Dissipative soliton mode fiber based optical parametric oscillator

A fiber optic parametric amplifier that includes a resonating cavity. The resonating cavity includes linear fiber optic gain medium, with negative chromatic dispersion; and a nonlinear fiber optic gain medium with positive chromatic dispersion.

BACKGROUND

1. Field of Art

This disclosure generally relates to fiber based optical parametric oscillators.

2. Description of the Related Art

Light sources based on optical parametric interaction are interesting since they provide access to laser wavelengths that existing gain materials based on electronic transitions cannot provide. An optical parametric oscillator (OPO) can be realized by exploiting the χ(2)nonlinear optical response in a wide range of crystals or the χ(3)nonlinear response in optical fibers.

Optical fiber based OPO (FOPO) are particularly attractive owning to their potential in achieving low cost, alignment-free and compact laser systems while still providing very wide tuning range and high power operation.

The operation of FOPOs is in essence based on degenerated four-wave-mixing (FWM) wherein two pump photons interact with the fiber to generate a signal photon and an idler photon. The exact frequencies of the signal and idler photons are defined by the phase matching condition which depends on the pump laser wavelength, its peak power as well as the dispersion profile of the optical fiber. There are two common ways to pump an OPO. The first approach is continuous pumping where the pump laser is a continuous wave laser or a laser generating long pulses compared to the OPO cavity round trip time. The second approach is based on synchronous pumping of pulsed pump laser. In the second approach, the pump laser is synchronized with the OPO cavity.

Ultrafast synchronously pumped FOPOs have been demonstrated in the past. However, the output pulse energy and peak power of past FOPO have been quite modest. The best ultrafast FOPO that has been produced in the past in terms of output pulse energy and peak power was up to 2 nJ pulse energy and ˜12 kW peak power). The technique used to achieve this was to use only 2 cm fiber length and the rest of the cavity was free-space optics. This implementation avoided the adverse influence of nonlinear effects but required the use of mostly free-space components so free-space alignment was required, and the problems that go along with such a system. What is needed is a high power ultrafast FOPO in an all-fiber laser cavity or mostly all-fiber laser cavity. What is also needed is an ability to tune the emission wavelength of the FOPO over a wide rage while providing high output pulse energy.

SUMMARY

An embodiment is a fiber optic parametric amplifier comprising: an input port for receiving an optical pump pulse, with a first pulse duration, at a pump wavelength; and a resonating cavity. The resonating cavity comprising: a first coupler for coupling the optical pump pulse into the resonating cavity; a linear fiber optic gain medium, with negative chromatic dispersion, that increases the intensity of the pump pulse and increases the first pump pulse duration to a second pump pulse duration after exiting the linear fiber optic gain medium; a nonlinear fiber optic gain medium with positive chromatic dispersion, that provides parametric gain to light which exits the linear fiber optic gain medium, the nonlinear optical gain medium transfers energy from the pump pulse after exiting the linear optical gain medium to a signal pulse with a first signal pulse duration after exiting the nonlinear fiber optic gain medium; and a power splitter. The power splitter receives light that exits the nonlinear fiber optic gain medium and provides: a first portion of the light that exits the nonlinear fiber optic gain medium to an output port to exit the resonating cavity; and a second portion of the light that exits the nonlinear fiber optic gain medium is fed back into the resonating cavity so that it passes through the linear fiber optic gain medium. The linear fiber optic gain medium increases the pulse duration of the signal pulse with a first pulse duration to a second signal pulse duration after exiting the linear fiber optic gain medium.

In an alternative embodiment the average chromatic dispersion of the resonating cavity is within the normal dispersion range.

In an alternative embodiment the second portion of the light that exits the nonlinear fiber optic gain medium that is provided by the power splitter is fed back into the resonating cavity via the first coupler.

In an alternative embodiment further comprises a seed laser for providing the optical pump pulse.

In an alternative embodiment the resonating cavity further comprises an optical delay line.

In an alternative embodiment the linear gain medium is an Erbium doped fiber optic amplifier.

In an alternative embodiment the resonating cavity further comprises a non-gain fiber that does not provide optical gain and has a chromatic dispersion that has a chromatic dispersion that is greater than the chromatic dispersion of the nonlinear fiber optic gain medium.

In an alternative embodiment a peak wavelength of the light that exits the output port is changed by changing a repetition rate of the pump pulse.

In an alternative embodiment the resonating cavity further comprises: a first wavelength division multiplexer for splitting the light in the resonating cavity into signal light and idler light, wherein the signal light has a first wavelength range that includes the signal pulse and the idler wavelength has a second wavelength range different from the first wavelength range; a second wavelength division multiplexer for combining the signal light and the idler light, wherein the combined light travels back through the resonating cavity together; a first fiber coupling the signal from the first wavelength division multiplexer to the second wavelength division multiplexer; and a second fiber coupling the signal from the first wavelength division multiplexer to the second wavelength division multiplexer.

In an alternative embodiment the length of the first fiber is different from the length of the second fiber.

In an alternative embodiment a material property of the first fiber is different from a material property of the second fiber.

In an alternative embodiment a peak wavelength of the light that exits the output port is changed by changing at least two properties of the pump pulse selected from the group consisting of: repetition rate of the pump pulse; center wavelength of the pump pulse; peak power of the pump pulse; repetition frequency of the pump pulse.

In an alternative embodiment the signal pulse is a soliton and pulse breaking is prevented by dissipative soliton mode locking in the resonating cavity.

In an alternative embodiment the peak wavelength of the light that exits the output port is changed by changing the length of the resonating cavity and the there is no spectral filter within the resonating cavity.

Further features and aspects will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings.

DESCRIPTION OF THE EMBODIMENTS

Embodiments will be described below with reference to the attached drawings.

Nonlinear distortion leading to pulse breaking has been a dominant factor limiting the output peak power and pulse energy in standard ultrafast fiber lasers based on rare-earth-doped gain fibers. In order to reduce nonlinear distortion in the laser cavity of the OPO may be designed to have a highly chirped pulses within the resonator. Which effectively reduces the intra-cavity peak power and provides the pulse energy scaling capability. Operating the OPO in a normal dispersion regime can help ensure chirped pulse formation in the laser cavity of the OPO for either one or more, or all of the pump, idler, and signal. The normal dispersion regime refers to when the length averaged dispersion of entire OPO resonating cavity is less than zero in units of ps/nm/km. The anomalous dispersion regime refers to when the length averaged dispersion of entire OPO resonating cavity is greater than zero in units of ps/nm/km.

First Exemplary Embodiment

A first exemplary embodiment is a synchronously pumped fiber optical parametric oscillator (FOPO) operating in the normal dispersion regime. The FOPO generates chirped pulses at the output allowing significant pulse energy scaling potential without pulse-breaking. The average output power of the FOPO at 1600 nm was −60 mW; corresponding to 1.45 nJ pulse energy and ˜55% slope power conversion efficiency. The output pulses directly from the FOPO were highly chirped (˜3 ps duration) and could be compressed outside of the cavity to 180 fs by using a standard optical fiber compressor having anomalous dispersion.

A first exemplary embodiment100is a synchronously pumped FOPO operating in the normal dispersion regime and is pumped by a pump source (seed laser, pump laser)102which may be a compact Er-doped femtosecond fiber laser as illustrated inFIG. 1. The FOPO generated chirped pulses at an output104allowing significant pulse energy scaling potential without pulse-breaking. The obtained output average power from the FOPO at 1600 nm was ˜60 mW corresponding to ˜1.45 nJ pulse energy (taking into account the 42 MHz repetition rate) and ˜55% slope power conversion efficiency. The output pulses directly from the FOPO were highly chirped with duration of ˜3 ps but they could be compressed (by using a standard optical fiber having anomalous dispersion) to ˜180 fs outside of the FOPO cavity.

