Method and system for receiving an optical-duo-binary signal The invention relates to methods and devices for receiving an optical-duo-binary, ODB, signal having a predefined ODB- transmission bit-rate.

Background of the invention

Due to severe optical filtering performed by today's DWDM-ODB transmission systems, the ratio between bit-rate and optical signal bandwidth may reach values of up to 1.4 bit/Hz or even more. For instance, an optical signal having an optical band ^¬ width (FWHM) of 32 GHz may transmit a 44.6 Gb/s data bit stream.

Objective of the present invention

An objective of the present invention is to provide a method for receiving optical-duo-binary signals where an optimized sensitivity performance in case of a high ratio between bit- rate and optical bandwidth may be achieved.

A further objective of the present invention is to provide a photoreceiver which may achieve an optimized sensitivity per- formance in case of a high ratio between bit-rate and optical bandwidth .

A further objective of the present invention is to provide an optical system which comprises a photoreceiver and an optical pre-filter, and achieves an optimized sensitivity performance in case of a high ratio between bit-rate and optical band ^¬ width . Brief summary of the invention

An embodiment of the present invention relates to a method of receiving an optical-duo-binary, ODB, signal having a predefined ODB- transmission bit-rate, said method comprising the step of filtering the ODB signal using a filter which provides a frequency peak in the photoreceiver ' s frequency re ^¬ sponse located in the spectral range between 30% and 70% of the predefined ODB-transmission bit-rate. An advantage of this embodiment is that unsurpassed optical signal-to-noise (OSNR) values may be achieved. This is due to the inventive frequency peak. A novel modeling and simulation of the ODB transmission system as carried out by the inventor showed a correlation existing between a properly peaked fre- quency response of the photoreceiver and an optimized OSNR sensitivity for a given bit-rate and optical filter band ^¬ width. For simulation, an Adaptive Inverse Filter (AIF) has been introduced to compensate for the optical filter rolloff, accomplishing the quasi-optimum raised-cosine pulse sequence at the decision section. At low OSNR, the thermal noise, generated by the photoreceiver plays almost a negligible role and the consistent noise bandwidth increase due to the highly peaked AIF photoreceiver response almost does not degrade the SNR at the decision section, compared to the benefit in terms of the pulse reshaping and strong reduction of the intersym- bol interference. The optical noise converted into signal- spontaneous beat noise density at the photoreceiver input ex ^¬ tends over whatever is smaller between the half-width baseband optical bandwidth and the electrical noise bandwidth, and the total noise results almost independent from the photoreceiver noise bandwidth. For instance, with the proposed frequency peak in the photoreceiver ' s frequency re ^¬ sponse, 12 dB OSNR sensitivity at 44,6 Gb/s may be achieved. According to a preferred embodiment, the frequency peak level is between 5dB and 7dB over the DC-value. The DC-value refers to the frequency response for an input signal having a fre- quency of 0 Hz.

The received ODB signal may be filtered in the optical domain with a Delay Line Interferometer filter having a free spectral range corresponding to between 60% and 140%, more pref- erably between 80% and 120%, of the ODB- transmission bit- rate .

In order to reliably achieve the described peak in the photo- receiver's frequency response in a cost-efficient manufactur- ing process, a Mach- Zehnder-Interferometer, MZI, filter may be employed.

For instance, the optical ODB signal may be split into a first signal portion and a second signal portion, wherein the first signal portion is filtered by a Mach-Zehnder-

Interferometer, MZI, filter having a free spectral range corresponding to between 60% and 140%, more preferably between 80% and 120%, of the ODB- transmission bit-rate. Then, the filtered first signal portion and the second signal portion may be added by an optical coupler having two exit ports, each of which being connected to a photodiode. The added sig ^¬ nal portions at each exit port may be detected with the re ^¬ spective photodiode. In this embodiment, a frequency peak level of 6dB may be achieved at a frequency corresponding to half of the predefined ODB-transmission bit-rate.

The anode- terminals of both photodiodes may be connected with each other and with the input terminal of a transimpedance amplifier. As such, the transimpedance amplifier may amplify the added signals.

