Method and arrangement in connection with optical bragg-reflectors

Abstract

A method of establishing transmission of light through a chirped Bragg-reflector, a method of analyzing the power spectrum of a light signal using a chirped Bragg-reflector, and an arrangement for analyzing the power spectrum of a light signal. The Bragg-reflector reflects, in an unperturbed state, essentially all incident light within a predefined wavelength range. The methods include the steps of directing the light to be analyzed into an input end of a light-guiding structure, such as an optical fiber, which light-guiding structure is provided with a Bragg-reflector, and sending an acoustic pulse along the light-guiding structure, thereby effectively lowering the reflectance of the Bragg-reflector for a certain wavelength at a certain time. By monitoring the light thus transmitted through the Bragg-reflector, a power spectrum analysis of the incident light is obtained.

What is claimed is:  
     
         1 . A method of establishing transmission of light, comprising the steps of: 
 providing a light-guiding structure incorporating a chirped Bragg-reflector,    directing light into an input end of said light-guiding structure, and    sending a longitudinal acoustic pulse into said light-guiding structure for propagation along the same, the acoustic pulse being such that, for each location of said acoustic pulse in said Bragg-reflector, the reflectivity for a wavelength associated with said location in said Bragg-reflector is altered.    
     
     
         2 . The method of  claim 1 , wherein the propagation of the acoustic pulse along said light-guiding structure causes an alteration of a local grating period of said chirped Bragg-reflector to provide, for each location of said acoustic pulse in said Bragg-reflector, transmission of a wavelength associated with said location, thereby providing a wavelength scan of transmitted wavelengths as the acoustic pulse propagates along said Bragg-reflector.  
     
     
         3 . The method of  claim 1 , wherein the acoustic pulse has a first part and a second part, said first part causing a temporary increase of the local grating period of the chirped Bragg-reflector, and said second part causing a temporary decrease of the local grating period of said Bragg-reflector.  
     
     
         4 . The method of  claim 3 , wherein the portion of said chirped Bragg-reflector containing the first part of said acoustic pulse has, in its unperturbed state, a longer grating period than the portion of said Bragg-reflector containing the second part of said acoustic pulse.  
     
     
         5 . The method of  claim 1 , wherein the step of sending a longitudinal acoustic pulse through the light-guiding structure is performed repeatedly, whereby a plurality of acoustic pulses is sent through said light-guiding structure, of which plurality of acoustic pulses only one acoustic pulse is present in the chirped Bragg-reflector at any one instant.  
     
     
         6 . The method of  claim 1 , further comprising the step of detecting, at an output end of the light-guiding structure, the intensity of the light transmitted, the light detected at a certain instant in time corresponding to a certain wavelength, thereby allowing analysis of the power spectrum of the light directed into said light-guiding structure.  
     
     
         7 . The method of  claim 6 , further including the step of prefiltering the light to be directed into the light-guiding structure by means of a broadband filter, said filter passing essentially the same wavelength range as the chirped Bragg-reflector reflects.  
     
     
         8 . The method of  claim 3 , wherein either one of the first and the second part of the acoustic pulse is predominant, the acoustic pulse thereby causing mainly an increase or mainly a decrease of the local grating period of the Bragg-reflector.  
     
     
         9 . The method of  claim 1 , wherein said light-guiding structure is the core of an optical fiber.  
     
     
         10 . An arrangement for analyzing the power spectrum of a light signal, comprising: 
 a light-guiding structure, having an input end and an output end;    a chirped Bragg-reflector in said light-guiding structure, which chirped Bragg-reflector is provided between the input end and the output end;    an acoustic actuator connected to said light-guiding structure; and    a detector provided at the output end of said light-guiding structure, said detector delivering a detector output signal,    wherein the input end of said light-guiding structure is arranged to receive the light signal to be analyzed, said acoustic actuator is operative to emit a longitudinal acoustic pulse for propagation along the light-guiding structure, and said detector is operative to detect the light thus transmitted through said Bragg-reflector and to provide a detector output signal associated with the power spectrum of said light signal.    
     
     
         11 . The arrangement of  claim 10 , further including a broadband filter provided in the light path prior to the input end of the light-guiding structure, said filter being arranged to pass only light of a desired wavelength range to said light-guiding structure.  
     
     
         12 . The arrangement of  claim 11 , wherein the broadband filter is arranged to pass essentially the same wavelength range as the chirped Bragg-reflector reflects.  
     
     
         13 . The arrangement of  claim 10 , further comprising a logic unit connected to said acoustic actuator, for supplying actuating pulses to said acoustic actuator.  
     
