Large power cladding pumping single mode fiber Raman laser
阅读说明:本技术 大功率包层泵浦单模光纤拉曼激光器 (Large power cladding pumping single mode fiber Raman laser ) 是由 瓦伦丁·盖庞特瑟夫 伊格尔·山马尔特瑟夫 尼古拉·普拉特诺夫 于 2018-05-14 设计创作,主要内容包括:一种拉曼光纤激光源被配置有馈送光纤,馈送光纤将MM泵浦辐射传送到双包层MM拉曼光纤激光器的内包层。MM泵浦光束辐射具有足够的功率以在MM拉曼光纤中产生拉曼散射,从而将泵浦辐射转换为拉曼频移波长λram的MM信号辐射,该波长λram长于泵浦辐射的波长λpump。拉曼激光源还具有一对间隔开的反射器,在反射器之间限定了针对第一阶斯托克斯波长的信号辐射的谐振器,并位于拉曼光纤的MM纤芯的至少一部分处的侧面,MM纤芯设置有中心纤芯区域,所述中心纤芯区域掺杂有杂质以增强拉曼过程。反射器和中心纤芯区域的尺寸被确定为对应于拉曼光纤输出的MM信号辐射的基模,该MM信号辐射的功率范围在几kW至几十kW之间且M<Sup>2</Sup>因子≤1.1。(A raman fiber laser source is configured with a feed fiber that delivers MM pump radiation to the inner cladding of a double-clad MM raman fiber laser. The MM pump beam radiation has sufficient power to generate raman scattering in the MM raman fiber to convert the pump radiation into MM signal radiation at a raman shifted wavelength λ ram, which is longer than the wavelength λ pump of the pump radiation. The raman laser source further has a pair of spaced apart reflectors defining between them a resonator for the signal radiation of the first-order stokes wavelength and flanking at least a portion of the MM core of the raman fiber, the MM core being provided with a central core region doped with impurities to enhance the raman process. The reflector and central core region being dimensioned to correspond to the output of the Raman fiberFundamental mode of MM signal radiation having a power ranging from several kW to several tens of kW and M 2 The factor is less than or equal to 1.1.)
1. A high power single mode SM raman laser source comprising:
an end-pumped multi-clad raman fiber comprising:
an inner cladding receiving multimode MM pump radiation propagating along the path at a wavelength λ pump, and
an MM core surrounded by the inner cladding and provided with a central core region, the pump radiation generating Raman scattering causing conversion of the pump radiation into signal radiation at a Raman-shifted wavelength λ ram, wherein the Raman-shifted wavelength λ ram is larger than the wavelength λ pump,
the central core region being dimensioned to substantially confine only a fundamental mode FM of the signal radiation and doped with an impurity that enhances the Raman scattering; and
spaced apart wavelength selective reflectors defining therebetween a resonator for FM of wavelength λ ram, the resonator at least partially including the central core region,
ruler of the wavelength selective reflectorIs determined to match the FM of the signal radiation, wherein the Raman fiber output power ranges between several kilowatts kW and several tens kW, M2FM signal radiation less than or equal to 1.1.
2. The SM raman laser source of claim 1 further comprising a MM feed fiber located upstream of the raman fiber and delivering the pump radiation to an upstream end of the raman fiber.
3. An SM raman laser source according to claim 1 or 2, wherein the MM feed fiber and raman fiber are directly fused to each other, the wavelength selective reflector being a fiber bragg grating, FBG, formed in the central core region.
4. An SM raman laser source according to claim 1 or 2 further comprising a MM intermediate passive fiber fused to opposite ends of the respective feed and raman fibers and configured with a MM core supporting substantially only propagation of FM of the intermediate fiber, wherein the FM of the intermediate and raman fibers having the respective mode field diameters are matched to each other.
5. The SM raman fiber laser of any preceding claim further comprising a SM output passive fiber fused to a downstream end of the raman fiber, the wavelength reflectors being respective fiber bragg gratings written into the cores of the respective SM input and output passive fibers and the wavelength reflectors being optically aligned with the central core region.
6. An SM raman laser source according to claim 1 or 2, further comprising: a collimating lens and a focusing lens between the spaced apart feed fiber and the raman fiber; and a tilting mirror between the lenses to deflect back reflected light away from the path.
