Abstract:
Techniques for characterizing the response of an optical device comprising modulating at least one signal using the device; coupling the modulated signal with a reference signal in a variety of ways; detecting the coupled signals; and obtaining the response of the modulator by analyzing the detected signals, are described. In a heterodyne embodiment, the method includes modulating a first optical signal using the optical device to produce a modulated first optical signal, the modulated first optical signal is combined with a second optical signal in a different spectral region; and the response of the optical device is determined from the intensity of the combined optical signal. A homodyne method using various splitting and recombining of the modulated optical signal with a reference signal is also described.

Description:
FIELD OF THE INVENTION 
     The invention is related to the field of optical telecommunications, and in particular, to method and apparatus for analyzing optical components. 
     BACKGROUND OF THE INVENTION 
     The measurement of the properties of optical devices is of high interest for the telecommunications community. Temporal modulators are routinely used to carve trains of optical pulses and encode data via amplitude or phase modulation. Devices such as semiconductor optical amplifiers are also used for amplification and processing of optical waves. The accurate description of the properties of a device, such as the amplitude and phase response versus the electrical or optical drive signal or the dependence between transmission and phase, is important to the understanding of the properties of the generated optical signals. 
     SUMMARY 
     Various deficiencies of the prior art are addressed by the present invention of methods and apparatus for characterization of the response of optical devices. In one embodiment, the invention includes modulating a first optical signal using an optical device to produce a modulated first optical signal. The modulated first optical signal is combined with a second optical signal. The first and second optical signals are associated with respective spectral regions. The intensity of the combined signals is measured and processed to yield the properties of the modulation. 
     Another embodiment of the invention provides for modulating a first optical signal using the optical device to produce a modulated first optical signal. The modulated first signal and a second signal are inputted into an optical hybrid structure. Two pairs of optical signals are outputted from the optical hybrid structure. Each pair of optical signals is sent to a balanced photodetector unit, and the response of the optical device is determined by combining the electrical signals generated by the two balanced photodetector units. 
     The invention further provides other methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below. 
     The foregoing, together with other aspects of this invention, will become more apparent when referring to the following specification, claims, and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a block diagram of a heterodyne measuring arrangement according to an embodiment of the invention; 
         FIG. 2  depicts a flow diagram of a method according to an embodiment of the invention; 
         FIG. 3  depicts a block diagram of a measuring unit; 
         FIG. 4  depicts a block diagram of a homodyne measuring arrangement according to an embodiment of the invention; and 
         FIG. 5  depicts a flow diagram of a method according to an embodiment of the invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be primarily described within the context of measuring arrangements for temporal modulators in the art of optical telecommunications; however, those skilled in the art and informed by the teachings herein will realize that the invention is also applicable to any apparatus and method that measures communications components of a communications network. 
       FIG. 1  depicts a block diagram of a heterodyne measuring arrangement according to an embodiment of the invention. The heterodyne arrangement  100  of  FIG. 1  includes a first optical signal source  110 , a second optical signal source  120 , a temporal modulator under test  130 , a coupler  140 , and a photodetector and electronics  150 . 
     The first optical signal source  110  is a monochromatic source at optical frequency ω 1  with electric field E 1 ·exp(iω 1 t+φ 1 (t)). The second signal source is another monochromatic source at optical frequency ω 2  with electric field E 2 ·exp(iω 2 t+φ 2 (t)). The signal sources are monochromatic lasers. In one embodiment, the monochromatic lasers at ω 1  and ω 2  can be provided by two tunable lasers. An example of such a laser is the Agilent 81689A tunable laser module. The source  110  with optical frequency ω 1  is sent into the modulator  130 . 
     The modulator  130  is driven by a stimulus (for example a RF sinewave), so that its complex transmission is R(t). The modulated output is represented by E 1 ·R(t)·exp(iω 1 t+ω 1 (t)). In one embodiment, the modulator is an electroabsorption modulator (EAM) such as an OKI C-band electroabsorption modulator (OM5642W-30B). The EAM can be biased and driven by a 10 MHz sinewave that spans voltages from −4 V to 0V. Other examples of modulators and devices that could be characterized using the teachings of the present invention include a Mach-Zehnder modulator or a phase modulator driven by a time-varying electrical signal, a semiconductor optical amplifier driven by a time-varying electrical signal or modulated by a time-varying optical signal propagating jointly with the first signal source in the semiconductor optical amplifier. These examples should not be considered as limiting the application of the present invention in any way, and it should be apparent that the concepts presented here can be applied to any situation where the properties of a time-varying device need to be measured. 
     The coupler  140  combines the modulated signal with the monochromatic reference source at optical frequency ω 2 . The coupler is any device that outputs a linear combination of the two input signals. This includes fiber-based and waveguide-based couplers. As the first and second signals have different optical frequencies ω 1  and ω 2 , wavelength-dependent couplers such as those used in wavelength multiplexed systems could also be used. 
     The output signal from the coupler  140  is transmitted to photodetector and electronics  150 . In one embodiment, photodetection is implemented using a 15 GHz photodetector followed by an AC-coupled 1 GHz amplifier. Any assembly of photodetector and electronics can be used provided that its bandwidth is sufficient to measure the intensity of the output signal from the coupler  140 . The photodetector and electronics  150  temporally resolve the intensity S(t) of the output signal, leading to the signal
 
