Fiber-laser-pumped high-power mid-infrared optical parametric oscillator based on MgO:PPLN crystal

Wavelength tunable picosecond pulse bunch laser around 3 µm through PPMgLN –based DFG with temperature insensitive high power scaling

The research reports broadband, continuous-wave (CW), mid-infrared generation based on an optical parametric oscillator (OPO), pumped by an amplified spontaneous emission (ASE) fiber source. The ASE fiber source has a ytterbium (Yb)-doped fiber master oscillator power amplifier (MOPA) structure. By injecting another fiber laser with high power and a narrow linewidth into the nonlinear crystal, the phase-matching difference frequency generation (DFG) can be realized at a low pumping threshold. It is shown that during the down-conversion of the ASE fiber source, the intracavity DFG can steadily and effectively generate a broadband mid-infrared laser, with a central wavelength of 3713 nm. The bandwidth of the mid-infrared laser is 122 nm. The pump power of 2 W generates a 0.238 W mid-infrared laser with a pump-idler conversion efficiency of 11.6%. It has tremendous application potential in the research of broadband CW mid-infrared lasers.

The goal of the presented work is the development of a laser system suitable for intracavity phase interferome-try (IPI), allowing to detect a phase shifts in the order of 0.1 µrad, based on beat-note detection. The key part of a such system is a laser oscillator in which two intracavity pulses circulate independently but with the same repe-tition rate. Our solution is based on an intracavity pum-ped PPLN linear optical parametrical oscillator (OPO), operating at the wavelength tunable in the range from 1.4 up to 1.6 µm. It was used a SESAM-modelocked, picosecond, diode pumped Nd:YVO4 linear oscillator, operating at 1.06 µm for synchronous pumping of OPO. To obtain a long-term stable generation of two indepen-dent pulse trains inside the OPO, the OPO cavity was set to be twice longer than the pumping Nd:YVO4 la-ser cavity. Simultaneously, the pumping laser system, containing the PPLN crystal, was set in such manner that the parametric gain inside the PPLN overcame the OPO threshold only for one direction of pumping pulses propagation. Since the both pulse trains inside the OPO were pumped by the same spatial mode of the pump laser, the possible pointing instability of the pump laser does not affect the system performance and the beat-note bandwidth. The OPO output beams were derived be semireflected mirror, placed close to the OPO cros-sing point of pulse trains. After proper delay of one pulse train both trains interfered on a slow InGaAs detector, where the beat note was detected. In order to verify the system performance a LiNbO3 electro-optic phase mo-dulator was placed inside the OPO. To modulate one of the pulse trains, RF-signal obtained from pumping pulse train detected by fast InGaAs, divided by two, and am-plified to desired value (maximum 10 V), was applied on the modulator. The beat-note signal with frequency from 1 up to 20 kHz was successfully measured in de-pendency on RF-signal amplitude.The goal of the presented work is the development of a laser system suitable for intracavity phase interferome-try (IPI), allowing to detect a phase shifts in the order of 0.1 µrad, based on beat-note detection. The key part of a such system is a laser oscillator in which two intracavity pulses circulate independently but with the same repe-tition rate. Our solution is based on an intracavity pum-ped PPLN linear optical parametrical oscillator (OPO), operating at the wavelength tunable in the range from 1.4 up to 1.6 µm. It was used a SESAM-modelocked, picosecond, diode pumped Nd:YVO4 linear oscillator, operating at 1.06 µm for synchronous pumping of OPO. To obtain a long-term stable generation of two indepen-dent pulse trains inside the OPO, the OPO cavity was set to be twice longer than the pumping Nd:YVO4 la-ser cavity. Simultaneously, the pumping laser system, containing the PPLN crystal, was set in such manner that the parametric gain inside the PPLN overcame the OPO threshold only for one direction ...

