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A largely simplified and highly efficient all-fiber-based synchronously pumping scheme is proposed. The synchronization between pump light and the cavity round-trip can be achieved by adjusting the repetition rate of pumping light without the requirement of altering the cavity length. Based on this scheme, we achieved generating narrow linewidth highly efficient 1120 nm pulse directly from an all-fiber Raman cavity. By pump repetition rate detuning and pump duration adjustment, the duration of the 1120 nm pulse can be widely tuned from 18 ps to ~1 ns, and the repetition rate can be adjusted from 12.41 MHz to 99.28 MHz by harmonic pumping. Up to 4.3 W high power operation is verified based on this scheme. Owing to the compact all-fiber configuration, the conversion efficiency of the 1066 nm pump light to the 1120 nm Stokes light exceeds 80% and the overall conversion efficiency (976 nm-1066 nm-1120 nm) is as high as 53.7%. The nonlinear output dynamics of the Raman laser are comprehensively explored. Two distinct operation regimes are investigated and characterized.
© 2015 Optical Society of America
1. Introduction
Utilizing the nonlinear process of stimulated Raman scattering (SRS) in optical fibers to access new wavelengths has been a widely investigated technique, and it has been employed in a variety of configurations to generate versatile output of various wavelength and temporal characteristics. Benefiting from the strong confinement and the long interaction length of optical fibers, SRS-induced Stokes light can be generated efficiently with just a single pass through a section of optical fiber. However, placing a resonating cavity around the Raman gain fiber to resonate the Stokes wavelength(s) can further reduce conversion threshold, increase the conversion efficiency and control the cascading of the SRS process. The conversion efficiency of more than 85% had been demonstrated in a continuous wave (CW) Raman laser [1]. However, for the pulsed lasers with duration of picosecond to nanosecond, which are shorter than the transit time through the Raman fiber, to achieve efficient pulsed Stokes light conversion within a Raman resonator, the resonator must be synchronously pumped with the round-trip of the resonator matched to the inter-pulse period of the pump laser. There were plenty of works featuring CW-pumped mode locked Raman fiber laser, which usually exhibit a low conversion efficiency, mainly caused by the ultra-short excited state lifetime of Raman effect.
Synchronously pumping had been widely used in the practice of nonlinear frequency conversion in the pulsed regime to raise the conversion efficiency, especially the optical parametric oscillator [2–4 ] and the pulsed Raman laser [5–13 ]. In both cases, the commonly employed synchronously pumping scheme is pumping a cavity-length-adjustable resonator with a mode-locked pump laser. There were plenty of works employing synchronously pumping to generate pulsed Raman laser in fiber cavity [5–7,9,11,13 ]. Among those works, there are three commonly used techniques to tune the cavity length to achieve synchronously pumping, space optical path adjustment [5–7 ], piezoelectric-ceramics-based fiber stretcher [9], and fiber coupled optical delay line [11]. However, space optical path adjustment requires complex and precise cavity mirror arrangement and bulk-to-fiber coupling, which largely reduces the conversion efficiency. And the adjustment range of the fiber coupled optical delay line and the piezoelectric ceramics-based fiber stretcher are usually limited, and the cavity fiber length still requires precisely cropping to achieve the match between the round-trip of the cavity and the inter-pulse period of the pump laser.
Here, we propose a largely simplified all-fiber-integrated scheme of synchronously pumped short-pulse Raman fiber laser. In this scheme, the pulsed pump laser comes from a rare-earth–doped fiber amplifier, which is seeded by an electrically controlled gain-switched laser diode. Based on the commonly available pulse electrical driver, the temporal specifications, like pulse repetition rate and pulse duration, of the pulsed pump laser are adjustable in large ranges. Thus, the synchronization between the pumping light and the cavity round-trip can be achieved by adjusting the repetition rate of the pumping light without the need of altering the cavity length. This electrically controlled pumping scheme has the particular advantage of easy-to-synchronization, which provides the facility to further synchronously pumping amplification. This synchronous pumping scheme can be adopted in many other applications, such as all-fiberized pulse-pumped optical parametric oscillator. Besides, the previously-published lasers were all pumped with ultrafast mode locked pulses with relatively wide spectrum bandwidth [5–7,9,11,13 ]. By contrast, in our scheme, the laser is pumped with modulated laser diode with longer pulse durations and much narrower linewidth. The difference in the duration and the spectrum of the pump laser may make some new observation, which is different than the prior lasers.