 S ( t )=| E   1   ·R ( t )·exp( iω   1   t+φ   1 ( t ))+ E   2 ·exp( iω   2   t+φ   2 ( t ))| 2   (1)
 
     This signal can be developed as
 
 S ( t )=| E   2 | 2   +|E   1   ·R ( t )| 2 +2· Re[E   1   ·E*   2   ·R ( t )·exp( iΩ   1,2   t+φ   1 ( t )−φ 2 ( t ))]  (2)
 
with Ω 1,2 =ω 1 −ω 2 . In equation (2), the first term is a constant, the second term is proportional to the time-varying transmission of the modulator, and the third term has quickly varying fringes with period 2π/Ω 1,2 . The difference frequency Ω 1,2 =ω 1 −ω 2  is chosen so that the spectrum of the interferometric component does not overlap with the spectrum of the transmission of the modulator. This non-overlapping property allows the extraction of the interferometric component via Fourier processing by Fourier transforming the measured intensity, applying a filter on one of the sidebands located around the frequency Ω 1,2  and inverse Fourier transforming to the time domain.
 
       FIG. 2  depicts a flow diagram  200  of a method according to an embodiment of the invention. The signal S(t) represents the intensity of the combined sources at the output of the coupler  140 , which is measured at step  210 . 
     At step  220 , a Fourier transform is performed on the intensity measured at step  210 , and the representation of S(t) is obtained in the frequency domain. In the frequency domain, the transformed S(t) includes the Fourier transform of the various terms of Eq. (2). While the Fourier transform of the first two terms is located around the zero frequency, the Fourier transform of the last terms is composed of two sidebands around the frequency Ω 1,2  and −Ω 1,2 . 
     At step  230 , Filtering is performed on the transformed signal of step  220  to extract the sideband at frequency Ω 1,2 . This process is done through software or hardware. 
     At step  240 , the filtered sideband signal in the frequency domain is Fourier transformed back into the time domain, i.e. one performs an inverse Fourier transform on the filtered sideband. This newly transformed signal yields the response of the modulator R(t). The filtered processed intensity signal leads to the complex quantity E 1 ·E* 2 ·R(t)·exp(iΩ 1,2 t+φ 2 (t)−φ 2 (t)). The linear temporal phase Ω 1,2 t is usually of no interest in identifying the response of the modulator. If the relative phase of the two sources φ 1 −φ 2  is stable during the measurement time, the signal S(t) leads to the direct measurement of the complex function R(t) after removal of a linear fit to its phase. In case of instability in this phase, the complex function R(t) can still be retrieved after averaging of various determinations of the quantity E 1 ·E* 2 ·R(t)·exp(iΩ 1,2 t+φ 2 (t)). 
     When performing these measurements, the photodetector and electronics need to have sufficient bandwidth to resolve the temporal fringes, and must therefore have bandwidth higher than Ω 1,2 . However, the photodetector and electronics  150  can be AC-coupled as the signal of interest is encoded on a carrier frequency. The relative phase between the two interfering sources  110 ,  120  must be stable during the measurement time, which means that their coherence time must be longer than the measurement time if the sources at ω 1  and ω 2  are not correlated. The use of Fourier processing on the measured signal S allows for increased versatility. 
       FIG. 3  depicts a block diagram of the measuring unit  300 . The processing unit  300  includes an interface  310  for measuring signals ( 320 - 1 ,  320 - 2 ,  320 - n ) inputted into the measuring unit  300 , a data processing unit  330  which includes a central processing unit (CPU)  340  and memory  350 , and a display  360 . The data processing unit  330  performs calculations on the measured signals, such as performing Fourier transform on the measured input. The calculation or other information from the data processing unit  330  can be graphically or symbolically shown using the display  360 . For example, measurement of the signal of Eq. 1 and displaying of the response of the modulator may be achieved with an Infinium continuous sampling scope with a 1.5 GHz bandwidth, while the processing is performed with a personal computer. Other appropriate measurement units include analog-to-digital converter boards. 
       FIG. 4  depicts a block diagram of a homodyne measuring arrangement according to an embodiment of the invention. The homodyne arrangement  400  includes a signal source  410 , a splitter S  420 , a temporal modulator under test  430 , a 90° optical hybrid structure  440 , and two balanced photodetector units  450 ,  460 . 
     The optical signal source  410  is a monochromatic source at an optical frequency ω 3 . The signal produced by the source has an electric field represented by E 3 ·exp(iω 3 t+φ 3 (t)). In one embodiment, the optical source  410  is implemented using an Agilent 81689A laser. 
     The source is sent to a splitter module  420  which includes a splitter S. Such splitting can be performed for example using fiber-based or waveguide-based couplers. Part of the light is sent to the modulator  430  to generate a modulated signal. The remaining part is sent directly to one of the input ports of the 90° optical hybrid structure  440  to act as a reference signal. 
