News Column

Patent Issued for Ultraviolet Fiber Laser System

July 23, 2014

By a News Reporter-Staff News Editor at Journal of Engineering -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventor Ter-Mikirtychev, Valeri V. (Mountain View, CA), filed on August 17, 2012, was published online on July 8, 2014.

The assignee for this patent, patent number 8774236, is Veralas, Inc. (Mountain View, CA).

Reporters obtained the following quote from the background information supplied by the inventors: "The invention relates generally to lasers and laser systems, and more particularly to fiber lasers operating in the ultraviolet (UV) spectral range.

"Ultraviolet laser sources having high power and high pulse energy for short nanosecond pulse widths are required for many important applications such as laser material processing and medicine, e.g., eye surgery. Presently, these applications primarily use excimer gas lasers because of the lack of other suitable laser sources. Excimer lasers can produce laser emissions at several main wavelengths, such as nanometer (nm) wavelengths of 351 nm, 308 nm, 248 nm, 193 nm and 157 nm. While being reliable laser sources, excimer lasers are bulky, require periodic service and, therefore, can have high ongoing costs of ownership. In addition high power excimer lasers often exhibit poor beam quality, use high voltage electronics and have low wall plug efficiency.

"Fiber lasers are a relatively new type of laser source that are capable of delivering laser emission in a wide spectral range from infrared (IR) to UV. Fiber lasers can operate in continuous wave (CW) to ultra-short pulse modes with output power levels exceeding tens of kilowatts (kW). Commercially available fiber lasers operate in pulse and CW regimes, and at fundamental operating frequencies with micrometer (.mu.m) wavelengths in the 1 .mu.m, 1.5 .mu.m and 2 .mu.m ranges. Nonlinear frequency conversion can be used to generate higher order harmonics and shorter wavelengths. One approach to obtaining UV spectral wavelengths in the 190-196 nm range has been to use a fiber laser operating at a fundamental wavelength in the 1.5 .mu.m range and non-linearly convert the fundamental frequency to its harmonic. However, obtaining the desired average power level and pulse energy for nanosecond pulses in the higher harmonic spectral emissions has been a problem.

"Several nonlinear optical effects, mainly Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and Self Phase Modulation (SPM), as well as the bulk optical damage in a fiber core limit the pulse energy of a fiber laser. The fiber laser pulse energy is usually restricted to a fraction of a millijoule (mJ) for nanosecond (ns) pulse widths and the pulse peak power is restricted to tens of kW.

"This peak power limitation may be overcome using ultra short pulse generation in the picosecond (ps) and femtosecond (fs) ranges due to higher optical damage threshold of the glass fiber in these ranges of laser pulse width. Ultra short pulse, high peak power fiber lasers can generate high average powers with close to transform limited pulses. However, even for transform limited operation, the ultra short pulse fiber lasers have relatively broad spectral line widths (this is a basic quantum mechanics uncertainty principle). For example, a 1 ps transform limited Gaussian shaped optical pulse has approximately a 3 nm spectral line width at a 1.5 .mu.m wavelength. This broad spectral bandwidth limits efficient nonlinear frequency conversion to higher optical harmonics. In addition increasing the ultra-short laser pulse energy significantly increases the already high peak power which, in turn, triggers the detrimental nonlinear optical effects that impact the fiber laser power scaling.

"Thus, power and pulse energy scaling of a fiber laser is a challenging task, particularly when applications require close to diffraction limited beam quality, a pulse width less than 10 ns, tens of watts of average power, and a polarized beam. Scaling a fiber laser pulse energy in the visible and UV spectral range through nonlinear frequency conversion is likewise limited by the low available pulse energy of the fundamental wavelength. With sub-mJ pulse energy at a fundamental wavelength of 1 .mu.m, the pulse energy in the or higher harmonics is of the order of a microjoule (.mu.J).

"One approach to overcome these problems and scale the power and pulse energy to required levels in the UV range has been to use beam combining architectures (fiber bundling) that combines the outputs of several fiber laser sources to form a composite beam having increased average power and pulse energy with subsequent higher order nonlinear frequency conversion to the UV spectral range. For example, MOPA (master oscillator power amplifier) systems comprising erbium (Er) doped fibers synchronously seeded by the same master oscillator (MO) operating in the 1.5 .mu.m range have been bundled to form a composite beam, and subsequently converted to the 193 nm spectral range. While fiber bundling can increase the pulse energy in the fundamental beam and high order harmonics, combining the outputs from bundled individual fiber lasers deteriorates the resulting laser beam and requires fine (nanosecond or sub-nanosecond) gating of the individual laser pulses to overlap. Additionally, the polarizations of the beams have to be aligned for efficient nonlinear harmonic generation which complicates spatial alignment of the individual fiber laser sources in the bundle. Another possibility is to combine multiple laser beams that have already been converted by the nonlinear frequency conversion to the UV range. However, this approach also requires several individual fiber laser sources, and is bulky, complicated and costly.