Based on this scheme, we achieved generating narrow linewidth highly efficient 1120 nm pulse directly from all-fiber-based Raman cavity. The duration of the 1120 nm pulse can be widely tuned from 18 ps to sub-ns, and the repetition rate can be adjusted from 12.41 MHz to 99.28 MHz by harmonic pumping. Up to 4.3 W high power operation is verified based on this scheme. To the best of our knowledge, it is the most powerful 1120 nm picosecond narrow linewidth Raman pulse directly generated from an all-fiberized Raman cavity. Owing to the compact all-fiber configuration, the conversion efficiency of the 1066 nm pump light to the 1120 nm Stokes light exceeds 80% and the overall conversion efficiency (976 nm-1066 nm-1120 nm) is as high as 54%. Its flexible output dynamics, narrow linewidth, and widely scalable pulse energy make the laser applicable to a wide range of applications. Particularly, different from the ultrashort broadband dissipative soliton presented in [11], the narrow linewidth property makes the laser a suitable source to be efficiently frequency-doubled to 560 nm, which may find applications in biological fields, like multi-photon microscopy [14].
2. Experimental setup
The experimental setup is depicted in Fig. 1 , which is composed of an electrically driven pulsed laser diode seed, a two-stage Yb-doped fiber amplifier and a single piece of fiber Bragg grating (FBG) based fiber Raman cavity. The pulsed laser diode seed is based on an electrical-pulse-driven 1066 nm distributed feedback (DFB) laser diode with fiber pigtail, whose pulse duration and repetition rate are tunable in large ranges. The gain medium of the first stage amplifier is a section of single-cladding Yb-doped fiber, which is pumped with 400 mW 976 nm single mode laser diode. The gain of the 2nd stage is provided by a section of 5-meters-long double-cladding Yb-doped fiber with a core/cladding ratio of 10/130, which is pumped with an 8 W 976 nm multi-mode laser diode. Three in-line optical isolators are located among the seed laser, the amplifiers and the Raman cavity to prevent backward light and ensure the system stability. A bandpass filter is located after the second amplifier to filter the noise and prevent the backward 1120 nm Stokes light from the Raman cavity. The Raman cavity is a section of 10-meters single mode Raman gain fiber (Corning Hi 1060) with a pair of FBGs (99.6% and 11% reflectivity, 1.5 nm and 0.45 nm reflection bandwidth, respectively) directly written on the fiber with a spacing of 8 meters. The reflection spectra of the FBGs are centered at 1120 nm, which is near the 1st Raman gain peak of silica fiber with respect to the pump wavelength 1066 nm. The output laser from the Raman cavity is launched into an in-line 1060/1120-1178 nm filter WDM to separate the 1120 nm Stokes light and the residue 1066 nm pump light. To be mentioned, no optical path adjusting bulk element, like optical delay line or piezoelectric ceramics, is required in this configuration. The output spectra are measured by an optical spectrum analyzer (measurement scale of 600-1700nm, 0.02nm minimum resolution). The temporal waveforms of the output pulse are probed with a high speed InGaAs photodetector (45 GHz bandwidth), then analyzed by a 60 GHz-bandwidth sampling oscilloscope and a radio frequency spectrum analyzer (RSA). The trigger signal of the oscilloscope is feed by the electrical pulse driver, where the trigger signal is synchronized with the laser diode seed’s driving signal.
