Optical delay lines for electrical skew compensation

Abstract

A skew compensation apparatus and method. In an optical system that uses optical signals, skew may be generated as the optical signals are processed from an input optical signal to at least two electrical signals representative of the phase-differentiated optical signals. A compensation of the skew is provided by including an optical delay line in the path of the optical signal that does not suffer the skew (e.g., that serves as the time base for the skew measurement). The optical delay line introduces a delay Tskew equal to the delay suffered by the optical signal that is not taken as the time base. The two signals are thereby corrected for skew.

What is claimed is: 
     
         1 . An optical receiver system, comprising:
 a receiver configured to receive an input optical signal comprising a first optical component signal and a second optical component signal from a transmitter via an optical medium, and to convert the first optical component signal and the second optical component signal into a first electrical component signal and a second electrical component signal, respectively; the second electrical component signal is subject to a first timing delay relative to the first electrical component signal caused by at least one of the transmitter, the optical medium, and the receiver;   a first skew compensation element comprising a first delay line comprising a first single fixed length of waveguide configured to apply a first fixed predetermined compensation timing delay to the second optical component signal, to achieve a first net skew with the first timing delay in the second electrical component signal.   
     
     
         2 . The system according to  claim 1 , wherein the input optical signal also comprises a third optical component signal and a fourth optical component signal;
 wherein the receiver is also configured to convert the third optical component signal and the fourth optical component signals into a third electrical component signal and a fourth electrical component signal;   wherein the third electrical component signal and the fourth electrical component signal are subject to a second timing delay and a third timing delay, respectively, relative to the first electrical component signal caused by at least one of: the transmitter, the optical medium, and the receiver; and   further comprising:
 a second skew compensation element comprising a second delay line comprising a second fixed length of waveguide configured to apply a second fixed predetermined compensation timing delay to the third optical component signal, to achieve a second net skew with the second timing delay in the third electrical component signal; and 
 a third skew compensation element comprising a third delay line comprising a third fixed length of waveguide configured to apply a third fixed predetermined compensation timing delay to the fourth optical component signal, to achieve a third net skew with the third timing delay in the fourth electrical component signal. 
   
     
     
         3 . The system according to  claim 2 , wherein the receiver comprises:
 an input port for inputting the input optical signal;   a polarization beam splitter (PBS) for splitting the input optical signal into a first polarized component and a second polarized component;   a local oscillator and a beam splitter for generating a first oscillator component and a second oscillator component;   a first hybrid mixer for generating the first optical component signal and the second optical component signals, which are phase differentiated, from the first polarized component and the first oscillator component; and   a second hybrid mixer for generating the third optical component signal and the fourth optical component signal, which are phase differentiated, from the second polarized component and the second oscillator component.   
     
     
         4 . The system according to  claim 3 , wherein the first delay line is disposed between the PBS and the first hybrid mixer or the second hybrid mixer;
 and wherein the second delay line is disposed between the local oscillator and the first hybrid mixer or the second hybrid mixer.   
     
     
         5 . The system according to  claim 3 , further comprising respective photodiodes and electrical amplifiers for converting each of the first optical component signal, the second optical component signal, the third optical component signal and the fourth optical component signal into the first electrical component signal, the second electrical component signal, the third electrical component signal, and the fourth electrical component signal, respectively, and thereby contributing to generation of the first timing delay, the second timing delay and the third timing delay. 
     
     
         6 . The system according to  claim 5 , wherein the first delay line is disposed between the first hybrid mixer and a first of the respective photodiodes; and wherein the second delay line is disposed between the second hybrid mixer and a second of the respective photodiodes. 
     
     
         7 . The system according to  claim 2 , wherein the first delay line, the second delay line and the third delay line each consists of a single mode waveguide on a substrate. 
     
     
         8 . The system according to  claim 7 , wherein each of the first delay line, the second delay line and the third delay line consists of a silicon waveguide. 
     
     
         9 . The system according to  claim 8 , wherein at least one of the first delay line, the second delay line, and the third delay line is about 75 μm long providing about 1 ps of delay. 
     
     
         10 . The system according to  claim 8 , wherein at least one of the first delay line, the second delay line and the third delay line is about 225 μm long providing about 3 ps of delay. 
     
     
         11 . A method of compensating skew in an optical receiver, comprising the steps of:
 receiving an input optical signal comprising a first optical component signal and a second optical component signal from a transmitter via an optical medium in the optical receiver;   converting the first optical component signal and the second optical component signal into a first electrical component signal and a second electrical component signal, respectively, wherein the second electrical component signal is subject to a first timing delay relative to the first electrical component signal caused by at least one of the transmitter, the optical medium, and the receiver;   passing the second optical component signal through a first skew compensation element comprising a first delay line, comprising a first fixed length of waveguide configured to apply a first fixed predetermined compensation timing delay to the second optical component signal, to achieve a first net skew with the first timing delay.   
     
