High power picosecond green and deep ultraviolet generations with an all-fiberized MOPA.

We demonstrate high power picosecond green and deep ultraviolet (DUV) lasers based on an all-fiberized master oscillator power amplifier (MOPA). The main power amplifier is fabricated with a highly Yb-doped large mode area (LMA) silicate glass fiber. It delivers 75.2-W laser output at 1029 nm with a pulse repetition rate of 10 MHz and a pulse duration of 70 ps. With a lithium triborate (LBO) crystal, a 43.0-W green output at 514.5 nm has been achieved with a pulse duration of 55 ps. With a caesium lithium borate (CLBO) crystal, a 14.5-W picosecond DUV output at 257 nm has been generated, which is the highest power for the all-fiber based DUV laser, to the best of our knowledge.

Single frequency tunable UV laser at 308†nm based on all-fiberized master oscillator power amplifiers.

A tunable narrow linewidth UV laser near 308 nm is necessary for highly sensitive hydroxyl (OH) radical measurement. We demonstrated a high-power fiber-based single frequency tunable pulsed UV laser at 308 nm. The UV output is generated from the sum frequency of a 515 nm fiber laser and a 768 nm fiber laser, which are harmonic generations from our proprietary high-peak-power silicate glass Yb- and Er-doped fiber amplifiers. A 3.50 W single frequency UV laser with 100.8 kHz pulse repetition rate, 3.6 ns pulse width, 34.7 µJ pulse energy, and 9.6 kW peak power has been achieved, which represents the first demonstration, to the best of our knowledge, of a high-power fiber-based 308 nm UV laser. With temperature control of the single frequency distributed feedback seed laser, the UV output is tunable for up to 792 GHz at 308 nm.

Rare Earth Doped Optical Fibers with Multi-section Core.

The gain bandwidth of a single-mode fiber is limited by the atomic transitions of one rare earth gain element. Here we overcome this long-standing challenge by designing a new single-mode fiber with multi-section core, where each section is doped with different gain element. We theoretically propose and experimentally demonstrate that this configuration provides a gain bandwidth well beyond the capability of conventional design, whereas the inclusion of multiple sections does not compromise single-mode operation or the quality of the transverse modal profile. This new fiber will be beneficial in realizing all fiber laser systems with few-cycle pulse duration or octave tunability.

1.8 mJ, 3.5 kW single-frequency optical pulses at 1572 nm generated from an all-fiber MOPA system.

High-energy single-frequency optical pulses at 1572 nm were generated from an all-fiber MOPA system for atmospheric CO2 LIDAR system application. We report the experimental demonstration of 1.8 mJ, a peak power of 3.5 kW at the pulse repetition of 2.5 kHz, as well as 1.3 mJ, a peak power of 2.5 kW at the pulse repetition of 7.5 kHz single-frequency optical pulses at 1572 nm using single-mode large-core polarization-maintaining Er-Yb co-doped silicate glass fiber amplifiers pumped at 976 nm. To the best of our knowledge, this is the highest pulse energy of single frequency at 1572 nm from an all-fiber amplifier system.

High-efficiency ytterbium-free erbium-doped all-glass double cladding silicate glass fiber for resonantly-pumped fiber lasers.

A highly efficient ytterbium-free erbium-doped silicate glass fiber has been developed for high-power fiber laser applications at an eye-safe wavelength near 1.55 µm. Our preliminary experiments show that high laser efficiency can be obtained from a relatively short length of the gain fiber when resonantly pumped at 1535 nm in both core- and cladding-pumping configurations. With a core-pumping configuration as high as 75%, optical-to-optical efficiency and 4 W output power were obtained at 1560 nm from a 1 m long gain fiber. When using a cladding-pumping configuration, approximately 13 W output power with 67.7% slope efficiency was demonstrated from a piece of 2 m long fiber. The lengths of silicate-based gain fiber are much shorter than their silica-based counterparts used in other experiments, which is significantly important for high-power narrow-band and/or pulsed laser applications.

Single-frequency gain-switched Ho-doped fiber laser.

