WO2019117761A1

WO2019117761A1 - A coherent optical receiver, a control arrangement and method therein for controlling the frequency of a local oscillator

Info

Publication number: WO2019117761A1
Application number: PCT/SE2017/051242
Authority: WO
Inventors: Boris Dortschy; Adriana ROSA DORTSCHY
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2017-12-11
Filing date: 2017-12-11
Publication date: 2019-06-20

Abstract

Embodiments herein relate to a method performed by a control arrangement (220) for controlling the frequency of a local oscillator (230) in a coherent optical receiver (200). The control arrangement (220) obtains two or more at least partially different frequency 5 bands of a signal corresponding to a first optical signal (ES+LO) in the coherent optical receiver (200). The control arrangement (220) also determines the power content of the signal in each of the two or more at least partially different frequency bands. The control arrangement (220) then controls the frequency of the local oscillator (230) based on the determined power content of the signal in each of the two or more at least partially 10 different frequency bands. Embodiments of a control arrangement (220) for controlling the frequency of a local oscillator (230) in a coherent optical receiver (200) is also described. Embodiments herein also relate to a coherent optical receiver (200) comprising the control arrangement (220).

Description

A COHERENT OPTICAL RECEIVER, A CONTROL ARRANGEMENT AND METHOD THEREIN FOR CONTROLLING THE FREQUENCY OF A LOCAL OSCILLATOR

TECHNICAL FIELD

Embodiments herein relate to optical transmissions in an optical fiber network. In particular, embodiments herein relate to a control arrangement and method therein for controlling the frequency of a local oscillator in a coherent optical receiver. Also, embodiments herein relate to a coherent optical receiver comprising a control

arrangement.

BACKGROUND

Optical transmission in an optical fiber network is commonly performed by optical transmitters capable of converting electronic signals into optical signals and transmitting the optical signals via an optical fiber, and by optical receivers capable of receiving the optical signals via the optical fiber and converting the optical signals into electronic signals. An optical transmitter and an optical receiver may also be comprised in a single optical transceiver unit or modules for transmitting and receiving optical signals over the same or separate optical fibers.

One category of optical receivers are referred to as coherent optical receivers. Coherent optical receivers implement the radio receiver concept of down-mixing of high- frequency carrier signals to frequencies located in a lower frequency range. In this case, down-mixing a high-frequency optical carrier signal to frequencies in the radio-frequency domain. In order to do so, a coherent optical receiver requires a coherent reference field, which may be provided by a local oscillator, LO, that generates the coherent reference field locally in the coherent optical receiver by using a laser source. An advantage of using coherent optical receivers is that it provides an improved detection performance. Coherent optical receiver may operate at shot-noise limits, and provide magnitudes higher affordable loss budgets as compared to non-coherent receivers. This is because the LO in coherent optical receivers ideally imposes a gain equal to the square-root of the LO signal power that is not attenuated in the same way as the transmitted optical signal after the optical signal transmission over an attenuating optical distribution channel, i.e. after transmission via the optical fiber.

Conventionally, coherent optical receivers may be divided into three different detection types: homodyne detection, heterodyne detection and intradyne detection, wherein the latter may be considered a kind of overlapping case of the before mentioned types.“Coherent detection in optical fiber systems”, Ezra Ip, Alan Pak Tao Lau, Daniel J.

F. Barros, Joseph M. Kahn, OPTICS EXPRESS, vol. 16, no. 2, pp. 753-791 , Jan. 21 ,

2008 provides an extensive overview about coherent detection in optical fiber systems, and in particular, section 3.4 explains homodyne and heterodyne detection.

For example, assume that fe and to are the optical carrier frequencies of the optical signal and the locally generated LO signal, respectively. In heterodyne detection, it is assumed that fe ¹ to. This means that the resulting electrical signal after photo detection will appear at an intermediate frequency (IF) carrier having a carrier frequency at†IF = abs(f_Es - to). In homodyne detection, it is instead assumed that fe = to. This means that the resulting electrical signal after photo detection will have a carrier frequency,† = 0. In this case, the signal may be referred to as being mixed down to the baseband. In case of intradyne detection, it is assumed fe ~ to. This means that the resulting electrical signal after photo detection will have a low carrier frequency,† .

Homodyne detection in a coherent optical receiver has some advantages over heterodyne detection in terms of received information signal power, required bandwidth for Analog-to-Digital, A/D, conversion, and in the further digital signal processing.

