SINGLE PASS AMPLIFICATION OF DISSIPATIVE SOLITON-LIKE SEED PULSES

Abstract

A system for single pass amplification of dissipative soliton-like seed pulses of 1-20 ps to produce output pulses of 50-200 fs, without requiring a stretcher. Such an amplifier relies on the inherent chirp of the seed pulse out of the oscillator instead of pulse stretching.

Description

SUMMARY OF THE INVENTION

It is an object of the present invention to provide apparatus and methods for generation of high-energy (>1 μJ) ultrafast laser pulses through an oscillator-amplifier configuration employing single pass amplification of dissipative soliton-like seed pulses of 1-20 ps to produce output pulses of 50-200 fs, in a compact configuration with a minimum of free-space components. Such system ideally makes use of a mode-locked “oscillator” seed laser that generates pulses that are inherently chirped, avoiding the need for a pulse stretcher before the amplifier.

An embodiment of an amplifier according to the present invention is fed with a seed pulse train with the pulses having a pulse duration in excess of 1 ps. The oscillator is tuned to produce a pulse appropriate for amplification, having a typical spectral width of 8 nm near 1040 nm wavelength, emerging from the model-locked oscillator with a spectral phase “chirp” sufficient to stretch the pulse to 10 ps duration. The bat ears shaped pulse produced by an ANDi laser is ideal.

The pulse train is then directed into one or more fiber amplifiers via a pump combiner that combines pump light and seed light into a first gain fiber, which is. doped with rare earth ions with one core size and doping level, and amplifies the seed pulses by a factor of 10-1000. The majority of the pump light is transmitted to the second gain fiber. The gain fibers can include single mode (SM) fiber, large mode area (LMA) fiber, or photonic crystal fiber (PCF), and can be single or double clad.

The seed source may comprise an oscillator configured to produce appropriate shaped seed pulses directly, such as an ANDi oscillator, or may comprise a conventional oscillator and a phase conditioning stage, which could consist of an optical fiber, grating pair, or other dispersive element to control the phase and achieve the desired phase. Other potential elements could include a spectral filter, a pulse picker, etc. A compressor then generates compressed output pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D (Prior Art) illustrate prior art approaches to fiber amplification. FIG. 1A illustrates Chirped Pulse Amplification. FIG. 1B illustrates cubicon amplification. FIG. 1C illustrates parabolic amplification. FIG. 1D is a schematic diagram of an ANDi oscillator.

FIG. 2 is a block diagram illustrating an embodiment of an amplifier according to the present invention.

FIGS. 3A and 3B are block diagrams illustrating use of the amplifier of FIG. 2 with oscillator seed pulse generators.

FIG. 4 is a plot showing the spectrum of the seed pulse and the output pulse of the system of FIG. 3.

FIG. 5 is a plot of the spectral phase of the output pulse of FIG. 4.

FIG. 6 is a plot of the temporal intensity of the output pulse of FIG. 4.

FIG. 7A is a plot illustrating a second embodiment of the present invention with an 8 nm wide bandpass filter.

FIG. 7B is a simulated result using the same parameters as the experimental result of 7A.

FIG. 8 is a simulated result of propagation through the system of FIG. 3B using a Gaussian pulse in place of the prepared dissipative soliton pulse.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a block diagram illustrating an embodiment 200 of an amplifier according to the present invention. The amplifier system begins with a seed pulse train 202 with the pulses having a pulse duration in excess of 1 ps. In the preferred embodiment, this oscillator is tuned to produce a pulse appropriate for amplification, having a typical spectral width of 8 nm near 1040 nm wavelength, emerging from the model-locked oscillator with a spectral phase “chirp” sufficient to stretch the pulse to 10 ps duration. The bat ears shaped pulse produced by an ANDi laser is ideal (see FIG. 1D).

Alternative to directly adjusting the modelocked laser operating parameters to obtain the optimum pulse, a conventional pulse may optionally be shaped in other ways, as shown in FIG. 3.

The pulse train 202 is then directed into one or more fiber amplifiers via a pump combiner 206 that combines pump light 204 and seed light 202 into a passive (non-doped) fiber (not shown), or into the first gain fiber 208.

First gain fiber 208 is doped with rare earth ions with one core size and doping level, with the seed amplified by a factor of 10-1000, and the majority of the pump light being transmitted. The first gain fiber can include single mode (SM) fiber, large mode area (LMA) fiber, or photonic crystal fiber (PCF), and can be single or double clad.

Second gain fiber 214 has a second core size and doping level, where both the seed and the pump light from the first gain fiber are coupled in sequence into the second fiber, can also be added. Additional gain in the second gain fiber is from 10-1000. The second gain fiber can include single mode (SM) fiber, large mode area (LMA) fiber or photonic crystal fiber (PCF), including rod type fibers, and can be single or double clad. Mode adapter 210 is optional.

Seed source 202 may comprise an oscillator configured to produce appropriate shaped seed pulses directly. FIG. 3A is a schematic diagram illustrating a system in which pulses from a conventional oscillator 320 are propagated through a phase conditioning step 322, which could consist of an optical fiber, grating pair, or other dispersive element to control the phase and achieve the desired phase. The pulses can then optionally be directed to a spectral filter 324, which can narrow the bandwidth if the oscillator is set to produce a bandwidth that is larger than the optimal width. Given the highly chirped nature of the pulses, this spectral narrowing causes some change to the temporal duration of the pulse. For this reason, it is sometimes desirable to spectrally filter in advance of any phase conditioning or stretching. The prepared pulse can then optionally be directed to a pulse picker 326 that selectively passes only some of the pulses from the modelocked laser 320 to amplifier 200, reducing the repetition rate of the resulting source laser output. Compressor 314 then generates compressed output pulses 316.