     The temporal modulator  430  has a complex transmission R(t). In one embodiment, the modulator can be an electroabsorption modulator (EAM) such as an OKI C-band electroabsorption modulator (OM5642W-30B). The EAM can be biased and driven by a 10 MHz sinewave that spans voltages from −4 V to 0V. The modulated signal is sent to another input port of the optical hybrid structure  440 . Other examples of modulators and devices that could be characterized using the teachings of the present invention include a Mach-Zehnder modulator or a phase modulator driven by a time-varying electrical signal, a semiconductor optical amplifier driven by a time-varying electrical signal or modulated by a time-varying optical signal propagating jointly with the first signal source in the semiconductor optical amplifier. These examples should not be considered limiting the application of the present invention in any way, and it should be apparent that the concepts presented here can be applied to any situation where the properties of a time-varying device need to be measured. 
     In one embodiment, the 90° optical hybrid structure  440  is a structure made of silica waveguides on a silicon substrate. Other implementations of such structure leading to substantially identical function include for example a polarization-based hybrid and a fiber-based hybrid. The hybrid structure  440  is configured to split the modulated signal at splitter S MOD  into first and second modulated signals and configured to split the reference signal at splitter S REF  into a first and second reference signals. The second reference signal is phase shifted by a π/2 phase shifter. Those skilled in the art will recognize that such phase shifter could be located in the optical path of any of the two modulated signals or in the optical path of any of the two reference signals. Additionally, a variable phase shifter might be used for more versatility. The hybrid structure  440  then recombines the first modulated beam and first reference beam at combiner C A  and recombines the second modulated beam and second reference beam at combiner C B . Each combiner C A  and C B  has two outputs that are sent to the two photodetectors of a balanced photodetector unit, respectively balanced photodetector unit  450  and  460 , in order to perform dual-quadrature detection. 
     Two balanced photodetectors ( 450 ,  460 ) are connected to the two outputs of the two combiners (C A  and C B ). An example of the balanced photodetector is the New Focus 800 MHz balanced detectors, but substantially identical operation would be obtained with any other balanced photodetector units having sufficient bandwidth. Each balanced photodetector unit includes two detectors (i.e.  452  and  454  for balanced photodetector unit  450  and  462  and  464  for balanced photodetector unit  460 ) and a subtracter (i.e.  456  and  466  respectively for the balanced photodetector unit  450  and  460 ). In an embodiment, the balanced photodetector  450  using detector  452  detects the sum of the intensity of the modulated signal, the intensity of the reference signal and the real part of the interference between the first modulated signal and first reference signal. The other detector  454  detects the sum of the intensity of the modulated signal, the intensity of the reference signal and the opposite of the real part of the interference between the modulated signal and reference signal. The difference of the signals from these two detectors is determined at the subtracter  456  where the real part of the interference between the first modulated signal and first reference signal S A (t) is outputted. The other balanced photodetector unit  460  functions in the same manner as the balanced photodetector unit  450 . However, because of the phase shift of π/2, the subtracter  466  outputs the imaginary part of the interference between the modulated signal and reference signal S B (t). 
     When the phase of one of the signals of the hybrid is properly adjusted by a phase shifter  445 , the signals measured by the two balanced photodetector units are respectively proportional to the real and imaginary part of the interference between the modulated signal and reference signal given by 
                       S   A     ⁡     (   t   )       =     Re   ⁢     ⌊              E   3          2     ·     R   ⁡     (   t   )       ·     exp   ⁡     (       ⅈ   ⁡     (         φ   3     ⁡     (   t   )       -       φ   3     ⁡     (     t   -   τ     )         )       +   φ     )         ⌋               (   3   )                   S   B     ⁡     (   t   )       =     Im   ⁢     ⌊              E   3          2     ·     R   ⁡     (   t   )       ·     exp   ⁡     (       ⅈ   ⁡     (         φ   3     ⁡     (   t   )       -       φ   3     ⁡     (     t   -   τ     )         )       +   φ     )         ⌋               (   4   )               
where τ is a delay due to the difference of the optical modulation path and the reference path and φ is a phase due to the difference of the modulation signal and reference signal. As mentioned above, the second reference signal is phase shifted. However, any one of the signal paths can be shifted by π/2 and get the desired outputs.
 