However, the homodyne detection comprises a direct down-conversion of the information signal that is modulated on the optical carrier signal. This means that any remaining and impairing offset between optical carrier signal and frequency of the locally generated LO signal is difficult, if not impossible, to deal with in homodyne receivers. On the contrary, heterodyne detection in a coherent optical receiver may comprise digital signal processing on RF frequencies after the optical detectors, and thus have the ability to correct impairments, such as, for example, distortion and imperfect offsets. Heterodyne detection in a coherent optical receiver, however, requires bandwidths that increase with the information symbol rate. This means that heterodyne detection is limited to very low bandwidth systems or comes at a very high cost.

However, it should be noted that independent of the detection type of the coherent optical receiver, rather intense digital signal processing is required after the A/D conversion of the output signal of the coherent optical receiver in order to be able to gain from the advantages of using a coherent systems, such as, e.g. loss budget and spectral/wavelength locality.

Fig. 1 shows an example of a coherent optical receiver 100. An optical signal, Es, is received in the coherent optical receiver 100 over an optical fiber 101. The coherent optical receiver 100 also locally generates a LO signal, ELO, via a local oscillator laser 120. The optical signal, Es, and the LO signal, ELO, are inputted into a 3x3 optical coupler device 110. The 3x3 optical coupler device 110 outputs the combined optical signal,

ES+LO, to a number of photo detectors 1 1 1 . The resulting electrical signal outputted by the photo detectors 111 is then low-pass filtered and squared by the low-pass filters 121 and squaring units 122, respectively, before being summarized by the summation unit 123.

The resulting summarized signal is then low-pass filtered again by another low-pass filter 124 before being outputted. This type of coherent optical receivers, which relies on simple Amplitude Shift Keying, ASK, modulation, was suggested as part of a project referred to as“FP7 COCONUT project for optical access solutions over standard passive optical networks (PON)”. This is described in more detail in, for example,“Polarization- Independent Receivers for Low-Cost Coherent OOK Systems”, Ernesto Ciaramella, IEEE PHOTONICS TECHNOLOGY LETTERS, vol. 26, no. 6, pp. 548-551 , March 15, 2014.

The coherent optical receiver 100 shown in Fig. 1 has been found to be very cost efficient compared to other conventional coherent optical receivers. This is mainly due to the purely analogue implementation of its post-processing, which means that no intensive digital signal processing after the A/D conversion of the output signal of the coherent optical receiver, normally required in other conventional coherent optical receivers, is used.

It is noted that it would be advantageous further improve the performance in conventional coherent optical receivers, preferably without incurring any significant additional costs.

SUMMARY

It is an object of embodiments herein to provide an improved performance of a coherent optical receiver.

According to a first aspect of embodiments herein, the object is achieved by a method performed by a control arrangement for controlling the frequency of a local oscillator in the coherent optical receiver. The control arrangement obtains two or more at least partially different frequency bands of a signal corresponding to a first optical signal in the coherent optical receiver. The control arrangement also determines the power content of the signal in each of the two or more at least partially different frequency bands.

Furthermore, the control arrangement controls the frequency of the local oscillator based on the determined power content of the signal in each of the two or more at least partially different frequency bands. According to a second aspect of embodiments herein, the object is achieved by a control arrangement for controlling the frequency of a local oscillator of the coherent optical receiver. The control arrangement comprises obtaining means configured to obtain two or more at least partially different frequency bands of a signal corresponding to a first optical signal in the coherent optical receiver. The control arrangement also comprises determination and control means configured to determine the power content of the signal in each of the two or more at least partially different frequency bands. The control means is further configured to control the frequency of the local oscillator based on the determined power content of the signal in each of the two or more at least partially different frequency bands.

According to a third aspect of embodiments herein, the object is achieved by an coherent optical receiver comprising a control arrangement as described above.

By having a control arrangement in a coherent optical receiver capable of obtaining different frequency spectral parts of a signal and differentiating between the power contribution of each of the different frequency spectral parts of the signal, the control arrangement is able to control the frequency of the local oscillator in the coherent optical receiver such that the performance of the coherent optical receiver is consistently maintained. For example, in case the signal corresponds to an optical signal comprising a combination of a received optical signal and the optical signal of the local oscillator, the control arrangement is able to determine whether the frequency of the optical signal of the local oscillator is too high or too low compared to the received optical signal in order to maintain the performance of the coherent optical receiver, and thus adjust the frequency of the local oscillator signal accordingly. Hence, in consistently maintaining performance of the coherent optical receiver, the performance of coherent optical receivers is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the embodiments will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the accompanying drawings, wherein:

Fig. 1 is a schematic block diagram illustrating a coherent optical receiver

arrangement according to prior art, Fig. 2 is a schematic block diagram illustrating embodiments of a coherent optical receiver and a control arrangement therein,

Fig. 3 is a flowchart depicting embodiments of a method in a control

arrangement,

Fig. 4 is a graph illustrating an example of a power spectral distribution of a

received signal in embodiments of a control arrangement, and

Fig. 5 is a graph illustrating another example of a power spectral distribution of a received signal in embodiments of a control arrangement, and

Fig. 6 is a schematic block diagram depicting embodiments of a control

arrangement.

