Source: http://www.google.com/patents/US6373867?dq=6150774
Timestamp: 2017-10-17 16:17:21
Document Index: 469102987

Matched Legal Cases: ['art 810', 'art 810', 'art 810', 'art 810', 'art 810', 'art 810', 'art 810', 'art 810', 'art 810', 'art 810']

Patent US6373867 - Generation of a wavelength-tunable laser oscillation in a wave-guiding gain ... - Google Patents
A passive mode-locked linear-resonator fiber laser using polarization-maintaining fibers and a saturable absorber to produce ultra short pulses and a long-term reliable operation with reduced maintenance. Such a fiber laser can be configured to produce tunable pulse repetition rate and tunable laser...http://www.google.com/patents/US6373867?utm_source=gb-gplus-sharePatent US6373867 - Generation of a wavelength-tunable laser oscillation in a wave-guiding gain medium based on passive mode lock
Publication number US6373867 B1
Application number US 09/575,675
Also published as US6097741, US6570892
Publication number 09575675, 575675, US 6373867 B1, US 6373867B1, US-B1-6373867, US6373867 B1, US6373867B1
Inventors Hong Lin, Katherine Y. Lin
Original Assignee Calmar Optcom, Inc.
Patent Citations (12), Non-Patent Citations (4), Referenced by (93), Classifications (13), Legal Events (3)
US 6373867 B1
a pump light source which includes a semiconductor light source to produce a pump beam at a pump wavelength;
a wave-guiding gain medium to absorb light at said pump wavelength to emit light at a laser wavelength within a spectral gain profile and is different from said pump wavelength;
an optical coupler to couple said pump beam from said pump light source into said wave-guiding gain medium;
first and second reflective elements disposed to form a linear optical resonator that encloses said wave-guiding gain medium and supports a plurality of longitudinal modes;
a saturable absorber disposed in said optical resonator and formed of a material to exhibit an intensity-dependent absorption to effectuate a mode-locking mechanism that locks said longitudinal modes in phase to produce optical pulses at said laser wavelength, said material in said saturable absorber selected to exhibit a slow saturation process with a low saturation intensity to initiate said optical pulses and a fast saturation process with a high saturation intensity to shorten a temporal width of said pulses; and
a tuning element located in said optical resonator to change at least one property of said optical pulses.
2. The laser as in claim 1, wherein said wave-guiding gain medium includes a fiber segment that is doped with rare-earth ions.
a first fiber segment coupled in an optical path between said first reflective element and said wave-guiding gain medium; and
a second fiber segment coupled in an optical path between said second reflective element and said wave-guiding gain medium,
wherein an intensity of said pump beam is less than a first threshold level above which a nonlinear optical effect occurs in said first and said second fiber segments, and is above a second threshold level above which an optical absorption in said saturable absorber is saturated.
9. The laser as in claim 1, wherein said optical coupler is positioned in an optical path of said optical resonator between said wave-guiding medium and said saturable absorber.
a wavelength-tuning mechanism in said optical resonator configured to (1) select radiation at said laser wavelength to travel in said optical resonator between said first and said second reflective elements and to suppress radiation at other wavelengths in said optical resonator and (2) to tune said laser wavelength from one wavelength to another within said spectral gain profile; and
a resonator-length tuning mechanism in said optical resonator configured to change an optical path length between said first and said second reflective elements of said optical resonator so as to alter a temporal repetition rate of said optical pulses.
12. The laser as in claim 1, wherein an intensity of said pump beam is selected and said optical resonator is configured so that said optical pulses are soliton optical pulses.
a photo detector positioned to receive a portion of said optical pulses from said optical resonator; and
an electronic control, coupled to receive a detector signal from said photo detector and operable to determine said pulse repetition rate from said detector signal and to produce an error signal indicative of a difference between said pulse repetition rate and a reference clock rate,
wherein said electronic control is operable to control said resonator-length tuning element to adjust and maintain said pulse repetition rate at said reference clock rate.
