We report a novel quasi-synchronously pumped PPMgLN-based high power mid-infrared (MIR) laser with picosecond pulse bunch output. The pump laser is a linearly polarized MOPA structured all fiberized Yb fiber laser with picosecond pulse bunch output. The output from a mode-locked seed fiber laser was directed to pass through a FBG reflector via a circulator to narrow the pulse duration from 800 ps to less than 50 ps and the spectral FWHM from 9 nm to 0.15 nm. The narrowed pulses were further directed to pass through a novel pulse multiplier through which each pulse was made to become a pulse bunch composing of 13 sub-pulses with pulse to pulse time interval of 1.26 ns. The pulses were then amplified via two stage Yb fiber amplifiers to obtain a linearly polarized high average power output up to 85 W, which were then directed to pass through an isolator and to pump a PPMgLN-based optical parametric oscillator via quasi-synchronization pump scheme for ps pulse bunch MIR output. High MIR output with average power up to 4 W was obtained at 3.45 micron showing the feasibility of such pump scheme for ps pulse bunch MIR output.
© 2013 Optical Society of America
Mid-infrared (MIR) laser sources around 3-5 μm are especially important in application fields such as environmental monitoring, missile counter-measurement and medical diagnostics [1–3]. Optical parametric oscillators (OPOs) based on the periodically poled magnesium-oxide doped lithium niobate (PPMgLN) are among the most popular schemes because PPMgLN can offer characteristics of high effective nonlinear coefficient, long interacting length, good laser power sustainability and wide spectral transmissive range [4,5]. The pump laser of the PPMgLN based OPO can be pulsed or even continuous wave (CW) solid state lasers or fiber lasers. Fiber lasers, famous for their compactness, robustness, excellent beam quality, simple thermal management capability and ultrahigh electrical-optical (E-O) conversion efficiencies [6–9], have brought great progresses to OPOs including the enhanced wall-plug efficiency and the better beam quality as well as the reduced dimensions and the cooling difficulties, which are particularly favorable in practical outdoor experiments. Therefore, the fiber laser pumped OPOs have been extensively investigated both in CW or pulse scheme [10–18]. Due to the specific application advantages of ultra-short pulse MIR lasers, such as in biomedical interactions and missile counter-measurement, investigations on the development of fiber laser pumped ultra-short pulse MIR laser has long been an important issue in the past decade [15–18]. Normally, the synchronization pump scheme should be adopted in the OPO to obtain ultra-short pulse MIR laser outputs. As the synchronization pump scheme needs a carefully adjusted cavity length to match the pump pulse trains precisely, however, the cavity structure is usually complicated such as using the four mirror cavity [14–18]. Though giga-Hertz repetition rate mode-locked laser can be used to lessen the cavity length, the pulse energy as well as the peak power is normally limited. Early in 2000, Robertson et al  reported a singly resonant OPO synchronously pumped by a 2.5 GHz mode-locked diode laser system, in which an average idler output power of 78 mW was realized at 2100 nm. In 2005, Lecomte et al  reported a synchronously pumped OPO with an extremely high repetition rate of 81.8 GHz, in which up to 0.9 W of signal average output power was generated at 1569.7 nm. In the above two works, pump sources with several GHz repetition rates were applied to shorten the OPO cavity length. However, as a compromise, the peak powers of the OPO output were relatively low. Later in 2009, Lamour et al  reported a synchronously pumped OPO with repetition rate of 15.3 MHz and signal pulse energies greater than 70 nJ at pulse durations of 1.5 ps. As a pity, the cavity length of the OPO was as long as 9.8 m, which inevitably made the cavity structure complicated. In 2010, Kokabee et al  reported a high power picosecond OPO synchronously pumped by a high power Yb fiber laser with 20.8 ps pulse durations, in which high average idler powers exceeding 4 W were obtained at wavelengths about 3 µm. In the same year, a MIR OPO synchronously pumped by a femtosecond erbium fiber laser was reported , in which the idle wavelengths tunable from 3.7 to 4.7 μm were demonstrated.