DETAILED DESCRIPTION

The figures are schematic and simplified for clarity, and they merely show details which are essential to the understanding of the embodiments presented herein, while other details have been left out. Throughout, the same reference numerals are used for identical or corresponding parts or steps.

As part of the developing of the embodiments described herein, it has been noted that the modulation formats and detection in coherent optical receivers normally requires the locally generated LO signal to be locked in frequency, and phase, to the signal carrier frequency, i.e. the carrier wavelength of the received optical signal. This is performed in order to reduce complexity or make further signal processing, such as, e.g. equalization and detection, feasible by means of standard digital signal processors, DSPs. However, locking the frequency and phase of a locally generated LO signal to the carrier frequency of the received optical signal usually requires an accurate, complicated and complex optical or electrical Phase-Locked Loop, PLL, in order for high-level modulation formats to be implemented in the coherent optical receiver. This is because a trade-off for a reasonable complexity in the electrical post-processing and detection domain is required. On the other hand, for lower-level modulation formats, and therefore often more robust modulation formats, such a high complexity in the coherent optical receiver may not be justified.

In addressing this issue, it was realized that in order for a coherent optical receiver to maintain its performance for some modulations formats, the frequency of the local oscillator in the coherent optical receiver need only be located within a certain frequency range. This does not mean, however, that it is required to be directly linked or locked to the carrier frequency or phase of the received optical signal as in conventional coherent optical receivers. For example, for the coherent optical receiver shown in Fig. 1 , it has been demonstrated that an optimal performance for a 1.25 Gbit/s optical signal transmission may be obtained with an IF frequency of about 1.5GHz, but may also be located within a certain IF range thereabout in order to maintain the performance of the coherent optical receiver.

Hence, this issue is addressed by the embodiments herein by providing a control arrangement in a coherent optical receiver that is capable of obtaining different frequency spectral parts of a signal in the coherent optical receiver, differentiating between the power contribution of each of the different frequency spectral parts of the signal, and control the frequency of the local oscillator in the coherent optical receiver based on the power distributions of each of the different frequency spectral parts of the signal. This may be performed such that the performance of the coherent optical receiver is consistently maintained, without having to directly link or lock the frequency of the local oscillator to the carrier frequency, and phase, of the received optical signal as in conventional coherent optical receivers.

An advantage of the embodiments herein is that a low-cost coherent optical receiver with reduced local oscillator frequency and phase lock complexity is obtained.

The embodiments herein may also advantageous be implemented by using various low- cost analogue signal processing means, such as, for example, analogue circuitry components or low-complexity microcontrollers, which also may be integrated with the photonic parts of the coherent optical receivers. A further advantage of the embodiments herein is a low power consumption.

Particularly, the embodiments herein advantageously also provide for a trade-off between complexity and measurement accuracy of the local oscillator frequency relative to the carrier frequency of the received optical signal, such as, e.g. the coherent optical receiver shown in Fig. 1. The trade-off may, for example, be dependent on the number of frequency bands that are measured when determining the different power distributions of the signal. The embodiments herein are also advantageously applicable to other modulation formats than ASK modulated signals, since the spectrum for other modulated optical carrier signals, such as, e.g. ASK/NRZ, PAM-4 and Duobinary modulated optical carriers, are similar.

Fig. 2 shows an example of an embodiment of a coherent optical receiver 200. The coherent optical receiver 200 arranged to be connected to an optical fiber 201. The optical fiber 201 may be part of a fiber optical network, such as, a Passive Optical Network, PON. The coherent optical receiver 200 is arranged to receive a transmitted optical signal Es via the optical fiber 201 .

The coherent optical receiver 200 comprises a local oscillator (LO) 230. The local oscillator 230 is configured to locally generate an optical signal ELO. The optical signal ELO may also be referred to as a coherent reference field signal. The local oscillator 230 may be an optical signal generator, such as, e.g., a laser.