Even though the splitting ratio of the optical couplers may range between 20% and 80%, a splitting ratio of 50% or at least approximately 50% is preferred. In other words, the couplers may be 3dB- couplers .

Before filtering the ODB signal, the ODB signal may be pre- filtered using an optical line pre-filter having a full- width-half-maximum bandwidth of 75% or less of the ODB- transmission bit-rate.

A further embodiment of the present invention relates to a photoreceiver adapted for receiving an optical -duo-binary, ODB, signal having a predefined ODB-transmission bit-rate. The photoreceiver comprises: at least one photodetector , and a filter providing a frequency peak in the photoreceiver ' s frequency response located in the spectral range correspond- ing to between 30% and 70% of the predefined ODB-transmission bit-rate .

The frequency peak level is preferably between 5dB and 7dB, more preferably 6dB, over the DC-value.

The frequency peak is preferably located at a frequency cor ^¬ responding to half of the predefined ODB-transmission bit- rate . The filter may comprise a Delay Line Interferometer structure having a free spectral range corresponding to between 60% and 140%, more preferably between 80% and 120%, of the ODB- transmission bit-rate. The filter may comprise a Mach-Zehnder-Interferometer having a free spectral range, wherein the free spectral range of the Mach- Zehnder-Interferometer corresponds to the ODB- transmission bit-rate.

The filter preferably comprises a first coupler having a first output and a second output, said first output being connected to an input of the Mach-Zehnder-Interferometer, a second coupler having a first input and a second input, said first input being connected to the output of said Mach- Zehnder-Interferometer, and said second input being connected to said second output of the first coupler. The first and second couplers preferably have coupling ratios between 20% and 80%. For instance, the couplers are 3dB- couplers .

A further embodiment of the present invention relates to a photoreceiver adapted for receiving an optical -duo-binary, ODB, signal having a predefined ODB-transmission bit-rate, said photoreceiver comprising at least one photodetector, and a at least one filter, wherein said filter comprises

- a first coupler having a first output and a second output, said first output being connected to an input of a Mach- Zehnder-Interferometer, said Mach-Zehnder-Interferometer having a free spectral range,

- a second coupler having a first input and a second input, said first input being connected to the output of said Mach- Zehnder-Interferometer, and said second input being connected to said second output of the first coupler. The free spectral range of the Mach-Zehnder-Interferometer preferably corresponds to the ODB-transmission bit-rate.

A further embodiment of the present invention relates to an optical system comprising a photoreceiver adapted for receiving optical-duo-binary signals, and an optical pre-filter, wherein said photoreceiver comprises at least one photodetec- tor, and a filter providing a frequency peak in the photore ^¬ ceiver 's frequency response, said frequency peak being lo- cated in the spectral range between 50% and 80% of the pre- filters bandwidth and having a peak level between 5dB and 7dB over the DC-value.

Brief description of the drawings

In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily un ^¬ derstood, a more particular description of the invention briefly described above will be rendered by reference to spe ^¬ cific embodiments thereof which are illustrated in the ap- pended drawings. Understanding that these drawings depict on ^¬ ly typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which

Figure 1 shows a first exemplary embodiment of a photoreceiver according to the present invention ;

Figure 2 shows an exemplary embodiment of an in ^¬ terferometer filter for the photoreceiver shown in Figure 1 ; Figure 3 shows the modulus of the transfer func ^¬ tion ^H » of the photoreceiver of Figure 1 in an exemplary fashion;

Figure 4 shows the simulated spectrum of the re ^¬ sponse of the photoreceiver of Figure 1 in an exemplary fashion;

Figure 5 shows an eye-diagramm of the photore ^¬ ceiver of Figure 1 in an exemplary fashion; shows the simulated OSNR sensitivity ver sus response of the photoreceiver of Fig ure 1 in an exemplary fashion; and

Figure 7 shows an exemplary embodiment of an opti ^¬ cal system according to the present invention .

Detailed description of the preferred embodiment

The preferred embodiment of the present invention will be best understood by reference to the drawings, wherein identi ^¬ cal 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 in ^¬ vention, as represented in Figures 1-7, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the in ^¬ vention .

