Dual-wavelength passively q-switched single - OSA Publishing

Vol. 24, No. 14 | 11 Jul 2016 | OPTICS EXPRESS 16149

Dual-wavelength passively q-switched singlefrequency fiber laser YUANFEI ZHANG,1, 2 CHANGSHENG YANG,1 ZHOUMING FENG,1 HUAQIU DENG,4 MINGYING PENG,1 ZHONGMIN YANG,1, 3, 4 AND SHANHUI XU1, 3, 4, * 1

State Key Laboratory of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China 2 School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510640, China 3 Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, Guangzhou 510640, China 4 Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510640, China *[email protected]

Abstract: We propose a compact dual-wavelength Q-switched single-frequency fiber laser based on a 17-mm-long home-made highly Er3+/Yb3+ co-doped phosphate fiber (EYDPF) and a semiconductor saturable absorber mirror (SESAM). The short cavity length and a polarization-maintaining fiber Bragg grating (PM-FBG) ensure that only one longitudinal mode is supported by each reflection peak. The maximum pulse energy of more than 34.5 nJ was realized with the shortest pulse duration of 110.5 ns and the Q-switched fiber laser has a repetition rate reaching over 700 kHz with a temporal synchronization of pulses at two wavelengths. Besides, the optical signal-to-noise ratio (OSNR) of larger than 64.5 dB was achieved. © 2016 Optical Society of America OCIS codes: (140.3540) Lasers, Q-switched; (140.3570) Lasers, single-mode; (140.3510) Lasers, fiber.

References and Links 1.

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#266542 Journal © 2016

http://dx.doi.org/10.1364/OE.24.016149 Received 19 May 2016; revised 2 Jul 2016; accepted 4 Jul 2016; published 8 Jul 2016 Corrected: 3 August 2016


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1. Introduction Airborne differential Lidar system has been proven as a reliable and accurate apparatus to capture rupture of earthquake [1]. To implement such a system, a dual-wavelength (DW) laser source with single-frequency operation and pulsing mode would be more robust as considering long coherent length and significant attenuation while through the detection process [2,3]. Besides, a compact size would make it ideal for integration in airborne system, where constraints on weight and bulk are often extreme. However, to ensure a stable operation of this type of fiber laser, there are several issues need to be carefully addressed. As to pulsing operation, a straightforward method is to externally modulate continuous-wave laser by acoustic-optical modulation (AOM) or electricoptical modulation (EOM) [4–6]. Despite the merit of easily controllable to pulsing parameters, this method suffers from a significant attenuation to laser signal and several amplified stages are required for application. Q-switching is a suitable apparatus to obtain considerable output power and high-beam-quality laser, and it has attracted the research attention in the past two decades [7–11]. Compared to actively Q-switched fiber lasers which require an extra controller to generate modulating signal, passively Q-switched fiber lasers possess attractive advantages of compactness, simplicity, and flexibility in design [10, 11]. Secondly, a fundamental problems that haunts DW lasers is mode competition, which tends to destabilize the laser system. To overcome it, one of approach is to utilize polarization maintaining (PM) components, and lasers utilizing polarization-maintaining fiber Bragg gratings (PM-FBG) or PM gain fibers are believed to be stable for the divergence induced by the polarization hole burning effect [12, 13]. Besides, the phenomenon of multiplelongitudinal-mode oscillation would be caused by long cavity length for narrow longitudinal mode spacing [10, 11]. Thus, a mode-selecting mechanism should be employed to eliminate multiple longitudinal mode oscillation [14], but the integration of the laser turns out to be even more difficult. A laser source with a short cavity length corresponding to a large longitudinal mode spacing could ensure a stably single-longitudinal-mode (SLM) operation. The following problem is that the output power is limited with a shorter gain fiber in the cavity. Thanks to the development of heavily rare earth (RE) ions doped multi-component glass fiber, the high gain phosphate fiber has brought about high power fiber lasers with very short cavities [15–19]. In this letter, we present a compact passively Q-switched DW-SLM fiber laser based on a self-developed 17-mm-long phosphate fiber and a semiconductor saturable