The coherent optical receiver 200 further comprises an optical coupling 210. The optical coupling 210 may also be referred to as an optical coupler or hybrid. The optical coupling 210 is configured to receive the optical signal Es, from the optical fiber 201. The optical coupling 210 is also configured to receive the optical signal ELO from the local oscillator 230. The optical coupling 210 is configured to combine the received optical signal Es from the optical fiber 201 with the received optical signal ELO from the local oscillator 230 into a combined signal ES+LO. The optical coupling 210 may also be configured to provide the optical signal ES+LO to one or more optical detectors 211. The one or more optical detectors 211 may be photo diodes (PDs), photonic detectors, or other optical sensors configured to detect and receive optical signals. The one or more optical detector(s) 211 is arranged to convert the received optical signal ES+LO into a corresponding electrical signal E’S+LO, and output the corresponding electrical signal,

E’S+LO. The coherent optical receiver 200 further comprises a control arrangement 220 according to the embodiments described herein. The control arrangement 220 is arranged to receive the electrical signal E’S+LO, corresponding to the combined optical signal ES+LO, from the one or more optical detectors 21 1 . The control arrangement 220 is also arranged to control the frequency of the local oscillator 230 based on the received electrical signal E’S+LO corresponding to the optical signal ES+LO, from the one or more optical detectors 21 1 . This may comprise transmitting information and/or control signals 227 to the local oscillator 230. Embodiments of the control arrangement 220 is explained and described in more detail below with reference to Figs. 3-6.

The coherent optical receiver 200 is further arranged to output the electrical signal, E’S+LO, corresponding to the combined optical signal, ES+LO, for further signal processing and/or transmission.

Example of embodiments of a method performed by a control arrangement 220 for controlling the frequency of a local oscillator 230 in a coherent optical receiver 200 will now be described with reference to the flowchart depicted in Fig. 3. Fig. 3 illustrates an example of actions or operations which may be taken by the control arrangement 220 as shown in Fig. 2. The method may comprise the following actions.

Action 301

The control arrangement 220 obtains two or more at least partially different frequency bands of a signal E’S+LO corresponding to a first optical signal ES+LO in the coherent optical receiver 200. This means that the control arrangement 220 may differentiate the signal E’S+LO into different spectral parts or slices. Acquiring the suitable spectral parts or slice, i.e. copies of the received signal components, may depend on the structure of the coherent optical receiver 200. In general, and as shown in Fig. 2, the suitable spectral parts may be derived from the signal output of one or more optical detector(s) 211.

In some embodiments, the first optical signal ES₊LO may be a combined signal of a second optical signal Es and an optical reference field signal ELO. The second optical signal Es may be an optical signal received by the coherent optical receiver 200 via an optical fiber 201. The optical reference field signal ELO may be generated by the local oscillator 230 in the coherent optical receiver 200. For example, as shown in Fig. 2, the second optical signal Es and the optical reference field signal ELO may be combined via an optical coupling 210. Here, it should also be noted that the optical carrier frequency of the optical reference field signal ELO is determined by the frequency of the local oscillator 230.

Action 302

After obtaining the two or more at least partially different frequency bands in Action 301 , the control arrangement 220 determines the power content of the signal in each of the two or more at least partially different frequency bands. This means that the control arrangement 220 is able to differentiate between the power contribution of the different spectral parts of the received signal E’S+LO.

In some embodiments, the control arrangement 220 may compare the determined power content of the signal E’S+LO in a first frequency band of the two or more at least partially different frequency bands with the power content of the signal E’S+LO in at least one second frequency band of the two or more at least partially different frequency bands. Based on the comparison, the control arrangement 220 may determine if the frequency of the local oscillator 230 is to be increased, unchanged or decreased. Advantageously, the control arrangement 220 may, for example, measure the power content within the different spectrum parts using amplitude after envelope detection, perform the comparison using established and standard analogue components. This will preserve the cost- efficiency of the coherent optical receiver 200 by not having to use complex digital signal processing requiring costly Digital Signal Processors (DSPs) to be employed. An example of how the control arrangement 220 may perform the comparison and determine if the frequency of the local oscillator 230 is to be increased, unchanged or decreased is described below with reference to Figs. 4-5.

