Intensity modulation of optical signals

Abstract

A wavelength converter or demultiplexer device comprising first and second stages ( 30, 40 ). For wavelength converter action, the first stage impresses an intensity modulation (IM) and phase modulation (PM) on a CW input signal (λ 1 ) carried by an input DATA signal (λ 2 ) using cross phase modulation in a non-linear optical element ( 46 ). The first stage in an embodiment is a semiconductor laser amplifier loop optical mirror (SLALOM) which has a semiconductor optical amplifier (SOA) as the non-linear optical element. The second stage has a non-linear transfer function and impresses a further IM on the optical signal responsive to the PM impressed in the first stage. In an embodiment, the second stage is a polarization maintaining fiber (PMF) optical loop. The transfer function of the overall device is thus improved by making it steeper and more time confined. This is achieved by combining a fast first stage with a second stage that has a non-linear transfer function, so that the residual PM from the first stage synchronously drives a nonlinear process in the second stage.

1 . A device for intensity modulating an optical data signal, the device comprising: 
 a first stage arranged to impress an intensity modulation that is non-inverting as well as a phase modulation on a first optical signal of a first frequency responsive to an intensity modulation carried by a second optical signal of a second frequency using cross phase modulation in a non-linear optical element; and    a second stage arranged to impress a further intensity modulation that is non-inverting on the first optical signal, after the cross phase modulation, responsive to the phase modulation impressed on the first optical signal in the first stage.    
     
     
         2 . A device according to  claim 1 , wherein the second stage is arranged separate from the first stage, to act on the first optical signal subsequent to output of the first optical signal from the first stage.  
     
     
         3 . A device according to  claim 1 , wherein the second stage is arranged within the first stage in series with the non-linear optical element.  
     
     
         4 . A device according to any one of the preceding claims, wherein the second stage has a transfer function of transmission versus frequency that has a low transmission value (T low ) at the first frequency (ω 1 /2π) and a high transmission value (T high ) at a frequency equal to the first frequency plus the maximum instantaneous frequency deviation (ω 1 /2π+Δf max ).  
     
     
         5 . A device according to claims  2  and  4 , wherein the low transmission value is substantially zero.  
     
     
         6 . A device according to  claim 5 , wherein the non-linear transfer function has a frequency width Δν defined between a minimum transmission value at the first frequency (ω 1 /2π) and a maximum transmission value (T max ) at frequencies greater than the first frequency plus the frequency width (ω 1 /2π+Δν), wherein the first and second stages are matched so that the high transmission value is equal to at least half the maximum transmission value (T high ≧0.5T max ).  
     
     
         7 . A device according to  claim 5 , wherein the non-linear transfer function has a frequency width Δν defined between a minimum transmission value at the first frequency (ω 1 /2π) and a maximum transmission value (T max ) at frequencies greater than the first frequency plus the frequency width (ω 1 /2π+Δν), wherein the first and second stages are matched so that Δν≦Δf max , whereby the high transmission value attains the maximum transmission value (T high =T max ).  
     
     
         8 . A device according to any one of  claims 4  to  7 , wherein the transfer function has a non-linear section between the first frequency (ω 1 /2π) and the maximum instantaneous frequency deviation (ω 1 /2π+Δf max ).  
     
     
         9 . A device according to any one of  claims 1  to  8 , wherein the non-linear optical element is a semiconductor optical amplifier (SOA).  
     
     
         10 . A device according to any one of  claims 1  to  8 , wherein the non-linear optical element comprises a Kerr-effect inducing medium.  
     
     
         11 . A device according to any one of  claims 1  to  8 , wherein the non-linear optical element is a section of non-linear optical fiber.  
     
     
         12 . A device according to any one of  claims 1  to  11 , wherein the first stage has a Sagnac interferometer configuration comprising a loop in which the non-linear optical element is asymmetrically arranged.  
     
     
         13 . A device according to any one of  claims 1  to  11 , wherein the first stage has a Mach-Zehnder interferometer configuration comprising first and second arms having differing path lengths, the non-linear optical element being arranged in the second arm and a further non-linear optical element being arranged in the first arm.  
     
     
         14 . A device according to any one of the preceding claims, wherein the first stage has a frequency response of 10 Gbit/s or higher.  
     
     
         15 . A device according to any one of  claims 1  to  14 , wherein the non-linear element has a Sagnac interferometer configuration comprising a loop of polarization maintaining optical fiber.  
     
     
         16 . A device according to any one of  claims 1  to  14 , wherein the non-linear element comprises a fiber Bragg grating.  
     
     
         17 . A device according to any one of  claims 1  to  14 , wherein the non-linear element comprises a ring resonator device.  
     
     
         18 . A method of intensity modulating an optical data signal, the method comprising: 
 (a) impressing an intensity modulation that is non-inverting, as well as a phase modulation, on a first optical signal responsive to an intensity modulation carried by a second optical signal, using cross phase modulation; and    (b) impressing a further intensity modulation that is non-inverting on the first optical signal, after the cross phase modulation, responsive to said phase modulation.    
     
     
         19 . A method of wavelength converting a return-zero (RZ) optical signal, the method comprising: 
 (a) receiving a first optical signal at a first frequency (ω 1 /2π);    (b) receiving a second optical signal at a second frequency carrying RZ intensity modulated data;    (c) impressing an intensity modulation that is non-inverting, as well as a phase modulation, on the first optical signal by cross phase modulation with the second optical signal, wherein the intensity modulation carries the data, and the phase modulation is associated with a maximum instantaneous frequency deviation (Δf max ); and    (d) impressing a further intensity modulation that is non-inverting on the first optical signal, after the cross phase modulation, responsive to said phase modulation.    
     
     
         20 . A method of demultiplexing a time division multiplexed return-zero (RZ) optical signal, the method comprising: 
 (a) receiving a first optical signal at a first frequency (ω 1 /2π) carrying RZ intensity modulated data organized in time slots for respective time division multiplexed channels;    (b) receiving a second optical signal at a second frequency carrying a sequence of intensity modulated clock pulses;    (c) impressing an intensity modulation that is non-inverting, as well as a phase modulation, on the first optical signal by cross phase modulation with the second optical signal, wherein the intensity modulation serves to select at least one of the time slots, and the phase modulation is associated with a maximum instantaneous frequency deviation (Δf max ); and    (d) impressing a further intensity modulation that is non-inverting on the first optical signal, after the cross phase modulation, responsive to said phase modulation.    
     
     
         21 . A method according to any one of  claims 18  to  20 , wherein the further intensity modulation is impressed using a transfer function of transmission versus frequency that has a low transmission value (T low ) at the first frequency (ω 1 /2π) and a high transmission value (T high ) at a frequency equal to the second frequency plus the maximum instantaneous frequency deviation (ω 1 /2π+Δf max ).  
     
     
         22 . A method according to  claim 21 , wherein the transfer function has a non-linear section between the first frequency (ω 1 /2π) and the maximum instantaneous frequency deviation (ω 1 /2π+Δf max ).  
     
     
         23 . A method according to any one of  claims 18  to  22 , wherein the intensity and phase modulation is impressed on the first optical signal using the Kerr effect.  
     
     
         24 . A method according to any one of  claims 18  to  23 , wherein the intensity and phase modulation is impressed on the first optical signal using a component having a frequency response of 10 Gbit/s or higher