Method and system for characterizing an optical device

An embodiment of the invention relates to a system comprising an optical device (10) and an evaluation device (20) for characterizing the optical device. The optical device comprising a 90° optical hybrid unit (30) having a first and second optical input (30E1, 30E2) and at least two optical outputs (30A1-30A4) wherein optical output signals (So1-So4) leaving the optical outputs have optical phase differences between each other of 90° or multiple thereof; a first photodetector (P1) connected to a first optical output (30A1) and a second photodetector (P2) connected to a second optical output (30A2), wherein the first optical output emits a first optical output signal (So1) and the second optical output emits a second optical output signal (So2), said second optical output signal having an optical phase difference of 180° relative to the first optical output signal; and a first transimpedance amplifier (Tr1) connected to the first and second photodetectors (P1, P2). The evaluation device (20) is characterized by a signal source (110) configured to generate a first and second intensity-modulated optical input signal (Sin1, Sin2); an optical phase shifter (120) adapted to phase-shift the optical phase of the second intensity-modulated optical input signal relative to the optical phase of the first intensity-modulated optical input signal; a measurement unit (160) adapted to measure the electrical output signal of the first transimpedance amplifier (Tr1); and a control unit (130) adapted to control the optical phase shifter.

The invention relates to a method and a system for characterizing an optical device.

BACKGROUND OF THE INVENTION

OBJECTIVE OF THE PRESENT INVENTION

An objective of the present invention is to provide a method which allows characterizing an optical device having a 90° optical hybrid unit, at least two photodetectors, and a transimpedance amplifier.

A further objective of the present invention is to provide a method which allows measuring the amplitude and phase response of an optical device having a 90° optical hybrid unit, at least two photodetectors, and a transimpedance amplifier, for specific intensity-modulation frequencies.

A further objective of the present invention is to provide a system for characterizing an optical device having a 90° optical hybrid unit, at least two photodetectors, and a transimpedance amplifier for a specific intensity-modulation frequency.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the present invention relates to a method for characterizing an optical device, which comprises: a 90° optical hybrid unit having a first and a second optical input and at least two optical outputs wherein signals leaving the optical outputs have optical phase differences between each other of 90° or multiple thereof; a first photodetector connected to a first optical output and a second photodetector connected to a second optical output, wherein the first optical output emits a first optical output signal and the second output emits a second optical output signal, said second optical output signal having an optical phase difference of 180° relative to the first optical output signal; and a first transimpedance amplifier connected to the first and second optical outputs of the 90° optical hybrid unit. A first intensity-modulated optical input signal and a second intensity-modulated optical input signal are generated. The first intensity-modulated optical input signal is inputted into the first optical input and the second intensity-modulated optical input signal is inputted into the second optical input. The optical phase of the second intensity-modulated optical input signal is phase-shifted relative to the first intensity-modulated optical input signal, and the electrical output signal of the first transimpedance amplifier is used to characterize the optical device.

In a preferred embodiment, the optical device comprises a third photodetector connected to a third optical output of the 90° optical hybrid unit, and a fourth photodetector connected to a fourth optical output of the 90° optical hybrid unit, the third optical output emitting a third optical output signal and the fourth optical output emitting a fourth optical output signal, said fourth optical output signal having an optical phase difference of 180° relative to the third optical output signal. The electrical output signals of the third and fourth photodetector may be low-pass filtered and the low-pass filtered signals may be inputted into an operational amplifier. The output signal of the operational amplifier may be used to control a phase-shifter which controls the optical phase-difference between the first and second intensity-modulated optical input signals in order to provide that the low-pass filtered signals have the same amplitude.

Preferably an optical reference signal is intensity-modulated at an intensity-modulation frequency. The intensity-modulated optical reference signal may be inputted into a 3 dB-splitter which generates the first and second intensity-modulated optical input signals for the 90° optical hybrid unit.

According to a preferred embodiment, the intensity-modulation frequency is higher than the bandwidth of the low-pass filtered signals.

