News Column

Patent Application Titled "Optical Scanning and Imaging Systems Based on Dual Pulsed Laser Systems" Published Online

August 28, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventors FERMANN, Martin E. (Dexter, MI); HARTL, Ingmar (Hamburg, DE), filed on April 8, 2014, was made available online on August 14, 2014.

The assignee for this patent application is Imra America, Inc.

Reporters obtained the following quote from the background information supplied by the inventors: "The invention relates to scanning pulsed laser systems for optical imaging.

"Dual pulsed laser systems comprising two modelocked lasers operating at two slightly different repetition rates f.sub.1 and f.sub.2, such that .delta.=|(f.sub.1-f.sub.2)|

"The use of dual modelocked lasers can be replaced for probing the optical response functions by implementing dual electronic circuit systems, as has been suggested in U.S. Pat. No. 5,748,309 by van der Weide. The approach has some benefit for the interrogation of the spectral dependence of signal transmission in the THz spectral range. Two pulsed signal sources also operating at two slightly different repetition rates f.sub.1 and f.sub.2 were disclosed, which produce emission in the THz spectral range made up of frequency lines comprising pure harmonics of the two repetition rates. Detection of the beat signal at .delta., 2.delta., . . . n.delta. is then used to infer the signal transmission at the harmonics of the repetition rate f.sub.1, 2f.sub.1, . . . nf.sub.1. Note that in this scheme a beat signal at a difference frequency as low as .delta. is used, which is not ideal, since .delta. typically has a small value where acoustic noise can corrupt the signal.

"The use of mode locked lasers was again later disclosed by Keilmann et al., in 'Time domain mid-infrared frequency-comb spectrometer', Opt. Lett., vol. 29, pp. 1542-1544 (2004), who suggested the use of a dual scanning laser system for Fourier Transform Spectroscopy (FTS) and the analysis of the spectral transmission of materials in the infrared spectral range.

"In order to improve the scan rate of dual laser scanning FTS, Keilmann et al., in International Patent Application Publication WO2007/045461, further suggested to dither the repetition rate of one laser versus the other using techniques similar to the ones described in the '016 patent.

"The use of lasers for spectroscopy has also been suggested by Haensch et al. in U.S. Pat. No. 7,203,402, where a single frequency comb laser based on a mode-locked laser was used for the measurement of certain properties of optical elements. Here the measurement was performed either simultaneously or sequentially at each individual frequency line of the comb laser.

"A frequency comb laser was recently also combined with a conventional Fourier transform spectrometer to obtain an improved signal/noise ratio for spectral measurements (J. Mandon et al., 'Fourier transform spectroscopy with a laser frequency comb', in Nature Photonics, 2009)

"Prior dual scanning laser systems have a number of limitations when applied to spectroscopy. The low repetition rate of implemented laser sources leads to excessively long data acquisition times. and the techniques for signal generation in the near IR to mid-IR spectral range are relatively cumbersome. Systems implemented with bulky solid-state lasers are not well suited for instrumentation applications and require a large components count. Other systems (P. Giaccari et al., 'Active Fourier-transform spectroscopy combining the direct RF beating of two fiber-based mode-locked lasers with a novel referencing method', Opt. Express., vol. 16, pp. 4347 (2008)) and (I. Coddington et al., 'Coherent Multiheterodyne Spectroscopy Using Stabilized Optical Frequency Combs,' Phys. Rev. Lett. 100, 13902 (2008)) provide only very limited spectral coverage."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "In the following we refer to dual scanning laser systems that exploit the discrete frequency spectrum, i.e. the comb spectrum, of modelocked lasers but that do not require or do not rely on precision comb control inside the laser oscillator as coherent dual scanning lasers, CDSLs.

"Here we disclose a new CDSL for applications in spectroscopy, micro-spectroscopy, microscopy, Fourier transform spectroscopy (FTS), optical and THz imaging, and/or similar applications. The CDSLs are based on modelocked fiber lasers designed for operation at high repetition rates allowing for large scanning speeds. Efficient spectroscopic measurements are enabled by the implementation of low noise, phase controlled fiber lasers, which are designed to provide broad spectral coverage via the implementation of nonlinear spectrally broadening optical elements. Various compact designs are described. In various embodiments a reduction of component count is further accomplished via simultaneous use of nonlinear spectral broadening elements and the use of appropriate time delays between the lasers.

"We further disclose the use of highly nonlinear waveguides in conjunction with coherent supercontinuum generation for generating an optical output from the visible to the mid-infrared spectral region. Difference frequency generation (DFG) produces output in the mid-IR spectral region and simplifies the implementation of FTS. DFG eliminates variations of the carrier envelope offset frequency external to the laser cavity and thus produces an output spectrum comprising true harmonics of the laser repetition rates.