Fig. 4 shows a graph illustrating an example of the Power Spectral Density (PSD) of the received signal E’S+LO in the control arrangement 220. In this case, the received signal E’S+LO may, for example, be an ASK modulated signal such as an On-Off keying modulated signal. In this example, the control arrangement 220 has obtained three partially different frequency bands of the signal E’S+LO, i.e. a first, second and third frequency region. These may also be referred to as frequency or spectral parts or slices. Optionally, these may also be referred to as filter passband regions, since it is the frequencies within these frequency band that is separately filtered out and passed. The control arrangement 220 may then measure the signal power of each of the different spectral contents in each of the three frequency regions. This is illustrated by the arrows in each of the three frequency regions. Since it is assumed in this example that the ideal frequency operation range or point, i.e. the ideal IF frequency or target carrier frequency of the received signal E’S+LO, is in the centre or within the second frequency region, it can easily be seen in the graph in Fig. 4 that the frequency of the local oscillator 230 is currently suitable since the main part of the detected signal power of the received signal E’S+LO, is located within the second frequency region. Hence, in this case, there is no need for the control arrangement 220 to adjust the frequency of the local oscillator 230.

Fig. 5 shows another graph illustrating an example of the Power Spectral Density (PSD) of the received signal E’_S+LO in the control arrangement 220 similar to that in Fig. 4. However, in this case, since it is still assumed in this example that the ideal frequency operation range or point, i.e. the ideal IF frequency or target carrier frequency of the received signal E’S+LO, is in the centre or within the second frequency band, it can easily be seen in the graph in Fig. 5 that the frequency of the local oscillator 230 is currently unsuitable since the main part, or at least a large part, of the detected signal power of the received signal E’S+LO, is located outside of the second frequency region, namely in the third frequency region. Hence, this indicates to the control arrangement 220 that the frequency of the local oscillator 230 need to be adjusted in order to reduce the resulting or real IF frequency back towards the ideal IF frequency. Action 303

After the determination in Action 302, the control arrangement 220 controls the frequency of the local oscillator 230 based on the determined power content of the signal E’S+LO in each of the two or more at least partially different frequency bands. This means that the control arrangement 220 is able to adjust the frequency of the local oscillator 230 based the power contribution in each of the different spectral parts of the received signal E’S+LO such that the performance of the coherent optical receiver 200 is consistently maintained, i.e. decide whether to increase or decrease (or leave unchanged) the frequency of the local oscillator 230 in order to operate the coherent optical receiver 200 in its most performing operation range.

In other words, the control arrangement 220 is able to differentiate between the power contribution of particular spectral parts of the received signal E’S+LO, and determine if the power contribution of particular spectral parts of the received signal E’S+LO is lower or higher than, for example, a determined threshold value or compared to power

contributions in other spectrum parts. Thus, the control arrangement 220 is able to differentiate whether the frequency of the local oscillator 230 is too high or too low by comparing the power contributions in the different spectrum parts. The control

arrangement 220 may then use this information to adjust the frequency of the local oscillator 230 accordingly. As described in reference to Fig. 6 below, this may

advantageously be enabled via a slow and low complexity decision circuit. The resulting accuracy of this slow and low complexity decision circuit may not be high, but will be sufficient enough for most coherent optical receivers.

In some embodiments, the control arrangement 220 may control the frequency of the local oscillator 230 so as to maintain the carrier frequency of the signal E’_S+LO about a target carrier frequency. Here, the carrier frequency of the signal E’S+LO may correspond to the difference between the optical carrier frequency of the second optical signal Es and the optical carrier frequency of the optical reference field signal ELO, wherein the optical carrier frequency of the optical reference field signal ELO is determined by the frequency of the local oscillator 230. Here, it should be noted that the target carrier frequency, i.e. the ideal operating point or region of the frequency of the local oscillator 230, will depend on the determined modulation format and data rate of the second optical signal Es received via the optical fiber 201 in the coherent optical receiver 200. This may be determined and configured in the control arrangement 220 before starting up and operating the coherent optical receiver 200. To perform the method actions for controlling the frequency of a local oscillator 230 of the coherent optical receiver 200, the control arrangement 220 may comprise the following arrangements depicted in Fig. 6. Fig. 6 shows a schematic block diagram of embodiments of the control arrangement 220. The embodiments of the control arrangement 220 described herein may be implemented in a coherent optical receiver 200 as shown in Fig. 2.

