WO2017004473A1

WO2017004473A1 - Multimodal imaging source based on femtosecond lasers for picosecond pulse generation

Info

Publication number: WO2017004473A1
Application number: PCT/US2016/040593
Authority: WO
Inventors: Erin Stranford LAMB; Frank W. Wise
Original assignee: Cornell University
Priority date: 2015-06-30
Filing date: 2016-06-30
Publication date: 2017-01-05

Abstract

The technology disclosed in this patent document includes pulsed fiber laser technologies for producing high energy femtosecond laser pulses and picosecond pulses as multimodal pulsed laser sources for various applications including applications involving nonlinear optical interactions and other applications desiring high pulses peak power and energy with different laser wavelengths. Implementations of the disclosed technology include use of spectral compression of a high-power femtosecond fiber laser as a route to producing transform-limited picosecond pulses to optically pump a fiber optical parametric oscillator to yield a robust fiber source capable of providing the synchronized picosecond pulse trains needed for Raman scattering microscopy and other applications using laser pulses at different laser wavelengths. Thus, this system can be used as a multimodal platform for nonlinear microscopy techniques and various sensing applications.

Description

MULTIMODAL IMAGING SOURCE BASED ON FEMTOSECOND LASERS FOR

PICOSECOND PULSE GENERATION

PRIORITY CLAIM AND RELATED PATENT APPLICATION

[0001] This patent document claims the priority and benefits of U.S. Provisional Patent Application No. 62/187,220 entitled "MULTIMODAL IMAGING SOURCE BASED ON FEMTOSECOND LASERS FOR PICOSECOND PULSE GENERATION" and filed on June 30, 2015, which is incorporated by reference as part of this document.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under grant EB002019 awarded by the National Institutes of Health, along with grants BIS-0967949 and ECCS-1306035, awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

[0003] This patent document relates to pulsed laser technologies including pulsed fiber lasers.

[0004] Pulsed lasers can be configured to produce coherent laser pulses with durations in picoseconds and femtosecond ranges and with high pulse energy and high pulse peak power that are desirable in a wide range of applications including telecommunications, materials processing, biological imaging and surgery, environmental monitoring, metrology and scientific research. Fiber pulsed lasers use doped optical fiber as the laser gain media to achieve advantages over solid-state laser gain materials. For example, the waveguide nature of optical fiber enables good optical overlap and alignment of the pump and signal in the fiber gain medium; some fiber laser implementations use all fiber optical components with a near alignment free operations to avoid technical issues associated with free space components; the large ratio of the surface area to the volume of the fiber can largely eliminate the need for water cooling of the fiber gain medium; the single mode properties of optical fiber can be used to ensure excellent beam quality; in addition, fiber lasers tend to be less expensive than solid-state lasers, enabling laser technology to be more widely adopted, especially outside of specialized laboratories. [0005] The waveguide confinement of the fiber gain medium in pulsed fiber lasers leads to high concentration of laser energy in or near the core of the fiber, a small spatial volume when compared to counterparts in the solid-state pulsed lasers. This waveguide confinement can lead to large enhancements of nonlinear effects that may be undesirable in some aspects to laser performance.

SUMMARY

[0006] The technology disclosed in this patent document includes pulsed fiber laser technologies for producing high energy femtosecond laser pulses and picosecond pulses as multimodal pulsed laser sources for various applications including applications involving nonlinear optical interactions and other applications desiring high pulses peak power and energy with different laser wavelengths. Implementations of the disclosed technology include use of spectral compression of a high-power femtosecond fiber laser as a route to producing transform- limited picosecond pulses to optically pump a fiber optical parametric oscillator to yield a robust fiber source capable of providing the synchronized picosecond pulse trains needed for Raman scattering microscopy and other applications using laser pulses at different laser wavelengths.

Thus, this system can be used as a multimodal platform for nonlinear microscopy techniques and various sensing applications.

[0007] In one aspect, a pulsed laser is provided for producing femtosecond and picosecond laser pulses and includes a femtosecond pulse laser that produces femtosecond laser pulses at a first laser wavelength; an anomalous dispersion element that receives the femtosecond laser pulses to produce first modified laser pulses by inducing a negative chirp; and a nonlinear optical material located to receive the first modified laser pulses and to induce spectral compression in laser pulses, thus producing picosecond laser pulses at the first laser wavelength. In

implementations, a first optical output port is provided to output femtosecond laser pulses produced by the femtosecond pulse laser and a second optical output port is provided to output the picosecond laser pulses. In implementations, an optical amplifier can be coupled between the second optical output port and the nonlinear optical material to amplify the picosecond laser pulses at the first laser wavelength and such an optical amplifier may include a divided pulse amplifier. In addition, the pulsed laser can include an optical conversion device coupled to receive a portion of the picosecond laser pulses at the first laser wavelength output by the nonlinear optical material and configured to produce, via a nonlinear optical process, new light of picosecond laser pulses at a second laser wavelength that is different from the first laser wavelength. A third optical output port can be coupled to the optical conversion device to output the picosecond laser pulses at the second laser wavelength.