Fig. 1 Schematic of the experiment configuration. DFB-LD: distribute feedback laser diode; GS: gain switching; ISO: optical isolator. BPF: bandpass filter; FBG: fiber Bragg grating; HR: high reflectivity; LR: low reflectivity; WDM: wavelength-division multiplexer; PD: photodetector; OSA: optical spectrum analyzer; RSA: radio frequency analyzer.
Figures 2(a) and 2(b) exhibit the optical spectrum and the pulse waveform of the amplified 1066 nm pump pulses, respectively, when the pump power is 3 W. Owing to the narrow linewidth of the DFB laser diode seed, the spectrum bandwidth of the 1066 nm pump laser is as narrow as 0.06 nm. And 98 ps is the narrowest pulse width can be generated from the laser diode seed. As long as the driving signal is properly set, the pulse width can be further increased to above 10 ns. Besides, with the maximum available 8 W pump power from the 976 nm laser diode, the 1066 nm pulse can be amplified to 5.1 W (before the Raman cavity), which is the maximum available pump power for 1120 nm pulse generation.
Fig. 2 The optical spectrum (a) and pulse waveform (b) of the amplified 1066 nm pump pulses, when the pump power is 3 W.
3. Experimental results and discussion
3.1 Output characteristics of two operation regimes
The 1120 nm Raman laser pulses can be readily generated as soon as the inter-pulse period of the pump pulse is adjusted near the round trip of the Raman fiber cavity and the pump power exceeds the threshold. When the pulse repetition rate of the pump laser is fixed at 12.41 MHz, and the pulse duration is set at minimum 98 ps. Figures 3(a)-3(d) exhibit the basic characteristics of the output 1120 nm pulses. Firstly, the blue line in Fig. 3(a) shows the variations of the 1120 nm output power versus the increasing of the pump power (the 0.4 dB insertion loss of the 1060/1120-1178 nm WDM is always compensated in the measurement of the output power). The pump threshold is 0.6 W and the conversion efficiency reaches 81% at the pump power of 2.1 W. As a nonlinear characteristic of the stimulated Raman scattering [1], the slope efficiency exceeds 158% near the threshold, but reduces to 84% when the power exceeds 1.2 W. However, the pulses become splitting and unstable when the pump power is increased above 1.6 W. Figures 3(b) and 3(c) exhibit the comparison of the stable and unstable pulse waveforms and the corresponding optical spectra of output 1120 nm pulses under the pump power of 1.2 W and 1.8 W, respectively. The waveforms are acquired on a 60 GHz sampling oscilloscope with a synchronized signal as the trigger. As we can see, at the pump power of 1.2 W, the 1120 nm pulse waveform can be clearly acquired and displayed by the sampling oscilloscope, which means the 1120 nm pulse train is stable and repetitive, and the pulse duration is measured to be 78 ps. In the frequency domain, the 3 dB bandwidth of the optical spectrum is as narrow as 0.08 nm and the −20 dB bandwidth is measured to be 1.1 nm. The time-bandwidth product is calculated to be 0.93, which indicates that the pulse is near Fourier transformation. The narrow linewidth feature of the laser can be advantageous in plenty of applications, such as frequency doubling [15]. By contrast, at the pump power of 1.8 W, the 1120 nm pulse waveform is fluctuating and disorderly on the top, and the spectrum largely broadens. The 3 dB bandwidth broadens to 1.2 nm and the −20 dB bandwidth broadens to 5 nm. There are two symmetric side bands rising alongside the main spectrum peak, which resembles the features of the modulation instability [16]. Higher output power can be achieved by increasing the pump power, but the splitting and destabilization of the pulse waveforms will worsen along with the broadening of the spectrum, which are mainly attributed to the iterative accumulation of nonlinear phase shift in the cavity [11,17 ]. The maximum average output power when the laser has stable output was 1.3 W (corresponding to 104 nJ pulse energy) at 1.6 W pump power. Attribute to the all-fiberized configuration, the laser can immune the environmental influences and operate stably as long as the pump parameters are properly set. The pulse stability is verified with the RSA, Fig. 3(d) exhibits the radio frequency spectrum and the oscilloscope trace (inset figure) of the 1120 nm output pulse with the pump power of 1.2 W. There is a narrow line appearing at the frequency of 12.41 MHz without sidebands. It is revealed that the signal-to-noise ratio (SNR) is suppressed better than 58 dB, indicating a high temporal stability.
