2nd-order random lasing in a multimode diode-pumped graded-index fiber

Abstract

Raman lasing in a graded-index fiber (GIF) attracts now great deal of attention due to the opportunity to convert high-power multimode laser diode radiation into the Stokes wave with beam quality improvement based on the Raman clean-up effect. Here we report on the cascaded Raman generation of the 2nd Stokes order in the 1.1-km long GIF with 100-μm core directly pumped by 915-nm diodes. In the studied all-fiber scheme, the 1st Stokes order is generated at 950–954 nm in a linear cavity formed at GIF ends by two fiber Bragg gratings (FBGs) securing beam quality improvement from M2 ≈ 30 to M2 ≈ 2.3 due to special transverse structure of FBGs. The 2nd Stokes wave is generated either in linear (two FBGs) or half-open (one FBG) cavity with random distributed feedback via Rayleigh backscattering. Their comparison shows that the random lasing provides better beam quality and higher slope efficiency. Nearly diffraction limited beam (M2 ≈ 1.6) with power up to 27 W at maximum gain (996 nm), and 17 W at the detuned wavelength of 978 nm has been obtained, thus demonstrating that the 2nd-order random lasing in diode-pumped GIF with FBGs provides high-efficiency high-quality beam generation in a broad wavelength range within the Raman gain spectral profile.

Cascaded Generation in Multimode Diode-Pumped Graded-Index Fiber Raman Lasers

“…Two different LD sets with wavelengths of 915 or 940 nm with a total input power in GIF up to ~200 W have been used for pumping. The corresponding conversion of highly multimode (M 2 ~34) pump radiation at 915 nm into the first/second Stokes waves with wavelengths 950 (954)/976 (996) nm selected by corresponding FBG sets have been explored in the first case [16], and 940 nm LD pump conversion into the first/second Stokes waves with wavelengths 976/1019 nm in the second case [17]. Cascaded Raman laser scheme based on multimode LD-pumped fiber: LD1-3 are laser diodes pigtailed by step-index fiber (SIF) with 100 µm core and combined by a pump combiner; GIF is 1 km Raman graded-index fiber with 100 µm core in which an UV-inscribed highly reflective (R~90%) fiber Bragg grating (UV FBG) and a fs-inscribed output coupler (R~4%) (FS FBG) form a cavity at wavelengths corresponding to the 1st and the 2nd Stokes waves.…”

“…As a first step, we compared cascaded generation in a linear cavity formed by highly reflective UV FBG and a mode-selective FS FBG with low reflection (R~4%) for both the first (S1) and the second (S2) Stokes wave-see Figure 3a-with the configuration in which output FS FBG for the second Stokes wave is absent so that it starts generating in a half-open cavity configuration due to the random distributed feedback via Rayleigh backscattering; see Figure 3b. We tested [16,17] and compare here two different wavelength sets-(1) 940 nm (LD)-976 nm (S1)-1019 nm (S2);…”

“…Here, we report on an opportunity to further broaden the operating wavelength range via cascaded Raman conversion in graded-index fibers of multimode LD pump radiation into higher Stokes orders, analyzing the results of papers [16,17] with a corresponding discussion of the key physical process and adding new results that help to clarify Raman laser operation in cascaded schemes based on multimode fibers. Linear and half-open (with RDFB) cavities for high-order Stokes generation are compared.…”

High-brightness all-fiber Raman lasers directly pumped by multimode laser diodes

“…It appears also possible to obtain random lasing at significantly shifted wavelength (978 nm) with relatively good parameters, which are measured to be much better than the normal 2nd-order Raman lasing in the linear cavity formed by two FBGs; for more details of this study, see Ref. [22]. The obtained high beam quality made it possible to realize efficient frequency doubling in PPLN nonlinear crystal, thus generating blue laser light near 489 nm; the details of this study will be published elsewhere.…”

Микро- И Наноструктурирование Оптических Материалов С Помощью Фемтосекундного ИК Излучения

“…Это же свойство используется для формирования ВБР в разных областях поперечного сечения многомодовых световодов, что позволяет селектировать отдельные поперечные моды: как фундаментальную (LP 01 ), так и высших (например, LP 11 ). На этой основе созданы эффективные ВКР-лазеры с прямой диодной накачкой пассивного многомодового градиентного световода, генерирующие в области 950-980 нм [5], где традиционные волоконные лазеры на волокнах, легированных редкоземельными элементами, не работают.…”