The control arrangement 220 comprises obtaining means or modules 222, 223A, 223B, ... , 223N . The obtaining means or modules 222, 223A, 223B, ... , 223N may also be referred to as signal splitting and filtering means or modules. The obtaining means or modules 222, 223A, 223B,... , 223N are configured to obtain two or more at least partially different frequency bands of a signal E’S+LO corresponding to a first optical signal ES+LO in the coherent optical receiver 200. The control arrangement 220 further comprises determination and control means or modules 226, 224A, 224B,... , 224N, 225. The determination and control means or modules 226, 224A, 224B,... , 224N, 225 are configured to determine the power content of the signal in each of the two or more at least partially different frequency bands and control the frequency of the local oscillator 230 based on the determined power content of the signal E’S+LO in each of the two or more at least partially different frequency bands.

In some embodiments, the first optical signal ES₊LO may be a combined signal of a second optical signal Es and an optical reference field signal ELO, wherein the second optical signal Es is received by the coherent optical receiver 200 via an optical fiber 201 and the optical reference field signal ELO is generated by the local oscillator 230 in the coherent optical receiver 200. Also, in some embodiments, the signal E’S₊LO is an electrical signal originating from the signal output of one or more optical detectors 21 1 in the coherent optical receiver 200 configured to convert optical signals into electrical signals.

In some embodiments, the determination and control means 226, 224A, 224B,... , 224N, 225 may be further configured to compare the determined power content of the signal E’S+LO in a first frequency band of the two or more at least partially different frequency bands with the power content of the signal E’S+LO in at least one second frequency band of the two or more at least partially different frequency bands, and determine if the frequency of the local oscillator 230 is to be increased, unchanged or decreased based on the comparison.

Further, in some embodiments, the determination and control means 226, 224A, 224B,... , 224N, 225 may be further configured to control the frequency of the local oscillator 230 so as to maintain the carrier frequency of the signal E’S+LO about a target carrier frequency. In this case, according to some embodiments, the carrier frequency of the signal E’S+LO corresponds to the difference between the optical carrier frequency of the second optical signal Es and the optical carrier frequency of the optical reference field signal ELO, wherein the optical carrier frequency of the optical reference field signal ELO is determined by the frequency of the local oscillator 230.

As shown in Fig. 6, the obtaining means 222, 223A, 223B, ... , 223N may, in some embodiments, comprise a signal splitter unit 222 and two or more frequency filters 223A, 223E3, ... , 223N, and the determination and control means or modules 226, 224A, 224E3,... , 224N, 225 may, in some embodiments, comprise a microcontroller 226 or a circuit of analogue logic components 224A, 224E3, ... , 224N, 225. These embodiments are exemplified and described in more detail with reference to Fig. 6 below.

In Fig. 6, the control arrangement 220 is configured to receive a signal, E’S+LO, corresponding to a first optical signal, ES+LO, in the coherent optical receiver 200. The signal, E’S+LO, may be received as separate individual signals from each of the outputs of the one or more optical detector(s) 211 in the coherent optical receiver 200 or as an aggregated signal combining the output signals from the outputs of the one or more optical detector(s) 21 1 in the coherent optical receiver 200. The signal, E’S+LO, may be receive via an input 221 of the control arrangement 220.

The signal splitter unit 222 is configured to split or direct the received signal,

E’S+LO, towards each of the two or more frequency filters 223A, ... , 223N. The signal splitter unit 222 may consist of analogue components only.

Each of the two or more frequency filters 223A, ... , 223N are configured to filter the received signal E’_S+LO such that only the content of the signal E’S+LO having

frequencies within a determined frequency band of each of the two or more frequency filters 223A, ... , 223N is outputted from each of the two or more frequency filters 223A, ... , 223N. This means that each of the two or more frequency filters 223A, ... , 223N allows passing of only a certain part of the spectrum. The two or more frequency filters 223A, ... , 223N, may also be referred to as an filter bank; or, alternatively, as a parallel filter. The determined frequency bands of the two or more frequency filters 223A, ... , 223N filters are at least partially different. This means that the frequency bands of the two or more frequency filters 223A, ... , 223N filters may comprise partially overlapping frequencies and/or comprise non-overlapping frequencies. The two or more frequency filters 223A, ... , 223N may consist of analogue components only. For example, the two or more frequency filters 223A, ... , 223N be realized by a simple LC resonator combination. The LC resonator combination may also be integrated, especially if the inductance of the LC resonator combination may be kept low, such as, e.g. for high centre frequencies.

It should also be noted that the two or more frequency filters 223A, ... , 223N may comprise any suitable combination of Low-Pass (LP) filters, High-Pass (HP) filters and Band-Pass (BP) filters. A LP filter is configured to remove all frequency content of the signal E’S+LO above a specific frequency, a HP filter is configured to remove all frequency content of the signal E’S+LO below a specific frequency, and a BP filter is configured to remove all frequency content of the signal E’S+LO below a specific frequency and above a specific frequency. For example, as illustrated in Figs. 4-5, the first frequency region may be implemented using a LP or BP filter, the second frequency region may be implemented using a BP filter, and the third frequency region may be implemented using a HP or BP filter.