We demonstrate a single-frequency gain-switched Ho-doped fiber laser based on heavily doped silicate glass fiber fabricated in-house. A Q-switched Tm-doped fiber laser at 1.95 µm was used to gain-switch the Ho-doped fiber laser via in-band pumping. Output power of the single-frequency gain-switched pulses has been amplified in a cladding-pumped Tm-Ho-codoped fiber amplifier with 1.2 m active fiber pumped at 803 nm. Two different nonlinear effects, i.e., modulation instability and stimulated Brillouin scattering, could be seen in the 10 µm-core fiber amplifier when the peak power exceeds 3 kW. The single-frequency gain-switched fiber laser was operated at 2.05 µm, a popular laser wavelength for Doppler lidar application. This is the first demonstration of this kind of fiber laser.

High-spectral-flatness mid-infrared supercontinuum generated from a Tm-doped fiber amplifier.

Broadband mid-infrared supercontinuum pulses were generated directly from a short piece of active fiber in a single-mode Tm-doped fiber amplifier. The broadband mid-infrared pulses have an extremely high spectral flatness with ~600 nm FWHM bandwidth (from 1.9 µm to 2.5 µm), >15 kW peak power, and >20 GW/cm(2) laser peak intensity. This new approach exhibits a significantly different physical mechanism from other supercontinuum generation demonstrations in the literature, in which usually a piece of passive fiber was used for nonlinear spectral broadening. The physical mechanism for the broadband mid-infrared supercontinuum generation in this approach has been attributed to a combined effect of two superradiative processes of Tm(3+) ions (i.e., the (3)F(4)-(3)H(6) transition covering the 1.8~2.1 µm spectral region and the (3)H(4)-(3)H(5) transition covering the 2.2~2.5 µm spectral region), and also nonlinear optical processes as well in the Tm-doped gain fiber. The spectra of the mid-infrared supercontinuum pulses were further broadened in a 2 m chalcogenide fiber with 20 dB bandwidth ~1100 nm and a 3 m fluoride fiber with 20 dB bandwidth ~2600 nm.

All-fiber wavelength-swept laser near 2 µm.

We report, for the first time to our knowledge, an all-fiber wavelength-swept Tm-doped laser based on a fiber Fabry-Perot tunable filter in the 2 µm spectral region. The laser wavelength can be continuously tuned over 200 nm from 1840 to 2040 nm in a short period of time. The demonstrated tuning speed was 12.5 µm/s with a tuning efficiency of 17.5 nm/V. The spectral linewidth of the laser was measured to be approximately 300 MHz or 0.01 cm(-1). This kind of laser can find potential applications in both high-resolution laser spectroscopy and tunable mid-IR generation via nonlinear frequency conversion.

Kilowatt-peak-power, single-frequency, pulsed fiber laser near 2 µm.

We generated single-frequency pulses at kilowatt peak power from an all-fiber Tm-doped master oscillator power amplifier system, which is the first report of this kind (to the best of our knowledge) of a laser in the 2 µm region. Compared with the laser linewidth of seed pulses, spectral broadening by a factor of 3 was observed with the amplified pulses. This was attributed to self-phase modulation in passive pigtail fibers of the components (isolator and wavelength division multiplexing) that were placed after the fiber amplifier. The short pulse width (~7 ns) of the kilowatt-level pulses prevents an onset of stimulated Brillouin scattering in the long fiber. When launching the pulses into several-meter single-mode fiber, significant nonlinear spectral broadening occurs due to modulation instability in the fiber. This reaction is beneficial for generation of a mid- and long-wavelength IR supercontinuum in nonlinear IR fibers.

Single-mode low-loss optical fibers for long-wave infrared transmission.

In this Letter, we report single-mode fibers made of chalcogenide glasses with low loss in the 5-12µm range. Glasses from the Ge-As-Te-Se system were optimized to prevent nucleation and to exhibit low density of charge carriers. Single-mode fibers were obtained through the rod-in-tube method by substituting 2% Te/Se between the core and cladding glasses. The resulting single-mode fibers had a core diameter of 30µm and exhibited losses of ~6 dB/m at 10.6µm, and as low as 3-4dB/m in the 6-10µm range.

All-fiber Q-switched single-frequency Tm-doped laser near 2 mum.