Fig. 3 (a). The 1120 nm output power as the function of the pump power in the operation regime I and II; (b). The comparison of the output pulse waveforms under the pump power of 1.2 W and 1.8 W in regime I; (c).The optical spectra of output pulses under the pump power of 1.2 W, 1.8 W and 2.1 W in regime I, and the inset shows the zoomed spectra on the top; (d). The radio frequency spectrum and the oscilloscope trace (inset figure) of the 1120 nm output pulse with the pump power of 1.2 W in regime I; The comparison of (e) the pulse waveforms and (f) the corresponding optical spectra of output 1120 nm pulses under the pump power of 0.5 W, 1.2 W and 2.1 W in regime II, and the inset shows the zoomed spectra on the top.
Interestingly, there is another operation regime exhibiting distinct output dynamics, which can be achieved by slightly raising the pump repetition rate. For the convenience of description, we call this operation regime as regime II and the above-mentioned regime as regime I. In regime II, the output characteristics of the Raman laser are sensitive to the pump repetition rate and the pump power. At the pump power of 0.6 W, when the pump repetition rate is tuned between 12.4734 MHz and 12.4662 MHz, the output pulses operate in regime II, and the tuning range expands with the increasing pump power. To exhibit the basic output characteristics of this operation regime, we fixed the pump repetition rate at 12.4710 MHz, where the laser has the lowest threshold. As we can see from the orange line in Fig. 3(a), in regime II, the pump threshold is 0.2 W. The output power gains almost linearly with the pump power and the slope efficiency is calculated to be 90%. Similar to regime I, when the pump power exceeds 0.7 W, the output pulses become splitting and unstable, while the threshold of destabilization is lower. With the pump repetition rate fixed at 12.4710 MHz, Fig. 3(e) and 3(f) exhibit the comparison of the stable and unstable pulse waveforms and the corresponding optical spectra of output 1120 nm pulses under the pump power of 0.5 W and 1.2 W, respectively. Interestingly, as depicted in Fig. 3(e), there is a stable periodic modulation pattern carried on the top of the pulse, which is a distinct feature of regime II. Also, the spectrum broadens when the pulses become splitting and unstable, but the spectrum shape evolves in a different way from the first operation regime. The phenomena of spectrum broadening and pulse distortion under high pump power were also observed in [11]. The spectrum broadening is believed to be mainly induced by the quasi-degenerate four-wave-mixing between different longitudinal modes [18]. This unstable state resembles the so-called noise-like pulse [19], which have been widely observed and investigated in mode locked fiber lasers. Additionally, there is a gradual transition between regime I and II, and the transition state between them is unstable.
Briefly speaking, the output characteristics of the Raman laser are sensitive to the pump repetition rate and pump power. There are two different operation regimes exhibiting distinct output dynamics, which are separately achieved in the different pump repetition rate ranges. The two operation regimes are mainly distinguished by their different pump thresholds and the different output pulse waveform patterns. Meanwhile, in both of the two operation regimes, output 1120 nm pulse will experience pulse waveform destabilization and optical spectrum nonlinear broadening with high power pumping. The unstable state resembles the noise like regime reported in [11].