"Other approaches to achieve UV fiber laser operation use optical frequency mixing of different pulsed MOPAs having different fundamental wavelengths, for example, one having a fundamental wavelength of 10YY nm and another having a fundamental wavelength of 15YY nm (or 21YY nm), to provide trains of optical pulses. The 10YY-nm pulses are frequency quintupled to a wavelength of 213 nm, and the 15YY nm (or 21YY nm) pulses are mixed with the 213 nm pulses to provide pulses having a wavelength of 193 nm. The 10YY nm and 21YY nm MOPAs include a fiber-laser and a bulk amplifier. However, this still requires two laser systems which need to be synchronized, properly triggered, and spatially overlapped in nonlinear crystals.

"It is desirable to provide efficient power and energy scalable hybrid fiber laser/bulk crystalline solid-state amplifier systems capable of yielding a high pulse energy scalable to over 10 mJ, high average power scalable to over 100 W, output pulse widths controllable and variable from sub-ns to hundreds of ns pulse duration, and pulse repetition rates controllable from tens of Hz to over a MHz. It is to these ends that the present invention is directed."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventor's summary information for this patent: "The invention affords new laser systems, based on fiber laser technology and laser active crystalline materials, which provide laser emission lines close to the 193 nm and 248 nm spectral lines, power and pulse energy scaling with close to diffraction limited laser beam quality, reduced laser volume, improved laser efficiency, controlled pulse repetition rate, controlled pulse duration and reduced cost. Laser systems in accordance with the invention can replace excimer lasers.

"In one aspect, the invention affords laser systems operating in the ultraviolet and visible light ranges that have high pulse energy, high average power, and controllable pulse width and pulse repetition rate by using a hybrid fiber-bulk master oscillator--power amplifier system comprising a low power seed laser and a hybrid fiber-bulk laser amplifier and subsequent nonlinear frequency conversion.

"The invention further affords high power, pulsed, diffraction limited Ytterbium (Yb.sup.3+) doped fiber lasers operating in the spectral vicinity of 980 nm which corresponds to the zero-phonon emission line of Yb.sup.3+ in glass fiber between 972 nm and 985 nm in the near infrared spectral range, with subsequent pulse energy amplification using a matching crystalline laser amplifier and non-linear frequency conversion to the visible region between 486 nm to 492.5 nm, the UV region of 243 nm to 246.25 nm, and to the 194.4 nm to 197 nm spectral range.

"The invention, in another aspect, affords a high power, pulsed diffraction limited Er.sup.3+ doped or Er.sup.3+--Yb.sup.3+ doped fiber laser system operating in approximately the 1550 nm spectral range corresponding to the Er.sup.3+ emission line in optical fiber glass between 1530 nm and 1610 nm in the near infrared region, with subsequent pulse energy amplification using a matching crystalline laser amplifier and non-linear frequency conversion to the 191.25 nm to 201.25 nm UV spectral range.

"The invention, in yet a further aspect, affords high power, pulsed, diffraction limited Thulium (Tm.sup.3+) doped fiber lasers system operating in the 2 .mu.m spectral range which corresponds to the Tm.sup.3+ emission line in optical fiber glass between about 1800 nm and 2150 nm in the near infrared with subsequent pulse energy amplification using a matching crystalline laser amplifier and non-linear frequency conversion to the 450 nm to 537.5 nm blue-green spectral range.

"The invention, in still another aspect, affords simultaneous scaling of the laser pulse energy, and control of the laser pulse duration and the laser pulse repetition rate using a matched hybrid fiber--bulk laser crystalline amplifier comprising laser materials doped with an element selected from the group Yb.sup.3+, Er.sup.3+, Er.sup.3+--Yb.sup.3+, Tm.sup.3+ and Ho.sup.3+ (Holmium), and nonlinear frequency conversion of the amplified pulses to produce laser lines in the visible and UV spectral ranges.

"Applications of laser systems in accordance with the invention include materials processing, laser communication, and laser medicine, especially for treatment of the eye."

For more information, see this patent: Ter-Mikirtychev, Valeri V.. Ultraviolet Fiber Laser System. U.S. Patent Number 8774236, filed August 17, 2012, and published online on July 8, 2014. Patent URL:

Keywords for this news article include: Veralas Inc.

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Source: Journal of Engineering

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