[0008] The above and other aspects, associated technical features, and their implementations are described in greater detail in the drawings, the description and the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 shows an example of a multimodal pulsed fiber laser showing some of functional modules based on the disclosed technology.

[0010] FIG. 2 includes FIGS. 2A, 2B and 2C showing simulation of the spectral compression of femtosecond laser pulses to produce transform-limited Gaussian picosecond laser pulses: (a) Simulation trends for 4 nJ transform limited Gaussian pulses with the initial pulse duration given in the legend. Compression is performed in fiber lengths of 50-100 m with the length selected to give the minimum spectral bandwidth (FIG. 2A). (b) Simulated spectral compression of a 4 nJ dissipative soliton (FIG. 2B). (c) Experimental spectral compression of a 1 nJ dissipative soliton (FIG. 2C).

[0011] FIG. 3 shows an example of a multimodal pulsed fiber laser based on the laser design in FIG. 1 that includes an all normal dispersion mode locked pulsed laser or a dissipative soliton laser for producing femtosecond laser pulses (purple), a spectral compression stage that compresses the pulse spectral width and increases the pulse duration of the laser pulses from the mode locked pulse laser (yellow), a divided-pulse amplifier (DP A) for amplifying the laser pulse power of laser pulses from the spectral compression stage to produce picosecond laser at the same laser wavelength as the fs laser pulses (green), and an optical parametric oscillator (OPO) that converts the picosecond laser pulses to laser pulses at a longer laser wavelength than the laser wavelength of the mode locked pulsed laser (red). Legends include "col." for optical collimator; QWP for quarter- wave plate; HWP for half-wave plate; PBS for polarizing beam splitter; B.P. for birefringent plate; ISO for optical isolator; M for mirror; PCF for photonic crystal fiber. [0012] FIG. 4 includes FIGS. 4A, 4B, 4C and 4D showing spectrally compressed dissipative soliton laser: (a) spectrum and (b) autocorrelation. Signal pulses from optical parametric oscillator: (c) spectrum and (d) autocorrelation

[0013] FIG. 5 shows examples of measurements of intensity noise spectra for the 1040 nm and 800 nm pulse trains. Data taken with a Signal Hound SA44B analyzer with a resolution bandwidth of 6.5 kHz.

[0014] FIG. 6 includes FIG. 6A showing energy levels and transitions in an example of coherent anti-Stokes Raman scattering (CARS) microscopy and FIG. 6B showing energy levels and transitions in an example of stimulated Raman scattering (SRS) microscopy. DETAILED DESCRIPTION

[0015] The technology disclosed in this document can be used to provide a fiber pulsed laser platform for generating high power and synchronized laser pulses in two different picosecond pulse trains at two different laser wavelengths from a high power femtosecond laser pulse train. Therefore, such a fiber pulsed laser platform is multimodal for outputting 3 different pulse outputs for a wide range of applications. In various implementations, the laser wavelengths, the pulse durations and other parameters of the generated laser pulses can be adjusted or tunable to meet specific requirements in applications.

