# US 2014/0192403 A1 · Multi-mode fiber amplifier

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- Number: US 2014/0192403 A1
- Document ID: US20140192403A1
- Status: Published application
- Kind: A1
- Application number: 14/174,269
- Filed: 2014-02-06
- Published: 2014-07-10
- Earliest claimed: 1998-11-25
- Assignees: Imra America, Inc.
- Applicants: IMRA America, Inc.
- Inventors: Martin E. Fermann; Donald J. Harter
- Web page: https://patentgrep.com/patent/US20140192403A1

## CPC

- H01S 3/06708: {Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering (optical fibres as passive waveguides G02B6/02)}
- G02B 6/26: Optical coupling means (G02B6/36, G02B6/42 take precedence)
- H01S 3/067: Fibre lasers
- H01S 3/06704: {Housings; Packages}
- H01S 3/06712: {Polarising fibre; Polariser}
- H01S 3/06716: {Fibre compositions (per se C03C13/04)or doping with active elements (lasing materials in general H01S3/14)}
- H01S 3/06729: {Peculiar transverse fibre profile}
- H01S 3/06733: {Fibre having more than one cladding}
- H01S 3/06737: {Fibre having multiple non-coaxial cores, e.g. multiple active cores or separate cores for pump and gain}
- H01S 3/06745: {Tapering of the fibre, core or active region}
- H01S 3/06783: {Amplifying coupler}
- H01S 3/08045: Single-mode emission
- H01S 3/08054: {Passive cavity elements acting on the polarization, e.g. a polarizer for branching or walk-off compensation (quarter-wave plates in a Q-switch laser H01S3/1124, H01S3/115)}
- H01S 3/094007: {Cladding pumping, i.e. pump light propagating in a clad surrounding the active core}
- H01S 3/094019: {Side pumped fibre, whereby pump light is coupled laterally into the fibre via an optical component like a prism, or a grating, or via V-groove coupling}
- H01S 3/094042: {of a fibre laser}
- H01S 3/094069: {Multi-mode pumping}
- H01S 3/1106: Mode locking
- H01S 3/1109: Active mode locking
- H01S 3/1115: using intracavity saturable absorbers
- H01S 3/1118: Semiconductor saturable absorbers, e.g. semiconductor saturable absorber mirrors [SESAMs]; Solid-state saturable absorbers, e.g. carbon nanotube [CNT] based
- H01S 3/1608: {erbium}
- H01S 3/1618: {ytterbium}
- H01S 3/0092: {Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity (nonlinear frequency conversion per se G02F1/35)}
- H01S 3/06725: {Fibre characterized by a specific dispersion, e.g. for pulse shaping in soliton lasers or for dispersion compensating [DCF]}
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

A laser utilizes a cavity design which allows the stable generation of high peak power pulses from mode-locked multi-mode fiber lasers, greatly extending the peak power limits of conventional mode-locked single-mode fiber lasers. Mode-locking may be induced by insertion of a saturable absorber into the cavity and by inserting one or more mode-filters to ensure the oscillation of the fundamental mode in the multi-mode fiber. The probability of damage of the absorber may be minimized by the insertion of an additional semiconductor optical power limiter into the cavity.

## Claims

### Claim 1

An optical apparatus comprising:
a length of multi-mode optical fiber comprising a gain medium and configured to propagate a fundamental mode;
a length of single mode optical fiber configured to propagate a single mode; and
a splice between the length of multi-mode optical fiber and the length of single-mode optical fiber,
wherein the single mode of the length of single mode optical fiber is matched to the fundamental mode of the length of multi-mode optical fiber.

### Claim 2 (depends on 1)

The optical apparatus of claim 1, wherein an output end of the length of single mode optical fiber is spliced to an input end of the length of multi-mode optical fiber.

### Claim 3 (depends on 1)

The optical apparatus of claim 1, wherein a tapered portion of the length of single mode optical fiber and the length of multimode optical fiber comprises the splice.