Moreover, it was found that when the laser operates in regime II, the output 1120 nm pulse waveforms will exhibit periodic modulation patterns. Repetition rate detuning and the change of pump power can instantly affect the modulation patterns. Figures 4(a)-4(f) display the different 1120 nm pulse waveforms when the pump repetition rate is detuned from 12.4710 MHz by + 2.4 kHz, + 1.2 kHz, 0 kHz, −1.2 kHz, −2.4 kHz, −4.8 kHz, respectively. Additionally, Figs. 4(g)-4(h) exhibit the corresponding waveforms of 1066 nm pump residue when the pump detuning is set at + 1.2 kHz, 0 kHz. As we can see, as the pump repetition rate is detuned away from 12.4698 MHz (−1.2 kHz detuned from 12.4710 MHz), the duration of the 1120 nm pulse narrows down, and the number of the fringes on the top of the pulse decreases accordingly. On one hand, because of the ultrafast response time of SRS in silica fiber, the 1120 nm Stokes pulse can only be amplified when it is temporally synchronized with the pump pulse. The pump repetition rate detuning can reduce the temporal overlapping of the pump pulse and the Stokes pulse. Only the part of the Stokes pulse (rising or falling edges of the pulse) can be amplified in the oscillation. Hence the duration of 1120 nm Stokes pulse is reduced when the repetition rate is detuned from the cavity free spectral range (the inverse round-trip time). On the other hand, the modulation pattern on the top of the pulse waveform may be interpreted as the oscillation relaxation effect in the process of the SRS-based Stokes pulse generation. As we can see from the comparison of Figs. 4(b) and 4(g) as well as the comparison of Figs. 4(c) and 4(h), the pulse waveform of the pump pulse and the Stokes pulse are basically mutually complementary. There are fast interactions between the pump pulse and the Stokes pulse in the process of the intra-cavity amplification and oscillation, and this interaction behavior resembles the oscillation relaxation phenomenon that happens in many nonlinear gain system [20,21 ]. As far as we know, the modulation patterns observed in regime II are firstly observed in synchronously pumped fiber Raman laser. The wide tunability of the pump repetition rate and the employment of high speed photodetector and oscilloscope make the observations possible. Although, the exact physical nature of the modulation pattern requires further specific theoretical investigations and experimental verifications.
Fig. 4 The different 1120 nm pulse waveforms when the pump repetition rate is detuned from 12.4710 MHz by + 2.4 kHz, + 1.2 kHz, 0 kHz, −1.2 kHz, −2.4 kHz, −4.8 kHz, and the corresponding waveforms of 1066 nm pump residue when the pump detuning is set at + 1.2 kHz, 0 kHz.
To be mentioned, by the method of the pump repetition rate detuning, with the pump duration fixed at 98 ps, the duration of the generated 1120 nm pulse can be adjusted to as short as 18 ps, as illustrated in Fig. 4(a).
3.2 Harmonic pumping and pulse duration tunability
Besides the aforementioned fundamental pumping, harmonic pumping can also be easily achieved by adjusting the repetition rate of the pump pulse to the integer multiples of the cavity free spectral range. With harmonic pumping, the average output power of stable operation can be further scaled up by limiting the peak power and circumventing the nonlinearity-induced pulse splitting.
Limited by the available pump power, up to the 8th order harmonic pumping (99.28 MHz) can be achieved by multiplying the pump repetition rate. Figure 5(a) exhibits the radio frequency spectrum and the oscilloscope trace (inset figure) of the 1120 nm output pulse with the 2nd harmonic pumping under the pump power of 1.5 W, and Fig. 5(b) shows the case of the 8th harmonic pumping under the pump power of 5.1 W. The clean frequency spectrum and high SNR indicate the high temporal stability. To be mentioned, in both cases, the laser is tuned to operate in the aforementioned regime I.