[0016] Numerous fiber-based systems can be used to produce laser pulses at different wavelengths using various techniques Raman imaging, including, e.g., using fiber lasers to pump solid-state optical parametric oscillators (OPOs), and using fiber lasers to as pump in various nonlinear processes in fiber to generate the second color or laser wavelength, using four-wave mixing (FWM) in the normal dispersion region of photonic crystal fiber (PCF) to frequency- convert picosecond pulses from ytterbium-doped fiber lasers. In addition to matching the pulse parameters needed for a particular applications such as CARS, certain applications also need to have ultra-low intensity noise laser pulses at different wavelengths such as laser sources for SRS microscopy. Using all-fiber laser designs to achieve low intensity noise for video-rate SRS microscopy with direct detection can be difficult. The technology disclosed in this document provides a multimodal pulsed laser platform to achieve such a low noise performance by providing both synchronized picosecond pulses for the Raman microscopies and the high energy femtosecond pulses for multiphoton techniques. [0017] FIG. 1 shows an example of a multimodal pulsed fiber laser showing some of functional modules based on the disclosed technology. A femtosecond pulsed laser is provided as the base laser for the multimodal pulsed fiber laser. Next, an anomalously dispersive material is used to induce a negative chirp on the laser pulses to produce the femtosecond laser pulses as the first output. A nonlinear optical medium to induce spectral compression of each laser pulse and thus to increase the laser pulse duration to generate the picosecond laser pulses at the same laser wavelength of the femtosecond laser pulses. This is the second laser output of the system for producing the first picosecond laser pulse train. In some implementations, spectral filtering can be introduced between the laser and the spectral compression stage for extra tunability of the pulse duration of the first picosecond laser pulse train. In the above processes, the compression may be performed in a passive or active material. This technique could be implemented with various suitable femtosecond lasers, anomalously dispersive media, and nonlinear media.

Downstream from the nonlinear medium after producing the first picosecond laser pulse train, an OPO can be used to generate a second picosecond laser pulse train at a different laser wavelength that can be tuned to a desired frequency difference from the laser wavelength of the first picosecond laser pulse train.

[0018] Various features are provided in this disclosure for laser designs and laser methods for producing high power picosecond pulses through spectral compression of a femtosecond laser, such as a dissipative soliton laser. In addition to providing a route to high power picosecond pulses, this source is capable of providing both femtosecond and picosecond pulses, making it an ideal platform for multimodal imaging that incorporates Raman scattering microscopy with other multiphoton techniques. An additional frequency conversion stage, such as an optical parametric oscillator, can be used in conjunction with the spectrally compressed femtosecond laser to generate the second color for the Raman techniques. Additionally, other applications in spectroscopy, microscopy, materials processing, and other areas require transform limited picosecond pulses. Soliton-based lasers are currently the most reliable source for picosecond pulse generation but various designs for fiber soliton lasers may be limited to low energy pulses and thus exhibit higher intensity noises. The designs based on the disclosed technology can produce high power picosecond pulses at potentially lower intensity noise levels in a fiber platform. Products such as the Insight DeepSee from Newport's Spectra-Physics serve as two color multimodal imaging sources. However, the pulse durations are on the order of 120 fs, so the user must perform additional pulse shaping to achieve the picosecond pulse durations for Raman microscopy, highlighting the advantage of the disclosed technology's ability to easily generate both durations.

[0019] The technology described in this patent document can be implemented in specific ways that provide one or more of the following features. For example, a fiber laser source can produce high power picosecond pulses that serve as the pump pulses for a frequency conversion stage to create a two-color source for Raman microscopy in a fiber platform. Since this technology starts from a high power femtosecond laser, it can naturally produce both pulse durations, giving it a large advantage over other multimodal imaging sources incorporating Raman microscopy with imaging modalities that favor femtosecond pulses. Many other multimodal sources provide either picosecond or femtosecond pulses, but not both. Notably, the disclosed technology can be used to provide picosecond pulses at higher energies, and thus potentially lower relative intensity noise levels, than competing fiber microscopy sources. This is advantageous for applications, such as stimulated Raman scattering microscopy, that require very low-noise picosecond pulses.

[0020] Referring back to FIG. 1, in some implementations, dissipative soliton lasers with all normal dispersion elements inside the laser cavity can be configured as a reliable source of high- power femtosecond laser pulses. Launching a down-chirped pulse into a nonlinear medium induces spectral compression, thus creating longer pulses. This transforms the high-power femtosecond pulses from a dissipative soliton laser into picosecond pulses by inducing a negative chirp with diffraction gratings and then launching the pulses back into fiber. This creates a simple and robust source of high-power picosecond pulses.

[0021] The output from a dissipative soliton laser (or other suitable source for femtosecond pulses) is down-chirped through diffraction gratings (e.g., a grating pair), prisms, or other anomalously dispersive material. The down-chirped pulses are then coupled into passive fiber, active fiber, or other nonlinear material to induce spectral compression to generate picosecond pulses. Various applications, such as spectroscopy, microscopy, materials processing, or machining, requiring high power and/or low noise picosecond pulses may benefit from this method of picosecond pule generation. Specifically, the disclosed technology provides a route to the highest power and thus potentially the lowest noise picosecond pulses from a fiber-source, making it an ideal candidate for use in a multimodal fiber-source incorporating Raman microscopy techniques. Additional information pertaining to the disclosed technology is described in the U.S. Provisional Patent Application No. 62/187,220 entitled "MULTIMODAL IMAGING SOURCE BASED ON FEMTOSECOND LASERS FOR PICOSECOND PULSE GENERATION" and filed on June 30, 2015 and Dr. Erin Lamb's dissertation entitled

"Development Of Fiber Lasers And Devices For Coherent Raman Scattering Microscopy" at Cornell University in 2015 (which is available online at

http8://ecomtnons.comeli.edu b.aadle/^ 81.3/41086?8how^::::fuil ), both of which are incorporate by reference as part of the disclosure of this patent document.