7. An SM raman fiber laser according to any preceding claim further comprising a plurality of fiber laser pumps having respective output fibers coupled together in a beam combiner such that the outputs of the respective fiber laser pumps constructively interfere with one another to produce high power MM pump radiation at a pump wavelength λ pump.
8. An SM raman laser according to any preceding claim, wherein the pump wavelength λ pump is about 1070nm and the signal wavelength λ ram is about 1120 nm.
9. An SM raman fiber laser according to any preceding claim, wherein the inner cladding and MM core of the raman fiber are made of pure silica, the inner cladding and MM core having respective indices of refraction matched to one another, the inner cladding region being doped with fluorine to provide the depression.
Technical Field
The present disclosure relates to a high power Continuous Wave (CW) raman fiber laser system operable to output a Single Mode (SM) laser beam having a power ranging between several kilowatts (kW) to tens of kW. In particular, the present invention discloses a high power fiber laser pump outputting multimode (MM) pump light, which is end-coupled to a pump capable of outputting M2The cladding of the multimode (MM) Raman fiber of kW-level SM signal light less than or equal to 1.1.
Background
Fiber lasers are used to efficiently convert poor quality pump radiation delivered by multimode Laser Diodes (LDs) into high quality laser beams. In high power fiber lasers, several powerful multimode LDs are typically coupled (e.g., by pump combiners) to the silica cladding of a double-clad active fiber with a core doped with a rare earth element such as (Yb), erbium (Er), etc. Pump radiation guided by the fiber cladding excites the dopants in the core, providing amplification to the core-guided light which is emitted as fundamental transverse mode radiation and has an approximately gaussian beam profile if the geometry of the fiber meets certain conditions. A feature of known all-fiber laser configurations is the production of high quality laser beams over a wide range of output powers.
It is known that lasing in passive fibers is possible due to inelastic raman scattering of pump radiation resulting in amplification of the displaced scattered light. When two laser beams of different wavelengths, pump (pump) light and signal light, propagate together through a raman-active medium, the longer wavelength light (i.e., stokes wave) can undergo optical amplification at the expense of the shorter wavelength pump beam — a phenomenon known as stimulated raman radiation (SRS). One of the unique characteristics of SRS includes: beam clean-up, i.e. brightness enhancement by SRS in MM fiber; and rapid energy transfer between the pump and the raman signal light. SRS thus provides an attractive solution to optically convert MM to SM laser output in both CW and pulsed formats.
Since fiber raman lasers/amplifiers (FRL and FRA) are based on the raman gain induced by pumping in passive fibers, respectively, the lasing characteristics of these devices are fundamentally different compared to rare earth doped fibers, i.e. small quantum defects characterized by first-order stokes, fast response of gain to pumping variations, low background spontaneous emission and lack of photodarkening effects, which is particularly severe in doped active fibers at short wavelengths.
The output power of conventional core-pumped (SM raman) fiber lasers is limited by the availability of high power SM Diode Lasers (DL). The power level is significantly improved by using the MM multi-clad Raman fiber as a gain medium and high-power MM pump light which is end-pumped into the inner cladding of the Raman fiber. One of the many configurations using cladding pumped Raman fibers is reported by Codemard et al in "High power CW clamped pumped Raman fiber laser" optics letters/Vol.31, No.15, 8/1/2006. The RFL disclosed herein is characterized by a double-clad raman fiber having an MM core supporting only the Fundamental Mode (FM) of the desired stokes wavelength, a germanium-doped inner cladding, and a silica outer cladding, by laser pumping using an MM fiber. Raman gain occurs throughout the MM core and inner cladding. FM selection is achieved by Fiber Bragg Gratings (FBGs), where the pitch is adjusted for the effective index of FM. As is conventional in the art, the core has an increased refractive index due to the high concentration of dopants, such as germanium, known to enhance the raman process.
The RFL disclosed herein can output several watts in SM. Single mode is achieved by using a true SM output fiber fused to the end of the raman fiber. The cores of the respective raman and output fibers are configured such that the respective Mode Field Diameters (MFDs) of the single and fundamental modes substantially match each other. The raman resonator is defined between strong and weak FBGs and provides gain substantially only to the fundamental mode of the first-order stokes wavelength. However, the SM output is obtained by filtering out the undesirable high-order modes present at the output of the raman fiber using an SM output fiber.