"In conjunction with photoconductive antennas spectral emission in the THz spectral region can be obtained.

"In order to use difference frequency generation effectively, the mode locked lasers can be configured with two outputs each. Amplifiers can be further implemented to amplify those outputs. Supercontinuum generation can then be implemented for spectral broadening of these fiber laser outputs. Difference frequency generation can be implemented between spectral components of the supercontinuum or between a spectral component of the supercontinuum and another fiber laser output.

"Nonlinear signal interference in nonlinear frequency broadening elements from overlapping pulses can be eliminated by using separate nonlinear frequency broadening elements for each laser. Alternatively, an optical delay line can be inserted at the output of the CDSL to produce an interference signal only from pulses that do not overlap in any nonlinear optical elements. Electronic gating can further be implemented for optimum signal conditioning.

"In at least one embodiment the carrier envelope offset frequencies in coherent dual scanning femtosecond modelocked fiber lasers can be adjusted by control of various intra-cavity optical elements such as intra-cavity loss, saturable absorber temperature, fiber temperature and fiber grating temperature. In some embodiments carrier envelope offset frequency control can be averted by the implementation of DFG.

"In at least one embodiment the carrier envelope offset frequencies and repetition rates in coherent dual scanning femtosecond modelocked fiber lasers can further be controlled by phase locking the two lasers to external cavities.

"In at least one embodiment the carrier envelope offset frequencies and repetition rates in coherent dual scanning femtosecond modelocked fiber lasers can further be controlled by phase locking the two lasers to two external single-frequency lasers.

"In another embodiment the difference in carrier envelope offset frequencies and the repetition rates in coherent dual scanning femtosecond modelocked fiber lasers can further be controlled by phase locking the two lasers to one external single-frequency laser.

"For improved spectral resolution coherent dual scanning femtosecond modelocked fiber lasers can also be constructed with lasers where the repetition rate of one laser is an approximate harmonic of the repetition rate of the other laser.

"The noise of the carrier envelope offset frequencies can be minimized by an appropriate adjustment of the intra-cavity laser dispersion, and the pulse width injected into the supercontinuum fibers.

"Any drift in carrier envelope offset frequency between the two lasers in the CDSLs can be monitored and corrected for by external optical means. Also, an f-2f interferometer can be implemented for carrier envelope offset frequency monitoring.

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 is a diagram illustrating an example of a CDSL.

"FIG. 2 is a schematic diagram of an optically integrated dispersion compensator, and non-linear frequency conversion section as used for supercontinuum generation.

"FIG. 3 is a schematic diagram of a CDSL as used for optical imaging applications.

"FIG. 4 is a schematic diagram of a CDSL designed with a reduced number of components.

"FIG. 5 is a schematic diagram of yet another CDSL based on carrier envelope offset frequency monitoring.

"FIG. 6a is a schematic diagram of an intra-cavity assembly of a modelocked fiber oscillator for resistive heating of an intra-cavity fiber Bragg grating for carrier envelope offset frequency control.

"FIG. 6b is a schematic diagram of an intra-cavity assembly of a modelocked fiber oscillator for modulating the pressure applied to an intra-cavity fiber Bragg grating for carrier envelope offset frequency control.

"FIG. 6c is a schematic diagram of an assembly including an intra-cavity modulator as used for modulating the intra-cavity loss of a modelocked laser for carrier envelope offset frequency control.

"FIG. 6d is a schematic diagram of an intra-cavity assembly of a modelocked fiber oscillator for modulating the residual pump power impinging on an intra-cavity saturable absorber for carrier envelope offset frequency control.

"FIG. 7 is a plot of the RF spectrum of a carrier envelope offset frequency locked Yb fiber laser operating at a repetition rate of 1 GHz measured after a nonlinear f-2f interferometer.

"FIG. 8 is a plot of the spectral output of Yb fiber laser based coherent supercontinuum source operating at a repetition rate of 1 GHz.

"FIG. 9 is a schematic diagram of a dual scanning laser system which is locked to two external cavities for repetition rate and carrier phase control.

"FIG. 10 is a schematic diagram of a dual scanning laser system which is locked to two narrow linewidth lasers for repetition rate and carrier phase control.

"FIG. 11 is a schematic diagram of an ultra-compact dual scanning laser system which uses one external narrow linewidth laser for repetition rate and carrier phase control."

For more information, see this patent application: FERMANN, Martin E.; HARTL, Ingmar. Optical Scanning and Imaging Systems Based on Dual Pulsed Laser Systems. Filed April 8, 2014 and posted August 14, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=4328&p=87&f=G&l=50&d=PG01&S1=20140807.PD.&OS=PD/20140807&RS=PD/20140807

Keywords for this news article include: Laser System, Medical Devices, Imra America Inc..

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