We present an all-fiber Q-switched single-frequency laser oscillator operating in the eye-safe region at 1950 nm. It is based on the stress-induced polarization modulation in a Tm-doped distributed Bragg reflector fiber laser. The laser emits Q-switched single-frequency laser pulses with a pulse repetition rate ranging from tens of hertz to hundreds of kilohertz and an average power of several milliwatts. Pulse duration and laser spectral linewidth have been characterized. This is, to our best knowledge, the first demonstration of a Q-switched single-frequency fiber laser near 2 microm.

Single-frequency narrow-linewidth Tm-doped fiber laser using silicate glass fiber.

Single-frequency laser operation near 2 microm has been demonstrated in an all-fiber short-cavity (2-6 cm) distributed feedback laser cavity using both cladding- and core-pump configurations in a newly developed heavily Tm-doped multicomponent silicate glass fiber. Using a single-mode Er-doped fiber laser at 1575 nm as a core-pump source, a 2-cm-long distributed Bragg reflector fiber laser delivers single-frequency output at 1950 nm with laser linewidth less than 3 kHz, which is, to the best of our knowledge, the narrowest linewidth demonstrated to date from any 2 microm single-frequency laser.

Dual-frequency Brillouin fiber laser for optical generation of tunable low-noise radio frequency/microwave frequency.

We demonstrate a new approach, i.e., a cw dual-frequency Brillouin fiber laser pumped by two independent single-frequency Er-doped fiber lasers, for the generation of tunable low-noise rf/microwave optical signals. Its inherent features of both linewidth narrowing effect in a Brillouin fiber cavity and common mode noise cancellation between two laser modes sharing a common cavity allow us to achieve high frequency stability without using a supercavity. Beat frequency of the dual-frequency Brillouin fiber laser can be tuned from tens of megahertz up to 100 GHz by thermally tuning the wavelengths of the two pump lasers with tuning sensitivity of approximately 1.4 GHz/ degrees C. Allan variance measurements show the beat signals have the hertz-level frequency stability.

Distributed fiber temperature and strain sensor using coherent radio-frequency detection of spontaneous Brillouin scattering.

A novel technique that enables coherent detection of spontaneous Brillouin scattering in the radio-frequency (<500 MHz) region with excellent long-term stability has been demonstrated for distributed measurements of temperature and strain in long fiber. An actively stabilized single-frequency Brillouin fiber laser with extremely low phase noise and intensity noise is used as a well-defined, frequency-shifted local oscillator for the heterodyne detection, yielding measurements of spontaneous Brillouin scattering with high frequency stability. Based on this approach, a highly stable real-time fiber sensor for distributed measurements of both temperature and strain over long fiber has been developed utilizing advanced digital signal processing techniques.

Efficient operation of diode-pumped single-frequency thulium-doped fiber lasers near 2 micro m.

Efficient operation of diode-pumped single-frequency fiber lasers at wavelengths from 1740 to 2017 nm has been demonstrated by using a very short piece of newly developed single-mode active fiber, i.e., heavily thulium-doped germanate glass fiber. At 1893 nm, the single-frequency fiber laser has a pump threshold of 30 mW, a slope efficiency of 35%, and maximum output power of 50 mW with respect to the launched power of single-mode pump diodes at 805 nm. To the best of our knowledge, this is the highest lasing efficiency achieved in single-frequency fiber lasers operating near 2 micro m. Frequency noise of the single-frequency fiber laser at 1893 nm has been characterized and compared with that of single-frequency fiber lasers at 1 and 1.55 micro m.

Pump-to-Stokes transfer of relative intensity noise in Brillouin fiber ring lasers.

We have experimentally investigated pump-to-Stokes intensity noise transfer in both the frequency domain and the time domain in all-fiber single-frequency Brillouin ring lasers. In the high-frequency region (>1MHz), the pump-to-Stokes noise transfer function can be much smaller than unity, indicating that the Brillouin fiber lasers act as an efficient low-pass filter. The maximum noise reduction of 40-60 dB was observed at antiresonant frequencies that are multiples of half the cavity free spectral range. This is the first experimental demonstration, to the authors' knowledge, of intensity noise reduction in Brillouin fiber lasers.