Once the signal spectrum of the received signal E’S+LO has been differentiated into different parts/slices by the two or more frequency filters 223A, ... , 223N, the contained power content in each part/slice may be determined by the power measurement means or modules 224A, 224B,... , 224N. The power measurement means or modules 224A, 224B,... , 224N may measure the power content in each part/slice by determining amplitudes using, e.g. envelope detection. It should be noted, however, that other values indicative of the power contained in each part/slice of the spectrum of the received signal E’S+LO may be used as long as the measured values scale with the contained power in each spectrum slice.

Based on the power content in each of the parts/slices of the spectrum of the received signal E’S+LO received from the power measurement means or modules 224A, 224B,... , 224N, a decision logic means or module 225 may consist of a set of analogue comparators and logic gates configured to, for example, implement a truth table which may indicate when the control arrangement 220 should increase or decrease the frequency of the local oscillator 230. The decision logic means or module 225 may also be configured to output a control signal via the output 227 to the local oscillator 230 indicating whether the local oscillator 230 is to increase or decrease the frequency of the local oscillator 230 or whether to leave the frequency of the local oscillator 230 unchanged. Hence, it should here be noted that the power measurement means or modules 224A, 224B,... , 224N and the decision logic means or module 225 may be implemented as a circuit of analogue logic components 224A, 224B,... , 224N, 225.

However, it should also be noted that the power measurement means or modules 224A, 224B,... , 224N and the decision logic means or module 225 may be implemented in a microcontroller 226. In this case, the power measurement means or modules 224A, 224B, ... , 224N and the decision logic means or module 225 may be implemented in the microcontroller 226 as computer program code or program modules for performing the functions and/or method actions of the embodiments herein.

The microcontroller 226 may also be referred to as a microprocessor or micro processing circuitry. Several of the functional means or modules of the microcontroller 226 discussed may be provided through the use of dedicated hardware or analogue logic components, while others are provided with hardware or analogue logic

components for executing appropriate software or firmware. Thus, the term“processor” or“controller” as may be used herein does not exclusively refer to hardware capable of executing software. In some embodiments, several or all of the various functions of the control arrangement 220 may be implemented together, such as in a single application- specific integrated circuit (ASIC). From the above it may be seen that the embodiments may further comprise a computer program product, comprising instructions which, when executed on at least one microcontroller 226 cause the at least one

microcontroller 226 to carry out the method for controlling the frequency of a local oscillator 230 of the coherent optical receiver 200 in said control arrangement 220.

As mentioned earlier, the embodiments described above provide a control of the frequency of the local oscillator 230 in a low-cost coherent optical receiver with reduced frequency and phase lock complexity for the local oscillator 230. The embodiments advantageously also enable a control of the frequency of the local oscillator 230 without any analogue-to-digital, A/D, conversion by providing for a full low-cost analogue implementation.

It should also be noted that the embodiments herein provide for higher operational limits, i.e. symbol rates, than A/D based solutions at a lower cost.

The terminology used in the detailed description of the particular embodiments illustrated in the accompanying drawings is not intended to be limiting of the described optical receiver 200, control arrangement 220 and method therein which instead should be construed in view of the enclosed claims. As used herein, the term "and/or" comprises any and all combinations of one or more of the associated listed items.

Further, as used herein, the common abbreviation "e.g.", which derives from the Latin phrase "exempli gratia," may be used to introduce or specify a general example or examples of a previously mentioned item, and is not intended to be limiting of such item. If used herein, the common abbreviation "i.e.", which derives from the Latin phrase "id est," may be used to specify a particular item from a more general recitation. The common abbreviation“etc.”, which derives from the Latin expression "et cetera" meaning "and other things" or "and so on” may have been used herein to indicate that further features, similar to the ones that have just been enumerated, exist.

As used herein, the singular forms "a", "an" and "the" are intended to comprise also the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms "includes," "comprises," "including" and/or "comprising," when used in this specification, specify the presence of stated features, actions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, actions, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms comprising technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the described embodiments belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The embodiments herein are not limited to the above described preferred embodiments. Various alternatives, modifications and equivalents may be used.

Therefore, the above embodiments should not be construed as limiting.

