589 nm Light Source Based on Raman Fiber Laser

Harmonic generation using nonlinear optical crystals is currently the most efficient approach of obtaining visible and ultraviolet lasers. Diode-pumped Nd-doped solid-state lasers can operate in the blue, green, and red spectral regions by intracavity frequency doubling. However, there are few solid-state laser sources that can efficiently produce laser emission in a region covering 550 to 650 nm for the absence of fundamental lasers that can operate efficiently there. Laser sources in yellow-orange spectra are of interest for many applications in metrology, remote sensing, and medicine. Several approaches have been used to generate all-solidstate sources in yellow-orange wavelength region: solid dye lasers, frequency doubling of crystal Raman lasers or LiF:F 2 lasers, 4) and sum-frequency mixing of two Nd laser lines at 1064 nm and 1319 nm. The most impressive study to date is on a 20W continuous wave (CW) laser at 589.159 nm by sum-frequency mixing two injection-locked Nd:YAG lasers in lithium triborate (LBO) in a doubly resonant external cavity. This study was aimed at building a laser source for sodium D2 laser guided star for adaptive optical systems, where high-duty-cycle sources such as CW are preferred to avoid saturation. However, the entire system was very complex and expensive. Our approach is to generate a 589 nm laser source by frequency doubling of a Raman fiber laser at 1178 nm. Fiber lasers are easy to handle and excellent beam quality can be achieved with single-mode fibers. Moreover, CW Raman fiber laser in a broad wavelength range can be generated despite the low Raman gain in glass, because of the large interaction length available in fibers. These characteristics make Raman fiber lasers potentially attractive CW laser sources for sodium laser-guided star systems. A laser at 1178 nm by stimulated Raman scattering in phosphosilicate single-mode fibers has already been investigated. In this letter, yellow emission at 589 nm by intracavity frequency doubling of a Raman fiber laser emitting at 1178 nm is described. A sketch of the experimental setup is shown in Fig. 1. An ytterbium-doped double-clad fiber laser emitting at 1100 nm was used as the pump source. The pump source fiber end was spliced to a fiber Bragg grating (FBG) (reflectivity > 99% at 1178 nm and bandwidth of 1.2 nm). Since both the pump fiber and the FBG are made of Flexcor-1060 fiber, a very low loss splicing was achieved between them. The Raman gain fiber was a 300-m-long phosphorous-doped single-mode optical fiber (PDF), which had 12mol% of P2O5 and a refractive index difference between the core and clad of 0.0107. A small mismatch in the mode field diameters of the PDF and Flexcor-1060 fiber resulted in a splicing loss of only about 0.2 dB between them. Aspheric lens L2 (focal length: 8mm) was used to collimate the Raman laser. The beam was focused once again into LBO crystal using lens L1 (focal length: 50mm). Both lenses were coated with antireflection coating for 1178 nm. Frequency doubling was achieved in a type-I noncritically phase-matched LBO crystal (3 3 20mm) mounted in an oven with temperature control. Concave mirror M1 (50mm radius of curvature and HR at around 1178 nm) was used as the end-mirror. Dichroic mirror M2 (highly reflecting at 1178 nm and transmitting at 589 nm) was used as the output coupler for 589 nm. The visible light escaping from the cavity through M2 was separated from infrared radiation leaking through M2 by a Brewster prism. Spectra of the output were measured with an AQ-6315A optical spectrum analyzer (ANDO Co.) directly after dichroic mirror M2. The Raman spectrum of the PDF contains two peaks: one at 1330 cm 1 is sharp and corresponds to the vibration mode of double-bonded oxygen with phosphor atoms and the other at 490 cm 1 is broader and corresponds to the vibration of oxygen with silicon atoms. A 602 cm 1 Raman shift is required to convert the pump laser of 1100 nm to 1178 nm laser emission. The Raman gain at around 602 cm 1 is not the peak value but a high-efficiency Raman laser has been demonstrated with such a configuration. Intracavity doubling was used to increase the efficiency of second harmonic generation (SHG). However, one will see in the following paragraphs that this approach gives rise to other difficulties. The LBO crystal was cut at 1⁄4 90 and Yb-doped fiber laser 1100nm FBG 1178nm HR PDF 300 m LBO in oven