18. The laser as in claim 6, wherein said wavelength-tuning element includes a tunable optical bandpass filter.
27. The laser as in claim 12, further comprising a wavelength-tuning mechanism in said optical resonator configured to (1) select radiation at said laser wavelength to travel in said optical resonator between said first and said second reflective elements and to suppress radiation at other wavelengths in said optical resonator and (2) to tune said laser wavelength from one wavelength to another within said spectral gain profile.
33. The laser as in claim 30, further comprising a fiber grating controller coupled to said tunable fiber grating and operable to produce said control signal to adjust said tunable fiber grating.
first and second reflective elements disposed relative to each other to form an optical resonator having a plurality of longitudinal modes;
a fiber gain medium having optical transitions operable to absorb photons at said pump wavelength and to emit photons at a laser wavelength within a spectral gain profile that is different from said pump wavelength, said fiber gain medium configured to maintain light polarization in a specified direction perpendicular to an optic axis of said fiber gain medium;
a semiconductor absorber disposed in said optical resonator and configured to exhibit a saturable intensity-dependent absorption and to have a bandgap equal to or less than a photon energy corresponding to said laser wavelength, said semiconductor absorber operable to lock a plurality of oscillating longitudinal modes in phase to produce optical pulses at said laser wavelength and configured to exhibit a slow saturation process with a low saturation intensity to initiate said optical pulses and a fast saturation process with a high saturation intensity to shorten a temporal width of said pulses;
an optical coupler, disposed relative to said fiber gain medium to couple said pump beam into said fiber gain medium; and
a first polarization-maintaining fiber segment disposed to transmit light between said first reflective element and said fiber gain medium; and
a second polarization-maintaining fiber segment disposed to transmit light between said fiber gain medium and said second reflective element, wherein each of said first and said second fiber segments has a principal polarization axis aligned with said specified direction defined by said fiber gain medium.
36. The fiber laser as in claim 35, wherein said optical coupler is positioned in an optical path between said fiber gain medium and said semiconductor absorber to direct said pump beam towards said fiber gain medium.
64. The fiber laser as in claim 60, wherein said end facet of said first polarization-maintaining fiber segment is coated with an anti-reflection coating.
The present disclosure includes a passive mode-locked fiber laser with a simple linear cavity and a saturable absorber to generate femtosecond pulses with a peak power up to and greater than tens of watts.
Polarization-maintaining single-mode (“PM”) fibers or polarizing single-mode fibers are preferable for any fibers in the optical path of the laser pulses, i.e., the doped fiber 130 and other undoped fiber segments linking various optical elements in the resonator. Fiber segments 140 a, 140 b, 160 a, 160 b, and 121 a for linking optical couplers 140, 160 and optical collimator 121 are such examples. Preferably, fiber segments 144 and 164 in the couplers 140 and 160 are also PM or polarizing fibers although regular fibers may be used. A polarization-maintaining fiber is configured to have well-defined principle axes for two mutually orthogonal polarizations. A polarizing fiber has a single principal polarization axis. These two types of fibers can be configured so that a principal axis is essentially not influenced by environmental conditions, such as fiber position, temperature, and stress. Therefore, the polarization of a beam propagating in such a fiber can be maintained. In the following description, “polarization-maintaining fiber” fiber will be used to include any fiber or optical waveguide that can preserve an optical polarization of a beam in a resonator.
The pump optical coupler 140 is disposed relative to the doped fiber segment 130 to couple the pump beam in the fiber 142 into the doped fiber segment 130. Two undoped fiber segments 140 a and 140 b may be used to optically coupled the coupler 140 into the resonator. The fiber 140 a is connected to one end of the doped fiber 130 by, for example, fusion splicing. The pump beam is directed into the doped fiber 130 to produce population inversion at a desired optical transition. This initiates spontaneous emission at the desired laser wavelength defined by the optical transition and build up the laser oscillation due to the optical feedback as the output power of the pump light source 150 exceeds a threshold level. Since the laser wavelength is different from the pump wavelength, the pump optical coupler 140 should be a wavelength-division multiplexer (“WDM”). For Er-doped fiber laser, the WDM 140 is configured for coupling energy between two different channels near a 1.55-μm lasing region (e.g., from 1520 nm to 1580 nm) and near a 0.98-μm pump region (e.g., from 965 nm to 995 nm).