In this paper, we report a novel quasi-synchronously pumped PPMgLN-based high power MIR laser with picosecond pulse bunch output. The purpose of this work is mainly to develop an ultra-short pulse MIR laser with comparatively high peak power and high energy output but with compact OPO cavity structure. The pump laser is a linearly polarized master oscillator power amplifier (MOPA) structured all fiberized Yb fiber laser with picosecond pulse bunch output. A mode-locked Yb fiber laser was developed to be the seed laser, which emitted a comparatively lower repetition rate output pulse train of 2.72 MHz and was constructed in a figure-of-eight cavity structure. The output from the mode-locked seed fiber laser was first directed to pass through a fiber Bragg grating (FBG) reflector via a fiber circulator to narrow the pulse duration from 800 ps to less than 50 ps and the spectral full width at half maximum (FWHM) from 9 nm to 0.15 nm. The narrowed pulses were further directed to pass through a novel pulse multiplier through which each pulse was made to become a pulse bunch composing of 13 sub-pulses with pulse to pulse time interval of 1.26 ns. The pulses were then amplified via two stage Yb fiber amplifiers to obtain a linearly polarized high average power output with a maximum output of 85 W, which were then directed to pass through an isolator and to pump a PPMgLN-based optical parametric oscillator via quasi- synchronization pump scheme for ps pulse bunch MIR output. High MIR output with average power up to 4 W was obtained at 3.45 micron, showing the feasibility of such pump scheme for low repetition rate and high peak power ps pulse bunch MIR output.
2. Fiber laser with picosecond pulse bunch output
A high power linearly polarized Yb fiber laser with picosecond pulse bunch output was constructed as the pump source. The schematic diagram of the whole fiber laser system is illustrated in Fig. 1. The MOPA structured fiber laser consisted of a mode-locked seed fiber laser, a spectrum purifier, a pulse multiplier, and two stages of fiber amplifiers.
The seed laser was a figure-of-eight cavity structured mode-locked Yb fiber laser, in which the mode-locking mechanism was based on the nonlinear optical loop mirror (NOLM) consisting of 50 m passive fiber and a 20/80 coupler. The role of the NOLM here was just as a saturable absorber with intensity dependent transmission. A 2.5 m of double-cladding Yb-doped fiber (with the core and inner clad diameter of 9/125 µm and NA of 0.1) with a peak absorption of 5 dB/m at 976 nm was used as the gain medium. A fiber-pigtailed 976 nm laser diode (LD) was used as the pump source, which was spliced to a (1 + 1) × 1 multi-mode fiber combiner. A fiber-pigtailed optical isolator was employed to ensure optical single direction transmission. The coupler3 acted as the output coupler with a dividing ratio of 20/80. A 50/50 optical coupler (coupler1) was used to connect the nonlinear loop and the linear loop. An approximately 50 m long normal dispersion fiber (core diameter of 6.2 µm, NA of 0.14) was used to obtain the nonlinear phase-shift as a result of self phase modulation (SPM).
The spectra and the output pulse shapes from the seed laser under different pump levels were measured by using an optical spectrum analyzer (OSA, AQ6317C from ANDO with wavelength resolution of 0.02 nm around 1060 nm) and a digital oscilloscope (DSA71254 from Tektronix with bandwidth of 12.5 GHz and sampling rate of 50 GS/s) together with a 20 GHz InGaAs detector and are demonstrated in Fig. 2. Under the pump power of 780 mW, a stable mode-locked pulse trains could be obtained from the laser oscillator with an average power of 45 mW and a pulse repetition rate of 2.72 MHz, corresponding to the total cavity length of 75 m. As shown in Fig. 2, it could be found that the seed pulses had a complex pulse shape with a pulse width of 800 ps and the seed spectrum was measured spanning from 1063 nm to 1083 nm with central wavelength at 1072 nm.