Here we report a high power, pulsed opti cal parametric oscillator (OPO) at 3.5 µm by using a MgO:PPLN crystal as the gain medium. The OPO itself was pumped by a semiconductor diode-seeded, Yb 3+ -doped fiber Master Oscillator Power Amplifier (MOPA) operating at 1062nm. An OPO output power as high as 11W at an overall slope efficiency of 67% was achieved, with nearly 2.7W and 8.2W of optical power obtained at 3.5µm and 1.5µm respectively. Due to the fast response time of the external modulator, it is possible to implement active pulse shaping on a nanosecond time-scale. Using adaptive pulse shaping of the seed laser (using an external modulator) we demonstrated a reduction in the impact of dynamic gain saturation and optical Kerr/Raman nonlinear ities within the fibre MOPA obtaining shaped signal and idler pulses at the OPO output and reduced spectral bandwidt hs. We have also investigat ed the dependence of the OPO build-up time and energy transfer efficiency on pump pulse peak power and shape. The build-up time shows an exponential dependence on the pulse peak power and as expected decreases with an increase in pulse peak power. Analyzing the shift in spectral peak at 1.5µm it is possible to estimate the internal temperature of the crystal for various pump powers. Our experiments were pump-power limited and considerable scope remains for further power-scaling of the OPO output using this approach. Keywords: Optical fiber lasers, pulsed lasers, optical parametric oscillators, PPMgLN

We recently experimentally demonstrated a high-power, high-repetition-rate mid-infrared (mid-IR) radiation at 4.0 µm, based on the fiber-laser pumped optical-parametric-oscillator (OPO) configuration. To ensure the excellent characteristics of the OPO output, the master oscillator power amplifier (MOPA) structure fiber laser was developed as the pump source and the homemade periodically poled magnesium-oxide-doped lithium niobate (PPMgLN) crystal (60 mm in length) was employed in the oscillator. Under the pump power of 18.5 W, the maximum 2.03 W average power of 4.0 µm idler laser was finally achieved at a high repetition rate of 200 kHz. In addition, the overall OPO output power of 8.8 W was also realized, corresponding to the total conversion efficiency as high as 48%.

A 71 mJ, 300 Hz mid-infrared source based on ZnGeP2 MOPA system

We report a periodically poled magnesium-oxide-doped lithium niobate (PPMgLN) based optical parametric oscillator (OPO) pumped by a diode-seeded, linearly polarized, high-power, pulsed, ytterbium fiber master oscillator power amplifier (MOPA). Using adaptive pulse shaping of the seed laser (using an external modulator), we demonstrate a reduction in the impact of dynamic gain saturation and optical Kerr/Raman nonlinearities within the fiber MOPA, obtaining shaped signal and idler pulses at the OPO output and reduced spectral bandwidths. A maximum average output power of 26.5 W was obtained from the MOPA at 1062 nm. An output power as high as 11 W from the OPO at an overall slope efficiency of 67% was achieved, with 2.7 W of output power obtained at a wavelength of 3.5 mum. Our experiments were pump-power-limited and considerable scope remains for further power scaling of such OPOs using this approach.

The remarkable progress in nonlinear materials and laser pump sources continues to advance optical parametric oscillators (OPOs) to new frontiers in spectral and temporal coverage and performance capability, with no end in sight. The advent of a new class of mid-infrared (mid-IR) birefringent nonlinear materials such as CdSiP2 (CSP) and quasi-phase-matched (QPM) crystals such as orientation-patterned GaP (OP-GaP) has finally paved the way for the practical development of OPOs into the deep-IR at wavelengths beyond 4 μm, where multiphonon absorption in oxide-based QPM materials such as MgO:PPLN has proved a persistent barrier to direct wavelength generation for more than two decades. By exploiting the newly developed nonlinear crystals in combination with well-established and widely available Nd-based solid-state and Yb-doped fiber lasers near ∼1 μm, new OPO sources in different time-scales from pulsed nanosecond to ultrafast picosecond and few-cycle femtosecond domain have been realized, accessing spectral regions across 5–12 μm in deep-IR, and offering unprecedented performance capabilities. On the other hand, the exploitation of cascaded pumping techniques in combination with CSP has made possible the generation of femtosecond pulses out to ∼8 μm in the deep-IR using the Kerr-lens-mode-locked Ti:sapphire laser as the primary pump source (Fig. 1). At the same time, to enable the exploitation of alternative materials for deep-IR generation, recent efforts have led to the development of practical high-power near-degenerate Yb-fiber-laser-based picosecond OPOs at ∼2.1 μm (Fig. 2) as alternative pump sources for birefringent crystals such as ZnGeP2 and QpM crystal such as OP-GaAs.