### Claim 4 (depends on 3)

The optical apparatus of claim 3, wherein the tapered portion comprises an adiabatic taper.

### Claim 5 (depends on 3)

The optical apparatus of claim 3, wherein the length of multi-mode optical fiber comprises a core, and a diameter of the core at an input end is sufficiently small to provide single mode operation of the length of multi-mode fiber at the tapered portion.

### Claim 6 (depends on 1)

The optical apparatus of claim 1, wherein the length of multi-mode optical fiber comprises a core having a core diameter, and said gain medium is concentrated centrally within a fraction of the core diameter.

### Claim 7 (depends on 6)

The optical apparatus of claim 6, wherein mode coupling into higher-order modes is reduced by gain guiding and the fundamental mode is preferentially amplified.

### Claim 8 (depends on 1)

The optical apparatus of claim 1, wherein the length of multi-mode optical fiber comprises a fiber core having a total fiber core cross-sectional area, and the gain medium is confined in a cross-sectional area of the fiber core which is substantially smaller than the total fiber core cross-sectional area.

### Claim 9 (depends on 1)

The optical apparatus of claim 1, wherein the optical apparatus is disposed in an optical cavity formed between a first mirror and a second mirror.

### Claim 10 (depends on 9)

The optical apparatus of claim 9, wherein the optical cavity further comprises a pair of Faraday rotators.

### Claim 11 (depends on 1)

The optical apparatus of claim 1, wherein the length of multi-mode optical fiber comprises a core having a diameter that is several tens of microns.

### Claim 12 (depends on 1)

The optical apparatus of claim 1, wherein the optical apparatus is configured as a portion of a multi-mode fiber amplifier.

### Claim 13 (depends on 1)

The optical apparatus of claim 1, wherein the splice is used to match the mode of the length single mode fiber to the fundamental mode of the length of multimode fiber.

### Claim 14 (depends on 13)

The optical apparatus of claim 13, wherein the single mode of the length of single mode fiber is matched to the fundamental mode of the length of multimode fiber with an efficiency of approximately 90% or higher.

### Claim 15 (depends on 1)

The optical apparatus of claim 1, wherein a core of the single mode fiber is thermally tapered to match the single mode of the length of single mode fiber to the fundamental mode of the length of multimode fiber.

### Claim 16 (depends on 1)

An optical amplifier comprising the optical apparatus of claim 1, said optical amplifier configured to provide an amplified output substantially in a fundamental mode.

### Claim 17 (depends on 16)

The optical amplifier of claim 16, wherein the length of multi-mode optical fiber comprises a double clad structure, the gain medium is cladding pumped, and the optical amplifier provides output power greater than 10 kW and a near diffraction limited output beam.

### Claim 18 (depends on 17)

The optical amplifier of claim 17, wherein a core diameter of said length of multimode fiber is several tens of microns.

### Claim 19 (depends on 17)

The optical amplifier of claim 17, wherein a core diameter of said length of multi-mode fiber is larger than a core diameter of said length of single mode fiber.

### Claim 20

An optical apparatus for amplifying light, the apparatus comprising:
a length of multi-mode optical fiber, said multi-mode optical fiber including a gain medium;
an output for receiving amplified light from said length of multi-mode optical fiber;
an energy source for exciting said gain medium; and
an input for providing a beam of light to be amplified within said length of multi-mode fiber, said input comprising a near-diffraction limited laser source, said input optically coupled to said length of multi-mode fiber, said length of multi-mode fiber providing substantially single mode light at the output of said apparatus.

### Claim 21

A light amplifier comprising:
a length of multi-mode optical fiber, said multi-mode optical fiber including a gain medium;
an energy source for exciting said gain medium; and
an input for providing a near-diffraction limited beam of light to be amplified within said length of multi-mode fiber, said input optically coupled to said length of multi-mode fiber to provide substantially diffraction limited light at the output of said amplifier, said input comprising a near-diffraction limited laser source.

### Claim at position 22

-40. (canceled)