     
         12 . The method according to  claim 11 , further comprising:
 generating a third optical component signal and a fourth optical component signal for the input optical signal;   receiving the third optical component signal and the fourth optical component signal in the optical receiver;   converting the third optical component signal and the fourth optical component signal into a third electrical component signal and a fourth electrical component signal, wherein the third electrical component signal and the fourth electrical component signal are subject to a second timing delay and a third timing delay, respectively, relative to the first electrical component signal caused by at least one of the transmitter, the optical medium and the receiver;   passing the third optical component signal through a second skew compensation element comprising a second delay line, comprising a second fixed length of waveguide configured to apply a second fixed predetermined compensation timing delay to the third optical component signal, to achieve a second net skew with the second timing delay in the third electrical component signal; and   passing the fourth optical component signal through a third skew compensation element comprising a third delay line, comprising a third fixed length of waveguide configured to apply a third fixed predetermined compensation timing delay to the fourth optical component signal, to achieve a third net skew with the third timing delay in the fourth electrical component signal.   
     
     
         13 . The method according to  claim 12 , further comprising:
 splitting the input optical signal into a first polarized component and a second polarized component in a polarization beam splitter (PBS);   generating a first oscillator component and a second oscillator components with a local oscillator and a beam splitter;   generating the first optical component signal and the second optical component signal, which are phase differentiated, from the first polarized component and the first oscillator component in a first hybrid mixer; and   generating the third optical component signal and the fourth optical component signal, which are phase differentiated, from the second polarized component and the second oscillator component in a second hybrid mixer.   
     
     
         14 . The method according to  claim 13 , wherein the first delay line is disposed between the PBS and the first hybrid mixer or the second hybrid mixer; and wherein the second delay line is disposed between the local oscillator and the first hybrid mixer or the second hybrid mixer. 
     
     
         15 . The method according to  claim 14 , wherein the receiver further comprises respective photodiodes and electrical amplifiers for converting each of the first optical component signal, the second optical component signal, the third optical component signal and the fourth optical component signal into the first electrical component signal, the second electrical component signal, the third electrical component signal, and the fourth electrical component signal, respectively, and thereby contributing to generation of the first timing delay, the second timing delay and the third timing delay. 
     
     
         16 . The method according to  claim 15 , wherein the first delay line is disposed between the first hybrid mixer and a first of the respective photodiodes;
 and wherein the second delay line is disposed between the second hybrid mixer and a second of the respective photodiodes.   
     
     
         17 . The method according to  claim 12 , wherein the first skew compensation element, the second skew compensation element, and the third skew compensation element are disposed in the transmitter. 
     
     
         18 . The method according to  claim 12 , further comprising:
 i) determining the first timing delay in the second electrical component signal, the second timing delay in the third electrical component signal, and the third timing delay in the fourth electrical component signal based on experience or measurement;   ii) fabricating the first delay line, the second delay line and the third delay line based on step i).   
     
     
         19 . A transmitter system, comprising:
 a transmitter configured for generating an input optical signal comprising a first optical component signal and a second optical component signal, and transmitting the input optical signal via an optical medium to a receiver, which is configured for converting the first optical component signal and the second optical component signal into a first electrical component signal and a second electrical component signal, respectively;   the second electrical component signal is subject to a first timing delay relative to the first electrical component signal caused by at least one of the transmitter, the optical medium and the receiver;   a first skew compensation element comprising a first delay line, comprising a first fixed length of waveguide configured to apply a first fixed predetermined compensation timing delay to the second optical component signal, to pre-compensate for the first timing delay in the second electrical component signal.   
     
     
         20 . The system according to  claim 19 , wherein the input optical signal also comprises a third optical component signal and a fourth optical component signal;
 wherein the receiver is also configured to convert the third optical component signal and the fourth optical component signals into a third electrical component signal and a fourth electrical component signal;   wherein the third electrical component signal and the fourth electrical component signal are subject to a second timing delay and a third timing delay, respectively, relative to the first electrical component signal caused by at least one of: the transmitter, the optical medium, and the receiver; and   further comprising:
 a second skew compensation element comprising a second delay line comprising a second fixed length of waveguide configured to apply a second fixed predetermined compensation timing delay to the third optical component signal, to pre-compensate for the second timing delay in the third electrical component signal; and 
 a third skew compensation element comprising a third delay line comprising a third single fixed length of waveguide configured to apply a third fixed predetermined compensation timing delay to the fourth optical component signal, to pre-compensate for the third timing delay in the fourth electrical component signal.