Doping at the reported high concentration levelsThe necessity of the entire MM core of the raman fiber disclosed above adds complexity and cost to the fiber manufacturing process. The reported signal power is far from meeting the current industry demand. However, the applicant is aware of the existence of 1.3kW raman lasers operating in continuous mode (CW). However, to the applicant's knowledge, the laser outputs light M2M with a factor significantly higher than the required quality output2A factor.
Therefore, there remains a need for a MM raman cladding pumped fiber laser source operable to output a beam of mass M2SM bright signal light less than or equal to 1.1 and with power range of several kilowatts.
Disclosure of Invention
The laser source of the present invention meets this need by utilizing a high power fiber laser based pump that outputs MM pump light that is end-coupled into the cladding of the MM raman fiber.
According to one aspect of the present disclosure, a high power Single Mode (SM) raman laser source of the present disclosure is configured with an end-pumped multi-clad raman fiber having an inner cladding that receives multi-mode (MM) pump radiation propagating along a path at a wavelength λ pump and an MM core provided with a central core region. The pump radiation is sufficiently powerful to generate raman scattering to convert the pump radiation into signal radiation at a raman-shifted wavelength or a signal radiation at a signal wavelength λ ram longer than wavelength λ pump.
The disclosed raman laser source also has spaced apart wavelength selective reflectors, such as Fiber Bragg Gratings (FBGs), between which a resonator for the signal radiation is defined. The discriminator and the central core channel are optically aligned and sized to match the FM of the signal radiation output from the Raman fiber, which may reach tens of kilowatts (kW) and M21.1 or less (preferably 1.05 or less).
The SM raman laser source is further configured with a MM feed fiber located upstream of the raman fiber and delivering pump radiation thereto. The feed fiber and the raman fiber may be directly fused to each other or spaced apart from each other.
In configurations featuring spaced apart feed and MM fibers, the disclosed laser source further includes bulk optics that shape the pump radiation so that it is coupled into the inner cladding of the raman fiber. In addition, tilted mirrors can be placed between the lenses to deflect the back-reflected radiation of the signal wavelength λ ram away from the path to protect the pump lasers.
Drawings
The above and other features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
fig. 1 is a schematic diagram of an all-fiber raman light source of the present invention according to one structural modification.
Fig. 2 is a raman source of the present invention utilizing free space communication between a feed fiber and a raman fiber.
Fig. 3 shows the refractive index profile of the raman fiber of the present invention.
Fig. 4 and 5 show the respective field distribution and intensity distribution of the fundamental mode in the exemplary inventive structures of fig. 1 and 2.
Fig. 6 shows a fiber laser source of the present invention featuring a fiber raman amplifier.
Detailed Description
Reference will now be made in detail to embodiments of the invention. Features of the invention may be used alone or in combination with selected inventive features or all other inventive features in each disclosed configuration of the raman light source of the invention. Wherever possible, the same or similar reference numbers are used in the drawings and the description to refer to the same or like parts or steps. The figures are in simplified form and are not drawn to precise scale. The term "coupled" and similar terms do not necessarily denote a direct or immediate connection, but also include a connection through intermediate elements or devices.
Fig. 1 shows a raman
The output passive fibres 4 of the
The
The diameters of the
To ensure signal light near the diffraction limit at the desired signal wavelength (e.g., the first-order stokes wave), the
Optionally, an intermediate MM passive fiber, not shown, may be fused to the opposite ends of the respective feed fiber and raman fiber. Although having an MM core, the intermediate fiber may be configured to support only propagation of FM having an MFD substantially matching the MFD of FM supported in the
Fig. 2 shows an alternative architecture of the disclosed raman
Fig. 3 shows an example of the
Fig. 6 illustrates another aspect of the present disclosure, wherein a fiber raman source 50 is configured with a MM Fiber Raman Amplifier (FRA) 32. What is needed to do so is a SM
The
Alternatively, element 42 of fig. 6 may be configured as a seed laser that outputs light at a first-order stokes wavelength (e.g., 1120 nm). The remaining configuration, except for the
As the accumulated pump beam continues to pass through the MM core of
Although the present disclosure has been described in terms of disclosed examples, various modifications and/or additions to the above disclosed embodiments will be apparent to those skilled in the laser art without departing from the scope and spirit of the appended claims.