Claims

1. A method performed by a control arrangement (220) for controlling the frequency of a local oscillator (230) in a coherent optical receiver (200), the method comprising obtaining (301) two or more at least partially different frequency bands of a signal (E’S+LO) corresponding to a first optical signal (ES+LO) in the coherent optical receiver (200);

determining (302) the power content of the signal in each of the two or more at least partially different frequency bands; and

controlling (303) the frequency of the local oscillator (230) based on the determined power content of the signal (E’_S+LO) in each of the two or more at least partially different frequency bands.

2. The method according to claim 1 , wherein the first optical signal (ES+LO) is a

combined signal of a second optical signal (Es) and an optical reference field signal (ELO), wherein the second optical signal (Es) is received by the coherent optical receiver (200) via an optical fiber (201) and the optical reference field signal (E_LO) is generated by the local oscillator (230) in the coherent optical receiver (200).

3. The method according to any of claims 1 or 2, wherein the controlling (303) further comprises comparing the determined power content of the signal (E’S+LO) in a first frequency band of the two or more at least partially different frequency bands with the power content of the signal (E’S+LO) in at least one second frequency band of the two or more at least partially different frequency bands, and determining if the frequency of the local oscillator (230) is to be increased, unchanged or decreased based on the comparison.

4. The method according to any of claims 1 -3, wherein the controlling (303) is

performed so as to maintain the carrier frequency of the signal (E’S+LO) about a target carrier frequency.

5. The method according to claim 4, wherein the carrier frequency of the signal

(E’S+LO) corresponds to the difference between the optical carrier frequency of the second optical signal (Es) and the optical carrier frequency of the optical reference field signal (ELO) , wherein the optical carrier frequency of the optical reference field signal (ELO) is determined by the frequency of the local oscillator (230).

6. A control arrangement (220) for controlling the frequency of a local oscillator (230) in a coherent optical receiver (200), the control arrangement (220) comprising

obtaining means (222; 223A, 223B,... , 223N) configured to obtain two or more at least partially different frequency bands of a signal (E’S+LO) corresponding to a first optical signal (ES+LO) in the coherent optical receiver (200), and

determination and control means (226; 224A, 224E3,... , 224N, 225) configured to determine the power content of the signal in each of the two or more at least partially different frequency bands and control the frequency of the local oscillator (230) based on the determined power content of the signal (E’_S+LO) in each of the two or more at least partially different frequency bands.

7. The control arrangement (220) according to claim 6, wherein the first optical signal (ES+LO) is a combined signal of a second optical signal (Es) and an optical reference field signal (ELO), wherein the second optical signal (Es) is received by the coherent optical receiver (200) via an optical fiber (201) and the optical reference field signal (ELO) is generated by the local oscillator (230) in the coherent optical receiver (200).

8. The control arrangement (220) according to claim 6 or 7, wherein the determination and control means (226; 224A, 224E3,... , 224N, 225) is further configured to compare the determined power content of the signal (E’S+LO) in a first frequency band of the two or more at least partially different frequency bands with the power content of the signal (E’S+LO) in at least one second frequency band of the two or more at least partially different frequency bands, and determine if the frequency of the local oscillator (230) is to be increased, unchanged or decreased based on the comparison.

9. The control arrangement (220) according to any of claims 6-8, wherein the

determination and control means (226; 224A, 224E3,... , 224N, 225) is further configured to control the frequency of the local oscillator (230) so as to maintain the carrier frequency of the signal (E’S+LO) about a target carrier frequency.

10. The control arrangement (220) according to claim 9, wherein the carrier frequency of the signal (E’S+LO) corresponds to the difference between the optical carrier frequency of the second optical signal (Es) and the optical carrier frequency of the optical reference field signal (ELO), wherein the optical carrier frequency of the optical reference field signal (ELO) is determined by the frequency of the local oscillator (230).

11. The control arrangement (220) according to any of claims 6-10, wherein the signal

(E’S+LO) is an electrical signal originating from the signal output of one or more optical detectors (211) configured to convert optical signals into electrical signals.

12. The control arrangement (220) according to any of claims 6-11 , wherein the

obtaining means (222; 223A, 223B,... , 223N) comprises a signal splitter unit (222) and two or more frequency filters (223A, 223B,... , 223N), and the determination and control means (226; 224A, 224B,... , 224N, 225) comprises a microcontroller (226) or a circuit of analogue logic components (224A, 224B,... , 224N; 225).

13. An coherent optical receiver (200) comprising a control arrangement (220)

according to any of claims 7-12.