FIG. 1is a schematic diagram of the normal dispersion FOPO cavity. The pump source was an Er-doped femtosecond fiber laser working at 1560 nm with a fiber taper embedded in carbon nanotube/polymer composite saturable absorber. The repetition rate of the laser in this embodiment is 42 MHz corresponding to a cavity length of around 4.9 m. The seed laser102emitted nearly transform-limited pulses with pulse duration of 400 fs and about ˜1 mW output average power. The seed laser102was coupled to a first polarization controller106a. The first polarization controller106amay have been spliced to a C port of a wavelength division multiplexer (WDM) coupler108a. The first WDM coupler108amay be a C/L band fused fiber coupler.

The pulse train from the seed laser is coupled into the FOPO cavity using the first WDM coupler108a. The multiplexed port of the first WDM coupler108ais spliced to a signal port of a second WDM coupler108b. The second WDM coupler108bmay be a 980/1550 fused fiber coupler. A first 980 nm pump laser110ais connected to a pump port of the second WDM coupler108b. The multiplexed port of the fused fiber coupler is coupled to an Er-doped fiber (EDF)112. The EDF112is spliced to a multiplexed port of a third WDM coupler108c. The third WDM coupler108cmay be a980/1550fused fiber coupler. A second 980 nm pump laser110bis connected to a pump port of the third fused fiber coupler108c.

The pulse train from the seed laser is amplified by an Er-doped fiber amplifier (EDFA), which is described above is located inside the FOPO cavity. A pulse enters the EDFA as a seed and leaves as a pump. The length of the EDF112was 24 m and it was pumped from both sides as described above. The dispersion of the EDF is about −20 ps/nm/km at 1560 nm and −17 ps/nm/km at 1600 nm. The maximum average power of the amplified pump pulse train after the EDFA was measured to be about 175 mW (limited by the available 980 nm pump power).

The output of the EDFA exits the signal port of the third WDM108c, which is spliced to dispersion shifted fiber (DSF)114which is used as the parametric gain medium. The DSF114may a standard DSF manufactured by Corning Inc. as the parametric gain medium (˜5 m in length). The zero dispersion wavelength (ZDW) of the DSF114may be 1548 nm. The dispersion slope at the ZDW of the DSF114was 0.07 ps2/nm/km. The output of the DSF114is spliced to an input port of a power splitter116. The power splitter116may be a fused fiber coupler. The power splitter may have a 90%/10% split. The 90% output port may produce the output signal104. The 10% output port may be spliced to an optical delay line (ODL)118. The cavity length may be adjusted within a few centimeters using the fiber-coupled ODL118. The output of the ODL118may be connected to a polarization controller106bwhich may then be spliced to an L port of the first WDM coupler108a, thus completing a resonant ring cavity for the FOPO.

Based on the theory of parametric phase matching conditions, the generated signal and idler wavelengths were calculated to be around 1510 nm and 1610 nm, respectively as illustrated inFIG. 2.

FIG. 3is an illustration of a dispersion map of the OPO cavity. The average cavity dispersion at the idler wavelength (1600 nm) was estimated to be around −9 ps/nm/km, which is in the normal dispersion regime. A dichroic filter may also be inserted at the ODL118to remove any residual pump light to improve the stability of the FOPO. The dichroic filter may also block the generated signal wave so that only the idler wave makes the round trip in the resonating ring cavity and resonates. Due to the long length of the EDF and the DSF needed in this cavity, we operated the FOPO at 7 times of the cavity length of the pump laser (besides the EDF and the DSF the rest of the cavity consisted of standard SMF-28 fiber). Therefore, there were 7 idler pulses circulating in the cavity at the same time.

In an alternative embodiment the cavity maybe use a shorter EDF112to make the total cavity length shorter so that there will be only one idler pulse per cavity round trip.

The two polarization controllers106a-bwere used to optimize the polarization states of the pump and the resonating idler waves to achieve the best conversion efficiency. In an alternative embodiment, polarization maintaining fiber may be used throughout.

The introduction of the EDFA within the FOPO cavity is a novel feature which provides several important advantages. The EDF provides the required normal dispersion for the chirped pulse operation. The 1560 nm pump pulses also acquired a similar positive chirp by propagating through the EDF. Due to the interplay between gain, Kerr nonlinearity, and normal dispersion, the pump pulses were stretched to picosecond range which ensures good chirp synchronization and temporal overlap with the resonating idler in the cavity. The FOPO cavity length could be measured accurately by removing the pump blocking dichroic filter and switching off the external pump pulse train so that the FOPO could lase on its own (around 1570 nm) from the gain that the EDF provided. The exact longitudinal mode spacing, which is directly connected to the laser cavity length, can then be measured by using a fast photodetector and an RF spectrum analyzer. Generally, it is quite time consuming to match the cavity length of an OPO with that of the pump laser since the OPO would oscillate only when perfect cavity length matching is achieved. This approach enables quick and precise cavity length estimation reducing the time needed to construct a FOPO.

An alternative embodiment may include an isolator to ensure that there ring cavity operates in a single direction. The isolator may be a fiber coupled isolator located between the first WDM coupler108aand the second WDM coupler108b. The isolator may be a free space isolator located in the ODL118. The isolator may be located between the 10% port of the power splitter116and the first WDM coupler108a. The isolator may be optimized for the C-band, C+L band or the L-Band. The isolator may include a dichroic filter.

Experimental Results of First Exemplary Embodiment

During the operation of the FOPO100the idler pulses are generated as soon as the pump power is increased above the threshold level of about 65 mW. The evolution of the FOPO output power104as function of the pump power is illustrated inFIG. 4. The evolution of the optical spectra of the pump, signal, and idler waves at different pumping powers (70 mW, 120 mW, and 175 mW) is also illustrated inFIG. 4. A clear square-like shape of the spectrum of the FOPO idler wave, which is a typical spectral shape for mode-locked lasers operating in the normal dispersion regime, was observed. At the highest available pump power, the full width at half maximum (FWHM) spectral bandwidth of the FOPO was measured to be 20 nm which could support ˜170 fs pulses after de-chirping. At high pump powers, several FWM peaks (due to interaction between the signal/pump and idler/pump waves) were also observed. The FWM peaks in the shorter wavelength sides were more pronounced because the ZDW of the DSF was at 1548 nm making it more favorable for phase matching at the shorter wavelength side of the 1560 nm pump.

At ˜175 mW of pump power, the highest generated output power of the idler alone was measured to be ˜60 mW (after using a dichroic filter to remove the residual pump light at 1560 nm as well as the generated signal light at ˜1525 nm). This corresponded to the idler pulse energy of ˜1.45 nJ taking into account the 42 MHz repetition rate. The maximum power conversion efficiency, which is defined as the ratio between the converted output power to the total pump power, was calculated to be 34% at 175 mW pump power. The estimated slope conversion efficiency was ˜55%. We attribute this achieved high conversion efficiency to the degenerated FWM gain (from the pump to idler/signal) and an additional FWM gain (from pump and idler to signal). This will be clarified more in the next section.

To investigate the robustness of the FOPO against cavity detuning, we adjusted the cavity length and observed the output spectral shift. We could change the FOPO cavity length up to 6 mm in total and still observed good power output. The resonating idler signal wavelength shifted from 1585 nm to 1615 nm for the whole tuning range. The output spectra's envelop reflected well the calculated shape of the DSF gain spectrum.

The idler pulses were highly chirped at the output of the FOPO. We used a standard single mode fiber (SMF-28) to compensate for the accumulated chirp (or to de chirp). The pulse duration close to the output of the FOPO was ˜3 ps. And the pulsed could be compressed to close to transform-limited duration ˜180 fs using ˜10 m of SMF-28. The peak power was estimated to be about 8 kW given the 1.45 nJ pulse energy.