Because the spectral width of the pump laser plays an important role in the fiber laser pumped OPO, the mode-locked seed fiber laser had too large a spectral width that should be unsuitable to pump a PPMgLN based OPO. We demonstrated years ago that a narrow spectral pump generally results in much higher gain than a broad band pump in a PPMgLN based OPO . To purify the spectrum of the laser, the output pulses from the mode-locked seed laser were directed to pass through a FBG reflector (centering at 1067nm) via a three port fiber circulator. A narrowed laser spectrum with a FWHM of 0.15 nm was obtained after the circulator at a price of power reduction from 45 mW to 0.8 mW. The change of the spectrum narrowing is illustrated in Fig. 3(a). Fortunately, the narrowing of the spectrum also resulted in much narrower a pulse duration of less than 50 ps in the meantime (see Fig. 3(b) for the pulse shape comparison before and after the spectrum purification), indicating existence of strong linear chirping in the original mode-locked output pulses. Such a strong chirping can be further confirmed by displaying the original pulse and the processed pulse simultaneously in an oscilloscope. Streching the FBG would result in the position change of the narrowed pulse within the original pulse envelope. It could be found from the OSA and the oscilloscope that a spectrum shift of 0.5 nm would result in an estimated 10 ps pulse position change. It is clear that suitable dispersion management in the mode-locked fiber laser would result in less chirping and then the higher average power after the same spectrum purifier. This will be considered in the further work.
The narrowed pulses with purified spectrum were further directed to pass a novel pulse multiplier. As shown in Fig. 1, the pulse multiplier was composed of three cascaded 2 × 2 fiber couplers, which were all set in loop structure with loop lengths of 104 cm, 52 cm and 26 cm respectively. The corresponding coupling ratios of three couplers were 3:7, 4:6, and 4:6 respectively. In this way, the input pulses could be multiplied. Each pulse was enabled to become a pulse bunch composing of thirteen sub-pulses with similar pulse shape. The time interval between each sub-pulse was about 1.26 ns, which was decided by the loop length of the third coupler. Figure 4 illustrates the detailed pulse structure of the pulse bunch.
Owing to the insertion loss of the spectrum purifier, the power from the seed fiber laser was decreased to 0.8 mW, which was not sufficient for direct power amplification to tens of watts. Thus, two stages of fiber amplifiers were applied to raise the average power to over 80W. The fiber pre-amplifier was constructed as illustrated in Fig. 1. The output fiber of the seed was spliced to a random-polarization isolator (ISO), followed by a (1 + 1) × 1 multi-mode combiner. A home-made double-clad Yb doped fiber of 5 m in length with a peak absorption of 2.8 dB/m at 976 nm was pumped by a LD working at 976 nm. When the pump power was up to 3 W, the output laser power from the pre-amplifier was about 1.6 W. The spectral bandwidth after the preamplifier was 0.19 nm.
The pre-amplified pulse train was directed to pass a polarization maintaining isolator (PM-ISO) before it entered the main power amplifier. The schematic diagram of the power amplification stage is also shown in Fig. 1. The fast-axis blocked PM-ISO, in which the input and the output fibers were the passive LIEKKI PM fibers of 10/125 µm (NA of 0.08) and 25/250 µm (NA of 0.07) respectively, output a linearly polarized signal with an average power of about 0.7 W. Such a signal power was strong enough to saturate the final stage power amplifier. As a result, the signal power variation due to the polarization state change in the random polarized seed fiber laser and pre-amplifier would not strongly affect the output power of the final amplifier. After the ISO, there was a PM multi-mode (6 + 1) × 1 fiber combiner. Five fiber-pigtailed LDs (each with a maximum output power of 30 W at 915 nm) were used as the pump sources. The gain fiber applied was 4.5 meters of large mode area (LMA) Yb doped PM DCF from LIEKKI with core and inner clad diameter of 25/250 µm, core and inner clad NA of 0.07 and 0.46 respectively. The peak absorption of the LMA PM fiber was 2.3 dB/m at 920 nm. The output end surface of the gain fiber was cut into 8° to avoid the fiber amplifier from the backward laser emission. After collimation using a convergent lens with a focal length of 8 mm, the collimated laser output was directed to pass through two dichroic mirrors at first to eliminate the undepleted pump laser and then to pass through a free space structured PM ISO, which consisted of a half wave plate (HWP), a polarization beam splitter (PBS), a Faraday rotator (FR) and a quartz rotator (QR).