[0022] Simulations and experiments were conducted on the basic multimodal design in FIG. 1. The femtosecond pulsed laser in FIG. 1 can be implemented by an all-normal-dispersion femtosecond fiber laser disclosed in Cornell University's U.S. Patent No. 8,416,817 issued on April 9, 2013, which is incorporated by reference as part of this patent document. Such a fiber laser for facilitating generation of femtosecond duration pulses can include a gain fiber to increase the energy of a pulse in a laser cavity that is free of anomalous dispersion and whose dispersion elements inside the laser cavity consist only of elements with normal group velocity dispersion. The laser cavity includes a fiber gain medium to produce the laser gain at the laser wavelength of the pulsed laser (e.g., doped fiber media) and one or more dispersion elements, each of which provides only normal group velocity dispersion. For example, the laser cavity can include a chirping element for broadening a multiple frequency component pulse in the laser cavity and spreading the frequency components in a laser pulse apart over time, and a spectral bandpass filter for passing a portion of a chirped pulse including only frequency components in a narrow range around a selected center frequency, thereby producing self-amplitude modulation of the pulse. The laser can include a pulse dechirping element outside the laser cavity to narrow the pulse duration of the laser pulses output by the laser cavity to a femtosecond pulse duration. Such a laser is also referred to as a dissipative solution laser.

[0023] Such a dissipative solution laser can designed to provide very high-energy pulses for a given fiber core size. The performance of these lasers has been well documented, e.g., by A. Chong, W. H. Renninger, and F. W. Wise in "Properties of normal-dispersion femtosecond fiber lasers," J. Opt.Soc. Am. B 25, 140-148 (2008). The generate chirped femtosecond pulses from such a laser can be processed to compress their spectral range to create transform-limited picosecond pulses. Spectral compression occurs when a down-chirped pulse is launched into a nonlinear material. FIG. 2 A shows the calculated spectral compression ratio versus impressed anomalous dispersion for 4 nJ transform limited Gaussian pulses with the indicated initial durations. The spectral compression is performed in passive fiber with a 10 mm core diameter up to 100 m long; the length is chosen to minimize the compressed bandwidth. These trends indicate that larger compression ratios can be achieved by impressing larger amounts of negative chirp on the pulse and then compressing the spectrum in correspondingly longer lengths of fiber in order to reach the transform-limited duration. FIG. 2B shows compression of a simulation of a dissipative soliton. As the compressed pulse becomes longer, its peak power and thus nonlinear phase accumulation is reduced, which results in the saturation of the compression ratio with increasing anomalous dispersion. As indicated in FIG. 2A, the compression ratio can be limited to 10-20 with typical laser parameters. FIG. 2C further shows experimental spectral

compression of a 1 nJ dissipative soliton.

[0024] FIG. 3 shows an example of a multimodal pulsed fiber laser based on the laser design in FIG. 1. Details of various components are described below and additional information on the laser designs and components are provided in Dr. Erin Lamb's dissertation entitled

"Development Of Fiber Lasers And Devices For Coherent Raman Scattering Microscopy." This multimodal pulsed fiber laser includes an all normal dispersion mode locked pulsed laser or a dissipative soliton laser for producing femtosecond laser pulses as indicated by the dashed box. The output laser pulses are direct through a dechirping device to produce the femtoseocnd laser pulse output as the first laser output of this device at a first laser wavelength λΐ, e.g., 1040 nm. The multimodal pulsed fiber laser includes a spectral compression stage that is optically downstream from the femtosecond laser to compress the pulse spectral width and increase the pulse duration of the laser pulses. An optical amplifier, such as a divided-pulse amplifier (DP A), for amplifying the laser pulse power of laser pulses from the spectral compression stage, is provided to produce picosecond laser pulses at the same laser wavelength λΐ as the fs laser pulses (e.g., 1040 nm). This produces a picosecond laser pulse train as the second laser output of the multimodal pulsed fiber laser.