Second Exemplary Embodiment

A second exemplary embodiment700is described with reference toFIG. 7. Configurations common to those of the first exemplary embodiment100will be denoted by the same reference numerals as those of the first exemplary embodiment and the description thereof will be omitted.

The second exemplary embodiment700is identical the first exemplary embodiment100except that it does not include the EDFA. The second exemplary embodiment700further includes single mode fiber720.

Numerical Simulation of First Exemplary Embodiment

A numerical simulation using realistic experimental parameters can be used to simulate the formation as well as the evolution of the idler pulses around the FOPO cavity. A model maybe built based on the nonlinear Schrodinger equation. A fourth order Runge-Kutta in the interaction picture (RK4-IP) method may be used to solve the equation numerically. The pump pulses were assumed to have a Gaussian shape. The resonating idler wavelength maybe calculated using the phase matching curve for estimating the DSF. The delay between the pump and idler pulses may then be calculated using the dispersion profiles of all the fibers used in the cavity to satisfy the cavity synchronization requirement. Depending on the pump power, the resonating idler wave in the FOPO cavity reached steady state after about a hundred roundtrips. With 175 mW pump power, a clean Gaussian time profile with time duration of 3.7 ps idler wave was obtained at the output of the simulated FOPO. Its wavelength was −1600 nm. The applicant has found that these results were in good agreement with the experimental results

The evolution of output power of the FOPO as function of pump power as well as the output spectra corresponding to the three different output powers corresponds well with what was observed above experimentally. High conversion efficiency can be achieved via an assisted Raman process. However, the applicant has determined that this is not the case for the first exemplary embodiment100since an intra-cavity filter was not used to produce the experimental and numerical results above, and the peak of the Raman gain in silica fiber is around 1670 nm (when pumped at 1560 nm) whereas our generated idler (1600 nm) was located quite far away from the peak of the Raman gain. The applicants have concluded that a new mechanism involved in the setup of the first exemplary embodiment. Looking closely at the experimental and simulation results, the applicant determined that the number of photons in the idler and the signal are unequal. The applicants have concluded that the high conversion efficiency observed was due to an additional FWM process which happens between the pump photon and the generated signal photon which actually supplied additional energy to the idler wave. To confirm this new mechanism, we plotted the simulated evolution of the pulse energy contained in the pump, the signal, the idler, and the generated FWM waves in the DSF fiber when the pump power was fixed at 175 mW as shown inFIG. 5. At the first 1 m of the DSF, degenerated FWM occurred due to the pump pulse and the signal/idler waves grew gradually. After about 1.5 m of DSF, the growing speed of the idler was higher than that of the signal because of the appearance of another FWM process between the pump and the signal waves. From 1.5 m to 3 m of the DSF, the signal wave was growing and transferring energy simultaneously to the 1st FWM wave and the signal at around 1600 nm. After 3 m of the DSF, the signal wave stopped growing. All of the energy received from the pump was immediately transferred to the signal and the 1st FWM wave. This explains why the idler wave had much higher energy than the signal wave at the end of the DSF.

To have a better understanding of the evolution of the laser pulses around the FOPO cavity, the pulse energy and pulse duration of the generated idler and the pump waves around the cavity are illustrated inFIG. 6. The pulse profile at 1560 nm and 1600 nm are illustrated at points A, B, and C. The results show that both the generated idler and the pump pulses acquired a significant amount of chirp and stretched in time when passing through the EDF, from point A to point B. The duration (at FWHM) of the idler pulse increased from 2.7 ps to 8.5 ps while the pump pulse stretched from 0.4 ps to 5.5 ps by propagating through the EDF, from point A to point B. In the numerical simulations, only the pump pulses were amplified significantly in this stage due to the gain of the EDF. In the DSF, from point B to point C, the pump energy transferred to the idler via the parametric interaction. At the first half of the DSF, the central part of the idler grew faster than other parts leading to a narrower pulse width (from 8.5 ps down to 3.5 ps). Whereas the pump pulse duration increased because the central part of the pump went down. At the second half of the DSF, when the central part of the pump transferred most of its energy to the idler, the duration of the idler started to increase slightly since its leading edge and trailing edge were amplified more than the central part of the pulse. The pump duration decreased slightly in this segment. At the end of the DSF, the central part of the pump is completely converted to the idler wave making a hole in its temporal profile as illustrated by the power profiles at point C, while the temporal profile of the amplified idler wave was quite close to a Gaussian shape. 90% of the idler was then coupled to the output of the cavity after the DSF. In the SMF-28, only 10% of the idler propagates. Its duration decreases from 3.7 ps to 2.7 ps due to the anomalous dispersion of the SMF-28. The same evolution repeated again in the next round trip.

Wavelength Tuning of the Output of the FOPO

It is often desirable to build FOPO such as the one described in the first exemplary embodiment100and the second embodiment700which is tunable. Such that the output wavelength of the FOPO is adjustable. Prior art methods of tuning have included placing a filter in the resonating cavity. The applicant's have discovered a variety of wavelength tuning methods that can be used to tune the FOPO. Wavelength tuning can be achieved by changing the length of the resonating cavity of the FOPO, changing the repetition rate of the seed pulse, or changing the wavelength of the pump pulse. These methods can be used separately or in combination with each other.

Since the parametric gain is narrower as the length of the fiber used for parametric gain is longer, the longer fiber works as a spectral filter. In this case, the wavelength tuning can be achieved by changing the wavelength or power of the seed pulse.

The pump pulse used in a FOPO is typically very narrow and is described in equation (0).
P0=P0(t)  (0)

However, the pulse can be treated as a continuous wave when the spectrum width is narrow enough. Therefore, in the following discussion, we can treat the pump light as a CW light source. Gain for the signal or idler is described by Giin equations (1)-(5). An example of the waveguide described below is the DSF114.

γ: Nonlinear coefficient of the waveguide114

P0: Pump power

L: Length of waveguide114

n2: Nonlinear index of waveguide114

Aeff: Effective area of waveguide114

β2, β4,: are the second and fourth order dispersion parameters of the waveguide114.

λp: Center wavelength of pump light

ωp: Angular frequency of pump light

ωs: Angular frequency of signal light

ωi: Angular frequency of idler light

Since it is reasonable to assume that the parameter g is a rational number, therefore the limits for Δβ may be derived from equation (3) to be equation (6).
−4γP0<Δβ<0  (6)

According to equations (7) and (4), the frequency differences Δω between the pump and the signal or the idler at the maximum and at the gain edges are calculated as follows. Equation (8) describes the frequency differences at the maximum gain ΔωmaxEquation (9) describes the frequency differences at one of the gain edges Δωedge1. Equation (10) describes the frequency differences at one of the gain edges Δωedge2.

As described in equation (1), the gain spectrum is a function of the length of the waveguide. As the length of waveguide increases, the gain spectrum width decreases. However, if the intensity of the pump light is kept constant, then the peak wavelength of the gain spectrum does not change, because the parameter “g” is not changed.

Meanwhile, the gain spectrum width at the full width at half maximum can be calculated by following equations (11)-(12). The edge wavelengths calculated by Equation (11) are nearer to the center wavelength than the edge wavelengths as calculated by Equations (6)-(10).

FIG. 8is an illustration of a relationship between the pump light and the parametric gain spectrum in the waveguide114. The gain spectrum region is limited by Equations (6)-(10). Such that the edge wavelengths calculated by Equations (11)-(12) are farther from the center wavelength than the wavelengths calculated by Equations (6)-(10). This is indicative of a possible square-like shape to the spectrum. In addition to that, since the wavelength of pump light is close to the zero-dispersion wavelength of the waveguide114, the pump pulse shape is kept. It is expected that as intensity of the pulse becomes smaller along the waveguide114, energy is transferred from the pump to the signal and/or the idler.