FIG. 3B is a block diagram illustrating use of the amplifier of FIG. 2 with an ANDi oscillator seed pulse generator 302. ANDi oscillator 302 provides a continuous train of chirped pulses 304 with spectral bandwidth >˜10 nm to isolator 306. Isolator 306 prevents feedback from the amplifier chain back to the oscillator. The pulses 308 then pass through a pulse picker 310. Pulse picker 310 can optionally be used to pass (“pick”) some of the pulses while blocking others from the oscillator, effectively reducing the repetition rate of pulses 312 passing into amplifier 200 (see FIG. 2), which generates output pulses 202. Compressor 314 then generates compressed output pulses 316.

FIG. 4 is a plot showing the spectrum of the seed pulse (narrow) and the output pulse (broad) of the system of FIG. 3. The seed pulse is generated from an ANDi oscillator tuned to produce pulses of width appropriate for amplification in this regime. The pulses are coupled into a 10 m long section of fiber which serves to shape the phase such that the pulses compress well using a grating compressor after amplification. The pulses are then directed into a pulse picker to reduce the pulse train repetition rate from approximately 60 MHz to 1 MHz. The output of the pulse picker is then directed into a pump combiner where the signal pulse train is injected into the core of double clad fiber and the 976 nm pump light is directed into the cladding of the fiber. This passive double clad fiber is spliced to a matching active fiber of 10 um core and 125 um cladding. The Yb doping level is a few %. The length of this pump fiber is 13 cm. This active fiber is then spliced to a short length of passive double clad fiber of the same dimensions as the active fiber. This fiber is then spliced to a larger mode area double clad photonic crystal fiber, with a 230 um cladding diameter and 40 um core diameter.

FIG. 5 is a plot of the spectral phase of the output pulse of FIG. 4.

FIG. 6 is a plot of the temporal intensity of the output pulse of FIG. 4.

FIG. 7A illustrates a second embodiment of the present invention with an 8 nm wide bandpass filter. In this embodiment, the dissipative soliton pulse is spectrally filtered before amplification. The spectrum directly from the oscillator is shown, as is the filtered spectrum at the output of the birefringent spectral filter. The dashed line shows the calculated transmission function of the spectral filter. The spectrally filtered pulse is directed into the pulse picker described above. The pulse then propagates through the pump combiner described above, and further through the 2 stage fiber amplifier as described above. The pump power is set to 30 W and the output power of the resulting pulse train is 10 W. The output spectrum is as given in the bottom panel of FIG. 7A. The two curves again correspond to the output spectrum of the amplifier seeded by filtered and unfiltered input spectra. While the characteristic “bat-eared” spectral shape is shown in both cases, the pulse background was reduced by the inclusion of the spectral filter at the input. This output spectrum agrees well with the simulated result created using the same parameters as described in the experiment. This simulated output spectrum is shown in the second panel of FIG. 7B. The top panel shows the temporal pulse out of the amplifier after it has been compressed using 1000/mm ruled gratings with ˜5 cm separation.

FIG. 8 is a simulated result of propagation through the system of FIG. 3B using a Gaussian pulse in place of the prepared dissipative soliton pulse.

While the exemplary preferred embodiments of the present invention are described herein with particularity, those skilled in the art will appreciate various changes, additions, and applications other than those specifically mentioned, which are within the spirit of this invention. For example, a birefringent filter spectral filter has been used in this embodiment, but other methods of filtering could be used to produce pulses appropriate for amplification. In an example embodiment, an optical fiber is used to apply appropriate phase, but any dispersive element, for example a diffraction grating stretcher or volume or fiber bragg grating, could be used. The source used here is a dissipative soliton laser, while any laser producing chirped pulses possessing at least the desired bandwidth could be used in an alternate embodiment. Furthermore, the pump light could be introduced into the fiber amplifier in either the co-propagating or the counter-propagating direction. An important characteristic is that the spectrum maintains a dissipative-soliton type character that allows for compression of the pulse to short duration.

Claims

1. The method of amplifying dissipative soliton-like seed pulses to produce output pulse comprising the steps of: (a) providing dissipative soliton-shaped seed pulses of 1-20 ps from an oscillator tuned to produce inherently chirped pulses of appropriate spectral shape and phase;(b) providing a pump beam;(c) combining the seed pulses and the pump beam and providing them to an amplifier;(d) amplifying the combined seed pulses and pump beam using two stages of gain fiber to generate amplified pulses; and(c) temporally compressing the amplified pulses and generating output pulses of 50-200 fs;

2. An amplifier comprising: a seed source;a pump source;a pump combiner that combines the pump light and seed light and provides them to a first gain fiber;a first gain fiber doped with rare earth ions with one core size and doping level, with the seed amplified by a factor of 10-1000, and the majority of the pump light being transmitted; The first gain fiber can include single mode (SM) fiber, large mode area (LMA) fiber, or photonic crystal fiber (PCF), and can be single or double clad;a second gain fiber with a second core size and doping level, where both the seed and the pump light from the first gain fiber are coupled in sequence into the second fiber, wherein additional gain in the second gain fiber is from 10-1000; The second gain fiber can include single mode (SM) fiber, large mode area (LMA) fiber or photonic crystal fiber (PCF), including rod type fibers, and can be single or double clad; anda compressor.

3. The apparatus of claim 2 wherein the pump source is a single co-propagating pump that is used to provide energy for both gain segments.