[0025] In addition, the multimodal pulsed fiber laser in FIG. 3 includes an optical conversion device that converts the picosecond laser pulses at the first laser wavelength λΐ to a longer laser wavelength λ2. This optical conversion device can be, in some implementations, an optical parametric oscillator (OPO) as shown by a fiber OPO in FIG. 3. This fiber OPO produces the third laser output for the multimodal pulsed fiber laser in FIG. 3. The first picosecond laser pulses at the first laser wavelength λΐ and second picosecond laser pulses at the second laser wavelength λ2 are synchronized to each other and the difference between their wavelengths can be tuned by the operation of the OPO to be resonant with specific vibrational transitions for Raman imaging or spectroscopic measurements.

[0026] The specific example in FIG. 3 is one of various implementations of a pulsed laser for producing femtosecond and picosecond laser pulses. In connection with the features in FIG. 3, such a pulsed laser can include the following components to produce three different optical trains of pulses. First, a femtosecond pulse laser is provided to produce femtosecond laser pulses at a first laser wavelength and may include, for example, a laser cavity that is free of anomalous dispersion and whose dispersion elements inside the laser cavity consist only of elements with normal group velocity dispersion. The laser cavity includes a fiber gain medium to provide an optical gain in a spectral range covering the first laser wavelength. Outside this laser cavity, an anomalous dispersion element can be provided to receive laser pulses output by the laser cavity and to induce a negative chirp on the received laser pulses as the femtosecond laser pulses. A first optical output port is coupled to the femtosecond laser to produce a first laser output of a train of the femtosecond laser pulses at the first laser wavelength. Next, such a pulsed laser includes a nonlinear optical material coupled to the femtosecond laser to receive a portion of light of the femtosecond laser pulses and to induce spectral compression in the received laser pulses to picosecond laser pulses at the first laser wavelength and an optical amplifier coupled to receive the picosecond laser pulses at the first laser wavelength and to amply the picosecond laser pulses. A second optical output port is then coupled to receive a portion of the picosecond laser pulses amplified by the optical amplifier as the to produce a second laser output of a train of the amplified picosecond laser pulses at the first laser wavelength. In addition, such a pulsed laser includes the following to generate the third output: an optical conversion device coupled to receive a portion of the picosecond laser pulses amplified by the optical amplifier and configured to produce, via a nonlinear optical process, picosecond laser pulses at a second laser wavelength that is different from the first laser wavelength. The optical conversion device can be configured to be tunable to adjust the second laser wavelength. A third optical output port is also coupled to the optical conversion device to output a third laser output of a train of picosecond laser pulses at the second laser wavelength. [0027] In the example in FIG. 3, the limit on achievable compression ratio implies a tradeoff in the pulse durations. A test multimodal pulsed fiber laser shown in FIG. 3 was built to target roughly 300 fs and 5 ps for the multimodal source where the dissipative soliton laser is based on double-clad Yb-doped fiber with a 10 mm core diameter (Liekki). The laser can be operated to generate pulses with over 20 nm bandwidth and over 10 nJ pulse energy, providing a source of pulses around 100 fs after dechirping with a grating pair. The repetition rate of the cavity is 21.4 MHz, which is selected for ease of producing spectral bandwidths near 10 nm (corresponding to a pulse duration of 300 fs). These bandwidths are ideal for spectral compression to picosecond pulse durations as shown in the simulations described in FIG. 2. The 10-nm-wide spectra are easily obtained with minor adjustment of the waveplates and no other change to the cavity. The fiber leads on the collimators and combiner are passive fiber with 10 mm core size. Around 5.5 m of HI1060 (Corning) fiber is added before the gain fiber to reduce the repetition rate to 21.4 MHz; the smaller-core fiber is used to reduce possible multimode content in the laser. The HI1060 fiber is spliced on both ends to SMF28e+ (Corning), which is then spliced to the 10 mm fiber to reduce splicing loss. A quartz plate is used for the birefringent filter, which provides a bandwidth around 8 nm.

[0028] To generate the picosecond pulses, the chirp on the output pulse is reversed with a grating pair (LightSmyth Technologies, 1600 lines/mm) that provides -3.3 ps2 of anomalous dispersion, and then spectral compression is performed in around 55 m of passive fiber with 10 mm core diameter.

