulse duration, the repetition rate and the part of the energy located in the pedestals, the output pulse peak power is about 3.3 kW. The decrease of the peak power due to the shaping device is thus only 5 dB, including the 3 dB contribution of the 1064 nm interferential filter. The spectrum of the 32 ps shortened pulse is shown in the inset of Fig. 3. It exhibits 340 pm FWHM, giving a time bandwidth product DtDn 2.9, far higher than the value obtained for transform limited pulses. The spectrum profile with two maxima is very similar to that of pulses which undergo self phase modulation. This shape suggests that the pulse suffers from a significant chirp. Therefore, temporal compression could be operated in order to reach additional duration reduction. Shortened pulses temporal shape stability over a long time has also been studied. The upper and lower envelops of the pulses, recorded over 1 h are displayed in Fig. 4. This figure shows that
We realized a compact picosecond laser source using a microchip oscillator fibered with a standard Corning HI 980 optical fiber. The 500 ps pulses emitted from a bulk Nd:YAG laser are shortened down to 32 ps by means of an interplay between SRS and NPR processes occurring in normal dispersion regime (l ¼1064 nm). The output pulse has a peak power of 3.3 kW with peak to peak fluctuations lower than 4%. The output pulses duration is tunable, offering additional potentiality to this pulse source. The overall volume of the source is less than 1 l, making it attractive for portable applications. Improvements of the device are under study. First, the timebandwidth product of 32 ps pulse being high (close to 2.9), we plan to perform pulse compression and pulses as short as 11 ps are expected. Thus, the source should be able to cover the picosecond range with kilohertz repetition rate. In addition, because of the moderate repetition rate, the amplification of output pulses with reasonable mean power can also be realized. We performed preliminary experiments suggesting that MW peak power is achievable. This system could advantageously replace mode-locked laser source coupled with regenerative amplifier producing megawatt picoseconds pulses.
Acknowledgements The authors thank the French ANRT (National Association for Technical Research) for its financial support to Florent Doutre via a convention with the CILAS Company. References [1] Okuno M, Kano H, Leproux P, Couderc V, Hamaguchi H. Ultrabroadband multiplex CARS microspectroscopy and imaging using a subnanosecond supercontinuum light source in the deep near infrared. Opt Lett 2008;33: 923–5. [2] Zayhowski JJ. Microchip lasers. Opt Mater 1999;11:255–67. ¨ [3] Spuhler GJ, Paschotta R, Fluck R, Braun B, Moser M, Zhang G, Gini E, Keller U. Experimentally confirmed design guidelines for passively Q-switched microchip lasers using semiconductor saturable absorbers. J Opt Soc Am B 1999;16: 376–88. [4] Alfano RR. The supercontinuum laser source. 2nd ed. Springer; 2006. [Chapter 10]. [5] Iwashita K, Nakagawa K, Nakano Y, Suzuki Y. Chirp pulse transmission through a singlemode fiber. Electron Lett 1982;18:873–4. [6] Doutre F, Pagnoux D, Couderc V, Tonello A, Vergne B, Jalocha A. Large temporal narrowing of subnanosecond pulses in a low-birefringence optical fiber. Opt Lett 2008;33:1789–91. [7] Doutre F, Pagnoux D, Couderc V, Tonello A, Jalocha A. Shortening pulses from subnanosecond to picosecond by means of ultrafast temporal filtering in an optical fiber. Opt Lett 2009;34:2087–9.