The 1st order dispersion is considered to derive a relationship between the spectrum of the signal or the idler pulse, total dispersion of the FOPO and repetition rate of pump pulse. For a fiber parametric oscillator such as in the second embodiment200the round trip time of signal and idler pulse is estimated by equation (13).

nHNLF(ω): Index at the frequency ω of highly nonlinear fiber (HNLF)114for parametric gain.

nODCF(ω): Index at the frequency ω of oscillator dispersion controlling fiber (ODCF)720for building the oscillator.

β1,HNLF(ω): 1st order dispersion at the frequency ω of high nonlinear fiber (HNLF). The 1st order dispersion is equivalent to group velocity, i.e. velocity of the pulse propagating in the waveguide.

β1,ODCF(ω): 1st order dispersion at the frequency ω of oscillator dispersion controlling fiber (ODCF). Wherein the ODCF are one more fibers which in combination with the HNLF are used to control the dispersion of the entire cavity.

c: Optical velocity in vacuum.

ω0: Angular frequency at zero-dispersion of the waveguide.

LHNLF: Length of HNLF.

LODCF: Length of ODCF.

The relationship between index n(ω) and dispersion β(ω) is described by equation (14). Wherein the index n(ω) is the total refractive index including both linear and non-linear terms.

Thus equation (13) can be transformed to be written in terms of beta as described in equation (15).

For the ODCF, the ODCF can consist of several kinds of fibers. Thus, the 2nd term of Equation (15) is described in equation (16). As described in equation (16) there are multiple fibers of multiple types, and lengths.

Given a pump pulse produced by the pump source102with a repetition rate frepand a temporal width Δτ. The gain pulse has the same temporal width as the pump pulse width Δτ. And the gain pulse at the optical angular frequencies around ωs/ishould move at the same speed as the pump pulse of

1β1,HNLF⁡(ωp),
even though the 1st order dispersion of β1,HNLF(ωs/i) is different from β1,HNLF(ωp). Therefore, the length of HNLF is calculated based on the period of time that the pulse, signal, and idler overlap as described in equation (17).

However, the gain spectrum region is limited by Equations (6)-(10) and is a limited by the length as calculated in equation (17). If the non-linear fiber is longer, then the pump, signal, and idler no longer overlap and equations (6)-(10) are no longer as relevant.

Relationship Between the Repetition Rate and the Round Trip in the FOPO

The temporal width of the gain pulse is assumed to be Δτ. Thus, equation (18) can be derived to describe amplifying the oscillating pulse.

In accordance with equations (15), (16), and (18) equation (19) further describes amplifying the oscillating pulse.

From Equation (19) we can calculate the center wavelength and edge wavelengths of the signal or idler pulses as described in equations (20)-(22).

ωs/i—c: Center angular frequency of the signal light/idler light.

ωs/i—fwhm1: An angular frequency of the signal light/idler light at FWHM.

ωs/i—fwhm2: Another angular frequency of signal light/idler light at FWHM.

As described by Equation (20), the center wavelength of signal and idler light are determined by total dispersion of the FOPO and the repetition rate of pump pulse. Therefore, the wavelength of the signal or idler can be controlled by changing the repetition rate of pump pulse.

In addition, the spectral width of the signal and idler light are determined by dispersion or temporal pulse width of pump pulse. Thus, the spectral width can be controlled by design of oscillator dispersion and/or pump pulse width. This effectively means that the temporal shape of the pump pulse can work as a spectral filter. Equations (21) and (22) do not completely describe all of forces shaping the spectral width of the signal and idler pulses there are also other nonlinear effects in the oscillator.

Third Exemplary Embodiment

A third exemplary embodiment is a specific example of the second embodiment700described above. In which the zero-dispersion wavelength is 1050 nm, a 3rd order dispersions of the HNLF114for parametric gain is 6.71776E-41 s3/m, and 4th order dispersion of HNLF114for parametric gain is −9.83483E-56 s4/m. For the pump pulse produced by the pump laser102, the output wavelength is 1040 nm, the average power is 4 W, the repetition rate is 15 MHz, the temporal pulse width is 10 ps, and the spectral width is 0.11 nm.

FIGS. 9A-Bshow the simulated results of the evolution of the gain spectrum width depending on the HNLF114length LHNLFas the wavelength of pump pulse is changed from 1040 nm to 1060 nm. As illustrated inFIG. 9Awhen the center wavelength of pump pulse is 1040 nm and LHNLFis 0.01 m the signal gain spectrum is 763 nm to 804 nm. As illustrated inFIG. 9Bwhen the center wavelength of pump pulse is 1040 nm and LHNLFis 0.01 m the idler gain spectrum is 1469 nm to 1632 nm.

Fourth Exemplary Embodiment

A fourth exemplary embodiment is a specific example of the second embodiment700described above. In which fiber720is a single mode fiber (SMF) with a zero-dispersion wavelength of 1300 nm, a 3rd order dispersion of is 6.61859E-41 s3/m, and 4th order dispersion is −4.59295E-56 s4/m. This fiber has normal negative second order dispersion at the range of from 776 nm to 824 nm. As8at 1300 nm is 7,125,000 [1/m] and the length of the fiber720LQDCFis 13.52 m, the ODL118may be adjusted from 0 to about 4 cm for a repetition rate of 15 MHz for wavelength of 763 nm to 804 nm. The HNLF114is identical to the fiber used in the third exemplary embodiment. The pump pulse duration is 10 ps, the output spectral width is calculated as about 6 nm by Equations (21) and (22).

Fifth Exemplary Embodiment

A fifth exemplary embodiment is another specific example of the second embodiment700described above. It is similar to the fourth exemplary embodiment except that the fiber720is normal dispersion fiber (NDF). In this fifth exemplary embodiment, the fiber720has normal dispersion in the wavelength range of the idler, and is used for the oscillator at the wavelength region of idler light. The fiber720is a NDF fiber with a zero-dispersion wavelength of 1350 nm, a 3rd order dispersion is −2.50339E-40 s3/m, and 4th order dispersion is −3.43249E-55 s4/m. As β0—ODCFat 1350 nm is 6,735,000 1/m and the length of the fiber720LODCFis 13.8 m, the ODL118may be adjusted from 0 to about 4 cm for a repetition rate of 15 MHz for wavelength of 1469 nm to 1632 nm. The pump pulse duration is 10 ps, the output spectral width is calculated as about 25 nm at 1469 nm and 15 nm 1632 nm by equations (21) and (22).

Sixth Exemplary Embodiment

A sixth exemplary embodiment1000is described with reference toFIG. 10. Configurations common to those of previous embodiments will be denoted by the same reference numerals the descriptions thereof will be omitted. Compared to previous embodiments the signal and idler pulses are divided by a fourth WDM coupler1024aand recombined later with a fifth WDM coupler1024b. The signal light is coupled into the SMF1020fiber described in the fourth embodiment and the idler light is coupled into NDF1022used in fifth embodiment. The fiber lengths for the SMF1020is 13.52 m and the NDF1022are 13.8 m.

Seventh Exemplary Embodiment

The seventh exemplary embodiment is another specific example of second embodiment700described above and is similar to the fourth exemplary embodiment except for the differences described below. The pump laser102is a tunable laser whose wavelength can be tuned from 1040 nm to 1060 nm. The length of the HNLF114is 1 m. Thus, the center wavelength is tuned from 778 nm to 835 nm, and the spectral width of the signal is changed from 3.5 nm to 5 nm. In the FOPO, the fiber720is SMF and the length is 12.7 m. The length of the ODL118is tuned from 4 cm to 6 cm.