Figure 1 shows a first exemplary embodiment of a photore- ceiver 5 which is capable of receiving an optical-duo-binary, ODB, signal S. The optical signal S carries a predefined ODB- transmission bit-rate.

The photoreceiver 5 comprises an interferometer filter 10, a back-to-back sum photodiode pair unit SPD and a single-ended transimpedance amplifier TIA.

The photodiode pair unit SPD comprises two photodiodes Dl and D2. The anode-terminals A of both photodiodes Dl and D2 are connected with each other and with the input terminal I of the transimpedance amplifier TIA. Thus, the transimpedance amplifier may amplify the sum of the signals that are pro ^¬ vided by both photodiodes Dl and D2. The interferometer filter 10 provides a frequency peak of 6dB in the photoreceiver ' s frequency response located at a fre ^¬ quency which is equal to half of the predefined ODB- transmission bit-rate. An exemplary embodiment of the interferometer filter 10 of Figure 1 is shown in Figure 2. The interferometer filter 10 is a delay line interference filter having a Mach- Zehnder- Interferometer 20, a first coupler 30, and a second coupler 40.

As shown in Figure 2, the first coupler 30 comprises a first output 31 and a second output 32. The first output 31 is con ^¬ nected to an input 21 of the Mach-Zehnder-Interferometer 20. The second coupler 40 comprises a first input 41 and a second input 42. The first input 41 is connected to the output 22 of the Mach-Zehnder-Interferometer 20. The second input 42 is connected to the second output 32 of the first coupler 30.

The Mach-Zehnder-Interferometer 20 comprises two interferome ^¬ ter arms 23 and 24. One of those interferometer arms, for instance interferometer arm 23, comprises a delay element 25 which provides a delay time T and thus a wavelength-dependent phase shift between both interferometer arms 23 and 24. The free spectral range FSR (FSR=1/T) of the Mach-Zehnder- Interferometer 20 is preferably equal to the predefined ODB- transmission bit-rate B. For instance, for a 44,6 Gb/s photo- receiver, the free spectral range would preferably be FSR = 1/T = B = 44,6 GHz .

A first signal portion SP1 of the optical signal S is fil ^¬ tered by the Mach-Zehnder-Interferometer 20 according to the free spectral range. The filtered first signal portion and a second signal portion SP2, which passes through waveguide 50, are added by the second coupler 40 which forwards the added signals SA1 and SA2 to the photodiodes Dl and D2 in Figure 1. The optical couplers 30 and 40 provide a low frequency path to the optical field. Without the optical couplers 30 and 40, the frequency response would exhibit a sine profile with a null at DC and every multiple of the FSR-value. In the following, it is assumed that all optical couplers are balanced without insertion loss. The baseband equivalent (slowly varying envelope) field transfer function between input port 11 (see Figures 1 and 2) of the interferometer fil- ter 10 and both output ports 12 or 13 (see Figures 1 and 2) of interferometer filter 10 are respectively:

Η ₂₁ (ω) = -j ^~ H ₂₁ (ω) +ΐ

(1)

The transfer function of the optical intensity envelope (OIE) at each output 12 and 13 is given by the square modulus of the expressions in equation (1) : («)f = ^[l+ 3 sin r/2)] , H ₂₂ (w) =- ¹ _COS ² (wT/2)

The transfer function Ι(ω) between the sum of the output photocurrents and the optical intensity envelope applied at the input port 11 is obtained by multiplying and in equation (2) by the respective responsivities and summing the resulting photocurrents. In the following, the same responsivity is assumed for both photodiodes :

Ι(ω) =

At DC, the interferometer loses 6dB. The first peak is at

wh _ere the oiE transfer function reaches the unit value. Subsequent peaks and valleys are interleaved by the same frequency interval. Any consecutive peaks or valleys are separated by the free spectral range FSR of the interferome- ter, hence FSR = \/T . The transimpedance amplifier TIA as shown in Figure 1 pro ^¬ vides a transimpedance gain with a relatively flat frequency response ^ζ { ^ω ) . The resulting transfer function ^Η *( ^ω of the interferometric photoreceiver 5 may then be obtained by mul ^¬ tiplying the transfer function Ι(ω) in equation (3) with the transimpedance gain Z (ω) of the transimpedance amplifier TIA:

Η-(ω)= -R ri+ sin ² (a) 772)1z (a))

2 V < V > ₍₄₎

Figure 3 shows the modulus of the transfer function ^Η *( ^ω ) of the interferometric photoreceiver 5. The peak occurs at half FSR and is 6dB high with respect to the DC value. The trans- impedance amplifier TIA cutoff is supposed to equal B=l/T.