[0029] The measurements in Fig. 2C were obtained from the above test multimodal pulsed fiber laser to show good agreement with simulation. A second compressed spectrum and the corresponding autocorrelation are shown in FIGS. 4 A and 4B to highlight the fact that spectra with different shapes, but similar bandwidths near the base of the spectrum, yield similar results. Although the initial pulse energies are over 10 nJ, only 1-1.5 nJ remains after compression. Most of the loss occurs in coupling the beam back into fiber for the compression, owing to the use of sub-optimal components in this proof of-concept experiment. The efficiency of the compression in this test model can be improved.

[0030] Referring back to FIG. 3, the 7-ps laser pulses output from the spectral compression stage are amplified in a divided-pulse amplifier (DP A). The combination of 10 mm core fiber and division by 16 corresponds to direct amplification in a fiber with 40 mm mode-field diameter. The DPA used in the test model produced an amplification to 40 nJ using standard single-mode fiber compatible with fiber-format combiners and collimators. FIG. 4B shows a triangular autocorrelation trace, which indicates that the spectral compression is creating square- shaped pulses. These pulses are used to pump the OPO. A Fabry-Perot filter (OZ optics) replaces a grating-based filter in the feedback loop; this serves to decouple the OPO path length from the resonant wavelength. The long-wavelength FWM product is resonated in the cavity. With 15-20 nJ coupled into 28 cm of PCF (5 mm core, zero dispersion wavelength near 1051 nm), 3 nJ of signal pulses can be generated around 800 nm. This corresponds to conversion efficiencies of 15- 20%. In conducted tests, over 10 nJ of the 1040 nm pulses can be picked-off before the OPO to serve as the Stokes light for CARS or SRS microscopy while maintaining similar performance from the OPO. Signal pulse energies up to 4-5 nJ can be generated from the OPO at higher pump powers, but the spectra become more structured and the intensity fluctuations increase. If desired, higher pulse energies can be achieved by optimizing the PCF length and the pump pulse duration. The pulse parameters for both beams are well suited for CARS microscopy in the popular C-H stretch region of the spectrum. The tuning range of the OPO signal wavelength is around 790-820 nm, and is currently limited by the 1450-1500 nm tuning range of the Fabry- Perot filter in the feedback loop. In this experiment, free-space coupling of light into and out of the PCF was used for ease of optimization; an all-fiber version of this OPO was recently demonstrated by Cornell University by E. S. Lamb, H. Pei, and F. W. Wise in "Towards low- noise fiber sources for coherent Raman microscopy," Proc. SPIE 9329-70 (2015). Greater wavelength tunability can be provided to increase the usefulness of this source. Currently, the femtosecond pulses are tunable over the bandwidth of the ytterbium gain medium. Fiber OPOs have been designed to achieve tunable femtosecond pulses with up to 2 nJ of pulse energy, as demonstrated in references such as by C. Gu, H. Wei, S. Chen, W. Tong, and J. E. Sharping, "Fiber optical parametric oscillator for sub-50 fs pulse generation: optimization of fiber length," Opt. Lett. 35, 3516-3518 (2010) and by J. E. Sharping, C. Pailo, C. Gu, L. Kiani, and J. R.

Sanborn, "Microstructure fiber optical parametric oscillator with femtosecond output in the 1200 to 1350 nm wavelength range," Opt. Express 18, 3911-3916 (2010). Recent results from Kieu and coworkers show that high energy femtosecond pulses can be created from a picosecond- pumped fiber OPO by exploiting dissipative-soliton formation in the OPO cavity. See K. Q.

Kieu, D. E. Churin, R. Gowda, T. Ota, Y. Inoue, S. Uno, and N. Peyghambarian, "All-PM-fiber normal dispersion femtosecond optical parametric oscillator pumped by Yb-doped fiber laser," Proc. SPIE 9344-97 (2015). Since the pulse duration from the spectrally compressed

dissipative-soliton laser presented here is tunable over a wide range through mode-locking and compressing different spectra, it is capable of providing pump pulses for various fiber OPOs optimized for femtosecond pulse generation. Thus, the addition of a femtosecond fiber OPO to the set-up demonstrated here would provide a natural route for extending the wavelength tunability of the femtosecond pulses.