In order to characterize the amplitude response of the optical device, a ratio between the amplitude of the electrical output signal of the first transimpedance amplifier and the amplitude of the intensity-modulation of the optical reference signal, or a signal proportional thereto, may be determined for the intensity-modulation frequency, said ratio characterizing the amplitude response of the optical device for the intensity-modulation frequency with respect to the first optical output.

In order to characterize the phase response of the optical device, a phase difference value, which indicates the phase difference between the phase of the electrical output signal of the first transimpedance amplifier and the phase of the intensity-modulation of the optical reference signal, or a signal proportional thereto, may be determined for the intensity-modulation frequency, said phase difference value characterizing the phase shift of the optical device for the intensity-modulation frequency with respect to the first optical output.

After determining said ratio and/or said phase difference value, the optical phase difference between the first and second intensity-modulated optical input signals may be changed by 180° and may be then further controlled such that the low-pass filtered signals have the same amplitude. A second ratio between the amplitude of the electrical output signal of the first transimpedance amplifier and the amplitude of the intensity-modulation of the optical reference signal, or a signal proportional thereto, may be determined for the intensity-modulation frequency, said second ratio characterizing the amplitude response of the optical device for the intensity-modulation frequency with respect to the second optical output.

Additionally or alternatively, a second phase difference value between the phase of the electrical output signal of the first transimpedance amplifier and the phase of the intensity-modulation of the optical reference signal, or a signal proportional thereto, may be determined for the intensity-modulation frequency, said second phase difference value characterizing the phase shift of the optical device for the intensity-modulation frequency with respect to the second optical output.

A further embodiment of the invention relates to a system comprising an optical device and an evaluation device for characterizing the optical device, said optical device comprising: a 90° optical hybrid unit having a first and second optical input and at least two optical outputs wherein optical signals leaving the optical outputs have optical phase differences between each other of 90° or multiple thereof; a first photodetector connected to a first optical output and a second photodetector connected to a second optical output, wherein the first optical output emits a first optical output signal and the second optical output emits a second optical output signal, said second optical output signal having an optical phase difference of 180° relative to the first optical output signal; and a first transimpedance amplifier connected to the first and second photodetectors. The evaluation device is characterized by a signal source configured to generate a first and second intensity-modulated optical input signal; an optical phase shifter adapted to phase-shift the optical phase of the second intensity-modulated optical input signal relative to the optical phase of the first intensity-modulated optical input signal; a measurement unit adapted to measure the electrical output signal of the first transimpedance amplifier; and a control unit adapted to control the optical phase shifter.

According to a preferred embodiment, the optical device further comprises a third photodetector connected to a third optical output of the 90° optical hybrid unit, and a fourth photodetector connected to a fourth optical output of the 90° optical hybrid unit, the third optical output emitting a third optical output signal and the fourth output emitting a fourth optical output signal, said fourth optical output signal having an optical phase difference of 180° relative to the third optical output signal. The control unit may comprise low-pass filters adapted to low-pass filter the electrical output signals of the third and fourth photodetectors and to provide low-pass filtered signals. The control unit may further comprise an operational amplifier to which said low-pass filtered signals are coupled, said operational amplifier being adapted to control the optical phase of the phase-shifter in order to provide that said low-pass filtered signals have the same amplitude.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout.

It will be readily understood that the present invention, as generally described and illustrated in the figures herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, as represented inFIGS. 1-2, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.

FIG. 1shows an exemplary embodiment of a system comprising an optical device10and an evaluation device20which is configured to characterize the optical device10.

The optical device10comprises a 90° optical hybrid unit30having a first optical input30E1and second optical input30E2and four optical outputs30A1-30A4. The optical output signals So1-So4which leave the optical outputs30A1-30A4have optical phase differences between each other of 90° or multiple thereof.