FIGS. 3A-3F show alternative embodiments of the first reflective element 110. FIG. 3A uses a lens 114 a in place of the collimator 114 in FIG. 1. The lens 114 a is placed away from the grating 112 with a spacing larger than the lens focal length f to image the free end facet of the fiber 111 to the grating 112. Since the beam is a Gaussian beam, the beam on the grating surface is essentially collimated within the Rayleigh length range. The output facet of the fiber 111 as shown is perpendicular to the fiber core. This facet is preferably coated with an anti-reflection coating at the pump wavelength to reduce the reflection of the pump beam coupled into the resonator from the coupler 140. This coating may also be anti-reflective at the laser wavelength to reduce a sub-cavity effect due to intra-cavity reflection at the laser wavelength. FIG. 3B is similar to FIG. 3A except that the output facet 111 a of the fiber 111 is polished at an angle of several degrees (typically, about 60˜80) to reduce optical reflection at both laser wavelength and pump wavelength. The angle-polished fiber 111 is tilted at an angle with respect to the optic axis of the lens 114 a in order to achieve proper optical coupling. FIG. 3C shows an embodiment that uses a reflector 113 with a high reflectivity at the laser wavelength and a low reflectivity at the pump wavelength. FIG. 3D places a lens 114 a away from the output facet of the fiber 111 by a focal length to collimate the beam incident to the high reflector 113. This optical configuration can also replace the focusing configuration shown in FIGS. 3A-3C.
For example, the first reflective element 110 can be designed to reflect only the light at the laser wavelength and to significantly reduce or eliminate any light reflection at the pump wavelength. This reduces the adverse feedback to the pump light source 150. One implementation uses an angle cleaved or angle polished facet (e.g., larger than 6°) at the free end of the PM fiber 140 a of the WDM coupler 140. Another implementation applies an anti-reflection coating designed for the pump wavelength on the facet of the facet at the free end of the PM fiber 140 a of the WDM coupler 140.
A fiber laser in accordance with the invention may be operated either in a soliton mode or in non-soliton mode. Soliton is a special nonlinear phenomenon in which an optical pulse maintains its shape and spectral profile essentially unchanged during propagation in fibers. An optical pulse traveling in a fiber is subject to the fiber dispersion so that different frequency components in the pulse travel at different group velocities. This dispersion causes pulse broadening in the time domain. In addition, a pulse also experiences a nonlinear effect “self phase modulation” (“SPM”) caused by the intensity dependence of the refractive index of the fiber. SPM can lead to new frequency components in high intensity pulses, thus effectively broadening the pulse in the frequency domain. Soliton pulses are generated from a fiber laser when the fiber dispersion is negative, i.e., the group velocity of a high frequency component is higher than that of a low frequency component, and the group velocity dispersion compensates for SPM.
FIG. 8A shows a side view of an adjustable mount 800 for holding the optical collimator 114 and 121 in the fiber laser 100 in FIG. 1. The mount 800 has a base 810 with a first base part 810 a and a second base part 810 b in a shape of the letter “L”. The base 810 is made of a rigid material such as aluminum or steel. A slot 812 is formed in the first base part 810 to change the position of the second base part 810 b upon being compressed by a set of screws 814 a and 814 b. Screws 814 a adjust the position of the second base part 810 b in a direction opposite to that controlled by screws 814 b. A slot 812 b is formed in the second base part 810 b to split a portion thereof to form an adjustable part 810 c. The second base part 810 b has a through hole 820 to hold a collimator 802. Screws 816 on the adjustable part 810 c are used to adjust the relative tilt of the part 810 c in order to control the orientation of the collimator 802. FIGS. 8B and 8C show different side views of the mount 800 along the lines 8B—8B and 8C—8C, respectively. This design can also be used to eliminate the conventional ball bearing.
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U.S. Classification 372/18, 372/75, 372/6, 372/98
International Classification H01S3/098, H01S3/067
Cooperative Classification H01S3/1118, H01S3/1394, H01S3/067, H01S3/0675, H01S3/1305
European Classification H01S3/067D, H01S3/067