The output power dependence of the main fiber amplifier on the pump power is illustrated in Fig. 5. When the laser was directly collimated with an aspherical lens, a maximum output power of 85 W was obtained under the pump power of 125 W (measured by a power meter from Ophir with measurement range up to 150 W). The slope efficiency was computed to be about 68%. With increasing the output power, the spectrum broadened correspondingly due to SPM. At the maximum output power of 85 W, the FWHM was measured to be 0.28 nm. When the average output power exceeded 80 W, the stimulated Raman scattering (SRS) spectrum could be observed obviously, which deterred the further increase of the output power under the current fiber laser configuration.
The polarization extinction ratio of the fiber laser was measured to be 14 dB after HWP and PBS under the maximum power output of 85 W. After the ISO, the maximum 1067 nm laser power was reduced to 60 W due to the 1.5 dB loss of ISO.
3. Fiber laser pumped OPO
The schematic diagram of the OPO setup is shown in Fig. 6. The OPO resonator was configured in a single pass singly resonant (SPSR) linear cavity structure. It was composed of a PPMgLN wafer and two cavity mirrors. The homemade PPMgLN wafer was in single period domain grating structure with period of 30 micron and size of 50 × 10 × 1 mm. Both end surfaces were finely polished and coated with anti-reflection film covering wavelength of 1.03-1.08 micron, 1.4-1.7 micron and 3-4 micron. The PPMgLN wafer was mounted on a metal holder. The plane input mirror (M1) was coated with high-reflection (HR) film for signal (R>99.7% over 1.4-1.7 μm) and idler band (3-4 μm), and high transmission coating for pump (T>98%). The output coupler (M2) was a concave mirror with a radius of curvature 200 mm and coated with HR for signal (R>99.7% over 1.4-1.7 μm) and high transmission for the pump and the idler (3-4 μm) (T>98%). To meet the requirement of quasi-synchronously pumping, the cavity length was chosen to be approximately 12 cm, corresponding to the sub-pulse time interval of 1.26 ns within the pulse bunch, and slightly adjustable by placing the output coupler onto an one-dimension translation stage. The undepleted pump and the OPO signal were highly reflected by M3 (R>99.8%). The signal output was extracted through M2, while the idler power was measured after M3.
One convergent lens with a focal length of 250 mm was applied as the beam condenser. The fiber laser output beam through the ISO was focused onto the middle of the PPMgLN wafer with a waist radius of 160 mm. By carefully adjusting the longitudinal position of M2, stable parametric oscillation could be realized under pump power greater than 4.5W. With increasing pump power, the idler power output at 3.45 μm increased at an initial slope efficiency of ~10% but gradually roll-over until it reached 2 W at pump power of 25 W.
It was found that the idler power roll-over of the OPO at high pump power level was mainly due to the focused pump fiber laser waist position change under different power levels. There seems existed some thermal lens effect at the output surface of the main fiber amplifier. With increasement of the pump power, the focused beam waist left the PPMgLN wafer gradually and resulted in the conversion efficiency roll-over. In order to obtain higher idler output under higher pump power, the distance between the condensing lens and OPO was shortened by 25 mm. As a result, the OPO had much higher a threshold of 16 W. However, the output idler power increased quickly with a slope efficiency of ~16%. A maximum idler power of 4 W at 3.45 μm was obtained under the pump power of 43 W (see Fig. 7 for the idler output power dependence on the pump power under two different lens positions).