We demonstrate a lithium triborate (LBO) optical parametric oscillator (OPO), which is synchronously pumped with a pulse-compressed and frequency-doubled master-oscillator power-amplifier (MOPA) system consisting of a gain-switched laser diode and a series of ytterbium-doped fiber amplifiers. The 20ps pulses from the MOPA were compressed in a transmission grating compressor down to 4.4ps with a throughput efficiency of ~70% and subsequently frequency-doubled with an efficiency of ~60% in a 20mm long LBO to a maximum of ~25W. With a typical pump power of 17W for the OPO, we obtained a maximum combined signal and idler output power of 2.5W (at 877nm) and 1.7W (at 1.3µm). Individually, a maximum signal power of up to 3.7W at 740 nm was obtained with a signal pulse duration of ~3.2ps. The OPO was widely tunable from 651nm-1040nm (signal) and from 1081nm-2851nm (idler). To the best of our knowledge, this is the highest output power from a green-pumped LBO OPO. The fiber-based pump source can potentially be operated between 100MHz and 1GHz, which in combination with the few-picosecond pulses and the near-IR tunability of the OPO is a very attractive source for nonlinear microscopy.

The concept of system for intracavity interferometry based on the beat note detection in subharmonic synchronously intracavity pumped optical parametrical oscillator (OPO) is presented. The system consisted of SESAM-modelocked, picosecond, diode pumped Nd:YVO 4 laser, operating at wavelength 1.06 μ m and tunable linear intracavity pumped OPO based on MgO:PPLN crystal, widely tunable in 1.5 μ m able to deliver two independent trains of picosecond pulses. The optical length of the OPO cavity was set to be exactly twice the pumping cavity length. In this configuration the OPO produces signal pulses with the same repetition frequency as the pump laser but the signal consists of two completely independent pulse trains. For purpose of pump probe measurements the setup signal with half repetition rate and scalable amplitude was derived from the OPO signal using RF signal divider, electropotical modulator and fiber amplifier. The impact of one pump beam on the sample is detected by one probing OPO train, the other OPO train is used as a reference. The beat note measured using the intracavity interferometer is proportional to phase modulation caused by the pump beam. The bandwidth of observed beat-note was less than 1 Hz (FWHM), it corresponds to a phase shift measurement error of less than 1.5 × 10 -7 rad without any active stabilization. Such compact low-cost system could be used for ultra-sensitive phase-difference measurements (e.g. nonlinear refractive index measurement) for wide range of material especially in spectral range important for telecom applications.

A high-power fiber laser with a master oscillator power amplifier (MOPA) structure is constructed by using pump sources and large mode area ytterbium doped fiber (LMA). Lasers include three parts: seed laser source, preamplifier and main amplifier. The fiber laser is integrated with air cooling structure. When the pulse is set to 200 ns 100 kHz, the fiber laser has an air cooling structure, the maximum pump power is 210 W, the 124 W is obtained. The wavelength is the stable output of the 1064 nm laser, the conversion efficiency is 59 %, the single pulse energy is 1.24 mJ, the peak power is 6.2 kW, the laser beam quality is M2 = 1.38, and the power stability is within 2 %. At present, the output power of laser is limited by pump power, and the increase of pump source is expected to further improve output power.