[0031] One motivation for replacing the low-power soliton laser typically used to generate the seed pulses in fiber-based systems for Raman imaging is the potential to start with a higher energy pulse, which has a lower shot noise limit. This lower noise feature can be beneficial for various applications such as SRS microscopy, which requires a nearly shot-noise limited source in the 10-20 MHz frequency range used for lock-in detection. Many fiber OPOs provide a route to low-noise pulses that is superior to filtering a supercontinuum, so optimization of the pump laser is important to increasing the relative intensity noise (RIN) performance of fiber systems for Raman microscopy. In the source presented here, the spectrally compressed pulses have 20- 30 mW of average power. After amplification, the 1040 nm pulses have relative intensity noise (RIN) around -150 dBc/Hz (FIG. 5). The OPO itself adds around 10 dB of noise, yielding a RIN around -140 dBc/Hz at 800 nm. This RIN is comparable to the RIN achieved by using a soliton seed pulse, and would be suitable for SRS microscopy with balanced detection. The higher starting average power in this system makes it possible to improve the noise performance of this source by optimization of the laser. Eventually, it may be advantageous to construct a

dissipative-soliton laser with a large core fiber, which can directly produce 40 nJ pulses for pumping the OPO without amplification. See, e.g., S. Lefrancois, K. Kieu, Y. Deng, J. D. Kafka, and F. W. Wise, "Scaling of dissipative soliton fiber lasers to megawatt peak powers by use of large-area photonic crystal fiber," Opt. Lett. 35, 1569-1571 (2010) and M. Baumgartl, F. Jansen, F. Stutzki, C. Jauregui, B. Ortac,, J. Limpert, and A. T ^'unnermann, "High average and peak power femtosecond large-pitch photonic-crystal-fiber laser," Opt. Lett. 36, 244-246 (2011). This would reduce the shot noise limit of the pump pulses even further, and eliminate the RIN added during amplification. As a practical benefit, elimination of the amplifier would also reduce the number of parts and complexity of the set-up. [0032] The simulation and test results have demonstrated a fiber source based on a spectrally-compressed dissipative-soliton laser and a fiber OPO capable of producing the two picosecond pulse trains desired for CARS and SRS microscopy. Therefore, dissipative soliton lasers can be used to provide a route to a Raman scattering microscopy source. In addition, the dissipative soliton laser can produce high energy femtosecond pulses at 1040 nm that are suitable for other imaging modalities, such as TPEF and SHG microscopy, and the wavelength tenability could be extended by using the pulses to pump a fiber OPO optimized for femtosecond pulse generation. The disclosed device provides RIN levels comparable to some of the best performance achieved by fiber sources to date and could be further optimized for low-noise operation through the design of the laser and through energy scaling with large core fibers. A fiber source able to provide the correct pulse parameters for Raman and multiphoton

microscopies could provide a tremendous cost advantage and extend the application of these techniques.

[0033] Referring back to FIG. 3, the two picosecond laser pulse trains at the first laser wavelength λΐ (e.g., 1040nm) and at the second laser wavelength λ2 (e.g., 800nm) are synchronized to each other and the difference between their wavelengths can be tuned by the operation of the OPO to be resonant with specific vibrational transitions for Raman imaging or spectroscopic measurements. FIG. 6A shows energy levels and transitions in an example of coherent anti-Stokes Raman scattering (CARS) microscopy where col is the pump frequency, co2 is the Stokes frequency, co3 is the anti-Stokes frequency, and coR is the Raman vibrational frequency, which is the frequency difference between col and co2. The col and co2 can be the two laser wavelengths in FIG. 3. The laser in FIG. 3 can be used for performing stimulated Raman scattering (SRS) microscopy shown in FIG. 6B in which SRG is the stimulated Raman gain and SRL is the stimulated Raman loss.

[0034] While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple

embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

[0035] Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document and the attached appendices.

Claims

CLAIMS What is claimed is:

1. A pulsed laser for producing femtosecond and picosecond laser pulses, comprising:

a femtosecond pulse laser that produces femtosecond laser pulses at a first laser wavelength;

an anomalous dispersion element that receives the femtosecond laser pulses to produce first modified laser pulses by inducing a negative chirp; and

a nonlinear optical material located to receive the first modified laser pulses and to induce spectral compression in laser pulses, thus producing picosecond laser pulses at the first laser wavelength.

2. The pulsed laser in claim 1, further comprising:

a first optical output port that outputs femtosecond laser pulses produced by the femtosecond pulse laser; and

a second optical output port that outputs the picosecond laser pulses.

3. The pulsed laser in claim 2, wherein:

the first optical output port is coupled to an output of the anomalous dispersion element to receive dechirped laser pulses as the output femtosecond laser pulses; and

the second optical output port is coupled to an output of the nonlinear optical material.

4. The pulsed laser in claim 3, further comprising:

an optical amplifier coupled between the second optical output port and the nonlinear optical material to amplify the picosecond laser pulses at the first laser wavelength.