Eighth Exemplary Embodiment

The eighth exemplary embodiment is a specific example of the second embodiment700described above and is similar to fifth exemplary embodiment except for the differences described below. Like the seventh exemplary embodiment, the pump laser102is a tunable laser whose wavelength can be tuned from 1040 nm to 1060 nm. The center wavelength of the FOPO is tuned from 1451 nm to 1568 nm, and the spectral width is changed from 14.8 nm to 13.6 nm. In the FOPO, the fiber720is a NDF and the length is 12.9 m. The length of the ODL is tuned from 11 cm to 14 cm.

Ninth Exemplary Embodiment

The ninth exemplary embodiment is another example of the sixth embodiment1000described above except for the differences described below. Like the seventh exemplary embodiment, the pump laser102is a tunable laser whose wavelength can be tuned from 1040 nm to 1060 nm. The fiber length for the HNLF114is 1 m. The fiber lengths for the SMF1020is 13.52 m and the NDF1022is 12.98 m. The length of the ODL118is tuned from 0 cm to 3 cm.

The second through ninth embodiments described above prevent pulse breaking based on the nonlinear effect in fiber because of dissipative soliton mode locking. Therefore, it is possible to achieve high power output based on dissipative soliton mode locking without a spectral filter. By removing the spectral filter, it can be possible to tune the wavelength only by adjusting the length of the oscillator as the wavelength tunability is kept.

Tenth Embodiment

A tenth embodiment1100is a wide tuning range, normal dispersion FOPO using a femtosecond, fixed wavelength fiber laser as the pump source102. A tuning range from 1360 nm to 1835 nm with a pulse energy of up to 1.3 nJ was achieved by operating the FOPO in the normal dispersion regime. The output pulse duration reached ˜200 fs after de-chirping using a few meters of standard single mode fiber (SMF-28) outside of the FOPO cavity.

A tenth embodiment1100is described with reference toFIG. 11. Configurations common to those of the previous embodiments will be denoted by the same reference numerals as those of the first exemplary embodiment and the description thereof will be omitted.FIG. 11is a schematic diagram of the tenth embodiment which is a normal dispersion FOPO source1100. The source1100can be divided into two parts, the FOPO resonating cavity, and a seed laser102. The seed laser102includes an oscillator stage1126and one or more amplifier stages.

Oscillator

The oscillator1126is a tunable laser. The oscillator1126may be a fiber optic ring laser that includes one or more fiber optic, fused fiber, and fiber coupled micro-optic components. The oscillator1126may include a first linear gain medium1130a, which may be Erbium doped fiber. The first linear gain medium1130amay be another gain medium or fiber with another dopant. A first end of the first linear gain medium1130amay be coupled to a first port of a first WDM coupler1132a. The first WDM coupler1132amay be similar or identical the WDM coupler108b. The first WDM coupler1132amay include a signal port and a pump port, the first WDM coupler1132amay be operated as a multiplexer/de-multiplexer. A first pump laser1134ais coupled to the pump port of the first WDM coupler1132awhich passes the pump light to the first port and on into the first linear gain medium1130awhich induces the first linear gain medium1130ato emit signal light. The signal light is coupled from the first linear gain medium1130ainto the first port of the coupler1132a. The coupler1132athen passes the signal to the signal port. The first pump laser1134aand the first linear gain medium1130amay be configured in a reverse pump configuration. The first pump laser1134amay be a 980 nm fiber coupled laser. The coupler1132amay be a fused fiber coupler. Alternatively, the first pump laser1134amay be a 1480 nm laser, and the first WDM coupler1132amay be a dichroic filter based fiber-coupled micro-optic component.

The signal port of the first WDM coupler1132ais connected to an input port of an isolator1136a. The isolator1136apasses the signal light to an output port of the isolator1136a. The isolator1136ablocks light from passing from the output port to the input port of the isolator1136a. This ensures that the ring of the oscillator1126operates in a one direction. The isolator1136aand the first WDM coupler1132amay be combined into a single component.

The output port of the isolator1136ais then coupled to an input of a saturable absorber1138. Light from the saturable absorber1138is then coupled to an input of an optical delay line1140. Light from the optical delay line is then coupled to an input port of a coupler1142. The coupler1142is a power splitter. The power splitter1142splits light coupled into the input port into a second port which is coupled back into the first linear gain medium1130athus providing feedback for the laser cavity, and an output port which is passed on to the one or more amplifiers of the pump source102. The power splitter1142may have 5%/95% splitting ratio. The power splitter1142may be a fused fiber optic coupler. The power splitter1142may be a micro optic based fiber coupler splitter. The power splitter1142may be optimized for the wavelength range of the oscillator.

One or more of the components described herein in the oscillator1126may be combined together to improve efficiency or reduce cost. The components of the oscillator may be replaced with free space optical components. The repetition rate of the oscillator1126is adjusted by changing the ODL1140.

Linear Amplifier

The light exiting the oscillator102through the output port of the power splitter1142is than coupled to an input port of second isolator1136b. The light is then passed to a signal port of a second WDM coupler1132b. A second pump1134bis coupled to a pump port of the second WDM coupler1132b. The second WDM coupler1132bmultiplexes the signal light and the pump light and outputs the multiplexed light via an output port of the second WDM coupler1132b. The output port of the WDM coupler is coupled to an input of a second linear gain medium1130b. The second isolator blocks stimulated from the second linear gain medium1130bfrom entering the oscillator1126. The second linear gain medium amplifies the light produced by the oscillator1126. The amplified light exiting the second linear gain medium1130bthen passed through a bandpass filter1128. The bandpass filter1128may fiber coupled micro-optic filter which only lets light within the wavelength range produced by the oscillator through.

The light that has passed through the bandpass filter1128is coupled to a signal port of a third WDM coupler1132c. A third pump1134cis coupled to a pump port of the third WDM coupler1132c. The third WDM coupler1132cmultiplexes the signal light and the pump light and outputs the multiplexed light via an output port of the third WDM coupler1132c. The output port of the third WDM coupler1132cis coupled to an input of a third linear gain medium1130c. The output of the third linear gain medium1130cis coupled to a fourth WDM coupler1134d. A fourth pump1134dis coupled to a pump port of the fourth WDM coupler1134d. The fourth WDM coupler1132dcouples the amplified light from the third linear gain medium1130cto a signal port of the fourth WDM coupler1132d. The fourth WDM coupler1132dalso couples the pump light to the third linear gain medium.

The amplified light is then passed through a dispersion compensating fiber1136, to modify the chirp of the seed pulse before it exits the seed laser102and enters the parametric amplifier.

Parametric Amplifier

The output of the seed laser is coupled to the Parametric Amplifier via a first polarization controller106a. The first polarization controller106ais coupled to a C/L band WDM coupler108a. The pulse train from the seed laser is coupled into the FOPO cavity using the C/L band coupler108a. The multiplexed port of the first WDM coupler108ais coupled to a HNLF114such as DSF which is used as the parametric gain medium. The output of the HNLF114is coupled to an input port of a power splitter116. A first output port of the power splitter116produces the output signal104. A second output port of the power splitter116is coupled to an input of a dispersion management fiber1120. The dispersion management fiber1120has a specific dispersion and length which is used to control the total dispersion of the optical parametric oscillator. The output of the dispersion management fiber1120is coupled to a second polarization controller106bwhich is coupled to an L port of the C/L band coupler108a, thus completing a resonant ring cavity for the FOPO.

A key feature of the FOPO is that the average dispersion/unit length over the entire resonating ring cavity is in the normal dispersion regime. Another feature of the FOPO is that the total length of the FOPO is constant, yet the peak output wavelength of the FOPO can be tuned by adjusting the pump laser102.