absorber mirror (SESAM). A reliable DW-SLM pulsed fiber laser is achieved while the maximum pulse energy of more than 34.5 nJ was realized with the shortest pulse duration of 110.5 ns, and the wavelength spacing of the obtained dual lasing line is 0.37 nm. 2. Experimental setup The schematic drawing of DW pulsed fiber laser is shown in Fig. 1. The two reflectors of laser cavity are composed of one PM-FBG and a SESAM. The PM-FBG as output coupler of the cavity was inscribed with a 3-dB bandwidth of 0.05 nm and a reflectivity of 50% at 1540 nm. A home-made 17-mm-long Er3+/Yb3+ co-doped phosphate fiber (EYDPF) were fusionspliced to the PM-FBG, and this home-made gain fiber was drawn using a fiber-drawing tower through a phosphate glass preform fabricated by the rod-in-tube technique, more detail about this home-made phosphate fiber was discussed in [17]. The SESAM, which has an absorbance of 4% at around 1540 nm and a modulation depth of 2.4%, is butt-coupled to the other end facet of EYDPF. The total resonant cavity was assembled into a copper tube, which is thermally controlled by a precision thermoelectric cooler with a resolution of 0.01°C. With a proper temperature control, a robustly single-frequency laser could be obtained without mode-hop and mode-competition phenomena. The DW-SLM fiber laser was pumped by a commercially available single-mode 980-nm laser diode (LD) through a 980/1550-nm wavelength division multiplexer (WDM). The WDM output was passed through an isolator (ISO) to prevent back reflections, which destabilized the fiber laser. The deliver fiber and optical device are polarization-maintaining components (both axis working) to maintain good stability of output laser.

Fig. 1. Schematic drawing of DW passively Q-switched single-frequency fiber laser.

3. Results and discussion Since the different refractive index along the fast and slow axis of the fiber, the PM-FBG has two reflection peaks, one for each polarization. And the two reflection peaks are within the reflection bands of the well-design SESAM. The reflective spectrum of PM-FBG pumped by an amplitude spontaneous emission (ASE) light source in 1.5-μm region is shown in Fig. 2(a). It can be observed that the wavelength spacing of two reflective peaks of slow and fast axis is 0.37 nm. As shown in Fig. 2(b), the spectrum of DW fiber laser was measured by an optical spectrum analyzer (OSA) with the span of 5 nm and a resolution of 0.02 nm. The two lasing wavelengths are 1540.25 nm and 1540.62 nm respectively corresponding to the two reflection peak wavelengths of the PM-FBG. Besides, the optical signal-to-noise ratio (OSNR) of > 64.5 dB was obtained. The effective length of the cavity including 17-mm active fiber and a half of the 15-mm PM-FBG is only less than 25 mm, giving a longitudinal mode spacing of 4.0 GHz.


Besides, the PM-FBG has a reflection bandwidth of less than 6.3 GHz, and it is clear that only one longitudinal mode is supported within the laser cavity for each reflection peak. The short cavity length and the PM-FBG ensure that only one longitudinal mode is supported by each reflection peak. Besides, the spectrums for each polarization were measured by using a polarization beam splitter (PBS) and OSA with a span of 4 nm in Fig. 2(c) and Fig. 2(d). It can be observed that only one lasing wavelength for each polarization. In addition, the singlefrequency characteristics for each lasing wavelength were measured by a scanning Fabry– Pérot interferometer (FPI) with a resolution of 7.5 MHz and a free spectral range of 1.5 GHz, and the SLM operations were confirmed in the inset of Fig. 2(c) and Fig. 2(d).

Fig. 2. (a) Reflective spectrum of the PM-FBG pumped by an ASE light source. (b) Spectrum of output pulse with a span of 5 nm. Laser spectrums at 1540.25 nm (c) and 1540.62 nm (d) with a span of 4 nm. Inset: Single-frequency performance of the laser at 1540.25 nm (c) and 1540.62 nm (d).

Figure 3(a) demonstrates pulse duration and repetition rate as a function of pump power. It can be observed that the pulse duration was narrowed from 387 to 110.5 ns by increasing pump power. For passively Q-switched laser, pulse duration is related to cavity length and modulation depth of saturable absorber (SA) [20]. We would expect to reduce pulse duration by using an SA with higher modulation depth because pulse duration is expected to be inversely proportional to the modulation depth of the SA [21]. By increasing the pump power from 95.8 to 216.7 mW, the pulsing repetition rate could be varied over a wide range of frequencies from 216 to 728 kHz. The average power and pulse energy versus different pump powers are shown in Fig. 3(b). And the lasing threshold of laser pulse is around 95.8 mW as indicated in Fig. 3(b). When the pump power is above lasing threshold, the average power shows a trend of linear enhancement with increasing of pump power. And a maximum average power of more than 25.1 mW was obtained at the pump power of 216.7 mW. The single pulse energy, which could be calculated utilizing the measured average power and repetition rate, enhances linearly as the pump power increases. It is observed that the maximum pulse energy reaches 34.5 nJ at the pump power of 216.7 mW, and the calculated peak power is 0.29 W. There are some parameters that influence the peak power of passively Q-switched laser, as shown in the formula in [20],


Ppeak  S p

Ep

p



 R 

2

TR L

A

oc .

