Patent Publication Number: US-2002001125-A1

Title: Optical device for generating a plurality of optical signals

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to an optical fiber amplifier; and, more particularly, to an optical fiber amplifier incorporating therein a wavelength division multiplexer and a plurality of diffraction grating pairs for generating a number of optical signals with a pumping light of a single wavelength, each optical signal having a different wavelength.  
       DESCRIPTION OF THE PRIOR ART  
       [0002] In recent years, many researches for an optical fiber amplification technology in a ultra-wide range of which a wavelength is 1.4 μm˜1.6 μm, are being advanced to achieve tens of tetra-bit speed in an optical communication. An optical fiber Raman amplifier that is expected to contribute to the long distance optical communication has an advantage of expanding a gain bandwidth with ease in case of using a multi-wavelength pumping source because the range of amplification wavelength is determined by a pumping wavelength.  
       [0003] Referring to FIG. 1, there is provided a schematic view of a prior art optical fiber  100 , comprising a pumping source  110  for generating a pumping light, an optical fiber  130  for guiding the pumping light and the optical signals, three pairs of diffraction gratings  150 A and  150 B,  160 A and  160 B,  170 A and  170 B for forming oscillators to generate different optical signals, e g., wavelengths of a first Stokes frequency shift, a second Stokes frequency shift and a third Stokes frequency shift in sequence, an unpaired grating  190  for reflecting the pumping light and transmitting an output signal of a third Stokes frequency shift, the other unpaired gratings  180  for reflecting the wavelength of the third Stokes frequency shift and inducing to output the optical signal of the third Stokes frequency shift, and an optical gain fiber  185  for transforming the pumping light into a light with wavelength of the first Stokes frequency shift and the light with wavelength of the first Stokes frequency shift into a light with wavelength of the second Stokes frequency shift subsequently.  
       [0004] In the conventional optical amplifier as described above, one pumping light from the pumping source  110  makes only one output signal.  
       [0005] Therefore, in order to generate optical signals having a plurality of wavelengths, the conventional optical amplifier needs pumping sources corresponding to the number of output signals which, in turn, a system is more complicated and has an increased manufacturing cost thereof.  
       SUMMARY OF THE INVENTION  
       [0006] It is, therefore, an object of the present invention to provide an optical fiber amplifier for generating a number of optical signals with a pumping light of a single wavelength by incorporating therein a wavelength division multiplexer and a plurality of diffraction grating pairs.  
       [0007] In accordance with one aspect of the present invention, there is provided an optical device for generating a plurality of optical signals with a first pumping light of a wavelength, each optical signal having a different wavelength, comprising: a first optical means for guiding the first pumping light and the optical signals; a second optical means for generating a second pumping light with the first pumping light and generating the plurality optical signals with the second plumping light; and an optical coupler for introducing the first pumping light into the second optical means and outputting the optical signals. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008] The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:  
     [0009]FIG. 1 illustrates a schematic view of a prior art Raman laser for a single wavelength;  
     [0010]FIG. 2 presents a schematic view showing an optical device in accordance with a first preferred embodiment of the present invention;  
     [0011]FIGS. 3A and 3B are graphs showing a relationship between a coupling ratio of a wavelength division multiplexer (WDM) and wavelengths;  
     [0012]FIG. 4 shows an exemplary spectrum of the optical device in accordance with the first preferred embodiment of the present invention;  
     [0013]FIG. 5 is a graph showing a variation of an output spectrum by translating mechanically in the two-wavelength optical device in accordance with the preferred embodiment of the present invention;  
     [0014]FIG. 6 depicts a graph showing an output spectrum to modulate intensity according to a variation of the reflective feature in accordance with the preferred embodiment of the present invention; and  
     [0015]FIG. 7 represents a schematic view showing a four-wavelength optical device in accordance with a second preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0016] Referring to FIG. 2, there is provided a schematic view of an optical device  300 , e.g., Raman laser, for outputting two optical signals in accordance with a preferred embodiment of the present invention, comprising a pumping source  310  for generating a first pumping light, a first optical fiber  320  for guiding the first pumping light and the optical signals, a second optical fiber  330 , an optical gain fiber  334  for transforming the first pumping light into a second pumping light, an optical element  332  for introducing the first pumping light into the second optical fiber  330  and outputting the optical signals, two pairs of diffraction gratings  338 A and  338 B,  340 A and  340 B for forming oscillators to the two optical signals respectively, and an unpaired grating  336  for reflecting the first pumping light back to the optical element  332 . The diffraction gratings  338 A and  340 A are placed in the first optical fiber  320  and the diffraction gratings  338 B and  340 B are placed in the second optical fiber  330 . The number of the pairs is determined by the number of optical signals to be generated.  
     [0017] In the optical amplifier  300 , the first pumping light from the pumping source  310  introduces into the optical element  332 , e.g., a wavelength division multiplexer (WDM), after passing through the diffraction gratings  338 A,  340 A. In the preferred embodiment, the first pumping light has a center wavelength of 1,313 nm. The first pair of diffraction gratings  338 A,  338 B reflect a first optical signal, e.g., having a center wavelength of 1,480 nm, and transmit a light having other wavelengths. It should be noted that the optical element  332  could be replaced with a pair of optical fiber diffraction gratings.  