Experimental Results

The applicants have produced experimental results based upon the tenth embodiment. The repetition rate of the seed oscillator1126was adjustable from ˜18.8 MHz to ˜19.0 MHz by using the ODL1140. The seed oscillator1126delivered ˜400 fs pulses at 1560 nm with an average power of ˜1 mW. The amplifier was based on Er-fibers1130band1130cand had 2 stages. The pulse train from the oscillator was amplified to ˜20 mW in the first stage1130b. The band-pass filter1128had a bandwidth of 1 nm centered at 1560 nm and was used to avoid excessive nonlinear spectral broadening and ASE noise. At the output of the seed laser102, the signal was boosted to ˜180 mW. The pump pulse duration at the input of the FOPO was estimated to be around 5 ps and the pulses were highly chirped. The HNLF was 1.5 m long. The power splitter116was a 90/10% fused fiber output coupler. The dispersion management fiber1120was 7.5 m of DCF-38.

The dispersions at 1560 nm of fibers used in the FOPO are illustrated inFIG. 17. Owing to the relatively long length of the dispersion compensating fiber (DCF-38)1120used in the cavity, the average dispersion of the ring cavity is ˜−25 ps/nm/km. This normal dispersion will enable dissipative solitons to form inside the FOPO cavity. By adjusting the cavity length of the seed oscillator to match the cavity length of the FOPO, parametric oscillation was observed at pump powers >40 mW.

An optical spectrum analyzer was used to record the output spectrum from the FOPO as the cavity length of the seed oscillator was tuned with the ODL1140. The measured spectra for some of the offsets is shown inFIG. 12A. The peak wavelength for the signal and idler spectra as a function of the offset of the ODL1140is illustrated in FIG.12B.FIG. 12Care illustrations of the power and bandwidth of signal and idler produced by the FOPO.

The applicants observed that the pump at 1560 nm has transferred its energy to two symmetric sidebands (signal and idler) on both side of the pump wavelength. Note that when the generated waves were not far from the pump, additional four wave mixing (FWM) effects occur which generated several higher order of FWM peaks. We obtained ˜400 nm total tuning range (by cavity length adjustment over 16 mm) with a wavelength span of 475 nm (from 1360 nm to 1835 nm). The maximum output pulse energy was ˜1.3 nJ.

Eleventh Exemplary Embodiment

A eleventh exemplary embodiment1300is described with reference toFIG. 13. Configurations common to those of the previous embodiments will be denoted by the same reference numerals as those of the first exemplary embodiment and the description thereof will be omitted. The eleventh exemplary embodiment1300includes a pump laser102and a FOPO. The peak output104is controlled by adjusting the repetition rate of the pump laser102. The repetition rate of seed laser102is controllable. The repetition rate controllable seed laser102may be an active mode-locked laser, an active Q-switched laser, a pulse laser generated from a continuous wave (CW) laser by intensity modulation, or a pulse laser generated from CW laser by phase modulation.

The pump laser102is coupled to a first input port of the WDM108a. The WDM108apasses the pump light to an output port of the WDM108a. The output port of the WDM108ais coupled to an input of the HNLF114. The HNLF provides parametric gain. The output of the HNLF114is coupled to an input port of a filter1338. The output of the filter1338is coupled to an input port of the power splitter116. A first output of the power splitter116provides the output signal104. A second output of the power splitter116is coupled to a second input port of the WDM108awhich is mixed with the pump signal and sent out of the WDM108ato provide feedback for the FOPO.

In a specific example of the eleventh embodiment an output wavelength of the seed laser102may be stable at 1550 nm. The repetition rate of the seed laser102may be tuned from 15.363 MHz to 15.366 MHz. Output from the seed laser is coupled into a fiber optical parametric oscillator (FOPO) via WDM coupler108a. The FOPO resonating cavity includes: 10 m SMF with a 0 dispersion and 0.092 ps/2km dispersion slope at 1310 nm; and 5 m HNLF114with 0 dispersion and 0.07 ps/nm2/km dispersion slope at 1548 nm. The total dispersion in this FOPO resonating cavity is in the anomalous dispersion region. Thus, the round trip frequency at 1580 nm and 1620 nm are 15.366 MHz and 15.363 MHz, respectively.

In the HNLF114, parametric processes occur and the signal light and idler light are generated. After that, the signal and pump lights are cut off by the filter1338, which in this case may be a long pass filter. Then, the power splitter116sends 90% of idler light to the output104and 10% of idler light is returned to the FOPO and overlapped with next seed laser pulse.

The advantages of this example are high speed wavelength tuning and no free space alignment at the wavelength region of the idler. In addition, since the oscillated pulse is operated in soliton regime, the pulse shape can be sech2like.FIG. 14Ais an illustration of the tuning range in which the filter1338is a long pass filter in which the total dispersion of the FOPO1440is plotted vs the .wavelength. The signal wavelength is area by dashed line1442. The pump wavelength is shown by dashed line1444. The idler wavelength is within the area1446. The repetition rate of the seed laser102maps to the dispersion range1448a, which coincides with a wavelength range represented by range1450a. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440.

Twelfth Exemplary Embodiment

A twelfth exemplary embodiment is another specific example of the eleventh exemplary embodiment1300described above. An output wavelength of the seed laser102may be stable at 1550 nm. The repetition rate of the seed laser102may be tuned from 15.368 MHz to 15.369 MHz. Output from the seed laser is coupled into a fiber optical parametric oscillator (FOPO) via WDM coupler108a. The FOPO resonating cavity includes: 10 m single mode fiber (SMF) with 0 dispersion and 0.092 ps/nm2/km dispersion slope at 1310 nm; and 5 m HNLF with 0 dispersion and 0.07 ps/nm2/km dispersion slope at 1548 nm. The total dispersion in this FOPO resonating cavity is in the anomalous dispersion region. Thus, the round trip frequency at 1580 nm and 1620 nm are 15.369 MHz and 15.368 MHz, respectively.

In the HNLF114, parametric processes occur and the signal light and idler light are generated. After that, the idler light and pump light are cut off by the filter1338, which in this case may be a short pass filter. Then, the power splitter116sends 90% of idler light to the output104and 10% of idler light is returned to the FOPO and overlapped with next seed laser pulse.

FIG. 14Bis an illustration of the tuning range in which the filter1338is a long pass filter in which the total dispersion of the FOPO1440is plotted vs the .wavelength. The signal wavelength is area by dashed line1442. The pump wavelength is shown by dashed line1444. The idler wavelength is within the area1446. The repetition rate of the seed laser102maps to the dispersion range1448b. Which coincides with a wavelength range represented by range1450b. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440.

Thirteenth Exemplary Embodiment

A thirteenth exemplary embodiment1400is described with reference toFIG. 15. Configurations common to those of the previous embodiments will be denoted by the same reference numerals as those of the first exemplary embodiment and the description thereof will be omitted. This configuration is identical to the eleventh exemplary embodiment illustrated inFIG. 13, except that DCF1520is between the HNLF114and the filter1338.

In a specific example of the thirteenth exemplary embodiment an output wavelength of the seed laser102may be stable at 1550 nm. The repetition rate of the seed laser102may be tuned from 15.525 MHz to 15.529 MHz. Output from the seed laser is coupled into a fiber optical parametric oscillator (FOPO) via WDM coupler108a. The FOPO resonating cavity includes: 10 m dispersion compensating fiber (DCF) with ˜40 ps/nm/km dispersion and −0.1 ps/nm2/km dispersion slope at 1550 nm; 5 m highly nonlinear fiber (HNF) with 0 dispersion and 0.07 ps/nm2/km dispersion slope at 1548 nm. The total dispersion in this FOPO resonating cavity is in the normal dispersion region. Thus, the round trip frequency at 1580 nm and 1620 nm are 15.525 MHz and 15.529 MHz, respectively.

In the HNLF114, parametric processes occur and the signal light and idler light are generated. After that, the signal and pump lights are cut off by the filter1338, which in this case may be a long pass filter. Then, the power splitter116sends 90% of idler light to the output104and 10% of idler light is returned to the FOPO and overlapped with next seed laser pulse.