(1)

In Eq. (1), Ppeak is the peak power, Sp is the pulse shape factor, Ep is the pulse energy, τp is the pulse duration, ΔR is the modulation depth, A is the pumped area, TR is the round trip time of cavity, σL is emission cross section of the laser material, and ηoc is output coupling efficiency. To increase peak power, a large modulation depth is obviously important. Besides, a smaller round trip time and higher output coupling efficiency could also realize higher peak power. We expected that a higher peak power could be realized by further optimizing the cavity parameters (e.g. SESAM’s performance, cavity loss).

Fig. 3. (a) Pulse duration and repetition rate of the output laser corresponding to different pump powers. (b) Average power and pulse energy of the output laser versus pump power.

The pulsing shapes of the passively Q-switched laser with different pump powers are shown in Fig. 4(a). It can be observed that the single-pulse envelopes have symmetric Gaussian-like intensity profiles and no parasitic or multiple optical pulse exits. The narrowest pulse width of 110.5 ns was achieved with a pump power of 216.7 mW. As shown in Fig. 4(b), recorded traces of Q-switched pulse trains were measured by an InGaAs photodetector and a 100 MHz bandwidth oscilloscope. The stably pulsing trains with the different repetition rates were observed when the pump power was gradually increased from 132.3 to 216.7 mW. A simple express about the repetition rate is obtained for passively Q-switched laser in [20], f rep 

s  Pp  Pp ,th  Ep

 r  1.

(2)

frep is the repetition rate, ηs is the slope efficiency, Pp is the pump power, Pp,th is the threshold pump power, and Ep is the pulse energy. Here r = Pp / Pp,th is the pump parameter. The threshold pump power is constant for a particular cavity, the pulsing repetition rate of pulse would increase as the enhancement of pump power, and it is the typical feature of passively Q-switched laser. While increasing the pump power further over 216.7 mW, the Q-switched pulsing trains become unstable for a strong amplitude variation and timing jitter appearing. It is because the SESAM would degraded over time at higher powers, and thus deteriorate performance of laser, and using a SESAM with higher damage threshold could improve the stability of the Q-switched pulse at higher pump power. The temporal synchronization of pulse trains of two lasing wavelengths is shown in Fig. 5 with a pump power of 216.7 mW, and there is no obvious temporal delay between two wavelengths. It indicates that the two lasing wavelengths can be temporally synchronized in Q-switched process since the modulated parameters of SESAM are designed to have almost same values at two lasing wavelengths.


Fig. 4. (a) Pulsing shapes of the output laser with pump powers varying from 132.3 mW to 216.7 mW. (b) Traces of pulsing trains of the passively Q-switched fiber laser under different pump powers.

Fig. 5. Traces of pulsing trains of one single wavelength at 1540.25 nm, another single wavelength at 1540.62 nm with a pump power of 216.7 mW.

4. Conclusion In conclusion, by utilizing a 17-mm-long EYDPF, a compact structure cavity with a SESAM and a PM-FBG has been presented to achieve a DW pulsed fiber laser in SLM operation. The achieved OSNR of output pulse was larger than 64.5 dB while the two lasing wavelengths are


located at 1540.25 nm and 1540.62 nm. And the Q-switched pulses at two wavelengths can be temporally synchronized by using the well-designed SESAM. Moreover, the maximum pulse energy of more than 34.5 nJ was realized with the shortest pulse duration of 110.5 ns and the pulsing repetition rates ranged from 216 to 728 kHz by tuning the pump power. This type of compact DW pulsed fiber laser provides a promising candidate in airborne differential LIDAR system. 5. Funding China State 863 Hi-tech Program (2014AA041902); Natural National Science Foundation of China (NSFC) (61535014,51132004, and 51302086); Fundamental Research Funds for Central Universities (2015ZP013 and 2015ZM091); Guangdong Natural Science Foundation (S2011030001349 and S20120011380); China National Funds for Distinguished Young Scientists (61325024); Science and Technology Project of Guangdong (2013B090500028, 2014B050505007); The cross and cooperative science and technology innovation team project of the CAS (2012-119), China.