     [0018] On the other hand, the second pair of diffraction gratings  340 A and  340 B reflect a second optical signal, e.g., having a center wavelength of 1,500 nm, and transmits a light having other wavelengths.  
     [0019] The optical element  332  includes a first, a second, a third and a fourth ports A, B, C and D, wherein the first and the second ports A and B are connected to the first optical fiber  320  and the third and the fourth ports C and D are coupled to the second optical fiber  330 . It is preferable that the optical element  332  has a coupling ratio of approximately 100% between the first and the second optical fibers  320 ,  330  to the first pumping light as shown in FIGS. 3A and 3B. Therefore, the first pumping light is inputted to the second optical fiber  330  through the first and fourth ports A and D and outputted to the unpaired diffraction grating  336  through the third and the second ports C and B.  
     [0020] In the second optical fiber  330 , the first pumping light is transferred into a second pumping light, e.g., having a wavelength around 1,400 nm, by a first Stokes frequency shift after passing through the optical gain fiber  334 .  
     [0021] Referring back to FIGS. 3A and 3B, the optical element  332  as a very low coupling ratio at the wavelength of the second pumping light, In this result, the second pumping light is oscillated in the second optical fiber  330  and amplified after passing through the optical gain fiber  334 . In this embodiment, the second optical fiber  330  is in the form of a closed loop. After the amplified second pumping light becomes a predetermined amount, the amplified second pumping light is transferred into a first and a second optical signals by a second Stokes frequency shift, the first and the second optical signals having 1,480 nm, and 1,500 nm center wavelengths, respectively.  
     [0022] Since the optical element  332  has a coupling ratio of less than 100% and greater than 50% between the first and the second optical fibers  320 ,  330  to the first and the second optical signals, a major portion of the first optical signal is oscillated from the diffraction grating  338 A to the diffraction grating  338 B and a major portion of the second optical signal is oscillated from the diffraction grating  340 A to the diffraction grating  340 B. The remaining portions of the first and the second optical signals are outputted through the unpaired diffraction grating  336   
     [0023] Referring to FIGS.  4 , there is provided an experimental output spectrum data of the optical device in accordance with the preferred embodiment of the present invention. In this result, the center wavelength of the first Stokes frequency shift is generated approximately at 1,400 nm and those of the second stokes frequency shift are at 1,480 nm and 1,500 nm respectively.  
     [0024] Referring to FIG. 5, it is possible to modulate each output wavelength by stretching or compressing the diffraction gratings in each pair simultaneously. However, if the wavelength of only one grating is changed between a pair of gratings, it is impossible to generate laser oscillation and the intensity of the output light is reduced owing to the different reflective property of each grating. Therefore, this is utilized to control the intensity of the output light by unbalancing the reflective property of each grating.  
     [0025] Referring to FIG. 5, there is shown the experimental data of the multi-wavelength optical device  300  of the present invention, wherein the output light of 1,480 nm is varied from 1,480 nm to 1,485 nm by the mechanical translation. A pair of gratings with high reflective ratio to the wavelength of 1,480 nm are stretched simultaneously, thereby modulating the wavelength about 5 nm differentials. It is also possible to change the wavelength of the pair of gratings with high reflective ratio to the wavelength of 1,500 nm by stretching and compressing.  
     [0026]FIGS. 6A to  6 C are the experimental result of the multi-wavelength optical device  300  of the present invention shown the intensity relative to the modulation of each grating. FIG. 6A shows two output light of the wavelength in 1,480 nm and 1,500 nm in normal state, FIG. 6B shows the reduction of intensity by unbalancing the reflective property of a pair of gratings in the wavelength of 1,480 nm on purpose, and FIG. 6C shows the same result to the pair of gratings in the wavelength of 1,500 nm.  
     [0027] It is useful to control the property of the gain in the optical amplifier using the pumping source as the Raman laser for enabling to generate the oscillation of two-wavelengths and modulate the intensity.  
     [0028] The multi-wavelengths optical device  300  is implemented simply by adding the pairs of gratings in the wavelength correspondent to that of the output optical signals. But, in this case, the operating range of the multi-wavelengths Raman laser is only within the range of the Raman gain.  
     [0029] Referring to FIG. 7, there is shown the four-wavelength optical device  800 , e.g., Raman laser, to overcome the previous one. This scheme is implemented by adding a two pairs of diffraction gratings to the two-wavelength Raman laser of the preferred embodiment of the present invention as described in FIG. 2. Since the change of the reflective wavelength of the diffraction gratings has no influence on the transmission property of the other wavelength, the intensity of the wavelength can be controlled separately. However, the operating range of the multi-wavelengths optical device  800  is only within the range of the Raman gain as described above.  
     [0030] Besides the preferred embodiment of the present invention, the Raman lasers with over than third Stokes&#39; order is also implemented by adding, the diffraction gratings and he WDM to the Raman laser of the first Stokes shift. Furthermore, this present invention is applied to the Raman laser using an erbium dopped fiber also.  
     [0031] Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.