The advantages of this example are high speed wavelength tuning and no free space alignment at the wavelength region of the Idler. In addition, since the oscillated pulse is operated in the dissipative soliton regime, the pulse energy can be higher than the pulse generated in the soliton regime.FIG. 16Ais an illustration of the tuning range in which the filter1338is a long pass filter in which the total dispersion of the FOPO1440is plotted vs the wavelength. The signal wavelength is the area by dashed line1442. The pump wavelength is shown by dashed line1444. The idler wavelength is within the area1446. The repetition rate of the seed laser102maps to the dispersion range1648a, which coincides with a wavelength range represented by range1650a. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440.

Fourteenth Exemplary Embodiment

A twelfth exemplary embodiment is another specific example of the eleventh exemplary embodiment1300described above. An output wavelength of the seed laser102may be stable at 1550 nm. The repetition rate of the seed laser102may be tuned from 15.519 MHz to 15.521 MHz. Output from the seed laser is coupled into a fiber optical parametric oscillator (FOPO) via WDM coupler108a. The FOPO resonating cavity includes: 10 m DCF with −40 ps/nm/km dispersion and 0.07 ps/nm2/km dispersion slope at 1310 nm; and 5 m HNLF with 0 dispersion and 0.07 ps/nm2/km dispersion slope at 1548 nm. The total dispersion in this FOPO resonating cavity is in the normal dispersion region. Thus, the round trip frequency at 1520 nm and 1540 nm are 15.519 MHz and 15.521 MHz, respectively.

In the HNLF114, parametric processes occur and the signal light and idler light are generated. After that, the idler light and pump light are cut off by the filter1338, which in this case may be a short pass filter. Then, the power splitter116sends 90% of idler light to the output104and 10% of idler light is returned to the FOPO and overlapped with next seed laser pulse.

FIG. 16Bis an illustration of the tuning range in which the filter1338is a long pass filter in which the total dispersion of the FOPO1440is plotted vs the .wavelength. The signal wavelength is in the region1442. The pump wavelength is shown by dashed line1444. The idler wavelength is shown by dashed line1446. The repetition rate of the seed laser102maps to the dispersion range1648b, which coincides with a wavelength range represented by range1650b. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440.

Fifteenth Exemplary Embodiment

A fifteenth exemplary embodiment is a variation on the specific example of the eleventh exemplary embodiment1300described above, which is described with reference toFIG. 15. An output wavelength of the seed laser102may be stable at 1550 nm. The repetition rate of the seed laser102may be tuned from 44.4983 MHz to 44.499 MHz. Output from the seed laser is coupled into a fiber optical parametric oscillator (FOPO) via WDM coupler108a. The FOPO resonating cavity includes: 0.18 m DCF1420with −40 ps/nm/km dispersion and −0.1 ps/nm2/km dispersion slope at 1550 nm; and 5 m HNLF with 0 dispersion and 0.07 ps/nm2/km dispersion slope at 1548 nm. The total dispersion in this FOPO resonating cavity is in the normal dispersion region at wavelengths shorter than 1550 nm and anomalous dispersion in the wavelength region greater than 1550 nm. Thus, the round trip frequency at 1520 nm and 1540 nm are 44.5407 MHz and 44.5412 MHz, respectively. In addition, the round trip frequency at 1580 nm and 1620 nm are 44.5414 MHz and 44.5406 MHz, respectively.

In the HNLF114, parametric processes occur and the signal light and idler light are generated. After that, the idler light and pump light are cut off by the filter1338, which in this case may be a short pass filter. Then, the power splitter116sends 90% of idler light to the output104and 10% of idler light is returned to the FOPO and overlapped with next seed laser pulse.

FIG. 16Cis an illustration of the tuning range in which the filter1338is a band cut filter in which the total dispersion of the FOPO1440is plotted vs the .wavelength in which the dispersion is normal for the signal and anomalous for the idler. The signal wavelength is in the region1442. The pump wavelength is shown by dashed line1444. The idler wavelength is in area1446. The repetition rate of the seed laser102maps to the dispersion range1648b, which coincides with a wavelength ranges1650c1and1650c2. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440.

Sixteenth Exemplary Embodiment

A sixteenth exemplary embodiment is another specific example of the eleventh exemplary embodiment1300described above in which the seed laser is a tunable pulse laser with an adjustable repetition rate. The oscillator of the seed laser102may include a 15.005 m of large mode area fiber (LMAF) and a semiconductor optical amplified (SOA). The LMAF may have 38.25 ps/nm/km dispersion and 0.0733 ps/nm2/km dispersion slope at the wavelength of 1550 nm. The SOA is driven by a modulated current. The seed laser102may be tuned from 1555-1560 nm. The repetition rate of the seed laser102may be tuned from 15.367 MHz to 15.362 MHz. Output from the seed laser is coupled into a fiber optical parametric oscillator (FOPO) via WDM coupler108a. The FOPO resonating cavity includes: 10 m SMF with a 0 dispersion and 0.092 ps/2km dispersion slope at 1310 nm; and 5 m HNLF114with 0 dispersion and 0.07 ps/nm2/km dispersion slope at 1548 nm. The total dispersion in this FOPO resonating cavity is in the anomalous dispersion region. Thus, the round trip frequency at 1580 nm and 1620 nm are 15.366 MHz and 15.363 MHz, respectively. By synchronizing the repetition rates of seed oscillator and parametric oscillator, the wavelength tunable range will be expanded.

In the HNLF114, parametric processes occur and the signal light and idler light are generated. After that, the signal and pump lights are cut off by the filter1338, which in this case may be a long pass filter. Then, the power splitter116sends 90% of idler light to the output104and 10% of idler light is returned to the FOPO and overlapped with next seed laser pulse.

The advantages of this example are high speed wavelength tuning and no free space alignment at the wavelength region of the idler. Moreover, since the seed pulse at shorter wavelengths generates gain at longer wavelengths in the idler region, a fluctuation in the power of the seed oscillator, can be reduced. In addition, since the oscillated pulse is operated in soliton regime, the pulse shape can be sech2like.FIG. 14Cis an illustration of the tuning range in which the filter1338is a long pass filter and the seed laser is tunable in which the total dispersion of the FOPO1440is plotted vs the .wavelength. The signal wavelength is area by dashed line1442. The pump wavelength is shown by the region1444. The idler wavelength is within the area1446. The repetition rate of the seed laser102maps to the dispersion range1448c, which coincides with a wavelength range represented by range1450c. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440. The tuning of the seed laser102expands the wavelength range1450c. As the wavelength of pump laser is adjusted, the dispersion in the pump laser wavelength region is important, the relationship between the repetition rate pump and idler (or signal) is expands or suppresses the wavelength tunable range of FOPO.

Seventeenth Exemplary Embodiment

A seventeenth exemplary embodiment is another specific example of the eleventh exemplary embodiment1300described above, it is substantially similar to the sixteenth embodiment except that the idler light and pump light are cut off by the filter1338, which in this case may be a short pass filter instead of a long pass filter. Also, the repetition rate of the seed laser102may be tuned from 15.372 MHz to 15.367 MHz. The round trip frequency at 1520 nm and 1540 nm are 15.369 MHz and 15.368 MHz, respectively.

FIG. 14Dis an illustration of the tuning range in which the filter1338is a short pass filter in which the total dispersion of the FOPO1440is plotted vs the .wavelength and the seed laser is tunable. The signal wavelength is area1442. The pump wavelength is shown by area1444. The idler wavelength is within the area1446. The repetition rate of the seed laser102maps to the dispersion range1448d, which coincides with a wavelength range represented by range1450d. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440. The tunable range of the idler1450dis increased by tuning the wavelength and repetition rate of the seed laser102. It is important to tune both the repetition rate and wavelength of the seed laser, in order to expand the tuning range of FOPO.