The optical device10further comprises a first photodetector P1, which is connected to the first optical output30A1, and a second photodetector P2, which is connected to the second optical output30A2. The first optical output30A1emits the first optical output signal So1and the second optical output30A2emits the second optical output signal So2, the second optical output signal So2having an optical phase difference of 180° relative to the first optical output signal So1. A first transimpedance amplifier Tr1of the optical device10is connected to the first and second photodetectors P1and P2.

A third photodetector P3of the optical device10is connected to the third optical output30A3of the 90° optical hybrid unit, and a fourth photodetector P4is connected to a fourth optical output30A4of the 90° optical hybrid unit. The third optical output30A3emits the third optical output signal So3and the fourth optical output30A4emits the fourth optical output signal So4, said fourth optical output signal So4having an optical phase difference of 180° relative to the third optical output signal So3.

The evaluation device20includes a signal source110configured to generate a first intensity-modulated optical input signal Sin1and a second intensity-modulated optical input signal Sin2. As can be seen inFIG. 1, the signal source110comprises a laser111, an intensity modulator112and a 3 dB-splitter113.

The evaluation device20further includes an optical phase shifter120which can phase-shift the optical phase of the second intensity-modulated optical input signal Sin2relative to the optical phase of the first intensity-modulated optical input signal Sin1.

A control unit130of the evaluation device20controls the optical phase shifter120. The control unit130comprises low-pass filters140adapted to low-pass filter the electrical output signals I3and I4of the third and fourth photodetectors P3and P4, and to provide low-pass filtered signals I3′ and I4′. The frequency limit (cut-off-frequency) of the low-pass filter ranges preferably between 100 Hz and 10 kHz.

The control unit130further comprises an operational amplifier150to which the low-pass filtered signals I3′ and I4′ are coupled. The operational amplifier150controls the optical phase difference φ of the phase-shifter120and thus provides that the low-pass filtered signals I3′ and I4′ have the same amplitude.

The evaluation device20further includes a measurement unit160which measures the electrical output signal(s) of the first transimpedance amplifier TR1. The measurement unit160is connected to a central unit170of the evaluation device20. The central unit170evaluates the signals provided by the measurement unit160and controls the signal source110.

The system shown inFIG. 1may operate as follows:

The signal source110generates an optical reference signal Iref which is intensity-modulated at an intensity-modulation frequency by intensity modulator112. The intensity modulated reference signal Iref′ is inputted into 3 dB-splitter113which generates the first intensity-modulated optical input signal Sin1and the second intensity-modulated optical input signal Sin2.

The optical phase of the second intensity-modulated optical input signal Sin2is phase-shifted by optical phase shifter120. The optical phase difference between signals Sin1and Sin2decides about the intensity distribution of the optical output signals So1-So4as shown inFIG. 2. InFIG. 2, the intensity of the optical output signals So1-So4, which corresponds to the intensity of the electrical output signals I1-I4of the photodetectors P1-P4, is plotted versus the phase difference φ of the optical phase shifter120. Thus, by changing the phase difference φ, the optical intensity may be directed to a particular optical output30A1-30A4of the 90° optical hybrid unit30.

For instance, if the phase difference φ is 180°, the intensity of the optical output signal So1reaches its maximum and the intensity of the optical output signal So2reaches its minimum. This allows characterizing the amplitude and/or phase response of the optical device10with respect to the first optical output30A1by evaluating the output signal of the transimpedance amplifier Tr1.

If the phase difference φ is switched to 0°, the intensity of the optical output signal So2reaches its maximum and the intensity of the optical output signal So1reaches its minimum. Now, the amplitude and/or phase response of the optical device10may be evaluated with respect to the second optical output by evaluating the output signal of the transimpedance amplifier Tr1.

It is apparent that changing the phase difference φ allows characterizing the amplitude and/or phase response of the optical device either with respect to the first optical output or with respect to the second optical output, based on the output signal of transimpedance amplifier Tr1.

In order to determine an amplitude and/or phase response of the optical device at a particular amplitude (intensity) modulation frequency, the central unit170controls the intensity modulator112to intensity-modulate (i.e. amplitude-modulate) the reference signal Iref with said particular amplitude modulation frequency. The amplitude modulation frequency preferably ranges between 10 MHz and 100 GHz and is thus smaller than the optical frequency of the optical reference signal Iref.