     
         14 . The arrangement of  claim 13 , further comprising a processing unit, which is arranged to control the operation of the logic unit.  
     
     
         15 . The arrangement of  claim 13 , wherein the logic unit is arranged to perform basic signal analysis of the detector output signal.  
     
     
         16 . The arrangement of  claim 15 , wherein the basic signal analysis performed by the logic unit is utilized for the control of the acoustic actuator.  
     
     
         17 . The arrangement of  claim 10 , wherein the light-guiding structure is clamped at a first point and a second point thereof, thereby defining a first and a second reflection point, at which reflection points the acoustic pulse is reflected, and effectively providing multiple passages of the acoustic pulse through the Bragg-reflector.  
     
     
         18 . The arrangement of  claim 17 , wherein the acoustic actuator is connected to the light-guiding structure at either one of the first and the second reflection points.  
     
     
         19 . The arrangement of  claim 18 , wherein the acoustic actuator is arranged to emit a longitudinal acoustic pulse into the light-guiding structure at the time when a previous acoustic pulse, after its reflection, arrives at the point where said actuator is connected to the light-guiding structure.  
     
     
         20 . A method of analyzing the power spectrum of a light signal, comprising the steps of: 
 directing the light signal to be analyzed into a light-guiding structure incorporating a chirped Bragg-reflector;    sending a longitudinal acoustic pulse into said light-guiding structure for propagation along the same, the acoustic pulse being such that, for each location of said acoustic pulse in said Bragg-reflector, the reflectivity for a wavelength associated with said location in said Bragg-reflector is altered;    monitoring the light thus transmitted through said light-guiding structure; and    analyzing the monitored light, in order to obtain a power spectrum analysis of the light signal directed into the light-guiding structure.    
     
     
         21 . The method of  claim 20 , wherein the propagation of the acoustic pulse along said light-guiding structure causes an alteration of the local grating period of said chirped Bragg-reflector to provide, for each location of said acoustic pulse in said Bragg-reflector, transmission of a wavelength associated with said location, thereby providing a wavelength scan of transmitted wavelengths as the acoustic pulse propagates along said Bragg-reflector.  
     
     
         22 . The method of  claim 20 , wherein the acoustic pulse has a first part and a second part, said first part causing a temporary increase of the local grating period of the Bragg-reflector, and said second part causing a temporary decrease of the local grating period of said Bragg-reflector.  
     
     
         23 . The method of  claim 22 , wherein the portion of said chirped Bragg-reflector containing the first part of said acoustic pulse has, in its unperturbed state, a longer grating period than the portion of said Bragg-reflector containing the second part of said acoustic pulse.  
     
     
         24 . The method of  claim 20 , wherein the step of monitoring the light transmitted through the light-guiding structure includes detecting said transmitted light by means of a detector, the detector providing a detector output signal associated with the power of the transmitted light signal.  
     
     
         25 . The method of  claim 20 , wherein said light-guiding structure is the core of an optical fiber.  
     
     
         26 . The method of  claim 24 , wherein the step of sending a longitudinal acoustic pulse along the light-guiding structure is performed repeatedly, whereby a plurality of acoustic pulses is sent along said light-guiding structure, of which plurality of acoustic pulses only one acoustic pulse is present in the chirped Bragg-reflector at any one instant.  
     
     
         27 . The method of  claim 26 , wherein each acoustic pulse is reflected back and forth between two reflection points in the light-guiding structure, each successive pulse of said plurality of acoustic pulses being sent into the light-guiding structure at the time when the previous acoustic pulse, after its reflection, reaches the acoustic actuator.  
     
     
         28 . The method of  claim 27 , wherein the wavelength scan obtained by a passage of an acoustic pulse through the chirped Bragg-reflector is utilized together with a subsequent wavelength scan for spectrum analysis of the light signal directed into the light-guiding structure.  
     
     
         29 . The method of  claim 28 , wherein correlation between different wavelength scans in the detector output signal is performed in order to interpret said detector output signals, to ascribe the detector output at a certain instant to a specific wavelength.  
     
     
         30 . The method of  claim 20 , further including the step of pre-filtering the light to be directed into the light-guiding structure by means of a broadband filter, said filter passing essentially the same wavelength range as the chirped Bragg-reflector reflects.  
     
     
         31 . The method of  claim 24 , wherein the detector output signal is fed to a logic unit, said logic unit processing said detector output signal to provide a logic unit output signal indicative of the power spectrum of the light signal directed into the light-guiding structure.