Eighteenth Exemplary Embodiment

The eighteenth exemplary embodiment is substantially similar to the thirteenth exemplary embodiment except that it replaces the stable seed laser102with a tunable seed laser102similar to the one described in sixteenth embodiment. The oscillator of the seed laser102may include 14.848 m LMAF. The repetition rate of the seed laser102may be tuned from 15.529 MHz to 15.524 MHz depending on the wavelength.

The eighteenth exemplary embodiment includes a FOPO with the same fibers as the specific example of the thirteenth exemplary embodiment.FIG. 16Dis an illustration of the tuning range in which the filter1338is a long pass filter in which the total dispersion of the FOPO1440is plotted vs the wavelength and the seed laser102is tunable. The signal wavelength is the area by the dashed line1442. The pump wavelength is in the range1444. The idler wavelength is within the area1446. The repetition rate of the seed laser102maps to the dispersion range1648d, which coincides with a wavelength range represented by range1650d. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440. The range1650dis expanded by changing the wavelength and repetition rate of the seed laser102. It is important to tune both the repetition rate and wavelength of seed laser, in order to expand the tuning range of FOPO.

Nineteenth Exemplary Embodiment

The nineteenth exemplary embodiment is substantially similar to the fourteenth exemplary embodiment except that it replaces the stable seed laser102with a tunable seed laser102similar to the one described in sixteen embodiment. The oscillator of the seed laser102may include 14.855 m LMAF. The repetition rate of the seed laser102may be tuned from 15.522 MHz to 15.517 MHz depending on the wavelength. The round trip frequency at 1520 nm and 1540 nm are 15.519 MHz and 15.521 MHz, respectively.

The nineteenth exemplary embodiment includes a FOPO with the same fibers as the specific example of the thirteenth exemplary embodiment.FIG. 16Eis an illustration of the tuning range in which the filter1338is a short pass filter in which the total dispersion of the FOPO1440is plotted vs the wavelength and the seed laser102is tunable. The signal wavelength is the area by the dashed line1442. The pump wavelength is in the range1444. The idler wavelength is within the area1446. The repetition rate of the seed laser102maps to the dispersion range1648e, which coincides with a wavelength range represented by range1650e. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440. The range1650dis expanded by changing the wavelength and repetition rate of the seed laser102. It is important to tune both of the repetition rate and wavelength of seed laser, in order to expand the tuning range of FOPO.

Twentieth Exemplary Embodiment

The twentieth exemplary embodiment is substantially similar to the fifteenth exemplary embodiment except that it replaces the stable seed laser102with a tunable seed laser102similar to the one described in sixteenth embodiment. The oscillator of the seed laser102may include 5.175 m of LMAF. The repetition rate of the seed laser102may be tuned from 44.552 MHz to 44.537 MHz depending on the wavelength.

FIG. 16Fis an illustration of the tuning range in which the filter1338is a band cut filter in which the total dispersion of the FOPO1440is plotted vs the .wavelength in which the dispersion is normal for the pump and anomalous for the idler. The signal wavelength is in the region1442. The pump wavelength is in the region1444. The idler wavelength is in area1446. The repetition rate of the seed laser102maps to the dispersion range1648f, which coincides with a wavelength ranges1650f1and1650f2. The mapping between repetition rate and wavelength of the idler is controlled by the dispersion curve1440.

Twenty First Exemplary Embodiment

The twenty first exemplary embodiment is substantially similar to the sixteenth exemplary embodiment. Except for the oscillator of the seed laser102includes 15.228 m of DCF with −40 ps/nm/km dispersion and −0.1 ps/nm2/km dispersion slope at 1550 nm. The wavelength is tuned from 1550 nm to 1565 nm and the repetition rate is changed from 15.3634 MHz to 15.3657 MHz depending on the wavelength. The dispersion for the pump is in the normal dispersion regime while the dispersion of the idler is in the anomalous dispersion regime. The total dispersion of the FOPO is anomalous. The output of the wavelength of the FOPO may be tuned by adjusting the wavelength of the pump and the repetition rate of the pump.

Twenty Second Exemplary Embodiment

The twenty second exemplary embodiment is substantially similar to the seventeenth exemplary embodiment except that the oscillator of the seed laser is similar to oscillator used in twenty first exemplary embodiment except that 15.224 m of the same DCF is used and the repetition rate is changed from 15.3682 MHz to 15.3689 MHz depending on the wavelength. The dispersion for the pump is in the normal dispersion regime while the dispersion of the signal is in the anomalous dispersion regime. The total dispersion of the FOPO is anomalous. The output of the wavelength of the FOPO may be tuned by adjusting the wavelength of the pump and the repetition rate of the pump.

Twenty Third Exemplary Embodiment

The twenty third exemplary embodiment is substantially similar to the eighteenth exemplary embodiment except that the oscillator of the seed laser is similar to oscillator used in twenty first exemplary embodiment except that 15.069 m of the same DCF is used and the repetition rate is changed from 15.5255 MHz to 15.5278 MHz depending on the wavelength. The dispersion for the pump and idler are in the normal dispersion regime. The total dispersion of the FOPO is normal. As the wavelength of pump laser is tuned the dispersion in tuned pump laser wavelength region is important and the relationship between the repetition rate pump and idler (or signal) is important to expand or suppress the wavelength tunable range of FOPO. The output of the wavelength of the FOPO may be tuned by adjusting the wavelength of the pump and the repetition rate of the pump.

Twenty Fourth Exemplary Embodiment

The twenty fourth exemplary embodiment is substantially similar to the nineteenth exemplary embodiment except that the oscillator of the seed laser is similar to the oscillator used in the twenty first exemplary embodiment except that 15.076 m of the same DCF is used and the repetition rate is changed from 15.5814 MHz to 15.5206 MHz depending on the wavelength. The dispersion for the pump and signal are in the normal dispersion regime. As the wavelength of pump laser is tuned, the dispersion in pump laser wavelength region is important to the tunability of the FOPO. The total dispersion of the FOPO is normal. The output of the wavelength of the FOPO may be tuned by adjusting the wavelength of the pump and the repetition rate of the pump. The seed laser is same as Example 11. The oscillator of the seed laser is consisted of 15.076 m dispersion compensating fiber (DCF) with −40 ps/nm/km dispersion and −0.1 ps/nm2/km dispersion slope at 1550 nm and SOA. The wavelength is tuned from 1550 nm to 1565 nm and the repetition rate is changed from 15.5184 MHz to 15.5206 MHz depending on the wavelength.

Twenty Fifth Exemplary Embodiment

The twenty fifth exemplary embodiment is substantially similar to the twentieth exemplary embodiment except that the oscillator of the seed laser is similar to the oscillator used in the twenty first exemplary embodiment except that 5.253 m of the same DCF is used and the repetition rate is changed from 44.5394 MHz to 44.5416 MHz depending on the wavelength. The dispersion for the pump and signal are normal, while the dispersion for idler is anomalous. The output of the wavelength of the FOPO may be tuned by adjusting the wavelength of the pump and the repetition rate of the pump.

Output from the seed laser is coupled into a fiber optical parametric oscillator (FOPO). Since the FOPO is consisted of 0.18 m dispersion compensating fiber (DCF) with −40 ps/nm/km dispersion and −0.1 ps/nm2/km dispersion slope at 1550 nm, 5 m highly nonlinear fiber (HNF) with 0 dispersion and 0.07 ps/nm2/km dispersion slope at 1548 nm, total dispersion in this FOPO is normal dispersion at the wavelength range of shorter 1550 nm and anomalous dispersion at the wavelength range longer than 1550 nm. Thus, round trip frequency at 1520 nm and 1540 nm are 44.5407 MHz and 44.5412 MHz, respectively. On the other hand, round trip frequency at 1580 nm and 1620 nm are 44.5414 MHz and 44.5406 MHz, respectively.