Then, the phase difference φ of the optical phase shifter120is set to approximately 0° or to approximately 180° and further regulated by the operational amplifier150. The operational amplifier150regulates the optical phase difference φ such that the low-pass filtered signals I3′ and I4′ reach the same amplitude. As apparent fromFIG. 2, the low-pass filtered signals I3′ and I4′ will reach equal amplitudes of 0.5 at the same time when the optical output signal So1or the optical output signal So2reach the maximum amplitude of 1.0.

The phase regulation provided by operational amplifier150based on the low-pass filtered signals I3′ and I4′ makes sure that any unintentional phase shift in the optical path inside and outside the optical device10does not influence the evaluation of the amplitude and/or phase response of the optical device at the amplitude modulation frequency. For instance, temperature changes or mechanical influences may change the optical transmission between the signal source110and the optical device10and the optical transmission inside the optical device10and thus the optical intensities at the optical outputs30A1-30A4. Temperature changes or mechanical influences are usually very slow and do not exceed the cut-off frequency (preferably between 100 Hz and 10 kHz) of the low-pass filters140. Therefore, signal changes at the optical outputs30A3and30A4based on these influences will reach the operational amplifier150which counteracts. The operational amplifier150regulates the optical phase shifter120such that the low-pass filtered signals I3′ and I4′ maintain their constant amplitude of 0.5. As such the intensity at the optical outputs30A1and30A2may be measured and evaluated as mentioned above.

In order to evaluate the amplitude and/or phase response of the optical device10at a particular amplitude modulation frequency, the central unit170controls the intensity modulator112to intensity-modulate (i.e. amplitude-modulate) the reference signal Iref.

Then, the central unit170may determine a ratio between the amplitude of the electrical output signal of the first transimpedance amplifier Tr1and the amplitude of the intensity-modulation of the optical reference signal Iref′, or a signal proportional thereto, for the intensity-modulation frequency. The ratio characterizes the amplitude response of the optical device for the intensity-modulation frequency with respect to the first optical output30A1.

Alternatively or additionally, the central unit170may determine a phase difference value, which indicates the phase difference between the phase of the electrical output signal of the first transimpedance amplifier Tr1and the phase of the intensity-modulation of the optical reference signal Iref′, or a signal proportional thereto, for the intensity-modulation frequency. The phase difference value characterizes the phase shift of the optical device10for the intensity-modulation frequency with respect to the first optical output30A1.

After determining the ratio and/or the phase difference value with respect to the first optical output30A1, the input signals of the operational amplifier150may be swapped. This allows determining a second ratio between the amplitude of the electrical output signal of the first transimpedance amplifier Tr1and the amplitude of the intensity-modulation of the optical reference signal, or a signal proportional thereto, for the intensity-modulation frequency. The second ratio characterizes the amplitude response of the optical device10for the intensity-modulation frequency with respect to the second optical output30A2.

Alternatively or additionally, it is possible to determine a second phase difference value between the phase of the electrical output signal of the first transimpedance amplifier Tr1and the phase of the intensity-modulation of the optical reference signal, or a signal proportional thereto, for the intensity-modulation frequency. The second phase difference value characterizes the phase shift of the optical device10for the intensity-modulation frequency with respect to the second optical output30A2.

InFIG. 1the evaluation device20is used to characterize the optical device10with respect to the optical outputs30A1. Furthermore, the optical output30A2may be characterized if the connections of the photodiode P3and P4to the control unit130are swapped. In the same fashion, the optical outputs30A3and30A4may be characterized if the measurement unit160of the evaluation device20is connected to transimpedance amplifier Tr2and if the control unit130is connected to photodetectors P1and P2. The control unit130is used again to filter (or suppress) disturbing low-frequency phase shifts such as those generated by thermal or mechanical effects.

REFERENCE SIGNS