It was found difficult to increase the idler power further when increasing the pump power above 40W. It was possibly due to the back-conversion because the output coupler M2 was highly reflective at the signal band. The output signal spectrum was measured using an OSA. Figure 8 demonstrates the different signal spectra under the pump power of 18 W, 25 W, and 40 W respectively. Owing to the temperature rise and the idler absorption-induced thermal effect in PPMgLN, it is evident that the higher the pump power, the longer the signal wavelength and the broader the spectral bandwidth .
Under the maximum idler power, the pulse profiles of the OPO signal and the undepleted pump were measured simultaneously using two detectors and are shown together in Fig. 9. It was clear that both the signal and the pump had the bunch-like pulse profiles.
We tried to replace the HR output coupler (M2) using a lower reflective mirror (with signal reflectivity of ~78%). However, the OPO seemed to oscillate only near the threshold because we could only observe some red flicker of the cavity. As strong red light could always be observed when the PPMgLN based OPO oscillated due to the cascaded nonlinear conversion in the PPMgLN crystal, such a red flicker of the cavity indicated the OPO was sometimes oscillated and sometimes not.
It is a little puzzling that the high peak power of the pump fiber laser should be able to produce high enough gain in such a SPSR OPO to overcome the cavity loss. To understand the cause underneath this phenomenon, the signal photon lifetimes of the OPO cavity were calculated with different mirror reflectivities and round-trip losses. As shown in Fig. 10, it is easy to find that with increasing output mirror reflectivity, the signal photon lifetime become longer. As to the specific conditions in this experiment, the cavity signal round-trip loss was estimated to be ~4% (mainly considering the coating loss of the PPMgLN wafer in the signal band, about 1% for each surface) and then the photon lifetime should be 22 ns, which is close to the experimental value of 18 ns obtained from Fig. 9 (noting the decay of the pulse envelope). It seemed the cavity loss was at normal level.
The main reason of the high threshold and the relatively low conversion efficiency in this experiment may be the inaccurate sub-pulse intervals within each pulse bunch. When the sub-pulse intervals were not exactly the same, the quasi-synchronization pump scheme might not effectively work. That is to say that the signal pulse generated along the first pump pulse could not be fully amplified in the following pump pulses. Instead, it was only partially enhanced in the following pulses because the signal and the pump pulses were only partially overlapped and then the synchronization was not fully assured. Such an explanation was partly supported by the experimental data if we noted the evolution of the signal sub-pulses. The signal sub-pulses changed gradually from clear pulses in the left side to the splitted ones and then to many unclear sub-pulses in the right side as illustrated in Fig. 9.
In conclusion, we have successfully developed a novel fiber laser pumped mid-infrared OPO system with picosecond pulse bunch output. The output pulses from the mode-locked fiber laser were purified using a FBG reflector at first and then multiplied in three cascaded loop-structured fiber couplers. Each pulse was converted into a pulse bunch with about thirteen sub-pulses. After two stage amplification, the fiber laser could output a linearly polarized pulse output with an average power up to 85W (60 W after passing through the final ISO) with spectral width 0.28 nm, polarization extinction ratio higher than 14 dB and peak power up to 50 kW(33 kW after passing through the final ISO). The formation of ps pulse bunch in the fiber laser made the quasi-synchronously pumping scheme feasible using a short linear cavity OPO. Pulse bunch mid-infrared output was obtained with an average power up to 4 W at 3.45 micron. It was expected that optimization of the cascaded loop lengths with accuracy better than 1 mm in the pulse multiplier may further improve the OPO efficiency through better synchronization.
This work was partly supported by the National Natural Science Foundation of China (NSFC) (project 61078015) and the National Basic Research Program (973) of China (project 2011CB311803).
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