Fig. 5 The radio frequency spectrum and the oscilloscope trace (inset figure) of the 1120 nm output pulse with (a). The 2nd harmonic pumping under the pump power of 1.5 W, and (b). The 8th harmonic pumping under the pump power of 5.1 W. (c). The variations of the 1120 nm output power versus the increasing of the pump power under the 2nd, 3rd, 5th, and 8th harmonic pumping.
To further reveal the characteristics of the laser under harmonic pumping, Fig. 5(c) shows the variations of the 1120 nm output power versus the increasing of the pump power under the 2nd, 3rd, 5th, and 8th harmonic pumping, respectively. As we can see, the laser threshold increase with the harmonic pumping order, which is caused by the reduction of the pump peak power when the pump repetition rate is increased. The slope efficiency (especially near the threshold) is also increased. With the 3rd harmonic pumping and the most available pump power of 5.1 W, as high as 4.3 W stable 1120 nm pulse can be generated from the Raman cavity. The conversion efficiency is calculated to be 84.3%, and the overall conversion efficiency (976 nm-1066 nm-1120 nm) is as high as 53.7%. As far as we know this is the most powerful 1120 nm picosecond Raman pulse generated from an all-fiberized cavity. Besides, owing to the reduction of the pump peak power, the nonlinearities-induced spectral broadening is suppressed, the linewidth is as narrow as 0.10 nm. Except the repetition rate and the power characteristics, the other characteristics of the Raman laser (the optical spectrum and the temporal waveforms) are basically the same when the laser is synchronously pumped at the fundamental frequency and at its harmonics as long as the pump pulses have the similar peak power.
Besides the tunability of the repetition rate, the pulse duration can also be facilely tuned in wide range by adjusting the duration of the pump pulse. Increasing the pulse duration can also help to limit the peak power and circumvent the nonlinearity-induced pulse splitting. Figure 6 shows the evolution of the output 1120 nm pulse waveforms with the pump pulses of different durations. As the pump durations increase from 200 ps to 900 ps, and the corresponding durations of output 1120 nm pulses increase from 185 ps to 750 ps. The pulse waveforms are acquired with different pump power, because the threshold rises with the increasing duration of the pump pulses.
Fig. 6 The evolution of the output 1120 nm pulse waveforms with the pump pulses of different durations and powers.
4. Conclusion
Based on a versatile and efficient all-fiber-based synchronously pumping scheme, we achieved generating high power widely tunable picosecond narrow linewidth 1120 nm Raman laser directly from a single piece of fiber grating based fiber Raman cavity. Owing to the flexibility of the pump laser, the characteristics of the Raman laser are comprehensively explored. Two distinct operation regimes are investigated and characterized. The relaxation-oscillation-related modulation patterns on the Raman Stokes pulses are observed. Besides, the laser exhibits several distinguished features, such as high stability, wide temporal tunability, high conversion efficiency, and high power capability. The laser’s duration can be widely tuned from 18 ps to ~1 ns, and the repetition rate can be adjusted from 12.41 MHz to 99.28 MHz by harmonic pumping. The conversion efficiency of the 1066 nm pump light to the 1120 nm Stokes light exceeds 80% and the overall conversion efficiency (976 nm-1066 nm-1120 nm) is as high as 53.7%. Up to 4.3 W high power operation is verified, which is, as far as we know, the most powerful 1120 nm picosecond Raman pulse generated from an all-fiberized cavity. However, the exact physical mechanisms behind some of the intriguing phenomena have not been well understood, which require further investigations based on dependable numerical modeling and specifically designed experiments. The parameters of the Raman cavity used in the experiments, such as the cavity length and the reflectance of the output FBG, can be further optimized to alleviate the nonlinearities. And the optimal parameters may require specific numerical simulation to determine, which will be also covered by our future work.
Acknowledgments
This work was supported by the State Key Program of National Natural Science of China (Grant No. 61235008), the National High Technology Research and Development Program of China (863 Program) (Grant No. 2015AA021101), and the Innovation Project of National University of Defense Technology for graduate student (NO. B150701).
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