We experimentally demonstrated a compact fiber laser-pumped multichannel PPMgLN-based optical parametric oscillator (OPO) generating total OPO output power of 15.8, 15.2, 14.2, 12.9, and 8.8 W with idler output power of 4.7, 4.3, 4.1, 3.3, and 2.1 W at the wavelength of 3.43, 3.63, 3.72, 3.83, and 3.99 μm, respectively. The OPO was pumped by a fully fiberized polarization maintaining (PM) ytterbium-doped pulsed fiber master oscillation power amplifier (MOPA) operating at 1064 nm at a repetition rate of 65 kHz with effective pump power of 28.7 W. The MOPA system was constructed with an acousto-optic Q-switched fiber laser seed and only one stage PM fiber amplifier without any free space components, which makes the pump system compact and stable in the long-term. Comparisons on efficiencies and signal wavelength shifts between different channels showed that the idler absorption was the main factor preventing high average-power OPO operation with long idler wavelength.

We demonstrate a compact high power mid-infrared (MIR) optical parametric oscillator (OPO) pumped by a gain-switched linearly polarized, pulsed fiber laser. The gain-switched fiber laser was constructed with a piece of Yb doped polarization maintaining (PM) fiber, a pair of fiber Bragg gratings written into the matched passive PM fiber and 6 pigtailed pump laser diodes working at 915 nm with 30 W output peak power each. By modulating the pulse width of the pump laser diode, simple pedestal-free pulse shape or pedestal-free trailing pulse shape ("figure-of-h" as we call it) could be achieved from the gain-switched fiber laser. The laser was employed as the pump of a two-channel, periodically poled magnesium oxide lithium niobate-based OPO system. High power MIR emission was generated with average output power of 5.15 W at 3.8 μm channel and 8.54 W at 3.3 μm channel under the highest pump power of 45 W. The corresponding pump-to-idler conversion efficiency was computed to be 11.7% and 19.1%, respectively. Experimental results verify a significant improvement to signal-to-idler conversion efficiency by using "figure-of-h" pulses over simple pedestal-free pulses. Compared to the master oscillator power amplifier (MOPA) fiber laser counterpart, the presented gain switched fiber laser is more attractive in OPO pumping due to its compactness and simplicity which are beneficial to construction of OPO systems for practical MIR applications.

In this paper, we presented a widely tunable Mid-IR OPO based on a single multiple periods MgO:PPLN crystal, acousto-optical (AO) Q-switched Nd:YVO 4 laser pumped by 880 nm laser diode (LD) was used as the pumping source. The OPO was designed as an extracavity single resonant optical parametric oscillator. When the grating period of the MgO:PPLN crystal was 31.5 μm and working temperature of 100°C, and the pump power was 10.95 W with repetition rate of 50 kHz, the single wavelength of 1.72 μm and idler wavelength of 2.78 μm was obtained respectively, the maximum average output power of the parametric lasers was 3.02 W, the pulse width of 7.9 ns was achieved. The optic-optic conversion efficiency was 27.58% from the 1064 nm pump laser to the parametric lasers. By tuning the grating period (28.5~31.5 μm) and the working temperature (25~192.1°C) of the MgO:PPLN, the single laser can be tuned from 1.46 μm to 1.72 μm, the corresponding idler laser can be tuned from 2.40 μm to 4.17 μm.

This paper presents a specially designed optical parametric oscillator (OPO) which achieved high-efficiency mid-infrared laser of 2.83 μm. The cascaded nonlinear interactions of OPO and optical parametric amplifier (OPA) were simultaneously realized in a single MgO:PPLN crystal. The signal oscillation of 1.70 μm was used to pump a secondary parametric process that resulted in amplification of the idler laser of 2.83 μm. When the MgO:PPLN crystal with a grating period of 31.2 μm was pumped by a 1.064 μm laser and operated at 148°C, the quasi-phase-matching of both OPO and OPA could be simultaneously achieved. Average output power of 7.68 W at 2.83 μm was obtained for 25 W of pump at 7 kHz. The power conversion efficiency of 2.83 μm laser was 30.7%, which was evidently higher than common OPOs.