1. Field of the Invention
The present invention relates to a cascade Raman laser.
2. Description of Related Art
FIG. 9 is a block diagram showing one exemplary structure of a conventional cascade Raman laser outputting laser light with a wavelength of 1480 nm. (see S. G. Grubb, et al., “High-Power 1.48 μm Cascaded Raman Laser in Germanosilicate Fibers,” in Optical Amplifiers and Their Applications (1995), paper SaA4 for example). This cascade Raman laser 2000 has a structure in which a clad pump fiber laser (CPFL) 100 outputting laser light with a wavelength of 1117 nm as pumping light and a cascade Raman resonator (CRR) 200 that generates laser light with the wavelength of 1480 nm by using the laser light with the wavelength of 1117 nm are connected in cascade. It is noted that marks ‘x’ in the figure indicate fused connecting points. It is also noted that optical fibers represented by thin solid lines have an outer diameter of 125 μm and optical fibers represented by thick solid lines have an outer diameter of 250 μm.
The structure of the cascade Raman laser 2000 will be explained concretely from a back of the CPFL 100. Firstly, the CPFL 100 has a structure in which an extension optical fiber 101, a front pumping TFB (Tapered Fiber Bundle) 102, i.e., a multiplexer for multiplexing the pumping light, a highly reflective fiber Bragg grating (FBG, referred to as HR (High Reflection) hereinafter) 103 having reflectivity of about 100% for the wavelength of 1117 nm, a clad pump fiber (CPF) 104 having a double clad structure and Yb is doped in its core, a back pumping TFB 105 and an outputting fiber Bragg grating (FBG, referred to as an OC (Output Coupler) hereinafter) having reflectivity of about 30 to 60% for the wavelength of 1117 nm are connected in cascade by means of fusion splicing. The front pumping TFB 102 is connected with eighteen semiconductor pumping lasers 107 with a wavelength of 989 nm through eighteen pumping optical fibers 108. The back pumping TFB 105 is connected with eighteen semiconductor pumping lasers 111 through eighteen pumping optical fibers 112.
The CPFL 100 outputs laser light with the wavelength of 1117 nm by an amplifying action of the CPF 104 to which the pumping light is supplied and by an optical resonator composed of the HR 103 and the OC 106.
Meanwhile, the CRR 200 has a structure in which an input-side reflector (referred to as a CRRin hereinafter) 201, a Raman optical fiber 202 for Raman amplification, an output-side reflector (referred to as a CRRout hereinafter) 203 and an outputting optical fiber 204 are connected in cascade by means of fusion splicing.
The CRRin 201 is composed of five FBGs that selectively reflect lights of mutually different wavelengths. Reflection center wavelengths of the respective FBGs are, from an input side, about 1480 nm, 1390 nm, 1310 nm, 1239 nm and 1175 nm. Meanwhile, the CRRout 203 is composed of six FBGs that selectively reflect lights of mutually different wavelengths. Reflection center wavelengths of the respective FBGs are, from an input side, 1480 nm, 1175 nm, 1239 nm, 1310 nm, 1390 nm and 1117 nm. It is noted that reflectivity of the reflection center wavelengths of the respective FBGs of the CRRin 201 and the CRRout 203 is from 5 to 30% for the FBGs whose reflection center wavelength is 1480 nm and is about 100% for the other FGBs. (It is also noted that in order of the reflection center wavelengths of the respective FBGs of CRRin 201 and CRRout 203 is accordance with U.S. Pat. No. 5,815,518 titled “Article comprising a cascaded raman fiber laser” and is set up to get the high-efficiency optical power with the wavelength of 1480 nm within CRR 200. It is able to operate that in order of the FBGs is not same above-mentioned CRR. No matter what the FBGs is in different orders.)
Next, operations of the CRR 200 will be explained. When the laser light with the wavelength of 1117 nm is inputted to the Raman optical fiber 202 from the CPFL 100, Raman scattering light with a wavelength of 1175 nm corresponding to a first Stokes wavelength of Raman scattering (referred to as a first Stokes ray hereinafter) is generated and is Raman-amplified. The amplified first Stokes ray is enhanced in terms of its power by multiple reflection of the optical resonator composed of the CRRin 201 and the CRRout 203, functions as a pumping light in due course and generates a second Stokes ray with a wavelength of 1240 nm. Third to fifth Stokes rays of the wavelengths of 1310 nm, 1396 nm and 1480 nm are generated sequentially by the similar operation. Here, the reflectivity of the FBG reflecting the light with the wavelength of 1480 nm corresponding to the fifth Stokes ray is low, so that this light with the wavelength of 1480 nm outputs to an outside of the CRRout 203 through the outputting optical fiber 204. It is noted that because the CRRout 203 has the FBG whose reflecting wavelength is 1117 nm, the laser light with the wavelength of 1117 nm outputted out of the CPFL 100 is blocked from outputting to the outside of the CRR 200 and is efficiently utilized within the Raman optical fiber 202.
However, the cascade Raman laser 2000 having the conventional structure has a problem that there is a case when an output of the cascade Raman laser 2000 decreases even if an output of the semiconductor pumping laser of the CPFL 100 is increased to enhance power of output light.