5. The pulsed laser in claim 4, wherein:

the optical amplifier includes a divided pulse amplifier.

6. The pulsed laser in claim 3, comprising:

an optical conversion device coupled to receive a portion of the picosecond laser pulses at the first laser wavelength output by the nonlinear optical material and configured to produce, via a nonlinear optical process, new light of picosecond laser pulses at a second laser wavelength that is different from the first laser wavelength; and

a third optical output port coupled to the optical conversion device to output the picosecond laser pulses at the second laser wavelength.

7. The pulsed laser in claim 6, wherein:

the optical conversion device includes an optical parametric oscillator.

8. The pulsed laser in claim 7, wherein:

the optical parametric oscillator is a fiber-based optical parametric oscillator.

9. The pulsed laser in claim 7, wherein:

the optical parametric oscillator is tunable to adjust the second laser wavelength.

10. The pulsed laser in claim 1, wherein:

the femtosecond pulse laser includes a laser cavity that is free of anomalous dispersion and whose dispersion elements inside the laser cavity consist only of elements with normal group velocity dispersion.

11. The pulsed laser in claim 10, wherein:

the laser cavity includes a chirping element for broadening a multiple frequency component pulse in the laser cavity and spreading the frequency components in a laser pulse apart over time, and a spectral bandpass filter for passing a portion of a chirped pulse including only frequency components in a narrow range around a selected center frequency, thereby producing self-amplitude modulation of the pulse.

12. The pulsed laser in claim 1, wherein:

the anomalous dispersion element includes optical gratings or prisms.

13. The pulsed laser in claim 1, wherein:

the nonlinear optical material that induces spectral compression in laser pulses includes a length of passive fiber.

14. The pulsed laser in claim 1, wherein:

the nonlinear optical material that induces spectral compression in laser pulses includes an active nonlinear optical material.

15. The pulsed laser in claim 1, wherein:

the femtosecond pulse laser includes a fiber gain material formed by a double-clad Yb- doped fiber.

16. A pulsed laser for producing femtosecond and picosecond laser pulses, comprising:

a femtosecond pulse laser that produces femtosecond laser pulses at a first laser wavelength, the femtosecond laser including a laser cavity that is free of anomalous dispersion and whose dispersion elements inside the laser cavity consist only of elements with normal group velocity dispersion, the laser cavity includes a fiber gain medium to provide an optical gain in a spectral range covering the first laser wavelength, the femtosecond pulse laser further including an anomalous dispersion element outside the laser cavity to receive laser pulses output by the laser cavity and to induce a negative chirp on the received laser pulses as the femtosecond laser pulses;

a first optical output port coupled to the femtosecond laser to produce a first laser output of a train of the femtosecond laser pulses at the first laser wavelength;

a nonlinear optical material coupled to the femtosecond laser to receive a portion of light of the femtosecond laser pulses and to induce spectral compression in the received laser pulses to picosecond laser pulses at the first laser wavelength;

an optical amplifier coupled to receive the picosecond laser pulses at the first laser wavelength and to amply the picosecond laser pulses;

a second optical output port coupled to receive a portion of the picosecond laser pulses amplified by the optical amplifier as the to produce a second laser output of a train of the amplified picosecond laser pulses at the first laser wavelength;

an optical conversion device coupled to receive a portion of the picosecond laser pulses amplified by the optical amplifier and configured to produce, via a nonlinear optical process, picosecond laser pulses at a second laser wavelength that is different from the first laser wavelength, the optical conversion device being tunable to adjust the second laser wavelength; and

a third optical output port coupled to the optical conversion device to output a third laser output of a train of picosecond laser pulses at the second laser wavelength.

17. The pulsed laser in claim 16, wherein:

the optical conversion device includes an optical parametric oscillator.

18. The pulsed laser in claim 17, wherein:

19. The pulsed laser in claim 17, wherein:

the femtosecond pulse laser includes a chirping element for broadening a multiple frequency component pulse in the laser cavity and spreading the frequency components in a laser pulse apart over time, and a spectral bandpass filter for passing a portion of a chirped pulse including only frequency components in a narrow range around a selected center frequency, thereby producing self-amplitude modulation of the pulse.

20. The pulsed laser in claim 17, wherein:

the anomalous dispersion element includes optical gratings or prisms.

21. The pulsed laser in claim 17, wherein:

22. The pulsed laser in claim 17, wherein:

the nonlinear optical material that induces spectral compression in laser pulses includes active nonlinear optical material.