       FIG. 5  depicts a flow diagram  500  of a method according to an embodiment of the invention. The measured signals S A (t) and S B (t) are the real and imaginary components of the interference, respectively. The measuring unit  300  of  FIG. 3  also can be used to measure the response of the modulator  430  in the homodyne arrangement. It measures both components S A (t) and S B (t) of the interference  510 . Then, the processing unit  330  of the measuring unit  300  will combine signals S A  and S B . The signals of Eq. 3 and 4 can be combined to give |E 3 | 2 ·R(t)·exp(i(φ 3 (t)−(t−τ))+φ). If the delay τ is significantly shorter than the coherence time of the monochromatic laser, φ 3 (t)−φ 3 (t−τ) is equal to zero. If the phase φ is stable during the measurement time, such technique therefore directly leads to the complex function R(t) describing the response of the modulator to the drive signal up to a non-significant constant phase. Averaging of multiple acquisitions of the combined signals S A  and S B  can be performed to get accurate determination of the function R(t) even when the phase φ is not stable during the measurement time or the delay τ is not significantly shorter than the coherence time of the monochromatic laser. In an embodiment, the signals S A  and S B  can be measured with the Infinium oscilloscope similar to that used in the heterodyne measurement. Other appropriate measurement units include analog-to-digital converter boards. 
     While the foregoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.