Figures 4 and 5 show the simulated output of the interfer- ometric photoreceiver 5 of Figure 1. The simulation was carried out using a Matlab® based simulator which has been de- veloped for this purpose by the inventor. The ODB transmis ^¬ sion system operates at B=44.6 Gb/s and the interferometer has a free spectral range FSR=44.6 GHz. The transimpedance amplifier TIA is modeled in an exemplary fashion with a fourth-order Butterworth amplifier with 35 GHz cutoff fre- quency.

Figures 4 and 5 confirm that the frequency response of photo ^¬ receiver 5 shows the expected 6dB peak at half bit-rate fre ^¬ quency. At OSNR=16dB the photoreceiver shows a bit error rate BER=2.8e-6, corresponding to as low as OSNR=11.95dB at pre- FEC BER=2.0e-3. An interesting feature of the photoreceiver 5 is the highly insensitive behavior of the OSNR sensitivity versus the cut ^¬ off frequency of the transimpedance amplifier TIA. This prop- erty is attractive for increasing production yield while re ^¬ laxing costs.

Figure 6 shows the simulated OSNR sensitivity of the 44.6 Gb/s ODB photoreceiver 5 of Figure 1 (with FSR=B) , assuming a fourth-order Butterworth response profile of the transimpedance amplifier TIA. The calculations show that, when the cutoff exceeds 30 GHz, the OSNR sensitivity remains at the minimum value of 11.95 dB, even increasing the cutoff over 50GHz. This confirms that the sensitivity performances of the interferometric photoreceiver 5 are almost decoupled from the high frequency TIA response.

The graph further shows the baseline OSNR=16.9 dB referred to the received ODB pattern (RDB) and assuming infinite band- width photodetection process without any additional noise term (thermal, RIN,...) . It is apparent that a large OSNR sen ^¬ sitivity gain of about 5dB may be achieved with the peaked interferometric photoreceiver 5 of Figure 1, compared to an OSNR=16.9 dB obtained with a conventional infinite flat re- sponse and noiseless photodetection.

In summary, the interferometric photoreceiver 5 of Figure 1 provides a high spectral efficiency and a high OSNR sensitiv ^¬ ity. A gain of about 5dB may be achieved at a bit-rate of 44.6 Gb/s and an optical signal bandwidth of 32 GHz.

Figure 7 shows an exemplary embodiment of an optical system 100 which comprises the photoreceiver 5 of Figure 1. The in- terferometer filter 10 of photoreceiver 5 may correspond to the interferometer filter 10 as shown in Figure 2, for instance. As such the photoreceiver 5 is capable of receiving optical-duo-binary signals.

The optical system 100 further comprises a DWDM-filter unit 110 which has at least one optical line pre-filter 120. The optical line pre-filter 120 passes the optical channel S, which is meant to be received by the photoreceiver 5, and blocks all other channels. For receiving an optical-duo- binary signal S, which has an ODB-transmission bit-rate B (e.g. B = 44,6 Gb/s), the optical bandwidth fo of the optical line pre-filter 120 is preferably about 70% of B (e.g. fo = 0, 7 *44, 6 10 ⁹ 1/s = 32 GHz) .

The frequency response of the photoreceiver 5 preferably com ^¬ prises a frequency peak located in the spectral range between 50% and 80%, preferably between 65% and 75%, of the pre- filters bandwidth (e.g. 32 GHz) . This corresponds to about half of ODB-transmission bit-rate of 44,6 Gb/s. The peak level is preferably between 5dB and 7dB over the DC-value.